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1. Crystal structure of N -butyl-2,3-bis(dicyclohexylamino)cyclopropeniminium chloride benzene monosolvate

   https://doi.org/10.1107/S2056989022008076  

   Muñoz Sánchez, Gaby M.; Zdilla, Michael J. (September 2022, Acta Crystallographica Section E Crystallographic Communications)

   N -Butyl-2,3-bis(dicyclohexylamino)cyclopropenimine ( 1 ) crystallizes from benzene and hexanes in the presence of HCl as a monobenzene solvate of the hydrochloride salt, [ 1 H]Cl·C 6 H 6 or C 31 H 54 N 3 + ·Cl − ·C 6 H 6 , in the P 2 1 / n space group. The protonation of 1 results in the generation of an aromatic structure based upon the delocalization of the cyclopropene double bond around the cyclopropene ring, giving three intermediate C—C bond lengths of ∼1.41 Å, and the delocalization of the imine-type C—N double bond, giving three intermediate C—N bond lengths of ∼1.32 Å. Ion–ion and ion–benzene packing interactions are described and illustrated. 

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2. A hemilabile manganese( i )–phenol complex and its coordination induced O–H bond weakening

   https://doi.org/10.1039/d0dt00973c  

   Kadassery, Karthika J.; Crawley, Matthew R.; MacMillan, Samantha N.; Lacy, David C. (January 2020, Dalton Transactions)

   The known compound K[( PO ) 2 Mn(CO) 2 ] ( PO = 2-((diphenylphosphino)methyl)-4,6-dimethylphenolate) (K[ 1 ]) was protonated to form the new Mn( i ) complex ( HPO )( PO )Mn(CO) 2 ( H 1 ) and was determined to have a p K a approximately equal to tetramethylguanidine (TMG). The reduction potential of K[ 1 ] was determined to be −0.58 V vs. Fc/Fc + in MeCN and allowed for an estimation of an experimental O–H bond dissociation free energy (BDFE O–H ) of 73 kcal mol −1 according to the Bordwell equation. This value is in good agreement with a corrected DFT computed BDFE O–H of 68.0 kcal mol −1 (70.3 kcal mol −1 for intramolecular H-bonded isomer). The coordination of the protonated O-atom in the solid-state H 1 was confirmed using FTIR spectroscopy and X-ray crystallography. The phenol moiety is hemilabile as evident from computation and experimental results. For instance, dissociation of the protonated O-atom in H 1 is endergonic by only a few kcal mol −1 (DFT). Furthermore, [ 1 ] − and other Mn( i ) compounds coordinated to PO and/or HPO do not react with MeCN, but H 1 reacts with MeCN to form H 1 + MeCN . Experimental evidence for the solution-bound O-atoms of H 1 was obtained from 1 H NMR and UV-vis spectroscopy and by comparing the electronic spectra of bona fide 16-e − Mn( i ) complexes such as [{ PNP }Mn(CO) 2 ] ( PNP = − N{CH 2 CH 2 (P i Pr 2 )} 2 ) and [( Me3SiOP )( PO )Mn(CO) 2 ] ( Me3Si 1 ). Compound H 1 is only meta-stable ( t 1/2 0.5–1 day) and decomposes into products consistent with homolytic O–H bond cleavage. For instance, treatment of H 1 with TEMPO resulted in formation of TEMPOH, free ligand, and [Mn II {( PO ) 2 Mn(CO) 2 } 2 ]. Together with the experimental and calculated weakened BDFE O–H , these data provide strong evidence for the coordination and hemilability of the protonated O-atom in H 1 and represents the first example of the phenolic Mn( i )–O linkage and a rare example of a “soft-homolysis” intermediate in the bond-weakening catalysis paradigm. 

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3. Mechanistic Insights into Key Steps of Dinitrogen Reduction to Ammonia with Pincer ligated Mo Complexes: Case Study of Easily Reduced PSP Mo Complexes

   https://doi.org/10.26434/chemrxiv-2024-2v9z2  

   Malakar, Santanu; Mandal, Souvik; Zhou, Xiaoguang; Bruch, Quinton; Allen, Rachel; Giordano, Laurence; Walker, Nicholas; Emge, Thomas; Hasanayn, Faraj; Miller, Alexander; et al (May 2024, ChemRxiv)

   The thioether-diphosphine pincer-ligated molybdenum complex, (PSP)MoCl3 (1-Cl3, PSP = 4,5-bis(diisopropylphosphino)-2,7-di-tert-butyl-9,9-dimethyl-9H-thioxanthene) has been synthesized as a catalyst-precursor for N2 reduction catalysis, with a focus on an integrated experimental/computational mechanistic investigation. The (PSP)Mo unit is isoelectronic with the (PNP)Mo (PNP = 2,6-bis(di-t-butylphosphinomethyl)pyridine) fragment found in the family of catalysts for the reduction of N2 to NH3 first reported in 2011 by Nishibayashi and co-workers. Under an atmosphere of N2 the reaction of 1-Cl3 with three reducing equivalents yields the dinuclear penta-dinitrogen Mo complex [(PSP)Mo(N2)2](-N2), 2. Electrochemical studies reveal that 1-Cl3 is significantly more easily reduced than (PNP)MoCl3 (with a potential ca. 0.4 eV less negative). The bridging-nitrogen complex 2 shows no indication of undergoing N2 cleavage to Mo nitride complexes. The reaction of 1-Cl3 with only two reducing equivalents, however, under N2 atmosphere and in the presence of iodide, affords the product of N2 cleavage, the nitride complex (PSP)Mo(N)(I). DFT calculations implicate another N2-bridged complex, [(PSP)Mo(I)]2(N2), as a viable intermediate in facile N2 cleavage to yield (PSP)Mo(N)(I). Conversion of the nitride ligand to NH3 has been studied. If considering sequential addition of H atoms to the nitride, formation of the first N-H bond is by far the thermodynamically least favorable of the three N-H bond formation steps. The first N-H bond was formed by reaction of (PSP)Mo(N)(I) with [LutH]Cl, where coordination of Cl– to Mo plays an essential role. Computations suggest that a second protonation, followed by a rapid and very favorable one-electron reduction, and then a third protonation, furnishes ammonia. In agreement with calculations, ammonia can be generated using either mild H-atom transfer reagents or mild reductants/acids. This comprehensive analysis of the elementary steps of ammonia synthesis and the role of the central pincer donor and halide association provides guidance for future catalyst designs. 

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4. Diverse Coordination Geometries Derived from Trisaminocyclohexane Ligands with Appended Outer‐Sphere Hydrogen Bond Donors

   https://doi.org/10.1002/ejic.202300434  

   Ekanayake, Danushka_M; Sheridan, Patrick_E; Lindeman, Sergey_V; Fiedler, Adam_T (September 2023, European Journal of Inorganic Chemistry)

   Abstract With the aim of constructing hydrogen‐bonding networks in synthetic complexes, two new ligands derived fromcis,cis‐1,3,5‐triaminocyclohexane (TACH) have been prepared that feature pendant pyrrole or indole rings as outer‐sphere H‐bond donors. The TACH framework offers a facial arrangement of threeN‐donors, thereby mimicking common coordination motifs in the active sites of nonheme Fe and Cu enzymes. X‐ray structural characterization of a series of Cu^I‐X complexes (X=F, Cl, Br, NCS) revealed that these neutral ligands (H₃L^R, R=pyrrole or indole) coordinate in the intended facialN₃manner, yielding four‐coordinate complexes with idealizedC₃symmetry. The N−H units of the outer‐sphere heterocycles form a hydrogen‐bonding cavity around the axial (pseudo)halide ligand, as verified by crystallographic, spectroscopic, and computational analyses. Treatment of H₃L^pyrroleand H₃L^indolewith divalent transition metal chlorides (M^IICl₂, M=Fe, Cu, Zn) causes one heterocycle to deprotonate and coordinate to the M(II) center, giving rise to tetradentate ligands with two remaining outer‐sphere H‐bond donors. Further ligand deprotonation is observed upon reaction with Ni(II) and Cu(II) salts with weakly coordinating counteranions. The reported complexes highlight the versatility of TACH‐based ligands with pendant H‐bond donors, as the resulting scaffolds can support multiple protonation states, coordination geometries, and H‐bonding interactions. 

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5. Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical

   https://doi.org/10.1038/s41467-024-45721-1  

   Sinhababu, Soumen; Singh, Roushan Prakash; Radzhabov, Maxim R.; Kumawat, Jugal; Ess, Daniel H.; Mankad, Neal P. (February 2024, Nature Communications)

   Abstract Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals. However, coordination-induced bond weakening is difficult with earth’s most abundant metal, aluminum, because of its redox inertness under mild conditions. Here, we report a system that uses aluminum with a redox non-innocent ligand to achieve significant levels of coordination-induced bond weakening of O-H and N-H bonds. The multisite proton-coupled electron transfer manifold described here points to redox non-innocent ligands as a design element to open coordination-induced bond weakening chemistry to more elements in the periodic table. 

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