Bis(trimethylsilyl)amide - ACS Publications - American Chemical Society

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Silicon(II) Bis(trimethylsilyl)amide (LSiN(SiMe3)2, L = PhC(NtBu)2) Supported Copper, Silver, and Gold Complexes Shabana Khan,*,† Saurabh K. Ahirwar,‡ Shiv Pal,† Nasrina Parvin,† and Neha Kathewad† †

Department of Chemistry, Indian Institute of Science Education and Research, Dr. Homi Bhabha Road, Pashan, Pune 411 008, India Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India

‡

S Supporting Information *

ABSTRACT: The reactions of silicon(II) bis(trimethylsilyl)amide (1; LSiN(SiMe3)2, L = PhC(NtBu)2) with the readily available CuI, AgOTf (Tf = SO2CF3), AgNO3, and AuCl(SMe2) have been reported, which resulted in a series of silylene−coinage-metal complexes (2−5) with Si(II)→coinage-metal bonds. Each compound has been characterized by multinuclear NMR spectroscopy, mass spectrometry, elemental analysis, and single-crystal X-ray diffraction studies. X-ray analyses of 2 and 3 showed that they are dimeric in nature, featuring four-membered Cu2I2 and Ag2O2 rings, respectively. However, 4 and 5 are mononuclear complexes with a silylene ligand and possess only one SiII→Ag or SiII→Au bond, respectively. Prior to our contribution, there have been very few structurally authenticated silylene−silver and silylene−gold complexes known. Therefore, 2−5 are noteworthy additions to the Si(II)−coinage-metal complexes.

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complex with a diaminosilylene ligand (A) (Chart 1).7 Recently, the groups of Driess8 and Iwamoto9 described the

INTRODUCTION The use of N-heterocyclic silylenes (NHSis) as ligands for transition metals can be rationalized by their inherent σ-donor ability through a lone pair of electrons to the empty orbitals of the transition metals, which resemble the coordination characteristics of their lighter analogues, N-heterocyclic carbenes (NHCs). However, in contrast to a wide variety of NHC−metal complexes, the analogous silylene−metal complexes have been scarcely investigated.1 Fürstner et al. prepared a (NHSi)(PPh3)Pd0 complex to catalyze the Suzuki coupling of arylboronic acids with bromoarenes.2 This reaction exemplified the potential of NHSi as a ligand. Current research efforts in this area have focused on the isolation of well-defined transition-metal complexes with divalent Si atoms and their application in homogeneous catalysis.3 Very recently, Driess and Hartwig described the hydrosilylation of ketone, reduction of amide, catalytic borylation of arene, and C−C bond formation reactions, which are mediated by various NHSi− transition-metal complexes.3a−e However, the majority of these transition-metal complexes are with the metals Ni, Rh, Ir, and Fe. To the best of our knowledge, NHSi−coinage-metal complexes are still very scant, despite the theoretical impetus from Frenking et al. that NHSi forms strong bonds with the coinage metals.4 The trend of the silylene−coinage-metal bond strength is as follows: Au > Cu > Ag. Prior to Frenking’s publication, Jutzi et al. isolated a Fischer type SiII−Au complex [(η5-Me5C5)(η1-Me5C5)SiAuCl] from the reaction of [(η5Me5C5)2Si] with Au(CO)Cl under the elimination of CO.5 Moreover, the same group also reported the insertion of [(η5Me5C5)2Si] into a Au−Cl bond, resulting in a silylgold complex.6 Lappert et al. reported on a four-coordinate copper © XXXX American Chemical Society

Chart 1. Reported Silylene−Coinage-Metal Complexes

isolation of cationic Cu(I) complexes with functionalized silylenes (B−E) and dialkylsilylene (F), respectively. Although tBu2SiAg(OSO2CF3) was experimentally detected as a reactive intermediate in the silver-catalyzed silylene transfer reactions,10 the isolation of a NHSi−Ag complex was recently accomplished Received: September 1, 2015

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DOI: 10.1021/acs.organomet.5b00745 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Reactions of 1 with Various Coinage-Metal Salts

(G) by Iwamoto et al.9 The N-donor-stabilized benzamidinatochlorosilylene [PhC(NtBu)2SiCl]11 from the Roesky group has also enjoyed substantial attention in recent years for activating small molecules12 as well as for demonstrating rich coordination chemistry toward various transition metals.1e,g,h,3,13 Using the same N-tert-butyl C-phenyl substitution, Roesky et al. prepared another functionalized silylene, [PhC(NtBu)2SiN(SiMe3)2] (1),14 in high yield. However, 1 is yet to be explored as a ligand for transition metals. Keeping the paucity of NHSi− coinage-metal complexes in mind, we explored the reaction of 1 with the easily available group 11 metal salts, e.g. CuI, AgOTf, AgNO3, and AuCl(SMe2), and our results are described herein. It must be noted here that this is the first report where a single silylene forms a Lewis adduct with all three coinage metals.

in the respective 29Si NMR spectrum in comparison to that of the precursor 1 (δ −8.07 ppm). The 29Si NMR resonance of 2 is marginally upfield with respect to those of C and E (C, δ 32.9 ppm; E, δ 18.3 ppm), which is a reflection of the substituents attached to the Si and Cu centers. In the 1H NMR spectra of all four complexes the tBu protons resonate as a singlet, which are marginally shifted downfield (2, δ 1.33; 3, δ 1.25; 4, δ 1.28; 5, δ 1.32 ppm) in comparison to that of 1 (δ 1.23 ppm) (Table 1). Table 1. Key Chemical Shifts and Si−M Bond Distances Observed for Complexes 2−5 SiII−M (Cu, Ag, Au) bond length (Å)

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RESULT AND DISCUSSION Our efforts began by exploring the ability of 1 to react with a broad range of coinage-metal salts. As summarized in Scheme 1, the Lewis adducts [{PhC(NtBu)2}Si{N(SiMe3)2}]2Cu2I2 (2), [{PhC(NtBu) 2 }Si{N(SiMe 3 ) 2 }] 2 Ag 2 [OTf] 2 (3), [{PhC(NtBu)2}Si{N(SiMe3)2}]AgNO3 (4), and [{PhC(NtBu)2}Si{N(SiMe3)2}]AuCl (5), derived from the reaction of 1 with CuI, AgOSO2CF3, AgNO3, and AuCl(SMe2), were obtained under mild conditions in good yield. All four complexes are crystalline solids, soluble in common organic solvents, and stable under an inert atmosphere. These complexes were characterized by multinuclear NMR spectroscopy and singlecrystal X-ray structural analysis. 2 and 3 feature dinuclear complexes with a four-membered (TM)2X′2 (TM = Cu, Ag; X′ = I, O of OTf) ring, where each Cu and Ag atom is bound to the silylene ligand. In contrast, 4 and 5 feature mononuclear complexes with silylene ligand. It would not be that erroneous to state that [1→MX] (M = Cu, Ag; X = I, OTf) is not sufficiently stable, and as a result it led to the dimerized products. The coordination of the coinage metal to the Si(II) center was accompanied by the downfield shift of Si(II) resonances (2, δ 16.85; 3, δ 8.57; 4, δ 7.87; and 5, δ 8.76 ppm)

complex

δ(29Si{1H}) (ppm)

δ(tBu) (ppm)

1 2

−8.07 16.85

1.23 1.33

3

8.57

1.25

4 5

7.87 8.76

1.28 1.32

exptl

theor calcd

2.243(3), 2.250(3) 2.337(2), 2.346(2),

2.334, 2.340

2.265(1)

2.458, 2.465

2.358

A variable-temperature (−50 to +50 °C) 1H NMR spectroscopic analysis of these compounds revealed the same singlet resonances for the tBu protons, thereby indicating a rigid coordination of the silylene ligand toward group 11 metals. The molecular structure of compound 2 is shown in Figure 1, and the bond parameters are given in the legend of Figure 1. 2 crystallizes in the monoclinic space group C2/c15 and consists of a four-membered Cu2I2 ring, where each silylene ligand is coordinated to a Cu atom. The two Si−Cu bond lengths are 2.243(3) and 2.250(3) Å. The Si−Cu bond lengths lie within the range of the previously reported Si−Cu bond lengths in A (2.289(4) Å),7 C (2.1981(12) Å),8 and F (2.268 Å (mean)).9 Each Si(II) center has four coordination sites and features B


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Organometallics

the Si(II) center. The substantially longer Si→Ag bond length in 3 in comparison to the Si→Cu bond in 2 and the Si→Au bond in 5 can be attributed to the electron-withdrawing nature of the triflate groups. When AgNO3 was used instead of AgOTf, the 1:1 adduct 4 was obtained. The disparity in the product formations could be explained by the different coordination modes of triflate and nitrate ligands. In 3, the O atom of the triflate ligand is bound to the Ag atom in a monocoordinated fashion. However, the nitrate ligand behaves as a chelating ligand, where two O atoms are bound to the Ag center in 4 (Scheme 1). The additional electron donation presumably inhibits the dimerization and permits the isolation of the monocoordinated Si II→Ag complex 4. Although X-ray diffraction analysis on a single crystal unambiguously established the connectivity of 4, we refrain from the discussion of the bonding parameters because of the low quality of the data (see the Supporting Information for the figure). In agreement with the NMR data, the molecular structure of 4 is comprised of a silylene-bound four-membered ring with one Ag atom, two O atoms, and one N atom. The molecular structure of compound 5 is shown in Figure 3, and the bond parameters are given in the legend of Figure 3.

Figure 1. Molecular structure of 2 with anisotropic displacement parameters depicted at the 50% probability level. Hydrogen atoms and tBu groups are not shown for clarity. Selected bond lengths (Å) and bond angles (deg): Cu1−I1 2.608(2), Cu1−I2 2.645(2), Cu2−I1 2.598(2), Cu2−I2 2.652(2), Cu1−Si1 2.243(3), Cu2−Si4 2.250(3), Si1−N3 1.731(10), Si4−N6 1.731(9); I1−Cu1−I2 98.32(5), I1− Cu2−I2 98.38(5), Cu1−I1−Cu2 81.25(5), Cu1−I2−Cu2 79.55(5).

slightly distorted tetrahedral geometry. The Cu−I bond distances in 2 range from 2.599(2) to 2.652(2) Å. The solid-state structure of 3 is similar to that of 2 (Figure 2).15 Two silylene units are bound to the four-membered Ag2O2 core, in which the two O atoms from the OTf groups are not in the same plane of the Ag atoms. The Si−Ag bond lengths (2.337(2) and 2.346(2) Å) are shorter than those of G (2.4015(16) Å)9 and the reported silylsilver complex (2.483(2) Å),16 which are presumably a reflection of the substituents on

Figure 3. Molecular structure of 5 with anisotropic displacement parameters depicted at the 50% probability level. Hydrogen atoms are not shown for clarity. Selected bond lengths (Å) and bond angles (deg): Si1−Au1 2.265(1), Au1−Cl1 2.341(1), Si1−N1 1.824(3), Si1− N2 1.837(2), Si1−N3 1.716(3), Si2−N3 1.781(3), Si3−N3 1.772(2); Si1−Au1−Cl1 179.47(3), N3−Si1−Au1 120.13(9), N2−Si1−Au1 113.91(8), N1−Si1−Au1 113.27(9), N1−Si1−N2 71.55(11), N3− Si1−N1 114.24(12), N3−Si1−N2 114.11(12).

Compound 5 crystallizes in the monoclinic space group P21/c. 5 displays a two-coordinate Au(I) complex with one coordination site occupied by the silylene ligand and the other site occupied by the chloride ligand. The linearity of the Au(I) atom is proved by the Si1−Au1−Cl1 angle of 179.47(3)°. Like 2−4, the geometry around the central Si atom of 5 is distorted tetrahedral. The isolation of 5 indicates that the silylene ligand provides sufficient stabilization to stop the dimerization of [1→Au(Cl)]. The finding is in agreement with the theoretical results by Frenking et al.,4 which described that silylene forms the strongest coordination bond with Au among the group 11 metals. The Si−Au distance of 2.265(1) Å is close to the theoretically calculated Si(II)−Au(I) bond length of 2.227 Å4 and substantially shorter than those of structurally

Figure 2. Molecular structure of 3 with anisotropic displacement parameters depicted at the 50% probability level. Hydrogen atoms are not shown for clarity. Selected bond lengths (Å) and bond angles (deg): Ag1−O1 2.290(6), Ag1−O2 2.468(8), Ag2−O1 2.566(6), Ag2−O2 2.363(7), Ag1−Si1 2.337(2), Ag2−Si4 2.346(2), Si4−N3 1.729(6), Si1−N6 1.724(6); O1−Ag1−O2 76.4(2), O1−Ag2−O2 73.3(2), Ag1−O1−Ag2 96.8(2), Ag1−O2−Ag2 97.6(3), Ag1−Si1− N6 125.2(2), Ag2−Si4−N3 122.3(2). C


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Organometallics

−Au(I) complexes (4 and 5). All compounds were characterized by single-crystal X-ray studies and state of the art spectroscopic tools.

authenticated gold(I) silyl complexes (2.365 Å).6,17 The nature of the Si→Au bond in 5 is different from that of Jutzi’s silylene gold complex (Cp*2SiAu(Cl)), which possesses a formal SiAu double bond.5 To examine the bonding properties of complexes 2, 3, and 5, density functional theory (DFT) calculations were performed by using the Gaussian09 suite.18 The B3LYP exchangecorrelation functional was used throughout for all the calculations.19 For geometry optimization, a combination of basis sets, namely, 6-31g for C, H, N, P, O, Si, S, F, and Cl and LANL2DZ for Au, Cu, Ag, and I atoms,20 were used. In Figure 4, the HOMO and LUMO of the optimized structures for 2, 3,

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EXPERIMENTAL SECTION

All manipulations were carried out under an inert gas atmosphere of dinitrogen using standard Schlenk techniques and in a dinitrogen-filled glovebox. The solvents used were purified by a MBraun MB SPS-800 solvent purification system. Compound 1 was prepared by the literature method.14 All chemicals purchased from Aldrich were used without further purification. 1H, 13C, 31P, and 29Si NMR spectra were recorded in CD2Cl2 and CDCl3 using a Bruker 400 MHz spectrometer. Mass spectra were recorded using an AB Sciex 4800 plus MALDI TOF/TOF instrument. Synthesis of [{PhC(NtBu)2}Si{N(SiMe3)2}]2Cu2I2 (2). Toluene (30 mL) was added to a mixture of 1 (0.210 g, 0.5 mmol) and CuI (0.095 g, 0.5 mmol) in a Schlenk flask (50 mL), and the solution of the reaction mixture became green within 5 min. It was stirred overnight at room temperature, and the solution became gradually colorless. The solution was filtered, concentrated, and kept at 0 °C to afford colorless crystals of 2. Yield: 65% (0.200 g). Mp: 100−104 °C. 1 H NMR (400 MHz, CDCl3, 298 K): δ 0.37 (s, 18H, SiMe3); 0.79 (s, 18H, SiMe3); 1.33 (s, 36H, CMe3); 6.85−7.56 (m, 10H, Ph) ppm. 13 C{1H} NMR (100.613 MHz, CDCl3, 298 K): δ 4.74 (SiMe3), 5.88 (SiMe3), 31.41 (CMe3), 54.28 (CMe3), 125.30, 127.23, 128.17, 128.93, 129.63, 131.95 (Ph-C), 166.30 (NCN) ppm. 29Si{1H} NMR (79.495 MHz, CDCl3, 298 K): δ 16.85 (SiN(SiMe3)2), 3.51 (SiMe3), 3.29 (SiMe3) ppm. HRMS: m/z (C42H82Cu2I2N6Si6): 1221.7866 [M+]. Anal. Calcd: C, 41.33; H, 6.77; N, 6.89. Found: C, 41.65; H, 6.81; N, 6.98. Synthesis of [{PhC(NtBu)2}Si{N(SiMe3)2}]2Ag2[OTf]2 (3). Toluene (20 mL) was added to a mixture of 1 (0.210 g, 0.5 mmol) and silver triflate (0.128 g, 0.5 mmol) in a Schlenk flask (50 mL). It was stirred overnight at room temperature, and the solution became gradually colorless. The solution was filtered, concentrated, and kept at −30 °C overnight to afford colorless crystals of 3. Yield: 63% (0.210 g). Mp: 118−120 °C. 1H NMR (400 MHz, CDCl3, 298 K): δ 0.32 (s, 18H, SiMe3); 0.46 (s, 18H, SiMe3); 1.25 (s, 36H, CMe3), 7.25−7.58 (m, 10H, Ph) ppm. 13C{1H} NMR (100.613 MHz, CDCl3, 298 K): δ 4.75 (SiMe3), 5.91 (SiMe3), 31.92 (CMe3), 55.16 (CMe3), 127.07, 128.07, 128.39, 128.58, 130.31, 131.15 (Ph-C), 168.74 (NCN) ppm. 29 Si{1H} NMR (79.495 MHz, CDCl3, 298 K): δ 8.57 (SiN(SiMe3)2), 6.25 (SiMe3), 5.74 (SiMe3) ppm. HRMS m/z (C44H82Ag2F6N6O6S2Si6): 1350.3901 [M+]. Anal. Calcd: C, 39.04; H, 6.11; N, 6.21. Found: C, 39.56; H, 6.07; N, 5.91. Synthesis of [{PhC(NtBu)2}Si{N(SiMe3)2}]AgNO3 (4). Toluene (20 mL) was added to a mixture of 1 (0.210 g, 0.5 mmol) and AgNO3 (0.085 g, 0.5 mmol) in a Schlenk flask (50 mL), and the solution became pale yellow within 5 min. It was stirred for another 2 days at room temperature, and then the solution was filtered, concentrated to 5 mL, and stored at 0 °C overnight to afford colorless crystals of 4. Yield: 60% (0.177g). Mp: 123 °C. 1H NMR (400 MHz, CDCl3, 298 K): δ 0.36 (s, 9H, SiMe3), 0.51 (s, 9H, SiMe3), δ 1.28 (s, 18H, CMe3), 7.18−7.58 (br, 5H, Ph) ppm. 13C{1H} NMR (100.613 MHz, CDCl3, 298 K): δ 4.73 (SiMe3), 5.84 (SiMe3), 31.87 (CMe3), 54.98 (CMe3), 125.29, 127.05, 128.21, 129.02, 130.93, 137.85 (Ph-C), 167.93 (NCN) ppm. 29Si{1H} NMR (79.495 MHz, CDCl3, 298 K): δ 7.87 (SiN(SiMe3)2), 0.99 (SiMe3), 0.59 (SiMe3) ppm. MALDI MS: m/z 528.21 [M+ − (NO3)] (100%). HRMS m/z (C21H41AgN4O3Si3): 589.1571 [M+]. Anal. Calcd: C, 42.77; H, 7.01; N, 9.50. Found: C, 41.94; H, 6.87; N, 9.12. Synthesis of [{PhC(NtBu)2}Si{N(SiMe3)2}]AuCl (5). Toluene (20 mL) was added to a mixture of 1 (0.210 g, 0.5 mmol) and AuCl(SMe2) (0.147 g, 0.5 mmol) in a 50 mL Schlenk flask with stirring at room temperature. The reaction mixture was stirred overnight at room temperature. The solution was filtered, concentrated to 5 mL, and kept at 0 °C to afford colorless crystals of 5. Yield: 60% (0.197 g). Mp: 128−130 °C dec. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 0.39 (s, 9H, SiMe3); 0.63 (s, 9H, SiMe3); 1.32 (s, 18H,

Figure 4. DFT-derived surface diagrams of HOMOs and LUMOs: (A) 2; (B) 3; (C) 5.

and 5 are shown. The DFT optimized geometries of 2, 3, and 5 show a close resemblance with their experimentally obtained geometries. However, during the course of optimization, the Si→M (M = Cu, Ag, Au) bond lengths have been slightly increased (vide supra, Table 1). The HOMOs of 2 (−4.7611 eV) and 3 (−5.9206 eV) are largely composed of the Si→Cu and Si→Ag dative bonds, respectively (Figure 4). The LUMO of 2 (−0.7494 eV) is located on the phenyl ring of the amidinato ligand, while the LUMO of 3 (−1.7803 eV) is constricted to the triflate moieties, most likely due to their highly electron withdrawing nature. The HOMO (−5.9398 eV) in 5 is mainly located on the Au−Cl bond, and the LUMO (−1.1821 eV) is largely associated with the silylene moiety, which indicates the possibility of d−p back-bonding. The HOMO of 2 is higher in energy than those of 3 and 5, which presumably can be attributed to the more downfield shift in the 29 Si NMR spectrum of 2 in comparison to those of 3 and 5. In summary, we synthesized and characterized a range of Si(II)→coinage-metal complexes. It has been shown that the functionalized silylene 1 forms adducts with all three coinage metals, although the structures of the complexes depend on the coinage-metal precursors used as well as the strength of the Si(II)→coinage-metal bond. For example, the reactions of 1 with CuI and AgOTf led to silylene-supported Cu2I2 and Ag2O2 diamond cores (2 and 3), while the analogous reactions with AgNO3 and AuCl(SMe2) resulted in 1:1 silylene−Ag(I) and D


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Organometallics CMe3); 7.45−7.57 (m, 5H, Ph) ppm. 13C{1H} NMR (100.613 MHz, CD2Cl2, 298 K): δ 4.51 (SiMe3), 5.97 (SiMe3), 31.44 (CMe3), 53.43 (CMe3), 127.14, 127.86, 128.31, 129.46, 130.98, 131.79 (Ph-C), 171.67 (NCN) ppm. 29Si{1H} NMR (79.495 MHz, CD2Cl2, 298 K): δ 8.76 (SiN(SiMe3)2), 7.09 (SiMe3), 6.68 (SiMe3) ppm. MALDI MS: m/z 650.12 (M+) (25%). HRMS m/z (C21H41AuClN3Si3): 616.2271 [M+ − Cl] (100%). Anal. Calcd: C, 38.67; H, 6.34; N, 6.44. Found: C, 39.18; H, 6.38; N, 5.78. Crystal Structure Determination. Crystal reflections were collected on a Bruker Smart Apex Duo diffractometer using Mo Kα radiation (λ = 0.710 73 Å) for 2 (200 K), 3 (100 K), 4 (150 K), and 5 (150 K). The structures of 2−5 were solved by direct methods and refined by full-matrix least-squares methods against F2 (SHELXL2014/6).15,21 Crystal data for 2: C49H90Cu2I2N6Si6, FW 1312.68, monoclinic; space group C2/c, a = 34.742(5) Å, b = 11.2775(15) Å, c = 31.977(3) Å; α = 90°, β = 90.370(6), γ = 90°; V = 12528(3) Å3, Z = 8, R1 = 0.0911, wR2 = 0.1826. Crystal data for 3: C44H82Ag2F6N6O6S2Si6, FW 1353.55, monoclinic; space group Cc, a = 9.455(3) Å, b = 39.412(11) Å, c = 17.344(5) Å; α = 90°, β = 104.283(7)°, γ = 90°; V = 6264(3)Å3, Z = 4, R1 = 0.0407, wR2 = 0.1013. Crystal data for 4: C25H46AgN4O3Si3, FW 642.80, monoclinic; space group P21/c, a = 20.681(3) Å, b = 9.0586(13) Å, c = 18.489(3) Å; α = 90°, β = 110.715(3)°, γ = 90°; V = 3239.8(8) Å3, Z = 4, R1 = 0.1193, wR2 = 0.2666. Crystal data for 5: C21H41AuClN3Si3, FW 652.25, monoclinic; space group P21/c, a = 8.904(3) Å, b = 18.360(7) Å, c = 17.794(7) Å; α = 90°, β = 101.464(9)°, γ = 90°; V = 2851.1(18) Å3, Z = 4, R1 = 0.0210, wR2 = 0.0452.

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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.organomet.5b00745. Crystal structure of 4 and crystallographic data of 2−5 (PDF) Crystallographic data for 2 (CIF) Crystallographic data for 3 (CIF) Crystallographic data for 4 (CIF) Crystallographic data for 5 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for S.K.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K. thanks the SERB (India) and IISER Pune for financial support. S.K. also acknowledges Rajarshi Dasgupta for his valuable helps. S.P., N.P., and N.K. thank the UGC for providing fellowships.

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REFERENCES

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