A series of lithium complexes of anionic N-heterocyclic carbenes that contain a weakly coordinating borate moiety (WCA-NHC) was prepared in one step from free N-heterocyclic carbenes by deprotonation with n-butyl lithium followed by borane addition. The reaction of the resulting lithium-carbene adducts with [M(COD)Cl]2 (M = Rh, Ir; COD = 1,5-cyclooctadiene) afforded zwitterionic rhodium(I) and iridium(I) complexes of the type [(WCA-NHC)M(COD)], in which the metal atoms exhibit an intramolecular interaction with the N-aryl groups of the carbene ligands. For M = Rh, the neutral complex [(WCA-NHC)Rh(CO)2] and the ate complex (NEt4)[(WCA-NHC)Rh(CO)2Cl] were prepared, with the latter allowing an assessment of the donor ability of the ligand by IR spectroscopy. The zwitterionic iridium-COD complexes were tested as catalysts for the homogeneous hydrogenation of alkenes, which can be performed in the presence of nonpolar solvents or in the neat alkene substrate. Thereby, the most active complex showed excellent stability and activity in hydrogenation of alkenes at low catalyst loadings (down to 10 ppm).

Scheme 1. Reactivity of a frustrated carbene-borane Lewis pair. Scheme 2. Selected examples of anionic N-heterocyclic carbenes. Mes = 2,4,6-trimethylphenyl.

A variety of Lewis acid-base pairs consisting of tris(pentafluorophenyl)borane, B(C(6)F(5))(3), in combination with sterically demanding five- and six-membered N-heterocyclic carbenes (NHCs) of the imidazolin-2-ylidene, imidazolidin-2-ylidene, and tetrahydropyrimidin-2-ylidene types were investigated with respect to their potential to act as frustrated Lewis pairs (FLP) by reaction with dihydrogen (H(2)) and tetrahydrofuran (THF). A sufficient degree of "frustration" was usually established by introduction of a 1,3-di-tert-butyl or 1,3-diadamantyl carbene substitution pattern, which allows an unquenched acid-base reactivity and thus leads to heterolytic dihydrogen activation and ring-opening of THF. In contrast, 1,3-bis(2,6-diisopropylphenyl)-substituted carbenes showed ambiguous behavior, and the corresponding five-membered imidazolin-2-ylidene formed a stable carbene-B(C(6)F(5))(3) adduct, whereas fast C-F activation and formation of a zwitterionic pyrimidinium-fluoroborate was observed for the six-membered tetrahydropyrimidin-2-ylidene. A stable adduct was also isolated for the combination of the acyclic carbene bis(diisopropylamino)methylene with B(C(6)F(5))(3), and consequently no reactivity toward H(2) and THF was observed. To rationalize the reactivity of the carbene-borane Lewis pairs, the thermodynamics of adduct formation with B(C(6)F(5))(3) were calculated for 10 different carbenes; the stability (or instability) of these adducts can be used as a good measure of the degree of "frustration".

In addition to the known 1,3,4,5‐tetramethyl‐2‐methyleneimidazoline (1a), which exhibits a highly polarized exocyclic C–C bond, a series of novel 2‐alkylidene‐substituted 1,3,4,5‐tetramethylimidazolines 1b–e were synthesized and characterized. The molecular structures of 1b, 1d, and 1e were determined by X‐ray diffraction analysis and revealed an increase in the polarization of the exocyclic C–C bond with increasing steric demand of the 2‐substituent. On the basis of their ylidic nature, 1a–e show enhanced basicity and reactivity towards Lewis acidic centers. Treatment of 1a and 1b with [{RhCl(cod)}2] or B(C6F5)3 afforded complexes of the type [(L)RhCl(cod)] (4a,b), [(L)RhCl(CO)2] (7a,b) (L = 1a,b) or classical Lewis acid/base adducts [(1a)B(C6F5)3] (8a) and [(1b)B(C6F5)3] (8b). In contrast, complexes [(L)RhCl(cod)] with 1c and 1d as ligands are not stable, and imidazolium dichlororhodate salts 6a and 6b were isolated instead. Rhodium–alkyl complexes 5a and 5b are assumed to be intermediates in this decomposition process, and 5a was characterized by X‐ray diffraction analysis. Furthermore, treatment of 1c and 1d with B(C6F5)3 did not afford classical Lewis adducts, and instead imidazolium hydridoborate salts 9a and 9b are formed by hydride abstraction. Surprisingly, we found that 1e does not react with [{RhCl(cod)}2] and forms an abnormal Lewis adduct 10 when treated with B(C6F5)3.

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