Synthesis and Properties of Diphenoxo-Bridged Co^II, Ni^II, Cu^II, and Zn^IIComplexes of a New Tripodal Ligand: Generation and Properties of M^II-Coordinated Phenoxyl Radical Species

Abstract

Four dinuclear complexes of composition [MII2(L)2].xS [M=Co, x=0.5, S=1,4-dioxane (1.0.5 1,4-dioxane); Ni, x=0 (2) [single crystals have x=2, S=diethyl ether (2.2 diethyl ether)]; Cu, x=0 (3); Zn, x=0.5, S=1,4-dioxane (4.0.5 1,4-dioxane)] have been synthesized using a new tripodal ligand [2,4-di tert-butyl-6-{[(2-pyridyl)ethyl](2-hydroxybenzyl)-aminomethyl}-phenol (H2L)], in its deprotonated form, providing a N 2O 2 donor set. Crystallographic analyses reveal that the complexes have a similar diphenoxo-bridged structure. Each metal ion is terminally coordinated by 2,4-di tert-butyl-phenolate oxygen, a tertiary amine, and a pyridyl nitrogen. From each ligand, unsubstituted phenolate oxygen provides bridging coordination. Thus, each metal center assumes M (II)N 2O 3 coordination. Whereas the geometry around the metal ion in 1.0.5 1,4-dioxane, 2.2 diethyl ether and, 4.0.5 1,4-dioxane is distorted trigonal-bipyramidal, in 3 each copper(II) center is in a square-pyramidal environment. Temperature-dependent magnetic behavior has been investigated to reveal intramolecular antiferromagnetic exchange coupling for these compounds (-J=6.1, 28.6, and 359 cm(-1) for 1.0.5 1,4-dioxane, 2, and 3, respectively). Spectroscopic properties of the complexes have also been investigated. When examined by cyclic voltammetry (CV), all four complexes undergo in CH2Cl2 two reversible ligand-based (2,4-di tert-butylphenolate unit) one-electron oxidations [E1/2(1)=0.50-0.58 and E1/2(2)=0.63-0.75 V vs SCE (saturated calomel electrode)]. The chemically/coulometrically generated two-electron oxidized form of 3 rearranges to a monomeric species with instantaneous abstraction of the hydrogen atom, and for 4.0.5 1,4-dioxane the dimeric unit remains intact, exhibiting an EPR spectrum characteristic of the presence of ZnII-coordinated phenoxyl radical (UV-vis and EPR spectroscopy). To suggest the site of oxidation (metal or ligand-centered), in each case DFT calculations have been performed at the B3LYP level of theory.

Synthesis and structure of mono-, bi- and trimetallic amine-bis(phenolate) cobalt(ii) complexes

“…The moment decreases as T decreases, but the rate of decrease becomes smaller as it approaches 50 K. Below 50 K, the moment decreases abruptly, reaching a minimum value of 6.3 m B per mole of dimer (4.5 m B per Co) at 10 K. Below this temperature the moment begins to rise and reaches a maximum of 6.5 m B (4.6 m B per Co) at 2 K. These variations in magnetic moment over temperature are in fact relatively small, and a plot of c M -1 versus T reveals a line consistent with Curie-Weiss behaviour as shown in Fig. 11(b also included) 47 are obtained. Complex 3 shows no maximum over the temperature range examined, thereby implying magnetically isolated Co II centers.…”

Unique mononuclear MnIIcomplexes of an end-off compartmental Schiff base ligand: experimental and theoretical studies on their bio-relevant catalytic promiscuity

“…[62][63][64][65][66] Now to rationalize the exact pathway for our complexes we have performed DFT calculation (vide infra). [62][63][64][65][66] Now to rationalize the exact pathway for our complexes we have performed DFT calculation (vide infra).…”

Metal Coordinated Phenoxyl Radicals

“…20 [Cu II 2 (L 7 tBu ) 2 ] was also shown to dissociate upon oxidation into the radical monomer [Cu II (L 7 tBu ) • (H 2 O)] + that quickly decomposes. 112 There are only two examples of dicopper(II) complexes in which the bridging atom is a phenoxyl oxygen (only one being stable enough to be characterized). 110 Both examples were isolated from the dinucleating ligand H di L 4 OMe , depending on the amount of triethylamine added during the synthesis.…”

“…Its EPR spectrum thus contrasts sharply with that of [Zn II ( Me L 2 OMe ) •• ] 2+ as it is constituted by a single (S = 1 / 2 ) signal at g = 2.006. 112 Zinc(II)-phenoxyl radical complexes derived from salen ligands are rare. 140 Although the metal ion is tetracoordinated in the corresponding copper complexes, the zinc ion in [Zn II (L 9 )(EtOH)] is pentacoordinated and square pyramidal, with a solvent molecule that occupies the apical position.…”