Synthesis of a Ru(II) Complex with a Naphthoquinone-Annelated Imidazole Ligand Exhibiting Proton-Responsive Redox and Luminescent Behavior

A mononuclear ruthenium complex, [Ru(L)(bpy)2](PF6), with a naphthoquinone-annelated imidazole ligand HL (2-(pyridin-2-yl)-1H-naphtho[2,3-d]imidazole-4,9-dione) was synthesized and structurally characterized. Electrochemical study reveals that the Ru complex shows four reversible redox waves at +0.98 V, −1.13 V, −1.53 V, and −1.71 V versus SCE in acetonitrile, which are assigned to Ru(II)/Ru(III), L−/L•2−, and two bpy/bpy•− redox couples, respectively. The redox potential of Ru(II)/Ru(III) was positively shifted upon the addition of trifluoromethanesulfonic acid due to protonation of the L− moiety, leading to stabilization of the Ru 4d orbital. In UV-vis absorption measurements for the Ru complex in acetonitrile, a metal-to-ligand charge transfer (MLCT) band was observed at 476 nm, which was shifted to 450 nm by protonation, which might be due to a decrease in the electron delocalization and stabilization of the π orbitals in L−. The blue shift of the MLCT band by protonation was associated with a shift of an emission band from 774 nm to 620 nm, which could be caused by the decreased electronic delocalization in the MLCT excited state. These electrochemical and spectroscopic changes were reversible for the protonation/deprotonation stimuli.


Introduction
Stimuli-responsive materials have attracted considerable attention for their potential application as smart materials in the field of optoelectronics and life science [1][2][3]. Various external stimuli, such as temperature, pressure, and light, have been utilized to control the physical properties of metal complexes [4,5]. In addition, chemical stimuli, such as guest molecules/ions, humidity, and protons, have been employed as a trigger of the electronic and structural changes [6][7][8][9][10]. Among them, the proton is one of the easy-to-use triggers, inducing physical property changes, and many proton-responsive compounds have been intensively studied. pH-responsive luminescence materials have been widely investigated from the viewpoint of environmental and life science [11,12]. On the other hand, protons play an important role in biological systems, in which redox behavior is significantly altered through the proton-coupled electron transfer (PCET) process [13,14], and the pH-dependent redox reactions of ruthenium complexes have been intensively studied to understand the interplay between electrons and protons [15][16][17][18]. In the field of solid state chemistry, proton dynamics and modulation of hydrogen bonds have been extensively studied in various types of switching materials [19][20][21][22], and many interesting physical properties, such as protonation-affected spin transition behavior in metal complexes [23][24][25][26], solid state proton transfer induced by spin-crossover phenomena [27], and proton dynamics-directed phase transition in conducting organic molecules [28]. We have previously reported Fe(II) complexes with a naphthoquinone-annelated imidazolate ligand L − (HL = 2-(pyridin-2-yl)-1H-naphtho [2,3-d]imidazole-4,9-dione), [Fe II (L) 2 (L 1−4 ) 2 ] (L 1 = ethylenediamine, L 2 = cyclohexanediamine, L 3 = diamino benzoquinone, L 4 = iminosuccinonitrile), and their spin-crossover behavior in solid state was reported [29]. The naphthoquinone moiety in HL is expected to show a proton response and redox activities and to be a good building block for the proton-responsive coordination complex. On the other hand, ruthenium complexes have been afforded a family of luminescent and redox-active systems originating from their high thermal stability and strong spin-orbit coupling [30][31][32][33]. Various functional ligands, such as 1,10phenanthroline, 2,9-di(pyrid-2 -yl)-1,10-phenanthroline, 2,6-di(quinoline-8-yl)pyridine, and 2-(2 -pyridyl)naphthoimidazole, have been employed to modify the molar absorptivity, emission energy, and excited-state lifetime of the Ru complexes [34][35][36][37][38]. From the viewpoints described above, exploration of the functional ligands for Ru complexes is important. The naphthoquinone-annelated multidentate ligand could show various electronic states by protonation and redox process and be useful for switching the functions of the Ru complexes. We report here a mononuclear ruthenium complex, [Ru II (L)(bpy) 2 ](PF 6 ) (1), with naphthoquinone-annelated imidazole ligand L − (HL = 2-(pyridin-2-yl)-1H-naphtho [2,3d]imidazole-4,9-dione), exhibiting proton-responsive redox and emission properties. (1) Single crystals of complex 1 suitable for X-ray diffraction measurements were obtained by recrystallization of 1 in MeOH/EtOAc. Complex 1 crystallized in the monoclinic space group P2 1 /c and consists of one complex cation, [Ru(L)(bpy) 2 ] + , and one PF 6 − anion. In the crystal lattice, there is one AcOEt molecule and half of an MeOH molecule per one complex cation ( Figure 1, Table 1). In [Ru(L)(bpy) 2 ] + , deprotonated naphthoquinone-annelated imidazolate, L − , acts as a mono-anionic bidentate ligand and coordinates with the ruthenium ion. The ruthenium ion has an octahedral coordination geometry with N 6 donor atoms, of which the four are bipyridine nitrogen atoms and the two are nitrogen atoms of pyridine and imidazolate moieties donated by the L − ligand. Thus, the coordination environment can be regarded as five pyridines and one imidazolate functionality. The two C-N bond distances of the imidazolate moiety (C6-N2: 1.374 Å, C6-N3: 1.327 Å) suggests that the deprotonated imidazolate group coordinates to ruthenium ion. By considering the above structural features, 1 is a ruthenium complex containing a Ru(II) ion with a chemical formula of [Ru(L)(bpy) 2 ](PF 6 ) . AcOEt . 0.5MeOH. The ligand reveals a novel binding behavior of the imidazole entity. So far, the coordination of an imine nitrogen was observed, exemplarily found in the respective naphthyl analogue complex [Ru(II)(bpy) 2 (PYNI)](ClO 4 ) 2 (PYNI = 2-(2 -pyridyl)naphthoimidazole)) [37]. Instead, complex 1 has an imidazolate coordination site, formed upon deprotonation. This fact was confirmed by IR data, in which there is no N-H stretch band of the imidazole group.

Electrochemistry
The electrochemical properties of HL and complex 1 were investigated by cyclic voltammetry (CV). The resultant cyclic voltammograms are shown in Figure 2, and the redox potentials (E 1/2 ) and their assignments are provided in Table 2. HL shows two reversible redox waves at E 1/2 = −0.63 and −1.30 V versus SCE, which are assigned to the two-step reduction in naphthoquinone moieties through a formation of a semiquinone radical anion as an intermediate [39]. Complex 1 shows one oxidation wave at +0.98 V, and three reduction waves at −1.13 V, −1.53 V, and −1.71 V. The reversible oxidation wave corresponds to a Ru(II)/Ru(III) redox couple characteristic of Ru(II) complexes with diimine ligands [40]. It is well known that the Ru(II) complex with two bpy ligands, [Ru(bpy) 2 (L')] (L' = polypyridine-type ligands), exhibit two successive reduction process at ca. −1.4 and ca. −1.8 V due to the stepwise reduction of two bpy ligands [40][41][42][43][44]. The redox potentials of the second and third reduction waves in 1 are comparative to the reported reduction Inorganics 2021, 9, 24 3 of 10 potentials in [Ru(bpy) 2 (L')]; therefore, they can be assigned to the two bpy/bpy •− redox couples. Considering the redox potential of bpy in the [Ru(bpy) 3 ] 2+ complex (−1.36, −1.52, −1.76 V vs. SCE), the second and third reduction waves for complex 1 correspond to a two-step reduction of bpy/bpy •− . The first reduction wave at −1.13 V is assignable to a reduction of the naphthoquinone moiety in L − by considering the redox behavior of HL. Note that HL shows two large reduction peaks at −0.63 and −1.30 V and one small peak at −1.1 V. The main two reduction peaks can be assigned as HL/L •− and HL •− /HL 2− , while the small peak was originated from the existence of a dimer of the neutral-anion radical complex [45]. The first redox potential of the naphthoquinone moiety in 1 was negatively shifted compared with that in the protonated HL, which could be due to the deprotonation of the imidazole moiety in 1. Consequently, the three-step reduction process in 1 is originating from the reversible reduction reactions of L − /L •2− , and two bpy/bpy •− , respectively. These redox potentials of Ru(II)/Ru(III) and two bpy ligands are close to the values of the redox potentials for the naphthyl analogs [Ru(II)(bpy) 2 (PYNI)](ClO 4 ) 2 (Ru(II)/Ru(III) 1,16 V vs. SCE; bpy/bpy •− −1.44 and −1.68 V vs. SCE) [37].

Electrochemistry
The electrochemical properties of HL and complex 1 were investigated by cyclic voltammetry (CV). The resultant cyclic voltammograms are shown in Figure 2, and the redox potentials (E1/2) and their assignments are provided in Table 2. HL shows two reversible redox waves at E1/2 = −0.63 and −1.30 V versus SCE, which are assigned to the two-step reduction in naphthoquinone moieties through a formation of a semiquinone radical an-

UV-Vis and Emission Spectra
Absorption spectra of complex 1 in acetonitrile are shown in Figure 3a. T intense absorption bands were observed around 300 nm, with the shoulders in t of 300-350 nm, while two characteristic bands were observed at 444 and [Ru II (bpy)3] 2+ is reported to show a -* transition band at 285 nm and a Ru(d) → metal-to-ligand charge transfer (MLCT) band at 458 nm [35]. By considering the signments in [Ru(bpy)3] 2+ and relating complexes [35], the most intense absorptio around 300 nm in 1 originate from the -* transition bands in two bpy moietie lower energy absorption bands at 300-350 nm are assignable to the -* transitio in the L − moiety. HL and L − showed -* transition bands at 382 nm and 450 nm tively, which are lower in energy than that in bpy. These data supported the assi mentioned above. These assignments were supported by the electrochemical dat tion 2.2. The reduction potential of L − is more positive than that of bpy, indica lower-lying * orbitals in L − , which may be due to the extended  conjugated stru   3 ] 2+ (versus SCE). The difference between E p,c (peak potential of cathodic current) and E p,a (peak potential of anodic current) is shown in parenthesis.

UV-Vis and Emission Spectra
Absorption spectra of complex 1 in acetonitrile are shown in Figure 3a. The most intense absorption bands were observed around 300 nm, with the shoulders in the range of 300-350 nm, while two characteristic bands were observed at 444 and 476 nm. [Ru II (bpy) 3 ] 2+ is reported to show a π-π* transition band at 285 nm and a Ru(dπ) → bpy(π*) metal-toligand charge transfer (MLCT) band at 458 nm [35]. By considering the band assignments in [Ru(bpy) 3 ] 2+ and relating complexes [35], the most intense absorption bands around 300 nm in 1 originate from the π-π* transition bands in two bpy moieties, while lower energy absorption bands at 300-350 nm are assignable to the π-π* transition bands in the L − moiety. HL and L − showed π-π* transition bands at 382 nm and 450 nm, respectively, which are lower in energy than that in bpy. These data supported the assignments mentioned above. These assignments were supported by the electrochemical data in Section 2.2. The reduction potential of L − is more positive than that of bpy, indicating the lower-lying π* orbitals in L − , which may be due to the extended π conjugated structure of L − and cause the lower π-π* excitation energy in the L − moiety. The two absorption bands at 444 and 476 nm can be assigned to Ru(dπ) → bpy(π*) and Ru(dπ) → L − (π*) MLCT because the stabilized π* bands in L − should cause the lower energy MLCT band than that in bpy. This assignment is supported by the TD-DFT calculation on 1 ( Figure S4, Table S1). The calculated transition bands at 434 nm and 428 nm are assigned to MLCT Ru/bpy. On the other hand, the transition bands at 529 nm and 526 nm have a contribution of MLCT Ru/L − . These lower energy transition bands appeared in the observed UV spectra. Emission spectra of 1 in acetonitrile were recorded and displayed in Figure 3b. Excitation of the MLCT bands gives rise to a broad emission band at 774 nm, which could be due to luminescence from the 3 MLCT excited state originating from the lowest energy Ru(dπ) → L − (π*) excited state [35,37]. The relatively large Stokes shift (3.36 × 10 4 cm −1 , 298 nm) in 1 may be due to the charge delocalization on the naphthoquinone-annelated imidazole moiety in the excited state.
MLCT Ru/L − . These lower energy transition bands appeared in the observed UV spectra. Emission spectra of 1 in acetonitrile were recorded and displayed in Figure 3b. Excitation of the MLCT bands gives rise to a broad emission band at 774 nm, which could be due to luminescence from the 3 MLCT excited state originating from the lowest energy Ru(d) → L − (*) excited state [35,37]. The relatively large Stokes shift (3.36 × 10 4 cm −1 , 298 nm) in 1 may be due to the charge delocalization on the naphthoquinone-annelated imidazole moiety in the excited state.

Effects of Protonation on the Electronic Structure
UV-vis absorption spectra were measured upon the addition of trifluoromethanesulfonic acid (TfOH) to a solution of 1 in MeCN, and the spectral change is shown in Figure  4. It should be noted that the spectral change showed isosbestic points at 307, 408, and 462 nm, which were not changed when increasing the amount of TfOH up to 1.6 eq. This result suggested that the spectral change is caused by a one-step protonation of the Lmoiety to yield a singly protonated 1 (H + −1). With increasing the amount of TfOH, the -*(L − ) and Ru(d) → L − (*) MLCT absorption bands (290 nm and 478 nm) were shifted to higher energy region (286 nm and 450 nm), due to the protonation of the L − moiety in H + −1. The protonation may cause a decrease in electron delocalization due to a decrease in the resonance structures and stabilization of the  orbitals in L − , leading to a higher excitation energy in the -* and Ru(d) → L − (*) MLCT transitions. The results of the TD-DFT calculations for the geometry-optimized H + −1 support the shift of the transition bands (Figure S5, Table S2). The electronic structure change of 1 upon protonation was proved by emission spectroscopy measurements.

Effects of Protonation on the Electronic Structure
UV-vis absorption spectra were measured upon the addition of trifluoromethanesulfonic acid (TfOH) to a solution of 1 in MeCN, and the spectral change is shown in Figure 4. It should be noted that the spectral change showed isosbestic points at 307, 408, and 462 nm, which were not changed when increasing the amount of TfOH up to 1.6 eq. This result suggested that the spectral change is caused by a one-step protonation of the L − moiety to yield a singly protonated 1 (H + −1). With increasing the amount of TfOH, the π-π*(L − ) and Ru(dπ) → L − (π*) MLCT absorption bands (290 nm and 478 nm) were shifted to higher energy region (286 nm and 450 nm), due to the protonation of the L − moiety in H + −1. The protonation may cause a decrease in electron delocalization due to a decrease in the resonance structures and stabilization of the π orbitals in L − , leading to a higher excitation energy in the π-π* and Ru(dπ) → L − (π*) MLCT transitions. The results of the TD-DFT calculations for the geometry-optimized H + −1 support the shift of the transition bands ( Figure S5, Table S2). The electronic structure change of 1 upon protonation was proved by emission spectroscopy measurements. Inorganics 2021, 9, x FOR PEER REVIEW The 3 MLCT emission band at 774 nm was shifted to 620 nm upon addition (Figure 5a), corresponding to the blue shift of the MLCT absorption bands by pro of L − . It should be noted that the protonated state shows a smaller Stokes shift of 5 cm −1 (170 nm), which could be caused by the decreased electronic delocalization moiety of H + −1. The emission band of H + −1 is close to that of [Ru(bpy)3] 2+ , suggesti sion from MLCT Ru/bpy. The spectroscopic changes due to protonation and de tion were reversible upon additions of TfOH and Et3N ( Figure S6, Supplementa rials). In CV, the redox wave of Ru(II)/Ru(III) couple was positively-shifted from to +1.23 V by the addition of 1.6 eq. of TfOH (Figure 5b). The stabilization of th In CV, the redox wave of Ru(II)/Ru(III) couple was positively-shifted from +0.98 V to +1.23 V by the addition of 1.6 eq. of TfOH (Figure 5b). The stabilization of the Ru 4d orbital is ascribed to the electron-withdrawing effects from the protonated L − moiety in H + −1. CV data of H + −1 in the potential range of −2.0 − + 1.5 V is shown in Figure S7 (Supplementary Materials), in which H + −1 showed a complicated reduction processes due to adsorption of the reduced H + −1 on the electrode surface. Related works on pH-sensitive ruthenium(II) complexes [Ru(bpy) 2 L im ] n+ bearing imidazole-type ligands L im have been reported [39,40]. In these works, the emission intensity decrease, and a slight red shift is observed with increasing the pH value. In the present work, a similar responsivity was observed; however, detailed understandings require the isolation of H + −1 and theoretical calculations. The 3 MLCT emission band at 774 nm was shifted to 620 nm upon addition of TfOH (Figure 5a), corresponding to the blue shift of the MLCT absorption bands by protonation of L − . It should be noted that the protonated state shows a smaller Stokes shift of 5.88 × 10 4 cm −1 (170 nm), which could be caused by the decreased electronic delocalization in the L − moiety of H + −1. The emission band of H + −1 is close to that of [Ru(bpy)3] 2+ , suggesting emission from MLCT Ru/bpy. The spectroscopic changes due to protonation and deprotonation were reversible upon additions of TfOH and Et3N ( Figure S6, Supplementary Materials). In CV, the redox wave of Ru(II)/Ru(III) couple was positively-shifted from +0.98 V to +1.23 V by the addition of 1.6 eq. of TfOH (Figure 5b). The stabilization of the Ru 4d orbital is ascribed to the electron-withdrawing effects from the protonated L − moiety in H + −1. CV data of H + −1 in the potential range of −2.0 − + 1.5 V is shown in Figure S7 (Supplementary Materials), in which H + −1 showed a complicated reduction processes due to adsorption of the reduced H + −1 on the electrode surface. Related works on pH-sensitive ruthenium(II) complexes [Ru(bpy)2L im ] n+ bearing imidazole-type ligands L im have been reported [39,40]. In these works, the emission intensity decrease, and a slight red shift is observed with increasing the pH value. In the present work, a similar responsivity was observed; however, detailed understandings require the isolation of H + −1 and theoretical calculations.

Materials
All solvents and chemicals were reagent grade, purchased commercially, and used without further purification unless otherwise noted. The naphthoquinone-annelated lig-

Microwave Reaction
Reactions were performed using a CEM Discover microwave unit. Reactions were performed in 10 mL capacity sealed tubes. The contents of the reaction vessel were stirred using an electromagnet located below the floor of the microwave cavity and a Teflon-coated magnetic stir bar in the vessel. [Ru(L)(bpy) 2 ](PF 6 ) . 2H 2 O (1 . 2H 2 O) was prepared by microwave-assisted synthesis.
[RuCl 2 (bpy) 2 ] (34.69 mg, 0.067 mmol) and HL (18.35 mg, 0.067 mmol) were dissolved in ethylene glycol (5 mL) under nitrogen, and the mixture was irradiated by microwave for 15 min at 210 • C (200 W). The cooled reaction mixture was diluted with water (16.7 mL), followed by filtration to remove the insoluble precipitate to obtain a dark-red solution. The addition of an excess amount of NaPF 6 to the filtrate resulted in a reddish-brown suspension. The reddish-brown powder of the crude product was collected and washed with water (47.8 mg, yield: 86%). The obtained reddish-brown powder was dissolved in MeOH. The solution was filtered, and the filtrate was carefully layered with AcOEt. After several days, Xray quality red needle crystals of [Ru(L)(bpy) 2 ](PF 6 ) . AcOEt . 0.5MeOH (1 . AcOEt . 0.5MeOH) were collected by filtration, and dried in the air, affording [Ru(L)(bpy) 2 ](PF 6

X-ray Crystallography
Crystals were mounted on a micromount, and the data were collected at −173 • C (Bruker SMART APEXII diffractometer coupled with a CCD area detector with graphite monochromated MoKα (λ = 0.71073 Å radiation). The structure was solved using direct methods and expanded using Fourier techniques within the SHELXTL program [49]. Empirical absorption corrections were calculated using SADABS [50]. In the structure analyses, non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were included in the calculated positions and refined with the isotropic thermal parameters riding on those of the parent atoms. Further crystallographic details can be found in the corresponding CIF file provided in the ESI. Crystallographic data for the structure reported in this work were deposited at the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication number: CCDC-2058847. Copies of the data can be obtained online using https://summary.ccdc.cam.ac.uk/structure-summary-form (accessed on 30 January 2021).

Physical Measurements
Elemental analyses were performed using a Perkin-Elmer 2400 II CHN for the C, H, and N elements. Infrared absorption spectra were measured on KBr pellet samples using a SHIMADZU IRAffinity-1 spectrometer. UV-vis absorption spectra of MeCN solution in quartz cell with a path length of 1 cm were recorded on a SHIMADZU UV-3150 spectrometer. Fluorescence spectra were measured by a JASCO FP-8500 spectrofluorometer at room temperature. Measurements were performed using a degassed solvent. MALDI-TOF-MS measurements were performed using a time of flight (TOF) mass spectrometer (AB SCIEX TOF/TOF 5800 system) equipped with a matrix-assisted laser/desorption ionization (MALDI) source.
Titration experiments were performed as follows. A solution of 1 in MeCN was titrated by TfOH, and the spectra of the solution were recorded every 0.2 eq. of TfOH added.

Cyclic Voltammetry
Cyclic voltammetry measurements were carried out in a standard one compartment cell under N 2 at 20 • C equipped with a platinum wire counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and a glassy carbon (GC) working electrode using an ALS BAS 620A electrochemical analyzer. The measurements were performed in acetonitrile with 0.1 M tetra-n-butylammonium hexafluorophosphate (N n Bu 4 PF 6 ) as the supporting electrolyte. The concentration of the complex 1 was 1 mM. The scan rate was 100 mV/sec.

DFT Calculations
DFT calculations were carried out using the B3LYP exchange correlation function and implemented in the Gaussian 16 program package [51,52]. The electronic structures of the complexes were determined using a general basis set with the Los Alamos effective core potential LanL2DZ basis set [53][54][55]. Low-lying excitation energies were calculated with time-dependent DFT (TD-DFT). The conductor polarizable continuum model (CPCM) was used to account for solvent effects on the electronic transitions [56]. The Cartesian coordinates of the geometry-optimized 1 and H+−1 are shown in Tables S3 and S4.

Conclusions
We have synthesized a mononuclear ruthenium complex 1 with a naphthoquinoneannelated imidazole ligand HL. The single-crystal X-ray structure analysis revealed that 1 consists of a central Ru ion, a redox-active naphthoquinone moiety, and a protonsensitive imidazolate moiety. In the electrochemical study, 1 showed a reversible one-step Ru(II)/Ru(III) oxidation and a three-step reduction containing a reversible reduction of the naphthoquinone moiety. In addition, a characteristic broad emission from an excited state is generated by the Ru(II) to L − charge transfer. Consequently, 1 was proven to exhibit a characteristic redox and emission behavior based on the naphthoquinone moiety as an electron acceptor. Modulations of the redox and emission behavior were investigated by protonation. As a result, we found a significant change in the electronic structure of 1 by adding 1.6 eq. of TfOH, which is caused by the decrease in electronic delocalization in the L − moiety by protonation. These results demonstrated that the naphthoquinoneannelated imidazole ligand is useful to develop emission and redox switching materials by protonation. Taking advantage of molecular designs based on both redox-active and proton-responsive behavior, further work will pave the road towards the design and development of diverse molecular-based functional materials with stimuli response.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/inorganics9040024/s1, X-Ray crystallographic data for complex 1; the CIF and the checkCIF output files are included in the Supplementary Materials. Figure S1. 1 H and 13 C NMR spectra of 1 in CD 3 CN-d 3 . Figure S2. IR spectra of HL (green) and complex 1 (red). Figure S3. UV-Vis absorption spectra of HL and L − in DMF (left) and cyclic voltammograms of HL and L − in CH 2 Cl 2 (left). Figure  S4. Calculated frontier orbitals for the optical transitions observed in 1. Orbitals are shown with an iso-value of 0.06. Figure S5 Calculated frontier orbitals for the optical transitions observed in H + -1. Orbitals are shown with an iso-value of 0.06. Figure S6 UV-Vis absorption spectral changes of complex 1 with Et 3 N after addition of 1.6 eq. TfOH. Figure S7 Cyclic voltammograms of complex 1 in acetonitrile before and after the addition of 1.6 eq. of TfOH. Table S1. Main TD-DFT calculated transition of 1 compared with experimental data. Table S2. Main TD-DFT calculated transition of H + -1 compared with experimental data. Table S3. Cartesian coordinates of geometry optimized 1. Table S4 Cartesian coordinates of geometry optimized H + -1.