Simultaneous Introduction of Two Nitroxides in the Reaction: A New

Nov 13, 2017 - A new approach to the synthesis of multispin compounds has been developed, namely, the simultaneous introduction of two different stabl...
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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Simultaneous Introduction of Two Nitroxides in the Reaction: A New Approach to the Synthesis of Heterospin Complexes Victor Ovcharenko,*,† Olga Kuznetsova,† Elena Fursova,† Gleb Letyagin,† Galina Romanenko,† Artem Bogomyakov,† and Ekaterina Zueva‡ †

International Tomography Center, Institutskaya Street 3a, Novosibirsk 630090, Russian Federation Kazan National Research Technological University, 68 K. Marx Street, Kazan 420015, Russian Federation

‡

S Supporting Information *

ABSTRACT: A new approach to the synthesis of multispin compounds has been developed, namely, the simultaneous introduction of two dif ferent stable nitroxides (nitronyl nitroxide and imino nitroxide) in a reaction with a metal ion. An important characteristic of the new method is that nitronyl nitroxide and imino nitroxide introduced in the reaction with the metal are the products of different series; i.e., the nitronyl nitroxide molecule differs from the imino nitroxide molecule not only in one additional oxygen molecule per molecule but also in another substituent in the side chain of the organic paramagnet. This possibility was demonstrated on the synthesis of multispin compounds [Ni2(A1)(L2)2(Piv)(MeOH)], [Ni2(L1)(A2)2(Piv)(H2O)], [Co2(A1)(L2)2(Piv)(MeOH)], and [Co3(L1)2(A2)2(Piv)2], in which Ln and An differ in the substituent in the phenyl ring. The number of multispin compounds that can be synthesized by the proposed method is almost unlimited. The heterospin complexes of transition metals with coordinated nitronyl nitroxide and imino nitroxide in one molecule contain energy-different exchange interaction channels that differ in both magnitude and sign, as confirmed by the quantum-chemical analysis of exchange channels in [Ni(B1)(B2)2](NO3)2. The series of mixed-radical complexes may include compounds with nontrivial magnetic properties such as [Co2(A1)(L2)2(Piv)(MeOH)], which experiences bulk magnetic ordering below 3.5 K.

1. INTRODUCTION Redox processes using transition metal complexes with organic ligands are the object of constant interest for researchers who develop catalytic systems.1−18 They have also attracted much attention in the creation of redox-triggered molecular switches19−22 and as a basis of many biochemical processes.23,24 Redox processes are met in studies of molecular magnets25−30 and especially frequently in the development of syntheses of heterospin compounds from transition metal complexes with stable organic radicals.31−47 Over the past 20 years, the products that form as a result of redox reactions between nitroxides and transition metals were detected. For example, it was found that a radical can be reduced to the corresponding hydroxylamine and then form the product of cocrystallization of the starting radical and the complex with nitrone.48 The products in which hydroxylamine reduced the metal while having been oxidized itself to the corresponding nitroxide were isolated.49 The formation of complexes was recorded in which, as a result of interaction of the metal with dinitroxide biradicals, the ligands were unusual organic paramagnets, with one of the nitroxyl groups reduced to the hydroxylamine anion during the reaction.50,51 A nontrivial effect of transition metal oxidation induced by the reduction of one of the coordinated nitroxides (“reductively induced oxidation of the metal center”) was found.52,53 A complex containing the coordinated O atoms of © XXXX American Chemical Society

both nitroxide and the corresponding hydroxylamine anion was described.54 It was also found that polynuclear complexes of transition metals are capable of catalyzing the deep transformation of nitroxides.55 However, despite various possible complications resulting from the redox reaction, they can be purposefully used for the synthesis of multispin compounds, including those that cannot be obtained in any other way. In a recent study of the products of the reaction of cobalt pivalate with the imino nitroxide 2-(2-hydroxy-5-nitrophenyl)4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl (HA 2 ; Chart 1), a process was recorded that received the name “redox-induced change in the ligand coordination mode”. In this reaction, some molecules of the spin-labeled Schiff base HA2 were reduced to the amidine oxide 4-nitro-2-(4,4,5,5tetramethyl-3-oxy-4,5-dihydro-1H-imidazol-2-yl)phenol (HA3), and the solid product was the [Co3(A2)2(A3)2(Piv)2] mixedligand complex containing both the starting imino nitroxide and its reduced diamagnetic analogue. The redox process provoked a change not only in the electronic state of the ligand but also in its coordination mode.56 If the diamagnetic HA3 is replaced by the structurally related nitronyl nitroxide HA1, we can obtain [Co3(A1)2(A2)2(Piv)2], whose nontrivial peculiarity is the Received: September 7, 2017

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DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Chart 1. Structural Formulas of Nitronyl Nitroxides and Their Reduced Derivatives

presence of the two paramagnetic ligands in the coordination sphere: nitronyl nitroxide A1 and imino nitroxide A2.57 We have found that the synthetic potential of the approach that suggests the introduction of both nitronyl nitroxide and imino nitroxide in the reaction with the metal is much wider than previously assumed. Moreover, the nitroxides introduced in the reaction can differ not only in the oxidation state but also in the set of substituents in the molecule, i.e., be the representatives of different structural series. In the synthesis of heterospin complexes containing different paramagnetic ligands in the coordination sphere, it is possible to use nitronyl nitroxides and imino nitroxides that differ in substituents, for example, HA1 and HL2 or HA2 and HL1 (Chart 1). We also showed that the proposed synthetic approach is applicable to paramagnets of the 2-imidazoline series B1 and B2 containing a heterocyclic substituent in the side chain (Chart 1). In general, the obtained data suggest that the introduction of both nitronyl nitroxide and imino nitroxide in the reaction with a metal is a route to an almost unlimited range of previously unknown and inaccessible multispin compounds.

Figure 1 shows the molecular structures of trinuclear complex 1 (a) and mononuclear complex 2 (b). Both 1 and 2 contain the

2. RESULTS AND DISCUSSION Because the objects of investigation HL1−HL3 differ from the previously studied HA1−HA3 only in the methoxy substituent in the phenyl ring (i.e., they differ slightly from each other from a functional viewpoint), it was reasonable to assume that, upon reacting with binuclear cobalt(II) and nickel(II) pivalates, HL1−HL3 would behave in the same way as HA1−HA3 and lead to identical products. Indeed, this assumption was confirmed in some aspects. Thus, while investigating the products of the reaction of binuclear [Co2(Piv)4(HPiv)4(H2O)] with imino nitroxide HL2 in an acetone−heptane mixture at different reagent ratios, we found that the solid product was [Co3(L2)2(Piv)4(H2O)2(HPiv)2]·C7H16 (1·C7H16) at ratios of 1:1 to 1:2 [Co2(Piv)4(HPiv)4(H2O)]−HL2 and predominantly [Co(L2)2(HPiv)2] (2) at ratios of 1:4 to 1:6 of the same reagents. Pure 2 can be isolated with a high yield from a CH2Cl2−heptane mixture. The highest yield of 1·C7H16 was achieved at a reagent ratio of 3:4 [Co2(Piv)4(HPiv)4(H2O)]− HL2, which agrees with the stoichiometric coefficients of the reaction

Figure 1. Molecular structures of (a) 1 and (b) 2. Color code: magenta, Co; red, O; blue, N. H atoms, CH3, and But are omitted for clarity. Dashed lines: hydrogen bonds.

starting unchanged imino nitroxide molecule. The crystal and molecular structures of 1 and 2 are described in the Table S1 and Figure S3. Of greatest interest was the complex [Co3(L2)2(L3)2(Piv)2]· Me2CO·H2O (3·Me2CO·H2O) containing both the starting imino nitroxide and the product of its single-electron reduction: the corresponding hydroxylamine coordinated in the form of the prototropic isomer amidine oxide L3. It formed as an impurity in addition to 2 in an acetone−heptane mixture at a reagent ratio of ≥1:4 [Co2(Piv)4(HPiv)4(H2O)]−HL2. The 3· Me2CO·H2O single crystals were of extremely poor quality, which led to an unacceptably high R factor. The situation considerably improved under the conditions of counter synthesis, i.e., when the redox process was “suppressed” by introducing a mixture of HL2 and authentic HL3 into the

3[Co2(Piv)4 (HPiv)4 (H 2O)] + 4HL2 + H 2O → 2[Co3(L2)2 (Piv)4 (H 2O)2 (HPiv)2 ] + 12HPiv B

DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

the reaction of [Co2(Piv)4(HPiv)4(H2O)] with HL2 is direct evidence for this. When the structural formulas of HL1 and HL3 (Chart 1) are compared, it is easy to notice that the donor group L3 that forms seven-membered metallocycles in molecule 3 is identical with that in the nitronyl nitroxide HL1. This prompted us to study the product of the reaction of cobalt(II) pivalate with the equimolar mixture of HL1 and HL2. It was assumed that the isostructurality of the donor groups L1 and L3 would favor the purposeful introduction of L1 in the molecule of the complex to obtain a compound containing two different paramagnetic ligands. Indeed, the reaction of [Co2(Piv)4(HPiv)4(H2O)] with a mixture of nitroxides HL1 and HL2 leads to the trinuclear complex [Co3(L1)2(L2)2(Piv)2]·4MePh (4·4MePh) containing both radicals. To synthesize the product with a maximum yield (70%), the favorable molar ratio of reagents is 3:4:4 [Co2(Piv)4(HPiv)4(H2O)]−HL1−HL2, corresponding to the stoichiometric coefficients of the reaction

reaction with [Co2(Piv)4(HPiv)4(H2O)]. This approach allowed the synthesis of 3·Me2CO·H2O with an approximately 45% yield. The structure of 3·Me2CO·H2O is described in the Table S2 and Figure S4. Our experiments showed that a good reaction medium for the synthesis of the complex is acetonitrile, from which the compound was isolated in the form of perfect single crystals with a composition of 3· 1.5CH3CN. The highest yield of 3·1.5CH3CN (75%) was achieved at a molar ratio of reagents of 3:4:4 [Co2(Piv)4(HPiv)4(H2O)]−HL2−HL3, corresponding to the stoichiometric coefficients of the reaction 3[Co2(Piv)4 (HPiv)4 (H 2O)] + 4HL2 + 4HL3 → 2[Co3(L2)2 (L3)2 (Piv)2 ] + 20HPiv + 3H 2O

The structure of trinuclear molecule 3 is shown in Figure 2a. The terminal nitroxide L2 molecules are coordinated as

3[Co2(Piv)4 (HPiv)4 (H 2O)] + 4HL1 + 4HL2 1 2 → 2[Co3(L) 2 (L )2 (Piv)2 ] + 20HPiv + 3H 2O

Note that it is rather difficult to choose the “correct” reagent ratio. It is generally determined only after the structure solution of the single crystal of the compound. The trinuclear molecule of complex 4 is similar in topology to molecule 3 (Figure 2b). The terminal Co atoms form sixmembered metallocycles with the coordinated imino nitroxides. The central Co2 atom forms seven-membered metallocycles with nitronyl nitroxides that perform the bridging tetradentate cyclic function. The Co−O distances in the environment of all metal atoms are in the range of 1.925(2)−2.241(2) Å; the Co− N distances are 2.037(3) and 2.038(3) Å. The O atoms of the MeO groups are not coordinated. The environment of Co2 is distorted octahedral; for Co1 and Co3, it is close to a square pyramid with an OPiv atom at its apex. In the noncoordinated N−O groups, the interatomic distances are appreciably shorter [1.250(4)−1.264(4) Å] than those in the coordinated groups, whose O atoms are bridging [1.325(3) and 1.309(3) Å]. Thus, the reaction of cobalt(II) pivalate with a mixture of nitronyl and imino nitroxides is an effective method for synthesis of the heterospin complex 4 containing two different paramagnetic ligands. A favorable factor for redox transformation of the paramagnetic ligand is the ability of the metal to be at different oxidation levels.56 In contrast to CoII, for NiII the oxidation level does not change when it reacts with nitroxides; therefore, the difference in the product composition is mainly determined by the ratio of the starting reagents and the solubility of the reaction products. At a ratio of 1:1 [Ni2(Piv)4(HPiv)4(H2O)]− HL2, the solid product is the trinuclear complex [Ni3(L2)2(Piv)4(HPiv)2(H2O)2]·0.5C7H16 (5·0.5C7H16); at a ratio of 1:4, this is the mononuclear complex [Ni(L2)2(HPiv)2] (6). The structures of molecules 5 and 6 (Figures S7 and S8) are almost identical with those of 1 and 2. Note that for 6 we also found experimental procedures for synthesis of the compound in the form of both the trans and cis isomers (Figure S8b). The mixed-ligand [Ni2(L1)(L2)(Piv)2(HPiv)]·C7H16 (7· C7H16) complex was isolated only when authentic HL1 and HL2 were introduced to the reaction. It was impossible to obtain 7·C7H16 in any other way. The structure of molecule 7 is shown in Figure 2c. A comparison of mixed-ligand compounds

Figure 2. Structures of trinuclear molecules (a) 3 and (b) 4 and (c) binuclear molecule 7. Color code: yellow, H; green, Ni.

bidentate ligands via the imine N atom and the O atom of the deprotonated phenol group. The deprotonated amidine oxide L3 molecules perform the bridging tetradentate function because of the fact that each deprotonated O atom of the phenol group and each O atom of the nitrone fragment are coordinated by two Co atoms. They form seven-membered metallocycles with the “central” Co2 atom. The Co−N distances are 2.047(4)−2.089(5) Å, and the Co−O bond lengths are in the range of 1.919(3)−2.260(3) Å. The N−O distances in nitroxide L2 [1.255(6)−1.282(9) Å] are markedly shorter than those in the N-oxide group of L3 [1.322(5)− 1.353(5) Å]. Earlier, it was assumed that a change in the function of the paramagnetic ligand, including the reduction of nitroxide to hydroxylamine, proton addition, subsequent rearrangement to nitrone, and rotation of the 2-imidazoline heterocycle that changes the coordinated atom, must be a quite frequent effect in the reaction of the organic radical with the transition metal capable of changing the oxidation state.56 The formation of 3 in C

DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Mixed Nitroxide Complexes

and 10, respectively. The structure of the trinuclear molecule 11 is actually the same as that of 4 except that the function of L2 in 4 is performed by A2 (in 11). Thus, the introduction of both nitronyl nitroxide and imino nitroxide, which differ in the set of substituents in the paramagnetic molecule in the reaction with the metal, is a route to the significant expansion of the range of accessible multispin mixed-ligand complexes, whose number can be almost infinite. Thousands of compounds have been synthesized and studied over the more than 40-year history of studies of heterospin transition metal complexes with nitronyl or imino nitroxides. However, researchers always preferred to introduce in the reaction either individual nitronyl nitroxide or individual imino nitroxide because the reaction of a radical-containing reaction mixture often gave several different compounds, the same compound but in the form of different polymorphic modifications, or both.58,59 The introduction of mixtures of paramagnetic ligands in a reaction with a metal was indirectly hindered by the fact that further investigation of the synthesized compounds suggests studies of magnetic properties in a wide temperature range often performed using a SQUID magnetometer, which is very sensitive to magnetoactive impurities. For this reason, to avoid additional synthetic and magnetochemical complications, the reaction was generally performed with an individual paramagnetic organic ligand of high purity. The results of our study show that after the synthesis and isolation of some transition metal complexes with nitronyl nitroxide or its imino nitroxide analogue synthesized for the first time, it is useful to examine whether mixed-ligand complexes can form, i.e., the complexes that can form in the presence of both nitronyl nitroxide and imino nitroxide in the reaction medium. This is a separate class of multispin compounds. If the nitronyl nitroxide (imino nitroxide) synthesized for the first time is combined with known imino

4 and 7 shows that the structure of the nickel complex differs substantially from that of the cobalt complex (Figure 2b,c). In molecule 7, the Piv anions and HPiv perform different structural functions. The paramagnetic ligands are also coordinated differently: the O atom of the MeO group of L2 is involved in coordination to the metal, while the similar O atom of L1 is not. The successful synthesis of mixed-ligand compounds 4 and 7 prompted us to study the possibility of introducing not only a mixture of nitronyl nitroxide and iminio nitroxide of the same series in the reaction with a metal but also a mixture of nitronyl nitroxide and iminio nitroxide of different series, i.e., with different sets of substituents in the paramagnetic molecule. It was not obvious from the beginning that multispin complexes of metals containing nitroxides from different structural series could be obtained. However, our experiments showed that the cobalt and nickel mixed-ligand complexes in question can really form. Table 1 summarizes the results: it gives the formulas and molecular structures of the mixed-ligand complexes isolated in the form of solvates (8·MeOH), (9·2MePh·H2O), (10· MeOH), and (11·2MePh). Binuclear molecules 8 and 10 are similar in their structure (Table 1). One of the L2 ligands is coordinated as a bidentate ligand and forms a six-membered metallocycle with the metal. Piv performs the bridging bidentate function, and A1 performs the bridging cyclic tridentate function. In the second L2, the MeO group is involved in coordination by the metal atom. As a result, this L2 serves as a bridging tetradentate cyclic ligand. Coordination of the MeO group prevents the formation of a trinuclear complex similar to 4, in which all of the MeO groups are noncoordinated. On the other hand, the molecular structures of 8 and 10 are similar to that of binuclear 7, in which the bidentate function is performed by Piv instead of L2 in 8 and 10. The molecular structure of 9 is also similar to those of 8 and 10, with the only difference being that the structural functions are performed by L1 and A2 instead of L2 and A1 in 8 D

DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

12, appears exclusively because the data of magnetic measurements for 13 are used below in analysis of the magnetic properties of 12. Because the present paper concentrates on the new approach to the synthesis of heterospin compounds, the structural−magnetic correlations inherent in 1−15 will be described separately. Here we only note that in the series of mixed-ligand complexes of transition metals, there may undoubtedly appear compounds with interesting magnetic properties. For example, 10·MeOH undergoes magnetic bulk ordering below 3.5 K (Figure S26). In the majority of complexes with imino nitroxides, the ferromagnetic exchange interactions show themselves as an increase in the effective magnetic ordering at lowered temperatures (Figures S17−S22 and S27). Also note that, in all mixed-ligand complexes, there are different exchange channels whose energy can differ in both magnitude and sign. Table 2 gives the results of a quantum-

nitroxides (nitronyl nitroxides), the number of new multispin compounds will increase dramatically. The possibility of introducing both nitronyl nitroxide and imino nitroxide, which differ in the set of substituents in the paramagnetic molecule in the reaction with a metal, was demonstrated here on the pairs of paramagnetic Schiff bases HLn and HAn (Chart 1) containing a phenol fragment in the side chain of the radical. The potential of the suggested synthetic approach will evidently increase if we expand the scope of nitroxide derivatives by introducing, e.g., a heterocyclic fragment instead of the phenol fragment in the side chain of the radical. To confirm that there are no hindrances to this expansion, below we give the structures of our synthesized complexes that formed in the reactions of nickel(II) and zinc(II) nitrates with nitronyl nitroxide B1 and imino nitroxide B2 (Chart 1) in aqueous solutions at an initial ratio of reagents of 1:1:2 M(NO3)2·6H2O−B1−B2. They are close in composition, [Ni(B1)(B2)2](NO3)2·1.5H2O (12·1.5H2O) and [Zn(B1)(B2)2](NO3)2·H2O (13·H2O), and have the same structure in the solid state. In the isostructural coordinated cations [M(B1)(B2)2]2+ (Figure 3a), both imino nitroxides B2 are

Table 2. Calculated Exchange Parameters (in cm−1) for [M(B1)(B2)2]2+, Where M = Ni and Zn JAB Intracluster Exchange J12 J13 J14 J23 J24 J34 Intercluster Exchange J24

Figure 3. Structure of (a) the [M(B1)(B2)2]2+ cation and molecules (b) 14 and (c) 15.

M = Ni

M = Zn

−155 105 99 1 −13 −6

0 −17 −13

0

0

chemical analysis of the intracluster and intercluster exchange channels in 12·1.5H2O and 13·H2O. The calculated isotropic exchange parameters JAB characterize the pair exchange interactions in the crystal structures obtained at 293 K. The indices of the exchange parameters correspond to the numbering of paramagnetic fragments shown in Scheme 1. At

coordinated by the N atoms, forming a five-membered chelate ring, and nitronyl nitroxide B1 is coordinated by the N atom of the imidazole ring and the O atom of the nitronyl nitroxyl fragment. The Ni−N bond lengths are in the range of 2.057(3)−2.146(3) Å, Zn−N 2.096(2)−2.284(2) Å, Ni−O 2.076(2) Å, and Zn−O 2.148(2) Å. As in all of the abovediscussed compounds, in 12 and 13, the interatomic distances in the noncoordinated N−O groups are markedly shorter [1.259(3)−1.281(3) Å] than those in the coordinated groups [1.307(3) Å]. The data of Figure 3 show that, upon reaction with nitroxides containing a heterocyclic fragment in the side chain, as well as in reactions with the paramagnetic Schiff bases HLn and HAn studied earlier, NiII cannot provoke the redox transformations of the paramagnetic ligand. Therefore, the reactions of NiII salts with nitronyl nitroxide B1 form a complex containing nitronyl nitroxide [Ni(B1)2(NO3)2] (14) as a paramagnetic ligand (Figure 3b), while the reactions with imino nitroxide B2 give a complex with imino nitroxide [Ni(B2)2(H2O)2](NO3)2 (15; Figure 3c). A mixed-radical complex forms only when NiII reacts with a mixture of the paramagnetic ligands B1 and B2. There is no other way to obtain 12·1.5H2O. The structures of 12·1.5H2O, 13·H2O, 14, and 15 are described in detail in Figures S12 and S13 and Table S5. Here we mainly discussed the mixed-ligand complexes of the paramagnetic CoII and NiII ions. As mentioned above, CoII was chosen as a paramagnetic transition metal ion capable of easily changing its oxidation level; in contrast, NiII retains its oxidation level under the normal conditions. The zinc(II) complex 13, which is isostructural with the nickel(II) complex

Scheme 1. Scheme of the Paramagnetic Centers in 12· 1.5H2O and 13·H2O

293 K, the shortest molecular contacts (the distances between the NO atomic groups of fragments 2 and 4 of the neighboring [M(B1)(B2)2]2+ complexes) exceed 3 Å. The calculated exchange parameters indicate that the intercluster exchange interactions are weak. The intracluster exchange interactions between the nitronyl and imino nitroxide fragments (in the pairs of 23, 24, and 34) are also weak. They are comparable for both the nickel(II) and zinc(II) complexes. The unpaired electrons of the NiII ion form effective exchange channels with the coordinated organic paramagnetic fragments (the pairs of 12, 13, and 14); the exchange with the nitronyl nitroxide fragment is antiferromagnetic, and that with the imino nitroxide fragment is ferromagnetic. When describing the intracluster isotropic exchange in 12·1.5H2O, it suffices to consider only these exchange pairs because the inclusion of other intracluster exchange interactions in the spin Hamiltonian does not lead to any significant changes in its eigenvalues. The presence of exchange interactions with opposite signs in the exchange E

DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry cluster can lead to a “smoothing” of the μeff(T)60 dependence; indeed, it is observed in the temperature range from 50 to 300 K (Figure 4). At 300 K, μeff for 12·1.5H2O is 4.34 μB and

4. EXPERIMENTAL SECTION General Procedures. The IR spectrum of the compounds (4000− 400 cm−1) was recorded with a Bruker VECTOR 22 instrument in KBr pellets. Microanalysis was carried out on a HEKAtech GmbH EA3000 analyzer. The binuclear pivalates [Co2(Piv)4(HPiv)4(H2O)], [Ni2(Piv)4(HPiv)4(H2O)], and 2-(imidazol-4-yl)-4,4,5,5-tetramethyl2-imidazolin-3-oxide-1-oxyl (B1) and 2-(imidazol-4-yl)-4,4,5,5-tetramethyl-2-imidazolin-1-oxyl (B2) were prepared as described elsewhere.61−63 The synthesis of HA1, HA2, and HA3 is described elsewhere.64 Scheme 2 shows the stepwise synthesis of HL1, HL2, and HL3.

Scheme 2. Stepwise Synthesis of HL1, HL2, and HL3

Figure 4. Dependence of μeff on T for 12·1.5H2O. Solid line: theoretical curve.

gradually decreases at lower temperatures with a plateau at ∼4.03 μB below 100 K; when the temperature is lowered further below 50 K, μeff decreases to 2.01 μB at 2 K. The hightemperature value of μeff is close to the spin-only value of 4.12 μB for four noninteracting paramagnetic centers (three nitroxides with spin S = 1/2 and one NiII ion with spin S = 1) at g = 2. The theoretical curve constructed within the framework of the isotropic model for the four-center exchange cluster using the quantum-chemical values of exchange parameters reproduces the smooth decrease in μeff. Analysis of the experimental μeff(T) dependence using the isotropic ⃗ ·S⃗B1) − 2J2[(S⃗M·S⃗B2) + (S⃗M·S⃗B2′)] exchange model H = −2J1(SM for the four-center exchange cluster including intercluster exchange interactions (zJ′) in a molecular-field approximation gave the following optimum values of the g factor, J1/k, J2/k, and zJ′/k: 2.24, −97, 76, and −2.1 K, respectively. These values are effective because we neglected the temperature dependence of the exchange parameters determined by the thermally induced changes in the crystal structure. This also leads to a slightly exaggerated value of the g factor. Nevertheless, the optimum values of J1 and J2 agree well with the quantumchemical values obtained with the use of the high-temperature crystal structure.

Synthesis of 2-(2-Hydroxy-3-methoxy-5-nitrophenyl)4,4,5,5-tetramethylimidazolidine-1,3-diol. A mixture of 3-methoxy-5-nitrosalicylic aldehyde (0.25 g, 0.13 mmol) and 2,3bis(hydroxylamino)-2,3-dimethylbutane (0.2 g, 0.14 mmol) was dissolved at room temperature in methanol (MeOH; 5 mL). The resulting yellow solution was placed in a refrigerator, kept for 15−20 h, and then evaporated. The residue was washed with H2O, and the product was crystallized from toluene. Yellow crystals were collected, washed with toluene, and dried with an air current (0.27 g, 65% yield). Mp (in a sealed tube): 168−171 °C (dec). Anal. Calcd for C14H21N3O6: C, 51.4; H, 6.5; N, 12.8. Found: C, 51.6; H, 6.3; N, 12.7. IR (KBr): 3552, 3417, 3009, 2982, 2908, 1708, 1637, 1618, 1594, 1524, 1495, 1467, 1437, 1379, 1340, 1283, 1252, 1182, 1155, 1099, 1063, 1028, 1016, 991, 970, 915, 892, 856, 847, 814, 789, 745, 648, 616, 536, 480 cm−1. Synthesis of 2-(2-Hydroxy-3-methoxy-5-nitrophenyl)4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide-1-oxyl (HL1). NaIO4 (0.12 g, 0.56 mmol) was added to a solution of the 1,3dihydroxy derivative (0.13 g, 0.38 mmol) in a mixture of CH2Cl2 (10 mL), ethyl acetate (EtOAc; 10 mL), and H2O (5 mL). The reaction mixture was vigorously stirred at room temperature for 40 min. The end of the reaction was monitored by thin-layer chromatography on silica gel using EtOAc as an eluent. The organic layer was separated, and the solution was evaporated. The residue was washed with H2O and ethanol and purified by two recrystallizations from acetone. The blue needle crystals were filtered off and washed with cold acetone (0.07 g, 60% yield). Mp (in a sealed tube): 128−130 °C (dec). Anal. Calcd for C14H18N3O6: C, 51.8; H, 5.6; N, 12.9. Found: C, 51.7; H, 5.5; N, 12.8. IR (KBr): 3552, 3416, 3236, 3122, 2995, 2949, 2605, 2045, 1703, 1637, 1618, 1582, 1530, 1463, 1430, 1395, 1351, 1258, 1200, 1164, 1138, 1105, 1069, 981, 924, 886, 873, 852, 792, 761, 739, 667, 616, 539, 458 cm−1. The high-temperature value of the effective magnetic moment of HL1 (1.7 μB) agrees with the theoretical value, which confirms the magnetic purity of the phase (Figure S14). Synthesis of 2-(2-Hydroxy-3-methoxy-5-nitrophenyl)4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl (HL2 ). NaNO2 (0.025 g, 0.37 mmol) and glacial acetic acid (0.2 mL) were added, with vigorous stirring, to a solution of HL1 (0.12 g, 0.37 mmol) in CH2Cl2 (10 mL) and H2O (1 mL) at room temperature. The end of the reaction was monitored by thin-layer chromatography on silica gel using EtOAc as an eluent. The organic layer was separated, and the solution was evaporated. The residue was thoroughly washed with H2O from residual acetic acid; the product was extracted with CH2Cl2 and evaporated to dryness. The product was dissolved in CH2Cl2 and

3. SUMMARY The proposed method for the synthesis of multispin compounds with both nitronyl nitroxide and imino nitroxide introduced in the reaction with the metal has high synthetic potential. Importantly, the nitronyl nitroxide and imino nitroxide introduced in the reaction with the metal may have different sets of substituents in the paramagnetic organic molecule. This means that the number of multispin compounds that can be synthesized by using the suggested approach is almost infinite. This synthetic technique was demonstrated here on the synthesis of multispin compounds [Ni2(A1)(L2)2(Piv)(MeOH)], [Ni2(L1)(A2)2(Piv)(H2O)], [Co2(A1)(L2)2(Piv)(MeOH)], and [Co3(L1)2(A2)2(Piv)2], in which nitronyl nitroxide and imino nitroxide differ in the substituents in the phenyl ring. All of these compounds are the representatives of a new type of heterospin complexes, which had not been known before. F

DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

g, 0.037 mmol) in CH3CN (2 mL). The resulting solution was kept in an open flask for 2 or 3 days. The brown prismatic crystals of 3· 1.5CH3CN suitable for XRD analysis were filtered off, washed with CH3CN, and dried with an air current (0.025 g, 60% yield). Anal. Calcd for C69H92.5N13.5Co3O24: C, 49.6; H, 5.6; N, 11.3. Found: C, 49.3; H, 5.4; N, 11.4. Using an authentic mixture of HL2 and HL3 at a reagent ratio of 3:4:4 in the reaction with [Co2(Piv)4(HPiv)4(H2O)] under similar conditions, it is possible to obtain 3·1.5CH3CN with a 75% yield. In this case, however, the formation and crystallization of the complex are much faster; as a result, the product is isolated as a finely dispersed powder. Synthesis of [Co3(L1)2(L2)2(Piv)2]·4MePh (4·4MePh). A solution of a mixture of HL1 (0.018 g, 0.056 mmol) and HL2 (0.017 g, 0.056 mmol) in acetone (3 mL) was added at room temperature to a solution of [Co2(Piv)4(HPiv)4(H2O)] (0.04 g, 0.042 mmol) in acetone (1 mL). Heptane (2 mL) was slowly superimposed in layers on the reaction mixture without stirring, and this mixture was kept in a loosely closed flask for 2 days. The product was collected and recrystallized from an acetone−toluene mixture of solvents. The brown prismatic crystals of 4·4MePh were filtered off, washed with toluene, and dried in an air current (0.046 g, 80% yield). Anal. Calcd for the desolvated product C66H86N12Co3O26: C, 48.3; H, 5.3; N, 10.2. Found: C, 48.7; H, 4.7; N, 9.7. [Ni3(L2)2(Piv)4(H2O)2(HPiv)2]·0,5 C7H16 (5·0,5 C7H16). A solution of HL2 (0.021 g, 0.067 mmol) in acetone (3 mL) was added at room temperature to the solution of [Ni2(Piv)4(HPiv)4(H2O)] (0.05 g, 0.05 mmol) in heptane (2 mL). After 2 or 3 days, red crystals of 5·0,5 C7H16 suitable for XRD analysis were filtered off, washed with heptane, and dried in an air current (0.041 g, 80% yield). Anal. Calcd for the desolvated product C58H94N6Ni3O24: C, 48.5; H, 6.6; N, 5.9. Found: C, 48.9; H, 6.6; N, 5.8. Synthesis of trans-[Ni(L 2 ) 2 (HPiv) 2 ] (6). A mixture of [Ni2(Piv)4(HPiv)4(H2O)] (0.023 g, 0.024 mmol) and HL2 (0.03 g, 0.097 mmol) in acetone (4 mL) was dissolved at room temperature. Heptane (2 mL) was slowly superimposed in layers on the reaction mixture without stirring, and this mixture was kept in a loosely closed flask for 2 days. The wine-colored prismatic crystals of 6 suitable for XRD analysis were filtered off, washed with heptane, and dried in an air current (0.035 g, 80% yield). Anal. Calcd for C38H54N6NiO14: C, 52.0; H, 6.2; N, 9.6. Found: C, 51.9; H, 6.0; N, 9.3. The cis[Ni(L2)2(HPiv)2] isomer can be obtained if the same reaction is performed in a drier atmosphere, for example, in a desiccator with a 45% yield. Synthesis of [Ni2(L1)(L2)(Piv)2(HPiv)]·C7H16 (7·C7H16). A mixture of HL1 (0.014 g, 0.042 mmol) and HL2 (0.013 g, 0.042 mmol) in CH2Cl2 (2 mL) was added at room temperature to the solution of [Ni2(Piv)4(HPiv)4(H2O)] (0.04 g, 0.042 mmol) in CH2Cl2 (2 mL). Heptane (1.5 mL) was slowly superimposed in layers on the reaction mixture without stirring, and this mixture was kept in a loosely closed flask for 2 days. The wine-colored prismatic crystals of 7 suitable for XRD analysis were filtered off, washed with heptane, and dried in an air current (0.034 g, 70% yield). Anal. Calcd For C50H74N6Ni2O17: C, 52.3; H, 6.5; N, 7.3. Found: C, 51.5; H, 6.5; N, 7.5. Synthesis of [Ni2(A1)(L2)2(Piv)(MeOH)]·MeOH (8·MeOH). A mixture of HL2 (0.026 g, 0.084 mmol) and HA1 (0.012 g, 0.042 mmol) in CH2Cl2 (2 mL) was added at room temperature to a solution of [Ni2(Piv)4(HPiv)4(H2O)] (0.04 g, 0.042 mmol) in CH2Cl2 (2 mL). Then MeOH (1 mL) was added in layers to the reaction mixture. The solution was kept in an open flask for 2 or 3 days. The dark-brown crystals of 8·MeOH were filtered off, washed with MeOH, and dried in an air current (0.046 g, 90% yield). Anal. Calcd for C48H66N9Ni2O19: C, 48.4; H, 5.6; N, 10.6. Found: C, 47.8; H, 5.1; N, 10.6. Synthesis of [Ni2(L1)(A2)2(Piv)(H2O)]·2MePh·H2O (9·2MePh· H2O). A mixture of HL1 (0.012 g, 0.036 mmol) and HA2 (0.02 g, 0.072 mmol) in CH2Cl2 (2 mL) was added at room temperature to a solution of [Ni2(Piv)4(HPiv)4(H2O)] (0.034 g, 0.036 mmol) in CH2Cl2 (1 mL). Then toluene (2 mL) was added in layers to the reaction mixture, and this mixture was kept at −10 °C for 5 or 6 days. The dark-wine-colored platelike crystals of 9·2MePh·H2O suitable for XRD analysis were filtered off, washed with toluene, and dried in an air

transferred to a SiO2 column; then it was washed on a column with hexane and eluted with diethyl ether. The resulting solution was evaporated, and the product was recrystallized from an EtOAc−hexane mixture. The orange crystals of HL2 were filtered off and washed with hexane (0.06 g, 50% yield). Mp (in a sealed tube): 120−123 °C (dec). Anal. Calcd for C14H18N3O5: C, 54.5; H, 5.9; N, 13.6. Found: C, 54.4; H, 6.0; N, 14.2. IR (KBr): 3585, 3522, 3442, 3351, 3119, 2984, 2939, 1625, 1593, 1557, 1521, 1479, 1404, 1371, 1336, 1302, 1276, 1216, 1180, 1150, 1101, 1057, 979, 960, 894, 857, 794, 780, 743, 705, 681, 650, 583 cm−1. A complete X-ray diffraction (XRD) study was performed for HL2; its molecular and crystal structures were determined (Figure S1). The high-temperature value of the effective magnetic moment of HL2 (1.7 μB) confirms the magnetic purity of the phase because it agrees with the theoretical value for one electron (Figure S15). Synthesis of 2-(2-Hydroxy-3-methoxy-5-nitrophenyl)4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-3-oxide (HL3). A mixture of the 1,3-dihydroxy derivative (0.22 g, 0.67 mmol) and SeO2 (0.027 g, 0.24 mmol) in MeOH (50 mL) was stirred for 4 h at room temperature. The end of the reaction was monitored by thin-layer chromatography on silica gel using EtOAc as an eluent and aqueous NaIO4. The yellow spot corresponding to amidine oxide became orange (the color of HL2). After the reaction was completed, the solution was evaporated, and HL3 was purified on a silica gel column (EtOAc eluent). The solution containing the HL3 fraction was then evaporated, and the residue was recrystallized from EtOAc with a heptane addition (the EtOAc−heptane ratio was 2:1). The yellow crystals were filtered off and washed with heptane (0.14 g, 70% yield). Mp (in a sealed tube): 150 °C (dec). Anal. Calcd for C14H19N3O5: C, 54.3; H, 6.2; N, 13.6. Found: C, 54.1; H, 6.1; N, 13.5. IR (KBr): 3552, 3476, 3415, 3236, 3084, 2996, 2979, 2937, 2828, 2050, 1714, 1637, 1617, 1502, 1397, 1370, 1338, 1293, 1246, 1151, 1107, 1060, 985, 938, 911, 859, 826, 807, 781, 763, 747, 688, 622, 569, 472, 433 cm−1. For HL3, a complete XRD study was performed and its molecular and crystal structures were determined (Figure S2). Synthesis of [Co3(L2)2(Piv)4(H2O)2(HPiv)2]·C7H16 (1·C7H16). A solution of HL2 (0.017 g, 0.056 mmol) in acetone (3 mL) was added at room temperature to the solution of [Co2(Piv)4(HPiv)4(H2O)] (0.04 g, 0.042 mmol) in acetone (2 mL). Then heptane (2 mL) was slowly added in layers without stirring the solution. The reaction mixture was kept in an open flask for 2 days. The red platelike crystals of the product suitable for XRD analysis were filtered off, washed with heptane, and dried with an air current (0.038 g, 90% yield). Anal. Calcd for the partially desolvated product C60.3H99.3N6Co3O24: C, 49.3; H, 6.8; N, 5.7. Found: C, 49.3; H, 6.6; N, 5.8. Synthesis of [Co(L2)2(HPiv)2] (2). A solution of HL2 (0.039 g, 0.13 mmol) in CH2Cl2 (3 mL) was added at room temperature to the solution of [Co2(Piv)4(HPiv)4(H2O)] (0.03 g, 0.032 mmol) in CH2Cl2 (3 mL). Then heptane (2 mL) was slowly added in layers without stirring the reaction mixture. The resulting solution was kept in an open flask for 2 days. Red crystals of 2 suitable for XRD analysis were filtered off, washed with heptane, and dried with an air current (0.036 g, 65% yield). Anal. Calcd for C38H52N6CoO14: C, 52.1; H, 6.0; N, 9.6. Found: C, 51.7; H, 6.3; N, 9.4. Synthesis of [Co3(L2)2(L3)2(Piv)2]·Me2CO·H2O (3·Me2CO·H2O). A solution of a mixture of HL2 (0.02 g, 0.064 mmol) and HL3 (0.02 g, 0.064 mmol) in acetone (3 mL) was added at room temperature to a solution of [Co2(Piv)4(HPiv)4(H2O)] (0.061 g, 0.064 mmol) in acetone (2 mL). Then heptane (2 mL) was slowly added in layers without stirring the solution. The reaction mixture was kept in an open flask for 2 days. The resulting cluster crystals were filtered off and washed with heptane, and the product was recrystallized from a CH2Cl2−heptane mixture. The brown prismatic crystals of 3·Me2CO· H2O suitable for XRD analysis were filtered off, washed with heptane, and dried with an air current (0.033 g, 45% yield). Anal. Calcd for C69H96N12Co3O26: C, 49.1; H, 5.7; N, 10.0. Found: C, 49.2; H, 5.3; N, 10.2. Synthesis of [Co3(L2)2(L3)2(Piv)2]·1.5CH3CN (3·1.5CH3CN). A solution of HL2 (0.03 g, 0.098 mmol) in CH3CN (3 mL) was added at room temperature to a solution of [Co2(Piv)4(HPiv)4(H2O)] (0.035 G

DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry current (0.033 g, 70% yield). Anal. Calcd for the partially desolvated product C52H68N9Ni2O18: C, 51.0; H, 5.6; N, 10.3. Found: C, 50.9; H, 5.3; N, 9.5. Synthesis of [Co2(A1)(L2)2(Piv)(MeOH)]·MeOH (10·MeOH). The procedure for the synthesis of this compound (black prismatic crystals) is absolutely identical with that for 8·MeOH (0.047 g, 95% yield). Anal. Calcd for C48H66N9Co2O19: C, 48.4; H, 5.6; N, 10.6. Found: C, 47.3; H, 5.3; N, 10.4. Synthesis of [Co3(L1)2(A2)2(Piv)2]·2MePh (11·2MePh). The procedure for the synthesis of this compound (brown platelike crystals) is absolutely identical with that for 9·2MePh·H2O (0.037 g, 85% yield). Anal. Calcd for the partially desolvated product C71H90N12Co3O24: C, 51.0; H, 5.4; N, 10.1. Found: C, 50.5; H, 5.1; N, 9.9. Syntheses of [Ni(B1)(B2)2](NO3)2·1.5H2O (12·1.5H2O) and [Zn(B1)(B2)2](NO3)2·H2O (13·H2O). A solution of a mixture of B1 (0.077 g, 0.34 mmol) and B2 (0.143 g, 0.68 mmol) in H2O (5 mL) was added at room temperature to the solution of Ni(NO3)2·6H2O (0.1 g, 0.34 mmol) in H2O (2 mL). After 3 days, dark-blue crystals of 12· 1.5H2O (0.06 g, 20% yield) suitable for XRD analysis were filtered off. Anal. Calcd for C30H48N14NiO11.5: C, 42.5; H, 5.7; N, 23.1. Found: C, 41.6; H, 5.4; N, 22.9. A similar procedure gave blue crystals of 13·H2O using Zn(NO3)2·6H2O (0.08 g, 30% yield). Anal. Calcd for C30H47N14ZnO11: C, 42.6; H, 5.6; N, 23.2. Found: C, 42.2; H, 5.3; N, 22.6. Synthesis of [Ni(B1)2(NO3)2] (14). A solution of B1 (0.154 g, 0.68 mmol) in H2O (5 mL) was added through a paper filter at room temperature to a solution of Ni(NO3)2·6H2O (0.1 g, 0.34 mmol) in H2O (2 mL), and the mixture was kept for 3 days. Then crystals 14 (0.05 g, 25% yield) suitable for XRD analysis were filtered off. Anal. Calcd for C20H30N10NiO10: C, 38.2; H, 4.8; N, 22.3. Found: C, 37.8; H, 4.7; N, 22.1. [Ni(B2)2(H2O)2](NO3)2 (15) was prepared by a similar procedure. The results of the structural study of single crystals 1−15 are presented in the Supporting Information. Quantum-Chemical Calculations. The intra/intercluster isotropic exchange parameters were computed using the brokensymmetry methodology.65 The scheme proposed by Yamaguchi and co-workers66 was employed. All calculations were performed using the crystallographically determined geometries within the framework of the UB3LYP/TZVP computational procedure67−69 implemented in the Gaussian09 package.70 Magnetic Measurements. The magnetic susceptibility of the polycrystalline samples was measured with a Quantum Design MPMSXL SQUID magnetometer in the temperature range of 2− 300 K with a magnetic field of up to 5 kOe. Diamagnetic corrections were applied using Pascal constants. The effective magnetic moment was calculated as μeff(T) = [(3k/NAμB2)χT]1/2 ≈ (8χT)1/2. The magnetic properties were analyzed using the PHI program.71 XRD. XRD experiments were performed on SMART APEX II CCD and APEX DUO (Bruker AXS) diffractometers. To avoid the possible loss of solvate molecules during the XRD experiment, all of the single crystals were picked from under the layer of the mother solution and isolated from contact with the atmosphere using an epoxy resin. All of the structures were solved by direct methods and refined by full-matrix least-squares analysis in an anisotropic approximation for non-H atoms. The positions of the majority of H atoms were calculated. The methyl H atoms were refined isotropically in a rigid group approximation. All calculations were performed with the Bruker SHELXTL, version 6.14, and SHELXL-2014/7 program packages. The crystal data, details of the experiments, and selected bond lengths and angles for all compounds are given in Tables S1−S5.

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Detailed information regarding structural data and magnetic measurements including the field dependence of magnetization (PDF) Accession Codes

CCDC 1574492−1574508 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Victor Ovcharenko: 0000-0002-8280-8112 Artem Bogomyakov: 0000-0002-6918-5459 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the Russian Science Foundation (Project 15-13-30012). E.F. acknowledges the RFBR for financial support of the synthesis of nitroxides (Project 15-03-00488) and FANO RF. G.R. thanks FANO RF for partial support of the XRD studies. A.B. is grateful for partial support of the magnetochemical measurements (Grant MK8345.2016.3). E.Z. thanks the MES RF (Grant 4.5382.2017).

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02308. H

DOI: 10.1021/acs.inorgchem.7b02308 Inorg. Chem. XXXX, XXX, XXX−XXX

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