Nickel Derivatives Research Articles

5,10,15-Triferrocenylcorrole Complexes

Complexes of 5,10,15-triferrocenylcorrole were synthesized from the crude free-base corrole product obtained by the reaction of ferrocenyl aldehyde and pyrrole. Direct formation of the complex in this manner leads to an increase of the reaction yield by protecting the corrole ring toward oxidative decomposition. The procedure was successful and gave the expected product in the case of the copper and triphenylphosphinecobalt complexes, but an unexpected result was obtained in the case of the nickel derivative, where metal insertion led to a ring opening of the macrocycle at the 5 position, giving as a final product a linear tetrapyrrole nickel complex bearing two ferrocenyl groups. The purified 5,10,15-triferrocenylcorrole complexes have been fully characterized by a combination of spectroscopic methods, electrochemistry, spectroelectrochemistry, and density functional theory calculations. Copper derivatives of 10-monoferrocenyl- and 5,15-diferrocenylcorrole were prepared to investigate how the number and position of the ferrocenyl groups influenced the spectroscopic and electrochemical properties of the resulting complexes. A complete assignment of resonances in the (1)H and (13)C NMR spectra was performed for the cobalt and nickel complexes, and detailed electrochemical characterization was carried out to provide additional insight into the degree of communication between the meso-ferrocenyl groups on the conjugated macrocycle and the central metal ion of the ferrocenylcorrole derivatives.

Bis(methylborole) sandwich compounds of the first row transition metals

Bis(borole)nickel derivatives (η5-C4H4BR)2Ni, isoelectronic with the corresponding ferrocenes (η5-C5H4R)2Fe, have been synthesized and characterized structurally by X-ray crystallography as sandwich compounds with parallel borole rings. The geometries, energetics, and preferred spin states of the complete series of bis(methylborole)metal sandwich compounds (C4H4BCH3)2M of the first row transition metals (M=Ti to Ni) have now been investigated by density functional theory. The late transition metal derivatives (η5-C4H4BCH3)2M energetically prefer metallocene-like sandwich structures having parallel pentahapto borole rings with singlet, doublet, triplet, quartet, and quintet spin states for M=Ni, Co, Fe, Mn, and Cr, respectively. However, the early transition metal (η5-C4H4BCH3)2M derivatives energetically prefer bent sandwich structures having ∼30° dihedral angles between the pentahapto borole rings with singlet, doublet, and triplet spin states for M=Ti, V, and Cr, respectively. For (η5-C4H4BCH3)2Cr the closeness in energy (within ∼1kcal/mol) of the parallel quintet and bent triplet structures suggests interesting magnetic properties. The titanium and vanadium derivatives are predicted to form marginally stable [(C4H4BCH3)2M]2 dimers (M=Ti, V) with the two halves linked by a metal–metal bond as well as two bridging borole carbon atoms through agostic hydrogen atoms.

Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2]+ (1) (CR=2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [RuII(bpy)3]2+ (Ru) as photosensitizer and ascorbate/ascorbic acid (HA−/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [CoIII(CR)(H2O)2]3+ (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [ZnII(CR)Cl]+ (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05mM. Interestingly, the analogous nickel derivative, [NiII(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA−/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq=2.5×107M−1s−1) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket=1.4×109M−1s−1). The reduced catalyst can then enter into the cycle of hydrogen evolution.

An unusual hydroxy-substituted mononuclear nickel(II) complex with a tetradentate Schiff base: Synthesis, spectroscopy, electrochemistry, crystallography, DNA binding, and theoretical investigation

The 2:1 Schiff base condensates of 4-methyl-2,6-dibenzoylphenol (mdbpH) or 4-tert-butyl-2,6-dibenzoylphenol (bdbpH) with ethane-1,2-diamine, L1H2 and L2H2, respectively, were isolated and used to prepare nickel(II) derivatives. The resultant complexes, [Ni{OC6H2(CH3)(COC6H5)}2(C6H5CN(CH2)2NCC6H5)] (1) and [Ni{OC6H2(t-Bu)(COC6H5)}2(C6H5CN(CH2CHOH)NCC6H5)] (2), have been characterized by elemental analyses, conductivity measurements, mass spectrometry, IR, 1H and 13C NMR and electronic spectroscopy, cyclic voltammetry, and X-ray crystallography. The ligand precursors and complexes all crystallize in the P1¯ space group. In 1 and 2, the coordination at NiII, which shows a slight distortion from square planar towards tetrahedral geometry, comprises two phenolato O and two imine N atoms of the tetradentate (L2−) unit. Density Functional Theory (DFT) calculations provide a satisfactory description of the structural and electronic properties of the new compounds. Electrochemical studies of 1 and 2 are described. The interactions of complexes 1 and 2, and ligands L1H2 and L2H2, with calf thymus DNA were investigated by UV–Vis absorption titrations and cyclic voltammetric methods. Complexes 1 and 2, and ligands L1H2 and L2H2 bind to DNA via intercalation; the binding affinity of complex 1 is higher than that of complex 2 and of the ligand precursors L1H2 and L2H2.

Microwave and solvothermal methods for the synthesis of nickel and ruthenium complexes with 9-anthracene carboxylate ligand

Microwave and solvothermal activation processes have been explored as tools for the preparation of various nickel and ruthenium complexes. Different reaction conditions are tested using ethanol or water as solvents. Three nickel derivatives, [Ni(9-atc)2(OH2)2(py)2]·2EtOH (1), [Ni2(9-atc)4(OH2)(py)4]·2H2O (2·2H2O), and [Ni2(9-atc)4(py)2] (3), and two diruthenium compounds, {[Ru2Cl(9-atc)4]·2H2O}n (4) and [Ru2(9-atc)4(EtOH)2]·2EtOH (5), are obtained. The crystal structure determination of complexes 1–3 and 5 is also described. Compound 1 displays a 1D extended supramolecular structure with hydrogen bonds involving crystallization solvent molecules. Compound 2 is dimetallic, and both nickel centers show an octahedral coordination environment, whereas complexes 3 and 5 display a typical carboxylate-bridged paddlewheel-type structure with two metal atoms connected by four bridging carboxylate ligands. All compounds show weak antiferromagnetic interactions except 3, where a strong intra-dimer antiferromagnetic coupling is observed. Compound 4 also shows a strong zero field splitting.

Feasibility of an oxygen-getter with nickel electrodes in alkaline electrolysers

Alkaline electrolysis is the long-established technology for water splitting to produce hydrogen and has been industrially used since the nineteenth century. The most common materials used for the electrodes are nickel and derivatives of nickel (e.g. Raney nickel). Nickel represents a cost-effective electrode material due to its low cost (compared to platinum group metals), good electrical conductivity and exhibits good resistance to corrosive solutions. The steady degradation of the nickel electrodes over time is known as a result of oxide layer formation on the electrode surface. Reducing oxide layer growth on the electrode surface will increase the efficiency and lifetime of the electrolyser. Titanium has a higher affinity to oxygen than nickel so has been introduced to the electrolyser as a sacrificial metal to reduce oxide layer formation on the nickel. Two identical electrolysers were tested with one difference: Cell B had titanium chips present in the electrolyte solution, whilst Cell A did not have titanium present. SEM results show a reduction of 16 % in the thickness of the Cell B oxide layer on nickel compared to the Cell A nickel, which is supported by the large increase in oxide layer build-up on the titanium in Cell B. EDX on the same samples showed on average a 59 % decrease in oxygen on the Cell B nickel compared to Cell A. XPS surface analysis of the same samples showed a 17 % decrease in the oxygen on Cell B nickel. These results support the hypothesis that adding titanium to an alkaline electrolyser system with nickel electrodes can reduce the oxide layer formation on the nickel.

Unravelling mechanisms behind the biological activity of bis(S-citronellalthiosemicarbazonato)nickel(ii)

Bis(S-citronellalthiosemicarbazonato)nickel(II), [Ni(tcitr)2], is a compound that inhibits proliferation of tumour line U937 by inducing a G2/M block and leading the cancer cells to apoptosis. This nickel derivative shows no activity on non proliferating healthy cells. In this paper we report our studies on the action mechanisms of [Ni(tcitr)2]. Apoptosis in U937 cells exposed to [Ni(tcitr)2] takes place through activation of caspase-9, and therefore through an intrinsic triggering mechanism. Given the DNA damage observed in the Comet assay, the mutagenic activity of the metal complex was tested, including with the Ames test, micronuclei and DNA damage recovery, but neither mutagenicity nor recovery were detected. Nickel-complex-DNA interactions were analyzed by direct action of the compound on plasmidic and linear DNA by UV-vis and CD spectroscopy, gel electrophoresis and Atomic Force Microscopy. These experiments reveal that [Ni(tcitr)2] does not cause DNA breaks and does not intercalate, but significantly alters the DNA conformation creating knot-like structures and hairpins.

Zinc, copper and nickel derivatives of 2-[2-bromoethyliminomethyl]phenol as topoisomerase inhibitors exhibiting anti-proliferative and anti-metastatic properties

Transition metal (Zn, Cu and Ni) derivatives of (2-[2-bromoethyliminomethyl]phenol), were found to inhibit topoisomerase I activity, induce DNA cleavage and bind to calf thymus DNA. The compounds are also cytotoxic and anti-invasive against PC3.

Towards accurate structural characterization of metal centres in protein crystals: the structures of Ni and Cu T(6) bovine insulin derivatives.

Using synchrotron radiation (SR), the crystal structures of T6 bovine insulin complexed with Ni(2+) and Cu(2+) were solved to 1.50 and 1.45 Å resolution, respectively. The level of detail around the metal centres in these structures was highly limited, and the coordination of water in Cu site II of the copper insulin derivative was deteriorated as a consequence of radiation damage. To provide more detail, X-ray absorption spectroscopy (XAS) was used to improve the information level about metal coordination in each derivative. The nickel derivative contains hexacoordinated Ni(2+) with trigonal symmetry, whereas the copper derivative contains tetragonally distorted hexacoordinated Cu(2+) as a result of the Jahn-Teller effect, with a significantly longer coordination distance for one of the three water molecules in the coordination sphere. That the copper centre is of type II was further confirmed by electron paramagnetic resonance (EPR). The coordination distances were refined from EXAFS with standard deviations within 0.01 Å. The insulin derivative containing Cu(2+) is sensitive towards photoreduction when exposed to SR. During the reduction of Cu(2+) to Cu(+), the coordination geometry of copper changes towards lower coordination numbers. Primary damage, i.e. photoreduction, was followed directly by XANES as a function of radiation dose, while secondary damage in the form of structural changes around the Cu atoms after exposure to different radiation doses was studied by crystallography using a laboratory diffractometer. Protection against photoreduction and subsequent radiation damage was carried out by solid embedment of Cu insulin in a saccharose matrix. At 100 K the photoreduction was suppressed by ∼15%, and it was suppressed by a further ∼30% on cooling the samples to 20 K.

Further iminopyrrolyl complexes of nickel, cobalt, iron and copper: Synthesis and structural characterisation

The reaction between two equivalents of 2-(N-arylformimino)pyrrolyl sodium salts (NaNC4H3C(H) = N-Ar), with Ar = C6H5 (I), 2,6-iPr2-C6H3 (II) or 2-(N-arylformimino)phenanthro[9,10-c]pyrrolyl sodium salts (NaNC16H10C(H)N-Ar), with ArC6H5 (III), 2,6-iPr2-C6H3 (IV) and one equivalent of nickel(II), cobalt(II), iron(II) and copper(I) halides, in THF, at low temperatures, afforded bis(iminopyrrolyl) or bis(iminophenanthro[9,10-c]pyrrolyl) complexes with different geometries around the metal centres. Nickel(II) derivatives displayed distorted square planar geometries, whereas for cobalt a tetrahedral complex is proposed. The reactions with FeCl2 resulted in very unstable products, giving rise to dimers with pentacoordinated Fe(III) centres. The addition of two equivalents of pyridine to the reaction medium improved the stability of the resulting compound, enabling the formation of a 16-electron Fe(II) complex, which contained simultaneously two iminopyrrolyls and also a pyridine ligand. In the reaction of CuBr with 2-(N-2,6-diisopropylphenylformimino)pyrrolyl sodium salt, a complex with the general formulation [Cu(iminopyrrolyl)]x was obtained. When exposed to the air, this complex gives rise to the corresponding bis(iminopyrrolyl) Cu(II) complex.

Chiroptical properties of anionic and cationic porphyrins and metalloporphyrins in complex with left-handed Z-DNA and right-handed B-DNA

We report the chiroptical signature and binding interactions of cationic (meso-tetrakis(4-N-methylptridyl)porphyrin, 2HT4) and anionic (meso-tetrakis(4-sulfonatophenyl)porphyrin, 2HTPPS) porphyrins and their zinc(II) and nickel(II) derivatives (ZnT4, ZnTPPS, NiT4, and NiTPPS) with right-handed B-form and two forms of left-handed Z-form of alternating guanine–cytosine polydeoxynucleotide poly(dG–dC)2. NiTPPS is able to spectroscopically discriminate between spermine-induced Z-DNA and Co(III)-induced Z-DNA via new induced circular dichroism signal in the visible region of the electromagnetic spectrum.

A new type of sandwich compound: homoleptic bis(trimethylenemethane) complexes of the first row transition metals

The metal carbonyl complexes [η4-(CH2)3C]Fe(CO)3 and [η4-(CH2)3C]Cr(CO)4 have been synthesized containing the umbrella-shaped trimethylenemethane ligand. The prospect of using this ligand in the metal sandwich complexes [(CH2)3C]2M (M = Ti, V, Cr, Mn, Fe, Co, Ni) has now been investigated by density functional theory. The lowest energy structures of such complexes have the metal atom sandwiched between two tetrahapto trimethylenemethane ligands. Singlet spin state structures are strongly preferred for the titanium and nickel derivatives and doublet spin state structures for the vanadium and cobalt derivatives. Similarly, the triplet spin state is preferred for the iron derivative by more than 11 kcal mol−1. The preferred spin states for the chromium and manganese derivatives depend on the functional used for the structure optimization. Thus the B3LYP and B3LYP* methods predict the higher spin states, namely triplet for chromium and quartet for manganese. However, the BP86 method predicts the lower spin states of singlet for chromium and doublet for manganese. The higher spin state structures for the late transition metal derivatives, namely quintet [(CH2)3C]2Fe, quartet [(CH2)3C]2Co, and triplet [(CH2)3C]2Ni, have trihapto rather than tetrahapto trimethylenemethane ligands. The frontier molecular orbitals in the singlet [(CH2)3C]2M derivatives (M = Ti, Ni) suggest strong metal–ligand σ and π bonding but insignificant metal–ligand δ bonding.

Structural Features of Hexakis-DMSO Nickel(II) Complex Cations: <i>Pseudo</i>-<i>S</i><sub>6</sub>- and <i>Pseudo</i>-<i>S</i><sub>4</sub>-Propeller-Like Structures

It is important to study how the coordination geometry is distorted around a central metal ion in the coordination compounds, because the slight distortion influences the sign of the zero-field splitting, which will determine the magnetic behavior.1 Previously, a crystal structure of hexakis-DMSO cobalt(II) complex [Co(DMSO)6](BPh4)2 (DMSO = dimethylsulfoxide) was determined.2 The CoO6S6 unit in the complex cation was approximated as an S6 point group; however, the coordination geometry—the CoO6 unit—was best described as having a pseudo-C2v symmetry with elongation along an O–Co–O axis. In the present study, we prepared a nickel(II) derivative with the aim of revealing its structural features. The nickel(II) derivative was prepared by refluxing a DMSO solution (5.0 ml) of nickel(II) nitrate hexahydrate (1.0 mmol, 0.29 g) for 10 min; after adding a DMSO solution (1.0 ml) of sodium tetraphenylborate (2.0 mmol, 0.68 g), the solution was refluxed for 1 h; then, upon cooling, green microcrystals were precipitated. Single-crystals suitable for X-ray analysis were obtained by recrystallization of the microcrystals from DMSO. The crystal data are included in Table 1. The structure was solved by direct methods and expanded using Fourier X-ray Structure Analysis Online

Ni(ii) and Cu(ii) N4-ethylmorpholine citronellalthiosemicarbazonate: a comparative analysis of cytotoxic effects in malignant human cancer cell lines

In this paper we report a study conducted with two analogous complexes, bis(N(4)-ethylmorpholine citronellalthiosemicarbazonate) nickel(II) and -copper(II) on four tumour cell lines (U937, HL60, SK-N-MC and HT29). All cell lines appear to be sensitive to both metal complexes, but while in U937, HL60 and SK-N-MC, apoptosis is the main mode through which cell death occurs, HT29 cells undergo necrosis. Among the cell lines which undergo apoptosis, SK-N-MC response is characterized by the intrinsic pathway, whereas U937 and HL60 involve both the intrinsic and the extrinsic pathways. The redox activity of the two complexes provides experimental evidence that they can modulate reactive oxygen species (ROS) production as a function of both the metal and the cell line used. Among the four cell lines, HL60 does not seem to give a significant response to exposure to both compounds. In the case of the nickel derivative, ROS generation is a relatively early event, and ROS could be the mediator leading to cellular damage. HT29 shows a remarkable and rapid ROS increase and a significant induction of membrane peroxidation that could be correlated to the onset of necrosis.

Complex formation of Zn-, Ni-, and Pd-derivatives of purpurin-18 with serum albumin

We analyzed the spectral characteristics of the complexes of Zn2+, Ni2+, and Pd2+ derivatives of purpurin-18 with human serum albumin (HSA) in aqueous buffer at pH 7.0. Pd2+ in the coordination sphere of purpurin-18 decreased the affinity to HSA compared to the respective complexes of zinc and nickel derivatives. Since the formation of complexes with HSA is an important parameter of photodynamic activity of tetrapyrrolic compounds, the differential affinity of metal derivatives of purpurin-18 to this protein should be considered for the optimization of photosensitizers.

Diverse Topologies in Chiral Divalent Metal Camphorate Coordination Polymers Containing 4,4′-Dipyridylamine

Hydrothermal synthesis has afforded six chiral divalent metal coordination polymers containing d-camphorate (d-cmph) and 4,4′-dipyridylamine (dpa) ligands, whose topologies depend largely on metal coordination geometry preference. Both cadmium derivatives {[Cd(dpa)(d-cmphH)2(H2O)]·0.875H2O}n (1) and {[Cd(d-cmph)(dpa)(Hdpa)]ClO4}n (1a) exhibit one-dimensional (1-D) chains with protonated pendant ligands; 1 possesses sinusoidal chains with a “wavelength” of over 67 Å. [Zn2(d-cmph)2(dpa)(H2O)]n (2) exhibits a two-dimensional (2-D) 3,4-connected binodal net with (53)2(5482) topology. {[Ni(d-cmph)(dpa)]·3H2O}n (3) and {[M2(d-cmph)2(dpa)]·H2O}n (M = Co, 4; M = Cu, 5) manifest three-dimensional (3-D) networks. The nickel derivative has an uncommon 2-fold interpenetrated 4-connected 4284lvt topology, while the isostructural cobalt and copper complexes possess 6-connected pcu topologies based on {M2(OCO)4} paddlewheel dimers. Complexes 1/1a and 2 undergo visible light emission upon ultraviolet excitation. Variable temperature magnetic susceptibility experiments indicate weak (J = −3.6(2) cm–1) and strong (J = –202(5) cm–1) antiferromagnetic coupling for 4 and 5, respectively.

Bis(heptalene) “submarine” metal dimer sandwich compounds (C12H10)2M2 (M = Ti, V, Cr, Mn, Fe, Co, Ni)

The bis(heptalene)dimetal complexes (C12H10)2M2 of the first row transition metals from Ti to Ni are predicted by density functional theory to exhibit "submarine" sandwich structures with a pair of metal atoms sandwiched between the two heptalene rings. For the early transition metal derivatives (C12H10)2M2 (M = V, Cr) there are two types of such structures. In one structural type the metals are sandwiched between two heptahapto heptalene rings with metal-metal distances (3.5-3.8 Å) too long for direct metal-metal bonding. The other type of (C12H10)2M2 (M = V, Cr, Mn) structure has a pair of bonded metal atoms sandwiched between a fully bonded heptalene ligand and a heptalene ligand bonded to the metals only through an eight-carbon heptafulvene subunit, leaving an uncomplexed cis-1,3-diene unit. The formal metal-metal bond orders in these latter structures are 3, 2, and 1 for M = V, Cr, and Mn with predicted bond lengths of 2.5, 2.7, and 2.8 Å, respectively. For (C12H10)2Fe2 a singlet structure with each iron atom sandwiched between a hexahapto and a tetrahapto heptalene ring is energetically preferred over an alternate structure with ferrocene-like iron atoms sandwiched between two pentahapto heptalene rings. Partial bonding of each heptalene ring to the metal atoms occurs in the late transition metal derivatives (C12H10)2M2 (M = Co, Ni). This leads to an unsymmetrical structure for the cobalt derivative and a structure for the nickel derivative with each nickel atom sandwiched between a trihapto ligand and a tetrahapto ligand.

Triangular Nickel Complexes Derived from 2-Pyridylcyanoxime: An Approach to the Magnetic Properties of the [Ni3(μ3-OH){pyC(R)NO}3]2+ Core

A series of nickel complexes with nuclearity ranging from Ni(3) to Ni(6) have been obtained by treatment of a variety of nickel salts with the 2-pyridylcyanoxime ligand. The reported compounds have as a common structural feature the triangular arrangement of nickel cations bridged by a central μ(3)-oxo/alkoxo ligand. These compounds are simultaneously the first nickel derivatives of the 2-pyridylcyanoxime ligand and the first examples of isolated, μ(3)-O triangular pyridyloximate nickel complexes. Magnetic measurements reveal antiferromagnetic interactions promoted by the μ(3)-O and oximato superexchange pathways and comparison of the experimental structural and magnetic data with DFT calculations give an in-depth explanation of the factors that determine the magnetic interaction in this kind of system.

Binuclear methylborole nickel carbonyls

The existence of the bis(borole) nickel sandwich compound (η5-C4H4BCH3)2Ni suggests the possibility of mixed methylborole nickel carbonyls. The structures and thermochemistry of the binuclear methylborole nickel carbonyls (C4H4BCH3)2Ni2(CO)n (n=3, 2, 1) have been compared with those of the isoelectronic cyclopentadienylcobalt derivatives (C5H5)2Co2(CO)n and cyclobutadiene nickel derivatives (C4H4)2Ni2(CO)3. For the tricarbonyls (n=3) the lowest energy structures in all three systems have two terminal carbonyl groups, one bridging carbonyl group, and metal–metal single bonds. However, for the unsaturated derivatives (C4H4BCH3)2Ni2(CO)n (n=2, 1) the pendant methyl groups are not “innocent bystanders”. Instead structures with one of the methyl hydrogens functioning as an agostic hydrogen to the nickel atom are competitive in energy with structures analogous to those of the (C5H5)2Co2(CO)n and (C4H4)2Ni2(CO)n analogues. Thus for (C4H4BCH3)2Ni2(CO)2 a structure with two bridging carbonyl groups and a formal NiNi double bond of length ∼2.37Å and a structure with one bridging carbonyl group, one terminal carbonyl group, a C–H–Ni agostic hydrogen atom, and a Ni–Ni single bond of length ∼2.48Å have essentially the same energies. For (C4H4BCH3)2Ni2(CO) the two lowest energy structures also have agostic C–H–Ni hydrogen atoms and Ni–Ni single bonds. The thermochemistry for the synthesis of (C4H4BCH3)2Ni2(CO)n derivatives from Ni(CO)4+(η5-C4H4BCH3)2Ni is not favorable since such reactions are endothermic requiring, for example, 13kcal/mol of energy for the reaction Ni(CO)4+(η5-C4H4BCH3)2Ni→(C4H4BCH3)2Ni2(CO)3+CO.

Redox Trends in Terpyridine Nickel Complexes

A synthesis has been developed that allows the isolation of four-coordinate [(tpy)Ni-Br] (1, tpy = terpyridine) in high yield. Complex 1 has been structurally characterized, and the X-ray data reveal a square-planar geometry, unlike the known [(tpy')Ni-I] (tpy' = 4,4',4''-tri-tert-butyl-terpyridine) but similar to [(tpy)Ni-CH(3)]. In the solid-state, EPR spectroscopy indicates, however, that unlike [(tpy)Ni-CH(3)], the electronic structure of 1 is a metal-centered, not a ligand-centered radical. Density functional theory (DFT) analyses support this assignment. The preparation of 1 also facilitated the analysis of the redox potentials of a series of terpyridine nickel derivatives. It was found that the overall ligand sphere (one vs two coordinated terpyridine ligands) plays more of a role in determining the redox potentials of these derivatives than do the formal oxidation states of the nickel ions in the solution phase.