Synthesis, structure and reactivity with phosphines of Hg(ii) ortho-cyano-aminothiophenolate complexes formed via C–S bond cleavage and dehydrogenation of 2-aminobenzothiazoles

Abstract

Addition of 2-aminobenzothiazole (abt) and substituted derivatives to Hg(OAc)₂ leads to the high yield formation of ortho-cyano-aminothiophenolate (ocap) complexes [Hg{SC₆H₃XN(CN)}]_n (X = H, Me, Cl, Br, NO₂) resulting from dehydrogenation and C–S bond cleavage. The reaction appears to be unique to Hg(OAc)₂ and with HgCl₂ the product [HgCl₂(abt)]_n contains an intact abt ligand, but reacts with acetate to afford the ocap complex [Hg{SC₆H₄N(CN)}]_n. Binding of abt to Hg(II) has previously been probed in molecular structures of [Hg(sac)₂(abt)L] (L = MeOH, DMSO) and these have been reexamined to understand the perturbation of abt upon coordination. When the reaction of abt and Hg(OAc)₂ was carried out at low temperatures the intermediate [Hg(κ²-OAc)(EtOH)(μ-HNCNSC₆H₄)]₂ was isolated resulting from a single ligand deprotonation thus allowing a mechanism for ring-opening to be proposed. Reactions of [Hg{SC₆H₃XN(CN)}]_n with mono- and bidentate phosphines have been studied, affording a series of complexes in which the ocap ligands adopt four different binding modes in the solid state, as shown by a number of crystallographic studies. In all, the ligand chelates to a single mercury centre but spans to the second via either: (i) a simple S,N-chelate, (ii) coordination through nitrogen of the CN group, (iii) the sulfur acting as a thiolate-bridge, (iv) both thiolate bridging and cyanide coordination. With PPh₃ two different binding modes are seen in complexes [Hg{SC₆H₃XN(CN)}(PPh₃)]₂ being dependant upon the nature of the arene-substituent, while addition of excess PPh₃ affords mononuclear [Hg{SC₆H₃XN(CN)}(PPh₃)₂]. With dppm, binuclear [Hg{SC₆H₃XN(CN)}(κ¹-dppm)]₂ result in which the diphosphine binds in a monodentate fashion. With the more flexible diphosphines, dppe and dppb, coordination polymers [Hg{SC₆H₃XN(CN)}(κ¹,κ¹-diphosphine)]_n result in which ocap binds in a simple chelate fashion. Somewhat unexpectedly, with dppp, binuclear complexes [Hg₂{SC₆H₃XN(CN)}₂(μ,κ¹,κ¹-dppp)] result in which two diphosphines bridge the Hg₂ centre, while with dppf mononuclear chelates are proposed to result. Thus, the simple and high-yielding ring-opening of 2-aminobenzothiazole and substituted derivatives by mercuric acetate provides ready access to a range of novel ortho-cyano-aminothiophenolate complexes, being shown to be a highly versatile ligand that can adopt a number of different coordination modes.