Optical and EPR Detection of a Triplet Ground State Phenyl Nitrenium Ion

Nitrenium ions are important reactive intermediates participating in synthetic chemistry and biological processes. Phenyl nitrenium ions (Ph-NH+) typically have closed-shell singlet ground states with large singlet–triplet energy gaps, while little is known of triplet nitrenium ions regarding their reactivity, lifetimes, spectroscopic signatures, and electronic configurations. In this work, m-pyrrolidinyl-phenyl hydrazine hydrochloride (1) is synthesized as the photoprecursor to photochemically generate the corresponding m-pyrrolidinyl-phenyl nitrenium ion (2), which is computed to adopt a π,π* triplet ground state. A combination of femtosecond (fs-) and nanosecond (ns-) transient absorption (TA) spectroscopy, cryogenic continuous-wave electronic paramagnetic resonance (CW-EPR) spectroscopy, computational analysis, and photoproduct studies, elucidated the complete photolysis pathway of this photoprecursor and offers the first direct experimental detection of a ground state triplet nitrenium ion. Upon photoexcitation, 1 is optically pumped to singlet excited states, followed by internal conversion (IC) to S1 on the sub-picosecond timescale, where bond heterolysis occurs and the NH3 leaving group is extruded in 1.8 ps, generating a vibrationally-hot, spin-conserving closed-shell singlet phenyl nitrenium ion (12) that undergoes vibrational cooling in 19 ps. Subsequent intersystem crossing (ISC) takes place in 534 ps, yielding the ground state triplet phenyl nitrenium ion (32), with a lifetime of 0.8 μs. Unlike electrophilic singlet phenyl nitrenium ions, this triplet phenyl nitrenium reacts through sequential H atom abstractions, resulting in the eventual formation of the reduced m-pyrrolidinyl-aniline as the predominant stable photoproduct. Supporting the triplet ground state, continuous irradiation of 1 in a glassy matrix at 80 K forms a paramagnetic species consistent with the triplet nitrenium ion by cryogenic CW-EPR spectroscopy.