Ni-N Distances Research Articles

Dinuclear Cyclometalated Pincer Nickel(II) Complexes with Metal-Metal-to-Ligand Charge Transfer Excited States and Near-Infrared Emission.

Facile non-radiative decay of low-lying metal-centered (MC) dd excited states has been well documented to pose a significant obstacle to the development of phosphorescent NiII complexes due to substantial structural distortions between the dd excited state and the ground state. Herein, we prepared a series of dinuclear Ni2 II,II complexes by using strong σ-donating carbene-phenyl-carbene (CNHC Cphenyl CNHC) pincer ligands, and prepared their dinuclear Pt2 II,II and Pd2 II,II analogues. Dinuclear Ni2 II,II complexes bridged by formamidinate/α-carbolinato ligand exhibit short Ni-Ni distances of 2.947-3.054 Å and singlet metal-metal-to-ligand charge transfer (1MMLCT) transitions at 500-550 nm. Their 1MMLCT absorption energies are red-shifted relative to the Pt2 II,II and Pd2 II,II analogues at ~450 nm and ≤420 nm respectively. One-electron oxidation of these Ni2 II,II complexes produces valence-trapped dinuclear Ni2 II,III species, which are characterized by EPR spectroscopy. Upon photoexcitation, these Ni2 II,II complexes display phosphorescence (τ=2.6-8.6 μs) in the NIR (800-1400 nm) spectral region in 2-MeTHF and in the solid state at 77 K, which is insensitive to π-conjugation of the coordinated [CNHC Cphenyl CNHC] ligand. Combined with DFT calculations, the NIR emission is assigned to originate from the 3dd excited state. Studies have found that the dinuclear Ni2 II,II complex can sensitize the formation of singlet oxygen and catalyze the oxidation of cyclo-dienes under light irradiation.

Cyclonickelated complexes featuring a terminal or bridging triazole

Cyclometallated complexes can serve as models for studying the mechanistic details of CH functionalization processes. In this context, we have shown that the cyclonickelated complexes {κP,κC-(i-Pr)2PO-Ar}Ni(Br)(NCMe) can be isolated from CH nickelation of aryl phosphinites. Previous studies showed that treating these compounds with PhCH2Br or (i-Pr)2PCl allows CC and CP functionalized products, respectively, whereas reaction with hydroxylamines HO-N(CH2R)2 fails to promote the desired CN functionalization, giving instead the imine adducts {κP,κC-(i-Pr)2PO-Ar}Ni(Br)(κN-N(CH2R)CH = CHR) arising from net dehydration of the hydroxylamine substrate. The present report describes a study on the reactivities of the dimeric complexes [{κP,κC-(i-Pr)2P-OAr}Ni(μ-Br)]2 (Ar = C6H4, 1a;4-OMe-C6H3, 1b; 4-Cl-C6H3, 1c; 1-naphthyl, 1d; 4-OMe,1-naphthyl, 1e) with 4-Amino-4H-1,2,4-triazole featuring a potentially labile NN bond. As was observed previously with hydroxylamines, here too the hoped-for CN functionalization did not materialize, the reactions giving instead the mononuclear triazole adducts {κP,κC-(i-Pr)2P-OAr}Ni(Br)(κN-4-amino-4H-1,2,4-triazole) 2a and 2e and the triazole-bridged dinuclear adducts [{κP,κC-(i-Pr)2PO-Ar}Ni(Br)}2(μ,κN,κN’-4-amino-4H-1,2,4-triazole) 3b, 3c, and 3d. The solid-state structures of these new compounds reveal the following three features: (a) slightly distorted square planar geometries around the Ni center, (b) 50-65° angles between the plane of the triazole ligand and the coordination planes, and (c) significantly shorter Ni-N distances in the mononuclear adducts. Variable temperature NMR monitoring of the reactions between the Ni precursors and the triazole substrate pointed to a dynamic exchange process between the mono- and dinuclear triazole adducts.

On the Role of Water in the Formation of a Deep Eutectic Solvent Based on NiCl2·6H2O and Urea.

The metal-based deep eutectic solvent (MDES) formed by NiCl2·6H2O and urea in 1:3.5 molar ratio has been prepared for the first time and characterized from a structural point of view. Particular accent has been put on the role of water in the MDES formation, since the eutectic could not be obtained with the anhydrous form of the metal salt. To this end, mixtures at different water/MDES molar ratios (W) have been studied with a combined approach exploiting molecular dynamics and ab initio simulations, UV–vis and near-infra-red spectroscopies, small- and wide-angle X-ray scattering, and X-ray absorption spectroscopy measurements. In the pure MDES, a close packing of Ni2+ ion clusters forming oligomeric agglomerates is present thanks to the mediation of bridging chloride anions and water molecules. Conversely, urea poorly coordinates the metal ion and is mostly found in the interstitial regions among the Ni2+ ion oligomers. This nanostructure is disrupted upon the introduction of additional water, which enlarges the Ni–Ni distances and dilutes the system up to an aqueous solution of the MDES constituents. In the NiCl2·6H2O 1:3.5 MDES, the Ni2+ ion is coordinated on average by one chloride anion and five water molecules, while water easily saturates the metal solvation sphere to provide a hexa-aquo coordination for increasing W values. This multidisciplinary study allowed us to reconstruct the structural arrangement of the MDES and its aqueous mixtures on both short- and intermediate-scale levels, clarifying the fundamental role of water in the eutectic formation and challenging the definition at the base of these complex systems.

Ferro- vs. antiferromagnetic exchange between two Ni(II) ions in a series of Schiff base heterometallic complexes: what makes the difference?

Three new NiII/ZnII heterometallics, [NiZnL'2(OMe)Cl]2 (1), [NiZnL''(Dea)Cl]2·2DMF (2) and [Ni2(H3L''')2(o-Van)(MeOH)2]Cl·[ZnCl2(H4L''')(MeOH)]·2MeOH (3), containing three-dentate Schiff bases as well as methanol or diethanolamine (H2Dea) or o-vanillin (o-VanH), all deprotonated, as bridging ligands were synthesized and structurally characterized. The Schiff base ligands were produced in situ from o-VanH and CH3NH2 (HL'), or NH2OH (HL"), or 2-amino-2-hydroxymethyl-propane-1,3-diol (H4L'''); a zerovalent metal (Ni and Zn in 1, Zn only in 2 and 3) was employed as a source of metal ions. The first two complexes are dimers with a Ni2Zn2O6 central core, while the third compound is a novel heterometallic cocrystal salt solvate built of a neutral zwitterionic ZnII Schiff base complex and of ionic salt containing dinuclear NiII complex cations. The crystal structures contain either centrosymmetric (1 and 2) or non-symmetric di-nickel fragment (3) with NiNi distances in the range 3.146-3.33 Å. The exchange coupling is antiferromagnetic for 1, J = 7.7 cm-1, and ferromagnetic for 2, J = -6.5 cm-1 (using the exchange Hamiltonian in a form Ĥ = Jŝ1ŝ2). The exchange interactions in 1 and 2 are comparable to the zero-field splitting (ZFS). High-field EPR revealed moderate magnetic anisotropy of opposite signs: D = 2.27 cm-1, E = 0.243 cm-1 (1) and D = -4.491 cm-1, E = -0.684 cm-1 (2). Compound 3 stands alone with very weak ferromagnetism (J = -0.6 cm-1) and much stronger magnetic anisotropy with D = -11.398 cm-1 and E = -1.151 cm-1. Attempts to calculate theoretically the exchange coupling (using the DFT "broken symmetry" method) and ZFS parameters (with the ab initio CASSCF method) were successful in predicting the trends of J and D among the three complexes, while the quantitative results were less good for 1 and 3.

Co-catalyst free ethene dimerization over Zr-based metal-organic framework (UiO-67) functionalized with Ni and bipyridine

Ni functionalized metal organic frameworks (MOF) are promising heterogeneous ethene dimerization catalysts. Activities comparable to or higher than Ni-aluminosilicates have been reported in literature. However, unlike the Ni-aluminosilicates, those Ni-MOFs require a large excess of co-catalyst to initiate the dimerization process and some catalysts generate polymers which lead to catalyst deactivation. Herein, we report a series of Ni(II) and 2,2′-bipyridine-5,5′-dicarboxylate (bpy) functionalized UiO-67 MOF that catalyze the ethene dimerization reaction co-catalyst free. The catalysts were active for ethene dimerization (up to 850 mg butene gcat−1 h−1) after activation at 300 °C in 10 % O2 for 360 min and subsequent exposure to flowing ethene (P(ethene) =26 bar, 250 °C) for 240 min. The catalysts yielded up to 6 % conversion with 99 % selectivity to linear 1- and 2-butenes, which formed in non-equilibrated ratios. Overall, the test data indicate that all three linear butenes are formed on a single active site, in accordance with the Cossee-Arlman mechanism. Ex situ XAS and CO FT-IR spectroscopy studies point towards Ni monomers or, plausibly, low-nuclearity Ni-multimers, docked at bpy linkers with Ni-Ni distances > 4 Å, as the main active site for the ethene dimerization reaction.

Linear pentanuclear nickel(II) and tetranuclear copper(II) complexes with pyrazine-modulated tripyridyldiamine ligand: Synthesis, structure and properties

By using tripyridyldiamine ligand, N,N’-di(pyrazin-2-yl)pyridine-2,6-diamine (H2dpzpda), linear pentanuclear nickel(II) [Ni5(µ5-dpzpda)4Cl2] (1) and tetranuclear copper(II) [Cu4(Hdpzpda)2(CH3COO)6] (2) complexes were first synthesized and structurally characterized. This pentanickel linear metal chain is helically wrapped by four syn-syn-syn-syn type dpzpda2- ligands. There are two types of NiNi distances existing in the complex. The terminal NiNi distances bonded to the axial ligand are longer (2.3877(8) Å) affected by the axial ligands. The inner NiNi distances are very short and remain constant (2.3071(6) Å). Two terminal Ni(II) ions bonded to the axial ligands are in a square-pyramidal (NiN4Cl) environment and exhibit long NiN bonds (2.097(4) Å), which are consistent with a high-spin Ni(II) configuration. The inner three Ni(II) ions display short NiN (1.8915(8)–1.903(4) Å) bond distances, which are consistent with a square planar (NiN4), the diamagnetic arrangement of a low-spin Ni(II) configuration. Temperature-dependent magnetic research revealed an antiferromagnetic interaction between the two terminal atoms through a superexchange pathway along metal cores with a parameter of about −51.0 cm–1. Observation of the first oxidation peak in the cyclic voltammograms of 1 at +1.206 V revealed that this complex is the most resistant to oxidation among all known pentanuclear nickel strings with unmodulated and modulated oligo-α-pyridylamido ligands. The structure of 2 consists of four copper atoms linearly placed with a copper(II) acetate dimer in the centre. In complex 2 ligand H2dpzpda coordinates to the Cu(II) atoms as a tetradentate ligand in an all anti conformation. Copper complex 2 is built to a 1-D supramolecular chain structure through intermolecular hydrogen bond and strong π-π interaction (3.19 Å) between the pyridine and pyrazine rings. From the magnetic susceptibility data, 2 shows strong antiferromagnetic coupling, which was satisfactorily fitted using Hamiltonian H = J1(S1S2 + S3S4) + J2S2S3. The best fitting result were obtained with the parameters J1 = 10.64 cm−1, J2 = −175.36 cm−1, g = 2.11.

Electronic structure, pore size distribution, and sorption characterization of an unusual MOF, {[Ni(dpbz)][Ni(CN)4]}n, dpbz = 1,4-bis(4-pyridyl)benzene

The monoclinic (Ni(L)[Ni(CN)4] (L= 1,4-Bis(4-pyridyl) benzene) compound (defined as Ni-dpbz) is a flexible metal organic framework which assumes a pillared structure with layers defined by 2D Ni[Ni(CN)4]n nets and dpbz ligands as pillars. The structure features an entrapped dpbz ligand that links between the open ends of four-fold Ni sites from two neighboring chains. This arrangement results in an unusual 5-fold pseudo square-pyramid environment for Ni and a significantly long Ni-N distance of 2.369(4) Å. Using Density Functional Theory calculations, the different bonding characteristics between the 5-fold and 6-fold Ni's were determined. We found that there is weak covalent bonding between the 5-fold Ni and N in the entrapped ligand, and the 6-fold Ni-N bonds provide effective electronic conduction. The disordered dimethyl sulfoxide (DMSO) solvent molecules are not bonded to the framework. The material has a single pore with a diameter of 4.1 Å. This pore includes approximately 55% of the total free volume (based on a zero-diameter probe). The accessible pore surface area and pore volume were calculated to be 507 m2/g and 6.99 cm3/kg, respectively. The maximum amount of CO2 that can be accommodated in the pores after DMSO is removed was found to be 204 mg/g, agreeing with the results of adsorption/desorption experiments of about 220 mg/g.

Synthesis and Speciation-Dependent Properties of a Multimetallic Model Complex of NiSOD That Exhibits Unique Hydrogen-Bonding

The complex Na3[{NiII(nmp)}3S3BTAalk)] (1) (nmp2- = deprotonated form of N-(2-mercaptoethyl)picolinamide; H3S3BTAalk = N1,N3,N5-tris(2-mercaptoethyl)benzene-1,3,5-tricarboxamide, where H = dissociable protons), supported by the thiolate-benzenetricarboxamide scaffold (S3BTAalk), has been synthesized as a trimetallic model of nickel-containing superoxide dismutase (NiSOD). X-ray absorption spectroscopy (XAS) and 1H NMR measurements on 1 indicate that the NiII centers are square-planar with N2S2 coordination, and Ni-N and Ni-S distances of 1.95 and 2.16 Å, respectively. Additional evidence from IR indicates the presence of H-bonds in 1 from the approximately -200 cm-1 shift in νNH from free ligand. The presence of H-bonds allows for speciation that is temperature-, concentration-, and solvent-dependent. In unbuffered water and at low temperature, a dimeric complex (1A; λ = 410 nm) that aggregates through intermolecular NH···O═C bonds of BTA units is observed. Dissolution of 1 in pH 7.4 buffer or in unbuffered water at temperatures above 50 °C results in monomeric complex (1M; λ = 367 nm) linked through intramolecular NH···S bonds. DFT computations indicate a low energy barrier between 1A and 1M with nearly identical frontier MOs and Ni-ligand metrics. Notably, 1A and 1M exhibit remarkable stability in protic solvents such as MeOH and H2O, in stark contrast to monometallic [NiII(nmp)(SR)]- complexes. The reactivity of 1 with excess O2, H2O2, and O2•- is species-dependent. IR and UV-vis reveal that 1A in MeOH reacts with excess O2 to yield an S-bound sulfinate, but does not react with O2•-. In contrast, 1M is stable to O2 in pH 7.4 buffer, but reacts with O2•- to yield a putative [NiII(nmp)(O2)]- complex from release of the BTA-thiolate based on EPR.

Quasi-1D chains of dinickel lantern complexes and their magnetic properties.

Four new quasi-1D Ni2-lantern chain complexes of the form [Ni2(SOCR)4(L)]∞ (R = Ph, L = DABCO (1); R = Ph, L = pyz (2); R = CH3, L = DABCO (3); R = CH3, L = pyz (4)) were prepared from the reaction of [Ni2(SOCR)4(EtOH)], R = CH3 or Ph, with the N,N'-donor bridging ligands pyrazine (pyz) or 1,4-diazabicyclo[2.2.2]octane (DABCO). Reaction of [Ni2(tba)4(EtOH)], (tba = thiobenzoate) with the mono-N donor ligand quinuclidine (quin) gave the discrete Ni2-lantern complex [Ni2(tba)4(quin)] (5), whereas reaction with pyridine led to fragmentation of the lantern and formation of the known [Ni(tba)2(py)2] (6). Single-crystal X-ray diffraction reveals 2-4 to be 1D chain complexes comprising DABCO or pyz ligands which bridge the Ni2-lantern units. Complex 5 forms dimers through two equivalent NiS interactions. The Ni-Ni distances within the Ni2-lanterns are 2.5316(18)-2.595(2) Å for the 1D chain complexes 2-4, and 2.5746(4) Å in the dimeric complex 5, respectively. Comparing the solid state magnetism of 5 to precursor [Ni2(tba)4(EtOH)] demonstrates a change in coupling upon change of capping ligand. Meanwhile, chains 1-4 exhibit magnetic properties consistent with an S = 1 system, due to a mixed valent system where the two Ni centers differ in spin state, while 5 possesses two S = 1 Ni(ii) centers. DFT calculations confirm low-spin S = 0 {NiS4} and high-spin S = 1 {NiO4} centers in each lantern. Fits to the magnetic susceptibility data of the chains suggest a weak antiferromagnetic mean field interaction is present that is largely 1-D in nature, though neither pyrazine nor DABCO promote significant magnetic interaction between neighboring Ni2-lanterns.

A Comparative DFT and DFT+U Study on Magnetism in Nickel-Doped Wurtzite AlN

GGA and GGA+U formalisms have been implemented to comparatively study the electronic and magnetic properties of the Ni-doped AlN wurtzite semiconductor within density functional theory. Electronic structure calculations have been performed for ferromagnetic and antiferromagnetic states of Ni x Al1−x N (x = 0.056 and 0.0625) by GGA and GGA+U schemes. It has been found that the GGA+U method increases the repulsion of the impurity bands in the gap of semiconductors. The magnetic moment of the atom impurity in Ni x Al1−x N (x = 0.056 and 0.0625) is higher than those of the anions bonded to it for both GGA and GGA+U. Ni (N) contributions to the magnetic moment of Ni x Al1−x N (x = 0.056 and 0.0625) decrease (increase) in NiN4 tetrahedron when using GGA+U. A magnetic moment per Ni atom of about 2.10 μB is predicted in Ni x Al1−x N for x = 0.056 with GGA approach for the most stable system. However, it was found that the ground state nature for Ni x Al1−x N (x = 0.056 and 0.625) changes from ferromagnetic to antiferromagnetic with the GGA+U approach for the Ni-Ni closest configuration. On the contrary, for the Ni-Ni farthest configuration both GGA and GGA+U formalisms predict a ferromagnetic ground state for Ni x Al1−x N (x = 0.056 and 0.0625). For intermediate Ni-Ni distances, both GGA and GGA+U schemes present almost the same energy differences and the same ferromagnetic ground state. The ferromagnetic ground state originates from the strong hybridization between Ni-d and N-p states. Additionally, it was found that the GGA+U formalism increases the magnetic moment per Ni atom for all Ni x Al1−x N (x = 0.056) ferromagnetic states.

Stereochemical diversity of {MNO}(10) complexes: molecular orbital analyses of nickel and copper nitrosyls.

The great majority of {NiNO}(10) complexes are characterized by short Ni-N(O) distances of 1.60-1.65 Å and linear NO units. Against this backdrop, the {CuNO}(10) unit in the recently reported [Cu(CH3NO2)5(NO)](2+) cation (1) has a CuNO angle of about 120° and a very long 1.96 Å Cu-N(O) bond. According to DFT calculations, metal-NO bonding in 1 consists of a single Cu(dz(2))-NO(π*) σ-interaction and essentially no metal(dπ)-NO(π*) π-bonding, which explains both the bent CuNO geometry and the long, weak Cu-N(O) bond. This σ-interaction is strongly favored by a ligand trans to the NO; indeed such a trans ligand may be critical for the existence and stability of a {CuNO}(10) unit. By contrast, {NiNO}(10) complexes exhibit a strong avoidance of such trans ligands. Thus, a five-coordinate {NiNO}(10) complex appears to favor a trigonal-bipyramidal structure with the NO in an equatorial position, as in the case of [Ni(bipy)2(NO)](+) (6). An unusual set of Ni(d)-NO(π*) orbital interactions accounts for the strongly bent NiNO geometry for this complex.

Structure of bis(2-methylimino-3-penten-4-onato) nickel(II). properties of nickel(II) ketoiminates

A new volatile complex Ni(mi-aa)2 [mi-aa = CH3C(NCH3)CHC(O)CH3)] is synthesized and its crystal structure is determined (APEX DUO diffractometer with a 4C CCD detector, λMoKα, graphite monochromator, T = 100 K). Crystallographic data for Ni(mi-aa)2 are as follows: space group Pbca, a = 5.4058(3) A, b = 11.8684(6) A, c = 19.8472(10) A, V = 1273.4(1) A3, Z = 4. The Ni(II) atom is coordinated by the O and N atoms of two ligands. The Ni-O and Ni-N distances are 1.8390(9) A and 1.926(1) A; the chelate ONiN angle is 92.52(4)°. The complex has the molecular structure formed from the isolated molecules of Ni(mi-aa)2 bound by van der Waals interactions. For some complexes, the TG study is performed and the intermolecular interaction energy in the crystals is calculated.

Bis[1-methoxy-2,2,2-tris(pyrazol-1-yl-κN2)ethane]nickel(II) bis(trifluoromethanesulfonate) methanol disolvate

In the title salt, [Ni(C12H14N6O)2](CF3SO3)2·2CH3OH, the NiII ion is coordinated by six N atoms from two tridentate 1-meth­oxy-2,2,2-tris­(pyrazol-1-yl)ethane ligands in a distorted octa­hedral geometry. The NiII ion is situated on an inversion centre. The Ni—N distances range from 2.0589 (19) to 2.0757 (19) Å, intra-ligand N—Ni—N angles range from 84.50 (8) to 85.15 (8)°, and adjacent inter-ligand N—Ni—N angles range between 94.85 (8) and 95.50 (8)°. In the crystal, O—H⋯O hydrogen bonds between methanol solvent mol­ecules and tri­fluoro­methane­sulfonate anions are observed.

Molecular dynamics simulation of Al grain mixing in Fe/Ni matrices and its influence on oxidation

AlxNiyFe(1−x−y) alloys are structural materials with potential application in high-temperature oxidizing environments. These materials are of specific interest as they have the ability to develop an oxidation resistant surface layer. To study diffusion and oxidation processes related to this surface layer formation, the mixing behavior of different sized Al grains in pure Ni and Fe matrices, with approximate grain/matrix atom ratio of 1:3, at temperatures above and below the structure melting point, was studied using ReaxFF-based molecular dynamics simulations. The simulations have been carried out at constant pressure, with temperatures being stepwise ramped over the range of 300-3000 K. For the Ni matrix, our results indicated lower chemical strain energy for Al in the mixed alloy and completion of mixing at a lower temperature for the Fe matrix. These results confirm that the Al-Ni alloy is energetically more stable than the Al-Fe alloy, which is in agreement with experiment. Further, larger Al grains appear to be favorable for mixing with Fe matrix, whereas for Ni matrix, smaller Al grains appear to be favorable. We suggest that this Al grain size effect on mixing matrices is due to the differences in formation energies between Ni/Al and Fe/Al alloys and differences in Ni-Ni and Fe-Fe bond distances. We also performed additional cooling simulations over the temperature range of 3000-300 K. The simulations revealed that for the considered cooling rate Fe alloy solidifies at a lower temperature than Ni alloy. Moreover, both alloys solidify to chemically disordered crystalline structures, of which the Ni structure is less ordered than the Fe structure. Preliminary oxidation simulations of slab structures with single grain indicate that the dynamics of matrix/grain mixing processes have a pronounced influence on the oxidation reactions. We find that Al and Ni atoms in their unmixed state are the most active reactants towards oxygen, while the Al/Ni alloy and pure Fe layers show substantially slower oxidation kinetics.

Theoretical Design of Molecular Electrocatalysts with Flexible Pendant Amines for Hydrogen Production and Oxidation

The design of hydrogen oxidation and production electrocatalysts is important for the development of alternative renewable energy sources. The overall objective is to maximize the turnover frequency and minimize the overpotential. We use computational methods to examine a variety of nickel-based molecular electrocatalysts with pendant amines. Our studies focus on the proton-coupled electron transfer (PCET) process involving electron transfer between the complex and the electrode and intramolecular proton transfer between the nickel center and the nitrogen of the pendant amine. The concerted PCET mechanism, which tends to require a lower overpotential, is favored by a smaller equilibrium Ni-N distance and a more flexible pendant amine ligand, thereby decreasing the energetic penalty for the nitrogen to approach the nickel center for proton transfer. Our calculations provide predictions about designing catalysts that incorporate these properties. These design principles will be useful for developing the next generation of hydrogen catalysts.

The Stannides RE2Ni2Sn (RE = Pr, Ho, Er, Tm) – Structural Transition from the W2B2Co to the Mo2B2Fe Type as a Function of the Rare Earth Size

Abstract The stannides RE2Ni2Sn (RE=Pr, Ho, Er, Tm) were synthesized by arc-melting of the elements and characterized by powder X-ray diffraction. Pr2Ni2Sn crystallizes with the orthorhombic W2B2Co-type structure, Immm, a=443.8(1), b=572.1(1), c=855.1(2) pm, wR2=0.0693, 293 F2 values, 13 variables. A structural transition to the tetragonal Mo2B2Fe type occurs for the heavier rare earth elements. The structures of Ho2Ni2Sn (a=729.26(9), c=366.66(7) pm, wR2=0.0504, 250 F2 values, 12 variables), Er2Ni2Sn (a=727.2(2), c=364.3(1) pm, wR2=0.0397, 262 F2 values, 12 variables), and Tm2Ni2Sn (a=725.2(1), c=362.8(1) pm, wR2=0.0545, 258 F2 values, 12 variables) were refined from single-crystal diffractometer data. The switch in structure type is driven by the size of the rare earth element. The [Ni2Sn] substructures are composed of Ni2Sn2 squares and Ni4Sn2 hexagons in Pr2Ni2Sn, and of Ni3Sn2 pentagons in Er2Ni2Sn. The Ni4Sn2 hexagons and Ni3Sn2 pentagons exhibit Ni2 pairs with Ni-Ni distances of 247 pm in Pr2Ni2Sn, and of 250 pm in Er2Ni2Sn.

Influence of Axial Ligands on Diverse Properties in Three Trinickel String Complexes

Three new trinickel string complexes, [Ni3(dpa)4(3-nba)2](1), [Ni3(dpa)4(4-nba)2]·(CH3OH) (2), and [Ni3(dpa)4(3,5-dnba)2](3) where dpa− is the anion of 2,2′-dipyridylamine, 3-nba− is 3-nitrobenzoate anion, 4-nba− is 4-nitrobenzoate anion, and 3,5-dnba− is 3,5-dinitrobenzoate anion, were synthesized in good yield and characterized by X-ray crystallography, infrared spectra, elemental analysis, magnetic susceptibility, cyclic voltammogram (CV), UV-vis spectra, fluorescence spectra, and TG analysis. The magnetic susceptibilities suggested that the terminal Ni atoms in all three complexes are in high-spin state while the central Ni atom is in a low-spin state. The CVs of complexes 1–3 showed reversible one-electron redox waves at E1/2 = 1.1395 V for 1, 1.108 V for 2, and 1.109 V for 3 corresponding to Ni3 7+/Ni3 6+. The Ni-Ni distances in three complexes are somewhat different, indicating the axial nitrobenzoate ligands have significant effect on the structural assembly. Supplemental materials are available for this article. Go to the publisher's online edition of Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry to view the supplemental file.

Shuttling of Nickel Oxidation States in N4S2 Coordination Geometry versus Donor Strength of Tridentate N2S Donor Ligands

Seven bis-Ni(II) complexes of a N(2)S donor set ligand have been synthesized and examined for their ability to stabilize Ni(0), Ni(I), Ni(II) and Ni(III) oxidation states. Compounds 1-5 consist of modifications of the pyridine ring of the tridentate Schiff base ligand, 2-pyridyl-N-(2'-methylthiophenyl)methyleneimine ((X)L1), where X = 6-H, 6-Me, 6-p-ClPh, 6-Br, 5-Br; compound 6 is the reduced amine form (L2); compound 7 is the amide analog (L3). The compounds are perchlorate salts except for 7, which is neutral. Complexes 1 and 3-7 have been structurally characterized. Their coordination geometry is distorted octahedral. In the case of 6, the tridentate ligand coordinates in a facial manner, whereas the remaining complexes display meridional coordination. Due to substitution of the pyridine ring of (X)L1, the Ni-N(py) distances for 1~5 < 3 < 4 increase and UV-vis λ(max) values corresponding to the (3)A(2g)(F)→(3)T(2g)(F) transition show an increasing trend 1~5 < 2 < 3 < 4. Cyclic voltammetry of 1-5 reveals two quasi-reversible reduction waves that correspond to Ni(II)→Ni(I) and Ni(I)→Ni(0) reduction. The E(1/2) for the Ni(II)/Ni(I) couple decreases as 1 > 2 > 3 > 4. Replacement of the central imine N donor in 1 by amine 6 or amide 7 N donors reveals that complex 6 in CH(3)CN exhibits an irreversible reductive response at E(pc) = -1.28 V, E(pa) = +0.25 V vs saturated calomel electrode (SCE). In contrast, complex 7 shows a reversible oxidation wave at E(1/2) = +0.84 V (ΔE(p) = 60 mV) that corresponds to Ni(II)→Ni(III). The electrochemically generated Ni(III) species, [(L3)(2)Ni(III)](+) is stable, showing a new UV-vis band at 470 nm. EPR measurements have also been carried out.

Tetrakis(1-phenyl-1H-imidazole-κN3)bis(thiocyanato-κN)nickel(II)

The title compound, [Ni(NCS)2(C9H8N2)4], crystallizes with two independent half-mol­ecules in the asymmetric unit and the NiII ions situated on centres of symmetry. In both independent mol­ecules, the NiII ion displays a compressed octa­hedral environment formed by four N atoms from the 1-phenyl-1H-imidazole ligands, which define the equatorial plane, with a mean Ni—N distance of 2.119 (11) Å, and two axial N atoms from two NCS− anions, with a mean Ni—N distance of 2.079 (7) Å. The crystal packing exhibits weak inter­molecular S⋯S contacts of 3.411 (2) Å.

Crystal structure of a complex of nickel(II) with 2-amino-4-imino-2-pentene

The structure of a nickel(II) complex with 2-amino-4-iminopentane is determined by X-ray crystallography at 150 K. Crystal data for C10H18N4Ni are: a = 10.9802(3)A, b = 13.5780(4)A, c = 8.0935(2)A, β = 107.304(1)°, space group P21/c, V = 1152.04(5) A3, Z = 4, dx = 1.459 g/cm3, R = 0.0283. The structure is molecular; the metal atom is coordinated by four nitrogen atoms of two β-diimine ligands. The Ni-N distances in Ni(NacNac)2 fall within 1.8571–1.8623 A. The molecules in the crystal are joined by only van der Waals interactions.