Frontiers in spectroscopic techniques in inorganic chemistry

The reference to the “structure” of a chemical entity inevitably, in common practice, depicts its nuclear co-ordinates. Yet, through decades of research in synthetic bio-inorganic chemistry, an inherent fallacy of such a practice has revealed itself, and a question has repeatedly baffled chemists – if it looks like it, why doesn't it work like it? From the 1970s to 1990s, synthetic inorganic chemistry accomplished the synthesis of elegant structural models of several metalloenzymes, only to be baffled by their difference or lack of reactivity when compared to their natural counterparts. Alternatively, the same reactivity could, in certain cases, be derived from artificial analogues that hardly mimicked the exact design of the protein active site. The solution to the dilemma lay in the realization that chemical reactivity depended, largely, on the spatial distribution, spin, and energy of electrons in the molecule i.e. its electronic structure. The electronic structure of a molecule, while ingrained in its geometric structure, is not directly obtained from its molecular geometry. Rather a different set of experimental techniques are needed to extract the details of the electronic structure of a molecule – spectroscopy.

Electronic structure reflects the bonding that results in the overall structure, the orientation and energies of the valence electrons, molecular vibrations and spin density distribution. The first introduction to electronic structure for most students of science is likely the blue to pink transition of CoCl₂ upon dilution. In advanced curriculum, the structure of ferrocene, the intense color of Prussian blue, the magnetic behavior of the copper acetate dimer, the synergistic effect in metal carbonyls and the consequences of ligand field theory enthralled many of us. The realization that electron and spin density eventually decides reactivity and that normal modes in vibrational spectra describe reaction co-ordinates empowers a spectroscopist with a unique multi-dimensional view of chemical processes. Over the years, different spectroscopic techniques have emerged to investigate different aspects of the electronic structure of a molecule. Many of these developments have been prompted by an urge to understand the electronic structures of key transition metal active sites in nature. The spectroscopic techniques involved/developed span excitation energies from 10⁻¹ cm⁻¹ to 10⁸ cm⁻¹. These efforts have been aptly rewarded with the unraveling of small nuances in the immediate co-ordination environment or around the active site that play a determining role in the function of these metalloenzyme factors that went amiss in synthetic models prepared without the foreknowledge of these, apparently, small effects. These effects can cause 1 V differences in reduction potentials, orders of magnitude differences in O₂ binding, and radical changes in reactivity and selectivity. Gradually, structure function correlations evolved into geometric and electronic structure contributions. Current synthetic efforts have begun to take cognizance of these effects, and the phrase “2^nd sphere effect” has now transcended the metalloenzyme literature to artificial catalysis.

The goal of this issue is to highlight the work being done all over the world in which advanced spectroscopic methods are complementing conventional structural and reactivity probes to elucidate deeper understanding of complex chemical reactions and emulate the reactivity of transition metal active sites. A combination of spectroscopic techniques alongside conventional analytical tools can not only enable diagnosis of chemical processes, but also provide deeper insight into the physical details of the processes involved at a molecular level. This issue will focus on the recent developments in different contemporary areas of inorganic chemistry research which heavily rely on the application of different spectroscopic techniques like resonance Raman, FTIR, EPR (cw and pulsed), Mössbauer (zero field and magnetic), MCD/XMCD, and XAS/EXAFS in the investigation and analyses of inorganic, organometallic, and biological systems, gaining insight into unusual electronic structures and the ensuing reactivity which are relevant to catalysis, organometallics, sensing and energy solutions.

Garcia-Serres (DOI: 10.1039/C7DT00736A) uses a combination of EPR and Mössbauer to show the direct interaction of pyruvate and SAM with the two iron–sulfur clusters in a TW1 protein involved in Wybutosine synthase, which is involved in tRNA modification. The space between the two clusters is enough to accommodate both cofactors, and the data show that binding of pyruvate to the proximal cluster causes its oxidation. Schenk (DOI: 10.1039/C7DT01350G) uses a combination of MCD, EPR, ITC and detailed kinetics to probe the active site of CpsB, which is a player in antibiotic resistance. The VTVH data and the ground state parameters, the rates obtained with different metals and the EPR data clearly suggest that two Co²⁺ ions form the active site, and not three Mn²⁺ ions as reported in the crystal structure. John Enemark (DOI: 10.1039/C7DT01731F) shows how a combination of EPR and pulsed electron spin echo spectroscopy on different forms of the sulfur oxidizing enzyme containing Mo(V) has led to a consensus regarding the active site structure. Kirk (DOI: 10.1039/C7DT01728F) uses a combination of resonance Raman spectroscopy and DFT calculations to probe the nature of the active site and substrate interaction in xanthine oxidase. Resonance enhancement of the MLCT band involving the Mo and product directly implies the role of the Mo–O–C interaction in promoting the two-electron oxidation and lowering the transition state barrier of a very important reaction.

Scholes and Burger (DOI: 10.1039/C7DT01354J) use ENDOR spectroscopy to probe the local environment of ferric bleomycin produced by cryo-reduction of activated bleomycin, which is known to bind DNA and cleave it specifically. The resonances indicate that there is a solvent molecule bound to the ferric site in ferric bleomycin, while the hydrogen bonding to the hydroperoxide group in activated bleomycin from the neighboring peptide chain is clearly picked up. An abundance of protons are available to quench the high valency iron species produced on O–O bond cleavage. Ghosh Dey and co-workers (DOI: 10.1039/C7DT01700F) show that heme bound Aβ peptides, which could be potent sources of oxidative stress, generally favor a high spin iron active site. They use a combination of Raman, EPR and absorption spectroscopy to show that excess Aβ peptide seems to neutralize the threat of heme-Aβ by converting it to a less reactive low spin site. Yet on simultaneous binding of Cu²⁺, the resulting high spin heme and Cu²⁺ bound Aβ peptides are even more prone to producing reactive oxygen species than heme and Cu alone.

Spectroscopic investigations of electronic structure bridge metalloenzymology and biomimetic chemistry. This is wonderfully demonstrated by Fiedler (DOI: 10.1039/C7DT01600J), using a biomimetic Co complex, thoroughly characterized by X-ray diffraction, NMR and EPR, to understand the reaction catalyzed by cysteine dioxygenase. The reactivity of the Co(II) complex depends on the substituents on the ligand, and in one of these cases a Co(III)–O₂⁻ species was isolated and characterized with EPR, resonance Raman and DFT calculations. These results help us understand the factors that control the reactivity of the enzyme active site. Metal nitrosyls are key entities in biology, and their ground state wavefunctions represent some of the most complex known cases of electronic structure. Lehnert and Fujisawa (DOI: 10.1039/C7DT01565H) report a detailed investigation of cobalt nitrosyls using FTIR/far IR, MCD, NMR and DFT calculations. These nitrosyls react with O₂ to produce nitrido bound Co complexes. The Franck–Condon analysis of the Co–NO excited state shows substantial weakening of the Co–N bond, which is key to its observed reactivity with O₂. Lim (DOI: 10.1039/C7DT01489A) uses a combination of EPR, ¹H and ¹⁵N ENDOR and DFT calculations to discover a synthetic route to access, for the first time, the less known trans-III isomer of the Cu tetramethylcyclam complex, utilizing an anion effect. The spectroscopic parameters, in conjunction with DFT calculations, not only enabled the definitive assignment of the isomers, but also indicated factors that are responsible for their relative stability. Ghosh (DOI: 10.1039/C7DT01364G) uses elegant NMR and ESI-MS techniques to elucidate the dynamics of [2]-pseudorotaxanes and factors that can block and unblock the dynamics. A series of transition metal bound [2]-pseudorotaxanes are used as precursors to produce rotaxanes which are characterized using a series of techniques. Using ⁴⁵Sc NMR, Gauvin et al. (DOI: 10.1039/C7DT02415K) could demonstrate the complex nature of the interface of a catalyst and its solid support, here silica, in heterogeneous catalysts, stressing furthermore the need for spectroscopic analysis of these composites to get a clear picture.

The importance of using modern spectroscopic methods in organometallic chemistry has been steadily increasing. Neidig and Deng (DOI: 10.1039/C7DT01748K) compare the electronic structures of Co(II) phosphine and Co(II) NHC complexes, which are very efficient organometallic catalysts, using MCD and DFT calculations. The NHCs are found to be better donors than diamine and phosphine ligands. Substitution on the nitrogen in the NHC has a stronger effect on the electronic structure of the Co center than peripheral substitution. Apfel (DOI: 10.1039/C7DT01459G) reports the spectroscopy and reactivity of a series of triphos ligands. Apart from the effective synthetic strategy, the work also uses spectroelectrochemistry to show isomerization of CO complexes of ligands bearing both phosphines and thioethers. Their results, also backed up by DFT calculations, reveal the possibility of enhancing the known reactivity of metal complexes with triphos based ligands.

Ultrafast techniques have been amply used in gaining insight into the excited states of inorganic complexes. Gray (DOI: 10.1039/C7DT02632C) utilizes a nano-second pulsed laser system to derive the two-photon absorption cross section using two-photon luminescence spectroscopy. Using this new approach, he determined that the cross section of tungsten isocyanide is 300–400 times greater than that of a Ru bipyridine complex. Hammarström (DOI: 10.1039/C7DT02437A) uses transient spectroscopy to investigate low singlet–triplet mixing, which results in microsecond long triplet lifetimes that deviate from the energy gap law.

Raman is a powerful technique that has recently been used to probe electrocatalysts in situ. Weidinger and co-workers (DOI: 10.1039/C7DT01174A) demonstrate this approach by investigating iron Hangman porphyrins which are drop cast on electrodes. They propose new intermediates which were not observed in solution when they investigated the electrodes involved in O₂ reduction. These insights are very valuable in designing efficient catalysts for O₂ reduction. Tian and co-workers (DOI: 10.1039/C7DT01631J) show how Raman can be used to study the complexation of uranyl systems, which is an important process in the isolation and separation of these rare earth elements.

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