Synthesis and Characterization of Cr(III) Complexes with 3‐Hydroxy‐4‐Pyrones and 1,2‐Dimethyl‐3‐Hydroxy‐4‐Pyridinone (DMHP): X‐Ray Crystal Structures of Cr(DMHP)3 · 12H2O and Cr(ma)3

Abstract

This report describes the synthesis and characterization of three Cr(III) complexes: Cr(DMHP)3 · 12H2O (DMHP = 1,2‐dimethyl‐3‐hydroxy‐4‐pyridinone), Cr(ma)3 (Hma = maltol or 3‐hydroxy‐2‐methyl‐4‐pyrone), and Cr(ema)3 (Hema = ethylmaltol or 2‐ethyl‐3‐hydroxy‐4‐pyrone). All three Cr(III) complexes were prepared from reactions of chromium(III) chloride with DMHP, Hma, and Hema, respectively, in the presence of a base such as triethylamine. They have been characterized by elemental analysis, IR, ESI‐MS, UV‐visible and electrochemical methods, and in the cases of Cr(DMHP)3 · 12H2O and Cr(ma)3 by X‐ray crystallography. The X‐ray crystal structure of Cr(DMHP)3 · 12H2O shows that the Cr is coordinated by six oxygen donor atoms from three DMHP ligands in distorted octahedral coordination geometry, and is isostructural to the exoclathrate metal complexes M(DMHP)3 · 12H2O (M = Al, Ga, Fe, In and Mn). Although the Cr(III) in Cr(ma)3 is also coordinated by six oxygen atoms from three maltols, one of the coordinated maltol ligands is disordered to give two isomers for the complex Cr(ma)3, both of which have the mer‐configuration. UV/vis spectral and electrochemical data clearly showed that changing the bidentate ligand from 3‐hydroxy‐4‐pyrone (maltol and ethylmaltol) to DMHP has a significant impact on the electronic and electrochemical properties of their corresponding Cr(III) complexes. Since the electron density in the coordinated DMHPs is more delocalized over the whole pyridinone ring than that in coordinated maltols, DMHP is able to stabilize both the Cr(III) and Cr(IV) oxidation states, as demonstrated by a relatively low redox potential for the Cr(III)/Cr(IV) couple, while maltol and ethylmaltol can only stabilize the Cr(III) oxidation state. DMHP might be a useful chelator for treatment of chronic Cr toxicity. Acknowledgment is made to Dr. Phillip E. Fanwick, Department of Chemistry, Purdue University, for the X‐ray crystallography of Cr(DMHP)3 · 12H2O and Cr(em)3.