New copper(I) complexes bearing N-phosphino- and N-phosphinomethyl-functionalized NHC ligands (NHCP systems) were synthesized and fully characterized. Using halide-containing copper(I) precursors, strikingly different structural motifs were found, dependent on the substitution pattern of the NHCP moiety. An interesting tetranuclear μ4-bridged copper(I) cluster (4b) is formed when CuBr·SMe2 is reacted with the N-phosphino-substituted ligand 1a, whereas the dinuclear complex 4a is formed in an analogous reaction with 1b. Dinuclear metallacycles (2a,b) were isolated and characterized by X-ray diffraction when the halide-free copper(I) precursor [Cu(CH3CN)4]PF6 was reacted with NHCP ligands 1a,b using the N-phosphinomethyl-substituted NHCP ligands 5a,b, as well as employing the bis-NHC ligands BIM and BIM-BMe2 (10a,b), respectively. The observed copper(I)–copper(I) distances are directly correlated with the size of the metallacycle formed. An unexpected strong influence upon the metal–metal separation was found for the charged complexes in 10a,b. Using directly N-tBu-substituted NHCP ligands led to formation of an electronically less favorable trans-C,C coordination at the copper(I) centers (3 and 6a), which inhibited the formation of a dimeric species in the case of 3.

Mononuclear, neutral copper(I) carbenes [Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu=CPh(2) and [Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu=C(p-NO(2)C(6)H(4))(2) (4a,b) as well as the binuclear mu-carbene copper complex {[Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu}(2)(mu-CPh(2)) (5) have been synthesized. The solid-state structures of 4b and 5 are reported. Carbene 4b is active in cyclopropanation of styrene. Reaction of 4b with (p-MeC(6)H(4))C=N(2) afforded [Bu(t)(2)P(NSiMe(3))(2)-kappa(2)N]Cu=C(p-MeC(6)H(4))(2), the corresponding mixed azine Ar(2)C=N-N=CAr'(2) and the carbene dimer Ar(2)C=CAr'(2), demonstrating an important pathway for carbene loss in cyclopropanation of olefins via reaction of the copper(I) carbene with nucleophilic diazo compounds. Complex 5 reacted with C(2)H(4) or styrene to give 1:1 mixtures of the terminal carbene 4a and of the corresponding copper(I) olefin complexes, proving unambiguously that binuclear complexes like 5 are not relevant for copper-catalyzed cyclopropanation of olefins with diazo compounds. The electronic structure of 4b was investigated by means of DFT calculations.

Employment of bidentate, anionic bis(3-tert-butylimidazol-2-ylidene)dialkylborate N-heterocyclic ligands [BIMtBuBR2]− (R = Ph or Me) led to the isolation of two unprecedented lithium carbene complexes: monomeric chelating [(BIMtBuBPh2)Li(OEt2)] (4a) and nonchelating, dimeric μ2-κC 2:κC 2′ -[(BIMtBuBMe2)2Li2] (4b). Both have been characterized by spectroscopic methods and by single-crystal X-ray diffraction.

The reaction of [tBu 2 P(NSiMe 3 ) 2 -κ 2 N]Cu(η 2 -C 2 H 4 ) (1) with various aryl diazo esters [p-X-C 6 H 4 ]C(N 2 )[C(O)R] allowed us to synthesize the corresponding R-carbonyl copper(I) carbene complexes 8), where X ) OMe, NO 2 . The rotation barriers around the Cu-C carbene and C carbene -C Ar bonds or their low limits were determined for some of these compounds by 1 H-VT-NMR spectroscopy. Carbene 8g with X ) OMe and R ) OCH(p-Cl-C 6 H 4 ) 2 was isolated in analytically pure, crystalline form as the first stable representative of this important class of compounds. Its solid-state molecular structure revealed an orthogonal position of the carbene fragment relative to both the ligand plane and the ester CdO group and a remarkably short Cu-C carbene distance of 1.822(4) Å. Compound 8g reacted with styrene stereoselectively to give the corresponding trans-cyclopropane derivative and [tBu 2 P(NSiMe 3 ) 2 -κ 2 N]Cu(η 2 -CH 2 dCHPh). The stoichiometric cyclopropanation of styrene with 8g and the previously described diarylcarbene [tBu 2 P(NSiMe 3 ) 2 -κ 2 N]CudC[p-NO 2 -C 6 H 4 ] 2 (6) in toluene-d 8 revealed that the reactions are first order in both the copper carbenes and the alkene. The activation parameters for 8g (∆H q ) 51.5(9) kJ/mol and ∆S q ) -127.1(28) J/(mol K)] and for 6 (∆H q ) 53.4(8) kJ/mol and ∆S q ) -152.1(23) J/(mol K)) were derived from the kinetics of the cyclopropanation processes. Thermal decomposition of carbene 8g in toluene-d 8 displayed first-order kinetics until 20-25% conversion with activation parameters ∆H q ) 85.5(24) kJ/mol and ∆S q ) -49.0(76) J/(mol K). Solutions of 6 in toluene-d 8 also decompose in a first-order fashion with ∆H q ) 66.1(20) kJ/mol and ∆S q ) -125.5(56) J/(mol K). A Hammett study employing 8g and para-substituted styrenes afforded F ) -1.06(19), demonstrating the electrophilic nature of R-carbonyl copper(I) carbene (Fischer-type) complexes. The electronic structure of 8 with X ) R ) OMe was investigated by DFT methods.

Palladium complexes supported by a 2,3-diphenylcyclopropenylidene carbene ligand are efficient and robust catalysts for C−N coupling reactions.