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1. η2 Coordination of Electron-Deficient Arenes with Group 6 Dearomatization Agents

   https://doi.org/10.1021/acs.organomet.0c00277  

   Smith, J. A.; Simpson, S. R.; Westendorff, K. S.; Weatherford-Pratt, J.; Myers, J. T.; Wilde, J. H.; Dickie, D. A.; Harman, W. D. (June 2020, Organometallics)

   null (Ed.)

   The exceptionally π-basic metal fragments {MoTp(NO)(DMAP)} and {WTp(NO)(PMe3)} (Tp = tris(pyrazolyl)borate; DMAP = 4-(N,N-dimethylamino)pyridine) form thermally stable η2-coordinated complexes with a variety of electron-deficient arenes. The tolerance of substituted arenes with fluorine-containing electron withdrawing groups (EWG; −F, −CF3, −SF5) is examined for both the molybdenum and tungsten systems. When the EWG contains a π bond (nitriles, aldehydes, ketones, ester), η2 coordination occurs predominantly on the nonaromatic functional group. However, complexation of the tungsten complex with trimethyl orthobenzoate (PhC(OMe)3) followed by hydrolysis allows access to an η2-coordinated arene with an ester substituent. In general, the tungsten system tolerates sulfur-based withdrawing groups well (e.g., PhSO2Ph, MeSO2Ph), and the integration of multiple electron-withdrawing groups on a benzene ring further enhances the π-back-bonding interaction between the metal and aromatic ligand. While the molybdenum system did not form stable η2-arene complexes with the sulfones or ortho esters, it was capable of forming rare examples of stable η2-coordinated arene complexes with a range of fluorinated benzenes (e.g., fluorobenzene, difluorobenzenes). In contrast to what has been observed for the tungsten system, these complexes formed without interference of C–H or C–F insertion. 

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2. Arene non-innocence in dinuclear complexes of Fe, Co, and Ni supported by a para-terphenyl diphosphine

   https://doi.org/10.1039/c4cc00838c  

   Horak, Kyle T.; Velian, Alexandra; Day, Michael W.; Agapie, Theodor (January 2014, Chem. Commun.)

   Cofacial Fe 2 , Co 2 , and Ni 2 complexes supported by a para -terphenyl diphosphine ligand were prepared. Central arene deplanarization and a μ 2 :(η 3 ,η 3 ) coordination mode suggest partial bisallyl character in the Fe 2 and Co 2 complexes. An oxidation induced shift in Fe 2 –arene binding highlights the non-innocent nature of the arene ligand. 

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3. Partially Sulfated Pillar[5]Arenes: Synthesis and Molecular Recognition Properties

   https://doi.org/10.1002/chem.202404024  

   Goswami, Ayona; Çınaroğlu, Süleyman_Selim; Singh, Noor; Zavalij, Peter_Y; Mobley, David_L; Isaacs, Lyle (December 2024, Chemistry – A European Journal)

   Abstract We report the synthesis and characterization of sulfated pillar[5]arene hosts (P5S₂‐P5S₁₀) that differ in the number of sulfate substituents. All fiveP5S_nhosts display high solubility in water (73–131 mM) and do not undergo significant self‐association according to¹H NMR dilution experiments. The x‐ray crystal structures ofP5S₆,P5S₆ ⋅ Me₆HDA,P5S₈ ⋅ Me₆HDA, andP5S₁₀ ⋅ Me₆HDAreveal one intracavity molecule ofMe₆HDAand several external molecules ofMe₆HDAwhich form a network of close methonium ⋅ ⋅ ⋅ sulfate interactions. The thermodynamic parameters of complexation betweenP5S_nand the panel of guests was measured by direct or competitive isothermal titration calorimetry. We find that the binding free energy toward a guest becomes more negative as the number of sulfate substituents increase. Conversely, the binding free energy of a specific sulfated pillar[5]arene toward a homologous series of guests becomes more negative as the number of NMe groups increases. The ability to tune the host ⋅ guest affinity by changing the number of sulfate substituents will be valuable in supramolecular polymers, separation materials, and latching applications. 

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4.  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 substituents 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. 

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5. Pillar[ n ]MaxQ: A New High Affinity Host Family for Sequestration in Water

   https://doi.org/10.1002/anie.202005902  

   Xue, Weijian; Zavalij, Peter Y.; Isaacs, Lyle (June 2020, Angewandte Chemie International Edition)

   Abstract We report the synthesis, X‐ray crystal structure, and molecular recognition properties of pillar[n]arene derivativeP[6]AS, which we refer to as Pillar[6]MaxQ along with analoguesP[5]ASandP[7]AStoward guests1–18. The ultratight binding affinity ofP[5]ASandP[6]AStoward quaternary (di)ammonium ions renders them prime candidates for in vitro and in vivo non‐covalent bioconjugation, for imaging and delivery applications, and as in vivo sequestration agents. 

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