Abstract
The addition of tert-butyl hydro-peroxide ( t BuOOH) to two structurally related MnII complexes containing N,N-bis-(6-methyl-2-pyridyl-meth-yl)ethane-1,2-di-amine (6-Me-DPEN) and N,N-bis-(6-methyl-2-pyridyl-meth-yl)propane-1,2-di-amine (6-Me-DPPN) results in the formation of high-valent bis-oxo complexes, namely di-μ-oxido-bis-{[N,N-bis-(6-methyl-2-pyridylmeth-yl)ethane-1,2-di-amine]-manganese(II)}(Mn-Mn) bis-(tetra-phenyl-borate) dihydrate, [Mn(C16H22N4)2O2](C24H20B)2·2H2O or {[MnIV(N4(6-Me-DPEN))]2(μ-O)2}(2BPh4)(2H2O) (1) and di-μ-oxido-bis-{[N,N-bis-(6-methyl-2-pyridylmeth-yl)propane-1,3-di-amine]-manganese(II)}(Mn-Mn) bis-(tetra-phenyl-borate) diethyl ether disolvate, [Mn(C17H24N4)2O2](C24H20B)2·2C4H10O or {[MnIV(N4(6-MeDPPN))]2(μ-O)2}(2BPh4)(2Et2O) (2). Complexes 1 and 2 both contain the 'diamond core' motif found previously in a number of iron, copper, and manganese high-valent bis-oxo compounds. The flexibility in the propyl linker in the ligand scaffold of 2, as compared to that of the ethyl linker in 1, results in more elongated Mn-N bonds, as one would expect. The Mn-Mn distances and Mn-O bond lengths support an MnIV oxidation state assignment for the Mn ions in both 1 and 2. The angles around the Mn centers are consistent with the local pseudo-octa-hedral geometry.
Highlights
A heterometallic cubane cluster, MndangCaMn3O5, referred to as the oxygen-evolving complex (OEC), is involved in photosynthetic catalytic water oxidation (Umena et al, 2011)
The cluster is housed in the enzyme photosystem II (PSII) and consists of high-valent MnIII/IV ions linked by oxo bridges and one dangling MnIV/V ion
Developing a wide base of chemical information on a variety of Mn—O species similar to the fragments implicated in the key O—O bond-forming step should aid the development of a detailed understanding of photosynthetic water oxidation
Summary
A heterometallic cubane cluster, MndangCaMn3O5, referred to as the oxygen-evolving complex (OEC), is involved in photosynthetic catalytic water oxidation (Umena et al, 2011). Proposed mechanisms for O—O bond formation involve either nucleophilic attack by an M—OH group (M = Mn or Ca) at an electrophilic MnV O site, or radical coupling between two MnIV oxyl radicals to afford an unobserved peroxo intermediate (Hatakeyama et al, 2016; Lohmiller et al, 2017; Renger, 2011; Yano & Yachandra, 2014). Developing a wide base of chemical information on a variety of Mn—O species similar to the fragments implicated in the key O—O bond-forming step should aid the development of a detailed understanding of photosynthetic water oxidation. A key step in OEC-catalyzed water oxidation involves the formation of a peroxo O—O bond prior to dioxygen evolution. Peroxo, and reactive mixed-valent MnIIIMnIV bis-oxo intermediates were shown to form. The isolation and crystallographic characterization of the bis-oxo complexes 1 and 2 (Figs. 1 and 2, formed via alkylperoxo Mn—OOtBu intermediates (Coggins et al, 2020), further expands the available library of high-valent Mn–oxo dimers (Mullins & Pecoraro, 2008), demonstrating the stability of the metal–oxo diamond core described previously (Que & Tolman, 2002)
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