Synthesis of Phosphorus(V)-Stabilized Geminal Dianions. The Cases of Mixed PX/P→BH 3 (X = S, O) and PS/SiMe 3 Derivatives Hadrien Heuclin, † Marie Fustier-Boutignon, † Samuel Ying-Fu Ho, †,‡ Xavier-Fre ́ de ́ ric Le Goff, † Sophie Carenco, † Cheuk-Wai So,* ,‡ and Nicolas Me ́ zailles* ,†,§ † Laboratoire “Hé té roe ́ le ́ ments et Coordination”, Ecole Polytechnique, CNRS, Route de Saclay, 91128 Palaiseau Cedex, France ‡ Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore * S Supporting Information ABSTRACT: The monodeprotonation of [CH 2 (PPh 2 → BH 3 )(PPh 2 E)] (E = S (6), O (7)) afforded [CH(PPh 2 → BH 3 )(PPh 2 E)] - (E = S (6 ‑ ), O (7 - )), whose structures were confirmed by X-ray crystallography. The kinetics of the second deprotonation appeared to be crucial in efficient synthesis of the corresponding dianions. Thus, the double deprotonation of 6 only led to 6 2- ; the analogous reaction with 7 was slower and resulted only in the partial formation of 7 2- . Double deprotonation of the compound [CH 2 (SiMe 3 )- (PPh 2 S)] (8) also resulted in the partial formation of [C(SiMe 3 )(PPh 2 S)] 2‑ (8 2- ), whose structure was confirmed by X-ray crystallography. The rare monomeric Mg carbene compound [MgC(PPh 2 →BH 3 )(PPh 2 S)] (9) was obtained by the reaction of 6 with Mg(nBu) 2 . The X-ray structure of 9 is presented. ■ INTRODUCTION The groundbreaking syntheses of electrophilic carbene complexes by Fischer in 1964 1 and of nucleophilic carbene complexes by Schrock roughly 10 years after 2 have opened the way for a considerable number of studies. 3 The use of carbene complexes in organic synthesis, in stoichiometric as well as in catalytic processes, was then developed extensively. In particular, among the many processes involving carbene complexes as catalysts, the alkene metathesis reaction has seen a tremendous development over the past decades, leading to applications in various fields ranging from polymer science to total synthesis. 4 The almost infinite variations of both the substitution scheme of the carbene fragment “CR 1 R 2 ” and the metal fragment allow for a very fine tuning of the properties of the carbene complex, ranging from nucleophilic to electrophilic. In the past decade, a novel strategy relying on the use of geminal dianions to bring formally the four electrons of the MC bond has been devised (Scheme 1). This chemistry has been mainly developed with three geminal dilithiated compounds for which the carbon atom is substituted symmetrically by an iminophosphorane (PPh 2 NSiMe 3 ), a thiophosphinoyl (PPh 2 S), or a phosphonate moiety (P(OiPr) 2 O) (1 a 2- , 2 2- , and 3 2- ; Scheme 2). 5,6 These species have allowed the synthesis of a large variety of carbene complexes of transition metals, rare earths, and uranium. 7 In 2006, Le Floch et al. developed a general method allowing the introduction of other substituents at the nitrogen atom of the iminophosphorane moiety (1 b‑e 2- ) 8,9 and subsequently proved the influence of the nitrogen substituent in a catalytic process involving Nd carbene complexes. 10 In 2006 and 2008, the Henderson group published the synthesis of geminal dianions incorporating other alkali metals, 11,12 and in 2009, Harder and co-workers reported the synthesis of a bis(cesium) derivative of 1d. 13 This strategy is currently limited because of (i) the very small number of known geminal dianions (A-E; Scheme 2) and (ii) a lack of an efficient access to them (1 2- to 4 2- ) as, needless to say, these dianionic species are extremely water sensitive and cannot be readily purified. In this respect, it is noteworthy that double deprotonation of the ligand in the coordination sphere of the metal has been used in some instances as an alternative strategy to yield the same carbene complexes. 14 The underlying reason for the paucity of geminal dianions lies in the required efficient stabilization of two charges on the same carbon atom. We have shown using DFT calculations in the thiophosphinoyl case, 2 2- , that these two lone pairs interact in donor-acceptor type interactions with empty antibonding orbitals of appropriate energy and symmetry. Upon coordina- tion of the dianions with metal fragments, an electron transfer from the carbon center to the metal center occurs to lead to a formal MC interaction. The extent of this electron transfer depends on two factors: (i) the energy match between the orbitals of the dianion and those of the metal fragment and (ii) the orbital overlap. The electron transfer to the metal center is obviously in competition with the electron transfer from the Received: October 16, 2012 Published: January 11, 2013 Article pubs.acs.org/Organometallics © 2013 American Chemical Society 498 dx.doi.org/10.1021/om300954a | Organometallics 2013, 32, 498-508