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Article

Photooxidative Dehydrogenation of Chiral Ir (III) Amino Acid Complexes Based on [Λ-Ir(ppy)2(MeCN)2](PF6)

by
Suyang Yao
1,
Yanxi Pu
1,
Lulu Ren
1,
Manli Cao
1,* and
Baohui Ye
2
1
School of Chemistry and Materials Science, Guangdong University of Education, Guangzhou 510303, China
2
MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-sen University, Guangzhou 510275, China
*
Author to whom correspondence should be addressed.
Inorganics 2023, 11(10), 380; https://doi.org/10.3390/inorganics11100380
Submission received: 28 August 2023 / Revised: 18 September 2023 / Accepted: 21 September 2023 / Published: 25 September 2023
(This article belongs to the Section Coordination Chemistry)
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Abstract

:
Octahedral chiral-at-metal Ir(III) complexes exhibit excellent structural stability and stereoselectivity in asymmetric synthesis. Selectively oxidative dehydrogenation of amino acids could be achieved by exploiting such complexes as chiral templates. The obtaining stable imine complexes can then be utilized in nucleophilic additions to generate corresponding chiral amine compounds. In this study, a conveniently synthesized [Λ-Ir(ppy)2(MeCN)2](PF6) chiral complex (ppy is 2-phenylpyridine) was utilized as a chiral template. A series of chiral amino acid complexes Λ-[Ir(ppy)2(D/L-AA)] (AA is amino acid) were prepared in high yield and optical purity. The above amino acid complexes were then oxidized to their corresponding imino acid complexes Λ-[Ir(ppy)2(AA-2H)] under visible light. All these complexes exhibited high selectivity during the dehydrogenation process without the formation of C-N bond coupling byproducts. The photooxidative dehydrogenation rates of these complexes were studied, which show that D-configured amino acids exhibited faster dehydrogenation rates when using the Λ-configured complex as a chiral template and the substitution of electron-donating or bulky groups in the N-α position of the amino acid decreased their dehydrogenation rates. The crystal structures of Λ-Ir(ppy)2(D-Thr) (Thr is threonine) and its dehydrogenated complex Λ-Ir(ppy)2(Thr-2H) indicate the process of photooxidative dehydrogenation and the configuration stability of metal center throughout the process.

1. Introduction

Imines are important intermediates in chemical and biological processes that are widely used in synthetic chemistry [1,2,3]. Among them, α-imino esters are an important class of imine compounds [4], which are more reactive relative to other types of imines and can be further functionalized into chiral amino acids via nucleophilic addition to the imine bonds [5,6,7,8]. Exploring the synthesis of different types of imine compounds is of significant importance [9]. However, most imines are relatively unstable and prone to hydrolysis or being oxidized to nitriles [10]. After the coordination of amino acids and the metal center, obtaining amino complexes could facilitate the oxidation and dehydrogenation of amine ligands [11]. Meanwhile, the coordination of the lone pair electrons to the metal center prevents the overoxidation of the amine ligands [11,12,13,14,15,16,17]. Therefore, the oxidation of the coordinated amines into imines is an important protocol for the synthesis of imine complexes and chiral amines.
Most amino acid compounds can be easily oxidized to the corresponding imino acid when coordinated to the metal center. For instance, Yamaguchi et al. used trivalent cobalt as the metal center and obtained cobalt(III) amino acid complexes. The amino ligands of these complexes can be oxidized to imino ligands by potassium permanganate [18]. Subsequently, their group successfully employed electrochemical oxidation to convert ruthenium(II) amino acid complexes into the corresponding imino acid complexes [19,20]. The Bonnet group utilized proline as a ligand and obtained ruthenium(II) proline complexes. Their study first showed that the proline ligands of these complexes can be oxidized to imino ligands under photoirradiation [21].
However, the amino acid complexes used to be dehydrogenated in the above cases were all racemic. Due to the presence of two enantiomers in amino acids, the oxidative dehydrogenation of the two enantiomers often exhibits different reaction rates in chiral environments. The dehydrogenation property of different enantiomers of chiral amino acids still requires further investigation.
Ir(III) complexes exhibit excellent photophysical properties and can serve as photosensitizers in various types of redox reactions [22,23]. Additionally, the octahedral chiral-at-metal Ir(III) complex possesses a stable configuration center that can be used as an efficient chiral template in asymmetric synthesis. For instance, the Meggers research group has recently applied enantiopure Ir(III) complexes in hydrogen bonding and Lewis acid-based asymmetric catalytic reactions, demonstrating their configuration stability and remarkable stereoselectivity in asymmetric synthesis [24,25,26,27]. Moreover, these complexes can also be used as photosensitizers and catalyze various types of asymmetric reactions under visible light. This further underscores the stability of the configuration stability of the chiral Ir(III) complex and its favorable photophysical properties [28,29,30,31]. Furthermore, due to the high diastereoselective recognition between metal center and chiral compounds, these chiral Ir(III) complexes have been successfully utilized as chiral metal templates for the separation of a series of chiral sulfone compounds [32].
Recently, our research group utilized the chiral complex Δ-[Ir(pq)2(MeCN)2](PF6) (pq is 2-phenylquinoline) as a template in the oxidative dehydrogenation of chiral amino acids. The study revealed that after coordination with the chiral metal center, amino acid substrates with different stereo configurations show significant differences in dehydrogenation rates. Mechanism study shows that this is caused by interactions between the configuration of the metal center and the stereocenter of amino acids. For the Δ-configured metal center template, amino acids in the L-configuration show a faster rate of dehydrogenation compared to their enantiomeric counterparts (D-configured amino acids). Because of this difference in dehydrogenation rates among two amino acid enantiomers, enantioselective dehydrogenation of chiral amino acids was achieved, which further enabled the conversion of the configurations between two amino acid enantiomers [33]. Subsequently, we also investigated dehydrogenation reactions of a series of amino acids with different structures using the Δ-[Ir(pq)2(MeCN)2](PF6) complex as a chiral template. This research observed the selective dehydrogenation of amino acids at different positions and facilitated the understanding of the reaction mechanism [34,35].
However, during the investigation of oxidative dehydrogenation of amino acid based on the [Ir(pq)2(MeCN)2](PF6) complex, the coordinated amino acids exhibited oxidative dehydrogenation along with C-N coupling side reactions with the cyclic metal ligands in the complex, thereby making the reaction system more complicated [36]. Meanwhile, the dehydrogenation property of complexes with different kinds of amino acids is still uninvestigated. In order to enhance the efficiency of the oxidative dehydrogenation process, minimize unnecessary competitive reactions during dehydrogenation, and investigate the oxidative dehydrogenation properties of different amino acids, we employed the more conveniently synthesized [Λ-Ir(ppy)2(MeCN)2](PF6) (ppy is 2-phenylpyridine) as the chiral metal template. A series of amino acid complexes, Λ-[Ir(ppy)2(D/L-AA)] (AA representing the amino acid), were synthesized, and the oxidative dehydrogenation reactions of these amino acid complexes were studied, which provided us with a series of imino-coordinated complexes, Λ-[Ir(ppy)2(AA-2H)]. Meanwhile, dehydrogenation rates between two enantiomers of L/D-alanine (Ala) and L/D-serine (Ser) and three different D-configured amino acids were compared. We found that when using Λ configuration metal templates, the D-configured amino acids exhibited faster dehydrogenation rates. Additionally, amino acids with electron-donating groups at their N-α position show relatively slow dehydrogenation rates. Subsequently, the crystal structures of complexes based on threonine (Thr), Λ-Ir(ppy)2(D-Thr), and their corresponding dehydrogenated complexes Λ-Ir(ppy)2(Thr-2H) were obtained to further confirm the structure and composition of these complexes.

2. Results and Discussion

2.1. Synthesis of Amino Acid Complexes and Photooxidative Dehydrogenation Reaction

As shown in Scheme 1, we first utilized Λ-[Ir(ppy)2(MeCN)2](PF6) as a template and MeONa as a base to react with five different D-configured amino acids (D-Ala, D-Val, D-Leu, D-Ser, D-Thr). This led to a series of chiral Ir(III) complexes after simple purification, namely Λ-[Ir(ppy)2(D-AA)] (Λ-[Ir(ppy)2(D-Ala)], Λ-[Ir(ppy)2(D-Val)], Λ-[Ir(ppy)2(D-Leu)], Λ-[Ir(ppy)2(D-Ser)], and Λ-[Ir(ppy)2(D-Thr)] as depicted in Scheme 1). The reactions all exhibited high yields (greater than 90%). According to previous studies, complexes with Λ-configured metal centers and L-configured amino acids (Λ-L) are thermodynamically more stable than those with D-configured amino acids (Λ-D) [32]. This implies that amino acid complexes with Λ-L structure exhibit slower rates for oxidative dehydrogenation. As a comparison, we synthesized two Λ-[Ir(ppy)2(L-AA)] amino acid complexes with the two types of L-configured amino acids (alanine and serine). The yield of Λ-[Ir(ppy)2(L-Ala)] and Λ-[Ir(ppy)2(L-Ser)] are 96% and 93%, respectively.
Obtained complexes were characterized by NMR spectroscopy (1H and 13C NMR spectroscopy) as well as mass spectrometry (MS) to determine their composition and optical purity. Taking the alanine (Ala) complex as an example, the 1H-NMR spectra of the two diastereomer complexes Λ-[Ir(ppy)2(D-Ala)] and Λ-[Ir(ppy)2(L-Ala)] show distinct 1H-NMR signals (as shown in Figure 1a,b). According to the literature report, two resonance peaks at a low field in the spectra are assigned to α-H of the ppy ligand, as shown in Figure 1 [35,36]. The diastereomers can be distinguished in the resonance peaks of α-H of the ppy ring at 9.07 ppm(H1), 8.64 ppm(H2) for Λ-[Ir(ppy)2(L-ala)], and 9.16 ppm(H1), 8.58 ppm(H2) for Λ-[Ir(ppy)2(D-ala)], as shown in Figure 1. Through 1H-NMR spectra of two complexes, we found that the de values of both complexes are more than 98%, and the metal center configuration remained stable throughout the synthesis process. Analysis of the 1H-NMR spectra of other amino acid complexes indicated that the entire series of amino acid complexes exhibited high optical purity.
In a previous study, we found that when [Λ-Ir(pq)2(MeCN)2](PF6) serves as the metal template and coordinates amino acids with different configurations, two diastereomer complexes can be obtained. Under O2 conditions and visible light, the Λ-D diastereomer undergoes dehydrogenative oxidation to generate the corresponding imino acid complex, while the Λ-L diastereomer exhibits both oxidative dehydrogenation and a C-N cross-coupling reaction between the amino acid substrate and the 2-phenylquinoline ligand [33]. Referring to the previously mentioned reaction conditions, we employed [Λ-Ir(ppy)2(MeCN)2](PF6) as a template and investigated the dehydrogenation reactions of these amino acid complexes under visible light (λ = 450–470 nm).
First, by taking Ala as the standard amino acid substrate, we monitored the reaction process by 1H-NMR spectra. It was observed that the ethanol solution of the Λ-[Ir(ppy)2(D-Ala)] complex could be completely converted to the corresponding imino acid complex Λ-[Ir(ppy)2(Ala-2H)] in 8.5 h under oxygen and visible light conditions. In contrast, the other diastereomers, Λ-[Ir(ppy)2(L-Ala)], exhibited a slower dehydrogenative oxidation rate under the same conditions, and it took 28 h for the amino acid complex to be completely converted to the imino acid complex. However, for both diastereomers, only the dehydrogenations take place during the photooxidation. The separation yields of the dehydrogenation products are both greater than 95%. The reaction is straightforward and does not involve any C-N cross-coupling side reaction throughout the dehydrogenation process. As depicted in Figure 1c, after the oxidation of the amino acid complexes to imino acid complexes, two resonance peaks in the amino acid NH2 group shifted downfield from 5.21 ppm, 3.83 ppm (Λ-[Ir(ppy)2(L-Ala)]), and 4.56 ppm, 4.34 ppm (Λ-[Ir(ppy)2(D-Ala)]) to 12.18 ppm. This shift indicates the formation of the imino group (NH=C). On the other hand, the integration of the hydrogen NMR signals reveals the disappearance of one hydrogen signal from the amino acid NH2 group, indicating the occurrence of dehydrogenative oxidation of the amino acid. Subsequently, we conducted mass spectrometry analysis on these amino and imino acid complexes (as described in the Experimental section). As shown in Figure 2a, The complex Λ-[Ir(ppy)2(L/D-Ala)] exhibited strong signals at m/z = 590.08 [M+H]+, 612.26 [M+Na]+, and 628.22 [M+K]+, which corresponded to the simulated theoretical values of m/z = 589.70 [M+H]+, 611.68 [M+Na]+, and 627.80 [M+K]+. On the other hand, the imino acid complex after dehydrogenation showed peaks at m/z = 588.21 [M+H]+ and 610.33 [M+Na]+ (Figure 2b), which matched the simulated theoretical values of m/z = 587.68 [M+H]+ and 609.67 [M+Na]+. A comparison between the two spectra reveals a difference of two units, indicating the removal of two hydrogen atoms during the reaction.
In addition, we also investigated the dehydrogenative oxidation process of complexes based on serine. Unlike alanine, serine contains a hydroxyl group. As depicted in Figure 3a,b, the two diastereomers of this serine complexes, Λ-[Ir(ppy)2(D-Ser)] and Λ-[Ir(ppy)2(L-Ser)], exhibit distinct 1H-NMR signals. Similar to alanine, two serine complexes diastereomers can be distinguished in the resonance peaks of α-H of the ppy ring at 8.63 ppm(H2) for Λ- [Ir(ppy)2(L-ser)] and 8.58 ppm(H2) for Λ-[Ir(ppy)2(D-ser)], as shown in Figure 3. Both diastereomers can undergo a photooxidative dehydrogenation process under oxygen conditions. By monitoring the reaction process through 1H-NMR, it is observed that Λ-[Ir(ppy)2(D-Ser)] can be completely dehydrogenated to Λ-[Ir(ppy)2(Ser-2H)] in 12 h, while the other diastereomer, Λ-[Ir(ppy)2(L-Ser)], exhibited a slower reaction rate, requiring 25 h for complete conversion to Λ-[Ir(ppy)2(Ser-2H)]. The separation yields of the dehydrogenation products from two diastereomers are both greater than 95%. Similarly, for complexes based on serine, both diastereomers undergo dehydrogenation reactions of the amino acid during the photooxidation process exclusively. There is no occurrence of C-N cross-coupling reactions during the dehydrogenation process.
Subsequently, under the same reaction conditions, we investigate the photooxidative dehydrogenation reactions of other amino acid complexes. As shown in Table 1, the yields of imino acid complexes are above 95% after simple separation. And only dehydrogenation products are observed throughout the photoreaction process. After dehydrogenation, the imino acid complexes Λ-[Ir(ppy)2(Val-2H)], Λ-[Ir(ppy)2(Leu-2H)], and Λ-[Ir(ppy)2(Thr-2H)] all exhibit signals in the low-field region (>10 ppm) in 1H-NMR spectra. Additionally, the mass spectra of imino acid complexes also show character peaks, which are two mass units less than the corresponding amino acid complexes (see the Experimental section).
As shown in Scheme 2, early research showed some related studies about the oxidative rate constants of coordinated amino acids. Bonnet and co-workers have found that the proline ligand of rac-[Ru(bpy)2(L-Pro)](PF6) (bpy is 2,2′-bipyridine) was photooxidized upon visible light irradiation with a rate constant kobs = 2.0 × 10−3 s−1. In contrast, in our former study, we found that the rate constant of Δ-Ir-L-Pro (kobs = 6.0 × 10−4 s−1) is about 60 times larger than that of Δ-Ir-D-Pro (kobs =1.0 × 10−5 s−1) when utilizing Δ-[Ir(pq)2(MeCN)2](PF6) as a chiral metal template. Next, we investigated the configuration effect of amino acids during the dehydrogenation process when using [Λ-Ir(ppy)2(MeCN)2](PF6) as a template. As shown in Figure 4, when Λ-[Ir(ppy)2(L-Ala)] was reacted under visible light in O2, only a trace amount of the dehydrogenated product Λ-[Ir(ppy)2(Ala-2H)] at 12.17 ppm was observed after 2.5 h. After 5 h, dehydrogenated Λ-[Ir(ppy)2(Ala-2H)] became obvious in the spectrum with about 15% conversion and increased to about 21% when reacted at 7.5 h. After 30 h, there is still remaining Λ-[Ir(ppy)2(L-Ala)] in the reaction system. However, Λ-[Ir(ppy)2(D-Ala)] shows a much higher dehydrogenation rate under the same condition. As shown in Figure 5, the dehydrogenated product Λ-[Ir(ppy)2(Ala-2H)] was observed after 2.5 h with about 38% conversion. After 5 h, the conversion of Λ-[Ir(ppy)2(D-Ala)] is about 61% and increased to about 78% after 7.5 h, which is much higher than that of Λ-[Ir(ppy)2(L-Ala)]. After 30 h, no amino complex can be observed through the spectra.
We further studied the oxidative dehydrogenation rates of obtained amino acid complexes by monitoring the concentration of reactants in the system during the reaction process through 1H-NMR(see supporting information for details). For the alanine complex, the rate constant k for Λ-[Ir(ppy)2(D-Ala)] is 0.205 h−1, while the rate constant of the other diastereomer Λ-[Ir(ppy)2(L-Ala)] is 0.063 h−1, which is significantly lower than that of Λ-[Ir(ppy)2(D-Ala)]. The rate constants of complexes based on serine also show similar results. The reaction rate of the Λ-[Ir(ppy)2(L-Ser)] is notably slower than that of the Λ-[Ir(ppy)2(D-Ser)]. This result is in accordance with our former research when using [Ir(pq)2(MeCN)2](PF6) as a chiral template [33]. Subsequently, we continued to investigate the dehydrogenation rates of other D-configured amino acid complexes, Λ-[Ir(ppy)2(D-AA)], with different R substituents at the N-α position. As shown in Table 1, it can be observed that when R is a methyl group (alanine), the complex Λ-[Ir(ppy)2(D-Ala)] exhibits the fastest dehydrogenation rate. And, when the electron-donating ability and steric hindrance of the R group increase, the oxidative dehydrogenation rates of the amino acid complexes decrease. Among them, when R is an OH group with strong electron-donating ability, the corresponding complex Λ-[Ir(ppy)2(D-Ser)] shows the lowest oxidative dehydrogenation rate. It is worth noting that Λ-[Ir(ppy)2(D-Thr)] seems to deviate from the regular described above. Although the R group of threonine is the strongest electron-donating compared to amino acids, its corresponding complex Λ-[Ir(ppy)2(D-Thr)] exhibits a higher oxidative dehydrogenation rate. This might be due to the differences in the dehydrogenative oxidation mechanism of this amino acid complex compared to the other amino acid complexes, which are still under investigation.

2.2. Characterization of Crystal Structures

In order to better investigate the composition and structure of the amino acid complexes and their corresponding imino acid complexes, we obtained the single crystals of Λ-[Ir(ppy)2(D-Thr)] and Λ-[Ir(ppy)2(Thr-2H)] through the evaporation of an EtOH solution of corresponding complexes. The crystallographic data are presented in Table 2, while the bond lengths and bond angles are shown in Table S1.
The complex Λ-[Ir(ppy)2(D-Thr)]·CH3OH crystallizes in the orthorhombic system with the P212121 space group. Its unit cell parameters are a = 9.39429(11) Å, b = 11.15738(11) Å, c = 22.1602(2) Å, and V= 2322.73(4) Å3. Each asymmetric unit contains one Λ-[Ir(ppy)2(D-Thr)] molecule and one methanol molecule with the chemical formula C27H28IrN3O4. From the coordination environment diagram (Figure 6a), it can be observed that Ir1 adopts a six-coordinated mode, coordinated to two ppy molecules and one deprotonated D-Thr with Λ configuration. According to the bond length table of the complex (see Table S1), the Ir-C bond lengths range from 2.001(5) Å to 2.016(6) Å, the Ir-N bond lengths range from 2.047(4) Å to 2.183(5) Å, and the Ir-O bond length is 2.181(4) Å. The C24-N3 bond in the amino acid has a length of 1.485(7) Å, indicating a typical single bond, with two hydrogens on N3 in this case.
The dehydrogenated complex Λ-[Ir(ppy)2(D-Thr-2H)] also crystallizes in the orthorhombic system with the P212121 space group. Its unit cell parameters are a = 9.39429(11) Å, b = 11.15738(11) Å, c = 22.1602(2) Å, and V = 2322.73(4) Å3. Each asymmetric unit contains only Λ-[Ir(ppy)2(D-Thr-2H)], without any solvent, resulting in the chemical formula C26H22IrN3O3. As shown in the coordination environment diagram (Figure 6b), Ir1 adopts a six-coordinated mode, coordinated to two ppy molecules and one dehydrogenated imino acid ligand with Λ configuration. According to the bond length table (Table S1), the Ir-C bond lengths range from 2.014(7) Å to 2.038(7) Å, the Ir-N bond lengths range from 2.049(6) Å to 2.115(5) Å, and the Ir-O bond length is 2.194(5) Å. Upon comparison, it can be observed that the C24-N3 bond length changes from 1.485(7) Å to 1.261(9) Å, indicating the conversion of the C-N bond to a double bond. This suggests that the complex undergoes dehydrogenative oxidation, with only one hydrogen atom remaining on the N atom.
The Flack parameters of the crystals are −0.016(5) for Λ-[Ir(ppy)2(D-Thr)] and −0.020(6) for Λ-[Ir(ppy)2(Thr-2H)], which indicate that both complexes are chiral configurations and exhibit high enantiomeric purity. This indicates that during the dehydrogenative oxidation process, the configuration of the metal center remains stable, and no racemization occurs. In our previous research, we observed a C-N coupling side reaction between the α-C-H on the quinoline ring and the N atom on the ligand [35] when using [Ir(pq)2(MeCN)2](PF6) as a template. However, when changing to [Ir(ppy)2(MeCN)2](PF6) chiral complex as a template, we can only observe oxidative dehydrogenation product without other side reactions. As shown in the crystal structure of Λ-[Ir(ppy)2(D-Thr)], we observed that the pyridine ring of the chelating ligand ppy is far away from the nitrogen atom in the amino acid ligand compared to the quinoline ring. The distance between the α-C-H on the pyridine ring and the N atom is approximately 3.353 Å, which is insufficient to form a chemical bond. Therefore, the photoreaction process of Λ-[Ir(ppy)2(L/D-AA)] only leads to their oxidative dehydrogenation products.

3. Materials and Methods

All chemicals and reagents (iridium(III) chloride hydrate (IrCl3·3H2O), 2-phenylpyridine (ppy), chiral amino acids, sodium methoxide (NaOMe), anhydrous sodium sulfate (Na2SO4), trifluoroacetic acid (TFA), potassium hexafluorophosphate (KPF6), and relevant organic solvents) were of analytical grade and commercially available. The Ir(III) complex template Λ-[Ir(ppy)2(MeCN)2](PF6) was synthesized as referenced in the literature [37]. All complex synthesis reactions were conducted under a nitrogen atmosphere, while the oxidation and dehydrogenation reactions of amino acid complexes were performed under an oxygen atmosphere with a blue light-emitting diode (LED) corn lamp (λ = 450–470 nm, 45 W) obtained from Shenzhen Taoyuan Technology Co., Ltd. as a light source. Electrospray ionization mass spectrometry (ESI-MS) characterization was carried out using a Thermo LCQ DECA XP mass spectrometer. Elemental (C, H, and N) analyses were performed on an Elementar Vario EL analyzer. 1H and 13C NMR spectra were recorded with a Bruker 300 MHz or Bruker 400 MHz nuclear magnetic resonance spectrometer. Single crystal X-ray diffraction data were collected using a single crystal diffractometer (Bruker Smart Apex CCD).

3.1. Synthesis of Chiral Amino Acid Complexes Λ-[Ir(ppy)2(L/D-AA)]

A total of 118 mg (0.163 mmol) of Λ-[Ir(ppy)2(MeCN)2](PF6), corresponding chiral amino acid (0.326 mmol), and 22 mg (0.408 mmol) of NaOMe were dissolved in 45 mL of methanol. The reaction mixture was stirred under a nitrogen atmosphere at 45 °C for 12 h and monitored by TLC. After the reaction was completed, the solution was cooled to room temperature, and the methanol was removed under reduced pressure. Then, 20 mL of dichloromethane was added to dissolve the resulting mixture, followed by washing the organic phase three times with 10 mL of distilled water. Subsequently, anhydrous Na2SO4 was added to the organic phase for drying, and the solution was filtered. The dichloromethane was removed under reduced pressure, yielding a yellow powdered product, Λ-[Ir(ppy)2(L/D-AA)].
Λ-[Ir(ppy)2(L-Ala)]. Yield: 96% (92 mg), Anal. Calcd for C25H22IrN3O2: C 51.01, H 3.77, N 7.14; Found: C 51.33, H 3.78, N 7.22, 1H NMR (400 MHz, DMSO-d6) δ 9.07 (d, J = 5.8 Hz, 1H), 8.65 (d, J = 5.9 Hz, 1H), 8.16 (t, J = 8.0 Hz, 2H), 7.95 (t, J = 7.9 Hz, 2H), 7.71 (dd, J = 7.9, 4.4 Hz, 2H), 7.46 (t, J = 6.6 Hz, 1H), 7.40 (t, J = 6.4 Hz, 1H), 6.78 (d, J = 6.5 Hz, 2H), 6.61 (t, J = 7.1 Hz, 2H), 6.26 (d, J = 7.5 Hz, 1H), 5.95 (d, J = 7.5 Hz, 1H), 5.22 (t, J = 10.9 Hz, 7.5 Hz, 1H), 3.83 (t, J = 10.9 Hz, 1H), 2.01 (q, J = 7.1, 6.5 Hz, 1H), 1.24 (d, J = 7.0 Hz, 3H).13C NMR (151 MHz, DMSO-d6) δ 182.73, 169.17, 168.42, 154.29, 151.09, 148.25, 147.85, 145.34, 145.09, 137.95, 137.89, 132.72, 132.17, 128.94, 128.88, 124.56, 124.35, 123.08, 122.73, 120.75, 120.07, 119.38, 119.35, 50.99, 21.88. ESI-MS: m/z = 590.08 [M + H]+, 612.26 [M+Na]+, 628.22 [M+K]+; IR: (KBr, v[cm−1]): 3738 (w), 3423 (m), 3040 (w), 2924 (w), 1605 (vs), 1584 (vs), 1477 (s), 1418 (m), 1385 (m), 1298 (w), 1267 (w), 1224 (w), 1159 (w), 1103 (w), 1059 (w), 1030 (w), 919 (w), 846 (w), 796 (w), 760 (s), 735 (m), 671 (w), 550 (w), 420 (w).
Λ-[Ir(ppy)2(D-Ala)]. Yield:97% (93 mg), Anal. Calcd for C25H22IrN3O2: C 51.01, H 3.77, N 7.14; Found: C 51.36, H 3.74, N 7.23; 1H NMR (400 MHz, DMSO-d6) δ 9.17 (d, J = 5.8 Hz, 1H), 8.59 (d, J = 5.8 Hz, 1H), 8.18 (t, J = 9.4 Hz, 2H), 7.96 (t, J = 7.8 Hz, 2H), 7.72 (dd, J = 7.8, 3.4 Hz, 2H), 7.45 (t, J = 6.6 Hz, 1H), 7.39 (t, J = 6.6 Hz, 1H), 6.76 (q, J = 7.7 Hz, 2H), 6.60 (td, J = 7.3, 4.6 Hz, 2H), 6.23 (d, J = 7.6 Hz, 1H), 5.96 (d, J = 7.6 Hz, 1H), 4.56 (dd, J = 12.1, 8.6 Hz, 1H), 4.36 (dd, J = 12.4, 7.3 Hz, 1H), 1.24 (d, J = 7.3 Hz, 3H). 13C NMR (151 MHz, DMSO-d6) δ 183.40, 169.06, 168.70, 154.01, 150.74, 148.44, 148.14, 145.31, 145.09, 138.02, 137.98, 133.05, 132.14, 128.97, 128.93, 124.67, 124.32, 122.82, 122.72, 120.74, 120.13, 119.57, 119.25, 50.69, 21.58. ESI-MS: m/z = 590.08 [M + H]+, 612.26 [M + Na]+, 628.22 [M + K]+; IR: (KBr, v[cm−1]): 3738 (w), 3423 (m), 3040 (w), 2924 (m), 1605 (vs), 1584 (vs), 1477 (s), 1418(m), 1385 (m), 1298 (w), 1267 (w), 1224 (w), 1159 (w), 1103 (w), 1059 (w), 1030 (w), 919 (w), 846 (w), 796 (w), 760 (s), 735 (m), 671 (w), 550 (w), 420 (w).
Λ-[Ir(ppy)2(L-Ser)]. Yield:93% (92 mg), Anal. Calcd for C25H22IrN3O3 C 49.66, H 3.67, N, 6.95, Found: C 49.55, H 3.70, N 7.02; 1H NMR (400 MHz, DMSO-d6) δ 9.05 (d, J = 5.5 Hz, 1H), 8.64 (d, J = 5.3 Hz, 1H), 8.18 (t, J = 8.9 Hz, 2H), 7.96 (t, J = 8.6 Hz, 2H), 7.72 (t, J = 6.8 Hz, 2H), 7.42 (dd, J = 11.3, 6.4 Hz, 2H), 6.77 (q, J = 7.9, 7.5 Hz, 2H), 6.62 (t, J = 7.4 Hz, 2H), 6.25 (d, J = 7.3 Hz, 1H), 5.98 (d, J = 6.9 Hz, 1H), 5.22 (dd, J = 11.4, 7.5 Hz, 1H), 4.73 (t, J = 5.9 Hz, 1H), 3.69 (dt, J = 10.4, 5.4 Hz, 1H), 3.47–3.42 (m, 3H). 13C NMR (151 MHz, DMSO-d6) δ 181.26, 168.99, 168.19, 153.62, 151.07, 148.55, 148.00, 145.32, 144.87, 138.21, 138.00, 132.95, 132.33, 129.05, 129.04, 124.63, 124.40, 123.18, 122.66, 120.83, 120.24, 119.38, 119.32, 63.87, 56.89. ESI-MS: m/z = 606.08 [M + H]+, 628.23 [M + Na]+, 644.22 [M + K]+; IR: (KBr, v[cm−1]): 3741 (w), 3624 (w), 3410 (m), 3233 (w), 3039 (w), 2923 (w), 2851 (w), 1607 (vs), 1585 (vs), 1479 (s), 1420 (w), 1381 (w), 1267 (w), 1226 (w), 1161 (w), 1061 (w), 1032 (w), 985 (w), 847 (m), 795 (w), 758 (s), 733 (m), 669 (w), 561 (w), 420 (w).
Λ-[Ir(ppy)2(D-Ser)]. Yield:93% (92 mg), Anal. Calcd for C25H22IrN3O3 C 49.66, H 3.67, N, 6.95, Found: C 49.65, H 3.66, N 7.01; 1H NMR (400 MHz, DMSO-d6) δ 9.04 (d, J = 5.8 Hz, 1H), 8.58 (d, J = 5.6 Hz, 1H), 8.26–8.12 (m, 2H), 7.96 (t, J = 7.7 Hz, 2H), 7.71 (d, J = 5.8 Hz, 2H), 7.45 (t, J = 6.5 Hz, 1H), 7.37 (t, J = 6.5 Hz, 1H), 6.76 (q, J = 7.5 Hz, 2H), 6.60 (q, J = 7.1 Hz, 2H), 6.19 (d, J = 7.6 Hz, 1H), 5.98 (, J = 7.6 Hz, 1H), 4.95 (t, J = 5.1 Hz, 1H), 4.47 (t, J = 11.2 Hz, 1H), 4.35–4.23 (m, 1H), 3.76 (dt, J = 10.9, 5.4 Hz, 1H), 3.54 (dd, J = 10.2, 5.1 Hz, 1H), 3.11–2.93 (m, 1H). 13C NMR (151 MHz, DMSO-d6) δ 181.50, 169.06, 168.74, 153.71, 150.17, 148.08, 148.04, 145.20, 145.12, 138.08, 138.03, 132.98, 132.09, 129.03, 129.00, 124.66, 124.35, 122.88, 122.73, 120.75, 120.23, 119.64, 119.24, 63.57, 56.60. ESI-MS: m/z = 606.08 [M + H]+, 628.23 [M + Na]+, 644.22 [M + K]+; IR: (KBr, v[cm−1]): 3743 (w), 3420 (m), 3302 (m), 3256 (m), 3044 (w), 2924 (w), 2853 (w), 1607 (vs), 1585 (vs), 1477 (s), 1418 (m), 1267 (w), 1225 (w), 1159 (w), 1059 (m), 1034 (w), 985 (w), 847 (s), 794 (w), 760 (s), 735 (m), 671 (w), 559 (w), 418 (w).
Λ-[Ir(ppy)2(D-Thr)]. Yield:95% (92 mg), Anal. Calcd for C26H24IrN3O3 C 50.47, H 3.91, N 6.79; Found: C 50.47, H 3.88, N 6.72; 1H NMR (400 MHz, DMSO-d6) δ 9.04 (d, J = 5.8 Hz, 1H), 8.56 (d, J = 5.8 Hz, 1H), 8.16 (dd, J = 16.0, 8.1 Hz, 1H), 8.03–7.91 (m, 2H), 7.78–7.64 (m, 2H), 7.43 (t, J = 6.6 Hz, 1H), 7.33 (t, J = 6.5 Hz, 1H), 6.84–6.74 (m, 2H), 6.64–6.55 (m, 2H), 6.18 (d, J = 7.6 Hz, 1H), 5.96 (d, J = 7.6 Hz, 1H), 5.07 (d, J = 5.8 Hz, 1H), 4.49 (t, J = 10.0 Hz, 1H), 4.38–4.20 (m, 1H), 4.14–3.95 (m, 1H), 2.80 (t, J = 8.4 Hz, 1H), 1.09 (d, J = 6.5 Hz, 3H). 13C NMR (151 MHz, DMSO-d6) δ 181.78, 169.04, 168.93, 153.84, 150.15, 148.50, 147.92, 145.26, 145.14, 137.92, 132.99, 131.97, 129.00, 124.58, 124.30, 122.77, 122.57, 120.64, 120.13, 119.60, 119.10, 67.25, 59.84, 21.24. ESI-MS: m/z = 620.10 [M + H]+, 642.26 [M + Na]+, 658.26 [M + K]+; IR: (KBr, v[cm−1]): 3742 (w), 3420 (m), 3263 (w), 3039 (w), 2924 (w), 2852 (w), 1607 (vs), 1584 (vs), 1477 (s), 1420 (m), 1381 (w), 1308 (w), 1265 (w), 1159 (w), 1130 (w), 1059 (w), 1032 (w), 898 (w), 760 (s), 735 (m), 671 (w), 417 (w).
Λ-[Ir(ppy)2(D-Leu)]. Yield:94% (97 mg), Anal. Calcd for C28H28IrN3O2 C 53.32, H 4.47, N 6.66; Found: C 53.30, H 4.50, N 6.72; 1H NMR (400 MHz, DMSO-d6) δ 9.15 (d, J = 5.8 Hz, 1H), 8.57 (d, J = 5.8 Hz, 1H), 8.17 (dd, J = 13.8, 8.2 Hz, 2H), 7.95 (t, J = 7.8 Hz, 2H), 7.71 (t, J = 7.3 Hz, 2H), 7.41 (dt, J = 19.1, 6.6 Hz, 2H), 6.76 (q, J = 7.1 Hz, 2H), 6.63–6.50 (m, 2H), 6.26 (d, J = 7.6 Hz, 1H), 5.94 (d, J = 7.6 Hz, 1H), 4.77–4.54 (m, 1H), 4.18 (dd, J = 12.4, 6.7 Hz, 1H), 2.97 (d, J = 8.7 Hz, 1H), 1.93 (d, J = 41.6 Hz, 1H), 1.68 (t, J = 11.6 Hz, 1H), 1.57–1.41 (m, 1H), 0.87 (d, J = 6.6 Hz, 3H), 0.75 (d, J = 6.4 Hz, 3H). 13C NMR (151 MHz, DMSO-d6) δ 183.66, 169.05, 168.84, 153.89, 150.78, 148.82, 148.01, 145.30, 145.23, 137.94, 133.22, 132.10, 128.95, 128.82, 124.57, 124.27, 122.68, 122.62, 120.70, 120.02, 119.53, 119.18, 52.96, 44.83, 24.23, 23.88, 21.59. ESI-MS: m/z = 632.24 [M + H]+, 654.31 [M + Na]+, 670.30 [M+K]+; IR: (KBr, v[cm−1]): 3899 (w), 3743 (w), 3444 (m), 3424 (m), 3158 (w), 3041 (w), 2954 (m), 2924 (m), 2861 (w), 1605 (vs), 1584 (vs), 1475 (s), 1418 (m), 1373 (m), 1308 (w), 1267 (w), 1224 (w), 1159 (w), 1059 (w), 1030 (w), 979 (w), 851 (w), 794 (w), 758 (m), 733 (m), 671 (w), 558 (w), 419 (w).
Λ-[Ir(ppy)2(D-Val)]. Yield:97% (97 mg), Anal. Calcd for C27H26IrN3O2 C 52.58, H 4.25, N 6.81; Found: C 52.44, H 4.13, N 6.90; 1H NMR (400 MHz, DMSO-d6) δ 9.14 (d, J = 5.8 Hz, 1H), 8.50 (d, J = 5.6 Hz, 1H), 8.17 (dd, J = 16.0, 8.2 Hz, 2H), 7.94 (t, J = 7.8 Hz, 2H), 7.79–7.63 (m, 2H), 7.41 (t, J = 6.6 Hz, 1H), 7.36 (t, J = 6.6 Hz, 1H), 6.76 (q, J = 6.9 Hz, 2H), 6.60 (t, J = 7.4 Hz, 2H), 6.29 (d, J = 7.6 Hz, 1H), 5.91 (d, J = 7.6 Hz, 1H), 4.40 (t, J = 10.6 Hz, 1H), 3.96 (t, J = 10.7 Hz, 1H), 2.95–2.72 (m, 1H), 2.40–2.18 (m, 1H), 0.88 (dd, J = 12.3, 6.9 Hz, 6H). 13C NMR (151 MHz, DMSO-d6) δ 182.08, 169.08, 169.01, 153.68, 150.32, 148.84, 147.85, 145.31, 145.18, 138.05, 137.94, 133.28, 132.04, 128.95, 128.83, 124.63, 124.25, 122.62, 122.49, 120.76, 120.05, 119.64, 119.20, 60.27, 31.34, 19.48, 17.00. ESI-MS: m/z = 618.15 [M + H]+, 640.32 [M + Na]+, 628.22 [M + K]+ IR: (KBr, v[cm−1]): 3860 (w), 3742 (w), 3672 (w), 3648 (w), 3443 (w), 3429 (w), 3314 (w), 3250 (w), 3042 (w), 3040 (w), 2959 (m), 2924 (m), 2855 (w), 1607 (vs), 1584 (vs), 1477 (s), 1418 (m), 1391 (w), 1379 (w), 1309 (w), 1267 (w), 1224 (w), 1060 (w), 967 (w), 892 (w), 794 (w), 760 (s), 733 (m), 671 (w), 420 (w).

3.2. Synthesis of Chiral Imino Acid Complexes Λ-[Ir(ppy)2(AA-2H)]

A total of 0.100 mmol Λ-[Ir(ppy)2(L/D-AA)] was dissolved in 20 mL EtOH. The solution was stirred and exposed to the light from a 10W blue LED (λ = 450–470 nm) at room temperature under an atmosphere of O2 for an appropriate duration. The reactions were monitored by TLC. Afterward, the solvent was removed under reduced pressure, and the product was purified through recrystallization from ethanol.
Λ-[Ir(ppy)2(Ala-2H)]. Yield:95% (56 mg), Anal. Calcd for C25H20IrN3O2: C 51.18, H 3.44, N 7.16; Found: C 51.13, H 3.43, N 7.19; 1H NMR (400 MHz, DMSO-d6) δ 12.18 (s, 1H), 8.45 (d, J = 5.5 Hz, 1H), 8.32 (d, J = 5.5 Hz, 1H), 8.19 (d, J = 8.1 Hz, 2H), 7.97 (q, J = 6.7 Hz, 2H), 7.76 (d, J = 9.3 Hz, 2H), 7.47 (t, J = 6.4 Hz, 1H), 7.39 (t, J = 6.4 Hz, 1H), 6.82 (t, J = 7.4 Hz, 2H), 6.73–6.61 (m, 2H), 6.19 (d, J = 7.6 Hz, 1H), 5.97 (d, J = 7.4 Hz, 1H), 2.16 (s, 3H). ESI-MS: m/z = 588.21 [M + H]+, 610.33 [M + Na]+; IR: (KBr, v[cm−1]): 3742 (w), 3445 (m), 3040 (w), 2924 (m), 1647 (vs), 1611 (vs), 1584 (s), 1560 (m), 1477 (s), 1420 (m), 1304 (w), 1267 (w), 1161 (w), 1061 (w), 1032 (w), 850 (w), 797 (w), 760 (s), 735 (m), 669 (w), 417 (w).
Λ-[Ir(ppy)2(Thr-2H)]. Yield:97% (60 mg), Anal. Calcd for C26H22IrN3O3: C 50.64, H 3.60, N 6.81; Found: C 50.66, H 3.60, N 6.77; 1H NMR (400 MHz, DMSO-d6) δ 11.13 (s, 1H), 8.37 (dd, J = 11.9, 5.6 Hz, 2H), 8.20 (d, J = 8.1 Hz, 2H), 7.98 (q, J = 7.4, 6.9 Hz, 2H), 7.76 (dd, J = 7.5, 3.3 Hz, 2H), 7.47 (t, J = 6.4 Hz, 1H), 7.37 (t, J = 6.4 Hz, 1H), 6.83 (q, J = 7.1 Hz, 2H), 6.68 (q, J = 6.6 Hz, 2H), 6.18 (d, J = 7.5 Hz, 1H), 5.99 (d, J = 7.4 Hz, 1H), 5.59 (d, J = 4.6 Hz, 1H), 4.75–4.51 (m, 1H), 1.14 (d, J = 6.7 Hz, 3H). ESI-MS: m/z = 618.32 [M + H]+, 640.42 [M + Na]+; IR: (KBr, v[cm−1]): 3744 (m), 2926 (w), 1701 (m), 1678 (m), 1645 (s), 1609(vs), 1584(vs), 1560 (s), 1541 (m), 1524 (m), 1516 (m), 1477 (s), 1456 (m), 1420 (m), 1393 (m), 1267 (w), 1161 (w), 1061 (w), 1034 (w), 760 (s), 735 (w), 673 (w), 419 (m).
Λ-[Ir(ppy)2(Ser-2H)]. Yield:96% (58 mg), Anal. Calcd for C25H20IrN3O3: C 49.82, H 3.35, N 6.97; Found: C 49.76, H 3.36, N 7.01; 1H NMR (400 MHz, DMSO-d6) δ 11.20 (s, 1H), 8.45 (d, J = 5.8 Hz, 1H), 8.35 (d, J = 5.8 Hz, 1H), 8.19 (d, J = 8.2 Hz, 2H), 7.97 (q, J = 7.6 Hz, 2H), 7.76 (d, J = 7.8 Hz, 2H), 7.47 (t, J = 6.6 Hz, 1H), 7.37 (t, J = 6.6 Hz, 1H), 6.82 (td, J = 7.3, 3.6 Hz, 2H), 6.68 (q, J = 6.8 Hz, 2H), 6.18 (d, J = 7.7 Hz, 1H), 5.97 (d, J = 7.6 Hz, 1H), 5.56 (s, 1H), 4.53–4.20 (m, 2H). ESI-MS: m/z = 604.45 [M + H]+, 626.49 [M + Na]+; IR: (KBr, v[cm−1]): 3784 (w), 3433 (m), 3043 (w), 2926 (w), 2854 (w), 1632 (vs), 1609 (vs), 1585 (s), 1477 (s), 1420 (w), 1266 (w), 1119 (vs), 1063 (w), 1036 (w), 995 (w), 910 (w), 863 (w), 760 (m), 735 (m), 688 (w), 619 (m), 558 (w), 517 (w), 477 (w), 438 (w).
Λ-[Ir(ppy)2(Leu-2H)]. Yield:96% (60 mg), Anal. Calcd for C28H26IrN3O2: C 53.49, H 4.17, N 6.68; Found: C 53.52, H 4.12, N 6.71; 1H NMR (400 MHz, DMSO-d6) δ 12.12 (s, 1H), 8.43 (d, J = 5.8 Hz, 1H), 8.32 (d, J = 5.9 Hz, 1H), 8.24–8.13 (m, 2H), 7.97 (q, J = 7.6 Hz, 2H), 7.77 (d, J = 7.8 Hz, 2H), 7.46 (t, J = 6.6 Hz, 1H), 7.39 (t, J = 6.6 Hz, 1H), 6.82 (td, J = 7.4, 3.3 Hz, 2H), 6.67 (q, J = 7.3 Hz, 2H), 6.19 (d, J = 7.6 Hz, 1H), 6.01 (d, J = 7.5 Hz, 1H), 2.23 (dd, J = 12.0, 8.4 Hz, 1H), 1.46 (s, 1H), 1.30 (s, 1H), 0.81 (d, J = 6.7 Hz, 3H), 0.56 (d, J = 6.6 Hz, 3H). ESI-MS: m/z = 630.51 [M + H]+, 652.62 [M + Na]+; IR: (KBr, v[cm−1]): 3898 (w), 3820 (w), 3802 (w), 3744 (m), 3674 (w), 3651 (w), 3624 (w), 3590 (w), 3566 (w), 3040 (w), 2957 (m), 2926 (m), 1701 (w), 1632 (vs), 1611 (vs), 1584 (s), 1560 (m), 1541 (w), 1516 (w), 1477 (s), 1420 (m), 1308 (w), 1267 (w), 1225 (w), 1159 (w), 1061 (w), 1032 (w), 847 (w), 795 (w), 760 (s), 735 (m), 673 (w), 554 (w), 420 (w).
Λ-[Ir(ppy)2(Val-2H)]. Yield:97% (59 mg), Anal. Calcd for C27H24IrN3O2: C 52.75, H 3.94, N 6.84; Found: C 52.80, H 3.93, N 6.80; 1H NMR (400 MHz, DMSO-d6) δ 11.82 (s, 1H), 8.39 (d, J = 5.7 Hz, 1H), 8.27 (s, 1H), 8.19 (d, J = 8.2 Hz, 2H), 7.96 (q, J = 7.5 Hz, 2H), 7.76 (d, J = 7.7 Hz, 2H), 7.45 (t, J = 6.7 Hz, 1H), 7.40 (t, J = 6.7 Hz, 1H), 6.81 (q, J = 6.8 Hz, 2H), 6.66 (td, J = 7.3, 4.2 Hz, 2H), 6.20 (d, J = 7.6 Hz, 1H), 5.96 (d, J = 7.5 Hz, 1H), 3.13 (p, J = 6.9 Hz, 1H), 2.01 (q, J = 7.4 Hz, 1H), 1.14 (d, J = 7.0 Hz, 3H), 1.00 (d, J = 7.0 Hz, 3H). ESI-MS: m/z = 616.46 [M + H]+, 638.56 [M + Na]+; IR: (KBr, v[cm−1]): 3862 (w), 3743 (m), 3672 (w), 3649 (w), 3623 (w), 3042 (w), 2959 (m), 2924 (s), 2855 (m), 1701 (w), 1643 (m), 1607 (vs), 1584 (vs), 1477 (s), 1420 (m), 1393 (w), 1379 (w), 1309 (w), 1267 (w), 1225 (w), 1161 (w), 1128 (w), 1061 (w), 967 (w), 891 (w), 794 (w), 760 (s), 735 (w), 670 (w), 420 (m).

3.3. Kinetic Study of Photooxidative Dehydrogenation Reaction

A total of 0.05 mmol of the complex Λ-[Ir(ppy)2(L/D-AA)] was dissolved in 10 mL of ethanol. The solution was exposed to the light from a 10 W blue LED (λ = 450–470 nm) at room temperature under an atmosphere of O2. Samples of the reaction mixture were monitored at intervals of 2.5 h, and the solvent was removed under reduced pressure. The resulting solid was dissolved in deuterated DMSO, and the remaining amount of reactant at different time points in the system was monitored through 1H-NMR spectroscopy. The oxidative dehydrogenation reaction of amino acid complexes under visible light follows first-order kinetics [32], and it can be described by the rate equation ln(Ct/C0) = −k·t, where C0 is the initial concentration of the amino acid complex, Ct is the concentration of the amino acid complex at a specific time t, and Kobs is the rate constant obtained through regression analysis.

3.4. Crystal Structure of Complexes

The crystal data were collected using a Bruker Smart Apex CCD diffractometer equipped with a graphite monochromator and Mo Kα radiation (λ = 0.071 073 nm) at 293 K. The diffraction data were empirically corrected for absorption using the SADABS program. The crystal structures were solved by direct methods using the SHELXS [38] program and refined by least-squares methods using the SHELXL [39] program. The refinement included full-matrix least-squares corrections for all non-hydrogen atom coordinates and anisotropic thermal factors, with hydrogen atoms bonded to carbon added theoretically and those bonded to nitrogen and oxygen located based on the position of Q-peaks.

4. Conclusions

In this study, a series of chiral amino acid complexes Λ-[Ir(ppy)2(D/L-AA)] based on [Λ-Ir(ppy)2(MeCN)2](PF6) chiral template were synthesized in high a yield and optical purity. The above amino acid complexes could be oxidized under visible light to the corresponding imino acid complexes Λ-[Ir(ppy)2(AA-2H)], which show high selectivity in the dehydrogenation process without the formation of C-N bond byproducts. Crystal structure study shows that this is because the pyridine ring of the ppy ligand in the template complex is far from the nitrogen atom in the amino acid ligand. The photooxidative dehydrogenation rates of these complexes were studied, which show that D-configured amino acids exhibited faster dehydrogenation rates when using the Λ-configured complex as a chiral template and the substitution of electron-donating or bulky groups in the N-α position of the amino acid decreased their dehydrogenation rates. The crystal structures of the amino acid complexes based on threonine, Λ-Ir(ppy)2(D-Thr), and its corresponding dehydrogenated complex, Λ-Ir(ppy)2(Thr-2H), indicate the formation of dehydrogenated imino acid complex and the configuration stability of metal center during the reaction process. By synthesizing and studying the dehydrogenation property of these amino acid complexes, we can oxidize amino acids to the achiral imino acids more efficiently and further achieve the functionalization of amino acids or the kinetic resolution of two amino acid isomers.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/inorganics11100380/s1, Figure S1: ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(L/D-Ala)]; Figure S2: ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(L/D-Ser)]; Figure S3. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(D-Val)]; Figure S4. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(D-Leu)]; Figure S5. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(D-Thr)]; Figure S6. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(Ala-2H)]; Figure S7. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(Ser-2H)]; Figure S8. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(Val-2H)]; Figure S9. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(Leu-2H)]; Figure S10. ESI-MS characterization of chiral complexes Λ-[Ir(ppy)2(Thr-2H)]; Figure S11. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(L-Ala)]; Figure S12. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Ala)]; Figure S13. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(Ala-2H)]; Figure S14. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(L-Ser)]; Figure S15. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Ser)]; Figure S16. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(Ser-2H)]; Figure S17. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Val)]; Figure S18. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(Val-2H)]; Figure S19. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Leu)]; Figure S20. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(Leu-2H)]; Figure S21. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Thr)]; Figure S22. 1H NMR spectrum of chiral complexes Λ-[Ir(ppy)2(Thr-2H)]; Figure S23. 13C NMR spectrum of chiral complexes Λ-[Ir(ppy)2(L-Ala)]; Figure S24. 13C NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Ala)]; Figure S25. 13C NMR spectrum of chiral complexes Λ-[Ir(ppy)2(L-Ser)]; Figure S26. 13C NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Ser)]; Figure S27. 13C NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Val)]; Figure S28. 13C NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Leu)]; Figure S29. 13C NMR spectrum of chiral complexes Λ-[Ir(ppy)2(D-Thr)]; Table S1: Bond lengths (Å) and bond angles (°) for Complexes Λ-[Ir(ppy)2(D-Thr)]·CH3OH and Λ-[Ir(ppy)2(Thr-2H)]; Figure S30. (Left) Intercept 1H NMR (DMSO-d6) spectra of dehydrogenation process of Λ-[Ir(ppy)2(D-Ser)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(D-Ser)]; Figure S31. (Left) Intercept 1H NMR (DMSO-d6) spectra of dehydrogenation process of Λ-[Ir(ppy)2(L-Ser)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(L-Ser)]; Figure S32. (Left) Intercept 1H NMR (DMSO-d6) spectra of dehydrogenation process of Λ-[Ir(ppy)2(D-Val)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(D-Val)]; Figure S33. (Left) Intercept 1H NMR (DMSO-d6) spectra of dehydrogenation process of Λ-[Ir(ppy)2(D-Leu)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(D-Leu)]; Figure S34. (Left) Intercept 1H NMR (DMSO-d6) spectra of dehydrogenation process of Λ-[Ir(ppy)2(D-Thr)] at different time. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(D-Thr)]; Figure S35. IR spectra of threonine, [Λ-Ir(ppy)2(MeCN)2](PF6), Λ-Ir(ppy)2(Thr-2H) and [Λ-[Ir(ppy)2(D-Thr)].

Author Contributions

Conceptualization, B.Y. and S.Y.; methodology, S.Y.; formal analysis, M.C. and S.Y.; investigation, Y.P. and L.R.; data curation, Y.P. and L.R.; writing—original draft preparation, S.Y.; writing—review and editing, M.C. and B.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Science and Technology Research Project of Guangzhou, (grant number 202102021122); the Guangdong Provincial research platforms and projects of universities (no. 2019KQNCX101, no. 2021KTSCX086);the Fund for the Quality and Reform Project of Guangdong Province Undergraduate Teaching (no. XQSYS-2222873).

Data Availability Statement

The data presented in this study are available in this article and Supplementary Materials.

Conflicts of Interest

The authors declare no conflict of interest.

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Inorganics 11 00380 sch001
Scheme 1. Synthesis of chiral amino acid complexes Λ-[Ir(ppy)2(L/D-AA)] with different amino acids.
Scheme 1. Synthesis of chiral amino acid complexes Λ-[Ir(ppy)2(L/D-AA)] with different amino acids.
Inorganics 11 00380 sch001
Inorganics 11 00380 g001
Figure 1. Intercept 1H NMR (DMSO-d6) spectra of chiral complexes based on alanine: (a) Λ-[Ir(ppy)2(L-Ala)], (b) Λ-[Ir(ppy)2(D-Ala)] and (c) Λ-[Ir(ppy)2(Ala-2H)].
Figure 1. Intercept 1H NMR (DMSO-d6) spectra of chiral complexes based on alanine: (a) Λ-[Ir(ppy)2(L-Ala)], (b) Λ-[Ir(ppy)2(D-Ala)] and (c) Λ-[Ir(ppy)2(Ala-2H)].
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Figure 2. Mass spectra of chiral complexes based on alanine. (a) Λ-[Ir(ppy)2(L/D-Ala)], (b) Λ-[Ir(ppy)2(Ala-2H)].
Figure 2. Mass spectra of chiral complexes based on alanine. (a) Λ-[Ir(ppy)2(L/D-Ala)], (b) Λ-[Ir(ppy)2(Ala-2H)].
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Inorganics 11 00380 g003
Figure 3. Intercept 1H NMR (DMSO-d6) spectra of chiral complexes based on serine: (a) Λ-[Ir(ppy)2(L-Ser)], (b) Λ-[Ir(ppy)2(D-Ser)], and (c) Λ-[Ir(ppy)2(Ser-2H)].
Figure 3. Intercept 1H NMR (DMSO-d6) spectra of chiral complexes based on serine: (a) Λ-[Ir(ppy)2(L-Ser)], (b) Λ-[Ir(ppy)2(D-Ser)], and (c) Λ-[Ir(ppy)2(Ser-2H)].
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Inorganics 11 00380 sch002
Scheme 2. (a,b): Studies about the oxidative rate constants of coordinated amino acids.
Scheme 2. (a,b): Studies about the oxidative rate constants of coordinated amino acids.
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Figure 4. (Left) Intercept 1H NMR (DMSO-d6) spectra of dehydrogenation process of Λ-[Ir(ppy)2(L-Ala)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(L-Ala)].
Figure 4. (Left) Intercept 1H NMR (DMSO-d6) spectra of dehydrogenation process of Λ-[Ir(ppy)2(L-Ala)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(L-Ala)].
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Inorganics 11 00380 g005
Figure 5. (Left) Intercept 1H NMR (DMSO-d6) spectra of the dehydrogenation process of Λ-[Ir(ppy)2(D-Ala)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(D-Ala)].
Figure 5. (Left) Intercept 1H NMR (DMSO-d6) spectra of the dehydrogenation process of Λ-[Ir(ppy)2(D-Ala)] at different times. (Right) Dehydrogenation rate constant of Λ-[Ir(ppy)2(D-Ala)].
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Figure 6. Molecular structures of Λ-[Ir(ppy)2(D-Thr)]·CH3OH (a) and Λ-[Ir(ppy)2(Thr-2H)] (b) with 50% probability ellipsoids.
Figure 6. Molecular structures of Λ-[Ir(ppy)2(D-Thr)]·CH3OH (a) and Λ-[Ir(ppy)2(Thr-2H)] (b) with 50% probability ellipsoids.
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Table 1. Dehydrogenation yields and rates of different amino acid complexes.
Table 1. Dehydrogenation yields and rates of different amino acid complexes.
Amino AcidYield a (%)Kobs (h −1)
D-Ala970.205
L-Ala960.063
D- Ser930.111
L-Ser930.074
D-Val970.179
D-Leu940.169
D-Thr950.192
a Isolated yield.
Table 2. Crystallographic data for Λ-[Ir(ppy)2(D-Thr)]·CH3OH and Λ-[Ir(ppy)2(Thr-2H)].
Table 2. Crystallographic data for Λ-[Ir(ppy)2(D-Thr)]·CH3OH and Λ-[Ir(ppy)2(Thr-2H)].
ComplexΛ-[Ir(ppy)2(D-Thr)]·CH3OH (1)Λ-[Ir(ppy)2(Thr-2H)] (2)
Empirical formulaC27H28IrN3O4C26H22IrN3O3
Formula weight650.72616.66
Temperature293(2) K293(2) K
Wavelength1.54184 Å1.54184 Å
Crystal system, space groupOrthorhombic, P212121Orthorhombic, P212121
Unit cell dimensionsa = 10.3001(1) Å, α = 90°a = 9.3943(1) Å, α = 90°
b = 10.5102(1) Å, β = 90°b = 11.1574(1) Å, β = 90°
c = 23.0301(1) Å, γ = 90°c = 22.1602(2)Å, γ = 90°
Volume2493.15(4) Å32322.73(4) Å3
Z, calculated density4, 1.734 Mg/m34, 1.763 Mg/m3
Absorption coefficient10.682 mm−111.397 mm−1
F(000)12801200
Crystal size0.210 × 0.190 × 0.180 mm0.240 × 0.220 × 0.160 mm
Theta range for data collection3.839 to 76.808 deg.3.990 to 76.773 deg.
Limiting indices−9 ≤ h ≤ 12, −12 ≤ k ≤ 13, −28 ≤ l ≤ 28−11 ≤ h ≤ 10, −13 ≤ k ≤ 8, −27 ≤ l ≤ 27
Reflections collected/unique17,746/5008 [R(int) = 0.0380]16,219/4553 [R(int) = 0.0370]
Completeness to theta = 67.684100.00%100.00%
Absorption correctionSemi-empirical from equivalentsSemi-empirical from equivalents
Max. and min. transmission0.249 and 0.2130.263 and 0.171
Refinement methodFull-matrix least-squares on F2Full-matrix least-squares on F2
Data/restraints/parameters5008/0/3164553/0/299
Goodness-of-fit on F21.0021.011
Final R indices [I > 2 sigma(I)]R1 = 0.0230, wR2 = 0.0586R1 = 0.0254, wR2 = 0.0638
R indices (all data)R1 = 0.0234, wR2 = 0.0588R1 = 0.0262, wR2 = 0.0642
Absolute structure parameter−0.016(5)−0.020(6)
Largest diff. peak and hole0.952 and −0.627 e Å−30.952 and −0.627 e Å−3
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Yao, S.; Pu, Y.; Ren, L.; Cao, M.; Ye, B. Photooxidative Dehydrogenation of Chiral Ir (III) Amino Acid Complexes Based on [Λ-Ir(ppy)2(MeCN)2](PF6). Inorganics 2023, 11, 380. https://doi.org/10.3390/inorganics11100380

AMA Style

Yao S, Pu Y, Ren L, Cao M, Ye B. Photooxidative Dehydrogenation of Chiral Ir (III) Amino Acid Complexes Based on [Λ-Ir(ppy)2(MeCN)2](PF6). Inorganics. 2023; 11(10):380. https://doi.org/10.3390/inorganics11100380

Chicago/Turabian Style

Yao, Suyang, Yanxi Pu, Lulu Ren, Manli Cao, and Baohui Ye. 2023. "Photooxidative Dehydrogenation of Chiral Ir (III) Amino Acid Complexes Based on [Λ-Ir(ppy)2(MeCN)2](PF6)" Inorganics 11, no. 10: 380. https://doi.org/10.3390/inorganics11100380

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