Homogeneous CO Hydrogenation: Ligand Effects on the Lewis Acid-Assisted Reductive Coupling of Carbon Monoxide

Abstract

Structure−function studies on the role of pendent Lewis acids in the reductive coupling of CO are reported. Cationic rhenium carbonyl complexes containing zero, one, or two phosphinoborane ligands (Ph2P(CH2) n B(C8H14), n = 1−3) react with the nucleophilic hydride [HPt(dmpe)2]+ to reduce [M−CO]+ to M−CHO; this step is relatively insensitive to the Lewis acid, as both pendent (internal) and external boranes of appropriate acid strength can be used. In contrast, whether a second hydride transfer and C−C bond forming steps occur depends strongly on the number of carbon atoms between P and B in the phosphinoborane ligands, as well as the number of pendent acids in the complex: shorter linker chain lengths favor such reductive coupling, whereas longer chains and external boranes are ineffective. A number of different species containing partially reduced CO groups, whose exact structures vary considerably with the nature and number of phosphinoborane ligands, have been crystallographically characterized. The reaction of [(Ph2P(CH2)2B(C8H14))2Re(CO)4]+ with [HPt(dmpe)2]+ takes place via a “hydride shuttle” mechanism, in which hydride is transferred from Pt to a pendent borane and thence to CO, rather than by direct hydride attack at CO. Addition of a second hydride in C6D5Cl at −40 °C affords an unusual anionic bis(carbene) complex, which converts to a C−C bonded product on warming. These results support a working model for Lewis acid-assisted reductive coupling of CO, in which B (pendent or external) shuttles hydride from Pt to coordinated CO, followed by formation of an intramolecular B−O bond, which facilitates reductive coupling.

Oxidative‐Insertion Reactivity Across a Geometrically Constrained Metal→Borane Interaction

“…[25][26][27][28][29][30][31][32] However,w er ecently reported the singly buttressed platinum-borane complex [Pt(k 2 -N,B-Cy2 BIM)(CNAr Dipp2 )] (1), which features ab identate (boryl)iminomethane (BIM) ligand. [25][26][27][28][29][30][31][32] However,w er ecently reported the singly buttressed platinum-borane complex [Pt(k 2 -N,B-Cy2 BIM)(CNAr Dipp2 )] (1), which features ab identate (boryl)iminomethane (BIM) ligand.…”

Phosphinoborane-induced fragmentation of the unsaturated hydride H2Re2(CO)8: X-ray structure of HRe(CO)4(κB,P-Ph2PCH2CH2BR2) (where BR2 = 9-borabicyclo[3.3.1]nonanyl) and DFT Evaluation of hydride versus CO coordination by the ancillary borane

“…The rhenium center exhibits idealized octahedral symmetry, in keeping with the coordination of a hydride group, a phosphine moiety, and four terminal CO ligands in the primary coordination sphere of 3. The bond distances and angles within the framework of the ancillary phosphinoborane ligand in 3 are unremarkable in comparison with those bond distances and angles in the structure of 2 [14], the piano-stool compound (p-cymene) RuCl 2 (Ph 2 PCH 2 CH 2 BR 2 ) [11], and the eight structures of Re-based derivatives characterized by Labinger and Bercaw [12,13].…”

“…Within the expanse of such compounds, phosphinoboranes are unique because they contain tethered nucleophilic phosphorus and electrophilic boron centers, typically in close proximity [4,5,6,7,8,9,10,11]. The ambiphilic ligand properties of phosphinoboranes have been exploited in CO reduction and the modeling of alkyl-abstraction schemes related to ZieglereNatta catalysis [12,13,14,15]. In these studies, coordination of the phosphine donor to the metal center gives rise to a pendant or "free" borane moiety in the initially formed product.…”

“…In these studies, coordination of the phosphine donor to the metal center gives rise to a pendant or "free" borane moiety in the initially formed product. In terms of CO reduction, the Lewis acid can assist in the delivery of a hydride from an exogenous reducing agent to a coordinated CO ligand [12,13]. For alkyl-substituted metal compounds, the abstraction of a coordinated alkyl group generates a reactive zwitterionic borate species that is capable of coordinating an alkene [14,15,16].…”

Functionalization of Carbon Monoxide and tert‐Butyl Nitrile by Intramolecular Proton Transfer in a Bis(Phosphido) Thorium Complex

“…[61,62] Throughout the Fischer-Tropsch reaction chemistry literature,w e cannot find any report in which ap roton is transferred directly to the carbon of CO without an external agent (Lewis acid or proton source) or beginning from ah ydride source. [71] Other examples involving Frustrated Lewis Pairs (FLPs) have been noted, [72][73][74] but we can find no prior reports of unassisted functionalization of CO (other than when hydride is part of the starting material). [71] Other examples involving Frustrated Lewis Pairs (FLPs) have been noted, [72][73][74] but we can find no prior reports of unassisted functionalization of CO (other than when hydride is part of the starting material).…”