Theoretical study on second hyperpolarizabilities of singlet diradical square planar nickel complexes involving o-semiquinonato type ligands

Hybrid density functional theory method is applied for investigating the diradical character dependence of the second hyperpolarizability (gamma) of square planar nickel complexes involving several types of bidentate ligands [o-C6H4XY, where X = Y = O, NH, S, Se, and PH as well as (X, Y) = (NH, NH2) and (S, NH2)]. It is found that, as a function of the donor atoms, the diradical character of these complexes varies from 0.0 to 0.884 and is associated with substantial variations of gamma ranging from 14 x 10(3) to 819 x 10(3) au. In particular, the largest gamma values are associated with intermediate diradical characters in good agreement with the structure-property relationship obtained for pure hydrocarbon systems. Increasing the electronegativity of the X and Y donor groups of the ligands leads to larger diradical characters as a result of the enhancement of the double bond nature of the C=X(Y) bonds, which further stabilizes the diradicals on both-end benzene rings. This demonstrates that the electronegativities of the donor atoms of the ligands become a tuning parameter of the diradical character and then of the gamma values of these complexes.     

Enhancement of second hyperpolarizabilities in open-shell singlet slipped-stack dimers composed of square planar nickel complexes involving o-semiquinonato type ligands

Using the spin-unrestricted hybrid density functional theory method, we have investigated the intermolecular interaction effects on the longitudinal static second hyperpolarizability (γ) of open-shell singlet slipped-stack dimers composed of singlet diradical square planar nickel complexes involving o-semiquinonato type ligands, Ni(o-C(6)H(4)X(2))(2) (where X = O, NH, S, Se, PH). For comparison, we have also examined the γ values of a closed-shell singlet slipped-stack dimer composed of closed-shell monomers Ni[o-C(6)H(4)S(NH(2))](2). It is found that for interplanar distance ranging between 3.0 and 5.0 ? the slipped-stack dimers with intermediate monomer diradical characters exhibit larger γ values per monomer (γ(dimer)/2) than those with large monomer diradical characters or than the closed-shell dimer. These results extend the domain of validity of the relationship found between γ and the diradical character for individual molecules. It also turns out that the ratio R = (γ(dimer)/2)/γ(monomer) increases upon decreasing the interplanar distance and that this increase is larger for intermediate diradical character than for the other cases. These phenomena have been analyzed by considering the γ density distributions of the dimers, demonstrating a significant field-induced third-order charge transfer between the monomers in the case of intermediate diradical character. The present results indicate that open-shell singlet slipped-stack aggregates composed of monomers with intermediate diradical characters constitute another mean for achieving highly efficient and tunable third-order nonlinear optical materials.     

Origin of the enhancement of the second hyperpolarizability of singlet diradical systems with intermediate diradical character

The origin of the diradical character dependence of the second hyperpolarizability (gamma) of neutral singlet diradical systems is clarified based on the perturbation formula of gamma using the simplest diradical molecular model with different diradical characters, i.e., H2 under bond dissociation. The enhancement of gamma in the intermediate diradical character region turns out to originate from the increasing magnitude of the transition moment between the first and second excited states and the decrease of that between the ground and first excited states, respectively, with the increase in diradical character. This feature confirms that open-shell singlet conjugated molecules with intermediate diradical characters constitute a new class of third-order nonlinear optical systems, whose gamma values can be controlled by the diradical character in addition to the conjugation length.     

Dynamic behaviour attributed to chiral carbohydrate substituents of N-heterocyclic carbene ligands in square planar nickel complexes

Nickel complexes having acetylated glucopyranosyl group incorporated N-heterocyclic carbene (NHC) ligands with methyl or benzyl groups as an N-substituent exhibit two kinds of dynamic behaviours in solution (1)H NMR spectroscopy. One of the dynamic behaviours is attributed to the anti- and syn-rotamers, which occur by the rotation of the unsymmetrical NHC ligands around the axes of the Ni-C bonds. The other is attributed to the diastereomers of the syn-rotamers, which occur by opposite rotation of the imidazolylidene rings and the chiral carbohydrate group incorporated into the NHC ligands. Crystallographic analysis of the nickel complex having the NHC ligand with acetylated glucopyranosyl and benzyl groups as N-substituents showed CH-π interaction between the glucopyranosyl unit of each NHC ligand and the phenyl ring of the other NHC ligand in the complex in the solid state.     

Second hyperpolarizability (gamma) of singlet diradical system: dependence of gamma on the diradical character

The dependence of the second hyperpolarizability (gamma) on the diradical character (y) for singlet diradical systems is investigated using a model compound, the p-quinodimethane (PQM) molecule with different both-end carbon-carbon (C-C) bond lengths, by several ab initio molecular orbital and density functional theory methods. The diradical character based on UHF calculations indicates that at equilibrium geometry PQM is in a singlet ground state and primarily exhibits a quinoid structure, whereas the diradical character increases when increasing both-end C-C bond lengths. At the highest level of approximation, that is, using the UCCSD(T) method with the 6-31G+diffuse p (zeta = 0.0523) basis set, the longitudinal static gamma of PQM presents a maximum value for intermediate diradical character (y approximately 0.5) while the gamma values are larger for intermediate and large diradical character (y approximately 0.5-0.7) than for small diradical character (y < 0.2). This feature suggests that the gamma values of singlet diradical systems in the intermediate and somewhat strong correlation regimes are significantly enhanced as compared to those in the weak correlation regime. These results are substantiated by a complementary study of the variation in gamma upon twisted ethylene.     

Square planar vs tetrahedral coordination in diamagnetic complexes of nickel(II) containing two bidentate pi-radical monoanions

The reaction of three different 1-phenyl and 1,4-diphenyl substituted S-methylisothiosemicarbazides, H(2)[L(1-6)], with Ni(OAc)(2).4H(2)O in ethanol in the presence of air yields six four-coordinate species [Ni(L(1-6)(*))(2)] (1-6) where (L(1-6)(*))(1-) represent the monoanionic pi-radical forms. The crystal structures of the nickel complexes with 1-phenyl derivatives as in 1 reveal a square planar structure trans-[Ni(L(1)(-3)(*))(2)], whereas the corresponding 1,4-diphenyl derivatives are distorted tetrahedral as is demonstrated by X-ray crystallography of [Ni(L(5)(*))(2)] (5) and [Ni(L(6)(*))(2)] (6). Both series of mononuclear complexes possess a diamagnetic ground state. The electronic structures of both series have been elucidated experimentally (electronic spectra magnetization data). The square planar complexes 1-3 consist of a diamagnetic central Ni(II) ion and two strongly antiferromagnetically coupled ligand pi-radicals as has been deduced from correlated ab initio calculations; they are singlet diradicals. The tetrahedral complexes 4-6 consist of a paramagnetic high-spin Ni(II) ion (S(Ni) = 1), which is strongly antiferromagnetically coupled to two ligand pi-radicals. This is clearly revealed by DFT and correlated ab initio calculations. Electrochemically, complexes 1-6 can be reduced to form stable, paramagnetic monoanions [1-6](-) (S = (1)/(2)). The anions [1-3](-) are square planar Ni(II) (d,(8) S(Ni) = 0) species where the excess electron is delocalized over both ligands (class III, ligand mixed valency). In contrast, one-electron reduction of 4, 5, and 6 yields paramagnetic tetrahedral monoanions (S = (1)/(2)). X-band EPR spectroscopy shows that there are two different isomers A and B of each monoanion present in solution. In these anions, the excess electron is localized on one ligand [Ni(II)(L(4-6)(*))(L(4-6))](-) where (L(4-6))(2-) is the closed shell dianion of the ligands H(2)[L(4-6)] as was deduced from their electronic spectra and broken symmetry DFT calculations. Oxidation of 1 and 5 with excess iodine yields octahedral complexes [Ni(II)(L(1,ox))(2)I(2)] (7), [Ni(II)(L(1,ox))(3)](I(3))(2) (8), and trans-[Ni(II)(L(5,ox))(2)(I(3))(2)] (9), which have been characterized by X-ray crystallography; (L(1-)(6,ox)) represent the neutral, two-electron oxidized forms of the corresponding dianions (L(1-6))(2-). The room-temperature structures of complexes 1, 5, and 7 have been described previously in refs 1-5.     

Theoretical characterization of ruthenium complexes containing functionalized bithiophene ligands for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) have gained widespread interest for their potential as low-cost solar energy conversion devices. One of the key issues is the design of higher efficient light-absorbing dyes. In this paper, we present a theoretical characterization of ruthenium complexes containing functionalized bithiophene (btp) ligands (CYC-B1 and CYC-B11) based on density functional theory (DFT) calculations. Molecular geometries, electronic structures, and optical absorption spectra are investigated both in the gas phase and in dimethylformamide (DMF) solution. Frontier orbital analysis shows the three highest HOMOs are composed of nonbonding combinations of the Ru t_2g orbitals with the p orbital and lone pairs of the SCN ligands, while the six lowest LUMOs are the π* combinations of the 4,4’-dicarboxy-2,2’-bipyridine (dcbpy) and/or btp-functionalized bipyridine (bpy) ligands calculated in the gas phase. Inclusion of solvent results in great changes in energies and compositions of the molecular orbitals of these complexes. The spectra are assigned to the intraligand π → π* transitions of the dcbpy ligand in the ultraviolet region, whereas in the visible region the spectra show multitransition character of metal-to-ligand charge transfer (MLCT), interligand π → π*, and intraligand π → π*. Our results clarify the role of the functionalized btp ligands on the absorption properties of the dyes.     

The structure of the singlet tetramethylene diradical intermediate

Ab initio MC SCF geometry optimizations of the gauche and trans conformers of the singlet tetramethylene diradical have been carried out using MC SCF gradients with a minimal (STO-3G) and extended (4-31G) basis set. At both computational levels, it has been found that the tetramethylene diradical exists as a stable species in two different conformations, a gauche and a trans.     

Linkage isomerism of the oximato group: The characterization of some mono- and binuclear square planar nickel(II) complexes of vicinal oxime-imine ligands

The reactions of the Schiff-base N,N-ethylenebis-(isonitrosoacetylacetoneimine), (H₂L), with Ni(II) acetate led to the formation of the yellow-orange complex LNi (I) in water and the red complex LNi (II) in ethanol. Both oximato groups in I are coordinated to the metal through the oximino-oxygen whereas in II one group is similarly coordinated while the other is coordinated through the oximino-nitrogen. Complex I was converted to complex II by boiling in chloroform and the conversion was reversed by reacting complex II with either piperidine or ethylenediamine. H₂L neutralized by ammonia reacted with Ni(II) chloride (1:1) and the complex formed was characterized as the red square planar bis-(4-iminopentane-2,3-dione 3-oximato)Ni(II);(III). This trans complex reacted with piperidine (1:4) to produce its cis configuration (IV). Complex III reacted with ethylenediamine (2:1) and 1,3-diaminopropane (1:1) to produce complexes II and V respectively of the identical structure. Attempted similar reaction (1:1) with either 1,4-diaminobutane or 1,5-diaminopentane led to the formation of the binuclear complexes VI and VII in which two molecules of complex III are linked together by -(CH₂)₄- and -(CH₂)₅-Moieties respectively. The suggested structures of the square planar Ni(II) complexes are based on analytical, spectral and magnetic moment evidence.     

Combination of nickel and titanium complexes containing nitrogen ligands as catalyst for polyethylene reactor blending

Ethylene was polymerized using a combination of Ni(diimine)Cl₂ ( 1 ) (diimine = 1,4‐bis(2,6‐diisopropylphenyl)‐acenaphthenediimine) and {Tp^Ms*}TiCl₃ ( 2 ) (Tp^Ms* = hydridobis(3‐mesitylpyrazol‐1‐yl)(5‐mesitylpyrazol‐1‐yl)) compounds in the presence of methylaluminoxane (MAO) at 30°C. The productivity reaches a maximum at X_Ni = 0.75 (1 400 kg of PE/mol[M]· h), and the produced polyethylene (PE) showed maximal melt flow index (0.13 g/10 min) and minimal intrinsic viscosity (2.24 dL/g) compared to polyethylenes obtained with different values of nickel loading fractions (X_Ni). Productivity, intrinsic viscosity data, as well as melt flow index measurements markedly depend upon the content of the late transition metal, thus suggesting a synergic effect between nickel and titanium catalysts.     

Synthesis,crystal structures and spectral studies of square planar nickel(II) complexes containing an ONS donor Schiff base and triphenylphosphine

Five mononuclear nickel(II) complexes, viz. [Ni(L1)(PPh₃)] (1), [Ni(L2)(PPh₃)] (2), [Ni(L3)(PPh₃)] (3), [Ni(L4)(PPh₃)] (4) and [Ni(L5)(PPh₃)] (5) (where L1, L2, L3, L4 and L5 are dianions of N-(2-mercaptophenyl)salicylideneimine, 5-methyl-N-(2-mercaptophenyl)salicylideneimine, 5-chloro-N-(2-mercaptophenyl)salicylideneimine, 5-bromo-N-(2-mercaptophenyl)salicylideneimine and N-(2-mercaptophenyl)naphthylideneimine, respectively), have been synthesized and characterized by means of elemental analysis, electronic, IR, ¹H, ¹³C and ³¹P NMR spectroscopy. Single crystal X-ray analysis of two of the complexes (1 and 5) has revealed the presence of a square planar coordination geometry (ONSP) about nickel. The crystal structures of the complexes are stabilized by intermolecular π–π stacking between the ligands (L) and by various C–H···π interactions.     

A study of the mechanism for the electrocatalysis of carbon dioxide reduction by nickel and cobalt square planar complexes in solution

Cyclic voltammetry and controlled potential electrolysis have been used to investigate the mechanism by which some square planar complexes of nickel and cobalt with macrocyclic ligands catalyse the cathodic reduction of carbon dioxide in acetonitrile + water mixtures. The complexes can have a high turnover number and, in terms of a reduction in overpotential, the complexes can also be effective catalysts; the presence of the catalyst may reduce the overpotential by more than 0.5 V. On the other hand, the maximum current density which has been achieved for these catalysed reductions is less encouraging, although the rate of the catalytic cycle is enhanced by the presence of water. It has been confirmed that the product of the reduction of carbon dioxide is carbon monoxide, although bicarbonate is also formed due to the generation of base. The influence of added proton donor, the choice of base electrolyte cation and the presence of a heterocyclic compound able to act as a ligand to the octahedral sites of the complexes have also been investigated.     

Synthesis and characterization of square planar nickel(II)-arylthiolate complexes with the biphenyl-2,2'-dithiolate ligand

Two new NiIIS4 complexes with the biphenyl-2,2'-dithiolate ligand (L) are reported. The dinuclear complex 1, [Ni2L3]2-, was formed in the reaction of 2-3 equiv of Na2L and [NiCl4]2- and the mononuclear complex [NiL2]2- (2) by using 4-10 equiv of Na2L. Complexes 1 and 2 have been crystallographically characterized. (Et4N)2[1].0.5S2Ph2, CH3CN: C60H71N3Ni2S7, triclinic, P1, a = 13.806(2) A, b = 14.267(2) A, c = 16.873(2) A, alpha = 69.263(10) degrees, beta = 69.267(8) degrees, gamma = 83.117(10) degrees, Z = 2, R1 = 0.0752 (wR2 = 0.2011). (Et4N)(Na.CH3CN)[2]: C34H39N2NaNiS4, triclinic, P1, a = 9.9570(10) A, b = 13.2670(10) A, c = 13.9560(10) A, alpha = 108.489(7) degrees, beta = 90.396(6) degrees, gamma = 103.570(4) degrees, Z = 2, R1 = 0.0390 (wR2 = 0.0995). Both complexes are square planar about the nickel ion in the solid state as well as in solution. Most Ni(II)-thiolate complexes are square planar except the tetrahedral mononuclear complexes with monodentate arylthiolate ligands that cannot force a square planar geometry. The ligand (L) has some flexibility to change its bite angle via the phenyl-phenyl bond and should not force a planar geometry on its complexes either. Therefore, it is interesting that 2 has adopted a square planar structure. Complex 2 readily converts to 1 in solution when not in the presence of excess L in a process that is presumably similar to that known for other mononuclear, bidentate ligated Ni(II) complexes. Both complexes, at least in the solid state, appear to have an inclination to bind another metal ion on one face of the complex (Ni2+ in 1, Na+ in 2). We hope to take advantage of this in future work to synthesize relevant model complexes for the active sites of the nickel-iron hydrogenases after suitable modifications are made to L.     

Synthesis and characterization of nickel(II) maltolate complexes containing ancillary bisphosphine ligands

Cationic nickel(II) complexes containing chelating O,O′-donor maltolate or ethyl maltolate ligands in conjunction with bidentate bisphosphine ligands Ph₂P(CH₂) _n PPh₂ were prepared by a one-pot reaction starting from nickel(II) acetate, bisphosphine, maltol (or ethyl maltol), and trimethylamine, and isolated as their tetraphenylborate salts. An X-ray structure determination of [Ni(maltolate)(Ph₂PCH₂CH₂PPh₂)]BPh₄ shows that the maltolate ligand binds asymmetrically to the (slightly distorted) square-planar nickel(II) center. The simplicity of the synthetic method was extended to the synthesis of the known platinum(II) maltolate complex [Pt(maltolate)(PPh₃)₂]BPh₄ which was obtained in high purity.     

The thermal isomerisation of some four co-ordinate square planar complexes of palladium and platinum with phosphine ligands

Thermally induced cis—trans isomerisation is reported for square planar complexes of the type MX₂L₂, M = Pd or Pt, L = organophosphorus ligand and X = halide. However, the palladium dichloro-phenyldimethyl complex provides another example of the more unusual trans—cis isomerisation. A number of phosphine complexes of platinum have been vacuum-distilled, in some cases with accompanying isomerisation. The tendency for such isomeric behaviour is interpreted in terms of variation in metal to phosphorus bond strength.     

Mechanism of beta-hydrogen elimination from square planar iridium(I) alkoxide complexes with labile dative ligands

Mechanistic studies were conducted on beta-hydrogen elimination from complexes of the general formula [Ir(CO)(PPh(3))(2)(OR)], which are square planar alkoxo complexes with labile ligands. The dependence of rate, isotope effect, and alkoxide racemization on phosphine concentration revealed unusually detailed information on the reaction pathway. The alkoxo complexes were remarkably stable, including those with a variety of electronically and sterically distinct groups at the beta-carbon. These complexes were much more stable than the corresponding alkyl complexes. Thermolysis of these complexes in the presence of PPh(3) yielded the iridium hydride [Ir(CO)(PPh(3))(3)H] and the corresponding aldehyde or ketone with rate constants that were affected little by the groups at the beta-carbon. The reactions were first order in iridium complexes. At low [PPh(3)], the reaction rate was nearly zero order in PPh(3), but reactions at high [PPh(3)] revealed an inverse dependence of reaction rate on PPh(3). The rate constants were similar in toluene, THF, and chlorobenzene. The y-intercept of a 1/k(obs) vs [PPh(3)] plot displayed a primary isotope effect, indicating that the y-intercept did not simply correspond to phosphine dissociation. These data and a dependence of alkoxide racemization on [PPh(3)] showed that the elementary beta-hydrogen elimination step was reversible. A mechanism involving reversible beta-hydrogen elimination followed by associative displacement of the coordinated ketone or aldehyde by PPh(3) was consistent with all of our data. This mechanism stands in contrast with the pathways proposed recently for alkoxide beta-hydrogen elimination involving direct elimination, protic catalysts, or binuclear mechanisms and shows that alkoxide elimination can follow pathways similar to those for beta-hydrogen elimination from alkyl complexes.     

The electrochemistry of square planar macrocyclic nickel complexes and the reaction of Ni(I) with alkyl bromides: Nickel tetraamine complexes

The electrochemistry of nine cyclic tetraamine complexes of Ni(II) in aprotic solvents has been investigated. It is shown that all form Ni(I) complexes which react with alkyl bromides, but only with some complexes are the reactions catalytic with respect to the nickel complex. The dependence of the electrochemical parameters and the mechanism and kinetics of the coupled chemical reactions on the structure of the ligands are discussed.     

Spectral and thermal study on the adduct formation between square planar nickel(II) chelates and some bidentate ligands

Adducts of Ni(II)-square planar complexes [Ni(beta-dik)(Me(4)en)](+), with a series of bidentate ligands (L), where beta-dik=acetylacetonate (acac) and benzoylacetonate (bzac), Me(4)en=N,N,N',N'-tetramethylethylenediamine and L=Me(4)en, 2,2'-bipyridine (bipy), ethylenediamine (en) and oxalate (C(2)O(4)(2-)) have been synthesized and characterized by spectral, thermal and magnetic measurements. Formation constants of the adducts formed from a series of ternary mixed Ni(II) complexes with the general formula [Ni(beta-dik)(diam)](+) with 1,10-phenanthroline (phen), 2,2'-bipyridine (bipy) and pyridine were spectrophotometrically determined. Thermodynamic parameters of the adduct formation between nickel(II) square-planar chelates and pyridine (py), 2,2'-bipyridine (bipy) and acetylacetone (acac) were also spectrophotometrically determined in 1,2-dichloroethane. The thermal stability of the isolated adducts was studied using thermogravimetry and the decomposition schemes of the adducts were given.     

Thermal square planar-to-octahedral transformation of nickel(II) complexes containing butanediamines in a solid phase

Thermal reactions of the nickel(II) complexes, [Ni(m-bn or i-bn)₂]X₂ and [Ni(H₂O)₂(dl-bn)₂]X₂·n H₂O, where m-bn, i-bn, and dl-bn are meso-2,3-butanediamine, 2-methyl-1,2-propanediamine, and dl-2,3-butanediamine, respectively, X is Cl^?, Br^?, I^?, NO^?₃, or ClO^?₄, and n is 2 for bromide, and 0 for the others, were investigated in a solid phase before and after heating using thermal analyses (TG and DSC) and spectral and magnetic measurements. In the case of the chloride and bromide, the square planar bis(dl-bn) complexes obtained by dehydration of the respective diaqua complexes were transformed to the octahedral diacido bis(dl-bn) complexes upon further heating. The same structural transformation was observed in the thermal reactions of [Ni(m-bn)₂](NO₃)₂ and [Ni(i-bn)₂]Cl₂. It was summarily recognized that such square planar-to-octahedral transformation was favored in the order dl-bn > i-bn > m-bn complexes in the respective halides, and it was a reversible thermochromism from yellow to blue. The changes in enthalpy of the reactions were endothermic and fell in the range of about 10–20 kJ mole^?1. The possibility of such configurational change seems to be dependent mainly upon the ionic radius of the X anion, the orientation of two C-substituted methyl groups on butanediamines in the formation of the complexes, and the thermal stability of the complexes themselves.     

The electrochemistry of square planar macrocyclic nickel complexes and the reaction of Ni(I) with alkyl bromides: Tetradentate Schiff base complexes

The electrochemistry of nine square planar macrocyclic Schiff base complexes of Ni(II) has been investigated in aprotic solvents. It is shown that all form Ni(I) complexes, and with one exception these species may be reacted with alkyl bromides in processes which are catalytic with respect to the nickel complex. The dependence of the electrochemical parameters and the mechanism and kinetics of the coupled chemical reactions on the structure of the ligands is discussed.