Theoretical study of the effect of ligand topology on Fe(IV)O and Ru(IV)O complex reactivities

Abstract

Density functional theory calculations were carried out to clarify the effect of ligand topology on the stability and reactivity of cis-α-[FeIV(O)(BQCN)]2+ (Fe-2a), cis-β-[FeIV(O)(BQCN)]2+ (Fe-2b), cis-α-[RuIV(O)(BQCN)]2+ (Ru-2a) and cis-β-[RuIV(O)(BQCN)]2+ (Ru-2b) (BQCN=N,N′-dimethyl-N,N′-bis(8-quinolyl)-cyclohexanediamine). All the iron and ruthenium isomers possess the triplet ground states. The relative stability between the two iron isomers follows the order of Fe-2a>Fe-2b, which is in agreement with the conclusions of Hong et al. Moreover, the trend of the relative stability of the two ruthenium isomers is Ru-2a>Ru-2b. The Density-of-States spectrums represent that the contribution of BQCN ligand to 3Fe-2b is more than the corresponding contribution to 3Fe-2a. The iron isomers react with isopropylbenzene via a two-state reactivity pattern on competing triplet and quintet spin states, while the ruthenium isomers react with isopropylbenzene by a single-state mechanism, only on the triplet spin state. The H-abstraction is affected by the tunneling contribution, which can decrease the reaction barriers. By adding the ZPE correction, PCM model assessed by DFT-D3 (BJ) and the tunneling correction, the analysis of the trend of the hydrogen-abstraction reactions barriers shows that Fe-2a is the more reactive than Fe-2b with the higher FeIV/III redox potential. Moreover, for the ruthenium complexes, although Ru-2a is the more reactive than Ru-2b, only 0.6kcal/mol lower. Above all, the ligand topology has little effect on the reactivities of the [RuIV(O)(BQCN)]2+ complexes.