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

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(η5-C5H4DMEG)­(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 pKa of the protonated base, the free energy of
H2 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.