Probing the effects of steric bulk on the solution-phase behaviour and redox chemistry of cobalt-diimine complexes

Symes, Mark D.; Wilson, Claire

doi:10.1080/10610278.2017.1373195

“…Although complexes of Co II with pyridyl donors are usually able to undergo electrochemical oxidation to Co III , no Co III/II redox couple was observed for [Co( 6 )](ClO 4 ) 2 . The inaccessibility of the Co III oxidation state is consistent with Sauvage and co-workers’ original report in which a catenane ligand was found to prevent access to higher oxidation states by inhibiting ligand reorganization, and a similar effect was recently observed in the context of a Co II complex of the sterically hindered ligand neocuprine, but this is the first time to our knowledge that a similar effect has been observed in the context of a rotaxane…”

Section: Electronic and Redox Properties Of Rotaxane-based Metal Comp...supporting

confidence: 90%

Rotaxane-Based Transition Metal Complexes: Effect of the Mechanical Bond on Structure and Electronic Properties

Cirulli

¹

,

Kaur

²

,

Lewis

³

et al. 2018

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Early work by Sauvage revealed that mechanical bonding alters the stability and redox properties of their original catenane metal complexes. However, despite the importance of controlling metal ion properties for a range of applications, these effects have received relatively little attention since. Here we present a series of tri-, tetra-and pentadentate rotaxane-based ligands, a detailed study of their metal binding behavior and, where possible, compare their redox and electronic properties with their non-interlocked counterparts. The rotaxane ligands form complexes with most of the metal ions investigated and x-ray diffraction revealed that in some cases the mechanical bond enforces unusual coordination numbers and distorted arrangements as a result of the exclusion of exogenous ligands driven by the sterically crowded binding sites. In contrast, only the non-interlocked equivalent of the pentadentate rotaxane Cu II complex could be formed selectively, and this exhibited compromised redox stability compared to its interlocked counterpart. Frozen-solution EPR data demonstrate the formation of an interesting biomimetic state for the tetradentate Cu II rotaxane, as well as the formation of stable Ni I species and the unusual coexistence of high-and low-spin Co II in the pentadentate framework. Our results demonstrate that readily available mechanically chelating rotaxanes give rise to complexes the non-interlocked equivalent of which are inaccessible, and that the mechanical bond augments the redox behavior of the bound metal ion in a manner analogous to the carefully-tuned amino acid framework in metalloproteins.

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“…This arrangement is rather reminiscent of the geometry previously observed by Tanaka and co-workers for the complex [CuCl(Me 1 TMPA)] + , where the TMPA pyridine bearing the methyl group exhibited a much longer Cu–N interaction than the unsubstituted pyridines (2.337 Å vs ∼1.99 Å) . On account of the differing electronic properties of methyl and methyl ester substituents, it therefore seems likely that this common bond elongation effect is the result of steric crowding brought about by the close proximity of the substituents to the N-donor atom . Similarly long Cu–N interactions have also been noted before in Cu(II) tris(2-methylpyridyl)amine complexes bearing bulky substituents next to the N donors (albeit where the ligands were more symmetrically substituted) by Reinaud and co-workers .…”

Section: Resultssupporting

confidence: 66%

“…46 On account of the differing electronic properties of methyl and methyl ester substituents, it therefore seems likely that this common bond elongation effect is the result of steric crowding brought about by the close proximity of the substituents to the N-donor atom. 83 Similarly long Cu− N interactions have also been noted before in Cu(II) tris(2methylpyridyl)amine complexes bearing bulky substituents next to the N donors (albeit where the ligands were more symmetrically substituted) by Reinaud and co-workers. 84 The shortest distance between the Cu center in [3-CH 3 CN] 2+ and any of the oxygens on the nearest perchlorate anion is 2.745 Å, suggesting only a very weak interaction between the Cu and perchlorate and therefore that the metal center is best considered as five-coordinate.…”

supporting

confidence: 60%

Proton-Coupled Electron Transfer Enhances the Electrocatalytic Reduction of Nitrite to NO in a Bioinspired Copper Complex

Cioncoloni

¹

,

Roger

²

,

Wheatley

³

et al. 2018

Self Cite

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The selective and efficient electrocatalytic reduction of nitrite to nitric oxide (NO) is of tremendous importance, both for the development of NO-release systems for biomedical applications and for the removal of nitrogen oxide pollutants from the environment. In nature, this transformation is mediated by (among others) enzymes known as the copper-containing nitrite reductases. The development of synthetic copper complexes that can reduce nitrite to NO has therefore attracted considerable interest. However, there are no studies describing the crucial role of proton-coupled electron transfer during nitrite reduction when such synthetic complexes are used. Herein, we describe the synthesis and characterization of two previously unreported Cu complexes (3 and 4) for the electrocatalytic reduction of nitrite to NO, in which the role of proton-relaying units in the secondary coordination sphere of the metal can be probed. Complex 4 bears a pendant carboxylate group in close proximity to the copper center, while complex 3 lacks such functionality. Our results suggest that complex 4 is twice as effective an electrocatalyst for nitrite reduction than is complex 3 and that complex 4 is the best copper-based molecular electrocatalyst for this reaction yet discovered. The differences in reactivity between 3 and 4 are probed using a range of electrochemical, spectroscopic, and computational methods, which shed light on the possible catalytic mechanism of 4 and implicate the proton-relaying ability of its pendant carboxylate group in the enhanced reactivity that this complex displays. These results highlight the critical role of proton-coupled electron transfer in the reduction of nitrite to NO and have important implications for the design of biomimetic catalysts for the selective interconversions of the nitrogen oxides.

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“…46 On account of the differing electronic properties of methyl and methyl ester substituents, it therefore seems likely that this common bond elongation effect is the result of steric crowding brought about by the close proximity of the substituents to the N-donor atom. 83 Similarly long Cu− N interactions have also been noted before in Cu(II) tris(2methylpyridyl)amine complexes bearing bulky substituents next to the N donors (albeit where the ligands were more symmetrically substituted) by Reinaud and co-workers. 84 The shortest distance between the Cu center in [3-CH 3 CN] 2+ and any of the oxygens on the nearest perchlorate anion is 2.745 Å, suggesting only a very weak interaction between the Cu and perchlorate and therefore that the metal center is best considered as five-coordinate.…”

Section: Acs Catalysissupporting

confidence: 60%

Proton-Coupled-Electron Transfer Enhances the Electrocatalytic Reduction of Nitrite to NO in a Bioinspired Copper Complex

Symes,

Cioncoloni,

Roger

et al. 2019

Meet. Abstr.

Self Cite

0

View full text Add to dashboard Cite

The selective and efficient electrocatalytic reduction of nitrite to nitric oxide (NO) is of tremendous importance, both for the development of NO-release systems for biomedical applications and for the removal of nitrogen oxide pollutants from the environment. In nature, this transformation is mediated by (among others) enzymes known as the copper-containing nitrite reductases. The development of synthetic copper complexes that can reduce nitrite to NO has therefore attracted considerable interest. However, there are no studies describing the crucial role of proton-coupled electron transfer during nitrite reduction when such synthetic complexes are used. Herein, we describe the synthesis and characterization of two previously unreported Cu complexes (3 and 4) for the electrocatalytic reduction of nitrite to NO, in which the role of proton-relaying units in the secondary coordination sphere of the metal can be probed. Complex 4 bears a pendant carboxylate group in close proximity to the copper center, while complex 3 lacks such functionality. Our results suggest that complex 4 is twice as effective an electrocatalyst for nitrite reduction than is complex 3 and that complex 4 is the best copper-based molecular electrocatalyst for this reaction yet discovered. The differences in reactivity between 3 and 4 are probed using a range of electrochemical, spectroscopic, and computational methods, which shed light on the possible catalytic mechanism of 4 and implicate the proton-relaying ability of its pendant carboxylate group in the enhanced reactivity that this complex displays. These results highlight the critical role of proton-coupled electron transfer in the reduction of nitrite to NO and have important implications for the design of biomimetic catalysts for the selective interconversions of the nitrogen oxides.

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Probing the effects of steric bulk on the solution-phase behaviour and redox chemistry of cobalt-diimine complexes

Cited by 4 publications

References 53 publications

Rotaxane-Based Transition Metal Complexes: Effect of the Mechanical Bond on Structure and Electronic Properties

Rotaxane-Based Transition Metal Complexes: Effect of the Mechanical Bond on Structure and Electronic Properties

Proton-Coupled Electron Transfer Enhances the Electrocatalytic Reduction of Nitrite to NO in a Bioinspired Copper Complex

Proton-Coupled-Electron Transfer Enhances the Electrocatalytic Reduction of Nitrite to NO in a Bioinspired Copper Complex

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