A bridging side-on allenylidene dimolybdenum complex without carbonyl stabilization

Article abstract of DOI:10.1021/om060663a

The bis(nitrile) complex [Mo₂Cp₂(μ-SMe) ₃(NCCH₃)₂](BF₄) (1) reacts with HC≡CCPh₂(OH) to give the μ-alkyne product [Mo ₂Cp₂(μ-SMe)₃{HO≡CCPh ₂(OH)}](BF₄) (2). Sequential treatment with triethylamine and tetrafluoroboric acid converts 2 almost quantitatively, via the μ-alkynyl derivative 3, into [Mo₂Cp₂(μ-SMe)₃(μ- η¹:η²-C=C=CPh₂)] (BF₄) (4), the first example of a dinuclear μ-η¹:η²- allenylidene species without carbonyl ligands.

Full text of DOI:10.1021/om060663a

Organometallics 2006, 25, 5503-5505
5503
A Bridging Side-on Allenylidene Dimolybdenum Complex without
Carbonyl Stabilization
Wilfried-Solo Ojo,† Franc¸ois Y. Pe´tillon,*,† Philippe Schollhammer,*,† Jean Talarmin,† and
Kenneth W. Muir*,‡
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, and Chemistry Department,
UniVersity of Glasgow, Glasgow G12 8QQ, U.K.
ReceiVed July 24, 2006
Summary: The bis(nitrile) complex [Mo2Cp2(µ-SMe)3(NCCH3)2]-
(BF4) (1) reacts with HCtCCPh2(OH) to giVe the µ-alkyne
product [Mo2Cp2(µ-SMe)3{HCtCCPh2(OH)}](BF4) (2). Se-
quential treatment with triethylamine and tetrafluoroboric acid
conVerts 2 almost quantitatiVely, Via the µ-alkynyl deriVatiVe
3, into [Mo2Cp2(µ-SMe)3(µ-η1:η2-CdCdCPh2)] (BF4) (4), the
first example of a dinuclear µ-η1:η2-allenylidene species without
carbonyl ligands.
vast majority of known allenylidene complexes are mono-
nuclear.1 General methods of synthesis for binuclear or poly-
nuclear derivatives are unavailable, and consequently, there have
been only a few reports of such complexes.5,6 Two well-
established bridging modes have been found for allenylidenes
in binuclear complexes, the µ-η1:η1 (2e) (end-on)5a,c,e,g-i and
µ-η1:η2 (4e) (side-on)5b,d,f forms; however, there are also
examples of dinuclear complexes containing nonbridging alle-
nylidene ligands (2e).3a,7 All known dinuclear and polynuclear
complexes containing bridging allenylidene ligands also contain
carbonyl ligands;5,6 the only possible exception is [{Cp2ZrEt}2-
(µ-CdCdCMe2)], where one Zr atom appears to interact with
all three chain carbon atoms.5k Accordingly, in an attempt to
synthesize µ-allenylidene complexes which do not owe their
stability to the presence of carbonyl ligands, we have reacted
the bis(isonitrile) compound [Mo2Cp2(µ-SMe)3(NCCH3)2](BF4)
(1)8 with propargylic alcohols, followed by sequential treatment
with triethylamine and tetrafluoroboric acid. We now report that
Introduction
Recent interest1 in allenylidene complexes arises from their
importance in several developing areas of organometallic
chemistry. (i) They are involved in the growth of carbon chains
at metal surfaces.1b (ii) Their unsaturated carbon chains are
possible precursors of molecular wires or polymers which have
novel optical and electronic properties.1c,2 (iii) They can catalyze
synthetically useful organic reactions: for example, nucleophilic
substitution of propargylic alcohols3 and alkene metathesis.4 The
* To whom correspondence should be addressed. E-mail:
francois.petillon@univ-brest.fr (F.Y.P.); schollha@univ-brest.fr (P.S.);
ken@chem.gla.ac.uk (K.W.M.).
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Commun. 1999, 601. (c) Picquet, M.; Touchard, D.; Bruneau, C.; Dixneuf,
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† Universite´ de Bretagne Occidentale.
‡ University of Glasgow.
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10.1021/om060663a CCC: $33.50 © 2006 American Chemical Society
Publication on Web 09/22/2006

5504 Organometallics, Vol. 25, No. 22, 2006
Notes
acetylide derivative [Mo2Cp2(µ-SMe)3{µ-η1:η2-CtCCPh2-
(OH)}] (3) was isolated in moderate yield (Scheme 1). The
presence of the alkynyl ligand in 3 was established from both
the 13C{1H} NMR spectrum and the 1H-13C experiment, which
indicate the presence of only one resonance corresponding to
the acetylenic carbon atoms13 at 120.6 ppm and a singlet at 77.4
Scheme 1
1
ppm due to the CPh2(OH) carbon atom. H NMR spectra of 2
and 3 show only one Cp signal, which indicates that both
complexes are fluxional at room temperature. Addition of HBF4
to a solution of 3 in dichloromethane afforded exclusively the
µ-allenylidene complex [Mo2Cp2(µ-SMe)3(µ-η1:η2dCdCd
CPh2)](BF4) (4) (Scheme 1). Mechanistically, the allenylidene-
bridged derivative 4 most probably results from spontaneous
dehydration of the µ-η1:η2-hydroxyvinylidene intermediate A
(Scheme 1), generated by electrophilic attack of the proton of
HBF4 at the Câ carbon atom of the alkynyl complex 3. The 1H
NMR spectrum of 4 shows a single cyclopentadienyl resonance
at δ 6.00, which is integrated as 10 hydrogens. This is clearly
inconsistent with the solid-state structure established for this
complex (see below) and suggests that the molecule is fluxional
in solution at room temperature. This is in accord with the
observed line broadening of the Cp resonance when a dichlo-
romethane-d2 solution of 4 is cooled from 298 to 174 K. The
13C{1H} NMR spectrum contains a resonance at low field (δ
302.0) assignable to a carbenoid carbon atom (CR). This low-
field chemical shift is indicative of an asymmetric side-on
coordination mode of the allenylidene group bridging a bimetal-
lic core ([M]-[M]), for which a 13CR chemical shift range of
302-282 ppm has been observed in known µ-allenylidene (side-
on) dinuclear derivatives,5b,d,f and it is in contrast with the values
of 206.5-173 ppm reported for related end-on species.5a,c,e In
short, these spectroscopic data suggest that 4 has a highly
unsymmetrical structure in solution. Single-crystal X-ray analy-
sis confirms that the cation of 4 displays the unsymmetrical
side-on coordination of the allenylidene group (Figure 1),
bridging the metal-metal bond as a four-electron donor.
However, a typical disorder of the µ-SMe groups (occupancy
of minor sites 0.076(5)) affects the accuracy of the results. The
length of the Mo-Mo single bond in 4 (2.659(9) Å) is close to
the average value for [Mo2III(µ-SR)3] complexes (2.644 Å), and
the µ-Mo-S distances are also unexceptional.14 The short Mo2-
C4 distance (1.88(1) Å) formally represents a double bond;
comparable distances are found in related neutral side-on
allenylidene derivatives, such as [Mo2Cp2(CO)4(µ-η1:η2-CdCd
CMe2)] (1.912(3) Å)5f and [Mo2Cp2(CO)4(µ-η1:η2-CdCd
C6H10)] (1.90(1) Å),5d formally Mo(I) species, and in the cationic
vinylidene derivative [Mo2Cp2(µ-SMe)3(µ-η1:η2dCdCPh2)]-
(BF4) (1.894(5) Å).10a In comparison to µ-η1:η1 (2e) allenylidene
ligands, which are linear, the C3Ph2 fragment in 4 is kinked,
with C4-C5-C6 ) 144.0(1)°. The CR-Câ and Câ-Cγ
distances of 1.31(2) and 1.36(1) Å are comparable to those
observed in other side-on allenylidene complexes: i.e., 1.35(1)
and 1.33(1) Å in [Mo2Cp2(CO)4(µ-η1:η2-CdCdC6H10)]5d and
1.336(3) and 1.348(4) Å in [Mo2Cp2(CO)4(µ-η1:η2-CdCd
CMe2)].5f
the ultimate product of these reactions contains the desired C3
ligand bridging two molybdenum atoms in a µ-η1:η2 (4e)
manner.
Results and Discussion
Treatment of 1 with 1 equiv of HCtCCPh2(OH) in CH2Cl2
gave [Mo2Cp2(µ-SMe)3{HC2CPh2(OH)}](BF4) (2) as a brownish
green solid in 84% yield (Scheme 1). 2 is formulated as a
dimolybdenum complex with an alkyne ligand bonded in a
parallel µ-η1:η1 manner, since its spectra show obvious parallels
with those of the related species [Mo2Cp2(µ-SPri)2(µ-S)(µ-η1:
η1-C2Ph2)] and [Mo2Cp2(µ-SMe)3(µ-η1:η1-HC2CO2Me)](BF4),
whose molecular structures have been confirmed by X-ray
analysis.9,10 In particular, the low-field 13C chemical shift of
the alkyne carbon atoms (δ 243.4) is in the range (δ 272-
213)9,10a,11 observed for other dinuclear complexes of group 6
metals, in which the alkyne group lies parallel to the metal-
metal bond. This chemical shift differs strongly from the values
(δ 87-62) reported for related compounds where the alkyne
bridges two metals in the perpendicular µ-η2:η2 (4e) mode.12
Complex 2 was readily deprotonated in the presence of
triethylamine to give a green solution from which the neutral
(9) Adams, H.; Morris, M. J.; Mountford, P.; Patel, R.; Spey, S. E. Dalton
Trans. 2001, 2601.
In conclusion, the formation of the dinuclear µ-allenylidene
complex 4 involves the four steps shown in Scheme 1. A similar
(10) (a) Schollhammer, P.; Cabon, N.; Capon, J.-F.; Pe´tillon, F. Y.;
Talarmin, J.; Muir, K. W. Organometallics 2001, 20, 1230. (b) Ojo, W.-S.;
Pe´tillon, F. Y.; Schollhammer, P. To be submitted for publication.
(11) (a) Bott, S. G.; Clark, D. L.; Green, M. L. H.; Mountford, P. J.
Chem. Soc., Dalton Trans. 1991, 471. (b) Feng, Q.; Green, M. L. H.;
Mountford, P. J. Chem. Soc., Dalton Trans. 1992, 2171.
(12) See for example: (a) Bailey, W. I.,, Jr.; Chisholm, M. H.; Cotton,
F. A.; Rankel, L. A. J. Am. Chem. Soc. 1978, 100, 5764. (b) Capon, J. F.;
Cornen, S.; Le Berre-Cosquer, N.; Pichon, R.; Kergoat, R.; L’Haridon, P.
J. Organomet. Chem. 1994, 470, 137.
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Organomet. Chem. 1999, 585, 83.
(14) (a) Pe´tillon, F. Y.; Schollhammer, P.; Talarmin, J.; Muir, K. W.
Coord. Chem. ReV. 1998, 178-180, 203. (b) Orpen, A. G.; Brammer, L.;
Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. International Tables
for Crystallography; Kluwer Academic: Dordrecht, The Netherlands, 1992;
Vol. C (I.U.Cr.), Table 9.6.

Notes
Organometallics, Vol. 25, No. 22, 2006 5505
overall yield of 201 mg (84%) of 2. IR (KBr, cm-1): ν(OH) 3402
(vs), ν(CdC) 1635 (m). 1H NMR (CD2Cl2, 25 °C): δ 12.52 (s, 1H,
RCCH), 6.90-7.28 (m, 10H, C(C6H5)2), 6.42 (s, 10H, C5H5), 3.21
(s, 1H, OH), 2,04, 2.02, and 1.99 (s, 3H, SCH3). 13C{1H} NMR
(CD2Cl2, 25 °C): δ 243.4 (HCdC), 136.6 (C ipso Ph), 130.2-
127.5 (C6H5), 106.7 and 98.3 (C5H5), 78.2 (COH). 30.0 and 15.4
(SCH3). Anal. Calcd for C28H31BF4Mo2OS3: C, 44.34; H, 4.11.
Found: C, 43.95; H, 4.31.
Reaction of 2 with Et3N: Synthesis of 3. To a dark green
solution of 2 (201 mg, 0.26 mmol) in CH2Cl2 (20 mL) was added
an excess of triethylamine (2 equiv, 74 µL). The solution turned
readily to pale green. After the mixture was stirred for a few minutes
at ambient temperature, the solvent was evaporated and 3 was
extracted with diethyl ether (2 × 15 mL). The solvent was then
removed in vacuo from the pooled extracts. When the residue was
washed with cold pentane, 3 was obtained as a green powder (84
mg, 47% yield). IR (KBr, cm-1): ν(OH) 3412 (vs, br), ν(CtC)
1959 (m). 1H NMR (CDCl3, 25 °C): δ 7.82-6.98 (m, 10H,
C(C6H5)2), 5.16 (s, 10H, C5H5), 2.57 (sbr, 1H, OH), 1.64, 1.61,
and 1.49 (s, 3H, SCH3). 13C{1H} NMR (C6D6, 25 °C): δ 147.2 (C
ipso Ph), 130.1-126.8 (C6H5), 120.6 (CtCCPh2), 90.9 (C5H5), 77.4
(C(OH)Ph2), 17.0, 11.8, and 10.9 (SCH3). Anal. Calcd for C28H30-
Mo2OS3: C, 50.15; H, 4.51. Found: C, 50.21; H, 4.50.
Figure 1. View of the [Mo2Cp2(µ-SMe)3(µ-η1:η2-(CdCdCPh2)]+
cation in crystals of 4‚CH2Cl2. Each S atom is disordered over two
sites; only the major sites with occupancy 0.924(5) are shown here.
Ellipsoids at the 20% probability level are shown, except for
hydrogen atoms, which are represented by spheres of arbitrary
radius. Selected distances (Å) and angles (deg): Mo1-Mo2, 2.659-
(9); Mo1-C4, 2.27(1); Mo1-C5, 2.44(1); Mo1-S1, 2.468(3);
Mo1-S2, 2.482(3); Mo1-S3, 2.456(3); Mo2-C4, 1.88(1); Mo2-
S1, 2.480(3); Mo2-S2, 2.497(3); Mo2-S3, 2.439(3); C4-C5, 1.31-
(2); C5-C6, 1.36(1); C6-C31, 1.51(1); C6-C41, 1.50(1); Mo1-
C4-Mo2, 79.0 (4); C5-C4-Mo2, 160.1(9); C5-C4-Mo1, 81.3(7);
C6-C5-C4, 144.0(1); C6-C5-Mo1, 149.3(8); C4-C5-Mo1,
66.6(7); C31-C6-C5, 121.9(9); C31-C6-C41, 118.2(9); C41-
C6-C5, 119.8(9).
Synthesis of 4. One equivalent of H[BF4]‚Et2O in diethyl ether
(5 mL) was added with stirring to a solution of 3 (84 mg, 0.125
mmol) in dichloromethane (10 mL) at room temperature. A purple
solid readily precipitated from the solution and was collected by
filtration and washed with pentane (2 × 15 mL). Compound 4 was
obtained as a purple powder (78 mg, 75.5% yield). Crystals of 4,
suitable for X-ray analysis, were formed by crystallization at room
temperature from a CH2Cl2 solution layered with diethyl ether. IR
(KBr, cm-1): ν(CdCdC) 1653 (m). 1H NMR (CD2Cl2, 25 °C): δ
7.48-7.24 (m, 10H, C(C6H5)2), 6.00 (s, 10H, C5H5), 2.01, 1.78,
and 1.46 (s, 3H, SCH3). 13C{1H} NMR (CD2Cl2, 25 °C): δ 302.0
(Mo2CdCdCPh2), 170.2 (Mo2CdCdCPh2), 140.2, 130.3, 130.2,
and 129.1 (dC(C6H5)2), 135.7 (Mo2CdCdCPh2), 98.3 (C5H5), 25.6,
13.1, and 9.3 (SCH3). Anal. Calcd for C28H29BF4Mo2S3‚CH2Cl2:
C, 42.20; H, 3.79. Found: C, 42.28; H, 3.92.
four-stage pathway was previously proposed by Selegue15 for
mononuclear transition-metal derivatives. Remarkable features
of this work are that two of the three intermediates involved in
the formation of 4 have been isolated and that, finally, a non-
carbonyl µ-allenylidene dinuclear complex has been prepared
and structurally characterized. Further experiments are now in
progress: they extend the scope of the reactivity of these side-
on allenylidene species and should give a better understanding
of the factors which govern their behavior.
Crystallographic Data. X-ray crystal data for 4‚CH2Cl2: C29H31-
BCl2F4Mo2S3, fw ) 825.31, monoclinic, space group P21/c, a )
10.4870(7) Å, b ) 17.028(12) Å, c ) 19.060(16) Å, â ) 105.853-
(8)°, V ) 3274.1(4) Å3, T ) 293 K, Z ) 4, d(calcd) ) 1.674 g/cm3.
4888 unique, absorption-corrected intensities with θ(Mo KR) <
25.0°. R(F) ) 0.079 for 3201 reflections with I > 2σ(I), and Rw-
(F2, all data) ) 0.200 after refinement of 345 parameters. |∆F| <
Experimental Section
General Procedures. All reactions were routinely carried out
under a nitrogen atmosphere using standard Schlenk techniques.
Solvents were distilled immediately before use under nitrogen from
the appropriate drying agents. Literature methods were used for
the synthesis of [Mo2Cp2(µ-SMe)3(NCCH3)2](BF4).8 Other reagents
were purchased from the usual commercial suppliers and used as
received. Infrared spectra were recorded on a Nicolet-Nexus FT
IR spectrophotometer from KBr pellets. Chemical analyses were
performed by the Service de Microanalyse ICSN-CNRS, Gif sur
Yvette, France. The NMR spectra (1H, 13C) were recorded at room
temperature in CD2Cl2, CDCl3, or C6D6 solution with a Bruker
0.59 e Å-3
.
Acknowledgment. We are grateful to Dr. F. Michaud for
the crystallographic measurements of 4 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 Universities of Glasgow and Brest
for financial support. The Ministe`re de l’Enseignement Supe´rieur
et de la Recherche du Gabon is acknowledged for providing
studentships (W.-S.O.).
1
AMX 400 spectrometer and were referenced to SiMe4. H-13C
experiments were carried out on a Bruker DRX 500 spectrometer.
Synthesis of 2. A solution of complex 1 (200 mg, 0.32 mmol)
in CH2Cl2 (20 mL) was stirred with 1 equiv of HCtCCPh2(OH)
(66 mg) for 50 min at room temperature. The solution turned from
red to green. It was then concentrated, and 30 mL of diethyl ether
was added. A brownish green solid precipitated and was collected
by filtration and then washed with pentane (2 × 15 mL), giving an
Supporting Information Available: For 4, tables giving details
of structure determination, non-hydrogen atomic positional param-
eters, all bond distances and angles, anisotropic parameters and
hydrogen atomic coordinates; crystallographic data are also given
as a CIF file. This material is available free of charge via the Internet
at http://pubs.acs.org.
(15) Selegue, J. P. Organometallics 1982, 1, 217.
OM060663A