(Imido)vanadium(V)-alkyl, -alkylidene complexes exhibiting unique reactivity towards olefins and alcohols

Abstract

This minireview introduces recent results in the synthesis of a series of (imido)vanadium(V)-alkyl complexes, and some reactions with alcohols (phenols) that are proposed to proceed via intermediates involving coordination of the alcohols (phenols). (Imido)vanadium(V)-alkylidene complexes, prepared by α-hydrogen elimination in the presence of a neutral donor ligand (PMe₃, etc.), exhibited high activity for ring-opening metathesis polymerisation of cyclic olefins (norbornene) even at high temperature; these are promising for olefin metathesis by vanadium complex catalysts.

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Kotohiro Nomura

Kotohiro Nomura received his PhD in 1993 from Osaka University, before joining the group of Richard Schrock (MIT). He then moved to Sumitomo Chemical Company, Ltd. and then the NAIST as an associate professor in 1998. Recently he moved to Tokyo Metropolitan University as a full professor. His research focuses on the design of molecular catalysts. He received The CSJ Award for Young Chemists in Technical Development from The Chemical Society of Japan in 1996, and The Society Award (Chemical & Engineering) from the Catalysis Society of Japan in 2001. He has co-authored over 160 publications and is an editorial board member for J. Mol. Catal. A: Chem.

Wenjuan Zhang

Wenjuan Zhang joined the group of Wen-Hua Sun (ICCAS) for her PhD in olefin polymerisation. She then moved to the group of Kotohiro Nomura (NAIST) in 2006, as a JSPS postdoctoral fellow for two years. She focused on vanadium(V) chemistry for olefin coordination insertion and metathesis polymerisation and related organometallic chemistry. After finishing her fellowship, she returned to ICCAS as an associate professor in 2008. Her recent research has focused on the design of transition metal catalysts for olefin polymerisation, and related organometallic chemistry, and ring-opening polymerisation. She received the Award for Excellent Young Scientists from the Institute of Chemistry, Chinese Academy of Sciences, both in 2005 and 2009.

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Introduction

Efficient carbon–carbon bond formation is one of the most important reactions in organic synthesis as well as in polymer synthesis, and metal-alkyl and metal-alkylidene species are known to play essential roles. The synthesis and reaction chemistry of metal-alkyl complexes have thus been considered to be important not only for designing efficient catalysts, but also for a better understanding of the organic reactions, especially in terms of catalytic cycles or reaction pathways.¹ Polyolefins produced by metal catalysed olefin coordination/insertion polymerisation are important synthetic polymers in our daily life, and the market capacity (even of the conventional polyethylene and polypropylene) still increases every year. Catalyst development is considered as core technology, therefore considerable attention has been devoted to the design of efficient catalysts that are highly effective for the synthesis of new polymers which cannot be prepared using conventional catalysts.^2–5

Classical Ziegler-type vanadium catalysts (e.g. VOCl₃, VCl₄, VCl₃–AlBr₃, AlCl₃–AlPh₃, AlⁱBu₃, SnPh₄) are known to display unique characteristics in olefin polymerisation. For example, the catalyst system afforded (i) high molecular weight linear polyethylene with uniform molecular weight distribution,⁶ and (ii) high molecular weight amorphous polymers applied to the synthesis of ethylene/propylene/diene copolymers (called EPDM, synthetic rubbers)^7,8 as well as ethylene/cyclic olefin copolymers (COC, as commercialised as ‘APEL™’ in Mitsui Chemicals, Inc., information and electronic materials). Moreover, (iii) the catalyst system [V(acac)₃ (acac = acetylacetonato)–Et₂AlCl] polymerises propylene to give not only a syndiotactic “living” polymer with narrow molecular weight distribution (M_w/M_n = 1.05–1.20),⁹ but also diblock copolymers of propylene and methyl methacrylate (MMA).¹⁰

Due to the promising characteristics demonstrated above, the design and synthesis of new vanadium complex catalysts directed toward controlled polymerisation has thus been an attractive target.^3a,b,5 Although examples of the synthesis of vanadium complexes used as catalyst precursors for olefin polymerisation are known, successful examples which exhibit the above unique characteristics of vanadium were limited until recently.^3a,b,5a,b Moreover, examples concerning the synthesis and reaction chemistry of vanadium-alkyls were limited until recently,^11–13 probably due to these vanadium-alkyls tending to be reactive and/or thermally labile, and reductions to lower oxidation states often occurred in the reactions with organometallic reagents.¹³

Olefin metathesis [such as ring-opening metathesis polymerisation (ROMP), ring-closing metathesis and cross metathesis reactions etc.], introduces promising possibilities for the synthesis of functional polymers as well as valuable organic compounds,¹⁴ as demonstrated especially by molybdenum¹⁵ and ruthenium.¹⁶ Transition metal-alkylidene complexes, especially high-oxidation-state early transition metal alkylidene complexes, attract considerable attention^15,17 because they play essential roles as catalysts in olefin metathesis and Wittig-type or group transfer reactions,^14,15,17 as demonstrated especially by molybdenum (so called Schrock type complexes).^14,15 However, as described below, no examples that promote the olefin metathesis reaction using vanadium(V)-alkylidene complexes were known until recently.

In this minireview, our recent efforts concerning (i) the synthesis of a series of (imido)vanadium(V)-alkyl, and alkylidene complexes, including recent examples by others, and (ii) some reactions of these vanadium(V)-alkyl complexes with alcohols (phenols, ROH) that proceed via coordination of ROH to the vanadium metal center are discussed.¹⁶

1. Synthesis of (imido)vanadium(V)-dichloro complexes as effective catalyst precursors for olefin polymerisation

A series of (imido)vanadium(V)-dichloride complexes, V(NAr)Cl₂(X) [Ar = 2,6-Me₂C₆H₃; X = aryloxo (1),¹⁸ ketimide (2),¹⁹ phenoxy-imine (3),²⁰ (2-anilidomethyl)pyridine (4)²¹] or V(NAd)Cl₂(X) [Ad = 1-adamantyl; X = aryloxo (5), ketimide (6)],²² could be prepared by treating (ArN)VCl₃ with the corresponding lithium salts [such as LiN=C^tBu₂, LiOAr etc.] in Et₂O or toluene (Scheme 1). The crystallographic results (Fig. 1) revealed that the ketimide-dichloride analogue, V(NAr)Cl₂(N=C^tBu₂) (2a), folds in a distorted tetrahedral geometry around V, and the complex (2a) is a 14 electron species.¹⁹ As shown in Scheme 2, complex 2a reacted with one equivalent of PMe₃ to afford V(NAr)Cl₂(N=C^tBu₂)(PMe₃) that folds in a distorted trigonal bipyramidal structure around V; the reaction of 2a with bis(dimethylphosphino)ethane (dmpe) afforded V(NAr)Cl₂(N=C^tBu₂)(dmpe), which folds in a distorted octahedral geometry around V.¹⁹

Scheme 1 Selected examples for (imido)vanadium(V) dichloride complexes.

Fig. 1 Structures for V(NAr)Cl₂(N=C^tBu₂) (2a, top left), V(NAr)Cl₂(N=C^tBu₂)(PMe₃) (top right) and V(NAr)Cl₂(N=C^tBu₂)(dmpe) (bottom).¹⁹ All hydrogen atoms are omitted for clarity.

Scheme 2 Synthesis of (arylimido)vanadium(V)-dichloro complexes containing a ketimide ligand.¹⁹

As described in the introduction, these complexes were effective catalyst precursors for ethylene (co)polymerisation in the presence of Al cocatalysts.^18,20–24 In particular, the arylimido-aryloxo analogues (1) showed remarkable catalytic activities not only for ethylene polymerisation,^18,23 but also for ethylene/norbornene copolymerisation in the presence of halogenated Al alkyls (Et₂AlCl, Me₂AlCl, EtAlCl₂etc.), affording high molecular weight polymers with uniform distributions.²³ Selected results in ethylene polymerisation using V(NAr)Cl₂(O-2,6-Me₂C₆H₃)–Al cocatalyst systems are summarised in Table 1.^23b

Table 1 Selected results for the effect of Al cocatalyst and solvent in ethylene polymerisation by V(N-2,6-Me₂C₆H₃)Cl₂(O-2,6-Me₂C₆H₃)–Al cocatalyst systems^a

Complex/μmol	Al cocatalyst	Solvent	Activity^b kg-PE/mol-V h	TOF^c/h^−1	M w × 10^−5	M w/Mn^d	M η × 10^−5
a Selected data from ref. 23b. Conditions: solvent + cocatalyst solution = total 30 mL, ethylene 8 atm, 0 °C (25 °C with MAO), 10 min, Al cocatalyst 250 μmol (MAO 2.5 mmol). b Activity in kg-polymer/mol-V h. c TOF = (molar amount of ethylene consumed)/(mol-V h). d GPC data in o-dichlorobenzene vs. polystyrene standards. e Molecular weight by viscosity. f Insoluble in o-dichlorobenzene for GPC measurement. g Cp*TiCl2(O-2,6-^iPr2C6H3) was used, ethylene 4 atm, 25 °C, 10 min (cited from ref. 3d).
0.05	Me2AlCl	Toluene	27500	980000	—^f	—	89.8
0.05	Et2AlCl	Toluene	11700	415000	36.5	1.42	98.7
0.05	^iBu2AlCl	Toluene	52000	1850000	—^f	—	125
0.01	^iBu2AlCl	Toluene	64800	2310000	—^f	—
0.05	EtAlCl2	Toluene	37400	1330000	6.02	3.04
1.0	MAO	Toluene	2930	104000	28.7	1.64
0.2 (Ti)^g	MAO	Toluene	8400	298000	12.4	1.90
0.05	Et2AlCl	C6H5Cl	19000	677000	—^f	—	38.3
0.05	EtAlCl2	C6H5Cl	64400	2300000	24.4	3.14
0.05	Me2AlCl	CH2Cl2	19700	702000	24.2	3.38
0.05	Et2AlCl	CH2Cl2	13200	471000	13.4	3.93
0.05	^iBu2AlCl	CH2Cl2	45200	1610000	—^f	—	58.5
0.01	EtAlCl2	CH2Cl2	584000	20800000	—^f	—

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The catalytic activity in the ethylene polymerisation was highly dependent upon the Al cocatalyst employed, and the activities in toluene increased in the order: ⁱBu₂AlCl (52000 kg-PE/mol-V h) > EtAlCl₂ (37400) > Me₂AlCl (27500) > Et₂AlCl (11700) > MAO (2930) ≫ Et₂Al(OEt), Me₃Al, Et₃Al (trace or less). The activity did not decrease after 30 min. The activity was highly affected by the solvent employed; the activity of 584000 kg-PE/mol-V h (TOF 20800000 h⁻¹, 5780 s⁻¹) was attained in CH₂Cl₂ in the presence of EtAlCl₂. The resultant polymers prepared in toluene possessed ultrahigh molecular weights with unimodal molecular weight distributions (the M_η values in the resultant polymers prepared in the presence of ⁱBu₂AlCl and Me₂AlCl were 9.87–12.5 × 10⁶ and 8.98 × 10⁶, respectively). The activity decreased upon addition of CCl₃CO₂Et, which can be commonly used as an effective additive (for restarting the catalytic cycle from the deactivated catalyst by re-oxidation to a higher oxidation state) in the ethylene polymerisation using vanadium(III) and/or vanadium(IV) complexes such as V(acac)₃ (acac = acetylacetonato), V(β-diketonate)₃.^5,25 The results clearly suggest that the catalytically active species were thus different from those prepared from vanadium(III),(IV) complexes. We assumed that a plausible reason for the observed difference in the catalytic activities in the presence of MAO and Et₂AlCl cocatalysts might be due to the different catalytically-active species and catalyst/cocatalyst nuclearity effect²⁶ generated in the two catalyst systems.

2. Synthesis of (imido)vanadium(V)-alkyl complexes, and some reactions with alcohols, thiols, borates

2-1. Synthesis of (imido)vanadium(V)-dialkyl, trialkyl complexes

The reaction of 2a with LiCH₂SiMe₃ in n-hexane afforded V(NAr)(CH₂SiMe₃)₂(N=C^tBu₂) (7) in high yield (95%, Scheme 3).²¹ The coordination of PMe₃ to 7 was not observed even by addition of an excess amount (7.0 equiv.) at 25 °C, and this observation might be due to the steric hindrance of the rather bulky CH₂SiMe₃ group around the V metal center. The dialkyl complexes containing (2-anilidomethyl)pyridine ligand 8 were also isolated by adopting this approach (yields 60, 73% for R = Me, ⁱPr, respectively).²¹ However, attempts at isolating the dialkyl complexes from the other dichloro analogues (1, 3, 5, 6) failed due to difficulties in the isolation of the pure, desired complexes by recrystallisation in most cases. For example, reaction of V(NAr)Cl₂(O-2,6-ⁱPrC₆H₃) with 2.0 equiv. of LiCH₂SiMe₃ in n-hexane afforded the corresponding dialkyl complex (>90%) containing a tiny amount of unidentified by-product (in the ¹H NMR spectrum) which caused difficulties for separation.²⁷ As described below, these dialkyl complexes are key intermediates for the synthesis of the alkylidene complexes by α-hydrogen elimination, therefore, we had to find another possibility for the isolation of the dialkyl complexes.

Scheme 3 Synthesis of dialkyl complexes.^19,21,27

In contrast, the analytically pure trialkyl analogues, V(NAr)(CH₂SiMe₃)₃ (9) and V(NAd)(CH₂SiMe₃)₃ (10), could be isolated in high yields (Scheme 4).^27,28 Although these complexes (9, 10) are low coordinate unsaturated vanadium(V) complexes, the coordination of PMe₃ to V was not observed (by ⁵¹V NMR spectroscopy), even by addition of an excess amount (7.0 equiv.) in C₆D₆ at 25 °C, and this observation would be due to the steric hindrance of three CH₂SiMe₃ ligands around the vanadium(V) metal center. Resonance in the ⁵¹V NMR spectrum (δ 1070 ppm) is relatively close to that in V(N-2,6-ⁱPr₂C₆H₃)(CH₂Ph)₃ (1008 ppm),^12b and the resonances in a series of (imido)vanadium(V)-trialkyl complexes are found to be influenced by the imido substituent employed [ca. V(N-4-MeC₆H₄)(CH₂SiMe₃)₃ (δ 1028),²⁹ V(NAd)(CH₂SiMe₃)₃ (δ 895),²⁸ V(N^tBu)(CH₂SiMe₃)₃ (δ 877)³⁰]. The crystallographic result (Fig. 2)²⁸ revealed that 10 folds in a rather distorted tetrahedral geometry around V. The V–N–C bond angle is 180.00(17)° and the V–N distance is 1.6317(14) Å, and the three V–C bond distances and angles are the same [112.55(7)°, 2.0267(18) Å]; these are in unique contrast to those for the arylimido-tribenzyl complex, V(N-2,6-ⁱPr₂C₆H₃)(CH₂Ph)₃, [ex. V–N–C 169.0(5)° and 1.641(6) Å] reported by Turner and Murphy.^12b As described below, these complexes are important intermediates for the synthesis of various dialkyl analogues containing anionic ancillary donor ligands.

Scheme 4 Synthesis of (imido)trialkyl complexes (9, 10).^27,28

Fig. 2 ORTEP drawing of 10.²⁸ Thermal ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity.

2-2. Reaction of (imido)vanadium-alkyl complexes with various alcohols, thiols, and borates

In general, metal-alkyls, especially metal-alkyls with early transition metals, possess a more nucleophilic nature than those with late transition metals, and are thus highly reactive toward Brønsted/Lewis acids.^1,5,31,32 For instance, cationic alkyl complexes, which have been proposed to be the catalytically-active species in olefin coordination polymerisation, are generated from their dialkyl analogues by reacting them with cocatalysts via facile protonolysis or alkyl abstraction;³¹ some organometallic complexes can be thus grafted onto a silica surface by reaction of the alkyl compounds with silanol groups on the surface.^33,34

In contrast, some exceptions (that showed remarkable stability toward alcohols), exemplified by reactions of Zr(CH₂Ph)₄ or Zr(CH₂^tBu)₄ with ^tBu₃COH requiring longer hours even under refluxing conditions, are known.³² Although the reactions of early transition metal-alkyls with alcohols (phenols) should be important basic reactions in organometallic chemistry, a detailed mechanistic study, especially including direct isolation and/or observation of the intermediates, had not so far been explored.

Reactions of the (arylimido)vanadium(V) trialkyl analogue, V(N-2,6-ⁱPr₂C₆H₃)(CH₂Ph)₃, with 2,6-ⁱPr₂C₆H₃OH or (CF₃)₃COH in CH₂Cl₂ or n-hexane afforded the corresponding aryloxide/alkoxide in high yield.^12b The similar reaction of V(NAr)(CH₂SiMe₃)₃ (9), with 2,6-Me₂C₆H₃OH, 2,6-ⁱPr₂C₆H₃OH, C₆F₅OH (in n-hexane at 25 °C) cleanly afforded corresponding vanadium(V)-aryloxide complexes (11a–c) in high yields (90–93%, Scheme 5).²⁷ The reactions of 9 with 1.0 equiv. of ^tBuOH, (CF₃)₂MeCOH in n-hexane also gave the corresponding alkoxides (11d,e) in high yields (92–93%).²⁷ Substitution of the alkyl group in 9 with alcohol or phenol leads to upfield chemical shifts (δ 406–659 ppm) from the trialkyl complex (9, 1070 pm) in the ⁵¹V NMR spectra, and the degree of the upfield shift in the dialkyl complexes (11a–e) increased in the order: δ 406 (11d, O^tBu) > 506–507 (11a,b, O-2,6-Me₂C₆H₃, O-2,6-ⁱPr₂C₆H₃) > 595 [11e, OCMe(CF₃)₂] > 659 (11c, OC₆F₅). These resonances seem to be influenced mostly by an electronic factor in the anionic donor ligand employed. Reaction of V(NAd)(CH₂SiMe₃)₃ (10) with 1.0 equiv. of C₆F₅OH in n-hexane afforded a mixture of V(NAd)(CH₂SiMe₃)₂(OC₆F₅) (62% yield) and [V(NAd)(CH₂SiMe₃)(OC₆F₅)(μ₂-OC₆F₅)]₂ (4% yield).²⁸ The present route from the trialkyl analogues should thus be more suited for the synthesis of a series of vanadium(V) dialkyl derivatives containing aryloxo/alkoxo ligands, because, as described above, isolations of these complexes were difficult from the dichloro analogues (1, 5) by alkylation.

Scheme 5 Reaction of V(NAr)(CH₂SiMe₃)₃ (9) with phenols, alcohols.²⁷

Note that the reaction of 9 with 2,6-^tBu₂-4-MeC₆H₂OH did not take place under the same conditions; the reaction did not occur even if the C₂D₂Cl₄ solution containing 9 was heated at 50 °C in the presence of 2,6-^tBu₂-4-MeC₆H₂OH with an excess amount (2.6 equiv.).²⁷ The above fact is interesting, because V(NAr)Cl₂(O-2,6-^tBu₂-4-MeC₆H₂) was prepared from V(NAr)Cl₃ by treatment with 1.0 equiv. of 2,6-^tBu₂-4-MeC₆H₂OH (yield 94%).¹⁸ We believe that the observed fact would be due to the steric bulk of both the phenol and three CH₂SiMe₃ ligands in the coordination of oxygen to the vanadium(V) metal center, because, as described below, we propose that the reaction of V(NAr)Me(N=C^tBu₂)₂ with phenols proceeds via a pentacoordinated trigonal bipyramidal species by coordination of the phenol to the vanadium.^35,36

2-2-1. Reactions of V(NAr)Me(N=C^tBu₂)₂ with various phenols, alcohols. Reactions of V(NAr)Me(N=C^tBu₂)₂ (12), which was prepared by treating V(NAr)Cl(N=C^tBu₂)₂ (1a) with 1.0 equiv. of MeMgBr,³⁵ with 1.0 equiv. of phenols exclusively gave another vanadium(V)-methyl complex of the type, V(NAr)Me(N=C^tBu₂)(OAr′) [13, Ar′ = 2,6-Me₂C₆H₃ (a), 4-^tBu-2,6-ⁱPr₂C₆H₂ (b), Ph (c), C₆F₅ (d)], by replacement with the ketimide ligand (Scheme 6).^35,36 Note that the reaction with the methyl group did not proceed under these conditions, and the selectivity in the reaction was not dependent upon the kind of phenol employed. Treatment of 12 with ⁱPrOH, 3-buten-1-ol or 5-hexen-1-ol in n-hexane cleanly afforded V(NAr)Me(N=C^tBu₂)(OⁱPr) (13e), VMe(NAr)(N=C^tBu₂)[OCH₂(CH₂)_nCH=CH₂] [n = 1 (13f), 3 (13g)], respectively.^35,36 The reaction of 12 with 2-Me-6-{(2,6-ⁱPr₂C₆H₃)N=CH}C₆H₃OH (1.1 equiv. to V) in C₆D₆ at 25 °C reached completion after 3 days, although the above reaction with 2,6-ⁱPr₂C₆H₃OH, 2,6-Me₂C₆H₃OH completed within 3 h. The product was V(NAr)Me(N=C^tBu₂)[O-2-Me-6-{(2,6-ⁱPr₂C₆H₃)N=CH}C₆H₃] (13h) (yield 92%),²⁰ indicating that a unique reactivity toward alcohol in the vanadium-methyl complex was preserved even in the reaction with the imino-phenol.

Scheme 6 Reaction of V(NAr)Me(N=C^tBu₂)₂ (12) with various phenols, alcohols.^20,35,36

Mechanistic studies for reaction of V(NAr)Me(N=C^tBu₂)₂ (12) with alcohols. Exploration for unique reactivity of the vanadium-methyl bonds toward alcohols. The reaction of 12 with 1.0 equiv. of 2,6-Me₂C₆H₃OH or 2,6-ⁱPr₂-4-^tBuC₆H₂OH at 25 °C afforded a mixture of 13a,b and the corresponding bis(phenoxide) (14a,b), and the quantitative conversion of 14a,b could be achieved by reacting with 2.0 equiv. of Ar′OH at 25 °C after 24 h (Scheme 7).³⁶ Note that the formation of methane was not observed even under these conditions.

Scheme 7 Reaction of V(NAr)Me(N=C^tBu₂)(OAr′) (13) with phenol.³⁶

A rapid scrambling of 13a and 13b was observed when 13a was treated with 1.0 equiv. of 4-^tBu-2,6-ⁱPr₂C₆H₂OH in CDCl₃ at 25 °C or when 1.0 equiv. of 2,6-Me₂C₆H₃OH was added into a CDCl₃ solution containing 13b (Scheme 8); the resultant solution eventually gave a ca. 1 : 1 mixture of 13a and 13b within 30 min at 25 °C.³⁶ The solution then gave 3 species upon heating at 60 °C for 12 h which were assigned as 14a, 14b, and the mixed bis(aryloxo) complex, (ArN)VMe(O-4-^tBu-2,6-ⁱPr₂C₆H₂)(O-2,6-Me₂C₆H₃) (14c). These results clearly explain the unique reactivity of both 12 and 13a,b toward alcohols according to Scheme 8. Both phenol-scrambling and the phenol/ketimide exchange reaction should be preferred if the phenol approaches the electrophilic vanadium metal center trans to the Me group (NNN face in 12 or NNO face in 13a,b, and not the NNC faces in 12 or 13a,b), to give pentacoordinated trigonal bipyramidal intermediates (shown in brackets in Scheme 8).³⁶ The reaction with the Me group would not occur if the reaction takes place via this proposed intermediate. A similar assumption was also proposed by Schrock et al.³⁷ in the alkoxide exchange reaction of Mo(NAr)(CH^tBu)(CH₂^tBu)(OAr) with ROH (OR = OCMe₃, OAd, OC₆F₅) and the presence of a similar intermediate was assumed.

Scheme 8 Proposed reaction pathways.³⁶

Based on our experiments, we propose that the reaction with alcohols proceeds in the following steps: 1) the alcohols initially approaches the electron-deficient metal center trans to the methyl group to give a pentacoordinated trigonal bipyramidal species, and 2) proton transfer to the aryloxide/ketimide occurs to give ketimine/phenol dissociation.³⁶

2-2-2. Reaction of V(NAr)Me(N=C^tBu₂)₂ (12) with various thiols, borates. In contrast, the reaction of 12 with 1.0 equiv. of 2,6-Me₂C₆H₃SH afforded V(NAr)(N=C^tBu₂)₂(S-2,6-Me₂C₆H₃) (15a) via cleavage of the V–Me bond (Scheme 9).³⁶ The reaction with n-C₆H₁₃SH in n-hexane afforded V(NAr)(N=C^tBu₂)₂(S-n-C₆H₁₃) (15b) as the sole product.³⁶ These facts clearly indicate that the reaction mechanism should differ between alcohol and thiol.

Scheme 9 Reactions of V(NAr)Me(N=C^tBu)₂ (12) with thiols, borates.³⁶

The reaction of 12 with 1.0 equiv. of [PhN(H)Me₂][B(C₆F₅)₄] in THF afforded cationic [V(NAr)(N=C^tBu)₂(THF)][B(C₆F₅)₄] (16a) (Scheme 9). The crystallographic analysis of 16a (Fig. 3) indicates that 16a folds in a pseudo-tetrahedral geometry around the vanadium center with the coordination of one THF molecule, and the V–N(C^tBu₂) distances (1.802–1.808 Å) are comparable to those found in V(NAr)Cl(N=C^tBu₂)₂ and somewhat shorter than those in 12. The position of another THF molecule could not be defined/determined. The same compound (16a) could also be cleanly isolated (90%) by the reaction of 12 with 1.0 equiv. of [Ph₃C][B(C₆F₅)₄], and the reaction of 12 with B(C₆F₅)₃ also gave [V(NAr)(N=C^tBu)₂(THF)][MeB(C₆F₅)₃] (16b) exclusively.³⁶ Moreover, the reaction of 14a with 1.0 equiv. of [PhN(H)Me₂][B(C₆F₅)₄] also gave the corresponding cationic species [V(NAr)(N=C^tBu₂)(O-2,6-Me₂C₆H₃)(THF)₂][B(C₆F₅)₄] (16c). These results clearly indicate that cleavage of the V–Me bond was favored in all cases.³⁶

Fig. 3 ORTEP drawing (50% probability ellipsoids) for 16a.³⁶ All hydrogen atoms are omitted for clarity.

2-2-3. Reactions of (1-adamantylimido)vanadium(V)-alkyl complexes containing a chelate alkoxo(imino)pyridine ligand with alcohols (ROH) that proceed via intermediates formed by coordination of ROH. A (1-adamantylimido)vanadium(V)-dialkyl complex containing a chelate alkoxo(imino)pyridine ligand, V(NAd)(CH₂SiMe₃)₂(L′) [17, L′ = 6-OC(Me)₂-2-{(2,6-ⁱPr₂C₆H₃)N=CMe}C₅H₃N] was prepared from V(NAd)(CH₂SiMe₃)₃ (10) with the corresponding imino-phenol (1.0 equiv. in C₆D₆ at 50 °C for 3 days).³⁸ The complex (17) folds in a trans (not cis) geometry, and is thermally stable in C₆D₆ at 80 °C (even after 72 h). Dissociation of neutral donor(s) in the chelate ligand (L′) did not take place even in the presence of NHC [2.1 equiv. in C₆D₆ at 80 °C, 72 h, NHC = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene].³⁸

The reaction of 17 with 1.0 equiv. of (CF₃)₂CHOH (in n-hexane at 25 °C) did not take place even after >12 h, and the reaction with 2.0 equiv. of (CF₃)₂CHOH did not take place even after several hours. The fact should be an interesting contrast to the observed fact that the reaction of the other (imido)vanadium(V)-alkyl complexes [such as V(NAr)Me(N=C^tBu₂)₂ (12), V(NAr)(CH₂SiMe₃)₃ (9), V(NAd)(CH₂SiMe)₃ (10)] with alcohols (phenols) took place cleanly even at room temperature.

Monitoring the reaction mixture by NMR spectroscopy revealed that the reaction of 17 with (CF₃)₂CHOH first generated an intermediate, and was converted to the original complex (17) when the mixed solution was placed in vacuo (to remove volatiles). On the basis of ¹H, ¹⁹F, and ⁵¹V NMR spectra, the intermediate should be V(NAd)(CH₂SiMe₃)₂(L′)[(CF₃)₂CHOH] (T1) formed by coordination of oxygen in (CF₃)₂CHOH into the vanadium in 17, accompanied as well by dissociation of the two imino groups (Scheme 10). The reaction completed upon heating or by stirring for a longer time (48 h) to afford the monoalkyl-alkoxo species, V(NAd)(CH₂SiMe₃)(L′)[OCH(CF₃)₂] (18a). The structure determined by X-ray crystallography indicates that 18a folds in a distorted tetrahedral geometry around vanadium, and that the two neutral nitrogen donors in L′ are dissociated.³⁸

Scheme 10 Proposed scheme for reaction of V(NAd)(CH₂SiMe₃)₂(L′) (17) with alcohol.³⁸

Moreover, a reaction mixture of the dialkyl complex (17) and 2 equiv. of (CF₃)₂CHOH (25 °C after 48 h) afforded two resonances in the ⁵¹V NMR spectrum including a resonance ascribed to the monoalkyl complex (18a). Only one resonance due to 18a was observed when the reaction mixture was placed in vacuo (to remove volatiles). The fact also suggests the formation of another intermediate, assumed to be V(NAd)(CH₂SiMe₃)(L)[OCH(CF₃)₂][(CF₃)₂CHOH] (T2), generated by coordination of (CF₃)₂CHOH into 18a.³⁸ The reaction of 17 with 2.0 equiv. of (CF₃)₂(CH₃)COH in place of (CF₃)₂CHOH did not take place at 25 °C, and a similar observation (such as formation of an intermediate that could be returned to 17 after removal of volatiles) was made. The reaction completed to afford V(NAd)(CH₂SiMe₃)[OC(CH₃)(CF₃)₂] (18b), when the mixture was heated at 80 °C for 22 h.³⁸

On the basis of the above results, we propose that reactions of the dialkyl complex (17) with ROH [R = CH(CF₃)₂, C(CH₃)(CF₃)₂] proceeded via five coordinate intermediates formed by coordination of the oxygen in ROH (not via addition of H⁺) accompanied by dissociation of two neutral nitrogen donors (imino groups) in the chelate tridentate ligand (Scheme 10). Confirmation of the intermediates, V(NAd)(CH₂SiMe₃)₂(L′)[ROH] (T1), is potentially important not only for better understanding of the reactions of vanadium(V)-alkyls with alcohols, but also to explain the observed unique reactivities of the vanadium(V)-alkyls in V(NAr)(Me)(N=C^tBu₂)₂ (12),^35,36 V(NAr)(CH₂SiMe₃)₃ (9),²⁷ V(NAd)(CH₂SiMe)₃ (10)²⁸ with various alcohols. We believe that this hypothesis should be applied to the reactions of early transition metal alkyls (especially in high oxidation states) with alcohols.

3. Synthesis of vanadium(V)-alkylidene complexes and their use as olefin metathesis catalysts

As described in the introduction, transition metal-alkylidene complexes play important key roles in olefin metathesis, which introduces promising possibilities for the synthesis of functional polymers as well as valuable organic compounds,¹⁴ as demonstrated especially by molybdenum¹⁵ and ruthenium.¹⁶ High-oxidation-state early transition metal alkylidene complexes attract considerable attention,^15,17 because they play essential roles as catalysts in olefin metathesis and Wittig-type or group transfer reactions,^14,15,17 as demonstrated especially by molybdenum (so-called Schrock-type carbene complexes).^14,15,17

Although classical Ziegler type vanadium catalysts display unique characteristics such as use in the synthesis of high molecular weight polymers with rapid propagation in olefin coordination insertion polymerisation^3a,b,5 and the syntheses of some vanadium(III),(IV)-alkylidene^39,40 as well as vanadium(V)-alkylidene complexes⁴¹ are known, examples for olefin metathesis with vanadium-alkylidene complexes have been limited. Since no examples were known until recently concerning the synthesis of ‘olefin metathesis active’ vanadium(V)-alkylidenes,^39b their synthesis and reaction chemistry are thus promising subjects in terms of the basic understanding of organometallic chemistry as well in their application in catalysis.

3-1. Approaches for the synthesis of vanadium-alkylidene complexes: previous reports and selected recent examples

The most common method to prepare high-oxidation state metal alkylidenes is by promoting α-H abstraction or α-H deprotonation reactions from metal alkyl complexes lacking β-hydogens.¹⁷ Some additional assistance (that can stimulate photochemically and/or sterically, or with a base) is commonly used to promote/induce the abstraction/deprotonation. Scheme 11 shows selected reported examples for the synthesis of vanadium-alkylidene complexes till 2000. Vanadium(III)-alkylidene complex, CpV(CH^tBu)(dmpe), was prepared by α-hydrogen abstraction upon addition of dmpe (Scheme 11, top equation).^39b Reaction of CpV(CH^tBu)(dmpe) with norbornene was attempted but showed extremely low (catalytic) activity (TON 0.92 after 96 h at 20 °C).^39b

Scheme 11 Selected reported examples for synthesis of vanadium-alkylidenes.^39b,41b,c

Transfer of a benzylidene from phosphorus to vanadium(III) afforded the first vanadium(V)-alkylidene complex (Scheme 11, middle).^41b However, no reaction between CpV(CHPh)(N-2,6-ⁱPr₂C₆H₃)(PMe₃) and norbornene or acetone was observed. Reaction of a V(III) borohydride complex with diphenylacetylene yielded a vanadium(V) complex containing a V=C double bond, although the mechanism was not clear (Scheme 11, bottom).^41c No data concerning the reactivity of this carbene species were given.

Another approach recently employed for the synthesis of a vanadium(IV)-alkylidene was the so-called oxidatively induced α-hydrogen abstraction reported by Mindiola et al.^39c,e As shown in Scheme 12, a vanadium(III)-dialkyl complex, (nacnac)V(CH₂^tBu)₂ [nacnac⁻ = {(Ar′)NC(Me)CHC(Me)CN(Ar′)}⁻], was oxidised to afford neutral or cationic vanadium(IV)-alkylidene complexes, [(nacnac)V(CH^tBu)(I)] or [(nacnac)V(CH^tBu)(THF)]⁺[BPh₄]⁻.^39c Reaction of [(nacnac)V(CH^tBu)(I)] with LiPR (R = 2,4,6-ⁱPr₃C₆H₂, 2,4,6-^tBu₃C₆H₂) afforded the corresponding vanadium(IV)-alkyl, phosphinidene complex, [(nacnac)V(CH₂^tBu)(PR)],^39d and the vanadium(V)-alkylidyne complex, [(nacnac)V(C^tBu)(L)], was formed via the corresponding vanadium(IV)-alkyl,alkylidene complex upon oxidation (Scheme 12).^39e

Scheme 12 Synthesis of a vanadium-alkylidene by oxidatively induced α-hydrogen abstraction.^39c–e

More recently, the synthesis of a series of vanadium(V)-alkylidene complexes from a vanadium(III) dialkyl complex containing a monoanionic tridentate chelate ligand (PNP), [V(CH₂^tBu)₂(PNP)] PNP = N[4-Me-2-(PⁱPr₂)C₆H₃]₂⁻, by treating with π-acids or two electron oxidants was reported (Scheme 13).⁴² The reaction of [V(CH₂^tBu)₂(PNP)] with chalcogen sources resulted in the formation of the vanadium(V)-alkylidene, chalcogenide species, [V(X)(CH^tBu)(PNP)] (X = O, S, Se, Te). The vanadium(III)-alkylidene complex was also formed upon addition of 2,2′-bipyridine by α-hydrogen abstraction, and a similar reaction with N₂CPh₂ in Et₂O afforded the alkylidene-hydrazido complex, V(CH^tBu)(NNCPh₂)(PNP). It has been proposed that these reactions take place via a transient vanadium(III)-alkylidene complex.⁴²

Scheme 13 Synthesis of vanadium(V)-alkylidenes containing a PNP ligand.⁴²

3-2. Generation of catalytically-active species for ring-opening metathesis polymerisation (ROMP) from (imido)vanadium(V)-dialkyl complexes containing anionic ancillary donor ligands

Although the initial attempt at reaction of CpV(CHCMe₃)(dmpe) with norbornene (NBE) afforded polymers with extremely low (catalytic) activity (TON 0.92 after 96 h at 20 °C), it was revealed that ring-opening metathesis polymerisation (ROMP) of NBE took place in the presence of V(NAr)Cl₂(OAr′) (1) and AlMe₃.^18a Since 1 showed remarkable catalytic activities for ethylene polymerisation in the presence of halogenated Al alkyls, is was thus assumed that the aluminium promoters directly control the olefin coordination insertion or metathesis pathways (Scheme 14). Moreover, the dibenzyl complex (20) initiated ROMP of NBE catalytically at 25 °C, affording the ring-opened poly(NBE) with high molecular weight with uniform molecular weight distribution.^18a As far as we know, this is apparently the first catalytic example of vanadium-initiated ROMP of NBE, and formation of the alkylidene species was thus assumed in this catalysis.^18a The dibenzyl complex is, however, not stable and decomposed gradually; an attempted isolation of the assumed alkylidene was not successful.

Scheme 14 Ring-opening metathesis polymerisation (ROMP) of norbornene (NBE) by (arylimido)vanadium(V) complex catalysts containing an aryloxo ligand.^18a

Later, we reported that V(NAr)(CH₂SiMe₃)₂(N=C^tBu₂) (7), which showed better thermal stability than V(NAr)(CH₂Ph)₂(O-2,6-ⁱPr₂C₆H₃) (20), initiated ROMP of NBE in toluene.¹⁹ The activity increased especially by addition of PMe₃ (3 equiv.) at higher temperature (80 °C) but decreased on further addition (5 equiv.); the activity was extremely low in the absence of PMe₃ even at 80 °C.¹⁹ The resultant polymer possessed a ring-opened structure containing a mixture of cis- and trans-olefinic double bonds. These results clearly suggest the formation of vanadium(V)-alkylidene by α-hydrogen elimination from the dialkyl species in the presence of PMe₃.

3-3. Synthesis of (imido)vanadium(V)-alkylidene complexes and their use as catalysts for ring-opening metathesis polymerisation (ROMP)

It was then revealed that the reaction of 7 with PMe₃ (7.0 equiv.) in benzene-d₆ at 80 °C afforded the alkylidene complex, V(CHSiMe₃)(NAr)(N=C^tBu₂)(PMe₃) (21, a mixture of syn/anti forms in solution), by α-hydrogen elimination (Scheme 15).^19,43 This is the common method to prepare high-oxidation state metal alkylidenes by promoting α-hydrogen abstraction reactions from metal alkyl complexes lacking β-hydrogens, and addition of PMe₃ was required to promote the abstraction by steric crowding.¹⁷ The crystal structure (Fig. 4) indicates that 21 folds in a distorted tetrahedral geometry around the vanadium metal center, and the V–CHSiMe₃ bond distance (1.860 Å) is close to that in the bicyclic carbene-amide complex (1.876 Å)^41c and is shorter than that in the benzylidene complex (1.922 Å).^41b The ¹H and ¹³C NMR spectra showed resonances corresponding to V=CHSiMe₃ at 14.52, 302.0 ppm, respectively. These results clearly indicate that 21 is a nucleophilic 16-electron vanadium(V)-alkylidene complex.

Scheme 15 Synthesis of vanadium(V)-alkylidene complex (21).^19,43

Fig. 4 ORTEP drawing of 21 Thermal ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity.¹⁹

The complex (21) exhibited remarkable catalytic activity for the ROMP of NBE without a cocatalyst (Table 2).^19,43 The activity (turnover number, TON) was low when the ROMP was conducted at 25 °C, whereas a standard Schrock type initiator, Mo(CHCMe₂Ph)(N-2,6-ⁱPr₂C₆H₃)(O^tBu)₂ (Mo), exhibited especially remarkable catalytic activity under similar conditions.^43,44 The activity of 21 increased at 50 °C, whereas the activity of Mo was negligible due to the decomposition of the catalytically active species.⁴⁴ The activity markedly increased at higher temperature (80 °C), and the observed activity was higher than that of Ru(CHPh)(Cl)₂(PCy)₂ under the same conditions due to the improved thermal stability.^19,43

Table 2 Ring opening metathesis polymerisation (ROMP) of norbornene (NBE) by V(CHSiMe₃)(N-2,6-Me₂C₆H₃)(N=C^tBu₂)(PMe₃) (21), Ru(CHPh)Cl₂(PCy₃)₂ (Ru, Cy = cyclohexyl), and Mo(CHCMe₂Ph)(N-2,6-ⁱPr₂C₆H₃)(O^tBu)₂ (Mo).^a

Complex (μmol)	Solvent	Temperature/°C	Time/min	TON^b	M w × 10^−4	M w/Mn^c
a Selected data cited from ref. 43, conditions: catalyst 0.2 or 1.0 μmol, NBE 2.12 mmol, benzene or toluene 9.6 mL, initial NBE conc. 0.22 mmol mL^−1. b TON = NBE consumed (mmol)/V (mmol). c GPC data vs. polystyrene standards.
21 (1.0)	Benzene	25	360	267	46	2.3
Ru (1.0)	Toluene	25	60	1306	54	1.7
Mo (0.20)	Toluene	25	60	8550	160	1.2
21 (1.0)	Benzene	50	180	1275	49	1.6
21 (1.0)	Benzene	50	180	166
Mo (0.20)	Toluene	50	40	Trace
21 (1.0)	Benzene	80	30	967	140	1.3
21 (1.0)	Benzene	80	60	1583	133	1.4
21 (1.0)	Toluene	80	60	1244	32	2.8
Ru (1.0)	Toluene	80	60	350

---

The aryloxo-alkylidene analogue (22) was also isolated from the corresponding dialkyl complex (11b), which also showed remarkable catalytic activities for ROMP of NBE in the presence of PMe₃, upon heating in C₆D₆ in the presence of PMe₃ (Scheme 16).²⁷ PMe₃ was partially dissociated, as expressed in V(CHSiMe₃)(NAr)(O-2,6-ⁱPr₂C₆H₃)(PMe₃)_x (22, x = 0.89), from the metal center during the purification procedure (by placing the grown microcrystals in vacuo to remove solvent), probably due to the steric bulk of the two ⁱPr groups on the aryloxo ligand and the presence of both the PMe₃ coordinated and the PMe₃ free species in solution. This can also be confirmed by the observation that two resonances in both ⁵¹V and ¹H spectra (corresponding to V=CHSiMe₃) became one resonance on addition of PMe₃. Complex 22 could be prepared from the dichloro analogue without isolation of the dialkyl analogue (11b).

Scheme 16 Synthesis of (arylimido)vanadium(V)-alkylidene containing aryloxo ligand (22).²⁷

Other vanadium(V)-dialkyl complexes containing an aryloxo ligand like V(NAr)(CH₂SiMe₃)₂(O-2,6-Me₂C₆H₃) (11a) and other complexes containing a (2-anilidomethyl)pyridine ligand (8 in Scheme 3) showed notable catalytic activities for the ROMP of NBE in the presence of PMe₃, but attempts at isolating the corresponding vanadium(V)-alkylidene were not successful, although the presence of resonances that may be ascribed to the assumed alkylidene species was observed in the ¹H NMR spectra.^21,27 One probable reason for this difficulty could be assumed as due to the formed vanadium(V)-alkylidene species being highly reactive even in C₆D₆ and affording another species [such as V[CH(D)SiMe₃](C₆D₅) etc.] by C–H activation.⁴⁵ Some reactions with the isolated vanadium(V)-alkylidene are thus in progress, and will be introduced soon.

The aryloxo-alkylidene (22) showed higher catalytic activities at 25 °C than the ketimide-alkylidene (21), and the resultant polymers obtained at 25 °C possessed narrow molecular weight distributions and the M_n value increased over the time course (Fig. 5). The M_n values increased upon increasing the TON values (polymer yields) with consistently narrow molecular weight distributions (M_w/M_n = 1.1–1.2) during the polymerisation, clearly indicating that the ROMP of NBE by 22 proceeded in a living manner at 25 °C.

Fig. 5 Plots of TON vs M_n, M_w/M_n in the ring-opening metathesis polymerisation (ROMP) of norbornene by V(NAr)(CHSiMe₃)(O-2,6-ⁱPr₂C₆H₃)(PMe₃)_x (22) in benzene (conditions: 22 5.0 μmol, NBE 21.2 mmol, benzene 24 mL at 25 °C).²⁷

Synthesis of (imido)vanadium(V)-alkyl, alkylidene complexes containing N-heterocyclic carbene ligands from their trialkyl analogues. As described above, one of the methods commonly employed for generating metal-alkylidenes is the α-hydrogen abstraction promoted by heat or irradiation.¹⁷N-Heterocyclic carbenes (NHC) have been known to play essential roles as ligands in metal complex catalysts as well as organocatalysts,⁴⁶ and the NHC in VOCl₃(IMes) [IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] has shown its utility in stabilising the high-oxidation-state vanadium(V) complex.⁴⁷ We thus explored the reaction of (imido)vanadium(V) trialkyl analogues, V(NR)(CH₂SiMe₃)₃ [R = Ar (9), Ad (10)], in the presence of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (NHC).

Coordination of NHC was not observed if 10 was added with 1.0 equiv. of NHC in C₆D₆ at 25 °C, but a new resonance ascribed to the alkylidene proton (13.4 ppm) appeared when the solution was heated at 80 °C for 18 h.²⁸ The targeted alkyl-alkylidene complex, V(CHSiMe₃)(NAd)(CH₂SiMe₃)(NHC) (23), could be isolated (22% based on NHC, 15% based on V) when 10 was treated with 2/3 equiv. of NHC at 80 °C in C₆D₆ (Scheme 17).²⁸ Similarly, the reaction of the arylimido analogue (9) with NHC afforded V(CHSiMe₃)(NAr)(CH₂SiMe₃)(NHC) (24) in better yield (75% based on NHC, 60% based on V, Scheme 17). Thermolysis of the (adamantylimido)vanadium(V) trialkyl analogue, V(NAd)(CH₂SiMe₃)₃ (10), in C₆D₆ at 80 °C afforded a dimeric species, [V(μ₂-NAd)(CH₂SiMe₃)₂]₂ (25) determined by X-ray crystallography (Fig. 6). However, the reaction of 10 with (more bulky and stable) 1,3-di-tert-butylimidazole-2-ylidene did not show any changes in the ⁵¹V NMR spectra even at 80 °C for 3 days. The fact would thus suggest that this NHC stabilises 10 probably by weak coordination but did not promote α-hydrogen elimination under these conditions.

Scheme 17 Reactions of (imido)vanadium(V)-trialkyl complexes.²⁸

Fig. 6 ORTEP drawing of 25. Thermal ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity.²⁸

The (adamantylimido)vanadium(V)-alkyl, alkylidene complex containing NHC (23) folds in a rather distorted tetrahedral geometry around vanadium (Fig. 7), and the V–C bond distances for the alkyl, the alkylidene and the NHC are 2.069(3), 1.829(3) and 2.172(2) Å, respectively; these values (alkyl, alkylidene) are similar to those in previous reports.^19,47 The V–C(NHC) distance is close to that in VOCl₃(IMes) (2.137 Å); the coordination seems likely as neutral carbene and a similar explanation would thus be possible. Ring-opening metathesis polymerisation of norbornene by 23 (and 24) took place, but the activities (at 80 °C in benzene) were lower than that of V(CHSiMe₃)(NAr)(N=C^tBu₂)(PMe₃),^19,43 and the activities at 25 °C were negligible.

Fig. 7 ORTEP drawing of 23. Thermal ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity.²⁸

The present approaches should be widely applied for the preparation of various early transition metal-alkylidene complexes. Moreover, the present alkyl-alkylidenes would also be promising intermediates for syntheses of not only the alkylidynes but also of the cationic alkylidenes or the alkylidene-containing series of anionic donor ligands. Further studies concerning their application as catalysts, including the effect of the adamantyl imido ligand are now underway.

Conclusions

In this account, recent examples of the syntheses and reactions of vanadium(V)-alkyl, and alkylidene complexes containing imido ligands have been summarised. In particular, unique promising characteristics, such as the synthesis of ultra high molecular weight polymers in coordination polymerisation, unique reactivity with alcohols (ROH) that proceed via intermediates formed by coordination of ROH, and olefin metathesis active vanadium(V)-alkylidene complexes that exhibit better thermal stability, have been introduced. On the basis of these efforts, we believe this subject will expand the possibilities for the establishment of more efficient/precise catalytic processes as well as the better understanding of organometallic chemistry with vanadium.

Acknowledgements

K.N. expresses his heartfelt thanks to former/present group members who contributed to this project as coauthors, and Mr Shohei Katao (Nara Institute of Science and Technology, NAIST) for his assistance in the crystallographic analysis. The research by K.N. was partly supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 19028047, “Chemistry of Concerto Catalysis”) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. W.Z. expresses her sincere thanks to JSPS for a postdoctoral fellowship (P06349). The authors also express their thanks to Dr Shu Zhang (NAIST) for help in the preparation of this manuscript.

Notes and references

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15. For examples, see: (a) R. R. Schrock, Acc. Chem. Res., 1990, 23, 158; (b) R. R. Schrock, in Alkene Metathesis in Organic Synthesis, ed. A. Fürstner, Springer, Berlin, Germany, 1998, p. 1.
16. For examples, see: (a) T. M. Trnka and R. H. Grubbs, Acc. Chem. Res., 2001, 34, 18; (b) S. T. Nguyen, T. M. Trnka, in Handbook of Metathesis, ed. R. H. Grubbs, Wiley-VCH, Weinheim, 2003, vol. 3, p. 61.
17. For examples, see: (a) R. R. Schrock, Acc. Chem. Res., 1979, 12, 98; (b) R. R. Schrock, Chem. Rev., 2002, 102, 145; (c) R. R. Schrock, in Handbook of Metathesis, ed. R. H. Grubbs, Wiley-VCH, Weinheim, 2003, vol. 1, p. 8; (d) D. Mindiola, Acc. Chem. Res., 2006, 39, 813; (e) D. Mindiola, B. Bailey and F. Basuli, Eur. J. Inorg. Chem., 2006, 3135; (f) R. R. Schrock, Chem. Rev., 2009, 109, 3211.
18. (a) K. Nomura, A. Sagara and Y. Imanishi, Macromolecules, 2002, 35, 1583; (b) W. Wang, J. Yamada, M. Fujiki and K. Nomura, Catal. Commun., 2003, 4, 159.
19. J. Yamada, M. Fujiki and K. Nomura, Organometallics, 2005, 24, 2248.
20. Y. Onishi, S. Katao, M. Fujiki and K. Nomura, Organometallics, 2008, 27, 2590.
21. S. Zhang, S. Katao, H. W,-Sun and K. Nomura, Organometallics, 2009, 28, 5925.
22. W. Zhang and K. Nomura, Inorg. Chem., 2008, 47, 6482.
23. (a) W. Wang and K. Nomura, Macromolecules, 2005, 38, 5905; (b) W. Wang and K. Nomura, Adv. Synth. Catal., 2006, 348, 743.
24. K. Nomura, W. Wang and J. Yamada, Stud. Surf. Sci. Catal., 2006, 161, 123.
25. For example D. L. Christman, J. Polym. Sci., Part A-1, 1972, 10, 471.
26. For example (review): A. Macchioni, Chem. Rev., 2005, 105, 2039.
27. K. Nomura, Y. Onishi, M. Fujiki and J. Yamada, Organometallics, 2008, 27, 3818.
28. W. Zhang and K. Nomura, Organometallics, 2008, 27, 6400.
29. The data including syntheses of V(N-4-MeC₆H₄)Cl(CH₂SiMe₃) and V(N-4-MeC₆H₄)Cl₂(CH₂SiMe₃) are shown in ref. 13b. Calculation chemistry concerning the effect of the imido substituent in V(NR)Me₃ is as follows: M. Bühl, Organometallics, 1999, 18, 4894.
30. F. Preuss, G. Overhoff, H. Becker, H. J. Häusler, W. Frank and G. Reiss, Z. Anorg. Allg. Chem., 1993, 619, 1827.
31. For example (reviews): (a) R. F. Jordan, Adv. Organomet. Chem., 1991, 32, 325; (b) H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger and R. M. Waymouth, Angew. Chem., Int. Ed. Engl., 1995, 34, 1143; (c) E. Y-. X. Chen and T. J. Marks, Chem. Rev., 2000, 100, 1391.
32. (a) T. V. Lubben, P. T. Wolczanski and G. D. van Duyne, Organometallics, 1984, 3, 977 . In this report, the reaction with Zr(CH₂Ph)₄ with 1.14 equiv. of ^tBu₃COH in benzene under reflux conditions for 7 h afforded Zr(CH₂Ph)₃(OC^tBu₃), whereas synthesis of Zr(CH₂Ph)₃(OC^tBu₃) by the reaction of Zr(CH₂^tBu)₄ with ^tBu₃COH in benzene required 30 h at 93–95 °C; (b) it is also known that certain chromium(IV)-tetra(alkyl)s are stable even in alcohols under reflux conditions.
33. For related reviewing articles, see: (a) C. Copéret, M. Chavanas, R. P. Saint-Arroman and J. M. Basset, Angew. Chem., Int. Ed., 2003, 42, 156; (b) J. M. Thomas, R. Raja and D. W. Lewis, Angew. Chem., Int. Ed., 2005, 44, 6456; (c) for an example of a proposed mechanism for the reaction of Cr(CH₂^tBu)₄ with a silica surface, see: J. A. N. Ajou and S. L. Scott, Organometallics, 1997, 16, 86.
34. For recent examples, see: (a) P. Nicholas, H. S. Ahn and T. J. Marks, J. Am. Chem. Soc., 2003, 125, 4325; (b) M. W. McKittrick and C. W. Jones, J. Am. Chem. Soc., 2004, 126, 3052; (c) B. Rhers, A. Salameh, A. Baudouin, E. A. Quadrelli, M. Taoufik, C. Copéret, F. Lefebvre, J. -M. Basset, X. Solans-Monfort, O. Eisenstein, W. W. Lukens, L. P. H. Lopez, A. Sinha and R. R. Schrock, Organometallics, 2006, 25, 3554; (d) F. Blanc, J.-M. Basset, C. Copéret, A. Sinha, Z. J. Tonzetich, R. R. Schrock, X. Solans-Monfort, E. Clot, O. Eisenstein, A. Lesage and L. Emsley, J. Am. Chem. Soc., 2008, 130, 5886.
35. J. Yamada and K. Nomura, Organometallics, 2005, 24, 3621.
36. J. Yamada, M. Fujiki and K. Nomura, Organometallics, 2007, 26, 2579.
37. A. Sinha, L. P. H. Lopez, R. R. Schrock, A. S. Hock and P. Müller, Organometallics, 2006, 25, 1412.
38. W. Zhang, S. Katao, W.-H. Sun and K. Nomura, Organometallics, 2009, 28, 1558.
39. Examples for isolation of terminal vanadium(III),(IV) alkylidenes: (a) ref. 11c ; (b) B. Hessen, J.-K. F. Buijink, A. Meetsma, J. H. Teuben, G. Helgesson, M. Hakansson, S. Jagner and A. L. Spek, Organometallics, 1993, 12, 2268 . Reaction of CpV(CHCMe₃)(dmpe) with norbornene was attempted but showed extremely low (catalytic) activity (TON 0.92 after 96 h at 20 °C). No GPC data were described; (c) F. Basuli, U. J. Kilgore, X. Hu, K. Meyer, M. Pink, J. C. Huffman and D. J. Mindiola, Angew. Chem., Int. Ed., 2004, 43, 3156; (d) F. Basuli, B. C. Bailey, J. C. Huffman, M.-H. Baik and D. J. Mindiola, J. Am. Chem. Soc., 2004, 126, 1924. Synthesis of vanadium(IV)-phosphinidene complex from the vanadium(IV)-alkylidene; (e) F. Basuli, B. C. Bailey, D. Brown, J. Tomaszewski, J. C. Huffman, M.-H. Baik and D. J. Mindiola, J. Am. Chem. Soc., 2004, 126, 10506. Synthesis of vanadium(V)-alkylidyne via oxidatively induced α-hydrogen abstraction from vanadium(IV)-alkyl, alkylidene complex.
40. Related isolated examples for vanadium-carbene complexes: (a) G. Erker, R. Lecht, R. Schlund, K. Angermund and C. Krüger, Angew. Chem., Int. Ed. Engl., 1987, 26, 666; (b) R. Milczarek, W. Rüsseler, P. Binger, K. Jonas, K. Angermund, C. Kruger and M. Regitz, Angew. Chem., Int. Ed. Engl., 1987, 26, 908.
41. (a) B. Hessen, A. Meetsma, F. van Bolhuis and J. H. Teuben, Organometallics, 1990, 9, 1925; (b) J.-K. F. Buijink, J. H. Teuben, H. Kooijman and A. L. Spek, Organometallics, 1994, 13, 2922. No reaction between CpV(CHPh)(NAr)(PMe₃) and norbornene or acetone was observed (c) M. Moore, S. Gambarotta, G. Yap, L. M. Liable-Sands and A. L. Rheingold, Chem. Commun., 1997, 643.
42. (a) U. J. Kilgore, C. A. Sengelaub, M. Pink, A. R. Fout and D. J. Mindiola, Angew. Chem., Int. Ed., 2008, 47, 3769; (b) U. J. Kilgore, C. A. Sengelaub, H. Fan, J. Tomaszewski, J. A. Karty, M.-H. Baik and D. J. Mindiola, Organometallics, 2009, 28, 843.
43. W. Zhang, J. Yamada and K. Nomura, Organometallics, 2008, 27, 5353.
44. K. Nomura, T. Atsumi, M. Fujiki and J. Yamada, J. Mol. Catal. A: Chem., 2007, 275, 1.
45. S. Zhang, K. Nomura, unpublished results. Formation of the phenyl complex was confirmed by the crystallographic analysis. These results will be submitted soon.
46. For example: (a) N-Heterocyclic Carbenes in Synthesis, ed. S. P. Nolan, Wiley-VCH, Weinheim, Germany, 2006; (b) N-Heterocyclic Carbenes in Transition Metal Catalysis, ed. F. Glorius, Springer, Berlin, Germany, 2007; (c) D. Bourissou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem. Rev., 2000, 100, 39; (d) D. Enders, O. Niemeier and A. Henseler, Chem. Rev., 2007, 107, 5605.
47. C. D. Abernethy, G. M. Codd, M. D. Spicer and M. K. Taylor, J. Am. Chem. Soc., 2003, 125, 1128.

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