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 (E_a= 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 E_a 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.