Tuning the photoinduced O2-evolving reactivity of Mn4O47+, Mn4O46+, and Mn4O3(OH)6+ manganese-oxo cubane complexes

The manganese-oxo cubane core complex Mn4O4L61 (1, L-1 = Ph2PO2-), 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 (Mn4O2L51+). Herein, we investigate the photochemistry and electronic structure of a series of manganese-oxo cubane complexes: [Mn4O4L62] (2), 1(+)(ClO4-), 2(+)(ClO4-), and Mn4O3(OH)L-6(1) (1H). We report the atomic structure of [Mn4O4L62](ClO4), 2(+)(ClO4-) [L-2 = (4-MeOPh)(2)-PO2-] UV photoexcitation of a charge-transfer band dissociates one phosphinate, two core oxygen atoms, and the Mn4O2L5+ 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-(mu(3)-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.