Enantiomerisation of tetrahedral homochiral [M4L6] clusters: synchronised four Bailar twists and six atropenantiomerisation processes monitored by temperature-dependent dynamic 1H NMR spectroscopy

Temperature-dependent 1H NMR studies prove homochiral, racemic [([symbol: see text])/([symbol: see text])]-((NH4)4[symbol: see text] [Mg4(L1)6]) (1) to be kinetically stable on the NMR timescale. Due to steric reasons, rotation around the central C-C single bond in (L1)2- is blocked, which prevents 1 from enantiomerisation. Most interestingly, however, the 1H NMR spectrum of racemic 2a reveals dynamic temperature dependence. This phenomenon can be explained by simultaneous Bailar twists at the four octahedrally coordinated magnesium centres, synchronised with the sterically unhindered atropenantiomerisation processes around the C-C single bonds of the six ligands (L2)2-, leading to the unprecedented enantiomerisation ([symbol: see text])-2a [symbol: see text] ([symbol: see text])-2a. The profound nondissociative rearrangement occurs without the formation of diastereoisomers. Supplementary support for the interpretation of the temperature-dependent dynamic 1H NMR spectra of 2a is presented by additional studies of [([symbol: see text])/([symbol: see text])]-((EtNH3)4 [symbol: see text] [Mg4(L2)6]) (2b). In 2a and 2b, the ether methylene protons exhibit identical temperature dependence. However, with addition, the methylene protons of the ethyl ammonium groups of 2b display similar temperature dependence as the ligand ether methylene protons.     

Unprecedented stabilization of cobalt(II) in a tetrahedral S2O2 environment: the use of a redox-noninnocent ligand

The reaction of Zn(II) and Co(II) with thiosalicylic acid, o-HSC6H4COOH, and its methyl ester has led to the following complexes: [Zn(SC6H4COO)] (1), (NEt4)Na[Zn(SC6H4COO)2].H2O (2), (NEt4)2Na[Co(SC6H4COO)3].2H2O (3), (NEt4)3Na3[(Co(SC6H4COO)3)2].6MeOH (4), [Zn(SC6H4COOMe)2] (5), and [Co(SC6H4COOMe)n], n = 2 (6), 3 (7). These ligands have not allowed stabilization of Co(II) in a sulfur-oxygen coordination environment. The structures of complexes 2-4 and 7 have been determined crystallographically. Those of 2-4 show significant similarities such as the behavior of the -SC6H4COO- anion as chelating ligand and the involvement of sodium ions as a structural element. Thus, the structure of the [Na(Zn(SC6H4COO)2)(H2O)]- anion in complex 2 can be described as infinite chains of consecutive [Zn(SC6H4COO)2]2- metalloligands linked by [Na(H2O)]+ centers, that of the [Na(Co(SC6H4COO)3(H2O)2)]2(4-) anion in 3 as a centrosymmetric tetranuclear Co2Na2 dimer with a (CoIII(S[symbol: see text]O)3)Na(mu-H2O)2Na(CoIII(S[symbol: see text]O)3) core, and that of the pentanuclear [Na3(Co(SC6H4COO)3)2(MeOH)6]3- anion in 4 as two dinuclear [(CoIII(S[symbol: see text]O)3)Na(MeOH)3] fragments linked to a central sodium ion, which appears to be the first structurally characterized example of a NaS6 site. The use of the o-HSC6H4COOMe ligand allowed the synthesis of [Co(SC6H4COOMe)2] (6) but not its full structural characterization. Instead, [Co(SC6H4COOMe)3] (7) was obtained and structurally characterized. It consists of mononuclear molecules containing an octahedral CoIIIS3O3 core. The selection of 2,2-diphenyl-2-mercaptoacetic acid as ligand with reductive properties has afforded the first mononuclear complex containing a CoIIS2O2 core and thus an unprecedented model for Co(II)-substituted metalloproteins containing tetrahedral MS2O2 active sites. The synthesis and full structural characterization of the isostructural complexes (NEt4)2[Zn(Ph2C(S)COO)2] (8) and (NEt4)2[Co(Ph2C(S)COO)2] (9) show that they consist of discrete [M(Ph2C(S)COO)2]2- anions, with a distorted tetrahedral coordination about the metal. In addition, the stability conferred by the ligand on the CoIIS2O2 core has allowed its characterization in solution by paramagnetic 1D and 2D 1H NMR studies. The longitudinal relaxation times of the hyperfine-shifted resonances and NOESY spectra have led to the assignment of all resonances of the cobalt complex and confirmed that it maintains its tetrahedral geometry in solution. Magnetic measurements (2-300 K) for complex 9 and 9.2H2O are in good agreement with distorted tetrahedral and octahedral environments, respectively.     

Cobalt(II), nickel(II), copper(II), and zinc(II) complexes with [3(5)]adamanzane, 1,5,9,13-tetraazabicyclo[7.7.3]nonadecane, and [(2.3)(2).2(1)] adamanzane, 1,5,9,12-tetraazabicyclo[7.5.2]hexadecane

Isolation of the free bicyclic tetraamine, [3(5)]adamanzane.H(2)O (1,5,9,13-tetraazabicyclo[7.7.3]nonadecane.H(2)O), is reported along with the synthesis and characterization of a copper(II) complex of the smaller macrocycle [(2.3)(2).2(1)]adamanzane (1,5,9,12-tetraazabicyclo[7.5.2]hexadecane) and of three cobalt(II), four nickel(II), one copper(II), and two zinc(II) complexes with [3(5)]adamanzane. For nine of these compounds (2-8, 10b, and 12) the single-crystal X-ray structures were determined. The coordination geometry around the metal ion is square pyramidal in [Cu([(2.3)(2).2(1)]adz)Br]ClO(4) (2) and trigonal bipyramidal in the isostructural structures [Cu([3(5)]adz)Br]Br (3), [Ni([3(5)]adz)Cl]Cl (5), [Ni([3(5)]adz)Br]Br (6), and [Co([3(5)]adz)Cl]Cl (8). In [Ni([3(5)]adz)(NO(3))]NO(3) (4) and [Ni([3(5)]adz)(ClO(4))]ClO(4) (7) the coordination geometry around nickel(II) is a distorted octahedron with the inorganic ligands at cis positions. The coordination polyhedron around the metal ion in [Co([3(5)]adz)][ZnCl(4)] (10b) and [Zn([3(5)]adz)][ZnCl(4)] (12) is a slightly distorted tetrahedron. Anation equilibrium constants were determined spectrophotometrically for complexes 2-6 at 25 and 40 degrees C and fall in the region 2-10 M(-1) for the halide complexes and 30-65 M(-1) for the nickel(II) nitrate complex (4). Rate constants for the dissociation of the macrocyclic ligand from the metal ions in 5 M HCl were determined for complexes 2, 3, 5, 8, 10, and 12. The reaction rates vary from half-lives at 40 degrees C of 14 min for the dissociation of the Zn([3(5)]adz)(2+) complex (12) to 14-15 months for the Ni([3(5)]adz)Cl(+) ion (5).     

A family of cyanide-bridged molecular squares: structural and magnetic properties of [{MIICl2}2{CoII(triphos)(CN)2}2].xCH2Cl2, M = Mn, Fe, Co, Ni, Zn

The syntheses, structures, and magnetic properties of a series of tetranuclear cyanide-bridged compounds are reported. This family of molecular squares, [{M(II)Cl2}2{Co(II)(triphos)(CN)2}2] (M = Mn ([CoMn]), Fe ([CoFe]), Co ([CoCo]), Ni ([CoNi]), and Zn ([CoZn]), triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane), has been synthesized by the reaction of Co(II)(triphos)(CN)2 and MCl2 (M = Mn, Co, Ni, Zn) or Fe4Cl8(THF)6 in a CH2Cl2/EtOH mixture. These complexes are isostructural and consist of two pentacoordinate Co(II) and two tetrahedral M(II) centers. The resulting molecular squares are characterized by antiferromagnetic coupling between metal centers that generally follows the spin-coupling model S total = SM(II)1 - SCo1 + SM(II)2 - SCo2. Magnetic parameters for all the complexes were measured using SQUID magnetometry. Additionally, [CoZn] and [CoMn] were studied by both conventional and high-frequency and high-field electron paramagnetic resonance.     

X-ray spectroscopy of enzyme active site analogues and related molecules: bis(dithiolene)molybdenum(IV) and -tungsten(IV,VI) complexes with variant terminal ligands

The X-ray absorption spectra at the molybdenum and selenium K-edges and the tungsten L2,3-edges are acquired for a set of 14 Mo(IV) and W(IV,VI) bis(dithiolene) complexes related to the active sites of molybdo- and tungstoenzymes. The set includes square pyramidal [MoIVL(S2C2Me2)2]- (L = O2-, R3SiO-, RO-, RS-, RSe-) and [WIV(OR)(S2C2Me2)2]-, distorted trigonal prismatic [MoIV(CO)(SeR)(S2C2Me2)2]- and [WIV(CO)L(S2C2Me2)2]- (L = RS-, RSe-), and distorted octahedral [WVIO(OR)(S2C2Me2)2]-. The dithiolene simulates the pterin-dithiolene cofactor ligand, and L represents a protein ligand. Bond lengths are determined by EXAFS analysis using the GNXAS protocol. Normalized edge spectra, non-phase-shift-corrected Fourier transforms, and EXAFS data and fits are presented. Bond lengths determined by EXAFS and X-ray crystallography agree to < or = 0.02 A as do the M-Se distances determined by both metal and selenium EXAFS. The complexes [MoIV(QR)(S2C2Me2)2]- simulate protein ligation by the DMSO reductase family of enzymes, including DMSO reductase itself (Q = O), dissimilatory nitrate reductase (Q = S), and formate dehydrogenase (Q = Se). Edge shifts of these complexes correlate with the ligand electronegativities. Terminal ligand binding is clearly distinguished in the presence of four Mo-S(dithiolene) interactions. Similarly, five-coordinate [ML(S2C2Me2)2]- and six-coordinate [M(CO)L(S2C2Me2)2]- are distinguishable by edge and EXAFS spectra. This study expands a previous XAS investigation of bis(dithiolene)metal(IV,V,VI) complexes (Musgrave, K. B.; Donahue, J. P.; Lorber, C.; Holm, R. H.; Hedman, B.; Hodgson, K. O. J. Am. Chem. Soc. 1999, 121, 10297) by including a larger inventory of molecules with variant physiologically relevant terminal ligation. The previous and present XAS results should prove useful in characterizing and refining metric features and structures of enzyme sites.     

Ditopic hexaazamacrocycles containing pyridine: synthesis, protonation and complexation studies

Structural studies of metal complexes of five ditopic hexaazamacrocycles containing two pyridine rings ([n]py2N4 n= 18, 20, 22, 24 and 26) have been carried out. The synthesis of macrocycles [22]- to [26]-py2N4 are also reported. The protonation constants of the last three compounds and the stability constants of their complexes with Ni2+, Cu2+, Zn2+, and Pb2+ were determined at 25 degrees C in 0.10 mol dm(-3) KNO3 in aqueous solution. Our results with [22]py2N4 show significant differences from those described previously, while [24]py2N4 has not been studied before and [26]py2N4 is a new compound. Mononuclear and dinuclear complexes of the divalent metal ions studied with [22]- to [26]-py2N4 were found in solution. The stability constants for the ML complexes of the three ligands follow the Irving-Williams order: NiL2+ < CuL2+ > ZnL2+ > PbL2+, however for the dinuclear complexes the values for Pb2+ complexes are higher than the corresponding values for the Ni2+ and the Zn2+ complexes. The X-ray single crystal structures of the supramolecular aggregates [Cu2([20]py2N4)(H2O)4][Cu(H2O)6](SO4)3 x 3H2O and [Cu(2)([20]py(2)N4)(CH3CN)4][Ni([20]py2N4)]2(ClO4)8 x H2O, which are composed of homodinuclear [Cu2([20]py2N4])(H2O)4]4+ and [Cu2([20]py2N4])(CH3CN))4]4+, and mononuclear species, [Cu(H2O)6]2+ and [Ni([20]py2N4)]2+, respectively, assembled by an extensive network of hydrogen bonds, are also reported. In both homodinuclear complexes the copper centres are located at the end of the macrocycle and display distorted square pyramidal coordination environments with the basal plane defined by three consecutive nitrogen donors and one solvent molecule, water in and acetonitrile in . The macrocycle adopts a concertina-type conformation leading to the formation of macrocyclic cavities with the two copper centres separated by intramolecular distances of 5.526(1) and 5.508(7) A in 1a and 2a, respectively. The mononuclear complex [Ni([20]py2N4])]2+ displays a distorted octahedral co-ordination environment with the macrocycle wrapping the metal centre in a helical shape. EPR spectroscopy of the copper complexes indicated the presence of mono- and dinuclear species.     

Structural characterization of metallopeptides designed as scaffolds for the stabilization of nickel(II)-Fe(4)S(4) bridged assemblies by X-ray absorption spectroscopy

In earlier work, de novo designed peptides with a helix-loop-helix motif and 63 residues have been synthesized as potential scaffolds for stabilization of the [Ni(II)-X-Fe(4)S(4)] bridged assembly that is the spectroscopically deduced structure of the A-Cluster in clostridial carbon monoxide dehydrogenase. The 63mers contain a consensus tricysteinyl ferredoxin domain in the loop for binding an Fe(4)S(4) cluster and Cys and His residues proximate to the loop for binding Ni(II), with one Cys residue designed as the bridge X. The metallopeptides HC(4)H(2)-[Fe(4)S(4)]-Ni and HC(5)H-[Fe(4)S(4)]-M, containing three His and one Cys residue for Ni(II) coordination and two His and two Cys residues for binding M = Ni(II) and Co(II), have been examined by Fe-, Ni-, and Co-K edge spectroscopy and EXAFS. All peptides bind an [Fe(4)S(4)](2+) cubane-type cluster. Interpretation of the Ni and Co data is complicated by the presence of a minority population of six-coordinate species with low Z ligands, designated for simplicity as [M(OH(2))(6)](2+). Best fits of the data were obtained with ca. 20% [M(OH(2))(6)](2+) and ca. 80% M(II) with mixed N/S coordination. The collective XAS results for HC(4)H(2)-[Fe(4)S(4)]-Ni and HC(5)H-[Fe(4)S(4)]-M demonstrate the presence of an Fe(4)S(4) cluster and support the existence of the distorted square-planar coordination units [Ni(II)(S.Cys)(N.His)(3)] and [Ni(II)(S.Cys)(2)(N.His)(2)] in the HC(4)H(2) and HC(5)H metallopeptides, respectively. In the HC(5)H metallopeptide, tetrahedral [Co(II)(S.Cys)(2)(N.His)(2)] is present. We conclude that the designed scaffolded binding sites, including Ni-(mu(2)-S.Cys)-Fe bridges, have been achieved. This is the first XAS study of a de novo designed metallopeptide intended to stabilize a bridged biological assembly, and one of a few XAS analyses of metal derivatives of designed peptides. The scaffolding concept should be extendable to other bridged metal assemblies.     

A cross-coupling strategy for the synthesis of dimetallic assemblies containing mixed bipyridine-terpyridine bridging ligands: luminescence and energy transfer properties

Tris-bidentate complexes of the form [Ir(N[symbol: see text]C)2(N[symbol: see text]N)]+ and [Ru(N[symbol: see text]N)3]2+ incorporating a boronic acid substituent (N[symbol: see text]C=2-phenylpyridyl, N[symbol: see text]N=a 2,2'-bipyridyl ligand), are cross-coupled with bromo-substituted bis-terpyridyl ruthenium and iridium complexes to generate heterometallic assemblies comprising bpy-phi(n)-tpy bridges; they display efficient energy transfer from Ir(III) to Ru(II).     

Sulfur-bridged Co(III)Pt(II)Co(III) trinuclear complexes with mixed aliphatic and aromatic thiolate ligands: effect of nonbridging ligands on the homochiral linkage of cobalt(III) octahedrons by platinum(II)

The reaction of [Ni[Co(aet)(2)(pyt)](2)](2+) (aet = 2-aminoethanethiolate, pyt = 2-pyridinethiolate) with [PtCl(4)](2)(-) gave an S-bridged Co(III)Pt(II)Co(III) trinuclear complex composed of two [Co(aet)(2)(pyt)] units, [Pt[Co(aet)(2)(pyt)](2)](2+) ([1](2+)). When a 1:1 mixture of [Ni[Co(aet)(2)(pyt)](2)](2+) and [Ni[Co(aet)(2)(en)](2)](4+) was reacted with [PtCl(4)](2)(-), a mixed-type S-bridged Co(III)Pt(II)Co(III) complex composed of one [Co(aet)(2)(pyt)] and one [Co(aet)(2)(en)](+) units, [Pt[Co(aet)(2)(en)][Co(aet)(2)(pyt)]](3+) ([2](3+)), was produced, together with [1](2+) and [Pt[Co(aet)(2)(en)](2)](4+). The corresponding Co(III)Pt(II)Co(III) trinuclear complexes containing pymt (2-pyrimidinethiolate), [Pt[Co(aet)(2)(pymt)](2)](2+) ([3](2+)) and [Pt[Co(aet)(2)(en)][Co(aet)(2)(pymt)]](3+) ([4](3+)), were also obtained by similar reactions, using [Ni[Co(aet)(2)(pymt)](2)](2+) instead of [Ni[Co(aet)(2)(pyt)](2)](2+). While [Pt[Co(aet)(2)(en)](2)](4+) formed both the deltalambda (meso) and deltadelta/lambdalambda (racemic) forms in a ratio of ca. 1:1, the preferential formation of the deltadelta/lambdalambda form was observed for [1](2+) (ca. deltalambda:deltadelta/lambdalambda = 1:3) and [2](3+) (ca. delta(en)lambda(pyt)/lambda(en)delta(pyt):deltadelta/lambdalambda = 1:2). Furthermore, [3](2+) and [4](3+) predominantly formed the deltadelta/lambdalambda form. These results indicate that the homochiral selectivity for the S-bridged Co(III)Pt(II)Co(III) trinuclear complexes composed of two octahedral [Co(aet)(2)(L)](0 or +) units is enhanced in the order L = en < pyt < pymt. The isomers produced were separated and optically resolved, and the crystal structures of the meso-type deltalambda-[1]Cl(2).4H(2)O and the spontaneously resolved deltadelta-[4](ClO(4))(3).H(2)O were determined by X-ray analyses. In deltalambda-[1](2+), the delta and Lambda configurational mer(S).trans(N(aet))-[Co(aet)(2)(pyt)] units are linked by a square-planar Pt(II) ion through four aet S atoms to form a linear-type S-bridged trinuclear structure. In deltadelta-[4](3+), a similar linear-type trinuclear structure is constructed from the delta-mer(S).trans(N(aet))-[Co(aet)(2)(pymt)] and delta-C(2)-cis(S)-[Co(aet)(2)(en)](+) units that are bound by a Pt(II) ion with a slightly distorted square-planar geometry through four aet S atoms.     

Mono- and Dinuclear Transition Metal Complexes of the Hexadentate Ligand Tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane (L)

The hexadentate, pendant arm macrocycle 1,4,7-tris(4-tert-butyl-2-mercaptobenzyl)-1,4,7-triazacyclononane (H(3)L) has been synthesized and isolated as its trihydrochloride, H(3)L.3HCl, or sodium salt, Na(3)L, and its coordination chemistry with first-row transition metals has been studied. Mononuclear complexes of the type [LM(III)] (M = Ga (1), In (2), V (3), Cr (4), Mn (5), Fe,Co (6)) have been isolated as have the one-electron-oxidized forms [LM]PF(6) (M = V(IV) (3a), Mn(IV) (5a)). The crystal structure of 6 has been determined by single-crystal X-ray crystallography. Complex 6 crystallizes in the orthorhombic space group Iba2, with cell constants a = 14.206(8) ?, b = 22.53(1) ?, c = 26.07(1) ?, V = 8344.0(3) ?(3), and Z = 8. The cobalt(III) ion is in a distorted octahedral fac-N(3)S(3) donor set. The reaction of L with divalent metal chlorides in a 1:2 ratio in methanol affords the homodinuclear complexes [LM(II)(2)Cl] (M = Mn (7), Co (8), Ni (9), Zn (10), Cd (11)) where one metal is six- (N(3)MS(3)) and the other is four-coordinate (S(3)MCl); the two polyhedra are linked by three &mgr;(2)-thiolato bridges. Heterodinuclear complexes of the type [LM(1)M(2)Cl] have been obtained from [LM(2)Cl] species by abstraction of the four-coordinate metal ion and replacement by a different metal ion. The complexes [LZn(II)M(II)Cl] (M = Fe (12), Co (13), Ni (14)), [LNi(II)M(II)Cl] (M = Co (15), Zn (16)), and [LMn(II)M(II)Cl] (M = Fe (17), Co (18), Ni (19), Zn (20), Cd (21), Hg (22)) have been isolated as solid materials. The crystal structure of 14 has been determined by X-ray crystallography. Complex 14 crystallizes in the orthorhombic space group P2(1)2(1)2(1), with cell constants a = 15.45(1) ?, b = 17.77(1) ?, c = 17.58(1) ?, V = 4826.5(4) ?(3), and Z = 4. The linkage isomers 14 and 16 show characteristic electronic spectra for octahedrally and tetrahedrally coordinated Ni(II), respectively. The electronic structures of new complexes have been investigated by UV-vis spectroscopy; their magnetochemistry and electrochemistry are reported.     

Preparation and characterization of mononuclear Co, Ni, and Zn complexes of dinucleating macrocyclic hexaaza-dithiophenolate ligands and their open-chain derivatives

The preparation and characterization of mononuclear complexes of the dinucleating 24-membered hexazadithiophenolate macrocycles H2L2 and H2L3 and their open-chain N3S2 analogues H2L4 and H2L5 are reported. The highly crystalline compounds [Ni(L4)] (4), [Ni(L5)] (5), [Co(L5)] (6), [NiH2(L2)]2+ (7), [ZnH2(L2)]2+ (8), and [NiH2(L3)]2+ (9) could be readily prepared by stoichiometric complexation reactions of the hydrochlorides of the free ligands with the corresponding metal(II) dichlorides and NEt3 in methanolic solution. All complexes were characterized by X-ray crystallography. Monometallic complexes 4-6 of the pentadentate ligands H2L4 and H2L5 feature distorted square pyramidal MN3S2 structures (tau = 0.01 to 0.44). Similar coordination geometries are observed for the macrocyclic complexes 7-9 of the octadentate ligands H2L2 and H2L3. The two hydrogen atoms in 7-9 are attached to the noncoordinating benzylic amine functions and are hydrogen bonded to the metal-bound thiophenolate functions. A comparison of the structures of 4-9 reveals that the macrocycles L2 and L3 have a rather flexible ligand backbone that do not confer unusual coordination geometries on the metal ions. We also report on the ability of the monometallic complexes 7 and 8 to serve as starting materials for the preparation of dinuclear complexes.     

A 3,12-connected vertice sharing adamantoid hydrogen bonded network featuring tetrameric clusters of cyclotriveratrylene

The crystalline supramolecular complex [Sr(H2O)8][(CH3CN) [symbol: see text] (CTV)]4(H2O)4[Co(C2B9H11)2]2 features [(CH3CN) [symbol: see text] (CTV)] host-guest interactions, back-to-back tetrameric clusters of CTV host molecules and an extensive hydrogen bonding giving a 3,12-connected net.     

Bimetallic-induced tail-to-tail dimerization and C-H activation of methyl acrylate

An organometallic complex resulting from tail-to-tail dimerization and C-H activation of methyl acrylate (MA), [Mo(CO2Cp(eta 3-(MeO2C)CH[symbol: see text]CH[symbol: see text]CHCH2(CO2Me)] 2, has been fully characterized from the reaction of the heterobimetallic complex [Cp*Ni=Mo(mu-CO)(CO)2Cp] with MA and an exclusively eta 3-allyl bonding mode of the coupled ligand was established for the first time by X-ray diffraction; formation of 2 is accompanied by that of the mu 3-alkylidyne-capped cluster [NiMo2(mu 3-CCH2CO2Me)(CO)4Cp*Cp2] 3 which results from a double C-H activation of the CH2 group of MA; none of these reactions occur with the corresponding homodinuclear complexes.     

Co-ordination chemistry of amino pendant arm derivatives of 1,4,7-triazacyclononane

The binding properties of 1,4,7-triazacyclononane ([9]aneN3) to metal cations can be adapted through sequential functionalisation of the secondary amines with aminoethyl or aminopropyl pendant arms to generate ligands with increasing numbers of donor atoms. The new amino functionalised pendant arm derivative of 1,4,7-triazacyclononane ([9]aneN3), L1, has been synthesised and its salt [H2L1]Cl2 characterised by X-ray diffraction. The protonation constants of the ligands L1-L4 having one, two or three aminoethyl or three aminopropyl pendant arms, respectively, on the [9]aneN3 framework, and the thermodynamic stabilities of their mononuclear complexes with CuII and ZnII have been investigated by potentiometric measurements in aqueous solutions. In order to discern the protonation sites of ligands L1-L4, 1H NMR spectroscopic studies were performed in D2O as a function of pH. While the stability constants of the CuII complexes increase on going from L1 to L2 and then decrease on going from L2 to L3 and L4, those for ZnII complexes increase from L1 to L3 and then decrease for L4. The X-ray crystal structures of the complexes [Cu(L1)(Br)]Br, [Zn(L1)(NO3)]NO3, [Cu(L2)](ClO4)2, [Ni(L2)(MeCN)](BF4)2, [Zn(L4)](BF4)2.MeCN and [Mn(L4)](NO3)2.1/2H2O have been determined. In both [Cu(L1)(Br)]Br and [Zn(L1)(NO3)]NO3 the metal ion is five co-ordinate and bound by four N-donors of the macrocyclic ligand and by one of the two counter-anions. The crystal structures of [Cu(L2)](ClO4)2 and [Ni(L2)(MeCN)](BF4)2 show the metal centre in slightly distorted square-based pyramidal and octahedral geometry, respectively, with a MeCN molecule completing the co-ordination sphere around NiII in the latter. In both [Zn(L4)](BF4)2.MeCN and [Mn(L4)](NO3)2.1/2H2O the metal ion is bound by all six N-donors of the macrocyclic ligand in a distorted octahedral geometry. Interestingly, and in agreement with the solution studies and with the marked preference of CuII to assume a square-based pyramidal geometry with these types of ligands, the reaction of L4 with one equivalent of Cu(BF4)2.4H2O in MeOH at room temperature yields a square-based pyramidal five co-ordinate CuII complex [Cu(L6)](BF4)2 where one of the three propylamino pendant arms of the starting ligand has been cleaved to give L6.     

Designing binuclear transition metal complexes: a new example of the versatility of N,N'-bis(2-aminobenzyl)-4,13-diaza-18-crown-6

N,N'-Bis(2-aminobenzyl)-4,13-diaza-18-crown-6 (L) is a versatile receptor able to adapt to the coordinative preferences of different metal cation guests. With first-row transition metal ions, L tends to form binuclear complexes but, depending on the nature of the particular metal ion, the structure of the binuclear complex may be very different. Herein we report a study of the structure and magnetic properties of the corresponding nickel(II) and cobalt(II) complexes. The X-ray crystal structure of the nickel complex (1), with formula [Ni2(L)(CH3CN)4](ClO4)4.CH3CN, shows that this compound presents a symmetric coordination environment with L adopting an anti arrangement. Each Ni(II) ion is six-coordinate in a distorted octahedral environment, and both metal ions are quite far from each other. On the other hand, the X-ray crystal structure of the cobalt complex (2), with formula [Co(L)(mu-OH)Co(CH3CN)](ClO4)3, reveals a rather different structure. Coordination number asymmetry is found: one of the Co(II) is five-coordinate in a distorted trigonal-bipyramidal coordination environment, while the second Co(II) ion is six-coordinate in a distorted octahedral arrangement. Now L adopts a syn arrangement and a hydroxide group acts as a bridge between both cobalt ions. This hydroxo-bridged Co(II) binuclear complex shows structural features that mimic the active site of methionine aminopeptidases. The magnetic properties of 1 and 2 have been investigated in the temperature range 2.0-300 K. Whereas 1 displays a Curie law except for temperatures below 50 K where zero-field splitting of the S= 1 ground state is observed, antiferromagnetic exchange in the singular asymmetric binuclear Co(II) complex 2 has been observed. This magnetic behaviour has been fitted considering first-order spin-orbit coupling in the assumed axially distorted octahedral site and totally quenched orbital contribution in the five-coordinate site in which zero-field splitting of the S= 3/2 ground state is operative.     

Excellent chiralselectivity in sulfur-bridged CoIIIMCoIII (M = NiII and PdII) trinuclear complexes containing 1,2-cyclohexanediamine

The reaction of [Ni(aet)2] with [CoCl2(R,R-chxn)2]+ (aet = 2-aminoethanethiolate, R,R-chxn = 1R,2R-cyclohexanediamine) in water gave a CoIIINiIICoIII trinuclear complex, DeltaRRDeltaRR-[Ni(Co(aet)(2-)(R,R-chxn))2]4+ ([1a]4+), in which two cis(S)-[Co(aet)2(R,R-chxn)]+ units are linked by a central NiII ion through sulfur bridges. The two CoIII units in [1a]4+ uniformly adopt the Delta configuration, which is induced by the chirality of the terminal R,R-chxn ligands. The central NiII ion in [1a]4+ was replaced by a PdII ion to produce an analogous CoIIIPdIICoIII trinuclear complex, DeltaRRDeltaRR-[Pd(Co(aet)2(R,R-chxn))2]4+ ([2a]4+), with retention of the Delta configuration. When racemic R,R/S,S-chxn was employed instead of R,R-chxn, not only the chirality about two CoIII centers but also the chirality about two chxn ligands was unified in the S-bridged trinuclear structure, leading to the selective formation of a pair of enantiomers, DeltaRRDeltaRR/LambdaSSLambdaSS-[M(Co(aet)2(chxn))2]4+ (M = NiII ([1b]4+) and PdII ([2b]4+)). The stereochemical and spectroscopic features of these complexes are discussed on the basis of the electronic absorption, CD, and NMR spectroscopies, along with the crystal structures of [1a]4+ and [2a]4+.     

Structures and fragmentations of cobalt(II)-cysteine complexes in the gas phase

The electronebulization of a cobalt(II)/cysteine(Cys) mixture in water/methanol (50/50) produced mainly cobalt-cationized species. Three main groups of the Co-cationized species can be distinguished in the ESI-MS spectrum: (1) the cobalt complexes including the cysteine amino acid only (they can be singly charged, for example, [Co(Cys)n- H]+ with n = 1-3 or doubly charged such as [Co + (Cys)2]2+); (2) the cobalt complexes with methanol: [Co(CH3OH)n- H]+ with n = 1-3, [Co(CH3OH)4]2+; and (3) the complexes with the two different types of ligands: [Co(Cys)(CH3OH) - H]+. Only the singly charged complexes were observed. Collision-induced dissociation (CID) products of the [Co(Cys)2]2+, [Co(Cys)2 - H]+ and [Co(Cys) - H]+ complexes were studied as a function of the collision energy, and mechanisms for the dissociation reactions are proposed. These were supported by the results of deuterium labelling experiments and by density functional theory calculations. Since [Co(Cys) - H]+ was one of the main product ions obtained upon the CID of [Co(Cys)2]2+ and of [Co(Cys)2 - H]+ under low-energy conditions, the fragmentation pathways of [Co(Cys) - H]+ and the resulting product ion structures were studied in detail. The resulting product ion structures confirmed the high affinity of cobalt(II) for the sulfur atom of cysteine.     

Synthesis, structural characterization, gas sorption and guest-exchange studies of the lightweight, porous metal-organic framework alpha-[Mg3(O2CH)6

Unsolvated magnesium formate crystallizes upon reaction of the metal nitrate with formic acid in DMF at elevated temperatures. Single-crystal XRD studies reveal the formation of [Mg3(O2CH)6 [symbol: see text] DMF], 1, a metal-organic framework with DMF molecules filling the channels of an extended diamondoid lattice. The DMF molecules in 1 can be entirely removed without disruption to the framework, giving the guest-free material alpha-[Mg3(O2CH)6], 2. Compound 2 has been characterized by both powder and single-crystal XRD studies. Thermogravimetric analyses of 1 show guest loss from 120 to 190 degrees C, with decomposition of the sample at approximately 417 degrees C. Gas sorption studies using both N2 and H2 indicate that the framework displays permanent porosity. The porosity of the framework is further demonstrated by the ability of 2 to uptake a variety of small molecules upon soaking. Single-crystal XRD studies have been completed on the six inclusion compounds [Mg3(O2CH)6 [symbol: see text] THF], 3; [Mg3(O2CH)6 [symbol: see text] Et2O], 4; [Mg3(O2CH)6 [symbol: see text] Me2CO], 5; [Mg3(O2CH)6 [symbol: see text] C6H6], 6; [Mg3(O2CH)6 [symbol: see text] EtOH], 7; and [Mg3(O2CH)(6) [symbol: see text] MeOH], 8. Analyses of the metrical parameters of 1-8 indicate that the framework has the ability to contract or expand depending on the nature of the guest present.     

Connecting [NiFe]- and [FeFe]-Hydrogenases: Mixed-Valence Nickel-Iron Dithiolates with Rotated Structures

New mixed-valence iron-nickel dithiolates are described that exhibit structures similar to those of mixed-valence diiron dithiolates. The interaction of tricarbonyl salt [(dppe)Ni(pdt)Fe(CO)(3)]BF(4) ([1]BF(4), where dppe = Ph(2)PCH(2)CH(2)PPh(2) and pdt(2-) = -SCH(2)CH(2)CH(2)S-) with P-donor ligands (L) afforded the substituted derivatives [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) incorporating L = PHCy(2) ([1a]BF(4)), PPh(NEt(2))(2) ([1b]BF(4)), P(NMe(2))(3) ([1c]BF(4)), P(i-Pr)(3) ([1d]BF(4)), and PCy(3) ([1e]BF(4)). The related precursor [(dcpe)Ni(pdt)Fe(CO)(3)]BF(4) ([2]BF(4), where dcpe = Cy(2)PCH(2)CH(2)PCy(2)) gave the more electron-rich family of compounds [(dcpe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = PPh(2)(2-pyridyl) ([2a]BF(4)), PPh(3) ([2b]BF(4)), and PCy(3) ([2c]BF(4)). For bulky and strongly basic monophosphorus ligands, the salts feature distorted coordination geometries at iron: crystallographic analyses of [1e]BF(4) and [2c]BF(4) showed that they adopt "rotated" Fe(I) centers, in which PCy(3) occupies a basal site and one CO ligand partially bridges the Ni and Fe centers. Like the undistorted mixed-valence derivatives, members of the new class of complexes are described as Ni(II)Fe(I) (S = (1)/(2)) systems according to electron paramagnetic resonance spectroscopy, although with attenuated (31)P hyperfine interactions. Density functional theory calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the spin for [1e](+) is mostly localized in a Fe(I)-centered d(z(2)) orbital, orthogonal to the Fe-P bond. The PCy(3) complexes, rare examples of species featuring "rotated" Fe centers, both structurally and spectroscopically incorporate features from homobimetallic mixed-valence diiron dithiolates. Also, when the NiS(2)Fe core of the [NiFe]-hydrogenase active site is reproduced, the "hybrid models" incorporate key features of the two major classes of hydrogenase. Furthermore, cyclic voltammetry experiments suggest that the highly basic phosphine ligands enable a second oxidation corresponding to the couple [(dxpe)Ni(pdt)Fe(CO)(2)L](+/2+). The resulting unsaturated 32e(-) dications represent the closest approach to modeling the highly electrophilic Ni-SI(a) state. In the case of L = PPh(2) (2-pyridyl), chelation of this ligand accompanies the second oxidation.     

Raman scattering properties of a protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text], vacancy; x=0.7) with lepidocrocite-type layered structure

Raman scattering spectroscopy is employed to characterize a layered titanate HxTi2-x/4[symbol: see text]x/4O4.H2O ([symbol: see text]: vacancy; x=0.7) with lepidocrocite (gamma-FeOOH)-type layered structure. Nine Raman lines corresponding to (3Ag+3B1g+3B3g) Raman-active modes expected from this orthorhombic structure (space group D2h25-Immm) are recorded at 183, 270, 387, 449, 558, 658, 704, 803, and 908 cm(-1), which are assigned to Ti-O lattice vibrations within the two-dimensional (2D) lepidocrocite-type TiO6 octahedral host layers. These intrinsic Raman bands present a clear signature that can be used for probing the protonic titanate HxTi2-x/4[symbol: see text]x/4O4.H2O and the 2D titanate nanosheets, as well as their corresponding derivates.