Efficient and chemoselective ethene hydromethoxycarbonylation catalysts based on Pd-complexes of heterodiphosphines o-C₆H₄(CH₂P^tBu₂)(CH₂PR₂)

Abstract

The synthesis and properties of a series of unsymmetrical diphosphines o-C₆H₄(CH₂P^tBu₂)(CH₂PR₂) are reported where R = C₆H₅ (L_a); p-CH₃C₆H₄ (L_b); o-CH₃C₆H₄ (L_c); o-CH₃CH₂C₆H₄ (L_d); o-CH₃OC₆H₄ (L_e); 2,4,6-(CH₃)₃C₆H₄ (L_f); CH(CH₃)₂ (L_g); or PR₂ = PCg (L_h) where PCg is 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamant-6-yl. Hydromethoxycarbonylation of ethene under commercially relevant conditions is catalysed by the Pd complexes of each of the ligands L_a–ha–h to give methyl propanoate in >99% selectivity with catalytic activities comparable to those obtained with o-C₆H₄(CH₂P^tBu₂)₂ (L₁) or o-C₆H₄(CH₂PCg)₂ (L₅5). The catalysts derived from L_c, L_d and L_h are more active than the catalyst derived from L_a or L₁; these ligands have in common, a PR₂ donor that is more bulky than the PPh₂ present in L_a. Treatment of [PtCl(CH₃)(cod)] with L_a–ha–h gave [PtCl(CH₃)(L)] as mixtures of 2 isomers 1a–h and 2a–h. The major isomer in each case was 1a–h with the CH₃ ligand trans to the P^tBu₂ group; the diastereoselectivity of this reaction for products 1a–h ranges from 88% to over 99%. The crystal structures of 1b, 1c and 1e have been determined. Fluxionality associated with chelate ring inversion has been detected for 1c by variable temperature ³¹P NMR spectroscopy. The ³¹P NMR data are compared for the complexes [PtCl(CH₃)(L)], [Pt(CH₃)₂(L)] and [PtCl₂(L)] where L = L_h, L₁ or L₅5. The crystal structure of [Pt(CH₃)₂(L_h)] (4h) has been determined and shows that PCg is sterically less demanding than P^tBu₂ in this complex. Treatment of 4h with HCl gave a 1 : 1 mixture of 1h and 2h that equilibrated over 5 h to a 70 : 1 mixture. Treatment of an equilibrium mixture of 1h and 2h with isotopically labelled ¹³CO gas gave a single acyl complex [PtCl(¹³COCH₃)(L_h)] (5h) with retention of configuration at Pt, i.e. the ¹³COCH₃ is trans to the P^tBu₂ group. Mechanisms for the CO insertion are discussed which are consistent with the observed stereochemistry. The palladium complexes [PdCl(CH₃)(L_h)] (7/8) also reacted with labelled ¹³CO to give a single acyl species [PdCl(¹³COCH₃)(L_h)]. Treatment of [PdCl(¹³COCH₃)(L_h)] with MeOH rapidly gave CH₃¹³COOMe.

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Introduction

The Pd-catalysed conversion of ethene to methyl propionate (MeP in the reaction below)¹ is an elegant example of a recently commercialised application of homogeneous catalysis. It is the fruit of decades of research on ethene carbonylation.^2–6 Ethene to MeP conversion is the first stage of Lucite's successful two-stage process for the manufacture of methyl methacrylate.⁷

The catalyst for the Lucite Process (below) is based on palladium complexes of the bulky xylenyl diphosphine L₁.¹ The selectivity and activity of the Pd catalyst is a sensitive function of the phosphorus substituents and the ligand backbone, with the most active and MeP-selective ligands being those with bulky substituents on the P and/or a large bite angle.^5,6,8,9 The ligand effects are subtle, e.g. while the catalyst based on L₁ or L₂2 gives MeP efficiently in greater than 99.9% chemoselectivity, the catalyst based on L₃33 or L₄44 gives ethene/CO co-oligomers and polyketone.^3,10 Notably, ligand L₅5⁵ and other ligands containing the bulky but electron-poor phospha-adamantyl donors⁶ also give highly active, MeP-selective catalysts which suggests that ligand steric effects are more important than electronic effects in determining the efficiency of the catalyst.The culmination of many mechanistic studies^11–13 of Pd-catalysed ethene hydromethoxycarbonylation has been the generally accepted hydride-mechanism outlined in Scheme 1. The observation that bulky diphosphines produce MeP chemoselectively rather than ethene/CO co-oligomers and polymer can be explained using this mechanism by reasoning that the crowded intermediate A selectively binds MeOH over ethene because MeOH is smaller.^11,12

Scheme 1 Hydride mechanism when the diphos ligand is symmetrical.

From the results of screening many diphos ligands^{1,3–6,8,9,14} for hydromethoxycarbonylation catalysis, it was reasonable to deduce that the best MeP-selective diphos ligands should contain two very bulky PR₂ groups such as P^tBu₂. However, we recently showed¹⁰ that the catalyst derived from the mixed diphosphine L_a was comparable in selectivity and activity to the commercial catalyst derived from L₁. If a similar mechanism were assumed for the carbonylation with the heterodiphos catalyst then the mechanism shown in Scheme 2 would operate with two geometric isomers for each of the catalytic intermediates. The migration steps i and iii would involve inversion of stereochemistry at the Pd, although isomerisation between the geometric isomers might be rapid compared to the rate of the subsequent step. Iggo et al.¹² have shown that the ³¹P nuclei in [Pd(COCH₂CH₃)(THF)(L₁)]BF₄ are rendered equivalent on the NMR timescale at ambient temperature by rapid site exchange of the COCH₂CH₃ group. The unexpectedly high selectivity for MeP of the L_a-catalyst was rationalised by postulating the kinetic dominance of diastereoisomer A₁11 over A₂22 at the pivotal, chemoselectivity determining step (iv in Scheme 2).

Scheme 2 Hydride mechanism when the diphos ligand is unsymmetrical.

The discovery of the catalyst based on L_a prompted us to explore the generality of varying the second PR₂ donor. Thus the ditopic ligands L_b–hb–hb–h shown in Chart 1 are the theme of this article. Each of these ligands contains one ^tBu₂P group with the other P-donor being either a PAr₂ group (L_b–fb–fb–f) or a P(Alkyl)₂ group (L_g,hg,hg,h). The range of stereoelectronic effects spanned by L_a–ha–h has resulted in some tentative structure–activity relationships that may aid future ligand design for improved hydromethoxycarbonylation catalysts.

Chart 1

Results and discussion

Ligand synthesis

The general route to the substituted diarylphosphines L_b–gb–gb–g is shown in Scheme 3 and is based on the previously reported synthesis of the parent compound L_a.¹⁰ All of the ligands shown in Chart 1 with the exception of L_g¹⁵ are new and have been fully characterised (see Experimental). Their ³¹P NMR spectra each show two singlets for the inequivalent phosphorus nuclei and thus, as with ligand L_a, ⁵J(PP) is not resolved.

Scheme 3 Synthesis of unsymmetrical diphos ligands.

The diphosphine L_h was prepared by a similar procedure to that shown in Scheme 3 but from CgPH(BH₃) rather than CgPH. Ligand L_h features a rigid phospha-adamantane cage that behaves as a bulky, electron-poor P-donor.¹⁶

Carbonylation catalysis

Ethene hydromethoxycarbonylation to give MeP was carried out with palladium catalysts derived from L_b–hb–hb–h under the same conditions and the results are given in Table 1, along with those reported¹⁰ with ligand L_a and the symmetrical diphosphines L₁¹⁰ and L₅5^5,6 for comparison.

Table 1 Pd-catalysed hydromethoxycarbonylation of ethene^a

Entry	Ligand	Ar substituent	TON	Selectivity/%
a The data given in this Table are the average of 2 or more runs. For reaction conditions, see Experimental. TON (in mol/mol Pd) were calculated from the mass gain after 3 h assuming the product was pure MeP. The selectivity to MeP was measured by GC; the remainder was a mixture of co-oligomers.
1	L1	—	17 900	>99.9
2	La	H	16 300	99.5
3	Lb	p-CH3	9100	99.5
4	Lc	o-CH3	32 200	99.5
5	Ld	o-CH2CH3	30 800	99.5
6	Le	o-OCH3	3600	99.5
7	Lf	2,4,6-(CH3)3	4700	99.5
8	Lg	—	17 900	99.0
9	Lh	—	40 200	>99.9
10	L55	—	25 600	>99.9

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All of the new unsymmetrical diphosphines L_b–hb–hb–h give very active catalysts that are highly chemoselective for MeP (>99%). By comparing the activities of the catalysts derived from the substituted diarylphosphines L_b–fb–fb–f with the activity for the unsubstituted L_a (Entry 2), the following observations are made: (a) catalyst activity is approximately halved by the presence of the p–methyl substituent (Entry 3), suggesting that increasing the donor strength of the PAr₂ has a negative effect; (b) activity is approximately doubled by the presence of an o–methyl substituent (Entry 4), suggesting that increasing the bulk of the PAr₂ can have a positive effect although with the much bulkier mesityl group (Entry 7) the rate was substantially reduced; (c) an o–ethyl substituent (Entry 5) has a similar activity-promoting effect to the o–methyl substituent (Entry 4) whereas the approximately isosteric o–methoxy substituent (Entry 6) reduces the activity considerably, suggesting that incorporating a potentially weakly coordinating ortho substituent is detrimental.

The results obtained with L_g (Entry 8) are similar to those with L_a (Entry 2) indicating that the PⁱPr₂ and PPh₂ donors have remarkably similar effects on the catalysis; this reinforces the idea that sterics are more important than electronics for the PR₂ donor of the heterotopic ligands. Of the catalysts shown in Table 1, the best in terms of activity and selectivity was derived from L_h (Entry 9); it is highly chemoselective and 2.5 times as active as the catalyst based on L_a. We have previously characterised RPCg ligands as being similar sterically to a RP^tBu₂ ligand and akin electronically to RP(OR)₂ ligands.¹⁶ This unusual combination of stereoelectronic properties has been used to explain the wide range of catalytic processes that RPCg ligands promote.¹⁷ It is notable that the catalyst based on the unsymmetrical L_h (Entry 9) outperforms both of the catalysts derived from the symmetrical analogues, L₁ (Entry 1) and L₅5 (Entry 10).

What emerges from the catalysis results given in Table 1 is that the ligands L_c, L_d and L_h produce catalysts that are more efficient than the parent unsymmetrical ligand L_a and the commercialised catalyst based on L₁. The effect on the rate of the catalysis of varying the second P-donor is subtle: the highest and lowest rates reported in Table 1 only differ by an order of magnitude. Thus, while such differences can be critical for industrial applications, the interpretation of trends in terms of mechanism should be treated with caution. Nonetheless, the coordination chemistry of ligands L_a–ha–h with platinum(II) and palladium(II) has been investigated because of the potential insight model complexes may provide.

Platinum(II) and palladium(II) coordination chemistry

The high chemoselectivity of catalysts derived from L_b–hb–hb–h may be explained in the same way as previously proposed¹⁰ for the L_a–catalyst, i.e. the catalysis is dominated by the intermediate with the acetyl group trans to the P^tBu₂ (A₁11 in Scheme 2). The isomers of [PtCl(CH₃)(L_a)] (1a/2a) contain a strong σ-donor (CH₃) and a relatively weak σ-donor (Cl) and the CH₃ is larger than Cl.¹⁸ Thus 1a/2a can be viewed as models for intermediates A₁11/A₂22 and C₁1/C₂2 (Scheme 2, R = Ph). It was previously shown¹⁰ that the thermodynamic ratio of 1a : 2a was ca. 90 : 1 and the isomers were in dynamic equilibrium with each other. If these results with the model complexes were to reflect the situation with the real catalyst, it would be consistent with thermodynamic preferences of A₁11 over A₂22 and C₁1 over C₂2 as well as support the idea that these species can interconvert. It was also shown¹⁰ that the reaction of a 90 : 1 mixture of 1a and 2a with ¹³CO gave 1a′ and 2a′′ in the ratio of ca. 1 : 5 (Scheme 4). It was posited that the isomeric species 1a′′ and 2a′ were not observed in the ³¹P NMR spectrum of the reaction mixture because the CH₃ migration in 2a′ was very rapid due to the propensity of the adjacent bulky ^tBu₂P group to promote the migration; conversely the less bulky PPh₂ adjacent to the CH₃ in 1a′ does not promote its migration and so 1a′′ was not observed.

Scheme 4 CO insertion reactions of [PtCl(CH₃)(L_a)].

It was of interest to see if the structural correlations for the Pt model complexes with L_a held for the complexes of the ligands shown in Chart 1. Thus the complexes [PtCl(CH₃)(L_b–hb–hb–h)] (1b–h/2b–h) were prepared according to Scheme 5 and the isomers were readily distinguished from the characteristic chemical shifts and ¹J_PtP coupling constants; δ_P for the coordinated P^tBu₂ was in the +20 to +40 ppm region and ¹J_PtP for the P trans to CH₃ was less than 2000 Hz and trans to Cl was over 4000 Hz. In each case, the isomer 1 was identified as the major isomer and the ratios of 1 : 2 given in Scheme 5 were estimated from integration of the ³¹P NMR spectra of mixtures that had equilibrated over 24 h. The preference for 1a over 2a was ascribed to steric effects: CH₃ is larger than Cl¹⁸ and therefore prefers to be trans to the bulkier P^tBu₂. It might then be predicted that more bulky PAr₂ groups would produce lower 1 : 2 ratios. However, the data given in Scheme 5 show that the ratios of 1 : 2 are not simply a function of sterics and, as usual, it is difficult to separate steric and electronic effects. Perhaps the strongest evidence in support of a steric effect comes from a comparison of the complexes of the o-MeC₆H₄ ligand L_c and the o-EtC₆H₄ ligand L_d: the 1c : 2c ratio is at least an order of magnitude greater than the 1d : 2d ratio.

Scheme 5 Synthesis of [PtCl(CH₃)(L_a–ha–h)].

The crystal structures of the major isomers 1b, 1c and 1e, have been determined and are shown in Fig. 1–3 along with selected bond lengths and angles. In the crystal structure of 1b, the chloro and methyl groups on platinum are disordered which precludes making any detailed comparisons between the parameters for 1b and the other two structures; the major component (0.83) is shown in Fig. 1. In 1c, the torsion angles Pt-P-C_ipso-C_ortho(Me) of 61.0 and 176.1° show that a g⁺a conformation is present.¹⁹ In the o-anisyl structure 1e, the metrical parameters are almost the same as for the o-tolyl analogue 1c and the torsion angles Pt-P-C_ipso-C_ortho(OMe) of 63.1 and 178.4° show that the same g⁺a conformation is present. The structures of 1c and 1e show significant distortions from square planar geometry, as reflected by the trans P-Pt-Cl angles which are 15–17° short of 180° and the distances of the Cl and C of the methyl groups from the mean Pt, P(1), P(2) plane: ca. 0.12 Å for the C atoms and 0.27 Å for the Cl atoms. Thus the crystal structures of 1c and 1e provide no clue as to why catalysts derived from ligands L_c and L_e give such different catalytic performance (Table 1).

Fig. 1 Thermal ellipsoid plot of the structure of [PtCl(CH₃)(L_b)] (1b). Most hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°); Pt(1)-P(1) 2.3604(7), Pt(1)-P(2) 2.2194(6), Pt(1)-Cl(1A) 2.3416(7), Pt(1)-C(31A) 2.1711(19), P(2)-Pt(1)-P(1) 101.89(2), C(31)-Pt(1)-Cl(1) 80.55(6), Cl(1)-Pt(1)-P(1) 92.64(2), C(31)-Pt(1)-P(2) 84.99(6), P(1)-Pt(1)-C(31A) 171.53(6), P(2)-Pt(1)-Cl(1A) 165.47(3).

Fig. 2 Thermal ellipsoid plot of the structure of [PtCl(CH₃)(L_cc)] (1c). Hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°); Pt(1)-P(1) 2.3699(8), Pt(1)-P(2) 2.2201(8), Pt(1)-Cl(1) 2.3852(8), Pt(1)-C(31) 2.108(3), P(1)-Pt(1)-P(2) 103.83(3), C(31)-Pt(1)-Cl(1) 83.51(9), C(31)-Pt(1)-P(2) 81.57(9), P(1)-Pt(1)-Cl(1) 91.51(3), P(1)-Pt(1)-C(31) 173.70(8), P(2)-Pt(1)-Cl(1) 163.41(3).

Fig. 3 Thermal ellipsoid plot of the structure of [PtCl(CH₃)(L_e)] (1e). Most hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°); Pt(1)-P(1) 2.3757(5), Pt(1)-P(2) 2.2182(5), Pt(1)-Cl(1) 2.3874(5), Pt(1)-C(31) 2.1198(18), P(2)-Pt(1)-P(1) 101.95(2), C(31)-Pt(1)-Cl(1) 82.49(5), P(1)-Pt(1)-Cl(1) 92.66(2), C(31)-Pt(1)-P(2) 83.30(5), P(1)-Pt(1)-C(31) 173.90(5), P(2)-Pt(1)-Cl(1) 164.043(19).

The variable temperature ³¹P NMR spectra of 1c show that it is fluxional on the NMR timescale. At ambient temperatures, the two ³¹P NMR signals are very broad; these sharpen at elevated temperatures and at 100 °C, two doublets with satellites are observed, consistent with an averaged structure of 1c. At −80 °C the spectrum is resolved into two sets of two doublets consistent with a 2 : 1 mixture of two similar species. We have previously identified the 7-membered chelate conformational fluxionality in 1a on the NMR timescale.¹⁰ The crystal structure of 1c shows that the aryl groups on P are inequivalent due to them adopting gauche and anti orientations and the conformation of the chelate puts the phenylene of the backbone syn to the gauche aryl. The two structures detected in solutions of 1c at low temperatures could therefore be syn-gauche and syn-anti and the fluxionality is the interconversion of these conformers; it is also possible that the two species are associated with gg and ag conformers of the PAr₂ groups.¹⁹

The catalyst derived from the unsymmetrical CgP/P^tBu₂ ligand L_h is the most active of the catalysts described here (Table 1) and significantly more active than the catalysts derived from the symmetrical analogues L₁ and L₅5. Thus a comparison of the coordination chemistry of L_h with that of L₁ and L₅5 was of interest as it might shed light on the source of the synergic effect observed in the catalysis.²⁰ We have previously established that PCg has similar steric bulk to P^tBu₂ but differs greatly in its donor properties;¹⁶ complexes of ligands containing both of these donors have been previously reported in patents.⁵ The ³¹P NMR data for the complexes [PtCl(CH₃)(L)] (1h–j, 2h), [PtCl₂(L)] (3h–j), and [Pt(CH₃)₂(L)] (4h–j) where L = L_h, L₁ and L₅5 (as a rac/meso-mixture of diastereoisomers) are collected in Table 2.

Table 2 ³¹P NMR data

Complex	Ligand	δ(P^tBu2)	J PtP	δ(PR2)	J PtP
1h	Lh	25.7	1658	−13.0	4563
1i	L1	19.5	1773
		24.0	4384
1j	L55			−9.5	4328
				−14.1	1489
				−10.9	4401
				−15.2	1500
2h	Lh	28.1	4261	−17.7	1658
3h	Lh	18.2	3437	−25.1	3714
3i	L1	11.9	3644
3j	L55			−20.0	3515
				−20.2	3469
4h	Lh	23.4	1722	−16.0	1816
4i	L1	19.3	1853
4j	L55			−15.7	1688
				−13.5	1621

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It is notable that the values of J(Pt–P^tBu₂) are smaller in the complexes of the heterotopic L_h (i.e.1h–4h) than in the analogous complexes of the homotopic L₁ (i.e.1i, 3i, 4i) and the converse is true for the values of J(Pt–PCg)—these are larger in the complexes of the heterotopic L_h (i.e.1h–4h) than in the analogous complexes of the homotopic L₅5 (i.e.1j, 3j, 4j). This effect is particularly evident in the dimethylplatinum complexes where the value of J(Pt–P^tBu₂) in 4i is ca. 200 Hz greater than J(Pt–PCg) in 4j whereas in 4h the value of J(Pt–P^tBu₂) is ca. 100 Hz less than J(Pt–PCg). The inference from these data is that, in the mixed-donor complexes, the Pt–PCg and Pt–P^tBu₂ interactions are mutually perturbed resulting in an enhanced binding of the PCg donor. Further insight was provided by the crystal structure of [Pt(CH₃)₂(L_h)] (4h). Crystals of 4h were grown from CH₂Cl₂/hexane and its crystal structure is shown in Fig. 4. The Pt–PCg distance is ca. 0.05 Å shorter than the Pt–P^tBu₂ consistent with the conclusion from the NMR data that the PCg was more strongly bound than the P^tBu₂. One explanation for this effect is that the P^tBu₂ has greater steric bulk than the PCg and elongation of the Pt–P^tBu₂ bond relieves some of the steric congestion in the complex. The crystal structure of 4h provides support for this argument as the two calculated half cone angles for the unsymmetrical chelate²¹ show that the ^tBu₂P (136°) is larger than CgP (129°).

Fig. 4 Thermal ellipsoid plot of the structure of [Pt(CH₃)₂(L_h)] (4h). Most hydrogen atoms and the disordered carbon atoms on P(1) have been omitted for clarity. Selected bond lengths (Å) and angles (°); Pt(1)-P(1) 2.3502(10), Pt(1)-P(2) 2.2974(9), Pt(1)-C(27) 2.114(4), Pt(1)-C(28) 2.104(4), P(1)-Pt(1)-P(2) 101.76(3), C(27)-Pt(1)-C(28) 78.48(19), C(28)-Pt(1)-P(1) 89.77(14), C(27)-Pt(1)-P(2) 91.52(13), C(27)-Pt(1)-P(1) 164.76(14), C(28)-Pt(1)-P(2) 165.18(13).

When a solution of [Pt(CH₃)₂(L_h)] (4h) in CH₂Cl₂ was treated with 1 equiv. of HCl in Et₂O, a non-equilibrium mixture of isomers of [PtCl(CH₃)(L_h)] (1h/2h) was formed (Scheme 6). The solution was originally a 1 : 1 mixture of 1h and 2h but within 5 h equilibrium was achieved. We previously reported¹⁰ that non-equilibrium mixtures of complexes 1a and 2a took over 5 days to achieve equilibrium.

Scheme 6 CO insertion reaction of [PtCl(CH₃)(L_h)].

Treatment of a solution containing almost exclusively 1h with ¹³CO gas produced an acyl complex gradually over 5 h. No intermediates were observed and the acyl complex was assigned structure 5h (with retention of configuration at Pt) on the basis of ³¹P and ¹³C NMR spectroscopy. The ³¹P NMR data (δ 16.9, J_PC = 109, J_PtP = 1314 Hz; −9.4, J_PtP = 4973 Hz) unambiguously showed that the acyl is trans to P^tBu₂ and this assignment is further supported by the ¹³C NMR data for the labelled acyl (δ 226.9, J_PC = 109, J_PtC = 636 Hz).

Two mechanisms for the CO insertion into the Pt–CH₃ bonds of 1h are shown in Scheme 7. Both mechanisms are based on the formation of cationic intermediates, as has been shown previously by others²² for the reaction of CO with related complexes of the type [PtCl(CH₃)(diphos)]. Pathway A involves CO displacement of Cl to give X followed by migration of the CH₃ to give 6h (the undetected isomer of the product) which then rapidly isomerises to the observed product 5h. The alternative Pathway B involves isomerisation of 1h to minor isomer 2h followed by CO coordination to give Y and then CH₃ migration to give 5h. We previously suggested¹⁰ that the carbonylation of 1a went via slow isomerisation to 2a (i.e. a B-type mechanism). The key feature of the CO reaction with 1h (that is not the case with 1a) is that, in both of the proposed terminal carbonyl intermediates X and Y, the CH₃ is adjacent to a bulky P-donor that would promote rapid migration. Thus both Pathways A and B are plausible for the carbonylation of 1h.

Scheme 7 Mechanisms for CO insertion reaction of [PtCl(CH₃)(L_h)].

The observation that CO reacts more rapidly with 1h than with 1a may be associated with the more rapid equilibration of 1h and 2h, i.e. the promotion of Pathway B by the bulky PCg moiety. Overall these promoting features of PCg correlate with the higher catalytic activity of the Pd complexes of L_h than L_a (Table 1). Similar migration-promoting reasons may be responsible for the observed high activity of the catalysts derived from the bulky aryl ligands L_c and L_d, although model studies with 1c and 1d were not pursued partly because of the complexity of the ³¹P NMR spectra arising from the presence of mixtures of rotamers of these complexes (see above).

The complexes [PdCl(CH₃)(L_h)] (7/8) were prepared by the addition of L_h to [PdCl(CH₃)(cod)]. The ³¹P NMR spectrum of the product at room temperature consisted of broad signals which at low temperatures (−90 °C) resolved into four sets of doublets which are assigned to the diastereoisomeric conformers 7α/7β and 8α/8β; the C₁-symmetry of the PCg moiety renders the conformers diastereoisomeric. The fluxionality of 7 and 8 is associated with the ring inversion process shown in Scheme 8.²³

Scheme 8 Conformations of [PdCl(CH₃)(L_h)].

The mixture of isomers of [PdCl(CH₃)(L_h)] (7/8) reacted with ¹³CO to give an acyl species (ν_CO 1656 cm⁻¹, δ_C 234.8) which could have structure 9 or 10. Fluxionality was evident in the ³¹P NMR spectra of this acyl product and, as in the starting material (7/8), it is associated with ring flipping of the type shown in Scheme 8. At −90 °C, the observed ABX pattern (δ_A 22.0, J_AX = 98, J_AB = 56 Hz; δ_B 21.5, J_BX = 17 Hz) is consistent with the presence of a single acyl complex. The similarity of the chemical shifts precludes a definite assignment of the geometry on the basis of the ³¹P NMR data, although in the light of the analogous Pt chemistry described above, we tentatively assign the structure 9 to the acyl product. Addition of MeOH to a solution of 9 gave the labelled ester CH₃¹³CO₂Me immediately, as shown by the ¹³C NMR signal at δ_C 176.6 ppm. We have previously reported similar chemistry with [PdCl(CH₃)(L_a)].¹⁰

Conclusions

It has been shown that a range of ditopic diphosphines of the type o-C₆H₄(CH₂P^tBu₂)(CH₂PR₂) give very active and chemoselective Pd catalysts for ethene hydromethoxycarbonylation to methyl propanoate. The near-quantitative chemoselectivity for MeP of the Pd catalysts derived from the heterotopic ligands L_a–ha–h suggests that the explanation previously given for the chemoselectivity of Pd–L_a catalysts¹⁰ can be extrapolated; that is, the acyl species A₁11 (in Scheme 2) kinetically dominates over the isomer A₂22 for all Pd-L_a–ha–h. The preference for the acyl group to be trans to the ^tBu₂P correlates with the thermodynamic preference for the alkyl group to be trans to the ^tBu₂P in the model complexes [PtCl(CH₃)(L_a–ha–h)].

Some structure–activity relationships are derived from the catalytic data obtained with L_a–fa–f but these are tentative because of the subtlety of the effect the P-substituent has on the rate – the highest to the lowest activity observed spans only an order of magnitude, or just 6 kJ mol⁻¹ in activation energy. The best performing ligands L_c, L_d and L_h contain bulky PR₂ donors and this may be associated with the ability of bulky PR₂ to promote Et migration to CO to give acyls A₁11 and A₂22 (step iii, Scheme 2).

The heterotopic CgP/^tBu₂P ligand L_h produces a catalyst that is more active than either of the homotopic counterparts derived from L₁ and L₅5. The source of this synergic effect might be traced to the mutual perturbation of the CgP and ^tBu₂P bonding that has been inferred from crystallographic and spectroscopic studies of Pt complexes of L_h.

It has been emphatically demonstrated that only one bulky phosphine donor is required for xylenyl diphosphines to produce a very effective hydromethoxycarbonylation catalyst. This opens up many future opportunities for ligand design.

Experimental

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 system²⁴ in flame and vacuum-dried glassware. MeOH was dried over 3 Å molecular sieves, pentane was dried over 4 Å molecular sieves and both were deoxygenated by N₂ saturation. The complexes [PtCl₂(cod)],²⁵ [Pt(CH₃)₂(cod)],²⁶ [PtCl(CH₃)(cod)],²⁷ [PdCl₂(NCPh)₂]²⁸ and [PdCl(CH₃)(cod)],²⁹ were prepared by literature methods. ^tBu₂PH in THF was obtained from JCI USA Inc. and ^tBu₂PH.BH₃ from Lucite International. The precursor o-xylenediyl sulfate¹⁵ ligands L_a,¹⁰L_g¹⁵ and L₅5⁵ were prepared by literature methods. All phosphines were stored under nitrogen at room temperature. All other reagents were used as received from Aldrich, Strem or Lancaster. NMR spectra were recorded on a Jeol Delta 270, Jeol Eclipse 300, Jeol Eclipse 400, Varian 400, or Lambda 300. Infrared spectroscopy was carried out on a Perkin Elmer 1600 Series FTIR. Mass spectra were recorded on a MD800 by the Mass Spectrometry Service, University of Bristol. Elemental analyses were carried out by the Microanalytical Laboratory of the School of Chemistry, University of Bristol.

Preparation of ligand L_b

A 1.6 M solution of ⁿBuLi (7.60 mL, 11.8 mmol) in hexane was added to a solution of the borane adduct of di-t-butylphosphine (^tBu₂PH.BH₃, 1.60 g, 10.0 mmol) in THF (15 mL) at −78 °C. The mixture was allowed to reach room temperature, stirred for 30 min at this temperature, cooled to −78 °C and then added in portions dropwise via syringe (over 10 min in total) to a precooled (−78 °C) solution of the o-xylenediyl sulfate (2.00 g, 10.0 mmol) in THF (20 mL). The mixture was stirred for 30 min at this temperature, cooled again to −78 °C and treated with a solution of (p-tol)₂PLi {prepared from (p-tol)₂PH (1.73 mL, 10.0 mmol) in THF (15 mL) and ⁿBuLi (7.40 mL of a 1.6 M solution in hexane, 12.0 mmol) at −78 °C}. The mixture was allowed to warm to room temperature and then stirred overnight. The solvent was removed under reduced pressure and the residue was redissolved in Et₂O (50 mL). The solution was then quenched with water (30 mL) and the aqueous phase was extracted with Et₂O (3 × 20 mL). The combined organic phase was dried over MgSO₄, filtered and concentrated to dryness under reduced pressure. The residue was then redissolved in the minimum amount of pyrrolidine (15–20 mL) and then stirred overnight (if that was not enough to de-boronate the product, it was heated to 50 °C for 2 h). The pyrrolidine was removed under reduced pressure and the oily solid was recrystallised from the minimum amount (15–20 mL) of boiling MeOH affording a white solid (1.67 g, 36% yield). ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ 27.0 (P^tBu₂, s), −15.8 (PAr₂, s). ¹H NMR (270 MHz, CD₂Cl₂): δ 7.58-6.76 (12 H, m), 3.67 (CH₂, 2 H, d, J(HP) = 1.6), 2.83 (CH₂, 2 H, d, J(HP) = 2.4 Hz), 2.33 (CH₃Ph, 6 H, s), 1.08 (C(CH₃)₃, 18 H, d, J(HP) = 10.6 Hz). ¹³C{¹H} NMR (75 MHz, CD₂Cl₂): δ 140.0 (dd, J(CP) = 9.2 Hz, J(CP) = 3.8 Hz), 139.3 (s), 136.3 (dd, J(CP) = 6.8 Hz, J(CP) = 2.4 Hz), 135.7 (d, J(CP) = 14.7 Hz), 133.5 (d, J(CP) = 18.8 Hz), 131.6 (dd, J(CP) = 12.7 Hz, J(CP) = 1.7 Hz), 131.2 (d, J(CP) = 7.5 Hz), 129.6 (d, J(CP) = 6.6 Hz), 126.1 (d, J(CP) = 2.8 Hz), 125.7 (t, J(CP) = 2.0 Hz), 34.2 (CH₂, dd, J(CP) = 16.1 Hz, J(CP) = 8.4 Hz), 32.3 (C(CH₃)₃, d, J(CP) = 23.1 Hz), 30.2 (C(CH₃)₃, d, J(CP) = 13.3 Hz), 26.7 (CH₂, dd, J(CP) = 25.1 Hz, J(CP) = 5.5 Hz), 21.6 (CH₃Ph, s). Accurate mass spectrum (ESI): M_r = 463.2691 (M + H)⁺ (calcd for C₃₀H₄₁P₂ 463.2678). Elemental analysis (calcd for C₃₀H₄₀P₂): C: 78.22 (77.89), H: 8.89 (8.72).

Preparation of ligand L_c

Ligand synthesised using a similar procedure as followed for L_b. White solid (24% yield). ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ 28.3 (P^tBu₂, s), −36.7 (PAr₂, s). ¹H NMR (270 MHz, CD₂Cl₂): δ 7.60-6.54 (12 H, m), 3.64 (CH₂, 2 H, d, J(HP) = 1.6 Hz), 2.82 (CH₂, 2 H, d, J(HP) = 2.6 Hz), 1.97 (CH₃Ph, 6 H, s), 1.07 (C(CH₃)₃, 18 H, d, J(HP) = 10.6 Hz). ¹³C{¹H} NMR (75 MHz, CD₂Cl₂): δ 142.9 (d, J(CP) = 25.4 Hz), 139.9 (dd, J(CP) = 9.3 Hz, J(CP) = 3.9 Hz), 137.3 (d, J(CP) = 15.9 Hz), 135.2 (dd, J(CP) = 6.7 Hz, J(CP) = 2.6 Hz), 131.5 (s), 131.0 (dd, J(CP) = 12.1 Hz, J(CP) = 2.0 Hz), 130.7 (d, J(CP) = 22.7 Hz), 129.7 (d, J(CP) = 4.6 Hz), 128.4 (s), 126.0 (d, J(CP) = 0.5 Hz), 125.7 (d, J(CP) = 2.9 Hz), 125.2 (t, J(CP) = 2.0 Hz), 33.9 (CH₂, dd, J(CP) = 16.4 Hz, J(CP) = 9.2 Hz), 32.4 (C(CH₃)₃, d, J(CP) = 23.4 Hz), 30.2 (C(CH₃)₃, d, J(CP) = 13.6 Hz), 26.5 (CH₂, dd, J(CP) = 25.4 Hz, J(CP) = 5.3 Hz), 21.1 (CH₃Ph, d, J(CP) = 20.8 Hz). EI mass spectrum: m/z 405.1 (M − ^tBu)⁺.

Preparation of ligand L_d

Ligand synthesised using a similar procedure as followed for L_b. White solid (21% yield). ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ 27.4 (P^tBu₂, s), −40.1 (PAr₂, s). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.45-6.56 (12 H, m), 3.65 (CH₂, br s), 2.88 (CH₂, br s), 2.51 (CH₂, br s), 1.10-0.90 (C(CH₃)₃ + CH₃CH₂, br m). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 149.4 (ipso C, d, J(CP) = 24.6 Hz), 140.1 (ipso C, dd, J(CP) = 9.6 Hz, J(CP) = 3.8 Hz), 137.6 (ipso C, d, J(CP) = 16.5 Hz), 136.0 (ipso C, dd, J(CP) = 6.9 Hz, J(CP) = 2.7 Hz), 132.4 (s), 131.6 (dd, J(CP) = 13.1 Hz, J(CP) = 1.5 Hz), 131.2 (d, J(CP) = 6.9 Hz), 129.3 (s), 128.8 (d, J(CP) = 4.6 Hz), 126.6 (s), 126.3 (d, J(CP) = 3.1 Hz), 125.7 (t, J(CP) = 1.9 Hz), 34.4 (CH₂P, dd, J(CP) = 16.4 Hz, J(CP) = 8.6 Hz), 32.4 (C(CH₃)₃, d, J(CP) = 23.8 Hz), 30.3 (C(CH₃)₃, d, J(CP) = 13.1 Hz), 27.7 (CH₂CH₃, d, J(CP) = 20.4 Hz), 26.5 (CH₂P, dd, J(CP) = 25.4 Hz, J(CP) = 5.8 Hz), 15.5 (CH₃CH₂, d, J(CP) = 2.3 Hz). Accurate mass spectrum (ESI): M_r = 491.2997 (M + H)⁺ (calcd for C₃₂H₄₅P₂ 491.2991).

Preparation of ligand L_e

Ligand synthesised using a similar procedure as followed for L_b. White solid (14% yield). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 27.9 (P^tBu₂, s), −33.0 (PAr₂, s). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.52-6.66 (14 H, m), 3.54 (CH₂, 2 H, s), 3.43 (CH₃O, 6 H, s), 2.78 (CH₂, 2 H, s), 0.96 (C(CH₃)₃, 18 H, d, J(HP) = 10.5 Hz). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 162.0 (d, J(CP) = 13.2 Hz), 140.1 (br s), 137.2 (dd, J(CP) = 7.8 Hz), 133.9 (d, J(CP) = 7.8 Hz), 131.2 (dd, J(CP) = 14.8 Hz, J(CP) = 1.6 Hz), 130.5 (s), 125.5 (br s), 125.4 (br s), 121.2 (d, J(CP) = 3.1 Hz), 110.9 (s), 55.9 (CH₃O, s), 32.3 (C(CH₃)₃, br d, J(CP) = 22.6 Hz), 31.8 (CH₂, br dd, J(CP) = 17.1 Hz, J(CP) = 6.2 Hz), 30.2 (C(CH₃)₃, br d, J(CP) = 13.2 Hz), 26.2 (CH₂, br d, J(CP) = 26.5 Hz). Accurate mass spectrum (ESI): M_r = 495.2596 (M + H)⁺ (calcd for C₃₀H₄₁O₂P₂ 495.2576). Elemental analysis (calcd for C₃₀H₄₀O₂P₂): C: 73.99 (72.85), H: 8.35 (8.15).

Preparation of ligand L_f

Ligand synthesised using a similar procedure as followed for L_b. White solid (23% yield). ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): δ 29.4 (P^tBu₂, s), −20.7 (PAr₂, s). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.38-6.66 (8 H, m), 4.14 (CH₂, br s), 2.64 (CH₂, br d, J(HP) = 2.4 Hz), 2.21 (p-CH₃Ph, 3 H, s), 2.10 (o-CH₃Ph, 6 H, s), 1.07 (C(CH₃)₃, 18 H, d, J(HP) = 10.6 Hz). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 142.7 (ipso C o-Ph, d, J(CP) = 13.8 Hz), 140.5 (ipso C, dd, J(CP) = 9.2 Hz, J(CP) = 4.2 Hz), 138.1 (ipso C p-Ph, s), 136.5 (ipso C, dd, J(CP) = 8.1 Hz, J(CP) = 2.7 Hz), 133.3 (ipso C, d, J(CP) = 25.8 Hz), 132.0 (d, J(CP) = 7.3 Hz), 131.3 (dd, J(CP) = 13.8 Hz, J(CP) = 2.3 Hz), 130.3 (d, J(CP) = 2.7 Hz), 126.2 (d, J(CP) = 3.1 Hz), 125.4 (m), 33.0 (CH₂, dd, J(CP) = 20.8 Hz, J(CP) = 6.9 Hz), 32.3 (C(CH₃)₃, d, J(CP) = 23.5 Hz), 30.2 (C(CH₃)₃, d, J(CP) = 13.5 Hz), 25.6 (CH₂, dd, J(CP) = 24.8 Hz, J(CP) = 5.2 Hz), 23.1 (o-CH₃Ph, d, J(CP) = 13.1 Hz), 21.1 (p-CH₃Ph, s). Accurate mass spectrum (ESI): M_r = 519.3319 (M + H)⁺ (calcd for C₃₄H₄₉P₂ 519.3304).

Preparation of ligand L_h

A 1.6 M solution of ⁿBuLi (3.80 mL, 6.10 mmol) in hexane was added to a solution of the borane adduct of di-t-butylphosphine (^tBu₂PH.BH₃, 0.80 g, 5.00 mmol) in THF (15 mL) at −78 °C. The mixture was allowed to reach room temperature, stirred for 30 min at this temperature, cooled to −78 °C and then added dropwise via syringe over 10 min to a precooled (−78 °C) solution of the cyclic sulphate (1.00 g, 5.00 mmol) in THF (20 mL). The mixture was stirred for 30 min at this temperature, cooled again to −78 °C and treated with a solution of CgPLi.BH₃ {prepared from CgPH.BH₃ (1.14 g, 5.00 mmol) in THF (15 mL) and ⁿBuLi (3.70 mL of a 1.6 M solution in hexane, 5.90 mmol) at −78 °C and added to the reaction mixture immediately after formation). The mixture was allowed to warm to room temperature and then stirred overnight. The solvent was removed under reduced pressure and the foamy residue was redissolved in Et₂O (20 mL). The solution was then quenched with water (30 mL) and the aqueous phase was extracted with Et₂O (3 × 20 mL). The organic phase was dried over MgSO₄, filtered and concentrated to dryness under reduced pressure. The residue was then redissolved in the minimum amount of pyrrolidine (10 mL) and then stirred overnight. The pyrrolidine was removed under reduced pressure and the oily solid was recrystallised from the minimum amount (6 mL) of boiling MeOH affording a white solid (0.59 g, 43% yield). ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): δ 25.8 (P^tBu₂, s), −31.5 (PCg, s). ¹H NMR (400 MHz, CD₂Cl₂): δ 7.60 (1 H, d, J(HH) = 7.5 Hz), 7.26 (1 H, br d, J(HH) = 7.3 Hz), 7.13-7.05 (2 H, m), 3.31 (CHHP, 1 H, d, J(HH) = 14.4 Hz), 3.10 (CHHP, 1 H, d, J(HH) = 15.3 Hz), 2.97 (CHHP, 1 H, dd, J(HH) = 15.3 Hz, J(HP) = 2.9 Hz), 2.81 (CHHP, 1 H, dd, J(HH) = 14.4 Hz, J(HP) = 2.3 Hz), 1.99 (CH₂, 2 H, d, J(HH) = 13.3 Hz), 1.94 (CHH, 1 H, dd, J(HH) = 13.1 Hz, J(HP) = 6.5 Hz), 1.84-1.75 (CHH, 1 H, m), 1.61 (CHH, 1 H, dd, J(HH) = 13.3 Hz, J(HP) = 3.9 Hz), 1.39 (CH₃, 3 H, s), 1.31 (CH₃, 3 H, s), 1.27 (CH₃, 3 H, d, J(HP) = 11.7 Hz), 1.17 (C(CH₃)₃, 9 H, d, J(HP) = 10.8 Hz), 1.15 (CH₃, 3 H, d, J(HP) = 12.5 Hz), 1.09 (C(CH₃)₃, 9 H, d, J(HP) = 10.8 Hz). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 139.9 (dd, J(CP) = 9.7 Hz, J(CP) = 2.7 Hz), 136.9 (dd, J(CP) = 7.8 Hz, J(CP) = 2.7 Hz), 131.9 (dd, J(CP) = 15.6 Hz, J(CP) = 1.2 Hz), 131.1 (d, J(CP) = 8.6 Hz), 126.5 (d, J(CP) = 1.9 Hz), 126.1 (dd, J(CP) = 1.9 Hz, J(CP) = 1.2 Hz), 97.1 (CO₂CH₃, d, J(CP) = 0.8 Hz), 96.4 (CO₂CH₃, s), 73.2 (COCH₃, d, J(CP) = 10.5 Hz), 72.0 (COCH₃, d, J(CP) = 24.1 Hz), 45.5 (CH₂, d, J(CP) = 15.6 Hz), 37.7 (CH₂, s), 32.4 (C(CH₃)₃, d, J(CP) = 23.0 Hz), 30.3 (C(CH₃)₃, d, J(CP) = 13.2 Hz), 28.6 (CH₃, s), 28.4 (CH₃, s), 28.1 (CH₃, s), 27.6 (CH₂P, dd, J(CP) = 26.5 Hz, J(CP) = 5.8 Hz), 27.1 (CH₂P, dd, J(CP) = 25.5 Hz, J(CP) = 7.2 Hz), 25.2 (CH₃, s). Accurate mass spectrum (ESI): M_r = 465.2701 (M + H)⁺ (calcd for C₂₆H₄₃O₃P₂ 465.2682).

Carbonylation catalysis

Using standard Schlenk line techniques, reaction solutions were prepared by dissolving Pd(OAc)₂ (22 mg, 0.10 mmol) and ligand L_a–ha–h (0.50 mmol) in methanol (300 mL). The mixture was stirred for 30 min the addition of methane sulfonic acid (2.92 mL, 45 mmol) to complete the preparation of the catalyst solution. The catalytic solution was added to the pre-evacuated autoclave and heated to 100 °C at which point the pressure generated by the solvent was 2.3 bar. The autoclave was then pressured to 12.3 bar with CO : ethene (1 : 1) from a 10 L reservoir at higher pressure. A regulatory valve ensured that the pressure of the autoclave was maintained throughout the reaction at 12.3 bar through continual injection of gas from the reservoir. After 3 h, the autoclave was cooled and vented. The reaction solution was collected from the base of the vessel and immediately placed under a N₂ atmosphere. The solution was weighed so that the TON could be calculated.

Preparation of [PtCl(CH₃)(L_b)] (1b/2b)

(a) From [PtCl(CH₃)(cod)]. A solution of L_b (29 mg, 0.06 mmol) in toluene (1 mL) was added dropwise over 30 s to a stirred solution of [PtCl(CH₃)(cod)] (22 mg, 0.06 mmol) in toluene (1 mL). (b) From reaction of [Pt(CH₃)₂(L_b)] with HCl. A solution of L_b (35 mg, 0.08 mmol) in CH₂Cl₂ (1 mL) was added dropwise over 30 s to a stirred solution of [Pt(CH₃)₂(cod)] (25 mg, 0.08 mmol) in CH₂Cl₂ (1 mL). The product [Pt(CH₃)₂(L_b)] was confirmed by ³¹P{¹H} NMR spectroscopy: δ 27.6 (P^tBu₂, J(PPt) = 1884 Hz, J(PP) = 15 Hz), 3.8 (PAr₂, d, J(PPt) = 1938 Hz, J(PP) = 15 Hz) before continuing the reaction. HCl (1 eq, 39 μL, 0.08 mmol) in Et₂O was added to the reaction mixture. The product was obtained as a mixture of two isomers in a 30 : 1 ratio (1b : 2b) at room temperature. ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): For 1b: δ 30.8 (P^tBu₂, d, J(PPt) = 1809 Hz, J(PP) = 14 Hz), 5.4 (PAr₂, d, J(PPt) = 4505 Hz, J(PP) = 14 Hz). For 2b: δ 28.3 (P^tBu₂, d, J(PPt) = 4276 Hz, J(PP) = 15 Hz), 6.9 (PAr₂, d, J(PPt) = 1785 Hz, J(PP) = 15 Hz). ¹H NMR (400 MHz, CD₂Cl₂): For 1b: δ 7.81-6.75 (11 H, m), 5.97 (1 H, d, J(HH) = 6.4 Hz), 3.74-3.34 (CH₂, 4 H, br m), 2.31 (CH₃Ph, 6 H, s), 1.47-1.40 (C(CH₃)₃, 18 H, br m), 0.12 (CH₃-Pt, t, J(HPt) = 49.0 Hz, J(HPtrans) = J(HPcis) = 5.2 Hz). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 142.1-140.8 (br), 136.6 (d, J(CP) = 4.3 Hz), 134.4 (br d, J(CP) = 10.5 Hz), 133.5 (dd, J(CP) = 3.9 Hz, J(CP) = 2.7 Hz), 132.4 (dd, J(CP) = 4.7 Hz, J(CP) = 2.1 Hz), 131.2 (dd, J(CP) = 4.1 Hz, J(CP) = 3.3 Hz), 129.9-128.6 (br), 127.3 (dd, J(CP) = 3.3 Hz, J(CP) = 1.8 Hz), 126.5 (t, J(CP) = 2.7 Hz), 38.3 (C(CH₃)₃, d, J(CP) = 32.7 Hz), 38.2 (C(CH₃)₃, d, J(CP) = 31.9 Hz), 32.2 (C(CH₃)₃, br s), 30.5 (C(CH₃)₃, br s), 27.4 (CH₂, d, J(CP) = 19.1 Hz), 26.6 (CH₂, d, J(CP) = 13.2 Hz), 21.7 (CH₃Ph, s), 21.6 (CH₃Ph, s), 14.3 (CH₃-Pt, dd, J(CPtrans) = 88.8 Hz, J(CPcis) = 7.0 Hz). Accurate mass spectrum (ESI): M_r = 672.2510 (M − Cl)⁺ (calcd for C₃₁H₄₃P₂Pt 672.2482). Elemental analysis (calcd for C₃₁H₄₃ClP₂Pt): C: 52.88 (52.58), H: 6.38 (6.12).

Preparation of [PtCl(CH₃)(L_c)] (1c/2c)

A solution of L_c (46 mg, 0.10 mmol) in CH₂Cl₂ (1 mL) was added dropwise over 30 s to a stirred solution of [PtCl(CH₃)(cod)] (35 mg, 0.10 mmol) in CH₂Cl₂ (1 mL). The mixture was stirred overnight. The volatiles were removed under reduced pressure and the residue was recrystallised from CH₂Cl₂/hexane affording a white solid (65 mg, 92% yield). The spectrum was broad at ambient temperatures and signals for the minor isomer 2c were not observed. At −90 °C, the spectrum showed the presence of two rotamers in a 2 : 1 ratio (1c/1c′) associated with the presence of gauche and anti o-tolyl conformations and the relative orientation of the o-xylene backbone (see Results and Discussion). ³¹P{¹H} NMR (121 MHz, CD₂Cl₂, −90 °C): For 1c: δ 23.3 (P^tBu₂, d, J(PPt) = 1820 Hz, J(PP) = 13 Hz), 8.2 (PAr₂, d, J(PPt) = 4455 Hz, J(PP) = 13 Hz). For 1c′: δ 28.0 (P^tBu₂, d, J(PPt) = 1846 Hz, J(PP) = 13 Hz), 11.7 (PAr₂, d, J(PPt) = 4552 Hz, J(PP) = 13 Hz). ¹H NMR (300 MHz, C₂D₂Cl₄, 100 °C): δ 8.00 (2 H, br s), 7.39-7.11 (9 H, m), 6.92 (1 H, d, J(HH) = 7.7 Hz), 6.59 (1 H, d, J(HH) = 7.5 Hz), 4.19 (CH₂, 2 H, dt, J(HP) = 30.5 Hz, J(HH) = 12.0 Hz), 3.63 (CH₂, 2 H, dd, J(HP) = 21.0 Hz, J(HH) = 10.8 Hz), 2.64 (CH₃Ph, 6 H, s), 1.58 (C(CH₃)₃, 18 H, d, J(HP) = 12.5 Hz), 0.29 (CH₃-Pt, 3 H, dd, J(HPt) = 52.6, J(HPtrans) = 6.3 Hz, J(HPcis) = 5.6 Hz). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 143.9-143.7 (br), 143.0 (s), 136.8 (br s), 134.3-133.9 (br), 132.9-132.6 (br s), 132.2 (br s), 131.7 (br s), 130.8-130.5 (br), 129.1 (s), 127.4 (s), 127.1 (s), 126.3-126.0 (br), 125.0 (s), 123.9 (s), 38.7-37.7 (C(CH₃)₃, br), 32.2 (C(CH₃)₃, br s), 31.0 (C(CH₃)₃, br s), 28.6 (s), 27.3 (br s), 24.8-24.2 (br), 21.1 (br s). Accurate mass spectrum (ESI): M_r = 671.2447 (M − HCl)⁺(calcd for C₃₁H₄₂P₂Pt 671.2415). Elemental analysis (calcd for C₃₁H₄₃ClP₂Pt): C: 52.41 (52.58), H: 6.11 (6.12).

Preparation of [PtCl(CH₃)(L_d)] (1d/2d)

A solution of L_d (48 mg, 0.10 mmol) in CH₂Cl₂ (2 mL) was added dropwise over 30 s to a stirred solution of [PtCl(CH₃)(cod)] (35 mg, 0.10 mmol) in CH₂Cl₂ (2 mL). The mixture was stirred overnight. The volatiles were removed under reduced pressure and the residue was washed with pentane and dried under reduced pressure affording a white solid (57 mg, 78% yield). The product was obtained as a mixture of three isomers (two totamers of 1d and the geometric isomer 2d) in a 12 : 3 : 2 ratio (1d : 1d′ : 2d) at −90 °C. ³¹P{¹H} NMR (121 MHz, CD₂Cl₂, −90 °C): For 1d: δ 26.3 (P^tBu₂, d, J(PPt) = 1844 Hz, J(PP) = 13 Hz), 11.8 (PAr₂, d, J(PPt) = 4529 Hz, J(PP) = 13 Hz). For 1d′: δ 22.9 (P^tBu₂, d, J(PPt) = 1961 Hz, J(PP) = 11 Hz), 9.0 (PAr₂, d, J(PPt) = 4488 Hz, J(PP) = 11 Hz). For 2d: δ 25.1 (P^tBu₂, d, J(PPt) = 4257 Hz, J(PP) = 12 Hz), 8.8 (PAr₂, d, J(PPt) = 1907 Hz, J(PP) = 11 Hz). ¹H NMR (300 MHz, C₂D₂Cl₄, 120 °C): For 1d: δ 8.03 (1 H, br s), 7.47-7.11 (9 H, m), 6.94 (1 H, t, J(HH) = 7.8 Hz), 6.59 (1 H, d, J(HH) = 6.9 Hz), 3.70-3.59 (CH₂P, 2 H, m), 3.32-3.21 (CH₂P, 2 H, m), 2.81-2.68 (CH₂CH₃, 4 H, m), 1.61 (C(CH₃)₃, 18 H, d, J(HP) = 12.6 Hz), 1.28-1.20 (CH₃CH₂, 6 H, m), 0.33 (CH₃-Pt, 3 H, dd, J(HPt) = 52.3 Hz, J(HPtrans) = 6.4 Hz, J(HPcis) = 5.4 Hz). ¹³C{¹H} NMR (75 MHz, CD₂Cl₂): δ 148.3 (br s), 136.6 (br s), 134.7 (br s), 133.8 (br s), 132.0 (br s), 130.7 (br s), 129.1 (s), 127.6 (br s), 127.2 (br s), 126.1-125.9 (br), 124.3-124.1 (br), 38.6 (C(CH₃)₃, br s), 37.6 (C(CH₃)₃, br s), 32.2 (C(CH₃)₃, br s), 30.8 (C(CH₃)₃, br s), 30.0 (br s), 28.8 (br s), 28.5 (s), 27.5 (s), 27.0 (br s), 15.6 (CH₃CH₂, br s), 13.3(CH₃-Pt, br d, J(CPtrans) = 89.5 Hz, J(CPcis) not resolved). Accurate mass spectrum (ESI): M_r = 700.2799 (M − Cl)⁺ (calcd for C₃₃H₄₇P₂Pt 700.2795). Elemental analysis (calcd for C₃₃H₄₇ClP₂Pt): C: 55.13 (53.84), H: 6.69 (6.43).

Preparation of [PtCl(CH₃)(L_e)] (1e/2e)

A solution of L_e (65 mg, 0.13 mmol) in CH₂Cl₂ (1 mL) was added dropwise over 30 s to a stirred solution of [PtCl(CH₃)(cod)] (46 mg, 0.13 mmol) in CH₂Cl₂ (1 mL). The mixture was stirred for two days. The volatiles were removed under reduced pressure and the residue was washed with pentane and dried under reduced pressure affording an off-white solid (68 mg, 71% yield). The product was obtained as a mixture of isomers in an 80 : 1 ratio (1e : 2e) at room temperature. ³¹P{¹H} NMR (121 MHz, CD₂Cl₂): For 1e: δ 25.9 (P^tBu₂, d, J(PPt) = 1822 Hz, J(PP) = 15 Hz), 3.9 (PAr₂, d, J(PPt) = 4526 Hz, J(PP) = 15 Hz). For 2e: δ 27.6 (P^tBu₂, d, J(PPt) not discernible, J(PP) = 16 Hz), 4.6 (PAr₂, d, J(PPt) not discernible, J(PP) = 16 Hz). ¹H NMR (400 MHz, CD₂Cl₂): δ 8.31-6.51 (12 H, m), 4.47 (CH₂, 2 H, br s), 3.82-3.68 (CH₃O, 6 H, m), 2.43-2.05 (CH₂, 2 H, br m), 1.55-1.20 (C(CH₃)₃, 18 H, br m), 0.05 (CH₃-Pt, 3 H, dd, J(HPt) = 50.7 Hz, J(HPtrans) = 6.6 Hz, J(HPcis) = 5.1 Hz). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 161.1 (br s), 148.7 (br), 136.6 (dd, J(CP) = 4.6 Hz, J(CP) = 2.3 Hz), 135.4 (s), 133.9 (s), 132.0 (t, J(CP) = 3.8 Hz), 131.1 (dd, J(CP) = 5.4 Hz, J(CP) = 2.3 Hz), 129.1 (s), 126.7 (dd, J(CP) = 3.8 Hz, J(CP) = 1.5 Hz), 126.6 (dd, J(CP) = 3.1 Hz, J(CP) = 2.3 Hz), 120.3 (br s), 111.5 (d, J(CP) = 5.4 Hz), 55.2 (CH₃O, br s), 37.9 (C(CH₃)₃, d, J(CP) = 12.7 Hz), 31.1 (C(CH₃)₃, d, J(CP) = 3.8 Hz), 28.5 (CH₂, br d, J(CP) = 10.8 Hz), 27.0 (CH₂, br d, J(CP) = 9.2 Hz), 13.5 (CH₃-Pt, dd, J(CPtrans) = 89.9 Hz, J(CPcis) = 6.3 Hz). Accurate mass spectrum (ESI): M_r = 704.2382 (M − Cl)⁺ (calcd for C₃₁H₄₃O₂P₂Pt 704.2381). Elemental analysis (calcd for C₃₁H₄₃ClO₂P₂Pt): C: 50.96 (50.30), H: 5.88 (5.86).

Preparation of [PtCl(CH₃)(L_f)] (1f/2f)

A solution of L_f (44 mg, 0.09 mmol) in CH₂Cl₂ (2 mL) was added dropwise over 30 s to a stirred solution of [PtCl(CH₃)(cod)] (30 mg, 0.09 mmol) in CH₂Cl₂ (2 mL). The mixture was stirred overnight. The volatiles were removed under reduced pressure and the residue was recrystallised from CH₂Cl₂/hexane affording a white solid (59 mg, 86% yield). The product was obtained as a mixture of two isomers in a 40 : 1 ratio (1f : 2f) at room temperature. ³¹P{¹H} NMR (162 MHz, CD₂Cl₂): For 1f: δ 42.4 (P^tBu₂, d, J(PPt) = 1853 Hz, J(PP) = 10 Hz), 24.0 (PAr₂, d, J(PPt) = 4300 Hz, J(PP) = 10 Hz). For 2f: δ 40.8 (P^tBu₂, d, J(PPt) not discernible, J(PP) = 9 Hz), 24.2 (PAr₂, d, J(PPt) not discernible, J(PP) = 9 Hz). ¹H NMR (400 MHz, CD₂Cl₂): For 1f: δ 7.76-6.70 (8 H, m), 3.74-3.28 (CH₂, 4 H, m), 2.30 (CH₃Ph, 6 H, s), 2.23 (CH₃Ph, 6 H, s), 2.00 (CH₃Ph, 6 H, s), 1.27 (C(CH₃)₃, 9 H, br s), 1.03 (C(CH₃)₃, 9 H, d, J(HP) = 12.5 Hz), 0.89 (CH₃-Pt, 3 H, t, J(HPt) = 44.5 Hz, J(HPtrans) = J(HPcis) = 6.8 Hz). For 2f: δ 0.47 (CH₃-Pt, 3 H, t, J(HPt) = 48.0 Hz, J(HPtrans) = J(HPcis) = 6.4 Hz). ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 142.3 (br s), 136.1 (br s), 135.6 (d, J(CP) = 4.3 Hz), 134.5 (dd, J(CP) = 3.3 Hz, J(CP) = 2.1 Hz), 133.4 (dd, J(CP) = 14.9 Hz, J(CP) = 2.1 Hz), 132.3 (br s), 131.8 (br s), 130.0 (d, J(CP) = 9.0 Hz), 129.1 (s), 127.3 (dd, J(CP) = 2.7 Hz, J(CP) = 1.6 Hz), 126.8 (d, J(CP) = 1.2 Hz), 43.4 (s), 38.0 (C(CH₃)₃, d, J(CP) = 12.5 Hz), 36.5 (C(CH₃)₃, d, J(CP) = 14.0 Hz), 32.4 (C(CH₃)₃, br s), 30.9 (C(CH₃)₃, br s), 29.3 (d, J(CP) = 3.9 Hz), 23.1 (CH₃-Pt, br d, J(CPtrans) = 100.8 Hz, J(CPcis) not resolved), 21.1 (CH₂, dd, J(CP) = 18.3 Hz, J(CP) = 1.5 Hz), 20.3 (CH₂, d, J(CP) = 3.5 Hz). Accurate mass spectrum (ESI): M_r = 728.3127 (M − Cl)⁺ (calcd for C₃₅H₅₁P₂Pt 728.3108). Elemental analysis (calcd for C₃₅H₅₁ClP₂Pt): C: 55.89 (55.00), H: 6.61 (6.73).

Generation of [PtCl(CH₃)(L_g)] (1g/2g)

A solution of L_g (68 mg, 0.19 mmol) in toluene (1 mL) was added dropwise over 30 s to a stirred solution of [PtCl(CH₃)(cod)] (66 mg, 0.19 mmol) in toluene (1 mL). The product was characterised by ³¹P NMR and mass spectrometry only. The product was as a mixture of 1g/2g in a 70 : 1 ratio. ³¹P{¹H} NMR (121 MHz, C₇D₈): For 1g: δ 21.7 (P^tBu₂, d, J(PPt) = 1775 Hz, J(PP) = 13 Hz), 10.5 (PⁱPr₂, d, J(PPt) = 4201 Hz, J(PP) = 13 Hz). For 2g: δ 21.7 (P^tBu₂, J(PPt) = 4170 Hz, J(PP) = 15 Hz), 11.8 (J(PPt) = 1783 Hz, J(PP) = 15 Hz). Accurate mass spectrum (ESI): M_r = 576.2498 (M − Cl)⁺ (calcd for C₂₃H₄₃P₂Pt 576.2482).

Preparation of [PtCl(CH₃)(L_h)] (1h/2h)

A solution of L_h (100 mg, 0.22 mmol) in toluene (2 mL) was added dropwise over 30 s to a stirred solution of [PtCl(CH₃)(cod)] (78 mg, 0.22 mmol) in toluene (3 mL). The mixture was stirred overnight and the volatiles were removed under reduced pressure affording a light yellow solid (97 mg, 62% yield). The product was obtained as a mixture of two isomers in a 30 : 1 ratio (1h : 2h) at room temperature. ³¹P{¹H} NMR (121 MHz, C₇D₈): For 1h: δ 25.9 (P^tBu₂, d, J(PPt) = 1649 Hz, J(PP) = 11 Hz), −12.5 (PCg, d, J(PPt) = 4564 Hz, J(PP) = 11 Hz). For 2h: δ 28.7 (P^tBu₂, d, J(PPt) not discernible, J(PP) = 11 Hz), −9.3 (PCg, d, J(PPt) not discernible, J(PP) = 11 Hz). ¹H NMR (300 MHz, C₇D₈): δ 7.10-6.91 (4 H, m), 5.43 (CH₂, 2 H, br s), 3.29-3.11 (CH₂P, 2 H, m), 3.04-2.91 (CHHP, 1 H, m), 2.37 (CHHP, 1 H, br d, J(HH) = 13.4 Hz), 2.17 (CH₃, 3 H, s), 1.56-0.91 (C(CH₃)₃ + CH₃ + CH₂, 29 H, m), 0.75 (CH₃-Pt, dd, J(HPt) not discernible, J(HPtrans) = 6.7 Hz, J(HPcis) = 3.9 Hz). ¹³C{¹H} NMR (100 MHz, C₇D₈): δ 153.5 (s), 133.6 (dd, J(CP) = 4.6 Hz, J(CP) = 2.3 Hz), 131.9 (br s), 130.4 (s), 127.1 (s), 126.7 (s), 96.5 (CO₂CH₃, s), 96.3 (CO₂CH₃, s), 70.0 (COCH₃, s), 65.7 (COCH₃, s), 42.0 (CH₂, s), 31.7 (CH₂, s), 31.4 (C(CH₃)₃, d, J(CP) = 26.1 Hz), 31.3 (C(CH₃)₃, d, J(CP) = 26.9 Hz), 28.1 (CH₃, s), 26.9 (CH₂P, dd, J(CP) = 13.1 Hz, J(CP) = 4.6 Hz), 26.4 (CH₂P, d, J(CP) = 3.8 Hz), 25.6 (C(CH₃)₃, s), 24.7 (CH₃, s), 22.8 (CH₃, s), 22.6 (CH₃, s), 14.8 (CH₃-Pt, dd, J(CPtrans) = 114.5 Hz, J(CPcis) not resolved). ESI mass spectrum: m/z 694.2 (M − CH₃)⁺, 674.2 (M − Cl)⁺.

Preparation of [PtCl₂(L_h)] (3h)

A solution of L_h (100 mg, 0.22 mmol) in CH₂Cl₂ (2 mL) was added dropwise over 30 s to a stirred solution of [PtCl₂(cod)] (83 mg, 0.22 mmol) in CH₂Cl₂ (2 mL). The solution was stirred overnight and then the solvent was removed under reduced pressure. The residue was washed with pentane and dried under reduced pressure affording a light yellow solid (117 mg, 74% yield). ³¹P{¹H} NMR (121 MHz, C₇D₈): δ 18.7 (P^tBu₂, d, J(PPt) = 3450 Hz, J(PP) = 11 Hz), −24.5 (PCg, d, J(PPt) = 3725 Hz, J(PP) = 11 Hz). ESI mass spectrum: m/z 694.2 (M − Cl)⁺.

Preparation of [Pt(CH₃)₂(L_h)] (4h)

A solution of L_h (171 mg, 0.38 mmol) in toluene (2 mL) was added dropwise over 30 s to a stirred solution of [Pt(CH₃)₂(cod)] (125 mg, 0.38 mmol) in toluene (3 mL). The mixture was stirred overnight and the volatiles were removed under reduced pressure. The residue was recrystallised from CH₂Cl₂/hexane affording a light brown solid (64 mg, 39% yield). ³¹P{¹H} NMR (121 MHz, C₇D₈): δ 23.4 (P^tBu₂, d, J(PPt) = 1692 Hz, J(PP) = 11 Hz), −16.0 (PCg, d, J(PPt) = 1802 Hz, J(PP) = 11 Hz). ¹H NMR (270 MHz, C₇D₈): δ 7.17-6.92 (4 H, m), 3.67-3.08 (CH₂P, 4 H, m), 2.71 (CHH, 1 H, J(HH) = 12.9 Hz, J(HP) = 3.6 Hz), 2.37 (CHH, 1 H, d, J(HH) = 13.5 Hz), 2.11 (CHH, 1 H, br s), 1.80-1.71 (CHH, 1 H, m), 1.47-1.06 (C(CH₃)₃ + CH₃ + CH₃-Pt, 36 H, m). ¹³C{¹H} NMR (68 MHz, C₇D₈): δ 147.0 (br s), 138.1 (br s), 136.0 (br s), 132.3 (dd, J(CP) = 4.2 Hz, J(CP) = 2.1 Hz), 131.0 (dd, J(CP) = 6.6 Hz, J(CP) = 3.0 Hz), 126.3 (br s), 96.9 (CO₂CH₃, s), 96.4 (CO₂CH₃, s), 74.4 (COCH₃, dd, J(CP) = 15.3 Hz, J(CP) = 2.9 Hz), 74.1 (COCH₃, d, J(CP) = 17.7 Hz), 41.4 (CH₂, br s), 40.3 (CH₂, br s), 37.5 (C(CH₃)₃, d, J(CP) = 10.4 Hz), 31.4 (CH₃, s), 31.2 (C(CH₃)₃, d, J(CP) = 4.2 Hz), 31.1 (CH₃, s), 30.6 (CH₂P, dd, J(CP) = 44.5 Hz, J(CP) = 7.8 Hz), 28.2 (CH₃, s), 27.5 (CH₃, s), 26.7 (CH₂P, dd, J(CP) = 30.2 Hz, J(CP) = 5.2 Hz), 7.9 (CH₃-Pt, dd, J(CPtrans) = 99.1 Hz, J(CPcis) not resolved), −4.0 (CH₃-Pt, dd, J(CPtrans) = 91.9 Hz, J(CPcis) not resolved). ESI mass spectrum: m/z 674.2 (M − CH₃)⁺.

Preparation of [PdCl(CH₃)(L_h)] (7/8)

A solution of L_h (100 mg, 0.22 mmol) in CH₂Cl₂ (2 mL) was added dropwise over 30 s to a stirred solution of [PdCl(CH₃)(cod)] (58 mg, 0.22 mmol) in CH₂Cl₂ (3 mL). The solution was stirred overnight and then the solvent was removed under reduced pressure. The residue was washed with pentane and dried under reduced pressure affording a light orange solid (86 mg, 66% yield). The product was obtained as a mixture of four isomers in a 7 : 7 : 2 : 1 ratio (7α : 7β : 8α : 8β) at −90 °C. ³¹P{¹H} NMR (121 MHz, CD₂Cl₂, −90 °C): For 7α: δ 34.0 (P^tBu₂, d, J(PP) = 36 Hz), 25.3 (PCg, d, J(PP) = 36 Hz). For 7β: δ 32.5 (P^tBu₂, d, J(PP) = 34 Hz), 20.7 (PCg, d, J(PP) = 34 Hz). For 8α: δ 37.8 (P^tBu₂, d, J(PP) = 5 Hz), 17.0 (PCg, d, J(PP) = 5 Hz). For 8β: δ 42.5 (P^tBu₂, d, J(PP) = 5 Hz), 18.7 (PCg, d, J(PP) = 5 Hz). ¹H NMR (400 MHz, CD₂Cl₂): For 7α/7β: δ 7.69-6.23 (4 H, m), 4.34-3.51 (CH₂P + CH₂, 6 H, m), 3.35 (CH₃, br d, J(HH) = 7.6 Hz), 2.73-2.19 (CH₃, 9 H, m), 2.02-1.76 (CH₂, m), 1.42 (C(CH₃)₃, 18 H, br s), 0.80 (CH₃-Pd, 3 H, t, J(HPtrans) = J(HPcis) = 6.5 Hz). For 8α/8β: δ Signals obscured by 7α/7β. ¹³C{¹H} NMR (100 MHz, C₇D₈): δ 137.0 (d, J(CP) = 3.9 Hz), 132.7 (br s), 132.2 (br s), 132.0 (br s), 127.6 (br s), 127.9 (br s), 94.2 (CO₂CH₃, s), 90.3 (CO₂CH₃, s), 70.9 (COCH₃, s), 70.1 (COCH₃, s), 37.0 (C(CH₃)₃, br s), 34.7 (CH₂, s), 31.1 (C(CH₃)₃, s), 30.2 (CH₂P, d, J(CP) = 12.5 Hz), 27.5 (CH₂, br s), 25.1 (CH₂P, br s), 22.9 (CH₃, s), 21.3 (CH_3, s), 14.4 (CH₃, s), 3.2 (CH₃-Pd, dd, J(CPtrans) = 97.3 Hz, J(CPcis) = 8.6 Hz). ESI mass spectrum: m/z 585.20 (M − Cl)⁺.

Generation of [PdCl(¹³COCH₃)(L_h)] (9/10)

A three-necked flask provided with a stirring bar was connected to a ¹³CO gas cylinder and a Schlenk line via a T-shape connector with an oil bubbler. When the system was under ¹³CO atmosphere, a solution of [PdCl(CH₃)(L_h)] (54 mg, 0.09 mmol) in CD₂Cl₂ (0.7 mL) was placed and the mixture was stirred for 30 min. ³¹P{¹H} NMR (121 MHz, CD₂Cl₂, −90 °C): δ 22.0 (dd, J(¹³CPtrans) = 98 Hz, J(PP) = 56 Hz), 21.5 (dd, J(PP) = 56 Hz, J(¹³CPcis) = 17 Hz). ¹³C{¹H} NMR (75 MHz, CD₂Cl₂, −90 °C): δ 234.8({¹³C(O)(CH₃)}-Pd, dd, J(¹³CPtrans) = 97.6 Hz, J(¹³CPcis) = 17.0 Hz). IR (cm⁻¹): ν_CO 1656.

Crystal structure determinations

X-ray diffraction experiments on [PtCl(CH₃)(L_c)] (1c) and [PtCl(CH₃)(L_e)] (1e) were carried out at 100 K on a Bruker APEX II diffractometer using Mo-Kα radiation (λ = 0.71073 Å). Similarly, data for [PtCl(CH₃)(L_b)] (1b) was collected at 100 K on an Oxford Diffraction GEMINI diffractometer, also using Mo-Kα radiation and for [Pt(CH₃)₂(L_h)] (4h), at 100 K on a Bruker Proteum diffractometer using Cu-Kα radiation (λ = 0.71073 Å). In all cases, data collections were performed using a CCD area detector from a single crystal mounted on a glass fibre. Intensities were integrated^30,31 from several series of exposures measuring 0.5° in ω or ϕ. Absorption corrections were based on equivalent reflections using SADABS (1c, 1e and 4h)³² or CrysAlis RED³¹ (1b). The structures were solved using SHELXS and refined against all F_o² data with hydrogen atoms riding in calculated positions using SHELXL.³³ Some restraints/constraints were necessary in modelling disorder in order to ensure a smooth refinement. Crystal structure and refinement data are given in Table 3.

Table 3 Crystallographic data

Compound	1b	1c	1e	4h
Colour, habit	Colourless block	Colourless block	Colourless block	Colourless needle
Size/mm	0.17 × 0.17 × 0.14	0.32 × 0.23 × 0.18	0.24 × 0.15 × 0.14	0.45 × 0.22 × 0.18
Empirical formula	C31H43ClP2Pt	C31H43ClP2Pt	C31H43ClO2P2Pt	C28H48O3P2Pt
M	708.13	708.13	740.13	689.69
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	P21/c	P21/c	P21/c
a/Å	13.8369(5)	11.3652(2)	11.5134(3)	12.0593(3)
b/Å	12.7997(4)	16.6336(3)	16.4546(4)	14.6253(3)
c/Å	16.9040(6)	15.0322(3)	16.2779(4)	17.0475(4)
β (°)	103.561(5)	91.911(1)	105.978(1)	103.394(1)
V/Å^3	2910.37(17)	2840.17(9)	2964.69(13)	2924.90(12)
Z	4	4	4	4
μ/mm^−1	5.041	5.166	4.958	10.205
θ min, max	2.34, 30.08	4.09, 30.67	3.79, 27.48	3.77, 70.21
Completeness	1.000 to θ = 27.50°	0.995 to θ = 30.67°	0.997 to θ = 27.48°	0.955 to θ = 70.21°
Reflections: total/independent	69480/8173	37958/8754	74072/6769	22152/5322
R int	0.0376	0.0560	0.0327	0.0425
Final R1 and wR2	0.0229, 0.0549	0.0294, 0.0642	0.0155, 0.0411	0.0310, 0.0836
Largest peak, hole/eÅ^−3	1.322, −0.827	1.569, −0.908	0.571, −0.897	1.697, −0.808
ρ c/g cm^−3	1.616	1.656	1.658	1.566

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Acknowledgements

We are grateful to Lucite and EPSRC for studentships, the Royal Society, and COST action CM0802 “PhoSciNet” for supporting this work, and Johnson Matthey for a loan of precious metal compounds.

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Footnote

† CCDC reference numbers 847221–847224. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c1cy00409c

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