Bio-Inspired Homometallic and Heterometallic Clusters Relevant to the Oxygen-Evolving Complex of Photosystem II

Abstract

The following two chapters delineate several endeavors to isolate and characterize functional models of the oxygen-evolving complex (OEC) of photosystem II. Understanding the electronic structure and the precise mechanism of the O–O bond coupling step in the Kok cycle affords insight into this fundamental process and will guide the design of new earth-abundant catalysts to perform water oxidation under environmentally benign conditions. Nature performs this transformation by a heterometallic CaMn4O5 cluster arranged a tetra-metallic cubane bridged to a dangling manganese ion. Although a myriad of synthetic inorganic complexes are capable of water oxidation, these structures significantly underperform the OEC in terms of turnover number and turnover frequency. The objectives of this thesis are (i) to construct multimetallic clusters using the OEC as inspiration, (ii) to explore the reactivity of these clusters with oxygen-atom transfer reagents, and (iii) to identify intermediates responsible for oxygen-based chemistry. In Chapter 1, a series of pseudo-C3 symmetric tetra-manganese clusters with an interstitial µ4-oxygen was synthesized and characterized in several oxidation states. These clusters (of the general formula [LMn3(PhPz)3OMn][OTx]x; x = 1, 2) are supported by pyridine and alkoxide donors connected by a 1,3,5-triarylbenzene spacer. A µ4-oxygen coordinates all four metal centers that are also bridged by phenyl pyrazolate (PhPz) ligands. This arrangement furnishes a vacant coordination site at a site-differentiated (apical) metal center. Exposure of these clusters to oxygen-atom transfer reagents (OAT’s) results in the intramolecular oxygenation of a C(sp2)–H bond of the bridging phenyl pyrazolate. Similarly, using 2,6-difluorophenyl pyrazolate (F2ArPz) as the bridging ligand results in the oxygenation of the C–F bond with concurrent F-atom transfer. This reactivity represents an unprecedented C–F activation for molecular manganese complexes. All hydroxylated and fluorinated clusters were independently prepared to confirm the observed reactivity upon exposure to OAT’s. The pathways responsible for arene activation – postulated to proceed through an iodosobenzene adduct and subsequent formation of a transient high-valent manganese-oxo motif – are discussed. In Chapter 2, a series of pseudo-C3 symmetric heterometallic Fe3Mn clusters of the general formula [LMn3(PhPz)3OMn][OTf]x (x = 1–3) was synthesized and characterized. Similar to their homometallic tetra-manganese and tetra-iron analogs (Chapter 1), these clusters contain four metal centers with a central bridging interstitial µ4-oxygen atom and bridging phenyl pyrazolate ligands. These clusters are further supported by pyridine and alkoxide donors, linked through a 1,3,5-triarylbenzene spacer. All complexes were characterized by zero-field 57Fe Mossbauer spectroscopy to confirm the presence of a manganese metal center in the apical position, illustrating that these clusters are stable with respect to metal scrambling and/or decomposition. Treatment of these clusters with 1-(tert-butylsulfonyl)-2-iodosylbenzene (sPhIO) resulted in the oxygenation of the C(sp2)–H bond of the proximal phenyl pyrazolate motif to afford [LMn3(PhPz)2(OArPz)OMn][OTf]x (x = 2, 3). During these studies, an unusual iodosobenzene adduct of [FeIII3MnII]3+ was isolated prior to C–H activation. This adduct has been characterized both by single-crystal XRD and 1H-NMR spectroscopy. In order to gain insight into the C–H bond oxygenation by this iodosobenzene adduct, preliminary computational studies are presented to discuss the viability of a transient manganese-oxo species responsible for arene hydroxylation.