Regioselective [3+2] Cycloaddition Reactions of the Phosphorus and Arsenic Analogues of the Thiocyanate Anion

Salt metathesis reactions of the tris-amide zirconium iodide complex ([NRR’]3ZrI (1-I) (with NRR’ = 3,5-Xylyl-tert-butylamide, N(Xyl)(tBu)) and sodium 2-phosphaethynthiolate (Na(diox)3SCP) or 2-arsaethynthiolate (Na(diox)3SCAs) in THF result in the formal [3+2] cycloaddition of the two SCE (E = P, As) units forming novel five membered 2-thio-1,3,4-thiadiphosphole and 2-thio-1,3,4-thiadiarsole heterocyclic ligands with an exocyclic sulfur atom, bridging two zirconium fragments with the general formula (N(RR’)3Zr(-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. Switching the solvent to toluene, salt metathesis with NaSCP results in the selective formation of the other 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. Quantum chemical investigations to elucidate the mechanism and solvent dependency suggest a concerted ring formation (in line with a [3+2] cycloaddition reaction) with 2-SP and 3-AsAs being the thermodynamic products of the reaction.