Teruhiko Saito

New tantalum ligand-free catalyst system for highly selective trimerization of ethylene affording 1-hexene: new evidence of a metallacycle mechanism.

Mild and salt-free reduction of MCl(5) (M = Nb, Ta) has been achieved by using 3,6-bis(trimethylsilyl)-1,4-hexadiene derivatives as reducing reagents. The salt-contact-free low-valent TaCl(3) exhibits high catalytic activity for the trimerization of ethylene to 1-hexene (>1000 TOF) with excellent selectivity (>98%). This salt-free tantalum system enables us to observe new evidence for a metallacycle mechanism.

Carbon radical generation by d0 tantalum complexes with α-diimine ligands through ligand-centered redox processes.

High-valent tantalum complexes having redox-active α-diimine ligands, (α-diimine)TaCl(n) (n = 3, 4), are prepared by the reaction of TaCl(5), α-diimine ligands, and an organosilicon-based reductant, 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene. Reductive cleavage of the C-Cl bond of polyhaloalkanes is accomplished by trichlorotantalum complexes having dianionic α-diimine ligands via electron transfer from the dianionic ligands, whereas oxidative decomposition of tetraphenylborate is observed using tetrachlorotantalum complexes with monoanionic α-diimine ligands through electron transfer to the monoanionic ligands. Chemically oxidized or reduced complexes of (α-diimine)TaCl(4) are isolated as ligand-centered redox products, [Cp(2)Co][(α-diimine)TaCl(4)] and [(α-diimine)TaCl(4)][WCl(6)], where the α-diimine ligand coordinates to the metal center as a dianionic or neutral ligand, respectively. On the basis of EPR measurements of (α-diimine)TaCl(4) complexes (which are key intermediates for reductive cleavage of C-Cl bond and oxidative decomposition of tetraphenylborate), two redox isomers--a tantalum-centered radical and ligand-localized radical--are present in solution.

Salt-Free Reducing Reagent of Bis(trimethylsilyl)cyclohexadiene Mediates Multielectron Reduction of Chloride Complexes of W(VI) and W(IV)

We developed a salt-free reduction of WCl6 using 1-methyl-3,6-bis(trimethylsilyl)-1,4-cyclohexadiene (MBTCD) in toluene to give a low-valent trinulcear tungsten complex involving W(II) and W(III) centers, while in the presence of redox active ligands such as α-diketone and α-diimine the same reduction produced W(IV) complexes with the corresponding redox-active ligands, (α-diketone)WCl4 and (α-diimine)WCl4. A W(VI) complex with two α-diketone ligands, (α-diketone)2WCl2, was found to be synthetically equivalent to low-valent W(IV) species that trapped azopyridine to give (α-diketone)WCl2(azopyridine).

Reduction of (tBuN═)NbCl3(py)2 in a Salt-Free Manner for Generating Nb(IV) Dinuclear Complexes and Their Reactivity toward Benzo[c]cinnoline

The organosilicon reducing reagent 2,3,5,6-tetramethyl-1,4-bis(trimethylsilyl)-1,4-diaza-2,5-cyclohexadiene (1a) was used for the one-electron, salt-free reduction of ((t)BuN═)NbCl3(py)2 (2), resulting in the formation of a neutral, triply chloride-bridged dinuclear niobium(IV) complex, [((t)BuN═)ClNb(py)](μ-Cl)3[((t)BuN═)Nb(py)2] (3) in moderately high yield. Heating 3 in toluene at 80 °C caused a unique intramolecular rearrangement of 3 to another neutral dinuclear complex, [Cl2Nb(py)](μ-Cl)(μ-N(t)Bu)2[ClNb(py)2] (4), in which two niobium(IV) atoms were bridged by one chloride atom and two imido ligands. Reaction of complex 3 with benzo[c]cinnoline produced a benzo[c]cinnoline-bridged dinuclear niobium(V) complex 7 by an overall two-electron reduction of benzo[c]cinnoline through a disproportionation of 3 into a mixture of a niobium(V) complex 2 and a niobium(III) complex, the latter of which efficiently reduced benzo[c]cinnoline.

Rhodium Complexes Bearing PAlP Pincer Ligands

We report rhodium complexes bearing PAlP pincer ligands with an X-type aluminyl moiety. IR spectroscopy and single-crystal X-ray diffraction analysis of a carbonyl complex exhibit the considerable σ-donating ability of the aluminyl ligand, whose Lewis acidity is confirmed through coordination of pyridine to the aluminum center. The X-type PAlP-Rh complexes catalyze C2-selective monoalkylation of pyridine with alkenes.

CCDC 1485111: Experimental Crystal Structure Determination

Related Article: Teruhiko Saito, Haruka Nishiyama, Kento Kawakita, Michael Nechayev, Benjamin Kriegel, Hayato Tsurugi, John Arnold, and Kazushi Mashima|2015|Inorg.Chem.|54|6004|doi:10.1021/acs.inorgchem.5b00812