Basic methanolysis of a sterically hindered aminobis(S-arylthiocarbamate) affords a novel aminobis(thiophenolate) pincer-type ligand NS22–; the in situ generated dianion reacts cleanly with Ni2+ and Zn2+ resulting in dimeric complexes with bridging thiophenolate ligands, as determined spectroscopically and by X-ray crystallography. The C2-symmetric [Ni(NS2)]2 dimer (1) has a square planar coordination geometry around the Ni2+ ions, while the [Zn(NS2)]2 analogue (2) is characterized by a distorted tetrahedral geometry around each independent Zn2+ ion. Addition of the neutral monodentate donor L = 2,6-xylylisocyanide to [Ni(NS2)]2 affords the monomeric complex [LNi(NS2)] (3), which is characterized in the solid state by a square planar geometry with the isocyanide donor trans to the tertiary amine of NS2. The pincer NS2 ligand provides redox plasticity to 1, manifested in the accessibility of the putative Ni+Ni+ and Ni3+Ni3+ dimeric complexes, based on comparative cyclic voltammetry studies with 2 and 3. The redox properties of 1 endow it with hydrogenase-type activity, as evidenced in the electrocatalytic reduction of protons in a mixed aqueous/organic phase, as well as the oxidation of hydrides from NaBH(OAc)3. Both 1 and 3 are resilient under protic and oxidative conditions, as evidenced in reactivity tests monitored by UV–vis spectroscopy.
We have made a detailed analysis of the adequacy of different theoretical approaches to the study of uracil tautomerism. We have considered the effect of molecular relaxation and the ability of several methods to reproduce it. We have also considered their performance in predicting the tautomerization energy barrier. We found that semiempirical methods are not adequate for predicting the energy value but that they can be used for obtaining geometry optimizations. We also found a strong discrepancy between the energy values predicted by a minimal basis set and a 23 basis set, the last one being 24.9 kcal/mol. The implications of these findings are discussed and the results compared with experimental evidence.
Iron and molybdenum complexes supported by a pincer-type dianionic [NS2]2- donor were prepared to compare their structural, spectroscopic, and electrochemical properties. The versatility of the [NS2]2Mo(iv) complex (2) to access different oxidation states was evidenced in the activation of methanol and isopropanol, oxidising them to formaldehyde or acetone with concomitant reduction and protonation to afford [NHS2]2Mo(ii), complex (3). This redox behaviour contrasts with the null reactivity observed for the analogous ferric complex [NS2][NHS2]Fe(iii) (1). Complex 2 presents a quasi-reversible process at E1/2 = -0.80 V relative to the ferrocenium/ferrocene couple (Fc+/Fc), which is attributed to the Mo(iv)/Mo(v) redox couple. Two irreversible cathodic processes were observed at Ecp = -1.59 and -2.20 V, which are attributed to the Mo(iv)/Mo(iii) and Mo(iii)/Mo(ii) redox couples. Cyclic voltammetry and solid-state structures obtained by X-ray crystallography support a 2H+ and 2e- process, whereby the Mo(iv) centre in 2 is reduced sequentially to Mo(iii), and finally to Mo(ii) in 3. These redox events were observed at Ecp = -1.22 and -2.15 V (vs. Fc+/Fc) in the anodic cyclic voltammograms of 2 in THF in the presence of acid. A new reduction peak was detected under these conditions at Ecp = -2.30 V, consistent with electrocatalytic proton reduction. This was corroborated for 2 as a catalyst precursor in the presence of increasing amounts of p-toluenesulfonic acid, with the addition of 2 to 14 equivs resulting in an increase of the current measured.
Tripodal ligands designed to generate a local C3 symmetry have resulted in novel types of metal complexes that feature unusual bonding and electronic properties. However, most complexes reported to date are characterised by strong field ligands that enforce low or intermediate-spin states for the metal centres. Moreover, anionic sulfur-based tripodal ligands are particularly scarce due to their challenging synthesis. In this context, we herein report the synthesis, spectral characterization, structural, and electronic properties of an iron complex supported by the tripodal, trianionic ligand [N(CH2ArS)3](3-) as the trigonal-bipyramidal complexes [Fe{N(CH2ArS)3}(X)] ((X), X = DMSO, THF). The solid-state structures reveal local C3v symmetry around the Fe(3+) ions, while electron spin resonance measurements established a high-spin state (S = 5/2). Electrochemical studies demonstrate the redox flexibility of the FeS3 fragment by direct comparison with the oxygen-based analogue N(CH2ArOH)3, which displays an irreversible reduction; in contrast, (THF) has a reversible Fe(3+)/Fe(2+) redox process at -0.83 V (relative to the ferrocenium/ferrocene redox couple). The high spin and redox properties of (THF) are attributable to the weak ligand field provided by the NS3 fragment, as confirmed by the electronic structure calculated by density functional theory, which reveals substantial electronic delocalisation and covalency of the Fe-S bonds in (X).
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