Mössbauer studies on ferrocene complexes: XVIII. A comparative study of azaferrocenes and phosphaferrocenes and their derivatives by 57Fe Mössbauer and 13C NMR spectroscopy

Abstract

The 13C NMR chemical shifts (δ) for azaferrocenes, phosphaferrocenes and their precursor ligands have been used to determine coordination shifts, Δδ ( = δ complex −δ ligand) for the complexes. For azaferrocenes, Δδ values were similar to that of ferrocene, but the phosphaferrocenes showed much larger coordination shifts. These observations are discussed in terms of the difference in bonding in these complexes. The 57Fe Mössbauer data are reported for a number of azaferrocene derivatives and compared with those for the phosphaferrocene analogues in terms of orbital contributions to the electric field gradient that governs the quadrupole splitting (QS). An excellent correlation is found between QS and the orbital population parameter (2P 2 - p 1), where p 2 and p 2 are electronic populations of the e 2 and e 1 iron-based orbitals, respectively. Protonation and quaternisation of azaferrocene causes a lowering of QS, in contrast to the slight increase observed for monophosphaferrocene in CF 3CO 2H solution. The results confirm that protonation at the N atom takes place for azaferrocene. The W(CO) 5 adduct of azaferrocene shows little change in QS from that for azaferrocene, which is interpreted as evidence for back bonding by the tungsten.