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

Article abstract of DOI:10.1021/om201036z

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 °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) A) and π...π (3.41-3.79 A) 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 (π...π) and covalent (Pt...Pt) bonding interactions involving the 5d and 6p atomic orbitals of the Pt centers. Compounds 1-3 undergo a thermal (165 °C, 24 h) irreversible ligand rearrangement process in the solid state and also in solution at temperatures above 0 °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...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 |e|), 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)(μ₂- η¹,η¹-CN)Pt...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(μ₂- η¹,η¹-CN)Pt(bzq)(CN)].