Tl complexes: a structural and photophysical study.

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Cyclometalated heteronuclear Pt/Ag and Pt/Tl complexes: a structural and photophysical study† Sirous Jamali,*a Reza Ghazfar,a Elena Lalinde,b Zahra Jamshidi,c Hamidreza Samouei,d Hamid R. Shahsavari,b M. Teresa Moreno,b Eduardo Escudero-Adán,d Jordi Benet-Buchholzd and Dalibor Milice To investigate the factors influencing the luminescent properties of polymetallic cycloplatinated complexes a detailed study of the photophysical and structural properties of the heteronuclear complexes [Pt2Me2(bhq)2(μ-dppy)2Ag2(μ-acetone)](BF4)2, 2, [PtMe(bhq)(dppy)Tl]PF6, 3, and [Pt2Me2(bhq)2(dppy)2Tl]-PF6, 4, [bhq = benzo[h]quinoline, dppy = 2-(diphenylphosphino)pyridine] was conducted. Complexes 3 and 4 synthesized by the reaction of [PtMe(bhq)(dppy)], 1, with TlPF6 (1 or 1/2 equiv.) and stabilized by unsupported Pt–Tl bonds as revealed by multinuclear NMR spectroscopy and confirmed by X-ray crystallography for 3. DFT calculations for the previously reported butterfly Pt2Ag2 cluster 2 reveal that in the optimized geometry the bridging acetone molecule is removed and the metal core displays a planarshaped geometry in which according to a QTAIM calculation and natural bond orbital (NBO) analysis the Ag⋯Ag metallophilic interaction is strengthened. In contrast to the precursor 1, which is only emissive in glassy solutions (3MLCT 485 nm), all 2–4 heteropolynuclear complexes display intense emissions in the solid state and in glassy solutions. Time-dependent density functional theory (TD-DFT) is used to elucidate the origin of the electronic transitions in the heteronuclear complexes 2 and 3. The low energy absorption and intense orange emission for cluster 2 (solid 77 K and glass) are attributed to metal–metal to ligand charge transfer (MM’LCT) with a minor L’LCT contribution. For 3 and 4 two different bands are

Received 13th August 2013, Accepted 3rd October 2013

developed: the high energy band (602–630 nm) observed for 4 at 77 K (solid, glass) and in diluted glasses

DOI: 10.1039/c3dt52209a

for 3 is ascribed to emission from discrete Pt2Tl units of mixed 3L’LCT/3LM’CT origin. However, the low energy band (670–690 nm) observed at room temperature (solid) for both complexes and also in con-

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centrated glasses for 3 is assigned to 3ππ excited states arising from intermolecular interactions.

Introduction The study of complexes and solid-state structures containing metal–metal and ligand–ligand interactions has been the subject of significant interest in the field of inorganic chemistry.1 It is now well recognized that multinuclear metal assemblies constructed via these interactions display fascinating a Chemistry Department, Sharif University of Technology, P.O. Box 11155-3516, Tehran, Iran. E-mail: [email protected] b Departamento de Química-Centro de Síntesis Química de La Rioja, (CISQ), Universidad de La Rioja, 26006 Logroño, Spain c Chemistry and Chemical Engineering Research Center of Iran, Tehran, Iran d Institute of Chemical Research of Catalonia (ICIQ), Avgda. Països Catalans 16, 43007 Tarragona, Spain e Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102a, 10000 Zagreb, Croatia † Electronic supplementary information (ESI) available: NMR and ESI-Mass spectra, tables of TD-DFT results, crystal data and structure refinement parameters, diffuse reflectance UV-Vis spectra, optimized structures of 2, 4 and CIF files. CCDC 955571–955572. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3dt52209a

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and unique chemical and physical properties that were not observed for their monomeric components.2 In particular, these non-covalent interactions have been used as a tool for crystal engineering and have been shown to play an important role in their associated photophysical properties, both in solution and in the solid state in a wide array of systems.3 Illustrative examples are found in copper cubane clusters Cu4L4X4 having short Cu–Cu distances (less than 2.8 Å), which exhibit thermochromic luminescent behavior associated with a cluster-centered (3CC) emission that dominates at room temperature.4 Furthermore, in many bi- or multinuclear complexes containing closed-shell d10 (Au, Cu, Ag) ions their luminescence properties have been demonstrated to depend on the degree of metallophillic interactions, which can be controlled by the nature and length of the bridging ligand.5 It has been also shown that the formation of excimers by π–π stacking of the adjacent pyridyl rings of ppy ligands can significantly change the excited state properties of d6 Ir(III) complexes.6 Cyclometalated complexes have recently attracted a great deal of attention because of their potential applications

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in many fields such as catalysis and molecular electronic devices.7 In platinum chemistry, the tendency of cyclometalated Pt(II) systems to self-assemble into one-dimensional or nanoscale structures has been a widely studied field for many years.8 Within this framework, the particular case of homometallic Pt(II) species containing strong-field, planar, luminophorous cyclometalating ligands has recently attracted growing interest, due to the fact that their assembling and their rich excited states (MLCT, ILCT, LLCT) can be controlled by the simultaneous tuning of PtII⋯PtII and π⋯π ligand interactions, which led to low energy emissions coming from MMLCT or excimer-like excited states.8c,9 Pt(II) complexes containing chromophoric cyclometalating ligands have been also successfully used as building blocks for the design of heteropolynuclear (Pt-d10, s2) aggregates or supramolecular networks, many of them only stabilized by unsupported heteronuclear Pt–M bonds.10 However, the research carried out on these complexes has been mainly focused on structural characteristics and reactivity. The limited attention that has been given to the impact of these heteronuclear metallophillic bonds on their excited states10 contrasts with the extensive studies carried out on homometallic systems. Some recent reports have shown that their emissive properties are generally enhanced in relation to the precursors, and are primarily determined by the electronic characteristics of the platinum cyclometalated unit modulated by the nature of the heterometal and coligands. Thus, the formation of the so-called unsupported donor–acceptor Pt-d10 (Pt–Cd, Pt–Ag)10f,g,11 bonds has been shown to cause a significant hypsochromic shift on the 1MLCT absorption and 3MLCT emission bands in relation to the precursors in bimetallic complexes, attributed to a decrease in electronic density at the Pt center, which probably causes a larger HOMO–LUMO gap.11 Similar hypsochromic shifts were observed in bi and trinuclear cyclometalated-dithiolate Pt–AuI complexes and attributed to a gradual loss of the LL′CT character in the electronic transition, with a concomitant increase in the LC/MLCT contribution.10d In contrast, the formation of Pt–TlI bonds in complexes [PtTl(C^N)X2] (C^N = benzo[h]quinoline or 2-phenylpyridine, X = CN, CuuCR) produce bathochromic shifts that are tentatively attributed to an increase in the energy of the Pt–Tl based HOMO, which lowers the energy of the emission ascribed to metal–metal to ligand charge transfer (MM′LCT).12 However, in the solid state (and also in clusters containing additional auxiliary bridging ligands) the influence of the heteronuclear metallophilic bonds is less predictive due to the prevalence of Pt⋯Pt and/or ligand–ligand (π⋯π) interactions in some systems.10b,g,h In order to further develop our understanding of the impact of these metallophilic bonds on the optical properties of cycloplatinated-based clusters, additional comparative studies would be of interest. In this field, we recently reported the synthesis of the neutral benzoquinolate complex [PtMe(bhq)(dppy)], 1, (dppy = 2-diphenylphosphine-pyridine), containing both an electronrich platinum center and an uncoordinated pyridyl group, which was successfully employed to generate the tetranuclear complex [Pt2Me2(bhq)2(µ-dppy)2Ag2(µ-acetone)](BF4)2,

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2.13 Cluster 2 contains a rare butterfly metallic Pt2Ag2 core stabilized by both, Pt–Ag and Ag⋯Ag bonding interactions, and was found to undergo easy skeletal rearrangement in solution. In view of these findings and given our current research into complexes containing Pt–M bonds, we decided to examine the reactivity of 1 towards Tl+ and to carry out comparative photophysical and theoretical studies. Here we report: (a) the synthesis and characterization of two new complexes [PtMe(bhq)(dppy)Tl]PF6, 3, and [Pt2Me2(bhq)2(dppy)2Tl]PF6, 4, (b) a comparative study of the photophysical properties of 1–4, and (c) theoretical studies to elucidate the nature of electronic transitions in these heteronuclear complexes.

Results and discussion Synthesis and structures As described in Scheme 1, the reaction of complex [PtMe(bhq)(dppy)], 1, with 1 and 0.5 equiv. of TlPF6 at room temperature in acetone gave the hetero bi- and trinuclear complexes [PtMe(bhq)(dppy)Tl]PF6, 3, and [Pt2Me2(bhq)2(dppy)2Tl]PF6, 4, respectively. Complexes 3 and 4 are stable in dichloromethane solution for several hours and were fully characterized by multinuclear NMR spectroscopy, and ESI-Mass spectrometry. In the 31 1 P{ H} NMR spectra of 3 and 4, the P atoms of the dppy ligands appeared as singlet signals at δ 35.8 and 36, which were coupled to Pt atoms to give satellites with 1JPtP = 1934 and 2025 Hz, respectively. Reduction of the coupling constant 1JPtP from the value of 2096 Hz in the 31P{1H} NMR spectrum of 1 to the values of 1934 and 2025 Hz of 3 and 4 suggests an increase in the coordination number of the platinum atom upon Tl complexation.13,14 However, the observation of only one singlet signal in the 31P{1H}NMR spectra of 3 and 4 and no evidence of coupling to the Tl atom at ambient temperature suggests a rapid dissociation and formation of the Pt–Tl dative bond in these complexes on the NMR time scale. This type of dynamic processes has been observed in previously reported Pt–Tl complexes.3k,15 Therefore, the 31P{1H} NMR spectrum of complex 4 was monitored as a function of the

Scheme 1


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Fig. 1 31P{1H} VT-NMR spectra of 4 in CD2Cl2 (a) at room temperature show only a singlet with platinum satellites, (b) at −80 °C show P–Tl and long range Pt–P couplings and (c) at −90 °C.

temperature in the CD2Cl2 solution (Fig. 1). As the temperature is lowered, the phosphorus resonance is broad with unresolved Tl–P coupling. Upon lowering the temperature to −80 °C, the expected splitting of the phosphorus signal into a doublet with 2JTlP = 202 Hz and the simultaneous appearance of long range coupling between the platinum and phosphorus atoms, 3 JPtP = 858 Hz, is observed. Although variable-temperature 31P{1H} NMR studies on the binuclear complex 3 revealed broadening of the phosphorus resonance upon lowering the temperature to −90 °C, no Tl–P coupling was resolved at this temperature. It seems that the coordination of Tl by two platinum centers in the trinuclear complex 4 decreased the rate of the dynamic process in comparison to that observed for the binuclear complex 3. In the 13 C{1H} NMR spectra of 3, 4, the C atoms of the methyl ligands appeared as doublets at δ −10.5 and −12.7 with 2JPC = 5 and 6 Hz, that further coupled with Pt atoms with 1JPtC = 765 and 710 Hz, respectively. Other structural details of 3 and 4 can be inferred from one and two dimensional 1H NMR spectroscopy techniques. In the 1H NMR spectra of 3 and 4, the Me ligand protons appeared at δ 1.14 and 0.70 as doublets due to coupling with the P atoms with 3JPH = 7.9 and 8 Hz, which are further coupled to the Pt atoms with 2JPtH = 78 and 79 Hz, respectively. The 2D NOESY spectra of 3 and 4 (Fig. S1 and S2 in ESI†) show cross peaks between the signals of the methyl ligands and the resonances appeared at δ 8.15 and 7.8, respectively. We attribute the latter resonances to the hydrogen atoms adjacent to the coordinated C atoms of the bhq ligands (H1c protons in Fig. S3 in ESI†) of 3 and 4, respectively. This is in agreement with the crystal and calculated structures of 3 and 4 (see below), which show these hydrogen atoms


to the side of and in close proximity to the methyl ligands. All of the proton resonances on the bhq and dppy (except the PPh2 protons) ligands belonging to the complexes 1, 3 and 4 were assigned from the connectivities in 2D COSY-DQF and COSY-LR experiments (Fig. S3–S9 in ESI†). The ESI-MS spectrum of 3 ( positive ions) exhibited peaks at m/z 855 and 636 which are associated with [PtMe(bhq)(dppy)Tl]+ and [Pt(bhq)( ppy)]+ species, respectively (Fig. S10 in ESI†). The peak at m/z 855 assigned to [PtMe(bhq)(dppy)Tl]+ is the most intense one, which clearly demonstrates that the PtII–TlI unit remains intact in the gas phase. ESI-Mass spectrum of 4 ( positive ions mode in Fig. 2) shows a base peak at m/z 1507, assignable to the species [Pt2Me2(bhq)2(dppy)2Tl]+ that confirms the formation of cyclometalated heterotrinuclear complex 4. Crystals of 3 suitable for X-ray crystallographic analysis were obtained by slow diffusion of n-pentane into a dichloromethane solution of 3 at low temperature. A view of the structure of 3 is shown in Fig. 3 and some selected bond lengths and angles are summarized in Table 1. Complex 3 crystallizes as a monohydrate in a triclinic system ˉ. In this complex the square planar in the space group P1 platinum(II) unit is linked to a thallium(I) metal ion by a Pt–Tl bond and a weak interaction between the Tl center and the N atom of the dppy ligand [Tl–N(dppy) 2.719(3) Å]. The Pt(1)– Tl(1) distance is 2.8914(2) Å, which is comparable to the sum of the metallic radii of Pt and Tl atoms (2.99 Å), and the Tl–Pt vector is roughly perpendicular to the coordination plane of the Pt(II) atom [tilted by 2.8(1)° with respect to the normal line of the coordination plane]. The thallium center completes its electronic requirements making additional interactions with the oxygen atom of a water molecule and the fluorine atom of

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Fig. 2

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ESI mass spectrum of complex 4 Inset: expanded (blue) and simulated (red) isotope pattern.

a PF6 counterion found at distances of 2.780 and 3.236 Å from the Tl center, respectively. Further examination of the crystal structure of 3 also reveals that the molecules form an extended network through intermolecular CH⋯π, Tl⋯F and Tl⋯O interactions (Fig. 3b, c and S11†). All attempts to obtain suitable crystals for X-ray diffraction of 4 were fruitless. In fact, yellow crystals of the precursor 1 were obtained by crystallization of 4 via diffusion of n-pentane into a solution of the crude solid in methanol. However, we note that despite the lack of structural confirmation for 4, its optimized structure using DFT calculation (Fig. S12 in ESI†) confirms a trinuclear structure with longer Pt–Tl bond distance (2.934 Å) in comparison to that observed in the binuclear complex 3. Complex 1 crystallizes in a monoclinic system, in the space group P21/n and showed disorder of the pyridyl group of the dppy ligand. A view of the structure of 1 is shown in Fig. 4 and some selected bond lengths and angles are summarized in Table 1. As previously anticipated by NMR analysis,13 the crystal structure of 1 confirms that the N atom of the pyridyl group is uncoordinated. The bond lengths of the Pt atom with the other donor atoms in the coordination plane are found to be slightly shorter than those observed for 3. The molecules are stacked in a head to tail fashion into an extended columnar array through π⋯π interactions (∼3 Å, Fig. 4b). Crystal structure of 2 has been determined previously.13 It has a butterfly tetranuclear structure with a Pt2Ag2 core in which the Ag atoms occupy the edge-sharing bond and carry a bridging acetone molecule. Dissociation of the bridging acetone molecule and establishment of a dynamic equilibrium between butterfly and planar geometries in the solution state have been shown using NMR experiments (Fig. S13 in ESI†). The phase-sensitive NOESY/ EXSY spectrum of 2 acquired at 298 K (Fig. S14 in ESI†) indicates cross peaks arising from exchange in addition to the NOE cross peaks that confirmed that an exchange process between butterfly and planar geometry occurs on the experimental time scale. Crystal packing of 2 shows that the

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molecules are stacked in an extended chain array through intermolecular C–H⋯π interactions (Fig. S15 in ESI†). Theoretical studies The ground state geometry of 2 has been fully optimized by a density functional theory approach at the density functional level of theory (B3LYP) (Fig. S16 in ESI†). Interestingly, the optimized structure shows a large deviation from that observed in the X-ray crystallography result. The calculated structure is consistent with the NMR results and shows that the bridging acetone molecule is removed from the cluster complex and a planar-shaped geometry is established for the Pt2Ag2 metal core. It is interesting to note that the removal of the bridging acetone molecule has a significant influence on the argentophilic interaction. The crystal structure of 2 indicates that an Ag–Ag argentophilic interaction exists in the butterfly core geometry [the Ag–Ag distance amounts to 2.826 Å, which is shorter than the sum of the van der Waals radii of Ag atoms (3.44 Å)] and is greatly strengthened in the optimized planar geometry with an Ag–Ag separation of 2.624 Å. However, these data provide a qualitative description of the argentophilic interactions and more accurate measurements of the strength of argentophilic interactions can be obtained from the Quantum Theory of Atoms in Molecules (QTAIM) analysis. For both geometries, a topological density analysis carried out (at the B3LYP level using all-electron WTBS basis16 for metals and 6-31G(d,p) for other elements), indicating the presence of a bond path connecting the Ag atoms to each other (Table 2). The electron density at the Ag–Ag bond critical point is significantly greater for the planar geometry (3.8 × 10−2 au) than for the butterfly geometry (2.22 × 10−2 au). Analysis of the Laplacian of the electronic density also reveals that the Ag–Ag and Pt–Ag bonds are of the closed-shell and electrostatic type respectively. Additional insights into the electronic structure of 2 can be derived from an inspection of the natural charges (qAg = +0.438, qPt = −0.123 |e| in the butterfly geometry and


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Fig. 4 (a) ORTEP plot of 1, hydrogen atoms and disorder of the pyridyl ring of the dppy ligand are omitted for clarity and (b) packing diagram of 1 showing π⋯π interactions.

Table 2

AIM and NBO analysis of Ag–Ag and Pt–Ag bonds for 2

Complex geometry

BCP

ρ(r)a

∇2P(r)b

BOWBI c

Planar Pt2Ag2

Ag(1)–Ag(1A) Pt(1)–Ag(1) Pt(1)–Ag(1A) Pt(1A)–Ag(1) Pt(1A)–Ag(1A) Ag(1)–Ag(1A) Pt(1)–Ag(1) Pt(1)–Ag(1A) Pt(1A)–Ag(1) Pt(1A)–Ag(1A)

0.380 0.026 0.015 0.016 0.025 0.022 0.039 — — 0.039

0.144 0.073 0.053 0.058 0.065 0.089 0.115 — — 0.115

0.382 0.254 0.231 0.231 0.254 0.158 0.206 0.189 0.189 0.206

Butterfly Pt2Ag2

Fig. 3 (a) ORTEP plot of complex 3 and (b) packing diagram of 3 showing Tl⋯O and Tl⋯F interactions and (c) packing diagram of 3 showing CH⋯π interactions.

Table 1

Selected bond lengths (Å) and angles (°) for complexes 3 and 1

Complex 3 Pt(1)–Tl(1) N(1)–Pt(1) P(1)–Pt(1) C(1)–Pt(1) C(10)–Pt(1) N(2)–Tl(1) Tl(1)–O(1W)

2.8914(2) 2.140(4) 2.3142(9) 2.081(4) 2.057(4) 2.719(3) 2.780(4)

P(1)–Pt(1)–Tl(1) N(1)–Pt(1)–Tl(1) C(10)–Pt(1)–Tl(1) C(1)–Pt(1)–Tl(1) C(1)–Pt(1)–P(1) N(1)–Pt(1)–P(1) C(10)–Pt(1)–C(1)

89.67(3) 93.37(9) 89.14(1) 87.9(14) 90.4(1) 97.5(1) 91.3(2)

Complex 1 C14A–Pt1A C11A–Pt1A P1A–Pt1A N1A–Pt1A

2.048(8) 2.053(8) 2.285(2) 2.128(7)

C14A–Pt1A–C11A C11A–Pt1A–N1A C14A–Pt1A–P1A N1A–Pt1A–P1A

90.7(3) 80.9(3) 92.2(3) 96.2(2)


a

Electronic density (in au). b Laplacian of the electronic density (in au). c Bond order (Wiberg bond index).

qAg = +0.221, qPt = −0.114 |e| in the planar geometry), which indicate that there is more charge transfer from the platinum centers to the Ag atoms in the planar geometry than in the butterfly geometry. This natural bond orbital (NBO) analysis clearly shows that the positive charge reduction on the Ag atoms is greater in the planar complex in comparison to that of the butterfly, indicating a further tendency of the Ag atoms to form an argentophilic interaction in planar geometry. The calculated Wiberg bond indices for the Ag–Ag and Pt–Ag bonds are shown in Table 2. These values also confirm the proposed interactions. In light of the photophysical properties of the complexes, DFT and TD-DFT (see the Photophysical study section) calculations on the tetranuclear complex 2 and its binuclear

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analogue Pt–Tl, 3, were performed at the B3LYP level of density functional theory (ESI†). The planar complex 2 shows an important contribution of the Pt and Ag atoms in the HOMO (24% Pt, 38% Ag) and HOMO−1 (38% Pt, 18% Ag) orbitals, whereas the LUMO and LUMO+1 of 2 are located mainly in the bhq ligands (71% in LUMO and 51% in LUMO+1) and the rest in the dppy ligands (11% in LUMO and 31% in LUMO+1). On the other hand, the HOMO and HOMO−1 of the heterobinuclear Pt–Tl complex 3 reside in the cyclometalating bhq ligand (HOMO−1: 91% in bhq, 7% in Pt and HOMO: 82% in bhq, 12% in Pt) while the LUMO and LUMO+1 are distributed in the dppy (59%, 20%), Tl (20%, 21%), bhq (14%, 44%) and Pt (6%, 14%) respectively. Fig. 5 Normalized electronic absorption spectra of the complexes 1–4 in CH2Cl2 solution (5 × 10−5 M).

Photophysical study Absorption spectra The electronic absorption spectra of precursor complex 1 and heteronuclear complexes 2–4 were recorded in CH2Cl2 (5 × 10−5 M solutions, Table 3 and Fig. 5). The absorption spectra of heteronuclear Pt–Tl complexes 3 and 4 are quite similar to that of the precursor 1 in solution. They exhibit intense high-energy absorptions (λ < 350 nm), typically ascribed to intraligand 1IL transitions (bhq and dppy) and to two weaker absorptions (ε = 2–7 × 103 M−1 cm−1) at λ values ranging from 350 to 410 nm (378, 402 nm 1, 378, 404 nm 3, 377, 403 nm 4). By contrast, the absorption spectrum of the Pt2Ag2 cluster 2 displays two intense bands located at 388 and 413 nm. The low-energy band, with a long tail to 500 nm and clearly red-

Table 3

Photophysical parameters for complexes 1–4 −1

−1

Complex

λabsorption (10 ε/(dm mol

1

238 (53.1), 305 (13.0), 326 (10.6), 378 (4.4), 402 (3.4) (CH2Cl2) 329 (8.4), 378 (4.0) (Acetone) 260, 320, 370, 407 (solid ) 235 (59.6), 302 (13.5), 320sh (8.2), 388 (10.9), 413 (7.4) (CH2Cl2) 327 (13.5), 387 (8.3), 407 (8.5) (Acetone) 256, 307, 410, 445 with tail to 550 (solid )

2

3

4

a

shifted in relation to the platinum precursor 1, seems to be characteristic of the presence of the tetranuclear core Pt2Ag2 in solution. We noted that this complex shows a pattern different from that observed in related systems containing Pt–Ag dative bonds. Many of them show a blue-shift in the low-energy absorption maxima as a consequence of the donation of electron-density from Pt(II) to the Ag(I) ion, which increases the electrophilicity of the Pt centre, lowering the energy of the HOMO (mainly located on the platinum fragment) and raising the energy of the 1MLCT [5d(Pt) → π*(bhq)] absorption.10i,11b Furthermore, in tetranuclear Pt–Ag complexes a negligible, if any, shift in the low-energy absorption maxima with respect to those of the starting compounds has been observed,10b,e,h,11b

a

3

3

cm ))

Solid Ta: λem/nm (λexc/nm) [Ф/%]

Glass 77 K Medium: λem/nm (λexc/nm)

τ (μs)

CH2Cl2 5 × 10−5 M: 480max, 520, 550 (320–400)b Acetone 5 × 10−5 M: 485max, 525, 565 (340–420) CH2Cl2 5 × 10−5 M: 560max, 660sh (330–440); 560, 640max, (360–500) CH2Cl2 10−3 M: 560, 660max (360–500) 660 (540) Acetone 5 × 10−5 M: 560max, 660sh (340–440)b CH2Cl2 5 × 10−5 M: 630 (350–470)

Non-emissive at 298 and 77 K 298 K: non-emissive 77 K: 590 (360–480)

8.3

232 (29.7), 245sh (24.3), 268 (17.2), 301 (13.7), 332 (9.3), 378 (3.4), 404 (2.0) (CH2Cl2) 328 (7.9), 378 (3.8), 406sh (1.6) (Acetone) 246, 310, 385, 460 with tail to 600 (solid )

298 K: 675 (480) [24]

8.7

77 K: 620sh, 675max (460)

8.8 (46%), 3.8 (54%)

240 (58), 298 (33.7), 330 (22.3), 377 (8.9), 403 (6.7) (CH2Cl2) 327 (14.5), 379 (6.6), 408 (2.3) (Acetone) 221, 245, 317, 401 with tail to 550 (solid )

298 K: 665 (500) [17]

9.3

77 K: 630 (360–460)

9.4

CH2Cl2 10−3 M: 690 (380–510) Acetone 5 × 10−5 M: 630 (360–470) Acetone 10−3 M: 690 (390–500) CH2Cl2 5 × 10−5 M: 620 (360–460) CH2Cl2 10−3 M: 610 (350–460) Acetone 5 × 10−5 M: 625 (360–460) Acetone 10−3 M: 630 (360–460)

5 × 10−5 M solutions. b Same pattern at 10−3 M.

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being attributed in some cases to the rupture of the Pt–Ag bond in solution.11b In this context, the red-shift observed in 2 is uncommon and is likely to be due to the role played by the transannular Ag⋯Ag metallophilic bond in the butterfly metallic core. The solid-state diffuse reflectance spectra of the heterometallic complexes 2–4 (Fig. S17 in ESI†) display additional absorptions (450–550 nm) when compared to their corresponding solution UV–vis spectra and they are absent in the precursor 1. These low-energy bands are responsible for the deep orange colors of these complexes and could be related to the presence of Pt–M (Ag⋯Ag in 2) bonding interactions. To gain insight into the optical properties of 2 and 3, time-dependent density functional theory (TD-DFT) calculations were performed at the optimized geometry of the ground states of the complexes. The TD-DFT results of the singlet and triplet excited states of 2 and 3 are given in Tables S1 and S2 in ESI.† Table S7† summarizes the selected calculated energy levels and the experimental absorption parameters of 2 and 3. The calculated results are in good agreement with the experimental results. For the Pt2Ag2 cluster 2, the lowest-energy absorptions calculated (HOMO−n to LUMO) involve transitions from orbitals with a strong metallic Pt2Ag2 character (HOMO 24% Pt, 38% Ag; HOMO−1 38% Pt, 18% Ag) to a LUMO, with a remarkable contribution of the bhq ligand (71%) and a lesser contribution of the metals (20%). The calculated singlet excited state S2 found at 449 nm (S1 at 514 nm has low intensity with f < 0.002), derived from both the HOMO → LUMO (60%) and HOMO−1 → LUMO (34%) excitations (Fig. S18†), can be related to the experimental values seen both in the solid state (445 nm) and in solution (413 nm). In this tetranuclear cluster, the low-energy absorption moves electron density from the metallic core, with a remarkable contribution of the Ag⋯Ag bond towards a low-lying orbital with a predominant ligand (bhq) participation and minor metal character, being mainly characterized as metal–metal-to-ligand charge transfer 1 MM′LCT (M, M′ = Pt, Ag, L = bhq) with some minor 1L′LCT character. Therefore, the distinctive red-shift observed in the low-energy band of 2 in relation to the precursor 1 (413 vs. 402 nm) and contrary to the blue-shift observed in other PtAg cyclometalated complexes can be ascribed to the notable participation not only of the Pt centers but also of the two Ag atoms through the Ag⋯Ag metallophilic interactions in the HOMO and HOMO−1. For 3, according to computational results, the lowest singlet excited states S1 and S2 at 398 and 388 nm are derived from the HOMO → LUMO and HOMO → LUMO+1 excitations. Both S0→S1 and S0→S2 transitions originate from excitations from bhq-based HOMOs to dppy-based LUMOs (and partially on Tl atom). Therefore, the lowest singlet transitions observed in the UV-vis spectrum of 3 (Fig. 5) at 404 and 378 nm probably contain an admixture of ligand-to-ligand and ligand-to-metal charge transfer character [1LL′CT/1LM′CT (L = bhq, L′ = dppy; M′ = Tl)]. In view of the similar absorption spectrum of the trinuclear complex 4, the same assignment can be made for the lowest-lying absorptions of 4.


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Emission spectra Emission data for 1–4 are summarized in Table 3. The precursor mononuclear complex 1 is nonemissive in the solid state (298 K and 77 K) or in fluid solution, but becomes emissive in glassy solutions (CH2Cl2 or acetone) exhibiting a vibronic structured band (480 max, 520, 550 nm CH2Cl2; 485 max, 525, 565 nm acetone), which is ascribed to an admixture of 3 IL/3MLCT excited states, originated in the monomer species (Fig. 6). In contrast, the tetranuclear Pt2Ag2 cluster 2 displays a long-lived intense orange emission (590 nm, τ = 8.3 µs) in the solid state at 77 K, although it is not emissive at room temperature. The non-structured profile of the observed emission (Fig. 7, black lines) together with the lack of luminescence in precursor 1 are indicative of the involvement of the metallic Pt2Ag2 core in the excited state.

Fig. 6 Normalized excitation and emission spectra of 1 in CH2Cl2 (5 × 10−5 M) at 77 K.

Fig. 7 Emission spectra of complex 2 in the solid state at 77 K (black lines) and in the glassy solution (CH2Cl2 5 × 10−5 and 10−3 M at 77 K).

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The excitation profile resembles the solid reflectance diffuse spectrum of 2 (Fig. S17†). In glassy CH2Cl2 and acetone diluted solutions (5 × 10−5 M, blue lines in Fig. 7), the complex shows a similar structureless emission band slightly blueshifted (λmax = 560 nm) in relation to that observed in the solid state, with a concentration-dependent broad shoulder centered at 660 nm, which is clearly enhanced in concentrated solutions (10−3 M, red lines). The results of the TD-DFT calculations indicate that the lowest excited state T1, located at 549 nm, is mainly derived from HOMO−1 → LUMO (70%) and HOMO → LUMO (26%) contributions (Table S7†). This S0→T1 transition involves excitation from platinum–silver and silver–silver metal-based HOMOs to the bhq π* ligand-based LUMO and can be related to the emission observed in the solid state and the high-energy emission located at 560 nm in glassy solutions. These calculations suggest that this emission band has strong metal– metal-to-ligand charge transfer contributions (3MM′LCT) with a somewhat 3L′LCT character. In glassy solution, the increasing intensity of the low-energy manifold located at 660 nm by increasing the concentration (from 5 × 10−5 M to 10−3 M) and the different excitation profile monitoring in both maxima suggest that this low energy band arises from the occurrence of π⋯π (or Pt⋯Pt) intermolecular interactions in ground-state aggregates that are formed particularly at higher concentrations. It is well known that the presence of π⋯π interactions usually stabilizes the photoexcitated state causing a red-shift of the emission. As shown in Table 3 and illustrated in Fig. 8, the heteropolymetallic Pt–Tl derivatives 3 and 4 are emissive in rigid media (solid at 298 and 77 K and CH2Cl2 or acetone glass at 77 K). Thus, in the solid state at 298 K, both complexes 3 and 4 display unstructured bright red emissions at similar energies (λmax 675 nm, ϕ = 24 for 3; 665 nm, ϕ = 17% for 4). Upon cooling the solid at 77 K, 3 shows an additional shoulder at 620 nm, in addition to the main band at 675 nm. Whereas the lifetime at the peak maxima fits to one component at 298 K (8.7 µs), at 77 K the decay in this band comprises two components [8.8 (46%) and 3.8 µs (54%)], which indicates that the emission has some degree of mixed origin. In glassy solutions, the emission profile of 3 is concentration-dependent. Thus, at high concentration (10−3 M), 3 exhibits a similar low energy unstructured band centered at 690 nm, both in CH2Cl2 or acetone at 77 K, whereas at 5 × 10−5 M it shows a high energy band located at 630 nm. Excitation spectra monitored in both maxima show different patterns and are also somewhat different from those obtained in the solid state at 77 K, indicating the presence of two different emissions arising from two emissive species. The emissive behaviour of the trinuclear Pt2Tl complex 4 is slightly different from that of 3 (Fig. 8b). In particular, upon cooling the solid at 77 K, the emission band was found to shift to the blue and a similar emission with minor variation in its maximum (610–630 nm) was also observed in diluted or concentrated glassy (CH2Cl2 or acetone) solutions. For complex 3, the difference between the calculated energy of the lowest triplet state T1 (467 nm, Table S7†) and the

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Fig. 8 Normalized excitation and emission spectra of (a) 3 and (b) 4 in the solid state at 298 K and 77 K and in CH2Cl2 glasses at 77 K.

emission experimentally observed do not enable us to conclusively explain the nature of the emissions using these calculations. With reference to previous observations in benzoquinolate platinum complexes and in view of the resemblance and similar energy of the low-energy emission of 3 (675 nm) and that seen for 4 at room temperature (665 nm), as well as the extensive π⋯π network found in the crystal of 3, we suggest that the low energy emission originates from excited states arising from intermolecular π⋯π interactions in the solids. The high-energy band seen at 77 K for 4 or in glassy solutions (610–630 nm), which is not dependent on concentration, is ascribed to the emission coming from discrete trinuclear sandwich Pt2Tl entities. Curiously, this emission resembles that observed for complex 3 in diluted glassy solutions, suggesting the presence (or formation) of similar emissive species. Although the assignment is not conclusive, the emissive state is likely to derive from a mixed 3LL′CT/3LM′CT (M′ = Tl), modified by the Pt–Tl bonds.


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Conclusions In conclusion, we have reported the theoretical and optical properties of an unusual butterfly [Pt2Me2(bhq)2(µ-dppy)2Ag2(µ-acetone)](BF4)2 cluster 2 and the synthesis and properties of two heterometallic platinum–thallium derivatives 3 and 4. For 2, the DFT calculated structure is consistent with the NMR results and shows that the bridging acetone molecule is removed from the cluster complex establishing a planarshaped geometry for the Pt2Ag2 metal core and enhancing the Ag⋯Ag argentophilic interaction. Furthermore, calculations suggest that the low energy absorption involves transitions from the metallic Pt2Ag2 core, with a remarkable contribution of the Ag⋯Ag bond towards a low-lying orbital with a predominant contribution of the bhq ligand and minor metal character, being thus characterized as metal–metal-to-ligand charge transfer 1MM′LCT (M, M′ = Pt, Ag; L = bhq) with minor 1L′LCT character. The notable participation of the two Ag atoms through the Ag⋯Ag metallophilic interactions in the HOMOs could explain the unusual red-shift observed in the low energy band in relation to the other Pt–Ag cyclometalated systems in which the major metal participation in the HOMO comes from the Pt center. Along the same lines, the high energy emission band observed in the solid state and in glassy solutions has a strong 3MM′LCT (M, M′ = Pt, Ag; L = bhq) character, with something of a 1L′LCT character. The low energy manifold observed in glassy solution, which increases its intensity when the concentration is increased, arises from the formation of ground state aggregates, based on π⋯π(or Pt⋯Pt) intermolecular interactions. The computational results for the Pt–Tl complex 3 indicate that the low energy transitions originate from an admixture of 1 LL′CT and 1LM′CT (L = bhq, L′ = dppy; M′ = Tl) charge transfer. Upon formation of the Pt(II)–Tl(I) bond, the operating non-radiative process in the precursor seems to be reduced, giving intense emissions in 3 and 4 in relation to the non-emissive 1. The low energy unstructured bright emission observed in the solid state for 3 and 4 and in concentrated CH2Cl2 glasses for 3 is tentatively ascribed to excited states arising from intermolecular π⋯π interactions in the isolated solids. The high energy band seen in the solid state at 77 K and in glasses for 4 and in diluted glasses for 3 is ascribed to the emission coming from discrete trinuclear sandwich Pt2Tl entities, probably derived from a mixed 3LL′ CT/3LM′CT (M′ = Tl), modified by the Pt–Tl bonds.

Experimental section The 1H, 31P{1H}, 13C{1H}, COSY-DQF, COSY-LR, NOESY and LT-NMR spectra were recorded using Bruker Avance DRX 500 and 400 MHz spectrometers. The operating frequencies and references, respectively, are shown in parentheses as follows: 1 H NMR (500 MHz, tetramethylsilane, SiMe4), 13C{1H} NMR (126 MHz, tetramethylsilane, SiMe4) and 31P{1H} (203 MHz, 85% H3PO4). The chemical shifts and coupling constants are in ppm and Hz, respectively. Electrospray ion mass spectra


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(ESI-MS) were recorded using a Hewlett-Packard Series 1100 spectrometer or recorded using a HP-5989B spectrometer using methanol–water as the mobile phase. The UV-vis absorption spectra were carried out using a Hewlett-Packard 8453 spectrophotometer. Diffuse reflectance UV-vis (DRUV) data of pressed powder were recorded using a Shimadzu (UV-3600 spectrophotometer with a Harrick Praying Mantis accessory) and recalculated following the Kubelka-Munk function. Excitation and emission spectra were obtained using a Jobin-Yvon Horiba Fluorolog 3-11 Tau-3 spectrofluorimeter. The lifetime measurements were performed in a Jobin Yvon Horiba Fluorolog operating in the phosphorimeter mode (with an F1-1029 lifetime emission PMT assembly, using a 450WXe lamp). Benzo[h]quinoline and [Ag(CH3CN)4]BF4, AgPF6 and TlPF6 were purchased from commercial sources. The complexes [PtMe(bhq)(dppy)], 1, [Pt2Me2(bhq)2(µ-dppy)2Ag2(µ-acetone)](BF4)2, 2 and [PtMe(bhq)(SMe2)] were prepared as described previously.13,17 [PtMe(bhq)(dppy)Tl]PF6, 3 To a solution of complex 1 (200 mg, 0.31 mmol) in THF (15 mL) at room temperature was added 1 equiv. of TlPF6 (108.3 mg, 0.31 mmol) under an argon atmosphere. The mixture was stirred under these conditions for 1.5 h, and then the solvent was removed under reduced pressure. The residue was washed with ether (2 × 3 mL), and the red powder product was dried under vacuum. Yield: 85%. Anal. Calcd for C31H25F6N2P2PtTl: C, 37.2; H, 2.5; N, 2.8. Found: C, 37.5; H, 2.7; N, 2.8; 1H NMR (500 MHz, CD2Cl2) δ 8.69 (d, J = 4.8 Hz, 1 H), 8.38 (dd, J = 8.1, 1.3 Hz, 1H), 8.15 (m, 3JPtH = 44 Hz, 1H), 8.02–7.98 (m, 2H), 7.95 (d, J = 8.7 Hz, 1H), 7.84–7.79 (m, 3H), 7.73 (d, J = 8.7 Hz, 1H), 7.63 (m, 8H), 7.58–7.49 (m, 5H), 7.13–7.06 (dd, J = 8.1, 4 Hz, 1H), 1.14 (d, 2JPtH = 78 Hz, 3JPH = 7.9 Hz, 3H); 13C{1H} NMR (126 MHz, CD2Cl2) δ 157.0 (s), 155.0 (s), 153.0 (s), 150.5 (d, J = 17.2 Hz), 149.4 (s), 138.5 (s), 138.3 (s), 135 (s), 131.7 (s), 131.1 (s), 129.9 (s), 129.7–129.6 (m), 129.2 (d, J = 10.0 Hz), 126.1 (s), 124.9 (s), 123.8 (s), 122.2 (s), 29.6 (s), −10.5 (d, 2JPC = 5 Hz, 1JPtC = 765 Hz). 31P{1H} NMR (203 MHz, CD2Cl2) δ 35.8 (s, 1JPtP = 1934 Hz), −144 (septet, PF6). [Pt2Me2(bhq)2(dppy)2Tl]PF6, 4 According to the procedure for the synthesis of 3 with 0.5 equivalent TlPF6 provided the orange product 4 (60% yield). Anal. Calcd for C62H50F6N4P3Pt2Tl: C, 45.06; H, 3.03; N, 3.39. Found: C, 45.4; H, 2.9; N, 3.5; 1H NMR (500 MHz, CD2Cl2) δ 8.34 (dd, J = 8.0, 1.3 Hz, 2H), 7.94 (d, J = 8.7 Hz, 2H), 7.75–7.69 (m, 2H), 7.68–7.63 (m, 6H), 7.58 (dd, J = 7.5, 1.9 Hz, 2H), 7.56–7.49 (m, 14H), 7.43 (ddd, J = 7.7, 4.1, 2.3 Hz, 8H), 7.17–7.11 (m, 2H), 7.01–6.95 (dd, J = 8, 4 Hz, 2H), 0.70 (d, 3JPH = 8.0 Hz, 2JPtH = 79 Hz, 6H); 13C{1H} NMR (126 MHz, CD2Cl2) δ 159.4 (s), 158.5 (s), 155.7 (d, J = 7 Hz), 154.8 (s), 154.4 (s), 150.0 (d, J = 117.2 Hz), 149.5 (d, J = 4 Hz), 143.5(s, JPtC = 100 Hz), 167.3 (s), 137.1 (d, J = 5 Hz), 134.8–134.2 (m), 131.1 (s), 130.5 (s), 130.2–129.5 (m), 129.3 (d, J = 7.3 Hz), 128.6 (dd, J = 31.3, 9.8 Hz), 127.4 (s), 124.9 (s), 123.9 (s), 123.6 (s), 121.8 (s), −12.7 (d, 2JPC = 6 Hz, 1JPtC = 710 Hz). 31P{1H} NMR (203 MHz, CD2Cl2) δ 36 (s, 1JPtP = 2025 Hz), −141.39 (PF6).

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Crystal structure determination X-ray diffraction data were collected from single crystals of 3·H2O mounted in a stream of cold nitrogen (150 K) using an Oxford Diffraction Xcalibur 3 CCD diffractometer with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The data were reduced using the CrysAlis software package.18 Solution, refinement and analysis of the structures were done using the software integrated in the WinGX system.19 The structures were solved by direct methods (SHELXS)20 and refined by the fullmatrix least-squares method based on F2 against all data (SHELXL-97).20 The non-hydrogen atoms were refined anisotropically. One of the two crystallographically independent PF6– anions in 3 is disordered, so it was modeled in two discrete positions by using appropriate restraints on the geometric (SAME) and atomic displacement parameters (DELU, SIMU, ISOR). Although elongated displacement ellipsoids of fluorine atoms also suggested a disorder of PF6– in 3·H2O, modeling of this anion in discrete disorder positions was not successful. The hydrogen atoms of a water molecule in 3·H2O were refined using the restraints on the corresponding bond lengths and angle. The carbon-bonded hydrogen atoms in both structures were refined using the appropriate riding model. Geometrical calculations were made using PLATON21 and the figures were prepared using ORTEP-3.22 Crystal structure determinations for 1 were carried out using an Apex DUO Kappa 4-axis goniometer equipped with an APPEX 24 K CCD area detector, a Microfocus Source E025 IuS using MoKα radiation, Quazar MX multilayer Optics as a monochromator and an Oxford Cryosystems low temperature device Cryostream 700 plus (T = −173 °C). Crystal structure solution for 1 was achieved using direct methods as implemented in SHELXTL and visualized using the program XP. Missing atoms were subsequently located from difference Fourier synthesis and added to the atom list. Least-squares refinement on F2 using all measured intensities was carried out using the program SHELXTL. All non-hydrogen atoms were refined including anisotropic displacement parameters. The asymmetric unit contains three independent molecules of the same complex with different conformations. In one of the molecules the phosphine rest is disordered in two orientations (ratio 50 : 50). In the other two independent molecules the nitrogen atom corresponding to the pyridine ring is disordered and located in two different equivalent aromatic rings (ratio: 50 : 50). Crystal data and refinement parameters for 3 and 1 are given in Tables S3 and S4 in ESI† respectively, and full crystallographic data are provided in the ESI.†

Computational details for theoretical calculations DFT and TD-DFT calculations by hybrid Becke’s three-parameter exchange functional combined with the Lee, Yang, and Parr correlation (B3LYP)23 method and def2-TZVP basis set24 were performed for2 using ORCA 2.9 suite of programs.25 The NBO analysis26 was conducted in order to obtain the natural

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charges and Wiberg bond indices. Wave function analysis based on QTAIM calculations was performed with AIM2000 package27 to calculate the properties of bond critical points (BCPs).

Acknowledgements We thank Sharif University of Technology Research Council and the Iran National Science Foundation (Grant No. 91000888) for financial support. EL and MTM are grateful to the Spanish MICINN (Project CTQ2008-06669-C02-02/BQU).

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