Three new nickel(II) complexes of formula [Ni2(Me2cyclen)2ox](ClO4)2•2H2O (1), [Ni2(cyclen)2ox](NO3)2 (2), and [Ni(cyclen)](H2O)2]2(C4O4)(ClO4)2 (3), where Me2cyclen = 1,7-dimethyl-1,4,7,10-tetraazacyclododecane, cyclen = 1,4,7,10-tetraazacyclododecane, ox2− = the oxalate anion, and C4O42− = the dianion of 3,4-dihydroxy-3-cyclobutene-1,2-dione (squarate), have been synthesized. The crystal and molecular structures of 1 and 3 have been solved at 298 K by single-crystal X-ray analyses. 1 crystallizes in the monoclinic system, space group P21/n, with a = 22.815 (15) Å, b = 10.713 (1) Å, c = 7.506 (3) Å, β = 97.85 (3)°, Z = 2, and R = 0.0415. 3 crystallizes in the orthorhombic system, space group P21cn, with a = 11.124 (3) Å, b = 11.461 (3) Å, c = 5.735 (6) Å, Z = 4, and R = 0.0345. The structure of 1 consists of centrosymmetrical µ-oxalato-bridged nickel(II) binuclear units [Ni2(Me2cyclen)2ox]2+ with noncoordinated perchlorate anions and water molecules. The tetraazamacrocycle adopts a folded conformation around the nickel atom, which is 6-coordinated in an octahedral distorted arrangement: the Ni–N distances are in the range 2.050 (2)–2.154 (3) Å, and the Ni–O(ox) distances are 2.095 (2) and 2.102 (2) Å. The oxalate ion acts as a bis-bidentate ligand between two nickel(II) ions. The structure of 3 is made up of [Ni(cyclen)(H2O)2]2+ units and noncoordinated squarate and perchlorate anions. The tetraazamacrocycle adopts a folded conformation to produce a cis-pseudooctahedral geometry at each nickel atom. The configuration of the four nitrogen atoms is such that the hydrogen atoms attached to them are on the same side with respect to the idealized macrocyclic plane. Coordinated water molecules and squarate oxygen atoms are linked by hydrogen bonds. Intramolecular antiferromagnetic spin-exchange coupling between the two nickel(II) ions is observed for complexes 1 (J = −34 cm−1; g = 2.30) and 2(J = −35 cm−1; g = 2.15) (J being the parameter of the exchange Hamiltonian Ĥ = −7ŜA•ŜB). The thermodynamic parameters of the equilibrium [Ni(L)]2+ + ox2− = [Ni(L)ox] where L = cyclam, cyclen, and Me2cyclen have been determined by potentiometric and microcalorimetric measurements in aqueous solution at 25 °C in 0.1 mol dm−3 KNO3. The values of ΔG° found for the addition of the oxalate anion to [Ni(cyclen)]2+ and [Ni(Me2cyclen)]2+ are equal within experimental error (ΔG° = −5.6 (1) and −5.7 (1) kcal mol−1, respectively). A greater enthalpy of reaction has been found for [Ni(Me2cyclen)]2+ (ΔH° = −2.9 (1) kcal mol−1 ) than for [Ni(cyclen)]2+ (ΔH° = −2.4 (1) kcal mol−1 ). The addition of oxalate to [Ni(cyclam)]2+ (69% square, 29% trans-diaquo, 2% cis-diaquo forms) is an enthalpy-driven reaction (ΔG° = −3.8 (1) kcal mol−1 and ΔH° = −3.8. © 1990, American Chemical Society. All rights reserved.
