Abstract
Chemical functionalization of nanotubes, in which their properties can be combined with those of other classes of materials, is fundamental to improve the physicochemical properties of nanotubes for potential technological applications. In this work, we theoretically and experimentally examine the Pauson-Khand reaction (PKR) on zig-zag, armchair, and chiral single-walled carbon nanotubes (SWCNTs). Our benchmarked density functional theory (DFT) calculations show that an alternative pathway to the widely accepted Magnus reaction pathway has significantly lower energy barriers, thus suggesting the use of this alternative pathway to predict whether a PKR on SWCNTs is favored or hampered. Accessible energy barriers of up to 16 kcal mol-1 are estimated and our results suggest that semiconducting SWCNTs react faster than metallic ones, although both types can be functionalized. Guided by our theoretical predictions, cyclopentenones are successfully attached to SWCNTs by heating and are, subsequently, characterized in the laboratory.
Highlights
We conclude that the Magnus mechanism is only favored in those cases in which CP3 is more stable than CP1′; electronic factors like the double-bond delocalization in single-walled carbon nanotubes (SWCNTs) and the steric hindrance bring about a destabilization of CP3 resulting in a non-operative Magnus pathway in Pauson–Khand reaction (PKR) involving SWCNTs
As a matter of fact, our results suggest that the PKR in (5,5)-SWCNT with acetylene could be difficult to proceed in the laboratory in view of that an energy barrier of at least 29.4 kcal mol−1, via TS1′obl, must be overcome
Nanoscale semimetallic (9,0)-SWCNT, 22.6 kcal mol−1 for the metallic (5,5)-SWCNT, and only 15.8 kcal mol−1 for the semiconducting (6,5)- and (8,0)-SWCNT. These results suggest that semiconducting SWCNTs react faster than any other, metallic ones are prone to PK-functionalization because of the accessible energy barrier of 22.6 kcal mol−1
Summary
First reported in the early 1970s by Pauson and Khand et al, the [2 + 2 + 1] cycloaddition between an alkene, an alkyne, and a carbon monoxide unit (CO) leads to the formation of cyclopentenones mediated or catalyzed by a transition metal; originally a complex of cobalt(0) (see Scheme 1).[1,2,3] This chemical transformation, usually termed as the Pauson–Khand reaction (PKR), has gained attention within the field of organic chemistry since many natural products can be synthesized from it.[4,5,6,7,8,9,10,11,12,13,14,15,16,17,18] Besides cobalt, other transition metals perform well in the PKR such as rhodium,[19,20,21] ruthenium,[22,23] palladium,[24] iridium,[25,26] iron,[27,28] tungsten,[29] molybdenum,[30,31] and chromium.[32,33]The PKR depends upon steric and electronic factors. Structures like CP388,89 and CP485 had been previously discussed, supporting the original idea of Magnus, some CP3-like structures were found to hamper the course of the PKR due to an unfavorable relative orientation of the cobalt-coordinated alkyne and alkene.[84] we conclude that the Magnus mechanism is only favored in those cases in which CP3 is more stable than CP1′; electronic factors like the double-bond delocalization in SWCNTs and the steric hindrance bring about a destabilization of CP3 resulting in a non-operative Magnus pathway in PKRs involving SWCNTs. a favored or hindered PKR may be predicted by TS1′ rather than TS1 in terms of kinetic arguments.
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