Reactions of α-diimino ligands with the chloro-bridged dimer [RhCl(COD)] 2(COD=1,5-cyclooctadiene)

Abstract

The reactions of α-diimino ligands N- N′ [ N- N′= 2,2′-bipyridine (bipy), C 5H 4N2CHNR (R= C 6H 4OMe- p, PyCa), RNCHCHNR (R=C 6H 4- OMe- p, DAB)] with [RhCl(COD)] 2 give rise to stoichiometry, solvent, ligand, and temperature dependent equilibria. In general, the 1/1 ligand/dimer reaction yields the ionic product [Rh(COD)( N- N′)] [RhCl 2(COD)], at room temperature. For N- N′=DAB, the ionic form is in equilibrium with the binuclear compound [{RhCl(COD)} (μ-DAB){RhCl(COD)}] (containing a σ σ,'- N, N′ bridging α-diimine), which becomes the predominant species at low temperatures. In [Rh(COD)( N- N′)] [RhCl 2(COD)], a fast exchange of the Rh(COD) unit between the cation and anion occurs at 30 °C for N- N′=PyCa and DAB (but not for N- N′=bipy). The 1/0.5 reaction leads to a product, generally formulated as Rh(COD)( N- N′)Cl, which probably consists of an equilibrium mixture of the cationic [Rh(COD)( N- N′)]Cl and neutral [RhCl(COD)( N- N′)] species, in rapid interconversion even at −80 °C. The cationic complex largely predominates in polar solvents, such as methanol, from which it can be precipitated as a perchlorate salt ( N-N′=bipy, PyCa). For [Rh(COD)(PyCa)]ClO 4, a low-energy process occurs which involves ligand site exchange ( cis-trans isomerization) and cannot be frozen at the lowest explored temperature (−80 °C). Such dynamic behaviour is interpreted in terms of the formation of stereochemically non-rigid five- coordinate intermediates through association of the cation with the solvent or the counteranion. In [Rh(COD)( N-N′)] +, the σ,σ′- N,N′ chelating abilities of N-N′ appear to increase in the order: DAB< PyCa < bipy.