Synthesis and Chemistry of Dithiadiazole Free Radicals [4-(4'-C5H4N)CN2S2] and [4(3'-5H4N)CN2S2] X-ray crystal structures of Pd3 -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→μSNC(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.