Soft Scorpionate Hydridotris(2-mercapto-1-methylimidazolyl) borate) Tungsten-Oxido and -Sulfido Complexes as Acetylene Hydratase Models

A series of W^IV alkyne complexes with the sulfur-rich ligand hydridotris(2-mercapto-1-methylimidazolyl) borate) (Tm^Me) are presented as bio-inspired models to elucidate the mechanism of the tungstoenzyme acetylene hydratase (AH). The mono- and/or bis-alkyne precursors were reacted with NaTm^Me and the resulting complexes [W(CO)(C₂R₂)(Tm^Me)Br] (R=H 1 , Me 2 ) oxidized to the target [WE(C₂R₂)(Tm^Me)Br] (E=O, R=H 4 , Me 5 ; E=S, R=H 6 , Me 7 ) using pyridine-N-oxide and methylthiirane. Halide abstraction with TlOTf in MeCN gave the cationic complexes [WE(C₂R₂)(MeCN)(Tm^Me)](OTf) (E=CO, R=H 10 , Me 11 ; E=O, R=H 12 , Me 13 ; E=S, R=H 14 , Me 15 ). Without MeCN, dinuclear complexes [W₂O(μ-O)(C₂Me₂)₂(Tm^Me)₂](OTf)₂ ( 8 ) and [W₂(μ-S)₂(C₂Me₂)(Tm^Me)₂](OTf)₂ ( 9 ) could be isolated showing distinct differences between the oxido and sulfido system with the latter exhibiting only one molecule of C₂Me₂. This provides evidence that a fine balance of the softness at W is important for acetylene coordination. Upon dissolving complex 8 in acetonitrile complex 13 is reconstituted in contrast to 9 . All complexes exhibit the desired stability toward water and the observed effective coordination of the scorpionate ligand avoids decomposition to disulfide, an often-occurring reaction in sulfur ligand chemistry. Hence, the data presented here point toward a mechanism with a direct coordination of acetylene in the active site and provide the basis for further model chemistry for acetylene hydratase.     

Phenols as Novel Photocatalytic Platforms for Organic Synthesis

In recent years, organic chemists have devoted a great deal of effort towards the implementation of novel green photocatalytic synthetic protocols. To this end, the development of new effective, non-toxic, inexpensive photocatalysts, which are capable of driving value-added chemical transformations, is highly desirable. Interestingly, phenols fulfill all these requirements due to their outstanding physicochemical features, therefore emerging as promising metal-free photocatalytic platforms for organic synthesis. This Perspective aims at highlighting the most recent applications of phenols in organic photocatalysis. More specifically, phenolate anions, formed upon deprotonation of phenols, are photo-active organic intermediates that may absorb light within the visible region. Thus, when in the excited states, these anions may be used as reductants to generate reactive open shell species from suitable precursors under mild operative conditions. Alternatively, phenolate anions and suitable radical precursors can form electron donor-acceptor (EDA) complexes. Specifically, the photochemical activity of these molecular aggregates can be used to initiate organic radical reactions. Lastly, forward-looking opportunities within this research field have been discussed.     

Templated Synthesis of Copper(II) Azacyclam Complexes Using Urea as a Locking Fragment and Their Metal‐Enhanced Binding Tendencies towards Anions

Copper(II) azacyclam complexes 3 ²⁺ and 4 ²⁺ were obtained through a metal‐templated procedure involving the pertinent open‐chain tetramine, formaldehyde and a phenylurea derivative as a locking fragment. Both metal complexes can establish interactions with anions through the metal centre and the amide NH group. Equilibrium studies in DMSO by a spectrophotometric titration technique were carried out to assess the affinity of 3 ²⁺ and 4 ²⁺ towards anions. While the NH group of an amide model compound and the metal centre of the plain Cu^II(azacyclam)²⁺ complex do not interact at all with anions, 3 ²⁺ and 4 ²⁺ establish strong interactions with oxo anions, profiting from a pronounced cooperative effect. In particular, 1) they form stable 1:1 and 1:2 complexes with H₂PO₄^? ions in a stepwise mode with both hydrogen‐bonding and metal–ligand interactions, and 2) in the presence of CH₃COO^?, they undergo deprotonation of the amido NH group and thus profit from axial coordination of the partially negatively charged carbonyl oxygen atom in a scorpionate binding mode.     

Novel Porphyrin-Cholic Acid Conjugates as Receptors for Biologically Important Anions

A series of novel receptors showing high binding affinity in aqueous media for biologically important anions are reported. These naturally chromophoric porphyrin-based receptors contain cholic acids connected via quaternary alkyl ammonium amido linkages.     

S‐Alkylation of Soft Scorpionates

The alkylation reactions of soft scorpionates are reported. The hydrotris(S‐alkyl‐methimazolyl)borate dications (alkyl=methyl, allyl, benzyl), which were prepared by the reaction of Tm^Me anion and primary alkyl halides, have been isolated and structurally characterised. The reaction is, however, not universally successful. DFT analysis of these alkylation reactions (C?S versus B? H alkylation) indicates that the observed outcome is driven by kinetic factors. Extending the study to incorporate alternative imine thiones (mercaptobenzothiazole, bz; thiazoline, tz) led to the structural characterisation of di[aquo‐μ‐aquohydrotris(mercaptobenzothiazolyl)boratosodium], which contains sodium atoms in the κ³‐S,S,S coordination mode. Alkylation of Na[Tbz] and Na[tzTtz] leads to decomposition resulting in the formation of the simple S‐alkylated heterocycles. The analysis of the species involved in these reactions shows an inherent weakness in the B? N bond in soft scorpionates, which has implications for their use in more advanced chemistry.