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  • Title: Effects of solvent on the relative stability of mono and di-aluminium aryloxide complexes of bipyridines: anomalous behavior of [(tBu)2Al(OPh)]2(mu-4,4-bipy).
    Author: Ogrin D, van Poppel LH, Barron AR.
    Journal: Dalton Trans; 2005 May 05; (9):1722-6. PubMed ID: 15852124.
    Abstract:
    The temperature dependence of the solution equilibrium constants for [((t)Bu)(2)Al(OPh)]2(mu-4,4'-bipy)(1a), [((t)Bu)2Al(OPh)](2)(mu-bipetha)(2a, bipetha = 1,2-bis(4-pyridyl)ethane), and [((t)Bu)(2)Al(OPh)]2(mu-bipethe)(3a, bipethe =trans-1,2-bis(4-pyridyl)ethylene) in C6D6 and CDCl(3) allow for the determination of DeltaH and DeltaS for the dissociation of one Al(tBu)2OPh moiety from the bridging ligand, i.e., 2[(tBu)2AL(OPh)]2(mu-L)<==>(K1)2AL(OPh)(tBu)2(L)+[(tBu)2Al(mu-OPh)]2. For compounds and the DeltaH 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 DeltaH 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 2Al(OPh)(tBu)2(L)<==>(K2)[(tBu)2Al(mu-OPh)]2+2L, for which the DeltaH 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)((t)Bu)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.
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