The heteronuclear cluster chemistry of the group IB metals part XVII. Synthesis and X-ray crystal structure of the hexanuclear mixed-metal cluster [Ag2Ru4(μ3-H)2(CO)12{P(C6H4Me-2)3}2]. An investigation of the steric properties of the P(C6H4Me-2)3 ligand

Abstract

Treatment of a dichloromethane solution of the salt [N(PPh3)2]2[Ru4(μ-H)2(CO)12] with two equivalents of the complex [Ag(NCMe)4]PF6 at −30°C, followed by the addition of two equivalents of the very bulky phosphine ligand P(C6H4Me-2)3 (cone angle 194°) affords the mixed-metal cluster [Ag2Ru4(μ3-H)2(CO)12{P(C6H4Me-2)3}2] (I) in ca. 70% yield. A single-crystal X-ray diffraction study has revealed thatI has a capped trigonal bipyramidal metal framework structure, with the two silver atoms in close contact [AgAg 2.876(2)Å]. This skeletal geometry is very surprising in view of a previous observation that PtBu3 (cone angle 182°), which is supposedly a less sterically demanding phosphine ligand than P(C6H4Me-2)3, is sufficiently bulky to prevent the two Ag(PtBu3) units being adjacent in the metal core of the closely related cluster [Ag2Ru4(μ3-H)2(CO)12(PtBu3)2]. In the solid-state structure ofI, it appears that two of the three C6H4Me-2 rings in each of the two P(C6H4Me-2)3 ligands can adopt relative orientations which allow the phosphine to behave as a less sterically demanding ligand than its large cone angle might otherwise suggest. Remarkably, the formal replacement of the two PPh3 ligands attached to the silver atoms in the capped trigonal bipyramidal metal framework of the closely related cluster [Ag2Ru4(μ3-H)2(CO)12(PPh3)2] by two P(C6H4Me-2)3 groups inI causes very little change in most of the metal-metal distances, despite the fact that the cone angle of PPh3 (145°) is 49° smaller than that of P(C6H4Me-2)3. In solution, compoundI undergoes two types of dynamic behaviour at ambient temperatures. One process involves an intramolecular rearrangement of the metal core, which exchanges the two silver atoms between the two inequivalent sites, and the other process is an intermolecular exchange of P(C6H4Me-2)3 ligands between clusters.