Ladder-like heteropolynuclear assemblies via cyanido bridges and platinum(II)–thallium(I) bonds: structural and photophysical properties

Abstract

We describe the mononuclear anionic cyanido-pentafluorophenyl complexes, (NBu₄)[Pt(C^N)(C₆F₅)(CN)] [C^N = 7,8-benzoquinolate (bzq) 1, 2-(2,4-difluorophenyl)pyridinate (dfppy) 2] and the heteropolynuclear derivatives [{Pt(C^N)(C₆F₅)(CN)}Tl] (C^N = bzq 3, dfppy 4). These complexes were synthesized via a two-step modular synthesis by reaction of the corresponding potassium salts K[Pt(C^N)(C₆F₅)(CN)], prepared in situ from [Pt(C^N)(C₆F₅)(DMSO)] and KCN in acetone/H₂O, with TlPF₆. The structures of {[Pt(bzq)(C₆F₅)(CN)Tl]·THF}_n (3·THF)_n and [{Pt(dfppy)(C₆F₅)(CN)}Tl]₄·dioxane [4]₄·dioxane, determined by X-ray crystallography, confirm the presence of Pt(II)–Tl(I) bonds [2.9795(6)–3.0736(3) Å], but in the dfppy complex, the incorporation of dioxane, causes a significant structural change. Thus, whereas [3·THF]_n achieves a bent-ladder shape extended double chain Tl⋯[Pt⋯Tl]_n⋯Pt supported by lateral bridging [Pt](μ-CN)[Tl] ligands, [4]₄·dioxane is formed by discrete Pt₄Tl₄ rectangular aggregates stabilized by [Pt](μ-CN)[Tl] and Pt⋯Tl bonds, which are connected by dioxane bridging molecules through Tl⋯O(dioxane) additional contacts. Solid state emissions are redshifted compared with the mononuclear derivatives 1 and 2 and have been assigned, with the support of theoretical calculations on Pt₄Tl₄ models, to metal–metal′-to-ligand charge transfer (³MM′LCT [d/s σ*(Pt, Tl) → π*(C^N)]) for 3 and mixed ³MM′LCT/³IL for 4. In fluid THF solution, the complexes are not emissive. At 77 K, 3 and 4 exhibit bright emissions attributed to the formation of bimetallic [{Pt(C^N)(C₆F₅)(CN)}Tl(THF)_x], and anionic [Pt(C^N)(C₆F₅)(CN)]⁻ fragments. Furthermore, both 3 and 4 exhibit a reversible mechanochromism with a red shift of the emissions upon crushing, suggesting some degree of shortening of metal–metal separation. Finally, complex 3 shows solvatochromic behavior with color/luminescence changes by treatment with a drop of MeOH, CH₂Cl₂, THF or Et₂O, with shifts from 583 in 3-MeOH to 639 nm in 3-THF. However, 4 only demonstrates a bathochromic response to MeOH.

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Introduction

The study of metal–metal interactions between closed (d¹⁰, d¹⁰s²) and pseudoclosed (d⁸) shell metal ions has been of significant research interest in the design of a wide array of polymetallic discrete compounds or supramolecular networks.¹ Many of these metallophilic interactions involve dative bonds between a Lewis base (donor) and a Lewis acid (acceptor) to give the denominated Metal Only Lewis Pairs (MOLP) complexes.² In many cases, the presence of metal–metal interactions is associated with a luminescent behavior and this can be influenced by the metal–metal distances, the number and type of metals or ligands, the geometry, the orientation between the units and even the secondary contacts in the solid state.^1e,f,3 Furthermore, some of these linkages suffer modifications in the color and/or in the emission as a result of external stimuli, being perfect candidates for the design of smart materials.⁴

Since the earliest pioneering luminescent complex trans-[Tl₂Pt(CN)₄],⁵ heterometallic systems containing Pt–Tl bonds between Pt(II) with a high-lying filled 5d_z² donor orbital and Tl(I) as acceptor (6p_z, π*) are often associated with intense photoluminescence. In this regard, numerous Pt(II)–Tl(I) backbones have been documented with various structural configurations, supported and unsupported by bridging ligands, comprising binuclear (PtTl),⁶ trinuclear (Pt₂Tl,^4b,7 PtTl₂^5,8), tetranuclear [paired (PtTl),^6a,8c,9 and trigonal (Pt₃Tl)¹⁰], hexanuclear clusters (Pt₂Tl₄,¹¹ Pt₃Tl₃^10b) or infinite networks.^{4a,b,6a,7,8b,c,11a,c,12}

Among them, some luminescent cyanido heterometallic assemblies based on Pt(II)–Tl(I) backbones have been reported.¹³ Our group described cyanido-pentafluorophenyl extended complexes [trans,trans,trans-Tl₂{Pt(C₆F₅)₂(CN)₂}·(CH₃COCH₃)₂]_n (two-dimensional, 2D), {Tl[Tl{cis-Pt(C₆F₅)₂(CN)₂}]·(H₂O)}_n (one-dimensional, 1D) and a discrete dimer [Tl{cis-Pt(C₆F₅)₂(CN)(PPh₂C≡CPh)}]₂,^8c and the cyanido-alkynyl {trans-[PtTl₂(C≡CTol)₂(CN)₂]·2MeOH}_n 1D-chain.^8b Within this area, it has been found that the use of the strong field cyclometalated ligands coordinated to Pt favors the formation of strong Pt → Tl bonds by raising the energy of the 5d_z² orbitals on the Pt center.^{4a,b,6a,7b,c,10,12c} With cyclometalated-cyanido platinum motifs, extended 2D networks of composition [PtTl(C^N)(CN)₂] (C^N = bzq, ppy)^6a and [PtTl(Naph^∧E)(CN)₂] [Naph^∧N = 1-(naphthalene-2-yl)-1H-pyrazolyl; Naph^∧C* = 3-methyl-1-(naphthalene-2-yl)-1H-imidazolyl-2-ylidene]^12c have been published. In all of them, the formation of additional secondary inter or intramolecular Tl⋯NC contacts typically determines the final arrangement of the units. Our group has also reported the utilization of cyclometalated-pentafluorophenyl derivatives, (NBu₄)[Pt(C^N)(C₆F₅)₂] (C^N = bzq)^4b and the solvate [Pt(C^N)(C₆F₅)S] (C^N = bzq, S = acetone; C^N = ppy, S = DMSO)^4a to form extended Pt–Tl chains, in which the occurrence of intramolecular Tl⋯F_ortho contacts play a significant role in stabilizing the Pt–Tl bonds.

In this context, we decided to expand the investigation of this type of system to neutralization reactions of new heteroleptic anionic metalloligands [Pt(C^N)(C₆F₅)(CN)]⁻ (C^N = bzq, dfppy) with Tl⁺. We report the synthesis, structures and optical properties of new neutral heterometallic systems based on Pt(II)–Tl(I) bonded [{Pt(C^N)(C₆F₅)(CN)}Tl]_n units, distinctly connected by Pt(μ-CN)Tl bonds. We have found that the C^N moiety and the crystallization solvent have a remarkable effect on the final structures and photoluminescent behaviour, which has been analyzed by computational studies.

Results and discussion

Synthesis and characterization

The mononuclear complexes (NBu₄)[Pt(C^N)(C₆F₅)(CN)] (C^N = bzq 1, dfppy 2) were prepared, as yellow solids in high yield, by displacement of dimethyl sulfoxide by cyanido in the corresponding precursors [Pt(C^N)(C₆F₅)(DMSO)] using one equivalent of (NBu₄)CN in acetone at room temperature (Scheme 1i). The products were fully characterized by means of multinuclear [¹H, ¹⁹F{¹H}, ¹³C{¹H}] NMR and IR spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF), and elemental analysis (CHN). ¹H and ¹³C{¹H} NMR signals were assigned based on 2D experiments, including ¹H–¹H (COSY) and ¹H–¹³C (HSQC and HMBC) correlations.

Scheme 1 Synthesis of 1–4, (i) 1 equiv. of (NBu₄)CN, acetone, (ii) 1 equiv. of KCN, acetone/H₂O (3/1), 298 K, (iii) 1 equiv. of TlPF₆.

Their IR spectra displayed the characteristic ν(C≡N) stretching vibration at 2102 (1) and 2110 cm⁻¹ (2), consistent with a terminal CN⁻ bond trans to C_C^N.¹⁴ Their MALDI-TOF(−) MS spectra display in both complexes the molecular peak anion [M]⁻ as the parent peak together with the peak corresponding to the diplatinum anion [Pt₂(C^N)₂(C₆F₅)₂(CN)]⁻ in less abundance, which suggests the propensity of these cyanido mononuclear complexes to form binuclear complexes through a cyanido bridge under MALDI-TOF conditions.¹⁴ The ¹H NMR of 1 and 2 in CDCl₃ show the signals for the cyclometalated and NBu₄ groups with their expected integration (Fig. S1–S3†). The ¹⁹F{¹H} NMR spectra display the three typical signals (F_o, F_p and F_m, AA′MXX′ spin system) of the C₆F₅ ligand, and in the case of 2, additionally, the two expected fluorine resonances of the cyclometalated dfppy ligand (F¹⁰ and F⁸) with long-range Pt satellites (⁴J_Pt–F = 50 and 40 Hz). The coordination of the cyanido takes place with retention of the geometry, as is reflected in the high values of ³J_o–F–Pt (530 1, 515 Hz 2),^4g,15 in coherence with the relatively low trans influence of the N pyridinic atom. The ¹³C{¹H} NMR spectra show the resonance of the cyanido at δ 147.7 1, 146.3 2, further supported by the ¹³C{¹H} NMR spectrum of the in situ prepared mononuclear complex K[Pt(bzq)(C₆F₅)(¹³CN)] (1′) in CDCl₃ (Fig. S2†), which displays a sharp resonance at δ 146.5 with ¹⁹⁵Pt satellites (¹J_Pt–C = 881 Hz). These values compare well with those recently reported for K[Pt(ppy)(p-MeC₆H₄)(¹³CN)] (δ 157.4, ¹J_Pt–C = 882 Hz).¹⁴

With the aim of investigating the metallophilic affinity of the anionic platinum substrates toward Tl(I), we decided to examine the reactivity of the synthons “[Pt(C^N)(C₆F₅)(CN)]⁻” toward TlPF₆. Thus, the heteropolynuclear complexes, 3 and 4 were synthesized via a two-step modular synthesis (Scheme 1ii, iii). The reactions of [Pt(C^N)(C₆F₅)(DMSO)] with KCN in a mixture acetone/H₂O render “in situ” the anionic complexes K[Pt(C^N)(C₆F₅)(CN)] and their treatment with one equivalent of TlPF₆ gives rise to the final compounds of stoichiometry [{Pt(C^N)(C₆F₅)(CN)}Tl] (C^N = bzq 3, dfppy 4) as orange (3) or yellow (4) solids in high yields. This procedure avoids the presence of (NBu₄)PF₆, difficult to separate from the reaction media. Attempts to obtain Pt₂Tl complexes by treatment of the anionic Pt complexes (NBu₄)[Pt(C^N)(C₆F₅)(CN)] with TlPF₆ in a molar relation 2 : 1 results in the formation of solids of identical stoichiometry, [{Pt(C^N)(C₆F₅)(CN)}Tl]. Their molecular structures were determined by single-crystal X-ray diffraction (see below). In their IR spectra, the ν(C≡N) absorption appear slightly shifted to higher frequencies (2108 3, 2120 cm⁻¹4) in relation to the precursors, confirming the electronic perturbation of the platinum fragment and/or the coordination of the CN⁻ to the acidic Tl center. The MALDI-TOF(+) mass spectra in THF solution show the peak corresponding to the binuclear [Pt(C^N)(C₆F₅)(CN)Tl]⁺ fragment, although in low abundance, suggesting the partial retention of these Pt–Tl fragments in solution.

Both complexes are only soluble in donor solvents, in which this type of donor acceptor Pt → M bonds are usually broken or involved in dynamic processes. Conductivity measurements in THF solution are close to 1 : 1 electrolytes (suggesting a remarkable dissociation process into the anion [Pt(C^N)(C₆F₅)(CN)]⁻ and [Tl(THF)_x]⁺). Not surprisingly, the corresponding ¹H and ¹³C{¹H} NMR spectra of 3 and 4 in THF-d₈ show chemical shifts and coupling constants very close to those of precursors (Fig. S4 and S5†). In the ¹³C{¹H} NMR spectra, the cyanido signal was assigned at δ 145.2 (3) and 145.5 (4), confirmed by the ¹³C{¹H} NMR spectrum of the in situ prepared [{Pt(dfppy)(C₆F₅)(¹³CN)}Tl] (4′) in THF-d⁸, which displays a sharp resonance at δ 144.5 flanked by ¹⁹⁵Pt satellites (¹J_Pt–C = 887 Hz) (Fig. S6†). The most significant change was observed in the ³J_o–F–Pt coupling constants of the ortho-fluorine resonances of the C₆F₅ group in the ¹⁹F{¹H} NMR spectra. Thus, in both complexes (3 and 4) the value of the ³J_o–F–Pt coupling is notably smaller than in the corresponding precursors (435 3/530 Hz 1, Δ = 77; 445 4/515 Hz 2, Δ = 70 Hz). This fact is coherent with the retention to some extend of the Pt–Tl bond in solution, which reduces the electronic density at the Pt center. Notwithstanding, no F–Tl coupling is observed at room temperature, indicating that the Pt–Tl bond is involved in a fast dynamic process on the NMR time scale. In agreement with this behavior, no signal could be detected in the ¹⁹⁵Pt NMR spectra.

Deep red suitable crystals for X-ray diffraction studies of 3·THF were grown by slow evaporation of a concentrated THF–d₈ solution of 3. For the dfppy complex 4, orange crystals of stoichiometry [4]₄·dioxane were obtained by slow diffusion of n-hexane into a saturated THF solution of 4 with one drop of 1,4-dioxane at room temperature. The crystallographic parameters and selected bond lengths and angles are summarized in Table 1 and Tables S1 and S2.† Unfortunately, due to the limitation of their solubility, it was not possible to obtain crystals in other solvents. Both compounds (3 and 4) crystallize in the same space group (P1̄) of the triclinic crystal system. Crystallographic characterization of 3·THF shows the formation of a double chain framework of stoichiometry {[Pt(bzq)(C₆F₅)(CN)Tl]·THF}_n (3·THF)_n, in which each Pt is coordinated to two Tl centers and each Tl to two Pt centers and the chains are connected by bridging [Pt](μ-CN)[Tl] ligands. The asymmetric unit contains two [{Pt(bzq)(C₆F₅)(CN)}Tl] fragments and two tetrahydrofuran molecules (see Fig. 1a), one coordinated to Tl(2) and the other as crystallization solvent. The structure reveals a pseudooctahedral environment around the platinum center, formed by the C and N of the cyclometalated bzq group, the C₆F₅ ligand, a cyanido group (trans to C_bzq) and two trans donor–acceptor Pt–Tl bonds, with a non-linear sequence [Tl(1)–Pt(1)–Tl(2) 156.90(9), Pt(1)–Tl(2)–Pt(2) 156.74(9), Tl(2)–Pt(2)–Tl(1) 166.81(6), Pt(2)–Tl(1)–Pt(1) 166.98(6)°]. The Pt–Tl distances [Pt(1)–Tl(1) 3.0279(7), Pt(1)–Tl(2) 3.0402(7), Pt(2)–Tl(2) 3.0140(7), Pt(2)–Tl(1) 2.9795(6) Å] are in the usual range for Pt(II)–Tl(I) separations found in related compounds with unsupported Pt–Tl bonds between the metal centers.^{4a,b,6a,7a,7c,8c,12c,16} The cyanido coordinated to Pt(2) contacts through the N to the Tl(2) of the adjacent chain, whereas the cyanido coordinated to Pt(1) is simultaneously interacting with Tl(1), thus generating a bent ladder-like assembly (Fig. 1b). The angles C≡N⋯Tl deviated notably from linearity [C(34)–N(2)–Tl(2) 135.4(2)° and C(14)–N(4)–Tl(1) 141.4(2)°], indicating a notable electrostatic interaction between the Tl centers and the cyanido groups. Both Tl centers show different environments. The Tl(1) is coordinated to both trans platinum centers, Pt(1) and Pt(2), and to the nitrogen of a cyanido bridging group of the neighboring chain [Tl(1)–N(4) 2.669(3) Å], whereas Tl(2) is coordinated to both Pt centers and stablishes a longer bond with the N of a cyanido group [Tl(2)–N(2) 2.7223(3) Å], due to an additional bonding interaction to the oxygen of one THF molecule [Tl(2)–O(1) 2.667(2) Å]. Additionally, both Tl atoms interact with two ortho-fluorine atoms of the C₆F₅ ring of the two adjacent units with relatively short Tl⋯F intramolecular contacts [2.929–3.203 Å],^7a,8c giving rise to a final five (Tl(1)) and six (Tl(2)) coordination, respectively (Fig. 1c). These oF⋯Tl interactions contribute to the stability, but are likely responsible for the deviation from linearity of the chain. The resulting ladder-like assembly is extended along the “a” axis defined by the two chains with repeated Pt(1)/Tl(1)–Pt(1)/Tl(1)–Pt(2)/Tl(2)–Pt(2)/Tl(2) fragments. These double chains are separated by THF molecules (Fig. S7†).

Fig. 1 View of the molecular structure of {[Pt(bzq)(C₆F₅)(CN)Tl]·THF}_n (3·THF)_n. (a) Asymmetric unit of the unit cell, (b) supramolecular structure showing the ladder shape assembly and (c) views of the coordination environments of both Tl centers.

Table 1 Selected bond lengths (Å) and angles (°) for {[Pt(bzq)(C₆F₅)(CN)Tl]·THF}_n (3·THF)_n and [{Pt(dfppy)(C₆F₅)(CN)}Tl]₄·C₄H₈O₂ [4]₄·dioxane

{[Pt(bzq)(C6F5)(CN)Tl]·THF}n (3·THF)n
Pt(1)–Tl(1)	3.0279(7)	Pt(1)–N(1)	2.097(2)
Pt(1)–Tl(2)	3.0402(7)	Pt(1)–C(15)	2.030(3)
Pt(2)–Tl(2)	3.0140(7)	Pt(1)–C(14)	2.024(3)
Pt(2)–Tl(1)#1	2.9795(6)	Pt(2)–C(35)	2.024(3)
Tl(1)–N(4)	2.669(3)	Pt(2)–C(34)	2.027(3)
Tl(2)–N(2)	2.7223(3)	Pt(2)–C(21)	2.052(3)
Tl(2)–O(1)	2.667(2)	Pt(2)–N(3)	2.097(2)
Pt(1)–C(1)	2.051(3)
Tl(1)–Pt(1)–Tl(2)	156.90(9)	Pt(1)–Tl(2)–Pt(2)	156.74(9)
Tl(2)–Pt(2)–Tl(1)	166.81(6)	Pt(2)–Tl(1)–Pt(1)	166.98(6)
C(34)–N(2)–Tl(2)	135.4(2)	C(14)–N(4)–Tl(1)	141.4(2)

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[{Pt(dfppy)(C6F5)(CN)}Tl]4·C4H8O2[4]4·dioxane
Pt(1)–Tl(1)	3.0736(3)	Pt(1)–C(12)#1	2.021(5)
Pt(1)–Tl(2)	3.0518(3)	Pt(1)–C(13)#1	2.016(4)
Pt(2)–Tl(2)	2.9258(3)	Pt(2)–C(31)	2.014(4)
Tl(1)–N(4)	2.596(4)	Pt(2)–C(30)	2.007(4)
Tl(2)–N(2)	2.626(4)	Pt(2)–C(19)	2.025(4)
Pt(1)–C(1)#1	2.039(4)	Pt(2)–N(3)	2.075(4)
Pt(1)–N(1)#1	2.082(4)
Tl(1)–Pt(1)–Tl(2)	162.723(8)	Pt(1)–Tl(2)–Pt(2)	160.299(8)
C(12)–N(2)–Tl(2)	142.8(4)	C(30)–N(4)–Tl(1)	140.6(4)

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In the case of the 2-(2,4-difluorophenyl)pyridinate complex 4, the presence of dioxane originates that the one-dimensional polymeric structure breaks down into discrete heteroocta-metallic Pt₄Tl₄ rectangular aggregates (ca. 4.6 × 9.56 Å), which are also stabilized by Pt(II)–Tl(I) bonds and bridging cyanido ligands, as is shown in Fig. 2a. The structure contains two different Pt and Tl centers and one molecule of dioxane (C₄H₈O₂) per tetramer as [4]₄·dioxane. Thus, the two Pt(1) atoms, located on the long edges of the rectangle, exhibit a similar pseudooctahedral environment formed by the C and N of the dfppy ligand, the C_ipso of the C₆F₅ ring, a cyanido ligand and two trans thallium centers [Tl(1) and Tl(2)]. The Pt(1)–Tl(1,2) bonds are slightly longer than those found in 3·THF [Pt(1)–Tl(1) 3.0736(3), Pt(1)–Tl(2) 3.0518(3) Å; Tl(1)–Pt(1)–Tl(2) 162.723(8)°], which is in accordance with its blue shifted color in relation to the bzq complex, either in crystals (orange [4]₄·dioxanevs. red 3·THF) or as pristine solids (yellow 4, orange 3). However, the Pt(2) centers, located at the corner of the rectangle, exhibit a distorted square pyramid environment due to the formation of only one Pt–Tl bond [C,N_dfppy, C_C6F5, C_CN and Tl(2)]. Due to the lower coordination index, the Pt(2)–Tl distance [Pt(2)–Tl(2) 2.9258(3) Å] is the shortest one. In the same line, as shown in Fig. 2b, the Tl(2) centers, localized along the edges, achieve a distorted octahedral environment with the Pt centers [Pt(1), Pt(2)] located in the axial position and the equatorial plane formed by the N of a cyanido, N(2) bonded to Pt(1), two o-F of adjacent C₆F₅ groups [Tl(2)⋯F 2.894 and 3.103 Å] and one fluorine atom of a dfppy ligand [Tl(2)⋯F 3.022 Å] of a close octanuclear unit. The Tl(1) at the corners adopts a very distorted hexacoordination formed by the Tl(1)–Pt(1) bond, the bridging N_NCPt2, three fluorine contacts [two o-F_C6F5 (intra-unit 3.016 Å) (inter-unit 3.197 Å) and one F_dfppy of an adjacent unit (3.188 Å)] and the oxygen of the dioxane molecule [Tl(1)–O(1) 2.945 Å], which acts as a bridge between the Pt₄Tl₄ aggregates. Within the aggregates, the Tl(1)–N distance is slightly shorter than Tl(2)–N [Tl(1)⋯N(4) 2.596(4) vs. Tl(2)⋯N(2) 2.626(4) Å], whereas the C≡N⋯Tl angles are similar [C(12)–N(2)–Tl(2) 142.8(4) vs. C(30)–N(4)–Tl(1) 140.6(4)°]. The crystal packing reveals the presence of π⋯π interactions between dfppy ligands of neighboring Pt₄Tl₄ units (3.269–3.315 Å) (Fig. S8†), presumably contributing to the stability of the packing.

Fig. 2 (a) View of the molecular structure of [{Pt(dfppy)(C₆F₅)(CN)}Tl]₄·C₄H₈O₂[4]₄·dioxane showing the Tl⋯F and Tl⋯O(dioxane) contacts, (b) view of the coordination environment of both Tl centers.

In the case of the dfppy complex (4), although the disruption of the ladder-chain observed for the bzq system (3) and formation of the octanuclear aggregates could be attributed to the coordination of the dioxane, it cannot be ruled out that the presence of the electron-withdrawing fluorine atoms on the cyclometalated ligand weakens the Pt–Tl bonds due to the lower electron density on the Pt centers.

Photophysical properties

Absorption spectra. Electronic absorption data for 1–4 in (THF) solution (5 × 10⁻⁵ M) and in solid state (diffuse reflectance) are summarized in Table S3 and Fig. S9 and S10.† A comparison of the absorption spectra of the bimetallic complexes with their corresponding precursors in THF solution is provided in Fig. 3. Complexes 1 and 2 display intense high-energy bands (220–330/350 nm) attributed to metal perturbed π–π* intraligand transitions, located on the cyclometalated and C₆F₅ ligands, and a weaker low-energy feature (430 1, 382 nm 2) ascribed, according to previous assignments¹⁴ and TD-DFT calculations, to mixed ¹MLCT/¹IL transitions localized on the Pt(C^N) fragment. Dissolution of the orange (3) and the bright yellow (4) heteropolynuclear complexes in THF solvent produces yellow-pale solutions, indicating that their color in the solid state mainly comes from metallophilic (Pt–Tl) and π–π interactions present in the rigid media. The absorption spectra of 3 and 4 in THF solution are quite similar to their respective starting materials, although the LE band is slightly blue-shifted compared to the precursors (420 3vs. 430 nm 1 and 374 4vs. 382 nm 2), thus suggesting the contribution, to some extent, of bimetallic fragments in solution.^4b In point of fact, the presence of the donor–acceptor platinum–thallium bonds should decrease the electron density on the Pt metal center, increasing the band gap related to the platinum-to-ligand charge transition. In the bzq derivatives (1, 3), the LE absorption band is red-shifted in relation to that of the fluorinated derivatives (2, 4), in agreement with the higher electron delocalization and stabilization of the LUMO of the bzq group in relation to the dfppy.¹⁴

Fig. 3 Top: UV-vis absorption spectra of complexes 1–4 in THF (5 × 10⁻⁵ M) at 298 K. Down: schematic representation of the frontiers orbitals of 1, 2 and models 3·(THF)₃ and 4·(THF)₃ with selected excitations.

The solid-state diffuse reflectance spectra of all complexes are shown in Fig. S10 and Table S3.† Pristine solids 3 (orange) and 4 (yellow) display additional red-shifted absorptions (440–575 nm 3; 410–525 nm 4) in relation to their respective solution UV-vis spectra, in coherence with their color. These low-energy bands reflect the effect of the platinum-thallium bonds in the molecular transitions in the solid state and according to calculations, can be mainly ascribed to ¹MM'LCT transitions due to the strong contribution of the metals in the high occupied molecular orbitals and the involvement of the cyclometalated groups in the target low-lying orbitals.

To rationalize the experimental observations, DFT and TD-DFT calculations in THF solution and in solid state have been carried out for 1–4 (Tables S4–S9 and Fig. S11–S16†). For the mononuclear precursors 1 and 2, which are optimized based on the reported crystallographic structures,¹⁴ the calculated S₀ → S₁ transition in THF agrees with the experimentally observed transition, and arises from HOMO → LUMO (Fig. 3). The HOMOs are mainly centered on the Pt(C^N) units (Pt/bzq 30/63% 1, 41/50% 2) with small contribution of the CN⁻ (6% 1, 9% 2), whereas the LUMOs are located on the cyclometalated ligand (96% 1, 92% 2) (Tables S4, S5 and Fig. S11†). Therefore, the LE absorption band of mononuclear complexes can be assigned to a mixed ¹MLCT/¹IL transition, localized on the Pt(C^N) fragment, with a minor contribution of the cyanido. Similar results were obtained in solid state (Tables S8, S9 and Fig. S14†).

For the absorption of complexes 3 and 4 in THF solution, calculations for model systems of solvated bimetallic [Pt(C^N)(C₆F₅)(CN)Tl(S)₃] (S = THF) were performed. In these units, the Tl ion is coordinated to the Pt fragment through metal and one ortho-fluorine atom and to three oxygens atoms from solvent molecules, achieving a five coordination. Calculations reveal that, upon formation of the Pt–Tl bond, the contribution of the platinum to the HOMO decreases notably, with a minor Tl contribution, increasing the weight of the cyclometalated group [Pt/Tl/C^N 21/1/73% 3·(THF)₃; 30/1/63% 4·(THF)₃] and, as a consequence, the HOMO is stabilized in both complexes in relation to the precursor anionic fragments (Fig. 3-down and Fig. S13†). As shown in Fig. 3, which shows the frontiers orbitals, in both models, the Pt–Tl bonding is reflected in the H-1 with a remarkable contribution of the C₆F₅ ring in the dfppy complex [Pt/Tl/C₆F₅ 65/17/6% 3·(THF)₃; 46/8/35% 4·(THF)₃]. In both models, the target LUMO (and also L + 1) are mainly located on the C^N ligands with minor metal contribution [LUMO, Pt/Tl/C^N 4/2/93% 3·(THF)₃; 8/5/85% 4·(THF)₃]. The LUMOs are also stabilized in relation to the anionic precursors, but to a lesser extent than the HOMOs and, as a consequence, the calculated more intense low lying S₁ excitation, which arises from HOMO → LUMO, is blue shifted in relation to the precursors (ca. 10 nm) and has mainly mixed ¹IL/¹MLCT character. The close S₂ transition [363 3·(THF)₃; 344 nm 4·(THF)₃] has ¹IL/¹MM′CT character in the bzq compound, 3·(THF)₃, and mixed ¹IL/¹MM′CT/¹L′LCT (L′ = C₆F₅) contribution in 4·(THF)₃. Considering these results and the notable dynamic dissociation, suggested by the NMR data for the heterometallic complexes, the blue shifted low-lying electronic absorption in THF solution could be attributed to mixed ¹IL/¹MLCT with some ¹MM′CT and ¹L′LCT contribution in 3 and 4, respectively.

In the solid state, the presence of extended Pt–Tl–Pt–Tl bonds connected by CN⁻ bridges modifies the electronic structures. For complex 3, a short double chain Pt₄Tl₄ similar to that seen for 4 has been computed based on the X-ray (Tables S8, S9 and Fig. S15†). For model 3, the HOMOs (HOMO, HOMO−1), with antibonding character, are mainly located at the metal centers (HOMO/H-1 Pt 46/47%; Tl 32/30%) with low contribution of the ligands (bzq 12/12%; C₆F₅ 5/7%), whereas the low lying LUMO and L + 1 are placed at the adjacent benzoquinolate ligands (Fig. 4a) with minor contribution of metals (Pt/Tl LUMO 9/12%; L + 1 8/10). In the case of 4, the contribution of metals in the HOMOs decreases (Pt 34/32%; Tl 9/10%), whereas increases the weight of the dfppy ligand (50/53%) (Fig. 4b). For 3, the computed low-lying more intense excitations (S₁ 443 and S₃ 431 nm) involve transitions from HOMO and H-1 to LUMO and L + 1, having strong metal–metal′-to-ligand charge transfer (¹MM′LCT). For 4, the computed most intense low-lying absorption, S₂ (HOMO → LUMO, 384 nm), is blue shifted in relation to 3 and has mixed ¹MM′LCT/¹IL character.

Fig. 4 Selected frontier MOs and vertical excitations in the solid state for 3 (optimized model Pt₄Tl₄ based on the reported crystallographic structure) and 4 (optimized model Pt₄Tl₄).

Emission spectra. Complexes 1–4 were found to be non-emissive in solution. However, all complexes display intense emission in the solid state at 298 and 77 K, in deoxygenated glassy THF solution (5 × 10⁻⁵ M) and in doped polystyrene (PS) matrix (5% wt) at 298 K. The emission data are collected in Table 2 and representative spectra are given in Fig. 5 and Fig. S17–S19.†

Fig. 5 Normalized excitation and emission spectra of (a) 3, (b) 4 in the solid state at 298 and 77 K.

Table 2 Photoluminescence properties of complexes 1–4

	Solid 298 K			Solid 77 K		THF 77 K		PS^a
	λ em/nm (λex/nm)	Φ PL	τ/μs	λ em/nm (λex/nm)	τ/μs	λ em/nm (λex/nm)	λ em/nm (λex/nm)	τ/μs
a 10% wt. b with additional minor peak at 458 nm.
1	500, 566 (415)	0.05	3.1 (49%), 11.8 (51%)	496, 532, 570 (415)	186.6 (67%), 21.4 (33%)	480, 515, 555, 600 (415)	523, 568 (415)	41.5 (21.6%), 12.2 (78.4%)
2	472, 497, 530, 575 (405)	0.02	2.8 (46%), 13.7 (54%)	470, 497, 530, 575 (385)	15.1	457, 490, 520 (385)	472, 496, 526 (410)	11.7
3	595 (460)	0.13	14.2	658 (525)		480, 520, 560 (415)	546 (385)	21.9
4	546 (460)	0.17	11.5	563 (450)		559 (415)^b	523 (385)	12.7

---

Both precursors, 1 and 2, display in rigid media (solid, glassy) a typical vibronic-structured emission originating from ³IL/³MLCT emissive state with predominantly ³IL character, which is typical of cyclometalated complexes^14,17 and supported by TD-DFT calculations. In the case of the bzq derivative 1, the emission profile in the solid state at room temperature presents an additional excimeric low energy feature at 566 nm. In agreement with this assignment, the excitation spectra monitored at the lower and higher energy emission bands are similar (Fig. S17a†). The energy peak maximum is blue shifted in 2 compared to 1, in line with the presence electron-acceptor fluorine atoms in the phenyl ring of 2 (λ_max 457–472 2vs. 480–523 nm 1, Fig. S17†) and the measured lifetimes are significantly longer in the bzq complex. This fact, which has been previously observed in other Pt(bzq) compounds,^14,18 is attributed to a lower platinum contribution into the excited state and to the stronger structural rigidity of the bzq ligand compared to the dfppy group. As shown in Fig. S16,† the SOMO and SOMO−1 and the spin density for both anion complexes support a higher Pt and MLCT contribution into the excited state for complex 2 (SOMO/SOMO−1 5/24% 2vs. 1/16% 1).

The formation of Pt⋯Tl bonding interactions in both heterometallic complexes 3 and 4 are reflected in their optical properties. Both pristine solids are brightly emissive in solid state. The dfppy hetero-octanuclear aggregate 4 exhibits in solid state at room temperature an unstructured yellow emission centered at 546 nm (ϕ = 17%), which is slightly red shifted at low temperature (563 nm). For the double extended chain bzq derivative, 3, as shown in Fig. 5, the emission is somewhat broader and red shifted (595 nm) at room temperature compared to 4, which agrees with the presence of a shorter Pt⋯Tl separation (see Table 1) and a more delocalized low-lying cyclometalated ligand (bzq vs. dfppy). Furthermore, upon cooling at 77 K, the emission is significantly more red-shifted (658 nm) than in the case of 4 (595 → 658 nm 3; 546 → 563 nm 4), a behavior which is generally attributed to some degree of thermal contraction of the Pt⋯Tl separation at low temperature and has many precedents in metal⋯metal chain systems.^4b,19 In both complexes, the excitation profile resembles the absorption spectra in the solid state with a maximum notable red shifted in 3 in relation to 4 (520 3, 450 nm 4), indicating that the absorption in these low energy regions is responsible for the corresponding emission. The large Stokes shifts and the lifetimes measured at room and at low temperature in the range of microseconds (Table 2) confirm the occurrence of phosphorescence processes. For the bzq derivative 3, the computed first vertical excitation at the ground state geometry (T₁, 537 nm) for the short double chain Pt₄Tl₄, in the gas phase, corresponds to the HOMO–LUMO transition having ³MM′LCT character. In 4, the calculated lowest T₁ is blue shifted (439 nm) and involves a combination of several electronic transitions from HOMO and H-1 to LUMO-L+3, which are mainly generated from the metals and the cyclometalated group (HOMO, H-1 Pt 32–34% Tl 9–10%, dfppy 50–53%; LUMO–LUMO+3 Pt 15–7%, Tl 25–3%, dfppy 53–88%). The emission is therefore ascribed to phosphorescence with mixed ³MM′LCT/³IL character.

As noted before, complexes 3 and 4 are soluble in THF and in this solvent the emission is lost at room temperature, a behavior that can be attributed to partial and reversible rupture of the Pt–Tl bonds. Upon cooling the solutions (5 × 10⁻⁵ M) to 77 K (Fig. S18†), bright emission is developed mainly formed by an unstructured low energy band (560 3, 559 nm 4), which is attributed to the formation of bimetallic fragments [{Pt(C^N)(C₆F₅)(CN)}Tl(THF)_x] in the frozen solution, together with an overlapped minor high energy structured band (λ_max 480 nm 3, 458 nm 4), associated with the presence of anionic [Pt(C^N)(C₆F₅)(CN)]⁻ fragments. Similar behavior has been previously reported in related heteropolynuclear complexes featuring unsupported Pt–Tl bonds.^4b,8c

Mechanochromic and solvatochromic behavior. When 3-Pristine powder is crushed in the mortar, the color and luminescence change from orange to deep orange and in the emission profile, the broad emission band centered at λ_max 595 nm shifts to the red at room temperature (∼606 nm, 631_sh) and to a lesser extend upon cooling (∼660 nm), suggesting that some degree of shortening of metal–metal separation is caused upon crushing (Fig. 6 and Fig. S20†). In the case of the yellow pristine solid 4, after mechanical grinding in the mortar, its color switched to orange and the emission turn orange under an UV hand lamp. As shown in Fig. 7, the emission of the crushed powder 4 depend on the excitation wavelength extending from 558, upon exciting at 455 nm, to 650 with λ_ex 500 nm. The excitation spectra also differ (Fig. S21†), indicating the formation of different aggregates with shorter and distinct Pt⋯Tl separations. We note that similar results were obtained by using different times of crushing and in both complexes the emission and color could be recovered upon dissolving the ground powder with acetone.

Fig. 6 Normalized emission spectra at 298 K of the orange solids 3, 3-ground and the solvates (3-solvent) (λ_ex 405–415 nm).

Fig. 7 Normalized emission spectra at 298 K of the orange solid 4, 4-MeOH and 4-ground (with different λ_exc).

Interestingly, we observed that the treatment of powder 3 with a drop of MeOH, CH₂Cl₂, THF or Et₂O results in color and luminescence changes, thus exhibiting solvatochromic behavior (Table S10 and Fig. S20†). As shown in Fig. 6, when solid 3 was treated with a drop of Et₂O, CH₂Cl₂ and THF a red shift in its color and emission band takes place from orange to pink-red (630 nm), to crimson (632 nm) and to deep pink color (639 nm), respectively. This bathochromic shift from 595 nm in solvent-free solid 3 to 639 nm in 3-THF suggests that the solvent molecules allow the platinum–thallium bond to be shortened and to form a strong interaction, decreasing the gap associated with the metal–metal-to ligand charge transition ³MM′LCT, leading to change in the color of the compound. Curiously, treatment of pristine solid 3 with a drop of MeOH causes a color change from deep orange to bright yellow-orange under UV light (reflected in its absorption spectrum, Fig. S10†), and shows a blue shift in the emission band both, at 298 K (595 → 583 nm) and at 77 K (658 → 650 nm). However, in contrast to complex 3, complex 4 demonstrates a bathochromic response to MeOH. Thus, upon addition of a drop of MeOH solvent to the yellow solid 4, the color of the complex changes from yellow to orange and a notable red shift appears in the emission spectrum, both at room and at low temperature (298/77 K 590/662 nm Fig. 7 and Fig. S21†). This bathochromic response suggests that the incorporation of MeOH likely provokes a partial or total shortening of the Pt–Tl bonds. Surprisingly, no similar response was observed with other solvents (Et₂O, CH₂Cl₂ or THF).

Conclusions

The new mononuclear anionic (NBu₄)[Pt(C^N)(C₆F₅)(CN)] (C^N = bzq 1, dfppy 2) and heterometallic [{Pt(C^N)(C₆F₅)(CN)}Tl] (C^N = bzq 3, dfppy 4) compounds have been prepared and their photophysical properties investigated with the support of theoretical calculations. Structures of (3·THF)_n and [4]₄·dioxane contain Pt(II)–Tl(I) bonds, but they show different arrangements. In the bzq complex, (3·THF)_n, each Pt interacts with two Tl(I) ions and each Tl(I) with two Pt centers, generating a bent ladder-like extended assembly where two chains are connected by bridging cyanido ligands [Pt](μ-CN)[Tl]. To the best of our knowledge, this double chain network is unknown in PtTl systems. In the case of the dfppy complex, [4]₄·dioxane, the presence of dioxane breaks the polymeric structure into discrete heterooctametallic Pt₄Tl₄ rectangular aggregates, connected by one molecule of dioxane, also stabilized by Pt–Tl bonds and bridging cyanido ligands. Notwithstanding, the presence of electron-withdrawing fluorine atoms on the dfppy ligand produces lower electron density on the Pt centers, also contributing to the weakening of the Pt–Tl bonds. In both structures, the secondary interactions of the Tl(I) centers with the ortho-fluorine atoms of the C₆F₅ rings of adjacent units and with oxygen atoms of THF (3) or dioxane (4) also contributes to the stability.

In the solid-state, the PtTl complexes 3 and 4 exhibit unstructured emissions (595 3, 546 nm 4) red shifted in relation to the mononuclear derivatives (500 1, 472 nm 2), mainly attributed to ³MM′LCT [d/s σ*(Pt, Tl) → π*(C^N)] in 3, with mixed ³MM′LCT/³IL character in 4. However, in glassy THF solution, both derivatives exhibit a dual emission, corresponding the unstructured low energy band to the formation of bimetallic [{Pt(C^N)(C₆F₅)(CN)}Tl(THF)_x] fragments and the minor structured high energy band to the anionic [Pt(C^N)(C₆F₅)(CN)]⁻ units. Interestingly, complexes 3 and 4 show a reversible mechanochromic behavior with red shifted color and luminescence changes, likely due to some degree of shortening of Pt–Tl separations, when pressure is applied. In addition, 3 displays strong solvatochromism in the solid state with MeOH, CH₂Cl₂, THF or Et₂O, showing shifts from 583 in 3-MeOH to 639 nm in 3-THF, which can be attributed to modulation of the Pt–Tl separations resulting from the effect of the solvent molecules. However, 4 only demonstrates response to MeOH.

Experimental section

General comments

The neutral precursor complexes, [Pt(C^N)(C₆F₅)(DMSO)] [C^N = 7,8-benzoquinolate (bzq), (2-(2,4-difluorophenyl)pyridinate) (dfppy)] were prepared according to published procedure.^17b Other reagents were used as received. The reactions with Tl(I) salts were performed in the absence of light. The microanalyses were carried out with a CA FLASH 2000 (Thermo Fisher Scientific) microanalyzer. Mass spectra were recorded on a Microflex MALDI-TOF Bruker spectrometer operating in the linear and reflector modes using THF solutions of the complexes and dithranol as the matrix. NMR spectra were recorded on a Bruker AVANCE ARX 400 MHz spectrometer at 298 K. IR spectra were obtained on a Fourier Transform PerkinElmer Spectrum UATR Two spectrophotometer, with the diamond crystal ATR attachment, which covers the region between 4000 and 450 cm⁻¹. The chemical shifts (δ) are reported in units of parts per million (ppm) relative to external standards (TMS for ¹H and ¹³C{¹H} and CFCl₃ for ¹⁹F{¹H}) and coupling constant were given in hertz (Hz). The UV-vis absorption spectra were measured with a Hewlett-Packard 8453 spectrophotometer. Diffuse Reflectance UV-vis (DRUV) spectra were carried out in SiO₂ pellets, using a Shimazdu UV-3600 spectrophotometer with a Harrick Praying Mantis accessory, and recalculated following the Kubelka Munk function. Excitation and emission spectra were obtained in a Shimazdu RF-60000. The measurements in solid state and PS films were carried out on air and in solutions under N₂ atmosphere. The lifetime measurements up to 10 μs at 298 K at all samples at 77 K were performed with a Jobin Yvon Horiba Fluorolog operating in the phosphorimeter mode (with an F1–1029 lifetime emission PMT assembly, using a 450 W Xe lamp) and the Jobin Yvon software packing, that works with Origin 6.0. The decay data were analysed by tail fitting to the functions “one-phase exponential decay function with time constant parameter” (ExpDec1) and “two-phase exponential decay function with time constant parameters (ExpDec 2)”. The lifetimes below 10 μs at 298 K were measured with a Datastation HUB-B with a nanoLED controller, using the technique “Time Correlated Single Photon Counting” (TCSPC). Quantum yields were measured using a Hamamatsu Absolute PL Quantum Yield Measurement System C11347-11.

Synthesis of (NBu₄)[Pt(bzq)(C₆F₅)(CN)] (1). To a solution of [Pt(bzq)(C₆F₅)(DMSO)] (490 mg, 0.777 mmol) in Me₂CO (30 mL) at room temperature, (NBu₄)CN (209 mg, 0.777 mmol) was added. After 1 hour of stirring, the greenish-yellow solution was evaporated under vacuum to give an oily yellow residue, which was treated with a mixture of CH₂Cl₂/H₂O (20 : 20 mL), removing the aqueous layer. It was repeated for three times. Treatment of the organic layer with MgSO₄, filtration through Celite, evaporation to dryness and treatment with n-hexane (5 mL) allowed to obtain 1 as a yellow solid (yield: 437 mg, 68%). Elem. Anal. Calcd for C₃₆H₄₄F₅N₃Pt (808.85): C, 53.46; H, 5.48; N, 5.20. Found: C, 53.80; H, 5.48; N, 5.10%; IR (cm⁻¹): 2102 (s) ν(CN); MALDI-TOF (−) m/z (%): 565.97 [Pt(bzq)(C₆F₅)(CN)]⁻ (100), 1105.98 [Pt₂(bzq)₂(C₆F₅)₂(CN)]⁻ (54). ¹H NMR (400 MHz, CDCl₃, 298 K, δ): 9.80 (dd, ³J_H–H = 5.2, ⁴J_H–H = 1.0, ³J_Pt–H = 28.6, H²), 8.33 (dd, ³J_H–H = 8.0, ⁴J_H–H = 1.0, H⁴), 7.78 (d, ³J_H–H = 8.8, H⁵), 7.55–7.38 (m, 4H, H^3,6–8), 7.20 (d, ³J_H–H = 7.0, ³J_Pt–H = 46.2, H⁹), 3.03 (m, 8H, N-CH₂–), 1.45 (m, 8H, N-CH₂-CH₂–, NBu₄), 1.26 (sx, 8H, ³J_H–H = 8.0, –CH₂-CH₃, NBu₄), 0.88 (t, 12H, ³J_H–H = 8.0, –CH₃, NBu₄). ¹³C{¹H} NMR (100.6 MHz, CDCl₃, 298 K, δ): 158.3 (s, C¹³), 157.1 (s, C¹⁰), 150.8 (s, C²), 147.7 (s, CN), 144.3 (s, C¹⁴), 136.6 (s, C⁴), 134.2 (s, C⁹), 133.7 (s, C¹¹), 129.7 (s, C⁵), 129.5 (s, C⁸), 126.7 (s, C¹²), 122.6 (s, C⁶), 121.7 (s, C³), 121.4 (s, C⁷), 58.6 (s, C^1′), 23.7 (s, C^2′), 19.5 (s, C^3′), 13.5 (s, C^4′). ¹⁹F{¹H} NMR (376.5 MHz, CDCl₃, 298 K, δ): −115.4 (dm, ³J_Pt–Fo = 530, 2F_o), −165.8 (m, 2F_m, F_p).

Synthesis of (NBu₄)[Pt(dfppy)(C₆F₅)(CN)] (2). Complex 2 was prepared as a yellow solid following a similar procedure to complex 1 (yield: 77 mg, 82%) starting from [Pt(dfppy)(C₆F₅)(DMSO)] (72 mg, 0.11 mmol) and (NBu₄)CN (31 mg, 0.11 mmol). Elem. Anal. Calcd for C₃₄H₄₂F₇N₃Pt (820.80): C, 49.75; H, 5.16; N, 5.12. Found: C, 49.41; H, 5.48; N, 4.78%; IR (cm⁻¹): 2110 (s) ν(CN); MALDI-TOF (−) m/z (%): 577.92 [Pt(dfppy)(C₆F₅)(CN)]⁻(100), 1129.93 [Pt₂(dfppy)₂(C₆F₅)₂(CN)]⁻ (78). ¹H NMR (400 MHz,CDCl₃, 298 K, δ): 9.64 (dd, ³J_H–H = 5.2, ⁴J_H–H = 1.0, ³J_Pt–H = 25.7, H²), 8.10 (d, ³J_H–H = 8.0, H⁵), 7.95 (d, ³J_H–H = 8.0, H⁴), 7.13 (t, ³J_H–H = 8.0, H³), 6.46 (ddd, ³J_H–F = 12.8, ³J_H–F = 9.0, ⁴J_H–H = 2.0, H⁹), 6.36 (dd, ³J_H–F = 9.0, ³J_H–H = 2.0, ³J_Pt–H = 52.8, H¹¹), 3.03 (m, 8H, N-CH₂–), 1.47 (m, 8H, N-CH₂-CH₂–, NBu₄), 1.28 (sx, 8H, ³J_H–H = 8.0, –CH₂-CH₃, NBu₄), 0.87 (t, 12H, ³J_H–H = 8.0, –CH₃, NBu₄). ¹³C{¹H} NMR (100.6 MHz, CDCl₃, 298 K): 165.4 (m, C⁸), 163.9 (d, J_C–F = 7.5, C⁶), 162.3 (m, C¹²), 159.2 (d, J_C–F = 11.8, C¹⁰), 152.6 (s, ²J_Pt–C = 32.5, C²), 149.2 (m, C_C6F5), 146.3 (s, CN), 138.3 (s, C⁴), 134.8 (m, C_C6F5), 130.7 (s, C⁷), 122.7 (s, C³), 122.5 (s, C⁵), 118.2 (dd, ²J_C–F = 16.4, ⁴J_C–F = 2.3, ²J_Pt–C = 90.6, C¹¹), 99.1 (t, ²J_C–F = 26.5, C⁹), 58.6 (s, C^1′), 23.7 (s, C^2′), 19.5 (s, C^3′), 13.4 (s, C^4′). ¹⁹F{¹H} NMR (376.5 MHz, CDCl₃, 298K, δ): −109.0 (d, ⁴J_F–F = 9.7, ⁴J_Pt–F = 50.5, F¹⁰), −111.0 (d, ⁴J_F–F = 9.7, ⁴J_Pt–F = 40, F⁸), −116.2 (dm, ³J_Pt–Fo = 515, 2F_o), −165.2 (m, 2F_m, F_p).

Synthesis of [{Pt(bzq)(C₆F₅)(CN)}Tl] (3). KCN (23.5 mg, 0.361 mmol) was added to a suspension of [Pt(bzq)(C₆F₅)(DMSO)] (223 mg, 0.361 mmol) in a mixture of Me₂CO/H₂O (15/5 mL). After 30 minutes of stirring at room temperature, TlPF₆ (126 mg, 0.361 mmol) was added. Immediately, the yellow solution changed to a yellow suspension, which was stirred for 20 min. The acetone solvent was evaporated under vacuum, the aqueous suspension was filtered and the orange solid was obtained washed with H₂O and n-hexane to give 3 (yield: 261 mg, 95%). Elem. Anal. Calcd for C₂₀H₈F₅N₂PtTl (770.75): C, 31.17; H, 1.05; N, 3.63. Found: C, 31.55; H, 1.26; N, 3.22%; IR (cm⁻¹): 2108 (s) ν(CN); MALDI-TOF (+) (THF) m/z (%): 604.96 [Pt(bzq)(C₆F₅)(CN) K]⁺ (100), 770.91 [Pt(bzq)(C₆F₅)(CN)Tl]⁺ (2.6), 816.08 [Pt(bzq)(C₆F₅)Tl(THF)]⁺ (23.5). MALDI-TOF (−) (THF) m/z (%): 565.90 [Pt(bzq)(C₆F₅)(CN)]⁻ (100), 1105.84 [Pt₂(bzq)₂(C₆F₅)₂(CN)]⁻ (77.3), 1336.95 [{Pt(bzq)(C₆F₅)(CN)}₂Tl]⁻(4.5). MALDI-TOF (−) (solid) m/z (%): 566 [Pt(bzq)(C₆F₅)(CN)]⁻ (100), 1105.94 [Pt₂(bzq)₂(C₆F₅)₂(CN)]⁻ (77.3), 1336.95 [{Pt(bzq)(C₆F₅)(CN)}₂Tl]⁻ (4.5). Λ_M (acetone): 90.5 Ω⁻¹ cm⁻² mol⁻¹. ¹H NMR (400 MHz, THF-d₈, 298 K, δ): 9.82 (dd, ³J_H–H = 5.2, ⁴J_H–H = 1.0, ³J_Pt–H = 27.7, H²), 8.53 (dd, ³J_H–H = 8.0, ⁴J_H–H = 1.0, H⁴), 7.75 (d, ³J_H–H = 8.7, H⁵), 7.66–7.61 (m, 2H, H^3,6), 7.50 (d, ³J_H–H = 7.7, H⁷), 7.29 (d, ³J_H–H = 7.7, H⁸), 7.04 (d, ³J_H–H = 7.0, ³J_Pt–H = 43.7, H⁹). ¹³C{¹H} NMR (100.6 MHz, THF-d₈, 298 K, δ): 157.7 (s, C¹³), 156.1 (s, C¹⁰), 153.0 (s, C²), 145.2 (s, CN), 145.1 (s, C¹⁴), 138.8 (s, C⁴), 135.6 (s, C⁹), 135.1 (s, C¹¹), 130.6 (s, C⁵), 130.2 (s, C⁸), 128.5 (s, C¹²), 124.1 (s, C⁶), 123.4 (s, C³), 123.1 (s, C⁷). ¹⁹F{¹H} NMR (376.5 MHz, THF-d₈, 298 K, δ): −116.5 (dm, ³J_Pt–Fo = 435, 2F_o), −166.6 (m, 2F_m, F_p).

Synthesis of [{Pt(dfppy)(C₆F₅)(CN)}Tl] (4). Complex 4 was prepared as a yellow solid following a similar procedure to complex 3 (yield: 292 mg, 99%) starting from [Pt(dfppy)(C₆F₅)(DMSO)] (236 mg, 0.374 mmol), KCN (24.4 mg, 0.374 mmol) and TlPF₆ (131 mg, 0.374 mmol). Elem. Anal. Calcd for C₁₈H₆F₇N₂PtTl (782.71): C, 27.62; H, 0.77; N, 3.58. Found: C, 27.61; H, 0.80; N, 3.18; IR (cm⁻¹): 2120 (s) ν(CN); MALDI-TOF (+) (THF) m/z (%): 782.84 [Pt(dfppy)(C₆F₅)(CN)Tl]⁺ (5.8). MALDI-TOF (−) (THF) m/z (%): 577.82 [Pt(dfppy)(C₆F₅)(CN)]⁻ (100), 1129.78 [Pt₂(dfppy)₂(C₆F₅)₂(CN)]⁻ (59.8). MALDI-TOF (−) (solid) m/z (%): solid: 577.83 [Pt(dfppy)(C₆F₅)(CN)]⁻ (84.3), 1129.79 [Pt₂(dfppy)₂(C₆F₅)₂(CN)]⁻ (100), 1360.71 [{Pt(dfppy)(C₆F₅)(CN)}₂Tl]⁻ (11.2). Λ_M (acetone): 92.8 Ω⁻¹ cm⁻² mol⁻¹. ¹H NMR (400 MHz, THF-d₈, 298 K, δ): 9.60 (dd, ³J_H–H = 5.2, ⁴J_H–H = 1.0, ³J_Pt–H = 28.0, H²), 8.15 (d, ³J_H–H = 8.0, H⁵), 8.03 (t, ³J_H–H = 7.0, H⁴), 7.30 (t, ³J_H–H = 7.0, H³), 6.55 (ddd, ³J_H–F = 12.6, ³J_H–H = 9.0, ⁴J_H–H = 2.0, H⁹), 6.37 (dd, ³J_H–H = 8.5, ⁴J_H–H = 2.0, ³J_Pt–H = 57.2, H¹¹). ¹³C{¹H} NMR (100.6 MHz, THF-d₈, 298 K, δ): 165.7 (dd, ¹J_C–F = 275.3, ⁴J_C–F = 11.3, C⁸), 164.6 (d, ⁴J_C–F = 8.0, ³J_Pt–C = 70.0, C⁶), 164.3 (m, C¹²), 162.3 (dd, ¹J_C–F = 280.1, ⁴J_C–F = 13.1, C¹⁰), 154.3 (s, ²J_Pt–C = 31.1, C²), 148.6 (dm, ¹J_C–F = 228, C₆F₅), 145.5 (s, CN), 140.3 (s, C⁴), 137.6 (dm, ¹J_C–F = 243, C₆F₅), 131.8 (s, C⁷), 125.5 (s, ³J_Pt–C = 19.0, C³), 123.7 (d, ⁴J_C–F = 25.2, ³J_Pt–C = 22.0, C⁵), 119.4 (dd, ²J_C–F = 20.0, ⁴J_C–F = 2.2, ²J_Pt–C = 86.2, C¹¹), 100.3 (t, ²J_C–F = 28.6, C⁹). ¹⁹F{¹H} NMR (376.5 MHz, THF-d₈, 298 K, δ): −109.8 (d, ⁴J_F–F = 9.7, ⁴J_Pt–F = 49.7, F¹⁰_dfppy), −111.4 (d, ⁴J_F–F = 9.7, ⁴J_Pt–F = 36.9, F⁸_dfppy), −117.0 (dm, ³J_Pt–Fo = 445, 2F_o), −166.1 (m, F_p), −166.4 (m, 2F_m).

X-ray diffraction

X-ray intensity data were collected using Molybdenum graphite monochromatic (Mo-K_α) radiation with Bruker APEX-II diffractometer at a temperature of 100 K using APEX-II programs for all complexes. Structures were solved by Intrinsic Phasing using SHELXT²⁰ with the WinGX graphical user interface.²¹ Multi-scan absorption corrections were applied to all the data sets and refined by full-matrix least squares on F² with SHELXL.²² Hydrogen atoms were positioned geometrically, with isotropic parameters U_iso = 1.2U_eq (parent atom) for aromatic hydrogens and CH₂ and U_iso = 1.5U_eq (parent atom) for methyl groups. Finally, the structures show some residual peaks in the vicinity of the platinum atoms but with no chemical meaning.

Computational details

Calculations were performed with the program suite Gaussian 16²³ on complexes with Beckés three-parameter functional combined with Lee–Yang–Parŕs correlation functional (B3LYP).²⁴ Optimizations on the singlet state (S₀) in THF and in gas phase of the mononuclear models 1 and 2 were performed based on the reported crystallographic structures.¹⁴ For 3 and 4, calculations in THF solutions were performed for model systems of solvated bimetallic species [Pt(C^N)(C₆F₅)(CN)Tl(S)₃] (S = THF) [3·(THF)₃, 4·(THF)₃], using as starting point the molecular geometry obtained by X-ray diffraction analysis. Calculations in gas phase on heteropolynuclear structures of 3 and 4 (considered as Pt₄Tl₄) were made starting from the X-Ray data. No imaginary frequency was found in the vibrational frequency analysis of the final equilibrium geometries. The LANL2DZ basis set was used for the platinum and thallium centers and all-electron 6-31G (d, p) basis set was applied for other atoms.²⁵ Solvent effects were taken into consideration by the Polarizable Continuum Model (PCM)²⁶ implemented in the Gaussian 16 software, in the presence of THF. The TD-DFT method was employed for calculations of the electronic absorption spectra. The predicted emission wavelengths were calculated by the energy difference between the triplet state at its optimized geometry and the singlet state at the triplet geometry. The results were visualized with GaussView 6. Overlap populations between molecular fragments were calculated using the GaussSum 3.0 program.²⁷

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Spanish Ministerio de Ciencia e Innovación (Project PID2019-109742GB-I00) funded by MCIN/AIE/10.13039/501100011033. D. G. S. is grateful to UR for a PhD grant.

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Footnotes

† Electronic supplementary information (ESI) available: Characterization of complexes (NMR spectra, crystal data), photophysical properties and computational details and photocatalytic studies. CCDC 2337876 and 2337877. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4dt00674g

‡ These authors have contributed equally to this work.

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