Etienne V.
Brouillet
a,
Alan R.
Kennedy
a,
Konrad
Koszinowski
*b,
Ross
McLellan
a,
Robert E.
Mulvey
a and
Stuart D.
Robertson
*a
aWestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, G1 1XL, UK. E-mail: stuart.d.robertson@strath.ac.uk
bInstitut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstr. 2, 37077 Göttingen, Germany. E-mail: konrad.koszinowski@chemie.uni-goettingen.de
First published on 22nd February 2016
The cationic magnesium moiety of magnesium organohaloaluminate complexes, relevant to rechargeable Mg battery electrolytes, typically takes the thermodynamically favourable dinuclear [Mg2Cl3]+ form in the solid-state. We now report that judicious choice of Lewis donor allows the deliberate synthesis and isolation of the hitherto only postulated mononuclear [MgCl]+ and trinuclear [Mg3Cl5]+ modifications, forming a comparable series with a common aluminate anion [(Dipp)(Me3Si)NAlCl3]−. By pre-forming the Al–N bond prior to introduction of the Mg source, a consistently reproducible protocol is reported. Usage of the green solvent 2-methyltetrahydrofuran in place of THF in the context of Mg/Al battery electrolyte type complexes is also promoted.
While the dinuclear cation is the most common structurally characterized motif within these systems due to its thermodynamic stability, other aggregated cationic moieties have been implicated as playing an important role evidenced through techniques such as mass spectrometry, although they have never been isolated nor characterized crystallographically in a magnesium organoaluminate species. These include mononuclear [MgCl]+ and trinuclear [Mg3(μ3-Cl)2(μ2-Cl)3]+ (Fig. 1)16 that can all be found in a complicated equilibrium in solution which is difficult to resolve due to, for example, lack of appropriate NMR handles in the [MgxCl2x−1·nTHF] cations. Indeed, Muldoon has contended that this equilibrium can conceivably affect Mg electrochemistry in solution and consequently it should not be assumed that the crystallographically verified dinuclear complex is solely responsible,17 a hypothesis supported by an X-ray absorption near edge structure (XANES) study which suggests that it is a different (unidentified) magnesium species which is electrochemically active.18 The suggestion that these different cationic oligomers were involved was particularly interesting to us given our long standing interest in the bimetallic ‘ate’ chemistry of the main group metals, including magnesium and aluminium.19 We felt our synthetic expertise could be exploited to shed light on these different, important oligomeric cations in the presence of a common anion, the results of such a study we now present herein.
Fig. 1 THF solvated magnesium chloride cations implicated in magnesium aluminate solution chemistry. |
Scheme 1 Reactions accessing crystallographically authenticated magnesium aluminates (R, R′ = Dipp, Me3Si). Stoichiometries of reagents and identities of by-products are not shown for brevity. |
Fig. 2 Molecular structures of (a) anionic moiety of 1 [(Dipp)(Me3Si)NAlCl3]−; (b) cationic moiety of 1 [Mg2(μ2-Cl)3·6THF]+; (c) cationic moiety of 2 [MgCl·Me6TREN]+; (d) cationic moiety of 3 [Mg3(μ3-Cl)2(μ2-Cl)3·3TMEDA]+; (e) cationic moiety of 4 [Mg3(μ3-Cl)2(μ2-Cl)3·6MeTHF]+. All ellipsoids are displayed at 50% probability and H atoms and minor disordered components have been omitted for clarity. The anion is consistent across all four complexes so only that of 1 is shown for brevity. For selected bond parameters of the anion in all four cases see Table S1;† for bond lengths of all four cations see Table S2;† for bond angles of all four cations see Tables S3–S6.† Selected average bond parameters (Å/°) of cation 1: Mg–Cl, 2.5027; Mg–O, 2.0808; O–Mg–Cltrans, 176.91; O–Mg–Clcis, 92.98; Cl–Mg–Cl, 84.61; O–Mg–O, 89.36; Mg–Cl–Mg, 77.98; cation 2: Mg–Cl, 2.3229; Mg–Nax, 2.219; Mg–Neq, 2.182; Nax–Mg–Cl, 178.28; Neq–Mg–Cl, 98.98; Nax–Mg–Neq, 81.06; Neq–Mg–Neq, 117.61; cation 3: Mg–Cl(μ2), 2.489; Mg–Cl(μ3), 2.573; Mg–N, 2.190; N–Mg–Cl(μ3)trans, 178.32; N–Mg–Cl(μ3)cis, 95.48; N–Mg–Cl(μ2)cis, 98.38; Cl(μ2)–Mg–Cl(μ3), 81.67; Cl(μ2)–Mg–Cl(μ2), 157.31; Cl(μ3)–Mg–Cl(μ3), 84.92; N–Mg–N, 84.12; Mg–Cl(μ3)–Mg, 79.42; Mg–Cl(μ2)–Mg, 82.65; cation 4: Mg–Cl(μ2), 2.4929; Mg–Cl(μ3), 2.5555; Mg–O, 2.034; O–Mg–Cl(μ3)trans, 176.52; O–Mg–Cl(μ3)cis, 93.95; O–Mg–Cl(μ2)cis, 98.11; Cl(μ2)–Mg–Cl(μ3), 81.58; Cl(μ2)–Mg–Cl(μ2), 157.25; Cl(μ3)–Mg–Cl(μ3), 83.84; O–Mg–O, 88.28; Mg–Cl(μ3)–Mg, 80.24; Mg–Cl(μ2)–Mg, 82.69. |
As seen previously in other relevant magnesium aluminates, the cation of 1 has a non-crystallographic C3 axis of symmetry passing through the two magnesium atoms, which are in a virtually octahedral environment consisting of three bridging fac-chloride anions and three terminal THF molecules.
With respect to mononuclear species, the predominant form of the [MgCl]+ cation in the solid state is the penta-THF solvate, although to the best of our knowledge it has never been seen as the cationic moiety in a magnesium organohaloaluminate complex, which appear to prefer to adopt the thermodynamically more stable [Mg2(μ2-Cl)3]+ cationic structure.22 We do note that [MgCl·5THF]+ has been crystallographically characterized with an [AlCl4]− counteranion,23 suggesting that in the presence of THF, cation nuclearity can be dictated by crystal packing effects and is thus difficult to control or predict. Given that THF seems an unsuitable Lewis donor to stabilize [MgCl]+ on demand, we utilized the hemisphere-capping tripodal tetraamine Me6TREN [N(CH2CH2NMe2)3], previously exploited by us to trap sensitive mononuclear organometallic species24 and which Hazari has shown can stabilize [MgBr]+ and [MgMe]+ cations.25 Repeating the synthesis of 1 but with an equivalent of Me6TREN added prior to crystallization yielded [(Dipp)(Me3Si)NAlCl3]− [MgCl·Me6TREN]+, 2 (see Fig. 2c for the cationic moiety: the anion is the same as structure 1 so omitted for clarity). This unequivocally confirmed our view that a mononuclear MgCl cation could be prepared on demand by fully occupying one hemisphere of the metal and represents the first example of [MgCl·Me6TREN]+ to be synthesized and crystallographically characterized. Moreover, this represents potentially a key breakthrough for the solution study of mononuclear magnesium aluminate battery electrolyte complexes (vide infra) since the previously synthesized THF solvate displays poor solubility even in polar THF.9a Its Mg centre lies in a trigonal bipyramidal environment with the central nitrogen and chloride occupying axial positions with the three pendant arm N donor atoms occupying equatorial sites.
The number of crystallographically authenticated [Mg3(halide)5]+ cations is limited and none are THF solvated (we have identified five diethyl ether solvates26 and one TMEDA solvate in the CCDB,27 none of which have an aluminate counteranion). Some relevant THF solvated trinuclear Mg cations, with three μ2-Cl ligands and two μ2-alkoxide/aryloxide ligands were reported recently and shown to be promising in the battery electrolyte context,17,28 making a reproducible synthetic protocol for [Mg3Cl5]+ species particularly timely. We therefore moved away from THF and rather utilized bidentate chelating TMEDA. The resulting product [(Dipp)(Me3Si)NAlCl3]− [Mg3(μ3-Cl)2(μ2-Cl)3·3TMEDA]+, 3 was trinuclear as hoped (Fig. 2d) although its THF solubility was poor making it difficult to adequately characterize in solution (vide infra). We then moved to 2-methyltetrahydrofuran (MeTHF), whose slightly increased bulk vis-a-vis THF might prevent tris-solvation of the metal centre, and that enforced bis-solvation would consequently promote trinuclear cation formation. MeTHF is a greener alternative to THF with some similar properties29 and has previously found use as an electrolyte solvent for rechargeable lithium batteries30 so any progress using this Lewis donor would constitute a welcome development in the magnesium battery sector. Introducing MeTHF as the bulk solvent proved a successful protocol, with the resulting crystalline material shown by XRD to be [(Dipp)(Me3Si)NAlCl3]− [Mg3(μ3-Cl)2(μ2-Cl)3·6MeTHF]+, 4 (Fig. 2e). The cations contain a six atom (MgCl)3 ring capped on each side by another chloride anion. In 3 and 4, the Mg centres are in a distorted octahedral environment made up of two mutually cis Lewis donor atoms (N, 3; O, 4), two trans μ2 chloride anions and two mutually cis μ3 chloride anions. Looking at the synthetic work as a whole, the yields of the isolated O-donor complexes 1 and 4 were excellent (88/93% respectively with respect to magnesium) while those of N-donor complexes 2 (65%) and 3 (31%) were lower. In each case, elemental analysis confirmed the bulk purity of the samples.
Following the success of our reactions with polydentate N-donors TMEDA and Me6TREN, we repeated our protocol using the related ligand N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (PMDETA), expecting that this tridentate ligand could act as an isodentate surrogate for three molecules of THF, yielding a magnesium aluminate with a dinuclear N-solvated cation. However, the resulting product was shown by single crystal X-ray diffraction to be the neutral magnesium dichloride complex MgCl2·PMDETA (5, Fig. 3). We note that the related tridentate O-donor bis(2-methoxyethyl)ether (diglyme) also results in a neutral magnesium complex when used in this context, specifically dimeric [MgCl2·diglyme]2.10b This may suggest that acyclic tridentate donors do not possess the correct spatial conformation of their donor atoms to adequately protect one end of a [Mg2Cl3]+ fragment as they are aligned for mer rather than fac coordination to an octahedral metal centre, while they do not have the requisite number of donor atoms to protect a mononuclear or trinuclear cation.
1H and 13C NMR spectra, collected in d8-THF, confirmed the solvent separated ion pairing was maintained in solution with the anions giving identical spectra in all four cases (see Table S8† for a summary). The only 1H/13C containing fragments of the cations were the donor ligands making characterization challenging. However, the resonances of the Lewis donors in complexes 2 and 3 were noticeably deshielded with respect to the free ligand in the same solvent (see Fig. S1† for full details) showing that THF had not replaced Me6TREN or TMEDA. For complex 1, the 1H NMR spectroscopic THF resonances were very close to those of free THF, probably as a consequence of OC4H8/OC4D8 exchange while for 4, the MeTHF resonances were identical to those of free MeTHF (see Fig. S1 and S2†) suggesting that MeTHF is replaced by d8-THF, although this does not shed light on the aggregation state of the magnesium containing species in solution. 27Al NMR spectroscopy was uninformative, with well-resolved singlets absent due to the lack of high symmetry at the aluminium centre.
Complexes 1, 2 and 4 were also sufficiently soluble in C6D6 to obtain NMR spectra in the absence of bulk Lewis donor. Surprisingly, despite the anion being identical in all three cases, there were noticeable differences in their 1H NMR spectra (Fig. 4) suggesting strong ion-pairing in less polar solvent.
While the amido resonances of the di- and trinuclear complexes are fairly similar, those of the mononuclear complex (2) are clearly different. Specifically, the aryl resonances are well resolved into triplet (para) and doublet (meta), while the iPr and SiMe3 resonances are deshielded with respect to those in the di- and trinuclear complexes (1 and 4 respectively). Furthermore the iPr methyl resonance is resolved into a pair of doublets (1.64/1.53 ppm) suggesting inequivalence. There is a slight separation of these resonances in 1 and 4 but they still overlap at around 1.32 ppm in each. Temperature effects can be ruled out since all spectra were obtained at 300 K, while a variable concentration NMR experiment revealed identical spectra, showing that this change is not a consequence of concentration variation. The reason for these differences is not instantly clear, although one could perhaps speculate that the unique feature of mononuclear complex 2, namely a terminal Mg–Cl bond, is somehow able to interact with the anionic moiety in weakly-donating benzene solvent.
In positive ion mode, we looked first at the N-chelate containing complexes 2 and 3, in THF solution. For 2, the cations [MgCl(Me6TREN)]+ and [Mg2Cl3(Me6TREN)2]+ were observed (Fig. 5a, S10 and S11†). The former corresponds to the cationic component of the salt, the latter to its dinuclear homologue. It is not clear whether the dinuclear ion was already present in the original sample solution or whether it only formed during the ESI process. The ESI process produces charged nanodroplets, which permanently lose solvent molecules due to evaporation. The increased effective concentration in these nanodroplets can lead to shifts of aggregation equilibria and, thus, to formation of the observed dinuclear ions.32 Both [MgCl(Me6TREN)]+ and [Mg2Cl3(Me6TREN)2]+ exhibit a 1:1 stoichiometry of magnesium and ligand, which reflects the latter's polydentate nature. Likewise, the absence of any THF adducts points to the lack of empty coordination sites at the magnesium centre. In addition, ions containing the protonated ligand were detected (Fig. 5a and S12†). Because of its high Brønsted basicity, the ligand can easily react with traces of protic contaminants remaining in the used glassware or the ESI source. For 3, which was run at a lower concentration due to its poor solubility, vide supra, we observed ions belonging to the homologous series [MgnCl2n−1(TMEDA)n]+, n = 1–3 (Fig. 5b and S13–S15†). Like in the case of the Me6TREN-containing ions, these chelated ions display a 1:1, Mg:ligand stoichiometry and do not bind any THF.
Moving to the THF-solvated dinuclear complex [(Dipp)(Me3Si)NAlCl3]− [Mg2(μ2-Cl)3·6THF]+ (1) in THF, small amounts of [MgCl(THF)3]+ as well as [Mg2Cl3(THF)n]+, n = 3–5, and [Mg3Cl5(THF)n]+, n = 5 and 6, were detected (Fig. 5c and S16–S19†). Concentration-dependent measurements (Fig. 5d) showed that the relative abundance of the mononuclear ion decreased as a function of concentration, as expected on the basis of the law of mass action. The fraction of the dinuclear ions slightly increased with higher concentrations, whereas that of their trinuclear counterparts decreased slightly, the reason for this decrease not being obvious.
Analysis of solutions of the MeTHF-solvated complex [(Dipp)(Me3Si)NAlCl3]− [Mg3(μ3-Cl)2(μ2-Cl)3·6MeTHF]+ (4) in THF gave similar mass spectra (Fig. S20†). This finding proves that the MeTHF molecules coordinating to Mg centres are easily exchanged by excess of less bulky THF and corroborates our NMR findings. Repeating this experiment in MeTHF resulted in the detection of the trinuclear ions [Mg3Cl5(MeTHF)n]+, n = 4 and 5 (Fig. 5e and S21†). In comparison with THF-solvated 1 in THF, the nuclearity of the observed complexes was significantly shifted toward higher aggregation states. Accordingly, the behavior of these salts in solution appears to parallel their behavior in the solid state.
Next we performed the reverse control experiment and dissolved the THF-containing salt 1 in MeTHF (Fig. 5f and S22†). In this case, the recorded ESI mass spectrum showed mainly ions coordinated by MeTHF, but a few complexes retaining a single THF molecule as well. This incomplete exchange again indicates that THF binds to the magnesium cations more strongly than MeTHF.
Finally, further information was obtained from the gas-phase fragmentation experiments (Fig. S10–S20†). The Mg complexes binding THF and MeTHF exclusively dissociated by losing one or two solvent molecules (Fig. S28–S33†). For the larger and more fully solvated ions, the loss of one THF or MeTHF molecule occurred so easily that it proceeded even without the application of any extra excitation energy, as also the poorer mass resolution of the isotope patterns for these ions indicated.33 For the smaller and less solvated ions, the loss of one THF or MeTHF molecule occurred less easily and required the concomitant addition of one water molecule to avoid a decrease in the coordination number (the ion trap mass spectrometer inevitably contains a low partial pressure of background water). The TMEDA-containing ions exchanged a ligand for water only to a minor extent, but mainly decomposed by expulsion of a neutral [MgCl2(TMEDA)] fragment (Fig. S26 and S27†). The analogous loss of neutral [MgCl2(Me6TREN)] was also observed for the dinuclear complex [Mg2Cl3(Me6TREN)2]+ (Fig. S25†) whereas such a fragmentation reaction was not feasible for its mononuclear counterpart. This mononuclear ion only underwent partial decomposition of the ligand (Fig. S23†). This deviating behavior of the TMEDA- and Me6TREN-containing complexes reflects the significantly stronger binding energies of these chelating ligands in comparison with monodentate THF and MeTHF.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1431065–1431068 and 1443823. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt00531d |
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