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

Reaction of [Mo(CO)(4)(norbomadiene)] with 4,4',4 ''-tri-tert-butyl-2,2':6',2 ''-terpyridine afforded the complex [Mo(CO)(4)(4,4',4 ''-(t)Bu(3)terpy)] (2), which in solution shows fluxional behavior, observed by H-1 and C-13 NMR studies at between 25 and -70 degrees C. [Mo(CO)(4)(4,4'-(t)Bu(2)bipy)] (1) reacts with I-2 and SnBr4 by oxidative addition to give molybdenum(II) complexes, [Mo(CO)(3)(4,4'-(t)Bu(2)bipy)I-2] (3) and [Mo(CO)(3)(4,4'-(t)Bu(2)bipy)(SnBr3)Br] (4). The related compounds [Mo(CO)(4,4'-(t)Bu(2)bipy)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. Delta G double dagger 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 C-13 NMR studies of 4 have shown that this compound in solution has a different kind of fluxional behavior.