International Journal of M ass Spectrometry and Zon Processes, 325 101 (1990) 325-336 Elsevier Science Publishers B.V., Amsterdam FIRST STUDIES OF THE GAS PHASE ION CHEMISTRY METAL ION LIGANDS * ARTHUR T. BLADES, PALITHA PAUL KEBARLE** JAYAWEERA, OF M3+ MICHAEL G. IKONOMOU and Chemistry Department, University of Alberta, Edmonton T6G 2G2 (Canada) (First received 26 February 1990; in final form 3 May 1990) ABSTRACT Triply charged ion ligand complexes, M(L):+, were produced in the gas phase by electrospray of solutions of the M’+ salts and observed with a triple quadrupole mass spectrometer. (M = yttrium, lanthanum, cerium, neodymium or samarium.) Where L was dimethylsulfoxide or dimethylformamide M3+ resulted. However, Hz0 as L led only to the charge reduced ion MOH(H20);+. Collision induced dissociation with the triple quadrupole was used to confirm the assignments. The above M have third ionization energies which are, in relative terms, very low: IE”’ = 19-23 eV. A triply charged complex for the much higher IE”’ = 33.5 eV (cobalt) could be produced by using the hexadentate, tricyclic ligand sepulchrate. INTRODUCTION Determinations of the gas phase equilibria involving singly charged ions and solvent molecules such as H,O: zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPON M(H,O);-, + H20 eM(H,O),+(n - 1, n) (1) or other ligands providing the sequential bond enthalpies AZ_,,” and entropies AS&_, were initiated some 20 years ago [1,2]. Such studies have provided a wealth of data [3] on ion/solvent and ligand interactions [4]. Extensive theoretical work [5] and modern experimental studies such as laser spectroscopy of ion-clusters in molecular beams [6] were also stimulated. The quoted work [l-6] only gives representative examples of a very large number of publications in each area. The above studies have been limited to singly charged ions, M+ , yet doubly and triply charged ions such as M&+, Ca*+ and Fe*+, Fe3+, Co’+, Co3+, * Dedicated to Dr Fred P. Lossing on the occasion of his 75th birthday. **Author to whom correspondence should be addressed. 0168-l 176/90/$03.50 0 1990 Elsevier Science Publishers B.V. 326 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Cu2+, Zn2+ >etc., are of paramount importance in condensed phase chemistry and in biochemistry. The general method used for ion-clusters such as M+ (H2 0), is to generate M+ in a gas phase in which ligand molecule vapour is present. The formation of M+(H,O), then proceeds spontaneously by third body dependent association reactions corresponding to reaction 1. This method may not work for M2+ species when the second ionization energy of M, IE”(M), is much larger than the ionization energy of the ligand, as is the case for Mg where IEli = 15.0eV and IE(H,O) = 12.6eV. Some results for reactions of the type M2+ + L have been reported [7]. These involved [7a] the relatively low IE”(M) species Mg, Ca and Ba. Reactions of Ti2+ with the lower alkanes have also been examined [7b] and similar work has also been reported [7c] for Nb2+ and La2+. The results are of considerable interest; however, for the above systems, the IE”(M) value is relatively low. Furthermore, most of the ion/ligand reactions observed [7b,7c] led to charge separation, i.e., among the products were two singly charged ions. It appears also that only a limited range of M2+ ions can be produced with the methods used [7], and that many low IE ligands will lead to charge separation even in the first collision. Since doubly and triply charged ions exist in solution, preparation of gas phase ions such as M(H;,O)z+ might be possible by transfer of multiply hydrated ions from the liquid to the gas phase. Until recently this alternative would have been considered impossible. However, mass spectrometric techniques, developed for analytical purposes, such as fast atom bombardment (FAB) [&lo], thermospray [l l-131, electrohydrodynamic ionization (EHI) [14] and electrospray [l&16], have demonstrated that inorganic ions, M+, attached to ligands can be produced in the gas phase when the corresponding ions are present as electrolytes in the solution. Some doubly charged ion/ligand species can be produced with all of these techniques. However, the most promising technique appears to be electrospray [15,16]. As reported recently [17-191, abundant doubly charged ions, M2+ (Mg2+, Ca2+, St’, Ba2+, Mn2+, Fe2+, Co2+, Ni2+, Zn2+) with a variety of ligands (H,O, CH,OH, dimethylsulfoxide (DMSO), dimethylformamide (DMF), acetone and the polydentate peptides, di- and poly-ketones and cyclic tetramines) can be produced by electrospray. While electrospray has been known for many years, it was Yamashita and Fenn [15] who first introduced mass spectrometric studies of electrospray produced ions. Mann et al. [20] and Covey et al. [21], who used a related technique, “ion spray”, have also reported multiply charged ions. However, these were organic systems such as peptides of molecular weight ca. 20 000 containing many basic residues. The basic groups can be protonated and lead to ions with as many as 20 positive charges [21]. While these findings are of the greatest analytical interest, the 327 protonated basic groups are very far apart and the species can be viewed as singly charged ions connected with peptide strings. In previous [17-191 and present work from this laboratory, attention is focused on multiply charged metal ions, where the charge originates from one center, the metal ion, even though the multiply charged metal ion/ligand cluster may owe its stability to charge delocalization onto the ligands. In previous work [17-191, which dealt with doubly charged ions, it was shown that thermal equilibrium populations for the ion hydrates: zyxwvutsrqponmlkjihgfedcb M(H,O):?, + Hz0 --‘M(H,O);+ (2) could be measured. At T = 300 K and p(H,O) E 2 x lop3 Torr, the equilibrium distribution peaked at n z 10. From the AG&_, values that were determined, binding energies AH&_, could be estimated and these were found to be A#,,,, z 15 kcalmol-’ for ions such as Co*+ or Ni*+. Inner shell energies, (n < 6) are expected to be much larger (30-80 kcal mol-‘) and thus less accessible to the ion equilibrium technique [2] due to the high temperature that would be required. However, information on such high energy processes can be obtained with collision induced dissociation (CID) in the triple quadrupole mass spectrometer used [17-191. Single ligand loss could be induced with CID only down to a given IZ.Below this n, charge separation reactions 2 and 3, became dominant: M(H20);+ -MOH(H,O): M(DMSO);+ e + H30(H20)n+-ck+g M(DMSO);_, + DMSO+ (3) (4) We will call reactions 3 and 4 charge separation or charge reduction reactions. The product containing the original core ion will be called the charge reduced species. Charge reduction with ligands such as H,O or NH, led to intracluster proton transfer followed by dissociation, see reaction 3, while other ligands, such as DMSO or DMF led to intra cluster charge transfer and dissociation, see reaction 4. Furthermore, these aprotic solvent molecules delayed charge separation, i.e. led to charge separation at lower yt than the protic solvent molecules H, 0 and NH, [ 17-191. Approximate information on M(L):+ bond energies, where L stands for a particular ligand molecule, could be obtained even for y1values which were below the charge reduction limit, by means of thermodynamic cycles [ 17,191. The present work reports results for M3+ ions. While the success with this group was much more limited, the results are of interest and enable some illuminating comparisons with the findings for the M2+ ion complexes to be made. 328 EXPERIMENTAL The electrospray ion source and the triple quadrupole mass spectrometer used for the ion detection have been described previously [ 16,181 in some detail. Therefore only a brief mention of the pertinent features will be given here. The electrospray was operated in much the same way as before [ 161.For the M3+ ions, solutions of the M3+ salts in methanol, at flow rates of 10 ,ulmin-‘, were passed through the capillary which was held at + 9OOOV[16]. The salts used for the triple charged ions were: Sc(NO,),, YCl,, LaCl,, CeCl,, Nd(NO,),, Sm(NO,), and CoCl,. The solutions were lop4 M in methanol. Water (1-5 wt.“/,) was added when the hydrate clusters were to be observed. To generate the DMSO clusters, 10-3-10-2 M DMSO solutions in methanol were used with or without the water addition and the same conditions were used for DMF. The electrospray was operated at atmospheric pressure and the electrospray capillary was mounted in the atmospheric pressure chamber of an atmospheric pressure triple quadrupole mass spectrometer, the SCIEX TAGA-6000E (SCIEX Ltd., Thornhill, Ontario) [22]. The ions produced by the electrospray, driven by an electric field, enter an interface chamber containing ultrapure nitrogen at atmospheric pressure. The ions drift across the interface chamber and those arriving in the vicinity of an orifice at the bottom of the chamber are entrained by N, gas escaping through the orifice into the vacuum chamber which contains the triple quadrupole mass spectrometer. There the ions can be mass separated and detected or mass selected with the first quadrupole, Ql, exposed to CID in Q2 and the ionic products detected with 43 [23]. RESULTS AND DISCUSSION Ions M(L):+, where L stands for some given ligand, were observed for M = SC, Y, La, Ce, Nd, Sm and Co. The position of these elements M in the periodic table and the values of the second and third ionization potential are given in Fig. 1. No triply charged hydrates, M(H,O)z+, could be observed with the M3+ salts for any of the M listed above, under experimental conditions where M2+ salts had led to abundant hydrates M(H, 0):’ in the previous work [ 18,191(see Fig. 1). The species observed with the M3+ salts were the charge reduced variety MOH(H,O), 2f . This was the case even when “mild” conditions were used. Mild conditions correspond to absence of applied electric fields between the orifice from which the gas enters the vacuum region and the following electrodes, down to the aperture electrode immediately in front of Ql and where Ql is operated only some 5 V below the orifice voltage [ 18,191. When 329 Fig. 1. Elements whose M*+ salts led to M(L):+ (previous work [ 16-181) shown in small circles. Elements whose M3+ salts led to M(L):+ (present work) shown with blackened top of circle. These led also to doubly charged ions. Second ionization energies and third ionization energies given in eV. accelerating voltages are applied to the above electrodes, CID occurs in this region due to collisions of the accelerated ions with gas molecules, since the gas density in this region is quite high. Such conditions will be called “harsh”. The third ionization energies of the M used in the present work are all above 19 eV, see Fig. 1. In the earlier work involving the alkaline earths and first row transition metals, and the M2+ salts, under mild conditions, doubly charged hydrates M(H20)i+ were observed when the second ionization potential was less than ca. 18 eV. For Be (IE” = 18.2 eV) and Cu (IEn = 20.3 eV) only the species MOH(H,O)T were observed. Thus, for both the M2+ and the M3+ species, the M2+ or M3+ hydrates can be observed only when the second or third IE is less than ca. 18 eV. The CID daughter spectrum with collision gas Ar in 42 for MOH(H20)i+ from yttrium is shown in Fig. 2. Successive loss of H,O leading to n = 2 is the only reaction observed. Similar results were also obtained for the other M. Triply charged ions, M(DMSO):+ and M(DMF);?+, were obtained with solutions to which low3 M DMSO or DMF was added. The actual populations under mild conditions were difficult to find, presumably because the ions 330 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1000' 6 800- 5 4 I3 g 3 200. 2 0 ,....,....,....!!....'. 60 70 40 50 80 90 100 ! 110 m/z Fig. 2. CID spectrum of parent ion, YOH(H20)i+, n = 2. Collision gas Ar. showing successive losses of H,O down to were clusters containing not only DMSO (or DMF) but also a large number of water and some methanol molecules and were therefore spread over many masses. The detected M3+ ions were obtained with CID in the acceleration region ahead of Ql. This led to M(DMSO)z+ ions with n from 4-8 for M = Y, La, Ce, Nd, Sm. This result establishes that for these core ions, DMSO is much more strongly held in the inner shell of the ion than water or methanol. Similar results were obtained with DMF although only Y and La were examined with this ligand. The CID spectrum with Ar collision gas in Q2 for La(DMSO)z+ is shown in Fig. 3. Successive loss of DMSO is observed down to y1= 3. Similar CID results were obtained for the other M(DMSO)%+ ions. In addition to zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDC M(DMSO)i+ , abundant doubly charged ions of the general type MX(DMSO)z+ were also present. X- was most often the counter ion of the salt used, i.e. Cl- for chlorides, NO; for nitrates or some accidental anionic impurity. Thus an X of mass 59 was traced to be due to the acetato complex formed due to the presence of CH,CO; impurity at the ca. 10e6 M level. Deliberate addition of sodium acetate at a 10e5 M level led to an increase of the mass 59 containing peaks by a factor of 3-4. The CH, CO; ion is a well known ligand leading to acetato complexes in aqueous solution. MX(DMSO)i+ ions where X- was OH- or CH,O- were also often found to be present. The CID spectrum of La(DMS0)3+ in Fig. 3 accidentally provides an example for the presence of CH,O-, due to a parent ion of the series LaOCH, (DMSO)?;+ . The spacings between the main daughter ions in 331 6 5 5000 L*pmSOq:, 4000 i zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 4 3000 i ; $ z *Ooo g 10001 3 Ol 60 100 120 140 160 160 200 m/z Fig. 3. CID spectrum of parent ion, La(DMSO)i+, showing successive DMSO (MW = 78) losses at zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA m/z = 26 intervals down to La(DMSO), 3+. The small peak at m/z = 39 units down from the parent ion is due to loss of DMSO from the extraneous doubly charged parent ion LaOCHJ(DMSO):’ of the same m/z (= 202) as La(DMSO)i+ . Fig. 3 are m/z = 26 which corresponds to loss of DMSO (MW = 78) from a triply charged ion, i.e., from La(DMSO)z+. A much smaller daughter peak, occurring m/z = 39 units below the parent corresponds to loss of DMSO from a doubly charged ion. The m/z value of this doubly charged ion (= 202) is the same as that of La(DMSO)i+, and the doubly charged ion tits the assignment: where no m/z LaOCH, (DMSO):+ . Ions of the series LaOCH,(DMSO)i+, coincidences occur, were also identified. The results in Fig. 3 do, however, indicate the value of the triple quadrupole CID for the identitication of the ion clusters. The observation that M(L):+ species could be observed with DMSO and DMF but not with H,O correlates with the earlier findings [17-191 of doubly charged M(L):+. For this series, Cu(H,O)z+ could not be observed but Cu(DMS0):’ could. As mentioned before, the second ionization energy of Cu is IE”(Cu) = 20.3eV and thus of the same magnitude as the third IE of the present series. The reasons why DMSO and DMF should resist charge separation more than HZ0 were discussed earlier [19]. The enthalpy change AH, for the charge separation reaction is as shown below M(H20);+ --MOH(H,O),+_, AH, = AH&M2+) + H,O+ - IE”(M) + IE(H,O) + AH, - D(M+-OH) - AHn_2,0(MOH+) (5) 332 Co(Sep-H) * 1 k;j- Co(Sep-2H)+ n HCI n H,O 1 2 zyxwvutsrqponmlkjihgfedc Co(Sep) +++ n Hz0 ‘246 1 ‘a11 ‘. 100 0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ I I23 If i l I 200 -7- 300 MI2 Fig. 4. Cobalt-containing ions from electrosprayed Co - sepulchrate C&, observed under harsh conditions, i.e., with some CID in acceleration region. where AH,,0 is the enthalpy change for the dissociation of all H,O from the indicated core ion, D is bond dissociation energy and AH6 is as shown below: H20+ + H,O eH,O + OH (6) AH, = - 26 kcal mol-’ Thermochemical data for reaction 6 were obtained from Lias et al. [24]. Charge separation, as in reaction 5, is favoured relative to simple charge transfer by the exothermicity of reaction 6 and the relatively high exothermicity of the M+-OH bond formation. For a table of M+-OH bond energies, see Magnera et al. [25]. For aprotic ligands like DMSO and DMF, the reaction equivalent to reaction 6, as well as the bond forming reaction to M+ are expected to be less exothermic and these ligands are therefore less prone to intra cluster proton transfer. Suitable multidentate ligands may be expected to protect the core ion even more from intracluster charge reduction reactions. For example, the triply charged (Co * sepulchrate)3+ ion was observed in the gas phase, even though the third ionization energy of Co, 1E”‘Co = 33.5eV, is some 1OeV higher than the third ionization energies of the other species of the present work. The mass spectrum of the Co *sepulchrate ion is shown in Fig. 4. The triply charged ion represents only a small fraction of the total Co ions. The spectrum shown was obtained with some CID in the acceleration region. The triply 333 Fig. 5. The Co-sepulchrate(H,O):+ ion observed under milder acceleration CID conditions. Notice higher n when compared to the same series in Fig. 4. charged ion under milder conditions is much more hydrated, i.e., n = 5-10 peaking at about 7, see Fig. 5. This high degree of hydration, even though the ion is stabilized by sixfold complexation with nitrogen, illustrates the power of the triple charge. The charge reduced species, where the core ion (n = 0) is Co(Sep-H)2+ or Co(Sep-2H)+, see Fig. 4, demonstrate the strongly acidifying effect of the high positive core charge. The charge reduction occurs probably through the hydrates Co(Sep)(H; 0):’ -- zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFE Co(Sep-H)(H,O)z + H,O(H, 0): _ m_ ,) The protons lost are probably from the secondary amino groups that are complexed to the Co. Also observed in the spectrum, Fig. 4, are singly charged cobalt ions, the dominant series being Co(Sep-2H)(HCl),f. The Cl obviously being due to the Cl- counter ions present in the solution. It was shown in earlier work [17,19] that charge reduction reactions, such as reaction 5, occur when the multiply charged parents are energized by collisions in Q2. Therefore the question may be asked: are the charge reduced species observed with Ql and with mild conditions, also due to CID induced by electric fields in the electrospray chamber, the interface chamber or by the quadrupole fields? It can be shown that, due to the high pressure used in the electrospray and interface chamber, the increase of internal energy of the ion 334 clusters is very small [ 16,181. Therefore, charge reduction in this region will occur only for clusters that are extremely prone to undergoing this reaction in preference to single ligand loss. The same holds true for CID due to the quadrupole oscillating fields in the path of the molecular beam. It is clear from the cases where counter ions X- originating from the solution (X- = Cl-, NO;, CH,CO; ) are observed (see preceding text) that charge reduction in these cases is most likely due to a process occurring in solution. Such ion pairing is expected in solution particularly when multiply charged ions are involved [26]. The evaporating electrosprayed droplets experience an increase of electrolyte concentration which will lead to increased ion pairing. Therefore, the mass spectrometrically observed charge reduction probably reflects the state of ion pairing in the very small droplets which lead to ions. On this basis, one is led to expect that charge reduction with Xcounterions from the solution will decrease as the concentration of the salt used is decreased. Experiments examining this premise are presently underway. CONCLUSIONS Triply charged M(L):’ ions can be observed from electrosprayed solutions of M3+ salts particularly when the third ionization energy of M is less than 23 eV. Ligands favouring charge retention are dipolar aprotic molecules such as DMSO and DMF. Multidentate polycyclic ligands with suitable cavities to accommodate the triply charged ion lead to triply charged species even when the third ionization energy is as high as 34 eV. The relative intensities of triply charged ions M(L):+ from M3+ salts are considerably lower than M(L):+ ions from M2+ salts and these are lower than M(L),f ions from M+ salts [16-181. These decreasing yields are due to an increased tendency for charge reduction as the ion charge increases. Because triply charged ion M(L):+ abundances are low, quantitative determinations of ion/ligand bond energies by ion equilibria or quantitative CID from kinetic energy thresholds would be more difficult than is the case for doubly charged ions [16-l 81. 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