Synthesis, Structure, Spectroscopy, and Reactivity of Azapentadienyl–Cobalt–Phosphine Complexes

We report the synthesis, structure, spectroscopy, and reactivity of the first examples of azapentadienyl–cobalt complexes. Treatment of (Cl)­Co­(PMe₃)₃ with potassium <i>tert</i>-butylazapentadienide produces ((1,2,3-η³)-5-<i>tert</i>-butylazapentadienyl)­Co­(PMe₃)₃ (<b>1</b>), a dark red solid. Compound <b>1</b> undergoes ligand substitution reactions with trimethyl phosphite and carbon monoxide, producing ((1,2,3-η³)-5-<i>tert</i>-butylazapentadienyl)­Co­(PMe₃)₂[P­(OMe)₃] (<b>2</b>) and ((1,2,3-η³)-5-<i>tert</i>-butylazapentadienyl)­Co­(PMe₃)₂(CO) (<b>3</b>), respectively. Compounds <b>1</b>–<b>3</b> react with 1 equiv of triflic acid (HO₃SCF₃) at the azapentadienyl nitrogen, yielding the corresponding η⁴-(<i>tert</i>-butylamino)­butadiene–cobalt complexes (<b>4</b>–<b>6</b>). These cations have unusually long Co–C4 bond distances, indicating that η³ resonance structures are contributing to the overall bonding picture. Treatment of compound <b>4</b> with P­(OMe)₃ or CO leads to displacement of (<i>tert</i>-butylamino)­butadiene (rather than PMe₃ ligand exchange) and formation of Co­(PMe₃)₂[P­(OMe)₃]₃⁺O₃SCF₃^– (<b>7</b>) or Co­(PMe₃)₃(CO)₂⁺O₃SCF₃^– (<b>8</b>), respectively. Compound <b>5</b> reacts readily with a another 1 equiv of triflic acid, again at the nitrogen center. The dicationic product, <b>9</b>, possesses an η⁴-(<i>tert</i>-butylammonium)­butadiene ligand with a strong Co–C4 interaction. The X-ray crystal structures of compounds <b>1</b>–<b>4</b>, <b>6</b>, <b>7</b>, and <b>9</b> have been obtained