Formation of Different Isomers of Phosphine–Imidazolyl and –Pyridyl Ruthenium(II) Complexes Affecting the Catalyst Activity in the Acceptorless Dehydrogenation of Alcohols

Abstract

The synthesis, reactivity, and catalytic activity of Ru II complexes with different pyridine-and imidazole-based P,N ligands are reported. The investigations reveal a strong influence of the N-heterocycle and the steric demand of the phosphine groups on the stability of different isomers of [L 2 RuX 2 ] (L = P,N ligand; X = Cl, H). The imidazole-based complex 5 with dicyclohexylphosphine groups was found to be the most active precatalyst for the acceptorless dehydrogenation of primary alcohols, whereas different phosphine groups at the imidazole ligand as well as pyridine-based ligands caused a[a]

Ruthenium complexes of diphenylphosphino derivatives of carboxylic amides: Synthesis and characterization of bidentate P,N- and P,O-chelate ligands and their reactivity towards [RuCl2(PPh3)3]

“…Thus, it is not surprising that we were able to isolate two of these isomers (3c I and 3c II ). Whereas Langer et al [12] reported these three isomers with a similar P,N-bidentate ligand, in our study of the coordination behavior of 1c towards Ru(II) we found the initial formation of 3c I (as a kinetic product) which slowly isomerizes into a mixture of 3c I and 3c II , where 3c II could be separated by crystallization. There were no spectroscopic hints at the formation of the N-trans isomer 3 V .…”

“…Tables: Table 1 Selected bond lengths (Å) and angles (°) of 1a, 1b and 1c. (10) 96.73 (13) 103.26 (12) 97.13 (11) 104.31 (12) 102.05 (10) 103.21(10) C-P2-N2 99.33 (12) 104.97 (11) 102.76 (9) 103.72(10) P-N-C carbonyl 115.81 (15) 117.69 (19) 115.08 (17) 117.86 (14) 116.17 (17) Selected bond lengths (Å) and angles (°) of 3c I and 3c II . …”

“…This compound has already been characterized crystallographically [26] and in the solid state each Ru atom carries two chemically different 7.53 (m, aryl, 2H). 13 1c: This compound was synthesized according to a procedure of Opatz et al [12] (modified procedure). To a solution of 2-hydroxypyridine (2.00 g, 21.0 mmol) and triethylamine (2.14 g, 21.0 mmol) in THF (30 mL) at 40°C chlorodiphenylphosphine (4.65 g, 21.0 mmol) was added with stirring.…”

“…In general, the P,N [7] and P,O [8] bidentate donor sets may exhibit mono-vs. bidentate ligand characteristics, and in combination with rather soft transition metal atoms the phosphorus atom represents the superior σ-donor over the N or O atom in case of monodentate binding in the transition metal coordination sphere (A, B and C in Scheme 1). This hemilabile property is supportive in many catalytic processes like oligo-and polymerization, carbonylation as well as hydro-and dehydrogenation [8][9][10][11][12].…”

“…In general, the P,N [7] and P, O [8] bidentate donor sets may exhibit mono-vs. bidentate ligand characteristics, and in combination with rather soft transition metal atoms the phosphorus atom represents the superior σ-donor over the N or O atom in case of monodentate binding in the transition metal coordination sphere (A, B and C in Scheme 1). This hemilabile property is supportive in many catalytic processes like oligo-and polymerization, carbonylation as well as hydro-and dehydrogenation [8][9][10][11][12].Moreover, phosphino-functionalized carboxylic amides, which still bear an acidic proton (N−H moiety) can be utilized as mono-anionic bidentate chelators upon deprotonation, and their coordination chemistry has been studied with a variety of transition metal compounds (some examples are shown in Scheme 1, C, D and E) [13,14]. Phosphino-functionalized carboxylic amides devoid of this acidic N−H feature (or in general lacking the capability of forming the corresponding anion) can thus only serve as neutral chelators, hence lacking some electrostatic binding force, which should render these ligands more labile.…”