Synthesis and Structural Characterization of Mixed-Valent Ytterbium

Apr 14, 2014 - The formation of mixed-valent ytterbium and europium complexes L4LnIII2LnII (Ln = Yb (1), Eu (2)) was observed for the first time in th...
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Synthesis and Structural Characterization of Mixed-Valent Ytterbium and Europium Complexes Supported by a Phenoxy(quinolinyl)amide Ligand Yinyin Jiang, Xi Zhu, Muzi Chen, Yaorong Wang,* Yingming Yao,* Bing Wu, and Qi Shen Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, People’s Republic of China S Supporting Information *

ABSTRACT: The formation of mixed-valent ytterbium and europium complexes L4LnIII2LnII (Ln = Yb (1), Eu (2)) was observed for the first time in the spontaneous reduction reaction system of quinolinyl aminophenol (H2L) with Ln[N(SiMe3)2]3 (Ln = Yb, Eu) in toluene at 90 °C, whereas the same reaction with Sm[N(SiMe3)2]3 gave the expected monoamido samarium complex LSmN(SiMe3)2(DME) (4). The isolation of the binuclear ytterbium complex L3Yb2 (3) under mild conditions demonstrates that the transformation from a trivalent ytterbium complex to the mixed-valent ytterbium species 1 may involve a ligand redistribution reaction and homolysis of the Yb−N bond.

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reduction reactions, while no mixed-valent lanthanide complexes were obtained, the latter of which might be useful for understanding the mechanism of spontaneous reductions for both cyclopentadiene- and non-cyclopentadiene-based systems. We herein report that the reaction of quinolinyl aminophenol with the lanthanide amides Ln[N(SiMe3)2]3 (Ln = Yb, Eu) resulted in the partial spontaneous reduction of Ln(III) to Ln(II) and gave unexpected mixed-valent ytterbium and europium complexes. The mechanism for the transformation from a trivalent ytterbium complex to a mixed-valent ytterbium species has also been elucidated.

he most stable oxidation state of all lanthanide metals is +3, and most divalent lanthanide complexes can be spontaneously oxidized to trivalent complexes because of their strongly reductive properties.1,2 In contrast, it is difficult for spontaneous reduction from Ln(III) to Ln(II) to occur unless reducing agents, such as alkali metals, are present. The first example of such a spontaneous reduction reaction was observed from the reaction of europium trichloride with sodium pentamethylcyclopentadienide, which gave the divalent europium complex Eu(C5Me5)2(THF).3 In recent years, important development has been achieved in this area. Systematic studies revealed that a large number of divalent lanthanide complexes bearing bulky substituted cyclopentadienyl or β-diketiminate ligands can be prepared by the sterically induced reduction (SIR) of trivalent precursors,4,5 which was first described by Evans in the chemistry of Ln(C5Me5)3.1,6 However, there are also some examples that spontaneous reduction of Ln(III) to Ln(II) occurred, in which the coordination environment around the lanthanide metals was not very crowded, and the driving force for the spontaneous reduction is unclear. For example, the bis(methylcyclopentadienyl)ytterbium methyl complex [(MeC5H4)2YbMe]2 was slowly reduced to the divalent ytterbocene (MeC5H4)2Yb in hot toluene.7 Reactions of lanthanide amides Ln[N(SiMe3)2]3(μ-Cl)Li(THF)3 (Ln = Yb, Eu) with indenes or fluorenes bearing pendant amine or ether functional groups resulted in spontaneous reduction to the corresponding divalent lanthanocene complexes.8 It is noteworthy that treatment of the europium amide Eu[N(SiMe3)2]3(μ-Cl)Li(THF)3 with pyrrolyl-functionalized secondary amines also led to the reduction of Eu(III) to Eu(II),9 which is a spontaneous reduction that occurred with a noncyclopentadiene-based system. Furthermore, only divalent lanthanide complexes were isolated from the aforementioned © 2014 American Chemical Society

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RESULTS AND DISCUSSION Very recently, we reported that the amine elimination reaction of Ln[N(SiMe3)2]3 or Ln[N(SiMe3)2]3(μ-Cl)Li(THF)3 with aminophenols in toluene at 90 °C gave a series of lanthanide aminophenoxy amides.10 Therefore, we expanded the exchange reaction to the quinolinyl aminophenol (H2L) system and attempted to elucidate the influence of the structure of the phenoxyamide ligand on the reactivity. Reactions of Ln[N(SiMe3)2]3 (Ln = Yb, Eu) with 1 equiv of H2L in toluene at 90 °C led to a dark red solution. After workup, dark red crystals were isolated from concentrated toluene solution. Elemental analysis and further solid-state structure determination showed that the dark red crystals were not the desired monoamido lanthanide complexes LLn[N(SiMe3)2](THF) but the mixedvalent trinuclear lanthanide complexes L4LnIII2LnII (Ln = Yb (1), Eu (2)), in which one lanthanide metal is in the +2 oxidation state and the others adopt the +3 oxidation state Received: January 10, 2014 Published: April 14, 2014 1972

dx.doi.org/10.1021/om5000198 | Organometallics 2014, 33, 1972−1976

Organometallics

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(Scheme 1). BVS (bond valence sum)11 calculations for lanthanide atoms LnII in complexes 1 and 2 gave values of

SmIII/SmII (−1.55 V).6a Indeed, the reaction of Sm[N(SiMe3)2]3 with 1 equiv of H2L in toluene at 90 °C yielded a dark red suspension, from which a dark red precipitate was obtained by centrifugation. Upon crystallization of the precipitate from a mixture of DME and n-hexane, the desired monoamido samarium complex LSmN(SiMe3)2(DME) (4) was isolated in 67% yield (Scheme 3). Complexes 1−3 are all readily soluble in THF and toluene but insoluble in n-hexane, whereas complex 4 is soluble in THF and slightly soluble in toluene.

Scheme 1. Synthesis of Complexes 1 and 2

Scheme 3. Synthesis of Complex 4 1.61 and 2.36, respectively, indicating that one of the lanthanide atoms is in the +2 oxidation state. Such mixed-valent trinuclear lanthanide complexes were reported previously, as the bridged bis(phenolate) samarium complex SmIII2SmII[BunN(CH2-2OC 6 H 2 -3,5-Bu t 2 ) 2 ] 4 1 2 and the ytterbocene complex YbIII2YbII(C5Me5)4(μ-CCPh)4.13 However, different from our system, they formed via single-electron transfer reactions of divalent lanthanide complexes with corresponding organic molecules. Obviously, the presence of Ln(II) species in complexes 1 and 2 indicated that partial reduction from Ln(III) to Ln(II) occurred. In order to gain further information, the stoichiometric reaction was conducted under mild condition. Treatment of Yb[N(SiMe3)2]3 with H2L in a 3:4 molar ratio in nhexane at room temperature, after workup, gave the trivalent ytterbium complex L3Yb2 (3) in 53% isolated yield (Scheme 2),

Refluxing the toluene solution of complex 3 for 3 days did not lead to the isolation of the mixed-valent trinuclear complex 1, indicating that complex 3 is not the real intermediate in the formation of complex 1. However, addition of 1 equiv of Yb[N(SiMe3)2]3 to the toluene solution of complex 3 and subsequent heating of the mixture at 90 °C for 1 day led to the isolation of the mixed-valent complex 1 (Scheme 2). These findings revealed that the formation of the mixed-valent complex 1 might result from the ligand redistribution reaction between Yb[N(SiMe3)2]3 and complex 3 and the subsequent partial reduction reaction. Thus, the formation pathway of the mixed-valent trinuclear complex is proposed as follows: the silylamine elimination reaction of quinolinyl aminophenol (H2L) with Yb[N(SiMe3)2]3 affords the binuclear lanthanide complex L3Yb2 (3), which further reacts with Yb[N(SiMe3)2]3 to give the trinuclear lanthanide amide (L2Yb)2YbN(SiMe3)2 (A) as the intermediate via a ligand redistribution reaction. Homolysis of the Yb−N bonds then occurred, resulting in the partial reduction of Yb(III) to Yb(II) to give the mixed-valent trinuclear complex 1 (Scheme 2). LC-MS characterization proved the formation of [(Me3Si)2N]2 during this reaction. Efforts to isolate the unstable intermediate A have not met with success, although the similar structure (L′2Ln)2LnN(SiMe3)2 (L′H2 = (S)-5,5′,6,6′,7,7′,8,8′-octahydro-2-(pyrrol-2-ylmethyleneamino)-2′-hydroxy-1,1′-binaphthyl) has been reported.14 The definitive molecular structures of all complexes 1−4 were confirmed by single-crystal structure analysis. Complexes 1 and 2 are isostructural and possess bridging aminophenoxy-O atoms (Figure 1 for complex 1 and Figure S1 (Supporting Information) for complex 2). There are six solvate toluene molecules for 1 and four and a half solvate toluene molecules for 2. The overall molecular structures are quite similar to that of SmIII2SmII[BunN{CH2-(2-OC6H2-3,5-But2)}2]4.12 In each molecule of L4LnIII2LnII, two of the lanthanide atoms are each ligated by four nitrogen atoms and two oxygen atoms from two phenoxy(quinolinyl)amide ligands in a distortedoctahedral geometry and the third lanthanide atom is bound to four oxygen atoms from four ligands in a distorted-tetrahedral geometry. The three lanthanide atoms have an almost linear arrangement with Ln(1)−Ln(2)−Ln(3) angles of 180.0° for 1 and 176.80(1)° for 2. The planes defined by Yb(2)O(1)O(1A) (Eu(2)O(1)O(2)) and Yb(2)O(2)O(2A) (Eu(2)O(3)O(4)) are almost orthogonal with a dihedral angle of 89.9(2)° for 1

Scheme 2. Mechanistic Study of Mixed-Valent Trinuclear Complex Formation

indicating that spontaneous reduction of Yb(III) to Yb(II) did not take place at room temperature. It is worth noting that the reaction of Yb[N(SiMe3)2]3 with H2L in a 1:1 molar ratio in nhexane at room temperature also gave complex 3, albeit in a lower yield, instead of the expected monoamido ytterbium complex LYb[N(SiMe3)2](THF). We postulated that the monoamido ytterbium complex first formed in this system, which reacted further with H2L to afford complex 3. In sharp contrast, Eu[N(SiMe3)2]3 with H2L in the ratio 3:4 at room temperature produced the mixed-valent trinuclear europium complex 2 in 84% isolated yield, instead of an Eu(III) complex analogous to complex 3 (Scheme 2). This finding is reasonable, because reduction of the Eu(III) ion to Eu(II) ion is easier than that for the Yb(III) ion, due to the lower reduction potential of EuIII/EuII (−0.35 V) in comparison to that of YbIII/YbII (−1.15 V).6a It is expected that the monoamido samarium complex may be available because of the high reduction potential of 1973

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Figure 1. Ortep diagram of complex 1 with 20% probability ellipsoids. Hydrogen atoms and the free toluene molecule are omitted for clarity.

Figure 2. Ortep diagram of complex 3 with 20% probability ellipsoids. Hydrogen atoms and the free toluene molecule are omitted for clarity.

and 88.8(1)° for 2, which result in a favorable staggered arrangement of the tBu group. The average Ln(1,3)−O bond distances of 2.214(5) Å for 1 and 2.327(4) Å for 2 are consistent with the corresponding values in SmIII2SmII[BunN{CH2-(2-OC6H2-3,5-But2)}2]4 (average 2.363(5) Å)12 and [LYbCl(THF)]2 (2.230(4), 2.251(4) Å),15 provided the difference in ionic radii is considered.16 However, the average Ln(2)−O bond lengths of 2.322(6) and 2.437(4) Å in complexes 1 and 2, respectively, are about 0.11 Å larger than the values of Ln(1,3)−O but close to those observed for the divalent ytterbium complexes [Et2 NCH 2 CH2 N{CH 2 -(2OC 6 H 2 -3,5-Bu t 2 )} 2 Yb] 2 (2.324(4), 2.328(3), 2.357(3), 2.371(4) Å)17 and [Me2NCH2CH2N{CH2-(2-OC6H2-3,5But2)}2Yb]2 (2.297(5), 2.349(5) Å)18 and the divalent europium complex [Me2NCH2CH2N{CH2-(2-OC6H2-3,5But2)}2Eu]2 (2.400(5), 2.406(6), 2.457(5), 2.459(6) Å).12 These bond length data suggest that the lanthanide atoms Ln(1) and Ln(3) are trivalent and Ln(2) is divalent, since the ionic radii of LnII are approximately 0.11 Å larger than that of LnIII.16 This finding provides additional evidence to validate that complexes 1 and 2 are indeed mixed-valent species. As expected, the Ln(1,3)−N(amido) and Ln(1,3)−N(quinoline) bond distances in complexes 1 and 2 are comparable to each other when the difference in the ionic radii between Yb and Eu is considered, and the values are also consistent with those found in [LYbCl(THF)]2.15 Complex 3 has an unsolvated bimetallic structure (Figure 2), in which the coordination chemistry around the two Yb atoms is different. The Yb(1) atom is five-coordinated with two nitrogen atoms from one phenoxy(quinolinyl)amide ligand and three oxygen atoms from three phenoxy(quinolinyl)amide ligands in a distorted-trigonal-bipyramidal geometry with the O(1) and N(6) atoms occupying the axial positions of the bipyramid. The Yb(2) atom is six-coordinated with four nitrogen atoms and two oxygen atoms from two phenoxy(quinolinyl)amide ligands in a distorted-octahedral geometry with O(2) and N(3) atoms occupying two sites of the axis. Yb(1) and Yb(2) are bridged by O(1) and O(2) atoms with an Yb(1)−Yb(2) distance of 3.5089(3) Å. The bridging Yb(1)− O(bridged) bond distances of 2.268(3) and 2.169(3) Å and the Yb(2)−O(bridged) bond lengths of 2.288(3) and 2.217(3) Å are in accordance with the corresponding values observed in another binuclear ytterbium complex, {L″YbN(SiMe3)2}2 (2.289(4), 2.305(3), 2.165(3), 2.172(3) Å; L″H2 = (S)-2-

(pyrrol-2-ylmethyleneamino)-2′-hydroxy-6,6′-dimethyl-1,1′-biphenyl).14 As expected, the terminal Yb(1)−O(3) bond length of 2.065(3) Å is apparently shorter than the bridging Yb(1)− O(bridged) bond distances but is in agreement with the values of the terminal bond in bridged bis(phenolate) ytterbium complexes.19 The average Yb−N(quinoline) bond distance of 2.403(4) Å is consistent with those in complex 1 but longer than those in Yb(thqtcn) (H3thqtcn = (1,4,7-tris[2-(8hydroxyquinolinyl)methyl]-1,4,7-triazacyclononane),20 if a subtraction of the difference in radii between six- and ninecoordinate Yb3+ is made.16 Complex 4 has a DME-solvated monomeric structure, and the central metal samarium atom is coordinated by one phenoxy(quinolinyl)amide ligand, one DME molecule, and one amino N(SiMe3)2 group to form a highly distorted octahedral geometry with O(3) and N(3) atoms occupying two sites of the axis (Figure S2, Supporting Information). The Sm− N(amido) bond distances of 2.308(4) and 2.314(4) Å and the terminal Sm−O(Ar) bond length of 2.178(3) Å compare well with the corresponding bond lengths in complex 3 if the difference in ionic radii between Sm and Yb is considered. Obviously, the Sm−N(quinoline) bond length of 2.578(5) Å and the Sm−O(DME) bond distances of 2.543(4) and 2.535(4) Å are consistent with dative bonding. In conclusion, the mixed-valent trinuclear complexes L4LnIII2LnII (Ln = Yb (1), Eu (2)), binuclear ytterbium complex L3Yb2 (3), and monoamido samarium complex LSmN(SiMe3 ) 2(DME) (4) were synthesized by amine elimination reactions of Ln[N(SiMe3)2]3 with quinolinyl aminophenol (H2L). The mechanism of the spontaneous reduction chemistry has also been studied, which may involve a silylamine elimination reaction, a ligand redistribution reaction, and homolysis of the Ln−N bonds resulting in reduction of LnIII to LnII. This finding provides additional information for the spontaneous reduction reactions in the non-cyclopentadiene-based system, which may also provide some hints for the cyclopentadiene-based system. Further study on factors that influence the spontaneous reduction is ongoing in our laboratory.

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EXPERIMENTAL SECTION

Materials and Methods. All of the manipulations were performed under an argon atmosphere using standard Schlenk techniques. 1974

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Quinolinyl aminophenol (H2L = 3,5-But2-2-(OH)C6H2CH2NH-8C9H6N)21 and Ln[N(SiMe3)2]322 were prepared according to the literature procedures. GC-MS analysis was carried out on a Bruker SCION SQ/436GC instrument. Carbon, hydrogen, and nitrogen analyses were performed by direct combustion with a Carlo-Erba EA1110 instrument. L4YbIII2YbII·6C7H8 (1·6C7H8). A solution of quinolinyl aminophenol H2L (0.74 g, 2.04 mmol) in toluene was slowly added to a stirred solution of Yb[N(SiMe3)2]3 (1.33 g, 2.04 mmol) in toluene at room temperature. The reaction mixture was stirred at 90 °C for 3 days to give a dark red solution. After a small amount of precipitate was removed by centrifugation, the resulting clear solution was concentrated to about 8 mL. Complex 1 was isolated as dark red crystals after this solution stood at room temperature for several days (0.71 g, 55% based on aminophenol). Anal. Calcd for C138H160N8O4Yb3: C, 65.93; H, 6.42; N, 4.46; Yb, 20.65. Found: C, 66.01; H, 6.53; N, 4.52; Yb, 20.98. IR (KBr, cm−1): 2954 (s), 2902 (w), 2864 (m), 2364 (w), 1611 (s), 1543 (m), 1511 (s), 1475 (s), 1439 (w), 1383 (m), 1362 (w), 1303 (w), 1233 (w), 1201 (w), 1161 (m), 1087 (w), 977 (w), 834 (s), 790 (m). L4EuIII2EuII·4.5C7H8 (2·4.5C7H8). The synthesis of complex 2 was carried out in the same way as that described for complex 1, except that quinolinyl aminophenol H2L (0.73 g, 2.01 mmol) and Eu[N(SiMe3)2]3 (1.27 g, 2.01 mmol) were used. Dark red crystals were isolated from a concentrated toluene solution at 0 °C (0.55 g, 47% based on aminophenol). Anal. Calcd for C127.5H148N8O4Eu3: C, 66.22; H, 6.45; N, 4.85; Eu, 19.71. Found: C, 65.91; H, 6.63; N, 4.62; Eu, 20.28. IR (KBr, cm−1): 2954 (s), 2902 (w), 2863 (m), 2365 (w), 1608 (s), 1585 (m), 1511 (s), 1473 (s), 1424 (w), 1381 (m), 1360 (w), 1302 (w), 1232 (w), 1200 (w), 1160 (m), 1087 (w), 978 (w), 833 (s), 790 (m). L3Yb2·C7H8 (3·C7H8). An n-hexane solution of H2L (0.74 g, 2.04 mmol) was slowly added to a solution of Yb[N(SiMe3)2]3 (1.0 g, 1.53 mmol) in n-hexane at room temperature. The mixture was stirred at room temperature for 1 day. The precipitate that formed was obtained by centrifugation. Dark red crystals were obtained from a concentrated toluene solution at 0 °C after several days. Yield: 0.55 g (53% based on aminophenol). Anal. Calcd for C79H92N6O3Yb2: C, 62.44; H, 6.10; N, 5.53; Yb, 22.77. Found: C, 62.01; H, 6.23; N, 5.52; Yb, 22.98. IR (KBr, cm−1): 2954 (s), 2902 (w), 2863 (m), 2367 (w), 1609 (s), 1585 (m), 1511 (s), 1478 (s), 1424 (w), 1381 (m), 1358 (w), 1302 (w), 1239 (w), 1209 (w), 1165 (m), 1087 (w), 978 (w), 834 (s), 792 (m). LSmN(SiMe3)2(DME)·0.5DME (4·0.5DME). A solution of quinolinyl aminophenol H2L (0.74 g, 2.04 mmol) in toluene was slowly added to a stirred solution of Sm[N(SiMe3)2]3(THF) (1.44 g, 2.04 mmol) in toluene at room temperature. The reaction mixture was stirred at 90 °C for 3 days to give a dark red suspension. Removing the toluene solution by centrifugation led to dark red solids. The solids were crystallized from a mixture of DME (10 mL) and n-hexane (4 mL) at room temperature to afford dark red crystals of 4 after several days (1.10 g, 67%). Anal. Calcd for C36H61N3O4Si2Sm: C, 53.62; H, 7.62; N, 5.21; Sm, 18.65. Found: C, 54.24; H, 7.48; N, 5.47; Sm, 18.97. IR (KBr, cm−1): 2957 (s), 2896 (w), 2861 (m), 2345 (w), 1620 (s), 1548 (m), 1509 (s), 1476 (s), 1436 (w), 1378 (m), 1358 (w), 1304 (w), 1271 (w), 1198 (w), 1164 (m), 1015 (w), 936 (w), 835 (s), 740 (m). GC-MS Analysis. A small portion of the reaction mixture for the preparation of 2 was hydrolyzed and analyzed by GC-MS. A small amount of coupling product [(Me3Si)2N]2 was detected. Retention time for [(Me3Si)2N]2: 9.95 min. MS: m/z calcd for {[(Me3Si)2N]2 − 4Me}+ 264.13, found 264.2. X-ray Crystallography. Suitable single crystals of complexes 1−4 were sealed in thin-walled glass capillaries. Intensity data were collected with a Rigaku Mercury or Bruker APEXII CCD area detector in ω scan mode using Mo Kα radiation (λ = 0.071073 or 0.071075 nm). Details of the intensity data collection and crystal data are given in Table S1 (Supporting Information).

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ASSOCIATED CONTENT

S Supporting Information *

Figures giving the structures of complexes 2 and 4 and CIF files and a table giving crystallographic data for 1−4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (Grants 21072146, 21174095, and 21132002) and the Key Laboratory of Organic Synthesis of Jiangsu (KJS1008) for financial support.

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REFERENCES

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Organometallics

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dx.doi.org/10.1021/om5000198 | Organometallics 2014, 33, 1972−1976