Abstract
The compounds bis(acetylacetonato)chromium(II) and bis(dipivaloyl-methanato)chromium(II) have been prepared, and the knowledge of their properties extended by magnetic measurements down to liquid nitrogen temperature, and by preparation of their pyridine adducts. Investigations of complexes of chromium(II) with Schiff bases derived from 2-hydroxyarylcarbonyl compounds and from acetylacetone have been carried out. Several attempts to prepare the unknown chromium(Il)-salicyl-aldimine complexes by various routes have failed, giving chromium(III) compounds of uncertain composition. During investigations to explain this failure, synthesis of several complexes of chromium(II) with 2-hydroxyarylcarbonyl compounds was attempted. While bis(salicyl-aldehydato)chromium(II) and bis(2-hydroxyacetophenonato)chromium(II) could not be isolated, CrMeOSal2 (MeOSalH = methyl salicylate) was obtained, though in an impure state. A study of the products from these experiments indicates that complexes with chromium(II) are not formed when the ligand is easily reduced, and provides an explanation of the failure to obtain chromium(II)-salicylaldimine complexes. In confirmation of this, preparations of chromium(II) complexes of B-ketoamines and salicylketimines were all successful. The new compounds Cr(N-R-acetylacetoneimine)[2] (R = Me, Et, Pr, i-Pr, Bu, i-Bu, Ph), N,N-ethylenebis(acetylacetoneiminato)chromium(II), and Cr(N-propyl-2-hydroxyacetophenoneimine)[2] have thus been isolated. X-ray studies and electronic spectroscopy indicate gross square planar geometry. The marked decrease in magnetic moment with decreasing temperature has been interpreted as due either to spin-isomerism or to strong antiferromagnetic interaction. Although the experimental evidence in favour of spin-isomerism is shown to be inconclusive, further, confirmatory, experiments are suggested. The bis(pyridine) adducts of two of the complexes have been isolated and characterised. These new compounds contain chromium(II) in the low spin configuration, providing further evidence in favour of the spin-isomerism explanation of the magnetic behaviour of the unadducted compounds. The reaction of nitric oxide with some of the chromium(II)-Schiff base complexes gave chromium(III) products of indefinite composition, but three [15]N-nitrosyls of cobalt(II)-Schiff base complexes have been isolated. It has been shown that a very low field [15]N nmr signal clearly indicates the presence of a bent MNO group.