Heterobimetallic complexes with highly flexible 1,1′-bis(phospholanoalkyl)ferrocene ligands

Abstract

The two highly flexible ligands 1,1′-bis(4-phospholanobutyl)ferrocene (5a) and 1,1′-bis(5-phospholanopentyl)ferrocene (5b) reacted with [PtCl₂(cod)] (cod = 1,5-cyclooctadiene), [AuCl(tht)] (tht = tetrahydrothiophene) and [{RhCl(CO)₂}₂] to give the respective chelate complexes cis-[PtCl₂(5a,b-κ²P,P′)] (7a,b), [AuCl(5a,b-κ²P,P′)] (8a,b) and trans-[RhCl(CO)(5b-κ²P,P′)] (9b). Treatment of 5a,b with selenium gave the corresponding selenides 6a,b. All compounds have been fully characterised by NMR (¹H, ¹³C, ³¹P) and IR spectroscopy and mass spectrometry. In addition, crystal structures of 6a, 7b, 8a,b and 9b were determined by X-ray crystallography. Furthermore, the rhodium-catalysed hydroformylation of 1-octene has been studied with ligand 5b.

---

Introduction

Bis(phosphino)ferrocenes are commonly used ligands in a variety of homogeneously catalysed reactions.¹ Enormous progress has been made in the chemistry of 1,1′-bis(diphenylphosphino)ferrocene (dppf)² and its achiral³ and chiral^4,5 derivatives. However, less rigid ligands in which the ferrocene moiety and the phosphine are separated by a flexible carbon-based spacer are rare,⁶ and the preparation of the corresponding metal complexes often suffers from low selectivity. Usually, complex product mixtures resulting from intra- and intermolecular reactions are obtained due to insufficient differentiation caused by the high conformational flexibility of 1,1′-substituted ferrocene derivatives. Thus, no metal complexes of the type [Fe{C₅H₄(CH₂)_nPR₂}₂] (n = 2, R = Ph, tBu; n = 3, R = Ph) could be isolated.⁷ However, flexibility of a bidentate ligand may be important for the acceleration of specific transitions within a catalytic cycle and, therefore, may have a significant effect on the catalyst's efficiency.⁸

Recently, we have shown that highly flexible bis-phospholane ligands based on solely aliphatic backbones react selectively with [AuCl(tht)] (tht = tetrahydrothiophene) to give macrocyclic complexes [Au₂Cl₂{μ-(C₄H₈P)(CH₂)_n(PC₄H₈)-κ²P,P′}₂] (n = 5, 7, 9, 11),⁹ without using high-dilution techniques or templates to avoid polymerisation.¹⁰

As the phospholane moiety apparently increases the selectivity of metal complex formation and bisphospholanes proved to be privileged structures in catalysis,¹¹ we have prepared the ferrocene-based bis-phospholanes [Fe{C₅H₄(CH₂)_nP(C₄H₈)}₂] [n = 4 (5a), 5 (5b)] and studied their coordination behaviour in platinum(II), gold(I) and rhodium(I) complexes.

Results and discussion

Synthesis of bis-phospholane ligands and selenides

Two bis-phospholane ligands having highly flexible alkylene spacers with four or five methylene groups between the ferrocene moiety and the phospholane were synthesised by a modification of a strategy published by Haddow et al. in 2009 (Scheme 1).¹² The starting materials, 1,1′-bis(ω-bromoalkyl)ferrocenes, were prepared by Friedel–Crafts acylation of ferrocene with ω-bromoacyl chlorides and subsequent reduction of the obtained 1,1′-acylferrocenes 1a,b.¹³ The crystal structure of 1,1′-bis(5-bromopentanoyl)ferrocene (1b) shows no unusual features and can be found in the ESI.‡ Treatment of two equivalents of 1-phenylphospholane with 1,1′-bis(ω-bromoalkyl)ferrocenes 2a,b gave the corresponding phosphonium salts 3a and 3b in quantitative yield as orange solids. Basic hydrolysis of the P−C(phenyl) bond by aqueous sodium hydroxide solution led to the formation of the phosphine oxides 4a and 4b. From these, the bis-phospholane ligands 5a and 5b were obtained as viscous, orange oils by reduction with a mixture of lithium aluminium hydride and trimethylsilyl chloride.

Scheme 1 Synthesis of bis-phospholane ligands 5a and 5b. (i) AlCl₃, 2 equiv. Br(CH₂)_nCOCl, CHCl₃ (n = 3, 4); (ii) AlCl₃, LiAlH₄, Et₂O; (iii) 2 equiv. 1-phenylphospholane, CH₃CN, 80 °C; (iv) NaOH_aq, 95 °C; (v) LiAlH₄, SiMe₃Cl, THF.

The corresponding selenides 6a and 6b were prepared by treating 5a and 5b with two equivalents of grey selenium. Crystals of 6a suitable for X-ray diffraction (crystal structure shown in the ESI‡) could be obtained from ethyl acetate/n-hexane. In contrast, 6b is a yellow oil at room temperature. In the ³¹P{¹H} NMR spectra of both compounds, a sharp singlet at δ = 45.6 ppm is observed (¹J_P–Se = 693 Hz, determined from the ⁷⁷Se satellites). Since ¹J_P–Se is directly related to the s character of the lone pair of electrons,¹⁴6a and 6b appear to be stronger σ donors in comparison with FerroLANE (FerroLANE = 1,1′-bis(phospholano)ferrocene) ligands (cf. J_P–Se = 738 Hz in (S)-1,1′-bis(2,5-dimethylphospholanoselenide)ferrocene).⁵

Metal complexes

Treatment of [PtCl₂(cod)] (cod = 1,5-cyclooctadiene) with one equivalent of the bis-phospholanes 5a and 5b in dichloromethane afforded the corresponding complexes [PtCl₂(5a-κ²P,P′)] (7a) and [PtCl₂(5b-κ²P,P′)] (7b) (Scheme 2). While 7a is formed selectively, the reaction of 5b with [PtCl₂(cod)] gave a mixture containing 7b and another platinum complex (two singlets, ¹J_P–Pt ≈ 3450 Hz, determined from the ¹⁹⁵Pt satellites, consistent with cis-coordinated phosphines, are observed in the ¹P{¹H} NMR spectrum of the reaction mixture). In analogy to observations made by Guino-o et al. on the formation of platinum complexes with FerroLANE ligands, the minor side product may contain 5b in a bridging binding mode.¹⁵ Compounds 7a and 7b, the latter isolated from the product mixture by crystallisation, were obtained as slightly hygroscopic orange solids and were fully characterised by NMR and IR spectroscopy, mass spectrometry, cyclic voltammetry and elemental analysis. In the ³¹P{¹H} NMR spectra, a downfield shift is observed on coordination, from δ ≈ −26.8 ppm (free ligand) to sharp signals at 17.4 (7a) and 15.7 ppm (7b). Crystal structures of both complexes were obtained (Fig. 1). 7a crystallises in the space group P1̄ from CH₂Cl₂/n-hexane, while 7b crystallises in space group P2₁/n. The platinum atoms in 7a and 7b are coordinated in a square-planar fashion by two phospholane moieties and two chlorido ligands in cis configuration, as expected from the ¹J_P–Pt values of 3426 and 3452 Hz, respectively. The Pt–P and Pt–Cl bond lengths as well as the P–Pt–P and Cl–Pt–Cl bond angles are in agreement with those of related bis-phospholane platinum(II) complexes (e.g., cis-[PtCl₂{PPh(C₄H₈)}₂]: Pt–P 223.9(1) pm, Pt–Cl 236.7(1) pm, P–Pt–P 92.60(6)° Cl–Pt–Cl 88.30(7)°).¹⁶

Scheme 2 Synthesis of metal complexes.

Fig. 1 Molecular structures of 7a (top) and 7b (bottom). Ellipsoids are drawn at the 50 and 30% probability level, respectively. Hydrogen atoms are omitted for clarity. Selected bond lengths (in pm) and angles (in °): 7a: Pt1–Cl1 234.9(1), Pt1–Cl2 237.1(2), Pt1–P1 223.2(1), Pt1–P2 223.8(1), P1–C19 184.7(3), P1–C22 182.0(3), P2–C23 182.4(4), P2–C26 184.0(4); P1–Pt1–P2 94.27(4), Cl2–Pt1–Cl1 87.71(4), C22–P1–C19 95.1(2), C23–P2–C26 94.1(2); dihedral angle C11–C12–C13–C14 71.2(3). 7b: Pt1–Cl1 234.7(2), Pt1–P1 223.8(2), Pt1–P2 222.7(2), P1–C21 182.5(8), Pt1–Cl2 234.3(3), P1–C24 184.7(8), P2–C25 180(1), P2–C28 183(1); P1–Pt1–P2 94.52(8), Cl2–Pt1–Cl1 87.62(9), C21–P1–C24 93.9(4), C25–P2–C28 94.2(5); dihedral angle C11–C12–C13–C14 167.5(8).

While the environment of the platinum atoms in 7a and 7b is very similar, differences are observed in the structural parameters of the ferrocene moieties and the aliphatic spacers (Table 1).

Table 1 Structural parameters of the ferrocene moiety

Compound	Θ (°)	τ (°)	Cp^R ring conformation
a Dihedral angle between mean planes through cyclopentadienyl (Cp^R) rings. b Torsional twist about Cp^R(centroid)⋯Fe⋯Cp^R(centroid) axis.
1b	1.40	142.12	Anticlinal
6a	1.55	146.83	Anticlinal
7a	2.10	10.50	Synperiplanar
7b	5.54	11.41	Synperiplanar
8b	2.81	4.32	Synperiplanar
9b	0.54	72.30	Synclinal

---

Although the cyclopentadienyl rings in both complexes adopt a synperiplanar conformation, the dihedral angle between the mean planes through the cyclopentadienyl rings is significantly larger in 7b (5.54 vs. 2.10°).

A gauche conformation (θ = 71.2(3)°) is observed in one of the two alkylene spacers of 7a. In contrast, all methylene groups in the aliphatic backbone of 7b have an almost perfect antiperiplanar conformation.

Addition of [AuCl(tht)] to a solution of one equivalent of 5a or 5b led to the selective formation of the corresponding complexes [AuCl(5a-κ²P,P′)] (8a) and [AuCl(5b-κ²P,P′)] (8b) (Scheme 2). The formation of polymeric products, known for the dppf ligand and several dppf derivatives, was not observed.¹⁷ The ³¹P{¹H} NMR spectra of 8a and 8b exhibit sharp singlets at δ = 34.3 and 37.4 ppm, respectively. In the mass spectra (ESI+ mode), [M − Cl]⁺ peaks are observed. All attempts to obtain single crystals of 8a suitable for structure determination were unsuccessful. The molecular structure of 8b, which crystallises in the orthorhombic space group P2₁2₁2₁, is shown in Fig. 2. In 8b, the gold atom is almost linearly coordinated by the two phospholane moieties (P1–Au1–P2 173.06(5)°). The distortion occurs due to an additional, weak coordination of a terminal chlorido ligand. The Au–Cl bond (287.1(1) pm vs. 250.0(4) pm in [AuCl(PPh₃)₂]¹⁸) is conspicuously long, but is in line with non-ferrocene-based analogues, as are the Au–P bond lengths (cf. [Au₂Cl₂{μ-(C₄H₈P)(CH₂)₇(PC₄H₈)}₂]: Au–Cl 290.3(1) pm, Au–P 229.7(1) and 229.4(1) pm).⁹ The cyclopentadienyl rings adopt a synperiplanar arrangement, and a gauche conformation is observed in analogy to 7a in one of the two alkylene spacers.

Fig. 2 Molecular structure of 8b. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (in pm) and angles (in °): Au1–P1 229.1(1), Au1–P2 229.5(1), Au–Cl1 287.1(1), P1–C21 183.6(6), P1–C24 181.9(6), P2–C25 183.3(5), P2–C28 183.1(5); P1–Au1–P2 173.06(5), P1–Au1–Cl1 96.20(5), P2–Au1–Cl1 90.66(5), C21–P1–C24 95.5(3), C25–P2–C28 95.2(2); dihedral angle C11–C12–C13–C14 70.10(3).

Treatment of [{RhCl(CO)₂}₂] with two equivalents of 5b in dichloromethane gave the corresponding complex [RhCl(CO)(5b-κ²P,P′)] (9b). All attempts to isolate pure [RhCl(CO)(5a-κ²P,P′)] (9a) from the obtained product mixture were unsuccessful. However, the appearance of an [M]⁺ peak in the mass spectrum (ESI+ mode) supports the formation of 9a.

A sharp doublet is observed at δ = 26.5 ppm in the ³¹P{¹H} NMR spectrum of 9b. The coupling constant of ¹J_P–Rh = 115.4 Hz indicates a trans configuration. The IR spectrum shows one absorption for ν_CO at 1952 cm⁻¹ (cf. 1968 cm⁻¹ in [Rh₂Cl₂(CO)₂{μ-(C₄H₈P)(CH₂)₃(PC₄H₈)}₂]).¹² Compound 9b crystallises from CH₂Cl₂/Et₂O in the monoclinic space group P2₁/n (Fig. 3). The rhodium atom is coordinated in a square-planar fashion by the trans-coordinated bis-phospholane, a carbonyl and a chlorido ligand. Bond lengths and angles are in agreement with those of structurally related bis-phospholane rhodium complexes.¹²

Fig. 3 Molecular structure of 9b. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (in pm) and angles (in °): Rh1–P1 231.10(8), Rh1–P2 230.42(8), Rh1–Cl1 236.38(6), Rh1–C29 180.1(2), P1–C21 184.5(2), P1–C24 183.7(2), P2–C25 184.0(2), P2–C28 184.7(2), C29–O1 114.9(2); P1–Rh1–P1 176.66(1), C21–P1–C24 94.43(7), Cl1–Rh1–C29 175.27(5), C25–P2–C28 94.36(8), Rh1–C29–O1 177.9(2); dihedral angle C11–C12–C13–C14 70.58(1).

The cyclopentadienyl rings in 9b adopt a synclinal conformation, in contrast to the obtained platinum and gold complexes. Similar to 7a and 8b, a gauche conformation is observed in one of the two alkylene spacers.

Cyclovoltammetry

In the cyclic voltammograms (see ESI‡), a single, reversible oxidation occurs in all isolated complexes (6a,b, 7a,b, 8a,b and 9b) at E_1/2 = −0.06, 0.10 or −0.13 V versus Fc⁺/FcH, attributed to the Fe^II/Fe^III redox couple.

Theoretical considerations

DFT calculations were conducted to comprehend the gauche conformation observed in 7a, 8b and 9b.

Three conformers of 9b in which the ferrocene moieties adopt a synclinal conformation were optimised (Fig. 4). One of the two alkylene chains exhibits a gauche conformation in C1 and C2, the latter similar to that in the crystal structure. In contrast, all methylene groups in C3 are antiperiplanar to each other. Furthermore, the ferrocene moiety in C3 exhibits a stronger tilt towards the plane that contains the chloride and the metal atoms (angle of slope 54° vs. 60° in C1). However, energy differences between the three conformers (G_rel) are negligible (see Fig. 4.). Therefore, we suggest that the gauche conformation arises from effects of crystal packing. The appearance of C2 instead of the energetically favoured conformer C3 supports this suggestion.

Fig. 4 Optimised structures of the most stable conformers of 9b.

Hydroformylation catalysis

Bis-phospholanes with a flexible aliphatic backbone are interesting ligands for the rhodium-catalysed hydroformylation of alkenes.¹² Total conversion of 1-octene is observed with 5b/[{RhCl(CO)₂}₂] as catalyst over 18 h at 40 bar CO/H₂ (1 : 1) pressure and 50 °C (Scheme 3). The regioselectivity (n : iso = 1.8) is similar to those of non-ferrocene-based bis-phospholane systems.¹²

Scheme 3 Hydroformylation of 1-octene. Reaction conditions: 50 °C, 40 bar CO/H₂ (1 : 1) for 18 h, 0.05 mol% [{RhCl(CO)₂}₂], 5b : Rh = 5 : 2. See Experimental part for details. Conversion 100%, n : iso = 1.8.

Conclusions

A new class of highly flexible 1,1′-bis(phospholano)ferrocene ligands was prepared. Their selective reaction with [PtCl₂(cod)], [{RhCl(CO)₂}₂] and [AuCl(tht)], independent of the number of methylene groups in the spacer molecule, was shown. Chelate complexes from intramolecular reactions were obtained exclusively. Furthermore, all complexes show a reversible Fe^II/Fe^III redox process. To the best of our knowledge, the reported metal complexes are the first 1,1′-bis(phosphino)ferrocene-based heterobimetallic complexes with highly flexible alkylene backbones that could be isolated and structurally characterised by X-ray diffraction.

The reported results are essential for a deeper understanding of the influence of the substituent at the phosphorus atom on the metal complex formation of flexible bis(phosphino)ferrocenes.

Experimental

General methods

All reactions were carried out in a nitrogen atmosphere by using standard Schlenk techniques¹⁹ and anhydrous solvents, which were purified with an MB SPS-800 solvent purification system from MBRAUN or as mentioned in the literature.²⁰ 1-Phenylphospholane,²¹ [AuCl(tht)],²² [PtCl₂(cod)]²³ and 2a,b¹³ were prepared according to the literature. All other chemicals were used as purchased. NMR spectra were recorded at 298 K on a Bruker AVANCE DRX 400 spectrometer or a Bruker VARIAN 300 spectrometer. The chemical shifts δ of ¹H, ¹³C, ³¹P are reported in parts per million (ppm) at 400.12 (300.23), 100.63 (75.49) and 162.02 MHz, respectively, with tetramethylsilane as an internal standard and referencing to the unified scale.²⁴ Coupling constants J are given in Hz. FTIR spectra were recorded on a Perkin-Elmer Spectrum 2000 FTIR spectrometer, scanning between 400 and 4000 cm⁻¹, using KBr pellets. Wavenumbers [small nu, Greek, tilde] are reported in cm⁻¹. Mass spectra were recorded on an ESQUIRE 3000 plus spectrometer. Elemental analyses were carried out with a Heraeus VARIO EL oven. Melting and decomposition points were measured in sealed capillaries by using a variable heater from Gallenkamp. Cyclic voltammetry experiments were conducted at ambient temperature with an SP-50 potentiostat from BioLogic Science Instruments. All scans were performed with 0.1 M tetrabutylammonium tetrafluoridoborate as supporting electrolyte in CH₂Cl₂ and a standard three-electrode cell (glassy carbon working electrode, platinum counter electrode, silver wire reference electrode). Ferrocene was used as internal reference.

Synthesis and characterisation

Preparation of phosphonium salts. Phenylphospholane (0.66 g, 4.0 mmol) was added to a stirred solution of the respective 1,1′-bis(ω-bromoalkyl)ferrocene (2.0 mmol) in acetonitrile (20 mL). The mixture was stirred for 2 days at 80 °C. Then volatile compounds were removed under reduced pressure. The resulting orange solids (3a,b) were used without further purification.
1,1′-Bis(4-(phenylphospholanium)butyl)ferrocene dibromide (3a). Yield 1.56 g (99%). Decomp. >156 °C. Found C, 58.4; H, 6.6. Calc. for C₃₈H₅₀Br₂FeP₂: C, 58.2; H, 6.4. IR [small nu, Greek, tilde] 3054w, 2935s, 2359w, 1636m, 1438s, 1404m, 1264m, 1165w, 1120s, 1020m, 879m, 805m, 751m, 692m, 516m. ¹H NMR (CD₃OD, 400 MHz): δ 7.93 (m, 4H), 7.83 (m, 2H), 7.74 (m, 4H), 3.95 (m, 4H), 3.93 (m, 4H), 2.74–2.58 (m, 12H), 2.35 (t, J = 7.2, 4H), 2.30–2.08 (m, 8H), 1.66–1.47 (m, 8H). ¹³C{¹H} NMR (CD₃OD, 100 MHz): δ 135.5 (d, J_C,P = 3.2), 133.2 (d, J_C,P = 9.5), 131.2 (d, J_C,P = 12.3), 121.0 (d, J_C,P = 77.1), 89.3 (s), 69.8 (s), 69.0 (s), 33.1 (d, J_C,P = 15.6), 29.6 (s), 27.2 (d, J_C,P = 5.9), 23.5 (d, J_C,P = 45.3), 23.3 (d, J_C,P = 52.3), 22.9 (d, J_C,P = 4.6). ³¹P{¹H} NMR (CD₃OD, 162 MHz): δ 50.1 (s). MS (ESI(+), CH₃OH) m/z 703.2 [M − Br]⁺, 312.1 [M − 2Br]⁺.
1,1′-Bis(5-(phenylphospholanium)pentyl)ferrocene dibromide (3b). Yield 1.61 g (99%). Decomp. >220 °C. Found C, 59.1; H, 6.7. Calc. for C₄₀H₅₄Br₂FeP₂: C, 59.3; H 6.8. IR [small nu, Greek, tilde] 2964m, 2932m, 2854w, 1456m, 1438m, 1404w, 1262m, 1097m, 1024m, 803m, 749w, 693w, 518w. ¹H NMR (CD₃OD, 400 MHz): δ 7.96 (m, 4H), 7.80 (m, 2H), 7.71 (m, 4H), 3.93 (m, 4H), 3.92 (m, 4H), 2.75–2.61 (m, 12H), 2.3 (t, J = 6.8, 4H), 2.27–2.08 (m, 8H), 1.60–1.40 ppm (m, 12H). ¹³C{¹H} NMR (CD₃OD, 100 MHz): δ 135.5 (d, J_C,P = 2.9), 133.1 (d, J_C,P = 9.5), 131.2 (d, J_C,P = 12.1), 121.1 (d, J_C,P = 77.1), 89.8 (s), 69.8 (s), 69.0 (s), 31.7 (s), 31.2 (d, J_C,P = 15.5), 30.2 (s), 27.2 (d, J_C,P = 5.8), 23.6 (d, J_C,P = 45.3), 23.3 (d, J_C,P = 52.3), 23.3 (d, J_C,P = 4.5). ³¹P{¹H} NMR (CD₃OD, 162 MHz): δ 50.0 (s). MS (ESI(+), CH₃OH) m/z 733.2 [M − Br]⁺, 326.1 [M − 2Br]⁺.

Preparation of phosphine oxides. 2.0 mmol of the respective phosphonium salt was dissolved in aqueous sodium hydroxide solution (20 wt%, 20 mL) and the mixture was stirred at 95 °C overnight. The reaction mixture was extracted with CHCl₃ (3 × 20 mL). The combined organic layers were washed with distilled water and brine and dried over MgSO₄. Removal of the solvent under reduced pressure gave the bis-phospholane oxides 4a,b as orange oils.
1,1′-Bis(4-(phospholano oxide)butyl)ferrocene (4a). Yield 1.28 g (96%). Found C, 62.3; H, 8.1. Calc. for C₂₆H₄₀FeO₂P₂: C, 62.2; H, 8.0. IR [small nu, Greek, tilde] 3092w, 2935s, 2864m, 1635m, 1451m, 1407m, 1266s, 1162s, 1111m, 1056m, 1023m, 862m, 803m, 729m, 518m. ¹H NMR (CDCl₃, 300 MHz): δ 3.91 (d, J = 1.6, 4H), 3.88 (d, J = 1.6, 4H), 2.29 (t, J = 7.2, 4H), 1.94 (m, 4H), 1.81–1.46 (m, 24H). ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 88.3 (s), 68.7 (s), 67.9 (s), 32.7 (d, J_C,P = 13.5), 30.8 (d, J_C,P = 61.6), 29.1 (s), 26.9 (d, J_C,P = 64.8), 24.5 (d, J_C,P = 7.8), 22.0 (d, J_C,P = 4.0). ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ 71.2 (s). MS (ESI(+), CH₂Cl₂, CH₃CN) m/z 503.5 [M + H]⁺.
1,1′-Bis(5-(phospholano oxide)pentyl)ferrocene (4b). Yield 1.38 g (99%). Found C, 63.4; H, 8.2. Calc. for C₂₈H₄₄FeO₂P₂: C, 63.4; H, 8.4. IR [small nu, Greek, tilde] 3094w, 2925s, 2860m, 1464m, 1405m, 1264s, 1169s, 1110s, 1053s, 1042s, 1021s, 858m, 816s, 803s, 752w, 728m, 514m, 498m. ¹H NMR (CDCl₃, 300 MHz): δ 3.96 (m, 4H), 3.94 (m, 4H), 2.32 (t, J = 7.2, 4H), 2.01 (m, 4H), 1.88–1.58 (m, 20H), 1.58–1.34 (m, 8H). ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 88.6 (s), 68.4 (s), 67.6 (s), 30.8 (s), 30.7 (d, J_C,P = 61.5), 30.7 (d, J_C,P = 14.0), 29.0 (s), 26.8 (d, J_C,P = 64.7), 24.3 (d, J_C,P = 7.8), 21.9 (d, J_C,P = 4.0). ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ 70.9 (s). MS (ESI(+), CH₂Cl₂, CH₃CN) m/z 530.2 [M]⁺.

Preparation of bis-phospholanes. Chlorotrimethylsilane (1.30 g, 12.0 mmol) was added to a stirred suspension of lithium aluminium hydride (0.46 g, 12.0 mmol) in THF (10 mL) at 0 °C. The mixture was stirred for 2 h at rt and a solution of the respective bis-phospholane oxide (3.0 mmol) in THF (10 mL) was added at 0 °C via cannula. The reaction mixture was stirred overnight at rt and quenched carefully with CH₃OH. All volatile compounds were removed under reduced pressure, and the residue was extracted with CH₂Cl₂ and filtered over vacuum-dried silica. Removal of the solvent under reduced pressure gave the bis-phospholanes 5a–b as orange oils.
1,1′-Bis(4-(phospholano)butyl)ferrocene (5a). Yield 0.97 g (69%). Found C, 66.1; H, 8.25. Calc. for C₂₆H₄₀FeP₂: C, 66.4; H, 8.6. IR [small nu, Greek, tilde] 3081w, 2930s, 2852m, 1458w, 1447w, 1262s, 1106s, 1040s, 1021s, 872m, 802s, 709w, 485m. ¹H NMR (CDCl₃, 400 MHz): δ 3.89 (m, 4H), 3.88 (m, 4H), 2.24 (t, J = 6.0, 4H), 1.77–1.56 (m, 12H), 1.55–1.23 (m, 16H). ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 89.0 (s), 68.6 (s), 67.7 (s), 33.9 (d, J_C,P = 10.8), 29.2 (s), 28.8 (d, J_C,P = 14.8), 27.8 (s), 26.8 (d, J_C,P = 14.7), 25.9 (d, J_C,P = 10.4). ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ −26.7 (s). MS (ESI(+), CHCl₃, CH₃CN) m/z 471.2 [M + H]⁺.
1,1′-Bis(5-(phospholano)pentyl)ferrocene (5b). Yield 0.97 g (65%). Found C, 67.45; H, 8.5. Calc. for C₂₈H₄₄FeP₂: C, 67.5; H, 8.9. IR [small nu, Greek, tilde] 3087m, 2925s, 2855s, 1447m, 1262s, 1095s, 1022s, 803s, 743w, 705m, 668m, 488m. ¹H NMR (CDCl₃, 400 MHz): δ 3.88 (m, 4H), 3.87 (m, 4H), 2.22 (t, J = 7.4, 4H), 1.77–1.53 (m, 12H), 1.45–1.27 (m, 16H), 1.21 (m, 4H). ¹³C{¹H} NMR (CDCl₃, 75 MHz): δ 89.2 (s), 68.2 (s), 67.7 (s), 31.3 (d, J_C,P = 9.1), 31.2 (s), 29.4 (s), 29.0 (d, J_C,P = 13.0), 27.9 (s), 26.8 (d, J_C,P = 13.0), 26.0 (d, J_C,P = 8.8). ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ −26.8 (s). MS (ESI(+), CHCl₃, CH₃CN) m/z 499.4 [M + H]⁺.

Preparation of selenides. Grey selenium (31.6 mg 0.4 mmol) was added to a solution of the respective bis-phospholane (0.2 mmol) in CHCl₃ (6 mL). The mixture was heated to reflux for 2 h and stirred overnight at rt. Unconsumed selenium was removed by filtration and the solvent was removed under reduced pressure to afford the selenides as yellow (6a) crystalline solid and viscous oil (6b), respectively.
1,1′-Bis(4-(phospholanoselenide)butyl)ferrocene (6a). Yield 94.2 mg (75%). mp 107 °C (n-hexane/ethyl acetate) Found C, 49.6; H, 6.6. Calc. for C₂₆H₄₀FeP₂Se₂: C, 49.7; H, 6.4. IR [small nu, Greek, tilde] 3084w, 2928s, 2861m, 1656w, 1649w, 1460m, 1401m, 1336w, 1301w, 1256w, 1178w, 1114m, 1101m, 1067m, 1040w, 1017m, 881m, 860m, 820m, 766w, 750w, 728m, 704m, 675m, 528m. ¹H NMR (CDCl₃, 400 MHz): δ 3.92 (m, 4H), 3.89 (m, 4H), 2.30 (t, J = 7.6, 4H), 2.25–1.89 (m, 16H), 1.83–1.61 (m, 8H), 1.55 (m, 4H). ¹³C{¹H} NMR (CDCl₃, 400 MHz): δ 88.3 (s), 68.1 (s), 67.8 (s), 33.8 (d, J_C,P = 45.3), 33.3 (d, J_C,P = 39.2), 32.1 (d, J_C,P = 14.8), 29.1 (s), 26.5 (d, J_C,P = 5.1), 23.7 (d, J_C,P = 3.2). ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ 45.6 (s, ⁷⁷Se satellites d, ¹J_P,Se = 693). MS (ESI(+), CH₂Cl₂, CH₃CN) 630.0 [M]⁺.
1,1′-Bis(5-(phospholanoselenide)pentyl)ferrocene (6b). Yield 99.8 mg (76%). Found C, 51.1; H, 6.65. Calc. for C₂₈H₄₄FeP₂Se₂: C, 51.2; H, 6.8. IR [small nu, Greek, tilde] 3085w, 2929s, 2856m, 1656w, 1631w, 1446m, 1409m, 1262s, 1108s, 1068s, 1023s, 919w, 860m, 802s, 699w, 661w, 527m. ¹H NMR (CDCl₃, 400 MHz): δ 3.91 (m, 4H), 3.89 (m, 4H), 2.26 (t, J = 7.4), 2.22–1.88 (m, 16H), 1.83–1.57 (m, 8H), 1.51–1.31 (m, 8H). ¹³C{¹H} NMR (CDCl₃, 100 MHz): δ 88.9 (s), 68.8 (s), 67.9 (s), 33.9 (d, J_C,P = 45.4), 33.3 (d, J_C,P = 39.2), 30.8 (s), 30.2 (d, J_C,P = 15.0), 29.2 (s), 26.5 (d, J_C,P = 5.1), 23.7 (d, J_C,P = 3.4). ³¹P{¹H} NMR (CDCl₃, 162 MHz): δ 45.6 (s, ⁷⁷Se satellites d, ¹J_P,Se = 693). MS (ESI(+), CH₂Cl₂, CH₃CN) 658.1 [M]⁺.

Preparation of platinum complexes. [PtCl₂(cod)] (74.8 mg, 0.2 mmol) was added to a solution of the respective bis-phospholane (0.2 mmol) in CH₂Cl₂ (6 mL) and the mixture was stirred for 18 h at rt. All volatile compounds were removed under reduced pressure. Crystallisation from diethyl ether/CH₂Cl₂ afforded the platinum complexes 7a,b as yellow, crystalline solids. Crystals suitable for X-ray diffraction were grown by liquid-phase diffusion from CH₂Cl₂/n-hexane (7a) or vapour diffusion of diethyl ether into a solution of 7b in CH₂Cl₂ over two weeks at rt.
cis-Dichlorido-[1,1′-bis(4-(phospholano)butyl)ferrocene-κ²P,P′]-platinum(II) (7a). Yield 123.7 mg (84%). Decomp. >180 °C. Found C, 42.55; H, 5.4. Calc. for C₂₆H₄₀Cl₂FeP₂Pt: C, 42.4; H, 5.5. IR [small nu, Greek, tilde] 3078w, 2932s, 2858m, 1697w, 1636m, 1448m, 1262m, 1111s, 1074s, 1024s, 882m, 808s, 714w, 685w, 514m, 498m, 418m. ¹H NMR (CD₂Cl₂, 400 MHz): δ 4.08 (m, 8H), 2.40 (m, 4H), 2.33–2.08 (m, 8H), 2.05–1.77 (m, 12H), 1.78–1.55 (m, 8H). ¹³C{¹H} NMR (CD₂Cl₂, 75 MHz): 88.9 (s), 69.1 (s), 68.2 (s), 32.4 (m), 29.5–28.5 (m), 27.9–26.0 (m), 25.4 (s). ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ 17.4 (s, ¹⁹⁵Pt satellites d, ¹J_P,Pt = 3426). MS (ESI(−), CH₂Cl₂, CH₃OH) m/z 771.0 [M + Cl]⁻.
cis-Dichlorido-[1,1′-bis(5-(phospholano)pentyl)ferrocene-κ²P,P′]-platinum(II) (7b). Yield 85.6 mg (56%). Decomp. >160 °C. Found C, 43.6; H, 6.1. Calc. for C₂₈H₄₄Cl₂FeP₂Pt: C, 44.0; H, 5.8. IR [small nu, Greek, tilde] 3084w, 2964m, 2828m, 2854m, 1456m, 1419w, 1263s, 1096s, 1024s, 804s, 689w, 661w, 498m. ¹H NMR (CD₂Cl₂, 400 MHz): δ 4.04 (m, 4H), 3.96 (m, 4H), 2.31 (m, 4H), 2.12–1.96 (m, 8H), 1.93–1.76 (m, 12H), 1.68 (m, 4H), 1.47–1.58 (m, 8H). ¹³C{¹H} NMR (CD₂Cl₂, 75 MHz): δ 90.4 (s), 69.8 (s), 68.8 (s), 31.4–30.8 (m), 29.8 (s), 29.7–29.0 (m), 28.1–27.5 (m), 27.2–26.5 (m), 26.3–25.6 (m). ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ 15.7 (s, ¹⁹⁵Pt satellites d, ¹J_P,Pt = 3452). MS (ESI(−), CH₂Cl₂, CH₃CN) m/z 800.1 [M + Cl]⁻. MS (ESI(+), CH₂Cl₂, CH₃CN) m/z 764.2 [M − Cl]⁺.

Preparation of gold complexes. [AuCl(tht)] (64.1 mg, 0.2 mmol) was added to a solution of the respective bis-phospholane (0.2 mmol) in CH₂Cl₂ (6 mL) and the mixture was stirred for 18 h at rt. Volatile compounds were removed under reduced pressure to afford the gold complexes 8a,b as yellow solids.

Crystals suitable for X-ray diffraction could be obtained through vapour diffusion of diethyl ether into a solution of 8b in dichloromethane over two weeks at 4 °C.

[1,1′-Bis(4-(phospholano)butyl)ferrocene-κ²P,P′]gold(I) chloride (8a). Yield 136.3 mg (97%). Decomp. >170 °C. Found C, 44.2; H, 5.4. Calc. for C₂₈H₄₄AuClFeP₂: C, 44.4; H, 5.7. IR [small nu, Greek, tilde] 2928m, 2865m, 1653m, 1631s, 1456m, 1262s, 803m, 694w, 516m, 490m, 417w. ¹H NMR (CD₂Cl₂, 400 MHz): δ 3.99 (m, 8H), 2.44–2.22 (m, 8H), 2.10–1.75 (m, 16H), 1.75–1.56 (m, 8H). ¹³C{¹H} NMR (CD₂Cl₂, 75 MHz): δ 88.9 (s), 68.9 (s), 68.0 (s), 32.2 (bs), 29.1 (s), 28.5 (m), 27.1 (s), 27.0–26.2 (m). ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ 34.3 (s). MS (ESI(+), CH₂Cl₂, CH₃CN) m/z 667.1 [M − Cl]⁺.
[1,1′-Bis(5-(phospholano)pentyl)ferrocene-κ²P,P′]gold(I) chloride (8b). Yield 143.3 mg (98%). Decomp. >175 °C. Found C, 45.7; H, 6.1. Calc. for C₂₈H₄₄AuClFeP₂: C, 46.0; H, 6.1. IR [small nu, Greek, tilde] 3084w, 2963m, 2927m, 2854m, 1437w, 1419w, 1263s, 1099s, 1023s, 803s, 701w, 668m, 488m. ¹H NMR (CD₂Cl₂, 400 MHz): δ 4.02 (m, 4H), 3.92 (m, 4H), 2.42 (m, 4H), 2.31 (m, 4H), 2.05 (m, 4H), 1.96–1.77 (m, 12H), 1.70 (m, 4H), 1.60 (m, 4H), 1.50 (m, 4H). ¹³C{¹H} NMR (CD₂Cl₂, 75 MHz): δ 90.0 (s), 69.1 (s), 67.5 (s), 31.7–31.4 (m), 31.4 (s). 29.5–28.9 (m), 28.8 (s), 27.4 (bs), 27.2 (m). ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): δ 37.4 (s). MS (ESI(+), CH₂Cl₂, CH₃CN) m/z 695.2 [M − Cl]⁺. MS (ESI(−), CH₂Cl₂, CH₃CN) m/z 765.4 [M + Cl]⁻.

Preparation of rhodium complexes. [{RhCl(CO)₂}₂] (38.9 mg, 0.1 mmol) was added to a solution of the respective bis-phospholane (0.2 mmol) in CH₂Cl₂ (6 mL) and the mixture was stirred for 18 h at rt. The solution was concentrated and filtered. Diethyl ether was slowly added. The resulting precipitate was filtered and washed with diethyl ether. Crystallisation from diethyl ether/CH₂Cl₂ afforded 9b as orange, crystalline solid. Crystals suitable for X-ray diffraction were obtained by vapour diffusion of diethyl ether into a saturated solution of 9b in dichloromethane over 4 days at rt.
trans-Carbonylchlorido-[1,1′-bis(5-(phospholano)butyl)ferrocene-κ²P,P′]rhodium(I) (9a). 9a could not be isolated in pure form and was therefore only characterised by ³¹P{¹H} NMR and ESI-MS. ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): 26.9 (d, ¹J_P,Rh = 115.6). MS (ESI(+), CH₂Cl₂, CH₃CN) m/z 636.2 [M]⁺.
trans-Carbonylchlorido-[1,1′-bis(5-(phospholano)pentyl)ferrocene-κ²P,P′]rhodium(I) (9b). Yield 109.2 mg (82%). Decomp. >125 °C. Found C, 52.6; H, 6.4. Calc. for C₂₉H₄₄ClFeOP₂Rh: C, 52.4; H, 6.7. IR [small nu, Greek, tilde] 3079w, 2928s, 2854m, 1952w (CO), 1656m, 1627m, 1558w, 1506m, 1498w, 1405m, 1263m, 1111m, 1053m, 1022m, 853w, 802m, 658w, 517w. ¹H NMR (CD₂Cl₂, 400 MHz): δ 3.91(m, 4H), 3.87 (m, 4H), 2.30 (m, 4H), 2.20 (m, 4H), 1.96–1.66 (m, 20H), 1.62–1.41 (m, 8H). ¹³C{¹H} NMR (CD₂Cl₂, 75 MHz): δ 188.9–187.3 (m, CO), 89.7 (s), 68.6 (s), 66.8 (s), 31.5–30.8 (m), 29.5–29.0 (m), 28.0 (s), 27.0 (bs), 26.3–25.5 (m). ³¹P{¹H} NMR (CD₂Cl₂, 162 MHz): 26.5 (d, ¹J_P,Rh = 115.4). MS (ESI(+), CH₂Cl₂, CH₃CN) m/z 629.2 [M − Cl]⁺.

Crystal structure determinations

Data for compounds 1b, 6a, 7a, 7b, 8b and 9b were collected on an Oxford Diffraction CCD Xcalibur-S diffractometer (data reduction with CrysAlis Pro,²⁵ including the program SCALE 3 ABSPACK²⁶ for empirical absorption correction) by using Mo_Kα irradiation (λ = 71.073 pm) and ω-scan rotation. Structures were solved with the SIR tool.²⁷ Refinement was performed with SHELXL97.²⁸ Non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined by constrained methods using the riding model. The refinement was carried out with the least-squares method on F². Final R indices were calculated as follows: R₁ = ∑||F_o|−|F_c||/∑|F_o| and wR₂ = {∑[w(F_o² − F_c²)²]/∑w(F_o²)²}^1/2. Figures were drawn with ORTEP.²⁹ CCDC 1410621 (1b), 1410623 (6a), 1410618 (7a), 1410619 (7b), 1410620 (8b), 1410622 (9b) contain the supplementary crystallographic data for this paper. Table 2 summarises the details of the data collection, structure solution and refinement for 7ab, 8b, and 9b.

Table 2 Summary of data collection, structure solution and refinement details for 7a,b, 8b, 9b

Compound	7a	7b	8b	9b
Formula	C26H40Cl2FeP2Pt	C28H44Cl2FeP2Pt	C28H44AuClFeP2	C29H44ClFeOP2Rh
Formula weight	736.36	764.41	730.84	664.79
Temperature [K]	130(2)	130(2)	130(2)	130(2)
Crystal system	Triclinic	Monoclinic	Orthorhombic	Monoclinic
Space group	P1̄	P21/n	P212121	P21/n
Unit cell
a [pm]	885.8(5)	1986.9(5)	1229.6(5)	1077.3(5)
b [pm]	1049.0(5)	777.4(5)	1352.6(5)	1218.3(5)
c [pm]	1540.0(5)	2021.6(5)	1697.4(5)	2216.6(5)
α [°]	72.024(5)	90	90	90
β [°]	85.158(5)	112.348(5)	90	97.477(5)
γ [°]	76.207(5)	90	90	90
V [Å^3]	1322(1)	2888(2)	2823(2)	2885(2)
Z	2	4	4	4
ρ calcd. [g cm^−3]	1.850	1.758	1.720	1.531
μ [mm^−1]	6.172	5.653	5.928	1.300
F(000)	728	1520	1456	1376
Crystal size [mm]	0.15 × 0.10 × 0.10	0.10 × 0.10 × 0.02	0.25 × 0.15 × 0.04	0.30 × 0.30 × 0.30
Θ Min–ΘMax [°]	2.09–26.37	2.45–25.35	2.54–26.37	2.54–36.32
Collected reflections	10 398	13 600	24 262	64 818
Indep. reflections (Rint)	5383 (0.0287)	5292 (0.0510)	5759 (0.0386)	13 973 (0.0359)
Completeness to ΘMax	100.0%	99.9%	99.9%	100.0%
Restraints/parameters	0/289	0/307	0/298	0/492
GooF (F^2)	0.917	1.045	1.050	1.041
R 1, wR2 (I > 2σ(I))	0.0229, 0.0452	0.0520, 0.0931	0.0268, 0.0603	0.0315, 0.0668
R 1, wR2 (all data)	0.0319, 0.0478	0.0795, 0.1027	0.0301, 0.0615	0.0427, 0.0714
Residual electron density [e Å^−3]	0.768/−1.375	1.063 /−0.679	0.866/−0.366	1.632/−0.725
Absolute structure parameter	—	—	−0.010(5)	—

---

Hydroformylation catalysis

Catalytic reactions were performed with an AMTEC SPR16 generation 2 slurry-phase reactor system and stock solutions in toluene. Typical procedure: 15 mL steel autoclaves were purged with argon. 0.25 μmol [{RhCl(CO)₂}₂] and 1.25 μmol 5b were transferred into the reactor and stirred for 30 min at rt. Toluene (to achieve a bulk volume of 3 mL) and 0.5 mmol 1-octene were added. The autoclave was then pressurised with 40 bar of CO/H₂ (1 : 1) and heated to 50 °C. The mixture was stirred vigorously for 18 h. Analysis of the reaction products was performed by GC-MS with a GCMS-QP2010 spectrometer from Shimadzu.

Computational studies

For all quantum chemical calculations, geometry optimisations as well as vibrational analysis, the programme suite ORCA³⁰ version 3.0.2 was used. DFT calculations were performed using the B3LYP functional³¹ with the basis set Def2-TZVP.³² The convergency criterion was set to 10⁻⁸ Hartree in all calculations. The vibrational analyses were carried out numerically. All structures were verified as local energy minima by only positive eigenvalues of the Hessian matrix. The simulation of the solvent environment was performed using the COSMO model³³ for which the dielectric constant ε was set to 8.93 (CH₂Cl₂). The picture was generated with PYMOL.³⁴

Acknowledgements

We gratefully acknowledge financial support from the Fonds der Chemischen Industrie (FCI, doctoral grant to A. Schmied), the Studienstiftung des deutschen Volkes (doctoral grant for T.G.) and the Graduate School Leipzig School of Natural Sciences – Building with Molecules and Nano-objects (BuildMoNa). We thank S. Märcker, M. Röckl, B. Fritzsche, and R. Zäbe for measurements of IR and NMR spectra and M. Schmied-Tobies for help with GC-MS analysis.

Notes and references

1. (a) P. Štěpnička, Ferrocenes. Ligands, Materials and Biomolecules, John Wiley & Sons Ltd., West Sussex, 2008; (b) A. Togni and T. Hayashi, Ferrocenes. Homogeneous Catalysis. Organic Synthesis. Material Science, VCH, New York, Weinheim, 1995.
2. (a) G. Bandoli and A. Dolmella, Coord. Chem. Rev., 2000, 209, 161–196; (b) I. R. Butler, W. R. Cullen, T.-J. Kim, S. J. Rettig and J. Trotter, Organometallics, 1985, 4, 972–980; (c) C. Nataro, A. N. Campbell, M. A. Ferguson, C. D. Incarvito and A. L. Rheingold, J. Organomet. Chem., 2003, 673, 47–55; (d) G. Pilloni, B. Longato and B. Corain, J. Organomet. Chem., 1991, 420, 57–65; (e) P. J. Stang, B. Olenyuk, J. Fan and A. M. Arif, Organometallics, 1996, 15, 904–908; (f) D. A. Clemente, G. Pilloni, B. Corain, B. Longato and M. Tiripicchio-Camellini, Inorg. Chim. Acta, 1986, 115, L9; (g) O. J. Curnow, T. Jones, T.-J. Kim and F. W. B. Einstein, Organometallics, 1985, 4, 346–351.
3. (a) A. Acosta-Ramírez, M. Muñoz-Hernández, W. D. Jones and J. J. García, Organometallics, 2007, 26, 5766–5769; (b) F. N. Blanco, L. E. Hagopian, W. R. McNamara, J. A. Golen, A. L. Rheingold and C. Nataro, Organometallics, 2006, 25, 4292–4300; (c) A. Fihri, P. Meunier and J.-C. Hierso, Coord. Chem. Rev., 2007, 251, 2017–2055; (d) L. E. Hagopian, A. N. Campbell, J. A. Golen, A. L. Rheingold and C. Nataro, J. Organomet. Chem., 2006, 691, 4890–4900; (e) T. Hayashi, M. Konishi, Y. Kobori, M. Kumada, T. Higuchi and K. Hirotsu, J. Am. Chem. Soc., 1984, 106, 158–163; (f) S. L. Kahn, M. K. Breheney, S. L. Martinak, S. M. Fosbenner, A. R. Seibert, W. S. Kassel, W. G. Dougherty and C. Nataro, Organometallics, 2009, 28, 2119–2126; (g) J.-F. Ma and Y. Yamamoto, J. Organomet. Chem., 1999, 574, 148–154; (h) J. H. L. Ong, C. Nataro, J. A. Golen and A. L. Rheingold, Organometallics, 2003, 22, 5027–5032; (i) R. C. J. Atkinson, V. C. Gibson and N. J. Long, Chem. Soc. Rev., 2004, 33, 313–328; (j) P. Štěpnička, I. Císařová and J. Schulz, Organometallics, 2011, 30, 4393–4403; (k) M. Trivedi, S. K. Ujjain, G. Singh, A. Kumar, S. K. Dubey and N. P. Rath, J. Organomet. Chem., 2014, 772–773, 202–209; (l) M. A. Zuideveld, B. H. G. Swennenhuis, M. D. K. Boele, Y. Guari, G. P. F. van Strijdonck, J. N. H. Reek, P. C. J. Kamer, K. Goubitz, J. Fraanje, M. Lutz, A. L. Spek and P. W. N. M. van Leeuwen, J. Chem. Soc., Dalton Trans., 2002, 2308; (m) M. J. Burk, T. P. Harper, J. R. Lee and C. Kalberg, Tetrahedron Lett., 1994, 35, 4963–4966.
4. (a) U. Berens, M. J. Burk, A. Gerlach and W. Hems, Angew. Chem., Int. Ed., 2000, 39, 1981–1983; (b) R. Gómez Arrayás, J. Adrio and J. C. Carretero, Angew. Chem., Int. Ed., 2006, 45, 7674–7715; (c) D. Heller, H.-J. Drexler, J. You, W. Baumann, K. Drauz, H.-P. Krimmer and A. Börner, Chem. – Eur. J., 2002, 8, 5196–5203; (d) J. You, H.-J. Drexler, S. Zhang, C. Fischer and D. Heller, Angew. Chem., Int. Ed., 2003, 42, 913–916; (e) M. J. Burk and M. F. Gross, Tetrahedron Lett., 1994, 35, 9363–9366.
5. C. L. Mandell, S. S. Kleinbach, W. G. Dougherty, W. S. Kassel and C. Nataro, Inorg. Chem., 2010, 49, 9718–9727.
6. (a) N. J. Goodwin, W. Henderson and B. K. Nicholson, Inorg. Chim. Acta, 1999, 295, 18–24; (b) J. J. Adams, O. J. Curnow, G. Huttner, S. J. Smail and M. M. Turnbull, J. Organomet. Chem., 1999, 577, 44–57; (c) T. Höcher, S. Blaurock and E. Hey-Hawkins, Eur. J. Inorg. Chem., 2002, 1174–1180; (d) Y. Yamamoto, T. Tanase, I. Mori and Y. Nakamura, J. Chem. Soc., Dalton Trans., 1994, 3191–3192; (e) D. M. Bensley and E. Mintz, J. Organomet. Chem., 1988, 353, 93–102; (f) O. J. Curnow, G. Huttner, S. J. Smail and M. M. Turnbull, J. Organomet. Chem., 1996, 524, 267–270; (g) T. Kauffmann, J. Ennen, H. Lhotak, A. Rensing, S. Fritz and A. Woltermann, Angew. Chem., Int. Ed. Engl., 1980, 19, 328–329; (h) N. E. Schore and B. E. LaBelle, J. Organomet. Chem., 1981, 46, 2306–2310.
7. R. T. Kettenbach, W. Bonrath and H. Butenschön, Chem. Ber., 1993, 126, 1657–1669.
8. P. W. N. M. van Leeuwen, P. C. J. Kamer, J. N. H. Reek and P. Dierkes, Chem. Rev., 2000, 100, 2741–2770.
9. M. Streitberger, A. Schmied and E. Hey-Hawkins, Inorg. Chem., 2014, 53, 6794–6804.
10. J. W. Steed and J. L. Atwood, Supramolecular Chemistry, Wiley, Chichester, UK, 2nd edn, 2009.
11. T. P. Yoon and E. N. Jacobsen, Science, 2003, 299, 1691–1693.
12. M. F. Haddow, A. J. Middleton, A. G. Orpen, P. G. Pringle and R. Papp, Dalton Trans., 2009, 202–209.
13. J. Bhatt, B. M. Fung and K. M. Nicholas, Liq. Cryst., 1992, 12, 263–272.
14. (a) D. W. Allen, I. W. Nowell and B. F. Taylor, J. Chem. Soc., Dalton Trans., 1985, 2505–2508; (b) D. W. Allen and B. F. Taylor, J. Chem. Soc., Dalton Trans., 1982, 51–54.
15. M. A. Guino-o, A. H. Zureick, N. F. Blank, B. J. Anderson, T. W. Chapp, Y. Kim, D. S. Glueck and A. L. Rheingold, Organometallics, 2012, 31, 6900–6910.
16. R. A. Baber, M. F. Haddow, A. J. Middleton, A. G. Orpen, P. G. Pringle, A. Haynes, G. L. Williams and R. Papp, Organometallics, 2007, 26, 713–725.
17. (a) M. C. Gimeno, A. Laguna, S. Cristina and P. G. Jones, Inorg. Chem., 1993, 32, 5926–5932; (b) A. Houlton, M. D. P. Mingos, D. M. Murphy, D. J. Williams, L.-T. Phang and A. T. S. Hor, J. Chem. Soc., Dalton Trans., 1993, 3629–3630; (c) P. Štěpnička and I. Císařová, J. Organomet. Chem., 2012, 716, 110–119.
18. G. A. Bowmaker, J. C. Dyason, P. C. Healy, L. M. Engelhardt, C. Pakawatchai and A. H. White, J. Chem. Soc., Dalton Trans., 1987, 1089–1097.
19. S. Herzog and J. Dehnert, Z. Chem., 1964, 4, 1–11.
20. D. D. Perrin and W. L. F. Armarego, Purification of Laboratory Chemicals, Pergamon Press, Oxford, U.K., 3rd edn, 1988.
21. (a) G. Grüttner and E. Krause, Chem. Ber., 1916, 49, 437–444; (b) G. Grüttner and M. Wiernik, Chem. Ber., 1915, 48, 1473–1486.
22. G. R. Bourret, P. J. G. Goulet and R. B. Lennox, Chem. Mater., 2011, 23, 4954–4959.
23. J. X. McDermott, J. F. White and G. M. Whitesides, J. Am. Chem. Soc., 1976, 98, 6521–6528.
24. R. K. Harris, E. D. Becker, S. M. Cabral de Menezes, R. Goodfellow and P. Granger, Pure Appl. Chem., 2001, 73, 1795–1818.
25. CrysAlis Pro, Oxford Diffraction Ltd., Oxfordshire, U.K., 2010.
26. SCALE3 ABSPACK, Oxford Diffraction Ltd., Oxfordshire, U.K., 2010.
27. A. Altomare, G. Cascarano, C. Giacovazzo and A. Guagliardi, J. Appl. Crystallogr., 1993, 26, 343–350.
28. G. M. Sheldrick, SHELX97: Programs for Crystal Structure Analysis (Release 97–2), Universität Göttingen, Göttingen, 1997.
29. L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
30. F. Neese, Wiley Interdiscip. Rev.: Comput. Mol. Sci., 2012, 73–78.
31. (a) A. D. Becke, J. Chem. Phys., 1993, 5648–5652; (b) C. Lee, W. Yang and R. G. Parr, Phys. Rev. B: Condens. Matter, 1988, 785–789.
32. (a) A. Schaefer, H. Horn and R. Ahlrichs, J. Chem. Phys., 1992, 2571–2577; (b) F. Weigand and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 3297–3305.
33. S. Sinnecker, A. Rajendran, A. Klamt, M. Diedenhofen and F. Neese, J. Phys. Chem. A, 2006, 2235–2245.
34. The PyMOL Molecular Graphics System, Version 1.7.4 Schrödinger, LLC.

---

Footnotes

† Dedicated to Professor Christian Robl on the occasion of his 60^th birthday.

‡ Electronic supplementary information (ESI) available: Crystal structures of 1b and 6a, crystallographic data, and cyclic voltammograms of 5ab, 7ab, 8ab and 9b. CCDC 1410618–1410623. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt02567b

---