A hemilabile ligand approach to ruthenium-based luminescent molecular sensors
Creators: name: Rogers, Cerrie Wenlyn
Publisher: name: University of British Columbia
Year: 2009

description: In this thesis, luminescent Ru(II) complexes that contain a metal-based receptor site for nucleophilic small molecules were studied, with emphasis on their potential utility as luminescent chemosensors. Two approaches to achieving small-molecule dependent luminescence were investigated: one exploited a metal-based lumophore (Part 1) and the other an organic-based lumophore (Part 2). In Part 1 of this thesis, molecular sensors based on the well-known [cis,-Ru(bpy)₂(LL')]²⁺ lumophore were investigated. Complexes in which L'-L' is a hemilabile phosphine-ether ligand (POR) were shown to bind small-molecule analytes (e.g., water, acetonitrile, thiols, dimethylsulfoxide) via displacement of the labile ether moiety, which was readily monitored via ¹H and ³¹P{¹ H} NMR spectroscopy. The resulting changes in the coordination sphere of the metal caused the metal-to-ligand charge-transfer derived absorption and emission spectra to be shifted. For example, complex 1, [cis-Ru(bpy)₂(POMe-.P,0)]²⁺ (POMe = 2-methoxyphenyldiphenylphosphine), yields orange luminescence at low temperatures when the ether is metalbound. In response to analytes, the emission of 1 changes colour as follows: yellow for acetonitrile, cherry red for triethylamine, poppy red for dimethylsulfoxide, brownish yellow for thiols. Thus, [cis-Ru(bpy)₂(POR)]²⁺ complexes are potential candidates for development into sensory materials in multianalyte sensor applications, although their luminescence is limited to low temperatures. A side reaction encountered during the preparation of 1 was also studied. Specifically, the metal-bound ether in 1 was found to be prone to dealkylation by free phosphine-ether ligands to form phosphino-phenoxide complex 4, [cis-Ru(bpy)₂(Ph2PPhO-P,0)]⁺. Similar P,O-complexes with the ligands POEt (POEt = 2-ethoxyphenyldiphenylphosphine) and PC20Me (2-methoxyethyldiphenylphosphine) showed lower propensities toward ether dealkylation. The trend in reactivity was determined to be related to the electrophilicity of the ether's C[sub α] as well as resonance stabilization of 4. Changes in the ether substituent in the P,0-ligands were found to influence the reactivity of the [cis-Ru(bpy)₂(POR)]²⁺ complexes. The POEt complex, 2, showed reactivity and photophysical properties very similar to those of 1 but had slightly higher affinities for most analytes. Isopropyl- and trifluoromethyl-substituted ligands yielded complexes with such enhanced ether lability that their P,0-chelate complexes (6 and 7) were not isolated free from coordinated solvents. These types of modifications may be useful in future studies on tuning the selectivity and specificity of hemilabile molecular sensors. An unsuccessful attempt to introduce room temperature luminescence into a smallmolecule reactive [cis-Ru(bpy)₂(L-L')]²⁺ lumophore involved replacement of the phosphine in the hemilabile ligand with a nitrogen-based donor. The pyridine-ether ligands PyClOMe (PyClOMe = 2-(methoxymethyl)pyridine) and PyC20Me (PyC20Me = 2-(2- methoxyethyl)pyridine) led to complexes 8 and 9, which were shown to be luminescent only at low temperature. Furthermore, the pyridine-ether ligands in these complexes did not exhibit the hemilabile behaviour desired for chemosensory applications. In Part 2 of this thesis, the hemilabile approach was used to effect analyte-induced displacement of an organic lumophore away from a metal-based analyte receptor. Specifically, a pyrene lumophore was incorporated into a phosphine-ether ligand (POC4Pyr = 4-(2- (diphenylphosphino)phenoxy)butylpyrene) and used to prepare trans, cis, cis,-RuCl₂(POC4Pyr- P,0)₂, 10. Complex 10 was shown to exhibit a rapid, dramatic luminescence response to carbon monoxide at room temperature. The resulting dicarbonyl complex trans,trans,trans- RuCl₂(CO)₂(POC4Pyr-P)₂,11, was observed to slowly convert into the cis,cis,trans-isomev 12, which showed very strong blue-green pyrene excimer-based emission.