DOI: 10.1002/chem.201501126
Communication
& P,N Ligands
Synthesis and Coordination Chemistry of Iminophosphanes Tom van Dijk,[a] Sebastian Burck,[a] Amos J. Rosenthal,[a] Martin Nieger,[b] Andreas W. Ehlers,[a] J. Chris Slootweg,*[a] and Koop Lammertsma*[a] In memory of Paul von Ragu¦ Schleyer
Abstract: Iminophosphanes are a new group of 1,3-P,N-ligands, readily obtainable from secondary phosphanes and nitrilium ions, having a tunable N-donor site by means of varying the imine substituents. These ligands give, in high yields, monodentate gold complexes and bidentate rhodium and iridium complexes. Crystal structures are reported for both the ligands and the complexes.
Hybrid ligands with hard nitrogen and soft phosphorus donor sites are valuable in coordination chemistry,[1] but the availability of readily tunable 1,3-P,N-ligands is limited. The best known one, 2-pyridyldiphenylphosphane (A; see below for structure), acts as a P-, N-, and P,N-chelating ligand[2] and is used in a plethora of catalytic transformations, such as Ru-,[3] Rh-,[3] and Ir-catalyzed[4] hydroformylations of alkenes; alkyne methoxycarbonylation by Pd-complexes;[5] Ru-catalyzed hydrogena[6] tions of styrene, phenylacetylene,[6] and benzoic aldehydes;[7] and the hydration of nitriles with Ru-complexes[8] and that of 1-pentyne by Au-complexes.[9] Given these important applications it is unfortunate that the pyridine ring hampers the electronic tuning.[10] Iminophosphanes (B; IUPAC: c-phosphanylimines) would be ideal 1,3-P,N-ligands because all of the P, N, and C substituents can be varied, but currently access is limited to salt metatheses (i.e., phosphides and imidoyl chlorides),[11] radical reactions (e.g., phosphanes and isocyanides),[12] and thermal decompositions (e.g., (amino)(phosphino)carbenes).[13] This study reports the convenient access to new neutral 1,3P,N-ligands, now fully tunable, particularly at the N-site, and [a] T. van Dijk, Dr. S. Burck, A. J. Rosenthal, Dr. A. W. Ehlers, Dr. J. C. Slootweg, Prof. Dr. K. Lammertsma Department of Chemistry and Pharmaceutical Sciences VU University Amsterdam, De Boelelaan 1083 1081 HV Amsterdam (The Netherlands) E-mail: [email protected] [email protected] [b] Dr. M. Nieger Laboratory of Inorganic Chemistry Department of Chemistry University of Helsinki, 00014 Helsinki (Finland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501126. Chem. Eur. J. 2015, 21, 9328 – 9331
their coordination chemistry. The key reagent is nitrilium triflate 1, which is obtained from imidoyl chlorides by Cl-abstraction. Earlier, its reaction with primary phosphanes was shown to give access to the anionic phosphaamidinates via the tautomeric mixture of iminophosphanes and aminophosphaalkenes.[14] Herein, secondary phosphanes are used to generate non-tautomerizable iminophosphanes 3–5 (Scheme 1) of which, importantly, the ratio of the E/Z conformations has no influence on the coordination chemistry due to ease of isomerization of the imine bond.
Scheme 1. Synthesis of iminophosphanes 3–5.
We start with the synthesis of P,N-ligands 3-5. Adding Ph2PH to 1 a (R1 = Ph, R2 = tBu) in DCM at ¢78 8C and deprotonating the iminium ion adduct (2 a; d31P = 14.5 ppm, d1H = 13.11 ppm, no 1JPH) gave Z-iminophosphane 3 a (80 %; d31P = ¢0.4 ppm; Scheme 1). Ion 2 a illustrates that the proton transfers from phosphorus to nitrogen on adding Ph2PH to 1 a. 31P NMR showed full conversion to only Z-3 a, which is also the favored isomer at wB97X-D/6-31 + G(d,p) (DG (E-3 a–Z-3 a) = 3.5 kcal mol¢1).[15] The same reaction with 1 b (R1 = Ph, R2 = Ph) gave E-3 b and Z-3 b (72 %; d31P = 8.0 and ¢1.4 ppm, respectively) in a 2:1 ratio; the assignment is based on the 2.4 kcal mol¢1 energetic preference for the E-isomer. Likewise, reaction with 1 c (R1 = Mes, R2 = Ph; Mes = mesityl) gave a 3:1 mixture of E-3 c and Z3 c (94 %; d31P = 8.6 and 0.8 ppm, respectively). Reaction with 1 d (R1 = iPr, R2 = Ph), which is labile above ¢20 8C,[14] gave isolable 2 d (d31P = 3.7 ppm) and on its deprotonation a 1.3:1.0 mixture of E-3 d and Z-3 d (59 %; d31P = 5.1 and ¢9.7 ppm, respectively; DG(Z-3 d–E-3 d) = 2.6 kcal mol¢1). The molecular structure of 3 d shows an imine bond (1.278(5) æ), a C1¢P1 single bond (1.856(4) æ),[11d] and an E-configuration (Figure 1, left).[16] Apparently, rapid isomerization occurs as the crystals
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Figure 1. Molecular structures of imino-phosphanes E-3 d (left) and E-5 a (right) at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (æ) and angles (8) of E-3 d (values of the second crystallographic independent molecule in the asymmetric unit are in brackets): N1¢C1 1.278(5) [1.279(6)], N1¢C2 1.479(6) [1.475(6)], C1¢C5 1.496(6) [1.496(6)], P1¢C1 1.857(4) [1.857(4)], P1¢C11 1.830(4) [1.833(5)], P1¢C17 1.830(5) [1.815(4)]; N1-C1-P1 119.1(3) [118.6(3)]. E-5 a: N1¢C1 1.269(2), N1¢C6 1.411(2), C1¢C2 1.541(2), P1¢C1 1.9000(15), P1¢C12 1.8624(18), P1¢C18 1.8801(17); N1-C1-P1 113.98(13).
were grown from an E/Z-mixture, indicating the absence of cocrystallization, and the same mixture was regained on redissolving the crystals of E-3 d. Also reacting Ph2PH with 1 e (R1 = Cy, R2 = Ph; labile > ¢20 8C; Cy = cyclohexyl) gave a 1.1:1.0 mixture of E-3 e and Z-3 e (87 %; d31P = 6.0 and ¢9.7 ppm, respectively; DG (Z-3 e–E-3 e) = 2.4 kcal mol¢1). Reacting tBu2PH, instead of Ph2PH, with 1 a gave only E-4 a (95 %; d31P = 32.1 ppm; DG(Z-4 a–E-4 a) = 6.9 kcal mol¢1). Likewise, Cy2PH afforded only E-5 a (87 %; d31P = 9.4 ppm) based on the 1.7 kcal mol¢1 energy preference over the Z-isomer, NOESY experiments (see the Supporting Information), and the molecular structure (Figure 1, right).[16] Its shorter imine (C1¢N1 = 1.2690(20) æ) and elongated C1¢P1 (1.9000(15) æ) bonds compared to those of E-3 d (Figure 1) illustrate the influence of the substituents on the NCP frame. Having established that iminophosphanes are readily accessible from simple starting materials, their coordination behavior was explored. Gold(I) complexation was studied first to confirm the preference of phosphorus over nitrogen coordination.[17] The recent influx in ‘gold catalysis’[18] makes these complexes of interest as the pendant imine group may facilitate proton shuttling in hydroaminations and hydroalkoxylations.[18c] AuI complex 6 a (d31P = 28.3 ppm) resulted quantitatively from N-phenyl-substituted iminophosphane 3 a and (tht)AuCl in DCM (Scheme 2; tht = tetrahydrothiophene). The molecular structure shows coordination of only phosphorus to the gold center (Figure 2, left)[16] and reveals full Z- to E-isomerization of the ligand. The P¢Au (2.2321(11)/2.2356(11) æ) and Au¢Cl (2.2839(11)/2.2890(12) æ) bonds are of similar lengths to those
Scheme 2. Coordination of iminophosphanes with AuCl. Chem. Eur. J. 2015, 21, 9328 – 9331
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Figure 2. Molecular structures of gold complexes E-6 a (left) and E-7 a (right) at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (æ) and angles (8) of E-6 a (values of the second crystallographic independent molecule in the asymmetric unit are in brackets): N1¢ C1 1.267(6) [1.266(5)], N1¢C2 1.421(5) [1.413(5)], C1¢C8 1.535(6) [1.533(6)], P1¢C1 1.874(4) [1.878(4)], P1¢C18 1.813(4) [1.817(4)], P1¢Au1 2.2321(11) [2.2356(11)], Au1¢Cl1 2.2839(11) [2.2890(12)]; N1-C1-P1 110.1(3) [110.6(3)], P1-Au1-Cl1 177.29(4) [173.02(4)]. E-7 a: N1¢C1 1.264(4), N1¢C14 1.412(4), C1¢C2 1.545(4), P1¢C1 1.887(3), P1¢C6 1.885(3), P1¢Au1 2.2462(9), Au1¢Cl1 2.2862(9); N1-C1-P1 108.0(2), P1-Au1-Cl1 178.95(3).
of 2-pyridyldiphenylphosphane gold(I) chloride (A¢AuICl; P¢Au 2.234(4), Au¢Cl 2.286(4) æ),[17] suggesting comparable P-donor properties. As expected, the N-sites differ, as exemplified by the C=N stretching frequency of 1639 cm¢1 for 6 a and the pyridine C=N stretch of 1571 cm¢1 for A¢AuICl).[9] The N-imine substituent is readily modified as illustrated by the reaction of N-alkyl-substituted 3 d with (tht)AuCl that affords AuI complex 6 d (90 %; d31P = 34.3 ppm, n(C=N) = 1612 cm¢1; Scheme 2). The P substituents are equally readily modifiable. Illustrative is the reaction of tBu2P-substituted 4 a with (Me2S)AuCl that quantitatively yielded AuI complex 7 a (d31P = 60.1 ppm). Its molecular structure (Figure 2, right)[16] shows that ligand 4 a has retained its E-conformation. Rhodium and iridium coordination of the iminophosphanes was explored next (Scheme 3). Mixing 3 a with 0.5 equiv of
Scheme 3. Coordination chemistry of iminophosphanes with [Cp*MCl2]2.
[Cp*RhCl2]2 in DCM gave two products in a 6.3:1.0 ratio. The major one (d31P = ¢4.4 ppm; 1J(P,Rh) = 113.9 Hz) is bidentate complex 10 a-Cl, in which a Rh-chloride is displaced for the imine nitrogen. The minor product (d31P = 43.3 ppm; 1J(P,Rh = 141.9 Hz) is assigned to monodentate complex 8 a,[19] which, like A, coordinates only the phosphane.[20] Adding AgOTf (OTf = triflate) enables its conversion to the P,N-bidentate complexes, as the triflate anion coordinates only weakly,[3b, 20, 21] 9329
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Scheme 4. Influence of the imine substituents on the N-donor capacity. [a] E (N-LP) is the energy difference between the N-lone pair containing orbitals of [10*] + and [A-RhIIICp*Cl] + .
Figure 3. Molecular structures of bidentate rhodium complexes 10 a-OTf (left) at the 50 % probability level and 10 d-OTf (right) at the 30 % probability level. Hydrogen atoms and the triflate counter ion are omitted for clarity. Selected bond lengths (æ) and angles (8) of 10 a-OTf: N1¢C1 1.298(8), N1¢C6 1.430(7), N1¢Rh1 2.106(5), C1¢C2 1.513(8), P1¢C1 1.870(6), P1¢C12 1.805(6), P1¢Rh1 2.2931(16), Rh1¢Cl1 2.3952(15); N1-C1-P1 97.4(4), P1-Rh1-N1 66.29(15). 10 d-OTf (values of the other two crystallographic independent molecules in the asymmetric units are in brackets): N1¢C1 1.281(9) [1.300(10), 1.276(9)], N1¢C2 1.485(9) [1.470(10), 1.497(9)], N1¢Rh1 2.175(6) [2.187(6), 2.159(6)], C1¢C5 1.484(10) [1.467(10), 1.486(10)], P1¢C1 1.836(7) [1.826(8), 1.845(7)], P1¢C11 1.818(8) [1.804(9), 1.797(7)], P1¢Rh1 2.325(2) [2.339(2), 2.314(2)], Rh1¢Cl1 2.387(2) [2.384(2), 2.389 (2)]; N1-C1-P1 102.9(5) [103.8(5), 101.7(5)], P1-Rh1-N1 66.25(16) [66.38(18), 66.23(16)].
The impact of changing the imine substituents became even more evident when [Cp*RhCl2]2 was reacted with a 1.3:1.0 E/Z-mixture of 3 d to give only a single isomer of P1 monodentate 8d (d31P = 30.6 ppm, J(P,Rh) = 144.3 Hz; Scheme 3). Not only does this underscore the ease of E/Z-isomerization of the ligand, it also reveals a reduced N-donor strength as evidenced by the absence of bidentate 10 d-Cl. Full conversion to 10 d-OTf (76 %; d31P = ¢6.2 ppm, 1J(P,Rh) = 117.0 Hz; Scheme 3) resulted on adding AgOTf. The molecular structure (Figure 3, right)[16] shows imine (1.276(7) æ) and P1¢ C1 (1.845(5) æ) bonds of similar lengths as those of the free ligand, but the N1-C1-P1 angle compression from 119.2(3)8 to 101.7(4)8) illustrates the effect of forming the Rh-cycle. The reaction of iridium complex [Cp*IrCl2]2 with 3 a yielded only bidentate 11 a-Cl (67 %; d31P = ¢26.5 ppm; no monodentate 9 aCl was detected) and on ion exchange 11 a-OTf (83 %) with virtually identical NMR spectra. The question then arises: how tunable is the N-donor site of iminophosphane and how does it compare to pyridyldiphenylphosphane A? An impression is obtained by comparing the Rh¢N bond strengths of the experimentally ascertained complexes 10 a, 10 d, and [A-RhIIICp*Cl] + ,[20] which can be estimated from their ring opening to the corresponding P-monodentate complexes as shown in Scheme 4.[22] Using fully optimized geometries, the respective bond strengths are 21.1, 16.0, and 12.1 kcal mol¢1 at wB97X-D/6-31 + G(d,p) (Def2-TZVP for Rh). Chem. Eur. J. 2015, 21, 9328 – 9331
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This underscores that 3 a is a better N-donor than both 3 d and A, which is corroborated by the relative energies of 1.8 and 0.9 eV for the N-lone pair containing orbitals of the respective P-monodentate complexes [10 a*] + and [10 d*] + compared to that of [A-RhIIICp*Cl] + (see also the Supporting Information). To delineate the influence of aryl versus alkyl N- and C-substituents on the N-donor strength of the iminophosphane ligand, we modeled complexes 10 b and 10 f–h bearing only Me and Ph substituents (lower half of Scheme 4). The relative energies illustrate that changing the N-imine substituent significantly impacts the N-donor capacity of the iminophosphane ligands, which is further fine-tuned by changing the C-substituent. In summary, iminophosphanes with diverse substituents at the core P, N, and C centers are readily obtained from nitrilium ions and secondary phosphanes. These electronically tunable 1,3-P,N-ligands give selectively, and in high yield, mono- or bidentate complexes, depending on the choice of the coordinating metal. Their potential in homogeneous catalysis will be substantial.
Acknowledgements This work was supported by the Council for Chemical Sciences of The Netherlands Organization for Scientific Research (NWO/ CW). We thank E. Janssen (VU University Amsterdam) and J. W. H. Peeters (University of Amsterdam) for measuring high resolution mass-spectra. Keywords: ligand design · coordination modes · synthetic methods · N,P ligands · density functional calculations
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[1] For a review on hybrid ligands, see: W.-H. Zhang, S. W. Chien, T. S. A. Hor, Coord. Chem. Rev. 2011, 255, 1991. [2] a) For a review on A, see: Z.-Z. Zhang, H. Cheng, Coord. Chem. Rev. 1996, 147, 1; b) P. Braunstein, D. G. Kelly, A. Tiripicchio, F. Ugozzoli, Bull. Soc. Chim. Fr. 1995, 132, 1083; c) C. Tejel, M. A. Ciriano, S. Jim¦nez, L. A. Oro, C. Graiff, A. Tiripicchio, Organometallics 2005, 24, 1105; d) M. J. Calhorda, C. Ceamanos, O. Crespo, M. C. Gimeno, A. Laguna, C. Larraz, P. D. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[3]
[4] [5]
[6] [7] [8] [9] [10]
[11]
[12] [13] [14]
[15]
Vaz, M. D. Villacampa, Inorg. Chem. 2010, 49, 8255; e) For a review on bimetallic P,N complexes see: S. Maggini, Coord. Chem. Rev. 2009, 253, 1793. a) K. Kurtev, D. Ribola, R. A. Jones, D. J. Cole-Hamilton, G. Wilkinson, J. Chem. Soc. Dalton Trans. 1980, 55; b) D. Drommi, F. Nicolý, C. G. Arena, G. Bruno, F. Faraone, Inorg. Chim. Acta 1994, 221, 109. G. Franciý, R. Scopelliti, C. G. Arena, G. Bruno, D. Drommi, F. Faraone, Organometallics 1998, 17, 338. a) E. Drent, P. Arnoldy, P. H. M. Budzelaar, J. Organomet. Chem. 1993, 455, 247; b) E. Drent, P. Arnoldy, P. H. M. Budzelaar, J. Organomet. Chem. 1994, 475, 57; c) J. J. M. de Pater, C. E. P. Maljaars, E. de Wolf, M. Lutz, A. L. Spek, B.-J. Deelman, C. J. Elsevier, G. van Koten, Organometallics 2005, 24, 5299. I. Moldes, E. de La Encarnaciûn, J. Ros, A. Alvarez-Larena, J. F. Piniella, J. Organomet. Chem. 1998, 566, 165. B. A. Garcia, D. Y. Gin, J. Am. Chem. Soc. 2000, 122, 4269. T. Oshiki, H. Yamashita, K. Sawada, M. Utsunomiya, K. Takahashi, K. Takai, Organometallics 2005, 24, 6287. C. Khin, A. S. K. Hashmi, F. Rominger, Eur. J. Inorg. Chem. 2010, 2010, 1063. For selected examples of 6-substituted pyridylphosphanes, see: a) L. Hintermann, T. T. Dang, A. Labonne, T. Kribber, L. Xiao, P. Naumov, Chem. Eur. J. 2009, 15, 7167; b) A. C. Laungani, M. Keller, J. M. Slattery, I. Krossing, B. Breit, Chem. Eur. J. 2009, 15, 10405; c) D. B. Grotjahn, D. A. Lev, J. Am. Chem. Soc. 2004, 126, 12232; d) U. Gellrich, J. Huang, W. Seiche, M. Keller, M. Meuwly, B. Breit, J. Am. Chem. Soc. 2011, 133, 964. a) K. Issleib, O. Lçw, Z. Anorg. Allg. Chem. 1966, 346, 241; b) J. Heinicke, A. Tzschach, Z. Chem. 1978, 18, 452; c) X. Li, H. Song, C. Cui, Dalton Trans. 2009, 9728; d) D. Walther, S. Liesicke, R. Fischer, H. Gçrls, J. Weston, A. Batista, Eur. J. Inorg. Chem. 2003, 4321; e) D. Walther, S. Liesicke, L. Bçttcher, R. Fischer, H. Gçrls, G. Vaughan, Inorg. Chem. 2003, 42, 625; f) For the use of silylphosphanes, see: K. Issleib, H. Schmidt, H. Meyer, J. Organomet. Chem. 1978, 160, 47. T. Saegusa, Y. Ito, N. Yasuda, T. Hotaka, J. Org. Chem. 1970, 35, 4238. N. Merceron, K. Miqueu, A. Baceiredo, G. Bertrand, J. Am. Chem. Soc. 2002, 124, 6806. T. van Dijk, S. Burck, M. K. Rong, A. J. Rosenthal, M. Nieger, J. C. Slootweg, K. Lammertsma, Angew. Chem. Int. Ed. 2014, 53, 9068; Angew. Chem. 2014, 126, 9214. DFT calculations on ligands 3 (see the Supporting Information) were carried out with Gaussian 09; Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
Chem. Eur. J. 2015, 21, 9328 – 9331
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[16]
[17] [18]
[19]
[20] [21] [22]
G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ©. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc. Wallingford CT, 2009. CCDC 1009957 (3 d), 1009958 (5 a), 1009959 (6 a), 1009960 (7 a), 1009961 (10 a-OTf), and 1009962 (10 d-OTf) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. N. W. Alcock, P. Moore, P. A. Lampe, K. F. Mok, J. Chem. Soc. Dalton Trans. 1982, 207. a) A. S. K. Hashmi, G. J. Hutchings, Angew. Chem. Int. Ed. 2006, 45, 7896; Angew. Chem. 2006, 118, 8064; b) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180; c) A. Arcadi, Chem. Rev. 2008, 108, 3266; d) D. J. Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351; e) Z. Li, C. Brouwer, C. He, Chem. Rev. 2008, 108, 3239; f) A. S. K. Hashmi, Angew. Chem. Int. Ed. 2010, 49, 5232; Angew. Chem. 2010, 122, 5360. Coordination of iminophosphanes 3 to Cp*RhCl2 favors P over N (DE P vs. N coordination for 3 a = 29.3 kcal mol¢1 and for 3 d = 30.1 kcal mol¢1) at wB97X-D/6-31 + G(d,p) (Def2-TZVP for rhodium); see the Supporting Information. M. L. Clarke, A. M. Z. Slawin, M. V. Wheatley, J. D. Woollins, J. Chem. Soc. Dalton Trans. 2001, 3421. K. Wajda-Hermanowicz, Z. Ciunik, A. Kochel, Inorg. Chem. 2006, 45, 3369. DFT calculations on Rh complexes 8, 10, 12, and 13, and Ir complex 11 a were carried out with Gaussian09 (Revision D.01); see the Supporting Information.
Received: March 23, 2015 Published online on May 15, 2015
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& P,N Ligands
Synthesis and Coordination Chemistry of Iminophosphanes Tom van Dijk,[a] Sebastian Burck,[a] Amos J. Rosenthal,[a] Martin Nieger,[b] Andreas W. Ehlers,[a] J. Chris Slootweg,*[a] and Koop Lammertsma*[a] In memory of Paul von Ragu¦ Schleyer
Abstract: Iminophosphanes are a new group of 1,3-P,N-ligands, readily obtainable from secondary phosphanes and nitrilium ions, having a tunable N-donor site by means of varying the imine substituents. These ligands give, in high yields, monodentate gold complexes and bidentate rhodium and iridium complexes. Crystal structures are reported for both the ligands and the complexes.
Hybrid ligands with hard nitrogen and soft phosphorus donor sites are valuable in coordination chemistry,[1] but the availability of readily tunable 1,3-P,N-ligands is limited. The best known one, 2-pyridyldiphenylphosphane (A; see below for structure), acts as a P-, N-, and P,N-chelating ligand[2] and is used in a plethora of catalytic transformations, such as Ru-,[3] Rh-,[3] and Ir-catalyzed[4] hydroformylations of alkenes; alkyne methoxycarbonylation by Pd-complexes;[5] Ru-catalyzed hydrogena[6] tions of styrene, phenylacetylene,[6] and benzoic aldehydes;[7] and the hydration of nitriles with Ru-complexes[8] and that of 1-pentyne by Au-complexes.[9] Given these important applications it is unfortunate that the pyridine ring hampers the electronic tuning.[10] Iminophosphanes (B; IUPAC: c-phosphanylimines) would be ideal 1,3-P,N-ligands because all of the P, N, and C substituents can be varied, but currently access is limited to salt metatheses (i.e., phosphides and imidoyl chlorides),[11] radical reactions (e.g., phosphanes and isocyanides),[12] and thermal decompositions (e.g., (amino)(phosphino)carbenes).[13] This study reports the convenient access to new neutral 1,3P,N-ligands, now fully tunable, particularly at the N-site, and [a] T. van Dijk, Dr. S. Burck, A. J. Rosenthal, Dr. A. W. Ehlers, Dr. J. C. Slootweg, Prof. Dr. K. Lammertsma Department of Chemistry and Pharmaceutical Sciences VU University Amsterdam, De Boelelaan 1083 1081 HV Amsterdam (The Netherlands) E-mail: [email protected] [email protected] [b] Dr. M. Nieger Laboratory of Inorganic Chemistry Department of Chemistry University of Helsinki, 00014 Helsinki (Finland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201501126. Chem. Eur. J. 2015, 21, 9328 – 9331
their coordination chemistry. The key reagent is nitrilium triflate 1, which is obtained from imidoyl chlorides by Cl-abstraction. Earlier, its reaction with primary phosphanes was shown to give access to the anionic phosphaamidinates via the tautomeric mixture of iminophosphanes and aminophosphaalkenes.[14] Herein, secondary phosphanes are used to generate non-tautomerizable iminophosphanes 3–5 (Scheme 1) of which, importantly, the ratio of the E/Z conformations has no influence on the coordination chemistry due to ease of isomerization of the imine bond.
Scheme 1. Synthesis of iminophosphanes 3–5.
We start with the synthesis of P,N-ligands 3-5. Adding Ph2PH to 1 a (R1 = Ph, R2 = tBu) in DCM at ¢78 8C and deprotonating the iminium ion adduct (2 a; d31P = 14.5 ppm, d1H = 13.11 ppm, no 1JPH) gave Z-iminophosphane 3 a (80 %; d31P = ¢0.4 ppm; Scheme 1). Ion 2 a illustrates that the proton transfers from phosphorus to nitrogen on adding Ph2PH to 1 a. 31P NMR showed full conversion to only Z-3 a, which is also the favored isomer at wB97X-D/6-31 + G(d,p) (DG (E-3 a–Z-3 a) = 3.5 kcal mol¢1).[15] The same reaction with 1 b (R1 = Ph, R2 = Ph) gave E-3 b and Z-3 b (72 %; d31P = 8.0 and ¢1.4 ppm, respectively) in a 2:1 ratio; the assignment is based on the 2.4 kcal mol¢1 energetic preference for the E-isomer. Likewise, reaction with 1 c (R1 = Mes, R2 = Ph; Mes = mesityl) gave a 3:1 mixture of E-3 c and Z3 c (94 %; d31P = 8.6 and 0.8 ppm, respectively). Reaction with 1 d (R1 = iPr, R2 = Ph), which is labile above ¢20 8C,[14] gave isolable 2 d (d31P = 3.7 ppm) and on its deprotonation a 1.3:1.0 mixture of E-3 d and Z-3 d (59 %; d31P = 5.1 and ¢9.7 ppm, respectively; DG(Z-3 d–E-3 d) = 2.6 kcal mol¢1). The molecular structure of 3 d shows an imine bond (1.278(5) æ), a C1¢P1 single bond (1.856(4) æ),[11d] and an E-configuration (Figure 1, left).[16] Apparently, rapid isomerization occurs as the crystals
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Figure 1. Molecular structures of imino-phosphanes E-3 d (left) and E-5 a (right) at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (æ) and angles (8) of E-3 d (values of the second crystallographic independent molecule in the asymmetric unit are in brackets): N1¢C1 1.278(5) [1.279(6)], N1¢C2 1.479(6) [1.475(6)], C1¢C5 1.496(6) [1.496(6)], P1¢C1 1.857(4) [1.857(4)], P1¢C11 1.830(4) [1.833(5)], P1¢C17 1.830(5) [1.815(4)]; N1-C1-P1 119.1(3) [118.6(3)]. E-5 a: N1¢C1 1.269(2), N1¢C6 1.411(2), C1¢C2 1.541(2), P1¢C1 1.9000(15), P1¢C12 1.8624(18), P1¢C18 1.8801(17); N1-C1-P1 113.98(13).
were grown from an E/Z-mixture, indicating the absence of cocrystallization, and the same mixture was regained on redissolving the crystals of E-3 d. Also reacting Ph2PH with 1 e (R1 = Cy, R2 = Ph; labile > ¢20 8C; Cy = cyclohexyl) gave a 1.1:1.0 mixture of E-3 e and Z-3 e (87 %; d31P = 6.0 and ¢9.7 ppm, respectively; DG (Z-3 e–E-3 e) = 2.4 kcal mol¢1). Reacting tBu2PH, instead of Ph2PH, with 1 a gave only E-4 a (95 %; d31P = 32.1 ppm; DG(Z-4 a–E-4 a) = 6.9 kcal mol¢1). Likewise, Cy2PH afforded only E-5 a (87 %; d31P = 9.4 ppm) based on the 1.7 kcal mol¢1 energy preference over the Z-isomer, NOESY experiments (see the Supporting Information), and the molecular structure (Figure 1, right).[16] Its shorter imine (C1¢N1 = 1.2690(20) æ) and elongated C1¢P1 (1.9000(15) æ) bonds compared to those of E-3 d (Figure 1) illustrate the influence of the substituents on the NCP frame. Having established that iminophosphanes are readily accessible from simple starting materials, their coordination behavior was explored. Gold(I) complexation was studied first to confirm the preference of phosphorus over nitrogen coordination.[17] The recent influx in ‘gold catalysis’[18] makes these complexes of interest as the pendant imine group may facilitate proton shuttling in hydroaminations and hydroalkoxylations.[18c] AuI complex 6 a (d31P = 28.3 ppm) resulted quantitatively from N-phenyl-substituted iminophosphane 3 a and (tht)AuCl in DCM (Scheme 2; tht = tetrahydrothiophene). The molecular structure shows coordination of only phosphorus to the gold center (Figure 2, left)[16] and reveals full Z- to E-isomerization of the ligand. The P¢Au (2.2321(11)/2.2356(11) æ) and Au¢Cl (2.2839(11)/2.2890(12) æ) bonds are of similar lengths to those
Scheme 2. Coordination of iminophosphanes with AuCl. Chem. Eur. J. 2015, 21, 9328 – 9331
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Figure 2. Molecular structures of gold complexes E-6 a (left) and E-7 a (right) at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (æ) and angles (8) of E-6 a (values of the second crystallographic independent molecule in the asymmetric unit are in brackets): N1¢ C1 1.267(6) [1.266(5)], N1¢C2 1.421(5) [1.413(5)], C1¢C8 1.535(6) [1.533(6)], P1¢C1 1.874(4) [1.878(4)], P1¢C18 1.813(4) [1.817(4)], P1¢Au1 2.2321(11) [2.2356(11)], Au1¢Cl1 2.2839(11) [2.2890(12)]; N1-C1-P1 110.1(3) [110.6(3)], P1-Au1-Cl1 177.29(4) [173.02(4)]. E-7 a: N1¢C1 1.264(4), N1¢C14 1.412(4), C1¢C2 1.545(4), P1¢C1 1.887(3), P1¢C6 1.885(3), P1¢Au1 2.2462(9), Au1¢Cl1 2.2862(9); N1-C1-P1 108.0(2), P1-Au1-Cl1 178.95(3).
of 2-pyridyldiphenylphosphane gold(I) chloride (A¢AuICl; P¢Au 2.234(4), Au¢Cl 2.286(4) æ),[17] suggesting comparable P-donor properties. As expected, the N-sites differ, as exemplified by the C=N stretching frequency of 1639 cm¢1 for 6 a and the pyridine C=N stretch of 1571 cm¢1 for A¢AuICl).[9] The N-imine substituent is readily modified as illustrated by the reaction of N-alkyl-substituted 3 d with (tht)AuCl that affords AuI complex 6 d (90 %; d31P = 34.3 ppm, n(C=N) = 1612 cm¢1; Scheme 2). The P substituents are equally readily modifiable. Illustrative is the reaction of tBu2P-substituted 4 a with (Me2S)AuCl that quantitatively yielded AuI complex 7 a (d31P = 60.1 ppm). Its molecular structure (Figure 2, right)[16] shows that ligand 4 a has retained its E-conformation. Rhodium and iridium coordination of the iminophosphanes was explored next (Scheme 3). Mixing 3 a with 0.5 equiv of
Scheme 3. Coordination chemistry of iminophosphanes with [Cp*MCl2]2.
[Cp*RhCl2]2 in DCM gave two products in a 6.3:1.0 ratio. The major one (d31P = ¢4.4 ppm; 1J(P,Rh) = 113.9 Hz) is bidentate complex 10 a-Cl, in which a Rh-chloride is displaced for the imine nitrogen. The minor product (d31P = 43.3 ppm; 1J(P,Rh = 141.9 Hz) is assigned to monodentate complex 8 a,[19] which, like A, coordinates only the phosphane.[20] Adding AgOTf (OTf = triflate) enables its conversion to the P,N-bidentate complexes, as the triflate anion coordinates only weakly,[3b, 20, 21] 9329
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Scheme 4. Influence of the imine substituents on the N-donor capacity. [a] E (N-LP) is the energy difference between the N-lone pair containing orbitals of [10*] + and [A-RhIIICp*Cl] + .
Figure 3. Molecular structures of bidentate rhodium complexes 10 a-OTf (left) at the 50 % probability level and 10 d-OTf (right) at the 30 % probability level. Hydrogen atoms and the triflate counter ion are omitted for clarity. Selected bond lengths (æ) and angles (8) of 10 a-OTf: N1¢C1 1.298(8), N1¢C6 1.430(7), N1¢Rh1 2.106(5), C1¢C2 1.513(8), P1¢C1 1.870(6), P1¢C12 1.805(6), P1¢Rh1 2.2931(16), Rh1¢Cl1 2.3952(15); N1-C1-P1 97.4(4), P1-Rh1-N1 66.29(15). 10 d-OTf (values of the other two crystallographic independent molecules in the asymmetric units are in brackets): N1¢C1 1.281(9) [1.300(10), 1.276(9)], N1¢C2 1.485(9) [1.470(10), 1.497(9)], N1¢Rh1 2.175(6) [2.187(6), 2.159(6)], C1¢C5 1.484(10) [1.467(10), 1.486(10)], P1¢C1 1.836(7) [1.826(8), 1.845(7)], P1¢C11 1.818(8) [1.804(9), 1.797(7)], P1¢Rh1 2.325(2) [2.339(2), 2.314(2)], Rh1¢Cl1 2.387(2) [2.384(2), 2.389 (2)]; N1-C1-P1 102.9(5) [103.8(5), 101.7(5)], P1-Rh1-N1 66.25(16) [66.38(18), 66.23(16)].
The impact of changing the imine substituents became even more evident when [Cp*RhCl2]2 was reacted with a 1.3:1.0 E/Z-mixture of 3 d to give only a single isomer of P1 monodentate 8d (d31P = 30.6 ppm, J(P,Rh) = 144.3 Hz; Scheme 3). Not only does this underscore the ease of E/Z-isomerization of the ligand, it also reveals a reduced N-donor strength as evidenced by the absence of bidentate 10 d-Cl. Full conversion to 10 d-OTf (76 %; d31P = ¢6.2 ppm, 1J(P,Rh) = 117.0 Hz; Scheme 3) resulted on adding AgOTf. The molecular structure (Figure 3, right)[16] shows imine (1.276(7) æ) and P1¢ C1 (1.845(5) æ) bonds of similar lengths as those of the free ligand, but the N1-C1-P1 angle compression from 119.2(3)8 to 101.7(4)8) illustrates the effect of forming the Rh-cycle. The reaction of iridium complex [Cp*IrCl2]2 with 3 a yielded only bidentate 11 a-Cl (67 %; d31P = ¢26.5 ppm; no monodentate 9 aCl was detected) and on ion exchange 11 a-OTf (83 %) with virtually identical NMR spectra. The question then arises: how tunable is the N-donor site of iminophosphane and how does it compare to pyridyldiphenylphosphane A? An impression is obtained by comparing the Rh¢N bond strengths of the experimentally ascertained complexes 10 a, 10 d, and [A-RhIIICp*Cl] + ,[20] which can be estimated from their ring opening to the corresponding P-monodentate complexes as shown in Scheme 4.[22] Using fully optimized geometries, the respective bond strengths are 21.1, 16.0, and 12.1 kcal mol¢1 at wB97X-D/6-31 + G(d,p) (Def2-TZVP for Rh). Chem. Eur. J. 2015, 21, 9328 – 9331
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This underscores that 3 a is a better N-donor than both 3 d and A, which is corroborated by the relative energies of 1.8 and 0.9 eV for the N-lone pair containing orbitals of the respective P-monodentate complexes [10 a*] + and [10 d*] + compared to that of [A-RhIIICp*Cl] + (see also the Supporting Information). To delineate the influence of aryl versus alkyl N- and C-substituents on the N-donor strength of the iminophosphane ligand, we modeled complexes 10 b and 10 f–h bearing only Me and Ph substituents (lower half of Scheme 4). The relative energies illustrate that changing the N-imine substituent significantly impacts the N-donor capacity of the iminophosphane ligands, which is further fine-tuned by changing the C-substituent. In summary, iminophosphanes with diverse substituents at the core P, N, and C centers are readily obtained from nitrilium ions and secondary phosphanes. These electronically tunable 1,3-P,N-ligands give selectively, and in high yield, mono- or bidentate complexes, depending on the choice of the coordinating metal. Their potential in homogeneous catalysis will be substantial.
Acknowledgements This work was supported by the Council for Chemical Sciences of The Netherlands Organization for Scientific Research (NWO/ CW). We thank E. Janssen (VU University Amsterdam) and J. W. H. Peeters (University of Amsterdam) for measuring high resolution mass-spectra. Keywords: ligand design · coordination modes · synthetic methods · N,P ligands · density functional calculations
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[1] For a review on hybrid ligands, see: W.-H. Zhang, S. W. Chien, T. S. A. Hor, Coord. Chem. Rev. 2011, 255, 1991. [2] a) For a review on A, see: Z.-Z. Zhang, H. Cheng, Coord. Chem. Rev. 1996, 147, 1; b) P. Braunstein, D. G. Kelly, A. Tiripicchio, F. Ugozzoli, Bull. Soc. Chim. Fr. 1995, 132, 1083; c) C. Tejel, M. A. Ciriano, S. Jim¦nez, L. A. Oro, C. Graiff, A. Tiripicchio, Organometallics 2005, 24, 1105; d) M. J. Calhorda, C. Ceamanos, O. Crespo, M. C. Gimeno, A. Laguna, C. Larraz, P. D. Ó 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Communication
[3]
[4] [5]
[6] [7] [8] [9] [10]
[11]
[12] [13] [14]
[15]
Vaz, M. D. Villacampa, Inorg. Chem. 2010, 49, 8255; e) For a review on bimetallic P,N complexes see: S. Maggini, Coord. Chem. Rev. 2009, 253, 1793. a) K. Kurtev, D. Ribola, R. A. Jones, D. J. Cole-Hamilton, G. Wilkinson, J. Chem. Soc. Dalton Trans. 1980, 55; b) D. Drommi, F. Nicolý, C. G. Arena, G. Bruno, F. Faraone, Inorg. Chim. Acta 1994, 221, 109. G. Franciý, R. Scopelliti, C. G. Arena, G. Bruno, D. Drommi, F. Faraone, Organometallics 1998, 17, 338. a) E. Drent, P. Arnoldy, P. H. M. Budzelaar, J. Organomet. Chem. 1993, 455, 247; b) E. Drent, P. Arnoldy, P. H. M. Budzelaar, J. Organomet. Chem. 1994, 475, 57; c) J. J. M. de Pater, C. E. P. Maljaars, E. de Wolf, M. Lutz, A. L. Spek, B.-J. Deelman, C. J. Elsevier, G. van Koten, Organometallics 2005, 24, 5299. I. Moldes, E. de La Encarnaciûn, J. Ros, A. Alvarez-Larena, J. F. Piniella, J. Organomet. Chem. 1998, 566, 165. B. A. Garcia, D. Y. Gin, J. Am. Chem. Soc. 2000, 122, 4269. T. Oshiki, H. Yamashita, K. Sawada, M. Utsunomiya, K. Takahashi, K. Takai, Organometallics 2005, 24, 6287. C. Khin, A. S. K. Hashmi, F. Rominger, Eur. J. Inorg. Chem. 2010, 2010, 1063. For selected examples of 6-substituted pyridylphosphanes, see: a) L. Hintermann, T. T. Dang, A. Labonne, T. Kribber, L. Xiao, P. Naumov, Chem. Eur. J. 2009, 15, 7167; b) A. C. Laungani, M. Keller, J. M. Slattery, I. Krossing, B. Breit, Chem. Eur. J. 2009, 15, 10405; c) D. B. Grotjahn, D. A. Lev, J. Am. Chem. Soc. 2004, 126, 12232; d) U. Gellrich, J. Huang, W. Seiche, M. Keller, M. Meuwly, B. Breit, J. Am. Chem. Soc. 2011, 133, 964. a) K. Issleib, O. Lçw, Z. Anorg. Allg. Chem. 1966, 346, 241; b) J. Heinicke, A. Tzschach, Z. Chem. 1978, 18, 452; c) X. Li, H. Song, C. Cui, Dalton Trans. 2009, 9728; d) D. Walther, S. Liesicke, R. Fischer, H. Gçrls, J. Weston, A. Batista, Eur. J. Inorg. Chem. 2003, 4321; e) D. Walther, S. Liesicke, L. Bçttcher, R. Fischer, H. Gçrls, G. Vaughan, Inorg. Chem. 2003, 42, 625; f) For the use of silylphosphanes, see: K. Issleib, H. Schmidt, H. Meyer, J. Organomet. Chem. 1978, 160, 47. T. Saegusa, Y. Ito, N. Yasuda, T. Hotaka, J. Org. Chem. 1970, 35, 4238. N. Merceron, K. Miqueu, A. Baceiredo, G. Bertrand, J. Am. Chem. Soc. 2002, 124, 6806. T. van Dijk, S. Burck, M. K. Rong, A. J. Rosenthal, M. Nieger, J. C. Slootweg, K. Lammertsma, Angew. Chem. Int. Ed. 2014, 53, 9068; Angew. Chem. 2014, 126, 9214. DFT calculations on ligands 3 (see the Supporting Information) were carried out with Gaussian 09; Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
Chem. Eur. J. 2015, 21, 9328 – 9331
www.chemeurj.org
[16]
[17] [18]
[19]
[20] [21] [22]
G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, ©. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian, Inc. Wallingford CT, 2009. CCDC 1009957 (3 d), 1009958 (5 a), 1009959 (6 a), 1009960 (7 a), 1009961 (10 a-OTf), and 1009962 (10 d-OTf) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. N. W. Alcock, P. Moore, P. A. Lampe, K. F. Mok, J. Chem. Soc. Dalton Trans. 1982, 207. a) A. S. K. Hashmi, G. J. Hutchings, Angew. Chem. Int. Ed. 2006, 45, 7896; Angew. Chem. 2006, 118, 8064; b) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180; c) A. Arcadi, Chem. Rev. 2008, 108, 3266; d) D. J. Gorin, B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351; e) Z. Li, C. Brouwer, C. He, Chem. Rev. 2008, 108, 3239; f) A. S. K. Hashmi, Angew. Chem. Int. Ed. 2010, 49, 5232; Angew. Chem. 2010, 122, 5360. Coordination of iminophosphanes 3 to Cp*RhCl2 favors P over N (DE P vs. N coordination for 3 a = 29.3 kcal mol¢1 and for 3 d = 30.1 kcal mol¢1) at wB97X-D/6-31 + G(d,p) (Def2-TZVP for rhodium); see the Supporting Information. M. L. Clarke, A. M. Z. Slawin, M. V. Wheatley, J. D. Woollins, J. Chem. Soc. Dalton Trans. 2001, 3421. K. Wajda-Hermanowicz, Z. Ciunik, A. Kochel, Inorg. Chem. 2006, 45, 3369. DFT calculations on Rh complexes 8, 10, 12, and 13, and Ir complex 11 a were carried out with Gaussian09 (Revision D.01); see the Supporting Information.
Received: March 23, 2015 Published online on May 15, 2015
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