First-Row Transition-Metal Complexes of the 1-Methoxycyclobutenedionate(1−) Ion

The synthesis and characterization by single-crystal X-ray crystallography of a series of monomeric first-row
transition-metal complexes with the 1-methoxycyclobutenedionate(1−) ligand are described. The isomorphous
compounds [M(CH3OC4O3)2(H2O)4] (M = Mn, Co, Ni, Zn) are C2 symmetric and crystallize in the monoclinic
space group C2/c. The metal atom in each of these complexes is six-coordinate with two cis 1-methoxycyclobutenedionate(1−) ligands, the methoxy substituent being oriented cis with respect to the ligating oxygen
atom. The remaining coordination sites are filled by four aqua ligands. Monomers are linked by O−H···O hydrogen
bonds to form arrays of stepped tapes. Hydrolysis of the methoxy group on the ligand occurs during the formation
of the copper complex and {[Cu(C4O4)(H2O)2]·0.25H2O}n is produced. This complex crystallizes in the tetragonal
space group P4/n and has a structure similar to, but differing significantly from, those of a series of 3-dimensional
cage squarates which have been reported previously.