Electrocatalytic hydrogen evolution at low overpotentials by cobalt macrocyclic glyoxime and tetraimine complexes

Cobalt complexes supported by diglyoxime ligands of the type Co(dmgBF2)2(CH3CN)2 and Co(dpgBF2)2(CH3CN)2 (where dmgBF2 is difluoroboryl-dimethylglyoxime and dpgBF2 is difluoroboryl-diphenylglyoxime), as well as cobalt complexes with [14]-tetraene-N4 (Tim) ligands of the type [Co(TimR)X2]n+ (R=methyl or phenyl, X=Br or CH3CN; n=1 with X=Br and n=3 with X=CH3CN), have been observed to evolve H2 electrocatalytically at potentials between -0.55 V and -0.20 V vs SCE in CH3CN. The complexes with more positive Co(II/I) redox potentials exhibited lower activity for H2 production. For the complexes Co(dmgBF2)2(CH3CN)2, Co(dpgBF2)2(CH3CN)2, [Co(TimMe)Br2]Br, and [Co(TimMe)(CH3CN)2](BPh4)3, bulk electrolysis confirmed the catalytic nature of the process, with turnover numbers in excess of 5 and essentially quantitative faradaic yields for H2 production. In contrast, the complexes [Co(TimPh/Me)Br2]Br and [Co(TimPh/Me)(CH3CN)2](BPh4)3 were less stable, and bulk electrolysis only produced faradaic yields for H2 production of 20-25%. Cyclic voltammetry of Co(dmgBF2)2(CH3CN)2, [Co(TimMe)Br2]+, and [Co(TimMe)(CH3CN)2]3+ in the presence of acid revealed redox waves consistent with the Co(III)-H/Co(II)-H couple, suggesting the presence of Co(III) hydride intermediates in the catalytic system. The potentials at which these Co complexes catalyzed H2 evolution were close to the reported thermodynamic potentials for the production of H2 from protons in CH3CN, with the smallest overpotential being 40 mV for Co(dmgBF2)2(CH3CN)2 determined by electrochemistry. Consistent with this small overpotential, Co(dmgBF2)2(CH3CN)2 was also able to oxidize H2 in the presence of a suitable conjugate base. Digital simulations of the electrochemical data were used to study the mechanism of H2 evolution catalysis, and these studies are discussed.     

Theoretical analysis of mechanistic pathways for hydrogen evolution catalyzed by cobaloximes

The mechanistic pathways for hydrogen evolution catalyzed by cobalt complexes with supporting diglyoxime ligands are analyzed with computational methods. The cobaloximes studied are Co(dmgBF(2))(2) (dmg = dimethylglyoxime) and Co(dpgBF(2))(2) (dpg = diphenylglyoxime) in acetonitrile. The reduction potentials and pK(a) values are calculated with density functional theory in conjunction with isodesmic reactions, incorporating the possibility of axial solvent ligand loss during the reduction process. The solvent reorganization energies for electron transfer between the cobalt complex and a metal electrode and the inner-sphere reorganization energies accounting for intramolecular rearrangements and the possibility of ligand loss are also calculated. The relative reduction potentials agree quantitatively with the available experimental values. The pK(a)s and reorganization energies agree qualitatively with estimates based on experimental data. The calculations suggest that a peak measured at ca. -1.0 V vs SCE in cyclic voltammetry experiments for Co(dmgBF(2))(2) is more likely to correspond to the Co(II)H/Co(I)H reduction potential than the Co(III)H/Co(II)H reduction potential. The calculations also predict pK(a) values of Co-hydride complexes and reduction potentials for both cobaloximes that have not been determined experimentally. The results are consistent with a mechanism in which the Co(III) and Co(II) complexes have two axial solvent ligands and the Co(I) complex has a single axial ligand along the reaction pathway. Analysis of the free energy diagrams generated for six different monometallic and bimetallic hydrogen production pathways identified the most favorable pathways for Co(dmgBF(2))(2) and tosic acid. The thermodynamically favored monometallic pathway passes through a Co(III)H intermediate, and Co(II)H reacts with the acid to produce H(2). The thermodynamically favored bimetallic pathways also pass through the Co(III)H intermediate, but the pathways in which two Co(III)H or two Co(II)H complexes react to produce H(2) are not thermodynamically distinguishable with these methods. On the basis of the electrostatic work term associated with bringing the two cobalt complexes together in solution, the preferred bimetallic pathway involves the reaction of two Co(III)H complexes to produce H(2). This mechanistic insight is important for designing more effective catalysts for solar energy conversion.     

Oxidation of mixed-valence Co(III)/Fe(II) complexes reversed at high pH: a kinetico-mechanistic study of water oxidation

The outer-sphere oxidation of Fe(II) in the mixed-valence complex trans-[L(14S)Co(III)NCFe(II)(CN)(6)](-), being L(14S) an N(3)S(2) macrocylic donor set on the cobalt(III) center, has been studied. The comparison with the known processes of N(5) macrocycle complexes has been carried out in view of the important differences occurring on the redox potential of the cobalt center. The results indicate that the outer-sphere oxidation reactions with S(2)O(8)(2-) and [Co(ox)(3)](3-) involve a great amount of solvent-assisted hydrogen bonding that, as a consequence from the change from two amines to sulfur donors, are more restricted. This is shown by the more positive values found for DeltaS(#) and DeltaV(#). The X-ray structure of the oxidized complex has been determined, and it is clearly indicative of the above-mentioned solvent-assisted hydrogen bonding between nitrogen and cyanide donors on the cobalt and iron centers, respectively. trans-[L(14S)Co(III)NCFe(III)(CN)(6)], as well as the analogous N(5) systems trans-[L(14)Co(III)NCFe(III)(CN)(6)], trans-[L(15)Co(III)NCFe(III)(CN)(6)], and cis-[L(13)Co(III)NCFe(III)(CN)(6)], oxidize water to hydrogen peroxide at pH > 10 with a rather simple stoichiometry, i.e., [L(n)()Co(III)NCFe(III)(CN)(5)] + OH(-) --> [L(n)()Co(III)NCFe(II)(CN)(5)](-) + (1)/(2)H(2)O(2). In this way, the reversibility of the iron oxidation process is achieved. The determination of kinetic and thermal and pressure activation parameters for this water to hydrogen peroxide oxidation leads to the kinetic determination of a cyanide based OH(-) adduct of the complex. A second-order dependence on the base concentration is associated with deprotonation of this adduct to produce the final inner-sphere reduction process. The activation enthalpies are found to be extremely low (15 to 35 kJ mol(-1)) and responsible for the very fast reaction observed. The values of DeltaS(#) and DeltaV(#) (-76 to -113 J K(-1) mol(-1) and -5.5 to -8.9 cm(3) mol(-1), respectively) indicate a highly organized but not very compressed transition state in agreement with the inner-sphere one-electron transfer from O(2-) to Fe(III).     

Phenylthiyl radical complexes of gallium(III), iron(III), and cobalt(III) and comparison with their phenoxyl analogues.

Three hexadentate, asymmetric pendent arm macrocycles containing a 1,4,7-triazacyclononane-1,4-diacetate backbone and a third, N-bound phenolate or thiophenolate arm have been synthesized. In [L(1)](3)(-) the third arm is 3,5-di-tert-butyl-2-hydroxybenzyl, in [L(2)](3)(-) it is 2-mercaptobenzyl, and in [L(3)](3)(-) it is 3,5-di-tert-butyl-2-mercaptobenzyl. With trivalent metal ions these ligands form very stable neutral mononuclear complexes [M(III)L(1)] (M = Ga, Fe, Co), [M(III)L(2)] (M = Ga, Fe, Co), and [M(III)L(3)] (M = Ga, Co) where the gallium and cobalt complexes possess an S = 0 and the iron complexes an S = (5)/(2) ground state. Complexes [CoL(1)].CH(3)OH.1.5H(2)O, [CoL(3)].1.17H(2)O, [FeL(1)].H(2)O, and [FeL(2)] have been characterized by X-ray crystallography. Cyclic voltammetry shows that all three [M(III)L(1)] complexes undergo a reversible, ligand-based, one-electron oxidation generating the monocations [M(III)L(1)(*)](+) which contain a coordinated phenoxyl radical as was unambiguously established by their electronic absorption, EPR, and M?ssbauer spectra. In contrast, [M(III)L(2)] complexes in CH(3)CN solution undergo an irreversible one-electron oxidation where the putative thiyl radical monocationic intermediates dimerize with S-S bond formation yielding dinuclear disulfide species [M(III)L(2)-L(2)M(III)](2+). [GaL(3)] behaves similarly despite the steric bulk of two tertiary butyl groups at the 3,5-positions of the thiophenolate, but [Co(III)L(3)] in CH(2)Cl(2) at -20 to -61 degrees C displays a reversible one-electron oxidation yielding a relatively stable monocation [Co(III)L(3)(*)](+). Its electronic spectrum displays intense transitions in the visible at 509 nm (epsilon = 2.6 x 10(3) M(-)(1) cm(-)(1)) and 670sh, 784 (1.03 x 10(3)) typical of a phenylthiyl radical. The EPR spectrum of this species at 90 K proves the thiyl radical to be coordinated to a diamagnetic cobalt(III) ion (g(iso) = 2.0226; A(iso)((59)Co) = 10.7 G).     

Microsolvation of Co2+ and Ni2+ by acetonitrile and water: photodissociation dynamics of M(2+)(CH3CN)n(H2O)m

The microsolvation of cobalt and nickel dications by acetonitrile and water is studied by measuring photofragment spectra at 355, 532 and 560-660 nm. Ions are produced by electrospray, thermalized in an ion trap and mass selected by time of flight. The photodissociation yield, products and their branching ratios depend on the metal, cluster size and composition. Proton transfer is only observed in water-containing clusters and is enhanced with increasing water content. Also, nickel-containing clusters are more likely to undergo charge reduction than those with cobalt. The homogeneous clusters with acetonitrile M(2+)(CH(3)CN)(n) (n = 3 and 4) dissociate by simple solvent loss; n = 2 clusters dissociate by electron transfer. Mixed acetonitrile/water clusters display more interesting dissociation dynamics. Again, larger clusters (n = 3 and 4) show simple solvent loss. Water loss is substantially favored over acetonitrile loss, which is understandable because acetonitrile is a stronger ligand due to its higher dipole moment and polarizability. Proton transfer, forming H(+)(CH(3)CN), is observed as a minor channel for M(2+)(CH(3)CN)(2)(H(2)O)(2) and M(2+)(CH(3)CN)(2)(H(2)O) but is not seen in M(2+)(CH(3)CN)(3)(H(2)O). Studies of deuterated clusters confirm that water acts as the proton donor. We previously observed proton loss as the major channel for photolysis of M(2+)(H(2)O)(4). Measurements of the photodissociation yield reveal that four-coordinate Co(2+) clusters dissociate more readily than Ni(2+) clusters whereas for the three-coordinate clusters, dissociation is more efficient for Ni(2+) clusters. For the two-coordinate clusters, dissociation is via electron transfer and the yield is low for both metals. Calculations of reaction energetics, dissociation barriers, and the positions of excited electronic states complement the experimental work. Proton transfer in photolysis of Co(2+)(CH(3)CN)(2)(H(2)O) is calculated to occur via a (CH(3)CN)Co(2+)-OH(-)-H(+)(NCCH(3)) salt-bridge transition state, reducing kinetic energy release in the dissociation.     

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.     

NMR spectroscopic and X-ray crystallographic study of methylcobalt(III) compounds with saturated amine ligands

The (13)C chemical shifts of methylcobalt(III) compounds with saturated amine ligands in cis positions to the methyl group and a monodentate ligand, L = CN(-), NH(3), NO(2)(-), N(3)(-), H(2)O, or OH(-), in the trans position are reported. The amine ligands used, 1,2-ethanediamine (en), 1,3-propanediamine (tn), N,N'-bis(2-aminoethyl)-1,3-propanediamine (2,3,2-tet), N,N'-bis(3-aminopropyl)-1,2-ethanediamine (3,2,3-tet), and 1,4,8,11-tetraazacyclotetradecane (cyclam), all exert an apparent cis influence on the (13)C resonance signal of the coordinated methyl group. In the trans-[Co(en)(2)(CH(3))(L)](n+) series the (15)N resonance frequency of the coordinated en has also been measured. The influence of L on the en (15)N chemical shifts is reverse the influence on the methyl (13)C chemical shifts except in the case of L = NO(2)(-), which affects a further deshielding of the amine nitrogen nucleus. The methyl (1)J(CH) coupling constants in the trans-[Co(en)(2)(CH(3))(L)](n+) series range from 128.09 Hz (L = CN(-)) to 134.11 Hz (L = H(2)O). The crystal structures of trans-[Co(en)(2)(CH(3))(ClZnCl(3))], trans-[Co(3,2,3-tet)(CH(3))(N(3))]ClO(4), trans,trans-[(CH(3))(en)(2)Co(CN)Co(en)(2)(CH(3))](PF(6))(3)(CH(3)CN), and cis-[Co(en)(2)(CH(3))(NH(3))]ZnCl(4) were determined from low-temperature X-ray diffraction data.     

Coordination algorithms control molecular architecture: [CuI4(L2)4]4+ grid complex versus [MII2(L2)2X4]y+ side-by-side complexes (M=Mn,Co, Ni,Zn; X=solvent or anion) and [FeII(L2)3][Cl3FeIIIOFeIIICl3

The synthesis and characterisation of a pyridazine-containing two-armed grid ligand L2 (prepared from one equivalent of 3,6-diformylpyridazine and two equivalents of p-anisidine) and the resulting transition metal (Zn, Cu, Ni, Co, Fe, Mn) complexes (1-9) are reported. Single-crystal X-ray structure determinations revealed that the copper(I) complex had self-assembled as a [2 x 2] grid, [Cu(I) (4)(L2)(4)][PF(6)](4).(CH(3)CN)(H(2)O)(CH(3)CH(2)OCH(2)CH(3))(0.25) (2.(CH(3)CN)(H(2)O)(CH(3)CH(2)OCH(2)CH(3))(0.25)), whereas the [Zn(2)(L2)(2)(CH(3)CN)(2)(H(2)O)(2)][ClO(4)](4).CH(3)CN (1.CH(3)CN), [Ni(II) (2)(L2)(2)(CH(3)CN)(4)][BF(4)](4).(CH(3)CH(2)OCH(2)CH(3))(0.25) (5 a.(CH(3)CH(2)OCH(2)CH(3))(0.25)) and [Co(II) (2)(L2)(2)(H(2)O)(2)(CH(3)CN)(2)][ClO(4)](4).(H(2)O)(CH(3)CN)(0.5) (6 a.(H(2)O)(CH(3)CN)(0.5)) complexes adopt a side-by-side architecture; iron(II) forms a monometallic cation binding three L2 ligands, [Fe(II)(L2)(3)][Fe(III)Cl(3)OCl(3)Fe(III)].CH(3)CN (7.CH(3)CN). A more soluble salt of the cation of 7, the diamagnetic complex [Fe(II)(L2)(3)][BF(4)](2).2 H(2)O (8), was prepared, as well as two derivatives of 2, [Cu(I) (2)(L2)(2)(NCS)(2)].H(2)O (3) and [Cu(I) (2)(L2)(NCS)(2)] (4). The manganese complex, [Mn(II) (2)(L2)(2)Cl(4)].3 H(2)O (9), was not structurally characterised, but is proposed to adopt a side-by-side architecture. Variable temperature magnetic susceptibility studies yielded small negative J values for the side-by-side complexes: J=-21.6 cm(-1) and g=2.17 for S=1 dinickel(II) complex [Ni(II) (2)(L2)(2)(H(2)O)(4)][BF(4)](4) (5 b) (fraction monomer 0.02); J=-7.6 cm(-1) and g=2.44 for S= 3/2 dicobalt(II) complex [Co(II) (2)(L2)(2)(H(2)O)(4)][ClO(4)](4) (6 b) (fraction monomer 0.02); J=-3.2 cm(-1) and g=1.95 for S= 5/2 dimanganese(II) complex 9 (fraction monomer 0.02). The double salt, mixed valent iron complex 7.H(2)O gave J=-75 cm(-1) and g=1.81 for the S= 5/2 diiron(III) anion (fraction monomer=0.025). These parameters are lower than normal for Fe(III)OFe(III) species because of fitting of superimposed monomer and dimer susceptibilities arising from trace impurities. The iron(II) centre in 7.H(2)O is low spin and hence diamagnetic, a fact confirmed by the preparation and characterisation of the simple diamagnetic iron(II) complex 8. M?ssbauer measurements at 77 K confirmed that there are two iron sites in 7.H(2)O, a low-spin iron(II) site and a high-spin diiron(III) site. A full electrochemical investigation was undertaken for complexes 1, 2, 5 b, 6 b and 8 and this showed that multiple redox processes are a feature of all of them.     

Direct Lanthanide-Transition Metal Interactions: Synthesis of (NH(3))(2)YbFe(CO)(4) and Crystal Structures of {[(CH(3)CN)(3)YbFe(CO)(4)](2).CH(3)CN}(infinity) and [(CH(3)CN)(3)YbFe(CO)(4)](infinity)

The heterometallic complex (NH(3))(2)YbFe(CO)(4) was prepared from the reduction of Fe(3)(CO)(12) by Yb in liquid ammonia. Ammonia was displaced from (NH(3))(2)YbFe(CO)(4) by acetonitrile in acetonitrile solution, and the crystalline compounds {[(CH(3)CN)(3)YbFe(CO)(4))](2).CH(3)CN}(infinity) and [(CH(3)CN)(3)YbFe(CO)(4)](infinity) were obtained. An earlier X-ray study of {[(CH(3)CN)(3)YbFe(CO)(4)](2).CH(3)CN}(infinity) showed that it is a ladder polymer with direct Yb-Fe bonds. In the present study, an X-ray crystal structure analysis also showed that [(CH(3)CN)(3)YbFe(CO)(4)](infinity) is a sheetlike array with direct Yb-Fe bonds. Crystal data for {[(CH(3)CN)(3)YbFe(CO)(4)](2).CH(3)CN}(infinity): monoclinic space group P2(1)/c, a = 21.515(8) ?, b = 7.838(2) ?, c = 19.866(6) ?, beta = 105.47(2) degrees, Z = 4. Crystal data for [(CH(3)CN)(3)YbFe(CO)(4)](infinity): monoclinic space group P2(1)/n, a = 8.364(3) ?, b = 9.605(5) ?, c = 17.240(6) ?, beta = 92.22(3) degrees, Z = 4. Electrical conductivity measurements in acetonitrile show that these acetonitrile complexes are partially dissociated into ionic species. IR and NMR spectra of the solutions reveal the presence of [HFe(CO)(4)](-). However, upon recrystallization, the acetonitrile complexes show no evidence for the presence of [HFe(CO)(4)](-) on the basis of their IR spectra. The solid state MAS (2)H NMR spectra of deuterated acetonitrile complexes give no evidence for [(2)HFe(CO)(4)](-). It appears that rupture of the Yb-Fe bond could occur in solution to generate the ion pair [L(n)Yb](2+)[Fe(CO)(4)](2-), but then the highly basic [Fe(CO)(4)](2-) anion could abstract a proton from a coordinated acetonitrile ligand to form [HFe(CO)(4)](-). However, upon crystallization, the proton could be transferred back to the ligand, which results in the neutral polymeric species.     

The monoanionic pi-radical redox state of alpha-iminoketones in bis(ligand)metal complexes of nickel and cobalt

The electronic structures of nickel and cobalt centers coordinated by two alpha-iminoketone ligands have been elucidated using density functional theory calculations and a host of physical methods such as X-ray crystallography, cyclic voltammetry, UV-vis spectroscopy, electron paramagnetic resonance spectroscopy, and magnetic susceptibility measurements. In principle, alpha-iminoketone ligands can exist in three oxidation levels: the closed-shell neutral form (L)0, the closed-shell dianion (L(red))(2-), and the open-shell monoanion (L*)(-). Herein, the monoanionic pi-radical form (L*)(-) of alpha-iminoketones is characterized in the compounds [(L*)2Ni] (1) and [(L*)2Co] (3), where (L*)(-) is the one-electron-reduced form of the neutral ligand (t-Bu)N=CH-C(Ph)=O. The metal centers in 1 and 3 are divalent, high-spin, and coupled antiferromagnetically to two ligand pi radicals. These bis(ligand)metal complexes can be chemically oxidized by two electrons to give the dications [trans-(L)2Ni(CH3CN)2](PF6)2 (2) and [trans-(L)2Co(CH3CN)2](PF6)2 (4), wherein the ligands are in the neutral form.     

Molecular and electronic structures of tetrahedral complexes of nickel and cobalt containing N,N'-disubstituted, bulky o-diiminobenzosemiquinonate(1-) pi-radical ligands

The reaction of 2 equiv of the bulky ligand N,N'-bis(3,5-di-tert-butylphenyl)-1,2-phenylenediamine, H2[3L(PDI)], excess triethylamine, and 1 equiv of M(CH3CO2)2.4H2O (M = Ni, Co) in the presence of air in CH3CN/CH2Cl2 solution yields violet-black crystals of [Ni(II)(3L(ISQ))2] CH3CN (1) or violet crystals of [Co(3L)2] (3). By using Pd(CH3CO2)2 as starting material, green-blue crystals of [Pd(II)(3L(ISQ))2].CH3CN (2) were obtained. Single-crystal X-ray crystallography revealed that 1 and 3 contain (pseudo)tetrahedral neutral molecules [M(3L)2] (M = Ni, Co) whereas in 2 nearly square planar, neutral molecules [Pd(II)(3L(ISQ))2] are present. Temperature-dependent susceptibility measurements established that 1 and 2 are diamagnetic (S = 0) whereas 3 is paramagnetic with an S = 3/2 ground state. It is shown that 1 contains two pi radical benzosemiquinonate(1-)-type monoanions, ((3L(ISQ))(1-*), S(rad) = 1/2), and a central Ni(II) ion (d8; S = 1) which are antiferromagnetically coupled yielding the observed S(t) = 0 ground state. This result has been confirmed by broken symmetry DFT calculations of 1. In contrast, the S(t) = 3/2 ground state of 3 is more difficult to understand: the two resonance structures [Co(III)(3L(ISQ))(3L(PDI))] <--> [Co(II)(3L(PDI))(3L(IBQ))] might be invoked (for tetrahedral [Co(II)(3L(ISQ))2] containing an S(Co) = 3/2 with two antiferromagnetically coupled pi-radical ligands an S(t) = 1/2 is anticipated). Complex 2 is diamagnetic (S = 0) containing a Pd(II) ion (d8, S(Pd) = 0 in an almost square planar ligand field) and two antiferromagnetically coupled ligand radicals (S(rad) = 1/2). The electrochemistry and spectroelectrochemistry of 1, 2, and 3 have been studied, and electron-transfer series comprising the species [M(L)2]z (z = 2+, 1+, 0, 1-, 2-) have been established. All oxidations and reductions are ligand centered.     

Reaction of an osmium(VI) nitrido complex with cyanide: formation and reactivity of an osmium(III) hydrogen cyanamide complex

Reaction of [Os(VI)(N)(L(1))(Cl)(OH(2))] (1) with CN(-) under various conditions affords (PPh(4))[Os(VI)(N)(L(1))(CN)(Cl)] (2), (PPh(4))(2)[Os(VI)(N)(L(2))(CN)(2)] (3), and a novel hydrogen cyanamido complex, (PPh(4))(2)[Os(III){N(H)CN}(L(3))(CN)(3)] (4). Compound 4 reacts readily with both electrophiles and nucleophiles. Protonation and methylation of 4 produce (PPh(4))[Os(III)(NCNH(2))(L(3))(CN)(3)] (5) and (PPh(4))[Os(III)(NCNMe(2))(L(3))(CN)(3)] (6), respectively. Nucleophilic addition of NH(3), ethylamine, and diethylamine readily occur at the C atom of the hydrogen cyanamide ligand of 4 to produce osmium guanidine complexes with the general formula [Os(III){N(H)C(NH(2))NR(1)R(2)}(L(3))(CN)(3)](-) , which have been isolated as PPh(4) salts (R(1) = R(2) = H (7); R(1) = H, R(2) = CH(2)CH(3) (8); R(1) = R(2) = CH(2)CH(3) (9)). The molecular structures of 1-5 and 7 and 8 have been determined by X-ray crystallography.     

Redox-active nickel and cobalt tris(pyrazolyl)borate dithiocarbamate complexes: air-stable Co(II) dithiocarbamates

A series of new cobalt(II) and nickel(II) tris(3,5-diphenylpyrazolyl)borate (Tp(Ph2)) dithiocarbamate complexes [Tp(Ph2)M(dtc)] (M = Co, dtc = S?CNEt? 1, S?CNBz? 2 and S?CN(CH?)? 3; M = Ni, dtc = S?CNEt? 4, S?CNBz? 5 and S?CN(CH?)? 6) have been prepared by the reaction of [Tp(Ph2)MBr] with Nadtc in CH?Cl?. IR spectroscopy indicates that the Tp(Ph2) ligand is κ3 coordinated while the dithiocarbamate ligand is κ2 coordinated. 1H NMR and UV-Vis spectroscopy are consistent with high spin, five-coordinate metal centres. X-ray crystallographic studies of 1, 3 and 6 confirm the κ3 coordination of the Tp(Ph2) ligand and reveal an intermediate five-coordinate geometry with an asymmetrically coordinated dithiocarbamate ligand. Electrochemical studies of 1-6 reveal a metal centred reversible one-electron oxidation to M(III). Attempted oxidation of [Tp(Ph2)Co(dtc)] with [FeCpCp(COMe)]BF? yields [Co(dtc)?], Hpz(Ph2) and a further product which may be [Tp(Ph2)CoBp(Ph2)]. DFT calculations indicate that the low redox potentials in these complexes result from a strongly antibonding M-S σ* HOMO.     

Hydrogen storage in the dehydrated prussian blue analogues M3[Co(CN)6]2 (M = Mn, Fe, Co, Ni, Cu, Zn)

The porosity and hydrogen storage properties for the dehydrated Prussian blue analogues M3[Co(CN)6]2 (M = Mn, Fe, Co, Ni, Cu, Zn) are reported. Argon sorption isotherms measured at 87 K afford BET surface areas ranging from 560 m2/g for Ni3[Co(CN)6]2 to 870 m2/g for Mn3[Co(CN)6]2; the latter value is comparable to the highest surface area reported for any known zeolite. All six compounds show significant hydrogen sorption at 77 K and 890 Torr, varying from 1.4 wt % and 0.018 kg H2/L for Zn3[Co(CN)6]2 to 1.8 wt % and 0.025 kg H2/L for Cu3[Co(CN)6]2. Fits to the sorption data employing the Langmuir-Freundlich equation give maximum uptake quantities, resulting in a predicted storage capacity of 2.1 wt % and 0.029 kg H2/L for Cu3[Co(CN)6]2 at saturation. Enthalpies of adsorption for the frameworks were calculated from hydrogen isotherms measured at 77 and 87 K and found to increase with M varying in the order Mn < Zn < Fe < Co < Cu < Ni. In all cases, the binding enthalpies, which lie in the range of 5.3-7.4 kJ/mol, are higher than the 4.7-5.2 kJ/mol measured for Zn4O(1,4-benzenedicarboxylate)3.     

Unusual iron(II) and cobalt(II) complexes derived from monodentate arylamido ligands

The coordination chemistry of the N-substituted arylamido ligands [N(R)(C6H3R'2-2,6)] [R = SiMe3, R' = Me (L1); R = CH2But, R' = Pri (L2)] toward FeII and CoII ions was studied. The monoamido complexes [M(L1)(Cl)(tmeda)] [M = Fe (1), Co (2)] react readily with MeLi, affording the mononuclear, paramagnetic iron(II) and cobalt(II) methyl-arylamido complexes [M(L1)(Me)(tmeda)] [M = Fe (3), Co (4)]. Treatment of 2:1 [Li(L2)(THF)2]/FeCl2 affords the unusual two-coordinate iron(II) bis(arylamide) [Fe(L2)2] (5).     

Co[HO2C(CH2)3NH(CH2PO3H)2]2: a new canted antiferromagnet

A new cobalt(II) carboxylate-phosphonate, namely, Co[HO2C(CH2)3NH(CH2PO3H)2]2, with a layered architecture has been synthesized by hydrothermal reactions. The Co(II) ion in the title compound is octahedrally coordinated by six phosphonate oxygen atoms from four carboxylate phosphonate ligands. Neighboring CoO6 octahedra are interconnected by phosphonate groups into a 2D layer with a 4,4-net topology. Adjacent layers are further cross-linked via hydrogen bonds between the noncoordinate carboxylate groups and noncoordinate phosphonate oxygens. The ac and dc magnetic susceptibility and magnetization measurements indicate that Co[HO2C(CH2) 3NH(CH2PO3H)2]2 is a canted antiferromagnet with T(c) = 8.75 K.     

New linear tricobalt complex of di(2-pyridyl)amide (dpa), [Co3(dpa)4(CH3CN)2][PF6]2

Reaction of the linear tricobalt compound Co3(dpa)4Cl2 (1) (dpa = di(2-pyridyl)amide) with silver hexafluorophosphate in acetonitrile yields [Co3(dpa)4(CH3CN)2][PF6]2 (2). Two crystalline forms are obtained from the same solution, namely, a monoclinic (P2(1)) form 2xCH3CNx2Et2O and a triclinic (P1) form, 2x3CH3CN. The tricobalt units in both crystals are essentially symmetrical, though this is not required by crystal symmetry, with Co-Co distances in the range 2298-2304 A. Each of the two terminal Co atoms is coordinated to an acetonitrile molecule with Co-N distances in the range 2068-2111 A at 213 K. The spiral arrangement of ligands gives an overall idealized D4 point group symmetry for the cation [Co3(dpa)4(CH3CN)2]2+ . Chiral crystals of both delta and lambda configurations in the P2(1) form have been isolated. The absolute configurations were determined by X-ray crystallography and their mirror-image circular dichroism spectra measured. The D4 symmetry of the cation appears to be preserved in solution as judged by the presence of only five proton resonance signals in the 1H NMR spectrum. Magnetic susceptibility measurements in the solid state indicates that 2 has a doublet ground state and exhibits an increase of the effective moment at high temperature (approximately 160 K) due to a spin crossover process.     

Tuning redox potentials of bis(imino)pyridine cobalt complexes: an experimental and theoretical study involving solvent and ligand effects

The structure and electrochemical properties of a series of bis(imino)pyridine Co(II) complexes (NNN)CoX(2) and [(NNN)(2)Co][PF(6)](2) (NNN = 2,6-bis[1-(4-R-phenylimino)ethyl]pyridine, with R = CN, CF(3), H, CH(3), OCH(3), N(CH(3))(2); NNN = 2,6-bis[1-(2,6-(iPr)(2)-phenylimino)ethyl]pyridine and X = Cl, Br) were studied using a combination of electrochemical and theoretical methods. Cyclic voltammetry measurements and DFT/B3LYP calculations suggest that in solution (NNN)CoCl(2) complexes exist in equilibrium with disproportionation products [(NNN)(2)Co](2+) [CoCl(4)](2-) with the position of the equilibrium heavily influenced by both the solvent polarity and the steric and electronic properties of the bis(imino)pyridine ligands. In strong polar solvents (e.g., CH(3)CN or H(2)O) or with electron donating substituents (R = OCH(3) or N(CH(3))(2)) the equilibrium is shifted and only oxidation of the charged products [(NNN)(2)Co](2+) and [CoCl(4)](2-) is observed. Conversely, in nonpolar organic solvents such as CH(2)Cl(2) or with electron withdrawing substituents (R = CN or CF(3)), disproportionation is suppressed and oxidation of the (NNN)CoCl(2) complexes leads to 18e(-) Co(III) complexes stabilized by coordination of a solvent moiety. In addition, the [(NNN)(2)Co][PF(6)](2) complexes exhibit reversible Co(II/III) oxidation potentials that are strongly dependent on the electron withdrawing/donating nature of the N-aryl substituents, spanning nearly 750 mV in acetonitrile. The resulting insight on the regulation of redox properties of a series of bis(imino)pyridine cobalt(II) complexes should be particularly valuable to tune suitable conditions for reactivity.     

Chemistry of cobalt acetate. 7. Electrochemical oxidation of mu3-oxo-centered cobalt(III) acetate trimers

Electrochemical and spectroelectrochemical properties of five cobalt(III) acetate complexes [CoIII3(mu3-O)(CH3CO2)5(OR)(py)3][PF6] are described, where py=pyridine and R=OCCH3 (A), H (B), CH3 (C), CH2CH=CH2 (D), and CH2C6H5 (E). Each is reduced irreversibly as observed by cyclic voltammetry at room temperature and at -40 degrees C in acetonitrile at scan rates up to 20 V s(-1), but oxidized reversibly to a mixed-valence Co(III)2Co(IV) species at approximately 1.23 V vs the ferrocenium/ferrocene couple. Controlled potential coulometry confirmed a one-electron-oxidation process. Spectroelectrochemical oxidation of A at 5 degrees C showed isosbestic points in the electronic absorption spectrum that showed the oxidized complex to be stable in solution for at least 1 h.     

Theoretical studies of the mechanism of catalytic hydrogen production by a cobaloxime

Our theoretical studies of the standard reduction potentials of the molecular complex [Co(II)(dmgBF(2))(2)](0) (dmgBF(2) = difluoro-boryldimethylglyoximate) in acetonitrile solution shed light on its electrocatalytic mechanism for hydrogen production. Three such mechanisms have been proposed, all proceeding through the formation of Co(III)H. Our results indicate that the mechanism involving a Co(II)H intermediate is the most likely.