Communication Cite This: Organometallics XXXX, XXX, XXX−XXX
pubs.acs.org/Organometallics
Optical pKa Control in a Bifunctional Iridium Complex Ivan Demianets,† Jonathan R. Hunt,‡ Jahan M. Dawlaty,*,‡ and Travis J. Williams*,† †
Donald P. and Katherine B. Loker Hydrocarbon Institute and Department of Chemistry, University of Southern California, Los Angeles, California 90089-1661, United States ‡ Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
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S Supporting Information *
ABSTRACT: There are few ways to switch a catalyst’s reactivity on or off, or change its selectivity, with external radiation; many of these involve photochemical activation of a catalyst. In the case of homogeneous late-transition-metal catalysts, the metal complex itself is frequently the chromophore involved in such reactivity switching. We show here a base-pendant ligand−metal bifunctional scaffold wherein a photobase, a compound that becomes more basic in the excited state (pKa < pKa*), is used to switch the proton acceptor ability on an active site of the complex. The system differs from those with metal-centered chromophores, because the photobase operates independently of the metal. While excellent progress has been made in photoacid chemistry, neither a photoacid nor a photobase has been designed into the structure of a transition-metal catalyst where the metal is not part of the chromophore. We find that quinoline is an efficient photobase that preserves its unique properties in the close proximity of an iridium center: the efficacy of the photobase (9.3 < pKa* < 12.4) in the iridium complex is unhindered relative to the free quinoline. We apply this notion to successful photodriven deprotonation of an aliphatic alcohol, thus showing the first case of metal-orthogonal optical pKa control in a transition-metal complex.
P
carboxylic acids (1-naphtholate) and azoles (quinoline), with the latter exhibiting a relatively high pKa* of 11.5, with ΔpKa = 6.7.4 Despite outstanding recent progress in photoredox chemistry based on iridium, ruthenium, and nickel systems, we see a dearth of examples of photoacids and -bases in transition-metal catalysis, particularly as applied to organic synthesis. Here we report the synthesis and characterization of two iridium complexes that contain a pendant quinoline. We demonstrate that the photobasicity of the quinoline is retained in these complexes and metal-orthogonal optical control over pKa of the photobase is feasible. We also show that one of the newly synthesized complexes can deprotonate hexafluoroisopropyl alcohol (HFIPA) (pKa = 9.3)51 upon photoexcitation. This demonstrates for the first time that a photobase can be designed into a late-metal complex, where it can effect proton transfers independent of the metal center. We designed our ligands (6 and 7, Scheme 1) to avoid a direct conjugation connection between the metal and the photobase. Thus, the quinoline moiety is placed on a carbon intervening the two chelating pyridine rings. Tris-heterocyclic fragments 6 and 7 are readily available in three and four steps, respectively, from the corresponding bromoaniline, and each participates in facile ligation to cyclooctadiene iridium chloride dimer to afford complexes 1 and 2, respectively. Crystals of 1 suitable for X-ray structure analysis were obtained by layering n-heptane over a concentrated dichloro-
hotoacids and photobases are compounds that change their pKa upon photoexcitation. Photon absorption in a photoacid results in an increased acidity, with an excited state pKa* being lower than the ground state; pKa > pKa*. Compounds with this class of photoreactivity, e.g. 1-naphthol, 1-naphthylamine, etc., have been understood since the 1930s.1 Förster characterized this phenomenon as excited state proton transfer (ESPT)2 and described the Förster cycle as a strategy to determine pKa*.3 Photophysical processes in photoacids have been investigated for more than 40 years. 4−17 Applications of these photoacids vary from probes for solvent environment18−22 and molecular switches23 to tools for local pH control24 and markers for protein folding studies.25 Inorganic photoacids first appeared in 1976. These compounds generally feature ruthenium and a functionalized polypyridine ligand.26−32 In transition-metal-containing photoacids, the metal is part of the chromophore. The supporting ligand tends to contain amines28 or carboxylates,29,30 which are common in photobases, or phenols,31 which are common in photoacids. Examples of photobases and photoacids containing transition metals Re(I),33 Fe(II),34 Os(II),35,36 Pt(II),37 Ir(III),38 and Rh(III)39 are known, as are select hetero- and homobimetallic39 and trimetallic34 complexes. In fact, Meyer et al.40 recently demonstrated that in ruthenium polypyridine the same functional group can behave as either a photoacid or a photobase depending on the ancillary ligands. Photobasicity has been characterized more recently than photoacidity,4,41−50 and there are fewer applications of this phenomenon in reaction chemistry. Common examples of organic compounds that exhibit photobasicity include aromatic © XXXX American Chemical Society
Received: October 25, 2018
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DOI: 10.1021/acs.organomet.8b00778 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
studied, quinoline absorbs from the unprotonated form (see the Supporting Information), as is expected from the solvents’ respective pKa values. The emission of quinoline in dichloromethane and high-pKa alcohols appears to be negligible, which is consistent with no proton transfer in the excited state. However, in HFIPA significant emission from protonated quinoline was observed (Figure 2A), which indicates that
Scheme 1. Synthesis of Quinoline−Iridium(I) Conjugates 1 and 2
methane solution of 1. An X-ray structure affirmed that the iridapyrimidine ring prefers the expected boat confirmation (Figure 1). Importantly, iridium is not bound to the quinoline
Figure 2. Emission spectra of quinoline (A, B) and complex 2 (C, D) in various solvents. Spectra B and D are enlarged versions of spectra A and C excluding HFIPA data for clarity. The concentration of 2 was 1.4 × 10−4 M, and the concentration of quinoline was 2.5 × 10−5 M. Spectra were collected at room temperature under ambient conditions. Figure 1. ORTEP drawing of 1. The triflate counterion is omitted for clarity. Ellipsoids are drawn at the 50% probability level.
quinoline deprotonates HFIPA after it has been photoexcited. Similar emission from protonated quinoline was observed in 2,2,2-trifluoroethanol (TFE), but with far less intensity (Figure 2A,B). The difference in intensity is most probably related to the higher pKa of TFE, as discussed in the Supporting Information. We use the method above to approximate the pKa* of the quinoline moiety when it is imbedded in complex 2. The emission behavior of the quinoline moiety is more complicated when it is attached to a transition metal than it is in free quinoline; nevertheless, we observe increased emission from complex 2 when it is irradiated in HFIPA solvent (Figure 2C). This observation implies that the quinoline moiety in complex 2 can deprotonate HFIPA and that quinoline, therefore, retains its photobasicity when it is tethered in complex 2. The ability of the tethered quinoline to deprotonate HFIPA means that its pKa* is near the pKa of HFIPA (9.3) or higher. However, unlike molecular quinoline, the same was not observed in TFE solvent (pKa 12.4, Figure 2D). This indicates that the pKa* of the quinoline moiety is lower than the pKa of TFE or the pKa* of molecular quinoline (Table 1). The two lowest energy absorption bands of complex 2 can be assigned to dσ-MLCT (472 nm) and dπ*-MLCT to the N-
moiety of 6, which is in contrast with an analogous zinccontaining complex (12; see the Supporting Information). Solution NMR data for both 1 and 2 are consistent with a dynamic structure in which both the ring flip of the iridapyrimidine ring52 and rotation of the quinoline−carbinol bond are rapid on the NMR time scale. Thus, while the quinoline nitrogen is not bound to the iridium center, it has access to the space around the metal. While we have previously enjoyed success using the Förster thermodynamic cycle to measure the excited state pKa* values of molecular quinoline species,48 analysis of organometallic complexes 1 and 2 is frustrated by the overlap of broad iridium absorption features with those of the quinoline chromophore. We were thus unable to perform a clean Förster cycle analysis. We have previously used the emission of 5-methoxyquinoline in organic solvents of varying pKa values to bracket its excited state pKa* as an alternative to Förster cycle analysis.53 Here we will take a similar approach. In this work, we begin by studying the absorption and emission spectra of quinoline in a set of solvents. Previous studies suggest that unsubstituted quinoline displays rapid (
