Preparation and protonation reactions of aryl complexes of manganese and rhenium

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Abstract

Aryl M(κ1-Ar)(CO)nP5−n [M = Mn, Re; Ar = C6H5, 4-CH3C6H4; n = 2, 3; P = P(OEt)3, PPh(OEt)2, PPh2OEt] and Re(κ1-C6H5)(CO)3[Ph2PO(CH2)3OPPh2] complexes were prepared by allowing hydrides MH(CO)nP5−n to react first with triflic acid and then with the appropriate aryl lithium (LiAr) compounds. The complexes were characterized spectroscopically (IR and 1H, 31P, 13C NMR) and by the X-ray crystal structure determination of Re(κ1-C6H5)(CO)3[Ph2PO(CH2)3OPPh2] derivative. Protonation reaction of the aryl complexes with HBF4 · Et2O lead to free hydrocarbons Ar–H and the unsaturated [M(CO)nP5−n]+ cations, separated as solids in the case of [Re(CO)3P2]BF4 derivatives.

Graphical abstract

The preparation of aryl complexes M(η1-Ar)(CO)nP5−n of manganese and rhenium is presented. The structural parameters of the Re(η1-C6H5)(CO)3[Ph2PO(CH2)2OPPh2] derivative was also determined. The protonation of the aryl complexes with Brønsted acid was studied and led to free hydrocarbons and the unsaturated [M(CO)nP5−n]+ cations.

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Introduction

Aryl complexes of transition metals are an important class of organometallic compounds which has been widely studied in the last forty years [1], [2]. A number of metal complexes have been prepared using different methods, and the reactivity mode of both the coordinate aryl group and the metal fragment investigated.

However, an interesting but unexplored reaction of the aryl complexes [M]–Ar (Ar = Ph, p-tolyl) may be the protonation with Brønsted acid, which may involve the coordinate aryl group giving the aromatic hydrocarbons. The formation of the arene molecule in the coordination sphere of the metal may favour its η2-coordination [3], [4] on the metal center giving the corresponding derivatives. Complexes containing an η2-arene ligand are known, and are generally prepared by reducing appropriate complexes in the presence of hydrocarbons [3], [4]. Protonation of a σ-bonded κ1-aryl complex may be an alternative synthetic method, which finds a support in the parallel reaction of metal hydride with Brønsted acid to prepare η2-H2 dihydrogen derivatives [5], [6].

We are interested in the synthesis and protonation of transition metal hydrides to obtain η2-H2 dihydrogen complexes and have reported several studies on manganese, rhenium, iron, ruthenium and platinum as a central metal [7], [8]. Now we have extended these studies to aryl complexes and their protonation reaction, with the aim of synthesising new σ-bonded κ1-aryl complexes and testing whether η2-coordination of an aromatic ring can take place. The results of these studies, which involve the synthesis and some reactivity of rare examples of aryl complexes of manganese and rhenium, are reported here.

Section snippets

General considerations and physical measurements

All synthetic work was carried out under an appropriate atmosphere (Ar, H2) using standard Schlenk techniques or a Vacuum Atmosphere dry-box. Once isolated, the complexes were found to be relatively stable in air, but were stored under an inert atmosphere at −25 °C. All solvents were dried over appropriate drying agents, degassed on a vacuum line and distilled into vacuum-tight storage flasks. The Re2(CO)10 was a Pressure Chem (USA) product, while Mn2(CO)10 an Aldrich one, both used as received.

Preparation of κ1-aryl derivatives

Aryl complexes of rhenium of the Re(κ1-Ar)(CO)nP5−n types were prepared by allowing hydride ReH(CO)nP5−n species to react first with triflic acid (HOTf) and then with an excess of LiAr, as shown in Scheme 1.

Also the Re(κ1-C6H5)(CO)3[Ph2PO(CH2)3OPPh2] (3d) complex, containing the bidentate phosphite ligand, was prepared from the reaction of the triflate complexes with phenyllithium, as shown in Scheme 2. Furthermore, related aryl complexes of manganese Mn(κ1-C6H5)(CO)3P2 can be prepared using

Conclusions

In this paper we report the synthesis of a series of κ1-aryl complexes of manganese and rhenium stabilized by carbonyl and phosphite ligands. The structural parameters for the Re(κ1-C6H5)(CO)3[Ph2PO(CH2)3OPPh2] derivative were also obtained. Studies on the protonation reactions of the aryl complexes with Brønsted acid highlighted the formation of free hydrocarbons and the unsaturated [M(CO)nP5−n]+ cations.

Acknowledgement

The financial support of MIUR (Rome)—Programmi di Ricerca Scientifica di Rilevante Interesse Nazionale, PRIN 2004—is gratefully acknowledged. We thank Daniela Baldan for technical assistance.

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