Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals

Abstract

AbstractFive bis(tert‐butylnitroxide) diradicals connected by a silole (7 a–d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna21 with a = 20.752(5), b = 5.826(5), and c = 34.309(5) Å. X‐ray crystal structure determination, electronic spectroscopy, variable‐temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6‐31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a π‐conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a “Bleaney–Bowers” model that gives values of J = −142.0 cm−1 for 7 a, −1.8 cm−1 for 7 b, −1.3 cm−1 for 7 c, −4.2 cm−1 for 7 d, and −248.0 cm−1 for 12. The temperature dependence of the EPR half‐field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with ΔET–S values of 3.48, 2.09, and 8 cm−1, respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the ΔET–S and J values (ΔET–S = −2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a “Karplus–Conroy”‐type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5‐positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon π system of the silole.

Autoren

Nans Roques
Philippe Gerbier
Ulrich Schatzschneider
Jean‐Pascal Sutter
Philippe Guionneau
José Vidal‐Gancedo
Jaume Veciana
Eva Rentschler
Christian Guérin

DOI

10.1002/chem.200501280

eISSN

1521-3765

ISSN

0947-6539

Ausgabe der Veröffentlichung

21

Zeitschrift

Chemistry – A European Journal

Sprache

en

Online publication date

2006

Paginierung

5547 - 5562

Datum der Veröffentlichung

2006

Status

Published

Herausgeber

Wiley

Herausgeber URL

http://dx.doi.org/10.1002/chem.200501280

Datum der Datenerfassung

2023

Titel

Experimental and Theoretical Studies of Magnetic Exchange in Silole‐Bridged Diradicals

Ausgabe der Zeitschrift

12

Datenquelle: Crossref

Abstract

Five bis(tert-butylnitroxide) diradicals connected by a silole (7 a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a "Bleaney-Bowers" model that gives values of J = -142.0 cm(-1) for 7 a, -1.8 cm(-1) for 7 b, -1.3 cm(-1) for 7 c, -4.2 cm(-1) for 7 d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with DeltaE(T-S) values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the DeltaE(T-S) and J values (DeltaE(T-S) = -2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a "Karplus-Conroy"-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.

Addresses

Laboratoire de Chimie Moléculaire et Organisation du Solide, Université Montpellier 2, C.C.007, Place E. Bataillon, 34095 Montpellier Cedex 5, France.

Autoren

Nans Roques
Philippe Gerbier
Ulrich Schatzschneider
Jean-Pascal Sutter
Philippe Guionneau
José Vidal-Gancedo
Jaume Veciana
Eva Rentschler
Christian Guérin

DOI

10.1002/chem.200501280

eISSN

1521-3765

Externe Identifier

PubMed Identifier: 16680786

Open access

false

ISSN

0947-6539

Ausgabe der Veröffentlichung

21

Zeitschrift

Chemistry (Weinheim an der Bergstrasse, Germany)

Schlüsselwörter

Free Radicals
Butanes
Organosilicon Compounds
Electron Spin Resonance Spectroscopy
Models, Chemical
Models, Molecular

Sprache

eng

Medium

Print

Paginierung

5547 - 5562

Datum der Veröffentlichung

2006

Status

Published

Datum der Datenerfassung

2006

Titel

Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals.

Sub types

Research Support, Non-U.S. Gov't
Journal Article

Ausgabe der Zeitschrift

12

Datenquelle: Europe PubMed Central

Abstract

Five bis(tert-butylnitroxide) diradicals connected by a silole (7 a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2(1) with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ray crystal structure determination, electronic spectroscopy, variable-temperature EPR spectroscopy, SQUID measurements and DFT computations (UB3LYP/6-31+G*) were used to study the molecular conformations and electronic spin coupling in this series of molecules. Whereas compounds 7 b, 7 c, and 7 d are quite stable both in solution and in the solid state, 7 a and 12 undergo a partial electronic rearrangement to both a diamagnetic quinonoidal form and a monoradical species owing to the fact that they correspond to the open form of a pi-conjugated Kekulé structure. In the solid state, magnetic measurements indicate that the diradicals are all antiferromagnetically coupled, as expected from their topology. These interactions are best reproduced by means of a "Bleaney-Bowers" model that gives values of J = -142.0 cm(-1) for 7 a, -1.8 cm(-1) for 7 b, -1.3 cm(-1) for 7 c, -4.2 cm(-1) for 7 d, and -248.0 cm(-1) for 12. The temperature dependence of the EPR half-field transition in frozen dichloromethane solutions is consistent with singlet ground states and thermally accessible triplet states for diradicals 7 b, 7 c, and 7 d with DeltaE(T-S) values of 3.48, 2.09, and 8 cm(-1), respectively. No evidence of a populated triplet state was found for diradicals 7 a and 12. Similarities between the DeltaE(T-S) and J values (DeltaE(T-S) = -2 J) clearly show the intramolecular origin of the observed antiferromagnetic interaction. Analyses of the data with a "Karplus-Conroy"-type equation enabled us to establish that the silole ring, as a whole, allows a more efficient magnetic coupling of the two nitroxide radicals attached to its 2,5-positions than the thiophene ring. This superiority probably originates from the nonaromaticity of the silole which thus permits a better magnetic interaction through it. DFT calculations also support the experimental results, indicating that the magnetic exchange pathway preferentially involves the carbon pi system of the silole.

Autoren

Nans Roques
Philippe Gerbier
Ulrich Schatzschneider
Jean-Pascal Sutter
Philippe Guionneau
José Vidal-Gancedo
Jaume Veciana
Eva Rentschler
Christian Guérin

Autoren-URL

https://www.ncbi.nlm.nih.gov/pubmed/16680786

DOI

10.1002/chem.200501280

ISSN

0947-6539

Ausgabe der Veröffentlichung

21

Zeitschrift

Chemistry

Schlüsselwörter

Butanes
Electron Spin Resonance Spectroscopy
Free Radicals
Models, Chemical
Models, Molecular
Organosilicon Compounds

Sprache

eng

Country

Germany

Paginierung

5547 - 5562

Datum der Veröffentlichung

2006

Status

Published

Datum, an dem der Datensatz öffentlich gemacht wurde

2007

Titel

Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals.

Sub types

Journal Article
Research Support, Non-U.S. Gov't

Ausgabe der Zeitschrift

12

Datenquelle: PubMed

Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals

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