Synthesis, properties, and structure of tethered molybdenum alkylidenes

Article abstract of DOI:10.1021/om800650v

A new class of molybdenum alkylidenes has been prepared where the alkylidene is tethered to an imido ancillary ligand. The amine required for the synthesis is accessible in 38% yield in five steps from 1,3-diisopropylbenzene. The amine is then installed to generate the tethered alkylidene bis(triflate) complex, which was structurally characterized as its DME adduct. The Inflates are replaced by hexafluoro-terfbutoxide groups using the thallium salt of the alkoxide, and the bis(alkoxide) was characterized as its quinuclidine adduct. For comparison, an alkylidene bis(alkoxide) was prepared without the tether and having a formula similar to that of the tethered system. The structures from X-ray diffraction and NMR spectroscopy of the two complexes with and without the tether but with similar formulas are compared. The tether has the apparent effect, judging from J_CH couplings in the alkylidene and angles in the solidstate structure, of reducing the strength of the a-agostic interaction. Four complexes were structurally characterized during this study: Mo[=N-2,4-Pr₂ⁱC₆H₂-2-CH ₂CH₂CMe₂CH=](DME)(OTf)₂, Mo(OBf)₂, Mo(OBu_F6^t)₂(quin)-[=N-2,4- Pr₂ⁱC₆H₂-2-CH₂CH ₂CMe₂CH=], Mo[N(2,4-Pr₂ⁱ-6-MeC ₆H₂)]₂(neopentyl)₂, and Mo(OBu _F6^t)₂(quin)-[N(2,4-Pr₂ ⁱ-6-MeC₆H₂)] [=C(H)Bu^t].

Full text of DOI:10.1021/om800650v

5130
Organometallics 2008, 27, 5130–5138
Synthesis, Properties, and Structure of Tethered Molybdenum
Alkylidenes
Kapil S. Lokare, Richard J. Staples, and Aaron L. Odom*
Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824
ReceiVed July 9, 2008
A new class of molybdenum alkylidenes has been prepared where the alkylidene is tethered to an
imido ancillary ligand. The amine required for the synthesis is accessible in 38% yield in five steps from
1,3-diisopropylbenzene. The amine is then installed to generate the tethered alkylidene bis(triflate) complex,
which was structurally characterized as its DME adduct. The triflates are replaced by hexafluoro-tert-
butoxide groups using the thallium salt of the alkoxide, and the bis(alkoxide) was characterized as its
quinuclidine adduct. For comparison, an alkylidene bis(alkoxide) was prepared without the tether and
having a formula similar to that of the tethered system. The structures from X-ray diffraction and NMR
spectroscopy of the two complexes with and without the tether but with similar formulas are compared.
The tether has the apparent effect, judging from JCH couplings in the alkylidene and angles in the solid-
state structure, of reducing the strength of the R-agostic interaction. Four complexes were structurally
characterized during this study: Mo[dN-2,4-Pri2C6H2-2-CH2CH2CMe2CHd](DME)(OTf)2, Mo(OButF6)2(quin)-
[dN-2,4-Pri2C6H2-2-CH2CH2CMe2CHd], Mo[N(2,4-Pri2-6-MeC6H2)]2(neopentyl)2, and Mo(OButF6)2(quin)-
[N(2,4-Pri2-6-MeC6H2)][dC(H)But].
alkylidene tethered to an N-heterocyclic carbene.4 This catalyst
was employed by Grubbs and co-workers in the cyclooligo-
Introduction
Olefin metathesis continues to grow as an important meth-
odology for the generation of new carbon-carbon bonds with
applications to the synthesis of small molecules and polymers.1
Two catalyst types have risen to preferred status for this
important reaction (Chart 1): one based on ruthenium developed
by the Grubbs group and one based on molybdenum by the
Schrock group. Extensive synthetic effort has gone into the
elaboration of both catalyst types from the groups of the
catalyst’s progenitors and others. At present there is a wide
selection of derivatives available for specialized applications:
e.g., asymmetric ring-closing reactions.2 The R group in
Schrock’s catalyst is often varied in this system for various
applications with R ) But often used for ring-opening metathesis
polymerization (ROMP)3 and R ) C(CF3)2Me often used for
ring-closing metathesis (RCM), but many others are of utility.
The effect of imido substituents on molybdenum alkylidene
reactivity has also been extensively examined.1
(2) For applications in asymmetric olefin metathesis: (a) La, D. S.;
Alexander, J. B.; Cefalo, D. R.; Graf, D. D.; Hoveyda, A. H.; Schrock,
R. R. J. Am. Chem. Soc. 1998, 120, 9720. (b) Fujimura, O.; Grubbs, R. H.
J. Org. Chem. 1998, 63, 824. (c) Weatherhead, G. S.; Ford, J. G.; Alexanian,
E. J.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc. 2000, 122, 1828.
(d) Dolman, S. J.; Sattely, E. S.; Hoveyda, A. H.; Schrock, R. R. J. Am.
Chem. Soc. 2002, 124, 6991. (e) Tsang, W. C. P.; Hultzsch, K. C.;
Alexander, J. B.; Bonitatebus, P. J., Jr.; Schrock, R. R.; Hoveyda, A. H.
J. Am. Chem. Soc. 2003, 125, 2652. (f) Lee, A.-L.; Malcolmson, S. J.;
Puglisi, A; Schrock, R. R.; Hoveyda, A. H J. Am. Chem. Soc. 2006, 128,
5153. (g) Sattely, E. S.; Cortez, G. A.; Moebius, D. C.; Schrock, R. R.;
Hoveyda, A. H. J. Am. Chem. Soc. 2005, 127, 8526. (h) Dolman, S. J.;
Schrock, R. R.; Hoveyda, A. H. Org. Lett. 2003, 5, 4899. (i) Funk, T. W.;
Berlin, J. M.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 1840. (j) Jernelius,
J. A.; Schrock, R. R.; Hoveyda, A. H. Tetrahedron 2004, 60, 7345. (k)
Hultzsch, K. C.; Jernelius, J. A.; Hoveyda, A. H.; Schrock, R. R. Angew.
Chem., Int. Ed. 2002, 41, 589. (l) Cortez, G. A.; Schrock, R. R.; Hoveyda,
A. H. Angew. Chem., Int. Ed. 2007, 46, 4534. (m) Funk, T. W.; Berlin,
J. M.; Grubbs, R. H. J. Am. Chem. Soc. 2006, 128, 1840. (n) Berlin, J. M.;
Goldberg, S. D.; Grubbs, R. H. Angew. Chem., Int. Ed. 2006, 45, 7591.
For some selected applications to organic synthesis: (o) Nicolaou, K. C.;
Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005, 44, 4490. (p) Deiters,
A.; Martin, S. F. Chem. ReV. 2004, 104, 2199. (q) Mohapatra, D. K.;
Ramesh, D. K.; Giardello, M. A.; Chorghade, M. S.; Gurjar, M. K.; Grubbs,
R. H. Tetrahedron Lett. 2007, 48, 2621. For application of olefin metathesis
in the synthesis of specialized materials: (r) Anders, U.; Nuyken, O.;
Buchmeiser, M. R.; Wurst, K. Macromolecules 2002, 35, 9029. (s)
Buchmeiser, M. R. J. Mol. Catal. A 2002, 190, 145. (t) Kro¨ll, R. M.; Schuler,
N.; Lubbad, S.; Buchmeiser, M. R. Chem. Commun. 2003, 21, 2742. (u)
Buchmeiser, M. R. New J. Chem. 2004, 28, 549. (v) Buchmeiser, M. R.
Prog. Polym. Sci. 2005, 176. (w) Choi, T. L.; Rutenberg, I. M.; Grubbs,
R. H. Angew. Chem., Int. Ed. 2002, 41, 3839. (x) Gorodetskaya, I. A.; Choi,
T.-L.; Grubbs, R. H. J. Am. Chem. Soc. 2007, 42, 12672. (y) Gabert, A. J.;
Verploegen, E.; Hammond, P. T.; Schrock, R. R. Macromolecules 2006,
39, 3993. (z) Mayershofer, M. G.; Nuyken, O.; Buchmeiser, M. R.
Macromolecules 2006, 39, 2452. (aa) Singh, R.; Czekelius, C.; Schrock,
R. R. Macromolecules 2006, 39, 1316. (bb) Boyd, T. J.; Schrock, R. R.
Macromolecules 1999, 32, 6608. (cc) Lee, J. K.; Schrock, R. R.; Baigent,
D. R.; Friend, R. H. Macromolecules 1995, 28, 1966. (dd) Tassoni, R.;
Schrock, R. R. Chem. Mater. 1994, 6, 744. (ee) Albagli, D.; Bazan, G. C.;
Schrock, R. R.; Wrighton, M. S. J. Am. Chem. Soc. 1993, 115, 7328. (ff)
Cortez, G. A.; Baxter, C. A.; Schrock, R. R.; Hoveyda, A. H. Org. Lett.
2007, 9, 2871. (gg) Singh, R.; Schrock, R. R. Macromolecules 2008, 41,
2990–2993. (hh) Hoveyda, A. H.; Zhugralin, A. R. Nature 2007, 450, 243.
Our group has been developing catalysts based on the Schrock
framework for selective cyclooligomerization of cyclic olefins.
In those efforts, we sought to develop a synthesis for covalent
attachment of the alkylidene CR carbon to an ancillary ligand
on the metal center. The position of attachment decided upon
was through the imido ancillary ligand. During these efforts, a
catalyst was reported by Fu¨rstner and co-workers having an
* To whom correspondence should be addressed. E-mail: odom@
cem.msu.edu.
(1) (a) Grubbs, R. H., Ed. Handbook of Metathesis; Wiley-VCH:
Chichester, U.K., 2003. (b) Ivin, K. J.; Mol, J. C. Olefin Metathesis and
Metathesis Polymerization; Academic Press: San Diego, CA, 1997. (c)
Schrock, R. R. J. Mol. Catal. A 2004, 213, 21. (d) Schrock, R. R.; Hoveyda,
A. H. Angew. Chem., Int. Ed. 2003, 42, 4592. (e) Schrock, R. R. Angew.
Chem., Int. Ed. 2006, 45, 3748.
10.1021/om800650v CCC: $40.75
2008 American Chemical Society
Publication on Web 09/04/2008

Tethered Molybdenum Alkylidenes
Organometallics, Vol. 27, No. 19, 2008 5131
merization of cyclooctene.5 Our group reported a tethered
carbene of molybdenum based on Schrock’s catalyst (1).6 In
this paper, we will discuss the synthesis, structure, and properties
of these tethered alkylidenes. These complexes are unusual
metallacycles bearing two different metal-ligand multiple bonds
in a ring.
Chart 1. Structures of Grubbs’ “Second Generation”
Catalyst, Schrock’s Catalyst, and Tethered Derivatives
The tethered molybdenum catalyst architecture shown in
Chart 1 was successful in the sense that it was stable and
polymerization active as the bis(triflate). However, the bis(tri-
flate) was quite a slow catalyst for ROMP, but this does begin
to highlight the differences made by the tether, considering the
untethered versions of the bis(triflate) do not seem to be as active
for polymerization of norbornene.
The usual method for increasing catalyst activity in the
Schrock system is to generate the alkoxide.7 However, replace-
ment of the triflates with alkoxides using several different
techniques led to uncharacterized paramagnetic products due
to decomposition. Coupled with this, the previously designed
synthesis was plagued with several regiochemical issues regard-
ing the aromatic ring and the synthesis of the tether containing
a quaternary center adjacent to an olefin (Scheme 1). The result
was an amine (A) that could be prepared on multigram scales
but required careful column chromatography for purification.
The tethered bis(triflate) complex 1 is prepared using the
Schrock protocol with an intramolecular olefin metathesis to
generate the metallacycle.6
Scheme 1. Previous Tethered Alkylidene Synthesis
Results and Discussion
Synthesis of the Tethered Catalyst. In order in increase the
stability of the resulting complex, we sought to develop a new
synthetic protocol for the tethering amine with more steric
protection on the aromatic ring. In addition, we sought to
redesign the synthesis so that no tedious column separations
were necessary and larger scales were possible.
(3) (a) Crowe, W. E.; Mitchell, J. P.; Gibson, V. C.; Schrock, R. R.
Macromolecules 1990, 23, 3534. (b) Sankaran, V.; Cummins, C. C.;
Schrock, R. R.; Cohen, R. E.; Silbey, R. J. J. Am. Chem. Soc. 1990, 112,
6858. (c) Cummins, C. C.; Schrock, R. R.; Cohen, R. E. Chem. Mater.
1992, 4, 27. (d) Fox, H. H.; Lee, J. K.; Park, L. Y.; Schrock, R. R.
Organometallics 1993, 12, 759. (e) McConville, D. H.; Wolf, J. R.; Schrock,
R. R. J. Am. Chem. Soc. 1993, 115, 4413. (f) Schrock, R. R.; Krouse, S. A.;
Knoll, K.; Feldman, J.; Murdzek, J. S.; Yang, D. C. J. Mol. Catal. 1988,
46, 243.
(4) (a) Fu¨rstner, A.; Krause, H.; Ackermann, L.; Lehmann, C. W. Chem.
Commun. 2001, 2240. (b) Fu¨rstner, A.; Ackermann, L.; Gabor, B.; Goddard,
R.; Lehmann, C. W.; Mynott, R.; Stelzer, F.; Thiel, O. R. Chem. Eur. J.
2001, 7, 3236.
(5) (a) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. Science 2002, 297,
2041. (b) Bielawski, C. W.; Benitez, D.; Grubbs, R. H. J. Am. Chem. Soc.
2003, 125, 8424.
(6) Ciszewski, J. T.; Xie, B.; Cao, C.; Odom, A. L. Dalton Trans. 2003,
4226.
The new synthetic sequence is shown in Scheme 2, which
takes advantage of recent developments in transition metal
catalysis. The first step is selective Smith borylation8 of 1,3-
diisopropylbenzene catalyzed by iridium to generate arene boryl
B; no other regioisomers of the product are observed. After
conversion to the boronic acid C, Suzuki coupling using the
(7) For replacement of triflate with alkoxide: (a) Schrock, R. R.
Polyhedron 1995, 14, 3177. Recently, other replacements have been found
to be highly reactive as well. For replacement of triflate to alkyl alkoxide: (b)
Sinha, A.; Schrock, R. R. Organometallics 2004, 23, 1643. (c) Sinha, A.;
Lopez, L. P. H.; Schrock, R. R.; Hock, A. S.; Muller, P. Organometallics
2006, 25, 1412. (d) Rhers, B.; Salameh, A.; Baudouin, A.; Quadrelli, E. A.;
Taoufik, M.; Coperet, C.; Lefebvre, F.; Basset, J.-M.; Solans-Monfort, X.;
Eisenstein, O.; Lukens, W. W.; Lopez, L. P. H.; Sinha, A.; Schrock, R. R.
Organometallics 2006, 25, 3554. For replacement of triflate to amido: (e)
Sinha, A.; Schrock, R. R.; Muller, P.; Hoveyda, A. H. Organometallics
2006, 25, 4621. (f) Hock, A. S.; Schrock, R. R.; Hoveyda, A. H. J. Am.
Chem. Soc. 2006, 128, 16373. (g) Kreickmann, T.; Arndt, S.; Schrock, R. R.;
Muller, P. Organometallics 2007, 26, 5702. (h) Singh, R.; Czekelius, C.;
Schrock, R. R.; Muller, P.; Hoveyda, A. H. Organometallics 2007, 26, 2528.
For replacement of triflate to ketiminate: (i) Tonzetich, Z. J.; Jiang, A. J.;
Schrock, R. R.; Muller, P Organometallics 2007, 26, 3771. For replacement
of triflate to pyrrolyl alkoxide: (j) Singh, R.; Schrock, R. R.; Muller, P.;
Hoveyda, A. H. J. Am. Chem. Soc. 2007, 129, 12654.
(8) (a) Paul, S.; Chotana, G. A.; Holmes, D.; Reichle, R. C.; Maleczka,
R. E.; Smith, M. R. J. Am. Chem. Soc. 2006, 128, 15552. (b) Holmes, D.;
Chotana, G. A.; Maleczka, R. E.; Smith, M. R. Org. Lett. 2006, 8, 1407.
(c) Chotana, G. A.; Rak, M. A.; Smith, M. R. J. Am. Chem. Soc. 2005,
127, 10539. (d) Maleczka, R. E.; Shi, F.; Holmes, D.; Smith, M. R. J. Am.
Chem. Soc. 2003, 125, 7792. (e) Cho, J. Y.; Tse, M. K.; Holmes, D.;
Malecka, R. E.; Smith, M. R. Science 2002, 295, 305. (f) Tse, M. K.; Cho,
J. Y.; Smith, M. R. Org. Lett. 2001, 3, 2831. (g) Cho, J. Y.; Iverson, C. N.;
Smith, M. R. J. Am. Chem. Soc. 2000, 122, 12868. (h) Ishiyama, T.; Tkagi,
J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.; Hartwig, J. F. J. Am. Chem.
Soc. 2002, 124, 390. (i) Boller, T. M.; Murphy, J. M.; Hapke, M.; Ishiyama,
T.; Miyaura, N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 14263.

5132 Organometallics, Vol. 27, No. 19, 2008
Lokare et al.
Scheme 2. Redesigned Amine Synthesis Used To Generate
the Tethering Amino-Olefin
Scheme 3. Synthesis of the Tethered Carbene Alkoxide
Catalyst 5
protocol developed by Fu and co-workers installs the tethering
olefin.9 The new coupling conditions allow high yields of the
hydrocarbon D using what might otherwise be a problematic
alkyl bromide containing ꢀ-hydrogens. Due to the position and
size of the sterics on the aromatic ring, nitration proceeds to
give the single observed product E with the nitro group ortho
to the olefinic tethering group as desired. Lithium aluminum
hydride reduction of nitro to amine provides the desired aniline
derivative F in 38% overall yield for the five steps. The amino-
olefin F has been prepared on multigram scales using this
protocol, and the purification procedures essentially only involve
a flush through a plug of silica gel or extractions.
Aniline derivative F was installed on the metal to generate
bis(imido)dichloro(DME)molybdenum(VI) (2), which was syn-
thesized from (NH4)2Mo2O7 using the procedure of Schrock and
co-workers (Scheme 3).10 Replacement of the chlorides with
neopentyl groups occurs in high yield to provide Mo(NAr)2(Np)2
(3).
these molybdenum bis(triflates) exhibit spectra indicative of
several isomers being present in solution. An X-ray diffraction
study on the bis(triflate) 4 revealed an isomer different from
that in previous structural studies with cis triflate ligands (Figure
1).6,10
The distances and angles for the metal-ligand multiple bonds
in 4 are quite similar to those for the two other reported
molybdenum bis(triflate) structures. The differences between
the structure of 4 and these previously reported derivatives
largely reside in the triflate and DME ligands. In the previously
reported complexes the triflates are mutually trans, with
O(triflate)-Mo-O(triflate) angles of 152.3(4) and 153.8(3)°.
In 4, the two triflates are in cis positions in the pseudo-octahedral
compound with an O(triflate)-Mo-O(triflate) angle of 84.0(5)°.
Reaction of 3 with triflic acid (HOTf) presumably produces
an unobserved intermediate neopentylidene bis(triflate) complex.
Formation of the metallacycle occurs by intramolecular olefin
metathesis on the neopentylidene, providing 4. In other words,
triflic acid addition can be seen as initiating imido protolytic
cleavage, R-abstraction on neopentyl to form neopentylidene,
and intramolecular olefin metathesis in a single step.
Properties of Tethered Alkylidene Complexes. Currently,
there are two molybdenum imido alkylidene bis(triflate) deriva-
tives in the Cambridge Structural Database:11 one untethered
derivative reported by Schrock and co-workers10a and our
previously reported tethered derivative 1.6 Both of these
complexes have the two triflate ligands mutually trans. Often,
(9) Kirchhoff, J. H.; Netherton, M. R.; Hills, I. D.; Fu, G. C. J. Am.
Chem. Soc. 2002, 124, 13662.
(10) (a) Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Robbins, J.;
DiMare, M.; O’Regan, M. J. Am. Chem. Soc. 1990, 112, 3875. For a
different approach to a similar complex using aryl isocyanates, see for
example: (b) Bryson, N.; Youinou, M.-T.; Osborn, J. A. Organometallics
1991, 10, 3389.
Figure 1. ORTEP representation of the structure of 4 from an X-ray
diffraction study with hydrogens and ether solvent omitted.
(11) Cambridge Structural Database, version 5.28 (November 2006).

Tethered Molybdenum Alkylidenes
Organometallics, Vol. 27, No. 19, 2008 5133
Scheme 4. Synthesis of Mo(OButF6)2(NAr′)(dCHBut)(quin) (9)
Figure 2. ORTEP representation for the structure of 5 from an
X-ray diffraction study with hydrogens omitted.
One of the triflates is trans to the imido group, and the other is
trans to a DME oxygen. As would be expected, the strongly
trans-influencing imido group provides an Mo-O(11) distance
of 2.226(11) Å, and the triflate oxygen trans to a DME oxygen
is significantly shorter at 2.101(11) Å. Likewise, the DME
oxygen coordinated trans to the alkylidene has a much longer
Mo-O bond, 2.313(12) Å, relative to that trans to triflate,
2.143(12) Å.
In solution, bis(triflate) 4 has access to several different
isomers, as judged by its NMR spectroscopy. For example, at
-60 °C in the 19F NMR spectrum there are three pairs of
resonances, which suggests the presence of three different
isomers with inequivalent triflates. The other nuclei examined
show similar behavior, with H and 13C NMR showing two
different alkylidene resonances at room temperature, for example.
1
distances are typical at 1.743(4) and 1.870(5) Å, respectively.
The angle subtended at N(1) is essentially linear at 175.9(4) Å.
For comparison with 5, we prepared a close isomer not
bearing the tether, Mo(OButF6)2(quin)(NAr′)[dC(H)But] (9),
where Ar′ ) C6H2-2,4-Pri2-6-Me, using the protocol shown in
Scheme 4. The pseudotetrahedral bis(neopentyl) intermediate
7 was also structurally characterized; see the Supporting
Information for details. While 9 only differs in formula from 5
by two hydrogens, its structure (Figure 3) is different in the
solid state in several ways. The five-coordinate complex is best
described as a pseudo trigonal bipyramid: τ ) 0.78 versus 0.13
for 5. The alkylidene carbon occupies one of the pseudoequa-
torial sites, and the axis is the quinuclidine nitrogen and an
alkoxide with an angle subtended at Mo of 162°. The imido
bends somewhat from essentially linear in 5 to 156.6(4)° in 9.
The aryl ring of the imido rotates to place the larger group
toward the syn-alkylidene; the bending of the imido is away
from this unfavorable steric interaction between imido aryl and
alkylidene tert-butyl. However, it has been well documented
that imido angles, especially of heavier congeners like molyb-
denum and tungsten, often have very flat potential energy
surfaces associated with imido bending,14 and it is unlikely that
this imido bending results in a large energetic change relative
to the linear variety found in the tethered complex.
In contrast to imido alkylidenes of molybdenum not contain-
ing the tether, on replacement of the triflates in 4 by hexafluoro-
tert-butoxide (OButF6) using TlOButF6 we were unable to isolate
a stable product. However, TlOButF6 triflate metathesis (Scheme
3) in the presence of quinuclidine (quin) provides isolable
Mo(OButF6)2(quin)(N-2,4-Pri2C6H2-6-CH2CH2CMe2CHd) (5).12
Complex 5 has been structurally characterized, and an ORTEP
representation is shown in Figure 2. The five-coordinate complex
is best described as a pseudo square pyramid with τ ) 0.13,
where τ ) 0 is square pyramidal and τ ) 1 is trigonal
bipyramidal.13 The alkylidene carbon occupies the pseudoaxial
site with angles to the remaining ligands ranging from 96 to
109°. This largest angle to the alkylidene is with the alkoxide
ligand trans to the imido nitrogen, and the angle may be opened
slightly to allow an alkylidene R-agostic interaction. The Mo-O
distances are not significantly different, despite one being trans
to imido and the other trans to quin. The ModN and ModC
(12) For similar adduct formation by alkylidene complexes: (a) Schrock,
R. R.; Crowe, W. E.; Bazan, G. C.; DiMare, M.; O’Regan, M. B.; Schofield,
M. H. Organometallics 1991, 10, 1832. (b) Schrock, R. R.; Shifang, L.;
Zanetti, N. C.; Fox, H. H. Organometallics 1994, 13, 3396. (c) Cantrell,
G. K.; Geib, S. J.; Meyer, T. Y. Organometallics 1999, 18, 4250.
(13) The τ value is calculated using the equation t ) (R-ꢀ)/60, where
R and ꢀ are the largest and next to largest angles around the metal center.
For an ideal square pyramid τ ) 0, and τ ) 1 for an ideal trigonal
bipyramid: (a) Alvarez, S.; Llunell, M. Dalton Trans. 2000, 3288. (b)
Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn, J.; Verschoor, G. C.
Dalton Trans. 1984, 1349. (c) Zabrodsky, H.; Peleg, S.; Avnir, D. J. Am.
Chem. Soc. 1992, 114, 7843.
Of greater possible consequence are the angles associated with
the alkylidene (Figure 4). The alkylidene CH in Schrock’s
(14) (a) Ciszewski, J. T.; Harrison, J. F.; Odom, A. L. Inorg. Chem.
2004, 43, 3605. (b) Gibson, V. C.; Redshaw, C.; Clegg, W.; Elsegood,
M. R. J. Polyhedron 2007, 26, 3161.

5134 Organometallics, Vol. 27, No. 19, 2008
Lokare et al.
the tethered derivative has a slightly attenuated R-agostic
interaction. The applications of these new tethered derivatives
of Schrock’s catalyst to various forms of olefin metathesis are
currently being explored and are very effective catalysts for
common applications such as ring-closing metathesis.16 The
alternative architecture found here may provide interesting
differences from untethered analogues and provide new op-
portunities to study the effects of these structure types on
reactivity and electronic structure.
Experimental Section
General Considerations. All manipulations of air-sensitive
materials were carried out in an MBraun glovebox under an
atmosphere of purified nitrogen. Ethereal solvents, pentane, and
toluene were purchased from Aldrich Chemical Co. and purified
through alumina columns to remove water after sparging with N2
to remove oxygen. NMR solvents (C6D6 and CDCl3) were
purchased from Cambridge Isotopes Laboratories, Inc. Deuterated
benzene was distilled from purple sodium benzophenone ketyl.
Deuterated chloroform was distilled from CaH2 under dry N2. NMR
solvents were stored under sealed containers equipped with a Teflon
stopcock in the drybox prior to use. Spectra were taken on Varian
instruments located in the Max T. Rogers Instrumentation Facility
at Michigan State University. Routine coupling constants are not
reported. All NMR signals are given in ppm. Some of the
assignments are tentative, due to the large number of overlapping
peaks; in such cases the spectra are provided in the Supporting
Information. The 13C NMR assignments are based on decoupled
13C, peak heights for overlapping signals, and DEPT experiments.
Combustion analyses were performed by facilities in the Department
of Chemistry at Michigan State University. Alumina, Celite, and
silica were dried at a temperature >200 °C under dynamic vacuum
for at least 16 h and then stored under an inert atmosphere.
HB(Pin)17 and Ir(Indenyl)(COD)18 were prepared as described in
the literature. Most conveniently, HB(Pin) supplied by BASF in
NEt3-stabilized form was also employed, which can be used without
purification. 1,3-Diisopropylbenzene was purchased from Aldrich
Chemical Co. and was distilled from purple sodium benzophenone
ketyl. Sodium metaperiodate, acetic acid, acetic anhydride, triethy-
lamine, and aluminum chloride were purchased from Spectrum
Chemical Co. and used without purification. 5-Bromo-3,3-dimeth-
ylpent-1-ene was prepared as described in the literature.19 Potassium
tert-butoxide, Pd(OAc)2, P(tBu)2Me, fuming nitric acid, triflic acid,
LiAlH4, and dmpe were purchased from Aldrich Chemical Co. and
used without purification. tert-Amyl alcohol was purchased from
TCI Chemical Co., distilled over magnesium turnings, stored over
molecular sieves (3 Å, 1/16 in. pellets), and degassed under nitrogen
prior to use. Thallium ethoxide was purchased from Strem Chemical
Co. and was degassed before use. Quinuclidine hydrochloride was
purchased from Aldrich Chemical Co., was basified using K2CO3,
and was crystallized from ether/pentane at -35 °C. 2-Chloropropane
was purchased from Acros Chemical Co. and was used without
purification. Neopentyllithium was prepared as described in the
literature.20
Figure 3. ORTEP representation for the structure of 9 from an
X-ray diffraction study with hydrogens and ether solvent omitted.
Figure 4. Structural comparisons between tethered 5 and untethered
9. L ) quinuclidine.
catalyst has an R-agostic interaction leading to larger than
normal Mo-CR-R angles and depressed JCH couplings relative
to common sp2-hybridized carbons. The tether appears to reduce
the Mo-CR-R angle by about 12° relative to the untethered
derivative. In addition, there is an 11° change in the N(imido)-
Mo-C(alkylidene) angle, with this parameter for the cyclic
derivative being significantly smaller. This leads to a rise in
the alkylidene JCH coupling of about 4 Hz, presumably due to
a slightly reduced R-agostic interaction in the tethered com-
plex.15
Concluding Remarks
The synthesis of tethered molybdenum alkylidenes has been
improved to the point where these specialized catalysts can be
made on relatively large scales. The required tethering aniline
can be prepared as a single isomer with workup procedures not
involving rigorous column chromatography. The synthesis of
the bis(imido)bis(neopentyl)molybdenum(VI) proceeded through
the usual route. On addition of triflic acid, an imido group was
protolytically cleaved with concomitant R-abstraction to form
an unobserved neopentylidene, which was trapped by the
pendant olefin to generate the metallacyclic bis(triflate). Me-
tathesis of the triflates to alkoxides was best accomplished with
the thallium salts. This occurred to give an isolable hexafluoro-
tert-butoxide complex in the presence of quinuclidine.
Preparation of Thallium(I) Hexafluoro-tert-butoxide. In a 120
mL Erlenmeyer flask was loaded HOButF6 (1.50 g, 8.24 mmol), a
stir bar, and pentane (8 mL). To the stirred solution of the alcohol
was added TlOEt (2.055 g, 8.24 mmol) in pentane (10 mL). The
reaction mixture was capped with a septum and stirred for 14 h.
Structurally characterized examples of electronically similar
tethered and untethered hexafluoro-tert-butoxide complexes
provided evidence, supported by JCH couplings from NMR, that
(16) Lokare, K. S.; Odom, A. L. Manuscript in preparation.
(17) Tucker, E. C.; Davidson, J.; Knochel, P. J. Org. Chem. 1992, 57,
3482.
(15) For examples of five-coordinate alkylidene JCH couplings see ref
12. For general references on agostic interactions, see: Brookhart, M.; Green,
M. L. H.; Parkin, G. Proc. Natl. Acad. Sci. 2007, 104, 6908. Brookhart,
M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 1988, 36, 1.
(18) Merola, J. S.; Kacmarcik, R. T. Organometallics 1989, 8, 778.
(19) Srikrishna, A.; Dethe, D. H.; Kumar, P. R. Tetrahedron Lett. 2004,
45, 2939.
(20) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359.

Tethered Molybdenum Alkylidenes
Organometallics, Vol. 27, No. 19, 2008 5135
Volatiles were removed under vacuum. The resulting white solid
was crystallized from ether-pentane (1:1) at -35 °C, which
provided 2.54 g (80%) of purified thallium alkoxide.21 1H NMR
(300 MHz, acetone-d6): 1.60 (sept, CH3, JHF ) 1.2 Hz). 13C{1H}
NMR (75.6 MHz, acetone-d6): 128 (q, CF3, JCF ) 288.45 Hz), 77.18
(sept, CMe(CF3)2, JCF ) 27.55 Hz). 19F NMR (282 MHz, acetone-
d6): -78.06. Mp: 155-157 °C.
CH(CH3)2, JCH ) 6.9 Hz), 2.60-2.54 (m, 2 H, CH2), 1.71-1.65
(m, 2 H, CH2), 1.32 (d, 12 H, CH3, JCH ) 6.9 Hz), 1.15 (s, 6 H,
CH3). 13C{1H} NMR (75.6 MHz, CDCl3): 149.07, 148.53, 143.31,
124.15, 122.23, 110.9, 45.10, 37.02, 34.45, 31.58, 27.01, 24.39.
Anal. Calcd for C19H30: C, 88.28; H, 11.72. Found: C, 88.35; H,
12.10. MS (EI): m/z 258 (M+). Rf ) 0.82 (SiO2, hexane-ethyl
acetate 8:2).
Preparation of 3,5-Diisopropylphenylborane Pinacolate (B). In
a glovebox, Ir(Indenyl)(COD) (416 mg, 1 mmol, 2 mol%), dmpe
(146 mg, 1 mmol, 2 mol%), HBPin (6.4 g, 0.05 mol), and 1,3-
diisopropylbenzene (170.4 g, 1.05 mol) were placed in a 1 L
Schlenk flask equipped with a stir bar. The flask was closed with
a septum, taken outside the glovebox, and stirred at room temper-
ature for 20 min. The flask was purged with a continuous flow of
purified N2 and heated in an oil bath at 130 °C for 28 h. The reaction
mixture was cooled to room temperature, poured into CH2Cl2 (200
mL), filtered through a short pad of silica with copious washings
(CH2Cl2, 250 mL), and concentrated in vacuo. 1,3-Diisopropyl-
benzene was removed by distillation in vacuo, leaving essentially
pure product, which could be crystallized from ether at 0 °C as
colorless crystals (11.3 g, 0.039 mol, 78.4%). 1H NMR (300 MHz,
CDCl3): 7.48 (s, 2 H, o-H), 7.17 (s, 1 H, p-H), 2.89 (sept, 2 H,
CH(CH3)2, JCH ) 6.9 Hz), 1.33 (s, 12 H, C(CH3)2), 1.24 (d, 12 H,
CH(CH3)2, JCH ) 6.9 Hz). 13C{1H} NMR (75.6 MHz, CDCl3):
148.11, 130.42, 127.65, 83.57, 34.18, 24.84, 24.06. One aryl carbon,
which we believe to be that adjacent to boron, was not located.
11B NMR (96.2 MHz, CDCl3): 31.15. Anal. Calcd for C18H29BO2:
C, 74.99; H, 10.16. Found: C, 74.65; H, 10.01. Mp: 120-122 °C.
MS (EI): m/z 288 (M+). Rf ) 0.84 (SiO2, CH2Cl2).
Preparation of 1-(3,3-Dimethylpent-4-enyl)-3,5-diisopropyl-2-
nitrobenzene (E). To a flask was added fuming HNO3 (1.6 mL,
90%, d ) 1.5), HOAc (1.5 mL), and Ac2O (1.2 mL), and the
solution was cooled to room temperature before proceeding. This
solution was added dropwise to D (5.8 g, 0.022 mol) in 2 mL of
Ac2O. The reaction mixture was maintained at 0 °C during the
addition. After the addition was complete, the mixture was stirred
at 0 °C for 6 h. The reaction mixture was poured into ice-cold
water (50 mL). The product was extracted with diethyl ether (4 ×
25 mL), and the combined organic layers were washed with portions
of NaHCO3 (250 mL) until no gas formed on addition of the basic
aqueous solution. The organic solution was filtered, and the
separated solids were washed with ether (5 × 40 mL). The
combined ether solutions were dried with MgSO4. The volatiles
were removed in vacuo, providing the product as a yellow oil (5.85
1
g, 0.019 mol, 86%). H NMR (300 MHz, CDCl3): 7.01 (s, 1 H,
aromatic H), 6.95 (s, 1 H, aromatic H), 5.93 (dd, 1 H, CHdCH2,
JCH ) 10.5 Hz, JCH ) 17.7 Hz), 5.03-5.00 (m, 1 H, CHdCH2),
4.96-4.99 (m, 1 H, CHdCH2), 2.93 (sept, 2 H, CH(CH3)2, JCH
)
6.9 Hz), 2.66-2.59 (m, 2 H, CH2), 1.70-1.65 (m, 2 H, CH2), 1.28
(d, 6 H, CH3, JCH ) 6.9 Hz), 1.22 (d, 6 H, CH3, JCH ) 6.9 Hz),
1.08 (s, 6 H, CH3). 13C{1H} NMR (75.6 MHz, CDCl3): 150.99,
147.44, 139.46, 133.64, 125.71, 123.86, 122.12, 111.22, 44.13,
36.69, 34.16, 29.02, 27.08, 26.68, 26.48, 23.81. Anal. Calcd for
C19H29NO2: C, 75.18; H, 9.65; N, 4.62. Found: C, 75.09; H, 10.03;
N, 4.99. MS (EI) m/z ) 302(M+). Rf ) 0.73 (SiO2, hexanes:ethyl
acetate 8:2).
Preparation of 3,5-Diisopropylphenylboronic Acid (C). In a 250
mL round-bottom flask equipped with a stir bar was added B (11.0
g, 0.038 mol), THF-H2O (4:1, 80:20 mL), and NaIO4 (25 g, 0.117
mol, 3 equiv). The mixture was stirred until homogeneous, and
then 2 N HCl (2 mL) was added. The reaction mixture was stirred
at room temperature for 12 h. After 12 h, the reaction mixture was
extracted with ethyl acetate (5 × 30 mL), and the combined organic
extracts were washed with water and brine. The solution was dried
with Na2SO4 and concentrated in vacuo to give a white solid. The
solid was washed with ice-cold pentane to give the desired product
as white flakes (6.97 g, 0.034 mol, 89%). The compound was used
Preparation of 2-(3,3-Dimethylpent-4-enyl)-4,6-diisopropyla-
niline (F). In a glovebox, LiAlH4 (2.93 g, 0.077 mol, 4 equiv) and
diethyl ether (100 mL) were placed in a 250 mL Schlenk flask
equipped with a stir bar. The flask was closed with a rubber septum,
taken outside the glovebox, and kept in a water bath to maintain
the temperature between 16 and 25 °C. To the slurry was slowly
added E (5.85 g, 0.019 mol) over a period of 1 h. After the addition
was complete, the water bath was removed, and the reaction mixture
was stirred overnight at room temperature. The mixture was cooled
to 0 °C using an ice bath, and the excess hydride was quenched by
the dropwise addition of a saturated solution of MgSO4 solution.
The precipitated salts were removed by filtration through Celite
and washing with chloroform (200 mL). The filter cake was washed
again with chloroform (3 × 50 mL). The combined organic
solutions were dried to afford the desired product as a red oil (4.23
1
without further purification. H NMR (300 MHz, CD3CN): 7.46
(s, 1 H, o-H), 7.45 (s, 1 H, o-H), 7.20 (s, 1 H, p-H), 6.04 (s, 2 H,
OH), 2.89 (sept, 2 H, CH(CH3)2, JCH ) 6.9 Hz), 1.23 (d, 6 H
CH(CH3)2, JCH ) 6.9 Hz). 13C{1H} NMR (75.6 MHz, CD3CN):
149.02, 130.39, 128.23, 118.31, 34.95, 24.39. 11B NMR (96.2 MHz,
CD3CN): 29.60. Mp: 142-144 °C.
Preparation of 1-(3,3-Dimethylpent-4-enyl)-3,5-diisopropylben-
zene (D). In a glovebox, Pd(OAc)2 (316 mg, 1.41 mmol, 5 mol%)
and PBut2Me (452 mg, 2.82 mmol, 10 mol%) were placed in a
250 mL Schlenk flask equipped with a stir bar. The flask was closed
with a septum and taken outside the glovebox. To this was added
tert-amyl alcohol (20 mL), C (6.97 g, 0.034 mmol, 1.2 equiv), and
KOBut (9.48 g, 0.084 mmol, 3 equiv). The reaction mixture was
stirred at room temperature for 10 min. To this was added 5-bromo-
3,3-dimethylpent-1-ene18 (4.99 g, 0.028 mmol), and the resulting
heterogeneous reaction mixture was stirred vigorously for 6 h at
room temperature. The reaction was poured into hexanes (200 mL),
filtered through a short pad of Celite with copious washings
(hexanes, 200 mL combined), concentrated, and passed though a
plug of silica gel (250-400 mesh, 400 g) to afford the desired
1
g, 0.015 mol, 80%). H NMR (300 MHz, CDCl3): 6.86 (s, 1 H,
aromatic H), 6.75 (s, 1 H, aromatic H), 5.93 (dd, 1 H, CHdCH2,
JCH ) 10.5 Hz, JCH ) 17.7 Hz), 5.02-5.00 (m, 1 H, CHdCH2),
4.95-4.97 (m, 1 H, CHdCH2), 3.49 (br s, 2 H, NH2), 2.88 (sept,
1 H, CH(CH3)2, JCH ) 6.9 Hz), 2.77 (sept, 1 H, CH(CH3)2, JCH
)
6.9 Hz), 2.41-2.35 (m, 2 H, CH2), 1.59-1.53 (m, 2 H, CH2), 1.23
(d, 6 H, CH3, JCH ) 6.9 Hz), 1.19 (d, 6 H, CH3, JCH ) 6.9 Hz),
1.06 (s, 6 H, CH3). 13C{1H} NMR (75.6 MHz, CDCl3): 147.86,
138.77, 138.64, 132.39, 126.95, 124.59, 121.05, 111.17, 41.84,
36.74, 33.58, 27.95, 27.18, 26.62, 24.35, 22.47. Anal. Calcd for
C19H31N: C, 83.45; H, 11.43; N, 5.12. Found: C, 83.35; H, 11.18;
N, 4.98. MS (EI): m/z 273 (M+). Rf ) 0.35 (SiO2, hexanes-ethyl
acetate 8:2).
1
product as a colorless oil (5.8 g, 0.022 mol, 80%). H NMR (300
MHz, CDCl3): 6.97 (s, 1 H, p-H), 6.93 (s, 2 H, o-H), 5.96 (dd, 1
H, CHdCH2, JCH ) 10.4 Hz, JCH ) 17.7 Hz), 5.06-5.09 (m, 1 H,
CHdCH2), 5.02-5.05 (m, 1 H, CH)CH2), 2.93 (sept, 2 H,
Preparation of Mo(NAr)2Cl2(DME) (2). In a glovebox, in a 250
mL Schlenk flask was loaded (NH4)2MoO4 (0.621 g, 1.827 mmol),
DME (20 mL), and a stir bar. To the suspension was added NEt3
(1.48 g, 0.015 mol), ClSiMe3 (3.37 g, 0.031 mmol), and F (2 g,
0.0073 mmol). The mixture was stirred at room temperature inside
(21) For similarly prepared thallium alkoxides see: Zechmann, C. A.;
Boyle, T. J.; Pedrotty, D. M.; Alam, T. M.; Lang, D. P.; Scott, B. L. Inorg.
Chem. 2001, 40, 2177.

5136 Organometallics, Vol. 27, No. 19, 2008
Lokare et al.
the glovebox for 1 h. After 1 h, the flask was closed with a septum,
partially evacuated, and heated in an oil bath at 70 °C for 3 days.
The reaction was monitored periodically by cooling the reaction
mixture to room temperature, evacuating the headspace of the
Schlenk flask, and taking the flask inside the glovebox. An aliquot
was taken and filtered, the solvent removed, and an NMR spectrum
recorded to follow the ArNHSiMe3/ArN(SiMe3)2 peaks formed
during the reaction. The reaction was allowed to proceed until these
silyl amine peaks all but disappeared from the spectrum. After it
was cooled to room temperature, the flask was partially evacuated
and taken inside the glovebox. The solution was filtered though
Celite. The volatiles of the filtrate were removed in vacuo to give
a dark red viscous oil, which can be crystallized from hexameth-
yldisiloxane to give a brick red powder of the desired product (1.73
the multitude of overlapping peaks. The spectra are included in
the Supporting Information, and some of the key identifiable
resonances are given here. 1H NMR (500 MHz, CDCl3): 14.49 (s,
JCH ) 126 Hz), 13.74 (s, JCH ) 121 Hz). 13C{1H} NMR (126 MHz,
CDCl3): 333 (CH), 324 (CH). 19F NMR (470 MHz, CDCl3): -76.77
(major isomer), -77.88 (minor isomer). Anal. Calcd for
C24H37NO8S2F6Mo: C, 38.87; H, 5.03; N, 1.89 Found: C, 38.74;
H, 5.23; N, 2.21. Mp: 110-112 °C dec.
Preparation of Mo[dN-2,4-Pri2C6H2-2-CH2CH2CMe2CHd]-
(quin)(OButF6)2 (5). In a glovebox, to a frozen solution of 4 (100
mg, 0.135 mmol) in ether-THF (9:1, 1 mL) was added a solution
of quinuclidine (30 mg, 0.269 mmol) in ether-THF (9:1, 1 mL).
The reaction mixture was stirred for 10 min. After 10 min, a
TlO(CF3)2CH3 (104 mg, 2 equiv, 0.269 mmol) solution was added
and stirring continued for 3 h. The solvent then was removed, and
the product was dissolved in pentane. The salts were removed by
filtration through Celite. The product was crystallized at -35 °C
from pentane as yellow crystals (56 mg, 0.07 mmol, 50.2%). Due
to the fluxionality of the resulting tether and multiple isomers15 in
solution, the NMR spectrum is broad and complex. The alkylidene
resonance for the quinuclidine adduct is sufficiently separated from
other resonances to be assigned definitively. The spectra were
scanned and are included in the Supporting Information. The
assignable peaks due to the alkylidene in the major isomer are
provided here. 1H NMR (500 MHz, C6D6): 13.10 (JCH ) 125 Hz).
13C{1H} NMR (126 MHz, C6D6): 294.43. Anal. Calcd for
C33H46N2O2F12Mo: C, 47.98; H, 5.62; N, 3.39. Found: C, 48.50;
H, 5.90; N, 3.42.
Preparation of 1-Methyl-3,5-diisopropylbenzene (G). A two-
necked flask was loaded with toluene (9.21 g, 0.1 mol) and AlCl3
(26.27 g, 0.2 mol, 2 equiv). This mixture was chilled to -40 °C in
a dry ice/acetonitrile bath and stirred vigorously using a mechanical
stirrer. To this slurry was slowly added 2-chloropropane (31.42 g,
0.4 mol, 4 equiv), and the mixture was further stirred vigorously
for another 3 h. The slurry was added to ice-cold water (500 mL),
and this mixture was stirred vigorously for 2 h. The product was
extracted with ether (5 × 120 mL). The combined ether extracts
were washed with water (2 × 100 mL) and brine (2 × 75 mL) and
dried over anhydrous MgSO4. After filtration, the volatiles were
removed in vacuo to afford the desired product as a yellow oil
(16.7 g, 0.094 mol, 94%).22 1H NMR (300 MHz, CDCl3): 6.90 (s,
3 H, aromatic H), 2.86 (sept, 2 H, CH(CH3)2, JCH ) 6.9 Hz), 2.34
(s, 3 H, CH3), 1.26 (d, 12 H, CH3, JCH ) 6.9 Hz). 13C{1H} NMR
(75.6 MHz, CDCl3): 148.81, 137.62, 124.64, 121.78, 34.11, 24.07,
21.51. Anal. Calcd for C13H20: C, 88.54; H, 11.45. Found: C, 88.95;
H, 11.80. MS (EI): m/z 176 (M+). Rf ) 0.65 (SiO2, hexanes-ethyl
acetate 8:2).
Preparation of 2-Methyl-4,6-diisopropyl-1-nitrobenzene (H). To
a flask, was added fuming nitric acid (3.6 mL, 90%, d ) 1.5), acetic
acid (3.5 mL), and acetic anhydride (2.7 mL). The solution was
cooled to room temperature. This solution was added dropwise to
1-methyl-3,5-diisopropylbenzene (8.9 g, 0.05 mol) in 4 mL of acetic
anhydride. The reaction was maintained at 0 °C during the addition
using an ice water bath. After the addition was complete, the
mixture was stirred at 0 °C for 12 h. The reaction mixture was
poured into ice-cold water (100 mL). The product was extracted
with diethyl ether (5 × 50 mL), and the combined organic layers
were washed with a saturated solution of NaHCO3 (500 mL) until
no gas formed on addition of the solution. The organic solution
was filtered, and the separated solids were washed with ether (5 ×
50 mL). The combined ether solutions were dried with anhydrous
MgSO4. The volatiles were removed in vacuo, providing the product
as a light yellow oil. The compound was used without further
purification (10.5 g, 0.047 mol, 95%). The compound was isolated
1
g, 2.17 mmol, 60%). H NMR (300 MHz, CDCl3): 6.85 (s, 2 H,
aromatic H), 6.77 (s, 2 H, aromatic H), 5.81 (dd, 2 H, CHdCH2,
JCH ) 10.5 Hz, JCH ) 17.7 Hz), 4.91-4.82 (m, 4 H, CHdCH2),
3.92 (s, 4 H, OCH2), 3.80 (s, 6 H, OCH3), 3.83 (sept, 2 H,
CH(CH3)2, JCH ) 6.6 Hz), 2.89-2.84 (m, 4 H, CH2), 2.79 (sept, 2
H, CH(CH3)2, JCH ) 6.6 Hz), 1.57-1.51 (m, 4 H, CH2), 1.16 (d,
12 CH3, JCH ) 6.6 Hz), 0.97 (d, 12 CH3, JCH ) 6.6 Hz), 0.94 (s,
12 CH3). 13C{1H} NMR (75.6 MHz, CDCl3): 152.80, 148.87,
147.85, 145.28, 138.99, 122.86, 120.76, 110.05, 71.15, 63.07, 42.57,
36.65, 34.23, 27.35, 26.54, 26.04, 24.65, 23.97. Anal. Calcd for
C42H68N2O2Cl2Mo: C, 63.07; H, 8.57; N, 3.50 Found: C, 63.35;
H, 8.41; N, 3.72. Mp: 145-147 °C dec.
Preparation of Mo(NAr)2(Np)2 (3). In a glovebox, to a near
frozen solution of 2 (1.73 g, 2.169 mmol) in ether 5 mL was added
8.7 mL of a 0.5 M solution of neopentyllithium (4.77 mmol, 2.2
equiv). The solution was warmed to room temperature and stirred
for 6 h. An aliquot of the reaction mixture was filtered through
Celite to remove LiCl and added to dilute nitric acid (0.25 M)
solution. This solution was added to a 20 mL vial containing 50
mg of silver nitrate in 1 mL of distilled water. The absence of a
white precipitate corresponding to AgCl indicated the completion
of the reaction. The volatiles were removed in vacuo, and the
product was redissolved in pentane and filtered through Celite to
remove the lithium chloride. The volatiles of the filtrate were
removed in vacuo, to give a bright red viscous oil (1.93 g, 2.131
mmol, 98%). The oil was used without further purification. 1H NMR
(300 MHz, CDCl3): 6.79 (s, 2 H, aromatic H), 6.74 (s, 2 H, aromatic
H), 5.65 (dd, 2 H, CHdCH2, JCH ) 10.5 Hz, JCH ) 17.7 Hz),
4.77-4.85 (m, 4 H, CHdCH2), 3.44 (sept, 2 H, CH(CH3)2, JCH
)
6.6 Hz), 2.77 (sept, 2 H, CH(CH3)2, JCH ) 6.6 Hz), 2.48-2.36 (m,
4 H, CH2), 2.02 (s, 4 H, CH2), 1.45-1.42 (m, 4 H, CH2), 1.16 (d,
12 CH3, JCH ) 6.6 Hz), 1.12 (s, 18 H, CH3), 0.97 (d, 12 CH3, JCH
) 6.6 Hz), 0.82 (s, 12 CH3). 13C{1H} NMR (75.6 MHz, CDCl3):
152.19, 148.38, 145.37, 142.24, 136.60, 123.14, 120.25, 110.46,
79.17, 42.55, 36.50, 34.18, 33.47, 33.34, 27.91, 26.89, 26.53, 24.06,
23.27.
Preparation of Mo[dN-2,4-Pri2C6H2-2-CH2CH2CMe2CHd]-
(DME)(OTf)2 (4). In a glovebox, a near frozen solution of triflic
acid (225 mg, 1.50 mmol, 3 equiv) in DME (2 mL) was added
dropwise to a near frozen orange solution of 3 (453 mg, 0.5 mmol)
in DME (10 mL). This solution was stirred for 24 h, and then
volatiles were removed in vacuo to give a dark yellow oil. The oil
was then extracted with about 15 mL of chilled toluene and filtered
through Celite. The filtrate was concentrated in vacuo, and the
resulting dark yellow oil was dissolved in ether and layered with
pentane to obtain a bright yellow precipitate. This bright yellow
precipitate was dissolved again in a minimum amount of ether and
layered with an equal amount of pentane to obtain essentially pure
product (190 mg, 0.256 mmol, 51%). The NMR spectroscopic data
are consistent with an approximately 1.8:1 mixture of two major
isomers in CDCl3. The spectra are further complicated, due to the
broadening of some resonances. As a result, the spectra are more
complex than expected, and the assignments are difficult due to
(22) The literature procedure led to an out-of-control exothermic reaction
when attempted. The procedure was modified for a more controlled
reaction: Ghiaci, M.; Asghari, J. Synth. Commun. 1998, 28, 2213.

Tethered Molybdenum Alkylidenes
Organometallics, Vol. 27, No. 19, 2008 5137
Table 1. Structural Parameters for Mo[dN-2,4-Pri2C6H2-2-CH2CH2CMe2CHd](DME)(OTf)2 (4),
Mo(OButF6)2(quin)[dN-2,4-Pri2C6H2-2-CH2CH2CMe2CHd] (5), Mo[N(2,4-Pri2-6-MeC6H2)](neopentyl)2 (7), and
Mo(OButF6)2(quin)[N(2,4-Pri2-6-MeC6H2)][dC(H)But] (9)
4 · 1/2OEt2
5
7 · 1/2(pentane)
38.5H66MoN2
9 · OEt2
formula
formula wt
space group
a (Å)
b (Å)
c (Å)
C26H41F6MoNO8.5S2
777.66
C33H46F12MoN2O2
826.66
C
C37H56F12MoN2O3
900.78
652.87
j
j
P1
Pbcn
P21/c
P1
34.32(2)
14.820(10)
14.379(9)
17.949(2)
10.2839(12)
20.216(2)
9.820(3)
14.543(4)
15.017(4)
94.018(5)
105.146(5)
107.555(5)
1947.8(10)
2
10.550(3)
13.514(3)
15.842(4)
102.763(5)
90.020(5)
107.753(5)
2092.4(8)
2
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
102.473(3)
7314(8)
8
0.546
3643.5(7)
4
0.453
µ (mm-1
)
0.362
1.113
0.403
1.430
Dcalcd (g cm-3
)
1.412
1.507
total no. of rflns
no. of unique rflns (Rint
extinction coeff
23075
4689 (0.1372)
30575
18888
6837 (0.0712)
18236
)
5247 (0.1280)
0.00057(13)
0.0428
6051 (0.0847)
0.0010(5)
0.0535
R(Fo) (I > 2σ)
0.1401
0.2882
0.0702
0.1806
Rw(Fo2) (I > 2σ)
0.0837
0.1159
as a mixture of two isomers with the desired isomer favored 9:1.
1H NMR (300 MHz, CDCl3): 7.02 (s, 1 H, aromatic H), 6.92 (s, 1
H, aromatic H), 2.86 (overlapping sept, 2 H, CH(CH3)2), 2.24 (s,
3 CH3), 1.22 (d, 6 CH3, JCH ) 6.9 Hz), 1.21 (d, 6 CH3, JCH ) 6.9
Hz). 13C{1H} NMR (75.6 MHz, CDCl3): 151.02, 139.61, 128.69,
126.53, 124.66, 122.23, 34.12, 28.99, 23.79, 23.77, 17.41. MS (EI):
m/z 221 (M+). Rf ) 0.73 (SiO2, hexanes-ethyl acetate 8:2).
Preparation of 2-Methyl-4,6-diisopropylaniline (I). In a glove-
box, LiAlH4 (3.6 g, 0.094 mol, 2 equiv) and diethyl ether (300
mL) were placed in a 500 mL Schlenk flask equipped with a stir
bar. The flask was closed with a rubber septum, taken outside the
glovebox, and kept in a water bath to maintain the temperature
between 16 and 25 °C. To the slurry was slowly added 2-methyl-
4,6-diisopropyl-1-nitrobenzene (10.5 g, 0.047 mol, 9:1 mixture of
isomers from the previous step) over a period of 1 h. After the
addition was complete, the water bath was removed, and the reaction
mixture was stirred overnight at room temperature. The mixture
was cooled to 0 °C using an ice bath, and the excess lithium
aluminum hydride was quenched by the dropwise addition of a
saturated solution of MgSO4 solution. The precipitated salts were
removed by filtration through Celite and washed with chloroform
(400 mL). The filter cake was again washed with chloroform (5 ×
50 mL). The combined organic solutions were dried to afford the
mixture as an orange oil. Column chromatography (silica gel,
250-400 mesh, 8:2 hexane-ethyl acetate) afforded the desired
a dark red viscous oil, which can be crystallized from ether/pentane
to give a brick red powder of Mo(NAr′)2(DME)Cl2 (7.6 g, 11.96
mmol, 83%). 1H NMR (300 MHz, CDCl3): 6.86 (s, 2 H, aromatic
H), 6.76 (s, 2 H, aromatic H), 3.96 (s, 4 H, OCH2), 3.85 (s, 6 H,
OCH3), 3.78 (sept, 2 H, CH(CH3)2, JCH ) 6.9 Hz), 2.79 (sept, 2 H,
CH(CH3)2, JCH ) 6.9 Hz), 2.41 (s, 6 CH3), 1.17 (d, 12 H, CH(CH3)2,
JCH ) 6.9 Hz), 1.03 (d, 12 CH(CH3)2, JCH ) 6.9 Hz). 13C{1H}
NMR (75.6 MHz, CDCl3): 153.31, 147.79, 144.46, 134.72, 125.38,
120.83, 71.12, 63.11, 33.94, 27.61, 24.45, 23.87, 18.97. Anal. Calcd
for C30H48N2O2Cl2Mo: C, 56.68; H, 7.63; N, 4.41. Found: C, 56.70;
H, 7.29; N, 4.41. Mp: 175-177 °C dec.
Preparation of Mo(NAr′)2(CH2tBu)2 (7). In a glovebox, to a near
frozen solution of Mo(NAr′)2(DME)Cl2 (2.2 g, 3.46 mmol) in 50
mL of ether was added neopentyllithium (0.541 g, 6.92 mmol, 2
equiv). The solution was warmed to room temperature and stirred
for 5 h. The reaction was monitored in a fashion similar to that
t
discussed previously for Mo(NAr)2(CH2 Bu)2. The reaction mixture
was filtered through Celite to remove LiCl. The volatiles of the
filtrate were removed in vacuo to give a red viscous oil, which
t
was crystallized from ether/pentane to afford Mo(NAr′)2CH2 Bu)2
as orange microcrystals (1.78 g, 2.88 mmol, 83%). 1H NMR (300
MHz, CDCl3): 6.83 (s, 2 H, aromatic H), 6.76 (s, 2 H, aromatic
H), 3.47 (sept, 2 H, CH(CH3)2, JCH ) 6.9 Hz), 2.78 (sept, 2 H,
CH(CH3)2, JCH ) 6.9 Hz), 2.09 (s, 6 H, CH3), 2.01 (s, 4 H, CH2),
1.18 (d, 12 H, CH(CH3)2, JCH ) 6.9 Hz), 1.11 (s, 18 H, CH3), 1.03
(d, 12 H, CH(CH3)2, JCH ) 6.9 Hz). 13C{1H} NMR (75.6 MHz,
CDCl3): 152.84, 145.09, 141.61, 132.37, 125.01, 120.52, 78.89,
33.92, 33.47, 33.44, 28.00, 23.99, 23.27, 19.48. Anal. Calcd for
C36H60N2Mo: C, 70.08; H, 9.82; N, 4.54. Found: C, 70.27; H, 9.88;
N, 4.55. Mp: 153-155 °C dec.
1
product as a yellow oil (5.5 g, 0.029 mol, 62%). H NMR (300
MHz, CDCl3): 6.92 (s, 1 H, aromatic H), 6.84 (s, 1 H, aromatic
H), 3.55 (s, 2 H, NH2), 2.93 (sept, 1 H, CH(CH3)2, JCH ) 6.9 Hz),
2.81 (sept, 1 H, CH(CH3)2, JCH ) 6.9 Hz), 2.20 (s, 3 CH3), 1.29
(d, 6 H, CH(CH3)2, JCH ) 6.9 Hz), 1.24 (d, 6 H, CH(CH3)2, JCH
)
6.9 Hz). 13C{1H} NMR (75.6 MHz, CDCl3): 139.25, 138.68,
131.94, 125.72, 122.17, 121.15, 33.52, 27.93, 24.35, 22.40, 18.07.
MS (EI): m/z 191 (M+). Rf ) 0.70 (SiO2, hexanes-ethyl acetate
8:2).
Preparation of Mo(NAr′)(CHBut)(DME)(OTf)2 (8). In a glove-
box, a near frozen solution of triflic acid (1.3 g, 8.64 mmol, 3 equiv)
in DME (8 mL) was added to a near frozen solution of Mo-
(NAr′)2(CH2But)2 (1.8 g, 2.88 mmol) in DME (30 mL). The reaction
mixture was stirred for 12 h. During this period, the color changed
from bright orange to dark yellow. The volatiles were removed in
vacuo to give a dark yellow oil. The resulting oil was extracted
with cold toluene, and the extract was filtered through a plug of
Celite. The filtrate was concentrated in vacuo. The resulting dark
yellow oil was dissolved in ether, layered with pentane, and allowed
to stand at -35 °C until a bright yellow powder was obtained. The
powder was collected by filtration and recrystallized from layered
1:1 ether-pentane to obtain the pure product (1.3 g, 1.75 mmol,
61%). By NMR spectroscopy there were two isomers visible.
However, one isomer was in much higher concentration than the
Preparation of Mo(NAr′)2Cl2(DME) (6). In a glovebox, a 250
mL Schlenk flask was loaded with (NH4)2MoO4 (2.44 g, 7.18
mmol), 100 mL of DME, and a stir bar. To the suspension was
added NEt3 (5.8 g, 57 mmol), ClSiMe3 (13.25 g, 122 mmol), and
Ar′NH2 (5.5 g, 29 mmol). The mixture was stirred at room
temperature inside the glovebox for 1 h. The flask was closed with
a septum, partially evacuated, and heated in an oil bath at 70 °C
for 12 h. The reaction mixture gradually changed color from light
yellow to orange to dark red in the first couple of hours. The reaction
was monitored in a fashion similar to that discussed previously for
2. After it was cooled to room temperature, the flask was evacuated
and taken inside the glovebox. The solution was filtered though
Celite. The volatiles of the filtrate were removed in vacuo to give
1
other. The spectral resonances for the major isomer are given. H
NMR (300 MHz, CDCl3): 14.08 (s, 1 H, ModCH, JCH ) 121 Hz),

5138 Organometallics, Vol. 27, No. 19, 2008
Lokare et al.
6.97 (s, 1 H, aromatic H), 6.84 (s, 1 H, aromatic H), 4.28 (br s, 3
H, OCH3), 4.07 (br s, 2 H, OCH2), 3.84 (br s, 2 H, OCH2), 3.68
(sept, 1 H, CH(CH3)2, JCH ) 6.9 Hz), 3.52 (br s, 3 H, OCH3), 2.84
(sept, 1 H, CH(CH3)2, JCH ) 6.9 Hz), 2.43 (s, 3 H, CH3), 1.26 (s,
9 H, CMe3), 1.21 (d, 12 H, CH(CH3)2, JCH ) 6.9 Hz). 13C{1H}
NMR (75.6 MHz, CDCl3): 327.88, 151.98, 147.95, 142.26, 130.43,
130.04, 128.28, 128.22, 126.32, 126.12, 124.68, 124.03, 121.30,
73.18, 70.55, 66.07, 62.67, 58.56, 30.56, 28.10, 27.93, 25.26, 23.47,
22.49. 19F NMR (CDCl3): -76.77. Anal. Calcd for
C24H39NO8S2F6Mo: C, 38.75; H, 5.30; N, 1.88. Found: C, 38.32;
H, 5.18; N, 2.09.
65.88, 52.67, 49.54, 46.45, 46.40, 32.39, 31.50, 29.16, 28.61, 27.92,
26.14, 26.02, 25.58, 24.66, 24.26, 24.06, 23.94, 20.60, 19.83, 19.14,
18.60, 16.48, 15.28. Anal. Calcd for C33H48N2O2F12Mo: C, 47.83;
H, 5.84; N, 3.38. Found: C, 47.72; H, 5.72; N, 3.39. Mp: 168-170
°C.
General Considerations for X-ray Diffraction. Crystals grown
from concentrated solutions at -35 °C quickly were moved from
a scintillation vial to a microscope slide containing Paratone N.
Samples were selected and mounted on a glass fiber in wax and
Paratone. The data collections were carried out at a sample
temperature of 173 K on a Bruker AXS platform three-circle
goniometer with a CCD detector. The data were processed and
reduced utilizing the program SAINTPLUS supplied by Bruker
AXS. The structures were solved by direct methods (SHELXTL
v5.1, Bruker AXS) in conjunction with standard difference Fourier
techniques. Structural parameters for 4, 5, 7, and 9 are given in
Table 1.
Preparation of Mo(NAr′)(CHBut)(quin)(OButF6 2
) (9). In a
glovebox, to a frozen solution of Mo(NAr′)(CHBut)(dme)(OTf)2
(700 mg, 0.942 mmol) in ether-THF (9:1, 1 mL) was added a
solution of TlOButF6 (726 mg, 2 equiv, 1.883 mmol) in ether-THF
(9:1, 1 mL). The reaction mixture was stirred for 3 h. Then, the
solvent was removed, and the product was dissolved in pentane.
The salts were filtered away through Celite, and the solvent was
removed in vacuo. The product was redissolved in pentane (2 mL).
To this solution was added a solution of quinuclidine (105 mg,
0.0941 mmol) in pentane (1 mL), and the reaction mixture was
stirred for 40 min. After 40 min, the solution was concentrated to
1 mL and 3-4 drops of THF were added. The solution was kept
Acknowledgment. We thank the Office of Naval Re-
search, the National Science Foundation, the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, the Department of Energy - Defense
Programs, and Michigan State University for financial
support of our research group. We also thank BASF for a
generous gift of HBPin.
in the freezer at -35 °C to afford Mo(NAr′)(CHBut)(quin)OButF6 2
)
as yellow-orange crystals in 11% isolated yield (78 mg, 0.094
mmol). The NMR spectroscopic data are consistent with an
approximately 1:1 mixture of two major isomers in CDCl3; there
1
are at least three minor isomers present. H NMR (300 MHz, 25
Supporting Information Available: Tables, figures, and CIF
files giving results of X-ray diffraction studies and NMR spectra
for Mo(NAr)2(Np)2 (3), Mo[dN-2,4-Pri2C6H2-2-CH2CH2CMe2-
CHd](DME)(OTf)2 (4), Mo[dN-2,4-Pri2C6H2-2-CH2CH2CMe2-
CHd](quin)(OButF6)2 (5), 2-methyl-4,6-diisopropyl-1-nitrobenzene
(H), 2-methyl-4,6-diisopropylaniline (I), and Mo(NAr′)(CHBut)-
(quin)(OButF6)2 (9). This material is available free of charge via
the Internet at http://pubs.acs.org.
°C, CDCl3): 13.10 (s, ModCH, JCH ) 139.06 Hz, anti), 12.40 (s,
ModCH, JCH ) 121 Hz, syn), 7.26-7.12 (m, aromatic), 4.46 (sept,
1 H, JCH ) 7.2 Hz, CH(CH3)2), 3.51 (sept, JCH ) 6.9 Hz,
CH(CH3)2), 3.14 (t, JCH ) 8.1 Hz, CH2), 2.85 (t, JCH ) 7.5 Hz,
CH2), 1.80 (s, CH), 1.46-1.60 (m, CH2), 1.44 (br s, CH2),
1.35-1.32 (m, CH2), 1.30 (s, CH3), 1.26 (s, CH2), 1.24 (s, CH2),
1.22-1.21 (m, CH2), 1.19 (s, CH3), 1.16 (s, CH3). 13C{1H} NMR
(75.6 MHz, CDCl3): 311.04 (anti), 298.75 (syn), 153.32, 151.97,
148.15, 147.57, 146.07, 128.84, 127.85, 124.30, 123.63, 123.57,
OM800650V