Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Electrospray Ionization Mass Spectrometric Study of the Gas-Phase Coordination Chemistry of Be2+ Ions with 1,2- and 1,3-Diketone Ligands Onyekachi Raymond,† Penelope J. Brothers,‡ Magnus R. Buchner,§ Joseph R. Lane,† Matthias Müller,§ Nils Spang,§ William Henderson,*,† and Paul G. Plieger*,∥ †
Chemistry, School of Science, University of Waikato, Private Bag 3105, Hamilton 3216, New Zealand School of Chemical Sciences, University of Auckland, Private Bag 92019, Auckland 1142, New Zealand § Anorganische Chemie, Nachwuchsgruppe Berylliumchemie, Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße 4, 35032 Marburg, Germany ∥ School of Fundamental Sciences, Massey University, Private Bag 11222, Palmerston North 4410, New Zealand
Inorg. Chem. Downloaded from pubs.acs.org by UNIV OF MELBOURNE on 04/10/19. For personal use only.
‡
S Supporting Information *
ABSTRACT: Electrospray ionization mass spectrometry (ESI MS) is a powerful technique for the study of coordination complexes because of its ability to analyze solution systems involving very low concentrations of metal complexes. In this work, the coordination chemistry of Be ions with a selection of well-known 1,3-diketone and related 1,2-diketone ligands has been investigated using ESI MS. With acetylacetone (Hacac), a range of acac-containing ions is observed, including [Be(acac)2H]+, [Be(acac)(MeOH)n]+ (n = 1, 2), and polynuclear species such as the dinuclear [Be2(acac)3]+ and trinuclear [Be3O(acac)3]+. Density functional theory calculations indicate that the latter species has a central Be3(μ3-O) core, with each Be chelated (as opposed to being bridged) by an acac ligand. The effect of changing the substituents on 1,3-diketone was explored by an investigation of mixtures of Be2+ with other 1,3-diketones such as dibenzoylmethane (Hdbm), where the [Be(dbm)2H]+ ion showed a lesser tendency to undergo fragmentation and aggregation processes. Comparisons with the corresponding aluminum acetylacetone system were also made. In contrast, mixtures of Be2+ and the 1,2-diketones diacetyl and phenanthrenequinone showed poor metal−ligand interactions. Be2+ interacted with the 1,2-diketone benzil [PhC(O)C(O)Ph], forming the [Be(benzil)n]2+ (n = 2−4) ions. The synthesis (from BeCl2) and X-ray structures of the dibenzoylmethanato (dbm) complex Be(dbm)2 and the benzil complex [BeCl2(benzil)] are also reported.
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INTRODUCTION
technique is amenable for rapid microscale screening of solution species. This advantage has been well harnessed in a variety of “combinatorial-type” approaches toward the ESI MS survey of metalloligand chemistry in inorganic and organometallic systems.2 Worthy of mention is the utilization of this strategy in the conservation of rather expensive metals and starting materials prior to macroscale characterization using other techniques such as X-ray crystallography and NMR spectroscopy.2,4,10,11 Indeed, a similar limitation is encountered in the coordination chemistry of beryllium as a result of its high toxicity. Therefore, it is even more desirable to work on a microscale as a prelude for further investigation when navigating the relatively uncharted territory of beryllium coordination chemistry.12,13
The technique of electrospray ionization mass spectrometry (ESI MS) has gained significant popularity as a means of probing solution speciation, with a broad spectrum of applications across many scientific fields including inorganic and organometallic chemistry.1−3 An essential feature of this technique is its ability to directly provide stoichiometric information on metal−ligand complexation and reactivity in solution.2,4,5 In addition, the ESI MS technique can also be employed to quantify the abundance of representative ions in the mass spectra as a means of examining metal−ligand binding affinity and selectivity in solution.5,6 While this requires careful consideration of the (coordination) chemistry principles of the system, numerous studies have revealed good agreement of ESI MS speciation data with other solution-based techniques.1,7−9 However, the greatest advantage of the ESI MS technique lies in its ability to handle complex mixtures involving tiny amounts of sample, to the extent that the © XXXX American Chemical Society
Received: February 26, 2019
A
DOI: 10.1021/acs.inorgchem.9b00578 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 1. 1,3-Diketone and 1,2-diketone ligands used in this study.
Table 1. Ion Assignments for the Positive-Ion ESI MS Analysis of 1:2 Molar Mixtures of BeSO4/Hacac and BeSO4/Hdbm in a 1:1 Methanol/Water Solution Be2+/Hacac
Be2+/Hdbm
relative ion intensity (%) with CEV m/z 101 108 126 136 190 140 158 172 166 208 212 233 315 340 420 445 461
ion [H2acac]+ [Be(acac)]+ [Be(acac)(H2O)]+ [Be3O3(CH3CO)(H2O)]+ [Be3O3(CH3CO)(H2O)4]+ [Be(acac)(CH3OH)]+ [Be(acac)(CH3OH) (H2O)2]+ [Be(acac)(CH3OH)2]+ [Be5O4(CH3COCH2)]+ [Be(acac)2H]+ [Be2(acac)(SO4)]+ [Be2OH(acac)2]+ [Be2(acac)3]+ [Be3O(acac)3]+ [Be3(acac)3SO4]+ [Be3(acac)3(SO4)BeO]+ [Be4O(OH) (HSO4)2(acac)2]+
low 40 V
medium 80 V
high 180 V
2
40 4
10 35 3 10
2 6 100 45
10 75
25 40
100
6 100 2 2 2 2
2 6 3 40 25
relative ion intensity (%) with CEV m/z
ion
105 225 250 264 282 296 456
[C6H5CO]+ [H2dbm]+ [Be(dbm)(H2O)]+ [Be(dbm)(CH3OH)]+ [Be(dbm)(CH3OH)(H2O)]+ [Be(dbm)(CH3OH)2]+ [Be(dbm)2H]+
513 687 481 712 792 260
[Be2OH(dbm)(CH3OH)]+ [Be2(dbm)3]+ [Be2OH(dbm)2]+ [Be3O(dbm)3]+ [Be3(dbm)3SO4]+ [Be5O4(C6H5COCH2) (CH3OH)]+
low 40 V
medium 80 V
high 180 V
2
95
3 17
15
100
35
3 15
5 15 2
1
3 40
16 50 13 5 20 100
5 43 5 4 100
6 2 2
Like the grossly understudied chemistry of beryllium,14 the mass spectrometry of beryllium compounds is equally sparse, although older ionization techniques such as electron ionization and fast-atom-bombardment mass spectrometry have previously been employed in a few fragmentation studies.15−19 The limited use of mass spectrometry is not unexpected because, in addition to its toxicity, beryllium is monoisotopic and exhibits only one stable oxidation state (2+). The isotopic pattern of beryllium complexes with organic ligands essentially involves only one major peak except in the presence of heavier isotopes of anions such as chloride or bromide. Consequently, very little is portrayed in terms of isotopic information in comparison to other isotope-rich metals, where the presence of the distinctive isotopic signature of the element provides unequivocal evidence for its presence. Nevertheless, general research interest in the chemistry of beryllium is currently being rejuvenated in response to burgeoning production output and diversification of applica-
tions involving this element across many industries.20,21 Because an area of frontline interest in this field is the interaction of beryllium with important classes of ligands, our current work aims to project the technique of ESI MS as an alternative and safe methodology suitable for investigating the solution speciation of the toxic Be ion. However, the correspondence between solution speciation and ions observed in the (gas-phase) mass spectrum is not always general,22 so caution must always be exercised during their comparison. Following our recent investigation into the ESI MS behavior of the Be ion in the presence of simple inorganic ligands such as sulfate,13 we have recently exploited ESI MS in order to screen the coordination chemistry of the Be2+ ion with aminopolycarboxylate ligands for potential selective binding.23 Herein we report investigations on the ESI MS ionization and fragmentation behavior of beryllium complexes with a range of diketone ligands (see Figure 1). These complexes, which were synthesized in situ and subjected to detailed characterization B
DOI: 10.1021/acs.inorgchem.9b00578 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
presence of beryllium hydrolysis species at a low concentration of Hacac. A further increase of the ratio of Hacac increased the abundance of [Be(acac)2H]+ (m/z 208), but only a few differences were observed between the spectra of Be2+/Hacac at 1:2 and 1:4 ratios. The ESI MS spectra of Be2+/Hacac at a 1:1 molar ratio reveals a base peak at m/z 140, which can be assigned to either the hydroxido species [Be3(OH)3(OCH3)2]+ or the diketonate species [Be(acac)(CH3OH)]+. However, the behavior of the ion signal at m/z 140 when the solvent is changed to an acetonitrile/water solvent mixture (Figure 3)
by ESI MS, serve to identify any ligand exchange and/or solvolysis processes (because ESI MS transfers preexisting solution ions into the gas phase) and add to the present knowledge of factors that influence the stability of the beryllium complexes. For example, the chelate effect and polynuclear binding via bridging phenolic groups have already been identified as playing a major role in stabilizing beryllium complexes.24
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RESULTS AND DISCUSSION Investigation of Mixtures of Be2+ and Acetylacetone by ESI MS. 1,3-Diketones such as acetylacetone [pentane-2,4dione, CH3C(O)CH2C(O)CH3, Hacac] and its corresponding acetylacetonate (acac−) anion are important ligands in beryllium chemistry because they are widely employed as chelating agents in extraction of the Be ion in numerous analytical procedures and mineral processing.25−27 Given the many practical applications of beryllium diketonato complexes, extensive ESI MS investigation has been carried out on these complexes especially with the parent acetylacetonate ligand, in order to provide insight into metal−ligand binding in this system. Positive- and negative-ion ESI MS spectra were recorded at a range of low (40 V), medium (80 V), and high (180 V) capillary exit voltages (CEVs) for mixtures of BeSO4 and Hacac in molar ratios of 1:0.5, 1:1, 1:2, and 1:4 in a 1:1 methanol/water solution. The CEV is a useful parameter allowing simple manipulation of the ionization conditions, with higher CEVs typically resulting in a greater propensity to observe singly charged ions. Data are summarized in Table 1, while illustrative spectra at various metal−ligand molar ratios are shown in Figure 2. The spectra suggest the predominant
Figure 3. Positive-ion ESI MS spectra of a 1:2 molar ratio BeSO4 and Hacac in (a) 1:1 methanol/water and (b) 1:1 acetonitrile/water solutions at a CEV of 40 V, displaying a change in the ion signals corresponding to the solvated species. L = [CH3COCHCOCH3]−.
points to the latter species being the most probable assignment. Thus, ESI MS analysis of Be2+ and Hacac in an acetonitrile/water solution (Figure 3b) reveals the corresponding [Be(acac)(CH3CN)]+ species at m/z 149. [Be(acac)(CH3CN)2]+ was also observed at m/z 190. It is interesting that acetonitrile competes very successfully with water for the hard Be2+ center. In this regard, previous studies have shown that, in the gas phase, the uranyl (UO22+) cation is preferentially solvated by acetonitrile over water (in contrast to the solution behavior) because of the higher dipole moment and polarizability of acetonitrile,28 and it is likely that the same effect is operating with Be2+. These observations also support the caveat that care must be taken during interpretation of the ESI MS spectra. The bis(acetylacetonato) complex Be(acac)2 is well-known and has been structurally characterized.29 The assignment of the [Be(acac)2H]+ (m/z 208) ion is further supported by spiking the analyte solution with Na and K ions, which revealed the corresponding [Be(acac)2Na]+ and [Be(acac)2K]+ ions at m/z 230 and 246, respectively. An additional interesting feature in these mass spectra is the dominance of the peak at m/z 315 corresponding to the polynuclear species [Be2(acac)3]+. While this complex has not been isolated, a possible route to its formation involves Be(acac)2 acting as a metalloligand toward the [Be(acac)]+ species. A possible structure, maintaining four-coordination at each beryllium, is shown by 1. This ion is observed even at high CEVs, suggesting that it is relatively stable toward CEV-induced fragmentation. The geometry of 1 was optimized using density
Figure 2. Positive-ion ESI MS spectra for 1:0.5, 1:1, 1:2, and 1:4 molar mixtures of BeSO4 and Hacac in a 1:1 methanol/water solution at a low CEV of 40 V. L = [CH3COCHCOCH3]−. For detailed ion assignments, refer to Table 1.
species to be [Be(acac)(CH3OH)]+ (m/z 140), [Be(acac)(CH3OH)2]+ (m/z 172), [Be(acac)2H]+ (m/z 208), and [Be2(acac)3]+ (m/z 315). The small size of the Be2+ cation and the likelihood of a maximum coordination number of 4 implies that the solvent and acetylacetonate ligands must compete for the four available coordination sites. The ESI MS spectra of Be2+/Hacac at ratios below 1:0.5 reveal the species [Be(OCH3)(CH3 OH)3]+, [Be(OH)(H 2 O) 2 (CH 3 OH)] + , [Be 2 (OH)(SO 4 )(CH 3 OH) 3 ] + , [Be3(OH)3(SO4)]+, and [Be(acac)(CH3OH)]+, reflecting the C
DOI: 10.1021/acs.inorgchem.9b00578 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Figure 4. (Top) Proposed structures of 2a and 2b. (Bottom) Optimized structures of 1 (bottom left) and 2 (bottom right) obtained with DFT using the ωB97X-D/6-311++G(2d,2p) method.
functional theory (DFT) and is shown in Figure 4. Despite consideration of several quite different initial starting geometries, this structure was consistently found to be the lowestenergy structure, exhibiting all positive vibrational frequencies, i.e., a true minimum. Although 1 has not been previously reported, other compounds containing the {M2(acac)3} moiety with chelating bridging acac ligands are well-known in the literature,30,31 including an X-ray structure of the related aluminum species [Al2Cl2(acac)3][AlCl4],32 which has the same core structure of 1, with each aluminum additionally bearing a terminal chloride ligand. The species [Ni2(acac)3]+ has been studied in detail using ESI MS, where it undergoes a range of reactions in the gas phase, leading to it being given the “superatomic” descriptor.33
Structural information (through fragmentation) can be obtained by changing the CEV. Positive-ion ESI MS spectra for a 1:2 Be2+/Hacac molar mixture recorded at a range of CEVs are shown in Figure 5. At a low CEV, the spectrum is dominated by the bischelated beryllium complex [Be(acac)2H]+ at m/z 208, in addition to [Be2(acac)3]+ at m/z 315. There is also a significant abundance of the [Be(acac)(CH3OH)2]+ ion at m/z 172. However, a solvent molecule in this complex is readily dissociated so that this signal disappears entirely at a moderate CEV of 80 V. At a medium CEV of 80
Figure 5. Positive-ion ESI MS behavior of 1:2 molar mixtures of BeSO4 and Hacac in a 1:1 methanol/water solution at a range of CEVs of 40, 80, and 180 V. See Table 1 for detailed ion assignments.
V, the [Be(acac)(CH3OH)]+ ion at m/z 140 dominates the spectrum. At a high CEV (180 V; Figure 5), new peaks emerge, revealing ligand fragmentation and other aggregates, as shown D
DOI: 10.1021/acs.inorgchem.9b00578 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Table 2. Assignment of Ions Observed in the Negative-Ion ESI MS of 1:2 Be2+/Hacac and 1:2 Be2+/Hdbm mixtures Be2+/Hacac, L = acac
Be2+/Hdbm, L = dbm peak abundance (%)
peak abundance (%)
negative ion
experimental m/z
calculated m/z
80 V
160 V
experimental m/z
calculated m/z
80 V
160 V
[HSO4]− [L]− [BeOHSO4]− [BeO(L)]− [BeSO4(L)]− [BeSO4(L)(H2O)]− [BeSO4(L)(CH3OH)]− [Be(HSO4)2(L)]− [Be2(L)(SO4)2]− [Be3(L)3(SO4)2]− unidentified
96.9962 98.9898 122.0092 123.9663 204.0738 222.0888 236.1081 302.0645 309.0553 516.2120 194.9893
96.9596 99.0446 121.9667 124.0512 204.0085 222.0191 236.0347 301.9759 308.9724 516.0738
100 20 10 0 70 0 0 20 10 0 15
100 10 20 70 35 10 10 15 10 10 0
96.9951 223.1445
99.9596 223.0759
30 30
10 15
247.9130 328.1340 346.1488 360.1683
248.0830 328.0498 346.0504 360.0660
0 100 0 0
10 100 30 30
433.1243
433.0167
15
0
such species could be identified with the Al3+ cation (although fragmentation patterns were closely related and in agreement with previously reported electron ionization mass spectra).19 This includes fragment species [Al(acac)(CH3COCH2)]+ at m/z 183 and [Al(CH3COCH2)2]+ at m/z 141. The absence of aluminum oxido or hydroxido species suggests either a lesser tendency for aluminum to hydrolyze in solution or its stronger binding with the acetylacetonate ligand in comparison to beryllium. Noteworthy in the comparison of the ESI MS behavior of these two metals is that no aluminum-containing ions were observed in the negative-ion ESI MS spectra of Al3+/Hacac mixtures, whereas the ESI MS of a 1:2 Be2+/Hacac solution gave a range of negative ions, as summarized in Table 2, the most important of which was the sulfate species [Be(acac)SO4]− at m/z 204. Investigation of Mixtures of Be2+ and Other 1,3Diketones Using ESI MS. Representative ESI MS spectra of 1:2 molar mixtures of Be2+ and the substituted diketones HL (L = dbm, tta, and tfac; refer to Figure 1 for structures) are shown in Figure 6. Observed ions are summarized in Tables 1
in Table 1. A high CEV is expected to dissociate solvent molecules to give a dominant peak for the species [Be(acac)]+, but the signal corresponding to this species at m/z 108 remained insignificant, suggesting that the acetylacetonate ligand cannot sufficiently stabilize the high charge density of the Be2+ cation to form a gas-phase ion. Instead, a higherintensity ion, [Be2(OH)(acac)2]+ (m/z 233), is formed, presumably by hydroxido bridging between two [Be(acac)]+ species. An increase in CEV thus appears to produce aggregation in the beryllium species observed. The reasons for this are not clear at present but may be related to the high charge density of beryllium, with a concomitant strong tendency to retain a full inner coordination sphere. Comparison with the ESI MS Behavior of the Aluminum Acetylacetone System. The Be2+ and Al3+ ions have many similarities in their chemistry because of their diagonal relationship in the Periodic Table.34 However, there are notable differences, specifically the tendency for aluminum to readily adopt six-coordination, which is absent in beryllium chemistry, as exemplified by the existence and stability of (six-coordinate) Al(acac)3. The ESI MS spectra of aluminum sulfate and Hacac mixtures were recorded under the same experimental conditions as those for beryllium (vide supra). The mass spectra reveal simpler speciation for aluminum in comparison to beryllium. The species [Al(acac)2]+ (m/z 225) appears as the base peak, while the [Al(acac)3H]+ ion (m/z 325) displayed a low intensity. Aluminum is able to form a four-coordinate species, [Al(acac)2]+. However, for beryllium, the analogous species formed by dissociation of an acac− ligand would only be two-coordinate and aggregates, via a bridging acac ligand, to form polynuclear species, thereby retaining the normal fourcoordination for beryllium. This may be attributable to the larger size and coordination number of aluminum. Although aluminum and beryllium have similar size-to-charge ratios, aluminum is tricationic, and while it prefers a coordination number of 6, four-coordination is well-documented, thereby allowing aluminum to accommodate two bidentate monoanionic acetylacetonate ligands in [Al(acac)2]+. The ESI MS spectra of Al3+/Hacac mixtures do not reveal any prominent solvated species, which could be an indication that the Be2+ cation is solvated more strongly in the gas phase than Al3+ or, alternatively, Al3+ might have a stronger affinity for anionic ligands. A more distinct difference in the ESI MS behavior of these two metal cations is observed at high CEVs, where several beryllium oxido species were observed, while no
Figure 6. Positive-ion ESI MS spectra for 1:2 molar mixtures of beryllium sulfate and (a) Hdbm, (b) Htta, and (c) Htfac in a 1:1 methanol/water solution at a CEV of 100 V. E
DOI: 10.1021/acs.inorgchem.9b00578 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
Table 3. Ion Assignments for the Positive-Ion ESI MS Analysis of 1:2 Molar Mixtures of Beryllium Sulfate and Fluorinated Diketone Ligands (L = tta, tfac)a at CEV of 100 V Be2+/Htta ion
calculated m/z
[BeLBeO]+ [BeL(CH3OH)]+ [BeL(H2O)2]+ [BeL(CH3OH)(H2O)]+ [BeL(CH3OH)2]+ [BeL2H]+ [BeL2H(CH3OH)]+ [Be2L3]+ [C4H3SCO]+ [BeF(L-CF3)]+ [BeL(L-CF3)]+ [Be2OH(L)2]+ [Be3L3O]+ [Be3L3SO4]+
255.0077 262.0268 266.0217 280.0368 294.0525 451.9963 484.0225 680.9896 110.9905 180.0038 381.9938 477.0039 705.9967 785.9530
Be2+/Htfac
experimental m/z (peak abundance) 255.0813 262.0631 266.0610 280.0821 294.0972 452.0639 484.0863 681.0732 111.0190 180.0417 382.0534 477.0769 706.0944 786.0583
(
