The first 1,3,2-diazabora-[3]ferrocenophanes - molecular structures and dynamic behaviour in solution

Dilithiated 1,1^′-bis(trimethylsilylamino)ferrocene (1) reacts with aminoboron dihalides (2) X₂B-N(R^′)R X=Br, R^′=R=Et (2a); X=Cl, R^′=Me, R=CH₂Ph (2b), X=Cl, R^′=Et, R=Ph (2c)] to give 2-amino-1,3,2-diazabora-3]ferrocenophanes (3a-c) for the first time. The steric constraints exerted by the 3]ferrocenophane unit and the presence of the N-SiMe₃ groups cause rather different B-N bonding situations in these tri(amino)boranes. The boron atom has the choice between three nitrogen atoms for BN(pp)π bonding: in the cases of 3a and 3b, it prefers the NEt₂ and the N(Me)CH₂Ph group, respectively, over the N-SiMe₃ groups, whereas in 3c the N(Et)Ph group appears to be the weaker π-donor. This can be concluded from the X-ray structural analyses carried out for 3a and 3c, and from the low temperature ¹H, ¹³C, and ²⁹Si NMR spectra of 3a-3c.