Ytterbium and Europium Complexes of Redox-Active Ligands

Aug 8, 2017 - Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism. Igor L. Fedushkin†‡ , Dmitriy S. Yambulatov...
0 downloads 0 Views 1MB Size
Download PDF
Article pubs.acs.org/IC

Ytterbium and Europium Complexes of Redox-Active Ligands: Searching for Redox Isomerism Igor L. Fedushkin,*,†,‡ Dmitriy S. Yambulatov,† Alexandra A. Skatova,† Evgeny V. Baranov,† Serhiy Demeshko,§ Artem S. Bogomyakov,∇ Victor I. Ovcharenko,∇ and Ekaterina M. Zueva⊥,∥ †

G.A. Razuvaev Institute of Organometallic Chemistry of the Russian Academy of Sciences, Tropinina Street 49, Nizhny Novgorod 603137, Russian Federation ‡ Koz’ma Minin Nizhny Novgorod State Pedagogical University, Ul’yanova Street 1, Nizhny Novgorod, 603002, Russian Federation § Institut für Anorganische Chemie, Universität Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany ∇ International Tomography Center, Siberian Branch of the Russian Academy of Sciences, Institutskaya Street 3a, Novosibirsk 630090, Russian Federation ⊥ A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientific Centre of the Russian Academy of Sciences, Arbuzov Street 8, Kazan 420088, Russian Federation ∥ Kazan Federal University, Kremlyovskaya Street 18, Kazan 420008, Russian Federation S Supporting Information *

ABSTRACT: The reaction of (dpp-Bian)EuII(dme)2 (3) (dpp-Bian is dianion of 1,2-bis[(2,6diisopropylphenyl)imino]acenaphthene; dme is 1,2-dimethoxyethane) with 2,2′-bipyridine (bipy) in toluene proceeds with replacement of the coordinated solvent molecules with neutral bipy ligands and affords europium(II) complex (dpp-Bian)EuII(bipy)2 (9). In contrast the reaction of related ytterbium complex (dpp-Bian)YbII(dme)2 (4) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in (dpp-Bian)YbIII(bipy)(bipy−̇ ) (10) − ytterbium(III) derivative containing both neutral and radical-anionic bipy ligands. Noteworthy, in both cases dianionic dpp-Bian ligands retain its reduction state. The ligand-centered redox-process occurs when complex 3 reacts with N,N′-bis[2,4,6trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad). The reaction product (dpp-Bian)EuII(mes-dad)(dme) (11) consists of two different redox-active ligands both in the radical-anionic state. The reduction of 3,6-ditert-butyl-4-(3,6-di-tert-butyl-2-ethoxyphenoxy)-2-ethoxycyclohexa-2,5-dienone (the dimer of 2-ethoxy-3,6di-tert-butylphenoxy radical) with (dpp-Bian)EuII(dme)2 (3) caused oxidation of the dpp-Bian ligand to radical-anion to afford (dpp-Bian)(ArO)EuII(dme) (ArO = OC6H2-3,6-tBu2-2-OEt) (12). The molecular structures of complexes 9−12 have been established by the single crystal X-ray analysis. The magnetic behavior of newly prepared compounds has been investigated by the SQUID technique in the range 2−310 K. The isotropic exchange model has been adopted to describe quantitatively the magnetic properties of the exchange-coupled europium(II) complexes (11 and 12). The best-fit isotropic exchange parameters are in good agreement with their density functional theorycomputed counterparts.

■

Bian),3 while treatment with Me2NC(S)S−S(S)CNMe2 affords GaIII derivative (dpp-Bian)Ga(S2CNMe2).2b Also, the isostructural rare earth metal complexes (dppBian)LnII(dme)2 (Ln = Sm, Eu, Yb) react in a different manner with the same reagents. Thus, one-electron oxidation of samarium complex (dpp-Bian)SmII(dme)2 (2)4 with halogen containing reagents occurs exclusively at the metal and results in SmIII derivatives. In contrast, the reactions of related europium compound (dpp-Bian)EuII(dme)2 (3)5 with halogen containing reagents resulted always in oxidation of the ligand, while the metal remains divalent. Ytterbium complex (dppBian)YbII(dme)2 (4)6a represents a special case. Oxidation of complex 4 with iodine affords the YbII derivative,6b similar to the europium analogue, while oxidation of complex 4 with

INTRODUCTION A use of redox-active ligands in coordination chemistry allows a significant extension of reactivity of the metal complexes, which might be useful in molecular catalysis.1 In a series of papers we have shown that a combination of redox-inactive metal ions such as MgII, CaIII, AlIII, and GaIII with dianion of 1,2-bis[(2,6diisopropylphenyl)imino]acenaphthene (dpp-Bian) results in unique redox-active metal complexes, whose reactivity is mainly ligand-centered.2 Further, the redox-active ligands can substantially expand a reactivity of the metal−metal bonded species as well as of the complexes, whose central metal ion possesses several oxidation states. For instance, digallane (dpp-Bian)Ga− Ga(dpp-Bian) (1) exhibits a dual redox behavior: oxidizing reagents can withdraw electron either from the metal−metal bond, or from the dpp-Bian dianions resulting in their oxidation to the radical-anions. Thus, oxidation of 1 with iodine results in a metal−metal bonded biradical (dpp-Bian)GaI−GaI(dpp© 2017 American Chemical Society

Received: May 26, 2017 Published: August 8, 2017 9825

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry

deep red crystal in 57% yield (Scheme 1). In contrast, related ytterbium complex (dpp-Bian)YbII(dme)2 (4) reacts in dme

chlorine containing reagents, as in the case of the samarium complex, produces the YbIII derivative.6c The latter crystallizes from benzene in three different crystalline morphologies. One of them at 293 K consists of mixed-valence dimer (dppBian)(dme)YbII(μ-Cl)2YbIII(dme)(dpp-Bian) (5a).6c Cooling down of the crystal to 150 K causes an intramolecular metal-toligand electron transfer to result in another redox-isomer, (dppBian)(dme)YbIII(μ-Cl)2YbIII(dme)dpp-Bian) (5b).6c This temperature-induced intramolecular electron transfer is accompanied by a remarkable alteration of (i) the magnetic moment of the complex; (ii) the metal-to-ligand bond lengths; (iii) the unit cell parameters; and (iv) the heat capacity. It is worth nothing that this intramolecular electron transfer is completely reversible. To date this is a sole example of redox-isomerism phenomenon in the lanthanide series,7 although intramolecular electron transfer in the lanthanide complexes of the redoxactive ligands in solution has been observed several times.8 For instance, for the bromine derivative [(dpp-Bian)YbII(μ-Br)(dme)]2 (6) a complete interconversion between YbII/YbII and YbIII/YbIII redox-isomers has been observed in a dme solution in the range from +5 to +95 °C.6a Searching for new redox-isomeric systems among the lanthanide series we prepared two derivatives [(dpp-Bian)EuII(μ-Cl)(dme)]2 (7) and [(dpp-Bian)EuII(μ-Br)(dme)]2 (8). Their behavior in solution (273−368 K) has been examined by electron absorption spectroscopy, in the solid state (0−300 K)by the measurement of magnetic susceptibility.5 Unfortunately, neither complex 7 nor compound 8 exhibits in solution and in the solid state any dynamic behavior that might be indicative for the intramolecular metal-to-ligand electron transfer. Continuing our study on reactivity of the rare earth metal complexes of the redox-active ligands and searching for new redox-isomeric systems in the lanthanide series we reacted complexes (dpp-Bian)EuII(dme) 2 (3) 5 and (dpp-Bian)YbII(dme)2 (4)6a with the substrates that are known to serve as redox-active ligands in the metal complexes. Here we report on the synthesis and characterization of europium and ytterbium complexes containing simultaneously of two different redox-active ligands(dpp-Bian)EuII(bipy)2 (9), (dpp-Bian)YbII(bipy)2 (10), (dpp-Bian)EuII(mes-dad)(dme) (11), and (dpp-Bian)(ArO)EuII(dme) (ArO = OC6H2-3,6-tBu2-2-OEt) (12). The charge distributions between the ligands and metals in these molecules have been deduced on the basis of structural and magnetic data.

Scheme 1. Different Reactivity of Complexes 3 and 4 towards bipy

with bipy instantly on mixing the reagents. Thus, an addition of even one molar equivalent of bipy causes the color change of the reaction mixture from red-brown to blue-green. This color is indicative for the presence in the product of dpp-Bian dianion coordinated to a trivalent metal ion. Addition of a second molar equivalent of bipy does not cause any alteration of the color of the mixture. Complex (dpp-Bian)YbII(bipy)(bipy−̇ ) (10) has been isolated from dme as green crystals in 75% yield (Scheme 1). Hence, the reaction of complex (dpp-Bian)YbII(dme)2 (4) with bipy in dme proceeds with the electron transfer from the metal to bipy and results in YbIII derivative containing both neutral and radical-anionic bipy ligands besides dpp-Bian dianion. The reduction potentials (E°) for LnIII + e → LnII increase on going from Eu to Yb and Sm (−0.35, −1.15, and −1.55 V correspondingly).10 In the hydrophobic room temperature melts [TFSI]−[EMI]+ and [TFSI]−[BMP]+ (TFSI = bistrifluoromethylsulfonylimide; EMI = 1-ethyl-3-methylimidazolium; BMP = 1-n-butyl-1-methylpyrrolidinium) the redox reactions LnIII/LnII for the salts LnIII(TFSI)3 (Ln = Eu, Yb, Sm) were observed at −0.3, −1.0, and −1.6 V (vs Ag/AgI) respectively.11 For comparison, the first reduction potential of bipy is −2.1 V in DMF (vs SCE)12 and −2.63 V in CH3CN (vs Fc+/Fc),13 while the first reduction potential of the dpp-Bian is −1.82 (vs Fc+/Fc in CH3CN).13 Paramagnetic character of compounds 9 and 10 prevents its characterization by the NMR spectroscopy. The EPR spectroscopy cannot be used for the investigation of complex 10 containing the radical-anionic bipy ligand due to the presence of the paramagnetic YbIII ion. However, the IR spectroscopy provides for some useful information on the products 9 and 10. In the IR spectra of complexes 9 and 10 (see Supporting Information) the absorption bands at 1314 and 1310 cm−1 correspond to stretching vibrations of single C−N bonds in the dianion of dpp-Bian. In both spectra the bands associated with stretching vibrations of double CN bonds in neutral dppBian (1642, 1652, and 1671 cm−1) as well as sesquialteral C−N bonds in radical-anion of dpp-Bian (1500−1550 cm−1) are absent. The presence of the neutral bipy ligand in complex 9 is confirmed by the absorption at 1576 and 1594 cm−1 (ring

■

RESULTS AND DISCUSSION Synthesis and Characterization of (dpp-Bian)EuII(bipy)2 (9), (dpp-Bian)YbII(bipy)(bipy−̇ ) (10), (dppBian)Eu II (mes-dad)(dme) (11), and (dpp-Bian)Eu II (OC6H2-3,6-But2-2-OEt)(dme) (12). As far as we know compound (tBu-dad)2SmIII(bipy)9 is the only example of the lanthanide complex containing two different redox-active ligands. Besides the redox-isomerism associated with reversible metal-to-ligand electron transfer, another type of electronic transformation may be expected in such systemsintramolecular redox process between two ligands. Furthermore, a variety of heterospin systems can be designed by combining different radical-anionic ligands in lanthanide complexes. Addition of two molar equivalents of 2,2′- bipyridine (bipy) to the brown solution of (dpp-Bian)EuII(dme)2 (3) in dme at 293 K does not cause any visible changes. However, replacement of dme with toluene leads to a color change to red. Complex (dpp-Bian)EuII(bipy)2 (9) has been isolated as a 9826

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry

In 2005 Abakumov and co-workers have shown15 that 4-oxo3-ethoxy-2,5-di-tert-butyl-2,5-cyclohexadienyl ether of 2-ethoxy3,6-di-tert-butylphenol [2ArO] in solution in the temperature range 200−350 K can reversibly dissociate to aroxyl radicals ArO·. The latter serve well as oxidizing reagents for cathecholate,16 diazadiene,17 and amidophenolate18 metal derivatives. For instance, the reactions of tin cathecholate (3,6-tBu2C6H2O2)SnII and tin amidophenolate [3,6-tBu2C6H2(NAr)O]SnII (Ar = 2,6-iPr2C6H3) with [2ArO] afford ArO-SnII derivatives supported by radical-anionic semiquinonato and amidophenolato ligands.18 Bearing in mind a high reactivity of aroxyl radicals ArO· toward metal complexes of redox-active ligands we reacted dimer [2ArO] with complex 3. Addition of 0.5 mol equiv of [2ArO] to solution of complex 3 at 278 K caused an instant color change from brown to red, thus indicating formation of dpp-Bian radical-anion. Crystallization from Et2O affords complex (dpp-Bian)(ArO)EuII(dme) (ArO = OC6H2-3,6-tBu2-2-OEt) (12) (52%) (Scheme 3). In contrast to the above-mentioned tin complexes, that undergo symmetrization upon crystallization, compound 12 is stable in solution as well as in the solid state. It is worth noting that compound 12 is the first structurally characterized europium complex, which consists simultaneously of redoxactive and aryloxide ligands. Molecular Structures. The molecular structures of the complexes 9−12 were determined by single-crystal X-ray diffraction and are depicted in Figures 2−5. The crystal data collections and structure refinement details are listed in Table 1. Selected bond lengths and bond angles are presented in Table 2. Coordination environment of metal atoms in complexes 9 and 10 is very similar: the six nitrogen atoms form an irregular octahedron, which tends toward trigonal prismatic geometry. The Ln−N bonds in pairs in each N-ligand in 9 and 10 are very close to each other, thus indicating the chelating character of the ligands. Despite a similarity of the coordination environment, complexes 9 and 10 differ in electronic structure. The reduction state of the ligands dpp-Bian and bipy can be deduced from their metrical parameters. The N(1)−C(1) and N(2)−C(2) bonds in dpp-Bian ligands in 9 (av. 1.383 Å) and 10 (av. 1.400 Å) are well compared with those in other metal complexes of dpp-Bian dianion, for instance, in starting (dppBian)EuII(dme)2 (av. 1.387 Å)5 and (dpp-Bian)YbII(dme)2 (1.386 Å).6 These values are longer in comparison with those bonds in lanthanide complexes with the dpp-Bian radical-anion, for instance, in europium complexes [(dpp-Bian)EuII(μ-Cl)(dme)]2 (av. 1.33 Å)5 and Cp*2EuII(p-MeO-Bian) (av. 1.340 Å).19 The C−C and C−N distances in two bipy ligands in complex 9 are the same within the estimated standard deviations and indicate the neutral state of both ligands. For instance, the central C−C bonds in bipy ligands in 9 (1.493(5)

deformation vibrations). Besides this absorption the IR spectrum of compound 10 consists of the band at 948 cm−1, which corresponds to the ring deformation vibrations of the radical-anion of bipy.12 Further, the difference in the reduction states of the ligands in complex 10 is confirmed by the UV VIS spectroscopy (Figure 1).

Figure 1. Absorption spectrum of complex 10 (dme, 298 K).

The absorption in the range 450−550 nm indicates the presence of the bipy radical-anion, while dpp-Bian dianion has absorption with maxima at ca. 700 nm. The long-wave absorption (750−1100 nm) is indicative of the ligand-to-ligand charge transfer (LLCT) between two different bipy ligands. This low energy charge transfer has been observed earlier in the bis-bipyridine lanthanum complex LaIIII2(bipy)2(dme).14 As for the reaction with 2,2′-bipyridyl, complex 3 reacts with MesNC(H)C(H)NMes (mes-dad) in noncoordinating toluene, but not 1,2-dimetoxyethane. The reaction is accompanied by the color change from brown to red. Crystallization of the product from hexane affords red crystals of (dpp-Bian)EuII(mes-dad)(dme) (11) (39%). The spectroscopic and crystallographic data indicate that in the course of the reaction the electrons are redistributed between two Nligands, while the metal retained its divalent state (Scheme 2). Interestingly, the related diazadiene ArNC(Me)C(Me) NAr (Ar = 2,6-iPr2C6H3) is not reactive toward 3 in dme as well as in toluene. In the IR spectrum of compound 11 the absorption associated with the stretching vibrations of CN bonds in neutral dpp-Bian (1671 and 1652 cm−1) and mes-dad (1620 cm−1) are absent. The presence of the dpp-Bian radical-anion in complex 11 is manifested by the absorption at 1515 cm−1. No color alteration of toluene solution of complex 11 could be observed in the range 173−373 K. Also, the differential scanning calorimetry (100−300 K) of the solid sample of compound 11 does not disclose any effects associated with the phase transition in the course of intramolecular electron transfer. Scheme 2. Reactivity of Complex 3 towards mes-dad

9827

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry Scheme 3. Reactivity of Complex 3 towards [2ArO]

in EuII(PBI)3(bipy)(H2O) (PBI = 3-phenyl-4-benzoyl-5-isoxazolone) (2.627(3) and 2.686(2) Å), which contains neutral bipy.21 Relatively short Eu−N(dpp-Bian) bonds (2.454(2) and 2.472(2) Å) reflect a stronger interaction of EuII cation with the dianionic dpp-Bian ligand in comparison to interaction with neutral bipy ligands. In contrast to compound 9, complex 10 consists of three chelating ligands in different reduction states. As a consequence the Yb−N distances become longer on going from dianion of dpp-Bian to radical-anionic bipy and further to neutral bipy: the differences in Yb−N bonds on going from the dpp-Bian ligand to the bipy ligands in complex 10 are 0.12 Å ([dpp-Bian]2−/[bipy]1−) and 0.21 Å ([dpp-Bian]2−/[bipy]0). The Yb−N(5) and Yb−N(6) bonds in complex 10 (2.364(3) and 2.337(3) Å correspondingly) are close to those values in Cp*2YbIII(bipy) (Yb−N, av. 2.32 Å), which consists of the bipy radical-anion.12 The different reduction state of two bipy ligands in complex 10 is further confirmed by the ligand geometries. Thus, the C(51)−C(52) bond (1.437(5) Å) in the bipy radical-anion is shorter than the C(41)−C(42) bond in

Figure 2. Molecular structure of 9. Thermal ellipsoids are drawn at 40% probability level. Hydrogen atoms are omitted.

and 1.492(4) Å) correspond to that bond in free 2,2′-bipyridine (1.490(3) Å).20 Further, the Eu−N(bipy) bonds in complex 9 (av. 2.694(2) Å) are only slightly elongated in comparison with those bonds

Table 1. Crystal Data and Structure Refinement Details for Compounds 9, 10, 11, and 12

formula Mr [g mol−1] crystal system space group a [Å] b [Å] c [Å] α [°] β [°] γ [°] V [Å3] Z ρcalc, [g cm−3] μ [mm−1] F(000) crystal size, [mm3] θmin/θmax [°] index ranges

reflections collected independent reflections Rint max/min transmission data/restraints/parameters GOF on F2 final R indices [I > 2σ(I)] R indices (all data) largest diff. peak/hole [e Å−3]

9×C7H8

10×C4H10O2

11

12×0.5C4H10O

C63H64EuN6 1057.16 trigonal R3̅ 34.1488(3) 34.1488(3) 24.1302(2) 90 90 120 24369.3(5) 18 1.297 1.203 9846 0.40 × 0.40 × 0.20 3.00−30.00 −47 ≤ h ≤ 48 −48 ≤ k ≤ 48 −33 ≤ l ≤ 33 172081 15749 0.0609 1.00000/0.83829 15749/54/671 1.038 0.0406/0.1039 0.0589/0.1117 1.517/−1.011

C60H66N6O2Yb 1076.23 monoclinic P21/c 16.8084(8) 15.0286(7) 21.1934(10) 90 106.183(1) 90 5141.5(4) 4 1.390 1.868 2216 0.46 × 0.27 × 0.04 1.85−26.00 −19 ≤ h ≤ 20 −18 ≤ k ≤ 17 −26 ≤ l ≤ 20 30282 10068 0.0377 0.9290/0.4803 10068/1/630 0.993 0.0335/0.0769 0.0566/0.0828 1.615/−0.531

C60H74EuN4O2 1035.19 monoclinic P21/c 12.3792(9) 23.7452(17) 19.1167(14) 90 108.357(1) 90 5333.3(7) 4 1.289 1.221 2164 0.45 × 0.15 × 0.10 2.40−28.00 −16 ≤ h ≤ 16 −31 ≤ k ≤ 31 −25 ≤ l ≤ 25 64583 12866 0.0301 0.8876/0.6095 12866/0/626 1.023 0.0240/0.0580 0.0287/0.0597 1.158/−0.377

C58H80EuN2O4.5 1029.20 monoclinic P21/c 14.0694(8) 20.5044(11) 19.6151(11) 90 107.4574(10) 90 5398.0(5) 4 1.266 1.208 2164 0.31 × 0.24 × 0.10 2.39−28.00 −18 ≤ h ≤ 18 −27 ≤ k ≤ 27 −25 ≤ l ≤ 25 64543 12986 0.0375 0.8240/0.6432 12986/99/676 1.048 0.0265/0.0613 0.0361/0.0639 1.129/−0.470

9828

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry Table 2. Selected Bond Lengths [Å] and Angles [°] in 9, 10, 11, and 12 C(1)−C(2) N(1)−C(1) N(2)−C(2) Ln(1)−N(1)a Ln(1)−N(2) Ln(1)−N(3) Ln(1)−N(4) Ln(1)−N(5) Ln(1)−N(6) Ln(1)−O(1) Ln(1)−O(2) N(1)−Ln(1)−N(2) a

9

10

11

12

1.405(3) 1.376(3) 1.390(3) 2.454(2) 2.472(2) 2.703(2) 2.676(2) 2.741(2) 2.654(2)

1.389(5) 1.403(4) 1.398(4) 2.215(3) 2.239(3) 2.456(3) 2.426(3) 2.364(3) 2.337(3)

1.4485(19) 1.3287(19) 1.3339(18) 2.6124(12) 2.6162(12) 2.5801(13) 2.5834(12)

1.443(3) 1.326(2) 1.322(2) 2.5820(15) 2.5652(15)

72.32(7)

79.76(10)

66.05(4)

2.7241(12) 2.3077(13) 67.00(5)

Ln = Eu for 9, 11, and 12; M = Yb for 10.

Figure 3. Molecular structure of 10. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted. Figure 4. Molecular structure of 11. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted.

the second bipy ligand (1.481(5) Å) as well as in free bipy (1.490(3) Å).20 In complex 11 the europium atom reveals a distorted octahedral environment with atoms N(1) and O(2) located in axial positions (angle N(1)−Eu(1)−O(2) is 158.41°). The C(1)−N(1) and C(2)−N(2) bond lengths (1.3287(19) and 1.3339(18) Å) point out a radical-anionic state of the dpp-Bian ligand in complex 11 (cf. [(dpp-Bian)EuII(μ-Cl)(dme)]2, av. 1.33 Å5; Cp*2EuII(p-MeO-Bian), av. 1.340 Å19). As far as we know EuII complexes with radical-anionic dad ligands have not yet been reported. The C−N(dad) bonds in 11 (N(3)−C(37) 1.323(2) and N(4)−C(38) 1.331(2) Å) are longer compared to those bonds in neutral dad ligand in Cp*2EuII(tBu-dad) (1.249(5) and 1.256(6) Å)22 but shorter than respective C−N bonds in the dianionic dad ligand in [(dpp-dad)YIII(μCl)(thf)2]2 (1.416(4) and 1.419(5) Å)23 and thus fall into the range of the bond lengths expected for the dad radicalanion. Besides, the Eu−N(dad) and Eu−N(dpp-Bian) distances are close to each other and, thus, indicate that both N-ligands in complex 11 act as radical-anions (see Table 2). Compound 12 represents an octahedral EuII complex. The electron transfer from the dpp-Bian dianion to [2ArO] generates an aryloxide group that acts as a chelating ligand to the europium center. An inspection of the bond lengths within the NCCN fragment (C(1)−N(1), 1.326(2); C(2)−N(2), 1.322(2); C(1)−C(2) 1.443(3) Å) clearly indicates the radicalanionic state of the dpp-Bian ligand. As expected Eu(1)−O(2) bond (2.3077(13) Å) is much shorter than the Eu(1)−O(1) bond (2.7241(12) Å), thus reflecting covalent and coordinative bonding correspondingly. The Eu−N bonds in complex 12 are

Figure 5. Molecular structure of 12. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted.

well compared with those bonds in complex 11, which also contains dpp-Bian radical-anion. Solid State Magnetic Susceptibilities. Figures 6−9 show the μeff vs. T dependencies for the complexes 9, 10, 11, and 12 correspondingly. The μeff value for the complex 9 is about 7.77 μB in the temperature range 15−310 K and is in a good agreement with theoretical spin-only one 7.94 μB for one EuII ion (4f7, 8S7/2 ground state with g = 2). Small decreasing of the μeff value below 10 K can be attributed to saturation effects and/or weak antiferromagnetic intermolecular interaction. Indeed, analysis of the μeff(T) dependence, taking into account 9829

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry

dependence indicates the presence of antiferromagnetic exchange interactions between paramagnetic centers.

Figure 6. Solid state magnetic susceptibility, μeff vs. T plot of complex 9. The solid line shows the calculated curve fit. Figure 8. Solid state magnetic susceptibility, μeff vs. T plot of complex 11. The solid line shows the calculated curve fit.

intermolecular exchange interactions (zJ′) in the mean-field approximation, allows the best fit parameter values g = 1.95 and zJ′ = −0.002 cm−1 to be obtained. Density functional theory (DFT)-computed spin density distribution confirms the presence of divalent europium in complex 9: the spin density is concentrated on the metal center (atomic spin density on europium is equal to 6.97); that is, the intramolecular metal-toligand electron transfer is absent. In contrast, the magnetic moment of ytterbium complex 10 at 300 K (4.30 μB) indicates the presence of the YbIII center (4f13, 2F7/2) and radical-anionic 2,2′-bipy ligand (S = 1/2), since the alternative explanation with the YbII center (4f14, 1S0) and neutral ligand (S = 0) would lead to the zero magnetic moment. For a situation with noninteracting YbIII ion and ligand radical, the predicted magnetic moments should vary from 4.8 to 4.2 μB between 300 and 5 K.12 However, the μeff for ytterbium complex 10 decreases very strongly from 4.3 to 1.3 μB on cooling down from 300 to 2 K, indicating a significant influence of the antiferromagnetic coupling between YbIII and ligand radical on magnetic behavior.24

To assess the relative importance of exchange interactions we performed DFT analysis of the intramolecular exchange pathways. According to the computed J values (JEu‑R1 = −9 cm−1, JEu‑R2 = −13 cm−1, JR1‑R2 = −31 cm−1), the energies of exchange interactions between the metal ion and coordinated radicals (R1 = dpp-Bian, R2 = mes-dad) are close to each other, but much smaller than that between the two radicals. Although the JR1‑R2 pathway is a dominant exchange interaction, magnetic functions are rather insensitive to the value of JR1‑R2. In contrast, the JEu‑R pathways have a significant impact on magnetic behavior, especially in the low-temperature region. The computed J values generate an S = 5/2 ground state, while a decrease in |JEu‑R| leads to the inversion of low-lying S = 5/2 and S = 7/2 states, generating a minimum on the μeff(T) curve in the low-temperature region. In accordance with the results of DFT calculations, the experimental μeff(T) dependence was ⃗ analyzed using the isotropic model H = −2JEu−R(S⃗R1 · S⃗Eu + SEu · S⃗R1) − 2JR−R (S⃗R1 · S⃗R2) for the three-center exchange cluster where JEu‑R1 and JEu‑R2 are adopted to be equal to each other. The set of best-fit parameters that emerges from this analysis of the μeff(T) dependence is g = 1.90, JEu‑R = −2.4 cm−1, JR1‑R2 = −16.1 cm−1. The μeff value for the complex 12 is 7.83 μB at 300 K and decreases with lowering the temperature down to 6.25 μB at 2 K with small inflection at ∼10 K. The high-temperature value of μeff is somewhat lower than the theoretical spin-only one 8.12 μB for two uncoupled paramagnetic centersone EuII ion with S = 7/2 and one radical with S = 1/2. The decreasing of μeff with lowering the temperature indicates a domination of antiferromagnetic exchange. The experimental μeff(T) dependence is well described within the framework of the isotropic model H = −2JEu−R (S⃗Eu · SR⃗ ) for the two-center exchange cluster with the best-fit values of g = 1.94 and JEu‑R = −3.7 cm−1. DFT calculations lead to JEu‑R = −11 cm−1. Note that DFTcomputed values of JEu‑R for the dpp-Bian ligand (−9 cm−1 in 11 and −11 cm−1 in 12) correlate with the Eu−N(dpp-Bian) distances in both complexes (see Table 2)the shorterdistance exchange pathway offers a stronger coupling. Thus, the μeff(T) dependencies for the complexes 9, 11, and 12 provide unambiguous evidence for the presence of divalent europium because the μeff values of EuII and EuIII are 7.94 and 3.5 μB, correspondingly.10a

Figure 7. Solid state magnetic susceptibility, μeff vs. T plot of complex 10.

The μeff value for the complex 11 is 7.81 μB at 300 K and decreases with lowering the temperature down to 7.22 μB at 17 K. Below 17 K μeff slightly increases to 7.34 μB at 6 K and drops to 6.90 at 2 K. The high-temperature value of μeff is somewhat lower than the theoretical spin-only one 8.31 μB for three uncoupled paramagnetic centers−one EuII ion with S = 7/2 and two radicals with spins S = 1/2. The character of the μeff(T) 9830

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry

acenaphthenequinone with 2,6-diisopropylaniline (both from Aldrich) in acetonitrile under reflux. 4-Oxo-3-ethoxy-2,5-di-tert-butyl-2,5-cyclohexadienyl ether of 2-ethoxy-3,6-di-tert-butylphenol was obtained according to the literature procedure.15 N,N′-Bis[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad) was synthesized as described earlier.26 2,2′-Bipyridine was purchased from Aldrich and purified by sublimation in a vacuum. Complexes (dpp-Bian)EuII(dme)2 (3)5 and (dpp-Bian)YbII(dme)2 (4)6a were synthesized reacting dpp-Bian with an excess of the corresponding metal in dme at reflux as previously reported. The yields of the products were calculated from the amount of the dpp-Bian used (0.5 g, 1.0 mmol) in the syntheses. Synthesis of (dpp-Bian)EuII(bipy)2 (9). To a solution of (dppBian)EuII(dme)2 (in situ from 0.5 g (1.0 mmol) of dpp-Bian and europium shavings) in dme (30 mL) 0.31 g (2.0 mmol) of bipy was added. Replacement of the solvent from dme to toluene (45 mL) caused the color change of solution from brown to red. The reaction mixture was heated at 90 °C for 30 min and then kept at ambient temperature for 24 h. Compound 9 was isolated from toluene solution as deep-red cubic crystals (0.6 g, 57%). Mp: 205 °C. Anal. Calcd for C63H64EuN6 (1057.16): C, 71.58; H, 6.10. Found: C, 70.32; H, 6.06. IR (Nujol): 1643 w, 1594 m, 1576 m, 1514 w, 1463 s, 1438 m, 1412 s, 1314 vs, 1253 m, 1211 w, 1171 m, 1155 w, 1142 w, 1099 m, 1057 w, 1041 w, 1006 m, 997 m, 920 m, 893 w, 849 w, 812 w, 793 w, 756 vs, 695 w, 678 w, 653 w, 639 w, 620 m, 600 w, 538 w, 510 w, 492 w, 466 w cm−1. (dpp-Bian)YbII(bipy)(bipy−̇ ) (10). To a dme solution (40 mL) of compound (dpp-Bian)YbII(dme)2 (in situ from 0.3 g (0.6 mmol) of dpp-Bian and ytterbium shavings) 0.19 g (1.2 mmol) of bipy was added. The color of the reaction mixture turned instantly from redbrown to blue-green. Crystallization from dme at ambient temperature afforded complex 8 (0.65 g, 75%) as green crystals. Mp: >290 °C. Anal. Calcd for C56H56N6Yb × C4H10O2 (1076.23): C, 66.96; H, 6.18. Found: C, 66.78; H, 6.09. IR (Nujol): 1613 w, 1597 m, 1580 m, 1547 m, 1498 s, 1428 s, 1356 w, 1310 s, 1296 s, 1282 m, 1252 s, 1217 m, 1176 m, 1148 m, 1133 w, 1120 w, 1102 m, 1082 w, 1055 w, 1016 w, 1004 m, 948 s, 918 s, 889 w, 866 w, 813 m, 800 m, 780 m, 760 s, 698 w, 681 w, 667 w, 646 w, 626 w, 621 w, 607 w, 550 w, 512 w, 480 w cm−1. UV−vis (293 K, dme): 445, 480, 500, 700, 780, 877 nm. (dpp-Bian)EuII(mes-dad)(dme) (11). To a solution of (dppBian)EuII(dme)2 (in situ from 0.5 g (1.0 mmol) of dpp-Bian and europium shavings) in dme (30 mL) 0.29 g (1.0 mmol) of N,N′bis[2,4,6-trimethylphenyl]-1,4-diaza-1,3-butadiene (mes-dad) was added. Replacement of the solvent from dme to toluene (40 mL) caused the color change of solution from brown to red. The reaction mixture was heated at 90 °C for 30 min. Then toluene was evaporated under reduced pressure. To the residue left hexane (80 mL) was added and the mixture was frozen in liquid nitrogen within 5 min. After that the frozen mixture slowly warmed up at ambient temperature. In 48 h compound 11 was isolated from hexane solution as red needle-like crystals (0.4 g, 39%). Mp: 270 °C. Anal. Calcd for C60H74EuN4O2 (1035.19): C, 69.61; H, 7.21. Found: C, 68.40; H, 7.10. IR (Nujol): 1674 w, 1590 w, 1576 w, 1543 w, 1515 s, 1426 w, 1410 m, 1351 w, 1340 w, 1311 m, 1260 vs, 1185 s, 1148 w, 1113 m, 1069 s, 1013 w, 959 w, 935 m, 904 w, 884 w, 860 m, 842 w, 819 m, 795 m, 770 s, 756 s, 667 w, 601 w, 536 w, 513 w cm−1. (dpp-Bian)(ArO)EuII(dme) (12). To a solution of (dpp-Bian)EuII(dme)2 (in situ from 0.5 g (1.0 mmol) of dpp-Bian and europium shavings) in dme (30 mL) 0.25 g (0.5 mmol) of 4-oxo-3-ethoxy-2,5-ditert-butyl-2,5-cyclohexadienyl ether of 2-ethoxy-3,6-di-tert-butylphenol was added. The color of the reaction mixture turned instantly from brown to red. A complete evaporation of toluene and dissolving the residue in diethyl ether (45 mL) afforded a red solution. Compound 12 was isolated from the concentrated (20 mL) ether solution as red rhombic crystals (0.54 g, 52%). Mp: 185 °C. Anal. Calcd for C58H80EuN2O4.5 (1029.20): C, 67.69; H, 7.83. Found: C, 66.55; H, 7.49. IR (Nujol): 1585 w, 1578 w, 1520 vs, 1443 w, 1414 m, 1356 w, 1318 m, 1297 m, 1281 w, 1251 m, 1216 w, 1207 w, 1185 vs, 1160 w, 1143 w, 1101 m, 1078 w, 1059 s, 1031 w, 1015 w, 985 w, 971 m, 933 m, 921 w, 854 m, 842 w, 821 m, 795 m, 791 m, 774 s, 760 s, 676 m, 657 w, 618 w, 601 w, 545 w, 534 w cm−1.

Figure 9. Solid state magnetic susceptibility, μeff vs. T plot of complex 12. The solid line shows the calculated curve fit.

■

CONCLUSION Here we presented a new illustration of how redox-active ligands can extend reactivity of the lanthanide complexes. Depending on the substrate complex (dpp-Bian)EuII(dme)2 (3) can be either involved in simple ligand exchange reactions, e.g., the reaction between 3 and bipy, or it can act as reducing agent using for reduction of the substrate the dpp-Bian dianion, as for instance in the reaction of 3 with mes-dad or [2ArO]. Although the EuII center itself can be oxidized to the trivalent state, in the system [(dpp-Bian)2−Eu2+] the oxidation always begins with the ligand. This situation is opposite to that observed with the [(dpp-Bian)2−Sm2+] system,4 where the electron within the first oxidation step is withdrawn from the metal. The ytterbium system [(dpp-Bian)2−Yb2+] represents a special case: due to the closeness of the reduction potentials [YbIII + e → YbII] and [(dpp-Bian)1− + e → (dpp-Bian)2−] this system can be oxidized with X0 to two redox-isomers [(dppBian)1−Yb2+X1−] and [(dpp-Bian)2−Yb3+X1−]. We demonstrated here that oxidation of (dpp-Bian)YbII(dme)2 (4) with bipy results in YbIII derivative 10, which does not reveal the intramolecular ligand-to-metal electron transfer to result in the YbII redox isomer. Finally, the redox-active ligands are useful instruments for preparation of the lanthanide complexes with radical ligands, as for instance complexes 11 and 12. Their magnetic behavior is not as simple as that of complexes containing of the lanthanide ion as a sole magnetic center. The use of isotropic exchange model provides a reasonable description of the magnetic behavior for the exchange-coupled europium(II) complexes (11 and 12). All exchange interactions are antiferromagnetic, and the best-fit isotropic exchange parameters are in a good agreement with their DFT-computed counterparts.

■

EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out under a vacuum using glass ampules. Toluene, diethyl ether, and 1,2dimethoxyethane were dried over sodium/benzophenone, hexane− over sodium. The IR spectra were recorded on an FSM-1201 spectrometer in a Nujol. The magnetic susceptibilities in the solid state were determined using a SQUID MPMSXL (Quantum Design) at 5 kOe in the range from 2 to 310 K for the complexes 9, 11, and 12, and from 2 to 300 K for compound 10. The magnetic susceptibility data were corrected for the diamagnetic contribution. The effective magnetic moments were calculated as μeff = [3kχT/(NAμB2)]1/2. The PHI program was used to fitting and simulation of the experimental magnetic properties.25 Melting points were measured in sealed capillaries. The dpp-Bian was prepared by the condensation of 9831

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry Quantum-Chemical Calculations. For europium complexes, the UB3LYP calculations were performed using the def2-TZVP basis set27 with the Stuttgart-Dresden effective core potential for europium28 as implemented in the ORCA 4.0.0 package.29 The isotropic exchange parameters were computed using the broken symmetry methodology. The scheme proposed by Yamaguchi and co-workers was employed.30 The use of the second-order Douglas−Kroll−Hess Hamiltonian with the SARC-DKH-TZVP basis set (for europium)31 and the DKH-def2TZVP basis set (for other elements) leads to similar results, but J values are larger in magnitude (JEu‑R1 = −19 cm−1, JEu‑R2 = −26 cm−1, JR‑R = −60 cm−1 for 11 and JEu‑R = −22 cm−1 for 12). All calculations were performed using the crystallographically determined geometries. X-ray Crystallography. The X-ray diffraction data were collected on an Agilent Xcalibur E (9), a SMART APEX (for 10) and a Bruker D8 Quest (for 11 and 12) diffractometers (MoKα radiation, ω-scan technique, λ = 0.71073 Å). The intensity data were integrated by CrysAlisPro32 for 9 and by SAINT33,34 for 10 and 11, 12 respectively. All structures 9−12 were solved by direct methods and were refined on Fhkl2 using the SHELXL package.35 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions and were refined in the riding model. SCALE3 ABSPACK36 for 9 and SADABS37 for 10−12 were used to perform area-detector scaling and absorption corrections. Crystals of 9, 10, and 12 contain disordered solvate molecules of toluene, DME, and diethyl ether correspondingly. The main crystallographic data and structure refinement details for 9−12 are presented in Table 1. CCDC 1544865 (9), 1544866 (10), 1544867 (11), and 1544868 (12) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/data_request/cif from the Cambridge Crystallographic Data Centre.

■

Chem. Rev. 2010, 254, 1580−1588. (c) Dzik, W. I.; van der Vlugt, J. I.; Reek, J. N. H.; de Bruin, B. Ligands that Store and Release Electrons during Catalysis. Angew. Chem., Int. Ed. 2011, 50, 3356−3358. (d) Chirik, P. J. Preface: Forum on Redox-Active Ligands. Inorg. Chem. 2011, 50, 9737−9740. (e) van der Vlugt, J. I. Cooperative Catalysis with First-Row Late Transition Metals. Eur. J. Inorg. Chem. 2012, 2012, 363−375. (f) Lyaskovskyy, V.; de Bruin, B. Redox NonInnocent Ligands: Versatile New Tools to Control Catalytic Reactions. ACS Catal. 2012, 2, 270−279. (g) Broere, D. L. J.; Plessius, R.; van der Vlugt, J. I. New avenues for ligand-mediated processes − expanding metal reactivity by the use of redox-active catechol, o-aminophenol and o-phenylenediamine ligands. Chem. Soc. Rev. 2015, 44, 6886− 6915. (h) Fedushkin, I. L.; Morozov, A. G.; Rassadin, O. V.; Fukin, G. K. Addition of Nitriles to Alkaline Earth Metal Complexes of 1,2Bis[(phenyl)imino]acenaphthenes. Chem. - Eur. J. 2005, 11, 5749− 5757. (2) (a) Fedushkin, I. L.; Morozov, A. G.; Chudakova, V. A.; Fukin, G. K.; Cherkasov, V. K. Magnesium(II) Complexes of the dpp-BIAN Radical-Anion: Synthesis, Molecular Structure, and Catalytic Activity in Lactide Polymerization. Eur. J. Inorg. Chem. 2009, 2009, 4995− 5003. (b) Fedushkin, I. L.; Nikipelov, A. S.; Skatova, A. A.; Maslova, O. V.; Lukoyanov, A. N.; Fukin, G. K.; Cherkasov, A. V. Reduction of Disulfides with Magnesium(II) and Gallium(II) Complexes of a Redox-Active Diimine Ligand. Eur. J. Inorg. Chem. 2009, 2009, 3742− 3749. (c) Fedushkin, I. L.; Skatova, A. A.; Lukoyanov, A. N.; Chudakova, V. A.; Dechert, S.; Hummert, M.; Schumann, H. Reactions of (dpp-BIAN)Mg(thf)3 complex (dpp-BIAN is 1,2-bis{(2,6diisopropylphenyl)imino}acenaphthene) with halogen-containing reagents. Russ. Chem. Bull. 2004, 53, 2751−2762. (d) Fedushkin, I. L.; Skatova, A. A.; Cherkasov, V. K.; Chudakova, V. A.; Dechert, S.; Hummert, M.; Schumann, H. Reduction of Benzophenone and 9(10H)-Anthracenone with the Magnesium Complex [(2,6-iPr2C6H3Bian)Mg(thf)3]. Chem. - Eur. J. 2003, 9, 5778−5783. (e) Fedushkin, I. L.; Skatova, A. A.; Hummert, M.; Schumann, H. Reductive Isopropyl Radical Elimination from (dpp-Bian)Mg-iPr(Et2O). Eur. J. Inorg. Chem. 2005, 2005, 1601−1608. (f) Fedushkin, I. L.; Khvoinova, N. M.; Skatova, A. A.; Fukin, G. K. Oxidative addition of phenylacetylene through C-H bond cleavage to form the MgII-dpp-Bian Complex: Molecular structure of [Mg{dpp-Bian(H)}(C°CPh)(thf)2] and its diphenylketone insertion product [Mg(dpp-Bian){OC(Ph2)C°CPh}(thf)]. Angew. Chem., Int. Ed. 2003, 42, 5223−5226. (g) Fedushkin, I. L.; Skatova, A. A.; Fukin, G. K.; Hummert, M.; Schumann, H. Addition of Enolisable Ketones to (dpp-Bian)Mg(THF)3 [dpp-Bian = 1,2Bis{(2,6-diisopropylphenyl)imino}acenaphthene]. Eur. J. Inorg. Chem. 2005, 2005, 2332−2338. (h) Fedushkin, I. L.; Makarov, V. M.; Rosenthal, E. C. E.; Fukin, G. K. Single-Electron-Transfer Reactions of α-Diimine dpp-BIAN and Its Magnesium Complex (dppBIAN)2−Mg2+(THF)3. Eur. J. Inorg. Chem. 2006, 2006, 827−832. (i) Razborov, D. A.; Lukoyanov, A. N.; Baranov, E. V.; Fedushkin, I. L. Addition of phenylacetylene to a magnesium complex of monoiminoacenaphtheneone (dpp-mian). Dalton Trans. 2015, 44, 20532− 20541. (j) Dodonov, V. A.; Skatova, A. A.; Cherkasov, A. V.; Fedushkin, I. L. Synthesis and structure of bischelate gallium complexes (dpp-Bian)Ga(acac) and (dpp-Bian)Ga(2,2′-bipy) (dppBian is 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene. Russ. Chem. Bull. 2016, 65, 1171−1177. (3) (a) Fedushkin, I. L.; Skatova, A. A.; Dodonov, V. A.; Chudakova, V. A.; Bazyakina, N. L.; Piskunov, A. V.; Demeshko, S.; Fukin, G. K. Digallane with Redox-Active Diimine Ligand: Dualism of ElectronTransfer Reactions. Inorg. Chem. 2014, 53, 5159−5170. (b) Fedushkin, I. L.; Skatova, A. A.; Dodonov, V. A.; Yang, X.-J.; Chudakova, V. A.; Piskunov, A. V.; Demeshko, S.; Baranov, E. V. Ligand “Brackets” for Ga−Ga Bond. Inorg. Chem. 2016, 55, 9047−9056. (4) Fedushkin, I. L.; Maslova, O. V.; Hummert, M.; Schumann, H. One- and Two-Electron-Transfer Reactions of (dpp-Bian)Sm(dme)3. Inorg. Chem. 2010, 49, 2901−2910. (5) Fedushkin, I. L.; Skatova, A. A.; Yambulatov, D. S.; Cherkasov, A. V.; Demeshko, S. V. Europium complexes with 1,2-bis(arylimino)-

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01344. IR spectra and electrochemical data (PDF) Accession Codes

CCDC 1544865−1544868 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.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Igor L. Fedushkin: 0000-0003-2664-2266 Artem S. Bogomyakov: 0000-0002-6918-5459 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The research was supported by the Russian Foundation for Basic Research (Grant No. 15-43-02568). V.I.O. and A.S.B. thank the RSF for particular support of magnetochemical measurements (Grant 15-13-30012).

■

REFERENCES

(1) (a) Grützmacher, H. Cooperating Ligands in Catalysis. Angew. Chem., Int. Ed. 2008, 47, 1814−1818. (b) Kaim, W.; Schwederski, B. Non-innocent ligands in bioinorganic chemistry: An overview. Coord. 9832

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833

Article

Inorganic Chemistry acenaphthenes: a search for redox isomers. Russ. Chem. Bull. 2015, 64, 38−43. (6) (a) Fedushkin, I. L.; Maslova, O. V.; Baranov, E. V.; Shavyrin, A. S. Redox Isomerism in the Lanthanide Complex [(dpp-Bian)Yb(DME)(μ-Br)]2 (dpp-Bian = 1,2-Bis[(2,6-diisopropylphenyl)imino]acenaphthene. Inorg. Chem. 2009, 48, 2355−2537. (b) Fedushkin, I. L.; Maslova, O. V. et al. unpublished results. (c) Fedushkin, I. L.; Maslova, O. V.; Morozov, A. G.; Dechert, S.; Demeshko, S.; Meyer, F. Genuine Redox Isomerism in a Rare-Earth-Metal Complex. Angew. Chem., Int. Ed. 2012, 51, 10584−10587. (7) (a) Tezgerevska, T.; Alley, K. G.; Boskovic, C. Valence tautomerism in metal complexes: Stimulated and reversible intramolecular electron transfer between metal centers and organic ligands. Coord. Chem. Rev. 2014, 268, 23−40. (b) Sato, O. Dynamic molecular crystals with switchable physical properties. Nat. Chem. 2016, 8, 644− 656. (8) Trifonov, A. A. Reactions of Ytterbocenes with Diimines: Steric Manipulation of Reductive Reactivity. Eur. J. Inorg. Chem. 2007, 2007, 3151−3167. (9) Petrovskaya, T. V.; Fedyushkin, I. L.; Bochkarev, M. N.; Shumann, G.; Veimann, R. Synthesis and crystal structure of the Sm(ButNCHCHNBut)2(bpy) complex. Russ. Chem. Bull. 1997, 46, 1766−1768. (10) (a) Bunzli, J.-C. G. Benefiting from the Unique Properties of Lanthanide Ions. Acc. Chem. Res. 2006, 39, 53−61. (b) Mikheev, N. B.; Auerman, L. N.; Rumer, I. A.; Kamenskaya, A. N.; Kazakevich, M. Z. The anomalous stabilisation of the oxidation state 2+ of lanthanides and actinides. Russ. Chem. Rev. 1992, 61, 990−998. (11) Yamagata, M.; Katayama, Y.; Miura, T. J. Electrochemical behavior of samarium, europium, and ytterbium in hydrophobic roomtemperature molten salt systems. J. Electrochem. Soc. 2006, 153, E5− E9. (12) Schultz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.; Andersen, R. A. Coordination of 2,2′-Bipyridyl and 1,10-Phenanthroline to Substituted Ytterbocenes: An Experimental Investigation of Spin Coupling in Lanthanide Complexes. Organometallics 2002, 21, 460− 472. (13) Budnikova, Y. G. et al. unpublished results (for details see Supporting Information). (14) Bochkarev, M. N.; Fedushkin, I. L.; Nevodchikov, V. I.; Cherkasov, V. K.; Schumann, H.; Hemling, H.; Weimann, R. Synthesis and ESR-characterization of radical anion complexes of lanthanum. Xray crystal structure of the mixed bipy, bipy−1 complex of lanthanum(III) [LaI2(bipy)(bipy)(DME)]: evidence for an interligand charge transfer. J. Organomet. Chem. 1996, 524, 125−131. (15) Abakumov, G. A.; Cherkasov, V. K.; Nevodchikov, V. I.; Druzhkov, N. O.; Fukin, G. K.; Kursky, Y. A.; Piskunov, A. V. The oxidation of 2-alkoxy-3,6-di-tert-butylphenols. The reversible dimerization of 2-alkoxy-3,6-di-tert-butylphenoxy radicals. Tetrahedron Lett. 2005, 46, 4095−4097. (16) Abakumov, G. A.; Cherkasov, V. K.; Piskunov, A. V.; Lado, A. V.; Fukin, G. K.; Abakumova, L. G. New lead(II) catecholate and osemiquinone complexes. Russ. Chem. Bull. 2006, 55, 1146−1154. (17) Piskunov, A. V.; Aivaz’yan, I. A.; Cherkasov, V. K.; Abakumov, G. A. New paramagnetic N-heterocyclic stannylenes: An EPR study. J. Organomet. Chem. 2006, 691, 1531−1534. (18) Chegerev, M. G.; Piskunov, A. V.; Maleeva, A. V.; Fukin, G. K.; Abakumov, G. A. Multiple Reactivity of SnII Complexes Bearing Catecholate and o-Amidophenolate Ligands. Eur. J. Inorg. Chem. 2016, 2016, 3813−3821. (19) Vasudevan, K. V.; Cowley, A. H. Synthesis and structures of 1,2bis(imino)acenaphthene (BIAN) lanthanide complexes that involve the transfer of zero, one, or two electrons. Chem. Commun. 2007, 3464−3466. (20) Chisholm, M. N.; Huffman, J. C.; Rothwell, I. P.; Bradley, P. G.; Kress, N.; Woodruff, W. H. Bis(2,2′-bipyridyl)diisopropoxymolybdenum(II). Structural and spectroscopic evidence for molybdenum-to-bipyridyl π* bonding. J. Am. Chem. Soc. 1981, 103, 4945−4947.

(21) Biju, S.; Ambili Raj, D. B.; Reddy, M. L. P.; Jayasankar, C. K.; Cowley, A. H.; Findlater, M. Dual emission from stoichiometrically mixed lanthanide complexes of 3-phenyl-4-benzoyl-5-isoxazolonate and 2,2′-bipyridine. J. Mater. Chem. 2009, 19, 1425−1432. (22) Moore, J. A.; Cowley, A. H.; Gordon, J. C. Mediating Oxidation States in Decamethyleuropocene Complexes. The Role of the Diazabutadiene Fragment. Organometallics 2006, 25, 5207−5209. (23) Panda, T. K.; Kaneko, H.; Pal, K.; Tsurugi, H.; Mashima, K. Salt Metathesis and Direct Reduction Reactions Leading to Group 3 Metal Complexes with a N,N-Bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene Ligand and Their Solid-State Structures. Organometallics 2010, 29, 2610−2615. (24) (a) Booth, C. H.; Walter, M. D.; Kazhdan, D.; Hu, Y.-J.; Lukens, W. W.; Bauer, E. D.; Maron, L.; Eisenstein, O.; Andersen, R. A. Decamethylytterbocene Complexes of Bipyridines and Diazabutadienes: Multiconfigurational Ground States and Open-Shell Singlet Formation. J. Am. Chem. Soc. 2009, 131, 6480−6491. (b) Lukens, W. W.; Magnani, N.; Booth, C. H. Application of the Hubbard Model to Cp*2Yb(bipy), a Model System for Strong Exchange Coupling in Lanthanide Systems. Inorg. Chem. 2012, 51, 10105−10110. (25) Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S. PHI: A powerful new program for the analysis of anisotropic monomeric and exchange-coupled polynuclear d- and fblock complexes. J. Comput. Chem. 2013, 34, 1164−1175. (26) Arduengo, A. J.; Krafczyk, R.; Schmutzler, R.; Craig, H. A.; Goerlich, J. R.; Marshall, W. J.; Unverzagt, M. Imidazolylidenes, imidazolinylidenes and imidazolidines. Tetrahedron 1999, 55, 14523− 14534. (27) (a) Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297−3305. (b) Weigend, F. Accurate Coulomb-fitting basis sets for H to Rn. Phys. Chem. Chem. Phys. 2006, 8, 1057−1065. (28) Dolg, M.; Stoll, H.; Preuss, H. Energy adjusted abinitio pseudopotentials for the rare earth elements. J. Chem. Phys. 1989, 90, 1730−1734. (29) Neese, F. The ORCA program system. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2012, 2, 73−78. (30) Shoji, M.; Koizumi, K.; Kitagawa, Y.; Kawakami, T.; Yamanaka, S.; Okumura, M.; Yamaguchi, K. A general algorithm for calculation of Heisenberg exchange integrals J in multispin systems. Chem. Phys. Lett. 2006, 432, 343−347. (31) Pantazis, D. A.; Neese, F. J. All-Electron Scalar Relativistic Basis Sets for the Lanthanides. J. Chem. Theory Comput. 2009, 5, 2229− 2238. (32) CrysalisPro, Software Package Data Collection, Reduction, and Correction Program; Agilent Technologies, 2012. (33) SAINT Plus, Data Reduction and Correction Program, v. 6.45a; Bruker AXS: Madison, WI, 2003. (34) SAINT, Data Reduction and Correction Program, v. 8.34b; Bruker AXS: Madison, WI, 2014. (35) Sheldrick, G. M. SHELXTL, Structure Determination Software Suite, v. 6.14; Bruker AXS: Madison, WI, 2003. (36) SCALE3 ABSPACK, Empirical absorption correction, CrysalisPro− Software Package; Agilent Technologies: Yarnton, U.K, 2012. (37) Sheldrick, G. M. SADABS, Bruker/Siemens Area Detector Absorption Correction Program, v. 2014/5; Bruker AXS: Madison, WI, 2014.

9833

DOI: 10.1021/acs.inorgchem.7b01344 Inorg. Chem. 2017, 56, 9825−9833