Article pubs.acs.org/JPCB

Ferromagnetic Spin Coupling through the 3,4′-Biphenyl Moiety in Arylamine OligomersExperimental and Computational Study Vincent Maurel,*,† Lukasz Skorka,‡ Nicolas Onofrio,† Ewa Szewczyk,‡ David Djurado,§ Lionel Dubois,† Jean-Marie Mouesca,† and Irena Kulszewicz-Bajer*,‡ †

University Grenoble Alpes, INAC, SCIB, F-38000 Grenoble and CEA, INAC, SCIB, F-38054 Grenoble, France Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warsaw, Poland § University Grenoble Alpes, INAC, SPrAM, F-38000 Grenoble and CEA, INAC, SPrAM, F-38054 Grenoble, France ‡

S Supporting Information *

ABSTRACT: This report describes the study of a dimer d2+ and a linear trimer t3+ of amminium radical cations coupled by 3,4′-biphenyl spin coupling units. The synthesis of the parent diamine and triamine and their optical and electrochemical properties obtained by UV−visible and cyclic voltammetry are presented. The chemical doping of the parent diamine d and triamine t was performed quantitatively to obtain samples containing the corresponding dimer d2+ and trimer t3+ in almost pure high-spin states as evidenced by pulsed EPR nutation spectroscopy. The J coupling constants of the corresponding S = 1 and S = 3/2 spin states were measured (J/k = 135 K) and compared quantitatively to DFT calculations.

■

INTRODUCTION Thanks to their high chemical stability,1−5 amminium radical cations are considered promising candidates for the design of high-spin molecules (for general reviews on high-spin organic materials, see refs 6−13) with the ultimate goal of obtaining a purely organic magnetic material. However, up to now, polymers of amminium radical cations reported in the literature exhibited no collective magnetic behavior and the best systems reported to date showed high-spin states up to S = 5.14−16 The failure in obtaining polymers with higher spin states pointed out the need to further investigate the strategy used to link together the amminium radical cations and to couple magnetically their spins. Consistently, several groups have designed well-defined oligomers of amminium radical cations. These studies allowed one to obtain high-spin linear,1,4,17−25 star-shaped,1,26−33 and cyclic compounds such as cyclophanes,18,20,34−39 connected or fused cyclophanes,31,40−44 double- and triple-decker,45−47 that can be used as building blocks for high-spin systems and to identify several factors that help or hinder the formation of polyradicals with ferromagnetic spin coupling.19−21 The first factor is the nature of the “spin coupling unit” that binds amminium radical cations together. It is well-known that, for topological reasons,8,12,48,49 the spins of two free radicals connected by a 1,3-benzene unit, a 3,4′-biphenyl unit,20,50 or a 4,4″-meta-terphenyl19 are ferromagnetically coupled. Using this property, several examples of high-spin diradicals and polyradicals were designed by connecting free radicals acting as “spin bearing units” to these “spin coupling units” (see refs © 2014 American Chemical Society

20, 22, 26, 51, and 52 for examples based on amminium radical cations and examples reviewed in refs 7, 9, and 11 for examples based on other free radical moieties). The efficiency of the ferromagnetic coupling, measured by the intensity of the magnetic exchange constant J, depends strongly on the chosen spin coupling unit. The general trend expected from quantum chemistry calculations19,52−54 is that smaller coupling units yield higher J values, i.e., J(1,3-benzene) > J(3,4′-biphenyl) > J(4,4″-meta-terphenyl) for a given type of free radical. All three simple dimers of amminium radical cations in the series pictured in Scheme 1 were synthesized and characterized as S = 1 ground state by EPR spectroscopy. However, only one J value (J/k = 42 K) could be measured for the 4,4″-meta-terphenyl spin coupling unit.19 It would be thus highly desirable to obtain the experimental J value for a dimer of amminium radical cation coupled by a 3,4′-biphenyl moiety in order to rely (or not) on this spin coupling unit to design a high-spin polymer with amminium radical cations. The second factor that can prevent the formation of a highspin state with several amminium radical cations coupled ferromagnetically is the electrostatic repulsion between the positive charges. Several attempts aimed to prepare high-spin polymers by binding amminium radical cations with 1,3benzene units.55−57 The corresponding poly(m-aniline)s were synthesized, but the oxidation was difficult and the polyradical Received: April 30, 2014 Revised: June 12, 2014 Published: June 13, 2014 7657

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

Scheme 1. Examples of Amminium Diradical Dications Connected by 1,3-Benzene, 3,4′-Biphenyl, and 4,4″-Meta-Terphenyl Spin Coupling Units Reported by Bushby et al.19,20 and Sato et al.55

cations exhibited a magnetic behavior corresponding to a main S = 1/2 spin state with a fraction of S = 1 state. This result was attributed to an incomplete chemical oxidation due to the electrostatic repulsion between the amminium radical cation moieties. This hypothesis was confirmed by the studies of linear oligomers based on meta-aniline derivatives: all attempts to obtain the fully oxidized tri- or tetra(radical cations) failed.17,20 A smaller Coulombic electrostatic repulsion between adjacent holes of radical cations can be obtained by using larger “spin coupling units”, such as 3,4′-biphenyl. The electrochemical properties of a dimer of amminium radical cations coupled by this spin coupling unit were studied in detail by Bushby et al.,21 who established that, during the oxidation of the parent diamine, the second oxidation wave occurs at a potential only ∼50 mV higher than the first one. According to these results, systems based on 3,4′-biphenyl spin coupling units and amminium radical cations are promising for the formation of high-spin states in the corresponding oligomers and polymers. However, to the best of our knowledge, no study of a welldefined trimer (or a longer oligomer) of amminium radical cations connected by 3,4′-biphenyl spin coupling units was reported in the literature. This report describes the study of a dimer (d) and a linear trimer (t) of amminium radical cations coupled by 3,4′biphenyl spin coupling units (see Scheme 2). In a first section, the synthesis of the diamine and triamine parents and their optical and electrochemical properties obtained by UV−vis and cyclic voltammetry measurements are presented and qualitatively rationalized with the help of preliminary DFT calculations. A second section deals with the investigation of the magnetic properties by pulsed EPR and SQUID magnetometry. The J coupling constants of the corresponding S = 1 and S = 3/2 spin states of d2+ and t3+, respectively, were measured and compared quantitatively to J coupling constants calculated by DFT.

Scheme 2. Chemical Structures of the Studied Dimer d and Trimer t and the Numbering of Nitrogen Atoms

a four-step procedure (Scheme 3). The coupling reaction of di(4-butylphenyl)amine (1) to 1,3-dibromobenzene or to 1,4dibromobenzene according to the conditions established by Jorgensen et al.58 for monoamination gave compounds 2 and 3 with yields of 73 and 63%, respectively. The bromo-derivative 3 was converted to the corresponding pinacol borate derivative 4 with 86% yield in a classical manner. The key step was a Suzuki coupling reaction of compounds 2 and 4 which allows one to obtain dimer d with a yield of 60%. The synthesis of trimer t is presented in Scheme 4. In this case, the dibromo-derivative 6 was converted to diborate derivative 7 with 55.5% yield. Then, the Suzuki coupling reaction between 7 and 3-bromoaniline afforded compound 8. It should be emphasized that in the conditions used primary amine groups were not active and compound 8 was obtained with a satisfactory yield of 69%. The reaction of 8 with 4-

■

RESULTS AND DISCUSSION Synthesis, Electrochemical, and Optical Properties of Parent Compounds. Synthesis. The model compounds containing 3,4′-biphenyl coupler, namely, dimer d and trimer t, were obtained using palladium catalyzed Buchwald−Hartwig and Suzuki coupling reactions. The dimer d was synthesized in 7658

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

Scheme 3. Synthesis of Dimer d: (a) 1,3-Dibromobenzene, Pd2(dba)3, BINAP, t-BuONa, Toluene, 90 °C; (b) 1,4Dibromobenzene, Pd2(dba)3, BINAP, t-BuONa, Toluene, 110 °C; (c) BuLi, THF, −78 °C, Isopropyl Pinacol Borate; (d) Pd2(dba)3, (tolyl)3P, K3PO4, BTEAC, Dioxane, Toluene, H2O, 100 °C

Scheme 4. Synthesis of Trimer t: (a) 4-tert-Butylbromobenzene, Pd(OAc)2, t-Bu3P, t-BuONa, Toluene, 110 °C; (b) NBS, DMF; (c) BuLi, THF, −78 °C, Isopropyl Pinacol Borate; (d) 3-Bromoaniline, Pd(OAc)2, t-Bu3P, BTEAC, K3PO4, Toluene, Dioxane, H2O, 110 °C; (e) 4-Butylbromobenzene, Pd(OAc)2, t-Bu3P, t-BuONa, Dioxane, 100 °C

processes. Both oxidation potential values were higher than those reported by Bushby et al.21 for a similar dimer but containing methoxy substituents. The increased values of the oxidation potentials observed for our dimer d can be related to the lesser electro-donating effect of butyl groups with respect to methoxy ones studied by Bushby.21 The cyclic voltammetry of trimer t (Figure 2) shows also two reversible waves at 0.55 and 0.70 V vs Ag/Ag+ (0.40 and 0.55 V vs Fc/Fc+) and appeared at higher potentials than that registered for dimer d. The observed oxidation peak current

butylbromobenzene in the presence of palladium catalyst gave trimer t with a yield of 76%. Electrochemical Properties. The electrochemical properties of dimer d and trimer t were studied by cyclic voltammetry in dichloromethane with 0.1 M Bu4NBF4 as a supporting electrolyte. The cyclic voltammogram of dimer d (Figure 1) showed two reversible oxidation waves at 0.52 and 0.64 V vs Ag/Ag+ (0.36 and 0.48 V vs Fc/Fc+). The second oxidation potential increased by 0.12 V from the first one which is related to Coulombic repulsion between holes created in the oxidation 7659

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

Figure 1. (a) Cyclic voltammogram obtained for dimer d in CH2Cl2 solution (the concentration of d was c = 10−4 M) containing an electrolyte - 0.1 M Bu4NBF4 (reference electrode - Ag/0.1 M AgNO3 in acetonitrile, scan rate - 100 mV/s). (b) Differential pulse voltammogram.

Figure 2. (a) Cyclic voltammogram obtained for trimer t in CH2Cl2 solution (the concentration of t was c = 10−4 M) containing an electrolyte - 0.1 M Bu4NBF4 (reference electrode - Ag/0.1 M AgNO3 in acetonitrile, scan rate - 100 mV/s). (b) Differential pulse voltammogram.

of the first oxidation wave was ca. 2 times as large as that for the second oxidation process, thus suggesting that the first (2e−) oxidation takes place on both lateral amine groups. The oxidation of the central amine group of the trimer was more difficult than the second oxidation of the dimer. The ΔE between the first and second oxidation steps was equal to 0.15 V and reflects Coulombic repulsion of adjacent holes in the molecule. UV−vis−NIR Spectroscopy. The chemical oxidation of dimer d and trimer t was followed by UV−vis−NIR spectroscopy. The absorption bands for dimer d and trimer t in their neutral (nonoxidized) state are located at 308 and 303 nm, respectively. The compounds were oxidized with tris(4bromophenyl)ammonium hexachloroantimonate (TBA·SbCl6) in dichloromethane solution. After the oxidation of dimer d and trimer t to one radical cation per molecule, their absorption spectra changed significantly and new bands appeared at the vis−NIR region. These new bands were located at 488 and 1625 nm in the dimer’s spectrum and at 483 and ca. 2000 nm in the trimer’s spectrum (Figure 3). Both monocations d+ and t+ exhibit an intervalence charge transfer (IV-CT) band (1625 and 2000 nm, respectively), making them belong to either class II (symmetry-broken/double minimum) or class III (symmetrically delocalized/single minimum) systems of the Robin− Day classification (class I fully localized systems are excluded).

The oxidation of trimer t with 2 equiv of the oxidant (i.e., to the intermediate oxidation state) caused the intensity increase of the NIR band with simultaneous displacement of its maximum to 1902 nm. Additionally, new bands centered at 570 and 680 nm appeared. Finally, the absorption spectra of the oligomers oxidized to their highest oxidation states changed significantly. Namely, the spectrum of dimer d2+ revealed a new very intense band located at 701 nm with the shoulders at ca. 580 and 800 nm, whereas the spectrum of trimer t3+ showed the band centered at 697 nm with the shoulders at ca. 580 and 820 nm. These new bands can be attributed to the transition between orbitals centered on carbon atoms toward empty orbitals centered on nitrogen atoms. It can also be emphasized that the NIR band related to the IV-CT transition almost disappeared, confirming the oxidation of the oligomers to their highest oxidation states. However, the stability of radical cations is rather limited; thus, the character of the spectra changed with time,, indicating that d2+ and t3+ can decompose to lower oxidation states. Preliminary DFT Calculations for Rationalizing Optical and Electrochemical Studies. In the light of the previous electrochemical properties, it would be highly interesting to compute by DFT all redox potentials for both d and t. For both monocations d+ and t+, the proper DFT description of the (de)localization of the hole/electron is related to three main 7660

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

start to explain the fact that the first two oxidation waves of trimer t occur at the same potential? (all technical details are reported in Supporting Information, the DTF Methodology section). In the case of d, we report in Table 1 an orbital analysis based on B3LYP (with 20% Hartree−Fock) Mülliken spin populations of the (minority) spin beta orbitals (HOMO and LUMO) for both N1 (4′-side) and N2 (3-side) amine sites of d+, in the absence of solvent (in vacuo, ε = 1) (top right) and in its presence (bottom left, ε = 9 for dichloromethane). We also report equivalent data for the neutral d (top left) and d2+ (bottom right) for the sake of comparison (ε = 9). To ease discussion, we propose to define the following localization index xloc = [(pop(N2) − pop(N1)]/[pop(N2) + pop(N1)] varying between 0 (fully delocalized) and 1 (fully localized), with all values computed for the same orbital # 128. First, in the neutral d dimer, xloc = 0.47 (semilocalized: Table 1, top left). By contrast, it can be seen for d+ that the LUMO hole in vacuo is shared by both amine sites (xloc = 0.13, i.e., delocalized) (Table 1, top right). This drastically changes as soon as an even moderate (ε = 9) polar dielectric continuum environment is introduced (Table 1, bottom left): the hole is localized on N1, with xloc(ε = 9) = 0.60. This behavior reproduces trends found in the literature,58 and more will be presented elsewhere. These preliminary results show that the N1 site is more prone to oxidation than the other N2 site and d+ appears as a class II system, an identification substantiated by the fact that two reversible (one-electron) oxidation waves are observed at different values: 0.36 and 0.48 eV (vs Fc/Fc+), respectively. In the case of d2+, both holes are semilocalized in the solvent (xloc = 0.38; Table 1, bottom right) as hole−hole repulsion dominates. This last result is important, as it shows that the computation of exchange coupling constants J for the highest oxidized state (d2+) will be little affected by the environment. Both HS (S = 1) and BS (Ms = 0) spin states, necessary for the computation of J, exhibit the same two localized holes, thus giving credence to the values we predict (see below the DFT Calculations of J Coupling Constants Subsection and the Magnetic Properties Section). A similar analysis performed for the t trimer turned out to be much more delicate and has to be reported elsewhere. Suffice to say here that, without a symmetry-breaking environment, the (beta spin) hole in t+ is located on the central amine para-N1para (ε = 9), in agreement with what was observed for d+. Experimentally, however, the first reversible oxidation wave (0.40 eV vs Fc/Fc+) is twice as large as the second wave (0.55 eV), suggesting that both (one-electron) oxidations occur at the same potential (0.40 eV) at both (N2 and N3) ends of the trimer. However, preliminary results (see the Supporting Information) show that the presence of a lateral counteranion near one of the N2 sites breaks the trimer’s electronic

Figure 3. (a) UV−vis−NIR spectra of dimer d oxidized with TBA· SbCl6 in CH2Cl2 solution (the concentration of d, c = 8 × 10−4 M); the Ox/d molar ratio: (a) 1, (b) 2. (b) UV−vis−NIR spectra of trimer t oxidized with TBA·SbCl6 in CH2Cl2 solution (the concentration of t, c = 8 × 10−4 M); the Ox/t molar ratio: (a) 1, (b) 2, (c) 3.

factors:58,59 (i) the size of the monocation, a small size favoring delocalization; (ii) the amount of Hartree−Fock (HF) exchange within the exchange-correlation (XC) potential used to polarize the electronic structure; and (iii) the presence of a (polar) solvent. These factors turned out to be very sensitive for d+ and t+ and strongly impact both the first two 0/+1 and +1/+2 redox potentials as well as their computed UV−visible spectra. To explore the interplay between these factors is therefore beyond the scope of the present paper; a full theoretical (TD-)DFT study focusing on these IV-CT systems is underway. We would like to address here two basic questions by preliminary DFT calculations: (i) which amine site is first oxidized in the asymmetrically bridged d dimer? (ii) how to

Table 1. B3LYP Mülliken Population Analysis (%) Computed for the Frontier (Beta Spins) Orbitals for (a) d (ε = 9): #128 = HOMO−1 and #129 = HOMO; (b) d+ in vacuo (ε = 1): #128 = HOMO and #129 = LUMO; (c) the Same as b with ε = 9; (d) d2+ (ε = 9): #128 = LUMO and #129 = LUMO+1 (Triplet State)a models d(ε = 9)a d+(ε = 9)c

a

orbitals # # # #

129 128 129 128

N1

N2

models

N1

N2

18.4 7.5 20.1 (h) 5.6

7.0 20.7 4.8 (h) 22.5

d+(ε = 1)b

13.7 (h) 11.9 17.8 (h) 8.6 (h)

11.4 (h) 15.6 8.1 (h) 19.2 (h)

d2+(ε = 9)d

The label (h) indicates a hole. 7661

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

symmetry by localizing the hole on that first N2 site near the negative charge while simultaneously raising the energy of the opposite N3 site which becomes the next oxidized site. This may start to explain the experimental findings regarding the first reversible oxidation wave and illustrate that taking into account the counteranion will be crucial for the computation of the 0/ 1+ and 1+/2+ redox potentials of the trimer. Note that this counteranion effect is expected to be less crucial for d+ whose hole is already localized in the presence of the solvent. Magnetic Properties of the Diradical Dication d2+ and the Triradical Trication t3+. Pulsed EPR Nutation Spectroscopy. The chemical oxidation of the compounds d and t by TBA·SbCl6 was also studied by pulsed-EPR nutation spectroscopy. The nutation frequencies measured by this technique are directly related to the spin states of the paramagnetic species in the sample by the relationship (1) (see the Experimental Section). Under the conditions of the experiments reported in Figure 4, S = 1/2 species are expected at νnut = νS=1/2 = 23 MHz

CH2Cl2. All attempts to carry out the doping reaction by mixing solutions of TBA·SbCl6 in acetonitrile with solutions of parent compounds d and t in CH2Cl2 failed. SQUID Magnetometry. Samples prepared from the same doped solutions of d and t as those studied by pulsed-EPR nutation spectroscopy were studied by SQUID magnetometry. These measurements are corrected for diamagnetism by using the experimental values of diamagnetic susceptibility of the sample holder and the solvent (see the Experimental Section). The experimental M = f(H) curves recorded at T = 2 K for oxidized samples of d and t are shown in Figure 5 (left upper and lower frames, respectively). The M = f(H) curve recorded for d is very well fitted by a Brillouin function corresponding to the S = 1 pure spin state and including a T − θ term corresponding to mean field analysis of small antiferromagnetic intermolecular interactions. The value of θ = −0.14 K was deduced from the analysis of the χT = f(T) experiment (see below). From this analysis, the number of S = 1 species can be obtained and it appears that 89% of the initial molecules of d were doped up to diradical dications with S = 1 spin state. The same analysis was performed for trimer t. Again the M = f(H) curve recorded for t is very well fitted by a Brillouin function corresponding to the S = 3/2 pure spin state, with θ = −0.09 K deduced from the analysis of the χT = f(T) experiment (see below and the Supporting Information, Figure S1, for a comparison of M = f(H) with a pure Brillouin function, i.e., θ = 0 K). From this fitting, the number of S = 3/2 species can be estimated to 89% of the initial molecules of t. However, one must keep in mind that in this analysis the small contributions of S = 1 and S = 1/2 species observed by pulsed-EPR nutation are neglected. The variations of magnetic susceptibility with temperature were recorded for oxidized samples of d and t at low magnetic field (H = 0.05 T) and produced the χT = f(T) curves shown in Figure 5 (right upper and lower frames, respectively). For the dimer d, the curve could be modeled with eq 1 corresponding to the Bleaney−Bowers equation,60 which is classical for systems with two coupled S = 1/2 electron spins. This equation was affected by a T − θ term corresponding to the mean field analysis mentioned previously.

Figure 4. 2D-pulsed EPR nutation spectra at T = 7 K for samples obtained by chemical oxidation of d and t. Lower frame: the dimer d ([d] = 7.0 × 10−3 M) was oxidized with TBA·SbCl6 in CH2Cl2 solution with the molar ratios: Ox/d = 2. Upper frame: the trimer t ([t] = 3.2 × 10−3 M) was oxidized with TBA·SbCl6 in CH2Cl2 solution with the molar ratios: Ox/t = 3.

χT =

nutation frequency, S = 1 at νnut(S=1) = √2·νS=1/2 = 32 MHz, and S = 3/2 at νnut = √3·νS=1/2 = 40 MHz (for the |3/2, 1/2⟩ ↔ |3/2, 3/2⟩ and |3/2, −3/2⟩ ↔ |3/2, −1/2⟩ EPR transitions of an S = 3/2 state). The 2D pulsed-EPR nutation spectra shown in Figure 4 were obtained at T = 7 K for samples d (lower frame) and t (upper frame) doped in dichloromethane solution with 2 and 3 equiv of TBA·SbCl6, respectively. In the case of the dimer d, an almost pure S = 1 spin state is detected, with only a very faint signal corresponding to the S = 1/2 state. In the case of the trimer t, a strongly dominant S = 3/2 spin state is detected, with a much weaker signal due to the S = 1 spin state and a very faint signal corresponding to the S = 1/2 state. From these spectra, one can conclude that d was quantitatively doped up to the corresponding high-spin (S = 1) diradical dication and that t was almost quantitatively doped to the corresponding highspin (S = 3/2) triradical trication. However, it should be mentioned that these almost pure high-spin states could be obtained only by performing the doping reaction in pure

2Ng 2β 2 T 1 (T − θ ) k 3 + exp( −J /kT )

(1)

β stands for the Bohr magneton, g for the Landé factor, k for the Boltzman’s constant, N for the number of dication diradicals, and J for the exchange coupling constant between the electron spins in the dication diradicals and corresponding to the Heisenberg Hamiltonian H = −J·S1·S2. From this analysis, the exchange coupling constant was estimated to be J/ k = 136 ± 9 K and the Weiss temperature of the mean field analysis, to be θ = −0.14 K. A similar analysis was performed for the χT = f(T) curve corresponding to trimer t. This curve was modeled by eq 2 derived from the Van Vleck formula corresponding to a linear trimer of S = 1/2 spins.61−63 It is assumed that, in the triradical trications obtained from t, two S = 1/2 electronic spins are localized in both extremities (spins noted S2 and S3) and one in the central part of the molecule (spin noted S1) and that, because of the symmetry of the spin distribution, it can be modeled by the Heisenberg Hamiltonian H = −(J·S1·S2 + J·S1· 7662

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

Figure 5. Magnetization measurements (after subtraction of diamagnetism) of the samples obtained by chemical oxidation of d and t with the molar ratios: Ox/d = 2 and Ox/t = 3 (the same solutions as for Figure 4). Upper frame: (left) M = f(H) curve recorded for d at T = 2 K, (right) χT = f(T) curve recorded for d at H = 0.05 T. Lower frame: (left) M = f(H) curve recorded for t at T = 2 K. (right) χT = f(T) curve recorded for t at H = 0.05 T (see the Experimental Section for more details).

S3), leading to the following equation for fitting the χT = f(T) curve: χT =

Table 2. High-Spin (HS) Magnetic State (S = 1 for d2+ and S = 3/2 for t3+) and Broken Symmetry (BS) Magnetic States (Ms = 1/2: BS1 ≡ ↑↓ for d2+, BS1 ≡ ↑↓↑ and BS2 ≡ ↓↑↑ for t3+) Used to Compute Exchange Coupling Constants JBS (See the Methodology Section)a

Ng 2β 2 10 + exp( − J /2kT ) + exp( − 3J /2kT ) T (T − θ) 4k 2 + exp( − J /2kT ) + exp( − 3J /2kT ) (2)

From this analysis, the exchange coupling constant in the triradical trication derived from t was estimated to be J/k = 134 ± 10 K and the Weiss temperature of the mean field analysis, to be θ = −0.09 K. DFT Calculations of J Coupling Constants in d2+ and 3+ t . As expected in view of our previous work on chemically similar oligomers, the geometries computed for d2+ and t3+ radical cations both exhibit (almost) planar NC3 motifs around nitrogen atoms with phenyl groups distributed around a given nitrogen atom in a propeller-like fashion which does not hinder ferromagnetic coupling between adjacent amine sites. DFT calculated exchange coupling constants for d2+ and t3+ are in good agreement with the values measured experimentally, though larger by about 30% (see Table 2). The DFT values have been obtained here without recourse to either counteranions or solvent, as done in our previous work.18 Comparison of the Magnetic Properties of d2+ and t3+ with Other Oligomers of Amminium Radical Cations

systems

d2+

t3+

HS (eV) BS1 (eV) BS2 (eV) JBS (K) Jexp (K)

−481.3267 −481.3187 n.a. 185 136 ± 9

−716.1596 −716.1450 −716.1523 170 134 ± 10

a

Bonding energies are reported in eV and J values (JBS and Jexp) in kelvin (K).

Previously Reported in the Literature. To the best of our knowledge, this study reports the first experimental measurement of the exchange coupling constant due to the connection of free radical moieties by the 3,4′-biphenyl coupling unit. The exchange coupling constant in the following analogue carboncentered diradical (structure I, Scheme 5) was evaluated by a simple model based on Huckel molecular orbitals19 to 0.66 kcal/mol (330 K) and variable temperature EPR measurements reported by Rajca and Rajca showed a constant product T × I 7663

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

Scheme 5. Structures of Diphenylmethyl Radicals Linked via 3,4′-Biphenyl (I)50 and 4,4″-Meta-Terphenyl (II)19

Scheme 6. Chemical Structures of Bi- and Tri-Nitroxides Containing 1,3-Benzene Coupler (Structures III and IV)65,66 and of a Dimer Previously Reported by Our Team (Structure V)18

(I, EPR intensity; T, temperature) up to 80 K50 consistent with J/k > 80 K. The experimental values J/k ∼ 135 K obtained is this study for amminium radical cations coupled through a 3,4′-biphenyl coupling unit can be compared with the values measured by Busbhy for the coupling of the same kind of amminium radical cations through a 4,4″-meta-terphenyl spin coupling unit (J/k = 42 K). This quantitative comparison shows that J coupling constants are typically 3−4 times higher when using a 3,4′biphenyl rather than 4,4″-meta-terphenyl spin coupling unit for amminium diradical dications. This ratio is close to the ratio of 4.4 between the J values estimated by Bushby19 for similar (Ph)2C• radicals coupled by 3,4′-biphenyl (J = 0.66 kcal.mol−1, J/k = 330 K) and 4,4″-meta-terphenyl spin coupling unit (structure II, Scheme 5, J = 0.15 kcal·mol−1, J/k = 75 K) using the simple model based on Huckel molecular orbitals (see Table 3).

there, the magnetic orbitals overlapped poorly at the level of the coupler, related to the spin delocalization occurring between formal spin bearers (conjugated p−p segments) and couplers, therefore withdrawing/diluting much spin away from the meta-couplers. By contrast, in the present work, the size of the spin bearing units is reduced to its minimum for amminium radical cations, whereas that of the spin coupler is increased to form a 3,4′-biphenyl unit. The size of this bearer reduces the direct magnetic orbital overlap (thus inhibiting the antiferromagnetic contribution to J) while allowing substantial spatial contact over two phenyl rings (thus exalting the ferromagnetic contribution). As shown by the DFT calculations reported here, the asymmetry of the 3,4′-biphenyl spin coupler unit in d and t allows for partial localization of the electron/charge, as already shown at the level of the d monocation. This opens the way to finely controlling the energy difference between both amine sites (and therefore partial localization trends) via appropriate substituents, in addition to tuning local redox potentials. For higher oxidation states (2+ for both systems, 3+ for t), this adds a second player in the (de)localization game, mainly controlled by hole−hole repulsion in symmetrical oligomers (and polymers, for which conformation is also important). The additional difficulty, only hinted at by the present DFT calculations, that is the significant role played by the location of the counteranions for the electronic structure of the mixedvalence states, is expected to be less important for higher oxidation states. With all that in mind, therefore, the use of 3,4′biphenyl spin coupler units in locally symmetry-broken polymers looks very promising. We can also notice that the diradical dication d2+ and the triradical trication t3+ are obtained with a very high (89%) doping efficiency (as estimated by SQUID magnetometry), which compares well to doping efficiencies reported for amminium radical cation dimers, oligomers, and polymers reported in the literature, which are generally in the 65−80% range (see ref 11 and references therein). At last, the pulsed-EPR and SQUID data reported here clearly demonstrate that one can obtain a pure S = 3/2 highspin ground state for the doped trimer t in the t3+ oxidation state. This result should be emphasized, since previous attempts

Table 3. Experimental and Calculated Exchange Coupling Constants (J/k in K) for Diradicals and Diradical Dications according to Their Spin Coupling Unit and Spin Bearing Units: (a) from This Study; (b) from ref 19, JHuckel Stands for an Estimate Made by Bushby et al. Based on Huckel Theory; (c) from refs 65 and 66; (d) from ref 50 coupling unit

(Ar)3N•+ JExp/JDFT

(Ar)3C• JExp/JHuckel

(Ar)2NO• JExp

1,3-benzene 3,4′-biphenyl 4,4″-meta-terphenyl

.../590(a) 135(a)/185(a) 42(b)/...

.../1400(b) >80(d)/330(b) .../75(b)

480(c) ... ...

It would be highly desirable to compare the J coupling constants reported here for d2+ and t3+ with the J coupling constants in similar di- and triradicals based on a 1,3-benzene spin coupling unit. However, to the best of our knowledge, no experimental data were reported in the literature. By performing DFT calculations at the same level as that for d2+ and t3+, we obtain J/k = 590 K. It suggests that the J coupling constants obtained for d2+ and t3+ are approximately 3−4 times smaller than the J coupling constants of two amminium radical cations linked through two 1,3-benzene spin coupling units. The corresponding J value was predicted by Ito et al. to be as high as 350 K by ab initio MO study at the ROHF, GVB, and CASSCF levels.64 Let us mention that comparable high J coupling constants were measured only once for dimers and trimers of nitroxides connected by 1,3-benzene unit(s) (J/k = 480 K, Scheme 6, structures III and IV).65,66 In a previous work,18 we studied a dimer, in which two amino-p-phenylenediamine units were linked via a 1,3-benzene spin coupling unit (cf. Scheme 6, structure V). The measured J coupling constant (J/k = 33 K) was 1 order of magnitude lower than that for the trinitroxide mentioned above. As it turned out 7664

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

methodology, and preliminary studies of t. This material is available free of charge via the Internet at http://pubs.acs.org.

to obtain a high-spin ground state from doped linear oligomers of amminium radical cations were not successful.1,20 When doping a trimer of p-phenylenediamine coupled by two 1,3benzene moieties as spin coupling units (Scheme 7, structure

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone: 33-438783598. *E-mail: [email protected]. Phone: 48-22-2345584.

Scheme 7. Structures of Linear Arylamine Oligomers

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS I.K.-B. and L.S. wish to acknowledge financial support from National Centre of Science in Poland (NCN, Grant No. UMO2011/01/B/ST5/03903).

■

(1) Ito, A.; Sakamaki, D.; Ino, H.; Taniguchi, A.; Hirao, Y.; Tanaka, K.; Kanemoto, K.; Kato, T. Polycationic States of Oligoanilines Based on Wurster’s Blue. Eur. J. Org. Chem. 2009, 4441−4450. (2) Chung, Y. C.; Su, Y. O. Effects of Phenyl- and MethylSubstituents on p-Phenylenediamine, an Electrochemical and Spectral Study. J. Chin. Chem. Soc. 2009, 56, 493−503. (3) Connelly, N. G.; Geiger, W. E. Chemical Redox Agents for Organometallic Chemistry. Chem. Rev. 1996, 96, 877−910. (4) Ito, A.; Taniguchi, A.; Yamabe, T.; Tanaka, K. Triplet State of Würster’s Blue-Based Di(cation radical). Org. Lett. 1999, 1, 741−743. (5) S. Amthor, S.; Noller, B.; Lambert, C. UV/Vis/NIR Spectral Properties of Triarylamines and Their Corresponding Radical Cations. Chem. Phys. 2005, 316, 141−152. (6) Aoki, T.; Kaneko, T.; Teraguchi, M. Synthesis of Functional πConjugated Polymers from Aromatic Acetylenes. Polymer 2006, 47, 4867−4892. (7) Crayston, J. A.; Devine, N. J.; Walton, J. C. Conceptual and Synthetic Strategies for the Preparation of Organic Magnets. Tetrahedron 2000, 56, 7829−7857. (8) Dougherty, D. A. Spin Control in Organic Molecules. Acc. Chem. Res. 1991, 24, 88−94. (9) Rajca, A. Organic Diradicals and Polyradicals: From Spin Coupling to Magnetism? Chem. Rev. 1994, 94, 871−893. (10) Rajca, A. From High-Spin Organic Molecules to Organic Polymers with Magnetic Ordering. Chem.Eur. J. 2002, 8, 4834− 4841. (11) Bujak, P.; Kulszewicz-Bajer, I.; Zagorska, M.; Maurel, V.; Wielgus, I.; Pron, A. Polymers for Electronics and Spintronics. Chem. Soc. Rev. 2013, 42, 8895−8999. (12) Bushby, R. J.; Gooding, D.; Vale, M. E. High-Spin Polymeric Arylamines. Philos. Trans. R. Soc., A 1999, 357, 2939−2957. (13) Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. p-Doped High Spin Polymers. J. Mater. Chem. 1997, 7, 2343−2354. (14) Fukuzaki, E.; Nishide, H. Room-Temperature High-Spin Organic Single Molecule: Nanometer-Sized and Hyperbranched Poly[1,2,(4)-phenylenevinylene anisylaminium]. J. Am. Chem. Soc. 2006, 128, 996−1001. (15) Michinobu, T.; Inui, J.; Nishide, H. m-Phenylene-Linked Aromatic Poly(aminium cationic radical)s: Persistent High-Spin Organic Polyradicals. Org. Lett. 2003, 5, 2165−2168. (16) Michinobu, T.; Inui, J.; Nishide, H. Two-Dimensionally Extended Organic High-Spin Poly(aminium cationic radical)s and Their Magnetic Force Microscopic Images. Polymer J. 2010, 42, 575− 582. (17) Ito, A.; Ino, H.; Tanaka, K.; Kanemoto, K.; Kato, T. Facile Synthesis, Crystal Structures, and High-Spin Cationic States of Allpara-Brominated Oligo(N-phenyl-m-aniline)s. J. Org. Chem. 2002, 67, 491−498. (18) Maurel, V.; Jouni, M.; Baran, P.; Onofrio, N.; Gambarelli, S.; Mouesca, J. M.; Djurado, D.; Dubois, L.; Desfonds, G.; KulszewiczBajer, I. Magnetic Properties of a Doped Linear Polyarylamine Bearing

VI), mainly S = 1/2 with a small fraction of S = 1 state was observed by pulsed-EPR,1 while a triradical trication of the corresponding star-branched trimer exhibited mainly the S = 3/ 2 high-spin state. In a study of dimers and tetramers of meta-aniline (Scheme 7, structure VII), Bushby et al.20 demonstrated that oxidizing arylamine moieties connected by a 1,3-benzene moiety is very difficult: due to electrostatic repulsion, the oxidation of a second neighboring arylamine can be obtained only at potentials higher by 0.4 V compared with the oxidation of the first amine moiety. Thus, the tetraradical cation derived from a tetra-meta-aniline could not be obtained neither by electrochemistry nor by chemical oxidation. The fact that the trimer t could be chemically and electrochemically oxidized up to t3+ indicates that the 3,4′-biphenyl coupling spin unit makes the electrostatic repulsion much smaller than in the case of the 1,3-benzene coupler. In this view, the results achieved for the trimer t seem to be very promising and 3,4′-biphenyl can be considered an effective ferromagnetic coupler for arylamine spin bearing units.

■

CONCLUSIONS This study shows that the 3,4′-biphenyl spin coupling unit provides high coupling constants (J/k ∼ 135 K) between amminium radical cations in the dimer d and the linear trimer t. Moreover, it shows that the increase of the oxidation potential required to reach t3+ compared with d2+ is small (only 0.07 V) due to moderate Coulombic repulsion between amminium radical cations connected by a 3,4′-biphenyl unit. At last, as suggested by preliminary calculations, the asymmetry of the 3,4′-biphenyl coupling unit can be used as an additional parameter to (de)localize spins of radical cations in locally symmetry-broken polymers. These features make polymers based on the 3,4′-biphenyl spin coupler unit and amminium radical cations very promising. The study of such polymers is currently underway in the laboratory.

■

REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The synthesis procedures and characterization of d and t, details of pulsed EPR nutation experiments, DFT calculation 7665

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

a High Concentration of Coupled Spins (S = 1). Phys. Chem. Chem. Phys. 2012, 14, 1399−1407. (19) Bushby, R. J.; Taylor, N.; Williams, R. A. Ferromagnetic SpinCoupling 4,4″-through Metaterphenyl: Models for High-Spin Polymers. J. Mater. Chem. 2007, 17, 955−964. (20) Bushby, R. J.; Kilner, C. A.; Taylor, N.; Vale, M. E. Disjoint and Coextensive Amminium Radical Cations: a General Problem in Making Amminium Radical Cation Based High-Spin Polymers. Tetrahedron 2007, 63, 11458−11466. (21) Bushby, R. J.; McGill, D. R.; Ng, K. M.; Taylor, N. Disjoint and Coextensive Diradical Diions. J. Chem. Soc, Perkin Trans. 2 1997, 1405−1414. (22) Wienk, M. M.; Janssen, R. A. J. Stable Triplet-State Di(Cation Radicals) of a Meta−Para Aniline Oligomer by “Acid Doping. J. Am. Chem. Soc. 1996, 118, 10626−10628. (23) Wienk, M. M.; Janssen, R. A. J. High-Spin Cation Radicals of Meta−Para Aniline Oligomers. J. Am. Chem. Soc. 1997, 119, 4492− 4501. (24) van Meurs, P. J.; Janssen, R. A. J. Ferromagnetic Spin Alignment in Head-to-Tail Coupled Oligo(1,4-phenyleneethynylene)s and Oligo(1,4-phenylenevinylene)s Bearing Pendant p-Phenylenediamine Radical Cations. J. Org. Chem. 2000, 65, 5712−5719. (25) Michinobu, T.; Takahashi, M.; Tsuchida, E.; Nishide, H. Robust Triplet Molecule: Cationic Diradical of 3,4′-Bis(diphenylamino)stilbene. Chem. Mater. 1999, 11, 1969−1971. (26) Yoshizawa, K.; Chano, A.; Ito, A.; Tanaka, K.; Yamabe, T.; Fujita, H.; Yamauchi, J.; Shiro, M. ESR of the Cationic Triradical of 1,3,5-Tris(diphenylamino)benzene. J. Am. Chem. Soc. 1992, 114, 5994−5998. (27) Yoshizawa, K.; Chano, A.; Ito, A.; Tanaka, K.; Yamabe, T.; Fujita, H.; Yamauchi, J. Electron Spin Resonance of the Quartet State of 1,3,5-Tris(diphenylamino)benzene. Chem. Lett. 1992, 21, 369−372. (28) Ito, A.; Ino, H.; Matsui, Y.; Hirao, Y.; Tanaka, K.; Kanemoto, K.; Kato, T. A Bindschedler’s Green-Based Arylamine: Its Polycations with High-Spin Multiplicity. J. Phys. Chem. A 2004, 108, 5715−5720. (29) Hirao, Y.; Ino, H.; Ito, A.; Tanaka, K.; Kato, T. High-Spin Radical Cations of a Dendritic Oligoarylamine. J. Phys. Chem. A 2006, 110, 4866−4872. (30) Hirao, Y.; Ishizaki, H.; Ito, A.; Kato, T.; Tanaka, K. The Poly(Radical Cation) of a Star-Shaped Oligoarylamine − Detection of Excited High-Spin States. Eur. J. Org. Chem. 2007, 186−190. (31) Ito, A.; Yamagishi, Y.; Fukui, K.; Inoue, S.; Hirao, Y.; Furukawa, K.; Kato, T.; Tanaka, K. Trimacrocyclic Arylamine and Its Polycationic States. Chem. Commun. 2008, 6573−6575. (32) Selby, T. D.; Stickley, K. R.; Blackstock, S. C. Triamino-striazine Triradical Trications. An Experimental Study of Triazine as a Magnetic Coupling Unit. Org. Lett. 2000, 2, 171−174. (33) Fukuzaki, E.; Nishide, H. 2,6,10-Tris(dianisylaminium)-3,7,11tris(hexyloxy) triphenylene: A Robust Quartet Molecule at Room Temperature. Org. Lett. 2006, 8, 1835−1838. (34) Kulszewicz-Bajer, I.; Maurel, V.; Gambarelli, S.; Wielgus, I.; Djurado, D. Ferromagnetic Spins Interaction in Tetraaza- and Hexaazacyclophanes. Phys. Chem. Chem. Phys. 2009, 11, 1362−1368. (35) Hauck, S. I.; Lakshmi, K. V.; Hartwig, J. F. Tetraazacyclophanes by Palladium-Catalyzed Aromatic Amination. Geometrically Defined, Stable, High-Spin Diradicals. Org. Lett. 1999, 1, 2057−2060. (36) Ito, A.; Ono, Y.; Tanaka, K. The Tetraaza[1.1.1.1]m,p,m,pcyclophane Dication: A Triplet Diradical Having Two m-Phenylenediamine Radical Cations Linked by Twisted Benzenes. Angew. Chem., Int. Ed. 2000, 39, 1072−1075. (37) Nelsen, S. F.; Li, G.; Schultz, K. P.; Tran, H. Q.; Guzei, I. A.; Evans, D. H. Alkylated Trimethylene-Bridged Bis(p-Phenylenediamines). J. Am. Chem. Soc. 2008, 130, 11620−11622. (38) Sakamaki, D.; Ito, A.; Furukawa, K.; Kato, T.; Tanaka, K. Meta− para-Linked Octaaza[18]cyclophanes and Their Polycationic States. J. Org. Chem. 2013, 78, 2947−2956. (39) Vale, M.; Pink, M.; Rajca, S.; Rajca, A. Synthesis, Structure, and Conformation of Aza[1n]metacyclophanes. J. Org. Chem. 2008, 73, 27−35.

(40) Sakamaki, D.; Ito, A.; Furukawa, K.; Kato, T.; Tanaka, K. HighSpin Polycationic States of an Alternate Meta-para-linked Oligoarylamine Incorporating Two Macrocycles. Chem. Commun. 2009, 4524− 4526. (41) Ito, A.; Inoue, S.; Hirao, Y.; Furukawa, K.; Kato, T.; Tanaka, K. An N-substituted Aza[14]metacyclophane Tetracation: a Spin-Quintet Tetraradical with Four Para-phenylenediamine-Based Semi-Quinone Moieties. Chem. Commun. 2008, 3242−3244. (42) Yan, X. Z.; Pawlas, J.; Goodson, T.; Hartwig, J. F. Polaron Delocalization in Ladder Macromolecular Systems. J. Am. Chem. Soc. 2005, 127, 9105−9116. (43) Ito, A.; Ono, Y.; Tanaka, K. N-Methyl-Substituted Aza[1n]metacyclophane: Preparation, Structure and Properties. J. Org. Chem. 1999, 64, 8236−8241. (44) Ito, A.; Ono, Y.; Tanaka, K. Tetraaza[1.1.1.1]metacyclophane. New J. Chem. 1998, 22, 779−781. (45) Sakamaki, D.; Ito, A.; Tanaka, K.; Furukawa, K.; Kato, T.; Shiro, M. 1,3,5-Benzenetriamine Double- and Triple-Decker Molecules. Angew. Chem., Int. Ed. 2012, 51, 8281−8285. (46) Sakamaki, D.; Ito, A.; Furukawa, K.; Kato, T.; Shiro, M.; Tanaka, K. A Polymacrocyclic Oligoarylamine with a Pseudobeltane Motif: Towards a Cylindrical Multispin System. Angew. Chem., Int. Ed. 2012, 51, 12776−12781. (47) Yokoyama, Y.; Sakamaki, D.; Ito, A.; Tanaka, K.; Shiro, M. A Triphenylamine Double-Decker: From a Delocalized Radical Cation to a Diradical Dication with an Excited Triplet State. Angew. Chem., Int. Ed. 2012, 51, 9403−9406. (48) Misurkin, I. A.; Ovchinnikov, A. A. The Electronic Structures and Properties of Polymeric Molecules with Conjugated Bonds. Russ. Chem. Rev. 1977, 46, 967−987. (49) Ovchinnikov, A. A. Multiplicity of the Ground State of Large Alternant Organic Molecules with Conjugated Bonds. Theor. Chem. Acc. 1978, 47, 297−304. (50) Rajca, A.; Rajca, S. Intramolecular Antiferromagnetic vs Ferromagnetic Spin Coupling through the Biphenyl Unit. J. Am. Chem. Soc. 1996, 118, 8121−8126. (51) Galecka, M.; Wielgus, I.; Zagorska, M.; Pawlowski, M.; Kulszewicz-Bajer, I. High-Spin Radical Cations of Poly(m−p-anilines) and Poly(m−p−p-anilines): Synthesis and Spectroscopic Properties. Macromolecules 2007, 40, 4924−4932. (52) Dobrzynska, E.; Jouni, M.; Gawrys, P.; Gambarelli, S.; Mouesca, J. M.; Djurado, D.; Dubois, L.; Wielgus, I.; Maurel, V.; KulszewiczBajer, I. Tuning of Ferromagnetic Spin Interactions in Polymeric Aromatic Amines via Modification of Their π-Conjugated System. J. Phys. Chem. B 2012, 116, 14968−14978. (53) Li, S. H.; Ma, J.; Jiang, Y. S. Strength of Spin Coupling in HighSpin Organic Molecules. J. Phys. Chem. A 1997, 101, 5567−5573. (54) Li, S. H.; Ma, J.; Jiang, Y. S. Electron Correlation and Magnetism: A Simple Molecular Orbital Approach for Predicting Ground-State Spins of Conjugated Hydrocarbons. J. Phys. Chem. A 1997, 101, 5587−5592. (55) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.; Takui, T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. Polycationic High-Spin States of One- and Two-Dimensional (Diarylamino)benzenes, Prototypical Model Units for Purely Organic Ferromagnetic Metals as Studied by Pulsed ESR/Electron Spin Transient Nutation Spectroscopy. J. Am. Chem. Soc. 1997, 119, 6607−6613. (56) Ito, A.; Ota, K.; Tanaka, K.; Yamabe, T.; Yoshizawa, K. n-Alkyl Group-Substituted Poly(m-aniline)s: Syntheses and Magnetic Properties. Macromolecules 1995, 28, 5618−5625. (57) Yoshizawa, K.; Tanaka, K.; Yamabe, T.; Yamauchi, J. Ferromagnetic Interaction in Poly(m-aniline): Electron Spin Resonance and Magnetic Susceptibility. J. Chem. Phys. 1992, 96, 5516− 5522. (58) Larsen, S. B.; Bang-Andersen, B.; Johansen, T. N.; Jorgensen, M. Palladium-Catalyzed Monoamination of Dihalogenated Benzenes. Tetrahedron 2008, 64, 2938−2950. (59) Renz, M.; Theilacker, K.; Lambert, C.; Kaupp, M. A Reliable Quantum-Chemical Protocol for the Characterization of Organic 7666

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667

The Journal of Physical Chemistry B

Article

Mixed-Valence Compounds. J. Am. Chem. Soc. 2009, 131, 16292− 16302. (60) Heckmann, A.; Lambert, C. Organic Mixed-Valence Compounds: A Playground for Electrons and Holes. Angew. Chem., Int. Ed. 2012, 51, 326−392. (61) Bleaney, B.; Bowers, K. D. Anomalous Paramagnetism of Copper Acetate. Proc. R. Soc. A 1952, 214, 451−465. (62) Belorizky, E.; Fries, P. H. Exact-Solutions for Simple Spin Clusters with Isotropic Heisenberg Exchange Interactions. J. Chim. Phys. Phys.-Chim. Biol. 1993, 90, 1077−1100. (63) Veit, R.; Girerd, J. J.; Kahn, O.; Robert, F.; Jeannin, Y. Copper(II) and Nickel(II) Trinuclear Species with Dithiooxamide Derivative Ligands: Structural, Magnetic, Spectroscopic, and Electrochemical Properties. Inorg. Chem. 1986, 25, 4175−4180. (64) Ito, A.; Miyajima, H.; Yoshizawa, K.; Tanaka, K.; Yamabe, T. Ferromagnetic Interaction between Aminium Radical Centers through m-Phenylene and m-1,3,5-Triazinediyl. J. Org. Chem. 1997, 62, 38−43. (65) Mukai, K.; Nagai, H.; Ishizu, K. The Proof of a Triplet Ground State in the N,N′-di -t-butyl-m-phenylenebinitoxide Biradical. Bull. Chem. Soc. Jpn. 1975, 48, 2381−2382. (66) Ishida, T.; Iwamura, H. Bis[3-tert-butyl-5-(N-oxy-tertbutylamino)phenyl] nitroxide in a Quartet Ground State: a Prototype for Persistent High-Spin Poly[(oxyimino)-1,3-phenylenes]. J. Am. Chem. Soc. 1991, 113, 4238−4241.

7667

dx.doi.org/10.1021/jp504223v | J. Phys. Chem. B 2014, 118, 7657−7667