the 1.92-1.96 A range corresponding to a bond order of unity'" and falls between the 1.859(3) A seen in the monomeric phosphinoborane Mes2BPPh2, 2,12' and the 1.82-1.83 A in the phosphinideneborate species [Mes2BPMesIQ, 3, and [Li(Et20)Mes2BPR], 4 (R=Ph, C6H1,, or Me~).[~l The monomer 2, on the basis of its structure, has a significant interaction between the phosphorus lone pair and the empty boron p orbital, while the species 3 and 4 are thought to have a large amount of double-bond character. The B3P3 ring has thus at least three aromatic characteristics; ( I ) planarity, (2) equal BP bond lengths, (3) shortened BP bonds. It is also significant that the electronegativities of B and P are almost identical so that the B3P3 array should be much less polar than the corresponding B3N3 compound. Unfortunately, in the absence of comparative data, neither 'H-, I3C-, "B-, nor 3'P-NMR spectroscopy could provide evidence supporting the presence of a ring current.
3'P-and "B-NMR data are consistent with the X-ray structure. The "P-NMR shift (6= 51.85) of the singlet seen for 1 appears at much lower field than that of Mes2BPPh2 (2, 6= 30.2) or M ~s B ( P P ~, ) ~, [ ~] (6= -0.02) suggesting less electron density at P, consistent with lone-pair delocalization. The broad singlet at 6=52.6 in the "B-NMR spectrum is very close to the value in 2 (6=51.7). It is, however, further downfield than that found for M ~S B ( P P ~, ) ~ (6 = 30. I), the closest compound available for comparison. In contrast, borazines generally absorb at higher (A6 = 10 ppm) field than monoaminoboranes whereas they absorb at similar fields to diaminoboranes.[I0' Efforts to synthesize other derivatives related to 1 involving different substituents and heavier congeners are in progress.
Experimental Procedure
1 : MesBBr2 [I I] (3.55 g, 12 mmol) in hexane (20 mL) was added to a solution of C6HIIPHLi (from C6H,,PH2 (2.86g, 25mmol) and nBuLi in hexane (15.5 mL of a 1.6 M solution)) in E t 2 0 (80 mL) at 0°C. Stirring for 12 h at ambient temperature afforded a yellow solution. Removal of volatiles under reduced pressure and extraction into hexane at 50°C gave, after filtration and concentration under reduced pressure and cooling to O"C, pale yellow crystals of the title compound, 1. Yield: 1.45 g, 48%; m.p. >275"C.