Palladium(II) ortho-cyano-aminothiophenolate (ocap) complexes.

A series of Pd(II) complexes containing ortho-cyano-aminothiophenolate (ocap) ligands have been prepared and their molecular structures elucidated. Hg(II) ocap complexes, [Hg{SC6H3XN(CN)}]n (X = H, Me) (1), react with Na2S to afford HgS and Na2[ocap] which reacts in situ with K2[PdCl4] to afford palladium ocap complexes [Pd{SC6H3XN(CN)}]n (2). A second route to these coordination polymers has also been developed from reactions of 2-aminobenzothiazole (abt) complexes, trans-PdCl2(abt)2 (3), with NaOH. We have not been able to crystallographically characterise coordination polymers 2, but addition of PPh3, a range of phosphines and cyclic diamines affords mono and binuclear complexes in which the ocap ligand adopts different coordination geometries. With PPh3, binuclear [Pd(µ-κ2,κ1-ocap)(PPh3)]2 (4) results, in which the ocap bridges the Pd2 centre acting as an S,N-chelate to one metal centre and binding the second via coordination of the cyanide nitrogen. In contrast, with diphosphines, Ph2P(CH2)nPPh2 (n = 1-4), mononuclear species predominate as shown in the molecular structures of Pd(κ2-ocap){κ2-Ph2P(CH2)nPPh2} (5-7; n = 1-3). With 2,2'-bipy and 1,10-phen we propose that related monomeric chelates Pd(κ2-ocap)(κ2-bipy) (9) and Pd(κ2-ocap)(κ2-phen) (10) result but we have been unable to substantiate this crystallographically. Addition of HgCl2(phen) to 9a (generated in situ) affords heterobimetallic Pd(κ2-phen)(µ-κ2,κ1-ocap)HgCl2(κ2-phen) (11), in which Hg(II) is coordinated through the ring sulfur.

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

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(CN)}]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(CN)}]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(CN)}]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 CN 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(CN)}(PPh3)]2 being dependant upon the nature of the arene-substituent, while addition of excess PPh3 affords mononuclear [Hg{SC6H3XN(CN)}(PPh3)2]. With dppm, binuclear [Hg{SC6H3XN(CN)}(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(CN)}(κ1,κ1-diphosphine)]n result in which ocap binds in a simple chelate fashion. Somewhat unexpectedly, with dppp, binuclear complexes [Hg2{SC6H3XN(CN)}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.

Formation of ortho-cyano-aminothiophenolate ligands with versatile binding modes via facile carbon-sulfur bond cleavage of 2-aminobenzothiazoles at mercury(II) centres.

Addition of 2-aminobenzothiazole and substituted derivatives to mercuric acetate in warm ethanol leads to the high yield formation of [Hg{SC6H3XN(C[triple bond, length as m-dash]N)}]n resulting from loss of hydrogen and sulfur-carbon bond cleavage. Addition of phosphines affords a series of complexes in which the new ortho-cyano-aminothiophenolate ligands adopt three different binding modes.

Bioinspired Hydrogenase Models: The Mixed-Valence Triiron Complex [Fe3(CO)7(µ-edt)2] and Phosphine Derivatives [Fe3(CO)7-x (PPh3) x (µ-edt)2] (x = 1, 2) and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2] as Proton Reduction Catalysts.

The mixed-valence triiron complexes [Fe3(CO)7-x (PPh3) x (µ-edt)2] (x = 0-2; edt = SCH2CH2S) and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2] (diphosphine = dppv, dppe, dppb, dppn) have been prepared and structurally characterized. All adopt an anti arrangement of the dithiolate bridges, and PPh3 substitution occurs at the apical positions of the outer iron atoms, while the diphosphine complexes exist only in the dibasal form in both the solid state and solution. The carbonyl on the central iron atom is semibridging, and this leads to a rotated structure between the bridged diiron center. IR studies reveal that all complexes are inert to protonation by HBF4·Et2O, but addition of acid to the pentacarbonyl complexes results in one-electron oxidation to yield the moderately stable cations [Fe3(CO)5(PPh3)2(µ-edt)2]+ and [Fe3(CO)5(κ2-diphosphine)(µ-edt)2]+, species which also result upon oxidation by [Cp2Fe][PF6]. The electrochemistry of the formally Fe(I)-Fe(II)-Fe(I) complexes has been investigated. Each undergoes a quasi-reversible oxidation, the potential of which is sensitive to phosphine substitution, generally occurring between 0.15 and 0.50 V, although [Fe3(CO)5(PPh3)2(µ-edt)2] is oxidized at -0.05 V. Reduction of all complexes is irreversible and is again sensitive to phosphine substitution, varying between -1.47 V for [Fe3(CO)7(µ-edt)2] and around -1.7 V for phosphine-substituted complexes. In their one-electron-reduced states, all complexes are catalysts for the reduction of protons to hydrogen, the catalytic overpotential being increased upon successive phosphine substitution. In comparison to the diiron complex [Fe2(CO)6(µ-edt)], [Fe3(CO)7(µ-edt)2] catalyzes proton reduction at 0.36 V less negative potentials. Electronic structure calculations have been carried out in order to fully elucidate the nature of the oxidation and reduction processes. In all complexes, the HOMO comprises an iron-iron bonding orbital localized between the two iron atoms not ligated by the semibridging carbonyl, while the LUMO is highly delocalized in nature and is antibonding between both pairs of iron atoms but also contains an antibonding dithiolate interaction.