Comparison of two MnIVMnIV-bis-μ-oxo complexes {[MnIV(N4(6-Me-DPEN))]2(μ-O)2}2+ and {[MnIV(N4(6-Me-DPPN))]2(μ-O)2}2+

The addition of tert-butyl hydroperoxide (tBuOOH) to two MnII complexes, differing by a small synthetic alteration from an ethyl to a propyl linker in the ligand scaffold, results in the formation of the high-valent bis-oxo complexes {[MnIV(N4(6-Me-DPEN))]2(μ-O)2}2+ (1) and {[MnIV(N4(6-Me-DPPN))]2(μ-O)2}2+ (2).


Chemical context
A heterometallic cubane cluster, Mn dang CaMn 3 O 5 , 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 Mn III/IV ions linked by oxo bridges and one dangling Mn IV/V ion. Water oxidation is thermodynamically unfavorable, and requires an energy input of 359 kJ mol À1 that is provided by sunlight (Yano & Yachandra, 2014). Although the exact details of the mechanism for water oxidation are unknown, two water molecules are thought to bind to the cluster to produce one equivalent of dioxygen, four electrons, and four protons (Kok et al., 1970). Sequential oxidation of the cluster, starting with the Ca II Mn IV Mn 3 III O 5 core, generates partially oxidized states, S i (where i = number of stored oxidizing equivalents), which store oxidizing equivalents in preparation for O-O bond formation and O 2 release (Hatakeyama et al., 2016;Lohmiller et al., 2017;Renger, 2011;Yano & Yachandra, 2014). Very little is known about the key OEC-catalyzed O-O bond-forming step, because it occurs following the rate-determining step ISSN 2056-9890 (Retegan et al., 2016). Proposed mechanisms for O-O bond formation involve either nucleophilic attack by an M-OH group (M = Mn or Ca) at an electrophilic Mn V O site, or radical coupling between two Mn IV 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. Fundamental concepts obtained from these studies can then be applied towards the maintenance of stable energy reserves and improve the world's energy economy by storing solar energy in chemical bonds (Lewis, 2016).

Complex 2
Complex 2 also sits on an inversion center (1 À x, 2 À y, 1 À z), making the two Mn atoms crystallographically equivalent. There is disorder in the position of the propyl linker carbon atoms (C1, C2, C3). The site occupancies of N3, C1-C3 and N3B, C1B-C3B refined to 0.804 (5) and 0.196 (5), respectively, with the constraint of both together giving 100% occupancy. The Mn ion of 2 is again in a pseudo-octahedral environment, with small deviations in O-Mn-N angles relative to ideal octahedral geometry: O1-Mn1-N1 = 106.39 (7), O1-Mn1-N2 = 174.90 (7), O1-Mn1-N3 = 89.11 (13), and O1-Mn1-N4 = 103.70 (6) . Again, as is true for all diamond cores, the O1-Mn1-O1 0 angle of 2 is slightly compressed at 85.98 (7) , and is similar to that in 1. Metrical parameters, Mn-O1 = 1.8325 (15) and Mn-O1 0 = 1.8349 (15) Å , are also similar to those found in 1, and fall within the reported range (1.8 to 1.9 Å ) for oxo-bridged Mn IV complexes. The pyridine nitrogen atoms are once again further from the Mn ions than expected for a formal Mn-N bond, but are oriented towards Mn at distances of Mn1-N1 = 2.3251 (18) Å and Mn1-N4 = 2.3522 (18) Å . This bond elongation is likely to be due to steric interference from the methyl groups at the 6-position of the pyridine rings. The nitrogens on the amine arms are much closer to the Mn center, and fall within the normal Mn-N range (1.9 to 2.1 Å ) for Mn IV . The Mn-N distance involving the tertiary amine [Mn1-N2 = 2.1828 (18) Å ] is noticeably longer than that involving the secondary amine [Mn1-N3= 2.133 (6) Å ]. The large difference between these bond lengths in 2, relative to those of 1, likely reflects the increased flexibility of the propyl linker in 2. The Mn1-Mn1 0 distance [2.6825 (7) Å ] in 2 is essentially the same as that found in 1, and falls within the normal range (2.6 to 2.8 Å ) for bis-oxo-bridged Mn IV Mn IV dimers containing a diamond core. Complex 2 crystallizes with two tetraphenylborate counter-ions and two diethyl ether molecules per cation.

Database survey
The structures of 1 and 2 are analogous to other reported Mn IV Mn IV (-O) 2 dimers. The Mn1-Mn1 0 distances of 2.6899 (15) Å in 1 and 2.6825 (7) Å in 2 are comparable to other literature examples (Krewald et al., 2013;Mullins & Pecoraro, 2008;Torayama, et al., 1998). The Mn-O bond lengths of 1.829 (3) and 1.835 (2) Å for 1 and 1.8350 (15) and 1.8325 (15) Å for 2 are also similar to literature reported values for Mn IV Mn IV (-O) 2 dimers (Krewald et al., 2013;Mullins & Pecoraro, 2008;Torayama et al., 1998). The octahedral geometry of the Mn centers of both structures are very similar in terms of bond angles, all of which are close to the ideal 90 and 180 . The similarities in bond lengths and angles show that 1 and 2 contain a metal-oxo diamond core motif, previously observed in manganese, iron and copper complexes (Que & Tolman, 2002).

General methods
All syntheses were performed using Schlenk-line techniques or under an N 2 atmosphere in a glovebox. Reagents and solvents were purchased from commercial vendors, were of highest available purity and were used without further purification unless otherwise noted. MeOH (Na), MeCN (CaH 2 ), and CH 2 Cl 2 (CaH 2 ) were dried and distilled prior to use. Et 2 O was rigorously degassed and purified using solvent purification columns housed in a custom stainless steel cabinet and dispensed by a stainless steel Schlenk-line (GlassContour). Complexes 3 and 4 were synthesized as described by Coggins et al. (2020).

Synthesis of 1 and 2
The addition of 1.5 equivalents of t BuOOH to CH 2 Cl 2 solutions of alkoxide-ligated 3 and 4 in an anaerobic cell at room temperature results in the formation of 1 and 2, respectively. Single crystals of the isolated compounds in the form of brown plates for 1 and purple plates for 2 were obtained in up to 40% yield via slow evaporation and crystallization from CH 2 Cl 2 . Both reactions result in the loss of the Schiff-base arm present in the starting Mn II complexes 3 and 4, most probably because the reactions were performed in moist air (Coggins et al., 2020).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Scattering factors are taken from Waasmaier & Kirfel (1995). Hydrogen atoms were placed in geometrically idealized positions and constrained to ride on their parent atoms with C-H distances in the range 0.95-1.00 Å . Isotropic displacement parameters U eq were fixed at 1.2U eq (C) or 1.5U eq (C-methyl). For the disordered water molecule in complex 1, the water was set-up as a rigid group free to rotate and move during refinement, with DFIX restraints between O and H and between both H per water. The displacement parameters of O2 and O2B were made the same with the EADP constraint. Hydrogen-atom isotropic displacement parameters were fixed at 1.5 times that of the water oxygen atoms. For the disorder in complex 2, the geometry of both groups was set to be similar with the 'SAME' option. Displacement parameters of N3-N3B, C1-C1B, C2-C2B, and C3-C3B were restrained with the SIMU command at 0.005 strength.

Di-µ-oxido-bis{[N,N-bis(6-methyl-2-pyridilmethyl)ethane-1,2-diamine]manganese(II)}(Mn-Mn) bis(tetraphenylborate) dihydrate (Complex1)
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.42 e Å −3 Δρ min = −0.46 e Å −3 Special details Experimental. 20 seconds exposure, 0.5 degree steps Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Special details
Experimental. 20 seconds exposure, 0.5 degree steps, 40mm distance Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.