Luca Bertini

Chemical information from the source function

The source function, which enables one to equate the value of the electron density at any point within a molecule to a sum of atomic contributions, has been applied to a number of cases. The source function is a model‐independent, quantitative measure of the relative importance of an atoms or groups contribution to the density at any point in a system, and it represents a potentially interesting tool to provide chemical information. It is shown that the source contribution from H to the electron density ρb at the bond critical point in HX diatomics decreases with increasing Xs electronegativity, and that this decrease is a result of significant changes in the Laplacian distribution within the H‐basin. It is also demonstrated that the source function from Li to ρb in LiX diatomics is a more sensitive index of atomic transferability than it is the lithium atomic energy or population. The observed changes are such as to ensure a constant percentage source contribution from Li to ρb throughout the LiX series, rather than a constant source as one would expect in the limit of perfect atomic transferability. Application of the source function to planar lithium clusters has revealed that the source function clearly discriminates between a nonnuclear electron density maximum and a maximum associated to a nucleus, on the basis of the relative weight of the source contributions from the basin associated to the maximum and from the remaining basins in the cluster. The source function has also allowed for a classification of hydrogen bonds in terms of characteristic source contributions to the density at the H‐bond critical point from the H involved in the H‐bond, the H‐donor D, and the H‐acceptor A. The source contribution from the H appears as the most distinctive marker of the H‐bond strength, being highly negative for isolated H‐bonds, slightly negative for polarized assisted H‐bonds, close to zero for resonance‐assisted H‐bonds, and largely positive for charge‐assisted H‐bonds. The contributions from atoms other than H, D, and A strongly increase with decreasing H‐bond strength, consistently with the parallel increased electrostatic character of the interaction. The correspondence between the classification provided by the Electron Localization Function topologic approach and by the source function has been highlighted. It is concluded that the source function represents a practical tool to disclose the local and nonlocal character of the electron density distributions and to quantify such a locality and nonlocality in terms of a physically sound and appealing chemical partitioning.

Nanostructured Co1−xNixSb3 skutterudites: Synthesis, thermoelectric properties, and theoretical modeling

Nanostructured skutterudite Co1-xNixSb3 has been synthesized by chemical alloying with Ni substitution for Co up to 27.5 at. %. High concentration of grain boundaries provided by nanostructuring is ...

Nanostructured Co1-xNix(Sb1-yTey)(3) skutterudites : Theoretical modeling, synthesis and thermoelectric properties

The properties of Te-doped Co(Sb1-yTey)(3) and Te-Ni double-doped Co1-xNix(Sb1-yTey)(3) nanostructured skutterudites were evaluated by means of x-ray powder diffraction, and transport properties me ...

The local form of the source function as a fingerprint of strong and weak intra- and intermolecular interactions

This work introduces a local form for the source function, from each atom, for the electron-density value at a given point. The source function enables one to equate the value of the electron density at any point within a molecule to a sum of atomic contributions and thus to view properties of the density at representative points, such as the bond critical points, from a new perspective. The local form of the function introduces further detail. When plotted along a bond path and with reference to the bond critical point (b.c.p.), the source function shows which regions of the atoms involved in the bonding are accumulating or removing electronic charge at the b.c.p. The local form of the source function therefore represents an interesting fingerprint of a given bonding interaction. The local source may be expressed as a sum of two contributions, related to the kinetic energy density and electronic potential energy density, respectively. This approach gives further physical insight into why an atomic region is accumulating or removing charge at the b.c.p. The local form of the source function is applied to the study of the second-row diatomic hydride series and of a number of prototypical hydrogen-bonded systems. Differences in the local source contributions to the density at bond critical points due to chemical bonding (deformation density) and crystallization (interaction density) are also explored and found to be more informative and experimentally detectable than are the corresponding changes for the bond-critical-point properties of weak intermolecular interactions. This result might be of potential interest when judging the data quality of a charge-density experimental determination. Although the present paper deals with electron densities derived from theoretical computations only, both the source function and its local form should also be easily obtainable from a charge-density quality X-ray diffraction experiment.

Unsensitized Photochemical Hydrogen Production Catalyzed by Diiron Hydrides

The diiron hydride [(μ-H)Fe(2)(pdt)(CO)(4)(dppv)](+) ([H2](+), dppv = cis-1,2-C(2)H(2)(PPh(2))(2)) is shown to be an effective photocatalyst for the H(2) evolution reaction (HER). These experiments establish the role of hydrides in photocatalysis by biomimetic diiron complexes. Trends in redox potentials suggests that other unsymmetrically substituted diiron hydrides are promising catalysts. Unlike previous catalysts for photo-HER, [H2](+) functions without sensitizers: irradiation of [H2](+) in the presence of triflic acid (HOTf) efficiently affords H(2). Instead of sacrificial electron donors, ferrocenes can be used as recyclable electron donors for the photocatalyzed HER, resulting in 4 turnovers.

CO Disrupts the Reduced H-Cluster of FeFe Hydrogenase. A Combined DFT and Protein Film Voltammetry Study

Carbon monoxide is often described as a competitive inhibitor of FeFe hydrogenases, and it is used for probing H(2) binding to synthetic or in silico models of the active site H-cluster. Yet it does not always behave as a simple inhibitor. Using an original approach which combines accurate electrochemical measurements and theoretical calculations, we elucidate the mechanism by which, under certain conditions, CO binding can cause permanent damage to the H-cluster. Like in the case of oxygen inhibition, the reaction with CO engages the entire H-cluster, rather than only the Fe(2) subsite.

Functionally relevant interplay between the Fe(4)S(4) cluster and CN(-) ligands in the active site of [FeFe]-hydrogenases.

[FeFe]-hydrogenases are highly efficient H(2)-evolving metalloenzymes that include cyanides and carbonyls in the active site. The latter is an Fe(6)S(6) cluster (the so-called H-cluster) that can be subdivided into a binuclear portion carrying the CO and CN(-) groups and a tetranuclear subcluster. The fundamental role of cyanide ligands in increasing the basicity of the H-cluster has been highlighted previously. Here a more subtle but crucial role played by the two CN(-) ligands in the active site of [FeFe]-hydrogenases is disclosed. In fact, QM/MM calculations on all-atom models of the enzyme from Desulfovibrio desulfuricans show that the cyanide groups fine-tune the electronic and redox properties of the active site, affecting both the protonation regiochemistry and electron transfer between the two subclusters of the H-cluster. Despite the crucial role of cyanides in the protein active site, the currently available bioinspired electrocatalysts generally lack CN(-) groups in order to avoid competition between the latter and the catalytic metal centers for proton binding. In this respect, we show that a targeted inclusion of phosphine ligands in hexanuclear biomimetic clusters may restore the electronic and redox features of the wild-type H-cluster.

Structural study of Fe doped and Ni substituted thermoelectric skutterudites by combined synchrotron and neutron powder diffraction and ab initio theory

We present neutron and synchrotron powder-diffraction investigations as well as ab initio calculations to elucidate delicate structural features in doped skutterudites. Samples with assumed Fe dopi ...

DFT Dissection of the Reduction Step in H2 Catalytic Production by [FeFe]-Hydrogenase-Inspired Models: Can the Bridging Hydride Become More Reactive Than the Terminal Isomer?

Density functional theory has been used to study diiron dithiolates [HFe2(xdt)(PR3)n(CO)5-nX] (n = 0, 2, 4; R = H, Me, Et; X = CH3S(-), PMe3, NHC = 1,3-dimethylimidazol-2-ylidene; xdt = adt, pdt; adt = azadithiolate; pdt = propanedithiolate). These species are related to the [FeFe]-hydrogenases catalyzing the 2H(+) + 2e(-) ↔ H2 reaction. Our study is focused on the reduction step following protonation of the Fe2(SR)2 core. Fe(H)s detected in solution are terminal (t-H) and bridging (μ-H) hydrides. Although unstable versus μ-Hs, synthetic t-Hs feature milder reduction potentials than μ-Hs. Accordingly, attempts were previously made to hinder the isomerization of t-H to μ-H. Herein, we present another strategy: in place of preventing isomerization, μ-H could be made a stronger oxidant than t-H (E°μ-H > E°t-H). The nature and number of PR3 unusually affect ΔE°t-H-μ-H: 4PEt3 models feature a μ-H with a milder E° than t-H, whereas the 4PMe3 analogues behave oppositely. The correlation ΔE°t-H-μ-H ↔ stereoelectronic features arises from the steric strain induced by bulky Et groups in 4PEt3 derivatives. One-electron reduction alleviates intramolecular repulsions only in μ-H species, which is reflected in the loss of bridging coordination. Conversely, in t-H, the strain is retained because a bridging CO holds together the Fe2 core. That implies that E°μ-H > E°t-H in 4-PEt3 species but not in 4PMe3 analogues. Also determinant to observe E°μ-H > E°t-H is the presence of a Fe apical σ-donor because its replacement with a CO yields E°μ-H < E°t-H even in 4PEt3 species. Variants with neutral NHC and PMe3 in place of CH3S(-) still feature E°μ-H > E°t-H. Replacing pdt with (Hadt)(+) lowers E° but yields E°μ-H < E°t-H, indicating that μ-H activation can occur to the detriment of the overpotential increase. In conclusion, our results indicate that the electron richness of the Fe2 core influences ΔE°t-H-μ-H, provided that (i) the R size of PR3 must be greater than that of Me and (ii) an electron donor must be bound to Fe apically.

Excited state properties of diiron dithiolate hydrides: implications in the unsensitized photocatalysis of H2 evolution.

Density functional theory (DFT) and time-dependent DFT (TDDFT) have been used to investigate how visible light photons can excite an asymmetrically substituted diiron hydride, [Fe2(pdt)(μ-H)(CO)4dppv](+) (1(+), dppv = cis-1,2-C2H2(PPh2)2; pdt = 1,3-propanedithiolate), as well as the symmetric species [Fe2(pdt)(μ-H)(CO)4(PMe3)2](+) (2(+)), which are the first photocatalysts of proton reduction operating without employing sensitizers (Wang, W.; Rauchfuss, T. B.; Bertini, L.; Zampella, G.; J. Am. Chem. Soc., 2012, 134, 4525). Theoretical results illustrate that the peculiar reactivity associated to the excited states of 1(+) and 2(+) is compatible with three different scenarios: (i) it can arise from the movement of the hydride ligand from fully bridging to semibridging/terminal coordination, which is expected to be more reactive toward protons; (ii) reactivity could be related to cleavage of a Fe-S bond, which implies formation of a transient Fe penta-coordinate species that would trigger a facile turnstile hydride isomerization, if lifetime excitation is long enough; (iii) also in line with a Fe-S bond cleavage is the possibility that after excited state decay, a highly basic S center is protonated so that a species simultaneously containing S-H(δ+) and Fe-H(δ-) moieties is formed and, once reduced by a suitable electron donor, it can readily afford H2 plus an unprotonated form of the FeFe complex. This last possibility is consistent with (31)P NMR and IR solution data. All the three possibilities are compatible with the capability of 1(+) and 2(+) to perform photocatalysis of hydrogen evolving reaction (HER) without sensitizer. Moreover, even though it turned out difficult to discriminate among the three scenarios, especially because of the lack of experimental excitation lifetimes, it is worth underscoring that all of the three pathways represent a novelty regarding diiron carbonyl photoreactivity, which is usually associated with CO loss. Results provide also a rationale to the experimental observations which showed that the simultaneous presence of donor ligands (dppv in the case of 1(+)) and a H ligand in the coordination environment of diiron complexes is a key factor to prevent CO photodissociation and catalyze HER. Finally, the comparison of photoexcitation behavior of 1(+) and 2(+) allows a sort of generalization about the functioning of such hydride species.