Synthesis and Characterization of the Double Salts [Pt(bzq)(CNR)2] [Pt(bzq)(CN)2] with Significant Pt•••Pt and π•••π Interactions. Mechanistic Insights into the Ligand Exchange Process from Joint Experimental and DFT Study

Double complex salts (DCSs) of stoichiometry [Pt(bzq)(CNR)(2)][Pt(bzq)(CN)(2)] (bzq = 7,8-benzoquinolinate; R = tert-butyl (1), 2,6-dimethylphenyl (2), 2-naphtyl (3)) have been prepared by a metathesis reaction between [Pt(bzq)(CNR)(2)]ClO4 and [K(H2O)][Pt(bzq)(CN)(2)] in a 1:1 molar ratio under controlled temperature conditions (range: -10 to 0 degrees C). Compounds 1-3 have been isolated as air-stable and strongly colored solids [purple (1), orange (2), red-purple (3)]. The X-ray structure of 2 shows that it consists of ionic pairs in which the cationic and anionic square-planar Pt(II) complexes are almost parallel to each other and are connected by Pt-Pt (3.1557(4) angstrom) and pi center dot center dot center dot pi (3.41-3.79 angstrom) interactions. Energy decomposition analysis calculations on DCSs 1-3 showed relatively strong ionic-pair interactions (estimated interaction energies of -99.1, -110.0, and -108.6 kcal/mol), which are dominated by electrostatic interactions with small contributions from dispersion (pi center dot center dot center dot pi) and covalent (Pt center dot center dot center dot Pt) bonding interactions involving the 5d and 6p atomic orbitals of the Pt centers. Compounds 1-3 undergo a thermal (165 degrees C, 24 h) irreversible ligand rearrangement process in the solid state and also in solution at temperatures above 0 degrees C to give the neutral complexes [Pt(bzq)(CN)(CNR)] as a mixture of two possible isomers (SP-4-2 and SP-4-3). The mechanism of this process has been thoroughly explored by combined NMR and DFT studies. DFT calculations on 1-3 show that the existing Pt center dot center dot center dot Pt interactions block the associative attack of the Pt(II) centers by the coordinated cyanide and/or isocyanide ligands. Moreover, they support a significant transfer of electron density from the anionic to the cationic component (0.20-0.32 lel), which renders the isocyanide ligand dissociation more feasible than that in the "free-standing" cationic [Pt(bzq)(CNR)(2)](+) components as well as the dissociation of the CN- in trans position to the C-bzq in the anionic [Pt(bzq)(CN)(2)](-) component. Therefore, the first step in the ligand rearrangement pathway is the dissociation of the isocyanide in trans position to the C-bzq, yielding the [(RNC)(bzq)(mu(2)-eta(1),eta(1)-CN)Pt center dot center dot center dot Pt(bzq)(CN)] intermediates. The rate-limiting step corresponds to the transformation of these intermediates to the neutral [Pt(bzq)(CN)(CNR)] complexes following a synchronous mechanism involving rupture of the Pt-Pt and formation of the Pt-CN bonds through transition states formulated as [(RNC)(bzq)Pt(mu(2)-eta(1),eta(1)-CN)Pt(bzq)(CN)].