Stereoelectronic effects in a homologous series of bidentate cyclic phosphines. A clear correlation of hydroformylation catalyst activity with ring size

Abstract

The homologous series of diphosphines (CH₂)_n−1P(CH₂)₃P(CH₂)_n−1 where n = 5 (L₅), 6 (L₆), or 7 (L₇) have been synthesized from the corresponding PhP(CH₂)_n−1. Treatment of [PtCl₂(cod)] with L_5–7 gave the 6-membered chelates cis-[PtCl₂(L_5–7)], the crystal structures for which reveal that L_5–7 have very similar steric bulk and bite angles. Treatment of [Rh₂Cl₂(CO)₄] with L_5–7 gave the binuclear trans-[Rh₂Cl₂(CO)₂(μ-L_5–7)₂] with syn and anti orientations of the CO and Cl ligands suggested by the ³¹P NMR spectra and the crystal structures of syn–trans-[Rh₂Cl₂(CO)₂(μ-L₅)₂] and anti–trans-[Rh₂Cl₂(CO)₂(μ-L₇)₂]. The ν(CO) values for trans-[Rh₂Cl₂(CO)₂(μ-L_5–7)₂] indicate that the donor strength increases in the order L₅ < L₆ < L₇. A study of rhodium-catalysed hydroformylation of 1-octene using diphosphines L_5–7 is described. The catalyst activity decreases with increasing phosphacycle ring size: L₅ > L₆ > L₇.

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Introduction

Monodentate and bidentate phosphacycles of a variety of types and sizes are excellent ligands for hydroformylation catalysis as illustrated by the examples L_A–H given in Fig.1.

Fig. 1 Examples of phosphacyclic ligands used in hydroformylation catalysis.

Phosphacyclic ligands often give more active, selective and stable catalysts than their linear analogues. Delineating factors that influence the efficiency of catalysts for hydroformylation is apparently easy, understanding them remains a challenge.^1,2 In the case of phosphacycles, the features that lead to their advantages include: (i) the entropic stabilisation of the P–X bonds in phosphacycles, (ii) the conformational rigidity of the ring and (iii) the constraints on the X–P–X angle imposed by the ring. Thus, the kinetic stabilisation of the P–O bonds in cyclic phosphites,³ such as L_A or the P–N bonds in cyclic phosphamides,⁴ such as L_B make them less susceptible to hydrolysis than acyclic analogues and this has greatly contributed to their utility. The rigidity of ligands such as L_C,⁵L_D⁶ and L_E,⁷ make the α-substituents sterically demanding, which may contribute to the high hydroformylation catalytic activity of their rhodium complexes. The ring rigidity in L_F results in a highly defined chiral space around the metal centre, which may account for the success of L_F and related ligands in asymmetric hydroformylation.⁸ The constrained C–P–C angle imposed by the phosphacycle influences the frontier orbital energies⁹ thereby modulating the σ-donor and π-acceptor characteristics of the ligand and this may partly explain the high activity of hydroformylation catalysts derived from phosphacycles such as L_G¹⁰ and L_H.¹¹

As is evident from the examples in Fig. 1, phosphacycles with ring sizes from 5–7 are important for hydroformylation catalysis. We are interested in understanding the effect the size of the ring has on the coordination chemistry and catalytic performance of P-ligands.^7,12,13 Recently, we reported a systematic study of the homologous series of cyclic monophosphines 1a–c and their application in rhodium-catalysed hydroformylation.¹² From spectroscopic measurements, it was found that the donor order was 1a < 1b < 1c, which is in line with the prediction that the more acute the C–P–C angle is made, the lower the HOMO and LUMO energies on the ligand become.⁹ The steric properties of the monophosphacycles (particularly 1c) were shown to depend critically on the ring conformation but no clear trend emerged from the hydroformylation catalysis study with 1a–c and related ligands.¹² Part of the problem with discerning structure–activity relationships with monodentate ligands is the variable number (between 0 and 3) of ligands that may be coordinated to the metal and the cis/trans/axial/equatorial geometric relationships to each other that they adopt in the catalytic intermediates. We reasoned that the number of variables would be reduced, and therefore clearer trends may emerge, using the cyclic diphosphines L_5–7.

Results and discussion

Ligand synthesis

The series of bidentate cyclic phosphines L_5–7 were synthesised by the route shown in Scheme 1. The phosphacycles¹²1a–c were quaternized with 1,3-dibromopropane to give the corresponding phosphonium salts 2a–c as air-stable solids. Refluxing these salts in aqueous sodium hydroxide¹⁴ yielded the phosphine oxides 3a–c as white waxy solids. The reduction of 3a–c with phenylsilane, followed by distillation directly from the hot reaction mixture, produced L_5–7. Diphosphines L₅ and L₆ are colourless liquids and L₇ is a low-melting solid (see Experimental for the characterising data); L₅ has been previously reported¹⁵ but L₆ and L₇ are new.

Scheme 1

Platinum(II) complexes

The addition of one equivalent of diphosphines L_5–7 to [PtCl₂(cod)] in dichloromethane afforded the corresponding complexes cis-[PtCl₂(L_5–7)] (4a–c) as air-stable, white solids in quantitative yields (eqn (1), see Experimental for the characterising data).

Crystal structures for all three chelates 4a–c were obtained (see Fig. 2–4 and Tables 1–3). The chloroform solvate of 4a crystallises in the space groupP2₁2₁2₁ from a saturated CHCl₃ solution. The molecule of 4a (see Fig. 2) has approximate mirror symmetry. Selected bond lengths and angles are given in Table 1.

Table 1 Selected bond distances (Å) and angles (°) for 4a·CHCl₃

Bond distances/Å
Pt1–P1	2.2143(11)	Pt1–P2	2.2089(11)
Pt1–Cl1	2.3604(10)	Pt1–Cl2	2.3533(10)
P1–C1	1.835(4)	P1–C4	1.828(4)
P1–C5	1.807(4)	P2–C7	1.819(4)
P2–C8	1.822(5)	P2–C11	1.838(4)

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Bond angles/°
P1–Pt1–P2	96.02(3)	C1–P1–C5	104.1(2)
C1–P1–C4	96.26(19)	C4–P1–C5	104.1(2)
C7–P2–C8	104.5(2)	C7–P2–C11	103.5(2)
C8–P2–C11	95.81(19)

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Fig. 2 Thermal ellipsoid plot of the structure of 4a in 4a·CHCl₃. Displacement ellipsoids are shown at the 50% probability level. All hydrogen atoms have been removed for clarity.

Fig. 3 Thermal ellipsoid plot of the structure of 4b. Displacement ellipsoids are shown at the 50% probability level. All hydrogen atoms have been removed for clarity.

Fig. 4 Thermal ellipsoid plot of the ordered molecule (of three) in the asymmetric unit of the structure of 4c. Displacement ellipsoids are shown at the 50% probability level. All hydrogen atoms have been removed for clarity.

The five-membered phospholane rings can adopt a range of conformations of which four idealised forms may usefully be identified (Fig. 5). If the phosphorus atom and the two neighbouring carbon atoms are considered to define a reference plane, the other two carbons in the ring can be: (A) in plane with these atoms (planar), (B) one in plane and one out of the plane (asymmetric envelope), (C) both above or both below the plane (envelope) or (D) one above and one below the plane (twist). In 4a, both rings adopt an asymmetric envelope conformation, with the torsion angle C–C–C–C = ± 47.6°.

Fig. 5 Conformations of five-membered phospholane rings.

Crystals of 4b crystallised from a saturated MeOH solution in the space groupPbca with one molecule in the asymmetric unit. Selected bond lengths and angles are given in Table 2. The PC₅ rings adopt a chair conformation with the C₃ chelate backbone and the metal in axial and equatorial sites, respectively (see Fig. 3).

Table 2 Selected bond distances (Å) and angles (°) for 4b

Bond distances/Å
Pt1–P1	2.2228(12)	Pt1–P2	2.2289(13)
Pt1–Cl1	2.3716(13)	Pt1–Cl2	2.3774(13)
P1–C1	1.825(5)	P1–C5	1.822(5)
P1–C6	1.823(4)	P2–C8	1.817(5)
P2–C9	1.827(5)	P2–C13	1.827(5)

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Bond angles/°
P1–Pt1–P2	96.48(5)	C8–P2–C9	104.8(2)
C8–P2–C13	104.0(2)	C9–P2–C13	104.0(2)
C1–P1–C5	104.5(2)	C1–P1–C6	104.0(2)
C5–P1–C6	105.6(2)

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Crystals of 4c crystallised from a saturated MeCN solution in the space groupPc with three molecules in the asymmetric unit. In two of the molecules, significant disorder is present in the PC₆ rings. The molecular geometry (see Table 3) of the only ordered molecule in the crystal structure is shown in Fig. 4. The seven-membered rings in the molecule containing Pt1 adopt twist-chair and boat conformations. In the molecule containing Pt2 they adopt chair conformations with the P atom in a different position in each of the rings and in the molecule containing Pt3 they adopt twist-chair, and chair conformations.^12,16

Table 3 Selected bond distances (Å) and angles (°) for 4c.^a Parameters for the atoms that were restrained have been omitted

Bond distances/Å
a Equivalent distances for the second and third independent molecules in the asymmetric unit are given in columns 4 and 6. Atoms which are disordered are labelled A or B.
Pt1–P1	2.2275(17)	Pt2–P3	2.2258(16)	Pt3–P5	2.2303(18)
Pt1–P2	2.2330(15)	Pt2–P4	2.2293(15)	Pt3–P6	2.2347(18)
Pt1–Cl1	2.3719(16)	Pt2–Cl3	2.3740(16)	Pt3–Cl5	2.3636(18)
Pt1–Cl2	2.3884(16)	Pt2–Cl4	2.3898(15)	Pt3–Cl6	2.3921(18)
P1–C1	1.837(6)	P3–C16	1.835(6)	P5–C31	1.811(7)
P1–C4	1.821(6)	P3–C19	1.829(6)
P1–C9	1.825(6)	P3–C24	1.821(6)
P2–C3	1.818(6)	P4–C18	1.810(6)	P6–C33	1.814(6)
P2–C10	1.827(6)	P4–C25	1.823(6)	P6–C40	1.823(7)
P2–C15	1.831(6)	P4–C30	1.825(6)	P6–C45	1.827(7)

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Bond angles/°
P1–Pt1–P2	97.09(6)	P3–Pt2–P4	97.00(6)	P5–Pt3–P6	95.24(6)
C1–P1–C4	102.6(3)	C16–P3–C24	102.2(3)
C1–P1–C9	103.3(3)	C16–P3–C19	103.5(3)
C4–P1–C9	109.5(3)	C19–P3–C24	110.0(3)
C3–P2–C10	104.1(3)	C18–P4–C25	103.4(3)	C33–P6–C40	102.6(3)
C3–P2–C15	103.6(3)	C18–P4–C30	103.7(3)	C33–P6–C45	102.2(3)
C10–P2–C15	110.4(3)	C25–P4–C30	110.5(3)	C40–P6–C45	108.6(3)

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From the crystallographic parameters collected in Table 4, it can be deduced that: (1) the intracyclic C–P–C angles increases significantly with increasing ring size, (2) the ligand bite angles increase slightly with increasing ring size and (3) the ligand cone angles (calculated by Tolman's method¹⁷) show that the ligands have essentially the same bulk.

Table 4 Selected angles (°) from the crystal structures of 4a–c

Angles/°	4a	4b	4c
a Average of two. b Average of five. c Average of three.
Intracyclic C–P–C	96.0^a	104.3^a	109.8^b
P–Pt–P	96.0	96.5	97.0^a
Tolman cone angle	228	229	225^c

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Rhodium(I) complexes

Addition of two equivalents of L_5–7 to [Rh₂Cl₂(CO)₄] in CH₂Cl₂ gave two species in each case, which are assigned to the binuclear complexes trans-[Rh₂Cl₂(CO)₂(μ-L_5–7)₂] as a mixture of anti-5a–c and syn-5a–c isomers (eqn (2), see Experimental for characterising data). These structures are assigned on the basis of the ³¹P NMR spectra, which showed, in each case, two doublets with similar δ(P) and J(RhP) having values typical of a trans-RhCl(CO)(PR₃)₂ structure.¹⁸ A similar ligand-bridged binuclear structure has been reported for trans-[Rh₂Cl₂(CO)₂(μ-Ph₂P(CH₂)₃PPh₂)₂].¹⁹

The crystal structures of syn-5a and anti-5c have been determined (see Fig. 6 and 7 and Tables 5 and 6). Crystals of syn-5a were grown by slow diffusion of diethyl ether into a saturated CH₂Cl₂ solution. The crystals form in the space groupP2₁/c with one binuclear molecule in the asymmetric unit. The phospholane rings all adopt distorted asymmetric envelope conformations with four out of five atoms in the ring (including phosphorus) coplanar. The angle between the mean planes of Rh1, P1, P4 and Cl1 and Rh2, P2, P3 and Cl2 is 15.1(1)°. Selected geometric parameters are given in Table 5.

Table 5 Selected bond distances (Å) and angles (°) for syn-5a

Bond distances/Å
Rh1–C23	1.805(4)	Rh1–Cl1	2.3714(9)
Rh1–P1	2.3183(9)	Rh1–P4	2.3049(9)
Rh2–C24	1.811(4)	Rh2–Cl2	2.3746(9)
Rh2–P2	2.3017(9)	Rh2–P3	2.3077(9)
P1–C1	1.839(3)	P1–C4	1.852(3)
P1–C5	1.833(4)	P2–C7	1.831(3)
P2–C8	1.839(4)	P2–C11	1.836(4)
P3–C12	1.838(3)	P3–C15	1.847(3)
P3–C16	1.835(3)	P4–C18	1.838(3)
P4–C19	1.836(3)	P4–C22	1.844(3)

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Bond angles/°
C1–P1–C4	94.56(16)	C1–P1–C5	103.68(17)
C4–P1–C5	103.21(17)	C7–P2–C11	104.51(17)
C7–P2–C8	104.57(18)	C8–P2–C11	93.59(18)
C12–P3–C15	94.05(16)	C12–P3–C16	105.84(16)
C15–P3–C16	104.71(17)	C18–P4–C22	103.57(16)
C18–P4–C19	106.55(16)	C19–P4–C22	94.05(16)

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Table 6 Selected bond distances (Å) and angles (°) for anti-5c

Bond distances/Å
Rh1–C31	1.927(9)	Rh1–Cl1	2.381(2)
Rh1–P1	2.3200(17)	Rh1–P3	2.3160(17)
Rh2–C32	1.807(8)	Rh2–Cl2	2.376(2)
Rh2–P2	2.3111(18)	Rh2–P4A	2.429(10)
Rh2–P4B	2.242(9)
P1–C1	1.833(6)	P1–C4	1.830(6)
P1–C9	1.833(6)	P2–C3	1.836(5)
P2–C10	1.819(6)	P2–C15	1.822(6)
P3–C16A	1.813(7)	P3–C19	1.837(6)
P3–C24	1.826(6)	P4A–C25A	1.812(18)
P4A–C30A	1.81(3)

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Bond angles/°
C1–P1–C4	104.4(3)	C1–P1–C9	99.7(3)
C4–P1–C9	101.8(4)	C3–P2–C10	101.8(3)
C3–P2–C15	102.9(3)	C10–P2–C15	101.7(4)
C16A–P3–C19	102.8(3)	C16A–P3-C24	101.2(3)
C19–P3–C24	101.5(3)	C18A–P4A–C25A	106.5(10)
C18A–P4A–C30A	101.7(13)	C25A–P4A–C30A	101.5(19)
C18B–P4B–C25B	104.2(8)	C18B–P4B–C30B	101.9(10)
C25B–P4B–C30B	101.8(14)

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Fig. 6 Thermal ellipsoid plot of the structure of syn-5a. Displacement ellipsoids are shown at the 50% probability level. All hydrogen atoms have been removed for clarity.

Fig. 7 Thermal ellipsoid plot of the structure of anti-5c. Displacement ellipsoids are shown at the 50% probability level. All hydrogen atoms have been removed for clarity.

Crystals of anti-5c were grown by slow diffusion of hexane into a saturated CH₂Cl₂ solution, and form in the space groupP2₁2₁2₁ with one binuclear molecule in the asymmetric unit (see, Fig. 7). There is disorder in one of the seven membered rings (involving P4) which was modelled as lying over two conformations in the ratio 0.44 : 0.56. The angle between the mean planes of Rh1, P1, P3 and Cl1 and Rh2, P2, P4 and Cl2 is 41.0(1)°. Selected geometric parameters are given in Table 6.

The IR spectra of 5a–c showed single absorptions for ν(CO) at 1968 (5a), 1963 (5b) and 1960 cm⁻¹ (5c), which are consistent with the donor properties²⁰ of the ligands being in the order L₅ < L₆ < L₇. This is the trend expected⁹ on the basis of the crystallographically determined intracyclic C–P–C angles for the phosphines (see below).

Hydroformylation catalysis

Diphosphines with a propane backbone have previously been used in rhodium-catalysed hydroformylation and it is assumed that mononuclear 6-membered chelates are involved in the catalysis.²¹ The diphosphines L_5–7 were screened for the rhodium-catalysed hydroformylation of 1-octene and the results are given in Table 7. The regioselectivity is similar for the three catalysts which is consistent with the ligands having similar steric bulk.²² The activity of the catalysts is in the order L₅ > L₆ > L₇, which parallels the order of increasing electronic donation of the three ligands. Thus for this series of isosteric diphosphine ligands containing 5-, 6- or 7-membered phosphacycles, the smaller the ring, the higher the hydroformylation catalytic activity of the derived rhodium complex.

Table 7 Hydroformylation of 1-octene^a

Entry	Ligand	% Conversion to nonanals	n : iso
a Reactions conditions: 90 °C, 10 bar CO–H2 (1 : 1) for 4 h in toluene, L–Rh = 5 : 1. See Experimental for details. No decomposition to metallic rhodium was apparent when the autoclaves were opened at the end of a catalytic run. Under more forcing conditions (140 °C, 80 bar), quantitative conversion to aldehyde was observed for all three ligands. No 2-octene was detected in the product of any run and under the same conditions (90 °C, 10 bar, 4 h) the conversions when 2-octene was the substrate were < 1%. The conversions of 1-octene to nonanals are the average of 2 or 3 runs; the differences between individual runs were within 5% for entries 1 and 2 and within 0.5% for entry 3.
1	L 5	69	2.3
2	L 6	48	2.2
3	L 7	3	2.0

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Diphosphine ligands are widely used for hydroformylation and it has been shown that the ligand bite angle and the bulk of the ligand have a significant effect on the efficiency of the catalyst.^1,23 However, the crystal structures of 4a–c described above show that chelated ligands L_5–7 are essentially isosteric and have very similar bite angles. Moreover, the ligand giving the most active catalyst (L₅), has the smallest bite angle while generally, increasing the ligand bite angle above 90° increases the activity of the derived hydroformylation catalysts.²³

Electronegative substituents on the P-donor often increase the hydroformylation catalytic activity²⁴ of the Rh complex although this is not always the case.²⁵ Therefore, we conclude that the phosphacycle ring effect observed here is electronic in origin, confirming the prediction of Orpen and Connelly⁹ that reducing the C–P–C angle has the effect of reducing the σ-donor and increasing the π-acceptor properties of the tertiary phosphine.

Experimental

General procedures

Unless otherwise stated, all reactions were carried out under a dry nitrogen atmosphere using standard Schlenk line techniques. Dry N₂-saturated solvents were collected from a Grubbs solvent system²⁶ in flame and vacuum dried glassware. MeOH was dried over 3 Å molecular sieves and deoxygenated by N₂ saturation. Commercial reagents were used as supplied unless otherwise stated. All phosphines were stored under nitrogen at room temperature. Most complexes were stable to air in the solid state and were stored in air at room temperature. Starting materials [PtCl₂(cod)]²⁷ and [Rh₂Cl₂(CO)₄]²⁸ were prepared by literature methods. Elemental analyses were carried out by the Microanalytical Laboratory of the School of Chemistry, University of Bristol. Electron Impact and Fast Atom Bombardment mass spectra were recorded by the Mass Spectrometry Service, University of Bristol on a MD800 and an Autospec. Infrared spectroscopy was carried out on a Perkin Elmer 1600 Series FTIR. NMR spectra were measured on a Joel GX 300, Jeol Eclipse 400 or Jeol GX 400. ³¹P{¹H}, ¹³C{¹H} and ¹H NMR spectra were recorded at ambient temperature of the probe at 300, 100 and 121 MHz, respectively, using deuterated solvent to provide the field–frequency lock.

Synthesis of 1,3-bis(phenylphospholanium)propane dibromide (2a). 1-Phenylphospholane 1a (2.11 g, 12.9 mmol) and 1,3-dibromopropane (0.65 cm³, 1.30 g, 6.45 mmol) in acetonitrile (20 cm³) were heated at 60 °C for 48 h. The solvent was removed under reduced pressure to give 2a as a white solid (3.40 g, 6.45 mmol, 100%). Elemental analysis, found (calcd for C₂₃H₃₂Br₂P₂·H₂O): C 49.85 (50.38), H 6.28 (6.25). ³¹P NMR (CD₂Cl₂) δ_P/ppm: 49.7 (s). ¹H NMR (CD₂Cl₂) δ_H/ppm: 8.16–8.03 (m, 4H, ArH), 7.74–7.58 (m, 6H, ArH), 3.48–3.23 (m, 8H), 2.60–2.44 (m, 4H), 2.36–1.94 (m, 10H). ¹³C NMR (CD₂Cl₂) δ_C/ppm: 134.7 (s), 132.6 (m), 130.4 (m), 119.6 (d, ArC_ipso, ¹J(PC) 77.6 Hz), 26.9 (m), 23.5 (m), 23.5 (d, PCH₂, ¹J(PC) 51.5 Hz), 17.6 (t, PCH₂CH₂CH₂P, ²J(PC) 2.3 Hz).

1,3-Bis(phenylphosphinanium)propane dibromide (2b). Compound 2b was made in a similar fashion to 2a from 1-phenylphosphinane 1b in 84% yield. Elemental analysis, found (calcd for C₂₅H₃₆Br₂P₂): C 53.53 (53.78), H 6.74 (6.50). ³¹P NMR (CD₂Cl₂) δ_P/ppm: 21.9 (s). ¹H NMR (CD₂Cl₂) δ_H/ppm: 8.20–8.10 (m, 4H, ArH), 7.73–7.66 (m, 2H, ArH), 7.66–7.58 (m, 4H, ArH), 3.21–2.94 (m, 12H), 2.24–2.06 (m, 4H), 2.06–1.90 (m, 2H), 1.87–1.72 (m, 2H), 1.70–1.52 (m, 6H). ¹³C NMR (CD₂Cl₂) δ_C/ppm: 134.6 (s), 133.1 (m), 130.4 (m), 117.3 (d, ArC_ipso, ¹J(PC) 79.2 Hz), 24.9 (m), 24.4 (dd, PCH₂CH₂CH₂P, ¹J(PC) 48.4 Hz, ³J(PC) 16.1 Hz), 21.4 (m), 18.5 (d, PCH₂, cyclo, ¹J(PC) 47.7 Hz), 16.1 (t, PCH₂CH₂CH₂P, ²J(PC) 3.1 Hz).

1,3-Bis(phenylphosphepanium)propane dibromide (2c). Compound 2c was made in a similar fashion to 2a from 1-phenylphosphepane 1c in 90% yield. Elemental analysis, found (calcd for C₂₇H₄₀Br₂P₂): C 54.71 (55.31), H 7.24 (6.88). ³¹P NMR (CDCl₃) δ_P/ppm: 33.9 (s). ¹H NMR (CDCl₃) δ_H/ppm: 8.02–7.92 (m, 4H, ArH), 7.63–7.51 (m, 6H, ArH), 3.27–3.12 (m, 4H), 3.06–2.95 (m, 4H), 2.93–2.79 (m, 4H), 2.17–1.93 (m, 6H), 1.81–1.62 (m, 8H), 1.56–1.41 (m, 4H). ¹³C NMR (CDCl₃) δ_C/ppm: 134.1 (s), 131.9 (m), 130.1 (m), 118.7 (d, ArC_ipso, ¹J(PC) 80.0 Hz), 28.7 (s), 25.0 (m), 21.7 (m), 21.4 (d, J(PC) 46.9 Hz), 16.4 (m).

Synthesis of 1,3-diphospholanopropane dioxide (3a). To 2a (3.40 g, 6.45 mmol) was added NaOH (50 cm³, 20 wt% in distilled water) and the resulting suspension was heated under reflux for 16 h. The product was extracted into CHCl₃ (3 × 25 cm³) and the resulting solution dried over MgSO₄. The solvents were removed under reduced pressure to give 3a as a white solid (1.43 g, 5.76 mmol, 90%). ³¹P NMR (CDCl₃) δ_P/ppm: 70.4 (s). ¹H NMR (CDCl₃) δ_H/ppm: 2.24–1.66 (m). ¹³C NMR (CDCl₃) δ_C/ppm: 31.5 (dd, PCH₂CH₂CH₂P, ¹J(PC) 60.7 Hz, ³J(PC) 11.5 Hz), 26.9 (d, PCH₂, cyclo, ¹J(PC) 65.3 Hz), 24.2 (m), 15.7 (t, PCH₂CH₂CH₂P, ²J(PC) 3.8 Hz).

1,3-Diphosphinanopropane dioxide (3b). Compound 3b was made in a similar fashion to 3a from 2b in 100% yield. ³¹P NMR (CD₂Cl₂) δ_P/ppm: 39.0 (s). ¹H NMR (CD₂Cl₂) δ_H/ppm: 1.97–1.36 (m). ¹³C NMR (CD₂Cl₂) δ_C/ppm: 28.9 (dd, PCH₂CH₂CH₂P, ¹J(PC) 64.6 Hz, ³J(PC) 12.3 Hz), 28.0 (d, PCH₂, cyclo, ¹J(PC) 62.3 Hz), 26.9 (m), 23.1 (m), 14.2 (t, PCH₂CH₂CH₂P, ²J(PC) 3.9 Hz).

1,3-Diphosphepanopropane dioxide (3c). Compound 3c was made in a similar fashion to 3a from 2c in 99% yield. ³¹P NMR (CDCl₃) δ_P/ppm: 52.9 (s). ¹H NMR (CDCl₃) δ_H/ppm: 2.10–1.52 (m). ¹³C NMR (CDCl₃) δ_C/ppm: 31.1 (dd, PCH₂CH₂CH₂P, ¹J(PC) 63.68 Hz, ³J(PC) 11.5 Hz), 29.8 (s) 29.8 (d, PCH₂, cyclo, ¹J(PC) 62.3 Hz), 21.1 (m), 14.6 (t, PCH₂CH₂CH₂P, ²J(PC) 3.9 Hz).

Synthesis of 1,3-diphospholanopropane (L₅). A mixture of 3a (1.43 g, 5.76 mmol) and phenylsilane (0.98 cm³, 0.88 g, 8.1 mmol) was heated at 100 °C for 2 h and the evolution of gas was observed. The resulting opalescent viscous liquid was distilled (0.01 mmHg, fraction collected and boiled over the range 100–140 °C) to give L₅ as a clear viscous liquid (0.58 g, 2.69 mmol, 46%). Elemental analysis, found (calcd for C₁₁H₂₂P₂): C 60.78 (61.10), H 10.02 (10.25). ³¹P NMR (CDCl₃) δ_P/ppm: −26.7 (s). ¹H NMR (CDCl₃) δ_H/ppm: 1.89–1.61 (m, 12H), 1.59–1.33 (m, 10H). ¹³C NMR (CDCl₃) δ_C/ppm: 30.4 (dd, PCH₂CH₂CH₂P, ¹J(PC) 16.6 Hz, ³J(PC) 10.9 Hz), 27.6 (d, J(PC) 4.2 Hz), 25.7 (d, PCH₂, cyclo, ¹J(PC) 11.4 Hz), 24.0 (t, PCH₂CH₂CH₂P, ²J(PC) 16.6 Hz).

1,3-Diphosphinanopropane (L₆). L₆ was made in a similar fashion to L₅ in 85% yield. b.p. = 112–113 °C, 0.2 mmHg. Elemental analysis, found (calcd for C₁₃H₂₆P₂) C 64.38 (63.91), H 11.05 (10.73). ³¹P NMR (CDCl₃) δ_P/ppm: −41.9 (s). ¹H NMR (CDCl₃) δ_H/ppm: 1.87–1.73 (m, 14H), 1.73–1.62 (m, 4H), 1.58–1.44 (m, 12H), 1.33–1.17 (m, 6H). ¹³C NMR (CDCl₃) δ_C/ppm: 28.7 (dd, PCH₂CH₂CH₂P, ¹J(PC) 13.1 Hz, ³J(PC) 11.5 Hz), 27.8 (d, J(PC) 2.3 Hz), 24.5 (d, PCH₂, cyclo, ¹J(PC) 10.8 Hz), 23.2 (d, J(PC) 2.3 Hz), 22.1 (t, PCH₂CH₂CH₂P, ²J(PC) 15.4 Hz).

1,3-Diphosphepanopropane (L₇). L₇ was made in a similar fashion to L₅ in 78% yield. b.p. = 117–126 °C, 0.2 mmHg. Elemental analysis, found (calcd for C₁₅H₃₀P₂): C 66.45 (66.15), H 11.10 (11.27). ³¹P NMR (CDCl₃) δ_P/ppm: −33.2 (s). ¹H NMR (CDCl₃) δ_H/ppm: 1.90–1.65 (m, 8H), 1.63–1.20 (m, 22H). ¹³C NMR (CDCl₃) δ_C/ppm: 31.0 (t, J(PC) 10.7 Hz), 29.0 (d, J(PC) 13.1 Hz), 28.3 (d, 4.6 Hz), 25.8 (d, J(PC) 7.7 Hz), 22.6 (t, PCH₂CH₂CH₂P, ²J(PC) 14.6 Hz).

Synthesis of [PtCl₂(L₅)] (4a). To a solution of L₅ (0.090 g, 0.416 mmol) in dichloromethane (2 cm³) was added [PtCl₂(cod)] (0.140 g, 0.375 mmol). After stirring the reaction for 12 h, the solvent was removed and the resulting white solid was washed with methanol (10 cm³) to give the desired product (0.142 g, 0.294 mmol, 79% yield). Elemental analysis, found (calcd for C₁₁H₂₂Cl₂P₂Pt): C 27.82 (27.40), H 4.56 (4.60). ³¹P NMR (CDCl₃) δ_P/ppm: 2.8 (s, J(PtP) 3323 Hz). ¹H NMR (CDCl₃) δ_H/ppm: 2.88–2.57 (m, 4H), 2.40–1.59 (m, 18H). ¹³C NMR (CDCl₃) δ_C/ppm: 27.7 (m), 27.3 (m), 23.8 (m), 20.8 (s). EI mass spectrum: m/z 482 (M⁺), 446 (M⁺− Cl), 409 (M⁺− 2Cl).

[PtCl₂(L₆)] (4b). Compound 4b was made in a similar fashion to 4a from L₆ in 64% yield. Elemental analysis, found (calcd for C₁₃H₂₆Cl₂P₂Pt): C 30.42 (30.78), H 5.07 (5.14). ³¹P NMR (CDCl₃) δ_P/ppm: −17.1 (s, J(PtP) 3343 Hz). ¹H NMR (CDCl₃) δ_H/ppm: 2.99–2.81 (m, 4H), 2.05–1.76 (m, 14H), 1.75–1.65 (m, 2H), 1.60–1.38 (m, 6H). ¹³C NMR (CDCl₃) δ_C/ppm: 25.7 (m), 23.2 (m), 22.2 (m), 19.3 (s), 17.8 (m). EI mass spectrum: m/z 474 (M⁺− Cl), 437 (M⁺− 2Cl).

[PtCl₂(L₇)] (4c). Compound 4c was made in a similar fashion to 4a from L₇ in 70% yield. Elemental analysis, found (calcd for C₁₅H₃₀Cl₂P₂Pt): C 33.43 (33.47), H 5.48 (5.62). ³¹P NMR (CDCl₃) δ_P/ppm: −8.3 (s, J(PtP) 3333 Hz). ¹H NMR (CDCl₃) δ_H/ppm: 2.98–2.78 (m, 4H), 2.14–1.73 (m, 14H), 1.72–1.46 (m, 12H). ¹³C NMR (CDCl₃) δ_C/ppm: 29.3 (s), 28.1 (m), 23.1 (m), 22.1 (m), 18.9 (m). FAB mass spectrum: m/z 501 (M⁺− Cl-1).

Synthesis of [Rh₂Cl₂(CO)₂(μ−L₅)₂] (5a). To a solution of L₅ (0.034 g, 0.157 mmol) in dichloromethane (3 cm³) was added [RhCl(CO)₂]₂ (0.030 g, 0.079 mmol) as a solid. After stirring the reaction for 1 h, the solvent was reduced to ca. 1 cm³ and hexane (20 cm³) was added. The resulting precipitate was filtered off, washed with hexane and dried under reduced pressure to afford the desired product as a light brown powder (0.027 g, 0.071 mmol, 45% yield). Elemental analysis, found (calcd for C₂₄H₄₄Cl₂O₂P₄Rh₂): C 37.20 (37.67), H 5.27 (5.80). ³¹P NMR (CD₂Cl₂) δ_P/ppm: 27.9 (br d, J(RhP) 108 Hz), 27.0 (d, J(RhP) 113 Hz). ¹H NMR (CD₂Cl₂) δ_H/ppm: 2.50–1.30 (br m, 22H, CH₂). ¹³C NMR (CD₂Cl₂) δ_C/ppm: 32.0–24.4 (m). IRν_CO (CH₂Cl₂): 1968 cm⁻¹. EI mass spectrum: m/z 708 (M⁺− 2CO).

[Rh₂Cl₂(CO)₂(μ-L₆)₂] (5b). Compound 5b was made in a similar fashion to 5a from L₆ in 64% yield. Elemental analysis, found (calcd for C₂₈H₅₂Cl₂O₂P₄Rh₂): C 41.24 (40.95), H 7.06 (6.38). ³¹P NMR (CD₂Cl₂) δ_P/ppm: 6.7 (d, J(RhP) 117 Hz), 6.0 (d, J(RhP) 117 Hz). ¹H NMR (CD₂Cl₂) δ_H/ppm: 2.50–1.10 (m, 26H, CH₂). ¹³C NMR (CD₂Cl₂) δ_C/ppm: 188.7–187.6 (m, CO), 30.9–20.5 (m). IRν_CO (CH₂Cl₂): 1963 cm⁻¹. EI mass spectrum: m/z 764 (M⁺− 2CO).

[Rh₂Cl₂(CO)₂(μ-L₇)₂] (5c). Compound 5c was made in a similar fashion to 5a from L₇ in 88% yield. Elemental analysis, found (calcd for C₃₂H₆₀Cl₂O₂P₄Rh₂): C 43.75 (43.80), H 6.98 (6.89). ³¹P NMR (CD₂Cl₂) δ_P/ppm: 18.5 (d, J(RhP) 115 Hz), 17.0 (d, J(RhP) 115 Hz). ¹H NMR (CD₂Cl₂) δ_H/ppm: 2.60–2.29 (m, 2H), 2.13–1.36 (m, 28H). ¹³C NMR (CD₂Cl₂) δ_C/ppm: 188.8–187.6 (m, CO), 32.5–23.9 (m). IRν_CO (CH₂Cl₂): 1960 cm⁻¹. FAB mass spectrum: m/z 877 (M⁺), 720 (M⁺− 2CO).

Hydroformylation catalysis. The phosphine (0.060 mmol) and [Rh(CO)₂(acac)] (3.0 mg, 0.012 mmol) were dissolved in toluene (10 cm³) under nitrogen in a Schlenk tube. The resulting solution was transferred by cannula to a 100 mL autoclave, which had been flushed 3 times with 3 bar CO–H₂ (1 : 1). The autoclave was then pressurized with 2 bar of CO–H₂ (1 : 1) at room temperature. The reaction mixture was then stirred vigorously with a sparging stirrer and heated to 90 °C over a period of 30 min. After this pre-activation of the catalyst, the 1-octene (10 g) was introduced into the autoclavevia a lock by means of CO–H₂ pressure. The pressure was then immediately raised to 10 bar and maintained at this pressure throughout the catalysis by introduction of further CO–H₂ (1 : 1) via a pressure regulator. After running the reactions for 4 h, the autoclave was cooled, vented and emptied. Analysis of the reaction products was carried out by GC.

Crystal structure determinations

X-Ray diffraction experiments of 4b, 4c, syn-5a and anti-5c were carried out at 173K on a Bruker SMART diffractometer; an experiment on 4a (as its chloroform solvate) was carried out at 100K on a Bruker SMART APEX diffractometer, all using MoKa radiation (λ = 0.71073 Å). All data collections were performed using a CCD area detector from a single crystal coated in perfluoroalkylether and mounted on a glass fibre. Intensities were integrated²⁹ from several series of exposures measuring 0.3° in ω. Absorption corrections were based on equivalent reflections using SADABS,³⁰ and structures were refined against all F_o² data with hydrogen atoms riding in calculated positions using SHELXTL.³¹ Crystal structure and refinement data are given in Table 8. In 4a·CHCl₃ the molecular complex has approximate mirror symmetry, as such that the crystal structure may also be approximately described in the space groupPnma, albeit with disorder in the chloroform molecule. The crystal structure refines satisfactorily in P2₁2₁2₁ as an inversion twin (Flack parameter = 0.461(5)). Coupled with the fact that there are a significant number of (albeit weak) reflections present in the diffraction data, which should be systematically absent in Pnma, and that the weighted R-factor is lower and residual density map is cleaner in the lower symmetry space group, this suggests that P2₁2₁2₁ is the correct choice. In 4c, the ring containing P(4) has three of the carbon atoms disordered over two positions in proportions 0.66/0.34, and the ring containing P(5) has all of the carbon atoms disordered over two positions (ration 0.57/0.43). The use of restraints on carbon–carbon bond lengths was necessary to ensure convergence of the refinement.

Table 8 Crystallographic data

Compound	4a·CHCl3	4b	4c	5a	5c
Colour, habit	Colourless, block	Colourless, plate	Colourless, block	Yellow, block	Yellow, plate
Size/mm	0.14 × 0.07 × 0.07	0.25 × 0.07 × 0.03	0.14 × 0.10 × 0.06	0.40 × 0.30 × 0.20	0.40 × 0.26 × 0.02
Empirical Formula	C12H23Cl5P2Pt	C13H26Cl2P2Pt	C15H30Cl2P2Pt	C24H44Cl2O2P4Rh2	C32H60Cl2O2P4Rh2
M r/g mol^−1	601.58	510.27	538.32	765.19	877.40
Crystal system	Orthorhombic	Orthorhombic	Monoclinic	Monoclinic	Orthorhombic
Space group	P212121	Pbca	Pc	P21/c	P212121
a/Å	12.165(2)	12.723(3)	34.324(7)	11.7519(7)	11.0204(18)
b/Å	12.425(3)	11.954(3)	6.3850(13)	11.9472(7)	11.9691(16)
c/Å	12.614(3)	22.007(6)	12.625(3)	22.4102(13)	28.877(3)
β/°	90.00	90.00	92.05(3)	91.1220(10)	90.00
V/Å^3	1906.4(7)	3347.0(15)	2765.1(10)	3145.8(3)	3809.0(9)
Z	4	8	6	4	4
μ/mm^−1	8.217	8.878	8.065	1.442	1.202
T/K	100	173	173	173	173
Total reflections	21 202	20 486	28 297	20 348	25 010
Independent reflections	4334	3843	12 421	7198	8754
R int	0.0291	0.0602	0.0296	0.0518	0.0811
Final R1 (I > 2σ)	0.0185	0.0292	0.0281	0.0364	0.0444
Final wR2	0.0468	0.0706	0.0693	0.0795	0.1079
Largest peak, hole (e Å^−3)	1.06, −0.66	1.65, −1.61	1.76, −1.57	0.65, −0.65	0.62, −0.95
ρ calcd/g cm^−3	2.096	2.025	1.940	1.616	1.530
Flack parameter	0.461(5)	—	-0.001(4)	—	−0.01(4)

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Acknowledgements

We thank BASF and the EPSRC for financial support, Johnson-Matthey for a loan of precious metal compounds and the Leverhulme Trust for a Research Fellowship (to PGP).

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25. M. L. Clarke, D. Ellis, K. L. Mason, A. G. Orpen, P. G. Pringle, R. L. Wingad, D. A. Zaher and R. T. Baker, Dalton Trans., 2005, 1294.
26. A. B. Pangborn, M. A. Giardello, R. H. Grubbs, R. K. Rosen and F. J. Timmers, Organometallics, 1996, 15, 1518.
27. J. X. McDermott, J. F. White and G. M. Whitesides, J. Am. Chem. Soc., 1976, 98, 6521.
28. J. A. McCleverty and G. Wilkinson, Inorg. Synth., 1990, 28, 212.
29. Bruker SAINT v6.02A (4b, 4c, syn-5a, anti-5c) or v6.28A (4a), Siemens Analytical X-ray Instruments Inc., Madison, WI, 1994 or 1998.
30. G. M. Sheldrick, SADABS v2.03 (4b, syn-5a), v2.05 (4a) or v2.10 (4c, anti-5c), University of Göttingen, Germany.
31. SHELXTL program system version v5.1, Bruker Analytical X-ray instruments Inc., Madison, WI, 1998.

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Footnote

† CCDC reference numbers 700268–700272. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b815056g

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