Coordination Polymers of Polyphenyl-Substituted Potassium Cyclopentadienides

A series of potassium salts of di- and tri-arylsubstituted cyclopentadienes has been obtained by the metalation of the corresponding cyclopentadienes with benzylpotassium in THF media. Crystals of all compounds, afforded by recrystallization from THF/hexane, diglyme-THF/hexane and toluene/hexane mixtures, have been studied by X-ray diffraction. All studied potassium cyclopentadienides exhibit the luminescence at room temperature and overall quantum yield of photoluminescence for potassium salt of diarylsubstituted cyclopentadiene is 18%.


Introduction
The cyclopentadienyl anion, its substituted derivatives and various analogs (e.g., indenyl, fluorenyl, etc.) are probably the most frequently used ligands in organometallic chemistry since the middle of the last century [1][2][3][4]. The synthesis of cyclopentadienyl complexes of d-and f-metals usually requires the use of cyclopentadienyl derivatives of alkali metals. Sodium and lithium cyclopentadienides are the most popular, while potassium salts of cyclopentadienes are used much less frequently in the synthesis of f-element cyclopentadienyl complexes [5]. Metalation of various cyclopentadienes, using organopotassium compounds, is more attractive due to the greater basicity of organopotassium compounds compared to RLi and RNa compounds [6]. Despite this, potassium cyclopentadienides still find limited use in synthesis, probably due to traditional ideas about the low availability of organopotassium compounds compared to lithium and sodium derivatives. Meanwhile, recently introduced convenient methods for the synthesis of different organopotassium compounds, for example, benzyl potassium, have significantly increased the availability of potassium cyclopentadienides, which makes them increasingly popular in the synthesis of Cp derivatives of d-and f-organometallic compounds [7][8][9]. However, little is known about the structure of these precursors, sometimes even the exact composition of these reagents (for example, the number of coordinated donor ligands) is questionable. The availability of information on the structure and properties of various potassium cyclopentadienides should promote the development of methods for the synthesis of these important precursors.
In this work, we studied the structural features of a number of polyaryl-substituted potassium cyclopentadienides (Scheme 1), as well as their main photophysical properties, which may be useful in comparative studies of the photophysical properties of lanthanide cyclopentadienyl complexes obtained from the corresponding potassium salts discussed here.
Molecules 2022, 27, x FOR PEER REVIEW 2 of 20 increased the availability of potassium cyclopentadienides, which makes them increasingly popular in the synthesis of Cp derivatives of d-and f-organometallic compounds [7][8][9]. However, little is known about the structure of these precursors, sometimes even the exact composition of these reagents (for example, the number of coordinated donor ligands) is questionable. The availability of information on the structure and properties of various potassium cyclopentadienides should promote the development of methods for the synthesis of these important precursors.
In this work, we studied the structural features of a number of polyaryl-substituted potassium cyclopentadienides (Scheme 1), as well as their main photophysical properties, which may be useful in comparative studies of the photophysical properties of lanthanide cyclopentadienyl complexes obtained from the corresponding potassium salts discussed here.

Scheme 1.
Structures of the cyclopentadienyl-anions discussed in this article.

Synthesis and Crystallization
Synthesis of polyarylcyclopenadienyl potassium tetrahydrofuranates (KCp'(THF)x, 1-4, Scheme 2) is straightforward and involves a reaction between benzyl potassium and corresponding aryl-substituted cyclopentadiene in THF. The use of benzylpotassium for the metalation of polyarylcyclopentadienes has significant advantages over other reagents, such as potassium hydride, since it allows the reaction to be carried out in a homogeneous system with 100% cyclopentadiene conversion to a target salt. The 1 H and 13 C{ 1 H} NMR spectra for in-situ-generated K(1,3-Ph2C5H3) in DMSOd6 [10] have been reported earlier and revealed non-rigid behavior of the anion, namely, free rotation of Ph-groups around Cipso-Cp-Cipso-Ph bonds. The 1 H and 13 C NMR spectral data for potassium derivatives 1a, 2, 3, 4a in THFd8 media fully confirm this idea for all studied polyarylcyclopentadienide anions (Supplementary Materials: see Figures S9-S27 for 1D and 2D spectra). The 1 H NMR data for 2 and 3 additionally indicate that vacuum drying Scheme 1. Structures of the cyclopentadienyl-anions discussed in this article.

Synthesis and Crystallization
Synthesis of polyarylcyclopenadienyl potassium tetrahydrofuranates (KCp'(THF) x , 1-4, Scheme 2) is straightforward and involves a reaction between benzyl potassium and corresponding aryl-substituted cyclopentadiene in THF. The use of benzylpotassium for the metalation of polyarylcyclopentadienes has significant advantages over other reagents, such as potassium hydride, since it allows the reaction to be carried out in a homogeneous system with 100% cyclopentadiene conversion to a target salt. increased the availability of potassium cyclopentadienides, which makes them increasingly popular in the synthesis of Cp derivatives of d-and f-organometallic compounds [7][8][9]. However, little is known about the structure of these precursors, sometimes even the exact composition of these reagents (for example, the number of coordinated donor ligands) is questionable. The availability of information on the structure and properties of various potassium cyclopentadienides should promote the development of methods for the synthesis of these important precursors.
In this work, we studied the structural features of a number of polyaryl-substituted potassium cyclopentadienides (Scheme 1), as well as their main photophysical properties, which may be useful in comparative studies of the photophysical properties of lanthanide cyclopentadienyl complexes obtained from the corresponding potassium salts discussed here. Scheme 1. Structures of the cyclopentadienyl-anions discussed in this article.

Synthesis and Crystallization
Synthesis of polyarylcyclopenadienyl potassium tetrahydrofuranates (KCp'(THF)x, 1-4, Scheme 2) is straightforward and involves a reaction between benzyl potassium and corresponding aryl-substituted cyclopentadiene in THF. The use of benzylpotassium for the metalation of polyarylcyclopentadienes has significant advantages over other reagents, such as potassium hydride, since it allows the reaction to be carried out in a homogeneous system with 100% cyclopentadiene conversion to a target salt.  [10] have been reported earlier and revealed non-rigid behavior of the anion, namely, free rotation of Ph-groups around C ipso-Cp -C ipso-Ph bonds. The 1 H and 13 C NMR spectral data for potassium derivatives 1a, 2, 3, 4a in THF d8 media fully confirm this idea for all studied polyarylcyclopentadienide anions (Supplementary Materials: see Figures S9-S27 for 1D and 2D spectra). The 1 H NMR data for 2 and 3 additionally indicate that vacuum drying of these compounds leads to partial loss of coordinating THF molecules to provide formulas K(1,2,4-Ph 3 C 5 H 2 )(THF) 0.4 and K[1,2-Ph 2 -4-(2-MeOC 6 H 4 )C 5 H 2 ](THF) 0.6 , respectively.
The potassium derivatives studied in this work display 1D and 2D-coordination polymer structures (1a, 2b, 3, 4a and 2a, correspondingly, see Scheme 3). It might be noted that in 4a coordinated diglyme is partially replaced with THF (not shown in Scheme 3, see Section 2.2.5 below). The following description is arranged according to similarity of their crystal structures.

Luminescent Studies
All the studied compounds exhibit luminescence at 77 and 300 K. The emission bands are very wide, their maxima are ~435 nm, 460 nm, 455 nm and 465 nm for 1, 2b, 3 and 4a, respectively, at 300 K (Figure 8). Compared to emission maximum of 1, the other maxima are redshifted (2b, 3 and 4a). These data are expected considering that the increase in the number of phenyl substituents leads to decrease in the energy of the emission band, which

Luminescent Studies
All the studied compounds exhibit luminescence at 77 and 300 K. The emission bands are very wide, their maxima are~435 nm, 460 nm, 455 nm and 465 nm for 1, 2b, 3 and 4a, respectively, at 300 K ( Figure 8). Compared to emission maximum of 1, the other maxima are redshifted (2b, 3 and 4a). These data are expected considering that the increase in the number of phenyl substituents leads to decrease in the energy of the emission band, which we observed earlier for lanthanide complexes with di-, triand tetraphenyl substituted Cp [9]. Interestingly, these maxima are relatively similar in the case of 2b, 3 and 4a (where Cp ring contains three phenyl substituents or two phenyl-and one methoxyphenyl substituents, respectively). The overall quantum yields of photoluminescence for compounds 1, 2b, 3 and 4a are 18, 5, 3 and 1%, respectively. To understand the difference obtained in these values, it should be noted that in 1, the dihedral angle between the Cp and Ph planes is the smallest (16.07(8) • ), which reflects the coplanarity of substitutions with the Cp plane and, as a consequence, extends the length of π-system. The luminescence excitation spectra of the compounds 1, 2b, 3 and 4a exhibit broad bands in the region 250-450 nm ( Figure 9). Namely, a broad band centered at ~270 nm is tentatively assigned to unsubstituted Cp, while the band at ~370-415 nm is probably due to the attached phenyl rings and intraligand charge transfer (ILCT) state [9,30,31]. As it was shown earlier, the energy of the band assigned to the phenyl ring of substituted Cp depends on the number of these rings and their coplanarity with Cp. The most intense ILCT band is observed in 4a where the dihedral angle of one Ph ring is relatively small 17.1(3)° and Cp-(4-MeOC6H4) dihedral angles range from 1.9(4)° to 8.6(2)°. These data point to high degree of coplanarity of substitutions with Cp. Thus, the presence of one phenyl with methoxy group and two unsubstituted phenyls leads to significant nonequivalence of aryl groups in this ligand and promotes the ILCT. The luminescence excitation spectra of the compounds 1, 2b, 3 and 4a exhibit broad bands in the region 250-450 nm ( Figure 9). Namely, a broad band centered at~270 nm is tentatively assigned to unsubstituted Cp, while the band at~370-415 nm is probably due to the attached phenyl rings and intraligand charge transfer (ILCT) state [9,30,31]. As it was shown earlier, the energy of the band assigned to the phenyl ring of substituted Cp depends on the number of these rings and their coplanarity with Cp. The most intense ILCT band is observed in 4a where the dihedral angle of one Ph ring is relatively small 17.1(3) • and Cp-(4-MeOC 6 H 4 ) dihedral angles range from 1.9(4) • to 8.6(2) • . These data point to high degree of coplanarity of substitutions with Cp. Thus, the presence of one phenyl with methoxy group and two unsubstituted phenyls leads to significant non-equivalence of aryl groups in this ligand and promotes the ILCT.
depends on the number of these rings and their coplanarity with Cp. The most intense ILCT band is observed in 4a where the dihedral angle of one Ph ring is relatively small 17.1(3)° and Cp-(4-MeOC6H4) dihedral angles range from 1.9(4)° to 8.6(2)°. These data point to high degree of coplanarity of substitutions with Cp. Thus, the presence of one phenyl with methoxy group and two unsubstituted phenyls leads to significant nonequivalence of aryl groups in this ligand and promotes the ILCT.

General Experimental Remarks
The potassium arylcyclopentadienyl complexes are extremely sensitive, even to the traces of oxygen and moisture. All of these compounds completely or significantly decompose when exposed to air for less than 1-2 s. All operations with potassium arylcyclopentadienides, including their synthesis, isolation, crystallization, preparation of samples for NMR spectroscopy, luminescence studies and X-ray diffraction analysis, were carried out inside a Specs-GB2 argon glove box. The box atmosphere contained less than 1 ppm of oxygen and water. Samples for NMR spectroscopy were prepared in J.Young NMR tubes, THF-d8 was vacuum transferred into the J.Young NMR-tube.
Samples for luminescence studies were placed in cuvettes, which were quartz tubes 4 mm in diameter, sealed at one end and equipped with a stopcock. The filled cuvettes were removed from the glove box, attached to a vacuum line and evacuated (10 −2 torr). Then, they were sealed using a quartz-blowing torch. To prepare samples for X-ray diffraction analysis, crystals of the obtained compounds were placed in liquid paraffin in the glove box into the screw-capped vials. Liquid paraffin was distilled over sodium in high vacuum (2−5 × 10 −2 torr) prior to use.

X-ray Structure Determination
Experimental intensities of reflections were measured on a Bruker SMART APEX II platform, using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) in an ω-scan mode. The collected data were integrated with the SAINT program [36]. Absorption corrections based on measurements of equivalent reflections were carried out by SADABS (multi-scan methods) [37]. The structures were solved by direct methods with the SHELXT program [38] and refined by full matrix least-squares on F 2 with SHELXL [39]. Positions of all non-H atoms were found from electron difference density maps and refined with individual anisotropic displacement parameters. Positions and individual isotropic displacement parameters for phenyl and cyclopentadienyl H-atoms in 1 and 3 were refined to obtain better quality crystallographic models. The other H-atoms in 1 and 3 and all H-atoms in 2a, 2b and 4a were positioned geometrically and refined as riding atoms with relative isotropic displacement parameters. A rotating group model was applied for methyl groups. The SHELXTL program suite [38] and the Mercury program [40] were used for molecular graphics. Since the presence of poorly resolved disordered lattice solvent molecules along with a rather large volume of the unit cell in 2b resulted in a poor crystallographic model; therefore, the non-coordinating molecules were removed by the SQUEEZE method [28] implemented in the PLATON program [41], which substantially improved the model. However, insufficient residual electron density did not allow us to resolve disorder of Oand C-atoms for some of coordinated THF molecules, resulting in level B alerts.
Crystal data, data collection and structure refinement details for 1a, 2a, 2b, 3 and 4b are summarized in Table 6. Selected bond distances and angles, as well as more detailed X-ray data refinement, are presented in the Supplementary Materials for this paper. The structures have been deposited at the Cambridge Crystallographic Data Center with the reference CCDC numbers 2206799-2206803, and they also contain the supplementary crystallographic data. These data can be obtained free of charge from the CCDC via http://www.ccdc.cam.ac.uk/data_request/cif (accessed on 11th August 2022).

Optical Measurements
All samples studied were solid stated. Steady-state luminescence and excitation measurements in the visible region were performed with Fluorolog FL 3-22 spectrometer from Horiba-Jobin-Yvon-Spex which has a 450 W xenon lamp as the excitation source and R-928 photomultiplier at 77 and 300 K. The quantum yield measurements were carried out on solid samples with a Spectralone-covered G8 integration sphere (GMP SA, Fällanden, Switzerland), according to the absolute method of Wrighton [42][43][44]. Each sample was measured several times under slightly different experimental conditions. The estimated error for quantum yields is ±10%.

Conclusions
The main result of the work is the demonstration of the synthetic availability of potassium cyclopentadienyl derivatives, which are convenient starting compounds for the preparation of arylcyclopentadienyl complexes of f-and d-elements, as well as determination and interpretation of their structures. These results open the way to well-defined organopotassium precursors for organometallic synthesis. The obtained data on the photophysical properties of these compounds can be useful in a comparative analysis of the photophysical properties of arylcyclopentadienyl complexes of transition and rare earth metals. Indeed, the presented results shed light on the influence of the degree coplanarity of substituents with Cp plane on the luminescence efficiency and allow the role of intra ligand charge transfer states in the energy transfer process to be highlighted.