Reversible cyclometalation at RhI as a motif for metal-ligand bifunctional bond activation and base-free formic acid dehydrogenation
Reversible cyclometalation at RhI as a motif for metal-ligand bifunctional bond activation and base-free formic acid dehydrogenation
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DOI：
10.1039/c5cy01505g
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发表时间：
2016-01-01
影响因子：
5
通讯作者：
van der Vlugt, J. I.
中科院分区：
文献类型：
--
作者：
Jongbloed, L. S.;de Bruin, B.;van der Vlugt, J. I.
可逆环金属化被证明是一种活化质子小分子的策略，并在没有外源碱的情况下对甲酸的脱氢反应进行了原理验证催化应用。明确的Rh-I络合物Rh(CO)(L)1，带有反应性的环金属化Pn(C)配体L(L-H=Pnch=2-di.tert-butylphosphinomethyl)-6-phenylpyridine)，)，与硫醇和三氟甲磺酰胺等弱质子剂发生质子化反应。该体系还表现出与甲酸的双功能金属-配体质子化反应和随后甲酸盐络合物的脱羧基反应。密度泛函理论(DFT)计算表明，从假定的Rh(CO)(H)(L-H)配合物A演化为H-2是非常容易的，可能包括在Rh上的正式C-H氧化加成，通过无核中间体B得到C，然后H-2被还原消除。配合物1是无碱甲酸脱氢的催化活性物种，甲酸盐配合物4是中间体。密度泛函理论计算揭示了侧翼苯基参与甲酸初始活化和H-2释放的可及障碍，支持合作途径。因此，可逆的C-H活化是金属-配体双功能催化的一种可行机制。
Reversible cyclometalation is demonstrated as a strategy for the activation of small protic molecules, with a proof-of-principle catalytic application in the dehydrogenation of formic acid in the absence of an exogenous base. The well-defined Rh-I complex Rh(CO)(L) 1, bearing the reactive cyclometalated PN(C) ligand L (L-H = PNCH = 2-di.tert-butylphosphinomethyl)-6-phenylpyridine), undergoes protonolysis of the Rh-C-Ph bond with weak protic reagents, such as thiols and trifluoromethanesulfonamide. This system also displays bifunctional metal-ligand protonolysis reactivity with formic acid and subsequent decarboxylation of the formate complex. Density functional theory (DFT) calculations show that H-2 evolution from putative Rh(CO)(H)(L-H) complex A is very facile, proposedly encompassing formal C-H oxidative addition at Rh to give C via agostic intermediate B and subsequent reductive elimination of H-2. Complex 1 is a catalytically competent species for base-free formic acid dehydrogenation, with the intermediacy of formate complex 4. DFT calculations reveal accessible barriers for involvement of a flanking phenyl group for both initial activation of formic acid and release of H-2, supporting a cooperative pathway. Reversible C-H activation is thus a viable mechanism for metal-ligand bifunctional catalysis.