Electronic Structure of 2,2′-Bipyridine Organotransition-Metal Complexes. Establishing the Ligand Oxidation Level by Density Functional Theoretical Calculations

Abstract

A density functional theoretical (DFT) study (B3LYP) has been carried out on 20 organometallic complexes containing η(5)- and/or η(3)-coordinated cyclopentadienyl anions (Cp(-)) and 2,2'-bipyridine (bpy) ligand(s) at varying oxidation levels, i.e., as the neutral ligand (bpy(0)), as the π-radical monoanion (bpy(•-))(-), or as the diamagnetic dianion (bpy(2-))(2-). The molecular and electronic structures of these species in their ground states and, in some cases, their first excited states have been calculated using broken-symmetry methodology. The results are compared with experimental structural and spectroscopic data (where available) in order to validate the DFT computational approach. The following electron-transfer series and complexes have been studied: [(Cp)(2)V(bpy)](0,+,2+) (1-3), [(Cp)(2)Ti(bpy)](-,0,+,2+) (4-7), [(Cp)(2)Ti(biquinoline)](0,+) (8 and 9), [(Cp*)(2)Ti(bpy)](0) (10) (Cp* = pentamethylcyclopentadienyl anion), [Cp*Co(bpy)](0,+) (11 and 12), [Cp*Co(bpy)Cl](+,0) (13 and 14), [Fe(toluene)(bpy)](0) (15), [Cp*Ru(bpy)](-) (16), [(Cp)(2)Zr(bpy)](0) (17), and [Mn(CO)(3)(bpy)](-) (18). In order to test the predictive power of our computations, we have also calculated the molecular and electronic structures of two complexes, A and B, namely, the diamagnetic dimer [Cp*Sc(bpy)(μ-Cl)](2) (A) and the paramagnetic (at 25 °C) mononuclear species [(η(5)-C(5)H(4)(CH(2))(2)N(CH(3))(2))Sc((m)bpy)(2)] (B). The crystallographically observed intramolecular π-π interaction of two N,N'-coordinated π-radical anions in A leading to an S = 0 ground state is reliably reproduced. Similarly, the small singlet-triplet gap of ~600 cm(-1) between two antiferromagnetically coupled (bpy(•-))(-) ligands in B, two ferromagnetically coupled radical anions in the triplet excited state of B, and the structures of A and B is reproduced. Therefore, we are confident that we can present computationally obtained, detailed electronic structures for complexes 1-18. We show that N,N'-coordinated neutral bpy(0) ligands behave as very weak π acceptors (if at all), whereas the (bpy(2-))(2-) dianions are strong π-donor ligands.