Fluxional behavior of tert-butyl-substituted 2,2′: 6′2″-terpyridylmolybdenum(0) and 2,2′-bipyridylmolybdenum(II) complexes: enhanced solubility and ease in NMR study caused by the tert-butyl group

Abstract

Reaction of [Mo(CO) 4(norbornadiene)] with 4,4′,4″-tri-tert-butyl-2,2′: 6′,2″-terpyridine afforded the complex [Mo(CO) 4(4,4′,4″- tBu 3terpy)] ( 2), which in solution shows fluxional behavior, observed by 1H and 13C NMR studies at between 25 and −70°C. [Mo(CO) 4(4,4′- tBu 2bipy)] ( 1) reacts with I 2 and SnBr 4 by oxidative addition to give molybdenum(II) complexes, [Mo(CO) 3(4,4′- tBu 2bipy)I 2] ( 3) and [Mo(CO) 3(4,4′- tBu 2bipy)(SnBr 3)Br] ( 4). The related compounds [Mo(CO) 3(4,4′- tBu 2bipy)X 2] (X = Br( 5) or Cl( 6) have been prepared by oxidation of 1 with two equivalents of CuX 2. The IR spectra of 3 in different polar solvents suggest that this compound exists in solution in two isomeric forms, which are in equilibrium. The NMR spectra of 3, 5 and 6 are temperature dependent, indicating their dynamic behavior in solution. ΔG ‡ values for the fluxional process of about 47 kJ mol −1 and about 53 kJ mol −1 have been obtained for 5 and 6 respectively. Low temperature 13C NMR studies of 4 have shown that this compound in solution has a different kind of fluxional behavior.