SYNTHESIS AND STRUCTURE OF THE FIRST BIS-HYDROTRIS(3-(T)BU-5-ME PYRAZOLYL)BORATE COMPLEXES, LN(TP(T)BU,ME)(2) (LN=SM, YB) - FLUXIONALITY, BONDING MODE EXCHANGE AND B-H-LN BRIDGE BONDING

The syntheses of bis-hydrotris(3-(t) Bu-5-Mepyrazolyl)borate lanthanide(II) complexes, Ln(Tp(tBu,Me))(2) (Ln=Sm, 1, Yb, 2), are reported, The X-ray crystal structure of the Sm complex (1) has been determined and shows two different Tp(tBU,Me) ligands; one is bonded via classical eta(3)-bonding mode, the other is coordinated via two pyrazolyl nitrogens and an agostic B-H-Sm interaction, The spectroscopic data are consistent with a solution structure which is identical with the solid stale form, In particular, the observation of Yb-171-HB coupling of 85 Hz confirms the persistence of the agostic interaction in solution, Variable temperature NMR studies show that the molecules are fluxional in solution and execute two distinct and specific rearrangement processes, The low temperature process involves exchange of the pyrazolyl rings within individual Tp(tBU,Me) ligands wifhout exchanging the bonding modes of the ligands, it is also suggested that the process preserves the B-H- Ln interaction. Above room temperature the two different Tp(tBU,Me) ligands also begin to slowly exchange their bonding modes, Estimates of the activation barriers for the exchange processes are 8.5 and 16.5 kcal/mol, respectively.