Dinuclear Center Research Articles

Ditungsten hexaalkoxides: templates for organometallic chemistry and catalysis

The reactions between W2(OR)6 compounds and carbon monoxide, ethene, acetylene, allene, 1,3-dienes, ketones, aldehydes and eneones are described. The selection of R allows for control of steric access to the (WW)6+ centre and the relative reducing power of the W–W π electrons and Lewis acidity of the W atoms. With CO, W2(OBut)6 gives W2(OBut)6(µ-CO), (CO)= 1575 cm–1, while W2[OCMe2(CF3)]6 yields W2[OCMe2(CF3)]6(CO)2 with (CO)= 2081 and 2050 cm–1. Similarly, whereas MeCN and W2(OBut)6 react rapidly in hydrocarbons to yield products of metathesis (ButO)3 WN and (ButO)3 WCMe, W2[OCMe2(CF3)]6 reversibly forms the Lewis-base adduct W2[OCMe2(CF3)]6(NCMe)2. Allene and W2(OBut)6 react to give a 1:1 adduct having a µ-C–C–C fragment that is parallel to the W–W axis and a 2:1 adduct wherein one allene forms a metallaallyl that is η1 bound to one W atom and η3 to the other. Novel modes of binding to the dinuclear centre lead to novel modes of substrate activation. Examples include the reductive cleavage of ketone and aldehyde C–O bonds which forms the basis of an olefination reaction. Also co-ordination of 1,3-dienes as a µ-η1:η4 carbon fragment to the dinuclear centre facilitates the specific hydrogenation to give 3-enes.

Investigation of the Dinuclear Fe Center of Methane Monooxygenase by Advanced Paramagnetic Resonance Techniques: On the Geometry of DMSO Binding

We report an extensive advanced paramagnetic resonance characterization of the mixed-valence dinuclear Fe center of methane monooxygenase hydroxylase (MMOHmv) from Methylococcus capsulatus (Mc) (Bath) and of binding to it by the exogenous ligand DMSO. We employ continuous wave and pulsed electron nuclear double resonance (ENDOR) spectroscopy, both at Q-band microwave frequencies, to examine 14,15N, 1,2H, 13C, and 57Fe nuclei. Preliminary 1H ENDOR results were communicated previously (DeRose, V. J.; Liu, K. E.; Hoffman, B. M.; Lippard, S. J. J. Am. Chem. Soc. 1993, 115, 6440−6441). ENDOR-derived 14,15N hyperfine tensors are interpreted in terms of the spin distribution on histidyl ligands bound to the dinuclear center. Determination of the 57Fe hyperfine tensors gives a complete picture of the spin-coupled Fe2+ and Fe3+ ions. The 1,2H ENDOR results disclose the presence of a bridging hydroxide and an aqua ligand in both native and DMSO-treated enzyme. A novel procedure for describing the 1H hyperfine tenso...

Identification of the ferroxidase centre of Escherichia coli bacterioferritin

The bacterioferritin (BFR) of Escherichia coli takes up iron in the ferrous form and stores it within its central cavity as a hydrated ferric oxide mineral. The mechanism by which oxidation of iron (II) occurs in BFR is largely unknown, but previous studies indicated that there is ferroxidase activity associated with a site capable of forming a dinuclear-iron centre within each subunit [Le Brun, Wilson, Andrews, Harrison, Guest, Thomson and Moore (1993) FEBS Lett. 333, 197-202]. We now report site-directed mutagenesis experiments based on a putative dinuclear-metal-ion-binding site located within the BFR subunit. The data reveal that this dinuclear-iron centre is located at a site within the four-alpha-helical bundle of each subunit of BFR, thus identified as the ferroxidase centre of BFR. The metal-bound form of the centre bears a remarkable similarity to the dinuclear-iron sites of the hydroxylase subunit of methane mono-oxygenase and the R2 subunit of ribonucleotide reductase. Details of how the dinuclear centre of BFR is involved in the oxidation mechanism were investigated by studying the inhibition of iron (II) oxidation by zinc (II) ions. Data indicate that zinc (II) ions bind at the ferroxidase centre of apo-BFR in preference to iron (II), resulting in a dramatic reduction in the rate of oxidation. The mechanism of iron (II) oxidation is discussed in the light of this and previous work.

Iron(II) oxidation by H chain ferritin: evidence from site-directed mutagenesis that a transient blue species is formed at the dinuclear iron center.

The iron storage molecule, ferritin, consists of an iron core surrounded by a shell of 24 protein subunits, which, in mammals, are of two types, H and L. Prior to storage of iron as a hydrous ferric oxide within the protein shell, Fe(II) is catalytically oxidized at dinuclear centers within H chains. When 48 Fe(II) atoms/molecule were added to 1 microM recombinant human H chain apoferritin (apo-HuHF), in 0.1 M Mes (pH 6.5), oxidation was 80% complete within about 0.2 s while 99% of the Fe(II) was oxidized within 10 s. A broad visible absorption band (400-800 nm, with a maximum at 650 nm) appeared during the fast phase of Fe(II) oxidation. It reached a plateau at 0.2-0.3 s and then declined while Fe(II) oxidation proceeded to completion and absorbance in the near-UV (300-400 nm) increased. The transient visible species was not observed when Tyr-34 was replaced by phenylalanine or when other conserved amino acids at the ferroxidase centers were substituted by residues which are unable to bind iron or which alter the charge balance. When a second increment of 48 iron atoms was added, 10 min after the first, the visible absorbance was absent and the rate of oxidation slower. Restoration of full oxidative activity took over 24 h. The data indicate that the fast oxidation of Fe(II) by apo-HuHF and the transient visible absorbance associated with it are due to Fe(II) oxidation at the ferroxidase centers.

Structure and function of dinuclear metal centres in proteins: optical and magnetic studies

Structure and function of dinuclear metal centres in proteins: optical and magnetic studies

The highly conserved methionine of subunit I of the heme-copper oxidases is not at the heme-copper dinuclear center: Mutagenesis of M110 in subunit I of cytochrome bo3-type ubiquinol oxidase from Escherichia coli

A common feature within the heme-copper oxidase superfamily is the dinuclear heme-copper center. Analysis via extended X-ray absorption fine structure (EXAFS) has led to the proposal that sulfur may be bound to CU B, a component of the dinuclear center, and a highly conserved methionine (M110 in the E. coli oxidase) in subunit I has been proposed as the ligand. Recent models of subunit I, however, suggest that this residue is unlikely to be near CU B, but is predicted to be near the low spin heme component of the heme-copper oxidases. In this paper, the role of M110 is examined by spectroscopic analyses of site-directed mutants of the bo 3-type oxidase from Escherichia coli. The results show that M110 is a non-essential residue and suggest that it is probably not near the heme-copper dinuclear center.

Allene adducts of ditungsten hexaalkoxides. Three modes of allene coordination to dinuclear centers as seen in the structures of W2(OBu-tert)6(C3H4), W2(OBu-tert)6(C3H4)2, and W2(OBu-tert)6(C3H4)(CO)2

Allene adducts of ditungsten hexaalkoxides. Three modes of allene coordination to dinuclear centers as seen in the structures of W2(OBu-tert)6(C3H4), W2(OBu-tert)6(C3H4)2, and W2(OBu-tert)6(C3H4)(CO)2

Cytochrome oxidase: a perspective

Abstract

Reactions of co-ordinated ligands. Part 37. Synthesis and structure of the dimolybdenum µ-allylidene complex [Mo2(µ-σ:η3-CHCHCMe2)(CO)4(η-C5H5)2]; formation and crystal structure of the oxo-µ-allylidene complex [Mo2O(µ-σ:η3-CHCHCMe2)(µ-CO)(CO)(η-C5H5)2]. Thermal rearrangement to alkyne and 1,3-diene dimolybdenum species and structure of [Mo3(µ3-η4-CCHCMeCH)(CO)4(η-C5H5)3

Reaction of the unsaturated dinuclear complex [Mo2(CO)4(η-C5H5)2] with 3,3-dimethylcyclopropene leads to ring opening and formation of the µ-allylidene complex [Mo2(µ-σ,η3-CHCHCMe2)(CO)4–(η-C5H5)2](1), which has been structurally identified by X-ray crystallography. The molecule contains two molybdenum atoms singly bonded [Mo(1)–Mo(2) 3.1 057(9)A]. A CHCHCMe2 fragment bonds σ to Mo(1) and as an allyl, η3, to Mo(2), i.e. as a dimethyl substituted µ-allylidene where the two olefinic protons of the original cyclopropene retain a relative cis configuration. The mechanism of formation of (1) is discussed in terms of a reaction at a dinuclear centre. In refluxing wet acetonitrile (1) is transformed into the mixed-valence µ-allylidene complex [Mo2O(µ-σ,η3-CHCHCMe2)(µ-CO)(CO)(η-C5H5)2](2). An X-ray crystal-structure determination shows that the oxygen ligand is co-ordinated to Mo(1) which is σ bonded to the µ-allylidene fragment; the Mo–Mo bond is reduced to 2.881(1)A. In refluxing toluene–hexane (1) rearranges to form the alkyne complex [Mo2(µ-PriC2H)(CO)4(η-C5H5)2](3), an isoprene complex [Mo2{CH2CHC(Me)CH2}(CO)2(η-C5H5)2](4), and a tinuclear complex [Mo3(µ3-η4-CCHCMeCH)(CO)4(η-C5H5)3](5), the last being identified by X-ray crystallography. The molecule is a triangular array of three molybdenum atoms, which show typical Mo–Mo single bond separations. The µ-allylidene fragment present in (1) loses three hydrogen atoms in the formation of (5), generating a methyl substituted molybdacyclopentadiene. The mechanism of formation of (3), (4), and (5) is discussed.

Interconverting multiple bonds between molybdenum and tungsten atoms: oxidative-additions and reductive-eliminations from dinuclear centers

Analogies between the chemistry of mononuclear group 8, 9 and 10 transition elements and the chemistry of dinuclear molybdenum and tungsten compounds are noted. Just as the d 8- and d 6-configurations determine preferred coordination numbers, geometries, substitution patterns and the propensity for oxidative-addition or reductive-elimination, so does the MM bonding configuration influence the coordination chemistry and reactivity of dinuclear compounds. The presence of two metal atoms united by a multiple bond provides an electron source or sink for multi-electron redox reactions and a template for substrate assembly and coupling. The presence of two metal centers poses fascinating mechanistic questions concerning cooperative interactions between the metal atoms and the detailed reaction profile for even simple addition-elimination sequences. These general principles are illustrated in studies of the formation of M-M quadruple bonds by reductive-elimination from (MM) 6+ centers and oxidative-additions leading to (MM) 8+ and (M|M) 10+ centers.

Structure of 1,1,2-tricarbonyl-μ-(dimethyl 2-butynedioate-μ-C2,μ-C3)-(2-3-η-dimethyl 2-butynedioate)-μ-(1–5:1'–5'-η-fulvalene)-dimolybdenum(Mo–Mo), [Mo2(C10H8)(C6H6O4)2(CO)3

M r=687.9, monoclinic, P2Jc, a= 9.6984 (6), b= 15.4379 (21), c= 16.4863 (17)A, fl = 95.880 (7) °, V= 2455.4 (8) A 3, Z = 4, D x = 1.86gem -3, 2(MoKa)=0.7107A, g= 10.6cm -1, F(000)= 1368, T= 298 K, final R = 0.022, wR = 0.029 for 2692 (I > 3e(/)) unique reflections. Impor- tant features include the lack of multiple alkyne coupling, a contracted Mo-Mo distance of 2.906 (1)A, a bridging (4e--donating) alkyne C-C distance of 1.374 (4)A and a singly coordinating (2e--donating) alkyne C-C distance of 1.265 (4)A. The bridging alkyne is slightly displaced towards the singly coordinating alkyne ligand, in the more sterically hindered direction. Introduction. The unusual structural constraints im- posed by a fulvalene ligand anchored to a molybdenum dinuclear center should result in interesting structural and chemical properties relative to analogous r/5- CsHsMo dimers. The multiple alkyne coupling in r/5-CsHsMo dimers (Beck, Knox, Stansfield, Stone, Winter & Woodward, 1982; Green, Norman & Orpen, 1981; Knox, Stansfield, Stone, Winter & Woodward, 1978) is nearly always present with rare exceptions