FP(μ-N)2S: A Sulfur-Pnictogen Four-Membered Ring with 6π Electrons

The new 6 & pi;-electron four-membered ring compound 3-fluoro-1 & lambda;(2),2,4,3 & lambda;(3)-thiadiazaphosphetidine, FP(& mu;-N)(2)S, has been generated in the gas phase through high-vacuum flash pyrolysis (HVFP) of thiophosphoryl diazide, FP(S)(N-3)(2), at 1000 K. Subsequent isolation of FP(& mu;-N)(2)S in cryogenic matrices (Ar, Ne, and N-2) allows its characterization with matrix-isolation IR and UV-vis spectroscopy by combination with N-15-isotope labeling and computations at the CCSD(T)-F12a/VTZ-F12 level of theory. Upon visible-light irradiation at 550 nm, this cyclic compound undergoes ring-opening to the thiazyl isomer FPNSN, followed by dissociation to FP and SN2 under subsequent UV-irradiation at 365 nm. In sharp contrast to the square planar structure for the isolobal four-membered ring S2N2, a puckered structure with significant biradical character has been found for FP(& mu;-N)(2)S.