Covalent Linkage of the Type-2 and Type-3 Structural Mimics to Model the Active Site Structure of Multicopper Oxidases: Synthesis and Magneto- Structural Properties of Two Angular Trinuclear Copper(II) Complexes

Two new angular trinuclear copper(II) complexes of formulation [Cu3(HL)LL‘](ClO4), where L‘ is imidazole (Him, 1)
or 1-methylimidazole (1-MeIm, 2) and H3L is a Schiff base obtained from the condensation of salicylaldehyde and
1,3-diaminopropan-2-ol (2:1 mole ratio), are prepared from a reaction of [Cu2L(μ-Br)] and [Cu(HL)] in the presence
of L‘ and isolated as perchlorate salts. The crystal structures of 1 and 2 consist of a trinuclear copper(II) unit
formed by the covalent linkage of monomeric type-2 mimic and dimeric type-3 mimic precursor complexes to give
an angular arrangement of the metal atoms in the core which is a model for the active site structure of blue
multicopper oxidases. In 1 and 2, the coordination geometry of two terminal copper atoms is distorted square-planar. The central copper has a distorted square-pyramidal (4 + 1) geometry. The mean Cu···Cu distance is
∼3.3 Å. The complex has a diphenoxo-bridged dicopper(II) unit with the phenoxo oxygen atoms showing a planar
geometry. In addition, the complex has an endogenous alkoxo-bridged dicopper(II) unit showing a pyramidal geometry
for the oxygen atom. The 1:1 electrolytic complexes show a d−d band at 607 nm. Cyclic voltammetry of the
complexes in MeCN containing 0.1 M TBAP using a glassy carbon working electrode displays a Cu3(II)/Cu2(II)Cu(I)
couple near −1.0 V (vs SCE). The variable temperature magnetic susceptibility measurements in the range 300−18 K show antiferromagnetic coupling in the complexes giving magnetic moments of ∼3.0 μB at 300 K and ∼2.1
μB at 18 K for the tricopper(II) unit. The experimental susceptibility data are theoretically fitted using a model with
Heisenberg spin-1/2 Hamiltonian for a trimer of spin-1/2 copper(II) ions having two exchange parameters involving
the alkoxo-bridged dicopper(II) (J1) and the diphenoxo-bridged dicopper(II) (J2) units, giving J1 and J2 values of
−82.7, −73 cm-1 for 1 and −98.3, −46.1 cm-1 for 2, respectively. The structural features indicate a higher magnitude
of anitiferromagnetic coupling in the alkoxo-bridged unit based on the greater value of the Cu−O−Cu angle in
comparison to the diphenoxo-bridged unit. The core structures of 1 and 2 compare well with the first generation
model complexes for the active site structure of multicopper oxidases in the oxidized form. The crystal structure of
1 exhibits a lamellar structure with a gap of ∼7 Å containing water molecules in the interlamellar space. Complex
2 forms a hexanuclear species due to intermolecular hydrogen bonding interactions involving two trimeric units.
The crystal packing diagram of 2 displays formation of a three-dimensional framework with cavities containing the
perchlorate anions.