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Keywords:

  • Macrocyclic ligands;
  • Carbenes;
  • Phosphanes;
  • Iron

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements

Complex [Fe(η5-C5H5)(η6-PhMe)]PF6 reacts with 1,2-bis[bis(2-fluorophenyl)phosphanyl]benzene (2) in acetonitrile under irradiation for 6 h to give complex [Fe(η5-C5H5)(2)(CH3CN)]PF6 ([3]PF6). Reaction of [3]PF6 with 1 equiv. of 2-azidoethyl isocyanide (4) yields complex [Fe(η5-C5H5)(2)(4)]X ([5]X) (X = Br, PF6). Staudinger reaction with PPh3 at the azido function followed by hydrolysis of the iminophosphorane with HBr yields compound [6]X (X = Br, PF6) with an NH,NH-stabilized NHC ligand. The facially coordinated NH,NH-stabilized NHC and the fluorinated diphosphane ligands were linked by N,N′-deprotonation of the NHC and nucleophilic attack of the nitrogen atoms at two C–F bonds, which gave – after anion exchange with NH4PF6 – complex [1]PF6 bearing the macrocyclic [11]ane-P2CNHC ligand.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements

The inherent stability of metal complexes with macrocyclic ligands makes them suitable for applications in which the macrocyclic framework has to withstand vigorous conditions.1 N-Heterocyclic carbene (NHC)2 and phosphane ligands3 are known to form stable metal complexes, and this stability can be further enhanced by incorporation of these donor groups into macrocyclic ligands. We have prepared complexes of type A (Figure 1) bearing the facially coordinated mixed diphosphane–NHC macrocyclic ligand [11]ane-P2CNHC in a template synthesis at template centers such as {ReI(CO)3},4 {MnI(CO)3},4 and {CpRuII}.5 A similar template strategy also enables the synthesis of complexes with the macrocyclic [16]ane-P2CNHC2 ligand.6

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Figure 1. Macrocyclic P2CNHC and P3 ligands.

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So far the template synthesis of macrocycles with carbene donors involved the use of costly template metals like rhenium or ruthenium, wereas liberation of the macrocycle from the metal center proved difficult. In our search for more cost-efficient template metals we became interested in iron. Iron(II) has been used as template for the generation of P3 macrocycles (B, Figure 1), which can be liberated from the metal center.7 Here we describe the template-controlled formation of an [11]ane-P2CNHC macrocycle at a {CpFeII} template leading to complex [1]PF6.

Results and Discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements

Our approach to the template-controlled synthesis of an [11]ane-P2CNHC ligand requires the coordination of a suitably substituted diphosphane and a reactive cyclic NH,NH-stabilized diaminocarbene in facial positions to the template metal center and their subsequent connection while coordinated.4,5 A suitable diphosphane is 1,2-bis[bis(2-fluorophenyl)phosphanyl]benzene (2),4a,8 which reacts with [Fe(η5-C5H5)(η6-PhMe)]PF69 under irradiation in acetonitrile for 6 h to give complex [Fe(η5-C5H5)(2)(CH3CN)]PF6 ([3]PF6) (Scheme 1). A similar reaction has been described with [Fe(η5-C5H5)(η6-PhMe)]PF6 and a related diphosphane.10

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Scheme 1. Preparation of complexes [5](PF6) and [1](PF6) (the numbering refers to the assignment of the NMR resonances, see Experimental Section).

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Complex [3]PF6 was not isolated but instead was directly treated with 1 equiv. of 2-azidoethyl isocyanide (4)11 in dichloromethane to give complex [5]PF6 in 95 % yield (note that the reaction of [3]PF6 with isocyanide 4 in acetonitrile does not yield [5]PF6). Complex [3]PF6 was fully characterized by 1H, 13C, 19F, and 31P NMR, IR spectroscopy and MALDI mass spectrometry.

The signal for the isocyanide carbon atom (C-16) in the 13C{1H} NMR spectrum of [3]PF6 appears at δ = 164.2 ppm with a 2J(P,C) coupling constant of 28.0 Hz. All other resonances have been assigned based on 2D NMR spectroscopy. The IR spectrum of [5]PF6 (in KBr) shows the isocyanide stretching frequency at equation image = 2134 cm–1, a lower wavenumber than observed for the free ligand 4 (equation image = 2152 cm–1 in benzene).11

Such a decrease in wavenumber indicates backbonding from the metal center to the isocyanide and a concurrent slight deactivation of the isocyanide for a nucleophilic attack. A similar decrease in the wavenumbers has been observed in pentacarbonyl complexes of coordinated 2-hydroxyphenyl isocyanide, where enhanced d[RIGHTWARDS ARROW]π* backbonding can lead to complete deactivation of the isocyanide group for an intramolecular nucleophilic attack by the hydroxy group.12 The amino groups in coordinated 2-aminoethyl5,11,13 or 2-aminophenyl isocyanides14 are much more nucleophilic compared to hydroxy groups in 2-hydroxyphenyl isocyanide, and we were therefore convinced that in spite of the slight decrease in the C≡N wavenumber by coordination of the isocyanide, the conversion [5]PF6 [RIGHTWARDS ARROW] [6]PF6 would proceed upon reduction of the azido group in [5]PF6 to the primary amine.

Various methods for the reduction of the azido group to a primary amine in coordinated 2-azido-functionalized isocyanides have been described.[5–7,13] From these we selected the Staudinger reaction for the conversion [5]PF6 [RIGHTWARDS ARROW] [6]X (X = PF6, Br) employing the sterically demanding phosphane PPh3 to prevent substitution of the isocyanide in [5]PF6. Complex [5]PF6 was treated with PPh3 in methanol, and the resulting iminophosphorane group was then hydrolyzed with HBr/H2O to lead to the complex with the 2-aminoethyl isocyanide ligand, which is not stable but reacts under intramolecular nucleophilic attack of the amino group at the isocyanide carbon atom with formation of complex [6]X. Starting from the hexafluorophosphate salt [5]PF6 and using HBr for the hydrolysis we obtained [6]X with both types of anions present. Due to this situation, compound [6]X was not isolated and purified. The crude reaction product was characterized instead. The MALDI mass spectrum showed a peak at m/z = 709 amu, which can be assigned to the cation [6]+ or the corresponding complex with a non-cyclized 2-aminoethyl isocyanide ligand. However, the expected intramolecular cyclization of the 2-aminoethyl isocyanide ligand can be concluded from the absence of the absorption for the isocyanide stretching mode in the IR spectrum of [6]X. The resonance for the carbene carbon atom of the formed NH,NH-stabilized carbene ligand was observed at δ = 218 ppm [t, 2J(P,C) = 24.6 Hz] in the 13C{1H} NMR spectrum of [6]X (in [D6]dmso).

After removing the methanol solvent used for the generation of [6]X, the solid residue was dissolved in thf. Ring closure to give the [11]ane-P2CNHC macrocycle was accomplished by deprotonation of the nitrogen atoms of the NH,NH-stabilized carbene ligand in [6]+ with KOtBu (Scheme 1). Ring-closure was completed within 24 h. After anion exchange with NH4PF6, compound [1]PF6 was obtained in an overall yield of 74 %.

Formation of the [11]ane-P2CNHC macrocycle can be concluded from the MALDI mass spectrum, which showed the formal loss of 2 equiv. of HF from [6]+ and formation of cation [1]+ with a peak at m/z = 669. The characteristic resonance for the carbene carbon atom was detected at δ = 222.3 ppm [t, 2J(P,C) = 31.0 Hz] in the 13C{1H} NMR spectrum, which falls in the range described for [Fe(η5-C5H5)(NHC)L2]+ cations.15 In addition, the signal for the phosphorus atoms of the macrocyclic ligand was observed in the 31P{1H} NMR spectrum at δ = 89.8 ppm, and only one signal for the two remaining fluorine atoms was found in the 19F NMR spectrum at δ = –96.1 ppm.

An X-ray diffraction analysis, performed with a crystal of the composition [1]PF6·MeOH (obtained by evaporation of the solvent from a methanolic solution of [1]PF6) unambiguously confirmed the formation of the [11]ane-P2CNHC macrocycle (Figure 2). Bond lengths and angles fall in the range described for related iron(II)–NHC complexes.15,16

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Figure 2. Structure of the cation [1]+ in [1]PF6·MeOH (with hydrogen atoms omitted for clarity). Selected bond lengths [Å] and angles [°]: Fe−P1 2.1277(6), Fe−P2 2.1282(6), Fe−C1 1.895(2), range Fe−CCp 2.088(2)−2.097(2), N1−C1 1.363(3), N2−C1 1.361(3); P1−Fe−P2 88.13(2), P1−Fe−C1 86.90(7), P2−Fe−C1 85.99(7), N1−C1−N2 106.8(2).

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Compared to transition metal complexes bearing the[11]ane-P2CNHC macrocycle (M = Re,4 Mn,4 Ru5), equivalent M−Ccarbene and M−P bond lengths reach a minimum in complex cation [1]+ (Table 1) caused by the small iron(II) cation. This geometric situation actually facilitates the formation of the macrocycle. The short Fe−Ccarbene and Fe−P bond lengths in [1]+ and presumably in the precursor complex cation [6]+ force the phosphane and carbene ligands closer together, thereby facilitating the formation of the two Ncarbene−Cphosphane bonds. Consequently, macrocycle formation [6]+ [RIGHTWARDS ARROW] [1]+ proceeds with good yield in 1 d, whereas the same reaction takes much longer at the {Re(CO)}+ (5 d) or {Mn(CO)3}+ (3 d) templates.

Table 1. Selected bond lengths and angles for complexes of the[11]ane-P2CNHC macrocycle.
Metal templateM–CNHC [Å]M–P [Å]P–M–P [°]Ref.
{Re(CO)3}+2.172(4)2.3957(13)81.58(5)4b
  2.3985(14)  
{Mn(CO)3}+2.033(2)2.2469(7)84.64(2)4b
  2.2386(8)  
{Ru(Cp)}+1.996(5)2.2221(12)84.89(4)5b
  2.2257(12)  
{Fe(Cp)}+1.895(2)2.1277(6)88.13(2)this study
  2.1282(6)  

Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements

We have demonstrated that the [11]ane-P2CNHC macrocycle can be generated at the low-cost [Fe(η5-C5H5)]+ template. In addition, short Fe−Ccarbene and Fe−P bond lengths at the iron(II) template facilitate the macrocycle formation compared to {Re(CO)3}+ and {Mn(CO)3}+ templates containing larger metal atoms. Given the current resurgence in iron-catalyzed reactions,17,18 we believe that water-stable compounds like [1]PF6 will find useful applications in catalysis. In addition, related triphosphane macrocycles have been generated at and liberated from the {Fe(η5-C5H5)}+ template. Therefore, it appears feasible to liberate the [11]ane-P2CNHC macrocycle from the {Fe(η5-C5H5)}+ template under generation of a new type of neutral, facially coordinating 6π-electron donor ligand.

Experimental Section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements

General Remarks:Caution! Azide compound 4 can decompose vigorously upon uncontrolled heating! Complex [Fe(η5-C5H5)(PhMe)]PF69 and ligand 411 were prepared according to published procedures. Diphosphane 24a was obtained as described for similar compounds by Kyba.8 See Scheme 1 for assignment of NMR spectra for complexes [5]PF6 and [1]PF6. Consistent microanalytical data were difficult to obtain due to the presence of fluoride (PF6 and fluorinated phenyl groups) in all complexes.

Synthesis of [Fe(Cp)(2)(4)]PF6 ([5]PF6): A solution of [Fe(η5-C5H5)(η6-PhMe)]PF6 (33 mg, 0.09 mmol) and diphosphane 2 (50 mg, 0.10 mmol) in acetonitrile (30 mL) was irradiated with a high-pressure mercury lamp at ambient temperature for 6 h. The reaction solution, presumably containing the diphosphane complex [3]PF6, was then transferred to a 100 mL Schlenk flask, and the solvent was removed under reduced pressure. The resulting solid was dissolved in dichloromethane (20 mL), and isocyanide 4 (13 mg, 0.14 mmol) was added. The reaction mixture was stirred for 12 h, after which the solvent was removed under reduced pressure. The residure was dissolved in dichloromethane (1 mL). Addition of diethyl ether (10 mL) to this solution led to precipitation of [5]PF6 as an orange solid. Yield: 77 mg (0.09 mmol, 95 %). 1H NMR (400 MHz, CDCl3): δ = 7.89–7.77 (m, 4 H, 2-H, 3-H), 7.81–7.71 (m, 2 H, 15-H), 7.71–7.61 (m, 2 H, 9-H), 7.59–7.45 (m, 2 H, 6-H), 7.49–7.33 (m, 2 H, 14-H), 7.45–7.32 (m, 2 H, 12-H), 7.44–7.26 (m, 2 H, 13-H), 7.25–7.09 (m, 2 H, 8-H), 6.88–6.71 (m, 2 H, 7-H), 4.72 (s, 5 H, Cp-H), 3.08 (t, 3J(H,H) = 5.2 Hz, 2 H, 17-H), 3.00 (t, 3J(H,H) = 5.2 Hz, 2 H, 18-H) ppm. 13C{1H} NMR (100 MHz, CDCl3): δ = 164.2 [t, 2J(C,P) = 28.0 Hz, C-16], 162.4 [d, 1J(C,F) = 246.5 Hz, C-11], 161.8 [dd, 1J(C,F) = 251.5 Hz, 2J(C,P) = 3.8 Hz, C-5], 141.3 (m, C-1), 134.2 (s, C-15), 134.1 (m, C-9), 133.1 (m, C-2, C-7), 132.4 (s, C-3), 132.0 (s, C-13), 125.2 (br. s, C-14), 124.9 (br. s, C-8), 121.5−120.1 (m, C-4, C10), 116.6 (s, C-6), 116.5 (s, C-12), 82.6 (s, Cp-C), 48.7 (s, C-18), 44.1 (s, C-17) ppm. 31P{1H} NMR (162 MHz, CDCl3): δ = 84.2 (s, ligand 2-P), –144.2 (sept, PF6) ppm. 19F NMR (376 MHz, CDCl3): δ = –98.8 (Ar-F), –99.8 (Ar-F), –70 (d, PF6) ppm. IR (KBr): equation image = 2134 (CN), 2136 (N3), 2099 (N3) cm–1. MALDI-MS (positive ions): m/z (%) = 735 (100) [M – PF6]+.

Synthesis of [Fe(η5-C5H5)([11]ane-P2CNHC)]PF6 ([1]PF6): To a solution of [5]PF6 (56 mg, 0.64 mmol) in methanol (20 mL) was added dropwise a solution of PPh3 (17 mg, 0.64 mmol) in methanol (20 mL) over 4 h. After stirring for 12 h, water (0.5 mL) and two drops of concentrated aqueous HBr were added. The mixture was stirred for 2 d. Subsequently, all solvents were removed under reduced pressure. An IR spectrum of the crude material obtained showed no resonances for isocyanide or azide groups anymore, and a MALDI mass spectrum showed a peak for the cation [6]+ at m/z = 709 amu. The resonance for the carbene carbon atom of [6]+ was detected at δ = 218 ppm [t, 2J(P,C) = 24.6 Hz]. These data confirm the formation of [6]X, which was, however, not isolated due to the presence of two different anions (PF6 and Br). To the crude dry [6]X were added KOtBu (28 mg, 0.25 mmol) and thf (20 mL). The mixture was stirred for 1 d. The thf was then removed under reduced pressure. The solid residue was dissolved in dichloromethane and filtered. Removal of the dichloromethane solvent gave an orange solid. This solid was dissolved in MeOH (5 mL), and a solution of NH4PF6 (32 mg, 0.20 mmol) in H2O (1 mL) was added. After stirring for 1 min, complex [1]PF6 was precipitated by addition of water (15 mL). The solid was collected by filtration and dried under reduced pressure to give [1]PF6 as a yellow-orange powder. Yield: 38 mg (0.53 mmol, 82 %). 1H NMR (400 MHz, [D6]dmso): δ = 7.73–7.64 (m, 2 H, 3-H), 7.69–7.59 (m, 2 H, 7-H), 7.64–7.57 (m, 2 H, 9-H), 7.56–7.48 (m, 2 H, 2-H), 7.54–7.46 (m, 2 H, 13-H), 7.52–7.39 (m, 2 H, 6-H), 7.49–7.40 (m, 2 H, 12-H), 7.41–7.32 (m, 2 H, 14-H), 7.27–7.18 (m, 2 H, 8-H), 6.89–6.74 (m, 2 H, 15-H), 4.59 [dd, 2J(H,H) = 7.0 Hz, 3J(H,H) = 2.7 Hz, 2 H, NCHHCHHN], 4.19 (s, 5 H, Cp-H), 3.51 [dd, 2J(H,H) = 7.0 Hz, 3J(H,H) = 2.7 Hz, 2 H, NCHHCHHN] ppm. 13C{1H} NMR (100 MHz, [D6]dmso): δ = 222.3 [t, 2J(C,P) = 31 Hz, C-16], 162.3 [d, 1J(C,F) = 246.9 Hz, C-5], 144.5 [t, 2J(C,P) = 6.0 Hz, C-11], 143.4 (pseudo-t, C-1), 134.2 [d, 3J(C,F) = 8.7 Hz, C-7], 132.8 (br. s, C-15), 132.8 (s, C-3), 131.7 (m, C-9), 131.2 (m, C-2), 129.3 (s, C-13), 125.7 (br. s, C-8), 124.6 (s, C-14), 120.2 (s, C-12), 119.4 (m, C-4), 116.8 [d, 2J(C,F) = 22.9 Hz, C-6], 81.1 (s, Cp-C), 49.1 (s, NCH2CH2N) ppm. 31P{1H} NMR (162 MHz, [D6]dmso): δ = 89.8 (s, [11]ane-P2CNHC-P), –144.2 (sept, PF6) ppm. 19F NMR (376 MHz, [D6]dmso): δ = –96.1 ([11]ane-P2CNHC-F), –70 (d, PF6) ppm. MALDI-MS: m/z (%) = 669 (100) [M – PF6]+.

Crystal Data for [1]PF6·MeOH: C39H33F8FeN2OP3, M = 846.43, monoclinic, P21/n, a = 13.5480(6), b = 15.6393(7), c = 18.6503(8) Å, β = 110.8560(10)°, V = 3692.7(3) Å3, T = 153(2) K, λ = 0.71073 Å, Z = 4, ρ = 1.522 g cm–3, μ = 0.614 mm–1, 28638 data measured, 10658 unique data (Rint = 0.0258), R = 0.0479, wR = 0.1326 for 8212 contributing reflections [I ≥ 2σ(I)], refinement against |F2| with anisotropic thermal parameters for all non-hydrogen atoms and hydrogen atoms on calculated positions (no hydrogen position were calculated for the methanol molecule in the asymmetric unit, which is also disordered). CCDC-CCDC-771185http://www.ccdc.cam.ac.uk/cgi-bin/catreq.cgi contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Results and Discussion
  5. Conclusion
  6. Experimental Section
  7. Acknowledgements

A. F.-F. thanks the NRW Graduate School of Chemistry Münster and the Direccion General de Relaciones Internacionales – Secretaria de Educacion Publica (DGRI-SEP), Mexico for predoctoral grants. Our studies are also supported by the Deutsche Forschungsgemeinschaft (IRTG1444).

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