Coordination Chemistry DOI: 10.1002/ange.200900981 A Rhenium Tricarbonyl 4’-Oxo-terpy Trimer as a Luminescent Molecular Vessel with a Removable Silver Stopper** Michael P. Coogan,* Vanesa Fernµndez-Moreira, Benson M. Kariuki, SimonJ. A. Pope, and Flora L. Thorp-Greenwood One of the great challenges for the synthetic chemist is the preparation of large, hollow, molecular species capable of hosting smaller molecules. In contrast to solid-state materials, which have no true existence in solution, there are only a very small number of truly discrete families of molecular species that provide a shell enclosing a void, such as calixarenes, [1a] cyclodextrins, [1b] and cucurbiturils. [1c] In the field of coordina- tion chemistry, this area has been dominated by the self- assembly of cage complexes. [1d] Multidentate ligands that may bind more than one metal center provide a route to cage complexes, [1e–i] and the use of preformed complexes with additional binding sites (complex-as-ligand approach) has led to a variety of coordination networks with a range of complex topologies, including examples of cage complexes appropriate for host–guest applications. [2] Coordination complexes con- taining cylindrical or psuedo-cylindrical cavities analogous to cyclodextrins or calixarenes are, however, very rare, with very few examples of metallacalixarenes, in which arene ligands mimic calixarene walls. [3] These species potentially offer more modes of binding than the organic analogues: both organic- based ligand–guest interactions similar to those in pure organic systems, and contributions from metal–guest or charge–guest interactions. Furthermore, coordination com- plexes acting as hosts may, with judicious choice of metal and ligand, be capable of responding to the presence of the guest by modulation of a detectable property, such as luminescence. Amongst the most widely studied luminescent metal com- plexes are the rhenium tricarbonyl bisimine cations [Re(CO) 3 (bisim)X] + (bisim = 2,2’-bipyridine or derivative thereof; X = pyridine or derivative) owing to their high quantum yields, long luminescence lifetimes, and large Stokes shifts arising from emission from a 3 MLCT excited state. [4] Recently these species have been applied in the fluorescence microscopy imaging of cells, [5] and many have been proposed as sensors in the past. [6] No examples of rhenium metal- lacalixarenes, and indeed no 3 MLCT emissive metallacalixar- enes have been reported to date, and no rational design for such species exists. [3] Herein we report the development of a luminescent rhenium bisimine-based cylindrical trimer syn- thesized by the complex-as-ligand approach: the cavity has the correct dimensions for hosting linear diatomics, and a pendant tripyridine unit can bind metal ions, closing one end of the cylinder to form a cup and simultaneously triggering a modulation of the luminescence of the core trimer. The reaction of 1 [2c] with rhenium pentacarbonyl bromide or chloride gave the expected product [Re(4’oxo-h 2 -terpy)- (CO) 3 ]X 2 (terpy = 2,2’:6’,2’’-terpyridyl, X = Cl, Br), in which one of the terminal pyridine rings is uncoordinated [7] and the central ring has tautomerized to the 4-hydroxypyridine form (Scheme 1). Typically, halide abstraction activates the rhe- nium tricarbonyl bisimine complexes, allowing attack by pyridines or other ligands, which could lead to a range of interesting oligomers if the pendent pyridine or the oxo unit then linked rhenium cores (Figure 1). Abstraction of the halide from 2 with silver tetrafluoroborate in acetonitrile gave [Re(4’oxo-h 2 -terpy)(CO) 3 (MeCN)]BF 4 3 (Scheme 2). This behavior, although typical in simple examples of these systems, was unexpected: such acetonitrile complexes are reactive, and it could be expected that upon halide abstrac- tion, either the pendant pyridine or hydroxy group would compete with (or subsequently displace) acetonitrile to give a higher nuclearity complex. The stability of 3 may be explained by the ability of the ligand to lose a proton and generate the neutral species, which could be expected to be of lower reactivity than the cationic acetonitrile complexes derived Scheme 1. Coordination of bispyridylpyridone 1 to rhenium pentacar- bonyl halides. Conditions: 1) [Re(CO) 5 X], toluene, reflux; X = Cl, Br. Figure 1. Possible structures from oligomerization of rhenium terpy: A cyclization, B chain formation, C dimerization. [*] Dr. M. P. Coogan, Dr. V. Fernµndez-Moreira, Dr. B. M. Kariuki, Dr. S. J. A. Pope, F. L. Thorp-Greenwood Department of Chemistry, Cardiff University Cardiff CF10 3AT (UK) Fax: (+ 44) 29-2087-4030 E-mail: cooganmp@cf.ac.uk [**] We thank the EPSRC National Mass Spectrometry Service Centre, Swansea, for HRMS. Terpy = 2,2’:6’,2’’-terpyridyl. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.200900981. Angewandte Chemie 5065 Angew. Chem. 2009, 121, 5065 –5068  2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim