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1. Synthesis of SCO FeII complexes with novel π-extended 2,2'-biimidazole ligands

   Yergeshbayeva, S.; Shatruk, M. (April 2021, National Meeting of the American Chemical Society)

   null (Ed.)

   Spin crossover (SCO) is a phenomenon observed for certain transition metal complexes with electronic configuration 3d4-3d7. The conversion between the low-spin (LS) and high-spin (HS) states is usually driven by a variety of external perturbations, such as temperature, pressure, or light. The switching between the enthalpically preferred LS state and entropically favorable HS state is accompanied by dramatic changes in the metal-ligand bond lengths, unit cell volume, optical absorption spectrum, and magnetic susceptibility.1 These changes make SCO materials suitable for applications in sensors, memory, and display devices. One of the central challenges in the SCO research is to initiate strongly cooperative interactions known to lead to abrupt spin transitions and thermal hysteresis that can be harvested as a memory effect. One of the strategies to enhance the cooperativity is to design SCO complexes with supramolecular interactions such as π-stacking of aromatic fragments or hydrogen bonding.2 In this work, we report syntheses and characterization of heteroleptic complexes of [Fe(tpma)(L)](ClO4)2 (tpma = tris(pyridin-2-ylmethyl)amine) with novel π-extended biimidazole-type ligands (L) bearing 2,3-dimethyl-naphthalene-, 6,7-dimethyl-2,3-diphenyl-quinoxaline, and 2,3-dimethyl-anthracene pendant fragments. Solvent-free naphthalene-functionalized complex [Fe(tpma)(xnap-bim)](ClO4)2 exhibits abrupt spin transition at T1/2 = 127K with a narrow 1 K hysteresis loop. In contrast, polymorph of this complex that contains one interstitial molecules of pyridine exhibits gradual SCO. Anthracene-functionalized complex [Fe(tpma)(anthra-bim)](ClO4)2 also crystallizes as two polymorphs. Structural studies at 100, 230, and 300 K revealed dramatic changes in the N-Fe-N biting angles and Fe-N distances, indicating the occurrence of temperature-induced SCO. Complex [Fe(tpma)(quin-bim)](ClO4)2 (quin-bim = 6,7-dimethyl-2,3-diphenyl-quinoxaline-2,2’-biimidazole) showed only HS state at 100 and 230 K. In the crystal packing the mononuclear cations form stacks along b axis. We discuss how the observed magnetic behavior correlates with changes in the crystal packing and interactions between the pendant aromatic substituents on the aforementioned complexes. 

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2. Palladium(0) complexes of diferrocenylmercury diphosphines: synthesis, X-ray structure analyses, catalytic isomerization, and C–Cl bond activation

   https://doi.org/10.1039/d1dt00641j  

   Tagne Kuate, Alain C.; Lalancette, Roger. A.; Bockfeld, Dirk; Tamm, Matthias; Jäkle, Frieder (April 2021, Dalton Transactions)

   Palladium(0) phosphine complexes are of great importance as catalysts in numerous bond formation reactions that involve oxidative addition of substrates. Highly active catalysts with labile ligands are of particular interest but can be challenging to isolate and structurally characterize. We investigate here the synthesis and chemical reactivity of Pd 0 complexes that contain geometrically adaptable diferrocenylmercury-bridged diphosphine chelate ligands (L) in combination with a labile dibenzylideneacetone (dba) ligand. The diastereomeric diphosphines 1a (p S p R , meso -isomer) and 1b (p S p S -isomer) differ in the orientation of the ferrocene moieties relative to the central Ph 2 PC 5 H 3 –Hg–C 5 H 3 PPh 2 bridging entity. The structurally distinct trigonal LPd 0 (dba) complexes 2a ( meso ) and 2b (p S p S ) are obtained upon treatment with Pd(dba) 2 . A competition reaction reveals that 1b reacts faster than 1a with Pd(dba) 2 . Unexpectedly, catalytic interconversion of 1a ( meso ) into 1b ( rac ) is observed at room temperature in the presence of only catalytic amounts of Pd(dba) 2 . Both Pd 0 complexes, 2a and 2b , readily undergo oxidative addition into the C–Cl bond of CH 2 Cl 2 at moderate temperatures with formation of the square-planar trans -chelate complexes LPd II Cl(CH 2 Cl) ( 3a , 3b ). Kinetic studies reveal a significantly higher reaction rate for the meso -isomer 2a in comparison to (p S p S )- 2b . 

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3. Crystal structures of two dioxomolybdenum complexes stabilized by salan ligands featuring phenyl and cyclohexyl backbones

   https://doi.org/10.1107/S2056989022000524  

   Trieu-Tran, Tristhan; Martinez, Stephenie N.; Brannon, Jacob P.; Stieber, S. Chantal; John, Alex (March 2022, Acta Crystallographica Section E Crystallographic Communications)

   Two cis -dioxomolybdenum complexes based on salan ligands with different backbones are reported. The first complex, dioxido{2,2′-[l,2-phenylenebis(iminomethylene)]bis(phenolato)}molybdenum(VI) dimethylformamide disolvate, [Mo(C 20 H 18 N 2 O 2 )O 2 ]·2C 3 H 7 NO ( Ph LMoO 2 , 1b ), features a phenyl backbone, while the second complex, (6,6′-{[(cyclohexane-1,2-diyl)bis(azanediyl)]bis(methylene)}bis(2,4-di- tert -butylphenolato))dioxidomolybdenum(VI) methanol disolvate, [Mo(C 36 H 56 N 2 O 2 )O 2 ]·2CH 3 OH ( Cy LMoO 2 , 2b ), is based on a cyclohexyl backbone. These complexes crystallized as solvated species, 1b ·2DMF and 2b ·2MeOH. The salan ligands Ph LH 2 ( 1a ) and Cy LH 2 ( 2a ) coordinate to the molybdenum center in these complexes 1b and 2b in a κ 2 N ,κ 2 O fashion, forming a distorted octahedral geometry. The Mo—N and Mo—O distances are 2.3475 (16) and 1.9567 (16) Å, respectively, in 1b while the corresponding measurements are Mo—N = 2.3412 (12) Å, and Mo—O = 1.9428 (10) Å for 2b . A key geometrical feature is that the N—Mo—N angle of 72.40 (4)° in Cy LMoO 2 is slightly less than that of the Ph LMoO 2 angle of 75.18 (6)°, which is attributed to the flexibility of the cyclohexane ring between the nitrogen as compared to the rigid phenyl ring in the Ph LMoO 2 . 

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4. p ‐Substituted Tris(2‐pyridylmethyl)amines as Ligands for Highly Active ATRP Catalysts: Facile Synthesis and Characterization

   https://doi.org/10.1002/anie.202004724  

   Enciso, Alan E.; Lorandi, Francesca; Mehmood, Arshad; Fantin, Marco; Szczepaniak, Grzegorz; Janesko, Benjamin G.; Matyjaszewski, Krzysztof (June 2020, Angewandte Chemie International Edition)

   Abstract A facile and efficient two‐step synthesis ofp‐substituted tris(2‐pyridylmethyl)amine (TPMA) ligands to form Cu complexes with the highest activity to date in atom transfer radical polymerization (ATRP) is presented. In the divergent synthesis,p‐Cl substituents in tris(4‐chloro‐2‐pyridylmethyl)amine (TPMA^3Cl) were replaced in one step and high yield by electron‐donating cyclic amines (pyrrolidine (TPMA^PYR), piperidine (TPMA^PIP), and morpholine (TPMA^MOR)) by nucleophilic aromatic substitution. The [Cu^II(TPMA^NR2)Br]⁺complexes exhibited larger energy gaps between frontier molecular orbitals and >0.2 V more negative reduction potentials than [Cu^II(TPMA)Br]⁺, indicating >3 orders of magnitude higher ATRP activity. [Cu^I(TPMA^PYR)]⁺exhibited the highest reported activity for Br‐capped acrylate chain ends in DMF, and moderate activity toward C−F bonds at room temperature. ATRP ofn‐butyl acrylate using only 10–25 part per million loadings of [Cu^II(TPMA^NR2)Br]⁺exhibited excellent control. 

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5. Mechanistic Insights into Photocatalytic Reduction and Triplet Sensitization of 4-Azido-1-phenylbutan-1-ones to Selectively Form 2-Phenyl-1-pyrrolines

   https://doi.org/10.1021/acs.joc.5c01550  

   Gatlin, DeVonna M; Abdelaziz, Nayera M; Wasson, Fiona J; Borah, Anindya; Etz, Brian D; Abbas, Sulaiman Umar; Perry, Anntonette N; Hudak, Mikayla A; Diaw, Khadyjah; Vyas, Shubham; et al (October 2025, The Journal of Organic Chemistry)

   Organic azides are valuable precursors in synthetic chemistry, particularly for nitrogen-based functionalization through photochemical activation. In this study, the photoreactivities of 4-azido-1-phenylbutan-1-one (1a) and 4-azido-(4-methoxy)phenylbutan-1-one (1b) were investigated using visible-light photocatalysts [Ir(dF(CF3)ppy)2(dtbpy)]PF6 and [Ru(bpy)3]Cl2 to elucidate the mechanistic differences between triplet energy transfer and photoreductive electron transfer pathways. Direct irradiation of 1a in methanol favors the formation of a biradical species via intramolecular H atom abstraction to generate its lowest triplet ketone (T1K) with an (n,π*) configuration, which selectively yields 2-phenyl-1-pyrroline derivative 2a. However, 1b reacts through its less reactive T1K, which has a (π,π*) configuration, to form 2-phenyl-1H-pyrrole as the major product. When sensitized by [Ir(dF(CF3)ppy)2(dtbpy)]PF6, selective excitation of the triplet azido moiety (TA) of both 1a and 1b yields the corresponding pyrroline (2a and 2b) via triplet alkylnitrene (31aN and31bN) formation. In contrast, photoactivation of [Ru(bpy)3]Cl2 in the presence of diisopropylethylamine (DIPEA) results in photoreductive electron transfer, forming azido radical anion intermediates, which cyclize to also yield 2a and 2b. Product studies, cyclic voltammetry, laser flash photolysis, and DFT calculations supported these mechanistic assignments. This work demonstrates complementary approaches to control alkyl azide photoreactivity and unlock new strategies for visible-light-induced nitrogen incorporation. 

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