Nitrile Imines: Matrix Isolation, IR Spectra, Structures, and Rearrangement to Carbodiimides

Abstract

The structures and reactivities of nitrile imines are subjects of continuing debate. Several nitrile imines were generated photochemically or thermally and investigated by IR spectroscopy in Ar matrices at cryogenic temperatures (Ph-CNN-H <b>6</b>, Ph-CNN-CH₃ <b>17</b>, Ph-CNN-SiMe₃ <b>23</b>, Ph-CNN-Ph <b>29</b>, Ph₃C-CNN-CPh₃ <b>34</b>, and the boryl-CNN-boryl derivative <b>39</b>). The effect of substituents on the structures and IR absorptions of nitrile imines was investigated computationally at the B3LYP/6-31G* level. IR spectra were analyzed in terms of calculated anharmonic vibrational spectra and were generally in very good agreement with the calculated spectra. Infrared spectra were found to reflect the structures of nitrile imines accurately. Nitrile imines with IR absorptions above 2200 cm^–1 have essentially propargylic structures, possessing a CN triple bond (typically PhCNNSiMe₃ <b>23</b>, PhCNNPh <b>29</b>, and boryl-CNN-boryl <b>39</b>). Nitrile imines with IR absorptions below ca. 2200 cm^–1 are more likely to be allenic (e.g., HCNNH <b>1</b>, PhCNNH <b>6</b>, HCNNPh <b>43</b>, PhCNNCH₃ <b>17</b>, and Ph₃C-CNN-CPh₃ <b>34</b>). All nitrile imines isomerize to the corresponding carbodiimides both thermally and photochemically. Monosubstituted carbodiimides isomerize thermally to the corresponding cyanamides (e.g., Ph-NCN-H <b>5</b> → Ph-NH-CN <b>8</b>), which are therefore the thermal end products for nitrile imines of the types RCNNH and HCNNR. This tautomerization is reversible under flash vacuum thermolysis conditions