Synthesis, spectroscopy, fluorescence and crystal structures of cyclopalladated dinuclear complexes with C, N-chelating azobenzenes and N, S-bridging heterocyclic-2-thiolates

Reaction of palladium(II) chloride with azobenzene (azb) in acetonitrile-dimethyl formamide mixture followed by the addition of pyridine-2-thione (pySH) yielded dark reddish brown compound [Pd2(κ2-μ-N, S-pyS)2(κ2-C, N-azb)2] 1. Similarly, dark reddish brown compound [Pd2(κ2-μ-N, S-pymS)2(κ2-C, N-azb)2] 2 incorporating pyrimidine-2-thiolate (pymS−) was obtained. The p-methoxyazobenzene (p-mazb) has formed similar complexes, [Pd2(κ2-μ-N, S-pyS)2(κ2-C, N-p-mazb)2] 3 and [Pd2(κ2-μ-N, S-pymS)2(κ2-C, N-p-mazb)2] 4. The method of preparation of 3 and 4 involved reaction of pyridine-2-thione (3)/pyrimidine-2-thione (4) with orange [Pd(κ2-C, N-p-mazb)(κ2-μ-Cl)]2 precursor suspended in dichloromethane followed by the addition of Et3N base. Complexes crystallized in triclinic (1, P-1) and monoclinic (P21/n, 2; C2/c, 3 and 4) systems. Complex 1 exists as a polymorph to a reported structure and has two crystallographically independent dimeric molecules in the asymmetric unit unlike one molecule each in 2–4. Both the thio-ligands act as N, S-bridging in the dimers prepared while the azb and p-mazb are C, N-chelating. It is the p-methoxyphenyl ring of p-mazb which ortho-palladates in 3 and 4, unlike the phenyl ring palladation in 1 and 2. The metal–metal separation were relatively short in complexes 2 {2.851(1) Å} and 4 {2.854(1) Å} having pymS− bridging anions and were long in 1 {2.898(2) Å} and 3 {2.882(1) Å} complexes having pyS− bridging anions.