Supramolecular Assembly of Organophosphonate Diesters Using Paddle-Wheel Complexes: First Examples in Porphyrin Series

The reactions of dicopper tetrapivalate complex Cu-2(mu-OOC-t-Bu)(4)(NCMe)(2) (1) with triphenylphosphine oxide and diethyl phosphite allow paddle-wheel (PW) copper(II) complexes with phosphorus-containing axial ligands (2, 3) to be obtained. When meso-bis(diethoxyphosphoryl)porphyrins 4M were employed in this ligand exchange reaction, a series of one-dimensional (1D) homo- and heterometallic coordination polymers 5M composed of PW subunits and organophosphonate diesters were prepared and characterized by means of single crystal X-ray analysis. Planar porphyrinate 4Pd and nonplanar metalloporphyrinates 4Cu and 4Ni proved to be appropriate molecular structural blocks for assembly of coordination polymers. The structural parameters of the tetrapyrrolic macrocycles incorporated into the polymer chain are determined by the nature of the metal center of the porphyrin moiety. While the geometry of palladium(II) and nickel(II) porphyrinates 4Pd and 4Ni does not change significantly in the polymer chain, saddle-shaped Cu(II) porphyrinate 4Cu exhibits a nearly planar core configuration, being coordinated to the copper centers of PW fragments by two peripheral phosphoryl groups in the polymer chain. The geometry of the tetrapyrrolic core is a key parameter influencing the structural properties of the polymeric materials. For 5Pd and for isostructural 5Cu, all metal centers of the polymeric chain are aligned. The planar macrocycles of adjacent chains are parallel and are shifted one to another in such a way that the angle between the Pd center dot center dot center dot P and Pd center dot center dot center dot Pd directions is 40.4 degrees, and the distance between the nearest palladium(II) atoms of neighboring chains is 11.668 angstrom. There is no free volume in these crystals. In the crystals of 5Ni, formed by nonplanar porphyrinates, only copper atoms of the PW pivalate moiety are located in one plane, and zigzag chains are formed so that two adjacent tetrapyrrolic macrocycles are located in alternating positions with respect to this plane, the nickel atoms being displaced from this plane by 1.548 angstrom. This arrangement naturally leads to the formation of regular pores. The resulting channels have an effective cross-section of about 10 X 12 angstrom and represent ca. 18% of the volume of the crystal. The exchange reaction between the free-base porphyrin 4H2 and an excess of copper(II) pivalate complex 1 is accompanied by the metalation of the porphyrin core affording the polymer 5Cu. Moreover, self-assembly of metalloporphyrinate 4Zn is observed under studied experimental conditions, which interferes with the formation of the target mixed coordination polymers.