Unsaturated dinickel-molybdenum clusters with N-heterocyclic carbene ligands

Article abstract of DOI:10.1039/b800747k

The first examples of mixed metal trinuclear clusters carrying N-heterocyclic carbene (NHC) ligands were isolated from reactions of the complexes [Ni(NHC)ClCp] [NHC = bis-(2,6-diisopropylphenyl)- or bis-(2,4,6-trimethylphenyl)-imidazol-2-ylidene] with [Mo

Full text of DOI:10.1039/b800747k

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www.rsc.org/dalton | Dalton Transactions
Unsaturated dinickel–molybdenum clusters with N-heterocyclic carbene
ligands†
Sandra Milosevic, Eric Brenner, Vincent Ritleng and Michael J. Chetcuti*
Received 15th January 2008, Accepted 21st January 2008
First published as an Advance Article on the web 14th February 2008
DOI: 10.1039/b800747k
The first examples of mixed metal trinuclear clusters carrying
N-heterocyclic carbene (NHC) ligands were isolated from
reactions of the complexes [Ni(NHC)ClCp] [NHC = bis-(2,6-
diisopropylphenyl)- or bis-(2,4,6-trimethylphenyl)-imidazol-
2-ylidene] with [Mo(CO)3Cp]−; the unsaturated 46-electron
clusters have triangular MoNi2 cores and the reaction path-
way activates usually inert Ni–Cp and Ni–NHC bonds.
and structurally characterized species [Ni(CO)3((Pr2Ar)2NHC)].10
While 3a displays a H NMR spectrum in agreement with that
1
expected for the compound [NiMo(CO)3((Pr2Ar)2NHC)Cp2], its
IR spectrum exhibits a suspiciously low energy m(CO) stretching
frequency at 1747 cm−1 (other absorptions at 1892 and 1813 cm−1
were also seen).¶ The low frequency absorption, inconsistent with
terminal or even l2-CO ligands, suggests the presence of a l3-CO
group spanning a trinuclear cluster. Scheme 1 gives the reaction
and structures of 2a and 3a.
N-Heterocyclic carbenes (NHCs) are an important manifold of
ligands that are increasingly being used in synthesis and catalysis.
NHC complexes are often more air and thermally stable than their
isoelectronic trialkylphosphine analogues.1 Nevertheless, only a
handful of clusters which incorporate these ligands are known, and
heterobimetallic (htb) species as well as tri- or higher nuclearity
mixed metal clusters are almost unknown.2,3
Many htb complexes activate small molecules in a unique
manner.4 Our group has long been interested in the chemistry
and reactions of such species.5 We have reported the synthesis
of l-methylene complexes in which the ligand spans nickel–
molybdenum or –tungsten bonds6 and have observed a variety
of coupling reactions between CH2 groups spanning these metal–
metal bonds and other ligands. Similar couplings have also been
observed with other htb systems.7 Despite the high stability of
NHC ligands and the significant electronic and steric differences
that exist between methylene and NHC groups, we believed
that if htb complexes with NHC ligands anchored onto the
bimetallic framework were available, coupling reactions between
NHC groups and other ligands, hitherto quite rare, might be
promoted by the htb centers.8
Scheme 1 Reactions of [Ni(NHC)ClCp] with [Mo(CO)3Cp]−.
The structure of 3a was established unambiguously by a single
crystal X-ray diffraction study.* Fig. 1 shows the molecular struc-
ture of 3a, [MoNi2(CO)4((Pr2Ar)2NHC)Cp2] and establishes its
nature as a trinuclear MoNi2 46-electron unsaturated cluster. One
nickel atom (Ni1) has been stripped of its Cp group and bears a
(Pr2Ar)2NHC group as its only terminal ligand. The observed Ni1–
We have reported that the reaction of [W(CO)3Cp]− with
[Ni(PMe3)ICp*]‡ afforded the tungsten-bound phosphine com-
plex [NiW(CO)3(PMe3)Cp*Cp] (Ni–W), in a phosphine migra-
tion reaction.6c We attempted the synthesis of [NiMo(CO)3-
(NHC)Cp2] species in similar fashion, by reacting [Ni((Pr2Ar)2-
NHC)ClCp]9a 1a in a 1 : 1 molar ratio with the molybdenum
anion [Mo(CO)3Cp]−.§ However the expected [NiMo(CO)3-
((Pr2Ar)2NHC)Cp2] (Ni–Mo) was not obtained. Instead, 2a and
3a (see below) were harvested from this reaction, together with
some unreacted 1a and traces of [Mo2(CO)6Cp2] (Mo–Mo). The
mixture was separated by column chromatography, and pure 2a
and 3a were isolated.
˚
C1 distance of 1.907(1) A is slightly longer than the corresponding
1.875(1) A distance noted in [Ni((Pr2Ar)2NHC)ClCp].9a The other
nickel atom, Ni2, is still bonded to a Cp group but has no NHC
ligand linked to it. The metallic triangular core is asymmetric,
˚
˚
with Ni–Mo distances of 2.5300(6) (Ni1–Mo) and 2.7108(7) A
(Ni2–Mo). The Ni1–Mo bond distance is the shortest such value
known and the relatively close proximity of the two bonded metals
probably reflects the partial multiple bond character of this bond.
No Ni Mo bond distances have been reported, but Ni–Mo bonds
in clusters normally lie in the range 2.62–2.72 A.11 A molecule with
˚
a formal Ni W bond has a nickel–tungsten distance of 2.46 A.
All four CO groups in the cluster are interacting with more
than one metal. The normal Ni1–Ni2 bond of 2.4264(7) A is
spanned by an asymmetrically l2-bridging CO ligand while one of
the three CO groups on the Mo atom interacts in a l2-semibridging
fashion with Ni1. The other two Mo-bound CO groups interact
(weakly in one case, and more significantly in the other) with both
nickel atoms, so that they are perhaps best regarded as semi-triply
bridging CO ligands.
=
12
˚
=
Elemental analysis, NMR spectroscopy and a single crystal X-
ray diffraction study established 2a as the previously reported
˚
Laboratoire de Chimie Organome´tallique Applique´e, UMR CNRS 7509,
ECPM, Universite´ Louis Pasteur, 25 Rue Becquerel, 67087, Strasbourg,
France. E-mail: chetcuti@chimie.u-strasbg.fr; Tel: +33 3 9024 2631
† CCDC reference number 667976. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b800747k
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The Royal Society of Chemistry 2008
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©

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which cleavage of normally robust Ni–Cp bonds has occurred
in diamagnetic nickel complexes. Formation of 3 also entails the
rupture of the metal–NHC bonds originally present in complexes
1. We have previously noted that Ni–Cp* bonds are not inert
and hence, Ni–Cp* cleavage and subsequent Cp* ligand transfer
is not unusual in NiCp* chemistry.13 However the rupture of
Ni–Cp bonds under relatively mild conditions is very unusual
for diamagnetic 18-electron nickel complexes. Metal–NHC bond
cleavage is also a relatively rare phenomenon.8a–d,14
Further investigations into reactions leading to complexes 3 and
related reactions are in progress.
We gratefully acknowledge the Universite´ Louis Pasteur and
the CNRS for funding and Dr Lydia Brelot of the X-ray
crystallography service at the Institut de Chimie, Universite´ Louis
Pasteur for the structural study. S.M. thanks the CNRS for a
summer fellowship.
Notes and references
5
5
‡ Throughout this manuscript, Cp = g -C5H5, Cp’ = g -C5H4Me, Cp* =
5
g -C5Me5, (Pr2Ar)2NHC = bis-(2,6-diisopropylphenyl)imidazol-2-ylidene,
Mes2NHC = bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene.
Fig. 1 Molecular structure of 3a showing all non-H atoms. Ellipsoids are
§ 3a was prepared by reacting Na[Mo(CO)3Cp]·2DME (275 mg,
0.61 mmol) with [Ni((Pr2Ar)2NHC)ClCp], 1a (336 mg, 0.61 mmol) in
THF at 45 ◦C for 8.5 h. Silica gel chromatography of the concentrated
mixture using toluene as an eluting solvent separated unreacted 1a from
traces of [Mo2(CO)6Cp2] (Mo–Mo), pale pink 2a and dark green 3a. Green
needles of 3a (173 mg, 0.205 mmol, 34% based on Mo) were obtained from
toluene–pentane solutions at −20 ◦C. 3b was prepared similarly from
[Ni(Mes2NHC)ClCp], 1b (17% based on Mo).
shown at the 50% probability lev◦el and key atoms are labelled. Pertinent
˚
bond lengths (A) and angles ( ): Ni1–Ni2 = 2.4264(7), Ni1–Mo =
2.5300(6), Ni2–Mo = 2.7108(7), Ni–C1 = 1.907(4); Ni1–Ni2–Mo =
58.97(2), Ni2–Ni1–Mo = 66.27(2) and Ni1–Mo1–Ni2 = 55.029(17).
While it is not easy to rationalise the formal electron count in
3a, it is clear that Ni1, which carries the (Pr2Ar)2NHC ligand, is
unsaturated. It is this nickel atom which forms the short bond to
molybdenum. The nickel atom is electronically stabilized by the
strong electron-donating properties of the NHC ligand and, in
addition, is shielded from its environment by the enormous bis-
2,6-diisopropylphenyl pendant groups of this ligand, as shown by
space filling models. These steric and electronic effects stabilize and
protect the unsaturated metal. The centroids of the two Cp ligands
essentially lie in the MoNi2 plane, maximizing their distance from
the massive NHC ligand.
The reaction of [Mo(CO)3Cp]− with [Ni(Mes2NHC)ClCp]9b
1b under similar reaction conditions analogously afforded a
mixture of complexes 2b10 and 3b (Scheme 1), together with
some unreacted 1b and traces of [Mo2(CO)6Cp2] (Mo–Mo).
Structural data are not available for the bis-mesityl analogue
[MoNi2(CO)4(Mes2NHC)Cp2] 3b. However, solid-state IR and
solution NMR spectroscopic data¶ of this species parallel those
of 3a and suggest that the two complexes have similar structures.
The isolated yields of 3b were lower than for 3a.§ The lower yield
of 3b may reflect the smaller steric footprint of the (Mes)2NHC
ligand which thus offers less steric protection to the cluster.
Excess 1 does not increase the yield of clusters 3 as even at 1 :
1 (Ni : Mo) stoichiometry, unreacted 1 is recovered. We have also
demonstrated that room temperature benzene-d6 solutions of 3b
decompose slowly to yield 2b as a reductive elimination product.
This decomposition route may indeed be the mechanistic pathway
to complexes 2 in this reaction. Note that the nickel atom bearing
the NHC ligand [Ni1 in the X-ray study of 3a] is already interacting
with three CO ligands (and weakly with a fourth).
¶ Selected physical data for 3a and 3b: 3a; 1H NMR (C6D6, 300 MHz,
d/ppm, J/Hz): 1.05 (d, 12H, J = 8.8, CHMe2); 1.52 (d, 12 H, J = 8.8,
12H, CHMe2); 3.13 (m, 4H, CHMe2); 4.99 and 5.00 (2 × 5H, 2Cp); 6.69
(2H, NCH); 7.26 (m, 4H, m-H); 7.29 (m, 2H, p-H). 13C NMR (C6D6,
75 MHz, d/ppm): 194.6 (N–C–N); 146.6, 136.1, 130.3 and 125.0 (CAr),
=
124.3 (NCH NCH); 94.1 and 91.9 (2 Cp), 28.4 (CHMe2), 26.7 and 22.9
(CHMe2). IR [ATR, m(CO)/cm−1]: 1892 (m), 1813 (s, br), 1747 (s, br). 3b;
1H NMR (C6D6, 300 MHz, d/ppm, J/Hz): 2.12 (6H, p-Me); 2.30 (12H,
o-Me); 4.96 and 5.03 (2 × 5H, 2 Cp); 6.29 (2H, NCH); 6.90 (4H, m-H). 13
C
NMR (C6D6, 75 MHz, d/ppm): 192.0 (N–C–N), 138.9 (ipso-CAr), 136.2
=
(p-CAr), 135.7 (o-CAr), 129.8 (m-CAr), 123.4 (NCH NCH), 94.0 and 92.1
(2Cp), 21.1 (p-Me), 18.9 (o-Me). IR [ATR, m(CO)/cm−1]: 1882 (m), 1797
(s, vbr), 1747 (s, br).
* Crystal data for 3a, C41H46MoN2Ni2O4, Z = 4, M 844.16, Dc=
1.466 g cm−3, k = 0.71069 A (Mo-Ka), monoclinic, space group P21/c, a =
˚
◦
˚
˚
˚
11.9951(7) A, b = 17.6958(8) A, c = 20.1788(10) A, b = 116.754(3) , V =
3
3824.7(3) A , T = 172(2) K, l = 1.34 mm−1, 23780 measured reflections,
˚
8702 independent, 5001 with I > 2r(I). R = 0.0542, gof = 0.975.
1 Recent reviews of NHC ligands and their applications in catalysis
include: (a) R. B. Kissling, M. S. Viciu, G. A. Grasa, R. F. Germaneau,
T. Gueveli, M.-C. Pasareanu, O. Navarro-Fernandez and S. P. Nolan,
ACS Symp. Ser., 2003, 856, 323; (b) E. Peris and R. H. Crabtree, Coord.
Chem. Rev., 2004, 248, 2239; (c) E. A. B. Kantchev, C. J. O’Brien
and M. G. Organ, Angew. Chem., Int. Ed., 2007, 46, 2768; (d) S.
Diez-Gonzalez and S. P. Nolan, Top. Organomet. Chem., 2007, 21, 47;
(e) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290; (f) R. H.
Crabtree, J. Organomet. Chem., 2005, 690, 5451; (g) O. Kuhl, Chem.
Soc. Rev., 2007, 36, 592.
2 Few bimetallic NHC complexes exist and we are only aware of
homo-bimetallic examples. Fe2 mono and bis-NHC complexes: (a) D.
Morvan, J-F. Capon, F. Gloaguen, A. Le Goff, M. Marchivie, F.
Michaud, P. Schollhammer, J. Talarmin, J-J. Yaouanc, R. Pichon and
N. Kervarecand, Organometallics, 2007, 26, 2042; (b) J. W. Tye, J. Lee,
H.-W. Wang, R. Mejia-Rodriguez, J. H. Reibenspies, M. B. Hall and
M. Y. Darensbourg, Inorg. Chem., 2005, 44, 5550; (c) J.-F. Capon,
S. El Hassnaoui, F. Gloaguen, P. Schollhammer and J. Talarmin,
Organometallics, 2005, 24, 2020; (d) S. Jiang, J. Liu, Y. Shi, Z. Wang, B.
The reaction pathway leading to the formation of clusters
3 is highly unusual as it gives rise to molecules (2 and 3) in
˚
Akermark and L. Sun, Polyhedron, 2007, 26, 1499; (e) bimetallic species
in which there are no direct M–M interactions include: P. L. Arnold
1974 | Dalton Trans., 2008, 1973–1975
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and S. T. Liddle, Organometallics, 2006, 25, 1485 (Sm2 complexes); U.
Scheele, D. Juergen, S. Dechert and F. Meyer, Inorg. Chim. Acta, 2006,
359, 4891 (Hg2 complexes).
9 (a) R. A. Kelly III, N. M. Scott, S. D´ıez-Gonza´lez, E. D. Stevens and
S. P. Nolan, Organometallics, 2005, 24, 3442; (b) C. D. Abernethy, A. H.
Cowley and R. A. Jones, J. Organomet. Chem., 2000, 596, 3.
3 (a) Ag–Os polynuclear species provide one of the only known examples
of NHC ligands in mixed metal clusters: C. E. Cooke, T. Ramnial,
M. C. Jennings, R. K. Pomeroy and J. A. C. Clyburne, Dalton Trans.,
2007, 1755; other trinuclear clusters with such ligands include homo-
trimetallic Ru3 and Os3 examples; (b) J. A. Cabeza, I. del R´ıo, D. Miguel
and M. G. Sa´nchez-Vega, Chem. Commun., 2005, 3956; (c) J. A. Cabeza,
I. da Silva, I. del R´ıo and M. G. Sa´nchez-Vega, Dalton Trans., 2006,
3966; Ag4 and Au4 NHC clusters have also been reported recently: (d) Y.
Zhou and W. Chen, Organometallics, 2007, 26, 2742.
4 Hydrocarbyl ligand coupling mediated by heterobimetallic complexes
has been reviewed recently: V. Ritleng and M. J. Chetcuti, Chem. Rev.,
2007, 107, 797.
5 Key aspects of our work are summarized in: E. Brenner and M. J.
Chetcuti, Comments Inorg. Chem., 2006, 27, 145.
6 (a) M. J. Chetcuti, B. E. Grant and P. E. Fanwick, Organometallics,
1990, 9, 1345; (b) M. J. Chetcuti, P. E. Fanwick and B. E. Grant,
Organometallics, 1991, 10, 3003; (c) M. J. Chetcuti, K. J. Deck, J. C.
Gordon, B. E. Grant and P. E. Fanwick, Organometallics, 1992, 11,
2128.
7 Examples include: (a) S. J. Trepanier, J. N. L. Dennett, B. Sterenberg, R.
McDonald and M. Cowie, J. Am. Chem. Soc., 2004, 126, 8046; (b) M.
Cowie, Can. J. Chem., 2005, 83, 1043. See also ref. 4.
8 Examples of NHC groups coupling with other ligands include:
(a) E. Becker, V. Stingl, G. Dazinger, K. Mereiter and K. Kirchner,
Organometallics, 2007, 26, 1531; (b) E. Becker, V. Stingl, G. Dazinger,
M. Puchberger, K. Mereiter and K. Kirchner, J. Am. Chem. Soc., 2006,
128, 6572; (c) D. S. McGuinness, N. Saendig, B. F. Yates and K. J.
Cavell, J. Am. Chem. Soc., 2001, 123, 4029; (d) D. S. McGuinness and
K. J. Cavell, Organometallics, 2000, 19, 4918; (e) A. A. Danopoulos, N.
Tsoureas, J. C. Green and M. B. Hursthouse, Chem. Commun., 2003,
756.
10 R. Dorta, E. D. Stevens, N. M. Scott, C. Costabile, L. Cavallo, C. D.
Hoff and S. P. Nolan, J. Am. Chem. Soc., 2005, 127, 2485.
11 Clusters
with
Ni–Mo
bonds
include:
(a)
[Mo2Ni(l3-
CCH2C(O)OMe)(CO)4Cp2Cp*]: P. Braunstein, M. J. Chetcuti and R.
Welter, Chem. Commun., 2001, 2508; (b) [Mo2Ni2(CO)2(l3-S)4Cp’2]:
M. D. Curtis, P. D. Williams and W. M. Butler, Inorg. Chem., 1988, 27,
2853; (c) [Ni2Mo2(l3-CPh)2(l3-CO)2Cp4]: A. D. Shaposhnikova, M. V.
Drab, G. L. Kamalov, A. A. Pasynskii, I. L. Eremenko, S. E. Nefedov,
Y. T. Struchkov and A. I. Yanovsky, J. Organomet. Chem., 1992, 429,
i
109; (d) [FeMoNi(CO)4(PhC2CO2 Pr)2Cp2]: M. Mlekuz, P. Bougeard,
B. G. Sayer, S. Peng, M. J. McGlinchey, A. Marinetti, J.-Y. Saillard, J. B.
Naceur, B. Mentzen and G. Jaouen, Organometallics, 1985, 4, 1123;
(e) [MoNi3(l3-CO)3Cp3Cp’]: M. J. Chetcuti, S. R. McDonald and
J. C. Huffman, Inorg. Chem., 1989, 28, 238; (f) [FeMoNi(CO)7CpCp*]:
P. Braunstein, M. J. Chetcuti and R. Welter, C. R. Chim., 2002,
5, 67.
=
=
12 The complex [Cp*Ni W(CO)3Cp] has Ni W distances of 2.457(2)
˚
and 2.475(2) A for the two independent molecules in the unit cell: M. J.
Chetcuti, B. E. Grant, P. E. Fanwick, M. J. Geselbracht and A. M.
Stacy, Organometallics, 1990, 9, 1343.
13 (a) M. J. Chetcuti, B. E. Grant and P. E. Fanwick, J. Am. Chem. Soc.,
1989, 111, 2743; (b) M. J. Chetcuti, L. A. Deliberato, P. E. Fanwick
and B. E. Grant, Inorg. Chem., 1990, 29, 1295; (c) M. J. Chetcuti,
P. E. Fanwick and B. E. Grant, J. Organomet. Chem., 2001, 630,
215.
14 Examples of sometimes reversible M–NHC bond cleavage or substitu-
tion reactions include: (a) D. P. Allen, C. M. Crudden, L. A. Calhoun
and R. Wang, J. Organomet. Chem., 2004, 689, 3203; (b) R. W. Simms,
M. J. Drewitt and M. C. Baird, Organometallics, 2002, 21, 2958; (c) R.
Dorta, E. D. Stevens, C. D. Hoff and S. P. Nolan, J. Am. Chem. Soc.,
2003, 125, 10490.
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 1973–1975 | 1975
©