Reduced early transition metal (ETM) compounds are inherently reactive and undergo distinct transformations compared to late TM analogs. This divergence is advantageous for designing new and complementary stoichiometric and catalytic reactions. State-of-the-art synthetic routes toward low-valent ETM complexes often involve strong chemical reductants, thermal activation of high-valent organometallic precursors, or high-energy UV and broad-spectrum light source in the case of organozirconium. Our prior work with zirconium complexes containing electron-rich pyridine monopyrolate (PMP) ligands proved the importance of ligand-to-metal charge transfer (LMCT) bands for visible light excitation promoting C-C bond formation by formal reductive elimination at zirconium. These studies prompt the synthesis and photochemical investigation of (MePMPMe)2ZrBn2 (MePMPMe = 3,5-dimethyl-2-(2-pyridyl)pyrrolide, Bn = benzyl) as a precursor to low-valent intermediates. Absorption spectroscopy in benzene showed one strong band in the visible region at 415-nm maximum (ε = 12 300 M-1 cm-1). Time-dependent density functional theory (TD-DFT) calculations revealed the hybrid nature of the absorption band: LMCT/ligand-to-ligand charge transfer (LLCT). Ligand contribution came from either MePMPMe or Bn. Within minutes of exposure to 462-nm LED, the bibenzyl coupling product was formed and opened the coordination sphere for further reactivity. Photolysis in excess diphenylacetylene lead to the unexpected half-sandwich complex (MePMPMe)2Zr(η4-C4Ph4), confirmed by X-ray crystallography. The same reaction condition using (MePMPMe)2ZrBn2 and 4-methylbenzyl bromide generated (MePMPMe)2ZrBr2. Bibenzyl by-products (bibenzyl, 4-methylbibenzyl and 4,4’-dimethylbibenzyl) confirmed by GC-MS and various NMR experiments suggested a radical mechanism. Catalytic trials using phenylacetylene and 1-phenyl-1-propyne lead to trimerization of the organic substrates that was selective toward symmetric and asymmetric coupling products, respectively. Preliminary photoreactions probing steric and electronic influence were done using 2-, 3-, and 4-ethynylanisole.

Blue light photochemistry in alkyne trimerization at a zirconium center

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.