A potential case of the rare Cluster Core Isomerism for phosphido-bridged Pt3 units suggested by DFT calculations

Abstract

Triangular platinum clusters with phosphido bridging ligands and 44 valence electrons may adopt two structural types with either an equilateral or isosceles Pt3 triangle, hence with a closed or an open ring formulation in relation with the nature of the terminal coligands. The most genuinely documented case of such a rare Cluster Core Isomerism (CCI), namely (PPh3)2PhPt3(μ-PPh2)3, was apparently induced by the nature of the solvent and its co-crystallization effects. This paper addresses another CCI example based on the in silico optimizations of both the closed and open Pt3 triangles of unique formula H(CNR)2Pt3(μ-PH2)3 for R = H. Moreover, the DFT approach to these simplified models has been extended to other R substituents at the isocyanide terminal ligands, namely CH3−xFx (x = 0 → 3), with the aim of studying the effects of variable electron-withdrawing properties. The isomerism of the species is always operative except for the species with R = CHF2 and CF3, which are uniquely open. In general, the energy differences between isomers are very small (of the order of 1 kcal mol–1), but importantly imply that they may in principle stand alone, not excluding additionally favourable solid state packing effects. The paper highlights some qualitative electronic aspects, such as the accumulation of the electron density at two separating Pt atoms, in a clear relationship with progressive repulsion on opening.