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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Tri- and Tetranuclear Copper Hydride Complexes Supported by Tetradentate Phosphine Ligands Takayuki Nakajima,* Yoshia Kamiryo, Kanae Hachiken, Kanako Nakamae, Yasuyuki Ura, and Tomoaki Tanase* Department of Chemistry, Faculty of Science, Nara Women’s University, Kitauoya-nishi-machi, Nara 630-8506, Japan

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ABSTRACT: Three types of tetradentate phosphine ligands with different central methylene chains and configurations, meso- and rac-Ph2PCH2P(Ph)(CH2)nP(Ph)CH2PPh2 (n = 2, meso- and rac-dpmppe; n = 3, meso-dpmppp) were utilized to synthesize a new series of tri- and tetranuclear copper hydride complexes. Reactions of meso-dpmppe or meso-dpmppp with CuCl/NH4PF6 or [Cu(CH3CN)4]PF6 in the presence of NaBH4 afforded trinuclear copper hydride complexes, [Cu3(μ3-H)(meso-dpmppe)2](PF6)2 (1) and [Cu3(μ3-H)(meso-dpmppp)2](PF6)2 (2), while a similar reaction with rac-dpmppe resulted in the formation of a tetranuclear copper dihydride complex, [Cu4(μ3-H)2(rac-dpmppe)2](PF6)2 (5). Complexes 1 and 5 further reacted with RNC (R = tBu, Cy, Xyl) to give [Cu3(μ3-H)(meso-dpmppe)2(XylNC)](PF6)2 (3), [Cu4(μ3-H)2(meso-dpmppe)2(RNC)2](PF6)2 (R = tBu (4a), Cy (4b)) and [Cu4(μ3-H)2(rac-dpmppe)2(RNC)2](PF6)2 (R = t Bu (6a), Cy (6b), Xyl (6c)), respectively. Complexes 1−6 were characterized by ESI-MS and 1H and 31P NMR spectroscopy and X-ray diffraction analyses, demonstrating that a hydride ligand is located at the center of triangular Cu3 plane of 1−3, while two μ3-hydride-capped Cu3 planes are fused to result in rhombic Cu4H2 structures in 4a,b, 5, and 6a−c. Complexes 1−6 in CD3CN solutions notably showed high thermal stability and no reactivity toward H2O and CO2. DFT calculations indicated an interesting correlation between the Wiberg bond indices (WBI) of Cu−H bonds and their natural atomic charge (NAC), where the isocyanide ligands had an appreciable influence on the Cu−H interactions.

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INTRODUCTION For the past two decades, mononuclear copper hydride species have been of considerable interest due to their ability to catalyze numerous organic reactions1−4 such as selective hydrogenation5,6 and hydrosilylation7−10 of various carbonyl compounds, including CO2,11−14 and hydroalkylation,15,16 hydroamination,17,18 and hydroboration19 of carbon−carbon multiple bonds. The reactivity and selectivity of these reactions are tuned by the nature of the copper hydride intermediates generated in situ. Despite the recent progress in catalytic reactions promoted by mononuclear Cu−H species, detailed molecular studies on synthesizing discrete copper hydride clusters have been limited to monophosphine-stabilized hexanuclear clusters of [Cu6H6(PR3)6], the so-called Stryker’s reagent and related complexes.20−27 In recent years, however, several Cu−H multinuclear complexes stabilized by di- and triphosphine ligands, such as 1,2-bis(diphenylphosphino)benzene (dppbz), bis(diphenylphosphino)methane (dppm), bis(diphenylphosphino)amine (dppa), and 1,1,1-tris(diphenylphosphinomethyl)ethane, have also been synthesized with the aim of establishing reactivity of multinuclear Cu−H species.25,28−32 These studies strongly suggested that multidentate phosphines have the potential to stabilize copper hydride clusters with control of the structures and properties of hydride ligands. © XXXX American Chemical Society

We have recently reported the synthesis of di- and tetranuclear copper hydride complexes [Cu2(μ-H)(mesodpmppm)2]+ and [Cu4(μ-H)3(meso-dpmppm)2]+ supported by a new linear tetraphosphine ligand, meso-bis[(diphenylphosphinomethyl)phenylphosphino]methane (dpmppm) (Figure 1), and these complexes show facile

Figure 1. Structures of tetradentate phosphine ligands.

reactivity with CO2 (1 atm, room temperature) to give formate-bridged complexes via insertion of CO2 into the Cu− H bond.33 DFT calculations revealed that CO2 insertion occurred on the Cu2(μ-H) dinuclear unit under mild conditions. Furthermore, by using dppm, the octanuclear copper hydride cluster [Cu8(μ-H)6(μ-dppm)5]2+ was synthesized by treating [Cu6H6(PPh3)6] with dppm in the presence Received: June 12, 2018

A

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of meso- and rac-dpmppe

Scheme 2. (a) Preparations of [Cu3(μ3-H)(meso-dpmppe)2](PF6)2 (1) and [Cu3(μ3-H)(meso-dpmppp)2](PF6)2 (2) and (b) Transformation of 1 to [Cu3(μ3-H)(meso-dpmppe)2(XylNC)](PF6)2 (3) and Preparations of [Cu4(μ3-H)2(mesodpmppe)2(RNC)2](PF6)2 (R = tBu (4a), Cy (4b))

of [Cu(CH3CN)4]PF6 and showed interesting fluxional behaviors in solution where the hydride and dppm ligands were scrambling around the trans-bicapped octahedral Cu8

framework, which readily reacted with CO2 (1 atm, room temperature) to afford the trinuclear complex [Cu3(μ-H)(μO2CH)(μ-dppm)3]+.34 B

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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complex, [Cu3(μ3-H)(meso-dpmppe)2(XylNC)](PF6)2 (3), was obtained in 51% yield (Scheme 2b, method A), while similar treatment with bulky aliphatic isocyanides (tBuNC and CyNC) afforded the tetranuclear copper complexes [Cu4(μ3H)2(meso-dpmppe)2(RNC)2](PF6)2 (R = tBu (4a), Cy (4b)) (Scheme 2b; vide infra), instead of the Cu3H complexes analogous to 3. Insertion of XylNC into the Cu−H bonds did not proceed at all, which contrasts with Sadighi’s report on the insertion of BnNC into a copper hydride dimer.37 In remarkable contrast, complex 2 containing meso-dpmppp ligands did not afford any stable isocyanide adducts of triand tetranuclear Cu−H complexes. The crystal structure of 1 contains a planar Cu3(μ3-H) core bridged by two meso-dpmppe ligands to possess a crystallographic C2 symmetry (Figure 2a). It should be noted that structurally characterized trinuclear hydride complexes with a planar M3(μ3-H) core are limited to a few compounds,29,38,39 involving [Cu3(μ3-H)(Cy2PCH2PCy2)3]2+ as a sole example of a copper hydride complex.29 The Cu1 center adopts a Yshaped trigonal-planar geometry, and the other copper ions

In the present study, we have utilized three linear tetradentate phosphine ligands with different central methylene chains and configurations at the inner P atoms, meso- and rac-Ph2PCH2P(Ph)(CH2)nP(Ph)CH2PPh2 (n = 2, meso- and rac-dpmppe; n = 3, meso-dpmppp) (Figure 1) to elucidate the effects of multidentate phosphine ligands on the structures and reactivity of copper hydride complexes. These ligands were found to react with CuCl/NH4PF6 or [Cu(CH3CN)4]PF6 in the presence of NaBH4 to afford tri- and tetranuclear copper hydride complexes, [Cu3(μ3-H)(meso-dpmppe)2](PF6)2 (1), [Cu 3 (μ 3 -H)(meso-dpmppp) 2 ](PF 6 ) 2 (2), and [Cu 4 (μ 3 H)2(rac-dpmppe)2](PF6)2 (5). Complexes 1 and 5 further reacted with RNC (R = tBu, Cy, Xyl) to give the isocyanide adducts [Cu3(μ3-H)(meso-dpmppe)2(XylNC)](PF6)2 (3), [Cu4(μ3-H)2(meso-dpmppe)2(RNC)2](PF6)2 (R = tBu (4a), Cy (4b)), and [Cu4(μ3-H)2(rac-dpmppe)2(RNC)2](PF6)2 (R = tBu (6a), Cy (6b), Xyl (6c)), respectively. The site selectively incorporated isocyanides exhibited an appreciable influence on the Cu−H interactions, which was evaluated and discussed on the basis of DFT calculations. The solution behaviors of the tri- and tetranuclear complexes are also described.

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RESULTS AND DISCUSSION Synthesis of Tetradentate Phosphine Ligands. New tetraphosphines, meso- and rac-dpmppe, were synthesized by a modification of the procedure applied for the synthesis of meso-dpmppp.35 Lithiation of 1,2-bis(phenylphosphino)ethane with nBuLi in the presence of TMEDA and subsequent treatment with Me3SiCH2Cl afforded bis[(trimethylsilylmethyl)phenylphosphino]ethane (tmsppe) in 88% yield. Addition of Ph2PCl to tmsppe, followed by precipitation from MeOH, afforded dpmppe in 74% yield as a 1:1 mixture of meso and rac isomers, which was determined by NMR spectroscopic methods (Scheme 1). The pure meso-dpmppe was isolated by recrystallization from CH3CN in 43% yield (vs meso-/rac-dpmppe mixture). The other isomer, rac-dpmppe, was obtained from the mother liquor containing a mixture of two isomers (meso:rac = ca. 1:2) after deposition of meso-dpmppe. To the mother liquor was added [PdCl2(cod)], and crystallization from CH2Cl2/Et2O afforded [Pd(rac-dpmppe)2]Cl2 (Figure S1) as yellow crystals, which was treated with excess NaCN in H2O and CH2Cl2, and the released phosphine was extracted with CH2Cl2 to give pure rac-dpmppe in 61% yield. Recently, the synthesis of meso- and rac-dpmppe ligands was reported by Chen’s group using some different procedures.36 Synthesis and Structures of Trinuclear Copper Hydride Complexes. Reactions of meso-dpmppe or mesodpmppp with 1.5 equiv of CuCl and NH4PF6 (method A) or [Cu(CH3CN)4]PF6 (method B) in the presence of NaBH4 in a CH2Cl2/CH3OH mixed solvent afforded colorless crystals of the trinuclear copper complexes [Cu3(μ3-H)(meso-dpmppe)2](PF6)2 (1) and [Cu3(μ3-H)(meso-dpmppp)2](PF6)2 (2) in 68% (method A) and 49% (method B) yields for 1 and 38% (method A) and 61% (method B) yield for 2 (Scheme 2a). With the aim of tuning the steric and electronic factors of the hydride ligand, reactions of 1 and 2 with various isocyanide ligands were investigated, in light of their crystal structures showing the presence of a coordinatively unsaturated, threecoordinate copper center (vide infra). When complex 1 with meso-dpmppe ligands was reacted with 5 equiv of XylNC in CH2Cl2 at room temperature, a trinuclear copper hydride

Figure 2. Perspective views for the complex cations of 1 (a) and 2* (b). Carbon atoms are drawn with capped stick models, and the C−H hydrogen atoms, counteranions, and solvent molecules are omitted for clarity. C

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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The reaction mixture of 1 and tBuNC was investigated by ESIMS to show the presence of dinuclear copper species with {Cu 2 H(meso-dpmppe)( t BuNC)} + and {Cu 2 H(mesodpmppe)}+ cation peaks together with mononuclear species with a {Cu(meso-dpmppe)}+ peak. Although some attempts to isolate the intermediate species were not successful, and thus detailed mechanisms for the change in copper nuclearity are not clear, plausible mechanisms could be assumed on the basis of the in situ ESI-MS data; The first step would be the coordination of RNC to the three-coordinated Cu center as observed in 3. Then, steric repulsion of the bulky isocyanide and phenyl rings of meso-dpmppe may lead to decomposition into a postulated intermediate complex of [Cu2(μ-H)(mesodpmppe)(RNC)]+, which may further be coupled to form the tetranuclear complexes 4a,b. In order to elucidate influences of the P configurations of diphosphines on the Cu−H complexes, rac-dpmppe was used instead of meso-dpmppe. Treatment of rac-dpmppe with 2 equiv of CuCl in the presence of NaBH4 and NH4PF6 in a CH2Cl2/CH3OH mixed solvent gave colorless crystals of a tetranuclear copper complex, [Cu4(μ3-H)2(rac-dpmppe)2](PF6)2 (5), in 27% yield (Scheme 3a). Even though 1.5

Cu2 and Cu2* have a tetrahedral geometry. Each mesodpmppe ligand chelates to Cu2 with inner P2 and P3 atoms to form a five-membered ring (P2−Cu2−P3 = 88.75(4)°) and coordinates to Cu1 and Cu2* with outer P1 and P4 atoms, respectively. While crystals of 2 suitable for X-ray analysis were not obtained, an analogous complex with BF4− counterions, [Cu3(μ3-H)(meso-dpmppp)2](BF4)2 (2*), was synthesized and utilized for structural determination. The structure of 2* is essentially similar to that of 1 with a pseudo-C2 axis passing through the Cu1 and H1 atoms (Figure 2b). The Cu···Cu distances of 2.8989(7)−2.9751(7) Å for 1 and 2.886(1)− 3.054(1) Å for 2* are slightly longer than those found in [Cu3(μ3-H)(Cy2PCH2PCy2)3]2+ (average 2.882 Å).29 The hydride positions were determined by difference Fourier syntheses and located at the center of the triangular Cu3 plane with sums of Cu−H−Cu angles of 360.2° (1) and 360° (2*). The distances of the three-coordinate Cu1 center to hydride (1.56(7) (1) and 1.48(6) Å (2*)) are shorter than the other two Cu−H bonds (average 1.78(4) (1) and 1.86 Å (2*)). The average Cu−H distances of 1.71 (1) and 1.73 Å (2*) are slightly longer than those of 1.67 Å in [Cu3(μ3H)(Cy2PCH2PCy2)3]2+,29 although accurate hydride positions can hardly be determined by X-ray crystallography. The bite angles P2−Cu2−P3 = 95.27(6) o and P6−Cu3−P7 = 98.82(6)o in 2* are much larger than those of 1 due to elongation of the central methylene chains from ethylene to propylene units. The difference between meso-dpmppe and meso-dpmppp ligands brought about slight structural changes in the three-coordinate Cu1 geometry in 1 and 2*; the two outer P1 and P5 atoms bound to Cu1 in 2* are located nearly in the Cu3 plane (dihedral angles P1−Cu1−H1−Cu2 = 5(5)°, P5−Cu1−H1−Cu3 = 5(5)°), while the corresponding P1 and P1* atoms in 1 exist out of the Cu3 plane (dihedral angle P1− Cu1−H1−Cu2 = 32.04(2)°). In addition, the cis angle of P1− Cu1−P5 (112.80(6)°) in 2* is significantly smaller than the corresponding angle (P1−Cu1−P1* = 123.74(4)°) in 1. These structural differences around the Cu1 center between 1 and 2* may lead to entirely different reactivity with isocyanides as mentioned above. The structure of 3 is essentially similar to that of 1 except for the coordination of XylNC to the Cu1 center (Cu1−C1 = 1.946(10) Å) and is composed of trinuclear copper ions supported by two meso-dpmppe, one μ3-hydride, and one terminal XylNC (Figure S2). The coordination of XylNC caused a slight structural change of the Cu3(μ3-H) core; i.e., the Cu···Cu distances (2.932(2)− 3.002(2) Å) in 3 are slightly longer than those in 1. The μ3hydride H1 was located by difference Fourier syntheses just deviating below the reverse side of the Cu3 plane with respect to XylNC (∑Cu−H−Cu = 353.0°), which could be discriminated from the typical face-capping Cu3(μ3-H) structures as observed in [Cu3(μ3-H)(OAc)2(dppm)3], [Cu3(μ3-H)(O2CH)(dppm)3]+, and [Cu3(μ3-H)(X)(dppa)3]+ (X = BH4, Cl, S2CH).28,30,31,34 Synthesis and Structures of Tetranuclear Copper Hydride Complexes. Treatment of 1 with 5 equiv of RNC (R = tBu, Cy) in CH2Cl2 at room temperature gave the tetranuclear copper dihydride complexes [Cu4(μ3-H)2(mesodpmppe)2(RNC)2](PF6)2 (R = tBu (4a), Cy (4b)) in 39% and 52% yields (vs dpmppe), respectively, which should be contrasted to the reaction of 1 with XylNC forming the trinuclear complex 3 (Scheme 2b). Notably, no isocyanide adducts of trinuclear copper hydride complexes similar to 3 were obtained with the bulky isocyanides tBuNC and CyNC.

Scheme 3. (a) Synthesis of [Cu4(μ3-H)2(racdpmppe)2](PF6)2 (5) and (b) Transformation of 5 to [Cu4(μ3-H)2(rac-dpmppe)2(RNC)2](PF6)2 (R = tBu (6a), Cy (6b), Xyl (6c))

equiv of CuCl was used, the trinuclear complex [Cu3(μ3H)(rac-dpmppe)2](PF6)2 was not formed at all. Since complex 5 contains three-coordinate Cu centers, further reactions of 5 with ca. 3−5 equiv of RNC (R = tBu, Cy, Xyl) successfully afforded the tetranuclear complexes [Cu 4 (μ 3 -H) 2 (racdpmppe)2(RNC)2](PF6)2 (R = tBu (6a), Cy (6b), Xyl (6c)) in 43%, 53%, and 28% yields, respectively (Scheme 3b, method A). Notably, the tetracopper complexes 4a,b and 6a−c were also obtained by the reactions of CuCl, rac-dpmppe, NaBH4, RNC, and NH4PF6 in an appropriate ratio (see method B in the Experimental Section). The solid-state structures of 4a,b were determined by X-ray analyses (Figure 3 for 4a, Figure S3 for 4b). The asymmetric D

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Perspective view for the complex cation of 4a. Carbon atoms are drawn with capped stick models, and the C−H hydrogen atoms, counteranions, and solvent molecules are omitted for clarity.

unit of 4a,b contains two independent but chemically identical complex cations with very similar structural parameters, the small differences being attributed to packing effects. The following description refers to only one of the nearly identical complex cations. Complexes 4a,b are composed of coplanarly arranged, rhombic, tetranuclear copper centers supported by two meso-dpmppe, two μ3-hydrides, and two terminal isocyanides. It is noteworthy that only one example of a tetranuclear copper hydride complex, [Cu 4 (μ 4 -H)(μH)2(meso-dpmppm)2]+, has been reported so far,33 and the {Cu4H3} rectangular structure with meso-dpmppm is entirely different from the present {Cu4H2} framework of 4. Each mesodpmppe chelates to the Cu center (Cu3 and Cu4) of a wing position with two inner phosphorus atoms (P2/P3 and P6/P7) and bridges to Cu1 and Cu2 centers of a body position with two outer P atoms (P1/P4 and P5/P8) so as to possess a pseudo-C2 symmetry around the Cu1−Cu2 axis. The Cu4 framework is C2 distorted from a rhombus along the Cu1−Cu2 axis, where the distances Cu1···Cu3 (2.8126(8) Å (4a), 2.8474(8) Å (4b)) and Cu1···Cu4 (2.8803(8) Å (4a), 2.7837(8) Å (4b)) are significantly shorter than those of Cu2···Cu3 (3.1765(8) Å (4a), 3.1514(8) Å (4b)) and Cu2··· Cu4 (3.1714(8) Å (4a), 3.2083(8) Å (4b)). The two hydride ligands were determined from DF maps to be located at 0.31(5)/0.38(4) Å (4a) and 0.40(7)/0.15(6) Å (4b) apart from the triangular Cu3 planes. The Cu−H distances are 1.60(4)−1.83(3) Å (4a) and 1.56(7)−1.89(7) Å (4b), and the sums of Cu−H−Cu angles are 349° (H1) and 345° (H2) for 4a and 343° (H1) and 358° (H2) for 4b. The μ3-H structure of 4 is thus recognized to be appreciably deviated from the Cu3 plane on the same side of the isocyanide ligands, due presumably to steric rigidness of the bridging meso-dpmppe. The crystal structure of 5 possesses a crystallographically imposed inversion center, consisting of a Ci distorted rhombic tetranuclear copper core bridged by two μ3-hydride ligands and an enantiomeric pair of R,R- and S,S-dpmppe ligands (Figure 4a). Four copper ions are arranged in a coplanar fashion with Cu···Cu distances of 2.4651(7) Å (Cu1···Cu1*), 2.6851(6) Å (Cu1···Cu2), 2.8159(4) Å (Cu1···Cu2*), and 4.9195(5) Å (Cu2···Cu2*). The Cu1···Cu2 distance 2.6851(6) Å is significantly shorter than the Cu1···Cu2* distance 2.8159(4)

Figure 4. Perspective views for the complex cations of (a) 5 and (b) 6a. Carbon atoms are drawn with capped stick models, and the C−H hydrogen atoms, counteranions, and solvent molecules are omitted for clarity.

Å, which leads to a parallelogram distortion of the tetranuclear rhombic plane. The Cu1 ion exhibits a tetrahedral geometry ligated by two inner phosphorus atoms (P2 and P3) of racdpmppe and two μ3-hydride ligands (Cu1−P2 = 2.2661(7) Å, Cu1−P3 = 2.2699(7) Å, Cu1−H1= 1.70(3) Å, Cu1−H1*= 1.74(3) Å). On the other hand, the Cu2 center has a Y-shaped trigonal-planar geometry surrounded by two outer P atoms (P1 and P4*) of two rac-dpmppe ligands and one hydride ligand. The rac-dpmppe chelates to the Cu1 ion with inner P2 and P3 atoms and bridges Cu2 and Cu2* centers with outer P1 and P4 atoms, respectively (Cu−P = 2.2641(7)−2.2787(7) Å). The hydride ligand H1 was determined from DF maps and located at 0.78(3) Å above the Cu3 plane with Cu−H distances of 1.70(3)−1.74(3) Å. The sum of Cu−H−Cu angles (302.6°) is fairly small and is characteristic of a μ3-capping mode on the Cu3 triangle as observed in [Cu3(μ3-H)(OAc)2(dppm)3]28 and [Cu3(μ3-H)(X)(L)3] (L = dppm, X = O2CH;33 L = dppa, X = BH4, Cl, S2CH).30,31 The structure of 6a is quite similar to that of 5, containing a Ci distorted Cu4 rhombus with Cu···Cu distances of 2.4462(7) Å (Cu1···Cu1*), 2.8556(6) Å (Cu1··· Cu2), 3.0443(8) Å (Cu1···Cu2*), and 5.372(1) Å (Cu2··· Cu2*) (Figure 4b). Owing to the coordination of the tBuNC ligand to the Cu2 center, Cu···Cu distances in 6a are longer by ca. 0.2−0.4 Å in comparison to the corresponding values in 5, except for the body Cu1···Cu1* distance. The Cu−H distances of 1.72(2)−1.76(3) Å in 6a are also slightly longer than those of 5 (1.70(3)−1.74(3) Å), and the sum of Cu−H−Cu angles (322.3°) is appreciably larger than that of 5 (302.6°) due to E

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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spectrum of 4a in CD3CN at −40 °C was well resolved and exhibited a singlet peak for two μ3-H hydride ligands at −1.04 ppm and four doublet signals for two sets of germinal methylene protons between two phosphorus atoms (Ha/Hb and Ha′/Hb′) observed at 3.45, 1.53 ppm and 3.27, 1.57 ppm (Figure 5 and Figures S9 and S10), which were assigned by

steric repulsion between the hydride and tBuNC ligands, but the structural parameters for the Cu3(μ3-H) system still suggested a μ3-capping feature of the [Cu1Cu2Cu1*] triangle with a distance of 0.4837(3) Å from the plane. Solution Behaviors of Tri- and Tetranuclear Copper Hydride Complexes. As reference compounds to analyze the solution structures of 1 and 2, the deuterated complexes [Cu3(μ3-D)(meso-dpmppe)2](PF6)2 (1-D) and [Cu3(μ3-D)(meso-dpmppp)2](PF6)2 (2-D) were synthesized by reactions using NaBD4 similar to those for 1 and 2. The ESI-MS spectra of 1-D and 2-D showed monovalent cation peaks at m/z 1622.200 (1-D) and 1650.207 (2-D), corresponding to {[Cu3(μ3-D)L2](PF6)}+ (L = meso-dpmppe, meso-dpmppp), which shifted by 1 mass unit from the signals of 1 and 2 at m/z 1621.229 (1) and 1649.276 (2) (Figures S23 and S24), clearly demonstrating that the complex cations as found in the crystal structures are retained in solution. The 1H{31P} NMR spectra of 1 and 2 in CD3CN exhibited a broad singlets at −0.65 and −0.20 ppm, respectively, assignable to hydride ligands, as confirmed by comparing the spectra with those for the deuterated complexes (1-D and 2-D) (Figures S4 and S5). The 1H{31P} NMR spectra of 1-D and 2-D in CD3CN are the same as those of 1 and 2 except for the absence of the hydride peak, and the 2H NMR spectra showed the hydride signal at −0.65 (1-D) and −0.20 ppm (2-D). In the 31P{1H} spectra in CD3CN, two broad resonances at 3.9 (6P) and −2.8 ppm (2P) were observed for 1 (Figure S17a), while four broad resonances were observed at −0.1 (2P), −5.4 (2P), −9.3 (2P), and −12.5 ppm (2P) for 2. The NMR spectral features of 1 and 2 in CD3CN are not altered in CD2Cl2. The 1H{31P} NMR spectrum of 3 in CD2Cl2 was similar to that of 1 except for additional signals for a XylNC ligand, where a broad resonance for a μ3-H hydride ligand (−0.54 ppm) shifted slightly to lower frequency in comparison with 1 (−0.50 ppm) (Figure S6). The chemical shifts of the μ3-hydride in 1−3 (Table S12) fall within the range from 3.10 to −1.28 ppm observed for the structurally characterized copper clusters containing μ3-H ligands,28−30,34,40−44 and notably they shifted appreciably upfield in comparison with the hydride signals of phosphine-supported copper hydride multinuclear complexes observed from 3.10 to 2.04 ppm.28−30,34,43 In the 31P{1H} NMR spectra of 3 in CD2Cl2, two broad resonances were observed at 2.8 and −3.9 ppm in 3:1 intensity ratio as in 1 (Figure S17d). The ESI-MS spectra of [Cu4(μ3-D)2(mesodpmppe)2(tBuNC)2](PF6)2 (4a-D) and [Cu4(μ3-D)2(mesodpmppe)2(CyNC)2](PF6)2 (4b-D) showed divalent cation peaks at m/z 854.134 (4a-D) and 880.192 (4b-D) corresponding to [Cu4(μ3-D)2(meso-dpmppe)2(RNC)]2+, which shifted by 1 m/z unit (z = 2) from the signals of 4a,b at m/z 853.185 (4a) and 879.196 (4b) (Figures S25 and S26). The 1H{31P} NMR spectra of 4a,b in CD2Cl2 at room temperature showed a very broadened feature, in which a resonance for two μ3-hydrides was observed at −0.82 (4a) and −0.79 ppm (4b); the assignment was confirmed by comparison to the deuterated complexes (Figures S7 and S8). The 31P{1H} NMR spectra of 4a,b in CD2Cl2 at room temperature also showed two broad peaks around 4.9 and −7.0 ppm for 4a and 5.6 and −7.0 ppm for 4b in a 1:1 ratio (Figure S17b,c). These spectral features indicated fluxional behaviors of the Cu4 core at around room temperature, and hence, variable-temperature (VT) NMR measurements of 4a were carried out in CD3CN from −40 to 60 °C. The 1H{31P} NMR

Figure 5. VT 1H{31P} NMR spectra of 4a in CD3CN from −40 to 60 °C, showing methylene (red color ●) and ethylene protons (blue color ●) of rac-dpmppe and hydride peak (green color ●). Asterisks indicate solvent impurity.

using 1H−1H COSY techniques (Figures S11 and S12). The P{1H} NMR spectrum of 4a in CD3CN at −40 °C showed three broad signals at 6.46, −3.89, and −9.28 ppm in 2:1:1 ratio, the first of which was assignable to inner phosphorus atoms and the other two to the outer phosphine units by 1 H−1H COSY and 1H−31P HMQC techniques (Figures S11− S16). These NMR data at −40 °C are consistent with the crystal structure of 4a with a C2 distorted Cu4(μ3-H)2 rhombic core. Upon an increase in temperature, the μ3-H hydride signal shifted to −0.70 ppm at 60 °C. The two sets of two doublet signals coalesced at −10 °C as very broad signals centered at 3.33 and 1.66 ppm, corresponding to an averaged methylene unit (Ha/Hb), and they were further broadened and came closer to each other centered at 2.57 ppm at 60 °C (Figure 5 and Figure S9). In the VT 31P{1H} spectra of 4a in CD3CN, two signals for the outer phosphines coalesced as a broad signal centered at −6.6 ppm with an increase in temperature from −40 to 60 °C, while signals for the inner phosphines became broadened and were observed at 5.2 ppm at 60 °C (Figure S15). These 1H{31P} and 31P{1H} VT NMR spectral changes indicated that a dynamic exchange between the C2 31

F

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(Figure 6b), in which the two wing coppers CuC and CuD move reversibly with respect to the body CuA and CuB atoms. Although VT 31P{1H} NMR spectra remained almost unchanged with a broad signal around −1.6 ppm, the two sharp doublets for Ha and Hb protons in the 1H{31P} NMR spectrum at 20 °C became broadened with a decrease in temperature and split into very broad resonances at −40 °C and, on the other hand, were very sharpened at 60 °C (Figure S22). These spectral changes might be suggestive of fluxional behavior in solution as shown in Figure 6b, although other fluxional pathways cannot be excluded due to the broadened 31 P signals. In the ESI-MS spectra of 6a−c, an intense divalent cation peak for [Cu4(μ3-H)2(rac-dpmppe)2]2+ was found at m/ z 770.097−770.155 (z = 2) together with weak peaks for [Cu4(μ3-H)2(rac-dpmppe)2(RNC)]2+ at m/z 811.640 (6a), 824.682 (6b), and 879.196 (6c) (Figures S28−S30). Complexes 1, 2, 4a, 5, and 6a in CD3CN are remarkably stable up to 60 °C for 24 h even in air, and furthermore, these complexes do not react with excess H2O in CD3CN at room temperature. Intramolecular H2 evolution did not occur from tetranuclear copper dihydride complexes 4−6, which contrasts with other polynuclear copper hydride clusters.40,41 The high stability can be explainable by bulky phenyl rings of the tetraphosphines sterically protecting the μ3-H ligand. Notably, all the present complexes showed no reactivity toward CO2, unlike di- and tetranuclear copper complexes with mesodpmppm ligands, [Cu2(μ-H)(meso-dpmppm)2]+ and [Cu4(μ4H)(μ-H)2(meso-dpmppm)2]+,33 suggesting that the reactivity of multinuclear copper hydride complexes can be tuned by varying the multidentate phosphine ligands. Theoretical Calculations on Complexes 1, 2, and 5, and Model Complexes for 3 (M3), 4a (M4), and 6a (M6). Since the hydride positions are not accurately determined by single-crystal X-ray analysis and large crystals suitable for neutron diffraction study were not obtained for the present complexes, the hydride positions in 1−3, 4a, 5, and 6a were evaluated by DFT optimization with B3LYP functionals and lanl2dz (for Cu), 6-311+G(d,p) (for hydride H), and 631G(d) (for other atoms) basis sets on the real structures of 1, 2, and 5 and the model compounds [Cu3(μ3-H)(mesodpmppe)2(CNH)]2+ (M3), [Cu4(μ3-H)2(mesodpmppe) 2 (CNH) 2 ] 2 + (M4), and [Cu 4 (μ 3 -H) 2 (racdpmppe)2(CNH)2]2+ (M6), which correspond to the real structures of 3, 4a, and 6a, respectively, where the isocyanides are replaced by CNH. All atoms were fully optimized to converge with the structures 1opt, 2opt, 5opt, M3opt, M4opt, and M6opt, which are in good agreement with the crystal structures except for some elongations usually observed for Cu···Cu distances (Table 1 and Figures S31−S36). The μ3-H ligand in 1opt and 2opt is located at the center of the Cu3 triangle with Cu−H distances of 1.698−1.760 Å (1opt) and 1.686−1.815 Å (2opt) and sums of Cu−H−Cu angles of 360° (1opt) and 360° (2opt), while μ3-H ligands in 5opt, M3opt, M4opt, and M6opt deviate from the Cu3 plane with Cu−H distances of 1.687− 1.804 Å (5opt), 1.744−1.774 Å (M3opt), 1.559−1.857 Å (M4opt), and 1.550−1.801 Å (M6opt) and average sums of Cu−H−Cu angles of 306° (5opt), 352° (M3opt), 345° (M4opt), and 344° (M6opt). These structural features of the Cu3(μ3-H) interaction are consistent with those determined by X-ray crystallography, given that large experimental errors of hydride positions must be involved in the X-ray analyses. In order to understand the electronic states with respect to Cu−H interaction, natural bond orbital (NBO) analyses were

distorted Cu4 rhombic cores (Figure 6a) occurred above room temperature, in which the two wing coppers CuC and CuD

Figure 6. Fluxional behaviors of (a) the C2 distorted Cu4 core in 4 and (b) the Ci distorted Cu4 core in 6 in the solution state. Cu−Cu and Cu- - -Cu indicate relatively shorter and longer Cu···Cu distances, respectively. L = isocyanide.

moved up and down concertedly with respect to the body CuA and CuB atoms within the NMR time scale. The spectral patterns in CD2Cl2 are essentially similar to those in CD3CN, implying no solvent effect on the fluxional processes. The ESI-MS spectrum of [Cu4(μ3-D)2(rac-dpmppe)2](PF6)2 (5-D) in CH3CN showed a divalent cation peak at m/z 771.143 corresponding to [Cu4(μ3-D)2(rac-dpmppe)2]2+, which shifted by one m/z unit (z = 2) from the signal of 5 at m/z 770.104 (Figure S27). The 1H{31P} NMR spectrum of 5 in CD3CN exhibited a broad singlet at −0.63 ppm, assignable to hydride ligands in comparison with 5-D (Figure S18). It should be noted that the hydride peak in CD2Cl2 appeared at 0.62 ppm, significantly shifted to higher frequency in comparison with that in CD3CN (Figure S18d). In the 31 1 P{ H} spectra in CD3CN, two broad signals were observed at 1.2 and −3.7 ppm in a 1:1 ratio, which shifted markedly upfield from those in CD2Cl2 at 5.8 and −2.6 ppm. These solvent effects strongly suggested that an acetonitrile molecule was likely to attach on the three-coordinate Cu1 center in 5. The 1H{31P} NMR spectrum of 6a in CD2Cl2 at 20 °C was well resolved, exhibiting a broad signal at −1.14 ppm for two μ3-hydride ligands and a set of two doublets at 3.03 and 2.87 ppm for methylene protons between two phosphorus atoms (Ha/Hb) and two doublets at 2.34 and 0.87 ppm for ethylene protons (Figure S19). The hydride signal (−1.14 ppm) significantly shifted to lower frequency in comparison with that of 5 (0.62 ppm in CD2Cl2) due to the coordination of t BuNC to the three-coordinate wing copper centers. In the 31 1 P{ H} NMR spectrum of 6a in CD2Cl2 at 20 °C, a broad signal for inner and outer P atoms was observed at −2.0 ppm (Figure S21a). The 31P{1H} spectra of 6b,c at 20 °C are almost identical with each other and are similar to those of 6a (Figure S21b,c). These symmetrical NMR spectra indicated a dynamic interconversion between the Ci distorted rhombic or parallelogram Cu4 cores as found in the crystal structure G

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Inorganic Chemistry Table 1. Results of DFT Structural Optimization and NBO Analyses

tetraphosphine L CuA···CuB/Å CuA···CuC/Å CuB···CuC/Å CuA···HA/Å CuB···HA/Å CuC···HA/Å ∑Cu−H−Cu/degb NAC (CuA) NAC (CuB) NAC (CuC) NAC (HA) WBI (CuA···HA) WBI (CuB··· HA) WBI (CuC··· HA) ∑WBI(Cu−H)c tetraphosphine L CuA···CuB/Å CuA···CuC/Å CuA···CuD/Å CuB···CuC/Å CuB···CuD/Å CuA···HA/Å CuB···HA/Å CuC···HA/Å CuA···HB/Å CuB···HB/Å CuD···HB/Å av ∑Cu−H−Cu/degb NAC (CuA) NAC (CuB) NAC (CuC) NAC (CuD) NAC (HA) NAC (HB) WBI (CuA···HA) WBI (CuB··· HA) WBI (CuC··· HA) WBI (CuA···HB) WBI (CuB··· HB) WBI (CuD··· HB) av ∑WBI(Cu−H)c

1opt

2opt

M3opt

meso-dpmppe none 3.035 (2.975)a 3.055 (2.975) 2.926 (2.899) 1.70 (1.56) 1.75 (1.78) 1.76 (1.78) 360 (360) +0.467 +0.131 +0.131 −0.508 0.133 0.253 0.250 0.636 5opt

meso-dpmppp none 3.043 (2.886) 3.043 (3.045) 3.119 (3.054) 1.69 (1.48) 1.82 (1.86) 1.82 (1.86) 360 (360) +0.485 +0.157 +0.157 −0.522 0.143 0.236 0.236 0.615 M4opt

meso-dpmppe CNH (XylNC)a 3.116 (3.002) 2.971 (2.932) 2.912 (2.978) 1.74 (1.65) 1.76 (1.69) 1.77 (1.88) 352 (353.0) +0.200 +0.172 +0.192 −0.460 0.235 0.227 0.219 0.681 M6opt

meso-dpmppp CNH (tBuNC) 2.613 (2.547) 2.931 (2.813) 2.903 (2.880) 3.201 (3.177) 3.295 (3.171) 1.56 (1.77) 1.83 (1.61) 1.70 (1.73) 1.77 (1.75) 1.86 (1.83) 1.71 (1.60) 345 (347)d +0.171 +0.361 +0.009 +0.030 −0.425 −0.427 0.239 0.151 0.337 0.245 0.146 0.332 av. 0.725d

rac-dpmppe CNH (tBuNC)a 2.447 (2.446) 2.967 (2.856) 3.160 (3.044) 3.160 (3.044) 2.967 (2.856) 1.55 (1.72) 1.75 (1.76) 1.72 (1.73) 1.80 (1.73) 1.75 (1.72) 1.72 (1.76) 344 (322.3)d +0.140 +0.140 +0.090 +0.090 −0.386 −0.386 0.262 0.232 0.277 0.232 0.262 0.276 av. 0.771d

rac-dpmppe none 2.485 (2.465)a 2.762 (2.685) 2.894 (2.816) 2.880 (2.816) 2.778 (2.685) 1.77 (1.70) 1.79 (1.74) 1.69 (1.74) 1.80 (1.74) 1.78 (1.70) 1.69 (1.74) 306 (302.6)d −0.060 −0.062 +0.502 +0.494 −0.371 −0.370 0.328 0.303 0.139 0.299 0.325 0.143 av. 0.769d

a

Information from X-ray analyses is given in parentheses. bSum of Cu−H−Cu angles for Cu3(μ3-H) structure. cSum of WBI values of Cu−H for Cu3(μ3-H) structure. dAverage.

carried out on 1opt, 2opt, 5opt, M3opt, M4opt, and M6opt, and the obtained values of natural atomic charge (NAC) and Wiberg

bond indices (WBI) for Cu−H interaction together with the structural metric parameters are summarized in Table 1 H

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Figure 7. Simplified interaction diagrams for bonding and antibonding overlaps between 3d orbitals of Cu3/4 fragments and s orbitals of hydrides in 1opt (a) and 5opt (b). Isosurface drawings of MO365 (2a*) in 1opt (c) and MO376 (2b*) (d) and MO372 (2a*) (e) in 5opt (cutoff value is 0.04).

of M3opt (Figure 7a,c and Figure S43a), which are derived from antibonding interaction between an in-phase d orbital of Cu3 fragment and an s orbital of the hydride, and is stabilized to some extent by in-phase mixing of 4p orbitals of the Cu3 fragment, generating a partial nonbonding character (Figure 7). In the Cu4(μ3-H)2 core for 5opt, the NAC values of −0.060 and −0.062 for CuA and CuB (body position) are much smaller than the values of +0.502 and +0.494 for CuC and CuD (wing position) due to stronger electron donation from inner P atoms in comparison with outer atoms. The WBIs of Cu−H for CuA and CuB (0.299−0.328) are larger than those of CuC and CuD (0.139 and 0.143), whereas CuC/D−H bond lengths are shorter than CuA/B−H distances and are seemingly established by ionic interaction (Table 1). For M6opt, coordination of CNH ligands to CuC/D centers results in a considerable decrease in the NAC values to +0.090, while those of CuA/B slightly increase to +0.140. This charge distribution enhances the strength of CuC/D−H bonds with WBI values of 0.277 and 0.276 and reduces those of CuA/B−H bonds with WBI values of 0.232−0.262. In this case, the sum of WBIs for Cu−H (average 0.771) is almost same as that for 5 (average 0.769), and the NAC value of H (−0.386) is not altered from that of 5 (−0.371). In M4opt, the NAC value of CuB is the largest among the four Cu ions, and thus, the WBI value of Cu−H for CuB is very small (0.151), which obviously results from the C2 distortion of the Cu4 rhombic core, as is also found in the crystal structure. The correlation between the NAC and WBI values observed for the Cu4(μ3-H)2 interaction is essentially identical with that for the Cu3(μ3-H) system and

(Figures S37−S42). In the Cu3(μ3-H) core of 1opt, the NAC value for four-coordinate CuB and CuC ions (+0.131) is much smaller than that of three-coordinate CuA ion (+0.467), due to electron donation from three phosphorus atoms to the former ions, and the WBI values of CuB/C−H (0.253, 0.250) are significantly larger than that of CuA−H (0.133) (see the schematic structures in Table 1). A similar propensity is observed in 2opt and demonstrates that the WBI of Cu−H increases with a decrease in NAC charge separation (vice versa). It should be surprising that the CuA−H bond distance is shorter than those of CuB/C−H for 1opt and 2opt, which is suggested by the theoretical optimization as well as X-ray analyses, despite the fact that CuA−H covalent bond interactions are weaker than those of CuB/C−H on the basis of their WBIs. From these results, the Cu3(μ3-H) bonding is assumed to arise from extra covalent interactions in addition to ionic interactions for CuB/C−H bonds, and in contrast, to be established for CuA−H bond predominantly by ionic interaction with much smaller covalent character. In M3opt, coordination of an isocyanide (CNH) decreases the NAC of CuA (+0.200), resulting in a delocalized charge distribution of the Cu3 ions and almost identical WBIs of Cu−H (0.219− 0.235). As a result, the sum of WBIs for CuA/B/C−H (0.681) increases from the value for 1opt (0.636) and the NAC value of H (−0.460) also increases from that of 1opt (−0.508). These results implied that electron density is transferred from the hydride to Cu centers as their bonding interaction is enhanced. The additional covalent character to the primarily ionic Cu−H bonding schemes could be recognized in the high-lying occupied MOs (HOMO-1), i.e. MO365 of 1opt and MO372 I

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(9.45 g, 38.4 mmol) in THF (200 mL) was added dropwise nBuLi (53.5 mL, 84.5 mmol, 1.57 M in hexane) at −78 °C. At the same temperature, N,N,N′,N′-tetramethyl-1,2-diaminoethane (TMEDA; 1.15 mL, 7.68 mmol) was added. Then, the cooling bath was removed and the mixture was stirred at room temperature for 1.5 h. To the mixture was added (chloromethyl)trimethylsilane (21.5 mL, 154 mmol) at −78 °C. The resultant solution was stirred overnight at room temperature before degassed H2O (160 mL) and Et2O (60 mL) were added. The organic phase was separated, and the aqueous phase was extracted with Et2O (30 mL × 3). The combined organic extract was dried over Na2SO4, filtered, and concentrated under reduced pressure to give tmsppe (14.2 g, 33.9 mmol, 88%). 1H NMR (CDCl3): δ 7.40−7.19 (m, 10H, Ph), 1.68−1.32 (m, 4H), 0.94−0.71 (m, 4H), −0.17 (s, 18H, SiCH3). 31P NMR (CDCl3): δ −25.7 (s, 2P), −26.3 (s, 2P). meso-Bis[(diphenylphosphinomethyl)phenylphosphino]ethane (meso-dpmppe). To tmsppe (14.2 g, 33.9 mmol) was added PPh2Cl (15.0 g, 67.8 mmol) at room temperature, and the mixture was heated at 120 °C for 1 h. By addition of dry MeOH (140 mL) a white precipitate was formed, which was filtered off and washed with Et2O (20 mL × 3) to give dpmppe as a mixture of meso and rac isomers (16.2 g, 25.2 mmol, 74%) in 1:0.9 ratio. The meso isomer was obtained as a white solid (0.51 g) from crystallization of meso- and rac-dpmppe (1.19 g) in MeCN, which was used for the synthesis of copper complexes. The mother liquor from recrystallization was used for the synthesis of rac-dpmppe (vide infra). Anal. Calcd for C40H38P4: C, 74.76; H, 5.96. Found: C, 73.85; H, 5.87. IR (KBr): 3050 (m), 1480 (w), 1433 (m, P−C), 1093 (m), 1025 (w), 784 (m), 742 (s), 694 (s). 1H NMR (acetone-d6): δ 7.49−7.29 (m, 30H, Ph), 2.52 (m, 4H, PCH2P), 1.87−1.58 (m, 4H, PCH2CH2P). 31P{1H} NMR (acetone-d6): δ −22.5 (m, 2P), −25.6 (m, 2P). ESI-MS (in acetone): m/z 643.23 (z1, [dpmppe + H]+ (643.20)). rac-Bis[(diphenylphosphinomethyl)phenylphosphino]ethane (rac-dpmppe). The mother liquor obtained from recrystallization of meso-dpmppe was used for the synthesis of racdpmppe. The solvent was removed under reduced pressure to give a mixture of meso- and rac-dpmppe in a ratio of ca. 1:2. To the mixture of meso- and rac-dpmppe (1.62 g, 2.52 mmol) were added CH2Cl2 (10 mL) and [PdCl2(cod)] (0.360 g, 1.26 mmol). The resulting solution was stirred at room temperature for 6 h, and the solvent was removed under reduced pressure. The residue was washed with Et2O (7.5 × 3) and extracted with CH2Cl2 (12 mL). The extract was concentrated to ca. 7 mL, to which Et2O (2 mL) was added. The solution was allowed to stand in a refrigerator to afford pale yellow crystals of [Pd(rac-dpmppe)2]Cl2·0.75CH2Cl2 (0.747 g, 51.1 μmol, 61%) based on rac-dpmppe. Anal. Calcd for C80.75H77.5Cl3.5P8Pd: C, 63.54; H, 5.12. Found: C, 63.64; H, 4.84. IR (KBr): 1481 (m), 1434 (s, P−C), 1106 (m), 876 (m), 736 (s), 695 (s), 510 (m), 486 (m). 1H NMR (CDCl3): δ 8.52−6.82 (m, 60H, Ph), 4.70 (br, 4H, PCH2P), 1.42 (br, 4H, PCH2P), 1.96 (br, 4H, PCH2CH2P), 0.69 (br, 4H, PCH2P). 31P{1H} NMR (CDCl3): δ 50.8 (4P), −27.4 (4P). ESI-MS (in MeOH): m/z 1427.36 (z1, [Pd(dpmppe)Cl]+ (1427.26)). To a solution of [Pd(rac-dpmppe)2]Cl2·0.75CH2Cl2 (0.521 g, 0.356 mmol) in degassed H2O (15 mL) and CH2Cl2 (7.5 mL) was added NaCN (0.353 g, 7.21 mmol), and the resulting solution was stirred at room temperature for 10 min. The organic phase was separated, and the aqueous phase was extracted with CH2Cl2 (5 mL × 3). The combined organic extract was dried over Na2SO4, filtered, and concentrated under reduced pressure to give rac-dpmppe as a white solid (0.324 g, 0.505 mmol, 71%). Anal. Calcd for C40H38P4: C, 74.76; H, 5.96. Found: C, 74.51; H, 5.97. IR (KBr): 3050 (m), 1479 (w), 1433 (m, P−C), 1094 (m), 785 (m), 741 (s), 694 (s). 1H NMR (acetone-d6): δ 7.67−7.11 (m, 30H, Ph), 2.48 (m, 4H, PCH2P), 1.93−1.56 (m, 4H, PCH2CH2P). 31P{1H} NMR (acetone-d6): δ −22.6 (m, 2P), −25.9 (m, 2P). ESI-MS (in acetone): m/z 643.23 (z1, [dpmppe + H]+ (643.20)). [Cu3(μ3-H)(meso-dpmppe)2](PF6)2 (1). Method A. To a solution of meso-dpmppe (121 mg, 0.189 mmol) and CuCl (27.7 mg, 0.280 mmol) in CH2Cl2 (5 mL) and CH3OH (5 mL) was added NH4PF6 (46.6 mg, 0.289 mmol) and NaBH4 (36.2 mg, 0.957 mmol), and the

implies that the Cu−H interactions involve covalent bonding character to some extent in addition to the primarily Cu−H ionic interaction. The additional covalent character is assumed to arise from partial in-phase mixing of Cu 4p orbitals into the high-lying Cu−H antibonding MOs, HOMO (MO376) and HOMO-4 (MO372) of 5 and HOMO (MO390) and HOMO7 (MO383) of M6opt (Figure 7b,d,e and Figure S43b), which are derived from antibonding interactions of the in-phase d orbitals of Cu4 fragment with s orbitals of hydride ligands.

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CONCLUSION We have utilized three types of tetradentate phosphine ligands Ph2PCH2P(Ph)(CH2)nP(Ph)CH2PPh2 (n = 2, meso- and racdpmppe; n = 3, meso-dpmppp) with different central methylene chains and configurations, to synthesize a new series of multinuclear copper hydride complex. The meso isomers of dpmppe and dpmppp ligands reacted with CuCl/ NH4PF6 or [Cu(CH3CN)4]PF6 in the presence of NaBH4 to afford the trinuclear copper hydride complexes [Cu3(μ3H)(meso-dpmppe) 2 ](PF 6 ) 2 (1) and [Cu 3 (μ 3 -H)(mesodpmppp)2](PF6)2 (2), where a hydride ligand is located at the center of the triangular Cu3 plane. On the other hand, the rac isomer of dpmppe gave the tetranuclear copper dihydride complex [Cu4(μ3-H)2(rac-dpmppe)2](PF6)2 (5), where each hydride bridges on the Cu3 triangle. Complexes 1 and 5 further reacted with isocyanides RNC (R = tBu, Cy, Xyl), to give [Cu3(μ3-H)(meso-dpmppe)2(XylNC)](PF6)2 (3), [Cu4(μ3H)2(meso-dpmppe)2(RNC)2](PF6)2 (R = tBu (4a), Cy (4b)), and [Cu4(μ3-H)2(rac-dpmppe)2(RNC)2](PF6)2 (R = t Bu (6a), Cy (6b), Xyl (6c)), respectively, while complex 2 with meso-dpmppp did not afford any isocyanide adducts. These results clearly demonstrate that the central methylene chains and configurations of tetradentate phosphine ligands have a significant influence on the structure and reactivity of copper hydride complexes. Furthermore, DFT calculations revealed an interesting correlation between Wiberg bond indices (WBIs) of Cu−H interactions and their natural atomic charges (NACs); i.e. when the separation of NAC value for a Cu−H pair becomes smaller, the WBI value of the Cu−H bond becomes larger (and vice versa). The observed propensity implied a partial involvement of covalent bonding character in the Cu3(μ3-H) and Cu4(μ3-H)2 systems into the predominant ionic interaction, which is recognized by a nonbonding character of the high-lying occupied antibonding MOs for Cu3(μ3-H) and Cu4(μ3-H)2 systems. The present results unveiled that the structures and reactivity of multinuclear copper hydride complexes could be tuned by varying the multidentate phosphine supporting ligands.

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EXPERIMENTAL SECTION

General Procedure and Materials. All manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques. meso-dpmppp was prepared by the reported procedure.35 Reagent grade solvents were dried by the standard procedures and were freshly distilled prior to use. IR spectra were recorded on a Jasco FT/IR-410 spectrophotometer. 1H and 31P{1H} NMR spectra were recorded on a Bruker AV-300N spectrometer at 300 and 121 MHz, respectively. 1H NMR spectra were referenced to TMS as external standard, and 31P{1H} NMR spectra were referenced to 85% H3PO4 as external standard. ESI-TOF MS spectra were recorded on a JEOL JMS-T100LC high-resolution mass spectrometer with positive ionization mode. meso-/rac-Bis[(trimethylsilylmethyl)phenylphosphino]ethane (tmsppe). To a solution of 1,2-bis(phenylphosphino)ethane J

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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[Cu4(μ3-H)2(meso-dpmppe)2(tBuNC)2](PF6)2 (4a). Method A. To a solution of 1 (28.0 mg, 15.8 μmol) in CH2Cl2 (5 mL) was added t BuNC (10.0 μL, 89.0 μmol), and the reaction mixture was stirred at room temperature for 3 h. The solvent was removed under reduced pressure, and the residue was extracted with CH3CN (5 mL). The extract was passed through a membrane filter and was concentrated to ca. 3 mL, to which Et2O was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 4a (9.3 mg, 4.7 μmol, 39%). Method B. To a solution of meso-dpmppe (30.5 mg, 47.5 μmol) and CuCl (9.6 mg, 97.0 μmol) in CH2Cl2 (5 mL) and CH3OH (2 mL) was added tBuNC (25.0 μL, 223 μmol), NH4PF6 (16.0 mg, 98.2 mmol), and NaBH4 (9.1 mg, 241 μmol) in CH3OH (5 mL) and the reaction mixture was stirred for 4 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (10 mL). The extract was passed through a membrane filter. The solvent was removed under reduced pressure, and the residue was extracted with CH3CN (1.5 mL). To the extract was carefully added Et2O (6.5 mL). The solution was allowed to stand in a refrigerator to afford colorless crystals of 4a (20.2 mg, 10.1 μmol, 43%). Anal. Calcd for C90H96Cu4F12N2P10: C, 54.11; H, 4.84; N, 1.40. Found: C, 53.90; H, 4.83; N, 1.44. IR (KBr): 2163 (s, CN), 1968 (w), 1894 (w), 1829 (w), 1098(m), 840 (s, PF6), 775 (m), 695 (m), 556 (m). 1H NMR (CD3CN, −40 °C): δ 8.42−5.24 (m, 60H, Ph), 3.47−3.44 (m, 2H), 3.29−3.26 (m, 2H), 2.18−2.07 (m, 2H), 1.71− 1.66 (m, 4H), 1.56−1.46 (m, 2H), 1.13 (s, 18H), 0.42−0.34 (m, 2H), −1.04 (s, 2H). 31P{1H} NMR (CD3CN, −40 °C): 6.5 (m, 4P), −3.9 (m, 2P), −9.3 (m, 2P), −144.4 (sep, JPF = 705 Hz, 2P). 1H NMR (CD2Cl2, −60 °C): δ 8.34−5.05 (m, 60H, Ph), 3.16−3.04 (m, 4H), 2.08 (m, 2H), 1.75 (m, 4H), 1.59 (m, 2H), 1.25 (m, 2H), 1.12 (s, 18H), 0.35 (m, 2H), −1.12 (s, 2H). 31P{1H} NMR (CD2Cl2, −60 °C): 6.7 (m, 4P), −3.8 (m, 2P), −9.9 (m, 2P), −144.7 (sep, JPF = 712 Hz, 2P). ESI-MS (in MeCN): m/z 853.185 (z2, [Cu 4 H 2 (dpmppe) 2 ( t BuNC) 2 ] 2 + (853.133)), 811.655 (z2, [Cu 4 H 2 (dpmppe) 2 ( t BuNC)] 2 + (811.596)), 770.104 (z2, [Cu4H2(dpmppe)2]2+ (770.059)). [Cu4(μ3-H)2(meso-dpmppe)2(CyNC)2](PF6)2 (4b). By a procedure similar to that for 4a, colorless crystals of 4b were obtained in 52% (method A), 44% (method B). Anal. Calcd for C90H96Cu4F12N2P10: C, 55.08; H, 4.92; N, 1.37. Found: C, 54.98; H, 4.86; N, 1.51. IR (KBr): 3054 (m), 2939 (m), 2859 (m), 2170 (s, CN), 1436 (m), 1098 (m), 840 (s, PF6), 775 (m), 695 (m), 556 (m). 1H NMR (CD3CN, −40 °C): δ 8.49−5.22 (m, 60H, Ph), 3.46 (m, 2H), 3.29 (m, 2H), 2.11−2.35 (m, 4H), 1.69−1.05 (m, 24H), 0.66 (m, 3H), 0.40 (m, 3H), −1.00 (s, 2H, hydride). 31P{1H} NMR (CD3CN, −40 °C): δ 7.1 (s, 4P), −3.6 (2P), −8.3 (2P), −144.5 (sep, JPF = 705 Hz, 2P). ESI-MS (in MeCN): m/z 879.196 (z2, [Cu 4 H 2 (dpmppe) 2 (CyNC) 2 ] 2 + (879.149)), 824.651 (z2, [Cu 4 H 2 (dpmppe) 2 (CyNC)] 2 + (824.604)), 770.104 (z2, [Cu4H2(dpmppe)2]2+ (770.059)). [Cu4(μ3-H)2(rac-dpmppe)2](PF6)2 (5). To a solution of racdpmppe (61.2 mg, 95.2 μmol) and CuCl (18.8 mg, 190 μmol) in CH2Cl2 (5 mL) and CH3OH (5 mL) were added NH4PF6 (30.8 mg, 189 μmol) and NaBH4 (19.0 mg, 502 μmol), and the reaction mixture was stirred for 4 h at room temperature to give a yellow solution. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (10 mL). The extract was passed through a membrane filter. The solvent was removed under reduced pressure, and the residue was extracted with acetone (5 mL). The extract was concentrated to ca. 1.5 mL, to which Et2O was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 5 (23.8 mg, 13.0 μmol, 27%). Anal. Calcd for C81H80Cl2Cu4F12P10 (5·CH2Cl2): C, 50.77; H, 4.21. Found: C, 50.37; H, 4.11. IR (KBr): 1436 (m), 1100 (m), 838 (s, PF6), 739 (m), 692 (m), 556 (m). 1H NMR (CD3CN, 20 °C): δ 7.95−6.56 (m, 60H, Ph), 3.02−2.93 (m, 8H), 2.32 (br, 4H), 0.83 (m, 4H), −0.63 (s, 2H, hydride). 1H NMR (CD2Cl2, 20 °C): δ 7.76−6.64 (m, 60H, Ph), 2.95 (br, 8H), 2.58 (br, 4H), 1.13 (m, 4H), 0.62 (s, 2H). 31P{1H} NMR (CD3CN, 20 °C): δ 1.2 (m, 4P), −3.7 (m, 4P), −144.3 (sep, J = 706 Hz, 2P). 31P{1H} NMR (CD2Cl2, 20 °C): δ 5.8 (s, 4P), −2.6

reaction mixture was stirred for 3 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (10 mL). The extract was passed through a membrane filter. The solvent was removed under reduced pressure, and the residue was extracted with CH3CN (5 mL). The extract was concentrated to ca. 3 mL, to which Et2O was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 1 (112.9 mg, 93.9 μmol, 68%). Method B. To a solution of meso-dpmppe (112.9 mg, 0.078 mmol) and [Cu(CH3CN)4]PF6 (44.2 mg, 0.119 mmol) in CH3CN (5 mL) and CH3OH (3 mL) was added NaBH4 (15.0 mg, 0.397 mmol), and the reaction mixture was stirred for 6 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (10 mL). The extract was passed through a membrane filter and concentrated to ca. 3 mL, to which Et2O was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 1 (33.8 mg, 19.1 μmol, 49%). Anal. Calcd for C80H77Cu3F12P10: C, 54.38; H, 4.39. Found: C, 54.21; H, 4.27. IR (KBr): 1484 (w), 1436 (m, P−C), 1097 (m), 1001 (w), 836 (s, PF6), 740 (m), 693 (m), 557 (m), 515 (m). 1H NMR (CD3CN at 20 °C): δ 7.98−6.68 (m, 60H), 3.66 (d, J = 14 Hz, 2H), 3.31 (d, J = 14 Hz, 2H), 2.75−2.43 (m, 4H), 2.62 (d, J = 14 Hz, 2H), 2.02 (d, J = 13 Hz, 2H), 1.82−1.52 (m, 5H), −0.65 (s, 1H, hydride). 1H NMR (CD2Cl2 at 20 °C): 7.90−6.79 (m, 60H), 3.60 (d, J = 15 Hz, 2H), 3.25 (d, J = 13 Hz, 2H), 2.87 (m, 2H), 2.62 (m, 4H), 1.88 (d, J = 14 Hz, 2H), 1.77−1.57 (m, 4H), −0.50 (s, 1H, hydride). 31P{1H} NMR (CD3CN at 20 °C): δ 3.9 (6P), −2.8 (2P), −144.3 (sep, JPF = 706 Hz, 2P). 31 1 P{ H} NMR (CD2Cl2 at 20 °C): δ 2.9 (6P), −3.9 (2P), −144.5 (sep, JPF = 711 Hz, 2P). ESI-MS (in MeCN): m/z 738.086 (z2, [Cu3H(dpmppe)2]2+ (738.091)). [Cu3(μ3-H)(meso-dpmppp)2](PF6)2 (2). By a procedure similar to that for 1, colorless crystals of 2·0.3CH2Cl2 (38% (method A), 61% (method B)) were isolated. Anal. Calcd for C82.3H81.6Cl0.6Cu3F12P10 (3·0.3CH2Cl2): C, 54.30; H, 4.52. Found: C, 54.15; H, 4.51. IR (KBr): 1484 (s, P−C), 1435 (s, P−C), 837 (s, P−F), 829 (m), 739 (m), 694(m), 557 (m). 1H NMR (CD3CN, 20 °C): δ 6.91−7.94 (m, 60 H, Ph), 3.23−3.27 (m, 6H, CH2), 3.00−3.10 (m, 2H, CH2), 1.64 (br, 2H, CH2), 1.43 (br, 4H, CH2), 0.71−0.78 (m, 4H, CH2), 0.38− 0.44 (m, 2H, CH2), −0.20 (s, 1H, hydride). 1H NMR (CD2Cl2, 20 °C): 6.91−7.73 (m, 60H, Ph), 3.00−3.13 (m, 6H, CH2), 3.00−3.13 (m, 2H, CH2), 1.65−1.80 (br, 4H, CH2), 1.01−1.14 (br, 4H, CH2), 0.69−0.82 (m, 4H, CH2), −0.06 (s, 1H, hydride). 31P{1H} NMR (CD3CN, 20 °C): δ −0.1 (2P), −5.4 (2P), −9.3 (2P), −12.5 (2P), −144.4 (sep, JPF = 711 Hz, 2P). 31P{1H} NMR (CD2Cl2, 20 °C): δ −0.7 (2 P), −6.4 (2 P), −10.1 (2 P), −12.9 (2 P), −144.5 (sep, JPF = 708 Hz, 2P). ESI-MS (MeCN): m/z 1649.276 (z1, {[Cu3H(dpmppp)2]PF6}+ (1649.277)), 783.100 (z1, [Cu2H(dpmppp)]+ (783.083)), 719.155 (z1, [Cu(dpmppp)]+ (719.138)). Crystals suitable for X-ray analysis were obtained from [Cu3(μ3-H)(mesodpmppp)2](BF4)2 (2*), which was synthesized by a similar reaction with NH4BF4 in 27% yield. [Cu3(μ3-H)(meso-dpmppe)2(XylNC)](PF6)2 (3). To a solution of 1 (29.2 mg, 16.5 μmol) in CH2Cl2 (5 mL) was added XylNC (6.8 mg, 51.8 μmol), 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 CH2Cl2 (5 mL). The extract was passed through a membrane filter. The solvent was removed under reduced pressure. The residue was extracted with acetone (1 mL), to which Et2O (2.5 mL) was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 3 (14.6 mg, 8.3 μmol, 51%). Inorganic impurity slightly involved in the sample prevented us from obtaining satisfactory results of elemental analysis, and the formula was determined by an X-ray analysis. IR (KBr): 2130 (s, CN), 1435 (m), 1097 (m), 838 (s, PF6), 737 (m), 693 (m), 557 (m), 511 (m). 1H NMR (CD2Cl2 at 20 °C): δ 7.89−6.81 (m, 63H, Ph), 3.56 (m, 2H), 3.25 (m, 2H), 2.82−2.61 (m, 6H), 2.30 (br, 6H), 1.96 (m, 2H), 1.72 (m, 4H), −0.54 (s, 1H, hydride).31P{1H} NMR (CD2Cl2 at 20 °C): δ 2.8 (s, 6P), −3.9 (2P), −144.5 (sep, JPF = 724 Hz, 1P). K

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

extract was passed through a membrane filter. The solvent was removed under reduced pressure, and the residue was dissolved with CH3CN (1 mL), to which Et2O (3 mL) was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 6c (5.1 mg, 2.5 μmol, 28%). Method B. To a solution of rac-dpmppe (37.2 mg, 57.9 μmol) and CuCl (11.6 mg, 117 μmol) in CH3CN (5 mL) and CH3OH (2 mL) were added NH4PF6 (18.9 mg, 116 μmol), NaBH4 (11.5 mg, 304 μmol), and CH3OH (5 mL). The reaction mixture was stirred for 1 h at room temperature before addition of XylNC (11.6 mg, 88.4 μmol), and the resulting solution was further stirred for 3 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with CH3CN (10 mL). The extract was passed through a membrane filter was concentrated to ca. 2.0 mL, to which Et2O was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 6c (11.5 mg, 5.5 μmol, 19%). Anal. Calcd for C98H96Cu4F12N2P10 (6c): C, 56.22; H, 4.62; N, 1.34. Found: C, 55.82; H, 4.36; N, 1.35. IR (KBr): 2138 (s, CN), 1435 (m), 1099 (m), 839 (s, PF6), 740 (m), 693 (m), 556 (m). 1H NMR (CD3CN at 20 °C): δ 8.03−6.54 (m, 66H, Ph), 3.05 (m, 8H), 2.39 (m, 4H), 1.11 (br, 12H), 0.77 (br, 4H), −0.89 (s, 2H, hydride). 1 H NMR (CD2Cl2 at 20 °C): δ 7.98−6.54 (m, 66H, Ph), 3.12 (m, 4H), 2.87 (m, 4H), 2.45 (br, 4H), 1.80 (s, 12H), 0.84 (br, 4H), −0.84 (s, 2H, hydride). 31P{1H} NMR (CD3CN at 20 °C): δ −0.6 (br, 8P), −144.3 (sep, J = 706 Hz, 2P). 31P{1H} NMR (CD2Cl2 at 20 °C): δ −2.0 (br, 8P), −144.7 (sep, J = 712 Hz, 2P). ESI-MS (in MeCN): m/z 835.734 (z2, [Cu 4 H 2 (dpmppe) 2 (XylNC)] 2+ (835.596)), 770.059 (z2, [Cu4H2(dpmppe)2]2+ (770.059)). X-ray Crystallography. Crystals of 1−3, 4a,b, 5, 6a, and [Pd(racdpmppe)2]Cl2 were quickly coated with Paratone N oil and mounted on top of a loop fiber at room temperature. Crystal and experimental data are summarized in Tables S1−S11. All data were collected at low temperature with a Rigaku VariMax Mo/Saturn CCD diffractometer equipped with graphite-monochromated confocal Mo Kα radiation using an RA-Micro7 rotating-anode X-ray generator (50 kV, 24 mA). A total of 1080 oscillation images, covering a whole sphere of 6° < 2θ < 55°, were corrected by the ω-scan method (−110° < ω < 70° (1−3, 5, 6a), −115° < ω < 65° (4b), −118° < ω < 62° (4a, [Pd(racdpmppe)2]Cl2)) with a Δω value of 0.5°. The crystal-to-detector distance was set at 45 mm (1−3, 5, 6a), 55 mm (4b), and 60 mm (4a, [Pd(rac-dpmppe)2]Cl2). The data were processed using the Crystal Clear 1.3.5 program (Rigaku/MSC)45 and corrected for Lorentz− polarization and absorption effects.46 The structures of complexes were solved by SHELXL-9747 (2, 5) and SIR-9248 (1, 3, 4a, 4b, 6a, [Pd(rac-dpmppe)2]Cl2) and were refined on F2 with full-matrix leastsquares techniques with SHELXL-2014/749 using the Crystal Structure 4.2 package.50 All non-hydrogen atoms were refined with anisotropic thermal parameters, and the C−H hydrogen atoms were calculated at ideal positions and refined with riding models. The positions of hydride H atoms were determined by difference Fourier syntheses and were refined isotropically. All calculations were carried out on a Windows PC with the Crystal Structure 4.2 package.50 Theoretical Calculations. Gas-phase DFT calculations were performed on the real structures of 1, 2, and 5 and the model compounds of [Cu 3 (μ 3 -H)(meso-dpmppe) 2 (CNH)] 2+ (M3), [Cu 4 (μ 3 -H) 2 (meso-dpmppe) 2 (CNH) 2 ] 2+ (M4), and [Cu 4 (μ 3 H)2(rac-dpmppe)2(CNH)2]2+ (M6), which correspond to the real structures of 3, 4a, and 6a, respectively, and the isocyanides are replaced by CNH. The structures were optimized using B3LYP51,52 functionals with LANL2DZ53,54 (for Cu), 6-311+G(d,p) (for hydride H), and 6-31G(d) (for other atoms) basis sets, to result in the optimized ground state structures of 1opt, 2opt, 5opt, M3opt, M4opt, and M6opt. The initial coordinates of the hydrides were determined by Xray crystallography. Natural population analyses were carried out on the optimized structures with the NBO program package.55−57 All calculations were carried out using Research Center for Computational Science, Okazaki, Japan with Gaussian 09 program packages.58

(m, 4P), −144.5 (sep, J = 711 Hz, 2P). ESI-MS (in MeCN): m/z 770.104 (z2, [Cu4H2(dpmppe)2]2+ (770.059)). [Cu4(μ3-H)2(rac-dpmppe)2(tBuNC)2](PF6)2 (6a). Method A. To a solution of 5 (16.0 mg, 8.7 μmol) in CH2Cl2 (5 mL) was added t BuNC (5.0 μL, 44.5 μmol), and the reaction mixture was stirred at room temperature for 3 h. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (5 mL). The extract was passed through a membrane filter. The solvent was removed under reduced pressure, and the residue was dissolved with acetone (1 mL), to which Et2O (2 mL) was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 6a (7.6 mg, 3.8 μmol, 43%). Method B. To a solution of rac-dpmppe (59.2 mg, 92.1 μmol) and CuCl (18.1 mg, 183 μmol) in CH2Cl2 (5 mL) and CH3OH (2 mL) were added NH4PF6 (20.0 mg, 123 μmol), NaBH4 (17.4 mg, 460 μmol), and CH3OH (5 mL). The reaction mixture was stirred for 1 h at room temperature before addition of tBuNC (30.0 μL, 267 μmol), and the resulting solution was further stirred for 3 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (10 mL). The extract was passed through a membrane filter. The solvent was removed under reduced pressure, and the residue was extracted with acetone (5 mL). The extract was concentrated to ca. 2.0 mL, to which Et2O was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 6a (24.8 mg, 12.4 μmol, 27%). Samples for elemental analysis were obtained by crystallization from CH2Cl2/ Et2O. Anal. Calcd for C91H98Cl2Cu4F12N2P10 (6a·CH2Cl2): C, 52.48; H, 4.74; N, 1.35. Found: C, 52.57; H, 4.59; N, 1.31. IR (KBr): 2158 (s, CN), 1435 (m), 1103 (m), 839 (s, PF6), 736 (m), 692 (m), 557 (m). 1H NMR (CD3CN, 20 °C): δ 7.93−6.54 (m, 60H, Ph), 3.02 (m, 4H), 2.32 (m, 4H), 1.11 (s, 18H), 0.85 (br, 4H), −1.20 (s, 2H, hydride). 1H NMR (CD2Cl2, 20 °C): δ 7.93−6.55 (m, 60H, Ph), 3.02 (m, 4H), 2.86 (m, 4H), 1.11 (s, 18H), 0.86 (br, 4H), −1.14 (s, 2H, hydride). 31P{1H} NMR (CD3CN, 20 °C): δ −0.9 (s, 8P), −144.3 (sep, J = 706 Hz, 2P). 31P{1H} NMR (CD2Cl2, 20 °C): δ 2.0 (s, 8P), −144.7 (sep, J = 711 Hz, 2P). ESI-MS (in MeCN): m/z 811.640 (z2, [Cu 4 H 2 (dpmppe) 2 ( t BuNC)] 2 + (811.596)), 770.097 (z2, [Cu4H2(dpmppe)2]2+ (770.059)). [Cu4(μ3-H)2(rac-dpmppe)2(CyNC)2](PF6)2 (6b). Method A. To a solution of 5 (8.2 mg, 4.5 μmol) in CD2Cl2 (0.5 mL) was added CyNC (1.2 μL, 9.8 μmol), and the reaction mixture was allowed to stand at room temperature for 3 h, to which Et2O (1.5 mL) was added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 6b (4.9 mg, 2.4 μmol, 53%). Method B. To a solution of rac-dpmppe (44.5 mg, 69.2 μmol) and CuCl (13.8 mg, 139 μmol) in CH2Cl2 (5 mL) and CH3OH (2 mL) were added NH4PF6 (22.4 mg, 137 μmol), NaBH4 (13.2 mg, 349 μmol), and CH3OH (5 mL). The reaction mixture was stirred for 1 h at room temperature before addition of CyNC (26.0 μL, 212 μmol), and the resulting solution was further stirred for 3 h at room temperature. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (10 mL). The extract was passed through a membrane filter. The solvent was removed under reduced pressure. The residue was extracted with CH3CN (1.5 mL), to which Et2O (2 mL) was carefully added. The solution was allowed to stand in a refrigerator to afford colorless crystals of 6b (11.0 mg, 15.9 μmol, 16%). Anal. Calcd for C94H100Cu4F12N2P10 (6b): C, 55.08; H, 4.92; N, 1.37. Found: C, 54.72; H, 4.64; N, 1.43. IR (KBr): 2168 (s, CN), 1435 (m), 1103 (m), 839 (s, PF6), 742 (m), 694 (m), 556 (m) . 1H NMR (CD2Cl2 at 20 °C): δ 7.86−6.46 (m, 60H, Ph), 3.57 (m, 2H), 3.03 (d, J = 14 Hz, 4H), 2.86 (d, J = 13 Hz, 4H), 2.33 (br, 4H), 1.17 (br, 20H), −1.10 (s, 2H, hydride). 31P{1H} NMR (CD2Cl2 at 20 °C): δ −1.8 (s, 8P), −144.7 (sep, J = 711 Hz, 2P). ESI-MS (in MeCN): m/z 824.682 (z2, [Cu4H2(dpmppe)2(CyNC)]2+ (824.604)), 770.114 (z2, [Cu4H2(dpmppe)2]2+ (770.059)). [Cu4(μ3-H)2(rac-dpmppe)2(XylNC)2](PF6)2 (6c). Method A. To a solution of 5 (16.0 mg, 8.7 μmol) in CH2Cl2 (4 mL) was added XylNC (3.5 mg, 26.7 μmol), and the reaction mixture was stirred at room temperature for 3 h. The solvent was removed under reduced pressure, and the residue was extracted with CH2Cl2 (5 mL). The L

DOI: 10.1021/acs.inorgchem.8b01628 Inorg. Chem. XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01628. Structural parameters of 1−6, ORTEP diagrams of 2, 3, 4b, and [Pd(rac-dpmppe)2]Cl2, NMR spectra of 1−6, ESI-MS of 1−6, and optimized structures, natural population analyses, natural charge and Wiberg bond indices, and atomic coordinates of 1opt, 2opt, 5opt, M4opt, M4opt, and M6opt (PDF) Accession Codes

CCDC 1847693−1847699 and 1849981 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for T.N.: [email protected]. *E-mail for T.T.: [email protected]. ORCID

Yasuyuki Ura: 0000-0003-0484-1299 Tomoaki Tanase: 0000-0003-1838-0112 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (no. 18H03914) and on Priority Area 2107 (no. 22108521, 24108727) and 2802 (no. 17H05374) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. The computations were performed using the Research Center for Computational Science, Okazaki, Japan. The authors thank Prof. Kohtaro Osakada of Tokyo Institute of Technology for help in MS spectral measurements. T.N. is grateful to Tokuyama Science Foundation, Kurata Memorial Hitachi Science and Technology Foundation, and Nara Women’s University for a research project grant.

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REFERENCES

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