J O U R N A L O F C H E M I S T R Y Materials Two spin-containing fragments connected by a two-electron one-center heteroatom p spacer. A new open-shell organic molecule with a singlet ground state Ll. Viadel,a J. Carilla,a E. Brillas,b A. Labartac and L. Julia�*a aDepartament de Quý�mica Orga`nica Biolo`gica, Centre d’Investigacio� i Desenvolupament (CISC), Jordi Girona 18–26, 08034 Barcelona, Spain bDepartament de Quý�mica Fý�sica, Universitat de Barcelona, Avgda. Diagonal 647, 08028 Barcelona, Spain cDepartament de Fý�sica Fonamental, Universitat de Barcelona, Avgda.Diagonal 647, 08028 Barcelona, Spain The synthesis of 4,4-iminobis(2,2¾,2,4¾,4,6,6¾,6-octachlorotriphenylmethyl) diradical 3, a stable organic magnetic molecule consisting of two spin-containing fragments linked by a nitrogen atom, is reported.Electron paramagnetic resonance (EPR) analysis in methyltetrahydrofuran solution (~10-3 M) at low temperatures showed a typical fine structure (gxx=2.0042; gyy=2.0048; gzz=2.0027) of Dms=±1 transition (|D/hc|=0.0071 cm-1; |E/hc|=0.0006 cm-1) as well as a broad (DHpp=7.5 G) and weak signal of Dms=±2 transition (g=4.124), due to an asymmetric and excited triplet state corresponding to an intramolecular spin–spin interaction which diminishes with decreasing temperature.It also showed a pair of small peaks, that might be associated with a weaker dipole–dipole interaction which diminishes with increasing temperature, emerging at both sides of a central and single peak due to a doublet state resonance corresponding to 2,2¾,2,4¾,4,6,6¾,6-octachloro-4-{3,5-dichloro-4- [bis(2,4,6-trichlorophenyl)methylene]cyclohexa-2,5-dienylideneamino}triphenylmethyl radical 4, obtained by smooth oxidation of 3.From magnetic susceptibility measurements of the sample in the solid, a linear four-spin model was applied to establish that the singlet is the ground state of the molecule and that two triplets (Jintra=-286±30 K, J¾inter=-160±50 K) were the low-lying excited states.Organic solutions of 3 in air slowly oxidize to give 4, a much more persistent monoradical which is also obtained by a smooth oxidation of 3 with AgNO3 in CHCl3. Cyclic voltammograms for the reduction of 3 and 4 in dimethylformamide (DMF) with tetra-n-butylammonium perchlorate exhibited three consecutive redox couples with standard potentials of -0.23,-0.58 and -0.74 V vs.SCE, indicating a reasonable stability of anions 3 - and 32- in DMF solution. There is great current interest in the preparation of organic molecular materials with new magnetic properties.1 Organic molecules with paramagnetic behavior are those with openshell electronic structures where one or more electrons are unpaired.These molecules are called radicals or polyradicals, and they are normally transient and very reactive to air, moisture and, in general, to their environment. Thus, if free radicals are to be good candidates for magnetic materials, it has to be possible to prepare, handle and store them without diYculty. The stability of the carbon-centered radicals is mainly achieved by steric protection.2,3 In highly chlorinated triphenylmethyl radicals, this protection is mainly accomplished by the six aromatic chlorines in the ortho-positions surrounding the trivalent carbon atom.3 Thus, this kind of very stable free radical is completely unassociated in crystalline solids with no appreciable decomposition either in solid or in solution. The synthesis of new polyradical molecules of great persistence and their physical properties involving magnetic behavior have recently been published.4,5 The preparation of radical amino 1,6 a new polychlorotriphenylmethyl radical of the TTM [tris (2,4,6-trichlorophenyl)- C• Cl Cl Cl Cl Cl Cl Cl Cl Cl TTM C• NH2 Cl Cl Cl Cl Cl Cl Cl Cl 1 C+ Cl Cl Cl Cl Cl Cl Cl Cl Cl 2 SbCl6 – methyl] series, as a potential radical intermediate in the synthesis of many new polyradicals which use the characteras a diradical of the TTM series, the amino-diradical 3, whose istics and extensive reactivity of the amino group, has also two spin-bearing moieties, the triphenylmethyl radicals, are recently been published. The isolation of a secondary reaction held together by the amine function, bonded in the para- product which could be identified, from preliminary analysis position of a phenyl of each moiety.The radical 3 is easily results, as a secondary amine, resulting from the condensation oxidized in solution to a new and very persistent paramagnet, of two molecules of the hydrocarbon salt 25a with ammonia the imino-radical 4. followed by reduction with SnCl2, was also mentioned.6 It is now possible to fully describe this new secondary amine Lahti et al.have carried out semi-empirical calculations to J. Mater. Chem., 1998, 8(5), 1165–1172 1165predict the ground state spin multiplicity of a large number of Cyclic voltammetry (CV) systems composed of two doublets, organic oxyl radicals, or A cyclic voltammogram for the reduction of a solution of two triplets, methylenes or organic nitrenes, connected by a 0.5 mM amino-diradical 3 in dimethylformamide (DMF) with spacer consisting of a magnetic coupling unit.7 When the 0.1 M tetra-n-butylammonium perchlorate (TBAP) is presented spacer is a heteroatom,7b oxygen or nitrogen, they predicted in Fig. 1. In the potential range between 0 and -1.2 V three an antiferromagnetic coupling in the para,para¾ connectivity, consecutive redox couples, O1/R1, O2/R2 and O3/R3, with that was stronger with a nitrogen atom as spacer than with respective standard potentials E0 of-0.23,-0.58 and-0.74 V an oxygen atom.In non-planar geometries, this interaction vs. SCE (NaCl-saturated calomel electrode) can be observed. dropped substantially to give nearly degenerate high-spin and No more peaks were found for 3 at potentials higher than low-spin in the ground-state. -1.2 V.The above redox couples were also recorded in the In Lahti’s terminology, the magnetic material 3 consists of cyclic voltammograms of a saturated solution of the iminotwo spin-containing (SC) fragments, two carbon-centered radradical 4 in DMF (Fig. 2), along with a further irreversible icals, connected by a two-electron one-center heteroatom peak R4.The diVerent height of the peaks in Fig. 1 and 2 is p spacer, a nitrogen atom. Due to the relative stability of 3, due to the lower concentration of 4 than 3, whereas the described below, its properties could be tested and the predicpresence of the additional peak R4 in Fig. 2 can be ascribed tions regarding the multiplicity of its electronic ground-state to reaction of 4 with electrogenerated radicals at the electrode corroborated.reaction layer. Fig. 1 also shows that O2/R2 and O3/R3 couples partially overlapped, and for this reason the height of peaks Results R2, R3 and O2 cannot be accurately determined. In fact, the O1R1, O2/R2 and O3/R3 couples behaved as reversible one- The reaction of the hydrocarbon salt 25a with an excess of electron systems controlled by diVusion.So the diVerences ammonia in CH2Cl2 followed by treatment with SnCl2 gave between the anodic and cathodic peak potentials (Epa-Epc) the amino-radical 16 (58%), the diradical 3 (17%) and a low for each of these three couples was close to 60 mV in all scan yield of the TTM radical8 (9%) as a direct reduction of salt 2 rates (n) considered, whereas the height of peaks R1, R2+R3, (Scheme 1).The amino-diradical 3 is a green microcrystalline solid with a visible spectrum in cyclohexane as follows: l/nm (e/dm3 mol-1 cm-1), 375 (40 600), 440 (15 200), 633 (17 700). It is stable in the solid state (its decomposition in 13 d in air is practically nil, checked by electronic spectroscopy), and it oxidizes in solution to the more persistent imino-radical 4.A smooth oxidative treatment of 3 with a basic aqueous solution of silver nitrate in chloroform gave 4 quantitatively, which, by the action of SnCl2 in tetrahydrofuran, reverted to 3. Radical 4 is an extremely persistent dark brown solid, even in solution ark. Its visible spectrum in cyclohexane is as follows; l/nm (e/dm3 mol-1 cm-1), 377 (37 350), 409 (29 400), 573 (15 100), 619 (12 700).Fig. 1 Cyclic voltammogram of a 0.5 mM amino-radical 3 solution in 0.1 M TBAP+DMF. Scan rate 50 mV s-1 and temperature 25 °C. Starting and final potential 0 V; reverse potential -1.2 V. Fig. 2 Cyclic voltammogram of a saturated imino-radical 4 solution in 0.1 M TBAP+DMF under the same experimental conditions as NH Cl Cl (C6H2Cl3)2C• + 1 + TTM 2 3 2 (1) NH3, CH2Cl2 (2) SnCl2, THF Cl Cl C6H2Cl3 = Cl C • Cl Cl Cl Cl NH Cl 2 2 3 AgNO3, NaOH, HCl3 SnCl2, THF C Cl Cl Cl Cl Cl C Cl Cl Cl Cl N Cl • 2 2 4 Scheme 1 indicated in Fig. 1 1166 J. Mater. Chem., 1998, 8(5), 1165–1172O1 and O2+O3 increased linearly with the square root of the from coplanarity, due to the presence of the six ortho-chlorines in the phenyl rings. The stability of 32- is then mainly scan rate.9 Note that the height of peaks R2+R3 was approxiattributed to steric protection.mately equal to that of peaks O2+O3 and twice that of peak As shown in Fig. 2, a saturated solution of 4 in DMF with R1. However, the |Ipa|/Ipc ratio (Ipa=anodic peak current, TBAP (0.1 M) displayed the same O1/R1, O2/R2 and O3/R3 Ipc=cathodic peak current) for the O1/R1 pair was ca. 0.6 pairs as 3. In this case, the height of peaks R2+R3 is only 1.4 (Fig. 1). times that of peak R1. In addition, the |Ipa|/Ipc ratio for the The CV behavior for 3 described above helps to establish corresponding O1/R1 couple was ca. 1 in all n tested, as that the O1/R1 couple corresponds to the equilibrium reaction expected if all amino-diradical 3 formed in peak R1 is also between the imino-radical 4 and the amino-diradical 3, i.e.oxidized to imino-radical 4 in peak O1. The height of peaks eqn. (1), R1 and O1 for compound 4 in a given n value was similar to 4+H++1 e-P3 (1) that of peak O1 found for compound 3. Since these peaks are diVusion-controlled, their heights must be proportional to the where H+ proceeds from a proton donor such as water, always solubility of 4, i.e.its concentration in the saturated solution. present in small amounts in DMF. The O2/R2 pair can then This suggests that the |Ipa|/Ipc ratio of ca. 0.6 found for the be ascribed to the reversible conversion of 3 into its radical- O1/R1 couple of compound 3 (Fig. 1) is due to the loss of anion 3 -: eqn.(2), compound 4 near the electrode towards the bulk solution 3+1 e-P3·- (2) during the oxidation of 3 in peak O1 until reaching its saturation in the reaction layer. So, by comparing the height and the O3/R3 couple to the equilibrium (3) between 3 - and of the diVusion-controlled peak R1 for compounds 3 and 4, its dianion 32-. which must be directly proportional to their concentrations, 3 -+1 e-P32- (3) the solubility of 4 in DMF was found to be 0.31 mM.The presence of peak R4 in Fig. 2 is more diYcult to explain. The presence of three consecutive reversible one-electron redox This irreversible peak has a height similar to that of peak R1, pairs for compound 3 (Scheme 2) indicates a good stability of which means it is diVusion-controlled.In addition, the diVeranions 3 - and 32- in DMF, without the existence of any chemience between its cathodic half-peak and peak potentials, cal reaction involving their disappearance from the medium. (Ep/2c-Epc), was 70–80 mV, whereas its Epc value varies lin- In addition, the small diVerence between the second and early with -log n with a slope close to 30 mV per decade. All third cathodic peaks (160 mV) in 3 is a measure of the small these parameters agree with the behavior expected for a firstinteractions between the two negative charges in the dianion order one-electron EC mechanism [theoretical values at 32-, which must be situated apart from each other to prevent 25.0 °C:9 (Ep/2c-Epc)=59.6 mV, slope of Epc vs.-log n plot= Coulombic repulsions between them.By analogy with the 29.6 mV per decade]. electronic structure in the neutral radical, in 32- the charges The fact that the height of peaks R2+R3 in Fig. 2 is will be located mainly in the aliphatic carbon atoms of each significantly lower than twice that of peak R1, indicates that triphenylmethyl moiety adopting a stable conformation far not all of the compound 3 formed in this last peak is completely converted into 3 - and 32-, i.e.part of these ions disappears at the reaction layer. This phenomenon is not observed in cyclic voltammograms of 3 (Fig. 1) and suggests a reaction of 3 - and 32- with 4, present in the reaction layer by its diVusion from bulk solution, yielding the electroactive species of peak R4. A reversible one-electron reduction of this species, followed by an irreversible chemical decomposition of the resulting compound, could explain the EC process found for this peak.Electron paramagnetic resonance (EPR) Recently, the X-band EPR spectrum of the amino-radical 1 recorded in CH2Cl2 solution at 173 K was reported.6 It displayed an overlapping triplet of septets, centered at g= 2.0030, corresponding to the hyperfine splitting of the free electron with the nitrogen atom (aN=1.10 G) and the six aromatic meta-hydrogens (aH=1.10 G).The magnetic interaction with the a-amino hydrogen ones gave negligible splitting and most probably contributes to line broadening. The spectrum found for the isotropic solution (~10-3 M) of amino-diradical 3 in 2-methyltetrahydrofuran (MTHF) at room temperature contained a single broad line centered at g=2.0036 with a peak-to-peak linewidth of the derivative line, DHpp=6.2 G.This large linewidth value and the existence of abnormally intense tails in the derivative line, which increase with decreasing temperature, are accounted for by the modulation of the energies of the triplet spin levels due to the anisotropic part of the intramolecular electron–electron magnetic interaction.These large absorptions probably hamper the observation of satellite lines corresponding to the coupling with 13C nuclear spins of carbons in the molecule, mainly those from a-carbons where the majority of the spin density resides,10 and preclude any information in fluid solution about the strong or weak electron–electron exchange coupling (the scalar part of the magnetic interaction between two unpaired electrons) which will be expressed by a normal or a half value C Cl Cl Cl Cl Cl C Cl Cl Cl Cl N Cl • 2 2 4 C Cl Cl Cl Cl Cl C Cl Cl Cl Cl NH Cl • 2 2 3 • C Cl Cl Cl Cl Cl C Cl Cl Cl Cl NH Cl • 2 2 3–• C Cl Cl Cl Cl Cl C Cl Cl Cl Cl NH Cl 2 2 32– – – – +1e– +1e– +1e– +1H+ Scheme 2 of those coupling constants, respectively.11 J.Mater. Chem., 1998, 8(5), 1165–1172 1167At low temperatures (~150 K), in a very viscous solution near to glassy MTHF, the spectrum showed three pairs of lines in the Dms=±1 region (Fig. 3), typical of a randomly oriented ensemble of immobilized triplet species without axial symmetry, described by the zero-field splitting parameters |D/hc|=0.0071 cm-1 and |E/hc|=0.0006 cm-1, with the principal values of the g tensor being gxx=2.0032, gyy=2.0037 and gzz=2.0029.Further confirmation of the triplet state configuration was provided by the observation of the Dms=±2 region of a broad line centered at g=4.124, with a peak-to-peak spacing, DHpp=7.5 G (Fig. 3). Although the fine structure in the Dms=±1 remains at lower temperatures in the rigid glass MTHF, the intensity rapidly decreases with decreasing temperature, being hardly detected at temperatures lower than 70 K, which is consistent with the fact that the triplet is an excited state.In addition, an intense central line corresponding to S=1/2 species (g=2.0032) appears, which can be ascribed to the imino-radical 4 always present in the sample as an impurity, resulting from the smooth oxidation of aminodiradical 3.From parameter D, the average distance between the two unpaired electrons has been estimated12 as 7.15 A° , a smaller value than the theoretical one of 9.1 A ° .13 At lower temperatures and in conditions of microwave saturation of the doublet state resonance (power: 5.02 mW), a pair of signals (g=2.0034) emerging from the edges of the central line in the region Dms=±1 appears.In Fig. 4, the Fig. 5 (a) A series of EPR spectra of a solution of 3 in MTHF glass spectrum recorded at 90 K shows the presence of both inter- from 4 to 90 K (microwave power, 0.2 mW), showing the intensity dependence of the weak dipole–dipole interaction as explained in the actions, the strong one with a fine structure of three pairs of text, and (b) the EPR spectrum at 4 K lines of low intensity and the weak one with a closer pair of lines emerging from the wings of the strong single line.In Fig. 5, a series of spectra recorded from 4 to 90 K, irradiating ance. At much lower microwave power (10-3 mW), the intenthe sample at low microwave power (0.2 mW) in conditions sity of the central line gradually decreases from 4 K upwards, of non-saturation of these lateral signals is displayed.In this following Curie’s law in the temperature range 37–90 K, where series, while the intensity of the lateral signals diminishes from the signal amplitude is inversely proportional to the tempera- 4 K upwards, in such a way that they practically disappear at ture (Fig. 6). So, both signals, the doublet and the central 59 K, the intensity of the central signal increases from 4 to single line, must correspond to diVerent species, as shown by 90 K, due to microwave saturation of the doublet state resonselective microwave saturation, and the idea that the doublet might correspond to a weak ferromagnetic interaction in the ground state between pairs of molecules, since its intensity increases with decreasing temperature is not discarded.The EPR spectrum of imino-radical 4 in tetrachloroethylene solution at room temperature consisted of a single line, DHpp= 2.6 G, centered at g=2.0037 [Fig. 7(a)]. At higher gain, the isotropic coupling with the 13C nuclear spins of the a-carbon atoms in the molecule appeared in the spectrum with a coupling constant, a#15 G.This value is practically half the value corresponding to the 13C hyperfine coupling in radicals of the TTM series5a,c,6 (a#29.5 G), which indicates that the spin density on these carbons is also half the normal value and, consequently, the electronic structure of 4 can be depicted Fig. 3 EPR spectrum of 3; Dms=±1 transition in MTHF glass at 150 K. Insert shows the signal corresponding to Dms=±2 forbidden transition Fig. 6 A series of EPR spectra of a solution of 3 in MTHF glass from Fig. 4 EPR spectrum of 3; Dms=±1 transition in MTHF glass at 90 K, showing the presence of the two dipole–dipole interactions, as 4 to 90 K (microwave power, 10-3 mW), showing the normal intensity dependence of the doublet state resonance, as explained in the text explained in the text 1168 J.Mater. Chem., 1998, 8(5), 1165–1172Fig. 8 Thermal variation of meff/mB for amino-diradical 3 ($) from sample measured in a Faraday balance operating with a field-strength of 17 kOe and (#) from sample measured in a SQUID magnetometer operating with a field-strength of 20 kOe. The solid lines are theoretical ones, as described in the text. As shown in Fig. 8, when the temperature decreases from 300 K, the meff value of both microcrystalline samples decreases continuously. This behavior is associated with a strong antiferromagnetic interaction. In the range 12–75 K, there is a plateau of meff=0.73 and meff=1.04 mB for the two samples, respectively, which suggests that some monoradical impurities exist in greater proportions in one of the samples.After applying diVerent models to account for these results, the model which best fits the experimental data is of a system composed of a couple of diradical molecules with an intramolecular interaction characterized by J, interacting with each other at a strength given by J¾ (Fig. 9).14 In such a system, Fig. 7 EPR spectrum of 4 (a) in tetrachloroethylene at room temperature and (b) in CH2Cl2 at 213 K there is a proportion of monoradical impurities, most probably the oxidized imino-radical 4, which diVers from one sample to the next.as a structure in resonance between the two canonical structures shown in Scheme 3. A hyperfine splitting of an overlapping multiplet of lines appeared in the spectrum when recorded in CH2Cl2 at low temperatures (213 K and lower) [Fig. 7(b)]. Magnetic susceptiblity Fig. 9 Model system for diradical interaction The molar magnetic susceptibility (xS) of diradical 3 was The spin Hamiltonian for such a system is given by eqn. (4), measured in two diVerent samples in the temperature range 4–300 K, one of them with a SQUID magnetometer and the H=-2JS1S2-2J¾S2S3-2JS3S4 (4) other one with a Faraday balance operating in a field-strength where Si corresponds to the spin angular momentum vector of 20 and 17 kOe, respectively.The data (x¾S =xS-xdia- for the single electron in each center. xholder) were corrected for the magnetization of the sample The total spin states and energies, li, corresponding to this holder and for the diamagnetic susceptibility of the molecule system are obtained from the diagonalization of the (-608.7×10-6 cm3 mol-1, using Pascal’s constants).The ther- Hamiltonian [eqn. (5)] mal variation of the molar eVective magnetic moment in Bohr magnetons shown in Fig. 8 is given by meff=2.828(x¾ST )1/2. Quintuplet (S=2) l1=-J- J¾ 2 Triplet (S=1) l2=J- J¾ 2 Triplet (S=1) l3= J¾ 2 +EJ¾2+J2 Triplet (S=1) l4= J¾ 2 -EJ¾2+J2 Singlet (S=0) l5= J¾ 2 +J+2SAJ¾ 2 B2 - J¾ 2 J+J2 Singlet (S=0) l6= J¾ 2 +J-2SAJ¾ 2 B2 - J¾ 2 J+J2 (5) C Cl Cl Cl Cl Cl C Cl Cl Cl Cl N Cl • 2 2 C Cl Cl Cl Cl Cl C Cl Cl Cl Cl N Cl 2 2 • Scheme 3 The expression for the susceptibility, using the van Vleck J.Mater. Chem., 1998, 8(5), 1165–1172 1169states of the molecule into a ground state singlet and an excited triplet. On the other hand, weaker interactions either in the glassy solution or in the solid state are also predicted by EPR and susceptibility measurements, respectively.At present, we are not able to attribute the first one which increases with decreasing temperature, but the second one is ascribed to an intermolecular antiferromagnetic interaction. Fig. 10 Diagram of levels for the singlet ground state and two triplet Concerning the intramolecular interaction, Lahti et al.preexcited states dicted, by using AM1-CI semi-empirical procedures, a strongly antiferromagnetic coupling in the para,para¾ connectivity in this kind of system in their planar conformation, with a formula,15 is as in eqn. (6), dominant closed-shell configuration in their ground state, best described by a pair of equivalent zwitterionic Kekule� resonance xd= xD 2 = Nm2Bg2 6kBT structures.All these predictions are confirmed in amino-diradical 3. Results from EPR analysis and susceptibility measurements, × 30 e-b1+6 e-b2+6 e-b3+6 e-b4 5 e-b1+3 e-b2+3 e-b3+3 e-b4+e-b5+e-b6 (6) as shown above, established the singlet character of the ground state of 3, which is best described as a resonance of the where: bi=li/kBT , N is the Avogadro number, mB is the Bohr canonical structures shown in Scheme 4.magnetron, g is the Lande� factor, kB is the Boltzmann constant, However, the remarkable persistence of polychlorotriphenylxD is the susceptibility of a cluster compound of two diradical methyl radicals is mainly attributed to steric shielding by the molecules and xd is the susceptibility per molecule.six chlorines surrounding the trivalent carbon, which leads to The expression for the susceptibility of the monoradical a torsion of the phenyl rings around their bond with the impurity is eqn. (7). central carbon. As a result, these radicals adopt a stable propeller-like conformation with a significant inhibition of the xM= Nm2B g2 4kBT (7) delocalization of the free electron into the three rings.In the stable non-planar conformation most of the spin density is in Then, the thermal dependence of meff for the whole system the central carbon. becomes as in eqn. (8hese findings suggest that in amino-diradical 3, the diradical structure plays an important role in the configuration of meff/mB=2.828 its ground state if the twisted geometry is the most stable conformation, with the free electron mainly confined to the ×S TFm T-hm 3 8 +(1-Fm) T T-hd central carbon atom of each triphenylmethyl moiety.In such a case, the dipolar spin–spin interaction is much weaker than × 15 eb1+3 e-b2+3 e-b3+3 e-b4 10 e-b1+6 e-b2+6 e-b3+6 e-b4+2 e-b5+2 e-b6 (8) in the planar conformation, as also predicted by Lahti et al.,7b and the triplet–singlet energy gap is not very high, 1.38 kcal mol-1 (1 cal=4.184 J) as stated above.assuming g=2, where: Fm is the fraction of monoradical impurity and (1-Fm)=Fd is the fraction of diradical molecules On the other hand, the fact that the average distance between the unpaired electrons is smaller than the theoretical one, as and hm and hd account for the eVect of residual path interactions among molecules which are not considered in the model.described in the EPR section, favours the closed-shell electronic structures, where the molecule is forced to adopt a more planar This equation was fitted to the experimental data obtained for both samples to give the following parameters: (a) from conformation between the two a-carbons. These conformations make the delocalization of the electronic spin on the phenylene sample analyzed in the SQUID: Fm=0.19±0.02; Fd= 0.81±0.02; hm=-1.50±0.5 K; hd=-1±1 K; J= rings easier than on the four extreme phenyls.Concerning the intermolecular interaction in the solid state, -290±20 K; J¾=-203±40 K; (b) from sample analyzed in the Faraday balance: Fm=0.37±0.02; Fd=0.63±0.02; hm= it is obvious that the proposed model of a weak interacting dimer from magnetic susceptibility measurements can only -1.1±0.8 K; hd=-1±1 K; J=-296±30 K; J¾= -160±50 K.The values of J and J¾ are quite similar for both provide a simplified picture of the real situation, but it does involve enough radical centers and interactions among them samples although they display diVerent impurity content.The diagram of levels for the singlet ground state, l6, and to reproduce the low lying energy terms with suYcient accuracy. If the detailed treatment of the interactions with other the first two triplet excited states of the system, the lowest triplet l4 and the next l2 considering the above values for J radical centers (e.g. the tendency to form a linear chain) is and J¾ and their intervals of error, can be established as shown in Fig. 10. The highest excited states, one singlet, one triplet and one quintuplet, the three of energy too high to be significantly populated in the normal range of temperatures, are not considered. The low negative values for hm and hd indicate the existence of small residual antiferromagnetic interactions among molecules. Discussion Amino-diradical 3 is a clear and stable example of a system composed of two spin-bearing units linked by a two-electron one-center heteroatom p spacer.Both the EPR analysis and the results from magnetic susceptibility measurements predict a strong dipole–dipole interaction, either in dilute solution (EPR) or in the solid (susceptibility), which is attributed to the C Cl Cl R R C H N Cl Cl R R + C Cl Cl R R C H N Cl Cl R R + C Cl Cl R R C H N Cl Cl R R – • • – Scheme 4 intramolecular electron spin–spin interaction, splitting the spin 1170 J.Mater. Chem., 1998, 8(5), 1165–1172included in the model, neither the spin multiplicity of the three 4-Amino-2,2=,2==,4=,4==,6,6=,6==-octachlorotriphenylmethyl radical 1 and 4,4-iminobis(2,2=,2==,4=,4==,6,6=,6==- low-lying levels varies nor the energy values change significantly.octachlorotriphenylmethyl ) diradical 3 So, it can be concluded that a ground singlet state with a Dry NH3 was passed slowly in the dark, through a solution total pairing of the spins and, at least, two triplets as the of salt 2 (2.12 g) in CH2Cl2 (500 ml) until the blue color of nearest excited states, is the low-lying energy diagram which the solution suddenly changed to red.Then argon was passed achieves a good fit of the experimental data for amino-diradical through to eliminate the NH3 and the resulting mixture was 3 in the solid state. filtered. The filtrate was evaporated to dryness and the red residue was dissolved in THF (100 ml). Anhydrous SnCl2 (0.56 g) was added to the solution, and the mixture was stirred in the dark at room temperature for 30 min.The resulting Experimental mixture was filtered and evaporated to dryness. The residue General procedures in diethyl ether (100 ml), washed with aqueous NaHCO3 and with water, then dried and evaporated, gave a new residue All melting points are uncorrected. Solvents were dried and which was chromatographed (silica gel flash chromatography, purified before use.THF was freshly distilled from sodium CCl4–CHCl3, 151) to give the following: (a) radical 1 (0.26 g; benzophenone ketyl. Magnetic susceptibility data for microcry- 9%) identified by mp and IR. (b) Amino-diradical 3 (0.14 g; stalline samples of amino-diradical 3 were measured from 4 to 17%), mp 268–271 °C; n/cm-1 (KBr) 3400 (w), 3100 (w), 1565 298 K with a Manics DSM8 susceptometer operating with a (s), 1525 (m), 1370 (m), 1310 (m), 1180 (m), 1130 (m), 1075 (w), field strength of 17 kOe and with a SQUID magnetometer 1055 (w), 980 (w), 920 (w), 850 (m), 820 (m), 805 (m), 790 (m); operating with a field strength of 20 kOe.UV–VIS (cyclohexane) lmax/nm (e/dm3 mol-1 cm-1) 375 (40 600), 440 (15 200), 633 (13 700).Anal. Calc. for C38H13Cl16N: 43.4; H, 1.25; N, 1.3; Cl, 54.0. Found: C, 43.9; Electrochemical measurements H, 1.4; N, 1.3; Cl, 53.9%. (c) Radical TTM (0.74 g; 58%), identified by its mp and IR. The cyclic voltammetric experiments were carried out in a three-electrode cell under an argon atmosphere. A platinum Oxidation of 3. Synthesis of 2,2=,2==,4=,4==,6,6=,6==-octachloro-4- sphere with a surface area of 0.093 cm2 was used as the working {3,5-dichloro-4-[bis(2,4,6-trichlorophenyl )methylene]cyclo- electrode and a Pt wire as the counter electrode.The reference hexa-2,5-dienylidenamino}triphenylmethyl radical 4 electrode was an SCE (NaCl-saturated aqueous solution) connected to the cell through a salt bridge containing a 0.1 M To a solution of 3 (20 mg) in CHCl3 (5 ml) at room temperature TBAP–DMF solution.The temperature of test solutions and was added a basic aqueous solution of AgNO3 (5 ml, AgNO3 of the SCE was kept at 25 °C. In all experiments, the cell 2%, NaOH 2%) and the resulting mixture was vigorously was maintained in darkness to avoid the photochemical stirred for 5 min in the dark. The organic solution was washed decomposition of substrates in solution.with water, dried and evaporated to dryness, giving a residue CV measurements were performed with standard equipment which was chromatographed (silica gel, CHCl3) to give 4 consisting of a PAR 175 universal programmer, an Amel 551 (18 mg; 90%), mp>350 °C; n/cm-1 (KBr) 3080 (w), 1550 (s), potentiostat and a Philips 8043 X-Y recorder. Cyclic voltam- 1530 (s), 1370 (s), 1285 (w), 1180 (m), 1130 (s), 1075 (w), 960 mograms of all solutions were recorded at a scan rate (n) of (w), 920 (w), 885 (w), 850 (s), 820 (m), 800 (s), 785 (s); UV–VIS 20–200 mV s-1.(cyclohexane) lmax/nm (e/dm2 mol-1 cm-1) 210 (128 000), 376 A solution of amino-diradical 3 (0.5 mM) in DMF containing (37 000), 413 (31 000), 573 (15 000), 621 (12 000).Anal. Calc. TBAP (0.1 M) as the background electrolyte was studied. Since for C38H12Cl16N: C, 43.5; H, 1.1; N, 1.3; Cl, 54.0. Found: C, imino-radical 4 showed lower solubility in DMF, its CV 43.0; H, 1.1; N, 1.3; Cl, 54.3%. measurements were carried out using a saturated solution in DMF with TBAP (0.1 M). The volume of all test solutions Reduction of 4 to give 3 was 25 ml.Anhydrous SnCl2 (5 mg) was added to a solution of 4 (20 mg) in CHCl3 (5 ml) and the mixture was vigorously stirred at room temperature and in the dark (30 min). Then, the solvent EPR experiments was evaporated oV and the residue in CHCl3 was filtered (silica EPR spectra were recorded with a Varian E-109 spectrometer gel ) to give 3 (19 mg; 97%), identified by IR and UV–VIS working in the X band and using a Varian E-257 temperature- spectra.controller to obtain spectra at temperatures as low as 130 K. A Bruker ESP 300 spectrometer with a Bruker ER 4112 HV Support of this research by DGICYT of MEC (Spain) through continuous-flow liquid helium cryostat and an Oxford project PB92-0031 is acknowledged. The authors express their Instruments temperature-controller system was used to obtain gratitude to the EPR services of the Centre d’Investigacio� i EPR spectra at lower temperatures (4 K).Samples of amino- Desenvolupament (CSIC) and the Universitat de Barcelona. diradical 3 and imino-radical 4 were prepared in quartz EPR tubes and degassed by three freeze–pump–thaw cycles before References being inserted into the EPR cavity. Handling of radicals in solution was performed in the dark. 1 For reviews see: J. S. Miller, A. J. Epstein and W. M. ReiV, Chem. Rev., 1988, 88, 201; Acc. Chem. Res., 1988, 21, 114; H. Iwamura, Adv. Phys. Org. Chem., 1990, 26, 179; D. A. Dougherty, Acc. Chem. Res., 1991, 24, 88; H. Iwamura and N. Koga, Acc. Chem. Res., 1993, Tris(2,4,6-trichlorophenyl )carbenium hexachloroantimonate 2 26, 346; H.Kurreck, Angew. Chem., Int. Ed. Engl., 1993, 32, 1409; J. S. Miller and A. J. Epstein, Angew. Chem., Int. Ed. 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