Synthesis, Structure, and Electrochemical Properties of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene Derivatives

Heteroleptic 2,3,4,5-tetraphenyl-1-monophosphaferrocene [FeCp(η5-PC4Ph4)] was obtained at a 62% yield through the reaction of lithium 2,3,4,5-tetraphenyl-1-monophosphacyclopentadienide Li(PC4Ph4) (1) with [FeCp(η6-C6H5CH3)][PF6]. The structure of 1-monophosphaferrocene 2 and its W(CO)5-complex 3 were confirmed by multinuclear NMR and single-crystal X-ray diffraction study and further supported by DFT calculations. Cyclic voltammetry demonstrated that [FeCp(η5-PC4Ph4)] 2 has a quasi-reversible oxidation wave. The comparison of the properties of phosphaferrocene 2 with those of W(CO)5-complex 3 shows the possibility of changing the coordination type during oxidation.


Introduction
The discovery of ferrocene [Fe(η 5 -C 5 H 5 ) 2 ] approximately seventy years ago significantly influenced chemical research and provided a key boost for establishing and expanding organometallic chemistry, which has continued to develop rapidly. Over the years of intensive research, the ferrocene unit has been recognized as an extremely versatile platform for ligand design, materials research, medicinal chemistry, and many other research fields [1]. Among the various heterometallocenes reported to date, monophosphaferrocenes are by far the most investigated [2][3][4]. Recently, a facile one-step method for the synthesis of "fully inorganic" ferrocene analogue was reported and [Fe(P 4 ) 2 ] 2− represents the closest allphosphorus derivatives of iron to ferrocene [Fe(η 5 -C 5 H 5 ) 2 ] so far [5]. Phosphaferrocenes are commonly regarded as phosphorus ligands with weaker σ-donor properties than classical tertiary phosphines and stronger π-acceptor ability similar to that of phosphites P(OR) 3 [6]. From a practical standpoint, monophosphaferrocenes have been utilized as chiral ligands in homogeneous and asymmetric catalysis [7][8][9][10][11][12][13][14], as building blocks for multidentate ligand systems [15][16][17][18], and as functional materials for self-assembled monolayers [19,20].
At present, two main protocols have been developed for the preparation of monophosphaferrocenes. The first is the reaction between P-phenyl-phosphole and [CpFe(CO) 2 ] 2 at high temperatures, which was developed by the Mathey workgroup in 1977 [21,22]. The phosphaferrocenes obtained through this route have a tendency to be contaminated by the 2-phenylated derivative appearing through the thermal [1,5]-sigmatropic shift of the P-phenyl substituent onto the phosphole ring. Therefore, this procedure provides a desired product with low yields [23]. The second method of the synthesis of monophosphaferrocenes is the reaction between monophospholide anion and cationic (π-arene)iron(II) complex. In 1986, Wells demonstrated that [(η 6 -mesitylene)FeCp]PF 6 complex playing the role of CpFe + synthon is an excellent precursor for the synthesis of monophosphaferrocenes [24]. Generally, phospholide anions are prepared by the reductive cleavage of the exocyclic C-P bond in P-phenyl-1-monophospholes with lithium metal. However, phenyllithium PhLi is an undesirable by-product, the deactivation of which is necessary. This method has recently been modified by the use of inexpensive aluminum chloride as an in situ-generated phenyllithium scavenger, and thus a 50% yield of desired [(η 6 -mesitylene)FeCp]PF 6 was attained [25].
The structure of 2 was undoubtedly confirmed by the single-crystal X-ray diffraction. Appropriate single crystals were obtained by crystallization from a toluene solution. Complex 2 crystallizes in the orthorhombic space group Pbca with a single molecule in the Scheme 1. Synthesis of 2,3,4,5-tetraphenyl-1-monophosphaferrocene 2 and its tungsten complex 3. The structure of 2 was undoubtedly confirmed by the single-crystal X-ray diffraction. Appropriate single crystals were obtained by crystallization from a toluene solution. Complex 2 crystallizes in the orthorhombic space group Pbca with a single molecule in the asymmetric cell ( Figure 1). The phospholyl (PC 4 ) and cyclopentadienyl (C 5 ) ligands of 2 are almost eclipsed with a turning angle P1-Cnt(PC 4 )-Cnt(C 5 )-C5 of 12.38(6) • (Cnt is centroid), and their two planes form an angle ∠(PC 4 )(C 5 ) equal to 3.14(4) • (Table 1). Selected internuclear distances characterizing the coordination sphere are listed in the caption. The phenyl substituents exhibit a propeller-like arrangement with torsion angles varying from 120.9 • to 140.5 • . All geometrical parameters (bond angles and bond lengths) of 2 are similar to those of the related monophosphaferrocenes with alkyl substituents ( Table 1). It is worth noting that in this series, compound 2 has the shortest Fe-Cnt(PC 4 ) distance, while Fe-Cnt(C 5 ) distances are quite close. Despite the steric volume of four phenyl substituents, the smallest Fe-P distance is also observed for 2. Scheme 1. Synthesis of 2,3,4,5-tetraphenyl-1-monophosphaferrocene 2 and its tungsten complex 3.
The structure of 2 was undoubtedly confirmed by the single-crystal X-ray diffraction. Appropriate single crystals were obtained by crystallization from a toluene solution. Complex 2 crystallizes in the orthorhombic space group Pbca with a single molecule in the asymmetric cell ( Figure 1). The phospholyl (PC4) and cyclopentadienyl (C5) ligands of 2 are almost eclipsed with a turning angle P1-Cnt(PC4)-Cnt(C5)-C5 of 12.38(6)° (Cnt is centroid), and their two planes form an angle ∠(PC4)(C5) equal to 3.14(4)° (Table 1). Selected internuclear distances characterizing the coordination sphere are listed in the caption. The phenyl substituents exhibit a propeller-like arrangement with torsion angles varying from 120.9° to 140.5°. All geometrical parameters (bond angles and bond lengths) of 2 are similar to those of the related monophosphaferrocenes with alkyl substituents (Table 1). It is worth noting that in this series, compound 2 has the shortest Fe-Cnt(PC4) distance, while Fe-Cnt(C5) distances are quite close. Despite the steric volume of four phenyl substituents, the smallest Fe-P distance is also observed for 2.
is the angle between planes of phospholyl and cyclopentadienyl ligands. d The data are given for the first and second (disordered) symmetry-independent molecules, respectively.

Electrochemical Properties of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene Derivatives
The electrochemical properties of monophosphaferrocenes, especially those containing aryl substituents, remain poorly investigated. According to the literature data, the introduction of one phosphorus atom instead of the CH-fragment in ferrocene leads to higher oxidation potentials compared to ferrocene [Fe(η 5 -C 5 H 5 ) 2 ] [41,42]. At the same time, the presence of two or more methyl groups has a slight effect on the HOMO-LUMO gap of monophosphaferrocenes (Table 2). In this work, compounds 2 and 3 were studied by cyclic voltammetry. Compound 2 has a quasi-reversible oxidation wave at a potential of 0.55 V vs. FcH/FcH + , which is 0.49 V more anodic than the literary analogue [(Me 5 Cp)Fe(η 5 -PC 4 Ph 4 )] ( Figure 2). Despite the paucity of literature data on phosphaferrocenes, it is generally accepted that the presence of one phosphorus atom in the structure of the cyclopentadienide ring does not lead to irreversible oxidation processes in a phosphaferrocene solution.
The quasi-reversibility during the oxidation of structure 2 can be associated with the fact that during the formation of Fe II in Fe III , the P-atom could be coordinated to the Fe-atom, as a result of which the re-reduction potential (−0.28 V vs. FcH/FcH + ) is shifted to the negative region (Scheme 2). Previously, the formation of such complexes was demonstrated in the case of phosphanickelocene [43]. A change in the type of coordination can also lead to intramolecular disproportionation, where the charge may not necessarily be stored on the Fe-atom or phospholide ring (Scheme 2). This assumption is visually confirmed by comparing the electrochemical properties of compound 2 and its complex 3 with tungsten, in this case of which quasi-reversibility disappears in cyclic voltammetry. Since the lone pair of P-atom is bounded to W-atom, the intramolecular rearrangement of the phospholide becomes impossible, and thus the stabilization of the oxidized Fe-atom becomes unlikely. Additionally, bulky phenyl fragments do not allow the electrolyte anion to move close enough to stabilize the positive charge, as a result of which an irreversible oxidation wave is observed.
In this work, compounds 2 and 3 were studied by cyclic voltammetry. Compound 2 has a quasi-reversible oxidation wave at a potential of 0.55 V vs. FcH/FcH + , which is 0.49 V more anodic than the literary analogue [(Me5Cp)Fe(η 5 -PC4Ph4)] (Figure 2). Despite the paucity of literature data on phosphaferrocenes, it is generally accepted that the presence of one phosphorus atom in the structure of the cyclopentadienide ring does not lead to irreversible oxidation processes in a phosphaferrocene solution. The quasi-reversibility during the oxidation of structure 2 can be associated with the fact that during the formation of Fe II in Fe III , the P-atom could be coordinated to the Featom, as a result of which the re-reduction potential (−0.28 V vs. FcH/FcH + ) is shifted to the negative region (Scheme 2). Previously, the formation of such complexes was demonstrated in the case of phosphanickelocene [43]. A change in the type of coordination can also lead to intramolecular disproportionation, where the charge may not necessarily be stored on the Fe-atom or phospholide ring (Scheme 2). This assumption is visually confirmed by comparing the electrochemical properties of compound 2 and its complex 3 with tungsten, in this case of which quasi-reversibility disappears in cyclic voltammetry.
Since the lone pair of P-atom is bounded to W-atom, the intramolecular rearrangement of the phospholide becomes impossible, and thus the stabilization of the oxidized Fe-atom becomes unlikely. Additionally, bulky phenyl fragments do not allow the electrolyte anion to move close enough to stabilize the positive charge, as a result of which an irreversible oxidation wave is observed.
It is well known that the oxidation of the ferrocene [Fe(η 5 -C5H5)2] molecule leads to the appearance of a Fe III cation with 3d 5 configuration in a low spin state [44]. Although low spin state Fe III complexes are often observed by ESR (electron paramagnetic resonance) [45,46] and have a g-factor close to the g-factor of the free electron of 2.0023, the ferrocenium cation is ESR-silent at temperatures above 78 K, which is due to the short relaxation time. Indeed, the oxidation of [Fe(η 5 -C5H5)2] in the electrochemical ESR cell did not lead to the appearance of any signals. At the same time, the oxidation of phosphaferrocene 2 leads to the appearance of a single line with magnetic resonance parameters g = 2.0019 and ΔH = 7 G at a potential of 0.55 V (vs. FcH/FcH + ) (Figure 3). We attribute this signal to the phosphaferrocenium cation of 2 in the low-spin state since complexes with high-spin Fe III have a much larger line width [47,48]. The oxidation of the W(CO)5 complex 3 does not lead to the appearance of an ESR signal, which does not provide an unambiguous answer to the question about the state of Fe III in the oxidized form of 3. Such behavior of complex 3 can be explained by the assumption that the relaxation time of the cation of 3 is shorter than that of the cation of 2. It is well known that the oxidation of the ferrocene [Fe(η 5 -C 5 H 5 ) 2 ] molecule leads to the appearance of a Fe III cation with 3d 5 configuration in a low spin state [44]. Although low spin state Fe III complexes are often observed by ESR (electron paramagnetic resonance) [45,46] and have a g-factor close to the g-factor of the free electron of 2.0023, the ferrocenium cation is ESR-silent at temperatures above 78 K, which is due to the short relaxation time. Indeed, the oxidation of [Fe(η 5 -C 5 H 5 ) 2 ] in the electrochemical ESR cell did not lead to the appearance of any signals. At the same time, the oxidation of phosphaferrocene 2 leads to the appearance of a single line with magnetic resonance parameters g = 2.0019 and ∆H = 7 G at a potential of 0.55 V (vs. FcH/FcH + ) (Figure 3). We attribute this signal to the phosphaferrocenium cation of 2 in the low-spin state since complexes with high-spin Fe III have a much larger line width [47,48]. The oxidation of the W(CO) 5 complex 3 does not lead to the appearance of an ESR signal, which does not provide an unambiguous answer to the question about the state of Fe III in the oxidized form of 3. Such behavior of complex 3 can be explained by the assumption that the relaxation time of the cation of 3 is shorter than that of the cation of 2.
The preference for the low-spin state of oxidized species 2 and 3 was also shown quantum-chemically. Thus, geometries of monophosphaferrocene 2 and its tungsten complex 3 have been optimized quantum-chemically together with their cations (Supplementary Materials Tables S1-S6). For cations, two possible spin states have been considered, namely S = 1/2 (low-spin) and S = 5/2 (high-spin). For both low-spin cations, computations predict the elongation of distances between cyclopentadienyl (C 5 ) and phospholyl (PC 4 ) rings and the Fe-atom. The optimization of the high-spin states of 2 and 3 leads to the notable distortion of structures ( Table 3). The substituted phospholyl rings (PC 4 ) "tilt" from the initial position. Energetically, for both cationic forms, the low-spin state is more stable compared to the high-spin state.
signal to the phosphaferrocenium cation of 2 in the low-spin state since complexes with high-spin Fe III have a much larger line width [47,48]. The oxidation of the W(CO)5 complex 3 does not lead to the appearance of an ESR signal, which does not provide an unambiguous answer to the question about the state of Fe III in the oxidized form of 3. Such behavior of complex 3 can be explained by the assumption that the relaxation time of the cation of 3 is shorter than that of the cation of 2. The preference for the low-spin state of oxidized species 2 and 3 was also shown quantum-chemically. Thus, geometries of monophosphaferrocene 2 and its tungsten  The presence of four phenyl rings also significantly lowers the reduction potential of the phospholide ring, and as a result, the HOMO-LUMO gap decreases, which makes them thermodynamically more stable. The tungsten complex 3 has two reduction waves, unlike the phosphaferrocene 2 ( Figure 4). In the literature [49], the reduction of the W(CO) 5 complex of 3,3 ,4,4 -tetramethyl-1,1 -diphosphaferrocene was accompanied by an electrochemicalchemical mechanism. In our case, with only one phospholide ligand, this mechanism is not implemented, although two reduction waves are also observed, because, in this case, the second reduction wave does not coincide with phosphaferrocene 2. The first reduction wave can be attributed to the formation of a radical anion on the phospholide anion (Scheme 3). The shift of the potential in comparison with 2 to the anodic region is associated with the shift in the electron density from the phosphaferrocene fragment to the W(CO) 5 fragment. The second reduction wave probably refers to the reduction of the W(CO) 5 fragment and, under experimental conditions, has time to be fixed without decomposition.

General
The NMR spectra were recorded on a Bruker MSL-400 ( 1 H 400 MHz, 31 P 161.7 M 13 C 100.6 MHz). SiMe4 was used as an internal reference for 1 H and 13 C NMR chem shifts, and 85% H3PO4 as an external reference for 31 P NMR. All experiments were ca out using standard Bruker pulse programs. The infrared (IR) spectra were recorded

General
The NMR spectra were recorded on a Bruker MSL-400 ( 1 H 400 MHz, 31 P 161.7 MHz, 13 C 100.6 MHz). SiMe4 was used as an internal reference for 1 H and 13 C NMR chemical shifts, and 85% H3PO4 as an external reference for 31 P NMR. All experiments were carried out using standard Bruker pulse programs. The infrared (IR) spectra were recorded on a Bruker Vector-22 spectrometer.

DFT Calculations
All calculations were performed with the Gaussian 16 suite of programs [50]. The hybrid PBE0 functional [51] and the Ahlrichs' triple-ζ def-TZVP AO basis set [52] were used for the optimization of all structures. In all geometry optimizations, the D3 approach [53] was applied to describe the London dispersion interactions, as implemented in the Gaussian 16 program.

Electrochemical Measurements
Electrochemical measurements were conducted with a BASi Epsilon EClipse electrochemical analyzer. The program concerned Epsilon-EC-USB-V200 waves. A conventional three-electrode system was used with glassy carbon (GC) or carbon paste electrode (CPE) solutions for powder samples as the working electrode, the Ag/AgCl (0.01 M) electrode as the reference electrode, and a Pt wire as the counter electrode. A 0.1 M Et4NBF4 was used as the supporting electrolyte to determine the current-voltage characteristics.

General
The NMR spectra were recorded on a Bruker MSL-400 ( 1 H 400 MHz, 31 P 161.7 MHz, 13 C 100.6 MHz). SiMe 4 was used as an internal reference for 1 H and 13 C NMR chemical shifts, and 85% H 3 PO 4 as an external reference for 31 P NMR. All experiments were carried out using standard Bruker pulse programs. The infrared (IR) spectra were recorded on a Bruker Vector-22 spectrometer.

DFT Calculations
All calculations were performed with the Gaussian 16 suite of programs [50]. The hybrid PBE0 functional [51] and the Ahlrichs' triple-ζ def-TZVP AO basis set [52] were used for the optimization of all structures. In all geometry optimizations, the D3 approach [53] was applied to describe the London dispersion interactions, as implemented in the Gaussian 16 program.

Electrochemical Measurements
Electrochemical measurements were conducted with a BASi Epsilon EClipse electrochemical analyzer. The program concerned Epsilon-EC-USB-V200 waves. A conventional three-electrode system was used with glassy carbon (GC) or carbon paste electrode (CPE) solutions for powder samples as the working electrode, the Ag/AgCl (0.01 M) electrode as the reference electrode, and a Pt wire as the counter electrode. A 0.1 M Et 4 NBF 4 was used as the supporting electrolyte to determine the current-voltage characteristics.

ESR Measurements
ESR measurements were carried out on an X-band ELEXSYS E500 ESR spectrometer. Samples in a cell of combined electrochemistry-ESR were inserted into an ER 4102ST cavity, after which the spectrometer was tuned and the ESR spectra were recorded. Oxygen was removed from liquid samples through three cycles of "freezing in liquid nitrogenevacuation-thawing" and, after the last cycle, the cell was filled with gaseous helium. The material of the auxiliary electrode was platinum, the reference electrode was Ag/AgCl, and a platinum plate served as a working electrode. A Bruker E 035M teslameter was used to accurately determine the g-factor.

Single Crystal X-ray Diffraction
The X-ray diffraction data for the single crystal 2 were collected on a Bruker D8 QUEST diffractometer with a PHOTON III area detector and an IµS DIAMOND microfocus X-ray tube, using Mo Kα (0.71073 Å) radiation. The diffractometer was equipped with an Oxford Cryostream LT device for low-temperature experiments. The data reduction package APEX4 v2021.10-0 was used for data collecting and processing. The analysis of the integrated data did not show any decay. The data were corrected for systematic errors and absorption: numerical absorption correction based on integration over a multifaceted crystal model and empirical absorption correction based on spherical harmonics according to the mmm point group symmetry using equivalent reflections. The structures were solved by the direct methods using SHELXT-2018/2 [54] and refined by the full-matrix leastsquares on F 2 using SHELXL-2018/3 [55]. Non-hydrogen atoms were refined anisotropically. The hydrogen atoms were inserted at the calculated positions and refined as riding atoms.
Crystallographic data for 2.
3.6.1. Synthesis of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene (2) [FeCp(η 6 -C 6 H 5 CH 3 )][PF 6 ] (0.54 g, 1.51 mmol) was added to lithium 2,3,4,5-tetraphenyl-1-monophospholide (1) (0.82 g, 1.52 mmol) in 20 mL of diglyme. The reaction mixture was stirred at 25 • C for 1 h and then heated to 160 • C for additional 2 h. Then, the reaction mixture was cooled to 25 • C, filtered, and the solvent was evaporated and the remaining solid was dissolved in 30 mL toluene. The toluene solution was kept at −20 • C for 2 days, filtered, and passed through a layer of silica (4-5 cm), and the silica was additionally washed with toluene (3 × 15 mL). After the removal of the solvent, compound 2 was obtained as a reddish powder (0.62 g, 72% yield), and recrystallization from hot toluene gave crystals 2,3,4,5-tetraphenyl-1-monophosphaferrocene (2) with m.p. 180 • C. 1  A solution of W(CO) 6 (0.35 g, 1.0 mmol) in THF (100 mL) was exposed to UV light (365 nm) in a quartz reaction vessel under argon at 0 • C for 3 h. The color of the resulting solution was yellow. A solution of 2 (0.56 g, 1.0 mmol) in THF was added and the reaction mixture was stirred for 20 h at 25 • C. The color changed to brown-red. The solvent was removed in vacuo and the product was extracted with toluene. The solvent was evaporated to give 0.76 g (86%) 3 as an orange powder with m.p. 204 • C. 1

Conclusions
In this paper, we described the rational synthetic method of novel 2,3,4,5-tetraphenyl-1-monophosphaferrocene 2 and its W(CO) 5 -complex 3 and elucidated their electrochemical properties. The structures were extensively studied from experimental (NMR and IR spectroscopies and X-ray diffraction) and theoretical points of view. Chemical properties and IR study showed a high π-acceptor with poor σ-donor ability of 2,3,4,5-tetraphenyl-1monophosphaferrocene (2). Cyclic voltammetry showed that [CpFe(η 5 -PC 4 Ph 4 )] 2 has a quasi-reversible oxidation wave and a potential more positive by 0.49 V than its literary analogue [(Me 5 Cp)Fe(η 5 -PC 4 Ph 4 )]. A comparison of electrochemical properties with the tungsten complex 3 showed the possibility of changing the type of coordination upon oxidation.

Data Availability Statement:
The data presented in this study are contained within the article or are available upon request from the first author, A.A.Z.