Rhodium(I) Complexes of New Ferrocenyl Benzimidazol‐2‐ylidene Ligands: The Importance of the Chelating Effect for Ketone Hydrosilylation Catalysis

Abstract

N‐[(1‐Phosphanylferrocen‐1′‐yl)methyl]‐N′‐[(2,4,6‐trimethylphenyl)methyl]‐5,6‐di‐X‐benzimidazolium tetrafluoroborate salts (X = H, 5a and Me, 5b), precursors of new phosphane‐benzimidazol‐2‐ylidene bifunctional ligands, and related ferrocenyl 5,6‐di‐X‐benzimidazol‐2‐ylidene iodide salts (X = H, 6a and Me, 6b), precursors of monodentate benzimidazol‐2‐ylidene ligands, have been prepared for the first time. Cationic rhodium(I) complexes 7a and 7b and neutral rhodium(I) complexes 8a and 8b have been obtained in good yields and have been fully characterised. Cationic rhodium(I) complexes 10a and 10b were prepared from 8a and 8b, for comparison with complexes 7, but not isolated. All complexes showed good activities for the catalytic hydrosilylation of acetophenone derivatives. The activities are much greater than for related imidazol‐2‐ylidene systems, and the cationic complexes are more active than the neutral complexes; the highest activity is observed for the more soluble complex 7b. The use of bidentate ligands proved to be essential for obtaining good selectivities of the desired alcohol.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Diether functionalized rhodium(I)–N-heterocyclic carbene complexes and their catalytic application for transfer hydrogenation reactions

“…Values of d( 13 C carbene ) were in the 195. 4-197.4 ppm range and coupling constants J( 103 Rh-13 C) were in the range 49.2-50.3 Hz for the new rhodium(I) complexes 3 and 4, and these values were similar to those found for other benzimidazol-2-ylidene rhodium(I) complexes [30][31][32][33][34][35][36][37][47][48][49][50][51]. Table 2 Selected bond lengths (Å) and angles (°) for 3a and 3b.…”

“…Benzannulated carbenes derived from benzimidazolium precursors exhibit interesting properties due to their intermediate position between saturated and unsaturated analogs [28,29], a few groups have reported the catalytic activities of rhodium-benzimidazol-2-ylidene complexes [30][31][32][33][34][35][36][37]. Some related imidazolium salts bearing oligoether substituents [38,39], were reported while this work was in progress.…”

Rhodium(iii) and ruthenium(ii) complexes of redox-active, chelating N-heterocyclic carbene/thioether ligands

“…The 40 coordination of the carbene on rhodium was evidenced by 13 C NMR by a doublet at δ 166.3, with a coupling constant J Rh-C of 54 Hz, typical of rhodium(III)-NHC complexes. [13][14]20 All three methyl groups from the mesityl moiety are now distinct, whereas the two ortho methyl groups had the same chemical shift in 3b. 45 This means that the rotation of the mesityl group along the C-N bond is blocked, presumably because of close contact with the Cp* ligand.…”

“…We have shown that NHCs bearing thioether or phosphine donors give very efficient and robust catalysts. 13,14 The work reported 40 here describes the preparation of redox-active NHC-thioether ligands incorporating the ubiquitous ferrocene group and the study of their coordination chemistry and electrochemical behaviour. …”

Kinetic study and modeling of the Rh-catalyzed hydrosilylation of acetophenone in a batch reactor and in a microreactor

“…Evans et al [9] investigated the effect of mixed phosphorus/sulfur-containing ligands on the Rhcatalyzed hydrosilylation of size-differentiated ketones with a wide variety of substrates giving an enantiomeric excess of more than 90%. Recently, Schneider et al [10] and Gülcemal et al [11] reported a very efficient enantioselective Rh catalyst system for the ketone hydrosilylation by means of novel nitrogen-based chiral ligands. Both the relative complexity of the ligand synthesis and the low rate of the hydrosilylation reaction on such catalysts present a disadvantage for applied chemistry.…”