Syntheses, structures and fluxional behaviour of η⁴-1,4-di-tert-butylbuta-1,3-diene complexes of iron and ruthenium

Abstract

The complexes [M(CO)3(η4-diene)](M = Fe or Ru; diene = 1,4-di-tert-butylbuta-1,3-diene) have been synthesised and their crystal structures and solution fluxional behaviour determined. While they adopt square-pyramidal geometries in the solid state, the carbonyl ligands show fluxional behaviour in solution (Ea= 44.6, ΔH‡= 42.2 kJ mol–1, ΔS‡=–29.8 J K–1 mol–1 where M = Fe; 52.6, 50.0 kJ mol–1 and –30.5 J K–1 mol–1 where M = Ru). These activation parameters are the largest reported for simple [M(CO)3(diene)] complexes. An inverse correlation between Ea and the diene ‘bite’ angle θ1 for a range of [M(CO)3(diene)] complexes is described and used to account for variations in activation barriers.