Influence of Cis and Trans Ligands in Platinum(II) Complexes on the Ability of the Platinum Center to Activate C−H Bonds. A Density Functional Theory Study

Abstract

We have studied the influence of different ligands X (X = F, Cl, Br, I, NO2, and CN) on the C−H bond activation of CH4 in trans-PtCl2X(CH4)−, 1, and trans-PtClX2(CH4)−, 2, where X is either trans (1) or cis (2) to methane. For 1 with X in the trans position, the trans-PtCl2X− fragment interacts with CH4 through the overlap between the empty dσ-based orbital 2a1 pointing along the Pt−X direction and σCH on CH4. An interaction also takes place between an occupied dσ-based orbital 1b1 and the empty σ*CH orbital on CH4, where the dπ metal orbital is positioned perpendicular to the PtCl2X− plane. The dσ metal orbital contribution in 2a1 is antibonding with respect to σx on X, whereas dπ in 1b1 is antibonding with respect to πx. Through the series F, Cl, Br, I, NO2, and CN, the energies of σx and πx increase. This is mostly an electronegativity effect. The increase in energy causes an increase in the contribution from σx and πx to 2a1 and 1b1, respectively. As a consequence, the bonding overlaps 〈 σ CH | 2 a 1 〉 and 〈 σ CH * | 1 b 1 〉 will diminish, as only the d-component in 2a1 and 1b1 contributes to the overlap. As a result of the decreasing bonding overlaps, the Pt−CH4 bond strength will decline. It is thus shown that the experimentally established order of trans-labilizing power for the series of ligands X studied here, F < Cl < Br < I < NO2 < CN, can be related to the orbital energies of σx and πx and the electronegativity of the elements that are involved in these orbitals. The labilization of the Pt−CH4 bond in the C−H activation transition state is even larger than in the adduct 1, leading to an increase in the C−H activation barrier along the series F < Cl < Br < I < NO2 < CN. For 2 with X in the cis position, solvation has the largest influence on trends in the Pt−CH4 bond for both 2 and the transition state. However C−H activation barriers are quite similar for different X.