Guanidine-Functionalized Rhenium Cyclopentadienyl Carbonyl Complexes: Synthesis and Cooperative Activation of H–H and O–H Bonds

Abstract

Catalytic reactions utilizing carbon monoxide as a substrate are numerous, and they typically involve selective functionalization of a metal-bound CO. We have developed group 7 carbonyl complexes where secondary coordination sphere, Lewis acidic functionalities can assist in the activation of substrate molecules, mainly in the context of syngas conversion. This work describes a new class of cyclopentadienyl (Cp) rhenium carbonyl compounds of the type [Re(η 5 -C 5 H 4 DMEG)(CO) 3−n (NO) n ] n (DMEG = dimethylethyleneguanidine, n = 0, 1), where a tethered guanidine base is appended to the Cp ring to participate in cooperative substrate activation with the electrophilic carbonyl. A reliable synthetic route for these complexes is presented, with crystallographic characterization of the free-base and protonated forms for both the carbonyl and mixed carbonyl-nitrosyl complexes. The latter are employed as platforms to study heterolytic H−H and O−H bond cleavage reactions that result in nucleophilic CO functionalization. The corresponding formyl complex is prepared by hydride transfer, and by measuring its hydricity (ΔG°H − ) and pK a of the protonated base, the free energy of H 2 cleavage is found to be +3.3(6) kcal/mol. The activation of methanol to form methoxycarbonyl complexes is found to be more favorable, with ΔG°≈ 0 for the intramolecular addition of methanol to the guanidine-appended carbonyl complex. A detailed thermodynamic study is described for both the intramolecular methanol activation reaction and related intermolecular reactions with external bases. The results highlight some tangible thermodynamic benefits of tethering the base in the secondary coordination sphere.

Main Group Lewis Acid-Mediated Transformations of Transition-Metal Hydride Complexes

“…In an effort to improve the thermodynamics of H 2 cleavage, a strong guanidine base was appended to a cyclopentadienyl rhenium carbonyl/nitrosyl complex, which is a substantially better hydride acceptor than complex 85 . As summarized in Scheme , the heterolytic cleavage of H 2 and CH 3 OH was studied for guanidine-appended complex 87 . In this case, tethering the base to the Cp ligand attenuates the electrophilicity of the carbonyl ligand, and complex 87 is a ca.…”

“…As summarized in Scheme 44, the heterolytic cleavage of H 2 and CH 3 OH was studied for guanidine-appended complex 87. 117 In this case, tethering the base to the Cp ligand attenuates the electrophilicity of the carbonyl ligand, and complex 87 is a ca. 3 kcal/ mol worse hydride acceptor than the parent complex [Re(Cp)(CO) 2 (NO)] + .…”

Thermodynamic Hydricity of Transition Metal Hydrides

“…In the case of the monophosphine pentacarbonyl rhenium cations, the formyl species had Δ G ° H– ≈ 45 kcal/mol and pendant acids with p K a ≈ 15, which leads to H 2 heterolysis being unfavorable by 8–11 kcal/mol . The free energy for H 2 addition was improved to ∼3 kcal/mol by moving to a [Re­(Cp DMEG )­(NO)­(CO) 2 ] + , a rhenium carbonyl platform with an improved hydride acceptor ability (Δ G ° H– = 51.7 kcal/mol for the formyl complex) . Despite the thermodynamic accessibility of the H 2 addition product shown at the bottom of Chart , no evidence for H 2 heterolysis was observed at P H2 = 3 atm over prolonged time periods, indicating a substantial kinetic barrier for the process.…”

The chemistry of the carbon-transition metal double and triple bond: Annual survey covering the year 2014

“…Additional electrochemical studies of metal acyl complexes were also reported. Related synthetic and thermochemical studies on acylrhenium complexes that contain protonated guanidine-substituted Cp ligands were also reported [950]. A computational study of methyl trioxorhenium-catalyzed cyclopropanation favored a mechanism where the carbene unit is bound primarily to an oxo ligand (formally a rhenium formaldehyde complex) over a metal carbene mechanism [951].…”