Rhoda- and iridacarborane halide complexes: Synthesis, structure and application in homogeneous catalysis

This work is dedicated to the 120th anniversary of academician A.N. Nesmeyanov, who was a great chemist and organizer. In 1954 he founded the Nesmeyanov Institute of Organoelement Compounds of Russian Academy of Science.
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Highlights

  • Synthesis and structure of rhoda- and iridacarborane halide complexes.

  • The structure is strongly dependent on the nature of substituents in carborane cage.

  • These complexes catalyze C–H activation and reductive amination reactions.

Abstract

The review covers the synthesis, structural features and catalytic activity of rhoda- and iridacarborane halide complexes containing monoanionic dicarbollide [L-C2B9H10] (L = SMe2, NMe3) and tricarbollide [tBuHN-C3B8H10] ligands. These complexes are structural analogs of the well-known cyclopentadienyl complexes [Cp*MCl2]2, which are widely used as catalysts for various C–H activation processes. Although the search for applications of halide metallacarboranes started not a long time ago, they have already proved to be efficient catalysts for oxidative coupling of benzoic acids with internal alkynes and reductive amination of carbonyl compounds without external hydrogen source. The influence of substituents in the carborane cage on the structure and catalytic activity is also discussed.

Introduction

(Cyclopentadienyl)halide complexes of rhodium and iridium [Cp*MX2]2 (M = Rh, Ir; X = Cl, Br, I) have been known since 1967 [1][1], [1]a), [1]b), [1]c). Their dimeric structure involving two terminal and two bridging halide ligands was elucidated by Churchill with co-workers 10 years later [[2], [2]a), [2]b), [2]c)]. These compounds have been widely used as synthons of the Cp*M moieties in organometallic synthesis [[3], [3]a), [3]b), [3]c), [3]d), [3]e), [3]f), [3]g), [3]h)], while their catalytic application has been presented by only a few examples for many years. For example, halides [Cp*MCl2]2 proved to be efficient catalysts for olefin hydrogenation in the presence of triethylamine [4]. Severin with co-workers showed that the dinuclear complex Cp*Rh(μ-Cl)3RuCl(PPh3)2 is one of the best catalyst precursors for the oxidation of secondary alcohols by 2-butanone [5].

In recent years, complexes [Cp*MCl2]2 have been widely employed as catalysts for C–H activation processes, which are atom- and step-economy approach for the construction of the carbon-carbon and carbon-heteroatom bonds [[6], [6]a), [6]b), [6]c), [6]d), [6]e), [6]f), [6]g), [6]h)]. The Cp* ligand acts as a supporting ligand to stabilize potential catalytic species and intermediates. It has been found that the use of chiral or other modified Cp ligands may improve catalytic activity and selectivity [[7], [7]a), [7]b), [7]c), [7]d), [7]e), [7]f), [7]g)]. In particular, Shibata and Tanaka showed that the introduction of two electron-deficient ester groups (COOEt) into the cyclopentadienyl ligand leads to a considerable increase in catalytic activity [[8], [8]a), [8]b), [8]c), [8]d)]. Recently, we have found that the rhodium triple-decker complexes (η-C4H4BR)Rh(μ-η:η-C4H4BR)Rh(η-C4H4BR) with borole ligand, which is formed as a result of the formal replacement of one CH unit in the Cp ligand by the BR moiety, effectively catalyze the oxidative coupling of benzoic acid with alkynes selectively giving naphthalene derivatives [9].

The dicarbollide anion [C2B9H11]2− is another prospective supporting ligand for catalyst design with unique electronic and steric properties [[10]e), [10]f), [10]g), [10]h), [10]i), [10]j), [10]k), [10], [10]a), [10]b), [10]c), [10]d)]. Although many complexes of this ligand with transition metals have been synthesized, their catalytic applications are known to date mainly for Group 4 metals [[11]a), [11]b), [11]c), [11]d), [11]]. At the same time, to the best of our knowledge, there are only a few catalytic processes catalyzed by dicarbollide complexes of other metals [12]. In particular, Teixidor with co-workers employed ruthenacarboranes [3-H-3,3-(PPh3)2-8-SR1R2-1-R-3,1,2-RuC2B9H9] as catalysts for the Kharasch addition of CCl4 to methyl methacrylate and styrene [13]. Chizhevsky and Grishin showed that the related ruthenacarboranes effectively catalyze the atom transfer radical polymerization of methyl methacrylate [[14]a), [14]b), [14]c), [14]d), [14]].

In the last two decades, we have developed general methods for the synthesis of rhodium and iridium halide complexes with di- and tricarbollide ligands, which are related with the halide complexes [Cp*MX2]2, and demonstrated their application as catalysts for reductive amination and C–H activation. This review summarizes data on synthesis, reactivity, structure and catalytic activity of these compounds. We focus special attention to elucidate a correlation between the nature of substituents in the carborane cage, the structure of the complex and its catalytic activity.

Section snippets

Synthesis, reactivity and structure

Only two general approaches to rhoda- and iridacarborane halide complexes are known to date. They are based on the oxidative replacement of olefin ligands in metallacarboranes by hydrogen halides or halogens. Therefore, this chapter is divided into two sections accordingly to the nature of halogenation agents.

Catalytic applications

Both mono- and dimeric halide metallcarboranes show catalytic activity in the oxidative coupling of benzoic acids with internal alkynes and in the reductive amination of aldehydes and ketones without external hydrogen source. The first reaction is an efficient direct approach to isocoumarin and naphthalene derivatives, which are of great interest for application in the development of photoactive materials [6a,g,[32]c), [32]d), [32]e), [32], [32]a), [32]b)]. The second reaction is a

Conclusions

The present review demonstrates that reactions of olefin rhoda- and iridacarboranes with hydrogen halides or halogens are an efficient approach to halide compounds. Both di- and tricarbollide derivatives are suitable for this procedure. The structure of products is strongly dependent on the nature of substituents in the carborane cage. For example, C-methylated dicarbollides form monomeric halides with pseudocloso geometry, while tricarbollides and non-methylated dicarbollides lead to dimeric

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The work was supported by the Russian Science Foundation (Grant No. 17-73-30036). A literature search was performed with the financial support from Ministry of Science and Higher Education of the Russian Federation.

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