Oxidative Addition of Disulfides, Alkyl Sulfides, and Diphosphides to

Aug 16, 2016 - The aluminum(I) compound NacNacAl (1) reacts with diphenyl disulfide and diethyl sulfide to form the respective four-coordinate bis(phe...
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Oxidative Addition of Disulfides, Alkyl Sulfides, and Diphosphides to an Aluminum(I) Center Terry Chu,† Yaroslav Boyko,† Ilia Korobkov,‡ Lyudmila G. Kuzmina,§ Judith A. K. Howard,∥ and Georgii I. Nikonov*,† †

Department of Chemistry, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S 3A1, Canada X-ray Core Facility, Faculty of Science, University of Ottawa, 150 Louis Pasteur, Ottawa, Ontario K1N 6N5, Canada § N. S. Kurnakov Institute of General and Inorganic Chemistry, 31 Leninskii Prospekt, Moscow 119991, Russia ∥ Department of Chemistry, Durham University, South Road, Durham DH1 3LE, United Kingdom ‡

S Supporting Information *

ABSTRACT: The aluminum(I) compound NacNacAl (1) reacts with diphenyl disulfide and diethyl sulfide to form the respective four-coordinate bis(phenyl sulfide) complex NacNacAl(SPh)2 (2) and alkyl thiolate aluminum complex NacNacAlEt(SEt) (3). As well, reaction of 1 with tetraphenyl diphosphine furnishes the bis(diphenyl phosphido) complex NacNacAl(PPh2)2 (4). Production of 3 and 4 are the first examples of C(sp3)−S and R2P−PR2 activation by a main-group element complex. All three complexes were characterized by multinuclear NMR spectroscopy and X-ray crystal structure analysis. Furthermore, a variable-temperature NMR spectroscopic study was undertaken on 4 to study its dynamic behavior in solution.

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INTRODUCTION Many chemical transformations are mediated by catalysts, typically in the form of a late transition-metal complex.1 However, because of the recognized toxicity of the heavy metals and the significant costs associated with them,2 recent efforts have been directed toward developing alternative catalysts based on main-group metals.3 Activation of robust σ-bonds is typically the start of many catalytic cycles. However, this reactivity is uncommon for main-group elements, posing a significant obstacle to further the development of catalysts based on main-group elements. Transition metals readily mediate such bond activations, commonly via oxidative addition.4 Recently, significant advances have been made by utilizing low-valent main-group element complexes5 as well as frustrated Lewis pairs6 (FLPs) for the activation of a variety of H−X bonds (where X can be H, B, Si, N, and P, for example). In 2014, we demonstrated that Roesky’s aluminum(I) compound, NacNacAl (1, NacNac = [ArNC(Me)CHC(Me)NAr]− and Ar = 2,6-iPr2C6H3),7 can undergo facile oxidative addition with a wide array of H−X σ-bonds (X = H, B, Al, C, Si, N, P, O).8 We9 and others10 have since extended the scope of activation to more robust C(sp3)−F, C(sp2)−F, C(sp3)−O, and C(sp2)−O bonds. Related activations of C(sp3)−X bonds (where X = Cl, Br, and I) have been observed with the gallium11 and indium12 derivatives of 1, NacNacGa, and NacNacIn, respectively, and silylenes.13 Schulz and co-workers have also recently demonstrated the activation of C(sp3)−Bi, Sb−Sb, and Bi−Bi bonds with 1 as well as cleavage of the C(sp3)−Te and Te−Te bonds using NacNacGa.14 Inspired by these recent reports, we were interested in the ability of 1 to © 2016 American Chemical Society

activate the carbon and homoelement bonds of the lighter congeners, sulfur, and phosphorus. Activation of the carbon− sulfur bond by transition metals has become increasingly relevant in the petroleum and synthetic chemical industry.15 Such reactions with organosulfur compounds may provide promising routes to the dehydrosulfurization of petroleum feedstocks as well as offer potentially useful synthetic protocols for cross-coupling reactions and biomimetic organic synthesis. Significant progress has been accomplished in the last few decades and recently reviewed.16 While considerably less wellstudied, P−P bond activation of functionalized phosphorus dimers and oligomers by transition metals has been reported by Stephan and co-workers.17 Interest in the products of these reactions is due to the potential of utilizing them in novel routes for the synthesis of new phosphorus-based derivatives.18 In this contribution, we further expand the scope of bond activations by 1 to include the RS−SR, C(sp3)−S, and R2P− PR2 bonds, the latter two being the first examples by a maingroup element complex.

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RESULTS AND DISCUSSION

Our investigation began with the reaction of diphenyl disulfide with 1. The sulfur−sulfur bond was readily cleaved at room temperature within 3 h to cleanly furnish the symmetrically substituted bis(phenyl sulfide) aluminum complex, NacNacAl(SPh)2 (2; Scheme 1). Received: July 19, 2016 Published: August 16, 2016 9099

DOI: 10.1021/acs.inorgchem.6b01668 Inorg. Chem. 2016, 55, 9099−9104

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Inorganic Chemistry

overnight resulted in formation of the unsymmetrically substituted alkyl thiolate aluminum complex NacNacAlEt(SEt), 3 (Scheme 2). The formation of 3 is, to the best of our knowledge, the first example of C(sp3)−S oxidative addition mediated by a main-group element.

Scheme 1. Oxidative Addition of Diphenyl Disulfide by 1

Scheme 2. C(sp3)−S Bond Cleavage of Diethyl Sulfide by 1 This reactivity is consistent with the activation of disulfides with other low-valent main-group species such as silylenes,19 germylenes,20 stannylenes,20a,e,21 and plumbylenes22 as well as low-valent antimony and bismuth complexes.23 FLPs have also effected the cleavage of the S−S bond in disulfides.24 The 1H NMR spectrum of 2 is consistent with C2v symmetry in solution with a single apparent septet, due to coupling with diastereotopic methyl groups, at 3.33 ppm (3JH−H = 6.7 Hz) that is coupled to two doublets at 1.44 and 1.02 ppm (3JH−H = 6.6 and 6.8 Hz, respectively). The structure of 2, shown in Figure 1, was confirmed by X-ray diffraction analysis. The

Complex 3 was fully characterized by multinuclear NMR spectroscopy and X-ray analysis. Two distinct iPr groups are observed in the 1H NMR spectrum at 3.79 ppm (3JH−H = 6.8 Hz) and 3.31 ppm (3JH−H = 6.8 Hz), consistent with Cs symmetry. The ethyl group gives rise to a quartet and triplet at 0.06 (3JH−H = 8.1 Hz) and 0.94 ppm (3JH−H = 8.1 Hz), respectively, while the quartet at 2.68 ppm (3JH−H = 7.4 Hz) and triplet at 1.35 ppm (overlapped with a doublet) is assigned to the ethyl sulfide moiety. The crystal structure of 3 (Figure 2)

Figure 1. Molecular structure of 2 (thermal ellipsoids are shown at 30%; hydrogen atoms are omitted for clarity). Selected bond lengths (Å) and angles (deg): Al(1)−S(1) 2.2220(6), Al(1)−S(2) 2.2199(6), Al(1)−N(1) 1.895(1), Al(1)−N(2) 1.893(1), S(1)−Al(1)−S(2) 102.77(2), N(1)−Al(1)−N(2) 97.09(6). Figure 2. Molecular structure of 3 (thermal ellipsoids are shown at 30%; hydrogen atoms are omitted for clarity). Selected bond lengths (Å) and angles (deg): Al(1)−S(1) 2.2239(8), Al(1)−C(18) 1.995(2), Al(1)−N(1) 1.911(1), S(1)−Al(1)−C(18) 105.89(7), N(1)−Al(1)− N(1a) 96.61(7). The aluminum−ethyl and sulfur−ethyl groups are disordered over two positions related by the crystallographic mirror plane, but only one is shown.

coordination geometry at the aluminum center is a distorted tetrahedral with the atom sitting 0.420 Å below the plane defined by the N2C3 skeleton of the NacNac ligand. The Al−S bond lengths of 2.2219(6) and 2.2197(6) Å are essentially identical to each other and are comparable to the distances found in the related bis-thiolate compound NacNacAl(SH)2 (2.223(1) and 2.217(1) Å)25 and the corresponding Al−SPh distance in a three-coordinate (2.225(1) Å)26 and fivecoordinate (2.274(3) Å)27 aluminum complex. The metrical parameters are also similar to the respective bond distances of 2.237(1) and 2.245(1) Å in the dimeric bis-sulfide species (NacNacAlS)228 as well as the Al−S linkages in [NacNacAl(μS3)2AlNacNac] (2.223(1) and 2.248(1) Å), obtained upon reaction of 1 with S8.29 The Al−N distances of 1.894(1) and 1.893(1) Å are akin to the average Al−N distance (1.912 Å) in other four-coordinate NacNac ligated aluminum complexes reported previously by us.8,9 Encouraged by the ease of sulfur−sulfur bond cleavage, we then attempted the activation of the more challenging C(sp3)− S bond. Stirring a mixture of 1 and diethyl sulfide at 50 °C

closely resembles that of 2 with similar Al−S and Al−N bond distances of 2.2239(8) and 1.911(1) Å, respectively, with a crystallographically imposed mirror plane bisecting the molecule. The Al−C distance of 1.995(2) Å is comparable to the corresponding distances of 1.980(4) and 1.974(4) Å measured in the related complex tBuNacNacAlEt2 (where tBu NacNac is [ArNC(tBu)CHC(tBu)NAr]−) reported by Cui and co-workers.30 After successfully activating both the S−S and C(sp3)−S bonds, we turned our attention toward the activation of the corresponding bonds to phosphorus in diphosphines. Reaction of 1 with the bulky tetraphenyl diphosphine resulted in cleavage of the P−P bond over the course of 3 d at 70 °C to give cleanly 9100

DOI: 10.1021/acs.inorgchem.6b01668 Inorg. Chem. 2016, 55, 9099−9104

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Inorganic Chemistry the novel aluminum bis(diphenyl phosphido) complex NacNacAl(PPh2)2 (4; Scheme 3). While there are several Scheme 3. Addition of Tetraphenyl Diphosphine Across 1

examples of low-valent main-group complexes activating white phosphorus,31 including 1,32 the production of 4 is the first example of a main-group complex activating the P−P bond in a diphosphine. Complex 4 was fully characterized by multinuclear NMR spectroscopy and a single-crystal X-ray diffraction study. Interestingly, despite the symmetric substitution pattern at the aluminum center, two different iPr groups are present in the molecule, as two apparent septets at 3.79 and 3.65 ppm (3JH−H = 6.5 and 6.6 Hz, respectively) are observed in the 1H NMR spectrum. As well, two singlets at −36.8 and −50.0 ppm were detected in the proton-decoupled 31P NMR spectrum. The nonequivalency of the phosphido groups in bis(phosphide) complexes has precedent, for example, in the structure of Cp2Hf(PEt2)2 reported by Baker et al. A significant difference in the Hf−P bond distances as well as the geometry around the phosphorus atom was observed (2.488(1) Å, trigonal planar vs 2.682(1) Å, pyramidal) that was attributed to the donation of the lone pair on one phosphorus center into a vacant orbital on hafnium.33 However, such a difference in coordination geometry around the two phosphorus atoms was not observed in 4. Therefore, the apparent Cs symmetry in complex 4 is due to the significant steric bulk afforded by the diphenyl phosphide ligands, resulting in restricted movement and rotation around the aluminum center. The molecular structure of 4 as determined by X-ray diffraction is shown in Figure 3. Similar to the structures of 2 and 3, a distorted tetrahedral geometry is seen at the aluminum center. Because of the large diphenyl phosphide groups, the aluminum atom sits 0.685 Å below the plane defined by the N2C3 framework of the ligand, even further than the corresponding distance of 0.571 Å8 in the closely related hydrido phosphido complex NacNacAlH(PPh2). Similarly, an expansion of the bite angle is noted in 4 compared to NacNacAlH(PPh2) (98.80(5) vs 95.42(6)°) due to the increased steric pressure.8 The Al−P bonds in 4 (2.3775(5) and 2.3979(5) Å) are both comparable to the Al−P distance of 2.3971(6) Å8 found in NacNacAlH(PPh2). To probe whether dynamic behavior in 4 can be observed and studied, a variable-temperature NMR spectroscopy study was performed. As illustrated in Figure 4, two apparent septets are observed for the inequivalent methine protons on the isopropyl group in the slow-exchange regime. As the temperature is raised, the signals merge, and the pattern of the isopropyl moieties morphs into a single apparent septet, consistent with an average C2v symmetry for the molecule in the fast-exchange regime. From the 1H NMR data, the barrier for this process was found to have a ΔG‡ value of 17.0 kcal mol−1 at the coalescence temperature of 70 °C. Similar dynamic behavior was observed by Piers and co-workers in bis(alkyl) scandium complexes tBuNacNacScR2 (R = Cl, Me, Et, Bz, CH2CMe3, CH2SiMe3); however lower coalescence temperatures ranging from −60 to 30 °C were found.34

Figure 3. Molecular structure of 4 (thermal ellipsoids are shown at 30%; hydrogen atoms are omitted for clarity). Selected bond lengths (Å) and angles (deg): Al(1)−P(1) 2.3775(5), Al(1)−P(2) 2.3979(5), Al(1)−N(1) 1.9254(10), Al(1)−N(2) 1.902(1), P(1)−Al(1)−P(2) 124.87(2), N(1)−Al(1)−N(2) 98.80(5). One phenyl ring at P(1) is disordered over two positions, but only one is shown.

Figure 4. Series of 1H NMR spectra of 4 (300 MHz, C7H8), depicting the coalescence temperature and dynamic behavior of the molecule.

Finally, cleavage of the C(sp3)−P bond was attempted. Unfortunately, even after prolonged heating (days) at 90 °C, no productive chemistry toward bond cleavage was observed in our hands in the reaction between 1 and triethylphosphine, likely due to significant steric congestion in the possible transition state.

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CONCLUSION In conclusion, we have demonstrated the ability of 1 to activate the S−S bond in disulfides under mild conditions along with 9101

DOI: 10.1021/acs.inorgchem.6b01668 Inorg. Chem. 2016, 55, 9099−9104

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Inorganic Chemistry the first examples of C(sp3)−S and P−P bond cleavage in a diphosphine mediated by a main-group element. Because of the presence of two bulky diphenylphosphine groups in 4, Cs symmetry is observed in solution at room temperature, while the more symmetric C2v structure is observed upon heating the sample. A variable-temperature NMR study was undertaken, and the energy barrier was calculated to be 17.0 kcal mol−1 with a coalescence temperature of 70 °C.

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12H, C6H3 and C6H5), 6.77 (m, 10H, C6H3 and C6H5), 4.99 (s, 1H, CH), 3.79 (hept, 2H, CH(CH3)2, 3JH−H = 6.5 Hz), 3.65 (hept, 2H, CH(CH3)2, 3JH−H = 6.6 Hz), 1.61 (d, 6H, CH(CH3)2, 3JH−H = 6.7 Hz), 1.53 (s, 6H, NCCH3), 1.09 (d, 6H, CH(CH3)2, 3JH−H = 6.8 Hz), 1.03 (d, 6H, CH(CH3)2, 3JH−H = 6.5 Hz), 0.98 (d, 6H, CH(CH3)2, 3 JH−H = 6.6 Hz). 13C{1H} (101 MHz, C6D6): δ 171.7 (NCCH3), 146.0, 144.3, 125.9, 125.0 (C6H3), 141.6 (Cipso in C6H3), 137.9, 135.4, 125.8, 125.6, (C6H5), 135.1 (p-C C6H5), 101.0 (CH), 29.5, 28.7 (CH(CH3)2), 25.6, 25.6, 25.3, 25.0 (CH(CH3)2), 24.6 (NCCH3). 31 1 P{ H} (162 MHz, C6D6): δ −36.8 (s, P−Ph2), −50.0 (s, P−Ph2). Anal. Calcd for C53H61AlN2P2: C, 78.11; H, 7.54; N, 3.44. Found: C, 78.10; H, 7.65; N, 3.51%. X-ray Crystallography. The crystals were mounted in a film of perfluoropolyether oil on a glass fiber and transferred to a diffractometer. Intensity data for 2 were collected on a Bruker SMART 1K diffractometer, and for 3 and 4 they were collected on a Bruker AXS SMART single-crystal diffractometer equipped with a sealed Mo tube source APEX II CCD detector, using Mo Kα radiation (0.710 73 Å).36 Semiempirical absorption corrections based on equivalent reflections were applied.37 The presence of systematic absences in the diffraction data set together with unit cell parameters suggested the monoclinic space group P2(1)/c (No. 14) for compound 2, orthorhombic Pnma (No. 62) for compound 3, and monoclinic space group P2(1)/n (No. 14) for compound 4. Solutions in these space groups yielded a chemically reasonable and computationally stable result of refinement. All structures were solved by direct methods38 and refined by full matrix least-squares on F2,39 with anisotropic thermal parameters for all non-hydrogen atoms. The positions of the hydrogen atoms attached to noncarbon atoms (Al, N, or S) were found as residual electron density peaks from the Fourier maps. The hydrogen atoms directly attached to carbon atoms were restrained to riding models and were consecutively treated as idealized contributions during the refinement. Refinement of the structural model for 3 revealed that the independent molecule lie on the mirror symmetry element of the space group. In the final stages of refinement, thermal displacement coefficients and locations of residual density peaks suggested the presence of positional disorder. Such a disorder appeared due to the location of the molecule in the special position. Two fragments of the model S(1)−C(16)−C(17) and C(18)−C(19) exhibited symmetry-related positional disorder in the positions of C(17) and C(19) atoms, respectively. The disorder was successfully modeled with 50%/50% occupation coefficients for both fragments. To achieve the least-constrained configuration geometry, it was decided to orient the groups belonging to the same disordered part in opposite directions. The asymmetric unit for the crystallographic model of 4 consists of one target compound molecule located in the general position. In the final stages of refinement, the thermal motion coefficients and locations of the residual electron density peaks suggested the presence of a nonsymmetry-related positional disorder for the phenyl ring comprised of C(30) to C(35) carbon atoms. The above-mentioned moiety was split into two independent positions. The occupancy coefficients were refined at the early stages, but, based on the refined values, were later constrained to 50%/50% at the latest stage of refinement, to preserve the data-to-parameter ratio. To ensure a reasonable fragment geometry and acceptable thermal motion parameter coefficients, a set of geometry restraints (AFIX 66) and thermal motion coefficients constraints (EADP) were applied to the disordered fragments. All scattering factors were contained in several versions of the SHELXTL program library, with the latest version used being v.6.12.40 Crystal and structure refinement data are given in Table SI1.

EXPERIMENTAL SECTION

General. Unless stated otherwise, all manipulations were performed using standard inert atmosphere (N2 gas) glovebox and Schlenk techniques. Benzene, hexanes, and diethyl ether were dried and purified using a Grubbs-type solvent purification system. Benzened6 was predried and distilled from K/Na alloy and stored in a glass vessel in the glovebox. NMR spectra were obtained with a Bruker DPX-300 and Avance III HD 400 MHz spectrometer (1H, 300 and 400 MHz; 13C, 75 and 101 MHz; 31P, 121 and 162 MHz) at room temperature, unless stated otherwise, then processed and analyzed with MestReNova software (v10.0.2−15465). Elemental analyses were obtained by the analytical laboratory of the London Metropolitan University. Diethyl sulfide and diphenyl disulfide were purchased from Sigma-Aldrich. Tetraphenyldiphosphine was prepared by the reaction of chlorodiphenylphosphine with 1 equiv of lithium metal.35 Compound 1 was prepared according to a literature procedure.7 NacNacAl(SPh2) (2). A solution of 1 (0.099 g, 0.223 mmol) in benzene (4 mL) was added to a vial followed by the addition of diphenyl disulfide (0.049 g, 0.223 mmol) dissolved in 3 mL of benzene. The reaction was stirred for 3 h at room temperature to yield a yellow solution. Volatiles were removed under vacuo and taken up in diethyl ether. Colorless crystals of 2 were obtained upon cooling to −30 °C (0.073 g, 0.110 mmol, 49%). 1H NMR (400 MHz, C6D6): δ 7.19 (m, 4H, C6H3 and C6H5), 7.14 (m, 2H, C6H3 and C6H5), 7.06 (m, 4H, C6H3 and C6H5), 6.89 (m, 6H, C6H3 and C6H5), 5.15 (s, 1H, CH), 3.33 (hept, 4H, CH(CH3)2, 3JH−H = 6.7 Hz), 1.53 (s, 6H, NCCH3), 1.44 (d, 12H, CH(CH3)2, 3JH−H = 6.6 Hz), 1.02 (d, 12H, CH(CH3)2, 3JH−H = 6.8 Hz). 13C{1H} (101 MHz, C6D6): δ 172.2 (NCCH3), 145.2, 125.1 (C6H3), 139.8 (Cipso in C6H3), 136.7, 134.1, 129.3, 127.3, 124.4 (C6H3 and C6H5), 99.4 (CH), 28.9 (CH(CH3)2), 25.2, 25.0 (CH(CH 3 ) 2 ), 24.0 (NCCH 3 ). Anal. Calcd for C41H51AlN2S2: C, 74.28; H, 7.75; N, 4.23. Found: C, 74.61; H, 7.52; N, 4.27%. NacNacAlEt(SEt) (3). Diethyl sulfide (0.022 mL, 0.202 mmol) was added to a flask containing 1 (0.090 g, 0.202 mmol) in benzene (7 mL). The mixture was heated with stirring at 50 °C overnight to give a yellow solution. Removal of the solvent in vacuo yielded a yellow solid. The product was recrystallized from hexanes at −30 °C to give yellow crystals of 3 (0.044 g, 0.082 mmol, 41%). 1H NMR (400 MHz, C6D6): δ 7.18 (m, 4H, C6H3), 7.09 (m, 2H, C6H3), 4.80 (s, 1H, CH), 3.79 (hept, 2H, CH(CH3)2, 3JH−H = 6.8 Hz), 3.31 (hept, 2H, CH(CH3)2, 3 JH−H = 6.8 Hz), 2.68 (q, 2H, S−CH2, 3JH−H = 7.4 Hz), 1.60 (d, 6H, CH(CH3)2, 3JH−H = 6.6 Hz), 1.54 (s, 6H, NCCH3), 1.35 (m, 9H, CH(CH3)2 and S−CH2−CH3), 1.25 (d, 6H, CH(CH3)2, 3JH−H = 6.9 Hz), 1.08 (d, 6H, CH(CH3)2, 3JH−H = 6.8 Hz), 0.94 (t, 3H, Al−CH2− CH3, 3JH−H = 8.1 Hz), 0.06 (q, 2H, Al−CH2, 3JH−H = 8.1 Hz). 13C{1H} (101 MHz, C6D6): δ 170.7 (NCCH3), 145.8, 143.8, 127.6, 125.3, 124.2 (C6H3), 140.6 (Cipso), 98.6 (CH), 28.8, 28.3 (CH(CH3)2), 26.9, 25.0, 25.0, 24.6 (CH(CH3)2), 23.5 (NCCH3), 20.9 (S−CH2), 20.1 (S−CH2−CH3), 9.3 (Al−CH2−CH3), −0.3 (Al−CH2). Anal. Calcd for C33H51AlN2S: C, 74.11; H, 9.61; N, 5.24. Found: C, 73.93; H, 9.73; N, 5.33%. NacNacAl(PPh2)2 (4). A flask containing 1 (0.101 g, 0.227 mmol) in benzene (7 mL) was charged with tetraphenyl diphosphine (0.084 g, 0.227 mmol) dissolved in benzene (2 mL) and heated with stirring for 3 d at 70 °C. Solvent was removed from the resulting orange solution, and the residue was dissolved in hexanes. Cooling the solution to −30 °C afforded orange crystals of 4 (0.107 g, 0.131 mmol, 58%). 1H NMR (400 MHz, C6D6): δ 7.57 (d, 4H, p-H C6H5, 3JH−H = 7.4 Hz), 7.04 (m,

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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.6b01668. 9102

DOI: 10.1021/acs.inorgchem.6b01668 Inorg. Chem. 2016, 55, 9099−9104

Article

Inorganic Chemistry

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NMR spectra and crystal and structure refinement data (PDF) Crystallographic data for 2 (CIF) Crystallographic data for 3 (CIF) Crystallographic data for 4 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.I.N. acknowledges the financial support of the Petroleum Research Fund administered by the American Chemical Society. L.G.K. thanks the Russian Fund for Basic Research. T.C. is grateful to the Government of Ontario for an Ontario Graduate Scholarhip.

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DOI: 10.1021/acs.inorgchem.6b01668 Inorg. Chem. 2016, 55, 9099−9104

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DOI: 10.1021/acs.inorgchem.6b01668 Inorg. Chem. 2016, 55, 9099−9104