Strongly Luminescent Pt(IV) Complexes with a Mesoionic N-Heterocyclic Carbene Ligand: Tuning Their Photophysical Properties

Abstract

The synthesis, electrochemistry, and photophysical properties of a series of bis-cyclometalated Pt(IV) complexes that combine the mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with either 1-phenylpyrazole or 2-arylpyridine (C^∧N) are reported. The complexes (OC-6-54)-[PtCl₂(C^∧N)(trz)] bearing cyclometalating 2-arylpyridines present phosphorescent emissions in the blue to yellow color range, which essentially arise from ³LC(C^∧N) states, and reach quantum yields of ca. 0.3 in fluid solutions and almost unity in poly(methyl methacrylate) (PMMA) matrices at 298 K, thus representing a class of strong emitters with tunable properties. A systematic comparison with the homologous C₂-symmetrical species (OC-6-33)-[PtCl₂(C^∧N)₂], which contains two equal 2-arylpyridine ligands, shows that the introduction of a trz ligand leads to significantly lower nonradiative decay rates and higher quantum efficiencies. Computational calculations substantiate the effect of the carbene ligand, which raises the energy of dσ* orbitals in these derivatives and results in the higher energies of nonemissive deactivating ³LMCT states. In contrast, the isomers (OC-6-42)-[PtCl₂(C^∧N)(trz)] are not luminescent because they present a ³LMCT state as the lowest triplet.

Short abstract

Bis-cyclometalated Pt(IV) complexes bearing an aryl-1,2,3-triazolylidene ligand and heteroaromatic C^∧N ligands with different energies for the lowest π−π* transition are reported, constituting a class of strongly phosphorescent compounds with tunable properties. A comparison with bis-cyclometalated complexes bearing only C^∧N ligands provides the first systematic demonstration of the electronic effects of a cyclometalating aryl-NHC ligand on the luminescence of Pt(IV) complexes.

Introduction

Transition-metal complexes featuring long-lived emissive triplet excited states are at the core of numerous technological, analytical, biomedical, and synthetic developments, including chemosensing,^1,2 cell imaging,³ photodynamic therapy,⁴ photocatalysis,⁵ and light-emitting materials.⁶⁻⁸ Over the past decades, most research in this area has focused on luminescent Ir(III)⁸⁻¹³ and Pt(II)¹⁴⁻¹⁸ complexes with cyclometalating heteroaromatic ligands because of the high tunability and adaptability of their excited states, whereas Pt(IV) complexes have only started to be systematically explored as strong emitters in recent years.¹⁹⁻²⁷

In previous contributions, we have shown that several types of Pt(IV) complexes bearing cyclometalating 2-arylpyridines may exhibit efficient and long-lived luminescence from essentially ligand-centered triplet excited states (³LC) that possess a very low metal-to-ligand charge-transfer (MLCT) admixture.^{19,20,22,23,27} These characteristics make them promising candidates for applications that take advantage of relatively long excited-state lifetimes, such as sensing, singlet-oxygen sensitization, or photocatalysis. Although small, the extent of the MLCT contribution to the emissive state has been observed to fluctuate depending on the coordination environment, causing variations in the radiative rates.^21,22,28 Thus, shorter Pt–C bonds from metalated aryls or the presence of suitable π-donor ancillary ligands, e.g., the fluoride ion, result in occupied dπ orbitals with higher energies and greater MLCT admixtures, leading to higher radiative rates.²¹ However, a more critical factor that influences the emission properties of cyclometalated Pt(IV) complexes is the presence of thermally accessible ligand-to-metal charge-transfer (LMCT) excited states originating from electronic promotions to dσ* orbitals, which can provide the effective nonradiative deactivation of the emissive excited state. Higher-energy LMCT states can be achieved by introducing strong σ-donor ligands, which usually lead to lower nonradiative rates and increased emission efficiencies.²³

N-Heterocyclic carbenes (NHCs) have emerged as very valuable ligands for the design of highly efficient luminescent complexes of late-transition-metal ions, such as Ir(III),^{29,30,39,31−38} Pt(II),^{40,41,50,42−49} or Au(III).⁵¹ The beneficial effects exerted by these ligands can be attributed to their exceptional σ-donor capabilities,^52,53 which lead to strong ligand–field splittings and an increased energy of nonemissive excited states that arise from electronic transitions to dσ* orbitals, which could otherwise become thermally populated and cause nonradiative deactivation or even degradation via ligand–metal σ-bond labilization. Consequently, the use of NHCs brings about improved stabilities and emission efficiencies, which are particularly important for the development of blue emitters. Diverse types of NHC ligands have been used to synthesize luminescent complexes, including chelating dicarbenes (C*^∧C*),⁵⁴⁻⁵⁶ pyridyl-NHCs (N^∧C*),^57,58 and cyclometalated aryl-NHCs (C^∧C*).^43,50,59 Most incorporate normal Arduengo-type NHC moieties, whereas the use of mesoionic NHCs is rather infrequent.^{46,47,60−62} Although cyclometalating aryl-NHCs have been demonstrated as chromophoric ligands in homoleptic Ir(III) emitters,³⁷⁻³⁹ mixed-ligand systems have also been developed in which they act as supporting ligands while other chelating heteroaromatic ligands, such as arylpyridines^34,63 or bipyridines,³² are responsible for the emission.

We have recently developed a synthetic method that allowed the preparation of the first examples of Pt(IV) complexes bearing a cyclometalated aryl-NHC ligand.⁶⁴ The reported complexes combined a mesoionic carbene of the 1,2,3-triazolylidene subclass (C^∧C*) and either a monocyclometalating 2,6-diarylpyridine or a dicyclometalating 2,6-diarylpyridine (C^∧N^∧CH or C^∧N^∧C, respectively) and were found to display exceptionally intense phosphorescence in poly(methyl methacrylate) (PMMA) matrices at 298 K, which originated from a ³LC state involving the C^∧N^∧CH or C^∧N^∧C ligand. In addition, the complex with a monocyclometalating C^∧N^∧CH ligand showed an intense luminescence in a fluid solution, which was marginally enhanced compared to that of similar bis-cyclometalated Pt(IV) complexes containing only C^∧N ligands. However, no systematic evidence of the effects of supporting C^∧C* ligands on the emission efficiencies of Pt(IV) complexes has been gathered so far.

In this work, we present a family of bis-cyclometalated Pt(IV) complexes bearing an aryl-1,2,3-triazolylidene ligand and cyclometalating C^∧N ligands of different energies for the lowest π–π* transition, which exhibit strong phosphorescent emissions in fluid solutions and can reach quantum efficiencies of almost unity in PMMA matrices. Their emission properties are compared to those of bis-cyclometalated complexes bearing only C^∧N ligands with the aim of providing a clear and general demonstration of the electronic effects of the C^∧C* ligand.

Results and Discussion

Synthesis

Scheme 1 shows the synthetic route to the targeted bis-cyclometalated complexes (OC-6-54)-[PtCl₂(C^∧N)(trz)] (3a–e), where trz = cyclometalating 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene and C^∧N = cyclometalating 1-phenylpyrazole (ppz, a), 2-(2,4-difluorophenyl)pyridine (dfppy, b), 2-phenylpyridine (ppy, c), 2-(p-tolyl)pyridine (tpy, d), or 2-(2-thienyl)pyridine (thpy, e), based on the previously described methodology for (OC-6-54)-[PtCl₂(dtpyH)(trz)] (dtpyH = monocyclometalating 2,6-di(p-tolyl)pyridine).⁶⁴ The reaction of dichlorido-bridged dimers [Pt₂(μ-Cl)₂(C^∧N)₂] (1a–e) with the in situ-generated silver carbene “AgI(trzH)” led to the selective formation of complexes trans-C,C*-[PtCl(C^∧N)(trzH)] (trans-C,C*-2a–e), where the carbene coordinates in a trans-configuration to the metalated aryl of the C^∧N ligand. The photoisomerization to the corresponding cis-C,C*-2a–e complexes was achieved by either irradiating acetone solutions of trans-C,C*-2b–e with visible light (λ = 454 nm) or irradiating a MeCN solution of trans-C,C*-2a with UV light (λ = 310 nm). In contrast to the related Pt(II) species cis-N,N-[PtCl(C^∧N)(N^∧CH)], which undergo a photochemical cyclometalation of the coordinated N^∧CH ligand to give a bis-cyclometalated Pt(IV) hydride,⁶⁵ the cyclometalation of the coordinated trzH ligand did not occur in any of the studied cases.

Scheme 1.

(i) Ag₂O, (ii) [Pt₂(μ-Cl)₂(C^∧N)₂] (1), (iii) blue LEDs, and (iv) PhICl₂.

The photoisomerization process can be easily followed by ¹H NMR spectrometry because the resonance of the proton ortho to the metalated carbon of the C^∧N ligand is significantly shielded in the cis-isomers due to the diamagnetic current of the triazolylidene ring (e.g., 7.83 vs 6.31 ppm for trans- and cis-C,C*-2d, respectively). Further confirmation of the isomerization was provided by the X-ray diffraction study of complex cis-C,C*-2d (Figure 1).

Figure 1.

Structure of cis-C,C*-2d (thermal ellipsoids at 50% probability). Hydrogen atoms are omitted. Selected bond distances (Å) and angles (°) are as follows: Pt–C1, 1.9825(17); Pt–N1, 2.0669(15); Pt–C13, 1.9670(18); Pt–Cl, 2.4091(5); C1–Pt–N1, 81.13(7); and N1–Pt–C13, 174.67(6).

The treatment of cis-C,C*-2a–e with PhICl₂ led to the bis-cyclometalated complexes 3a–e, respectively, as the major products in all cases. However, the trichlorido complex (OC-6-41)-[PtCl₃(C^∧N)(trzH)] (4a–e) was also formed as a minor product (Scheme 1). This result differs from the reported reactions of the related cis- or trans-N,N-[PtCl(C^∧N)(N^∧CH)] complexes with PhICl₂, which exclusively led to bis-cyclometalated Pt(IV) complexes.^19,24,66,67 Complexes 3 and 4 were obtained in different molar ratios depending on the C^∧N ligand (Table S2). The most favorable outcome was obtained with the dfppy ligand in a 95:5 ratio (3b:4b), whereas the tpy ligand led to the lowest molar proportion of the bis-cyclometalated complex in a 60:40 ratio (3d:4d). The formation of these mixtures can be attributed to two competing processes that take place from the pentacoordinate Pt(IV) intermediate arising from the formal addition of a Cl⁺ ion to cis-C,C*-2a–e (Scheme 2). The electrophilic metalation of the phenyl ring of the trzH ligand (path A) leads to 3, whereas the coordination of the Cl^– ion released from the PhICl₂ reagent in the vacant coordination site produces 4 (path B). Apparently, the electrophilic metalation is less favored for the present triazolylidene ligand compared to that for N-coordinated 2-arylpyridines. The fact that derivative 3b was obtained in a higher molar proportion can be explained by the higher electrophilic character of the metal center in this case because of the diminished electron-donating ability of the dfppy ligand.

Scheme 2.

The isolation of complexes 3 from the above mixtures was only possible after several recrystallizations, resulting in low to moderate yields (from 13% for 3d to 50% for 3b). Complexes 4 could not be obtained in pure forms except for the tpy derivative 4d; nevertheless, the ¹H NMR spectra of enriched fractions allowed us to unequivocally establish their identities. In the case of the tpy derivative 4d, the considerable shielding of the proton ortho to the metalated tolyl carbon (6.71 ppm, J_HPt = 33 Hz) indicates that it is directed toward the triazolylidene moiety and is affected by its ring current, implying that the mutual disposition of the tpy and carbene ligands is retained after the oxidative addition of PhICl₂. In view of this configuration, we considered forcing the metalation of the phenyl group of the carbene ligand in complexes 4a and 4c–e at a high temperature in the presence of a base. Thus, by heating mixtures of 3 and 4 at 130 °C in 1,2-dichlorobenzene in the presence of Na₂CO₃, complexes 4 produced the corresponding complexes 3, which could then be isolated in improved yields (38–64%).

The ¹H NMR spectra of complexes 3 corroborated the presence of two metalated aryl groups, each of which gives a considerably shielded resonance flanked by ¹⁹⁵Pt satellites arising from the proton ortho to the metalated carbon, which is affected by the diamagnetic current of an orthogonal ring. The crystal structures of 3d and 3e (Figures 2 and 3, respectively) are compatible with the NMR data, further confirming that the metalated aryls are mutually cis, while the carbene and pyridine moieties are trans to each other.

Figure 2.

Thermal ellipsoid representation (50% probability) of the crystal structure of 3d. Hydrogen atoms and solvent molecules are omitted. Selected bond distances (Å) and angles (°) are as follows: Pt–C1, 2.011(2); Pt–N1, 2.0866(19); Pt–C20, 1.987(2); Pt–C41, 2.019(2); Pt–Cl1, 2.4361(6); Pt–Cl2, 2.4295(6); C1–Pt–N1, 80.84(8); C20–Pt–N1, 173.32(8); and C20–Pt–C41, 80.80(10).

Figure 3.

Thermal ellipsoid representation (50% probability) of the crystal structure of 3e. Hydrogen atoms are omitted. Selected bond distances (Å) and angles (°) are as follows: Pt–C1, 1.992(2); Pt–N1, 2.1031(18); Pt–C20, 1.983(2); Pt–C41, 2.024(2); Pt–Cl1, 2.4227(5); Pt–Cl2, 2.4178(6); C1–Pt–N1, 80.16(8); C20–Pt–N1, 172.66(8); and C20–Pt–C41, 80.77(9).

We also attempted the preparation of bis-cyclometalated complexes with a trans-arrangement of the carbene and aryl moieties to compare their photophysical properties with those of the isomeric complexes 3. The reaction of trans-C,C*-2a with PhICl₂ afforded a mixture from which complex (OC-6-42)-[PtCl₂(ppz)(trz)] (5a; Scheme 3) could be isolated in a 23% yield, while the other products could not be identified. In the case of trans-C,C*-2d, the same reaction gave a mixture of the desired complex (OC-6-42)-[PtCl₂(tpy)(trz)] (5d) and the trichlorido complex (OC-6-43)-[PtCl₃(tpy)(trzH)] (6d) in a ca. 18:82 molar ratio; the mixture could be separated thanks to their different solubilities in MeOH, and the complexes were isolated in 11 and 67% yields, respectively. The ¹H NMR spectrum of complex 6d shows that the protons ortho to either the metalated p-tolyl or the coordinated N atom are not shielded by the triazolylidene ring, implying that the carbenic carbon is not coplanar with the tpy ligand. Reasonably, the cationic Pt(IV) intermediate complex isomerizes to avoid the trans-arrangement of the tolyl and triazolylidene groups, which explains the low yield in 5d. A similar result was previously found upon the oxidation of an analogous complex bearing monocyclometalating 2,6-di(p-tolyl)pyridine with PhICl₂.⁶⁴

Scheme 3.

The crystal structures of 5a and 5d (Figures 4 and 5, respectively) confirmed the expected ligand arrangement. In both cases, the Pt–C* bond is nearly 0.1 Å longer than those in complexes 3 because of the high trans-influence exerted by the metalated aryl ring of the C^∧N ligand. Additionally, significantly longer Pt–C1 bond lengths were found, e.g., 2.044 Å for 5d vs 2.011 Å for 3d, as a consequence of the higher trans-influence of the triazolylidene ring relative to that of the chlorido ligand.

Figure 4.

Thermal ellipsoid representation (50% probability) of the crystal structure of 5a. Hydrogen atoms are omitted. Selected bond distances (Å) and angles (°) are as follows: Pt–C1, 2.050(2); Pt–N1, 2.006(2); Pt–C20, 2.078(2); Pt–C41, 2.025(2); Pt–Cl1, 2.4227(6); Pt–Cl2, 2.3177(6); C1–Pt–N1, 80.09(9); and C20–Pt–C41, 80.71(10).

Figure 5.

Thermal ellipsoid representation (50% probability) of the crystal structure of 5d. Hydrogen atoms are omitted. Selected bond distances (Å) and angles (°) are as follows: Pt–C1, 2.0444(15); Pt–N1, 2.0363(13); Pt–C28, 2.0832(15); Pt–C12, 2.0285(15); Pt–Cl1, 2.4196(4); Pt–Cl2, 2.3293(4); C1–Pt–N2, 80.79(6); and C12–Pt–C28, 80.43(6).

Photophysical Properties

The electronic absorption spectra of 3a–e, 5a, and 5d were registered in a CH₂Cl₂ solution at 298 K (Table 1 and Figure 6). Structured absorption bands are observed for complexes 3a–e in the 250–360 nm range that can be ascribed to essentially ¹LC transitions involving the cyclometalating ligands.^20,23,24 The lowest-energy band resembles those observed for complexes [PtMe(Cl)(C^∧N)₂];²³ its lowest maximum shifts from 320 to 354 nm along the sequence 3a → 3e in accordance with the decreasing energies of the lowest π–π* transition of the C^∧N ligands and can therefore be ascribed to a primarily ¹LC(C^∧N) excitation. The absorption spectra of 5a and 5d differ from their respective isomeric complexes 3a and 3d mainly in regard to the lowest-energy feature, which appears to enclose additional absorptions. This is particularly evident for 5d, whose lowest-energy feature is significantly red-shifted with respect to that of 3d. We attribute these differences to LMCT transitions on the basis of TDDFT calculations (see below).

Table 1. Electronic Absorption Data for the Studied Complexes in a CH₂Cl₂ Solution (ca. 5 × 10^–5 M) at 298 K.

complex	λmax (nm) (ε × 10–2 (M–1 cm–1))
3a	272 (111), 303 (54), 320 (21)
3b	255 (189), 303 (94), 311 (99), 321 (86)
3c	259 (218), 269 (sh, 184), 306 (103), 320 (95), 328 (79)
3d	262 (254), 271 (sh, 228), 310 (125), 322 (134), 333 (109)
3e	260 (170), 269 (171), 283 (168), 297 (sh, 135) 346 (91), 354 (87)
5a	264 (179), 299 (86), 314 (43)
5d	267 (200), 303 (94), 315 (sh, 87), 336 (71), 347 (65)

Figure 6.

Electronic absorption spectra of complexes 3a–e, 5a, and 5d in a CH₂Cl₂ solution at 298 K.

The photoluminescence of 3a–e was examined in a deaerated CH₂Cl₂ solution and poly(methyl methacrylate) (PMMA) matrices (2 wt %) at 298 K. Air-equilibrated samples were also examined to evaluate the luminescence quenching of these complexes by atmospheric oxygen. The obtained emission data are compiled in Table 2, and the spectra in the deaerated CH₂Cl₂ solution are shown in Figure 7. In all cases, the excitation spectra match the corresponding absorption profiles (Figure S29). The excitation and emission spectra in PMMA are almost identical to those in CH₂Cl₂ (Figure S30). In the absence of molecular oxygen, complexes 3b–e show intense emissions in both media, whereas 3a is not emissive in CH₂Cl₂ and only weakly so in PMMA. In all cases, the bands are vibronically structured, and the energy of the highest-energy peak correlates with the triplet emission of the respective N^∧CH ligand (ppzH, 378 nm;⁶⁸ dfppyH, 424 nm;⁶⁹ ppyH, 430 nm;⁷⁰ tpyH, 437 nm;⁶⁹ and thpyH, 485 nm⁷⁰). Therefore, the C^∧N ligand is the chromophoric one in all cases, while the cyclometalating trz acts as a supporting ligand. The radiative lifetimes range from tens to hundreds of microseconds, which is consistent with triplet emissive states of an essentially ligand-centered character (³LC). The complexes bearing ppy-based ligands (3b–d) are the most efficient emitters, with quantum yields around 0.3 in solution that increase to almost unity in PMMA matrices; the latter are the highest values ever observed for Pt(IV) complexes. An analysis of their radiative and nonradiative rate constants (k_r and k_nr, respectively) shows that the large increases in the quantum yields in PMMA are a consequence of the inhibition of molecular motion, which leads to dramatic decreases in the value of k_nr. The significantly weaker emission of the ppz derivative 3a can be attributed to the thermal population of a nonemissive ³LMCT excited state because of the high energy of the ³LC(ppz) state; a similar behavior has been reported for the tris-cyclometalated complexes fac-[Ir(ppz)₃]³⁷ and fac-[Pt(ppz)₃]⁺.¹⁹ The lower efficiency of the thpy complex 3e compared with those of 3b–d can be explained by its lower emission energy, which must result in an increased nonradiative deactivation via a vibrational overlap with the ground state.

Table 2. Emission Data of Complexes 3a–e.

complex	mediuma	λem (nm)b	Φc	τ (μs)d	kr × 10–3 (s–1)e	knr × 10–3 (s–1)f
3a	PMMA	417, 467, 487	0.044	46	0.96	21
3b	CH2Cl2	436, 466, 494 (sh), 554 (sh)	0.27	133	2.0	5.5
	PMMA	435, 464, 493	0.97	295	3.3	0.088
	CH2Cl2 (air)	436, 465, 495	0.012	4.3	2.8	233
	PMMA (air)	436, 465, 494	0.23	88 (48%), 207 (52%)
3c	CH2Cl2	447, 478, 504 (sh)	0.31	110	2.8	6.2
	PMMA	446, 478, 504 (sh)	0.93	258	3.6	0.26
	CH2Cl2 (air)	448, 478, 503 (sh)	0.009	3.4	2.6	291
	PMMA (air)	447, 478, 504 (sh)	0.23	80 (41%), 178 (59%)
3d	CH2Cl2	453, 485, 514 (sh)	0.26	140	1.8	5.3
	PMMA	452, 484, 511 (sh)	0.86	319	2.7	0.43
	CH2Cl2 (air)	453, 484, 513 (sh)	0.007	2.5	2.8	397
	PMMA (air)	453, 484, 513 (sh)	0.20	88 (52%), 213 (48%)
3e	CH2Cl2	513, 530, 552	0.046	45	1.0	21
	PMMA	512, 528, 551	0.49	304	1.6	1.7
	PMMA (air)	513, 530, 552	0.11	86 (41%), 196 (39%)

^aUnder the exclusion of oxygen, except where noted.

^bThe most intense peak is italicized.

^cQuantum yield.

^dEmission lifetime; relative amplitudes are given in parentheses for biexponential decays.

^eRadiative rate constant, k_r = Φ/τ.

^fNonradiative rate constant, k_nr = (1 – Φ)/τ.

Figure 7.

Emission spectra of complexes 3b–e in deaerated CH₂Cl₂ solutions at 298 K.

Air-equilibrated samples of 3b–d showed measurable luminescence, with quantum yields around 0.01 in CH₂Cl₂ and 0.20 in PMMA. However, no emission could be detected from 3a in any medium, and 3e was emissive only in PMMA in the presence of atmospheric oxygen. Lifetimes dropped to a few microseconds in CH₂Cl₂. The calculated k_r values in this medium remained the same order of magnitude as those obtained from deaerated samples, but k_nr values increased by two orders of magnitude as a consequence of oxygen quenching. Much longer lifetimes were observed for samples in PMMA matrices; however, they could only be fitted to biexponential decays, which is probably because of the inhomogeneous oxygen distribution. The present data show that complexes of this kind could be used for luminescence-based applications in the presence of atmospheric oxygen as well as for the development of oxygen sensors.

For comparison purposes, the luminescence of C₂-symmetrical complexes (OC-6-33)-[PtCl₂(C^∧N)₂] (Chart 1) with C^∧N = ppz,¹⁹ dfppy,¹⁹ ppy,⁶⁶ tpy¹⁹ and thpy²⁰ was also studied in a deaerated CH₂Cl₂ solution and PMMA matrices (2 wt %) (Table 3). These compounds show moderate or weak emissions except for the thpy derivative, which was not emissive in solution, and the ppz derivative, which did not show an emission in any medium. The observed emission spectra are almost identical in shape to those of the corresponding complexes 3, although they are slightly red-shifted (Figures S31 and S32). Single-exponential decays were observed for the ppy-based derivatives in the CH₂Cl₂ solution, whereas double-exponential decays were obtained in PMMA. Where possible, comparisons with complexes 3 show that lifetimes are significantly shorter for (OC-6-33)-[PtCl₂(C^∧N)₂]. In all cases, the measured quantum yields are much lower than those of complexes 3, implying that the replacement of one of the C^∧N ligands by a cyclometalated trz mainly results in an enhancement of the emission efficiencies. This beneficial effect is primarily reflected in the k_nr values, which are generally one order of magnitude lower for complexes 3 relative to the value of the corresponding (OC-6-33)-[PtCl₂(C^∧N)₂] complex, whereas variations in the value of k_r are much less significant. Reasonably, the stronger σ-donating ability of the carbene compared with that of the C^∧N ligand pushes the metal dσ*-orbitals to higher energies in complexes 3, implying both that the deactivating LMCT states lie at higher energies and that their thermal population from the emitting state is more difficult than those in (OC-6-33)-[PtCl₂(C^∧N)₂] complexes, leading to lower nonradiative decay rates.

Chart 1.

Table 3. Emission Data of C₂-Symmetrical Complexes (OC-6-33)-[PtCl₂(C^∧N)₂].

C∧N	mediuma	λem (nm)b	Φc	τ (μs)d	kr × 10–3 (s–1)e	knr × 10–3 (s–1)f
dfppy	CH2Cl2	440, 470, 506 (sh)	0.058	44	1.3	21
	PMMA	439, 469, 500 (sh)	0.21	76.6 (29%), 218 (71%)
ppyg	CH2Cl2	450, 481, 509 (sh)	0.11	27	4.1	33
	PMMA	450, 481, 509 (sh)	0.18	57.0 (28%), 157 (72%)
tpy	CH2Cl2	457, 489, 524 (sh)	0.15	40	3.8	21
	PMMA	457, 487, 521 (sh)	0.22	84.7 (20%), 208 (80%)
thpy	PMMA	515, 530 (sh), 553	0.054	111	0.5	8.5

^aUnder the exclusion of oxygen.

^bThe most intense peak is italicized.

^cQuantum yield.

^dEmission lifetime; relative amplitudes are given in parentheses for biexponential decays.

^eRadiative rate constant, k_r = Φ/τ.

^fNonradiative rate constant, k_nr = (1 – Φ)/τ.

^gData in the CH₂Cl₂ solution from ref (21).

In contrast to complexes 3, the isomeric 5a and 5d, where the carbene moiety is trans to the metalated aryl of the C^∧N ligand, are not emissive in either the CH₂Cl₂ solution or PMMA films at room temperature. A similar behavior was previously observed for the homologous unsymmetrical (OC-6-32)-[PtCl₂(C^∧N)₂] complexes (Chart 1), which was attributed to a thermally accessible and nonemissive ³LMCT excited state that provides an effective nonradiative deactivation pathway.²¹

Electrochemistry

The redox properties of the bis-cyclometalated complexes 3a–e, 5a, and 5d were examined by means of cyclic voltammetry in a MeCN solution. The voltammograms are depicted in Figure 8, and the potentials of the most important redox processes and highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO, respectively) energy estimations are compiled in Table 4. An irreversible oxidation peak was observed in the range from 1.71 to 2.03 V vs SCE except for the dfppy derivative 3b, where the oxidation must fall outside the accessible potential range. The associated HOMO energies vary according to the sequence 3c < 3a < 3d < 3e and agree with previously determined C^∧N-based π-orbital energies in cyclometalated Pt(IV) complexes.^19,20,27 The isomeric pairs 3a/5a and 3d/5d possess identical HOMO energies, suggesting that the HOMO is also primarily a π-orbital of the C^∧N ligand in complexes 5a and 5d. The first reduction is irreversible for all complexes and is visible in the range from −1.54 to −1.69 V vs SCE for 3a–e, whereas for 5a and 5d it appears at distinctively less negative potentials (−1.41 or −1.43 V vs SCE, respectively). The LUMO energies are almost identical for 3a, 3c, and 3d and somewhat lower for 3b and 3e. Since the ppz-, ppy-, and tpy-based LUMOs have been previously shown to have higher energies,¹⁹ it is likely that the LUMO in derivatives 3a, 3c, and 3d is a π*-orbital of the trz ligand, as predicted by the DFT calculations for 3d (see below). The LUMO energies found for 3b and 3e are compatible with a dfppy- and thpy-based orbital, respectively. In contrast, the significantly lower LUMO energies found for 5a and 5d imply that the LUMO is no longer ligand-localized in these complexes. Instead, it is assigned as a dσ*-orbital on the basis of DFT calculations (see below). In all cases, the first reduction is followed by a reversible or quasi-reversible wave in the E_1/2 range from −1.95 to −2.16 V vs SCE, and additional irreversible reductions were also observed. The reversible wave was observed at identical E_1/2 values for each of the pairs 3a/5a and 3d/5d, suggesting that the species produced as a consequence of the first irreversible reduction is not dependent on the ligand arrangement; however, its identity cannot be unambiguously established.

Figure 8.

Cyclic voltammograms of complexes 3a–e, 5a, and 5d in MeCN at 100 mV s^–1.

Table 4. Electrochemical Data^a and HOMO and LUMO Energy Estimations^b for Complexes 3a–e, 5a, and 5d.

complex	Ep,ac	Ep,cd	E1/2e	EHOMO	ELUMO	ΔEHOMO–LUMO
3a	1.98	–1.67, –2.59	–2.15	–6.54	–3.15	3.39
3b	–f	–1.56, –2.25	–1.95		–3.25
3c	2.03	–1.65, –2.30, –2.48	–2.01	–6.89	–3.15	3.74
3d	1.93	–1.69, –2.30, –2.53	–2.06	–6.48	–3.13	3.35
3e	1.71, 1.91	–1.54, –2.27, –2.44	–1.97	–6.32	–3.20	3.12
5a	1.90	–1.41, –2.60	–2.16	–6.50	–3.48	3.02
5d	1.86	–1.43, –2.30, –2.53	–2.06	–6.44	–3.42	3.02

^aIn volts versus SCE, registered in a 0.1 M solution of (Bu₄N)PF₆ in dry MeCN at 100 mV s^–1.

^bIn electronvolts.

^cIrreversible anodic peak potentials.

^dIrreversible cathodic peak potentials.

^eFor the reversible wave.

^fOutside the solvent window.

Computational Study

DFT and TDDFT calculations were performed for complexes 3d, (OC-6-33)-[PtCl₂(tpy)₂], and 5d (see the Supporting Information for details). Frontier orbital energies and their main characters are presented in Figure 9. In the three cases, the HOMO is essentially a π-orbital of the tpy ligand(s), with some contribution from metal dπ*-orbitals (4% for 3d, 7% for (OC-6-33)-[PtCl₂(tpy)₂], and 3% for 5d). The LUMO and LUMO + 1 in 3d are π-orbitals of the trz and tpy ligands, respectively, and those in (OC-6-33)-[PtCl₂(tpy)₂] correspond to π*-orbitals delocalized over the two tpy ligands. In contrast, the LUMO in 5d is essentially a dσ*-orbital that is mostly distributed along the N–Pt–Cl axis and lies at a noticeably lower energy with respect to the trz-based LUMO of 3d, which agrees with the electrochemical results. Notably, the lowest molecular orbital with a primarily dσ* character in complexes 3d and (OC-6-33)-[PtCl₂(tpy)₂] is LUMO + 2, which has a significantly higher energy for the trz complex, implying that a major effect of the carbene is to increase the ligand-field splitting.

Figure 9.

Orbital energy diagrams from DFT calculations for complexes 3d, (OC-6-33)-[PtCl₂(tpy)₂], and 5d.

The TDDFT calculations reveal a ligand-to-ligand charge transfer (LLCT) from the tpy ligand to the trz ligand, π(tpy)−π*(trz), and a LC transition within the tpy ligand, π(tpy)−π*(tpy), as the lowest singlet excitations in complex 3d (S₁ and S₂, respectively), whereas in (OC-6-33)-[PtCl₂(tpy)₂] only LC(tpy) excitations are predicted to contribute to the lowest-energy absorptions. The three lowest singlet excitations in complex 5d are predicted to be weak and involve transitions from π(tpy), π(trz), or p(Cl) orbitals to dσ*-orbitals that can hence be designated as LMCT or LMCT/XMCT; a more intense excitation of primarily LC(tpy) character is predicted at a higher energy (S₄). Therefore, the presence of low-energy LMCT absorptions explains the red-shifted lowest-energy feature in the absorption spectrum of 5d.

The lowest triplet excitation energies are represented in Figure 10. The first triplet (T₁) corresponds to an essentially LC(tpy) transition in complexes 3d and (OC-6-33)-[PtCl₂(tpy)₂], with a somewhat lower energy for the latter complex that is in agreement with the observed variation in emission energies. The lowest ³LMCT excitations are T₄ in 3d and T₃ in (OC-6-33)-[PtCl₂(tpy)₂]; the energy difference with respect to T₁ is higher for 3d (0.78 eV) than for (OC-6-33)-[PtCl₂(tpy)₂] (0.64 eV), implying that, consistent with the observed lower k_nr values, the thermal population from the emitting state should be less favorable for the carbene complex. In the case of 5d, the first triplet excitation is a LMCT transition, which explains the lack of emission of this complex.

Figure 10.

Lowest triplet excitation energies from TDDFT calculations at the ground-state geometry. The red lines correspond to essentially LMCT excitations.

For further insight, a geometry optimization of the lowest triplet excited state (T₁) was carried out for the three studied complexes. The corresponding spin density distribution (Figure 11) matches the topology of a π–π* excitation within the tpy ligand in 3d or one of the tpy ligands in (OC-6-33)-[PtCl₂(tpy)₂], which is consistent with an essentially ³LC(tpy) emitting state in these complexes, and the associated geometry variations relative to the ground state are mostly limited to the affected ligand (Table S13). The computed adiabatic energy differences with respect to the ground state are 2.78 eV for 3d and 2.75 eV for (OC-6-33)-[PtCl₂(tpy)₂], which are a good match with the observed emission energies. The natural spin densities on the Pt atom are 0.0187 for 3d and 0.0221 for (OC-6-33)-[PtCl₂(tpy)₂], indicating a small degree of metal orbital contribution and therefore a certain MLCT admixture in the essentially LC emitting state²¹ that is slightly higher for (OC-6-33)-[PtCl₂(tpy)₂]. This fact is consistent with both the increased metal orbital contribution to the HOMO in the latter complex and its lower emission energy that imply a higher energy of metal dπ orbitals, probably because the arylpyridine is a weaker π-acceptor than the cyclometalated trz.⁵² In the case of 5d, the spin density distribution in the relaxed T₁ state clearly corresponds to a LMCT state involving an electronic transition to a dσ* orbital, which causes severe geometry distortions that mostly result from Pt–ligand bond elongations (Table S14).

Figure 11.

Spin-density distributions (0.001 e bohr^–3) of the optimized lowest triplet excited states of 3d, (OC-6-33)-[PtCl₂(tpy)₂], and 5d.

Conclusions

Mixed-ligand Pt(IV) derivatives containing cyclometalating trz and C^∧N ligands of different energies for the lowest π–π* transition have been synthesized by the oxidative addition of PhICl₂ to Pt(II) precursors of the type cis- or trans-C,C*-[PtCl(C^∧N)(trzH)]. The electrophilic metalation of the pendant phenyl group of the trz ligand upon oxidation proved to be more difficult in comparison to analogous reactions involving 2-arylpyridines and compete with the coordination of a chlorido ligand. The complexes (OC-6-54)-[PtCl₂(C^∧N)(trz)] that contain cyclometalating 2-arylpyridines (3b–e) exhibit strong phosphorescent emissions that originate from ³LC states primarily localized on the C^∧N ligand, which can reach quantum yields of ca. 0.3 in a fluid solution and almost unity in PMMA matrices; the latter are the highest efficiencies ever observed for Pt(IV) complexes. Therefore, they constitute a class of strongly emissive compounds whose emission energies can be tuned by varying the C^∧N ligand. A comparison between the photophysical properties of 3 and those of the homologous C₂-symmetrical complexes (OC-6-33)-[PtCl₂(C^∧N)₂] showed that the replacement of one of the C^∧N ligands for trz results in lower nonradiative decay rates and higher quantum efficiencies. The computational results substantiate a higher energy of dσ* orbitals and deactivating ³LMCT states in complexes 3, which are attributed to the strong σ-donor character of the trz ligand. In contrast, the isomeric complexes (OC-6-42)-[PtCl₂(C^∧N)(trz)] (5), featuring a trans arrangement of the carbene and aryl moieties, are not emissive because they present a ³LMCT state as the lowest triplet, which involves a low-lying dσ* orbital along the N–Pt–Cl axis.

Experimental Section

General Considerations

Unless otherwise noted, procedures were performed at room temperature under atmospheric conditions using synthesis-grade solvents. Reactions involving silver reagents were conducted under a N₂ atmosphere in the dark. The dichlorido-bridged dimers 1a(71) and 1b–e,²⁸ PhICl₂,⁷² and the triazolium iodide salt⁷³ were synthesized following published procedures. NMR spectra were registered on Bruker Advance 300, 400, or 600 MHz spectrometers. Chemical shifts (δ) are given in parts per million downfield from tetramethylsilane. Elemental analyses were determined using a LECO CHNS-932 microanalyzer. The irradiation of trans-C,C*-2a was carried out using a 36 W Philips UVB Narrowband lamp centered at 310 nm. Complexes trans-C,C*-2b–e were irradiated with Blue LEDs following the previously described experimental setup.⁷⁴

General Procedure for the Synthesis of trans-C,C*-[PtCl(C^∧N)(trzH)] (trans-C,C*-2)

The triazolium salt (100 mg, 0.29 mmol) and Ag₂O (37 mg, 0.16 mmol) were suspended in CH₂Cl₂ (10 mL), and the mixture was stirred for 14 h. The suspension was filtered through Celite, and [Pt₂(μ-Cl)₂(C^∧N)₂] (1) (0.15 mmol) was immediately added to the filtrate. The mixture was stirred in the dark for 1 h and filtered through Celite. The filtrate was evaporated to dryness, and the residue was washed with Et₂O (3 × 5 mL) and vacuum-dried to give trans-C,C*-2.

trans-C,C*-2a

White solid, obtained from 1a (109 mg). Yield: 100 mg, 58%. ¹H NMR (300 MHz, CD₂Cl₂): δ 8.38 (dd, J_HH = 8.2, 1.7 Hz, 2H, H_arom), 7.93 (dd, J_HH = 7.5, 1.8 Hz, 1H, H_arom), 7.88 (d, J_HH = 3.0 Hz, 1H, H_arom), 7.49–7.39 (m, 3H, H_arom), 7.17–7.09 (m, 3H, H_arom), 6.89 (d with satellites, J_HH = 2.3 Hz, J_HPt = 16 Hz, 1H, H_arom), 6.26 (br t, J_HH = 2.6 Hz, 1H, H_arom), 4.16 (s, 3H, NCH₃), 3.23 (ddd, J_HH = 14.6, 9.6, 6.2 Hz, 1H, CH₂), 2.90 (ddd, J_HH = 14.6, 9.6, 6.2 Hz, 1H, CH₂), 1.91–1.69 (m, 2H, CH₂), 1.45–1.33 (m, 2H, CH₂), 0.87 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C NMR (75 MHz, CD₂Cl₂): δ 165.6 (C), 149.0 (C), 145.0 (C), 144.6 (C), 140.2 (C), 139.6 (CH), 134.9 (J_CPt = 34 Hz, CH), 129.8 (CH), 129.4 (CH), 126.2 (J_CPt = 35 Hz, CH), 125.6 (J_CPt = 38 Hz, CH), 124.7 (CH), 124.0 (CH), 110.5 (J_CPt = 16 Hz, CH), 107.4 (J_CPt = 40 Hz, CH), 36.8 (NCH₃), 31.6 (CH₂), 25.4 (CH₂), 23.0 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₂H₂₄ClN₅Pt: C, 44.79; H, 4.27; N, 11.87. Found: C, 44.81; H, 4.19; N, 11.80.

trans-C,C*-2b

Yellow solid, obtained from 1b (116 mg). Yield: 141 mg, 62%. ¹H NMR (600 MHz, CD₂Cl₂): δ 8.38–8.35 (m, 2H, H_arom), 8.05–7.96 (m, 2H, H_arom), 7.78 (br t, J_HH = 8.0 Hz, 1H, H_arom), 7.65 (dd, J_HH = 8.8, 2.5 Hz, 1H, H_arom), 7.48–7.38 (m, 3H, H_arom), 6.72 (ddd, J_HH = 7.4, 6.0, 1.5 Hz, 1H, H_arom), 6.56 (ddd, J_HH = 12.7, 8.9, 2.5 Hz, 1H, H_arom), 4.19 (s, 3H, NCH₃), 3.22 (ddd, J_HH = 14.7, 9.6, 6.1 Hz, 1H, CH₂), 2.88 (ddd, J_HH = 14.7, 9.6, 6.1 Hz, 1H, CH₂), 1.91–1.81 (m, 1H, CH₂), 1.80–1.70 (m, 1H, CH₂), 1.43–1.33 (m, 2H, CH₂), 0.85 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 169.2 (C), 166.9 (d, J_CF = 7 Hz, C), 165.6 (C), 164.2 (dd, J_CF = 256, 11 Hz, C), 161.2 (dd, J_CF = 260, 12 Hz, C), 151.6 (CH), 148.7 (C), 140.2 (C), 138.3 (CH), 130.1 (CH), 129.5 (CH), 124.7 (CH), 123.2 (d, J_CF = 20 Hz, CH), 122.9 (CH), 115.7 (d, J_CF = 17 Hz, CH), 99.4 (t, J_CF = 27 Hz, CH), 37.0 (NCH₃), 31.5 (CH₂), 25.6 (CH₂), 23.1 (CH₂), 14.1 (CH₃). ¹⁹F NMR (282 MHz, CD₂Cl₂): δ −108.16 (m, 1H), −112.45 (m, 1H). Anal. Calcd for C₂₄H₂₃ClF₂N₄Pt: C, 45.32; H, 3.65; N, 8.81. Found: C, 45.14; H, 3.56; N, 8.74.

trans-C,C*-2c

Yellow solid, obtained from 1c (112 mg). Yield: 135 mg, 78%. ¹H NMR (600 MHz, CD₂Cl₂): δ 8.46–8.43 (m, 2H, H_arom), 8.02 (dd, J_HH = 7.5, 1.2 Hz, 1H, H_arom), 7.96 (ddd with satellites, J_HH = 5.9, 1.6, 0.8 Hz, J_HPt = 48 Hz, 1H, H_arom), 7.74 (ddd, J_HH = 8.7, 7.4, 1.5 Hz, 1H, H_arom), 7.64 (d, J_HH = 8.1 Hz, 1H, H_arom), 7.49 (d, J_HH = 7.7 Hz, 1H, H_arom), 7.46–7.42 (m, 2H, H_arom), 7.42–7.38 (m, 1H, H_arom), 7.25 (td, J_HH = 7.3, 1.2 Hz, 1H, H_arom), 7.09 (td, J_HH = 7.5, 1.3 Hz, 1H, H_arom), 6.68 (ddd, J_HH = 7.4, 5.9, 1.5 Hz, 1H, H_arom), 4.18 (s, 3H, NCH₃), 3.25 (ddd, J_HH = 14.7, 9.6, 6.0 Hz, 1H, CH₂), 2.89 (ddd, J_HH = 14.7, 9.5, 6.1 Hz, 1H, CH₂), 192–1.83 (m, 1H, CH₂), 1.80–1.72 (m, 1H, CH₂), 1.43–1.33 (m, 2H, CH₂), 0.85 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 171.4 (C), 170.3 (C), 160.1 (C), 151.3 (CH), 148.7 (C), 146.3 (C), 140.3 (C), 137.6 (CH), 134.1 (CH), 130.1 (CH), 129.9 (CH), 129.4 (CH), 124.5 (CH), 123.7 (CH), 123.3 (CH), 122.7 (CH), 119.5 (J_CPt = 43 Hz, CH), 36.9 (NCH₃), 31.5 (CH₂), 25.5 (CH₂), 23.0 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₄H₂₅ClN₄Pt: C, 48.04; H, 4.20; N, 9.34. Found: C, 48.02; H, 4.16; N, 9.47.

trans-C,C*-2d

Yellow solid, obtained from 1d (116 mg). Yield: 127 mg, 59%. ¹H NMR (300 MHz, CD₂Cl₂): δ 8.47–8.42 (m, 2H, H_arom), 7.92 (ddd with satellites, J_HH = 5.9, 1.5, 0.7 Hz, J_HPt = 53 Hz, 1H, H_arom), 7.83 (br s with satellites, J_HPt = 26 Hz, 1H, H_arom), 7.70 (ddd, J_HH = 8.2, 7.3, 1.6 Hz, 1H, H_arom), 7.58 (ddd, J_HH = 8.3, 1.6, 0.7 Hz, 1H, H_arom), 7.47–7.35 (m, 4H, H_arom), 6.91 (ddd, J_HH = 7.9, 1.8, 0.7 Hz, 1H, H_arom), 6.63 (ddd, J_HH = 7.4, 5.9, 1.6 Hz, 1H, H_arom), 4.17 (s, 3H, NCH₃), 3.25 (ddd, J_HH = 14.7, 9.5, 6.1 Hz, 1H, CH₂), 2.88 (ddd, J_HH = 14.6, 9.4, 6.2 Hz, 1H, CH₂), 2.36 (s, 3H, CH₃), 1.93–1.66 (m, 2H, CH₂), 1.44–1.27 (m, 2H, CH₂), 0.84 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (75 MHz, CD₂Cl₂): δ 171.5 (C), 170.3 (C), 159.8 (C), 151.2 (J_CPt = 18 Hz, CH), 148.6 (C), 143.6 (C), 140.2 (C), 137.5 (CH), 134.8 (J_CPt = 50 Hz, CH), 129.8 (CH), 129.4 (CH), 124.6 (CH), 124.4 (CH), 123.3 (J_CPt = 28 Hz, CH), 122.2 (J_CPt = 46 Hz, CH), 119.2 (J_CPt = 45 Hz, CH), 36.8 (NCH₃), 31.4 (CH₂), 25.5 (CH₂), 23.0 (CH₂), 22.3 (CH₃), 14.1 (CH₃). Anal. Calcd for C₂₅H₂₇ClN₄Pt: C, 48.90; H, 4.43; N, 9.12. Found: C, 48.86; H, 4.52; N, 8.83.

trans-C,C*-2e

Orange solid, obtained from 1e (117 mg). Yield: 141 mg, 80%. ¹H NMR (600 MHz, CD₂Cl₂): δ 8.42–8.40 (m, 2H, H_arom), 7.72 (ddd with satellites, J_HH = 5.9, 1.5, 0.8 Hz, J_HPt = 50 Hz, 1H, H_arom), 7.61 (ddd, J_HH = 8.1, 7.4, 1.5 Hz, 1H, H_arom), 7.48–7.40 (m, 4H, H_arom), 7.35 (d, J_HH = 4.6 Hz, 1H, H_arom), 7.27 (ddd, J_HH = 8.0, 1.5, 0.8 Hz, 1H, H_arom), 6.51 (ddd, J_HH = 7.4, 5.9, 1.5 Hz, 1H, H_arom), 4.18 (s, 3H, NCH₃), 3.24 (ddd, J_HH = 14.7, 9.6, 6.0 Hz, 1H, CH₂), 2.90 (ddd, J_HH = 14.7, 9.6, 6.1 Hz, 1H, CH₂), 1.91–1.83 (m, 1H, CH₂), 1.80–1.72 (m, 1H, CH₂), 1.42–1.35 (m, 2H, CH₂), 0.86 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 168.5 (C), 168.0 (C), 165.2 (C), 151.3 (CH), 148.6 (C), 141.6 (C), 140.2 (C), 138.5 (CH), 133.9 (J_CPt = 78 Hz, CH), 130.0 (CH), 129.4 (CH), 128.0 (CH), 124.6 (CH), 120.3 (CH), 118.2 (CH), 36.9 (NCH₃), 31.5 (CH₂), 25.5 (CH₂), 23.0 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₂H₂₃ClN₄PtS: C, 43.60; H, 3.83; N, 9.24; S, 5.29. Found: C, 43.50; H, 3.87; N, 9.28; S, 5.09.

General Procedure for the Synthesis of cis-C,C*-[PtCl(C^∧N)(trzH)] (cis-C,C*-2)

A deaerated solution of trans-C,C*-2 in acetone (20 mL) was irradiated with blue LEDs for 16–72 h under a N₂ atmosphere. The solvent was evaporated to dryness, and the residue was washed with Et₂O (3 × 3 mL) and vacuum-dried to give cis-C,C*-2. For the ppz-derivative 2a, MeCN (5 mL) and UV light were used.

cis-C,C*-2a

White solid, obtained from trans-C,C*-2a (50 mg) after 16 h of UV irradiation. Yield: 34 mg, 68%. ¹H NMR (400 MHz, CD₂Cl₂): δ 8.35–8.28 (m, 2H, H_arom), 8.06 (d, J_HH = 2.2 Hz, 1H, H_arom), 7.94 (d, J_HH = 2.6 Hz, 1H, H_arom), 7.46–7.39 (m, 3H, H_arom), 7.11 (d, J_HH = 7.6 Hz, 1H, H_arom), 6.98 (t, J_HH = 7.4 Hz, 1H, H_arom), 6.69 (t, J_HH = 7.3 Hz, 1H, H_arom), 6.55 (t, J_HH = 2.5 Hz, 1H, H_arom), 6.46 (d, J_HH = 7.6 Hz, 1H, H_arom), 4.18 (s, 3H, NCH₃), 3.11 (ddd, J_HH = 15.0, 9.6, 5.8 Hz, 1H, CH₂), 2.89 (ddd, J_HH = 15.0, 9.6, 6.3 Hz, 1H, CH₂), 1.91–1.67 (m, 2H, CH₂), 1.43–1.33 (m, 2H, CH₂), 0.86 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (100 MHz, CD₂Cl₂): δ 146.6 (C), 146.5 (C), 144.6 (C), 140.4 (C), 139.0 (J_CPt = 48 Hz, CH), 135.8 (J_CPt = 86 Hz, CH), 129.9 (CH), 129.2 (CH), 128.1 (C), 126.3 (CH), 126.0 (CH), 125.4 (CH), 123.2 (CH), 111.6 (J_CPt = 28 Hz, CH), 107.1 (CH), 37.1 (NCH₃), 31.1 (CH₂), 25.8 (J_CPt = 23 Hz, CH₂), 23.1 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₂H₂₄ClN₅Pt: C, 44.79; H, 4.27; N, 11.87. Found: C, 44.77; H, 4.25; N, 11.69.

cis-C,C*-2b

Yellow solid, obtained from trans-C,C*-2b (54 mg) after 72 h of irradiation. Yield: 25 mg, 47%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.60 (br d, J = 5.8 Hz, 1H, H_arom), 8.29–8.24 (m, 2H, H_arom), 8.05 (d, J = 8.2 Hz, 1H, H_arom), 7.87 (td, J = 8.1, 1.3 Hz, 1H, H_arom), 7.46–7.41 (m, 3H, H_arom), 7.28 (ddd, J = 7.3, 5.6, 1.4 Hz, 1H, H_arom), 6.48 (ddd, J = 12.7, 9.2, 2.4 Hz, 1H, H_arom), 6.04 (dd with satellites, J = 9.1, 2.4 Hz, J_HPt = 78 Hz, 1H, H_arom), 4.19 (s, 3H, NCH₃), 3.09 (ddd, J_HH = 15.0, 9.7, 5.8 Hz, 1H, CH₂), 2.87 (ddd, J_HH = 14.7, 9.7, 6.3 Hz, 1H, CH₂), 1.91–1.82 (m, 1H, CH₂), 1.79–1.70 (m, 1H, CH₂), 1.43–1.33 (m, 2H, CH₂), 0.87 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 164.1 (dd, J_CF = 254, 13 Hz, C), 162.9 (d, J_CF = 7 Hz, C), 161.0 (dd, J_CF = 258, 13 Hz, C), 149.4 (CH), 147.5 (d with satellites, J_CF = 7 Hz, J_CPt ∼ 1160 Hz, C), 147.1 (C), 146.5 (C), 140.3 (C), 139.4 (CH), 130.0 (CH), 129.3 (CH), 129.1 (C), 125.3 (CH), 122.6 (CH), 122.5 (d, J_CF = 18 Hz, CH), 117.1 (d with satellites, J_CF = 18 Hz, J_CPt = 101 Hz, CH), 98.7 (t, J_CF = 27 Hz, CH), 37.2 (NCH₃), 31.0 (CH₂), 25.7 (CH₂), 23.1 (CH₂), 14.0 (CH₃). ¹⁹F NMR (282 MHz, CD₂Cl₂): δ −109.98 (m, 1H), −110.97 (m, 1H). Anal. Calcd for C₂₄H₂₃ClF₂N₄Pt: C, 45.32; H, 3.65; N, 8.81. Found: C, 45.51; H, 3.75; N, 9.05.

cis-C,C*-2c

Yellow solid, obtained from trans-C,C*-2c (60 mg) after 16 h of irradiation. Yield: 45 mg, 75%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.54 (ddd, J_HH = 5.6, 1.7, 0.8 Hz, 1H, H_arom), 8.35–8.31 (m, 2H, H_arom), 7.85–7.82 (m, 1H, H_arom), 7.69 (d, J_HH = 8.1 Hz, 1H, H_arom), 7.46 (dd, J_HH = 7.8, 1.2 Hz, 1H, H_arom), 7.44–7.38 (m, 3H, H_arom), 7.25 (ddd, J_HH = 7.1, 5.7, 1.3 Hz, 1H, H_arom), 6.97 (td, J_HH = 7.5, 1.2 Hz, 1H, H_arom), 6.80 (td, J_HH = 7.4, 1.4 Hz, 1H, H_arom), 6.48 (dd, with satellites, J_HH = 7.6, 1.2 Hz, J_HPt = 67 Hz, 1H, H_arom), 4.18 (s, 3H, NCH₃), 3.09 (ddd, J_HH = 14.6, 9.7, 5.8 Hz, 1H, CH₂), 3.33 (ddd, J_HH = 14.7, 9.7, 6.3 Hz, 1H, CH₂), 1.90–1.83 (m, 1H, CH₂), 1.78–1.71 (m, 1H, CH₂), 1.42–1.33 (m, 2H, CH₂), 0.86 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 166.1 (C), 149.2 (CH), 148.2 (C), 146.4 (C), 145.4 (C), 143.3 (J_CPt ∼ 1138 Hz, C), 140.5 (C), 138.9 (CH), 135.2 (J_CPt = 105 Hz, CH), 130.5 (J_CPt = 80 Hz, CH), 129.8 (CH), 129.2 (CH), 125.3 (CH), 124.0 (J_CPt = 42 Hz, CH), 122.8 (CH), 122.6 (CH), 118.6 (CH), 37.1 (NCH₃), 31.0 (CH₂), 25.8 (CH₂), 23.1 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₄H₂₅ClN₄Pt: C, 48.04; H, 4.20; N, 9.34. Found: C, 48.07; H, 4.34; N, 9.41.

cis-C,C*-2d

Yellow solid, obtained from trans-C,C*-2d (70 mg) after 16 h of irradiation. Yield: 52 mg, 75%. ¹H NMR (300 MHz, CD₂Cl₂): δ 9.50 (ddd with satellites, J_HH = 5.7, 1.7, 0.8 Hz, J_HPt = 21 Hz, 1H, H_arom), 8.35–8.28 (m, 2H, H_arom), 7.79 (ddd, J_HH = 8.2, 7.4, 1.7 Hz, 1H, H_arom), 7.63 (dt, J_HH = 8.1, 1.2 Hz, 1H, H_arom), 7.47–7.38 (m, 3H, H_arom), 7.35 (d with satellites, J_HH = 7.9 Hz, J_HPt ∼ 7 Hz, 1H, H_arom), 7.20 (ddd, J_HH = 7.2, 5.7, 1.4 Hz, 1H, H_arom), 6.79 (ddd, J_HH = 7.9, 1.8, 0.7 Hz, 1H, H_arom), 6.31 (s with satellites, J_HPt = 70 Hz, 1H, H_arom), 4.18 (s, 3H, NCH₃), 3.13 (ddd, J_HH = 14.9, 9.4, 5.7 Hz, 1H, CH₂), 2.88 (ddd, J_HH = 14.5, 9.4, 6.6 Hz, 1H, CH₂), 2.12 (s, 3H, CH₃), 1.97–1.65 (m, 2H, CH₂), 1.44–1.27 (m, 2H, CH₂), 0.85 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (75 MHz, CD₂Cl₂): δ 166.1 (C), 149.0 (J_CPt = 20 Hz, CH), 148.3 (C), 146.4 (C), 143.1 (C), 142.7 (C), 140.5 (C), 138.8 (CH), 136.0 (J_CPt = 107 Hz, CH), 129.7 (CH), 129.1 (CH), 125.3 (CH), 123.9 (J_CPt = 45 Hz, CH), 123.7 (CH), 122.1 (J_CPt = 20 Hz, CH), 118.2 (J_CPt = 29 Hz, CH), 37.2 (NCH₃), 30.9 (CH₂), 25.8 (J_CPt = 24 Hz, CH₂), 23.0 (CH₂), 21.8 (CH₃), 14.0 (CH₃). Anal. Calcd for C₂₅H₂₇ClN₄Pt: C, 48.90; H, 4.43; N, 9.12. Found: C, 48.70; H, 4.41; N, 8.86.

cis-C,C*-2e

Orange solid, obtained from trans-C,C*-2e (70 mg) after 40 h of irradiation. Yield: 68 mg, 98%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.28 (d, J_HH = 5.8 Hz, 1H, H_arom), 8.31–8.27 (m, 2H, H_arom), 7.71 (td, J_HH = 7.7, 1.6 Hz, 1H, H_arom), 7.45–7.41 (m, 3H, H_arom), 7.31 (d, J_HH = 7.8 Hz, 1H, H_arom), 7.14 (br d, J_HH = 4.7 Hz, 1H, H_arom), 7.09 (br t, J_HH = 6.6 Hz, 1H, H_arom), 6.15 (d, J_HH = 4.8 Hz, 1H, H_arom), 4.16 (s, 3H, NCH₃), 3.09 (ddd, J_HH = 15.0, 9.6, 5.8 Hz, 1H, CH₂), 2.87 (ddd, J_HH = 15.0, 9.5, 6.2 Hz, 1H, CH₂), 1.88–1.79 (m, 1H, CH₂), 1.76–1.67 (m, 1H, CH₂), 1.42–1.33 (m, 2H, CH₂), 0.87 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 161.8 (C), 149.1 (CH), 146.8 (J_CPt = 90 Hz, C), 145.8 (J_CPt = 1150 Hz, C), 143.3 (C), 140.4 (C), 139.8 (C), 139.4 (CH), 133.3 (J_CPt = 150 Hz, CH), 129.9 (CH), 129.2 (CH), 128.3 (J_CPt = 94 Hz, CH), 125.2 (CH), 120.1 (CH), 117.2 (CH), 37.2 (NCH₃), 31.1 (CH₂), 25.8 (CH₂), 23.0 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₂H₂₃ClN₄PtS: C, 43.60; H, 3.83; N, 9.24; S, 5.29. Found: C, 43.73; H, 3.98; N, 9.19; S, 5.19.

General Procedure for the Synthesis of (OC-6-54)-[PtCl₂(C^∧N)(trz)] (3)

To a solution of cis-C,C*-2 in CH₂Cl₂ (5 mL) was added PhICl₂, and the mixture was stirred for 30 min. Partial evaporation under reduced pressure (2 mL) and the addition of Et₂O (20 mL) led to the precipitation of a pale-yellow solid, which was filtered off and vacuum dried. The obtained product was placed in a Carius tube with 1,2-dichlorobenzene (1 mL) and Na₂CO₃ (30 mg), and the suspension was heated at 130 °C for 16 h under a N₂ atmosphere. After cooling to room temperature, Et₂O (10 mL) was added, and the resulting suspension was filtered. The collected solid was extracted with CH₂Cl₂ (5 × 5 mL). The partial evaporation of the resulting solution under reduced pressure (2 mL) and the addition of Et₂O (10 mL) led to the precipitation of a solid, which was filtered off and vacuum dried to give 3.

(OC-6-54)-[PtCl₂(ppz)(trz)] (3a)

White solid, obtained from cis-C,C*-2a (55 mg, 0.093 mmol) and PhICl₂ (31 mg, 0.112 mmol). Yield: 28 mg, 48%. ¹H NMR (600 MHz, CD₂Cl₂): δ 8.44 (d, J_HH = 2.4 Hz, 1H, H_arom), 8.26 (d, J_HH = 2.8 Hz, 1H, H_arom), 7.61 (d, J_HH = 7.8 Hz, 1H, H_arom), 7.34 (d, J_HH = 8.0 Hz, 1H, H_arom), 7.14 (t, J_HH = 7.6 Hz, 1H, H_arom), 7.11 (t, J_HH = 7.7 Hz, 1H, H_arom), 6.97–6.93 (m, 1H, H_arom), 6.87 (s, 1H, H_arom), 6.84–6.79 (m, 1H, H_arom), 6.41 (d with satellites, J_HH = 7.8 Hz, J_HPt = 49 Hz, 1H, H_arom), 6.21 (d with satellites, J_HH = 7.9 Hz, J_HPt = 43 Hz, 1H, H_arom), 4.27 (s, 3H, NCH₃), 3.51–3.43 (m, 1H, CH₂), 3.36–3.30 (m, 1H, CH₂), 1.86–1.74 (m, 2H, CH₂), 1.57–1.47 (m, 2H, CH₂), 0.97 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 145.4 (J_CPt = 85 Hz, C), 140.8 (C), 139.6 (C), 138.6 (J_CPt = 22 Hz, CH), 132.2 (J_CPt = 41 Hz, CH), 130.7 (CH), 130.4 (J_CPt = 44 Hz, CH), 128.4 (J_CPt = 797 Hz, C), 127.9 (CH), 127.7 (CH), 126.4 (2CH), 124.9 (C), 116.0 (J_CPt = 31 Hz, CH), 113.7 (J_CPt = 21 Hz, CH), 109.2 (CH), 37.5 (NCH₃), 31.8 (CH₂), 24.1 (CH₂), 23.1 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₂H₂₃Cl₂N₅Pt: C, 42.38; H, 3.72; N, 11.23. Found: C, 42.23; H, 3.82; N, 10.94.

(OC-6-54)-[PtCl₂(dfppy)(trz)] (3b)

In this case, a different synthetic procedure from the general method was followed. To a solution of cis-C,C*-2b (60 mg, 0.078 mmol) in CH₂Cl₂ (5 mL) was added PhICl₂ (26 mg, 0.094 mmol), and the mixture was stirred for 30 min. Partial evaporation under reduced pressure (2 mL) and the addition of Et₂O (20 mL) led to the precipitation of a white solid, which was filtered off, washed with MeOH (2 × 1 mL) and Et₂O (2 × 3 mL), and vacuum dried to give 3b. Yield: 26 mg, 50%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.84 (ddd with satellites, J = 5.7, 1.7, 0.8 Hz, J_PtH = 15 Hz, 1H, H_arom), 8.39 (d, J = 8.2 Hz, 1H, H_arom), 8.14 (td, J = 8.0, 1.7 Hz, 1H, H_arom), 7.63 (dd with satellites, J = 7.9, 1.3 Hz, J_PtH = 10 Hz, 1H, H_arom), 7.60 (ddd, J = 7.3, 5.7, 1.4 Hz, 1H, H_arom), 7.15 (td, J = 7.7, 1.2 Hz, 1H, H_arom), 6.92 (td with satellites, J = 7.7, 1.4 Hz, J_PtH ∼ 8 Hz, 1H, H_arom), 6.64 (ddd, J = 12.3, 8.9, 2.4 Hz, 1H, H_arom), 6.15 (dd with satellites, J = 7.9, 1.2 Hz, J_PtH = 43 Hz, 1H, H_arom), 6.04 (ddd with satellites, J = 8.1, 2.4, 0.9 Hz, J_PtH = 54 Hz, 1H, H_arom), 4.29 (s, 3H, NCH₃), 3.44–3.34 (m, 2H, CH₂), 1.90–1.82 (m, 1H, CH₂), 1.82–1.73 (m, 1H, CH₂), 1.55–1.48 (m, 2H, CH₂), 0.98 (t, J_HH = 7.4 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 163.2 (dd, J_CF = 360, 12 Hz, C), 161.5 (dd, J_CF = 364, 13 Hz, C), 159.9 (d, J_CF = 7 Hz, C), 148.8 (CH), 145.6 (J_CPt = 78 Hz, C), 142.2 (d, J_CF = 8 Hz, C), 141.1 (CH), 141.0 (J_CPt = 12 Hz, C), 131.7 (C), 130.3 (J_CPt = 45 Hz, CH), 129.9 (CH), 128.2 (J_CPt = 798 Hz, C), 126.6 (CH), 124.8–124.5 (m, 2CH), 116.2 (J_CPt = 32 Hz, CH), 114.6 (d, J_CPt ∼ 52 Hz, J_CF = 23 Hz, CH), 102.0 (t, J_CF = 27 Hz, CH), 37.6 (NCH₃), 31.8 (CH₂), 24.1 (CH₂), 23.1 (CH₂), 14.1 (CH₃). ¹⁹F NMR (282 MHz, CD₂Cl₂): δ −105.90 (m, 1H), −107.80 (m, 1H). Anal. Calcd for C₂₄H₂₂Cl₂F₂N₄Pt: C, 43.00; H, 3.31; N, 8.36. Found: C, 43.06; H, 3.28; N, 8.31.

(OC-6-54)-[PtCl₂(ppy)(trz)] (3c)

White solid, obtained from cis-C,C*-2c (60 mg, 0.100 mmol) and PhICl₂ (33 mg, 0.120 mmol). Yield: 32 mg, 48%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.80 (d with satellites, J_HH = 6.5 Hz, J_HPt = 14 Hz, 1H, H_arom), 8.11 (td, J_HH = 7.8, 1.6 Hz, 1H, H_arom), 8.05 (d, J_HH = 8.1 Hz, 1H, H_arom), 7.70 (dd, J_HH = 7.8, 1.6 Hz, 1H, H_arom), 7.61 (dd with satellites, J_HH = 7.9, 1.4 Hz, J_HPt = 9 Hz, 1H, H_arom), 7.57 (ddd, J_HH = 7.3, 5.7, 1.5 Hz, 1H, H_arom), 7.14–7.09 (m, 2H, H_arom), 6.92 (td with satellites, J_HH = 7.6, 1.4 Hz,, J_HPt ∼ 8 Hz, 1H, H_arom), 6.89 (td with satellites, J_HH = 7.6, 1.3 Hz, J_HPt ∼ 8 Hz, 1H, H_arom), 6.43 (dd with satellites, J_HH = 7.8, 0.6 Hz, J_HPt = 46 Hz, 1H, H_arom), 6.16 (dd with satellites, J_HH = 7.9, 1.2 Hz, J_HPt = 44 Hz, 1H, H_arom), 4.27 (s, 3H, NCH₃), 3.46 (ddd, J_HH = 14.3, 10.4, 5.6 Hz, 1H, CH₂), 3.33 (ddd, J_HH = 14.3, 10.4, 5.8 Hz, 1H, CH₂), 1.88–1.74 (m, 2H, CH₂), 1.51 (h, J_HH = 7.4 Hz, 2H, CH₂), 0.97 (t, J_HH = 7.4 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 162.8 (J_CPt = 52 Hz, C), 148.6 (CH), 145.5 (J_CPt = 82 Hz, C), 141.3 (C), 141.1 (C), 140.6 (CH), 139.9 (J_CPt = 792 Hz, C), 132.5 (J_CPt = 1112 Hz, C), 131.9 (J_CPt = 53 Hz, CH), 131.2 (J_CPt = 53 Hz, CH), 130.1 (J_CPt = 46 Hz, CH), 129.9 (CH), 128.6 (C), 126.3 (CH), 125.8 (2 × CH), 124.6 (J_CPt = 16 Hz, CH), 120.9 (J_CPt = 21 Hz, CH), 115.9 (J_CPt = 33 Hz, CH), 37.5 (NCH₃), 31.7 (CH₂), 24.1 (CH₂), 23.1 (CH₂), 14.1 (CH₃). Anal. Calcd for C₂₄H₂₄Cl₂N₄Pt·0.33CH₂Cl₂: C, 44.10; H, 3.75; N, 8.45. Found: C, 44.08; H, 3.82; N, 8.70.

(OC-6-54)-[PtCl₂(tpy)(trz)] (3d)

White solid, obtained from cis-C,C*-2d (74 mg, 0.121 mmol) and PhICl₂ (36 mg, 0.133 mmol). Yield: 50 mg, 56%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.76 (d with satellites, J_HH = 6.2 Hz, J_HPt = 14 Hz, 1H, H_arom), 8.10–8.06 (m, 1H, H_arom), 8.00 (d, J_HH = 8.2 Hz, 1H, H_arom), 7.62 (dd with satellites, J_HH = 7.9, 1.2 Hz, J_HPt = 10 Hz, 1H, H_arom), 7.59 (d, J_HH = 7.9 Hz, 1H, H_arom), 7.52 (ddd, J_HH = 7.2, 5.7, 1.3 Hz, 1H, H_arom) 7.15–7.11 (m, 1H, H_arom), 6.94 (d, J_HH = 7.9 Hz, 1H, H_arom), 6.89 (td with satellites, J_HH = 7.7, 1.3 Hz, J_HPt = 8 Hz, 1H, H_arom), 6.26 (s with satellites, J_HPt = 46 Hz, 1H, H_arom), 6.16 (dd with satellites, J_HH = 7.8, 1.2 Hz, J_HPt = 44 Hz, 1H, H_arom), 4.29 (s, 3H, NCH₃), 3.52 (ddd, J_HH = 14.4, 10.2, 5.6 Hz, 1H, CH₂), 3.32 (ddd, J_HH = 14.4, 10.2, 5.9 Hz, 1H, CH₂), 2.14 (s, 3H, CH₃), 1.91–1.75 (m, 2H, CH₂), 1.56–1.47 (m, 2H, CH₂), 0.98 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 163.0 (J_CPt = 50 Hz, C), 148.5 (CH), 145.5 (J_CPt = 82 Hz, C), 142.8 (J_CPt = 53 Hz, C), 141.2 (C), 140.5 (CH), 140.0 (C), 138.6 (C), 132.6 (C), 131.9 (J_CPt = 53 Hz, CH), 130.1 (J_CPt = 45 Hz, CH), 130.0 (CH), 128.7 (J_CPt = 817 Hz, C), 126.7 (CH), 126.2 (CH), 125.6 (J_CPt = 32 Hz, CH), 124.1 (J_CPt = 17 Hz, CH), 120.6 (J_CPt = 22 Hz, CH), 116.0 (J_CPt = 33 Hz, CH), 37.6 (NCH₃), 31.8 (CH₂), 24.1 (CH₂), 23.1 (CH₂), 21.9 (CH₃), 14.2 (CH₃). Anal. Calcd for C₂₅H₂₆Cl₂N₄Pt·CH₂Cl₂: C, 42.58; H, 3.85; N, 7.61. Found: C, 42.21; H, 3.96; N, 7.57.

(OC-6-54)-[PtCl₂(thpy)(trz)] (3e)

Beige solid, obtained from cis-C,C*-2e (70 mg, 0.116 mmol) and PhICl₂ (38 mg, 0.139 mmol). Yield: 28 mg, 38%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.63–9.58 (m, 1H, H_arom), 8.01 (td, J_HH = 7.8, 1.6 Hz, 1H, H_arom), 7.68 (d, J_HH = 7.9 Hz, 1H, H_arom), 7.59 (dd with satellites, J_HH = 7.9, 1.4 Hz, J_HPt ∼ 10 Hz, 1H, H_arom), 7.42 (ddd, J_HH = 7.3, 5.2, 1.2 Hz, 1H, H_arom), 7.29 (d with satellites, J_HH = 4.8 Hz, J_HPt = 9 Hz, 1H, H_arom), 7.14 (ddd, J_HH = 8.4, 7.5, 1.0 Hz, 1H, H_arom), 6.93 (td, J_HH = 7.7, 1.3 Hz, J_HPt ∼ 7 Hz,1H, H_arom), 6.26 (dd with satellites, J_HH = 7.9, 1.2 Hz, J_HPt ∼ 43 Hz, 1H, H_arom), 6.22 (d with satellites, J_HH = 4.9 Hz, J_HPt = 18 Hz, 1H, H_arom), 4.26 (s, 3H, NCH₃), 3.51 (ddd, J_HH = 14.3, 10.1, 5.9 Hz, 1H, CH₂), 3.29 (ddd, J_HH = 14.4, 10.2, 6.1 Hz, 1H, CH₂), 1.83–1.73 (m, 2H, CH₂), 1.55–1.47 (m, 2H, CH₂), 0.98 (t, J_HH = 7.4 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 158.8 (J_CPt = 39 Hz, C), 148.5 (CH), 146.1 (J_CPt = 86 Hz, C), 140.9 (CH), 138.1 (C), 130.3 (J_CPt = 43 Hz, CH), 130.2 (CH), 129.7 (J_CPt = 64 Hz, CH), 128.8 (J_CPt = 83 Hz, CH), 128.3 (C), 127.6 (C), 126.4 (CH), 122.3 (J_CPt = 15 Hz, CH), 119.9 (J_CPt = 17 Hz, CH), 115.8 (J_CPt = 33 Hz, CH), 37.5 (NCH₃), 32.0 (CH₂), 24.0 (CH₂), 23.1 (CH₂), 14.1 (CH₃) (two of the quaternary carbons were not observed). Anal. Calcd for C₂₂H₂₂Cl₂N₄PtS: C, 41.26; H, 3.46; N, 8.75; S, 5.01. Found: C, 41.26; H, 3.37; N, 8.67; S, 5.07.

(OC-6-41)-[PtCl₃(tpy)(trzH)] (4d)

To a solution of cis-C,C*-2d (73 mg, 0.098 mmol) in CH₂Cl₂ (5 mL) was added PhICl₂ (30 mg, 0.109 mmol), and the mixture was stirred at room temperature for 30 min. The mixture was concentrated under reduced pressure (2 mL). To the mixture was added Et₂O (20 mL), whereupon a pale-yellow precipitate formed that was filtered off. Extraction with MeOH (2 × 1 mL) and evaporation to dryness led to an analytically pure sample of 4d. ¹H NMR (400 MHz, CD₂Cl₂): δ 9.72–9.67 (1H, H_arom), 7.95–7.80 (4H, H_arom), 7.64–7.59 (1H, H_arom), 7.53–7.46 (1H, H_arom), 7.43–7.32 (3H, H_arom), 7.07–7.02 (1H, H_arom), 6.71 (J_PtH = 33 Hz, 1H, H_arom), 4.26 (3H, NCH₃), 3.36–3.27 (1H, CH₂), 2.82–2.72 (1H, CH₂), 2.37 (3H, CH₃), 2.14–1.89 (2H, CH₂), 1.76–1.60 (1H, CH₂), 1.40–1.30 (1H, CH₂), 0.95–0.85 (3H, CH₃) (signal multiplicity could not be determined for this complex because its very low solubility resulted in a poorly resolved spectrum). Anal. Calcd for C₂₅H₂₇Cl₃N₄Pt·CH₂Cl₂: C, 40.56; H, 3.80; N, 7.28. Found: C, 40.66; H, 3.79; N, 7.37.

(OC-6-42)-[PtCl₂(ppz)(trz)] (5a)

To a solution of trans-C,C*-2a (100 mg, 0.170 mmol) in CH₂Cl₂ (10 mL) was added PhICl₂ (47 mg, 0.170 mmol), and the mixture was stirred for 30 min. The partial evaporation of the solvent under reduced pressure (2 mL) and the addition of Et₂O (20 mL) led to the precipitation of a white solid, which was filtered off, washed with MeOH (3 × 1 mL) and Et₂O (2 × 3 mL), and vacuum dried to give 5a. Yield: 25 mg, 23%. ¹H NMR (600 MHz, CD₂Cl₂): δ 8.17 (dd, J_HH = 7.5, 1.2 Hz, 1H, H_arom), 8.06 (d, J_HH = 2.9 Hz, 1H, H_arom), 7.65 (dd, J_HH = 7.9, 1.5 Hz, 1H, H_arom), 7.44–7.36 (m, 3H, H_arom), 7.11–7.05 (m, 2H, H_arom), 6.87 (td, J_HH = 7.6, 1.5 Hz, 1H, H_arom), 6.47 (t, J_HH = 2.7 Hz, 1H, H_arom), 6.39 (dd with satellites, J_HH = 7.9, 1.2 Hz, J_PtH = 43 Hz, 1H, H_arom), 4.25 (s, 3H, NCH₃), 3.45 (ddd, J_HH = 14.3, 9.9, 6.1 Hz, 1H, CH₂), 3.34 (ddd, J_HH = 14.3, 9.9, 6.0 Hz, 1H, CH₂), 1.85–1.76 (m, 2H, CH₂), 1.56–1.50 (m, 2H, CH₂), 0.99 (t, J_HH = 7.4 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 150.8 (C), 149.2 (C), 142.4 (C), 142.1 (C), 141.3 (C), 138.7 (J_CPt = 45 Hz, CH), 134.3 (CH), 132.6 (CH), 130.1 (J_CPt = 46 Hz, CH), 128.4 (J_CPt = 24 Hz, CH), 127.8 (J_CPt = 19 Hz, CH), 126.7 (CH), 126.1 (CH), 122.6 (C), 116.4 (J_CPt = 25 Hz, CH), 112.7 (CH), 109.0 (J_CPt = 25 Hz, CH), 37.0 (NCH₃), 32.5 (CH₂), 23.9 (CH₂), 23.1 (CH₂), 14.2 (CH₃). Anal. Calcd for C₂₂H₂₃Cl₂N₅Pt·0.25CH₂Cl₂: C, 41.45; H, 3.67; N, 10.86. Found: C, 41.39; H, 3.91; N, 11.04.

(OC-6-42)-[PtCl₂(tpy)(trz)] (5d) and (OC-6-43)-[PtCl₃(tpy)(trzH)] (6d)

To a solution of trans-C,C*-2d (160 mg, 0.261 mmol) in CH₂Cl₂ (15 mL) was added PhICl₂ (79 mg, 0.287 mmol), and the mixture was stirred for 30 min. The partial evaporation of the solvent under reduced pressure (2 mL) and the addition of Et₂O (20 mL) led to the precipitation of a pale-yellow solid, which was collected by filtration. The ¹H NMR spectrum of this material revealed a mixture of complexes 5d and 6d in a 1:4 molar ratio, which was stirred in MeOH (20 mL) for 10 min to give a suspension. The insoluble white solid was collected by filtration, washed with Et₂O (2 × 3 mL), and vacuum-dried to give 5d. The filtrate was evaporated to dryness, and the pale-yellow residue was washed with Et₂O (3 × 3 mL) and vacuum-dried to give 6d.

Data for 5d

Yield: 18 mg, 11%. ¹H NMR (600 MHz, CD₂Cl₂): δ 8.06 (br s, 1H, H_arom), 7.94 (d with satellites, J_HH = 5.9 Hz, J_HPt = 35 Hz, 1H, H_arom), 7.85 (d, J_HH = 7.5 Hz, 1H, H_arom), 7.75 (ddd, J_HH = 8.5, 7.3, 1.5 Hz, 1H, H_arom), 7.67 (d, J_HH = 7.9 Hz, 1H, H_arom), 7.64 (dd, J_HH = 7.9, 1.5 Hz, 1H, H_arom), 7.21 (dd, J_HH = 7.9, 1.2 Hz, 1H, H_arom), 7.07 (td, J_HH = 7.6, 1.2 Hz, 1H, H_arom), 6.89–6.83 (m, 2H, H_arom), 6.37 (dd with satellites, J_HH = 8.0, 1.2 Hz, J_HPt = 42 Hz, 1H, H_arom), 4.26 (s, 3H, NCH₃), 3.42–3.38 (m, 2H, CH₂), 1.92–1.76 (m, 2H, CH₂), 1.56–1.50 (m, 2H, CH₂), 0.99 (t, J_HH = 7.4 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 167.7 (C), 158.7 (C), 156.3 (C), 149.9 (CH), 148.6 (C), 143.2 (C), 142.7 (C), 140.4 (C), 140.1 (CH), 134.2 (CH), 132.1 (CH), 129.9 (J_CPt = 44 Hz, CH), 126.6 (CH), 126.5 (CH), 125.4 (CH), 123.4 (CH), 121.1 (CH), 116.3 (CH), 37.0 (NCH₃), 32.3 (CH₂), 24.1 (CH₂), 23.2 (CH₂), 22.5 (CH₃), 14.2 (CH₃). Anal. Calcd for C₂₅H₂₆Cl₂N₄Pt: C, 46.30; H, 4.04; N, 8.64. Found: C, 46.19; H, 4.16; N, 8.34.

Data for 6d

Yield: 120 mg, 67%. ¹H NMR (600 MHz, CD₂Cl₂): δ 9.56 (d with satellites, J_HH = 6.0 Hz, J_HPt = 31 Hz, 1H, H_arom), 7.71 (t, J_HH = 8.0 Hz, 1H, H_arom), 7.63 (s with satellites, J_HPt = 28 Hz, 1H, H_arom), 7.31 (d, J_HH = 8.0 Hz, 1H, H_arom), 7.24–7.16 (m, 2H, H_arom), 7.01–6.96 (m, 2H, H_arom), 6.84 (br t, J_HH = 7.6 Hz, 1H, H_arom), 6.77 (br t, J_HH = 7.6 Hz, 1H, H_arom), 6.38 (d, J_HH = 7.6 Hz, 1H, H_arom), 6.14 (d, J_HH = 7.9 Hz, 1H, H_arom), 4.08 (s, 3H, NCH₃), 3.85 (ddd, J_HH = 14.6, 12.3, 4.4 Hz, 1H, CH₂), 3.28 (ddd, J_HH = 14.6, 12.3, 4.5 Hz, 1H, CH₂), 2.41 (s, 3H, CH₃), 1.83–1.68 (m, 2H, CH₂), 1.66–1.58 (m, 2H, CH₂), 1.07 (t, J_HH = 7.3 Hz, 3H, CH₃). ¹³C APT NMR (151 MHz, CD₂Cl₂): δ 164.9 (C), 150.1 (C), 149.0 (CH), 142.7 (C), 142.6 (C), 140.7 (CH), 139.9 (C), 137.4 (C), 131.8 (J_CPt = 26 Hz, CH), 129.8 (CH), 129.2 (CH), 129.0 (CH), 127.3 (CH), 127.0 (CH), 126.1 (J_CPt = 27 Hz, CH), 125.8 (CH), 123.4 (J_CPt = 27 Hz, CH), 121.4 (J_CPt = 30 Hz, CH), 117.6 (C), 37.9 (NCH₃), 31.9 (CH₂), 27.1 (CH₂), 23.4 (CH₂), 22.3 (CH₃), 14.2 (CH₃). Anal. Calcd for C₂₅H₂₇Cl₃N₄Pt: C, 43.84; H, 3.97; N, 8.18. Found: C, 43.79; H, 4.11; N, 8.01.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00410.

• Photophysical characterization, X-ray structure determinations, electrochemical measurements, crystallographic data, NMR spectra, excitation and emission spectra, and computational methods and data (PDF)

Accession Codes

CCDC 2061761–2061762 and 2061767–2061769 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

The authors declare no competing financial interest.