A CCC-NHC pincer Ni(II)Cl complex was prepared according to the metallation/transmetallation methodology. It was fully characterized by electrochemical, NMR spectroscopic, theoretical, and X-ray crystallographic methods. The complex and its cation were evaluated for electrocatalytic reduction of CO2 under a variety of conditions and found to provide some of the fastest catalytic rates and highest substrate selectivities (CO2vs. H+) reported. Rates improved in the presence of water and, significantly, catalysis occurred at the first reduction potential, presumably at the Ni(I) state. Controlled potential electrolysis (CPE) was found to yield CO at 34% and formate at 47% Faradaic efficiency (FE).

•CCC-NHC nickel pincer complexes were synthesised by metallation/transmetallation.•Raman was obtained for the chloride complex along with a simulated spectrum.•Cationic CCC-NHC pincer nickel complexes were made which exhibited polymorphism.•Coupling of NHC's from two different ligands, which were oxidized.The metallation/transmetallation strategy has been successfully applied to the preparation of CCC-NHC pincer nickel complexes. Manipulation of the coordination sphere lead to cationic CCC-NHC pincer nickel complexes. These were found to exhibit polymorphism in the solid state with significant differences in the intermolecular distances observed by X-ray crystallography. Three polymorphs were observed for the PF6− salt, and two were observed for the BF4− salt. When BPh4− was the counterion only one morphology was observed, and crystals grown of the triflate salt were too small for analysis. Details of the differences are documented for comparison. Additionally, during crystallization it was found that a rearrangement of the CCC-NHC ligand had occurred, involving coupling of an NHC from two different ligands, which were oxidized.Polymorph packing differences.Download high-res image (164KB)Download full-size image

The coupling of N-heterocyclic azoles (imidazoles, benzimidazoles, triazoles) to bromobenzenes (1,2-; 1,3-; or 1,4) in a step-wise, sequential manner was accomplished by manipulation of reaction time and stoichiometry, which provided straight-forward access to unsymmetrical bis(azolium) salts in only three isolation steps from commercially-available starting materials. Eight mono(azole) substituted bromobenzenes, four mono(azolium)bromobenzene salts, twelve unsymmetrical bis(azole)benzenes, and fourteen unsymmetrical bis(azolium) salts, which are precursors for pincer ligand complexes, are reported.Download high-res image (95KB)Download full-size image

Development of CCC-NHC pincer Co complexes via transmetalation from Zr is reported. Formation of these air-stable Co(III) complexes was achieved through use of a CoCl2 or Co(acac)3in situ or with a discrete CCC-NHC pincer Zr transmetallating agent. Preliminary activity in the hydroboration of styrene is reported. This facile methodology will further the development of CCC-NHC pincer first-row transition metal complexes.

•A pincer complex catalyzed 1-4-addition reactions at room temperature.•Cyclic and acyclic enone substrates were reactive under optimized conditions.•Eco-friendly solvents were applicable with air- and water-stable CCC-NHC pincer Rh complexes.Quantitative β-boration of α,β-unsaturated carbonyl compounds was achieved utilizing the eco-friendly solvent EtOH along with MeOH at room temperature in 1 h, by a CCC-NHC pincer Rh complex mixture. Substrates with β-substituents were successfully converted yielding challenging, quaternary C–B bonds. The air- and water-stable pre-catalyst A, identified as a mixture of iodo and chloro CCC-NHC pincer Rh amine complexes, was evaluated for catalytic activity. This report is the first example of a pincer Rh complex demonstrating catalytic activity in a 1,4-addition at room temperature.

A rare Ta bis(imido) complex, which has unique reactivity, was prepared by manipulating the coordination sphere of a CCC-NHC pincer Ta complex. The reaction of lithium tert-butylamide with complex 1 yielded (1,3-bis(3′-butylimidazol-2′-yl-1′-idene)-2-phenylene)bis(tert-butylimido)tantalum(V) (2) as a lithium iodide bridged dimer, as determined by the X-ray structure. Complex 2 catalytically cyclized α,ω-aminoalkenes to effect an oxidative amination of alkenes (dehydrogenation by C–H activation) and produced a cyclic imine, an equivalent of reduced substrate, and varying proportions of hydroamination. Various additives and concentration impact the catalytic results. Computational and experimental observations have led to an initial mechanistic hypothesis. Based upon it, precatalyst 2 appears to be the first example of a bifunctional catalyst (MH-NHR) that is highly selective for nonpolar C═C bonds in preference to polar C═X bonds for outer-sphere hydrogenation.

A report that demonstrated an efficient methodology for the arylation of imidazoles has been extended to bis(N-heterocyclic) compounds. Using bis(aryl) iodonium salts provides high-yielding access to CCC-NHC ligand precursors in a single step. Examples of arylation using various iodonium salts are reported herein with an investigation into the factors governing their relative rate of reactivity. The metalation of one of these compounds using Zr(NMe2)4 and its subsequent treatment with [Pt(COD)Cl2] to yield a transmetalated product are reported.

An air- and water-stable CCC–NHC pincer Rh complex catalyzed the 1,4-addition of aryl boronic acids to α,β-unsaturated ketones and aldehydes. This bench top method proceeds in eco-friendly solvents including methanol and water. The scope of boronic acids was expanded to include heterocyclic examples.

•Four new CCC-NHC Zr or Hf pincer complexes are reported.•Hydroamination/cyclization of unactivated aminoalkenes with the complexes was explored.•Complexes with iodide ligands gave the fastest reaction rates (I > Br > Cl).•The Zr complexes provided faster hydroamination/cyclization rates than the Hf analogs.1,3-Bis(3′-butylimidazol-1′-yl)benzene dibromide (2b) or 1,3-bis(3′-butylimidazol-1′-yl)benzene dichloride (2c) was reacted with a stoichiometric amount of Zr(NMe2)4 or Hf(NMe2)4 yielding four new early transition metal CCC-N-heterocylic carbene (CCC-NHC) pincer complexes. Two of the CCC-NHC pincer complexes were synthesized via a new methodology, which allowed for a room temperature reaction, shorter reaction times, and slightly less Zr(NMe2)4 or Hf(NMe2)4. The molecular structure of 2-(1,3-bis-3′-butylimidazol-1′-yl-2′-ylidene)phenylene)(dimethylamido)(dibromo)zirconium(IV) (3b) was determined by X-ray crystallography. The complexes were evaluated for hydroamination/cyclization of unactivated aminoalkenes yielding pyrrolidines or piperidines. The hydroamination/cyclization rates were dependent upon the halogen or the metal in the complex. The complexes with iodide ligands gave the fastest hydroamination/cyclization rates (I > Br > Cl). The Zr complexes provided faster hydroamination/cyclization rates than the Hf analogs. Use of CCC-NHC ZrI3 complex 5 enhanced the rate of reaction.CCC-NHC Zr and Hf amido complexes of I, Br, and Cl catalyze the hydroamination/cyclization of unactivated aminoalkenes.

Unprecedented Ta bis(NHC) pincer complexes have been synthesized and characterized by extension of the early-transition-metal amido methodology. The reaction of 1,3-bis(3-butylimidazol-1-yl)benzene diiodide (1) with stoichiometric and substoichiometric amounts of (tert-butylimido)tris(dimethylamido)tantalum(V) yielded (1,3-bis(3-butylimidazol-1-yl-2-idene)-2-phenylene)(tert-butylimido)diiodotantalum(V) (2) and (1,3-bis(3-butylimidazol-1-yl-2-idene)-2-phenylene)(tert-butylimido)(dimethylamido)iodotantalum(V) (3). Use of excess (tert-butylimido)tris(dimethylamido)tantalum(V) to metalate 1,3-bis(3-butylimidazol-1-yl)benzene diiodide (1) yielded (1,3-bis(3-butylimidazol-1-yl-2-idene)-2-phenylene)(tert-butylimido)(dimethylamido)iodotantalum(V) (3) exclusively. Furthermore, the first early-transition-metal (group 3–5) triazole-based NHC complex, (1,3-bis(3-butyltriazol-1-yl-2-idene)-2-phenylene)(tert-butylimido)(dimethylamido)iodotantalum(V) (5), has been synthesized via amine elimination of 1,3-bis(3-butyltriazol-1-yl)benzene diiodide (4) with (tert-butylimido)tris(dimethylamido)tantalum(V).

Development of CCC-NHC pincer Co complexes via transmetalation from Zr is reported. Formation of these air-stable Co(III) complexes was achieved through use of a CoCl2 or Co(acac)3in situ or with a discrete CCC-NHC pincer Zr transmetallating agent. Preliminary activity in the hydroboration of styrene is reported. This facile methodology will further the development of CCC-NHC pincer first-row transition metal complexes.