Trigonal Bipyramidal Rhodium(I) Methyl and Phenyl Complexes: Precursors of Oxidative Methyl and Phenyl Radical Generation

Urs Fischbach,Peter Coburger,Monica Trincado,Matthias Vogt,Hansjörg Grützmacher

doi:10.3390/inorganics10030028

Urs Fischbach, Peter Coburger + Show 3 more

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https://doi.org/10.3390/inorganics10030028

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Abstract

The new complexes [Rh(Me)(trop3P)] (2) and [Rh(Ph)(trop3P)] (3) (trop = 5H-dibenzo[a,d]cyclohepten-5-yl) were synthesised by addition of organolithium reagents (MeLi and PhLi) to the parent pentacoordinated chloride complex [RhCl(trop3P)]. The compounds have a trigonal bipyramidal structure with olefin-only ligands in the equatorial position and the methyl or phenyl substituent in the axial position. Oxidation of complexes 2 and 3 leads to the liberation of methyl and phenyl radicals, which were indirectly detected by reaction with common spin trapping reagents.

Highlights

A large number of methods for the generation of C-centred radicals have been discovered in the past few years
The photolytic generation is often reported to be favourable with respect to the thermal generation because of the more controllable and less drastic conditions. Another important method for the generation of methyl radicals in aqueous media is the reaction of sulphoxides with hydroxyl radicals, produced either by redox chemistry (Fenton), hydroperoxide photolysis, or pulse radiolysis [7,8,9]
For the unsubstituted phenyl radical, for example, an radical stabilisation energy (RSE) value of +32.9 kJ mol−1 has been determined (Ph-H BDE = 472.2 kJ mol−1, H3C-H BDE = 439.3 kJ mol−1) [11], indicating that phenyl radicals are significantly less stable than alkyl radicals. This is due to the fact that the C–H bonds in benzene are significantly stronger than the primary C–H bonds in alkanes

Summary

Introduction

A large number of methods for the generation of C-centred radicals have been discovered in the past few years. The Rh–P bond in complexes such as [RhCl(trop3P)] (1) proved to be especially strong and, exhibited a large trans-influence on the opposing axial ligands. The oxidation of these complexes gives rise to the controlled release of methyl or phenyl radicals, respectively.

Results

Conclusion