Insertions of Cyclic and Acyclic Germanium and Tin Heterocarbenoids into Phosphorus-Chlorine Bonds: Syntheses, Structures, and Reactivities Joseph K. West and Lothar Stahl* Department of Chemistry, University of North Dakota, Grand Forks, North Dakota 58202-9024 United States * S Supporting Information ABSTRACT: Chloro(organo)phosphines are important pre- cursors to diphosphenes and cyclic oligophosphines. Although chloro(organo)phosphines are commonly reduced with bulk metals (e.g., Na, Mg, and Zn), these reactions are much more selective when done homogeneously. To test whether group 14 heterocarbenoid reductions yield isolable insertion pro- ducts, the mono- and dichloro(organo)phosphines PhPCl 2 (A), Ph 2 PCl (B), t BuPCl 2 (C), t Bu 2 PCl (D), and bis(dichloro- phosphino)methane (PCl 2 ) 2 CH 2 (E) were treated with the cyclic Me 2 Si(μ-N t Bu) 2 El (El = Ge (1), Sn (2)) and the acyclic [(Me 3 Si) 2 N] 2 El (El = Ge (3), Sn (4)) heterocarbenoids. The sterically least-encumbered phosphines were more reactive, while the cyclic stannylene 2 was the most reactive and the acyclic germylene 3 was the least reactive. All but one of the products were either mono- or diinsertion compounds, the lone exception being a distannane derived from 2. Semikinetic and structural data suggest that the tetravalent group 14 compounds are formed via an insertion mechanism whose rate depends on the steric bulk of the reaction partners and the nature of the group 14 elements. 1. INTRODUCTION Phosphorus is sometimes referred to as the “carbon copy” on account of the somewhat similar chemical properties that exist between these two elements. 1 This diagonal relationship, however, is less well developed than that between members of the earlier groups of the periodic table: cf. lithium/ magnesium or beryllium/aluminum. Unlike carbon, phosphorus(III) has a lone pair of electrons that renders it Lewis basic and makes it susceptible to redox reactions. The group 15 element does, however, have an undeniable tendency to form phosphorus-phosphorus single and double bonds that mirror certain aspects of organic chemistry. Phosphorus- phosphorus bonds, 2 just like carbon-carbon bonds, can be generated by the reduction of the corresponding halides. A variety of reducing agentsmainly alkali metals, magnesium, and zinchave been used for this purpose, although the nature of the metal does seem to matter in the outcomes of the reductions. 3-7 In contrast to alkyl halide reductions, where species such as R-Mg-X and R-Zn-X are formed, 8 no such intermediates are obtained for bulk metal reductions of the more reactive halophosphines. The isolation of oxidative addition products in chlorophosphine reduction requires both milder reducing agents and milder conditions. As long ago as 1983 Bertrand and Veith showed that the cyclic stannylene Me 2 Si(μ- t BuN) 2 Sn reduces dichlorophosphines to diphosphenes under extremely mild conditions, Me 2 Si(μ- t BuN) 2 - SnCl 2 being the only oxidation product. 9 The subsequent isolation of [Me 2 Si(μ- t BuN) 2 Ge(Cl)] 3 P, 10 upon treatment of PCl 3 with Me 2 Si(μ- t BuN) 2 Ge, provided clear evidence that such reductions proceed with the intermediacy of insertion products. This conclusion was supported by the work of du Mont et al., who proved that even GeCl 2 can insert into the P-Cl bonds of chloro(organo)phosphines. 11-13 Despite these well-documented cases of group 14 hetero- carbenoid insertions into P-Cl bonds, there have been no systematic investigations into these reactions. This dearth of interest is surprising, because although phosphorus-halogen bond transformations lack the general utility of carbon- halogen bond transformations, there are numerous applications for such reactions. For example, the making of phosphorus- phosphorus bonds by milder and hence more selective routes is important in the syntheses of diphosphenes and oligophos- phines. Detailed studies of heterocarbenoid insertions into P-Cl bonds may also provide insight into the selective generation of the long-sought phosphinidenes, 14-16 which are, incidentally, valence isoelectronic with germylenes and stannylenes. Prior to our investigations no stannylene insertion product into a phosphorus-halogen bond had ever been isolated; they had merely been identified by NMR spectroscopy. A few of the more stable germylene insertion products had been charac- terized by in situ 1 H and 31 P NMR techniques, 17 but only two of these (vide supra) had been structurally characterized by X-ray techniques. 10,12 Herein we report on insertions of cyclic and acyclic germanium and tin heterocarbenoids into the phosphorus-chlorine bonds of chloro(organo)phosphines. The emphasis in these studies was on the structures of the products, as well as the relative reactivities of the hetero- carbenoids and the phosphine substrates. It will be seen that P-Cl reductions with homologous heterocarbenoids did not Received: January 8, 2012 Published: February 22, 2012 Article pubs.acs.org/Organometallics © 2012 American Chemical Society 2042 dx.doi.org/10.1021/om3000168 | Organometallics 2012, 31, 2042-2052