Reaction of a 2 , 4 , 6-triphenylphosphinine ferrate anion with electrophiles : a new route to phosphacyclohexadienyl complexes †

A novel, versatile route to phosphorusand carbon-substituted η-phosphacyclohexadienyl complexes was developed. Reaction of the anionic 2,4,6-triphenylphosphinine iron complex [K([18]crown-6) (thf)2][Cp*Fe(PC5Ph3H2)] (1) with selected main group element electrophiles afforded the new complexes [Cp*Fe(2-endo-H-PC5Ph3H2)] (endo-3), [Cp*Fe(2-exo-H-PC5Ph3H2)] (exo-3), [Cp*Fe(1-Me-PC5Ph3H2)] (4), [Cp*Fe(1-Me3Si-PC5Ph3H2)] (5), [Cp*Fe(1-PPh2-PC5Ph3H2)] (6) and [Cp*Fe(2-BCat-PC5Ph3H2)] (7, BCat = 2-benzo[d][1,3,2]dioxaborol-2-yl). Initial attack of the electrophile at phosphorus was observed, leading to a P-substitued phosphinine ligand. A subsequent rearragement occured in some cases, resulting in C-substituted phosphinine complexes endo-3, exo-3 and 7. The new complexes were characterized by H, P{H}, and C{H} NMR spectroscopy, UV-vis spectroscopy and elemental analysis; their molecular structures were determined by X-ray crystallography.


Introduction
Phosphacyclohexadienyls are six-membered heterocycles, which can be distinguished from the related purely carboncontaining cyclohexadienyls by their ambidentate donor properties and versatile coordination behaviour (Fig. 1). 1 The donation of six π-electrons through an η 5 -coordinated anionic carbocyclic backbone of the phosphacyclohexadienyl ligand (C 5 mode) is the most common motif. [2][3][4][5][6][7] Additionally, the phosphorus atom may also become part of the coordinated π-system (C 4 P unit and CPC 3 mode, Fig. 1). 8 While η 5 -coordination is the most frequent coordination mode, η 1 -coordination can be found less often. The two electron donation of the P lone pair may be induced by additional chelating donor moieties such as pyridyl and phosphasulfide in the periphery of 1-substituted phosphacyclohexadienyl complexes. 1,2,[9][10][11][12][13][14][15][16] Rare η 2 -coordination was found in Pd II and Pt II complexes, 17 while a bridging η 3 :η 1 -mode was observed for dinuclear Ni and Zr complexes. 1,18 Phosphacyclohexadienyls thus are versatile ligands that bind to a range of transition metals and have successfully been applied in homogeneous catalysis, e.g. catalytic olefin polymerization and hydroformylation. 1,5 Nevertheless, preparative methods are limited to merely three routes (Fig. 2). The conventional method (exemplified in Fig. 2a) is based on the initial synthesis of phosphacyclohexadienyl anions by reacting a phosphinine with an organometallic nucleophile and subsequent salt metathesis. 2,9,11,17 This method was first established by Märkl in 1974. 3 Nief and Fischer developed the reduction of the phosphinine oxide complex C with HSiCl 3 as a more specialized approach (Fig. 2b) to the synthesis of hydrophosphacyclohexadienyl complexes. 8 The P-H functionalized complex D was formed as the kinetic product with an excess of HSiCl 3 at room temperature. The thermodynamically more stable carbon-protonated isomer exo-F was obtained by reflux- Fig. 1 Overview of the versatile coordination modes of the ambidentate 1-substituted, 2-substituted and 3-substituted phosphacyclohexadienyl complexes; M = transition metal; R = organic residue; further substituents on the phosphinine rings are omitted for clarity.
ing a solution of C with an excess of HSiCl 3 in toluene for three hours. When the reaction was stopped after 55 min, complex E was identified as the main product. Isolated E undergoes a quantitative isomerization to endo-F by refluxing overnight.
To our knowledge, endo-F is the only crystallographically characterized example of a 2-substituted phosphacyclohexadienyl complex with the C 4 P coordination mode so far. Using a different approach, we synthesized η 5 -phosphacyclohexadienyl iron complexes of type H by oxidizing the anionic pentamethylcyclopentadienyl complex 1 with iodine, followed by the conversion of the resulting cationic complex G with nucleopiles (Fig. 2c). 7 Nucleophilic attack occurs at the phosphorus atom, giving 1-substituted λ 3 σ 3 -phosphacyclohexadienyl complexes. While this route can in principle give access to a large family of complexes, a disadvantage is the required two-step reaction sequence. In this paper, we report a new, complementary route to phosphacyclohexadienyl complexes that is based on the direct reaction of the anion 1 with electrophiles. The application of this new method resulted in the synthesis and structural characterization of six new phosphacyclohexadienyl complexes endo-3, exo-3, and 4-7. The formation of 1-phosphacyclohexadienyls (where the substituents are attached to phosphorus) and 2-substituted phosphacyclohexadienyls (where the substituent is connected to an adjacent carbon atom) is observed. Consequently, the molecular structures of the complexes display the C 5 and the C 4 P coordination mode, respectively (Fig. 1).

Synthesis of novel phosphacyclohexadienyl complexes
We recently reported that the 1-hydrophosphacyclohexadienyl complex 2 can be synthesized by reacting the cationic phosphinine complex G with one equivalent LiBHEt 3 (Fig. 2c). Assuming that the protonation of the anionic complex 1 might give the same product, 1 was treated with one equivalent of HCl-(OEt 2 ) in THF. The reaction affords a mixture of compounds, including 2 and the new compounds endo-3 and exo-3. The latter are isomers of 2 and display 2-hydrophosphacyclohexadienyl ligands. In the case of endo-3, the hydrogen atom in the 2-position of the phosphinine is attached to the metalcoordinated face, causing the phenyl substituent to point to the bottom. The diastereomer exo-3 formally results from protonation of the phosphinine ring at the remote face to the iron center. Isomers 2, endo-3 and exo-3 are analogous to D, endo-F and exo-F previously prepared by Nief and Fisher via a completely different route (Fig. 2b). 8 31 P{ 1 H} NMR monitoring ([D 8 ]THF, Fig. 3) revealed the signal of 2 (−80 ppm) at −80°C. Two additional signals at 10 ppm and −64 ppm arise from unknown intermediates, which disappear at higher temperature. The signal of the starting material 1 (−49 ppm) continuously decreased on slow warming to 0°C, whereas the signal of 2 increased. The signals of the 2-H-substituted species exo-3 (−162 ppm) and endo-3 (−137 ppm) were observed in low intensity at −30°C; their intensity increased significantly at 0°C, whereas the signal of 2 decreased. An additional signal corresponding to an unidentified species became apparent at −14 ppm at −40°C. This signal could plausibly arise from a by-product similar to complex E (−20.4 ppm) 8 or a decomposition product. The 31 P{ 1 H} NMR spectrum of the reaction mixture recorded at room temperature displays the signals of 2, exo-3, endo-3 as well as a few weak singlets of further unidentified species. The signal intensities did not change further after one day. Stirring the raw product mixtures of 2, endo-3 and exo-3 at 50°C for several days ( 31 P{ 1 H} NMR monitoring) also did not lead to a further change of the integral ratios.
Even though 2 appears to be formed selectively at low temperature, we were not able to isolate it as a pure material from reactions performed at −40°C. However, 2 slowly converts to exo-3 upon treatment with HCl(OEt 2 ) (10 mol%) at room temperature in [D 8 ]THF. This indicates the rearrangement to be acid-catalysed. Attempts to optimise the reaction gave poorly reproducible product mixtures. Thus, it appears difficult to access 2, exo-3 and endo-3 as pure compounds by protonationg 1 with HCl(OEt 2 ).
The results of the monitoring experiment indicate that the 1-hydrophosphacyclohexadienyl complex 2 is formed as the main kinetic product along with two unidentified species (marked with an asterisk in Fig. 3). The 2-hydrophosphacyclohexadienyl complexes endo-3 and exo-3 appear to be thermodynamic products that form at higher temperatures. Indeed, gas-phase DFT calculations performed at the BP86/def2-TZVP level (see the Experimental section for details) indicate that endo-3 and exo-3 are close in energy, while 2 was calculated to be +7.0 kcal mol −1 less stable than endo-3 (Fig. 3, see the Experimental section for details).
Gratifyingly, the reaction of 1 with one equiv. isopropyl chloride in THF at room temperature (Scheme 1) proceeded cleanly, reproducibly affording a mixture of endo-3 and exo-3 in a 65 : 35 ratio (NMR integration). The formation of 2 as an intermediate was not observed by 31 P{ 1 H} NMR in this case, which indicates that the reaction proceeds via a different mechanism. Purification by column chromatography gave NMR-spectroscopically pure exo-3 and endo-3 after crystallization.
Exo-3 was isolated as orange rods in 25% yield, whereas pure endo-3 crystallized as orange plates in 41% yield. Both compounds are air-sensitive and dissolve well in n-hexane, diethyl ether, toluene and THF.
Complexes 4-6 are accessible in a similar fashion in moderate yields by reacting 1 with one equiv. of MeI, Me 3 SiCl, and Ph 2 PCl (Scheme 2a-c). ‡ The compounds are deeply coloured crystalline solids that dissolve well in polar and apolar solvents such as n-pentane, n-hexane, diethyl ether, toluene and THF. The 2-substituted phosphacyclohexadienyl complex 7 was obtained as bright orange crystals by a similar reaction with one equiv. of chlorocatecholborane (Scheme 2d). Compound 7 is moderately soluble in n-pentane and n-hexane, but dissolves well in more polar solvents such as diethyl ether, toluene and THF. 31  The 31 P{ 1 H} NMR spectrum recorded at room temperature shows the presence of 7, the diphoshinine complex [Cp* 2 Fe 2 (µ-{PC 5 Ph 3 H 2 } 2 )], the hydrophosphinine complex endo-3 and a small signal for an unidentified by-product (singlet at −50 ppm). The observation of this mixture shows that other processes than borylation may also occur, explaining the modest isolated yield (26%).
An analogous reaction with Ph 3 SnCl in THF produced the P-Sn functionalized complex 8 (Scheme 2e), but the reaction was unselective. According to 31 P{ 1 H} NMR integration complex 8 is only present in a low amount (26% of the total P content) in the reaction mixture after stirring for 17 h at room temperature. Several attempts to isolate it as a pure compound were not successful due to its low stability. Diphospinine [Cp* 2 Fe 2 (μ-{PC 5 Ph 3 H 2 } 2 )] and hexaphenyldistannane were identified as decomposition products by 31 P{ 1 H} and 119 Sn{ 1 H} NMR, suggesting decomposition by a radical pathway.
with less strongly electron-donating substituents at boron, enabling the formation of a new frustrated Lewis pair type system.  ]. 25 The spectra of 1-substituted 4-6 overall resemble those of related complexes of type H (Fig. 2c). 7 (293 Hz) in the typical range for a covalent P-P single bond. 26 The signal at 12.8 ppm is assigned to the PPh 2 group, because it splits into a doublet of quintets in the 31 P NMR spectrum ( 3 J PH = 6.5 Hz).

NMR and UV-Vis spectroscopic characterization
Complex 7, which features a 2-substituted phosphacyclohexadienyl moiety, gives rise to a similar high-field 31 P{ 1 H} NMR singlet (−126.7 ppm) as endo-3 (−136.3 ppm); the 1 H NMR data ( Table 2) are also similar in agreement with the similar structures. § The UV/vis spectra of endo-3-7 were recorded in n-hexane. The spectra of 2-H-substituted endo-3 and exo-3 are similar and display a weak shoulder at 450 nm; three stronger bands are found in the UV range (endo-3 220, 260 and 320 nm; exo-3 230, 275 and 325 nm). The spectrum of the structurally related complex 7 is analogous, showing slightly bathochromically shifted bands at 260, 290sh, 360sh and 460sh nm. The UV/vis spectra of the 1-substituted species 4-6 are distinct from those of the aforementioned complexes and feature two visible absorptions each with moderate intensities in the ranges λ max = 550-580 nm and λ max = 480-580 nm, respectively. Similar spectra were observed for other complexes of this type (type H, Fig. 2c). 7 Previous TD-DFT calculations indicated that these bands predominantly arise from excitations from filled metalcentered MOs into the ligand-based unoccupied MOs (MLCT). 7

Conclusions
The reaction of the anionic phosphinine complex 1 with diverse electrophiles represents a novel and straightforward synthetic pathway to phosphacyclohexadienyl iron complexes. Protonation of 1 using HCl(OEt 2 ) initially affords the 1-substituted complex 2 at low temperature, which appears to undergo an acid catalyzed rearrangement and converts to a mixture of isomers, including the 2-H-substituted compounds endo-3 and exo-3. The latter complexes were conveniently isolated in good yields from the reaction of 1 with isopropyl chloride. An analogous 2-substitued complex 7 formed in the reaction with chlorocatecholborane. Similar to the hydrophosphinine complexes, an initial formation of a phosphorus-substituted complex followed by a subsequent 1,2-shift of the substituent was observed. Using MeI, Me 3 SiCl, Ph 2 PhCl and Ph 3 SnCl, 1-substituted complexes 4-6 and 8 were obtained. Thus, HCl(OEt 2 ), isopropyl chloride and chlorocatecholborane result in products substituted at the 2-carbon atom, whereas MeI, Me 3 SiCl, Ph 3 SnCl and Ph 2 PhCl provide phosphorus-substituted products.
An extensive family of related compounds could become accessible via this route. In addition, the reactivity and possible catalytic activity of the new complexes presented here needs to be examined, where the unusually long P-P bond in 6 and the FLP type motif in 7 will be of particular interest. Investigations in these directions are underway in our laboratory. . A solution of isopropyl chloride in THF (1.0 mL, c = 0.108 mol L −1 ) was added to a dark orange solution of 1 (104 mg, 0.108 mmol) in THF (5 mL). The solution was stirred at room temperature for 24 hours. The resulting dark orange brown mixture was subjected to column chromatography (silica gel, 22 × 1 cm, n-hexane/toluene gradient, 100/1 to 5/1). Two bright orange bands were obtained: exo-3 was eluted first (R f (n-hexane/toluene, 5/1) = 0.42), slightly overlapping with endo-3, which followed immediately (R f (n-hexane/ toluene, 5/1) = 0.32). Removal of the solvent gave exo-3 and endo-3 as pure bright orange solids. Yield of exo-3: 14 mg (25%), yield of endo-3: 23 mg (41%), total including mixed fractions: 45 mg (80%). X-ray quality crystals formed upon storage of concentrated n-hexane solutions at room temperature for three days. Variable elemental analyses were obtained for exo-3 and endo-3. Traces of silica gel can be removed by taking up the product in n-hexane, filtration and removal of the solvent. endo-3.  A solution of methyl iodide in THF (1 mL, c = 0.106 mol L −1 ) was added to a dark orange solution of 1 (102 mg, 0.106 mmol) in THF (5 mL) at room temperature. The reaction mixture turned burgundy red immediately, and was stirred for four hours at room temperature. After removing the solvent in vacuo, the remaining dark red residue was extracted with n-hexane (10 × 1 mL). The burgundy red extracts were com-bined and the solution was concentrated to 5 mL. A solution of trimethylsilyl chloride in toluene (0.9 mL, c = 0.152 mol L −1 ) was added to a dark orange solution of 1 (132 mg, 0.137 mmol) in THF (7 mL) and stirred at room temperature for 16 h. The resulting dark greenish brown mixture was dried in vacuo, and the residue was extracted with n-pentane (16 × 0.5 mL). The fractions were combined and dried in vacuo. [18]crown-6 was sublimed at 60°C and <1.0 × 10 −3 mbar. The remaining residue was dissolved in n-hexane (8 mL). The greenish black solution was filtered and concentrated to 5 mL. 5 was isolated as dark green to black crystals after storage at −30°C for three days.