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 PPh₃. The precursor chloroborane ClB­{1,8-(NH)₂C₁₀H₆} (<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_3–<i>n</i>P­(SiMe₃)_<i>n</i> gave the corresponding Ph_3–<i>n</i>P­(B­{1,8-(NH)₂C₁₀H₆})_<i>n</i>, {<b>L</b>_<b>1</b> (<i>n</i> = 1), <b>L</b>_<b>2</b> (<i>n</i> = 2), and <b>L</b>_<b>3</b> (<i>n</i> = 3)}. The crystal structures of <b>L</b>_{<b>1</b>–<b>3</b>} 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)₂C₁₀H₆} 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 BN₂-containing ring is slightly antiaromatic. The complexes <i>cis-</i>[Mo­(<b>L</b>_{<b>1</b>–<b>3</b>})₂(CO)₄] (<b>1</b>–<b>3</b>) are prepared from [Mo­(nbd)­(CO)₄] (nbd = norbornadiene) and <b>L</b>_{<b>1</b>–<b>3</b>}. From the position of the ν­(CO) (<i>A</i>₁) 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