(1,2,3,4,7-Pentamethylindenyl)rhodium complexes with arene ligands
Graphical abstract
Arene complexes [(η5-C9H2Me5)Rh(arene)]2+ were prepared from iodide [(η5-C9H2Me5)RhI2]2 via intermediate generation of labile solvates [(η5-C9H2Me5)Rh(MeNO2)3]2+. The electrochemical behaviour of new complexes was studied. The rhodium–benzene bonding in series of the related cations [(ring)Rh(C6H6)]2+ (ring = Cp, Cp∗, C9H7, C9H2Me5) was analyzed using energy and charge decomposition schemes.
Introduction
Dicationic rhodium and iridium arene complexes [Cp∗M(arene)]2+ are known for more than 35 years [1]. They have attracted much attention because of high reactivity of the arene ligands toward nucleophilic reagents. In particular, they were used as catalysts for nucleophilic substitution of the fluorine atom in the benzene ring [2] and as reagents in a closed cycle of benzene reduction to cyclohexene [3].
Recently, we have synthesized analogous arene complexes with unsubstituted cyclopentadienyl ring [CpM(arene)]2+ (M = Rh, Ir) by reaction of solvate complexes [CpM(MeNO2)3]2+ (generated in situ by treatment of [CpMI2]2 with Ag+ in nitromethane) with arenes [4]. The iridium cation [CpIr(η-C6H6)]2+ exhibits a catalytic activity in reactions of alkane oxidation by m-chloroperoxybenzoic acid [5]. In a similar manner, we have prepared metallacarborane derivatives [(η-9-SMe2-7,8-C2B9H10)M(arene)]2+ (M = Rh, Ir) from bromides [(η-9-SMe2-7,8- C2B9H10)MBr2]2 [6].
The related arene complexes of the (indenyl)rhodium fragment are presently unknown, in spite of rather well study of chemistry of this fragment [7], [8], [9], [10]. Herein, we report the synthesis of (1,2,3,4,7-pentamethylindenyl)rhodium arene complexes [(η5-C9H2Me5)Rh(arene)]2+ as well as the results of their structural and electrochemical study.
Section snippets
Synthesis
Similar to the previously reported synthesis of iodide [CpRhI2]2 [4], the pentamethylindenyl analogue [(η5-C9H2Me5)RhI2]2 (2) was prepared by treatment of the cyclooctadiene complex (η5-C9H2Me5)Rh(1,5-C8H12) (1) with I2 (Scheme 1). The starting compound 1 was synthesized by reaction of [(1,5-C8H12)RhCl]2 with 1,2,3,4,7-pentamethylindenyl lithium.
We found that the reaction of the solvate complex [(η5- C9H2Me5)Rh(MeNO2)3]2+ (generated in situ by treatment of 2 with Ag+ in nitromethane) with
Conclusion
The iodide complex 2 can be used as a convenient synthon of the fragment (η5-C9H2Me5)Rh either directly or via intermediate transformation into labile nitromethane solvate. The arene–indenyl complexes 3a–c proved to be much less reactive towards arene replacement by solvent molecules compared with cyclopentadienyl analogues. Complexes 3a–c undergo reduction at more positive potential than Cp∗ derivatives in accordance with higher NBO charges at the rhodium atom.
General
The reactions were carried out under an inert atmosphere in dry solvents, unless otherwise stated. The isolation of products was conducted in air. Starting materials [(1,5-C8H12)RhCl]2 [36], AgBF4 · 3dioxane [37], and CpRh(η5-C4H4BPh) [38] were prepared as described in the literature. 1,2,3,4,7-Pentamethylindene was synthesized similar to 1,2,3-trimethylindene [39]. 1H NMR spectra (δ in ppm) were recorded on a Bruker AMX-400 spectrometer (400.13 MHz) relative to residual protons of the solvents.
Synthesis of 1,2,3,4,7-pentamethylindene
p
Acknowledgements
Financial support from the Division of General Chemistry and Material Sciences of RAS is gratefully acknowledged. P.Z. gratefully acknowledges the financial support of the University of Siena (PAR PROGETTI 2005).
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