Unsaturated and Benzannulated N-Heterocyclic Carbene Complexes of Titanium and Hafnium: Impact on Catalysts Structure and Performance in Copolymerization of Cyclohexene Oxide with CO2

Tridentate, bis-phenolate N-heterocyclic carbenes (NHCs) are among the ligands giving the most selective and active group 4-based catalysts for the copolymerization of cyclohexene oxide (CHO) with CO2. In particular, ligands based on imidazolidin-2-ylidene (saturated NHC) moieties have given catalysts which exclusively form polycarbonate in moderate-to-high yields even under low CO2 pressure and at low copolymerization temperatures. Here, to evaluate the influence of the NHC moiety on the molecular structure of the catalyst and its performance in copolymerization, we extend this chemistry by synthesizing and characterizing titanium complexes bearing tridentate bis-phenolate imidazol-2-ylidene (unsaturated NHC) and benzimidazol-2-ylidene (benzannulated NHC) ligands. The electronic properties of the ligands and the nature of their bonds to titanium are studied using density functional theory (DFT) and natural bond orbital (NBO) analysis. The metal–NHC bond distances and bond strengths are governed by ligand-to-metal σ- and π-donation, whereas back-donation directly from the metal to the NHC ligand seems to be less important. The NHC π-acceptor orbitals are still involved in bonding, as they interact with THF and isopropoxide oxygen lone-pair donor orbitals. The new complexes are, when combined with [PPN]Cl co-catalyst, selective in polycarbonate formation. The highest activity, albeit lower than that of the previously reported Ti catalysts based on saturated NHC, was obtained with the benzannulated NHC-Ti catalyst. Attempts to synthesize unsaturated and benzannulated NHC analogues based on Hf invariably led, as in earlier work with Zr, to a mixture of products that include zwitterionic and homoleptic complexes. However, the benzannulated NHC-Hf complexes were obtained as the major products, allowing for isolation. Although these complexes selectively form polycarbonate, their catalytic performance is inferior to that of analogues based on saturated NHC.publishedVersio

Methyltransferase Activity of an Iridium Center with Methylpyridinium as Methylene Source

An iridium center transfers a methyl group from pyridinium to an aryl unit, using exclusively the pyridine-bound methyl group as a mild methylene source. The reaction also involves cleavage of an unactivated C(aryl)[BOND]H bond and nitrile solvent activation. The process is reminiscent of DNA methylation and entails the formation of two new C(sp2)[BOND]C(sp3) bonds within the metal coordination sphere (see scheme).European Research CouncilScience Foundation Irelan

Monoanionic NSiN-type ligands in transition metal coordination chemistry and catalysis

Dedicated to Prof. Pierre Braunstein on the occasion of his 70th birthday with our most sincere congratulations for his leadership and outstanding contributions to the field of Coordination Chemistry and Catalysis, and best wishes.The number of late transition metal complexes bearing tridentate monoanionic silyl-based NSiN-type ligands has grown in the last few years. This review describes the synthetic methodologies that allow preparation of NSiN ligands precursors as well as the chemical and structural behavior of the transition metal complexes resulting from the reaction of these ligand precursors with different metallic starting materials. In addition, a description of the catalytic processes based on Rh- and Ir-NSiN catalysts so far reported is included. Moreover, an analysis of the chemistry of iridium complexes with different fac-κ3-(Si,N,N)-NSiN ligands allows to conclude that the nature of the silyl group along with the donor ability of the nitrogen atoms at the NSiN ligands influence on the reactivity and potential catalytic activity of the respective complexes.Financial support from MINECO/FEDER project CTQ2015-67366-P and DGA/FSE group E07 is gratefully acknowledged. Dr. R. Lalrempuia acknowledged ARAID-Foundation for funding parts of this work.Peer reviewe

Osmium−allenylidene complexes containing an N-Heterocyclic carbene ligand

4 pages, 3 figures.Complex [(η6-p-cymene)OsCl(IPr)]OTf (1) reacts with 1,1-diphenyl-2-propyn-1-ol to afford the hydroxyvinylidene derivative [(η6-p-cymene)OsCl{CCHC(OH)Ph2}(IPr)]OTf (2), which dehydrates to form [(η6-p-cymene)OsCl(CCCPh2)(IPr)]OTf (3). Treatment in acetonitrile under reflux of the BF4-salt of 3 with AgBF4 leads to the tetrakis(solvento) derivative [Os(CCCPh2)(CH3CN)4(IPr)][BF4]2 (4). At 95 °C, the addition of PiPr3 to an acetonitrile solution of 4 yields the dicationic mixed PiPr3-IPr osmium−allenylidene complex [Os(CCCPh2)(CH3CN)3(IPr)(PiPr3)][BF4]2 (5), which in 2-propanol and in the presence of NaCl evolves into the five-coordinate hydride−alkenylcarbyne [OsHCl(CCHCPh2)(IPr)(PiPr3)][BF4] (6). The X-ray structures of 2, 5, and 6 are also reported.Financial support from the MCYT of Spain (Projects CTQ2005-00656 and Consolider Ingenio 2010 CSD2007-00006) and Diputación General de Aragón (E35) is acknowledged. R.L. thanks the Spanish Ministerio de Educación y Ciencia for his postdoctoral fellowship, and R.C. thanks CSIC and MEC for funding through the “Ramon y Cajal” Program.Peer reviewe

Regioselective Electrophilic C-H Bond Activation in Triazolylidene Metal Complexes Containing a N-Bound Phenyl Substituent

Transmetalation of a 1,4-diphenyl-substituted 1,2,3-triazolylidene silver complex with an electrophilic metal center, e.g., RuII, IrIII, or RhIII, induces spontaneous and chemoselective cyclometalation involving C–H bond activation of the N-bound phenyl group exclusively. Less electrophilic metals such as IrI, RhI, and PtII yield a monodentate triazolylidene complex, while cyclometalation with borderline cases (PdII) or the activation of the C-bound phenyl ring requires acetate as a promoter.European Research CouncilIrish Research CouncilScience Foundation Irelan