Communication Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Formation and Properties of the Trichloroberyllate Ion Magnus R. Buchner,*,† Nils Spang,† Matthias Müller,† and Stefan S. Rudel‡ †
Anorganische Chemie, Nachwuchsgruppe Berylliumchemie,, and ‡Anorganische Chemie, Arbeitsgruppe Fluorchemie, Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, 35032 Marburg, Germany
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formation of phosphonium salts (Scheme 1). In the case of compound 1a in dichloromethane, the known [Me3PCH2Cl]Cl
ABSTRACT: The activation of C−Cl bonds in dichloromethane and chloroform was observed by BeCl2 in the presence of PMe3 and PCy3. This leads to the formation of [Me3PCH2Cl]Cl and [Cy3PCHCl2][BeCl3]. The latter compound is the first example of a tricoordinated beryllium species with nonbulky ligands and proof of the existence and stability of the long-predicted [BeCl3]− ion. In analogy to the isoelectronic BCl3, the trichloroberyllate anion exhibits Lewis acidic behavior toward electron-pair donors and was probed for the electronic and steric influence of the Lewis base. [BeCl3]− can also act as a chloride ion donor or acceptor, leading to the formation of neutral phosphane adducts to BeCl2 and [BeCl4]2−. The existing equilibria between these species were investigated and showed high chloride mobility.
Scheme 1. Reaction Pathway of the C−Cl Bond Activation in Dichloromethane with 1a and in Chloroform with 5a
I
n the past decade, interest in alkaline-earth-metal chemistry has grown immensely because of the evolution of Lewis acidic molecular catalysts based on magnesium, calcium, strontium, and barium.1 For example, CaI2, in combination with a strong organic Schwesinger base, is able to activate unreactive CC double bonds,2 while simple alkaline-earth-metal (Mg−Ba) bisamides can be used as precatalysts for the 100% atom-efficient assembly of imidazolidin-2-ones.3 Magnesium and calcium βdiketiminato compounds have also been shown to reduce CO2 in the presence of the strong Lewis acid B(C6F5)3.4 Despite all of these advances, beryllium has been excluded from any catalytic research, presumably because of its alleged extreme toxicity.5 However, beryllium compounds have been proven to exhibit high reactivity toward bond activation, as is evident from the beryllium insertion into C−N6 and Si−O7 bonds. Also, the chemistry of beryllium is closely related to that of aluminum.8 Considering that aluminum compounds have been widely employed as Friedel−Crafts catalysts9 and (co)catalysts in olefin polymerization reactions,10 the potential for beryllium compounds is immense. Especially because the Be2+ cation is the smallest known metal cation and has the highest charge density.8 Here we report on the beryllium-induced activation of C−Cl bonds and subsequent formation of the trichloroberyllate anion. It is known that PMe3 partly dissociates from (Me3P)2BeCl2 (1a) in solution.11 This leads presumably to the presence of small amounts of tricoordinated monophosphane adducts to BeCl2 (2) in solution, which act as Lewis acids and may form adducts to chlorinated solvents (3) via coordination to a chlorine atom. The electron-deficient beryllium atom polarizes the C−Cl bond, which enables nucleophilic attack of the dissociated phosphane at the carbon atom, leading to the © XXXX American Chemical Society
(4)12 was generated after several weeks in the form of colorless columns, analyzed via X-ray diffraction, and found to match the previously reported crystal parameters.13 To check if this Lewis acid catalyzed reaction could be employed more broadly and the reaction rate could be accelerated, we decided to investigate a more bulky phosphane (PCy3) and a more reactive solvent (CHCl3). PCy3 is known to form monophosphane adducts to BeCl2, which dimerize in the solid state to [(Cy3P)BeCl2]2 (5a).14 Therefore, it was probable that dissociation into compound 2 would occur in solution because this species should be significantly more stable with a more electrondonating and more bulky ligand than PMe3. Upon the addition of CDCl3 to a 1:1 mixture of BeCl2 and PCy3, a white precipitate formed immediately, and only signals of known [Cy3PCHCl2]+ could be observed in the 31P NMR spectrum.15 Upon standing for several days, crystals of [Cy3PCHCl2][BeCl3] (6a; CCDC 1853083) formed in the shape of colorless blocks, which were suitable for single-crystal X-ray diffraction analysis. Salt 6a crystallizes in the monoclinic space group C2/c (Figure 1). The beryllium atom is trigonal-planar-coordinated Received: July 11, 2018
A
DOI: 10.1021/acs.inorgchem.8b01934 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
one signal at 11.9 ppm is observed. This is in the typical range of three-coordinated beryllium compounds26 however at relatively high field due to the negative charge. The reaction of 2 equiv of [Cy3PCHCl2]Cl with BeCl2 gave [Cy3PCHCl2]2[BeCl4] [7a; Scheme 2(ii)], which shows one signal at 7.6 ppm in 9Be NMR spectroscopy. This chemical shift is typical for electron-deficient, tetrahedrally coordinated beryllium compounds like (PhC(O)H)2BeCl2 (7.2 ppm)27 or (HC(O)OMe)2BeCl2 (5.7 ppm)28 and suggests that the Be−Cl bond is significantly weaker than that in [BeCl3]−. This is supported by the IR spectrum, where one weak band and one strong band at 628 and 502 cm−1, respectively, were found for 7. These bands are in good agreement with a report by Neumüller and Dehnicke for [PPh4]2[BeCl4] (7b; 500 and 251 cm−1).18 The 1H, 13C, and 31P NMR spectra of compounds 6a and 7a show the expected signals and coupling patterns of the phosphonium cation.15 For comparison, 7b and [PPh4]2[Be2Cl6] (8) were synthesized according to literature procedures.18 As expected, the tetrachloroberyllate 7b showed the same 9Be NMR spectrum as that of salt 7a. However, to our surprise, the solution of the hexachlorodiberyllate 8 only showed the signal at 11.9 ppm in 9 Be NMR spectroscopy, which was also observed for compound 6a. Variable-temperature 9Be NMR spectroscopy showed a nonlinear temperature-dependent chemical shift toward higher field and increasing line broadening upon cooling. This is evidence for an equilibrium with a four-coordinated beryllium species at low temperature. We therefore deduce that [Be2Cl6]2− is only present in the solid state or in solution at low temperature, whereas it readily dissociates into [BeCl3]− at ambient temperature [Scheme 2(iii)]. This is in accordance with equilibria observed between different monomeric and dimeric trisazidoberyllates at elevated temperatures.29 Because [BeCl3]− is still an electron-deficient compound and strong Lewis base adducts to it were already known,19,30 we probed the electron-acceptor properties by the reaction of compound [PPh4][BeCl3] (6b) with PMe3, PCy3, and PPh3. If [BeCl3]− is treated with 1 equiv of phosphane, an upfield shift of the signal in the 9Be NMR spectra is observed, which is indicative for more electron density at the beryllium nucleus and therefore electron donation from the phosphane. The shift is strongest for PMe3 (δ = 6.5 ppm) and is in the typical range of four-coordinated beryllium nuclei, which suggests the formation of [PPh4][(Me3P)BeCl3] (9a), although the chemical shifts and line widths in the presence of PCy3 (δ = 9.7 ppm) and PPh3 (δ = 11.5 ppm) indicate an equilibrium between coordinated and free phosphane, which lies far on the noncoordinated side in the case of PPh3 (Scheme 3A; see also Table 1). The inferior bonding properties of PCy3 and especially PPh3 can be attributed to the steric bulk at the phosphanes and, for the latter, additionally to the significantly lower σ-donor capability. To further explore this equilibrium, [BeCl3]− was titrated with excess phosphane in CD2Cl2 to check whether the 9Be NMR signal is shifted to higher field. However, this is only the case for PPh3, while excess PCy3 leads to decomposition of the phosphane under the formation of [Cy 3 PCHCl 2 ] + , [Cy3PCH2Cl]+, Cy3PCl2, and [Cy3PCl]+, as is evident from their 31P NMR signals.15 A total of 2 equiv or more of PMe3 resulted in a sharp signal at 4.6 ppm (ω1/2 = 2.2 Hz) in the 9Be NMR spectrum, which is very close to the signal of 1a (4.1 ppm) and suggests the presence of a highly fluxional diphosphane adduct.11 Additionally, we observed the formation of 7b crystals from the reaction solutions of 6b with 1 equiv of PPh3 or PCy3 and of [PPh4]Cl from the reaction of 6b with excess PMe3.
Figure 1. Molecular structure of compound 6a in the solid state.
by three chlorine atoms, with Cl−Be−Cl angles between 119.2(1)° and 121.5(1)°. The Be−Cl distances are between 1.906(2) and 1.921(2) Å, which is in good agreement with the calculated ones (1.922−1.939 Å)16,17 but is significantly shorter than those in [Be2Cl6]− [terminal Cl, 1.952(3)−1.969(3) Å; bridging Cl, 2.102(3)−2.108(3) Å], [BeCl4]2− [7; 2.024(3)− 2.049(3) Å], and Lewis base adducts to [BeCl3]− [2.008(3)− 2.022(5) Å].18,19 This is expected because of the lower coordination number and therefore lower electron density at the beryllium atom in [BeCl3]−, which leads to stronger Be−Cl bonds. In accordance with this, shorter Be−Cl distances are only observed for three-coordinated neutral beryllium compounds like N-heterocyclic carbene adducts of BeCl2 [1.881(6)− 1.884(9) Å].20 The corresponding Zn−Cl/Br distances in [ZnCl3]− [2.260(2)−2.205(2) Å]21 and [ZnBr3]− [2.356(2)− 2.552(3) Å]22 are significantly longer, while the B−Cl distance in the isoelectronic BCl3 is significantly shorter [1.75(2) Å].23 The structural parameters of the cation show no extraordinary features in comparison to the known [Cy3PCHCl2]+ salts.15 To investigate the properties of the trichloroberyllate ion further, salt 6a was synthesized in dichloromethane directly from 1 equiv of BeCl2 and [Cy3PCHCl2]Cl, respectively, in quantitative yield [Scheme 2(i)]. Besides the bands of the Scheme 2. Reactions of (i) [Cy3PCHCl2]Cl with BeCl2 to 6a, (ii) 2 equiv of [Cy3PCHCl2]Cl or [PPh4]Cl with BeCl2 to 7a and 7b, Respectively, and (iii) [PPh4]Cl with BeCl2 to 6b in Solution, Which Dimerizes to 8 in the Solid State
cation, the IR spectrum of compound 6a shows one strong band at 714 cm−1, which belongs to BeCl3−. This is in good agreement with the calculated values (716−770 cm−1)17,24 and the assignments from matrix isolation studies (678.8−710.8 cm−1).24 In comparison to neat BeCl2, which exhibits bands in the IR spectrum at 583 and 450 cm−1, the band of BeCl3− is found at significantly higher wavenumbers, which is further proof for the strong Be−Cl bond.25 In 9Be NMR spectroscopy, B
DOI: 10.1021/acs.inorgchem.8b01934 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
both species. In contrast to this, in the presence of 2 equiv of beryllate 6b and 1 equiv of 7b, only one signal at 11.3 ppm was observed in the 9Be NMR spectrum. This is evidence for fast exchange between both species on the NMR time scale and proves a high chloride mobility (Scheme 3B). This is also in line with dissociation of the tetraazidoberyllate dianion into triazidoberyllate and azide ions at elevated temperatures.29 The reaction mixture of 1 equiv of beryllate 7b and compound 1a showed one signal at 8.8 ppm in the 9Be NMR spectrum, which indicates that not only adduct 9a but also compound 6b is formed. Therefore, some of PMe3 has to have been removed from the equilibrium. This is also evident from the presence of further signals in the 31P NMR spectra. The solution of a 1:1 mixture of complex 1a and [PPh4]Cl showed the same 9Be NMR spectrum as [BeCl3]− with an excess of PMe3, which proves fast exchange at the phosphane complex if Cl− is present (Scheme 3C). These experiments confirm that exchange between all observed and isolated species occurs. These equilibria are then shifted toward 7b and [PPh4]Cl because of their low solubility and consequent precipitation. We showed that BeCl2 is capable of activating C−Cl bonds, enabling nucleophilic attack of the carbon atom by phosphanes at ambient temperature. This led to the formation of 4 and 6a if PMe3 in dichloromethane or PCy3 in chloroform were used, respectively. [BeCl3]− in salt 6a is the first example of a stable, tricoordinated beryllium compound that does not bear bulky ligands. Surprisingly, also [Be2Cl6]2− dissociates into [BeCl3]− in solution, which proves the high stability of this species. [BeCl3]− acts as a Lewis acid, as is evident from the formation of phosphane adducts 9 and shows amphoteric behavior regarding chloride ions because it can act as a chloride acceptor or donor.
Scheme 3. Equilibria of (A) Phosphane and 6b with 9, (B) Phosphane and 6b with 7b and 1 or 5, and (C) 9 and PMe3 with [PPh4]Cl and 1a
Table 1. Observed 9Be NMR Chemical Shifts δ/ppm (ω1/2/Hz)a (PMe3)2BeCl2 (1a) [Cy3PCHCl2][BeCl3] (6a) [PPh4][BeCl3] (6b) [Cy3PCHCl3]2[BeCl4] (7a) [Ph4P]2[BeCl4] (7b) [Ph4P][(PMe3)BeCl3] (9a) 6b + PCy3 6b + PMe3
4.1 (11.9)11 11.9 (9.9) 11.9 (9.9) 7.6 (8.4) 7.4 (10.1) 6.5 (3.5) 9.7 (9.6) 11.5 (9.6)
a
All spectra were recorded in CD2Cl2.
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Therefore, it was apparent, that additional equilibria had to be present. To investigate these equilibria, we reacted the compounds assumed to be present in these equilibria in pairs (Figure 2). In the presence of 1 equiv of complex 1a and 2 equiv of beryllate 6b, signals for both compounds could be observed in CD2Cl2 solution via 9Be NMR spectroscopy. A slight shift of these signals toward each other indicates a slow equilibrium between
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01934. Full details on the synthesis, experimental setup, and characterization of the described compounds (PDF) Accession Codes
CCDC 1853082−1853084 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
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Magnus R. Buchner: 0000-0003-3242-6797 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS M.R.B. thanks Prof. F. Kraus for moral and financial support as well as the provision of laboratory space. We thank the NMR and X-ray department of Philipps-Universität Marburg for their assistance and the DFG for financial support.
Figure 2. 9Be NMR spectra of (a) 1a, (b) a 1:1.9 mixture of compounds 1a and 6b, (c) 6b, (d) a 2.3:1 mixture of compounds 6b and 7b, (e) 7b, and (f) a 1:1.4 mixture of compounds 1a and 7b in CD2Cl2. C
DOI: 10.1021/acs.inorgchem.8b01934 Inorg. Chem. XXXX, XXX, XXX−XXX
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Reactions, and Electronic Properties of 16 π-Electron Octaisobutyltetraphenylporphyrin. J. Am. Chem. Soc. 2010, 132, 12627−12638. (22) Santoro, O.; Nahra, F.; Cordes, D. B.; Slawin, A. M.; Nolan, S. P.; Cazin, C. S. Synthesis, characterization and catalytic activity of stable [(NHC)H][ZnXY2] (NHC = N-Heterocyclic carbene, X, Y = Cl, Br) species. J. Mol. Catal. A: Chem. 2016, 423, 85−91. (23) Atoji, M.; Lipscomb, W. N. B−Cl Distance in Boron Trichlorid. J. Chem. Phys. 1957, 27, 195−195. (24) Yu, W.; Andrews, L.; Wang, X. Infrared Spectroscopic and Electronic Structure Investigations of Beryllium Halide Molecules, Cations, and Anions in Noble Gas Matrices. J. Phys. Chem. A 2017, 121, 8843−8855. (25) Müller, M.; Pielnhofer, F.; Buchner, M. R. A facile synthesis for BeCl2, BeBr2 and BeI 2. Dalton Trans 2018, DOI: 10.1039/ C8DT01756E. (26) (a) Niemeyer, M.; Power, P. P. Synthesis, 9Be NMR Spectroscopy, and Structural Characterization of Sterically Encumbered Beryllium Compounds. Inorg. Chem. 1997, 36, 4688−4696. (b) Plieger, P. G.; John, K. D.; Keizer, T. S.; McCleskey, T. M.; Burrell, A. K.; Martin, R. L. Predicting 9Be Nuclear Magnetic Resonance Chemical Shielding Tensors Utilizing Density Functional Theory. J. Am. Chem. Soc. 2004, 126, 14651−14658. (c) Perera, L. C.; Raymond, O.; Henderson, W.; Brothers, P. J.; Plieger, P. G. Advances in beryllium coordination chemistry. Coord. Chem. Rev. 2017, 352, 264−290. (27) Müller, M.; Buchner, M. R. Beryllium Complexes with BioRelevant Functional Groups: Coordination Geometries and Binding Affinities. Angew. Chem., Int. Ed. 2018, 57, 9180. (28) Scheibe, B.; Buchner, M. R. Carboxylic Acid Ester Adducts of Beryllium Chloride and Their Role in the Synthesis of Beryllium Nitrates. Eur. J. Inorg. Chem. 2018, 20−21, 2300−2308. (29) Naglav, D.; Tobey, B.; Lyhs, B.; Römer, B.; Bläser, D.; Wölper, C.; Jansen, G.; Schulz, S. Synthesis, Solid-State Structure, and Bonding Analysis of a Homoleptic Beryllium Azide. Angew. Chem., Int. Ed. 2017, 56, 8559−8563. (30) Petz, W.; Dehnicke, K.; Neumüller, B. About the Reaction of BeCl2 with the Carbodiphosphorane Addition Compound O2CC(PPh3)2 and its Hydrolysis Product Ph3PCHP(O)Ph2. Z. Anorg. Allg. Chem. 2011, 637, 1761−1768.
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DOI: 10.1021/acs.inorgchem.8b01934 Inorg. Chem. XXXX, XXX, XXX−XXX
