Activation of C–Cl by ground-state aluminum atoms: an EPR and DFT investigation

Abstract

The reaction of ground-state Al atoms with dichloromethane (CH₂Cl₂) in an adamantane matrix at 77 K yielded two mononuclear Al species. The magnetic parameters, extracted from the axial EPR spectrum of Species A/A′ (g₁ = 2.0037, g₂ = g₃ = 2.0030, a_Al,1 = 1307 MHz, a_Al,2 = a_Al,3 = 1273 MHz, a_35Cl = 34 MHz and a_37Cl = 28 MHz) were assigned to the Al-atom insertion product, ClCH₂AlCl. Density functional theory (DFT) calculations of the values of the Al and Cl hyperfine interaction (hfi) of the Cl₁–Cl₂gauche conformer were in close agreement with the experimental values of ClCH₂AlCl. The second species, B/B′, had identical magnetic parameters to those of ClCH₂AlCl with the exception that the Al hfi was 15% smaller. Coordination of a ligand, possessing a lone pair of electrons, to the Al atom of the insertion product, [ClCH₂AlCl]:X, could cause the a_Al to decrease by 15%. Alternatively, it is possible that the Cl₁–Cl₂ anti conformer of ClCH₂AlCl is also isolated in the matrix. Support for the spectral assignments is given by calculation of the nuclear hfi of [ClCH₂AlCl]:H₂O and the Cl₁–Cl₂ anti conformer of ClCH₂AlCl using a DFT method. The potential energy hypersurface for an Al atom approaching CH₂Cl₂, calculated at the B3LYP level, suggests that Al atom abstraction of Cl forming AlCl and CH₂Cl is favoured in the gas phase. When produced in a matrix, the close proximity of AlCl and CH₂Cl could account for the formation of ClCH₂AlCl. EPR evidence was also found for the formation of the CHCl₂ radical.