A Thermodynamic Analysis of Rhenium(I)–Formyl C–H Bond Formation via Base-Assisted Heterolytic H₂Cleavage in the Secondary Coordination Sphere

Abstract

Conversion of synthesis gas, a mixture of carbon monoxide and hydrogen, into value-added C n≥2 products requires both C−H and C−C bond-forming events. Our group has developed a series of molecular complexes, based on group 7 (manganese and rhenium) carbonyl complexes, to interrogate the elementary steps involved in the homogeneous hydrogenative reductive coupling of CO. Here, we explore a new mode of H 2 activation, in which strong bases in the secondary coordination sphere are positioned to assist in the heterolytic cleavage of H 2 to form a formyl C−H bond at a rhenium-bound carbonyl. A series of cationic rhenium(I) complexes of the type [Re I (P∼B:-κ 1 -P)(CO) 5 ] n (n = 0, +1), where P∼B: is a phosphine ligand with a tethered strong base, are prepared and characterized; measurement of their protonation equilibria demonstrates a pronounced attenuation of the basicity upon coordination. Formyl complexes supported by these ligands can be prepared in good yield by hydride delivery to the parent pentacarbonyl complexes, and several of the free-base formyl complexes can be protonated, generating observable [Re I (P∼BH-κ 1 -P)(CHO)(CO) 4 ] n complexes. Intramolecular hydrogen bonding is evident for one of the complexes, providing additional stabilization to the protonated formyl complex. By measuring both the hydricity of the formyl, ΔG°H − , and its pK a , the overall free energy of H 2 cleavage is calculated from an appropriate cycle and found to be thermodynamically uphill in all cases (in the best case by only about 8 kcal/mol), although significantly dependent upon the properties of the supporting ligand.

H2Oxidation over Supported Au Nanoparticle Catalysts: Evidence for Heterolytic H2Activation at the Metal–Support Interface

“…This conclusion has been supported by DFT calculations and substantial synthetic modeling and is fundamentally similar to what we observe for the Au catalysts. Several systems of transition metal complexes have been shown to activate H 2 heterolytically. − Metal hydrides have long been known to be acidic, , so these systems might be considered as operating via the traditional oxidative addition mechanism followed by rapid deprotonation. There is also now a fairly extensive literature of heterolytic H 2 dissociation using frustrated Lewis pairs (FLPs), which can be used to perform a variety of organic hydrogenations without the use of transition metals. − Heterolytic H 2 activation has also been implicated at the Ru/TiO 2 MSI during hydrodeoxygenation of phenols.…”

Probing the Protonation State and the Redox-Active Sites of Pendant Base Iron(II) and Zinc(II) Pyridinediimine Complexes

“…In similar systems that contain arylphosphine ligands tethered with pendant bases, the pKa values of the pendant groups are severely attenuated once the phosphine is bound to a transition metal ion. 60 For example, the reported dimethylamine appended triphenylphosphine has a pKa of 18.0 when free and 13.0 when bound to a rhenium(I) center. This suggests that the base is able to communicate electronically through the aryl group, leading to severe attenuation in the pKa values, which is not observed in our systems.…”

Small Molecule Activation with Intramolecular “Inverse” Frustrated Lewis Pairs

“…The reaction of n BuLi with 2‐bromo‐phenyl guanidine 1 at −78 °C in hexanes, gives a white precipitate of 2‐lithio‐phenyl guanidine 2 , which was isolated in yields of ca. 70–80 % [12] . 2 is a pyrophoric but thermally stable powder, which can be stored in the glove box for months without signs of degradation.…”

“…70-80 %. [12] 2 is ap yrophoric but thermally stable powder, which can be stored in the glove boxf or months without signs of degradation.I ts hows good solubility in THF,i ss paringly soluble in toluene and benzene and insoluble in hexanes.…”