Trans-to-cis isomerization of a platinum(II) complex with two triphosphine ligands via coordination with gold(I) ion

The reaction of a square-planar platinum(II) complex having two bis(2-diphenylphosphinoethyl)phenylphosphine (triphos), [Pt(triphos) 2 ](NO 3 ) 2 ([ 1 ](NO 3 ) 2 ), with [Au(tu) 2 ]Cl (tu = thiourea) gave a new trinuclear Au I2 Pt II complex, [{Pt(triphos) 2 }{Au(tu)} 2 ]Cl 2 (NO 3 ) 2 ([ 2 ]Cl 2 (NO 3 ) 2 ), through the Au-P coordination bond formation. While the [Pt(triphos) 2 ] 2+ unit in [ 1 ](NO 3 ) 2 adopted the trans-meso configuration, only the cis-racemic isomer was observed in [ 2 ]Cl 2 (NO 3 ) 2 . The 31 P NMR spectroscopy indicated a rapid equilibrium among the possible isomers of [ 1 ] 2+ , facilitating the unique trans-to-cis transformation at the Pt II center in this system. Additionally, we observed that this structural transformation leads to an enhancement of the emission intensity.


Introduction
Square-planar platinum(II) complexes possess a kinetic inertness, often leading to the formation of geometric isomers, namely cis/trans or E/Z isomers [1].The presence of these isomers plays a crucial role in influencing biological activity [2], optical/electronic properties [3], and chemical reactivity [4,5].Consequently, the controlled synthesis of specific isomers is needed to develop functional platinum(II) coordination compounds.The structural manipulation of platinum(II) complexes, especially those with phosphine ligands, has garnered the longstanding interest of coordination chemists.This interest is primarily due to the fascinating isomerization behavior and photoluminescent features of these complexes [6].In 1970, Mastin and Haake reported that a square-planar platinum(II) complex with two triphenylphosphine and two chloride ligands, [Pt(PPh3)2Cl2], shows cis-trans equilibrium in solution and can be directed to the trans-isomer by irradiating UV light [7].After this finding, the cis-trans isomerism of platinum(II) complexes with monophosphine or diphosphine ligands was widely studied and utilized for stimuli-responsible systems, such as molecular gear and anion-receptor [8][9][10][11].
Typically, these compounds feature a platinum(II) center surrounded by two phosphine groups and two heteroatoms, represented as [Pt II (P)2(X)2].This category of compounds exhibits distinct electronic states, with the total energy varying between the cis and trans isomers [12].
Such variability facilitates the preferential formation of one isomer or enables the induction of isomerization through chemical/physical stimuli.Platinum (II) complexes bound by four phosphine groups, when modified asymmetrically by substituent groups (i.e., [Pt II (PA)2(PB)2]), can also form cis-trans isomers [13,14].These isomers tend to have nearly equal energy levels for the cis and trans isomers, making it more challenging to regulate their isomerism.While several studies have reported successful crystallization of these isomers [15][16][17][18], achieving structural control of geometric isomers through chemical reactions remains elusive.

Materials.
The starting complexes, [Pt(triphos)Cl]Cl [24] and [Au(tu)2]Cl [25], were prepared according to the method described in the literature.Other chemicals were purchased and used without further purification.

Preparation of the complexes. [Pt(triphos)2](NO3)2•CH2Cl2 ([1](NO3)2•CH2Cl2).
To a colorless solution containing [Pt(triphos)Cl]Cl (81 mg, 0.10 mmol) in 30 mL of methanol was added 54 mg (0.099 mmol) of triphos and 30 mL of methanol, which gave a yellow suspension.After stirring for 2 h, to the resulting clear yellow solution was added 1.0 mL of an aqueous solution of 1 M NaNO3 and 15 mL of water, then slowly evaporated in a draft for 3 days.The crude product of [1]

Physical Measurements.
The IR spectrum was recorded with a JASCO FT/IR-4100 infrared spectrophotometer using the ATR method at room temperature.Elemental analyses (C, H, N) were performed at Osaka University using a YANACO CHN Corder MT-5.X-ray fluorescence spectrometry was performed with a SHIMADZU EDX-7000 spectrometer.The TG and DTA measurements were measured using a SHIMADZU DTG-60 analyzer.The PXRD patterns were recorded using a BRUKER D2 PHASER at room temperature. 1H and 31 P NMR spectra were recorded with a JEOL ECS400 (400 MHz) spectrometer in methanol-d4 with tetramethylsilane (TMS) as an internal standard for 1 H and triphenylphosphine as an external standard for 31 P. Powder X-ray diffraction (PXRD) measurements were performed under ambient conditions using a BRUKER D2 PHASR.The powder simulation patterns were generated from the single-crystal X-ray structures using Mercury 2023.2 [26].The diffuse reflection spectra in the solid-state were measured with a JASCO V-670 UV/VIS spectrometer at room temperature using MgSO4.The photoluminescence spectra were recorded with a JASCO FP-8500 spectrometer.The internal emission quantum yield (Φ) was obtained via the absolute measuring method using an integrating sphere unit (JASCO ILFC-847); the internal surface was coated with highly reflective Spectralon.An ESC-842 calibrated light source (WI) and an ESC-843 calibrated light source (D2) were used to calibrate the emission intensities to measure the absolute quantum yields.

X-ray Structure Determinations.
The single-crystal X-ray diffraction dataset for [1](NO3)2•CH2Cl2 was collected at 100 K using a Synergy Custom X-ray diffractometer equipped with a Hypix-6000HE hybrid photon counting detector and a Rigaku VariMax rotating-anode X-ray source with a Mo target (λ = 0.71073 Å).The dataset for [2]Cl2(NO3)2 was collected using a PILATUS3 X CdTe 1M detector with a synchrotron X-ray source at the BL02B1 beamline at Spring-8.The intensity data were collected via the ω-scan technique and empirically corrected for absorption.All structures were solved by the intrinsic phasing method within the SHELXT program [27] and were refined on F 2 by the full-matrix least-squares technique using the SHELXL program [28] via the Olex2 interface [29].The hydrogen atoms, with the exception of those on the water molecules, were calculated and placed using riding models.All nonhydrogen atoms were refined anisotropically, while the H atoms were refined isotropically.Nitrate anions and water molecules could not be modeled and were, therefore, removed from the electron density map using the Olex2 solvent mask command [29].ISOR instructions were applied for C35 in [1](NO3)2•CH2Cl2 and C35 and N2 in [2]Cl2(NO3)2.
The crystallographic data are summarized in Table 1.The selected bond distances and angles are summarized in Tables 2 and 3.
Single-crystal X-ray analysis of [1](NO3)2 revealed that the asymmetric unit comprises half of a mononuclear platinum(II) complex cation with two triphos ligands, [Pt(triphos)2] 2+ , situated at the crystallographic inversion center, and half of a solvated CH2Cl2 molecule.
Although the NO3 -ions could not be modeled, they are presumed to occupy the crystal void spaces in a disordered manner.As illustrated in Fig. 2 Å, Pt-PPh = 2.23(1) Å) [24].The elongation of Pt-P bonds in [1](NO3)2 results from the larger trans influence of phosphine groups than Cl - [31,32], which is the reason why [1](NO3)2 can show solution equilibrium in the NMR timescale (vide infra).
It should be noted that the powder X-ray diffraction pattern of [1](NO3)2 matched well with the simulated pattern of the crystal structure, which indicated that only the trans-meso isomer was selectively formed in the bulk sample of [1](NO3)2 (Fig. 3a).In the packing structure of [1](NO3)2, complex cations form two kinds of intermolecular C-H•••π interactions between the phenyl groups of uncoordinated PPh2 and coordinated PPh2 groups (CH•••Cg = 2.53 Å and 3.07 Å), creating a 2D sheet supramolecular structure in the crystallographic ab plane (Fig. 4a).The 2D structures were stacked along c axis with accommodating the CH2Cl2 molecule in the interstitial space (Fig. 4b).We assumed that the crystal packing adjustable for accommodating CH2Cl2 is the key of the selective crystallization of the trans-meso isomer of [1](NO3)2.Similar solvent-directed crystallization of one of four isomers has been observed in a Pt II complex with chlorophosphineamides [16].
In the crystal packing structure, two Cl -anions are wrapped by the NH2 groups of tu ligands and the ethylene and phenyl groups of triphos ligands via NH•••Cl (2.25 Å) and CH•••Cl (2.64, 2.62, and 2.94 Å) hydrogen bonds.This interaction results in the formation of a discrete supramolecule {Cl2@[2]} 2+ (Fig. 5).We assumed that the interaction between [2] 4+ and Cl - leads to the preferable formation of the cis-racemic isomer of [2]Cl2(NO3)2.Moreover, these supramolecules are interconnected by additional CH•••Cl hydrogen bonds (2.63 Å) between the phenyl groups and Cl -anion from adjacent supramolecules, forming a 1D chain structure (Fig. 6a).The 1D chains further interacted with neighboring chains via CH•••π interactions (CH•••Cg = 3.21 and 2.71 Å) culminating in a 3D supramolecular structure with large void space (34.2%, 3102.3Å 3 per unit cell) (Fig. 6b).We could not model the nitrate anion and solvated water molecules in the crystal structure because they are severely disordered in the void space.
To investigate the molecular structure in solution, we performed NMR spectroscopic measurements of these compounds in methanol-d4.The 1 H NMR spectra were complicated (Fig. S3); therefore, our analysis primarily focused on the 31  groups, as typically seen in the literature [36].Additionally, complex, broad signals appeared between δ 15-50 ppm.We hypothesized that all or some of the four possible stereoisomers of [1] 2+ (cis-meso, cis-racemic, trans-meso, and trans-racemic) are in equilibrium in solution, and an averaged structure is observed at 25 °C.The exchange among the isomers requires the Pt-P bond cleavage.The elongated Pt-P bonds, attributed to the substantial trans influence of phosphine groups, may facilitate this bond cleavage within the system.
The 31 P NMR spectrum of [2]Cl2(NO3)2 in methanol-d4 at 25 °C showed two singlet signals with platinum satellites at 92.21 (JP-Pt = 1342 Hz) and 43.37 (JP-Pt = 1211 Hz) ppm ascribed to the Pt-PPh2 and Pt-PPh groups, respectively.In addition, the spectrum showed two singlet signals without platinum satellites at 34.6 and 33.2 ppm due to the Au-PPh2 group (Fig. 7b).
The overall spectral feature is consistent with the C2 symmetric trinuclear Au I 2Pt II structure observed in the single-crystal X-ray analysis, except for the appearance of two signals due to Au-PPh2 groups.The signal splitting presumably results from the partial exchange of the tu ligands with solvent in the solution state.Note that the VT NMR spectra of [2]Cl2(NO3)2 from 25 °C down to -80 °C showed no drastic spectral change, unlike the case of [1](NO3)2.The Au-P bond formation may suppress the Pt-P bond exchange, which maintains the cis-racemic isomer of [2]Cl2(NO3)2 even in the solution state.
Moreover, the emission band of [2]Cl2(NO3)2 revealed a structured pattern with a separation of ca.1600 cm -1 .This value is similar to the C=S stretching energy (1635 cm -1 ) of tu ligand in [2]Cl2(NO3)2 observed in the IR spectra (vide supra).Therefore, we propose that the emission of [1](NO3)2 is originated from the charge transfer from Pt II to phosphine ( 3 MLCT) of the [PtP4] moiety, while the emission of [2]Cl2(NO3)2 is ascribed to the charge transfer from S to P ( 3 LLCT) or S to Au ( 3 LMCT) [37].The large Stokes shift in [2]Cl2(NO3)2 could be explained by the intramolecular energy transfer from [PtP4] to [AuPS] moieties and the structural distortion of the Au I ion in the triplet state.The quantum yields (Φ) of the two complexes were also evaluated using an integration sphere to be 6.9 % for [1](NO3)2 and 16 % for [2]Cl2(NO3)2.
Multiple H bonds between the complex cation and Cl -may reduce the molecular vibration, which may contribute to the enhancement of photoluminescence in the Au I 2Pt II complex [2]Cl2(NO3)2.

Concluding Remarks
In this study, we succeeded in the crystallization and structural elucidation of [1](NO3)2, in which two triphos ligands cheleted to the square-planar Pt II center using two of three P atoms.
We also showed that the complex [1] 2+ can act as a P-donating metalloligand for Au I ions, utilizing the two remaining P atoms.This was evidenced by the formation of the trinuclear

Supporting Information
Fluorescence X-ray spectra (Fig. S1), TG profile (Fig. S2), and 1 H NMR spectra (Fig. S3) are included in the supplementary material in PDF format.
, each triphos ligand binds to the squareplanar Pt II center in a bidentate-P,P' fashion, engaging one of the two terminal PPh2 groups and the central PPh group.The Pt II center is coordinated by two PPh2 and PPh groups from the triphos ligands, forming a [Pt(PA)2(PB)2]-type chromophore with the trans configuration, thereby positioning the uncoordinated PPh2 groups on opposite sides.In addition to the cistrans isomerism around the Pt II center, the coordinated PPh group, becoming stereogenic phosphorous atoms, exhibit R-S configurations, leading to the meso(RS)-racemic(RR/SS) isomerism in[1] 2+ .Of possible four isomers illustrated in Chart 1,[1](NO3)2 was found to adopt the trans-meso isomer in the crystal structure.The Pt-PPh2 (2.3190(9) Å) and Pt-PPh (2.3268(9) Å) bond distances were found to be longer than those in a relating Au I Pt II complex having a similar asymmetric triphos coordination mode, [AuPt(triphos)Cl3] (Pt-PPh2 = 2.23(1) P NMR spectra.The 31 P NMR spectrum of[1](NO3)2 in methanol-d4 at 25 °C exhibited two singlet signals with platinum satellites at δ 46.28 (signal A) and 19.02 ppm (signal B) with an integration intensity ratio of 1:2 (Fig.7a).The coupling constant JP-Pt of signal A (1087 Hz) was approximately twice that of signal B (622.6 Hz).These spectral features are inconsistent with the Cs symmetric mononuclear structure identified in the single-crystal X-ray analysis, which should display two singlet signals with similar JP-Pt values alongside one singlet signal without platinum satellites.We tentatively assigned the31 P signals A and B to the central PPh and terminal PPh2 groups of triphos ligands, respectively, and we presumed the coordinated and uncoordinated PPh2 groups are rapidly exchanged in the NMR timescale.To verify this dynamic behavior, we measured the variabletemperature (VT) NMR spectra from 25 °C down to -80 °C.Upon cooling, signals A and B became broad at -40 °C and disappeared at lower temperatures.At -80 °C, a new singlet signal without platinum satellites appeared at -15.23 ppm (signal C), indicative of uncoordinated PPh2

Figure 5 .
Figure 5.A perspective view of the complex cation accommodating Cl -anion in [2]Cl2(NO3)2.The thermal ellipsoids are illustrated in the 50% probability.Color code: Pt: white, Au: pink, P: orange, Cl: green, N: blue, C: gray, H: pale blue.Red dotted lines indicate the hydrogen bonds.

Figure 6 .
Figure 6.Perspective views of [2]Cl2(NO3)2.(a) The 2D chain structure viewed from b axis.(b) The packing structure viewed from a axis.Red and blue dotted lines indicate the hydrogen bonds and CH•••π interactions.Color code: Pt: white, Au: pink, P: orange, Cl: green, N: blue, C: gray, H: pale blue.