Aliphatic carbon-carbon bond cleavage reactivity of a mononuclear Ni(II) cis-beta-keto-enolate complex in the presence of base and O2: a model reaction for acireductone dioxygenase (ARD)

The synthesis, characterization, and reactivity properties of a mononuclear Ni(II) cis-beta-keto-enolate complex, [(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1) (6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) are reported. Complex 1 was characterized by X-ray crystallography, elemental analysis, 1H NMR, and electronic absorption and infrared spectroscopy. Treatment of 1 with 1 equiv of Me4NOH.5H2O in the presence of O2 results in oxidative carbon-carbon bond cleavage and the formation of a new Ni(II) dicarboxylate complex, [(6-Ph2TPA)Ni(O2CPh)2(H2O)] (2). Complex 2 has been characterized by X-ray crystallography, 1H NMR, UV-vis, IR, and elemental analysis. Use of 18O2 in the reaction of 1 to produce 2 results in the incorporation of one 18O atom per carboxylate ligand in the majority of the sample. Production of CO in the oxidative conversion of 1 to 2 was detected using the palladium chloride method. This is the first functional model system of relevance to acireductone dioxygenase (ARD), a novel Ni(II)-containing enzyme that catalyzes a reaction that is a shunt out of the methionine salvage pathway in K. pneumoniae.     

Neutral acetohydroxamic acid coordination to a mononuclear Ni(II) center stabilized by an intramolecular hydrogen-bonding interaction

Treatment of a new chelate ligand having both amide- and phenyl-appended pyridyl moieties with Ni(ClO4).6H2O and acetohydroxamic acid in methanol solution results in the production of a novel pseudo-octahedral Ni(II) complex having a neutral acetohydroxamic acid ligand stabilized by a hydrogen-bonding interaction.     

Stereoselective electrochemical thioalkylation of glycine in Ni(II) coordination environment

One-pot method for direct stereoselective (S)-α-thioalkylation of glycine in Ni(II) coordination environment was elaborated. The reaction of electrochemically deprotonated Ni(II) glycine/Schiff-base complex containing (S)-o-[N-(N′-benzylprolyl)amino]benzophenone as an auxiliary chiral moiety with alkyl thiocyanates leads to the α-thioalkylated derivatives in a practical ca.70% chemical yield and stereoselectivity (de 80%).     

Synthesis and properties of one-dimensional Ni(II) and Ni(II)Cu(II) complexes linked by hydrogen bond

Four dithiooxalato (Dto) bridged one-dimensional Ni(ll) and Ni(ll)Cu(ll) complexes (Me₆[14]dieneN₄)Ni₂(Dto)₂) (1), (Me₆[14]dieneN₄)CuNi(Dto)₂ (2), (Me₆[14]aneN₄)Ni₂(Dto)₂ (3), and (Me₆[14]aneN₄)CuNi(Dto)₂ (4), were synthesized. These complexes have been characterized by elemental analysis, IR, UV and ESR spectra. The crystal structure of complex3 was determined. It crystallizes in the monoclinic system, space group C2/c with a = 2. 2425(4) nm,b = 1.0088(2) nm,c= 1.4665(3) nm, β= 125.32(3)δ Z = 4;R = 0.076, R_w = 0.079. In the complex, Ni(1) coordinates four sulphur atoms of two Dto ligands in plane square environment. Ni(2) lies in the center of macrocyclic ligand. For Dto ligand, two sulphur atoms coordinate Ni(1), and O(1) coordinates Ni(2) and forms weak coordination bond. O(2) is linked to N(2) of macrocyclic ligand through hydrogen bond.     

Comparing a mononuclear Zn(II) complex with hydrogen bond donors with a dinuclear Zn(II) complex for catalysing phosphate ester cleavage

Introducing ligand based hydrogen bond donors to increase the activity of a mononuclear Zn(II) complex for catalysing phosphate ester cleavage can be a more effective strategy than making the dinuclear analogue.     

Multiple photochemical reaction pathways in a Ni(II) coordination compound

The gas-phase photofragmentation of the mixed-ligand coordination compound trans-bis(trifluoroacetato)bis(N,N'-dimethylethylenediamine)nickel(II) (Ni(tfa)2(dmen)2) detected via time-of-flight mass spectrometry is reported. In contrast to most gas-phase studies of metal-containing compounds where fragmentation of weak metal-ligand bonds dominates, the data here show that the dmen ligands fragment while still coordinated to nickel. The manner in which these ligands fragment is highly specific, leading to mono- and diimine species that remain coordinated to nickel. Uncoordinated mono- and diimine species and various small dmen fragments are also observed with high intensities in the low mass region of the spectra. NiF+, a fragment that is formed by fluorine abstraction, is always observed, even though no Ni-F bonds exist in the starting material.     

Monodentate N-coordination of a 1,2,3,5-dithiadiazolyl to Mn(II), Co(II) and Ni(II): A new coordination mode

The reaction of neutral radical ligand 4-(2′-cyanofuryl-5′)-1,2,3,5-dithiadiazolyl with a stoichiometric amount of bis(hexafluoroacetylacetonato)-manganese(II), -cobalt(II) or nickel(II) generates alternating metal-ligand chains in the solid state. The coordination mode of this ligand to the metal dication is unprecedented, providing the first evidence that the nitrogen atoms of a 1,2,3,5-dithiadiazolyl are capable of coordination to a metal dication in the absence of chelation.     

Self-assembly using dynamic coordination chemistry and hydrogen bonding: mercury(II) macrocycles, polymers and sheets

The self-assembly of extended metal-containing arrays is described based on dynamic coordination chemistry at mercury(II) with bis(amidopyridyl) ligands to form macrocycles, polymers, or sheets which can be further organized by hydrogen bonding between amide substituents. The ligands 1,2-C6H4[NHC(O)-4-C5H4N]2, 1, 1,2-C(6)H(4)[C(O)NHCH(2)-4-C(5)H(4)N](2), 2, and 1,2-C(6)H(4)[CH(2)C(O)NHCH(2)-4-C(5)H(4)N]2, 3 can adopt polar conformations and so can confer helicity in their complexes. Several macrocycles of formula [(HgX(2))(2)(micro-LL)(2)] (LL = 1, 2), with tetrahedral mercury(II) centers, were prepared in which individual molecules are further self-assembled via hydrogen bonding in the solid state to form one- or two-dimensional polymers or sheets. In one case, a one-dimensional polymer [((HgX2)-(mu-3))n] was formed. It is shown that the mercury(II) centers can be six-coordinate in forming the sheet structure [((HgX2)(mu-2)2)n], in which there are particularly large pores.     

Solution chemistry of copper(II)-gentamicin complexes: relevance to metal-related aminoglycoside toxicity

The adverse effect to the inner ear of aminoglycosides, drugs widely administered for the treatment of serious infections, appears to result from the interaction of these drugs with Cu(II) or Fe(II)/Fe(III) ions. To understand more completely the metal-induced side effects of one such antibiotic, gentamicin, we studied copper(II) coordination to gentamicin C1a by potentiometry, UV-vis, CD, and EPR spectroscopies, and ESI mass spectrometry. Only monomeric complexes of the CuH(n)L stoichiometry, with n ranging from 3 to -2, were detected over the pH range of 4-12. CuH(3)L and CuH(2)L complexes exhibit the same coordination mode, binding copper(II) through the amino nitrogen atom and a deprotonated alcoholic oxygen atom of the garosamine ring. In the CuHL and CuL complexes a second amino nitrogen atom of the purpurosamine ring participates in central ion coordination. Finally, the additional axial binding of the deprotonated oxygen of the hydroxyl group of the 2-deoxystreptamine moiety occurs in the CuH(-)(1)L and CuH(-)(2)L complexes. Interactions of the Cu(II)-gentamicin-H(2)O(2) system at pH 7.4 with N,N-dimethyl-p-nitrosoaniline, arachidonic acid, and plasmid DNA confirmed that gentamicin complexes facilitate oxidative reactions leading to peroxidation of arachidonic acid and scission of double-stranded DNA mediated by copper-bound reactive oxygen species. However, the stability constants of Cu(II)-gentamicin complexes are inferior to the binding constants of copper(II) complexes with other components of human serum or cells. Computer simulations of copper(II) distribution in the human blood plasma showed that the concentration of gentamicin would have to be at impossible levels (100 M) before a significant fraction of Cu(II) ions would be bound to gentamicin. Further, once introduced into aqueous solution, histidine replaces gentamicin in Cu(II)-gentamicin complexes. Therefore, Cu(II)-gentamicin complexes might not exist under physiological conditions.     

Experimental evidence for the noninnocence of o-aminothiophenolates: coordination chemistry of o-iminothionebenzosemiquinonate(1-) pi-radicals with Ni(II), Pd(II), Pt(II)

The ligand 2-mercapto-3,5-di-tert-butylaniline, H[L(AP)], an o-aminothiophenol, reacts with metal(II) salts of Ni and Pd in CH3CN or C2H5OH in the presence of NEt3 under strictly anaerobic conditions with formation of beige to yellow cis-[M(II)(L(AP))2] (M = Ni (1), Pd (2)) where (L(AP))1- represents the o-aminothiophenolate(1-) form. The crystal structure of cis-[Pd(II)(L(AP))2][HN(C2H5)3][CH3CO2] has been determined by X-ray crystallography. In the presence of air the same reaction produces dark blue solutions from which mixtures of the neutral complexes trans/cis-[M(II)(L(ISQ))2] (M = Ni (1a/1b), Pd (2a/2b), and Pt (3a/3b)) have been isolated as dark blue-black solid materials. By using HPLC the mixture of 3a/3b has been separated into pure samples of 3a and 3b, respectively; (L(ISQ))1- represents the o-iminothionebenzosemiquinonate(1-) pi-radical. The structures of 1a.dmf and 3a.CH2Cl2 have also been determined. All compounds are square-planar and diamagnetic. 1H NMR spectroscopy established the cis <==> trans equilibrium of 1a/1b, 2a/2b, and 3a/3b in CH2Cl2 solution where the isomerization rate is very fast for the Ni, intermediate for the Pd, and very slow for the Pt species. It is shown that the electronic structures of 1a/1b, 2a/2b, 3a, and 3b are best described as diradicals with a singlet ground state. The spectro- and electrochemistries of all complexes display the usual full electron transfer series where the monocation, the neutral species, the mono- and dianions have been spectroscopically characterized. X-band EPR spectra of the monocations [1a/1b]+ and [3a]+ support the assignment of an oxidation-state distribution as predominantly [M(II)(L(ISQ))(L(IBQ))]+ where (L(IBQ))0 represents the o-iminothionequinone level. In contrast, the EPR spectra of the monoanions [1a/1b]- and [3a]- indicate an [M(II)(L(ISQ))(L(AP)-H)]- distribution but with a significant contribution of the [M(I)(L(ISQ))(2)]- resonance hybrid; (L(AP)-H)2- represents the o-imidothiophenolato(2-) oxidation level. Analysis of the geometric features of 120 published structures of complexes containing ligands of the o-aminothiophenolate type show that high precision X-ray crystallography allows to discern the differing protonation and oxidation levels of these ligands. o-Aminothiophenolates are unequivocally shown to be noninnocent ligands; the (L(ISQ))1- radical form is quite prevalent in coordination compounds and the electronic structure of a number of published complexes must be reconsidered.     

Tuning the coordination chemistry of a Cu(II) complex at high-pressure

Application of pressure on the Cu-complex CuAsp causes Jahn-Teller Cu-O bonds to be compressed, increasing the coordination environment from [4 + 1] to [4 + 2], highlighted by a discontinuity on compression of these bonding interactions.     

The coordination chemistry of simple phospholes: Complexes of nickel(II), palladium(II) and platinum(II)

The reactions between four very simply substituted phospholes and the chlorides of Ni(II), Pd(II) and Pt(II) are described. The phospholes 1-phenylphosphole, 3-methyl-1-phenyl-phosphole and 3,4dimethyl-1-phenylphosphole all readily form bis-complexes of formula L₂MCl₂ [L = phosphole ligand and M = Ni(II), Pd(II) or Pt(II)] or tris-complexes of formula L₃MCI₂. 1-n-Butyl-3,4-dimethylphosphole appears to form stable complexes only with Ni(II). Evidence is put forward which indicates that the L₂MCl₂ complexes exist in a four-coordinate, square-planar monomeric/five coordinate equilibrium while the L₃MCl₂ complexes are primarily the ionic species [L₃MCl]⁺ Cl^? in solution. Comparisons are made with the behaviour of other simple phospholes which do not form Ni(II) complexes and the results are discussed briefly in terms of both aromatic and non-aromatic phosphole models.     

From a paramagnetic, mononuclear supersulfidonickel(II) complex to a diamagnetic dimer with a four-sulfur two-electron bond

The first isolable "side-on" supersulfidonickel(II) complex 1(1) with the elusive S2-. ligand has been synthesized by facile oxidation of the corresponding nickel(I) precursor [LNi] with elemental sulfur. Remarkably, paramagnetic 1(1) associates voluntarily to give the diamagnetic dimer 1(2) with a four-sulfur two-electron bond as proven by X-ray diffraction analysis, spectroscopic measurements (1H NMR, EPR, SQUID), and DFT calculations. Gentle sulfur transfer of 1(1) to Ph3P or its reaction with [LNi] affords solely the genuine disulfide complex 3 having a Ni2(mu-mu2:mu2-S2) core.     

Effects of the intramolecular NH...S hydrogen bond in mononuclear platinum(II) and palladium(II) complexes with 2,2'-bipyridine and benzenethiol derivatives

A series of complexes, [M(bpy)(SAr)2] (M = platinum(II) or palladium(II), bpy = 2,2'-bipyridine, SAr = 2- or 4-(acylamino)benzenethiolate, or 2-(alkylcarbamoyl)benzenethiolate), were synthesized and characterized on the basis of 1H NMR, IR, and electrochemical properties. The structures of [Pt(bpy)(S-2-Ph3CCONHC6H4)2] (1) and [Pt(bpy)(S-2-t-BuNHCOC6H4)2] (3) were determined by X-ray analysis. The complexes have intramolecular NH...S hydrogen bonds between the amide NH group and the sulfur atom. A weak NH...S hydrogen bond in these complexes and [Pd(bpy)(S-2-Ph3CCONHC6H4)2] (4) is detected from the 1H NMR spectra and the IR spectra in chloroform and in the solid state. [Pt(bpy)(S-2-Ph3CCONHC6H4)2] (1) exhibits a remarkably high-energy-shifted lowest-energy band in UV-visible spectra and has a positively shifted oxidation potential. The blue-shift of 42 nm and the positive shift of +0.24 V, as compared to those of [Pt(bpy)(SC6H5)2), are due to the effect of the NH...S hydrogen bond.     

Ni(III) vs. Ni(II)-thiyl radical: charge-delocalisation in a binuclear Ni(III)Ni(II)-dithiolate complex

Multi-frequency EPR spectroscopy on 61Ni-labelled samples of [Ni2(L)]3+ confirms extensive charge-delocalisation between the Ni(III) centre and thiolate donors in the Ni(II)Ni(III) complex.     

The boron-boron single bond in diborane(4) as a non-classical electron donor for hydrogen bonding

An ab initio study of an isomer of diborane(4) [B(2)H(4)] has been carried out at MP2/aug-cc-pVTZ to investigate the ground-state properties of this unusual molecule, a derivative of which has been described in the recent literature. The geometric, electronic and orbital characteristics of B(2)H(4)(4) have been analyzed using AIM, NBO, and ELF methodologies. A region with a high concentration of electron density is located near and along the B-B bond, on the opposite side of this bond relative to the bridging H atoms. This site serves as an electron-donor site to electrophiles, resulting in hydrogen-bonded complexes of B(2)H(4) with proton donors HF, HNC, HCl, HCN, and HCCH, and a van der Waals complex with H(2). These complexes have C(2v) symmetry and stabilization energies that vary from 2 to 27 kJ mol(-1). The SAPT2 energy decomposition analysis shows that the relative importance of the various terms that contribute to the interaction energy depends on the strength of the interaction.     

Chiral Ni(II) coordination polymers: structure-driven effects of temperature and polyvinylpyrrolidone

Chiral Ni(II) coordination compounds with structures of [NiL(H(2)O)(3)] (1) and {[NiL(H(2)O)]·0.5H(2)O}(n) (2) (H(2)L = thiazolidine 2,4-dicaboxlic acid) have been successfully synthesized by the reaction of Ni(CH(3)COO)(2)·4H(2)O and H(2)L in aqueous solution at 25 and 80 °C, respectively. From the same procedure with polyvinylpyrrolidone (PVP) as a surfactant, another corresponding micrometer-scale Ni(II) coordination polymer, {[NiL(H(2)O)(2)]·H(2)O}(n) (3), has been obtained at both 25 and 80 °C, which shows a different structure (one-dimensional, 1D) than both 1 (discrete molecule) and 2 (3D). The conversions of structures and conformations are directed by temperature and surfactant (PVP), as confirmed by powder and single-crystal X-ray diffraction. Circular Dichroism (CD) and Second Harmonic Generation (SHG) measurements of the products have been investigated as well, which indicate the potential applications of these products in chiral and nonlinear optical (NLO) areas.     

Estimation of coordination bond energies of NH3, H2O and Et2NH ligands in the Ni(II) and Cu(II) complexes

Tridentate ligands 2-hydroxyphenylsalicylaldimine (SAPH₂), 2-hydroxyphenyl-2-hydroxy-1-naphtalaldimine (NAPH₂) and Ni(II) complexes with multidentate ligand Bis-N·N′-(salicylidene)-1,3-propanediamine (LH₂) as well as mononuclear complex of Cu(II) were prepared using the same multidentate ligand. Diethylamine (Et₂NH), NH₃ and H₂O monodentate ligands were bound to these complexes coordinatively. The heat absorbed at the temperatures where these ligands thermally dissociated from the complexes were measured using the TG and DSC methods. It is assumed that the states both of the complexes with and without the monodentate ligands are solid and coordination bond energy for the monodentate ligand is calculated. It is seen that these calculated coordination bond energies are comparable with hydrogen bond energies.     

Magnetic anisotropy in Ni(II)-Y(III) binuclear complexes: on the importance of both the first coordination sphere of the Ni(II) ion and the Y(III) ion belonging to the second coordination sphere

The synthesis of a new Ni(II)-Y(III) binuclear complex with a marked elongation axis in the first coordination sphere of the Ni(II) ion is presented. Its zero-field splitting (ZFS) is studied by means of magnetic data and state-of-the-art ab initio calculations. A good agreement between the experimental and theoretical ZFS parameter values is encountered, validating the whole approach. The magnetic anisotropy axes are extracted from the ab initio calculations, showing that the elongation axis around the Ni(II) ion corresponds to the hard axis of magnetization and that the sign of the axial D parameter is imposed by this axis. The Ni-Y axis is found to be an easy axis of magnetization, which is, however, not significant according to the sign of D. The already reported [(H(2)O)Ni(ovan)(2)(μ-NO(3))Y(ovan)(NO(3))]·H(2)O (ovan = o-vanillin) complex is then revisited. In this case, the elongation axis in the Ni(II) coordination sphere is less marked and the ZFS is dominated by the effect of the Y(III) ion belonging to the second coordination sphere. As a consequence, the D parameter is negative and the low-temperature behavior is dominated by the Ni-Y easy axis of magnetization. A competition between the first coordination sphere of the Ni(II) ion and the electrostatic effect of the Y(III) ion belonging to the second coordination sphere is then evidenced in both complexes, and the positive and negative D parameters are then linked to the relative importance of both effects in each complex.