COMMUNICATION DOI: 10.1002/asia.201402900
Facile Insertion of Carbon Dioxide into Cu2ACHTUNGRE(m-H) Dinuclear Units Supported by Tetraphosphine Ligands Kanako Nakamae, Bunsho Kure, Takayuki Nakajima, Yasuyuki Ura, and Tomoaki Tanase*[a]
Abstract: Reactions of meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) with CuI species in the presence of NaBH4 afforded di- and tetranuclear copper hydride complexes, [Cu2ACHTUNGRE(m-H)ACHTUNGRE(m-dpmppm)2]X (1) and [Cu4ACHTUNGRE(m-H)2ACHTUNGRE(m4-H)ACHTUNGRE(m-dpmppm)2]X (2) (X = BF4, PF6). Complex 1 undergoes facile insertion of CO2 (1 atm) at room temperature, leading to a formate-bridged dicopper complex [Cu2ACHTUNGRE(m-HCOO)ACHTUNGRE(dpmppm)2]X (3). The experimental and DFT theoretical studies clearly demonstrate that CO2 insertion into the Cu2ACHTUNGRE(m-H) unit occurred with the flexible dicopper platform. Complex 2 also undergoes CO2 insertion to give a formate-bridged complex, [Cu4ACHTUNGRE(m-HCOO)3 ACHTUNGRE(dpmppm)2]X, during which the square Cu4 framework opened up to a linear tetranuclear chain.
Strykers finding of the use of the hexanuclear copper hydride cluster [Cu6H6ACHTUNGRE(PPh3)6] as a Cu H precursor.[4] Recently, copper-catalyzed hydrosilylation and hydroboration of CO2 were established by the mononuclear copper complex of 1,2-bis(diphenylphosphino)benzene (dppbz)[5a] and an Nheterocyclic carbene,[5b,c] respectively. Despite the recent striking progress in catalytic reactions by using in-situ generated Cu H species, detailed molecular studies on synthesizing discrete copper hydride complexes have still been limited mainly to the traditional hexanuclear clusters of [Cu6H6ACHTUNGRE(PR3)6] (R = phenyl, p-tolyl, and NMe2)[6a–c] and the related compounds [Cu5H5ACHTUNGRE(PPh3)5][6d] and [Cu8H8ACHTUNGRE(dppp)4] (dppp = 1,3-bis(diphenylphosphino)propane);[6c] however, it is important to note that Liu et al. have recently developed high-nuclearity chalcogen-stabilized copper clusters, [Cu28H15ACHTUNGRE(S2CNR2)12]PF6 (R = nPr, R2 = aza15-crown-5), which confine 15 hydride anions in the rhombicuboctahedral Cu24 cage as a sort of hydrogen reservoir.[7] By contrast, as to copper hydride complexes with lower nuclearity, which are potentially important as hydrogen-donor key reductants, very few isolated and characterized examples of di- and trinuclear complexes, [Cu2ACHTUNGRE(m-H)2ACHTUNGRE(h2-CH3CACHTUNGRE(CH2PPh2)3)2],[8] [Cu2ACHTUNGRE(m-H)2(L)2] (L = N-heterocyclic carbene (NHC),[9a] cyclic alkylACHTUNGRE(amino)carbene (CAAC)),[10] [Cu3ACHTUNGRE(m3-H)(Cy2PCH2PCy2)3]2 + ,[11] and [Cu3ACHTUNGRE(m-H)3ACHTUNGRE(dppbz)2][12] have been reported. Although these complexes are stable, even in solution, they are unfortunately inactive toward reductions of CO2. Recently, Sadighi et al. reported a dicopper hydride complex with NHC ligands, [Cu2ACHTUNGRE(m-H)ACHTUNGRE(IDipp)2] + (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), which contains a hydridic bent Cu2ACHTUNGRE(m-H) core and reacts with CO2 under mild conditions to afford a formate-bridged dicopper complex via the mononuclear complex [CuACHTUNGRE(HCOO)ACHTUNGRE(IDipp)].[9b] The formate-generating process from insertion of CO2 into M H bond has been recognized as a crucial step in the initial stage of CO2 hydrogenations.[1b] In the present study, we have tried to synthesize multinuclear copper hydride complexes supported by a linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm),[13] and we were successful in isolating di- and tetranuclear copper hydride complexes, [Cu2 ACHTUNGRE(m-H)ACHTUNGRE(dpmppm)2] + (1) and [Cu4ACHTUNGRE(m-H)3ACHTUNGRE(dpmppm)2] + (2), which are quite stable in the solution states but nevertheless readily undergo insertion of CO2 to afford formate-bridged complexes. The experimental and DFT theoretical studies
Carbon dioxide is a ubiquitous C1 resource that can be recovered from a variety of processes including industrial chemistry, agricultural productions, and electric power generations, and with this respect, reduction of CO2 under lowcost catalytic conditions is one of the most important subjects in modern chemistry.[1] Especially, homogeneous reductions of CO2 under mild conditions catalyzed by inexpensive, 3d base metal complexes such as iron and copper are highly desired innovations. In homogeneous systems, hydrogenations of CO2 have been developed mostly with precious organometallic species such as Ru, Rh, and Ir complexes,[1a–c] and those by Fe and Cu complexes have still been unexplored,[2] although metallic copper is well known to promote hydrogenation of CO2 to CO and CH3OH etc. in many heterogeneous systems.[1b] On the basis of these backgrounds, copper hydride complexes have attracted widely and intensively growing interests, and the use of in-situ generated Cu H species for chemoselective reductions of unsaturated organic compounds[3] have been developed since [a] K. Nakamae, Dr. B. Kure, Prof. T. Nakajima, Prof. Y. Ura, Prof. T. Tanase Department of Chemistry Faculty of Science Nara Womens University Kitauoya-nishi-machi, Nara, 630-8506 (Japan) Fax: (+ 81) 742-20-3847 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402900.
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for 1 conclusively demonstrated a novel stoichiometric CO2 insertion into Cu2ACHTUNGRE(m-H) dinuclear units, which should be highly important in relation to the insertion promoted on the surface of metallic copper materials. Reactions of [CuACHTUNGRE(CH3CN)4]X (X = BF4, PF6) with 1 equivalent of dpmppm in the presence of NaBH4 in a CH3CN/ CH3OH (1:1) mixed solvent afforded colorless needle crystals of [Cu2ACHTUNGRE(m-H)-(m-dpmppm)2]X (X = BF4 (1 a), PF6 (1 b)) in high yields. When the copper(I) salts were treated with dpmppm in a ratio of 2:1 in the presence of NaBH4 in CH3CN/EtOH (1:1) solvent, pale yellow tetranuclear complexes [Cu4ACHTUNGRE(m-H)2ACHTUNGRE(m4-H)ACHTUNGRE(m-dpmppm)2]X (X = BF4 (2 a), PF6 (2 b)) were isolated in about 20 % yield together with the formation of dicopper complexes 1. The crystal structure of 1 a (Figure 1 (a)) contains a dinuclear CuI center bridged by a hydride and two dpmppm li-
theses at Cu H = 1.62 and Cu-H-Cu = 1208. The most conspicuous feature is that the bridging hydride is sterically well protected by the phenyl groups of the chelating terminal P atoms (P1, P1*) (Figure 3 (a),(c)). The 1H NMR spectrum of 1 in CD3CN showed a broad singlet at d 0.16 ppm assignable to the hydride, which is confirmed by comparing the spectra with the deuterated complex [Cu2ACHTUNGRE(m-D)ACHTUNGRE(mdpmppm)2]BF4 (1 a-D). Complex 1 a-D was prepared by the similar reaction with NaBD4. The 1H NMR spectrum of 1 aD is identical to that of 1 a except for the absence of the hydride peak, and instead, the 2H NMR spectrum showed the hydride signal at d 0.16 ppm (Figure S7 in the Supporting Information). The 31P{1H} NMR spectra of 1 a exhibited three broad resonances at around 1.6, 4.9, and 8.4 ppm in a 1:1:1 ratio, corresponding to the coordinated P atoms, and a sharp doublet of doublets at 39.2 ppm (2JPP’ = 81, 66 Hz) corresponding to the uncoordinate P atoms. The structure of 2 b consists of a rectangular Cu4 framework bridged by two dpmppm ligands as well as two m2- and one m4-hydride anions to result in a crystallographically imposed Ci symmetry (Figure 1 (b)). The hydride positions were determined from difference Fourier maps (Cu1 H37 = 1.52 , Cu2 H37 = 1.57 , Cu1 H38 = 1.82 , Cu2 H38 = 1.90 ). The Cu1···Cu2 distance (2.465(2) ) with m2-hydride is significantly shorter than the Cu1···Cu2* distance without hydride (2.789(1) ). It should be noted that complex 2 b is the first characterized example of a tetranuclear copper hydride complex. The ESI-MS spectrum of [Cu4ACHTUNGRE(m-D)2ACHTUNGRE(m4-D)ACHTUNGRE(m-dpmppm)2]PF6 (2 b-D3) exhibits a prominent complex cation peak at m/z 1516.20, which is shifted by three mass units from the signal of 2 b at m/z 1513.22 (Figure S10 in the Supporting Information). The 1HACHTUNGRE{31P} NMR spectrum of 2 b showed two broad peaks at d 1.44 and 1.08 ppm in a 2:1 ratio, which are assigned to the m2- and m4-hydride, respectively, by comparing with the 1H and 2H NMR spectra of 2 b-D3 (Figure S8 in the Supporting Information). These crystal and spectral data clearly demonstrated that the diand tetranuclear copper hydride complexes 1 and 2 are significantly stable in the solution states. When a CH3CN solution containing 1 a was exposed to CO2 (1 atm) at room temperature, the hydride complex 1 a was readily converted quantitatively into the formatebridged dicopper complex [Cu2ACHTUNGRE(m-HCOO)ACHTUNGRE(m-dpmppm)2]BF4 (3 a), as monitored by 31P{1H} NMR spectroscopy (Scheme 1 and Figure 2). Complex 3 a was isolated in 78 % yield and characterized by X-ray crystallography (Figure S3 in the Supporting Information) to be composed of a dinuclear copper core bridged by a formate and two dpmppm ligands. The formate bridges the two copper atoms in a pseudo C2 symmetrical m-h1:h1-fashion (O1 C1 = 1.244(5) , O2 C1 = 1.243(4) , O1-C1-O2 = 128.7(3)8). The Cu···Cu distance is dramatically elongated to 3.8111(5) from 2.7948(7) of 1 a, indicating that the phenyl groups of the terminal P atoms were unfolded outside in accord with CO2 insertion. In the 1HACHTUNGRE{31P} NMR spectrum, the hydride signal of 1 a disappeared and a new signal was observed at 8.88 ppm corresponding to the formate proton. The deuterated complex,
Figure 1. The structures for the complex cations of 1 a (a) and 2 b (b) with the atomic numbering schemes. The carbon atoms are drawn as capped-stick models and the hydrogen atoms except the hydrides are omitted for clarity.
gands to possess a crystallographic C2 symmetry.[14] Each dpmppm ligand chelates to Cu1 with P1 and P3 atoms, forming a six-membered ring, and bridges two copper ions (Cu1, Cu1*) with P3 and P4 atoms, with the remaining inner P2 atom uncoordinated. The Cu···Cu distance of 2.7948(7) is considerably longer than those observed in the Cu2ACHTUNGRE(m-H)2 complexes (2.3058(5) ,[8] 2.348(2) ,[9a] and 2.371(2) [10]) and [Cu2ACHTUNGRE(m-H)ACHTUNGRE(IDipp)2] + (avg. 2.543 ),[9b] but is appreciably shorter than those found in [Cu3ACHTUNGRE(m3H)(Cy2PCH2PCy2)3]2 + (avg. 2.882 ),[11] thus indicating no direct metal metal bonding interaction in the Cu2ACHTUNGRE(m-H) unit. The hydride was determined by difference Fourier syn-
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Figure 3. Perspective views of 1 a (a) and 1 a* (b) showing the steric protections of the hydride by the phenyl groups on the terminal P atoms (P1/P1* and P4/P8). The H label of 1 a* just shows a tentative position of the hydride. Space-filling views of 1 a (c) and 1 a* (d) projected along the Cu-H-Cu plane.
dinuclear unit (Scheme 1). All CO2 insertions into the Cu H bond reported thus far are promoted by a mononuclear copper species, and no such reaction on a dicopper center has been disclosed yet. To elucidate the effects of the uncoordinate phosphine units of 1 a, an analogous dicopper hydride complex with the phosphine oxide moieties, [Cu2ACHTUNGRE(m-H)(m-dpmppmO)2]BF4 (1 a*: dpmppmO = Ph2PCH2P(Ph)CH2P(=O)(Ph)CH2PPh2), was prepared through NaBH4 reduction of [Cu2ACHTUNGRE(m-Cl)(mdpmppmO)2]BF4 (4 a*), which was generated by treatment of 4 a with H2O2. The structure of 1 a* was confirmed by Xray crystallography to show an almost identical structure to 1 a except that the uncoordinate phosphine units are converted into phosphine oxides, which causes a slight but important structural deformation: the Cu···Cu distance of 1 a* (2.7581(4) ) is slightly shorter than that of 1 a (2.7948(7) ) and the hydride is further hindered by the phenyl groups of the terminal P atoms so as to avoid steric repulsions around the phosphine oxide moieties (Figure 3 (b),(d) and Figure S4 in the Supporting Information). Interestingly, the CO2 insertion of 1 a* (1 atm, 25 8C) in CD3CN proceeded more slowly in comparison with that of 1 a (Figure 2), presumably because a larger hindrance of the phenyl groups reduced the hydride reactivity. The pseudofirst-order rate constant (kobs) for the initial part of the reaction of 1 a (4.3 10 4 s 1) is appreciably larger than that of 1 a* (7.1 10 5 s 1). To investigate the reaction mechanism of the CO2 insertion into 1 a, DFT calculations were performed on a model complex, [Cu2ACHTUNGRE(m-H)ACHTUNGRE(m-PhP(H)CH2P(H)CH2P(H)CH2P(H)Ph)2] + (M1), with B3LYP functionals. In the model complex M1, four phenyl groups on the terminal P atoms are included to elucidate their steric effects, and other phenyl groups are replaced with H atoms. The ground states GS0 and GS1 were optimized at reasonable structures in agreement with the real structures of
Scheme 1. Sequential transformations of the Cu2ACHTUNGRE(m-H) complex 1 a.
Figure 2. Time-dependent ratios of the hydride complexes 1 a (*) and 1 a* (*) and the formate complexes 3 a (*) and 3 a* (*), monitored by 31 P{1H} NMR spectroscopy for the reactions of 1 a and 1 a* (1.4 10 2 m) with CO2 (1 atm) in CD3CN at 25 8C.
[Cu2ACHTUNGRE(m-DCOO)ACHTUNGRE(m-dpmppm)2]BF4 (3 a-D), was prepared by reacting 1 a-D with CO2, and showed a resonance at 8.87 ppm in the 2H NMR spectrum, demonstrating that the formate proton is derived from the hydride of 1 a via CO2 insertion. In the 31P{1H} NMR spectrum of 3 a, the characteristic signal corresponding to the uncoordinate P atoms shifted from 39.2 ppm (1 a) to 39.5 ppm (2JPP’ = 75, 67 Hz), which was used to monitor the reaction. Complex 3 a reacted with HCl to afford [Cu2ACHTUNGRE(m-Cl)ACHTUNGRE(mdpmppm)2]BF4 (4 a) with releasing 1 equivalent of formic acid (Scheme 1). When 3 a was treated with HBF4, the dicopper complex without a mono-atom bridge, [Cu2ACHTUNGRE(mdpmppm)2]ACHTUNGRE(BF4)2 (5 a), was obtained. Notably, 4 a and 5 a were converted by treatment with NaBH4 into the starting hydride complex 1 a, establishing a sequential cycle that involves an unprecedented CO2 insertion into the Cu2ACHTUNGRE(m-H)
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adopt linear Cu4 structures containing two terminal acetonitrile ligands in CH3CN. These results revealed that the square Cu4 framework of 2 b opened up to a linear Cu4 chain upon insertion of CO2 into Cu2ACHTUNGRE(m-H) units. In summary, by using the tetraphosphine dpmppm we have synthesized di- and tetranuclear copper hydride complexes that are highly reactive for insertion of CO2 into Cu2 ACHTUNGRE(m-H) dinuclear units. We will try to develop catalytic reactions that include the facile CO2 insertion by the dicopper hydride complexes in combination with various hydrogen sources.
Experimental Section Preparation of [Cu2ACHTUNGRE(m-H)ACHTUNGRE(m-dpmppm)2]BF4 (1 a): To a CH3CN/MeOH (1:1) solution (10 mL) of dpmppm (295 mg, 469 mmol) was added [CuACHTUNGRE(CH3CN)4]BF4 (150 mg, 476 mmol) and NaBH4 (165 mg, 4.35 mmol), and the reaction mixture was stirred for 3 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with 10 mL of CH2Cl2. The extract was passed through a membrane filter and concentrated to ca. 3 mL, to which diethyl ether was carefully added. The solution was allowed to stand in a refrigerator to afford colorless needle crystals of 1 a (290 mg, 197 mmol, 83 %). The detailed synthetic procedures and analytical data for all the compounds are described in the Supporting Information.
Figure 4. Simplified Gibbs free-energy profiles showing reaction pathway for insertion of CO2 into M1. Numbers in parentheses are differences in Go298 relative to the starting compounds (GS0 + CO2) in kcal mol 1.
1 a and 3 a, indicating a thermodynamically downhill process (Figure 4). The structure of GS1 with a m-h1:h1-formate bridge is appreciably stable compared with that of GS2 with a m-h2-formate bridge, and a transition state TS1 between GS0 and GS1 is successfully determined to reveal that a charge transfer from m-H to CO2 in a symmetrical fashion is a crucial step for the insertion. The activation of CO2 by a Cu···O electrostatic interaction is not involved in TS1 because pre-coordination of CO2 to one of the Cu atoms in {HCu···CuACHTUNGRE(OCO)} asymmetric structure (X in Figure S17 in the Supporting Information) would generate an extremely large steric distortion as indicated by its unreasonably high energy. The pathway as shown in Figure 4 is essentially similar to that proposed by Milstein et al. for decarboxylation of formate by an Fe PNP-pincer complex[15a] and that by Yang for CO2 insertion into an Ir H bond.[15b] Sakaki et al. also noted in their pioneering work that a nucleophilic attack of Cu H on CO2 carbon is more important rather than an electrophilic activation by Cu OCO coordination for insertion of CO2 into mononuclear Cu H complexes with phosphine ligands.[16] These experimental and theoretical results clearly demonstrate that the CO2 insertion into the Cu2ACHTUNGRE(m-H) unit proceeded on the flexible dinuclear scaffold supported by dpmppm ligands. Complex 2 b also reacted with CO2 (1 atm, 25 8C) to yield [Cu4ACHTUNGRE(m-HCOO)3ACHTUNGRE(m-dpmppm)2]PF6 (6 b) in 69 % yield (Scheme S1, Supporting Information). By treatment with HCl, 6 b was transformed into a chloride-bridged tetracopper complex, [Cu4ACHTUNGRE(m-Cl)3ACHTUNGRE(m-dpmppm)2]PF6 (7 b), generating 3 equivalents of formic acid. The analogous complex, [Cu4 ACHTUNGRE(m-Cl)3ACHTUNGRE(m-dpmppm)2ACHTUNGRE(CH3CN)]BF4 (7 a·CH3CN) was crystalized from CH3CN/Et2O and its crystal structure indicated a linear Cu4ACHTUNGRE(m-Cl)3 core bridged by two dpmppm ligands with a terminal acetonitrile ligand, implying that 6 b and 7 b
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Acknowledgements This work was partly supported by Grants-in-Aid for Scientific Research and that on Priority Area 2107 (no. 22108521, 24108727) from the Ministry of Education, Science, Sports, and Culture of Japan. KN is grateful to JSPS fellowship.
Keywords: CO2 insertion · copper hydride complex · dinuclear complex · formate complex · tetraphosphine ligand
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Received: July 30, 2014 Published online: September 9, 2014
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Facile Insertion of Carbon Dioxide into Cu2ACHTUNGRE(m-H) Dinuclear Units Supported by Tetraphosphine Ligands Kanako Nakamae, Bunsho Kure, Takayuki Nakajima, Yasuyuki Ura, and Tomoaki Tanase*[a]
Abstract: Reactions of meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) with CuI species in the presence of NaBH4 afforded di- and tetranuclear copper hydride complexes, [Cu2ACHTUNGRE(m-H)ACHTUNGRE(m-dpmppm)2]X (1) and [Cu4ACHTUNGRE(m-H)2ACHTUNGRE(m4-H)ACHTUNGRE(m-dpmppm)2]X (2) (X = BF4, PF6). Complex 1 undergoes facile insertion of CO2 (1 atm) at room temperature, leading to a formate-bridged dicopper complex [Cu2ACHTUNGRE(m-HCOO)ACHTUNGRE(dpmppm)2]X (3). The experimental and DFT theoretical studies clearly demonstrate that CO2 insertion into the Cu2ACHTUNGRE(m-H) unit occurred with the flexible dicopper platform. Complex 2 also undergoes CO2 insertion to give a formate-bridged complex, [Cu4ACHTUNGRE(m-HCOO)3 ACHTUNGRE(dpmppm)2]X, during which the square Cu4 framework opened up to a linear tetranuclear chain.
Strykers finding of the use of the hexanuclear copper hydride cluster [Cu6H6ACHTUNGRE(PPh3)6] as a Cu H precursor.[4] Recently, copper-catalyzed hydrosilylation and hydroboration of CO2 were established by the mononuclear copper complex of 1,2-bis(diphenylphosphino)benzene (dppbz)[5a] and an Nheterocyclic carbene,[5b,c] respectively. Despite the recent striking progress in catalytic reactions by using in-situ generated Cu H species, detailed molecular studies on synthesizing discrete copper hydride complexes have still been limited mainly to the traditional hexanuclear clusters of [Cu6H6ACHTUNGRE(PR3)6] (R = phenyl, p-tolyl, and NMe2)[6a–c] and the related compounds [Cu5H5ACHTUNGRE(PPh3)5][6d] and [Cu8H8ACHTUNGRE(dppp)4] (dppp = 1,3-bis(diphenylphosphino)propane);[6c] however, it is important to note that Liu et al. have recently developed high-nuclearity chalcogen-stabilized copper clusters, [Cu28H15ACHTUNGRE(S2CNR2)12]PF6 (R = nPr, R2 = aza15-crown-5), which confine 15 hydride anions in the rhombicuboctahedral Cu24 cage as a sort of hydrogen reservoir.[7] By contrast, as to copper hydride complexes with lower nuclearity, which are potentially important as hydrogen-donor key reductants, very few isolated and characterized examples of di- and trinuclear complexes, [Cu2ACHTUNGRE(m-H)2ACHTUNGRE(h2-CH3CACHTUNGRE(CH2PPh2)3)2],[8] [Cu2ACHTUNGRE(m-H)2(L)2] (L = N-heterocyclic carbene (NHC),[9a] cyclic alkylACHTUNGRE(amino)carbene (CAAC)),[10] [Cu3ACHTUNGRE(m3-H)(Cy2PCH2PCy2)3]2 + ,[11] and [Cu3ACHTUNGRE(m-H)3ACHTUNGRE(dppbz)2][12] have been reported. Although these complexes are stable, even in solution, they are unfortunately inactive toward reductions of CO2. Recently, Sadighi et al. reported a dicopper hydride complex with NHC ligands, [Cu2ACHTUNGRE(m-H)ACHTUNGRE(IDipp)2] + (IDipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), which contains a hydridic bent Cu2ACHTUNGRE(m-H) core and reacts with CO2 under mild conditions to afford a formate-bridged dicopper complex via the mononuclear complex [CuACHTUNGRE(HCOO)ACHTUNGRE(IDipp)].[9b] The formate-generating process from insertion of CO2 into M H bond has been recognized as a crucial step in the initial stage of CO2 hydrogenations.[1b] In the present study, we have tried to synthesize multinuclear copper hydride complexes supported by a linear tetraphosphine, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm),[13] and we were successful in isolating di- and tetranuclear copper hydride complexes, [Cu2 ACHTUNGRE(m-H)ACHTUNGRE(dpmppm)2] + (1) and [Cu4ACHTUNGRE(m-H)3ACHTUNGRE(dpmppm)2] + (2), which are quite stable in the solution states but nevertheless readily undergo insertion of CO2 to afford formate-bridged complexes. The experimental and DFT theoretical studies
Carbon dioxide is a ubiquitous C1 resource that can be recovered from a variety of processes including industrial chemistry, agricultural productions, and electric power generations, and with this respect, reduction of CO2 under lowcost catalytic conditions is one of the most important subjects in modern chemistry.[1] Especially, homogeneous reductions of CO2 under mild conditions catalyzed by inexpensive, 3d base metal complexes such as iron and copper are highly desired innovations. In homogeneous systems, hydrogenations of CO2 have been developed mostly with precious organometallic species such as Ru, Rh, and Ir complexes,[1a–c] and those by Fe and Cu complexes have still been unexplored,[2] although metallic copper is well known to promote hydrogenation of CO2 to CO and CH3OH etc. in many heterogeneous systems.[1b] On the basis of these backgrounds, copper hydride complexes have attracted widely and intensively growing interests, and the use of in-situ generated Cu H species for chemoselective reductions of unsaturated organic compounds[3] have been developed since [a] K. Nakamae, Dr. B. Kure, Prof. T. Nakajima, Prof. Y. Ura, Prof. T. Tanase Department of Chemistry Faculty of Science Nara Womens University Kitauoya-nishi-machi, Nara, 630-8506 (Japan) Fax: (+ 81) 742-20-3847 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402900.
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for 1 conclusively demonstrated a novel stoichiometric CO2 insertion into Cu2ACHTUNGRE(m-H) dinuclear units, which should be highly important in relation to the insertion promoted on the surface of metallic copper materials. Reactions of [CuACHTUNGRE(CH3CN)4]X (X = BF4, PF6) with 1 equivalent of dpmppm in the presence of NaBH4 in a CH3CN/ CH3OH (1:1) mixed solvent afforded colorless needle crystals of [Cu2ACHTUNGRE(m-H)-(m-dpmppm)2]X (X = BF4 (1 a), PF6 (1 b)) in high yields. When the copper(I) salts were treated with dpmppm in a ratio of 2:1 in the presence of NaBH4 in CH3CN/EtOH (1:1) solvent, pale yellow tetranuclear complexes [Cu4ACHTUNGRE(m-H)2ACHTUNGRE(m4-H)ACHTUNGRE(m-dpmppm)2]X (X = BF4 (2 a), PF6 (2 b)) were isolated in about 20 % yield together with the formation of dicopper complexes 1. The crystal structure of 1 a (Figure 1 (a)) contains a dinuclear CuI center bridged by a hydride and two dpmppm li-
theses at Cu H = 1.62 and Cu-H-Cu = 1208. The most conspicuous feature is that the bridging hydride is sterically well protected by the phenyl groups of the chelating terminal P atoms (P1, P1*) (Figure 3 (a),(c)). The 1H NMR spectrum of 1 in CD3CN showed a broad singlet at d 0.16 ppm assignable to the hydride, which is confirmed by comparing the spectra with the deuterated complex [Cu2ACHTUNGRE(m-D)ACHTUNGRE(mdpmppm)2]BF4 (1 a-D). Complex 1 a-D was prepared by the similar reaction with NaBD4. The 1H NMR spectrum of 1 aD is identical to that of 1 a except for the absence of the hydride peak, and instead, the 2H NMR spectrum showed the hydride signal at d 0.16 ppm (Figure S7 in the Supporting Information). The 31P{1H} NMR spectra of 1 a exhibited three broad resonances at around 1.6, 4.9, and 8.4 ppm in a 1:1:1 ratio, corresponding to the coordinated P atoms, and a sharp doublet of doublets at 39.2 ppm (2JPP’ = 81, 66 Hz) corresponding to the uncoordinate P atoms. The structure of 2 b consists of a rectangular Cu4 framework bridged by two dpmppm ligands as well as two m2- and one m4-hydride anions to result in a crystallographically imposed Ci symmetry (Figure 1 (b)). The hydride positions were determined from difference Fourier maps (Cu1 H37 = 1.52 , Cu2 H37 = 1.57 , Cu1 H38 = 1.82 , Cu2 H38 = 1.90 ). The Cu1···Cu2 distance (2.465(2) ) with m2-hydride is significantly shorter than the Cu1···Cu2* distance without hydride (2.789(1) ). It should be noted that complex 2 b is the first characterized example of a tetranuclear copper hydride complex. The ESI-MS spectrum of [Cu4ACHTUNGRE(m-D)2ACHTUNGRE(m4-D)ACHTUNGRE(m-dpmppm)2]PF6 (2 b-D3) exhibits a prominent complex cation peak at m/z 1516.20, which is shifted by three mass units from the signal of 2 b at m/z 1513.22 (Figure S10 in the Supporting Information). The 1HACHTUNGRE{31P} NMR spectrum of 2 b showed two broad peaks at d 1.44 and 1.08 ppm in a 2:1 ratio, which are assigned to the m2- and m4-hydride, respectively, by comparing with the 1H and 2H NMR spectra of 2 b-D3 (Figure S8 in the Supporting Information). These crystal and spectral data clearly demonstrated that the diand tetranuclear copper hydride complexes 1 and 2 are significantly stable in the solution states. When a CH3CN solution containing 1 a was exposed to CO2 (1 atm) at room temperature, the hydride complex 1 a was readily converted quantitatively into the formatebridged dicopper complex [Cu2ACHTUNGRE(m-HCOO)ACHTUNGRE(m-dpmppm)2]BF4 (3 a), as monitored by 31P{1H} NMR spectroscopy (Scheme 1 and Figure 2). Complex 3 a was isolated in 78 % yield and characterized by X-ray crystallography (Figure S3 in the Supporting Information) to be composed of a dinuclear copper core bridged by a formate and two dpmppm ligands. The formate bridges the two copper atoms in a pseudo C2 symmetrical m-h1:h1-fashion (O1 C1 = 1.244(5) , O2 C1 = 1.243(4) , O1-C1-O2 = 128.7(3)8). The Cu···Cu distance is dramatically elongated to 3.8111(5) from 2.7948(7) of 1 a, indicating that the phenyl groups of the terminal P atoms were unfolded outside in accord with CO2 insertion. In the 1HACHTUNGRE{31P} NMR spectrum, the hydride signal of 1 a disappeared and a new signal was observed at 8.88 ppm corresponding to the formate proton. The deuterated complex,
Figure 1. The structures for the complex cations of 1 a (a) and 2 b (b) with the atomic numbering schemes. The carbon atoms are drawn as capped-stick models and the hydrogen atoms except the hydrides are omitted for clarity.
gands to possess a crystallographic C2 symmetry.[14] Each dpmppm ligand chelates to Cu1 with P1 and P3 atoms, forming a six-membered ring, and bridges two copper ions (Cu1, Cu1*) with P3 and P4 atoms, with the remaining inner P2 atom uncoordinated. The Cu···Cu distance of 2.7948(7) is considerably longer than those observed in the Cu2ACHTUNGRE(m-H)2 complexes (2.3058(5) ,[8] 2.348(2) ,[9a] and 2.371(2) [10]) and [Cu2ACHTUNGRE(m-H)ACHTUNGRE(IDipp)2] + (avg. 2.543 ),[9b] but is appreciably shorter than those found in [Cu3ACHTUNGRE(m3H)(Cy2PCH2PCy2)3]2 + (avg. 2.882 ),[11] thus indicating no direct metal metal bonding interaction in the Cu2ACHTUNGRE(m-H) unit. The hydride was determined by difference Fourier syn-
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Figure 3. Perspective views of 1 a (a) and 1 a* (b) showing the steric protections of the hydride by the phenyl groups on the terminal P atoms (P1/P1* and P4/P8). The H label of 1 a* just shows a tentative position of the hydride. Space-filling views of 1 a (c) and 1 a* (d) projected along the Cu-H-Cu plane.
dinuclear unit (Scheme 1). All CO2 insertions into the Cu H bond reported thus far are promoted by a mononuclear copper species, and no such reaction on a dicopper center has been disclosed yet. To elucidate the effects of the uncoordinate phosphine units of 1 a, an analogous dicopper hydride complex with the phosphine oxide moieties, [Cu2ACHTUNGRE(m-H)(m-dpmppmO)2]BF4 (1 a*: dpmppmO = Ph2PCH2P(Ph)CH2P(=O)(Ph)CH2PPh2), was prepared through NaBH4 reduction of [Cu2ACHTUNGRE(m-Cl)(mdpmppmO)2]BF4 (4 a*), which was generated by treatment of 4 a with H2O2. The structure of 1 a* was confirmed by Xray crystallography to show an almost identical structure to 1 a except that the uncoordinate phosphine units are converted into phosphine oxides, which causes a slight but important structural deformation: the Cu···Cu distance of 1 a* (2.7581(4) ) is slightly shorter than that of 1 a (2.7948(7) ) and the hydride is further hindered by the phenyl groups of the terminal P atoms so as to avoid steric repulsions around the phosphine oxide moieties (Figure 3 (b),(d) and Figure S4 in the Supporting Information). Interestingly, the CO2 insertion of 1 a* (1 atm, 25 8C) in CD3CN proceeded more slowly in comparison with that of 1 a (Figure 2), presumably because a larger hindrance of the phenyl groups reduced the hydride reactivity. The pseudofirst-order rate constant (kobs) for the initial part of the reaction of 1 a (4.3 10 4 s 1) is appreciably larger than that of 1 a* (7.1 10 5 s 1). To investigate the reaction mechanism of the CO2 insertion into 1 a, DFT calculations were performed on a model complex, [Cu2ACHTUNGRE(m-H)ACHTUNGRE(m-PhP(H)CH2P(H)CH2P(H)CH2P(H)Ph)2] + (M1), with B3LYP functionals. In the model complex M1, four phenyl groups on the terminal P atoms are included to elucidate their steric effects, and other phenyl groups are replaced with H atoms. The ground states GS0 and GS1 were optimized at reasonable structures in agreement with the real structures of
Scheme 1. Sequential transformations of the Cu2ACHTUNGRE(m-H) complex 1 a.
Figure 2. Time-dependent ratios of the hydride complexes 1 a (*) and 1 a* (*) and the formate complexes 3 a (*) and 3 a* (*), monitored by 31 P{1H} NMR spectroscopy for the reactions of 1 a and 1 a* (1.4 10 2 m) with CO2 (1 atm) in CD3CN at 25 8C.
[Cu2ACHTUNGRE(m-DCOO)ACHTUNGRE(m-dpmppm)2]BF4 (3 a-D), was prepared by reacting 1 a-D with CO2, and showed a resonance at 8.87 ppm in the 2H NMR spectrum, demonstrating that the formate proton is derived from the hydride of 1 a via CO2 insertion. In the 31P{1H} NMR spectrum of 3 a, the characteristic signal corresponding to the uncoordinate P atoms shifted from 39.2 ppm (1 a) to 39.5 ppm (2JPP’ = 75, 67 Hz), which was used to monitor the reaction. Complex 3 a reacted with HCl to afford [Cu2ACHTUNGRE(m-Cl)ACHTUNGRE(mdpmppm)2]BF4 (4 a) with releasing 1 equivalent of formic acid (Scheme 1). When 3 a was treated with HBF4, the dicopper complex without a mono-atom bridge, [Cu2ACHTUNGRE(mdpmppm)2]ACHTUNGRE(BF4)2 (5 a), was obtained. Notably, 4 a and 5 a were converted by treatment with NaBH4 into the starting hydride complex 1 a, establishing a sequential cycle that involves an unprecedented CO2 insertion into the Cu2ACHTUNGRE(m-H)
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adopt linear Cu4 structures containing two terminal acetonitrile ligands in CH3CN. These results revealed that the square Cu4 framework of 2 b opened up to a linear Cu4 chain upon insertion of CO2 into Cu2ACHTUNGRE(m-H) units. In summary, by using the tetraphosphine dpmppm we have synthesized di- and tetranuclear copper hydride complexes that are highly reactive for insertion of CO2 into Cu2 ACHTUNGRE(m-H) dinuclear units. We will try to develop catalytic reactions that include the facile CO2 insertion by the dicopper hydride complexes in combination with various hydrogen sources.
Experimental Section Preparation of [Cu2ACHTUNGRE(m-H)ACHTUNGRE(m-dpmppm)2]BF4 (1 a): To a CH3CN/MeOH (1:1) solution (10 mL) of dpmppm (295 mg, 469 mmol) was added [CuACHTUNGRE(CH3CN)4]BF4 (150 mg, 476 mmol) and NaBH4 (165 mg, 4.35 mmol), and the reaction mixture was stirred for 3 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with 10 mL of CH2Cl2. The extract was passed through a membrane filter and concentrated to ca. 3 mL, to which diethyl ether was carefully added. The solution was allowed to stand in a refrigerator to afford colorless needle crystals of 1 a (290 mg, 197 mmol, 83 %). The detailed synthetic procedures and analytical data for all the compounds are described in the Supporting Information.
Figure 4. Simplified Gibbs free-energy profiles showing reaction pathway for insertion of CO2 into M1. Numbers in parentheses are differences in Go298 relative to the starting compounds (GS0 + CO2) in kcal mol 1.
1 a and 3 a, indicating a thermodynamically downhill process (Figure 4). The structure of GS1 with a m-h1:h1-formate bridge is appreciably stable compared with that of GS2 with a m-h2-formate bridge, and a transition state TS1 between GS0 and GS1 is successfully determined to reveal that a charge transfer from m-H to CO2 in a symmetrical fashion is a crucial step for the insertion. The activation of CO2 by a Cu···O electrostatic interaction is not involved in TS1 because pre-coordination of CO2 to one of the Cu atoms in {HCu···CuACHTUNGRE(OCO)} asymmetric structure (X in Figure S17 in the Supporting Information) would generate an extremely large steric distortion as indicated by its unreasonably high energy. The pathway as shown in Figure 4 is essentially similar to that proposed by Milstein et al. for decarboxylation of formate by an Fe PNP-pincer complex[15a] and that by Yang for CO2 insertion into an Ir H bond.[15b] Sakaki et al. also noted in their pioneering work that a nucleophilic attack of Cu H on CO2 carbon is more important rather than an electrophilic activation by Cu OCO coordination for insertion of CO2 into mononuclear Cu H complexes with phosphine ligands.[16] These experimental and theoretical results clearly demonstrate that the CO2 insertion into the Cu2ACHTUNGRE(m-H) unit proceeded on the flexible dinuclear scaffold supported by dpmppm ligands. Complex 2 b also reacted with CO2 (1 atm, 25 8C) to yield [Cu4ACHTUNGRE(m-HCOO)3ACHTUNGRE(m-dpmppm)2]PF6 (6 b) in 69 % yield (Scheme S1, Supporting Information). By treatment with HCl, 6 b was transformed into a chloride-bridged tetracopper complex, [Cu4ACHTUNGRE(m-Cl)3ACHTUNGRE(m-dpmppm)2]PF6 (7 b), generating 3 equivalents of formic acid. The analogous complex, [Cu4 ACHTUNGRE(m-Cl)3ACHTUNGRE(m-dpmppm)2ACHTUNGRE(CH3CN)]BF4 (7 a·CH3CN) was crystalized from CH3CN/Et2O and its crystal structure indicated a linear Cu4ACHTUNGRE(m-Cl)3 core bridged by two dpmppm ligands with a terminal acetonitrile ligand, implying that 6 b and 7 b
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Acknowledgements This work was partly supported by Grants-in-Aid for Scientific Research and that on Priority Area 2107 (no. 22108521, 24108727) from the Ministry of Education, Science, Sports, and Culture of Japan. KN is grateful to JSPS fellowship.
Keywords: CO2 insertion · copper hydride complex · dinuclear complex · formate complex · tetraphosphine ligand
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Received: July 30, 2014 Published online: September 9, 2014
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