Formation and characterization of polymetallic {Cr_xM_y} rings in vacuo

Abstract

Understanding the (dis)assembly mechanisms of large metallosupramolecules is critical in their design, stability and application. The inherent complexity of these structures leads to many potential pathways for combining (or separating) the constituent building blocks, which makes this task difficult. Here we use collision-induced dissociation mass spectrometry to study the disassembly of heterometallic complexes. Collisional activation leads to the formation of a series of previously unknown smaller ring products and we characterize their geometry using ion mobility. The disassembly of both {Cr_xCu₂} hourglass structures (x = 10, 12) and of a {Cr₁₂Gd₄} cluster shows the formation of rare closed, heptametallic species {Cr₆Cu}, {Cr₅Cu₂} and {Cr₅Gd₂} as dominant products, as well as other closed ions such as {Cr₅Cu}, {Cr₁₀Cu}, {Cr₁₂Cu}, {Cr₁₀}, {Cr₁₂} and {Cr₆Gd₂}. The collision cross-section of cyclic products and precursors has a linear correlation with ion mass—a relationship that does not hold for acyclic systems. As these rings are non-trivial to synthesize individually in solution, we propose the presented workflow to identify and characterize feasible molecules for bulk phase synthesis.

Main

Tandem mass spectrometry (MS²) involves isolation of target ions in the gas phase and often their subsequent dissociation to smaller ions. The most common fragmentation method is collision-induced dissociation (CID), in which ions of interest are subjected to collisions with an inert gas at user-defined kinetic energies. The structure of newly formed products as well as non-fragmented precursor ions can be accessed when CID is combined with ion mobility mass spectrometry (IM–MS), which allows the mass as well as size and shape to be measured in the same experiment. Structural information is provided in the form of collision cross-sections (CCS), which can be compared with literature data or to theoretical values computed from candidate geometries. Due to the high energies involved in collisional activation, analytes tend to disrupt and undergo major structural change, which becomes visible in the ion mobility spectra of the precursor and/or product ions. Particularly for proteins, the term ‘collision-induced unfolding’ was coined to describe this behaviour similar to denaturation¹. Although the disassembly of biomacromolecules is commonly investigated^2,3,4,5, the impact of collisional activation on large synthetic molecules is not well explored.

We have been studying a family of cyclic polymetallic supramolecules proposed as qubits in quantum information processing^6,7,8,9, and related compounds have also been used as resists for lithography^10,11,12. As the interest in these and other metallosupramolecular complexes increases, and similarly the complexity of their structures^13,14,15, the synthesis and analysis of their building blocks could aid in rationalizing the preference for certain compounds and predict the formation of others.

One prominent example where mass spectrometry was used to find a molecule stable enough to be produced in bulk phase was the discovery of the C₆₀ buckminsterfullerene by Kroto et al. in 1985 (ref. ¹⁶), which led to the award of the Nobel Prize in Chemistry 11 years later^17,18,19. More recently, Cronin and co-workers have used cryospray and IM–MS to identify polyoxometalate targets for bulk synthesis^20,21,22,23 and to unravel their assembly mechanism^21,23,24. We have recently applied CID and IM–MS to investigate the disassembly mechanisms and energetics of heterometallic rings and [2]-rotaxanes with the general formula [NH₂RR′][Cr₇MF₈(O₂C^tBu)₁₆] (M = Mn^II, Fe^II, Co^II, Ni^II, Cu^II, Zn^II and Cd^II), showing that both the metal M and the R,R′ groups can be used to tune the stability and conformational dynamics of these systems²⁵.

In this Article, we investigate the disassembly dynamics of more complex heterometallic systems, two of which have hourglass structures, [NH₂ⁿPr₂]₂[Cr₁₀Cu₂F₁₄(O₂C^tBu)₂₂] = ‘{Cr₁₀Cu₂}’ (Fig. 1a) and [NH₂ⁱPr₂]₂[Cr₁₂Cu₂F₁₆(O₂C^tBu)₂₄] = ‘{Cr₁₂Cu₂}’ (Fig. 1b), and a third that involves a lanthanide tetrahedron bound to two {Cr₆} chains, [NH₂ⁿPr₂]₂[Cr₁₂Gd₄F₂₁(O₂C^tBu)₂₉] = ‘{Cr₁₂Gd₄}’ (Fig. 1c). We demonstrate how CID, aided by IM–MS, can discriminate different disassembly pathways, which lead to the formation of closed polymetallic species that have not been made previously via solution synthesis (Fig. 1d). We suggest that this workflow could therefore inspire new synthetic targets and inform the tools available for the formation of metallosupramolecular compounds. Using information from the CID–IM–MS measurements, we also propose a workflow for the topological assignment of polymetallic systems, which applies to structures synthesized in solution as well as those formed in the gas phase.

Fig. 1: Crystal structures of the studied {Cr_xM_y} complexes and applied CID–IM–MS workflow.

a–c, Single crystal X-ray structure of {Cr₁₀Cu₂} (a), {Cr₁₂Cu₂} (ref. ²⁷) (b) and {Cr₁₂Gd₄} (c); Cr: dark green, Cu: brown, Gd: purple, F: yellow, O: red, N: blue, C: grey. Solvent molecules and hydrogen atoms have been removed for clarity. d, CID–IM–MS workflow following nano-electrospray of the precursor {Cr_xM_y} from solution and m/z-selection of an appropriate precursor ion. Upon collisional activation, several fragment ions are observed with different masses, and their CCS_N2 distribution can be extracted as shown for an ion of the type {Cr_x-aM_y-b}. Different structures of {Cr_x-aM_y-b} are possible, such as closed (low CCS_N2, narrow CCS_N2 distribution) or open forms (high CCS_N2, wide CCS_N2 distribution), of which only a closed {Cr_x-aM_y-b} species was found in this illustrative example.

Results

{Cr₁₀Cu₂} and {Cr₁₂Cu₂} hourglasses

The hourglass structures {Cr_xCu₂} (x = 10, 12) consist of two Cu^II and x Cr^III centres, which are bridged via pivalate ligands (⁻O₂C^tBu = Piv⁻) on the outside and fluorides on the inside of the hourglass structure (Fig. 1a,b). The coordination environment of the chromium ions involves four pivalates and two cis-fluorides except for those located at the hourglass bottleneck, where three pivalates and three mer-fluorides are present. These centres are adjacent to the two copper ions, which are penta-coordinated by three pivalates and two fluorides. Additionally, one secondary ammonium cation is located in each half of the hourglass (for {Cr₁₀Cu₂}: [NH₂ⁿPr₂]⁺ and for {Cr₁₂Cu₂}: [NH₂ⁱPr₂]⁺), where it exhibits hydrogen bonds to the fluorides, particularly to the terminal fluorides attached to the two bottleneck chromiums^26,27. Following optimization of solvent and nano-electrospray ionization (nESI) source conditions, mass spectra of {Cr_xCu₂} (x = 10, 12) were recorded from solutions of sodium iodide in positive mode. Cations of the type [{Cr_xCu₂} + Na]⁺ were obtained for both hourglasses (Supplementary Figs. 1 and 2).

Disassembly of {Cr ₁₀ Cu ₂} and {Cr ₁₂ Cu ₂}

We isolated the ions [{Cr₁₀Cu₂} + Na]⁺ and [{Cr₁₂Cu₂} + Na]⁺ and ramped the energy of collisional activation with nitrogen gas, while recording the arrival time distributions (ATD) of both analytes and their products. The ATD of all ions were converted to CCS distributions in nitrogen gas (CCS_N2) as a function of collision energy^28,29. The MS² spectra of both precursor ions show the loss of one secondary ammonium cation along with an anionic ligand, predominantly a pivalate, as the first dissociation step (Fig. 2). E₅₀ values, known as a relative measure of ion stability, were determined for both precursor ions and show a slightly higher stability for [{Cr₁₂Cu₂} + Na]⁺ (Supplementary Fig. 3).

Fig. 2: Tandem mass spectra of [{Cr_xCu₂} + Na]⁺ (x = 10, 12) at different collision energies.

a,b, MS² spectra of [{Cr₁₀Cu₂} + Na]⁺ (m/z = 3,365) at collision energies of 0 eV, 100 eV and 140 eV (a) and [{Cr₁₂Cu₂} + Na]⁺ (m/z = 3,910) at collision energies of 0 eV, 120 eV and 150 eV (b). Occurring fragmentation pathways are labelled: (i) – [NH₂^n/iPr₂]⁺, – Piv⁻; (ii) –Cu^II, – 2 Piv⁻; (iii) – [NH₂^n/iPr₂]⁺, – F⁻ and (iv) – Cu^II, – Piv⁻, – F⁻. Satellite peaks (labelled with ‘s’) correspond to the labelled main peak via the exchange of one anionic ligand for another one (Piv⁻ for F⁻ or F⁻ for Piv⁻; mass difference 82 Da). The spectra of both ions present two regions, one at lower m/z and one at higher m/z. Insets: schematics of {Cr₁₀Cu₂} and {Cr₁₂Cu₂} (Cr, green; Cu, brown).

Source data

Accurate mass and isotopic distributions were used to assign the hourglass fragments in the subsequent dissociation steps at higher collision energies. A variety of dissociation channels occur and ions are observed in two regions of the mass spectrum, one at lower m/z (x = 10, <1,800 m/z; x = 12, <2,200 m/z) and one at higher m/z (x = 10, 2,500–3,200 m/z; x = 12, 2,000–3,700 m/z). All of the observed species are singly charged cations, and between these two regions only minor ion populations were found (Fig. 2). For [{Cr₁₂Cu₂} + Na]⁺, more ions were found in the low mass region compared with [{Cr₁₀Cu₂} + Na]⁺, for which the high mass region clearly dominates. The ratio between the precursor and product ions in both regions depends both on the collision energy (Fig. 2) and the instrument used (Supplementary Fig. 4).

The dominant fragment ions of the low mass region contain seven transition metal ions. From the precursor [{Cr₁₀Cu₂} + Na]⁺, the dominant fragment is {Cr₅Cu₂} (mainly [Cr₅Cu₂F₇Piv₁₂Na]⁺ at 1,756 m/z). We also observe in minor amounts {Cr₅} that probably is the other direct fragment of the {Cr₁₀Cu₂} dissociation, at least partially (Fig. 2a top and Supplementary Dataset). {Cr₅Cu} was also found in low numbers (mainly [Cr₅CuF₇Piv₁₀Na]⁺ at 1,491 m/z); however, this and other low abundance ions are presumably secondary fragments from {Cr₅Cu₂} as they appear at higher energies than the latter (for {Cr₅Cu}: loss of Cu^II and two Piv⁻; Fig. 2a). In contrast, the precursor [{Cr₁₂Cu₂} + Na]⁺ dissociates to {Cr₆Cu} (mainly [Cr₆CuF₈Piv₁₂Na]⁺ at 1,764 m/z; Fig. 2b). The species in the high mass region follow fragmentation channels where Cu^II dissociates along with two anionic ligands, before the second ammonium cation and another ligand are lost. This in turn is followed by the dissociation of the second Cu^II centre, again along with two anionic ligands (Fig. 2). Overall, this yields species of the type Cr_x (x = 10, 12) as the product ions of the hourglass ions [{Cr_xCu₂} + Na]⁺, which was confirmed by their isotopic distributions as illustrated for [Cr₁₂F₁₅Piv₂₁Na]⁺ (m/z = 3,056; Supplementary Fig. 5).

Ion mobility allows us to probe the structure of both precursor and product ions via their CCS_N2 distributions (Fig. 3). The majority of products exhibit narrow, unimodal conformations, although slightly wider distributions were found for {Cr₁₂Cu} and {Cr₁₂} fragment ions of [Cr₁₂Cu₂ + Na]⁺ (Fig. 3b) as well as for some {Cr₅} species formed in the disassembly of [{Cr₁₀Cu₂} + Na]⁺ (Supplementary Dataset). The CCS_N2 values of the hourglass precursor ions and the presented products were quantified and compared with data of similar polymetallic closed species (Table 1). The data for {Cr₅Cu₂} and {Cr₆Cu} agree well with the CCS_N2 of a seven-membered [Cr₆MnF₈Piv₁₃]⁻ ring, formed from a {Cr₇Mn} ring via CID–MS²⁵, and the small difference can be explained with the different number of bulky pivalate ligands ({Cr₅Cu₂}, {Cr₆Cu}: 12 Piv⁻, {Cr₆Mn}: 13 Piv⁻). This and their unimodal, narrow distributions clearly suggest a limited conformational flexibility and hence the presence of heptametallic {Cr₅Cu₂} and {Cr₆Cu} species that are closed, which is distinct from other open {Cr₆Mn} horseshoe fragments observed previously (Table 1)²⁵. Other products and precursor ions cannot be directly compared with previous data; however, their similar CCS_N2 values and similarly unimodal and narrow peak shapes indicate cyclic structures as well for these species (Fig. 3 and Supplementary Dataset). For the minor {Cr₅} fragments, depending on the exact chemical composition of the cations, the conformational landscape is more diverse (Supplementary Dataset).

Fig. 3: CCS_N2 distributions of [{Cr_xCu₂ + Na]⁺} (x = 10, 12) and their fragment ions.

a,b, [{Cr₁₀Cu₂} + Na]⁺ (m/z = 3,365) including selected product ions of the types {Cr₁₀Cu} (m/z = 2,896), {Cr₁₀} (m/z = 2,509), {Cr₅Cu₂} (m/z = 1,756) (a), and [{Cr₁₂Cu₂} + Na]⁺ (m/z = 3,910) including selected product ions of the types {Cr₁₂Cu} (m/z = 3,443), {Cr₁₂} (m/z = 3,056), {Cr₆Cu} (m/z = 1,764) (b). Data were recorded at collision energies of 0 eV (both precursor ions) as well as 120 eV (fragments of [{Cr₁₀Cu₂} + Na]⁺) and 160 eV (fragments of [{Cr₁₂Cu₂} + Na]⁺), respectively. Insets: schematics of {Cr₁₀Cu₂}, {Cr₁₂Cu₂} and their products (Cr, green; Cu, brown). Fragment ions that are assigned as closed, due to the ion mobility data, are presented schematically; as the IM–MS experiment informs on the stoichiometry but not on the exact connectivity between the metal centres.

Source data

Table 1 ^TWCCS_N2 values of the precursors [{Cr_xCu₂} + Na]⁺ (x = 10, 12), [{Cr₁₂Gd₄} – Piv]⁺ and their fragment ions as well as ^TWCCS_N2 values from our previous works^25,32

{Cr₁₂Gd₄} cluster

We further investigated whether collision-induced disassembly would also lead to polymetallic rings when activating a different type of precursor. We chose the lanthanide cluster {Cr₁₂Gd₄} (Fig. 1c), which consists of a tetrahedral {Gd₄F₇Piv} cage, in which the fluorides and one pivalate bridge the four Gd^III centres. Each of the octa-coordinated gadolinium ions is attached to a terminus of one of two {Cr₆} chains, and a [NH₂ⁿPr₂]⁺ cation is located at each {Cr₆Gd₂} ring centre³⁰. The chromium atoms are each connected via two pivalate ligands and one fluoride, as in both {Cr_xCu₂} hourglasses.

The mass spectrum of {Cr₁₂Gd₄}, sprayed from methanol in the presence of sodium iodide using nESI, yielded only a small number of largely intact analyte ions (Supplementary Fig. 6), and we assigned the dominant one to [{Cr₁₂Gd₄} – Piv]⁺ (m/z = 4,688; Supplementary Fig. 7 for isotopic distribution). Several other polymetallic species were observed in the mass spectrum, including {Cr₆Gd} ions, which are possibly produced by decomposition reactions with the solvent. Using ion mobility, the {Cr₆Gd} clusters were identified as heptametallic rings (CCS_N2 values in Supplementary Table 1 including discussion of their disassembly; Supplementary Dataset), which are isostructural to a {Cr₆Ce} ring reported previously, which was only synthesized in very low yield (Discussion)³⁰. We computed the density functional theory (DFT) optimized structure of the neutral {Cr₆Gd} (Supplementary Fig. 8) as well as the corresponding sodium adduct (Supplementary Dataset), supporting their stabilities in the gas phase.

Disassembly of {Cr ₁₂ Gd ₄}

We used the MS² ion mobility workflow (Fig. 1d) to study [{Cr₁₂Gd₄} – Piv]⁺. Similar to the studies of [{Cr_xCu₂ + Na]⁺, the tandem mass spectra show the loss of [NH₂ⁿPr₂]⁺ along with one fluoride as the main fragmentation channel, although at higher energies (Fig. 4a). The E₅₀ value of [{Cr₁₂Gd₄} – Piv]⁺ was determined and is more than 50% higher than those of the hourglass ions (Supplementary Fig. 9). Upon increasing the energy further (Fig. 4a centre), two main regions were found in the tandem mass spectra: one at 1,800–2,500 m/z and the other at 3,200–4,500 m/z; however, the separation between regions is blurred at higher collision energies (Fig. 4a top). All species also occur as singly charged cations. In the lower mass region, the cluster type {Cr₆Gd₂} was found as the main product (mainly [Cr₆Gd₂F₈Piv₁₆(NH₂ⁿPr₂)]⁺ at m/z = 2,499; Supplementary Fig. 10 for isotopic distribution). This cluster is one half of the {Cr₁₂Gd₄} precursor, and we previously reported the solid-state structure of a cyclic {Cr₆Y₂} complex, albeit in low yield¹⁴. The {Cr₆Gd₂} ions undergo further fragmentation at even higher energies, with losses of [NH₂ⁿPr₂]⁺ or Cr^III, along with anionic ligands (mainly pivalates, Fig. 4a top). This, for example, leads to ions of the type {Cr₅Gd₂} (such as [Cr₅Gd₂F₈Piv₁₂]⁺ at m/z = 1,940). In contrast, the high mass region shows the stepwise disruption of the {Cr₆} chains with losses of [NH₂ⁿPr₂]⁺ along with Piv⁻, followed by several dissociations of {Cr(Piv)₃} units (Fig. 4a centre). This infers that the {Gd₄} tetrahedron remains intact.

Fig. 4: Tandem mass spectra of [{Cr₁₂Gd₄} – Piv]⁺ at different collision energies and CCS_N2 distributions of [{Cr₁₂Gd₄} – Piv]⁺ and fragment ions.

a, MS² spectra of [{Cr₁₂Gd₄} – Piv]⁺ at collision energies of 0 eV, 210 eV and 250 eV. Occurring fragmentation pathways are labelled: (i) – [NH₂^n/iPr₂]⁺, – Piv⁻; (iii) – [NH₂^n/iPr₂]⁺, – F⁻ and (v) – Cr^III, – 3 Piv⁻. Satellite peaks (labelled with ‘s’) correspond to the labelled main peak via the exchange of one anionic ligand for another one (Piv⁻ for F⁻ or F⁻ for Piv⁻; mass difference 82 Da). The spectrum at 210 eV shows two regions, one at lower m/z and one at higher m/z. Minor contaminant features were observed that were m/z-selected in the quadrupole along with [{Cr₁₂Gd₄} – Piv]⁺, and hence produce different fragments (labelled with ‘c’). b, CCS_N2 distributions of [{Cr₁₂Gd₄} – Piv]⁺ (m/z = 4,688) and the fragment ions {Cr₆Gd₂} (m/z = 2,499) and {Cr₅Gd₂} (m/z = 1,940). Data were recorded at collision energies of 0 eV (precursor), 130 eV for {Cr₆Gd₂} and 190 eV for {Cr₅Gd₂}, respectively. Insets: schematics of {Cr₁₂Gd₄} and the two products {Cr_xGd₂} (x = 5, 6; Cr, green; Gd, purple). Fragment ions that are assigned as closed, due to the ion mobility data, are presented schematically; as the IM–MS experiment informs on the stoichiometry but not on the exact connectivity between the metal centres.

Source data

The structure of [{Cr₁₂Gd₄} – Piv]⁺ and fragment ions was investigated via their CCS_N2 distributions (Fig. 4b for selected ions). The majority of the ions exhibit narrow, unimodal conformations (Supplementary Dataset). We quantified absolute CCS_N2 values of the precursor [{Cr₁₂Gd₄} – Piv]⁺ and products of the type {Cr₆Gd₂} and {Cr₅Gd₂} (Table 1). The CCS_N2 value of {Cr₆Gd₂} agrees well with a related {Cr₇Cu} ring²⁵, while the value for {Cr₅Gd₂} is similar to those of the seven-membered hourglass products {Cr₅Cu₂} and {Cr₆Cu}, as well as to the [Cr₆MnF₈Piv₁₃]⁻ closed species²⁵. These comparisons suggest closed, cyclic structures for the complexes of the type {Cr₆Gd₂} and {Cr₅Gd₂}.

Density functional theory and CCS_N2 calculations

To assess the stability of the closed fragments formed in the disassembly of {Cr₁₀Cu₂}, {Cr₁₂Cu₂} and {Cr₁₂Gd₄}, DFT optimized structures were generated for {Cr₅Cu₂}, {Cr₅Cu}, {Cr₆Cu}, {Cr₆Gd₂} and {Cr₅Gd₂} (Supplementary Figs. 11–15). At the level of theory used, all closed cations are considerably more stable with respect to their monometallic fragments [CrFPiv₂], [CrF₂Piv], [CuFPiv₂]⁻, [CuF₂Piv]⁻ and Piv⁻, by at least 1,400 kJ mol⁻¹. For the heptametallic species {Cr₅Cu₂}, {Cr₆Cu} and {Cr₅Gd₂}, different isomers were calculated, and the DFT energies suggest that hexametallic rings involving an additional metal bridge might even be more stable than the corresponding heptametallic rings (Supplementary Figs. 11, 13 and 15 and Supplementary Dataset). We cannot assign the observed fragments unambiguously to the computed candidate structures since the kinetics of the dissociation reaction and its mechanism are unknown; in addition, our sampling of candidate structures is not exhaustive.

Theoretical ^THCCS_N2 values were enumerated for {Cr₅Cu} and {Cr₆Gd₂} as well as the different isomers of {Cr₅Cu₂}, {Cr₆Cu} and {Cr₅Gd₂}, using the trajectory method implemented in IMoS³¹. This yielded 3–8% larger values than found experimentally (Supplementary Table 2), and we previously observed this discrepancy for similar polymetallic complexes^25,32, and have discussed possible explanations in detail²⁵. An exception to this was found for the six-membered ring {Cr₅Cu}, where the experimental and theoretical CCS_N2 values are in good agreement. The reason for this is not known, but given that this is the most compact and dense structure (Supplementary Fig. 12), it suggests that the cavities in the larger rings are not well represented by the computational ^THCCS_N2 methodology with the trajectory method, as previously discussed²⁵. The ^THCCS_N2 values of the different heptametallic isomers {Cr₅Cu₂}, {Cr₆Cu} and {Cr₅Gd₂} are similar (Supplementary Table 2), and the fragment structures can hence not be resolved via comparisons of experimental and theoretical CCS_N2.

We examined the high mass loss from {Cr₁₀Cu₂} in more detail. Experiment suggests that the cleavage does not occur at the hourglass bottleneck, as observed for {Cr₁₂Cu₂} and resulting in {Cr₆Cu}, but counterintuitively at the adjacent Cr–Cu edge. This leads to the formation of {Cr₅Cu₂} instead of {Cr₅Cu}, which agrees with the thermodynamic stabilities of the DFT optimized structures (Supplementary Figs. 11 and 12 and Supplementary Dataset). Here the disassembly of {Cr₅Cu₂} → {Cr₅Cu} + [CuPiv₂] is endothermic (ΔE ≈ 334 kJ mol⁻¹), and this fragmentation channel is experimentally only observed at higher collision energies (Fig. 2a top).

Discussion

The first major finding from this work is the formation, separation and isolation of closed cyclic products in the gas phase from the CID of the {Cr_xCu₂} hourglasses (x = 10, 12) and from {Cr₁₂Gd₄}. The observation that polymetallic rings are formed from all three precursors suggests that this workflow can produce smaller rings of varying sizes, including new types, as long as appropriate larger precursors are available. These rings form after collisions on a millisecond timescale, which suggests that both rearrangements and new bond formations occur fast. Also noteworthy is the occurrence of gaps in the tandem mass spectra of all three parent ions (Figs. 2 and 4a), which suggests that potential products in these regions are considerably less stable than those obtained at both lower and higher masses, akin to the magic numbers seen in the spectra for monatomic and molecular cluster ions^{16,33,34,35,36,37}. For the low m/z region, the primary and kinetically most stable products from the precursor ions are {Cr₅Cu₂}, {Cr₆Cu} and {Cr₆Gd₂}, whereas others such as {Cr₅Cu} and {Cr₅Gd₂} are secondary products from these fragments.

The formation of the closed, heptametallic species {Cr₅Cu₂}, {Cr₆Cu} and {Cr₅Gd₂} is striking, for which DFT optimizations indicate several plausible structures (Supplementary Figs. 11, 13 and 15). In all three cases, the lowest-energy conformer of those sampled involves a hexametallic ring with an additional metal bridge. In the family of polymetallic chromium rings³⁸, the only closed seven-metal ring synthesized so far is {Cr₆Ce}, which only forms as a very minor side product³⁰, and this is one of three structurally characterized heptametallic rings known^39,40. Given the sparsity of known heptametallic compounds it might be argued that these products are unlikely; however, in the synthesis of heterometallic chromium rings the difficulty is avoiding the formation of [Cr₈F₈Piv₁₆], which is achieved by adding a cationic template that occupies the centre of the cavity and leads to an anionic heterometallic ring²⁶. This allows formation of larger rings by choice of larger templates, but for closed seven-metal species there is insufficient space for any organic template. Applying the presented CID–MS approach on these larger complexes therefore offers a route for the formation of closed heptametallic and smaller species, and we have confirmed their gas phase stability using DFT²⁵.

The discussed approach can inspire supramolecular chemistry outside the gas phase. In some cases, rings similar to those produced by CID–MS have been previously synthesized and characterized. For example, several {Cr₁₀} rings have been reported by our group; however, these involved different bridging ligands^41,42,43. We have also structurally characterized a {Cr₆Y₂} ring of the formula [Cr₆Y₂F₈Piv₁₇(NH₂Et₂)(H₂O)], which is highly similar to the ion [Cr₆Gd₂F₈Piv₁₆(NH₂ⁿPr₂)]⁺ observed upon fragmentation of {Cr₁₂Gd₄}. The {Cr₆Y₂} was formed only in low yield, which correlates with the observation that the solution synthesis leads to {Cr₁₂Ln₄} cages as the major products³⁰.

Another aim for future endeavours is to connect gas phase experiments with bulk phase via the so-called preparative mass spectrometry (‘ion soft-landing’), in which ions produced in the mass spectrometer are transferred and gently deposited on a surface^44,45,46. This surface can in turn be analysed by a range of microscopy^47,48, spectroscopy^49,50 or mass spectrometry techniques⁵¹, among others. As the field of soft landing is still in its infancy and rapidly evolving, we hypothesize that the presented CID–MS formation strategy will be of practical significance for synthetic chemists in the future.

Our second major observation is that ion mobility can be used to identify whether a given polymetallic complex of this family is cyclic or not, and this applies both to structures formed via CID–MS in the gas phase, and also to those synthesized in solution that are just transferred to the gas phase (such as {Cr₁₀Cu₂}, {Cr₁₂Cu₂} and {Cr₁₂Gd₄}). The two main advantages here, compared with other bulk phase methods, are small measurement times and low amounts of samples needed. When directly assigning structures using ion mobility, theoretical calculations such as DFT are commonly applied to compare theoretical CCS values to those found experimentally; however, this can be non-trivial and requires careful structural and conformational sampling. On the basis of DFT calculations, we have benchmarked the following workflow to theoretical CCS_N2 values from this and previous works, yielding good agreement^25,32.

We plot the CCS_N2 of all species examined here and from previous work (Table 1) against their adjusted masses (Fig. 5). This correlation represents the packing density in a given ion^28,52,53, which is highest for ions with small CCS and large mass. Previously, Bleiholder et al. tracked the self-assembly of peptides and, using an analogous relationship, distinguished between the formation of larger globular peptides and β-sheets relevant for amyloid fibril formation⁵³. Later our group showed that the CCS/m slope of unfolded and intrinsically disordered proteins is significantly higher (lower density) than for native proteins²⁸. In the case of cyclic species from this and previous works, we observe a linear correlation, which does not hold for the acyclic {Cr₁₂Gd₄} cluster and the extended {Cr₆Mn} horseshoes²⁵. These are located above the line as their estimated spherical densities (ESD) are lower than for the polymetallic cyclic structures (Fig. 5 inset). ESD were derived from CCS_N2 values, following previously published calculations based on the assumption of a spherical ion⁵², and cannot be directly compared with the macromolecular density in the crystal structures, where available. The latter is typically higher (1.2–1.4 g cm⁻³) and represents the packing density of the molecules in the crystal lattice (together with solvent molecules), whereas the ESD discussed here inform on how dense the atoms are in the gas phase ion.

Fig. 5: Correlation between CCS_N2 and adjusted mass (m_ad) for all species in Table 1.

The cyclic complexes show a linear correlation, which does not hold for the acyclic ions ({Cr₁₂Gd₄} and {Cr₆Mn} horseshoes). 1: {Cr₅Cu}; 2: {Cr₅Gd₂}; 3: {Cr₅Cu₂}; 4: {Cr₆Cu}; 5, 6: {Cr₆Mn} rings. The mass was adjusted so that every metal centre is accounted for with the mass of chromium (52 Da). This model is reasonable as the nature of the metal in the polymetallic complex has a low overall impact on the CCS_N2, although some have high masses, for example in the case of Gd. Inset: schematics of selected cyclic and acyclic complexes (Cr, green; Cu, brown; Gd, purple; Mn, black). Fragment ions that are assigned as closed, due to the ion mobility data, are presented schematically; the IM–MS experiment informs on the stoichiometry but not on the exact connectivity between the metal centres. ESD derived from CCS_N2 values versus m_ad (black circles, cyclic; blue squares, acyclic), showing that the acyclic {Cr₆Mn} horseshoes and {Cr₁₂Gd₄} have a lower ESD than the cyclic species.

Source data

Disassembly of {Cr ₁₀ Cu ₂} and {Cr ₁₂ Cu ₂}

The first dissociation step of both hourglass ions [{Cr_xCu₂} + Na]⁺ (x = 10, 12) is the loss of the secondary ammonium cation, along with an anionic ligand, predominantly a pivalate. This is probably because the cation is the only non-covalently bound species in the system. As the primary fragmentation pathway is the same for both [{Cr_xCu₂} + Na]⁺ ions, their E₅₀ values are also similar. The same dissociation channel was observed for the heterometallic rotaxanes [NH₂RR′][Cr₇MF₈Piv₁₆], where R and R′ include bulky phenyl and tert-butyl groups²⁵. The secondary ammonium cations are smaller here, so the relative ease of dissociation from the hourglass ions is not surprising.

The further disassembly can be rationalized by considering the sites in the hourglass structure that are most prone to CID. These are the two Cu^II centres and their ligands, because of the lower oxidation state (Cu^II versus Cr^III), and hence a weaker electrostatic attraction between ligand and metal, as well as the coordination number (Cu^II: 5, Cr^III: 6; Fig. 1a,b). It seems likely that both the initially lost ammonium cation and pivalate are adjacent to the same Cu^II centre. This would weaken the hourglass scaffold further in this region, which is the reason for its disruption in the second dissociation step (rather than the loss of the second ammonium cation). As discussed above, the hourglass disruption occurs via different pathways, yielding product ions either with low m/z or high m/z. It is also possible that some initial fragments at high m/z fragment further to smaller products.

Fragmentation associated with higher mass loss leads to rings of the type {Cr₅Cu₂} (from [{Cr₁₀Cu₂} + Na]⁺) and {Cr₆Cu} (from [{Cr₁₂Cu₂} + Na]⁺; Figs. 2 and 3 and Table 1). In the disassembly of {Cr₁₂Cu₂} the dissociation occurs at the hourglass bottleneck, and this is probably the favoured site as Cu^II is far more reactive than Cr^III, and only two bridging ligands are present. As a result, the hourglass predominantly dissociates symmetrically to {Cr₆Cu}, which probably rearranges to a closed structure. For the fragmentation of {Cr₁₀Cu₂}, this same mechanism would lead to a six-membered {Cr₅Cu} species, but interestingly this ring, characterized by ion mobility, is only seen at higher energies. This suggests that it is a secondary fragment (Fig. 2a and Supplementary Dataset). By contrast, we predominantly observe {Cr₅Cu₂} products due to an asymmetric hourglass disruption, and here three bonds need to be broken instead of two. This preference may be due to the stability given by the formation of the heptametallic species, which implies it is more stable than the alternative hexametallic {Cr₅Cu}. The comparison of their energetics using DFT supports these findings, showing that {Cr₅Cu₂} is the thermodynamically more stable product than {Cr₅Cu} (ΔE ≈ 334 kJ mol⁻¹; Supplementary Figs. 11 and 12). Hence, the secondary and endothermic disassembly from {Cr₅Cu₂} to {Cr₅Cu} and [CuPiv₂] occurs only at higher collision energies. Notably, the lowest energy isomer found for {Cr₅Cu₂} is a {Cr₅Cu} ring bridged by the second Cu^II centre, which may inspire future investigations on the stability of different bridging situations in polymetallic rings.

No ammonium cation is present in the dominant {Cr₅Cu₂} or {Cr₆Cu} ring products, which suggests that these are formed from the part of the hourglass where the ammonium cations and Piv⁻ are lost in the primary fragmentation step. This is possibly due to insufficient space in the central cavity of a closed seven-metal species to bind an ammonium cation. Instead, Na⁺ was found in the heptametallic species observed, where it is probably located in the centre of the ring as the DFT optimized structures suggest (Supplementary Figs. 11 and 13). We have previously found that Na⁺ is too small for a good fit for the cavity of an octametallic ring, both in the bulk and the gas phase^32,54, and might hence be better suited for heptametallic species. The formation of {Cr₅Cu₂} from {Cr₁₀Cu₂}, probably occurring in a concerted step, implies that {Cr₅} is also formed. As discussed above, we have a low number of {Cr₅} ions (Fig. 2a top), which is probably due to the relatively higher stability of heptametallic rings and the preference of the charge-carrier Na⁺ to remain with {Cr₅Cu₂}, which complicates the detection of {Cr₅} cations. Our data suggest that {Cr₅} can occur in different conformations and/or topologies, potentially either as chains and/or rings (Supplementary Dataset).

The mechanism associated with smaller mass losses shows the stepwise disruption of the hourglasses, starting with the loss of one Cu^II centre along with two ligands. The formed {Cr₁₀Cu} and {Cr₁₂Cu} products are probably also closed due to their narrow and unimodal CCS_N2 distributions (Fig. 3 and Table 1), and require the formation of new bonds on the experimental timescale. The {Cr₁₂Cu} distribution is significantly wider than for {Cr₁₀Cu}, indicating a higher conformational flexibility. The same qualitative trend was observed in the next fragmentation step, where the second ammonium cation is lost, predominantly along with a pivalate. After that, the second Cu^II centre dissociates along with two anionic ligands, leading to ions of the type {Cr₁₀} and {Cr₁₂}. Here both ions exhibit narrow CCS_N2 distributions, once more suggesting cyclic structures.

Disassembly of {Cr ₁₂ Gd ₄}

If similar disassembly mechanisms take place for [{Cr₁₂Gd₄} – Piv]⁺ as for the hourglass ions, we would also expect smaller closed structures, of the type {Cr₆Gd} (as discussed above and in Supplementary Table 1) or {Cr₆Gd₂}. For the second route involving low mass loss, we would expect the stepwise disruption of the {Cr₆} chains, again followed by rearrangements to closed structures, as the {Cr₆} chains are more accessible for gas collisions and less strongly bound than the {Gd₄} unit.

The fragmentation of the ion [{Cr₁₂Gd₄} – Piv]⁺ follows similar routes as [{Cr_xCu₂} + Na]⁺ (x = 10, 12) and confirms our hypothesis that the mechanisms observed for the hourglass ions can be transferred to other polymetallic complexes. Here, after the loss of one secondary ammonium cation along with a fluoride, the disassembly associated with higher mass loss leads to {Cr₆Gd₂} and subsequently to {Cr₅Gd₂} complexes, for which the comparisons with previously reported CCS_N2 values clearly indicate closed structures.

The second disassembly mechanism of the {Cr₁₂Gd₄} cluster shows the loss of the second [NH₂ⁿPr₂]⁺ and a pivalate, followed by multiple dissociation steps in which {CrPiv₃} is lost. Here the loss of five {CrPiv₃} units, occurring as the most abundant pathways, exceeds the number of pivalates present in a {Cr₆} chain and suggests either major rearrangements, or the disruption of the second {Cr₆} unit before the complete dissociation of the first. The CCS_N2 distributions of the occurring products indicate more flexible structures (larger peak widths) than the precursor [{Cr₁₂Gd₄} – Piv]⁺; however most of the products still appear with unimodal conformations (similarly to the high mass fragments of [{Cr₁₂Cu₂} + Na]⁺). This suggests that no major ring perturbation takes place and both horseshoe units are possibly disrupted simultaneously. In either case, the {CrPiv₃} leaving group seems to be the driving force of this disassembly route, similar to previous observations^25,32.

Conclusions

For the three studied compounds, {Cr_xCu₂} (x = 10, 12) and {Cr₁₂Gd₄}, the formation of smaller rings via rearrangements and newly established connectivities suggests a ‘self-healing’ capability upon CID, driven by the stability of closed, polymetallic species. Several of the latter were formed including {Cr₆Cu}, {Cr₅Cu₂}, {Cr₅Cu}, {Cr₁₀Cu}, {Cr₁₀}, {Cr₁₂Cu}, {Cr₁₂}, {Cr₆Gd₂} and {Cr₅Gd₂}, whose topology was identified by ion mobility. This workflow can potentially inspire applications in materials science, if these or similar complexes can be made in isolable quantities, and in general aid the design process of polymetallic complexes by identifying feasible targets. We also propose a simple method to assign whether a polymetallic complex exists as a closed or open structure, and this approach can in the future be adapted for other compound families. This workflow, with some modifications, has great promise for the structural characterization of larger metallosupramolecular compounds, and so has the diagnostic use of the tandem mass spectra to predict the precursor structure. As reliable computations and X-ray crystallography are often not feasible, IM–MS is an important expansion of the available analytical methods for these systems⁵⁵.

Methods

Synthesis

{Cr₁₂Cu₂} was synthesized according to our previous work²⁷. {Cr₁₀Cu₂} was made similarly to [NH₂Et₂]₂[Cr₁₀Cu₂F₁₄(O₂C^tBu)₂₂] (ref. ²⁶), but the precursor diethylamine was replaced with dipropylamine. Elemental analysis (%) calculated for {Cr₁₀Cu₂}: C 43.84, H 6.94, N 0.84, Cr 15.56, Cu 3.80; found: C 44.13, H 6.99, N 0.89, Cr 15.42, Cu 3.86. {Cr₁₂Gd₄} was synthesized similarly to [NH₂Et₂]₂[Cr₁₂Gd₄F₂₁(O₂C^tBu)₂₉] (ref. ³⁰), by using the previously reported⁵⁶ [(NH₂ⁿPr₂)₃Cr₆F₁₁Piv₁₀]₂ as the reactant instead of [(NH₂Et₂)₃Cr₆F₁₁Piv₁₀]₂. Elemental analysis (%) calculated for {Cr₁₂Gd₄}: C 39.38, H 6.17, N 0.58, Cr 13.03, Gd 13.13; found: C 39.54, H 6.21, N 0.63, Cr 12.85, Gd 13.45. All reagents and solvents were purchased from Alfa, Fisher Scientific, Fluorochem or Sigma-Aldrich and used without further purification.

Sample preparation

Samples were prepared in 4:1 toluene/methanol with 500 µM NaI. Analyte concentrations of 200–500 µM were typically used for IM–MS and MS measurements.

nESI

Samples were ionized and transferred to the gas phase with an nESI source and were sprayed from borosilicate glass capillaries (World Precision Instruments). The latter were pulled on the Flaming/Brown P-2000 laser puller (Sutter Instrument Company). The capillary voltage (typically 1.0–1.8 kV) was applied through a platinum wire (diameter 0.125 mm, Goodfellow) inserted into the nESI capillaries. Source temperatures of 23 °C (Cyclic) or 30 °C (Q Exactive UHMR) were applied.

MS²

The Q Exactive Ultra-High-Mass-Range (UHMR) Hybrid Quadrupole-Orbitrap Mass Spectrometer (Thermo Fisher) was used for the derivation of the E₅₀ values via MS² experiments, and more precisely CID⁵⁷. Target ions were m/z-isolated in a quadrupole filter, accelerated to a user-defined kinetic energy (E_lab: 0–300 eV) and injected into the higher-energy C-trap dissociation cell, which contained nitrogen gas (trapping gas pressure parameter, 2.0). Non-fragmented precursor ions and fragment ions were transferred to the Orbitrap mass analyser (maximum inject time, 100 ms; resolution, 25,000).

IM–MS experiments were performed on a Select Series Cyclic IMS (Waters)⁵⁸. Following ionization (cone voltage, 20–60 V; source offset, 10–20 V; and purge gas, 0–300 l h⁻¹), ions were transferred to the trap and activated via collisions with nitrogen gas, if appropriate (trap voltage 0–200 V and gas flow 5 ml min⁻¹). Ions are further injected to the cyclic ion mobility drift ring (Stepwave Ion Guide RF: 300–700 V) and separated by using a non-uniform electric field under a constant nitrogen pressure with travelling waves (TW; height, 20–22 V; velocity, 375 m s⁻¹; and gas flow, 40 ml min⁻¹), pushing the ions through the drift ring. Ions travelled one pass in the cyclic drift ring (‘single path’, separation time: 2–60 ms) and were then transferred (transfer voltage: 4–15 V) to a time-of-flight mass analyser.

Data processing

E₅₀ values were obtained from a method described in our previous works^25,32. Mass spectra were recorded at different collision energies, and the share of the precursor ion count, relative to the total ion count (‘survival yield’), was plotted versus the collisional energy in the centre-of-mass frame (E_com, Supplementary Figs. 3 and 9). Survival yield plots were fitted with a sigmoidal Hill function (Hill1 function in OriginPro 2020b), yielding the point (E₅₀) at which the survival yield reaches 0.5 or 50%. This E₅₀ value is known as a relative measure of precursor ion stability^59,60,61.

Experimentally obtained ATD were converted to collisional cross-section distributions ^TWCCS_N2 via published calibration procedures⁶². The Agilent tune mix was used as a calibrant⁶³.

Density functional theory and CCS calculations

Analogous to our previous works^25,32, density functional calculations were carried out using the same effective core potentials and basis sets for Cu, Cr, C, N, O, F and H, assuming high-spin ferromagnetically coupled metals. For Gd^III, it was necessary to use an alternative effective core potential, including 7f electrons in core⁶⁴, with the corresponding (7s6p5d)/(5s4p3d) contracted valence basis set⁶⁵.

Theoretical CCS values (^THCCS_N2, TH: theoretical) were obtained from the software IMoS by using the trajectory method in nitrogen gas including quadrupole potential (number of orientations 3, gas molecules per orientation 300,000, temperature 298 K, and pressure 101,325 Pa = 1 atm)³¹.

Crystallographic data

Single crystals of {Cr₁₀Cu₂} and {Cr₁₂Gd₄} were grown by slow diffusion of acetonitrile into toluene solutions of the respective complexes. ORTEP structures of {Cr₁₀Cu₂} (Supplementary Fig. 16) and {Cr₁₂Gd₄} (Supplementary Fig. 17) can be found in Supplementary Information along with crystallographic refinement details (Supplementary Table 3). Crystal structures are also shown in Fig. 1a,c as well as in Supplementary Dataset, respectively.

X-ray diffraction data were collected using a dual wavelength Rigaku FR-X rotating anode diffractometer using MoKα (λ = 0.71073 Å) radiation, equipped with an AFC-11 4-circle goniometer, VariMAX microfocus optics, a Hypix-6000HE detector and an Oxford Cryosystems 800 plus nitrogen flow gas system, at a temperature of 100 K. Data were collected and reduced using Rigaku CrysAlisPro v42 (ref. ⁶⁶), and absorption correction was performed using empirical methods (SCALE3 ABSPACK) based upon symmetry-equivalent reflections combined with measurements at different azimuthal angles. The phase problem was solved using SHELXT and the structural model refined against all F² values using SHELXL^67,68, implemented through Olex2 v1.5 (ref. ⁶⁹). All non-solvent atoms were refined anisotropically. Hydrogen atoms were placed in calculated positions and refined using idealized geometries and assigned fixed isotropic displacement parameters.