Coordination chemistry in and of sulfur dioxide

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Abstract

The review describes the syntheses of metal salts with weakly interacting anions (AlCl4 −, AsF6 −, SbF6 −) in the poorly coordinating solvent SO2. The metal centers in the resulting complexes might be considered as almost ‘naked’, their unusual coordination chemistry is discussed.

Introduction

One topical field of research in fluorine chemistry is the synthesis of large, weakly interacting anions. In combination with poorly coordinating solvents, routes to almost ‘naked’, highly reactive cations are made possible by this approach.

An ideal combination of poorly interacting anions, weakly coordinating solvents and ready preparative availability is the system AF6 −SO2Mn+ (A=As, Sb), introduced into synthetic chemistry by Gillespie and his group [1], [2]. The versatility of the oxidizing properties of the combinations AsF5/SO2 and SbF5/SO2 was demonstrated in the reaction with mercury. Alchemists’ dreams seemed to have been realized with the most exciting result, [Hg2.86(AsF6)]n (Fig. 1) [3], [4], where liquid ‘quick silver’ was transformed into gold, and thrilled not only the scientific community, as reports in newspapers and journals documented. Depending on the stoichiometry, a series of polyatomic mercury cations was isolated [5]. Although first reports on the oxidation of chalcogens and other non-metal systems by AsF5/SO2 and SbF5/SO2 date back 30 years and extensive review articles have been published by Gillespie and Passmore [6], [7], [8], many areas of this field are still unexploited.

Metal chemistry is not restricted to mercury, Gillespie’s school extended it to Groups 11, 12, 14 and transition metals [9], [10]. The possibilities of ‘naked’ metal centers in coordination chemistry have been demonstrated by Dean [11], our group [12] and by Roesky [13], [14].

Weakly coordinating fluoroanions (AsF6 −, SbF6 −, [(SbF5)nF], etc.) are readily available and, in combination with other suitable solvents, e.g. HF and FSO3H, will lead to the formation and stabilization of unique metal-, non-metal and organometallic cations. ‘Naked’ metal cations, generated in the superacidic systems AF5/HF or AF5/HSO3F will coordinate even weak σ-donors, e.g. CO to give unprecedented highly-charged carbonyl cations even of the noble metals [15], [16]. However, due to the aggressive nature of the solvent-system no broad application in coordination chemistry is possible.

General considerations for the construction of ideal anions for this purpose have been recently discussed in detail: large size; low charge, uniformly distributed over the surface of the anion; strong element-fluorine bonds in the anion to prevent F transfer [17], [18]. The newest development in this direction seems to be multiteflate anions e.g. [Sb(OTeF5)6] [19] and the older [B(OTeF5)4] [20] or [Nb(OTeF5)6] and [Ta(OTeF5)6] [21]. The ultimate and almost unexploited development in this direction is the very recent generation of the [C2B10(CF3)12] anion [22].

This paper is restricted to the coordination chemistry of ‘naked’ metal cations Mn+ in and of SO2. Earlier general discussions of the solvent properties of SO2 are available [23], [24], [25]. Reports on the ligand properties of SO2 in organometallic chemistry, the chemistry of organometallic SO2 complexes [26], [27] and the behavior of organometallic complexes towards SO2 [28] can be found in the literature.

Section snippets

Syntheses of metal hexafluoroantimonates, hexafluoroarsenates and tetrachloroaluminates in liquid SO2

The most simple and straightforward method for the preparation of metal salts with weakly interacting anions should be the reactions of metal fluorides and chlorides with strong fluoro and chloro Lewis acids:A:MFn+nAF5SO2MSO2xAF6nA=As,SbMCln+nAlCl3SO2MSO2xAlCl4nThe success of this method is dependent upon the donor properties of the metal halides, and on the halide acceptor strength of the halo Lewis acid. According to the acid strength scale recently reported by Christe et al. [29], the most

Properties of the SO2 ligand

The bonding situation and the properties of the SO2 ligand in organometallic compounds have been discussed in detail [26], [82], [83]. η1-S or η2-SO coordination is exclusively observed by interaction of the soft metal centers with the 4a1-HOMO and/or 2b1-LUMO of the SO2 ligand. For a long time the only examples for O-coordination of the SO2 ligand were SbF5·OSO [79] and CH3OSO+ [84], [85], but the present paper shows that hard and borderline metal centers can also act in the same way. The

Coordination chemistry in liquid SO2

According to the present results, the coordination chemistry of metal hexafluoroarsenates, and of hexafluoroantimonates, is almost unrestricted, even such weak σ-donors as CO and PF3 could be introduced as ligands [88]. Since the metal centers are practically ‘naked’ almost no activation barriers have to be overcome; reactions proceed rapidly indeed at low temperatures. Thus, even extremely poor donors, thermally labile species and reactive intermediates might be introduced as ligands and

Summary and outlook

Since Gillespie’s first report on the oxidation of mercury by AsF5 in liquid SO2 [2], coordination chemistry in this solvent has developed into an independent research area.

Metal ions in the Mn+/SO2/AF6 − (A=As, Sb) system are almost ‘naked’ and this review article shows that even extremely weak donor ligands, thermally unstable ligands, multidentate heterocycles etc. may be added to the metal centers. Although several recent reports on ‘better’ anions for the generation of ‘naked’ cations have

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    Dedicated to Professor R.J. Gillespie on the occasion of his 75th birthday.

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