Tetrazole-Functionalized Organoboranes Exhibiting Dynamic Intramolecular N→B-Coordination and Cyanide-Selective Anion Binding.

Starting from two different cyano-functionalized organoboranes, we demonstrate that 1,3-dipolar [3+2] azide-nitrile cycloaddition can serve to generate libraries of alkyl-tetrazole-functionalized compounds capable of intramolecular N→B-Lewis adduct formation. Due to the relatively low basicity of tetrazoles, structures can be generated that exhibit weak and labile N→B-coordination. The reaction furnishes 1- and 2-alkylated regio-isomers that exhibit different effective Lewis-acidities at the boron centers, and vary in their optical absorption and fluorescence properties. Indeed, we identified derivatives capable of selectively binding cyanide over fluoride, as confirmed by 11B NMR. This finding demonstrates the potentialities of this synthetic strategy to systematically fine-tune the properties of lead structures that are of interest as chemical sensors.

Preparation of Structurally and Electronically Diverse N → B-Ladder Boranes by [2 + 2 + 2] Cycloaddition.

We report the synthesis of a series of eight N → B-ladder boranes through cobalt-mediated cyclotrimerization of (2-cyanophenyl)-dimesitylborane with different dialkynes. The resulting tetracoordinate boranes show variable electrochemical and optical properties depending on the substitution pattern in the backbone of the coordinating pyridine-derivatives. While boranes containing alkyl-substituted pyridines show lower electron affinities than the known parent compound, boranes featuring π-extended pyridine derivatives show higher electron affinities in the range of acceptor substituted triarylboranes. All derivatives show larger Stokes shifts (8790-6920 cm-1) compared to the N → B-ladder borane coordinated by an unsubstituted pyridine.

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 (Mes2B-H), or Piers' borane ((C6F5)2B-H, BPF-H) furnished new π-extended boranes capable of forming intramolecular six- or seven-membered N → B-heterocycles (tBuBBN, SiBPF), and, in the case of Mes2BH, formation of a sterically crowded styrylborane (SiBMes2) 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 (≈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.