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

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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 ]ClO 4 and [K(H 2 O)][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) Å) and π···π (3.41–3.79 Å) 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)(μ 2 -η 1,η 1 -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(μ 2 -η 1,η 1 -CN)Pt(bzq)(CN)].