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

Reaction of 1-methylcyclobutenedione with M(NO 3 ) 2 · x H 2 O [M = Co ( 1 ) and Ni ( 2 )] produces isomorphous polymeric complexes of formula M(CH 3 C 4 O 3 )(NO 3 )(H 2 O) 4 ·2H 2 O that crystallize in the monoclinic space group P 2 1 / m (No. 11) with, for 1, a = 6.409(5) Å, b = 14.717(10) Å, c = 7.259(6) Å, β = 114.73(4)°, and Z = 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 3 C 4 O 3 ) 2 ·4H 2 O ( 4 ). Isomorphous Mn ( 3 ), Cu ( 5 ), and Zn ( 6 ) analogues have also been synthesized. These crystallize in the monoclinic space group P 2 1 / n (No. 14) with, for 3, a = 8.8258(8) Å, b = 16.1527(14) Å, c = 9.9084(8) Å, β = 93.230(7)°, and Z = 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.