Bis(2,6-dinitroaryl)platinum(II) Complexes. Cis/Trans Isomerization^†

The complexes cis-[Pt(κ²-Aryl)(κ¹-Aryl)L] (κ²-Aryl = κ²-(C,O)-C₆(NO₂)₂-2,6-(OMe)_3, κ¹-Aryl = κ¹(C)-C₆(NO₂)₂-2,6-(OMe)₃, L = S-dmso (2cis), XyNC (4cis; Xy = 2,6-Me₂C₆H₄), CO (5cis), PPh₃ (7cis)) have been obtained by reacting at room temperature cis-[Pt(κ²-Aryl)(κ¹-Aryl)(OH₂)] (1cis) with dimethyl sulfoxide (dmso; 1:1 or excess), XyNC (1:1), or CO (excess) or cis-(Me₄N)[Pt(κ²-Aryl)(κ¹-Aryl)Cl] with PPh₃ (1:1), respectively. The room-temperature reaction of 1cis with XyNC (1:2), of 5cis with PPh₃ (1:1), or of cis-(Me₄N)[Pt(κ²-Aryl)(κ¹-Aryl)Cl] with PPh₃ (1:2) gives cis-[Pt(κ¹-Aryl)₂LL‘] (L = L‘ = XyNC (3cis), PPh₃ (8cis); L = CO, L‘ = PPh₃ (6cis)). Complexes 3cis, 4cis, and 5cis isomerize on heating in solution or in the solid state to give 3trans, 4trans, and 5trans, respectively, while 6cis and 8cis decompose to give 7cis instead of their trans isomers; these, however, can be prepared by reacting 5trans or 6trans with PPh₃ in molar ratios of 1:1 or 1:2, respectively. When they are heated, 6trans and 8trans also descompose to 7cis, while 2cis and cis-[Pt(κ²-Aryl)(κ¹-Aryl)L] (L = H₂O (1cis), PhCN, tht) decompose to a mixture of unidentified products (1cis) or are recovered unchanged. These results, and others reported in the literature, on the stability of cis- and trans-diaryl complexes of platinum or palladium can be explained as the result of two competing factors, transphobia and the steric requirements of the ligands. The X-ray crystal structures of 2cis, 3cis, 3trans, 4trans·CHCl₃, 5cis·CH₂Cl₂, 5cis·0.5hexane, 6cis, 6trans·CHCl₃, 7cis·Me₂CO, and 8trans·0.5CHCl₃ have been determined.