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)

Abstract

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.