Allan G. Blackman

Copper-Dioxygen and Copper-Oxo Species Relevant to Copper Oxygenases and Oxidases

Copper proteins mediate both the transport and activation of dioxygen in a number of biological systems. The active sites of these proteins comprise mononuclear, dinuclear, and trinuclear copper centers, with the copper ions displaying a variety of coordination numbers and stereochemistries. Information regarding the mechanism by which these proteins activate dioxygen has been obtained by studies of the reactions of small molecule model copper complexes with dioxygen and its derivatives. Superoxo, peroxo, and bis(μ-oxo) intermediates in these reactions have recently been characterized by X-ray crystallography and this article concentrates on the structures of these intermediates, along with several Cu/ O2 complexes that have been well characterized by spectroscopic methods. The oxygenase-type reactivities of a number of copper complexes on reaction with dioxygen are also discussed.

Effect of sulfur-based substituents on the electronic properties of Re(I) dppz complexes.

A series of sulfur-substituted dppz-based ligands and their Re(I)(CO)(3)Cl complexes are reported. The sulfur-substituted ligands and complexes show interesting electronic properties atypical of dppz-type systems. Substitution of dppz with thiocyanate (SCN) groups results in behavior typical of an electron withdrawing group. However, substitution of dppz with the electron donating trithiocarbonate (S(2)CS) or deca-alkylthioether (Sdec) groups confer intraligand charge-transfer (ICT) from the S adduct to the phenazine lowest unoccupied molecular orbital (LUMO). Upon complexation of the substituted dppz ligand to Re(CO)(3)Cl this ICT red-shifts and increases in intensity. Analysis of these observations using density functional theory (DFT) calculations and resonance Raman spectroscopy reveals that these transitions are a mixture of metal-to-ligand charge-transfer (MLCT) and S --> phenazine ICT in nature. The synthesized compounds are also characterized using (1)H NMR spectroscopy, IR spectroscopy, and electrochemistry. Single-crystal X-ray analysis was performed on dppz(SCN)(2) (C(20)H(18)N(6)S(2) a = 8.780 A, b = 9.792 A, c = 10.400 A, alpha = 95.95 degrees , beta = 112.13 degrees , gamma = 95.38 degrees , triclinic, P1, Z = 2, R1 = 0.0306, wR2 = 0.0829.

Complete Family of Mono-, Bi-, and Trinuclear ReI(CO)3Cl Complexes of the Bridging Polypyridyl Ligand 2,3,8,9,14,15-Hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene: Syn/Anti Isomer Separation, Characterization, and Photophysics

The syn and anti isomers of the bi- and trinuclear Re(CO)(3)Cl complexes of 2,3,8,9,14,15-hexamethyl-5,6,11,12,17,18-hexaazatrinapthalene (HATN-Me(6)) are reported. The isomers are characterized by (1)H NMR spectroscopy and X-ray crystallography. The formation of the binuclear complex from the reaction of HATN-Me(6) with 2 equiv of Re(CO)(5)Cl in chloroform results in a 1:1 ratio of the syn and anti isomers. However, synthesis of the trinuclear complex from the reaction of HATN-Me(6) with 3 equiv of Re(CO)(5)Cl in chloroform produces only the anti isomer. syn-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} can be synthesized by reacting 1 equiv of Re(CO)(5)Cl with syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} in refluxing toluene. The product is isolated by subsequent chromatography. The X-ray crystal structures of syn-{(Re(CO)(3)Cl)(2)(μ-HATN-Me(6))} and anti-{(Re(CO)(3)Cl)(3)(μ-HATN-Me(6))} are presented both showing severe distortions of the HATN ligand unit and intermolecular π stacking. The complexes show intense absorptions in the visible region, comprising strong π → π* and metal-to-ligand charge-transfer (MLCT) transitions, which are modeled using time-dependent density functional theory (TD-DFT). The energy of the MLCT absorption decreases from mono- to bi- to trinuclear complexes. The first reduction potentials of the complexes become more positive upon binding of subsequent Re(CO)(3)Cl fragments, consistent with changes in the energy of the MLCT bands and lowering of the energy of relevant lowest unoccupied molecular orbitals, and this is supported by TD-DFT. The nature of the excited states of all of the complexes is also studied using both resonance Raman and picosecond time-resolved IR spectroscopy, where it is shown that MLCT excitation results in the oxidation of one rhenium center. The patterns of the shifts in the carbonyl bands upon excitation reveal that the MLCT state is localized on one rhenium center on the IR time scale.

Reactions of Coordinated Ligands

Publisher Summary This chapter reviews the reactions of transition metal-bound aromatic heterocycles, focusing on the ways in which metal coordination affects the reactivity of the bound heterocycle in comparison to that of the free ligand. The coordination of a ligand to a metal ion has long been known to alter its chemical properties. The coordination of a ligand to a transition metal perturbs both the d -orbitals of the metal and the molecular orbitals of the ligand via a combination of electronic and steric effects, and these interactions can significantly alter the reactivity of the bound ligand relative to its free counterpart. The coordination to a metal ion also introduces steric constraints that are not present in the free ligand. Consequently, ligand atoms adjacent to the metal ion are sterically hindered and reactivity at these sites may be reduced or indeed totally eliminated.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)]n+ Complexes in Aqueous Solution

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

Trinuclear copper(I) complex containing 3,4,9,10,15,16-hexamethyl-1,6,7,12,13,18-hexaazatrinaphthylene: a structural, spectroscopic, and computational study.

The compound [(Cu(PPh(3))(2))(3)(HATNMe(6))](BF(4))(3) has been synthesized and characterized by X-ray crystallography, resonance Raman spectroscopy, and density functional theory (DFT) calculations. The X-ray structure of solvated [(Cu(PPh(3))(2))(3)(HATNMe(6))](BF(4))(3) [rhombohedral, R3, a = b = 21.6404(4) A, c = 53.188(3) A, alpha = beta = 90 degrees, gamma = 120 degrees] shows that the HATNMe(6) ligand is very slightly twisted. The electronic absorption spectrum of the complex in chloroform shows two bands in the visible region attributed to ligand-centered (LC) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. Time-dependent DFT calculations show good agreement with experiment, with two MLCT and one LC transition predicted in the visible region (641, 540, and 500 nm). Resonance Raman spectra of the complex using discrete excitation energies between 647 and 406 nm showed a variation in enhancement patterns consistent with at least two distinct transitions. The absolute Raman cross sections have been evaluated and, through a wavepacket analysis, the amount of distortion along each vibrational mode across the Franck-Condon surface is established from the calculated dimensionless displacement (Delta) values as well as other electronic parameters. The pattern of Delta values shows good agreement with the observed calculated modes, with the MLCT transition, showing much larger Delta values for outer ring modes such as nu(93) and nu(205) than in the LC transition. This is consistent with the molecular orbitals involved in the two transitions; the donor orbitals for the LC transition have similar outer-ring bonding characteristics compared to the MLCT transition, which has no donor orbital bonding characteristics on the ligand because the donor molecular orbitals are dpi orbitals.

Synthesis, characterization, and X-ray crystal structures of metal complexes with new Schiff-base ligands and their antibacterial activities

Two new potentially hexadentate Schiff bases, [H2L1] and [H2L2], were prepared by condensation of 2-(3-(2-aminophenoxy)naphthalen-2-yloxy)benzenamine with 3,5-di-tert-butyl-2-hydroxy benzaldehyde and o-vanillin, respectively. Reaction of these ligands with cobalt(II) chloride, copper(II) perchlorate, and zinc(II) nitrate gave complexes ML. The ligands and their complexes have been characterized by a variety of physico-chemical techniques. The solid and solution state investigations show that the complexes are neutral. Molecular structures of [CuL1], [CoL1] · C7H8, and [ZnL2] · CH3CN, which have been determined by single-crystal X-ray diffraction, indicate that [CuL1] and [ZnL2] · CH3CN display distorted square planar and distorted trigonal-bipyramidal geometry, respectively; the geometry around cobalt in [CoL1] · C7H8 is almost exactly between trigonal bipyramidal and square pyramidal. The synthesized ligands and their complexes were screened for their antibacterial activities against eight bacterial strains and the ligands and complexes have antibacterial effects. The most effective ones are [CuL2] against Proteus vulgaris, Serratia marcescens, Staphylococcus subtilis, [H2L1] against S. subtilis, and [H2L2] against S. subtilis.

The nature of the [Pt(bipy)2]2+ ion in aqueous alkaline solution: a new look at an old problem

It has been known since 1973 that addition of NaOH(aq) to an aqueous solution of [Pt(bipy)2](NO3)2·H2O gives rise to changes in both the solution UV/Vis and solution 1H and 13C NMR spectra of the complex. These spectral changes have been variously interpreted as arising either from attack of OH− at C-6 of a bipyridine ligand to form a covalent hydrate, or coordination of OH− to the Pt ion to form a higher coordinate complex. In this paper, we utilise 195Pt NMR spectroscopy for the first time to study this system. The broad 195Pt NMR peak due to [Pt(bipy)2]2+ at −2273 ppm decreases in intensity on sequential addition of less than stoichiometric amounts of NaOH(aq) and a significantly narrower peak appears at −2094 ppm in proportion to the amount of NaOH(aq) added. Only the latter peak is observed after addition of one mol equivalent of NaOH(aq). Neither NaCl(aq) nor Na2SO4(aq) affect the 195Pt NMR spectrum of [Pt(bipy)2]2+ but addition of half a mol equivalent of Na2S2O3(aq) gives a peak ∼430 ppm upfield of that due to [Pt(bipy)2]2+. The 195Pt chemical shift of the structurally characterised pseudo-five-coordinate complex [Pt(phen)2(CN)]+ is −2726 ppm. Our results rule out deprotonation of a water molecule weakly coordinated to [Pt(bipy)2]2+ by the added NaOH(aq) and are consistent with formation of a conformationally mobile pseudo-five-coordinate complex.

The donor ability of the chelated carbonate ligand: protonation and metallation of [(L)Co(O2CO)]+ complexes in aqueous solution

The syntheses and X-ray structures of [Co(Me-tpa)O2COZnCl3], [Co(pmea)O2COZnCl3].H2O [Co(trpyn)O2COZn(OH2)4OCO2Co(trpyn)](ZnCl4)2.H2O, [Co(trpyn)(O2COH)]ZnCl(4).3H2O and [Co(trpyn)(O2CO)]ClO4 are reported (Me-tpa=[(6-methyl-2-pyridyl)methyl]bis(2-pyridylmethyl)amine, pmea=bis(2-pyridylmethyl)-2-(2-pyridylethyl)amine, trpyn=tris(2-(1-pyrazolyl)ethyl)amine). The chelated bicarbonate complex [Co(trpyn)(O2COH)]ZnCl(4).3H2O is isolated as a crystalline solid from an acidic solution of the parent carbonate [Co(trpyn)(O2CO)]ClO4, and X-ray structural analysis shows that lengthening of the C=Oexo bond and shortening of the C-Oendo bond accompanies protonation. The bimetallic complex [Co(Me-tpa)O2COZnCl3] results from the unexpected coordination of ZnCl3- to the exo O atom of a chelated carbonate ligand. This complex is obtained from both acidic and neutral solutions in which [Zn2+]=1.0 M, while the structurally similar complex [Co(pmea)O2COZnCl3].H2O is isolated from an analogous neutral solution. The trimetallic complex [Co(trpyn)O2COZn(OH2)4OCO2Co(trpyn)](ZnCl4)2.H2O crystallises on prolonged standing of [Co(trpyn)(O2CO)]ClO4 in a neutral solution having [Zn2+]=1.0 M. The Zn-O bond lengths in all three complexes are indicative of bonds of significant strength. DFT calculations show that the nature of the bonding interaction between the Co(III) ion and the endo O atoms of the carbonate ligand remain essentially unaffected by coordination of Zn2+ to the exo O atom. They also show that such coordination of Zn2+ decreases the C-Oexo bond order.

A computational study of the electronic structure, bonding, and spectral properties of tripodal tetraamine Co(III) carbonate complexes

Density functional calculations have been carried out on the experimentally characterized Co(III) [Co(N4)(O2CO)]+ carbonate complexes containing a tripodal tetraamine ligand (N4 = tpa, Metpa, Me2tpa, Me3tpa, pmea, pmap, tepa) and also the model [Co(NH3)4(O2CO)]+ system. Calculations on the model species, performed using both gas-phase and solvent-corrected procedures, have revealed that the inclusion of a condensed-phase environment is necessary to obtain generally satisfactory results for the structural and bonding properties in these systems. Using the solvent-corrected approach, the observed trends in structural parameters for the metal-ligand bonds, 59Co chemical shifts, and changes in visible absorption wavelengths have been satisfactorily reproduced for the [Co(N4)(O2CO)]+ complexes. A time-dependent density functional analysis of the electronic excitations indicates that the overall composition and character of the relevant (d-d) transitions remain similar throughout the series, indicating that the changes in the Co-N interactions, associated with the structural variations occurring as the N-donor ligand identity and size change, appear most likely responsible for the particular spectroscopic features displayed by these species. These observations are further supported by molecular orbital and energy decomposition analyses. The results from the present calculations confirm recent findings that the inclusion of a treatment for solvent effects plays a critical role in the computational modelling of coordination complexes involving mixed (anionic and neutral) ligands.