Bio-inspired iron pincers : from [Fe]-hydrogenase mimics to hydrogen activation reactivity

Date

2019-05

Authors

Xie, Zhu-Lin

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Abstract

The enzyme [Fe]-hydrogenase catalyzes the heterolytic cleavage of H₂ and hydride transfer to the substrate methenyl-tetrahydromethanopterin (methenyl-H₄MPT⁺), a C₁ carrier during the methanogenic carbon dioxide (CO₂) reduction. This metalloenzyme (also called Hmd: H₂-forming H₄MPT dehydrogenase) plays an obligate role in the ‘nickel-free’ metabolism of CO₂ to methane (CO₂→CH₄) in the absence of bio-available nickel ([NiFe] hydrogenase) and is the only known biological example of H₂ activation by a mononuclear iron site. The active site of [Fe]-hydrogenase exhibits a distinctive array of non-proteinaceous ligands (except for Cys₁₇₆), including the cis-dicarbonyl, the bidentate pyridone-acyl unit that presents a unique (to biology) organometallic Fe–C bond, a cysteine thiolate, and a substrate binding site weakly occupied by H₂O in the resting state. Although computational studies of [Fe]-hydrogenase have shown that H₂ splitting is achieved by metal-ligand cooperation between iron and the pyridone-oxygen, there are few synthetic models that have sufficiently investigated this process. In order to shed light on the mechanism of [Fe]-hydrogenase, we developed three families of models that mimic different aspects the enzyme active site. The first family consisted of carbamoyl thioether pincer complexes ([superscript O=] C [superscript py] NS [superscript Me]) wherein the carbamoyl group (-NH-C [superscript =O] -) mimics the acyl unit in the enzyme. The reactivities of the [superscript O=] C [superscript py] NS [superscript Me] complexes with hydride sources and strong base were investigated. The H₂ activation reaction with the pentacoordinate [superscript O=] C [superscript py] NS [superscript Me] complex revealed that the fac-C, N, S arrangement is a critical factor to reactivity with H₂. The second family was Schiff base [superscript py] N [superscript C=] NS [superscript H] complexes, in which a variety of iron Schiff base thiol complexes were synthesized with the non-bulky thiolate ligands and bulky thiolate ligand. The thermal stability of the complexes and the role of anionic thiolate donors in stabilizing the cis-Fe(CO)₂ unit were investigated. The third family includes the carbamoyl phosphine pincer complexes ([superscript O=] C [superscript py] NP [superscript R2]), in which a phosphine donor was incorporated in place of the biomimetic sulfur donor. The phosphine donor is ideal for stabilizing Fe(II) carbonyl core, and promises the best path forward to a functional catalyst. The reactivity toward H₂ activation and catalytic efficacy of the carbamoyl Fe(II) phosphine complexes were investigated

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