Photophysical Properties and Kinetic Studies of 2-Vinylpyridine-Based Cycloplatinated(II) Complexes Containing Various Phosphine Ligands ^†

Abstract

A series of cycloplatinated(II) complexes with general formula of [PtMe(Vpy)(PR₃)], Vpy = 2-vinylpyridine and PR₃ = PPh₃ (1a); PPh₂Me (1b); PPhMe₂ (1c), were synthesized and characterized by means of spectroscopic methods. These cycloplatinated(II) complexes were luminescent at room temperature in the yellow–orange region’s structured bands. The PPhMe₂ derivative was the strongest emissive among the complexes, and the complex with PPh₃ was the weakest one. Similar to many luminescent cycloplatinated(II) complexes, the emission was mainly localized on the Vpy cyclometalated ligand as the main chromophoric moiety. The present cycloplatinated(II) complexes were oxidatively reacted with MeI to yield the corresponding cycloplatinated(IV) complexes. The kinetic studies of the reaction point out to an S_N2 mechanism. The complex with PPhMe₂ ligand exhibited the fastest oxidative addition reaction due to the most electron-rich Pt(II) center in its structure, whereas the PPh₃ derivative showed the slowest one. Interestingly, for the PPhMe₂ analog, the trans isomer was stable and could be isolated as both kinetic and thermodynamic product, while the other two underwent trans to cis isomerization.

1. Introduction

Luminescent cyclometalated platinum(II) complexes have been significantly studied by many researchers in the last decades [1,2,3,4,5,6,7,8,9,10,11]; publications in this field of research are increasing [12,13,14,15]. The light-emitting devices [16,17,18] dye-sensitized solar cell [19], photo-switches [20,21], photocatalysts [22], and chemical or biochemical sensors [23] are various applications related to the luminescent cycloplatinated complexes. The cyclometalated ligand is the main chromophoric part in the creation of room-temperature phosphorescence. In addition, heavy metal like Pt induces high spin–orbit coupling and allows singlet-triplet intersystem crossing. In addition to the factors mentioned, the ancillary ligands play a very important role in such complexes’ photophysical properties. These ligands control the electron density at the metal center and consequently the degree of Metal to Ligand Charge Transfer (MLCT) in the lowest energy transition. Some different arrangements for the ancillary ligands are expected depending on the charge (neutral or anionic) or binding mode (chelating and non-chelating) of the ancillary ligands, such as L^X [24,25,26], L^L [6,27,28], L/X [2,29,30,31,32], L/L [33,34] and X/X [35,36].

Kinetic and mechanistic investigations of the oxidative addition reactions of organoplatinum(II) complexes have clearly proved that these reactions follow an S_N2 mechanism [37,38,39,40,41,42,43,44,45] with the exception of a few cases [46,47]. The S_N2 mechanism with a large negative ∆S^≠ value can be observed for the small organic molecules, such as alkyl halides [48,49,50,51]. In these cases, theoretical approaches for mechanistic studies remarkably helped to certify the obtained experimental data [52,53,54,55]. The cycloplatinated(II) complexes with monophosphine ligands are the appropriate complexes for the study of the oxidative addition of alkyl halides on the Pt(II) center [56,57,58,59,60,61,62]. The phosphine ligands can affect electronically and sterically the rate constants and activation parameters. Electron-withdrawing or -donating properties of the substituents together with the Tolman cone angle significantly control the properties of monophosphine ligand [56]. In a comparison between PPh₂Me and PPh₃ ligands, the rate constants attributed to the PPh₂Me derivative are 3‒5-fold more than those for PPh₃, indicating the electronic and steric differences [63]. In this manner, it is expected that the PPhMe₂ derivative reactions should be considerably faster than those of PPh₂Me.

In the framework of our experiences on the photophysical [30,31,64,65,66,67,68,69] and the kinetico-mechanistic studies [70,71,72,73,74] of cyclometalated platinum complexes, some cycloplatinated(II or IV) complexes with 2-vinylpyridine (Vpy) and phosphine ligands (PPh₃ [75], PPh₂Me [76] and PPhMe₂) were designed. The kinetic and photophysical properties of the complex bearing PPh₂Me were previously reported by us [76]. Therefore, herein, we added the PPh₃ and PPhMe₂ derivatives to make a meaningful trend and have a complete picture for comparison in terms of photophysical and kinetic studies. In addition, density functional theory (DFT) calculations were performed for both investigations and support the experimental data.

2. Result and Discussion

2.1. Synthesis and Characterization

All the synthetic steps are demonstrated in Scheme 1. In this scheme, the precursor cycloplatinated(II) complex [PtMe(Vpy)(DMSO)], A, [75,76,77] Vpy = 2-vinylpyridine, reacted with several monophosphines to give the complexes [PtMe(Vpy)(PR₃)], PR₃ = PPh₃ (1a) [75]; PPh₂Me (1b) [76]; PPhMe₂ (1c). These complexes treated with MeI to give the corresponding cycloplatinated(IV) complexes of trans-[PtMe₂I(Vpy)(PR₃)], PR₃ = PPh₃ (2a); PPh₂Me (2b) [76] PPhMe₂ (2c), and then cis-[PtMe₂I(Vpy)(PR₃)], PR₃ = PPh₃ (3a); PPh₂Me (3b) [76]; PPhMe₂ (3c). We have previously reported 1b and its reaction with MeI (to form 2b and 3b) kinetically investigated [76]. The Pt(IV) complex 2b was not stable and gradually converted to 3b isomer, and it was decomposed to the known Pt(II) complex and an organic compound [76]. The same interconversion happened for 1a with this difference that 2a (trans isomer) cannot be detected due to the very fast trans to cis isomerization. The cis isomer 3a is similarly decomposed and forms the same products (Scheme 2; trans-[PtMeI(PPh₃)₂], 4, [{PtMe₂(Vpy)}₂(µ-I)₂], 5, and Z/E-[C₈H₉N], 6) [76]. On the other hand, for 1c, the corresponding cycloplatinated(IV) 2c is stable and not converted to 3c, and consequently, no decomposition process occurs (Scheme 2). The above-mentioned observations decisively point out the determining role of the phosphine ligand in determining the stability of the complexes.

The formation of the new complexes 1c, 2c and 3a confirmed by ¹H- and ³¹P{¹H}-NMR spectroscopy and their spectra have all been shown in Figures S1–S8 from the Supplementary Material. The structures of 1c and 2c were further characterized by X-ray crystallography (Table S1 from the Supplementary Material), and their Oak Ridge Thermal Ellipsoid Plot (ORTEP) plots are shown in Figure 1, while their selected geometrical parameters are presented in its caption. The structure of 1c clearly confirms the proposed structure, and as expected, the C ligating atom of Vpy is located trans to the P of PPhMe₂. The Vpy bite angle in 1c (N1-Pt1-C11) is 78.76°, which is very close to those observed for the other cyclometalated ligands in Pt(II) complexes [6]. Upon oxidative addition reaction, this angle in 2c does not show a meaningful change (79.09°). It can vividly be observed that the trans isomer is stable for 2c [72].

2.2. Photophysical Properties

The electronic transitions of the cycloplatinated(II) complexes 1a‒c were initially investigated by the UV-vis spectroscopy, and their spectra were characterized in detail by the help of density functional theory (DFT) and time dependence-DFT (TD–DFT) calculations. In the first step, the ground states of all these three complexes were optimized with the consideration of CH₂Cl₂ as the solvent (the same solvent for experimental absorption spectra); the optimized structures are shown in Figure S9 from the Supplementary Material, and the selected geometrical parameters are listed in Table S2 from the Supplementary Material. Using the optimized structures in CH₂Cl₂, the frontier molecular orbitals (MO), including “HOMO to HOMO−5” (Highest Occupied Molecular Orbital) and “LUMO to LUMO+5,” (Lowest Unoccupied Molecular Orbital) were obtained for all the complexes. The compositions of the selected molecular orbitals in terms of Pt and ligands are listed in Tables S3–S5 from the Supplementary Material, and the corresponding visual plots are depicted in Figures S10–S12 from the Supplementary Material. For all the cases, HOMO is predominantly localized on Pt and Vpy fragments, while in LUMO, Vpy is significantly dominated over the other fragments. In addition, the contribution of phosphine ligand is considerably increased in lower HOMOs and higher LUMOs.

As observed in Figure 2, although there is an acceptable agreement between the experimental UV-vis spectra and corresponding TD–DFT bars, an overestimation for the transition energies is observed in the visible region for all the cases.

Table 1. Wavelengths and nature of transitions of 1a–c (M = Pt, L = Vpy, L′ = PR₃).

Complex	Excited State	Oscillator Strength	Calculated λ (nm)	Transitions (Major Contribution)	Assignment
1a	S0 → S1	0.0569	372.78	HOMO→LUMO (89%)	ILCT/MLCT
	S0 → S2	0.0197	364.76	H-1→LUMO (99%)	MLCT
	S0 → S3	0.1341	326.71	H-2→LUMO (84%)	MLCT
	S0 → S4	0.0285	300.17	HOMO→L+1 (93%)	ML′CT/LL′CT
	S0 → S10	0.1398	285.59	HOMO→L+3 (86%)	ILCT/ML′CT
1b	S0 → S1	0.0465	373.00	HOMO→LUMO (86%)	ILCT/MLCT
	S0 → S2	0.0231	363.42	H-1→LUMO (95%)	MLCT
	S0 → S3	0.1182	329.57	H-2→LUMO (85%)	MLCT
	S0 → S8	0.1654	286.30	HOMO→L+2 (80%)	ILCT/MLCT/MLʹCT
1c	S0 → S1	0.0436	374.56	HOMO→LUMO (90%)	ILCT/MLCT
	S0 →S2	0.0206	364.26	H-1→LUMO (99%)	MLCT
	S0 →S3	0.1151	330.04	H-2→LUMO (84%)	MLCT
	S0 → S5	0.1056	289.38	HOMO→L+1 (76%) HOMO→L+2 (18%)	ML′CT/LL′CT ILCT/MLCT/ML′CT
	S0 → S6	0.0953	286.15	HOMO→L+2 (73%) HOMO→L+1 (19%)	ILCT/MLCT/ML′CT ML′CT/LL′CT

indicates that the low-energy bands (the wavelengths used for yielding the emission bands) in all the complexes are attributed to the electronic transitions in the cyclometalated ligand and also charge transfer from Pt to the cyclometalated ligand (mixed ¹ILCT/¹MLCT (Intra Ligand Charge Transfer), L = Vpy). In other words, the first excited state (S1) is majorly related to the HOMO → LUMO transitions, which are assigned as ¹MLCT/¹ILCT. However, in higher energy bands (higher excited states) and the ¹ILCT and ¹MLCT, ¹ML′CT (L′ = PR₃) and ¹LL′CT (Ligand to Ligand Charge Transfer) electronic transitions are present, which involve the transitions with the phosphine ligand as the target.

The photophysical properties of 1a–c were examined using photoluminescence spectroscopy. All three complexes are luminescent in solid state at room temperature and low-temperature, having the bands in yellow-orange area (see

Table 2. Emission data for 1a–c ^a.

Complex	State (Temp.)	λem/nm	τ/μs	Φ
1a	Solid (298 K) Solid (77 K)	550, 581max 530max, 575	0.9 4.1	0.01 0.04
1b	Solid (298 K) Solid (77 K)	550, 579max 542max, 577	3.4 7.1	0.11 0.27
1c	Solid (298 K) Solid (77 K)	550, 585max 543, 585max	5.3 15.5	0.17 0.44

^a The emissions were recorded upon excitation at 365 nm; lifetime values were measured at the peak maxima.

for the numerical emission parameters and Figure 3 for the emission spectra). Expectedly, the complexes exhibit brighter emissions at 77 K in relation to 298 K, which is due to the more rigidity of the structures at low-temperature (see Table 2 for the quantum yield (QY) values). The lifetime values measured for the complexes at both temperature conditions are in microsecond scale, indicating phosphorescence character in the emissive states. The absolute QY values of the complexes obey the trend 1c > 1b > 1a. This is probably related to the more electron-donating character of PPhMe₂ compared to PPh₂Me and PPh₃, which makes the Pt(II) center and consequently the chromophoric ligand of Vpy to be more electron-rich. As can be seen in Figure 3, the emission spectra for the entire cases exhibit structured emission bands, which always point out that the emission is majorly localized on the cyclometalated ligands (large amount of ³ILCT in the emissive state together with the small amount of ³MLCT) [6,30]. The mixed ³ILCT/³MLCT character for the emission bands reflects the ¹ILCT/¹MLCT character the in absorption spectra obtained by the TD–DFT calculations. The wavelength of emissions, being 550 nm at 298 K for all the complexes, is another evidence to confirm the identical emission nature in all the complexes (Vpy ligand). Upon lowering the temperature, a marginal blue shift is observed for the low-temperature bands in relation to their parents at room temperature, and also no tangible change is observed in the color or character of emissions. It should be noted that this blue shift is slightly larger for 1a compared to the other derivatives.

2.3. Kinetic Studies

The oxidative addition of an excess amount of MeI on 1a and 1c in acetone solvent was kinetically studied by following the disappearance of the ¹MLCT band of the cycloplatinated(II) complexes in the absorption spectra (see Figure 4 for the changes in the UV-vis spectra of 1a and 1c at room temperature). The parameter k_obs (pseudo-first-order rate constant) was calculated by the nonlinear least-squares fitting of the absorbance-time curves to a first-order equation. The k_obs/MeI concentration graphs were plotted for at least five temperatures (Figure 4), giving desirable straight-line plots with no intercept (no involvement of solvent or any dissociative pathway). The slope of the straight lines gives the second-order rate constant (k₂). To calculate the activation parameters, the Eyring equation (Equation (1)) was employed at different temperatures (Figure 5). The large negative Entropy of Activation ΔS^# values in

Table 3. Second-order rate constants ^a and activation parameters ^b for the reaction of 1 with MeI in acetone ^c.

Complex	L	102 k2/L·mol−1·s−1 at Different Temperatures					ΔH#/kJ·mol−1	ΔS#/J·K−1·mol−1
		10 °C	15 °C	20 °C	25 °C	30 °C
1a	PPh3	0.40	0.58	0.82	1.11	1.58	45.7 ± 0.1	−129 ± 1
1bc	PPh2Me	2.80	3.63	4.38	5.57	7.16	30.5 ± 0.1	−166 ± 1
1c	PPhMe2	24.4	28.6	34.0	39.1	44.3	19.1 ± 0.1	−189 ± 1

^a Estimated errors in k₂ values are ± 3%. ^b Activation parameters were calculated from the temperature dependence of the second-order rate constant in the usual way using the Eyring equation. ^c From ref [76].

are normally indicative of second-order kinetics (first-order for Pt(II) complex and MeI) and S_N2 mechanism. We have previously suggested an S_N2 mechanism for 1b, and the present reactions for 1a and 1c obey the previously reported mechanism.⁷⁶ In this mechanism, the trans isomer is considered as the kinetic product, while the thermodynamic product is attributed to the cis isomer. At all the temperatures, the reaction of 1c (containing PPhMe₂) with MeI is the fastest reaction, and 1b and 1a are the lower ranks, respectively. Therefore, the Enthalpy of Activation ΔH^# value for the reaction of 1c with MeI is the lowest value (the lowest energy barrier). However, the ΔS^# values are controlled by the steric hindrance induced by phosphine ligands. The highest ΔS^# value is attributed to the 1c having PPhMe₂ with the lowest steric hindrance. This is due to the associative mechanism with a penta-coordinated intermediate in which the lowest steric hindrance makes the highest ΔS^# value.

From Figure 5 and Table 3, it can be understood that the rate of the reactions obeys the trend 1c (PPhMe₂) > 1b (PPh₂Me) > 1a (PPh₃). For example, at 25 °C, the rate for 1b is almost 5 times larger than that for 1a, while the reaction of 1c is 7 times faster than that of 1b. This is absolutely related to the electron-donating trend of the present monophosphine ligands (PPhMe₂ > PPh₂Me > PPh₃). The more electron-donating ability makes more electron-rich Pt(II) center, which is favorable for oxidative addition reaction and makes it to be faster. Therefore, in the formation of kinetic products, the electronic factors are effective. However, in trans to cis isomerization (kinetic product to thermodynamic product), the steric effects play a determining role. Additionally, the trans isomer can be stabilized by the electron-donating phosphine ligands like PPhMe₂ [78] For the reaction of 1a with MeI, the kinetic product (2a) cannot be observed, and only the cis isomer (3a) as the thermodynamic product can be isolated. This trans to cis isomerization is attributed to a large steric hindrance and low electron-donating ability of the PPh₃ ligand. In the case of the reaction of 1b with MeI [76] having a phosphine (PPh₂Me) with less steric hindrance and more electron-donating ability compared with PPh₃, the trans isomer can be isolated and even completely characterized, but it will gradually be converted to the cis product and eventually decomposed [74]. However, if PPh₂Me is replaced by PPhMe₂, due to the smaller steric hindrance and large electron-donating ability imposed by PPhMe₂, the trans to cis isomerization is practically prohibited and trans isomer 2c as the kinetic product is completely stable so that it can be crystallized and characterized by X-ray crystallography.

3. Experimental

3.1. General Procedures and Materials

¹H-NMR (400 MHz) and ³¹P{¹H}-NMR (162 MHz) spectra were recorded on a Bruker Avance III instrument (Ettlingen, Germany) and are referenced to the external standards, i.e., SiMe₄ and 85% H₃PO₄, respectively. The chemical shifts (δ) being reported as ppm and coupling constants (J) expressed in Hz. UV-vis absorption spectra and kinetic studies were carried out by using an Ultrospec 4000 Pro, UV-vis spectrometer (Little Chalfont, UK) with temperature control using a Pharmacia Biotech constant-temperature bath. Excitation and emission spectra were obtained on a PerkinElmer LS45 fluorescence spectrometer (Beaconsfield, UK) with the lifetimes measured in phosphorimeter mode, and the quantum yields of the complexes were measured using an integrating sphere. 2-Vinylpyridine (Vpy), triphenylphosphine (PPh₃), dimethylphenylphosphine (PPhMe₂) and the other chemicals were purchased from Sigma Aldrich (St. Louis, MO, USA). All of the reactions were carried out under an argon atmosphere, and all of the common solvents were purified and dried according to standard procedures. The complexes [PtMe(Vpy)(DMSO)], A, [PtMe(Vpy)(PPh₃)], 1a, [PtMe(Vpy)(PPh₂Me)], 1b, and [PtMe₂I(Vpy)(PPh₂Me)], 2b, were prepared according to literature methods [75,76,77]. The NMR labeling for the Vpy ligand for clarifying the chemical shift assignments is shown in Scheme 3. New NMR data for 1a, ¹H-NMR (400 MHz, acetone-d₆, 295 K): δ 0.69 (d, ³J_PH = 8.1, ²J_PtH = 84.2 Hz, 3H, PtMe), 6.57 (t, ³J_HH = 6.3 Hz, 1H, H⁵), 7.25 (d, ³J_HH = 7.8 Hz, 1H, H³), 7.33 (dd, ³J_HH = 9.2 Hz, ⁴J_PH = 15.2 Hz, ³J_PtH = 94.3 Hz, 1H, H^α), 7.44–7.49 (m, 9H, H^m and H^p PPh₃), 7.58 (d, ³J_HH = 5.3 Hz, ³J_PtH = 16.4 Hz, 1H, H⁶), 7.68–7.74 (m, 7H, H⁴ and H^o PPh₃), 7.76 (dd, ³J_HH = 9.2 Hz, ³J_PH = 7.8 Hz, ²J_PtH = 163.4 Hz, 1H, H^β); ³¹P{¹H}-NMR (162 MHz, acetone-d₆, 295 K): δ 31.8 (s, ¹J_PtP = 2021 Hz, 1P).

3.2. Synthesis of Complexes

3.2.1. [PtMe(Vpy)(PPhMe₂)], 1c

To a solution of [PtMe(Vpy)(DMSO)], A, (150 mg, 0.382 mmol) in acetone (15 mL) was added PPhMe₂ ligand (60 μL, 0.420 mmol, 1.1 equivalent). The mixture was stirred at room temperature for 2 h. The deep orange solution was concentrated to a small volume (~2 mL), and n-hexane was added (3 mL) to give an orange solid identified as 1c. Yield: 93%; Anal. Calcd for C₁₆H₂₀NPPt (452.39); C, 42.48; H, 4.46; N, 3.10. Found: C, 42.91; H, 4.56; N, 3.17. ¹H-NMR (400 MHz, acetone-d₆, 295 K): δ 0.95 (d, ³J_PH = 8.7, ²J_PtH = 85.3 Hz, 3H, PtMe), 1.71 (d, ²J_PH = 7.7, ³J_PtH = 20.9 Hz, 6H, Me of PPhMe₂), 6.77 (td, ³J_HH = 6.5 Hz, ⁴J_HH = 1.4 Hz, 1H, H⁵), 7.20 (d, ³J_HH = 7.9 Hz, 1H, H³), 7.24 (dd, ³J_HH = 9.4 Hz, ⁴J_PH = 15.7 Hz, ³J_PtH = 87.1 Hz, 1H, H^α), 7.45–7.48 (m, 3H, H⁴ and H^m PPhMe₂), 7.72 (t, ³J_HH = 8.2 Hz, 1H, H^p PPhMe₂), 7.73 (t, ³J_HH + ³J_PH = 9.1 Hz, 7.8 Hz, ²J_PtH = 149.1 Hz, 1H, H^β), 7.89–7.95 (m, 3H, H⁶ and H^o PPh₃); ³¹P{¹H}-NMR (162 MHz, acetone-d₆, 295 K): δ −4.6 (s, ¹J_PtP = 1967 Hz, 1P).

3.2.2. cis-[PtMe₂I(Vpy)(PPh₃)], 3a

To solution of [PtMe(Vpy)(PPh₃)], 1a, (100 mg, 0.173 mmol) in acetone (15 mL) were added 250 μL (excess, 25-fold) of MeI. The solution was stirred for 3 h at room temperature, then diethyl ether was added to give a precipitate, which was filtered, washed with diethyl ether to give the product as a pale yellow solid identified as 3a. The product was dried in vacuum. Yield: 79%; Anal. Calcd for C₂₇H₂₇INPPt (718.47); C, 45.14; H, 3.79; N, 1.95. Found: C, 45.32; H, 3.87; N, 1.82. ¹H-NMR (400 MHz, acetone-d₆, 295 K): 1.04 (d, ³J_PH = 7.8, ²J_PtH = 60.7 Hz, 3H, Me ligand trans to PPh₃, PtMe), 1.75 (d, ³J_PH = 7.9, ²J_PtH = 69.8 Hz, 3H, Me ligand trans to N of Vpy, PtMe), 6.65 (dd, ³J_HH = 6.9 Hz, ⁴J_PH = 2.6 Hz, ³J_PtH = 104.2 Hz, 1H, H^α), 6.95 (t, ³J_HH = 6.7 Hz, 1H, H⁵), 7.31–7.45 (m, 9H, H^m and H^p PPh₃), 7.51–7.96 (m, 9H, H^β, H³, H⁴ and H^o PPh₃), 9.07 (d, ³J_HH = 5.1 Hz, ³J_PtH = 13.3 Hz, 1H, H⁶); ³¹P{¹H}-NMR (162 MHz, acetone-d₆, 295 K): δ −7.8 (s, ¹J_PtP = 1036 Hz, 1P).

3.2.3. trans-[PtMe₂I(Vpy)(PPhMe₂)], 2c

To solution of [PtMe(Vpy)(PPhMe₂)], 1c, (100 mg, 0.221 mmol) in acetone (15 mL) were added 150 μL (excess, 10-fold) of MeI. The solution was stirred for 3 h at room temperature, then diethyl ether was added to give a precipitate, which was filtered, washed with diethyl ether to give the product as a pale yellow solid identified as 2c. The product was dried in vacuum. Yield: 81%; Anal. Calcd for C₁₇H₂₃INPPt (594.33); C, 34.36; H, 3.90; N, 2.36. Found: C, 34.25; H, 3.92; N, 2.33. ¹H-NMR (400 MHz, acetone-d₆, 295 K): 0.65 (d, ³J_PH = 7.6, ²J_PtH = 70.3 Hz, 3H, Me ligand trans to I, PtMe), 1.26 (d, ³J_PH = 8.2, ²J_PtH = 69.4 Hz, 3H, Me ligand trans to N of Vpy, PtMe), 1.98 (d, ²J_PH = 8.9, ³J_PtH = 12.6 Hz, 3H, Me of PPhMe₂), 2.21 (d, ²J_PH = 9.0, ³J_PtH = 11.1 Hz, 3H, Me of PPhMe₂), 6.83 (td, ³J_HH = 6.6 Hz, ⁴J_HH = 1.3 Hz, 1H, H⁵), 7.03 (dd, ³J_HH = 8.0 Hz, ⁴J_PH = 21.7 Hz, ³J_PtH = 66.9 Hz, 1H, H^α), 7.44 (d, ³J_HH = 7.8 Hz, 1H, H³), 7.52–7.59 (m, 3H, H^p and H^m PPhMe₂), 7.72 (td, ³J_HH = 7.7 Hz, ⁴J_HH = 1.5 Hz, 1H, H⁴), 7.81 (d, ³J_HH = 5.1 Hz, ³J_PtH = 12.0 Hz, 1H, H⁶), 7.86–7.92 (m, 2H, H^o PPh₃), 8.03 (dd, ³J_HH = 8.0 Hz, ³J_PH = 9.2 Hz, ²J_PtH = 75.6 Hz, 1H, H^β); ³¹P{¹H}-NMR (162 MHz, acetone-d₆, 295 K): δ −37.8 (s, ¹J_PtP = 1278 Hz, 1P).

3.3. Kinetic Study

A solution of 1a or 1c in acetone (3 mL, 2.5 × 10⁻⁴ M) in a cuvette was thermostated at 25 °C, and a known excess of MeI (150 μL, 3200-fold for 1a; 15 μL, 320-fold for 1c) was added using a microsyringe. After rapid stirring, the absorbance at λ = 387 (1a) or 386 (1c) nm was collected with time. The absorbance-time curves were analyzed by the pseudo-first-order method (A_t = (A₀ − A_∞) exp(−k_obst) + A_∞). The same method was used at other concentrations, and temperatures (10, 15, 25, and 30 °C) and activation parameters (ΔS^# and ΔH^#) were obtained from the Eyring equation (Equation (1)) and the full data are collected in Table 1. Ref. [79]

$$\ln \left(\frac{k_2}{\mathrm{T}}\right) = \ln \left(\frac{k_{\mathrm{B}}}{h}\right) + \frac{\Delta {\mathrm{S}}^{‡}}{\mathrm{R}} - \frac{\Delta {\mathrm{H}}^{‡}}{\mathrm{RT}}.$$

(1)

3.4. X-ray Crystallography

The X-ray diffraction measurement was carried out on a STOE IPDS2T diffractometer (STOE & Cie GmbH, Darmstadt, Germany) with graphite-monochromated Mo Kα radiation. The single crystals suitable for X-ray analysis were obtained from CH₂Cl₂/n-hexane solution (at room temperature) and mounted on glass fiber, and used for data collection. Cell constants and an orientation matrix for data collection were obtained by least-square refinement of the diffraction data for 1c and 2c. Diffraction data were collected in a series of ω scans in 1° oscillations and integrated using the Stoe X-AREA software package (Stoe & Cie GmbH, Darmstadt, Germany) [80]. Numerical absorption correction was applied using X-Red32 software (Stoe & Cie GmbH, Darmstadt, Germany). The structure was solved by direct methods and subsequent difference Fourier maps and then refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters. Atomic factors are from the International Tables for X-ray Crystallography. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. All refinements were performed using the X-STEP32, SHELXL-2014 and WinGX-2013.3 programs [81,82,83,84,85] Crystallographic data for the structural analysis was deposited with the Cambridge Crystallographic Data Centre, No. CCDC-2057874 (for 1c) and CCDC-2057873 (for 2c).

3.5. Computational Details

Density functional calculations were performed with the program suite Gaussian09 (Wallingford, CT, USA) [86] using the B3LYP level of theory [87,88,89]. The LANL2DZ basis set was chosen to describe Pt [90,91] and the 6-31G(d) basis set was chosen for other atoms. The geometries of complexes were fully optimized by employing the density functional theory without imposing any symmetry constraints. To ensure the optimized geometries, frequency calculations were performed employing analytical second derivatives. CH₂Cl₂ as solvent was introduced using the conductor-like polarizable continuum model (CPCM) [92,93]. The calculations for the electronic absorption spectra by time-dependent DFT (TD–DFT) were performed at the same level of theory. The compositions of molecular orbitals and theoretical absorption spectra were plotted using the “Chemissian” software (Petersburg, Russia) [94].

4. Conclusion

We have reported synthesis, photophysical properties of 1b, and kinetic investigation of its reaction with MeI [76]. In this work, we completed the trend of phosphine ligands by synthesis of 1a and 1c. The effect of phosphines on the photophysical properties of the cycloplatinated(II) complexes and also on the rates of oxidative addition reactions with MeI studied. These complexes show emission bands in the yellow-orange region with the same wavelength of 550 nm. The emission bands exhibit a structured shape, which is indicative of the mixed character ³ILCT/³MLCT and emission from the Vpy cyclometalated ligand. However, 1c shows the strongest emission, while 1a shows the weakest emission, which is probably related to the electron-donating ability of the phosphine ligands, which obeys the trend PPhMe₂ > PPh₂Me > PPh₃. On the other hand, the nature of phosphine ligands affects the rate of oxidative addition reactions of cycloplatinated(II) complexes with MeI. The expected rate trend of 1c > 1b > 1a was obtained, which is attributed to the electron-donating ability of the phosphine ligands (PPhMe₂ > PPh₂Me > PPh₃), being favorable for the oxidative addition reaction. In this regard, trans to cis isomerization is also observed for two cases of 2a (very fast) and 2b (slow), which is controlled by the steric hindrance induced by the phosphine. However, 2c as the trans isomer and kinetic product is stable and can be isolated and crystallized, which is due to the small steric hindrance of the PPhMe₂ ligand.