Bonding along a Linear B···N···B Triad

The synthesis of tris(2,6-dihydroxyphenyl)amine diborate, 4, is reported. This compound contains
a linear B···N···B array for which a symmetrical three-center two-electron (3c-2e) bond is possible.
The X-ray crystal structure of 4 shows that 3c-2e bonding is, in fact, absent. Rather, the B−N−B
array of 4 is unsymmetrical, having a 2c-2e B−N dative bond with the remaining boron
pyramidalized outward and bonded to the oxygen of THF, i.e., 4·THF. In THF solution, 4 displays
temperature-dependent 13C NMR spectra from which a ΔG⧧ of 11.6 kcal/mol at 262 K may be
calculated. The dynamic process observed in solution corresponds to a bond-switching equilibrium
in which the B−N bond oscillates between the two borons (“bell clapper”). Ab initio calculations
indicate that the most likely pathway for the bond switch does not involve a 3c-2e B···N···B bond,
but rather occurs by nucleophilic attack of THF on the datively bonded boron to generate 4·(THF)2,
lacking any B−N interactions, followed by loss of one THF. The B−N−B system of 4 sans the
perturbing effect of solvent was also investigated computationally. The form of 4 containing a
3c-2e bond is found to be a transition state in the solvent-free bond-switch reaction of 4, lying 2.66
kcal/mol above 4. The stability of three-center bonds to in-line distortion (viz., X···Y···X →
X−Y·········X) is discussed from the point of view of the second-order Jahn−Teller effect.