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 Ph3–nP­(SiMe3)n gave
the corresponding Ph3–nP­(B­{1,8-(NH)2C10H6})n, {L1 (n =
1), L2 (n =
2), and L3 (n = 3)}. The crystal structures of L1–3 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(L1–3)2(CO)4] (1–3) are prepared from [Mo­(nbd)­(CO)4] (nbd = norbornadiene)
and L1–3.
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.