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Article

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

by
Almaz A. Zagidullin
1,
Alena R. Lakomkina
1,2,
Mikhail N. Khrizanforov
1,2,
Robert R. Fayzullin
1,
Kirill V. Kholin
1,3,
Tatiana P. Gerasimova
1,
Ruslan P. Shekurov
1,
Ilya A. Bezkishko
1 and
Vasili A. Miluykov
1,*
1
Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Arbuzov Street, 420088 Kazan, Russia
2
A.M. Butlerov Chemistry Institute of the Kazan Federal University, 18 Kremlevskaya Street, 420008 Kazan, Russia
3
Department of Physics, Kazan National Research Technological University, 68 Karl Marx Street, 420015 Kazan, Russia
*
Author to whom correspondence should be addressed.
Molecules 2023, 28(6), 2481; https://doi.org/10.3390/molecules28062481
Submission received: 31 January 2023 / Revised: 23 February 2023 / Accepted: 3 March 2023 / Published: 8 March 2023
(This article belongs to the Section Organometallic Chemistry)
Browse Figures
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Abstract

:
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.

Graphical Abstract

1. Introduction

The discovery of ferrocene [Fe(η5-C5H5)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(P4)2]2− represents the closest all-phosphorus derivatives of iron to ferrocene [Fe(η5-C5H5)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 [(η6-mesitylene)FeCp]PF6 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 [(η6-mesitylene)FeCp]PF6 was attained [25].
From the atom-economical point of view, it is better to use ready-made monophospholide anion uncontaminated with phenyllithium, since nucleophilic PhLi reacts with phosphaferrocenes [26,27]. Existing synthetic methods allow various phospholide anions to be obtained in their pure form, without PhLi impurities [4,28]. We have recently reported a convenient and effective method for the preparation of heteroleptic 1,2-diphosphaferrocenes [29,30] and 1,2,3-triphosphaferrocenes [31,32] through the reaction of appropriate 1,2-diphospholide- or 1,2,3-triphospholide anions in their pure form with a [(η6-toluene)FeCp]PF6 complex. In the present article, we describe the rational and atom-economical synthesis of 2,3,4,5-tetraphenyl-1-monophosphaferrocene and its W(CO)5 complex and compare their structural and electrochemical properties with the previously known analogues.

2. Results and Discussion

2.1. Synthesis and Structure of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene Derivatives

The target 1-monophosphaferrocene was prepared via a classical two-step sequence. At the first step, the highly moisture-sensitive lithium 2,3,4,5-tetraphenyl-1-monophosphacyclopentadienide (1) was obtained by straightforward synthesis from elemental phosphorus P4 and in situ-generated 1,4-dilithio-1,2,3,4-tetraphenylbutadiene. Compound 1 was characterized by the 31P NMR resonance at +99 ppm. This direct procedure based on elemental (white) phosphorus activation has advantages such as step-economy (two steps in one flask), mild conditions (+25 °C, 2 days), and good yields (up to 63%) [33,34].
In the next step, the lithium phospholide 1 was converted into phosphaferrocene 2 upon reaction with (toluene)cyclopentadienedienyl-iron(II) hexafluorophosphate salt [FeCp(η6-C6H5CH3)][PF6] at a 1:1 ratio in boiling diglyme in 2 h (Scheme 1). Diglyme was evaporated and the product was extracted with toluene. The subsequent filtration of toluene solution through a silica gel layer gave pure 2,3,4,5-tetraphenyl-1-monophosphaferrocene (2) as an air-stable powder in satisfactory yields (68–72%). Novel monophosphaferrocene 2 was characterized by multinuclear NMR spectroscopy and elemental analysis (Supplementary Materials, Figures S1–S3). The 31P{1H} NMR spectrum of 2 shows the singlet at –61 ppm shifted upfield in comparison to 1-monophospholide lithium 1 by ca. 160 ppm. In the 1H NMR spectrum, the characteristic signals of the aryl substituents at 7.06–7.20 ppm and the cyclopentadienyl ring at 4.43 ppm can be observed. The 13C{1H} NMR spectrum of 2 shows doublets at 99 ppm (1JPC = 57.7 Hz) and 100 ppm (2JPC = 4.5 Hz) for the carbon atoms of the 1-monophosphacyclopentadienyl ligand and a singlet at 76 ppm for the cyclopentadienyl ligand. The NMR data for 2,3,4,5-tetraphenyl-1-monophosphaferrocene 2 are comparable to those of related compounds bearing alkyl [35,36] or aryl [37,38] substituents.
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.
Tungsten complex 3, 2,3,4,5-tetraphenyl-1-monophosphaferrocene-1-tungstenpentacarbonyl, was obtained through the reaction of 2,3,4,5-tetraphenyl-1-monophosphaferrocene (2) with labile complex W(CO)5(THF) at 25 °C with a yield of 86% (Scheme 1). It is worth noting that the reaction of 2 with stable complex W(CO)5(CH3CN) did not proceed at temperatures from 25 to 110 °C. Phosphaferrocene 2 behaves as a weak σ-donor ligand.
In the 31P{1H} NMR spectrum of 3, a singlet at –30 ppm with coupling constant 1JWP = 262 Hz was observed. Both 1H and 13C NMR spectra confirm a definite structure and purity of complex 3 (Supplementary Materials, Figures S4–S6). In the IR spectrum of 3 recorded in a KBr pellet, four absorption bands ν(CO) were observed at 1930, 1948, 1966, and 2074 cm−1, which are characteristic for the W(CO)5L complexes. The IR-spectroscopic investigation of 3, in comparison with [(PPh3)W(CO)5] and [(2,4,6-triphenylphosphinine)W(CO)5], reveals the expected trend of the donor–acceptor capabilities of the corresponding ligands. The CO stretching frequencies ν(CO) in the IR spectra clearly indicated that the 2,3,4,5-tetraphenyl-1-monophosphaferrocene (2) (highest ν(CO) = 2074 cm−1) and 2,4,6-triphenylphosphinine (highest ν(CO) = 2073 cm−1) is the poorest electron pair donors while tripheylphosphine showed CO stretching frequencies at ν(CO) = 2071 cm−1 [40]. As expected, the IR studies of these complexes showed that 2 is a better π-acceptor than the 2,4,6-triphenylphosphinine and PPh3. These results display a high π-acceptor with poor σ-donor ability of 2.

2.2. 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(η5-C5H5)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 [(Me5Cp)Fe(η5-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 FeII in FeIII, 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.
It is well known that the oxidation of the ferrocene [Fe(η5-C5H5)2] molecule leads to the appearance of a FeIII cation with 3d5 configuration in a low spin state [44]. Although low spin state FeIII 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(η5-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 FeIII 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 FeIII 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 (C5) and phospholyl (PC4) 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 (PC4) “tilt” from the initial position. Energetically, for both cationic forms, the low-spin state is more stable compared to the high-spin state.
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 electrochemical–chemical 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.

3. Materials and Methods

3.1. General

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

3.2. 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.

3.3. 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.

3.4. 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 nitrogen–evacuation–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.

3.5. 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 least-squares on F2 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. C33H25FeP, orange prism (0.434 × 0.380 × 0.367 mm3), formula weight 508.35 g mol−1; orthorhombic, Pbca (No. 61), a = 12.8590(3) Å, b = 14.8048(3) Å, c = 25.9145(5) Å, V = 4933.47(18) Å3, Z = 8, Z′ = 1, T = 100(2) K, dcalc = 1.369 g cm−3, μ(Mo Kα) = 0.696 mm−1, F(000) = 2112; Tmax/min = 0.6842/0.6168; 227694 reflections were collected (2.231° ≤ θ ≤ 32.060°, index ranges: −19 ≤ h ≤ 19, −21 ≤ k ≤ 22 and −38 ≤ l ≤ 38), 8559 of which were unique, Rint = 0.0485, Rσ = 0.0194; completeness to θ of 32.060° 99.3%. The refinement of 316 parameters with no restraints converged to R1 = 0.0310 and wR2 = 0.0762 for 7234 reflections with I > 2σ(I) and R1 = 0.0424 and wR2 = 0.0810 for all data with goodness-of-fit S = 1.039 and residual electron density ρmax/min = 0.410 and –0.544 e Å−3, rms 0.064; max shift/e.s.d. in the last cycle 0.004. Deposition number 2218908 contains the supplementary crystallographic data for compound 2. These data are provided free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures (deposited on 10 November 2022).

3.6. Synthesis

All reactions and manipulations were carried out under dry pure N2 in the standard Schlenk apparatus. All solvents were distilled from sodium/benzophenone or phosphorus pentoxide and stored under nitrogen before use. Starting materials [FeCp(η6-C6H5CH3)][PF6] [56] and Li(THF)2-2,3,4,5-tetraphenyl-1-monophosphacyclopentadienide (1) [34] were prepared according to literature procedures. W(CO)6 was purchased from Aldrich and used without additional purification.

3.6.1. Synthesis of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene (2)

[FeCp(η6-C6H5CH3)][PF6] (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. 1H NMR (CDCl3, δ, ppm, J, Hz): 4.43 (s, 5H, Cp), 7.06–7.20 (m, 20H, Ph). 31P{1H} NMR (CDCl3, δ, ppm, J, Hz): −60.8 (s). 13C{1H} (CDCl3, δ, ppm, J, Hz): 75.5 (s, Cp), 99.2 (d, 1JPC = 57.7, C2/C5), 99.8 (d, 2JPC = 4.5, C3/C4), 126.0 (s, Ph), 126.5 (s, Ph), 127.2 (s, Ph), 127.4 (s, Ph), 130.5 (d, 3JPC = 7.1, Ph), 132.4 (s, Ph), 137.0 (s, Ph), 139.2 (d, 3JPC = 16.8, Ph). IR (KBr, cm−1): 460 (w), 493 (w), 562 (w), 591 (w), 697 (s), 718 (s), 747 (m), 759 (w), 825 (w), 916 (w), 1026 (w), 1074 (w), 1156 (w), 1388 (w), 1492 (s), 1597 (w), 1871 (w), 1945 (w), 2345 (w), 2926 (w), 2963 (w), 3054 (w), 3077 (w). Calculated for C37H37FeP (M 568): C 78.17, H 6.56, Fe 9.82, P 5.45. Found: C 78.33, H 6.72, P 5.71.

3.6.2. Synthesis of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene-1-Tungstenpentacarbonyl (3)

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. 1H NMR (CDCl3, δ, ppm, J, Hz): 4.70 (s, 5H, Cp), 6.94–7.30 (m, 20H, Ph). 31P{1H} NMR (CDCl3, δ, ppm, J, Hz): −30.1 (s, 1JPW = 263.3). 13C{1H} (CDCl3, δ, ppm, J, Hz): 77.3 (s, Cp), 93.4 (s, C2/C5), 97.2 (s, C3/C4), 126.9 (s, Ph), 127.1 (s, Ph), 127.6 (s, Ph), 127.7 (s, Ph), 132.6 (s, Ph), 132.6 (s, Ph), 132.7 (s, Ph), 136.2 (s, Ph), 136.7 (s, Ph). IR (KBr, cm−1): 491 (w), 513 (w), 574 (w), 593 (w), 663 (w), 669 (w), 800 (s), 865 (m), 1020 (br.s.), 1098 (br.s.), 1262 (s), 1414 (w), 1445 (w), 1470 (w), 1496 (w), 1930 (m), 1948 (m), 1966 (w), 2074 (w). Calculated for C42H37FePO5W (M 892): C 56.53, H 4.18, Fe 6.26, P 3.47, W 20.60. Found: C 56.49, H 4.32, P 3.68.

4. 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-1-monophosphaferrocene (2). Cyclic voltammetry showed that [CpFe(η5-PC4Ph4)] 2 has a quasi-reversible oxidation wave and a potential more positive by 0.49 V than its literary analogue [(Me5Cp)Fe(η5-PC4Ph4)]. A comparison of electrochemical properties with the tungsten complex 3 showed the possibility of changing the type of coordination upon oxidation.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules28062481/s1, Figure S1–S3: NMR spectra of 2; Figure S4–S6: NMR spectra of 3; Figure S7–S8: Semi-derivative of CV for oxidation of 2 and 3; Table S1–S6: Cartesian coordinates of the optimized ground state structure of neutral and cationic forms of 2 and 3 with S =1/2 and S =5/2.

Author Contributions

Chemical synthesis, A.A.Z., A.R.L. and I.A.B.; electrochemistry, M.N.K. and R.P.S.; X-ray diffraction analysis, R.R.F.; DFT calculations, T.P.G.; ESR method, K.V.K., project administration, V.A.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the grant from the Russian Science Foundation, No. 21-73-10204.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

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.

Acknowledgments

The measurements were carried out using the equipment of the Distributed Spectral-Analytical Center of Shared Facilities for Study of Structure, Composition and Properties of Substances and Materials of the FRC Kazan Scientific Center of RAS.

Conflicts of Interest

The authors declare no conflict of interest.

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Molecules 28 02481 sch001 550
Scheme 1. Synthesis of 2,3,4,5-tetraphenyl-1-monophosphaferrocene 2 and its tungsten complex 3.
Scheme 1. Synthesis of 2,3,4,5-tetraphenyl-1-monophosphaferrocene 2 and its tungsten complex 3.
Molecules 28 02481 sch001
Molecules 28 02481 g001 550
Figure 1. ORTEP of 2,3,4,5-tetraphenyl-1-monophosphaferrocene (2) for nonhydrogen atoms at 70% probability level according to the single-crystal X-ray diffraction data. Two different projections are shown. Selected internuclear distances [Å] are as follows: Fe1–Cnt(PC4) 1.6336(2), Fe1–Cnt(C5) 1.6600(2), Fe1–P1 2.2761(3), Fe1–C1 2.0675(10), Fe1–C2 2.0643(10), Fe1–C3 2.0599(10), Fe1–C4 2.0792(11), Fe1–C5 2.0668(11), Fe1–C6 2.0645(11), Fe1–C7 2.0492(11), Fe1–C8 2.0430(11), Fe1–C9 2.0469(11), P1–C1 1.7819(11), and P1–C4 1.7824(11).
Figure 1. ORTEP of 2,3,4,5-tetraphenyl-1-monophosphaferrocene (2) for nonhydrogen atoms at 70% probability level according to the single-crystal X-ray diffraction data. Two different projections are shown. Selected internuclear distances [Å] are as follows: Fe1–Cnt(PC4) 1.6336(2), Fe1–Cnt(C5) 1.6600(2), Fe1–P1 2.2761(3), Fe1–C1 2.0675(10), Fe1–C2 2.0643(10), Fe1–C3 2.0599(10), Fe1–C4 2.0792(11), Fe1–C5 2.0668(11), Fe1–C6 2.0645(11), Fe1–C7 2.0492(11), Fe1–C8 2.0430(11), Fe1–C9 2.0469(11), P1–C1 1.7819(11), and P1–C4 1.7824(11).
Molecules 28 02481 g001
Molecules 28 02481 g002 550
Figure 2. Cyclic voltammograms for oxidation of compound 2 and complex 3 in CH3CN on a glassy carbon (GC) electrode (0.5 mM complex). The potentials vs. Ag/AgCl. Scan rate = 100 mV s−1, room temperature.
Figure 2. Cyclic voltammograms for oxidation of compound 2 and complex 3 in CH3CN on a glassy carbon (GC) electrode (0.5 mM complex). The potentials vs. Ag/AgCl. Scan rate = 100 mV s−1, room temperature.
Molecules 28 02481 g002
Molecules 28 02481 sch002 550
Scheme 2. Proposed oxidation mechanism of 2.
Scheme 2. Proposed oxidation mechanism of 2.
Molecules 28 02481 sch002
Molecules 28 02481 g003 550
Figure 3. ESR signal of phosphaferrocenium cation of 2 in CH3CN in the electrochemical ESR cell at 0.55 V vs. FcH/FcH+.
Figure 3. ESR signal of phosphaferrocenium cation of 2 in CH3CN in the electrochemical ESR cell at 0.55 V vs. FcH/FcH+.
Molecules 28 02481 g003
Molecules 28 02481 g004 550
Figure 4. Cyclic voltammograms for reduction of compound 2 and complex 3 in CH3CN on a glassy carbon (GC) electrode (0.5 mM complex). The potentials vs. Ag/AgCl. Scan rate = 100 mV s−1, room temperature.
Figure 4. Cyclic voltammograms for reduction of compound 2 and complex 3 in CH3CN on a glassy carbon (GC) electrode (0.5 mM complex). The potentials vs. Ag/AgCl. Scan rate = 100 mV s−1, room temperature.
Molecules 28 02481 g004
Molecules 28 02481 sch003 550
Scheme 3. Proposed reduction mechanism of complex 3.
Scheme 3. Proposed reduction mechanism of complex 3.
Molecules 28 02481 sch003
Table
Table 1. Comparison of some geometrical parameters of compound 2 and the related phosphaferrocenes according to the single-crystal X-ray diffraction data a.
Table 1. Comparison of some geometrical parameters of compound 2 and the related phosphaferrocenes according to the single-crystal X-ray diffraction data a.
CompoundFe–Cnt(PC4)Fe–Cnt(C5)Turning Angle b∠(PC4)(C5) cFe–PReference
Molecules 28 02481 i0011.6336(2)1.6600(2)12.38(6)3.14(4)2.2761(3)this work
Molecules 28 02481 i0021.6393(10)1.6609(14)–15.34(12)2.60(9)2.2805(11)[38]
Molecules 28 02481 i0031.6433(9)/1.6467(9)1.6584(11)/1.6614(11)8.30(15)/–5.35(15)3.42(8)/3.17(8)2.2858(6)/2.2895(6)[25] d
Molecules 28 02481 i0041.6440(16)1.660(2)–1.4(3)3.83(16)2.2864(12)[25]
Molecules 28 02481 i0051.666(2)1.690(3)16.2(3)3.1(2)2.2821(16)[39]
a All distances and angles are given in angstroms [Å] and degrees [°], respectively; the word “centroid” is abbreviated as Cnt. b Turning angle corresponds to the torsion angle P–Cnt(PC4)–Cnt(C5)–C. c Angle ∠(PC4)(C5) 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.
Table
Table 2. Electrochemical data for redox properties of 2, 3, and other monophosphaferrocenes.
Table 2. Electrochemical data for redox properties of 2, 3, and other monophosphaferrocenes.
CompoundEox1(1/2Eox1), V vs. FcH/FcH+;
{Ia/Ic}		1/2Ered1, V vs. FcH/FcH+1EHOMO, eV1ELUMO, eVGap, eV
Ferrocene [42]0.03 (0 *);
{1}		−3.18 *−4.79 *−1.61 *3.18 *
Molecules 28 02481 i0060.15 (0.17 **);
{1}		−3.04 **−4.97−1.763.21
Molecules 28 02481 i0070.55 (0.44—semidiffE);
{0.6}		−2.25−5.24−2.552.69
Molecules 28 02481 i0080.06 (0.1 ***);
{››1}		n.a.n.a.n.a.n.a.
Molecules 28 02481 i009n.a. (0.07 **);
{≈1}		−3.06 **−4.87 **−1.74 **3.13
Molecules 28 02481 i0100.01 (−0.03 ***);
{≈1}		n.a.n.a.n.a.n.a.
Molecules 28 02481 i0110.40 (0.35);
{irrev}		−1.34
1/2Ered2 = −2.06V		−5.15−3.461.69
* Conditions: −50 °C, glassy carbon working electrode, the potentials vs. Ag/AgCl recalculated to FcH/FcH+, 0.5 mM concentration, Bu4NBF4, DMF, 100 mV s−1. ** Conditions: 1 mM solutions of a monophosphaferrocene on a mercury electrode in propylene carbonate containing 0.1 M TEAP at 10 V s−1. The potentials vs. SCE recalculated to FcH/FcH+ [42]. *** Conditions: 1 mM solutions of a monophosphaferrocene on a Pt electrode. Solvent CH2Cl2, 0.1 M Bu4NBF4; the potentials vs. SCE recalculated to FcH/FcH+ [41].
Table
Table 3. Some structural parameters of optimized structures of neutral and cationic forms of 2 and 3.
Table 3. Some structural parameters of optimized structures of neutral and cationic forms of 2 and 3.
Compound 2Compound 3
NeutralCation (+1)NeutralCation (+1)
S = 1/2S = 5/2 S = 1/2S = 5/2
Molecules 28 02481 i012Molecules 28 02481 i013Molecules 28 02481 i014Molecules 28 02481 i015Molecules 28 02481 i016Molecules 28 02481 i017
Fe-P, Å2.302.302.382.282.332.55
Fe-C(PC4), Å2.06–2.082.102.30–2.682.06–2.082.08–2.112.22–2.63
Fe-C(C5), Å2.052.082.27–2.402.07–2.092.07–2.092.22–2.32
ΔE, kcal mol−1 012 07.3
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Zagidullin, A.A.; Lakomkina, A.R.; Khrizanforov, M.N.; Fayzullin, R.R.; Kholin, K.V.; Gerasimova, T.P.; Shekurov, R.P.; Bezkishko, I.A.; Miluykov, V.A. Synthesis, Structure, and Electrochemical Properties of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene Derivatives. Molecules 2023, 28, 2481. https://doi.org/10.3390/molecules28062481

AMA Style

Zagidullin AA, Lakomkina AR, Khrizanforov MN, Fayzullin RR, Kholin KV, Gerasimova TP, Shekurov RP, Bezkishko IA, Miluykov VA. Synthesis, Structure, and Electrochemical Properties of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene Derivatives. Molecules. 2023; 28(6):2481. https://doi.org/10.3390/molecules28062481

Chicago/Turabian Style

Zagidullin, Almaz A., Alena R. Lakomkina, Mikhail N. Khrizanforov, Robert R. Fayzullin, Kirill V. Kholin, Tatiana P. Gerasimova, Ruslan P. Shekurov, Ilya A. Bezkishko, and Vasili A. Miluykov. 2023. "Synthesis, Structure, and Electrochemical Properties of 2,3,4,5-Tetraphenyl-1-Monophosphaferrocene Derivatives" Molecules 28, no. 6: 2481. https://doi.org/10.3390/molecules28062481

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