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

A density functional theoretical (DFT) study (B3LYP) has been carried out on 20 organometallic complexes containing η5- and/or η3-coordinated cyclopentadienyl anions (Cp–) and 2,2′-bipyridine (bpy) ligand(s) at varying oxidation levels, i.e., as the neutral ligand (bpy0), as the π-radical monoanion (bpy•–)−, or as the diamagnetic dianion (bpy2–)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)2V(bpy)]0,+,2+ (1–3), [(Cp)2Ti(bpy)]−,0,+,2+ (4–7), [(Cp)2Ti(biquinoline)]0,+ (8 and 9), [(Cp*)2Ti(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)2Zr(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 [(η5-C5H4(CH2)2N(CH3)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 bpy0 ligands behave as very weak π acceptors (if at all), whereas the (bpy2–)2– dianions are strong π-donor ligands.