Synthesis and structural analysis of palladium biscarbene complexes derived from bisimidazolium ligand precursors

Abstract

Dedicated to Gerard van Koten with deep appreciation for his outstanding contribution to the organometallic chemistry of late transition metals. AbstractThe palladation of potentially chelating bisimidazolium ligand precursors with palladium acetate gives bridging bimetallic, chelating monometallic, and homoleptic tetracarbene complexes. The coordination mode of the biscarbene ligand has been identified by spectroscopic analysis and crystallographic characterization of representative complexes, including the first example of a biscarbene A-frame structure. Substantial concentrations of free acetate favor the formation of tetracarbene over biscarbene palladium complexes, while in the absence of a base, the concentration of reactants influences the selectivity for bridging bimetallic versus chelating monometallic species. Preliminary kinetic and mechanistic studies indicate that chelating biscarbene palladium acetate complexes are intermediates in the formation of the homoleptic tetracarbene complexes. Probably due to the high trans effect of the biscarbene ligand, such complexes are more efficient palladating agents for bisimidazolium salts than palladium acetate.

Neutral Ligands with Exceptional Donor Ability for Palladium‐Catalyzed Alkene Hydrogenation

“…[11] The bite angle of the dicarbene is 83.8(5)8, very similar to that of the analogous C(2)-bound dicarbene complex 3 b. [12] The PdÀC bond lengths in 2 b are 1.932(12) and 1.963(11) , which are at the shorter end of the range typically observed for C(2)-bound analogues. [12,13] The Pd À I bonds are slightly longer than in classical carbenes (2.68 versus 2.65 ) and indicate a higher trans influence in the C(4)-bound carbenes.…”

“…[12] The PdÀC bond lengths in 2 b are 1.932(12) and 1.963(11) , which are at the shorter end of the range typically observed for C(2)-bound analogues. [12,13] The Pd À I bonds are slightly longer than in classical carbenes (2.68 versus 2.65 ) and indicate a higher trans influence in the C(4)-bound carbenes. Most significantly, the heterocyclic CÀC bond in 2 b corresponds to a delocalized double bond (ca.…”

“…Comparative experiments have been performed with the bissolvento complex of 3 b, that is, the C(2)-bound complex 6. [12] Irrespective of the solvent, the conversions are substantially lower with 6 than with the C(4)-bound complex 5 ( Table 1, entries 1-5). The hydrogenation of cyclooctene with 5 in polar solvents such as EtOH is complete within less than 5 h. Longer reaction times are required to achieve full conversion when the catalyst loading is reduced below 1 mol % (Table 1, entries 6-8).…”

“…[a] General conditions: cyclooctene (2.0 mmol), Pd complex (1 mol %), EtOH (6 mL), RT, 1 atm H 2 ; catalyst precursor 6 is the product from an AgBF 4 -mediated halide abstraction from 3 b (Scheme 1, see also Ref. [12] …”

Synthesis of palladium(0) and -(ii) complexes with chelating bis(N-heterocyclic carbene) ligands and their application in semihydrogenation

“…It is reported that the mode of coordination (chelating or bridging) of bis-carbenes is dictated by a combination of linker, N-substituents, counter ion and reaction conditions. 22,31 It has even been doubted whether chelated methylene linked bis-NHC ligands would be able to support a zero-valent palladium center. 32 However, we observed formation of only the chelate species for all wingtip substituents and for both linker lengths, as was concluded from the NMR-spectra.…”