Jiajia
Yang
ab,
Mohan
Bhadbhade
c,
William A.
Donald
b,
Hasti
Iranmanesh
b,
Evan G.
Moore
d,
Hong
Yan
a and
Jonathon E.
Beves
*ab
aState Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
bSchool of Chemistry, The University of New South Wales (UNSW), Sydney, NSW 2052, Australia. E-mail: j.beves@unsw.edu.au
cMark Wainwright Analytical Centre, The University of New South Wales (UNSW), Sydney, NSW 2052, Australia
dSchool of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia
First published on 13th February 2015
Substitution-inert, redox- and photo-active ruthenium(II) complexes based on 2,2′,6′,2′′-terpyridine ligands were self-assembled into discrete supramolecular cages via coordination to palladium(II) centres and characterised by NMR, ESI-MS and X-ray crystallography.
Fig. 1 Structure of complexes that are building blocks of more complex supramolecular cages, showing the numbering scheme adopted. |
Fig. 2 Synthesis of molecular tetrameric cage 324+ from complex [Ru(1)2](PF6)2 and Pd(dppp)(OTf)2 in nitromethane or acetonitrile at room temperature within minutes. |
Fig. 3 1H-NMR (CD3NO2, 600 MHz) spectra of (a) Ru(1)2(PF6)2 298 K; (b) cage 3 298 K; (c) cage 3 348 K; (d) Ru(2)2(PF6)2 298 K and (e) cage 4 298 K. See Fig. 1 for labelling scheme. Peaks not labelled correspond to the two non-equivalent phenyl rings of the dppp ligand. |
Electrospray ionisation mass spectrometry (ESI-MS) of a nitromethane solution of the product resulted in the formation of a relatively abundant distribution of ions that were charged from +5 to +10 (Fig. 4). The difference in m/z values between adjacent ions in the distribution and the isotopic patterns were assigned to [3(PF6−)24−n]n+ (n = 5 to 10), formed by the sequential loss of PF6 counter anions from [3(PF6)24] (11742 Da; Fig. 4). This Ru4Pd8 tetrameric cage structure, [3](PF6)24, is the smallest least-strained structure possible for this system.24 Additional peaks were observed corresponding to the loss of one Pd(dppp)2+ unit which were confirmed by collision-induced dissociation (CID) experiments. This structure is also consistent with the broad 1H NMR signals observed in solution, as the {Ru(tpy)2} units rotate slowly on the NMR timescale and the environment inside and outside of the cage is non-equivalent. This restricted rotation is due to the close contact between the {Ru(tpy)2} groups forming something resembling a poorly assembled gear box (see later X-ray structure discussion). Although stable in acetonitrile at high concentrations (>1 mM), based on 1H NMR, the cage disassembles upon dissolution in acetonitrile (see Fig. S25, ESI†), which is not surprising given this solvent can act as an excellent ligand for Pd(II) centres. However, the complex was observed to be stable in pure nitromethane (Fig. S26, ESI†), a polar but very weakly coordinating solvent, over the same concentration range, confirming this solvent as an excellent choice for this class of metallosupramolecular systems.
The analogous reaction with complex [Ru(2)2](PF6)2, which features alkyne spacers between the phenyl and pendant pyridyl rings, and Pd(dppp)(OTf)2 also formed a single major product in solution (Fig. 5). The 31P{1H} spectrum (Fig. S29, ESI†) again revealed a single sharp singlet at 8.91 ppm, effectively identical to that observed for [3]24+, and consistent with the formation of a single major product. However, in contrast to cage 324+, the 1H NMR signals (Fig. 3e) of this new species were not significantly broader than the parent Ru(2)2+ complex, suggesting the rotation of {Ru(tpy)2} units was effectively unhindered in this structure. The 1H NMR signals corresponding to the pendant pyridine protons were shifted relative to [Ru(2)2](PF6)2 (Δδ HD2 + 0.05; HD3 − 0.32), but as was the case for cage 324+, the signals of the terpyridine group were significantly affected, all being shifted upfield (Δδ HB3 − 0.31; HA3 − 0.19; HA4 − 0.26; HA5 − 0.24; HA6 − 0.28) but to a much less extent than in cage 324+. These peak shifts correspond to a cage environment considerably less shielded than in cage 324+. The 13C{1H} NMR spectrum (Fig. S28, ESI†) reveals relatively small changes in peak shifts for most signals, with the notable exceptions of the pendant pyridyl signals (Δδ CD2 − 0.3; CD3 + 2.7; CD4 + 3.6 ppm), the alkynes (Δδ CC–CC + 4.5 ppm; CD–CC − 1.5 ppm) and the central phenyl ring (Δδ CC1 − 1.6; CC2 + 0.2; CC4 + 2.0; CC5 + 0.1 ppm). These peak shifts reflect not only the electronic effect of Pd(II) coordination to the pyridyl group, but also the strain introduced to the alkyne upon bending to form a smaller cyclic structure, hence the observed changes are significantly different to that observed for cage 324+.
Fig. 5 Synthesis of molecular trimeric cage 424+ from complex [Ru(2)2](PF6)2 and Pd(dppp)(OTf)2 in nitromethane or acetonitrile at room temperature within minutes. |
The ESI-MS of a solution of this product (Fig. S36, ESI†) revealed a trimeric, rather than tetrameric structure, consistent with the structure of cage 4(PF6)18 (Fig. 5) indicating the additional flexibility of the alkyne spacers was sufficient to allow a smaller structure to form and simple molecular modelling (MMFF, Fig. S40, ESI†) supports this assignment. This structure was found to precipitate from solution (acetonitrile) over time to form an insoluble red powder, presumably a coordination polymer. In nitromethane the cage appears stable over several months in solution.
Slow diffusion of toluene into a nitromethane solution of cage 3(PF6)24 gave red block crystals suitable for X-ray diffraction.‡ The molecule crystallises in the P space group with the asymmetric unit containing half of one cage molecule and disordered solvents and anions (Fig. 6). The complex forms a box-like structure approximately 21 × 21 × 32 Å in dimensions with Pd(II) centres at each end forming near perfect squares (Pd–Pd–Pd angles of 86.0–92.8° and Pd⋯Pd distances of 13.2–13.4 Å). The centre of the cage is occupied by {Ru(tpy)2} units with alternating Ru⋯Ru distances of 11.82 and 8.78 Å and inter-{Ru(tpy)2} pyridine⋯pyridine separations of (centroid⋯centroid) 3.86 and 5.34 Å forming portals which are occupied by PF6− counterions. The cavities and each end of the cage are sufficiently large to potentially accommodate large guests such as a C60 molecule. The {Ru(tpy)2} groups form pairs of terpyridine ‘embraces’25 (Fig. S37, ESI†), a type of favourable edge-to-face and face-to-face aromatic interactions, as often seen in solid state packing of simple {M(tpy)2}n+ complexes. These interactions reveal the origin of the restricted rotation of these units in solution. Although not requiring a concerted rotation of all the {Ru(tpy)2} units, it appears this type of favourable π–π stacking interactions are more significant than simply steric crowding and result in the hindered rotation of these units. The cages are assembled together in the crystal structure via additional intermolecular terpyridine embraces to form 1D chain along the crystallographic a axis as well as extensive π–π interactions between the pyridine and phenyl rings along the b axis (Fig. S39, ESI†).
Fig. 6 The single crystal X-ray crystal structure of [3](PF6)17.5·CH3NO2. Viewed down the (a) crystallographic a axis, and (b) c* axis. Solvent and anions omitted for clarity. |
Preliminary investigations of the photophysical properties of cage [3](PF6)24 indicate the functionality of the parent complex is retained in the cage structure. The 1MLCT absorption maxima of the [Ru(1)2](PF6)220 complex and cage [3](PF6)24 were both located at 490 nm, while the 3MLCT emission spectra were essentially superimposable centred at 640 nm. The excited state lifetimes (1.26 ± 0.01 ns and 1.21 ± 0.01 ns respectively) were similarly identical, and are comparable to those of related [Ru(4′-tolyl-tpy)(tpytpy)]2+ complexes (see ESI† for details).26
Three dimensional molecular cages containing [Ru(tpy)2]2+ units are reported and characterised in solution and the solid state. The photophysical properties of the parent Ru(II) complex [Ru(1)2]2+ are retained in the cage 324+, suggesting this new class of molecular cages may be potential candidates to act as photosensitizers for bound guest molecules. The introduction of an alkyne spacer, producing a larger ligand, resulted in the formation of a smaller, trimeric cage highlighting the flexibility of these spacer units and the subtlety of the assembly process.
This work was supported by a National Science Foundation of China (NSFC) Research Fund for International Young Scientists Project (No. 21450110060 and 21271102). The Australian Research Council is acknowledged for Future Fellowship (EGM, FT100100795) and Discovery Early Career Research Awards (WAD, DE130100424). Photophysical measurements were undertaken at the Photochemistry and Ultrafast Laser Spectroscopy (PULS) facility, School of Chemistry and Molecular Biosciences, with financial support from the University of Queensland (MEI-2013000106). Crystallographic data was collected in-house at UNSW, or at the MX1 beamline at the Australian Synchrotron under a Collaborative Access Program (AS143_MXCAP_8503).
Footnotes |
† Electronic supplementary information (ESI) available: Synthetic procedures, 1H, 13C, 31P, COSY, HSQC, HMBC, NOESY and variable temperature and variable concentration NMR spectra, X-ray crystal refinement details, ESI-MS and photophysical data. CCDC 1040284 and 1045515. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc10292d |
‡ Crystal data and refinement: C232H188N20P8Pd4Ru2·8.75(F6P)·0.5(CNO2), M = 5429.01, T = 150(2) K, λ = 0.71073 Å, triclinic, space group P, a = 23.8865 (13), b = 25.4288 (13), C = 31.7818 (17) (Å), α = 96.150 (3), β = 107.106 (3), γ = 113.591 (2) (°), V (Å3) = 16341.8 (16), Z = 2, m (mm−1) = 0.46, F(000) = 5460.5, data/restr/param. 57493/1988/3008. GOOF on F2 = 1.11, R[F2 > 2s(F2)] = 0.113, wR(F2) = 0.320. CCDC reference number 1040284. |
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