Five-coordinate nickel(II) complexes with carboxylate anions and derivatives of 1,5,9-triazacyclododec-1-ene: structural and 1H NMR spectroscopic studies

The hydroxo complexes [Ni2(mcN3)2(mu-OH)2]2(PF6)2[mcN3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (Me3-mcN3) or 2,4,4,9-tetramethyl-1,5,9-triazacyclododec-1-ene (Me4-mcN3)] react with the corresponding carboxylic acid [HA = benzoic (Hbz), salicylic (Hsal) or acetylsalicylic (Hacsal) acid] to give five-coordinate nickel(II) complexes of the type [Ni(mcN3)(A)](PF6). The complexes have been studied by spectroscopic methods (IR, UV-Vis and 1H NMR). In acetone solution they exhibit isotropically shifted 1H NMR resonances. The full assignment of these resonances has been made using one- and two-dimensional 1H NMR techniques. The single-crystal structures of [Ni(Me4-mcN3)(bz)](PF6), [Ni(Me4-mcN3)(sal)](PF6) and[Ni(Me4-mcN3)(acsal)](PF6) have been established by X-ray diffraction.     

Synthesis and characterization of nickel inverse 9-metallacrown-3, palladium-silver, and dinuclear platinum complexes containing pyrazole-functionalized NHC ligands

Three metallacrown nickel complexes [Ni(3)(μ-OH)(L1)(3)](PF(6))(2) (1, L1 = 3-((N-methylimidazolylidenyl)methyl)-5-methylpyrazolate), [Ni(3)(μ-OH)(L2)(3)](PF(6))(2) (2, L2 = 3-((N-mesitylimidazolylidenyl)methyl)-5-methylpyrazolate), and [Ni(3)(μ-OH)(L3)(3)](PF(6))(2) (3, L3 = 3-((N-pyrimidin-2-ylimidazolylidenyl)methyl)-5-methylpyrazolate) were obtained by the reactions of corresponding silver-NHC complexes with Raney nickel powder at 45 °C. The same reaction at 80 °C afforded [Ni(3)(L2)(4)](PF(6))(2) (4). The carbene-transfer reaction of the silver-carbene complex with [(η(3)-C(3)H(5))PdCl](2) yielded the heterotrimetallic complex [AgPd(2)(η(3)-C(3)H(5))(2)(L2)(2)](PF(6)) (5), whereas the carbene-transfer reaction with Pt(cod)Cl(2) gave [Pt(2)(L3)(2)](PF(6))(2) (6). All of these complexes have been fully characterized by ESI-MS, NMR spectroscopy, and elemental analysis. The molecular structures of 1-6 were also studied by X-ray diffraction analysis. In 1-3, three nickel centers are bridged together by three pyrazole-NHC ligands and a hydroxide group, forming a 9-metallacrown-3 topology. Complex 4 is paramagnetic, consisting of two square-planar nickel(II) ions and one tetrahedral nickel ion in which three Ni ions are bridged by four pyrazolate units. In the mixed Pd-Ag complex 5, two palladium and one silver centers are bridged by two pyrazole-NHC ligands. Complex 5 showed good catalytic activity in the Sonogashira coupling reaction of aryl bromides and phenylacetylene under mild conditions typically catalyzed by Pd-Cu systems.     

Bis(alpha-diimine)nickel complexes: molecular and electronic structure of three members of the electron-transfer series [Ni(L)(2)](z)() (z = 0, 1+, 2+) (L = 2-Phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene). A combined experimental and theoretical study

From the reaction of Ni(COD)(2) (COD = cyclooctadiene) in dry diethylether with 2 equiv of 2-phenyl-1,4-bis(isopropyl)-1,4-diazabutadiene (L(Ox))(0) under an Ar atmosphere, dark red, diamagnetic microcrystals of [Ni(II)(L*)(2)] (1) were obtained where (L*)(1-) represents the pi radical anion of neutral (L(Ox))(0) and (L(Red))(2-) is the closed shell, doubly reduced form of (L(Ox))(0). Oxidation of 1 with 1 equiv of ferrocenium hexafluorophosphate in CH(2)Cl(2) yields a paramagnetic (S = 1/2), dark violet precipitate of [Ni(I)(L(Ox))(2)](PF(6)) (2) which represents an oxidatively induced reduction of the central nickel ion. From the same reaction but with 2 equiv of [Fc](PF(6)) in CH(2)Cl(2), light green crystals of [Ni(II)(L(Ox))(2)(FPF(5))](PF(6)) (3) (S = 1) were obtained. If the same reaction was carried out in tetrahydrofuran, crystals of [Ni(II)(L(Ox))(2)(THF)(FPF(5))](PF(6)) x THF (4) (S = 1) were obtained. Compounds 1, 2, 3, and 4 were structurally characterized by X-ray crystallography: 1 and 2 contain a tetrahedral neutral complex and a tetrahedral monocation, respectively, whereas 3 contains the five-coordinate cation [Ni(II)(L(Ox))(2)(FPF(5))](+) with a weakly coordinated PF(6)(-) anion and in 4 the six-coordinate monocation [Ni(II)(L(Ox))(2)(THF)(FPF(5))](+) is present. The electro- and magnetochemistry of 1-4 has been investigated by cyclic voltammetry and SQUID measurements. UV-vis and EPR spectroscopic data for all compounds are reported. The experimental results have been confirmed by broken symmetry DFT calculations of [Ni(II)(L*)(2)](0), [Ni(I)(L(Ox))(2)](+), and [Ni(II)(L(Ox))(2)](2+) in comparison with calculations of the corresponding Zn complexes: [Zn(II)((t)L(Ox))(2)](2+), [Zn(II)((t)L(Ox))((t)L*)](+), [Zn(II)((t)L*)(2)](0), and [Zn(II)((t)L*)((t)L(Red))](-) where ((t)L(Ox))(0) represents the neutral ligand 1,4-di-tert-butyl-1,4-diaza-1,3-butadiene and ((t)L*)(1-) and ((t)L(Red))(2-) are the corresponding one- and two-electron reduced forms. It is clearly established that the electronic structures of both paramagnetic monocations [Ni(I)(L(Ox))(2)](+) (S = 1/2) and [Zn(II)((t)L(Ox))((t)(L*)](+) (S = 1/2) are different.     

Dinuclear silver(I) complexes for the design of metal-ligand networks based on triazolopyrimidines

Silver(I) coordination complexes with the versatile and biomimetic ligands 1,2,4-triazolo[1,5-a]pyrimidine (tp), 5,7-dimethyl-1,2,4-triazolo[1,5-a]pyrimidine (dmtp) and 7-amine-1,2,4-triazolo[1,5-a]pyrimidine (7atp) all feature dinuclear [Ag(2)(μ-tp)(2)](2+) building units (where tp is a triazolopyrimidine derivative), which are the preferred motif, independently of the counter-anion used. According to AIM (atoms in molecules) and ELF (electron localization function) analyses, this fact is due to the great stability of these dinuclear species. The complexes structures range from the dinuclear entities [Ag(2)(μ-tp)(2)(CH(3)CN)(4)](BF(4))(2) (1), [Ag(2)(μ-tp)(2)(CH(3)CN)(4)](ClO(4))(2) (2), [Ag(2)(μ-7atp)(2)](ClO(4))(2) (3) and [Ag(2)(μ-dmtp)(2)(CH(3)CN)](PF(6))(ClO(4)) (4) over the 1D polymer chain [Ag(2)(μ-CF(3)SO(3))(2)(μ-dmtp)(2)](n) (5) to the 3D net {[Ag(2)(μ(3)-tp)(2)](PF(6))(2)·～6H(2)O}(n) (6) with NbO topology.     

Nickel complexes of carboxylate-containing polydentate ligands as models for the active site of urease

Two new carboxylate-containing polydentate ligands have been synthesized, the symmetric ligand 2,6-bis[N-(N-(carboxylmethyl)-N-((1-methylimidazol)methyl)amine)methyl]-4-methylphenolate (BCIMP) and the corresponding asymmetric ligand 2-(N-isopropyl-N-((1-aminomethyl)-4-methylphenol (ICIMP). The ligands have been used to prepare model complexes for the active site of the dinuclear nickel enzyme urease, viz. [Ni(2)(BCIMP)Ac(2)](-) (6), [Ni(2)(BCIMP)(Ph(2)Ac)(2)](-) (7), [Ni(2)(ICIMP)(Ph(2)Ac)(2)] (14), [Ni(4)(ICIMP)(2)(Ph(2)Ac)(2)][ClO(4)](2) (15), [Ni(4)(ICIMP)(2)(Ph(2)Ac)(2)(DMF)(2)][ClO(4)](2) (16), and [Ni(4)(ICIMP)(2)(Ph(2)Ac)(2)(urea)(H(2)O)][ClO(4)](2) (17), where the latter complex contains urea coordinated in a unidentate fashion through the carbonyl oxygen. The N(2)O-N(2)O(2) donor set of ICIMP provides a good framework for the preparation of urease models, but in some cases tetranuclear nickel complexes are formed due to coordination of the carboxylate moiety of one dinickel-ICIMP unit to one or both of the nickels of a second Ni(2) unit. Reactivity and kinetics studies of 7 and 15 show that these model complexes catalyze hydrolysis of 2-hydroxypropyl p-nitrophenyl phosphate (HPNP) at basic pH. In this assay, complexes based on the asymmetric ligand ICIMP exhibit a significantly faster rate of hydrolysis than the corresponding BCIMP complexes. Magnetic measurements indicate that there are weak antiferromagnetic interactions between the nickel ions in complex 16.     

Fe2(SR)2(mu-CO)(CNMe)6]2+ and analogues: a new class of diiron dithiolates as structural models for the H(ox)Air state of the fe-only hydrogenase

Low-temperature oxidation of Fe(2)(S(2)C(n)H(2n)(CNMe)(6-x)(CO)x (n = 2, 3; x = 2, 3) affords a family of mixed carbonyl-isocyanides of the type [Fe(2)(S(2)C(n)H(2n)(CO)x(CNMe)(7-x)](2+). The degree of substitution is controlled by the RNC/Fe ratio, as well as the degree of initial substitution at iron, with tricarbonyl derivatives favoring more highly carbonylated products. The structures of the monocarbonyl derivatives [Fe(2)(S(2)C(n)H(2n))(mu-CO)(CNMe)(6)](PF(6))(2) (n = 2,3) established crystallographically and spectroscopically, are quite similar, with Fe---Fe distances of ca. 2.5 A, although the mu-CO is unsymmetrical in the propanedithiolate derivative. Isomeric forms of [Fe(2)(S(2)C(3)H(6))(CO)(CNMe)(6)](PF(6))(2) were characterized where the CO is bridging or terminal, the greatest structural difference being the 0.1 A elongation of the Fe---Fe distance when MeNC (vs CO) is bridging. In the dicarbonyl species, [Fe(2)(S(2)C(2)H(4))(mu-CO)(CO)(CNMe)(5)](PF(6))(2), the terminal CO ligand is situated at one of the basal sites, not trans to the Fe---Fe vector. Oxidation of Fe(2)(S(2)C(2)H(4))(CNMe)(3)(CO)(3) under 1 atm CO gives the deep pink tricarbonyl [Fe(2)(S(2)C(2)H(4))(CO)(3)(CNMe)(4)](PF(6))(2). DFT calculations show that a bridging CO or MeNC establishes a 3-center, 2-electron bond within the two Fe(II) centers, which would otherwise be nonbonding.     

Enhancing the stability of trinickel molecular wires and switches: Ni(3)(6+)/Ni(3)(7+)

This paper describes in detail four new compounds that contain extended metal atom chains (EMACs) of three nickel atoms wrapped by either di(2-pyridyl)amide (dpa) or the new homologous ligand with an ethyl group at the para position of each pyridyl group, depa, and compares them to the precursor Ni(3)(dpa)(4)Cl(2) (1) and the oxidized and rather unstable Ni(3)(dpa)(4)(PF(6))(3) (2). The new molecules are Ni(3)(depa)(4)Cl(2) (3), Ni(3)(depa)(4)(PF(6))(3) (4), [Ni(3)(dpa)(4)(CH(3)CN)(2)](PF(6))(2) (5), and [Ni(3)(depa)(4)(CH(3)CN)(2)](PF(6))(2) (6). These compounds are fully described as to preparation, elemental composition, structure, infrared spectra, (1)H NMR spectra (where possible), electrochemistry, magnetic susceptibility, and an EPR spectrum for 4. The effects of (a) introducing the ethyl substituents on the ligands, (b) replacing axial anions by neutral axial ligands, and (c) oxidizing the Ni(3) chains are reported and discussed. The point of major interest is how oxidation profoundly alters the electronic structure of the EMAC.     

Nickel(II) and iron(II) triple helicates assembled from expanded quaterpyridines incorporating flexible linkages

In the present study the interaction of Fe(II) and Ni(II) with the related expanded quaterpyridines, 1,2-, 1,3- and 1,4-bis-(5'-methyl-[2,2']bipyridinyl-5-ylmethoxy)benzene ligands (4-6 respectively), incorporating flexible, bis-aryl/methylene ether linkages in the bridges between the dipyridyl domains, was shown to predominantly result in the assembly of [M(2)L(3)](4+) complexes; although with 4 and 6 there was also evidence for the (minor) formation of the corresponding [M(4)L(6)](8+) species. Overall, this result contrasts with the behaviour of the essentially rigid 'parent' quaterpyridine 1 for which only tetrahedral [M(4)L(6)](8+) cage species were observed when reacted with various Fe(II) salts. It also contrasts with that observed for 2 and 3 incorporating essentially rigid substituted phenylene and biphenylene bridges between the dipyridyl domains where reaction with Fe(II) and Ni(II) yielded both [M(2)L(3)](4+) and [M(4)L(6)](8+) complex types, but in this case it was the latter species that was assigned as the thermodynamically favoured product type. The X-ray structures of the triple helicate complexes [H(2)O?Ni(2)(4)(3)](PF(6))(4)·THF·2.2H(2)O, [Ni(2)(6)(3)](PF(6))(4)·1.95MeCN·1.2THF·1.8H(2)O, and the very unusual triple helicate PF(6)(-) inclusion complex, [(PF(6))?Ni(2)(5)(3)](PF(6))(3)·1.75MeCN·5.25THF·0.25H(2)O are reported.     

Rhenium(V) oxo complexes with N-heterocyclic carbenes

Air-stable rhenium(V) oxo complexes are formed when [ReOCl(3)(PPh(3))(2)] is treated with N-heterocyclic carbenes of the 1,3-dialkyl-4,5-dimethylimidazol-2-ylidene type, L(R) (R = Me, Et, i-Pr). Complexes of the compositions [ReO(2)(L(R))(4)](+), [ReOCl(L(R))(4)](2+), or [ReO(OMe)(L(R))(4)](2+) can be isolated depending on the alkyl substituents at the nitrogen atoms of the ligands and the reaction conditions applied. Despite the steric overcrowding of the equatorial coordination spheres of the metal atoms by each of the four carbene ligands, stable complexes with six-coordinate rhenium atoms are obtained. Steric demands of the alkyl groups allow control of the stability of the mono-oxo intermediates. Air-stable cationic complexes of the compositions [ReOCl(L(Me))(4)](2+), [ReOCl(L(Et))(4)](2+), and [ReO(OMe)(L(Me))(4)](2+) have been isolated, whereas reactions of [ReOCl(3)(PPh(3))(2)] or other rhenium(V) precursors with the more bulky 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (L(i)(-)(Pr)) directly yield the dioxo complex [ReO(2)(L(i)(-)(Pr))(4)](+). X-ray structures of [ReO(2)(L(i)(-)(Pr))(4)][ReO(4)], [ReO(2)(L(i)(-)(Pr))(4)][PF(6)], [ReO(2)(L(Me))(4)][ReO(4)](0.45)[PF(6)](0.55), [ReO(MeOH)(L(Me))(4)][PF(6)](2), and [ReOCl(L(Et))(4)][PF(6)](2) show that the equatorial coordination spheres of the rhenium atoms are essentially planar irrespective of the steric demands of the individual carbene ligands.     

New nickel(II) and iron(II) helicates and tetrahedra derived from expanded quaterpyridines

As an extension of prior studies involving the linear quaterpyridine ligand, 5,5'-dimethyl-2,2':5',5':2',2'-quaterpyridine 1, the synthesis of the related expanded quaterpyridine derivatives 2 and 3 incorporating dimethoxy-substituted 1,4-phenylene and tetramethoxy-substituted 4,4'-biphenylene bridges between pairs of 2,2'-bipyridyl groups has been carried out via double-Suzuki coupling reactions between 5-bromo-5'-methyl-2'-bipyridine and the appropriate di-pinacol-diboronic esters using microwave heating. Reaction of 2 and 3 with selected Fe(II) or Ni(II) salts yields a mixture of both [M(2)L(3)](4+) triple helicates and [M(4)L(6)](8+) tetrahedra, in particular cases the ratio of the products formed was shown to be dependent on the reaction conditions; the respective products are all sufficiently inert to allow their chromatographic separation and isolation. Longer reaction times and higher concentrations were found to favour tetrahedron formation. The X-ray structures of solvated [Ni(2)(2)(3)](PF(6))(4), [(PF(6)) ? Fe(4)(2)(6)](PF(6))(7), [Fe(4)(3)(6)](PF(6))(8) and [Ni(4)(3)(6)](PF(6))(8) have been determined, while the structure of the parent Fe(II) cage in the series, [(PF(6)) ? Fe(4)(1)(6)](PF(6))(7), was reported previously. The internal volumes of the Fe(II) tetrahedral cages have been calculated and increase from 102 ?(3) for [Fe(4)(1)(6)](8+) to 227 ?(3) for [Fe(4)(2)(6)](8+) to 417 ?(3) for [Fe(4)(3)(6)](8+) and to an impressive 839 ?(3) for [Ni(4)(3)(6)](8+). The corresponding void volume in the triple helicate [Ni(2)(2)(3)](4+) is 29 ?(3).     

Structures and magnetism of {Ni2Na2}, {Ni4} and {Ni6(II)Ni(III)} 2-hydroxy-3-alkoxy-benzaldehyde clusters

Three polynuclear complexes, [NiNa(μ(1,1,1)-N(3))(μ-hmb)(2)(DMF)](2), (1), [Ni(4)(μ(3)-OMe)(4)(heb)(4)(MeOH)(1.05)(H(2)O)(2.95)], (2) and [Ni(III)(OH)(6)(hmb)(6)Ni(II)(6)]·(ClO(4))(3) (3) (Hhmb = 2-hydroxy-3-methoxy-benzaldehyde; Hheb = 2-hydroxy-3-ethoxy-benzaldehyde), were prepared by reaction of the appropriate ligand with nickel(II) perchloride hexahydrate under solvothermal conditions. All compounds were characterized by elemental analysis, IR spectroscopy and X-ray single-crystal diffraction. Compound 1 exhibits a centrosymmetric heterotetranuclear cluster which represents the first nickel complex to possess two connected face-sharing cubes structure {Ni(2)Na(2)N(2)O(4)}. Compound 2 has a tetranuclear Ni cluster with a cubane topology in which the Ni(II) and the oxygen atoms from the methanol ligands occupying alternate vertices of the cube. Compound 3 consisits of a mixed-valence [Ni(III)(OH)(6)(hmb)(6)Ni(II)(6)](3+) subunits and it represents the first nickel {Ni(II)(6)Ni(III)} complex to possess a planar hexagonal disc-like structure. The results show that the minor ligand modifications or solvent change have a key role in the structural control of the self-assembly process. Magnetic properties of 1-3 in the 300-2 K have been discussed. The {Ni(2)Na(2)} (1) and {Ni(4)} (2) core display dominant ferromagnetic interactions from the nature of the binding modes through μ(3)-N(3)(-) or μ(3)-OCH(3)(-), while {Ni(II)(6)Ni(III)} core (3) displays dominant anti-ferromagnetic interactions from the nature of the binding modes through μ(3)-OH(-).     

Mono- and dinuclear ruthenium(II) 1,6,7,12-tetraazaperylene complexes

We report the synthesis of free 1,6,7,12-tetraazaperylene (tape). Tape was obtained from 1,1'-bis-2,7-naphthyridine by potassium promoted cyclization followed by oxidation with air. Mono- and dinuclear ruthenium(II) 1,6,7,12-tetraazaperylene complexes of the general formulas [Ru(L-L)(2)(tape)](PF(6))(2), [1](PF(6))(2)-[5](PF(6))(2), and [{Ru(L-L)(2)}(2)(μ-tape)](PF(6))(4), [6](PF(6))(4)-[10](PF(6))(4), with{L-L = phen, bpy, dmbpy (4,4'-dimethyl-2,2'-bipyridine), dtbbpy (4,4'-ditertbutyl-2,2'-bipyridine) and tmbpy (4,4'5,5'-tetramethyl-2,2'-bipyridine)}, respectively, were synthesized. The X-ray structures of tape·2CHCl(3) and the mononuclear complexes [Ru(bpy)(2)(tape)](PF(6))(2)·0.5CH(3)CN·0.5toluene, [Ru(dmbpy)(2)(tape)](PF(6))(2)·2toluene and [Ru(dtbbpy)(2)(tape)](PF(6))(2)·3acetone·0.5H(2)O were solved. The UV-vis absorption spectra and the electrochemical behavior of the ruthenium(ii) tape complexes were explored and compared with the data of the analogous dibenzoeilatin (dbneil), 2,2'-bipyrimidine (bpym) and tetrapyrido[3,2-a:2',3'-c:3',2'-h:2',3'-j]phenazin (tpphz) species.     

Site specificity of metal ions in heterodinuclear complexes derived from an "end-off" compartmental ligand

A phenol-based "end-off" compartmental ligand, 2-[N-[2-(dimethylamino)ethyl]iminomethyl]-6-[N,N-di(2-pyridylmethyl)aminomethyl]-4-methylphenol (HL), having a bidentate arm and a tridentate arm attached to the 2 and 6 positions of the phenolic ring, has afforded the following heterodinuclear M(a)(II)M(b)(II) complexes: [CuM(L)(AcO)(2)]ClO(4) (M = Mn (1), Fe (2), Co (3), Ni (4), Zn (5)), [ZnM(L)(AcO)(2)]ClO(4) (M = Co (6), Ni (7)), and [CuNi(L)(AcO)(NCS)(2)] (8). 1.MeOH (1'), 2.MeOH (2'), 3.MeOH (3'), 4.MeOH (4'), 5.MeOH (5'), and 7.MeOH (7') are isostructural and have a heterodinuclear core bridged by the phenolic oxygen atom of L(-) and two acetate groups. In 1'-5' the Cu(II) is bound to the bidentate arm and has a square-pyramidal geometry with one acetate oxygen at the apical site. The M(II) is bound to the tridentate arm and has a six-coordinate geometry together with two acetate oxygen atoms. In the case of 7' the Zn is bound to the bidentate arm and the Ni is bound to the tridentate arm. 8.2-PrOH (8') has a dinuclear core bridged by the phenolic oxygen atom of L(-) and one acetate group. The Cu bound to the bidentate arm has a square-pyramidal geometry with an isothiocyanate group at the apical site. The Ni bound to the tridentate arm has a six-coordinate geometry with further coordination of an isothiocyanate group. The site specificity of the metal ions is discussed together with the crystal structure of [Cu(4)(L)(2)(AcO)(3)](ClO(4))(3).H(2)O (9) prepared in this work.     

Non-oxo 5-coordinate and 6-coordinate vanadium(IV) complexes with their precursor [LV(III)(CH3OH)](0), where L = a trianionic aminetris(phenolate)-[N,O,O,O] donor ligand: a magnetostructural and EPR study

Ligating properties of a tripodal, potentially tetradentate aminetris(phenol) ligand, tris(2-hydroxy-3,5-di-tert-butylbenzyl)amine, H(3)L, containing [N,O,O,O] donor atoms toward the vanadium ions in +III and IV oxidation states have been studied. The structures of complexes 1 [LV(III)(CH(3)OH)](0), 2 [LV(IV)(OCH(3))](0) and 3 [LV(IV)(acac)](0) were determined by X-ray diffraction methods as having five-coordinate V(III), 1, five-coordinate non-oxo-vanadium(IV), 2, and six-coordinate non-oxo-vanadium(iv) 3, respectively. Compounds 1-3 were also studied with electrochemical methods, variable-temperature (2-295 K) magnetic susceptibility measurements and X-band electron paramagnetic resonance (EPR) spectroscopy. The electrochemical results of 2 and 3 suggest metal-centered oxidation, i.e. the generation of a V(V)-phenolate species. EPR investigations indicate a (d(xy))(1) ground state showing a considerable increase in the in-plane π-bonding, as is expected for a phenolate ligand.     

Novel dinuclear luminescent compounds based on iridium(III) cyclometalated chromophores and containing bridging ligands with ester-linked chelating sites

The syntheses and study of the spectroscopic, redox, and photophysical properties of a new set of species based on Ir(III) cyclometalated building blocks are reported. This set includes three dinuclear complexes, that is, the symmetric (with respect to the bridging ligand) diiridium species [(ppy)(2)Ir(mu-L-OC(O)-C(O)O-L)Ir(ppy)(2)][PF(6)](2) (5; ppy = 2-phenylpyridine anion; L-OC(O)-C(O)O-L = bis[4-(6'-phenyl-2,2'-bipyridine-4'-yl)phenyl]-benzene-1,4-dicarboxylate), the asymmetric diiridium species [(ppy)(2)Ir(mu-L-OC(O)-L)Ir(ppy)(2)][PF(6)](2) (3; L-OC(O)-L = 4-([(6'-phenyl-2,2'-bipyridine-4'-yl)benzoyloxy]phenyl)-6'-phenyl-2,2'-bipyridine), and the mixed-metal Ir-Re species [(ppy)(2)Ir(mu-L-OC(O)-L)Re(CO)(3)Br][PF(6)] (4). Syntheses, characterization, and spectroscopic, photophysical, and redox properties of the model mononuclear compounds [Ir(ppy)(2)(L-OC(O)-L)][PF(6)] (2) and [Re(CO)(3)(L-COOH)Br] (6; L-COOH = 4'-(4-carboxyphenyl)-6'-phenyl-2,2'-bipyridine) are also reported, together with the syntheses of the new bridging ligands L-OC(O)-L and L-OC(O)-C(O)O-L. The absorption spectra of all the complexes are dominated by intense spin-allowed ligand-centered (LC) bands and by moderately intense spin-allowed metal-to-ligand charge-transfer (MLCT) bands. Spin-forbidden MLCT absorption bands are also visible as low-energy tails at around 470 nm for all the complexes. All the new species exhibit metal-based irreversible oxidation and bipyridine-based reversible reduction processes in the potential window investigated (between +1.80 and -1.70 V vs SCE). The redox behavior indicates that the metal-based orbitals are only weakly interacting in dinuclear systems, whereas the two chelating halves of the bridging ligands exhibit noticeable electronic interactions. All the complexes are luminescent both at 77 K and at room temperature, with emission originating from triplet MLCT states. The luminescence properties are temperature- and solvent-dependent, in accord with general theories: emission lifetimes and quantum yields increase on passing from acetonitrile to dichloromethane fluid solution and from room-temperature fluid solution to 77 K rigid matrix. In the dinuclear mixed-chromophore species 3 and 4, photoinduced energy transfer across the ester-linked bridging ligands seems to occur with low efficiency.     

New class of diiron dithiolates related to the Fe-only hydrogenase active site: synthesis and characterization of [Fe2(SR)(2)(CNMe)7]2+

Electron-rich polyisocyano derivatives Fe(2)(S(2)C(n)H(2n)(CO)(6-x)(CNMe)(x) (x approximately 4) undergo oxidatively induced (FeCp(2)(+)) reaction with additional CNMe to give [Fe(2)(SR)(2)(CNMe)(7)](PF(6))(2), a new class of iron thiolates. Crystallographic characterization established that the 34 e(-) dinuclear core resembles the oxidized (H(2)-binding) form of the active sites of the Fe-only hydrogenases, key features being the face-sharing bioctahedral geometry, the mu-CX ligand, and an Fe-Fe separation of 2.61 A. Oxidation of the phenylthiolate Fe(2)(SPh)(2)(CO)(2)(CNMe)(4) led to mononuclear [Fe(SPh)(CNMe)(5)](PF(6)), which is analogous to [Fe(2)(SR)(2)(CNMe)(10)](PF(6))(2) formed upon treatment of [Fe(2)(S(2)C(3)H(6))(CNMe)(7)](PF(6))(2) with excess CNMe.     

Dinuclear Nickel(II) Complexes as Models for the Active Site of Urease

Dinuclear nickel(II) complexes of the ligands 2,6-bis[bis((2-benzimidazolylmethyl)amino)methyl]-p-cresol (bbapOH), N,N,N',N'-tetrakis(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane (tbpOH), N-methyl-N,N',N'-tris(2-benzimidazolylmethyl)-2-hydroxy-1,3-diaminopropane (m-tbpOH) and 1-[N,N-bis(2-benzimidazolylmethyl)amino]-3-[2-(3,5-dimethyl-1H-pyrazol-1-yl)ethoxy]-2-hydroxypropane (bpepOH) were prepared in order to model the active site of urease. The novel asymmetric structures of the dinuclear complexes were characterized by X-ray structure analysis. The complex [Ni(2)(bbapO)(ClO(4))(H(2)O)(MeOH)](ClO(4))(2).Et(2)O, 1, crystallizes in the monoclinic space group P2(1)/c, with a = 10.258(2) ?, b = 19.876(3) ?, c = 25.592(4) ?, and beta = 97.12(2) degrees. The nickel ions in 1 are bridged by the phenoxy donor of the ligand and a perchlorate anion. The complexes [Ni(2)(tbpO)(MeCOO)(H(2)O)](ClO(4))(2).H(2)O.Et(2)O, 2, [Ni(2)(m-tbpO)(PhCOO)(EtOH)(2)](ClO(4))(2).EtOH, 3, and [Ni(2)(bpepO)(MeCOO)(H(2)O)(2)](ClO(4))(2).H(2)O.Et(2)O.2EtOH, 4, also crystallize in the monoclinic crystal system with the following unit cell parameters: 2, C2/c, a = 35.360(13) ?, b = 10.958(3) ?, c = 24.821(10) ?, beta = 103.55(3) degrees; 3, Cc, a = 14.663(5) ?, b = 32.630(13) ?, c = 9.839(3) ?, beta = 92.49(2) degrees; 4, C2/c, a = 27.689(13) ?, b = 12.187(5) ?, c = 31.513(14) ?, beta = 115.01(3) degrees. The dinuclear centers of all these complexes are bridged by the alkoxy donor of the ligand and a carboxylate function. Compounds 2 and 3 have one of the nickel ions in a five-coordinated, trigonal bipyramidal coordination environment and thus show a high structural similarity to the dinuclear active site of urease from Klebsiella aerogenes. Furthermore, their magnetic and spectroscopic properties were determined and related to those of the urease enzymes. Activity toward hydrolysis of test substrates (4-nitrophenyl)urea, 4-nitroacetanilide, 4-nitrophenyl phosphate or bis(4-nitrophenyl) phosphate by the dinuclear complexes were examined by UV spectroscopic measurements.     

Formation of N2O from a nickel nitrosyl: isolation of the cis-[N2O2]2- intermediate

Addition of 2,2'-bipyridine (bipy) to [Ni(NO)(bipy)][PF(6)] (1) results in formation of a rare five-coordinate nickel nitrosyl [Ni(NO)(bipy)(2)][PF(6)] (2). This complex exhibits a bent NO(-) ligand in the solid state. On standing in acetonitrile, 2 furnishes the NO coupled product, [Ni(κ(2)-O(2)N(2))(bipy)] (8) in moderate yield. Subsequent addition of 2 equiv of acetylacetone (H(acac)) to 8 results in formation of [Ni(acac)(2)(bipy)], N(2)O, and H(2)O. Preliminary mechanistic studies suggest that the N-N bond is formed via a bimetallic coupling reaction of two NO(-) ligands.     

Evolution of nickel speciation during preparation of Ni-SiO(2) catalysts: effect of the number of chelating ligands in [Ni(en)x(H2O)6-2x]2+ precursor complexes

The evolution of nickel speciation during the successive preparation steps of Ni-SiO(2) catalysts is studied by UV-Vis-NIR, FT-IR, DTG, TPR and TEM. The study focuses on the effect of the number of chelating ligands in the precursor complexes [Ni(en)(x)(H(2)O)((6-2x))](2+) (en = ethylenediamine, x = 1, 2, 3) on the adsorption on silica, and on nickel speciation after thermal treatment. When the en:Ni ratio in solution increases from 1 to 3, the most abundant complex is [Ni(en)(H(2)O)(4)](2+) (64% of all Ni complexes), [Ni(en)(2)(H(2)O)(2)](2+) (81%) and [Ni(en)(3)](2+) (61%), respectively. Equilibrium adsorption of [Ni(en)(x)(H(2)O)((6-2x))](2+) on SiO(2) results in the selective grafting of [Ni(en)(H(2)O)(4)](2+) and [Ni(en)(2)(H(2)O)(2)](2+), through the substitution of two labile H(2)O ligands by two surface SiO(-) groups. The surface [Ni(en)(H(2)O)(2)(SiO)(2)] complex formed by the grafting of [Ni(en)(H(2)O)(4)](2+) onto silica tends to transform into NiO and nickel phyllosilicate after calcination, which consequently leads to large and heterogeneously distributed metallic Ni particles upon reduction. In contrast, [Ni(en)(2)(SiO)(2)], resulting from the grafting of [Ni(en)(2)(H(2)O)(2)](2+) onto silica, no longer has aqua ligands able to react with other nickel complexes or silicium-containing species. Calcination transforms these complexes into isolated Ni(2+) ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.     

Synthesis, molecular structure and properties of the [H6-nNi30C4(CO)34(CdCl)2]n- (n=3-6) bimetallic carbide carbonyl cluster: a model for the growth of noncompact interstitial metal carbides

Reaction of the [Ni(9)C(CO)(17)](2-) dianion with CdCl(2)2.5 H(2)O in THF affords the novel bimetallic Ni--Cd carbide carbonyl clusters [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) (n=3-6), which undergo several protonation-deprotonation equilibria in solution depending on the basicity of the solvent or upon addition of acids or bases. Although the occurrence in solution of these equilibria complicates the pertinent electrochemical studies on their electron-transfer activity, they clearly indicate that the clusters [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) (n=3-6), as well as the structurally related [H(6-n)Ni(34)C(4)(CO)(38)](n-) (n=4-6), undergo reversible or partially reversible redox processes and provide circumstantial and unambiguous evidence for the presence of hydrides for n=3, 4 and 5. Three of the [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-) anions (n=4-6) have been structurally characterized in their [NMe(3)(CH(2)Ph)](4)[H(2)Ni(30)C(4)(CO)(34)(CdCl)(2)]2 COMe(2), [NEt(4)](5)[HNi(30)C(4)(CO)(34)(CdCl)(2)]2 MeCN and [NMe(4)](6)[Ni(30)C(4)(CO)(34)(CdCl)(2)]6 MeCN salts, respectively. All three anions display almost identical geometries and bonding parameters, probably because charge effects are minimized by delocalization over such a large metal carbonyl anion. Moreover, the Ni(30)C(4) core in these Ni-Cd carbide clusters is identical within experimental error to those present in the [HNi(34)C(4)(CO)(38)](5-) and [Ni(35)C(4)(CO)(39)](6-) species, suggesting that the stepwise assembly of their nickel carbide cores may represent a general pathway of growth of nickel polycarbide clusters. The fact that the [H(6-n)Ni(30)C(4)(CO)(34)(micro(5)-CdCl)(2)](n-)(n=4-6) anions display two valence electrons more than the structurally related [H(6-n)Ni(34)C(4)(CO)(38)](n-) (n=4-6) species has been rationalized by extended Hückel molecular orbital (EHMO) analysis.