Synthesis, Characterization, and Dynamic Behavior of Os2Pt(CO)8(PPh3)2:  A Trinuclear Osmium−Platinum Cluster with Flexible Metal Framework

Reaction of Os2(CO)8(μ-η1,η1-C2H4) with (η2-C2H4)Pt(PPh3)2 gives predominantly the heterotrinuclear cluster Os2Pt(CO)8(PPh3)2, 1. The reaction is accompanied by redistribution of the PPh3 ligand, and as a result, compound 1 exists in solution as three isomers (1a−c) due to the presence and disposition of a phosphine ligand on both Os and Pt. The nature of the isomers was deduced from a combination of 13C, 31P, and 195Pt NMR spectroscopies, and the solid-state structure of the major isomer was corroborated by a single-crystal X-ray analysis. The isomers interconvert on the NMR time scale, and the mechanisms and energetics of the isomerization processes were determined by 31P NMR selective inversion magnetization transfer experiments. Two pathways for isomer exchange were identified. Isomers 1a and 1b exchange via an “olefin type” rotation of the diosmium fragment about the platinum center, whereas interconversion between 1a and 1c is accomplished by a restricted trigonal twist at the phosphine-substituted osmium center; the energetics for the processes are ΔH ⧧ = 10.1(2) kcal·mol-1, ΔS ⧧ = −13.1(11) eu and ΔH ⧧ = 10.7(3) kcal·mol-1, ΔS ⧧ = −5.2(12) eu, respectively. The mechanism of the 1a ⇄ 1c exchange was confirmed by 13C NMR spin saturation transfer experiments.