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Isomerization Mechanism in Hydrazone-Based Rotary Switches: Lateral Shift, Rotation, or Tautomerization?

Landge, Shainaz M. and Tkatchouk, Ekaterina and Benítez, Diego and Lafranchi, Don Antoine and Elhabiri, Mourad and Goddard, William A., III and Aprahamian, Ivan (2011) Isomerization Mechanism in Hydrazone-Based Rotary Switches: Lateral Shift, Rotation, or Tautomerization? Journal of the American Chemical Society, 133 (25). pp. 9812-9823. ISSN 0002-7863. https://resolver.caltech.edu/CaltechAUTHORS:20110803-120632649

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Abstract

Two intramolecularly hydrogen-bonded arylhydrazone (aryl = phenyl or naphthyl) molecular switches have been synthesized, and their full and reversible switching between the E and Z configurations have been demonstrated. These chemically controlled configurational rotary switches exist primarily as the E isomer at equilibrium and can be switched to the protonated Z configuration (Z-H^+) by the addition of trifluoroacetic acid. The protonation of the pyridine moiety in the switch induces a rotation around the hydrazone C═N double bond, leading to isomerization. Treating Z-H^+ with base (K_(2)CO_3) yields a mixture of E and “metastable” Z isomers. The latter thermally equilibrates to reinstate the initial isomer ratio. The rate of the Z → E isomerization process showed small changes as a function of solvent polarity, indicating that the isomerization might be going through the inversion mechanism (nonpolar transition state). However, the plot of the logarithm of the rate constant k vs the Dimroth parameter (E_T) gave a linear fit, demonstrating the involvement of a polar transition state (rotation mechanism). These two seemingly contradicting kinetic data were not enough to determine whether the isomerization mechanism goes through the rotation or inversion pathways. The highly negative entropy values obtained for both the forward (E → Z-H^+) and backward (Z → E) processes strongly suggest that the isomerization involves a polarized transition state that is highly organized (possibly involving a high degree of solvent organization), and hence it proceeds via a rotation mechanism as opposed to inversion. Computations of the Z ↔ E isomerization using density functional theory (DFT) at the M06/cc-pVTZ level and natural bond orbital (NBO) wave function analyses have shown that the favorable isomerization mechanism in these hydrogen-bonded systems is hydrazone–azo tautomerization followed by rotation around a C–N single bond, as opposed to the more common rotation mechanism around the C═N double bond.


Item Type:Article
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URLURL TypeDescription
http://dx.doi.org/10.1021/ja200699vDOIUNSPECIFIED
http://pubs.acs.org/doi/abs/10.1021/ja200699vPublisherUNSPECIFIED
ORCID:
AuthorORCID
Goddard, William A., III0000-0003-0097-5716
Additional Information:© 2011 American Chemical Society. Published In Issue June 29, 2011; Article ASAP June 02, 2011; Just Accepted Manuscript May 17, 2011; Received: January 24, 2011. This work was supported by Dartmouth College, the Burke Research Initiation Award, the Centre National de la Recherche Scientifique (CNRS) and the University of Strasbourg (UMR 7509 CNRS-UdS) in France. We thank Wayne T. Casey for the help with NMR spectroscopy and Dr. Richard Staples (Michigan State University) for X-ray analysis. Computational facilities were funded by grants from ARO-DURIP and ONR-DURIP.
Funders:
Funding AgencyGrant Number
Dartmouth CollegeUNSPECIFIED
Burke Research Initiation AwardUNSPECIFIED
Centre National de la Recherche Scientifique (CNRS)UNSPECIFIED
University of StrasbourgUMR 7509 CNRS-UdS
ARO-DURIPUNSPECIFIED
ONR-DURIPUNSPECIFIED
Issue or Number:25
Record Number:CaltechAUTHORS:20110803-120632649
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20110803-120632649
Official Citation:Isomerization Mechanism in Hydrazone-Based Rotary Switches: Lateral Shift, Rotation, or Tautomerization? Shainaz M. Landge, Ekatarina Tkatchouk, Diego Bentez, Don Antoine Lanfranchi, Mourad Elhabiri, William A. Goddard III, Ivan Aprahamian Journal of the American Chemical Society 2011 133 (25), 9812-9823
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:24656
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:03 Aug 2011 22:43
Last Modified:26 Nov 2019 11:15

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