Mainly two groups have advanced model chemistry of diiron dinitrosyl complexes over the last couple of years. The very first model system of a [Fe(NO)7]2 intermediate (XI) was published by Lippard et al. in 1996.[81] The diiron core in this example was stabilized by a dinucleating ligand N-Et-hptb providing a {N3O} donorset for each iron atom. As coligand a carboxylate bridge was introduced. The two nitrosyl ligands coordinate in a syn fashion and are spatially quite close. Mössbauer data confirmed an S= 3/2 spin state for each iron ion and the two metal ions were antiferromagnetically coupled. Upon oxygenation of the diiron dinitrosyl complex the formation of a corresponding diiron dini-trate species was observed. Further insights in the complex’ behavior upon reduction were not provided until 2015.[102] Upon exposure to light at room temperature and at 15 K, N2O formation was observed. Mechanistically the authors propose that upon irradiation a mononitrosyl intermediate is formed as they observed a new band in the infrared spec-trum (1695 cm–1) compared to the signature of the initial dinitrosyl stretching frequency (1784 cm–1). In a subsequent step a putative hyponitrite species is possibly formed via an electrophilic attack of the second NO molecule, which finally yielded N2O.
A second model (XII) that is capable of producing N2O upon reduction was presented by the Lehnert group in 2013.[103] A similar dinucleating ligand system with {N3O}
donor set was chosen. Instead of benzimidazole moieties, the ligand system contained pyridine side arms. Propionate or benzoate was employed as bridging coligand.[83,103]In the diiron dinitrosyl complex both iron atoms were found to be high spin with S= 3/2 and antiferromagnetically coupled. Upon chemical reduction of the propionate bridged version of these complexes with cobaltocene or KC8, complete conversion to N2O was observed.[103] As a mechanism the so called super reduced pathway has been suggested.
This assumption was supported by electrochemical studies as the reduction of the Fe–NO sites occurred in a single 2 e– transfer process and not sequentially.
Scheme 1.15 depicts the two described systems.
O
Scheme 1.15: Diiron dinitrosyl model complexes for the reduction of NO to N2O.
A detailed comparison of their structural and spectroscopic signatures is given in Table 1.5.
Table 1.5: Comparison of structural and spectroscopic parameters of models XIand XIIwith those of FprA.
FprA-NO reacted[94,98]
XI[81] XII[103]
Fe···Fe [Å] 3.23-3.57a 3.44 3.47
N–O[Å] 1.16a 1.15 1.16, 1.17
Fe–N–O [◦] 167a 167 156, 145
N···N (NO) [Å] 2.80a 2.82 2.80
λ[nm] 453 520, 620 410
[M–1cm–1] 2000 395, 579 ∼2000
δ [mm s–1] 0.71 0.67
-|∆EQ| [mm s–1] 1.85 1.44
-ν˜[cm–1] - 1785 1760
aBond distances and angles from calculations.
2 Aim of this Work
Low molecular weight analogues for enzymatic non heme diiron sites are important for understanding the function of these enzymes and in particular the potential mechanisms for the activation of small molecules. Efforts in this field have been made apart from elucidating biological functions to develop efficient catalysts for controlled reactions under mild conditions that would otherwise require immense energy resources.
The aim if this work is to develop new diiron complexes with monoanionic, pyrazolate bridged, bis(pentadentate), entirely nitrogen based chelating ligands to obtain functional models of non heme diiron sites for the activation of nitric oxide and dioxygen. The spectroscopic and structural characterization of precursors and intermediates during this process is pursued to obtain a deeper understanding of reaction pathways and catalytic events.
The anticipated diiron complexes with nitrogen based bis(pentadentate) ligand scaffolds differ from the active sites of biological of non heme diiron proteins. However, it is known that nitrogen donors impart stability while carboxylates are prone to dissociate or feature variable coordination modes. Moreover, high valent intermediates in diiron oxo model chemistry have been realized until today with largely nitrogen based ligands.
N N N
N
N N
N
N
N
N
Fe Fe
L L
L = Solvent, O2, NO N
= N
N N
Scheme 2.1:Targeted pyrazolate based diiron complexes with nitrogen based amine side arms.
In the last couple of years new pyrazolate/pyridine based scaffolds have been developed which have been shown to bridge two metal sites via the nitrogen atoms of the
pyrazo-late.[104,105] By the introduction of amine based side arms with additional pyridine or imidazolyl moieties, chelating nitrogen based ligands that should incorporate two iron centers can be obtained. The new diiron complexes should bear at least one vacant coor-dination site to bind labile exogenous coligands such as solvent molecules and allow for an access and binding of small molecules and/or substrates.
The diferrous form of these new molecules should serve as a platform for new reactivity and thus should be fully understood spectroscopically, structurally and electronically.
Consequently, these new models should also be investigated towards their ability to bind small molecules, in particular dioxygen and nitric oxide. Provided that this reactivity is accessible with these novel diiron complexes, a thorough analysis is anticipated to gain in-sight into their redox behavior and the catalytical transformation of these small molecules.
Ultimately, their reactivity with respect to substrate activation and applicability should be tested.
3 Design and Synthesis of Pyrazolate based Bis(pentadentate) Ligand Scaffolds
3.1 Inspiration
Ligand scaffolds that can host two metal sites are widely established in nature and model chemistry. The simplest ligands that can bridge two metal sites are single atom bridges such as halides, oxides or sulfides (e.g. as shown in the previously described diamond core structures).[106] Di- and triatomic bridging units as, e.g., pyrazolates or carboxylates can be used to tune the metal···metal distance, which has an influence on many factors such as reactivity, cooperativity and stability. Pyrazolates, however, outrival simple single atom bridges as they can be functionalized in the 3, 4, and 5-position to introduce additional donor sites that can stabilize the metal core, contribute to the overall chelation of the metal centers and thus enhance the chelate effect.[107–110] Pyrazoles in their deprotonated form serve as monoanionic ligands and are therefore beneficial to stabilize metals in higher oxidation states.
Scheme 3.1: Examples for single atom, diatomic (pyrazolate) and triatomic bridging ligands.
Moreover, small nitrogen containing heterocycles are close mimics for nitrogen donor sites in non-heme diiron proteins such as the imidazole group of the amino acid histidine.[111]
A clear advantage for the use of a pyrazolate over a carboxylate as bidentate bridging ligand in model chemistry lies in the lability and flexibility of carboxylate binding modes known as the carboxylate shift or twist.[7,112–114]The carboxylate shift may be beneficial in enzymatic oxygenation reactions and substrate uptake, in model chemistry, however, the binding mode of carboxylates is hardly controllable.
In theMeyergroup, pyrazolate based compartmental ligands were established for a series of polynuclear metal complexes.[115–117] Pyrazoles functionalized in the 3- and 5-position are covalently connected with a huge library of side arms. The design of a ligand for a certain bimetallic site involves a couple of considerations. The ligand framework can be altered at basically two positions. One is the variation of the spacer between the two metals, which would result in the modification of the pyrazole building unit. Another option that may alter the denticity is the choice of a specific side arm.
The bis(pentadentate) ligand scaffolds used in this work are inspired by a series of preced-ing complexes. Stable, oxidatively rugged ligands connectpreced-ing two metal sites cooperatively via a bridging pyrazolate were developed for the application in Ru-based water oxidation catalysis[118–121] (XIII, Scheme 3.2) and even before for the self assembly of [2×2]-Fe grid complexes (XIV, Scheme 3.2).[122–124]
Scheme 3.2:Water oxidation catalystXIIIand [2×2] grid complexXIVwith oxidatively rugged pyrazolate based ligand backbones.
A robust ligand design can be obtained by connecting the pyrazole in the 3- and 5-position with small aromatic heterocycles such as pyridines. This is advantageous to prevent ligand degradation if metals in high oxidation states under rather harsh reaction conditions need to be stabilized.[125] For the activation of small molecules at diiron sites with subsequent catalytic events, high valent intermediates might have to be stabilized, in which case a robust ligand backbone might be beneficial.
Dinuclear iron complexes favor the coordination by five or six donor atoms for each metal site. Hence to design a chelating ligand that can fulfill this requirement, aside from the pyrazolate backbone additional side arms need to be introduced.
Ligands with tetradentate chelating moieties for dinuclear pyrazolate based iron com-plexes[113,126–128] and with pentadentate moieties for dinuclear pyrazolate based ruthe-nium complexes[120] have been presented before (Scheme 3.3). The systems shown in Scheme 3.3 in particular comprise ligand scaffolds with amine side arms bearing small N-donating heterocycles for the stabilization of the diiron core.
The bis(pentadentate) pyrazolate based ligandXVIpreviously used for bimetallic ruthe-nium complexes will be employed in this work for the synthesis of diiron complexes.
N N N
Scheme 3.3: Left: Pyrazolate bridged diiron complex with bis((methylimidazolyl)methyl)amine side arms for the activation of small molecules. Right: Ruthenium based bimetallic complex with picolylamine side arms.