Bis(1,2,4-triphenylcyclopentadienyl) Terbium 4,4,4-trifluoro-1-phenylbutane-1,3-dionate

Abstract

A new bis(cyclopentadienyl) terbium(III) complex with 1,2,4-triphenylcyclopentadienyl and 4,4,4-trifluoro-1-phenylbutane-1,3-dionate ligands was synthesized. Single-crystal X-ray analysis revealed a mononuclear bis(cyclopentadienyl) complex with a diketonate ligand in the bisector plane. The compound under study exhibits a ligand ligand charge transfer state (LLCT), according to optical spectroscopy and crystallographic data.

1. Introduction

Lanthanide cyclopentadienyl complexes are among the most prevalent classes of rare earth organometallics. The use of complexes with substituted cyclopentadienyl (Cp) ligands in rare-earth chemistry has expanded even further [1,2,3,4,5]. Nevertheless, alkyl- and silyl-substituted cyclopentadienyl ligands are the primary limit of the spectrum of such cyclopentadienyl derivatives. There is significantly less research on aryl-substituted cyclopentadienyl complexes of lanthanides [6,7,8]. It should be mentioned that polyarylcyclopentadienyl ligands seem to be even more promising in the design of organolanthanides, allowing for the implementation of fundamental stability principles for these compounds [1,9]. We recently reported on the utilization of aryl-substituted cyclopentadienyl ligands in the design of several types of rare earth cyclopentadienyl complexes [10,11] (Scheme 1). One of the most fascinating is the bis(triphenycyclopentadienyl)lanthanide chloride complexes, which enable the substitution of a chloride for an organic ligand in a straightforward salt metathesis reaction. Numerous organolanthanide compounds can be easily synthesized using this reaction.

In addition, due to their conjugated π-system, high denticity (or coordination capacity), and an energy-level alignment that is very close to ideal, aryl-substituted cyclopentadienyl (Cp) ligands have become attractive options for light-harvesting units in Ln³⁺ ion photosensitization [10]. Despite these advantageous features, the intrinsic quantum yields of terbium complexes containing aryl-substituted Cp ligands range from 40% to 90%, whereas the photoluminescence quantum yield varies greatly, typically between 10% and 60%. A traditional sensitizing organic ligand, 4,4,4-trifluoro-1-phenylbutane-1,3-dionate (phdk), was chosen as an auxiliary ligand for the synthesis of new lanthanide complexes in order to prevent the formation of dinuclear or tetranuclear structures and to enhance the luminescence characteristics. The molecular structure and synthesis of bis(1,2,4-triphenylcyclopentadienyl)terbium(III) 4,4,4-trifluoro-1-phenylbutane-1,3-dionate are presented here, along with the luminescence sensitization peculiarities in this complex.

2. Results and Discussion

2.1. Synthesis and Structure

The reaction of [Cp^Ph3₂TbCl₂K]₂ (1) [10] with the stoichiometric amount of potassium 4,4,4-trifluoro-1-phenylbutane-1,3-dionate, obtained in situ from 4,4,4-trifluoro-1-phenylbutane-1,3-dione and potassium bis(trimethylsilyl)amide, gave the bis(1,2,4-triphenylcyclopentadienyl) terbium complex [Cp^Ph3₂Tb(phdk)] (2), where phdk = 4,4,4-trifluoro-1-phenylbutane-1,3-dionate (Scheme 2).

Complex 2 was obtained as yellow-orange crystals of X-ray quality by crystallization from a toluene solution by slow addition of hexane. The compound demonstrates stability for months in a glovebox under anaerobic conditions, with no observed discoloration of the crystals. At the same time, when exposed to air, it undergoes decomposition within seconds due to oxidation and hydrolysis.

According to the X-ray diffraction analysis, complex 2, which has 4,4,4-trifluoro-1-phenylbutane-1,3-dionate in the bisector plane, is built as a bent sandwich complex (Figure 1). The Tb(O-C-CH-C-O) fragment is flat within 0.028 Å. The primary structural parameters of 2 are shown in

Table 1. Selected structural parameters of complex 2.

Disnances and Angles	2	1 [10]
Tb-Cpcent (Å)	2.385, 2.400	2.422, 2.429
Cpcent-Tb-Cpcent (◦)	133.9	132.0
Tb-CCp(range) (Å)	2.645(1)–2.721(1)	2.656(3)–2.751(3)
Tb-CCp(average) (Å)	2.680(1)	2.691(3)
Tb-O (Å)	2.2126(9), 2.2428(8)	-
O-Tb-O, (Cl–TbCl) (◦)	76.64(3)	91.72(2)

. For comparison, the initial chloride complex 1 structural parameters are given.

In compound 2, the metallocene fragment {Cp^Ph3₂Tb} exhibits structural characteristics that are comparable to those of its precursor 1. The magnitude of the X-Tb-X angle (X = O, Cl) is the only notable difference between the metrics of the structures of these complexes. It is apparent that the rather rigid structure of the planar, conjugated six-membered ring Tb(O-C-CH-C-O) is the reason for the decrease in the value of this angle for 2. In the structure of 2, there is a short contact (3.301(1) Å) between the carbonyl carbon atom (C50) of the diketonate ligand and a carbon atom (C7) of one of the phenyl groups of the Cp^Ph3 ligand. We found a relatively small number of weak intermolecular interactions when we examined the structure of 2: F1..C54 (3.158Å), F2..H16 (2.60Å), C1..H53 (2.78Å), C2..H53 (2.74Å), C8..H5 (2.83 Å), C16..H55 (2.78Å), C17..H55 (2.83Å), C22..H15 (2.90Å), C31..H17 (2.85Å), C44..H31 (2.88Å). Calculations of the Hirshfeld surface [12,13] indicate large contributions of C..H/H..C (28.2%) and H..H (57.8%) contacts into the overall balance of non-covalent intermolecular interactions. Both facts point out that the metal center and its environment (Tb-X bonds) of the complex are sufficiently sterically shielded.

2.2. Photophysical Properties

According to the theory of sensitized luminescence of the Ln ion in complexes, the energy absorbed by the ligands is transferred to the excited levels of the Ln ion via singlet (S₁) and triplet (T₁) states (triplet-mediated scheme S₁ → T₁ → Ln³⁺ ion). The energy of the lowest excited single and triplet states is therefore crucial. The UV-Vis absorption spectra of 4,4,4-trifluoro-1-phenylbutane-1,3-dione in acetonitrile solution demonstrated two broad absorption bands with peaks at 256 and 326 nm assigned to the β-diketone moiety’s π–π* transition and the phenyl group’s ¹π–π* transition, respectively [14]. The energies of the S₁ and T₁ states of coordinated Cp^Ph3 are 360 and 435 nm, respectively, according to the previous study [10,15]. The energy of the lowest excited T₁ state of coordinated phdk-ligand is 467 nm [16] while the energy of the resonant ⁵D₄ level of the Tb³⁺ ion is 490 nm.

From the other side, the Tb complexes containing only one type of the π-bonded antenna ligands (Cp^Ph2, Cp^Ph3, and Cp^Ph4) demonstrate excellent luminescence characteristics, including the high quantum yield of photoluminescence (up to 60%) [10,11] (Figures S2 and S3). It was unexpected that complex 2 lacked the effective luminescence sensitization of the Tb ion. We need to emphasize two key factors in this context. According to X-ray data in the structure of 2, there is a short contact (3.301(1) Å) between the carbonyl carbon atom of the diketonate ligand and a carbon atom of one of the phenyl groups of the Cp^Ph3 ligand. Such strong contact can induce the appearance of low-lying charge transfer states, namely ligand–ligand charge transfer (LLCT) one [17,18]. A similar LLCT state observed in Ln complexes (Ln = Tb, Gd, Nd) containing Cp^Ph3 and 2,2′-bipyridine (bipy) ligands was caused by the strong intramolecular π^…π stacking interaction between the phenyl ring of Cp^Ph3 ligand and the phenyl ring of the bipy ligand (the shortest intramolecular C···C and N···C contacts are 3.18 and 3.20 Å in complex [Cp^Ph3LnCl₂(bipy)(THF)] [17]. This interaction leads to the strong luminescence quenching. The second fact is the relatively low energy of the T1 state of the phdk-ligand, which promotes the luminescence quenching of the Tb³⁺ ion due to the back energy transfer process that was observed in other Tb complexes with phdk-ligands [19,20]. Therefore, the peculiarities found in the sensitization luminescence of the Tb ion in the complex studied open a new challenge for using this type of compound for the luminescence sensitization of the Ln ions in the NIR region [17,20]. Indeed, the presence of a low-lying charge transfer state can be used for the effective sensitization by means of the scheme S₁ → CT state → Ln³⁺ ion. It is important to emphasize that, in addition to the classical triplet-mediated scheme S₁ → T₁ → Ln³⁺ ion, the singlet-mediated pathway involving the charge transfer excited states for the Ln sensitization is also well known [18,21].

3. Materials and Methods

3.1. General Considerations

All synthetic manipulations were carried out in prepurified argon atmosphere in anhydrous solvent media, using a glovebox. Tetrahydrofuran was predried over NaOH and distilled from potassium/benzophenone ketyl. Hexane was distilled from Na/K alloy/benzophenone ketyl. Toluene was distilled from sodium/benzophenone ketyl. KHMDS was synthesized from HN(SiMe₃)₂ and KH. [Cp^Ph3₂TbCl₂K]₂ and 4,4,4-trifluoro-1-phenylbutane-1,3-dione were prepared according to published procedures [10,22]. Elemental analyses were performed with a Thermo Scientific FLASH 2000 elemental CHNS/O analyzer (Milan, Italy). The IR spectrum was acquired using an IROS 05 Multipurpose FTIR spectrometer (Moscow, Russia) equipped with an ATR accessory. The solid sample was covered with Nujol to transfer the compound from the glovebox to the spectrometer to prevent oxidation.

3.2. Synthesis

Bis(1,2,4-triphenylcyclopentadienyl) Terbium 4,4,4-trifluoro-1-phenylbutane-1,3-dionate (2)

A solution of KHMDS (0.030 g, 0.15 mmol) in 2 mL of THF was added dropwise to the 2 mL THF solution of 4,4,4-trifluoro-1-phenylbutane-1,3-dione (0.032 g, 0.15 mmol). The reaction mixture was stirred for 4 h. The resultant solution was then added dropwise to a solution of [Cp^Ph3₂TbCl₂K]₂ (0.128 g, 0.075 mmol) in 3 mL of THF. The reaction mixture was stirred for 5 h. The precipitate was separated by centrifugation, and the solution was evaporated to dryness. The obtained sticky paste was dissolved in 4 mL of toluene and stirred for 30 min. During this time, a slightly colored powder, presumably Cp^Ph3K, precipitated. The precipitate was removed by centrifugation, and the yellow supernatant solution was layered with 6 mL of hexane to initiate crystallization. Yellow-orange crystals of Tb1 were obtained after one week. The crystals were dried under dynamic vacuum, yielding 0.023 g (0.024 mmol, 16%) of Tb2. Calcd for C₅₆H₄₀O₂F₃Tb: C, 70.01%; H, 4.20%. Found: C, 67.50%; H, 3.83%. Despite numerous attempts, including an examination of triply recrystallized materials, elemental analysis for 2 gives an underestimated result for carbon. It is commonly known that incomplete combustion and/or carbide formation are the causes of the poor combustion analysis of lanthanide organometallics [23]. IR (Nujol, cm⁻¹): ν = 1595 (s, C=O), 1142 (s, C–F), 756 (vs, CF₃ symm.)

3.3. X-Ray Diffraction Studies

Crystals of 2 (C₅₆H₄₀F₃O₂Tb, M = 960.861) are monoclinic, space group P2_1/c, at 100.0 (3) K: a = 17.51855(14) Å, b = 13.95895(12) Å, c = 17.77056(15) Å, α = 90°, β = 95.9470(8)°, γ = 90°, V = 4322.24(6) ų, Z = 4, dcalc = 1.477 g_cm⁻³, μ(MoK_α) = 1.689 mm⁻¹, and F(000) = 1936. Intensities of 129,500 reflections were collected. A total of 18,484 independent reflections [Rint = 0.0204] were used in further refinement.

X-ray diffraction data were collected at 100K on a four-circle Rigaku XtaLAB Synergy-S diffractometer (Tokyo, Japan) equipped with a HyPix-6000HE area-detector (kappa geometry, shutterless ω-scan technique), using monochromatized Mo K_α-radiation. The intensity data were integrated and corrected for absorption and decay by the CrysAlisPro program (Version 1.171.42 2023) [24]. The structure was solved by direct methods using SHELXT-2014/5 [25] and refined on F² using the OLEX2 program [26]. All non-hydrogen atoms were refined with individual anisotropic displacement parameters. All hydrogen atoms were placed in ideal calculated positions and refined as riding atoms with relative isotropic displacement parameters. Atomic coordinates, bond lengths and angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Center with deposition number CCDC 2499341.

3.4. Optical Measurements

Luminescent measurements in the visible region were performed with a Horiba-Jobin-Yvon-Spex Fluorolog FL 3–22 spectrometer, which has a 450 W xenon arc lamp as the excitation source for steady-state measurements and a 150 W xenon pulse lamp for kinetic experiments. The technique involved the use of specially designed, sealed cylindrical quartz cuvettes for manipulation with air-sensitive compounds. The complex studied was a polycrystalline powder.