The metalloclusters of carbon monoxide dehydrogenase/acetyl-CoA synthase: a story in pictures

Eight Ni proteins are known and three of these, CO dehydrogenase (CODH), acetyl-CoA synthase (ACS), and hydrogenase, are Ni-Fe-S proteins. In the last three years, the long-awaited structures of CODH and ACS have been solved. The bioinorganic community was shocked, as the structures of the active sites of CODH and ACS, the C- and A-cluster, respectively, which each had been predicted to consist of a [Fe₄S₄] cluster bridged to a single Ni, revealed unexpected compositions and arrangements. Crystal structures of ACS revealed major differences in protein conformation and in A-cluster composition; for example, a [Fe₄S₄] cluster bridged to a binuclear center in which one of the metal binding sites was occupied by Ni, Cu, or Zn. Recent studies have revealed Ni-Ni to be the active state, unveiled the source of the heterogeneity that had plagued studies of CODH/ACS for decades, and produced a metal-replacement strategy to generate highly active and nearly homogeneous enzyme.Abbreviations CFeSP corrinoid iron-sulfur protein - CH₃H₄folate methyltetrahydrofolate - CODH/ACS carbon monoxide dehydrogenase/acetyl-CoA synthases - ENDOR electron nuclear double resonance - MeTr methyltransferase     

Structural Assignment of Spectra by Characterization of Conformational Substates in Bound MbCO

Residue motions of the distal heme pocket and bound CO ligand of carbonmonoxy Myoglobin are studied using a combination of molecular dynamics simulations and quantum chemical methods. Using mixed quantum mechanics/molecular mechanics calculations together with sampling from molecular dynamics simulations (QM/MM(MD)), the experimentally observed spectroscopic A₀ and A₁ substates of the bound CO ligand are assigned to the open and closed conformation of His⁶⁴ and the His_ɛ⁶⁴ tautomer, respectively. Several previously proposed origins of the A₃ substate, including rotamers of the doubly protonated His⁶⁴H⁺ side chain, His⁶⁴H⁺ inside the distal pocket, and cooperative motions with Arg⁴⁵, are investigated with QM/MM(MD). However, the signatures of the calculated infrared spectra do not agree with the experimentally observed ones. For additional insight on this, extensive molecular dynamics simulations are used together with improved electrostatics for the bound ligand. A CO fluctuating charge model is developed to describe the ab initio dipole and quadrupole moments of the bound ligand. CO absorption spectra are then obtained directly from the dynamics simulations. Finally, the electrostatics of the heme pocket is examined in detail in an attempt to determine the structural origins of the observed spectroscopic A-states from MD simulations. However, contrary to related simulations for unbound CO in myoglobin, the shifts and splittings for carbonmonoxy Myoglobin are generally small and difficult to relate to structural change. This suggests that coupling of the CO motion to other degrees of freedom, such as the Fe-CO stretching and bending, is important to correctly describe the dynamics of bound CO in myoglobin.     

Acetyl-coenzyme A synthase: the case for a Ni<Subscript>p</Subscript><Superscript>0</Superscript>-based mechanism of catalysis

Acetyl-CoA synthase (also known as carbon monoxide dehydrogenase) is a bifunctional Ni-Fe-S-containing enzyme that catalyzes the reversible reduction of CO₂ to CO and the synthesis of acetyl-coenzyme A from CO, CoA, and a methyl group donated by a corrinoid iron-sulfur protein. The active site for the latter reaction, called the A-cluster, consists of an Fe₄S₄ cubane bridged to the proximal Ni site (Ni_p), which is bridged in turn to the so-called distal Ni site. In this review, evidence is presented that Ni_p achieves a zero-valent state at low potentials and during catalysis. Ni_p appears to be the metal to which CO and methyl groups bind and then react to form an acetyl-Ni_p intermediate. Methyl group binding requires reductive activation, where two electrons reduce some site on the A-cluster. The coordination environment of the distal Ni suggests that it could not be stabilized in redox states lower than 2+. The rate at which the [Fe₄S₄]²⁺ cubane is reduced is far slower than that at which reductive activation occurs, suggesting that the cubane is not the site of reduction. An intriguing possibility is that Ni_p²⁺ might be reduced to the zero-valent state. Reinforcing this idea are Ni-organometallic complexes in which the Ni exhibits analogous reactivity properties when reduced to the zero-valent state. A zero-valent Ni stabilized exclusively with biological ligands would be remarkable and unprecedented in biology.Electronic Supplementary Material Supplementary Material is available in the online version of this article at     

Preparation,properties and crystal structures of nickel(II) complexes with acyclic schiff bases derived from 2,6-diformyl-4-chlorophenol and 1,5-diamino-3-thiapentane or 3,3′-diamino-N-methyl-dipropylamine

Nickel(II) complexes with the compartmental Schiff bases derived from 2,6-diformyl-4-chlorophenol and 1,5-diamino-3-thiapentane (H₂L¹) or 3,3′-diamino-N-methyl-dipropylamine (H₂L²) were synthesized, and the crystal structures of [Ni(L¹)- (py)₂] and [Ni(L²)(dmf)]·H₂0 were determined by X-ray crystallography.Ni(L¹)(py)₂ is monoclinic, space group C2/c, with a= 18.457(6), b = 11.116(7), c= 16.098(6) Å, and β = 115.79(5)°; D_c = 1.49 g cm⁻³ for Z = 4. The structure was refined to the final R of 6.9%. The molecule has C₂ symmetry. The nickel atom is six-coordinated octahedral. Selected bond lengths are: NiO 2.04(1) Å, NiN (L¹) 2.08(1) Å, NiN(py) 2.17(1) Å.[Ni(L²)(dmf)]·H₂O is monoclinic, space group P2₁/n, with a = 17.329(6), b = 13.322(7), c = 12.476(7) Å and β = 95.43(5)°; D_c = 1.45 g cm⁻³ for Z = 4. The structure was refined to the final R of 5.1%. The nickel atom is bonded in the octahedral geometry to the bianionic pentadentate ligand L² and to one molecule of dimethylformamide. Selected bond lengths are: NiO (charged) 2.063(3) Å (mean value), NiO (neutral) 2.120(3) Å, NiN (planar) 2.050(3) Å (mean value), NiN (tetrahedral) 2.177(3) Å.     

Evidence that NiNi acetyl-CoA synthase is active and that the CuNi enzyme is not

The bifunctional CO dehydrogenase/acetyl-CoA synthase (CODH/ACS) plays a central role in the Wood-Ljungdahl pathway of autotrophic CO(2) fixation. One structure of the Moorella thermoacetica enzyme revealed that the active site of ACS (the A-cluster) consists of a [4Fe-4S] cluster bridged to a binuclear CuNi center with Cu at the proximal metal site (M(p)) and Ni at the distal metal site (M(d)). In another structure of the same enzyme, Ni or Zn was present at M(p). On the basis of a positive correlation between ACS activity and Cu content, we had proposed that the Cu-containing enzyme is active [Seravalli, J., et al. (2003) Proc. Natl. Acad. Sci. U.S.A. 100, 3689-3694]. Here we have reexamined this proposal. Enzyme preparations with a wider range of Ni (1.6-2.8) and Cu (0.2-1.1) stoichiometries per dimer were studied to reexamine the correlation, if any, between the Ni and Cu content and ACS activity. In addition, the effects of o-phenanthroline (which removes Ni but not Cu) and neocuproine (which removes Cu but not Ni) on ACS activity were determined. EXAFS results indicate that these chelators selectively remove M(p). Multifrequency EPR spectra (3-130 GHz) of the paramagnetic NiFeC state of the A-cluster were examined to investigate the electronic state of this proposed intermediate in the ACS reaction mechanism. The combined results strongly indicate that the CuNi enzyme is inactive, that the NiNi enzyme is active, and that the NiNi enzyme is responsible for the NiFeC EPR signal. The results also support an electronic structure of the NiFeC-eliciting species as a [4Fe-4S](2+) (net S = 0) cluster bridged to a Ni(1+) (S = (1)/(2)) at M(p) that is bridged to planar four-coordinate Ni(2+) (S = 0) at M(d), with the spin predominantly on the Ni(1+). Furthermore, these studies suggest that M(p) is inserted during cell growth. The apparent vulnerability of the proximal metal site in the A-cluster to substitution with different metals appears to underlie the heterogeneity observed in samples that has confounded studies of CODH/ACS for many years. On the basis of this principle, a protocol to generate nearly homogeneous preparations of the active NiNi form of ACS was achieved with NiFeC signals of approximately 0.8 spin/mol.     

Function of the tunnel in acetylcoenzyme A synthase/carbon monoxide dehydrogenase

Acetylcoenzyme A synthase/carbon monoxide dehydrogenase (ACS/CODH) contains two Ni–Fe–S active-site clusters (called A and C) connected by a tunnel through which CO and CO₂ migrate. Site-directed mutants A578C, L215F, and A219F were designed to block the tunnel at different points along the region between the two C-clusters. Two other mutant proteins F70W and N101Q were designed to block the region that connects the tunnel at the ββ interface with a water channel also located at that interface. Purified mutant proteins were assayed for Ni/Fe content and examined by electron paramagnetic resonance spectroscopy. Analyses indicate that same metal clusters found in wild-type (WT) ACS/CODH (i.e., the A-, B-, C-, and probably D-clusters) are properly assembled in the mutant enzymes. Stopped-flow kinetics revealed that these centers in the mutants are rapidly reducible by dithionite but are only slowly reducible by CO, suggesting an impaired ability of CO to migrate through the tunnel to the C-cluster. Relative to the WT enzyme, mutant proteins exhibited little CODH or ACS activity (using CO₂ as a substrate). Some ACS activity was observed when CO was a substrate, but not the cooperative CO inhibition effect characteristic of WT ACS/CODH. These results suggest that CO and CO₂ enter and exit the enzyme at the water channel along the ββ subunit interface. They also suggest two pathways for CO during synthesis of acetylcoenzyme A, including one in which CO enters the enzyme and migrates through the tunnel before binding at the A-cluster, and another in which CO binds the A-cluster directly from the solvent.     

Ligand exchange reactions between copper(II) and nickel(II) chelates of different sulphur and selenium containing ligands. VII. Single-crystal ESR study on the mixed ligand chelate system Ph4As[Cu/Zn(mnt)(Et2dtc)] and crystal and molecular structure of Ph4As[Zn(mnt)(Et2dtc)]

The synthesis and crystal and molecular structure of the first zinc(II) mixed ligand chelate containing a dithiolene ligand (maleonitriledithiolate) and N,N- diethyldithiocarbamate are reported. The compound Ph₄As[Zn(mnt)(Et₂dtc)] crystallizes monoclinic, space group P2₁/c with four molecules in the unit cell; a=17.834(3), b=12.056(2), c=16.171(4) Å, β=93.73(2)°.The coordination geometry of the ZnS₄ unit is nearly tetrahedral, with a dihedral angle of 87.6° between the chelate rings. The structure is compared with those of both the patent compounds (Ph₄- As)₂[Zn(mnt)₂] and Zn₂(Et₂dtc)₄. Ph₄As[Zn(mnt)(Et₂dtc)] could be used as the host lattice in single- crystal ESR investigations of the planar Cu mixed ligand complex. The rhombic spin-Hamiltonian parameters g and A^Cu are indicative of a low symmetry of the incorporated [Cu(mnt)(Et₂dtc)]⁻ complex anions. In order to substantiate the experimental findings about the actual structure of the copper molecules, the principal values of g and A^Cu were recalculated by means of Extended Hückel MO calculations. However, the calculations performed for the dihedral angles between the ligand planes varying between 0° and 90° suggest that the rotation of the ligands is not larger than 10°     

Synthesis and characterization of divalent metal complexes containing the heteroscorpionate ligand dihydrobis(3-carboxyethyl-5-methylpyrazolyl)borate

The dihydrobis(3-carboxyethyl-5-methylpyrazolyl)borate ligand, Bp^COOET,Me, reacts with divalent metals to yield complexes of general type [(Bp^COOET,Me)₂M], where M = Mn(II), Fe(II), Co(II), Ni(II), Zn(II), Cu(II), Pb(II) and Cd(II). All complexes have been fully characterized by elemental analyses and FT-IR in the solid state and by NMR (¹H and ¹¹³Cd NMR) spectroscopy and electrospray ionization mass spectrometry in solution. A single crystal structural characterization is reported for [Cu(Bp^COOET,Me)₂] and [Zn(Bp^COOET,Me)₂]. In the two complexes, both metals are four-coordinated and they are only bound to the nitrogen atoms of the bis(pyrazolyl)borate ligand; however, while the environment of the copper atom is square planar, that of the zinc center shows a tetrahedral distorted conformation.     

The Crystal Structure of the [NiFe] Hydrogenase from the Photosynthetic Bacterium Allochromatium vinosum: Characterization of the Oxidized Enzyme (Ni-A State)

The crystal structure of the membrane-associated [NiFe] hydrogenase from Allochromatium vinosum has been determined to 2.1 Å resolution. Electron paramagnetic resonance (EPR) and Fourier transform infrared spectroscopy on dissolved crystals showed that it is present in the Ni-A state (> 90%). The structure of the A. vinosum [NiFe] hydrogenase shows significant similarities with [NiFe] hydrogenase structures derived from Desulfovibrio species. The amino acid sequence identity is ∼ 50%. The bimetallic [NiFe] active site is located in the large subunit of the heterodimer and possesses three diatomic non-protein ligands coordinated to the Fe (two CN⁻ , one CO). Ni is bound to the protein backbone via four cysteine thiolates; two of them also bridge the two metals. One of the bridging cysteines (Cys64) exhibits a modified thiolate in part of the sample. A mono-oxo bridging ligand was assigned between the metal ions of the catalytic center. This is in contrast to a proposal for Desulfovibrio sp. hydrogenases that show a di-oxo species in this position for the Ni-A state. The additional metal site located in the large subunit appears to be a Mg²⁺ ion. Three iron-sulfur clusters were found in the small subunit that forms the electron transfer chain connecting the catalytic site with the molecular surface. The calculated anomalous Fourier map indicates a distorted proximal iron-sulfur cluster in part of the crystals. This altered proximal cluster is supposed to be paramagnetic and is exchange coupled to the Ni³⁺ ion and the medial [Fe₃S₄]⁺ cluster that are both EPR active (S = 1/2 species). This finding of a modified proximal cluster in the [NiFe] hydrogenase might explain the observation of split EPR signals that are occasionally detected in the oxidized state of membrane-bound [NiFe] hydrogenases as from A. vinosum.     

The reactivity of tetracarbonylnickel encapsulated in zeolite X. A case history of intrazeolite coordination chemistry.

Zeolite X shows a high capacity for tetracarbonylnickel (up to 28 weight percent) such that complete pore filling with ‘liquid like’ material takes place. The adsorbed material may be removed simply by evacuation at room temperature. Partial decomposition of the Ni(CO)₄ occurs on standing at room temperature under N₂. The resultant orange species is highly reactive and has spectroscopic properties consistent with a coordinatively unsaturated ‘Ni(CO)₃’. Complete and irreversible decomposition by heating to 200 °C in vacuo gives a black zeolite, with an undefined metal phase, which is unreactive towards carbon monoxide. Reaction of the zeolite supported Ni(CO)₄ with various phosphorus ligands is highly dependent on the original loading level as well as the physical size of the ligands involved. At low loadings two kinds of reactivity are observed: 1) With ligands too large to gain access to the zeolite crystal interior, reaction occurs only in solution and so drags the Ni(CO)₄ from the zeolite: 2) With smaller ligands, reaction takes place inside the zeolite cages leading to well-defined, encapsulated, ship-in-bottle complexes which have a stoichiometry dictated by the available space in the cages. At high loading levels, pore blocking phenomena lead to inhomogeneous distributions of encapsulated complexes wherein a complete shell of phosphorous ligand substituted nickel carbonyl species forms at the crystal surface layers and prevents further reaction deeper inside the crystal. The reactivity with large phosphines has been used to study the diffusion of Ni(CO)₄ from the zeolite. Monitoring the appearance of the Ni(CO)₃L (where L = phosphine) by 31-P NMR of the supernatant solution shows that Ni(CO)₄ leaves the zeolite with a first order rate constant of at least 2 × 10^?2 sec^?1 at 298 K.     

Antibacterial,antifungal and in vitro antileukaemia activity of metal complexes with thiosemicarbazones

1‐phenyl‐3‐methyl‐4‐benzoyl‐5‐pyrazolone 4‐ethyl‐thiosemicarbazone (HL) and its copper(II), vanadium(V) and nickel(II) complexes: [Cu(L)(Cl)]·C₂H₅OH·( 1 ), [Cu(L)₂]·H₂O ( 2 ), [Cu(L)(Br)]·H₂O·CH₃OH ( 3 ), [Cu(L)(NO₃)]·2C₂H₅OH ( 4 ), [VO₂(L)]·2H₂O ( 5 ), [Ni(L)₂]·H₂O ( 6 ), were synthesized and characterized. The ligand has been characterized by elemental analyses, IR, ¹H NMR and ¹³C NMR spectroscopy. The tridentate nature of the ligand is evident from the IR spectra. The copper(II), vanadium(V) and nickel(II) complexes have been characterized by different physico‐chemical techniques such as molar conductivity, magnetic susceptibility measurements and electronic, infrared and electron paramagnetic resonance spectral studies. The structures of the ligand and its copper(II) ( 2 , 4 ), and vanadium(V) ( 5 ) complexes have been determined by single‐crystal X‐ray diffraction. The composition of the coordination polyhedron of the central atom in 2 , 4 and 5 is different. The tetrahedral coordination geometry of Cu was found in complex 2 while in complex 4 , it is square planar, in complex 5 the coordination polyhedron of the central ion is distorted square pyramid. The in vitro antibacterial activity of the complexes against Escherichia coli, Salmonella abony, Staphylococcus aureus, Bacillus cereus and the antifungal activity against Candida albicans strains was higher for the metal complexes than for free ligand. The effect of the free ligand and its metal complexes on the proliferation of HL‐60 cells was tested.     

Synthetic,structural and molecular mechanics investigation of some mono- and dinuclear copper(I) and copper(II) complexes of macrocyclic and related acyclic ligands

A number of di-Cu(II) complexes of the new tetraimine macrocyclic ligand derived from the Schiff base [2 + 2] condensation of 2,5-diformylfuran with 3-oxa-pentane-1,5-diamine have been prepared by methods which employ the heavier alkaline earth metal ions as templates followed by transmetallation. The complexes have been characterised by spectroscopic and other physical methods. Several of the di-Cu(I) complexes react reversibly with CO in solution and irreversibly with O₂ in a 4:1 Cu:O₂ stoicheiometry. Depending on conditions the oxidation product may be a dinuclear Cu(II) complex of the macrocycle or a mononuclear Cu(II) complex of a new ring-opened ligand. The single crystal X-ray structure of the latter complex has been determined.[CuL](BPh₄)₂ is monoclinic, space group C2/c with a=20.12(1), b=14.48(1), c=22.37(2) Å, β=110.1(1)°, Z=4. 1389 Independent reflections above background were measured on a diffractometer and the structure refined to R=0.108. The cation has imposed C₂ symmetry. The copper atom is bonded to four nitrogen atoms in the ‘outer’ compartment of the ligand with unique CuN distances of 2.050(17) and 1.977(17) Å. The geometry of the copper atom is intermediate between square planar and tetrahedral with an angle of 39.7° between two CuN₂ planes. Molecular mechanics calculations show that this distortion is due to steric effects.     

(1,3-Bis(2′-pyridylimino)isoindolinato)nickel(II) complexes of phenylcyanamido ligands: crystal structure, electronic absorption spectroscopy and solvent adduct studies

Complexes of the formula Ni(L)X, where L=1,3-bis(2′-pyridylimino)isoindolinato and X=Cl, Br, N₃, NCS, 2-Clpcyd, 4-Clpcyd, 2,3-Cl₂pcyd,2,6-Cl₂pcyd, 2,4,5-Cl₃pcyd and 2,3,5,6-Cl₄pcyd, have been synthesized and characterized by elemental analysis, and IR, ¹H NMR and UV---Vis spectroscopies. A crystal structure determination of Ni(L)(2-Clpcyd) showed nickel in a distorted square planar coordination sphere of nitrogen donor atoms in which the phenylcyanamido ligand is coordinated to Ni(II) via the terminal nitrogen. The solvent coordination equilibria of Ni(L)(pcyd) complexes was also investigated and the results suggest that both electronic and steric factors play important roles in determining the stability of the solvated complex.     

Tridentate N(2)S ligand from 2,2'-dithiodibenzaldehyde and N,N-dimethylethylenediamine: Synthesis, structure, and characterization of a Ni(II) complex with relevance to Ni Superoxide Dismutase

Nickel Superoxide Dismutase (NiSOD) and the A-cluster of Carbon Monoxide Dehydrogenase/Acetyl Coenzyme A Synthase (CODH/ACS) both feature active sites with Ni coordinated by thiolate and amide donors. It is likely that the particular set of donors is important in tuning the redox potential of the Ni center(s). We report herein an expansion of our efforts involving the use of 2,2′-dithiodibenzaldehyde (DTDB) as a synthon for metal-thiolate complexes to reactions with Ni complexes of N,N-dimethylethylenediamine (dmen). In the presence of coordinating counterions, these reactions result in monomeric square-planar complexes of the tridentate N₂S donor ligand derived from the Schiff-base condensation of dmen and DTDB. In the absence of a coordinating counterion, we have isolated a Ni(II) complex with an asymmetric N₂S₂ donor set involving one amine and one imine N donor in addition to two thiolate donors. This latter complex is discussed with respect to its relevance to the active site of NiSOD.     

Synthesis and characterization of 6-(p-chlorobenzoyl)-5,7,12,14-tetramethyl-benzo[b]-1,4,8,11-tetraazacyclo[14]tetradecinato(2-)nickel(II)

A nickel(II) complex with 6-(p-chlorobenzoyl)-5,7,12,14-tetramethyl-benzo[b]-1,4,8,11- tetraazacyclo[14]tetradecine was synthesized and characterized by measurements of NMR, IR and an electronic spectra. The X-ray crystal structure shows that the coordination geometry around the Ni atom is a square planar with the unsymmetrical tetraaza[14]annulene.     

1H nmr studies of complexes of ferric leghemoglobin with substituted pyridines and nicotinic acids

The ¹H nuclear magnetic resonance (nmr) spectra of complexes of soybean ferric leghemoglobin with 3-substituted pyridines and 5-substituted nicotinic acids have been recorded in order to determine the influence of axial ligands on heme electronic structure. The hyperfine shifted resonances of the heme group were assigned by analogy to previous assignments for the pyridine and nicotinic acid complexes of leghemoglobin. The spectra are characteristic of predominantly low-spin ferric heme complexes. For the pyridine complexes, the rate of ligand exchange was found to increase with decreasing ligand pK_A. For many of the complexes, optical and nmr spectra reveal the presence of an equilibrium mixture of high- and low-spin states of the iron atom. The percentage of high-spin component increases with decreasing ligand pK_A Smaller hyperfine shifts are noted for leghemoglobin complexes with ligands capable of weak ligand → metal π bonding. The pattern of hyperfine shifted resonances is similar for all complexes studied and indicates that the overall heme electronic structure is dominated by the bonding to the proximal histidine.     

Studies on the metalamide bond. XIX. A comparison of molecular distortions in the crystal structures of [N,N′-bis(2′-pyridinecarboxamido)-1,2-benzene] nickel(II) with its 6′-methyl-substituted Analogue

[N,N′-Bis(pyridine-2′-carboxamide)-1,2-benzene]nickel(II) monohydrate, C₁₈H₁₄N₄O₃Ni, crystallizes in the monoclinic space group C2/c with a = 14.240(4), b = 20.071(3), c = 16.275(2) Å,β = 97.25(2)^o, Z = 12 and its crystal structure has been refined to R = 0.033 for 3597 diffractometer data. [N,N′-Bis(6′-methylpyridine-2′-carboxamide)-1,2- benzene]nickel(II) monohydrate, C₂₀H₁₈N₄O₃Ni, crystallizes in the orthorhombic space group Pbca with a = 10.14(2) b = 17.12(2), c= 21.11(5) Å, Z = 8 and its crystal structure has been refined to R = 0.088 for 1979 photographic data. In both structures the nickel atoms are four coordinate with the ligands acting as N₄ tetradentates. For the first mentioned complex the structure consists of two independent molecules one of which is constrained, by space group requirements, to have C₂ (2) symmetry. These two molecules are closely similar and both exhibit nearly planar molecular arrangements with a small tetrahedral twist of up to 4^o at the nickel atoms. In the second complex the methyl substitution at the 6′-pyridyl positions causes severe steric strain in the molecule which gives rise to a 14.9^o tetrahedral twist at the nickel atom and approximately 25% pyramidal distortion at both amide nitrogen atoms. The resulting methyl-methyl separation of 3.26(1) Å is considerably less than the sum of the van der Waals radii for two such groups. This close separation leads to carbon-acid character for the methyl group protons which are shown to exchange for deuterons in NMR studies. A full analysis of the out-of-plane distortions and torsion angles of the two structures and a comparison with the previously reported analogous copper structures are made.     

Synthesis, structure and characterization of a new tetrameric tungsten-oxo complex [WO2Cl2(THF)]4

WOCl₄ reacts with (Me₃Si)₂O and excess THF to give [WO₂Cl₂(THF)]₄ (1), a new tetrameric tungsten(VI)-oxo complex, which was characterized and crystal structure was determined by X-ray crystallography. Complex 1 has a roughly square planar tetranuclear structure bridged by μ-oxo ligands. Each tungsten atom is coordinated by two bridging oxygens, one terminal oxygen, two “axial” chlorine atoms and one “equatorial” O-bonded THF ligand. One of the two μ-oxo ligands is similar to the terminal oxygen atom and the other one is similar to the coordinated oxygen atom of the THF ligand, respectively, which confirmed a previous proposal. Four WO₃Cl₂(THF) octahedral are associated by sharing corners. Complex 1 is different from three known tetrameric tungsten analogues in its structural arrangement and properties.     

Synthesis of heterodinuclear FeRu(CO)6(R-DAB(6e))(R-DABRNC(H)C(H)NR) Part XV. Comparison of the reactivities of heterodinuclear FeRu(CO)6(R-DAB(6e)) and homodinuclear analogues M2(CO)6(R-DAB(6e)) (M = Fe,Ru). Single-crystal x-ray structures of FeRu(CO)6(i-Pr-DAB(6e)) and FeRu(CO)5(i-Pr-DAB(4e))(C3H4)

The reactions of Fe(CO)₃(R-DAB; R¹, H(4e)) (1a: R = i-Pr, R¹ = H; 1b: R = t-Bu, R¹ = H; 1c: R = c-Hex, R¹ = H; 1e: R = p-Tol, R¹ = H; 1f: R = i-Pr, R¹ = Me) with Ru₃(CO)₁₂ and of Ru(CO)₃(R-DAB; R¹, H(4e)) (2a: R = i-Pr, R¹ = H; 2d: R = CH(i-Pr)₂, R¹ = H) with Fe₂(CO)₉ in refluxing heptane both afforded FeRu(CO)₆(R-DAB; R¹, H(6e)) (3) in yields between 50 and 65%.The coordination mode of the ligand has been studied by a single crystal X-ray structure determination of FeRu(CO)₆(i-Pr-DAB(6e)) (3a). Crystals of 3a are monoclinic, space group P2₁/a, with four molecules in a unit cell of dimensions: a = 22.436(3), b = 8.136(3), c = 10.266(1) Å and β = 99.57(1)°. The structure was refined to R = 0.049 and R_w = 0.052 using 3045 reflections above the 2.5σ(I) level. The molecule contains an FeRu bond of 2.6602(9) Å, three terminally bonded carbonyls to Fe, three terminally bonded carbonyls to Ru and bridging 6e donating i-Pr-DAB ligand. The i-Pr-DAB ligand is coordinated to Ru via N(1) and N(2) occupying an apical and equatorial site respectively (RuN(1) = 2.138(4) RuN(2) = 2.102(3) Å). The C(2)N(2) moiety of the ligand is η²-coordinated to Fe with C(2) in an apical and N(2) in an equatorial site (FeC(2) = 2.070(5) and FeN(2) = 1.942(3) Å).The ¹H and ¹³C NMR data indicate that in all FeRu(CO)₆(R-DAB(6e)) complexes (3a to 3f) exclusively η²-CN coordination to the Fe atom and not to the Ru atom is present irrespective of whether 3 was prepared by reaction of Fe(CO)₃(R-DAB(4e)) (1) with Ru₃(CO)₁₂ or by reaction of Ru(CO)₃(R-DAB(4e)) (2) with Fe₂(CO)₉. In the case of FeRu(CO)₆(i-Pr-DAB; Me, H(6e)) (3f) the NMR data show that only the complex with the C(Me)N moiety of the ligand σ-N coordinated to the Ru atom and the C(H)N moiety η²-coordinated to the Fe atom was formed. Variable temperature NMR experiments up to 140 °C showed that the α-diimine ligand in 3a is stereochemically rigid bonded.FeRu(CO)₆(R-DAB(6e)) (3a and 3e) reacted with allene to give FeRu(CO)₅(R-DAB(4e))(C₃H₄) (4a and 4e). A single crystal X-ray structure determination of FeRu(CO)₅(i-Pr-DAB(4e))(C₃H₄) (4a) was performed. Crystals of 4a are triclinic, space group P1, with two molecules in a unit cell of dimensions: a = 9.7882(7), b = 12.2609(9), c = 8.3343(7) Å, α = 99.77(1)°, β = 91.47(1)° and γ = 86.00(1)°. The structure was refined to R = 0.028 and R_w = 0.043 using 4598 reflections above the 2σ(I) level. The molecule contains an FeRu bond of 2.7405(7) Å and three terminally bonded carbonyls to iron. Two carbonyls are terminally bonded to the Ru atom together with a chelating 4e donating i-Pr-DAB ligand [RuN = 2.110(1) (mean)]. The allene ligand is coordinated in an η³-allylic fashion to the Fe atom while the central carbon of the allene moiety is σ-bonded to the Ru atom (FeC(14) = 2.166(3), FeC(15) = 1.970(2), FeC(16) = 2.127(3) and RuC(15) = 2.075(2) Å). The ¹H and ¹³C NMR data show that in solution the coordination modes of the R-DAB and the allene ligands are the same as in the solid state.Thermolysis reactions of 3a with R-DAB or carbodiimides gave decomposition and did not afford C(imine)C(reactant) coupling products. Thermolysis reactions of 3a with M₃(CO)₁₂ (M = Ru, Os) and Me₃NO gave decomposition. When the reaction of 3a with Me₃NO was performed in the presence of dimethylacetylenedicarboxylate (DMADC) the known complex FeRu(CO)₄(i-Pr-DAB(8e))(DMADC) (5a) was formed in low yield. In 5a the R-DAB ligand is in the 8e coordination mode with both the imine bonds η²-coordinated to iron. The acetylene ligand is coordinated in a bridging fashion, parallel with the FeRu bond.     

Two new end-to-end single dicyanamide bridged Cu(II) complexes with Schiff base ligands: Structural, electrochemical and magnetic properties

The synthesis, crystal structures and magnetic properties of two different copper(II) complexes of formula [Cu(L¹)(dca)]_n · nClO₄ (1) and [Cu(L²)]₂(dca)(ClO₄) (2) [L¹ = N,N-dimethylethylene-N′-(pyridine-2-carbaldiiminato), HL² = N,N-dimethylethylene-N′-salicylaldiiminato, dca = dicyanamide anion] are described. Spectroscopic and electrochemical properties have also been discussed. A one-dimensional chain structure with single, symmetrical, μ_1,5-dca bridges is found in compound 1. The copper atom in 1 has a square pyramidal geometry. A tridentate Schiff base ligand, having NNN donor sites, and one nitrogen atom from dca occupy the basal plane. N(18) of a neighbouring unit occupies the apical site. The Schiff base used in compound 2 is a tridentate anion with NNO donor sites, which changes the structure in a dinuclear unit of copper atoms bridged by single end-to-end dicyanamide ion. The environment around copper in 2 is square planar. Magnetic susceptibility measurements for 1 and 2 reveal the occurrence of weak antiferromagnetic interaction through the dca ligand.