Intramolecular magnetic coupling between two nitrene or two nitroxide units through 1,1-diphenylethylene chromophores. Isomeric dinitrenes and dinitroxides related in connectivity to trimethylenemethane, tetramethyleneethane, and pentamethylenepropane

Matsumoto, Takuya; Ishida, Takayuki; Koga, Noboru; Iwamura, Hiizu

doi:10.1021/ja00051a030

Cited by 68 publications

(37 citation statements)

References 3 publications

(5 reference statements)

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“…A purity factor, f, was introduced to account for the possible presence of diamagnetic fractions caused by the instability of these diradicals. [56] The best fit to the experimental data yielded J = À1.8 cm À1 with f = 0.84 for 7 b, J = À1.3 cm À1 with f = 0.81 for 7 c, and J = À4.2 cm À1 and f = 0.96 for 7 d. The introduction of a Weiss constant, q, in the analytical expression did not significantly modify the resulting parameters. These exchange parameters are very similar to the singlet-triplet gaps (DE T-S = À2 J) obtained from the glassy matrix EPR studies and, therefore, they can be attributed to an intramolecular interaction taking place between the two radical units.The magnetic behavior for compounds 7 a and 12 has also been analyzed by a dimer model, but including a paramagnetic contribution …”

Section: Wwwchemeurjorgmentioning

confidence: 82%

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

Roques

Gerbier

Schatzschneider

et al. 2006

Chemistry A European J

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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.

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Section: Wwwchemeurjorgmentioning

confidence: 82%

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

Roques

Gerbier

Schatzschneider

et al. 2006

Chemistry A European J

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“…As a consequence, spin transfer from one radical unit to another through the P is certainly hindered but does take place as clearly shown by EPR and by the degree of spin localized at P. This could lead to a pseudo-disjoint system with almost degenerated doublet and quartet states favoring the lowspin ground state. [37,38] While these considerations would explain the sign of the exchange interaction among the spin carriers they do not account for the striking contradiction that exists between the spin distribution found and the effective magnetic interaction between the radicals units. Consequently, what appears as a discrepancy may simply reveal that the observed exchange interaction is not governed by a spin polarization mechanism.…”

mentioning

confidence: 88%

Discrepancy between the Spin Distribution and the Magnetic Ground State for a Triaminoxyl Substituted Triphenylphosphine Oxide Derivative

Borobia

Guionneau

Heise

et al. 2004

Chemistry A European J

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The magnetic interaction and spin transfer via phosphorus have been investigated for the tri-tert-butylaminoxyl para-substituted triphenylphosphine oxide. For this radical unit, the conjugation existing between the pi* orbital of the NO group and the phenyl pi orbitals leads to an efficient delocalization of the spin from the radical to the neighboring aromatic ring. This has been confirmed by using fluid solution high-resolution EPR and solid state MAS NMR spectroscopy. The spin densities located on the atoms of the molecule could be probed since (1)H, (13)C, (14)N, and (31)P are nuclei active in NMR and EPR, and lead to a precise spin distribution map for the triradical. The experimental investigations were completed by a DFT computational study. These techniques established in particular that spin density is located at the phosphorus (rho=-15x10(-3) au), that its sign is in line with the sign alternation principle and that its magnitude is in the order of that found on the aromatic C atoms of the molecule. Surprisingly, whereas the spin distribution scheme supports ferromagnetic interactions among the radical units, the magnetic behavior found for this molecule revealed a low-spin ground state characterized by an intramolecular exchange parameter of J=-7.55 cm(-1) as revealed by solid state susceptibility studies and low temperature EPR. The X-ray crystal structures solved at 293 and 30 K show the occurrence of a crystallographic transition resulting in an ordering of the molecular units at low temperature.

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“…Experimentally, a triplet ground state has not been observed for this system. 7 For the p,p 0 -isomer, each benzyl radical unit is connected at the carbon which has NBMO coefficients, which is non-disjoint. Thus, S ¼ 1 is predicted.…”

Section: 2mentioning

confidence: 99%

“…This has also been verified experimentally. 7 In other words, when two spin systems are connected to a bridging linker so that they are able to interact ferromagnetically, they should be connected at an atom that has NBMO coefficients (Scheme 2). Based on these determinants, many attempts have been made to prepare organic ferromagnetic materials.…”

Section: 2mentioning

confidence: 99%

From Persistent Triplet Carbenes to Persistent High-Spin Polycarbenes

Itoh¹,

Hirai²,

Tomioka³

2007

Bulletin of the Chemical Society of Japan

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Triplet carbenes are regarded as one of the most effective spin sources for organic ferromagnetic materials since the magnitude of the exchange coupling between the neighboring centers is large. However, those systems have two disadvantages that prevent their use as practical magnetic materials. First, a triplet carbene unit is highly unstable and lacks the stability for practical applications under ambient conditions. Second, diazo groups, which are precursors for triplet carbenes, are generally labile and, hence, cannot be used as building blocks to prepare a more complicated poly(diazo) compound. After great efforts to stabilize the triplet carbenes, we have succeeded in preparing fairly stable triplet carbenes that survive for days. We found that a diazo precursor for a persistent triplet carbene was also persistent and, hence, can be further modified into a more complicated diazo compound that can generate persistent high-spin polycarbenes. In this account, we would like to summarize our efforts along this line. We employed the following three approaches to prepare poly(diazo) compounds; (1) synthesis of dendritic molecules having peripheral diazo groups starting from diazo compounds, (2) preparation of a poly(phenylacetylene) with a diazo unit using Rh-complex-catalyzed polymerization of diazo compounds with p-ethynyl substituents, and (3) preparation of a polymer chain as a result of complexation of the pyridine ligand on diazo compounds with coordinatively unsaturated metal ions. We also characterized the magnetic properties of photoproducts obtained by photolysis of those poly(diazo) compounds. Although spin-states observed for those photoproducts were not so high, the results suggest that our approach using persistent triplet carbenes will eventually lead us to a persistent high-spin polycarbenes by taking advantage of the stability of our diazo compounds.An increasing amount of interest is being paid to molecular magnetism, in which spins of unpaired electrons in -orbitals of light atoms such as carbon, nitrogen, and oxygen are mainly responsible for the magnetic properties. Many attempts have been made to prepare organic ferromagnetic materials. 1,2There are two major structural determinants for constructing high-spin organic molecules. The first one is based on a valence bond model, which assumes that electrons are perfectly correlated. Thus, the ground state of any molecule is determined by which set of possible distribution of -and -electron spins allows the formation of the maximum number of bonds. 4 This can be simply expressed by Eq. 1, known as Ovchinnikov equation, 3 where the atoms can be divided into two sets, traditionally called starred and unstarred and n Ã and n are the numbers of the starred and unstarred atoms. If the numbers of starred and unstarred atoms are equal, the number of -and -spin electrons will also be the same, and the ground state will be a singlet. However, if there are more starred atoms than unstarred atoms, the number of -spin electrons will exceed the number of -spi...

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Intramolecular magnetic coupling between two nitrene or two nitroxide units through 1,1-diphenylethylene chromophores. Isomeric dinitrenes and dinitroxides related in connectivity to trimethylenemethane, tetramethyleneethane, and pentamethylenepropane

Cited by 68 publications

References 3 publications

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

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

Discrepancy between the Spin Distribution and the Magnetic Ground State for a Triaminoxyl Substituted Triphenylphosphine Oxide Derivative

From Persistent Triplet Carbenes to Persistent High-Spin Polycarbenes

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