Imidotitanium Tris(pyrazolyl)hydroborates: Synthesis, Solution Dynamics, and Solid-State Structure

Reaction of [Ti(NBu(t))Cl(2)(py-Bu(t))(2)] (1; py-Bu(t) = 4-tert-butyl pyridine) with 1 equivalent of K[Tp(Me2)], K[Tp(Pri)] or K[Tp(Pri,Br)] affords the corresponding complexes [Tp(Me2)Ti(NBu(t))Cl(py-Bu(t))] (2), [Tp(Pri)Ti(NBu(t))Cl(py-Bu(t))] (3), and [Tp(Pri,Br)Ti(NBu(t))Cl(py-Bu(t))] (4), respectively, which are the first examples of imido Group 4 tris(pyrazolyl)hydroborates [Tp(Me2) = tris(3,5-dimethylpyrazolyl)hydroborate; Tp(Pri) = tris(3-isopropylpyrazolyl)hydroborate; Tp(Pri,Br) = tris(3-isopropyl-4-bromopyrazolyl)hydroborate]. Complexes 2-4 are fluxional on the (1)H and (13)C NMR time scales, the spectra indicating restricted rotation about the Ti-py-Bu(t) bond. Activation parameters for this dynamic process have been determined both by (13)C NMR lineshape analysis and by coalescence measurements. The solution-state structure for 2 has been unambiguously assigned from a low temperature, phase-sensitive (1)H NOESY DQF spectrum and the solid-state X-ray crystal structure of the dichloromethane solvate of 3 has been determined (space group P2(1)/n; a = 12.539(3), b = 14.686(3), c = 21.747(4) Å; beta = 91.28(3) degrees; R(1) = 0.0694 and wR(2) = 0.154 for 1578 observed reflections). (13)C NMR Deltadelta values (Deltadelta = delta(C(alpha)) - delta(C(beta))) for the tert-butyl imido ligand in 2-4 suggest that the donor ability of the tris(pyrazolyl)hydroborate ligands increases in the order Tp(Pri,Br) < Tp(Pri) < Tp(Me2). None of these ligands, however, is as effective a donor to the metal center as either eta-C(5)H(5) or eta-C(5)Me(5).