Effects of solvent on the relative stability of mono and di-aluminium aryloxide complexes of bipyridines: anomalous behavior of [(tBu)2Al(OPh)]2(μ-4,4-bipy)

Abstract

The temperature dependence of the solution equilibrium constants for [(^tBu)₂Al(OPh)]₂(μ-4,4′-bipy) (1a), [(^tBu)₂Al(OPh)]₂(μ-bipetha) (2a, bipetha = 1,2-bis(4-pyridyl)ethane), and [(^tBu)₂Al(OPh)]₂(μ-bipethe) (3a, bipethe = trans-1,2-bis(4-pyridyl)ethylene) in C₆D₆ and CDCl₃ allow for the determination of ΔH and ΔS for the dissociation of one Al(^tBu)₂OPh moiety from the bridging ligand, i.e., . For compounds 2a and 3a the ΔH values in C₆D₆ [99(2) kJ mol⁻¹ (2a) and 109(5) kJ mol⁻¹ (3a)] and CDCl₃ [115(5) kJ mol⁻¹ (2a) and 139(7) kJ mol⁻¹ (3a)] were found to be inversely proportional with the dielectric constant of the solvent. In contrast, the ΔH value for 1a in CDCl₃ is surprisingly small [14.9(7) kJ mol⁻¹] and does not fit with the trends adopted by the bipetha and bipethe derivatives or the value obtained in C₆D₆ [110(2) kJ mol⁻¹]. Unlike the other compounds and the C₆D₆ solutions, the CDCl₃ solution of 1a allows for the observation of a second equilibrium , for which the ΔH has been determined [4.5(3) kJ mol⁻¹]. This result suggests that in CDCl₃ bonding of the second Al(^tBu)₂OPh moiety to Al(OPh)(^tBu)₂(4,4′-bipy) (1b) is stabilized by the presence of the first aluminium, which is counter to ab initio calculations that predicts the aluminium in Al(OPh)(^tBu)₂(L) should destabilize the Al–N interaction with a second Al(^tBu)₂OPh group. The BDE for dissociation of both Al(^tBu)₂OPh moieties from 1a–3a, and the energy of formation of hydrogen bond interactions with CHCl₃, has been calculated by ab initio methods, and no unusual effects are inherent in 1a.