Structural systematics of some trinuclear alkynyl and diynyl Group 11 complexes containing dppm [dppm = CH₂(PPh₂)₂]

Abstract

In this review the molecular structures of a series of trinuclear alkynyl and diynyl Group 11 cations [{M3(μ-dppm)3}(X)n](3−n)+ (M = Cu, Ag; n = 1, 2; where X is an alkynyl or diynyl group, an inorganic anion or solvent) are considered from the points of view of (i) the dimensions and geometries of the M3(P–P)3 cores, (ii) the conformations of the dppm ligands, and (iii) the attachment of the alkynyl and diynyl ligands. In the crowded [M3(μ-dppm)3]3+ core, the dppm ligands are arranged so that there is always one CH2 group up and two down, to give pseudo mirror symmetry perpendicular to the M3 plane (crystallographic in some cases). Attachment of the alkynyl or diynyl substituent(s) occurs roughly normal to the M3 plane; according to their perpendicularity, the C(1) atom may be μ2 or μ3. In most cases where only one alkynyl or diynyl ligand is present, a second ligand is also attached to the M3 core. Unusual and interesting dispositions/conformations of the dppm ligands are widespread, among the mono–diynyl complexes in particular, whereby some phosphorus donor atoms lie at unusual distances out of the M3 planes, a concomitant of strong agostic interactions between phenyl H atoms and the atoms of the open M3 face, and weak M⋯M interactions. With one X group, C–H⋯M interactions persist on the other face, with C–H⋯X interactions with the alkyne affecting the inclination of the alkyne and the conformation of the Ph rings. With two substituents (one of which may be a loosely bound anion), similar interactions may occur, accompanied by twisting of the dppm chelate ring to displace P atoms from the M3 plane. These factors possibly inhibit formation of the bis(diyndiyl) complexes, which are only obtained under more strongly basic conditions.