NICKEL(III) PEPTIDE AND CYANO COMPLEXES: EPR CHARACTERIZATION OF THEIR MIXED LIGAND COMPLEXES AND DYNAMIC JAHN-TELLER DISTORTIONS IN FROZEN AQUEOUS GLASSES
Abstract
Nickel(III) tripeptides readily form polypyridine and polyamine mixed ligand complexes in aqueous solution. These tetragonally elongated complexes are characterized by their EPR spectral shifts and are much more stable in regard to redox decomposition in neutral or basic solution than are the parent trans-diaqua complexes. Cyanide ion substitutes for the axial waters of trans-diaquatripeptidonickel(III) complexes, Ni('III)(H(,-2)L)(H(,2)O)(,2), in a stepwise manner to form the mono and bis cyano complexes Ni('III)(H(,-2)L)(H(,2)O)CN('-) and Ni('III)(H(,-2)L)(CN)(,2)('2-), with g(,av) = 2.156 and g(,av) = 2.136, respectively. Higher cyanide concentrations lead to the displacement of the peptide and the formation of hexacyanonickelate(III) (g(,av) = 2.056) without loss of the trivalent oxidation state. These assignments are based in part on EPR studies with isotopically enriched ('61)Ni, ('13)CN('-), and C('15)N('-). Oxidation of Ni(CN)(,4)('2-) in aqueous solution gives the trans-diaquatetracyanonickelate(III) ion, Ni('III)(CN)(,4)(H(,2)O)(,2)('-), which is moderately stable in acidic solution (11 min t(, 1/2) at 25(DEGREES)C, pH 1 to 3), but decays rapidly in base (0.5 s t(, 1/2) at pH 10). The Ni('III)(CN)(,4)(H(,2)O)(,2)('-) complex undergoes rapid axial substitution with ammonia, imidazole, pyridine, azide, and cyanate ions to form bis trans complexes which are more stable with regard to redox decomposition than is the parent complex. Addition of cyanide to the diaqua complex forms Ni('III)(CN)(,6)('3-), which gives temperature-dependent EPR spectra in frozen aqueous solution. At 60 K the EPR spectrum is indicative of a tetragonally-elongated geometry, while at 238 K an isotropic signal is observed. This change is reversible with temperature and is indicative of a dynamic Jahn-Teller distortion. Six-nitrogen bis(tripeptido)nickelate(III) complexes are shown to undergo a temperature dependent change in geometry as observed by EPR spectral changes in frozen aqueous glasses. A static Jahn-Teller distortion is observed at 60 K which results in a tetragonally elongated geometry (g(,(PERP)) > g(,(PARLL))). At 240 K a tetragonally compressed signal is observed (g(,(PARLL)) > g(,(PERP))), which is due to vibrational interchange of two equivalent elongated axes. This phenomenon is observed for several bis(tridentate) nickel(III) peptide complexes.
Degree
Ph.D.
Subject Area
Chemistry
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