Bulky N-Phosphinomethyl-Functionalized N-Heterocyclic Carbene Chelate Ligands: Synthesis, Molecular Geometry, Electronic Structure, and Their Ruthenium Alkylidene Complexes

Abstract

A new, extremely bulky, and electron-rich N-phosphinomethyl-functionalized N-heterocyclic carbene ligand, 5a ( tBuNHCP tBu), and a somewhat less bulky congener, 5b (MesNHCP tBu), forming five-membered chelate rings with metal centers, have been synthesized in four steps starting from the easily accessible di-tert-butyl(hydroxymethyl)phosphine oxide (1). 5a was isolated and fully characterized by spectroscopic methods including UV-photoelectron spectroscopy and X-ray diffraction. The reaction of 5a with [Ru(COD)Cl2] n under hydrogen pressure or with [Ru(p-cymene)Cl2]2 led to the formation of the unsaturated dinuclear complex [Ru( tBuNHCP tBu)(μ-Cl)(Cl)]2 (6), which serves as a precursor for a series of ruthenium carbene complexes (7a–f) using substituted phenyldiazomethanes (p-X-C6H4(CH)N2; X = H (a), Br (b), CF3 (c), NO2, (d), CH3 (e)) and trimethylsilyldiazomethane (f). Treatment of 6 with phosphine or pyridine ligands led to the formation of the mononuclear adducts, [Ru( tBuNHCP tBu)(Cl2)(PR3)] (R = Me (8), Ph (9), Cy (10)) and [Ru( tBuNHCP tBu)(Cl2)(py) n ] (n = 1 (11), 2 (12); if (py)2 = bipy (13)), which were synthesized in order to find alternative precursor complexes because the dimer 6 showed very low solubility in most organic solvents. Complex 7a was obtained analytically pure on a different route via transmetalation from a silver complex bearing tBuNHCP tBu (15) to the first-generation Grubbs catalyst as the ruthenium precursor. Complexes 7a–c and 7e were characterized by X-ray diffraction analysis, revealing a geometry that can be viewed as both a distorted square pyramid and a distorted trigonal bipyramid with the two chloro ligands in a cis configuration. The steric bulk, especially of 5a with its N-tBu moiety, stabilizes 16 VE Ru complexes. In contrast to ligand 5a, the somewhat less bulky MesNHCP tBu ligand 5b has allowed its direct metalation with two ruthenium alkylidene precursors, affording the two new carbene complexes 17 and 18.

Di‐ and Trinuclear Gold Complexes of Diphenylphosphinoethyl‐Functionalised Imidazolium Salts and their N‐Heterocyclic Carbenes: Synthesis and Photophysical Properties

“…Although a few phosphine-substituted NHCs were published at the time we started this work, [13] none of those ligands were applied as a polydentate gold ligand. Meanwhile, a few macrocyclic cationic gold complexes, [39][40][41] as well as Cu, Ag, Ru, Ni, Pd and Pt compounds with N-phosphorylated or N-phosphinomethyl-substituted NHCs, were reported by Braunstein, Kostyuk and Hofmann (Scheme 1), [42][43][44][45] but to the best of our knowledge, neutral gold complexes with phosphorus-containing NHCs are still unknown. Herein, we present a full account of phosphinoethyl-tethered NHC ligands in gold(I) chemistry.…”

Organometallic chemistry

“…In the mechanism of the Dötz reaction, he found that chromacyclobutene structures were unrealistic intermediates and instead proposed vinylcarbene complexes as much more stable isomers [64,68,88]. Other research projects of his group included the synthesis of ruthenium–carbene complexes with cis -dichloro ligands for olefin metathesis catalysis [86–87 90–91 98–100 108,132,138,147], the characterization of copper–carbene complexes as intermediates in the cyclopropanation of alkenes [102,106,113,119–120], as well as detailed studies into the mechanism of the hydroformylation of alkenes. An improved understanding of this industrially important process was achieved by a combination of ligand design for the rhodium catalysts, kinetic studies, and high-level quantum-chemical calculations [126,135,142,145–146 152,155].…”

A Mild One-Pot Reduction of Phosphine(V) Oxides Affording Phosphines(III) and Their Metal Catalysts

“… 19 Progress has been severely hampered due to difficulties accessing azolium salt 5 , with the problematic reduction of 4 being achievable only with a large excess of trichlorosilane (27.0 equiv) in anhydrous degassed chlorobenzene at elevated temperature over 2 days. 19 As well as the lengthy reaction time, we experienced some reproducibility issues, with the unsuccessful reduction being accompanied by the decomposition of the precious azolinium 4 , previously obtained via a multistep synthesis. 19 In light of this, a simple procedure for the conversion of 4 to 5 would be a great advantage.…”

“… 19 As well as the lengthy reaction time, we experienced some reproducibility issues, with the unsuccessful reduction being accompanied by the decomposition of the precious azolinium 4 , previously obtained via a multistep synthesis. 19 In light of this, a simple procedure for the conversion of 4 to 5 would be a great advantage. Such a process might also permit access to other challenging phosphine(III) and metal catalysts as well as permitting the recovery of the valuable phosphine(III) ligands: “closing the phosphorus cycle” is of increasing importance due to environmental and availability concerns.…”