Mono‑, Di‑, and Triborylphosphine Analogues of Triarylphosphines

Diazaborinylphosphines based on the 1,8-diaminonaphthylboronamide heterocycle are prepared by a chlorosilane-elimination reaction, and their structural and bonding properties are compared to those of PPh<sub>3</sub>. The precursor chloroborane ClB­{1,8-(NH)<sub>2</sub>C<sub>10</sub>H<sub>6</sub>} (<b>I</b>) is fully characterized including its crystal structure, which features intermolecular π–π stacking, B···N interactions, and N–H···Cl hydrogen bonding. Treatment of <b>I</b> with Ph<sub>3–<i>n</i></sub>P­(SiMe<sub>3</sub>)<sub><i>n</i></sub> gave the corresponding Ph<sub>3–<i>n</i></sub>P­(B­{1,8-(NH)<sub>2</sub>C<sub>10</sub>H<sub>6</sub>})<sub><i>n</i></sub>, {<b>L</b><sub><b>1</b></sub> (<i>n</i> = 1), <b>L</b><sub><b>2</b></sub> (<i>n</i> = 2), and <b>L</b><sub><b>3</b></sub> (<i>n</i> = 3)}. The crystal structures of <b>L</b><sub><b>1</b>–<b>3</b></sub> reveal an increase in the planarity at P as a function of <i>n</i>, and the steric bulk of the diazaborinyl substituent B­{1,8-(NH)<sub>2</sub>C<sub>10</sub>H<sub>6</sub>} is similar to that of a phenyl. Nucleus-independent chemical shift calculations were carried out that suggest that the 14 π-electron diazaborinyl substituent can be described as aromatic overall, though the BN<sub>2</sub>-containing ring is slightly antiaromatic. The complexes <i>cis-</i>[Mo­(<b>L</b><sub><b>1</b>–<b>3</b></sub>)<sub>2</sub>(CO)<sub>4</sub>] (<b>1</b>–<b>3</b>) are prepared from [Mo­(nbd)­(CO)<sub>4</sub>] (nbd = norbornadiene) and <b>L</b><sub><b>1</b>–<b>3</b></sub>. From the position of the ν­(CO) (<i>A</i><sub>1</sub>) band in the IR spectra of <b>1</b>–<b>3</b>, it is deduced that the diazaborinyl substituent has a donating capacity similar to an alkyl group.