Synthesis of dimethylmanganese(II) complexes bearing N-heterocyclic carbenes and nucleophilic substitution reaction of tetraalkoxysilanes by diorganomanganese(II) complexes

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Abstract

Reactions of manganese(II) dichlorides bearing a N-heterocyclic carbene ligand (L), [MnCl(μ-Cl)(L)]2 (1a, L = 1,3-diisopropyl-4,5-dimethylimidazole-2-ylidene (IiPr); 1b, L = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene (IMes); 1c, L = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr)) with MeLi afford the dinuclear dimethylmanganese(II) complexes, [MnMe(μ-Me)(L)]2 (2a, L = IiPr; 2b, L = IMes; 2c, L = IPr). Complexes 2a-c achieve nucleophilic substitution of Si(OEt)4 to selectively form MeSi(OEt)3. Related arylmanganese(II) complexes analogously react with Si(OEt)4 to afford ArSi(OEt)3 and Ar2Si(OEt)2 (Ar = Ph, 2,6-Me2(C6H3)). Kinetic studies support an associative mechanism for the observed transformation of Si(OEt)4, in which both the manganese species and Si(OEt)4 are involved in the rate-limiting step.

Introduction

Manganese(II) complexes possess five unpaired 3d electrons and exhibit strong tendency of being a high spin state. Owing to this unique electronic structure, manganese(II) complexes are expected to exhibit characteristic reactivity. Thus, exploration of their reaction chemistry is of great interest in terms of the investigation of new types of reactions. Since the first discovery of organomanganese complex in 1937 [1], reactivity of various organomanganese species, such as organomanganese halides (RMnX), diorganomanganeses (R2Mn), and organomanganates (R3Mn and R4Mn2-), have been thus far investigated more than half a century [2], [2](a), [2](b), [2](c), [2](d), [2](e), [3], [3](a), [3](b), [3](c), [3](d), [3](e), [3](f), [3](g), [3](h), [3](i), [3](j). These studies have been performed mainly using RMnX, R3Mn and R4Mn2− and examples of detailed study on well-defined diorganomanganese(II) complexes were still limited [4], mainly due to their extremely high instability. In this study, the dinuclear dimethylmanganese(II) complexes bearing N-heterocyclic carbene ligands have been synthesized. It was also revealed that the nucleophilic character of diorganomanganese(II) successfully attain nucleophilic substitution of tetraalkoxysilanes to form organosilicates.

Section snippets

Synthesis and structures of dimethylmanganase(II) complexes bearing N-heterocyclic carbenes

Synthesis of N-heterocyclic carbene-supported manganese(II) dichloride [MnCl(μ-Cl)(L)]2 (1b, L = 1,3-bis(2,4,6-trimethylphenyl)imidazole-2-ylidene (IMes); 1c, L = 1,3-bis(2,6-diisopropylphenyl)imidazole-2-ylidene (IPr)) were recently reported by Tonzetich et al. [5]. We also synthesized a manganese(II) dichloride bearing smaller N-heterocyclic carbene ligands, [MnCl(μ-Cl)(L)]2 (1a, L = 1,3-diisopropyl-4,5-dimethylimidazole-2-ylidene (IiPr)) by a similar procedure from MnCl2 and the

Conclusions

In this study, dimethylmanganese(II) complexes 2a-c supported by N-heterocyclic carbene ligand(s) have been synthesized and their structures were fully determined. Reactivity investigation of 2a-c as well as related diarylmanganese(II) complexes revealed their utility towards the direct conversion of tetraalkoxysilane Si-O bonds to Si-C bonds to give organosilicates. Kinetic studies supported the associative mechanism for the reaction of diphenylmanganese complex 5 with Si(OEt)4, in which both 5

General procedures

All reactions were carried out using standard Schlenk techniques or a glove box under a nitrogen atmosphere. Toluene, benzene, diethyl ether, THF, and hexane were purified by a solvent purification system (MBraun SPS-800). Other solvents were degassed and distilled from sodium benzophenone ketyl. Si(OEt)4 and Si(OMe)4 were purchased from commercial sources and distilled prior to use. C6D6 and THF-d8 were dried by sodium and distilled prior to use. MnCl2 was purchased from Kojundo Chemical

Acknowledgements

This work was supported by the “Development of Innovative Catalytic Processes for Organosilicon Functional Materials” project (PL: K.Sato) from the New Energy and Industrial Technology Development Organization (NEDO).

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