Rotaxane synthesis exploiting the M(I)/M(III) redox couple

Abstract

In the context of advancing the use of metal-based building blocks for the construction of mechanically interlocked molecules, we herein describe the preparation of late transition metal containing [2]rotaxanes (1). Capture and subsequent retention of the interlocked assemblies are achieved by the formation of robust and bulky complexes of rhodium(III) and iridium(III) through hydrogenation of readily accessible rhodium(I) and iridium(I) complexes [M(COD)(PPh₃)₂][BAr^F₄] (M = Rh, 2a; Ir, 2b) and reaction with a bipyridyl terminated [2]pseudorotaxane (3·db24c8). This work was underpinned by detailed mechanistic studies examining the hydrogenation of 1 : 1 mixtures of 2 and bipy in CH₂Cl₂, which proceeds with disparate rates to afford [M(bipy)H₂(PPh₃)₂][BAr^F₄] (M = Rh, 4a[BAr^F₄], t = 18 h @ 50 °C; Ir, 4b[BAr^F₄], t < 5 min @ RT) in CH₂Cl₂ (1 atm H₂). These rates are reconciled by (a) the inherently slower reaction of 2a with H₂ compared to that of the third row congener 2b, and (b) the competing and irreversible reaction of 2a with bipy, leading to a very slow hydrogenation pathway, involving rate-limiting substitution of COD by PPh₃. On the basis of this information, operationally convenient and mild conditions (CH₂Cl₂, RT, 1 atm H₂, t ≤ 2 h) were developed for the preparation of 1, involving in the case of rhodium-based 1a pre-hydrogenation of 2a to form [Rh(PPh₃)₂]₂[BAr^F₄]₂ (8) before reaction with 3·db24c8. In addition to comprehensive spectroscopic characterisation of 1, the structure of iridium-based 1b was elucidated in the solid-state using X-ray diffraction.

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Introduction

Coordination chemistry is an increasingly prominent feature of contemporary methods for preparing mechanically interlocked molecules.^1,2 In most cases metal ions are employed as well-defined templates, pre-organising the fusion of heteroatom-based molecular building blocks, but thereafter jettisoned to confer an interwoven organic product. An alternative but comparatively underdeveloped approach, resulting instead in retention of the metal complex within the final framework, involves capture of the entangled topology itself through formation of a persistent metal–ligand bond. Such an approach represents a potentially versatile means for preparing new and interesting metal-containing interlocked systems, as exemplified through the recent emergence of polyrotaxane metal–organic frameworks.³

In the context of archetypical [2]rotaxane systems, a straight forward scheme employing metal-based building blocks involves coordination of a bulky metal fragment to a donor functionalised “axle” interpenetrated within a macrocycle, viz. metal complex capping of a [2]pseudorotaxane (Scheme 1).¹ Indeed, the successful application of such a synthesis was demonstrated as early as 1981, with Ogino using cobalt(III) fragments to capture [2]rotaxanes comprised of a cyclodextrin macrocycle and diaminoalkane axle (A).⁴ With the notable exception of Sauvage's bis(terpyridine)-ruthenium(II) system B,⁵ the overwhelming majority of subsequent [2]rotaxanes prepared through metal complex capping (e.g.C–E) have, however, involved relatively weak dative bonding of mono-dentate pyridine donors.⁶

Scheme 1 Synthesis of [2]rotaxanes through capping methodology.

Seeking to further develop synthetic methods involving metal complex capping, we targeted the construction of [2]rotaxanes though formation of robust complexes of chelating ligands with bulky late transition metal fragments. To this end, we herein report the preparation of 1 though installation of rhodium(III) and iridium(III) fragments {M(PPh₃)₂H₂}⁺, generated in situ by hydrogenation of readily accessible rhodium(I) and iridium(I) complexes [M(COD)(PPh₃)₂][BAr^F₄] (M = Rh, 2a; Ir, 2b; COD = 1,5-cyclooctadiene; Ar^F = 3,5-(CF₃)₂C₆H₃), upon bipyridyl terminated [2]pseudorotaxane 3·db24c8 (Chart 1; db24c8 = dibenzo-24-crown-8). Mechanistic and structural aspects relevant to this process are first detailed using 2,2′-bipyridine (bipy) as a model.

Chart 1 Components of target [2]rotaxanes 1.

Results and discussion

The hydrogenation of [M(diene)(PPh₃)₂]⁺ (M = Rh, Ir) to afford metal fragments of the type {M(PPh₃)₂H₂}⁺ is well established and evidenced through characterisation as adducts of solvent, e.g. [M(PPh₃)₂H₂(OCMe₂)₂]⁺ and [Ir(PPh₃)₂H₂(ClCH₂CH₂Cl)]⁺,^7,8 or the counter anion, e.g. [Ir(PPh₃)₂H₂(1-closo-CB₁₁H₆X₆)] (X = Cl, I).⁹ Acetone adducts in particular are well-defined M(III) synthons and have enabled isolation of [M(bipy)H₂(PPh₃)₂]⁺ (M = Rh, 4a⁺; Ir, 4b⁺) through subsequent reaction with bipy in acetone solution.¹⁰ In the context of our envisioned synthesis of 1, where robust retention of 3·db24c8 in solution is critical, we sought instead to adapt a procedure reported by Crabtree for the preparation of 4b⁺ by hydrogenation of a mixture of [Ir(COD)(PPh₃)₂]⁺ and bipy in dichloromethane solution (for a range of anions).¹¹ As such we first studied hydrogenation reactions (1 atm) of 1 : 1 mixtures of 2 (20 mM) and bipy in CD₂Cl₂, using NMR spectroscopy, to gauge the viability of this procedure for enacting metal complex capping of 3·db24c8 (Scheme 2 and vide infra). Satisfyingly both the rhodium(I) and iridium(I) complexes were hydrogenated quantitatively in the presence of bipy to afford 4[BAr^F₄], although curiously the former required significantly more forcing conditions (t = 18 h @ 50 °C) than the latter (t < 5 min @ RT). In each case the identity of the dihydride products was confirmed by isolation on a preparative scale and full structural characterisation. Spectroscopic data for 4[BAr^F₄] are unsurprisingly in close agreement with precedents bearing different counter anions;^10,11 useful markers include low frequency hydride signals at δ −15.66 (app. q, ¹J_RhH ≈ ²J_PH = 14 Hz; M = Rh)/−19.48 (t, ²J_PH = 16.6 Hz; M = Ir) and ³¹P resonances at δ 47.1 (d, ¹J_RhP = 115 Hz; M = Rh)/20.1 (M = Ir).

Scheme 2 Preparation of 4[BAr^F₄].

Crystallographic data was also obtained in both cases for 4a[BAr^F₄] and 4b[BAr^F₄]. For the iridium(III) complex two polymorphs were identified and studied, one of which features two independent cations in the asymmetric unit (Z′ = 2). All cations bear the expected pseudo-octahedral metal geometries with trans-phosphine ligands (Fig. 1).^12–14 Illustrating the subtle effects of crystal packing, originating though variation of the anion and in some cases presence of solvent molecules, a wide range of geometries are observed for 4⁺ in the solid-state: including both staggered and eclipsed conformations of the phosphine substituents (cf.4a[BAr^F₄] vs.4b[OTf]), and alternative orientations of the phosphines about the metal–phosphine vector (cf.4a[BAr^F₄] vs.4b[BAr^F₄] (Z′ = 1)).

Fig. 1 Solid-state structures of 4⁺. Left: Selected independent cation from the X-ray structure of 4b[BAr^F₄] (Z′ = 2) with thermal ellipsoids drawn at the 50% probability level; non-hydridic hydrogen atoms omitted for clarity. Right: Conformations of 4⁺ viewed along the P–M–P vector, with phosphine ligands differentially coloured to aid comparison. ^a This work, polymorph with Z′ = 2; ^b this work, polymorph with Z′ = 1; ^c this work; ^d ref. 12; ^e ref. 13; ^f ref. 14. Selected bond lengths (Å) and angles (°): 4a[BAr^F₄], Rh1–P31, 2.3119(6); Rh1–N2, 2.154(2); P31–Rh1–P31*, 174.05(4); *1 − x, +y, 1 − z; 4b[BAr^F₄] (Z′ = 2), Ir1–P31, 2.2924(8); Ir1–P51, 2.3009(7); Ir1–N2, 2.118(2); Ir1–N13, 2.151(2); P31–Ir1–P51, 166.53(3); Ir11–P131, 2.3048(7); Ir11–P151, 2.3035(7); Ir11–N102, 2.115(2); Ir11–N113, 2.145(2); P131–Ir11–P151, 166.40(3); 4b[BAr^F₄] (Z′ = 1), Ir1–P31, 2.3000(8); Ir1–P51, 2.2945(8); Ir1–N2, 2.138(2); Ir1–N13, 2.128(3); P31–Ir1–P51, 164.08(3).

Despite both resulting in formation of the desired products, the disparate rates prompted us to interrogate the mechanism of 2 + bipy hydrogenation in dichloromethane. To this end the kinetics of reactions of 2 (20 mM) with bipy and H₂ (1 atm) were examined individually in CD₂Cl₂ at RT using ¹H and ³¹P NMR spectroscopy (Scheme 3). Presumably driven by the chelate effect, reaction between 2 and bipy resulted in irreversible formation of five coordinate [M(bipy)(COD)(PPh₃)][BAr^F₄] (M = Rh, 5a; Ir, 5b) alongside concomitant liberation of PPh₃, although under vastly different kinetic regimes (t_1/2 = 1.3 h, 2a; 34 h, 2b). Moreover dynamic exchange between bound/free phosphine is observed on the NMR timescale in the rhodium system, leading to a single broad ³¹P signal at δ 10.6 (fwhm = 55 Hz) at 298 K that decoalesced into two sharp resonances at δ 33.3 (d, ¹J_RhP = 129 Hz) and −8.3 in a 1 : 1 ratio on cooling to 200 K. Equivalent reaction mixtures are obtained from reactions between [M(bipy)(COD)][BAr^F₄] (M = Rh, 6a; Ir, 6b) and two equivalents of PPh₃ in CD₂Cl₂, but on a much faster timescale (t < 5 min). Indeed, 5 were subsequently isolated from reactions between 6 and one equivalent of PPh₃ for independent structural verification in solution and the solid-state (see Experimental section, Fig. 2 and CIF). While no further reaction of 5b with PPh₃ was observed in solution, COD is slowly and reversibly displaced from the lighter congener by PPh₃ resulting in equilibrium formation of [Rh(bipy)(PPh₃)₂][BAr^F₄] 7 (t_1/2 = 107 h; K = 2.4) at RT.¹⁵

Scheme 3 Hydrogenation of 2 in the presence of bipy. Conditions: 20 mM, CD₂Cl₂, RT and 1 atm H₂ (where relevant). Counter anion = [BAr^F₄]⁻ throughout. COE = cyclooctene.

Fig. 2 Solid-state structure of 5b. Thermal ellipsoids drawn at the 50% probability level; anion and hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (°): 5a, Rh1–P31, 2.3306(7); Rh1–N2, 2.262(2); Rh1–N13, 2.161(2); Rh1–Cnt(C14,C15), 1.996(3); Rh1–Cnt(C18,C19), 2.072(3); P31–Rh1–C18, 179.36(9); 5b, Ir1–P31, 2.3426(6); Ir1–N2, 2.213(2); Ir1–N13, 2.121(2); Ir1–Cnt(C14,C15), 2.013(2); Ir1–Cnt(C18,C19), 2.017(2); P31–Ir1–C18, 171.56(8).

Hydrogenation of 2a in CD₂Cl₂ proceeded at a moderate rate (t_1/2 = 0.8 h) to produce Rh(I) dimer [Rh(PPh₃)₂]₂[BAr^F₄]₂ (8) as the exclusive organometallic product alongside cyclooctane (COA).¹⁶ Complex 8 was subsequently isolated on a preparative scale and reacted with bipy (under an argon atmosphere) to afford 7 (t < 5 min).¹⁵ Placing isolated 7 under hydrogen (1 atm) thereafter quantitatively produced 4a[BAr^F₄] (t < 5 min).¹⁵ Reaction of 2b with hydrogen rapidly (t < 5 min) gave rise to a mixture of Ir(III) complexes of the formulation [Ir(PPh₃)₂H₂L₂][BAr^F₄] (9; L = H₂, CD₂Cl₂), with concomitant generation of COA, which then afforded 4b[BAr^F₄] quantitatively upon addition of bipy under hydrogen (t < 5 min).^8,17

Together these mechanistic experiments allow the disparate rates of 2 + bipy hydrogenation in dichloromethane to be reconciled by (a) the inherently slower reaction of 2a with H₂ compared to that of third row congener 2b,¹⁸ and (b) competing and irreversible reaction of 2a with bipy, leading to a very slow hydrogenation pathway, involving rate-limiting substitution of COD by PPh₃. Consistent with these proposals intermediate presence of 5a is observed in situ during the formation of 4a[BAr^F₄] from hydrogenation of 2a + bipy in dichloromethane at 50 °C (Schemes 2 and 3). Under these conditions a (COA + COE) : COD ratio of 42 : 58 was established at reaction completion by GC analysis,^19,20 broadly in line with the relative rates of reaction of 2a with H₂ (t_1/2 = 0.8 h) and bipy (t_1/2 = 1.3 h) determined in dichloromethane at RT. In contrast, no COD was detected by GC analysis on hydrogenation of 2b + bipy in dichloromethane at RT (Schemes 2 and 3).

Moving on from the mechanistic studies associated with metal complex capping reaction, the construction of 1 itself began with preparation of amine 10, which was readily obtained following a straightforward four step synthesis starting from previously reported 5-chloromethylbipyridine (Scheme 4; 66% yield over 4 steps).²¹ Subsequent protonation and halide exchange, using citric acid and Na[BAr^F₄] under biphasic conditions (CH₂Cl₂/H₂O), enabled isolation of the corresponding ammonium derivative 3 in high yield (84%). Under our chosen conditions, employing anhydrous dichloromethane and strategic incorporation of the weakly coordinating [BAr^F₄]⁻ counter anion,²² analysis by ¹H NMR spectroscopy indicated [2]pseudorotaxane 3·db24c8 was assembled rapidly (t < 5 min) and quantitatively on dissolution of a 1 : 1 mixture of 3 and db24c8 in CD₂Cl₂ at RT. Distinctive features in the ¹H NMR spectrum associated with the formation of 3·db24c8 include methylene resonances at δ 4.82 (bipyCH̲₂) and 4.71 (ArCH̲₂) coupled to a partially obscured ammonium resonance at δ 7.70, and significantly perturbed db24c8 resonances (see ESI† for stack plots). Additional experiments involving variation of the components (2 : 1, 1 : 2) confirmed that the system is under slow exchange on the NMR timescale (500 MHz, 298 K) consistent with robust retention of 3·db24c8 in solution.

Scheme 4 Synthesis of [2]rotaxanes 1. Reagents and conditions: i. Potassium phthalimide, DMF (40 °C); ii. hydrazine monohydrate, EtOH (reflux); iii. 3,5-di-tert-butylbenzaldehyde, 3 Å molecular sieves, MeOH (RT); iv. Na[BH₄] (0 °C to reflux); v. Na[BAr^F₄], citric acid monohyrate, CH₂Cl₂/H₂O (RT).

Guided by the aforementioned mechanistic studies, rhodium-based 1a was prepared in high isolated yield (75%) by reaction between 3·db24c8 and 8, individually prepared in situ from 3 + db24c8 and 2a + H₂ respectively, under hydrogen (1 atm) in dichloromethane at RT (t = 2 h). A more operationally straightforward procedure was possible for iridium-based 1b, involving addition of 2b to a solution 3·db24c8 and subsequently placing the reaction mixture under H₂ (1 atm, t = 1 h), enabling isolation of the analytically pure product in high yield (78%). Employing this hydrogenation procedure, but heating at elevated temperature (50 °C, t = 27 h; cf. Scheme 2), with 2a and 3·db24c8 did result in the formation of 1a, but as the major component of an intractable mixture (ca. 60%). Such an observation is not unexpected given the greater complexity associated with the hydrogenation of 2a + bipy.

The capture of 1 was conviently established by ESI-MS, with strong [M]²⁺ signals observed at 732.7929 (1a, calcd 732.7937) and 777.8218 (1b, calcd 777.8228) m/z with the expected half integer spacing. The structures of 1 were additionally fully corroborated in solution using by a combination of ¹H, ¹³C and ³¹P NMR spectroscopy. With the exception of the expected loss of symmetry, the metrics associated with the metal fragment are directly comparable to those of the model systems 4[BAr^F₄]. For instance the hydride resonances of 1a/1b are observed at δ −15.50 and −15.85 (cf. −15.66)/−19.30 and −19.64 (cf. −19.48) with associated ²J_HH (11.8/7.4 Hz) coupling, while the ³¹P resonances of 1a/1b are observed at δ 46.2 (¹J_RhP = 115 Hz; cf. 47.1, ¹J_RhP = 115 Hz)/19.8 (cf. 20.1). Likewise the spectroscopic indicators associated with interpenetration of the ammonium axle within db24c8 in 1 are very similar to those seen in the [2]pseudorotaxane 3·db24c8: for example, the methylene ¹H resonances bipyCH̲₂ (1a, 4.31; 1b, 4.30; cf. 4.82) and ArCH₂ (1a, 4.68; 1b, 4.69; cf. 4.71). Gratifyingly, we were also been able to grow single crystals of 1b and elucidate its solid-state structure using X-ray diffraction. [2]Rotaxane 1b crystallises from dichloromethane/hexane in the triclinic space group P1̄ with one full molecule in the asymmetric unit (Fig. 3). The iridium centre adopts the expected distorted octahedral geometry, with comparable metrics to those of 4b[BAr^F₄] and analogues thereof bearing different counter anions (Fig. 1).^12,13 Notably there is no meaningful steric buttressing apparent from close proximity of the interlocked db24c8 to the metal fragment (Ir1⋯N15, 6.540(2) Å), evident for example by the similarity of P31–Ir1–P51 bond angles in 1a (165.76(3)°) to those observed in 4b[BAr^F₄] (166.53(3)°, 166.40(3)°, 164.08(3)°). Other salient features include the location of the hydride ligands off the Fourier difference map, and the presence of two strong hydrogen bonds between the ammonium cation and ether linkages of db24c8 (N15–O71, 3.057(3); N15–O77, 3.030(3) Å).

Fig. 3 Solid-state structure of 1b. Thermal ellipsoids drawn at the 50% probability level; anions, most hydrogen atoms, and minor disordered components omitted for clarity. Selected bond lengths (Å) and angles (°): Ir1–P31, 2.3103(5); Ir1–P51, 2.2950(5); Ir1–N2, 2.151(2); Ir1–N13, 2.137(2); Ir1⋯N15, 6.540(2); N15–O71, 3.057(3); N15–O77, 3.030(3); P31–Ir1–P51, 165.76(2).

Summary and perspectives

We have presented a new reaction manifold for preparing [2]rotaxanes (1) using coordination chemistry. Our scheme involves topology capture through construction of bulky and robust rhodium(III) and iridium(III) complex stoppering groups from a bipyridyl terminated [2]pseudorotaxane (3·db24c8) and synthetically convenient metal precursors [M(COD)(PPh₃)₂][BAr^F₄] (M = Rh, 2a; Ir, 2b) in CH₂Cl₂ under hydrogen (1 atm). Guided by detailed mechanistic studies, examining the hydrogenation of 1 : 1 mixtures of 2 and bipy, operationally convenient and mild conditions were developed for both metal-based systems. In the case of rhodium-based 1a, pre-hydrogenation of 2a to form [Rh(PPh₃)₂]₂[BAr^F₄]₂ (8, t = 4 h, RT, 1 atm H₂) before reaction with 3·db24c8 (t = 2 h, RT, 1 atm H₂) is necessary to avoid an exceptionally slow hydrogenation pathway involving substitution of COD by PPh₃. The chemistry associated with the iridium(I)/iridium(III) redox couple is much more attractive; in comparison to 2a the iridium precursor 2b shows sluggish reaction with bipy at RT (t_1/2 = 34 h), whilst introduction of hydrogenation results in rapid and quantitative stoppering of 3·db24c8 (t < 1 h). Together this work showcases a facile route to the construction of new metal-containing interlocked molecules via formation of robust metal–ligand bonds. In addition to synthetic ease, the installation of {M(PPh₃)₂H₂}⁺ metal fragments in entangled architectures proffers a number of advantages, including (a) useful spectroscopic handles (distinctive low frequently hydride and ³¹P signals); (b) large steric profile (trans-phosphine ligands); (c) the presence of heavy transition metals amenable for routine analysis by X-ray diffraction (e.g.Fig. 3); and perhaps more importantly, (d) the possibly to exploit interesting reactivity and spectroscopic properties of the metal fragment. We are particularly interested in exploring the latter as part of our on-going research in this area.

Experimental

General methods

Manipulations were performed under an argon atmosphere using Schlenk and glove box techniques unless otherwise stated. Glassware was oven dried at 150 °C overnight and flame-dried under vacuum prior to use. Molecular sieves (3 Å) were activated by heating at 300 °C in vacuo overnight. Anhydrous solvents (Et₂O, CH₂Cl₂, CHCl₃, pentane, hexane, THF and toluene) were purchased from Acros Organics or Sigma-Aldrich and stored over molecular sieves (3 Å). CD₂Cl₂ was dried over molecular sieves (3 Å) and stored under argon. 5-Chloromethylbipyridine,²¹ Na[BAr^F₄]²³ and [M(COD)₂Cl]₂ (M = Rh, Ir)²⁴ were synthesised using established procedures. All other reagents are commercial products and were used as received. NMR spectra were recorded on Bruker spectrometers at 298 K unless otherwise stated. Chemical shirts are quoted in ppm and coupling constants in Hz. ESI-MS were recorded on Bruker Maxis Plus (HR) or Agilent 6130B single Quad (LR) instruments. Gas chromatography analyses were performed on an Agilent 7820A GC system fitted with a 7693A auto-injector. Microanalyses were performed at the London Metropolitan University by Mr Stephen Boyer.

Preparation of 1a

A solution of 2a (50.8 mg, 32.0 μmol) in CH₂Cl₂ (0.9 mL) was freeze–pump–thaw degassed and placed under dihydrogen (1 atm). After stirring at RT for 4 h, the resulting deep red solution was added under dihydrogen to a solution of 3 (40.0 mg, 32.0 μmol) and db24c8 (14.4 mg, 32.1 μmol) in CH₂Cl₂ (0.9 mL). The solution was stirred at RT for 2 h and then added to pentane (ca. 30 mL) under hydrogen with stirring, whereupon an orange-red gum precipitated. The supernatant was decanted away and the product dried in vacuo for 5 min to afford the product as a dark-yellow foam. Yield = 76.7 mg (75%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.17 (s, 1H, bipy), 7.98 (d, ³J_HH = 5.0, 1H, bipy), 7.80 (d, ³J_HH = 9.0, 1H, bipy), 7.71–7.76 (m, 16H, Ar^F), 7.67 (vbr, fwhm = 26 Hz, 2H, NH̲₂), 7.50–7.61 (obscured, 3H, bipy), 7.55 (br, 8H, Ar^F), 7.45 (br, 1H, C₆H₃), 7.23–7.32 (m, 18H, Ph), 7.14–7.22 (m, 14H, C₆H₃ + Ph), 6.84–6.96 (m, 9H, bipy + C₆H₄), 4.65–4.72 (m, 2H, ArCH̲₂), 4.28–4.36 (m, 2H, bipyCH̲₂), 4.05–4.12 (m, 4H, OCH₂), 3.94–4.02 (m, 4H, OCH₂), 3.52–3.63 (m, 8H, OCH₂), 3.33–3.47 (m, 8H, OCH₂), 1.17 (s, 18H, ^tBu), −15.50 (app. p, J = 14 (¹J_RhH = 15.2, ²J_HH = 11.8), 1H, RhH), −15.85 (app. p, J = 14 (¹J_RhH = 15.6, ²J_HH = 11.8), 1H, RhH). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.4 (q, ¹J_BC = 50, Ar^F), 155.2 (s, bipy), 154.2 (s, bipy), 153.7 (s, bipy), 153.3 (s, bipy), 152.8 (s, C₆H₃), 147.8 (s, C₆H₄), 138.2 (s, bipy), 136.3 (s, bipy), 135.4 (s, Ar^F), 133.6 (app. t, J_PC = 6, Ph), 132.5 (app. t, J_PC = 24, Ph), 131.1 (s, Ph), 131.0 (s, bipy), 130.2 (s, C₆H₃), 129.4 (qq, ²J_CF = 32, ²J_CB = 3, Ar^F), 129.2 (app. t, J_PC = 5, Ph), 127.1 (s, bipy), 125.4 (s, C₆H₃), 125.2 (q, ¹J_FC = 272, Ar^F), 124.8 (s, C₆H₃), 123.21 (s, bipy), 123.16 (s, C₆H₄), 122.7 (s, bipy), 118.1 (sept., ³J_FC = 4, Ar^F), 113.8 (s, C₆H₄), 71.1 (s, OCH₂), 70.7 (s, OCH₂), 68.8 (s, OCH₂), 55.1 (s, ArC̲H₂), 49.3 (s, bipyC̲H₂), 35.3 (s, ^tBu), 31.6 (s, ^tBu). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ 46.2 (d, ¹J_RhP = 115). HR ESI-MS (positive ion, 4 kV): 732.7929 [M]²⁺ (calcd 732.7937) m/z. Anal. calcd for C₁₅₀H₁₂₂B₂F₄₈P₂Rh (3193.04 g mol⁻¹): C, 56.42; H, 3.85; N, 1.32. Found: C, 56.50; H, 4.02; N, 1.34.

Attempted preparation of 1a by hydrogenation of 2a + 3·db24c8

To a solution of 3 (20.2 mg, 16.1 μmol) and db24c8 (7.2 mg, 16.1 μmol) in CH₂Cl₂ (0.8 mL) was added 2a (25.7 mg, 16.1 μmol). The resulting solution was freeze–pump–thaw degassed, placed under dihydrogen (1 atm), and stirred at 50 °C for 27 h. Volatiles were removed in vacuo and the solid redissolved in CH₂Cl₂ (ca. 2 mL) and layered with hexane (ca. 15 mL) to afford the crude product as a dark yellow gum, which was isolated through decantation of the supernatant and dried in vacuo. Analysis by ¹H NMR spectroscopy in CD₂Cl₂ indicated a mixture of products with the major constituent being 1a in ca. 60% purity.

Preparation of 1b

To a solution of 3 (40.0 mg, 32.0 μmol) and db24c8 (14.4 mg, 32.1 μmol) in CH₂Cl₂ (1.8 mL) was added 2b (54.1 mg, 32.0 μmol). The resulting solution was freeze–pump–thaw degassed and placed under dihydrogen (1 atm), and stirred at RT for 1 h. Volatiles were removed in vacuo and the solid redissolved in Et₂O (ca. 2 mL) and layered with hexane (ca. 15 mL). Storage at ambient temperature overnight afforded a yellow oil, which was isolated through decantation of the supernatant and dried in vacuo overnight to afford the product as a bright yellow foam. Yield = 82.2 mg (78%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.26 (s, 1H, bipy), 8.16 (d, ³J_HH = 5.4, 1H, bipy), 7.82 (d, ³J_HH = 9.7, 1H, bipy), 7.71–7.75 (m, 16H, Ar^F), 7.67 (vbr, fwhm = 43 Hz, 2H, NH̲₂), 7.54–7.59 (obscured, 1H, bipy), 7.55 (br, 8H, Ar^F), 7.51 (d, ³J_HH = 8.6, 1H, bipy), 7.47 (d, ³J_HH = 8.2, 1H, bipy), 7.45 (s, 1H, C₃H₃), 7.22–7.31 (m, 18H, Ph), 7.13–7.22 (m, 14H, C₆H₃ + Ph), 6.86–6.97 (m, 8H, C₆H₄), 6.81–6.86 (m, 1H, bipy), 4.66–4.73 (m, 2H, ArCH̲₂), 4.26–4.34 (m, 2H, bipyCH̲₂), 4.05–4.14 (m, 4H, OCH₂), 3.95–4.04 (m, 4H, OCH₂), 3.52–3.64 (m, 8H, OCH₂), 3.35–3.46 (m, 8H, OCH₂), 1.17 (s, 18H, ^tBu), −19.30 (td, ²J_PH = 16.9, ²J_HH = 7.4, 1H, IrH), −19.64 (td, ²J_PH = 16.1, ²J_HH = 7.4, 1H, IrH). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_BC = 50, Ar^F), 156.9 (s, bipy), 155.3 (s, bipy), 154.9 (s, bipy), 154.8 (s, bipy), 152.7 (s, C₆H₃), 147.8 (s, C₆H₄), 137.6 (s, bipy), 135.5 (s, bipy), 135.4 (s, Ar^F), 133.5 (app. t, J_PC = 6, Ph), 131.9 (s, bipy), 131.8 (t, ¹J_PC = 27, Ph), 131.1 (s, Ph), 130.1 (s, C₆H₃), 129.4 (qq, ²J_CF = 32, ²J_CB = 3, Ar^F), 129.1 (app. t, J_PC = 5, Ph), 127.9 (s, bipy), 125.4 (s, C₆H₃), 125.2 (q, ¹J_FC = 272, Ar^F), 124.9 (s, C₆H₃), 123.7 (s, bipy), 123.20 (s, bipy), 123.17 (s, C₆H₄), 118.0 (sept., ³J_FC = 4, Ar^F), 113.8 (s, C₆H₄), 71.0 (s, OCH₂), 70.7 (s, OCH₂), 68.7 (s, OCH₂), 55.1 (s, ArC̲H₂), 49.1 (s, bipyC̲H₂), 35.3 (s, ^tBu), 31.5 (s, ^tBu). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ 19.8 (vbr, fwhm = 43 Hz). HR ESI-MS (positive ion, 4 kV): 777.8218 [M]²⁺ (calcd 777.8228), 1554.6401 [M − H]⁺ (calcd 1554.6384) m/z. Anal. calcd for C₁₅₀H₁₂₂B₂F₄₈P₂Ir (3285.14 g mol⁻¹): C, 54.89; H, 3.75; N, 1.28. Found: C, 54.83; H, 3.74; N, 1.31.

Preparation of 2

General procedure. A suspension of [M(COD)Cl]₂ (M = Rh, Ir; 25.0 μmol), PPh₃ (26.2 mg, 100 μmol) and Na[BAr^F₄] (44.3 mg, 50.0 μmol) in CH₂Cl₂ (5 ml) was stirred overnight at RT. Hexane (1 mL) was added and the solution filtered and layered with excess hexane (45 mL) to afford the products on diffusion.

2a

Yield = 69.9 mg (87%, orange solid).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.70–7.75 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 7.42 (t, ³J_HH = 7.4, 6H, Ph), 7.35–7.40 (m, 12H, Ph), 7.27 (t, ³J_HH = 7.2, 12H, Ph), 4.55 (br, 4H, CH=CH), 2.54–2.42 (m, 4H, CH₂), 2.28–2.20 (m, 4H, CH₂). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 135.4 (s, Ar^F), 134.5 (app. t, J_PC = 6, Ph), 131.7 (s, Ph), 130.5–131.3 (m, Ph), 129.4 (qq, ²J_CF = 32, ²J_CB = 3, Ar^F), 129.2 (app. t, J_PC = 5, Ph), 125.2 (q, ¹J_FC = 272, Ar^F), 118.1 (sept., ³J_FC = 4, Ar^F), 99.6 (app. dt, J = 8, J = 5, CH=CH), 31.2 (s, CH₂). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 26.5 (d, ¹J_RhP = 145). HR ESI-MS (positive ion, 4 kV): 735.1812 [M]⁺ (calcd 735.1811) m/z. Anal. calcd for C₇₈H₅₄BF₂₄P₂Rh (1589.89 g mol⁻¹): C, 57.09; H, 3.40; N, 0.00. Found: C, 57.18; H, 3.56; N, 0.00.

2b

Yield = 49.0 mg (58%, red solid). Spectroscopic data are in agreement with the literature values.²⁵

¹ H NMR (400 MHz, CD₂Cl₂): δ 7.70–7.76 (m, 8H, Ar^F), 7.56 (s, 4H, Ar^F), 7.42 (t, ³J_HH = 7.4, 6H, Ph), 7.35–7.40 (m, 12H, Ph), 7.27 (t, ³J_HH = 7.2, 12H, Ph), 4.20 (br, 4H, CH=CH), 2.40–2.20 (m, 4H, CH₂), 2.06–1.85 (m, 4H, CH₂). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 17.6 (s). LR ESI-MS (positive ion): 825.2 [M]⁺ (calcd 825.2) m/z.

NMR scale reactions of 2

Hydrogenation of 2 + bipy. CD₂Cl₂ (0.5 mL) was condensed into a J. Young's NMR tube containing 2 (10 μmol) and bipy (1.6 mg, 10 μmol) cooled in a Dewar of liquid nitrogen, then thawed to RT under dihydrogen (1 atm). The NMR tube was sealed, inverted several times, and then left at the desired reaction temperature. At the conclusion of the reaction the solution was passed through a short plug of SiO₂ (CH₂Cl₂) and analysed by GC.

The hydrogenation of 2a + bipy was monitored in situ by ¹H and ³¹P NMR spectroscopy at 50 °C. After 1 h, 2a was consumed with concomitant formation of 4a[BAr^F₄] (ca. 58% [Rh]) and 5a (ca. 42% [Rh]). Complete conversion to 4a[BAr^F₄] was observed after 18 h. Analysis by GC gave a hydrocarbon distribution of: COA (6%), COE (36%), COD (58%).

Immediate analysis of the hydrogenation of 2b + bipy at RT (<5 min) by ¹H and ³¹P NMR spectroscopy indicated quantitative conversion to 4b[BAr^F₄]. Analysis by GC gave a hydrocarbon distribution of: COA (14%), COE (86%), COD (0%).

Reaction between 2 and bipy. A solution of 2 (10 μmol) and bipy (1.6 mg, 10 μmol) in CD₂Cl₂ (0.5 mL) was prepared in J. Young's NMR tube and then left to stand in a water bath at 25 °C. The ensuing reaction was monitored periodically in situ by ¹H and ³¹P NMR spectroscopy.

Reaction between 2a and bipy resulted in formation of 5a (t_1/2 = 1.3 h) and subsequently slow equilibrium formation of 7, alongside liberation of COD. Modelling the kinetics of the approach to equilibrium over 8 days,²⁶ enabled determination of the equilibrium constant (K = 2.4) and rate of forward reaction (t_1/2 = 107 h).

Reaction between 2b and bipy resulted in the slow and quantitative formation of 5b (t_1/2 = 34 h). No subsequent reaction was observed.

Hydrogenation of 2. A solution of 2 (10 μmol) in CD₂Cl₂ (0.5 mL) was prepared in J. Young's NMR tube and freeze–pump–thaw degassed three times and placed under dihydrogen (1 atm) at RT. The ensuing reaction was monitored periodically in situ by ¹H and ³¹P NMR spectroscopy.

The smooth conversion of 2a to 8 was observed over 4 h with a t_1/2 = 0.8 h, alongside concomitant generation of COA. No COE or COD was evident by ¹H NMR spectroscopy.

Hydrogenation of 2b resulted in the immediate colour change from red to colourless within 5 min. Analysis by ¹H and ³¹P NMR spectroscopy indicated the formation of a mixture of hydride species 9, alongside concomitant generation of COA. No COE or COD was evident by ¹H NMR spectroscopy. Subsequent transfer of this solution into a J. Young's NMR tube charged with bipy (1.6 mg, 10.0 μmol) resulted in the quantitative formation of 4b[BAr^F₄] within 5 min as gauged by ¹H and ³¹P NMR spectroscopy.

Selected data for 9.

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.71–7.76 (m, 8H, Ar^F), 7.56 (s, 4H, Ar^F), 7.47–7.58 (m, 30H, Ph), −26.24 (vbr, fwhm = 190 Hz). ³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ 23.4 (vbr, fwhm = 130 Hz). ¹H NMR (500 MHz, CD₂Cl₂, 185 K): δ 7.70–7.76 (m, 8H, Ar^F), 7.52 (br, 4H, Ar^F), 7.34–7.53 (m, 30H, Ph), −1.75 (br, T₁ = 54 ms, IrH₂(H̲₂)₂), −12.43 (br, T₁ = 51 ms, IrH₂(H̲₂)(CD₂Cl₂)₂), −23.59 (br, T₁ = 49 ms, IrH̲₂(H₂)₂),* −25.11 (t, ²J_PH = 14.6, T₁ = 401 ms, IrH₂(CD₂Cl₂)₂), −26.72 (br, T₁ = 361 ms, IrH̲₂(H₂)(CD₂Cl₂)₂), −28.13 (br, T₁ = 358 ms, IrH̲₂(H₂)(CD₂Cl₂)₂). * This resonance is assigned to a hydride despite the short T₁ value, which we attribute to chemical exchange on the NMR timescale. ³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 185 K): δ 27.0 (s, IrH₂(H₂)(CD₂Cl₂)₂), 23.2 (s, IrH₂(H₂)₂), 15.7 (s, IrH₂(CD₂Cl₂)₂).

Preparation of 3

A suspension of 10 (100 mg, 258 μmol), Na[BAr^F₄] (229 mg, 258 μmol) and citric acid monohydrate (54.2 mg, 258 μmol) in CH₂Cl₂/H₂O 1 : 1 (6 mL) was stirred vigorously at RT for 10 min (air). The organic phase was separated, washed with H₂O (5 × 5 mL) and dried over MgSO₄. The solvent was removed in vacuo to give the product as an off-white solid. Yield = 270 mg (84%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.64 (s, 1H, bipy), 8.57 (d, ³J_HH = 5.0, 1H, bipy), 8.08 (app. t, ³J_HH = 8, 1H, bipy), 7.97 (d, ³J_HH = 7.6, 1H, bipy), 7.93 (d, ³J_HH = 7.9, 1H, bipy), 7.81 (d, ³J_HH = 7.6, 1H, bipy), 7.69–7.75 (m, 8H, Ar^F), 7.62 (dd, ³J_HH = 7.6, 5.1, 1H, bipy), 7.54 (br, 4H, Ar^F), 7.43 (t, ⁴J_HH = 1.8, 1H, C₆H₃), 7.22 (d, ⁴J_HH = 1.8, 2H, C₆H₃), 4.26 (br, 2H, CH₂), 4.24 (br, 2H, CH₂), 1.24 (s, 18H, ^tBu). The NH₂ resonance was not unambiguously located. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 152.8 (s, C₆H₃), 148.4 (s, bipy), 142.9 (s, bipy), 141.8 (s, bipy), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_BC = 3, Ar^F), 127.5 (s, bipy), 125.1 (q, ¹J_FC = 272, Ar^F), 124.0 (s, bipy), 123.7 (s, C₆H₃), 123.5 (s, bipy), 123.1 (s, C₆H₃), 118.0 (sept., ³J_FC = 4, Ar^F), 54.9 (s, CH₂), 50.2 (s, CH₂), 35.3 (s, ^tBu), 31.6 (s, ^tBu). Not all resonances unambiguously located. HR ESI-MS (positive ion, 4 kV): 1252.3484 [M + H]⁺ (calcd 1252.3484) m/z.

Reaction of 3 with db24c8: preparation of 3·db24c8

A solution of 3 (6.3 mg, 5.0 μmol) and db24c8 (2.3 mg, 5.1 μmol) in CD₂Cl₂ (0.5 mL) was prepared within a J. Young's NMR tube. Analysis in situ by NMR spectroscopy indicated the quantitative formation of 3·db24c8 (slow exchange at 500 MHz).

Repeating the reaction using 0.5 and 2 equiv. db24c8 resulted in the formation of a 1 : 1 mixture of 3 and 3·db24c8, and 1 : 1 mixture of 3·db24c8 and db24c8, respectively (both slow exchange at 500 MHz).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.60 (d, ³J_HH = 4.6, 1H, bipy), 8.48 (d, ⁴J_HH = 1.6, 1H, bipy), 8.18 (d, ³J_HH = 7.9, 1H), 8.04 (d, ³J_HH = 8.2, 1H, bipy), 7.79 (app. td, ³J_HH = 8, ⁴J_HH = 1.8, 1H, bipy), 7.70–7.76 (m, 8H, Ar^F), 7.70 (obscured, 2H, NH₂), 7.63 (dd, ³J_HH = 8.2, ⁴J_HH = 2.0, 1H, bipy), 7.56 (br, 4H, Ar^F), 7.47 (t, ⁴J_HH = 1.8, 1H, C₆H₃), 7.30–7.34 (m, 3H, C₆H₃ + bipy), 6.69–6.79 (m, 8H, C₆H₄), 4.79–4.85 (m, 4H, bipyCH̲₂), 4.67–4.74 (m, 4H, ArCH̲₂), 4.01–4.19 (m, 8H, OCH₂), 3.71–3.87 (m, 8H, OCH₂), 3.58–3.68 (m, 4H, OCH₂), 3.45–3.56 (m, 4H, OCH₂), 1.23 (s, 18H, ^tBu).¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_BC = 50, Ar^F), 157.2 (s, bipy), 155.5 (s, bipy), 152.5 (s, C₆H₃), 150.4 (s, bipy), 149.6 (s, bipy), 147.6 (s, C₆H₄), 138.0 (s, bipy), 137.3 (s, bipy), 135.4 (s, Ar^F), 131.4 (s, C₆H₃), 129.4 (qq, ²J_CF = 32, ²J_CB = 3, Ar^F), 127.8 (s, bipy), 125.2 (q, ¹J_FC = 272, Ar^F), 124.6 (s, bipy), 124.4 (s, C₆H₃), 124.0 (s, C₆H₃), 122.3 (s, C₆H₄), 121.6 (s, bipy), 120.5 (s, bipy), 118.0 (sept. ³J_FC = 4, Ar^F), 113.0 (s, C₆H₄), 71.2 (s, OCH₂), 70.9 (s, OCH₂), 68.4 (s, OCH₂), 54.0 (s, ArC̲H₂), 50.6 (s, bipyC̲H₂), 35.4 (s, ^tBu), 31.6 (s, ^tBu).

Preparation of 4[BAr^F₄]

General procedure. A solution of [M(COD)Cl]₂ (20.0 μmol) and bipy (3.1 mg, 20 μmol) in CH₂Cl₂ (1 mL) was freeze–pump–thaw degassed three times then placed under dihydrogen (1 atm). The solution was agitated (M = Rh, 18 h at 50 °C; M = Ir; 10 min at RT), then layered with hexane (20 mL) to afford the products as crystalline solids on diffusion.

4a[BAr^F₄]

Yield = 17.6 mg (53%, off white solid).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.06 (d, ³J_HH = 5.2, 2H, bipy), 7.71–7.75 (m, 8H, Ar^F), 7.71 (obscured, 2H, bipy), 7.64 (t, ³J_HH = 7.5, 2H, bipy), 7.56 (br, 4H, Ar^F), 7.30–7.38 (m, 18H, Ph), 7.23 (t, ³J_HH = 7.4, 12H, Ph), 6.82 (ddd, ³J_HH = 7.6, 5.5, ⁴J_HH = 1.0, 2H, bipy), −15.66 (app. q, J = 14 (¹J_RhH = 15.5), 2H, RhH). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 154.5 (s, bipy), 154.3 (s, bipy), 137.8 (s, bipy), 135.4 (s, Ar^F), 133.7 (app. t, J_PC = 7, Ph), 132.5 (app. t, J_PC = 24, Ph), 130.8 (s, Ph), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 129.0 (app. t, J_PC = 5, Ph), 126.4 (s, bipy), 125.2 (q, ³J_FC = 272, Ar^F), 122.6 (s, bipy), 118.0 (sept., ³J_FC = 4, CH, Ar^F). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 47.1 (d, ¹J_RhP = 115). HR ESI-MS (positive ion, 4 kV): 785.1723 [M]⁺ (calcd 785.1716) m/z. Anal. calcd for C₇₈H₅₂BF₂₄N₂P₂Rh (1648.91 g mol⁻¹): C, 56.82; H, 3.18; N, 1.70. Found: C, 57.04; H, 2.91; N, 1.80.

4b[BAr^F₄]

Yield = 11.2 mg (34%, yellow solid).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.16 (d, ³J_HH = 5.3, 2H, bipy), 7.71–7.56 (m, 8H, Ar^F), 7.71 (obscured, 2H, bipy), 7.64 (t, ³J_HH = 7.8, 2H, bipy), 7.56 (br, 4H, Ar^F), 7.28–7.36 (m, 18H, Ph), 7.22 (t, ³J_HH = 7.4, 12H, Ph), 6.75 (ddd, ³J_HH = 7.5, 5.5, ⁴J_HH = 1.0, 2H, bipy), −19.48 (t, ²J_PH = 16.6, 2H, IrH). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 156.0 (s, bipy), 155.8 (s, bipy), 137.2 (s, bipy), 135.4 (s, Ar^F), 133.6 (app. t, J_PC = 6, Ph), 131.8 (app. t, J_PC = 27, Ph), 130.9 (s, Ph), 129.4 (qq, ²J_FC = 31, ³J_CB = 3, Ar^F), 128.9 (app. t, J_PC = 5, Ph), 127.3 (s, CH, bipy), 125.2 (q, ¹J_FC = 272, Ar^F), 123.2 (s, bipy), 118.0 (sept., ³J_FC = 4, Ar^F). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 20.1 (s). HR ESI-MS (positive ion, 4 kV): 875.2286 [M]⁺ (calcd 875.2293) m/z. Anal. calcd for C₇₈H₅₂BF₂₄IrN₂P₂ (1738.22 g mol⁻¹): C, 53.90; H, 3.02; N, 1.61. Found: C, 54.03; H, 2.85; N, 1.69.

Attempted hydrogenation of COD mediated by 4a[BAr^F₄]

A solution of 4a[BAr^F₄] (16.5 mg, 10.0 μmol) in CD₂Cl₂ (0.5 mL) was freeze–pump–thaw degassed, placed under dihydrogen (1 atm), COD (1.2 μL, 10 μmol) added under a hydrogen atmosphere, and finally heated at 50 °C for 18 h. The solution was passed through a short plug of SiO₂ (CH₂Cl₂). Analysis by GC gave a hydrocarbon distribution of: COA (0%), COE (1%), COD (99%).

Preparation of 5a

A solution of 6a (24.6 mg, 20.0 μmol) and PPh₃ (5.2 mg, 20 μmol) in CH₂Cl₂ (5 mL) was stirred for 30 min at RT. The solvent was removed in vacuo to give an orange solid, which was washed with hexane (10 mL). Yield = 12.0 mg (40%, orange powder).

¹ H NMR (500 MHz, CD₂Cl₂): δ 9.00 (d, ³J_HH = 5.1, 2H, bipy), 7.90 (t, ³J_HH = 7.4, 2H, bipy), 7.77 (d, ³J_HH = 8.1, 2H, bipy), 7.70–7.75 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 7.55 (obscured, 2H, bipy), 7.38 (t, ³J_HH = 7.3, 3H, Ph), 7.26 (br app. t, J = 8, 6H, Ph), 7.21 (t, ³J_HH = 8.4, 6H, Ph), 3.80 (s, 4H, CH=CH), 2.55–2.67 (m, 4H, CH₂), 2.03–2.15 (m, 4H, CH₂). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 154.5 (s, bipy), 151.7 (s, bipy), 139.6 (s, bipy), 135.4 (s, Ar^F), 133.9 (d, ²J_PC = 12, Ph), 131.3 (d, ¹J_PC = 28, Ph), 130.8 (s, Ph), 129.5 (qq, ³J_FC = 32, ³J_CB = 3, Ar^F), 129.1 (d, ³J_PC = 9, Ph), 127.1 (s, bipy), 125.2 (q, ¹J_FC = 272, Ar^F), 123.4 (s, bipy), 118.0 (sept., ³J_FC = 4, Ar^F), 82.8 (d, ¹J_RhC = 11, CH=CH), 31.8 (s, CH₂). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 22.31 (vbr, fwhm = 45 Hz). HR ESI-MS (positive ion, 4 kV): 367.0679 [M − PPh₃]⁺ (calcd 367.0676) m/z. Anal. calcd for C₆₈H₄₇BF₂₄N₂PRh·CH₂Cl₂ (1577.72 g mol⁻¹): C, 52.53; H, 3.13; N, 1.78. Found: C, 52.91; H, 3.17; N, 2.15.

5b

A solution of 6b (10.0 mg, 7.58 μmol) and PPh₃ (2.2 mg, 8.4 μmol) in CH₂Cl₂ (1 mL) was agitated for 10 min at RT then layered with excess hexane (ca. 20 mL) to afford the product as red crystals on diffusion. Yield = 6.7 mg (56%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 9.13 (br, 2H, bipy), 7.89 (t, ³J_HH = 7.6, 2H, bipy), 7.81 (d, ³J_HH = 7.9, 2H, bipy), 7.70–7.76 (s, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 7.50 (t, ³J_HH = 6.4, 2H, bipy), 7.38 (t, ³J_HH = 7.3, 3H, Ph), 7.25 (br app t, J = 8, 6H, Ph), 7.13 (t, ³J_HH = 8.9, 6H, Ph), 3.23 (br, 4H, CH=CH), 2.37–2.51 (m, 4H, CH₂), 1.81–1.94 (m, 4H, CH₂). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 155.6 (HMBC, bipy)152.5 (s, bipy), 138.9 (s, bipy), 135.4 (s, Ar^F), 133.6 (d, ²J_PC = 10, Ph), 131.1 (s, Ph), 130.8 (HMBC, Ph), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 129.1 (d, ³J_PC = 9, Ph), 127.7 (s, bipy), 125.2 (q, ¹J_FC = 272, Ar^F), 123.9 (s, bipy), 118.0 (sept., ³J_FC = 5, Ar^F), 65.2 (br, CH=CH), 33.0 (s, CH₂). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 10.1 (s). HR ESI-MS (positive ion, 4 kV): 719.2168 [M]⁺ (calcd 719.2163) m/z. Anal. calcd for C₆₈H₄₇BF₂₄N₂PIr (1582.10 g mol⁻¹): C, 51.62; H, 2.99; N, 1.77. Found: C, 51.43; H, 2.87; N, 1.84.

Preparation of 6

General procedure. A suspension of [M(COD)Cl]₂ (50.0 μmol), bipy (15.6 mg, 100 μmol) and Na[BAr^F₄] (88.6 mg, 100 μmol) in CH₂Cl₂ (5 mL) was stirred overnight at RT. Hexane (1 mL) was added, the solution filtered and then layered with excess hexane (ca. 45 mL) to afford the products on diffusion.

6a

Yield = 91.9 mg (75%, red solid).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.11 (app. td, ³J_HH = 8, ⁴J_HH = 1.5, 2H, bipy), 8.07 (d, ³J_HH = 8.2, 2H, bipy), 7.79 (d, ³J_HH = 5.5, 2H, bipy), 7.70–7.75 (m, 8H, Ar^F), 7.59 (ddd, ³J_HH = 7.2, 5.5, ⁴J_HH = 1.5, 2H, bipy), 7.55 (br, 4H, Ar^F), 4.57 (br, 4H, CH=CH), 2.54–2.65 (m, 4H, CH₂), 2.14–2.23 (m, 4H, CH₂). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 156.8 (s, bipy), 149.1 (s, bipy), 141.7 (s, bipy), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 128.1 (s, bipy), 125.2 (q, ¹J_FC = 272, Ar^F), 123.4 (s, bipy), 118.0 (sept., ³J_FC = 4, Ar^F), 86.5 (d, ¹J_RhC = 12, CH=CH), 30.7 (s, CH₂). HR ESI-MS (positive ion, 4 kV): 367.0677 [M]⁺ (calcd 367.0676) m/z. Anal. calcd for C₅₀H₃₂BF₂₄N₂Rh (1230.50 g mol⁻¹): C, 48.81; H, 2.62; N, 2.28. Found: C, 48.89; H, 2.52; N, 2.46.

6b

Yield = 91.5 mg (67%, dark green solid).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.20 (app. td, ³J_HH = 8, ⁴J_HH = 1.2, 2H, bipy), 8.14 (d, ³J_HH = 8.1, 2H, bipy), 8.11 (d, ³J_HH = 5.6, 2H, bipy), 7.71–7.75 (m, 8H, Ar^F), 7.69 (ddd, ³J_HH = 7.8, 5.6, ⁴J_HH = 1.0, 2H, bipy), 7.56 (s, 4H, Ar^F), 4.40 (br, 4H, CH=CH), 2.36–2.48 (m, 4H, CH₂), 2.00–2.12 (m, 4H, CH₂). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 158.4 (s, bipy), 149.6 (s, bipy), 142.6 (s, bipy), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 128.9 (s, bipy), 125.2 (q, ¹J_FC = 272, Ar^F), 123.7 (s, bipy), 118.0 (sept., ³J_FC = 4, Ar^F), 72.2 (s, CH=CH), 31.5 (s, CH₂). HR ESI-MS (positive ion, 4 kV): 457.1250 [M]⁺ (calcd 457.1251) m/z. Anal. calcd for C₅₀H₃₂BF₂₄IrN₂ (1319.81 g mol⁻¹): C, 45.50; H, 2.44; N, 2.12. Found: C, 45.83; H, 2.56; N, 2.19.

NMR scale reactions of 6

Reaction between 6a and PPh₃. In a J. Young's NMR tube, 6a (12.3 mg, 10.0 μmol) and PPh₃ (5.2 mg, 10 μmol) was dissolved in CD₂Cl₂ (0.5 mL) resulting in the immediate formation of fast exchanging mixture of 5a and free PPh₃, which was analysed by VT-NMR spectroscopy.

³¹P{¹H} NMR (202 MHz, CD₂Cl₂): δ 10.6 (vbr, fwhm = 55 Hz).³¹P{¹H} NMR (202 MHz, CD₂Cl₂, 200 K): δ 33.3 (d, ¹J_RhP = 129, 5a), −8.3 (s, PPh₃).

Reactions between 6 and H₂. A solution of 6 (10.0 μmol) in CD₂Cl₂ (0.5 mL) within a J. Young's NMR tube was freeze–pump–thaw degassed three times then placed under dihydrogen (1 atm). The tube was sealed, inverted several times then left to stand at ambient temperature for 24 h. No significant reaction was observed by ¹H NMR spectroscopy for 6a. In the case of 6b, the presence of small amount of a dihydride complex can be detected initially (<15%), however, this species does not grow in further over 48 h (or even extended heating at 50 °C). Consistent with an unfavourable equilibrium reaction with dihydrogen, the only species observed by ¹H NMR spectroscopy after freeze–pump–thaw degassing the solution is 6b.

Preparation of 7

A solution of 8 (14.9 mg, 5.00 μmol) and bipy (1.6 mg, 10 μmol) in CH₂Cl₂ (1 mL) was agitated for 10 min at RT. Addition of excess hexane (ca. 20 mL) afforded the product as an orange-red oil that solidified on extended drying in vacuo. Yield = 2.4 mg (15%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.97 (d, ³J_HH = 8.1, 2H, bipy), 7.89 (br d, ³J_HH = 5, 2H, bipy), 7.82 (app. t, ³J_HH = 8, 2H, bipy), 7.71–7.75 (m, 8H, Ar^F), 7.62–7.68 (m, 12H, Ph), 7.56 (br, 4H, Ar^F), 7.30 (t, ³J_HH = 7.4, 6H, Ph), 7.15 (t, ³J_HH = 7.5, 12H, Ph), 6.82 (dd, ³J_HH = 7.5, 5.8, 2H, bipy). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 156.5 (s, bipy), 153.8 (s, bipy), 138.6 (s, bipy), 135.4 (s, Ar^F), 135.2 (app. t, J_PC = 6, Ph), 133.0–134.1 (m, Ph), 130.8 (s, Ph), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 128.6 (app. t, J_PC = 5, Ph), 126.3 (s, bipy), 125.2 (q, ¹J_CF = 272, Ar^F), 122.5 (s, bipy), 118.0 (sept., ³J_FC = 4, Ar^F). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 46.6 (d, ¹J_RhP = 182). HR ESI-MS (positive ion, 4 kV): 815.1458 [M + MeOH]⁺ (calcd 815.1458) m/z.

Hydrogenation of 7

A solution of 7 (16.5 mg, 10.0 μmol) in CD₂Cl₂ (0.5 mL) within a J. Young's NMR tube was freeze–pump–thaw degassed three times and placed under dihydrogen (1 atm) resulting in quantitative formation of 4a[BAr^F₄] within 5 min as gauged by ¹H and ³¹P NMR spectroscopy.

Preparation of 8

A solution of 2a (64.0 mg, 40.0 μmol) in CH₂Cl₂ (5 mL) was freeze–pump–thaw degassed three times and placed under dihydrogen (1 atm). After stirring for 5 h the solution was freeze–pump–thaw degassed three times and placed under argon. The solution was layered with excess hexane (ca. 45 mL) to afford the product as a dark orange crystalline solid on diffusion. Yield = 33.7 mg (57%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.72–7.77 (m, 16H, Ar^F), 7.56 (s, 8H, Ar^F), 7.43–7.50 (m, 4H, Ph), 7.32–7.40 (m, 6H, Ph), 7.15–7.26 (m, 40H, Ph), 6.87 (t, ³J_HH = 6.9, 4H, η-Ph), 6.37 (t, ³J_HH = 6.4, 4H, η-Ph), 5.51 (t, ³J_HH = 6.5, 2H, η-Ph). ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 162.3 (q, ¹J_CB = 50, Ar^F), 135.4 (s, Ar^F), 135.1 (d, ²J_PC = 12, Ph), 134.1 (d, ²J_PC = 11, Ph), 132.9 (s, Ph), 132.1 (d, ⁴J_PC = 3, Ph), 129.5 (d, ³J_PC = 8, C, Ph), 129.4 (qq, ²J_FC = 32, ³J_CB = 3, Ar^F), 129.2 (d, ³J_PC = 11, Ph), 129 (obscured, Ph), 125.2 (q, ¹J_CF = 272, Ar^F), 123.1 (dd, ¹J_PC = 33, ²J_RhC = 7, η-Ph), 118.0 (sept., ³J_FC = 4, Ar^F), 106.5 (br d, ²J_PC = 10, η-Ph), 105.4 (br, η-Ph), 102.4–102.6 (m, η-Ph). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 45.9 (ddd, ¹J_RhP = 217, ²J_PP = 38, ²J_RhP = 5), 43.2 (dd, ¹J_RhP = 198, ²J_PP = 38). HR ESI-MS (positive ion, 4 kV): 627.0869 [½M]⁺ (calcd 627.0872) m/z.

Reaction of 8 with bipy

In a J. Young's NMR tube, 8 (14.9 mg, 5.0 μmol) and bipy (1.6 mg, 10.0 μmol) was dissolved in CD₂Cl₂ (0.5 mL) resulting in quantitative formation of 7 within 5 minutes as gauged by ¹H and ³¹P NMR spectroscopy.

Preparation of 5-phthalimidomethylbipyridine

A stirred solution of 5-chloromethylbipyridine (3.00 g, 14.7 mmol) and potassium phthalimide (4.07 g, 21.9 mmol) in DMF (70 mL) under N₂ was heated at 40 °C for 18 h. The solvent was removed in vacuo and the mixture dissolved in CH₂Cl₂ (100 mL) and washed with H₂O (2 × 100 mL), brine (100 mL), and then dried over MgSO₄. The solvent was removed in vacuo and the product recrystallised from hot EtOH (ca. 450 mL). Yield = 4.19 g (94%, white crystals).

¹ H NMR (400 MHz, CDCl₃): δ 8.76 (dd, ⁴J_HH = 2.3, ⁵J_HH = 0.8, 1H, bipy), 8.65 (ddd, ³J_HH = 4.8, ³J_HH = 1.8, ⁵J_HH = 0.9, 1H, bipy), 8.36 (app. dt, ³J_HH = 7.9, J = 1, bipy), 8.35 (dd, ³J_HH = 8.2, ⁵J_HH = 0.8, 1H, bipy), 7.88 (dd, ³J_HH = 8.2, ⁴J_HH = 2.3, 1H, bipy), 7.83–7.88 (m, 2H, Phth), 7.79 (app. td, ³J_HH = 8, ⁴J_HH = 1.8, 1H, bipy), 7.69–7.74 (m, 2H, Phth), 7.28 (ddd, ³J_HH = 7.6, 4.8, ⁴J_HH = 1.2, 1H, bipy), 4.91 (s, 2H, CH₂). ¹³C{¹H} NMR (101 MHz, CDCl₃): δ 168.0, 155.9, 155.9, 149.6, 149.3, 137.5, 137.0, 134.3, 132.1, 132.1, 123.9, 123.6, 121.3, 121.1, 39.1. HR ESI-MS (positive ion, 4 kV): 338.0903 [M + Na]⁺ (calcd 338.0900) m/z.

Preparation of 5-aminomethylbipyridine

A solution of 5-phthalimidomethylbipyridine (1.50 g, 4.76 mmol) and hydrazine monohydrate (2.31 mL, 47.6 mmol) in EtOH (100 mL) under N₂ was heated at reflux for 18 h. An aqueous solution of NaOH (2 M, 50 mL) was added and the mixture extracted with CH₂Cl₂ (3 × 100 mL). The organic phase was dried over MgSO₄ and the solvent removed in vacuo to give the compound as a waxy off-white solid. Yield = 0.794 g (90%).

¹ H NMR (400 MHz, (CD₃)₂SO): δ 8.67 (ddd, ³J_HH = 4.8, ⁴J_HH = 1.9, ⁵J_HH = 0.9, 1H, bipy), 8.63 (d, ⁴J_HH = 2.3, 1H, bipy), 8.37 (app. dt, ³J_HH = 8.0, J = 1, 1H, bipy), 8.33 (d, ³J_HH = 8.1, 1H, bipy), 7.92 (app. td, ³J_HH = 8, ⁴J_HH = 1.9, 1H, bipy), 7.90 (dd, ³J_HH = 8.1, ⁴J_HH = 2.3, 1H, bipy), 7.42 (ddd, ³J_HH = 7.5, 4.8, ⁴J_HH = 1.2, 1H, bipy), 3.82 (s, 2H, CH₂). ¹³C{¹H} NMR (101 MHz, (CD₃)₂SO): δ 155.3, 153.6, 149.2, 148.4, 139.3, 137.2, 136.0, 123.9, 120.2, 119.9, 42.8. HR ESI-MS (positive ion, 4 kV): 208.0846 [M + Na]⁺ (calcd 208.0845) m/z.

Preparation of 10

A solution of 5-aminomethylbipyridine (730 mg, 3.94 mmol) and 3,5-di-tert-butylbenzaldehyde (860 mg, 3.94 mmol) in MeOH (30 mL) was stirred at RT for 18 h over 3 Å molecular sieves under N₂. The resulting mixture was cooled to 0 °C and NaBH₄ (298 mg, 7.88 mmol) added in portions. The mixture was warmed to ambient temperature and stirred for 30 min, and then heated at reflux for 2 h. The solvent was removed in vacuo and the crude extracted with CH₂Cl₂ (30 mL). The organic phase was washed with H₂O (3 × 30 mL) and dried over MgSO₄. The solvent was removed in vacuo and the product purified by column chromatography (SiO₂, CH₂Cl₂/MeOH 95 : 5) to give a colourless oil. The product was freeze-dried in vacuo to give an off-white solid. Yield = 1.185 g (78%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 8.63–8.67 (obscured, 1H, bipy), 8.65 (br, 1H, bipy), 8.44 (d, ³J_HH = 8.0, 1H, bipy), 8.41 (d, ³J_HH = 8.2, 1H, bipy), 7.85 (dd, ³J_HH = 8.2, ⁴J_HH = 2.2, 1H, bipy), 7.82 (app. td, ³J_HH = 8, ⁴J_HH = 1.8, 1H, bipy), 7.35 (t, ⁴J_HH = 1.9, 1H, C₆H₃), 7.31 (ddd, ³J_HH = 7.5, 4.8, ⁴J_HH = 1.2, 1H, bipy), 7.22 (d, ⁴J_HH = 1.9, 2H, C₆H₃), 3.90 (s, 2H, CH₂), 3.82 (s, 2H, CH₂), 1.35 (s, 18H, ^tBu). The NH resonance was not unambiguously located. ¹³C{¹H} NMR (126 MHz, CD₂Cl₂): δ 156.7, 155.4, 151.4, 149.7, 149.7, 140.0, 137.3, 137.2, 136.9, 124.1, 122.9, 121.5, 121.2, 121.0, 54.4, 51.0, 35.3, 31.8. HR ESI-MS (positive ion, 4 kV): 388.2747 [M + H]⁺ (calcd 388.2747) m/z.

Crystallography

Full details about the collection, solution and refinement are documented in the CIF, which have been deposited with the Cambridge Crystallographic Data Centre under CCDC 1563158 (1b), 1563159 (4a[BAr^F₄]), 1563160 (4b[BAr^F₄]; Z′ = 1), 1563161 (4b[BAr^F₄]; Z′ = 2), 1563162 (5a), 1563163 (5b) and 1563164 (8).†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the EPSRC (J. E.-K.), Leverhulme Trust (RPG-2012-718, R. C. K), ERC (M. R. G., grant agreement 637313), and Royal Society (A. B. C.) for financial support. Crystallographic data for 4a[BAr^F₄] and high-resolution mass-spectrometry data were collected using instruments purchased through support from Advantage West Midlands and the European Regional Development Fund. Crystallographic data for 1b, 4b[BAr^F₄], 5, and 8 were collected using an instrument that received funding from the ERC under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 637313).

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Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C{¹H} and ³¹P{¹H} NMR and ESI-MS spectra of new compounds and selected reactions. CCDC 1563158–1563164. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7dt02648j

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