Synthesis, Magnetic Properties, and Electronic Spectra of Octahedral Mixed-Ligand (β-Diketonato)nickel(II) Complexes with a Chelated Nitronyl Nitroxide Radical

Abstract

Two series of new mixed-ligand nitronyl nitroxide Ni(II) complexes of [Ni(beta-diketonato)(2)(NIT2-py)] and [Ni(beta-diketonato)(tmen)(NIT2-py)](+) types with various kinds of beta-diketonates have been synthesized and structurally and magnetically characterized, where NIT2-py is 2-(2-pyridyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxyl 3-oxide and tmen is N,N,N ',N '-tetramethylethylenediamine. The X-ray structural analysis for three tmen complexes with 2,4-pentanedionate (1b), 1,3-diphenyl-1,3-propanedionate (2b), and 4,4,4-trifluoro-1-phenyl-1,3-butanedionate (5b) demonstrated that the beta-diketonato complexes assume a mer (cis,trans) N(3)O(3) geometrical configuration. The structural parameters are as follows: 1b (C(23)H(39)F(6)N(5)NiPO(4).(1)/(3)CH(2)Cl(2)), trigonal, R&thremacr;, a = 33.299(5) Å, c = 15.007(9) Å, Z = 18; 2b (C(28)H(41)F(6)N(5)NiPO(4)), orthorhombic, Pbca, a = 23.732(6) Å, b = 18.504(5) Å, c = 15.345(4) Å, Z = 8; 5b (C(28)H(38)F(9)N(5)NiPO(4)), triclinic, P&onemacr;, a = 12.068(2) Å, b = 16.942(2) Å, c = 9.161(1) Å, alpha = 105.26(1) degrees, beta = 103.72(1) degrees, gamma = 71.62(1) degrees, Z = 2. The antiferromagnetic interactions between Ni(II) and NIT2-py were found within ranges of J = -207 to -224 cm(-)(1) for the bis(beta-diketonato) complexes and of J = -35 to -150 cm(-)(1) for the (beta-diketonato)(tmen) complexes. The displacement of the beta-diketonates with tmen in bis(beta-diketonato) complexes decreases the J values, and the effect of the 1,3-substituents in the beta-diketonates on the J values is observed in a systematic manner for all the bis(beta-diketonato) complexes and for the (methyl- and phenyl-substituted-beta-diketonato)(tmen) complexes but not for the trifluoromethyl-substituted complex. The room-temperature electronic spectra of the bis(beta-diketonato) and the (beta-diketonato)(tmen) complexes exhibit enhanced spin-forbidden d-d transitions at 13.0 x 10(3) cm(-)(1) and new metal-ligand charge-transfer (MLCT) transitions around (16.0-19.0) x 10(3) cm(-)(1). The variation of the spectroscopic characteristics with modification of the beta-diketonato ligands is discussed through the exchange mechanism in connection with the antiferromagnetic interactions in terms of the substituent effects or the Hammett sigma(m) constants.