Acylamino boranes

Amides are derivatives of carboxylic acids, so that their coordination behavior to boranes might be similar to that of their parent compounds. B-NMR spectroscopic studies have shown that compounds 31 and 32 are monomeric species in solution, while compounds 33 and 34 with the more Lewis acidic 9-borabicyclo[3.3.1]nonyl unit form aggregates that may be dimeric, oligomeric, or polymeric. The grade of association could not be determined by mass spectrometric analyses, because in all cases only the monomer is liberated into the gas phase [65]. 

The solid state structure of 33 reveals a dimeric molecule with an eight-membered B2C2N2O2 heterocyclic ring in a chair conformation. The N—B and B—O bond lengths of 1.585(2) and 1.541(2) A, respectively, as well as the C—N and C—O bond lengths of 1.294(3) and 1.285(3) A, respectively, indicate delocalization of the Ti-electron density in the acylamino moiety [65]. 

During the study of the reaction products between six different co-lactams and triethylborane, monomeric (for 36, 39), dimeric (for 38), tetrameric (for 40) and oligomeric (for 35) species have been detected in solution by cryoscopic methods. For 37 [X = -CH2)5-] only lateral reaction products could be observed (Fig. 14) [66]. 

Some similar bimetallic acylamino complexes are also known with transition metal ions, e.g., with vanadium(II) [67], palladium(II) [68], and especially platinum(II) [69]. In the Cambridge Structural Database [39] only one trimetallic structure is found in which three iron(II) ions are bridged by a total number of six acylamino ligands [70]. 

---