Square planar cobalt(II) complexes of salen ligands N,N'-bis(3-tert-butyl-5R-salicylidene)-1,2-cyclohexanediamine), where R = OMe (1) and tert-butyl (2), were prepared. 1 and 2 were electrochemically reversibly oxidized into cations [1-H(2)O](+) and [2-H(2)O](+) in CH(2)Cl(2). The chemically generated [1-H(2)O](SbF(6))·0.68 H(2)O·0.82CH(2)Cl(2) and [2-H(2)O](SbF(6))·0.3H(2)O·0.85CH(2)Cl(2) were characterized by X-ray diffraction and NIR spectroscopy. Both complexes are paramagnetic species containing a square pyramidal cobalt ion coordinated at the apical position by an exogenous water molecule. They exhibit remarkable NIR bands at 1220 (7370 M(-1) cm(-1)) and 1060 nm (5560 M(-1) cm(-1)), respectively, assigned to a CT transition. DFT calculations and magnetic measurements confirm the paramagnetic (S = 1) ground spin state of the cations. They show that more than 70% of the total spin density in [1-H(2)O](+) and [2-H(2)O](+) is localized on the metal, the remaining spin density being distributed over the aromatic rings (30% phenoxyl character). In the presence of N-methylimidazole 1 and 2 are irreversibly oxidized by air into the genuine octahedral cobalt(III) bis(phenolate) complexes [1-im(2)](+) and [2-im(2)](+), the former being structurally characterized. Neither [1-im(2)](+) nor [2-im(2)](+) exhibits a NIR feature in its electronic spectrum. 1 and 2 were electrochemically two-electron oxidized into [1](2+) and [2](2+). The cations were identified as Co(III)-phenoxyl species by their characteristic absorption band at ca. 400 nm in the UV-vis spectrum. Coordination of the phenoxyl radical to the cobalt(III) metal ion is evidenced by the EPR signal centered at g = 2.00.

Using the dinucleating phenol-based ligand 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol] (HL(2)), in its deprotonated form, the six new dinuclear complexes [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][PF(6)] (M = Mn (2a), Co (3a), Zn (4a)) and [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][BPh(4)] (M = Mn (2b), Co (3b), Zn (4b)) have been synthesized. Crystallographic analyses on 2b·2MeCN, 3b·2MeCN, and 4b·2MeCN reveal that these complexes have closely similar μ-phenoxo bis(μ-carboxylato) structures. The physicochemical properties (absorption and ESI-MS spectral data, 2a,b, 3a,b, and 4a,b; (1)H NMR, 4a,b) of the cations of 2a-4a are identical with those of 2b-4b. Each metal ion is terminally coordinated by a pyrazole nitrogen and a pyridyl nitrogen from a 3-(pyridin-2-yl)pyrazole unit and a solvent molecule (MeCN). Thus, each metal center assumes distorted-octahedral M(II)N(3)O(3) coordination. Temperature-dependent magnetic studies on Mn(II) and Co(II) dimers reveal the presence of intramolecular antiferromagnetic (J = -8.5 cm(-1)) for 2b and ferromagnetic exchange coupling (J = +2.51 cm(-1)) for 3b, on the basis of the Hamiltonian H = -JS(1)·S(2). The exchange mechanism is discussed on the basis of magneto-structural parameters (M···M distance). Spectroscopic properties of the complexes have also been investigated. The pH titration and kinetics of phosphatase (transesterification) activity on 2-hydroxypropyl-p-nirophenylphosphate (HPNP) were studied in MeOH/H(2)O (33%, v/v) with 2a-4a, due to solubility reasons. This comparative kinetic study revealed the effect of the metal ion on the rate of hydrolysis of HPNP, which has been compared with what we recently reported for [Ni(II)(2)(L(2))(μ-O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (1a). The efficacy in the order of conversion of substrate to product (p-nitrophenolate ion) follows the order 4a > 3a > 2a > 1a, under identical experimental conditions. Notably, this trend follows the decrease of pK(a) values of M(II)-coordinated water (7.95 ± 0.04 and 8.78 ± 0.03 for 1a, 7.67 ± 0.08 and 8.69 ± 0.06 for 2a, 7.09 ± 0.05 and 8.05 ± 0.06 for 3a, and 6.20 ± 0.04 and 6.80 ± 0.03 for 4a). In this work we demonstrate that the stronger the Lewis acidity (Z(eff)/r) of the metal ion, the more acidic is the M(II)-coordinated water and the greater is the propensity of the metal ion to catalyze hydrolysis of the activated phosphate ester HPNP. Notably, the observed k(2) values (M(-1) s(-1)) for Mn(II) (2a, 0.152), Co(II) (3a, 0.208), and Zn(II) (4a, 0.230) complexes (1a, 0.058; already reported) linearly correlate with Z(eff)/r values of the metal ion. In each case a pseudo-first-order kinetic treatment has been done. Kinetic data analysis of complexes 2a-4a were also done following Michaelis-Menten treatment (catalytic efficiency k(cat)/K(M) values 0.170 M(-1) s(-1) for 2a, 0.194 M(-1) s(-1) for 3a and 0.161 M(-1) s(-1) for 4a; for 1a the value is 0.089 M(-1) s(-1)). Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transeste...

The ligands N,N′-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-ethanediamine and N,N′-bis(3-tert-butyl-5-methoxysalicylidene)-1,3-propanediamine were chelated to V(IV)O (1, 2), Cu(II) (3, 4), Co(II) (5), and Co(III) (6). The Xray crystal structures of 1−6 were solved. The vanadium center in 1−2 resides in square pyramidal geometry, with an axially bound oxo ligand, whereas the metal ion displays a tetrahedrally distorted square planar geometry in 3−5. The extent of distortion is correlated to the length of the diamine spacer: The longer the linker, the larger the tetrahedral distortions. Complex 6 is octahedral with a bidentate acetate molecule that completes the coordination sphere. All the complexes were characterized by UV−vis and EPR spectroscopies, as well as DFT calculations and electrochemistry. Complexes 1−6 exhibit a reversible one-electron oxidation wave in the range −0.11− 0.26 V vs Fc + /Fc. The cations 1 + and 2 + were structurally characterized, showing an octahedral V(V) ion with one oxo and one water molecule coordinated in axial positions. Their vis−NIR spectra are dominated by a band at 727 and 815 nm, respectively, which is assigned to a phenolate-tovanadium(V) charge transfer (CT) transition. The crystal structures of 3 + and 4 + are congruent with Cu(II)-radical species, wherein the metal center remains four-coordinated. Both feature a Class II (Robin-Day classification scale) IVCT transition at around 1200 nm (ε > 1 mM cm −1 ), indicative of partial localization of the radical. The structure of 5 + displays a square pyramidal cobalt ion, where the fifth (axial) coordination is occupied by a water molecule. It displays a NIR feature at 1244 nm and is described as intermediate between high spin Co(III) and Co(II) radical. In the presence of acetate the dimer [( 5) 2 (μ-OAc)] + forms, which was structurally characterized and shows a blue shift and lowering in intensity of the NIR absorption band in comparison to 5 + . Complex 6 + is a genuine Co(III) radical complex, wherein the phenoxyl moiety is localized on one side of the molecule.