Synthesis and group 9 complexes of macrocyclic PCP and POCOP pincer ligands

Abstract

The synthesis of macrocyclic variants of commonly employed phosphine-based pincer (pro)ligands derived from meta-xylene (PCP-14) and resorcinol (POCOP-14) is described, where the P-donors are trans-substituted with a tetradecamethylene linker. The former was accomplished using a seven-step asymmetric procedure involving (−)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis. A related, but non-diastereoselective route was employed for the latter, which consequently necessitated chromatographic separation from the cis-substituted by-product. The proligands are readily metalated and homologous series of M^I(CO) and M^IIICl₂(CO) derivatives (M = Rh, Ir) have been isolated and fully characterised in solution and the solid state. Metal hydride complexes are generated during the synthesis of the former and have been characterised in situ using NMR spectroscopy.

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Introduction

Conferring thermal stability whilst permitting a broad range of metal-based reactivity, the application of mer-tridentate “pincer” ligands in organometallic chemistry and homogeneous catalysis has had a profound impact.¹ Phosphine-based examples bearing a central aryl donor were first reported by Shaw in the late 1970s,² but these archetypical PCP ligands continue to attract contemporary interest, with iridium derivatives noteworthy for their ability to catalyse the dehydrogenation of alkanes.³ The predictable and modular composition of pincer ligands enables the steric and electronic properties of metal derivatives to be tuned through changes to the constituent donor groups, their substituents or the backbone configuration itself, and these adaptions have been extensively explored over the intervening four decades.⁴ Motivated by the potential to exploit additional reaction control though their unique steric profile and their use in the construction of interlocked assemblies, we have become interested in exploring the chemistry of macrocyclic pincer ligands, with our initial efforts devoted to NHC-based CNC ligands.⁵ We herein describe the synthesis and coordination chemistry of meta-xylene- and resorcinol-derived macrocyclic (pro)ligands PCP-14 and POCOP-14, where the chiral P-donors are trans-substituted with a tetradecamethylene linker (Chart 1). The preparation of analogous lutidine- and 2,6-dihydroxypyridine-derived PNP-14 and PONOP-14 ligands was described in a preceding contribution.⁶

Chart 1

Results and discussion

Proligand synthesis

Circumventing problems associated with the generation of the unwanted cis-substituted diastereoisomer, the proligand PCP-14′ was prepared using an adapted asymmetric procedure developed by Riera and Verdaguer that employs (−)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis (Scheme 1).⁷ The known oxazaphospholidine borane 1 (δ_31P 167.9) was first stereoselectively ring-opened by treatment with octen-7-yl-magnesium bromide, affording 2 (δ_31P 76.5). The auxiliary was then removed by acidolysis using an excess of H₂SO₄ in 3 : 1 MeOH–water to generate phosphinous acid borane 3 (δ_31P 125.6), which was subsequently converted into phosphine borane 4 (δ_31P 22.2, ¹J_PH = 352 Hz) by reduction of the mesylate with tetrabutylammonium borohydride and isolated in 65% overall yield from 1. Isolated 4 is unstable at room temperature, presumably due to a propensity to undergo hydrophosphination,⁸ and is best converted directly into the corresponding phosphide by deprotonation with nBuLi at −78 °C. Reaction thereafter with 1,3-bis(bromomethyl)benzene afforded 5 (δ_31P 32.3), which was cyclised by portion-wise treatment with Grubbs’ 1^st generation catalyst (totalling 10 mol%) under dilute conditions in CH₂Cl₂ (5 mmol L⁻¹) to afford the corresponding unsaturated macrocycle 6 (δ_31P 31.3–33.4). Subsequent hydrogenation using Wilkinson's catalyst (producing PCP-14′·2BH₃7, δ_31P 32.5) and deprotection by borane transfer in neat diethylamine afforded the proligand (δ_31P 3.2) as the (R,R)-diastereomer with a respectable overall yield of 48% and >95% ee, as determined by ¹H and ³¹P NMR spectroscopic analysis of the oxide derivative using the chiral shift reagent (−)-3,5-dinitro-N-(1-phenylethyl)benzamide (see ESI†).⁹

Scheme 1 Asymmetric synthesis of PCP-14′.

Attempts to develop an asymmetric procedure for the preparation of the proligand POCOP-14′ starting from enantiopure 3, involving reaction of the mesylate with disodium resorcinolate or nucleophilic aromatic substitution of 1,3-difluorobenzene by the conjugate base, were unsuccessful (see ESI† for details). Likewise, attempted conversion of 2 to the corresponding borane protected chlorodialkylphosphine by acidolysis with HCl was unsuccessful. Instead the five step racemic synthesis outlined in Scheme 2, adapted from our work with PNP-14 and PONOP-14 and also involving ring-closing olefin metathesis,⁶ was employed with the borane protected (saturated) macrocycle 12 proving to be the most conducive to resolution of the two diastereoisomers by column chromatography (cis-12, δ_31P 143.7; trans-12, δ_31P 143.8) and enabling determination of their respective configurations by single crystal X-ray diffraction (Fig. 1). In this way POCOP-14′ was obtained as an analytically pure racemate in 30% overall yield (δ_31P 141.6). Use of the chiral shift reagent (−)-3,5-dinitro-N-(1-phenylethyl)benzamide enabled the component phosphine oxide enantiomers to be resolved by ³¹P NMR spectroscopy (see ESI†).⁹

Scheme 2 Preparation of POCOP-14′.

Fig. 1 Solid-state structures of cis-12 (left) and trans-12 (right; not unique, Z′ = 2). Thermal ellipsoids drawn at 30% and 50% probability, respectively; minor disordered components omitted (cis-12; tBu groups and methylene chain). Selected bond lengths (Å): cis-12, P2–B2, 1.915(8); P3–B3, 1.896(7); P2–O107, 1.617(4); P3–O108, 1.630(4); trans-12, B2–P2, 1.908(2); B3–P3, 1.905(2); P2–O107, 1.6323(9); P3–O108, 1.6270(9); B12–P12, 1.901(2); B13–P13, 1.904(2); P12–O207, 1.6284(10); P13–O208, 1.6274(10).

Rhodium complexes

With the new macrocyclic proligands in hand, the synthesis of rhodium derivatives was targeted (Scheme 3). Using an adapted literature procedure,¹⁰ rhodium dihydrogen complexes [Rh(pincer)(H₂)] (pincer = PCP-14, 13a; POCOP-14, 13b) were prepared in quantitative spectroscopic yield using a one-pot procedure involving metalation of the proligands with [Rh(COE)₂Cl]₂ (COE = cyclooctene) in toluene at elevated temperature and subsequent treatment with KOtBu at 120 °C under an atmosphere of dihydrogen. Complexes 13a/b were characterised in situ and are notable for the adoption of C₂ symmetry, ³¹P resonances at δ 70.4 (¹J_RhP = 153 Hz)/198.4 (¹J_RhP = 165 Hz) displaying large coupling to ¹⁰³Rh, and broad 2H hydride signals at δ −4.36/−2.87 (298 K), which exhibit fast spin–lattice relaxation (T₁ = 74 ± 4/42 ± 2 ms at 298 K; T₁ = 117 ± 15/89 ± 11 ms at 200 K; 600 MHz, Ar) consistent with the assignment as dihydrogen complexes.^11,12 The formulation of 13a/b as σ-complexes is further substantiated by their instability to vacuum and onward reactivity with CO (1 atm), conferring C₂ symmetric Rh(I) carbonyl complexes [Rh(pincer)(CO)] (pincer = PCP-14, 14a; POCOP-14, 14b), which are characterised by ³¹P resonances at δ 75.0 (¹J_RhP = 146 Hz)/201.6 (¹J_RhP = 156 Hz) with ¹J_RhP coupling constants of similar magnitude. Complexes 14a/b were thereafter oxidised to the corresponding C₂ symmetric Rh(III) carbonyl complexes 15a/b by reaction with PhICl₂, as evidenced by ³¹P resonances at δ 64.9 (¹J_RhP = 87 Hz)/181.0 (¹J_RhP = 92 Hz) with the expected reductions in ¹J_RhP values, isolated as analytically pure materials in 80/48% overall yield, following purification by silica chromatography (CH₂Cl₂–pentane), and fully characterised (Fig. 2, Table 1).¹³ Analytically pure samples of very alkane soluble 14a/b were secured from the reaction of 15a/b with 3 equivalents iPrMgCl·LiCl in THF and subsequent recrystallisation from SiMe₄ in 85/94% yield; as confirmed by combustion analysis.

Scheme 3 Synthesis of rhodium PCP-14 and POCOP-14 complexes 13–15.

Fig. 2 Solid-state structures of 14 and 15. Thermal ellipsoids drawn at 50% probability; minor disordered components omitted (15a/b; methylene chains). Selected metrics provided in Table 1.

Table 1 Selected bond lengths (Å) and angles (°) for 14, 15, 18 and 19

	M = Rh				M = Ir
	14a	14b	15a	15b	18a	18b	19a	19b
a Angle between the least-squares mean planes of the aryl group and the MP2C(aryl) atoms.
M1–P2	2.2844(6)	2.2704(15)	2.3615(7)	2.3324(12)	2.2817(7)	2.2694(11)	2.3637(9)	2.336(2)
M1–P3	2.2731(6)	2.2856(13)	2.3938(6)	2.3768(11)	2.2760(7)	2.2798(10)	2.3922(8)	2.377(2)
M1–C4	1.865(3)	1.879(6)	2.005(3)	1.996(6)	1.862(3)	1.879(4)	1.940(3)	1.926(9)
M1–C101	2.083(3)	2.037(6)	2.061(3)	2.018(5)	2.085(3)	2.048(4)	2.089(3)	2.034(10)
M1–Cl6	—	—	2.3656(5)	2.3616(12)	—	—	2.3828(7)	2.380(2)
M1–Cl7	—	—	2.3591(6)	2.3521(12)	—	—	2.3793(8)	2.371(2)
P2–M1–P3	160.83(2)	155.98(5)	162.17(2)	156.80(5)	161.16(2)	156.21(3)	160.81(3)	156.18(9)
C101–M1–C4	171.38(10)	165.3(3)	177.22(11)	174.6(2)	171.91(12)	165.6(2)	177.24(14)	174.9(4)
Cl6–M1–Cl7	—	—	176.92(2)	176.32(5)	—	—	178.81(3)	177.37(9)
Aryl twist^a	14.30(6)	6.7(2)	18.52(7)	12.34(13)	13.95(8)	6.49(14)	17.93(9)	11.3(3)

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The solid-state structures of 14a/b have been determined by single crystal X-ray diffraction and demonstrate the adoption of distorted square planar metal coordination geometries, with the tetradecamethylene linker of the pincer ligands considerably skewed to one side of the coordination plane conferring overall C₁ symmetry and inducing an appreciable deviation of the C101–Rh1–C4 angles from linearity, especially in the case of the phosphinite pincer (171.38(10)°, 14a; 165.3(3)°, 14b). The methylene chains of the macrocycles are also non-symmetrically positioned in the Rh(III) congeners, but in this case contortion is counteracted by buttressing with the ancillary chloride ligands and more ideal C101–Rh1–C4 angles are observed (177.22(11)°, 15a; 174.6(2)°, 15b). These asymmetric configurations are not retained in solution, where time averaged C₂ symmetry is observed at 298 K (500–600 MHz) consistent with the tetradecamethylene linker being sufficiently large and flexible to accommodate the carbonyl ligand within the annulus of the macrocycle. The more obtuse P2–Rh1–P3 bite angles and rigid backbone conformations observed in the complexes of POCOP-14 in the solid state, compared to those of PCP-14, are fully in line with expectations for these pincer architectures.¹⁴ This homologous series of complexes showcases the effect of increased steric crowding on the latter, where C₂ symmetric twisting of the central aryl donor is considerably more pronounced in the octahedral Rh(III) congeners. Finally, the observed differences in Rh–P, Rh–CO, and Rh–aryl contacts for the Rh(I) and Rh(III) congeners are consistent with the nature of associated donors (vide infra for further discussion on the carbonyl ligand).

Iridium complexes

In a similar manner to that employed for the rhodium analogues, iridium tetrahydride complexes [Ir(pincer)H₄] (pincer = PCP-14, 16a; POCOP-14, 16b) were prepared using a one-pot procedure involving metalation of the macrocyclic proligands with [Ir(COE)₂Cl]₂ (16a) or [Ir(COD)Cl]₂ (16b, COD = cyclooctadiene) in toluene at elevated temperature and subsequent treatment with KOtBu at 120 °C under an atmosphere of dihydrogen (Scheme 4). Complexes 16a/b were characterised in situ and are notable for the adoption of C₂ symmetry, ³¹P resonances at δ 49.9/165.0, and relatively sharp triplet 4H hydride signals at δ −8.99 (²J_PH = 9.8 Hz)/−8.26 (²J_PH = 9.9 Hz) at 298 K that only broaden slightly on cooling to 200 K, with moderate spin–lattice relaxation (T₁ = 300 ± 10/194 ± 4 ms at 298 K; T₁ = 626 ± 15/242 ± 15 ms at 200 K; 600 MHz, Ar). These data are consistent with fluxional hydride ligands, where dynamic interconversion between classical tetrahydride and dihydride dihydrogen formulations occurs on the NMR timescale.^11,15 Evidencing the presence of the latter, reaction with CO (1 atm) resulted in immediate substitution of one equivalent of dihydrogen and formation of cis-17a/b [δ_1H(hydride) −10.66, −11.59; δ_31P 51.0, 45.8 (²J_PP = 286 Hz)/δ_1H(hydride) −9.82, −10.75; δ_31P 160.4, 154.6 (²J_PP = 305 Hz)]. Prolonged thermolysis at 120 °C under CO (1 atm) in the presence of tert-butylethylene (TBE) followed by removal of CO resulted in the formation of C₂ symmetric Ir(I) carbonyl complexes [Ir(pincer)(CO)] (pincer = PCP-14, δ_31P 68.1, 18a; POCOP-14, δ_31P 186.7, 18b). Transient formation of trans-17a/b [δ_1H(hydride) −9.65; δ_31P 54.1/δ_1H(hydride) −9.76; δ_31P 167.3] and generation of [Ir(pincer)(CO)₂] (pincer = PCP-14, δ_31P 51.6; POCOP-14, δ_31P 164.8) were noted during this transformation. Complexes 18a/b were thereafter oxidised to the corresponding C₂ symmetric Ir(III) carbonyl complexes 19a/b (δ_31P 33.8/144.4) by reaction with PhICl₂ and isolated as analytically pure and air-stable materials in 51/73% overall yield, following purification by silica chromatography (CH₂Cl₂–hexane), and fully characterised. Analytically pure samples of very alkane soluble 18a/b were secured from the reaction of 19a/b with 10 equivalents of KC₈ in pentane and subsequent recrystallisation from neohexane/SiMe₄ in 84/79% yield; as confirmed by combustion analysis.¹⁶

Scheme 4 Synthesis of iridium PCP-14 and POCOP-14 complexes 16–19.

The solid-state structures of 18a/b and 19a/b have all been determined by single crystal X-ray diffraction and are isomorphous to the corresponding rhodium congeners 14a/b and 15a/b, respectively (Table 1).

Carbonyl stretching frequencies

The ν(CO) bands of metal carbonyl derivatives are established reporter groups for donor properties of ligands.¹⁷ In the case of pincer ligands, group 9 examples have an established track record in the literature.¹⁸ The carbonyl stretching frequencies of 14, 15, 18 and 19 and those of known acyclic pincer complexes [M(pincer)(CO)] (M = Rh, Ir; pincer = 2,6-(tBu₂PCH₂)₂C₆H₃=PCP-tBu, 2,6-(tBu₂PO)₂C₆H₃=POCOP-tBu) have all been determined by IR spectroscopy under the same conditions and compiled in Table 2. These data suggest that PCP-14 and POCOP-14 are marginally weaker donors than PCP-tBu and POCOP-tBu, respectively, consistent with similar findings for the neutral PNP and PONOP analogues and attributed to changes in the phosphine/phosphinite substituents alone.⁶ Other trends apparent in the IR data associated with the extent of π-backbonding increasing in the order Ir > Rh, M(I) > M(III) and PCP > POCOP are fully in line with expectation and reinforced by similar trends in the X-ray derived metrics (Table 1) and NMR data (see Experimental section).

Table 2 Carbonyl stretching frequencies for rhodium and iridium complexes of macrocyclic PCP-14 and POCOP-14, and acyclic PCP-tBu and POCOP-tBu (toluene, cm⁻¹)

	Macrocyclic	Acyclic
[Rh(PCP)(CO)]	1939	1933
[Rh(POCOP)(CO)]	1958	1954
[Rh(PCP)Cl2(CO)]	2069	—
[Rh(POCOP)Cl2(CO)]	2083	—
[Ir(PCP)(CO)]	1925	1918
[Ir(POCOP)(CO)]	1943	1939
[Ir(PCP)Cl2(CO)]	2034	—
[Ir(POCOP)Cl2(CO)]	2049	—

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Conclusions

Procedures that enable the preparation of two phosphine-based macrocyclic pincer (pro)ligands PCP-14 and POCOP-14, where the chiral P-donors are trans-substituted with a tetradecamethylene linker, have been developed. The synthesis of meta-xylene-based proligand PCP-14′ was achieved using a seven-step asymmetric protocol involving (−)-cis-1-amino-2-indanol as a chiral auxiliary and ring-closing olefin metathesis, affording the (R,R)-diastereomer in 48% overall yield and >95% ee. Whilst we were unable to apply this methodology to the synthesis of resorcinol-based POCOP-14′, this proligand was obtained as analytically pure racemate in 30% overall yield using a non-diastereoselective route, where separation from the unwanted cis-substituted by-product was achieved by chromatography. Rhodium and iridium complexes of these new macrocyclic pincer ligands have been obtained through metalation of the proligands with dimeric rhodium(I) and iridum(I) halide precursors and subsequent dehydrohalogenation under a dihydrogen atmosphere. The resulting hydride complexes were characterised in situ by NMR spectroscopy and reacted on to generate isolatable M^I(CO) and M^IIICl₂(CO) derivatives that have been extensively characterised in solution and the solid state, including all eight complexes by single crystal X-ray diffraction. By comparison of the ν(CO) bands of the rhodium(I) and iridum(I) carbonyl adducts, determined by IR spectroscopy in toluene, PCP-14 and POCOP-14 can be considered to be marginally weaker net donors than their respective homoleptic tert-butyl substituted congeners PCP-tBu and POCOP-tBu, respectively. Work exploring the organometallic chemistry of these novel macrocyclic pincer ligands is ongoing in our laboratories and will be reported in due course.

Experimental

General methods

All manipulations were performed under an atmosphere of argon using Schlenk and glove box techniques unless otherwise stated. Hydrogen was dried by passage through a stainless-steel column of activated 3 Å molecular sieves. Carbon monoxide was used as received. Glassware was oven-dried at 150 °C overnight and flame-dried under vacuum prior to use. Molecular sieves were activated by heating at 300 °C in vacuo overnight. CD₂Cl₂ was freeze–pump–thaw degassed and dried over 3 Å molecular sieves. Toluene-d₈ and C₆D₆ were dried over sodium overnight, distilled, freeze–pump–thaw degassed and stored over 3 Å molecular sieves. THF was distilled from sodium and benzophenone and stored over 3 Å molecular sieves. Et₂NH was distilled from CaH₂. tert-Butylethylene was freeze–pump–thaw degassed and stored over 3 Å molecular sieves. SiMe₄ was distilled from liquid Na/K alloy and stored over a potassium mirror. Neohexane (2,2-dimethylbutane) was distilled from liquid Na/K alloy and stored over 3 Å molecular sieves. Other anhydrous solvents were purchased from Acros Organics or Sigma-Aldrich, freeze–pump–thaw degassed and stored over 3 Å molecular sieves. The concentration of nBuLi was titrated by ¹H NMR spectroscopy before use.¹⁹ Oxazaphospholidine 1,⁷ MgBr(C₈H₁₅),²⁰8,⁶ PhICl₂,²¹ [Rh(PPh₃)₃Cl],²² [Rh(COE)₂Cl]₂,²³ [Ir(COD)Cl]₂,²⁴ [Ir(COE)₂Cl]₂,²⁴ [M(pincer)(CO)] (M = Rh, Ir; pincer = PCP-tBu, POCOP-tBu)^2,14d,25 were synthesised according to published procedures. NMR spectra were recorded on Bruker spectrometers under argon at 298 K unless otherwise stated. Chemical shifts are quoted in ppm and coupling constants in Hz.²⁶ NMR spectra in toluene-d₀ were recorded using an internal capillary of C₆D₆. HR ESI-MS were recorded on a Bruker MaXis mass spectrometer. IR spectra were recorded on a Bruker Alpha Platinum ATR FT-IR spectrometer at RT (for full details see ESI†). Microanalyses were performed at the London Metropolitan University by Stephen Boyer in all cases, except for samples of 14a/b and 18a/b, which were analysed by Elemental Microanalysis Ltd.

Synthesis of PCP-14′

Preparation of 2. A solution of MgBr(C₈H₁₅) in THF (0.37 M, 45 mL, 17 mmol) was added dropwise in three equal portions separated by 30 minutes to a solution of 1 (1.39 g, 5.57 mmol) in toluene at 80 °C. The solution was concentrated under reduced pressure and the residue re-dissolved in a minimal amount of toluene (20 mL) and heated at reflux for 3 days. The reaction mixture was cooled to 0 °C, exposed to air and quenched with saturated aqueous NH₄Cl (10 mL). The aqueous phase was extracted with EtOAc (3 × 20 mL) and the combined organic fractions dried (MgSO₄), filtered, and concentrated under reduced pressure to give a pale-yellow oil. Purification by flash column chromatography (silica, 8% EtOAc in hexane, R_f = 0.15) afforded 2 as a colourless oil. Yield: 1.92 g (95%).

¹ H NMR (500 MHz, CDCl₃): δ 7.35–7.42 (m, 1H, Ar), 7.23–7.28 (m, 3H, Ar), 5.81 (ddt, ³J_HH = 16.9, 10.1, 6.5, 1H, CH̲=CH₂), 5.01 (app dq, ³J_HH = 16.9, J_HH = 2, 1H, CH=CH̲₂), 4.95 (ddt, ³J_HH = 10.1, ²J_HH = 2.2, ⁴J_HH = 1.2, 1H, CH=CH̲₂), 4.77 (app td, J = 10, ³J_HH = 4.5, 1H, NCH̲), 4.51 (app q, ³J_HH = 5, 1H, OCH̲), 3.10 (dd, ²J_HH = 16.6, ³J_HH = 4.7, 1H, OCHCH̲₂), 2.90 (d, ²J_HH = 16.6, 1H, OCHCH̲₂), 2.11 (d, ³J_HH = 10.0, NH), 2.05 (app q, ³J_HH = 7, 2H, CH̲₂CH=CH₂), 1.81 (d, ³J_HH = 4.4, 1H, OH), 1.56–1.83 (m, 4H, CH₂), 1.30–1.49 (m, 6H, CH₂), 1.26 (d, ³J_PH = 13.6, 9H, tBu), 0.55 (partially collapsed quartet, fwhm = 310 Hz, 3H, BH₃).

¹³ C{ ¹ H} NMR (126 MHz, CDCl₃): δ 142.9 (d, ³J_PC = 7, Ar{C}), 139.9 (s, Ar{C}), 139.1 (s, C̲H=CH₂), 128.1 (s, Ar), 127.1 (s, Ar), 125.6 (s, Ar), 124.3 (s, Ar), 114.5 (s, CH=C̲H₂), 74.8 (s, OCH), 62.3 (d, ²J_PC = 3, NCH), 39.4 (s, OCHC̲H₂), 33.8 (s, C̲H₂CH=CH₂), 31.9 (d, ¹J_PC = 39, tBu{C}), 31.5 (d, ²J_PC = 13, CH₂), 28.9 (s, CH₂), 28.8 (s, CH₂), 25.1 (d, ²J_PC = 2, tBu{CH₃}), 24.1 (d, ¹J_PC = 36, PCH₂), 23.0 (s, CH₂).

³¹ P{ ¹ H} NMR (121 MHz, CDCl₃): δ 76.5 (partially collapsed quartet, fwhm = 200 Hz).

HR ESI-MS (positive ion, 4 kV): 384.2603, [M + Na]⁺ (calcd 384.2602) m/z.

Preparation of 3. H₂SO₄ (98%, 5.4 mL, 99 mmol) was added dropwise to a solution of 2 (4.52 g, 12.5 mmol) in a MeOH–water mixture (140 mL : 45 mL). The reaction mixture was heated at 70 °C for 4 days, with periodic monitoring by TLC. The mixture was allowed to cool to RT, exposed to air, and CH₂Cl₂ (40 mL) and saturated aqueous NaCl (20 mL) added. The aqueous phase was extracted with CH₂Cl₂ (3 × 15 mL) and the combined organic extracts dried (MgSO₄), filtered, and concentrated under reduced pressure to give a colourless opaque oil. The crude material was dissolved in a minimal amount of CH₂Cl₂ and run through a short silica plug, eluting with CH₂Cl₂ (R_f = 0.6). The volatiles were removed under reduced pressure to afford 3 as a colourless oil. Yield: 2.51 g (87%).

¹ H NMR (500 MHz, toluene-d₈): δ 5.75 (ddt, ³J_HH = 17.0, 10.2, 6.7, 1H, CH̲=CH₂), 5.02 (br d, ³J_HH = 17.1, 1H, CH=CH̲₂), 4.97 (br d, ³J_HH = 10.1, 1H, CH=CH̲₂), 3.14 (br, 1H, OH), 1.95 (app q, ³J_HH = 7, 2H, CH̲₂CH=CH₂), 1.60–1.75 (m, 1H, CH₂), 1.39–1.56 (m, 2H, CH₂), 1.03–1.38 (m, 7H, CH₂), 0.95 (d, ³J_PH = 13.7, 9H, tBu), 0.63–1.30 (obscured, 3H, BH₃).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 139.1 (s, C̲H=CH₂), 114.6 (s, CH=C̲H₂), 34.2 (s, C̲H₂CH=CH₂), 31.5 (d, ²J_PC = 12, CH₂), 31.0 (d, ¹J_PC = 38, tBu{C}), 29.24 (s, CH₂), 29.15 (s, CH₂), 24.5 (d, ¹J_PC = 35, PCH₂), 24.0 (d, ²J_PC = 3, tBu{CH₃}), 22.8 (s, CH₂).

³¹ P{ ¹ H} NMR (121 MHz, toluene-d₈): δ 125.6 (partially collapsed quartet, fwhm = 200 Hz).

Preparation of 4. Triethylamine (3.79 mL, 27.2 mmol) was added dropwise to a solution of 3 (2.51 g, 10.9 mmol) and methanesulfonic anhydride (2.84 g, 16.3 mmol) in dichloromethane (15 mL) at −20 °C and the resulting solution stirred at this temperature for 1 h. A solution of tetrabutylammonium borohydride (2.81 g, 10.9 mmol) in CH₂Cl₂ (10 mL) was added at −20 °C and the reaction mixture stirred for a further 16 h at −20 °C. The mixture was allowed to warm to 0 °C and HCl (0.5 M, 20 mL) added slowly. The mixture was exposed to air, diluted with distilled water, and the aqueous phase extracted with CH₂Cl₂ (3 × 15 mL). The combined organic extracts were dried (MgSO₄), filtered and concentrated under reduced pressure to give a colourless oil. Purification by flash column chromatography (silica, 5% EtOAc in hexane, R_f = 0.42) afforded 4 as a colourless oil. Yield: 1.83 g (79%).

¹ H NMR (500 MHz, CDCl₃): δ 5.80 (ddt, ³J_HH = 17.0, 10.0, 6.5, 1H, CH̲=CH₂), 4.99 (app dq, ³J_HH = 17.0, J_HH = 2, 1H, CH=CH̲₂), 4.94 (ddt, ³J_HH = 10.0, ²J_HH = 2.3, ⁴J_HH = 1.2, 1H, CH=CH̲₂), 4.23 (dm, ¹J_PH = 350.6, 1H, PH), 2.04 (app q, ³J_HH = 7, 2H, CH̲₂CH=CH₂), 1.65–1.90 (m, 2H, CH₂), 1.45–1.60 (m, 2H, CH₂), 1.24–1.40 (m, 6H, CH₂), 1.21 (d, ³J_PH = 14.2, 9H, tBu), 0.46 (partially collapsed quartet, fwhm = 330, 3H, BH₃).

¹³ C{ ¹ H} NMR (126 MHz CDCl₃): δ 139.0 (s, C̲H=CH₂), 114.5 (s, CH=C̲H₂), 33.8 (s, CH₂), 30.9 (d, ²J_PC = 11, CH₂), 28.8 (s, CH₂), 28.7 (s, CH₂), 26.9 (d, ²J_PC = 2, tBu{CH₃}), 26.8 (d, ¹J_PC = 35, tBu{C}), 25.0 (d, ³J_PC = 2, CH₂), 17.6 (d, ¹J_PC = 32, PCH₂).

³¹ P NMR (202 MHz, CDCl₃): δ 22.2 (br d, ¹J_PH = 352).

³¹ P{ ¹ H} NMR (202 MHz, CDCl₃): δ 22.2 (partially collapsed quartet, fwhm = 165 Hz).

Preparation of 5. nBuLi (1.60 M, 4.11 mL, 6.59 mmol) was added dropwise to a solution of 4 (1.41 g, 6.59 mmol) in THF (15 mL) at −78 °C. The reaction mixture was stirred for 30 min at this temperature and a solution of 1,3-bis(bromomethyl)benzene (870 mg, 3.30 mmol) in THF (3 mL) was added at −78 °C. The reaction mixture was stirred at −78 °C for 1 h, allowed to warm to RT over 3 h, then quenched by slow addition of water (10 mL) at 0 °C in air. THF was removed under reduced pressure and aqueous mixture extracted with CH₂Cl₂ (3 × 20 mL). The combined organic extracts were dried (MgSO₄), filtered and concentrated under reduced pressure to give a colourless oil. Purification by flash column chromatography (silica, 5% EtOAc in hexane, R_f = 0.33) afforded 5 as a colourless oil, which was carried forward directly. Yield: 1.60 g (91%).

¹ H NMR (300 MHz, CDCl₃): δ 7.12–7.24 (m, 4H, Ar), 5.78 (ddt, ³J_HH = 16.9, 10.2, 6.8, 2H, CH̲=CH₂), 4.88–5.03 (m, 4H, CH=CH̲₂), 2.85–3.10 (m, 4H, PCH₂), 2.01 (app q, ³J_HH = 7.0, 4H, CH̲₂CH=CH₂), 1.10–1.65 (m, 24H, CH₂), 1.17 (d, ³J_PH = 13.1, 18H, tBu), 0.44 (partially collapsed quartet, fwhm = 270 Hz, 6H, BH₃).

³¹ P{ ¹ H} NMR (162 MHz, CDCl₃): δ 32.3 (partially collapsed quartet, fwhm = 140 Hz).

HR ESI-MS (positive ion, 4 kV): 553.4469, [M + Na]⁺ (calcd 553.4416) m/z.

Preparation of 6. A solution of 5 (1.60 g, 3.01 mmol) in CH₂Cl₂ (5 mmol L⁻¹, 600 mL) was treated with a solution of [Ru(PCy₃)₂Cl₂(CHPh)] (248 mg, 301 μmol, 10 mol%) in CH₂Cl₂ (2 mL) in 4 equal portions – every 12 h – over 2 days with periodic argon sparging (ca. 10 min). Volatiles were removed in vacuo and the crude mixture was purified by flash column chromatography in air (silica, 8% EtOAc in hexane, R_f = 0.26) to afford 6 (mixture of alkene isomers) as a colourless oil, which was carried forward directly. Yield: 1.37 g (91%).

¹ H NMR (500 MHz, CDCl₃): δ 7.14–7.25 (m, 4H, Ar), 5.21–5.29 (m, 0.6H, CH=CH minor), 5.21–5.29 (m, 1.4H, CH=CH major), 3.02 (dd, ²J_HH = 13.8, ²J_PH = 9.0, 2H, ArCH̲₂), 2.92 (app t, ²J = 14, 1.4H, ArCH̲₂ major), 2.92 (app t, ²J = 14, 1.4H, ArCH̲₂ major), 2.91 (app t, ²J = 14, 0.6H, ArCH̲₂ minor), 1.88–2.05 (m, 4H, CH₂), 0.93–1.70 (m, 20H, CH₂), 1.21 (d, ³J_PH = 13.0, 5.4H, tBu minor), 1.20 (d, ³J_PH = 13.0, 12.6H, tBu major), 0.45 (partially collapsed quartet, fwhm = 270 Hz, 6H, BH₃).

³¹ P{ ¹ H} NMR (162 MHz, CDCl₃): δ 31.3–33.4 (br m).

HR ESI-MS (positive ion, 4 kV): 525.4105, [M + Na]⁺ (calcd 525.4102) m/z.

Preparation of PCP-14′·2BH₃ (7). A suspension of [Rh(PPh₃)₃Cl] (126 mg, 136 μmol, 5 mol%) in benzene (5 mL) was added to a stirred solution of 6 (1.37 g, 2.73 mmol) in benzene (20 mL). The suspension was freeze–pump–thaw degassed, placed under dihydrogen and the resulting solution stirred at 50 °C for 2 days. Volatiles were removed under reduced pressure and the crude product purified by flash column chromatography in air (silica, 5% EtOAc in hexane, R_f = 0.25) to afford 7 as a colourless oil. Yield: 1.21 g (88%).

¹ H NMR (600 MHz, CDCl₃): δ 7.25 (s, 1H, Ar{2-CH}), 7.22 (t, ³J_HH = 7.6, 1H, Ar), 7.17 (d, ³J_HH = 7.6, 2H, Ar), 3.03 (dd, ²J_HH = 14.0, ²J_PH = 9.9, 2H, ArCH̲₂), 3.03 (app t, ²J = 14, 2H, ArCH̲₂), 1.50–1.61 (m, 4H, PCH₂), 1.40–1.50 (m, 2H, CH₂), 1.13–1.35 (m, 22H, CH₂), 1.17 (d, ³J_PH = 13.1, 18H, tBu), 0.46 (partially collapsed quartet, fwhm = 270 Hz, 6H, BH₃).

¹³ C{ ¹ H} NMR (151 MHz, CDCl₃): δ 133.9 (dd, ²J_PC = 5, ⁴J_PC = 2, Ar{C}), 132.0 (t, ³J_PC = 4, Ar{2-CH}), 128.7 (dd, ³J_PC = 4, ⁵J_PC = 3, Ar), 128.5 (t, ⁴J_PC = 2, Ar), 30.8 (d, ²J_PC = 12, CH₂), 28.8 (d, ¹J_PC = 31, tBu{C}), 28.7 (d, ¹J_PC = 27, ArC̲H₂), 27.8 (s, CH₂), 27.6 (s, 2 × CH₂), 27.4 (s, CH₂), 25.8 (d, ²J_PC = 1, tBu{CH₃}), 23.3 (s, CH₂), 20.2 (d, ¹J_PC = 31, PCH₂).

³¹ P{ ¹ H} NMR (243 MHz, CDCl₃): δ 32.5 (partially collapsed quartet, fwhm = 155 Hz).

HR ESI-MS (positive ion, 4 kV): 527.4250, [M + Na]⁺ (calcd 527.4259) m/z.

Anal. Calcd for C₃₀H₆₀B₂P₂ (508.32 g mol⁻¹): C, 71.44; H, 11.99; found: C, 71.37; H, 12.10.

Preparation of PCP-14′. A solution of 7 (26.3 mg, 52.1 μmol) in Et₂NH (2 mL) was heated at 80 °C for 3 days. Volatiles were removed in vacuo to afford PCP-14′ as a colourless oil. Yield: 24.8 mg (>99%).

¹ H NMR (600 MHz, toluene-d₈): δ 7.34 (s, 1H, Ar{2-CH}), 7.06 (br, 3H, Ar), 2.75 (d, ²J_HH = 13.5, 2H, ArCH̲₂), 2.51 (dd, ²J_HH = 13.5, ²J_PH = 3.0, 2H, ArCH̲₂), 1.10–1.50 (m, 28H, CH₂), 0.98 (d, ³J_PH = 10.9, 18H, tBu).

¹³ C{ ¹ H} NMR (151 MHz, toluene-d₈): δ 140.2 (d, ²J_PC = 8, Ar{C}), 130.9 (t, ³J_PC = 7, Ar{2-CH}), 128.5 (s, Ar), 127.1 (dd, ³J_PC = 7, ⁵J_PC = 2, Ar), 32.6 (d, ¹J_PC = 23, ArC̲H₂), 30.8 (d, ²J_PC = 12, CH₂), 28.6 (s, CH₂), 28.3 (s, CH₂), 28.23 (d, ¹J_PC = 15, tBu{C}), 28.21 (s, CH₂), 28.1 (s, CH₂), 27.5 (d, ²J_PC = 13, tBu{CH₃}), 27.3 (s, CH₂), 24.4 (d, ¹J_PC = 20, PCH₂).

³¹ P{ ¹ H} NMR (243 MHz, toluene-d₈): δ 3.2 (s).

Preparation of PCP-14′·O₂. Following an adapted literature procedure.²⁷ To a stirred solution of PCP-14′ (24.8 mg, 52.0 μmol) in EtOH (1 mL) cooled to −78 °C was added H₂O₂ (30% w/w in H₂O, 16 μL, 0.16 mmol). The mixture was allowed to warm to RT overnight before the volatiles were removed in vacuo. Toluene (0.5 mL) was added and the solution was stirred over 3 Å molecular sieves (15 mg) overnight. The solution was filtered, extracting the sieves with additional toluene (3 × 1 mL), and reduced to dryness to afford PCP-14′·O₂ as a colourless oil, which crystallised upon standing. Yield: 22.3 mg (84%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.37 (s, 1H, Ar), 7.23–7.28 (m, 1H, Ar), 7.21 (d, ³J_HH = 7.6, 2H, Ar), 3.12 (app t, ²J = 14, 2H, ArCH̲₂), 3.02 (dd, ²J_HH = 14.5, ²J_PH = 10.1, 2H, ArCH̲₂), 1.61–1.76 (m, 2H, PCH₂), 1.50–1.61 (m, 2H, PCH₂), 1.39–1.50 (m, 2H, CH₂), 1.10–1.50 (m, 22H, CH₂), 0.98 (d, ³J_PH = 10.9, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, CD₂Cl₂): δ 134.2 (dd, ²J_PC = 8, ⁴J_PC = 2, Ar{C}), 131.9 (t, ³J_PC = 5, Ar{2-CH}), 128.8 (t, ⁴J_PC = 2, Ar), 128.6 (app t, J_PC = 3, Ar), 33.2 (d, ¹J_PC = 65, tBu{C}), 32.9 (d, ¹J_PC = 55, ArC̲H₂), 30.9 (d, ²J_PC = 12, CH₂), 28.3 (s, CH₂), 28.2 (s, CH₂), 28.14 (s, CH₂), 28.08 (s, CH₂), 24.9 (s, tBu{CH₃}), 24.4 (d, 1J_PC = 62, PCH₂), 22.0 (d, ³J_PC = 5, CH₂).

³¹ P{ ¹ H} NMR (162 MHz, CD₂Cl₂): δ 52.2 (s).

HR ESI-MS (positive ion, 4 kV): 531.3490, [M + Na]⁺ (calcd 531.3491) m/z.

Preparation of POCOP-14′

Preparation of 9. A solution of resorcinol (3.53 g, 32.1 mmol) in THF (10 mL) was added to a suspension of NaH (1.62 g, 67.3 mmol) in THF (10 mL). The resulting suspension was heated at reflux for 1 h, allowed to cool to RT and a solution of 8 (16.6 g, 70.5 mmol) in THF (10 mL) added. The reaction mixture was then heated at reflux for a further 1 h. Volatiles were removed in vacuo and the residue extracted with hexane (3 × 10 mL) to afford 9 (1 : 1 mixture of diastereoisomers) as a colourless oil on drying. Yield: 16.1 g (95%).

¹ H NMR (300 MHz, C₆D₆): δ 7.43 (s, 1H, Ar{2-CH}), 7.04 (t, ³J_HH = 8.0, 1H, Ar), 6.95 (d, ³J_HH = 8.4, 2H, Ar), 5.76 (ddt, ³J_HH = 16.8, 11.8, 6.7, 2H, CH̲=CH₂), 4.95–5.10 (m, 4H, CH=CH̲₂), 1.95 (app q, ³J_HH = 7, 4H, CH̲₂CH=CH₂), 1.75–1.90 (m, 2H, CH₂), 1.61 (app sex, J = 8, 4H, CH₂), 1.15–1.40 (m, 14H, CH₂), 1.04 (d, ³J_PH = 12.0, 18H, tBu).

¹³ C{ ¹ H} NMR (75 MHz, C₆D₆): δ 161.0 (d, ²J_PH = 9.0, Ar{C}), 139.2 (s, C̲H=CH₂), 130.2 (s, Ar), 114.6 (s, CH=C̲H₂), 112.4 (d, ³J_PH = 11, Ar), 109.6 (t, ³J_PH = 12, Ar{2-CH}), 34.2 (s, C̲H₂CH=CH), 32.7 (d, ¹J_PH = 16, tBu{C}), 31.5 (d, ²J_PH = 12, CH₂), 29.3 (s, CH₂), 29.2 (s, CH₂), 29.2 (d, ¹J_PH = 24, PCH₂), 25.9 (d, ³J_PH = 16, CH₂), 25.4 (d, ²J_PH = 16, tBu{CH₃})

³¹ P{ ¹ H} NMR (121 MHz, C₆D₆): δ 142.40 (s, 1P), 142.36 (s, 1P).

HR ESI-MS (positive ion, 4 kV): 561.3231, [M + 2O + Na]⁺ (calcd 561.3233) m/z.

Preparation of 10. BH₃·SMe₂ (4.63 g, 5.78 mL, 61.0 mmol) was added dropwise to a stirred solution of 9 (15.4 g, 30.5 mmol) in THF (50 mL) at −78 °C. The reaction mixture was warmed to RT overnight and volatiles removed in vacuo to afford the crude material as an opaque colourless oil. Purification by flash column chromatography in air (silica, 8% EtOAc in hexane, R_f = 0.41) afforded 10 (1 : 1 mixture of diastereoisomers) as a colourless oil. Yield: 12.9 g (79%).

¹ H NMR (600 MHz, CDCl₃): δ 7.19 (t, ³J_HH = 8.2, 1H, Ar), 6.99 (app q, J = 2, 1H, Ar{2-CH}), 6.90 (app dt, ³J_HH = 8.4, J = 2, 2H, Ar), 5.79 (app ddt, ³J_HH = 16.9, 10.1, 6.7, 2H, CH̲=CH₂), 4.99 (app dq, ³J_HH = 17.0, J_HH = 2, 2H, CH=CH̲₂), 4.93 (app dq, ³J_HH = 10.2, J_HH = 2, 2H, CH=CH̲₂), 2.04 (app q, ³J_HH = 7, 4H, CH̲₂CH=CH₂), 1.92–2.01 (m, 2H, PCH₂), 1.68–1.79 (m, 4H, CH₂), 1.55–1.65 (m, 2H, CH₂), 1.28–1.44 (m, 12H, CH₂), 1.26 (d, ³J_PH = 14.0, 18H, tBu), 0.53 (partially collapsed quartet, fwhm = 300 Hz, 6H, BH₃).

¹³ C{ ¹ H} NMR (151 MHz, CDCl₃): δ 154.2 (d, ²J_PC = 6, Ar{C}), 139.1 (s, C̲H=CH₂), 129.6 (s, Ar), 117.0 (d, ³J_PC = 3, Ar), 114.5 (s, CH=C̲H₂), 114.1 (t, ³J_PC = 3.0, Ar{2-CH}), 33.8 (s, C̲H₂CH=CH₂), 32.9 (d, ¹J_PC = 37, tBu{C}), 31.4 (d, ²J_PC = 13.0, CH₂), 28.9 (d, ⁴J_PC = 3, CH₂), 28.7 (s, CH₂), 25.4 (d, ¹J_PC = 33, PCH₂), 24.9 (d, ²J_PC = 3, tBu{CH₃}), 22.8 (s, CH₂).

³¹ P{ ¹ H} NMR (121 MHz, CDCl₃): δ 143.7 (partially collapsed quartet, fwhm = 165 Hz).

HR ESI-MS (positive ion, 4 kV): 557.4000, [M + Na]⁺ (calcd 557.3990) m/z.

Preparation of 11. A solution of [Ru(PCy₃)₂Cl₂(CHPh)] (36.0 mg, 43.7 μmol, 2.5 mol%) in CH₂Cl₂ (3 mL) was added at RT to a solution of 10 (930 mg, 1.74 mmol) in CH₂Cl₂ (350 mL, 5 mmol L⁻¹). The solution was stirred at RT with periodic (every 12 h) argon sparging (ca. 10 min) for 2 days. Volatiles were removed in vacuo and the crude mixture was purified by flash column chromatography in air (silica, 10% EtOAc in hexane, R_f = 0.40) to afford 11 (1 : 1 mixture of diastereoisomers) as a colourless oil. Yield: 832 mg (94%).

¹ H NMR (500 MHz, CDCl₃): δ 7.20 (t, ³J_HH = 8.3, 0.5H, Ar cis), 7.19 (t, ³J_HH = 8.3, 0.5H, Ar trans), 7.03 (dd, ³J_HH = 8.3, ⁴J_HH = 2.4, 1H, Ar cis), 6.97 (t, ⁴J_HH = 2.3, 0.5H, Ar{2-CH} trans), 6.89 (dd, ³J_HH = 8.3, ⁴J_HH = 2.3, 1H, Ar trans), 6.74 (t, ⁴J_HH = 2.4, 0.5H, Ar{2-CH} cis), 5.26–5.36 (m, 2H, CH=CH), 1.93–2.10 (m, 6H, CH₂), 1.61–1.81 (m, 4H, CH₂), 1.24–1.60 (m, 14H, CH₂), 1.27 (d, ³J_PH = 14.0, 18H, tBu), 0.55 (partially collapsed quartet, fwhm = 300 Hz, 6H, BH₃).

¹³ C{ ¹ H} NMR (126 MHz, CDCl₃): δ 154.4 (d, ²J_PC = 6, Ar{C} cis), 154.1 (d, ²J_PC = 6, Ar{C} trans), 130.9 (s, CH=CH trans), 130.8 (s, CH=CH cis), 129.7 (s, Ar cis), 129.5 (s, Ar trans), 117.0 (d, ³J_PC = 4, Ar trans), 116.4 (d, ³J_PC = 3, Ar cis), 114.4 (t, ³J_PC = 3, Ar{2-CH} trans), 113.3 (t, ³J_PC = 4, Ar{2-CH} cis), 32.8 (d, ¹J_PC = 37, tBu{C} cis), 32.7 (d, ¹J_PC = 38, tBu{C} trans), 32.10 (s, C̲H₂CH=CH trans), 32.06 (s, C̲H₂CH=CH cis), 31.6 (d, ²J_PC = 13, CH₂trans), 31.2 (d, ²J_PC = 13, CH₂cis), 28.8 (s, CH₂trans), 28.7 (s, CH₂cis), 28.1 (s, CH₂trans), 27.9 (s, CH₂cis), 25.5 (d, ¹J_PC = 33, PCH₂cis), 25.4 (d, ¹J_PC = 33, PCH₂trans), 24.9 (d, ²J_PC = 3, tBu{CH₃} cis), 24.8 (d, ²J_PC = 3, tBu{CH₃} trans), 23.0 (d, ³J_PC = 2, CH₂trans), 22.7 (d, ³J_PC = 2, CH₂cis). Data for major alkene isomer only.

³¹ P{ ¹ H} NMR (162 MHz, CDCl₃): δ 142.0–144.6 (br m).

HR ESI-MS (positive ion, 4 kV): 529.3689, [M + Na]⁺ (calcd 529.3687) m/z.

Preparation of 12. A suspension of [Rh(PPh₃)₃Cl] (201 mg, 220 μmol, 5 mol%) in benzene (20 mL) was added to a stirred solution of 11 (2.20 g, 4.34 mmol) in benzene. The suspension was freeze–pump–thaw degassed and placed under a hydrogen atmosphere and the resulting solution stirred at 50 °C for 2 days. Volatiles were removed under reduced pressure and the crude mixture was eluted through a short silica plug in air (5% EtOAc in hexane) to afford 12 as a mixture of diastereomers (2.02 g, 3.97 mmol, 92%). Subsequent purification by repeated flash column chromatography in air (silica, 5% EtOAc in hexane) enabled separation of the cis- and trans-diastereomers.

POCOP-14′·2BH₃ (trans-12, R_f = 0.45). Yield: 951 mg (43%, colourless oil which slowly crystallised upon standing).

¹ H NMR (600 MHz, CDCl₃): δ 7.19 (t, ³J_HH = 8.2, 1H, Ar), 7.03 (t, ⁴J_HH = 2.5, 1H, Ar{2-CH}), 6.93 (dd, ³J_HH = 8.2, ⁴J_HH = 2.5, 2H, Ar), 1.94–2.04 (m, 2H, PCH₂), 1.67–1.76 (m, 4H, CH₂), 1.50–1.61 (m, 2H, CH₂), 1.36–1.44 (m, 4H, CH₂), 1.22–1.34 (m, 16H, CH₂), 1.26 (d, ³J_PH = 13.8, 18H, tBu), 0.58 (partially collapsed quartet, fwhm = 300 Hz, 6H, BH₃).

¹³ C{ ¹ H} NMR (151 MHz, CDCl₃): δ 154.2 (d, ²J_PC = 6, Ar{C}), 129.6 (s, Ar), 116.9 (d, ³J_PC = 3, Ar), 114.1 (t, ³J_PC = 3, Ar{2-CH}), 32.8 (d, ¹J_PC = 37, tBu{C}), 30.7 (d, ²J_PC = 12, CH₂), 28.1 (s, CH₂), 28.0 (s, CH₂), 27.6 (s, CH₂), 27.5 (s, CH₂), 25.1 (d, ¹J_PC = 33, PCH₂), 24.8 (d, ³J_PC = 3, tBu{CH₃}), 22.3 (s, CH₂).

³¹ P{ ¹ H} NMR (243 MHz, CDCl₃): δ 143.8 (partially collapsed quartet, fwhm = 165 Hz).

HR ESI-MS (positive ion, 4 kV): 531.3840, [M + Na]⁺ (calcd 531.3844) m/z.²⁸

Anal. Calcd for C₂₈H₅₆B₂O₂P₂ (508.32 g mol⁻¹): C, 66.16; H, 11.10; found: C, 66.09; H, 11.26.²⁸

cis-12 (R_f = 0.42). Yield: 984 mg (45%, white crystalline solid).

¹ H NMR (500 MHz, CDCl₃): δ 7.20 (t, ³J_HH = 8.3, 1H, Ar), 7.03 (dd, ³J_HH = 8.3, ⁴J_HH = 2.3, 2H, Ar), 6.82 (t, ⁴J_HH = 2.5, 1H, Ar{2-CH}), 1.90–2.06 (m, 2H, PCH₂), 1.67–1.81 (m, 4H, CH₂), 1.53–1.66 (m, 2H, CH₂), 1.20–1.50 (m, 20H, CH₂), 1.26 (d, ³J_PH = 14.0, 18H, tBu), 0.61 (partially collapsed quartet, fwhm = 285 Hz, 6H, BH₃).

¹³ C{ ¹ H} NMR (126 MHz, CDCl₃): δ 154.4 (d, ²J_PC = 6, Ar{C}), 129.7 (s, Ar), 116.6 (d, ³J_PC = 3, Ar), 113.4 (t, ³J_PC = 4, Ar{2-CH}), 32.9 (d, ¹J_PC = 37, tBu{C}), 30.5 (d, ²J_PC = 12, CH₂), 28.2 (s, CH₂), 27.9 (s, CH₂), 27.8 (s, CH₂), 27.6 (s, CH₂), 25.2 (d, ¹J_PC = 33, PCH₂), 24.9 (d, ³J_PC = 3, tBu{CH₃}), 22.0 (d, ³J_PC = 2, CH₂).

³¹ P{ ¹ H} NMR (162 MHz, CDCl₃): δ 143.7 (br, fwhm = 175 Hz).

HR ESI-MS (positive ion, 4 kV): 531.3840, [M + Na]⁺ (calcd 531.3844) m/z.²⁸

Anal. Calcd for C₂₈H₅₆B₂O₂P₂ (508.32 g mol⁻¹): C, 66.16; H, 11.10; found: C, 66.09; H, 11.26.²⁸

Preparation of POCOP-14′. A solution of trans-12 (231 mg, 455 μmol) in Et₂NH (7 mL) was heated at 60 °C for 2 days. Volatiles were removed in vacuo to afford POCOP-14′ as a colourless oil. Yield: 217 mg (>99%).

¹ H NMR (500 MHz, toluene-d₈): δ 7.30 (app p, J = 2, 1H, Ar{2-CH}), 6.97 (t, ³J_HH = 8.1, 1H, Ar), 6.87 (app dt, ³J_HH = 8.2, J = 2, 2H, Ar), 1.84 (dtd, ²J_HH = 14.4, ³J_HH = 7.3, ²J_PH = 3.2, 2H, PCH₂), 1.54–1.67 (m, 4H, CH₂), 1.15–1.50 (m, 22H, CH₂), 1.00 (d, ³J_PH = 12.1, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 160.9 (d, ²J_PC = 9, Ar{C}), 129.9 (s, Ar), 112.2 (d, ³J_PC = 11, Ar), 109.2 (t, ³J_PC = 12, Ar{2-CH}), 32.5 (d, ¹J_PC = 16, tBu{C}), 31.1 (d, ²J_PC = 11, CH₂), 29.0 (d, ¹J_PC = 24, PCH₂), 28.9 (s, CH₂), 28.5 (s, CH₂), 28.2 (s, CH₂), 28.0 (s, CH₂), 25.5 (d, ³J_PC = 15, CH₂), 25.3 (d, ²J_PC = 15, tBu{CH₃}).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈): δ 141.6 (s).

Preparation of the cis-diastereoisomer of POCOP-14′. A solution of cis-12 (10.0 mg, 19.7 μmol) in Et₂NH (0.5 mL) was heated at 80 °C for 2 days. Volatiles were removed in vacuo to afford the cis-diastereoisomer of POCOP-14′ as a colourless oil. Yield: 9.4 mg (>99%).

¹ H NMR (500 MHz, toluene-d₈): δ 7.21 (app p, J = 2, 1H, Ar{2-CH}), 6.97 (t, ³J_HH = 7.9, 1H, Ar), 6.91 (app dt, 3J_HH = 7.9, J = 2, 2H, Ar), 1.85 (dtd, ²J_HH = 14.2, ³J_HH = 7.4, ²J_PH = 3.1, 1H, PCH₂), 1.58–1.68 (m, 4H, CH₂), 1.41–1.52 (m, 2H, CH₂), 1.16–1.40 (m, 20H, CH₂), 1.01 (d, ³J_PH = 12.0, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 160.8 (d, ²J_PC = 9, Ar{C}), 129.9 (s, Ar), 111.7 (d, ³J_PC = 13, Ar), 109.4 (t, ³J_PC = 11, Ar{2-CH}), 32.5 (d, ¹J_PC = 16, tBu{C}), 30.9 (d, ²J_PC = 11, CH₂), 29.1 (d, ¹J_PC = 24, PCH₂), 28.9 (s, CH₂), 28.5 (s, CH₂), 28.2 (s, CH₂), 28.1 (s, CH₂), 25.4 (d, ³J_PC = 15, CH₂), 25.3 (d, ³J_PC = 15, tBu{CH₃}).

³¹ P{ ¹ H} NMR (162 MHz, C₆D₆): δ 140.5 (s).

Preparation of POCOP-14′·O₂. Following an adapted literature procedure.²⁷ To a stirred solution of POCOP-14′ (25.1 mg, 52.2 μmol) in EtOH (2 mL) cooled to −78 °C was added H₂O₂ (30% w/w in H₂O, 16 μL, 0.16 mmol). The mixture was allowed to warm to RT overnight before the volatiles were removed in vacuo. Toluene (1 mL) was added and the solution was stirred over 3 Å molecular sieves (45.2 mg) overnight. The solution was filtered, extracting the sieves with additional toluene (3 × 1 mL), and reduced to dryness to afford POCOP-14′·O₂ as a colourless oil, which crystallised upon standing. Yield: 26.2 mg (95%).

¹ H NMR (500 MHz, CD₂Cl₂): δ 7.23 (t, ³J_HH = 8.3, 1H, Ar), 7.19 (t, ⁴J_HH = 2.4, 1H, Ar{2-CH}), 7.04 (dd, ³J_HH = 8.3, ⁴J_HH = 7.1, 2H, Ar), 1.75–1.91 (m, 4H, CH₂), 1.16–1.73 (m, 24H, CH₂), 1.22 (d, ³J_PH = 15.5, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, CD₂Cl₂): δ 153.6 (d, ²J_PC = 10, Ar{C}), 130.2 (s, Ar), 116.4 (d, ³J_PC = 4, Ar), 113.3 (t, ³J_PC = 4, Ar{2-CH}), 33.9 (d, ¹J_PC = 91, tBu{C}), 30.9 (d, ²J_PC = 14, CH₂), 28.5 (s, CH₂), 28.3 (s, CH₂), 28.0 (s, CH₂), 27.7 (s, CH₂), 24.6 (s, tBu{CH₃}), 24.4 (d, ¹J_PC = 83, PCH₂), 21.9 (d, ³J_PC = 6, CH₂).

³¹ P{ ¹ H} NMR (162 MHz, CD₂Cl₂): δ 64.1 (s).

HR ESI-MS (positive ion, 4 kV): 535.3078, [M + Na]⁺ (calcd 535.3077) m/z.

Synthesis of rhodium complexes of PCP-14

A solution of PCP-14′ (118.2 mg, 248 μmol) and [Rh(COE)₂Cl]₂ (88.9 mg, 124 μmol) in toluene (5 mL) within a reaction flask fitted with a J Young's value was freeze–pump–thaw-degassed and placed under dihydrogen (1 atm), sealed, and heated at 100 °C for 18 h. The solution was cooled to RT and cannula transferred under dihydrogen into a reaction flask fitted with a J Young's value and charged with KOtBu (34.0 mg, 303 μmol). The flask was then sealed and heated at 120 °C for 2 h to generate 13a, which was characterised in situ. The solution was freeze–pump–thaw degassed and placed under carbon monoxide (1 atm) resulting in immediate formation of 14a, which was isolated by removal of the volatiles in vacuo and extraction of the residue with hexane. Crude 14a obtained in this way was dissolved in toluene (5 mL) and then slowly added to a suspension of PhICl₂ (68.7 mg, 250 μmol) in toluene (2 mL) at −78 °C. The reaction mixture was allowed to warm to RT overnight, the volatiles removed in vacuo, and the resulting residue extracted with hexane to afford the crude product, which was purified by alumina chromatography in air (20% CH₂Cl₂ in hexane, R_f = 0.55) to afford analytically pure 15a. Yield: 136.4 mg (80%). Complex 15a is stable in the solid state, but partial loss of CO was observed in solution over time.

[Rh(PCP-14)(H₂)] (13a). This complex is unstable to vacuum but was characterised in situ using a sample (ca. 10 μmol) prepared in a similar manner within a J Young's valve NMR tube following removal of volatiles, addition of toluene-d₈ (0.5 mL) under argon, freeze–pump–thaw-degassing, and placing under dihydrogen (1 atm). T₁ values were subsequently determined following rapid freeze–pump–thaw-degassing and placing under argon.

¹ H NMR (500 MHz, toluene-d₈, H₂): δ 7.12 (d, ³J_HH = 7.4, 2H, Ar), 7.05 (t, ³J_HH = 7.4, 1H, Ar), 3.08 (vt, J_PH = 3.9, 4H, ArCH̲₂), 1.87–2.00 (m, 2H, CH₂), 1.74–1.84 (m, 2H, CH₂), 1.17–1.67 (m, 24H, CH₂), 1.01 (vt, J_PH = 6.5, 18H, tBu), −4.36 (br, 2H, RhH).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈, H₂): δ 176.3 (dt, ¹J_RhC = 40, ²J_PC = 6, Ar{CRh}), 151.6 (vtd, J_PC = 12, ²J_RhC = 3, Ar{C}), 124.1 (s, Ar), 120.7 (vt, J_PC = 10, Ar), 39.9 (vtd, J_PC = 10, ²J_RhC = 4, ArC̲H₂), 31.9 (vt, J_PC = 11, tBu{C}), 29.5 (vt, J_PC = 4, CH₂), 29.2 (s, CH₂), 29.0 (s, CH₂), 28.6 (s, CH₂), 28.4 (vt, J_PC = 3, tBu{CH₃}), 27.7 (s, CH₂), 25.6 (vt, J_PC = 4, CH₂), 22.9 (vt, J_PC = 8, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈, H₂): δ 70.4 (d, ¹J_RhP = 153).

¹ H NMR (600 MHz, toluene-d₈, Ar, selected data): δ −4.36 (br d, ¹J_RhH = 17.9, 2H, RhH, T₁ = 74 ± 4 ms).

¹ H NMR (600 MHz, toluene-d₈, Ar, 200 K, selected data): δ −4.26 (br, 2H, RhH, T₁ = 117 ± 15 ms).

[Rh(PCP-14)Cl₂(CO)] (15a)

¹ H NMR (600 MHz, toluene-d₈, CO): δ 6.95 (t, ³J_HH = 7.4, 1H, Ar), 6.90 (d, ³J_HH = 7.4, 2H, Ar) 3.71 (dvt, ²J_HH = 15.6, J_PH = 4.8, 2H, ArCH̲₂), 2.96–3.04 (m, 2H, PCH₂), 2.91 (dvt, ²J_HH = 15.6, J_PH = 4.1, 2H, ArCH̲₂), 1.84–1.95 (m, 2H, CH₂), 1.25–1.78 (m, 24H, CH₂), 1.16 (vt, J_PH = 6.9, 18H, tBu).

¹³ C{ ¹ H} NMR (151 MHz, toluene-d₈, CO): δ 189.1 (dt, ¹J_RhC = 39, ²J_PC = 7, CO), 163.7 (d, ¹J_RhC = 21, Ar{CRh}), 147.4 (vt, J_PC = 8, Ar{C}), 125.6 (s, Ar), 122.6 (vt, J_PC = 9, Ar), 38.1 (vt, J_PC = 14, ArC̲H₂), 34.9 (vt, J_PC = 10, tBu{C}), 30.9 (vt, J_PC = 5, CH₂), 29.2 (s, CH₂), 29.1 (s, CH₂), 28.9 (s, CH₂), 28.6 (s, CH₂), 27.6 (s, tBu{CH₃}), 26.4 (s, CH₂), 22.3 (vt, J_PC = 11, PCH₂).

³¹ P{ ¹ H} NMR (243 MHz, toluene-d₈, CO): δ 64.9 (d, ¹J_RhP = 87).

IR (toluene): ν(CO) 2069 cm⁻¹.

Anal. Calcd for C₃₁H₅₃Cl₂OP₂Rh (677.52 g mol⁻¹): C, 54.96; H, 7.89; found: C, 54.99; H, 8.06.

Synthesis of rhodium complexes of POCOP-14

A solution of POCOP-14′ (217 mg, 452 μmol) and [Rh(COE)₂Cl]₂ (163 mg, 227 μmol) in toluene (5 mL) within a reaction flask fitted with a J Young's value was freeze–pump–thaw-degassed and placed under dihydrogen (1 atm), sealed, and heated at 120 °C for 3 days. The solution was cooled to RT and cannula transferred under dihydrogen into a reaction flask fitted with a J Young's value and charged with KOtBu (61.1 mg, 545 μmol). The flask was then sealed and heated at 120 °C for 1 h to generate 13b, which was characterised in situ. The solution was freeze–pump–thaw degassed and placed under carbon monoxide (1 atm) resulting in immediate formation of 14b, which was isolated by removal of the volatiles in vacuo and extraction of the residue with hexane. Crude 14b obtained in this way was dissolved in toluene (5 mL) and then slowly added to a suspension of PhICl₂ (125 mg, 455 μmol) in toluene (2 mL) at −78 °C. The reaction mixture was allowed to warm to RT overnight, the volatiles removed in vacuo, and the resulting residue extracted with hexane to afford the crude product, which was purified by alumina chromatography in air (20% CH₂Cl₂ in hexane, R_f = 0.45) to afford analytically pure 15b. Yield: 149.3 mg (48%). Complex 15b is stable in the solid state, but partial loss of CO was observed in solution over time.

[Rh(POCOP-14)(H₂)] (13b). This complex is unstable to vacuum but was characterised in situ using a sample (ca. 10 μmol) prepared in a similar manner within a J Young's valve NMR tube following removal of volatiles, addition of toluene-d₈ (0.5 mL) under argon, freeze–pump–thaw-degassing, and placing under dihydrogen (1 atm). T₁ values were subsequently determined following rapid freeze–pump–thaw-degassing and placing under argon.

¹ H NMR (500 MHz, toluene-d₈, H₂): δ 6.94 (t, ³J_HH = 7.9, 1H, Ar), 6.76 (d, ³J_HH = 7.9, 2H, Ar), 1.93–2.04 (m, 4H, CH₂), 1.83–1.92 (m, 2H, PCH₂), 1.17–1.67 (m, 22H, CH₂), 1.15 (vt, J_PH = 7.1, 18H, tBu), −2.87 (br d, ¹J_RhH = 18.8, 2H, RhH).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈, H₂): δ 167.3 (vt, J_PC = 10, Ar{C}), 143.1 (dt, ¹J_RhC = 35, ²J_PC = 10, Ar{CRh}), 126.7 (s, Ar), 104.8 (vt, J_PC = 7, Ar), 36.7 (vtd, J_PC = 12, ²J_RhC = 2, tBu{C}), 29.0 (vt, J_PC = 2, CH₂), 28.9 (s, CH₂), 28.7 (s, CH₂), 28.6 (s, CH₂), 28.1 (vtd, J_PC = 9, ²J_RhC = 2, PCH₂), 27.7 (s, CH₂), 26.8 (vt, J_PC = 4, tBu{CH₃}), 25.0 (vt, J_PC = 4, CH₂).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈, H₂): δ 198.4 (d, ¹J_RhP = 165).

¹ H NMR (600 MHz, toluene-d₈, Ar, selected data): δ −2.87 (br d, ¹J_RhH = 18.9, 2H, RhH, T₁ = 42 ± 2 ms).

¹ H NMR (600 MHz, toluene-d₈, Ar, 200 K, selected data): δ −2.72 (br, 2H, RhH, T₁ = 89 ± 11 ms).

[Rh(POCOP-14)Cl₂(CO)] (15b)

¹ H NMR (500 MHz, toluene-d₈, CO): δ 6.80 (t, ³J_HH = 7.9, 1H, Ar), 6.61 (d, ³J_HH = 7.9, 2H, Ar), 3.63 (app dp, ²J_HH = 14.1, J = 7, 2H, PCH₂), 1.87–1.94 (m, 2H, CH₂), 1.78–1.87 (m, 2H, PCH₂), 1.29 (vt, J_PH = 7.6, 18H, tBu), 1.11–1.72 (m, 22H, CH₂).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈, CO): δ 187.2 (dt, ¹J_RhC = 41, ²J_PC = 5, CO), 163.6 (vt, J_PC = 6, Ar{C}), 136.1 (dt, ¹J_RhC = 22, ²J_PC = 6, Ar{CRh}), 128.4 (s, Ar), 107.6 (vt, J_PC = 6, Ar), 41.5 (vt, J_PC = 11, tBu{C}), 30.7 (vt, J_PC = 6, CH₂), 29.5 (s, CH₂), 29.4 (s, CH₂), 29.2 (s, CH₂), 28.2 (s, CH₂), 26.5 (s, tBu{CH₃}), 24.1 (vt, J_PC = 3, CH₂), 23.8 (vt, J_PC = 13, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈, CO): δ 181.0 (d, ¹J_RhP = 92).

IR (toluene): ν(CO) 2083 cm⁻¹.

Anal. Calcd for C₂₉H₄₉Cl₂O₃P₂Rh (681.46 g mol⁻¹): C, 51.11; H, 7.25; found: C, 51.09; H, 7.31.

Synthesis of iridium complexes of PCP-14

A solution of PCP-14′ (57.6 mg, 121 μmol) and [Ir(COE)₂Cl]₂ (54.1 mg, 60.4 μmol) in toluene (5 mL) within a reaction flask fitted with a J Young's value was freeze–pump–thaw-degassed and placed under dihydrogen (1 atm), sealed, and heated at 70 °C for 18 h. The solution was cooled to RT and cannula transferred under dihydrogen into a reaction flask fitted with a J Young's value and charged with KOtBu (16.3 mg, 145 μmol). The flask was then sealed and heated at 120 °C for 18 h to generate 16a, which was characterised in situ. The solution was freeze–pump–thaw degassed and placed under carbon monoxide (1 atm) resulting in immediate formation of cis-17a, which was characterised in situ. To this solution tert-butylethylene (78 μL, 0.60 mmol) was added and the reaction mixture heated under carbon monoxide (1 atm, sealed flask) at 120 °C for 18 h, resulting in partial conversion to trans-17a but ultimately affording [Ir(PCP-14)(CO)₂] (δ_31P 51.6). The solution was freeze–pump–thaw degassed and placed under argon to generate 18a, filtered, and slowly added to a suspension of PhICl₂ (33.3 mg, 121 μmol) in toluene (1 mL) at −78 °C. The reaction mixture was allowed to warm to RT overnight, the volatiles removed in vacuo, and the resulting residue extracted with hexane to afford the crude product, which was purified by silica chromatography in air (40% CH₂Cl₂ in hexane, R_f = 0.43) to afford analytically pure 19a. Yield: 76.9 mg (51%).

[Ir(PCP-14)H₄] (16a). This complex is unstable to vacuum but was characterised in situ using a sample (ca. 10 μmol) prepared in a similar manner within a J Young's valve NMR tube following concentration in vacuo, placing under dihydrogen (1 atm), concentration to dryness in vacuo, addition of toluene-d₈ (0.5 mL) under argon, freeze–pump–thaw-degassing, and placing under dihydrogen (1 atm). NB: Long term storage in toluene-d₈ resulted in extensive H/D exchange of the hydride ligands, pincer backbone and some positions of the tetramethylene linker. T₁ values were subsequently determined following rapid freeze–pump–thaw-degassing and placing under argon.

¹ H NMR (600 MHz, toluene-d₈, H₂): δ 6.99–7.03 (m, 3H, Ar), 3.50 (dvt, ²J_HH = 16.1, J_PH = 3.6, 2H, ArCH̲₂), 2.89 (dvt, ²J_HH = 16.6, J_PH = 4.6, 2H, ArCH̲₂), 1.78–1.88 (m, 2H, PCH₂), 1.67–1.77 (m, 2H, CH₂), 1.53–1.63 (m, 2H, CH₂), 1.27–1.53 (m, 22H, CH₂), 0.94 (vt, J_PH = 6.8, 18H, tBu), −8.99 (t, ²J_PH = 9.8, 4H, IrH).

¹³ C{ ¹ H} NMR (151 MHz, toluene-d₈, H₂): δ 151.7 (s, Ar{CIr}), 147.7 (vt, J_PC = 8, Ar{C}), 123.3 (s, Ar), 120.1 (vt, J_PC = 8, Ar), 46.1 (vt, J_PC = 17, ArC̲H₂), 30.0 (vt, J_PC = 4, CH₂), 29.7 (s, CH₂), 29.3 (vt, J_PC = 15, tBu{C}), 29.1 (s, CH₂), 28.9 (s, CH₂), 28.2 (s, CH₂), 27.4 (br, CH₂), 27.2 (vt, J_PC = 15, PCH₂), 25.7 (vt, J_PC = 3, tBu{CH₃}).

³¹ P{ ¹ H} NMR (243 MHz, toluene-d₈, H₂): δ 49.9 (s).

¹ H NMR (600 MHz, toluene-d₈, Ar, selected data): δ −8.99 (t, ²J_PH = 9.8, 4H, IrH, T₁ = 300 ± 10 ms).

¹ H NMR (600 MHz, toluene-d₈, Ar, 200 K, selected data): δ −8.95 (br, 4H, IrH, T₁ = 626 ± 15 ms).

cis-[Ir(PCP-14)(H)₂(CO)] (cis-17a). This complex is most conveniently characterised in situ by placing a toluene-d₈ (0.5 mL) solution of analytically pure 18a (6.0 mg, 8.6 μmol) under dihydrogen (1 atm) in a J Young's valve NMR tube.

¹ H NMR (500 MHz, toluene-d₈): δ 6.99–7.04 (m, 3H, Ar), 3.50–3.65 (m, 2H, ArCH̲₂), 2.98 (dd, ²J_HH = 16.0, ²J_PH = 7.3, 1H, ArCH̲₂), 2.86 (dd, ²J_HH = 16.8, ²J_PH = 8.8, 1H, ArCH̲₂), 2.06–2.16 (m, 1H, PCH₂), 1.92–2.04 (m, 2H, CH₂), 1.14–1.91 (m, 25H, CH₂), 0.96 (d, ²J_PH = 13.3, 9H, tBu), 0.89 (d, ²J_PH = 13.6, 9H, tBu), −10.66 (ddd, ²J_PH = 23.4, ²J_PH = 10.5, ²J_HH = 3.0, 1H, IrH), −11.59 (app td, ²J_PH = 12, ²J_HH = 3.0, 1H, IrH).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 183.3 (m, CO), 155.1 (br, Ar{CIr}), 148.8 (dd, ²J_PC = 10, J_PC = 4, Ar{C}), 148.1 (dd, ²J_PC = 9, J_PC = 4, Ar{C}), 123.5 (s, Ar), 120.0 (d, ³J_PC = 14, Ar), 119.8 (d, ³J_PC = 16, Ar), 48.4 (d, ¹J_PC = 35, ArC̲H₂), 46.2 (d, ¹J_PC = 35, ArC̲H₂), 31.2 (dd, ¹J_PC = 24, ³J_PC = 3, tBu{C}), 30.5 (dd, ¹J_PC = 27, ³J_PC = 5, tBu{C}), 30.2 (d, ²J_PC = 10, CH₂), 29.9 (s, CH₂), 29.7 (d, ²J_PC = 9, CH₂), 29.4 (s, CH₂), 29.3 (s, CH₂), 29.1 (s, CH₂), 28.8 (s, CH₂), 28.5 (s, CH₂), 28.3 (s, CH₂), 28.2 (s, CH₂), 27.9 (dd, ¹J_PC = 29, ³J_PC = 4, PCH₂), 27.7 (s, CH₂), 27.3 (dd, ¹J_PC = 24, ³J_PC = 3, PCH₂), 27.2 (s, CH₂), 26.2 (d, ²J_PC = 4, tBu{CH₃}), 26.1 (d, ²J_PC = 4, tBu{CH₃}).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈): δ 51.0 (d, ²J_PP = 286, 1P), 45.8 (d, ²J_PP = 286, 1P).

trans-[Ir(PCP-14)(H)₂(CO)] (trans-17a)

¹ H NMR (400 MHz, toluene-d₀, CO, selected data): δ 1.01 (vt, ²J_PH = 7.1, 18H, tBu), −9.65 (t, ²J_PH = 14.4, 2H, IrH).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₀, CO): δ 54.1 (s).

[Ir(PCP-14)Cl₂(CO)] (19a)

¹ H NMR (500 MHz, toluene-d₈): δ 6.94–7.00 (m, 3H, Ar), 3.64 (dvt, ²J_HH = 15.8, J_PH = 5.0, 2H, ArCH̲₂), 3.05 (dvt, ²J_HH = 15.8, J_PH = 3.8, 2H, ArCH̲₂), 2.91–3.01 (m, 2H, PCH₂), 1.81–1.97 (m, 2H, CH₂), 1.02–1.81 (m, 24H, CH₂), 1.13 (br, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 175.7 (t, ²J_PC = 5, CO), 155.3 (s, Ar{CIr}), 148.5 (vt, J_PC = 8, Ar{C}), 126.0 (s, Ar), 121.6 (vt, J_PC = 8, Ar), 38.9 (vt, J_PC = 17, ArC̲H₂), 34.3 (vt, ¹J_PC = 13, tBu{C}), 30.8 (vt, J_PC = 5, CH₂), 29.2 (s, CH₂), 29.1 (s, CH₂), 29.0 (s, CH₂), 28.6 (s, CH₂), 27.5 (s, tBu{CH₃}), 26.4 (s, CH₂), 20.6 (vt, J_PC = 13, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈): δ 33.8 (s).

IR (toluene): ν(CO) 2034 cm⁻¹.

Anal. Calcd for C₃₁H₅₃Cl₂IrOP₂ (766.83 g mol⁻¹): C, 48.56; H, 6.97; found: C, 48.66; H, 6.92.

Synthesis of iridium complexes of POCOP-14

A solution of POCOP-14′ (69.1 mg, 144 μmol) and [Ir(COD)Cl]₂ (53.1 mg, 79.1 μmol) in toluene (5 mL) within a reaction flask fitted with a J Young's value was freeze–pump–thaw-degassed and placed under dihydrogen (1 atm), sealed, and heated at 100 °C for 3 days. The solution was cooled to RT and cannula transferred under dihydrogen into a reaction flask fitted with a J Young's value and charged with KOtBu (19.4 mg, 173 μmol). The flask was then sealed and heated at 120 °C for 2 h to generate 16b, which was characterised in situ. The solution was freeze–pump–thaw degassed and placed under carbon monoxide (1 atm) resulting in immediate formation of cis-17b, which was characterised in situ. To this solution tert-butylethylene (93 μL, 0.72 mmol) was added and the reaction mixture heated under carbon monoxide (1 atm, sealed flask) at 120 °C for 18 h, resulting in partial conversion to trans-17b but ultimately affording [Ir(POCOP-14)(CO)₂] (δ_31P 164.8, fwhm = 127 Hz). The solution was freeze–pump–thaw degassed and placed under argon to generate 18b, filtered, and slowly added to a suspension of PhICl₂ (39.5 mg, 144 μmol) in toluene (2 mL) at −78 °C. The reaction mixture was allowed to warm to RT overnight, the volatiles removed in vacuo, and the resulting residue extracted with hexane to afford the crude product, which was purified by silica chromatography in air (20% CH₂Cl₂ in hexane, R_f = 0.28) to afford analytically pure 19b. Yield: 107.9 mg (73%).

[Ir(POCOP-14)H₄] (16b). This complex is unstable to vacuum, ultimately resulting in irreversible decomposition. Attempts to prepare samples in toluene-d₈ resulted in extensive H/D exchange of hydride ligands, pincer backbone and some positions of the tetramethylene linker (see ESI†). T₁ values were determined following rapid freeze–pump–thaw-degassing and placing under argon.

¹ H NMR (500 MHz, toluene-d₀, H₂, selected data): δ 6.67 (d, ³J_HH = 7.9, 2H, Ar), 1.07 (vt, J_PH = 7.5, 18H, tBu), −8.26 (t, ²J_PH = 9.9, 4H, IrH).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₀, H₂): δ 165.0 (s).

¹ H NMR (600 MHz, toluene-d₀, Ar, selected data): δ −8.26 (t, ²J_PH = 10.0, 4H, IrH, T₁ = 194 ± 4 ms).

¹ H NMR (600 MHz, toluene-d₀, Ar, 200 K, selected data): δ −8.10 (br, 4H, IrH, T₁ = 242 ± 15 ms).

cis-[Ir(POCOP-14)(H)₂(CO)] (cis-17b). This complex is most conveniently characterised in situ by placing a toluene-d₈ (0.5 mL) solution of analytically pure 18b (6.0 mg, 8.6 μmol) under dihydrogen (1 atm) in a J Young's valve NMR tube.

¹ H NMR (500 MHz, toluene-d₈): δ 6.86 (t, ³J_HH = 7.9, 1H, Ar), 6.70 (d, ³J_HH = 7.9, 1H, Ar), 6.70 (d, ³J_HH = 7.9, 1H, Ar), 2.68–2.79 (m, 1H, PCH₂), 2.30–2.46 (m, 3H, CH₂), 1.85–1.97 (m, 1H, PCH₂), 1.12–1.70 (m, 23H, CH₂), 1.08 (d, ²J_PH = 14.8, 9H, tBu), 1.04 (d, ²J_PH = 15.0, 9H, tBu), −9.82 (app t, ²J_PH = 9, 1H, IrH), −10.75 (dd, ²J_PH = 21.8, ²J_PH = 14.7, 1H, IrH).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 180.6 (m, CO), 163.2 (dd, ²J_PC = 6, J_PC = 4, Ar{C}), 162.8 (dd, ²J_PC = 7, J_PC = 4, Ar{C}), 126.2 (s, Ar), 123.0 (app t, ²J_PC = 6, Ar{CIr}), 104.8 (d, ³J_PC = 12, Ar), 104.7 (d, ³J_PC = 16, Ar), 37.2 (dd, ¹J_PC = 28, ³J_PC = 7, tBu{C}), 37.0 (dd, ¹J_PC = 27, ³J_PC = 6, tBu{C}), 34.6 (dd, ¹J_PC = 30, ³J_PC = 5, PCH₂), 33.0 (dd, ¹J_PC = 27, ³J_PC = 5, PCH₂), 30.3 (s, CH₂), 29.8 (s, CH₂), 29.7 (d, J_PC = 6, CH₂), 29.5 (s, CH₂), 29.16 (s, CH₂), 29.15 (s, CH₂), 28.9 (s, CH₂), 28.6 (s, CH₂), 28.5 (s, CH₂), 27.7 (d, J_PC = 6, CH₂), 25.7 (d, J_PC = 5, CH₂), 25.2 (app vt, J_PC = 5, tBu{CH₃}), 25.1 (d, J_PC = 6, CH₂).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈): δ 160.4 (d, ²J_PP = 305, 1P), 154.6 (d, ²J_PP = 305, 1P).

trans-[Ir(POCOP-14)(H)₂(CO)] (trans-17b)

¹ H NMR (400 MHz, toluene-d₀, CO, selected data): δ 1.10 (vt, J_PH = 7.6, 18H, tBu), −9.76 (t, ²J_PH = 16.5, 2H, IrH).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₀, CO): δ 167.3 (s).

[Ir(POCOP-14)Cl₂(CO)] (19b)

¹ H NMR (500 MHz, toluene-d₈): δ 6.80 (t, ³J_HH = 8.0, 1H, Ar), 6.64 (d, ³J_HH = 8.0, 2H, Ar), 3.57 (app dp, ²J_HH = 14.3, J = 7, 2H, PCH₂), 1.85–1.99 (m, 2H, CH₂), 1.72–1.83 (m, 2H, PCH₂), 1.26 (vt, J_PH = 7.7, 18H, tBu), 1.11–1.71 (m, 22H, CH₂).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 175.6 (t, ²J_PC = 3, CO), 164.4 (vt, J_PC = 6, Ar{C}), 129 (obsc, Ar{CIr}), 128.9 (s, Ar), 106.8 (vt, J_PC = 6, Ar), 41.0 (vt, J_PC = 15, tBu{C}), 30.3 (vt, J_PC = 6, PCH₂C̲H₂), 29.5 (s, CH₂), 29.4 (s, CH₂), 29.1 (s, CH₂), 28.3 (s, CH₂), 26.5 (s, tBu{CH₃}), 24.2 (vt, J_PC = 2, CH₂), 22.0 (vt, J_PC = 16, PCH₂).

³¹ P{ ¹ H} NMR (121 MHz, toluene-d₈): δ 144.4 (s).

IR (toluene): ν(CO) 2049 cm⁻¹.

Anal. Calcd for C₂₉H₄₉Cl₂IrO₃P₂ (770.77 g mol⁻¹): C, 45.19; H, 6.41; found: C, 44.95; H, 6.30.

General procedure for the preparation of Rh(I) carbonyls

To a solution of 15 (ca. 30 μmol) in THF (2 mL) at RT was added iPrMgCl·LiCl (1.3 M in THF, 3 equivalents). The resulting solution was stirred at RT for 1 h, excess MeOH added and then reduced to dryness in vacuo. The residue was extracted with pentane to afford the crude product on removal of volatiles, which was subsequently recrystallised by slow evaporation of SiMe₄.

[Rh(PCP-14)(CO)] (14a). The product was obtained following the general procedure using 15a (20.0 mg, 29.5 μmol) and iPrMgCl·LiCl (68 μL, 89 μmol). Yield: 15.2 mg (85%).

¹ H NMR (600 MHz, toluene-d₈): δ 7.06 (d, ³J_HH = 7.3, 2H, Ar), 7.02 (t, ³J_HH = 7.3, 1H, Ar) 3.19 (dvt, ²J_HH = 16.3, J_PH = 3.4, 2H, ArCH̲₂), 3.15 (dvt, ²J_HH = 16.5, J_PH = 4.2, 2H, ArCH̲₂), 2.10–2.01 (m, 2H, CH₂), 1.77–1.36 (m, 26H, CH₂), 1.00 (vt, J_PH = 6.6, 18H, tBu).

¹³ C{ ¹ H} NMR (151 MHz, toluene-d₈): δ 201.5 (dvt, ¹J_RhC = 57, ²J_PC = 12, CO), 179.4 (dt, ¹J_PC = 29, ²J_PC = 7, Ar{CRh}), 152.2 (vt, J_PC = 12, Ar{C}), 125.7 (s, Ar), 120.5 (vt, J_PC = 9, Ar), 41.3 (vt, J_PC = 12, ArC̲H₂), 32.7 (vt, J_PC = 11, tBu{C}), 30.5 (vt, J_PC = 4, CH₂), 29.4 (s, CH₂), 29.2 (s, CH₂), 29.0 (s, CH₂), 28.52 (vt, J_PC = 4, tBu{CH₃}), 28.47 (s, CH₂), 26.4 (vt, J_PC = 4, CH₂), 24.2 (vt, ¹J_PC = 9, PCH₂).

³¹ P{ ¹ H} NMR (243 MHz, toluene-d₈): δ 75.0 (d, ¹J_RhP = 146).

IR (toluene): ν(CO) 1939 cm⁻¹.

Anal. Calcd for C₃₁H₅₃OP₂Rh (606.62 g mol⁻¹): C, 61.38; H, 8.81; found: C, 61.34; H, 8.74.

[Rh(POCOP-14)(CO)] (14b). The product was obtained following the general procedure using 15b (20.0 mg, 29.3 μmol) and iPrMgCl·LiCl (68 μL, 89 μmol). Yield: 16.8 mg (94%).

¹ H NMR (600 MHz, toluene-d₈): δ 6.91 (t, ³J_HH = 7.9, 1H, Ar), 6.70 (d, ³J_HH = 7.9, 2H, Ar), 2.00–2.06 (m, 4H, PCH₂), 1.73–1.85 (m, 4H, CH₂), 1.26–1.60 (m, 20H, CH₂), 1.15 (vt, J_PH = 7.2, 18H, tBu).

¹³ C{ ¹ H} NMR (151 MHz, toluene-d₈): δ 201.2 (dt, ¹J_RhC = 60, ²J_PC = 10, CO), 167.7 (vt, J_PC = 9, Ar{C}), 145.5 (dt, ¹J_RhC = 26, ²J_PC = 10, Ar{CRh}), 128.8 (obsc, Ar), 104.8 (vt, J_PC = 7, Ar), 38.1 (vt, ¹J_PC = 12, tBu{C}), 30.6 (br, CH₂), 29.4 (s, CH₂), 29.3 (s, CH₂), 29.24 (vt, J_PC = 9, PCH₂), 29.16 (s, CH₂), 28.8 (s, CH₂), 26.8 (vt, J_PC = 4, tBu{CH₃}), 25.8 (vt, J_PC = 4, CH₂).

³¹ P{ ¹ H} NMR (243 MHz, toluene-d₈): δ 201.6 (d, ¹J_RhP = 156).

IR (toluene): ν(CO) 1958 cm⁻¹.

Anal. Calcd for C₂₉H₄₉O₃P₂Rh (610.56 g mol⁻¹): C, 57.05; H, 8.09; found: C, 57.14; H, 8.07.

General procedure for the preparation of Ir(I) carbonyl complexes

A solution of 19 (ca. 20 μmol) in pentane (2 mL) was added to a flask charged with KC₈ (10 equivalents) and stirred at RT for 2 days. The solution was filtered, reduced to dryness in vacuo, and the analytically pure product obtained following recrystallisation.¹⁶

[Ir(PCP-14)(CO)] (18a). Following the general procedure using 19a (17.2 mg, 22.4 μmol) and KC₈ (30.4 mg, 225 μmol), 18a was obtained following recrystallisation by slow evaporation of neohexane. Yield: 13.1 mg (84%).

¹ H NMR (500 MHz, toluene-d₈): δ 7.14 (d, ³J_HH = 7.5, 2H, Ar), 7.02 (t, ³J_HH = 7.5, 1H, Ar), 3.35 (dvt, ²J_HH = 16.4, J_PH = 4.0, 2H, ArCH̲₂), 3.09 (dvt, ²J_HH = 16.4, J_PH = 3.8, 2H, ArCH̲₂), 2.03–2.19 (m, 2H, CH₂), 1.90–1.78 (m, 2H, PCH₂), 1.76–1.34 (m, 24H, CH₂), 1.01 (vt, J_PH = 6.7, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 198.6 (t, ²J_PC = 7, CO), 181.3 (t, ²J_PC = 4, Ar{CIr}), 153.8 (vt, J_PC = 11, Ar{C}), 126.1 (s, Ar), 120.2 (vt, J_PC = 8, Ar), 41.7 (vt, J_PC = 15, ArC̲H₂), 33.5 (vt, J_PC = 14, tBu{C}), 30.5 (vt, J_PC = 5, CH₂), 29.5 (s, CH₂), 29.3 (s, CH₂), 29.0 (s, CH₂), 28.5 (s, CH₂), 28.2 (vt, J_PC = 3, tBu{CH₃}), 26.3 (vt, J_PC = 3, CH₂), 24.2 (vt, J_PC = 13, PCH₂).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈): δ 68.1 (s).

IR (toluene): ν(CO) 1925 cm⁻¹.

Anal. Calcd for C₃₁H₅₃IrOP₂ (695.93 g mol⁻¹): C, 53.50; H, 7.68; found: C, 53.24; H, 7.76.

[Ir(POCOP-14)(CO)] (18b). Following the general procedure using 19b (12.3 mg, 16.0 μmol) and KC₈ (21.6 mg, 160 μmol), 18b was obtained following recrystallisation by slow evaporation of SiMe₄. Yield: 8.8 mg (79%).

¹ H NMR (500 MHz, toluene-d₈): δ 6.87 (t, ³J_HH = 7.9, 1H, Ar), 6.74 (d, ³J_HH = 7.9, 2H, Ar), 2.03–2.19 (m, 4H, CH₂), 1.73–1.87 (m, 4H, CH₂), 1.23–1.66 (m, 20H, CH₂), 1.15 (vt, J_PH = 7.4, 18H, tBu).

¹³ C{ ¹ H} NMR (126 MHz, toluene-d₈): δ 200.5 (t, ²J_PC = 5, CO), 168.3 (vt, J_PC = 8, Ar{C}), 148.7 (t, ²J_PC = 9, Ar{CIr}), 129.5 (s, Ar), 104.4 (vt, J_PC = 6, Ar), 39.5 (vt, J_PC = 16, tBu{C}), 30.8 (vt, J_PC = 2, CH₂), 29.6 (vt, J_PC = 14, PCH₂), 29.4 (s, CH₂), 29.3 (s, CH₂), 29.2 (s, CH₂), 28.9 (s, CH₂), 26.7 (vt, J_PC = 3, tBu{CH₃}), 26.0 (vt, J_PC = 3, CH₂).

³¹ P{ ¹ H} NMR (162 MHz, toluene-d₈): δ 186.7 (s).

IR (toluene): ν(CO) 1943 cm⁻¹.

Anal. Calcd for C₂₉H₄₉IrO₃P₂ (699.87 g mol⁻¹): C, 49.77; H, 7.06; found: C, 49.85; H 7.01.

Crystallographic details

Data were collected on a Rigaku Oxford Diffraction SuperNova AtlasS2 CCD diffractometer using graphite monochromated Mo Kα (λ = 0.71073 Å) or CuKα (λ = 1.54184 Å) radiation and an Oxford Cryosystems N-HeliX low temperature device [150(2) K]. Data were collected and reduced using CrysAlisPro and refined using SHELXL,²⁹ through the Olex2 interface.³⁰ Full details about the collection, solution, and refinement are documented in CIF format, which have been deposited with the Cambridge Crystallographic Data Centre under CCDC 1972811–1972820.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the European Research Council (ERC, grant agreement 637313) and Royal Society (UF100592, UF150675, A. B. C.) for financial support. High-resolution mass-spectrometry data were collected using instruments purchased through support from Advantage West Midlands and the European Regional Development Fund. Crystallographic data 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: Additional experimental details; NMR, IR and ESI-MS spectra of new compounds, and selected reactions (PDF); primary NMR data (MNOVA). CCDC 1972811–1972820. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt04835a

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