The mononuclear–dinuclear dance: Twisting the backbone in metalloligands operates a coordination switch

The copper(II) complex [Cu{(R,R)-1}] in which (R)-H21 is 1,6-bis(3-ethoxy-2-hydroxyphenyl)-(3R,4R)-(−)-cyclohexane-1,2-diyl-2,5-diazahexa-1,5-diene possesses an O4-donor cavity that can bind Pb2+, Cd2+ and Eu3+. The single crystal structures of [Cu(OH2){(R,R)-1}Pb(ONO2)2], {[Cu{(R,R)-1}Cd(ONO2)(OH2)2][NO3].MeOH}.[Cu{(R,R)-1}] and [Cu{(R,R)-1}Eu(O2NO)3] are presented. The co-crystallization of [Cu{(R,R)-1}Cd(ONO2)(OH2)2][NO3] and [Cu{(R,R)-1}] appears to be driven by hydrogen-bonded host–guest interactions between each axial water ligand in [Cu{(R,R)-1}Cd(ONO2)(OH2)2]+ with the O4-domain of [Cu{(R,R)-1}]. When the ligand scaffold is changed from cyclohexane-1,2-diyl to 1,1′-binaphthyl to give (R)-H22, the N2O2-cavity is unable to bind copper(II) in its preferred square planar environment. The single crystal structure of [Zn{(R)-2}] confirms the presence of tetrahedral zinc(II). As a result, the spatial properties of the ethoxy arms in [Zn{(R)-2}] and [Cu{(R)-2}] are not suited to the facile formation of dimetallic complexes.

Graphical abstractThe O4-domain in [Cu{(R,R)-1}] ((R)-H21 = 1,6-bis(3-ethoxy-2-hydroxyphenyl)-(3R,4R)-(−)-cyclohexane-1,2-diyl-2,5-diazahexa-1,5-diene) binds Pb2+, Cd2+ and Eu3+; switching the ligand scaffold from cyclohexane-1,2-diyl to 1,1′-binaphthyl results in the N2O2-cavity being more suited to a tetrahedral rather than square planar metal ion, and leaves the O4-domain poorly suited to binding a metal ion.Figure optionsDownload full-size imageDownload as PowerPoint slide