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(η^5-C_5H_4DMEG)(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.
Additional Information© 2014 American Chemical Society. Received: June 18, 2014; Publication Date (Web): July 30, 2014. We acknowledge BP for their support of this work through the XC^2 program. Dr. Glenn J. Sunley (BP) and Dr. Aaron Sattler (Caltech) are thanked for helpful discussions.
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Supplemental Material - om500650b_si_002.pdf