Synthesis and Structural Characterization of Porphyrinic Enediynes:  Geometric and Electronic Effects on Thermal and Photochemical Reactivity

We report the preparation of [5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenylethynyl)porphinato] complexes of Ni(II), H₂, Zn(II), Mg(II), and Cu(II), as well as select trimethylsilanylethynyl derivatives. The X-ray structures of the octakis(phenylethynyl) compounds show systematic deviations from planarity (Ni(II), 0.2851 Å; Zn(II), 0.0304 Å) as a function of the central cation. These geometric distortions are reflected in bathochromic shifts of the Soret and Q bands (Ni(II), 497, 604, and 650 nm; Mg(II), 515, 595, 642, and 705 nm) which loosely correlate with increasing planarity of the structure. Similarly, vibrational modes involving the octasubstituted porphyrin core exhibit shifts to lower frequency as a function of increasing planarity in the solution-state resonance Raman spectra (λ_exc = 501.7 nm) of these compounds. Analogous trends are also observed in their solid-state electronic and resonance Raman spectra, indicating that the structural distortions within the octakis(phenylethynyl) series are preserved in solution. Comparison of the saddle distortion of the octasubstituted Ni(II) compound with the ruffle/saddle distortions of the pentakis and hexakis Ni(II) derivatives reveals some influence of asymmetric peripheryl substitution on geometric structure. These Ni(II) derivatives also exhibit systematic red shifts in their electronic spectra as a function of the number of conjugated alkyne units (∼13 nm/alkyne), revealing participation of the enediyne units in the electronic ground and excited states. The solid-state Bergman cyclization temperatures of the phenylethynyl compounds vary markedly as a function of planarity, and correlate loosely with alkyne termini separation (Ni(PA)₈, 4.00 Å, 281 °C; MgP(PA)₈, 3.77 Å, 244 °C). In solution, both thermal and photochemical activation of the free-base octakis(phenylethynyl) compound lead to formal reduction of the porphyrin backbone via H-atom addition at opposing meso-positions. Generation of a common product suggests that both thermal and photochemical pathways to Bergman cyclization in solution contain significant activation barriers to formation of the 1,4-phenyl diradical intermediate, and under these solution conditions, alternate reaction channels are more thermodynamically favorable.