Disassembly Mechanisms and Energetics of Polymetallic Rings and Rotaxanes

Abstract

Understanding the fundamental reactivity of polymetallic complexes is challenging due to the complexity of their structures with many possible bond breaking and forming processes. Here, we apply ion mobility mass spectrometry coupled with density functional theory to investigate the disassembly mechanisms and energetics of a family of heterometallic rings and rotaxanes with the general formula [NH 2 RR'][Cr 7 MF 8 (O 2 C t Bu) 16 ] with

Formation and Characterisation of Polymetallic Rings in vacuo

“…We quantified the absolute CCSN2 values of the precursor [{Cr12Gd4} -Piv] + and products of the type {Cr6Gd2} and {Cr5Gd2} (Table 1). The CCSN2 value of {Cr6Gd2} agrees well with a related {Cr7Cu} ring, 23 while the value for {Cr5Gd2} is similar to those of the seven-membered hourglass products {Cr5Cu2} and {Cr6Cu}, as well as to the [Cr6MnF8Piv13]ring. 23 These comparisons suggest cyclic structures for the complexes of the type {Cr6Gd2} and {Cr5Gd2}.…”

“…The same dissociation channel was observed for the heterometallic rotaxanes [NH2RR'][Cr7MF8Piv16], where R and R' include bulky phenyl and tert-butyl groups. 23 The secondary ammonium cations are smaller here, so the relatively ease of dissociation from the hourglass ions is not surprising.…”

“…41 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} horseshoes. 23 These are located above the line as their estimated spherical densities (ESD) are lower than for the polymetallic cyclic structures (Figure 5 Inset).…”

“…E50 values were obtained a method described in our previous works. 23,26 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 vs. the collisional energy in the centre-of-mass frame (Ecom, Figures S3, S8). 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%.…”

“…21,22 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 t Bu)16] (M = Mn II , Fe II , Co II , Ni II , Cu II , Zn II , 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 systems. 23 Here, we investigate the disassembly dynamics of more complex heterometallic systems, two of which have hourglass structures, [NH2 n Pr2]2[Cr10Cu2F14(O2C t Bu)22] = "{Cr10Cu2}" (Figure 1a) and [NH2 i Pr2]2[Cr12Cu2F16(O2C t Bu)24] = "{Cr12Cu2}" (Figure 1b), and a third which involves a lanthanide tetrahedron bound to two {Cr6} chains, [NH2 n Pr2]2[Cr12Gd4F21(O2C t Bu)29] = "{Cr12Gd4}" (Figure 1c). We demonstrate how CID, aided by IM-MS, can discriminate different disassembly pathways, which lead to the production of polymetallic rings that have not been made previously via solution synthesis (Figure 1d).…”