1D and 2D coordination polymers based on ring-reduced freebase and copper-containing meso-tetra(4-pyridyl)porphyrins and M(III) (TMHD)3 units (M = Dy, Tb and Gd; TMHD: 2,2,6,6-tetramethyl-3,5-heptanedionate)

Abstract

The interaction of ring-reduced free-base tetra(4-pyridyl)porphyrin (H₂TPyP) and its copper(II) complex with an excess of MIII(TMHD)3 (TMHD = 2,2,6,6-tetramethyl-3,5-heptanedionate) afforded a new type of coordination polymers in which lanthanide ions are bridged by porphyrin anions. Unusual folded zig-zag 2D coordination polymers, {Cryptand(K+)}n{H2TPyP.[DyIII(TMHD)3]2-}n.3n(C6H14) (1) and {Cryptand(Cs+)}n{CuIITPyP.[TbIII(TMHD)3]2 - }n.3n(C6H5CH3).1.5n(C6H14) (2), contain paramagnetic singly reduced porphyrin ligands. 1D linear coordination polymers, {Cryptand(Cs+)}2n{H2TPyP.[TbIII(TMHD)3]2 2- }n.3n(C6H5CH3) (3) and the isostructural {Cryptand(Cs+)}2n{H2TPyP. [GdIII(TMHD)3]2 2- }n.3n(C6H5CH3) (4), contain diamagnetic doubly reduced porphyrin ligands. Each MIII(TMHD)3 unit forms M III -N(Py) coordination bonds (2.58-2.67 Å) with two porphyrins, resulting in a distorted square-antiprismatic coordination environment around the MIII ions. The folded zig-zag 2D polymers exhibit dihedral angles between porphyrin planes of 53.1-53.8° in compounds 1 and 2. These folds arise from the insertion of {Cryptand(M+)} cations between the porphyrin planes. In contrast, two such cations surround each porphyrin in compound 3, leading to the formation of a linear 1D polymer. The Cs⁺ ions are displaced from the cryptand cavity toward the porphyrin core, forming short Cs+-N(TPyP) contacts of 3.2-3.3 Å. Optical spectra confirm the formation of ring-reduced H2TPyP.- and TPyP.3- species in 1 and 2, whereas new bands attributed to H2TPyP2- dianions appear in the spectra of 3 and 4. Magnetic measurements suggest weak antiferromagnetic coupling in the 2D polymers of 1 and 2 mediated by paramagnetic porphyrin bridges. DFT calculations indicate weak localization of spin density on the nitrogen atoms of two of the four pyridyl rings due to their rotation relative to the porphyrin plane by 58.6-84.5°. The GdIII centers are separated by diamagnetic H2TPyP2- bridges in the linear polymers of 4 providing zero metal-metal magnetic coupling at small zero-field splitting.