Further Exploration of Aza-Cobalt-Cyclobutenes on Co^III(TIM) Complexes: Reactivity and Spectroelectrochemistry

Abstract

The reaction between [CoIII(TIM)Cl2]PF6 (TIM is 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) and electron-rich terminal alkynes (HC2Y, Y = 4-N,N-bis(4-methoxyphenyl)aniline (TPA) or Fc) yielded a pair of constitutional isomers: 1-aza-2-cobalt-cyclobutene-containing trans-[Co(TIM′)((HCC)Y)Cl]+ (1a (TPA) and 2a (Fc); TIM′ is the resultant derivative of TIM) and σ-coordinated trans-[Co(TIM)(C2Y)Cl]+ (1b (TPA) and 2b (Fc)). Additionally, the reactivity of the 1-aza-2-cobalt-cyclobutene species was explored based on trans-[Co(TIM′)((HCC)Fc)Cl]PF6 (2a) because of its good synthetic yield. The reaction of 2a with AgOTf in MeCN resulted in trans-[Co(TIM′)((HCC)Fc)(NCMe)](PF6)(OTf) (3), and the reaction with KCN in MeOH yielded trans-[Co(TIM′)((HCC)Fc)CN]PF6 (4). Similarly, trans-[Co(TIM)(C2Ph)(CN)]PF6 (5) was prepared from the reaction between KCN and trans-[Co(TIM)(C2Ph)Cl]PF6. Molecular structures of 1b and 3–5 were established through single crystal X-ray diffraction. Electrochemical studies of 2–4 revealed that these complexes display an irreversible CoIII reduction as well as a reversible one electron ferrocenyl oxidation between 0.11 and 0.18 V (vs Fc+1/0). UV–vis spectroelectrochemical studies revealed a CoIII to Fc+ charge-transfer upon the one electron oxidation of 2a and 2b.

Synthesis and Characterization of CoIII(TIM) Complexes Bearing Alkenyl or 1-Aza-2-cobalt-cyclopropane Moieties

“…Notably, the TIM-based reductions in previously reported aza-cobalt-cyclobutene-bearing Co­(TIM) complexes occur at highly negative potentials (ca. −2.4 V), indicating that the electronic influences imbued by the four-membered ring on the macrocycle are significantly different than those imparted on the macrocycle by the three-membered ring in 3a .…”

“… , Surprisingly, the reactions with more electron rich terminal alkynes, HC 2 R (R = 4- N , N -dimethylaniline and ferrocene) under weak or strong base conditions afford both the expected trans -[Co­(TIM)­(C 2 R)­Cl] + and an unexpected trans -[Co­(TIM′)­((HC = C)­R)­Cl] + species bearing a 1-aza-2-cobalt-cyclobutene unit (Path B in Scheme ). , The unique coordination of the alkynes in trans -[Co­(TIM′)­((HC = C)­R)­Cl] + was attributed to an unexpected reactivity involving the TIM ligand, which could be accessed upon the deprotonation of a TIM methyl group to trigger an intramolecular rearrangement generating the four-membered ring. Notably, the resulting products in these reactions were constitutional isomers of the anticipated α-alkynyl complexes, trans -[Co­(TIM)­(C 2 R)­Cl] + .…”

Fe(III) Bis-Alkynyls Supported by the TIM Macrocycle: Molecular and Electronic Structures and Altering the Nature of Charge Transfer Transitions through Reduction

“…7 During the investigation of the reaction between electron-rich alkynes, HC 2 Y (Y = 4-N,N-dimethylaniline (DMAP), ferrocene (Fc)) and [Co(TIM)X 2 ] + (III), an unusual aza-cobalt-cyclobutene was formed from the [2 + 2] addition of the terminal alkyne to a Co−N bond, which was aided by a formal two-electron donation through the deprotonation of one of the TIM's methyl groups. 8,9 These findings hint at the potential for new reactivity and novel electronic properties to be elicited from the 3d metal complexes of tetra/tri-imine macrocycles. The studies of iron alkynyl compounds have significantly enriched the chemistry of metal alkynyl compounds.…”

“…The potential for ligand noninnocence through storing electrons in the α-diimine units of the TIM macrocycle (TIM = 2,3,9,10-tetramethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene) was demonstrated in the study of Fe­(TIM) complexes. − In turn, Fe II (TIM) was used to probe the redox noninnocence of nitrosoarene ligands . During the investigation of the reaction between electron-rich alkynes, HC 2 Y (Y = 4- N,N -dimethylaniline (DMAP), ferrocene (Fc)) and [Co­(TIM)­X 2 ] + (III), an unusual aza-cobalt-cyclobutene was formed from the [2 + 2] addition of the terminal alkyne to a Co–N bond, which was aided by a formal two-electron donation through the deprotonation of one of the TIM’s methyl groups. , These findings hint at the potential for new reactivity and novel electronic properties to be elicited from the 3d metal complexes of tetra/tri-imine macrocycles.…”

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The bis-alkynyl complexes based on Fe III (TIM) (TIM = 2,3,9,4,8,3,8, are reported herein. A modified synthesis of trans-[Fe(TIM)Cl 2 ]PF 6 ([1]PF 6 ) in a 90% yield was developed.

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