Hemocyanin Models: Synthesis, Structure, and Magnetic Properties of a Binucleating Copper(II) System

Article abstract of DOI:10.1021/ja00329a021

A model compound approach to oxidized hemocyanin is described.The essential features of diamagnetism, 3.6-Angstroem copper-copper separation, approximate tetragonal stereochemistry, imidazole ligation, and bridging groups are reproduced in the azide complex (Cu2(L-Et)(N3))(BF4)2.HL-Et is the septadentate binucleating ligand N,N,N',N'-tetrakis(2-(1-ethylbenzimidazolyl))-2-hydroxy-1,3-diaminopropane prepared in good yield via condensation of 1,2-diaminobenzene with 2-hydroxy-1,3-diaminopropanetetraacetic acid.Related complexes (Cu2(L-Et)X)²⁺ where X^-=OAc^-, HCOO^-, NO2^-, and pyrazolate have quite different magnetic properties.The acetate is ferromagnetic (J ca. +12 1/cm) while the nitrite is antiferromagnetic (J=-139 1/cm).Crystal structures of the weakly coupled acetate complex (Cu2(L-Et)(OAc))(ClO4)2 (monoclinic, P2₁/n, a=14.190(3) Angstroem, b=22.707(4) Angstroem, c=15.883(4) Angstroem, β=97.20(2) deg, Z=4) and the diamagnetic azide complex (Cu2(L-Et)(N3))(BF4)2 (monoclinic, C2/m, a=19.082(3) Angstroem, b=23.896(3) Angstroem, c=13.230(2) Angstroem, β=116.21(1) deg, Z=4) suggest that these differences have their origin in the orientation of the ? magnetic orbitals with respect to the bridging ligands.The particular mode of 1,3-bridging azide in (Cu2(L-Et)(N3))²⁺, where it is supported by an alkoxide bridge from L-Et, has not been previously observed.It allows close approach of the copper(II) atoms (3.615(3) Angstroem) and thus becomes aviable stereochemistry for metazidohemocyanin.The present studies support the idea that tetragonally of the copper(II) stereochemistry is important for attaining diamagnetism in oxy- and methemocyanins and that alkoxide from serine or threonine would be well suited to the role of a bridging endogenous ligand.