Tridentate amido carbene ligands in early-transition-metal coordination chemistry

Article abstract of DOI:10.1021/om0498089

Coordination of the new tridentate N-hetero-cyclic carbene ligand precursor (p-Me-C₆H₄NHCH₂-CH₂) ₂N₂C₃H₂ to Zr(IV) generates an [NCN] donor set where the N represents

Full text of DOI:10.1021/om0498089

3372
Organometallics 2004, 23, 3372-3374
Tr id en ta te Am id o Ca r ben e Liga n d s in
Ea r ly-Tr a n sition -Meta l Coor d in a tion Ch em istr y
Liam P. Spencer, Scott Winston, and Michael D. Fryzuk*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver,
British Columbia, Canada V6T 1Z1
Received March 16, 2004
Summary: Coordination of the new tridentate N-hetero-
cyclic carbene ligand precursor (p-Me-C6H4NHCH2-
CH2)2N2C3H2 to Zr(IV) generates an [NCN] donor set
where the N represents an amido unit and the C corres-
ponds to the N-heterocyclic carbene. This ancillary
ligand allows for the isolation and structural charac-
terization of zirconium amido, chloro, and alkyl deriva-
tives; the carbene donor is rendered stable to dissociation
because of its central disposition between two anionic
amido units.
flanked by two amido ligands. This is modeled after
related diamidophosphine ligands that we have reported
previously.5 Two recent publications have described a
bidentate amidocarbene and a tridentate amidodicar-
bene.4 Samarium and yttrium complexes stabilized by
a bidentate amidocarbene did show displacement of the
carbene donor by Me2NCH2CH2NMe2 and Ph3PdO.4a
To construct an NHC with two pendant amine arms,
the reduction of a bis(amide) imidazolium chloride was
examined. The reaction of borane-dimethyl sulfide with
an easily assembled amide precursor6 (1) gives the
desired bis(amino)-imidazolium chloride in reasonable
yield. Subsequent deprotonation by KN(SiMe3)2 selec-
tively generates the expected free NHC 2 (eq 1). Both
Investigation into the use of N-heterocyclic carbene
(NHC) ligands in modern coordination chemistry and
homogeneous catalysis has become extremely popular,
almost to the point of being de rigueur. In a remarkably
short period of time, NHC donors have gone from
academic curiosities to essential ancillary ligands
throughout the periodic table.1 The ubiquity of NHC’s
is due to their strong donor properties, which are
enhanced by the diversity of N substituents and het-
erocyclic ring sizes that provide for some tunability of
stereoelectronic effects.2 Probably the aspect that has
most propelled the use of NHC’s to frenetic levels has
been their ability to replace phosphine ligands to
generate catalyst precursors more robust and versatile
than their phosphine congeners.3
With respect to the use of NHC’s with the early
transition elements (groups 3-6) and the lanthanides,
less is known about how susceptible these neutral
donors are to dissociation.4a For d0 systems such as
Zr(IV), neutral ligands are prone to exchange typically
via dissociation. In an effort to construct systems which
would reduce the tendency for ligand dissociation, we
have designed and synthesized a potentially tridentate
carbene donor system that has the neutral NHC donor
1H and 13C NMR spectra are consistent with a sym-
metrical molecule; diagnostic of the carbene moiety is
the absence of the resonance attributed to the iminium
proton (at C2), an upfield shift in the heterocycle protons
in the C4,5 positions, and a weak 13C resonance at 211.4
ppm for the carbene carbon.7
The incorporation of the [NCN] precursor 2 onto
zirconium is easily achieved by either amine or alkyl
elimination via protonolysis reactions. For example,
aminolysis of Zr(NEt2)4 with 2 proceeds smoothly at
room temperature to afford tol[NCN]Zr(NEt2)2 (3)
(tol[NCN] ) (p-Me-C6H4NCH2CH2)2N2C3H2) in good
* To whom correspondence should be addressed. E-mail: fryzuk@
chem.ubc.ca.
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Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. Rev.
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Commun. 1999, 241. (b) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J .
Am. Chem. Soc. 2003, 123, 13314. (c) Weiss, R.; Reichel, S.; Handke,
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Kariuki, B. M. Chem. Commun., in press.
10.1021/om0498089 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 06/09/2004

Communications
Organometallics, Vol. 23, No. 14, 2004 3373
yield. Complex 3 was characterized by 1H and 13C NMR
F igu r e 1. ORTEP view of tol[NCN]ZrCl2(py) (5), depicted
with 50% thermal ellipsoids; all hydrogen atoms have been
omitted for clarity. Selected bond distances (Å) and angles
(deg): Zr(1)-C(1) ) 2.391(5), Zr(1)-N(3) ) 2.167(4), Zr(1)-
N(4) ) 2.109(4); C(1)-Zr(1)-N(5) ) 84.02(18), N(4)-Zr(1)-
Cl(1) ) 94.77(14), N(4)-Zr(1)-N(3) ) 155.27(16), N(4)-
Zr(1)-Cl(1) ) 94.77(14), C(1)-Zr(1)-Cl(1) ) 170.38(14).
based NHC compounds (2.432(3)-2.456(3) Å),9 likely a
result of the ligand architecture that pulls the carbene
donor closer to the metal. The Zr-N amido bond lengths
average to 2.138(4) Å and are comparable to those of
other Zr-amide complexes.10 The Zr-Cl bond distances
are not unusual. The Zr-N bond length of the pyridine
donor is 2.398(5) Å and is normal.
The preparation of the bis((trimethylsilyl)methyl)
zirconium complex tol[NCN]Zr(CH2SiMe3)2 (6) was per-
formed by both alkylation and protonolysis methods.
spectroscopy, which showed resonances attributable to
equivalent NEt2 moieties along with equivalent amido
sidearms of the tol[NCN] unit; on the basis of these data
a C2v-symmetric geometry can be assigned to this five-
coordinate complex. The 13C NMR spectrum shows a
resonance at 188.8 ppm due to the coordinated carbene
carbon. Treatment of 3 with excess Me3SiCl in toluene
over 2 days gives the dichloride tol[NCN]ZrCl2 (4) in
quantitative yield. The dichloride is extremely insoluble
in common organic solvents, which may indicate a
dimeric structure in the solid state;8 although no solu-
tion spectroscopic data could be obtained, the empirical
formula of 4 was confirmed by mass spectrometry and
elemental analysis. To solubilize this material, pyridine
was added to a suspension of 4, and this resulted in the
benzene-soluble pyridine adduct tol[NCN]ZrCl2(py) (5),
which allowed full characterization by spectroscopic and
X-ray diffraction studies. The NMR data for 5 clearly
show the presence of resonances for coordinated pyri-
dine along with the [NCN] resonances; the Zr-carbene
carbon in 5 appears as a weak singlet at 187.9 ppm.
Crystals of 5 were grown from concentrated benzene
solution and analyzed by single-crystal X-ray diffraction;
the ORTEP diagram is shown in Figure 1. In the solid
state, the zirconium center is coordinated by the tri-
dentate tol[NCN] ligand in addition to pyridine. The two
chlorides adopt a mutually cis disposition, with the
[NCN] ligand in a meridional orientation to generate a
pseudo-octahedral arrangement around the central Zr
atom. Both of the six-membered chelate rings of [NCN]
are nearly planar, with N(2)-C(1)-Zr(1)-N(4) and
N(1)-C(1)-Zr(1)-N(3) torsion angles of -1.2 and -7.5°,
respectively; however, each ring has the carbon R to the
amido donor sitting above or below in the solid state.
The Zr(1)-C(1) bond length (2.391(5) Å) is significantly
shorter than previously characterized lengths of Zr-
The reaction of Zr(CH2SiMe3)4 with 2 led to the desired
product 6 via SiMe4 elimination. This complex could also
be synthesized from the reaction of dichloride 4 and 2
equiv of LiCH2SiMe3; however, a better yield is obtained
via the alkane elimination method at -30 °C. Complex
6 was isolated as air- and moisture-sensitive pale yellow
crystals. On exposure to light, solutions of 6 darkened
to unidentified products within ∼1 h. To our knowledge,
6 is the first reported zirconium dialkyl with an NHC-
based ancillary ligand. The X-ray diffraction study
(9) Niehues, M.; Kehr, G.; Erker, G.; Wibbeling, B.; Fro¨hlich, R.;
Blacque, O.; Berke, H. J . Organomet. Chem. 2002, 663, 192.
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Schrock, R. R.; Seidel, S. W.; Schrodi, Y.; Davis, W. M. Organometallics
1999, 18, 428. (c) Zhang, X.; Zhu, Q.; Guzei, I. A.; J ordan, R. F. J . Am.
Chem. Soc. 2000, 122, 8093. (d) Skinner, M. E. G.; Li, Y.; Mountford,
P. Inorg. Chem. 2002, 41, 1110.
(8) (a) Okuda, J . Dalton 2003, 2367. (b) Arndt, S.; Okuda, J . Chem.
Rev. 2002, 102, 1953. (c) Evans, W. J .; Drummond, D. K.; Grate, J .
W.; Zhang, H.; Atwood, J . L. J . Am. Chem. Soc. 1987, 109, 3928. (d)
Fagan, P. J .; Manriquez, J . M.; Marks, T. J .; Vollmer, S. H.; Day, C.
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3374 Organometallics, Vol. 23, No. 14, 2004
Communications
cal shift of the N-heterocyclic carbene carbon resonance
was monitored. In d5-pyridine this resonance was
observed at 181.0 ppm, while in d6-benzene, it appeared
at 187.9 ppm; the free carbene resonance observed for
2 is found at 211.4 ppm in d6-benzene. A second
experiment involved the addition of 10 equiv of Me2-
NCH2CH2NMe2 to a d6-benzene solution of 5; in this
case no change in the 13C NMR resonance of the carbene
carbon was observed. Both of these experiments are
consistent with the carbene carbon atom of the NHC in
the [NCN] ancillary ligand remaining bound to the Zr
center, confirming that the flanking amido donors do
anchor the NHC to the Zr(IV) center.
In conclusion, we have demonstrated the synthesis
and coordination chemistry of a potentially dianionic,
tridentate N-heterocyclic carbene system. By flanking
the NHC with two amine pendant donor groups, which
can be deprotonated to generate two anionic amido
donors, the carbene donor has been forced to bind to
Zr(IV) by virtue of its position in the chelate array.
Procedures that involve protonolysis and alkylation are
tolerated and have been used to synthesize both halide
and alkyl zirconium compounds in good yields. The
stability of the NHC donor to dissociation in this
particular tridentate motif suggests that its influence
on the chemistry of early metals will be measurable.
Further research in this area will involve not only
ancillary ligand modification but also the application
of these and related complexes in homogeneous catalysis
and dinitrogen activation.12
F igu r e 2. ORTEP view of tol[NCN]Zr(CH2SiMe3)2 (6),
depicted with 50% thermal ellipsoids; all hydrogen atoms
have been omitted for clarity. Selected bond distances (Å)
and angles (deg): Zr(1)-C(1) ) 2.415(3), Zr(1)-N(3) )
2.173(2), Zr(1)-N(4) ) 2.135(2), Zr(1)-C(23) ) 2.238(3),
Zr(1)-C(24) ) 2.254(3); N(3)-Zr(1)-C(1) ) 78.23(9), N(4)-
Zr(1)-C(1) ) 76.27(9), C(24)-Zr(1)-C(1) ) 115.29(10),
N(4)-Zr(1)-N(3) ) 148.73(9).
(Figure 2) of a single crystal of 6 revealed a distorted-
trigonal-bipyramidal geometry around the zirconium
metal center. The zirconium-carbene bond length (Zr(1)-
C(1)) is 2.415(3) Å, which is slightly longer than that
found in 5 but still shorter than the corresponding bond
distances reported for NHC zirconium complexes of the
type trans-ZrCl4L2 (L ) NHC).9 The [NCN] ligand in 6
is more distorted from meridional than that found in 5;
the N4-Zr-N3 bond angle is only 148.73(9)° in 6,
considerably less than the analogous angle of 155.27(16)°
found in 5. The only organometallic group 4 complexes
previously reported are the titanocene and zirconocene
methyl cations [Cp2MMe(IPr)]+ (M ) Ti, Zr; IPr ) 1,3-
diisopropylimidazol-2-ylidene), which contain a neutral
monodentate NHC; only the titanocene complex was
structurally characterized.11
Ack n ow led gm en t. We thank the NSERC of Canada
for funding (Discovery Grant to M.D.F. and an NSERC
PGS-B Scholarship to L.P.S.).
Su p p or tin g In for m a tion Ava ila ble: Text, tables, and
figures giving complete experimental details and information
on X-ray data collection and processing for 5 and 6 (PDF);
X-ray data for 5 and 6 are also given as CIF files. This
information is available free of charge via the Internet at
http://pubs.acs.org.
The aspect of carbene lability in tol[NCN]ZrCl2(py) (5)
was addressed in two ways: in the first, the 13C NMR
spectrum of 5 in d5-pyridine, a strongly coordinating
solvent, was measured and compared to that obtained
in noncoordinating d6-benzene; in particular, the chemi-
OM0498089
(11) Niehues, M.; Erker, G.; Kehr, G.; Schwab, R.; Fro¨hlich, R.;
Blacque, O.; Berke, H. Organometallics 2002, 21, 2905.
(12) (a) Shaver, M. P.; Fryzuk, M. D. Adv. Synth. Catal. 2003, 345,
1061. (b) MacKay, B. A.; Fryzuk, M. D. Chem. Rev. 2004, 104, 381.