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} (I) is fully characterized including its crystal structure, which features intermolecular π–π stacking, B···N interactions, and N–H···Cl hydrogen bonding. Treatment of I with Ph_3–nP­(SiMe₃)_n gave the corresponding Ph_3–nP­(B­{1,8-(NH)₂C₁₀H₆})_n, {L₁ (n = 1), L₂ (n = 2), and L₃ (n = 3)}. The crystal structures of L_1–3 reveal an increase in the planarity at P as a function of n, 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 cis-[Mo­(L_1–3)₂(CO)₄] (1–3) are prepared from [Mo­(nbd)­(CO)₄] (nbd = norbornadiene) and L_1–3. From the position of the ν­(CO) (A₁) band in the IR spectra of 1–3, it is deduced that the diazaborinyl substituent has a donating capacity similar to an alkyl group.