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1. Crystal structures of trans -acetyldicarbonyl(η ⁵ -cyclopentadienyl)(1,3,5-triaza-7-phosphaadamantane)molybdenum(II) and trans -acetyldicarbonyl(η ⁵ -cyclopentadienyl)(3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane)molybdenum(II)

   https://doi.org/10.1107/S2056989020003679  

   Anstey, Mitchell R.; Bost, John L.; Grumman, Anna S.; Kennedy, Nicholas D.; Whited, Matthew T. (April 2020, Acta Crystallographica Section E Crystallographic Communications)

   null (Ed.)

   The title compounds, [Mo(C 5 H 5 )(COCH 3 )(C 6 H 12 N 3 P)(CO) 2 ], (1), and [Mo(C 5 H 5 )(COCH 3 )(C 9 H 16 N 3 O 2 P)(C 6 H 5 ) 2 ))(CO) 2 ], (2), have been prepared by phosphine-induced migratory insertion from [Mo(C 5 H 5 )(CO) 3 (CH 3 )]. The molecular structures of these complexes are quite similar, exhibiting a four-legged piano-stool geometry with trans -disposed carbonyl ligands. The extended structures of complexes (1) and (2) differ substantially. For complex (1), the molybdenum acetyl unit plays a dominant role in the organization of the extended structure, joining the molecules into centrosymmetrical dimers through C—H...O interactions with a cyclopentadienyl ligand of a neighboring molecule, and these dimers are linked into layers parallel to (100) by C—H...O interactions between the molybdenum acetyl and the cyclopentadienyl ligand of another neighbor. The extended structure of (2) is dominated by C—H...O interactions involving the carbonyl groups of the acetamide groups of the DAPTA ligand, which join the molecules into centrosymmetrical dimers and link them into chains along [010]. Additional C—H...O interactions between the molybdenum acetyl oxygen atom and an acetamide methyl group join the chains into layers parallel to (101). 

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2. Proline C−H Bonds as Loci for Proline Assembly via C−H/O Interactions

   https://doi.org/10.1002/cbic.202200409  

   Daniecki, Noah J.; Bhatt, Megh R.; Yap, Glenn P.; Zondlo, Neal J. (December 2022, ChemBioChem)

   Abstract Proline residues within proteins lack a traditional hydrogen bond donor. However, the hydrogens of the proline ring are all sterically accessible, with polarized C−H bonds at Hα and Hδ that exhibit greater partial positive character and can be utilized as alternative sites for molecular recognition. C−H/O interactions, between proline C−H bonds and oxygen lone pairs, have been previously identified as modes of recognition within protein structures and for higher‐order assembly of protein structures. In order to better understand intermolecular recognition of proline residues, a series of proline derivatives was synthesized, including 4R‐hydroxyproline nitrobenzoate methyl ester, acylated on the proline nitrogen with bromoacetyl and glycolyl groups, and Boc‐4S‐(4‐iodophenyl)hydroxyproline methyl amide. All three derivatives exhibited multiple close intermolecular C−H/O interactions in the crystallographic state, with H⋅⋅⋅O distances as close as 2.3 Å. These observed distances are well below the 2.72 Å sum of the van der Waals radii of H and O, and suggest that these interactions are particularly favorable. In order to generalize these results, we further analyzed the role of C−H/O interactions in all previously crystallized derivatives of these amino acids, and found that all 26 structures exhibited close intermolecular C−H/O interactions. Finally, we analyzed all proline residues in the Cambridge Structural Database of small‐molecule crystal structures. We found that the majority of these structures exhibited intermolecular C−H/O interactions at proline C−H bonds, suggesting that C−H/O interactions are an inherent and important mode for recognition of and higher‐order assembly at proline residues. Due to steric accessibility and multiple polarized C−H bonds, proline residues are uniquely positioned as sites for binding and recognition via C−H/O interactions. 

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3. Anagostic Axial Interactions Inhibit Cross‐Coupling Catalytic Activity in Square Planar Pyridinophane Nickel Complexes

   https://doi.org/10.1002/cctc.202301677  

   Bouley, Bailey S; Garvey, Ian J; Na, Hanah; Chae, Ju_Byeong; Mirica, Liviu M (March 2024, ChemCatChem)

   Abstract Herein, we report for the first time the use of the nitrogen‐based bidentate molecule [2.2]pyridinophane (N2) as a ligand for metal complexes. Additionally, its improved synthesis allows for electronic modification of the pyridine rings to access the newpara‐dimethylamino‐[2.2]pyridinophane ligand (p‐NMe₂N2). These ligands bind nickel in an analogous fashion to other pyridinophane ligands, completing the series of tetra‐, tri‐, and bidentate pyridinophane‐nickel complexes. The new compounds exhibit geometrically enforced C−H anagostic interactions between the ethylene bridge protons and the nickel center that are not present in other pyridinophane systems. These ethylene bridge groups also act as an unusual form of steric encumbrance, enforcing square planar geometries in ligand fields that would otherwise adopt tetrahedral structures. In addition, these anagostic interactions inhibit the catalytic performance in Csp³–Csp³Kumada cross coupling reactions relative to other common bidentate N‐ligand platforms, possibly by preventing the formation of the 5‐coordinate oxidative addition intermediates. 

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4. Crystal structure of (1,4,7,10,13,16-hexaoxacyclooctadecane-κ ⁶ O )potassium-μ-oxalato-triphenylstannate(IV), the first reported 18-crown-6-stabilized potassium salt of triphenyloxalatostannate

   https://doi.org/10.1107/S2056989024007758  

   Song, Xueqing; Li, William; Torres, Yolanda; Greaves, Tazena (September 2024, Acta Crystallographica Section E Crystallographic Communications)

   The title complex, (1,4,7,10,13,16-hexaoxacyclooctadecane-1κ⁶O)(μ-oxalato-1κ²O¹,O²:2κ²O^1′,O^2′)triphenyl-2κ³C-potassium(I)tin(IV), [KSn(C₆H₅)₃(C₂O₄)(C₁₂H₂₄O₆)] or K[18-Crown-6][(C₆H₅)₃SnO₄C₂], was synthesized. The complex consists of a potassium cation coordinated to the six oxygen atoms of a crown ether molecule and the two oxygen atoms of the oxalatotriphenylstannate anion. It crystallizes in the monoclinic crystal system within the space groupP2₁. The tin atom is coordinated by one chelating oxalate ligand and three phenyl groups, forming acis-trigonal–bipyramidal geometry around the tin atom. The cations and anions form ion pairs, linked through carbonyl coordination to the potassium atoms. The crystal structure features C—H...O hydrogen bonds between the oxygen atoms of the oxalate group and the hydrogen atoms of the phenyl groups, resulting in an infinite chain structure extending alonga-axis direction. The primary inter-chain interactions are van der Waals forces. 

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5. Synthesis and characterization of a series of CpW(CO)2PR3H, [CpW(CO)2PR3]−, [CpW(CO)2PR3(CH3CN)]+, and [CpW(CO)2PR3]2 complexes

   https://doi.org/10.1016/j.ica.2024.122238  

   Isaacs, Diane P; Dempsey, Jillian L (October 2024, Inorganica Chimica Acta)

   A series of tungsten cyclopentadienyl carbonyl complexes were prepared and characterized to quantify their thermochemical properties and explore their reactivity. The PR3 ligand was systematically varied across a series of CpW(CO)2PR3H metal hydride complexes, where PR3 = P(OEt)3, P(Bu)3, and P(Cy)3. These complexes are known to undergo multiple proton, electron, and proton-coupled electron transfer reactions to access a variety of species including [CpW(CO)2PR3]–, [CpW(CO)2PR3(CH3CN)]+, and [CpW(CO)2PR3]2. Cyclic voltammograms of the CpW(CO)2PR3H•+/0 and [CpW(CO)2PR3]•0/– couples are chemically irreversible, indicating chemical reactivity upon oxidation; the anodic peak potential shifts to lower potentials as the donating ability of phosphine is increased, agreeing with previous literature on similar complexes. Additionally, voltammograms of [CpW(CO)2P(Cy)3]– become chemically reversible at scan rates above 500 mV/s, indicating that the dimerization of the [CpW(CO)2PR3]• product, formed by the oxidation of [CpW(CO)2PR3]–, is slower with the sterically bulky phosphine P(Cy)3, and at high scan rates the species can be reduced before dimerization occurs. Further, as the donating ability of the phosphine increases, the pKa of the CpW(CO)2PR3H complexes increases. This work shows how ligand sterics and electronics can tune the thermochemical properties that underpin proton, electron, and proton-coupled electron transfer reactivity of these complexes. 

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