Dinuclear Platinum(II) Terpyridyl Complexes with a para

Article pubs.acs.org/Organometallics

Dinuclear Platinum(II) Terpyridyl Complexes with a paraDiselenobenzoquinone Organometallic Linker: Synthesis, Structures, and Room-Temperature Phosphorescence Jamal Moussa,† Keith Man-Chung Wong,‡ Xavier F. Le Goff,§ Marie Noelle Rager,⊥ Carmen Ka-Man Chan,‡ Vivian Wing-Wah Yam,*,‡ and Hani Amouri*,† †

Institut Parisien de Chimie Moléculaire, IPCM, UMR CNRS 7201, Université Pierre et Marie Curie, Paris 6, 4 Place Jussieu, Case 42, 75252 Paris Cedex 05, France ‡ Institute of Molecular Functional Materials (Areas of Excellence Scheme, University Grants Committee, Hong Kong) and Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China § Laboratoire Heteroelements et Coordination, Ecole Polytechnique, CNRS, F-91128 Palaiseau Cedex, France ⊥ NMR Facilities of Ecole Nationale Supérieure de Chimie de Paris, 11 Rue Pierre et Marie Curie, 75231 Paris Cedex 05, France S Supporting Information *

ABSTRACT: We report the synthesis of a unique class of luminescent heterotrinuclear complexes of the general formulas [Pt(terpy){Cp*Ir-p-(η4-C6H4Se2)}Pt(terpy)][X]4 (X = OTf, 3a; PF6, 3b; BF4, 3c; ClO4, 3d; BPh4, 3e). In these coordination assemblies two Pt(terpy) moieties are held by a stable η4-diseleno-p-benzoquinone complex [Cp*Ir-p-(η4C6H4Se2)]. The molecular structures of solvates of 3a and 3b were ascertained by single-crystal X-ray diffraction study and confirmed the formation of the target molecules. The solidstate packing of two of these complexes confirms the presence of π−π and Pt···Pt interactions among individual units providing a 1D supramolecular chain for 3a, while a dimer species is obtained for 3b, illustrating the effect of the counterion on directing the crystal packing of the individual molecules. All compounds show phosphorescence in the red region (685−705 nm) in fluid solution and in the solid state at room temperature, unlike the analogous compound Pt(terpy){Cp*Ir-p-(η4-C6H4S2)}Pt(terpy)][CF3SO3]4 obtained with a dithiobenzoquinone organometallic linker, which is only luminescent at low temperature.

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that individual subunits exhibit π−π and Pt···Pt interactions to give a 1D chain in the solid state. The heterotrinuclear coordination assembly showed unusual UV/vis absorption and luminescence properties at low temperature, imparted from self-aggregation of the individual subunits mediated by π−π and Pt···Pt interactions. Pursuing our research in this area, we now wish to report the synthesis and luminescent properties of a unique class of Pt(terpy) complexes attached to a selenide organometallic linker, p-[Cp*Ir(η4-C6H4Se2)] (2) (Chart 1). To the best of our knowledge, these are the first compounds to be prepared displaying a selenoquinonoid π-complex as assembling ligand. The reason for the scarcity of such compounds is that unlike quinones, which are a prominent class of compounds that play an important role in chemistry and biology,7 the related selenium quinones “C6H4Se2” (1) (Chart 1) are, in contrast, unstable and consequently do not exist in nature, and hence their chemical properties remain unknown.8

INTRODUCTION

Platinum(II) terpyridyl complexes have received considerable attention due to their unique photophysical properties.1,2 Such compounds were found to form stable square-planar complexes with d8 configurations, which promote metal−metal and π−π interactions, which profoundly impact the properties of the electronic excited states.3 Thus, several mononuclear platinum(II) terpyridyl complexes were prepared and their luminescent properties have been tuned by modifying the nature and the electron property of the terpyridine and the coligand occupying the fourth position as well.4 In the course of preparing new coordination platinum(II) terpyridyl complexes with useful luminescent properties, efforts were devoted to the use of bridging linkers to attach two Pt(terpy) units, in the hopes that such a novel compound might promote inter- and/or intramolecular Pt···Pt and π−π interactions, which in turn would affect the luminescent properties.5 In this context we reported the synthesis of a luminescent heterotrinuclear coordination assembly of the formula [Pt(terpy){Cp*Ir-p-(η4C6H4S2)}Pt(terpy)][OTf]4, in which two Pt(terpy) units are attached to an organometallic linker [Cp*Ir(η4-C6H4S2)] through the sulfur centers.6 The molecular structure showed © 2013 American Chemical Society

Received: July 17, 2013 Published: August 22, 2013 4985

dx.doi.org/10.1021/om400700t | Organometallics 2013, 32, 4985−4992

Organometallics

Article

Chart 1. Schematic Drawings of the Hypothetical Selenoquinone (1), the Isolated Metalated Selenoquinone (2), and the Luminescent Heterotrinuclear Salts (3a−e)

Scheme 1. General Synthetic Procedure to Form Luminescent Heterotrinuclear Species (3a−e)

OTf (3a), X = PF6 (3b), X = BF4 (3c), X = ClO4 (3d), X = BPh4 (3e)). The molecular structures of solvates of [Pt(terpy){Cp*Ir-p-(η4-C6H4Se2)}Pt(terpy)][OTf]4 (3a) and [Pt(terpy){Cp*Ir-p-(η4-C6H4Se2)}Pt(terpy)][PF6]4 (3b) were determined by single-crystal X-ray diffraction and confirmed the formation of the target complexes. Remarkably the crystal packing shows that the Pt···Pt and π−π interactions are controlled by the nature of the counteranion. Moreover these complexes exhibit important luminescent properties in the solid state and in fluid solution at room temperature and in contrast to those observed with the related heterotrinuclear coordination assembly [Pt(terpy){Cp*Ir-p-(η4-C6H4S2)}Pt(terpy)][OTf]4 obtained with an organometallic thioquinone linker. This behavior might arise from the nature of the selenium ligand, which exhibits better donating properties and a higher spin−orbit coupling relative to the sulfur center. To the best of our knowledge, our compounds are the first examples of luminescent Pt(terpy) complexes bearing an ancillary ligand coordinated through a selenium center. Furthermore, the role of the counteranion on the luminescent properties16−19 of the above complexes 3a−e is presented.

Thus, we anticipated that metalated selenoquinone might be an appropriate assembling ligand for a new class of luminescent Pt(II) terpyridyl complexes. It is noteworthy, however, that when oxygen atoms in quinone are replaced by the heavier chalcogen atoms, sulfur or selenium, this leads to highly reactive intermediates that cannot be isolated in pure form due to the instability of the unnatural functional groups CE (E = S, Se).9 Thus, examples of isolated thioquinones are scarce, and the parent compound has been generated and characterized spectroscopically only at low temperature (10 K) in an argon matrix.10 The hypothetical selenoquinone molecule C6H4Se2 (1) (Chart 1) is even less stable, and to the best of our knowledge there is no report in the literature devoted to its preparation or even an attempt to isolate such a molecule. Pursuing our research program in the area of metal-stabilized reactive intermediates, we demonstrated that the Cp*Ir fragment is a powerful stabilizing entity.11 For instance, in 1998 we reported the synthesis of the first stable ortho-quinone methide metal complex, in which the Cp*Ir is attached to the internal diene moiety through η4-coordination.12 Later, in 2006 we described the synthesis of the first stable iridium pdithiobenzoquinone complex p-[Cp*Ir(η4-C6H4S2)] and also the related ortho congener o-[Cp*Ir(η4-C6H4S2)].13 Both compounds were used as successful organometallic linkers to construct functional coordination assemblies.14 More recently we reported the synthesis and the X-ray molecular structure of the p-diselenobenzoquinone as metal complex p-[Cp*Ir(η4C6H4Se2)] (2) (Chart 1).15 In this paper we describe the coordination properties of the organometallic linker p-[Cp*Ir(η4-C6H4Se2)] (2) toward the Pt(terpy) fragment, leading to the preparation of five different complex salts of luminescent heterotrinuclear species [Pt(terpy){Cp*Ir-p-(η4-C6H4Se2)}Pt(terpy)][X]4 (3a−e) (X =

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RESULTS AND DISCUSSION Synthesis of the Pt(II) Terpyridyl Complexes with the Metalated Selenobenzoquinone Linker (3a−e). Treatment of [Cp*Ir-p-(η4-C6H4Se2)] (2) with two equivalents of [Pt(terpy)Cl]Cl in CH3OH provided an orange-red solution, and subsequent treatment with a saturated methanol solution of NH4OTf gave the assembly {[Pt(terpy){Cp*Ir-p-(η4C6H4Se2)}Pt(terpy)][OTf]4}n (3a) (Scheme 1) in 85% yield. For instance, the 1H NMR spectrum of 3a recorded in CD3CN shows the presence of a singlet at δ 7.23 ppm attributed to the metalated p-diselenobenzoquinone and a singlet at δ 1.92 ppm 4986

dx.doi.org/10.1021/om400700t | Organometallics 2013, 32, 4985−4992

Organometallics

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Figure 1. (a) Crystal structure of the cationic part of 3a·2CH3CN. Selected bond distances (Å) and angles (deg) for 3a: Ir1···C16 = 2.323(7), Ir1··· C17 = 2.226(7), Ir1···C18 = 2.226(7), Ir1···C19 = 2.314(6), Ir1···C20 = 2.212(6), Ir1···C21 = 2.210(7), C16···Se = 1.890(6), C19···Se2 = 1.904(6). (b) 1D supramolecular chain formed through π−π and Pt···Pt contacts between individual tetracations.

C6H4Se2)]. The coordination geometry around each Pt center is distorted square planar, formed by three nitrogen atoms of the terpyridine ligand and one selenium center from the bridging organometallic linker 2. The aromatic ring of the organometallic linker (2) is almost planar, with the selenoquinone carbons bent out of the diene plane (hinge angles θ1 = 7.22°, θ2 = 7.44°), while acting as a bidentate ligand and connects two “Pt(terpy)” units. These angles are slightly bigger than those reported for the free organometallic linker [Cp*Ir-p-(η 4-C 6H 4Se 2)] (2).15 The C−Se bond distances in 3a are 1.890(6) and 1.904(6) Å, slightly longer than those reported for 2 but shorter than that reported for diselenocin, which has a C−Se single bond of 1.924(8) Å.20 The two Pt(terpy) units in 3a are not linearly disposed but are puckered and lean backward with Pt1···Pt2 distance in 3a of 8.202 Å. Analysis of the crystal packing of 3a reveals important information such as the molecules are stacked in a head-to-head and tail-to-tail fashion with two sets of π−π/Pt1···Pt1 or π−π/ Pt2···Pt2 interactions occurring at each side of one Pt(terpy) arm with the adjacent molecule. For instance, the first Pt(terpy) arm unit interacts with an adjacent Pt(terpy) fragment through a π−π interaction at d = 3.35(2) Å, θ = 29.3°, while the Pt1··· Pt1 distance is 3.961(6) Å, suggesting the absence of a weak interaction. The other Pt(terpy) arm forms a stronger π−π interaction with adjacent Pt(terpy) units with d = 3.36(2) Å, θ = 20.2° and exhibits a Pt2···Pt2 contact at 3.546(6) Å. These Pt···Pt/π−π interactions among individual subunits of 3a construct a 1D supramolecular chain (Figure 1b). We note also the presence of other Pt(terpy) complexes which show a combination of π−π/Pt···Pt interaction among the individual units.3a,b,6,17 Gratifyingly and after perseverance we were able to conveniently obtain crystals of the hexafluorophosphate salt [Pt(terpy){Cp*Ir-p-(η4-C6H4Se2)}Pt(terpy)][PF6]4 (3b) as a

assigned to the Cp*Ir moiety. Further we note the presence of several multiplets in the range δ 7.72−9.10 ppm assigned to the protons of the fragment “Pt(terpy)”. Complex 3a was fully characterized (see Experimental Section). Also crystals of 3a· 2CH3CN could be grown, and its X-ray structure was determined (vide infra). Of prime importance we note is the 77 Se NMR spectrum of 3a, which provides valuable information about the nature of the species in solution. Indeed the 77Se NMR spectrum of 3a recorded in CD3NO2 shows the presence of a singlet at δ 214.5 ppm with two satellites indicating JPt−Se = 416 Hz, which confirms that the integrity of the coordination assembly is maintained in solution. All other complexes were obtained following a similar synthetic procedure to 3a but using the appropriate ammonium salt NH4X {X = PF6 (3b), X = BF4 (3c)} or sodium salt NaX {X = ClO4 (3d), X = BPh4 (3e)} (Scheme 1). The 1H NMR data of complexes 3b−e show a similar trend relative to complex 3a. On the other hand the infrared spectra show unique absorptions appropriate to the different counteranions (see Experimental Section). X-ray Molecular Structures of {[Pt(terpy){Cp*Ir-p-(η4C6H4Se2)}Pt(terpy)][X]4} (X = OTf (3a); X = PF6 (3b)). Crystals of 3a for X-ray analysis were conveniently obtained after many attempts by slow diffusion of diethyl ether into a CH3CN solution of the complex. Complex 3a crystallizes as a bis(CH3CN) solvate in the triclinic unit cell, space group P1̅ with Z = 2. A view of the cationic part of the complex and selected bond distances and angles are shown in Figure 1. The structure of 3a shows that the p-diselenobenzoquinone iridium complex indeed bridges two “Pt(terpy)” units through the two selenium centers with Pt−Se bond distances of 2.4262(8) Å (Pt1−Se1) and 2.4311(8) Å (Pt2−Se2). In this coordination assembly “Pt−Ir−Pt”, the “Pt(terpy)” units can be described as two large wings of a butterfly where the main body is defined by the p-diselenobenzoquinone complex, [Cp*Ir-p-(η 44987

dx.doi.org/10.1021/om400700t | Organometallics 2013, 32, 4985−4992

Organometallics

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C6H4S2)}Pt(terpy)][OTf]4,6 as well as the starting material 2, which showed absorption bands at 342, 380, and 442 nm, the high-energy absorption is attributed to admixtures of [π → π*] intraligand (IL) transitions of terpyridine and the Cp*Ir(η4diselenobenzoquinone) system, while the low-energy absorptions are tentatively assigned as admixtures of [dπ(Pt) → π*(terpy)] metal-to-ligand charge transfer (MLCT) and metalperturbed [π → π*] IL transitions of the Cp*Ir(η4diselenobenzoquinone) system. An additional low-energy absorption shoulder was observed at 472−476 nm (Figure S1). On the basis of the previous spectroscopic studies on [Pt(terpy)Cl]+, which showed a lowenergy MLCT absorption band at 400 nm, as well as the strong electron-donating behavior of the diselenobenzoquinone group, such a low-energy absorption shoulder is assigned to the [pπ(Se) → π*(terpy)] ligand-to-ligand charge transfer (LLCT) transition with mixing of MLCT character. Compared to the pdithiobenzoquinone analogue, [Pt(terpy){Cp*Ir-p-(η 4 C6H4S2)}Pt(terpy)], no such additional low-energy absorption shoulder was observed, probably due to the lower electrondonating ability of the dithiobenzoquinone counterpart. Luminescence of 3a−e was observed in fluid solution and in the solid state at 77 K and room temperature upon photoexcitation. Their photophysical data are summarized in Table 1. A structureless low-energy emission band at 685−705 nm was observed in acetonitrile at room temperature. According to the lowest energy absorption band in the electronic absorption spectrum, the emission origin is tentatively assigned as derived from the triplet LLCT/MLCT excited state. The observation of an excitation band at around 495 nm, as revealed in their corresponding excitation spectra (Figure S2), further confirms the assignment of the lowest energy absorption shoulder at 472−476 nm as a singlet LLCT/ MLCT transition. The phosphorescence that resulted from the singlet LLCT/MLCT excitation is indicative of efficient intersystem crossing with Φisc close to unity, characteristic of complexes of heavy metal centers. Upon photoexcitation, 3a−e were found to give emission at 665−737 nm in the solid state. Figure 4 shows the normalized emission spectra of 3a−d in the solid state at room temperature. Since a triplet LLCT/MLCT excited state has been suggested to be responsible for the emission in fluid solution, a similar excited-state origin for the solid-state emission is proposed. Lower energy solid-state emission was observed in 3a (705 nm), compared to the pdithiobenzoquinone analogue, [Pt(terpy){Cp*Ir-p-(η 4 C6H4S2)}Pt(terpy)][OTf]4 (644 nm),6 which is in accordance with the trend of the assignment of LLCT/MLCT origin. However, the possible assignment of an origin of excimeric intraligand excited state and metal-metal-to-ligand charge transfer (MMLCT) excited state arising from terpyridyl π−π and Pt···Pt interactions cannot be completely excluded, on the basis of the short π−π and Pt···Pt contacts in some of their crystal packing, as well as the large difference in the emission maxima of the solid state with different counterions (Figure 4). The lower emission energy observed in 3a relative to that of 3b may also be suggestive of the involvement of an MMLCT emission origin given the presence of stronger π−π and Pt···Pt interactions in the crystal packing of 3a. For 3c and 3d, a strong involvement of MMLCT emission origin is anticipated at 77 K in the solid state in view of the substantial red shift upon cooling due to the lattice contraction. The π−π and Pt···Pt interactions in the crystal packing in the related platinum(II) terpyridyl systems have been reported to be influenced by

bis(CH3CN) solvate by crystallization from CH3CN/ether. Thus, we examined the structure of 3b·2CH3CN to probe the influence of the anion on the solid structure packing and eventually on the π−π and Pt···Pt contacts between individual molecules. This salt also crystallizes in the triclinic unit cell, space group P1̅ with Z = 2. A view of the cationic part of the complex and selected bond distances and angles are shown in Figure 2. The structure shows similar features to that observed

Figure 2. (a) View of the cationic part of the crystal structure of 3b· 2CH3CN. Selected bond distances (Å) and angles (deg) for 3b: Ir1··· C16 = 2.325(6), Ir1···C17 = 2.223(6), Ir1···C18 = 2.222(6), Ir1···C19 = 2.326(7), Ir1···C20 = 2.220(7), Ir1···C21 = 2.234(6), C16···Se = 1.891(6), C19···Se2 = 1.885(7). (b) Dimer complex formed through π−π and Pt···Pt contacts between two individual tetracations.

for 3a; that is, the organometallic linker [Cp*Ir-p-(η4C6H4Se2)] (2) holds two Pt(terpy) units through the two selenium centers. The Pt(terpy) units also lean backward in an analogous fashion to that of 3a. Of prime importance we note is the packing of the individual units in the crystal. In fact 3b is better described as a dimer and not a 1D chain as seen for 3a, where only one side of the molecule shows π−π interaction with d = 3.41(2) Å, θ = 20°, and a Pt1···Pt1 contact at 3.623(6) Å (Figure 2b). Although we were able to obtain crystal structures of only two salts of the above series 3a−e, it is obvious that the counterion affects the packing of these molecules in the solid state. We then examined their optical and luminescent properties. Photophysical Properties of 2 and 3a−e. The electronic absorption spectra of 3a−e in acetonitrile at room temperature showed an intense band at 278−346 nm and a low-energy absorption at 374−450 nm. Their photophysical data are summarized in Table 1, and the electronic absorption spectra of 2 and 3b are depicted in Figure 3. With reference to previous spectroscopic studies on [Pt(terpy)Cl][X]16 and the related pdithiobenzoquinone analogue, [Pt(terpy){Cp*Ir-p-(η 4 4988

dx.doi.org/10.1021/om400700t | Organometallics 2013, 32, 4985−4992

Organometallics

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Table 1. Photophysical Data for 2 and 3a−e emissiona

absorption complex 2 3a

3b

3c

3d

3e

medium (T[K]) acetonitrile (298) acetonitrile (298) solid (298) solid (77) thin film (298)c acetonitrile (298) solid (298) solid (77) thin film (298)c acetonitrile (298) solid (298) solid (77) thin film (298)c acetonitrile (298) solid (298) solid (77) thin film (298)c acetonitrile (298) solid (298) solid (77) thin film (298)c

λmax [nm] (εmax [dm mol 3

−1

−1

cm ])

342 (12 400), 380 (9800), 442 (4300) 280 (49 200), 330 (37 000), 342 (36 000), 374 (12 400), 448 (3700), 476 (2700)

282 (44 400), 328 (34 200), 342 (37 700), 374 (12 800), 450 (4400), 474 (3200)

282 (41 100), 332 (27300), 346 (20 600) 376 (10 500), 396 (8600), 472 (2500)

282 (47 700), 332 (36 200), 342 (37 100), 374 (12 900), 448 (3900), 476 (2900)

282 (51 200), 332 (34 000), 344 (29 100) 376 (10 500), 396 (7800), 476 (2600)

λmax [nm] (τo[μs]) − 700 705 680 578 685 675 675 578 685 700 750 578 700 665 702 578 705 737 715 578 b

Φlum −