The manganese-oxo "cubane" core complex Mn 4O 4L 1 6 (1, L 1 = Ph 2PO 2 -), a partial model of the photosynthetic water oxidation site, was shown previously to undergo photodissociation in the gas phase by releasing one phosphinate anion, an O 2 molecule, and the intact butterfly core cation (Mn 4O 2L 1 5 +). Herein, we investigate the photochemistry and electronic structure of a series of manganese-oxo cubane complexes: [Mn 4O 4L 2 6] (2), 1 +(ClO 4 -), 2 +(ClO 4 -), and Mn 4O 3(OH)L 1 6 (1H). We report the atomic structure of [Mn 4O 4L 2 6](ClO 4 -), 2 +(ClO 4 -) [L 2 = (4-MeOPh) 2-PO 2 -]. UV photoexcitation of a charge-transfer band dissociates one phosphinate, two core oxygen atoms, and the Mn 4O 2L 5 + butterfly as the dominant (or exclusive) photoreaction of all cubane derivatives in the gas phase, with relative yields: 1H ≫ 2 > 1 > 2 + > 1 +. The photodissociation yield increases upon (1) reducing the core oxidation state by hydrogenation of a corner oxo (1H), (2) increasing the electron donation from the phosphinate ligand (L 2), and (3) reducing the net charge from +1 to 0. The experimental Mn-O bond lengths and Mn-O bond strengths and the calculated ligand binding energy explain these trends in terms of weaker binding of phosphinate L 2 versus L 1 by 14.7 kcal/mol and stronger Mn-(μ3-O)(core) bonds in the oxidized complexes 2 + and 1 + versus 2 and 1. The calculated electronic structure accounts for these trends in terms of the binding energy and antibonding Mn-O(core) and Mn-O′(ligand) character of the degenerate highest occupied molecular orbital (HOMO), including (1) energetic destabilization of the HOMO of 2 relative to 1 by 0.75 eV and (2) depopulation of the antibonding HOMO and increased ionic binding in 1 + and 2 + versus 1 and 2.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry