Planar Paddlewheel

The complex has a magnetic moment of 7.95 pg, which is largely invariant from 150 to 300 K. CASSCF/PT2 calculations predict the ground spin state to be high spin, or S = 3. The calculations yield an effective bond order (EBO) of 1.19, suggesting a modest amount of multiple bonding despite the experimentally short Fe-Fe bond. The Mossbauer spectrum shows one quadrupole doublet with IS of 0.44 mm s and QS of 2.39 mm s .  

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Fig. 17 Commonly recurring structural motifs, (a) the paddlewheel dimer of Cu05 square pyramids, (b) the tetrahedron of Zn04 tetrahedra sharing a central oxygen, and (c) the trigonal planar trimer of OO, octahedra sharing a central oxygen. Gray spheres denote carbon and red oxygen CuO,. ZnO-. and CrC>6 polyhedra are shown in dark blue, pale blue and green, respectively.

Another noticeable example of an MOF constructed from a polytopic carboxylic acid and the binuclear metal-carboxylate paddlewheel-like cluster MBB, equivalent to a square planar SBU, is the one commonly referred to as HKUST-1 MOF, with the general formula [Cu3(TMA)2 (H20)3] where TMA denotes trimesic acid (1,3,5-benzene-tricarboxylic acid) (Figure 13, ). The MBB is defined by four carboxylate ions coordinating two Cu(II) ions in a paddlewheel-like structure with four points of extension defining a square-planar SBU. The authors reported the capability of exchanging the axial water molecules coordinated to the Cu(II) paddlewheel, postsynthetically, with pyridine molecules, opening the door for further investigations toward the viability of preferential sorption of small molecules and/or catalysis on coordinatively unsaturated metal centers. 

Temperature has long been used as a variable which influences the crystal form obtained. In the case of isomerism, the kinetic isomer at ambient temperature may become thermodynamically favored at a different temperature. One such example is provided by the coordination polymers with composition [Cu2(tcppda)(H20)2]n (tcppda = V, N, N, yV -tetrakis(4-carboxyphenyl)-l,4-phen-ylenediamine), 1. The tcppda ligand can adopt two conformations, either square-planar C2h or tetrahedral D2. On association with the square-planar Cu2 paddlewheels, these give rise to two supramolecular isomers in the form of... 

Figure 3 Temperature-dependent isomerism of [Cu2 (tcppda) (H20)2]n, 1- The tetrahedral conformation of Z)2-tccpda and square-planar C2/,-tccpda combine with the square-planar Cu2 paddlewheel secondary building unit (SBU) to form Ivt or pts networks.

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