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

The reaction of n‐C8H17B[N(Me)SiMe3]2 (1) with n‐C8H17BCl2 (2 a) yielded, instead of a linear poly(iminoborane), the aminoborane n‐C8H17B(Cl)N(Me)SiMe3 (4) and after cyclotrimerization the borazine cyclo‐(n‐C8H17BNMe)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‐C8H17BBr2 (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. Drawing a line under polymerization: Well‐defined oligo(iminoborane)s with up to 18 catenated BN units, on average, were prepared by Si/B or Sn/B exchange polycondensation and initiated polymerization of a dormant monomer (see scheme). The cyclolinear backbone imparts high stability and precludes the unwanted formation of borazine byproducts. The properties of the new materials are effectively tuned by variation of their side groups.