Square-planar Nickel Complexes Research Articles

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.

Square-planar NiII complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic PtII luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of 3MLCT states in square-planar NiII complexes by creating coordination environments that seemed particularly well adapted to the 3d8 valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the NiII center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into 1MLCT absorption bands and ultrafast intersystem crossing to the 3MLCT excited state, the latter relaxes onward into a metal-centered triplet state (3MC). A torsional motion of the tridentate ligand and a NiII-carbon bond elongation facilitate 3MLCT relaxation to the 3MC state. The 3MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar NiII complexes with enhanced photophysical properties. The longest 3MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5-10 compared to previously investigated square-planar NiII complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.

Synthesis, crystal structures, optical properties, DFT and TD-DFT studies of Ni (II) complexes with imine-based ligands

Two new square-planar nickel (II) complexes were synthesized from (2-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)phenol) and (1-((E)-(9-ethyl-9H-carbazol-7-ylimino)methyl)naphthalen-2-ol) ligands, for complex I and complex II, respectively. They were characterized by IR, UV-Vis spectroscopy and X-ray diffraction. Further measurements were also conducted as cyclic voltammetry and conductivity measurements. Both complexes crystallize in the monoclinic system with the space groups P21/n and P21/c for complex I and complex II, respectively. The electronic absorption spectrum of complex I exhibits the lowest-lying absorption band (LEB) at 381 nm and the highest energy absorption band (HEB) at 269 nm. For complex II, the LEB was located at 431 nm while the HEB was found to be at 260 nm. Theoretical structure geometries, vibrational frequencies and electronic properties of the two complexes were calculated, by means of the density functional theory (DFT). The basis set 6-31g+(d) was chosen, for non-metal atoms (C, H, N and O) and the Stuttgart/Dresden effective core potentials basis set (SDD), for Ni atoms. UV-Vis transitions nature was investigated via a time-dependent density functional theory (TD-DFT) calculation. Overall, theoretical calculations were in accordance with the experimental results.

Dynamic Covalent Chemistry Approach toward 18-Membered P4N2 Macrocycles and Their Nickel(II) Complexes.

A dynamic covalent chemistry approach was used for the stereoselective synthesis of 1,10-diaza-3,8,12,17-tetraphosphacyclooctadecanes via condensation reaction of 1,4-bis(organylphosphino)butane, formaldehyde, and primary amines. The obtained 18-membered P4N2 macrocycles were isolated in pure form as meso- (RPSPSPRP) or rac- (RPRPRPRP/SPSPSPSP) isomers. The structural features of the individual stereoisomers were revealed by NMR spectroscopy and X-ray structure analysis. All P4N2 macrocycles form square-planar nickel(II) complexes with the RPSPSPRP isomer only, in which the orientation of the lone pairs of electrons at phosphorus favors this coordination mode, independent of the initial configuration of the ligand, indicating the ability of the 18-membered P4N2 macrocycles to stereoisomerize in the course of the complexation.

Unconventional access to a solvatochromic nickel (II) dye featuring a coordination-induced spin crossover behavior

We report the synthesis of a square-planar nickel complex in which the metal center is located in a NOON environment. Promoting its axial ligation triggers a spin state switching process which is accompanied by a change of the absorption properties depending on the solvent. Indeed, a strong solvatochromic shifts could be observed in the presence of DMSO or pyridine, highlighting the key interplay between the donor strength of the solvent and the optical properties of the complex. The optical and magnetic properties of this compound in coordinating media have been carefully investigated through combined experimental/theoretical approaches.

Syntheses and structural characterization of new trans-bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato] nickel(II), the pyridine derivative thereof, and bis-{bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato]cobalt(II)} complexes

Abstract Three new metal complexes of ammonium n-butyl-(p-methoxyphenyl) dithiophosphinate (NH4L) were synthesized. NH4L was treated with NiCl2·6H2O and CoCl2·6H2O in ethanol to obtain a new trans-bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato]nickel(II) and bis-{bis-[n-butyl-(p-methoxyphenyl) dithiophosphinato]cobalt(II)} complexes, ([Ni(L)2] and [Co2(μ-L)2(L)2], respectively). Square-planar nickel(II) complex was furthermore reacted with excess pyridine (Py) and the octahedral {bis-pyridine-bis-[n-butyl-(p-methoxyphenyl)]}nikel(II) complex, ([Ni(L)2(Py)2]), was obtained. The structures of the complexes were analyzed by elemental analysis, 1H-, 13C-, 31P- NMR; and also by MS-, FTIR- and Raman spectroscopies. The magnetic susceptibilities of the Co(II) and the pyridine-Ni(II) complexes were measured to throw further light on the hybridization patterns. The solid-state structures of the nickel complexes were also done by X-ray crystallography. The P–S bonds in the Ni(II) complexes are essentially of the same lengths; indicating an effective electron delocalization around the central PS2 moiety.

Chameleon-like Nature of Anagostic Interactions and Its Impact on Metalloaromaticity in Square-Planar Nickel Complexes

Anagostic C–H···M (M = a metal center) intramolecular interactions, one of the most fundamental and elusive forces in organometallic chemistry, are intuitively considered as repulsive and purely el...

Tuning Semiconductor Performance of Nickel Complexes through Crystal Transformation.

Crystal transformation between two polymorphs (green, 1-G, and red, 1-R) of the square-planar nickel complex NiL2 (L = 2-ethoxy-6-( N-methyliminomethyl)phenolate) and their tuning effect to semiconductor properties were studied both experimentally and theoretically. When 1-G is heated to 413 K, it converts to 1-R, whereas soaking 1-R in several kinds of solvents causes it to revert to 1-G. Crystallographic and PXRD studies reveal the dramatic changes in crystal dimensions due to the changes of packing models. Heating device made from 1-G (D-1-G(298)) at 413 K significantly increases the electrical conductivity from 6.55 × 10-4 S cm-1 for D-1-G(298) to 1.11 × 10-3 S cm-1 for D-1-G(413), showing significant crystal form dependence. Heat-treating D-1-G and D-1-R devices at different temperatures clearly reveals the reason for the conductivity tuning. Thus, the conductivity of NiL2-based devices could be well tuned through crystal transformation by heating or by soaking in solvent. Theoretical calculations clearly revealed the reason for such conductivity changes and also predicted that both polymorphs are good p-type semiconductors with hole mobilities of 1.63 × 10-2 (1-G) and 2.11 × 10-1 cm2 V-1 s-1 (1-R).

Hydrogen-Atom Noninnocence of a Tridentate [SNS] Pincer Ligand.

Double deprotonation of bis(2-mercapto-4-methylphenyl)amine ([SNS]H3) followed by addition to NiCl2(PR3)2 in air-free conditions afforded [SN(H)S]Ni(PR3) (1a, R = Cy; 1b, R = Ph) complexes, characterized as diamagnetic, square-planar nickel(II) complexes. When the same reaction was conducted with 3 equiv of KH, the diamagnetic anions K{[SNS]Ni(PR3)} were obtained (K[2a], R = Cy; K[2b], R = Ph). In the presence of air, the reaction proceeds with a concomitant one-electron oxidation. When R = Cy, a square-planar, S = 1/2 complex, [SNS]Ni(PCy3) (3a), was isolated. When R = Ph, the bimetallic complex {[SNS]Ni(PPh3)}2 ({3b}2) was obtained. This bimetallic species is diamagnetic; however, in solution it dissociates to give S = 1/2 monomers analogous to 3a. Complexes 1-3 represent a hydrogen-atom-transfer series. The bond dissociation free energies (BDFEs) for 1a and 1b were calculated to be 63.9 ± 0.1 and 62.4 ± 0.2 kcal mol-1, respectively, using the corresponding p Ka and E°' values. Consistent with these BDFE values, TEMPO• reacted with 1a and 1b, resulting in the abstraction of a hydrogen atom to afford 3a and 3b, respectively.

Variation of M···H-C Interactions in Square-Planar Complexes of Nickel(II), Palladium(II), and Platinum(II) Probed by Luminescence Spectroscopy and X-ray Diffraction at Variable Pressure.

Luminescence spectra of isoelectronic square-planar d8 complexes with 3d, 4d, and 5d metal centers show d-d luminescence with an energetic order different from that of the spectrochemical series, indicating that additional structural effects, such as different ligand-metal-ligand angles, are important factors. Variable-pressure luminescence spectra of square-planar nickel(II), palladium(II), and platinum(II) complexes with dimethyldithiocarbamate ({CH3}2DTC) ligands and their deuterated analogues show unexpected variations of the shifts of their maxima. High-resolution crystal structures and crystal structures at variable pressure for [Pt{(CH3)2DTC}2] indicate that intermolecular M···H-C interactions are at the origin of these different shifts.

Self-organized Tetranickel(II) Molecular Square with Imidazolinyl-imidazole Bridging Ligand

A novel square-planar cyclic tetranuclear nickel(II) complex coordinated by im-H2bizn− ligand (Him-H2bizn = 4,5-bis(4,5-dihydro-1H-imidazol-2-yl)imidazole), [NiII4(im-H2bizn)4(MeOH)4(py)4](ClO4)4·2ether (py = pyridine), has been synthesized. The tetranickel(II) complex crystallized in C2/m space group and exhibited antiferromagnetic intramolecular coupling with J = −24 K. The complex arrests ClO4− ion by moderate O–H⋯O and weak multipoint C–H⋯O hydrogen bonds.

Nature and strength of weak intermolecular interactions with metal atoms in crystals of square-planar nickel(ii) complexes

The results of geometric analysis of 588 crystal structures of square-planar nickel complexes containing short intermolecular Ni…X contacts (X is a non-metal atom) are reported. The frequency of occurrence of Ni…X interactions of a particular type was evaluated depending on the nature of the X atom and geometric characteristics. The promolecular functions RDG and sign(λ2)•ρ(r) were used to analyze the character of Ni…X interactions in attractive/repulsive terms. The relative strength of these interactions was determined as the function of the nature of the X atom and the direction of charge transfer.

A nickel(II) thiosemicarbazonato complex: synthesis, structure, electrochemistry, and application in catalytic coupling of terminal alkynes with arylboronic acids

A simple route for the synthesis of a new square-planar nickel(II) complex of 2-((3-methylthiophen-2-yl)methylene)hydrazinecarbothioamide is described. Elemental analysis, solution conductivity, and spectral measurements were used to authenticate the composition of the complex. In addition, the X-ray crystal structure of the complex was obtained. Each of the two ligands of the centrosymmetric structure is coordinated to the metal center via the azomethine N and the thioimidate S atoms, forming five-membered chelate rings. Infrared and NMR spectroscopic features of the complex are consistent with its molecular structure. The complex displays a quasi-reversible reduction with E 1/2 of −0.83 V (ΔE p 160 mV), indicating that the ligand system can stabilize the Ni(I) oxidation state. The complex acts as an active catalyst for the C–C coupling of aryl acetylenes with arylboronic acids containing a variety of substituents. The influence of solvent, base, reaction temperature, and catalyst loading on the C–C coupling reaction was evaluated. A new square-planar nickel(II) complex bearing 2-((3-methylthiophen-2-yl)methylene)-hydrazinecarbothioamide was synthesized and characterized by elemental analysis, FTIR, NMR, and single-crystal X-ray crystallographic technique. Cyclic voltammetric study indicates stabilization of Ni(I) oxidation state. The complex acts as an effective catalyst for the carbon–carbon cross-coupling reaction of substituted phenylacetylenes with various arylboronic acids under optimized conditions.

A Readily Accessible Chiral NNN Pincer Ligand with a Pyrrole Backbone and Its Ni(II) Chemistry: Syntheses, Structural Chemistry, and Bond Activations.

A new class of chiral C2-symmetric N-donor pincer ligands, 2,5-bis(2-oxazolinyldimethylmethyl)pyrroles (PdmBox)H, was synthesized starting from the readily available ethyl 2,2-dimethyl-3-oxobutanoate (1). The synthesis of the ligand backbone was achieved by oxidative enole coupling with CuC12 followed by Paal-Knorr-type pyrrole synthesis. The corresponding protioligands (RPdmBox)H (R = iPr: 5a; Ph: 5b) were obtained by condensation with amino alcohols and subsequent zinc-catalyzed cyclization. Reaction of the lithiated ligands with [NiCl2(dme)] yielded the corresponding square-planar nickel(II) complexes [(RPdmBox)NiCl] (6a/b). Salt metathesis of 6a with the corresponding alkali or cesium salts in acetone led to the formation of air- and moisture-stable [(iPrPdmBox)NiX] (X = F (7), X = Br (8), X = I (9), X = N3 (10), X = OAc (11). Furthermore, the conversion of [(iPrPdmBox)NiF] (7) with hydride transfer reagents such as PhSiH3 led to the stable hydrido species [(iPrPdmBox)NiH] (27), the stoichiometric transformations of which were studied. Treatment of 6a with organometallic reagents such as ZnEt2, PhLi, PhC≡CLi, NsLi, or (4FBn)2Mg(THF)2 gave the corresponding alkyl, alkynyl, or aryl complexes. The availability of the new nonisomerizable PdmBox pincer ligands allowed the comparative study of their ligation to square-planar complexes as helically twisted spectator ligands as opposed to the enforced planar rigidity of their iso-PmBox analogues and the way this influences the reactivity of the Ni complexes.

Square-planar Ni(II) thiosemicarbazonato complex as an easily accessible and convenient catalyst for Sonogashira cross-coupling reaction

A simple route for the synthesis of a new square-planar nickel(II) complex bearing 2-((3-methylthiophen-2-yl)methylene)-N-phenylhydrazinecarbothioamide ligand has been described. The composition of the complex has been established by elemental analysis, FT-IR, NMR, and single crystal X-ray crystallographic technique. The complex acts as an active homogeneous catalyst for the Sonogashira reaction of phenylacetylene with electron deficient (activating) and electron rich (deactivating) aryl halides (iodides and bromides) under optimized conditions.

Catecholato complexes of cobalt and nickel with 1,4-disubstituted-1,4-diazabutadiens-1,3 and 1,2-bis(diphenylphosphino)ethane

Divalent cobalt and nickel form four-coordinate complexes with sterically hindered 3,6-di-tert-butylcatecholato dianion (3,6-DBCat) and neutral bidentate 1,4-disubstituted-1,4-diazabutadiens-1,3 (DAB). Structural study of (1,4-di-tert-butyl-1,4-diazabutadiene-1,3)(3,6-di-tert-butyl-catecolato)nickel and (1,4-bis-(2,6-di-iso-propylphenyl)-2,3-dimethyl-1,4-diazabutadiene-1,3)(3,6-di-tert-butyl-catecolato)cobalt indicates square-planar environment of metals. Chemical one-electron oxidation of nickel complexes proceeds through catecholate ligand and leads to o-semiquinonato adducts. EPR spectral parameters indicate preservation of square-planar configuration after oxidation. Complexes (DAB)M(Cat) (M = Ni, Co) undergo neutral ligand substitution reactions. A number of four-coordinated square-planar mono-catecholato complexes of cobalt and nickel of general formula (L)M(3,6-DBCat) were synthesized and characterized (L= N,N’-disubstituted 1,4-diazabutadienes-1,3 (DAB), dipyridine, bis(diphenylphosphino)ethane; M=Co, Ni; 3,6-DBCat =3,6-di-tert-butyl-catecholato dianion).

Asymmetric N2O2 complexes of iron(III) and nickel(II) obtained from acetylacetone-S-methyl-thiosemicarbazone: synthesis, characterization and electrochemistry

Iron(III) and nickel(II) complexes from acetylacetone-S-methyl-thiosemicarbazone were synthesized and characterized. The thiosemicarbazidato structures of N2O2 are N1-acetylacetone, N4-salicyl/3-methoxy/5-bromo-/3,5-dichlorosalicylidene. The square-pyramidal iron(III) and square-planar nickel(II) complexes of N1-acetylacetone-N4-3-methoxy thiosemicarbazidato were determined by X-ray diffraction. Electrochemical characterizations of the complexes were carried out by using cyclic voltammetry and square wave voltammetry. While nickel(II) complexes only give ligand based reduction and oxidation reactions, iron(III) complexes give reversible FeIII/FeII reduction in addition to the ligand based processes. Changing of the substituents on the ligand, shifts the redox processes and also affects the stabilities of the electrogenerated species. With respect to the reduction potential of FeIII/FeII couple, the most easily reduced complex was 3,5-dichloro substituted complexes and the most difficult to reduce was the 3-methoxy substituted one.

Donor-acceptor ligand-to-ligand charge-transfer coordination complexes of nickel(II).

A family of charge-transfer chromophores comprising square-planar nickel(II) complexes with one catecholate donor ligand and one α-diimine acceptor ligand is reported. The nine new chromophores were prepared using three different catecholate ligands and three different α-diimine ligands. Single-crystal X-ray diffraction studies on all members of the series confirm a catecholate donor-nickel(II)-α-diimine acceptor electronic structure. The coplanar arrangement of donor and acceptor ligands manifests an intense ligand-to-ligand charge-transfer (LL'CT) absorption band that can be tuned incrementally from 650 nm (1.9 eV) to 1370 nm (0.9 eV). Electrochemical studies of all nine complexes reveal rich redox chemistry with two one-electron reduction processes and two one-electron oxidation processes. For one dye, both the singly reduced anion and the singly oxidized cation were prepared, isolated, and characterized by EPR spectroscopy to confirm ligand-localization of the redox processes. The optical and electrochemical properties of these new complexes identify them as attractive candidates for charge-transfer photochemistry and solar-energy conversion applications.

Spectroscopic characterization, antioxidant and antitumour studies of novel bromo substituted thiosemicarbazone and its copper(II), nickel(II) and palladium(II) complexes

A new, slightly distorted octahedral complex of copper(II), square planar complexes of nickel(II) and palladium(II) with 2,4′-dibromoacetophenone thiosemicarbazone (DBAPTSC) are synthesized. The ligand and the complexes are characterized by FT-IR, FT-Raman, powder X-ray diffraction studies. The IR and Raman data are correlated for the presence of the functional groups which specifically helped in the confirmation of the compounds. In addition, the free ligand is unambiguously characterized by 1H and 13C NMR spectroscopy while the copper(II) complex is characterized by electron paramagnetic resonance spectroscopy (EPR). The g values for the same are found to be 2.246 (g1), 2.012 (g2) and 2.005 (g3) which suggested rhombic distortions. The HOMO–LUMO band gap calculations for these compounds are found to be in between 0.5 and 4.0eV and these compounds are identified as semiconducting materials. The synthesized ligand and its copper(II), nickel(II) and palladium(II) complexes are subjected to antitumour activity against the HepG2 human hepatoblastoma cell lines. Among all the compounds, nickel(II) complex is found to exert better antitumour activity with 57.6% of cytotoxicity.

Detailed Geometric and Electronic Structures of a One‐Electron‐Oxidized Ni Salophen Complex and Its Amido Derivatives

Square-planar nickel(II) complexes of 1,2-bis(2-hydroxy-3,5-di-tert-butylbenzimino)benzene ([1]), 1-(2-hydroxy-3,5-di-tert-butylbenzimino)-2-(2-hydroxy-3,5-di-tert-butylbenzamido)benzene ([2]–) and 1,2-bis(2-hydroxy-3,5-di-tert-butylbenzamido)benzene ([3]2–) were chemically oxidized by one-electron to afford nickel–radical species with non-negligible metal contribution to the singly occupied molecular orbital (SOMO). The X-ray crystal structure of [1][SbF6]shows an almost symmetric metal coordination sphere and phenoxyl pattern within the ligand framework. The spin population at the nickel ion calculated by DFT (B3LYP) does not evolve monotonically within the series, and the values are 18.7, 19.5, and 42.7 % for [1]+, [2], and [3]–, respectively. Complexes [1]+, [2], and [3]– each exhibit a weak near-IR (NIR) band at 3600, 4440, and 5900 cm–1, respectively, the origin of which differs. For [1]+, it corresponds to an intervalence charge transfer (IVCT) transition (β-HOMO → β-LUMO) typical for delocalized compounds (calcd. 3700 cm–1). For [2], which is a localized radical, it is assigned to an intraligand charge transfer (CT) transition (calcd. 5000 cm–1). Although the CT transition still involves orbitals mainly centered on the ligand, it acquires significant ligand-to-metal charge transfer (LMCT) character in the case of [3]– (calcd. 5700 cm–1).

SU-79: a novel germanate with 3D 10- and 11-ring channels templated by a square-planar nickel complex

An open-framework germanate SU-79 was synthesized using nickel complex and amine as the templates. The crystal structure was solved by the combination of rotation electron diffraction (RED) and synchrotron single crystal X-ray diffraction.