Cationic imido alkylidene complexes of molybdenum supported by β-diketonate and β-diketiminate ligands

Article abstract of DOI:10.1021/om060518x

Reaction of Mo(NAr)(CHCMe₂Ph)(OTf)₂(DME) (Ar = 2,6-i-Pr₂C₆H₃) with the lithium salt of various β-diketonates and β-diketiminates leads to complexes of the type Mo(NAr)-(CHCMe₂Ph)(L)(OTf) (L

Full text of DOI:10.1021/om060518x

Organometallics 2006, 25, 4725-4727
4725
Cationic Imido Alkylidene Complexes of Molybdenum Supported by
â-Diketonate and â-Diketiminate Ligands
Zachary J. Tonzetich, Annie J. Jiang, Richard R. Schrock,* and Peter Mu¨ller
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
ReceiVed June 13, 2006
Summary: Reaction of Mo(NAr)(CHCMe2Ph)(OTf)2(DME) (Ar
) 2,6-i-Pr2C6H3) with the lithium salt of Various â-diketonates
and â-diketiminates leads to complexes of the type Mo(NAr)-
(CHCMe2Ph)(L)(OTf) (L ) â-diketonate or â-diketiminate).
Treatment of these compounds with NaB[3,5-(CF3)2C6H3]4 in
dichloromethane affords rare examples of cationic imido
alkylidene complexes. The solid-state structures of two of these
compounds, {Mo(NAr)(CHCMe2Ph)(TMHD)(THF)}{B[3,5-
(CF3)2C6H3]4} and {Mo(NAr)(CHCMe2Ph)(ArCl-Nacnac)}-
{B[3,5-(CF3)2C6H3]4} (TMHD ) tetramethylheptanedionato,
ArCl-Nacnac ) [2,6-Cl2C6H3NC(Me)]2CH), haVe been deter-
mined by X-ray diffraction, and their reactiVity with ethylene
and other simple olefins has been explored.
yield is lower. The base-free species is less soluble than the
THF adduct perhaps as a consequence of formation of an
oligomeric structure.
Treatment of 1 with Na{BArF } (BArF4 ) B[3,5-(CF3)2C6H3]4)
4
in methylene chloride affords the soluble cationic complex
{Mo(NAr)(CHCMe2Ph)(TMHD)(THF)}{BArF } (2) and in-
4
soluble NaOTf (eq 2). Compound 2 is a yellow crystalline solid
Olefin metathesis has become a powerful tool for the synthesis
of new organic molecules and polymers.1 The most well-
developed catalysts that are capable of effecting this transforma-
tion are molybdenum or ruthenium alkylidene compounds.
Molybdenum compounds usually contain a bulky alkylidene
(initially, e.g., neopentylidene), a bulky imido group (e.g., 2,6-
diisopropylphenyl), and two bulky alkoxides (e.g., hexafluoro-
tert-butoxide).2 We sought to prepare cationic Mo alkylidene
complexes in order to study the effect of modification of the
traditional catalyst framework. High oxidation state cationic
olefin metathesis catalysts of tungsten3 and molybdenum4 are
rare, especially those that do not contain a coordinating anion.
There are several reports of catalytically active cationic ruthe-
nium alkylidenes.5
To produce cationic alkylidene compounds that resemble Mo
bis-alkoxide catalysts, we first explored â-diketonates. Reaction
of Mo(NAr)(CHCMe2Ph)(OTf)2(DME) with Li{TMHD} (TMHD
) 2,2,6,6-tetramethylheptane-3,5-dionato) in the presence of
THF produced Mo(NAr)(CHCMe2Ph)(TMHD)(OTf)(THF) (1)
in good yield (eq 1). Compound 1 exists as the syn isomer, as
that is soluble in THF and halogenated solvents. NMR spectra
(in CD2Cl2) of material crystallized from a mixture of dichlo-
romethane and pentane show one isomer with an alkylidene
HR resonance at 13.02 ppm and a CR resonance at 323.3 ppm.
The 1JCH value of 125 Hz is consistent with the formulation of
2 as a syn alkylidene.7,2a
Crystals of 2 suitable for X-ray diffraction were grown by
diffusion of pentane into a saturated CH2Cl2 solution at -25
°C. The solid-state structure of the cation is shown in Figure 1.
The geometry of the complex is best described as a square
pyramid with the alkylidene group occupying the apical position.
The mean deviation of the Mo atom out of the plane formed
by the four basal atoms (O1, O2, O1T, and N1) is 0.3 Å. The
Mo-C1 and Mo-N1 distances are within the range typically
encountered for four-coordinate alkylidene compounds possess-
ing two alkoxide ligands.2a,7b No close contacts were found
(3) (a) Schrock, R. R.; Rocklage, S. M.; Wengrovius, J. H.; Rupprecht,
G.; Fellmann, J. J. Mol. Catal. 1980, 8, 73. (b) Wengrovius, J. H.; Schrock,
R. R. Organometallics 1982, 1, 148. (c) Blosch, L. L.; Gamble, A. S.;
Abboud, K.; Boncella, J. M. Organometallics 1992, 11, 2342. (d) Kress,
J.; Osborn, J. A. J. Am. Chem. Soc. 1983, 105, 6346. (e) Kress, J.; Osborn,
J. A. Angew. Chem. 1992, 104, 1660.
(4) (a) Vaughan, W. M.; Abboud, K. A.; Boncella, J. M. Organometallics
1995, 14, 1567. (b) Vaughan, W. M.; Abboud, K. A.; Boncella, J. M. J.
Organomet. Chem. 1995, 485, 37.
shown through NMR studies and X-ray crystallography.6 It is
soluble in organic solvents and may be crystallized conveniently
from pentane. If THF is absent from the reaction, a base-free
(5) (a) Fu¨rstner, A. Chem. Commun. 1998, 1315. (b) Fu¨rstner, A.; Liebl,
M.; Lehmann, C. W.; Picquet, M.; Kunz, R.; Bruneau, C.; Touchard, D.;
Dixneuf, P. H. Chem. Eur. J. 2000, 6, 1847. (c) Hansen, S. M.; Volland,
M. A. O.; Rominger, F.; Eisentrager, F.; Hofmann, P. Angew. Chem., Int.
Ed. 1999, 38, 1273. (d) Hofmann, P.; Volland, M. A. O.; Hansen, S. M.;
Eisentrager, F.; Gross, J. H.; Stengel, K. J. Organomet. Chem. 2000, 606,
88. (e) Sanford, M. S.; Henling, L. M.; Grubbs, R. H. Organometallics
1998, 17, 5384.
(6) Tonzetich, Z. Unpublished results to be published in due course.
(7) (a) Schrock, R. R.; Crowe, W. E.; Bazan, G. C.; DiMare, M.;
O’Regan, M. B.; Schofield, M. H. Organometallics 1991, 10, 1832. (b)
Feldman, J.; Schrock, R. R. Prog. Inorg. Chem. 1991, 39, 1.
1
species (as judged by H NMR) can be isolated, although the
* Corresponding author. E-mail: rrs@mit.edu.
(1) (a) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003,
42, 4592. (b) Handbook of Metathesis; Grubbs, R. H., Ed.; 2003; Vols.
1-3.
(2) (a) Schrock, R. R. Chem. ReV. 2002, 102, 145. (b) Schrock, R. R.
Chem. Commun. 2005, 2773.
10.1021/om060518x CCC: $33.50 © 2006 American Chemical Society
Publication on Web 08/30/2006

4726 Organometallics, Vol. 25, No. 20, 2006
Communications
alkylidene complexes with ethylene.8 Destruction of alkylidene
complexes through formation of cyclopropanes could explain
the lack of significant catalytic activity in the metathesis
reactions noted above. However, the fact that 3 is observable
and apparently does not lose cyclopropane in the presence of
ethylene begs the question as to how cyclopropanes are formed
and why metathesis activity is not observed. So far we have
not been able to isolate 3.
We turned our attention to the more electron-rich and
sterically demanding â-diketiminates.9 As with the â-diketo-
nates, treatment of Mo(NAr)(CHCMe2Ph)(OTf)2(DME) with
Li{ArX-Nacnac} (ArX-Nacnac ) [2,6-X2C6H3NC(Me)]2CH, X
) Me, 4a; Cl, 4b) afforded the analogous triflate complexes
shown in eq 4. Unlike 1, the â-diketiminate complexes do not
Figure 1. POV-ray rendering (ellipsoids at 50%) of the cation of
2. Hydrogen atoms, solvent molecules, and the anion are not shown.
Selected bond distances (Å) and angles (deg): Mo-C1 ) 1.893(3);
Mo-N1 ) 1.745(2); Mo-O1 ) 2.0677(18); Mo-O2 ) 2.0540-
(18); Mo-O1T ) 2.1385(18); N(1)-Mo-C(1) ) 101.07(10); Mo-
C(1)-C(2) ) 142.4(2); Mo-N(1)-C(21) ) 167.89(18); O(1T)-
Mo-O(2) ) 158.79(8); N(1)-Mo-O(1) ) 156.49(9).
bind THF. Compounds 4a and 4b can be isolated in moderate
to good yield. Both 4a and 4b are soluble in ethers and aromatic
solvents, but are insoluble in pentane. NMR spectra of 4a show
predominantly the presence of the syn isomer in solution (98%),
but spectra of 4b indicate it to be a 5:1 mixture of syn and anti
isomers. The syn alkylidene resonance for 4b in C6D6 appears
at 12.87 ppm compared to 11.56 ppm for 4a. Treatment of 4a
between the cation and either the anion or the cocrystallized
solvent molecules.
Reaction of 2 with diallyl ether and norbornene resulted in
consumption of the original alkylidene, as judged by 1H NMR,
but did not lead to the products of ring-closing metathesis
(RCM) or ring-opening metathesis polymerization (ROMP),
respectively. The lack of productive metathesis of these olefins
led us to examine the reaction of 2 with ethylene. Exposure of
a CD2Cl2 solution of 2 to 1 atm of ethylene at room temperature
yielded several new species, one of which we propose to be a
cationic unsubstituted metallacycle complex (3; eq 3) on the
with NaBArF in CD2Cl2 did not lead to clean formation of a
4
cationic alkylidene complex (according to NMR spectra),
although NaOTf formed and precipitated from solution. In
contrast, treatment of 4b with NaBArF in CH2Cl2 afforded
4
{Mo(NAr)(CHCMe2Ph)(ArCl-Nacnac)}{BArF } (5) cleanly in
4
satisfactory yield (eq 5). The presence of a fifth (donor) ligand
appears necessary to stabilize the cationic complexes toward
decomposition (as with THF in complex 2), a requirement that
in this case can be fulfilled through intramolecular coordination
of the ortho chloride, as shown in an X-ray study (Vide infra).
Compound 5 is an orange-yellow crystalline solid that is soluble
in ethers and halogenated solvents. NMR spectra (CD2Cl2) of
5 are consistent with it being one major isomer (91%) in solution
with an HR resonance at 13.10 ppm and a CR resonance at 320.0
ppm. The JCH value of 116 Hz for the major isomer is consistent
with its formulation as a syn species. Two minor isomers are
also apparent with HR resonances at 15.15 (5.4%) and 14.91
(3.6%) ppm. The minor isomers are believed to be anti
basis of multiplets at 5.75 and 5.43 ppm for the R protons and
1
0.41 and 0.29 ppm for the â protons in its H NMR spectrum.
(The ratio of R to â protons is 2:1.) Similar reactions with 13C2H4
show a doublet resonance at 108.51 ppm (JCC ) 11.5 Hz) and
a triplet resonance at -3.51 ppm (JCC ) 11.5 Hz), consistent
with a metallacyclobutane complex.7b,8a Resonances for the
initial metathesis product, 3-phenyl-3-methylbutene, are present,
along with a singlet resonance at 0.24 ppm (JCH ) 155 Hz),
which we attribute to cyclopropane. In experiments involving
13CH2d13CH2 the singlet resonance for the cyclopropane carbon
atoms was observed at -2.90 ppm. A smaller singlet resonance
is also observable in the 13C NMR spectrum at -3.11 ppm that
we attribute to cyclo-PhCMe2C3H5 derived from the initial
metallacyclobutane. Traces of propylene (<5%) are also
observed in the 13C NMR spectra. The most interesting finding
is the formation of cyclopropanes, which typically are generated
only as minor products, if at all, in reactions of Mo imido
(8) (a) Tsang, W. C. P.; Jamieson, J. Y.; Aeilts, S. L.; Hultzsch, K. C.;
Schrock, R. R.; Hoveyda, A. H. Organometallics 2004, 23, 1997. (b) Wang,
S.-Y. S.; VanderLende, D. D.; Abboud, K. A.; Boncella, J. M. Organo-
metallics 1998, 17, 2628.
(9) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002,
102, 3031.

Communications
Organometallics, Vol. 25, No. 20, 2006 4727
base adducts of four-coordinate alkylidene species with the
alkylidene and imido groups residing in the equatorial plane.2a
Interestingly, cation 5 exists solely as the anti isomer in the
solid-state structure. We do not know whether the bulk
crystalline form consists entirely of the anti isomer or whether
we chose a crystal of an anti isomer for the X-ray experiment.
The Mo-C1 distance of 1.925(18) Å in 5 is slightly longer
than in 2, but in the range encountered for generally longer anti
ModC bonds in Mo(VI) alkylidene species.2a,7b The Mo-Cl4
contact of 2.581(5) Å is consistent with a dative interaction.
The reactivity of 5 with several different olefins in CD2Cl2
was examined. Unlike 2, compound 5 will polymerize 44 equiv
of norbornene at room temperature to give a 75:25 mixture of
cis to trans polynorbornene. Little of the initial 5 is consumed
in the process, consistent with rapid propagation relative to
initiation. Reaction of 5 with 44 equiv of diallyl ether did not
afford dihydrofuran in 24 h at room temperature; we believe
that coordination of the ether functionality to the electrophilic
metal center is responsible for a lack of productive metathesis
of diallyl ether. Compound 5 was found to react with allyltri-
methylsilane and diallyldimethylsilane to give a new alkylidene,
with triplet HR resonances at 13.57 and 14.42 ppm, respectively.
Exposure of 1 atm of propylene to a solution of 5 in CD2Cl2
yielded a mixture of cis and trans 2-butene, ethylene, and
propylene. These results suggest that complex 5 is a metathesis
initiator, although at this time we do not know how long-lived
various alkylidenes are in various reactions and what the
processes are that lead to catalyst decomposition and loss of
activity.
Figure 2. POV-ray rendering (thermal ellipsoids at 50%) of the
cation of 5. Hydrogen atoms, solvent molecules, the anion, the
phenyl group of the neophylidene, and the dichlorophenyl sub-
stituent bound to N2 are not shown for clarity. Selected bond
distances (Å) and angles (deg): Mo-C1 ) 1.925(18); Mo-N3 )
1.732(7); Mo-N1 ) 2.109(3); Mo-N2 ) 2.066(3); Mo-Cl4 )
2.581(5); N3-Mo-C1 ) 102.1(10); N1-Mo1-N2 ) 84.47(11);
Mo1-C1-C2 ) 135.3(1), Mo1-N3-C31 ) 166.2(15); Cl4-Mo-
N1 ) 164.3(3).
alkylidenes with relatively strong Mo-Cl interactions. (Bases
In conclusion, we have prepared the first cationic versions
of Mo imido alkylidene catalysts that contain a weakly
coordinating anion and have explored their initial metathesis
reactivity. Synthesis of compounds that contain other Nacnac
ligands and imido groups is currently underway. We suspect
that there will be some fundamental differences in reactions
that involve cationic species compared to the neutral bis-alkoxide
catalysts.
have been shown to bind more strongly to anti alkylidenes.2a,7a
)
The solution NMR spectrum of the major isomer of 5 at 20
°C in CD2Cl2 displays single resonances for both the neo-
phylidene methyl groups and the isopropyl groups of the aryl
imido; the structure in solution has a mirror plane of symmetry
and a freely rotating 2,6-diisopropylphenyl ring on the NMR
time scale. At -90 °C (see Supporting Information, Figure S1)
the NMR spectrum of 5 is consistent with the structure depicted
in eq 5, with no rotation of the 2,6-diisopropylphenyl ring about
the N-Cipso axis. Therefore if a donor chloride to metal bond
persists in solution, it must be too weak to maintain the structure
shown in eq 5 on the NMR time scale at room temperature.
Intramolecular weak coordination of a chloride is an interesting
way to create higher coordinate compounds that are more stable
to intermolecular reactions, yet react readily upon dissociation
of the chloride from the metal.
Acknowledgment. R.R.S. thanks the National Science
Foundation (CHE-0138495) for supporting this research. Z.J.T.
also acknowledges the NSF for a predoctoral fellowship and
the Alan Davison Fellowship fund for financial support. The
authors thank Mr. Rojendra Singh for insightful discussions.
Supporting Information Available: Complete experimental
details for all compounds, VT NMR spectrum of 5, and thermal
ellipsoid diagrams and crystallographic information files for 2 and
5. Supporting Information is available free of charge via the Internet
at http://pubs.acs.org. Data for the X-ray structure is also available
to the public at http://www.reciprocalnet.org/ (2 ) 05220; 5 )
06070).
Diffusion of pentane into a saturated CH2Cl2 solution afforded
crystals of 5 suitable for X-ray diffraction. The solid-state
structure of the cation of 5 is shown in Figure 2. The geometry
is best described as a trigonal bipyramid with Cl4 and one of
the Nacnac nitrogen atoms (N2) occupying the axial positions.
Unlike 2, the structure of 5 is similar to the structures of various
OM060518X