Variation on the π‐Acceptor Ligand within a Rh^I−N‐Heterocyclic Carbene Framework: Divergent Catalytic Outcomes for Phenylacetylene‐Methanol Transformations

Abstract

A series of neutral and cationic rhodium complexes bearing IPr {IPr=1,3‐bis‐(2,6‐diisopropylphenyl)imidazolin‐2‐carbene} and π‐acceptor ligands are reported. Cationic species [Rh(η4‐cod)(IPr)(NCCH3)]+ and [Rh(CO)(IPr)(L)2]+ (L=pyridine, CH3CN) were obtained by chlorido abstraction in suitable complexes, whereas the cod‐CO derivative [Rh(η4‐cod)(IPr)(CO)]+ was formed by the carbonylation of [Rh(η4‐cod)(IPr)(NCCH3)]+. Alternatively, neutral derivatives of type RhCl(IPr)(L)2 {L=tBuNC or P(OMe)3} can be accessed from [Rh(μ‐Cl)(η2‐coe)(IPr)]2. In addition, the mononuclear species Rh(CN)(η4‐cod)(IPr) was prepared by cyanide‐chlorido anion exchange, which after carbonylation afforded the unusual trinuclear compound [Rh{1κC,2κN‐(CN)}(CO)(IPr)]3. Divergent catalytic outcomes in the phenylacetylene‐methanol transformations have been observed. Thus, enol ethers, arisen from hydroalkoxylation of the alkyne, were obtained with neutral Rh−CO catalyst precursors whereas dienol ethers were formed with cationic catalysts. Variable amounts of alkyne dimerization, cyclotrimerization or polymerization products were obtained in the absence of a strong π‐acceptor ligand on the catalyst.

Iridium(i) complexes bearing hemilabile coumarin-functionalised N-heterocyclic carbene ligands with application as alkyne hydrosilylation catalysts

“…Moreover, the carbene carbon atom is more deshielded at high temperature (167.6 ppm, 213 K vs 187.0 ppm, 323 K) in agreement with an increment of electron density on iridium center in the square planar conformation of 5b'. 20 Similar thermodynamic parameters were calculated for the imidazolin-based complexes 4a (ΔHR = 22.6 ± 3 kJ•mol -1 , ΔSR = 72.3 ± 13 J•mol -1 •K -1 ) and 4e (ΔHR = 33.5 ± 8 kJ•mol -1 , ΔSR = 117.6 ± 26 J•mol -1 ) (see Supporting Information). The bis-carbonyl complex IrCl(BzI Tol Cou 7,8-Me2 )(CO)2 (6) was synthesised by bubbling carbon monoxide through a solution of 5a in THF and isolated in 54 % yield (Scheme 2).…”

Preparation of Mixed Bis-N-Heterocyclic Carbene Rhodium(I) Complexes

“…The appearance of two septuplets at 3.51 and 2.76 ppm, corresponding to the CH-isopropyl protons of the IPr, indicates a restricted rotation of the carbene ligands around the Rh-C axis [ 49 ]. In the 13 C{ 1 H}-APT spectrum, the most significant feature is the downfield shift of the carbene carbon atom to δ 191.8 ppm related to 1b (179.7 ppm), reflecting the increase of electron density at the metal center as a result of the coordination of a second powerful electron releasing IPr ligand [ 58 ]. Moreover, the reduction of the rhodium-carbon coupling ( J Rh-C = 41.6 Hz vs. 62.3 Hz in 1b ) is a consequence of an opposite disposition of two ligands with a strong trans influence.…”