Sodium-centered Dodecanuclear Co(II) and Ni(II) Complexes with 2-(phosphonomethylamino)succinic Acid: Studies of Spectroscopic, Structural, and Magnetic Properties

Two new isostructural cobalt(II) and nickel(II) polynuclear complexes with 2-(phosphonomethyl)aminosuccinic acid, H ₄ PMAS, namely, Na[Co ₁₂ (PMAS) ₆ (H ₂ O) ₁₇ (OH)] ·x2H ₂ O, 1·x2H ₂ O, and Na[Ni ₁₂ (PMAS) ₆ (H ₂ O) ₁₇ (OH)]·xH ₂ O, 2·xH ₂ O, have been synthesized for the first time from aqueous solutions and studied by single crystal X-ray diffraction, infrared, and UV-visible diffuse reflectance spectroscopy; TG/DTA analysis; and magnetochemistry. Both 1 and 2 crystallize in the rhombohedral crystal system with the R3Ì... space group with 1/6 of the Co ₁₂ (PMAS) ₆ or Ni ₁₂ (PMAS) ₆ moieties in the asymmetric unit. The X-ray refinements reveal the presence of 18 water sites, but unit cell charge balance requires that one water molecule must be an OH ^- anion, an anion which is disordered over the 18 sites. The PMAS ^4- ligand forms two five-membered and one six-membered chelation ring. Both 1 and 2 contain 24-membered metallacycles as a result of the bridging nature of the PMAS ^4- ligands. The resulting three-dimensional structures have one-dimensional channels with a sodium cation at the center of symmetry. The temperature dependence of the magnetic susceptibility reveals the presence of weak antiferromagnetic exchange coupling interactions in both 1 and 2. Two exchange coupling constants, J ₁ = -15.3(7) cm ^-1 and J ₂ = -1.06(2) cm ^-1 with S ₁ = S ₂ = 3/2 for the Co(1)···Co(1) and Co(1) ···Co(2) exchange pathways, respectively, are required for 1, and J ₁ = -1.17(6) cm ^-1 and J ₂ = -4.00(8) cm ^-1 with S ₁ = S ₂ = 1 for the Ni(1)···Ni(1) and Ni(1)···Ni(2) exchange pathways, respectively, are required for 2, in order to fit the temperature dependence of the observed magnetic susceptibilities.