Divalent Tetra- and Penta-phenylcyclopentadienyl Europium and

Article pubs.acs.org/Organometallics

Divalent Tetra- and Penta-phenylcyclopentadienyl Europium and Samarium Sandwich and Half-Sandwich Complexes: Synthesis, Characterization, and Remarkable Luminescence Properties Rory P. Kelly,† Toby D. M. Bell,† Rosalind P. Cox,† Daisy P. Daniels,† Glen B. Deacon,*,† Florian Jaroschik,*,‡ Peter C. Junk,*,§ Xavier F. Le Goff,⊥ Gilles Lemercier,‡ Agathe Martinez,‡ Jun Wang,†,‡ and Daniel Werner† †

School of Chemistry, Monash University, Clayton 3800, Australia Institut de Chimie Moléculaire de Reims, UMR CNRS 7312, Case Postale 44, UFR des Sciences Exactes et Naturelles, BP 1039, 56187 Reims Cedex 2, France § College of Science, Technology and Engineering, James Cook University, Townsville 4811, Australia ⊥ Laboratoire “Hétéroéléments et Coordination”, Ecole Polytechnique and CNRS, 91128 Palaiseau Cedex, France ‡

S Supporting Information *

ABSTRACT: The synthesis of the bulky divalent (polyphenylcyclopentadienyl)lanthanoid sandwich complexes [Ln(C5Ph5)2] (Ln = Sm, Eu) and [Ln(C5Ph4H)2(solv)] (Ln = Sm, solv = thf; Ln = Eu, solv = dme)], from redoxtransmetalation/protolysis (RTP) reactions, has been achieved. An analogous reaction with Yb afforded the solvent-separated ion pair [Yb(dme)4][C5Ph4H]2. In addition, rare divalent samarium halide complexes [Sm(C5Ph5)(μBr)(thf)2]2 and [Sm(C5Ph4H)I(thf)3], were also prepared, either by RTP or ligand rearrangement. X-ray studies showed that the [Ln(C5Ph5)2] complexes adopt highly symmetrical sandwich structures, whereas the [Ln(C5Ph4H)2(solv)] complexes have open sandwich structures. The unexpected, but limited, solubility of the [Ln(C5Ph5)2] complexes allowed for variabletemperature NMR spectra of [Sm(C5Ph5)2] to be obtained. Detailed 1D and 2D NMR studies were conducted on [Sm(C5Ph4H)2(thf)] to ascertain its structure in donor and nondonor solvents. During the course of these studies, the mixed tetraarylcyclopentadienyl sandwich complex [Sm{C5(2,5-Ph)2(3,4-p-tol2)H}2(thf)] was also prepared in order to fully assign the spectrum of [Sm(C5Ph4H)2(thf)]. The europium sandwich complexes [Eu(C5Ph5)2] and [Eu(C5Ph4H)2(dme)] exhibit remarkable luminescence properties with high quantum yields (45% and 41%, respectively) coupled with long emission lifetimes (approximately 800 and 1300 ns, respectively) in toluene.

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INTRODUCTION The predominantly ionic nature of the bonding in organolanthanoid complexes means that the steric bulk of ligands is often used to enhance solubility and effect kinetic stabilization of complexes. Cyclopentadienyl complexes have dominated research in organolanthanoid chemistry, and substituted cyclopentadienyl ligands have allowed for spectacular results over the past few decades, including the characterization of molecular divalent species for the entire lanthanoid series (with the exception of the radioactive promethium).1 In particular, the C5Me5 (Cp*) ligand has led to a flourishing of organolanthanoid chemistry. The decamethylsamarocenes, [Sm(Cp*)2]2 and [Sm(Cp*)2(thf)2]3 (thf = tetrahydrofuran) display remarkable reactivity, and [Sm(Cp*)2] allowed for the first characterized lanthanoid dinitrogen complex.4 Perhaps more unexpected was the behavior of [Sm(Cp*)3],5 which is strongly reducing despite having samarium in its highest accessible oxidation state.6 This chemistry has since been © XXXX American Chemical Society

extended to other [Ln(Cp*)3] (Ln = lanthanoid) complexes without a readily accessible divalent oxidation state,7 and the area has been labeled “sterically induced reduction”. Analogous [Ln(C5Me4H)3] complexes do not display this behavior, even though only one methyl group is missing from each Cp* ligand. On the other hand, [Lu(Cp*)(C 5 Me 4 H) 2 ] and [Y(Cp*)2(C5Me4H)] complexes reduce dinitrogen, whereas the bulkier complexes [Lu(Cp*)2(C5Me4H)] and [Y(Cp*)3] do not.8 It is not yet understood why these complexes behave so differently, but clearly small changes in the choice of ligand(s) can drastically affect reactivity. Such changes in the ligand can also have a pronounced effect on molecular structure. The first lanthanoid sandwich complex with parallel cyclopentadienyl rings, [Eu{C5(iPr)5}2],9 was reported only in 2000, shortly after the structures of the Received: October 6, 2015

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DOI: 10.1021/acs.organomet.5b00842 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of [Ln(C5Ph5)2] (Ln = Sm (1), Eu (2))

has been renewed interest in the field, and some exciting results have emerged in recent years17a,20 (see also relevant reviews).21 Of note is the study by Roesky et al. on a series of EuII borohydrides, with some complexes showing quantum yields greater than 70%.20b Herein, we report the syntheses and characterization of new divalent C5Ph4H and C5Ph5 sandwich and half-sandwich complexes of samarium and europium. The EuII compounds show excellent luminescence properties, with Φf values of >40% in toluene solution. Time-resolved photoluminescence studies on these large EuII sandwich complexes have also been conducted.

analogous heavy alkaline earth metal (i.e., calcium, strontium, and barium) complexes were published.10 In contrast, the reported octaisopropylmetallocenes [M{C5(iPr)4H}2] (M = Ca, Sr, Ba, Sm, Eu, Yb)9,11 all have slightly bent structures, despite the absence of coordinating solvent. Most alkaline earth and lanthanoid(II) metallocenes adopt bent structures, e.g., [Sm(Cp*)2],2 although a consensus view as to why this is the case has not yet been reached.12 In any case, interesting structural differences can arise from changing the number of substituents attached to the cyclopentadienyl ligand. Indeed, the effect of bulky ligands and secondary interactions on structure is also observed in complexes with noncyclopentadienyl ligands,13 demonstrating the rich coordination chemistry of the lanthanoids. In recent years, we have become particularly interested in the pentaphenylcyclopentadienyl (C5Ph5) ligand. It had previously attracted little attention in organolanthanoid chemistry due to reactivity and purification problems and the fact that decaphenylmetallocenes are reported to be highly insoluble.14 However, we were able to circumvent these issues by utilizing redox-transmetalation/protolysis (RTP) reactions to synthesize ytterbium half-sandwich complexes15 and the symmetrical decaphenylmetallocenes [M(C5Ph5)2] (M = Ca, Ba, Yb).16 These complexes are poorly soluble in nonpolar solvents, whereas in thf they form solvent-separated ion pairs (SSIPs), greatly restricting the exploration of their solution chemistry. Harder and co-workers eliminated the solubility problems of decaphenylmetallocenes by using the alkyl-substituted pentaphenylcyclopentadienyl ligand C5{C6H4(nBu)-4}5 (CpBIG), and they have achieved some impressive results.17 However, another approach to improve solubility is to use the tetraphenylcyclopentadienyl ligand (C5Ph4H). Indeed, we recently reported ytterbium and alkaline earth metal complexes bearing C5Ph4H ligands, and they show greatly increased solubility in aromatic solvents when compared with their C5Ph5 analogues.18 This is presumably related to the fact that the solvated ytterbium and calcium complexes [M(C5Ph4H)2(thf)] have open structures, as opposed to the flat, highly symmetric structures of analogous [M(C5Ar5)2] complexes. The recently described [Eu(CpBIG)2] complex (noted above) showed intense luminescence (measured from crystals), with emission in the red part of the visible spectrum (λmax = 606 nm) and a quantum yield (Φf) of 45%.17a This emission was attributed to 4f65d1 → 4f7 transitions, which are allowed, unlike the more familiar “atomic-like” EuIII emissions, which occur from disallowed transitions within the 4f-subshell. These are extremely weak without suitable sensitization, which is usually achieved through energy transfer from coordinating ligands. The luminescence properties of EuII complexes remain poorly studied, although there were some pioneering studies on divalent europium crown-ether complexes.19 However, there

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RESULTS Syntheses of Decaphenylmetallocene Complexes. Freshly filed samarium or europium metal, one equivalent of HgPh2, and two equivalents of C5Ph5H were sonicated in thf at 40 °C for several days. Filtration of the green (Sm) and yellow (Eu) solutions, respectively, followed by drying under vacuum then washing with toluene yielded the decaphenyllanthanocenes [Ln(C5Ph5)2] (Ln = Sm (1), Eu (2)) as dark red-violet and bright orange powders, respectively (Scheme 1). In addition, 2 can be synthesized using Hg(C6F5)2 in place of HgPh2 if the reaction is performed at room temperature (Scheme 1). In order to determine if this reaction also proceeded via a solvent-separated ion pair, as in the case of ytterbium,16 we conducted variable-temperature NMR studies on 1. Dissolution of the dark red-violet solid 1 in d8-thf in an NMR tube gives a green solution, showing one broad signal in the 1H NMR spectrum at 6.5−8.5 ppm, indicating the presence of mainly diamagnetic [Sm(d8-thf)n][C5Ph5]2. When the reaction mixture for the synthesis of 1 in thf is cooled to room temperature, a similar colored solution is observed. Heating the NMR sample resolves the broad peak into three paramagnetically shifted and broadened signals, corresponding to the para, meta, and ortho hydrogen atoms of the phenyl rings, and at 60 °C, they are observed at 8.76, 10.13, and 12.69 ppm, respectively. This is consistent with a samarium-bound C5Ph5 species, possibly [Sm(C5Ph5)2]. On cooling, interesting behavior is observed. The broad peak obtained at room temperature splits into signals that gradually resolve as the temperature is lowered further. However, in contrast to what is observed at high temperatures, this splitting consists of paramagnetically shifted components and components that are diamagnetic. Interestingly, the relative integration ratios of diamagnetic/paramagnetic components increase as the temperature goes down. At −25 °C, the color of the solution is dark blue-violet, and three broad singlets are observed at 6.76, 6.83, and 6.88 ppm, which are attributable to the para, meta, and ortho hydrogen atoms, respectively, of the phenyl rings of free C5Ph5 ligands. In B


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Organometallics Scheme 2. Synthesis of [Sm(C5Ph5)(μ-Br)(thf)2]2 (3)

Scheme 3. Syntheses of [Ln(C5Ph4H)2(solv)] (Ln = Sm (4), solv = thf; Ln = Eu (5), solv = dme) and [Yb(dme)4][C5Ph4H]2 (7)

for [Sm(CpBIG)2] (11.26 ppm at 20 °C in C6D6).17b Given the similarity between the structures of 1 and [Sm(CpBIG)2], it is surprising that the ortho hydrogen atoms are not observed in the spectrum of 1 at room temperature (they were reported at 15.67 ppm in the spectrum of [Sm(CpBIG)2] at 20 °C in C6D6). However, they are observed at 16.23 ppm after the addition of some drops of d8-thf to a C6D6 solution of 1 at room temperature, close to the value reported for the ortho hydrogen atoms in the spectrum of [Sm(CpBIG)2]. The 13C NMR data show five distinct peaks at −24.7 (Cpring), 130.7 (Cpara Ph), 136.0 (Cmeta Ph), 161.5 (Cortho Ph), and 182.3 ppm (Cipso Ph). Unsurprisingly, the Cpring and Cipso carbon atoms are the most paramagnetically affected. The combined NMR data strongly support a solid-state composition of [Sm(C5Ph5)2] for complex 1. The unexpected, but limited, solubility of these complexes allowed for the luminescence behavior of 2 to be explored in toluene (vide inf ra). Elemental analyses for both 1 and 2 were satisfactory, within the more generous range appropriate for organolanthanoids, and in particular divalent complexes, where combustion problems often lead to low % C values,1b,9,22 and this has been attributed to carbide formation.23 The IR spectra of both complexes in the solid state are essentially identical and correspond closely with reported [M(C5Ph5)2] complexes,16 showing characteristic absorptions at 1593 and 1501 cm−1 for both complexes. The similarity of the IR spectra offers further evidence that complex 1 is indeed [Sm(C5Ph5)2]. The composition of 1 was also confirmed by MALDI-TOF MS (matrix-assisted laser desorption/ionization mass spectrometry). In the MALDI-TOF mass spectrum of 1, the molecular ion peak, [Sm(C5Ph5)2]+, is observed with good intensity and the correct isotope pattern. Related ions such as [Sm(C5Ph5)]+ and [Sm2(C5Ph5)3]+ are seen as well, and the results are

addition, there are three paramagnetically shifted and broadened signals at 7.70, 8.21, and 8.96 ppm, which are attributable to the phenyl ring hydrogen atoms of a samariumbound C5Ph5 species. The relative integration ratio of diamagnetic/paramagnetic components is approximately 6:1, and this increases to 25:1 at −40 °C. These observations are in agreement with a dynamic equilibrium between a samariumbound C5Ph5 species and [Sm(d8-thf)n][C5Ph5]2. At higher temperatures, the equilibrium favors the samarium-bound C5Ph5 species, whereas at lower temperatures it favors [Sm(d8-thf)n][C5Ph5]2. This behavior can be rationalized as the thermally driven substitution of uncharged thf molecules by charged C5Ph5 ligands. In the case of decaphenyleuropocene (2), a yellow solution was obtained when the bright orange powder was dissolved in thf, indicating a similar equilibrium to when 1 was dissolved in thf. Unfortunately, attempts to crystallize [Ln(thf)n][C5Ph5]2 (Ln = Sm, Eu) complexes from thf failed. On the other hand, careful layering and slow diffusion of toluene into a concentrated solution of 2 in thf gave single crystals of [Eu(C5Ph5)2] that were suitable for X-ray diffraction studies. During the course of this work, it was determined that the decaphenylmetallocene complexes 1 and 2 are more soluble, albeit still sparingly, in aromatic solvents than was expected. Heating toluene or C6D6 suspensions of the decaphenylmetallocenes dissolved some of the precipitates, allowing for the characterization of 1 in C6D6. The 1H NMR spectrum at room temperature shows two broad peaks at 11.16 and 9.42 ppm in a 2:1 ratio, which correspond to the meta and para hydrogen atoms, respectively. At 70 °C, a third very broad peak is observed at 12.69 ppm, which integrates correctly for the ortho hydrogen atoms. The value at room temperature for the meta hydrogen atoms is in very good agreement with that reported C


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Organometallics consistent with previous MALDI-TOF MS studies of decaphenylmetallocenes.16,24 From one attempt to prepare 1, a few single crystals of [Sm(C5Ph5)(μ-Br)(thf)2]2·6thf (3·6thf) were isolated. A commercial sample of HgPh2 was used for the synthesis, and it is presumed that it was contaminated with small amounts of HgBr2 and/or PhHgBr. We have previously used PhHgBr in RTP reactions to synthesize ytterbium and calcium pseudoGrignard complexes bearing the C5Ph4H ligand.18a We attempted a similar reaction to prepare 3 (Scheme 2), but the yield was repeatedly very low. The previous results demonstrated that this method is much more reliable with calcium than with ytterbium, and this was attributed to the instability of YbII intermediates. The low success of the present reaction is congruent with this hypothesis since SmII is less stable than YbII. Although the use of sonication might increase the likelihood of decomposition of SmII intermediates, only very small amounts of product were detected when reactions were performed at room temperature. In the 1H NMR spectrum, the signals for the C5Ph5 ligands are observed at similar values to those of 1. Thus, at room temperature, only signals for the meta and para hydrogen atoms are observed, but at 70 °C a third very broad peak corresponding to the ortho hydrogen atoms appears at 12.68 ppm. The presence of residual lattice solvent in the sample was also confirmed. The IR spectrum shows absorptions at 1594 and 1501 cm−1 that are typical of coordinated C5Ph5 ligands. Elemental analysis of a thoroughly dried sample gave a hydrogen value consistent with loss of lattice solvent, but the carbon value was low. Solvent loss issues preventing satisfactory analyses are also well-known in lanthanoid and group 2 organometallic chemistry.11a,22c,25 Syntheses of Octaphenyllanthanocene Complexes. RTP reactions between an excess of freshly filed samarium or europium metal, one equivalent of HgPh2, and two equivalents of C5Ph4H2 were performed in thf. After stirring (or sonication at 40 °C in the case of samarium) the reaction mixtures for two to three days, the new divalent complexes [Sm(C5Ph4H)2(thf)] (4) and [Eu(C5Ph4H)2(dme)]·1.5dme (5·1.5dme) (dme = 1,2dimethoxyethane) were obtained after recrystallization of the crude products from toluene and dme, respectively (Scheme 3). Good elemental analyses were obtained for 5, whereas adequate elemental analyses were obtained for 4 given the problems with these systems (vide supra). The IR spectra are very similar, and both show absorptions corresponding to coordinated C5Ph4H ligands at 1594 and 1495 cm−1. 1 H and 13C NMR spectroscopy was used to characterize the C5Ph4H samarium complex 4, and the results are consistent with the single-crystal composition. The 1H NMR spectrum of 4 is well-resolved and shows relatively sharp paramagnetically shifted peaks that fall across the range −9.15 to 22.60 ppm (Figure 1a). The spectrum features nine distinct peaks, which are attributable to the C5Ph4H ring hydrogen atom(s) (one signal), the phenyl ring hydrogen atoms (six signals corresponding to two pairs of different phenyl groups), and the thf hydrogen atoms (two signals). Through a 1H−1HCOSY experiment, it was ascertained that one set of phenyl groups is much more influenced by the paramagnetic SmII center than the other set. As two-dimensional 1H−13C HMBC and 1H−1H NOESY experiments were not successful in distinguishing the two sets of phenyl groups, we decided to synthesize the new divalent samarium complex 6, carrying the 2,5-diphenyl-3,4-di(p-tolyl)cyclopentadienyl ligand (Scheme

Figure 1. 1H NMR data (δ): (a) complex 4 in C6D6, (b) complex 6 in C6D6, (c) complex 6 in d8-thf, and (d) complex 6 in d8-thf/C6D6 (1:1).

4). This complex was obtained through the same RTP protocol used for complex 4. In the 1H NMR spectrum of 6, the same Scheme 4. Synthesis of [Sm{C5(2,5-Ph2)(3,4-ptol2)H}2(thf)] (6)

pattern is observed as for 4, except there is a singlet corresponding to the p-methyl groups of the p-tolyl substituents in place of one set of p-hydrogen atoms (Figure 1b). From these studies, we ascertained that the phenyl groups in the 2 and 5 positions are more affected by the paramagnetic samarium center than the p-tolyl groups in the 3 and 4 positions (Scheme 4). Likewise, the cyclopentadienyl carbon attached to the hydrogen atom is the most paramagnetically shifted, followed by C2 and C5, and then C3 and C4. From the correspondence between the 2,5-Ph2 resonances of 6 and one set of phenyl resonances of 4, the two sets of phenyl resonances of 4 can be distinguished. In contrast to what was found with the europium complex 5, r ec r y s t a l l i z a t i o n o f t h e k n o w n c o m p o u n d [ Y b (C5Ph4H)2(thf)]18b from dme yielded single crystals of an SSIP, [Yb(dme)4][C5Ph4H]2·dme (7·dme) (Scheme 3). The difference between europium and ytterbium in these systems could be attributed to the greater oxophilicity of the smaller Yb2+ ion compared with the Eu2+ ion, but steric factors might also play a role. Addition of d8-thf to 4 does not lead to a color change, in strong contrast to what was observed for the decaphenylmetallocene complex 1. The influence of the paramagnetic SmII center is lessened, as shown by 1H NMR spectroscopy, but still present. The peaks for the phenyl groups are now in the range 6 to 10 ppm, in contrast to 8 to 23 ppm in C6D6, and the Cp−H ring hydrogen atoms are at 8.84 ppm, in contrast to −9.58 ppm in C6D6. A similar observation was made in the case of complex 6 (Figure 1c). Furthermore, when this complex was dissolved in a 1:1 mixture of d8-thf/C6D6, intermediate shifts were observed (Figure 1d). We therefore exclude the formation of SSIPs in the case of the samarium D


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Organometallics complexes 4 and 6 (and by extension, in the case of the europium complex 5). The fact that 5 crystallizes from dme as the intact sandwich complex supports this contention. Furthermore, on the basis of 1H NMR spectra of alkali metal cyclopentadienides, the C5Ph4H− anion in an SSIP would be expected to have a singlet at ca. 6.10 ppm for the Cp−H ring hydrogen atom and overlapping resonances at ca. 6.9 ppm for the phenyl ring hydrogen atoms.26 From these observations, it seems evident that for the present octaaryllanthanocene complexes, structural modifications of the sandwich structures occur in thf. Although complexes 4 and 5 are insoluble in aliphatic solvents, they show good solubility in polar and aromatic solvents, and this should allow for studies of their solutionbased reactivity. Reactivity studies of decaphenylytterbocene were hampered by its low solubility in aromatic solvents and the tendency of the C5Ph5 ligand to undergo oxidization to the C5Ph5• radical when strong oxidizing agents were added to [Yb(thf)6][C5Ph5]2, which forms when decaphenylytterbocene is dissolved in thf.16 It is also noteworthy that 4 and 5 display good thermal stability, in contrast to the poor thermal stability of [Ba(C5Ph4H)2(thf)].27 Similar behavior was observed for [Yb(C5 Ph4 H) 2 (thf)], which is also more stable than the isostructural calcium complex.18 The difference in thermal stability between the lanthanoid complexes and the barium complex presumably results from a greater degree of ionic bonding in the barium complex28 coupled with the larger size of the Ba2+ cation. Synthesis of [Sm(C5Ph4H)I(thf)3] (8). RTP reactions involving HgPh2 can be sluggish, so iodine activation, which has been used successfully to activate lanthanoid metals,29 was used in one attempt to synthesize 4. Instead of the exclusive formation of 4, a small number of crystals of [Sm(C5Ph4H)I(thf)3]·thf (8·thf) were isolated. Small amounts of SmI2(thf)n presumably reacted with the majority product 1 to form 8. Complex 8 could be synthesized quantitatively from the reaction between equimolar amounts of 4 and SmI2(thf)2 in thf (Scheme 5). The bulky C5Ph4H ligand inhibits the operation of a Schlenk-type equilibrium.

Figure 2. Normalized steady-state absorbance (dotted lines) and emission spectra (solid lines) of 2 (orange) and 5 (red) in toluene (λexc = 480 and 490 nm for 2 and 5, respectively; λem = 616 and 645 nm for 2 and 5, respectively).

nm. Much stronger absorption is present below ∼350 nm for 2 and from ∼450 nm for 5, bestowing a bright orange color to the solution of 2 and a more orange-red color to 5. Intense emission is observed from both compounds, centered at 616 nm for 2 (λexc = 480 nm) and 645 nm for 5 (λexc = 490 nm). Emission quantum yields are 45% and 41% for 2 and 5, respectively, similar to that reported by Harder et al., from crystals of [Eu(CpBIG)2] (Φf = 45%; λmax = 606 nm).17a Sitzmann et al. reported the fluorescence of divalent europium metallocenes [Eu{1,2,4-(tBu)3-C5H2}2] (red-orange), [Eu{C5(iPr)4H}2] (red-orange), and [Eu{C5(iPr)5}2] (light orange), but no λmax values were reported.9 [Eu(Cp*)2(OEt2)] emits at considerably longer wavelength (730 nm).30 The remarkable emission in toluene solution was further investigated using time-correlated single-photon counting (TCSPC), and fluorescence decay histograms and fitting data are shown in Figure 3. Long-lived decays on a microsecond time scale are evident for both compounds, and they can be well-fitted by a single-exponential decay function, with lifetimes of 810 ns for 2 (χ2 = 1.07) and 1270 ns (χ2 = 1.04) for 5. The lifetime of complex 5 in particular is impressively long compared with examples in the literature, e.g., EuCl2 (1.7 ns in anhydrous MeOH),19c [Eu(Cp*)2(OEt2)] (400 ns in toluene),30 and various EuII crown-ether complexes (∼300− 900 ns).19 There is also some short time scale emission present, particularly for 2, possibly due to scattered photons not fully accounted for by the instrument response function in the fitting process. Addition of a second, subnanosecond decay component to the fitting functions does not appreciably change the χ2-fitting parameters or alter the lifetimes of the long-lived component for either compound. The residuals to the fits shown in the insets in Figure 3 are symmetric around zero, indicative of well-fitted data. The radiative rate constants (Φf/ lifetime) were calculated to be 5.6 × 105 s−1 for 2 and 3.2 × 105 s−1 for 5. The corresponding value for the related complex [Eu(Cp*)2(OEt2)] is approximately 1 × 105 s−1.30 Related nonradiative rate constants (knr) can also be deduced (6.8 × 105 and 4.6 × 105 s−1 for 2 and 5, respectively), and the calculated kr/knr values (0.82 and 0.69 for 2 and 5, respectively) are consistent with the related quantum yield values. TCSPC was also performed on thin-film gel phase samples made by mixing each compound in Vaseline and smearing onto a microscope coverslip. The decay histograms recorded are also shown in Figure 3, and it can be seen that both compounds

Scheme 5. Synthesis of [Sm(C5Ph4H)I(thf)3] (8)

The 1H NMR spectrum of 8 is similar to that of 4, and it was assigned on the basis of 6. The IR spectrum is similar to those of 4 and 5, showing ν(CC) absorptions at 1595 and 1498 cm−1. Like 3, elemental analysis gave low carbon and hydrogen values. However, substantial loss of thf on standing does enable a reasonable fit for the carbon analysis, and thf loss on standing has been verified by 1H NMR spectroscopy. Luminescence Studies. The modest solubility of complex 2 and the good solubility of 5 in nonpolar solvents enabled us to study their luminescence properties both in solution and in the solid state. Steady-state absorption and emission spectra of 2 and 5 in toluene are shown in Figure 2. Weak absorption bands are present for both compounds extending to ∼550−600 E


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Organometallics

by which time both short components had completely decayed. Thus, the decays are well-fitted by the long-lived third decay component for the reminder of the decays, and the lifetimes recovered describe the data well for both compounds in Vaseline films. These remarkable microsecond-long lifetimes, in both solutions and Vaseline films, indicate excellent rigidity and stability of the complexes. In addition to measurements performed in toluene, measurements were also conducted on a solution of 2 in thf. Almost total quenching of emission was observed, and this supports NMR studies that indicate structural modification of the sandwich structures in donor solvents. Molecular Structures. Complex 2 crystallized in the monoclinic space group P21/n, and it is isomorphous with [Yb(C5Ph5)2].16 The molecular structure is shown in Figure 4,

Figure 3. Fluorescence decay histograms in toluene of 2 (orange) and 5 (red) with instrument response function (green) and fitted singleexponential decay functions (black). The upper panel shows decays in oxygen-free toluene solutions, and the lower panel shows decays recorded in Vaseline films under ambient conditions. Insets: Plots of residuals (data − fit).

Figure 4. Molecular structure of [Eu(C5Ph5)2] (2) shown with 50% probability thermal ellipsoids; hydrogen atoms have been removed for clarity.

and selected bond lengths and angles are listed in Table 1. The complex features two parallel C5Ph5 rings that are staggered with respect to each other. The phenyl rings of each Cp ring are bent out of the plane and interlock in the same manner as other decaphenylmetallocenes.16,17 When compared with the Yb−C bond lengths of [Yb(C5Ph5)2] (2.652(2)−2.680(2) Å), the Eu−C bond lengths of 2 (2.751(4)−2.788(3) Å) are shorter than would be expected given the difference in ionic radii between six-coordinate Yb2+ (1.02 Å) and six-coordinate Eu2+ (1.17 Å).31 This can presumably be attributed to greater steric crowding around the smaller Yb2+ ion. Moreover, subtraction of the ionic radius for six-coordinate Eu2+ from the ⟨Eu−C⟩ (2.770 Å) gives a value of 1.60 Å, at the low end of such values for (cyclopentadienyl)lanthanoid complexes.32 The sum of the steric coordination numbers (st.c.n.)33 of two C5Ph5 rings is 7.6 (calculated from the structure of [Yb(C5Ph5)2])34 and is not indicative of excessive crowding. Moreover, the Sm−C bond lengths of [Sm(CpBIG)2] (2.779(2)−2.791(2) Å)17b are similar in length to those of 2, in keeping with the similarity in ionic radii between Sm2+ and Eu2+. Complex 2 features the same characteristic C−H···C(π) interactions as observed in other decaphenylmetallocenes,16,17 and these are thought to bring the

display long-lived emission, although the count rate from 5 was quite low and short-lived emission is clearly present. The decay of 2 also shows some short-lived emission, and both decay profiles required three exponential decay components for satisfactory fitting: two short-lived (360 °C). IR (Nujol, cm−1): 1594s, 1501s, 1261w, 1178w, 1155w, 1143w, 1076m, 1025m, 915w, 841w, 802m, 776m, 739m, 731m, 711s, 700s, 679w, 668w. 1H NMR (C6D6, redviolet suspension, 300 MHz, 303 K): δ 9.42 (br s, 10H, p-H Ph), 11.16 (vbr s, 20H, m-H Ph). 1H NMR (C6D6, red-violet solution, 300 MHz, 343 K): δ 8.76 (br s, 10H, p-H Ph), 10.13 (br s, 20H, m-H Ph), 12.69 (vbr s, 20H, o-H Ph). 1H NMR (C6D6 + some C4D8O, red-violet solution, 250 MHz, 298 K): δ 9.85 (br s, 10H, p-H Ph), 11.62 (br s, 20H, m-H Ph), 16.23 (vbr s, 20H, o-H Ph). 1H NMR (C4D8O, green solution, 300 MHz, 303 K): δ 7.23 (vbr s, Ar-H Ph). 1H NMR (C4D8O, dark red solution, 300 MHz, 333 K): δ 8.56 (br s, 10H, p-H Ph), 9.60 (br s, 20H, m-H Ph), 11.75 (vbr s, 20H, o-H Ph). 13C{1H} NMR (C6D6, 125 MHz, 333 K): δ −24.7 (br s, Cring), 130.7 (s, Cpara

Ph), 136.0 (s, Cmeta Ph), 161.5 (vbr s, Cortho Ph), 182.3 (s, Cipso Ph). MALDI-TOF MS (m/z (%)): 446.06 ([C5Ph5H]+, calcd for C35H26 446.20, (100)), 596.90 ([Sm(C5Ph5)]+, calcd for C35H25Sm 597.11, (15)), 1041.98 ([Sm(C5Ph5)2]+, calcd for C70H50Sm 1042.31, (15)), 1638.92 ([Sm2(C5Ph5)3]+, calcd for C105H75Sm2 1639.42, (1)). Anal. Calcd (%) for C70H50Sm (1041.50): C, 80.72; H, 4.84. Found: C, 80.31; H, 5.28. Synthesis of [Eu(C5Ph5)2] (2). Method 1. Thf (10 mL) was added to a Schlenk flask charged with freshly filed europium metal (0.17 g, 1.1 mmol), HgPh2 (0.084 g, 0.24 mmol), and C5Ph5H (0.21 g, 0.47 mmol), and the suspension was sonicated at 40 °C for five days. The yellow solution was then filtered and concentrated under vacuum. Toluene (10 mL) was added, and the mixture was sonicated at 40 °C overnight, giving a bright orange solid. The supernatant was filtered off, and the solid was washed again with toluene (20 mL) and then dried under vacuum, giving pure 2 (0.16 g, 65%). Thf (2 mL) was added to the residue that remained in the flask after the solid had been removed, and then toluene (1 mL) was carefully layered on top of the yellow solution. Bright orange single crystals formed after three days. Mp: >360 °C. IR (Nujol, cm−1): 1593m, 1501s, 1261w, 1178w, 1154w, 1142w, 1075m, 1024w, 915w, 838w, 801m, 777m, 738m, 706s, 678w, 668w. Anal. Calcd (%) for C70H50Eu (1043.11): C, 80.60; H, 4.83. Found: C, 79.04; H, 4.88. A 1H NMR spectrum was not obtained due to the paramagnetism of the sample. Method 2. Thf (15 mL) was added to a Schlenk flask charged with freshly filed europium metal (0.27 g, 1.8 mmol), Hg(C6F5)2 (0.13 g, 0.24 mmol), and C5Ph5H (0.22 g, 0.49 mmol), and the suspension was stirred for two days at room temperature. The yellow solution then was filtered, and the solvent was removed under vacuum. Toluene (20 mL) was added, and the mixture was briefly sonicated at 40 °C, giving a bright orange solid. The solvent was decanted, and the solid was dried under vacuum, giving pure 2. Thf (2 mL) was added to the residue that remained in the flask after the solid had been removed, and then toluene (1 mL) was carefully layered on top of the yellow solution. Bright orange single crystals formed after three days, the solvent was decanted, and the crystals were dried under vacuum, giving pure 2 (combined yield: 0.15 g, 60%). Synthesis of [Sm(C5Ph5)(μ-Br)(thf)2]2·6thf (3·6thf). Thf (15 mL) was added to a Schlenk flask charged with samarium powder (0.37 g, 2.5 mmol), HgPhBr (0.27 g, 0.70 mmol), and C5Ph5H (0.31 g, 0.69 mmol). The mixture was sonicated at 40 °C for three days. The solution was concentrated under vacuum and stored at −30 °C overnight, affording a small batch of dark brown crystals. The yield of pure compound was consistently very low ( 2σ(I)), and wR2 was 0.1383 (all data). Crystal data for 3·6thf: C110H130Br2O10Sm2 (M = 2072.66 g/mol); monoclinic, space group P21/c (no. 14), a = 11.999(1) Å, b = 17.993(1) Å, c = 23.591(1) Å, β = 99.168(1)°, V = 5028.2(5) Å3, Z = 2, T = 150.0(1) K, μ(Mo Kα) = 2.008 mm−1, Dcalc = 1.369 g/cm3, 25 739 reflections measured (6.38° ≤ 2Θ ≤ 52.74°), 10 020 unique (Rint = 0.0297, Rsigma = 0.0317), which were used in all calculations. The final R1 was 0.0361 (I > 2σ(I)), and wR2 was 0.0892 (all data). Crystal data for 4: C62H50OSm (M = 961.37 g/mol); monoclinic, space group P21/n (no. 14), a = 10.323(2) Å, b = 40.667(8) Å, c = 11.096(2) Å, β = 105.38(3)°, V = 4491.3(17) Å3, Z = 4, T = 100 K, μ(Synchrotron) = 1.352 mm−1, Dcalc = 1.422 g/cm3, 84 424 reflections measured (2.002° ≤ 2Θ ≤ 57.506°), 11 632 unique (Rint = 0.1397, Rsigma = 0.0744), which were used in all calculations. The final R1 was 0.0513 (I > 2σ(I)), and wR2 was 0.1329 (all data). Crystal data for 5·1.5dme: C68H67EuO5 (M = 1116.17 g/mol); monoclinic, space group P21/c (no. 14), a = 12.2090(6) Å, b = 20.0023(11) Å, c = 44.869(3) Å, β = 97.081(2)°, V = 10873.8(10) Å3, Z = 8, T = 100 K, μ(Synchrotron) = 1.206 mm−1, Dcalc = 1.364 g/cm3, 116 013 reflections measured (3.424° ≤ 2Θ ≤ 60.628°), 32 305 unique (Rint = 0.0638, Rsigma = 0.0782), which were used in all calculations. The final R1 was 0.0537 (I > 2σ(I)), and wR2 was 0.1004 (all data). Crystal data for 7·dme, C222H126O24Yb3 (M = 3696.34 g/mol): monoclinic, space group P21/n (no. 14), a = 13.268(3) Å, b = 23.619(5) Å, c = 68.846(14) Å, β = 93.70(3)°, V = 21530(8) Å3, Z = 4, T = 100 K, μ(Synchrotron) = 1.351 mm−1, Dcalc = 1.140 g/cm3, 236 789 reflections measured (2.092° ≤ 2Θ ≤ 49.998°), 36 013 unique (Rint = 0.1092, Rsigma = 0.0657), which were used in all calculations. The final R1 was 0.0972 (I > 2σ(I)), and wR2 was 0.2790 (all data). Since 1xlattice DME was squeezed during refinement and H-atoms of coordinated DME were not included in the refinement due to high thermal motion of the C-atoms, thus causing refinement K


Article

Organometallics

Ed. 2009, 48, 1117−1121. (e) MacDonald, M. R.; Bates, J. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2013, 135, 9857−9868. (f) MacDonald, M. R.; Bates, J. E.; Fieser, M. E.; Ziller, J. W.; Furche, F.; Evans, W. J. J. Am. Chem. Soc. 2012, 134, 8420−8423. (g) Cassani, M. C.; Duncalf, D. J.; Lappert, M. F. J. Am. Chem. Soc. 1998, 120, 12958−12959. (h) Evans, W. J. Inorg. Chem. 2007, 46, 3435−3449. (2) Evans, W. J.; Hughes, L. A.; Hanusa, T. P. J. Am. Chem. Soc. 1984, 106, 4270−4272. (3) Evans, W. J.; Bloom, I.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1981, 103, 6507−6508. (4) Evans, W. J.; Ulibarri, T. A.; Ziller, J. W. J. Am. Chem. Soc. 1988, 110, 6877−6879. (5) Evans, W. J.; Gonzales, S. L.; Ziller, J. W. J. Am. Chem. Soc. 1991, 113, 7423−7424. (6) Evans, W. J.; Forrestal, K. J.; Ziller, J. W. J. Am. Chem. Soc. 1998, 120, 9273−9282. (7) (a) Evans, W. J.; Davis, B. L. Chem. Rev. 2002, 102, 2119−2136. (b) Evans, W. J.; Perotti, J. M.; Kozimor, S. A.; Champagne, T. M.; Davis, B. L.; Nyce, G. W.; Fujimoto, C. H.; Clark, R. D.; Johnston, M. A.; Ziller, J. W. Organometallics 2005, 24, 3916−3931. (8) Mueller, T. J.; Fieser, M. E.; Ziller, J. W.; Evans, W. J. Chem. Sci. 2011, 2, 1992−1996. (9) Sitzmann, H.; Dezember, T.; Schmitt, O.; Weber, F.; Wolmershäuser, G.; Ruck, M. Z. Anorg. Allg. Chem. 2000, 626, 2241−2244. (10) Sitzmann, H.; Dezember, T.; Ruck, M. Angew. Chem., Int. Ed. 1998, 37, 3113−3116. (11) (a) Sitzmann, H.; Weber, F.; Walter, M. D.; Wolmershäuser, G. Organometallics 2003, 22, 1931−1936. (b) Williams, R. A.; Tesh, K. F.; Hanusa, T. P. J. Am. Chem. Soc. 1991, 113, 4843−4851. (c) Visseaux, M.; Barbier-Baudry, D.; Blacque, O.; Hafid, A.; Richard, P.; Weber, F. New J. Chem. 2000, 24, 939−942. (12) (a) Hanusa, T. P. 2.02 - Alkaline Earth Organometallics. In Comprehensive Organometallic Chemistry III; Robert, H. C.; Mingos, D. M. P., Eds.; Elsevier: Oxford, 2007; pp 67−152. (b) Edelmann, F. T.. 4.01 - Complexes of Group 3 and Lanthanide Elements. In Comprehensive Organometallic Chemistry III; Robert, H. C.; Mingos, D. M. P., Eds.; Elsevier: Oxford, 2007; pp 1−190. (13) (a) Selikhov, A. N.; Mahrova, T. V.; Cherkasov, A. V.; Fukin, G. K.; Larionova, J.; Long, J.; Trifonov, A. A. Organometallics 2015, 34, 1991−1999. (b) Selikhov, A. N.; Cherkasov, A. V.; Fukin, G. K.; Trifonov, A. A.; del Rosal, I.; Maron, L. Organometallics 2015, 34, 555−562. (c) Momin, A.; Carter, L.; Yang, Y.; McDonald, R.; Essafi, S.; Nief, F.; Del Rosal, I.; Sella, A.; Maron, L.; Takats, J. Inorg. Chem. 2014, 53, 12066−12075. (14) Field, L. D.; Lindall, C. M.; Masters, A. F.; Clentsmith, G. K. B. Coord. Chem. Rev. 2011, 255, 1733−1790. (15) Forsyth, C. M.; Deacon, G. B.; Field, L. D.; Jones, C.; Junk, P. C.; Kay, D. L.; Masters, A. F.; Richards, A. F. Chem. Commun. 2006, 1003−1005. (16) Deacon, G. B.; Forsyth, C. M.; Jaroschik, F.; Junk, P. C.; Kay, D. L.; Maschmeyer, T.; Masters, A. F.; Wang, J.; Field, L. D. Organometallics 2008, 27, 4772−4778. (17) (a) Harder, S.; Naglav, D.; Ruspic, C.; Wickleder, C.; Adlung, M.; Hermes, W.; Eul, M.; Pöttgen, R.; Rego, D. B.; Poineau, F.; Czerwinski, K. R.; Herber, R. H.; Nowik, I. Chem. - Eur. J. 2013, 19, 12272−12280. (b) Ruspic, C.; Moss, J. R.; Schürmann, M.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 2121−2126. (c) Orzechowski, L.; Piesik, D. F. J.; Ruspic, C.; Harder, S. Dalton. Trans. 2008, 4742−4746. (18) (a) Deacon, G. B.; Jaroschik, F.; Junk, P. C.; Kelly, R. P. Organometallics 2015, 34, 2369−2377. (b) Deacon, G. B.; Jaroschik, F.; Junk, P. C.; Kelly, R. P. Chem. Commun. 2014, 50, 10655−10657. (19) (a) Starynowicz, P. Polyhedron 2003, 22, 337−345. (b) Higashiyama, N.; Nakamura, H.; Mishima, T.; Shiokawa, J.; Adachi, G. J. Electrochem. Soc. 1991, 138, 594−598. (c) Higashiyama, N.; Takemura, K.; Kimura, K.; Adachi, G.-y. Inorg. Chim. Acta 1992, 194, 201−206. (20) (a) Pan, C.-L.; Pan, Y.-S.; Wang, J.; Song, J.-F. Dalton. Trans. 2011, 40, 6361−6363. (b) Marks, S.; Heck, J. G.; Habicht, M. H.; Oña-Burgos, P.; Feldmann, C.; Roesky, P. W. J. Am. Chem. Soc. 2012,

issues, the correct formula and molecular weight for compound 7.dme are: C234 H276 O30 Yb3 and formula weight is 4087.88 g/mol. Crystal data for 8·thf: C45H53IO4Sm (M = 935.12 g/mol); monoclinic, space group P21/c (no. 14), a = 21.583(4) Å, b = 10.701(2) Å, c = 17.660(4) Å, β = 104.45(3)°, V = 3949.7(14) Å3, Z = 4, T = 100 K, μ(Synchrotron) = 2.310 mm−1, Dcalc = 1.573 g/cm3, 67 218 reflections measured (1.948° ≤ 2Θ ≤ 57.546°), 9322 unique (Rint = 0.0400, Rsigma = 0.0196), which were used in all calculations. The final R1 was 0.0323 (I > 2σ(I)), and wR2 was 0.0842 (all data).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00842. Crystallographic data for 2 (CIF) Crystallographic data for 3·6thf (CIF) Crystallographic data for 4 (CIF) Crystallographic data for 5·1.5dme (CIF) Crystallographic data for 7·dme (CIF) Crystallographic data for 8·thf (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail (G. B. Deacon): [email protected]. *E-mail (F. Jaroschik): fl[email protected]. *E-mail (P. C. Junk): [email protected]. Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.B.D. and P.C.J. gratefully acknowledge the ARC for funding (DP130100152), which supported the work of D.P.D., D.W., R.P.K., and J.W. T.D.M.B. would like to thank the ARC for funding (DP130101861/160101640), which supported the work of R.P.C. F.J. would like to acknowledge financial support from the CNRS and the Université de Reims, ChampagneArdenne, France. X.F.L.G. would like to thank Ecole Polytechnique and CNRS for supporting this research. G.L. acknowledges the Université de Reims, Champagne-Ardenne, France, for financial support. A.M. is grateful for funding from the following sources: PlAneT CPER project, CNRS, Conseil Regional Champagne Ardenne, Ministry of Higher Education and Research (MESR), and EU-program FEDER. D.P.D., R.P.C., and R.P.K. would like to thank the Faculty of Science for Dean’s Postgraduate Research Scholarships. D.W. is grateful for an APA. We gratefully acknowledge Dr. Simon Harris for MALDI-TOF MS studies. Parts of this research were undertaken on the MX1: Macromolecular Crystallography beamline at the Australian Synchrotron, Victoria, Australia.

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REFERENCES

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M


Divalent Tetra- and Penta-phenylcyclopentadienyl Europium and

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