New Tripodal Tridentate Ligands with {NS2} Donor Set and a Backbone Hydroxo Anchor, and Their Copper(I) and Copper(II) Complexes

A series of five tripodal tridentate ligands (HO)C[(CH2)(x)NR2](CH2SR)(2) (HL1-HL5) with {NS2} donor set has been synthesized via an efficient two-step protocol starting from 1,3-dichloroacetone. All five ligands bear a backbone hydroxo group that is available for further functionalization. The five ligands differ by the type of N-donor (NR2 = pyridine-2-yl or imidazol-2-yl), by the spacer between the backbone-C and the N-donor heterocycle (x = 0, 1), and with respect to their topology, viz. whether two thioether-S are part of a cyclic 1,4-dithiacycloheptane fragment or not (R = Et or -CH2-). Copper(I) and copper(II) complexes have been prepared and studied in solution (ESI-MS, UV/Vis, and NMR spectroscopy in case of the copper(I) complexes) as well as in solid state (single-crystal X-ray diffraction of [(HL2)Cu(MeCN)](n)(OTf)(n), [(HL3)Cu(MeCN)](PF6) and [(HL5)Cu(MeCN)](n)(PF6) n as well as [(L-4)(2)Cu-3(MeCN)(3)](PF6)(2) and [(L-2)(2)Cu2Cl2], [(L-3)(4)Cu-4](OTf)(4), and [(L-4)(2)Cu-2(MeCN)(2)](OTf)(2); SQUID magnetometry of [(L-2)(2)Cu2Cl2] and [(L-3)(4)Cu-4](OTf)(4),). The ligand scaffolds show ambidentate character depending on the metal ion oxidation state. All copper(I) ions are found in distorted tetrahedral environment comprising an {NS2} ligation of the tridentate ligand as well as an exogenous MeCN; either mononuclear complex cations or 1D polymeric structures are found. In case of all copper(II) complexes the backbone-alcohol group is deprotonated and bridges two metal ions, giving dinuclear or tetranuclear species with antiferromagnetic coupling. The Cu-S bond lengths are found to vary significantly, which reflects the plasticity of the Cu-thioether bond known from metalloproteins and synthetic analogs.