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.