Schiff Base in Ketoamine Form and Rh(η4-cod)-Schiff Base Complex with Z′ = 2 Structure from Pairwise C-H···Metallochelate-π Contacts

Condensation of 2-hydroxybenzaldehyde (salicylaldehyde) or 2-hydroxy-1-naphthaldehyde with 2-ethylaniline yields the Schiff base compound of (E)-2-(((2-ethylphenyl)imino)methyl)phenol (HL1) or (E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-ol (HL2), which in turn react with the dinuclear complex of [Rh(η4-cod)(µ-O2CCH3)]2 (cod = cycloocta-1,5-diene) to afford the mononuclear (η4-cod){(E)-2-(((2-ethylphenyl)imino)methyl)phenolato-κ2N,O}rhodium(I), [Rh(η4-cod)(L1)] (1) or (η4-cod){(E)-1-(((2-ethylphenyl)imino)methyl)naphthalen-2-olato-κ2N,O}rhodium(I), [Rh(η4-cod)(L2)] (2) (L1 or L2 = deprotonated Schiff base ligand). The X-ray structure determination revealed that the HL2 exists in the solid state not as the usual (imine)N···H-O(phenol) form (enolamine form) but as the zwitterionic (imine)N-H+···–O(phenol) form (ketoamine form). 1H NMR spectra for HL2 in different solvents demonstrated the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in solution. The structure for 1 and 2 showed that the deprotonated Schiff base ligand coordinates to the Rh(η4-cod)-fragment as a six-membered N^O-chelate around the rhodium atom with a close-to-square-planar geometry. Two symmetry-independent molecules (with Rh1 and Rh2) were found in the asymmetric unit in 1 in a structure with Z’ = 2. The supramolecular packing in HL2 was organized by π-π and C-H···π contacts, while only two recognized C-H···π contacts were revealed in 1 and 2. Remarkably, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetry-independent molecules in 1. This pairwise C-H contact to the Rh-N^O chelate (metalloaromatic) ring may be a reason for the two symmetry-independent molecules in 1. Differential scanning calorimetry (DSC) analyses revealed an irreversible phase transformation from the crystalline-solid to the isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds. Absorption spectra in solution were explained by excited state properties from DFT/TD-DFT calculations.

The present paper, in continuation, reports the results of synthesis, spectroscopy and molecular structures of the N,O-chelate Schiff bases (HL 1 or HL 2 ) and their complexes of [Rh(η 4 -cod)(L 1 )] (1) or [Rh(η 4 -cod)(L 2 )] (2) (Scheme 1). The molecular structures for HL 2 , 1 and 2 were elucidated by single-crystal X-ray diffraction and are discussed along with the supramolecular packing analysis. 1 H NMR studies revealed HL 2 to exhibit keto-enol tautomerism in solution, while at solid-state, it remains as the zwitterionic (imine)N-H + ··· -O(phenol) (ketoamine form). The optimized geometry and excited state properties were studied by DFT/TD-DFT and compared with the experimental results. Scheme 1. Formula of HL 1 , HL 2 (with NMR atom numbering), 1 and 2.

UV-Vis Spectra and Excited State Properties
Absorption spectra for the Schiff base (HL 1 ) and complexes (1 and 2) in chloroform are shown in Figure 2. The spectra for the complexes were almost identical, while they were a bit different from that of the Schiff base. The Schiff base showed three very strong bands below 400 nm with absorption maxima (λ max ) at 227 nm (ε max = 12,440 L mol −1 cm −1 ), 267 nm (ε max = 8000 L mol −1 cm −1 ) and 341 nm (ε max = 7030 L mol −1 cm −1 ) for different intra-ligand π→π * /n→π * transitions (LL) [17][18][19][20][21]. The complexes showed these intra-ligand transitions bands below 365 nm with absorption maxima at λ max = 323 nm (ε max = 8163 L mol −1 cm −1 ) and 275 nm (9413 L mol −1 cm −1 ) for 2, which were seen as a shoulder at ca. 310 nm for 1 due to shifting to the higher energy. The complexes further re-vealed a medium broad band at 365-500 nm with λ max = 399 nm (ε max = 2099 L mol −1 cm −1 ) for 1 and 417 nm (ε max = 2638 L mol −1 cm −1 ) for 2. This was due to metal-ligand charge transfer (ML) transitions based on the formation of [Rh(η 4 -cod)] + and [Rh(L 1 or L 2 )] species in [Rh(η 4 -cod)(L 1 or L 2 )] (1 or 2), as reported for the analogous Rh(η 4 -cod)-Schiff bases/amino-acids complexes [17][18][19][20][21]. The excited state properties (simulated UV-Vis spectra) for compound 1 or 2, c lated by TD-DFT at B3LYP/SDD, and the experimental spectra are shown in Figu Some selected excited state properties and simplified assignments associated with th perimental bands are listed in Table 2 and discussed herein. The results showed tha excitation occurred from the combination of several MM, ML and LL transitions at a ticular wavelength (excited state) (Tables S2 and S3). Indeed, excitation energies re to these transitions are very close, overlap with each other and make them diffic interpret independently and in a simple manner [19,23,25,26]. Hence, a combined consisting of MM (d-d) and ML transitions appeared at 475 (1) or 487 (2) nm, wit highest molecular orbital (MO) contribution of 98% (1) or 97% (2) and an osci strength (f) of 0.0022 or 0.0030 for the HOMO to LUMO transitions, respectively, (F 3, inset), which were not seen in the experimental spectra. Similarly, a combined comprising all three transitions (MM, ML and LL) was found at λmax = 402 (1) or 4 nm, with the second highest MO contribution of 71% (1) or 76% (2) (f = 0.0426 or 0. for the HOMO-1 to LUMO transitions, respectively, which were very close to the obse band at λmax = 399 (1) or 417 (2) nm in the experimental spectra. In fact, the simu spectra further indicated several very strong bands at shorter wavelengths with si cant MO contributions and oscillator strength in parallel with the experimental sp ( Figure 3 and Table 2). The excited state properties (simulated UV-Vis spectra) for compound 1 or 2, calculated by TD-DFT at B3LYP/SDD, and the experimental spectra are shown in Figure 3. Some selected excited state properties and simplified assignments associated with the experimental bands are listed in Table 2 and discussed herein. The results showed that the excitation occurred from the combination of several MM, ML and LL transitions at a particular wavelength (excited state) (Tables S2 and S3). Indeed, excitation energies related to these transitions are very close, overlap with each other and make them difficult to interpret independently and in a simple manner [19,23,25,26]. Hence, a combined band consisting of MM (d-d) and ML transitions appeared at 475 (1) or 487 (2) nm, with the highest molecular orbital (MO) contribution of 98% (1) or 97% (2) and an oscillator strength (f ) of 0.0022 or 0.0030 for the HOMO to LUMO transitions, respectively, (Figure 3, inset), which were not seen in the experimental spectra. Similarly, a combined band comprising all three transitions (MM, ML and LL) was found at λ max = 402 (1) or 410 (2) nm, with the second highest MO contribution of 71% (1) or 76% (2) (f = 0.0426 or 0.0662) for the HOMO-1 to LUMO transitions, respectively, which were very close to the observed band at λ max = 399 (1) or 417 (2) nm in the experimental spectra. In fact, the simulated spectra further indicated several very strong bands at shorter wavelengths with significant MO contributions and oscillator strength in parallel with the experimental spectra ( Figure 3 and Table 2).     Although the metal centred d-d (MM) electron transitions for the diamagnetic closed shell transition metals like Rh(I) and Zn(II) are not allowed, there were some d-electron clouds in the HOMO/HOMO-1. This is due to identical symmetry of the metal-d MO and ligand MOs, which provides a small amount of the electron clouds to the metal-d MO ( Figures 4 and S8). In fact, a very small amount of the electron clouds was observed in the LUMO, which reflected minor back donation again due to identical symmetry. As a result, excited state properties possibly deliver a very small d-d (MM) contribution in addition to the metal-ligand and/or ligand-ligand (ML/LL) π-transitions (Figures 4 and S8 and Table 2) [19,23,25,26]. The frontier HOMO-1, HOMO and LUMO are presented in Figures 4 and S8. The HOMO comprised mostly metal-d z 2 electron moieties, while the HOMO-1 comprised metal-d z 2 , sal/naphthal-π and (η 4 -cod)-π electron moieties. The LUMO comprised sal/naphthal-σ and -π with a small number of metal-d xy electrons moieties. The energy gap for HOMO to LUMO transitions is considerably low (∆E = 2.43 (1) or 2.26 (2) eV) and results in the highest MO contributions (e.g., 98 or 97%) to the excitation protocol.
Molecules 2023, 28, x FOR PEER REVIEW 9 of 21 Although the metal centred d-d (MM) electron transitions for the diamagnetic closed shell transition metals like Rh(I) and Zn(II) are not allowed, there were some d-electron clouds in the HOMO/HOMO-1. This is due to identical symmetry of the metal-d MO and ligand MOs, which provides a small amount of the electron clouds to the metal-d MO (Figures 4 and S8). In fact, a very small amount of the electron clouds was observed in the LUMO, which reflected minor back donation again due to identical symmetry. As a result, excited state properties possibly deliver a very small d-d (MM) contribution in addition to the metal-ligand and/or ligand-ligand (ML/LL) π-transitions (Figures 4 and S8 and  Table 2) [19,23,25,26]. The frontier HOMO-1, HOMO and LUMO are presented in Figure  4 and Figure S8. The HOMO comprised mostly metal-dz 2 electron moieties, while the HOMO-1 comprised metal-dz 2 , sal/naphthal-π and (η  -cod)-π electron moieties. The LUMO comprised sal/naphthal-σ and -π with a small number of metal-dxy electrons moieties. The energy gap for HOMO to LUMO transitions is considerably low (E = 2.43 (1) or 2.26 (2) eV) and results in the highest MO contributions (e.g., 98 or 97%) to the excitation protocol.

X-ray Analyses
The X-ray molecular structure revealed that the ligand HL 2 exists as the zwitterionic (imine)N-H + ··· -O(phenol) (ketoamine form) in the solid-state (Figure 5a), which is occasionally seen in Schiff base compounds [27]. The N-H proton could be found and refined. The molecular packing in HL 2 is organized by a π-π and a C-H···π contact ( Figure S4). The molecular structures for the rhodium complex 1 or 2 demonstrated that the deprotonated Schiff base ligand (L 1 or L 2 ) coordinated to the Rh(η 4 -cod)-fragment as a six-membered N,O-chelate ligand to the rhodium atom with a close-to-square-planar geometry if one considers the midpoints of the cod double bonds and the N,O donor atoms (Figure 5b,c).
There are two symmetry-independent molecules in the asymmetric unit in compound 1. This included a molecule with Rh1 and a molecule with Rh2 (Figure 5b), as reported in the analogous Rh(η 4 -cod)-Schiff base complexes [17,19]. Two symmetry-

X-ray Analyses
The X-ray molecular structure revealed that the ligand HL 2 exists as the zwitterionic (imine)N-H + ··· -O(phenol) (ketoamine form) in the solid-state (Figure 5a), which is occasionally seen in Schiff base compounds [27]. The N-H proton could be found and refined. The molecular packing in HL 2 is organized by a π-π and a C-H···π contact ( Figure S4). The molecular structures for the rhodium complex 1 or 2 demonstrated that the deprotonated Schiff base ligand (L 1 or L 2 ) coordinated to the Rh(η 4 -cod)-fragment as a six-membered N,O-chelate ligand to the rhodium atom with a close-to-square-planar geometry if one considers the midpoints of the cod double bonds and the N,O donor atoms (Figure 5b,c).
The molecular packing in 1 and 2 was largely controlled by van der Waals interactions between the C-H groups. There were no π-π interactions and only two recognized C-H···π contacts each between the molecules in 1 and 2 ( Figures S5 and S6). Remarkably, in 1, there were reciprocal or pairwise C-H···π contacts between a pair of each of the symmetryindependent molecules ( Figure 6). Both C-H···π contacts originated from the ortho-C-H atom of the ethylphenyl ring and pointed onto the six-membered Rh-NˆO chelate ring. This metallacycle can be regarded as metalloaromatic according to Masui, who had proposed an electron delocalization within a metal-heterocyclic chelate ring so that it exhibited metalloaromaticity [43][44][45][46][47][48]. This pairwise C-H contact to the Rh-NˆO chelate ring could be a reason for the two symmetry-independent molecules in 1. In 2, the C-H···π contacts were a normal interaction between two naphthyl C-H atoms onto either the naphthyl or the ethylphenyl ring ( Figure S6).  6. The reciprocal or pairwise C-H···metallochelate-π contacts between a pair of each of the symmetry-independent molecules in 1 with Rh1 (a) and Rh2 (b) (distance and angle details are given in Table S1). For clarity, the H atoms were omitted except for those of the C-H···π contact. Symmetry transformation i = -x, 1-y, 1-z; ii = 2-x, 1-y, 2-z.  Figure 6. The reciprocal or pairwise C-H···metallochelate-π contacts between a pair of each of the symmetry-independent molecules in 1 with Rh1 (a) and Rh2 (b) (distance and angle details are given in Table S1). For clarity, the H atoms were omitted except for those of the C-H···π contact. Symmetry transformation i = -x, 1-y, 1-z; ii = 2-x, 1-y, 2-z.

Keto-Enol Tautomerism
To  [24,48], we ran 1 H NMR spectra for HL 1 and HL 2 in CD 3 OD and DMSO-d 6 in addition to CDCl 3 ( Figure S3 and Table 1). A significant chemical shift downfield by ca. 0.12 ppm (CDCl 3 ) and 0.30 ppm (DMSO-d 6 ) for CH=N and ca. 0.44 ppm (DMSO-d 6 ) for OH was observed with increasing solvent polarity from CDCl 3 to CD 3 OD to DMSO-d 6 for HL 2 . Similarly, the CH=N peak shifted downfield by 0.28 ppm (DMSO-d 6 ) for HL 1 . The CH=N peak appeared in duplicate with a separation of 18.0 Hz in CD 3 OD, which corresponded to the presence of both the keto-and enol forms with an equilibrium of almost equimolar amounts in solution. However, the peak for OH (enol form) and/or NH (keto form) was not seen in CD 3 OD due to rapid proton-exchange with the alcoholic group (OD). In DMSO-d 6 , the spectrum also showed two peaks (separated by ca. 5.0 Hz) for CH=N or OH/NH. The exchange of the phenolic proton (OH) between the oxygen and nitrogen atoms is considerably slow on the 1 H NMR time scale, which results in the detection of both signals for keto-and enol-forms in CD 3 OD and DMSO-d 6 , which stabilize the ionic ketoamine form.

Keto-Enol Tautomerism
To check the existence of keto-enol tautomerism (i.e., keto ⇆ enol equilibrium) in the solution (Scheme 3) [24,48], we ran 1 H NMR spectra for HL 1 and HL 2 in CD3OD and DMSO-d6 in addition to CDCl3 ( Figure S3 and Table 1). A significant chemical shift downfield by ca. 0.12 ppm (CDCl3) and 0.30 ppm (DMSO-d6) for CH=N and ca. 0.44 ppm (DMSO-d6) for OH was observed with increasing solvent polarity from CDCl3 to CD3OD to DMSO-d6 for HL 2 . Similarly, the CH=N peak shifted downfield by 0.28 ppm (DMSO-d6) for HL 1 . The CH=N peak appeared in duplicate with a separation of 18.0 Hz in CD3OD, which corresponded to the presence of both the keto-and enol forms with an equilibrium of almost equimolar amounts in solution. However, the peak for OH (enol form) and/or NH (keto form) was not seen in CD3OD due to rapid proton-exchange with the alcoholic group (OD). In DMSO-d6, the spectrum also showed two peaks (separated by ca. 5.0 Hz) for CH=N or OH/NH. The exchange of the phenolic proton (OH) between the oxygen and nitrogen atoms is considerably slow on the 1 H NMR time scale, which results in the detection of both signals for keto-and enol-forms in CD3OD and DMSO-d6, which stabilize the ionic ketoamine form.

Phase Transformation and Thermal Stability
The differential scanning calorimetry (DSC) curves for HL 2 , 1 and 2 are show in Figure 7 (Figure S10) and data are listed in Table 4. The heating curves showed an endothermic peak with a considerable amount of heat of transformation (H/kJ mol −1 ), which corresponded to a phase transformation from the crystalline-solid to isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds, as reported for the Scheme 3. Keto-enol tautomerism of HL 2 in solution.

Phase Transformation and Thermal Stability
The differential scanning calorimetry (DSC) curves for HL 2 , 1 and 2 are show in Figure 7 ( Figure S10) and data are listed in Table 4. The heating curves showed an endothermic peak with a considerable amount of heat of transformation (∆H/kJ mol −1 ), which corresponded to a phase transformation from the crystalline-solid to isotropic-liquid phase and subsequently confirmed the thermal stability of the compounds, as reported for the analogous Rh(η 4 -cod)-Schiff base complexes [19]. The cooling curves showed no peak on the reverse direction, suggesting an irreversible phase transformation. The repeated heating curves in the second cycle for the same probe reproduced similar peaks for HL 2 , while no peak was observed for 1 or 2. The phase transformation temperature for 2 (ca. 221 • C) was considerably higher than 1 (ca. 185 • C), which corresponds to higher thermal stability in accordance with the high molecular weight. Similarly, the free ligand (HL 2 ) showed a low phase transformation temperature (ca. 92 • C) due to low molecular weight.

Materials and Characterization
All reactions were carried out under dry nitrogen gas using collecting flux. Solvents were dried and redistilled under nitrogen prior to use: benzene over Na metal and methanol over CaO. IR spectra were recorded on a Nicolet iS10 spectrometer as KBr discs at ambient temperature. UV-Vis spectra were measured with a Shimadzu UV 1800 spectrophotometer in chloroform at 25 °C. Differential scanning calorimeter (DSC) analyses were performed on a Shimadzu DSC-60 at the range of 30-240 °C (ca. 5 °C above the corresponding melting point) with a rate of 10 K min -1 . 1 H NMR spectra were recorded on a Bruker Avance DPX 400 spectrometer at 20 °C in CDCl3, DMSO-d6 or CD3OD. Electron impact (EI) mass spectra were recorded with a Thermo-Finnigan TSQ 700 mass spectrometer. Heat (2) Cool (2) Heat (1) Cool (

Materials and Characterization
All reactions were carried out under dry nitrogen gas using collecting flux. Solvents were dried and redistilled under nitrogen prior to use: benzene over Na metal and methanol over CaO. IR spectra were recorded on a Nicolet iS10 spectrometer as KBr discs at ambient temperature. UV-Vis spectra were measured with a Shimadzu UV 1800 spectrophotometer in chloroform at 25 • C. Differential scanning calorimeter (DSC) analyses were performed on a Shimadzu DSC-60 at the range of 30-240 • C (ca. 5 • C above the corresponding melting point) with a rate of 10 K min -1 . 1 H NMR spectra were recorded on a Bruker Avance DPX changed immediately from orange-red to bright yellow, and a precipitate formed within 30 min in the reaction with HL 2 . The solution was stirred for another ca. 6 h at room temperature. The precipitate was collected through filtration, washed three times with methanol (1 mL each) and dried in vacuo at 40 • C to obtain bright yellow microcrystals of [Rh(η 4 -cod)(L 2 )] (2). In the reaction with HL 1 , no precipitate was formed after 6 h of stirring, and the solvent was then evaporated to dryness in a rotary evaporator in vacuo at 40 • C. The residue was dissolved in 2 mL of a mixture of C 6 H 6 :MeOH (2:1, v/v), stirred for ca. 30 min and again dried in a rotary evaporator in vacuo at 40 • C. This process was repeated three times, and finally, bright yellow microcrystals of [Rh(η 4 -cod)(L 1 )] (1) were obtained. Single crystals suitable for X-ray diffraction measurements were grown by slow diffusion of n-hexane into a concentrated dichloromethane solution of 1 or 2 after 3-4 days at room temperature. [

Computational Method
Computations were performed with the Gaussian 09 program package [49]. The initial geometries for optimization were generated from the X-ray structures of compounds 1 and 2, respectively. The initial geometry was optimized with B3LYP/3-21G, which was then reoptimized with B3LYP/SDD [19,23,[50][51][52]. For calculations of excited state properties, time-dependent density functional theory (TD-DFT) was employed with different combinations of the functionals B3LYP and M06 and the basis sets SDD and DEF2SVP. The simulated spectra thus obtained were very similar with little shifting of band maxima and were also comparable with the experimental spectra ( Figure S9). These results further suggest the validity and reliability of the methods employed. The PCM (Polarization Continuum Model) in chloroform and 72 excited states (roots) were considered for the calculations (Tables S2 and S3) [19,23]. Assessments of excited state properties and molecular orbitals (MOs) calculations were carried out at the same level of theory. The simulated spectra were generated with the software SpecDis (version 1.71) [53,54] applying the Gaussian band shape with an exponential half-width σ = 0.16 eV.

X-ray Structure Determination
Suitable crystals were carefully selected under a polarized light microscope covered in protective oil and mounted on a cryo-loop. The single crystal diffraction data was collected using a Rigaku XtaLAB Synergy S four circle diffractometer with a Hybrid Pixel Array Detector and a PhotonJet X-ray source for Cu-Kα radiation (λ = 1.54184 Å) with a multilayer mirror monochromator. Data collection at 100.0 ± 0.1 K using ω-scans. Data reduction and absorption correction were performed with CrysAlisPro 1.171.41.105a [55]. The structures were solved using direct methods (SHELXT-2015), and full-matrix least-squares refinements on F 2 were carried out using the SHELXL-2017/1 program package in OLEX 2.1.3 [56][57][58]. All hydrogen atoms on C were positioned geometrically (with C-H = 0.95 Å for aromatic and aliphatic CH, C-H = 1.00 Å for ternary CH , C-H = 0.99 Å for CH 2 and C-H = 0.98 Å for CH 3 ) and were refined using riding models (AFIX 43, 13, 23 and 137 with U iso(H) = 1.2 U eq (CH, CH 2 ) and 1.5 U eq (CH 3 ). The protic hydrogen atom for NH in HL 2 was found and refined freely. Crystal data and details on the structure refinement are given in Table 5. Graphics were drawn with the program DIAMOND [59]. Computations on the supramolecular interactions were carried out with PLATON for Windows [60][61][62]. The CCDC numbers 2201154-2201156 for HL 2 , 1 and 2, respectively, contain the supplementary crystallographic data reported in this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Data Availability Statement:
The data presented in this study are available on request from the corresponding authors.