Regioselective [3 + 2] cycloaddition reactions of the phosphorus and arsenic analogues of the thiocyanate anion

Salt metathesis reactions of tris-amide zirconium iodide complex ([NRR′]$_3$ZrI (1-I) (with NRR′ = 3,5-Xylyl-tert-butylamide, N(Xyl)(tBu)) and sodium 2-phosphaethynthiolate (Na(diox)$_3$SCP) or 2-arsaethynthiolate (Na(diox)$_3$SCAs) in THF result in a [3 + 2] cycloaddition of two SCE (E = P, As) units forming five membered 2-thio-1,3,4-thiadiphosphole and 2-thio-1,3,4-thiadiarsole heterocycles with an exocyclic sulfur atom, bridging two zirconium fragments (general formula (N(RR′)$_3$Zr(κ-C,S-(SCE)$_2$-Zr(NRR′)$_3$ with E = P (2-PP) or As (3-AsAs)). The reactions are regioselective and only the P,P/As,As isomers are formed in THF and quantum chemical investigations suggest a concerted ring formation in line with a [3 + 2] cycloaddition reaction. Switching the solvent to toluene, salt metathesis with NaSCP results in the selective formation of the second possible [3 + 2] cycloaddition regioisomer 2-SP, with a 3-thio-1,2,4-thiadiphosphole bridge, while for NaSCAs both regioisomers with an As–As bond (3-AsAs) and an S–As bond (3-SAs; 3-thio-1,2,4-thiadiarsole bridge) are observed. Attempts to obtain the free diphosphole/diarsole rings using methyl triflate resulted in the cleavae of only one zirconium center, giving access to the triflate complex 1-OTf and the mono-metallated diphospholes/diarsoles 4-PP, 4-PS and 5-AsAs respectively.