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1. Molybdenum( vi ) tris(amidophenoxide) complexes

   https://doi.org/10.1039/C8DT03392G  

   Erickson, Alexander N. ; Brown, Seth N. ( November 2018 , Dalton Transactions)

   Tris(2-(arylamido)-4,6-di- tert -butylphenoxo)molybdenum( vi ) complexes ( R ap) 3 Mo can be prepared either from (cycloheptatriene)Mo(CO) 3 and the N -aryliminoquinone, or from MoO 2 (acac) 2 and the aminophenol. In contrast to all other reported unconstrained transition metal tris(amidophenoxide) complexes, the molybdenum complexes show a facial geometry in the solid state. In solution, the fac isomer predominates, though a small amount of mer isomer is detectable at room temperature. At elevated temperature the two species interconvert through Rây-Dutt trigonal twists, which are faster than Bailar twists in this system, presumably because of steric effects of the N -aryl groups. Substituents on the N -aryl ring shift the fac / mer equilibrium of the complex, with more electron-withdrawing substituents generally increasing the proportion of the mer isomer. The preference for fac over mer geometry is thus suggested to be due to enhanced π bonding in the fac isomer. In contrast to analogous catecholate complexes, the tris(amidophenoxide) complexes are not Lewis acidic and are inert to nucleophilic oxidants such as amine- N -oxides.
2.  Why Do N-Alkylated Anilides Bend Over? The Factors Dictating the Divergent Conformational Preferences of 2° and 3° N-Aryl Amides

   https://doi.org/DOI:10.1021/acs.jpca.9b04555  

   Pros, Gabrielle J. Bloomfield ( September 2019 , The journal of physical chemistry)

   The conformational preferences of 28 sterically and electronically diverse N-aryl amides were detd. using d. functional theory (DFT), using the B3LYP functional and 6-31G(d) basis set.  For each compd., both the cis and trans conformers were optimized, and the difference in ground state energy calcd.  For six of the compds., the potential energy surface was detd. as a function of rotation about the N-aryl bond (by 5° increments) for both cis and trans conformers.  A natural bond orbital (NBO) deletion strategy was also employed to det. the extent of the contribution of conjugation to the energies of each of the conformers.  By comparing these computational results with previously reported exptl. data, an explanation for the divergent conformational preferences of 2° N-aryl amides and 3° N-alkyl-N-aryl amides was formulated.  This explanation accounts for the obsd. relationships of both steric and electronic factors detg. the geometry of the optimum conformation, and the magnitude of the energetic difference between cis and trans conformers: except under the most extreme scenarios, 2° amides maintain a trans conformation, and the N-bound arene lies in the same plane as the amide unless it has ortho substituents; for 3° N-alkyl-N-aryl amides in which the alkyl and aryl substituentsmore »are connected in a small ring, trans conformations are also favored, for most cases other than formamides, and the arene and amide remain in conjugation; and for 3° N-alkyl-N-aryl amides in which the alkyl and aryl substituents are not connected in a small ring, allylic strain between the two N-bound substituents forces the aryl substituent to rotate out of the plane of the amide, and the trans conformation is destabilized with respect to the cis conformation due to repulsion between the π system of the arene and the lone pairs on the oxygen atom of the carbonyl.  The cis conformation is increasingly more stable than the trans conformation as electron d. is increased on the arene because the more electron-rich arenes adopt a more orthogonal arrangement, increasing the interaction with the carbonyl oxygen, while simultaneously increasing the magnitude of the repulsion due to the increased electron d. in the π system.  The trans conformation is favored for 2° amides even when the arene is orthogonal to the amide, in nearly all cases, because the C-N-C bond angle can expend at the expense of the C-N-H bond angles, while this is not favorable for 3° amides.« less
3. 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)

   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.
4. Crystal structures of [Cu(phen)(H 2 O) 3 ( M F 6 )]·H 2 O ( M = Ti, Zr, Hf) and [Cu(phen)(H 2 O) 2 F] 2 [HfF 6 ]·H 2 O

   https://doi.org/10.1107/S2056989021000645  

   Nisbet, Matthew L. ; Poeppelmeier, Kenneth R. ( February 2021 , Acta Crystallographica Section E Crystallographic Communications)

   The crystal structures of three bridged bimetallic molecular compounds, namely, triaqua-2κ 3 O -μ-fluorido-pentafluorido-1κ 5 F -(1,10-phenanthroline-2κ 2 N , N ′)copper(II)titanium(IV) monohydrate, [Cu(TiF 6 )(phen)(H 2 O) 3 ]·H 2 O (phen is 1,10-phenanthroline, C 12 H 8 N 2 ), (I), triaqua-2κ 3 O -μ-fluorido-pentafluorido-1κ 5 F -(1,10-phenanthroline-2κ 2 N , N ′)copper(II)zirconium(IV) monohydrate, [Cu(ZrF 6 )(phen)(H 2 O) 3 ]·H 2 O, (II), and triaqua-2κ 3 O -μ-fluorido-pentafluorido-1κ 5 F -(1,10-phenanthroline-2κ 2 N , N ′)copper(II)hafnium(IV) monohydrate, [Cu(HfF 6 )(phen)(H 2 O) 3 ]·H 2 O, (III), and one molecular salt, bis[diaquafluorido(1,10-phenanthroline-κ 2 N , N ′)copper(II)] hexafluoridohafnate(IV) dihydrate, [CuF(phen)(H 2 O) 2 ] 2 [HfF 6 ]·2H 2 O, (IV), are reported. The bridged bimetallic compounds adopt Λ-shaped configurations, with the octahedrally coordinated copper(II) center linked to the fluorinated early transition metal via a fluoride linkage. The extended structures of these Λ-shaped compounds are organized through both intra- and intermolecular hydrogen bonds and intermolecular π–π stacking. The salt compound [Cu(phen)(H 2 O) 2 F] 2 [HfF 6 ]·H 2 O displays an isolated square-pyramidal Cu(phen)(H 2 O) 2 F + complex linked to other cationic complexes and isolated HfF 6 2− anions through intermolecular hydrogen-bonding interactions.
5. Spectroscopic and Theoretical Studies of Hg(II) Complexation with Some Dicysteinyl Tetrapeptides

   https://doi.org/10.1155/2021/9911474  

   Springfield, Elliot ; Willis, Alana ; Merle, John ; Mazlo, Johanna ; Ngu-Schwemlein, Maria ( July 2021 , Bioinorganic Chemistry and Applications)

   Osella, Domenico (Ed.)

   Tetrapeptides containing a Cys-Gly-Cys motif and a propensity to adopt a reverse-turn structure were synthesized to evaluate how O-, N-, H-, and aromatic π donor groups might contribute to mercury(II) complex formation. Tetrapeptides Xaa-Cys-Gly-Cys, where Xaa is glycine, glutamate, histidine, or tryptophan, were prepared and reacted with mercury(II) chloride. Their complexation with mercury(II) was studied by spectroscopic methods and computational modeling. UV-vis studies confirmed that mercury(II) binds to the cysteinyl thiolates as indicated by characteristic ligand-to-metal-charge-transfer transitions for bisthiolated S-Hg-S complexes, which correspond to 1 : 1 mercury-peptide complex formation. ESI-MS data also showed dominant 1 : 1 mercury-peptide adducts that are consistent with double deprotonations from the cysteinyl thiols to form thiolates. These complexes exhibited a strong positive circular dichroism band at 210 nm and a negative band at 193 nm, indicating that these peptides adopted a β-turn structure after binding mercury(II). Theoretical studies confirmed that optimized 1 : 1 mercury-peptide complexes adopt β-turns stabilized by intramolecular hydrogen bonds. These optimized structures also illustrate how specific N-terminal side-chain donor groups can assume intramolecular interactions and contribute to complex stability. Fluorescence quenching results provided supporting data that the indole donor group could interact with the coordinated mercury. The results from this study indicate that N-terminal side-chain residues containingmore »carboxylate, imidazole, or indole groups can participate in stabilizing dithiolated mercury(II) complexes. These structural insights on peripheral mercury-peptide interactions provide additional understanding of the chemistry of mercury(II) with side-chain donor groups in peptides.« less