Activation of propargylic alcohols by dimolybdenum tris(μ-thiolate) complexes: Influence of the substituents R in HC{triple bond, long}CCR2(OH)-vinylidene/allenylidene transformation. Reactivity of allenylidene complexes

Available online at www.sciencedirect.com

Journal of Organometallic Chemistry 692 (2007) 5351–5367

www.elsevier.com/locate/jorganchem

Activation of propargylic alcohols by dimolybdenum

tris(l-thiolate) complexes: Inﬂuence of the substituents

R in HC„CCR₂(OH)-vinylidene/allenylidene transformation.

Reactivity of allenylidene complexes

a

a,

*

´

Wilfried-Solo Ojo , Jean-Franc¸ois Capon , Alan Le Goﬀ , Franc¸ois Y. Petillon

,

a,

a

b,

*

Philippe Schollhammer ^*, Jean Talarmin , Kenneth W. Muir

a

UMR CNRS 6521, Chimie, Electrochimie Mole´culaires et Chimie Analytique, UFR Sciences et Techniques,

Universite´ de Bretagne Occidentale, CS 93837, 29238 Brest-Cedex 3, France

b

Chemistry Department, University of Glasgow, Glasgow G12 8QQ, UK

Received 4 July 2007; received in revised form 12 August 2007; accepted 13 August 2007

Available online 28 August 2007

Abstract

Reaction of the bis(nitrile) complex [Mo₂Cp₂(l-SMe)₃(NCMe)₂](BF₄) (1) with dimethylpropargylic alcohol, HC„CCMe₂(OH), at

room temperature in dichloromethane produced good yields of the l-alkynol species [Mo₂Cp₂(l-SMe)₃{l-CHCCMe₂(OH)}](BF₄)

(2a) through replacement of the two acetonitrile ligands in 1 by the alkynol. The NMR spectra of 2a indicate a l-g¹:g¹coordination

mode for the alkyne which is thereby incorporated into a dimetallacyclobutene ring like that found here by X-ray diﬀraction (XRD)

analysis of the related complex [Mo₂Cp₂(l-SMe)₃(l-g¹:g¹-CHCCO₂Me)](BPh₄) (2b). When 2a was stirred with Et₃N at room temper-

ature in dichloromethane, deprotonation gave high yields of the l-3-hydroxyalkynyl derivative [Mo₂Cp₂(l-SMe)₃{l-g¹:g²-

CCCMe₂(OH)}] (3), together with small amounts of the already-known vinylacetylide [Mo₂Cp₂(l-SMe)₃{l-g¹:g²-CCC(Me)CH₂}] (4)

resulting from dehydration of 3. Treatment of 3 with 1 equiv. of HBF₄Æ OEt₂in diethyl ether at room temperature gave the 3-hydrox-

yvinylidene derivative [Mo₂Cp₂(l-SMe)₃{l-g¹:g²-CCHCMe₂(OH)}](BF₄) (5) as the major product, together with other minor products

[Mo₂Cp₂(l-SMe)₃{l-g¹:g²-CCHC(Me)CH₂}](BF₄) (6), [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CCCMe₂)](BF₄) (7), [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-

CCH₂)](BF₄) (8), [Mo₂Cp₂(l-SMe)₃{l-g¹:g²-CCH(CHMe₂)}](BF₄) (9) and [Mo₂Cp₂(l-SMe)₃(l-O)](BF₄) (10). The vinylidene (6) and

allenylidene (7) species resulted from dehydration of the 3-hydroxyvinylidene complex 5 whereas the vinylidene derivative 8 was formed

by deketonisation of 5. When 3 reacted with a large excess of HBF₄Æ OEt₂in dichloromethane, the 3-isopropylvinylidene complex 9 was

obtained nearly quantatively via a H^Åradical process. When left for several days CD₂Cl₂solutions of 5 aﬀorded mainly the vinylidene

species 8 by deketonisation and the side-oxoproduct [Mo₂Cp₂(l-SMe)₃(l-O)](BF₄) (10) by hydrolysis or reaction with oxygen. Addition

of nucleophiles (H^ꢀ, OMe^ꢀ, OH^ꢀ, SMe^ꢀ) to the allenylidene complex [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CCCPh₂)](BF₄) (11) resulted in the

formation of the corresponding l-acetylide derivatives [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CCCRPh₂)] [R = H (12), OMe (16a), OH (17),

SMe (16b)], which by further reaction with tetraﬂuoroboric acid aﬀorded either the vinylidene species [Mo₂Cp₂(l-SMe)₃{l-g¹:g²-

CCH(CRPh₂)}](BF₄) when R = H (13), or the starting complex 11 when R is a leaving group (OMe). Reaction of 13 with Na(BH₄) gave

the l-alkylidyne complex [Mo₂Cp₂(l-SMe)₃(l-g¹-CCH₂CPh₂H)] (14) by nucleophilic attack of H^ꢀat the C_bcarbon atom of the vinyl-

idene chain. Proton addition at C_ain 14 led to the formation of a l-vinylidene compound 15 containing an agostic C–H bond. New

complexes have been characterised by elemental analyses and spectroscopic methods, supplemented for 2b and 3 by X-ray diﬀraction

studies.

*

Corresponding authors.

´

E-mail addresses: francois.petillon@univ-brest.fr (F.Y. Petillon),

schollha@univ-brest.fr (P. Schollhammer), ken@chem.gla.ac.uk (K.W.

Muir).

doi:10.1016/j.jorganchem.2007.08.023

5352

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

Keywords: Dimolybdenum complexes; Thiolato-bridged complexes; Propargylic-alcohol activation; l-Acetylide, l-3-hydroxyvinylidene, l-3-isopropyl, l-

alkylidyne, l-alkylidene species; a-Agostic

1. Introduction

nuclear [12] derivatives of Fe, Ru and Os. Examples involv-

ing binuclear compounds of other transition metals have

until now been scarce [13]. We recently [2d] reported the

activation of the propargylic alcohol HC„CCPh₂(OH)

by the bis(nitrile) complex [Mo₂Cp₂(l-SMe)₃(NCCH₃)₂]-

(BF₄) (1) to give the allenylidene compound [Mo₂Cp₂(l-

SMe)₃(l-g¹:g²-C@C@CPh₂)](BF₄) (11) via a l-alkynyl

derivative, implying a four step process. However, though

two of the three postulated intermediates were isolated,

the third, a hydroxyvinylidene species, proved elusive. It

is reasonable to expect that the activation of propargylic

alcohols HC„CCRR⁰(OH) will change if the R and R⁰

substituents are electron-releasing alkyl groups rather than

p-acceptors such as phenyl. Accordingly, we now describe

the activation of HC„CCMe₂(OH) by the tris(thiolate)

dimolybdenum derivative [Mo₂Cp₂(l-SMe)₃(NCMe)₂](BF₄)

(1) and compare the results with those previously obtained

with HC„CCPh₂(OH) and 1 [2d]. We have also consid-

ered the eﬀect of the R/R⁰groups of the alkynol on the

reactivity of the corresponding allenylidene complexes with

various nucleophiles.

The activation of propargylic alcohols by transition

metal complexes generally leads, either directly or via the

resulting hydroxyvinylidene compounds, to the formation

of allenylidene complexes. These may in turn undergo fur-

ther reactions [1]. The facile dehydration of the hydroxy-

vinylidene derivatives often prevents their isolation and

characterisation [2]; exceptions to this rule include those

which cannot be dehydrated to give an allenylidene [3]

and those which are particularly resistant to dehydration

[4]. It is well established that for aliphatic alkyn-1-ols, as

opposed to their aromatic analogues, dehydration of the

corresponding hydroxyvinylidene cations, {[M]@C_a@

C_bHC_cRR⁰(OH)}⁺, can proceed in two distinct ways: (i)

across the C_b–C_cbond to give an allenylidene derivative,

or (ii) across a C_c–C_dbond to aﬀord alkenylvinylidene

species. The precise factors which govern the mode of

dehydration of hydroxyvinylidene derivatives [5] are still

unclear. Moreover, it should be noted that dehydration,

which most of the time is spontaneous, is the characteristic

reaction of hydroxyvinylidene complexes; in contrast,

elimination of ketones, which requires harsh conditions,

has rarely been observed [6].

2. Results and discussion

Though the synthesis and properties of hydroxy- and

alkenyl-vinylidene derivatives [5,7] have attracted much

attention, even more has been focused on the formation

and reactivity of allenylidene complexes [8]. The rapid

development of the chemistry of allenylidene derivatives

is mainly the result of the presence in the carbon chain of

both electrophilic and nucleophilic sites. Many types of

reaction are thereby made feasible. Thus, since transition

metal allenylidene species are delocalised systems, one

may expect that their properties will critically depend on

the electronic and steric characteristics of both the substit-

uents of the C_ccarbon atom and of the ancillary ligands at

the metal atoms. Indeed, it has been shown in theoretical

and experimental studies [1b,8c,9] that the reactivity of

these derivatives is mainly governed by the electron-deﬁ-

cient character of the C_aand C_ccarbon atoms which are

potential sites for nucleophilic attack. Moreover, it has

been calculated that the respective charges on C_a, C_band

C_care ꢀ0.48, ꢀ0.02 and ꢀ0.19 in a typical l-g²-allenylid-

ene dimolybdenum species [10]; this suggests that in similar

complexes nucleophilic attack on C_aand C_cis expected to

be orbital-controlled, whereas electrophilic attack on C_a

would be charge-controlled.

2.1. Activation of HC„CCMe₂(OH) by the dimolybdenum

tris(thiolate) complex 1: multi-step syntheses of vinylidene

and allenylidene derivatives

Schemes 1 and 2 summarise the activation reactions of

dimethylpropargylic alcohol discussed here. The ﬁrst step

of the synthesis of vinylidene and allenylidene derivatives

Me

Cp

Mo

Me

S

(BF₄)

S

Cp

Mo

S

Me

(BF₄)

Me

H

S

C

Me₂(OH)C-C CH

N

S

C

Mo

Cp

Me

S

Me

- 2NCMe

Mo

Cp

Me

C

(a)

N

C

Me

HO

1

2a

+ Et₃N

- [Et₃NH](BF₄)

(b)

Me

S

Cp

Mo

Me

Cp

Mo

S

Me

S

Me

C

- H₂O

S

C

Mo

Cp

C

Me

S

Mo

Cp

C

Me

C

The activation of propargylic alcohols by transition

metal complexes to give allenylidene derivatives via

3-hydroxyalkynyl and 3-hydroxyvinylidene intermediates

is predominantly a reaction of mononuclear [1,11] or tri-

C

H

HO

H

4

3

Scheme 1.

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

5353

3

+ excess HBF₄.OEt₂

- H₂O

(in CH₂Cl₂)

(d)

(c)

+ 1 equiv. HBF₄.OEt₂

(in Et₂O)

Me

S

Me

S

Cp

Mo

(BF₄)

Cp

Mo

(BF₄)

S

Me

S

Me

C

S

Mo

H

Mo

Cp

H

Me

Cp

C

OH

Me

H

Me

5

9

-H₂O

O

C

- Me

Me

S

(BF₄)

Me

Cp

Mo

Me

S

(BF₄)

Cp

Mo

(BF₄)

Cp

Mo

S

Me

S

Me

S

Me

9

+

C

S

Mo

Cp

C

S

H

Mo

Cp

Me

Mo

Cp

Me

C

Me

C

H

C

H

Me

C

Me

H

C

H

Me

8

7

6

Scheme 2.

consists in reacting the bis(nitrile) complex 1 with

HC„CCMe₂(OH) in dichloromethane at room tempera-

ture to give compound 2a in 60% yield after work-up.

Crystals of 2a suitable for X-ray analysis could not be

obtained. Nevertheless, 2a has been formulated as the

electron-deﬁcient Mo^III-Mo^IIIspecies [Mo₂Cp₂(l-SMe)₃-

{HCCCMe₂(OH)}](BF₄) by spectroscopy and elemental

analysis (see Section 4) though these methods did not allow

the mode of coordination of the alkyne to be reliably estab-

lished. In an attempt to address this problem, we have

examined by X-ray analysis the structure of the related

complex [Mo₂Cp₂(l-SMe)₃(HCCCO₂Me)](BPh₄) (2b)

which we previously synthesised [14]. The structural results

for 2b are complicated by the disorder of the cation and

only brief discussion is appropriate.

Each atom of the 2b cation is distributed 2:1 over two

sites, A and B. The more populated A sites deﬁne the

structure shown in Fig. 1 in which the well-known

{Mo₂Cp₂(l-SMe)₃} unit is bridged by a bidentate, doubly

bent HC„C–CO₂Me group r-bonded to the Mo₂unit

through both carbon atoms [Mo–C 2.03(2) and

Fig. 1. View of the [Mo₂Cp₂(l-SMe)₃(l-g¹:g¹-HCCCO₂Me)]⁺cation in

crystals of 2b. Each atom of the cation is disordered over two sites; only

the major sites with occupancy 0.67 are shown here. Ellipsoids at the 20%

probability level are shown, except for hydrogen atoms. Selected distances

(A) and angles (ꢁ): Mo1–Mo2 2.705(7); Mo1–C4 2.08(2); Mo2–C5 2.03(2);

C4–C5 1.26(2); C5–C6 1.55(2); Mo–S 2.40(1)–2.48(1); C5–C4–Mo1,

˚

2.08(2) A]. The alkyne lies parallel to the metal–metal bond

111(1); C4–C5–C6, 122(1); C4–C5–Mo2, 110(1); C6–C5–Mo2, 129(1).

with a Mo1–C4–C5–Mo2 torsion angle of 2ꢁ. The bending

5354

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

back angle of the carboxy substituent C4–C5–C6 [122(1)ꢁ]

is typical of alkynes bound in this fashion [15]. The l-g¹,g¹

bridge is unusual for molybdenum and only one similar

complex has been the subject of a diﬀraction study,

namely syn-[Mo₂Cp₂(l-S)(l-SPrⁱ)₂(l-g¹,g¹-C₂Ph₂)] [16].

˚

Its Mo1–Mo2 distance of 2.684 A is similar to that of

˚

2.705(7) A in 2b; both are typical for quadruply bridged

Mo(III)₂Cp₂(l-SMe)₃moieties and indicate bonds of unit

order [17]. It is apparent from Fig. 1 that the methyl groups

of the thiolate bridges have an anti-orientation in 2b. The B

sites deﬁne an equivalent cation with the alternative anti

tris(thiolate) arrangement (see Supplementary material).

We consider that the X-ray study establishes 2b as a

l-g¹,g¹-complex and therefore formulate 2a as

[Mo₂Cp₂(l-SMe)₃(l-g¹:g¹-HCCCMe₂(OH))](BF₄), with a

parallel orientation of the C–C axis of the alkyne relative

to that of the Mo–Mo bond, on the basis of the similarity

of the spectroscopic data of 2a and 2b.

Treatment of the isolated complex 2a with triethylamine

in dichloromethane at room temperature produced depro-

tonation of the g¹:g¹-alkyne ligand to give the alkynyl

derivative 3 as the major product (ꢁ76.5% yield), together

with small amount of the vinylacetylide species 4. The lat-

ter complex resulted from slow dehydration of 3, possible

for a C(CH₃)₂C(OH) system with deprotonable methyl

groups (Scheme 1). Compounds 3 and 4 were inseparable

by chromatography. However, they have been character-

ised by NMR spectroscopy and the molecular structure

of 3 was established by X-ray analysis of dark green crys-

tals which separated from a cold diethylether solution of

a mixture of 3 and 4 in 5:1 ratio. Complex 4 was identiﬁed

as the vinylacetylide [{Mo₂Cp₂(l-SMe)₃(l-g¹-CCC(Me)-

Fig. 2. View of a molecule of complex 3 showing 20% probability

ellipsoids for non-hydrogen atoms. Selected distances (A), angles (ꢁ) and

˚

torsion angles (ꢁ): Mo1–Mo2 2.622(2); Mo1–C4 2.066(4); Mo2–C4

2.365(4); Mo2–C5 2.701(4); C4–C5 1.212(6); C5–C6 1.481(5); Mo–S

2.440(2)–2.486(2); C4–C5–C6 161.2(4); C5–C4–Mo1 163.2(3); C5–C4–

Mo2 92.4(3); C4–C5–Mo2 61.0(2); C6–C5–Mo2 135.0(3); S3–Mo1–Mo2–

C4 175.3(1); S1–Mo1–Mo2–S2 176.1(1); C4–Mo1–Mo2–C5 ꢀ3.2(2).

linearity but the Mo1–C4–C5 angle [163.2(3)ꢁ] is within

the range expected for M₂(l-g¹:g²-CCR) groups [18]. The

C4–C5–C6 and Mo2–C5–C6 angles in 3 are 161.2(4)ꢁ and

135.0(3)ꢁ, respectively. They diﬀer slightly from the corre-

sponding values for the related p-tolylacetylide species

of 168.5(3)ꢁ and 125.2(2)ꢁ, respectively [14]. Lastly, the

S-methyl groups again show the anti-arrangement also

found in 2b.

1

CH₂}] by comparison of its H NMR data with those of

an authentic sample which we had made previously by a

diﬀerent route [14]. The ¹³C–{¹H} NMR spectra in solution

for 3 were highly informative about the structure: they

showed the low-ﬁeld resonance for the a-carbon at

131.1 ppm and the b-carbon at higher ﬁeld (d 120.1 ppm),

as expected for acetylenic carbon atoms [18], as well as a

singlet at d 66.2 ppm attributable to the CMe₂(OH) carbon

atom [2d].

While mononuclear transition metal complexes contain-

ing both 3-hydroxyalkynyl and hydrido ligands rearrange

spontaneously to their 3-hydroxyvinylidene isomers [1m],

such a transformation needs to begin with addition of pro-

tons to the acetylide ligand in the case of 3. Thus, treatment

of 3 with 1 equiv. of HBF₄Æ OEt₂in diethylether at room

temperature gave the 3-hydroxyvinylidene derivative 5 as

the major product, together with minor products 6–10

[Scheme 2 – reaction (c) and Section 4]. Unfortunately,

we have not been able to separate these complexes by chro-

matography and all attempts to isolate them as crystalline

solids from the reaction mixture failed. Despite this, the

characterisation of these compounds can be carried out

on the basis of the NMR [¹H and ¹³C–{¹H}] and 2D

NMR data recorded on the reaction mixtures. Comparison

of these data with those of authentic samples allows us to

identify safely the minor products 6, 8 and 10 as

[Mo₂Cp₂(l-SMe)₃{l-g¹:g²-C@CH(CMe@CH₂)}](BF₄) [14],

The X-ray analysis of 3 conﬁrms the unsymmetrical side-

on coordination of the bridging acetylide ligand (Fig. 2).

˚

The Mo–Mo separation [2.622(2) A] is typical for quadru-

ply bridged Mo₂^IIICp₂(l-SMe)₃systems with single metal–

metal bonds [17]. The structure of 3 strongly resembles that

of the related p-tolylacetylide derivative [Mo₂Cp₂(l-

SMe)₃(l-g¹:g²-C„C–C₆H₄Me)] which we have described

previously [14]. Thus, the three-electron-donor acetylide

in 3 r-bonds terminally to Mo1 and p-bonds unsymmetri-

cally via its C4–C5 triple bond to Mo2. Valuable back-

donation from Mo1 is responsible for the shortening of

the Mo1–C4 bond to 2.066(4) A and the lengthening

[Mo₂Cp₂(l-SMe)₃(l-g¹:g²-C@CH₂)](BF₄)

[14],

and

˚

of the C4–C5 bond to 1.212(6) A. For comparison, in the

[Mo₂Cp₂(l-O)(l-SMe)₃](BF₄), respectively. It has been

assumed that 6 is mainly formed by dehydration of the

vinylidene product 5 (Scheme 2). However, it should be

noted that the hydroxyl-alkynyl compound 3 cannot be

p-tolylacetylide species the corresponding distances are

˚

2.068(3) and 1.238(4) A [14]. In 3, the coordination of

the sp-hybridised C4 carbon atom deviates slightly from

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

5355

ined six times under the same conditions and gave similar

results. In a typical reaction the thermal transformation

of a mixture containing 5 (65%), 6 (8.5%), 7 (7.0%), 8

(2.0%), 9 (7.0%), 10 (7.0%) and 2a (3.5%) was undertaken

in CD₂Cl₂and was monitored by the changes in the room

temperature ¹H NMR spectra with time. The most striking

feature was the dramatic decrease in the relative amount of

5 from 65.0 to 19.0%, balanced by sharp increases in the

amounts of 8 (2.0 ! 27.0%) and acetone present. Evidently

vinylidene 5 is transformed into vinylidene 8 by loss of a

molecule of acetone, as indicated in Scheme 2. However,

another important feature of the results was the noticeable

increase in the amount of oxo-compound 10 (7.0 ! 26%).

This could be explained if the vinylidene derivatives reacted

with oxygen or water present in the CD₂Cl₂solution.

Otherwise, the small amounts of complexes 6, 7 and 9 also

present in the starting mixture did not change much with

time (see Section 4), suggesting that these three vinylidene

and allenylidene species are rather stable in solution at

room temperature. Finally, it should be noted that very

small amounts of the l-alkyne derivative 2a and an unchar-

acterised cyclopentadienyl compound were formed in some

Me

S

Cp

Mo

(BF₄)

S

Me

O

S

Mo

Cp

Me

10

Chart 1.

obtained in pure form (see Section 4), being contamined

with traces of the enynyl derivative 4, therefore part

amounts of complex 6 could also result from direct proton-

ation of 4. Complex 10 is identiﬁed with an oxo-derivative,

that was previously obtained by another route and charac-

1

terised by elemental analyses and H NMR spectroscopy

[19] (Chart 1). This side product probably results from

the reaction of either 3 or 5 with traces of either dioxygen

or water in the solvent. A more clear way to give 10 would

involve a vinylidene p-alkyne tautomerization and subse-

quent displacement of the coordinated alkyne by oxygen;

however, such a mechanism is not ﬁrmly conﬁrmed,

because no terminal alkyne was detected in the reaction

1

media by H NMR spectroscopy. The minor product 9

experiments; the latter was detected only by its H–Cp sig-

was identiﬁed on the basis of Hand ¹³C NMR data as

nal at d 6.57. The most notable feature of the chemistry of

2-methyl-3-butyn-2-ol coordinated to two molybdenum

atoms evident from these reactions was its transformation

in solution in a few hours at room temperature into acetone

and a vinylidene ligand. The latter species is a tautomer of

the g²-acetylene ligand. Elimination of acetone involves a

highly reactive hydroxyvinylidene complex 5, which simul-

taneously undergoes a C_b–C_cbond cleavage and an inter-

nal proton addition to the nucleophilic C_batom of the

unsaturated chain. Elimination of a ketone in this way is

rare, and we know of only one similar example which

involves the reaction of Fe₃(CO)₁₂with internal propargy-

lic alcohols to give uncharacterised products [6]. Interest-

ingly, the reaction observed here is the reverse of the

synthetic route to propargylic alcohols based on the con-

densation of an alkyne with a ketone [22]. However, it is

worth noting that reactions giving rise to ketones from

C–C bond scissions in vinylidene or allenylidene ligands

are already known, though they involve C_a–C_brather than

C_b–C_ccleavages. For example, Cadierno and Gimeno have

demonstrated that 1,1-diphenyl-2-propyn-1-ol can be isom-

erised into 3,3-diphenyl-2-propenal by hydrolysis of the

C_a–C_bdouble bond of the allenylidene ligand of a mono-

nuclear cationic ruthenium complex, acting in this case as

an active catalyst [1i]. It has also been shown that hydroly-

sis of coordinated terminal alkynes leads to alkyl-carbonyl

ruthenium or iridium complexes according to a mechanism

which involves a metal-assisted C_a–C_bbond cleavage, pos-

tulated to occur via vinylidene intermediates [23].

1

[Mo₂Cp₂(l-SMe)₃{l-g¹:g²-C@CH(CHMe₂)}](BF₄); it will

be discussed below. It should be noted that a separate

experiment showed that compound 7 is formed in higher

yields through the reaction of 3 with HBF₄Æ OEt₂in

dichloromethane instead of diethyl ether (see Section 4).

The new vinylidene (5) and allenylidene (7) compounds

were identiﬁed by NMR spectroscopy. The vinylidene

bridge l-g¹:g²-C_a@C_bHR (R = C(OH)Me₂) was charac-

1

terised by the observation of a H resonance of 7.01 ppm,

which was assigned to the @C_bHR group, and of two

resonances in the ¹³C NMR spectrum at low ﬁeld in

the typical carbene (360.9 ppm) and vinyl (125.7 ppm)

ranges: these were assigned to C_aand C_b, respectively.

The 2D experiments revealed for 5 the expected correlation

(¹J_C–H= 167.5 Hz) of the vinylidene group at d 7.01 with

C_bat d 125.7, conﬁrming the proposed structure. The

¹³C–{¹H} NMR spectra of 7 contain three resonances at

low or very low ﬁeld, d 307.6, 170.0 and 139.2, in the typ-

ical allenylidene chain range, which were assigned to C_a, C_b

and C_c, respectively. The very low ﬁeld chemical shift

(d 307.6), attributed to the carbenoid carbon atom (C_a)

of 7, is indicative of an asymmetric side-on coordination

mode of the allenylidene group bridging a bimetallic core

([M]-[M]) for which a ¹³C_achemical shift is usually

observed in the range 310-282 ppm [13a,18b,20], and it

contrasts with the values of 206.5–173 ppm reported for

related end-on species [13b,21]. Finally, a single resonance

was observed for the two cyclopentadienyl groups in the

vinylidene and allenylidene compounds 5 and 7, suggesting

that windshield wiper ﬂuxionality occurs in solution at

room temperature [18b].

Unexpectedly, the 3-hydroxyvinylidene derivative 5 was

not obtained when the protonation of the l-acetylide 3 was

performed with a large excess of HBF₄Æ OEt₂. Instead, a

new complex was formed. Thus, 3 reacted with about 10

equiv. of tetraﬂuoroboric acid in dichloromethane to give,

The evolution over time of solutions containing the mix-

ture of products from reaction (c) (Scheme 2) was exam-

5356

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

after work up, substantial amounts of a red powder

[see Section 4; Scheme 2 – reaction (d)]. Solutions of this

powder in dichloromethane-d solution contained only

one product 9 detectable by ¹H NMR. Its structure is based

Additional correlations of the proton of the vinylidene

group at d 6.68 with signals for the Cc [at 31.25 (²J_C–H)]

and C_methyl[at d 25.3 (³J_C–H)] carbon atoms conﬁrm the

presence of an isopropyl group in 9. In parallel, the obser-

1

on elemental analysis, 1D H, ¹³C–{¹H}, and 2D NMR

vation of a septuplet at d 2.42 in the H NMR spectrum

experiments. ¹H–¹³C HMBC experiments reveal the

expected correlations (²J_C–H) of the protons of the methyl

groups at d 1.05 with Cc (d 31.25) and those of the proton

at d 2.42 with signals for the C_b[at d 123.4 (²J_C–H)] and C_a

[at d 365.05 (³J_C–H)] carbon atoms of a vinylidene ligand.

indicates coupling of a proton with two methyls, as

expected for a CH(CH₃)₂group. In addition, with the

aim of verifying further the selective formation of an iso-

1

propyl moiety, a H–¹H COSY showed a correlation of

3

the proton at d 6.67 with that at 2.42 via a J_H–H

[Mo

Mo]

C

OH

C

Me

3

H⁺

Mo]⁺

[Mo

C

H

C

-H₂O

C

OH

Me

5

Me

-H₂O

Mo]⁺

H

Mo]⁺

C

[Mo

Mo]

[Mo

+

C

+

C

Me

CH₂

Me

7

6

Pathway(II)

Pathway(I)

H addition at C_γ

H addition at C_β

[Mo

Mo]

[Mo

Mo]

C

H

C

H

C

+

C

Me

B

A

Dismutation

H addition at C_γ

Mo]⁺

+ 0.5

[Mo

+ 0.5

Mo]

C

H

C

H

C

+

C

Me

H

Me

7

9

etc..

Scheme 3. Proposed routes for the formation of 9 from the acetylide 3. [Mo–Mo] = CpMo(l-SMe)₃MoCp.

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

5357

(8.0 Hz) coupling. These data conﬁrm the 3-isopropylviny-

lidene structure proposed for 9 in Scheme 2.

[Mo

Mo]

C

We have shown above that the addition of 1 equiv. of

HBF₄Æ OEt₂to a diethyl ether solution of the l-acetylide

complex 3 led mainly to the 3-hydroxyvinylidene derivative

5 by protonation of the C_bcarbon atom of the acetylide

ligand, together with 6, 7, 8 and 9 as minor products

[Scheme 2 – reaction (c)]. It is obvious that 5 was the initial

product of this reaction, partially evolving during the tak-

ing of the ¹H NMR spectrum at room temperature in

dichloromethane-d into 6, 7, 8 and 9. Substitution of

dichloromethane for diethyl ether (see Section 4) favoured

production of the allenylidene 7 at the expense of 5 but the

yield of 9 barely changed. In contrast, 9 was the only prod-

uct detected by ¹H NMR when 3 reacted with a large excess

of tetraﬂuoroboric acid in dichloromethane [Scheme 2 –

reaction (d)]. Evidently, both the amount of acid added

and the solvating properties of CH₂Cl₂and Et₂O help to

determine whether 5 or 9 is the major product of these reac-

tions. While the route to the l-3-hydroxyvinylidene com-

plex 5 from the l-acetylide 3 is well established, the

processes which produce the l-3-isopropylvinylidene deriv-

ative 9 are less clear. We therefore decided to monitor reac-

tions (c) and (d) (Scheme 2) by cyclic voltammetry. This

revealed that for both reactions several steps are probably

involved. Separate experiments showed that the l-acetylide

compound 3 was readily consumed in these reactions.

Thus, the main feature of the CV was the presence in the

solution of two products whose potentials, E_1/2^red= ꢀ0.60

and ꢀ0.90 V, are consistent with those of l-acetylide and

l-vinylidene derivatives [24], respectively. Several attempts

to get X-ray grade crystals of the new l-acetylide com-

C

Me

C

CH₂

4

H⁺

Mo]⁺

H

[Mo

Mo]

+

C

+

C

Me

CH₂

Me

6

7

+ H^.

9

Scheme 4. Proposed route for the formation of 9 from the acetylide 4.

[Mo–Mo] = CpMo(l-SMe)₃MoCp.

To examine further the role of 7 as an intermediate we

reacted the l-vinylacetylide compound 4 [14] with an excess

of HBF₄Æ OEt₂in dichloromethane. As hoped, the l-iso-

propylvinylidene species 9 was obtained as the major prod-

uct, together with moderate yields of the l-vinylidene

compound 6 (see Section 4). The ﬁrst step of this reaction

(Scheme 4) reasonably involves the formation of 7 (in its

propargyl carbocation form) by direct protonation at

C_d. Its subsequent reaction with H^Åradicals leads to 9 via

the route proposed in pathways (I) and (II) of Scheme 3.

Compound 9 was not obtained quantitatively, presumably

because of competition with the proton addition at C_cin 4

to aﬀord the minor product 6. These results are consistent

with our hypothesis that the route to 9 is through 7, as sug-

gested in Scheme 3.

pound (E ^red= ꢀ0.60 V) from the reaction mixture were

1/2

unsuccessful. Despite this, it is possible (see Scheme 3) to

propose two reaction paths from 3 to 9 consistent with

both the CV observations and the known structures though

some of the elementary steps involved remain conjectural.

The initial step, common to both pathways, is dehydration

of the l-acetylide complex 3 promoted by proton addition,

to give directly or via the l-3-hydroxyvinylidene 5 the inter-

mediate 7, shown in Scheme 3 in both its propargyl carbo-

cation and cationic allenylidene forms. In solution, 7 is

weakly stable and, like other mononuclear acetylides [25],

readily adds H^Åradicals, either at the C_ccarbon atom to

give either intermediate A (pathway (I)) or at the C_bcarbon

atom to give intermediate B (pathway (II)). Intermediate A

is probably the l-acetylide species detected in cyclic

voltammetry experiments. It ﬁnally dismutates into the l-

3-isopropylvinylidene product 9 and the propargyl carbo-

cation form of 7 (pathway (I)). Alternatively, the radical

species B adds a second H^Åradical at the C_ccarbon atom,

ﬁnally giving 9 (pathway (II)). This mechanism is also sup-

ported by the characterisation of the side-product 6 in reac-

tion (c). Low yields of 9 in reaction (c) may be attributed

both to the high stability of 5 in solution and the competi-

tion between the dehydration and deketonisation

processes.

2.2. Reactivity of the diphenylallenylidene compound 11

One of the aims of the present work was to see how dif-

ferent substituents aﬀect reactions of allenylidenes with

electrophiles and nucleophiles. The low stability of solu-

tions of the dimethylallenylidene derivative 7 has unhap-

pily precluded a complete study. Therefore, we mainly

consider here the reactivity of the diphenylallenylidene

complex 11, previously isolated as a pure derivative in

the reaction of the propargylic alcohol, HC„CCPh₂(OH),

with the bis(nitrile) complex [Mo₂Cp₂(l-SMe)₃(NCCH₃)₂]-

(BF₄) (1) [2d].

In an attempt to exploit the electron-deﬁcient character

of the C_aand C_ccarbon atoms of the unsaturated allenylid-

ene chain we added methanol to the l-diphenylallenylidene

compound 11, but no reaction was observed. Evidently,

nucleophilic attack of methanol on the electrophilic C_a

5358

Me

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

carbon atom did not occur, perhaps inhibited by the two

Cp

Mo

Me

S

(BF₄)

Ph

S

Cp

Mo

S

Me

C

phenyl substituents which tend to delocalise the positive

charge and thus to stabilise the allenylidene ligand see

Ref. [1o]. However, when 11 was treated with a strong

nucleophile such as sodium borohydride in acetonitrile,

good yields of the l-acetylide compound 12 were obtained

by selective addition of hydride at the electrophilic C_ccar-

bon atom [Reaction (e) in Scheme 5]. Similarly, 11 in dichlo-

romethane reacted with 1.5 equiv of Me₄N(OH) in MeOH

to give l-acetylide derivatives 16a (yield: 68.5%) and 17

(yield: 9%), together with low amounts of an oxochloro

by-product [Reaction (ij) in Scheme 6]. Formation of 16a

and 17 illustrates the ability of Me₄N(OH) in MeOH to

act as both a strong (OMe^ꢀ) and moderately strong

(OH^ꢀ) nucleophile, adding selectively to the electrophilic

C_callenylidene carbon atom. When sodium thiomethoxide

was added to a dichloromethane solution of 11, a mixture of

products inseparable by chromatography was obtained.

Despite this, the characterisation of a l-thioacetylide spe-

cies 16b, obtained in low yields (ꢁ27%), is possible from

¹H NMR data recorded on the reaction mixture [Reaction

(ii) in Scheme 6]. This result may suggest that the thiometh-

oxide anion is a weaker nucleophile than H^ꢀ, OMe^ꢀor

OH^ꢀ.

S

Me

+ Na(BH₄)

(e)

C

S

Mo

Cp

S

Me

Mo

Cp

C

Me

Ph

C

H

Ph

12

11

(f)

HBF₄.OEt₂

Me

S

Cp

Mo

Me

S

Cp

Mo

(BF₄)

S

Me

S

Me

+ Na(BH₄)

(g)

C

S

C

Mo

Cp

S

Me

Mo

Cp

H

CH₂

Me

C

H

Ph

14

HBF₄.OEt₂

13

(h)

Me

S

Cp

Mo

(BF₄)

S

Me

Products 12, 16a, 16b and 17 were characterised analyt-

ically and from multinuclear (¹H and ¹³C) NMR spectro-

scopic data given in Section 3. Complex 17 and the

oxochloro by-product obtained in reaction (ij) (Scheme 6)

C

CH₂CHPh₂

S

Mo

Cp

Me

H

15

1

were identiﬁed by comparing their H NMR spectra with

Scheme 5.

Me

S

Cp

Mo

S

Me

NaSMe

(ii)

C

S

Mo

Cp

C

Me

S

Ph

Cp

Mo

(BF₄)

Ph

C

S

Me

C

MeS

(i)

16b

C

S

Mo

Cp

Me

S

Me

S

Cp

Mo

Cp

Mo

C

S

Me

Me₄N(OH)

(ij)

S

Me

11

Ph

C

+

C

S

Mo

Cp

C

Mo

Cp

Me

Ph

C

Me

Ph

C

MeO

HO

16a

17

HBF₄.OEt₂

(BF₄)

(j)

Me

S

Cp

Mo

S

Me

C

S

- MeOH

Mo

Cp

H

Me

C

OMe

Ph

C

Scheme 6.

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

5359

those of authentic samples of [Mo₂Cp₂(l-SMe)₃{l-g¹:g²-

C„CCPh₂(OH)}] [2d] and [Mo₂Cp₂(O)(Cl)(l-SMe)₃]

[26]. The presence of the alkynyl ligand in 12 was

established from the ¹³C–{¹H} spectrum which has only

one resonance corresponding to the acetylenic carbon

atoms [18a,2d] at 113.45 ppm and a singlet at 49.0 ppm

two resonances at low ﬁeld in the carbene (365.55 ppm)

[27] and vinyl (118.3 ppm) ranges; they were assigned to

C_aand C_b, respectively, thus conﬁrming the structure

ascribed to this complex in Scheme 5. When [Mo₂Cp₂(l-

SMe)₃{l-CCCPh₂(OMe)}] (16a), where the alkynyl ligand

contains a good leaving group, reacted with 1 equiv. of

tetraﬂuoroboric acid in diethyl ether at room temperature,

11 was quantitatively obtained [Reaction (j) in Scheme 6]

instead of a vinylidene species. Compound 11 may be

formed by spontaneous loss of methanol from the unchar-

1

due to the CPh₂H. The close similarity of the H NMR

of 12 with those of 16a and 16b suggests that these three

compounds have similar geometries and that 16a and 16b

1

also contain alkynyl fragments. The H NMR spectra of

compounds 12, 16a and 16b diﬀer obviously by the reso-

nances attributable to the alkynyl substituents: 4.53 ppm

to CPh₂H in 12, 2.98 ppm to CPh₂(OMe) in (16a) and

acterised l-g¹:g²-methoxyvinylidene intermediate

C

(Scheme 6).

Reaction of 13 with an excess of Na(BH₄) in acetonitrile

at room temperature readily aﬀorded deep blue solutions

from which [Mo₂Cp₂(l-SMe)₃(l-CCH₂CPh₂H)] (14) was

isolated in good yield as a blue powder [Scheme 5 – reac-

tion (g) and Section 4]. The dinuclear complex 14 was

formed by a classical addition [28] of H^ꢀto the electro-

philic C_batom of the vinylidene C_a@C_bHCPh₂H bridge

in 13. It was characterised by elemental analysis and

NMR spectroscopy. Its ¹H NMR (C₆D₆) spectrum was

typical for derivatives with a {Mo₂Cp₂(l-SMe)₃} core; it

exhibited a single resonance at 5.22 ppm for the two cyclo-

pentadienyl ligands and three peaks for the three SMe

1

1.62 ppm to CPh₂(SMe) in 16b. The H NMR spectra of

12, 16a and 16b display only one Cp signal, indicating that

these complexes are ﬂuxional at room temperature.

Turning to allenylidene complexes with electron-releas-

ing substituents in their unsaturated chains, we have shown

[Section 2.1: Scheme 2 – reaction (d), and Scheme 3] that a

dichloromethane solution of the l-allenylidene complex

[Mo₂Cp₂(l-SMe)₃(l-g¹:g²-C@C@CMe₂)](BF₄) (7) under-

goes H^Åradical reaction in presence of an excess of

HBF₄Æ OEt₂. This involves hydrogen abstraction from

the solvent to give the l-isopropylvinylidene species 9. This

contrasts with the stabilisation of l-g¹:g²-(4e)(side-on)

allenylidene ligands by carbonyl groups in related di-

molybdenum and -tungsten complexes which add protons

to give propargylium cations [{M₂(CO)₄Cp₂}(l-g²-

HC₂CMe₂)]⁺(M = Mo, W) [10,18b] rather than the

isopropylvinylidene cations obtained here. These propargy-

lium cations are thought to be stabilised by the presence of

electron-donating substituents on the unsaturated chain

and by ancillary carbonyl ligands. In ﬁne, it should be

noted that two types of allenylidene complex are here

compared: the three l-thiolate donor ligands of 7 contrast

with the four p-acceptor carbonyl groups of the

[{M₂(CO)₄Cp₂}(l-g²-HC₂CMe₂)]⁺(M = Mo, W) com-

plexes [10,18b].

1

bridges [17]. Instead of the characteristic H NMR reso-

nance of the C_bHR group in 13 at d 7.37, two protons were

detected as a doublet (J_H–H= 8.0 Hz) between 4 and

5 ppm, conﬁrming that the hydride anion had added to

the outer carbon atom (C_b) of the vinylidene group to give

a new hydrocarbyl bridging compound. The ¹³C NMR

spectra of 14 displayed a typical l-alkylidyne signal at

extreme low ﬁeld (445.4 ppm) [29]. The equivalence of the

Cp groups suggests that the alkylidyne bridge is symmetri-

cally bonded to the two molybdenum atoms and that the

p-Mo–C overlap required by electron-counting rules, is

delocalised through the Mo–C–Mo bridge. A poor-quality

X-ray analysis of a single crystal of 14, obtained at room

temperature from a CH₂Cl₂solution layered with diethyl

ether, is consistent with these conclusions (see Supple-

mentary material). It shows a l-CCH₂CPh₂H group

symmetrically bridging the two molybdenum atoms of a

[Mo₂Cp₂{l-SMe)₃}] core through a single carbon atom.

Further reactions of the l-diphenylalkynyl derivative

12 with tetraﬂuoroboric acid allowed the isolation in high

yield of the stable l-vinylidene compound [Mo₂Cp₂(l-

SMe)₃(l-g¹:g²-C@CHCPh₂H)](BF₄) (13) [Reaction (f) in

Scheme 5 and Section 4]. Compound 13 was formed by

proton attack at the nucleophilic C_bcarbon atom of the

acetylide ligand. It should be noted that the correspond-

ing hydroxyvinylidene derivative, [Mo₂Cp₂(l-SMe)₃{l-

g¹:g²-C@CHCPh₂(OH)}](BF₄), was an elusive species

which readily dehydrated into allenylidene 11 [2d]. Com-

pound 13 was identiﬁed by analytical data and NMR

˚

The Mo–C distances [2.02(2) and 2.03(2) A] in 14 lie

˚

between the values of 1.894(5) A for the formally double

Mo@C bond in the vinylidene compound [Mo₂Cp₂(l-

1

2

˚

SMe)₃(l-g :g -C@CHTol)](BF₄) and 2.068(3) A for the

single Mo–C bond in the acetylide derivative [Mo₂Cp₂(l-

g¹:g²-C„CPh)] [14]. They indicate a symmetrical coordi-

nation of the alkylidene group through Mo–C bonds of

order 1.5. Several other l-alkylidyne-molybdenum species,

such as [Mo₂Cp₂(l-SMe)₃(l-CCH₂n-Pr)], [Mo₂Cp(l-SMe)₃-

{l-(g⁵-C₅H₄)(t-BuN)CN(t-Bu)C}] and [Mo₂Cp(l-SMe)₂-

{l-(g⁵-C₅H₄)(xylN)CN(xyl)C}{l-OCNRR⁰}] complexes,

have been structurally characterised [28,30]. The coordina-

tion of the bridging carbon atom in 14 resembles those of

the corresponding atoms in other l-alkylidyne bimetallic

complexes, for example [(C₅Me₅)WMe(l₂-CCH₃)]₂[31]

1

spectroscopy. The H NMR spectrum exhibited the two

sets of resonances expected for the {Mo₂Cp₂(l-SMe)₃}

core and for the l-g¹:g²-C_a@C_bHCPh₂H vinylidene

bridge. The latter gives rise to a multiplet between 7.45

and 7.33 ppm and two doublets at 7.37 and 4.66 (J_H–H

=

14.4 Hz) ppm; these were, respectively, assigned to the

two phenyl groups, the C_bHR group (R = CPh₂H) and

the CPh₂H alkyl group. The ¹³C NMR spectra showed

5360

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

and [Mo₂Cp₂(l-SMe)₃(l-CCH₂n-Pr)] [28] and the length of

the Mo–Mo single bond [2.578(2) A] is typical of the values

reported for other [Mo₂Cp₂(l-SMe)₃(l-X)] complexes [17].

The methyl groups of the thiolate bridges again adopt the

anti-orientation.

ꢀ4.78 ppm ascribed to a single hydrogen atom. This

high-ﬁeld shift strongly suggests the presence of a

{Ph₂CH₂C(l-H)Mo} backbone featuring an a-agostic

˚

1

interaction [33,34]. Comparison of the H and ¹³C NMR

data of 15 (see Section 4) with those of the closely related

compounds [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CHCH₂R)](BF₄)

(R = Tol, n-Pr) [28] supports this suggestion. Unfortu-

nately, several attempts to perform an X-ray analysis on

diﬀerent samples of crystals of 15, formed at room temper-

ature from a CH₂Cl₂solution layered with diethylether,

cannot be considered satisfactory (see Supplementary

material). The most that can be said is that the X-ray

results are consistent with the chemical and spectroscopic

evidence which indicates the formation of a [Mo₂Cp₂(l-

SMe)₃(l-CHCH₂CPh₂H)]⁺cation containing an asymmet-

rically bridging l-C carbon atom [Mo–C 2.02(2) and

We have previously shown that l-alkylidyne derivatives

[Mo₂Cp₂(l-SMe)₃(l-CCH₂R)] were readily protonated,

forming l-alkylidene species [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-

CHCH₂R)](BF₄) (R = Tol, n-Pr), which, according to

NMR evidence, exhibit a-agostic interactions [28]. A com-

puted structure of the l-alkylidene model [Mo₂Cp₂(l-

SH)₃(l-CHCH₂Tol)](BF₄) conﬁrmed the presence of an

a-agostic interaction [32]. Since this is yet to be conﬁrmed

by XRD analysis, we reacted 14 with 1 equiv. of tetraﬂuo-

roboric acid in diethyl ether at room temperature, obtain-

ing good yields of the desired l-alkylidene derivative 15

[Reaction (h) in Scheme 5 and Section 4]. Analytical data

and NMR spectroscopy clearly indicated that 15 arose

from a face-addition [33] of a proton to a Mo–C bond in

˚

2.29(2) A], in agreement with an agostic interaction. It

should be noted that the more common b-agostic structure

[35] is not adopted by 15, perhaps for steric reasons.

1

14. Thus, the H NMR spectrum displayed the resonances

expected for the phenyl groups, the {Mo₂Cp₂(l-SMe)₃}

core and the methylene group (at 4.20 ppm), as well as a

broad (unresolved triplet) high-ﬁeld resonance at

2.3. Electrochemistry of the allenylidene complex

[Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CCCPh₂)](BF₄) (11)

The cyclic voltammetry of 11 in CH₂Cl₂-[NBu₄][PF₆]

showed that the complex underwent two one-electron

red2

reductions steps with E_1/2^red1= ꢀ0.93 V and E_1/2

=

10

5

ꢀ1.32 V (Fig. 3a, Scheme 7), as well as two irreversible oxi-

dation peaks at E_p^ox1= 0.39 V and E_p^ox2= 0.99 V. Both

0

-5

35

30

25

20

15

10

5

-10

a

-15

-20

-25

-30

-35

b

-40

-45

0

-1.5

-1.0

-0.5

0.0

2

4

6

8

0

10

12

E / V

v (V.s^-1)

Fig. 3. Cyclic voltammetry of [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CCCPh₂)](BF₄)

(11) (ca. 0.6 mM) (a) in the absence of acid, and (b) after addition of 3

equiv. of CF₃CO₂H in CH₂Cl₂-[NBu₄][PF₆] (vitreous carbon electrode).

Fig. 4. Scan rate dependence of the current function [i_p^c/m^{1/2 red}

CH₂Cl₂-[NBu₄][PF₆] solution of 11 (ca. 0.3 mM) (a) (¤) in the absence

and (b) (n) in the presence of 1 equiv. of HBF₄Æ OEt₂.

]

for a

Me

Cp

Me

+

Me

S

-

Cp

Mo

Cp

Mo

S

Me

C

S

e^-

S

Me

C

S

Me

C

S

C

Mo

Cp

S

Me

S

Mo

Cp

Mo

Cp

Me

C

Ph

C

Ph

C

Ph

11

Ph

E_1/2^red1= -0.93 V

E_1/2^red2= -1.32 V

Scheme 7.

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

5361

Me

S

Cp

Mo

Cp

Mo

+

S

Me

S

Me

C

2e^-, H⁺

ECE

C

S

Mo

Cp

Mo

Cp

C

Me

Ph

Me

C

Ph

H

11

Ph

12

H⁺

Me

S

Me

S

Cp

Mo

+

Mo

S

Me

S

Me

2e^-, H⁺

ECE

C

S

Mo

Cp

H

Mo

Cp

CH₂

Me

C

H

Ph

14

13

Scheme 8.

reduction steps are essentially electrochemically reversible,

of 11 and protonation at Cc. This is conﬁrmed by the fact

that 12 was the product of the chemical reaction of hydride

with 11 (see above). However, the amount of 12 formed

during the reaction showed that a consecutive protonation,

occurring at the C_batom of 12, must be signiﬁcantly fast

and would produce the vinylidene species [Mo₂Cp₂(l-

SMe)₃{l-g¹:g²-CC(H)CPh₂H}](BF₄) (13). Previously, we

demonstrated that the electrochemical reduction in the

presence of acid of the [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-

CCPhH]⁺cation (at E_p= ꢀ1.1 V) gave quantitatively the

corresponding alkylidyne derivative [Mo₂Cp₂(l-SMe)₃(l-

CCH₂Ph)] by a two-electron ECE process [24]. Therefore,

the rapid reaction of the l-acetylide complex 12 with acid

accounted for the formation of 13, which was reduced at

the potential of the cathode (E_q= 1.1 V) to give the l-alkyli-

dyne species [Mo₂Cp₂(l-SMe)₃(l-CCH₂CPh₂H)] (14) by

reaction with acid (Scheme 8). Theoretically, 3 equiv. of acid

and 4 F mol^ꢀ113 are needed to form 14 quantitatively, but

this was not experimentally observed. Some acid might be

lost during the electrolysis (3 h) by either oxidation or slow

reaction involving solvents. The electrolysis performed with

larger excesses of acid gave the l-alkylidyne complex as the

major product.

with DE_p= 100 mV at m = 0.2 V s^ꢀ1. However, measure-

a

c red

ments of the peak current ratio [(i_p/i_p^c)^red< 1, (i_p/i_p)

increased with increasing scan rate] indicated that the elec-

tron transfers were followed by chemical reactions (EC

process) whose products were detected by the appearance

of an oxidation peak around ꢀ0.6 V on the reverse scan.

Controlled-potential reduction of 11, carried out at a

potential of 100 mV negative relative to that of the ﬁrst

reduction process, was complete after the passage of ca.

1

1 F mol^ꢀ1. Cyclic voltammetry and H NMR of the cato-

lyte conﬁrmed the instability of the reduced form of 11

and the presence of an EC process. Unfortunately,

attempts to determine the nature of the products formed

upon electrochemical reduction of 11 failed.

Upon addition of acid (HBF₄Æ OEt₂or CF₃CO₂H) to a

CH₂Cl₂-[NBu₄][PF₆] solution of 11, CV showed that the

current of the ﬁrst reduction increases while that of the

second decreases, consistent with the occurrence of an

ECE process (Fig. 3b). Furthermore, the variations of the

current function i_p^red/m^1/2against m deviate markedly from

linearity at slow scan rates (Fig. 4b), again conﬁrming that

the reduction of 11 in the presence of acid occurred accord-

ing to an ECE process (Scheme 8).

Controlled-potential reduction of 11 in the presence

of 3 equiv. of CF₃CO₂H (3.3 F mol^ꢀ111) led to the forma-

tion of [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CCCPh₂H)] (12) and

[Mo₂Cp₂(l-SMe)₃(l-CCH₂CPh₂H)] (14) as the major

products (45% and 55%, respectively), as shown by the

¹H NMR spectrum of the solid residue separated from

the supporting electrolyte. During the electrolysis the dark

purple solution turned blue. Electrolysis performed in the

presence of 1 equiv. of HBF₄Æ OEt₂produced (14) (ca.

30%) and unidentiﬁed product (ca. 70%) after transfer of

ca. 3 F mol^ꢀ111.

3. Conclusion

In summary, the activation of aromatic alkyn-1-ols by

mononuclear transition metal complexes has generally

resulted in the selective formation of allenylidene com-

pounds by dehydration of 3-hydroxyvinylidene intermedi-

ates via successively the proposed g²-alkynol and

hydroxyalkynyl derivatives [1m]. Here, in contrast, the

activation of aliphatic alkyn-1-ols (e.g. HC„CCMe₂(OH))

with the bis(nitrile) dimolybdenum compound [Mo₂Cp₂(l-

SMe)₃(NCCH₃)₂](BF₄) (1) aﬀorded either the l-3-hydrox-

yvinylidene (5) or the l-3-isopropylvinylidene (9) derivatives

depending on experimental conditions. In these reactions,

The formation of [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-CCCPh₂H)]

(12) could be expected by both electrochemical reduction

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W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

the formation of 9 would imply an unprecedented radical

process. The route to the two vinylidene species 5 and 9

has implicated the l-g¹:g¹-alkynol-bridged complex 2a

and the l-hydroxyalkynyl 3 as successive intermediates;

both were isolated and fully characterised by NMR spec-

troscopy and X-ray analysis. Further evolution of solutions

of the l-3-hydroxyvinylidene complex 5 involved either

dehydration across the C_b–C_cor C_c–C_dbonds to give the

dimethylallenylidene (7) and alkenylvinylidene (6) deriva-

tives, respectively, or deketonisation aﬀording the vinyli-

dene 8.

Cyclic voltammetric experiments were performed with a

PGSTAT 12 driven by an AUTOLAB software. Con-

trolled-potential electrolyses were performed using Tacus-

sel/Radiometer 6CU potentiostat and IG5-N integrator.

The cell and electrodes were as described previously [37].

All the potentials (text, Table 1, ﬁgures) are quoted against

the ferrocene-ferrocenium couple; ferrocene was added as

an internal standard at the end of the experiments.

4.2. Synthesis of the l-alkyne 2a

The properties of allenylidene complexes are mainly

based on both the electron-deﬁcient and electron-rich char-

acters of the C_cor C_bcarbon atoms of the unsaturated

chain of the allenylidene ligand [1b,8c–10].We have now

demonstrated that both the substituents of the allenylidene

chain and the ancillary ligands in our complexes help to

direct the reaction. For example, protonation of allenylid-

ene complexes with electron-donating substituents in the

unsaturated chain involved either H^Åradical reaction in

the case of compound 7, or formation of propargylium cat-

ions when the dialkylallenylidene derivative is stabilised by

ancillary carbonyl ligands [10,18b]. When the allenylidene

complexes contain electron-withdrawing substituents in

their unsaturated chain, e.g. 11, they display typical reac-

tivity with nucleophiles: thus, addition of H^ꢀ, OMe^ꢀ,

OH^ꢀor SMe^ꢀto 11 gave the expected hydroxyalkynyl

derivatives 12, 16a, 17 or 16b. Further addition to these

isolated acetylide complexes of H⁺, H^ꢀand again H⁺suc-

cessively aﬀorded the new l-diphenylvinylidene (13), l-

alkylidyne (14) and a-agostic l-alkylidene (15) derivatives.

Finally, it should be noted that the instability of solutions

of the l-dimethylallenylidene 7 makes it diﬃcult to com-

pare its reactivity with the l–diphenylallenylidene analogue

11.

A solution of complex 1 (200 mg, 0.316 mmol) in

CH₂Cl₂(20 mL) was stirred in the presence of 1 equiv. of

HC„CCMe₂(OH) (30 lL) for 50 min at room tempera-

ture. The solution turned from red to green. The solvent

was then concentrated, and diethyl ether (30 mL) was

added. A green solid precipitated, was collected by ﬁltra-

tion, was washed with pentane (2 · 15 mL) and then was

dried under vacuum. Compound 2a was obtained as a

1

green powder (120 mg) in 60% isolated yield. H NMR

[(CD₃)₂CO]: d 13.30 (s, 1H, HC@C), 6.74 (s, 10H, C₅H₅),

4.37 (s, 1H, OH), 2.26, 2.07 and 1.91 (s, 3H, SCH₃), 0.96

(s, 6H, CH₃). Anal. Calc. for C₁₈H₂₇BF₄Mo₂OS₃: C,

34.1; H, 4.2. Found: C, 33.8; H, 3.8%.

4.3. Synthesis of the l-acetylide 3

A solution of 2a (140 mg, 0.22 mmol) in dichlorometh-

ane (10 mL) was stirred in the presence of 2 equiv. of

Et₃N (62 lL) for 15 min at room temperature. The colour

of the solution turned from pale green to dark green. The

Table 1

Crystallographic data for complexes 2b and 3

Compound

2b

3

Empirical formula

Formula weight

Temperature (K)

Crystal system

Space group

C

866.62

170

Monoclinic

P2₁/c

11.1944 (16)

9.9876(6)

34.0784(14)

₄₁H₄₃BMo₂O₂S₃

C₁₈H₂₆Mo₂OS₃

546.45

295

4. Experimental

4.1. General procedures

Triclinic

ꢀ

P1

˚

a (A)

7.854 (3)

8.416(6)

16.0643(12)

93.08(2)

98.02(2)

104.10(5))

1015.6(8)

2

1.787

1.546

0.16 · 0.12 · 0.08

2.8–26.3

4983

0.019

4110/223

0.0363

0.0460

0.104

1.038

1.02, ꢀ1.12

All reactions were carried out using standard Schlenk

techniques under an inert atmosphere. Solvents were deox-

ygenated and dried according to standard procedures. The

starting materials [Mo₂Cp₂(l-SMe)₃(NCCH₃)₂](BF₄) (1)

[36] and [Mo₂Cp₂(l-SMe)₃(l-g¹:g²-C@C@CPh₂)](BF₄)

(11) [2d] were prepared as described previously. All other

reagents were purchased commercially. Elemental analyses

were obtained from the Service de Microanalyse I.C.S.N.,

Gif sur Yvette (France). The NMR spectra (¹H, ¹³C) were

recorded at room temperature in CD₂Cl₂, C₆D₆, CDCl₃, or

(CD₃)₂CO solution on either a Bruker AC 300 or AMX3

400 and DRX 500 spectrometers and were referenced to

SiMe₄. ¹H–¹Hand ¹H–¹³C experiments were performed

on a Bruker DRX 500 spectrometer. Column chromatog-

raphy was carried out with silica gel.

˚

b (A)

˚

c (A)

a (ꢁ)

b (ꢁ)

c (ꢁ)

97.669(5)

3

˚

V (A )

3776.1(3)

4

1.524

Z

q_calc(Mg mm^ꢀ3

)

l (mm^ꢀ1

)

0.865

Crystal size (mm)

Range of h (ꢁ)

Reﬂections measured

R_int

0.55 · 0.40 · 0.02

2.9–25.2

23460

0.040

5867/535

0.081

0.089

0.164

1.096

1.41, ꢀ1.34

Unique data/parameters

R₁[I > 2r(I)]

R₁(all data)

wR₂(all data)

Goodness-of-ﬁt on F²

The preparation and the puriﬁcation of the [NBu₄][PF₆]

supporting electrolyte were as described previously [37].

ꢀ3

˚

Dq_max, Dq_min(e A

)

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

5363

1

solvent was then removed under vacuum and organometal-

lic products were extracted with diethyl ether (2 · 15 mL).

The diethyl ether was removed and the crude products

(120 mg) were analysed in C₆D₆by ¹H NMR spectroscopy.

Compounds 3 and 4, and two uncharacterised by-products

were, respectively, formed in about the 10:2:1 ratio. On the

basis of ¹H NMR spectra of the mixture the yields of 3 and

4 were estimated at 76.5% and 15.5%, respectively.

Attempts to separate cleanly the complexes by chromatog-

raphy failed, only the by-products were eliminated from

the mixture of products. For this reason, no elemental

analysis is available for 3. However, the major product 3

has been fully characterised by X-ray analysis of dark green

crystals of this complex, which were picked from a cold

(ꢀ20 ꢁC) diethyl ether solution of a mixture of 3 and 4 in

a 5:1 ratio. Compound 4 has been characterised by com-

experiments. Compound 5: H NMR (CD₂Cl₂): d 7.01 (s,

1H, @CH), 6.21 (s, 10H, C₅H₅), 2.72 (s.br, 1H, OH),

1.86, 1.56 and 1.49 (s, 3H, SCH₃), 1.29 (s, 6H, CH₃).

NMR ¹³C–{¹H} (CDCl₃): d 360.9 (Mo@C@C), 125.7

(@CH), 97.4 (C₅H₅), 76.4 (C(OH)Me₂), 33.0 (C(OH)Me₂),

18.6, 10.5 and 6.7 (SCH₃).

Compound 7: ¹H NMR (CD₂Cl₂): d 6.16 (s, 10H, C₅H₅),

2.36 (s, 6H, CH₃), 1.92, 1.61 and 1.42 (s, 3H, SCH₃). ¹³C–

{¹H} NMR (CD₂Cl₂): d 307.6 (Mo@C@C@C), 170.0

(Mo@C@C@C), 139.2 (Mo@C@C@C), 96.8 (C₅H₅), 29.6

(@CMe₂), 22.2, 11.7 and 8.9 (SCH₃).

4.5. Reaction of 3 with HBF₄Æ OEt₂(1 equiv.) in

dichloromethane

In a similar manner, 1 equiv. of HBF₄Æ OEt₂(14 lL)

was added to a solution of 3 (58 mg, 0.10 mmol) in dichlo-

romethane (5 mL) instead of diethyl ether. The mixture

was then stirred for 5 min, at room temperature. A greenish

solid was precipitated by addition of diethyl ether (20 mL)

and was collected by ﬁltration and washed with diethyl

1

parison of its H NMR data with those of an authentic

sample [14]. Compound 3: ¹H NMR (C₆D₆): d 5.32 (s,

10H, C₅H₅), 1.66, 1.61 and 1.51 (s, 3H, SCH₃), 1.37 (s,

6H, CH₃); OH is not observed. ¹³C–{¹H} NMR (C₆D₆):

d 131.1 (Mo–C„C), 120.1 (Mo–C„C), 91.0 (C₅H₅), 66.2

(C(OH)Me₂), 25.1 (C(OH)(CH₃)₂), 17.0, 12.8 and 11.8

(SCH₃).

1

ether (3 · 15 mL). The H NMR spectrum of acetone-d

solution of the resultant powder (36 mg) indicated the pres-

ence of four products 5, 6, 7,and 9, in a molar ratio of

5.5:1:3:1.

4.4. Reaction of 3 with HBF₄Æ OEt₂(1 equiv.) in diethyl

ether

4.6. Thermal evolution of solutions containing a mixture of

the products obtained in reaction (d)

In a typical experiment, 1 equiv. of HBF₄Æ OEt₂(27 lL)

in diethyl ether (20 mL) was added with stirring to a solu-

tion of 3 (110 mg, 0.2 mmol) in diethyl ether (5 mL) at

room temperature. A brick-red solid readily precipitated

from the solution and was collected by ﬁltration and

washed with diethyl ether (3 · 15 mL). The ¹H NMR spec-

trum of the resultant powder (129 mg) indicated the pres-

ence mainly of six products 5, 6, 7, 9, 10 and 8, in a

molar ratio of 9.5:1.25:1:1:1 and 0.25. Attempts to separate

cleanly the complexes by either chromatography or crystal-

lisation failed, and therefore no analytical data are avail-

able for the main new products 5 and 7. However, all the

In a typical experiment (see Section 4.4), a NMR tube

was charged with a mixture of 5 (65.0%), 6 (8.5%), 7

(7.0%), 8 (2.0%), 9 (7.0%), 10 (7.0%) and 2a (3.5%)

(15 mg) in CD₂Cl₂(0.5 mL). The evolution of the com-

pounds with time was monitored by ¹H NMR spectro-

scopy at room temperature. Formation of acetone was

observed by means of ¹H NMR spectroscopy in a few

hours (18 h), with concomitant severe decrease of 5

(19.0%) and noticeable increase of 8 (27.0%) and 10

1

(26.0%). On the basis of the H NMR spectrum measured

1

new compounds have been properly characterised by H

at this time, the yields of the other compounds 6, 7, 9 and

2a were estimated at 7.5%, 8.0%, 8.5% and 4.0%, respec-

tively. All these compounds were identiﬁed by comparison

and ¹³C NMR spectroscopy and by 2D NMR experiments

(¹H, ¹³C HMQC and HMBC). The other complexes in

CD₂Cl₂[6 and 8 [14], 9 (see below), and 10 [19]] have been

identiﬁed by comparison of their NMR data with those of

pure samples. The above experiment was performed six

times and was shown to be reproducible, except for moder-

ate to notable changes in the amounts of products 5, 6, 7, 8,

9 and 10 (+ small trace of the starting compound 2a),

which were formed in 56.0–74.0%, 5.0–9.0%, 6.5–24.0%,

0.0–9.0%, 2.0–9.0% and 0.0–6.5% (%: relative to all dia-

magnetic products) yields, respectively. Caution: the tetra-

ﬂuoroboric acid-diethyl ether complex used for the

experimental should be rigorously anhydrous. ¹³C–{¹H}

1

of their H NMR data with those of pure samples.

4.7. Reaction of 3 with an excess of HBF₄Æ OEt₂in

dichloromethane: synthesis of 9

Complex 3 (51 mg, 0.093 mmol) and a large excess of

HBF₄Æ OEt₂(10 drops) were stirred in dichloromethane

(5 mL) at room temperature for 5 min. After ﬁltration,

diethyl ether (15 mL) was added to the solution to precip-

itate a red brown powder (56 mg), that was washed twice

1

with cold pentane (2 · 15 mL) and analysed by H NMR

2

NMR and D NMR experiments were undertaken on a

spectroscopy, which indicated the presence of only one

NMR detectable product 9 in dichloromethane-d solution.

Elemental analysis of this powder gave satisfactory

data, consistent with 9. However, by monitoring the above

sample containing 5 and 7 as major products, i.e. 60%

and 24%, respectively. Full ¹³C NMR assignments of com-

pounds 5 and 7 were based on ¹H–¹³C HMQC and HMBC

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W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

reaction by electrochemistry, C.V. of the catholyte showed

the formation of two products in these experiments, consis-

tent with those of the vinylidene complex 9 and of an acet-

ylide derivative, undetected by NMR. For this reason, the

analytical results for 9 are not enclosed here, because we

cannot exclude with assurance the presence in the powder

of some amounts of a paramagnetic derivative (A) of for-

mula diﬀering from that of 9 by only one hydrogen unit.9:

¹H NMR (CD₂Cl₂): d 6.67 (d, J_H–H= 8.0 Hz, 1H, C@CH),

6.12 (s, 10H, C₅H₅), 2.41 (spt, 1H, CHMe₂), 1.90, 1.55 and

1.48 (s, 3H, SCH₃), 1.05 (d. br, J_H–H= 8.0 Hz, 6H, CH₃).

¹³C–{¹H} NMR (CD₂Cl₂): 365.05 (Mo@C@C), 123.6

(@CH), 97.1 (C₅H₅), 31.25 (CHMe₂), 25.3 (CH₃), 21.9,

10.35 and 6.35 (SCH₃).

powder readily precipitated. After ﬁltration, the solid was

washed with cold pentane (3 · 15 mL), aﬀording 13 in high

1

yields (81 mg, 84%). Compound 13: H NMR (CDCl₂): d

7.45–7.33 (m, 10H, C₆H₅), 7.37 (d, J_H–H= 14.4 Hz, 1H,

@CH), 5.94 (s, 10H, C₅H₅), 4.66 (d, J_H–H= 14.4 Hz, 1H,

CHCPh₂), 1.93, 1.63 and 1.56 (s, 3H, SCH₃). ¹³C–{¹H}

NMR (CD₂Cl₂): d 365.55 (C_a), 143.3 (C ipso Ph), 129.3

(C ortho Ph), 128.1(C meta Ph), 127.9 (C para Ph), 118.3

(C_b), 97.5 (C₅H₅), 47.4 (C_c), 15.3, 10.5 and 6.6 (SCH₃).

Anal. Calc. for C₂₈H₃₁BF₄Mo₂S₃: C, 45.3; H, 4.2. Found:

C, 45.4; H, 4.8%.

4.11. Synthesis of the l-alkylidyne 14

A solution of complex 13 (138 mg, 0.185 mmol) in ace-

tonitrile (20 mL) was stirred in the presence of 2 equiv. of

Na(BH₄) (28 mg) for 30 min, at room temperature. The

colour of the solution turned from green to blue. The sol-

vent was then removed and the residue was chromato-

graphed on silica gel. Elution with a mixture of CH₂Cl₂/

hexane (1:1.5) removed a blue band. Evaporation of the

volatiles aﬀorded compound 14 as an analytically pure blue

powder (75 mg, 61.5% yield). Crystals of 14, suitable for X-

ray analysis, were formed by crystallisation at room tem-

perature from a CH₂Cl₂solution layered with diethyl ether.

Compound 14: ¹H NMR (C₆D₆): d 7.34–6.99 (m, 10H,

C₆H₅), 5.22 (s, 10H, C₅H₅), 4.82 (d, J_H–H= 8.0 Hz, 2H,

CH₂), 4.32 (t, J_H–H= 8.0 Hz, 1H, CHPh₂), 1.94, 1.60 and

1.43 (s, 3H, SCH₃). NMR ¹³C–{¹H} (CDCl₃): d 445.4

(Mo₂C), 146.1 (C ipso Ph), 128.4 (C ortho Ph), 127.9 (C

meta Ph), 126.0 (C para Ph), 67.0 (CH₂), 50.4 (CHPh₂),

30.5, 8.1 and 6.9 (SCH₃). Anal. Calc. for C₂₈H₃₂Mo₂S₃:

C, 51.2; H, 4.9. Found: C, 51.6; H, 5.1%.

4.8. Reaction of the vinyl-acetylide 4 with HBF₄Æ OEt₂

(excess): formation of 6 and 9

A solution of a pure sample of 4 [14] (105 mg, 0.20 mmol)

in dichloromethane (10 mL) was stirred in the presence of

an excess of HBF₄Æ OEt₂(10 drops) for 10 min at room tem-

perature. The solvent was then partially removed under vac-

uum, the products were precipitated by addition of diethyl

ether (20 mL) from the solution and then were collected

by ﬁltration and washed with cold pentane (2 · 15 mL).

The crude products were analysed in (CD₂Cl₂) by ¹H

NMR spectroscopy which showed that 4 wholly disap-

peared, and 9 and 6 were formed in the 2.5:1 ratio.

4.9. Reaction of the allenylidene 11 with hydride: synthesis of

12

A

solution of compound 11 Æ CH₂Cl₂(162 mg,

0.196 mmol) in acetonitrile (15 mL) was stirred in the pres-

ence of 2 equiv. of Na(BH₄) (44 mg) for 20 min at room

temperature. The colour of the solution turned from violet

to green. The solvent was then removed under vacuum and

the organometallic products were extracted with diethyl

ether (3 · 15 mL). Evaporation of the volatiles aﬀorded

complex 12 as an analytically pure, green solid (90 mg,

70% yield). However, it should be noted that in some

experiments small amounts of the l-carbyne complex 14

were formed together with the main product 12. Com-

4.12. Synthesis of the l-alkylidene 15

To a blue solution of 14 (57.6 mg, 0.088 mmol) in diethyl

ether (5 mL) was added a drop of HBF₄Æ OEt₂diluted in

diethyl ether (20 mL), at room temperature. A greyish solid

precipitated immediately. After ﬁltration, the powder was

washed with diethyl ether (3 · 15 mL) and then with cold

pentane (2 · 15 mL), aﬀording 15 in moderate yields

1

(41 mg, 63%). Compound 15: H NMR (CD₂Cl₂): d 7.37–

7.18 (m, 10H, C₆H₅), 5.98 (s, 10H, C₅H₅), 4.20 (m, 2H,

CH₂), 3.72 (t, J_H–H= 8.0 Hz, CHPh₂), 1.99, 1.75 and 1.51

(s, 3H, SCH₃), ꢀ4.78 (s. br, 1H, MoC_aH). ¹³C–{¹H} NMR

(CD₂Cl₂): d lCH not observed, 142.4 (C ipso Ph), 129.6 (C

ortho Ph), 128.1 (C meta Ph), 127.8 (C para Ph), 99.3

(C₅H₅), 61.0 (C_bH₂), 51.5 (C_cHPh₂), 37.1, 13.3 and 10.7

(SCH₃). Anal. Calc. for C₂₈H₃₃BF₄Mo₂S₃: C, 45.2; H, 4.5.

Found: C, 45.2; H, 4.9%.

1

pound 12: H NMR (C₆D₆): d 7.43–6.94 (m 10H, C₆H₅),

5.08 (s, 10H, C₅H₅), 4.53 (s, 1H, CHPh₂), 1.65, 1.56 and

1.53 (s, 3H,SCH₃). ¹³C–{¹H} NMR (CDCl₃): d 142.9 (C

ipso Ph), 128.2 (C ortho Ph), 127.8 (C meta Ph), 126.5 (C

para Ph), 113.45 (C_b), 90.3 (C₅H₅), 49.0 (C_c), 16.5, 11.2

and 8.6 (SCH₃). Anal. Calc. for C₂₈H₃₀Mo₂S₃: C, 51.4;

H, 4.6. Found: C, 51.9; H, 5.1%.

4.10. Synthesis of the l-vinylidene 13

4.13. Reaction of the allenylidene 11 with Me₄N(OH):

formation of the acetylide 16a

To a green solution of 12 (85 mg, 0.13 mmol) in diethyl

ether (5 mL) was added a drop of HBF₄Æ OEt₂diluted in

diethyl ether (20 mL), at room temperature. A greenish

To a solution of compound 11 (100 mg, 0.13 mmol) in

dichloromethane (20 mL) was added 83 lL of tetrameth-

W.-S. Ojo et al. / Journal of Organometallic Chemistry 692 (2007) 5351–5367

5365

ylammonium hydroxide (1.5 equiv.) in methanol. The

4.16. X-ray structural determinations

mixture was stirred for 1 h at room temperature. The

solvent was then removed and products were extracted

twice with diethyl ether (2 · 15 mL) as a green powder

Measurements for compounds 2b, 14 and 15 were made

in Brest on a Oxford Diﬀraction X-Calibur-2 CCD diﬀrac-

tometer equipped with a jet cooler device. Those for 3 were

made in Glasgow on a Nonius CAD4 diﬀractometer.

Graphite-monochromated Mo Ka radiation (k = 0.71073)

was used in all experiments. The structures were solved

and reﬁned by standard procedures [38]. H atoms were

positioned using stereochemical considerations.

The crystal specimens for 14 and 15 were of poor quality

and gave only weak, low-resolution diﬀraction patterns.

Despite the eﬀorts made to obtain them, the results of

the diﬀraction analyses for 14 and 15 do not, in our view,

provide independent proof of the proposed structures,

though they are consistent with our chemical and spectro-

scopic experiments. They are available as Supplementary

material.

1

(73 mg), and were analysed in CDCl₃by H NMR spec-

troscopy. Compounds 16a, [Mo₂Cp₂(l-SMe)₃{l-g¹:g²-

C„CC(OH)Ph₂}] (17) and [Mo₂Cp₂(O)(Cl)(l-SMe)₃]

were, respectively, formed in the ratio 9.5:1.25:1. On

1

the basis of H NMR spectra of the mixture and assum-

ing that no paramagnetic compound was formed, the iso-

lated yields of 16a and 17 were estimated at 68.5% and

9.0%, respectively. Unfortunately, compounds 16a, 17

and the other oxochloro by-product cannot be separated

by chromatography, because these derivatives decom-

posed on the column (silica gel). In ﬁne, 16a was sepa-

rated from the other complexes by crystallisation in

cold dichloromethane-hexane. Compounds 17 [2d] and

[Mo₂Cp₂(O)(Cl)(l-SMe)₃] [26] were identiﬁed by compar-

ison of their ¹H NMR spectra with those of authentic

samples. Compound 16a: ¹H NMR (CDCl₃) d 7.81–

6.20 (m, 10H, C₆H₅), 5.28 (s, 10H, C₅H₅), 2.98 (s, 3H,

OCH₃), 1.60, 1.45 and 1.39 (s, 3H, SCH₃). Anal. Calc.

for C₂₉H₃₂Mo₂OS₃: C, 50.9; H, 4.7. Found: C, 51.3;

H, 5.1%.

For 2a and 3 selected bond lengths and angles are given

in the captions to Figs. 1 and 2 and crystal data in Table 1.

Acknowledgements

We are grateful to Dr F. Michaud for assistance on X-

ray crystallography, and to Dr R. Pichon and N. Kervarec

for recording two dimensional NMR spectra on a Bruker

DRX 500 (500 MHz) spectrometer. We thank the CNRS,

the EPSCR, and the University of Glasgow for ﬁnancial

4.14. Reaction of the acetylide 16a with proton

One equivalent of HBF₄Æ OEt₂(10 lL) in diethyl ether

(5 mL) was added with stirring to a green solution of 16a

(48 mg 0.07 mmol) in diethyl ether (10 mL) at room tem-

perature. A purple solid readily precipitated from the solu-

tion and was collected by ﬁltration and washed with cold

pentane. The allenylidene complex 11 was thus obtained

quantitatively as a purple powder and was characterised

`

´

support. The Ministere de l’Enseignement Superieur et de

la Recherche du Gabon is acknowledged for providing stu-

dentships (W.-S. Ojo).

Appendix A. Supplementary material

1

CCDC 652779 and 652780 contain the supplementary

crystallographic data for 2b and 3. These data can be

obtained free of charge via http://www.ccdc.cam.ac.uk/

conts/retrieving.html, or from the Cambridge Crystallo-

graphic Data Centre, 12 Union Road, Cambridge CB2

1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-

it@ccdc.cam.ac.uk. Also deposited as supplementary data

are molecular drawings and tables of crystallographic data,

atomic parameters and bond lengths and angles for 2b, 3,

14 and 15. Supplementary data associated with this article

can be found, in the online version, at doi:10.1016/

j.jorganchem.2007.08.023.

by comparison of its H NMR spectrum with that of an

authentic sample [2d].

4.15. Reaction of the allenylidene 11 with sodium

thiomethoxide: formation of the acetylide 16b

To

a purple solution of compound 11 (83 mg,

0.10 mmol) in dichloromethane (20 mL) was added an

excess of Na(SMe) (12 mg, 0.17 mmol). The mixture was

stirred for 1 h at room temperature, after which time the

solution became greenish. The solvent was then removed

under vacuum and the organometallic products were

extracted from the residue with diethyl ether (3 · 15 mL).

Evaporation of the volatiles aﬀorded greenish solids which

were washed with cold pentane (2 · 10 mL) and shown by

¹H NMR analyses to be a mixture of inseparable products

(41.2 mg) by chromatography. However, the major com-

pound was clearly identiﬁed as the l-acetylide complex

16b. On the basis of the ¹H NMR spectrum of the mixture,

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Article Doi

Activation of propargylic alcohols by dimolybdenum tris(μ-thiolate) complexes: Influence of the substituents R in HC{triple bond, long}CCR2(OH)-vinylidene/allenylidene transformation. Reactivity of allenylidene complexes

DOI: 10.1016/j.jorganchem.2007.08.023

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