Application of molybdenum bis(imido) complexes in ethylene dimerisation catalysis

Abstract

In combination with EtAlCl₂ (Mo : Al = 1 : 15) the imido complexes [MoCl₂(NR)(NR′)(dme)] (R = R′ = 2,6-Prⁱ₂–C₆H₃ (1); R = 2,6-Prⁱ₂–C₆H₃, R′ = Bu^t (3); R = R′ = Bu^t (4); dme = 1,2-dimethoxyethane) and [Mo(NHBu^t)₂(NR)₂] (R = 2,6-Prⁱ₂–C₆H₃ (5); R = Bu^t (6)) each show moderate TON, activity, and selectivity for the catalytic dimerisation of ethylene, which is influenced by the nature of the imido substituents. In contrast, the productivity of [MoCl₂(NPh)₂(dme)] (2) is low and polymerisation is favoured over dimerisation. Catalysis initiated by complexes 1–4 in combination with MeAlCl₂ (Mo : Al = 1 : 15) exhibits a significantly lower productivity. Reaction of complex 5 with EtAlCl₂ (2 equiv.) gives rise to a mixture of products, while addition of MeAlCl₂ affords [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂]. Treatment of 6 with RAlCl₂ (2 equiv.) (R = Me, Et) yields [Mo({μ-N–Bu^t}AlCl₂)₂] (7) in both cases. Imido derivatives 1 and 3 react with Me₃Al and MeAlCl₂ to form the bimetallic complexes [MoMe₂(N{R}AlMe₂{μ-Cl})(NR′)] (R = R′ = 2,6-Prⁱ₂–C₆H₃ (8); R = 2,6-Prⁱ₂–C₆H₃, R′ = Bu^t (10)) and [MoMe₂(N{R}AlCl₂{μ-Cl})(NR′)] (R = R′ = 2,6-Prⁱ₂–C₆H₃ (9); R = 2,6-Prⁱ₂–C₆H₃, R′ = Bu^t (11)), respectively. Exposure of complex 8 to five equivalents of thf or PMe₃ affords the adducts [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂(L)] (L = thf (12); L = PMe₃ (13)), while reaction with NEt₃ (5 equiv.) yields [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂]. The molecular structures of complexes 5, 9 and 11 have been determined.

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Introduction

Selective transition metal-initiated alkene oligomerisation (in particular ethylene di- and trimerisation) has been the focus of a substantial industrial and academic research effort, providing simple and economical routes for the manufacture of higher olefinic feed stocks.^1–6 Of particular importance in this area is the manufacture of linear, terminal, short-chain materials (C₄–C₂₀) that are finding increasing uses as plasticiser co-monomers in polyethylene manufacture and as components of synthetic lubricant base oils.^7–9 A variety of different initiator systems have been developed for such applications, with those based on the Group 4, 6 and 10 metals being amongst the most successful and commercially relevant.^10–14

Recent years have seen a particular resurgence of interest in oligomerisation systems incorporating Group 6 metals especially those utilising chromium and tungsten. For example, building upon the pioneering studies of Manyik et al. and subsequently those of Briggs, chromium-based systems bearing nitrogen-containing ligands for selective alkene oligomerisation have been extensively probed.^15–19 A wide variety of mono-, bi-, and tri-dentate metal scaffolds have been employed, with pyrrolyide-supported initiators proving especially important from an industrial perspective.^6,20 The success of these latter systems is, in part, due to the variable η¹-/η⁵-coordination behaviour of the aza-heterocycle.²¹ Notably, a related phenomenon has been reported for Cr(I) bis(diphenylphosphino)propane pro-initiators, which upon activation by alkyl aluminium reagents, form diphosphine-bridged ansa-bis(arene) Cr(I) species.²² Together, these results highlight the potential importance of ligands that may adopt more than one binding mode (i.e. linkage isomerism) in the field of oligomerisation catalysis.

Attempts have also been made to utilise electronically-flexible N-donors to prepare chromium-based pro-initiators. Here efforts have focused on well-established imido ligands (NR²⁻) that can behave as 2- or 4-electron metal scaffolds.^23–25 However, in combination with a variety of aluminium co-activators these chromium imido systems (e.g. CrX₂(N–2,6-Prⁱ₂–C₆H₃)₂, X = Cl, Bu^t) yield high molecular weight polymer rather than the desired olefin oligomerisation products.²⁶

In contrast, related tungsten imido complexes such as [WCl₄(N–2,6-Prⁱ₂–C₆H₃)(thf)] and [WCl₂(N–2,6-Prⁱ₂–C₆H₃)₂(dme)] are both active for selective ethylene and propylene dimerisation when treated with EtAlCl₂.^27,28 Importantly, these observations support the proposed role of tungsten imido species in selective olefin dimerisation catalysis mediated by commercially attractive initiator systems derived in situ from sequential reaction of inexpensive WCl₆ with aniline, triethylamine, and EtAlCl₂ (W : aniline : NEt₃ : Al 1 : 2 : 4 : 15).^29–33 These multi-component systems are not only extremely selective to the dimer products for a range of α-olefins (>99% vs. higher oligomers), but also demonstrate high specificity for methyl-branched products.

Despite the importance of both chromium- and tungsten-based initiators in catalytic olefin dimerisation, no studies have been reported that probe the potential of analogous molybdenum systems. This is surprising since the utility of Mo-containing complexes in olefin metathesis chemistry has lead to the preparation of a wide variety of variously-substituted molybdenum imido, derivatives, something that could open the way for detailed structure/property correlations to be established.^34–36 Furthermore, a number of well-defined molybdenum imido complexes are commercially available, making use of such systems attractive from an industrial perspective. Consequently, herein, we report a fundamental study of the reactivity of a number of readily accessible molybdenum imido complexes with alkyl aluminium reagents and explore their potential as pro-initiators in selective, catalytic ethylene dimerisation.

Results and discussion

Catalytic ethylene dimerisation testing

The utility of a series of readily accessible molybdenum imido complexes [MoCl₂(NR)(NR′)(dme)] (R = R′ = 2,6-Prⁱ₂–C₆H₃ (1);^37,38 R = R′ = Ph (2);³⁹ R = 2,6-Prⁱ₂–C₆H₃, R′ = Bu^t (3);⁴⁰ R = R′ = Bu^t (4)⁴¹) as pro-initiators for ethylene dimerisation in combination with an aluminium activator RAlCl₂ (R = Me, Et) has been explored. Catalytic performance for each system was screened under conditions analogous to those used for the WCl₆-derived in situ systems, namely an Mo : Al ratio of 1 : 15, 40 bar ethylene pressure, and chlorobenzene as the solvent at 60 °C.^28,32

In combination with EtAlCl₂ the pro-initiators 1, 3, and 4 each achieve a moderate selectivity to but-1-ene of ∼90%, which compares favourably with the value established for the in situ WCl₆-derived system of ∼80% (Table 1, Fig. 1).^32,42 A noticeable attenuation in the product selectivity is observed for the more active well-defined pro-initiators consistent with the incorporation of the higher olefinic products generated during catalysis. The activity of the pro-initiators 1–4 varied according to the nature of the imido substituents, decreasing in the order 1 > 3 > 2 ≫ 4. However, a comparison of the TONs (turnover numbers) for each pro-initiator reveals that both complexes 1 and 3 perform well (∼60 000 (mol C₂H₄) (mol Mo)⁻¹), while the lifetimes of the bis(Ph)- and bis(Bu^t)-imido systems 2 and 4 are extremely low. Notably, the phenyl imido-substituted complex 2 also exhibits a very low selectivity to dimer products, instead affording significant quantities of polyethylene. The rapid deterioration in performance observed for pro-initiators 2 and 4 has tentatively been attributed to steric factors that open alternative, but potentially different reaction pathways for these Ph- and Bu^t-substituted imido systems. Although the steric demands of the N–Ph and N–Bu^t imido ligands are quite different, both motifs have a propensity to adopt bridging coordination modes, something that is prevented by the more sterically demanding N–2,6-Prⁱ₂–C₆H₃ moiety (vide infra).²³

Table 1 Ethylene dimerisation using pre-formed molybdenum bis(imido) complexes 1–6^a

Pro-initiator	Activator	Time (min)	TON^b	Activity^c	Mol % C4^d (of liq. prod.)	% 1-C4 in C4	% PE
a General conditions: 0.2 mmol pro-initiator and 3.0 mmol RAlCl2 (R = Me or Et); PhCl (solvent) 74 mL; 60 °C; ethylene pressure (40 bar); stirrer speed 1000 rpm; nonane standard (1.000 mL); catalytic runs were performed until consumption of C2H4 dropped below 0.2 g min^−1, at which time reaction was quenched by addition of dilute HCl. b TON is reported in (mol C2H4) (mol Mo)^−1. c Activity is reported in (mol C2H4) (mol Mo)^−1 h^−1. d In all runs C6 alkenes were produced (8.4–9.8% of liquid products) as well as trace ≥C8 fractions (0.1–2.4% of liquid products). PE = polyethylene.
1	EtAlCl2	55	58 500	63 820	88.9	63.7	0.1
1	MeAlCl2	55	24 790	27 250	97.5	78.6	0.5
2	EtAlCl2	1.5	770	34 350	94.1	97.9	69.6
2	MeAlCl2	3	2020	47 250	83.2	97.5	20.5
3	EtAlCl2	80	59 510	44 870	89.3	61.5	0.2
3	MeAlCl2	9	4420	28 500	90.9	87.0	0.2
4	EtAlCl2	74	14 030	11 360	88.9	63.7	3.0
4	MeAlCl2	2	1790	48 660	81.5	94.1	18.5
5	EtAlCl2	54	56 940	62 880	87.7	62.9	0.0
6	EtAlCl2	4	120	1900	100.0	100.0	0.0

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Fig. 1 Activity ((mol C₂H₄) (mol Mo)⁻¹ h⁻¹) and TON ((mol C₂H₄) (mol Mo)⁻¹) data for the dimerisation of ethylene initiated by complexes 1–6 in the presence of an aluminium activator (for experimental details see Table 1).

Previously, deuterium labelling studies have indicated that olefin dimerisation mediated by tungsten bis(imido) pro-initiators (in combination with EtAlCl₂ as activator) most likely occurs via a chain-growth pathway. Such reactions are believed to proceed through a tungsten hydride species, which is generated in situ following β-hydride elimination from a tungsten ethyl group, itself resulting from Al-to-W transmetallation.^28,32 Adoption of this type of reaction manifold for tungsten systems is supported by the observation that replacing EtAlCl₂ by MeAlCl₂ as activator results in a very significant drop in catalytic performance.³² Consequently, in order to probe the nature of the dimerisation path for molybdenum-based systems, the catalytic performance of complexes 1–4 was examined using MeAlCl₂ as activator. Although catalysis was initiated by each system, the productivities were significantly lower than those achieved using EtAlCl₂ as activator, although the levels of selectivity were retained (Table 1, Fig. 1). Together, these observations suggest that even when MeAlCl₂ is used tungsten hydride species are formed, albeit significantly less efficiently than with EtAlCl₂, as might be expected. Although the precise mechanistic trajectory by which MeAlCl₂ can lead to a metal hydride in such systems remains obscure, it could involve α-hydride elimination from a molybdenum or tungsten methyl species. Indeed, it is well known that for the WCl₆-based dimerisation systems use of lower loadings of aluminium activator results in a switch to an olefin metathesis pathway, although it should be noted that no metathesis products were detected in the catalysis performed here.²⁷

Reactions of [Mo(NHBu^t)₂(NR)₂] with R′_xAlCl_3−x

Previous studies have clearly exemplified the participation of imido species in catalytic olefin dimerisation using in situ WCl₆/aniline-derived systems (vide supra). However, the potential role of amido-ligated (R₂N⁻) species, which may equally as readily be generated during the formation of the active initiator, cannot be disregarded. Consequently, the catalytic performance of the mixed bis(amido) bis(imido) derivatives [Mo(NHBu^t)₂(NR)₂] (R = 2,6-Prⁱ₂–C₆H₃ (5)⁴³ and R = Bu^t (6)⁴⁴) was explored in combination with EtAlCl₂ (15 equiv.) under identical reaction conditions. Here, use of the aryl imido complex 5 gave rise to a system that initiated dimerisation with activity, productivity, and selectivity that is identical to that for the analogous dichloride complex [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)₂(dme)] (1). In contrast, the tert-butyl imido derivative 6 was barely active, despite its parent complex [MoCl₂(NBu^t)₂(dme)] (4) exhibiting moderate activity.

To further probe the significant differences observed in the catalytic performance of the 2,6-Prⁱ₂–C₆H₃- and Bu^t-substituted imido tert-butyl amido complexes 5 and 6, respectively, their reactions with EtAlCl₂ were probed in detail. Treatment of diamide 6 with two equivalents of EtAlCl₂ in C₆D₆ solution cleanly afforded the previously reported bimetallic species [Mo({μ-N–Bu^t}AlCl₂)₂] (7) and ethane, the latter resulting from deprotonation of the parent diamide, eqn (1).⁴⁴ Notably, this imido-bridged complex 7 was found to be inert to further alkylation and did not react with ethylene (10 equiv.) even in the presence of excess EtAlCl₂. In contrast, reaction of complex 5 with EtAlCl₂ (2 equiv.) under identical conditions afforded a mixture of unassignable products (according to ¹H and ¹³C NMR spectroscopy), although it was clear that no ethane evolution occurred. However, subsequent addition of ethylene (10 equiv.) to this complex mixture resulted in the formation of small quantities of butenes indicative of the presence of a dimerisation-active species.

Analogous reactions of complexes 5 and 6 with MeAlCl₂ (2 equiv.) were carried out (C₆D₆ solution) in order to probe the potential role of ethyl group β-hydride elimination, Scheme 1. Again, complex 6 was subject to deprotonation and afforded 7 and methane, while derivative 5 cleanly gave the known dimethyl complex [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂] via transmetallation; the formation of methane was not observed for the latter reaction.^45,53

Scheme 1 (i) MeAlCl₂ (2 equiv.), C₆D₆, room temperature.

In order to explore the differences in reactivity determined for complexes 5 and 6, attempts were made to determine their molecular structures. However, despite repeated attempts, no crystals of 6 suitable for X-ray diffraction studies could be obtained. In contrast, the molecular structure of complex 5 was verified by single-crystal X-ray diffraction (Fig. 2). This proved very similar to that of the previously reported complex [Mo(HN–2,6-Prⁱ₂–C₆H₃)₂(N–2,6-Prⁱ₂–C₆H₃)₂], with the Mo atom possessing a distorted tetrahedral coordination.⁴⁶

Fig. 2 Molecular structure of [Mo(NHBu^t)₂(N–2,6-Prⁱ₂–C₆H₃)₂] (5). H atoms are omitted for clarity and the thermal ellipsoids are shown at the 50% level. Selected bond distances (Å) and angles (°): Mo–N(1) 1.7609(14), Mo–N(2) 1.7543(15), Mo–N(3) 1.9537(16), Mo–N(4) 1.9495(16), N(1)–Mo–N(2) 112.32(7), N(3)–Mo–N(4) 111.88(7), N(1)–Mo–N(3) 105.53(7), N(1)–Mo–N(4) 111.25(7), Mo–N(1)–C(1) 155.32(13), Mo–N(2)–C(11) 170.28(13), Mo–N(3)–C(7) 130.80(14), Mo–N(4)–C(17) 133.68(13). Neither amido ligand forms any hydrogen bonds.

As expected, the imido Mo–N bond distances (1.7609(14) and 1.7543(15) Å) in 5 are intermediate between those of an LX₂ (4e⁻) and X₂ (2e⁻) donor and, although the two Mo=N–C^Ar bond angles differ significantly (155.3(1) and 170.3(1)°), they lie within the accepted range for quasi-linear imido motifs (150–180°).⁴⁰ The Mo–N amido bond distances are comparable with other molybdenum amido complexes (1.92–2.15 Å).^46,47 Both amide nitrogen atoms N(3) and N(4) are planar, within experimental error, and are orientated such that the N(3)–Mo–N(4) plane and the H–N–C planes for N(3) and N(4) are at 62.7 and 70.6°, respectively. This arrangement maximises the extent of N(pπ)→Mo(dπ) donation for each amido ligand about the pseudo-tetrahedral Mo atom, something facilitated by the pseudo-trans disposition of the two Bu^t substituents, which reduces steric constraints.^48–50

These solid-state data emphasize the electron-withdrawing nature of the aryl imido ligands, evidenced by appreciable N_amido→Mo lone pair donation. This is consistent with the ¹H NMR spectrum of complex 5 in which the amido NH proton appears to considerably higher frequency to that for complex 6 bearing electron-releasing tert-butyl imido ligands, δ 6.29 (5)⁴⁴versus 5.71 ppm (6). Further corroboration of this electronic disparity is given by a comparison of the chemical shift difference between the α and β carbon resonances (Δδ) of the tert-butyl amido substituents of 5 (Δδ = 22.1 ppm) and 6 (Δδ = 19.6 ppm). By analogy with the use of this parameter to probe the electron density at the imido nitrogen atom of d⁰tert-butyl imido complexes, the greater magnitude of Δδ for complex 5 is consistent with lower electron density at its amido nitrogen atoms, again resulting from greater N_amido→Mo donation. Together, these data are all indicative of a superior acidity of the amido NH group of 5versus that of 6.⁵¹ However, this clear difference in electronic nature of the imido substituents and consequently, in the relative acidities of the NH moieties of complexes 5 and 6, does not correlate with their observed reactivity with RAlCl₂ (R = Et, Me) since only the less acidic amido group of 6 is subject to deprotonation. Consequently, it is likely that the greater steric demands of the 2,6-diisopropylphenyl substituents of 5 prevent the formation of imido-bridged dimeric species such as 7, which result from amido deprotonation. Thus, the poor catalytic performance of pro-initiator 6 in combination with the necessary alkyl aluminium activator is attributed to the formation of complex 7in situ, which prevents access to the catalytically productive manifold and highlights the probable role of similarly-bridged imido species in catalyst deactivation pathways.

Reactions of [MoCl₂(NR)(NR′)(dme)] with Me_xAlCl_3−x

Understanding the subtle interplay between aluminium activator and transition metal pro-initiator remains key to the elucidation of the mechanism of catalytic olefin dimerisation. For example, it has been proposed from computational studies that coordination of R′AlCl₂ to tungsten bis(imido) fragments enhances ethylene dimerization via an oxidative coupling pathway due to a lowering of the energy barrier for reductive elimination.⁵² However, in a separate study, it has been revealed that although coordination of various Me_xAlCl_3−x species to tungsten bis(imido) moieties does indeed take place, the resulting bimetallic systems are inert towards both ethylene and propylene.²⁸ Hence, given the differences observed in catalytic performance between the different molybdenum imido complexes 1–4, the reactivity of each system with variously-substituted alkyl aluminium reagents has been studied.

Treatment of [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)₂(dme)] (1) with excess Me₃Al (6 equiv.) in hexane solution resulted in full conversion (by NMR spectroscopy) to [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8), Scheme 2. Here, the Me₂AlCl generated in situ by Al/Mo transmetallation has been sequestered by one of the Lewis basic imido nitrogens, presumably via an intermolecular pathway. In a similar fashion treatment of 1 with MeAlCl₂ (3 equiv.) in toluene cleanly afforded [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlCl₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (9), although further addition of MeAlCl₂ yielded other (unidentified) reaction products.‡ Complex 8 can also be formed directly via reaction of [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂] with Me₂AlCl, although the reaction also affords a significant number of other, unidentified side-products, eqn (2).

Scheme 2 (i) Me₃Al (6 equiv.), hexane, room temperature (r.t.); (ii) MeAlCl₂ (3.25 equiv.), toluene, r.t.

The ¹H NMR spectrum of 8 is consistent with the presence of a symmetrical MoMe₂ motif, which gives rise to a single resonance at δ 1.64 ppm, with the AlMe₂ moiety presenting a signal at δ − 0.09 ppm. Notably, the protons of the Mo–Me groups of 8 resonate to higher frequency of those reported previously for [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂] (δ 1.39 ppm), consistent with the electron-withdrawing effect of the coordinated Me₂AlCl.⁵³ Analogously, the ¹³C NMR spectrum for 8 shows two metal-bonded carbons at δ 41.5 and −7.0 ppm (Mo–Me and Al–Me, respectively), as expected. The ¹H NMR spectrum for complex 9 revealed a resonance at δ 1.67 ppm for the Mo–Me motif at a shift comparable to that determined for 8.

With the aim of better understanding the crucial alkyl aluminium chloride–metal imido interaction, single crystals of complex 9 were grown from CH₂Cl₂ solution and characterised by X-ray diffraction (Fig. 3, Table 2). The asymmetric unit of 9·½CH₂Cl₂ comprises four host molecules (A to D) of similar geometry, as well as two dichloromethane molecules (one of them chaotically disordered). The molecular structure of 9 reveals one terminal imido ligand, N(1)–2,6-Prⁱ₂–C₆H₃ (mean Mo–N distance 1.715(6) Å) and an asymmetrically-bridged imido unit, together an arrangement that closely resembles that of the related tungsten complex [WMe₂(N{2,6-Prⁱ₂–C₆H₃}AlCl₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)].²⁸ The Mo atom of 9 adopts a trigonal-bipyramidal coordination distorted in an ‘umbrella’ fashion,⁵⁴ whereby the equatorial ligands (bridging N(2)–2,6-Prⁱ₂–C₆H₃ and two methyl groups) are pushed away from the terminal Mo–N(1) bond and towards the bridging chloride ligand. The latter is rather weakly bonded, the Mo–Cl distance being longer even than previously reported Ar–Cl→Mo dative bonds (2.581(5) or 2.624(1) Å).⁵⁵ The Al atom of 9 adopts a near-tetrahedral geometry, with N(2) being trigonal planar (mean Σ_ang N(2) ∼ 360°). Arene ring (i) is roughly coplanar with the MoN(2)AlCl metallacycle (iii), while ring (ii) adopts a perpendicular orientation.

Table 2 Selected bond distances (Å) and angles (°) for complexes 9 and 11

	9, A	9, B	9, C	9, D	9, mean	11
Mo–Cl(1)	2.695(2)	2.708(2)	2.712(2)	2.695(2)	2.702(9)	2.7474(5)
Mo–N(1)	1.711(5)	1.718(5)	1.712(5)	1.723(5)	1.716(6)	1.7024(14)
Mo–N(2)	1.891(6)	1.851(6)	1.867(6)	1.865(6)	1.869(16)	1.8874(13)
Mo–C(1)	2.136(7)	2.123(7)	2.126(7)	2.127(7)	}2.124(11)	2.1191(17)
Mo–C(2)	2.141(6)	2.118(7)	2.118(7)	2.106(7)		2.1217(17)
Mo⋯Al	3.054(2)	3.056(2)	3.058(2)	3.045(2)	3.053(6)	3.1024(6)
Al–Cl(1)	2.205(3)	2.195(3)	2.194(3)	2.198(3)	2.198(5)	2.1887(7)
Al–N(2)	1.878(5)	1.885(6)	1.893(6)	1.880(6)	1.884(7)	1.8718(14)
Al–Cl(2)	2.087(3)	2.091(3)	2.096(4)	2.107(3)	} 2.099(9)	2.1094(7)
Al–Cl(3)	2.109(3)	2.110(3)	2.089(3)	2.099(3)		2.1118(8)
Mo–N(1)–C	162.6(5)	163.1(5)	164.2(5)	164.3(5)	163.6(8)	174.6(1)
Mo–Cl(1)–Al	76.39(8)	76.33(8)	76.31(8)	76.30(8)	76.33(4)	76.96(2)
Mo–N(2)–Al	108.2(3)	109.8(3)	108.9(3)	108.8(3)	108.9(7)	111.23(6)
Mo–N(2)–C(11)	129.4(4)	130.6(4)	131.7(4)	131.8(4)	131(1)	127.4(1)
Al–N(2)–C(11)	122.3(5)	119.6(4)	119.4(5)	119.3(4)	120(1)	121.4(1)
Interplanar angle
i/iii	10.4(3)	5.3(3)	4.1(3)	6.8(3)	6.7	—
ii/iii	86.6(1)	88.7(1)	88.2(2)	88.3(2)	87.9	89.0(1)

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Fig. 3 Molecular structures of [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlCl₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)]·½CH₂Cl₂ (9)—independent molecule A only, and [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlCl₂{μ-Cl})(NBu^t)] (11). H atoms are omitted for clarity and the thermal ellipsoids are shown at the 50% level.

The test data reported in Table 1 and Fig. 1 reveal a marked similarity in the catalytic performance of the bis(aryl) imido pro-initiator 1 and the mixed aryl/tert-butyl bis(imido) complex 3, while those for the bis(Bu^t) imido complex 4 were significantly poorer. Thus, it was of interest to examine in detail the interplay between the mixed imido complex [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)(NBu^t)(dme)] (3) and Me_xAlCl_(3−x) reagents. As was the case for 1, reaction of 3 with Me₃Al (6 equiv.) or MeAlCl₂ (3 equiv.) in toluene smoothly afforded the Lewis acid-containing complexes [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlX₂{μ-Cl})(NBu^t)] where X = Me (10) and X = Cl (11), respectively, Scheme 2.‡ Consistent with the proposed structure, complex 10 exhibits both Mo–Me and Al–Me resonances in its ¹H (δ 1.39 and −0.21 ppm, respectively) and ¹³C (δ 39.3 and −6.7 ppm, respectively) NMR spectra. The regiochemistry of complex 10 was confirmed via a ¹H/¹H NOESY experiment, which revealed a unique correlation between the Me groups of the 2,6-Prⁱ₂-C₆H₃ moiety and those bound to aluminium, consistent with a static structure in which there is coordination of Me₂AlCl to the aryl imido nitrogen only. Comparable NMR spectroscopic data were recorded for the Mo-methyl groups of complex 11 (¹H δ 1.38; ¹³C δ 45.1 ppm). The weaker Lewis acidity of the bound AlMe₂ residue compared to that of the AlCl₂ unit is reflected in the values of Δδ associated with the tert-butyl imido ligands of complexes 10 (45.6 ppm) and 11 (48.2 ppm), respectively, which indicate greater N_imido→Mo lone pair donation for the latter.⁵¹

Recrystallisation of complex 11 from toluene afforded light yellow crystals suitable for an X-ray diffraction study. The structure contains one independent molecule (Fig. 3, Table 2) geometrically similar to 9, except that the Mo–N–C bond angle of the terminal imido ligand in 11 is greater, presumably as a result of steric constraints.⁵⁶ The trigonal bipyramidal coordination of the Mo centre exhibits a comparable ‘umbrella-type’ distortion to that observed for 9, with the Mo atom being displaced from the N(2)C(1)C(2) plane towards N(1) by 0.31 Å (cf. 0.35 Å in 9). Specific coordination of the AlCl₃ moiety only to the aryl imido nitrogen N(2) rather than the tert-butyl imido nitrogen is apparent, which is in agreement with the inferences made for the analogous Me₂AlCl derivative 10 by NMR spectroscopy. As determined for complex 9, the aluminium-bound nitrogen N(2) in 11 is trigonal planar (Σ_ang N(2) 359.95°).

Complexes 9 and 11 result from regioselective coordination of the Me_xAlCl_3−x (x = 0 or 2), generated in situ following transmetallation, to the aryl imido ligand of the [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)(NBu^t)] fragment. The regioselectivity obtained here is consistent with observations that can be made from the molecular structure determination of the parent complex [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)(NBu^t)(dme)] (3). Here, the Mo–N bond distance associated with the NBu^t ligand is significantly shorter than that for the aryl imido ligand (1.728(2) vs. 1.753(2) Å), which reflects the greater π-donation of the NBu^t lone pair to the molybdenum centre.⁵⁷ Consequently, this suggests that in complexes 9 and 11 the NBu^t imido lone pair is likely to be less available for donation to the Lewis acidic Me_xAlCl_3−x fragments, giving rise to the observed regiochemistry in the bimetallic complex.

Reactions of [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] with Lewis bases

It has been established previously that tungsten imido-bound Al(III) species may be readily displaced through reaction with an appropriate Lewis base to afford the corresponding acid-free imido complexes, something that is of particular importance for the application of these molybdenum imido systems in olefin dimerisation.²⁸ Consequently, the reactivity of complex 8 with a variety of strong Lewis bases was probed in d₆-benzene and d₅-chlorobenzene solution (Scheme 3). Treatment of 8 with excess thf or PMe₃ (5 equiv.) cleanly afforded the known complexes [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)(L)] (L = thf (12); L = PMe₃ (13) together with the corresponding base adducts [Me₂AlCl(L)], while reaction of 8 with NEt₃ (5 equiv.) afforded the previously reported base-free complex [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂].^28,53 In contrast, no reaction occurred either between 8 or even its base-free analogue, [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂], and ethylene (5 equiv.) in C₆D₅Cl or C₆D₆ even at elevated temperature (60 °C), something that has been ascribed to the poor nucleophilicity of the olefin.

Scheme 3 (i) thf (5 equiv.) or PMe₃ (5 equiv.), C₆D₆, room temperature (r.t.); (ii) NEt₃ (5 equiv.), C₆D₆, r.t.

Analogous reactions with Lewis bases were attempted for the mixed bis(imido) complex 10. As was the case with 8 (d₆-benzene solution) no reaction was found to occur with excess ethylene, while reactions with thf, PMe₃ and NEt₃ (5 equiv.) afforded unattributable mixtures of products. This latter observation has been ascribed to the occurrence of imido ligand exchange, something that was reported during attempts to prepare the base-free complex [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)(NBu^t)].⁵³

Conclusions

We have shown in this study that preformed molybdenum bis(imido) complexes bearing at least one bulky 2,6-Prⁱ₂–C₆H₃ substituent, in combination with alkyl aluminium dichlorides RAlCl₂ (R = Me, Et), exhibit moderate activity and productivity for ethylene dimerisation. Here, it is believed that the sterically-demanding imido ligand is required to inhibit imido bridge formation, something that can not only prevent access to a catalytically productive manifold during pro-initiator activation, but is also a major contributing factor to initiator degradation.

Additionally, it is noted that the catalytic performance of such bis(imido) molybdenum systems is intimately linked to the nature of the aluminium activator, with the highest productivities and selectivities being achieved with EtAlCl₂. It is proposed that the significantly better catalytic performance observed when using EtAlCl₂ (compared with MeAlCl₂) can be attributed to ready access to a molybdenum hydride-based catalytic manifold as a result of β-hydride elimination in the molybdenum coordination sphere following Et–Al to Et–Mo transmetallation.

In contrast, the low levels of catalytic dimerisation activity determined for the MoCl₂(NR)₂/MeAlCl₂ systems is proposed to result from the formation of low concentrations of Mo–H species via a much less accessible α-hydride abstraction pathway. These proposed mechanistic differences are supported by the observation that although Me-group transmetallation from MeAlCl₂ to d⁰ molybdenum bis(imido) complexes is facile, the resulting new aluminium(III) species are sequestered by one of the Lewis basic imido nitrogen atoms to afford bimetallic complexes [MoMe₂(N{R}AlMe₂{μ-Cl})(NR′)] (8–11), which do not initiate ethylene dimerisation at low ethylene concentrations. Moreover, no reaction is observed between ethylene and the base-free derivative [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂] consistent with both the low nucleophilicity of the olefin and its low intrinsic kinetic facility for insertion. Together these observations are in agreement with studies of related aluminium-tungsten systems, which have also been demonstrated to be inert towards olefins, despite computational studies having highlighted such systems as potentially playing a crucial role in WCl₆/aniline-mediated olefin dimerisation.^28,52

Further studies exploring the behaviour of Group 6 imido complexes in alkene dimerisation are on-going.

Experimental

All operations were conducted under an atmosphere of dry nitrogen using standard Schlenk and cannula techniques, or in a nitrogen-filled Saffron Scientific glove box. NMR-scale reactions were conducted using NMR tubes fitted with Young's tap valves. Bulk solvents were purified using an Innovative Technologies SPS facility and degassed prior to use and NMR solvents (C₆D₆) were dried over P₂O₅, distilled and degassed before use. When appropriate, liquid reagents were dried, distilled and deoxygenated. Nitrogen gas was passed through a drying column (silica–CaCO₃–P₂O₅). The known complexes [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)₂(dme)] (1),^37,38 [MoCl₂(NPh)₂(dme)] (2),³⁹ [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)(NBu^t)(dme)] (3),⁴⁰ [MoCl₂(NBu^t)₂(dme)] (4),⁴¹ [Mo(NHBu^t)₂(NBu^t)₂] (6)⁴⁴ were prepared using standard literature procedures, while neat Me₃Al was purchased from Albermarle R & D centre and EtAlCl₂ from Aldrich. Solid MeAlCl₂ was obtained by slow evaporation of hexane from a hexane solution obtained from Aldrich. LiNHBu^t was prepared through addition of equimolar quantities of BuⁿLi to a cold (−78 °C) Et₂O solution of HNBu^t. Routine NMR spectra were collected on a Varian Unity 300 or 200, a Varian Mercury 400, Avance 400 MHz Bruker, Varian Inova 500, or a Varian 700 MHz spectrometers. Chemical shifts were referenced to residual protio impurities in the deuterated solvent (¹H) or the ¹³C shift of the solvent. Solvent proton shifts (ppm): C₆D₆, 7.15. Solvent carbon shifts (ppm): C₆D₆, 128.3. Elemental analyses were performed by the Analytical Services Department of the Chemistry Department, Durham University.

Catalysis was performed in 250 mL volume Buchi Miniclaves equipped with stainless steel vessels with integral thermal-fluid jackets, internal cooling coils, and mechanical mixing via gas-entraining stirrers. Ethylene (Grade 4.5) was supplied by Linde and passed through oxygen and moisture scrubbing columns prior to use; ethylene flow was measured using a Siemens Sitrans F C Massflo system (Mass 6000–Mass 2100) and the data logged. All catalytic tests were allowed to run until ethylene uptake had dropped below the 0.2 g min⁻¹ lower detection threshold. Gas phase sample GC-FID analysis was performed using an Agilent Technologies 6890 N GC System equipped with a 250 μL gas sample loop and GS GasPro column (30 m × 0.32 mm) using hydrogen as carrier gas. Liquid phase sample, GC-FID analysis was performed using an Agilent Technologies 6850 N GC System equipped with a PONA column (50 m × 0.20 mm × 0.50 μm) using hydrogen as carrier gas.

General procedure for evaluating ethylene dimerisation activity of pro-initiators 1–6

The autoclave was heated under vacuum at 90 °C for 1 h, then cooled and back-filled with Ar, before charging with PhCl (70 mL) and the required pro-initiator (20 μmol, as a 5 mM stock solution in PhCl). The vessel was then heated (60 °C), pressurized with ethylene (5 bar), and stirred (1000 rpm). Next, the autoclave was vented to enable addition of the desired activator, EtAlCl₂ or MeAlCl₂ (300 μmol). After activator addition, the vessel was pressurized with ethylene (40 bar). A constant pressure of 40 bar was maintained throughout the reaction period, whilst ethylene consumption was monitored via a flow-meter. After gas uptake was observed to cease, the autoclave was isolated from the ethylene supply and cooled to 0 °C. The reactor was carefully vented, with a portion of the vent gas being directly fed to a GC-FID instrument equipped with gas-sampling loop for analysis. Next, the reactor contents were treated sequentially with 1000 μL of nonane (GC internal standard) and a 10% aqueous HCl solution. A sample of the organic phase was taken for GC-FID analysis. Any solid formed was collected, washed repeatedly with 10% HCl and EtOH, dried overnight and weighed.

[Mo(NHBu^t)₂(N–2,6-Prⁱ₂–C₆H₃)₂] (5). To a cold (−78 °C) Et₂O (30 mL) solution of [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)₂(dme)] (1) (2.0 g, 3.28 mmol) was added a cold (−78 °C) Et₂O (100 mL) solution of LiNHBu^t (0.52 g, 6.57 mmol). The mixture was allowed to warm to r.t. and allowed to stir for a further 4 h. All volatile components were removed in vacuo and the residue extracted with hexane (3 × 30 mL) via filtration. Concentration of the resulting hexane solution under reduced pressure and subsequent recrystallisation at −35 °C afforded 5 as orange single crystals (0.36 g, 20%). Further concentration of the hexane solution resulted in precipitation of further quantities of 5 as an orange powder, which was collected by filtration; yield 0.45 g (45%). ¹H NMR (CDCl₃, 200 MHz): δ 6.96 (6H, br, H_meta and H_para), 6.29 (2H, br, NH), 3.57 (4H, septet, ³J_HH = 6.8 Hz, CH₃CH), 1.29 (18H, s, NCCH₃), 1.11 (24H, d, ³J_HH = 6.8 Hz, CH₃CH). ¹³C{¹H} NMR (CDCl₃, 125.6 MHz): δ 153.5 (C_ipso), 140.5 (C_ortho), 123.3 (C_meta), 122.1 (C_para), 55.2 (NCCH₃), 33.1 (NCCH₃), 28.3 (CH₃CH), 23.1 (CH₃CH). Anal. calcd for C₃₂H₅₄MoN₄: C, 65.06; H, 9.21; N, 9.48. Found C, 65.00; H, 9.34; N, 9.58.

[Mo({μ-NBu^t}₂AlCl₂)₂] (7)⁴⁴. [Mo(NHBu^t)₂(NBu^t)₂] (6) (50 mg, 0.13 mmol) was dissolved in C₆D₆ (0.8 mL), EtAlCl₂ (34 mg, 0.26 mmol) added and the reaction monitored by ¹H NMR spectroscopy. Within 10 min quantitative formation of previously reported complex 7 and ethane had occurred. ¹H NMR (C₆D₆, 500 MHz): δ 1.38 (s, NBu^t), 0.71 (s, C₂H₆).

Reaction of [Mo(NHBu^t)₂(N–2,6-Prⁱ₂–C₆H₃)₂] with EtAlCl₂ and C₂H₄. [Mo(NHBu^t)₂(N–2,6-Prⁱ₂–C₆H₃)₂] (5) (50 mg, 0.08 mmol) was dissolved in C₆D₆ (0.8 mL), EtAlCl₂ (22 mg, 0.17 mmol) added and the reaction monitored by ¹H NMR spectroscopy, which indicated the formation of a number of unattributable products. Subsequently, the NMR tube and its contents were cooled (−196 °C), evacuated, and ethylene (10 equiv., 0.80 mmol) added, which lead to the formation of butenes according to ¹H NMR spectroscopy.

Reaction of [Mo(NHBu^t)₂(NBu^t)₂] with MeAlCl₂. [Mo(NHBu^t)₂(NBu^t)₂] (6) (50 mg, 0.13 mmol) was dissolved in C₆D₆ (0.8 mL), MeAlCl₂ (29 mg, 0.26 mmol) added and the reaction monitored by ¹H NMR spectroscopy, which indicated the quantitative formation of 7.

Reaction of [Mo(NHBu^t)₂(N–2,6-Prⁱ₂–C₆H₃)₂] with MeAlCl₂. [Mo(NHBu^t)₂(N–2,6-Prⁱ₂–C₆H₃)₂] (5) (50 mg, 0.08 mmol) was dissolved in C₆D₆ (0.8 mL), MeAlCl₂ (19 mg, 0.17 mmol) added and the reaction monitored by ¹H NMR spectroscopy, which indicated the quantitative formation of [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂].⁵³

[MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8). [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)₂(dme)] (1) (1.0 g, 1.64 mmol) was dissolved in hexane (30 mL) and a solution of Me₃Al (0.80 g, 11.11 mmol) in hexane (30 mL) was added. This resulted in the immediate formation of a dark brown solution. The reaction was allowed to proceed for 16 h, after which time the solution was filtered to remove insoluble Me₃Al·dme adducts. The resulting solution was then cooled (−35 °C), which led to the formation of a small quantity of brown precipitate (Me₃Al–dme by-product), which was again removed via filtration. Further concentration of the hexane solution resulted in precipitation of 8 as a yellow solid, which was collected by filtration and recrystallised from hexane; yield 230 mg (24%). ¹H NMR (C₆D₆, 200 MHz): δ 6.93 (6H, s, H_meta and H_para), 3.53 (2H, septet ³J_HH = 6.8 Hz, CH₃CH), 2.59 (2H, septet ³J_HH = 6.6 Hz, CH₃CH), 1.64 (6H, s, MoCH₃), 1.17 (6H, d, ³J_HH = 6.8 Hz, CH₃CH), 1.02 (6H, d, ³J_HH = 6.8 Hz, CH₃CH), 0.87 (12 H, d, ³J_HH = 6.6 Hz, CH₃CH), −0.09 (6H, s, AlCH₃). ¹³C{¹H} NMR (C₆D₆, 125.6 MHz): δ 154.0 (C_ipso), 150.0 (C_ortho), 140.6 (C_meta), 125.0 (C_para), 41.5 (Mo(CH₃)₂), 30.1 (CH₃CH), 28.7 (CH₃CH), 26.2 (CH₃CH), 24.2 (CH₃CH), −7.0 (AlCH₃). Anal. calcd for C₂₈H₄₆AlClMoN₂: C, 59.10; H, 8.15; N, 4.92. Found C, 59.28; H, 8.14; N, 5.10.

[MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlCl₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (9). i) [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)₂(dme)] (1) (400 mg, 0.65 mmol) was dissolved in a minimal amount of toluene (20 mL), before addition of a solution of MeAlCl₂ (1.9 mL, 1.0 M in hexanes, 1.90 mmol). The reaction mixture was allowed to stir (16 h) before being filtered and the volatile components removed in vacuo. Recrystallisation from CH₂Cl₂ gave brown micro crystals of complex 9 of sufficient quality for single crystal X-ray diffraction analysis. However, ¹H NMR spectroscopy of the bulk reaction material consistently indicated the formation of multiple reaction products. Furthermore, all attempts to purify the bulk of the material by recrystallization repeatedly failed. Repetition of this procedure using a larger amount of 1 (3.0 g, 4.93 mmol) also failed to produce material of sufficient purity. ii) Addition of solid MeAlCl₂ (30 mg, 0.26 mmol) to a toluene solution (0.8 mL) of 1 (50 mg, 0.08 mmol) resulted in an immediate colour change to give a dark brown solution; which was then charged to a recrystallization tube. Over a 24 h period large red crystals of 9 evolved and were collected via filtration and then washed with hexane (3 × 2 mL). Dissolution of a sample of these crystals in C₆D₆ allowed 9 to be characterized using ¹H NMR spectroscopy; ¹H NMR (C₆D₆, 200 MHz): δ 6.90 and 6.79 (6H, aromatic), 3.70 (2H, septet, ³J_HH = 6.8 Hz, CH₃CH), 2.37 (2H, septet, ³J_HH = 6.8 Hz, CH₃CH), 1.67 (6H, s, MoCH₃), 1.33 (6H, d, ³J_HH = 6.8 Hz, CH₃CH), 0.98 (6H, d, ³J_HH = 7 Hz, CH₃CH), 0.82 (12H, d, ³J_HH = 6.6 Hz, CH₃CH). Complex 9 degrades slowly both in the solid state and solution, which precluded acquisition of meaningful ¹³C NMR spectroscopic and analytical data.

[MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(NBu^t)] (10). [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)(NBu^t)(dme)] (3) (75 mg, 0.14 mmol) was dissolved in C₆D₆ (0.8 mL) and to this solution was added Me₃Al (64 mg, 0.88 mmol). This resulted in the quantitative (by NMR spectroscopy) formation of complex 10. ¹H (C₆D₆, 500 MHz): δ 6.92 (3H, br s, aromatic), 3.45 (2H, septet, ³J_HH = 6.9 Hz, CH₃CH), 1.39 (6H, s, MoCH₃), 1.19 (6H, d, ³J_HH = 6.9 Hz, CH₃CH), 1.18 (6H, d, ³J_HH = 6.9 Hz, CH₃CH), 0.84 (6H, s, CCH₃), −0.21 (6H, s, AlCH₃). ¹³C{¹H} NMR (C₆D₆, 125.6 MHz): δ 156.0 (C_ipso), 140.4 (C_ortho), 127.2 (C_meta), 124.5 (C_para), 74.2 (NCCH₃), 39.3 (MoCH₃), 28.6 (NCCH₃), 28.2 (CH₃CH), 25.8 (CH₃CH), 25.0 (CH₃CH), and −6.7 (AlCH₃). In a ¹H–¹H NOESY spectrum of the reaction solution revealed a correlation between the CH₃CH (ⁱPr) resonances and Al(CH₃)₂Cl resonances.

[MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlCl₂{μ-Cl})(NBu^t)] (11). [MoCl₂(N–2,6-Prⁱ₂–C₆H₃)(NBu^t)(dme)] (3) (800 mg, 1.58 mmol) was dissolved in toluene (30 mL) and a hexane solution of MeAlCl₂ (4.8 mL, 1 M in hexanes, 4.76 mmol) was added. The reaction was allowed to stir for 16 h, after which time the solution was filtered. Concentration of the toluene solution followed by cooling (−5 °C) gave an initial crop of yellow micro crystals of complex 11 that were of sufficient quality for X-ray diffraction. After collection of the micro crystals the toluene washings were concentrated in vacuo resulting in further precipitation of complex 11, which was collected via filtration and washed with hexane (3 × 5 mL). This material was then recrystallized from CH₂Cl₂ to give a yellow powder, yield 330 mg (40%): ¹H NMR (C₆D₆, 500 MHz): δ 6.90 (3H, s, H_meta and H_para), 3.63 (2H, septet, ³J_HH = 7.0 Hz, CH₃CH), 1.38 (6H, s, MoCH₃), 1.32 (6H, d, ³J_HH = 7.0 Hz, CH₃CH), 1.11 (6H, d, ³J_HH = 7.0 Hz, CH₃CH), 0.68 (9H, s, NCCH₃).¹³C{¹H} NMR (C₆D₆, 125.6 MHz): δ 154.5 (C_ipso), 140.9 (C_ortho), 128.1 (C_meta), 124.9 (C_para), 76.3 (NCCH₃), 45.1 (MoCH₃), 28.6 (CH₃CH), 28.1 (NCCH₃), 25.7 (CH₃CH), 25.3 (CH₃CH). Anal. calcd for C₁₈H₃₃AlCl₃MoN₂: C, 42.66; H, 6.56; N, 5.53. Found: C, 42.87; H, 6.47; N, 5.56.

[MoMe₂(thf)(N–2,6-Prⁱ₂–C₆H₃)₂] (12). [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8) (50 mg, 0.07 mmol) was dissolved in C₆D₆ (0.8 mL) and thf (28 μL, 0.35 mmol) added. Subsequently, the reaction was monitored by ¹H NMR spectroscopy, which indicated the quantitative formation of [MoMe₂(thf)(N–2,6-Prⁱ₂–C₆H₃)₂] (12) together with a broad resonance from the aluminium-containing fragment.⁵³

[MoMe₂(PMe₃)(N–2,6-Prⁱ₂–C₆H₃)₂] (13). [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8) (50 mg, 0.07 mmol) was dissolved in C₆D₆ (0.8 mL) and PMe₃ (0.1 mL, 1.10 mmol) added via vacuum transfer at −196 °C. Following warming to r.t., the reaction was monitored by ¹H and ³¹P NMR spectroscopy, which indicated the quantitative formation of [MoMe₂(PMe₃)(N–2,6-Prⁱ₂–C₆H₃)₂] (13) together with broad resonances from the aluminium-containing residues.⁵³

Reaction of [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8) with NEt₃. [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8) (50 mg, 0.07 mmol) was dissolved in C₆D₆ (0.8 mL) and NEt₃ (50 μL, 0.35 mmol) added. Following warming to r.t., the reaction was monitored by ¹H NMR spectroscopy, which indicated the quantitative formation of [MoMe₂(N–2,6-Prⁱ₂–C₆H₃)₂] together with a broad resonance from the aluminium-containing fragment.⁵³

Reaction of [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8) with C₂H₄. [MoMe₂(N{2,6-Prⁱ₂–C₆H₃}AlMe₂{μ-Cl})(N–2,6-Prⁱ₂–C₆H₃)] (8) (50 mg, 0.08 mmol) was dissolved in C₆D₆ (0.8 mL) and the solution cooled (−196 °C), evacuated, and ethylene (10 equiv., 0.80 mmol) added, which lead to the formation of butenes according to ¹H NMR spectroscopy.

X-Ray crystallography

X-Ray diffraction experiments (Table 3) were carried out on Bruker 3-circle diffractometers with CCD area detectors ProteumM APEX (for 5) or SMART 1000 (for 9 and 11), using graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å) from a 60 W Mo-target microfocus Bede Microsource® X-ray generator with glass polycapillary X-ray optics (5) or a sealed tube (9, 11). The crystals were cooled using Cryostream 600 (Oxford Cryosystems) open-flow N₂ cryostats. Full sphere of the reciprocal space was covered with narrow-frame (0.3°) ω scans. Diffraction from 9 was very weak, mean I/σ(I) = 4.2. Reflection intensities were corrected for absorption by semi-empirical method based on Laue equivalents.⁵⁸ The structures were solved by direct methods and refined by full-matrix least squares against F² of all reflections, using SHELXTL 6.14,⁵⁹ PLATON⁶⁰ and OLEX2⁶¹ software. In the crystal of 9, two independent CH₂Cl₂ molecules of crystallisation occupied voids of 133 and 185 ų, respectively. The latter molecule showed chaotic disorder, which could not be modelled satisfactorily, therefore the refinement was completed with the observed structure factors corrected using BYPASS procedure.⁶² Full crystallographic data, excluding structure factors, have been deposited at the Cambridge Crystallographic Data Centre, CCDC-852768 (5), CCDC-852769 (9), and 852770 (11).

Table 3 Crystal data and experimental details

	5	9	11
a reflections with I≥2σ(I); b R 1 = Σ‖Fo| − |Fc‖/Σ|Fo|; c wR 2 = {Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]}^1/2.
Chemical formula	C32H54MoN4	C26H40AlCl3MoN2·½CH2Cl2	C18H32AlCl3MoN2
Fw	590.73	652.33	505.73
T/K	120	120	125
Crystal system	Orthorhombic	Monoclinic	Monoclinic
Space group (no.)	Pbca (# 61)	P21/n (# 14, non-standard)	P21/c (# 14)
a/Å	10.463(1)	16.424(2)	9.046(1)
b/Å	19.225(2)	19.713(2)	16.394(2)
c/Å	33.429(3)	39.485(4)	16.605(2)
β (°)	90.00	97.22(1)	101.76(1)
V/Å^3	6724(1)	12683(2)	2587.3(4)
Z, ρc/g cm^−3	8, 1.167	16, 1.367	4, 1.393
μ/mm^−1	0.41	0.80	0.92
No. of reflns, total	71726	68331	28594
No. of reflns, unique	8928, 6757^a	22278, 11385^a	6418, 5474^a
R int	0.047	0.119	0.035
R 1 ^a ^, ^b	0.033	0.066	0.025
wR2^c	0.079	0.135	0.066

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Acknowledgements

Durham University and Sasol Technology UK Ltd. (studentships WRHW and AMM) are warmly acknowledged for financial support of this work. We thank Dr A. M. Kenwright, Mrs. C. Heffernan, and Mr I. McKeag for assistance with NMR studies.

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Footnotes

† CCDC reference numbers 852768–852770. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt12061e

‡ For the preparation of complexes 7–10 excess alkyl aluminium reagent is required since the corresponding dme adduct of Me_xAlCl_3−x (x = 1 or 3) is obtained in each case; such species have been reported previously.⁶³

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