Cyclolinear Oligo- and Poly(iminoborane)s: The Missing Link in Inorganic Main-Group Macromolecular Chemistry

The reaction of n-C₈ H₁₇ B[N(Me)SiMe₃ ]₂ (1) with n-C₈ H₁₇ BCl₂ (2 a) yielded, instead of a linear poly(iminoborane), the aminoborane n-C₈ H₁₇ B(Cl)N(Me)SiMe₃ (4) and after cyclotrimerization the borazine cyclo-(n-C₈ H₁₇ BNMe)₃ (6). Side reactions that result in borazine formation were effectively suppressed if 1,3-bis(trimethylsilyl)-1,3,2-diazaborolidines 7 were employed as co-monomers in combination with dichloro- or dibromoboranes 2 or 8, respectively. Silicon/boron exchange polycondensation led to oligo(iminoborane)s 11 a,b,ac,d. Alternative synthetic routes to such species involve Sn/B exchange of 1,3-bis(trimethylstannyl)-2-n-octyl-1,3,2-diazaborolidine (16) and n-C₈ H₁₇ BBr₂ (8 a), and the initiated polycondensation of the dormant monomer 14 in the presence of a Brønsted acid (HCl, HOTf, or HNTf₂ ; Tf=trifluoromethylsulfonyl). Although an attempt to obtain an oligo-/poly(iminoborane) with phenyl side groups yielded only insoluble material, the incorporation of aryl groups was proven for a derivative with both phenyl and n-octyl boron substituents (11 ac), as well as for a derivative with 4-n-butylphenyl side groups (11 d). The highest-molecular-weight sample obtained was 11 ac. Featuring about 18 catenated BN units, on average, this is the closest approach to a poly(iminoborane) known.