Stepwise Approach to Bimetalic Porphyrin Hosts:  Spatially Enforced Coordination of a Nickel(II) Porphyrin

The synthesis of the Zn(3) 1,1,2-trimer Zn(3)()3 and the NiZn(2) 1,1,2-trimer NiZn(2)()3 by a stepwise convergent route is described. A large affinity for Py(3)()T by both cyclic 1,1,2-trimers, Zn(3)()3 or NiZn(2)()3, with respect to the linear trimers, provides a thermodynamic driving force for the templated cyclization of the new host molecules, which were fully characterized by NMR spectroscopy (COSY and NOESY). The binding properties of several pyridine-containing bidentate and tridentate ligands have been investigated in order to probe the shape and the size of the cavity of the host molecule. Both Py(2)()Pr and Py(2)()Py bind very strongly to Zn(3)()3 and NiZn(2)()3 (ligand affinities in CH(2)Cl(2) at 25 degrees C, >10(8) M(-)(1)), while the tridentate ligand Py(3)()T binds to both Zn(3)()3 and NiZn(2)()3 with a very high binding constant (ligand affinities in CH(2)Cl(2) at 25 degrees C, >10(9) M(-)(1)). This unexpected result for NiZn(2)()3 suggests that the cavity-enforced effective molarity of the third pyridyl of Py(3)()T at the Ni site is high enough to form the first example of a stable 1:1 Ni(II)-pyridyl complex. The resulting Ni(II) paramagnetic complex has been characterized by NMR; the temperature dependence deviates slightly from ideal Curie behavior.