The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to “traditional” precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal–ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands.

Catalytic hydrogenation and dehydrogenation reactions form the core of the modern chemical industry. This vast class of reactions is found in any part of chemical synthesis starting from the milligram-scale exploratory organic chemistry to the multi-ton base chemicals production. Noble metal catalysis has long been the key driving force in enabling these transformations with carbonyl substrates and their nitrogen-containing counterparts. This review is aimed at introducing the reader to the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

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Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

The sustainable use of the resources on our planet is essential. Noble metals are very rare and are diversely used in key technologies, such as catalysis. Manganese is the third most abundant transition metal of the Earth's crust and based on the recently discovered impressive reactivity in hydrogenation and dehydrogenation reactions, is a potentially useful noble-metal "replacement". The hope of novel selectivity profiles, not possible with noble metals, is also an aim of such a "replacement". The reactivity of manganese complexes in (de)hydrogenation reactions was demonstrated for the first time in 2016. Herein, we summarize the work that has been published since then and especially discuss the importance of homogeneous manganese catalysts in comparison to cobalt and iron catalysts.

Manganese Complexes for (De)Hydrogenation Catalysis: A Comparison to Cobalt and Iron Catalysts

Homogeneous catalysis of organic transformations by metal complexes has been mostly based on complexes of noble metals. In recent years, tremendous progress has been made in the field of base-metal catalysis, mostly with pincer-type complexes, such as iron, cobalt, nickel, and manganese pincer systems. Particularly impressive is the explosive growth in the catalysis by Mn-based pincer complexes, the first such complexes being reported as recently as 2016. This review covers recent progress in the field of homogeneously catalyzed reactions using pincer-type complexes of cobalt and manganese. Various reactions are described, including acceptorless dehydrogenation, hydrogenation, dehydrogenative coupling, hydrogen borrowing, hydrogen transfer, H–X additions, C–C coupling, alkene polymerization and N2 fixation, including their scope and brief mechanistic comments.

Homogeneous Catalysis by Cobalt and Manganese Pincer Complexes

Cobalt has become increasingly attractive in homogeneous catalysis because of its unique characteristics and outstanding catalytic performance in addition to being cheap and earth-abundant. Hydride transfer processes are involved in a broad range of organic transformations that allow the facile preparation of various useful chemicals and synthetic building blocks. These reactions have continuously received great attention both from academia and industry. In this perspective, we review homogeneous cobalt-catalyzed hydride transfer reactions according to the classified reaction types and provide a comprehensive overview of the design, synthesis, and reactivity of cobalt catalysts, their catalytic applications, and reaction mechanisms.

Hydride Transfer Reactions Catalyzed by Cobalt Complexes.

Here, we report an additive-free catalytic system for hydrogenation of carboxylic acid esters to alcohols with a well-defined cobalt pincer catalyst precursor. Various substrates, including methyl, ethyl, and benzyl esters, have been evaluated under hydrogenation conditions; however, methyl esters have low reactivity compared to those of the corresponding ethyl and benzyl esters. The biomass-derived γ-valerolactone successfully formed 1,4-pentanediol with a turnover number of 3890 with this system. Metal–ligand cooperativity is probed with the related [PN(Me)P] derivative of the cobalt catalyst, and the results suggest a nonbifunctional hydrogenation mechanism.

Additive-Free Cobalt-Catalyzed Hydrogenation of Esters to Alcohols

Guanine-rich DNA sequences are widely found in telomere regions and are prone to forming G-quadruplex structures through cyclic Hoogsteen base pairing between four guanine bases. According to the orientation of oligonucleotides, three formations of G-quadruplex have been suggested: all-parallel, antiparallel, and a hybrid of parallel/antiparallel, all of which have been reported as existing under different conditions. Conformations of the DNA quadruplex were deemed to be affected by several factors such as the composition of the loops, the number of stacked G-tetrads, and the presence of diverse cations or small molecules. Directly modified oligonucleotides, which could be utilized to form a G-quadruplex, have been found, for example, by Hartig and co-workers. The main aim of the study reported herein was to develop a new approach to form G-quadruplex complexes more easily and conveniently by modification of the sequence and to subsequently make use of these conformations as functional probes. Phosphorothioate oligonucleotides are a variant of normal DNA and are characterized by the replacement of a nonbridging oxygen group with a sulfur. This modification facilitates oligonucleotide stability in the presence of endoand exo-nucleases, which could be used in gene regulation through the antisense approach. In addition, recent investigations by Deng and co-workers have indicated that the phosphorothioation of DNA exist in some bacteria, which could be considered as the first known physiological modification on the DNA backbone. To develop this strategy, we combined the two principles that oligonucleotides could be modified with sulfur, and that the formation of phosphorothioate G-quadruplex could be regulated by metal ions, such as Hg , since Hg S forms a strongly covalent bond that would result in the folding of the G-quadruplex. Furthermore, the modified G-quadruplex could be utilized as a functional tool to identify potential targets of anticancer therapy, as well as the detection of specific metal ions or the identification of single-nucleotide polymorphisms (SNPs) as a biological probe. The toxicity of Hg to humans and the environment is associated with its disruption of cell membranes, its impairment of mitochondrial function, and its inhibition of DNA replication in a cell. Up to now, several methods have been developed for the detection of Hg under aqueous conditions with high sensitivity and selectivity, such as the application of DNAzymes and oligonucleotide–gold nanoparticles. The peroxidase-like DNAzyme comprising a G-quadruplex structure and hemin was first designed and developed by Sen and co-workers. Then Willner and co-workers applied it to detect small molecules, metal ions, proteins, and DNA as a catalytic label. Thereafter, Mikuma et al. reported that the tetramolecular G-quadruplex formed by dACHTUNGTRENNUNG(TTAGGG) possessed the ability to bind with hemin. In our strategy, the phosphorothioate modification was adopted within one of the T4G4 sequences, which were inclined to form G-quadruplex structures, and then utilized it to form a DNAzyme (Scheme 1). Compared to the former detection methods of Hg , the utilization of the DNAzyme operated by the phosphorothioate all-parallel G-quadruplex formed from T4G4-S3 combined the advantage of sensitive and selective optical detec[a] D. Zhang, M. Deng, L. Xu, Y. Zhou, J. Yuwen, Prof. X. Zhou College of Chemistry and Molecular Sciences Wuhan University, Hubei Wuhan 430072 (China) Fax: (+86) -27-87336380 E-mail : xzhou@whu.edu.cn [b] Prof. X. Zhou Key Laboratory of Biomedical Polymers of Ministry of Education College of Chemistry and Molecular Sciences Wuhan University, Hubei Wuhan, 430072 (China) [c] Prof. X. Zhou The State Key Laboratory of Natural and Biomimetic Drugs Beijing University (China) [] The two authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.200901268.

The Sensitive and Selective Optical Detection of Mercury(II) Ions by Using a Phosphorothioate DNAzyme Strategy

The active fragment [Tp′Rh(PMe3)], generated from a thermal precursor Tp′Rh(PMe3)(CH3)H, underwent oxidative addition of water and alcohols to give O–H adducts of the type Tp′Rh(PMe3)(OR)H (R = H, Me, Et, nPr, nBu, CH2Ph, iPr, c-pentyl, CH2CF3, CH2CH2OH) at ambient temperature. These activation products eliminate water or alcohols in benzene, which allows determination of the relative metal–oxygen bond energies by using previously established kinetics techniques. Analysis of the relationship between the relative M–O bond strengths and O–H bond strengths showed a linear correlation with RM–O/O–H of 0.97 (3) for aliphatic alcohols. The two extraordinary substrates (R = CH2CF3, CH2CH2OH) both have stronger M–O bonds than would be predicted from this trend, suggesting the stabilization of the M–O bond when an electron-withdrawing substituent is present as previously seen in M–C bond strengths. In addition, the O–H activation products of aliphatic alcohols are thermally unstable at 80 °C, as rearrangement to f...

Determination of Rhodium-Alkoxide Bond Strengths in Tp'Rh(PMe3)(OR)H.

A thermal reaction of amines, anilines, and amides with Tp′Rh(PMe3)(CH3)H (1, Tp′ = tris(3,5-dimethyl-pyrazolyl)borate) is described in this report. No N–H bond cleavage was observed for reactions between ammonia or unsubstituted aliphatic amines with the reactive fragment [Tp′Rh(PMe3)]. Instead, amine coordination products (κ2-Tp′)Rh(PMe3)(NHR1R2) (R1 = H, R2 = H, nPr, iPr, octyl; R1 = R2 = Et; R1, R2: pyrrolidine) were observed, and the crystal structure of (κ2-Tp′)Rh(PMe3)(NH2iPr) is reported. No coordination products were observed when 1 was reacted with 1,1,1,3,3,3-hexafluoropropan-2-amine, anilines, and amides. Instead, the oxidative addition products (κ3-Tp′)Rh(PMe3)(NHR)H (R = CH(CF3)2, C6H5, 3,5-dimethylbenzyl, C6F5, C(O)CH3, C(O)CF3) were observed. Both RhI–N coordination products (κ2-Tp′)Rh(PMe3)(NH2CH2CF3) and RhIII N–H addition products (κ3-Tp′)Rh(PMe3)(NHCH2CF3)H were generated when 1 was reacted with 2,2,2-trifluoroethylamine. Coordination products dissociate ammonia and amines in benzene m...

Coordination or Oxidative Addition? Activation of N-H with [Tp'Rh(PMe3)].

i ACKNOWLEDGEMENTS ii LIST	 OF	 TABLES v LIST	 OF	 FIGURES vi LIST	 OF	 SCHEMES vii LIST	 OF	 ABBREVIATIONS viii

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Jing Yuwen

Papers

Additive-Free Cobalt-Catalyzed Hydrogenation of Esters to Alcohols

The Sensitive and Selective Optical Detection of Mercury(II) Ions by Using a Phosphorothioate DNAzyme Strategy

Determination of Rhodium-Alkoxide Bond Strengths in Tp'Rh(PMe3)(OR)H.

Coordination or Oxidative Addition? Activation of N-H with [Tp'Rh(PMe3)].

Polymer-Based Photoactive Surface for the Efficient Immobilization of Nanoparticles, Polymers, Graphene and Carbohydrates