Lanthanum Complexes with a Diimine Ligand in Three Different

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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Lanthanum Complexes with a Diimine Ligand in Three Different Redox States Igor L. Fedushkin,*,‡ Anton N. Lukoyanov,‡ and Evgeny V. Baranov‡ ‡

G. A. Razuvaev Institute of Organometallic Chemistry, Russian Academy of Sciences, Tropinina 49, Nizhny Novgorod 603137, Russian Federation S Supporting Information *

ABSTRACT: The reduction of 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene (dpp-Bian) with an excess of La metal in the presence of iodine (dpp-Bian/I2 = 2/1) in tetrahydrofuran (thf) or dimethoxyethane (dme) affords lanthanum(III) complexes of dpp-Bian dianion: deep blue [(dpp-Bian)2−LaI(thf)2]2 (1, 84%) was isolated by crystallization of the product from hexane, while deep green [(dpp-Bian)LaI(dme)2] (2, 93%) precipitated from the reaction mixture in the course of its synthesis. A treatment of complex 1 with 0.5 equiv of I2 in thf leads to the oxidation of the dpp-Bian dianion to the radical anion and results in the complex [(dppBian)1−LaI2(thf)3] (3). Addition of 18-crown-6 to the mixture of 1 and NaCp* (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) in thf affords ionic complex [(dpp-Bian)2−La(Cp*)I][Na(18-crown-6)(thf)2] (4, 71%). In the absence of crown ether the alkali metal salt-free complex [(dpp-Bian)2−LaCp*(thf)] (5, 67%) was isolated from toluene. Reduction of complex 1 with an excess of potassium produces lanthanum−potassium salt of the dpp-Bian tetra-anion {[(dpp-Bian)4−La(thf)][K(thf)3]}2 (6, 68%). Diamagnetic compounds 1, 2, 4, 5, and 6 were characterized by NMR spectroscopy, while paramagnetic complex 3 was characterized by the electron spin resonance spectroscopy. Molecular structures of 2−6 were established by single-crystal X-ray analysis.

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transfer in compound [(dpp-Bian)GaI(Py)]19 in solution under removal of the coordinated pyridine is accompanied by the Ga−Ga bond formation (Scheme 1).

INTRODUCTION As ketiminate and amidinate ligands, diimines serve well for stabilization of metal−metal bonds as well as of metals low oxidation states. Using a ligand of diimine type 1,2-bis[(2,6diisopropylphenyl)imino]acenaphthene (dpp-Bian), we prepared a series of main-group metal complexes featuring metal−metal bonds, for example, Zn−Zn,1 Ga−Ga,2,3 Ga− Zn,2,4 Ga−La,4 Al−Al,5 and Ga−Li6 as well as Si7 and Ge8,9 analogues of N-heterocycling carbenes (NHCs). In contrast to ketiminate and amidinate ligands the dpp-Bian is redox-active: in the complexes with metals it may exist in several redox states. Generally, alteration of the redox states of redox-active ligands may occur either in the course of attack of the ligand by another reagent10−23 or within intramolecular electron transfer between the metal and the ligand as, for instance, in the reaction of digallane [(dpp-Bian)2−Ga−Ga(dpp-Bian)2−] with chromium hexacarbonyl: electron transfer from the ligand to the metal results in gallium carbenoide [(dpp-Bian)1−Ga:], which substitutes one of the CO groups to afford [(dppBian)1−Ga: → Cr(CO)5].24 When electron transfer occurs without changing of a composition of the complex it is called redox-isomerism or valence tautomerism. While the tautomeric transformations are well-documented for many transition-metal complexes the only example of a definitive solid-state redox isomerism in a rare-earth-metal complex is the dinuclear ytterbium complex [(dpp-Bian)Yb(μ-Cl)(dme)]225 (dme = dimethoxyethane). Interestingly, the ligand-to-metal electron © XXXX American Chemical Society

Scheme 1. Intramolecular Electron Transfer Accompanied with the Gallium−Gallium Bond Formation and Cleavage

An exceptional feature of the dpp-Bian is its ability to delocalize over its π systems (diimine plus naphthalene) up to four extra electrons without changing of an atom connectivity order. While the redox processes involving metal complexes of the dpp-Bian mono- and dianions are numerous, the reactions occurring with a participation of the dpp-Bian tri- and tetranions have not yet been reported except for the preparation of the sodium salts, [Na+nLm(dpp-Bian)n−] (n = 1, 2, 3 or 4; L = diethyl ether or tetrahydrofuran (thf)).26 Only few metal ions can sustain a strong reduction power of the dppBian tri- and tetra-anions. Among them are alkaline, alkaline Received: December 11, 2017

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DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

compound 2 was characterized by NMR and IR spectroscopy as well as by elemental analysis. A treatment of complex 1 with 0.5 equiv of I2 caused oxidation of the dpp-Bian dianion to radical anion resulting in compound [(dpp-Bian)LaI2(thf)3] (3) (Scheme 3). When the reagents are mixed at ambient temperature the solution turns initially black, but at 60 °C it changes gradually to red.

earth, and the rare-earth metals. Many transition-metal ions undergo reduction even by the dpp-Bian dianion.27−29 Because of the electropositive character and accessibility of two oxidation states several elements of the lanthanide series represent unique objects to investigate interplay between (i) low oxidation states and (ii) coordination of the organic ligands bearing a large negative charge, for instance, (dpp-Bian)3−Ln3+ versus (dpp-Bian)2−Ln2+. In this paper we report on a facile synthesis of the lanthanum(III) complexes of the dpp-Bian dianion, [(dppBian)LaI(L)2] (L = thf, 1; L = dme, 2). To test whether the low-valent lanthanum complexes may be obtained we studied reduction of compound [(dpp-Bian)LaI(thf)2]2 (1) with potassium metal. To demonstrate an ability of the dpp-Bian dianion to undergo oxidation being still coordinated to La(III) we reacted complex 1 with an iodine.

Scheme 3. Oxidation of Compound 1 with Iodine

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RESULTS AND DISCUSSION Synthesis and Characterization of Compounds 1−6. Complex [(dpp-Bian)LaI(thf)2]2 (1) was prepared reacting dpp-Bian with an excess of La metal (shavings) in the presence of iodine (dpp-Bian-to-I2 ratio is 1 to 0.5) in thf (Scheme 2). A

Compound 3 was isolated from the concentrated reaction mixture as large deep-red crystals. In thf in the presence of lanthanum metal compound 3 affords starting compound 1. Compound 3 is paramagnetic due to the presence of the dppBian radical anion. Besides the coupling of unpaired electron to protons the electron spin resonance (ESR) signal of compound 3 (Figure 1) reveals the coupling to the lanthanum (139La, I =

Scheme 2. Syntheses of Compounds 1 and 2

Figure 1. ESR spectrum of compound 3 (thf, 293 K): (a) experimental spectrum (g = 2.00145); (b) simulated spectrum (ai(139La) = 0.83, ai(2 × 14N) = 0.41, ai(4 × 1H) = 0.10, ai(2 × 127I) = 0.04 mT, ΔH = 0.10 mT).

synthesis of lanthanide complexes reacting metallic lanthanides with organic substrates in the presence of oxidizers as, for instance, of iodine, has been well-exploited.30−38 But, a preparation of the lanthanide derivatives of redox-active ligands directly from the lanthanide metals is limited to few examples.11,20,39 At ambient temperature, the the stirred mixture turned first brown. Heating at 60 °C resulted in blue solution. The product [(dpp-Bian)LaI(thf)2]2 (1) exhibits good solubility in thf and could not be isolated even from highly concentrated solution. A replacement of the solvent with hexane resulted in crystals of complex 1. Unfortunately, blue thin plates of complex 1 were not suitable for the single-crystal X-ray analysis. Nevertheless, compound 1 was well-characterized by NMR spectroscopy and elemental analysis (see Experimental Section). According to analytical data product 1 consists of two thf molecules per [(dpp-Bian)LaI] unit. We suggest compound 1 is dimeric and consists of bridging iodine ligands. The use of dme instead of thf as the solvent in the reaction afforded compound [(dppBian)LaI(dme)2] (2) (Scheme 2). In contrast to complex 1, compound 2 is poorly soluble in dme and precipitates from the reaction mixture already in the course of the synthesis. Also, addition of dme to the thf solution of complex 1 resulted in precipitation of compound 2. The X-ray quality crystals of compound 2 were isolated from toluene. Further diamagnetic

7/2, 99.91%), nitrogen (14N, I = 1, 99.64%), and iodine (127I, I = 5/2, 100%) nuclei. The 139La hyperfine coupling constant (0.83 mT) in the spectrum of 3 can be compared with those in the lanthanum 2,2′-bipyridyl complexes LaI2(bipy)2(dme) (0.74 mT), LaI2(bipy)2(thf)2 (293 K: 0.59 mT; 150 K: 0.87 mT), and LaI2(bipy)(dme)2 (0.70 mT).40 To set a coordination sphere of the metal in complex 1 free from solvent molecules we reacted complex 1 with Cp*Na (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl). The reaction mixture turned from deep blue to bright green. But, a lack of NaI precipitate allowed us to suggest the formation of ate complex. A treatment of the green solution with of 18-crown-6 did not cause any visible changes. Crystallization from thf resulted in complex [(dpp-Bian)LaCp*I][Na(18-crown-6)(thf)2] (4) (Scheme 4). Besides the signals of dpp-Bian dianion the 1H NMR spectrum of complex 4 consists of a singlet (1.95 ppm, 15 H) of CH3 groups of Cp* ligand as well as a singlet (3.47 ppm, 24 H) of 18-crown-6. Also, compound 4 was characterized by IR spectroscopy and elemental analysis. Molecular structure of 4 was determined by the single-crystal B

DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 4. Syntheses of Compounds 4 and 5

Scheme 5. Synthesis of Compound 6

In contrast, the reduction of dpp-Bian in diethyl ether with three molar equivalents of sodium metal results in the formation of paramagnetic [(dpp-Bian)Na3(Et2O)2]2, which can be easily isolated from solution in a crystalline form. A delocalization of the negative charge over the acenaphthene plane in complex 6 caused a strong high-field shift of the signals of the aromatic protons in the 1H NMR spectrum (4.78 (2 H), 3.45 (2 H), and 2.99 (2 H) ppm). To the best of our knowledge the coordination of lanthanide ion to a tetraanionic ligand having a negative charge delocalized over extended πsystem has not yet been reported. Thus, on the one hand, complex 6 is the first lanthanide complex with an organic tetraanion. On the other hand, currently there is a sole report on the synthesis of lanthanum(II) derivatives. Thus, reduction of Cp3La (Cp = η5-1,3-(Me3Si)2C5H3) by potassium in the presence of [18]crown-6 or [2,2,2]cryptand produced thermally stable mononuclear crystalline lanthanate(II) salts. The La+2 oxidation state in these complexes was confirmed both in solution (electron paramagnetic resonance (EPR)) and the solid state (EPR, superconducting quantum interference device (SQUID), and X-ray diffraction) and was supported by a computational study.44 Molecular Structures 2−6. Molecular structures of compounds 2, 3, 4, 5, and 6 were determined by the singlecrystal X-ray analysis and are depicted in Figures 2, 3, 4, 5, and 6, correspondingly. Crystal data and structure refinement details for 2−6 are collected in Table 1, and selected bond lengths and angles for 2−6 are listed in Table 2. The lanthanum complexes 2−6 consist of the dpp-Bian ligand in three different

X-ray analysis (vide infra). Further we attempted separation of sodium iodide from the product, formed initially on mixing complex 1 with Cp*Na. Thus, a replacement of thf with toluene is accompanied by a color change from green to blue. Then, sodium iodide precipitated slowly from the blue toluene solution. Within several days compound [(dpp-Bian)LaCp*(thf)] (5) crystallizes from the concentrated toluene solution in a form of deep blue crystals (Scheme 4). As for complex 4 the 1H NMR spectrum of compound 5 consists of two septets (3.75 and 3.14 ppm, each 2 H) and four doublets (1.45, 1.42, 1.20, and 1.05 ppm, each 6 H) of isopropyl groups of dpp-Bian ligand. Its aromatic protons (12 H) give rise to the signals in the expected region (7.36−6.34 ppm). A nonequivalence of the isopropyl groups (above and below the metallacycle) indicate that the Cp* and thf ligands are not exchanging their positions in solution (at least within NMR time scale). The same is true for Cp* and iodine ligands in complex 4. Accordingly, complexes 4 and 5 possess only one mirror plane that is orthogonal to the metallacycle and bisects the N−La−N ligand. In contrast, free dpp-Bian and many of its metal complexes possess also a mirror plane that is coinciding with the diimine plane. A unique feature of the dpp-Bian is its ability to exist in four different redox states (1−, 2−, 3−, and 4−), which are accessible in the course of reduction of dpp-Bian with alkali metals.26,41 Because of their electropositive character the lanthanides may also form complexes with ligands bearing a multiple negative charge, for example, naphthalene42 or bipyridyl43 dianions. These anions reveal strong reducing character toward transition and nontransition metal ions but not toward the lanthanides. Continuing the study of the metal complexes with organic polyanions, we aimed the preparation of the lanthanum complexes with dpp-Bian tri- and tetraanions. Before trying their synthesis in a preparative scale, we decided to follow by ESR spectroscopy the reduction of complex 1 with an excess of potassium metal. A treatment of complex 1 with potassium in thf caused a gradual color change from deep blue to deep brown. However, no paramagnetic species, for example, radical trianion of dpp-Bian, could be detected within the reduction process. We suggest that intermediate [(dppBian)3−LaI(thf)n]K(thf)n disproportionates rapidly to starting 1 and La/K salt of the dpp-Bian tetra-anion {[(dppBian)La(thf)][K(thf)3]}2 (6) (Scheme 5).

Figure 2. Molecular structure of 2. Thermal ellipsoids are drawn at 30% probability level. Hydrogen atoms are omitted. C

DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. Molecular structure of 3. Thermal ellipsoids are drawn at 50% probability level. Hydrogen atoms are omitted. Figure 5. Molecular structure of 5. Thermal ellipsoids are drawn at 30% probability level. Hydrogen atoms are omitted.

Figure 4. Molecular structure of anion [(dpp-Bian)La(Cp*)I] in compound 4. Thermal ellipsoids are drawn at 30% probability level. Hydrogen atoms are omitted.

redox states: radical anion (3), dianion (2, 4, and 5), and tetraanion (6). The reports on the lanthanide complexes with three different anionic forms of one ligand are rather rare. Among them are iminoquinone derivatives45 as well as 2,2′bipyridyl complexes.40,43,46 Since the lowest unoccupied molecular orbital (LUMO−1) in free dpp-Bian is localized mainly over the diimine fragment, the states [dpp-Bian]0, [dppBian]1−, and [dpp-Bian]2− can be recognized comparing the C−N and C−C bond lengths within metallacycles. On moving from neutral dpp-Bian to its radical anion and further to the dianion the C(1)−N(1) and C(2)−N(2) bonds become longer, while the C(1)−C(2) bond becomes shorter. Thus, in the free dpp-Bian both C−N bonds (1.282(3) Å)47 are ca. 0.055 Å shorter than in dpp-Bian radical anion in 3 (average 1.335 Å). In its turn the latter is ca. 0.055 Å shorter in comparison to those values in dpp-Bian dianion in compounds 2, 4, and 5 (average values: 1.408, 1.395, and 1.399 Å, correspondingly).

Figure 6. Molecular structure of 6. Thermal ellipsoids are drawn at 30% probability level. Hydrogen atoms are omitted.

The lengths of the La−N bonds in 2−6 are determined by two factors: (1) the redox state of the dpp-Bian ligand and (2) the coordination number (CN) of the metal. Noteworthy, the lanthanum has largest ionic radii (1.216 Å at CN = 9)48 among the lanthanides in the oxidation state +3. The coordination numbers vary notably in complexes 2, 3, 4, 5, and 6. Despite the difference in the redox states of the dpp-Bian ligands both 2 and 3 represent seven-coordinate lanthanum complexes. While a coordination polyhedron in complex 3 (Figure 3) can be wellD

DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Crystal Data and Structure Refinement Details for Compounds 2−6 2 formula Mr [g mol−1] crystal system space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z ρcalc, [g cm−3] μ [mm−1] F(000) crystal size, [mm3] θmin/θmax [deg] 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]

C44H60ILaN2O4 × C7H8 1038.88 orthorhombic Pna21 15.7948(7) 20.0597(9) 15.2409(7) 90 90 90 4828.9(4) 4 1.429 1.568 2120 0.17 × 0.12 × 0.03 2.12−26.00 −19 ≤ h ≤ 19 −24 ≤ k ≤ 24 −18 ≤ l ≤ 18 40 197 9447 0.0757 1.000/0.759 9447/32/557 1.006 0.0391/0.0587 0.0684/0.0635 0.667/−0.329

3 C48H64I2LaN2O3 1109.72 monoclinic P21 11.7628(3) 18.4239(4) 11.8970(3) 90 114.744(3) 90 2341.56(11) 2 1.574 2.271 1106 0.60 × 0.50 × 0.10 3.01−28.00 −15 ≤ h ≤ 15 −24 ≤ k ≤ 24 −15 ≤ l ≤ 15 42 115 11 296 0.0572 1.000 00/0.485 36 11 296/1/513 1.028 0.0291/0.0570 0.0322/0.0586 1.022/−0.731

4 C66H95ILaN2NaO8 × C4H8O 1405.34 monoclinic C2/c 37.174(2) 11.7544(6) 31.8247(17) 90 90.690(1) 90 13905.1(13) 8 1.343 1.119 5840 0.50 × 0.22 × 0.07 1.28−27.00 −47 ≤ h ≤ 47 −15 ≤ k ≤ 15 −40 ≤ l ≤ 40 62 371 15 112 0.0427 1.000/0.634 15 112/65/776 1.018 0.0498/0.1099 0.0715/0.1165 1.794/−0.933

5

6

C50H63LaN2O × C7H8 939.06 monoclinic P21/c 24.2809(9) 38.9056(14) 32.1948(12) 90 106.467(1) 90 29165.8(19) 24 1.283 0.919 11 808 0.47 × 0.24 × 0.15 1.46−27.00 −31 ≤ h ≤ 31 −49 ≤ k ≤ 49 −40 ≤ l ≤ 41 271 178 63 558 0.0704 1.000/0.663 63 558/681/3549 1.049 0.0590/0.1499 0.1092/0.1733 3.090/−2.398

C104H144K2La2N4O8 × 2C4H8O 2078.45 triclinic P1̅ 12.9829(9) 13.2673(9) 16.0100(11) 79.405(1) 78.704(1) 72.930(1) 2561.5(3) 1 1.347 0.964 1092 0.32 × 0.31 × 0.12 1.94−26.50 −14 ≤ h ≤ 16 −16 ≤ k ≤ 16 −11 ≤ l ≤ 20 15 821 10 537 0.0264 1.000/0.801 10 537/32/606 1.046 0.0414/0.0977 0.0552/0.1030 1.741/−1.030

Table 2. Selected Bond Lengths (Å) and Angles (deg) in Compounds 2−6 C(1)−N(1) C(2)−N(2) C(1)−C(2) La−N(1) La−N(2) La−I(1) La−I(2) La−C(Cp*)b La−O(1) La−O(2) La−O(3) La−O(4) N(1)−La−N(2) N(1)−C(1)−C(2)−N(2)/N(1)−La−N(2)c a

2

3

4

5a

6

1.411(17) 1.404(16) 1.400(9) 2.335(11) 2.390(11) 3.2307(5)

1.329(6) 1.341(6) 1.443(7) 2.556(4) 2.596(4) 3.1751(4) 3.1798(4)

1.405(5) 1.383(5) 1.414(5) 2.347(3) 2.367(3) 3.2105(4)

1.407(6) 1.391(6) 1.418(6) 2.345(4) 2.344(4)

1.416(4) 1.423(4) 1.447(4) 2.422(2) 2.381(3)

2.805(6) 2.659(8) 2.694(9) 2.760(9) 2.656(9) 79.06(18) 46.78

2.684(4) 2.545(4) 2.570(4)

2.786(5) 2.602(4)

2.591(2)

79.75(13) 51.46

77.17(9) 56.46

4.20

78.18(11) 48.19

For one of the crystallographically independent molecules. bAverage values. cCalculated with Mercury 1.4.2.

plane (the angle between the planes N(1)−C(1)−C(2)−N(2) and N(1)−La−N(2) are 46.8, 48.2, and 51.5°, correspondingly). In contrast, in compound 3 the metal atom deviates only 4.2° from a plane formed with atoms N(1), C(1), C(2), and N(2). Any π-bonding between the La atoms and the central double C(1)−C(2) bond in compounds 2, 4, and 5 seemed to be implausible, since in the absence of a back-donation from the metal π-interaction is too weak. The difference in the position of the La atoms relative radical-anionic and dianionic dpp-Bian ligands (3 vs 2, 4, and

described as pentagonal bipyramid, a coordination environment of the metal in complex 2 is irregular (Figure 2). The latter fact might be proven for a low degree of covalent bonding in the lanthanide complexes in general. Thus, a coordination polyhedron around the lanthanum center in 2 is assumed to be determined by the steric factors plus electrostatic interaction instead of the orbital interactions. Assuming the Cp* ligand is acting as six-electron donor the coordination numbers of La atoms in 4 and 5 are six. As for compound 2 the metal ions in 4 and 5 are positioned far away from the dimine E

DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

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CONCLUSION In this paper we reported on the preparation and structural characterization of five lanthanum complexes of a redox-active acenaphthene-1,2-diimine ligand, namely, dpp-Bian. Although the lanthanum(III) reveals one of the largest ionic radii among all the stable metal ions, a steric hindrance of the nitrogen atoms in dpp-Bian is effective enough to prevent the formation of the lanthanum complexes with the bridging ligand. Thus, all the complexes reported here are monomeric species with lanthanum atoms stabilized by coordination of the solvent donor molecules (thf or dme). The reduction of complex [(dpp-Bian)LaI(thf)2]2 (1) with potassium results in neither low-valent lanthanum species nor in metal−metal bonded derivatives. The reduction occurs exclusively at the ligand and affords the lanthanum complex of the dpp-Bian tetraanion, {[(dpp-Bian)La(thf)][K(thf)3]}2 (6), which represents the first lanthanide complex with an organic tetraanion. After treatment with oxidizers dpp-Bian ligand in 1 may change to radicalanionic state, which can be easily observed by the ESR spectroscopy. This study will be continued with other lanthanide ions that reveal more accessible divalent state, for example, thulium and dysprosium. We intend also preparation of heterolanthanide derivatives using dpp-Bian tetraanion as a ligand platform.

5) can be explained by electronic as well as steric effects. In compound 3 the nitrogen lone pairs (sp2) donate to the lanthanum empty orbitals (5d06s06p0), while four pz orbitals [two from C(1) and C(2) and two from N(1) and N(2)] combine to form a π system populated with five electrons. On the one hand, this suggestion is confirmed by an equality of the 14 N hyperfine constants in the ESR spectrum of complex 3 (vide supra). On the other hand, population of the LUMO of dpp-Bian with two electrons to result in dpp-Bian dianion causes an injury of the above-mentioned π system. Further, the shorter La−N(dpp-Bian dianion) distances compared to the La−N(dpp-Bian radical anion) distances cause some overcrowding around the metal centers in 2, 4, and 5 compared to that in 3. To minimize steric repulsion between the ligands, the lanthanum atoms in complexes 2, 4, and 5 occupy the positions out of the diimine plane. Molecular structures of complexes 2 and 3 can be compared with some other lanthanide halides containing radical-anionic or dianionic dpp-Bian ligand, [(dpp-Bian)Yb(μ-Cl)(dme)]2,25 [(dpp-Bian)Yb(μ-Br)(dme)] 2 , 49 [(dpp-Bian)Nd(μ-Cl)(thf)2]2,50 [(dpp-Bian)SmI2(thf)2],18 and [(dpp-Bian)Sm(μBr)(dme)]2.18 Except samarium iodide derivative, all the mentioned compounds represent halide-bridged dimers. The same is true for complex [(DAD)La(μ-Cl)(thf)2]2,51 which is a sole example of a lanthanum complex of this type. For the complex [(dpp-Bian)LaI(thf)2]2 (1), whose crystal structure could not be determined, we also assume dimeric structure. Compounds 4 and 5 are closely related to samarium complex [(dpp-Bian)SmCp*(thf)], which has been prepared by Vasudevan and Cowley reacting Cp*2Sm(Et2O) with dppBian.52 As in complex 5 in samarium complex metal atom is positioned 47.6° above the diimine plane. Complex 6 represents unusual dimer formed via bridging lanthanum atoms. The molecule possesses the inversion center located in the middle point of the line connecting two lanthanum atoms. Two dpp-Bian moieties are situated parallel to each other. Each lanthanum atom coordinates the diimine fragment of one of the dpp-Bian ligands and further is bonded to one of the naphthalene rings of another dpp-Bian ligand. The sandwich dianion [(dpp-Bian)La2(dpp-Bian)] is surrounded with two solvated potassium cations. As for the lanthanum complexes consisting of doubly reduced diimine fragment (complexes 2, 4, and 5) lanthanum atom is positioned out of the diimine plane. A count for the coordination number of the lanthanum atoms in 6 resulted in only number of four, which is too small for such a big ion as lanthanum. For comparison, in the lanthanum complex of the naphthalene dianion [μ2-η4:η4-C10H8][LaI2(thf)3]232 two lanthanum atoms are symmetrically situated from the opposite sides of sixmembered naphthalene rings and thus possess a coordination number of 7. Although the C(1)−N(1) and C(2)−N(2) bond lengths in complex 6 are somewhat elongated in comparison with those in compounds 2, 4, and 5 (Table 2), their values unambiguously confirm a total charge of 2− of the diimine moiety. Also, the naphthalene fragment in complex 6 bears a charge of 2−, which is delocalized over two six-membered rings. Thus, a remarkable elongation of the C−C bonds within the naphthalene rings in compound 6 (average C−C bond is 1.422 Å) in comparison, for instance, with those values in complex 2 (average C−C bond is 1.356 Å) indicates a population of the molecular oribital, which is antibonding for the carbon−carbon skeleton of the naphthalene fragment.

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EXPERIMENTAL SECTION

General Remarks. All manipulations were performed under vacuum using Schlenk technique. Solvents (tetrahydrofuran, 1,2dimethoxyethane, hexane, and toluene) were condensed into reaction flasks from sodium/benzophenone prior to use. Diimine dpp-Bian was prepared according to the literature procedures.47 The deuterated solvents thf-d8 and benzene-d6 (Aldrich) were dried at ambient temperature over sodium/benzophenone and, just prior to use, condensed under vacuum into NMR tubes containing a compound, for which the spectrum must be recorded. The NMR spectra were obtained on Bruker DPX 200 and Bruker Avance III spectrometers, while IR spectra (4000−400 cm−1) were recorded on FSM 1201 instrument in mineral oil. The ESR spectra were recorded using a Bruker EMX spectrometer (9.75 GHz), and the signals were referenced to the signal of diphenylpicrylhydrazyl (g = 2.0037). Synthesis of [(dpp-Bian)LaI(thf)2]2 (1). A solution of dpp-Bian (0.5 g, 1.0 mmol) and I2 (0.13 g, 0.5 mmol) in thf (40 mL) was added to lanthanum metal (10 g, 72 mmol), and the mixture was refluxed. In the course of ∼60 min reflux, the reaction mixture turned deep blue. The solution was then cooled to ambient temperature and decanted from the excess of lanthanum. The solvent was evaporated, and the residue was dried in vacuum at 80 °C. Hexane (60 mL) was added to the residual solid. Heating the mixture to 80 °C gave a deep blue solution. Solution was cooled to ambient temperature. After 3 d compound 1 was isolated as deep blue thin plate crystals. Yield 0.77 g (84%). mp 185−195 °C. Anal. Calcd for C44H56ILaN2O2 (910.74 g/ mol): C, 58.03; H, 6.20. Found: C, 57.29; H, 5.89%. 1H NMR (400 MHz, thf-d8): δ 7.14−7.08 (s, 6 H, Ar), 7.01 (d, 2 H, Ar, J = 8.0 Hz), 6.84 (dd, 2 H, Ar, J = 7.0; 8.0 Hz), 6.03 (d, 2 H, Ar, J = 7.0 Hz), 3.64 (m, thf), 3.61 (sept, 4 H, CH(CH3)2, J = 6.8 Hz), 1.79 (m, thf), 1.24 (d, 12 H, CH(CH3)2, J = 6.8 Hz), 0.99 (d, 12 H, CH(CH3)2, J = 6.8 Hz). IR (mineral oil): 3650 w, 2726 w, 1913 w, 1794 w, 1670 w, 1642 w, 1613 w, 1586 s, 1431 w, 1322 vs, 1252 s, 1240 s, 1098 m, 1057 w, 1034 m, 1009 m, 919 m, 868 s, 850 m, 818 s, 797 s, 758 vs, 723 m cm−1. Synthesis of [(dpp-Bian)LaI(dme)2] (2). A solution of dpp-Bian (0.5 g, 1.0 mmol) and I2 (0.13 g, 0.5 mmol) in dme (40 mL) was added to lanthanum metal in chips (10 g, 72 mmol), and the mixture was refluxed. In the course of ∼30 min reflux the reaction mixture turned deep green. The product [(dpp-Bian)LaI(dme)2] (2) is poorly soluble in dme. It was isolated as a green microcrystalline solid (0.77 g, 93%). F

DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry mp 228−235 °C. Anal. Calcd for C44H60ILaN2O4 × C7H8 (1038.88): C, 58.96; H, 6.60. Found: C, 58.18; H, 6.35%. 1H NMR (200 MHz, thf-d8): δ 7.20 (s, 2 H, Ar), 7.11 (d, 2 H, Ar, J = 8.3 Hz), 7.09 (s, 4 H, Ar), 6.83 (dd, 2 H, Ar, J = 6.8; 8.3 Hz), 6.02 (d, 2 H, Ar, J = 6.8 Hz), 3.60 (sept, 4 H, CH(CH3)2, J = 6.8 Hz), 3.44 (s, 8 H, dme), 3.28 (s, 12 H, dme), 1.24 (d, 12 H, CH(CH3)2, J = 6.8 Hz), 0.98 (d, 12 H, CH(CH3)2, J = 6.8 Hz). IR (mineral oil): 3044 w, 1927 w, 1860 w, 1796 w, 1613 m, 1581 vs, 1423 s, 1354 m, 1302 vs, 1242 s, 1206 w, 1190 m, 1110 s, 1060 vs, 1024 w, 926 s, 918 s, 859 vs, 837 w, 818 s, 798 s, 771 s, 765 s, 755 m, 703 w, 683 m, 665 w, 623 s cm−1. The crystals of 2 suitable for X-ray analysis were obtained by recrystallization of compound 2 from toluene. Synthesis of [(dpp-Bian)LaI2(thf)3] (3). A solution of dpp-Bian (0.5 g, 1.0 mmol) and I2 (0.13 g, 0.5 mmol) in thf (40 mL) was added to lanthanum metal (10 g, 72 mmol), and the mixture was refluxed. In the course of ∼60 min reflux, the reaction mixture turned deep blue. The solution was then cooled to ambient temperature and decanted from an excess of lanthanum. To this solution 0.13 g (0.5 mmol) of I2 was added. Color of the mixture turned instantly dirty brown-blue. A heating of the reaction mixture for 15 min at 60 °C resulted in cherryred solution. Concentration of this solution afforded deep red crystals of complex 3 (0.85 g, 77%). mp > 238 °C (dec). Anal. Calcd for C48H64I2LaN2O3 (1109.72): C, 51.95; H, 5.81. Found: C, 51.74; H, 5.75%. IR (mineral oil): 3050 w, 1933 w, 1879 w, 1814 w, 1671 m, 1642 w, 1593 m, 1517 vs, 1481 w, 1429 m, 1408 m, 1360 w, 1345 w, 1339 w, 1317 m, 1291 w, 1244 m, 1223 w, 1188 s, 1136 m, 1109 m, 1096 w, 1080 m, 1052 w, 1028 vs, 970 w, 964 w, 938 m, 924 m, 885 w, 860 m, 845 m, 836 w, 823 m, 802 m, 797 w, 791 m, 777 s, 764 s, 752 m, 688 w, 663 s, 622 w, 607 m, 592 w, 574 w, 540 s, 515 w cm−1. ESR (295 K, thf): g = 2.001 45, A(139La) = 0.83, A(14N) = 0.41 (2 × N), A(1H) = 0.10 (4 × H), A(127I) = 0.04 (2 × I) mT. Synthesis of [(dpp-Bian)LaCp*I][Na(18-crown-6)(thf)2] (4). To a solution of 1 (prepared in situ from 0.5 g of dpp-Bian) in thf (40 mL) 0.16 g (1.0 mmol) of NaCp* was added. Color of the mixture turned quickly from blue to green. Then 0.26 g (1.0 mmol) of 18-crown-6 was added to the reaction mixture. Concentration of the resulting solution afforded compound 4 as green crystals. Yield 1.00 g (71%). Anal. Calcd for C66H95ILaN2NaO8 × C4H8O (1405.34): C, 59.82; H, 7.39. Found: C, 60.01; H, 7.33%. 1H NMR (400 MHz, thf-d8): δ 7.09 (dd, 2 H, iPr2C6H3, J = 1.8; 7.5 Hz), 7.00 (dd, 2 H, iPr2C6H3, J = 1.8; 7.5 Hz), 6.91 (t, 2 H, iPr2C6H3, J = 7.5 Hz), 6.58 (d, 2 H, Ar, J = 8.0 Hz), 6.64 (dd, 2 H, Ar, J = 7.0; 8.0 Hz), 5.72 (d, 2 H, Ar, J = 7.0 Hz), 3.63 (m, thf), 3.55 (sept, 2 H, CH(CH3)2, J = 6.8 Hz), 3.47 (s, 24 H, 18crown-6), 3.42 (sept, 2 H, CH(CH3)2, J = 6.8 Hz), 1.95 (s, 15 H, C5(CH3)5), 1.79 (m, thf), 1.47 (d, 6 H, CH(CH3)2, J = 7.03 Hz), 1.19 (d, 6 H, CH(CH3)2, J = 6.8 Hz), 1.15 (d, 6 H, CH(CH3)2, J = 6.8 Hz), 0.72 (d, 6 H, CH(CH3)2, J = 6.8 Hz). IR (mineral oil): 3051 w, 1610 w, 1578 m, 1429 m, 1353 m, 1310 s, 1250 m, 1187 w, 1113 vs, 1069 w, 1057 m, 1001 w, 965 m, 933 w, 918 m, 864 w, 837 w, 816 w, 796 w, 768 m, 755 m cm−1. Synthesis of [(dpp-Bian)LaCp*(thf)] (5). Addition of 0.16 g (1.0 mmol) of NaCp* to a solution of 1 (prepared in situ from 0.5 g of dpp-Bian) in thf (40 mL) resulted in a quick change of color of the mixture from blue to green. Evaporation of the solvent in vacuum and treatment of the solid left with toluene (40 mL) afforded blue solution. Precipitated NaI was filtered off. Concentration of the filtered blue solution afforded complex 4 as blue crystals. Yield 0.57 g (67%). mp 232−235 °C. Anal. Calcd for C50H63LaN2O × C7H8 (939.07): C, 72.91; H, 7.62. Found: C, 72.64; H, 7.56%. 1H NMR (400 MHz, C6D6): δ 7.36−7.34 (m, 2 H, Ar), 7.20−7.26 (m, 4 H, Ar), 7.15−6.99 (m, 5 H, toluene) 7.06 (d, 2 H, Ar, J = 8.0 Hz), 6.87 (dd, 2 H, Ar, J = 7.0; 8.0 Hz), 6.34 (d, 2 H, Ar, J = 7.0 Hz), 3.75 (sept, 2 H, CH(CH3)2, J = 6.8 Hz), 3.14 (sept, 2 H, CH(CH3)2, J = 6.8 Hz), 2.98 (m, 4 H, thf), 2.1 (s, 3 toluene), 2.00 (s, 15 H, C5(CH3)5), 1.45 (d, 6 H, CH(CH3)2, J = 6.8 Hz), 1.42 (d, 6 H, CH(CH3)2, J = 6.8 Hz), 1.20 (d, 6 H, CH(CH3)2, J = 6.8 Hz), 1.05 (d, 6 H, CH(CH3)2, J = 6.8 Hz), 0.75 (m, 4 H, thf). IR (mineral oil): 3050 w, 2728 w, 1925 w, 1852 w, 1805 w, 1668 w, 1611 m, 1605 s, 1495 m, 1446 w, 1429 s, 1352 m, 1308 vs, 1252 s, 1235 m, 1190 w, 1178 w, 1158 w, 1144 w, 1103 s, 1081 w, 1057 w, 1036 w, 1015 s, 957 w, 932 m, 919 m, 885 w, 865 vs,

819 s, 796 s, 774 s, 756 vs, 729 vs, 695 s, 681 m, 667 w, 623 m, 592 w, 546 w, 511 w cm−1. Synthesis of {[(dpp-Bian)La(thf)][K(thf)3]}2 (6). A solution of compound 1 (in situ from 0.5 g of dpp-Bian) in thf (40 mL) was added to potassium metal (1 g, 25 mmol). Within 60 min at reflux the mixture turned red-brown. A filtered solution was concentrated to 10 mL and stored at room temperature. In 24 h deep brown crystals of compound 6 were isolated (0.71 g, 68%). mp > 180 °C; Anal. Calcd for C104H144K2La2N4O8 × 2C4H8O (2078.46): C, 64.72; H, 7.76. Found: C, 64.09; H, 8.10%. 1H NMR (200 MHz, thf-d8): δ 6.93 (dd, 2 H, Ar, J = 1.3; 7.3 Hz), 6.60 (dd, 2 H, Ar, J = 1.3; 7.5 Hz), 6.30 (dd, 2 H, Ar, J = 7.3; 7.5 Hz), 4.78 (dd, 2 H, Ar, J = 6.8; 8.0 Hz), 3.63 (m, 20 H, thf), 3.45 (d, 2 H, Ar, J = 8.0 Hz), 3.01 (sept, 2 H, CH(CH3)2, J = 6.3 Hz), 2.99 (d, 2 H, Ar, J = 6.8 Hz), 2.76 (sept, 2 H, CH(CH3)2, J = 6.3 Hz), 1.99 (d, 6 H, CH(CH3)2, J = 6.3 Hz), 1.78 (m, 20 H, thf), 0.82 (d, 6 H, CH(CH3)2, J = 6.3 Hz), 0.79 (d, 6 H, CH(CH3)2, J = 6.3 Hz), 0.68 (d, 6 H, CH(CH3)2, J = 6.3 Hz). IR (mineral oil): 3048 w, 1586 s, 1563 s, 1509 m, 1416 vs, 1342 s, 1302 m, 1244 s, 1227 s, 1207 w, 1194 m, 1175 w, 1142 w, 1115 m, 1073 m, 1050 w, 1030 s, 1011 w, 980 w, 937 w, 908 s, 876 m, 849 m, 816 w, 787 m, 777 w, 762 w, 750 s, 716 vs cm−1. X-ray Crystallography. The X-ray diffraction data were collected on an Agilent Xcalibur E (3) and a Bruker AXS SMART APEX (2, 4− 6) diffractometer (Mo-Kα radiation, ω-scan technique, λ = 0.710 73 Å). The intensity data were integrated by CrysAlisPro53 for 3 and by SAINT54 for 2, 4−6, respectively. All structures 2−6 were solved by direct methods and were refined on Fhkl2 using SHELXL package.55 All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions and were refined in the riding model. SCALE3 ABSPACK56 for 3 and SADABS57 for 2, 4−6 were used to perform area-detector scaling and absorption corrections. Crystals of 2 and 4−6 contain solvate molecules of toluene (2, 5) and thf (4, 6). The main crystallographic data and structure refinement details for 2−6 are presented in Table 1. CCDC 1589748 (2), 1589749 (3), 1589750 (4), 1589751 (5), and 1589752 (6) contain the supplementary crystallographic data for this paper. Additional crystallographic information is available in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03112. NMR spectra (PDF) Accession Codes

CCDC 1589748−1589752 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

Igor L. Fedushkin: 0000-0003-2664-2266 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by the Russian Foundation for Basic Research (Grant No. 16-03-00946). G

DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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(17) 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 redoxactive diimine ligand. Eur. J. Inorg. Chem. 2009, 2009, 3742−3749. (18) 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. (19) Fedushkin, I. L.; Skatova, A. A.; Dodonov, V. A.; Chudakova, V. A.; Bazyakina, N. L.; Piskunov, A. V.; Demeshko, S. V.; Fukin, G. K. Digallane with redox-active diimine ligand: dualism of electron-transfer reactions. Inorg. Chem. 2014, 53, 5159−5170. (20) Fedushkin, I. L.; Skatova, A. A.; Yambulatov, D. S.; Cherkasov, A. V.; Demeshko, S. V. Europium complexes with 1,2-bis(arylimino)acenaphthenes: a search for redox isomers. Russ. Chem. Bull. 2015, 64, 38−43. (21) Dodonov, V. A.; Skatova, A. A.; Cherkasov, A. V.; Fedushkin, I. L. Synthesis and structure of bischelate gallium complexes (dppBian)Ga(acac) and (dpp-Bian)Ga(2,2′-bipy) (dpp-Bian is 1,2-bis[(2,6diisopropylphenyl)imino]acenaphthene. Russ. Chem. Bull. 2016, 65, 1171−1177. (22) 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. (23) Fedushkin, I. L.; Yambulatov, D. S.; Skatova, A. A.; Baranov, E. V.; Demeshko, S.; Bogomyakov, A. S.; Ovcharenko, V. I.; Zueva, E. M. Ytterbium and Europium complexes of redox-active ligands: searching for redox isomerism. Inorg. Chem. 2017, 56, 9825−9833. (24) Fedushkin, I. L.; Sokolov, V. G.; Piskunov, A. V.; Makarov, V. M.; Baranov, E. V.; Abakumov, G. A. Adaptive behavior of a redoxactive gallium carbenoid in complexes with molybdenum. Chem. Commun. 2014, 50, 10108−10111. (25) Fedushkin, I. L.; Maslova, O. V.; Morozov, A. G.; Dechert, S.; Demeshko, S.; Meyer, F. Genuine redox isomerism in a rare-earthmetal complex. Angew. Chem., Int. Ed. 2012, 51, 10584−10587. (26) Fedushkin, I. L.; Skatova, A. A.; Chudakova, V. A.; Fukin, G. K. Four-step reduction of dpp-Bian with sodium metal: crystal structures of the sodium salts of the mono-, di-, tri- and tetraanions of dpp-Bian. Angew. Chem., Int. Ed. 2003, 42, 3294−3298. (27) Fedushkin, I. L.; Makarov, V. M.; Sokolov, V. G.; Fukin, G. K. Acenaphthene-1,2-diimine chromium complexes. Dalton Trans. 2009, 8047−8053. (28) Fedushkin, I. L.; Skatova, A. A.; Lukoyanov, A. N.; Khvoinova, N. M.; Piskunov, A. V.; Nikipelov, A. S.; Fukin, G. K.; Lysenko, K. A.; Irran, E.; Schumann, H. 1,2-Bis(imino)acenaphthene complexes of molybdenum and nickel. Dalton Trans. 2009, 4689−4694. (29) Fedushkin, I. L.; Makarov, V. M.; Sokolov, V. G.; Fukin, G. K.; Maslov, M. O.; Ketkov, S. Yu. Compounds of chromium, titanium, and zirconium with different reduced forms of acenaphthene-1,2-diimine. Russ. Chem. Bull. 2014, 63, 870−882. (30) Mashima, K.; Nakamura, A. Novel synthesis of lanthanoid complexes starting from metallic lanthanoid sources. J. Chem. Soc., Dalton Trans. 1999, 22, 3899−3907. (31) Mashima, K.; Fukumoto, H.; Nakayama, Y.; Tani, K.; Nakamura, A. Cationic monocyclooctatetraenyl-lanthanoid complexes derived from metallic lanthanoid: crystal structures of [Sm(η8C 8 H 8 )(hmpa) 3 ]I and [Sm(η 8 -C 8 H 8 )(hmpa) 3 ][Sm(η 8 -C 8 H 8 ) 2 ] (HMPA = hexamethylphosphoric triamide). Polyhedron 1998, 17, 1065−1071. (32) Nakayama, Y.; Nakamura, A.; Mashima, K. Lanthanoid complexes of azobenzene and benzo[c]cinnoline derived from metallic lanthanoids: crystal structures of binuclear [SmI(THF)3(μ-η2:η2-transPhNNPh) 2 SmI(THF) 3 ] and mononuclear [Yb(benzo[c]cinnoline)3(THF)2]. Chem. Lett. 1997, 26, 803−804. (33) Ali, S. H.; Deacon, G. B.; Junk, P. C.; Hamidi, S.; Wiecko, M.; Wang, J. Lanthanoid Pseudo-Grignard Reagents: A Major Untapped Resource. Chem. - Eur. J. 2018, 24, 230−242.

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DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.7b03112 Inorg. Chem. XXXX, XXX, XXX−XXX