Davit Zargarian

Group 10 metal indenyl complexes

Abstract This review summarises the synthesis, characterisation, and reactivities of Group 10 metal indenyl complexes reported up to the end of 2001. We begin with a brief summary of the various arguments which have been put forth over the years to explain the observed differences between the reactivities of transition metal indenyl and cyclopentadienyl complexes. The main discussion is then focused on the various metal–indenyl bonding modes and preliminary reactivity trends observed in the complexes (indenyl)2NiII, {(indenyl)PdIL}2, (indenyl)MIILX, and [(indenyl)MIILL′]+ (M=Ni, Pd, Pt; L and X are neutral and anionic ligands, 1 respectively).

New pincer-type diphosphinito (POCOP) complexes of NiII and NiIII

This communication reports the synthesis and characterization of the new, pincer-type, square-planar, 16-electron compounds {2,6-(OPPr(i)(2))(2)C(6)H(3)}Ni(II)Br, 1, and {(Pr(i)(2)POCH(2))(2)CH}Ni(II)Br, 2, and the square-pyramidal, 17-electron complex {(Pr(i)(2)POCH(2))(2)CH}Ni(III)Br(2), 3.

Monomeric and Dimeric Nickel Complexes Derived from a Pincer Ligand Featuring a Secondary Amine Donor Moiety

Reaction of NiBr(2)(CH(3)CN)(x) with the unsymmetrical pincer ligand m-(i-Pr(2)PO)(CH(2)NHBn)C(6)H(4) (Bn = CH(2)Ph) gives the complex (R,S)-kappa(P),kappa(C),kappa(N)-{2-(i-Pr(2)PO),6-(CH(2)NHBn)-C(6)H(3)}Ni(II)Br, 1, featuring an asymmetric secondary amine donor moiety. Deprotonation of the latter with methyl lithium gave a dark brown compound that could not be characterized directly, but fully characterized derivatives prepared from this compound indicate that it is the LiBr adduct of the 14-electron amido species [kappa(P),kappa(C),kappa(N)-{2-(i-Pr(2)PO),6-(CH(2)NBn)-C(6)H(3)}Ni], 2. Thus, 2.LiBr reacts with water to regenerate 1, while reaction with excess benzyl or allyl bromide gave the POCN-type pincer complexes 3 and 4, respectively, featuring tertiary amine donor moieties. On the other hand, heating 2.LiBr at 60 degrees C led to loss of LiBr and dimerization to generate the orange crystalline compound [mu(N);kappa(P),kappa(C),kappa(N)-{2-(i-Pr(2)PO),6-(CH(2)NBn)-C(6)H(3)}Ni](2), 5. Solid state structural studies show that 1, 3, and 4 are monomeric, square planar complexes involving one Ni-N interaction, whereas complex 5 is a C(2)-symmetric dimer involving four Ni-N interactions and a Ni(2)N(2) core featuring a short Ni-Ni distance (2.51 A). Preliminary reactivity tests have shown that 5 is stable toward weak nucleophiles such as acetonitrile but reacts with strong nucleophiles such as CO or 2,6-Me(2)(C(6)H(3))NC. Reactions with protic reagents showed that phthalimide appears to break the dimer to generate a monomeric species, whereas alcohols appear to leave the dimer intact, giving rise instead to adducts through N...H...O interactions. These ROH adducts of 5 were found to be active precatalysts for the alchoholysis of acrylonitrile with up to 2000 catalytic turnover numbers.

New derivatives of PCP-type pincer complexes of nickel.

The pincer-type complexes (PC(sp3)P(i-Pr))NiR (PC(sp3)P(i-Pr) = (i-Pr(2)PCH(2)CH(2))(2)CH) react with HBF(4) (R = C[triple bond]CMe, Ph, Me) or AgBF(4) (R = Br) to give (PC(sp3)P(i-Pr))Ni(BF(4)), 1, which was found to involve fluxional Ni-F-BF(3) interactions. Competition experiments revealed that the relative ease of protonation of the Ni-hydrocarbyl moiety follows the order Ni-Me > Ni-C[triple bond]CMe > Ni-Ph. Complex 1 reacts with water to give [(PC(sp3)P(i-Pr))Ni(H(2)O)][BF(4)], 2, that in turn undergoes H(2)O exchange with CH(3)CN, i-PrNH(2), and CO to give the corresponding cationic adducts 3, 4, and 5; alternatively, 3-5 can also be obtained directly from the reaction of 1 with CH(3)CN, i-PrNH(2), and CO, respectively. Deprotonation of complex 2 gives the neutral hydroxo complex (PC(sp3)P(i-Pr))Ni(OH), 6. All complexes have been characterized by NMR spectroscopy and, in the case of 2-6, by X-ray crystallography.

ECE-Type Pincer Complexes of Nickel

Pincer complexes of transition metals have demonstrated valuable catalytic reactivities and desirable properties as functional materials. Much more is known about pincer complexes of noble metals, but the pincer chemistry of nonprecious, 3d metals is poised for rapid growth over the next decade. This chapter presents a literature survey of nickel complexes based on tridentate ECE-type pincer ligands featuring a meridional coordination of the central metal atom through two dative E → Ni interactions and a covalent C–Ni linkage. The discussion is focused on the synthesis, characterization, and reactivities of complexes featuring both symmetrical and unsymmetrical ligands, ECE and ECE′. The material is organized into various sections according to the type of donor moiety E and E′ (phosphine, amine, phosphinite, phosphinimine, thioether, and N-heterocyclic carbene) and the hydrocarbyl linker (aromatic or aliphatic). Where possible, the presentation reflects the chronological order of the developments in this field of study. The review concludes with an overview of the current state of the chemistry of (ECE)Ni complexes and offers some predictions on the future prospects of this field.

Characterization of divalent and trivalent species generated in the chemical and electrochemical oxidation of a dimeric pincer complex of nickel.

The electrolytic and chemical oxidation of the dimeric pincer complex [κ(P),κ(C),κ(N),μ(N)-(2,6-(i-Pr(2)POC(6)H(3)CH(2)NBn)Ni](2) (1; Bn = CH(2)Ph) has been investigated by various analytic techniques. Cyclic voltammetry measurements have shown that 1 undergoes a quasi-reversible, one electron, Ni-based redox process (ΔE(0)(1/2) = -0.07 V vs Cp(2)Fe/[Cp(2)Fe](+)), and spectroelectrochemical measurements conducted on the product of the electrolytic oxidation, [1](+•), have shown multiple low-energy electronic transitions in the range of 10,000-15,000 cm(-1). Computational studies using Density Functional Theory (B3LYP) have corroborated the experimentally obtained structure of 1, provided the electronic structure description, and helped interpret the experimentally obtained absorption spectra for 1 and [1](+·). These calculations indicate that the radical cation [1](+·) is a dimeric, mixed-valent species (class III) wherein most of the spin density is delocalized over the two nickel centers (Ni(+2.5)(2)N(2)), but some spin density is also present over the two nitrogen atoms (Ni(2+)(2)N(2)·). Examination of alternative structures for open shell species generated from 1 has shown that the spin density distribution is highly sensitive toward changes in the ligand environment of the Ni ions. NMR, UV-vis, electron paramagnetic resonance (EPR), and single crystal X-ray diffraction analyses have shown that chemical oxidation of 1 with N-Bromosuccinimide (NBS) follows a complex process that gives multiple products, including the monomeric trivalent species κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH═NBn)}NiBr(2) (2). These studies also indicate that oxidation of 1 with 1 equiv of NBS gives an unstable, paramagnetic intermediate that decomposes to a number of divalent species, including succinimide and the monomeric divalent complexes κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH═NBn)}NiBr (3) and κ(P),κ(C),κ(N)-{2,6-(i-Pr(2)PO)(C(6)H(3))(CH(2)N(H)Bn)}NiBr(2) (4); a second equivalent of NBS then oxidizes 3 and 4 to 2 and other unidentified products. The divalent complex 3 was synthesized independently and shown to react with NBS or bromine to form its trivalent homologue 2. The new complexes 2 and 3 have been characterized fully.

Direct, one-pot synthesis of POCOP-type pincer complexes from metallic nickel

A one-pot procedure has been developed for the direct and atom-economical preparation of the pincer-type complexes (R′-POCOPR)NiCl (R′-POCOPR = κP,κC,κP-(2,6-(R2PO)2-R′n-C6H3−n); R = i-Pr, Ph; R′n = H, 4-OMe, 4-COOMe, 3,5-(t-Bu)2). This convenient synthetic protocol involves heating a 1:2:1–2 mixture of resorcinol (or its substituted derivatives), R2PCl, and nickel powder in toluene (at 100 °C) or acetonitrile (at 75 °C) for 18 h. The target nickel compounds can be obtained from this procedure in up to 93% yield. Using metallic palladium in this protocol instead of nickel gives the corresponding palladium complexes in low yields only (ca. 10%).

Allowed and Forbidden d-d Transitions in Poly(3,5-dimethylpyrazolyl)methane Complexes of Nickel(II)†

Abstract Absorption spectra of four nickel(II) complexes with poly(pyrazolyl)methane ligands are presented in the NIR-VIS-UV region and the band system corresponding to the lowest-energy spin-allowed and spin-forbidden transitions is analyzed. A quantitative theoretical model involving coupled electronic states provides precise energies for the lowest-energy triplet and singlet excited states and allows comparisons between complexes with a variable number of nitrogen and oxygen ligator atoms. Singlet energies between 12 840 and 13 000 cm−1 are determined for heteroleptic complexes. These energies are in an intermediate range between those for homoleptic complexes with either nitrogen or oxygen ligator atoms with singlet states at approximately 12 000 and 14 000 cm−1, respectively. The new theoretical approach is compared to the traditional ligand-field parameters obtained from the maxima of the broad, spin-allowed absorption bands.

Reactions of Phenylhydrosilanes with Pincer-Nickel Complexes: Evidence for New Si-O and Si-C Bond Formation Pathways.

This contribution presents evidence for new pathways manifested in the reactions of the phenylhydrosilanes PhnSiH4-n with the pincer complexes (POCsp(2)OP)Ni(OSiMe3), 1-OSiMe3, and (POCsp(3)OP)Ni(OSiMe3), 2-OSiMe3 (POCsp(2)OP = 2,6-(i-Pr2PO)2C6H3; POCsp(3)OP = (i-Pr2POCH2)2CH). Excess PhSiH3 or Ph2SiH2 reacted with 1-OSiMe3 to eliminate the disilyl ethers Ph(n)H(3-n)SiOSiMe3 (n = 1 or 2) and generate the nickel hydride species 1-H. Subsequent reaction of the latter with more substrate formed corresponding nickel silyl species 1-SiPhH2 or 1-SiPh2H and generated multiple Si-containing products, including disilanes and redistribution products. The reaction of 1-OSiMe3 with excess Ph2SiH2/Ph2SiD2 revealed a net KIE of ca. 1.3-1.4 at room temperature. Treating 1-OSiMe3 with excess Ph3SiH also gave 1-H and the corresponding disilyl ether Ph3SiOSiMe3, but this reaction also generated the new siloxide 1-OSiPh3 apparently via an unconventional σ-bond metathesis pathway in which the Ni center is not involved directly. The reaction of excess PhSiH3 and 2-OSiMe3 gave polysilanes of varying solubilities and molecular weights; NMR investigations showed that these polymers arise from Ni(0) species generated in situ from the reductive elimination of the highly reactive hydride intermediate, 2-H. The stoichiometric reactions of 2-OSiMe3 with Ph2SiH2 and Ph3SiH gave, respectively, siloxides 2-OSiPh2(OSiMe3) and 2-OSiPh3. Together, these results demonstrate the strong influence of pincer backbone and hydrosilane sterics on the different reactivities of 1-OSiMe3 and 2-OSiMe3 toward Ph(n)SiH(4-n) (dimerization, polymerization, and redistribution vs formation of new siloxides). The mechanisms of the reactions that lead to the observed Si-O, Si-C, and Si-Si bond formations are discussed in terms of classical and unconventional σ-bond metathesis pathways.

Antitumor, Antioxidant and Antimicrobial Studies of Substituted Pyridylguanidines

A series of N-pivaloyl-N′-(alkyl/aryl)-N″-pyridylguanidine of general formula C4H9CONHC(NR1R2)NPy have been synthesized and characterized using elemental analysis, FT-IR, multinuclear NMR spectroscopy, and in the case of compounds 7 and 11, by single crystal X-ray diffraction (XRD). The synthesized guanidines were tested for antitumor activities against potato tumor, and showed excellent inhibition against Agrobacterium tumefaciens (AT10)-induced tumor. The antioxidant and antimicrobial activities of these new compounds against various bacterial and fungal strains were also investigated.