Divergent\nReactivity of Azides and Diazoalkanes Toward\nFerrous Complexes and Isolation of a Fe<sup>III</sup> Carbene Radical\nComplex

Treatment of [(<sup><i>t</i>Bu</sup>pyrr<sub>2</sub>pyr)Fe(OEt<sub>2</sub>)] (<b>1</b>-OEt<sub>2</sub>)\n(<sup><i>t</i>Bu</sup>pyrr<sub>2</sub>pyr<sup>2–</sup> = 3,5-<sup><i>t</i></sup>Bu<sub>2</sub>-bis(pyrrolyl)pyridine)\nwith trimethylsilyl\nazide (N<sub>3</sub>SiMe<sub>3</sub>) and subsequent photolysis at\n390 nm results in clean formation of [(<sup><i>t</i>Bu</sup>pyrrpyrpyrrNHSiMe<sub>3</sub>)Fe] (<b>2</b>) as the result\nof a nitrene being inserted into a <i>tert</i>-butyl C–H\nbond of the <sup><i>t</i>Bu</sup>pyrr<sub>2</sub>pyr ligand.\nTreatment of <b>1</b>-OEt<sub>2</sub> with azidotrimethyltin\n(N<sub>3</sub>SnMe<sub>3</sub>), however, results in isolation of\nthe γ-bound azide adduct ferrous complex, [(<sup><i>t</i>Bu</sup>pyrr<sub>2</sub>pyr)Fe(N<sub>3</sub>SnMe<sub>3</sub>)] (<b>1</b>-N<sub>3</sub>SnMe<sub>3</sub>). When treated with diphenyl\ndiazomethane (Ph<sub>2</sub>CN<sub>2</sub>), complex <b>1</b>-OEt<sub>2</sub> converts to the iron carbene complex [(<sup><i>t</i>Bu</sup>pyrr<sub>2</sub>pyr)FeCPh<sub>2</sub>] (<b>1</b>-CPh<sub>2</sub>), and the X-ray structure revealed a Fe–CPh<sub>2</sub> bond length of 1.964(3) Å. A room temperature magnetic\nmoment of <b>1</b>-CPh<sub>2</sub> indicates an <i>S</i> = 2 spin state, consistent with a high-spin Fe<sup>III</sup> center\nantiferromagnetically coupled to a carbene radical anion (CPh<sub>2</sub><sup>•–</sup>). Zero-field <sup>57</sup>Fe Mössbauer\nspectroscopy and Fe K-edge X-ray absorption spectroscopy confirm this\nassignment. In solution, complex <b>1</b>-CPh<sub>2</sub> rearranges\nto [{<sup><i>t</i>Bu</sup>pyrrpyrC(CPh<sub>2</sub>)-C(CMe<sub>3</sub>)CH–C(CMe<sub>3</sub>)N}Fe]<sub>2</sub> (<b>3</b>) resulting from carbene insertion into the\n1-position of the pyrrolide arm of the <sup><i>t</i>Bu</sup>pyrr<sub>2</sub>pyr ligand and dimerization. Complex <b>3</b> possesses two high-spin Fe<sup>II</sup> centers according to <sup>57</sup>Fe Mössbauer spectroscopy, with antiferromagnetic\ncoupling between the spin centers. Monitoring the conversion of <b>1</b>-CPh<sub>2</sub> to <b>3</b> by UV–vis spectroscopy\nreveals this process to be first order in <b>1</b>-CPh<sub>2</sub> with a highly ordered transition state evidenced by activation parameters:\nΔ<i>S</i><sup>‡</sup> = −87.6 ±\n25.8 J·mol<sup>–1</sup>·K<sup>–1</sup> and\nΔ<i>H</i><sup>‡</sup> = 63.1 ± 8.3 kJ·mol<sup>–1</sup>.