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1. Synthesis of Chromium(II) Complexes with Chelating Bis(alkoxide) Ligand and Their Reactions with Organoazides and Diazoalkanes

   https://doi.org/10.3390/molecules25020273  

   Kurup, Sudheer S.; Staples, Richard J.; Lord, Richard L.; Groysman, Stanislav (January 2020, Molecules)

   Synthesis of new chromium(II) complexes with chelating bis(alkoxide) ligand [OO]Ph (H2[OO]Ph = [1,1′:4′,1′’-terphenyl]-2,2′’-diylbis(diphenylmethanol)) and their subsequent reactivity in the context of catalytic production of carbodiimides from azides and isocyanides are described. Two different Cr(II) complexes are obtained, as a function of the crystallization solvent: mononuclear Cr[OO]Ph(THF)2 (in toluene/THF, THF = tetrahydrofuran) and dinuclear Cr2([OO]Ph)2 (in CH2Cl2/THF). The electronic structure and bonding in Cr[OO]Ph(THF)2 were probed by density functional theory calculations. Isolated Cr2([OO]Ph)2 undergoes facile reaction with 4-MeC6H4N3, 4-MeOC6H4N3, or 3,5-Me2C6H3N3 to yield diamagnetic Cr(VI) bis(imido) complexes; a structure of Cr[OO]Ph(N(4-MeC6H4))2 was confirmed by X-ray crystallography. The reaction of Cr2([OO]Ph)2 with bulkier azides N3R (MesN3, AdN3) forms paramagnetic products, formulated as Cr[OO]Ph(NR). The attempted formation of a Cr–alkylidene complex (using N2CPh2) instead forms chromium(VI) bis(diphenylmethylenehydrazido) complex Cr[OO]Ph(NNCPh2)2. Catalytic formation of carbodiimides was investigated for the azide/isocyanide mixtures containing various aryl azides and isocyanides. The formation of carbodiimides was found to depend on the nature of organoazide: whereas bulky mesitylazide led to the formation of carbodiimides with all isocyanides, no carbodiimide formation was observed for 3,5-dimethylphenylazide or 4-methylphenylazide. Treatment of Cr2([OO]Ph)2 or H2[OO]Ph with NO+ leads to the formation of [1,2-b]-dihydroindenofluorene, likely obtained via carbocation-mediated cyclization of the ligand. 

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2. Aziridination Reactivity of a Manganese(II) Complex with a Bulky Chelating Bis(Alkoxide) Ligand

   https://doi.org/10.3390/molecules27185751  

   Kurup, Sudheer S.; Woodland, Natalie M.; Lord, Richard L.; Groysman, Stanislav (September 2022, Molecules)

   Treatment of Mn(N(SiMe3)2)2(THF)2 with bulky chelating bis(alkoxide) ligand [1,1′:4′,1′′-terphenyl]-2,2′′-diylbis(diphenylmethanol) (H2[O-terphenyl-O]Ph) formed a seesaw manganese(II) complex Mn[O-terphenyl-O]Ph(THF)2, characterized by structural, spectroscopic, magnetic, and analytical methods. The reactivity of Mn[O-terphenyl-O]Ph(THF)2 with various nitrene precursors was investigated. No reaction was observed between Mn[O-terphenyl-O]Ph(THF)2 and aryl azides. In contrast, the treatment of Mn[O-terphenyl-O]Ph(THF)2 with iminoiodinane PhINTs (Ts = p-toluenesulfonyl) was consistent with the formation of a metal-nitrene complex. In the presence of styrene, the reaction led to the formation of aziridine. Combining varying ratios of styrene and PhINTs in different solvents with 10 mol% of Mn[O-terphenyl-O]Ph(THF)2 at room temperature produced 2-phenylaziridine in up to a 79% yield. Exploration of the reactivity of Mn[O-terphenyl-O]Ph(THF)2 with various olefins revealed (1) moderate aziridination yields for p-substituted styrenes, irrespective of the electronic nature of the substituent; (2) moderate yield for 1,1′-disubstituted α-methylstyrene; (3) no aziridination for aliphatic α-olefins; (4) complex product mixtures for the β-substituted styrenes. DFT calculations suggest that iminoiodinane is oxidatively added upon binding to Mn, and the resulting formal imido intermediate has a high-spin Mn(III) center antiferromagnetically coupled to an imidyl radical. This imidyl radical reacts with styrene to form a sextet intermediate that readily reductively eliminates the formation of a sextet Mn(II) aziridine complex. 

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3. Efficient carbene transfer reactivity mediated by Fe( ii ) complexes supported by bulky alkoxides

   https://doi.org/10.1039/D4CC02108H  

   Kulathungage, Lakshani W; Kurup, Sudheer S; Browne, Edison A; Spalink, Gabriel H; Ward, Cassandra L; Lord, Richard L; Groysman, Stanislav (July 2024, Chemical Communications)

   The reaction of Fe(OR)₂(THF)₂(OR = bulky alkoxide ligand) with PhIC(CO₂Me)₂results in the formation of reactive remote carbene/vinyl radical intermediate that undergoes facile cyclopropanation or dimerization. 

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4. Synthesis and structure of a new bulky bis(alkoxide) ligand on a terphenyl platform

   https://doi.org/10.1107/S2056989021013438  

   Kurup, Sudheer S.; Nasser, Sandra; Ward, Cassandra L.; Groysman, Stanislav (January 2022, Acta Crystallographica Section E Crystallographic Communications)

   A new sterically bulky chelating bis(alkoxide) ligand 3,3′-([1,1′:4′,1′′-terphenyl]-2,2′′-diyl)bis(2,2,4,4-tetramethylpentan-3-ol), (H 2 [OO] tBu ), was prepared in a two-step process as the dichloromethane monosolvate, C 36 H 50 O 2 ·CH 2 Cl 2 . The first step is a Suzuki–Miyaura coupling reaction between 2-bromophenylboronic acid and 1,4-diiodobenzene. The resulting 2,2′′-dibromo-1,1′:4′,1′′-terphenyl was reacted with t BuLi and hexamethylacetone to obtain the desired product. The crystal structure of H 2 [OO] tBu revealed an anti conformation of the [CPh 2 (OH)] fragments relative to the central phenyl. Furthermore, the hydroxyl groups point away from each other. Likely because of this anti – anti conformation, the attempts to synthesize first-row transition-metal complexes with H 2 [OO] tBu were not successful. 

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5. Multinuclear iron–phenyl species in reactions of simple iron salts with PhMgBr: identification of Fe ₄ (μ-Ph) ₆ (THF) ₄ as a key reactive species for cross-coupling catalysis

   https://doi.org/10.1039/c8sc02915f  

   Carpenter, Stephanie H.; Baker, Tessa M.; Muñoz, Salvador B.; Brennessel, William W.; Neidig, Michael L. (October 2018, Chemical Science)

   The first direct syntheses, structural characterizations, and reactivity studies of iron–phenyl species formed upon reaction of Fe(acac) 3 and PhMgBr in THF are presented. Reaction of Fe(acac) 3 with 4 equiv. PhMgBr in THF leads to the formation of [FePh 2 (μ-Ph)] 2 2− at −80 °C, which can be stabilized through the addition of N -methylpyrrolidone. Alternatively, at −30 °C this reaction leads to the formation of the tetranuclear iron–phenyl cluster, Fe 4 (μ-Ph) 6 (THF) 4 . Further synthetic studies demonstrate that analogous tetranuclear iron clusters can be formed with both 4-F-PhMgBr and p -tolylMgBr, illustrating the generality of this structural motif for reactions of simple ferric salts and aryl Grignard reagents in THF. Additional studies isolate and define key iron species involved in the synthetic pathway leading to the formation of the tetranuclear iron–aryl species. While reaction studies demonstrate that [FePh 2 (μ-Ph)] 2 2− is unreactive towards electrophile, Fe 4 (μ-Ph) 6 (THF) 4 is found to rapidly react with bromocyclohexane to selectively form phenylcyclohexane. Based on this reactivity, a new catalytic reaction protocol has been developed that enables efficient cross-couplings using Fe 4 (μ-Ph) 6 (THF) 4 , circumventing the current need for additives such as TMEDA or supporting ligands to achieve effective cross-coupling of PhMgBr and a secondary alkyl halide. 

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