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Article

Binuclear Copper(I) Borohydride Complex Containing Bridging Bis(diphenylphosphino) Methane Ligands: Polymorphic Structures of [(µ2-dppm)2Cu2(η2-BH4)2] Dichloromethane Solvate

by
Natalia V. Belkova
1,
Igor E. Golub
1,2,
Evgenii I. Gutsul
1,
Konstantin A. Lyssenko
1,
Alexander S. Peregudov
1,
Viktor D. Makhaev
3,
Oleg A. Filippov
1,
Lina M. Epstein
1,
Andrea Rossin
4,
Maurizio Peruzzini
4 and
Elena S. Shubina
1,*
1
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences (INEOS RAS), 119991 Moscow, Russia
2
Inorganic Chemistry Department, Peoples’ Friendship University of Russia (RUDN University), 117198 Moscow, Russia
3
Institute of Problems of Chemical Physics, Russian Academy of Sciences (IPCP RAS), 142432 Moscow, Russia
4
Istituto di Chimica dei Composti Organometallici Consiglio Nazionale delle Ricerche (ICCOM CNR), 50019 Sesto Fiorentino, Italy
*
Author to whom correspondence should be addressed.
Crystals 2017, 7(10), 318; https://doi.org/10.3390/cryst7100318
Submission received: 18 September 2017 / Revised: 13 October 2017 / Accepted: 17 October 2017 / Published: 20 October 2017
(This article belongs to the Special Issue Crystal Structures of Boron Compounds)
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Versions Notes

Abstract

:
Bis(diphenylphosphino)methane copper(I) tetrahydroborate was synthesized by ligands exchange in bis(triphenylphosphine) copper(I) tetrahydroborate, and characterized by XRD, FTIR, NMR spectroscopy. According to XRD the title compound has dimeric structure, [(μ2-dppm)2Cu2(η2-BH4)2], and crystallizes as CH2Cl2 solvate in two polymorphic forms (orthorhombic, 1, and monoclinic, 2) The details of molecular geometry and the crystal-packing pattern in polymorphs were studied. The rare Twisted Boat-Boat conformation of the core Cu2P4C2 cycle in 1 is found being more stable than Boat-Boat conformation in 2.

1. Introduction

The concept of cooperative catalytic effects [1] in multinuclear transition metal systems led to the broad development and extensive investigation of the chemistry of transition metal complexes, bearing “short-bite” ligands that are able to lock two or more metallocenters in close proximity [2,3,4,5,6,7]. Such compounds are of great interest due to their catalytic activity including the transformation of small molecules on metal centres [8,9], they can also be used as synthetic models of enzyme action [10,11,12].
Phosphines are ubiquitous ligands in transition metal chemistry. Among various types of diphosphine ligands, bis-(diphenylphosphino)methane (dppm) is one of the very efficient bridging ligands [2]. As other diphosphine ligands it is able to chelate metals, but rarely acts as a bidentate ligand (η2-dppm), forming a strained four-membered cycle (Scheme 1) [13,14,15,16,17]. Rather, it has a tendency to act as either a monodentate (η1-dppm) or bridging bidentate ligand (μ2-dppm) [18]. Many examples of binuclear complexes containing the eight-membered ring M(μ2-dppm)2M' are known with a variety of metals and stereochemistries [18].
The Cu(I)-dppm complexes are emerging class of polynuclear complexes, that are drawing considerable attention because of their photophysical properties [19,20,21,22] and prospective use as a catalyst [23,24,25] and a sensor for various organic bases [26] and anions [27]. Binuclear Cu(I) species possess an enhanced reactivity toward organic azides in copper-catalysed azide-alkyne cycloaddition compared to monomeric copper complexes [28,29,30,31,32,33,34,35]. Copper(I) tetrahydroborates with phosphine ligands featuring relative stability to air oxygen and moisture are used as selective reducing agents [36,37,38,39,40], catalysts of photosensitized isomerization of dienes [41,42,43] and hydrolytic dehydrogenation of ammonia borane [44]. Since metal tetrahydroborates have great potential in hydrogen storage technology [45,46,47,48,49,50], as catalysts [51,52,53,54,55] and selective reducing agents [56,57,58,59,60,61] their structural and dynamic properties have been actively investigated [52,62,63,64]. These studies revealed different modes of BH4− coordination to the metal atom, which can behave as mono-, bi-, or tridentate ligand [64].
Our studies of intermolecular interactions of BH4− [65,66] and several metal tetrahydroborates [67,68,69,70] with proton donors have shown the versatility of dihydrogen bonded (DHB) complexes formed and their crucial role in the reactivity of these compounds. In particular, we have shown that the formation of bifurcate DHB complexes involving both bridging and terminal hydride hydrogens of (Ph3P)2Cu(η2-BH4) (Scheme 2) is prerequisite for the subsequent proton transfer and dimerization to occur [67]. Continuing these studies, we attempted the synthesis of (η2-dppm)Cu(η2-BH4) following the published recipe [71]. However, in our hands, it gave, instead, a binuclear dimer bearing two bridging μ2-dppm ligands between the two {Cu(η2-BH4)} moieties. Herein we describe its spectroscopic characterization and analysis of polymorphic structures of its dichloromethane solvate.

2. Experimental Section

All manipulations were performed under a dry argon atmosphere using the standard Schlenk technique. Commercially-available argon (99.9%) was additionally purified from traces of oxygen and moisture by sequential passage through Ni/Cr catalyst column and 4 Å molecular sieves.
The HPLC grade solvents (Acros Organics, Morris Plains, NJ, United States) were used for sample preparation after additional purification by standard procedures. Dichloromethane (DCM) and toluene were dehydrated over CaH2 and Na/benzophenone, respectively. All solvents were freshly distilled under argon prior to use. Deuterated solvent (CD2Cl2) was dehydrated over CaH2 and was distilled and degassed by three freeze-pump-thaw cycles prior to use. Bis(diphenylphosphino)methane (dppm) from Sigma Aldrich (St. Louis, MO, USA) was used without additional purification. Bis(triphenylphosphine) copper(I) tetrahydroborate was prepared following the previously-described procedure [67].
IR spectra were recorded on Shimadzu IR Prestige21 (Shimadzu Corporation, Kyoto, Japan) and Nicolet 6700 FTIR (Thermo Fisher Scientific, Waltham, MA, USA) spectrometers in KBr pellets and Nujol mull in thin polyethylene film. NMR spectra were recorded on a Bruker Avance II 500 and 600 MHz spectrometers (Bruker Corporation, Billerica, MA, USA). 1H and 13C{1H} chemical shifts are reported in parts per million (ppm) downfield to tetramethylsilane (TMS) and were calibrated against the residual solvent resonance, while 31P{1H} spectra were referenced to 85% H3PO4 with a downfield shift taken as positive; 11B spectra were referenced to BF3⋅Et2O. The 13C{1H} NMR spectra were registered using the JMODECHO mode; the signals for the C atoms bearing odd and even numbers of protons have opposite polarities.

2.1. Preparation of µ2-Bis(Diphenylphosphino)Methane Copper(I) Tetrahydroborate [(μ2-dppm)2Cu2][η2-BH4]2

The complex was synthesized through a slight modification of a previously described procedure [71]. Bis(diphenylphosphino)methane (dppm) (0.5 g, 1.32 mmol) were added to a solution of bis(triphenylphosphine) copper(I) tetrahydroborate (0.8 g, 1.32 mmol) in 50 ml toluene. The reaction mixture was stirred for 3 h at 60 °C, then cooled to room temperatures and refrigerated (−15 °C) to afford the white powder precipitate (0.3 g) of pure µ2-bis(diphenylphosphino)methane copper(I) tetrahydroborate (yield: 48%). The monocrystals suitable for XRD analysis were obtained by slow solvent evaporation from CH2Cl2 (DCM) solution under an argon stream.
1H NMR (500 MHz, CD2Cl2, 298 K, ppm): 1.26 (br d, BH4−), 2.88 (br q, CH2), 7.11 (t, meta Ph), 7.26 (t, para Ph), 7.33 (multiplet, ortho Ph). 31P{1H} NMR (202 MHz, CD2Cl2, 298 K, ppm): −14.6 ÷ −16.5 (s) depending on conc. 11B NMR (160 MHz, CD2Cl2, 298 K, ppm): −29.81 (broad multiplet). 13C NMR (126 MHz, CD2Cl2, 298 K, ppm) 25.74 (multiplet CH2), 129.93 (s) para Ph, 128.52 (s) meta Ph, 132.58 multiplet orto Ph, 132.99 multiplet ipso C Ph.
FTIR: 3075, 3049, 2382, 2360, 2294, 2249, 2019, 1967, 1934, 1484, 1433, 1384, 1368, 1331, 1312, 1278, 1187, 1158, 1133, 1095, 1025, 999, 918, 848, 777, 766, 741, 734, 719, 693, 516, 507, 477 cm−1 (KBr pellet); 521, 516, 507, 477, 430, 420, 412, 358 cm−1 (Nujol mull/polyethylene film).
Several attempts to obtain the pure complex by recrystallization from toluene gave samples containing the traces of this solvent. The satisfactory elemental analysis was obtained for the sample which contains according to 1H NMR approximately 0.7 molecules of toluene per one molecule of the copper dimer. Anal. calcd. for C50H52B2Cu2P4: C, 64.88; H, 5.66; B, 2.34; Cu, 13.73; P, 13.39. Found: C, 66.55; H, 5.83; B, 2.29; P, 12.71.

2.2. Computational Details

Full geometry optimization of 1 and 2 (with removed solvent molecules) was performed with the Gaussian09 (Revision D.01, Gaussian, Wallingford, CT, USA) [72] software package. The model was described by M06 [73], B3LYP [74,75,76], BP86 [77], and PBE0 [78] methods with spin-state-corrected s6-31G(d) [79] basis set for Cu atom and 6-311++G(d,p) for atoms of the BH4− and alcohol OH-groups [80,81]; 6-31G(d) for the phosphorus atoms [82]; and 6-31G for the carbon and hydrogen atoms of dppm ligand [80,83,84]. For B3LYP, BP86 and PBE0 functionals empirical dispersion correction suggested by Grimme (GD2 [85] and GD3BJ [86,87]) was applied. Frequency calculations were performed for all optimized complexes in the gas phase and are reported without the use of scaling factors. The nature of all the stationary points on the potential energy surfaces was confirmed by an absence of any imaginary frequencies in the vibrational analysis [88].
The inclusion of nonspecific solvent effects in the calculations was performed by using the SMD method [89]. The solute cavity was redefined with radii = UAHF, because this atomic cavity was found to be more suitable than the default atom cavity (radii = SMD-Coulomb) defined in the SMD model [70,90]. The interaction energies were calculated in CH2Cl2 (ε = 8.9) for the gas phase optimized geometries. Changes in Gibbs energies and enthalpies in the solvent were determined using corresponding corrections obtained for the gas phase [91]:
ΔHSolv. = ΔESolv. + ΔHcorrgas
ΔGSolv. = ΔESolv. + ΔGcorrgas

2.3. X-ray Crystallography

X-ray diffraction data were collected on an Bruker APEX II CCD diffractometer (Bruker Corporation, Billerica, MA, United States) using molybdenum radiation [λ(MoKα) = 0.71072 Å, ω-scans] for 1 and 2. The substantial redundancy in data allowed an empirical absorption correction to be applied with SADABS by multiple measurements of equivalent reflections. The structures were solved by direct methods and refined by the full-matrix least-squares technique against F2 in the anisotropic-isotropic approximation.
The positional and anisotropic displacement parameters of the disordered CH2Cl2 in 1 and 2 were refined with the constraints on the C–Cl bond length (DFIX) and anisotropic displacement parameters (EADP). C–H hydrogen atoms in all structures were placed in calculated positions and refined within the riding model. Hydrogen atoms of BH4− group in both structures were located from the Fourier density synthesis and refined in the riding model. All calculations were performed with the SHELXTL software package [92]. Crystal data and structure refinement parameters are listed in Table 1. Crystallographic data for the structures reported in this paper have been deposited to the Cambridge Crystallographic Data Centre as supplementary no.: CCDC-1572389 (for 1) and CCDC-1572388 (for 2). These data can be obtained free of charge from Cambridge Crystallographic Data via www.ccdc.cam.ac.uk/data_request/cif.

3. Results and Discussion

3.1. Experimental Characterization

The title copper(I) tetrahydroborate was synthesized by the ligand exchange in a manner similar to that given in [71], where of the reaction product was described as (η2-dppm)Cu(η2-BH4). In our hands the procedure described in the Experimental Section yielded, upon recrystallization from CH2Cl2, a bimetallic complex with two bridging dppm ligands [(μ2-dppm)2Cu2][η2-BH4]2, the possible formation of such compound was suggested earlier in [93].
The 1H NMR spectrum (Figure S1) of the complex obtained exhibits the signals of phenyl protons around δPh = 7.26 ppm, methylene protons δCH2 = 2.88 ppm, and broad multiplet at δBH4 = 1.26 ppm belonging to borohydride protons. The 11B NMR spectrum (Figure S2) consists of a broad multiplet at −29.81 ppm, which is similar to the resonance of the monometallic (Ph3P)2Cu(η2-BH4) (δB = −29.79 ppm) [67]. The 31P{1H} spectrum (Figure S3) shows only one singlet at −14.6 ÷ −16.5 (s) ppm (depending on concentration), which means the phosphorus atoms are magnetically equivalent. The signal is deshielded compared to the free dppm ligand (δP = −21.7); at the same time its position is significantly different from that reported for monometallic dppm compound [71] (−148 ppm relative to (MeO)3P as a reference which is 140 ppm relative to 85% H3PO4 used in this work). In the 13C{1H} NMR (see Figures S4–S7) hydrogen and carbon atoms of methylene bridge (−25.9 ppm) and carbon atoms in phenyl rings (ortho and ipso ones) give centrosymmetric multiplets (see Figures S5 and S6). For the sake of comparison the 31P{1H} and 1H NMR spectra of [(μ2-dppm)2Cu2][η2-BH4]2 were also measured in CDCl3 (Figure S10).
FTIR spectra in the KBr pellets of 1 and 2 (Figures S8 and S9) show two BHterm at 2382 and 2360 cm−1 and two BHbr stretching vibrations at 2019 and 1967 cm−1, BH2 deformation at 1133 cm−1 (Table 2) and a band at 358 cm−1, which can be attributed to the vibrations of the four-membered CuHBH cycle (νCuB) [67,94]. The positions of these bands are within the range reported for bis-phosphine {Cu(η2-BH4)} complexes (Table 2). Moreover, they coincide with those reported previously for the analogue dppm compound formulated as (dppm)Cu(η2-BH4) [94].
The spectral criteria allow determining the coordination mode: the hapticity of the BH4− ligand. IR spectra of {M(η1-BH4)} complexes show only one BHbr stretching vibration instead of two BHbr stretching vibrations observed for {M(η2-BH4)} complexes. The latter also exhibits two resolved or one broad BHterm stretching vibrations at a higher frequency than {M(η1-BH4)} (Table 2). Additionally, stretching vibration of CuHBH cycle is a unique feature of {M(η2-BH4)} complexes [52,63]. Analysis of the data for copper(I) tetrahydroborate complexes shows that the 11B NMR chemical shift of the BH4− group is slightly different for different coordination types (−29.1 ÷ −30.2 ppm for {Cu(η2-BH4)} and −30.2 ÷ −40.0 ppm for {Cu(η1-BH4)}). Thus, the spectral analysis can serve as a base for the initial assignment of the BH4 coordination mode. The X-ray data on the Cu∙∙∙B distance (vide infra) should allow to unambiguously distinguish the type of BH4− coordination even if the position of hydrogens could not be accurately determined [63].
The XRD analysis of monocrystals obtained for this copper(I) tetrahydroborate compound revealed it is a binuclear complex bearing two dppm ligands bridging two {Cu(η2-BH4)} fragments. Previously it was found that the addition of an excess anion able to act as a capping ligand (e.g., halogen anions) can yield not only binuclear, but tri-, or even tetranuclear Cu(I)-dppm complexes. However in our case, BH4− does not act as a capping ligand, and the trinuclear structure was found previously only once for (μ2-PPh2NHPPh2)3Cu3(μ3-H)-(μ3-BH4) [105].
Two solvatomorphic structures were identified (Table 1, Figure 1 and Figure 2): one of orthorhombic space group P212121 with one DCM molecule (1), and the second one of monoclinic space group P21/c with ½ molecule of DCM per molecule of the copper complex (2) (Figure S11). In both structures the solvent molecules are disordered in a 1:1 ratio across a crystallographic inversion centre. The copper atoms have distorted tetrahedral geometry, being ligated with two phosphorus atoms of dppm ligands and two hydrogen atoms from tetrahydroborate; the selected bond distances and angles are presented in Table 3 (for additional details see Tables S2 and S4). Copper atoms and ligands form eight-member cycles Cu2P4C2 that have Twisted Boat-Boat conformation in 1 and Boat-Boat conformation in 2 (Figure S11). The Cu(1)–Cu(2) distance is 3.2035(4) Å for 1 and 3.392(1) Å for 2, which is above the sum of van der Waals radii for copper (2.8 Å), and is within the range (2.679–4.797 Å) determined for eight-member Cu2P4C2 cycles (Table S1).
The DFT calculations (see below) of 2 revealed its possible structural instability, during the optimization the conformation changes from Boat-Boat to Twist Boat-Boat.
The non-covalent interactions apparently play an important role in the stabilization of both structures. In both crystals the π-π stacking interaction between the pairs of phenyl rings of dppm ligands is suggested by short inter-ring distance (3.723 Å for 1 and 3.888 Å for 2). The analysis of molecular packing of 1 (Figure 2a) reveals four short contacts between the C–H of phenyl rings of dppm ligands and the chlorine atom of DCM (C–H···Cl) per unit cell, which can be considered as weak hydrogen bonds. The angles (∠C–H···Cl) for these interactions vary from 145.9 to 149.6° and H···Cl distances are in the range 2.659–2.679 Å that is less than the sum of van der Waals radii for these two atoms (2.95 Å). There is also a short B–H···Cl–C distance 2.755 Å between BH and DCM with angle ∠C–Cl···H(B) = 158.2° that resembles a halogen bonding [106,107] and was referred to as a hydride-halogen bond [108,109,110,111,112,113]. This interaction has a donor-acceptor nature, where B–Hδ− acts as a donor of electron density and interacts with an electron deficient area (σ-hole) located on the halogen atom +δHal–R.
The [(μ2-dppm)2Cu2][η2-BH4]2 molecules in 2 are connected with each other and with the DCM molecules via hydrogen bonds [r(H···Cl) = 3.031 Å, ∠C–H···Cl = 149.2°] and dihydrogen bonds [r(H···H) = 2.246 Å, ∠C–H···H(B) = 168.2°] leading to the formation of a two-dimensional network (Figure 2b).
As mentioned above, the borohydride ligand is coordinated to copper via two hydrogen atoms, the distances Cu∙∙∙B (2.190–2.198 Å) are typical for structures of {Cu(η2-BH4)} complexes (according to previously suggested structural criteria) [63] found in CCDC (Table S6), but are slightly shorter if compared with the value 2.212 Å found for (PPh3)2Cu(η2-BH4) [67]. The {Cu(η1-BH4)} complexes are characterized by longer Cu∙∙∙B distances of 2.441–2.499 Å.

3.2. CCDC Analysis

The CCDC search for the structures containing eight-membered [(μ2-dppm)2Cu2]2+ moieties gave 110 entries, but none of them bears a BH4 ligand. The crystal structures found are gathered in Table S1, subdivided according to their conformation type (Figure 3) and arranged in ascending order of Cu(1)···Cu(2) distances. Eight of the structures found are characterized by strong copper-copper interaction, the Cu(1)···Cu(2) distances being less than the sum of van der Waals radii for two Cu atoms 2.679–2.789 Å. For each conformation (Figure 3) we detected the most representative structure among the entries found and the boundary conditions. According to the boundary conditions for basic conformation types (Boat-Boat, Boat-Chair, Chair-Chair and Plain) the difference between two angles ∠PCuP and between two dihedral angles χ(P,Cu,Cu,P) should be less or equal 5°. The Chair-Chair conformation is characterized by two straight dihedral angle χ(P,Cu,Cu,P). Twist-type conformations (Twisted Boat-Boat and Twisted Boat-Chair) should have one dihedral angle χ(P,Cu,Cu,P) close to 90° and another obtuse dihedral angle χ(P,Cu,Cu,P) (>117°). Remaining structures that have geometrical parameters between basic and twisted conformation types were named as distorted conformations (Distorted Boat-Boat and Distorted Boat-Chair). All conformational data are summarized in Table 4 and Table S1.
The analysis reveals that Boat-Boat and Distorted Boat-Boat conformations account for 48% of all found structures (Figure 4). Other conformation types are Chair-Chair–19%, Boat-Chair–14% and Twisted Boat-Boat, comprising 10% of conformations. The rest of the conformation types (Distorted Boat-Chair, Twisted Boat-Chair and Plain) account for 5% and less.
The majority of all three Boat-Boat structures (basic, distorted, and twisted) contains a bridging ligand (μ-R2S; μ-R2CO; RPy-O; μ-NO3; μ-RCOO) and is characterized by rather short Cu(1)···Cu(2) distances (2.679–3.852 Å). The Chair-Chair and Plain conformations feature the longest Cu(1)···Cu(2) distance (3.359–4.797 Å) because these complexes contain chelating or strongly-coordinating ligands, such as bipyridine, pyrazine, phenantroline, and their derivatives. Despite the difference in the solid state conformations, the 31P NMR chemical shifts of dppm ligand in eight-membered [(μ2-dppm)2Cu2]2+ moieties fall in the same, rather broad, range that does not allow discriminating of the conformation types on the basis of 31P NMR data in solution.

3.3. DFT Calculations

Since our attempt to synthesize the monomeric compound (η2-dppm)Cu(η2-BH4) (Figure 5a), previously described in [71] has failed, we attempted to optimize this structure by different DFT methods. However, these attempts were unsuccessful; instead of the proposed structure (Figure 5a), they gave the (η1-dppm)Cu(η2-BH4) complex stabilized by copper interaction with a phenyl ring (Figure 5b). The formation of the [(μ2-dppm)2Cu2][η2-BH4]2 dimer from two molecules of monomeric (η1-dppm)Cu(η2-BH4) is energetically favourable, ∆GDCM°form being −19.3 kcal/mol (M06) and −24.0 kcal/mol (B3LYP-D2) (Table S5). This is in agreement with only a few examples of Cu(I) complexes reported in which dppm acts as a chelate ligand (η2-dppm, Scheme 1) [114]; the dinuclear Cu(I) complexes with two bridging dppm ligands are by far more common [115].
The geometry optimizations by M06 and B3LYP-D2 methods (Table S7) reproduced quite well the X-ray determined geometry 1 of the binuclear copper complex; the difference between the calculated and experimentally determined Cu(1)∙∙∙Cu(2) distances (0.006 Å for M06 and 0.155 Å for B3LYP-D2) is the lowest among other DFT functionals used (Tables S2 and S3). The difference between the calculated and experimentally observed Cu–P bonds lengths is also rather small, 0.039–0.020 Å (M06) and less than 0.020 Å for B3LYP-D2. The difference in experimental and theoretical CuH bond length and Cu∙∙∙B distances is 0.008–0.101 Å (M06), 0.004–0.089 Å (B3LYP-D2), 0.001–0.005 Å (M06) and 0.011–0.023 Å (B3LYP-D2), respectively. The analogous performance for M06 and B3LYP-D2 methods was previously observed for calculations of the Cu(II)-silsesquioxane core [116]. When the optimization was attempted for the geometry of structure 2, it led to the conformation changes converting from Boat-Boat to Twisted Boat-Boat. No local minimum was found for the Boat-Boat conformation type.
The simulated IR spectra of 1 (Twisted Boat-Boat) optimized by M06 and B3LYP-D2 methods are in line with the experimental IR spectra of [(μ2-dppm)2Cu2][η2-BH4]2 in KBr pellets (Figure 6, Table 5). The M06 and B3LYP-D2-optimized (η1-dppm)Cu(η2-BH4) monomer gives similar positions of IR-active stretching vibrations but has a significant difference in the relative IR intensity of BHterm stretching vibrations.

4. Summary and Conclusions

The XRD analysis of monocrystals revealed the first example of the bimetallic complex [(μ2-dppm)2Cu2][η2-BH4]2 bearing two dppm ligands bridging two {Cu(η2-BH4)} fragments. Two solvatomorphic structures were identified: one of orthorhombic space group P212121 with one DCM molecule 1 and the second one of monoclinic space group P21/c with ½ molecule of DCM per molecule of complex 2. The former structure possesses the twisted boat-boat conformation, which is rather rare for eight-membered [(μ2-dppm)2Cu2]2+ moieties. Analysis of the literature data revealed that, despite the difference in conformations, the 31P NMR chemical shift of dppm ligand in eight-membered [(μ2-dppm)2Cu2]2+ moieties does not enable identification of conformation type in solution. On the other hand, the 11B NMR and IR spectra could be used to discriminate between the η1 and η2-BH4 coordination modes. However, the final assignment should come from the XRD analysis.
The DFT calculations by M06 and B3LYP-D2 methods reproduced, quite well, the geometry of 1 and observed experimental IR spectra. Optimization of 2 revealed structural instability during the optimization conformation changes from Boat-Boat to Twisted Boat-Boat (Table S4). This finding is surprising because Boat-Boat and Distorted Boat-Boat conformations account, together, for 48% of the reported CCSD structures bearing eight-membered [(μ2-dppm)2Cu2]2+ fragments, whereas the Twisted Boat-Boat conformation is revealed only for 10% of compounds.

Supplementary Materials

The following are available online at www.mdpi.com/2073-4352/7/10/318/s1. Figure S1. 1H NMR spectra (500 MHz, CD2Cl2, 298 K, ppm) of [(μ2-dppm)2Cu2][η2-BH4]2. Figure S2. 11B{1H} NMR spectra (160 MHz, CD2Cl2, 298 K, ppm) of [(μ2-dppm)2Cu2][η2-BH4]2. Figure S3. 31P{1H} NMR spectra (202 MHz, CD2Cl2, 298 K, ppm) of [(μ2-dppm)2Cu2][η2-BH4]2. Figure S4. 13C{1H} NMR spectra (126 MHz, CD2Cl2, 298 K, ppm) of [(μ2-dppm)2Cu2][η2-BH4]2. Figure S5. 13C{1H} NMR spectra (126 MHz, CD2Cl2, 298 K, ppm) of [(μ2-dppm)2Cu2][η2-BH4]2 (16,850–16,600 Hz). Figure S6. 13C{1H} NMR spectra (126 MHz, CD2Cl2, 298 K, ppm) of [(μ2-dppm)2Cu2][η2-BH4]2 (3290–3180 Hz). Figure S7. 13C{1H} NMR spectra (126 MHz, CD2Cl2, 298 K, ppm) in JMODECHO mode of [(μ2-dppm)2Cu2][η2-BH4]2. Figure S8. FTIR spectra of [(μ2-dppm)2Cu2][η2-BH4]2 in KBr pellet. Figure S9. FTIR spectra of [(μ2-dppm)2Cu2][η2-BH4]2 in Nujol mull/thin polyethylene film. Figure S10. 31P{1H} (202 MHz, 298 K, ppm) and 1H NMR spectra (500 MHz, 298 K, ppm) of [(μ2-dppm)2Cu2][η2-BH4]2 in CDCl3. Figure S11. General view of molecular structures of 1 and 2 conformations. The solvents molecules are omitted for clarity. Table S1. CCDC analysis of the structures, containing eight-membered [(μ2-dppm)2Cu2]2+ moieties. Table S2. Structural parameters of crystal structure 1 (Twisted Boat-Boat) and optimized structures. Table S3. The differences between structural parameters of crystal structure 1 (Twisted Boat-Boat) and optimized structures. Table S4. Structural parameters of crystal structure 2 (Boat-Boat) and optimized structures. Table S5. Energy of formation of DFT-optimized geometries binuclear complexes (1 and 2) computed relative monomer complexes. Table S6. CCDC analysis of the structures, containing {Cu(BH4)} moieties. Table S7. DFT-optimised geometries (Cartesian coordinates) and electronic energies.

Acknowledgments

Some parts of our work were financially supported by the Ministry of Education and Science of the Russian Federation (the Agreement number 02.a03.21.0008), the Russian-Italian bilateral project CNR-RFBR no. 15-53-78027 and RFBR project no. 16-03-00324. The X-ray diffraction and NMR spectroscopic data were obtained using the equipment of Centre for Molecule Composition Studies of INEOS RAS.

Author Contributions

Evgenii I. Gutsul and Andrea Rossin carried out the synthesis. Konstantin A. Lyssenko performed the XRD experiments. Alexander S. Peregudov measured NMR experiments. Evgenii I. Gutsul conducted the FTIR investigation. Viktor D. Makhaev contributed reagents. Lina M. Epstein analysed the literature data. Igor E. Golub and Oleg A. Filippov performed the DFT calculations. Natalia V. Belkova and Igor E. Golub analysed the data and wrote the paper. Maurizio Peruzzini and Elena S. Shubina conceived experiments and supervised the work.

Conflicts of Interest

The author declare no conflict of interest.

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Crystals 07 00318 sch001 550
Scheme 1. Possible coordination modes of dppm ligand in transition metal complexes.
Scheme 1. Possible coordination modes of dppm ligand in transition metal complexes.
Crystals 07 00318 sch001
Crystals 07 00318 sch002 550
Scheme 2. Possible structures of DHB complexes. Adapted with permission from ref [67]. Copyright 2012 American Chemical Society.
Scheme 2. Possible structures of DHB complexes. Adapted with permission from ref [67]. Copyright 2012 American Chemical Society.
Crystals 07 00318 sch002
Crystals 07 00318 g001 550
Figure 1. General view of molecular structures 1 (a) and 2 (b). Thermal ellipsoids are drawn at the 50% probability level.
Figure 1. General view of molecular structures 1 (a) and 2 (b). Thermal ellipsoids are drawn at the 50% probability level.
Crystals 07 00318 g001
Crystals 07 00318 g002 550
Figure 2. View of molecular packing of 1 (P212121) (a) and 2 (P21/c) (b) in a unit cell.
Figure 2. View of molecular packing of 1 (P212121) (a) and 2 (P21/c) (b) in a unit cell.
Crystals 07 00318 g002
Crystals 07 00318 g003 550
Figure 3. The conformation types found from CCDC search for [(μ2-dppm)2Cu2]2+ moieties.
Figure 3. The conformation types found from CCDC search for [(μ2-dppm)2Cu2]2+ moieties.
Crystals 07 00318 g003
Crystals 07 00318 g004 550
Figure 4. Conformational distribution of eight-membered [(μ2-dppm)2Cu2]2+ moieties.
Figure 4. Conformational distribution of eight-membered [(μ2-dppm)2Cu2]2+ moieties.
Crystals 07 00318 g004
Crystals 07 00318 g005 550
Figure 5. The tentative structure of (η2-dppm)Cu(η2-BH4) monomer (a) and its M06-optimized geometry (b).
Figure 5. The tentative structure of (η2-dppm)Cu(η2-BH4) monomer (a) and its M06-optimized geometry (b).
Crystals 07 00318 g005
Crystals 07 00318 g006 550
Figure 6. Experimental IR spectra of [(μ2-dppm)2Cu2][η2-BH4]2 and simulated IR spectra of M06 and B3LYP-D2 optimized (η1-dppm)Cu(η2-BH4) monomer and 1. No scaling factors applied.
Figure 6. Experimental IR spectra of [(μ2-dppm)2Cu2][η2-BH4]2 and simulated IR spectra of M06 and B3LYP-D2 optimized (η1-dppm)Cu(η2-BH4) monomer and 1. No scaling factors applied.
Crystals 07 00318 g006
Table
Table 1. Crystal data and structure refinement parameters for 1 and 2.
Table 1. Crystal data and structure refinement parameters for 1 and 2.
12
Brutto formulaC50H52B2Cu2 P4, CH2Cl2C50H52B2Cu2 P4, 0.5 CH2Cl2
Formula weight1010.42967.96
T, K120120
Space groupP212121P21/c
Z(Z’)4(1)4(1)
a/Å14.218(2)23.0884(18)
b/Å17.875(3)13.0448(10)
c/Å19.523(3)16.0830(13)
β/°90.0092.055(2)
Volume/Å34961.7(13)4840.8(7)
ρcalc, g/cm31.3531.328
μ/cm−111.2810.99
F(000)20882004
2θmax, °5858
Reflections collected (Rint)50,044 (0.0480)56,740 (0.0429)
Independent reflections1314012851
Reflections with I > 2σ(I)118749781
Parameters547576
R1 [I > 2σ (I)]0.06040.0368
wR20.15690.0974
GOF1.0941.018
Residual electron density, e·Å−3 (ρmin/ρmax)−2.080/1.142−0.730/0.840
Table
Table 2. Characteristic vibrations observed in IR spectra and 11B NMR chemical shifts of the BH4− group reported for copper(I) tetrahydroborate complexes.
Table 2. Characteristic vibrations observed in IR spectra and 11B NMR chemical shifts of the BH4− group reported for copper(I) tetrahydroborate complexes.
CompoundνBHtermνBHbrδBH2νCuBδBH4Ref.
[(EtO)3P]2Cu(η2-BH4)2380, 23501990, 19301135–−29.1 c[95]
[(μ2-dppm)2Cu2][η2-BH4]22382, 23602019, 19671133358−29.5 bThis work
(PPh3)2Cu(η2-BH4)2403, 23941994,19371142374−29.7 b[67,96]
[{Ph2P(CH2)2}2NCH2]2Cu(η2-BH4)236520101120–−30.2 c[97]
[(MeO)3P]2Cu(η2-BH4)2380, 23451990, 19351135–−30.4 c[95]
(dppm)Cu(η2-BH4)2382, 23602018, 19651130358–[94]
“[(μ2-dppm)2Cu2][η2-BH4]2”2391, 23451987, 19241144––[93]
“(η2-dppm)Cu(η2-BH4)”2370, 22291984, 19491185378 a–[71]
(dppe)Cu(η2-BH4)2384, 23411990, 19281141––[93]
(dppe)Cu(η2-BH4)2380, 23602010, 19501140355–[94]
(dppb)Cu(η2-BH4)2385, 23601985, 19501140––[94]
(dpph)Cu(η2-BH4)2388, 23601982, 19401140356–[94]
(FcPPh2)2Cu(η2-BH4)2398, 23602005, 19601140––[94,98]
(Fc2PPh)2(η2-BH4)2398, 23602005, 19501140368–[94]
(dppf)Cu(η2-BH4)2397, 23542013, 19701130376–[94]
(nBuPPh2)2Cu(η2-BH4)2404, 23941995, 19371139363–[96,99]
[(EtO)3P]2Cu(η2-BH4)2397, 23601994, 19331137386–[96]
[(iPrO)3P]2Cu(η2-BH4)2399, 23941999, 19321137384–[96]
[(Me2N)3P]2Cu(η2-BH4)2392, 23662023, 19461137356–[96]
[(p-MeOC6H4O)3P]2Cu(η2-BH4)2385, 23502005, 1961–––[100]
[(p-MeC6H4O)3P]2Cu(η2-BH4)2382, 23431990, 1930–––[100]
[(m-MeC6H4O)3P]2Cu(η2-BH4)2380, 23432018, 1944–––[100]
[(EtO)3P](phen)Cu(η2-BH4)2360, 23302080–––[101]
(PPh3)(phen)Cu(η2-BH4)2360, 23302070, 19101120––[102]
(dmdp)Cu(η2-BH4)2385, 235019821128398–[102]
(triphos)Cu(η1-BH4)2354, 23211988––−32.8 b[69]
(MePPh2)3Cu(η2-BH4)2335, 231520501075, 1060–−39.0 c[95,103]
[(MeO)3P]3Cu(η1-BH4)23402055––−39.0 c[95]
[(EtO)3P]3Cu(η1-BH4)23352055––−40.0 c[95]
(triphos)Cu(η1-BH4)2360, 23001980–––[104]
(NP3)Cu(η1-BH4)231020601130, 1060––[104]
(EtP3)Cu(η1-BH4)237520001130––[104]
For the ligands abbreviations see Supporting Information. a This stretching vibration was previously described as νCuP [71]; b CD2Cl2; c CDCl3.
Table
Table 3. Selected structural parameters for 1 and 2.
Table 3. Selected structural parameters for 1 and 2.
Distances, Å1Distances, Å2
Cu(1)···Cu(2)3.392(1)Cu(1)–Cu(2)3.2035(4)
Cu(1)–P(2)2.238(2)Cu(1)–P(2)2.2234(7)
Cu(2)–P(1)2.253(2)Cu(2)–P(1)2.2608(7)
Cu(1)–P(3)2.254(2)Cu(1)–P(3)2.2288(6)
Cu(2)–P(4)2.257(2)Cu(2)–P(4)2.2542(6)
Cu(1)–B(1)2.194(9)Cu(1)–B(1)2.198(2)
Cu(2)–B(2)2.190(7)Cu(2)–B(2)2.192(3)
H(19)A···Cl(1’)2.722H(10)A···Cl(2)D3.031
H(13)A···Cl(1’)2.727
H(29)A···Cl(1’)2.816
H(28)A···Cl(1’)2.627
H(1)BD···Cl(2’)2.814H(26)A···H(1)BD2.246
Angles, °1Angles, °2
P(2)–Cu(1)–P(3)112.93(7)P(2)–Cu(1)–P(3)117.74(2)
P(1)–Cu(2)–P(4)111.33(6)P(1)–Cu(2)–P(4)117.29(2)
P(1)–C(1)–P(2)112.6(3)P(1)–C(1)–P(2)110.6(1)
P(3)–C(2)–P(4)109.9(4)P(3)–C(2)–P(4)111.5(1)
C(19)–H(19)A···Cl(1’)150.9C(10)–H(10)A···Cl(2)D149.2
C(13)–H(13)A···Cl(1’)140.8
C(29)–H(29)A···Cl(1’)136.0
C(28)–H(28)A···Cl(1’)150.4
B(1)–H(1)BD···Cl(2’)142.6C(26)–H(26)A···H(1)BD168.2
Dihedral Angles, °1Dihedral Angles, °2
χ1(P,Cu,Cu,P)−92.41(6)χ1(P,Cu,Cu,P)117.04(2)
χ2(P,Cu,Cu,P)133.69(6)χ2(P,Cu,Cu,P)−118.44(2)
Table
Table 4. Summary of Cu(1)···Cu(2) distances, PCuP and dihedral χ(P,Cu,Cu,P) angles reported for eight-membered [(μ2-dppm)2Cu2]2+ moieties. N–number of CCSD structures of certain conformation.
Table 4. Summary of Cu(1)···Cu(2) distances, PCuP and dihedral χ(P,Cu,Cu,P) angles reported for eight-membered [(μ2-dppm)2Cu2]2+ moieties. N–number of CCSD structures of certain conformation.
ConformationNd[Cu(1)···Cu(2)], Å∠PCuP’, °χ[P,Cu(1),Cu(2),P’], °∠PCHP’, °δ31P{1H}, ppm
Boat-Boat302.679–3.651113–136113–136110–117−7.8 ÷ −15.2
Distorted
Boat-Boat		212.931–3.85295–13387–115/117–139111–117−7.9 ÷ −25.7
Twist
Boat-Boat		112.743–3.757110–14089–103/134–164109–115+2.1 ÷ −14.6
Boat-Chair152.735–3.901117–133119–138111–116−6.3 ÷ −10.9
Distorted
Boat-Chair		52.712–4.644115–146113–171110–122−6.6 ÷ −18.7
Twist
Boat-Chair		32.925/3.133120–122/130–132105–108/145–148110–115−7.7/−8.2
Plain14.277148/150170117–
Chair-Chair203.359–4.797130–145179–180110–147−5.6 ÷ −14.4
Table
Table 5. Experimental and the calculated values of IR-active vibration bands for crystal structure 1 and optimized structures.
Table 5. Experimental and the calculated values of IR-active vibration bands for crystal structure 1 and optimized structures.
1Monomer
Vibration typeexptM06B3LYP-GD2M06B3LYP-GD2
νCHas(Ph)3075, 30493228, 32253230, 32203229, 32273228, 3218
νCHas(CH2)–3109, 30243053, 304630583179
νBHtermas23822504250725612543
νBHterms23602459249324902497
νBHbr1as201920612127, 210119912035
νBHbr2as19672000205719682004
νCuH1433, 13841421, 14121417, 139614531444
δBH113311651187, 117811471168
νCuB358405392357362

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Belkova, N.V.; Golub, I.E.; Gutsul, E.I.; Lyssenko, K.A.; Peregudov, A.S.; Makhaev, V.D.; Filippov, O.A.; Epstein, L.M.; Rossin, A.; Peruzzini, M.; et al. Binuclear Copper(I) Borohydride Complex Containing Bridging Bis(diphenylphosphino) Methane Ligands: Polymorphic Structures of [(µ2-dppm)2Cu2(η2-BH4)2] Dichloromethane Solvate. Crystals 2017, 7, 318. https://doi.org/10.3390/cryst7100318

AMA Style

Belkova NV, Golub IE, Gutsul EI, Lyssenko KA, Peregudov AS, Makhaev VD, Filippov OA, Epstein LM, Rossin A, Peruzzini M, et al. Binuclear Copper(I) Borohydride Complex Containing Bridging Bis(diphenylphosphino) Methane Ligands: Polymorphic Structures of [(µ2-dppm)2Cu2(η2-BH4)2] Dichloromethane Solvate. Crystals. 2017; 7(10):318. https://doi.org/10.3390/cryst7100318

Chicago/Turabian Style

Belkova, Natalia V., Igor E. Golub, Evgenii I. Gutsul, Konstantin A. Lyssenko, Alexander S. Peregudov, Viktor D. Makhaev, Oleg A. Filippov, Lina M. Epstein, Andrea Rossin, Maurizio Peruzzini, and et al. 2017. "Binuclear Copper(I) Borohydride Complex Containing Bridging Bis(diphenylphosphino) Methane Ligands: Polymorphic Structures of [(µ2-dppm)2Cu2(η2-BH4)2] Dichloromethane Solvate" Crystals 7, no. 10: 318. https://doi.org/10.3390/cryst7100318

APA Style

Belkova, N. V., Golub, I. E., Gutsul, E. I., Lyssenko, K. A., Peregudov, A. S., Makhaev, V. D., Filippov, O. A., Epstein, L. M., Rossin, A., Peruzzini, M., & Shubina, E. S. (2017). Binuclear Copper(I) Borohydride Complex Containing Bridging Bis(diphenylphosphino) Methane Ligands: Polymorphic Structures of [(µ2-dppm)2Cu2(η2-BH4)2] Dichloromethane Solvate. Crystals, 7(10), 318. https://doi.org/10.3390/cryst7100318

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