Redox Tuning via Ligand-Induced Geometric Distortions at a YMn₃O₄ Cubane Model of the Biological Oxygen Evolving Complex

The function of proteins involved in electron transfer is dependent on cofactors attaining the necessary reduction potentials. We establish a mode of cluster redox tuning through steric pressure on a synthetic model related to Photosystem II. Resembling the cuboidal [CaMn₃O₄] subsite of the biological oxygen evolving complex (OEC), [Mn4O4] and [YMn₃O₄] complexes featuring ligands of different basicity and chelating properties were characterized by cyclic voltammetry. In the absence of ligand-induced distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. Contraction of Y-oxo/Y–Mn distances by 0.1/0.15 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strongly basic donors. Related protein-induced changes in Ca-oxo/Ca–Mn distances may have similar effects in tuning the redox potential of the OEC through entatic states and may explain the cation size dependence on the progression of the S-state cycle

Electronic Structure and Spectroscopy of Tetranuclear Mn4O4 and CaMn3O4 Complexes as Models of the Oxygen Evolving Complex in Photosystem II

This thesis describes a series of studies devoted toward the synthesis of model complexes that mimic aspects of structure, redox state, and spectroscopy of the oxygen evolving complex (OEC) of Photosystem II. The OEC is a unique metallocofactor featuring a heteronuclear CaMn4 core that catalyzes water oxidation. While advances in spectroscopic and structural techniques offer an ever more detailed view of the structure of the S-state catalytic intermediates, the precise mechanism of O−O bond formation remains debated. Aspects such as (1) role of Ca2+, (2) the location of the substrate waters, and (3) the (electronic) structure of the S-state intermediates remain unclear. To obtain a better understanding of the OEC, systematic structure−function(property) studies on relevant model complexes may be necessary. Despite significant efforts to prepare tetra- and pentanuclear complexes as models of the OEC, relevant complexes in terms of structure, redox state, spectroscopy, and reactivity are rare, likely due to the synthetic challenges of accessing a series of isolable clusters that are suitable for comparisons. Chapter 1 presents a survey of tetramanganese model compounds with an emphasis on redox state and electronic structure, as probed by magnetometry and EPR spectroscopy. Structurally characterized model complexes are grouped according to Mn oxidation states and the S-state that they are mirroring. In contrast to the vast number of spectroscopic studies on the OEC, studies that probe the effect of systematic changes in structure on the spectroscopy of model complexes are rare in the literature. Chapter 2 presents ongoing synthetic efforts to prepare accurate structural models of the OEC. The synthesis of accurate structural models is hampered by the low structural symmetry of the cluster, the presence of two types of metals, and the propensity of oxo moieties to form extended oligomeric structures. Desymmetrization of the previously reported trinucleating ligand leads to the formation of tetranuclear Mn4II precursors. Oxidation in the presence of Ca2+ leads to a CaMn4O2 model of the OEC, underscoring the utility of low-symmetry multinucleating ligands in the synthesis of hitherto unobserved oxo-bridged multimetallic core geometries related to the OEC. Chapter 3 presents a series of [MnIIIMn3IVO4] cuboidal complexes as spectroscopic models of the S2 state of the OEC. Such complexes resemble the oxidation state and EPR spectra of the S2 state, and the effect of systematic changes in the nature of the bridging ligands on spectroscopy was studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes needs to be carefully assessed. Chapter 4 presents a series of [YMn3O4] complexes as models of the [CaMn3O4] subsite of the OEC. The effect of systematic changes in the basicity and chelating properties of the bridging ligands on redox potential was studied. Results show that in the absence of ligand-induced geometric distortions that enforce a contraction of metal-oxo distances, increasing the basicity of the ligands results in a decrease of cluster reduction potential. A small contraction of metal-oxo/metal-metal distances by ~0.1 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strong basic donors. Such small, protein-induced changes in Ca-oxo/Ca-Mn distances may have a similar effect in tuning the redox potential of the OEC through entatic states, and may explain the cation size dependence on the progression of the S-state cycle. Chapter 5 presents a series of [CaMn3O4] and [YMn3O4] complexes as models of the [CaMn3O4] subsite of the OEC. The effect of systematic changes in cluster geometry, heterometal identity, and bridging oxo protonation on cluster spin state structure was studied. Results show that the electronic structure of the Mn3IV core is highly sensitive to small geometric changes, the nature of the bridging ligands, and the protonation state of the bridging oxos: the spin ground states of essentially isostructural compounds can be S = 3/2, 5/2, or 9/2. Interpretation of EPR signals and subsequent structural assignments based on an S = 9/2 spin state of the CaMn3O4 subsite of the OEC must be done very cautiously. While unfinished, appendices 1 and 2 present other important aspect in OEC model chemistry. Appendix 1 presents the synthesis of 17O-labeled [MnIIIMn3IVO4] and [CaMn3IVO4] complexes as models of the OEC. Ongoing characterization of μ3-oxos in such complexes provide valuable benchmarking parameters for future mechanistic studies. Appendix 2 presents the synthesis and characterization of [Mn4IVO4] cuboidal complexes as spectroscopic models of the S3 state of the OEC, the last observable intermediate prior to O−O bond formation at the OEC.</p

Redox Tuning via Ligand-Induced Geometric Distortions at a YMn₃O₄ Cubane Model of the Biological Oxygen Evolving Complex

The function of proteins involved in electron transfer is dependent on cofactors attaining the necessary reduction potentials. We establish a mode of cluster redox tuning through steric pressure on a synthetic model related to Photosystem II. Resembling the cuboidal [CaMn₃O₄] subsite of the biological oxygen evolving complex (OEC), [Mn4O4] and [YMn₃O₄] complexes featuring ligands of different basicity and chelating properties were characterized by cyclic voltammetry. In the absence of ligand-induced distortions, increasing the basicity of the ligands results in a decrease of cluster reduction potential. Contraction of Y-oxo/Y–Mn distances by 0.1/0.15 Å enforced by a chelating ligand results in an increase of cluster reduction potential even in the presence of strongly basic donors. Related protein-induced changes in Ca-oxo/Ca–Mn distances may have similar effects in tuning the redox potential of the OEC through entatic states and may explain the cation size dependence on the progression of the S-state cycle

New Phosgene-Free Route to Polycarbonates

A process to produce 100 million pounds per year of diphenyl carbonate (DPC) based on the phosgene-free oxidative carbonylation of phenol in US Patent 8,212,066 has been designed. In this reaction, phenol reacts directly with carbon monoxide in the presence of a palladium catalyst, an organic co-catalyst or ligand, a redox catalyst, and a base catalyst. The proposed design features a single-reactor system followed by a separation train that includes a vacuum distillation tower, and it produces only water and carbon monoxide as byproducts. Detailed process and unit descriptions, an in- depth economic analysis, and environmental concerns and other considerations are also included. The net present value of this project was determined to be $51,560,200 with a return on investment of 37.69% and an internal rate of return of 36.1%

S = 3 Ground State for a Tetranuclear Mn^(IV)₄O₄ Complex Mimicking the S₃ State of the Oxygen Evolving Complex

The S₃ state is currently the last observable intermediate prior to O–O bond formation at the oxygen-evolving complex (OEC) of Photosystem II, and its electronic structure has been assigned to a homovalent Mn^(IV)₄ core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S₃ state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S₃ state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a Mn^(IV)₄O₄ cuboidal complex as a spectroscopic model of the S₃ state. Results show that this Mn^(IV)₄O₄ complex has an S = 3 ground state with isotropic ⁵⁵Mn hyperfine coupling constants of −75, −88, −91, and 66 MHz. These parameters are consistent with an αααβ spin topology approaching the trimer–monomer magnetic coupling model of pseudo-octahedral Mn^(IV) centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S₂ state to the S₃ state. This same spin state change is observed following oxidation of the previously reported Mn^(III)Mn^(IV)₃O₄ cuboidal complex to the Mn^(IV)₄O₄ complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC

A CaMn_4O_2 model of the biological oxygen evolving complex: synthesis via cluster expansion on a low symmetry ligand

Using a new multinucleating ligand featuring two dipyridyl alkoxide moieties and a carboxylate moiety, low symmetry tetranuclear complexes 1-M (M = Mn, Fe, and Co) have been synthesized. Complex 1-Mn was used as a precursor for the synthesis of a pentanuclear CaMn_4O_2 cluster (3) with the same metal stoichiometry as the biological OEC

Ethylene Tetramerization Catalysis: Effects of Aluminum-Induced Isomerization of PNP to PPN Ligands

Diphosphinoamines (PNP) are commonly used to support Cr-catalyzed ethylene trimerization and tetramerization. Although isomerization of PNP to a PPN (iminobisphosphine) species has been established, such reactivity has not been studied in detail in the context of Cr-based selective ethylene oligomerization catalysis. Herein, we show that precursors that are stable as PNP frameworks can isomerize to PPN species in the presence of chlorinated aluminum activators relevant to ethylene oligomerization catalysis. Isomerization changes the pattern of reactivity of the ligands, making them more susceptible to nucleophilic attack by alkyl groups, resulting in a variety of degradation products. The isomerization-mediated degradation of PNP ligands leads to the formation of unwanted polymerization catalysts in ethylene tetramerization systems, thus providing insight into the formation of Cr species that affect the overall selectivity and activity values. For example, independently prepared [R_2PNR]^— leads to potent Cr polymerization catalysts. The susceptibility for isomerization is dependent on the nature of the N-substituent of the PNP precursor. Electron donating N-substituent i-Pr, which disfavors the PPN isomer compared to p-tolyl, and minimization of water contamination correlate with higher oligomerization activity and lower polymer byproducts. More broadly, the present study demonstrates the significant impact that Al-activators can have on the structure and behavior of the supporting ligand leading to detrimental reactivity

S = 3 Ground State for a Tetranuclear Mn^(IV)₄O₄ Complex Mimicking the S₃ State of the Oxygen Evolving Complex

The S₃ state is currently the last observable intermediate prior to O–O bond formation at the oxygen-evolving complex (OEC) of Photosystem II, and its electronic structure has been assigned to a homovalent Mn^(IV)₄ core with an S = 3 ground state. While structural interpretations based on the EPR spectroscopic features of the S₃ state provide valuable mechanistic insight, corresponding synthetic and spectroscopic studies on tetranuclear complexes mirroring the Mn oxidation states of the S₃ state remain rare. Herein, we report the synthesis and characterization by XAS and multifrequency EPR spectroscopy of a Mn^(IV)₄O₄ cuboidal complex as a spectroscopic model of the S₃ state. Results show that this Mn^(IV)₄O₄ complex has an S = 3 ground state with isotropic ⁵⁵Mn hyperfine coupling constants of −75, −88, −91, and 66 MHz. These parameters are consistent with an αααβ spin topology approaching the trimer–monomer magnetic coupling model of pseudo-octahedral Mn^(IV) centers. Importantly, the spin ground state changes from S = 1/2 to S = 3 as the OEC is oxidized from the S₂ state to the S₃ state. This same spin state change is observed following oxidation of the previously reported Mn^(III)Mn^(IV)₃O₄ cuboidal complex to the Mn^(IV)₄O₄ complex described here. This sets a synthetic precedent for the observed low-spin to high-spin conversion in the OEC

Calcium Valence-to-Core X-ray Emission Spectroscopy: A Sensitive Probe of Oxo Protonation in Structural Models of the Oxygen-Evolving Complex

Calcium is an abundant, nontoxic metal that finds many roles in synthetic and biological systems including the oxygen-evolving complex (OEC) of photosystem II. Characterization methods for calcium centers, however, are underdeveloped compared to those available for transition metals. Valence-to-core X-ray emission spectroscopy (VtC XES) selectively probes the electronic structure of an element’s chemical environment, providing insight that complements the geometric information available from other techniques. Here, the utility of calcium VtC XES is established using an in-house dispersive spectrometer in combination with density functional theory. Spectral trends are rationalized within a molecular orbital framework, and Kβ_(2,5) transitions, derived from molecular orbitals with primarily ligand p character, are found to be a promising probe of the calcium coordination environment. In particular, it is shown that calcium VtC XES is sensitive to the electronic structure changes that accompany oxo protonation in Mn₃CaO₄-based molecular mimics of the OEC. Through correlation to calculations, the potential of calcium VtC XES to address unresolved questions regarding the mechanism of biological water oxidation is highlighted

Tetranuclear [Mn^(III)Mn_3^(IV)O_4] Complexes as Spectroscopic Models of the S_2 State of the Oxygen Evolving Complex in Photosystem II

Despite extensive biochemical, spectroscopic, and computational studies, the mechanism of biological water oxidation by the oxygen evolving complex (OEC) of Photosystem II remains a subject of significant debate. Mechanistic proposals are guided by the characterization of reaction intermediates such as the S_2 state, which features two characteristic EPR signals at g = 2 and g = 4.1. Two nearly isoenergetic structural isomers have been proposed as the source of these distinct signals, but relevant structure–electronic structure studies remain rare. Herein, we report the synthesis, crystal structure, electrochemistry, XAS, magnetic susceptibility, variable temperature CW-EPR, and pulse EPR data for a series of [Mn^(III)Mn_3^(IV)O_4] cuboidal complexes as spectroscopic models of the S_2 state of the OEC. Resembling the oxidation state and EPR spectra of the S_2 state of the OEC, these model complexes show two EPR signals, a broad low field signal and a multiline signal, that are remarkably similar to the biological system. The effect of systematic changes in the nature of the bridging ligands on spectroscopy were studied. Results show that the electronic structure of tetranuclear Mn complexes is highly sensitive to even small geometric changes and the nature of the bridging ligands. Our model studies suggest that the spectroscopic properties of the OEC may also react very sensitively to small changes in structure; the effect of protonation state and other reorganization processes need to be carefully assessed