Gerhard Peters

The First Catalytic Tyrosinase Model System Based on a Mononuclear Copper(I) Complex: Kinetics and Mechanism†

Tyrosinase is a ubiquitous enzyme mediating the o-hydroxylation of monophenols to catechols and the subsequent twoelectron oxidation to quinones. Its physiological function is the conversion of tyrosine into dopaquinone, which constitutes the first step of melanine synthesis. 3] As evident from spectroscopic data and a recent X-ray crystal-structure determination, the active site of tyrosinase contains a binuclear copper center coordinated by six histidine residues. This type of active site (type 3) is analogous to the active sites of hemocyanin and catechol oxidase. All of these copper proteins in their oxy states bind dioxygen in a characteristic side-on bridging structure whereby the Cu centers in the deoxy states are converted into Cu in the oxy state. Starting from the oxy state of tyrosinase, monophenols are converted into o-diphenols (monophenolase reaction), presumably in an electrophilic substitution reaction. Subsequently, the odiphenol (catechol) intermediates are converted into o-quinones. A second reactivity of the oxy site is the two-electron oxidation of external catechol substrates to o-quinones (diphenolase reactivity). Whereas tyrosinase exhibits both monoand diphenolase reactions, the enzymatic activity of catechol oxidase is restricted to diphenolase reactivity. The reactivity of tyrosinase towards phenolic or aromatic substrates has successfully been reproduced with smallmolecule copper complexes. Whereas initially mainly aromatic hydroxylation reactions of the ligand framework by m-h:h-peroxo and bis-m-oxo dicopper cores were investigated, the hydroxylation of external monoand diphenolic substrates has more and more shifted into the focus of interest. An additional challenge in this research area has been the synthesis of catalytic tyrosinase-model systems. Apart from a patent, however, there are only two reports referring to this point. In 1990, R glier et al. synthesized the ligand bis-2,2’-[2-(pyrid-2-yl)ethyl]iminobiphenyl (BiPh(impy)2) which contains two pyridylethylimine sidearms bridged by a biphenyl spacer (Scheme 1). 26] It was shown

Spectroscopic Characterization of Molybdenum Dinitrogen Complexes Containing a Combination of Di-and Triphosphine Coligands : 31P NMR Analysis of Five-Spin Systems

The three molybdenum-N2 complexes [Mo(N2)(dpepp)(depe)] (1), [Mo(N2)(dpepp)(dppe)] (2), and [Mo(N2)(dpepp)(1,2-dppp)] (3), all of which contain a combination of a bi- and a tridentate phosphine ligand, were prepared and investigated by vibrational and (31)P NMR spectroscopy. As a tridentate ligand bis(2-diphenylphosphinoethyl)phenylphosphine (dpepp) has been employed. The three different bidentate ligands are 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis(diphenylphosphino)ethane (dppe), and R-(+)-1,2-bis(diphenylphosphino)propane (1,2-dppp). N-N as well as metal-N vibrations of 1-3 are identified and interpreted in terms of the geometric and electronic structures of the complexes. (31)P NMR spectra are recorded and fully analyzed. Moreover, correlation spectroscopy (COSY)-45 measurements are performed to determine the relative signs of coupling constants. Special attention is directed to a detection of different isomers and their (31)P NMR, as well as vibrational spectroscopic properties. The implications of the results for the area of synthetic nitrogen fixation with phosphine complexes are discussed.

The Hexahydro‐closo‐hexaborate Dianion [B6H6]2– and Its Derivatives

[B6H6]2– as the smallest known hydro-closo-borate is only moderately stable, and therefore its reaction chemistry remained unexplored for quite a long time. Since the mid 1980s some progress has been made in this field, and a considerable number of derivatives has been prepared. The provided structural and spectroscopic data together with the results of more sophisticated theoretical investigations by high level ab initio methods are a good basis for a systematic review on this class of compounds, and may encourage further synthetic and theoretical work. This is the aim of the present article giving a comprehensive survey up to the beginning of 1999.

Synthetic routes to the octahalogenoditechnetates [Tc2X8]2−,3− (X = Cl, Br), and normal coordinate analyses on the metal-metal multiple bond systems [Re2X8]2− (X = F, Cl, Br, I), [Tc2X8]2−,3− (X = Cl, Br)(D4h), and [Os2X8]2− (X = Cl, Br, I)(D4d)

Quick, facile, and high-yield syntheses of di- and trinegative octachloro- and octabromoditechnetates by reduction of tetrahalogenooxotechnetates(V) or hexahalogenotechnetates(IV) with tetrahydridoborate in organic solvents are described. The compounds are easily convertible by ligand exchange reactions. Under exclusion of air the octahaloditechnetates(lII, III) are stable in organic solvents and unstable in conc. hydrohalic acids, in contrast to the octahaloditechnetates(II, III) being stable in conc. hydrohalic acids and unstable in organic solvents. Based on a general valence force field, normal coordinate analyses on the metal-metal multiple bond systems [Re2X8]2− (X = F, Cl, Br, I), [Tc2X8]2−,3− (X = Cl, Br)(D4h), including35Cl and37Cl labeled species, and [Os2X8]2− (X = Cl, Br, I)(D4d) have been performed. With sets of seven or ten force constants very good agreement between observed and calculated frequencies has been achieved. The metal-metal valence force constants range from 2.7 to 4.9 mdyne/Å, the strength of the bonds depending on the bond order and the masses of the ligands. The bromo compounds in particular exhibit a strong vibrational coupling of the metal-metal stretching mode with the symmetric breathing vibration of the ligand sphere.

Alkyl derivatives of [B6H6]2−: NMR and vibrational spectra and crystal structure of (Ph4P)[B6H6CH2Ph]

Abstract In the reaction of [B6H6]2− with benzyl bromide in dichloromethane the protonated anion [B6H5HfacCH2Ph]− is formed. The crystal structure of (Ph4P)[B6H5HfacCH2Ph] has been determined by single crystal diffraction analysis: monoclinic, space group P21/c with a=13.795(5), b=11.133(5), c=19.231(5) A, β=110 659(5)°, Z=4, R=0.056. Typical distances of 1.59, 1.73 and 1.13 A are averaged out for B–C, B–B and B–H, respectively. One of the four facets, which shares the ipso boron atom, is significantly expanded suggesting that the additional proton Hfac is fixed above this facet by a 2e4c bond in the solid state. In solution and at room temperature the proton is fluctuating across the four ipso facets as shown by variable temperature 11B NMR spectroscopy, and the point of coalescence for its fixation is found at 234 K in dichloromethane solution in a field of 9.4 T. A second point of coalescence according to the fixation of the benzyl group is observed at 225 K. Additionally, a nuclear Overhauser interaction between Hfac and the alkyl H atoms has been proven by 1H–1H NOESY below the first coalescence temperature. All data from vibrational and 1H, 11B, 13C NMR spectroscopy are consistent with the structural features.

Molybdenum 17- and 18-Electron Bis- and Tris(Butadiene) Complexes: Electronic Structures, Spectroscopic Properties, and Oxidative Ligand Substitution Reactions

New results on the electronic structures, spectroscopic properties, and reactivities of the molybdenum tris(butadiene) and tris(2,3-dimethylbutadiene) complexes [Mo(bd)3] (1(bd)) and [Mo(dmbd)3] (1(dmbd)), respectively, are reported. Importantly, the metal ligand bonding interaction can be weakened by oxidizing the metal center with ferrocenium salts. The addition of the bidentate phosphine ligand 1,2-bis(diphenylphosphino)ethane then leads to a new type of stable 17-electron complex, [Mo(dmbd)2(dppe)](X) (2; X = BF4(-), PF6(-), BPh4(-)), where one of the butadiene ligands is exchanged by a chelating phosphine. Reduction of the cationic complexes 2 generates the corresponding 18-electron complex [Mo(dmbd)2(dppe)] (3), thus establishing a new strategy for ligand substitution reactions in [Mo(bd)3] complexes via one-electron oxidized intermediates. The new heteroleptic molybdenum complexes are characterized by X-ray structure analysis; vibrational, NMR, and EPR spectroscopy; and electrochemistry. DFT calculations are performed to explain the structural and specroscopic trends observed experimentally. For compound 1(bd), a normal coordinate analysis is presented, providing additional information on the bonding situation in this type of complex.