Aminophobanes: hydrolytic stability, tautomerism and application in Cr-catalysed ethene oligomerisation

Abstract

9-Amino-9-phosphabicyclo[3.3.1]nonanes, (PhobPNHR′; R = Me or ⁱPr) are readily prepared by aminolysis of PhobPCl and are significantly less susceptible to hydrolysis than the acyclic analogues Cy₂PNHR′. Treatment of Cy₂PNHMe with Cy₂PCl readily gave Cy₂PNMePCy₂. By contrast, treatment of PhobPCl with PhobPNHMe in the presence of Et₃N does not afford PhobPNMePPhob but instead the salt [PhobP(=NMeH)PPhob]Cl is formed which, upon addition of [PtCl₂(NC^tBu)₂] gives the zwitterionic complex [PtCl₃(PhobP(=NMeH)PPhob)]. The neutral PhobP(=NMe)PPhob is accessible from PhobNMeLi and is converted to the chelate [PdCl₂(PhobPNMePPhob)] by addition of [PdCl₂(cod)]. The anomalous preference of the PhobP group for the formation of PPN products is discussed. The unsymmetrical diphos ligands PhobPNMePAr₂ (Ar = Ph, o-Tol) are prepared, converted to [Cr(CO)₄(PhobPNMePAr₂)] and shown to form Cr-catalysts for ethene oligomerisation, producing a pattern of higher alkenes that corresponds to a Schulz-Flory distribution overlaid on selective tri/tetramerisation.

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Introduction

Significant differences between the donor properties of phosphacycles and their acyclic analogues are to be expected because of the effects that ring constraints can have on the frontier orbital energies and the steric properties of the P-donor.¹ The molecular manifestations of these ring effects include stability (thermodynamic and kinetic) and structural rigidity which can be desirable qualities when considering the design of ligands. As a result, the coordination chemistry of phosphacycles and their applications in catalysis have attracted much academic and industrial attention.²

Phobanes (PhobPZ, in Chart 1) are examples of rigid phosphacycles which have found important applications in homogeneous catalysis^3,4 most notably in Co-catalysed hydroformylation.⁵ We are interested in heterophobanes (PhobPZ where Z = a non-hydrocarbyl group) as ligands and particularly the effect that the phobyl group has on the reactivity of the P–Z bond. For example, fluorophobane (PhobPF) was shown to be a rare example of a fluorophosphine that is thermodynamically more stable to disproportionation and kinetically more stable to hydrolysis than acyclic fluorophosphine analogues; moreover PhobPF shows promise as a ligand for hydroformylation and hydrocyanation catalysis.⁶

Chart 1

Aminodiphosphines R₂PNR′PR₂ (known as PNP ligands, Chart 1) are excellent ligands for Cr-catalysed ethene tri/tetramerisation. As illustrated in Table 1, the characteristics of the R and R′ groups in R₂PNR′PR₂ have a decisive effect on the chemoselectivity, productivity and therefore the potential industrial utility of the oligomerisation catalyst.^7,8 Increased steric bulk serves to lower the ratio of 1-octene to 1-hexene obtained, whilst changing from aryl to alkyl substituents on phosphorus dramatically reduces activity and increases polymer formation. The data in Table 1 highlight the impact of process conditions such as solvent, temperature and pressure upon the catalysis.

Table 1 Catalytic data for Cr–PNP based tri/tetramerisation^a

Ligand	P/bar	T/°C	TON/kg per g Cr	TOF/kg per g Cr per h	1-C8 : 1-C6	C6 /wt%(% 1-C6)	C8 /wt%(% 1-C8)	C10+/wt%	PE/wt %
a Catalysis data taken from ref. 7 and 8. All wt% values are of total product slate. C6 and C8 refers to the entire C6 and C8 fractions and 1-C6 and 1-C8 refer to the proportion of the linear α-olefin within that fraction. b Catalysis conditions: 33 μmol CrCl3(THF)3; 2 eq. ligand; 300 eq. MMAO-3A; 100 mL toluene. c Catalysis conditions: 33 μmol Cr(acac)3; 2 eq. ligand; 300 eq. MMAO-3A; 100 mL toluene. d Catalysis conditions: 2.5 μmol Cr(acac)3; 1.2 eq. ligand; 300 eq. MMAO-3A; 100 mL methylcyclohexane.
Ph2PNMePPh2 ^b	30	65	27	54	5.7	24.8 (39.4)	59.0 (94.1)	n/r	1.4
Ph2PN^iPrPPh2 ^b	30	65	12	24	2.1	32.7 (86.5)	60.6 (99.2)	n/r	0.3
Ph2PN^iPrPPh2 ^c	45	45	272	544	5.7	16.9 (70.3)	68.3 (98.8)	n/r	1.1
Et2PNMePEt2 ^c	45	45	4	8	4.1	16.8 (64.6)	45.2 (97.4)	n/r	13.6
Ph2PNMePPh2 ^d	45	60	482	964	9.5	16.4 (33.4)	54.0 (95.9)	24.8	4.9
Ph2PN^iPrPPh2 ^d	45	60	552	1104	5.4	16.8 (75.5)	69.5 (99.0)	12.6	0.9

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The industrial interest in PNP ligands⁹ makes it important to have reliable methods for their preparation. As summarised in Scheme 1, the most general route to PNP ligands is the reaction of a primary amine with a chlorophosphine in the presence of a base.¹⁰ The monophos R₂PNHR′ species are presumed intermediates and when R or R′ is bulky, they are readily isolated and are potential intermediates to unsymmetrical PNP ligands.¹¹ When the substituents in either of the reactants R₂PCl or R′NH₂ are bulky, a complication is the formation of the phosphinimine PPN compounds (Scheme 1); Maumela et al.¹⁰ have shown that when R = Ph and R′ = ^tBu, the PPN product is the kinetic product whose isomerisation to the thermodynamic PNP product is catalysed by Ph₂PCl.

Scheme 1

We were interested in investigating PNP ligands such as L_a–L_d where a phobyl group has been incorporated (Chart 2). It is shown here that the monophosphines L_a and L_b are readily prepared but their conversions to L_c and L_d has not been achieved. However the mixed diphosphines L_f and L_g are accessible and are shown to be ligands for Cr-catalysed ethene tri/tetramerisation.

Chart 2 Aminophobanes targets; L_c and L_d (in parentheses) have not been observed.

Results and discussion

Stereoelectronically, a Cy₂P group can be viewed as an acyclic analogue of a PhobP group since ostensibly, they are similarly bulky dialkylphosphino groups. However, we have shown previously that the rigidity of the PhobP moiety leads to a larger steric profile than expected¹² and the approximately 90° C–P–C bridgehead angle in PhobP has the effect of lowering the HOMO and LUMO energies.¹³ Ligands L₁–L₄ (Chart 3) were targeted in the belief that a comparison of their chemistry with the phobane analogues L_a–L_d (Chart 2) would provide insight into the effect of the bicycle.

Chart 3 Aminodicyclohexylphosphines targets; L₄ (in parentheses) has not been observed.

Monodentate aminophobanes

The monophobanes L_a and L_b were readily prepared by aminolysis of PhobPCl. The relative lability of L_a and L_b to hydrolysis (eqn (1)) was gauged by treatment of L_a, L_b, L₁ and L₂ with aqueous solutions under the same conditions and monitoring the formation of R₂P(=O)H by ³¹P NMR spectroscopy. All four aminophosphines eventually underwent complete hydrolysis but at different rates. Comparison of the extents of hydrolysis after 16 h (Table 2) shows that the NHⁱPr group provides more protection from hydrolysis than the less bulky NHMe. Moreover, the bicyclic compounds PhobPNHR are significantly kinetically stabilised to hydrolysis with respect to the acyclic Cy₂PNHR analogues. The resistance to hydrolysis of PhobPNHR is consonant with the phobyl moiety behaving as a bulky group.¹²

Table 2 Comparison of some properties of aminophobanes and acyclic analogues

	% Hydrolysis at 16 h^a	ν CO ^b/cm^−1
a For the hydrolysis experiments, the aminophosphines (1.4 mmol) were dissolved in a 0.55 M solution of water in MeOH (25 mL) and stirred. The reactions were monitored by periodically taking aliquots of the solution and measuring the ^31P NMR spectrum. b The IR spectrum in the 2050–1850 cm^−1 region was measured in CH2Cl2 for the trans-[RhCl(CO)(L)2] complexes generated in situ by combining [Rh2Cl2(CO)4] with 4 equiv. of L. c L1 was 50% hydrolysed after 0.5 h.
PhobPNHMe (La)	5	1957
Cy2PNHMe (L1)	100^c	1955
PhobPNH^iPr (Lb)	1	1954
Cy2PNH^iPr (L2)	64	1951

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The donor properties of L_a and L_b can be compared quantitatively with L₁ and L₂ from the ν_CO values for their trans-[RhCl(CO)(L)₂] complexes.¹⁴ The rhodium complexes were made in situ (see Scheme 2) and the recorded ν_CO values (Table 2) are consistent with L_a and L_b being slightly poorer σ-donors/better π-acceptors than their acyclic analogues L₁ and L₂, as expected.¹³

Scheme 2 Reagents: (i) [PtCl₂(NC^tBu)₂] in CH₂Cl₂; (ii) [Cr(CO)₄(nbd)] (nbd = norbornadiene) in CH₂Cl₂.

Ligands L_a and L_b form trans-dichloroplatinum(II) complexes 1a and 1b, and trans-tetracarbonylchromium(0) complexes 2a and 2b (Scheme 2). The crystal structures of 1b and 2b have been determined and are shown in Fig. 1 and 2. In addition, the crystal structure of trans-[PtCl₂(L₂)₂] (3), an acyclic analogue of 1b has been determined (Fig. 3).

Fig. 1 Crystal structure of trans-[PtCl₂(PhobPNHⁱPr)₂] (1b). All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): Pt(1)–P(1) 2.3241(10), Pt(1)–Cl(1) 2.3102(8), P(1)–N(1) 1.6594(16), Pt(1)–P(1)–N(1) 109.40(6).

Fig. 2 Crystal structure of trans-[Cr(CO)₄(PhobPNHⁱPr)₂] (2b). All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): Cr(1)–P(1) 2.3434(3), Cr(1)–P(2) 2.3499(3), P(1)–N(1) 1.6910(9), P(2)–N(2) 1.6934(10), 114.06(4), Cr(1)–P(1)–N(1) 114.20(2), Cr(1)–P(2)–N(2).

Fig. 3 Crystal structure of trans-[PtCl₂(Cy₂PNHⁱPr)₂] (3). All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°): Pt(1)–P(1) 2.3178(7), Pt(1)–Cl(1) 2.3193(7), P(1)–N(1) 1.681(3), Pt(1)–P(1)–N(1) 113.91(10).

In aminophobane complex 1b and its acyclic analogue 3, the Pt metal centre is square planar. The Pt sits on a crystallographic inversion centre and the asymmetric unit consists of half of the complete molecule, consequently the N–P–P–N torsion angles are 180° in both cases, i.e. the anti conformer is adopted, as in other trans-[PtCl₂(PhobPZ)₂] complexes.^6,12,15 The cone angle of L_b in 1b is 111.8° and of L₂ in 3 is larger at 115.8°. In the structure of 2b, the asymmetric unit contains one complete molecule. The cone angle of L_b in 2b is 109.2° which is smaller than in 1b, the compression probably reflecting the greater crowding in the octahedral complex. The N–P–P–N torsion angle in 2b is 108.3(1)° indicating the amino groups are gauche to each other, a conformation not previously observed in PhobPZ complexes.

Bidentate aminophobanes

The previously reported¹⁶ diphosphinoamine L₃ is readily prepared from MeNH₂ and Cy₂PCl in the presence of Et₃N (Scheme 3). The intermediate in this reaction is presumably Cy₂PNHMe (L₁) and indeed treatment of the isolated L₁ with Cy₂PCl in the presence of Et₃N in CH₂Cl₂ gave L₃ quantitatively according to ³¹P NMR spectroscopy. The spectrum of the reaction mixture also revealed a transient PPN species (as evidenced by a large J_PP of 280 Hz) to which the tautomeric structure L′₃ is assigned. The PPN species L′₃ smoothly converted over 30 min to PNP ligand L₃ whose structure was confirmed by its conversion to the chelate complex 4 (Scheme 3), the crystal structure of which has been determined (Fig. 4).

Scheme 3

Fig. 4 Crystal structure of complex [PtCl₂(L₃)] (4). Only one of the three unique molecules is shown and all hydrogen atoms and six molecules of CHCl₃ have been omitted for clarity. Selected bond lengths (Å) and angles (°); Pt(1)–P(2) 2.206(6), Pt(1)–P(1) 2.257(6), Pt(1)–Cl(1) 2.363(5), Pt(1)–Cl(2) 2.376(7), P(1)–N(1) 1.745(17), P(2)–N(1) 1.668(18), P(2)–Pt(1)–P(1) 71.1(3), N(1)–P(1)–Pt(1) 92.8(6), N(1)–P(2)–Pt(1) 96.9(9), P(2)–N(1)–P(1) 99.0(8).

The asymmetric unit consists of three independent molecules of 4 along with six chloroform molecules. Although the PtP₂Cl₂ fragment is approximately planar (rms deviation ∼0.03 Å), the Pt has a distorted square planar geometry due to the constraints of the 4-membered PNP chelate. The three independent Pt–P–N–P rings are approximately planar with rms deviations of ∼0.03 Å.

In contrast to the ready reaction of L₁ with Cy₂PCl to give PNP ligand L₃ (Scheme 3), the reaction of PhobPNHMe (L_a) with PhobPCl in the presence of NEt₃ or N-methylpyrrolidine did not give the expected diphosphinoamine L_c. Instead, a PPN species (J_PP = 407 Hz) was the exclusive product; this was initially assigned structure L′_c but its ¹H NMR spectrum (which showed a multiplet at 7.01 ppm integrating for 1H) and mass spectrum (M⁺ at [L′_c + 1]) led to its assignment as the HCl adduct L′_c·HCl (Scheme 4). This was supported by its reaction with [PtCl₂(NC^tBu)₂] which yielded crystals of the insoluble, zwitterionic complex [PtCl₃(L′_c·H)] (5) whose X-ray crystal structure is shown in Fig. 5. The conditions under which L′_c·HCl was formed (Scheme 4) indicate that the iminophosphine L′_c is more basic than either NEt₃ or N-methylpyrrolidine.

Scheme 4

Fig. 5 Crystal structure of [PtCl₃(L′_c·H)] (5). All hydrogen (except H1N) atoms, a CH₂Cl₂ molecule (50% occupied) and one C₂H₂Cl₄ molecule have been omitted for clarity. Selected bond lengths (Å) and angles (°); Pt(1)–P(1) 2.2194(13), Pt(1)–Cl(1) 2.3085(12), Pt(1)–Cl(2) 2.3683(12), Pt(1)–Cl(3) 2.2988(12), P(1)–P(2) 2.2797(19), P(2)–N(1) 1.629(4), N(1)–C(17) 1.468(7), N(1)–H(1N) 0.8800, P(1)–Pt(1)–Cl(3) 88.86(5), P(1)–Pt(1)–Cl(1) 92.80(5), Cl(3)–Pt(1)–Cl(1)177.36(5), P(1)–Pt(1)–Cl(2) 177.89(5), Cl(3)–Pt(1)–Cl(2)89.39(5), Cl(1)–Pt(1)–Cl(2) 88.99(5), Pt(1)–P(1)–P(2) 108.26(6), N(1)–P(2)–P(1) 111.95(19).

The crystal structure of 5 has a square planar Pt with an rms deviation of the atoms from the square plane of ∼0.03 Å. The PPN ligand is rotated away from the PtCl₃ plane with torsion angles Cl1–Pt1–P1–P2 of −102.4(1)° and Cl3–Pt1–P1–P2 of 75.6(1)°.

Treatment of PhobPNMeH with ⁿBuLi at −78 °C followed by PhobPCl gave a PPN species with a J_PP = 327 Hz (significantly smaller than the J_PP of 407 Hz for L′_c·HCl) that is assigned to the neutral L_c′ which has been isolated. No reaction occurred upon addition of PhobPCl to L′_c in CH₂Cl₂, conditions that might have been expected to tautomerise L′_c to L_c.¹⁰

It has previously been shown that some neutral PPN compounds rearrange when they react with [MCl₂(cod)] (M = Pd or Pt)¹⁷ or [NiBr₂(dme)]¹⁸ to give PNP chelate complexes. Reaction of [PdCl₂(cod)] with L′_c gave the chelate [PdCl₂(L_c)] (6) whose crystal structure has been determined and is shown in Fig. 6.

Fig. 6 Crystal structure of complex [PdCl₂(L_c)] (6). All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°); Pd(1)–P(1) 2.2311(4), Pd(1)–P(2) 2.2353(4), Pd(1)–Cl(2) 2.3735(4), Pd(1)–Cl(1) 2.3748(4), P(1)–N(1) 1.6949(14), P(2)–N(1) 1.7016(14), P(1)–Pd(1)–P(2) 70.851(15).

The asymmetric unit contains one molecule of 6, with the PdP₂Cl₂ fragment being essentially planar (rms ∼ 0.07 Å) although the overall geometry is a distorted square planar due to the constraints of the 4-membered PNP chelate. As seen in the structure of analogue 4, the Pd1–P1–N1–P2 ring is also essentially planar with an rms deviation for the atoms of 0.01 Å.

From the homodiphos products obtained in the reactions of L_a and L₁ with R₂PCl (see Schemes 3 and 4), it appears that the PhobP group differs from Cy₂P and Ar₂P groups in promoting PPN over PNP formation; this raised the question of what would happen when the syntheses of the heterodiphos PNP ligands PhobPNMePR₂ where R = Cy (L_e), Ph (L_f) or o-Tol (L_g) were attempted?

The reaction between PhobPNHMe and Cy₂PCl was followed by ³¹P NMR spectroscopy and it was unambiguously shown that a PPN product was formed which, on the basis of its J_PP of 358 Hz, was tentatively assigned to the protonated species L′_e·HCl (Scheme 5); addition of Et₃N led to multiple P-containing species but there was no evidence for the formation of the neutral PPN (L′_e) or PNP (L_e) species. The reaction between PhobPCl and Cy₂PNHMe was also monitored and in this case, ³¹P NMR spectroscopy revealed that a PPN product was formed (J_PP = 403 Hz) which was assigned to the cationic species L′′_e·HCl (Scheme 5), an isomer of L′_e·HCl. It therefore appears that the PPN-promoting effect of the PhobP group dominates over the PNP-preference of the Cy₂P group.

Scheme 5

The unsymmetrical PNP ligands L_f and L_g (J_PP = 80 Hz in both) featuring PhobP groups were successfully prepared upon treatment of PhobPNHMe with Ar₂PCl (Ar = Ph or o-Tol) in the presence of Et₃N (eqn (2)). It therefore appears that the PPN formation promoted by the PhobP group is superseded by the PNP preference of the Ar₂P groups.

The reaction of PhobPNHMe with Tol₂PCl was monitored by ³¹P NMR spectroscopy. A PPN species (J_PP = 331 Hz), tentatively assigned to L′_g·HCl (Scheme 6) was formed rapidly which, upon treatment with NEt₃, was transformed to L_g (J_PP = 80 Hz).

Scheme 6

Treatment of the bulky R₂PNHⁱPr (L_b or L₂) with R₂PCl (R₂P = Cy₂P or PhobP) under the conditions that converted R₂PNHMe to the corresponding L₃ (Scheme 3) or L′_c·HCl/L′_c (Scheme 4) gave, according to in situ³¹P NMR spectroscopy, mixtures of unidentified products as well as the reactants.

Under the conditions that smoothly led to the mixed PNP ligands L_f and L_g (eqn (2)), L_b reacted with Ar₂PCl to give PPN species whose structures were assigned to the protonated L′_h·HCl and L′_i·HCl (eqn (3)) on the basis of the large J_PP values of 338 and 359 Hz respectively. Crystals of L′_h·HCl were obtained and the crystal structure shown in Fig. 7 confirms the PPN assignment. The N⋯Cl distance of 3.101(1) Å indicates the presence of hydrogen-bonding between the N–H and Cl.

Fig. 7 Crystal structure of complex L′_h·HCl. All hydrogen atoms, except on the N1, have been omitted for clarity. Selected bond lengths (Å) and angles (°); P(1)–N(1) 1.6241(0), P(1)–P(2) 2.2403(4), N(1)–H(1N) 0.863(16), N(1)–P(1)–P(2) 119.93(4).

PPN versus PNP preferences

The N- and P-substituents determine whether PNP (A) or PPN (A′) species are formed in the reaction of amines with chlorophosphines (Scheme 7). In some cases, it has been shown^10,19 that the PPN can be converted to the PNP tautomer using a R₂PCl catalyst and we have observed PPN species as transients en route to the PNP products (e.g. Cy₂PNMePCy₂ see Scheme 3) showing that the PNP is the thermodynamic product. In other cases (e.g. Cy₂PN{SO₂Ar}PCy₂) the neutral PPN tautomer appears to be the thermodynamic product.^17,18,20 An additional element observed in this work is the formation of a protonated A′·HCl product that is resistant to deprotonation by amines.

Scheme 7 Proposed pathway for the conversion of an amine to a PNP ligand. A dotted line is used for step iv since this step is not observed with the aminophobanes.

A pathway from chlorophosphine and primary amine to PNP/PPN products that encompasses these empirical observations is shown in Scheme 7. Nucleophilic attack by amine on chlorophosphine with loss of HCl would give the intermediate aminophosphine (step i). Reaction of a second chlorophosphine at the P site of the aminophosphine would give the salt A′·HCl (step ii) which can eliminate HCl to give the neutral A′ (step iii) and finally rearrangement to give PNP (step iv).

The formation of a PPN species when PhobPCl reacts with PhobPNHMe or PhobPNMeLi instead of PhobPNMePPhob contrasts with the smooth formation of Cy₂PNMePCy₂via a PPN intermediate; furthermore, PhobP(=NMe)PPhob does not isomerise to the PNP tautomer in the presence of PhobPCl. At present, it is not known whether these observations are due to PhobP(=NMe)PPhob being the thermodynamically preferred tautomer or slow kinetics of interconversion and therefore further investigation of this system is warranted.

Oligomerisation catalysis

The unsymmetrical PNP ligands L_f and L_g have been screened for Cr-catalysed ethene oligomerisation (see below) and it was therefore appropriate to explore their Cr coordination chemistry. The reaction of [Cr(CO)₄(nbd)] with L_f or L_g gave the corresponding Cr(0) complexes 7 and 8 (eqn (4)) which have been fully characterised and their crystal structures have been determined (Fig. 8 and 9).

Fig. 8 Crystal structure of complex [Cr(CO)₄(L_f)] (7). All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°); Cr(1)–P(3) 2.3163 (16), Cr(1)–P(2) 2.3474(16), P(2)–N(1) 1.714(5), P(3)–N(1) 1.701(5), P(3)–Cr(1)–P(2) 68.29(5), P(3)–N(1)–P(2) 100.1(2), N(1)–P(3)–Cr(1) 96.48(17), N(1)–P(2)–Cr(1) 94.99(16), C(17)–P(2)–C(16) 96.0(3), C(7)–P(3)–C(1) 103.5(2).

Fig. 9 Crystal structure of [Cr(CO)₄(L_g)] (8). All hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (°); Cr(1)–P(1) 2.3477(6), Cr(1)–P(2) 2.3669(6), P(1)–N(1) 1.7146(17), P(2)–N(1) 1.7035(17), P(1)–Cr(1)–P(2) 68.04(4), P(2)–N(1)–P(1) 101.01(9), N(1)–P(1)–Cr(1) 94.67(6), N(1)–P(2)–Cr(1) 94.29(6), C(16)–P(1)–C(13) 95.66(10), C(7)–P(2)–C(4) 105.17(9).

In combination with chromium, the ligands L_f and L_g gave moderate activities towards ethylene oligomerisation but the formation of polymer was high, as can be seen from Table 3. Within the liquid fraction, it is clear that a degree of selective oligomerisation to 1-hexene and 1-octene did occur (particularly for L_g) but concurrently with Schulz-Flory selectivity (Fig. 10). The 1-octene to 1-hexene ratios obtained for both ligands is high.

Fig. 10 The distribution of products in the liquid fraction formed in the ethene oligomerisation catalysed by Cr catalysts of (a) L_f and (b) L_g showing the preferential formation of C₆ and C₈ alkenes.

Table 3 Ethene oligomerisation results^a

L	Rxn time/min	TON/kg per g Cr	TOF/kg per g Cr per h	Liquid product/wt%	Solid product (PE)/wt%	C4/wt%	C6/wt%(% 1-C6)	C8/wt%(% 1-C8)	1-C8 : 1-C6	C10–14 /wt%	C15+/wt%
a Conditions: Cr(acac)3 (2.5 μmol), 1.2 eq. L, 960 eq. MMAO-3A (800 eq. added to pre-activation, 160 eq. added to autoclave vessel), PhCl solvent (70 mL), 53 bar ethylene, 60 °C. The wt% values for the carbon number fractions refer to the liquid fraction.
Lf	24.8	393	951	89.5	10.5	1.7	16.1 (20.7)	26.7 (93.0)	7.5	22.4	33.0
Lg	16.7	31	110	56.6	43.4	1.8	16.3 (70.2)	61.1 (99.4)	5.3	7.2	13.6

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Conclusions

The monodentate aminophobanes PhobPNHR (R = Me or ⁱPr) have been readily prepared and are more resistant to hydrolysis than their Cy₂PNHR analogues consistent with the PhobP group having a greater effective steric bulk than Cy₂P. Attempts to make the free ligand PhobPNMePPhob have been thwarted by formation of PPN species which resist tautomerisation although a rearrangement takes place in the presence of [PdCl₂(cod)] to give the desired PNP–Pd chelate. The readily prepared mixed diphos ligands PhobPNMePAr₂ (Ar = Ph or o-Tol) in combination with Cr, catalysed the oligomerisation of ethylene with a partial selectivity to tri/tetramerisation, the remainder of the selectivity appearing to be Schulz-Flory in nature; the activities were moderate, but the polymer formation was high.

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 system²¹ 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 starting materials PhobPCl,¹³ [Cr(CO)₄(η⁴-norbornadiene)],²² [PtCl₂(NC^tBu)₂],²³ [PdCl₂(cod)],²⁴ were prepared by literature methods. All other reagents were used as received from Aldrich, Strem or Lancaster. The aminophosphines were stored under nitrogen at room temperature. 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 PhobPNHMe (L_a)

PhobPCl (2.00 g, 11.3 mmol) was dissolved in a 2 M THF solution of MeNH₂ (22.9 mL, 45.3 mmol). The resulting suspension was stirred at room temperature for 16 h. The solvent was removed under reduced pressure to give a white residue which was extracted with diethyl ether (35 mL). The ethereal solution was filtered to remove the methylammonium chloride and then the solvent was evaporated to dryness to give a white solid (1.42 g, 75%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 26.8. ¹H NMR (270 MHz; CDCl₃) δ 1.48–2.20 (14H, m, phobane), 2.67 (3H, d, J_HP = 15.0 Hz, CH₃N). ¹³C{¹H} NMR (67 MHz; CDCl₃) δ 21.5 (d, J_CP = 2.0 Hz, CH₂), 22.9 (d, J_CP = 4.6 Hz, CH₂), 23.7 (d, J_CP = 3.1 Hz, CH₂), 28.1 (d, J_CP = 10.9 Hz, CH₃N), 31.2 (d, J_CP = 14.0 Hz, CH₂), 33.0 (d, J_CP = 26.9 Hz, CH). Elemental analysis: Found (Calc. for C₉H₁₈NP) C, 63.5 (63.1); N, 8.1 (8.1); H, 10.4 (10.6) %. MS (ESI: m/z 171 (M⁺).

Preparation of PhobPNHⁱPr (L_b)

PhobPCl (0.88 g, 5.0 mmol) was dissolved in a solution of ⁱPrNH₂ (1.54 mL, 20.1 mmol) in THF (9.0 mL). The resulting suspension was stirred at room temperature for 16 h and the filtered to remove the isopropylammonium chloride. The solvent was removed under reduced pressure to give an oily residue which was dissolved in toluene (10 mL) to precipitate any remaining isopropylammonium chloride. This toluene solution was filtered to remove the methylammonium chloride and then the solvent was evaporated to dryness to give a colourless oil (0.57 g, 63%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 15.5. ¹H NMR (270 MHz; CDCl₃) δ 1.47–2.15 (14H, m, phobane), 1.14 (6H, d, J_HP = 6.2 Hz, CH₃N), 3.1–3.3 (1H, m, CH). ¹³C{¹H} NMR (75 MHz; CDCl₃) δ 21.6 (s, CH₂), 22.9 (s, CH₂), 24.0 (d, J_CP = 3.1 Hz, CH₃), 26.7 (d, J_CP = 6.9 Hz, CH₂), 29.5 (d, J_CP = 9.2 Hz, CH), 31.4 (d, J_CP = 13.8 Hz, CH₂), 48.6 (d, J_CP = 24.3 Hz, CH). Elemental analysis: Found (Calc. for C₁₁H₂₂NP) C, 66.7 (66.3); N, 6.9 (7.0); H, 11.3 (11.1) %. HRMS (EI): Found (Calc. for C₁₁H₂₂NP 199.1496 (199.1490).

Preparation of [PhobP(=NHMe)PPhob]Cl (L′_c·HCl)

A solution of PhobPCl (0.530 g, 3.03 mmol) in CH₂Cl₂ (2.0 mL) was added in portions to a solution of L_a (0.510 g, 3.00 mmol)) and N-methylpyrrolidine (2.40 g, 28.2 mmol) in CH₂Cl₂ (2 mL). The reaction mixture was stirred for 3 h and then the solvent was removed under reduced pressure. The resulting white solid was recrystallised from hot MeCN to afford white crystals (0.43 g, 45%). ³¹P {¹H} NMR (121 MHz; CDCl₃) δ 46.5 (d, J_PP = 407 Hz), −24.2 (d, J_PP = 407 Hz). ¹H NMR (400 MHz; CDCl₃) δ 1.60–2.54 (28H, m, phobane), 2.66 (3H, dd, J_HP = 12.3 Hz, J_HP = 5.68 Hz, CH₃N), 7.01 (1H, m, HCl). ¹³C {¹H} NMR (100 MHz; CDCl₃) δ 20.4 (d, J_CP = 6.9 Hz), 20.7 (d, J_CP = 6.9 Hz), 20.8 (d, J_CP = 6.9 Hz), 21.2 (d, J_CP = 1.5 Hz), 21.9 (d, J_CP = 6.9 Hz), 24.9 (d, J_CP = 6.1 Hz), 25.1 (d, J_CP = 6.1 Hz), 26.2 (d, J_CP = 6.1 Hz), 26.9 (d, J_CP = 6.1 Hz), 27.3 (d, J_CP = 6.1 Hz), 27.5 (dd, J_CP = 9.2 Hz, J_CP = 1.5 Hz) 27.7 (d, J_CP = 3.8 Hz), 28.9 (t, J_CP = 3.84 Hz), 32.7 (d, J_CP = 14.6 Hz), 32.9 (d, J_CP = 15.3 Hz). Elemental analysis: Found (Calc. for C₁₇H₃₂ClNP₂) C, 59.1 (58.7); N, 3.8 (4.0); H, 9.4 (9.3) %. HRMS (EI): Found (Calc. for C₁₇H₃₂NP₂) 312.2004 (312.2011).

Preparation of PhobP(=NMe)PPhob (L′_c)

To the solution of L_a (0.45 g, 2.6 mmol) in THF (3.0 mL), a 1.6 M solution of ⁿBuLi (4.5 mL, 7.20 mmol) in hexane was added at −78 °C over 5 min. The reaction mixture was stirred at −78 °C for 40 min. PhobPCl (0.45 g, 2.6 mmol) in THF (2 mL) was added in portions to the cooled reaction mixture. The mixture was allowed to warm to room temperature and stirred for 2 h. The solvent was then removed under reduced pressure and the residue was triturated with diethyl ether (10 mL). The solid was filtered off and dried. satisfactory elemental analysis was not obtained and the product was used without further purification (0.52 g, 65%) ³¹P{¹H} NMR (121 MHz, C₆D₆) δ 30.7 (d, J_PP = 320 Hz), −32.3 (d, J_PP = 320 Hz). ¹H NMR (300 MHz, C₆D₆) δ 1.25–2.52 (14H, m, phobane), 2.62 (3H, dd, J_HP = 14.5 Hz, J_HP = 5.7 Hz, CH₃N).

Preparation of PhobPNMePPh₂ (L_f)

Aminophobane L_a (0.280 g, 1.64 mmol) and NEt₃ (0.220 g, 2.17 mmol) were dissolved in MeCN (1 mL). To this stirred mixture, Ph₂PCl (0.380 g, 2.12 mmol) was added dropwise over 5 min. The reaction mixture was stirred for 2 h to give a white solid, which was filtered off and recrystallised from hot MeCN to afford white needles (0.44 g, 69%). ³¹P {¹H} NMR (121 MHz; CD₂Cl₂) NMR δ 57.6 (d, J_PP = 80 Hz), 51.1 (d, J_PP = 80 Hz). ¹H NMR (270 MHz; CD₂Cl₂) δ 1.21–2.24 (14H, m, phobane), 2.32 (3H, dd, J_HP = 4.3 Hz, J_HP = 8.5 Hz CH₃N), 7.05–7.22, 7.55–7.69 (10H, 2 m, ArH). ¹³C {¹H} NMR (67 MHz; CD₂Cl₂) δ 21.8 (d, J_CP = 1.5 Hz, CH₂), 23.2 (d, J_CP = 2.6 Hz, CH₂), 28.7 (dd, J_CP = 20.7 Hz, J_CP = 11.9 Hz, CH₃N), 30.2 (d, J_CP = 17.1 Hz, CH₂), 33.0 (dd, J_CP = 16.6 Hz, J_CP = 7.2 Hz, CH), 24.1 (t, J_CP = 3.1 Hz, CH₂), 128.2 (d, J_CP = 5.7 Hz, CH), 128.4 (s, CH) 132.2 (d, J_CP = 19.2 Hz, CH), 138.4 (dd, J_CP = 16.6 Hz, J_CP = 1.0 Hz, C). Elemental analysis: Found (Calc. for C₂₁H₂₇NP₂) C, 71.8 (70.9); N, 4.5 (4.0); H, 7.8 (7.6) %. HRMS (EI): Found (Calc. for C₂₁H₂₇NP₂) 355.1613 (355.1619).

Preparation of PhobPNMeP(o-Tol)₂ (L_g)

Aminophobane L_a (0.200 g, 1.16 mmol) and NEt₃ (0.240 g, 2.32 mmol) were dissolved in MeCN (1 mL). To this stirred mixture, (o-Tol)₂PCl (0.580 g, 1.52 mmol) was added dropwise over 5 min. The reaction mixture was stirred for 16 h to give the white solid product, which was filtered off and recrystallized from hot MeCN to afford white needles (0.17 g, 38%). ³¹P {¹H} NMR (121 MHz; CD₂Cl₂) δ 60.2, (d, J_PP = 80 Hz), 36.8 (d, J_PP = 80 Hz). ¹H NMR (300 MHz; CD₂Cl₂) δ 1.44–2.59 (14H, m, phobane), 2.34 (3H, dd, J_HP = 3.9 Hz, J_HP = 8.9 Hz, CH₃N), 2.35 (6H, s, CH₃), 7.07–7.42 (8H, m, HAr). ¹³C {¹H} (100 MHz; CD₂Cl₂) 21.0 (d, J_CP = 5.3 Hz, CH₃), 21.2 (d, J_CP = 5.3 Hz, CH₃), 21.7 (d, J_CP = 1.5 Hz, CH₂), 23.1 (d, J_CP = 3.0 Hz, CH₂), 24.3 (t, J_CP = 2.8 Hz, CH₂), 28.7 (dd, J_CP = 20.7 Hz, J_CP = 11.5 Hz, CH₃N), 30.5 (dd, J_CP = 16.9 Hz, J_CP = 1.1 Hz CH₂), 33.9 (dd, J_CP = 16.1 Hz, J_CP = 6.1 Hz, CH), 128.5 (s, CH) 125.5 (s, CH), 130.3 (d, J_CP = 3.8 Hz, CH), 131.6 (d, J_CP = 2.3 Hz, CH), 136.7 (dd, J_CP = 17.6 Hz, J_CP = 1.5 Hz, C), 141.7 (d, J_CP = 26.9 Hz, C). Elemental analysis: Found (Calc. for C₂₃H₃₁NP₂) C, 72.2 (72.0); N, 3.8 (3.6); H, 8.1 (8.1) %. HRMS (EI): Found (Calc. for C₂₃H₃₁NP₂) 383.1931 (383.1932).

Preparation of [PhobP(=NHⁱPr)PPh₂]Cl (L′_h·HCl)

Ph₂PCl (0.90 g, 5.0 mmol) was added dropwise to a solution of L_b (0.83 g, 4.2 mmol) and N-methylpyrrolidine (0.39 g, 4.2 mmol) in CH₂Cl₂ (5 mL). The reaction mixture was stirred for 1 h and the solvent was then removed under reduced pressure. The resulting white solid was recrystallised from hot MeCN to yield white crystals (0.71 g, 44%) suitable for X-ray crystallography. ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 50.0 (d, J_PP = 338 Hz), 39.1 (d, J_PP = 338 Hz). ¹H NMR (400 MHz; CDCl₃) δ 0.94 (6H, d, J_HH = 6.2 Hz, CH₃), 1.35 (1H, d, J_HH = 6.4 Hz, CH), 1.74–2.94 (14H, m, phobane), 7.46–7.54, 7.64–7.77 (10H, 2 m, HAr). ¹³C{¹H} NMR (100 MHz; CD₂Cl₂) δ 20.5 (d, J_CP = 5.4 Hz), 20.9 (d, J_CP = 4.6 Hz), 25.1 (d, J_CP = 4.6 Hz), 26.6 (d, J_CP = 5.3 Hz), 27.7 (d, J_CP = 2.3 Hz), 28.1 (s), 46.6 (s), 128.5 (d, J_CP = 8.5 Hz), 130.2 (d, J_CP = 7.6 Hz), 131.4 (s), 134.9 (d, J_CP = 2.3 Hz). HRMS (EI): Found (Calc. for C₂₃H₃₂NP₂) 384.2004 (384.1999).

Preparation of [PhobP(=NHⁱPr)P(o-Tol)₂]Cl (L′_i·HCl)

A solution of (o-Tol)₂PCl (0.620 g, 2.51 mmol) in MeCN (5.0 mL) was added in portions to PhobPNH(ⁱPr) (L_b) (0.530 g, 2.67 mmol) and N-methylpyrrolidine (3.20 g, 37.5 mmol) in MeCN (5 mL). The reaction mixture was stirred for 1 h and then the solvent was removed under reduced pressure. The remaining white solid was recrystallised from hot MeCN to yield white crystals (0.71 g, 62%). ³¹P {¹H} NMR (161 MHz; CDCl₃) δ 49.3 (d, J_PP = 359 Hz), −39.8 (d, J_PP = 359 Hz). ¹H (400 MHz; CDCl₃) δ 0.89 (6H, d, J_HH = 6.36 Hz, CH₃ⁱPr), 1.65–2.40 (14H, m, phobane), 2.53 (6H, s, CH₃), 7.20–7.40 (8H, m, HAr). ¹³C{¹H} NMR (100 MHz; CDCl₃) δ 19.5 (dd, J_CP = 14.1 Hz, J_CP = 5.4 Hz), 21.1 (d, J_CP = 24.1 Hz), 23.8 (d, J_CP = 4.6 Hz), 25.8 (d, J_CP = 6.2 Hz), 26.8 (d, J_CP = 10.9 Hz), 28.5 (d, J_CP = 3.8 Hz), 28.9 (d, J_CP = 4.6 Hz), 45.6 (d, J_CP = 4.6 Hz), 123.8 (dd, J_CP = 14.0 Hz, J_CP = 3.8 Hz), 126.7 (s), 130.4 (d, J_CP = 7.8 Hz), 135.9 (d, J_CP = 7.0 Hz), 142.3 (dd, J_CP = 31.9 Hz, J_CP = 7.0 Hz). Elemental analysis: Found (Calc. for C₂₅H₃₆ClNP₂) C, 67.0 (67.0); N, 3.5 (3.1); H, 8.5 (8.1)%.

Preparation of Cy₂PNMePCy₂ (L₃)

Cy₂PCl (2.79 g, 11.9 mmol) was dissolved in CH₂Cl₂ (6.0 mL). To this, a 2 M THF solution of MeNH₂ (3.0 mL, 6.0 mmol) and NEt₃ (1.30 g, 12.8 mmol) were added dropwise over 5 min. The resulting suspension was stirred for 16 h. The solvent was then removed under reduced pressure and the residue was redissolved in toluene (30 mL). The [Et₃NH]Cl salt was filtered off to give a clear solution. The solvent was removed under reduced pressure to give the crude product, which was recrystallized from hot acetonitrile (0.77 g, 32%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 86.8. ¹H NMR (300 MHz; CDCl₃) δ 1.10–1.60 (44H, m, CH and CH₂), 2.64 (s, CH₂), 27.2 (s, CH₂), 29.5 (t, J_CP = 9.2 Hz, CH₃N). ¹³C{¹H} NMR (300 MHz; CDCl₃) δ 26.7 (d, J_CP 36.9 Hz, CH₃), 26.8 (s, CH₂), 24.9 (s, CH₂), 32.2 (t, J_CP 8.6 Hz, CH), 45.7 (s, CH₃). Elemental analysis: Found (Calc. for C₂₅H₄₇NP₂) C, 70.4 (70.9); N, 2.8 (3.3); H, 11.4 (11.2) %. HRMS (CI): Found (Calc. for [C₂₅H₄₇NP₂H]) 424.3262 (424.3262).

Preparation of trans-[PtCl₂(PhobPNHMe)₂] (1a)

PhobPNHMe (0.069 g, 0.45 mmol) was dissolved in CH₂Cl₂ (2.0 mL). [PtCl₂(NC^tBu)₂] (0.086 g, 0.23 mmol) was added and the resulting yellow solution was stirred for 2 h. The solvent was reduced to ca. 1 mL and then hexane (4 mL) was added to give the yellow precipitated product (0.080 g, 57%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 46.3 (J_PtP = 2638 Hz). ¹H NMR (300 MHz; CDCl₃) δ 1.52–2.16 (28H, m, phobane), 2.98 (6H, t, J_HP = 6.6 Hz, CH₃N). ¹³C{¹H} NMR (75 MHz; CDCl₃) δ 21.3 (s), 22.1 (s), 24.2 (t, J_CP = 6.6 Hz), 25.8 (s), 29.2 (s), 30.8 (s). Elemental analysis: Found (Calc. for C₁₈H₃₆Cl₂N₂P₂Pt) C, 35.4 (35.5); H, 5.5 (6.0); N, 4.6 (5.0) %. HRMS (EI): Found (Calc. for [C₁₈H₃₆Cl₂N₂P₂Pt]H) 608.1451 (608.1438).

Preparation of trans-[PtCl₂(PhobPNHⁱPr)₂] (1b)

PhobPNHⁱPr (0.18 g, 0.90 mmol) was dissolved in CH₂Cl₂ (2.0 mL). [PtCl₂(NC^tBu)₂] (0.20 g, 0.46 mmol) was added and the resulting yellow solution was stirred for 2 h. The solvent was reduced to ca. 1 mL and then hexane (4 mL) was added to give the yellow precipitated product (0.26 g, 85%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 40.4 (J_PtP = 2646 Hz). ¹H NMR (300 MHz; CDCl₃) δ 1.56–2.15, 2.55–2.76 (28H, m, phobane), 1.31 (12H, d, J_HH = 6.6 Hz, CH₃ⁱPr), 4.14–4.26 (2H, m, CH). ¹³C{¹H} NMR (75 MHz; CDCl₃) δ 21.4 (t, J_CP = 2.3 Hz), 22.0 (t, J_CP = 2.3 Hz), 25.4 (t, J_CP = 24.3 Hz). Elemental analysis: Found (Calc. for C₂₂H₄₄Cl₂N₂P₂Pt) C, 39.7 (39.7); N, 3.9 (4.2); H, 6.6 (6.6)%. MS (ESI): m/z 665.21 (M + 2H)⁺.

Preparation of trans-[Cr(CO)₄(PhobPNHMe)₂] (2a)

PhobPNHMe (0.048 g, 0.29 mmol) was dissolved in CH₂Cl₂ (4.0 mL). [Cr(CO)₄(η⁴-norbornadiene)] (0.048 g, 0.15 mmol) was added and the resulting yellow solution was stirred for 24 h. The solvent was removed under reduced pressure and the resulting yellow solid product was washed with hexane (0.023 g, 26%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 102.5. ¹H NMR (300 MHz; CDCl₃) δ 1.53–2.57 (28H, m, phobane), 2.70 (6H, d, J_HH = 8.4 Hz, CH₃N). ¹³C{¹H} NMR (75 MHz; CDCl₃) δ 21.5 (d, J_CP = 52.2 Hz, CH₃), 26.4 (s, CH₂), 28.3 (s, CH₂), 29.9 (t, J_CP = 8.6 Hz, CH). HRMS (ES): Found (Calc. for C₂₂H₃₆CrN₂O₂P₂) 506.1552 (506.1555). IR (CH₂Cl₂): v(CO) 1869, 1858 cm⁻¹.

Preparation of trans-[Cr(CO)₄(PhobPNHⁱPr)₂] (2b)

PhobPNHⁱPr (0.078 g, 0.39 mmol) was dissolved in CH₂Cl₂ (4.0 mL). [Cr(CO)₄(η⁴-norbornadiene)] (0.050 g, 0.19 mmol) was added and the resulting yellow solution was stirred for 24 h. The solvent was removed under reduced pressure and the resulting yellow solid product was washed with hexane (0.042 g, 36%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 94.9. ¹H NMR (300 MHz; CDCl₃) d 1.24 (12H, d, J_HH = 6.4 Hz, CH₃ⁱPr), 1.49–2.65 (12H, m, phobane). ¹³C{¹H} NMR (75 MHz; CDCl₃) δ 21.3 (d, J_CP = 36.9 Hz, CH₃), 26.8 (s, CH₂), 24.9 (s, CH₂), 32.2 (t, J_CP = 8.6 Hz, CH), 45.7 (s, CH₃). IR (CH₂Cl₂): v(CO) 1863 cm⁻¹.

Preparation of [PtCl₂(Cy₂PNMePCy₂)] (4)

To a solution of L₃ (0.032 g, 0.070 mmol) in CH₂Cl₂ (2 mL), [PtCl₂(NC^tBu)₂] (0.034 g, 0.070 mmol) in CH₂Cl₂ (2 mL) was added and the mixture was stirred for 2 h. The solvent was then reduced to ca. 2 mL, hexane was added (4 mL) and a yellow solid was obtained (0.023 g, 47%). ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 37.1 (J_PtP = 3211 Hz). ¹H NMR (300 MHz; CDCl₃) δ 1.20–1.97 (44H, m, CH and CH₂), 2.72 (3H, t, J_HP = 9.4 Hz, CH₃N). ¹³C {¹H} NMR (75 MHz; CDCl₃) δ 25.7 (s), 26.9 (m) 28.7 (s), 27.9 (s), 38.0 (t, J_CP = 15.2 Hz). Elemental analysis: Found (Calc. for C₂₅H₄₇Cl₂NP₂Pt) C, 43.3 (43.5); N, 2.3 (2.0); H, 6.8 (6.9) %. HRMS (EI): Found (Calc. for C₂₅H₄₇ClNP₂Pt) 653.2529 (653.2520).

Preparation of [PtCl₃(PhobP(=NHMe)PPhob)] (5)

A mixture of L′_c·HCl (0.025 g, 0.070 mmol) and [PtCl₂(NC^tBu)₂] (0.034 g, 0.070 mmol) were dissolved in CH₂Cl₂ (5 mL) and stirred for 2 h to give a yellow solution. Warming this solution to 40 °C led to the slow formation of yellow crystals of the product 5 suitable for X-ray crystallography. Satisfactory elemental analysis was not obtained and the crystals were insoluble in common organic solvents which precluded further characterisation by NMR spectroscopy.

Preparation of [PdCl₂(PhobPNMePPhob)] (6)

To a suspension of L′_c (0.031 g, 0.099 mmol) in toluene (3 mL), [PdCl₂(cod)] (0.031 g, 0.12 mmol) was added. The suspension was stirred at 50 °C for 5 min. The clear reaction mixture was then cooled to room temperature and the resulting yellow precipitate was filtered off and washed with hexane (0.010 g, 20%). Crystals suitable for X-ray crystallography were grown from CDCl₃ although satisfactory elemental analysis was not obtained. ³¹P{¹H} NMR (121 MHz; CDCl₃) δ 37.0. ¹H NMR (300 MHz; CDCl₃) δ 1.65–2.40 (28H, m, phobane), 2.58 (d, J_HP = 8.3 Hz). HRMS (EI): Found (Calc. for [C₁₇H₃₁NP₂ClPd]⁺) 452.0672 (452.0649).

Preparation of [Cr(CO)₄(PhobPNMePPh₂)] (7)

To a solution of L_f (0.030 g, 0.070 mmol) in CH₂Cl₂ (2 mL), [Cr(CO)₄(η⁴-norbornadiene)] (0.018 g, 0.070 mmol) in CH₂Cl₂ (2 mL) was added. The resulting yellow solution was stirred for 24 h. The solvent was then removed under reduced pressure to yield a yellow solid, which was washed with hexane (5 mL) (0.014 g, 27%). ³¹P{¹H} NMR (121 MHz, CDCl₃) δ 105.7 (d, J_PP = 42 Hz), 108.1 (d, J_PP = 42 Hz). ¹H NMR (300 MHz, CDCl₃) δ 1.50–2.65 (14H, m, phobane), 2.97 (3H, t, J_HP = 6.9 Hz, CH₃N), 7.33–7.59 (10H, m, HAr). ¹³C {¹H}NMR (125 MHz; CD₂Cl₂) δ 20.3 (d, J_CP = 7.8 Hz), 21.2 (d, J_CP = 6.3 Hz), 23.4 (d, J_CP = 4.4 Hz), 23.6 (d, J_CP = 8.3 Hz), 29.6 (m), 34.7 (m), 126.1 (d, J_CP = 10.7 Hz), 126.7 (d, J_CP = 12.2 Hz) 132.7 (m). HRMS (EI): Found (Calc. for C₂₅H₂₇CrNO₄P₂) 519.0812 (519.0820). IR (in CH₂Cl₂): ν(CO) 2003, 1910, 1885, 1873 cm⁻¹.

Preparation of [Cr(CO)₄(PhobPNMeP(o-Tol)₂)] (8)

To a solution of L_g (0.030 g, 0.060 mmol) in CH₂Cl₂ (2 mL), [Cr(CO)₄(η⁴-norbornadiene)] (0.017 g, 0.060 mmol) in CH₂Cl₂ (2 mL) was added. The resulting yellow solution was stirred for 24 h. The solvent was then removed under reduced pressure to yield a yellow solid, which was washed with hexane (5 mL) (0.021 g, 30%). ³¹P{¹H} NMR (121 MHz, CDCl₃) δ 104.1 (d, J_PP = 27 Hz), 104.6 (d, J_PP = 27 Hz). ¹H NMR (300 MHz, CDCl₃) δ 1.24 (6H, s, CH₃), 1.52–2.65 (14H, m, phobane), 2.97 (3H, dd, J_HP = 7.4 Hz, J_HP = 8.1 Hz, CH₃N), 7.11–7.56 (8H, m, HAr). HRMS (EI): Found (Calc. for C₂₇H₃₁CrNO₄P₂) 547.1130 (547.1133). IR (in CH₂Cl₂): ν(CO) 2003, 1908, 1885, 1869 cm⁻¹.

Oligomerisation catalysis

A rigorously cleaned autoclave was heated (130 °C) under vacuum for 60 min, then cooled to reaction temperature and back-filled with Ar (1 bar). Solvent was then added via syringe. The autoclave was pressurised with ethylene to 10 bar and vented. On a Schlenk line, a pre-activated catalyst solution was prepared by stirring the Cr source, ligand and modified methylaluminoxane (MMAO) together for 60 s, then transferred to the autoclave via syringe. The autoclave was pressurised and the pressure kept constant throughout the reaction by the continuous addition of ethylene, which was monitored via flow-meter. Once ethylene uptake had ceased or the autoclave was filled, the gas supply was closed and the reactor cooled to 5 °C. The reactor was carefully vented. The reactor contents were treated with 1000 μL of nonane (GC internal standard) and 10% HCl (aq). A sample of the organic phase was taken for GC-FID analysis. Any solid formed was collected, washed repeatedly with EtOH, then acetone and dried overnight and weighed.

GC-FID analysis was performed on an Agilent Technologies 6890N GC system equipped with PONA (50 m × 0.20 mm × 0.50 μm) and MDN-12 (60 m × 0.25 mm × 0.25 μm) columns. Catalysis was performed in a stainless steel 300 mL volume AE autoclave with Viton-ETP seals, equipped with a customised gas-entraining mechanical stirrer, internal cooling coil (tap water) and fluidised jacket (connected to an external thermostatic bath). Ethylene was passed through moisture and oxygen scrubbing columns prior to use and the flow measured using a Siemens Sitrans F C Massflo system (Mass 6000-Mass 2100) and the data logged.

Crystal structure determinations

X-ray diffraction experiments for 1b, 3, 4, 5, 6 and L_h·HCl were carried out at 100 K and for 2b at 173 K on a Bruker APEX II diffractometer using Mo-K_α radiation (λ = 0.71073 Å). 7 was collected at 120 K on a Bruker Nonius FR591 Rotating Anode using Mo-K_α radiation (λ = 0.71073 Å)²⁵ and 8 was collected on EH1 of Station I19 of Diamond Light Source (λ = 0.71073 Å) at 120 K.²⁶ Data collections were performed using a CCD area detector from a single crystal mounted on a glass fibre. Intensities were integrated using SAINT with a multi-scan absorption correction preformed using SADABS.²⁷ All structures were solved using SHELXS and refined against all F_o² using SHELXL and OLEX2.²⁸ All non-hydrogen atoms were refined anisotropically and hydrogen atoms were located geometrically and refined using a riding model. The structure of 4 was refined as a racemic twin and restraints were applied to the thermal displacement parameters to maintain sensible values. Crystal structure and refinement data are given in Table 4. The structures are shown in Fig. 1–8 with thermal ellipsoids drawn at the 50% probability level.

Table 4 Crystal data and structure refinement for all structures in the paper

Identification code	1b_LR302	2b_jj142	3_lr305	4_jj228	5_jj200
Empirical formula	C22H44Cl2N2P2Pt	C26H44CrN2O4P2	C30H60Cl2N2P2Pt	C27H49NP2Cl8Pt	C19.5H37Cl6NP2Pt
Formula weight	664.52	562.57	776.72	928.30	755.23
Temperature/K	99.99	173(2)	100.0	99.99	100.0
Crystal system	Monoclinic	Triclinic	Triclinic	Orthorhombic	Hexagonal
Space group	P21/n	P1̄	P1̄	Pna21	P65
a/Å	9.9057(7)	9.5739(2)	9.448(4)	21.9631(6)	16.7268(3)
b/Å	7.2983(5)	12.2783(2)	9.493(5)	27.7581(8)	16.7268(3)
c/Å	17.7434(13)	13.2759(2)	10.788(5)	17.7763(5)	17.0407(4)
α/°	90	69.2670(10)	109.190(10)	90	90.00
β/°	92.139(2)	89.1370(10)	107.174(9)	90	90.00
γ/°	90	71.601 (10)	93.167(15)	90	120.00
Volume/Å^3	1281.86(16)	1376.68(4)	860.5(7)	10 837.4(5)	4128.99(14)
Z	2	2	1	12	6
ρ calc/g cm^−3	1.722	1.357	1.499	1.707	1.822
μ/mm^−1	5.819	0.566	4.346	4.585	5.806
F(000)	664.0	600.0	396.0	5544.0	2226.0
Crystal size/mm^3	0.6 × 0.25 × 0.1	0.16 × 0.15 × 0.08	0.4 × 0.25 × 0.2	0.1 × 0.21 × 0.27	0.3 × 0.1 × 0.1
Radiation	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)	Mo Kα (λ = 0.71074)
2Θ range for data collection/°	4.594 to 66.432	6.328 to 61.144	4.238 to 66.332	2.72 to 55.062	3.7 to 68.3
Index ranges	−15 ≤ h ≤ 15, −11 ≤ k ≤ 10, −27 ≤ l ≤ 26	−13 ≤ h ≤ 13, −17 ≤ k ≤ 17, −18 ≤ l ≤ 18	−14 ≤ h ≤ 14, −14 ≤ k ≤ 14, −16 ≤ l ≤ 16	−28 ≤ h ≤ 28, −36 ≤ k ≤ 36, −23 ≤ l ≤ 23	−26 ≤ h ≤ 26, −26 ≤ k ≤ 26, −17 ≤ l ≤ 26
Reflections collected	46 419	56 825	36 290	300 715	137 345
Independent reflections	4710 [Rint = 0.0365, Rsigma = 0.0203]	8447 [Rint = 0.0291, Rsigma = 0.0188]	6208 [Rint = 0.0186, Rsigma = 0.0126]	24 919 [Rint = 0.0572, Rsigma = 0.0279]	9590 [Rint = 0.0716, Rsigma = 0.0407]
Data/restraints/parameters	4710/0/139	8447/0/328	6208/0/175	24 919/955/1060	9590/1/290
Goodness-of-fit on F^2	1.078	1.055	1.075	1.085	1.155
Final R indexes [I ≥ = 2σ (I)]	R 1 = 0.0304, wR2 = 0.0717	R 1 = 0.0282, wR2 = 0.0745	R 1 = 0.0167, wR2 = 0.0426	R 1 = 0.0561, wR2 = 0.1363	R 1 = 0.0278, wR2 = 0.0668
Final R indexes [all data]	R 1 = 0.0357, wR2 = 0.0745	R 1 = 0.0332, wR2 = 0.0777	R 1 = 0.0167, wR2 = 0.0426	R 1 = 0.0846, wR2 = 0.1581	R 1 = 0.0367, wR2 = 0.0909
Largest diff. peak/hole/e Å^−3	4.08/−1.91	0.45/−0.55	1.34/−1.00	4.17/−2.14	1.38/−1.76
Flack parameter	—	—	—	—	−0.021(6)

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Identification code	6_jj380	Lh·HCl _jj353	7_jj68	8_jj82
Empirical formula	C17H31Cl2NP2Pd	C23H32ClNP2	C25H27CrNO4P2	C27H31CrNO4P2
Formula weight	488.67	419.89	519.41	547.47
Temperature/K	100(2)	100(2)	120(2)	120.0
Crystal system	Monoclinic	Monoclinic	Monoclinic	Orthorhombic
Space group	P21/c	P21/c	P21/c	P212121
a/Å	11.9640(3)	16.8271(8)	11.5207(3)	10.05490(10)
b/Å	12.5989(4)	8.5130(4)	13.1975(5)	10.8831(2)
c/Å	13.8207(4)	15.6725(8)	16.7912(6)	23.6863(4)
α/°	90.00	90.00	90	90
β/°	112.2130(10)	100.569(2)	106.502(2)	90
γ/°	90.00	90.00	90	90
Volume/Å^3	1928.63(10)	2206.98(19)	2447.85(14)	2591.96(7)
Z	4	4	4	4
ρ calc/g cm^−3	1.683	1.264	1.409	1.403
μ/mm^−1	1.404	0.327	0.630	0.599
F(000)	1000.0	896.0	1080.0	1144.0
Crystal size/mm^3	0.26 × 0.11 × 0.08	0.35 × 0.28 × 0.28	0.1 × 0.06 × 0.02	0.24 × 0.2 × 0.15
Radiation	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)
2Θ range for data collection/°	3.68 to 55.08	2.46 to 55.08	5.902 to 55.066	6.376 to 55.032
Index ranges	−13 ≤ h ≤ 15, −12 ≤ k ≤ 16, −17 ≤ l ≤ 17	−21 ≤ h ≤ 21, −8 ≤ k ≤ 11, −19 ≤ l ≤ 20	−13 ≤ h ≤ 14, −16 ≤ k ≤ 17, −21 ≤ l ≤ 21	−12 ≤ h ≤ 13, −14 ≤ k ≤ 14, −30 ≤ l ≤ 30
Reflections collected	19 685	19 574	25 383	30 546
Independent reflections	4437 [Rint = 0.0231, Rsigma = 0.0191]	5066 [Rint = 0.0194, Rsigma = 0.0169]	5589 [Rint = 0.1046, Rsigma = 0.1035]	5929 [Rint = 0.0548, Rsigma = 0.0494]
Data/restraints/parameters	4437/0/220	5066/1/249	5589/0/299	5929/0/319
Goodness-of-fit on F^2	1.040	1.034	1.082	1.041
Final R indexes [I ≥ = 2σ (I)]	R 1 = 0.0178, wR2 = 0.0446	R 1 = 0.0277, wR2 = 0.0707	R 1 = 0.0979, wR2 = 0.1670	R 1 = 0.0328, wR2 = 0.0712
Final R indexes [all data]	R 1 = 0.0198, wR2 = 0.0458	R 1 = 0.0300, wR2 = 0.0724	R 1 = 0.1546, wR2 = 0.1940	R 1 = 0.0404, wR2 = 0.0742
Largest diff. peak/hole/e Å^−3	0.90/−0.30	0.43/−0.23	0.55/−0.58	0.27/−0.36
Flack parameter	—	—	—	0.020(10)

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Acknowledgements

We thank Sasol for a Ph.D. studentship (JJ). We thank the Bristol Chemical Synthesis Centre for Doctoral Training, funded by EPSRC (EP/G036764/1), and the University of Bristol, for a Ph.D. studentship (LR).

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Footnote

† CCDC 1433207–1433215. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt04394h

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