Expansion of the scope of alkylboryl-bridged N → B-ladder boranes: new substituents and alternative substrates.

A series of new boranes capable of forming intramolecular N -> B-heterocycles has been prepared and their properties have been studied by electrochemical methods and UV-vis-spectroscopy complemented by DFT calculations. A dimethylborane (BMe2), haloborane derivatives (BBr2, BF2, BI2) and mixed cyano/isocyano-borane (B(CN)(NC)) have been prepared by different techniques. Furthermore, 2 '-alkynyl-substituted 2-phenylpyridines bearing terminal tert-butyl- and trimethylsilyl-groups are introduced as a new class of substrates for hydroboration. Successful hydroboration with either 9H-borabicyclo[3.3.1]-nonane (9H-BBN), dimesitylborane (Mes(2)B-H), or Piers' borane ((C6F5)(2)B-H, BPF-H) furnished new pi-extended boranes capable of forming intramolecular six- or seven-membered N -> B-heterocycles (tBuBBN, SiBPF), and, in the case of Mes(2)BH, formation of a sterically crowded styrylborane (SiBMes(2)) incapable of intramolecular N -> B-coordination was observed. All the boranes listed above except BMe2 have been structurally characterized, and a study of their electrochemical properties showed that the systematic variation of the substituents on boron allows for the incremental variation of the electron affinity of the phenylpyridine-model system over a total range of >0.7 eV between alkylboranes (BMe2, BBN) and B(CN)(NC). B(CN)(NC) shows the strongest N -> B-bond (approximate to 175 kJ mol(-1)), and highest electron-affinity observed so far, and is the first example of a borane bearing an isocyano-substituent on boron.