SMALL HETEROBORANE CLUSTER SYSTEMS .2. PREPARATION OF PHOSPHABORANE SYSTEMS FROM THE REACTION OF SMALL BORANE CAGES WITH LOW-COORDINATE PHOSPHORUS-COMPOUNDS - REACTION CHEMISTRY OF PHOSPHAALKENES WITH PENTABORANE(9)

The reaction of (Me3Si)P = C(R)(OSiMe3) (where R = tert-butyl (1a) or adamantyl (1b) with 1 equiv of neutral pentaborane(9), B5H9, under mild conditions produces the synthetically versatile, small bridging phosphaboranes [mu-((R)(Me3SiO)HCP(SiMe3))B5H8] (2a,b) in excellent yields. Two possible mechanisms for the formation of 2a and 2b by this reaction are supported by both experimental and MNDO semiempirical theoretical data. These relatively thermal and air-stable compounds are also stable with respect to the elimination of Me3SiOSiMe3. They are quantitatively converted, however, to [mu-((R)(Me3SiO)CHP(X))B5H8] (where X = H (3a,b) and X = D (4a,b)) by electrophilic substitution reactions with water, D2O, or alcohol. Compound 2a is readily bridge-deprotonated by the action of NaH to produce the corresponding anion, [mu-((tert-butyl)(Me3SiO)CHP(SiMe3))B5H7]- (5a), while 2b is unreactive under similar conditions. Compound 5a was found to be unreactive toward metal cations in complex formation. Compound 3a slowly loses H-2 on standing at room temperature to form the bridged P = C system, [mu-((tert-butyl)(Me3SiO)C = P)B5H8] (6a). Compound 3a is readily cage-deprotonated to form the corresponding anion, [mu-((tert-butyl)-(Me3SiO)CHP(H))B5H7]- (7a), which, when reacted with metal halides, forms metallaphosphaborane complexes. Data from MNDO calculations for compounds 2a, 3a, 5a, and 6a show linear relationships between the calculated charge on the bridgehead boron atoms and both the phosphorus-apical boron bond distance and the bridgehead basal boron bond centroid-phosphorus-carbon bond angle. These trends have been rationalized by using semiempirical molecular orbital considerations. Characterization of the new compounds was by H-1, B-11, C-13, and P-31 NMR, infrared, mass spectral, and elemental analyses.