Syntheses and NMR studies of five-co-ordinate rhodium(I) complexes with α-diimines (RNC(H)C(H)NR): [RhCl(CO)(η 2-C 2H 4)(α-diimine)] and [RhCl(L) 2(α-diimine)] (R = t−Bu, EtMe 2C−; L = CO,PF 3)

Abstract

Interaction of α-diimines (RNCHCHNR abbreviated as R-dim) with [LL′Rh(μ-Cl) 2RhLL′] (LL' CO or PF 3 and LCO, L′η 2-C 2H 4) gives rise to various equilibrium mixtures whose nature depends on L. [RhCl(CO)(η 2-C 2H 4)(R-dim)], which can be isolated, has according to 1H and 13C NMR spectra a five-co-ordinate structure. The σ,σ-N,N′ chelate R-dim and η 2-olefin ligands are in the trigonal plane of a trigonal bipyramidal array with the Cl and CO groupings residing in the axial positions, a geometry similar to that of recently reported [PtCl 2(η 2-styrene)(t-Bu-dim)]. The dynamic behaviour of [RhCl(CO)(η 2-C 2H 4)(R-dim)] which involves rotation of the ethylene around the Rhη 2-C 2H 4 axis, has been studied. These complexes readily lose ethylene providing four-co-ordinate [RhCl(CO)(R-dim)]. Five-co-ordinate [RhCl(CO) 2(R-dim)] obtained in solution are in equilibrium (slow on the NMR time scale) with free R-dim, the ionic [Rh(CO) 2(R-dim)][RhCl 2(CO) 2] and the dinuclear species [{RhCl(CO) 2} 2(R-dim)] of which the latter two are four-co-ordinate Rh I species. 1H and 13C NMR spectra point to a trigonal bipyramidal structure of the five-co-ordinate species in which the intramolecular exchange between the axial and equatorial CO groups is fast on the NMR time scale. According to 1H, 13C, 19F and 31P NMR spectra analogous [RhCl(PF 3) 2(R-dim) complexes have a trigonal bipyramidal structure with cis-PF 3 groups of which one PF 3 group occupies the axial position trans to Cl. The stability of the five- vs. the four-co-ordinate geometry of these RhR-dim complexes is discussed in terms of the branching at C α of R. A comparison is made with iso-electronic and iso-structural platinum(II)η 2-olefinR-dim complexes. The [RhCl(CO)(η 2-C 2H 4)(R-dim)] complexes readily undergo reversible R-dim exchange with 2,4,6-Me 3pyridine but afforded [RhCl(CO)(PPh 3) 2] irreversibly with PPh 3.