Utilizing Raman Spectroscopy as a Tool for Solid- and Solution-Phase Analysis of Metalloorganic Cage Host-Guest Complexes

Abstract

The host-guest chemistry of coordination cages continues to promote significant interest, not least because confinement effects can be exploited for a range of applications, such as drug delivery, sensing, and catalysis. Often a fundamental analysis of noncovalent encapsulation is required to provide the necessary insight into the design of better functional systems. In this paper, we demonstrate the use of various techniques to probe the host-guest chemistry of a novel Pd₂L₄ cage, which we show is preorganized to selectively bind dicyanoarene guests with high affinity through hydrogen-bonding and other weak interactions. In addition, we exemplify the use of Raman spectroscopy as a tool for analyzing coordination cages, exploiting alkyne and nitrile reporter functional groups that are contained within the host and guest, respectively.

Figure 1

Chemical structures of (a) the previously studied quinone⊂1 host–guest cage complex and (b) the dicyanoarene⊂2 inclusion complex alongside the different guests investigated in this current study.

Figure 2

X-ray crystal structures of the cage structure 2 and dicyanoarene⊂2 host–guest complexes. (a) “Empty” cage structure 2 showing the partial ingress of BArF counteranions, with close contacts represented by the dashed black lines. (b) Host–guest complexes: (i) DCB⊂2; (ii) DCN⊂2; (iii) DCA⊂2; (iv) TCDCB⊂2. (c) Alternative views of (i and ii) DCB⊂2 and (iii and iv) DCA⊂2. The C atoms of the cage are shown in green, and the C atoms of the guests are shown in orange. Other color codes: Pd, blue; N, light blue; F, cyan; B, pink; H, white.

Figure 3

¹H NMR (500 MHz, CD₂Cl₂, 300 K) spectra of cage 2 and dicyanoarene⊂2 host–guest complexes. (a) ¹H NMR spectrum of “empty” cage 2. ¹H NMR spectra of a mixture of cage 2 and (b) DCB, (c) DCN, (d) DCA, and (e) TCDCB. The cage and dicyanoarene guest signals are highlighted in green and orange, respectively. The lettering refers to the assignments shown in Figure 1.

Figure 4

Solid-state analysis of the encapsulation of DCN within 2 using RS. Raman spectra were acquired from the free guest (DCN, black), “empty” cage (2, red), and host–guest complex (DCN⊂2, blue). Raman spectra were acquired using 785 nm excitation for 10 s with a 50× objective lens (0.18 mW). All assignments are in reciprocal centimeters. The full Raman spectra are provided in Figure S22.