The application of a sulfonyl-substituted dilithio methandiide in the synthesis of carbene complexes was examined. In all cases, the metal carbon interaction was found to be highly polar with only small π-contribution. Hence, the stability of these complexes was found to greatly rely on the coordination ability of the side-arms supporting the metal carbon interaction. As such, the sulfonyl moiety allowed the isolation of a carbene complex with the oxophilic zirconium, which is the first of its kind bearing no (bis)phosphonium ligand framework. On the contrary, complexes with the late transition metals ruthenium and palladium were found to be more labile due to the facile decoordination of the sulfonyl moiety. This results in the opening of a reactive coordination site at the metal center and hence in further reactions such as cyclometalation or sulfur transfer from the thiophosphoryl moiety to the carbenic carbon atom.
■ INTRODUCTIONTransition metal complexes with a metal carbon double bond have found wide-ranging applications in various catalytic and stoichiometric transformations. 1 The reactivity of these compounds was found to be strongly influenced by the substituents bound to the carbenic carbon atom, thus leading to their classification as Fischer-type carbene and Schrock-type alkylidene complexes. 2 However, in recent years carbene complexes derived from geminal dilithiated compounds have gained special interest, as they seem to contradict this general classification pattern. 3 Here, the metal carbon bond is formally formed by a four-electron donation from the ligand to the metal. 4 Yet, the bonding situation in these systems was found to vary strongly depending on the nature of the metal and its coligands. Thus, the metal carbon interaction in these complexes covers a range of bonding situations, starting from a kind of masked methandiide with mainly electrostatic M 2+ ··· C 2− interactions (A), to ylidic complexes with a carbanionic ligand, M + −C − (B), and complexes with a real MC double bond (C) (Figure 1). 5 These flexible bonding properties allowed the isolation of a series of carbene complexes incorporating early and late transition metals, but also lanthanides and actinides. 6 However, systematic studies on the bonding situation are strongly restricted, due to the limited number of dianionic starting compounds. 7 In particular, the impact of the α-substituents at the carbenic carbon atom on the metal carbon bond and the complex stability is virtually unexplored. So far, bis(phosphonium)-stabilized dianions, above all bis-(thiophosphoryl) and bis(iminophosphoryl) methandiides, have been the only dianionic ligand systems applied in this carbene chemistry. Supported by the pincer-type coordination mode, these methandiides allowed the isolation of quite unusual carbene-like complexes with metals in various oxidation states as reported by the groups of Cavell, Liddle, Le Floch, Meźailles, Harder, and others. Among others are the first cerium(IV)−carbon multiple bond, 8 uranium-carbene complexes with ...