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1. Intramolecular attack on coordinated nitriles: metallacycle intermediates in catalytic hydration and beyond

   https://doi.org/10.1039/D1DT02795F  

   Glueck, David S. (November 2021, Dalton Transactions)

   Hydration of nitriles is catalyzed by the enzyme nitrile hydratase, with iron or cobalt active sites, and by a variety of synthetic metal complexes. This Perspective focuses on parallels between the reaction mechanism of the enzyme and a class of particularly active catalysts bearing secondary phosphine oxide (SPO) ligands. In both cases, the key catalytic step was proposed to be intramolecular attack on a coordinated nitrile, with either an S-OH or S–O − (enzyme) or a P-OH (synthetic) nucleophile. Attack of water on the heteroatom (S or P) in the resulting metallacycle and proton transfer yields the amide and regenerates the catalyst. Evidence for this mechanism, its relevance to the formation of related metallacycles, and its potential for design of more active catalysts for nitrile hydration is summarized. 

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2. Improved Synthesis of Chiral 1,4,7‐Triazacyclononane Derivatives and Their Application in Ni‐Catalyzed Csp ³ −Csp ³ Kumada Cross‐Coupling

   https://doi.org/10.1002/hlca.202300170  

   Hu, Chi‐Herng; Byeong Chae, Ju; Mirica, Liviu M. (December 2023, Helvetica Chimica Acta)

   Abstract Herein, we report four new chiral 1,4,7‐triazacyclononane (TACN) derivatives and their corresponding nickel(II) chloride complexes. All TACN ligands are bearing one chiral N‐substituent and two alkyl (methyl ortert‐butyl) N‐substituents, and we have developed a new synthetic method for the dimethyl‐substituted TACN derivative, in order to prevent the rotational isomers that hinder the cyclization reaction. The nickel complexes change their coordination geometry significantly depending on the steric bulk of the N‐alkyl substituents, from a dinuclear tris(μ‐chloro)dinickel complex to mononuclear Ni‐dichloride and Ni‐chloride complexes. These complexes were then employed in the alkyl‐alkyl Kumada cross‐coupling reaction and revealed that the more sterically hindered ligands produced more homocoupled product rather than the cross‐coupled product, while the mononuclear Ni‐dichloride complex exhibited significantly lower catalytic activity. These chiral complexes were also employed in enantioconvergent cross‐coupling reactions as well, to afford significant enantioenrichment. Overall, the least sterically hindered Ni complex yields the best yields in the alkyl‐alkyl Kumada cross‐coupling reaction among the four complexes investigated, as well as the highest enantioselectivity. 

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3. trans -Diamminebis(1,2-dicyanoethene-1,2-dithiolato)platinum(IV)

   https://doi.org/10.1107/S2414314620009803  

   Siddiqui, Mahaa J.; Nesterov, Volodymyr V.; Steidle, Matthew T.; Smucker, Bradley W. (July 2020, IUCrData)

   The title compound, [Pt(C 4 N 2 S 2 ) 2 (NH 3 ) 2 ], represents an octahedral platinum(IV) complex with two trans- ammine and two mnt (mnt = 1,2-dicyanoethene-1,2-dithiolato) ligands. The Pt—N and Pt—S distances are consistent with those in other platinum(IV) complexes. As a result of a slight canting of the coordination of the mnt ligand to the platinum(IV) atom, the nitrile nitrogen atoms are positioned suitably to hydrogen-bond with adjacent ammines. 

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4. Bonding in nitrile photo-dissociating ruthenium drug candidates—A local vibrational mode study

   https://doi.org/10.1063/5.0094567  

   McCutcheon, Margaret; Freindorf, Marek; Kraka, Elfi (July 2022, The Journal of Chemical Physics)

   In this work, we investigated bonding features of 15 ruthenium(II) nitrile complexes of the type [Ru(tpy)(L)-(CH 3 CN)] n+ , containing the tridentate tpy ligand (tpy = 2,2′:6′,2″-terpyridine) and various bidentate ancillary ligands L; 12 compounds originally synthesized by Loftus et al. [J. Phys. Chem. C 123, 10291–10299 (2019)] and three new complexes. We utilized local vibrational force constants derived from the local mode theory as a quantitative measure of bond strength complemented with the topological analysis of the electron density and the natural bond orbital analysis. Loftus et al. suggested that nitrile dissociation occurs after light induced singlet–triplet transition of the original complexes and they used as a measure of nitrile release efficiency quantum yields for ligand exchange in water. They observed larger quantum yields for complexes with smaller singlet–triplet energy gaps. The major goal of this work was to assess how the Ru–NC and Ru–L bond strengths in these 15 compounds relate to and explain the experimental data of Loftus et al., particularly focusing on the question whether there is a direct correlation between Ru–NC bond strength and measured quantum yield. Our study provides the interesting result that the compounds with the highest quantum yields also have the strongest Ru–NC bonds suggesting that breaking the Ru–NC bond is not the driving force for the delivery process rather than the change of the metal framework as revealed by first results of a unified reaction valley approach investigation of the mechanism. Compounds with the highest quantum yield show larger electronic structure changes upon singlet–triplet excitation, i.e., larger changes in bond strength, covalency, and difference between the singlet and triplet HOMOs, with exception of the compound 12. In summary, this work provides new insights into the interplay of local properties and experimental quantum yields forming in synergy a useful tool for fine tuning of existing and future design of new nitrile releasing ruthenium compounds. We hope that this work will bring theoretical and experimental studies closer together and serves as an incubator for future collaboration between computational chemists and their experimental colleagues. 

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5. Pincer-Ligated Iridium Complexes with Low-Field Ancillary Ligands: Complexes of ( ^iPr PCP)IrCl ₂ and Comparison with ( ^iPr PCP)IrHCl

   https://doi.org/10.1021/acs.organomet.4c00162  

   Parihar, Ashish; Emge, Thomas J; Chakravartula, Srinivas_V S; Goldman, Alan S (June 2024, Organometallics)

   Pincer-ligated iridium complexes have been widely developed, and (pincer)Ir(III) complexes, particularly five-coordinate, are central to their chemistry. Such complexes typically bear two formally anionic ligands in addition to the pincer ligand itself. Yet despite the prevalence of halides as anionic ligands in transition metal chemistry there are relatively few examples in which both of these ancillary anionic ligands are halides or even other monodentate low-field anions. We report a study of the fragment (iPrPCP)IrCl2 (iPrPCP = 3 2,6 C6H3(CH2PiPr2)), and adducts thereof. These species are found to be thermodynamically disfavored relative to the corresponding hydridohalides. For example, DFT calculations and experiment indicate that one Ir-Cl bond of (iPrPCP)IrCl2 complexes will undergo reaction with H2 to give (iPrPCP)IrHCl or an adduct thereof. In the presence of aqueous HCl, (iPrPCP)IrCl2 adds a chloride ion to give an unusual example of an anionic transition metal complex ((iPrPCP)IrCl3–) with a Zundel cation (H5O2+). (iPrPCP)IrCl2 is not stable as a monomer at room temperature but exists in solution as a mixture of clusters which can add various small molecules. DFT calculations indicate that dimerization and trimerization of (iPrPCP)IrCl2 is more favorable than the analogous reactions of (iPrPCP)IrHCl, in accord with cluster formation being observed only for the dichloride complex. 

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