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.

Excited States and Optical Spectroscopy of Nitronyl Nitroxides and their Lanthanide and Transition Metal Complexes

“…Unusually intense and surprisingly broad metal centered transitions in the same energy range have been reported for chromium(III) complexes with nitronyl nitroxide and imino nitroxide radicals [24,25,31]. At higher energy, ligand-centered bands which sometimes have resolved vibronic structure are observed [25,31,47]. Other transition metal complexes investigated involve manganese(II), cobalt(II), cobalt(III), nickel(II), and zinc(II) metal centers.…”

“…The absorption spectrum of a complex with a semiquinone ligand shows a significant enhancement of the lowest-energy spin-forbidden d-d band at approximately 14,000 cm 1 [46]. Unusually intense and surprisingly broad metal centered transitions in the same energy range have been reported for chromium(III) complexes with nitronyl nitroxide and imino nitroxide radicals [24,25,31]. At higher energy, ligand-centered bands which sometimes have resolved vibronic structure are observed [25,31,47].…”

“…At 5 K, both spectra show well-resolved structure that is similar to the patterns observed for the uncoordinated radical, as summarized in Tables 1 and 2. The corresponding electronic transitions can be observed for many other complexes of lanthanide or d-block metal ions with radical ligands [24][25][26][27]30]. In general, the spectra for lanthanide complexes are very similar to those of the uncoordinated radicals.…”

“…Absorption spectra of nitroxides and their complexes in solution have been reported as short descriptions given when the compounds were first synthesized [21][22][23]. Over the past five years, several nitronyl nitroxide and related radicals and their transition metal and lanthanide complexes have been characterized with optical spectroscopic techniques [4,9,[24][25][26][27][28][29][30][31]. We illustrate in the following the low-temperature optical spectra of uncoordinated nitroxide radicals and of their complexes with lanthanide ions and conclude with a short outlook on the optical spectra of complexes with metals of the d series.…”

Crystal Structure of Bis(benzoylacetonato)methanolzinc(II)

“…The bite distances for O1·O2 and O3·O4 are 2.846(3)Å and 2.811(3)Å, respectively, which is slightly shorter than the value of 2.880 Å reported by Bubar & Mettrovic. 4,5 In addition, the structure has a hydrogen bond of the type O-H·O from the neighboring molecular, and follows accordingly: O5-H1·O2 with O5·O2 = 2.711(3)Å, ∠O5-H1·O2 = 173(3)˚ and the symmetry code is 1 -x, 2 -y, 1 -z. Table 2 Final coordinates and equivalent isotropic displacement parameters of the non-hydrogen atoms Table 3 The selected bond lengths (Å), angles (˚) and torsion angles (˚) for 1 …”