Copper 5-sulfoisophthalato quasi-planar squares in coordination polymers modulated by alkaline-earth metals: a way to molecular squares?

Abstract

Two copper-sip coordination polymers (sip³⁻ = 5-sulfoisophthalic anion), [Ca(H₂O)₃Cu₂(H₂O)₆(sip)₂]·2H₂O 2a, and [Sr(H₂O)₄Cu₂(H₂O)₆(sip)₂]·H₂O 2b, have been synthesized at room temperature. Single-crystal X-ray diffraction reveals that the structures of complexes 2a and 2b are 2-D coordination layer structures, containing [Cu₄(sip)₄] molecular squares linked by calcium (2a) or strontium (2b) bridges. The coordination layers display 3,6-connected topologies if the [Cu₄(sip)₄] molecular squares and alkaline-earth metals are considered as nodes, or (4,4) nets if alkaline-earth metals are considered to be part of the [Cu₄(sip)₄] squares. In comparison with the more commonly encountered Cu-sip zigzag chain subunits in copper-sip coordination polymers, the [Cu₄(sip)₄] square subunits in 2a and 2b represent similar coordination fashions and connecting modes while creating singular examples of new subunits in coordination complexes of the copper-sip system. Variable temperature magnetic susceptibility of 2a indicated that antiferromagnetic interactions dominate between the Cu(II) ions bridged by sip³⁻ ligands.