Chromium complexes of an isomeric N-donor ligand, 2-[(N-arylamino)phenylazo]pyridine: amination reactions, x-ray structure, and redox properties

Abstract

The chromium chemistry of two positional isomers of the ligand 2-[(N-arylamino)phenylazo]pyridine (HL¹and HL²) are described. While the ligand HL¹ coordinates as a bischelating tridentate N,N,N-donor, [L¹]-, with deprotonation of the amine nitrogen, its isomer HL² coordinates as a neutral bidentate N,N-donor. The amine nitrogen in this case remains protonated. Thus the reaction of CrCl₃.nH₂O with HL¹ produced the brown cationic complex, [Cr(L¹)₂]⁺, [1]⁺. The representative X-ray structure of [1a](ClO₄) is reported. The two azo nitrogens of the anioinc tridentate ligand approach the metal center closest with Cr(1)-N(azo) av 1.862(6) Å. There is a significant degree of ligand backbone conjugation in the coordinated ligands, which resulted in shortening of the C-N distances and also in lengthening of the diazo (N=N) distances. Two synthetic approaches for the synthesis of chromium complexes of HL² are investigated. The first approach is based on the substitution reaction, wherein all the coordinated CO ligands of Cr(CO)₆ were completely substituted by the three bidentate HL² ligands to produce a violet complex [Cr(HL²)₃]. The second approach is based on para-amination reaction of coordinated 2-(phenylazo)pyridine (pap). Thus the reaction of an inert complex, [CrCl₂(pap)₂], with ArNH₂ yields a mixed ligand complex, [CrCl₂(pap)(HL²)], 3. In this reaction one of the two coordinated pap ligands in [CrCl₂(pap)2] undergoes amination at the para carbon (with respect to the diazo function) to yield HL² in situ. This metal-promoted transformation is authenticated by the X-ray structure determination of a representative complex, [CrCl₂(pap)(HL^2a)], 3a. Notable differences in bond distances along the ligand backbones of the two coordinated ligands in 3a indicate different levels of metal-ligand overlap in this complex. All the chromium complexes of HL² are characterized by their intense blue-violet color. The frequencies of the visible range transitions in these complexes linearly correlate with the Hammett's substitution constant. Intraligand charge-transfer transitions in the visible region are believed to be responsible for the intense color. Redox properties of all these complexes are reported.