The goal in understanding the structure of the coordination environment of the Mn4cluster is to relate that structure to the function of each of the components and the mechanism of OO bond
formation. The prevelant proposals for H2O oxidation by the OEC are presented below.
8.20.5.1 Berkeley Model
The mechanism for water oxidation proposed by the group of Klein, Sauer, and Yachandra is shown inFigure 24.10,237The structure of the Mn4cluster is based on the results from EXAFS.30 The mechanism involves successive oxidation of the Mn4cluster via pathway A in Figure 23for all S-state transitions. The S2!S3transition, however, results in ligand-centered oxidation of a bridging -oxo rather than Mn oxidation. OO bond formation occurs by reaction of the bridging -oxyl radical with either outer-sphere H2O or another oxo bridge. Precedent for OO bond formation between bridging oxo groups comes from the bis(-oxo)dicopper com- plexes studied by Tolman and co-workers,308which form bridging-peroxy complexes. However, there is as yet no report of bridging -peroxy formation in model bis(-oxo)dimanganese complexes.
8.20.5.2 Babcock H-atom Abstraction Model
The mechanism of Babcock and co-workers was the first to suggest that pathway B inFigure 25is involved in YZ
reduction.223 Contrary to the Berkeley model, in this mechanism pathway B occurs in all S-state transitions, each time removing both an electron from Mn and proton from H2O. For this reason, the mechanism is known as the H-atom abstraction model, because the removal of both an electron from Mn and a proton from Mn-bound H2O is similar to abstraction
Table 4 Homolytic bond dissociation energies for substituted phenols and Mn complexes with water and hydroxide ligands.
Compound HBDE(kcal mol1)a References
Phenol 89.85 305
4-methyl-phenol 88.70 305
MnIIIMnIV(salpn)2(-O,-OH) 76.00 306
MnIIIMnIV((3,5-Cl-sal)pn)2(-O,-OH) 77.00 306
MnIIIMnIV((3,5-NO2-sal)pn)2(-O,-OH) 79.00 306
[MnIIIMnIII(bpy)4(-O,-OH)]3þ 84.00 307
MnIIIMnIII(2-OH(3,5-Cl-sal)pn)OH2 89.00 233
MnIIIMnIII(2-OHsalpn)OH2 85.00 233
MnIIIMnIII(2-OH(3,5-t-Bu-sal)pn)OH2 82.00 233
[MnIIIMnIV(2-OH(3,5-Cl-sal)pn)OH2]þ 94.00 233
MnIIIMnIV(2-OHsalpn)OH2þ 89.00 233
MnIIIMnIV(2-OH(3,5-t-Bu-sal)pn)OH2þ 86.00 233
a Values for phenols were measured in DMSO, while values for the Mn complexes were measured in 16 M H2O/CH3CN.
of an H-atom. Successive oxidations of the Mn4cluster by PCET path B leads to the formation of a terminal MnIVẳO species that undergoes a nucleophilic attack by a Mn-bound hydroxide. This results in a transient peroxide in S4that is oxidized to O2. Ca2þis proposed to act as a docking site for Cl, which migrates from Ca2þ to coordinate to the Mn4 cluster in the S2-state to neutralize the increasing positive charge on Mn.
8.20.5.3 Mn–oxyl Radical Model
In the mechanism put forth by Siegbahn,144 depicted in Figure 26, successive S-state advance- ments via PCET path B leads to the formation of a -oxyl radical. The mechanism is based largely upon density functional theory calculations. The role of Ca2þis to polarize the MnO
bonds and direct the placement of the radical. The OO bond-forming step is as yet unclear in the density functional theory (DFT) calculations, but it appears to involve reaction of the oxyl radical with outer-sphere H2O to form a hydroperoxide species that is oxidized to O2.
MnIV
O MnIII O O
MnIV MnIV
O O MnIII
O MnIII O O
MnIV MnIV
O O
S2 S1
MnIII
O MnII O O
MnIV MnIV
O O
H H
MnIV
O MnIII O O
MnIV MnIV
O O H2O
S0
MnIV
O MnIII O O
MnIV MnIV
O O S4
-OR- O2
2H2O
S3 MnIV
O MnIII O O
MnIV MnIV
O O
S4 e-, 2H+
e-
e-
e-
Figure 24 Proposed mechanism of water oxidation by Yachandra and co-workers.10,237Light-driven steps are indicated by solid arrows and spontaneous steps by dashed arrows. Reduction of YZproceeds via mechan- ism A inFigure 23. The key intermediate in OO bond formation is a bridging-oxyl radical formed in S3
that reacts with either another-oxo or an outer-sphere H2O molecule.
8.20.5.4 Electrophilic Mn–oxo Models
Based in part on an analysis of the reactivity of Ru and Mn oxidation catalysts, it seems that MnẳO centers are electron deficient and subject to nucleophilic attack by reductants.66 In the electrophilic MnẳO models, the OO bond-forming step involves nucleophilic attack by H2O/
OH on a MnẳO species. In the proposal by Pecoraro and co-workers,139 the S0!S1
transition involves PCET path A, but the transitions of S2!S3!S4 involve path B, as shown in Figure 27. This leads to the formation of Ca-bound OH and MnẳO species.
Nucleophilic attack of the OH on the MnẳO forms a terminal hydroperoxide, which then displaces Clon a second terminal Mn and is oxidized to O2.
The mechanism of Brudvig and co-workers, shown in Figure 28, also involves a MnẳO species.13 S-state advancement from S0!S1!S2 involves PCET path A. The large increases in activation energy and the deuterium kinetic isotope effect measured for the S2!S3transition are taken to mean that there is a switch from path A to path B in this step, triggered by the increased net positive charge on the Mn4cluster in S2. This leads to the formation of a MnVẳO species in S4. Nucleophilic attack on the MnVẳO by a Ca-bound H2O is prompted by
S3 MnIV
O MnIV O
O MnIV MnIV
O
OH O S4
MnIV O MnIV O
O MnIII MnIV
O O
O 2H2O
S2 O2
MnIV O MnIV O
O MnIII MnIV
O
OH OH S0
MnIV O MnII O
O MnIII MnIV
O
OH2 OH2
S1
MnIV O MnIII O
O MnIII MnIV
O
OH2 YZO OH
YZO YZOH
YZO YZOH YZO
YZOH
YZOH
Figure 25 Proposed mechanism of water oxidation by Babcock and co-workers.223 Light-driven steps are indicated by solid arrows and spontaneous steps by dashed arrows. In this cycle, YZis reduced according to pathway B in Figure 23in each S-state advancement. This forms a terminal-oxo MnẳO in S3that reacts
with Mn-bound hydroxide to form the OO bond.
contraction of the MnVCl bond. This forms a hydroperoxide moiety that hydrogen-bonds to a bridging -O, analogous to the binding of O2in hemerythrin. Oxidation of the hydroperoxide reduces Mn and releases O2.
S3
MnIV O O MnIV O MnIII
O O
Ca OH H2O
Cl H
O MnIV MnIV
O HO
MnIV O MnIII
O O
Ca O H2O
Cl H
O MnIV
S4 S2
MnIV O O MnIV O MnIII
O O
Ca OH H2O
Cl H H
O MnIV 2H2O
O2
S0 S1
MnII O HO
MnIV O MnIII
O O
Ca OH2
H2O Cl
H H
O MnIV
MnIII O O MnIV O MnIII
O O
Ca OH2
H2O Cl
H H
O MnIV
1) Possible transfer of radical to à-OH bridge between Mn and Ca
2) H-atom abstraction from outer sphere water and formation of à-OOH 3) Oxidation of à-OOH
S3'
e–
YZO YZOH
YZO YZOH
YZO YZOH
Figure 26 Proposed mechanism of water oxidation by Siegbahn.144Light-driven steps are indicated by solid arrows and spontaneous steps by dashed arrows. Reduction of YZby mechanism B inFigure 23leads to the formation of a bridging-oxyl radical in S3. Ca2þaids in polarizing the MnO bonds to direct proton and electron transfer. The OO bond-forming step involves reaction of the -oxyl radical with outer-sphere
H2O.