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.以一氧化碳（carbon monoxide）作为底物的催化反应数量众多，这类反应通常涉及金属键合CO的选择性官能化。我们开发了第7族羰基配合物，其中二级配位球上的路易斯酸性官能团可辅助底物分子的活化，相关研究主要围绕合成气转化展开。本工作报道了一类新型环戊二烯基（Cp）羰基铼化合物，其通式为[Re(η⁵-C₅H₄DMEG)(CO)₃–ₙ(NO)ₙ]ₙ（其中DMEG为二甲基亚乙基胍，n=0,1）；该类配合物的Cp环上锚连有胍碱基团，可与亲电性羰基协同参与底物活化。本文报道了这类配合物的可靠合成路线，并对游离碱形式以及质子化形式的羰基配合物、混合羰基-亚硝酰基配合物进行了晶体结构表征。上述混合羰基-亚硝酰基配合物被用作研究异裂H-H键与O-H键断裂反应的平台，这类反应可生成亲核性CO官能化产物。通过氢负离子转移反应制备了对应的甲酰基配合物，通过测定其氢负离子亲合能（ΔG°H–）以及质子化碱的酸解离常数（pKa），计算得到H₂异裂的标准自由能变化为+3.3(6) kcal/mol。研究发现，甲醇的活化过程更易生成甲氧基羰基配合物，胍基修饰的羰基配合物发生分子内甲醇加成反应的标准自由能变化ΔG°≈0。本文对分子内甲醇活化反应以及涉及外部碱的相关分子间反应开展了详细的热力学研究，结果证实了将碱基团锚定在二级配位球中所带来的切实热力学优势。