Synthesis and structural characterization of Mo2(μ-SC6H4–Cl-p)2(μ-dppm)(CO)6 and [(dppm)(CO)2Mo(μ-SC6H4–Cl-p)2Mo(O)]2(μ-SC6H4–Cl-p)2(μ-O) and electrochemical property of Mo2(μ-SC6H4–Cl-p)2(μ-dppm)(CO)6