Facile Arene Ligand Exchange in p-Cymene Ruthenium(II) Complexes of Tertiary P-Chiral Ferrocenyl Phosphines.

Half-sandwich arene-metal complexes are commonly used for specific applications. Herein, we report facile arene ligand exchange reactions of two ruthenium(II) complexes of tertiary P-stereogenic ferrocenyl phosphines. By mild photochemical activation, the displacement of p-cymene and subsequent tethering by η6-coordination of the terminal phenyl ring of a biphenylyl-substituted ferrocenyl phosphine were enabled. Furthermore, the spontaneous p-cymene displacement in a 2-methoxyphenyl-containing ferrocenyl phosphine and ensuing coordination of the ligand as a P,O chelate were examined. For both reactions, theoretical calculations of the general course of the reaction confirmed the experimental findings. The ease of the controlled arene displacement reported here can offer new pathways for the synthesis and design of novel tailor-made catalysts.

Introduction

Half-sandwich complexes of group 8 metals with a characteristic piano-stool architecture are privileged structures in organometallic and catalysis research,[1−5] and have found wide application in bioinorganic and medicinal chemistry.[3,6−9] In this context, 1-methyl-4-(propan-2-yl)benzene (para-cymene, p-cym) plays a predominant role because it represents an easily accessible formally neutral η6-coordinating arene ligand and its ruthenium(II) complex [{Ru(p-cym)Cl2}2] is commercially available.[10,11] The structural behavior of these complexes is crucial for their desired application. For example, transfer hydrogenation reactions proceeding via an outer sphere hydride transfer mechanism require the retention of the arene ligand on the metal center.[12] Furthermore, the interaction of half-sandwich arene–metal complexes with biomolecules usually takes place with the arene ligand remaining at the metal[6,7] (with only a few exceptions[13]). On the other hand, examples of hydrogen-borrowing catalysis or C–H arylation reactions were reported, where loss of the arene ligand is necessary for the desired reaction to proceed.[14−16] In addition, facile and controlled arene displacement became a key tool for the synthesis of new catalysts which is, for example, highlighted by the variety of transition-metal complexes of tethered arenes.[17]

Tethered complexes are an interesting subset of metal–arene complexes which emerge when the π-coordinating neutral arene ligand is connected with one of the coordinating donor groups. Here, phosphines with pendant aromatic groups in close proximity to the phosphorus atom are predominantly used as polydentate ligands for the formation of tethered ruthenium complexes.[18−26] Utilizing the chelate effect to stabilize the arene–metal interaction is one major benefit of tethered complexes compared to nontethered complexes.[21,27] In the case of chiral ligands, another advantage is a restrained stereochemical situation which raises the racemization barrier of the chiral tethered complex.[22,28−30] The arene coordination to form tethered complexes can be triggered thermally, but usually requires high temperatures.[27] In contrast, tethering triggered by light is a milder alternative in cases where thermal racemization needs to be avoided, for example, with P-stereogenic phosphines.[22]

For some time, we have been interested in developing new classes of ferrocenyl phosphines, where our focus moved from aryl-based ferrocenyl phosphine ligands for rhodium(I)-catalyzed hydroformylation of olefins[31,32] to immobilized ferrocenyl phosphine ligands for ruthenium(II)-catalyzed isomerization of allylic alcohols.[33] Following our report on the first example of redox-switchable catalysis involving dendritic phosphine-containing ligands,[34] our next target was the use of the corresponding P-stereogenic ferrocenyl phosphine ligands for asymmetric catalysis.

Results and Discussion

The P-stereogenic ferrocenyl phosphine boranes 1a,b were successfully synthesized from an unsymmetrically disubstituted 1,1′-ferrocene[34] and methyl(phenyl)phosphinite boranes (Scheme ) originating from an (−)-ephedrine-based oxazaphospholidine borane complex.[35,36] This auxiliary-based methodology provides a convenient strategy for a stepwise preparation of tertiary P-stereogenic phosphines (98.7 and 96.7% ee for 1a and 1b, respectively). In contrast to similarly prepared monodentate[37] or bidentate[38,39] ligands, 1a,b contain a 4-methoxyphenyl substituent in the 1′-position of the ferrocenyl group. Replacement of 4-methoxyphenyl in these model complexes with 4-hydroxyphenyl allows immobilization of these ligands on a variety of solid supports. Borane-deprotection of 1a,b was achieved by heating in diethylamine, and the free ferrocenyl phosphines 2a,b were reacted with [{Ru(p-cym)Cl2}2].

Scheme 1

Synthesis of P-Stereogenic Ferrocenyl Phosphine Boranes 1a,b

In the reaction of biphenylyl-containing ferrocenyl phosphine 2a, initially, the monodentate phosphine-ruthenium(II) complex 3a was formed after 12 h at room temperature, as indicated by a rather broad signal at 18.7 ppm in the 31P{1H} NMR spectrum (Figure , bottom). However, the reaction was not completed as revealed by a sharp peak at −22.9 ppm for the free ligand 2a. In addition, a small signal was observed at 41.5 ppm giving a first indication of the formation of a chelate complex.

Figure 1

31P{1H} NMR spectra of the reaction of 2a with [{Ru(p-cym)Cl2}2] yielding monodentate complex 3a and, after light irradiation, chelate complex 4a.

As in rare cases, when the 2-biphenylyl substituent of a phosphine in ruthenium complexes is able to undergo a tethering process by replacing an η6-coordinating arene ligand,[22] we anticipated a similar process here. Because P-stereogenic phosphines are generally known to be configurationally stable only under mild conditions and similar ferrocenyl-containing phosphines were reported to be thermally unstable,[28] we decided to trigger the tethering of 3a by mild photochemical activation, as described previously.[22,27−29] Indeed, the substitution reaction resulting in formation of the tethered complex 4a was already completed after 8 h just by irradiation with a conventional desk lamp (singlet at 41.5 ppm in the 31P{1H} NMR spectrum; Figure , top). This reaction is particularly remarkable because the displacement of the p-cymene fragment as the first step of tethering is generally known to proceed rather sluggishly or is even reported to be impossible.[22] Usually, arene displacements of this kind are achieved with labile η6-coordinated ligands, for example, electron-poor arenes such as alkyl benzoates.[40−42] The solid-state structure of 4a confirmed the η6-coordination of the terminal phenyl ring of the 2-biphenylyl substituent (Figure ).

Figure 2

Molecular structure of 4a in the solid state. Ellipsoids at 50% probability; hydrogen atoms are omitted for clarity.

In order to substantiate our experimental observations and understand why the p-cymene ligand can be easily displaced, DFT calculations on the complexation reaction with 2a were performed (Figure , see the Supporting Information for further details). The course of reaction for the tethering process giving 4a was calculated with a focus on key intermediates only. The first step, the coordination of ferrocenyl phosphine 2a to a [Ru(p-cym)Cl2] fragment yielding the monodentate complex 3a, is exergonic (−41.4 kJ mol–1). More interestingly, the subsequent displacement of the p-cymene ligand forming chelate complex 4a further stabilizes the system energetically by 10.3 kJ mol–1. This second step likely proceeds by stepwise hapticity decrease of the p-cymene ligand as reported previously.[16]

Figure 3

Differences in the energy levels of the reaction of 2a to 3a and 4a calculated by DFT using the ORCA program package: BP86/def2-TZVP (see the Supporting Information for further details).

In the reaction of 2-methoxyphenyl-containing ferrocenyl phosphine 2b with [{Ru(p-cym)Cl2}2], initially, the expected monodentate complex 3b was formed as indicated by a signal at 17.1 ppm in the 31P{1H} NMR spectrum (Figure , bottom). Additionally, single crystals of 3b were obtained, and X-ray crystallography confirmed the proposed structure (Figure ). However, when 3b was dissolved in CDCl3, a new phosphorus-containing compound was formed after several days at room temperature (Figure , top). A sharp singlet with a downfield shift to 59.8 ppm in the 31P{1H} NMR spectrum indicates the formation of a new phosphorus-containing species.

Figure 5

31P{1H} NMR spectra of the reaction of 2b with [{Ru(p-cym)Cl2}2], yielding the 1:1 complex 3b and, after several days at rt, formation of the 2:1 complex 4b by arene displacement.

Figure 4

Molecular structure of 3b in the solid state. Ellipsoids at 50% probability, hydrogen atoms are omitted for clarity.

Chromatographic separation of the newly formed compound followed by successful single-crystal X-ray structure analysis revealed the formation of the 2:1 complex 4b (Figure ). Accordingly, complex 4b can be obtained in high yield in a 2:1 (ligand/metal) reaction. In 4b, a RuCl2 fragment is coordinated by two ligand molecules 2b in a chelating bidentate fashion via phosphorus and oxygen with complete displacement of the p-cymene ligand. This corresponds well to the 1H NMR spectrum of 4b, showing a downfield shift for the associated methoxy groups from 3.57 ppm in 3b to 4.80 ppm upon coordination in 4b. The structure of the complex can be described as a distorted octahedral geometry with two trans-coordinating chlorido ligands and two coplanar P,O chelates with cis-coordinating oxygen and phosphorus donors. Out of five possible structural isomers, only the cis–cis–trans isomer shown in Figure is found because of steric crowding combined with the trans influence of the involved donor atoms, in line with previously reported examples.[43,44] While the coordination mode of ruthenium observed here in 4b is known, it is usually achieved by reacting a 2-methoxy-substituted phosphine with ruthenium(III) chloride.[45−47] Only a few examples are known where an arene displacement leads to this coordination motif.[44,48]

Figure 6

Molecular structure of 4b in the solid state. Ellipsoids at 50% probability, hydrogen atoms are omitted for clarity.

The unexpected ease of arene displacement is remarkable because it opens new pathways for the synthesis and design of novel metal complexes. These phosphine-ether ligands and their ruthenium(II) complexes have received much attention in homogeneous catalysis, mainly due to their hemilability.[44,49,50] The weakly coordinating oxygen atom readily dissociates generating a vacant site for substrate binding. Accordingly, the Ru1–O2 and Ru1–O3 bonds in 4b (2.271 and 2.248 Å, respectively) are much longer than the sum of the covalent radii.[45,51]

DFT calculations on the arene displacement of 3b were performed (Figure , see the Supporting Information for further details). Again, the general course of the reaction with key structures only was calculated. As expected, the reaction of ferrocenyl phosphine 2b with [{Ru(p-cym)Cl2}2] yielding the monodentate 1:1 complex 3b is exergonic by 93.1 kJ mol–1. The subsequent formation of the 2:1 complex 4b with loss of the p-cymene ligand is again exergonic by 44.4 kJ mol–1, which qualitatively resembles the experimental observations.

Figure 7

Differences in the energy levels of the reaction of 2b to 3b and 4b calculated by DFT using the ORCA program package: BP86/def2-TZVP (see the Supporting Information for further details).

Conclusions

In summary, we have reported two examples of ruthenium(II) complexes with P-stereogenic ferrocenyl phosphines which impressively demonstrate facile arene replacement, in the case of 4a, or displacement, in the case of 4b. Both compounds were isolated as follow-up products of the complexation reactions of the P-stereogenic ferrocenyl phosphine ligands with [{Ru(p-cym)Cl2}2], which initially gave the expected monodentate complexes 3a,b. For formation of 4a, we applied mild photochemical activation of 3a to induce tethering, which is unusual for p-cymene displacement. The ease with which p-cymene loss can be deliberately induced reveals that stable arene coordination to the metal center may not always be guaranteed and facile replacement opens new pathways for the synthesis of novel metal complexes.

Experimental Section

General Procedures and Analytical Methods

In the case of moisture or air sensitivity, the reactions were conducted under a nitrogen atmosphere using Schlenk techniques. Diethyl ether, dichloromethane, toluene, and hexane (isomeric mixture) were obtained from an MBraun Solvent Purification System SPS-800 and stored over 4 Å molecular sieves or potassium (in the case of hexane). Tetrahydrofuran (THF) was dried and distilled from potassium, and diethylamine was dried and distilled from potassium hydroxide. The starting material 1-bromo-1′-(4-methoxyphenyl)ferrocene[34] was synthesized from 1,1′-dibromoferrocene[52] according to literature procedures. The methyl(phenyl)phosphinite boranes[35] were synthesized by way of an (−)-ephedrine-based oxazaphospholidine borane complex. NMR spectra were recorded with a Bruker AVANCE III HD 400 or Bruker Ascend 400 spectrometer. Numbering schemes for the assignment of NMR signals are included in the Supporting Information. Mass spectra were obtained with a Bruker ESI-TOF micrOTOF, a Bruker ESI-qTOF Impact II, and a Bruker Esquire 3000plus spectrometer. Elemental analyses were determined with a Heraeus Vario EL Analyser. IR spectra were obtained with a PerkinElmer FT-IR Spectrum 2000 spectrometer. The samples were measured as KBr pellets. Chiral HPLC was performed with a Knauer HPLC system with a Smartline PDA 2800 detector (λ = 233 nm) and a 250 × 4.6 mm Lux 5 μm Amylose-1 column by Phenomenex. The specific rotations were measured with a Krüss Optronic P3002RS automatic digital polarimeter using a 1 dm micro polarimeter tube from Schmidt + Haensch. Melting points were determined using a Gallenkamp MPD 350 BM 2.5 capillary melting point apparatus and are reported uncorrected.

Synthesis of Ferrocenyl Phosphine Boranes 1a,b

At −80 °C, BuLi in hexane (1.10 equiv) was slowly added to a solution of 1-bromo-1′-(4-methoxyphenyl)ferrocene (1.05 equiv) in THF (0.25 mol L−1). After stirring for 1 h at −80 °C, the reaction mixture was slowly added to a solution of the corresponding methyl(phenyl)phosphinite borane (1.00 equiv) in THF (0.25 mol L−1) at −80 °C. The solution was allowed to warm to room temperature over a period of 12 h. Water was added, and the aqueous phase was extracted with diethyl ether. The combined organic layers were washed with saturated aqueous NaCl and dried over MgSO4. After removal of the solvent in vacuum, the crude product was purified by column chromatography on silica with hexane/DCM 4:1 grad. hexane/DCM 1:1.

1-((SP)-(2-Biphenylyl)(phenyl)phosphine P-borane)-1′-(4-methoxyphenyl)ferrocene (1a)

Ferrocenyl phosphine borane 1a was obtained from methyl (RP)-(2-biphenylyl)(phenyl)phosphinite P-borane (0.75 g, 2.43 mmol). Recrystallization from hexane/DCM gave orange crystals (0.54 g, 39%). R: 0.34 (hexane/DCM 2:1, v/v). mp 121–123 °C. 1H NMR (400.2 MHz, CDCl3): δ (ppm) 0.63–1.70 (br m, 3H, BH3), 3.79 (s, 3H, H1), 3.86–3.87 (m, 1H, H8/9), 3.89–3.90 (m, 1H, H12/13), 3.99–4.01 (m, 1H, H8/9), 4.03–4.05 (m, 1H, H7/10), 4.18–4.19 (m, 1H, H12/13), 4.21–4.23 (m, 1H, H11/14), 4.53–4.54 (m, 1H, H7/H10), 4.58–4.59 (m, 1H, H11/14), 6.74 (d, 2H, 3JHH = 8.7 Hz, H4), 6.87 (br s, 2H, H28), 6.99 (t, 2H, 3JHH = 7.5 Hz, H27), 7.05–7.15 (m, 2H, H24, H29), 7.10 (d, 2H, 3JHH = 8.8 Hz, H3), 7.16–7.24 (m, 2H, H21, H22), 7.31–7.41 (m, 3H, H18, H19), 7.43–7.49 (m, 1H, H23), 7.56–7.65 (m, 2H, H17). 13C{1H} NMR (100.6 MHz, CDCl3): δ (ppm) 55.4 (s, C1), 67.2 (s, C7/C10), 67.5 (s, C7/C10), 71.1 (s, C8/C9), 71.3 (s, C8/C9), 71.7 (d, 1JCP = 68.9 Hz, C15), 72.7 (d, 3JCP = 2.4 Hz, C12/13), 74.2 (d, 2JCP = 9.1 Hz, C11/C14), 74.4 (d, 3JCP = 6.2 Hz, C12/C13), 76.3 (d, 2JCP = 16.5 Hz, C11/C14), 87.1 (s, C6), 113.9 (s, C4), 126.9 (d, 2JCP = 9.1 Hz, C21), 127.0 (s, C3), 127.1 (s, C29), 127.2 (s, C27), 128.3 (d, 3JCP = 10.3 Hz, C18), 129.9 (s, C5), 130.1 (d, 4JCP = 1.6 Hz, C19), 130.2 (s, C28), 131.0 (d, 4JCP = 2.0 Hz, C23), 131.4 (d, 1JCP = 32.1 Hz, C16), 131.8 (s, C26), 131.9 (d, 3JCP = 7.8 Hz, C24), 133.2 (d, 2JCP = 9.5 Hz, C17), 133.6 (d, 3JCP = 8.8 Hz, C22), 140.5 (d, 2JCP = 3.1 Hz, C25), 146.8 (d, 1JCP = 9.4 Hz, C20), 158.3 (s, C2). 31P{1H} NMR (162.0 MHz, CDCl3): δ (ppm) 17.8 (br m). Chiral HPLC (n-hexane/isopropanol 90:10, 1.0 mL/min): ee = 98.7% (tR: 9.1 min [SP], 9.8 min [RP]). ESI(+)-MS: m/z (%) 589.2 (79.2) [M + Na]+, 575.1 (74.0) [M – BH3 + Na]+, 552.1 (100) [M – BH3]+. FT-IR (KBr): ν̃ (cm–1) 3426 w, 3053 w, 2995 w, 2927 w, 2934 w, 2414 m, 2251 w, 1607 w, 1523 m, 1457 m, 1439 m, 1426 w, 1387 w, 1304 w, 1288 w, 1263 m, 1242 s, 1177 m, 1108 w, 1065 m, 1030 m, 886 w, 866 w, 829 s, 806 m, 776 m, 748 m, 703 m, 678 w, 667 w, 638 w, 623 w, 610 w, 534 w, 521 w, 507 m, 498 m, 476 m, 431 w, 417 w. [α]D25 8.5° (c = 1.25 in CHCl3). Anal. Calcd for C35H32BFeOP (566.2): C, 74.24; H, 5.70. Found: C, 73.74; H, 5.65.

1-((SP)-(2-Methoxyphenyl)(phenyl)phosphine P-Borane)-1′-(4-methoxyphenyl)ferrocene (1b)

Ferrocenyl phosphine borane 1b was obtained from methyl (RP)-(2-methoxyphenyl)(phenyl)phosphinite P-borane (1.34 g, 5.13 mmol). Recrystallization from hexane/DCM gave orange crystals (1.45 g, 54%). R: 0.38 (hexane/DCM 2:1, v/v). mp 134–136 °C. 1H NMR (400.2 MHz, CDCl3): δ (ppm) 0.69–1.81 (br m, 3H, BH3), 3.41 (s, 3H, H26), 3.80 (s, 3H, H1), 4.14–4.16 (m, 1H, H8/9), 4.16–4.18 (m, 1H, H8/9), 4.27–4.29 (m, 1H, H12/13), 4.30–4.32 (m, 1H, H12/13), 4.40–4.42 (m, 2H, H7/H10, H11/14), 4.42–4.44 (m, 1H, H11/14), 4.46–4.47 (m, 1H, H7/H10), 6.74 (d, 2H, 3JHH = 8.7 Hz, H4), 6.84–6.88 (m, 1H, H24), 7.08–7.12 (m, 1H, H22), 7.13 (d, 2H, 3JHH = 8.7 Hz, H3), 7.30–7.40 (m, 3H, H18, H19), 7.46–7.54 (m, 3H, H17, H23), 7.84–7.89 (m, 1H, H21). 13C{1H} NMR (100.6 MHz, CDCl3): δ (ppm) 55.3 (s, C26), 55.4 (s, C1), 67.3 (s, C7/C10), 67.5 (s, C7/C10), 69.1 (d, 1JCP = 70.0 Hz, C15), 70.9 (s, C8/C9), 71.0 (s, C8/C9), 73.7 (d, 3JCP = 8.2 Hz, C12/13), 74.0 (d, 3JCP = 7.5 Hz, C12/13), 75.0 (d, 2JCP = 11.5 Hz, C11/C14), 75.1 (d, 2JCP = 6.5 Hz, C11/C14), 87.1 (s, C6), 112.0 (d, 3JCP = 4.4 Hz, C24), 114.0 (s, C4), 119.7 (d, 1JCP = 58.1 Hz, C20), 121.1 (d, 3JCP = 11.7 Hz, C22), 127.2 (s, C3), 128.0 (d, 3JCP = 10.5 Hz, C18), 130.0 (d, 4JCP = 2.5 Hz, C19), 130.1 (s, C5), 131.5 (d, 2JCP = 9.9 Hz, C17), 132.8 (d, 1JCP = 62.5 Hz, C16), 133.5 (d, 4JCP = 2.1 Hz, C23), 135.8 (d, 2JCP = 12.8 Hz, C21), 158.3 (s, C2), 161.0 (s, C25). 31P{1H} NMR (162.0 MHz, CDCl3): δ (ppm) 13.7 (br m). Chiral HPLC (n-hexane/isopropanol 90:10, 1.0 mL/min): ee = 96.7% (tR: 12.0 min [SP], 13.7 min [RP]). ESI(+)-MS: m/z (%) 543.1 (12.5) [M + Na]+, 519.1 (25.0) [M]+, 506.1 (100) [M – BH3]+. FT-IR (KBr): ν̃ (cm–1) 3134 w, 3065 w, 3036 w, 3000 w, 2967 w, 2935 w, 2836 w, 2397 m, 2363 m, 2338 m, 2278 w, 2261 w, 1611 w, 1588 m, 1571 w, 1523 m, 1479 m, 1456 m, 1438 m, 1428 m, 1388 w, 1278 m, 1249 s, 1177 m, 1168 w, 1139 w, 1107 w, 1082 w, 1061 m, 1042 s, 1026 m, 1012 m, 999 w, 886 w, 837 s, 814 w, 790 m, 760 m, 726 m, 702 w, 661 w, 628 w, 690 w. [α]D25 −24.5° (c = 1.25 in CHCl3). Anal. Calcd for C30H30BFeO2P (520.1): C, 69.27; H, 5.81. Found: C, 68.77; H, 5.74.

Synthesis of Ferrocenyl Phosphines 2a,b

A solution of 1 in diethylamine (0.025 mol L−1) was heated at 50 °C for 12 h. Diethylamine was removed by evaporation under reduced pressure, and the crude residue was purified by column chromatography on degassed, deactivated silica (pretreated with hexane/triethylamine 95:5) with degassed hexane/DCM 2:1 grad. hexane/DCM 1:2. In order to determine the enantiomeric excess, 2 was reprotected by reaction with BH3·SMe2 (2.0 mol L−1 in THF) prior analysis by chiral HPLC.

1-((SP)-(2-Biphenylyl)(phenyl)phosphine)-1′-(4-methoxyphenyl)ferrocene (2a)

Ferrocenyl phosphine 2a was obtained from ferrocenyl phosphine borane 1a (0.50 g, 0.88 mmol) as an orange solid (0.45 g, 93%). R: 0.41 (hexane/DCM 2:1, v/v). mp 46–48 °C. 1H NMR (400.2 MHz, CDCl3): δ (ppm) 3.68–3.70 (m, 1H, H12/H13), 3.79 (s, 3H, H1), 3.97–3.98 (m, 1H, H8/H9), 4.07–4.09 (m, 1H, H8/H9), 4.12–4.14 (m, 2H, H11/H14, H12/H13), 4.18–4.20 (m, 2H, H7/H10, H11/H14), 4.48–4.50 (m, 1H, H7/H10), 6.74 (d, 2H, 3JHH = 8.7 Hz, H4), 7.07–7.12 (m, 3H, H18, H19), 7.16 (d, 2H, 3JHH = 8.7 Hz, H3), 7.18–7.33 (m, 11H, H17, H21–H24, H27–H29). 13C{1H} NMR (100.6 MHz, CDCl3): δ (ppm) 55.4 (s, C1), 67.0 (s, C7/C10), 67.1 (s, C7/C10), 70.0 (s, C8/C9), 70.1 (s, C8/C9), 72.9 (s, C12/C13), 73.3 (s, C12/C13), 73.7 (d, 2JCP = 6.9 Hz, C11/C14), 76.0 (d, 2JCP = 29.9 Hz, C11/C14), 77.0 (d, 1JCP = 8.6 Hz, C15), 86.6 (s, C6), 113.9 (s, C4), 127.0 (s, C3), 127.3 (s, C27), 127.7 (s, C28), 127.9 (s, C24), 128.0 (s, C22), 128.2 (s, C29), 128.6 (s, C23), 129.8 (d, 3JCP = 3.9 Hz, C18), 129.9 (d, 2JCP = 3.9 Hz, C21), 130.7 (s, C5), 132.9 (s, C19), 134.5 (d, 2JCP = 20.5 Hz, C17), 138.5 (d, 2JCP = 8.6 Hz, C25), 139.2 (d, 1JCP = 15.2 Hz, C20), 141.9 (d, 3JCP = 4.9 Hz, C26), 146.9 (d, 1JCP = 24.8 Hz, C16), 158.2 (s, C2). 31P{1H} NMR (162.0 MHz, CDCl3): δ (ppm) −22.9 (s). Chiral HPLC (n-hexane/isopropanol 90:10, 1.0 mL/min): ee = 98.9% (tR: 9.5 min [SP], 10.2 min [RP]). ESI(+)-MS: m/z (%) 552.2 (100) [M]+, 553.1 (65.7) [M + H]+. FT-IR (KBr): ν̃ (cm–1) 3419 w, 3049 m, 2932 m, 2833 m, 1610 m, 1579 w, 1525 s, 1457 s, 1435 m, 1303 w, 1288 m, 1246 s, 1176 m, 1158 m, 1107 m, 1086 m, 1030 s, 884 w, 825 s, 774 w, 745 s, 698 s, 604 w, 490 m, 457 w, 418 w. Anal. Calcd for C35H29FeOP (552.1): C, 76.10; H, 5.29. Found: C, 76.07; H, 5.22.

1-((SP)-(2-Methoxyphenyl)(phenyl)phosphine)-1′-(4-methoxyphenyl)ferrocene (2b)

Ferrocenyl phosphine 2b was obtained from ferrocenyl phosphine borane 1b (1.00 g, 1.92 mmol) as an orange solid (0.92 g, 95%). R: 0.45 (hexane/DCM 2:1, v/v). mp 39–41 °C. 1H NMR (400.2 MHz, CDCl3): δ (ppm) 3.63 (s, 4H, H11/H14, H26), 3.72 (s, 3H, H1), 4.00 (br s, 1H, H11/H14), 4.08 (br s, 1H, H12/H13), 4.10 (br s, 1H, H8/9), 4.12 (br s, 1H, H8/9), 4.14 (br s, 1H, H12/H13), 4.35 (br s, 1H, H7/H10), 4.50 (br s, 1H, H7/H10), 6.70 (d, 2H, 3JHH = 8.5 Hz, H4), 6.73–6.82 (m, 2H, H22, H24), 7.20 (d, 2H, 3JHH = 8.4 Hz, H3), 7.23–7.27 (m, 5H, H17–H19), 7.37–7.41 (m, 2H, H21, H23). 13C{1H} NMR (100.6 MHz, CDCl3): δ (ppm) 55.4 (s, C1), 55.7 (s, C26), 67.1 (s, C7/C10), 67.4 (s, C7/C10), 70.0 (s, C8/C9), 70.1 (s, C8/C9), 72.7 (s, C12/13), 73.3 (s, C12/13), 73.4 (s, C11/C14), 75.9 (d, 2JCP = 24.8 Hz, C11/C14), 76.2 (d, 1JCP = 6.1 Hz, C15), 86.7 (s, C6), 110.3 (s, C24),113.9 (s, C4), 120.8 (s, C22), 127.4 (s, C3), 128.1 (d, 2JCP = 7.5 Hz, C17), 128.2 (d, 1JCP = 12.5 Hz, C20), 128.6 (s, C19), 130.1 (s, C18), 130.8 (s, C5), 133.7 (s, C21), 133.9 (s, C23), 138.2 (d, 1JCP = 8.3 Hz, C16),158.2 (s, C2), 160.8 (d, 2JCP = 15.3 Hz, C25). 31P{1H} NMR (162.0 MHz, CDCl3): δ (ppm) −29.6 (s). Chiral HPLC (n-hexane/isopropanol 90:10, 1.0 mL/min): ee = 97.2% (tR: 12.1 min [SP], 13.7 min [RP]). ESI(+)-MS: m/z (%) 507.1 (100) [M + H]+. FT-IR (KBr): ν̃ (cm–1) 3419 w, 3067 w, 3037 w, 3000 w, 2961 w, 2932 w, 2833 w, 2031 w, 1884 w, 1610 w, 1576 w, 1524 m, 1458 m, 1427 m, 1383 w, 1305 w, 1274 w, 1245 s, 1178 m, 1159 w, 1128 w, 1107 w, 1067 w, 1027 s, 884 w, 818 s, 793 m, 756 m, 745 m, 698 m, 632 w, 603 w, 529 m, 506 w, 484 m, 431 w. Anal. Calcd for C30H27FeO2P (506.1): C, 71.16; H, 5.37. Found: C, 71.11; H, 5.06.

Synthesis of Ferrocenyl Phosphine Ruthenium(II) Complexes 3b and 4a,b

A solution of di-μ-chlorobis-[(η6-p-cymene)chlororuthenium(II)] (0.50 equiv) in DCM (0.025 mol L−1) was added to a solution of 2 (1.00 equiv) in DCM (0.025 mol L−1) and stirred at room temperature until the 31P{1H} NMR spectrum indicated full conversion of the free phosphine (4a was exposed to the light of a conventional desk lamp while stirring). The volume of the reaction mixture was reduced under reduced pressure, and the resulting residue was purified by column chromatography on degassed silica with degassed pure DCM grad. DCM/ethyl acetate 1:1. The determination of the enantiomeric excess of the metal complexes was not successful. The results of the screening on a number of phases in different screening modes were inconclusive; no selectivity between the isomers was observed (chiral HPLC screening service by Phenomenex).

1-((SP)-(2-Methoxyphenyl)(phenyl)phosphino)[(η6-p-cymene)dichloro-ruthenium(II)]-1′-(4-methoxyphenyl)ferrocene (3b)

Complex 3b was obtained from ferrocenyl phosphine 2b (0.40 g, 0.79 mmol) after stirring at room temperature for 1 h. Redissolving in DCM (5 mL) and precipitation in hexane (100 mL) gave an orange solid (0.55 g, 86%). R: 0.27 (DCM/ethyl acetate 8:1, v/v). mp 146–148 °C. 1H NMR (400.2 MHz, CDCl3): δ (ppm) 0.37 (d, 3H, 3JHH = 6.8 Hz, H35/H36), 0.99 (d, 3H, 3JHH = 7.1 Hz, H35/H36), 2.12 (s, 3H, H27), 2.63 (sept, 1H, 3JHH = 6.9 Hz, H34), 3.57 (s, 3H, H26), 3.58–3.59 (m, 1H, H8/H9), 3.77 (s, 3H, H1), 3.79–3.80 (m, 1H, H7/H10), 3.85–3.87 (m, 1H, H8/H9), 3.88–3.90 (m, 1H, H11/H14), 4.01–4.02 (m, 1H, H12/H13), 4.13 (br s, 1H, H12/H13), 4.32–4.33 (m, 1H, H7/H10), 4.83 (d, 1H, 3JHH = 5.5 Hz, H31/32), 4.93–4.94 (m, 1H, H11/H14), 5.16–5.18 (m, 1H, H29/H30), 5.44 (d, 1H, 3JHH = 6.5 Hz, H31/32), 5.60 (d, 1H, 3JHH = 6.5 Hz, H29/H30), 6.74 (d, 2H, 3JHH = 8.7 Hz, H4), 6.88–6.92 (m, 2H, H22, H24), 7.18 (d, 2H, 3JHH = 8.7 Hz, H3), 7.28–7.37 (m, 2H, H21, H23), 7.45–7.49 (m, 3H, H18, H19), 7.94 (br s, 2H, H17). 13C{1H} NMR (100.6 MHz, CDCl3): δ (ppm) 17.4 (s, C27), 18.5 (s, C35/C36), 23.4 (s, C35/C36), 29.8 (s, C34), 54.8 (s, C26), 55.3 (s, C1), 66.5 (s, C7/C10), 67.5 (s, C7/C10), 72.4 (s, C8/C9), 72.7 (d, 3JCP = 8.6 Hz, C12/C13), 73.0 (s, C8/C9), 73.2 (d, 2JCP = 6.7 Hz, C11/C14), 74.1 (d, 3JCP = 4.6 Hz, C12/C13), 76.4 (d, 1JCP = 52.9 Hz, C15), 79.7 (d, 2JCP = 13.7 Hz, C11/C14), 80.6 (s, C31/C32), 86.1 (s, C6), 86.6 (s, C29/C30), 90.3 (d, 2JCP = 10.2 Hz, C31/C32), 93.4 (s, C28), 96.3 (d, 2JCP = 6.1 Hz, C29/C30), 108.4 (s, C33), 110.9 (d, 3JCP = 4.3 Hz, C24), 113.7 (s, C4), 120.9 (d, 3JCP = 9.6 Hz, C22), 126.3 (d, 1JCP = 44.0 Hz, C20), 127.1 (d, 3JCP = 8.9 Hz, C18), 127.1 (s, C3), 129.7 (d, 4JCP = 1.7 Hz, C19), 130.7 (s, C5), 131.6 (d, 4JCP = 0.9 Hz, C23), 132.8 (d, 2JCP = 9.1 Hz, C17), 137.3 (d, 1JCP = 47.1 Hz, C16), 137.5 (d, 2JCP = 7.9 Hz, C21), 157.9 (s, C2), 158.3 (d, 2JCP = 1.1 Hz, C25). 31P{1H} NMR (162.0 MHz, CDCl3): δ (ppm) 17.1 (s). ESI(+)-MS: m/z (%) 777.1 (62.5) [M – Cl]+, 507.1 (100) [M – Ru(η6-p-cymene)Cl2]+, 292.1 (51.5) [(4-methoxyphenyl)ferrocene]+. FT-IR (KBr): ν̃ (cm–1) 3422 w, 3052 w, 2958 m, 2931 m, 2869 w, 2833 w, 2219 w, 2035 w, 1609 w, 1584 m, 1573 m, 1524 s, 1458 s, 1433 s, 1384 m, 1303 w, 1277 m, 1246 s, 1201 w, 1177 m, 1158 m, 1131 w, 1093 w, 1057 w, 1028 s, 911 w, 889 w, 829 m, 798 m, 752 m, 728 m, 700 m, 673 w, 624 w, 604 w, 583 w, 535 m, 496 m. Anal. Calcd for C40H41Cl2FeO2PRu (812.1): C, 58.13; H, 5.09. Found: C, 57.59; H, 4.83.

1-((SP)-κP-η6-(2-Biphenylyl)(phenyl)phosphino)[dichloro-ruthenium(II)]-1′-(4-methoxyphenyl)ferrocene (4a)

Complex 4a was obtained from ferrocenyl phosphine 2a (0.25 g, 0.45 mmol). Light activation by a conventional desk lamp for 8 h enabled the tethering process while stirring. Redissolving in DCM (4 mL) and precipitation in hexane (100 mL) gave an orange solid (0.20 g, 61%). R: 0.32 (DCM/ethyl acetate 1:1, v/v). mp 121–123 °C. 1H NMR (400.2 MHz, CDCl3): δ (ppm) 3.82 (s, 3H, H1), 3.91–3.93 (m, 1H, H12/H13), 4.14–4.15 (m, 1H, H11/H14), 4.18–4.19 (m, 1H, H11/H14), 4.34–4.36 (m, 1H, H8/H9), 4.40–4.41 (m, 1H, H8/H9), 4.42–4.43 (m, 1H, H7/H10), 4.57–4.58 (m, 1H, H7/H10), 4.67–4.69 (m, 1H, H12/H13), 4.93 (d, 1H, 3JHH = 5.7 Hz, H27/H31), 5.18 (d, 1H, 3JHH = 5.6 Hz, H27/H31), 5.86 (t, 1H, 3JHH = 5.8 Hz, H28/H30), 6.00 (t, 1H, 3JHH = 5.9 Hz, H28/H30), 6.26 (t, 1H, 3JHH = 5.9 Hz, H29), 6.82 (d, 2H, 3JHH = 8.6 Hz, H4), 7.32 (d, 2H, 3JHH = 8.5 Hz, H3), 7.33–7.64 (m, 9H, H17–H19, H21–H24). 13C{1H} NMR (100.6 MHz, CDCl3): δ (ppm) 55.5 (s, C1), 67.8 (s, C7/C10), 68.5 (s, C7/C10), 71.5 (s, C8/C9), 72.1 (s, C8/C9), 72.4 (d, 2JCP = 7.6 Hz, C11/C14), 75.2 (d, 1JCP = 50.9 Hz, C15), 75.2 (d, 3JCP = 7.2 Hz, C12/C13), 75.4 (d, 2JCP = 13.2 Hz, C11/C14), 75.6 (d, 3JCP = 8.6 Hz, C12/C13), 81.0 (s, C27/C31), 82.0 (s, C27/C31), 87.6 (s, C6), 90.8 (d, 2JCP = 15.3 Hz, C29), 96.6 (d, 2JCP = 2.6 Hz, C28/C30), 98.8 (d, 2JCP = 3.9 Hz, C28/C30), 109.7 (d, 3JCP = 2.9 Hz, C26), 114.1 (s, C4), 127.1 (d, 3JCP = 12.2 Hz, C22), 127.7 (s, C3), 127.8 (d, 3JCP = 11.9 Hz, C18), 129.8 (d, 2JCP = 6.5 Hz, C21), 130.3 (s, C5), 130.5 (d, 3JCP = 2.5 Hz, C24), 131.4 (d, 4JCP = 1.4 Hz, C23), 132.2 (d, 2JCP = 9.1 Hz, C17), 132.2 (d, 1JCP = 51.2 Hz, C16), 133.3 (s, C19), 143.8 (d, 2JCP = 22.4 Hz, C25), 145.9 (d, 1JCP = 50.7 Hz, C20), 158.5 (s, C2). 31P{1H} NMR (162.0 MHz, CDCl3): δ (ppm) 41.5 (s). ESI(+)-MS: m/z (%) 723.9 (100) [M]+, 689.0 (19.9) [M – Cl]+. FT-IR (KBr): ν̃ (cm–1) 3445 m, 3054 m, 2954 m, 2927 m, 2833 w, 2035 w, 1609 m, 1576 w, 1525 s, 1483 w, 1458 s, 1435 s, 1287 w, 1287 m, 1247 s, 1177 m, 1163 m, 1130 w, 1102 m, 1059 w, 1030 s, 888 w, 830 s, 766 m, 746 m, 691 m, 619 m, 539 m, 496 s, 480 s, 415 w. Anal. Calcd for C35H29Cl2FeOPRu (723.9): C, 56.03; H, 4.04. Found: C, 55.93; H, 3.62.

trans-Dichlorobis[1-(RP)-κP-(2-(methoxy-κO)phenyl)(phenylphosphino)-1′-(4-methoxyphenyl)ferrocene]ruthenium(II) (4b)

Complex 4b was obtained as a byproduct of the reaction to 3b and was separated from 3b by column chromatography. Depending on the reaction time for 3b, different yields of 4b could be isolated. In solution, complex 3b slowly forms 4b at room temperature. R: 0.42 (DCM/ethyl acetate 8:1, v/v). mp 210–212 °C. 1H NMR (400.2 MHz, CD2Cl2): δ (ppm) 3.48 (br s, 2H, H11/H14), 3.52–3.53 (m, 2H, H8/H9), 3.55 (br s, 2H, H7/H10), 3.64–3.65 (m, 2H, H11/H14), 3.66 (br s, 2H, H12/H13), 3.68–3.69 (m, 2H, H12/H13), 3.75 (s, 6H, H1), 3.82–3.84 (m, 2H, H8/H9), 4.38–4.39 (m, 2H, H7/H10), 4.80 (s, 6H, H26), 6.72 (d, 4H, 3JHH = 8.8 Hz, H4), 7.10–7.15 (m, 8H, H17, H18), 7.13 (d, 4H, 3JHH = 8.7 Hz, H3), 7.24 (t, 4H, 3JHH = 7.3 Hz, H22, H23), 7.49–7.56 (m, 4H, H21, H24), 7.65–7.69 (m, 2H, H19). 13C{1H} NMR (100.6 MHz, THF-d8): δ (ppm) 55.4 (s, C1), 61.8 (s, C26), 67.0 (s, C7/C10), 68.5 (s, C7/C10), 71.3 (s, C8/C9), 71.9 (s, C8/C9), 72.7 (s, C12/C13), 73.2 (d, 2JCP = 11.0 Hz, C11/C14), 73.9 (s, C12/C13), 78.5 (d, 1JCP = 52.1 Hz, C15), 80.9 (d, 2JCP = 9.2 Hz, C11/C14), 87.1 (s, C6), 114.2 (s, C24), 114.4 (s, C4), 122.5 (s, C22), 127.4 (d, 3JCP = 6.5 Hz, C18), 127.9 (s, C3), 129.6 (d, 2JCP = 7.6 Hz, C17), 129.7 (d, 1JCP = 45.4 Hz, C20), 131.9 (s, C5), 132.5 (s, C21), 133.4 (s, C19), 135.4 (s, C23), 141.2 (d, 1JCP = 58.4 Hz, C16), 159.3 (s, C2), 164.5 (s, C25). 31P{1H} NMR (162.0 MHz, THF-d8): δ (ppm) 59.8 (s). ESI(+)-MS: m/z (%) 1190.1 (16.7) [M + Li]+, 598.1 (45.3) [2b + Na]+, 506.1 (100) [2b]+. FT-IR (KBr): ν̃ (cm–1) 3444 m, 3080 w, 2993 w, 2936 m, 2834 m, 2234 w, 2035 w, 1956 w, 1610 m, 1576 m, 1525 s, 1471 m, 1455 s, 1440 s, 1386 w, 1303 w, 1273 m, 1247 s, 1226 m, 1163 m, 1134 w, 1094 w, 1072 w, 1030 s, 1005 m, 908 m, 887 w, 829 m, 794 m, 757 m, 729 m, 695 m, 649 w, 629 w, 591 w, 575 w, 553 m, 533 m, 493 m, 450 w, 413 m. Anal. Calcd for C60H54Cl2Fe2O4P2Ru (1184.1): C, 60.83; H, 4.59. Found: C, 60.66; H, 4.26.

X-ray Crystallography

X-ray diffraction studies were performed with an Oxford Diffraction CCD Xcalibur-S diffractometer using Mo Kα radiation (λ = 0.71073 Å) and ω-scan rotation. Data reduction was performed with CrysAlis Pro[53] including the program SCALE3 ABSPACK[54] for empirical absorption correction. Structures were solved by direct methods and refined by full-matrix least-squares techniques against F2 by using the SHELX program package.[55] All nonhydrogen atoms were refined with anisotropic thermal parameters and all nonboron-bonded hydrogen atoms were assigned riding isotropic displacement parameters and constrained to idealized geometries. Structure figures were generated with Mercury 4.0.0.[56,57]

Computational Details

All calculations were carried out in the gas phase using the ORCA program package (version 4.0.0.1).[58,59] Geometry optimizations and frequency analysis, to prove the absence of imaginary frequencies, were obtained employing the BP86[60−62] functional in conjunction with a def2-TZVP[63,64] basis set. The energies from frequency analyses were used for the investigation of the reaction pathways. For all calculations, atom-pairwise dispersion corrections with the Becke–Johnson damping scheme (D3BJ) and density fitting techniques, also called resolution-of-identity approximation (RI), were utilized.[65,66]