Synthesis and Chemistry of Dithiadiazole Free Radicals [4-(4′-C5H4N)CN2S2] and [4-(3′-C5H4N)CN2S2]; X-ray Crystal Structures of [Pd3{μ-SNC(Ar′)NS–S,S′}2(PPh3)4] (Ar′ = 4′-C5H4N, 4′-C5H4NBEt3 and 3′-C5H4NBEt3)

Two new 1,2,3,5-dithiadiazoles, [4-(4′-C₅H₄N)CN₂S₂] (L¹) and [4-(3′-C₅H₄N)CN₂S₂] (L²), with different pyridyl groups at the 4-position were prepared. As Lewis bases, the dithiadiazoles reacted with Lewis acids via their pyridyl groups to form acid-base adducts with retention of the five-membered dithiadiazole ring. Ligands L¹ and L² reacted with excess triethylborane to give L¹BEt₃ and L²BEt₃ (1a and 1b) and with Mn(CO)₅Br to give the fac-Mn(CO)₃Br(L¹)₂ (2a) and fac-Mn(CO)₃Br(L²)₂ (2b) complexes. When reacted with Pd⁰ complexes, namely, Pd(PPh₃)₄ and a mixture of [Pd₂(dba)₃] and dppf, the dithiadiazoles underwent ring-opening reaction by reductive cleavage of the S–S bond of the dithiadiazole ring. The resulting formation of Pd–S bonds gave the corresponding trinuclear complexes [Pd₃{μ-SNC(4′-C₅H₄N)NS-S,S′}₂(PPh₃)₄] (3a), [Pd₃{μ-SNC(3′-C₅H₄N)NS-S,S′}₂(PPh₃)₄] (3b), and [Pd₃{μ-SNC(4′-C₅H₄N)NS-S,S′}₂(dppf)₂] (4) [dba = dibenzylideneacetone, dppf = 1,1′-bis(diphenylphosphanyl)ferrocene]. Complexes 3a and 3b reacted with excess triethylborane to give the corresponding acid-base adducts [Pd₃{μ-SNC(4′-C₅H₄NBEt₃)NS-S,S′}₂(PPh₃)₄] (5a) and [Pd₃{μ-SNC(3′-C₅H₄NBEt₃)NS-S,S′}₂(PPh₃)₄] (5b.) Complex 3a reacted with Mn(CO)₅Br to give {[Pd₃{μ-SNC(4′-C₅H₄N)NS-S,S′}₂(PPh₃)₄][MnBr(CO)₃]}_n (6). The structures of 3a, 5a, and 5b were established by X-ray crystallography.