Synthesis, spectroscopic characterization and dynamic behaviour of niobium complexes with poly(pyrazol-1-yl)methane ligands

The complex [{NbCl3(dme)}n]1(dme = 1,2-dimethoxyethane) reacted with an excess of bis-(pyrazol-1-yl) methane (bzpm), bis(3,5-dimethylpyrazol-1-yl)methane (bdmpzm) or bis(5-trimethylsilyl-pyrazol-1-yl) methane (btpzm) to give the binuclear complexes [{NbCl3(bpzm)}]2, 2, [{NbCl3-(bdmpzm)}2]3 and [{NbCl3(btpzm)}2]4, respectively. Mononuclear acetylene complexes [NbCl3(L–L)(RCCR′)](R = R′= Ph, L–L = bpzm 5, bdmpzm 6 or btpzm 7; R = R′= CO2Me, L–L = btpzm 8; R = R′= Me, L–L = bpzm 9, bdmpzm 10 or btpzm 11) have been preapared from either the reaction of 2–4 with acetylenes or the interaction of acetylene complexes [NbCl3(dme)(RCCR′)] with the appropriate L–L. This last reaction with tris(pyrazol-1-yl)methane (tpzm) or tris(3,5-dimethylpyrazol-1-yl) methane (tdmpzm) in the presence of 1 equivalent of AgBF4 afforded the cationic complexes [NbCl2(L–L–L)(RCCR′)]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 spectroscopic methods. Variable-temperature NMR studies of some of the complexes were carried out in order to establish their fluxional behaviour in solution and free activation energy values were calculated at the coalescence temperature.

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