Isolable Phosphanylidene Phosphorane with a Sterically Accessible Two-Coordinate Phosphorus Atom**

Phosphanylidene-s-phosphoranes (RP=PR’3) are phosphorus analogues of alkylidene-s-phosphoranes (R2C=PR’3), better known as Wittig reagents. Phosphanylidene-s-phosphoranes are synthetically accessible in the free form (RP= PR’3) and also in the transition-metal-stabilized form (LnM !P(R)=PR’3). [1] The latter complexes are commonly used in both P=C bond generation and as a source of the phosphinidene (R-P) moiety in the continuing pursuit of new terminal phosphinidene complexes R-P=MLn. In marked contrast, free phosphanylidene-s-phosphoranes have received little attention because isolable (i.e., thermally stable) examples remain rare. Herein we report the synthesis and structure of the stable cyclic phosphanylidenes-phosphorane 3 (see Scheme 1). Compound 3 possesses a sterically nonhindered phosphanylidene moiety, yet it is thermally stable enough to be isolated and stored at room temperature. The potential diversity of the coordination chemistry of 3 is illustrated by its transition-metal (Pd) complex and bis(borane) adduct. Recently, we synthesized the first “bottleable” (i.e., room temperature stable) phosphine–phosphine donor–acceptor (DA) complex 1. Because of their normally low thermal stability, the reactivity of phosphine–phosphine complexes (other than thermal decomposition pathways) is virtually unknown. Having access to 1, we set off to investigate its chemistry in detail. We were especially intrigued by the possibility of utilizing 1 as a precursor for compounds with a low-coordinate phosphorus atom in the peri position, since no compounds of this type had been reported in the literature. We postulated that the specific peri arrangement of the relatively basic PiPr2 group and the reactive dihalophosphine group should make 1 a good source of “across the peri gap” donor-stabilized, low-valent phosphorus species. The initial reactivity screen of 1 towards reducing reagents revealed that the reaction with BH3·Me2S proceeded cleanly, giving the bis(borane) adduct 2 (dP= 13.6 and 43.9 ppm, JPP= 198.5 Hz) in almost quantitative yield (Scheme 1). Although 2 does not possess a low-coordinate phosphorus center, the single-crystal X-ray structure (Figure 1) confirmed an interesting bonding situation in which a Lewisbase-stabilized phosphinidene moiety acts as a double donor towards two Lewis-acidic (borane) moieties. Two representative resonance forms of 2 (zwitterionic and with DA bonding) are shown in Scheme 2. The P1 P2 distance in 2 (2.2208(11) ), is consistent with a P P single bond. The P2 B1 and P2 B2 distances [1.943(5) and 1.943(4) , respectively] are equal and are also as expected for P B single bonds (typical range 1.90 to 1.95 ). As mentioned recently by Protasiewicz and co-workers, whilst of fundamental interest, no bis(borane) push-pull phosphinidene systems have been structurally characterized and compound 2 thus Scheme 1. Synthesis and selected reactivity of the phosphanylidene phosphorane 3.