Coordinatively Labile 18‐Electron Arene Ruthenium Iminophosphonamide Complexes

Abstract

The thermodynamics of chloride dissociation from the 18e arene ruthenium iminophosphonamides [(η -arene)RuCl{(R'N) PR }] (1 a-d) [previously known with arene=C Me , R=Ph, R'=p-Tol (a); R=Et, R'=p-Tol (b); R=Ph, R'=Me (c); and new with arene=p-cymene, R=Ph, R'=p-Tol (d)] was assessed in both polar and apolar solvents by variable-temperature UV/Vis, NMR, and 2D EXSY H NMR methods, which highlighted the influence of the NPN ligand on the equilibrium parameters. The dissociation enthalpy ΔH decreases with increasing electron-donating ability of the N- and P-substituents (1 a, 1 d>1 b>1 c) and solvent polarity, and this results in exothermic spontaneous dissociation of 1 c in polar solvents. The coordination of neutral ligands (MeCN, pyridine, CO) to the corresponding 16e complexes [(η -arene)Ru{(R'N) PR }] PF (2 a-d) is reversible; the stability of the 2⋅L adducts depends on the π-accepting ability of L. Carbonylation of 2 a and 2 d resulted in rare examples of cationic arene ruthenium carbonyl complexes (3 a, 3 d), while the monocarbonyl adduct derived from 2 c reacts further with a second equivalent of CO with conversion to carbonyl-carbamoyl complex 3 c, in which one CO molecule is inserted into the Ru-N bond. The new complexes 1 d, 2 d, 3 a, 3 c, and 3 d were isolated and structurally characterized.

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

“…This product has not been directly detected, but we note that related [(arene)Ru(µ‐OR) 3 Ru(arene)] + complexes have been described in the literature . Although we have shown earlier that even weak bases such as Et 2 NH can deprotonate NPNH during the synthesis of 1 ,, this acid‐base equilibrium is certainly solvent‐dependent and the reverse process of NPN ligand decoordination from the ruthenium atom may become favorable in polar protic solvents. The formation of 3 is accompanied by the generation of the diaminophosphonium salt [Ph 2 P(NH‐ p Tol) 2 ] + Cl – ( NPNH 2 + Cl – ), which can undergo rapid and degenerate proton transfer with NPNH , causing a broadening and a downfield shift for the 31 P NMR resonance with time (Figures S6 and S7).…”

“…A base (B) is required to make the generation of isopropoxide 3 from 1 quantitative and rapid at room temperature. In this case, however, a direct associative exchange of Cl – with O i Pr – seems more likely . The β‐H elimination in 3 to yield hydride 2 appears rather facile.…”

“…We have shown that the 16ē ruthenium iminophosphonamides complexes are very stable species due to strong σ,π‐donating ability of the NPN ligand . At the same time, the high negative charges located at the nitrogen atoms of the NPN ligand render the corresponding 18ē ruthenium iminophosphonamides coordinatively labile and reactive, as we demonstrated by the facile insertion of the CO into the Ru–N bond in the [(C 6 Me 6 )Ru(CO){(NMe) 2 PPh 2 }](PF 6 ) complex . Hence, the 18ē ruthenium iminophosphonamide complexes, being isoelectronic to Noyori's catalysts, are promising for the catalyzed TH of ketones.…”

“…The generation of these two ionic intermediates should be facilitated by the polarity of isopropyl alcohol. The involvement of the 16ē [ 7 ]Cl complex as intermediate in the dissociative chloride‐exchange process in polar solvents has previously been established and a salt of [ 7 ] + with the non‐coordinating PF 6 – anion has been fully characterized . Formation of the observed hydride product 2 requires deprotonation of 8 to generate 3 followed by β‐H elimination from the isopropoxide ligand.…”

“…In this paper we report the transfer hydrogenation of a model ketone (acetophenone) with isopropyl alcohol catalyzed by a specific [(η 6 ‐Cym)RuCl( NPN )] complex with NPN = ( p TolN) 2 PPh 2 ( 1 ) under various conditions, including kinetic studies and the identification of intermediates and an off‐loop species. A DFT study of the proposed catalytic cycle is also presented.…”