Synthesis and characterization of organometallic hydro-tris-pyrazolylborate derivatives of scandium via alkane elimination

Reaction between the potassium salts of the hydro-tris-pyrazolylborate ligands Tp^Me,Me and Tp^tBu,Me and the tris THF adduct of scandium trichloride lead to isolation of the scandium complexes (Tp^Me,Me)ScCl₂(THF), 1a, and (Tp^tBu,Me)ScCl₂, 1b, in 65 and 49% yield, respectively. Subsequent reactions between 1a and 1b and alkyl lithium reagents RLi (R = Me, CH₂SiMe₃, and CH(SiMe₃)₂) gave the lithium salts of the Tp ligands as the major isolated products in all cases but one. The complex (Tp^Me,Me)Sc(CH₂SiMe₃)₂(THF), 2a, could be obtained contaminated with at least 10% Tp^Me,MeLi via this methodology. A salt-free alkane elimination route to bis-trimethylsilylmethyl complexes 2a and the THF-free (Tp^tBu,Me)Sc(CH₂SiMe₃)₂, 2b, involving reaction between in situ generated Sc(CH₂SiMe₃)₃(THF)₂ and the protonated ligands, Tp^R,MeH, gave samples of the desired bis-alkyl derivatives in 67 and 87% yield, respectively. In solution, complexes 2a and 2b are both fluxional at room temperature. These dynamic processes were frozen out on the NMR time scale at lower temperatures and interpreted on the basis of the ¹H NMR spectra obtained as well as information obtained from the solid state structures of both compounds. 2a: monoclinic, space group P2₁/c, a = 11.765(4) Å, b = 14.8311(20) Å, c = 20.891(5) Å, β = 105.69(3)°, V = 3509.4(15) ų, Z = 4, R = 0.088, R_w = 0.082. 2b: triclinic, space group P1, a = 11.9427(2) Å, b = 20.3641(3) Å, c = 26.5602(2) Å, α = 112.192(1)°, β = 100.569(1)°, γ = 92.928(1)°, V = 5830.38(14) ų Z = 6, R = 0.0500, R_w = 0.1137.