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Synthesis, Structure, and Reactivity of Pentamethylcyclopentadienyl 2,4,6-Triphenylphosphinine Iron Complexes Babak Rezaei Rad,†,§ Uttam Chakraborty,† Bernd Mühldorf,† Julian A. W. Sklorz,‡ Michael Bodensteiner,† Christian Müller,*,‡ and Robert Wolf*,† †

Institute of Inorganic Chemistry, University of Regensburg, D-93040 Regensburg, Germany Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34/36, D-14195 Berlin, Germany

‡

S Supporting Information *

ABSTRACT: The potassium salt [K([18]crown-6)(THF)2][Cp*Fe(η4-2,4,6-triphenylphosphinine)}] (K1, Cp* = C5Me5) can be isolated in 68% yield by reacting the anionic naphthalene complex [K([18]crown-6){Cp*Fe(η4-C10H8)}] (C10H8 = naphthalene) with 2,4,6-triphenylphosphinine. Compound K1 reacts with water to afford [K([18]-crown6)]{Cp*Fe(η 4 -2,4,6-triphenyl-2,3-dihydrophosphinine 1oxide)}] (K2) with a novel 2,3-dihydrophosphinine 1-oxide ligand. Oxidation of K1 with one equivalent of ferrocenium hexafluorophosphate yields the P−P-bonded diphosphinine complex [Cp*Fe(η5-2,4,6-triphenylphosphinine)]2 (3), while the iodide salt [Cp*Fe(η6-2,4,6-triphenylphosphinine)]I (4) can be obtained by reacting K1 with one equivalent of iodine. Reactions of 4 with LiNMe2, Cp*Li, LiBHEt3, and Ga(nacnacDipp) (nacnacDipp = HC{C(Me)N(C6H3-2,6-iPr2)}2) afford [Cp*Fe(η5-1-dimethylamino-2,4,6-triphenylphosphacyclohexadienyl)] (5), [Cp*Fe(η 5 -1-(η 1 -Cp*)-2,4,6-triphenylphosphacyclohexadienyl)] (6), [Cp*Fe(η 5 -1-hydro-2,4,6-triphenylphosphacyclohexadienyl)] (7), and [Cp*Fe((η5-1-{Ga(nacnacDipp)I}-2,4,6-triphenylphosphacyclohexadienyl] (8). The molecular structures of 5−8 display η5-coordinated λ3σ3-phosphinine anions. All new complexes were fully characterized by spectroscopic techniques (1H, 13C, and 31P NMR, UV−vis, and IR spectroscopy), elemental analysis, and X-ray crystallography. The electronic structures of these new phosphinine complexes were investigated theoretically at the DFT level, using molecular orbital and population analyses. The nature of the electronic transitions observed in the UV−vis spectra was analyzed using TDDFT calculations.

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metal center.4 Moreover, the η6-coordination mode can be imposed by sterically demanding substituents, e.g., tert-butyl or Me3Si groups in the ortho-position with respect to the P atom, which impede σ-coordination via the lone-pair.5 A mixed η1−η6 binding mode, in which the phosphinine behaves as an 8e− donor, has been found in a few cases for metals in the center of the d-block, such as manganese and chromium.6 The coordination chemistry of phosphinines has been studied in depth during the last two decades, and a number of review articles on this topic have recently been published.4 It is thus surprising that only a few phosphinine-iron complexes have appeared in the literature, most of which feature iron in the zerovalent oxidation state. Elschenbroich et al. prepared the homoleptic η1-phosphinine complex [Fe(η1-PC5H5)5] (A), containing the parent phosphinine (Chart 1).7 The η6phosphinine-iron(0) complex B reported by Zenneck et al. was used in the catalytic [2+2+2] cyclotrimerization of dimethyl acetylenedicarboxylate to give C6(CO2Me)6.8 These authors also described the dinuclear phosphinine-iron(0) complex C, in which coordination to Fe(CO)3 and Fe(CO)4

INTRODUCTION The first preparation of 2,4,6-triphenylphosphinine by Märkl in 1966 represents a landmark in phosphorus chemistry. Since its discovery, this low-coordinate phosphorus compound has been regarded as an aromatic heterocycle with significantly different electronic properties compared to its nitrogen counterpart (pyridine), as well as classical trivalent P(III) species.1 In contrast to such phosphanes, phosphinines are ambidentate ligands and possess two different potential donor moieties, the lone pair at the phosphorus atom and the aromatic π-system.2 Quantum chemical calculations showed that the HOMO−2 of a phosphinine is suitable for σ-coordination toward a metal center. Its energy is, however, close to that of the HOMO−1 and HOMO orbitals that can participate in η6-coordination via the π-system.3 This situation leads to a range of distinct coordination modes, with the metal−ligand interaction through the phosphorus atom (σ-coordination, 2e− donation) as the most common one. It is generally observed in combination with late transition metals in low oxidation states, due to the strong π-acceptor properties of the aromatic phosphorus heterocycle.4 Although less frequent, the η6-coordination mode typically occurs in complexes with the earlier transition metals in order to compensate for the electron deficiency of the © XXXX American Chemical Society

Received: November 19, 2014

A

DOI: 10.1021/om501161y Organometallics XXXX, XXX, XXX−XXX

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Organometallics

the new phosphinine complexes K1, K2, and 3−8 are discussed. In addition, the nature of the spectroscopically observed electronic transitions and the bonding situation of selected species are rationalized using DFT calculations.

Chart 1

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RESULTS AND DISCUSSION The reaction of the iron(0) naphthalene complex [K([18]crown-6){Cp*Fe(η4-C10H8)}] (G) with one equivalent of 2,4,6-triphenylphosphinine proceeds below room temperature in THF, giving the potassium salt [K([18]crown-6)(THF)2][Cp*Fe(η4-2,4,6-triphenylphosphinine)}] (K1) via ligand exchange of naphthalene by the phosphinine (Scheme 1). Compound K1 can be isolated as a dark brown crystalline solid in 58% yield after diffusion of n-hexane into the THF solution. Scheme 1. Synthesis of the Salt K1 fragments occurs via both part of the aromatic system of the phosphinine heterocycle and the phosphorus lone-pair.9 The cationic iron(II) sandwich complex D, reported by Nief and Fischer, contains a neutral η6-coordinated 2,4,6-triphenylphosphinine ligand as well as an anionic cyclopentadienyl ligand.10 The complex was characterized as the hexafluorophosphate salt by multinuclear NMR spectroscopy and microanalysis, but a crystallographic verification of its molecular structure remained elusive. Due to the strong π-accepting properties of phosphinines, the preparation of anionic complexes is also possible. These are usually prepared starting from the chemically reduced ligands, which generally coordinate to the metal center in an η1coordination mode via the phosphorus lone-pair. This has been shown, for example, for the dianionic iron complex E, which contains two bisphosphinine ligands.11 Iron complexes of anionic derivatives have also been reported, of which complex F has been characterized crystallographically.12,13 In this compound, two P-substituted λ3σ3-phosphinine anions bind to the metal center in a cyclohexadienyl-like fashion. As part of a program studying the chemistry of anionic, lowvalent polyarene iron and cobalt complexes,14−16 we recently synthesized the naphthalene iron complex [K([18]crown6){Cp*Fe(η4-C10H8)}] (G, C10H8 = naphthalene).17 Starting from this “Cp*Fe−” equivalent, anionic sandwich complexes are accessible by ligand substitution of the labile naphthalene molecule.18 Given the known ability of phosphinines to act as π-acceptors in low-oxidation-state transition metal complexes (vide supra), we wondered how they might behave as ligands toward such a “Cp*Fe−” equivalent. Here, we report that the reaction of complex G with Märkl’s 2,4,6-triphenylphosphinine gives access to the first anionic iron complex with a πcoordinated phosphinine, [Cp*Fe(η4-2,4,6-triphenylphosphinine)}] (1−), which was isolated in the form of its potassium salt K1. Hydrolysis of K1 affords the complex K2, which displays the unusual η4-2,4,6-triphenyl-2,3-dihydrophosphinine 1-oxide ligand. Moreover, we have found that the neutral phosphinine dimer [Cp*Fe(η5-2,4,6-triphenylphosphinine)]2 (3) and the ionic iron(II) compound [Cp*Fe(η6-2,4,6triphenylphosphinine)]I (4) are formed by the stepwise chemical oxidation of anionic 1−. Finally, we describe the synthesis of the new P-substituted phosphacyclohexadienyl complexes 5−8, which were obtained by reacting 4 with selected nucleophiles. The solid-state molecular structures as well as the NMR, UV−vis, and IR spectroscopic properties of

Single-crystal X-ray diffraction revealed the molecular structure of K1. In the crystal, the asymmetric unit is composed of two [Cp*Fe(η4-2,4,6-triphenylphosphinine)}]− (1a−) anions, which are well separated from the [K([18]crown6)(THF)2]+ cations. The two crystallographically independent molecules feature very similar structures; thus, only one of them is displayed in Figure 1 and discussed below. The anions

Figure 1. Solid-state molecular structure of K1. Only one of two crystallographically independent molecules in the unit cell is shown; ellipsoids are drawn at the 30% probability level; H atoms and disorder in the phosphinine ligand and the THF molecules have been omitted for clarity; see Table 1 for important bond lengths and angles.

display one η5-coordinated Cp* ligand and one η4-coordinated phosphinine ligand. The phosphinine ring is bent along the P1−C3 axis with a large fold angle of 49°. This η4-binding of the phosphinine ligand is very rare, but was previously observed in the molecular structure of [Fe2(CO)7(η1,η4-2,4,6-triphenylphosphinine)] (C, Chart 1, fold angle 46.4°).9 In comparison, various [Fe(η4-polyarene)] complexes have been characterized, e.g., the naphthalene derivatives [K([18]crown-6){Cp*Fe(η4C10H8)}] (G) and [Fe(η4-naphthalene)L3] (L = P(OR)3; L3 = η6-arene).19 Only in the case of highly reactive, thermolabile [Fe(η6-arene)(η4-arene)] species has an η4-coordination of monocyclic arene ligands such as benzene or toluene been identified.20,21 Thus, the coordination mode of 2,4,6B


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Organometallics Table 1. Selected Bond Lengths (Å) and Angles (deg) of the Structures of Compounds 1−8a C1−C2 C2−C3 C3−C4 C4−C5 C1−P1 C5−P1 Fe1−C1 Fe−C2 Fe1−C3 Fe1−C4 Fe1−C5 Fe1−P1 C1−P1−C5 a

K1

K2

3

4

5

6

7

8

1.426(3); 1.428(5) 1.463(3); 1.456(5) 1.470(3); 1.474(5) 1.351(4); 1.357(7) 1.816(2); 1.801(3) 1.838(3); 1.833(5) 2.044(3); 2.048(3) 1.974(2); 1.980(4) 2.124(2); 2.104(4) 3.003(3); 3.004(4) 3.185(2); 3.204(4) 2.2739(7); 2.271(1) 99.0(1); 99.8(2)

1.422(4) 1.445(3) 1.532(5) 1.530(5) 1.792(2) 1.855(3) 2.099(3) 2.007(2) 2.124(2) 3.072(3) 3.317(3) 2.138(1) 100.4(1)

1.426(9)−1.42(1) 1.436(8)−1.590(9) 1.421(9)−1.41(1) 1.402(9)−1.42(1) 1.808(8)−1.826(8) 1.783(7)−1.829(8) 2.157(8)−2.135(8) 1.999(6)−1.993(6) 2.007(7)−2.245(7) 2.045(6)−2.084(7) 2.242(6)−2.080(7) 2.849(2)−2.815(2) 95.2(3)−85.3(3)

1.416(5) 1.414(5) 1.419(5) 1.408(5) 1.765(2) 1.774(3) 2.157(3) 2.074(4) 2.147(3) 2.095(4) 2.146(3) 2.392(1) 98.8(1)

1.408(3)−1.412(4) 1.423(4)−1.434(4) 1.416(4)−1.434(4) 1.412(4)−1.425(4) 1.801(2)−1.818(2) 1.793(2)− 1.821(3) 2.154(2)−2.173(3) 2.052(3)−2.064(2) 2.070(3)−2.085(3) 2.056(3)−2.069(2) 2.146(3)−2.177(3) 2.7923(7)−2.8284(9) 93.5(1)−94.2(1)

1.415(5) 1.415(5) 1.452(9) 1.507(9) 1.804(5) 1.716(4) 2.118(5) 2.036(6) 2.117(4) 2.056(9) 2.142(5) 2.843(2) 93.6(2)

1.414(4) 1.433(3) 1.427(3) 1.419(4) 1.810(2) 1.817(2) 2.164(2) 2.053(2) 2.095(2) 2.044(2) 2.164(2) 2.8814(6) 94.0(1)

1.423(4) 1.417(4) 1.429(4) 1.423(4) 1.832(3) 1.822(3) 2.151(3) 2.054(3) 2.084(3) 2.042(3) 2.150(3) 2.8915(8) 92.3(1)

Data for several crystallographically independent molecules are included for 1, 3, and 5.

triphenylphosphinine to iron in the molecular structure of K1 may be explained by the stabilizing effect exerted by the phosphorus atom, which is equivalent to that of the noncoordinated 6π-electron system of the naphthalene ligand. The Fe−C1/C2/C3 (1.974(2)−2.124(2) Å) and Fe−P (2.2739(7) Å) bond lengths in 1− (Table 1) are comparable to those of η4-coordinated monophospha- and 1,3-diphosphacyclobutadienes in [Fe(η4-PC3R4)(CO)3],22 [K([18]crown6)(THF)2][Cp*Fe(η4-P2C2tBu2)],17 and [Fe(η4P2C2tBu2)2].16b The C2−C3 and C3−C4 distances (1.463(3) and 1.470(3) Å, respectively) are identical within the standard deviations. The C1−C2 bond (1.426(3) Å) is slightly shorter, while the C4−C5 bond is the shortest one in the ring, with a C−C distance of 1.351(4) Å, which may indicate some doublebond character (Table 1). The C5−P1 bond (1.838(3) Å) is only marginally longer than C1−P1 (1.816(2) Å). DFT calculations at the BP86-D3/def2-TZVP level (see the Experimental Section for details) reproduce the crystallographically determined structure of the [Cp*Fe(η4-2,4,6triphenylphosphinine)}]− (1a−) anion. Inspection of the frontier Kohn−Sham molecular orbitals (Figure 2) shows that the three highest occupied molecular orbitals (HOMO to HOMO−2) are metal-centered MOs with predominant iron dorbital character, while lower-lying MOs have large contributions from ligand-based atomic orbitals. This suggests a d6 electron configuration for iron, similar to the situation observed for related formally zerovalent iron naphthalene and anthracene complexes.23 In THF-d8 solution, K1 gives rise to a broad 31P{1H} singlet at −46.4 ppm, which is shifted to higher field compared to that of the uncoordinated phosphinine molecule (δ = +184.2 ppm in CDCl3). A high-field shift of the 31P NMR signal of the phosphinine ligand upon π-complexation is well documented for η6-phosphinine complexes, e.g., [CpMn(η6-2,4,6-triphenylphosphinine)], 6b [CpFe(2,4,6-triphenylphosphinine)]PF 6 (D),10 and [(COD)M(η6-2,6-bis{trimethylsilyl}phosphinine)]+ (M = Rh, Ir, COD = 1,5-cyclooctadiene).5 The 1H NMR singlet for the Cp* ligand (δ = 1.24 ppm) is also considerably upfield shifted compared to that of the starting material (δ = 1.70 ppm). The 31P{1H} NMR signal and the 1H resonances for the phenyl groups of the 2,4,6-triphenylphosphinine unit of 1 are exceptionally broad at 300 K (Δν1/2(31P) = 137 Hz). The 31 1 P{ H} NMR line width increases slightly upon raising the temperature to +60 °C (Δν1/2 = 144 Hz) and decreases on cooling the solution to −60 °C (Δν1/2 = 21 Hz, Figure S4 in

Figure 2. Calculated frontier Kohn−Sham orbitals of the [Cp*Fe(η42,4,6-triphenylphosphinine)}]− anion (1a−), showing the large contributions of iron d atomic orbitals to the HOMO to HOMO− 2. The relative contributions from iron and phosphorus (reduced atomic orbital populations per MO) were obtained using a Löwdin population analysis.24

the Supporting Information). A possible explanation for the observed line broadening at room temperature might be the formation of a fast equilibrium between different coordination isomers in solution due to a haptotropic rearrangement of the 2,4,6-triphenylphosphinine moiety. Indeed, our DFT calculations indicate that the two isomers 1b− and 1c− (Figure 3) are viable species in addition to the crystallographically observed isomer 1a−. The structures of the three coordination isomers 1a−−1c− each feature an η4-bound 2,4,6-triphenylphosphinine ligand which shows a different relative position of the coordinated atoms with respect to the Cp*Fe moiety. The phosphinine ligand binds through three carbons and one C


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Organometallics Scheme 2. Synthesis of Complexes 2−4

Figure 3. DFT-calculated relative thermal enthalpies (ΔH at 298 K) of three viable coordination isomers of the [Cp*Fe(η4-2,4,6-triphenylphosphinine)}]− anion calculated at the BP86-D3/def2-TZVP level. Relative free enthalpies (ΔG at 298 K) are given in parentheses. Similar energy differences were obtained for single-point energies calculated at the BP86-D3/def2-QZVPP level on the BP86-D3/def2TZVP-optimized geometries (see Tables S3 and S4). The calculated Fe−P distances are 2.260 Å for 1a−, 2.241 Å for 1b−, and 3.292 Å for 1c−; see the Supporting Information for more details.

phosphorus atom in the case of 1a− and 1b−, but four carbon atoms in the case of 1c−. Note that the calculated relative enthalpy of the isomer 1b− is nearly the same as that of 1a−. In contrast, isomer 1c−, where phosphorus is not involved in the coordination to iron (Fe−P 3.292 Å), was calculated to be +12.9 kcal·mol−1 higher in energy than 1a−. Similar energy differences were obtained by single-point calculations on the BP86-D3/def2-QZVPP level (see Tables S3 and S4 of the Supporting Information). An equilibrium of different coordination isomers in solution might also explain the fact that the UV−vis spectrum of K1 in THF is rather featureless, showing a broad shoulder at λ = 324 nm with a long tail into the visible region up to 650 nm. This might be a consequence of the superposition of the UV−vis absorptions of the isomers 1a− and 1b− (see Figures S29 and S30 of the Supporting Information). Our TD-DFT calculations indicate that the transitions associated with these complexes in the visible region involve excitations from the metal-based HOMO, HOMO−1, and HOMO−2 orbitals into the ligandbased LUMO (Figure 2 and Table S5). Transitions at higher energy show a composed nature involving MLCT and intraligand charge transfer type excitations. Compound K1 is extremely moisture sensitive and reacts even with traces of water, producing the unusual ionic complex [K([18]-crown-6)]{Cp*Fe(η4-2,4,6-triphenyl-2,3-dihydrophosphinine 1-oxide)}] (K2, Scheme 2). Oxygen atom transfer to phosphorus and simultaneous hydrogenation of one carbon− carbon bond of the phosphinine is observed. The molecular structure of K2 differs from the known η5-complex [CpFe(η52,4,6-triphenyl-1-hydrophosphinine 1-oxide)] (H, Chart 2). The latter compound was obtained by hydrolysis of the cationic complex [CpFe(2,4,6-triphenylphosphinine)]AlCl4 (D) and shows a distinct 1-hydrophosphinine 1-oxide ligand.10 Dark red, crystalline K2 can be prepared deliberately in 42% isolated yield by treating K1 with one equivalent of water in THF. Multinuclear NMR spectra fully support the proposed molecular structure. The 1H NMR spectrum in THF-d8 displays the ABCD part of an ABCDX spin system (Figure 4) with multiplets at δ = 2.37, 2.57, and 3.28 ppm, which correspond to the aliphatic CH(Ph) and CH2 groups. The “olefinic” hydrogen atom HD gives rise to a doublet at 5.57 Hz (3J(HD,P) = 19.2 Hz). The proton-coupled 31P signal for K2 appears as a multiplet at δ = 123.8 ppm (in THF-d8).25 The aliphatic carbon atoms C2 and C3 resonate at δ = 47.2 and 49.1

Chart 2

ppm in the 13C{1H} NMR spectrum. The solid-state IR spectrum features a characteristic absorption band for the P O bond at ν = 1120 cm−1, which compares well to the PO stretch of complex H (ν = 1150 cm−1, Chart 2). A single-crystal X-ray structure determination confirmed that the Cp* and 2,4,6-triphenyl-2,3-dihydrophosphinine 1-oxide units are attached to iron in an η5- and η4-fashion, respectively (Figure 5). The potassium cation is coordinated by [18]crown6. It is additionally connected to the oxygen atom at the phosphorus atom with a K−O distance of 2.503(2) Å. The PC5 ring of the η4-coordinated 2,3-dihydrophosphinine 1-oxide ligand is folded by 54.6° along the P1−C3 axis. The iron carbon and iron phosphorus distances (Fe1−C1/C2/C3 2.007(2)−2.124(2) Å and Fe1−P1 2.138(1) Å) are similar to those in the structure of K1. The C1−C2 (1.422(4) Å) and C2−C3 (1.445(3) Å) bond lengths are also comparable to those of K1, while the C3−C4 (1.532(5) Å) and C4−C5 (1.530(5) Å) bond lengths agree well with the expected value for a C(sp3)−C(sp3) single bond. The P−O bond distance (1.509(2) Å) is similar to that in 2-(2′-pyridyl)-4,6diphenylphosphinine oxide complexes of iridium.26 In further studies, we investigated the chemical oxidation of our anionic iron(0) phosphinine complex [Cp*Fe(η4-2,4,6triphenylphosphinine)}]− (1−). Removing one electron from this anion should result in the neutral 17e− iron(I) complex [Cp*Fe(η4-2,4,6-triphenylphosphinine)}] (1). However, DFT computations suggest that the dimerization of this complex via P−P bond formation is highly exorgonic, with a calculated free enthalpy for the dimerization of −63.1 kcal mol−1 at 298 K (Table S6 of the Supporting Information). Indeed, the reaction D


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Organometallics

Figure 4. 1H NMR signals observed for C2−HC and C3−HAHB of K2 (top) and simulated spectrum (bottom). H−H and P−H coupling constants obtained by simulation: 2J(HA,HB) = 13.7 Hz, 3J(HA,HC) = 7.4 Hz, 3J(HB,HC) = 7.4 Hz, 3J(HA,P) = 2.3, 3J(HB,P) = 39.0, 2J(HC,P) = 15.2 Hz, 3 J(HD,P) = 19.2 Hz. The labeling scheme is given in Chart 2.

Chart 3

ppm, which is shifted downfield by 3.7 ppm compared to the resonance of K1. The solid-state molecular structure of 3 features two dimerized Cp*Fe(η5-2,4,6-triphenylphosphinine) units, which are covalently connected by the phosphorus atoms (Figure 6). The P−P bond length (2.249(2) and 2.251(2) Å) is in the range for a single bond and somewhat shorter than for the dianionic bis(phosphinine) compounds I (2.305(2) Å) and J (2.286(2) Å, Chart 3).27 The λ3σ3 phosphinine ligand is coordinated to both iron atoms in an η5 fashion through the carbon atoms, while the phosphorus atoms are noncoordinating and feature a pyramidalized environment (sum of bond angles around P1 = 308.1° and 294.6°, torsion angle C3−C2−C1−P1 = −21.9° and 26.8°). The Fe−C, C−C, and C−P bond lengths (Table 2) are similar to those of [Fe(η5-1-tert-butyl-2,4,6triphenylphosphinine)2] (F, Chart 1), in which the phosphinine ligand also adopts an η5-coordination mode.13 The subsequent oxidation of complex 3 with I2 (one equivalent) affords the cationic iron(II) phosphinine iodide [Cp*Fe(η6-2,4,6-triphenylphosphinine)]I (4) via P−P bond cleavage (Scheme 2). Direct oxidation of K1 with one equivalent of I2 also gives 4 as an orange solid in 61% isolated yield. Iodide salt 4 is sparingly soluble in THF, but dissolves well in acetonitrile and dichloromethane. Upon oxidation, the characteristic 1H NMR signal for the C3,5-H protons of 3 shifts to lower field and appears as a doublet at δ = 7.16 ppm (3JHP =

Figure 5. Solid-state molecular structure of K2. Ellipsoids are drawn at the 30% probability level, and H atoms except for those of C4 and C5 atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): P1−O7 1.509(2), K1−O7 2.533(2) Å; see Table 1 for additional structural data.

of K1 with [Cp2Fe]PF6 led to the P−P-bonded dimer [Cp*Fe(η5-2,4,6-triphenylphosphinine)]2 (3, Scheme 2). This type of coupling of a metal-phosphinine complex upon oxidation is unprecedented, although it should be noted that similar P−P coupling products (see Chart 3) form when uncoordinated phosphinines are reduced with alkali metals.27,28 Complex 3 was isolated as a dark green crystalline solid in 22% yield after sublimation of ferrocene and crystallization of the residue from THF/n-hexane. The 1H NMR spectrum of 3 in C6D6 displayed one set of resonances for the Cp* and the phosphinine moieties. The characteristic 1H NMR signals for the Cp* moiety and the meta-protons of the phosphinine ligand appear at δ = 0.79 and 5.59 ppm, respectively. The 31 1 P{ H} NMR signal was observed as a singlet at δ = −42.7 E


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Organometallics

Figure 7. Solid-state molecular structure of the [Cp*Fe(η6-2,4,6triphenylphosphinine)]+ (1+) cation in 4; ellipsoids are drawn at the 30% probability level; H atoms and the I− anion are omitted for clarity. Important bond lengths and angles are given in Table 1

the oxidation product 4, a quasi-reversible reduction process is found (E1/2,red = −2.018 V vs Fc*/Fc*+) as well as a quasireversible oxidation process at E1/2,ox = 0.698 V. Further irreversible processes are observed at EP,ox = 0.737 and 1.231 V (Fc*/Fc*+). An inspection of the frontier Kohn−Sham orbitals of the [Cp*Fe(η6-2,4,6-triphenylphosphinine)]+ (1+) cation reveals that the HOMO to HOMO−2 feature predominant metal dcharacter (Figure S37). The LUMO essentially corresponds to an antibonding combination of a metal d-orbital and Cp* πorbital, but also features a small contribution (9%) from a phosphorus p-orbital according to the Löwdin population analysis. The UV−vis spectrum of 4 in dichloromethane features a weak shoulder at λ = 358 nm and strong absorptions in the UV region at λ = 259 and 292 nm (Figure S31). TDDFT calculations reveal the composed nature of these transitions. To a first approximation, the absorption at λ = 358 nm may be characterized as an MLCT-type transition that arises from excitations from the metal-based HOMO−1 and HOMO orbitals to the LUMO and LUMO+1. The UV absorptions at λ = 256 and 292 nm may be assigned to transitions between ligand-based orbitals (see the Supporting Information for details). The reactivity of η6-phosphinine metal complexes is little explored. As shown in Scheme 3, solvolysis reactions with alcohol or water afford λ5σ4-phosphinine complexes presumably by nucleophilic attack at the electrophilic phosphorus center followed by trapping of H+.5,10 In addition, strong nucleophiles also attack free phosphinines at the phosphorus atom.30,31 In order to assess the reactivity of the coordinated 2,4,6triphenylphosphinine ligand in 1+, we sought to investigate its behavior toward selected nucleophiles.32 To our delight, the reactions with LiNMe2, LiBHEt3, LiCp*, and Ga(nacnacDipp) proceeded smoothly, yielding stable complexes with anionic λ3σ3-phosphinine ligands.31,32 Thus, the reaction with LiNMe2 gave the first η5-1-amino-λ3σ3phosphinine complex [Cp*Fe(η 5 -1-dimethylamino-2,4,6triphenylphosphacyclohexadienyl)] (5) by elimination of LiI (Scheme 4a). The reaction of 4 with LiCp* afforded [Cp*Fe(η5-1-(η1-Cp*)-2,4,6-triphenylphosphacyclohexadienyl)] (6, Scheme 4b). The 1-hydro-λ3σ3phosphinine analogue [Cp*Fe(η5-1-hydro-2,4,6triphenylphosphacyclohexadienyl)] (7) was isolated upon reaction of 4 with one equivalent of LiBHEt3. With respect to the latter reaction it is noteworthy that similar products

Figure 6. Solid-state molecular structure of 3. Only one of two crystallographically independent molecules in the unit cell is shown; ellipsoids are drawn at the 30% probability level; H atoms and disorder in the phosphinine ligand are omitted for clarity. Selected bond lengths (Å) and angles (deg): C1−P1 1.808(8) and 1.826(8), C5−P1 1.783(7) and 1.829(8), P1−P2 2.249(2) and 2.251(2), C1−P1−C5 95.2(3) and 85.3(3); see Table 1 for additional structural data.

Table 2. Characteristic 1H, 31P{1H}, and 13C{1H} NMR Data of Complexes 4−8 (solvent: THF-d8) 1

13

H NMR

31

complex 4 5 6 7 8

1

P{ H} NMR (ppm) −0.9 −15.1 −15.3 −80.7 −61.8

3,5

C -H (ppm) 7.16 6.20 6.06 6.33 5.98

(d) (d) (d) (d) (d)

C NMR

3

JHP (Hz)

C2,6 (ppm)

1 JCP (Hz)

4.7 2.8 2.6 2.8 2.5

115.1 (d) 58.8 (d) 47.5(d) 28.6(d) 31.2(d)

64.4 7.9 15.1 2.6 9.0

4.7 Hz). A downfield shift was also observed in the 31P{1H} NMR spectrum (δ = −0.9 ppm). In the 13C{1H} NMR spectrum, the Cring signals appearing in the range δ = 103.2− 129.3 ppm are also shifted downfield with respect to the spectrum of 3. The NMR data of 4 are similar to those of the previously reported ionic complex [CpFe(2,4,6triphenylphosphinine)]PF6 (D), which shows a 31P{1H} NMR singlet at δ = −10.4 ppm in CDCl3. The η6-coordination mode of the phosphinine ligand in 4 was confirmed by the single-crystal X-ray structure (Figure 7), which shows a planar phosphinine ring with structural parameters similar to those of η6-phosphinine complexes [Fe(η4-COD)(η6-2-trimethylsilyl-4,5-dimethylphosphinine)] (B, Chart 1) and [Cr(CO) 3(η 6 -2,4,6-triphenylphosphinine)].8,29 The iodide anion in 4 is separated from the phosphorus atom by 7.3558 Å. This long distance indicates that the interaction between the cation and anion is negligibly small. Cyclic voltammetry (CV) of 3 and 4 (Figure S28 in the Supporting Information) indicates that their electrochemical behavior differs significantly. The CV of compound 3 shows a quasi-reversible oxidation at E1/2,ox = 0.275 V vs Fc/Fc+ and a quasi-reversible reduction at E1/2,red = −2.317 V vs Fc/Fc+. An additional irreversible reduction is observed at −0.427 V. For F


Article

Organometallics Scheme 3. Solvolysis of η6-Phosphinine Complexes5,10

around the P−N bond on the 1H NMR time scale at room temperature. In case of the Cp* derivative 6, the η5-coordinated C5Me5 unit at iron shows a sharp 1H NMR signal at δ = 0.97 ppm, whereas the C5Me5 unit at the phosphorus atom gives rise to a broad singlet at δ = 1.26 ppm at room temperature. In the 13 C{1H} NMR spectrum at 300 K, a broad doublet at δ = 12.3 ppm (3JCP = 5.0 Hz) is observed for the methyl groups. However, no signal corresponding to the quarternary carbons of the C5Me5 unit was detected, probably due to the broadening of this signal. A low-temperature 1H NMR spectrum in THF-d8 revealed the decoalescence of the C5Me5 resonances at low temperature. Three methyl signals are observed for the P−C5Me5 unit in the aliphatic region in the expected 2:2:1 intensity ratio at 193 K (Figure S19). Moreover, the low-temperature spectrum displays the expected eight phenyl resonances, whereas only six phenyl signals are detected at room temperature due to the rapid 1,5-haptotropic rearrangement of the η1-coordinated C5Me5 moiety. Similar haptotropic rearrangements have been reported for other Cp*substituted trivalent phosphanes and phosphorus cations.33,34 The presence of a P−H moiety in 7 is evident from its characteristic 1H NMR doublet at δ = +4.91 ppm with a 1JHP coupling constant of 140.0 Hz. The corresponding 31P NMR doublet was detected at δ = −80.7 ppm. These characteristic values compare well with those of the previously characterized complex [CpFe(η5-1-hydro-2,4,6-triphenylphosphacyclohexadienyl)] (δ1H (P−H) = 5.30 ppm (d, 1JHP = 150 Hz), δ31P = −83.8 ppm in C6D6).10 The solid-state IR spectrum of 7 displays an absorption band at ν = 2119 cm−1, which we assign to the P−H stretch. The 1H NMR spectrum of 8 confirms the constitution of this complex, showing a set of four doublets and two septets for the characteristic diasterotopic methyl and methine group of the Dipp unit, respectively. Single-crystal X-ray diffraction studies of 5−8 (Figure 8) confirmed that both the Cp* and the λ3σ3-phosphinine units are attached to Fe center in an η5-fashion. The phosphorus atom is bent away from the iron center (angle between planes passing through C1, C2, C3, C4 and C1, C2, P1 = 28.1° for 5, 39.7° for 6, 38.2° for 7, and 39.4° for 8) with a strongly pyramidalized geometry (sum of angles around the P atom: 308.7−309.6° (5), 314.1° (6), 302.1° (7), 302.5° (8)). The Fe−C and C−C bond lengths are in the same range as observed in the diphosphinine complex 3 (Table 1). The crystal structure of complex 5 (Figure 8) features four crystallographically independent molecules in the asymmetric unit with largely similar structural parameters, but slightly

Scheme 4. Synthesis of Complexes 5−8a

a Reagents and byproducts: (a) +LiNMe2/−LiI; (b) +Li(C5Me5)/− LiI; (c) +LiBHEt3/−LiI, −BEt3, (d) +Ga(nacnacDipp); isolated yields of the products are given in parentheses.

result from hydrogen atom incorporation into different positions of the phosphinine unit when the triphenylphosphinine-1-hydro-1-oxide complex H (Chart 2) is reduced with HSiCl3, as reported by Nief and Fischer.10 Finally, we found that the gallium(I) species Ga(nacnacDipp) adds to the phosphorus atom of 4, yielding the gallium(III) complex [Cp*Fe(η5-1-{Ga(nacnacDipp)I}-2,4,6-triphenylphosphacyclohexadienyl)] (8, Scheme 4d). The λ3σ3-phosphinine iron complexes 5−8 were isolated as thermally stable, red crystalline solids in moderate yields. They are soluble in THF and acetonitrile. The 1H NMR signals for the C3,5-H protons of 5−8 appear in the range δ = 5.98−6.33 ppm (Table 1). The 31P{1H} NMR signals (δ = −15.1 to −80.7 ppm) and 13C{1H} NMR data for the carbon atom (C2,6) directly attached to the phosphorus atom in all four complexes 5−8 are upfield shifted with respect to those of the starting material (Table 2). The two methyl groups of the NMe2 moiety in 5 are diasterotopic and give two singlets at δ = 1.85 and 1.87 ppm in the 1H NMR spectrum. This reflects the slow rotation G


Article

Organometallics

Figure 8. Solid-state molecular structures of complexes 5−8. Ellipsoids are drawn at the 30% probability level; H atoms are omitted for clarity; one of four crystallographically independent molecules of 5 is displayed; the disorder of the structure of 6 is not shown; key bond lengths (Å) and angles (deg) for 5: C1−P1 1.801(2)−1.821(3), C5−P1 1.793(2)−1.810(3), P1−N1 1.697(3)−1.713(3) Å, C1−P1−C5 93.5(1)−94.2(1); for 6: C1−P1 1.804(5), C5−P1 1.716(4), P1−C24 2.264(6), P1−C23 2.699(5), P1−C25 2.910(5), P1′−C21 2.364(6), C1−P1−C5 93.6(2); for 7: C1−P1 1.810(2), C5−P1 1.817(2), P1−H1 1.42(2), C1−P1−C5 94.0(1); for 8: C1−P1 1.832(3), C5−P1 1.822(3), P1−Ga1 2.3673(8), Ga1−I1 2.6082(4), Ga1−N1 1.966(3), Ga1−N2 1.977(2), C1−P1−C5 92.3(1), N1−Ga1−N2 95.9(1); see Table 1 for additional structural data.

P−C bond length of 2.004 Å for the carbon atom directly attached to phosphorus. Similar results were obtained with hybrid functionals such as B3LYP. This carbon atom is somewhat pyramidalized in the calculated structure (sum of angles 341.8°), while the same atom appears to be planar in the X-ray structure of 6 (sum of angles around C24: 360.0(2)°), although this appears to be an artifact of the disorder of this Cp* moiety. A constrained-geometry DFT-optimized structure of 6 (D3-RI-BP86/def2-TZVP) with a fixed P−C bond length of 2.263 Å gave an electronic energy +2.8 kcal mol−1 higher than the energy of the freely optimized structure of 6. The molecular structure of 7 is similar to that of 5 and 6 (Figure 8). The hydrogen atom bound to phosphorus refined with an unexceptional P−H bond length of 1.42(2) Å, which is also reproduced by the DFT calculations (calculated P−H 1.45 Å). In the structure of complex 8 (Figure 8), the nacnacDipp ligand attached to the tetracoordinated gallium atom in 8 is bent away from the phosphinine ring, most probably due to steric repulsion. The gallium atom points out of the nacnacDipp ligand plane (angle between planes passing through N1, Ga1, N2 and N1, C34, C35, C36, N2 = 32.5°) and has a tetrahedral geometry with Ga−I and Ga−P single bond lengths of 2.6082(4) and 2.3673(8) Å, respectively. DFT calculations (BP86/def2-TZVP level) reveal a similar electronic structure for 5, 7, and 8.41 The highest occupied

different orientations of the NMe2 groups. The nitrogen atoms are in an essentially planar environment (sum of angles: 352.3− 359.7° for four crystallographically independent molecules). The P−N bond lengths (1.697(3)−1.713(2) Å) are comparable to those in Cp*2P(NMe2) (P−N 1.680(3) Å),35 PCl2(NMe2) (P−N 1.632(2) Å),36 and P(NMe2)3 (P−N 1.681(1)−1.699(3) Å).37,38 The structure refinement of 6 is affected by the disorder of the molecule over a crystallographic mirror plane, which can only partially be resolved, resulting in a modest quality of the refined structure. However, the single-crystal X-ray structure determination clearly shows that the Cp* substituent at phosphorus binds in an η1 mode (Figure 8). The Cp* ring attached to phosphorus is oriented parallel to the phosphinine C5 ring plane (C1−C2−C3−C4) with an interplanar angle of merely 3.7°. A similar structure with an η1-coordinated Cp* unit was observed for the 1-pentamethylcyclopentadienylphosphole, [Me2(CO2Et)2C4PCp*], which shows a P−C bond length of 1.897(2) Å.39 The P−C distances vary from 1.967(5) to 2.843(5) Å in [Cp*2P][μ-Cl].34 We believe that the apparent observation of unusually long P−C bonds for the P−Cp* moiety in 6 (P1−C24 2.264(6) and P1′− C21 2.364(6) Å, respectively) is partially due to the disorder of this moiety.40 DFT optimizations at the D3-RI-BP86/def2-TZVP level reproduced the basic structural features of 6, but gave a shorter H


Article

Organometallics

to the calculations, the same HOMO → LUMO and HOMO− 2 → LUMO excitations also contribute significantly to the shoulders at λ = 373 nm (5) and 392 nm (7).

molecular orbital corresponds to a combination of an iron dorbital and a π-orbital of the anionic λ3σ3-phosphinine ligand (see the representative MO scheme for complex 5 depicted in Figure 9). According to a Löwdin population analysis, three out

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CONCLUSIONS Previously established methods for the synthesis of transition metal phosphinine complexes include the use of cocondensation techniques,7,8 exchange of carbon monoxide in iron carbonyl complexes,9 Fischer−Hafner-type reactions,10 and salt metathesis of reduced phosphinine anions or anionic phosphinine derivatives.11−13 The preparation of the anionic iron(0) phosphinine iron complex K1 illustrates a new synthetic approach to this type of complex that is based on ligand exchange in a low-valent naphthalene iron complex. The fact that a range of related polyarene metalates has become accessible in recent years bodes well for a future extension of this methodology.14−17 The new anionic iron(0) phosphinine complex [Cp*Fe(η42,4,6-triphenylphosphinine)}] (1−) displays the scarce η4coordination mode for the phosphinine.9 This complex proved to be useful for the synthesis of new sandwich compounds with π-coordinated phosphinine ligands. Thus, the anion 1− can be readily oxidized by one and two electrons to the neutral η5phosphinine dimer [Cp*Fe(η5-2,4,6-triphenylphosphinine)]2 (3) and to the ionic sandwich compound [Cp*Fe(η6-2,4,6triphenylphosphinine)]I (4). The molecular structures of 3 and 4 highlight the ability of 2,4,6-triphenylphosphinine to adapt to the electronic requirements of the cation in a metal complex by changing its hapticity, acting as a 4e− donor in the structures of the anions 1− and 2−, a 5e− donor in the neutral 3 (as well as in 5−8), and a 6e− donor in the structure of 4. Reactions with selected nucleophiles have demonstrated the P-centered reactivity of the [Cp*Fe(η6-2,4,6-triphenylphosphinine)]+ (1+) cation present in the iodide salt 4, yielding the new λ3σ3-phosphinine complexes 5−8. An extensive family of related compounds ought to be accessible via this route in the future. Furthermore, it will be of interest to examine whether the electrophilictiy of the phosphorus atom in complex 4 can be exploited for new bond activation reactions. Investigations in these directions are under way.

Figure 9. Calculated frontier Kohn−Sham orbitals of [Cp*Fe(η5-1dimethylamino-2,4,6-triphenylphosphacyclohexadienyl)] (5) with reduced Löwdin atomic orbital populations per molecular orbital.24

of the four highest MOs (HOMO to HOMO−3) of 5, 7, and 8 have predominant (>50%) iron d character, showing that these molecules can be considered as d6 complexes. The HOMO−2 or HOMO−3 displays a partial P lone-pair character. The LUMO is an essentially ligand-based orbital with little more than 30% iron d-orbital contribution. The electronic structure of the Cp*-substituted complex 6 appears to be distinct from those of the remaining compounds by a more strongly ligandbased LUMO and additional contributions from the π*-orbitals of the Cp* substituent on phosphorus to HOMO−2 and HOMO−3 (see Figure S42). The similar electronic structures of complexes 5−8 are reflected by their UV−vis spectra. Each spectrum (recorded in THF) displays three bands in the near-UV and visible regions: two bands of moderate intensity in the ranges λmax = 465−489 nm and λmax = 523−572 nm and a strong shoulder approximately in the range λ = 350−400 nm. In order to gain insight into the nature of these observed transitions, we performed TD-DFT calculations (B3LYP/def2-TZVP) on the optimized geometries of complexes 5 and 7. The results show that the experimentally observed bands at λ = 465 and 523 nm for 5 and λ = 483 and 555 nm for 7 essentially correspond to metal-to-ligand charge transfer type transitions that predominantly arise from an excitation from the metal-centered HOMO into the ligand-based LUMO (Figure 9) with additional contributions from the HOMO−2 → LUMO excitations (Table S5 and Figures S41 and S43). According

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EXPERIMENTAL SECTION

General Considerations. All experiments were performed under an atmosphere of dry argon, by using standard Schlenk and glovebox techniques. Solvents were purified, dried, and degassed with an MBraun SPS800 solvent purification system. NMR spectra were recorded on Bruker Avance 300 and Avance 400 spectrometers at 300 K and internally referenced to residual solvent resonances. The assignment of the 1H and 13C NMR signals was confirmed by twodimensional (COSY, HSQC, and HMBC) experiments. Melting points were measured on samples in sealed capillaries on a Stuart SMP10 melting point apparatus. UV/vis spectra were recorded on a Varian Cary 50 spectrometer. Elemental analyses were determined by the analytical department of Regensburg University. The starting materials Cp*Li, 42 [K([18]crown-6)][Cp*Fe(C 10 H 8 )], 23 Ga(nacnacDipp),43 and 2,4,6-triphenylphosphinine (abbreviated as TPP)44 were prepared according to literature procedures. Iodine, ferrocenium hexafluorophosphate, and lithium dimethylamide were purchased from Sigma-Aldrich and used as received. [K([18]crown-6)][Cp*Fe(2,4,6-triphenylphosphinine)] (K1). A solution of 2,4,6-triphenylphosphinine (0.66 g, 2.03 mmol) in THF (20 mL) at −30 °C was added to a solution of [K([18]crown6){Cp*Fe(C10H8)}] (1.26 g, 2.03 mmol) in THF (50 mL) at −50 °C. The reaction mixture was stirred overnight and warmed to room temperature. Filtration and layering with n-hexane (100 mL) at room I


Article

Organometallics

300 K, C6D6): δ = 11.4 (s, C5(CH3)5), 40.9 (s, C4, TPP), 47.2 (d, 2JCP = 18.9, C3HAHB, TPP), 49.1 (d, 1JCP = 13.1 Hz, C2, TPP), 61.1 (d, 1JCP = 9.1 Hz, C6, TPP), 66.8 (d, 2JCP = 12.8 Hz, C5-H, TPP), 69.9 (s, CH2 of [18]crown-6), 82.7 (s, C5(CH3)5), 121.2 (s, C4-H of C4-Ph), 121.3 (s, C4-H of C2-Ph), 124.0 (s, C4-H of C2-Ph), 126.3 (s, C2,6-H of C6Ph), 127.4 (s, C3,5-H of C4-Ph), 127.76 (s, C3,5-H of C6-Ph), 127.81 (s, C3,5-H of C2-Ph), 128.0 (s, C2,6-H of C4-Ph), 129.2 (d, 3JCP = 5.9 Hz, C2,6-H of C4-Ph), 149.3 (s, C1 of C2-Ph), 151.5 (s, C1 of C4-Ph), 154.1 (s, C1 of C6-Ph). 31P{1H} NMR (161.98 MHz, 300 K, THF-d8): δ = 123.8 (s). 31P NMR (161.98 MHz, 300 K, THF-d8): δ = 123.8 (m, Xpart of the ABCDX spin system, 3J(P,HA) = 2.3 Hz, 3J(P,HB) = 39.0 Hz, 2J(P,HC) = 17.8 Hz, 2J(P,HD) = 19 Hz). Anal. Calcd for C45H58FeKO7P·OC4H8 (MW = 908.98 g·mol−1): C 64.74, H 7.32. Found: C 64.31, H 7.39. [Cp*Fe(2,4,6-triphenylphosphinine)]2 (3). A suspension of ferrocenium hexafluorophosphate (0.24 g, 0.73 mmol) in THF (20 mL)

temperature gave dark brown crystals. Yield: 1.13 g (1.37 mmol, 58%); mp >112 °C. UV/vis (THF) λmax (nm, (λmax/L·mol−1·cm−1)): 284 (2745), 324sh (1417). 1H NMR (400.13 MHz, 300 K, THF-d8): δ = 1.24 (s, 15H, C5(CH3)5), 1.78 (THF), 3.41 (s, 24H, CH2 of [18]crown-6), 3.62 (THF), 6.67 (t, 1H, C4-H of C4-Ph), 6.77−7.01 (m, 2H, C3,5-H of C4-Ph + 6H, C-H of C2,6-Ph + 2H, C3,5-H of TPP), 7.31 (d, 3JHH = 7.6 Hz, 2H, C2,6-H of C4-Ph), 7.85 (br s, 4H, C-H of C2,6-Ph). 13C{1H} NMR (100.61 MHz, 300 K, THF-d8): δ = 11.1 (s, C5(CH3)5), 70.9 (s, CH2 of [18]crown-6), 80.5 (s, C5(CH3)5), 119.6 (s, C4-H of C4-Ph), 121.9 (s, C4-H of C2,6-Ph), 125.6 (s, C2,6-H of C4Ph), 127.4 (s, C-H of C2,6-Ph), 127.5 (s, C3,5-H of C4-Ph), (signals for the TPP ring carbon atoms and C1 resonances of Ph were not observed). 31P{1H} NMR (161.98 MHz, 300 K, THF-d8): δ = −46.4 (s br); additional 31P{1H} signals for minor impurities are observed at δ = 29.9, −136.2, and −160.4 ppm, the combined integrals of these signals amount to 178 °C. UV/vis (THF) λmax (nm, (λmax/L·mol−1·cm−1)): 296sh (4423), 376sh (151), 433sh (65), 597 (390). IR (KBr): ν (cm−1) = 1150. 1H NMR (400.13 MHz, 300 K, THF-d8): δ = 0.79 (s, 30H, 2 × C5(CH3)5), 5.59 (vt, 4H, 2 × C3,5-H, TPP), 7.00−7.04 (m, 12H, 2 × C3,4,5-H of C2,6-Ph), 7.42−7.51 (m, 6H, 2 × C3,4,5-H of C4-Ph), 7.59− 7.61 (m, 8H, 2 × C2,6-H of C2,6-Ph), 7.83 (d, 4H, 2 × C2,6-H of C4Ph). 13C{1H} NMR (100.61 MHz, 300 K, THF-d8): δ = 8.7 (s, C5(CH3)5), 41.2 (vt, JCP = 6.0 Hz, C2,6, TPP), 80.6 (vt, JCP = 6.0 Hz, C3,5-H, TPP), 84.8 (s, C5(CH3)5), 93.8 (s, C4, TPP), 124.4 (s, C4-H, C2,6-Ph), 127.5 (s, C4-H of C4−Ph), 128.15 (s, C3,5-H of C2,6-Ph), 128.2 (s, C2,6-H of C4-Ph), 128.7 (t, 3JCP = 10.1 Hz, C2,6-H of C2,6-Ph), 128.9 (s, C3,5-H of C4-Ph), 142.0 (s, C1 of C4-Ph), 145.6 (vt, JCP = 11.7 Hz, C1 of C2,6-Ph). 31P{1H} NMR (161.98 MHz, 300 K, THF-d8): δ = −42.7 (s). Anal. Calcd for C66H64Fe2P2 (MW = 1030.88 g·mol−1): C 76.90, H 6.26. Found: C 76.96, H 6.02. [Cp*Fe(2,4,6-triphenylphosphinine)]I (4). Iodine (0.15 g, 0.61 mmol) was added to a solution of K1 (0.5 g, 0.61 mmol) in THF (50

was cooled to −30 °C. Water (0.01 mL, 0.56 mmol) was added to the solution with a microliter syringe. The dark red solution was stirred overnight at room temperature and was then concentrated to 10 mL. Dark red crystals were isolated after diffusion of n-hexane (60 mL) into this solution. The isolated compound contains one THF solvate molecule per formula unit after drying under vacuum. Yield: 0.092 g (42%); mp >140 °C. UV/vis (THF) λmax (nm, (λmax/L·mol−1·cm−1)): 239 (2873), 293 (2280), 324 (2226), 412 (395), 481 (197). IR (cm−1, KBr): ν = 1150. 1H NMR (400.13 MHz, 300 K, C6D6): δ = 1.85 (s, 15H, C5(CH3)5), 2.37 (m, 3J(HA,P) = 2.3 Hz, 2J(HA,HB) = 13.7 Hz, 3 J(HA,HC) = 7.4 Hz, 1H, C3HAHB, TPP), 2.57 (ddd, 3J(HB,P) = 39.0 Hz, 2J(HA,HB) = 13.7 Hz, 3J(HA,HC) = 7.4 Hz, 1H, C3HAHB, TPP), 3.03 (s, 24H, CH2 of [18]crown-6), 3.29 (dt, 2J(HC,P) = 17.8 Hz, 2 J(HA/B,HC) = 7.4 Hz, 1H, C2-HC, TPP), 5.57 (d, 3J(HD,P) = 19.2 Hz, 2H, C5-HD, TPP), 6.96 (t, 3JHH = 7.3 Hz, 2H, C4-H of C2-Ph), 7.08 (t, 3 JHH = 7.2 Hz, 2H, C4-H of C6-Ph), 7.18 (t, 3JHH = 7.6 Hz, 2H, C3,5-H of C2-Ph), 7.22−7.25 (m, 6H, C3,4,5-H of C4-Ph), 7.35 (t, 3JHH = 7.7 Hz, 4H, C3,5-H of C6-Ph), 7.66 (d, 3JHH = 7.5 Hz, 4H, C2,6-H of C2Ph), 7.90 (d, 3JHH = 7.7 Hz, 4H, C2,6-H of C6-Ph), 8.40 (dd, 3JHH = 7.3, 4 JHH = 2.0 Hz, 2H, C2,6-H of C4-Ph). 13C{1H} NMR (100.61 MHz,

mL) at room temperature. An orange solid precipitated after a few minutes. The reaction mixture was stirred overnight, and the solution was then removed by filtration. The solid residue was dissolved in hot acetonitrile. Orange crystals formed after slow cooling of this solution to room temperature. Yield: 0.24 g (0.37 mmol, 61%); mp >226 °C. UV/vis (CH2Cl2) λmax (nm, (λmax/L·mol−1·cm−1)): 259 (3454), 292 (2469), 358sh (233). 1H NMR (400.13 MHz, 300 K, CD3CN): δ = 1.26 (s, 15H, C5(CH3)5), 7.16 (d, 3JPH = 4.7 Hz, 2H, C3,5-H, TPP), J


Article

Organometallics

mol−1·cm−1)): 396sh (3132), 477 (1671), 548 (1187). 1H NMR (400.13 MHz, 300 K, THF-d8): δ = 0.97 (s, 15H, C5(CH3)5), 1.26 (s br, 15H, P-C5(CH3)5), 6.06 (d, 3JPH = 2.6 Hz, 2H, C3,5-H, TPP), 7.10 (t, 3JHH = 7.2 Hz, 2H, C4-H of C2,6-Ph), 7.28 (vt, 3JHH = 7.6 Hz, 4H, C3,5-H of C2,6-Ph), 7.45 (m, 1H, C4-H of C4−Ph), 7.55 (m, 2H, C3,5-H of C4-Ph), 8.12 (br, 4H, C2,6-H of C2,6-Ph), 8.23−8.26 (m, 2H, C2,6-H of C4-Ph). 13C{1H} NMR (100.61 MHz, 300 K, THF-d8): δ = 8.9 (s, C5(CH3)5), 12.3 (d br, 3JC,P = 5.0 Hz, P-C5(CH3)5), 47.5 (d, 1JCP = 15.1 Hz, C2,6, TPP), 79.9 (d, 2JCP = 7.0 Hz, C3,5-H, TPP), 85.3 (s, C5(CH3)5), 93.2 (d, 3JCP = 3.0 Hz, C4, TPP), 124.9 (s, C3,5-H of C2,6Ph), 125.0 (s, C4-H of C2,6-Ph), 128.0, 128.1 (singlets, C2,6-H and C4H of C4-Ph), 128.5 (s, C3,5-H of C2,6-Ph), 129.5 (s, C3,5-H of C4-Ph), 141.5 (s, C1 of C4-Ph), 145.8 (d, 2JCP = 24.1 Hz, C1 of C2,6-Ph), signals for P-C5(CH3)5 and for C2,6-H of C2,6-Ph were not observed due to fluxional behavior. 31P{1H} NMR (161.98 MHz, 300 K, THF-d8): δ = −15.3 (s). Anal. Calcd for C43H47FeP (MW = 650.65 g·mol−1): C 79.38, H 7.28. Found: C 78.48, H 7.14. [Cp*Fe(1-hydro-2,4,6-triphenylphosphacyclohexadienyl)] (7). A solution of LiBEt3H (0.13 mL, 1 M solution in THF) was added to a

7.60 (m, 6H, C3,4,5-H of C2,6-Ph), 7.71 (m, 3H, C3,4,5-H of C4-Ph), 8.14 (m, 4H, C2,6-H of C2,6-Ph), 8.34 (m, 2H, C2,6-H of C4-Ph). 13C{1H} NMR (100.61 MHz, 300 K, CD3CN): δ = 9.6 (s, C5(CH3)5), 86.5 (d, 2 JCP = 6.9 Hz, C3,5-H, TPP), 94.1 (s, C5(CH3)5), 103.2 (d, 3JCP = 3.0 Hz, C4, TPP), 115.1 (d, 1JCP = 64.4 Hz, C2,6, TPP), 129.3 (d, 3JCP = 18.1 Hz, C2,6-H of C2,6-Ph), 130.0 (s, C2,6-H of C4-Ph), 130.5 (s, C3,5-H of C2,6-Ph), 130.6 (s, C3,5-H of C4−Ph), 131.4 (s, C4-H of C4-Ph), 134.1 (s, C1 of C4-Ph), 136.6 (d, 2JCP = 22.1 Hz, C1 of C2,6-Ph). 31 1 P{ H} NMR (161.98 MHz, 300 K, CD3CN): δ = −0.9 (s). Anal. Calcd for C33H32FePI (MW = 642.34 g·mol−1): C 61.71, H 5.02. Found: C 62.12, H 5.00. [Cp*Fe(1-dimethylamino-2,4,6-triphenylphosphacyclohexadienyl)] (5). Solid lithium dimethylamide (0.01 g, 0.23 mmol) was added as

a solid to a solution of 2 (0.15 g, 0.23 mmol) in acetonitrile (20 mL). The color of the solution changed to red. The reaction mixture was stirred for 2 h at room temperature and was then concentrated to 5 mL in vacuo. Red crystals formed at 2 °C after 1 day. Yield: 0.058 g (0.01 mmol, 41%); mp > 200 °C (dec). UV/vis (THF) λmax (nm, (λmax/L·mol−1·cm−1)): 373sh (4729), 465 (1674), 523 (1035). 1H NMR (400.13 MHz, 300 K, THF-d8): δ = 1.07 (s, 15H, C5(CH3)5, 1.85 (s, 3H, N(CH3)A(CH3)B), 1.87 (s, 3H, N(CH3)A(CH3)B), 6.20 (d, 3JPH = 2.8 Hz, 2H, C3,5-H, TPP), 7.16 (t, 3JHH = 7.2 Hz, 2H, C4-H of C2,6-Ph), 7.31 (t, 3JHH = 7.2 Hz, 4H, C3,5-H of C2,6-Ph), 7.40−7.47 (m, 1H, C4-H of C4-Ph), 7.50−7.58 (m, 2H, C3,5-H of C4-Ph), 8.20− 8.30 (m, 6H, C2,6-H of C2,4,6-Ph). 13C{1H} NMR (400.13 MHz, 300 K, THF-d8): δ = 9.1 (s, C5(CH3)5), 40.7 (s, N(CH3)A(CH3)B), 40.8 (s, N(CH3)A(CH3)B), 58.8 (d, 1JPC = 7.9 Hz, C2,6, TPP), 70.9 (s, C5(CH3)5), 80.1 (d, 2JPC = 6.8 Hz, C3,5-H, TPP), 125.3 (s, 2 × C4-H of C2,6-Ph), 127.9 (s, C4-H of C4-Ph), 128.0, 128.1, 128.4 (m, C2,6-H of C2,4,6-Ph), 128.7 (s, C3,5-H of C2,6-Ph), 129.4 (s, C3,5-H of C4−Ph), 141.5 (s, C1 of C4-Ph), 145.6 (d, 2JCP = 26.3 Hz, C1 of C2,6-Ph). 31 1 P{ H} NMR (161.98 MHz, 300 K, THF-d8): δ = −15.1 (s). Anal. Calcd for C35H38FeNP (MW = 559.21 g·mol−1): C 75.13, H 6.85, N 2.50. Found: C 75.66, H 6.81, N 2.96. [Cp*Fe(1-pentamethylcyclopentadienyl-2,4,6-triphenylphosphacyclohexadienyl)] (6). Cp*Li (0.022 g, 0.16 mmol) was

solution of [Cp*Fe(2,4,6-triphenylphosphinine)]I (0.08 g, 0.13 mmol) in acetonitrile at −30 °C. The color of the solution changed to deep red. The mixture was warmed slowly to room temperature overnight, and the solvent was removed under vacuum until a dark purple solid residue precipitated. The solution was warmed to dissolve the solid residue and stored at 2 °C overnight. Compound 7 was isolated as a dark red crystalline solid by filtration. Yield: 0.024 g (0.046 mmol, 58%); mp >198 °C. UV/vis (THF) λmax (nm, (λmax/L· mol−1·cm−1)): 392sh (4255), 483 (1712), 555 (1395). IR (cm−1, KBr): ν = 2119. 1H NMR (400.13 MHz, 300 K, THF-d8): δ = 1.01 (s, 15H, C5(CH3)5), 4.91 (d, 1JHP = 140.0 Hz, 1H, P-H), 6.33 (d, 3JPH = 2.8 Hz, 2H, C3,5-H, TPP), 7.12 (m, 2H, C4-H of C2,6-Ph), 7.29 (m, 4H, C3,5-H of C2,6-Ph), 7.47 (m, 1H, C4-H of C4-Ph), 7.58 (m, 2H, C3,5-H of C4-Ph), 8.07−8.10 (m, 4H, C2,6-H of C2,6-Ph), 8.34−8.37 (m, 2H, C2,6-H of C4-Ph). 13C{1H} NMR (100.61 MHz, 300 K, THF-d8): δ = 9.0 (s, C5(CH3)5), 28.6 (d, 1JCP = 2.6 Hz, C2,6, TPP), 86.5 (d, 2JCP = 7.9 Hz, C3,5-H, TPP), 85.8 (s, C5(CH3)5), 97.5 (d, 3JCP = 1.1 Hz, C4, TPP), 124.8 (s, C3,5-H of C2,6-Ph), 124.9 (s, C4-H of C2,6-Ph), 128.3 (s, C2,6-H and C4-H of C4-Ph), 128.9 (s, C3,5-H of C2,6-Ph), 129.2 (d, 3JCP = 19.5 Hz, C2,6-H of C2,6-Ph), 129.6 (s, C3,5-H of C4-Ph), 141.6 (s, C1 of C4-Ph), 145.5 (d, 2JCP = 25.1 Hz, C1 of C2,6-Ph). 31P{1H} NMR (161.98 MHz, 300 K, THF-d8): δ = −80.7 (s). 31P NMR (161.98 MHz, 300 K, THF-d8): δ = −80.7 (d, 1JPH = 139 Hz). Anal. Calcd for C35H33FeP (MW = 516.43 g·mol): C 76.75, H 6.44. Found: C 76.26, H 6.48. [Cp*Fe(1-(nacnac D i p p )GaI-2,4,6-triphenylphosphacyclohexadienyl)] (8). Solid Ga(nacnacDipp) (0.078 g, 0.156 mmol) was added to a solution of [Cp*Fe(2,4,6-triphenylphosphinine)]I (0.10 g, 0.156 mmol) in THF (50 mL). The color of the mixture changed to dark brown. The mixture was stirred for 2 h at room temperature. The resulting solution was concentrated afterward under vacuum until a dark solid precipitated. The solution was warmed to dissolve the solid residue. A dark red crystalline solid of complex 6 formed after one night at room temperature. Yield: 0.106 g (0.094 mmol, 60%); mp >242 °C. UV/vis (THF) λmax (nm, (λmax/L·mol−1·cm−1)): 350 (2721), 489 (200), 571 (150). 1H NMR (400.13 MHz, 300 K, THF-d8): δ = 0.69 (s, 15H, C5(CH3)5), 0.85 (d, 3JHH = 6.6 Hz, 6H, CH3 of Dipp iPr), 0.92 (d, 3JHH = 6.6 Hz, 6H, CH3 of Dipp iPr), 0.98 (d, 3JHH = 6.6 Hz, 6H, CH3 of Dipp iPr), 1.22 (d, 3JHH = 6.6 Hz, 6H,

added to a solution of [Cp*Fe(2,4,6-triphenylphosphinine)]I (0.1 g, 0.16 mmol) in acetonitrile (20 mL) and stirred for 1 h, affording a red crystalline solid. The solid material was isolated by filtration and dissolved in THF. X-ray quality crystals formed after diffusion of diethyl ether into the THF solution at −20 °C. The isolated solid contained minor impurities with 31P NMR resonances at 15.5 and 13.8 ppm (140 °C. UV/vis (THF) λmax (nm, (λmax/L· K


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Organometallics

DFT Calculations. The calculations on 1a−, 1b−, 1c−, 1, 1+, 3, 4, 5, 6, 7, and 8 were performed using the ORCA program package (version 3.0.2.).48 The BP86 density functional and the Ahlrichs def2TZVP basis set were employed for all atoms.49,50 The RI approximation was used.51 The Ahlrichs Coulomb fitting basis for the TZVP basis for all atoms (TZV/J) and the atom-pair-wise dispersion correction to the DFT energy with Becke−Johnson damping (d3bj) were applied.52 The nature of the stationary points was verified by numerical frequency analyses. Due to the large size of the molecules, imaginary frequencies less than −10 cm−1 were ignored. The calculation of UV/vis spectra was performed with the B3LYP functional, and a triple-ξ basis set with one set of polarization functions (def2-TZVP), the D3 dispersion correction with Becke−Johnson damping (d3bj), and the RICOSX/J approximation53 for all atoms was employed. Reduced orbital charges and spin densities were calculated according to the Löwdin population analysis.24 Molecular orbitals and the spin density plots were visualized via the program Gabedit.54 The isosurface value is set to 0.05 for all figures, except for the plot of the spin density of 1 (Figure S38).

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CH3 of Dipp iPr), 1.52 (s, 6H, CH3 of nacnacDipp backbone), 2.92 (sept, 3JHH = 6.6 Hz, 2H, CH of Dipp iPr), 3.37 (sept, 3JHH = 6.6 Hz, 2H, CH of Dipp iPr), 5.13 (s, 1H, CH of nacnacDipp), 5.98 (d, 3JHP = 2.5 Hz, 2H, C3,5-H, TPP), 6.83 (d, 3JHH = 8.0 Hz, 2H, C3,5-H of Dipp), 6.88 (d, 3JHH = 8.0 Hz, 2H, C3,5-H of Dipp), 6.96−7.00 (m, 10H, C4-H of Dipp and C-H of C2,6-Ph TPP), 7.41 (m, 1H, C4-H of C4−Ph TPP), 7.49 (t, 3JHH = 8.0 Hz, 2H, C3,5-H of C4-Ph TPP), 7.98 (br, 2H, C-H of C2,6-Ph TPP), 8.27 (d, 3JHH = 8.0 Hz, C2,6-H of C4-Ph TPP). 13C{1H} NMR (100.61 MHz, 300 K, THF-d8): δ = 8.4 (s, C5(CH3)5), 23.95, 23.98 (each singlet, CH3 of Dipp iPr), 24.1 (s, CH3 of nacnacDipp backbone), 24.5 (s, CH3 of Dipp iPr), 28.8 (s, CH3 of Dipp iPr), 29.7, 29.8 (singlets, CH of Dipp iPr), 31.2 (d, 1JCP = 9.0 Hz, C2,6, TPP), 81.4 (d, 2JCP = 8.0 Hz, C3,5-H, TPP), 86.0 (s, C5(CH3)5), 97.9 (d, 3JCP = 1.3 Hz, C4 of TPP), 98.7 (s, CH of nacnacDipp), 124.25, 124.28, (singlets, C-H of C2,6-Ph TPP), 124.3, 125.5 (s, C3,5-H, Dipp), 127.75 (s, C4-H of C4-Ph TPP), 127.8 (s, C4-H, Dipp), 128.6 (br, C-H of C2,6-Ph TPP), 128.7 (s, C2,6-H of C4-Ph TPP), 128.9 (s, C3,5-H of C4-Ph TPP), 129.6 (s, C3,5-H of C4-Ph TPP), 141.9 (s, C1 of C4-Ph TPP), 142.2 (s, C1, Dipp), 142.9, 145.6 (singlets, C2,6 of Dipp), 144.9 (d, 2JPC = 25.0 Hz, C1 of C2,6-Ph TPP), 169.3 (s, NC-CH3 of nacnacDipp). 31P{1H} NMR (161.98 MHz, 300 K, THF-d8): δ = −61.8 (s). Anal. Calcd for C62H73FeGaIN2P (MW = 1129.73 g·mol−1): C 65.92, H 6.51, N 2.48. Found: C 65.99, H 6.58, N 2.37. Cyclic Voltammetry. Cyclic voltammetry measurements (MeCN, 0.1 M Bu4NPF6, ν = 100 mV s−1) were performed with a Metrohm Autolab potentiostat, using a glassy carbon working electrode (2 ± 0.1 mm), a Pt counter clectrode, and silver wire as a quasi-reference electrode. Ferrocene (Fc) and decamethylferrocene (Fc*) were employed as internal standards. The solvent acetonitrile (spectroscopic grade, Aldrich) was freshly distilled from CaH2 prior to use. X-ray Crystallography. The crystals were processed at an Agilent Technologies SuperNova Atlas CCD diffractometer with microfocus Cu radiation (K1, 3, 4, 5, 6, 7, and 8) or an Agilent Technologies SuperNova Eos CCD device employing microfocus Mo radiation (K2). The CrysAlis software was used to apply analytical (K1) or multiscan absorption corrections (K2, 3, 4, 5, 6, 7, and 8).45 Using Olex2,46 the structures were solved with direct methods by ShelXS and refined with ShelXL using least-squares minimization.47 Geometrical and displacement restraints were applied to the disordered parts of the structures if necessary. Two crystallographically independent molecules are present for K1 and 3, and four crystallographically independent molecules are present in the structure of 5. These crystallographically independent molecules display very similar structure parameters; hence, generally only one of them is discussed in the text. The phosphinine unit in the molecular structure of 6 was disordered over a crystallographic mirror plane, resulting in two sets of P−C bond lengths and angles for the P−Cp* unit. Details of the structure determinations are given in Table S1 of the Supporting Information. The crystallographic information files (CIF) have been deposited at the CCDC, 12 Union Road, Cambridge, CB21EZ, U.K., and can be obtained on request free of charge, by quoting the publication citation and deposition numbers 1032930−1032937.

ASSOCIATED CONTENT

S Supporting Information *

Selected NMR spectra of complexes 1−8, cyclic voltammograms of 3 and 4, crystallographic data of 1−8, calculated and experimental UV−vis spectra, and details of the DFT calculations (relative thermal and free enthalpies of 1a−, 1b−, and 1c−, dimerization energy of 1, calculated frontier molecular orbitals of 1+, calculated spin density for 1, calculated frontier molecular orbitals of 5−8, Cartesian coordinates of the structures 1a−, 1b−, 1c−, 1, 1+, 3, 4, 5, 6, 7, and 8). This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address §

University of Münster, MEET (Münster Electrochemical Energy Technology), Corrensstraße 46, D-48149 Münster, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We dedicate this paper to the memory of Prof. Gottfried Märkl. Dr. Eva-Maria Schnöckelborg is thanked for performing preliminary experiments. In addition, we thank Dr. Elizabeth Lupton for valuable discussions concerning the DFT calculations. Funding of this work by the Deutsche Forschungsgemeinschaft (DFG WO1496/4-1) is gratefully acknowledged.

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REFERENCES

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Organometallics

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