Doris C. Pestana

Multiple bonding, π-bonding contributions and aromatic character in isoelectronic boron-phosphorus, boron-arsenic, aluminum-nitrogen and zinc-sulfur compounds

Variable temperature 1H NMR studies of various boron-phosphorus (B-P) and boron-arsenic (B-As) compounds have afforded data on the strength of B-P and B-As x- bonds. For (RPBMes2)- and (RAsBMeszl- ions the energy of the x-bond can be as high as 24 kcal mol-1 and 21 kcal mol-1, respectively. Inversion barriers near 10 kcal mol-1 were determined for the pnictide center in simple monomeric phosphino or arsino boranes. The effects of substituent size and type on the B-P or B-As multiple bonds were also investigated. Compounds with extended linear arrays of up to five boron or phosphorus atoms were also synthesized and characterized both structurally and spectroscopically. In addition, the aromaticity of six-membered rings comprised of (B- Ph arrays was investigated. Structural spectroscopic and theoretical data support the view that the unsaturated B3P3 ring system has greater aromatic character than borazine. The range of quasi aromatic rings was also extended to include the isoelectronic and isolobal rings comprised of the arrays (Al-N)3 (alumazene) (:GeN)3, and (ZnS)3. The bonding and the relative aromatic character of these rings is also discussed.

Structural Characterization and Coordination Behavior of 1,2-Diphosphino and 1,2-Diarsino-Diborane(4) Compounds

Reaction of either LiPPh2 or LiAsMes2 with the diborane(4) compound (Me2N) BrBBBr(NMe2) affords the compounds (Me2N)(Ph2P)BB(PPh2)(NMe2), 1, and (Me2N) (Mes2As)BB(AsMes2)(NMe2)·0.5 PhMe, 2·0.5 PhMe (Mes = 2,4,6-Me3C6H2-). Both 1 and 2 feature normal B–B single bond lengths; however, the empty boron p-orbitals display strong π-interactions with the dimethylamino substituents. The B–P and B–As distances and the pyramidal phosphorus and arsenic geometries are consistent with single bonding. Furthermore, the stereochemical activity of the phosphorus and arsenic lone pairs is demonstrated by the ready formation of the complex cis-[Cr(CO)4(Me2N)(Ph2P)BB(PPh2) (NMe2)]·C6H6, 3·C6H6, by treatment of 1 with [Cr(CO)4(2,5-norbornadiene)]. Compounds 1–3 were characterized by X-ray crystallography, 11B and 31P NMR spectroscopy, and by IR spectroscopy in the case of 3.