First Row Transition Metal Complexes of 1-Methylcyclobutenedione, a Monosubstituted Squarate Ligand

Reaction of 1-methylcyclobutenedione with M(NO<sub>3</sub>)<sub>2</sub>·<i>x</i>H<sub>2</sub>O [M = Co (<b>1</b>) and Ni (<b>2</b>)] produces isomorphous polymeric complexes of formula M(CH<sub>3</sub>C<sub>4</sub>O<sub>3</sub>)(NO<sub>3</sub>)(H<sub>2</sub>O)<sub>4</sub>·2H<sub>2</sub>O that crystallize in the monoclinic space group <i>P</i>2<sub>1</sub>/<i>m</i> (No. 11) with, for <b>1</b>, <i>a</i> = 6.409(5) Å, <i>b</i> = 14.717(10) Å, <i>c </i>= 7.259(6) Å, β = 114.73(4)°, and <i>Z </i>= 2 (the metal atom being positioned on a center of symmetry). In both cases, adjacent metal centers are bridged μ-1,3 by the 1-methylcyclobutenedione ligand, which exhibits distinct multiple bond localization. The positive charges on the polymer chains are in each case balanced by nitrate ions. The metal atoms are six-coordinate with trans-oriented 1-methylcyclobutenedione ligands, and with four aqua ligands completing the coordination sphere. Reduction of the crystallization temperature, however, resulted, with Co, in the formation of the monomeric complex Co(CH<sub>3</sub>C<sub>4</sub>O<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O (<b>4</b>). Isomorphous Mn (<b>3</b>), Cu (<b>5</b>), and Zn (<b>6</b>) analogues have also been synthesized. These crystallize in the monoclinic space group <i>P</i>2<sub>1</sub>/<i>n</i> (No. 14) with, for <b>3</b>, <i>a</i> = 8.8258(8) Å, <i>b</i> = 16.1527(14) Å, <i>c </i>= 9.9084(8) Å, β = 93.230(7)°, and <i>Z</i> = 4. The central metal atom in each of these compounds is six-coordinate, being bound to two trans-oriented 1-methylcyclobutenedione ligands and four aqua ligands, the former coordinating in a monodentate fashion via the oxygen atom trans to the methyl substituent. In both the polymeric and monomeric complexes extensive three-dimensional hydrogen-bonded networks are formed in the solid state.