Facile Route to Rare Heterobimetallic Aluminum ... - ACS Publications

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Facile Route to Rare Heterobimetallic Aluminum−Copper and Aluminum−Zinc Selenide Clusters Bin Li,† Jiancheng Li,† Rui Liu,† Hongping Zhu,*,† and Herbert W. Roesky*,‡ †

State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China ‡ Institut für Anorganische Chemie, Georg-August-Universität, Tammannstraβe 4, 37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Heterobimetallic aluminum−copper and aluminum−zinc clusters were prepared from the reaction of LAl(SeH)2 [1; L = HC(CMeNAr)2 and Ar = 2,6iPr2C6H3] with (MesCu)4 and ZnEt2, respectively. The resulting clusters with the core structures of Al2Se4Cu4 and Al2Se4Zn3 exhibit unique metal−organic frameworks. This is a novel pathway for the synthesis of aluminum−copper and aluminum−zinc selenides. The products have been characterized by spectroscopic methods and single-crystal X-ray structural characterization.

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rganoaluminum chalcogenides have drawn much attention because of their significant role in both fundamental chemistry and industrial applications.1 Aluminum oxide is widely used as a support in heterogeneous catalysis. Moreover, bimetallic organoaluminum chalcogenides play an important role as efficient catalysts2 and, in addition, provide better insight into the intramolecular interaction of heterogeneous metal oxide catalysts. Furthermore, various potential applications of such species on electron conductivity,3 biomedical research,4 and chemical vapor deposition5 have attracted much interest. Therefore, different heterobimetallic aluminum oxides6 and sulfides7 have been well established. However, only a few corresponding selenides are known because of the instability of metal−selenium bonds, in addition to the limited synthetic precursors and methods so far known. In 2000, LAl(SeH)2 (1; L = HC(CMeNAr)2 and Ar = 2,6-iPr2C6H3)8 was reported, which is indicated as being an efficient precursor. By using the lithium aluminum selenide [LAl(SeLi)2(THF)2] (THF = tetrahydrofuran), several aluminum derivatives including LAl(μ-Se)2GeR2 (R = Me, Ph) and LAl(μ-Se)2MCp2 (M = Ti, Zr; Cp = C5H5) have been synthesized.9 However, no complex containing an Al−Se− Cu unit has been reported until now, although the ternary semiconductor CuAlSe2 is a promising material in applications of light-emitting diodes and semiconductor lasers.10 Recently, we successfully prepared the coinage metal aluminum sulfur species [LAl(SM)2]n (M = Cu, n = 2; M = Ag, n = 4)7e by the reaction of LAl(SH)2 with (MesCu)411 and (MesAg)4,12 respectively. Herein, we were interested in extending this synthetic method to selenide compounds. Starting from 1 and (MesCu)4, we successfully synthesized the heterobimetallic aluminum−copper selenide [LAl(SeCu)2]2 (2; Figure 1). However, the attempt to prepare silver derivatives gave an intractable mixture, which may be due to the instability of the Se−Ag bond. Moreover, we also © XXXX American Chemical Society

Figure 1. Molecular structure of 2 with thermal ellipsoids at the 50% probability level. The H atoms are omitted for clarity.

explored the reaction of 1 with ZnEt2, because RZnII (R = monovalent organic group) is an isolobal analogue of CuI, and the novel Al2Se4Zn3-containing cluster was isolated. These compounds are well-defined and indicate a new efficient route to the synthesis of heterobimetallic selenides. The reaction between 1 and (MesCu)4 in a 2:1 molar ratio was carried out in toluene and in the temperature range from −20 °C to room temperature. After workup, colorless crystalline solid 2 was obtained in a moderate yield (55%). Compound 2 is air- and moisture-sensitive and soluble in organic solvents like toluene, nhexane, and THF. It is stable in the solid state at ambient temperature, whereas it decomposes gradually in solution. The 1 H NMR spectrum of 2 shows characteristic resonances of the ligand backbone with one singlet (1.55 ppm), one septet (3.63 ppm), two doublets (1.06 and 1.65 ppm), and one singlet (4.87 ppm). In the 13C NMR spectrum of 2, a singlet at 98.66 ppm is observed, which corresponds to the γ-C of the ligand backbone and is quite close to those of LAl(SeLi)2(THF)2 (98.67 ppm) and LAl(μ-Se)2GeR2 (R = Me, 98.43 ppm; R = Ph, 98.63 ppm).9 The 27Al NMR spectrum of 2 exhibits a singlet at 103.43 ppm, which is much more upfield-shifted than those of 1 (122 ppm),8 LAlS4 (133.18 ppm), and LAlS6 (121.65 ppm)7e but slightly downfield-shifted compared to that of LAl(μ-Se)2GePh2 (94.61 ppm).9 The 77Se NMR spectrum of 2 gives a signal at −475.45 Received: January 7, 2017

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DOI: 10.1021/acs.inorgchem.7b00012 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry ppm, more upfield-shifted than those found in LAl(μ-Se)2GeMe2 (−180.65 ppm) and LAl(μ-Se)2TiCp2 (318.90 ppm). Furthermore, we investigated the reaction of 1 with ZnEt2 in 1:1 and 1:2 molar ratios. Unexpectedly, all of the attempts resulted in the formation of [(LAlSe2)2Zn3Et2] (3; Scheme 1).

To a large extent, the Se−Cu bond distances depend on the coordination number of the Cu atoms. The average bond distance of the Se−Cu bonds in 2 is 2.302 Å, considerably shorter than those of Se−Cu bonds in which Cu adopts a coordination number of 3 or 4. Examples are [Cu4(μ-Se[O]CFc)4(PPh3)4] [2.3770(13) and 2.4060(13) Å],16 [Cu[SeC5H3(Me-3)N]]4 (av. 2.395 and 2.400 Å for two molecules in one lattice),17 [Cu(bpy) (SeCF3)]2 [2.3967(6) and 2.4464(6) Å; bpy = bipyridine],18 and [(Me3P)4(CuSePh)2] [2.4799(4) and 2.5245(4) Å].19 Unlike the structure of 2, the structural analysis of 3 reveals that two of the Zn atoms each carry an ethyl group, while the third has eliminated both of the ethyl groups of the ZnEt2 precursor. It represents a unique Al2Se4Zn3 cluster core (Figure 2). Normally complexes of zinc feature a coordination number of

Scheme 1. Preparation of Compounds 2 and 3

Increasing the amount of 1 or the further reaction of 3 with 1 did not proceed under elimination of the residual ethyl group, which demonstrates that 3 is a kinetically stable product in this reaction. Compound 3 was isolated as colorless crystals with a yield of 60% when the molar ratio was modified to 2:3. Compound 3 is thermally stable and decomposes above 225 °C. In the 1H NMR spectrum of 3, characteristic resonances of the β-diketiminato ligand are observed, and the resonances at 0.03 and 1.24 ppm imply the existence of ethyl groups. The 27Al NMR spectrum displays a broad resonance at 111.97 ppm, slightly downfieldshifted when compared with that of 1. The 77Se NMR spectrum of 3 displays one signal at −336.10 ppm, demonstrating the structural flexibility in solution. To confirm the structures of 2 and 3, single-crystal X-ray diffraction was carried out. Compounds 2 and 3 both crystallize in the triclinic space group P1̅. Compared to the normal planar framework of copper siloxanes and alumoxanes reported before, compound 2 exhibits a rare metal−organic structure with an Al2Se2Cu4 structural motif that contains four chair-shaped AlSe3Cu2 rings (Supporting Information, Figure S3). A comparable structure was reported for [LAl(SCu)2]2.7e The angles between the AlSe2 and Cu4 planes are 76.42° and 72.19°, respectively, while both AlSe2 planes are perpendicular to each other. All of the Se atoms are three-coordinate, adopting a pyramidal geometry [the peripheral angles around each of the Se atoms are in the range of 248.29(4)−253.87(4)°]. The Se atoms exhibit a coordination tendency toward the inside of the cage structure, which contributes to the stability of 2. This character is remarkably different from that of [LAl(OCu·MesCu)2]2, in which the O atoms feature closely triangular geometry [the peripheral angles around each O atom range from 346.35(13) to 359.87(13)°] to form a planar framework.6c The internal Se− Al−Se angles (121.22° and 122.16°) are much larger than that of 1 (103.73°) but similar to those in (LAlSeH)2(μ-Se) (117.29° and 118.69°).8 All Se atoms display trigonal-pyramidal geometry with a similar angle around 253°. The Al−Se bond [av. 2.384(3) Å] is longer than the Al−Se bonds in 1 [2.331(3), 2.340(3), 2.356(1), and 2.367(1) Å] 8 and [Al 4 Se 5 (H) 2 (NMe 3 ) 4 ] [2.332(4)−2.361(4) Å],13 whereas it is shorter than those in [Al{k2-N,Se-[4,5-(P(Se)Ph2)2tz]}3] [2.557(1) Å; tz = 1,2,3triazole]14 and [(5η-C5Me5)AlSe]4 [2.461(6)−2.497(0) Å].15

Figure 2. Molecular structure of 3 with thermal ellipsoids at the 50% probability level. The H atoms are omitted for clarity.

4 at the Zn atom. The Zn atoms in the structure of 3 are threecoordinate as one Zn atom binds to three Se atoms while the other two Zn atoms bind to two Se atoms and one ethyl group. The Zn atoms adopt almost trigonal-planar coordination, whereas the three-coordinate Se centers exhibit trigonalpyramidal geometry with angle sums of 268.5° [Se(1)], 262.5° [Se(2)], and 325.6° [Se(3)], respectively. The other Se atom that binds only to two adjacent atoms displays a sharp angle of 82.2°. The Se−Zn bonds with three-coordinate Se atoms are in the range of 2.4647(4)−2.517(5) Å, which is comparable with the typical bond distance of the Se−Zn bonds observed in [Zn(CH2SiMe3)Se-2,4,6-tBu3C6H2] [2.429(2)−2.467(2) Å]20 and (LZnSePh)2 [2.4291(5) and 2.5951(6) Å].21 However, when the Se atom is two-coordinate, a considerably shorter Se− Zn bond distance of 2.3270(4) Å is observed, which is close to the corresponding examples reported before.22 In summary, the aluminum−copper and aluminum−zinc selenides 2 and 3 were prepared by the reaction of 1 with (MesCu)4 and ZnEt2, respectively. Instead of LAl(SeLi)2, aluminum diselenol was directly used as the starting material. During the reaction, the Mes group was removed under the formation of MesH and leads to formation of the Al−Se−Cu unit. In the sample of 3, the ethyl groups in one case were completely removed from ZnEt2, while the other two Zn atoms each still show one ethyl substituent. Compounds 2 and 3 exhibit uncommon metal−organic frameworks, which provide new insight into the coordination ability of selenium metal species. Moreover, a new method was discovered for the synthesis of heterobimetallic selenides. B

DOI: 10.1021/acs.inorgchem.7b00012 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00012. Experimental details, analytical and crystallographic data of 2 and 3, and the details of crystal structure refinements (PDF) X-ray crystallographic data in CIF format for 2 (CIF) X-ray crystallographic data in CIF format for 3 (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hongping Zhu: 0000-0002-9777-6731 Herbert W. Roesky: 0000-0003-4454-1434 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Nature Science Foundation of China (Grants 21473142 and 21673191), the Program for Innovative Research Team in Chinese Universities (Grants 2013B019 and IRT_14R31), and the Deutsche Forschungsgemeinschaft (Grant RO 224/64-1).

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DEDICATION Dedicated to Professor Ionel Haiduc on the occasion of his 80th birthday. REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00012 Inorg. Chem. XXXX, XXX, XXX−XXX