RESUMEN
A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp*MIIICl{(MeIm)2CHCOO}] and [MI(diene){(MeIm)2CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh, Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1,5-cyclooctadiene (cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)2CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the κ2C,C' coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the bowsprit position in the Cp*MIII complexes, whereas in the MI(diene) complexes it is at the flagpole position of the metallacycle. The complexes [RhI(diene){(MeIm)2CHCOO}] (diene = cod, nbd) exist as two conformational isomers in dichloromethane, bowsprit and flagpole, that interconvert through the boat-to-boat inversion of the metallacycle. An inversion barrier of â¼17 kcal·mol-1 was determined by two-dimensional exchange spectroscopy NMR measurements for [RhI(cod){(MeIm)2CHCOO}]. Reaction of zwitterionic Cp*MIII complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*MIIICl{(MeIm)2CHCOOMe}]+ or [Cp*MIIICl{(MeIm)2CHCOOH}]+ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [MI(diene){(MeIm)2CHCOOMe}]+ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic MI(diene) complexes with methyl triflate. In contrast, reaction of [IrI(cod){(MeIm)2CHCOO}] with HBF4·Et2O (Et = ethyl), CH3OTf, CH3I, or I2 gives cationic iridium(III) octahedral complexes [IrIIIX(cod){(MeIm)2CHCOO}]+ (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from κ2C,C' to κ3C,C',O coordination of the bis-NHC ligand accompanying the oxidative addition prevents the coordination of the anions eventually formed in the process that remain as counterions.
RESUMEN
The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for CC bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)-cyclooctadiene complexes having a NHC ligand with a O- or N-functionalised wingtip efficiently catalysed the oxidation and ß-alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH3 )(cod)(MeIm(2- methoxybenzyl))][BF4 ] (cod=1,5-cyclooctadiene, MeIm=1-methylimidazolyl) having a rigid O-functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF0 ) of 1283â h(-1) , and also in the ß-alkylation of 2-propanol with butan-1-ol, which gives a conversion of 94 % in 10â h with a selectivity of 99 % for heptan-2-ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross-aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new CC bond that involves the reaction of an O-bound enolate generated in the basic medium with the electrophilic aldehyde.
RESUMEN
The synthesis and structural characterization of zwitterionic [(η(6)-C6H5-BPh3)M(coe)2] (M = Rh, Ir) cyclooctene complexes is described. Both complexes exhibit an unusual exo-endo conformation of both cyclooctene ligands in the solid state. However, an equilibrium between the endo-endo and exo-endo rotational isomers arising from the hindered rotation about the metal-cyclooctene bond is observed in solution. Rotational barriers of around 65 kJ mol(-1) (Rh) and 84 kJ mol(-1) (Ir) have been determined by 2D EXSY NMR spectroscopy. The rotation process has also been studied by DFT calculations that showed that the dynamic behaviour is a consequence of the oscillation of the cyclooctene ligands about the metal-olefin bond instead of completing a full rotation.
RESUMEN
The elongated dihydrogen complex [formula: see text](1) reacts with 1,1-diphenyl-2-propyn-1-ol and 2-methyl-3-butyn-2-ol to give the hydride-hydroxyvinylidene-pi-alkynol derivatives [OsH{=C=CHC(OH)R2}{eta2-HC(triple bond)CC(OH)R2}(PiPr3)2]BF4 (R = Ph (2), Me (3)), where the pi-alkynols act as four-electron donor ligands. Treatment of 2 and 3 with HBF(4) and coordinating solvents leads to the dicationic hydride-alkenylcarbyne compounds [OsH((triple bond)CCH=CR2)S2(PiPr3)2][BF4]2 (R = Ph, S = H(2)O (4), CH(3)CN (5); R = Me, S = CH(3)CN (6)), which in acetonitrile evolve into the alkenylcarbene complexes [Os(=CHCH=CR2)(CH3CN)3(PiPr3)2][BF4](2) (R = Ph (7), Me (8)) by means of a concerted 1,2-hydrogen shift from the osmium to the carbyne carbon atom. Treatment of 2-propanol solutions of 5 with NaCl affords OsHCl2((triple bond)CCH=CPh2)(PiPr3)2 (10), which reacts with AgBF(4) and acetonitrile to give [OsHCl((triple bond)CCH=CPh2)(CH3CN)(PiPr3)2]BF(4) (11). In this solvent complex 11 is converted to [OsCl(=CHCH=CPh2)(CH3CN)2(PiPr3)2]BF(4) (12). Complex 5 reacts with CO to give [Os(=CHCH=CPh2)(CO)(CH3CN)2(PiPr3)2][BF(4)](2) (15). DFT calculations and kinetic studies for the hydride-alkenylcarbyne to alkenylcarbene transformation show that the difference of energy between the starting compounds and the transition states, which can be described as eta(2)-carbene species [formula: see text] increases with the basicity of the metallic center. The X-ray structures of 4 and 7 and the rotational barriers for the carbene ligands of 7, 8, and 12 are also reported.