Bond Dissociation Energies of Metallo-supramolecular Building Blocks

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Bond Dissociation Energies of Metallo-supramolecular Building Blocks: Insight from Fragmentation of Selectively Self-Assembled Heterometallic Metallo-supramolecular Aggregates Yvonne Lorenz,† Albert Gutiérrez,‡ Montserrat Ferrer,‡ and Marianne Engeser*,† †

Kekulé-Institute for Organic Chemistry and Biochemistry, University of Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany Departament de Quimica Inorgànica i Orgànica, Secció de Quimica Inorgànica, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain

‡

S Supporting Information *

ABSTRACT: A series of selectively self-assembled metallo-supramolecular square-like macrocycles with unsymmetric ditopic linkers and two different types of metal corners, i.e., {Pd(η3-2-Me-C3H4)} and {M(dppp)} with dppp = 1,3-bis(diphenylphosphino)propane and M = Pd2+ or Pt2+, have been studied in the gas phase using collision-induced dissociation. The aggregates show distinct fragmentation patterns determined by ligand length, i.e, aggregate size, and type of metal corner. Information on relative binding strength can be deduced. This is of particular interest for (methylallyl)Pd as a relatively new building block in metallo-supramolecular chemistry. The phosphane end of the unsymmetric ligand connected to (η3-2-Me-C3H4)Pd is bound significantly stronger than its pyridine end to (dppp)Pt and (dppp)Pd. These results are corroborated by DFT calculations.

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structure of the aggregate11d,14 and give insight into the relative binding strength of the respective subunits.10,15,16 Such information is highly valuable as it forms the basis for a rational design of complex functional supramolecular aggregates. We have previously reported the selective self-assembly of four homo- and heterometallic metallo-supramolecular squarelike macrocycles9 with unsymmetric edge ligands and two different cis-protected metal corners of the general composition [{Pd(η3-2-Me-C3H4)}2(Ln)4{M(dppp)}2](CF3SO3)6 with M = Pd, Pt; Ln = 4-Ph2Ppy (L1) or 4-Ph2PC6F4py (L2) (Scheme 1, dppp = 1,3-bis(diphenylphosphino)propane, py = pyridine, Me = methyl, Ph = phenyl) including inter alia their characterization by electrospray mass spectrometry. The aggregates show a complex dynamic behavior in solution:9 Equilibration due to associative solvent- and anion-assisted decoordination/rotation/coordination processes takes place. The (methylallyl)Pd− P connection seems to stay intact, whereas the (dppp)M−Py bond is remarkably labile even for the stable Pt-containing aggregates. Further, a very fast associatively activated exchange of building blocks is observed when using the longer bridging ligand L1. The tetrafluorophenylene unit decreases the basicity of the pyridine coordination site and thus destabilizes the (dppp)M−Py bonds. Associative exchange processes are impossible in the absence of putative reaction partners in the highly diluted gas phase of a mass spectrometer. This fact and the bimetallic nature of the species lead to interesting questions

INTRODUCTION Over the past decades, a vast diversity of self-assembled supramolecular structures based on an impressive variety of subcomponents has been designed, synthesized, and analyzed,1 among them the first metallo-supramolecular squares introduced by Fujita2 and Stang3 in the early 1990s. Going one step further beyond creating fascinating architectures, the implementation of function4 soon gained tremendous interest as visible in the 2016 Nobel Prize for Jean-Pierre Sauvage, Sir J. Fraser Stoddart, and Bernard L. Feringa “for the design and synthesis of molecular machines”.5 In contrast to assemblies consisting of symmetric monomers and one type of metal only, heterometallic assemblies selectively built from more than two different dynamically connected subcomponents still remain very challenging.6 To obtain a single species instead of a statistical mixture, the electronic and/or steric demands of all building blocks need to be adjusted very carefully to avoid unwanted interferences.7 Unsymmetric ligands can help in achieving this aim and provoke self-sorting due to electronic discrimination.8,9 Mass spectrometry under soft ionization conditions is not only essential to determine the stoichiometry and size of selfassembled aggregates.10−12 It is also ideal to analyze the intrinsic properties of weakly bound supramolecules in the absence of interferences from the environment.10 In solution, dynamic intermolecular exchange processes often complicate or even prevent studying one specific aggregate of choice. Further, induced fragmentation experiments13 in the gas phase can be used to probe the relative stability of noncovalent bonds. Analysis of the fragmentation pathways can confirm the © XXXX American Chemical Society

Received: April 13, 2018

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DOI: 10.1021/acs.inorgchem.8b00930 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Graphical Code Used To Depict the Four Metallo-supramolecular Aggregates Used for This Studya

a

(η3-2-Me-C3H4)Pd as triangle, (dppp)M as circle with M = Pd in green or M = Pt in red stripes, ditopic edge ligands L1 or L2 as rectangles with a white end for Ph2P and a black end for the pyridine side, triflate counter ion as black squares.

than their more stable Pt counterparts.16,20 This is visible also in the present case (Figures 1 and 2). The spectra of Pt-

on the nature of the weakest bond and the favored fragmentation pathways in the gas phase which we will report herein. Previous studies of gas-phase fragmentation of metallosupramolecular squares17 focused on the classic Stang-type3 formed of (elongated) 4,4′-bipyridine (“edges”) and cisprotected Pt-complexes with 1,3-bis(diphenylphosphino)propane (dppp) ligands (“corners”). All observed fragmentation pathways begin by breaking the weakest bond, the pyridine−Pt dative bond. Two fragmentation pathways then compete: loss of neutral edge ligands leaving the charge of the ion unchanged vs charge separation into two ions. The latter is more pronounced for higher charge states due to Coulomb repulsion and preferably occurs unsymmetrically,17 yielding [1:1] and [3:3] complexes out of [4:4] squares.18 Further, these results indicate that the nature and electronic properties of the building blocks determine the fragmentation more significantly than the aggregate’s topology (square).17 On the basis of this knowledge on the gas-phase behavior of symmetrical squares, we herein focus on the above-mentioned much more complex heterobimetallic macrocycles with a particular focus on the dependence of fragmentation patterns on the type of ligands and metal corners. Throughout this Article, the aggregates formed with the longer linking ligand L1 are denominated 1a with Pt and 1b with Pd as a dppp-ligated subunit. Similarly, the two variants with the shorter ligand L2 are referred to as 2a (Pt) and 2b (Pd). As a short nomenclature for aggregates and fragments, we use the form [A:B:C:D] with A = number of {Pd(η3-2-MeC3H4)} subunits, B = number of {M(dppp)} subunits with M = Pd or Pt, C = number of ligands L1 or L2, and D = number of triflate (OTf) anions. The neutral intact aggregates are thus assigned to [2:2:4:6] for all substances 1a−2b.

Figure 1. ESI(+) mass spectrum of 1a.

containing 1a and 2a show a series of intact squares in different charge states due to the abstraction of the respective number of triflate anions during the ESI process. Some fragmentation products are also present, but in much smaller abundances

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RESULTS AND DISCUSSION ESI of Metallo-supramolecular Aggregates. It is possible to transfer metallo-supramolecular aggregates from solution into the gas phase by applying very soft electrospray ionization (ESI) conditions.9−17,19 It is however hard to completely avoid fragmentation, particularly for Pd-connected aggregates which typically are much more prone to dissociation

Figure 2. ESI(+) mass spectrum of 2b. *: electronic noise. B


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Inorganic Chemistry

[1:1:2:2]+ at m/z 1889 is not observed, probably due to a rapid loss of L1, again leading to m/z 1478. Overall, the aggregate is split symmetrically into two dinuclear bimetallic fragments via elimination of two ligands L1. The preferred formation of the gas-phase ion [1:1:1:2]+ can be ascribed to an enhanced stability of a structure without open coordination sites: Both sides of the bridging ligand are coordinated to a metal, and the anions exactly fill the “free” coordinations sites at both metals, in contrast to all other putative ways of fragmenting the aggregate [2:2:3:4]2+. The observed fragmentation pattern does not allow a differentiation whether a Pd−P or a Pt−N bond is broken first. CID of the [1:1:1:2]+ fragmentation product (Figure 3c) unfortunately is not helpful in this respect either. The formation of the ligated platinum complex [0:1:1:1]+ at m/z 1167 is only possible by opening the Pd−P bond while keeping the Pt−N bond intact. However, significant amounts of a neutral ligated Pd complex [1:0:1:1] could also be formed (broken arrow in Figure 3). The corresponding product ion [0:1:0:1]+ at m/z 756 is strongly present in the spectra, but it is additionally generated via a secondary fragmentation pathway (loss of L1 from [0:1:1:1]+ at m/z 1167) and further fragments via loss of CF3SO3H as known from the literature.17b Unfortunately, one cannot observe the neutral molecules produced during fragmentation. Very similar to the fragmentation behavior of [1a-2OTf]2+, the main fragmentation route of homometallic [1b-2OTf]2+ is the loss of L1 (Figure 4a) with subsequent formation of the dinuclear complex [1:1:1:2]+ at m/z 1390 (Figure 4b).

when sufficiently soft ionization conditions are used (Figure 1). The spectra of their Pd analogues 1b and 2b also show signals for intact squares. But these are restricted to the lower charge states and many fragmentation products are additionally observed (Figure 2). The Pd derivatives obviously do not survive the enhanced Coulomb repulsion present in higher charge states. The nature of the fragments gives a first hint on the weakest bond in the aggregate: The ion [1:0:2:0]+, i.e., a mononuclear complex of (methylallyl)Pd with two coordinated ligands L2, forms the base peak (Figure 2). The preferred cleavage position in the aggregate thus seems to be the (dppp)Pd−pyridine bond. Some higher aggregation products are clearly visible in all cases, i.e., a signal for [4:4:8:9]3+, which are due to unspecific aggregation during the electrospray process. This is a result of the rather high concentration of the sprayed solutions necessary to avoid disassembly. Selfassembled supramolecular aggregates only exist in solution within an optimal concentration range which is limited by the so-called lowest self-assembly concentration (LSAC).21 The charge state of the species has a severe effect on the fragmentation patterns of metallo-supramolecular squares.17b For a comparison aiming at the influence of metal and ligand type, it is thus important to keep the charge state constant. All induced fragmentations reported herein thus start from doubly charged squares [2:2:4:4]2+ produced during the ESI process by stripping off two triflate counterions. Collision-Induced Dissociation (CID) of the Larger Aggregates [1a/b-2OTf]2+. The fragmentation cascade of the heterometallic [1a-2OTf]2+ (Figure 3) begins with loss of one ligand L1 to form an open-chain aggregate [2:2:3:4]2+ (m/z 1684), very similar to the findings for the doubly charged symmetric squares reported previously.17b Subsequent fragmentation of m/z 1684 leads to charge separation into singly charged ions. The spectrum predominantly shows the ion [1:1:1:2]+ at m/z 1478 (Figure 3b). The corresponding ion

Figure 4. Fragmentation cascade of [1b-2OTf]2+ monitored by MSn experiments: (a) CID of m/z 1801 with 12 eV, (b) CID/CID of m/z 1596 with 10 eV, (c) CID/CID of m/z 983 with 11 eV, (d) CID/ CID/CID of m/z 1390 with 7 eV, (e) CID/CID/CID of m/z 2206 with 10 eV, (f) CID/CID of m/z 1485 with 8 eV.22 Blue squares mark the mass-selected ions; *: electronic noise. Unlabeled gray arrows symbolize subsequent fragmentation pathways.

Figure 3. Fragmentation cascade of [1a-2OTf]2+ monitored by MSn experiments: (a) CID of m/z 1889 with 11 eV, (b) CID/CID of m/z 1685 with 11.2 eV,22 (c) CID/CID of m/z 1478 with 11.5 eV. Blue squares mark the mass-selected ions; *: electronic noise. C


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Inorganic Chemistry Fragmentation of this ion however is different (Figure 4d). The competitive cleavage of the ligand−Pd donor bond at the pyridine or phosphane side is now very clear. The only observed product is (dppp)Pd(OTf)+ at m/z 667. We will come back to this observation below. In the case of [1b-2OTf]2+, other fragmentation pathways are observed in minor abundances in addition to the strongly dominating ligand loss. These are two different unsymmetrical variations of a charge separating fragmentation of the doubly charged precursor ion into two singly charged fragment ions. Pathway A (broken green lines in Figure 4a) involves ruptures of two Py−Pd bonds and leads to the trinuclear complex [1:2:2:4]+ at m/z 2617 and the mononuclear [1:0:2:0]+ at m/z 983. Consecutive ligand loss from this complex is straightforward (Figure 4c). Whereas pathway A is rather low abundant, its analogue version A′ starting from m/z 1596 after the first ligand loss is an effectively competing alternative (Figure 4b). The trinuclear product ion [1:2:1:4]+ at m/z 2206 is interesting: Only one ambidentate ligand L1 faces three metal corners. Thus, a salt bridge structure is proposed in which the additional cationic metal center is bound by intercalated anions in accordance with the consecutive fragmentation of this ion (Figure 4e). Elimination of a neutral complex [1:0:1:1] leads to the solely anion bridged cluster [0:2:0:3]+ at m/z 1485 which then loses (dppp)Pd(OTf)2 (Figure 4f). The preference for path A′ which is visible in good intensities in contrast to path A which is only observed in trace amounts could be attributed to lower geometrical constraints in the Coulomb-connected saltbridged clusters in the absence of a second ligand L1. The very small signal at m/z 2111 in the fragmentation spectrum of [1b-2OTf]2+ (Figure 4a) finally is assigned to [2:1:2:3]+. According to its composition, it has to be the result of an additional asymmetric charge separating pathway B in which the (dppp)Pd building block is expelled (instead of the (methylallyl)Pd complex in path A). As we could not find ions that still bear the two missing ligands L1, we simply include it as a path B′ in Figure 4. As a first summary, the larger squares [1a/b-2OTf]2+ fragment rather similarly: The cascade starts with loss of a bridging ligand L1 and is followed by formation of the stable product ion [1:1:1:2]+. The less stable homometallic square with (dppp)Pd corners shows some additional chargeseparating fragmentations, however, in minor amounts. Collision-Induced Dissociation of the Smaller Aggregates [2a/b-2OTf]2+. Homometallic [2b-2OTf]2+ shows a tricky fragmentation pattern (Figure 5) which is rather different to the ones of the larger aggregates [1a/b-2OTf]2+. Initial expulsion of a ligand is not observed at all. At first sight, only two fragmentation pathways are present. Both include unsymmetrical cleavage under charge separation breaking the square into a single corner and a trinuclear aggregate. The first one, a very minor pathway, is the formation of [0:1:1:1]+ at m/z 930 and [2:1:3:3]+ at m/z 2078 which is similar to path B described for 1b above, but this time, the ligand L2 stays bound to the metal. The second one, path A, is slightly more intense (Figure 5a), occurs exactly like in 1b, and thus leads to [1:0:2:0]+ at m/ z 687 and [1:2:2:4]+ at m/z 2321. This ion fragments by loss of L2 (Figure 5c) to form the ion [1:2:1:4]+ at m/z 2058. The latter again consists of three cationic metal centers held together by four anions and only one bridging ligand. It subsequently expels the neutral complex [1:0:1:1] to form the same purely salt-bridged ion [0:2:0:3]+ at m/z 1485 as already described above (Figures 4f and 5d).

Figure 5. Fragmentation cascade of [2b-2OTf]2+ monitored by MSn experiments: (a) CID of m/z 1505 with 0 eV, (b) CID of m/z 1505 with 12.5 eV, (c) CID/CID of m/z 2321 with 40 eV, (d) CID/CID/ CID of m/z 1485 with 7 eV, (e) CID/CID of m/z 1242 with 10 eV. Blue squares mark the mass-selected ions; *: electronic noise. Unlabeled gray arrows symbolize subsequent fragmentation pathways.

A closer inspection of the spectra however reveals the real main fragmentation of [2b-2OTf]2+, labeled path C: The aggregate is exactly split into halves: [2:2:4:4]2+ fragments into two ions [1:1:2:2]+. m/z stays the same, but the cleavage is clearly obvious when the isotope pattern is inspected (Figure 5a). Actually, the whole square-like macrocycle is so fragile (in contrast to all other ones covered in this study) that it is difficult to tune the ESI conditions sufficiently soft to avoid fragmentation during normal ESI. Thus, already the starting spectrum contains some amount of fragments. Increasing the collision energy immediately changes the isotope pattern to the one of the fragment ion. [1:1:2:2]+ at m/z 1505 expectedly loses a ligand L1 to form the stable dinuclear ion [1:1:1:2]+ at m/z 1242 similarly to the behavior of the respective ion formed from 1b including its fragmentation into [0:1:0:1]+ at m/z 667. The observed charge-separating fragmentations are classified herein according to the following nomenclature:23 Path A: asymmetric cleavage resulting in a trinuclear aggregate and a mononuclear (methylallyl)Pd complex; Path B: asymmetric cleavage resulting in a trinuclear aggregate and a mononuclear (dppp)M complex; Path C: cleavage into two dinuclear aggregates. The complex fragmentation of the fourth aggregate, the small heterometallic [2a-2OTf]2+, can be analyzed as a superposition of all three types of charge-separating pathways A, B, and C (Figure 6). The major fragmentation pathways of [2a-2OTf]2+ go along path B: An asymmetric cleavage to form the mononuclear complex [0:1:1:1]+ at m/z 1019 and the trinuclear aggregate [2:1:3:3]+ at m/z 2167. This fragmentation resembles the one observed for triply charged normal Stang squares.17 However, the trinuclear product has the form of an open chain here as the alternating binding motifs of the D


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Inorganic Chemistry

The second intense charge separation pathway C of square [2a-2OTf]2+ leads to two binuclear aggregates, but occurs differently than for [2b-2OTf]2+. The aggregates are not split diagonally into two identical fragments incorporating two ligands L2 each. Instead, the spectra show the formation of the stable ion [1:1:1:2]+ at m/z 1330 with only one bridging ligand L2 and [1:1:3:2]+ at m/z 1856 with three molecules L2. The latter ion consecutively loses two times L2 (Figure 6f) until the former ion is reached again. Careful inspection of the spectra reveals the third charge-separating pathway A which is only visible in trace amounts (Figure 6a) and leads to the ions [1:0:2:0]+ at m/z 687 and [1:2:2:4]+ at m/z 2499. The intensities were too small for further MSn studies here, but the ion [1:0:2:0]+ with m/z 687 is slightly more abundant in the fragmentation spectra of [2b-2OTf]2+ and was already mentioned above (Figure 5a). All produced ions with an equal number of or more ligands L2 than metal centers fragment by ligand loss (Figure 6b,e,f), thus revealing a strong preference to form complexes with more corners than edges: The trinuclear product aggregate [2:1:2:3]+ with m/z 1903 is produced from [2:1:3:3]+; the dinuclear product [1:1:3:2] + consecutively yields [1:1:2:2]+ and [1:1:1:2]+. In the case of the mononuclear fragment, this leads to the “naked” corner [0:1:0:1]+ at m/z 756 already described above in the course of the fragmentation of [1a2OTf]2+. Implications for Bond Dissociation Energies. Collisioninduced dissociation typically occurs by breaking the weakest bond in the aggregate. The fragmentation of bimetallic aggregates can thus be used to gain information on relative bond dissociation energies.15 In particular, the ions [1:1:1:0]+ with two different metal centers connected by one ligand are of interest here. As described above, the Pd-containing ions [1:1:1:0]+ generated from 1b and 2b only result in the ion [1:0:0:1]+ for both ligands L1 and L2. In contrast, the Ptcontaining ions [1:1:1:0]+ generated from 1a and 2a do not show this preference as both [1:0:1:0]+ and [1:0:0:0]+ are detected. With the assumption that the (methylallyl)Pd−P bond is not significantly affected by the type of metal coordinated to the opposed binding site of the ligand (which is the case according to the DFT calculations), this indicates that the binding of the pyridine side of L to (dppp)Pd(OTf)+ is much weaker and cannot compete with the phosphane side of

Figure 6. Fragmentation cascade of [2a-2OTf]2+ monitored by MSn experiments: (a) CID of m/z 1593 with 9 eV, (b) CID/CID of m/z 2166 with 30 eV,22 (c) CID/CID of m/z 1903 with 9 eV, (d) CID/ CID of m/z 1329 with 15 eV,22 (e) CID/CID of m/z 1019 with 15 eV, (f) CID/CID of m/z 1856 with 15 eV.22 Blue squares mark the mass-selected ions;22 *: electronic noise. Unlabeled gray arrows symbolize subsequent fragmentation pathways.

unsymmetric ligand designed for the selective formation of heterobimetallic squares do not fit for a triangle. The consecutive fragmentations very well match the picture drawn so far: First, the dangling ligand L2 is split off (Figure 6b), which leads to the ion [2:1:2:3]+ at m/z 1903. Afterward, the neutral complex [1:0:1:1] is eliminated to yield the stable ion [1:1:1:2]+ at m/z 1330 (Figure 6c). Fragmentation of this ion (Figure 6d,e) very much resembles the one for the corresponding ion with m/z 1478 from [1a-2OTf]2+ discussed above.

Table 1. Calculated (TPSS-D3/def2-TZVP) Bond Dissociation Energies for Selected Metallo-supramolecular Building Blocksa ΔE/kJ/mol

ΔE/kJ/mol

ΔE/kJ/mol

M = Pt

M = Pd

M = Pt

M = Pd

L = L2

L = L2

complex

fragments

L = L1

L = L1

[(dppp)M(OTf)−L−Pd(MeAll)(OTf)]+

[(dppp)M(OTf)]+ L−Pd(MeAll)(OTf) [(dppp)M(OTf)−L]+ Pd(MeAll)(OTf) (dppp)M(OTf)2 [L−Pd(MeAll)]+ [(dppp)M(OTf)]+ L Pd(MeAll)(OTf) L [Pd(MeAll)]+ L

128

112

(dppp)M−Py

143

123

178

178

P−Pd(MeAll)

182

180

n.d.

n.d.

(dppp)M−Py with OTf− shift

204

187

143

127

(dppp)M−Py

157

140

[(dppp)M(OTf)−L−Pd(MeAll)(OTf)]+ [(dppp)M(OTf)−L−Pd(MeAll)(OTf)]+ [(dppp)M(OTf)−L]+ L−Pd(MeAll)(OTf) [L−Pd(MeAll)]+

a

ΔE/kJ/mol

type of broken bond

193

P−Pd(MeAll) neutral

197

n.d.

P−Pd(MeAll) cationic

284

n.d. = not determined; P = phosphane end of L; Py = pyridine end of L; MeAll = η3-2-Me-C3H4. E


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Inorganic Chemistry

ligand loss are observed, although the binding of L1 is slightly stronger than the one of L2. The fragmentation of the two smaller aggregates [2a-2OTf]2+ and [2b-2OTf]2+ is more complicated and dominated by charge-separating pathways. This highlights the importance of Coulomb repulsion in the fragmentation, similar to our observations with five-fold charged symmetrical Stang squares in comparison to triply and doubly charged ones.17b Accordingly, the least stable aggregate [2b-2OTf]2+ incorporates the small ligand L2 and M = Pd. Its fragmentation mainly is initiated by cleavage of two of the weakest (dppp)Pd−Py bonds, either unsymmetrically (path A) or symmetrically (path C). Changing the dppp-coordinated metal to Pt stabilizes the bonds to the pyridine side of the bridging ligands and induces a change in fragmentation: After or concomitant with cleavage of the (dppp)Pt−Py bond, a rupture of a (methylallyl)Pd−P bond now is viable (paths B and C′ in [2a-2OTf]2+). The weakness of the (dppp)Pd−Py bond most probably is also the reason for the tendency of homometallic species [1b-2OTf]2+ and [2b-2OTf]2+ to fragment into salt-bridged structures like [1:2:1:4]+ or [0:2:0:3]+ with more metal centers than can be connected by the actual number of linking ligands. In the course of all four fragmentation cascades, a strong preference for the formation of dinuclear ions [1:1:1:2]+ is observed. This reflects their particular stability as the number of “free” coordination sites exactly matches the number of anions. Whereas triflate is a noncoordinating anion in solution, coordination of anions in the gas-phase ions might be the reason for the observation that fragments with bridging ligands and “open” coordination sites at the metal are formed significantly more often than aggregates with edge ligands bound only on one side. The presented gasphase study beautifully matches and complements the NMR results on the dynamic behavior of these aggregates in solution9b and provides valuable data for the rather new (methylallyl)Pd building block and its further use in metallosupramolecular chemistry.

L binding to (methylallyl)Pd(OTf), whereas the respective bond strengths are less different for the Pt derivative described above. The fragmentation of the trinuclear aggregate [2:1:2:3]+ at m/z 1903 from [2a-2OTf]2+ is also informative. It exclusively consists of the expulsion of a neutral Pd complex [1:1:0:1] (Figure 6c). First of all, this confirms the structure of the aggregate as a chain with terminal Pd corners. Second, it is important to note that the Pd complex is not expelled without L2; i.e., there is no signal for [1:1:2:2]+ at m/z 1593. It is thus most probable that L2 remains coordinated over an intact Pd−P bond while the Pt−N bond is broken. The CID results thus indicate the following relative stabilities for the three types of labile bonds in the studied aggregates: the (dppp)Pd−Py bond is broken very easily; the (dppp)Pt−Py analogue is more stable, but not as strong as the (methylallyl)Pd−P bond. This is in line with the observed dynamics in solution9b and in full accordance with the results obtained from DFT calculations (Table 1). The bond dissociation energy for cleavage of the (methylallyl)Pd−P bond in the dimeric complexes is calculated to be the strongest one of the three labile bond types with 180 kJ/mol for both ligands and irrespective of the nature of the metal (Pd or Pt) bonded to the pyridine side of the ligand. Thus, there is no cooperative effect between the two binding sites of the bridging ligand. The (dppp)M−Py bond is significantly less stable and in all cases is 15−20 kJ/mol weaker for M = Pd compared to M = Pt in accordance with expectations. The binding at the pyridine side of the shorter ligand L2 is 10−15 kJ/mol stronger than the one of L1. This is however not reflected in the fragmentation spectra; in contrast, the squares 2a and 2b undergo facile fragmentation and show a rather complex fragmentation pattern. The weak increase in bond strength obviously is overcompensated by the smaller size and thus higher Coulomb repulsion in the multiply charged aggregates. In addition, a hypothetical fragmentation of the dinuclear aggregate was considered. Cleavage of the (dppp)M−Py bond concomitant with a triflate transfer to locate the charge on the other fragment needs 60 kJ/mol more energy than the simple cleavage, already without including a putative kinetic barrier. Thus, this pathway clearly can be ruled out and indeed is not observed at all. For comparison, the bond dissociation energies were also calculated for smaller complexes (second part of Table 1) with similar results. It is however important to incorporate the triflate anion at the (methylallyl)Pd complex to avoid an inadequate comparison of a cationic with a neutral complex.

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EXPERIMENTAL SECTION

ESI mass spectra were recorded on a commercial Thermo Fisher Scientific LTQ Orbitrap XL hybrid mass spectrometer equipped with an IonMax source with a heated electrospray ionization (HESI-II) probe. Acetone solutions in concentrations of approximately 100 μM were used and transferred into the ion source using the attached syringe pump at flow rates of 5−10 μL/min. Source parameters are individually optimized for best abundances for every sample. Typical parameters are source voltage: 4 kV, sheath gas flow rate: 15, aux. gas flow rate: 5, sweep gas flow rate: 0; capillary voltage: 15 V; capillary temp: 100 °C; tube lense voltage: 90 V. In all experiments, ion detection was achieved in the orbitrap analyzer with the resolution set to R = 30000. For collision-induced dissociation (CID) experiments in the He-filled ion trap, mass selection of the whole isotope pattern (selected mass window centered at the indicated value) was followed by a stepwise increase of collision voltage until fragmentation took place. Cascades of successive fragmentations were monitored by repeating multiple mass selection/induced fragmentation steps (MSn) for the major fragments. Mass labels in the spectra/schemes refer to the most intense signal of the measured/calculated isotope patterns. Some distortion of the isotope patterns was induced by the mass selection at a previous MS stage which leads to deviations between measured and calculated maximum in some cases. Signal assignments are based on accurate masses determined with the Orbitrap mass analyzer. Density functional theory calculations were performed with the Turbomole24 program package including D325 and the resolution-ofidentity (RI) approximation.26 All geometries were optimized using

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CONCLUSIONS Four homo- and heterometallic supramolecular square-like macrocycles with ditopic linkers Ph2P-C6F4-Py (L1) or Ph2P-Py (L2) were studied in the gas phase by CID experiments of the doubly charged species. The fragmentation patterns are determined by two factors: (1) the length of the linking ligand and thus the size of the aggregate accommodating the two charges, and (2) the type of metal in the (dppp)M building block. The latter defines the strength of the weakest bond in the aggregate which is the dative bond from the pyridine side of the bridging ligand to the (dppp)M complex. The dative bond from the phosphane side of the ligand to the (methylallyl)Pd complex is significantly more stable. For the two larger aggregates [1a-2OTf]2+ and [1b-2OTf]2+, relatively simple fragmentation patterns based on an initial F


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Inorganic Chemistry TPSS27-D3 with the triple-ζ basis set def2-TZVP.28 Harmonic vibrational frequency analyses were performed with def2-TZVP for all species containing L2 and with the basis set def2-SVP29 for the ones with L1. They confirm stationary points as minima. The energies given in Table 1 include zero-point energy correction.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00930. Calculated absolute energies and optimized geometries (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Marianne Engeser: 0000-0001-6987-4126 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft (SFB 813) is acknowledged. We thank Prof. Stefan Grimme for access to their excellent infrastructure for theoretical chemistry calculations. The authors are also grateful to the Ministerio de Economiá y Competividad (MINECO/FEDER) of Spain (Projects CTQ2015-65707-C2-1-P and CTQ2015-65040-P).

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