A New Method for the Synthesis of Heterospin Complexes

The interaction of binuclear Co(II) pivalate [Сo2(H2O)Piv4(HPiv)4] with nitronyl nitroxide HL 1


Results and Discussion
The synthesis of trinuclear [Сo3(Piv)2L 2 2L 3 2], where L 2 is the imino nitroxide 2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-1-oxyl anion and L 3 is the corresponding amidine oxide anion (Scheme 1), was described in [23].The isolation of this compound was a nontrivial problem, because [Сo3(Piv)2L 2 2L 3 2] crystallized from the reaction mixture only when the starting reagents were used in a strictly definite ratio.In addition, other compounds crystallized along with the desired product, from which they were sometimes separated mechanically.A good yield of [Сo3(Piv)2L 2 2L 3 2] in the individual state was obtained only when Co(II) pivalate and an equimolar mixture of HL 2 and HL 3 were used as reagents.This should be taken into account, because below, we make a certain analogy between the reaction of Co(II) pivalate with a mixture of HL 2 and HL 3 and the reaction of Co(II) pivalate with a mixture of HL 1 and HL 2 .
We noticed that the L 3 donor group that formed seven-membered metallocycles in the [Сo3(Piv)2L 2 2L 3 2] molecule (Scheme 2) was identical to that in nitronyl nitroxide HL 1 .This prompted us to study the product of the interaction of Co(II) pivalate with the equimolar mixture of HL 1 and HL 2 .The isostructural character of the L 1 and L 3 donor group (Scheme 3) was assumed to be favorable for purposeful introduction of L 1 in the complex molecule to ultimately obtain a multispin complex containing two different paramagnetic ligands.Scheme 3. The L 1 and L 3 donor group, which is favorable for the formation of seven-membered metallocycles.
The pentanuclear molecule has three different environments of Co atoms: tetrahedral for the "central" Co3, octahedral for the "internal" Co2 and Co4 and trigonal bipyramidal for the "terminal" Co1 and Co5 (Figure 2).The [Co5(Piv)4L 1 4L 2 2] molecule contains fragments similar to those of [Сo3(Piv)2L 1 2L 2 2]; namely, the environment of the Co1 and Co5 atoms in Figure 2 is the same as that of Co1 in Figure 1; the environment of Co2 and Co4 in Figure 2 is the same as that of Co2 in Figure 1.In [Co5(Piv)4L 1 4L 2 2] molecules, as well as in [Сo3(Piv)2L 1 2L 2 2] molecules, the "terminal" cobalt atoms (Co1 and Co5) form six-membered metallocycles typical for Schiff bases with the coordinated imino nitroxides, and the Co2 and Co4 atoms form seven-membered metallocycles with nitronyl nitroxides.All of the OPh atoms of the phenoxy groups L 1 perform the bridging function.Half of all nitroxyl ONO atoms (O1E and O1H) are also involved in the formation of bridging bonds, while the other half (O1B and O1F) are coordinated as monodentate ligands by the Co2 and Co4 atoms, respectively.The Co-N bond lengths are 2.039(5) and 2.041(7) Å; the Co-O bond lengths are 1.909(5)-2.269(5)Å.The Co-O1B and Co-O1F distances are long enough, 2.580(4) and 2.836(5) Å; as a result, the "central" Co3 atom has a tetrahedral environment.
Earlier, it was reported [23] that when HL 2 reacted with nickel pivalate [Ni2(H2O)(Piv)4(HPiv)4], imino nitroxide did not undergo any transformations.When Ni(II) pivalate reacted with HL 1 , nitronyl nitroxide also did not undergo any redox transformations.Moreover, no products of interaction of Ni(II) pivalate with L 1 were isolated irrespective of the starting reagent ratio and synthesis conditions (when the solution was concentrated, the solid product was primarily unchanged HL 1 ).The introduction of both HL 1 and HL 2 in the reaction system, however, led to the formation of a mixed-ligand complex [Ni3L 1 L 2 2(Piv)3(HPiv)3], as in the case of cobalt.The highest yield of the complex (60%) can be achieved when using the molar ratio of reagents [Ni2(H2O)(Piv)4(HPiv)4]:HL 1 :HL 2 = 3:2:4 corresponding to the stoichiometric coefficients of the reaction: The structure and composition of [Ni3L 1 L 2 2(Piv)3(HPiv)3] differ from those for the trinuclear Co(II) complex with L 1 and L 2 (cf.Figures 1 and 3  Thus, the factors that favor the synthesis of [Co5(Piv)4L 1 4L 2 2] by the reaction of Co(II) pivalate with L 1 are the ability of the metal to be at different oxidation levels and the kinetic stability of both the starting L 1 and the formed L 2 .When Ni(II) pivalate was used in the reaction with L 1 , the complexes containing L 2 were never recorded.This is fully consistent with the data of [23][24][25][26][27], where the authors also used the metals capable of changing the oxidation level.
Regarding the redox processes with nitroxides, it was noted that in the reaction with a transition metal, the nitroxide can be reduced to the corresponding hydroxylamine [28,29] and form the product of cocrystallization of the starting radical and the complex with nitrone [30] or complex with coordinated hydroxylamine [31].Co(hfac)2 is able to reduce the ferrocenyl bis(nitronyl nitroxide) with producing the diamagnetic ferrocenyl bis(amidine oxide) cation, which forms with the [Co(hfac)3] − , as the counterion, the complex salt [32].Several products are known in which hydroxylamine reduced the metal and was itself oxidized to the corresponding nitroxide [33].When the products of the interactions of metals with dinitroxides were studied, unusual compounds were isolated, in which one of the nitroxyl groups was reduced to the hydroxylamine anion during the reaction [24,25,34].The oxidation of the transition metal induced by the reduction of one of the coordinated nitroxides was described in [26,27].A specific copper(II)-nitroxide complex was described, which has a nontrivial structural peculiarity: it contains both the coordinated O atoms of nitroxide and those of the corresponding hydroxylamine anion [35].An interesting manganese(II)-nitroxide-hydroxylamine complex was known, as well [36].In the case of the interaction of transition metal compounds with L 2 [23] and of Co(II) pivalate with L 1 , we found that heterospin complexes could be obtained, which contained both the starting nitroxide and the product of its reduction as a result of the transition metal-catalyzed transformation of the starting radical.
The pathway of the ligand reduction has not been established, but it is probably not a simple process [28,31,32,34].However, the redox process can be suppressed in the synthesis of heterospin compounds of transition metals containing both nitronyl nitroxide and imino nitroxide or imino nitroxide and the corresponding nitrone (or hydroxylamine).Since nitronyl nitroxide, imino nitroxide and the corresponding nitrone are often kinetically stable and can be isolated in the individual state, a known mixture of components can be used in the synthesis, as was done in the case of the synthesis of [Сo3(Piv)2L 1 2L 2 2] and [Ni3L 1 L 2 2(Piv)3(HPiv)3].This is especially important in the latter case.Since Ni(II) does not initiate the transformation of L 1 into L 2 , [Ni3L 1 L 2 2(Piv)3(HPiv)3] cannot be synthesized by any other procedure.Therefore, this is actually a new method for the synthesis of heterospin complexes: the reaction of a metal compound with a mixture of nitronyl nitroxide and imino nitroxide or with a mixture of imino nitroxide and the corresponding nitrone.
The multispin molecules of the multinuclear complexes in question have a rather complex system of exchange channels, which requires a separate detailed study in each case.Since this study concentrated on the new approach to the synthesis of heterospin complexes, the magnetic properties of the isolated products are presented below in concise fractographic form.
For [Сo3(Piv)2L 1 2L 2 2]•2Me2CO, µeff, which is 8.09 µB at 300 K, gradually increased to 10.57 µB when the temperature decreased to 8 K (Figure 4a).In the temperature range 100-300 K, the dependence 1/χ(T) obeys the Curie-Weiss law.The optimum values of the Curie (C) and Weiss (θ) constants are 7.54 ± 0.03 cm 3 •K/mol and 25.4 ± 0.8 K, respectively.The positive value of the Weiss constant θ suggests that the ferromagnetic exchange interactions are dominant.For [Сo3(Piv)2L 1 2L 2 2]•2Me2CO, the dependence of magnetization on the strength of the external magnetic field below 14 K is nonlinear and cannot be described in terms of the Brillouin function (Figure 4b).This points to the transition of the substance into the magnetically-ordered state with spontaneous magnetization 26,000 cm 3 •G/mol at 2 K.The Curie temperature TC can be estimated at 5 K.For [Co5(Piv)4L 1 2L 2 4]•0.5Me2CO•0.5С7H16,µeff is 9.99 µB at 300 K and gradually increased to 14.85 µB when the temperature decreased to 8 K; then, it decreased abruptly to 11.17 µB at 2 K.Note that the high-temperature value of µeff is considerably higher than the theoretical pure spin value of 7.55 µB for nine non-interacting paramagnetic centers (three Co(II) ions (S = 3/2, g = 2) and four nitroxides (S = 1/2, g = 2)) due to the orbital contribution that is typical for Co(II) ions in an octahedral environment.In the temperature range 100-300 K, the dependence 1/χ(T) obeys the Curie-Weiss law.The optimum values of the Curie (C) and Weiss (θ) constants are 11.07 ± 0.04 cm 3 •K/mol and 32.8 ± 0.6 K, respectively.For [Co5(Piv)4L 1 2L 2 4]•0.5Me2CO•0.5С7H16,the dependence of magnetization on the strength of the external magnetic field below 10 K is nonlinear.A study of the magnetic susceptibility of [Co5(Piv)4L 1 2L 2 4]•0.5Me2CO•0.5С7H16 in an alternating magnetic field (Figure 5) showed that the temperature dependence of the out-of-phase component of magnetic-susceptibility χ"(T) has a maximum, which shifts toward low temperatures when the frequency of the alternating magnetic field decreases.The appearance of the out-of-phase component χ"(T) is one of the characteristics of single-molecule magnets (SMM).The maximum of χ" was not observed for all frequencies; the rough estimation of the energy barrier (Ea) based on the Arrhenius equation ln(2πν) = ln(1/τ0) + Ea/(kBT) gave Ea/kB = 3.1 K.The dependence µeff(T) for [Ni3L 1 2L 2 (Piv)3(HPiv)3] is presented in Figure 6.The µeff value at 300 K is 6.01 µB and does not change when the temperature decreases to 100 K. Below 100 K, µeff gradually increases, reaching 6.32 µB at 10 K, and then abruptly decreases to 5.28 µB at 2 K.In the temperature range 10-300 K, the dependence 1/χ(T) obeys the Curie-Weiss law.The optimum values of the Curie (C) and Weiss (θ) constants are 4.45 ± 0.01 cm 3 •K/mol and 2.4 ± 0.2 K, respectively.The high-temperature value of µeff agrees well with the theoretical pure spin value 5.74 µB for six non-interacting paramagnetic centers (three Ni(II) ions with spin S = 1 and three nitroxides with spins S = 1/2 at a g factor of two.The increased µeff at lower temperatures and the positive Weiss constant θ indicate that weak ferromagnetic exchange interactions between the spins of the paramagnetic centers dominate.

Crystal Structure Determination
The X-ray diffraction (XRD) experiments were performed on a SMART APEX II CCD and APEX DUO (Bruker AXS) diffractometer (Mo Kα for Co complexes and Cu Kα for the Ni complex).All of the structures were solved by direct methods and refined by full-matrix least-squares analysis in an anisotropic approximation for non-hydrogen atoms.The positions of the majority of H atoms were calculated.The methyl H atoms were refined isotropically in a rigid group approximation.Hydrogen atoms were refined isotropically with the use of geometrical constraints.Since the solvent molecules in {Co5} and {Ni3} complexes could not be modeled properly, they were squeezed out with PLATON [27,39].All calculations were performed with the Bruker SHELXTL (Version 6.14) and SHELXL (Version 2014/6) program packages [40].The crystal data and details of experiments are given in Table 1.Crystallographic data were deposited with the Cambridge Crystallographic Data Centre and can be obtained free of charge via www.ccdc.cam.ac.uk/getstructures.

Magnetic Measurements
The magnetic susceptibility of the polycrystalline samples was measured with a Quantum Design MPMSXL SQUID magnetometer in the temperature range 2-300 K with a magnetic field of up to 5 kOe.The diamagnetic corrections were made using the Pascal constants.The effective magnetic moment was calculated as µeff(T) = [(3k/NAµB 2 )T] 1/2  (8T) 1/2 .The AC magnetic susceptibility was measured in an oscillating AC field of 3.5 G and a zero DC field.The oscillation frequencies were in the range 98-1488 Hz.

Conclusions
Thus, our study showed that the reaction of Co(II) pivalate with nitronyl nitroxide HL 1 forms a pentanuclear complex [Co5(Piv)4L 1 4L 2 2], whose molecule has both the starting nitronyl nitroxide L 1 and its imino nitroxide analog L 2 .This prompted us to introduce a known mixture of HL 1 and HL 2 in the reaction.It appeared that this synthetic technique (the use of both the starting radical and the product of its reduction in the reaction with the metal) can serve as an independent method for the synthesis of heterospin complexes.It was shown that the interaction of Co(II) pivalate with nitroxides at a molar ratio of reagents of [Co2(H2O)(Piv)4(HPiv)4]:HL 1 :HL 2 = 3:4:4 gives the trinuclear heterospin complex [Co3(Piv)2L 1 2L 2 2] with a high yield.The replacement of Co(II) by Ni(II) completely suppresses the reduction of HL 1 into HL 2 .In addition, Ni(II) pivalate does not react with HL 1 .However, the use of the known mixture of HL 1 and HL 2 in the reaction with [Ni2(H2O)Piv4(HPiv)4] is an effective method for the synthesis of the heterospin complex [Ni3L 1 L 2 2(Piv)3(HPiv)3], which also contains both nitronyl and imino nitroxides.
Thus, the results of the present study open up a new opportunity in the synthesis of heterospin complexes.Since both nitronyl and imino nitroxides and the corresponding nitrone (the product of more profound reduction) are generally kinetically-stable products, their binary mixtures can readily be prepared.The use of these mixtures in reactions with transition metals can lead to multispin complexes, including [Сo3(Piv)2L 1 2L 2 2] and [Ni3L 1 L 2 2(Piv)3(HPiv)3], which were obtained only using the known mixture of nitronyl nitroxide and its imino nitroxide derivative as the starting reagent.
).All of the Ni atoms have an octahedral environment.The paramagnetic ligands perform the bridging cyclic tridentate function.Each monodentate coordinated HPiv molecule forms an H-bond with one of the O atoms of the neighboring μ2-O,O'-pivalate anion.The Ni-N distances are 2.062(4) and 2.106(3) Å; the Ni-O distances are 1.977(3) and 2.121(4) Å.

Table 1 .
Crystal data and the details of the experiments for the complexes.