FULL PAPER DOI: 10.1002/asia.201402280

Hydrogenation of Imines Catalyzed by Trisphosphine-Substituted Molybdenum and Tungsten Nitrosyl Hydrides and Co-Catalytic Acid Subrata Chakraborty, Olivier Blacque, Thomas Fox, and Heinz Berke*[a] Abstract: Hydride complexes Mo,W(CO)(NO)H(mer-etpip) (iPr2PCH2CH2)2PPh = etpip) (2 a,bACHTUNGRE(syn), syn and anti of NO and PhACHTUNGRE(etpip) orientions) were prepared and probed in imine hydrogenations together with co-catalytic [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] (140 8C, 60 bar H2). 2 a,bACHTUNGRE(syn) were obtained via reduction of syn/anti-Mo,W(NO)Cl3(meretpip) and syn,anti-Mo,W(NO)(CO)Cl(mer-etpip). [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] in THF converted the hydrides into THF complexes syn-[Mo,W(NO)(CO)-

(etpip)(THF)][BACHTUNGRE(C6F5)4]. Combinations of the p-substituents of aryl imines p-R1C6H4CH = N-p-C6H4R2 (R1,R2 = H,F,Cl,OMe,a-Np) were hydrogenated to amines (maximum initial TOFs of 1960 h 1 (2 aACHTUNGRE(syn)) and 740 h 1 (2 bACHTUNGRE(syn)) for N-(4-methoxybenzylidene)aniline). An ionic hydrogenation mechKeywords: hydrides · imines · ionic hydrogenation · molybdenum · tungsten

Introduction

for catalyses proceeding with more or less concerted proton and hydride transfers. The catalytic ionic hydrogenation with stepwise transfers is often also denoted as polar hydrogenation or protonic–hydridic catalysis.[3a] The polar H-atom transfers are mainly occurring in the secondary coordination sphere of transition metal centers, but were seen to also proceed “metal-free” at main group element centers of frustrated Lewis pairs.[3e–i] Such metal-induced reactions often take place in the secondary coordination sphere and are then found to be only moderately dependent on the type of the metal center. They hold therefore great potential for the catalytic application of a broad range of middle transition metal-based complexes and may eventually replace the platinum group metals Ru, Rh, and Ir, which still prevail in this area, in the respective homogeneous hydrogenations.[3a, 9] Recent years have seen tremendous efforts in the search for efficient inexpensive, abundant, and low-toxic middle transition element metal catalysts. In this regard and similar to iron-based catalytic systems,[2b, 10] molybdenum and tungsten catalysts deserve specific attention in the pursuit of the socalled “cheap metals for noble tasks”.[2a, 11] Catalytic hydrogenation employing Mo and W complexes started in 2000 by the pioneering work of Bullock and Voges,[12] who reported that MCp(CO)2ACHTUNGRE(PPh3)H (M = Mo, W) complexes can hydrogenate 3-pentanone in the presence of the trityl cation [Ph3C]ACHTUNGRE[BArF4] under mild conditions, albeit with a low turnover frequency (TOF) value of 2 h 1. Since then, there were several other reports on Mo- and Wcatalyzed polar hydrogenations of ketones.[13] However, these processes all lack high catalytic activities, particularly Mo- and W-complex-catalyzed homogeneous hydrogenations of the more challenging C=N functionality.[14, 15]

The development of efficient organometallic catalysts for homogeneous hydrogenation and their application in industrial processes have undoubtedly been notable achievements in the past decades. Most of the hydrogenation catalyses are dominated by Wilkinson- and Osborn-type reaction courses with the main characteristics of homolytic splitting of H2 to form dihydride complexes.[1] By contrast, ionic hydrogenation[2] catalyses proceed with heterolytic cleavage of H2 as the key step, forming chemically bound protons and hydrides followed by polar H-atom transfers to the unsaturated substrates. These H-atom transfers occur either stepwise or in a concerted or nearly concerted fashion.[3] Ionic hydrogenations are becoming progressively more effective and attractive as a hydrogenation method. In 1974, Kursanov[4] et al. first observed stoichiometric ionic hydrogenations using CF3COOH as a proton donor and Et3SiH as a hydride source. Since then several research groups utilized this concept for stoichiometric hydrogenations of various organic carbonyl compounds,[5a–c] alkenes,[5d,e] and alkynes,[5f] and the more challenging catalytic hydrogenation of C=N bonds.[6, 7] Noyori has coined the term metal-ligand bifunctionality[8]

[a] Dr. S. Chakraborty, Dr. O. Blacque, Dr. T. Fox, Prof. H. Berke Institute of Chemistry University of Zrich Winterthurerstrasse 190, CH-8057 Zrich (Switzerland) E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402280.

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anism based on linear Hammett plots (1 = 10.5, p-substitution on the C-side and 1 = 0.86, p-substitution on the Nside), iminium intermediates, linear P(H2) dependence, and DKIE = 1.38 is proposed. Heterolytic splitting of H2 followed by proton before hydride transfers are the steps in the ionic mechanism where H2 ligand addition is rate limiting.

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Recently, we employed the tridentate PNP ligand-based M(NO)(CO)ACHTUNGRE(PNP) {M = Mo, W; PNP = (iPr2PCH2CH2)2N)} system, which can split H2 heterolytically at room temperature and catalyze the hydrogenation of imines with a mechanism of polar H-atom transfer achieving a maximum TOF of 2912 h 1 in the hydrogenation of N-(4-methoxybenzylidene)aniline without using co-catalytic additives.[16] Furthermore, our group could demonstrate that complexes of the type Mo(NO)ACHTUNGRE(P^P)(CO)2H (P^P = bidentate ligand) perform moderately well in catalytic hydrogenations of imines at room temperature in the presence of the acid [HACHTUNGRE(Et2O)2] [BACHTUNGRE(C6F5)4], resulting in TOF values of up to 123 h 1.[15] An “ionic” mechanism was anticipated with stepwise proton before hydride H-atom transfers. These types of catalysts could not operate at temperatures higher than room temperature, thus restricting the optimization of the catalytic performance. Therefore, we set out to find new molybdenum and tungsten nitrosyl hydride catalysts stabilized by a thermally robust ancillary trisphosphine chelate ligand to approach ionic hydrogenation of imines that could potentially be conducted also at elevated temperature.

Scheme 1. Synthetic access to the syn/anti isomeric M(NO)Cl3(mer-etpip) (M = Mo, W) complexes and their reduction to M(0) carbonyl complexes.

conditions one chloride ligand stayed at the metal centers functioning as a replaceable anionic moiety. The 31P{1H} NMR spectra of the isomeric mixture of 1 aACHTUNGRE(syn,anti) revealed two conspicuous sets of doublets at d = 67.9 (2JPP = 7 Hz, 1 aACHTUNGRE(syn)) and 62.3 ppm (d, 2JPP = 9 Hz, PPh, 1 aACHTUNGRE(anti)) for terminal phosphorus atoms and two triplet sets at d = 79.5 (2JPP = 7 Hz, 1 aACHTUNGRE(syn)) and 58.9 ppm (t, 2JPP = 9 Hz, iPr2P, 1 aACHTUNGRE(anti)) for the central phosphorus atom, suggesting strongly the mer-arrangement of the etpip ligand and confirming the presence of the isomeric mixture. The signals in the 31P{1H} NMR spectra of the isomeric mixture of 1 bACHTUNGRE(syn,anti) were assigned as follows: d = 55.2 [2JPP = 5 Hz, 1 JPW = (d, satellites), 288 Hz, 1 bACHTUNGRE(syn)], 49 ppm [2JPP = 2.2 Hz, 1 JPW = (d, satellites), 281 Hz, 1 bACHTUNGRE(anti)], and two triplet sets at d = 72.6 [2JPP = 5 Hz,1JPW = (d, satellites), 215 Hz, 1 bACHTUNGRE(syn)] and 48.7 ppm [2JPP = 2.2 Hz, 1JPW = (d, satellites), 211 Hz, 1 bACHTUNGRE(anti)]. The 1H NMR spectra confirmed also the presence of the 1 a,bACHTUNGRE(syn,anti) isomers by exhibiting doubled, partly overlayed signals in the expected region for the methyl, methylene, and methyne protons. However, in the IR spectra the syn and anti isomers of 1 aACHTUNGRE(syn,anti) and 1 bACHTUNGRE(syn,anti) showed only one characteristic carbonyl and nitrosyl band at 1918 (nCO) and 1569 cm 1 (nNO) for 1 aACHTUNGRE(syn,anti) and at 1902 and 1559 cm 1 for 1 bACHTUNGRE(syn,anti) and could not be distinguished. Single crystals suitable for X-ray diffraction studies of 1 a,bACHTUNGRE(syn,anti) could be grown from toluene/pentane solutions at room temperature. The trans NO/Cl axes were found to be positionally disordered in the ratios of 0.76:0.24 (syn:anti) for 1 aACHTUNGRE(syn,anti) and of 0.83:0.17 (syn:anti) for 1 bACHTUNGRE(syn,anti). This NO/Cl disorder was interpreted in terms of the existence of distinct isomeric molecules in the crystal lattices. The isomers were taken up in the crystal roughly to the same relative amount as they exist in solution. The crystal lattice apparently does not significantly differentiate between the isomers of 1 a,bACHTUNGRE(syn) and 1 a,bACHTUNGRE(anti). The metal centers of the molecular structures possess pseudo-octahedral geometries with three

Results and Discussion In our previous work we have demonstrated access to hepta-coordinated syn/anti isomeric mixtures of complexes M(NO)Cl3(mer-etpip) (M = Mo, W) with the mer-tridentate (iPr2PCH2CH2)2PPh (etpip) ligand (syn and anti refers to the relative position of the NO ligand with respect to the phenyl ring at Pinternal of the etpip ligand), reacting M(NO)Cl3 ACHTUNGRE(NCMe)2 with 1 equivalent of etpip at room temperature in THF.[17] The syn/anti mixtures of the M(NO)Cl3(mer-etpip) complexes (Mo and W) could be fully characterized by NMR and IR spectroscopies as well as X-ray structures.[17] Olefin hydride complexes with this framework could be obtained and were found suitable catalyst precursors for efficient olefin hydrogenations. Herein we apply the syn/anti isomeric mixtures of M(NO)Cl3(mer-etpip) complexes (M = Mo, W) to prepare low-valent nitrosyl carbonyl chloride complexes through reductions in the presence of CO and to eventually prepare the related carbonyl hydride derivatives, which are supposed to be active in imine hydrogenations. Preparation of the Isomeric [M(NO)(CO)Cl(mer-etpip)] Complexes (M = Mo, 1aACHTUNGRE(syn,anti); W, 1bACHTUNGRE(syn,anti)) The isomeric mixtures of M(NO)Cl3(mer-etpip) (M = Mo, W) were reacted with excess of 1 % Na/Hg (5 equiv) in the presence of carbon monoxide (1 bar) at room temperature, which resulted in the formation of isomeric mixtures of the yellow diamagnetic M(0) complexes Mo(0)(NO)(CO)Cl(mer-etpip) (1 aACHTUNGRE(syn), 75 %; 1 aACHTUNGRE(anti), 25 % based on 31P{1H} NMR) and W(0)(NO)(CO)Cl(mer-etpip) (1 bACHTUNGRE(syn), 73 %; 1 bACHTUNGRE(anti), 27 % based on 31P{1H} NMR) in 65 % and 60 % overall isolated yields, respectively (Scheme 1). Under these

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reoselective reaction to the syn isomers was surprising since the starting complexes contained both isomers of 1 a,bACHTUNGRE(syn) and 1 a,bACHTUNGRE(anti) in a ratio of approximately 7:3. Pursuing the reactions in Et3N solvent and monitoring the 31P{1H} NMR spectra at 90 8C, we again found only product signals of 2 a,bACHTUNGRE(syn). Re-recording these 31P{1H} NMR spectra then at 60 8C confirmed the absence of signals for 2 a,bACHTUNGRE(anti). Thus, we have to assume that the anti isomer can either be converted into the corresponding thermodynamically more stable syn isomer in the higher temperature regime[17] or that the hydride formation is a kinetically controlled reaction with preference for the syn chloride/hydride exchange. The 1H NMR spectra of 2 a,bACHTUNGRE(syn) at room temperature revealed a doublet of triplet for the hydride ligands at d = 5.5 (2JPH = 41 Hz, 2JPH = 33 Hz) of 2 aACHTUNGRE(syn) and at 5.4 ppm 2 ( JPH = 36.8 Hz, 2JPH = 32 Hz) for 2 bACHTUNGRE(syn). The coupling pattern of the hydride signals, in addition to the multiplets of the methyl, methylene, and methyne protons of the meretpip ligand, resulted from coupling with the two chemically different types of phosphorus atoms, which were easily distinguishable due to the fact that there was no interference with signals of the anti isomers. In the IR spectra strong nCO bands appeared at 1888 cm 1 for 2 aACHTUNGRE(syn) and 1859 cm 1 for 2 bACHTUNGRE(syn), respectively. The broad bands at 1619 and 1637 cm 1 were assigned to the nM H vibrations of 2 aACHTUNGRE(syn) and 2 bACHTUNGRE(syn) along with the characteristic nNO bands at 1537 and 1526 cm 1, respectively, which were from their position in accord with the data of related complexes previously reported by our group.[18, 19] These hydride complexes were soluble in THF, CH2Cl2, toluene, and benzene, but sparingly soluble in pentane. The compositions of 2 a,bACHTUNGRE(syn) could be further substantiated by correct elemental analyses. Single crystals suitable for X-ray diffraction studies were obtained for 2 a,bACHTUNGRE(syn) from concentrated pentane solutions at 30 8C after several days. The molecular structures of 2 aACHTUNGRE(syn) and 2 bACHTUNGRE(syn) are depicted in Figure 2. Similar to the chloride isomeric complexes 1 a,bACHTUNGRE(syn) the hydride complexes 2 a,bACHTUNGRE(syn) revealed pseudo-octahedral geometries with the three phosphorus atoms arranged meridional and in plane with the carbonyl ligand. The nitrosyl

Figure 1. Molecular structures of [Mo(NO)(CO)Cl(mer-etpip)], 1 aACHTUNGRE(syn), (left) and W(NO)(CO)Cl(mer-etpip), 1 bACHTUNGRE(syn). (right). Thermal ellipsoids are drawn at the 30 % probability level. All H atoms and the toluene solvate were removed for clarity. Selected bond distances () and bond angles (8) for 1 aACHTUNGRE(syn): Mo(1) N(1A) 1.788(3), Mo(1) P(1) 2.5145(5), Mo(1) Cl(1A) 2.5178(10), N(1A) O(2A) 1.221(5), Mo(1) C(1) 2.020(2), C(1) O(1) 1.139(2) N(1A)-Mo(1)-Cl(1A) 176.40(8), P(1)-Mo(1)-P(3) 156.272(16), N(1A)-Mo(1)-P(2) 101.27(8). Selected bond distances () and bond angles (o) for 1 bACHTUNGRE(syn): W(1) N(1A) 1.836(4), W(1) P(1) 2.5002(7), W(1) Cl(1A) 2.5136(10), N(1A) O(2A) 1.172(6), W(1) C(1) 1.997(3), C(1) O(1) 1.156(3) N(1A)-W(1)-Cl(1A) 176.39(10), P(1)-W(1)P(3) 156.26(2), N(1A)-W(1)-P(2) 101.34(10).

phosphorus atoms and the carbonyl ligands in the mer-P3 plane (Figure 1). The average M–P distance of 2.511(5)  for 1 aACHTUNGRE(syn) and 2.500(7)  for 1 bACHTUNGRE(syn) are slightly larger than the related diphosphine M(NO)ACHTUNGRE(P^P)(CO)2H (P^P = bidentate ligand; M = Mo, W) system reported by our group.[15] Preparation of Syn-M(NO)(CO)H(mer-etpip) Complexes (M = Mo, 2aACHTUNGRE(syn); W, 2bACHTUNGRE(syn))

The isomeric chloride derivatives 1 aACHTUNGRE(syn,anti) and 1 bACHTUNGRE(syn,anti) were reacted with excess of LiBH4 in Et3N at 90 8C resulting in the formation of the syn hydride complexes Mo(NO)(CO)H(mer-etpip) 2 aACHTUNGRE(syn) and W(NO)(CO)H(mer-etpip) 2 bACHTUNGRE(syn) (Scheme 2) in 67 % and 58 % yields, respectively. The anti complexes could not be traced, also in the reaction solutions. The 31P{1H} NMR spectra of 2 aACHTUNGRE(syn) exhibited only one doublet at d = 92.9 ppm (2JPP = 5.9 Hz) and a triplet at d = 97.2 ppm (2JPP = 5.9 Hz) owing to the presence of two chemically different Pterminal and Pinternal phosphorus environments of the etpip ligand. The 31 P{1H} NMR spectra of 2 bACHTUNGRE(syn) showed the doublet and triplet resonances at d = 72.3 (2JPP = 5.2 Hz, 1JPW = (d, satellites), 295 Hz) and 79.5 ppm (2JPP = 5.2 Hz, 1JPW = (d, satellites), 221 Hz), respectively, which again indicated formation of only the syn isomers. At first glance, the apparently ste- Scheme 2. Synthetic access to the carbonyl hydrides 2 a,bACHTUNGRE(syn) and related derivatives.

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nCO and the nCO2 vibrations of the metal carbonyl and the formato groups, respectively. Finally, the compositions of the formato complexes 3 a,bACHTUNGRE(syn) were confirmed by correct elemental analyses. Reaction of 2a,bACHTUNGRE(syn) with [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] The [BACHTUNGRE(C6F5)4] anion is classified as a weakly coordinating or non-coordinating group large in size.[20, 21] Both properties support the stability of cationic dihydrogen or dihydride complexes obtained upon protonation of transition metal mono hydrides[22] using [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4]. The weakly coordinating H2 ligand can hardly be replaced by a [BACHTUNGRE(C6F5)4] anion. However, a weakly bound H2 ligand may be replaced by weakly coordinating, small solvent molecules, such as THF. Solvent complexes are often promising catalyst precursors in hydrogenations of ketones and imines, supporting the access of H2 or of the substrates to the metal centers by facile ligand exchanges. We observed that the reaction of the hydride 2 aACHTUNGRE(syn) with [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] led at room temperature in THF to instantaneous H2 evolution with the solution turning green, therby forming the ionic THF complex [Mo(NO)(CO)(mer-etpip)ACHTUNGRE(THF)][BACHTUNGRE(C6F5)4] 4 aACHTUNGRE(syn) (Scheme 2). The 1H NMR spectrum revealed the disappearance of the characteristic hydride signal of 2 aACHTUNGRE(syn) at 5.5 ppm, and the 31P{1H} NMR spectrum of the newly formed species showed upfield shifts of the phosphorus signals with two major singlets at d = 69.1 ppm and at 75.7 ppm (2:1 ratio). Compound 4 aACHTUNGRE(syn) was isolated as a green and not entirely pure solid. Its purity was estimated to 95 % as revealed from the 31P{1H} NMR spectrum. Additional, as yet unidentified, very weak signals appeared at d = 67.8 and 69.8 ppm. The solid-state IR spectrum of the green solid of 4 aACHTUNGRE(syn) revealed a band at 1943 cm 1 assignable to the nCO vibration. The shift of the nCO band to higher wave numbers in comparison with 2 aACHTUNGRE(syn) supported the formation of a cationic species. In the 1H NMR spectrum, the signals of the THF ligand were found at d = 3.1 and 1.1 ppm. THF coordination to the metal center was confirmed by 1D NOE experiments, which additionally established the exchange of the coordinated THF with free THF in solution. Our group has previously reported that the protonation of a MoACHTUNGRE(dippe)2(NO)H complex with [HACHTUNGRE(Et2O)2]ACHTUNGRE[BArF4] produced the 16 e cationic complex [MoACHTUNGRE(dippe)2(NO)]ACHTUNGRE[BArF4] stabilized by an agostic interaction of one of the HMe atoms of the dippe ligand with the metal center.[23] However, for 4 aACHTUNGRE(syn) evidence for an agostic stabilization by Hmethyl atoms of the etpip ligand could not be provided, since the protoncoupled 13C DEPT experiment showed no reduction of the JACHTUNGRE(C,H) coupling constants of the CMe atoms. The elemental analyses of 4 aACHTUNGRE(syn) confirmed the composition of [Mo(NO)(CO)(mer-etpip)ACHTUNGRE(THF)][BACHTUNGRE(C6F5)4]. In a similar way as for 2 aACHTUNGRE(syn) [D8]THF solutions of 2 bACHTUNGRE(syn) turned also green when mixed with equimolar amounts of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4]. The 1H NMR spectra revealed the disappearance of the typical hydride signal at 5.4 ppm with the formation of the [W(NO)(CO)(mer-

Figure 2. Molecular structures of [Mo(NO)(CO)H(mer-etpip)] 2 aACHTUNGRE(syn) (left) and [W(NO)(CO)H(mer-etpip)] 2 bACHTUNGRE(syn) (right) with thermal ellipsoids drawn at the 50 % probability level. Selected bond distances () and bond angles (8) for 2 aACHTUNGRE(syn): Mo(1) N(1) 1.8271(15), Mo(1) P(1) 2.4727(4), Mo(1) C(1) 1.9944(17), Mo(1) H 1.795(19), C(1) O(1) 1.1530(19), N(1) O(2) 1.2096(18) N(1)-Mo(1)-H 177.9(6), P(1)-Mo(1)P(3) 152.62(2), N(1)-Mo(1)-C(1) 97.10(7) Selected bond distances () and bond angles (8) for 2 bACHTUNGRE(syn): W(1) N(1) 1.830(2), W(1) P(1) 2.4635(6), W(1) C(23) 1.986(2), W(1) H 1.79(3), C(23) O(2) 1.163(3), N(1) O(1) 1.219(3), N(1)-W(1)-H 177.0(9), P(1)-W(1)-P(3) 152.88(2), N(1)-Mo(1)-C(23) 97.01(9).

and hydride ligands were in both cases of compounds disposed trans showing no positional disorder. Both crystals contain only syn isomers (2 a,bACHTUNGRE(syn)). The hydride ligand was located in a difference Fourier map and freely refined. The mean M–P distances of 2.4663(4)  for 2 aACHTUNGRE(syn) and 2.4636(6)  for 2 bACHTUNGRE(syn) are shorter than those in the corresponding precursors 1 a,bACHTUNGRE(syn). Preparation of the M(NO)(CO)ACHTUNGRE(h1-OCHO)(mer-etpip) Complexes (M = Mo, 3aACHTUNGRE(syn); W, 3bACHTUNGRE(syn)) The hydride reactivity of 2 a,bACHTUNGRE(syn) was tested by the reaction with CO2,[18, 19] which for 2 aACHTUNGRE(syn) resulted in rapid formation of the h1-formato complex Mo(NO)(CO)ACHTUNGRE(h1OCHO)(mer-etpip) 3 aACHTUNGRE(syn) at room temperature in THF. Insertion of CO2 into the Mo H bond had occurred. For the same reaction 2 bACHTUNGRE(syn) required a temperature of 60 8C to form W(NO)(CO)ACHTUNGRE(h1-OCHO)(mer-etpip) 3 bACHTUNGRE(syn) (Scheme 2). The 1H NMR spectra of 3 a,bACHTUNGRE(syn) revealed singlets at d = 7.9 for 3 aACHTUNGRE(syn) and at 7.75 ppm for 3 bACHTUNGRE(syn), respectively, attributed to the h1-formato protons. In the 13 C{1H} NMR spectra the Cformato atoms appeared at d = 164 (3 aACHTUNGRE(syn)) and 166.9 ppm (3 bACHTUNGRE(syn)) in addition to the CCO atoms at d = 232 and 233 ppm, respectively. The 31P{1H} NMR spectra of 3 aACHTUNGRE(syn) and 3 bACHTUNGRE(syn) exhibited one doublet each at d = 68.4 (2JPP = 7.5 Hz) and 57 ppm (2JPP = 4.5 Hz, 1 JPW (d, satellite) = 298 Hz) (Pterminal atoms of the etpip ligand) and a triplet each at d = 78.9 (2JPP = 7.5 Hz) and 73 ppm (2JPP = 4.5 Hz, 1JPW (d, satellite) = 226 Hz), respectively (Pcentral atom of the etpip ligand). Due to the presence of only the syn isomer of 2 a,bACHTUNGRE(syn) and under the assumption that CO2 insertion into the M H bond would proceed with retention of the configuration at the metal center, we expected 3 a,bACHTUNGRE(syn) to be formed as syn isomers. Strong bands in the IR spectra at 1917 and 1638 cm 1 for 3 aACHTUNGRE(syn) and at 1897 and 1644 cm 1 for 3 bACHTUNGRE(syn) were assigned to the &

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etpip)ACHTUNGRE(THF)][BACHTUNGRE(C6F5)4] 4 bACHTUNGRE(syn) complex (Scheme 2). In the 31 P{1H} NMR spectra, new doublet and triplet signals appeared at d = 56.9 (2JPP = 4.3 Hz, 1JPW = (d, satellites), 284 Hz) and 69.5 ppm (2JPP = 4.3 Hz, 1JPW = (d, satellites), 284 Hz) assigned to the terminal and central phosphorus atoms of the mer-etpip ligand attached to the metal center. The solid-state IR spectra of 4 bACHTUNGRE(syn) showed sharp bands at 1926 and 1613 cm 1, which were assigned to the nCO and nNO bands of the coordinated carbonyl and nitrosyl ligands. The 1 H NMR spectra showed several groups of signals in the methyl, methylene, and methyne regions of the etpip ligand along with two broad signals at d = 3.4 and 1.4 ppm observed in the chemical shift range of free THF, indicating again a THF ligand exchange fast on the NMR time scale. A satisfactory elemental analysis of 4 bACHTUNGRE(syn) supported the composition of [W(NO)(CO)(mer-etpip)ACHTUNGRE(THF)][BACHTUNGRE(C6F5)4]. As an alternative reaction mode, one could envisage formation of a cationic dihydrogen or dihydride complex, [W(NO)(CO)(H2)ACHTUNGRE(etpip)][BACHTUNGRE(C6F5)4] or [W(NO)(CO)(H)2 ACHTUNGRE(etpip)][BACHTUNGRE(C6F5)4], respectively, via protonation of 2 bACHTUNGRE(syn) at the hydride site to form an H2 ligand and subsequent oxidative addition.[23] However, this possibility was excluded since signals of dihydrogen or dihydride ligands could not be observed in the 1H NMR spectra or in the 31P–1H correlation spectra.

Table 1. Hydrogenation of various imines using the 2 a,bACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2] [BACHTUNGRE(C6F5)4] co-catalytic mixture.

Entry[a] Substrates

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2b

3 0.5 2 0.5 0.5 0.5 99

3

2a 2b

10 6

25 12

73 29

4

2a 2b

82 60

15 12

56 > 97

5

2a

10

13

11

6

2a 2b

90 110

25 14

64 32

7

2a 2b

810 268

0.5 3

81 85

8

2a

-

11

5

9

2a 2b

110 230

25 77 < 14 100

10

2a

400

2

56

11

2a

90

14

18

12

2a

490

3

74

1

A preliminary hydrogenation experiment of N-benzylideneaniline was carried out under 60 bar H2 at room temperature using 1 mol % of the catalyst 2 aACHTUNGRE(syn) in THF. A gas chromatographic analysis of the reaction mixture did not reveal conversion of the N-benzylideneaniline into the corresponding N-phenylbenzylamine after a reaction time of 16 h. An increase in the temperature to 140 8C could not initiate the catalytic turnover. Next, the effect of addition of co-catalytic acid was tested in the catalyses with 2 aACHTUNGRE(syn). Indeed, when 1 mol % of the catalyst 2 aACHTUNGRE(syn) and 1 mol % of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] were applied under the same reaction conditions as in the experiments before, hydrogenation of N-benzylideneaniline occurred at room temperature, revealing 40 % conversion after 3 h, which corresponded to a TOF of 13 h 1 (Table 1, entry 1). By carrying out the reaction in THF and by raising the temperature to 80 8C under a pressure of 60 bar H2, the loading of the catalyst 2 aACHTUNGRE(syn) could be lowered to 0.2 mol % and 1 equiv of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4]. The GC-MS analysis of the reaction mixture showed 38 % conversion after the initial 30 min, corresponding to an initial TOF of 380 h 1 (Table 1, entry 1). Further elevation of the temperature to 140 8C under otherwise identical conditions furnished a conversion of 89 % of N-phenylbenzylamine after a reaction time of 2 h and resulted in an initial TOF of 590 h 1 (Table 1, entry 1). Subsequently, various solvents were screened with the intention to improve the activity of the 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] co-catalytic 5

Conv.[c] [%]

13 380 590 155 130 290 160

2a

M(NO)(CO)H(mer-etpip) (M = Mo, 2aACHTUNGRE(syn); W, 2bACHTUNGRE(syn)) and [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] Co-catalyzed Imine Hydrogenation

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Cat TOF[b] t [h] ACHTUNGRE(syn) ACHTUNGRE[h 1]

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Table 1. (Continued) Entry[a] Substrates

Cat TOF[b] t [h] ACHTUNGRE(syn) ACHTUNGRE[h 1]

13[j]

2a

3.5

2

Conv.[c] [%]

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[a] Reaction conditions unless stated otherwise: 140 8C, 60 bar H2, 0.2 mol % catalyst 2 a or 2 b, and 1 equiv of the [HACHTUNGRE(Et2O)2][(BACHTUNGRE(C6F5)4] acid (relative to the catalyst), 1 mL of THF. [b] TOFs were determined after the initial 30 min by GC-MS. [c] Conversions determined by GCMS on the basis of the consumption of the substrates. [d] The reaction was carried out at room temperature using 1 mol % catalyst, and the TOF was determined after 3 h. [e] At 80 8C. [f] C6H5Cl solvent. [g] Toluene as solvent. [h] 25 bar H2. [i] TOFs determined after the initial 15 min by GC-MS. [j] Acetophenone as substrate and 1 mol % 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][(BACHTUNGRE(C6F5)4] co-catalytic mixture; TOF determined after 2 h.

Figure 3. Plot of the initial TOF (h 1) versus H2 pressure (bar). Conditions: N-benzylideneaniline (1.81 mmol), catalyst 2 aACHTUNGRE(syn) (2 mg, 0.2 mol %), [HACHTUNGRE(Et2O)BACHTUNGRE(C6F5)4] (5 mg ) in THF (1 mL) at 140 8C. TOFs were determined after the initial 30 min by GC-MS.

mixture. Changing the solvent to chlorobenzene or toluene in the hydrogenation of N-benzylideneaniline and applying a 0.2 mol % loading of the co-catalytic system 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] under 60 bar H2 and 140 8C led to initial TOF values of 155 h 1 and 130 h 1 (Table 1, entry 1). In the given non-polar solvents, precipitation of the iminium salt, which is assumed to be generated as a catalytic intermediate, seemed at least partly to occur. This had impact on the catalytic performance, causing a drastic decrease in the hydrogenation rates. Too donating solvent molecules were also expected to retard the reaction by competitive inhibition of the reactive sites through too strong coordination of the solvent molecules, forming species related to 4 aACHTUNGRE(syn). In the catalytic turnover, the access of H2 or of the substrate molecules to the metal center seemed to be hindered by this. Thus, THF seemed to be a good compromise, capable of appropriately stabilizing unsaturated reactive intermediates on the one hand and allowing facile exchange with catalytically relevant molecules on the other hand. In conjunction with this notion, we then wanted to quantify the dependency of an exemplary hydrogenation reaction in THF on the H2 pressure (Figure 3). We carried out hydrogenations of Nbenzylideneaniline in THF at 140 8C using 0.2 mol % of the catalyst 2 aACHTUNGRE(syn) in the presence of 5 mg of [HACHTUNGRE(Et2O)BACHTUNGRE(C6F5)4] at five different H2 pressures. The plot of the initial TOF values (h 1) as a function of P(H2) depicted in Figure 3 shows a linear dependency and no sign of a flattening out of the curve at higher H2 pressures, which would be indicative for H2 saturation in its coordination equilibrium. The scope of the imine hydrogenations was also explored with the tungsten-derived 2 bACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] cocatalytic system. Using 0.2 mol % 2 bACHTUNGRE(syn) in the presence of 1 equivalent of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] to hydrogenate N-benzylideneaniline in THF under 60 bar H2 and at 140 8C, a TOF of 160 h 1 was reached. The reaction went to completion in less than 8 h, as revealed by GC-MS analysis. Based on these various experiments of imine hydrogenations in THF, a temperature of 140 8C and a H2 pressure of 60 bar turned out to be optimal for the reactions catalyzed by the 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] co-catalytic system, &

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thus achieving a maximum initial TOF of 1960 h 1 (Table 1, entry 2) in the hydrogenation of N-(4-methoxybenzylidene)aniline, which indicated that substitution by an electrondonating group at the carbon side of the imine leads to higher activities. Under these conditions, the hydrogenation of N-(4-methoxybenzylidene)aniline with the 2 bACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] system showed also an enhanced catalytic performance, revealing an initial TOF of 740 h 1 and 99 % conversion in less than 3 h (Table 1, entry 2). Various other para-substituted aryl imines PhCH = N-(a-Np), pClC6H4CH = N-p-C6H4Cl, p-ClC6H4CH = NPh, PhCH = N-pC6H4Cl, p-MeOC6H4CH = N-p-C6H4OMe, p-ClC6H4CH = Np-C6H4OMe, p-MeOC6H4CH = N-p-C6H4Cl, PhCH = N-pC6H4OMe, p-FC6H4CH = NPh, and PhCH = N-p-C6H4F were then additionally tested in the hydrogenation with the 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] or 2 bACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] co-catalytic systems and were found to exhibit moderate to excellent activities and yields as listed in Table 1. It was surprising to see that some of the 2 a,bACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] co-catalyzed hydrogenation reactions (Table 1, entries 3 and 11) exhibited poor activities and conversions, and that longer reaction times could not compensate for the effect of incomplete conversions of the imines to corresponding amines. This can be explained by a too early decomposition of the catalyst. Furthermore, for chlorine-containing substrates (entries 5, 6, and 8) the lower activities and conversions could also be attributed to a blocking of the metal center occurring by chloride release during catalysis.[24] The hydrogenation of acetophenone was also attempted using the 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] system. A loading of 1 mol % of the catalyst 2 aACHTUNGRE(syn) in the presence of stoichiometric amounts of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] under 60 bar H2 and 140 8C in THF revealed only 7 % conversion of acetophenone into the corresponding product 1-phenylethanol after 2 h, revealing a TOF of 3.5 h 1 (Table 1, entry 13).

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Stoichiometric model experiments To provide more insight into the reaction course of the cocatalytic imine hydrogenations, stoichiometric reactions and deuterium labeling studies were carried out. First, the stoichiometric reaction between N-benzylideneaniline and the hydride 2 aACHTUNGRE(syn) was probed in a Young’s tap NMR tube; 2 aACHTUNGRE(syn) turned out to be stable under these conditions. The supposed insertion of the imine into the Mo H bond did not take place even after heating at 140 8C for a long time (> 3 h), as revealed by 31P{1H} NMR spectroscopy. This observation presumably excludes that the imine can directly insert into the M H bond in the catalytic reaction course, although a reversible insertion of the imine into the M H bond cannot be ruled out by this experiment. Furthermore, to probe a H2 heterolytic splitting mechanism with proton before hydride transfer, an iminium salt was prepared as protonation intermediate from the reaction of N-benzylideneaniline with [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] at room temperature.[26] The formation of the iminium salt [PhCH = NHPh][BACHTUNGRE(C6F5)4] was confirmed by 1H NMR spectroscopy, observing a typical downfield-shifted iminium HC proton, shifting from d = 8.6 (imine) to 9.6 (iminium salt) in [D8]THF. The salt was then suspended in THF and mixed with 2 aACHTUNGRE(syn). The hydride transfer from the metal center of 2 aACHTUNGRE(syn) to the electrophilic carbon atom of the iminium salt occurred instantaneously at room temperature, as indicated in the 1 H NMR spectrum by the disappearance of the signal of the iminium salt at d = 9.6 ppm and the appearance of a doublet at d = 4.3 ppm (amine CH2 proton).

Figure 4. Hammett plot for the hydrogenation of various para-substituted aryl imines catalyzed by the 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)BACHTUNGRE(C6F5)4] system extracted from initial TOF values: 1 = 10.5, hydrogenations with para substituents at the benzylidene side; squares with descending order: p-MeO, p-H, p-F, and p-Cl groups; 1 = 0.86, hydrogenations with para substituents at the aniline side; circles with ascending order: p-MeO, p-H, and p-F groups. The square and the circle points for the p-H substituent coincide because it is the same substrate.

Mechanistic Studies Hammett correlations for the imine hydrogenations In order to probe the mechanism and to understand the electronics of the imine hydrogenation process, we explored the influence of various para substituents of aromatic imines by Hammett correlations[16, 25] (Figure 4), employing the cocatalytic 2 aACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] system and a series of para-H-, methoxy-, fluoro-, and chloro-substituted N-benzylideneanilines substituted on the benzylidene side or on the aniline side. The initial rates (s 1) were obtained from the TOF values. Hammett correlations were established by plotting ln(hydrogenation rate) as a function of the substituent constant (s), which gave straight lines in the logarithmic plot with a negative slope (1) of 10.5 for the para substitution on the benzylidene side and a slightly positive slope (1) of 0.86 for the para substitution on the aniline side. The Hammett parameter (1) stands for the susceptibility of the respective chemical transformation with respect to electronic influences of the substituents. A negative 1 value larger than unity indicates a positive charge build-up at the reaction center in the transition state, and the rate of the reaction is greatly enhanced by the presence of electron-donating groups. By contrast, a positive 1 value stands for the built-up of a negative charge at the reaction center and, consequently, the reaction rates are then enhanced by electronwithdrawing groups. As expected for the benzylidene aniline hydrogenation, an electron-donating group on the benzylidene side increased the overall rates significantly. The parasubstituent influence on the aniline side was found to be practically negligible (Figure 4), which suggests a non-ratelimiting proton transfer to the N atom of the imine. Nevertheless, the large para-substituent influence of the hydride transfer to the C atom of the imine is presumed to indicate a stronger contribution to the rate-determining step.

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Deuterium isotope effect (DKIE) study An exemplary DKIE (deuterium kinetic isotope effect) experiment was also carried out. With a loading of 0.2 mol % of the catalyst 2 aACHTUNGRE(syn) and [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] (1 equiv relative to 2 aACHTUNGRE(syn)) in THF at 140 8C under 25 bar D2, a TOF of 210 h 1 was revealed as indicated by GC-MS analysis. Under similar conditions, a TOF of 290 h 1 was achieved when the reaction was carried out with 25 bar of H2. The DKIE value kH/kD = 1.38 was found to be relatively small. Due to this low DKIE value, heterolytic cleavage of the H2 molecule could not be taken into consideration as the rate-limiting step.[16, 27] Attempted detection of catalytic intermediates We also performed the hydrogenation reaction of N-benzylideneaniline at 140 8C and 60 bar H2 with a loading of 0.25 mol % of the catalyst 2 aACHTUNGRE(syn) (5 mg) and stoichiometric amounts of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] in THF to investigate the fate of the catalyst during catalysis and the species remaining at the end of these reactions under the relatively harsh reaction conditions. The reaction was stopped after 20 min before it went to full conversion. The GC-MS analysis revealed 67 % conversion of the N-benzylideneaniline to the corresponding N-phenylbenzylamine. The 31P{1H} NMR spectra of the reaction mixture showed several signals along with the identifiable resonances (30 %) of the cationic THF

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complex 4 aACHTUNGRE(syn). Furthermore, the 31P{1H} NMR spectra of the reaction mixture after completion of the reaction showed two major signals at d = 59 and 57 ppm at a ratio of 2:1, corresponding to as yet unidentified species. Further addition of N-benzylideneaniline (50 equiv) to the same reaction mixture (without isolating the unidentified species) did not reveal any conversion of the N-benzylideneaniline to the corresponding amine, thus supporting the catalytic inactivity of the unidentified species. Therefore, it became apparent that within the initial 30 min of imine hydrogenation, the reaction proceeded at a faster rate. Apparently, after a while the catalyst started to decompose, which considerably influenced the total catalytic activity, decreasing the rate of the reactions.

ther by deprotonation of the H2 ligand with the basic imine moiety. Through the proton-transfer step an iminium salt is thus generated,[14b] which corresponds to the heterolytic splitting step of H2. This step is assumed to be fast and it seems implausible that it participates in the rate-limiting step, based also on the low DKIE value and the Hammett substituent effect. The iminium cation I-3 is another key intermediate in the course of the hydrogenation reactions. Its presence was validated by the aforementioned separate stoichiometric experiments.[26] The next step comprises the hydride transfer to the highly electrophilic carbon center of the iminium cation I-3, closing the catalytic cycle by regeneration of the cationic intermediate I-1 and generation of the amine product. In related work of Norton[7] and Bullock[12] it was argued that the hydride transfer is the rate-limiting step of the catalysis, as high concentrations of the corresponding hydride complex were identified by NMR spectroscopy. But for our scheme of catalysis this seems not to be the case. For the M = Mo catalysis we could not verify a significant accumulation of the LnM-H complex during the course of the hydrogenation reaction by in-situ 31P{1H} NMR spectroscopy. For the system reported by Norton[7] it was also demonstrated that, in contrast to our catalyses, the overall reaction rate was independent of H2 pressure. The above-mentioned stoichiometric experiment of the hydride transfer from the corresponding metal hydride to the iminium salt, which was found to be fast on the NMR time scale at room temperature, also leads to the conclusion that other steps of the catalytic cycle should be rate-determining. Through this experiment, however, it cannot fully be excluded that the hydride transfer participates to a small extent in the rate-limiting step. Since our imine hydrogenation system exhibited a strong H2 pressure dependence, it is proposed that the H2 addition with formation of I-2 is rate-determining (Figure 3). It is worth mentioning that the crucial deprotonation of the dihydrogen complex to form the iminium cation I-3 is expected to compete with the same reaction to form the ammonium salt I-3’ from the product amines (Scheme 3). In general, aryl-substituted imine/amine pairs show low basicities, but the imine is in these cases always the stronger base. This makes it obvious that the hydrogenation of aniline derivatives could be readily accomplished. Their imines are stronger bases than the corresponding amines and the iminium cations are therefore available in kinetically relevant concentrations. By contrast, the basicities of alkyl-substituted imine/amine pairs are generally higher, with the amine being more basic than the corresponding imine derivatives. In the catalytic cycle this leads to consumption of a significant proportion of the protons by the product amine, which thus exerts a catalytic blocking effect the more of the amine is produced. Furthermore, the alkyl amines would also be stronger ligands and could also block the metal center of the intermediate I-1 in the very initial stage of the reaction. Presumably, due to these reasons, hydrogenations of an exemplary alkyl-substituted PhCH = NiBu substrate could not

Mechanistic proposal Based on the aforementioned observations, a general mechanistic scheme was proposed for the imine hydrogenations (Scheme 3). In the absence of a H2 atmosphere, the type 2

Scheme 3. General scheme for the ionic hydrogenation mechanism of imines occurring with heterolytic H2 splitting and proton before hydride H-atom transfers catalyzed by type 2/acid co-catalytic mixtures. The H2 addition step is assumed to be rate-determining.

pre-catalyst gets protonated, resulting in H2 evolution and formation of the weakly THF-coordinated cationic species 4 a,bACHTUNGRE(syn). By loss of THF, these species can generate the 16 e reactive intermediate of the type I-1 bearing a vacant site. In the actual catalytic transformations, I-1 is expected to be involved in competitive equilibria with the imine starting compounds and the product amines, H2, and THF, forming the LnML’ species (L’ = amine, imine, H2, and THF), which except for L’ = H2 take the role of resting states of the catalytic cycle. The actual H2 concentration controlled by the applied H2 pressure must be such that the formed dihydrogen complex I-2 is present in catalytically relevant concentrations,[28] enabling the catalytic cycle to be driven fur-

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be observed. The high difference in the pKb value of the PhCH = NiBu/PhCH2NHiBu pair further supports the notion that the amine is present in catalytic batches in the protonated form.[15]

ternal standard reference. Signal patterns are as follows: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. IR spectra were obtained by using either the ATR or KBr methods using a Bio-rad FTS-45 instrument. Elemental analyses were carried out at the Anorganisch-Chemisches Institut of the University of Zrich. GC-MS spectra were recorded on a Varian Saturn 2000 spectrometer equipped with a Varian 450-GC chromatograph {Phenomenex ZB-5ms (30 m), Brechbhler company; Gradient 70–2708}.

Conclusions

General Procedure for the Preparation of Isomeric Mixtures of [M(NO)Cl3(mer-etpip)] (M = Mo, syn,anti); W, syn,anti))[17]

To summarize, we have demonstrated a new synthetic route to low-valent Mo(0) and W(0) nitrosyl chloride complexes M(NO)(CO)ClACHTUNGRE(etpip) (M = Mo, 1 aACHTUNGRE(syn,anti); M = W, 1 bACHTUNGRE(syn,anti)) showing syn and anti isomerism designating the position of the NO ligand with respect to the Ph substituent. Two hydride complexes, M(NO)(CO)HACHTUNGRE(etpip) (M = Mo, 2 aACHTUNGRE(syn); W = 2 bACHTUNGRE(syn)), were obtained with the exclusive formation of the syn isomers. These hydrides 2 a,bACHTUNGRE(syn) were found to react with CO2 forming the formato complexes of the type M(NO)(CO)ACHTUNGRE(h1-OCHO)ACHTUNGRE(etpip), (M = Mo, 3 aACHTUNGRE(syn); W, 3 bACHTUNGRE(syn)). Complexes 2 aACHTUNGRE(syn) and 2 bACHTUNGRE(syn) exhibited catalytic performance in the presence of the [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] acid as a co-catalyst, thus allowing hydrogenation of various para-substituted imines to their corresponding amines. Maximum initial TOF values of 1960 h 1 and 740 h 1 were reached for the Mo and W catalysts, respectively, in the hydrogenation of N-(4-methoxybenzylidene)aniline employing the 2 a,bACHTUNGRE(syn)/[HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] co-catalytic mixtures under 60 bar H2 at 140 8C. Hammett correlation study revealed a negative 1 value of 10.5 for the para substitution on the benzylidene side and a slightly positive 1 value of 0.86 for the para substitution on the aniline side. Based on several kinetic observations {Hammett study, iminium salts as reaction intermediates, H2 pressure dependence, and deuterium kinetic isotope effect (DKIE) (kH/kD = 1.38)}, an “ionic” mechanism with heterolytic splitting of H2 was proposed followed by “proton before hydride” H-atom transfers, where the H2 addition step is proposed to be ratelimiting.

M(NO)Cl3ACHTUNGRE(NCMe)2 (1.6 mmol, M = Mo, W) was dissolved in 15 mL of THF at room temperature. Then a solution of the etpip ligand (1.6 mmol) in 5 mL THF was added. The resulting mixture turned immediately deep red and was stirred for 2 h. A solid precipitate was observed along with the solution turning green. The solid precipitate was separated from the solution, dried in vacuo, and washed with THF and pentane. Finally, the solid was extracted with CH2Cl2 and dried in vacuo. Recrystallization of the solid from a cold CH2Cl2/pentane mixture afforded the desired product as yellow crystals. Mo(NO)Cl3(mer-etpip) (syn,anti): Yield: 55 %; 1H NMR (500 MHz, CD2Cl2): d = 8.3–7.5 (m, Ph), 3.79–3.7 (m, PCH2), 2.9–2.6 (m, PCH), 2.5– 2.3 (m, PCH2), 1.52–1.17 ppm (m, CH3); 31P{1H} NMR (202.5 MHz, CD2Cl2): d = 76 (d, 2JPP = 72 Hz, ACHTUNGRE(syn)), 98 (t, 2JPP = 72 Hz, ACHTUNGRE(syn)), 77 (d, 2 JPP = 76 Hz, ACHTUNGRE(anti)), 89 ppm (t, 2JPP = 76 Hz, ACHTUNGRE(anti)); 13C{1H} NMR (125.8 MHz, CD2Cl2): d = 133.6 (m, Ph), 132 (m, Ph), 130 (m, Ph), 129.7 (m, Ph), 30 (m, PCH2), 29 (m, PCH), 28 (m, PCH), 22 (s, CH3), 20.3 (m, CH3), 20 (m, CH3), 19.7 (m, CH3), 18 ppm (m, PCH2); IR (ATR): n˜ NO = 1648 cm 1; elemental anal. calcd (%) for C22H41Cl3MoNOP3 : C 41.89, H 6.55, N 2.22; found: C 41.75, H 6.30, N 2.01. W(NO)Cl3(mer-etpip) (syn,anti): Yield: 52 %; 1H NMR (500 MHz, CD2Cl2): d 7.9–7.5 (m, Ph), 3.79–3.76 (m, PCH2), 3.0–2.9 (m, PCH2), 2.68–2.60 (m, PCH), 2.54–2.39 (m, PCH2), 1.48–1.15 (m, CH3); 13 C{1H} NMR (125.8 MHz, CD2Cl2): d 133.6 (m, Ph), 133.3 (m, Ph), 132.8 (m, Ph), 129.9 (m, Ph), 29.4 (m, PCH2), 29 (m, CH), 28 (m, CH), 22.1 (m, CH3), 20.6 (s, CH3), 20.1 (m, CH3), 19.6 (s, CH3), 17.5 ppm (m, -PCH2); 31 P{1H} NMR (202.5 MHz, CD2Cl2): d = 55 {d, 2JPP = 46 Hz, ACHTUNGRE(syn)}, 80 (t, 2 JPP = 46 Hz, ACHTUNGRE(syn)), 56 {d, 2JPP = 49 Hz, ACHTUNGRE(anti)}, 72 ppm {t, 2JPP = 49 Hz, ACHTUNGRE(anti)}; IR (ATR): n˜ NO = 1618 cm 1 (nNO); elemental anal. calcd (%) for C22H41Cl3NOP3W: C 36.77, H 5.75, N 1.95; found: C 36.65, H 5.71, N 1.99. General Procedure for the Preparation of the Isomeric Mixtures of M(NO)(CO)Cl(mer-etpip), (M = Mo, 1 aACHTUNGRE(syn,anti); W = 1 bACHTUNGRE(syn,anti)) The syn/anti isomeric mixture of M(NO)Cl3ACHTUNGRE(etpip) (0.48 mmol) (M = Mo, W) was added to a suspension of 1 % sodium amalgam (5 equiv, 2.4 mmol) in 15 mL THF in a Young Schlenk tube at room temperature. Then the nitrogen atmosphere was removed via a freeze–pump–thaw cycle and the tube was filled with CO (1 bar) and sealed. Stirring was continued overnight at room temperature to ensure the completion of the reaction. The final supernatant solution was filtered off from the mercury-containing residue and evaporated to dryness. The residue was washed with pentane and extracted with toluene. Recrystallization from a cold toluene/pentane solution afforded pure crystals in moderate yields.

Experimental Section General All manipulations were carried out under an atmosphere of nitrogen using either dry glove box or Schlenk techniques. Reagent-grade solvents benzene, tetrahydofuran, pentane, toluene, and diethyl ether were dried with sodium benzophenone ketyl and distilled prior to use under N2 atmosphere. CH2Cl2 and Et3N were dried over calcium hydride and distilled. Deuterated solvents were dried with sodium benzophenone ([D8]THF and C6D6) and calcium hydride (CD2Cl2) and distilled via freeze–pump–thaw cycle before use. M(NO)Cl3ACHTUNGRE(NCMe)3,[29a] etpip ligand[17] M(NO)Cl3(mer-etpip),[17] and [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4][29b] were prepared according to literature procedures. Other reagents were purchased and used without further purification. NMR spectra were measured with a Varian Mercury 200 spectrometer (at 200.1 MHz for 1H, at 81.0 MHz for 31P), a Varian Gemini-300 instrument (1H at 300.1 MHz, 13C at 75.4 MHz), a Bruker-DRX 500 spectrometer (500.2 MHz for 1H, 202.5 MHz for 31P, 125.8 MHz for 13C), and a Bruker-DRX 400 spectrometer (400.1 MHz for 1H, 162.0 MHz for 31P, 100.6 MHz for 13C). All chemical shifts for 1H and 13C{1H} NMR data are expressed in ppm relative to tetramethylsilane (TMS) and for 31P{1H} relative to 85 % H3PO4 as an ex-

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1 aACHTUNGRE(syn,anti): Yield: 65 %; 1H NMR (400 MHz, CD2Cl2): d = 7.7–7.3 (m, -Ph), 2.45–2.31 (m, PCH2), 2.25–2.19 (m, PCH2), 1.59–1.54 (m, CH), 1.41– 1.38 (m, CH3), 1.34–1.25 (m, CH3); 31P{1H} NMR (162 MHz, CD2Cl2): 79.5 {t, 2JPP = 7 Hz, PPh, 1 aACHTUNGRE(syn)}. 67.9 (d, 2JPP = 7 Hz, iPr2P, 1 aACHTUNGRE(syn)}. 62.3 (d, 2JPP = 9 Hz, PPh, 1 aACHTUNGRE(anti)}. 58.9 (t, 2JPP = 9 Hz, iPr2P, 1 aACHTUNGRE(anti)}; 13 C{1H} NMR (125.8 MHz, CD2Cl2, 1 aACHTUNGRE(syn)): d = 133.6 (d, 2JCP = 11.9 Hz, C-ortho), 132.9 (d, 1JCP = 29.8 Hz, C-ipso), 130.7 (s, C-para), 128.6 (d, 3 JCP = 8.3 Hz, C-meta), 26.5 (m, PCH2), 24.2 (m, PCH2), 25.8 (m, CH), 24.9 (m, CH), 20.9 (CH3), 19.6 (CH3), 19.5 (CH3), 17.4 ppm (s, CH3); 13CACHTUNGRE{1H NMR (125.8 MHz, CD2Cl2, 1 aACHTUNGRE(anti)): d = 133.9 (d, 2JCP = 10.7 Hz, Cortho), 132.4 (d, 1JCP = 26.2 Hz, C-ipso), 130.6 (s, C-para), 128.4 (d, 3JCP = 8.3 Hz, C-meta), 26.2 (m, PCH2), 24.0 (m, PCH2), 26 (m, CH), 24.8 (m, CH), 20 (CH3), 19.9 (CH3), 19.8 (CH3), 17.8 ppm (s, CH3); IR (ATR): n˜ = 1918 (nCO), 1569 cm 1 (nNO); elemental anal. calcd (%) for

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C23H41ClMoNO2P3 : C 46.99, H 7.03, N 2.38; found: C 47.14, H 7.15, N 2.29. The 1H NMR signals of 1 aACHTUNGRE(anti) overlapped with the signals of 1 aACHTUNGRE(syn).

upon shaking the Young NMR tube within 10 min in quantitative yield. 3 bACHTUNGRE(syn) was obtained in quantitative yield by heating 2 bACHTUNGRE(syn) under CO2 at 60 8C for 10 min.

1 bACHTUNGRE(syn,anti): Yield 60 %; 1H NMR (400 MHz, CD2Cl2): d = 7.9–7.3 (m, Ph), 2.72–2.61 (m, -PCH2), 2.51–1.25 (m, -PCH2), 1.84–1.54 (m, CH), 1.42–1.08 (m, CH3); 31P{1H} NMR (162 MHz, CD2Cl2): 72.6 {t, 2JPP = 5 Hz, PPh, 1JPW = (d, satellites), 215 Hz, 1 bACHTUNGRE(syn)}, 55.2 {d, 2JPP = 5 Hz, (PiPr) 1 JPW = (d, satellites), 288 Hz, 1 bACHTUNGRE(syn)}. 49 (d, 2JPP = 2.2 Hz, (PPh) 1JPW = (d, satellites), 281 Hz, 1 bACHTUNGRE(anti)}, 48.7 ppm {t, 2JPP = 2.2 Hz, (PiPr) 1JPW = (d, satellites), 211 Hz, 1 bACHTUNGRE(anti)}; 13C{1H} NMR (125.8 MHz, CD2Cl2, 1 bACHTUNGRE(syn)): d = 133.7 (s, Ph), 132.4 (s, Ph), 130.9 (s, Ph), 128.6 (d, 3JCP = 9.5 Hz, C-meta), 27.8 (m, PCH2), 27.1 (m, PCH2), 20.8 (m, CH), 20.1 (m, CH) 19.7 (m, CH3), 19.5 (m, CH3), 17.6 (m, CH3), 17.4 ppm (m, CH3); IR (ATR): n˜ = 1906 (nCO), 1561 cm 1 (nNO); elemental anal. calcd (%) for C23H41ClNO2P3W: C 40.88, H 6.12, N 2.07; found: C 41.04, H 6.20, N 2.08. The 1H NMR signals of 1 bACHTUNGRE(anti) overlapped with the signals of 1 bACHTUNGRE(syn). The 13C{1H} NMR signal of 1 bACHTUNGRE(anti) could not be distinguished owing to its low concentration in solution and overlapping with the signal of 1 bACHTUNGRE(syn).

3 aACHTUNGRE(syn): 1H NMR (400 MHz, 293 K, [D8]THF): d = 7.91(m, 2 H, Ph), 7.9 (s, OCHO), 7.5 (m, 3 H, Ph), 2.45–2.34 (m, 4 H, -PCH2), 2.23–2.16 (m, 4 H, CH), 1.97–1.60 (m, 4 H, PCH2), 1.32–1.10 ppm (m, 24 H, CH3); 31 P{1H} NMR (162 MHz, [D8]THF): 78.9 (t, 2JPP = 7.5 Hz, PPh), 68.4 ppm (d, 2JPP = 7.5 Hz, PiPr); 13C{1H} NMR (125.8 MHz, [D8]THF): d = 232 (m, CO), 164 ( d, 3JCP(trans to CO) = 4.8 Hz, OCHO), 132.7 (d, 2JCP = 11.9 Hz, Cortho), 131 (d, 1JCP = 31 Hz, C-ipso), 129 (s, C-para), 127 (d, 3JCP = 9.5 Hz, C-meta), 25 (m, PCH2), 24 (m, PCH2), 22.3 (m, PCH), 22.1 (m, PCH), 18.5 (m, CH3), 18 (m, CH3), 17.7 (m, CH3), 15.9 ppm (s, CH3); IR (KBr): n˜ = 1917 (nCO), 1638 (nOCHO), 1576 cm 1 (nNO); elemental anal. calcd (%) for C24H42MoNO4P3 : C 48.25, H 7.09, N 2.34; found: C 47.98, H 7.02, N 2.21. 3 bACHTUNGRE(syn): 1H NMR (400 MHz, [D8]THF): d = 7.93–7.89 (m, 2 H, Ph), 7.75 (s, OCHO), 7.46–7.44 (m, 3 H, Ph), 2.65–2.42 (m, 4 H, -PCH2), 2.31–2.29 (m, 2 H, -PCH), 2.13 (m, 2 H, PCH2), 1.99–1.96 (m, 2 H, -PCH), 1.58 (m, 2 H, PCH2), 1.34–1.12 (m, 24 H, CH3); 31P{1H} NMR (162 MHz, [D8]THF): 73 (t, 2JPP = 4.5 Hz, (PPh) 1JPW = (d, satellites), 226 Hz), 57 ppm (d, 2JPP = 4.5 Hz, (PiPr) 1JPW = (d, satellites), 298 Hz); 13 C{1H} NMR (125.8 MHz, [D8]THF): d = 233 (m, CO), 166.9 (d, 3JCP(trans OCHO), 134.7 (d, 2JCP = 11.9 Hz, C-ortho), 131(d, to CO) = 6 Hz, 1 JCP=37 Hz, C-ipso), 132 (s, C-para), 129.4 (d, 3JCP = 9.5 Hz, C-meta), 28.4 (m, PCH2), 28 (m, PCH), 25.4 (m, PCH2), 25 (m, PCH), 20.8 (s, CH3), 20.3 (s, CH3) 20.1 (s, CH3), 18 ppm (s, CH3); IR (KBr): n˜ = 1897 (nCO), 1644ACHTUNGRE(nOCHO), 1559 cm 1 (nNO); elemental anal. calcd (%) for C24H42NO4P3W: C 42.06, H 6.18, N 2.04; found: C 41.89, H 6.19, N 1.93.

General Procedure for the Preparation of M(NO)(CO)H(mer-etpip), {M = Mo, 2 aACHTUNGRE(syn); W, 2 bACHTUNGRE(syn)} The isomeric mixtures of M(NO)Cl(CO)(mer-etpip) (M = Mo, 1 aACHTUNGRE(syn,anti); W, 1 bACHTUNGRE(syn,anti); 0.51 mmol) and LiBH4 (2.5 mmol) were mixed in 20 mL of Et3N in a Young’s tap Schlenk tube. The resulting mixture was taken out from the Glove box and kept in a preheated (90 8C) oil bath with constant stirring for at least 24 h. The reaction was monitored by 31P{1H} NMR spectroscopy to ensure completion. After completion of the reaction, the solution was filtered off from a brown solid residue, and the Et3N was removed in vacuo. A sticky, red oily product was obtained, which was extracted with benzene at least twice (2  10 mL) and again dried in vacuo overnight. The obtained powdery solid was again first extracted with benzene and then with toluene (to ensure complete removal of excess LiBH4) and dried overnight. Then the solid was washed with a minimum amount of pentane to afford pure 2 aACHTUNGRE(syn) or 2 bACHTUNGRE(syn) as a yellow and red powder, respectively, after drying in vacuo.

Preparation of [Mo(NO)(CO)(mer-etpip)ACHTUNGRE(THF)][BACHTUNGRE(C6F5)4], 4 aACHTUNGRE(syn) A solution of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] (12 mg, 0.014 mmol) in 0.5 mL THF was added dropwise to a yellow solution of 2 aACHTUNGRE(syn) (8 mg, 0.014 mmol) in 1 mL THF. The resulting solution turned green immediately with the evolution of hydrogen gas. The resulting solution was kept stirring for 10 min. Finally, it was evaporated to dryness in vacuo, and the solid was washed with pentane (3  1 mL) to afford a green solid in 95 % yield based on the 31P{1H} NMR spectrum. 1H NMR (500 MHz, 223 K, Tol-d8): d = 7.8 (m, 2 H, Ph), 7.6 (m, 3 H, Ph), 3.1 (br s, THF), 2.42 (m, 4 H, -PCH2), 1.9–1.8 (m, 4 H, CH), 1.7–1.5 (m, 4 H, PCH2), 1.23–1.09 (m, CH3), 1.1 (br s, THF); 31P{1H} NMR (202 MHz, Tol-d8, 223 K): 69.1 (s, PiPr), 75.7 (s, PPh); IR (KBr): n˜ = 1943 (nCO), 1643 cm 1 (nNO); elemental anal. calcd (%) for C51H50BF20MoNO3P3 : C 46.95, H 3.86, N1.07; found: C 46.87, H 4.06, N 1.04.

2 aACHTUNGRE(syn): Yield: 67 %; 1H NMR (500 MHz, 293 K, C6D6): d = 7.7–7.04 (m, 5 H, -Ph), 2.2–2.1 (m, 4 H, PCH2), 1.94–1.86 (m, 4 H, CH), 1.7–1.5 (m, 4 H, PCH2), 1.35–0.93 (m, 24 H, CH3), 5.5 (2JPH = 41 Hz, 2JPH = 33 Hz); 31 P{1H} NMR (202.5 MHz, C6D6): 97.2 (t, 2JPP = 5.9 Hz, PPh). 92.9 ppm (d, 2 JPP = 5.9 Hz, iPr2P); 13C{1H} NMR (125.8 MHz, C6D6): d = 240 (m, CO), 136.4 (d, 1JCP = 17.6 Hz, C-ipso), 133.4 (d, 2JCP = 14.5 Hz, C-ortho), 130.4 (s, C-para), 128.5 (d, 3JCP = 8.8 Hz, C-meta), 31.8 (t, 1JCP = 8.4 Hz, PCH2), 33.6 (t, 1JCP = 8.4 Hz, PCH2), 28.2 (m, CH), 24.6 (m, CH), 19.3 (t, 2JCP = 2.4 Hz, CH3), 19.2 (t, 2JCP = 3.2 Hz, CH3), 18.8 (s, CH3), 18.5 ppm (s, CH3); IR (KBr): n˜ = 1888ACHTUNGRE(nCO), 1619 (nMo H), 1537 cm 1 (nNO); elemental anal. calcd (%) for C23H42MoNO2P3 : C 49.91, H 7.65, N 2.53; found: C 50.03, H 7.57, N 2.56.

Preparation of [W(NO)(CO)(mer-etpip)ACHTUNGRE(THF)][BACHTUNGRE(C6F5)4], 4 bACHTUNGRE(syn) To a solution of 2 bACHTUNGRE(syn) (8 mg, 0.0125 mmol) in 0.5 mL [D8]THF, [HCHTUNGRE(A Et2O)2][BACHTUNGRE(C6F5)4] (11 mg, 0.013 mmol) was added. The resulting solution turned green immediately with the evolution of hydrogen gas. It was evaporated to dryness and washed with pentane twice (2  1 mL) to afford a green solid in 90 % yield based on the 31P{1H} NMR spectrum. 1 H NMR (300 MHz, 293 K, [D8]THF): d = 7.9 (m, 2 H, Ph), 7.6 (m, 3 H, Ph), 3.03–2.9 (m, 4 H, -PCH2), 2.7–2.5 (m, 4 H, -PCH), 2.2–2.0 (m, 4 H, PCH2), 1.47–1.12 (m, 24 H, CH3); 31P{1H} NMR (202 MHz, Tol-d8): 56.9 (d, 2JPP = 4.3 Hz, 1JPW = (d, satellites), 284 Hz), 69.5 (t, 2JPP = 4.3 Hz, 1JPW = (d, satellites), 284 Hz); IR (ATR): n˜ = 1926 (nCO), 1613 cm 1 (nNO); elemental anal. calcd (%) for C47H42BF20NO2P3W.C4H8O: C 43.99, H 3.62, N 1.01; found: C 44.25, H 3.80, N 1.06.

2 bACHTUNGRE(syn): Yield: 58 %; 1H NMR (400 MHz, C6D6): d = 7.7–7.1 (m, 5 H, Ph), 2.31–2.29 (m, 4 H, -PCH2), 2.0–1.9 (m, 4 H, PCH), 1.74–1.63 (m, 4 H, PCH2), 1.23–0.9 (m, 24 H, CH3), 5.4 (2JPH = 36.8 Hz, 2JPH = 32 Hz); 31 P{1H} NMR (162 MHz, C6D6): 79.5 (t, 2JPP = 5.2 Hz, (PPh) 1JPW = (d, satellites), 221 Hz), 72.3 ppm (d, 2JPP = 5.2 Hz, (PiPr) 1JPW = (d, satellites), 295 Hz); 13C{1H} NMR (125.8 MHz, C6D6): d = 236 (m, CO), 135.6 (d, 1 JCP = 22.7 Hz, C-ipso), 133.9 (d, 2JCP = 14.3 Hz, C-ortho), 131 (s, C-para), 128.9 (d, 3JCP = 9.5 Hz, C-meta), 34.09 (t, 1JCP = 8.3 Hz, PCH2), 33.8 (t, 1 JCP = 8.3 Hz, PCH2), 29.2 (m, PCH), 26.9 (m, PCH), 20 (s, CH3), 19.8 (s, CH3) 19.5 (s, CH3), 19 ppm (s, CH3); IR (KBr): n˜ = 1859 (nCO), 1638 (nW H), 1526 cm 1 (nNO); elemental anal. calcd (%)for C23H42NO2P3W: C 43.07, H 6.60, N 2.18; found: C 43.27, H 6.56, N 2.20.

X-ray Diffraction Analyses Single-crystal X-ray diffraction data were collected at 183(2) K on an Agilent Technologies Xcalibur Ruby area-detector diffractometer using a single wavelength Enhance X-ray source with MoKa radiation (l = 0.71073 ).[30] The selected suitable single crystals were mounted using polybutene oil on a flexible loop fixed on a goniometer head and immediately transferred to the diffractometer. Pre-experiment, data collection, data reduction, and analytical absorption correction[31] were performed with the program suite CrysAlisPro.[32] The structures were solved by direct methods using SHELXS97.[33] The structure refinements were per-

General Procedure for the Preparation of M(NO)(CO)ACHTUNGRE(h1-OCHO)(meretpip), {M = Mo, 3 aACHTUNGRE(syn); W, 3 bACHTUNGRE(syn)} A solution of M(NO)(CO)H(mer-etpip), {M = Mo, 2 aACHTUNGRE(syn) or W, 2 bACHTUNGRE(syn)} (0.027 mmol) was frozen in a Young NMR tube. The nitrogen atmosphere was removed via a freeze–pump–thaw cycle. Then the tube was filled with 2 bar of CO2 and sealed. Formation of 3 aACHTUNGRE(syn) took place

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Table 2. Crystallographic data for compounds 1 aACHTUNGRE(syn), 1 bACHTUNGRE(syn), 2 aACHTUNGRE(syn), and 2 bACHTUNGRE(syn). CCDC empirical formula formula weight [g mol 1] T [K] wavelength [] crystal system, space group a [] b [] c [] a [deg] b [deg] g [deg] volume [3] Z, density (calcd) (Mg·m 3) abs coefficient (mm 1) FACHTUNGRE(000) crystal size [mm3] q range [deg] reflections collected reflections unique completeness to q [%] absorption correction max/min transmission data/restraints/parameters goodness-of-fit on F2 final R1 and wR2 indices [I > 2s(I)] R1 and wR2 indices (all data) largest diff. peak and hole (e  3)

1 aACHTUNGRE(syn)

1 bACHTUNGRE(syn)

2 aACHTUNGRE(syn)

2 bACHTUNGRE(syn)

956934 C23H41ClMoNO2P3 587.87 183(2) 0.71073 monoclinic, P21/c 12.8561(1) 12.3654(1) 17.5941(1) 90 96.729(1) 90 2777.68(4) 4, 1.406 0.761 1224 0.33  0.26  0.20 2.29 to 30.51 46 604 8490/[Rint=0.0570] 100.0 analytical 0.892 and 0.833 7085/2/316 1.073 0.0304, 0.0850 0.0388, 0.0876 0.735 and 0.619

956935 C23H41ClNO2P3W 675.77 183(2) 0.71073 monoclinic, P21/c 12.8278(1) 12.3530(1) 17.5831(2) 90 96.4620(1) 90 2768.55(4) 4, 1.621 4.462 1352 0.22  0.19  0.09 2.66 to 30.51 57 925 8458/[Rint=0.0732] 99.9 analytical 0.753 and 0.473 6585/0/298 0.948 0.0261, 0.0568 0.0394, 0.0585 1.415 and 0.799

956936 C23H42MoNO2P3 553.43 183(2) 0.71073 monoclinic, P21/c 16.5384(3) 10.4479(1) 15.9771(3) 90 101.325(2) 90 2706.95(8) 4, 1.358 0.681 1160 0.29  0.21  0.18 2.51 to 30.51 28 378 8249/[Rint=0.0356] 100.0 analytical 0.907 and 0.858 6357/0/283 0.947 0.0293, 0.0631 0.0438, 0.0657 0.589 and 0.472

956937 C23H42NO2P3W 641.33 183(2) 0.71073 monoclinic, P21/c 16.5197(3) 10.4466(1) 15.9539(2) 90 101.052(2) 90 2702.18(6) 4, 1.576 4.471 1288 0.29  0.25  0.15 2.51 to 30.51 48 781 8240/[Rint=0.0338] 100.0 analytical 0.603 and 0.389 6985/0/283 1.077 0.0221, 0.0585 0.0284, 0.0595 1.940 and 0.671

The unweighted R-factor is R1 = ACHTUNGRE(Fo Fc)/Fo; I > 2 s(I) and the weighted R-factor is wR2 = {wACHTUNGRE(Fo2 Fc2)2/wACHTUNGRE(Fo2)2}1/2. formed by full-matrix least-squares on F2 with SHELXL97.[33] PLATON[34] was used to check the result of the X-ray analysis. All programs used during the crystal structure determination process are included in the WINGX software.[35] DIAMOND[36] was used for the molecular graphics. CCDC 956934 (1 aACHTUNGRE(syn)), CCDC 956935 (1 bACHTUNGRE(syn)), CCDC 956936 (2 aACHTUNGRE(syn)), and CCDC 956937 (2 bACHTUNGRE(syn)) contain the supplementary crystallographic data (excluding structure factors) for this paper (Table 2). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

ture was cooled immediately and without further purification, conversion was determined by GC analysis on the basis of substrate consumption. In a similar way and under the same conditions, the reaction was also carried out with 25 bar H2 to determine the conversions, and initial TOFs were calculated. GC-MS data for Various Imines and Amines Formed during Hydrogenation Reaction Catalyses by 2 a,bACHTUNGRE(syn) and [HACHTUNGRE(Et2O)2] [BACHTUNGRE(C6F5)4] Co-catalytic Mixture. PhCH = NPh: tr = 9.083 min, m/z = 181; PhCH2NHPh: tr = 9.339 min, m/ z = 183; p-ClC6H4CH = N-p-C6H4Cl: tr = 11.790 min, m/z = 248; pClC6H4CH2NH-p-C6H4Cl: tr = 12.612 min, m/z = 251;PhCH = N(a-naphtyl): tr = 13.255 min, m/z = 231; PhCH2NH(a-naphtyl): tr = 13.662 min, m/ tr = 10.912 min, m/z = 211; pz = 233; p-MeOC6H4CH = NPh: MeOC6H4CH2NHPh: tr = 10.989 min, m/z = 213; p-ClC6H4CH = NPh: tr = 10.298 min, m/z = 215; p-ClC6H4CH2NHPh: tr = 10.722 min, m/z = 217; PhCH = N-p-C6H4Cl: tr = 10.379 min, m/z = 215; p-ClC6H4CH2NHPh: tr = 10.861 min, m/z = 217; p-MeOC6H4CH = N-p-C6H4OMe: tr = 13.219 min, m/z = 241; p-MeOC6H4CH2NH-p-C6H4OMe: tr = 12.916 min, m/z = 243; tr = 12.355 min, m/z = 245; pp-ClC6H4CH = N-p-C6H4OMe: ClC6H4CH2NH-p-C6H4OMe: tr = 12.499 min, m/z = 247; p-MeOC6H4CH = N-p-C6H4Cl: tr = 12.567 min, m/z = 245; p-MeOC6H4CH2NH-p-C6H4Cl: tr = 12.980 min, m/z = 247; p-FC6H4CH = NPh: tr = 8.925 min, m/z = 199; pFC6H4CH2NHPh: tr = 9.314 min, m/z = 201; PhCH = N-p-C6H4F: tr = 9.004 min, m/z = 199; PhCH2NH-p-C6H4F: tr = 9.334 min, m/z = 201; Ph(CO)CH3 : tr = 4.438 min, m/z = 120; PhCH(OH)CH3 : tr = 4.356 min, m/ z = 122.

General Procedure for the Catalytic Imine Hydrogenation Experiments A stock solution of freshly made catalyst 2 aACHTUNGRE(syn) or 2 bACHTUNGRE(syn) (10 mg in 5 mL of THF) was prepared. An aliquot (1 mL) of that stock solution was added to stoichiometric amounts of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] relative to catalyst (the solution turned light green). Subsequently, the imine (500 equiv relative to the catalyst) was added. The entire mixture was then transferred to a steel autoclave. The autoclave was charged with the required hydrogen pressure and kept in a preheated oil bath (140 8C). After an appropriate reaction time, the autoclave was immediately taken out and cooled to room temperature, and then the H2 pressure was released in the fume hood. The reaction mixture was removed from the autoclave, filtered through silica gel, and analyzed without further purification by GC-MS to determine the conversions of the hydrogenation products (on the basis of the substrate consumption) and to identify the products. General Procedure for the Deuterium Kinetic Isotope Effect Experiment In a 30 mL steel autoclave equipped with a stirring bar, molybdenum complex 2 aACHTUNGRE(syn) (2 mg from freshly prepared stock solution in THF) and [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] (1 equiv relative to the catalyst) were mixed, and THF was added to a final volume of 1 mL (the solution became green). N-benzylideneaniline (500 equiv relative to catalyst) was added to that solution. Subsequently, the system was charged with 25 bar of D2 and kept stirring at 140 8C. After 30 min of reaction time, the reaction mix-

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Acknowledgements Funds from the Swiss National Science Foundation, Lanxess AG and the University of Zrich are gratefully acknowledged.

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ÝÝ These are not the final page numbers!

FULL PAPER It’s important to stay active: Lowvalent M(0)(NO)(CO)HACHTUNGRE(etpip) {M = Mo, 2 aACHTUNGRE(syn); W = 2 bACHTUNGRE(syn), etpip = (iPr2PCH2CH2)2PPh)} complexes are active catalysts in the presence of [HACHTUNGRE(Et2O)2][BACHTUNGRE(C6F5)4] co-catalytic mixture in the hydrogenation of various para-substituted imines to their corresponding amines. An “ionic” mechanism with heterolytic splitting of H2 is proposed followed by “proton before hydride” H-atom transfers.

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Hydrogenation Subrata Chakraborty, Olivier Blacque, Thomas Fox, Heinz Berke* &&&&—&&&& Hydrogenation of Imines Catalyzed by Trisphosphine-Substituted Molybdenum and Tungsten Nitrosyl Hydrides and Co-Catalytic Acid

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