Synthesis, structure, and electrocatalysis of diiron C-functionalized propanedithiolate (PDT) complexes related to the active site of [FeFe]-hydrogenases

New C-functionalized propanedithiolate-type model complexes (1-8) have been synthesized by functional transformation reactions of the known complex [(mu-SCH2)2CH(OH)]Fe2(CO)6 (A). Treatment of A with the acylating agents PhC(O)Cl, 4-pyridinecarboxylic acid chloride, 2-furancarbonyl chloride, and 2-thiophenecarbonyl chloride in the presence of Et3N affords the expected C-functionalized complexes [(mu-SCH2)2CHO2CPh]Fe2(CO)6 (1), [(mu-SCH2)2CHO2CC5H4N-4]Fe2(CO)6 (2), [(mu-SCH2)2CHO2CC4H3O-2]Fe2(CO)6 (3), and [(mu-SCH2)2CHO2CC4H3S-2]Fe2(CO)6 (4). However, when A is treated with the phosphatizing agents Ph2PCl, PCl3 and PBr3, both C- and Fe-functionalized complexes [(mu-SCH2) 2CHOPPh2-eta1]Fe2(CO)5 (5), [(mu-SCH2) 2CHOPCl2-eta1]Fe2(CO)5 (6), and [(mu-SCH2) 2CHOPBr2-eta1]Fe2(CO)5 (7) are unexpectedly obtained via intramolecular CO substitution by P atoms of the initially formed phosphite complexes. The simplest C-functionalized model complex [(mu-SCH2) 2CO]Fe2(CO)6 (8) can be produced by oxidation of A with Dess-Martin reagent. While 8 is found to be an electrocatalyst for proton reduction to hydrogen, starting complex A can be prepared by another method involving the reaction of HC(OH)(CH2Br)2 with the in situ generated (mu-LiS) 2Fe2(CO)6. X-ray crystallographic studies reveal that the bridgehead C atom of 8 is double-bonded to an O atom to form a ketone functionality, whereas the bridgehead C atoms of A, 1, 3, and 4 are equatorially-bonded to their functionalities and those of 5-7 axially-bonded to their functionalities due to formation of the corresponding P-Fe bond-containing heterocycles.     

Preparation of stable alkyl complexes of Ni(I) and their one-electron oxidation to Ni(II) complex cations

The three-coordinate nickel(I) alkyl complexes (dtbpe)Ni(CH2CMe3) (2), (dtbpe)Ni(CH2SiMe3) (3), and (dtbpe)Ni(CH2CMe2Ph) (4) have been prepared by treatment of [(dtbpe)NiCl]2 with alkyllithium reagents. While thermally robust, they each undergo mild one-electron oxidation to give the corresponding Ni(II) complex cations [(dtbpe)Ni(CH2CMe3)+] (5), [(dtbpe)Ni(CH2SiMe3)+] (6), and [(dtbpe)Ni(CH2CMe2Ph)+] (7) as red-brown [PF6-] or [BArF4-] salts. In contrast to cationic amido and phosphido analogues that undergo alpha-deprotonation to afford imido and phosphinidene derivatives, deprotonation of 5-7 occurs at a gamma-CH3 group to give metallacyclobutane products (dtbpe)Ni(CH2CMe2CH2) (8), (dtbpe)Ni(CH2SiMe2CH2) (9), and (dtbpe)Ni(CH2CPhMeCH2) (10), not (dtbpe)Ni=CHR.     

Self-assembled polynuclear clusters derived from some flexible polydentate dihydrazide ligands

Polynuclear manganese(II), cobalt(II)/(III), iron(II)/(III) and nickel(II) complexes of a group of flexible polydentate dihydrazone ligands, based on pyridine-2,6-dipicolinic (A), oxalic (B) and malonic (C) subunits are described. Structural details are reported for the linear dinuclear complexes [Ni2(2poap)2(H2O)2](NO3)4 . 2CH3OH . 2.5H2O (1), [Mn2(pttp)(NO3)2(CH3OH)2(H2O)2](NO3)2 . H2O (2) and [Mn2(mapttp)2(NO3)2(H2O)2](NO3)2 . 10H2O (3), a square tetranuclear complex [Co4(pttp)4]Br6 . 9H2O (4), a tetranuclear tetrahedral complex [Ni4(pttp)6](BF4)6F2 . 14H2O (7), and a mixed spin state tetranuclear Ni(II) complex [(2pyoap)2Ni4(CH3OH)4] . 1.5CH3OH (10), with a diamond-like arrangement of metal ions. The paramagnetic metal centers are well separated in each case, leading to weak antiferromagnetic coupling or non-existent spin exchange.     

Bidentate phosphine ligand based Fe2S2-containing macromolecules: synthesis, characterization, and catalytic electrochemical hydrogen production

The reaction of [Fe2(CO)6(mu-SCH2)2NCH2CH2N(mu-SCH2)2Fe2(CO)6] (1) with 1,2-bis(diphenylphosphino)ethane in the presence of Me3NO.2H2O affords two structurally different metallomacromolecules: a dimer of the type [{Fe2(CO)5(mu-SCH2)2NCH2CH2N(mu-SCH2)2Fe2(CO)5}(Ph2PCH2)2] (2) and a tetramer species containing eight iron centers with an overall formula of [{Fe2(CO)6(mu-SCH2)2NCH2CH2N(mu-SCH2)2Fe2(CO)5}2(Ph2PCH2)2] (3). Their structures have been determined by X-ray crystallography, showing one macrocyclic complex (2) and one linear complex (3). Electrochemical hydrogen evolution catalyzed by these two complexes with ca. 80-90 single-run turnovers is observed, indicating good potential as catalysts for future applications.     

Homodinuclear iron thiolate nitrosyl compounds [(ON)Fe(S,S-C6H4)2 Fe(NO)2]- and [(ON)Fe(SO2,S-C6H4)(S,S-C6H4)Fe(NO)2]- with {Fe(NO)}7-{Fe(NO)2}9 electronic coupling: new members of a class of dinitrosyl iron complexes

Reaction of Fe(CO)2(NO)2 and [(ON)Fe(S,S-C6H3R)2]- (R = H (1), CH3 (1-Me))/[(ON)Fe(SO2,S-C6H4)(S,S-C6H4)]- (4) in THF afforded the diiron thiolate/sulfinate nitrosyl complexes [(ON)Fe(S,S-C6H3R)2 Fe(NO)2]- (R = H (2), CH3 (2-Me)) and [(ON)Fe(S,SO2-C6H4)(S,S-C6H4)Fe(NO)2]- (3), respectively. The average N-O bond lengths ([Fe(NO)2] unit) of 1.167(3) and 1.162(4) A in complexes 2 and 3 are consistent with the average N-O bond length of 1.165 A observed in the other structurally characterized dinitrosyl iron complexes with an {Fe(NO)2}9 core. The lower nu(15NO) value (1682 cm(-1) (KBr)) of the [(15NO)FeS4] fragment of [(15NO)Fe(S,S-C6H3CH3)2 Fe(NO)2]- (2-Me-15N), compared to that of [(15NO)Fe(S,S-C6H3CH3)2]- (1-Me-15N) (1727 cm(-1) (KBr)), implicates the electron transfer from {Fe(NO)2}10 Fe(CO)2(NO)2 to complex 1-Me/1 may occur in the process of formation of complex 2-Me/2. Then, the electronic structures of the [(NO)FeS4] and [S2Fe(NO)2] cores of complexes 2, 2-Me, and 3 were best assigned according to the Feltham-Enemark notation as the {Fe(NO)}7-{Fe(NO)2}9 coupling (antiferromagnetic interaction with a J value of -182 cm(-1) for complex 2) to account for the absence of paramagnetism (SQUID) and the EPR signal. On the basis of Fe-N(O) and N-O bond distances, the dinitrosyliron {L2Fe(NO)2} derivatives having an Fe-N(O) distance of approximately 1.670 A and a N-O distance of approximately 1.165 A are best assigned as {Fe(NO)2}9 electronic structures, whereas the Fe-N(O) distance of approximately 1.650 A and N-O distance of approximately 1.190 A probably imply an {Fe(NO)2}10 electronic structure.     

Chemistry of nitrosyliron complexes supported by a β-diketiminate ligand

Several nitrosyl complexes of Fe and Co have been prepared using the sterically hindered Ar-nacnac ligand (Ar-nacnac = anion of [(2,6-diisopropylphenyl)NC(Me)](2)CH). The dinitrosyliron complexes [Fe(NO)(2)(Ar-nacnac)] (1) and (Bu(4)N)[Fe(NO)(2)(Ar-nacnac)] (2) react with [Fe(III)(TPP)Cl] (TPP = tetraphenylporphine dianion) to generate [Fe(II)(NO)(TPP)] and the corresponding mononitrosyliron complexes. The factors governing NO transfer with dinitrosyliron complexes (DNICs) 1 and 2 are evaluated, together with the chemistry of the related mononitrosyliron complex, [Fe(NO)Br(Ar-nacnac)] (4). The synthesis and properties of the related cobalt dinitrosyl [Co(NO)(2)(Ar-nacnac)] (3) is also discussed for comparison to DNICs 1 and 2. The solid-state structures of several of these compounds as determined by X-ray crystallography are reported.     

Nitrosylated iron-thiolate-sulfinate complexes with {Fe(NO)}6/7 electronic cores: relevance to the transformation between the active and inactive NO-bound forms of iron-containing nitrile hydratases

The five-coordinated iron-thiolate nitrosyl complexes [(NO)Fe(S,S-C6H3R)2]- (R = H (1), m-CH3 (2)), [(NO)Fe(S,S-C6H2-3,6-Cl2)2]- (3), [(NO)Fe(S,S-C6H3R)2]2- (R = H (10), m-CH3 (11)), and [(NO)Fe(S,S-C6H2-3,6-Cl2)2]2- (12) have been isolated and structurally characterized. Sulfur oxygenation of iron-thiolate nitrosyl complexes 1-3 containing the {Fe(NO)}6 core was triggered by O2 to yield the S-bonded monosulfinate iron species [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]- (R = H (4), m-CH3 (5)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2(2-) (6), respectively. In contrast, attack of O2 on the {Fe(NO)}7 complex 10 led to the formation of complex 1 accompanied by the minor products, [Fe(S,S-C6H4)2]2(2-) and [NO3]- (yield 9%). Reduction of complexes 4-6 by [EtS]- in CH3CN-THF yielded [(NO)Fe(S,SO2-C6H3R)(S,S-C6H3R)]2- (R = H (7), m-CH3 (8)) and [(NO)Fe(S,SO2-C6H2-3,6-Cl2)(S,S-C6H2-3,6-Cl2)]2- (9) along with (EtS)2 identified by 1H NMR. Compared to complex 10, complexes 7-9 with the less electron-donating sulfinate ligand coordinated to the {Fe(NO)}7 core were oxidized by O2 to yield complexes 4-6. Obviously, the electronic perturbation of the {Fe(NO)}7 core caused by the coordinated sulfinate in complexes 7-9 may serve to regulate the reactivity of complexes 7-9 toward O2. The iron-sulfinate nitrosyl species with the {Fe(NO)}6/7 core exhibit the photolabilization of sulfur-bound [O] moiety. Complexes 1-4-7-10 (or 2-5-8-11 and 3-6-9-12) are interconvertible under sulfur oxygenation, redox processes, and photolysis, respectively.     

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.     

Two-coordinate d9 complexes. synthesis and oxidation of NHC nickel(I) amides

Reaction of the d9-d9 Ni(I) monochloride dimer, [(IPr)Ni(mu-Cl)]2 (1), with NaN(SiMe3)2 and LiNHAr (Ar = 2,6-diisopropylphenyl) gives the novel monomeric, 2-coordinate Ni(I) complexes (IPr)Ni{N(SiMe3)2} (2) and (IPr)Ni(NHAr) (3). Reaction of 2 with Cp2Fe+ results in its 1-e- oxidation followed by beta-Me elimination to give a base-stabilized iminosilane complex [(IPr)Ni(CH3){kappa1-N(SiMe3)=SiMe2.Et2O}][BArF4] (6). Oxidation of 3 gives [(IPr)Ni(eta3-NHAr)(THF)][BArF4] (4), which upon loss of THF affords dimeric [(IPr)Ni(N,eta3:NHC6iPr2H3)]2[BArF4]2 (5).     

Sulfur ligand substitution at the nickel(II) sites of cubane-type and cubanoid NiFe3S4 clusters relevant to the C-clusters of carbon monoxide dehydrogenase

Substitution reactions at the nickel site of the cubane-type cluster [(Ph3P)NiFe3S4(LS3)]2- (2) have been investigated in the course of a synthetic approach to the C-clusters of CODH. Reaction of 2 with RS- or toluene-3,4-dithiolate affords [(RS)NiFe3S4(LS3)]3- (R = Et (5), H (6)) or [(tdt)NiFe3S4(LS3)]3- (7), demonstrating that anionic sulfur ligands can be bound at the NiII site. Clusters 5 and 6 contain tetrahedral Ni(micro3-S)3(SR) sites. Cluster 7 is of particular interest because it includes a cubanoid NiFe3(micro2-S)(micro3-S)3 core and an approximately planar Ni(tdt)(micro3-S)2 unit. The cubanoid structure is found in all C-clusters, and an NiS4-type unit has been reported in C. hydrogenoformans CODH. Clusters 5/6 are formulated to contain the core [NiFe3S4]1+ identical with Ni2+ (S = 1) + [Fe3S4]1- (S = 5/2) and 7 the core [NiFe3S4]2+ identical with Ni2+ (S = 0) + [Fe3S4]0 (S = 2) on the basis of structure, 57Fe isomer shifts, and 1H NMR isotropic shifts. Also reported are [(EtS)CuFe3S4(LS3)]3- (9) and [Fe4S4(LS3)(tdt)]3- (11). The structures of 5-7, 9, and 11 are presented. Cluster 11, with a five-coordinate Fe(tdt)(micro3-S)3 site, provides a clear structural contrast with 7, which is currently the closest approach to a C-cluster but lacks the exo iron atom found in the NiFe4S4,5 cores of the native clusters. (CODH = carbon monoxide dehydrogenase, LS3 = 1,3,5-tris((4,6-dimethyl-3-mercaptophenyl)thio)-2,4,6-tris(p-tolylthio)benzene(3-), tdt = toluene-3,4-dithiolate).     

Family of carboxylate- and nitrate-diphenoxo triply bridged dinuclear Ni(II)Ln(III) complexes (Ln = Eu, Gd, Tb, Ho, Er, Y): synthesis, experimental and theoretical magneto-structural studies, and single-molecule magnet behavior

Seven acetate-diphenoxo triply bridged M(II)-Ln(III) complexes (M(II) = Ni(II) and Ln(III) = Gd, Tb, Ho, Er, and Y; M(II) = Zn(II) and Ln(III) = Ho(III) and Er(III)) of formula [M(μ-L)(μ-OAc)Ln(NO(3))(2)], one nitrate-diphenoxo triply bridged Ni(II)-Tb(III) complex, [Ni(μ-L)(μ-NO(3))Tb(NO(3))(2)]·2CH(3)OH, and two diphenoxo doubly bridged Ni(II)-Ln(III) complexes (Ln(III) = Eu, Gd) of formula [Ni(H(2)O)(μ-L)Ln(NO(3))(3)]·2CH(3)OH have been prepared in one pot reaction from the compartmental ligand N,N',N"-trimethyl-N,N"-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H(2)L). Moreover, Ni(II)-Ln(III) complexes bearing benzoate or 9-anthracenecarboxylate bridging groups of formula [Ni(μ-L)(μ-BzO)Dy(NO(3))(2)] and [Ni(μ-L)(μ-9-An)Dy(9-An)(NO(3))(2)]·3CH(3)CN have also been successfully synthesized. In acetate-diphenoxo triply bridged complexes, the acetate bridging group forces the structure to be folded with an average hinge angle in the M(μ-O(2))Ln bridging fragment of ~22°, whereas nitrate-diphenoxo doubly bridged complexes and diphenoxo-doubly bridged complexes exhibit more planar structures with hinge angles of ~13° and ~2°, respectively. All Ni(II)-Ln(III) complexes exhibit ferromagnetic interactions between Ni(II) and Ln(III) ions and, in the case of the Gd(III) complexes, the J(NiGd) coupling increases weakly but significantly with the planarity of the M-(O)(2)-Gd bridging fragment and with the increase of the Ni-O-Gd angle. Density functional theory (DFT) theoretical calculations on the Ni(II)Gd(III) complexes and model compounds support these magneto-structural correlations as well as the experimental J(NiGd) values, which were found to be ~1.38 and ~2.1 cm(-1) for the folded [Ni(μ-L)(μ-OAc)Gd(NO(3))(2)] and planar [Ni(H(2)O)(μ-L)Gd(NO(3))(3)]·2CH(3)OH complexes, respectively. The Ni(II)Dy(III) complexes exhibit slow relaxation of the magnetization with Δ/k(B) energy barriers under 1000 Oe applied magnetic fields of 9.2 and 10.1 K for [Ni(μ-L)(μ-BzO)Dy(NO(3))(2)] and [Ni(μ-L)(μ-9-An)Dy(9-An)(NO(3))(2)]·3CH(3)CN, respectively.     

Equilibrium of low- and high-spin states of Ni(II) complexes controlled by the donor ability of the bidentate ligands

Low-spin nickel(II) complexes containing bidentate ligands with modulated nitrogen donor ability, Py(Bz)2 or MePy(Bz)2 (Py(Bz)2 = N,N-bis(benzyl)-N-[(2-pyridyl)methyl]amine, MePy(Bz)2 = N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)methyl]amine), and a beta-diketonate derivative, tBuacacH (tBuacacH = 2,2,6,6-tetramethyl-3,5-heptanedione), represented as [Ni(Py(Bz)2)(tBuacac)](PF6) (1) and [Ni(MePy(Bz)2)(tBuacac)](PF6) (2) have been synthesized. In addition, the corresponding high-spin nickel(II) complexes having a nitrate ion, [Ni(Py(Bz)2)(tBuacac)(NO3)] (3) and [Ni(MePy(Bz)2)(tBuacac)(NO3)] (4), have also been synthesized for comparison. Complexes 1 and 2 have tetracoordinate low-spin square-planar structures, whereas the coordination environment of the nickel ion in 4 is a hexacoordinate high-spin octahedral geometry. The absorption spectra of low-spin complexes 1 and 2 in a noncoordinating solvent, dichloromethane (CH2Cl2), display the characteristic absorption bands at 500 and 540 nm, respectively. On the other hand, the spectra of a CH2Cl2 solution of high-spin complexes 3 and 4 exhibit the absorption bands centered at 610 and 620 nm, respectively. The absorption spectra of 1 and 2 in N,N-dimethylformamide (DMF), being a coordinating solvent, are quite different from those in CH2Cl2, which are nearly the same as those of 3 and 4 in CH2Cl2. This result indicates that the structures of 1 and 2 are converted from a low-spin square-planar to a high-spin octahedral configuration by the coordination of two DMF molecules to the nickel ion. Moreover, complex 1 shows thermochromic behavior resulting from the equilibrium between low-spin square-planar and high-spin octahedral structures in acetone, while complex 2 exists only as a high-spin octahedral configuration in acetone at any temperature. Such drastic differences in the binding constants and thermochromic properties can be ascribed to the enhancement of the acidity of the nickel ion of 2 by the steric effect of the o-methyl group in the MePy(Bz)2 ligand in 2, which weakens the Ni-N(pyridine) bond length compared with that of the nonsubstituted Py(Bz)2 ligand in 1.     

Structural and magnetic studies of Schiff base complexes of nickel(II) nitrite: change in crystalline state, ligand rearrangement and a very rare μ-nitrito-1κO:2κN:3κO' bridging mode

Four new nickel(II) complexes, [Ni(2)L(2)(NO(2))(2)]·CH(2)Cl(2)·C(2)H(5)OH, 2H(2)O (1), [Ni(2)L(2)(DMF)(2)(μ-NO(2))]ClO(4)·DMF (2a), [Ni(2)L(2)(DMF)(2)(μ-NO(2))]ClO(4) (2b) and [Ni(3)L'(2)(μ(3)-NO(2))(2)(CH(2)Cl(2))](n)·1.5H(2)O (3) where HL = 2-[(3-amino-propylimino)-methyl]-phenol, H(2)L(') = 2-({3-[(2-hydroxy-benzylidene)-amino]-propylimino}-methyl)-phenol and DMF = N,N-dimethylformamide, have been synthesized starting with the precursor complex [NiL(2)]·2H(2)O, nickel(ii) perchlorate and sodium nitrite and characterized structurally and magnetically. The structural analyses reveal that in all the complexes, Ni(II) ions possess a distorted octahedral geometry. Complex 1 is a dinuclear di-μ(2)-phenoxo bridged species in which nitrite ion acts as chelating co-ligand. Complexes 2a and 2b also consist of dinuclear entities, but in these two compounds a cis-(μ-nitrito-1κO:2κN) bridge is present in addition to the di-μ(2)-phenoxo bridge. The molecular structures of 2a and 2b are equivalent; they differ only in that 2a contains an additional solvated DMF molecule. Complex 3 is formed by ligand rearrangement and is a one-dimensional polymer in which double phenoxo as well as μ-nitrito-1κO:2κN bridged trinuclear units are linked through a very rare μ(3)-nitrito-1κO:2κN:3κO' bridge. Analysis of variable-temperature magnetic susceptibility data indicates that there is a global weak antiferromagnetic interaction between the nickel(ii) ions in four complexes, with exchange parameters J of -5.26, -11.45, -10.66 and -5.99 cm(-1) for 1, 2a, 2b and 3, respectively.     

Pyrrole-appended derivatives of O-confused oxaporphyrins and their complexes with nickel(II), palladium(II), and silver(III)

Condensation of 2,4-bis(phenylhydroxymethyl)furan with pyrrole and p-toluylaldehyde formed, instead of the expected 5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin, a pyrrole addition product [(H,pyr)OCPH]H(2); this product can formally be considered as an effect of hydrogenation of 3-(2'-pyrrolyl)-5,20-diphenyl-10,15-di(p-tolyl)-2-oxa-21-carbaporphyrin ([(pyr)OCPH]H). The new oxacarbaporphyrinoid presents the (1)H NMR spectroscopy features of an aromatic molecule, including the upfield shift of the inner H21 atom. Insertion of NiCl(2) or PdCl(2) into [(H,pyr)OCPH]H(2) gave two structurally related organometallic complexes, [(pyr)OCP]Ni(II)] and [(pyr)OCP]Pd(II)], in which the metal ions are bound by three pyrrolic nitrogens and the trigonally hybridized C21 atom of the inverted furan. The reaction of [(H,pyr)OCPH]H(2) with silver(I) acetate yields a stable Ag(III) complex [(C(2)H(5)O,pyr)OCP]Ag(III)] substituted at the C3 position by the ethoxy and pyrrole moieties. The macrocyclic frame of [(H,pyr)OCPH]H(2) is conserved. Addition of trifluoroacetic acid to [(C(2)H(5)O,pyr)OCP]Ag(III)] yielded a new aromatic complex [(pyr)OCP]Ag(III)](+). The structures of [(pyr)OCP]Ni(II)] and [(C(2)H(5)O,pyr)OCP]Ag(III)] have been determined by X-ray crystallography. In both molecules the macrocycles are only slightly distorted from planarity and the nickel(II) and silver(III) are located in the NNNC plane. The dihedral angle between the macrocyclic and appended-pyrrole planes of [(pyr)OCP]Ni(II)] reflects the biphenyl-like arrangement with the NH group pointing out toward the adjacent phenyl ring on the C5 position. Tetrahedral geometry around the C3 atom was detected for [(C(2)H(5)O,pyr)OCP]Ag(III)]. The Ni[bond]C and Ag[bond]C bond lengths are similar to other nickel(II) or silver(III) carbaporphyrinoids where the trigonal carbon atom coordinates the metal ion. The trend detected in the (13)C chemical shifts for the appended-pyrrole resonances has been rationalized by the extent of effective conjugation between the macrocycle and the appended pyrrole moiety controlled by the hybridization of the C3 atom and the metal ion oxidation state. The dianionic or trianionic macrocyclic core of the pyrrole-appended derivatives is favored to match the oxidation state of nickel(II), palladium(II), or silver(III), respectively.     

Anionic Roussin's red esters (RREs) syn-/anti-[Fe(mu-SEt)(NO)2]2(-): the critical role of thiolate ligands in regulating the transformation of RREs into dinitrosyl iron complexes and the anionic RREs

The anionic syn-/ anti-[Fe(mu-SEt)(NO) 2] 2 (-) ( 2a) were synthesized and characterized by IR, UV-vis, EPR, and X-ray diffraction. The geometry of the [Fe(mu-S) 2Fe] core is rearranged in going from [{Fe(NO) 2} (9)-{Fe(NO) 2} (9)] Roussin's red ester [Fe(mu-SEt)(NO) 2] 2 ( 1a) (Fe...Fe distance of 2.7080(5) A) to the [{Fe(NO) 2} (9)-{Fe(NO) 2} (10)] complex 2a (Fe...Fe distance of 2.8413(6) A) to minimize the degree of Fe...Fe interaction to stabilize complex 2a. On the basis of X-ray absorption (Fe K- and L-edge), EPR and SQUID, complex 2a is best described as the anionic [{Fe(NO) 2} (9)-{Fe(NO) 2} (10)] Roussin's red ester with the fully delocalized mixed-valence core. The complete bridged-thiolate cleavage yielded DNIC [(EtS) 2Fe(NO) 2] (-) ( 3a) in the reaction of 2 equiv of [EtS] (-) and complex 1a, whereas reaction of 2 equiv of [(t)BuS] (-) with [Fe(micro-S (t)Bu)(NO) 2] 2 (1b) gave DNIC [((t)BuS) 2Fe(NO) 2] (-) (3b) and the anionic Roussin's red ester [Fe(mu-S (t)Bu)(NO) 2] 2 (-) (2b) through bridged-thiolate cleavage in combination with reduction. In contrast to the inertness of DNIC 3b toward complex 1b, nucleophile DNIC 3a induces the reduction of complex 1a to produce the anionic Roussin's red ester 2a. Interestingly, dissolution of complex 3a in MeOH at 298 K finally led to the formation of a mixture of complexes 2a and 3a, in contrast to the dynamic equilibrium of complexes 3b and 1b observed in dissolution of complex 3b in MeOH. These results illustrate the aspect of how the steric structures of nucleophiles ([EtS] (-) vs [ (t)BuS] (-) and [(EtS) 2Fe(NO)2](-) vs [((t)BuS) 2Fe(NO)2] (-)) function to determine the reaction products.     

The first example of macrocycles containing butterfly transition metal cluster cores via novel tandem reactions

A novel type of double butterfly, two mu-CO-containing dianions {[(mu-CO)Fe2(CO)6]2[mu-SCH2(CH2OCH2)nCH2S-mu]}2- (m1, n = 2, 3), has been synthesized from dithiol HSCH2(CH2OCH2)nCH2SH (n = 2, 3), Fe3(CO)12, and Et3N in THF at room temperature. While dianions m1 react in situ with CS2 followed by treatment with dihalide 1,4-(BrCH2)2C6H4 or 1,4-I(CH2)4I to give macrocyclic clusters [mu-SCH2(CH2OCH2)nCH2S-mu](mu-CS2ZCS2-mu)[Fe2(CO)6]2 (1a, n = 2, Z = 1,4-(CH2)2C6H4; 1b, n = 3, Z = (CH2)4), reactions of dianions m1 with (mu-S2)Fe2(CO)6 followed by treatment with dihalide 1,4-I(CH2)4I afford macrocyclic clusters [mu-SCH2(CH2OCH2)nCH2S-mu]{[Fe2(CO)6]2(mu4-S)}2[mu-S(CH2)4S-mu] (2a, n = 2; 2b, n = 3). The crystal structures of 1a and 2b are described.     

Influence of Sulfur Metalation on the Accessibility of the Ni(II/I) Couple in [N,N'-Bis(2-mercaptoethyl)-1,5-diazacyclooctanato]nickel(II): Insight into the Redox Properties of [NiFe]-Hydrogenase

A redox model study of [NiFe] hydrogenase has examined a series of five polymetallics based on the metalation of the dithiolate complex [1,5-bis(mercaptoethyl)-1,5-diazacyclooctane]Ni(II), Ni-1. Crystal structures of three polymetallics of the series have been reported earlier: [(Ni-1)(2)()Ni]Cl(2)(), [(Ni-1)(2)()FeCl(2)()](2)(), and [(Ni-1)(3)()(ZnCl)(2)()]Cl(2)(). Two are described here: [(Ni-1)(2)()Pd]Cl(2)().2H(2)()Ocrystallizes in the monoclinic system, space group P2(1)/c with cell constants a = 12.212(4) ?, b = 7.642(2) ?, c = 16.625(3) ?, beta = 107.69(2) degrees, V = 1443.230(0) ?(3), Z = 2, R = 0.051, and R(w) = 0.056. [(Ni-1)(2)()CoCl]PF(6)() crystallizes in the triclinic system, space group P&onemacr;, with cell constants a = 8.14(2) ?, b = 13.85(2) ?, c = 15.67(2) ?, alpha = 113.59(10) degrees, beta = 101.84(14) degrees, gamma = 94.0(2) degrees, V = 1561.620(0)?(3), Z = 2, R = 0.072, and R(w) = 0.077. In all Ni-1 serves as a bidentate metallothiolate ligand with a "hinge" angle in the range 105-118 degrees and Ni-M distances of 2.7- 3.7 ?. The most accessible redox event is shown by EPR and electrochemistry to reside in the N(2)S(2)Ni unit and is the Ni(II/I) couple. Charge neutralization of the thiolate sulfurs by metalation can (dependent on the interacting metal) stabilize the Ni(I) state as efficiently as methylation forming a thioether. The implication of these results for the heterometallic active site of [NiFe]-hydrogenase as structured from Desulfovibrio gigas (Volbeda, A., et al. Nature, 1995, 373, 580), the generality of the Ni(&mgr;-SR)(2)M hinge structure, and a possible explanation for the unusual redox potentials are discussed.     

Mono- and dinuclear nickel(II) complexes of resolved Schiff-base ligands with extended quinoline substituents

The coordination chemistry of four enantiopure tetradentate bis(iminoquinoline) ligands with nickel(II) salts is reported. The previously reported ligands CBQ, CPQ, BBQ, and BPQ result from the condensation of (1R,2R)-cyclohexyldiamine or (R)-BINAM with two equivalents of 2-formylbenzo[h]quinoline or 8-isopropyl-2-quinolinecarboxaldehyde {CBQ = (1R,2R)-cyclohexanediamine-N,N'-bis(benzo[h]quinoline-2-ylmethylene), CPQ = (1R,2R)-cyclohexanediamine-N,N'-bis[[(8-isopropyl)-2-quinolinyl]methylene], BBQ = [(R)-1,1'-binaphthalene]-2,2'-diamine-N,N'-bis(benzo[h]quinoline-2-ylmethylene), BPQ = [(R)-1,1'-binaphthalene]-2,2'-diamine-N,N'-bis[[(8-isopropyl)-2-quinolinyl]methylene]}. Reaction of NiI(2) with the (1R,2R)-cyclohexyl ligands gives the mononuclear distorted trigonal-bipyramidal (TBP) complexes [Ni(N(3)-CBQ)I(2)] and [Ni(N(3)-CPQ)I(2)]. Incomplete iodide abstraction from [Ni(N(3)-CPQ)I(2)] with AgOTf leads to partial replacement of the iodide with hydroxide from adventitious water to give [Ni(N(3)-CPQ)I(1.6)(OH)(0.4)] (distorted TBP). The corresponding reaction with [Ni(N(3)-CBQ)I(2)] again fails to remove all of the iodide, resulting instead in conversion to the syn dinuclear [Ni(2)(CBQ)(μ-X)(2)I(2)] (X = Cl(0.925)I(0.075)) complex, where chloride abstraction from the solvent (CH(2)Cl(2)) has resulted in a mixed halide system and the metal centers are square-pyramidal. Reaction of Ni(OTf)(2) with CBQ leads to the isolation of the octahedral cation [Ni(CMBQ)(2)](2+), with CMBQ [(1R,2R)-cyclohexanediamine-mono-N-(benzo[h]quinoline-2-ylmethylene)] being the partial hydrolysis product of CBQ. [Ni(CMBQ)(2)][OTf](2) crystallizes as a 1:1 mixture of P and M helical diastereomers. The coordination of NiI(2) with the (R)-BINAM derived ligands yields the anti dinuclear P-helical complexes [Ni(2)(BBQ)(μ-I)(2)I(2)] and [Ni(2)(BPQ)(μ-I)(2)I(2)]: one nickel ion is coordinated in each bidentate iminoquinoline pocket and the geometry at the metal centers is distorted square-pyramidal. Characterisation by (1)H NMR, UV-Vis, electronic circular dichroism (ECD) spectroscopy, combustion analysis, and HRMS is reported in addition to structural and halide abstraction studies.     

A novel asymmetric di-Ni(II) system as a highly efficient functional model for phosphodiesterase: synthesis, structures, physicochemical properties and catalytic kinetics

A novel asymmetric phenol-based 'end-off' dinucleating ligand 2-{[(2-piperidylmethyl)amino]methyl}-4-bromo-6-[(1-methylhomopiperazine-4-yl)methyl]phenol (HL) and three dinuclear nickel(II) complexes, [Ni?L(μ-OH)] (ClO?)? (1), [Ni?L(DNBA)?(CH?CN)?]BPh? (2) and [Ni?L(BPP)?(CH?CN)?]BPh? (3) have been synthesized and characterized by a variety of techniques including: NMR, infrared and UV-vis spectroscopies, mass spectrometry, elemental analysis, molar conductivity, thermal analysis, magnetochemistry and single-crystal X-ray diffractometry. The UV-vis spectrum of complex 1 exhibits a strong peak at 510 nm, a characteristic absorption of a d-d transition of the square-planar four-coordinated Ni(II) center. Utilizing this feature, the stepwise formation of mono- and dinickel centers in solution can be monitored. Phosphodiesterase activity of a dinuclear Ni(II) system (complex 1), formed in situ by a 2?:?1 mixture of Ni(2+) ions and the ligand HL, was investigated using bis(4-nitrophenyl)phosphate (BNPP) as the substrate. The pH dependence of the BNPP cleavage in water-ethanol (1?:?1, v/v) reveals a bell-shaped pH-k(obs) profile with an optimum at about pH 8.3 which is parallel to the formation of the dinuclear species [Ni?L(μ-OH)](2+), according to the increase of the peak at 510 nm in the UV-vis absorption spectrum . These studies reveal that the di-Ni(II) system shows the highest catalytic activity reported so far, with an acceleration rate 1.28 × 10? times faster than the uncatalyzed reaction. The bridging hydroxyl group in [Ni?L(μ-OH)](2+) is responsible for the hydrolysis reaction. The possible mechanism for the BNPP cleavage promoted by di-Ni(II) system is proposed on the basis of kinetic and spectral analyses. This study provides a less common example of the asymmetric phosphodiesterase model, which is like the active sites of most native metallohydrolases.     

Coaggregation of paramagnetic d- and f-block metal ions with a podand-framework amine phenol ligand

This report covers initial studies in the coaggregation of nickel (Ni2+) and lanthanide (Ln3+) metal ions to form complexes with interesting structural and magnetic properties. The tripodal amine phenol ligand H3tam (1,1,1-tris(((2-hydroxybenzyl)amino)methyl)ethane) is shown to be particularly accommodating with respect to the geometric constraints of both transition and lanthanide metal ions, forming isolable complexes with both of these ion types. In the solid-state structure of [Ni(H2tam)(CH3CN)]PF6.2.5CH3CN.0.5CH3OH (1), the Ni(II) center has a distorted octahedral geometry, with an N3O2 donor set from the [H2tam]- ligand and a coordinated solvent (acetonitrile) occupying the sixth site. The reaction of stoichiometric amounts of H3tam with the Ni(II) ion in the presence of lanthanide(III) ions provides [LnNi2(tam)2]+ cationic complexes which contain coaggregated metal ions. These complexes are isolable and have been characterized by a variety of analytical techniques, with mass spectrometry proving to be particularly diagnostic. The solid-state structures of [LaNi2(tam)2(CH3OH)1/2(CH3CH2OH)1/2(H2O)]ClO4.0.5CH3OH.0.5CH3CH2OH.4H2O (2), [DyNi2(tam)2(CH3OH)(H2O)]ClO4.CH3OH. H2O(6), and [YbNi2(tam)2(H2O)]ClO4.2.58H2O(9) have been determined. Each complex contains two octahedral Ni(II) ions, each of which is encapsulated by the ligand tam3- in an N3O3 coordination sphere; each [Ni(tam)]-unit caps the lanthanide(III) ion via bridging phenoxy oxygen donor atoms. In 2, La3+ is eight-coordinated, while in 6, Dy(III) is seven- (to "weakly eight-") coordinated, and Yb(III) in 9 has a six-coordination environment. The complexes are symmetrically different, 2 possessing C2 symmetry and 6 and 9 having C1 symmetry. Magnetic studies of 2, 6, and 9 indicate that antiferromagnetic exchange coupling between the Ni(II) and Ln(III) ions increases with decreasing ionic radius of Ln(III).