SYNTHESIS, SPECTROSCOPIC CHARACTERIZATION AND DYNAMIC BEHAVIOR OF NIOBIUM COMPLEXES WITH POLY(PYRAZOL-1-YL)METHANE LIGANDS

The complex [{NbCl3(dme)}(n)] 1 (dme = 1,2-dimethoxyethane) reacted wi th an excess of bis(pyrazol-1-yl)methane (bzpm), bis(3.5-dimethylpyraz ol-1-yl)methane (bdmpzm) or bis(5-trimethylsilylpyrazol-1-yl)methane ( btpzm) to give the binuclear complexes [{NbCl3(bpzm)}(2)] 2, [{NbCl3(b dmpzm)}(3)] 3 and [{NbCl3(btpzm)}(2)] 4, respectively. Mononuclear ace tylene complexes [NbCl3(L-L)(RC drop CR')] (R = R' = Ph, L-L = bpzm 5, bdmpzm 6 or btpzm 7; R = R' = CO(2)Me, L-L = btpzm 8; R = R' = Me, L- L = bpzm 9, bdmpzm 10 or btpzm 11) have been prepared from either the reaction of 2-4 with acetylenes or the interaction of acetylene comple xes [NbCl3(dme)(RC drop CR')] with the appropriate L-L. This last reac tion with tris(pyrazol-1-yl)methane (tpzm) or tris(3,5-dimethylpyrazol -1-yl)methane (tdmpzm) in the presence of 1 equivalent of AgBF4 afford ed the cationic complexes [NbCl2(L-L-L)(RC drop CR')]BF4 (R = R' = Ph, L-L-L = tpzm 12 or tdmpzm 13; R = R' = Me, L-L-L = tpzm 14 or tdmpzm 15). The structures of these complexes have been determined by spectro scopic methods. Variable-temperature NMR studies of some of the comple xes were carried out in order to establish their fluxional behaviour i n solution and free activation energy values were calculated at the co alescence temperature.