Synthesis and structure of bis(azooximates) of dichlororhodium(III): the oxime–oximate O–H  · · ·  O bridge and the effect of its deprotonation

Abstract

The reaction of azooximes RC(NOH)NNPh (R = Me HL¹, Ph HL², or C₆H₄Me-p HL³) with RhCl₃·3H₂O afforded red trans-cis-cis(tcc)-[RhCl₂L(HL)] 1 which was converted into green [NEt₃H][tcc-RhCl₂L₂] 2 upon treatment with Et₃N; 2 isomerises to pink [NEt₃H][cct-RhCl₂L₂] 3 spontaneously in boiling benzene–toluene. The distorted octahedral co-ordination spheres are of type RhCl₂N^o₂N^a₂ (N^o = oximato N, N^a = azo N) with the relative arrangement of donor atom pairs as stated (e.g.tcc = trans-Cl₂-cis-N^o₂-cis-N^a₂). The crystal structures of tcc-[RhCl₂L²(HL²)] 1b, [NEt₃H][tcc-RhCl₂L²₂] 2b and [NEt₃H][cct-RhCl₂L³₂] 3c were determined. In 1b unsymmetrical hydrogen bonding is present, the O  · · ·  O distance being 2.515(5) Å. Upon deprotonation to 2b the distance increases to 2.833(8) Å. In 3c the distance is 4.207(10) Å. The NEt₃H⁺ cations in 2b and 3c are associated with oximato oxygen and cis-RhCl₂ chloride respectively. The isomerisation of 2b to 3b in hot toluene is characterised by a high enthalpy and entropy of activation.

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