Rhodium and ruthenium complexes of methylene-bridged, P-stereogenic, unsymmetrical diphosphanes

Abstract

Enantiopure P-stereogenic methylphosphane-boranes (S_P)-P(BH₃)PhArMe (ArMe; Ar = 1-naphthyl (NpMe), and 2-biphenylyl (BiphMe)) have been used to prepare diphosphanes of the type ArPhPCH₂PR₂ (R = Ph, iPr or tBu; ArR). The ligands have been reacted with [Rh(COD)₂]BF₄ to furnish the corresponding six monochelated [Rh(COD)(ArR)]BF₄ organometallic compounds (RhArR) or, depending on the reaction conditions, the bis(chelated) coordination compound [Rh(BiphiPr)₂]BF₄ as a mixture of cis and trans isomers. The crystal structure of cis-[Rh(BiphiPr)₂]BF₄ was obtained. The coordination of the BiphR with [RuCl(μ-Cl)(η⁶-p-cymene)₂]₂ under different conditions produced cationic chelated complexes of the type [RuCl(η⁶-p-cymene)(κ²-BiphR)]PF₆ (RuBiphR) and the neutral monocoordinated complex [RuCl₂(η⁶-p-cymene)(κ¹-BiphPh)] (RuBiphPh′) with the uncoordinated P-stereogenic moiety. The Rh(I) complexes were used in the catalytic hydrogenation of functionalized olefins and the Ru(II) complexes were tested in the transfer hydrogenation of acetophenone. Both precursors displayed good activities with moderate enantioselectivities.

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Introduction

Throughout the history of organometallic homogeneous catalysis, phosphorus has been and remains the most important donor group in ancillary ligands of catalytic precursors, due to its ability to coordinate to most of the catalytically more capable metals.¹ Among the many phosphorus-based ligands that have been synthesized, diphosphanes are still the most popular, especially in enantioselective catalysis.^2,3 The relatively high inversion energy of phosphorus allows for the enantioselective preparation of P-stereogenic ligands,^4,5 which have been very successful in several enantioselective reactions. These P-stereogenic diphosphanes shone especially bright in the early days of asymmetric catalysis specifically in enantioselective hydrogenation,^6–9 to the point that one of the awardees of the 2001 Nobel Prize in Chemistry was William S. Knowles¹⁰ for his research on the synthesis of L-DOPA —a drug used to treat Parkinson's disease— by enantioselective hydrogenation using the DIPAMP ligand, a P-stereogenic diphosphane.

After a long period with low activity, many more successful P-stereogenic diphosphanes have been developed,^5,11–15 affording excellent catalytic results. Some of the most successful have been those with a single atom linker between the two phosphorus atoms,¹⁶ like the MiniPHOS family,^17,18 TriChickenFootPHOS (TCPF),¹⁹ or MaxPHOS.^20–22 These short-bridged ligands (Fig. 1), despite their simple structures, have excelled in enantioselective hydrogenation.²³

Fig. 1 Structures of MiniPHOS, TCFP and MaxPHOS.

For these reasons, following our own research on P-stereogenic monophosphanes,^24–28 we recently reported an array of modular P-stereogenic diphosphanes with a methylene bridge and their Pd(II) complexes.²⁹ The ligands are rare examples of unsymmetrical (C₁-symmetric) diphosphanes, which have previously proved to be amongst the most efficient in metal-catalysed asymmetric hydrogenation.^{17–19,23,30–34}

In the present work, we present two new ligands bearing a P(tBu)₂ moiety on the non-stereogenic phosphorus, and the study of the coordination of this kind of ligands to Rh(I) and Ru(II), to further explore the coordinative variability they display. In addition, catalytic results in the hydrogenation and transfer hydrogenation reactions are presented.

Results and discussion

Unsymmetrical diphosphanes

The methylene-bridged diphosphanes ArPhP¹CH₂P²R₂ (ArR; Ar = 2-biphenylyl (Biph) and 1-naphthyl (Np); R = iPr, tBu or Ph) contain an enantiopure P-stereogenic moiety (denoted as P¹) and a non-P-stereogenic moiety (denoted as P²). The ligands were prepared from optically pure methylphosphane–boranes (ArMe) by deprotonation of the methyl group and phosphination of the resulting carbanion with the corresponding chlorophosphane²⁹ (Fig. 2). The diphosphanes could be conveniently stored as their bis(borane) adducts (ArR·BH₃), which can be easily deprotected when needed with morpholine.

Fig. 2 Synthesis of the unsymmetrical diphosphanes. NptBu was synthesized by a variation of method A, and BiphtBu with a variation of method B.

Given the good results that several P-stereogenic diphosphanes containing the bis(tert-butyl)phosphino group have produced in enantioselective catalysis,^13,22 the same procedure was followed to prepare the protected diphosphane BiphtBu·BH₃ (Fig. 2). Unexpectedly, it was found that this compound could not be isolated, but the monoboronated adduct BiphtBu·2-BH₃ —with only the P² phosphorus protected— was instead obtained (Fig. 3), a compound that was found to be stable in air for a few days.

Fig. 3 Synthesis of ArtBu diphosphanes.

In the ³¹P{¹H} NMR spectrum of the reaction mixture after workup, BiphtBu·2-BH₃ was observed as a broad singlet at δ_P = +45.1 ppm and a sharp doublet at −30.2 ppm (J_P¹–P² = 46.1 Hz), corresponding to P² and P¹ respectively. The formation of BiphtBu·BH₃ (with both phosphorus boronated) could only be observed by ³¹P{¹H} NMR spectroscopy in solution in the presence of an excess of BH₃·THF as two broad signals at δ_P +50.3 and +26.3 ppm, corresponding to P² and P¹, respectively. After following the aqueous work-up as in the previously reported procedure,²⁹ the species BiphtBu·2-BH₃ was obtained, meaning that P¹ had suffered a spontaneous deboronation reaction. This result can be explained first by the reduced basicity of the diarylphosphane P¹ compared to the trialkylphosphane P² and, second, by the steric hindrance exerted by the bis(tert-butyl)phosphane moiety of P², weakening the B–P¹ bond. It has to be noted that these two factors are necessary for the spontaneous cleavage of the B–P bond, since borane adducts of both alkyl,^18,19 and aryldiphosphanes^29,35,36 with a methylene bridge are known to be perfectly stable compounds.

The deprotection of BiphtBu·2-BH₃ with morpholine^29,37 was found to be very slow, requiring 72 h at 40 °C (Fig. 3). The synthesis was therefore adapted to furnish BiphtBu in a one-pot procedure, treating without isolation the intermediate compound BiphtBu·1-BH₃ —with only P¹ protected— with morpholine at room temperature for 24 h (Fig. 3). Following this same procedure, the ligand NptBu was also synthesized and fully characterized.

The ³¹P{¹H} NMR spectra of ligands BiphtBu (δ_P¹ = −24.1 ppm, δ_P² = +15.3 ppm; J_P¹–P² = 127.7 Hz) and NptBu (δ_P¹ = −28.1 ppm, δ_P² = +13.8 ppm; J_P¹–P² = 143.8 Hz) consist of two doublets while the hydrogen atoms of the tert-butyl groups appear in the ¹H NMR spectra as two sharp doublets (J_H–P² = 10.8 Hz) at around 0.8 ppm, in accordance with other methylene-bridged diphosphanes.^29,38,39

Rhodium complexes

Initially, we focused on obtaining the rhodium(I) complexes with the general formulae [Rh(ArR)(COD)]BF₄ (RhArR), which are typical hydrogenation catalytic precursors,^23,40 starting from the rhodium precursor [Rh(COD)₂]BF₄ (Fig. 4).

Fig. 4 Synthesis of the RhArR complexes.

However, upon adding one equivalent of [Rh(COD)₂]BF₄ to a solution of one equivalent of the deprotected diphosphane, a complex mixture of species was observed according to ³¹P{¹H} NMR, after a standard work-up in air. At lower field (50–20 ppm) some peaks probably corresponding to the oxidized diphosphanes^39,41,42 could be identified while at higher field (with a chemical shift between 0 and −40 ppm), where the signals of the Rh–P complexes tend to appear,^19,43–49 more than one species was observed. From the presence of oxidized diphosphanes, it was deduced that the complexes were air-sensitive and tended to decompose when exposed to air, an uncommon feature for compounds with this structure, which are often stable.¹⁹ However, some hints in the literature suggest a certain lability of the ligands.⁵⁰

The repetition of the syntheses and work-up under strictly inert atmosphere solved the oxidation issues, but a mixture of species was still observed at higher field in ³¹P{¹H} NMR spectra. From the great variety of coordination possibilities that the ligands had shown when coordinated to Pd(II),²⁹ it was assumed that more than one metal complex could be formed. In the literature, some examples of bischelated complexes were found for short-bridged ligands so we wondered whether mixtures of mono- and bischelated complexes could be formed (Fig. 5).^17,32,33

Fig. 5 General structure of the mono(chelated) and bis(chelated) complexes (top) and an example of a reported bis(chelated) complex with a methylene-bridged diphosphorus ligand (bottom).¹⁷

To favour the formation of the mono(chelated) structure, the order of the complexation was altered, adding a solution of the diphosphane to a solution of the rhodium precursor in a dropwise manner under vigorous stirring,^{19,32,33,50,51} to ensure a low local concentration of the ligand and thus avoid the formation of the bis(chelated) complexes. This new procedure afforded the pure mono(chelated) structures after recrystallisation under inert atmosphere. The complexes were found to be indefinitely stable if kept under nitrogen at low temperature. In Fig. 6 the ³¹P{¹H} NMR spectra of the complexes are displayed and the values of the chemical shifts and coupling constants are given in Table 1.

Fig. 6 ³¹P{¹H} NMR (CD₂Cl₂) spectra of the mono(chelated) Rh complexes. (a) RhBiphPh, (b) RhNpPh, (c) RhBiphiPr, (d) RhNpiPr, (e) RhBiphtBu, (f) RhNptBu. ^a At 162 MHz. ^b At 202 MHz.

Table 1 ³¹P{¹H} NMR (CD₂Cl₂) shifts (ppm) and coupling constants (Hz) for the rhodium complexes RhArR

Complex	δ P^1	δ P^2	^2 J P^1–P^2	^1 J P^1–Rh	^1 J P^2–Rh
a 202 MHz. b 162 MHz. c Values obtained using the software package Spinworks.^52
RhBiphiPr	−34.9	−13.4	76.1	129.4	126.9
RhBiphtBu	−35.8	−4.7	63.1	130.6	125.4
RhBiphPh ^,	−29.8	−29.1	75.7	130.6	126.6
RhNpiPr	−39.1	−12.3	77.4	131.7	126.7
RhNptBu	−38.3	−2.9	63.9	132.7	125.1
RhNpPh	−33.1	−28.9	78.0	130.1	130.2

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Each compound features two doublets of doublets, each one corresponding to one of the phosphorus atoms, coupled to the other one and to the rhodium nucleus. As it can be seen, the chemical shift for the P-stereogenic phosphorus (P¹) ranges between −40 and −33 ppm. The shift of the non-P-stereogenic phosphorus (P²) is defined by its substituents: ca. −30 ppm for PPh₂, −12 for PiPr₂ and −3 ppm for PtBu₂. This order is given by a combination of acidity and bulkiness of the substituents, influence of which on ³¹P{¹H} NMR shifts has been well studied and match with comparable complexes containing diphosphanes with the PPh₂, PiPr₂ and PtBu₂ groups.^{19,43,46–49,51,53}

In the case of RhBiphPh, the two phosphorus are so similar that they appear almost overlapped, appearing as a multiplet in the NMR spectrum due to a rather strong roofing effect on the peaks while in the case of complexes RhBiphtBu and RhNptBu the separation between the shifts of the phosphorus is more than 30 ppm and no roofing effect is observed whatsoever.

Obtaining single crystals of the complexes for X-ray diffraction measurements was found to be difficult, due to their tendency to form oily substances and their air sensitivity. Nonetheless, after many attempts we managed to obtain a single crystal of complex RhBiphtBu (Fig. 7) by vapor diffusion of hexane into a concentrated solution of the complex in tetrahydrofuran.

Fig. 7 ORTEP plot of RhBiphtBu with ellipsoids drawn at 50% probability level. Hydrogen atoms and the tetrafluoroborate anion have been omitted for clarity. Selected distances (Å) and angles (°): Rh1–P1 2.2861(7), Rh1–P2 2.3356(7), P1–C17 1.837(3), P2–C17 1.857(3), P1–Rh1–P2 72.35(2).

As it can be seen, the bond distance between the rhodium atom and the P-stereogenic phosphorus (P¹) is shorter than that of its non-P-stereogenic counterpart (P²), probably accounting for the bulkiness of the PtBu₂ moiety. The bite angle, 72.35(2)°, differs considerably from the ideal 90° that corresponds to square planar structures, but its value is within the range found for other Rh(I) complexes with diphosphane ligands with single-atom linkers.^{19,21,50,54–56}

After having obtained the mono(chelated) complexes we focused on obtaining a pure bis(chelated) complex. To this end, a solution of slightly less than two equivalents of ligand BiphiPr was treated with one equivalent of [Rh(COD)]₂BF₄ in dichloromethane. The apparently simple reaction posed a greater challenge than expected since the formation of some amount of mono(chelated) complex was observed (Fig. 8a and b), even when working at a stoichiometric ratio of ligand/Rh precursor of 2 : 1. This has been previously observed by Vidal-Ferran and coworkers in Rh(I) complexes with narrow bite-angle chiral phosphane–phosphite ligands.^32,33 Finally, adjusting the reaction conditions, it was possible to obtain the bis(chelated) complex Rh(BiphiPr)₂ containing only a small amount (∼1–2%) of RhBiphiPr. High-resolution ESI-MS analysis revealed the presence of the M⁺ cation at m/z = 887.2698 (requires 887.2701), while the ³¹P{¹H} NMR spectrum showed a complex spectrum (Fig. 8c).

Fig. 8 ³¹P{¹H} NMR spectra (CD₂Cl₂) of the mono- and bis(chelated) complexes. (a) Pure RhBiphiPr; (b) crude mixture of RhBiphiPr and Rh(BiphiPr)₂; (c) ∼99% pure Rh(BiphiPr)₂ as a mixture of cis- and trans isomers; (d) pure cis-Rh(BiphiPr)₂ from a monocrystal. ^a At 202 MHz. ^b At 162 MHz.

The compound features a complex ³¹P{¹H} NMR spectrum (Fig. 8c) comprising six highly symmetric multiplets with chemical shifts between 0 and −28 ppm. The high-field signals in the ³¹P{¹H} NMR spectrum are in the range expected for methylene-bridged diphosphanes (for dppm⁴³ and dcypm⁴⁸δ_P = −23.3 and −16.9 ppm, respectively) and the complexity of is not unexpected since a mixture of the cis and trans isomers (Fig. 5) could be present and both constitute second order AMM′XX′ spin systems^32,57 (M and M′ represent P², X and X′ P¹ and A, Rh). To confirm that an isomeric mixture was present, a bidimensional homonuclear COSY ³¹P{¹H}–³¹P{¹H} NMR experiment (Fig. 9) and also a HMBC ¹H–³¹P{¹H} (see ESI‡) were carried out.

Fig. 9 COSY ³¹P{¹H}–³¹P{¹H} NMR spectrum (202 MHz, CD₂Cl₂) of Rh(BiphiPr)₂.

The COSY spectrum clearly shows a correlation between the two central multiplets and the four external multiplets, but not between these two groups of signals. Therefore, it was deduced that the complex was present as a mixture of cis- and trans-isomers in an approximate 1 : 1.4 ratio.

Several attempts to separate the isomers by crystallization failed, but changes in the relative integrations of the two mentioned groups of signals in the recrystallized solids and the mother liquors were observed. This confirmed that two species were present and indicating that their separation could be possible. Luckily, it was possible to grow single crystals of the complex (Fig. 10), which corresponded to cis-Rh(BiphiPr)₂. The ³¹P{¹H} NMR spectrum of a crystal after its dissolution in CDCl₃ was recorded (Fig. 8d). From the figure it can be seen that the multiplets between −24 and −28 ppm and between 0 and −4 ppm correspond to cis-Rh(BiphiPr)₂. The structure of cis-Rh(BiphiPr)₂, obtained by X-ray diffraction, is to our knowledge the first reported example of a Rh(I) bis(chelated) complex with an unsymmetrical diphosphane (Fig. 10).

Fig. 10 ORTEP plot of the crystal structure of cis-Rh(BiphiPr)₂ with ellipsoids drawn at 50% probability level. Hydrogen atoms have been omitted for clarity. Selected distances (Å) and angles (°): P1A–Rh1A 2.295(1), P2A–Rh1A 2.328(1), P3A–Rh1A 2.292(1), P4A–Rh1A 2.297(1), P1A–Rh1A–P3A 72.60(4), P2A–Rh1A–P4A 72.72(4), P1A–P2A–P3A–P4A −1.40(3).

Interestingly, the isomer that crystallized was cis-Rh(BiphiPr)₂, exactly the same that had been observed with Pd(II).²⁹ As both isomers are formed in approximately equal ratio, it is reasonable to think that the cis isomer is more insoluble and easier to precipitate.

In the crystal structure, the bite angle matches that of the mono(chelated) structure with the same ligand and of other reported bis(chelated) complexes with similar ligands.^17,58 The near-zero torsion angle indicates that the Rh centre adopts a nearly perfect square-planar configuration, even though the bite angle differs greatly from the ideal 90°.

Despite many attempts, it was not possible to obtain, in a synthetically useful way, the pure isomers although the obtention of a crystal of cis-Rh(BiphiPr)₂ shows that it should be possible by simple recrystallisation.

Ruthenium complexes

After the study of the coordination chemistry of the ligands in the square-planar environments of d⁸ palladium(II)²⁹ and rhodium(I) we decided to study their behaviour with the well-known ruthenium(II)-p-cymene unit, to form octahedral, d⁶ Ru(II) complexes. Two ligands with a 2-biphenylyl moiety (BiphiPr and BiphPh) were chosen because it seems to lead to more stable and crystalline complexes.

Following previous studies,⁵⁹ 2 eq. of the BiphR ligand were added to a dichloromethane solution of [RuCl(μ-Cl)(η⁶-p-cymene)]₂ containing 5 equivalents of NH₄PF₆ in order to form the cationic complexes [RuCl(η⁶-p-cymene)(BiphR)]PF₆, (RuBiphR, Fig. 11, top). However, similarly to what had been observed with Rh(I) and Pd(II),²⁹ the reactions yielded several types of complexes, shown in Fig. 11 (bottom). It has to be noted that the complexes tend to form oils that had to be triturated with diethyl ether or pentane to give the final compounds as orange or reddish solids, a fact that has been mentioned in the literature for similar complexes.⁶⁰

Fig. 11 Top: reaction scheme for the complexation of the ligands to Ru(II) complexes. Bottom: structures of the different complexes.

Interestingly, the coordination behaviour is determined by the substituents of the non-P-stereogenic atom, P². The ligand BiphiPr, with the relatively bulky iPr groups preferentially forms the chelated structures,⁶¹ regardless of the addition of PF₆ as a counterion or not. Hence, RuBiphiPr was isolated with the standard procedure (Fig. 11, top) and when NH₄PF₆ was not added, RuBiphiPrCl was obtained instead, with chloride as a counterion.

For the two complexes, very clean ESI-MS spectra with a single peak at m/z = 663.2 were obtained, corresponding to the parent [RuCl(p-cymene)(BiphiPr)]⁺ cation. In the ³¹P{¹H} spectra NMR, both complexes featured two pairs of doublets at δ_P of approximately +21 and 0 ppm with a coupling constant of ca. 91 Hz. The chemical shifts can be compared to those of the related complexes with the simple κ²-coordinated ligand dppm, reported to occur at δ_P of +2.7 (PF₆),⁶⁰ +0.4 ppm (Cl)⁶² and similarly for other anions^63,64 in accordance with the expected upfield shift due to the ring effect of a four-membered metallacycle.^60,65–67 In addition, the spectrum of complex RuBiphiPr contains the diagnostic septet centred at −142.0 ppm of the hexafluorophosphate anion. It is interesting to note that a single set of signals was observed in ³¹P{¹H}, ¹³C{¹H} and ¹H spectra, suggesting that the complexes are present as single species in solution despite the fact that the ruthenium atom becomes a stereogenic centre and two configurations are possible. The rigidity of the rings probably accounts for this fact, as it happened with the short-bridge ligand MaxPHOS.⁵⁹

In the case of the ligand BiphPh, the standard conditions (Fig. 11, top) did not provide the expected chelated complex RuBiphPh, but the monocoordinated complex RuBiphPh′, with the BiphPh ligands coordinated in a κ¹ fashion (Fig. 11, bottom). Obviously, the very same complex was obtained if NH₄PF₆ was not added. There is only one related complex described in the literature, bearing the dppm ligand coordinated in κ¹ fashion.^68–70

The identity of RuBiphPh′ was confirmed by the ESI-MS spectrum, with the base peak at m/z = 767.1 corresponding to the [M + H]⁺ ion. Interestingly, a minor peak at m/z = 731.1 can also be detected, which can be assigned to the [M − Cl]⁺ cation, in which the ligand presumably acts as bidentate, most probably formed during the ionization process in the mass spectrometer chamber. The ³¹P{¹H} showed the absence of the hexafluorophosphate anion and the presence of two sharp doublets at δ_P of +20.9 and −36.9 ppm (in CDCl₃), with a coupling constant of 31.0 Hz. These values can be conveniently compared to those reported for the related complex with a κ¹-coordinated dppm (δ_P = +26.1, −27.6 ppm; J_PP = 31.8 Hz)^68,69 and for the free BiphPh (δ_P¹ = −29.7 ppm, δ_P² = −22.6 ppm; J_P¹–P² = 126.8 Hz). This comparison reveals that the BiphPh ligand is coordinated to the Ru(II) atom by the non-P-stereogenic phosphorus P², probably due to steric effects, preventing the coordination of the more crowded P-stereogenic atom P¹ bearing the 2-biphenylyl substituent. Interestingly, when the ³¹P{¹H} was recorded in CD₃OD the spectrum looked completely different (δ_P = +6.9, −2.4 ppm; J_P¹–P² = 99.5 Hz) because it corresponded to the chelated complex RuBiphPhCl, highlighting the subtle effects that govern the chelation of the methylene-bridged diphosphanes. A similar solvent effect was observed by Dyson and coworkers⁷¹ for dppm. After performing kinetic studies, they concluded that methanol favours ring-closing due to hydrogen bond formation with the chloride ligands, helping in their activation.

The synthesis and isolation of RuBiphPh was finally achieved using TlPF₆ instead of NH₄PF₆, the former being a much stronger halide scavenger.^28,72,73 This reaction was effective if the starting reagents were either [RuCl(μ-Cl)(η⁶-p-cymene)]₂ and the free BiphPh ligand, or the monodentate RuBiphPh′ complex, yielding the cationic complex after filtration of thallium chloride. The ESI-MS spectrum featured the expected peak centred at m/z = 731.1, corresponding to the organometallic cation. The ³¹P{¹H} NMR spectrum confirmed the identity of RuBiphPh, showing the expected coupled pair of doublets (δ_P¹ = −2.4 ppm, δ_P² = +6.9, J_P¹–P² = 99.6 Hz) apart from the septet of the hexafluorophosphate anion. The chemical shift values are very similar to those of the analogous complex with dppm.⁶⁰ It is interesting to note that the ²J_P¹–P² in the chelated complexes is around 100 Hz but only 30 Hz in the complexes with the monocoordinated ligands.

Finally, BiphPh was treated with an excess of [RuCl(μ-Cl)(η⁶-p-cymene)]₂ in order to obtain the bimetallic complex Ru₂BiphPh, as described in the literature a long time ago with dppm by Atwood, Dixneuf, Bonnet and coworkers^74,75 and by Lahuerta and coworkers⁷⁶ and much more recently by Blom and coworkers⁷⁷ using η⁶-benzene instead of η⁶-p-cymene. However, after a few attempts including heating a dichloromethane solution of the Ru dimer and the BiphPh ligand to reflux for several hours,⁷⁷ the desired complex was not formed, and only RuBiphPh′, impurified with RuBiphPhCl, free p-cymene and other species due to decomposition⁷⁸ was obtained. This unsuccessful synthesis shows that the steric crowding of the BiphPh ligand makes it rather different compared to dppm, despite having very similar electronic characteristics.

The crystal structure of RuBiphiPr was obtained (Fig. 12) after slow diffusion of hexane onto a saturated solution of the complex in dichloromethane. It appears that the complexes bearing the BiphiPr ligand are more prone to crystallize, since it is the only ligand for which it has been possible to elucidate at least one crystal structure for all the metals to which it has been coordinated (Pd,²⁹ Rh, and Ru), and also the only one to have an elucidated crystal structure in its boronated form.²⁹

Fig. 12 ORTEP plot of the crystal structure of RuBiphiPr with ellipsoids drawn at 50% probability level. Hydrogen atoms and the hexafluorophosphate anion have been omitted for clarity. Selected distances (Å) and angles (°): Ru1–centroid 1.760, Ru1–Cl1 2.3825(6), Ru1–P1 2.3492(5), Ru1–P2 2.3399(5), P1–C11 1.8341(19), P2–C11 1.8369(19); P1–Ru1–P2 70.997(17), Cl1–Ru1–P1 83.886(17), Cl1–Ru1–P2 85.467(18), centroid–Ru1–Cl1 122.51, centroid–Ru1–P1 139.43, centroid–Ru1–P2 134.66, P1–C11–P2 95.76(9).

As it can be seen in the figure, the complex forms a pseudo-tetrahedral structure, widely known as three-legged piano-stool, typical for this kind of complexes. The distance Ru-centroid (1.760 Å) is similar to other [RuCl(η⁶-p-cymene)(κ²-diphosphane)]⁺ complexes.⁶⁰ The bite angle of the diphosphane in the complex (71.00(2)°) is much smaller than that observed with the same ligand in a Pd(II) complex (74.266(18)°)²⁹ and in the Rh complex Rh(BiphiPr)₂ (72.60(4)° and 72.72(4)°) discussed before, but very similar to the bite angle of dppm^62,63,79 and a related ligand⁶⁶ in the few crystal structures reported for Ru(II) compounds containing the cation [RuCl(η⁶-p-cymene)(κ²-dppm)]. The smaller bite angle with Ru(II) is probably due to the octahedral coordination geometry enforced by a d⁶ metal, that allows the methylene-bridged diphosphane to reach its “natural” bite angle of 72°.^16,80,81 The absolute configuration of the Ru centre is S_Ru according to the priority sequence η⁶-p-cymene > Cl > P(Biph)Ph > P(iPr)₂ and the Cahn–Ingold–Prelog Rules.^82–84

Rh-catalysed hydrogenation

The rhodium complexes described in a previous section were explored as catalytic precursors in the reaction of asymmetric hydrogenation.²³ Two typically used benchmark substrates, Z-MAC ((Z)-methyl α-acetamido cinnamate) and DMI (dimethyl itaconate) were subjected to hydrogenation using 1% of Rh precursor at 20 bar of hydrogen pressure (Table 2).

Table 2 Asymmetric hydrogenation results with methanol

Entry	Catalyst	Substrate	Conversion (%)	ee (%)
1	RhBiphiPr	Z-MAC	>99	16 (R)
2		DMI	>99	29 (S)
3	RhBiphtBu	Z-MAC	61	rac
4		DMI	>99	rac
5	RhBiphPh	Z-MAC	>99	24 (R)
6		DMI	>99	6 (S)
7	RhNpiPr	Z-MAC	>99	29 (R)
8		DMI	>99	23 (S)
9	RhNptBu	Z-MAC	>99	50 (R)
10		DMI	>99	9 (S)
11	RhNpPh	Z-MAC	>99	16 (R)
12		DMI	>99	rac
13	Rh(BiphiPr)2	Z-MAC	12	rac
14		DMI	59	rac

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The precursors are active in the reaction, leading to full conversions except for the hydrogenation of Z-MAC with RhBiphiPr (entry 3) and the two catalytic runs with Rh(BiphiPr)₂ (entries 13 and 14). The lower activity of these precursors can be understood by the steric hindrance of the ligand and by the fact that the latter complex does not contain a labile ligand. Indeed, Imamoto and coworkers found a similar behaviour when studying their mono- and bis(chelated) complexes with MiniPHOS in hydrogenation.^17,18 Regarding the enantioselectivities, they are moderate, without a clear pattern and reaching a maximum value of 50% (entry 9), with Z-MAC usually giving slightly better enantioselectivities, as it is usually found.^5,23,40 As expected, the sense of enantioinduction is the same.⁸⁵

The moderate enantioselectivities obtained with the ligands of this manuscript and the availability of the crystal structures of RhBiphtBu (Fig. 7) and the analogous complex with the related ligand TriChickenFootPhos (TCFP, a highly enantioselective ligand in enantioselective hydrogenation),⁸⁶ reported by Hoge and coworkers,¹⁹ prompted us to evaluate the steric properties of these two ligands by calculation of the percent buried volume (%V_bur) parameter⁸⁷ and the topographic steric maps using the SambVca 2.1 web application^88–90 (Fig. 13).

Fig. 13 Topographic steric maps and %V_bur values (for each quadrant and average value) of BiphtBu using the crystal structure of RhBiphtBu (Fig. 7) (left) and TCFP using the crystal structure of [Rh(cod)₂TCFP]BF₄ ¹⁹ (right).

The averaged %V_bur values of the two ligands is indeed very similar with BiphtBu being slightly bulkier than TCFP. The spatial distribution of this bulkiness, however is rather different for the two ligands because the normalised %V_bur(NW + SW) : %V_bur(NE + SE) ratios are 100 : 94 and 100 : 91 for BiphtBu and TCFP respectively. This means that in TCFP the steric differentiation between the two phosphane moieties (i.e. between the eastern and western halves in the topographic maps of Fig. 13) is more pronounced than for BiphtBu. In addition, the NE quadrant is more sterically unhindered in TCFP than the comparable SE quadrant for BiphtBu.⁹¹ It can be concluded that the phenyl/2-biphenylyl substituents of P¹ are probably not distinct enough and hence the introduction at this phosphorus of a bulkier aromatic group instead of 2-biphenylyl could be a strategy to explore future generations of the ligands.⁹²

Ru-catalysed transfer hydrogenation

Ruthenium–arene complexes with P-stereogenic monophosphanes have been used by us^{26–28,93,94} and others⁹⁵ in the catalytic reduction of ketones under transfer hydrogenation reactions. Following these studies, ruthenium complexes RuBiphiPr, RuBiphiPrCl and RuBiphPh′ were studied as catalytic precursors, at 1% catalyst loading, in the transfer hydrogenation of acetophenone in refluxing isopropanol in the presence of base (Fig. 14).

Fig. 14 Conversion of acetophenone vs. time with complexes RuBiphiPr, RuBiphiPrCl, and RuBiphPh′.

The three complexes were found to be effective catalysts in the reaction with complexes RuBiphPh′ and RuBiphiPrCl being equally active furnishing over 90% conversion after 4 h, while RuBiphiPr is somewhat less active. The activity of the complexes is similar to the previously reported neutral complexes of the type [Ru(η⁶-p-cymene)Cl₂(P*)] (P* = P-stereogenic monophosphane, including BiphMe ^{26–28,93–95}) but in contrast to the moderate enantioselectivity obtained with some of the reported complexes with monophosphanes,²⁸ the enantioselectivity obtained with the complexes of Fig. 14 was very low (ee < 10%).

Conclusions

In this paper the rich coordination chemistry towards Rh(I) and Ru(II) of a group of six P-stereogenic, methylene-bridged, unsymmetrical diphosphanes has been studied. Two out of these six ligands, bearing two tert-butyl moieties on the non-P-stereogenic phosphorus, are described for the first time in this paper.

For Rh, mono(chelated) and bis(chelated) structures have been synthesized and isolated. The bis(chelated) complexes present the cis- and the trans-isomers, and it has been observed that the former crystallizes preferentially, which has allowed us to obtain its crystal structure by X-ray diffraction. This selectivity is in accordance with the previous results of the group when coordinating the same ligand to Pd.

For Ru, two different structures have been obtained for the BiphR ligands, namely a chelated (κ²-P¹,P²), and a monocoordinated (κ¹-P²) one, the latter with the ligand coordinated through the non-P-stereogenic phosphorus atom. The ligand BiphiPr shows a preference for the chelated structure, while BiphPh also forms the monocoordinated and the chelated complexes. The clean formation of the monocoordinated complex is very interesting, since it is an obvious entryway to heterobimetallic complexes with a variety of methylene-bridged diphosphanes, which up to now have only been studied with parent dppm, but already showing promising biological activities, especially the Ru(II)–Au(I) and Ru(II)–Fe(II) complexes.^{64,77,96–98}

Lastly, the applicability of the different complex structures has been tested in asymmetric catalysis. For the Rh complexes, asymmetric hydrogenation has been studied, while for the Ru ones the chosen reaction has been asymmetric transfer hydrogenation. The conversion has been excellent in both cases, although the enantioselectivity has ranged between moderate and low.

Experimental part

Generalities

All compounds were prepared under a purified nitrogen atmosphere using standard Schlenk and vacuum-line techniques or inside a glovebox. The solvents were purified by a solvent purification system or by standard procedures⁹⁹ and kept under nitrogen. ¹H, ¹³C{¹H}, ³¹P{¹H}, and bidimensional NMR spectra were recorded at room temperature with 400 or 500 MHz spectrometers. The fields are 400 or 500 MHz (¹H), 101 or 126 MHz (¹³C{¹H}) and 162 or 202 MHz (³¹P{¹H}). Chemical shifts are reported downfield from standards (SiMe₄ for ¹H and ¹³C and H₃PO₄ for ³¹P NMR) and the coupling constants are given in Hz. IR spectra were recorded with an ATR and the main absorption bands are expressed in cm⁻¹. High-resolution mass analyses (HRMS) were carried out in a time-of-flight instrument using electrospray ionisation. Diphosphanes BiphiPr, NpiPr, BiphPh and NpPh were prepared following the reported procedure.²⁹

Ligands

BiphtBu . (S)-(2-Biphenylyl)(methyl)(phenyl)phosphane–borane (BiphMe, 290 mg, 1.0 mmol) was dissolved in 5 mL of THF and the solution was cooled to −78 °C. s-BuLi (1.0 mL of a 1.3 M solution, 1.3 mmol) was added and the solution was stirred for 2 h. The solution was then cooled to −78 °C and chloroditertbutylphosphane (220 μL, 1.1 mmol) was added, allowing the mixture reach room temperature while stirring overnight.

The next day, 1 mL of methanol was added, and the mixture was brought to dryness. After that, it was dissolved in 5 mL of morpholine and stirred for 24 h at room temperature. The solvent was evaporated in vacuo, and the residue was subjected to column chromatography purification (alumina, toluene) under nitrogen to yield the desired product as a viscous white oil. Yield: 90% (379 mg).

¹ H NMR (400 MHz, C ₆ D ₆ ): 7.48–7.41 (m, 1H, Ar), 7.37–7.28 (m, 2H, Ar), 7.26–7.19 (m, 2H, Ar), 7.10–6.79 (m, 9H, Ar), 1.89–1.81 (m, 2H, CH_2(bridge)), 0.84 (d, J = 10.8, 9H, CH_3(tBu)), 0.76 (d, J = 10.5, 9H, CH_3(tBu)). ¹³C{¹H} NMR (101 MHz, C₆D₆): 147.8–122.1 (m, 18C, Ar), 30.7 (m, 2C, C_tBu), 28.7 (m, 6C, CH_3(tBu)), 20.1 (dd, J = 36.1, 24.1, 1C, CH_2(bridge)). ³¹P{¹H} NMR (162 MHz, C₆D₆): +13.8 (d, J = 129.8, P²), −25.1 (d, J = 129.8, P¹).

NptBu . This compound was prepared analogously to BiphtBu with (S)-(1-naphthyl)(methyl)(phenyl)phosphane–borane (NpMe). Yield: 87% (343 mg).

¹ H NMR (400 MHz, C ₆ D ₆ ): 8.66 (m, 1H, Ar), 7.64 (ddd, J = 7.1, 4.1, 1.3, 1H, Ar), 7.49–7.36 (m, 3H, Ar), 7.20–7.10 (m, 1H, Ar), 7.07–6.87 (m, 3H, Ar), 6.88–6.78 (m, 3H, Ar), 2.36–2.28 (m, 1H, CH_bridge), 2.06–1.96 (m, 1H, CH_bridge), 0.93 (d, J = 10.8, 9H, CH_3(tBu)), 0.80 (d, J = 10.8, 9H, CH_3(tBu)). ¹³C{¹H} NMR (101 MHz, C₆D₆): 133.3–123.1 (m, 16C, Ar), 30.8 (m, 2C, C_tBu), 29.6–27.8 (m, 6C, CH_3(tBu)), 20.6–18.7 (m, 1C, CH_2(bridge)). ³¹P{¹H} NMR (162 MHz, C₆D₆): +13.8 (d, ²J_P¹–P² = 144.0, P²), −28.1 (d, ²J_P¹–P² = 144.0, P¹).

Rhodium complexes

RhBiphiPr . Diphosphane BiphiPr (84.4 mg, 0.22 mmol) was dissolved in 10 mL of CH₂Cl₂ and added dropwise during 30 min to a solution of [Rh(COD)₂]BF₄ (82 mg, 0.20 mmol) in 5 mL of CH₂Cl₂. The resulting solution was left stirring for 1 h and the solvent was evaporated in vacuo. The obtained orange solid was recrystallized with 2 mL of CH₂Cl₂ and 15 mL of Et₂O and filtered through a Schlenk filter. The final product was transferred to a 5 mL scintillation vial and stored in the freezer inside a glovebox. Yield 33% (45 mg).

¹ H NMR (400 MHz, CD ₂ Cl ₂ ): 9.22–9.10 (m, 1H, Ar), 8.52–8.32 (m, 3H, Ar), 8.31–8.01 (m, 5H, Ar), 7.98–7.85 (m, 4H, Ar), 7.60–7.52 (m, 1H, Ar), 6.46 (s, br, 1H, CH_COD), 6.21 (s, br, 1H, CH_COD), 5.91 (s, br, 1H, CH_COD), 5.67 (s, br, 1H, CH_COD), 4.41–4.29 (m, 1H, CH_bridge), 3.98–3.86 (m, 1H, CH_bridge), 3.39–3.08 (m, 8H, CH_2(COD)), 3.07–2.96 (m, 1H, CH_iPr), 2.89–2.80 (m, 1H, CH_iPr), 2.12 (dd, J = 17.8, 7.2, 3H, CH_3(iPr)), 2.00–1.74 (m, 6H, CH_3(iPr)). ¹³C{¹H} NMR (101 MHz, CD₂Cl₂): 142.2–123.3 (m, 18C, CH_Ar), 99.5 (s, 1C, CH_COD), 95.0 (s, 1C, CH_COD), 93.8 (s, 1C, CH_COD), 93.6 (s, 1C, CH_COD), 27.8 (dd, J = 24.6, 19.0, 1C, CH_2(bridge)), 26.5–24.7 (m, 4C, CH_2(COD)), 21.2–20.4 (m, 2C, CH_iPr), 14.6 (s, 1C, CH_3(iPr)), 14.0 (s, 1C, CH_3(iPr)), 13.8 (s, 1C, CH_3(iPr)), 12.6 (s, 1C, CH_3(iPr)). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): −13.4 (dd, ¹J_P²–Rh = 126.9, ²J_P¹–P² = 76.1, P²), −34.9 (dd, ¹J_P¹–Rh = 129.4, ²J_P¹–P² = 76.1, P¹). HRMS: calcd for [M − BF₄ + 2O]⁺ 635.1715, found 635.1703. IR: 2962, 1629, 1435, 1258, 1037, 879, 795, 702.

RhBiphtBu . The same method used for RhBiphiPr was followed. The starting diphosphane was BiphtBu (95 mg, 0.23 mmol). Yield 64% (92 mg).

¹ H NMR (400 MHz, CD ₂ Cl ₂ ): 7.63–7.56 (m, 2H, Ar), 7.47–7.14 (m, 6H, Ar), 7.04–7.01 (m, 2H, Ar), 6.96–6.91 (m, 2H, Ar), 6.59–6.57 (m, 2H, Ar), 5.91 (s, br, 1H, CH_COD), 5.49 (s, br, 1H, CH_COD), 4.86 (s, br, 1H, CH_COD), 4.64 (s, br, 1H, CH_COD), 3.56–3.42 (m, 1H, CH_bridge), 3.21–3.07 (m, 1H, CH_bridge), 2.61–1.98 (m, 8H, CH_2(COD)), 1.30 (d, J = 14.5, 9H, CH_3(tBu)), 1.11 (d, J = 14.8, 9H, CH_3(tBu)). ¹³C{¹H} NMR (101 MHz, CD₂Cl₂): 146.7–126.2 (m, 18C, CH_Ar), 103.4–103.0 (m, 1C, CH_COD), 101.5–101.0 (m, 1C, CH_COD), 99.3–98.9 (m, 1C, CH_COD), 91.7–91.3 (m, 1C, CH_COD), 37.1 (dd, J = 7.5, 3.6, 1C, C_(tBu)), 36.0 (d, J = 7.2, 1C, C_(tBu)), 33.3 (dd, J = 23.6, 13.7, 1C, CH_2(bridge)), 31.5 (s, 1C, CH_2(COD)), 30.7 (s, 1C, CH_2(COD)), 29.4 (d, J = 4.9, 3C, CH_3(tBu)), 29.3 (d, J = 5.6, 3C, CH_3(tBu)), 29.1 (s, 1C, CH_2(COD)), 28.9 (s, 1C, CH_2(COD)). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): −4.7 (dd, ¹J_P²–Rh = 125.4, ²J_P¹–P² = 63.1, P²), −35.8 (dd, ¹J_P¹–Rh = 130.6, ²J_P¹–P² = 63.1, P¹). HRMS: calcd for [M − COD − BF₄]⁺ 523.1191, found 523.1183. IR: 2951, 2081, 2031, 1466, 1435, 1062, 736, 704.

RhBiphPh . The same method used for RhBiphiPr was followed. The starting diphosphane was BiphPh (104 mg, 0.23 mmol). Yield 22% (38 mg).

¹ H NMR (400 MHz, CD ₂ Cl ₂ ): 7.57–7.01 (m, 24H, Ar), 5.22 (s, br, 1H, CH_COD), 5.04 (s, br, 2H, CH_COD), 4.59 (s, br, 1H, CH_COD), 4.08–3.94 (m, 1H, CH_bridge), 3.70–3.58 (m, 1H, CH_bridge), 2.39–2.07 (m, 8H, CH_2(COD)). ¹³C{¹H} NMR (101 MHz, CD₂Cl₂): 146.9–126.3 (m, 30C, CH_Ar), 102.8–102.4 (m, 1C, CH_COD), 100.3–99.9 (m, 1C, CH_COD), 98.3–97.7 (m, 2C, CH_COD), 41.1 (t, ¹J_CP = 22.6, 1C, CH_2(bridge)), 30.8 (s, 1C, CH_2(COD)), 30.7 (s, 1C, CH_2(COD)), 29.5 (s, 1C, CH_2(COD)), 29.0 (s, 1C, CH_2(COD)). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): −29.1 (dd, ¹J_P²–Rh = 126.6, ²J_P¹–P² = 75.7, P²), −29.8 (dd, ¹J_P¹–Rh = 130.6, ²J_P¹–P² = 75.7, P¹). HRMS: calcd for [M − COD − BF₄ + O]⁺ 687.1453, found 687.1453. IR: 2886, 2837, 1481, 1433, 1345, 1314, 1281, 1164, 1091, 1032, 995, 863, 829, 784, 742, 692.

RhNpiPr . The same method used for RhBiphiPr was followed. The starting diphosphane was NpiPr (80 mg, 0.22 mmol). Yield 54% (72 mg).

¹ H NMR (400 MHz, CD ₂ Cl ₂ ): 8.43 (ddd, J = 16.8, 6.8, 1.2, 1H, Ar), 8.18 (d, J = 8.4, 1H, Ar), 8.02 (d, J = 8.0, 1H, Ar), 7.76 (d, J = 8.4, 1H, Ar), 7.67 (td, J = 7.4, 2.0, 1H, Ar), 7.57 (td, J = 7.6, 0.8, 1H, Ar), 7.53–7.40 (m, 6H, Ar), 5.75 (s, br, 1H, CH_COD), 5.57 (s, br, 1H, CH_COD), 5.15 (s, br, 1H, CH_COD), 4.92 (s, br, 1H, CH_COD), 4.16–4.06 (m, 1H, CH_bridge), 3.66–3.60 (m, 1H, CH_bridge), 2.54–2.26 (m, 8H, CH_2(COD)), 2.08–1.97 (m, 2H, CH_iPr), 1.55 (dd, J = 15.0, 6.9, 3H, CH_3(iPr)), 1.26 (dd, J = 15.0, 6.9, 3H, CH_3(iPr)), 1.10 (dd, J = 17.9, 7.1, 3H, CH_3(iPr)), 0.83 (dd, J = 17.3, 6.9, 3H, CH_3(iPr)). ¹³C{¹H} NMR (101 MHz, CDCl₃): 138.0–124.0 (m, 16C, CH_Ar), 102.9–102.7 (m, 1C, CH_COD), 99.6–99.4 (m, 1C, CH_COD), 99.1–99.0 (m, 1C, CH_COD), 96.2–96.1 (m, 1C, CH_COD), 33.8–33.3 (m, 1C, CH_2(bridge)), 30.8 (s, 1C, CH_2(COD)), 30.1 (s, 1C, CH_2(COD)), 30.0 (s, 1C, CH_2(COD)), 29.4 (s, 1C, CH_2(COD)), 25.5–25.2 (m, 2C, CH_(iPr)), 19.4 (d, J = 3.1, 1C, CH_3(iPr)), 18.3 (s, 1C, CH_3(iPr)), 18.6 (d, J = 4.0, 1C, CH_3(iPr)), 17.4 (s, 1C, CH_3(iPr)). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): −12.3 (dd, ¹J_PRh = 126.7, ²J_PP = 77.4, P²), −39.1 (dd, ¹J_PRh = 131.7, ²J_PP = 77.4, P¹). HRMS: calcd for [M − COD − BF₄ + 2CH₃CN]⁺ 567.1201, found 567.1206. IR: 2922, 1696, 1506, 1458, 1436, 1335, 1052, 881, 805, 694, 663, 639.

RhNptBu . The same method used for RhBiphiPr was followed, with the exception that the diphosphane was dissolved in 20 mL of CH₂Cl₂ and added over 2 h. The starting diphosphane was NptBu (78.9 mg, 0.2 mmol). Yield 42% (92 mg, 0.126 mmol).

¹ H NMR (500 MHz, CD ₂ Cl ₂ ): 8.69 (ddd, J = 17.2, 7.0, 1.3, 1H, Ar), 8.20 (d, J = 8.5, 1H, Ar), 8.02 (d, J = 8.0, 1H, Ar), 7.98 (d, J = 8.0, 1H, Ar), 7.73–7.34 (m, 8H, Ar), 6.07 (s, br, 1H, CH_COD), 5.74 (s, br, 1H, CH_COD), 4.98 (s, br, 1H, CH_COD), 4.68 (s, br, 1H, CH_COD), 4.25 (ddd, J = 16.7, 10.8, 8.3, 1H, CH_bridge), 3.79–3.68 (m, 1H, CH_bridge), 2.70–2.60 (m, 2H, CH_2(COD)), 2.57–2.46 (m, 1H, CH_2(COD)), 2.35 (m, 2H, CH_2(COD)), 2.28–2.18 (m, 3H, CH_2(COD)), 1.59 (d, J = 14.5, 9H, CH_3(tBu)), 1.07 (d, J = 14.9, 9H, CH_3(tBu)). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): 142.2–121.8 (m, 16C, Ar), 104.2 (s, 1C, CH_COD), 101.3 (s, 1C, CH_COD), 94.5 (s, 1C, CH_COD), 91.5 (s, 1C, CH_COD), 36.2 (m, 1C, CH_2(bridge)), 31.7 (s, 1C, CH_2(COD)), 31.2 (s, 1C, CH_2(COD)), 30.0 (d, J = 5.1, CH_3(tBu)), 29.7 (d, J = 6.0, CH_3(tBu)), 29.4 (s, 1C, C_tBu), 29.3 (s, 1C, C_tBu). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): −2.9 (dd, ¹J_P²–Rh = 125.1, ²J_P¹–P² = 63.9, P²), −38.3 (dd, ¹J_P¹–Rh = 132.7, ²J_P¹–P² = 63.9, P¹). IR: 2962, 2237, 1588, 1435, 1262, 1047, 886, 804, 775, 731, 694.

RhNpPh . The same method used for RhBiphiPr was followed. The starting diphosphane was NpiPr (80 mg, 0.22 mmol). Yield 42% (92 mg).

¹ H NMR (500 MHz, CD ₂ Cl ₂ ): 8.26 (d, J = 8.5, 1H, Ar), 8.05 (d, J = 8.0, 1H, Ar), 8.00 (d, J = 8.0, 1H, Ar), 7.87 (ddd, J = 16.0, 7.0, 1.5, 1H, Ar), 7.62–7.35 (m, 18H, Ar), 5.47 (s, br, 1H, CH_COD), 5.32 (s, br, 3H, CH_COD), 4.47–4.43 (m, 2H, CH_bridge), 2.42–2.21 (m, 8H, CH_2(COD)). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): 137.5–125.0 (m, Ar), 103.7 (s, CH_COD), 101.1 (m, CH_COD), 100.1 (s, CH_COD), 42.4 (t, J = 21.9, 1C, CH_2(bridge)), 30.5 (m, CH_2(COD)). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): −28.9 (dd, ¹J_P²–Rh = 130.2, ²J_P¹–P² = 78.0, P²), −33.1 (dd, ¹J_P¹–Rh = 130.1, ²J_P¹–P² = 78.0, P¹). HRMS: calcd for [M − COD − BF₄ + 2O]⁺ 677.1246, found 677.1238. IR: 2887, 2838, 1482, 1434, 1345, 1314, 1282, 1165, 1090, 1050, 992, 864, 828, 804, 776, 741, 962.

Rh(BiphiPr)₂ . Rh[COD₂]BF₄ (235 mg, 0.3 mmol) was dissolved in 5 mL of CH₂Cl₂ and added to a CH₂Cl₂ solution of the same volume containing the free diphosphane BiphiPr (123 mg, 0.6 mmol). The mixture was stirred for 1 h and the solvent was evaporated in vacuo. The resulting orange residue was recrystallized with 2 mL of CH₂Cl₂ and 15 mL of Et₂O, affording an oil that, after a night in the freezer, precipitated into a solid. Yield 21% (62 mg).

¹ H NMR (500 MHz, CD ₂ Cl ₂ ): 7.71–6.50 (m, 28H, Ar), 3.39–3.18 (m, 2H, CH_bridge), 2.77–2.64 (m, 2H, CH_bridge), 1.98–1.90 (m, 1H, CH_iPr), 1.82–1.72 (m, 3H, CH_iPr), 1.27–0.63 (m, 24H, CH_3(iPr)). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): 132.9–126.2 (m, Ar), 33.9 (m, CH_2(bridge)), 25.7 (m, CH_iPr), 24.9 (m, CH_iPr), 19.5–15.3 (m, CH_3(iPr)). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): 0.3−(−1.8) (m, P²_cis), −1.8−(−3.9) (m, P²_cis), −6.7−(−9.0) (m, P²_trans), −17.3−(−19.6) (m, P¹_trans), −24.2−(−26.1) (m, P¹_cis), −26.1−(−28.2) (m, P¹_cis). HRMS: calcd for [M − BF₄]⁺ 887.2701, found 887.2698. IR: 3054, 2957, 1460, 1438, 1049, 735, 694, 630.

Ruthenium complexes

RuBiphiPr . The precursor [RuCl(μ-Cl)Cl(p-cymene)₂]₂ (64 mg, 0.1 mmol) and NH₄PF₆ (945 mg, 5.9 mmol) was dissolved in 10 mL of CH₂Cl₂ and BiphiPr (98 mg, 0.25 mmol) was added. The solution was stirred for about 1 h at room temperature and, after that, the solvent was evaporated in vacuo and recrystallised with CH₂Cl₂ and Et₂O. Yield 90% (150 mg).

¹ H NMR (400 MHz, CDCl ₃ ): 8.10 (ddd, J = 17.2, 8.0, 0.8, 1H, Ar), 7.88 (dt, J = 8.0, 2.0, 1H, Ar), 7.62 (dt, J = 7.2, 1.6, 1H, Ar), 7.40–7.35 (m, 1H, Ar), 7.32–7.24 (m, 5H, Ar), 7.20 (ddd, J = 7.6, 4.0, 1.6, 1H, Ar), 7.16–7.12 (m, 2H, Ar), 6.66–6.58 (m, 2H, Ar), 6.26 (d, J = 6.1, 1H, Ar_(p-cymene)), 6.07 (d, J = 6.5, 2H, Ar_(p-cymene)), 5.90 (d, J = 6.1, 1H, Ar_(p-cymene)), 5.73 (d, J = 6.4, 1H, Ar_(p-cymene)), 3.57–3.42 (m, 1H, CH_bridge), 3.42–3.27 (m, 1H, CH_bridge), 2.76–2.62 (m, 1H, CH_iPr), 2.62–2.52 (m, 1H, CH_{iPr(p-cymene)}), 2.35–2.22 (m, 1H, CH_iPr), 2.10 (s, 3H, CH_3(p-cymene)), 1.30 (dd, J = 7.2, 2.6, 3H, CH_3(iPr)), 1.26 (dd, J = 7.3, 2.0, 3H, CH_3(iPr)), 1.22 (d, J = 7.3, 3H, CH_{3(iPr, p-cymene)}), 1.03 (dd, J = 13.9, 6.9, 6H, CH_3(iPr)), 0.96 (dd, J = 15.7, 7.2, 3H, CH_3(iPr)). ¹³C{¹H} NMR (101 MHz, CDCl₃): 145.1–128.0 (m, 18C, Ar), 120.1 (s, 1C, Ar_(p-cymene)), 101.8 (s, 1C, Ar_(p-cymene)), 94.0 (d, J = 7.1, 1CH, Ar_(p-cymene)), 89.6 (d, J = 6.0, 1CH, Ar_(p-cymene)), 89.2 (d, J = 6.3, 1CH, Ar_(p-cymene)), 87.7 (s, 1CH, Ar_(p-cymene)), 35.0 (dd, J = 27.3 21.2, 1C, CH_2(bridge)), 31.2 (s, 1C, CH_{iPr(p-cymene)}), 27.6 (d, J = 21.5, 1C, CH_iPr), 26.1 (dd, J = 18.4, 9.9, 1C, CH_iPr), 21.9 (d, J = 43.8, 2C, CH_{3(iPr, p-cymene)}), 19.6 (s, 3C, CH_3(iPr)), 19.2 (s, 1C, CH_3(iPr)), 19.0 (s, 1C, CH_3(p-cymene)). ³¹P{¹H} NMR (162 MHz, CDCl₃): +20.9 (d, ²J_P¹–P² = 91.0, P²), −0.2 (d, ²J_P¹–P² = 91.0, P¹), −144.2 (hept, ¹J_P–F = 712.8, PF₆). HRMS: calcd for [M − PF₆]⁺ 663.16502, found 633.1643. IR: 2966, 2935, 2880, 1462, 1436, 1387, 1096, 1059, 877, 832, 758, 737, 706, 692, 556, 556.

RuBiphiPrCl . The precursor [RuCl(μ-Cl)Cl(p-cymene)₂]₂ (197 mg, 0.3 mmol) was dissolved in 10 mL of CH₂Cl₂ and BiphiPr (235 mg, 0.60 mmol) was added dropwise with a dropping funnel. The solution was stirred for about 5 h at room temperature and, after that, the solvent was evaporated in vacuo and recrystallised with CH₂Cl₂ and Et₂O. Yield: 211 mg (50%).

¹ H NMR (400 MHz, CDCl ₃ ): 8.36 (dd, J = 15.6, 7.2, 1H, Ar), 8.11 (t, J = 6.8, 1H, Ar), 7.61 (t, J = 6.0, 1H, Ar), 7.39–7.08 (m, 9H, Ar), 6.65–6.57 (m, 2H, Ar), 6.47 (s, 1H, Ar_(p-cymene)), 6.43 (s, 1H, Ar_(p-cymene)), 6.32 (d, J = 6.5, 1H, Ar_(p-cymene)), 5.77 (d, J = 6.5, 1H, Ar_(p-cymene)), 3.46 (ddd, J = 15.2, 12.8, 11.6, 1H, CH_bridge), 3.30 (ddd, J = 15.3, 10.5, 9.1, 1H, CH_bridge), 2.75–2.63 (m, 1H, CH_iPr), 2.65–2.53 (m, 1H, CH_{iPr(p-cymene)}), 2.37 (m, 1H, CH_iPr), 2.20 (s, 3H, CH_3(p-cymene)), 1.33–1.14 (m, 9H, CH_3(iPr)), 1.03 (d, J = 6.1, 6H, 2CH_3(p-cymene)), 0.93 (dd, J = 15.9, 7.2, 3H, CH_3(iPr)). ¹³C{¹H} NMR (101 MHz, CDCl₃): 144.8–127.9 (m, 18C, Ar), 117.8 (d, 1C, Ar_(p-cymene)), 102.4 (s, 1C, C_p-cymene), 94.4 (d, 1CH, Ar_(p-cymene)), 90.4 (d, 1CH, Ar_(p-cymene)), 89.9 (d, 1CH, Ar_(p-cymene)), 88.6 (s, 1CH, Ar_(p-cymene)), 34.7 (dd, J_CP = 27.1, 20.9, 1C, CH_2(bridge)), 31.4 (s, 1C, CH_{(iPr, p-cymene)}), 27.6 (d, J = 21.3, 1C, CH_(iPr)), 26.2 (dd, J = 18.3, 9.9, 1C, CH_(iPr)), 22.2 (s, 1C, CH_{3(iPr, p-cymene)}), 22.1 (s, 1C, CH_{3(iPr, p-cymene)}), 19.8 (s, 1C, CH_3(p-cymene)), 19.8 (s, 1C, CH_3(iPr)), 19.6–19.3 (m, 3C, CH_3(iPr)). ³¹P{¹H} NMR (162 MHz, CDCl₃): +21.5 (d, ²J_P¹–P² = 91.4, P²), +0.1 (d, ²J_P¹–P² = 91.4, P¹). HRMS: calcd for [M − Cl]⁺ 663.1650, found 663.1645. IR: 3357, 3051, 2961, 1625, 1459, 1434, 1259, 1090, 1008, 877, 799, 690, 603.

RuBiphPh . RuBiphPh′ (100 mg, 0.13 mmol) was dissolved in 10 mL of DCM, of TlPF₆ (52 mg, 0.14 mmol) were added and the solution was vigorously stirred for 48 h. The solution was filtered through a silica pad with Celite® and evaporated to dryness. Yield 44% (59 mg).

¹ H NMR (400 MHz, CDCl ₃ ): 8.36 (dd, J = 17.2, 7.6, 1H, Ar), 7.92 (t, J = 7.6, 1H, Ar), 7.63 (t, J = 6.0, 1H, Ar), 7.42–7.01 (m, 17H, Ar), 7.79 (d, J = 8.4, 1H, Ar), 7.76 (d, J = 7.2, 1H, Ar), 6.58 (d, J = 5.6, 1H, Ar_(p-cymene)), 6.44–6.42 (m, br, 2H, Ar), 6.17 (d, J = 6.3, 1H, Ar_(p-cymene)), 5.75 (d, J = 6.3, 1H, Ar_(p-cymene)), 5.70 (d, J = 5.7, 1H, Ar_(p-cymene)), 4.06 (dt, J = 15.0, 12.8, 1H, CH_bridge), 3.49 (dt, J = 15.2, 10.1, 1H, CH_bridge), 2.40–2.26 (m, 1H, CH_{iPr(p-cymene)}), 1.37 (s, 3H, CH_3(p-cymene)), 0.95 (dd, J = 7.1, 2.3, 6H, CH_{3(iPr, p-cymene)}). ¹³C{¹H} NMR (101 MHz, CDCl₃): 143.8–123.3 (m, 30C, Ar), 117.3 (s, 1C, Ar_p-cymene), 101.5 (s, 1C, Ar_p-cymene), 95.4 (d, 1C, Ar_(p-cymene)), 89.8 (d, 1C, Ar_(p-cymene)), 89.3 (d, 1C, Ar_(p-cymene)), 87.3 (s, 1C, Ar_(p-cymene)), 39.5 (t, J = 27.3, 1C, CH_2(bridge)), 30.0 (s, 1C, CH_3(p-cymene)), 22.0 (s, 1C, CH_{iPr(p-cymene)}), 19.7 (s, 1C, CH_3(p-cymene)), 16.8 (s, 1C, CH_3(p-cymene)). ³¹P{¹H} NMR (162 MHz, CDCl₃): +6.5 (d, ²J_P¹–P² = 98.6, P²), −2.8 (d, ²J_P¹–P² = 98.6, P¹), −144.12 (hept, ¹J_PF = 713.0, PF₆). HRMS: calcd for [M − PF₆]⁺ 731.1337, found 731.1326. IR: 3057, 2966, 1436, 1302, 1098, 1058, 911, 832, 723, 690, 556.

RuBiphPh′ . The same method used for RuBiphiPr was followed. The starting diphosphane was BiphPh (122 mg, 0.25 mmol). Yield 123 mg (84%).

¹ H NMR (400 MHz, CDCl ₃ ): 7.80–7.71 (m, 2H, Ar), 7.59–7.45 (m, 2H, Ar), 7.38–7.15 (m, 12H, Ar), 7.09–6.95 (m, 4H, Ar), 6.94–6.81 (m, 4H, Ar), 5.28 (d, J = 6.2, 1H, Ar_(p-cymene)), 5.16 (d, J = 5.8, 2H, Ar_(p-cymene)), 4.93 (d, J = 6.0, 1H, Ar_(p-cymene)), 3.52 (dd, J = 16.4, 8.4, 1H, CH_bridge), 3.13 (dd, J = 16.0, 8.8, 1H, CH_bridge), 2.45 (hept, J = 7.2, 1H, CH_{iPr(p-cymene)}), 1.82 (s, 3H, CH_3(p-cymene)), 0.87 (d, J = 6.8, 3H, CH_{3(iPr, p-cymene)}), 0.70 (d, J = 6.9, 3H, CH_{3(iPr, p-cymene)}). ¹³C{¹H} NMR (126 MHz, CDCl₃): 135.1–124.4 (m, Ar), 106.8 (s, Ar_(p-cymene)), 93.9–79.1 (m, Ar_(p-cymene)), 28.9 (s, CH_{iPr(p-cymene)}), 22.6 (s, CH_3(p-cymene)), 20.7 (s, CH_2(bridge)), 19.8 (s, CH_3(p-cymene)). ³¹P{¹H} NMR (162 MHz, CDCl₃): +26.0 (d, ²J_P¹–P² = 30.8, P²), −36.9 (d, ²J_P¹–P² = 31.0, P¹). ¹H NMR (400 MHz, CD₃OD): 8.36–6.85 (m, 24H, Ar), 6.53 (d, J = 6.4, 1H, Ar_(p-cymene)), 6.43 (d, J = 6.4, 1H, Ar_(p-cymene)), 5.87 (d, J = 6.4, 1H, Ar_(p-cymene)), 5.79 (d, J = 6.0, 1H, Ar_(p-cymene)), 4.27 (dt, J = 15.3, 12.8, 1H, CH_bridge), 3.93 (dt, J = 15.4, 10.1, 1H, CH_bridge), 2.34 (septet, J = 6.7, 1H, CH_{iPr(p-cymene)}), 1.42 (s, 3H, CH_3(p-cymene)), 0.97 (d, J = 7.0, 3H, CH_{3(iPr, p-cymene)}), 0.76 (d, J = 6.9, 3H, CH_{3(iPr, p-cymene)}). ¹³C{¹H} NMR (101 MHz, CD₃OD): 146.8–125.3 (m, Ar), 94.8 (d, J = 7.5, Ar_(p-cymene)), 91.8 (d, J = 6.4, Ar_(p-cymene)), 89.5 (d, J = 6.5, Ar_(p-cymene)), 88.9 (s, Ar_(p-cymene)), 40.7 (t, J = 27.7, CH_2(bridge)), 30.9 (s, CH_{iPr(p-cymene)}), 21.9 (s, CH_{3(iPr, p-cymene)}), 19.7 (s, CH_{3(iPr, p-cymene)}), 16.4 (s, CH_3(p-cymene)). ³¹P{¹H} NMR (162 MHz, CD₃OD): +5.8 (d, ²J_P¹–P² = 100.4, P²), −0.78 (d, ²J_P¹–P² = 100.4, P¹). HRMS: calcd for [M + H]⁺ 767.1103, found 767.1097. IR: 3057, 2966, 1436, 1302, 1098, 1058, 910, 832, 723, 690, 556.

Catalytic runs

General procedure for the Rh-catalysed hydrogenation. A solution with the required amount of the rhodium complex (1 mol%) and the corresponding substrate (165 μmol) in anhydrous and degassed THF was prepared in a flask under a nitrogen atmosphere in the glovebox. In all cases, the concentration of the substrate in the reaction medium was adjusted to 0.2 M. Once the reaction mixture had been prepared, the glass vessel was placed into a steel reactor. The autoclave was purged three times with H₂ gas (at 10 bar without stirring), and finally, the autoclave was pressurized under 20 bar of H₂. The reaction mixture was stirred at 800 rpm at room temperature for 24 h. The autoclave was then slowly depressurized, and the reaction mixture was filtered through a short pad of SiO₂ into a 5 mL scintillation vial, eluting with EtOAc (3 mL). The resulting solution was concentrated under vacuum, and the conversion was determined by ¹H NMR spectroscopy. The enantiomeric excess was determined by GC chromatography on chiral stationary phases. The GC machines used were HP5890 Series II using helium as a carrier gas and with a FID detector. For the analysis of the hydrogenations of MAC, a CP-chirasil-D-Val column (25 m × 0.25 mm × 0.12 μm; 180 °C, isothermal) was used. For the hydrogenations of DMI, a Chiraldex β-DM column (30 m × 0.25 mm × 0.12 μm; 80 °C, isothermal) was used. The reactions were performed twice to ensure reproducibility.

General procedure for the Ru-catalysed transfer hydrogenation. Transfer hydrogenation reactions of acetophenone were carried out in 100 mL Schlenk flasks and in pairs to improve the reproducibility of the data. Under an inert atmosphere, the Schlenk flask was charged with 0.04 mmol (1%) of ruthenium complexes and 0.20 mmol (5%) of potassium tert-butoxide. Both solids are dissolved in 25 mL of isopropanol and the mixture was left stirring for 15 min at reflux (82 °C) to activate the catalyst. At this point, 4.0 mmol of acetophenone were added to start the reaction. At the allotted times, aliquots were extracted and analysed by gas chromatography to study the conversion of the reaction. For the GC analysis, the same chromatograph and column used for DMI were employed, but at 120 °C (isothermal).

Author contributions

J. E.: methodology, formal analysis, investigation, validation, data curation, writing-original draft, visualisation. Y. M. M.: investigation, validation, writing-original draft. J. C. C.: investigation. A. V.-F.: methodology, resources, writing-review & editing, supervision, funding acquisition. A. G.: investigation, validation. D. S.: resources, funding acquisition. M. F.-B.: investigation, formal analysis. A. G.: methodology, resources, writing-original draft, supervision, project administration, funding acquisition.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

We thank the MICINN (PID2020-115658GB-I00) and AGAUR (2021-SGR-01107) for financial support. Y. M. M. thanks IN2UB for a Master fellowship.

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Footnotes

† In memoriam of Prof. Paul Kamer (1960–2020).

‡ Electronic supplementary information (ESI) available: NMR spectra of the new compounds. CCDC 2223261–2223263 for complexes RhBiphtBu, Rh(BiphiPr)₂ and RuBiphiPr. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2dt04026c

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