Robert M. Buchanan

New tripodal Cu(II) complexes containing imidazole ligands

Abstract A new class of imidazole containing tripodal ligands has been synthesized containing from 0–3 imidazole pendants. Copper(II) complexes of the formulas [Cu(L)Cl] + and [Cu(L)(1-MeIm)] 2+ have been prepared and studied by UV-Vis and EPR spectroscopies and cyclic voltammetry. The X-ray crystal structures of [Cu(bipa)Cl] + and [Cu(tmima)Cl] + have been determined. Electrochemical and electronic spectral data suggest that the donor strengths of the tripods decrease in the following order: tpa > bpia > bipa > tmima. Results from EPR suggest that the complexes with tmima are more distorted than those of tpa.

Synthesis and properties of a binuclear (μ-oxo) diiron(III) complex containing a tripodal polybenzimidazole ligand

Several iron(III) complexes of the polybenzimidazole ligand tris((benzimidazol-2-yl)methyl)amine(ntb) have been synthesized and characterized. The μ-oxo bridged dinuclear complex, [Fe 2 O(ntb) 2 Cl 2 ](PF 6 ) 2 ·4THF, has been isolated and structurally characterized by X-ray crystallography. The complex crystallizes in the triclinic space group P with one complex molecule per unit cell. The cell dimensions are a =13.758(3), b =13.913(2), c =11.526(1) A, α=110.52(1), β=90.56(2), γ=60.65(2)° and V =1765.2 A 3 . The structure refined to a final agreement factor ( R ) of 0.057 using 5092 reflections with 1>3σ( I ). The complex is a centrosymmetric dimer and the coordination environment around each iron(III) center is pseudooctahedral. One benzimidazole pendant is bonded trans to the bridging oxo group, while the other benzimidazole pendants, the tertiary amine nitrogen atom and Cl − ion are bonded cis to the oxo ligand. The FeOFe bridging angle is 180°, as required by the crystallographically imposed center of symmetry. The FeFe separation is 3.610(1) A. A mononuclear form of complex 1 , [Fe(ntb)Cl 2 ](PF 6 ) 0.5 (Cl) 0.5 ( 2 ), has been prepared and shown to form 1 in aqueous ethanol solutions containing NEt 4 Cl. Compounds 1 and 2 can be used to prepare compound 3 which contains a μ-oxo-μ-acetato bridged core structure, [Fe 2 O(ntb) 2 (OAc)](ClO 4 ) 3 . Compound 3 was recrystallized from an acetonitrile solution containing THF. The dinuclear complex 1 exhibits antiferromagnetic exchange with a J value of −100 cm −1 . The room temperature solution effective magnetic moments for compounds 1 (μ eff =2.0) and 3 (μ eff =1.85 BM) are consistent with antiferromagnetically coupled diiron(III) complexes and indicate that the μ-oxo bridged structure is retained in solution. The room temperature effective magnetic moment of compound 2 is 5.9 BM. The electronic spectra of compounds 1–3 are similar to those observed for related complexes. The cyclic voltammetry of compound 1 indicates that the dinuclear complex can stabilize redox levels higher than +3. Compound 2 exhibits a single one electron redox process at E 1/2 =0.468 V NHE. The proton NMR spectra of compounds 1 and 3 confirm the stability of the μ-oxo bridged structures in solution. Inequivalent benzimidazole NH proton signals are observed for compounds 1 and 3 , consistent with the X-ray crystal structure of 1 .

Synthesis, structure and properties of a N3 tridentate bis-imidazolyl ligand with copper(II)

The synthesis of a new tridentate ligand, bis[2-(1-methylimidazolyl)methyl] amine, [B-MIMA], is described. The compound has been characterized by 1 H and 13 C NMR spectroscopy and mass spectrometry, and forms complexes with various copper(II) salts. Crystallographic studies have been completed on the blue product from the reaction of B-MIMA with copper(II) acetate, [Cu(B-MIMA)(CH 3 COO)](ClO 4 ) ( 6 ). Crystal data for 6 are: triclinic, P 1, a = 5.176(2), b = 8.489(3), c = 10.066(3) A, α = 95.09(5), β = 95.72(3), γ = 102.37(4), V = 427.1 A and Z = 1. The structure was solved using the Patterson method which revealed the position of the copper atom and successive difference Fourier syntheses which revealed the location of the remaining atoms. The atom positions were refined by least-squares methods producing a final agreement factor of R = 0.021 and R w = 0.029 for 1897 observed reflections. The structure consists of a polymeric acetate bridged network of distorted square-pyramidal copper(II) ions. The five coordinate geometry around each metal ion is a result of the bonding of three nitrogen atoms from the B-MIMA ligand and two oxygen atoms from different acetate ions. The acetate ions act as bidentate ligands bridging different copper ions and the resulting extended bridging network extends out the crystallograhic a axis. Spectroscopic, magnetic and electrochemical properties of the complex are described and compared with related compounds. The solution spectroscopic properties of 6 suggest that the square-pyramidal geometry of the metal ion in the solid-state is not retained in solution.

Synthesis and sulfur oxygenation of a (N3S)Ni complex related to nickel-containing superoxide dismutase.

A nickel(II) thiolate complex incorporating three N-donor types (amino, amido, and imidazole) has been synthesized and characterized. The (N(3)S)Ni complex, [N-{2-[(2-mercapto-2-methylpropyl)amino]ethyl}-1-methylimidazole-2-carboxamido]nickel(II) (1), is stable in the presence of O(2) but readily forms the sulfinato (RSO(2)(-)) derivative 2 upon the addition of H(2)O(2). Electrochemical investigations of 1 reveal an irreversible sulfur-based oxidation at +0.17 V vs Fc(+)/Fc (200 mV/s) that shifts to +0.81 V upon oxidation to 2. Density functional theory investigations of 1 reveal a highest occupied molecular orbital that is predominantly sulfur-based, consistent with the observed sulfur-based oxidation and O(2) stability.

Biomimetic oxidation studies. 10. Cyclohexane oxidation reactions with active site methane monooxygenase enzyme models and t-butyl hydroperoxide in aqueous micelles: Mechanistic insights and the role of t-butoxy radicals in the CH functionalization reaction

Abstract The oxidation of cyclohexane (CyH) in an aqueous micelle system with t -butyl hydroperoxide (TBHP) in the presence of biomimetic methane monooxygenase enzyme complexes, [Fe 2 O( η 1 -H 2 O)( η 1 -OAc)(TPA) 2 ] 3+ , 1 , [Fe 2 O( η 1 -H 2 O)( η 1 -OAc)(BPIA) 2 ] 3+ , 2 , and O 2 , was studied and found to provide cyclohexanol (CyOH), cyclohexanone (CyONE), and cyclohexyl- t -butyl peroxide (CyOO t -Bu). The mechanistic aspects of this oxidation reaction in aqueous micelles were studied and included the effects of the surfactant concentration, cetyltrimethylammonium hydrosulfate; concentration of CyH and TBHP; and a trapping reagent, CCl 4 . Several factors allowed us to conclude that a t -butoxy radical ( t -BuO) was generated from the favorable redox chemistry of the biomimetic complexes with TBHP, and was responsible for the free radical initiation process with CyH in the aqueous micelle system.

Synthesis, crystal structure and properties of (1,3-(bis(N-methylimidazolimine)propan-2-ol)Cu(II)•perchlorate

Abstract The synthesis, crystal structure and physical properties of a mononuclear Cu(II) complex of 1,3-bis(N-methylimidazolimine)propan-2-ol is described. The complex crystallizes in the triclinic space group P 1 with a = 11.343(5), b = 11.770(4), c = 8.452(2) A, α = 109.71(2)°, β = 91.25(2)°, γ = 98.24(3)°, V = 1048.4 A3 and Z = 2. The structure of the complex is a distorted square pyramid consisting of the N4 basal plane resulting from coordination of the 1,3-bis(N-methylimidazolimine)propan-2-ol ligand and a weak coordinated apical water molecule. The complex displays quasireversible electrochemical behavior in both aqueous and acetonitrile solutions. In addition, the complex has been studied by UV-Vis and EPR spectroscopies.

Beyond Metal-Hydrides: Non-Transition-Metal and Metal-Free Ligand-Centered Electrocatalytic Hydrogen Evolution and Hydrogen Oxidation

A new pathway for homogeneous electrocatalytic H2 evolution and H2 oxidation has been developed using a redox active thiosemicarbazone and its zinc complex as seminal metal-free and transition-metal-free examples. Diacetyl-bis(N-4-methyl-3-thiosemicarbazone) and zinc diacetyl-bis(N-4-methyl-3-thiosemicarbazide) display the highest reported TOFs of any homogeneous ligand-centered H2 evolution catalyst, 1320 and 1170 s(-1), respectively, while the zinc complex also displays one of the highest reported TOF values for H2 oxidation, 72 s(-1), of any homogeneous catalyst. Catalysis proceeds via ligand-centered proton-transfer and electron-transfer events while avoiding traditional metal-hydride intermediates. The unique mechanism is consistent with electrochemical results and is further supported by density functional theory. The results identify a new direction for the design of electrocatalysts for H2 evolution and H2 oxidation that are not reliant on metal-hydride intermediates.

Synthesis, X-ray structure and properties of a new nitrite-bound copper(II) complex with 2-(3,5- dimethylpyrazol-1-ylmethyl)pyridine in a CuN4(O) coordination

Abstract Synthesis and characterization of a nitrite-bound copper(II) compound [CuL4)2(ONO)]ClO4 have been achieved (L4 = 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine]. The bidentate ligand L4 provides a pyridine and a pyrazole donor site; however, they are separated by a methylene spacer. The complex has been structurally characterized and it belongs to only a handful of complexes having nitrito-bound mononuclear copper(II) centre. The metal atom has a distorted square pyramidal geometry with the copper atom displaced from the equatorial plane by 0.25 A. In MeCN solution the green complex exhibits a broad ligand-field transition at 655 nm with a shoulder at 675 nm and in dichloromethane-toluene glass (80 K) it exhibits an EPR spectral feature characteristic of the unpaired electron in the dx2−y2 orbital. Variable-temperature (80–300 K) magnetic susceptibility measurements in the solid state as well as room temperature measurement in MeCN solution reveal mononuclear magnetically dilute copper(II) centre. When examined by cyclic voltammetry (MeCN solution) it displays electrochemically irreversible CuIICuI response [cathodic peak potential, Epc (V vs saturated calomel electrode (SCE)): −0.32]. An oxidative response is observed at 1.14 V, probably due to bound-nitrite oxidation and is partially removed to generate a solvated complex at the electrode surface. The latter species gives rise to reversible CuIICuI redox response [ E 1 2 = 0.41 V vs SCE ].

Reversible methanol addition to copper Schiff base complexes: a kinetic, structural and spectroscopic study of reactions at azomethine CN bonds

The reversible methanolysis of an azomethine C[double bond, length as m-dash]N in a series of copper(ii) Schiff base complexes has been investigated through combined spectroscopic, structural, and kinetic studies. Pentadentate copper(ii) complexes [L1-Cu(X)]Y (L1 = 1,2-bis[(1-methyl-2-imidazolyl)methyleneamino]ethane; X = Y = ClO4- (1); X = Y = TfO- (2); X = Y = BF4- (3); X = H2O, Y = (ClO4-)2 (4) spontaneously add methanol in a ligand centered reaction to yield stable, isolable hemiaminal ether product complexes 5-8. In methanol free solution, 5-8 spontaneously release alcohol to regenerate 1-4. The methanol addition reaction is first-order in methanol and first-order in complex with second-order rate constants varying from 1.1 × 10-4 to 187 × 10-4 M-1 s-1 dependent on the donor ability of the axial ligand. Rate constants for methanol elimination vary from 0.67 to 3.7 × 10-4 s-1 with dependence on the counterion and water content of the solvent. Equilibrium constants for methanolysis range from 1.5 to 51 M-1. Structural comparisons of the Schiff base complexes 1-4 and the hemiaminal ether complexes 5-8 suggest methanol addition is favored by the release of ligand strain associated with three planar five-membered chelates in 1-4.

Binuclear schiff base complexes

Abstract Two new binucleating ligands, pentyl-bis(3-(bae)) and octyl-bis(3-(bae)) and their copper(II) and nickel(II) complexes have been prepared and characterized. The binuclear Cu(II) and Ni(II) complexes display magnetic and spectroscopic behavior characteristic of square-planar monomeric compounds. No magnetic exchange interactions between copper centers was detected at either room or liquid nitrogen temperatures. Both of the Cu(II) and Ni(II) ions in these complexes undergo two quasi-reversible one-electron reductions at nearly the same potentials.