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1. The Chemistry of Phenylimidotechnetium(V) Complexes with Isocyanides: Steric and Electronic Factors

   https://doi.org/10.3390/molecules27238546  

   Claude, Guilhem; Zeh, Laura; Roca Jungfer, Maximilian; Hagenbach, Adelheid; Figueroa, Joshua S.; Abram, Ulrich (December 2022, Molecules)

   Organometallic approaches are of ongoing interest for the development of novel functional 99mTc radiopharmaceuticals, while the basic organotechnetium chemistry seems frequently to be little explored. Thus, structural and reactivity studies with the long-lived isotope 99Tc are of permanent interest as the foundation for further progress in the related radiopharmaceutical research with this artificial element. Particularly the knowledge about the organometallic chemistry of high-valent technetium compounds is scarcely developed. Here, phenylimido complexes of technetium(V) with different isocyanides are introduced. They have been synthesized by ligand-exchange procedures starting from [Tc(NPh)Cl3(PPh3)2]. Different reactivity patterns and products have been obtained depending on the steric and electronic properties of the individual ligands. This involves the formation of 1:1 and 1:2 exchange products of Tc(V) with the general formulae [Tc(NPh)Cl3(PPh3)(isocyanide)], cis- or trans-[Tc(NPh)Cl3(isocyanide)2], but also the reduction in the metal and the formation of cationic technetium(I) complex of the formula [Tc(isocyanide)6]+ when p-fluorophenyl isocyanide is used. The products have been studied by single-crystal X-ray diffraction and spectroscopic methods, including IR and multinuclear NMR spectroscopy. DFT calculations on the different isocyanides allow the prediction of their reactivity towards electron-rich and electron-deficient metal centers by means of the empirical SADAP parameter, which has been derived from the potential energy surface of the electron density on their potentially coordinating carbon atoms. 

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2. Tuning π-Acceptor/σ-Donor Ratio of the 2-Isocyanoazulene Ligand: Non-Fluorinated Rival of Pentafluorophenyl Isocyanide and Trifluorovinyl Isocyanide Discovered

   https://doi.org/10.3390/molecules26040981  

   Hart, Mason D.; Meyers, John J.; Wood, Zachary A.; Nakakita, Toshinori; Applegate, Jason C.; Erickson, Nathan R.; Gerasimchuk, Nikolay N.; Barybin, Mikhail V. (February 2021, Molecules)

   null (Ed.)

   Isocyanoazulenes (CNAz) constitute a relatively new class of isocyanoarenes that offers rich structural and electronic diversification of the organic isocyanide ligand platform. This article considers a series of 2-isocyano-1,3-X2-azulene ligands (X = H, Me, CO2Et, Br, and CN) and the corresponding zero-valent complexes thereof, [(OC)5Cr(2-isocyano-1,3-X2-azulene)]. Air- and thermally stable, X-ray structurally characterized 2-isocyano-1,3-dimethylazulene may be viewed as a non-benzenoid aromatic congener of 2,6-dimethyphenyl isocyanide (2,6-xylyl isocyanide), a longtime “workhorse” aryl isocyanide ligand in coordination chemistry. Single crystal X-ray crystallographic {Cr–CNAz bond distances}, cyclic voltametric {E1/2(Cr0/1+)}, 13C NMR {δ(13CN), δ(13CO)}, UV-vis {dπ(Cr) → pπ*(CNAz) Metal-to-Ligand Charge Transfer}, and FTIR {νN≡C, νC≡O, kC≡O} analyses of the [(OC)5Cr(2-isocyano-1,3-X2-azulene)] complexes provided a multifaceted, quantitative assessment of the π-acceptor/σ-donor characteristics of the above five 2-isocyanoazulenes. In particular, the following inverse linear relationships were documented: δ(13COtrans) vs. δ(13CN), δ(13COcis) vs. δ(13CN), and δ(13COtrans) vs. kC≡O,trans force constant. Remarkably, the net electron withdrawing capability of the 2-isocyano-1,3-dicyanoazulene ligand rivals those of perfluorinated isocyanides CNC6F5 and CNC2F3. 

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3. Isocyanide Substituent Influences Reductive Elimination versus Migratory Insertion in Reaction with an [Fe ₂ (μ-H) ₂ ] ²⁺ Complex

   https://doi.org/10.1021/acs.inorgchem.4c03256  

   John, Titto Sunil; Dobrzycki, Łukasz; Catalano, Vincent J; Murray, Leslie J (November 2024, Inorganic Chemistry)

   Iron hydrides are proposed reactive intermediates for N2 and CO conversion in industrial and biological processes. Here, we report a reactivity study of a low-coordinate di(μ-hydrido)diiron(II) complex, Fe2(μ-H)2L, where L2– is a bis(β-diketiminate) cyclophane, with isocyanides, which have electronic structures related to N2 and CO. The reaction outcome is influenced by the isocyanide substituent, with 2,6-xylyl isocyanide leading to H2 loss, to form a bis(μ-1,1-isocyanide)diiron(I) complex, whereas all of the other tested isocyanides insert into the Fe–H bond to give (μ-1,2-iminoformyl) complexes. Steric bulk of the isocyanide substituent determines the extent of insertion (i.e., into one or both Fe–H–Fe units) with tert-butyl isocyanide reacting to yield the mono-(μ-1,2-iminoformyl)diiron(II) complex, exclusively, and isopropyl- and methyl isocyanides affording the bis(μ-1,2-iminoformyl)diiron(II) products. Treatment of Fe2(μ-1,2-CHNtBu)(μ-H)L with 2,6-xylyl isocyanide (or XylNC) yields Fe2(μ-XylNC)2L and tert-butylaldimine as one of the organic products. 

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4. High-valent nitridorhenium( v ) complexes containing PNP ligands: implications of ligand flexibility

   https://doi.org/10.1039/c7dt03615a  

   Lambic, Nikola S.; Sommer, Roger D.; Ison, Elon A. (January 2018, Dalton Transactions)

   The synthesis of (PNP)Re(N)X (PNP = [2-P(CHMe 2 ) 2 -4-MeC 6 H 3 ] 2 N, X = Cl and Me) complexes is described. The methylnitridorhenium complex 3 was found to react differently with CO and isocyanides, leading to the isolation of a Re( v ) acyl complex 4 and an isocyanide adduct 6 . Two parallel pathways were observed for the reaction of 3 with CO: (1) CO inserts into the Re–Me bond to afford 4 , and (2) 3 isomerizes by distortion of the aryl backbone of the PNP ligand to afford the isomer 3′ . This is followed by the reaction of 3′ with CO to afford the tricarbonyl complex 5 , which was fully characterized. The contrasting reaction of 3 with 2,6-dimethylphenyl isocyanide lends further support for the proposed isomerization pathway. DFT (M06) calculations suggest that insertion of CNR into the Re–Me bond (27.2 kcal mol −1 ) is inaccessible at room temperature. Instead the substrate adds to the metal center via the most accessible face i.e. syn to the rhenium–nitrido bond, to afford 6 . The addition of CO to isomer 3′ is proposed to proceed with a similar mechanism to 2,6-dimethylphenyl isocyanide. 

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5. Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

   https://doi.org/10.3390/molecules25020273  

   Kurup, Sudheer S.; Staples, Richard J.; Lord, Richard L.; Groysman, Stanislav (January 2020, Molecules)

   Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]Ph (H2[OO]Ph = [1,1′:4′,1′’-terphenyl]-2,2′’-diylbis(diphenylmethanol)) and their subsequent reactivity in the context of catalytic production of carbodiimides from azides and isocyanides are described. Two different Cr(II) complexes are obtained, as a function of the crystallization solvent: mononuclear Cr[OO]Ph(THF)2 (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr2([OO]Ph)2 (in CH2Cl2/THF). The electronic structure and bonding in Cr[OO]Ph(THF)2 were probed by density functional theory calculations. Isolated Cr2([OO]Ph)2 undergoes facile reaction with 4-MeC6H4N3, 4-MeOC6H4N3, or 3,5-Me2C6H3N3 to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO]Ph(N(4-MeC6H4))2 was confirmed by X-ray crystallography. The reaction of Cr2([OO]Ph)2 with bulkier azides N3R (MesN3, AdN3) forms paramagnetic products, formulated as Cr[OO]Ph(NR). The attempted formation of a Cr–alkylidene complex (using N2CPh2) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]Ph(NNCPh2)2. Catalytic formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various aryl azides and isocyanides. The formation of carbodiimides was found to depend on the nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or 4-methylphenylazide. Treatment of Cr2([OO]Ph)2 or H2[OO]Ph with NO+ leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand. 

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