Asymmetric Cyclometalated RuII Polypyridyl-Type Complexes with π

Article pubs.acs.org/IC

Asymmetric Cyclometalated RuII Polypyridyl-Type Complexes with π‑Extended Carbanionic Donor Sets Tina Schlotthauer,†,‡ Giovanny A. Parada,§,⊥ Helmar Görls,∥ Sascha Ott,§ Michael Jag̈ er,*,†,‡ and Ulrich S. Schubert*,†,‡ †

Laboratory of Organic and Macromolecular Chemistry (IOMC), Friedrich Schiller University Jena, Humboldtstraße 10, 07743 Jena, Germany ‡ Center for Energy and Environmental Chemistry Jena (CEEC Jena), Friedrich Schiller University Jena, Philosophenweg 7a, 07743 Jena, Germany § Department of Chemistry, Ångström Laboratory, Uppsala University, Box 523, SE-75120 Uppsala, Sweden ∥ Laboratory of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Lessingstraße 8, 07743 Jena, Germany S Supporting Information *

ABSTRACT: A series of novel cyclometalated RuII complexes were investigated featuring the tridentate dqp ligand platform (dqp is 2,6di(quinolin-8-yl)pyridine), in order to utilize the octahedral coordination mode around the Ru center to modulate the electrochemical and photophysical properties. The heteroleptic complexes feature C1 symmetry due to symmetry breaking by the peripheral five- or sixmembered carbanionic chelate (phenyl, naphthyl, or anthracenyl units). The chelation mode is controlled by the steric effects and C−H activation selectivity of the ligand, which prompted the development of a general synthesis protocol. The optimized conditions to achieve high overall yields (55−75%) involve NaHCO3 as the base and an simplified purification protocol: i.e., facile chromatographic separation using commercially available amino-functionalized silica applying nonaqueous salt-free conditions to omit the necessity of counterion exchange. The structural, photophysical, and electrochemical properties were studied in depth, and the results were corroborated by density functional theory (DFT) calculations. Steady state and time-resolved spectroscopy revealed red-shifted absorption (up to 750 nm) and weak IR emission (800−1000 nm) combined with prolonged emission lifetimes (up to 20 ns) in comparison to classical tpy-based (tpy is 2,2′:6′,2″-terpyridine) complexes. An enhanced stability was observed by blocking the reactive positions of the carbanionic ligand framework, while the reactive positions may be exploited for further functionalization.

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INTRODUCTION Ru polypyridyl-type complexes exhibit an extraordinary combination of photophysical and redox properties,1−3 which have led to their successful applications as photosensitizers in photovoltaic or photosynthetic devices,4 functional building blocks in molecular machines, phototherapeutics, and sensing applications. The ground and excited state properties of the complexes are controlled by the ligand sphere, which usually consists of six donor atoms embedded in aromatic subunits that can be further connected to form chelating ligands. The resulting enormous number of conceivable ligand sets has fueled the exploration of polypyridyl-type RuII complexes by rational ligand design, aiming to modulate the inherent geometric and electronic features. In parallel, theoretical methodologies based on density functional theory (DFT) have evolved to become a valuable tool for chemists to assist in such rational ligand design:5 e.g., to design complexes with tailored absorption to closely match the solar spectrum or to modulate and corroborate the experimentally determined excited state properties. In general, the frontier molecular orbitals of the typical polypyridyl-type complexes are composed

of Ru d orbitals to constitute the highest molecular orbitals (HOMOs) and ligand-based MOs to form the lowest unoccupied molecular orbitals (LUMOs). Due to the large HOMO−LUMO gap of the archetypical [Ru(bpy)3]2+ or [Ru(tpy)2]2+ complexes, the resulting absorption profile omits an integral portion of the solar spectrum in the red region. One powerful strategy is to raise the HOMO energy by virtue of anionic donors: e.g., carbanionic or N-containing aromatic units.6−12 This strategy has enabled the success of such Ru complexes in dye-sensitized solar cells since the seminal report by Grätzel,13 aiming at high energy conversion efficacy and improved chemical stability as reported by the groups of van Koten, Berlinguette, and others.8−12 In the case of cyclometalated complexes, the triplet excited state is dominated by metal-to-ligand charge transfer (3MLCT) character with some admixing of the cyclometalating donor,2 as exemplified by plots of the HOMO (Ru-cyclometalating fragment) and the LUMO (polypyridyl-type ligands).14

II

© 2017 American Chemical Society

Received: February 14, 2017 Published: July 5, 2017 7720

DOI: 10.1021/acs.inorgchem.7b00392 Inorg. Chem. 2017, 56, 7720−7730

Inorganic Chemistry

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Article

RESULTS AND DISCUSSION Synthesis. The novel ligands were prepared by a modified Suzuki−Miyaura cross-coupling protocol reported for the synthesis of the symmetric cyclometalating dqPhH ligand (dqPhH is 2,6-di(quinolin-8-yl)benzene).25 However, using an excess of 2,6-dibromopyridine with respect to 8-quinolineboronic acid (4/1) leads to low yields (∼20%) of the desired monofunctionalized intermediate 1.29 Presumably, the catalyst fragment is chelated by the formed product and undergoes the second coupling step, as corroborated by the formation of significant amount of dqp during our initial efforts to optimize this route.30 Hence, the original monophosphine SPHOS (SPHOS is 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl) was replaced by the bidentate phosphine ligand dppp (dppp is 1,3-di(diphenylphosphino)propane), which led to significantly increased yields of 1 (59%) even in a 1/1 ratio of the educts. The subsequent second Suzuki−Miyaura cross-coupling step employing the commercially available arylboronic acids of benzene, naphthalene, and anthracene yielded the asymmetric cyclometalating ligands 2−4 (Scheme 1). The ligands 2 and 4

Although the excited state is best described by a metal ligand to ligand CT state (3MLLCT), the term 3MLCT will be used synonymously for simplicity. Notably, different donating/ accepting organic subunits contribute to the HOMO/LUMO, which leads to the charge separation character of the excited state. In addition to the spectral properties, the lifetime of the excited state is an important parameter for efficient subsequent photochemical reactions (e.g., electron transfer) and is controlled by nonradiative and radiative deactivation pathways.14,15 In the case of thermally activated pathways, the deactivation via accessible triplet metal-centered (3MC) states has been identified. In order to diminish such a pathway, strong σ donors or an improved octahedral geometry has been utilized to increase the ligand field splitting to raise the barrier.8,16−19 In the case of direct coupling with the ground state, the extent of spin−orbit coupling becomes important and structural features also become important.20 The photoredox properties of the complexes are further tuned by systematically adjusting the frontier MO energies via functionalization of the ligand scaffold, e.g. of the carbanionic fragment by bromination via Nbromosuccinimide21 or CuBr2,22,23 as well as nitration with Cu(NO3)222 or with AgNO3 in the presence of PhCOCl.24 Such “chemistry-on-the-complex” modifications do not only diversify the set of photophysical and electrochemical properties but also offer the possibility to incorporate the moiety into molecular systems aiming, for example, at energy conversion. In this contribution, the synthesis of a series of novel cyclometalated RuII complexes was explored and the electrochemical and photophysical properties were investigated, including structural analysis by X-ray crystallography and a theoretical assessment based on density functional theory (DFT) (Figure 1). The cyclometalating ligands are derived

Scheme 1. Schematic Representation of the Ligand Synthesis (1−5) via Suzuki−Miyaura Cross Couplinga

a

Experimental conditions: (i) Pd(dba)2, dppp, K2CO3, CH3CN/H2O, 2 h, 130 °C; (ii) arylboronic acid, Pd(dba)2 SPHOS, K2CO3, CH3CN/ H2O, 1−2 h, 130−140 °C. Metalation sites are indicated by CH.

lead specifically to five- and six-membered chelates, respectively, whereas ligand 3 featuring the naphthalene unit can adopt two coordination modes (vide infra). Since more forcing reaction conditions are generally required to promote sixmembered vs five-membered chelates,28 the coordinating position for the latter was blocked by a methyl group (ligand 5). Excellent isolated yields were achieved except for 5, which is attributed to the difficulties in removing detrimental impurities from the crude 2-methylnaphthyl-1-ylboronic acid following the same route as for 8-quinolineboronic acid. Such residual impurities may cause side reactions during the second coupling step and suggest further optimization; however, sufficient amounts of 5 were obtained for the subsequent complexation reactions. With the cyclometalating ligands in hand, we investigated the synthesis of the corresponding complexes (Figure 2). Note that the order of the presented complexes reflects the main results from our optimization developments (Table 1), whereas a detailed discussion is provided in section 2 and Table S1 in the Supporting Information. The cyclometalation reaction typically requires higher temperatures in comparison to the coordination of related nitrogen donors. The symmetric complex 6 was readily prepared in 73% yield from [Ru(dqp)(CH3CN)3](PF6)2 and dqPhH in ethylene glycol using microwave heating (200 °C for 90 min) (Table 1, entry 1). In contrast, applying

Figure 1. Representation of reported cyclometalating ligands based on the tpy framework by replacing one N by CH (a) in a symmetrical (top) and symmetry-broken arrangement (bottom), as well as the related cyclometalating dqp-based ligands featuring enhanced bite angles in a symmetrical form (b) and symmetry-broken arrangement (c, this work). Typical anionic fragments for ArX− and Ar′X− comprise C-based and N-based (hetero)cycles: e.g., benzene, triazoles, and pyrazole moieties. Ar′′X− denotes carbanionic donors based on benzene, naphthalene, and anthracene (this work); dqp is 2,6di(quinolin-8-yl)pyridine, and tpy is 2,2′:6′,2″-terpyridine.

from 2,6-di(quinolin-8-yl)pyridine (dqp), which has been shown to provide a larger bite angle that leads to prolonged excited state lifetimes of RuII polypyridyl-type complexes and also to one related cyclometalated complex.25−28 In this work, one peripheral quinoline group of dqp is replaced by a phenyl, naphthyl, or anthracenyl moiety. Hence, such peripheral cyclometalation leads to a range of ligands that feature differently sized π systems and five- and six-membered chelates, as well as different mutual orientation and stacking of the aromatic subunits. 7721

DOI: 10.1021/acs.inorgchem.7b00392 Inorg. Chem. 2017, 56, 7720−7730

Article

Inorganic Chemistry

Figure 2. Representations of complexes prepared in this study featuring central cyclometalation (6), peripheral cyclometalation with five-membered chelates (7 and 8) or six-membered chelates (9−11), and a related bidentate complex (12, see text).

Table 1. Optimized Reaction Conditions for Complexes 6−12a entry

ligand

solvent

T (°C)

time (h)

1 2 3 4 5 6 7

dqPhH 3 1 2 3 4 5

EG EG EG EG EG EG EG

200b 200b 120 160d 160d 160d 160d

1.5 1.5 16 16 0.7 2.5 3

base

product

isolated yield (%)

lutidine NaHCO3 NaHCO3 NaHCO3

6 9h 12 7 8 11 10

73c 10c 49 74c 55e 75e 68e

a For full details see Table S1 in the Supporting Information. EG is ethylene glycol. bMicrowave heating. cPurification by two consecutive silica columns: CH2Cl2/CH3OH and CH3CN/H2O/aqueous KNO3 adapted from ref 25. dOil bath heated before synthesis (thermally equilibrated). e Purification by single column on amino-decorated silica. See text for further explanation.

Figure 3. (a) 1H NMR spectra of complexes 7−11 (CD3CN, 600 MHz, aromatic region) showing signal broadening (marked by asterisk) of the five-membered chelates (7 and 8) and a characteristic high-field shift of the six-membered chelates around 6.2 ppm (9−11). (b) Schematic representation of structural motifs (pyridine in blue, carbanionic fragment in green, interannular C−C bond in orange) leading to Signal broadening in five-membered chelates assigned to conformational exchange (bottom, see text). (c) High-field shift of the o-H in the cyclometalating ring due to shielding by the nearby pyridine π system of dqp (bottom). See the X-ray structures for more details.

oxidative cyanation of related 2-pyridyl aromatics, which occurs readily except for the anthracene moiety, ascribed to steric limitations during the C−H activation.31 This hypothesis is further corroborated by the improved yields using the smaller base NaHCO3 (Table 1, entries 5−7), which afforded the remaining complexes 8 (55%), 10 (68%), and particularly 11 (75%). The high yields are also attributed to the developed simplified purification protocoli.e., performing only a single chromatographic run using amino-functionalized silica and a CH2Cl2/CH3OH eluentwithout the need of subsequent counterion exchange. Structural Analysis. The 1H NMR spectra of the cyclometalated complexes 7−11 are depicted in Figure 3. Although the proton resonances are generally well resolved, some proton signals show an unusual signal broadening. This feature is observed for the five-membered chelates (7 and 8), while the six-membered congeners 9 and 10 exhibit the typical well-resolved spectra as for [Ru(dqp)2]2+ complexes (see also 2D NMR data in the Supporting Information).32,33 This

similar conditions for the chelation of the new chelating ligands gave significantly lower yields (