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 (λexc
= 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.