Synthesis and characterization of novel cobalt aluminum cofacial porphyrins. First crystal and molecular structure of a heterobimetallic biphenylene pillared cofacial diporphyrin

Article abstract of DOI:10.1021/ja00051a021

The synthesis of the novel family of heterodinuclear complexes (DP)CoAl(OR) (where DP^4- is the tetraanion of the diporphyrin biphenylene DPB or the diporphyrin anthracene DPA, and R = CH₃, CH₂CH₃, or CH₂C₆H₅) is reported. These complexes were obtained by selective metalation of the cofacial diporphyrins with cobalt and aluminum. Each (DP)CoAl(OR) complex was characterized by mass spectrometry and UV-vis, IR, ESR, and ¹H NMR spectroscopies. Unusually large paramagnetic shifts were observed for the protons on axial ligands bound to aluminum and for the porphyrinic N-H protons of the monocobalt DPB and DPA complexes. Analysis of the paramagnetic shifts indicates that the main contribution to isotropic shifts arises from a through space (or dipolar) interaction of an unpaired electron on cobalt(II). Structural data were deduced from the ¹H NMR study. In addition, the molecular structure of the cobalt(II) aluminum(III) ethoxide diporphyrin biphenylene (DPB)CoAl(OCH₂CH₃) was determined by X-ray diffraction. This is the first crystal structure of a heterobimetallic cofacial diporphyrin. (DPB)CoAl(OCH₂CH₃) (C₇₈H₈₁N₈OCoAl·C₇H ₈) crystallizes in the triclinic system, space group P1?. Its lattice constants are as follows: a = 13.095 (4) A?, b = 16.836 (3) A?, c = 16.986 (3) A?, α = 87.47 (1)°, β= 70.40 (3)°, γ = 85.90 (2)°, V = 3516 A?³, Z = 2, R(F) = 5.30%, R_w(F) = 5.27%, GOF = 2.5 for 6800 reflections with I ≥ 3σ(I). No metal-metal interaction occurs (Co-Al = 4.370 (1) A?), and both porphyrin moieties are slipped by α = 29.8°. Finally, the tedious synthesis of the (DPA)H₄ free-base porphyrin has been extensively modified, and several new reaction steps are presented leading to significant increases in both scale and yield.