Unexpected Coordination Complexes of the Metal Tris-silylamides M{N(SiMe₃)₂}₃ (M = Ti, V)

Abstract

The synthesis, molecular structures, and spectroscopic details of a series of isocyanide and nitrile complexes of the early first-row transition-metal tris­(silyl)­amides M­{N­(SiMe3)2}3 (M = Ti, V) are reported. Previously, first-row transition-metal tris­(silyl)­amides were generally thought to be incapable of forming complexes with Lewis bases due to their excessive steric crowding. However, it is now shown that simple treatment of the base-free trisamides with 2 equiv of an isocyanide or nitrile base at room temperature results in the formation of the trigonal bipyramidal complexes Ti­{N­(SiMe3)2}3(1-AdNC)2 (1), Ti­{N­(SiMe3)2}3(CyNC)2 (2), Ti­{N­(SiMe3)2}3(Bu t NC)2 (3), Ti­{N­(SiMe3)2}3(PhCN)2 (4), V­{N­(SiMe3)2}3(1-AdNC)2 (5), V­{N­(SiMe3)2}3(CyNC)2 (6), V­{N­(SiMe3)2}3(Bu t NC)2 (7), and V­{N­(SiMe3)2}3(PhCN)2 (8), which incorporate two donor ligands (1-AdNC = 1-adamantyl isocyanide, CyNC = cyclohexyl isocyanide, Bu t NC = tert-butyl isocyanide, PhCN = benzonitrile). All complexes display a characteristic increase in the frequency of the multiple bonded C–N stretching mode which is observed to be in the range of 2170–2190 cm–1 for the isocyanide complexes 1–3 and 5–7 and at 2250 cm–1 for the nitrile complex 8. This effect was not observed for the titanium nitrile complex 4, suggesting weak binding of the donor to titanium. Paramagnetic 1H NMR studies showed these complexes to have detectable, though extremely broadened, signals attributable to the trimethylsilyl groups of the amide ligands (δ = ca. 2.8 ppm for titanium isocyanide complexes, ca. 4.5–4.7 ppm for vanadium isocyanide complexes). A variable-temperature 1H NMR study showed that in solution these complexes exist as mixtures of the five-coordinate species and a putative four-coordinate species coordinating a single Lewis basic ligand. Electronic spectroscopy indicated that the vanadium complexes 5–8 bind the Lewis bases more strongly than the corresponding titanium complexes, where the spectra of complexes 1–4 are essentially identical to the base-free Ti­{N­(SiMe3)2}3 at the temperatures and concentrations studied. In contrast to these results, no corresponding complexes were detected for the metal silylamides M­{N­(SiMe3)2}3 (M = Cr, Mn, Fe, or Co) when treated with the isocyanide or nitrile bases.

Hydrides, Halides, and Polymers: Some Unexpected Intermediates on the Routes to First-Row Transition Metal M{N(SiMe3)2}n (n = 2, 3) Complexes

“…[Ti­(μ-Cl)­{N­(SiMe 3 ) 2 } 2 ] 2 ( 1 ), [V­(μ-Cl)­{N­(SiMe 3 ) 2 } 2 ] 2 ( 2 ), and [Co­(Br)­{μ-N­(SiMe 3 ) 2 }] 2 ( 5 ) have simple 1 H NMR spectra, with single broad signals attributed to the trimethylsilyl protons at 3.38 ( 1 ), 7.13 ( 2 ), and −5.12 ppm ( 5 ) ppm. A second, less intense, broadened signal also appeared at 19.81 ppm in the 1 H NMR spectrum of 2 , which is indicative of the presence of a slight impurity of the trisamide V­{N­(SiMe 3 ) 2 } 3 . , The effective magnetic moments of dimeric 1 , 2 , and 5 were determined by Evans’ method to be 3.0, 5.0, and 3.8 μ B , respectively. For 1 and 2 , these values are only slightly lower than the predicted spin only values, suggesting little antiferromagnetic coupling or orbital contribution to their magnetic moments.…”

“…A second, less intense, broadened signal also appeared at 19.81 ppm in the 1 H NMR spectrum of 2, which is indicative of the presence of a slight impurity of the trisamide V{N(SiMe 3 ) 2 } 3 . 11,31 The effective magnetic moments of dimeric 1, 2, and 5 were determined by Evans' method to be 3.0, 5.0, and 3.8 μ B , respectively. For 1 and 2, these values are only slightly lower than the predicted spin only values, suggesting little antiferromagnetic coupling or orbital contribution to their magnetic moments.…”

“…The structural and spectroscopic characterization and exploration of the redox chemistry of V­{N­(SiMe 3 ) 2 } 3 (which had been previously mischaracterized) were also described . A coordination chemistry was initiated for the titanium and vanadium tris­(silylamide) complexes (previously thought not possible due to steric crowding) to give M­{N­(SiMe 3 ) 2 } 3 L 2 (L = a nitrile or isocyanide ligand) complexes . Additionally, the isolation of a new vanadium­(V) oxy silylamide complex, e.g., V­(=O)­{N­(SiMe 3 ) 2 } 3 , that was absent from the literature despite several attempts to isolate it, and the synthesis of several unusual metal hydride complexes that are available by reduction of M­{N­(SiMe 3 ) 2 } 3 complexes were described …”

“…11 A coordination chemistry was initiated for the titanium and vanadium tris(silylamide) complexes (previously thought not possible due to steric crowding) to give M{N(SiMe 3 ) 2 } 3 L 2 (L = a nitrile or isocyanide ligand) complexes. 31 Additionally, the isolation of a new vanadium(V) oxy silylamide complex, e.g., V(=O){N-(SiMe 3 ) 2 } 3 , that was absent from the literature despite several attempts to isolate it, 32 and the synthesis of several unusual metal hydride complexes that are available by reduction of M{N(SiMe 3 ) 2 } 3 complexes were described. 33 During this work, two observations for these silylamide complexes became a focus of interest.…”

Reductions of M{N(SiMe3)2}3 (M = V, Cr, Fe): Terminal and Bridging Low-Valent First-Row Transition Metal Hydrido Complexes and “Metallo-Transamination”

“…21,22 Our recent finding 10 that the complex V{N(SiMe 3 ) 2 } 3 had been initially mischaracterized in the 1970s 3,4,7 led us to reexamine first-row transition metal M{N(SiMe 3 ) 2 } 3 complexes. Advances in techniques for the handling and characterization of such complexes has allowed the development of the coordination chemistry for the titanium and vanadium tris(silylamide) complexes, 23 as well as the isolation of several high-valent vanadium complexes that were unknown despite earlier efforts to synthesize them. 24−26 As described in these reports, attempts to extend this chemistry to other first-row transition metal M{N(SiMe 3 ) 2 } 3 complexes (M = Cr, Mn, Fe, Co) were largely unsuccessful.…”