Ruthenium p‐Cymene Iminophosphonamide Complexes: Activation under Basic Conditions and Transfer Hydrogenation Catalysis

Abstract

Complex [(η 6 -Cym)RuCl(NPN)] {Cym = p-cymene; NPN = (pTolN) 2 PPh 2 } (1) yields a thermally sensitive hydride derivative [(η 6 -Cym)RuH(NPN)] (2) by reaction with iPrOH in the presence of a strong base, via an observable isopropoxide intermediate [(η 6 -Cym)Ru(OiPr)(NPN)] (3), or with NaBHEt 3 in toluene. Partial conversion also occurs in iPrOH in the absence of base. 2 is stabilized by dihydrogen bonding with isopropyl alcohol, but attempts to isolate it induce isomerization by hydride migration to a ring CH position to yield a 16-electron cyclohexadienyl derivative [{η 5 -p-C 6 H 5 (Me)(iPr)}Ru(NPN)], which has been crystallographically characterized as a disordered mixture of two regioisomers (4/4′). Complex 2 is able to release H 2 upon treatment with medium strength proton donors (fluorinated alcohols), but also slowly with iPrOH. 2 is an active catalyst for [a]

The Origin of the MNXN Metallacycle Flexibility in the Chelate Iminophosphonamide and Amidinate Transition Metal Complexes

“…However, the chemistry of transition metal iminophosphonamides (NPN) that had been considered to be heteratomic analogs of the corresponding metal amidinates has been developing slowly. Nevertheless, in the last two decades transition metal iminophosphonamides have been reported as active catalysts for alkene polymerization (NiNPN, ZrNPN), 3,4‐selective polymerization of isoprene (rare earth NPN complexes), alkene oligomerization (CrNPN), ethene dimerization (NiNPN),[5c] cyclopropanation and aziridination (CuNPN), as well as for Tsuji–Trost cross‐coupling (PdN 2 P) Recently we have demonstrated that arene ruthenium iminophosphonamide [( p ‐cymene)RuCl{Ph 2 P(N‐ p Tol) 2 }] can efficiently reduce acetophenone under transfer hydrogenation conditions …”

Mechanistic diversity in acetophenone transfer hydrogenation catalyzed by ruthenium iminophosphonamide complexes

“…Complex 2c can alternatively be obtained by reacting 1c with 1 equiv. of NaBHEt3 in benzene-d6 (Figure S9a), 25 presenting the same characteristic resonances at δ 49.8 in 31 P NMR and at δ -4.09 in 1 H NMR (Figure S9b). Thus the hydride complex 2c is indeed generated in the reaction of 1c with isopropoxide, however due to the highly basic Natoms 27 it has very low stability in the presence of isopropanol, perhaps because of its facile protonation and further P-N bond alcoholysis to form the aminophosphine oxide.…”

“…We have recently described the generation of a hydride complex [(p-Cymene)RuH(NPN)] (2f, NPN = Ph2P(N-p-Tol)2) from the corresponding chloride 1f and sodium isopropoxide via β-H elimination in the isopropoxide intermediate [(p-Cymene)Ru(O i Pr)(NPN)] (3f). 25 Here we show that this procedure is operational for the generation of a wider array of ruthenium hydrides [(Arene)RuH(NPN)] (2a-e) from the corresponding chloride complexes 1a-e, although the stability of the products is very different.…”

“…Recently we have demonstrated for the first time that ruthenium iminophosphonamide [(p-Cymene)RuCl(Ph2P(N-p-Tol)2)] (1f) can catalyse transfer hydrogenation of acetophenone in isopropanol under basic conditions. 25 In contrast to isoelectronic Noyori-Ikariya catalysts, the NPN ligand in 1f is not deprotonatable and therefore no ligand assistance in the hydrogen transfer step is possible. We have proposed a catalytic cycle that involves an outer-sphere transfer of the hydridic atom from the in situ generated hydride complex [(p-Cymene)RuH(Ph2P(N-p-Tol)2)] (2f) to the carbonyl atom via the zwitterionic intermediate [(p-Cymene)Ru + (NPN)(H…C (O -)MePh], while the proton is delivered by the alcohol solvent.…”

Synthesis of Nonsymmetric Iminophosphonamines by Kirsanov Condensation

“…The latter have been reported as active catalysts for alkene polymerization, 2 3,4-selective polymerization of isoprene, 3 alkene oligomerization, 4 ethene dimerization, 2c cyclopropanation, 5 and aziridination, 6 as well as for Tsuji-Trost cross-coupling 7 and acetophenone transfer hydrogenation. 8 Nonsymmetric iminophosphonamines bearing different N,N′-substituents have been efficiently utilized in the Kumada cross-coupling of aryl Grignard reagents with aryl chlorides and fluorides 9 and in controlling regio-and stereoselectivity in the polymerization of isoprene. 10 Generally, there are two major approaches to the synthesis of iminophosphonamines, i.e.…”