Cleavage of Formamidinate Ligands on a Ta=Ta Double Bond: Formation of H_xCNAryl (x = 0 and 1) and Arylimido-Bridged Complexes

Three complexes which resulted from the C-N bond cleavage reactions of formamidinate anions, [ArNC(H)NAr]^-, have been synthesized and structurally characterized. The reaction of N.N'-diphenylformamidinate, DPhF^-, with (Me₂S)Cl₂Ta(μ-Me₂S)(μ-Cl) ₂TaCl₂(SMe₂) (abbreviated as Ta₂Cl₆(SMe₂)₃) in the presence of Zn metal produces the CNPh-containing dinuclear compound (η²-DPhF)₂Ta(μ-NPh)(μ-η ²-CNPh)Ta(η²DPhF)₂·C ₇H₈, 1·C₇H₈. By using DTolF^- and Na/Hg as reducing agent, the isolated product is (η²-DTolF)Ta(μ-CNTol)(μ-η ²CNTol)(μDTolF)₂·Ta(η ²-DTolF·1.5C₆H₁₄, where DTolF^- = N.N'-(p-tolyl)formamidinate. Both 1 and 2 are isolated in low yields. There is a formal single bond between the tantalum atoms which are bonded to intact formamidinate ions as well as the two fragments which result from the cleavage of a formamidinate ligand, namely arylimido (NAr^2-) and (CNAr). In the absence of a reducing agent, the reaction of LiDTolF and Ta₂Cl₆(SMe₂)₃ produces Cl₂(η²-DTolF)Ta(μ-NTol)(μ-η², η²-HCNTol)Ta(η²-DTolF)₂, 3, in high yield. In addition to the tolylimido group, a tolylformimidoyl fragment (HCNTol) binds to the tantalum atom through both carbon and nitrogen atoms in a η²,η² mode. Because of this and the relatively long C-N bond distance (1.42(1) Å), the ligand can be formulated as [HCNTol]^3-. Thus the tantalum atoms in 3 have a formal oxidation state of +5, which precludes the presence of a metal-metal bond. Crystal data for 1·C₇H₈: monoclinic, 12/a. a = 19.437(2) Å, b = 15.587(1) Å, c = 20.401(1) Å, and β= 97.840(6)°. Crystal data for 2·1.5C₆H₁₄: triclinic, P1̄, a = 13.990(2) Å, b = 15.841(1) Å, c = 19.442(4) Å, α = 98.98(1)°, β= 104.07(1)°; γ = 109.59(1)°. Crystal data for 3: monoclinic, P2₁/n, a = 17.962(3) Å, b = 15.539(4) Å, c = 19.973(3) Å, β = 90.126(7)°.