Synthetic and Structural Investigations of Monomeric Dilithium Boraamidinates and Bidentate NBNCN Ligands with Bulky N-Bonded Groups

The dilithiated boraamidinate complexes [Li2{PhB(NDipp)2}(THF)3] (7a) (Dipp = 2,6-diisopropylphenyl) and
[Li2{PhB(NDipp)(NtBu)}(OEt2)2] (7b), prepared by reaction of PhB[N(H)Dipp][N(H)R‘] (6a, R‘ = Dipp; 6b, R‘ =
tBu) with 2 equiv of nBuLi, are shown by X-ray crystallography to have monomeric structures with two terminal and
one bridging THF ligands (7a) or two terminal OEt2 ligands (7b). The derivative 7a is used to prepare the spirocyclic
group 13 derivative [Li(OEt2)4][In{PhB(NDipp)2}2] (8a) that is shown by an X-ray structural analysis to be a solvent-separated ion pair. The monoamino derivative PhBCl[N(H)Dipp] (9a), obtained by the reaction of PhBCl2 with
2 equiv of DippNH2, serves as a precursor for the synthesis of the four-membered BNCN ring {[R‘ ‘‘N(H)](Ph)B(μ-NtBu)2CnBu} (10a, R‘ ‘‘ = Dipp). The X-ray structures of 6a, 9a, and 10a have been determined. The related
derivative 10b (R‘ ‘‘ = tBu) was synthesized by the reaction of {Cl(Ph)B(μ-NtBu)2CnBu} with Li[N(H)tBu] and
characterized by 1H, 11B, and 13C NMR spectra. In contrast to 10a and 10b, NMR spectroscopic data indicate that
the derivatives {[DippN(H)](Ph)B(NR‘)2CR(NR‘)} (11a:  R = tBu, R‘ = Cy; 11b:  R = nBu, R‘ = Dipp) adopt
acyclic structures with three-coordinate boron atoms. Monolithiation of 10a produces the novel hybrid boraamidinate/amidinate (bamam) ligand {Li[DippN]PhB(NtBu)CnBu(NtBu)} (12a).