Reduced early transition metal (TM) 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 early TM complexes often involve strong chemical reductants or thermal activation of high-valent organometallic precursors. For organozirconium compounds, alternative photochemical means are attractive but mostly confined to high energy UV light or broad-spectrum light source. Our prior work with zirconium complexes containing electron-rich pyridine monopyrolate (PMP) ligands explored the importance of ligand-to-metal charge transfer (LMCT) bands for visible light excitation that facilitates C-C bond formation by formal reductive elimination at zirconium. These studies prompted 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 with a maximum at 417 nm (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 being exposed to a 462-nm LED, the bibenzyl coupling product was formed and opened the coordination sphere for further reactivity. Photolysis in the presence of diphenylacetylene lead to the unexpected half-sandwich complex (MePMPMe)2Zr(η4-C4Ph4), confirmed by X-ray crystallography. Under the same reaction condition, (MePMPMe)2ZrBn2 and 4-methylbenzyl bromide generated (MePMPMe)2ZrBr2. Bibenzyl by-products were identified by GC-MS and various NMR experiments. The presence of bibenzyl, 4-methylbibenzyl and 4,4’-dimethylbibenzyl suggested a radical mechanism.

Blue light photochemistry in the curious case of cyclobutadiene formation at a zirconium center

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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