Zirconium alkyl complexes supported by ureate ligands: Synthesis, characterization, and precursors to metal-element multiple bonds

Article abstract of DOI:10.1021/om100381d

A series of bis(ureate) zirconium complexes bearing reactive alkyl ligands have been prepared and fully characterized. A sterically demanding, nontethered ureate ligand was successfully employed in the synthesis of a dibenzyl complex; however synthesis can be complicated by suspected ligand disproportionation and redistribution. In contrast, tethered bis(ureate) ligands, including a chiral, C₂-symmetric ligand, are reliable supports for sterically accessible, mononuclear dibenzyl and bis(neopentyl) complexes. These coordinatively unsaturated species react with pyridine to form seven-coordinate adducts that exhibit remarkable thermal and photochemical stability. The reactive nature of the dibenzyl derivative can be exploited in the synthesis of imido complexes via protonolysis with 2,6-diisopropylaniline. A monometallic imido, supported by a sterically open ureate ligand, can be prepared in this manner with the use of excess pyridine; however, NMR spectroscopy indicates that this species undergoes dimerization in solution. When the reaction is performed in the absence of pyridine, only the dimeric complex is obtained.

Full text of DOI:10.1021/om100381d

5162 Organometallics 2010, 29, 5162–5172
DOI: 10.1021/om100381d
Zirconium Alkyl Complexes Supported by Ureate Ligands: Synthesis,
Characterization, and Precursors to Metal-Element Multiple Bonds†
David C. Leitch and Laurel L. Schafer*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada, V6T 1Z1
Received April 30, 2010
A series of bis(ureate) zirconium complexes bearing reactive alkyl ligands have been prepared and fully
characterized. A sterically demanding, nontethered ureate ligand was successfully employed in the
synthesis of a dibenzyl complex; however synthesis can be complicated by suspected ligand disproportion-
ation and redistribution. In contrast, tethered bis(ureate) ligands, including a chiral, C2-symmetric ligand,
are reliable supports for sterically accessible, mononuclear dibenzyl and bis(neopentyl) complexes. These
coordinatively unsaturated species react with pyridine to form seven-coordinate adducts that exhibit
remarkable thermal and photochemical stability. The reactive nature of the dibenzyl derivative can be
exploited in the synthesis of imido complexes via protonolysis with 2,6-diisopropylaniline. A monometallic
imido, supported by a sterically open ureate ligand, can be prepared in this manner with the use of excess
pyridine; however, NMR spectroscopy indicates that this species undergoes dimerization in solution.
When the reaction is performed in the absence of pyridine, only the dimeric complex is obtained.
Introduction
our research group has developed a series of modular, mixed
donor ligands for the generation of very electropositive early
transition metal centers.6 We have used one such ligand class,
amidates, in the synthesis of group 3, 4, and 5 amido complexes,
many of which exhibit remarkable reactivity in catalytic C-N,
C-O, and C-C bond forming reactions.7-9
Transition metal alkyl complexes represent a broad and
versatile class of organometallic compounds that are widely
used throughout synthetic chemistry.1 Group 4 alkyl derivatives
in particular possess a rich chemistry, being extensively used as
initiators in the production of polyolefins2 and as reagents and/
or catalysts for organic synthesis.3 The range of known struc-
tures spans from simple, homoleptic tetra(alkyl) species4 to
group 4 alkyl fragments supported by a variety of ancillary
ligand frameworks.2,5 A plethora of research has been directed
toward developing the latter category, with the goal of inducing
new reactivity patterns through steric and electronic manipula-
tion. Within this theme of exploring ligand-driven reactivity,
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† Part of the Dietmar Seyferth Festschrift. Dedicated to Professor Dietmar
Seyferth for his leadership and outstanding contributions to organometallic
chemistry.
*To whom correspondence should be addressed. Fax: 1-604-822-
2847. E-mail: schafer@chem.ubc.ca.
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pubs.acs.org/Organometallics
Published on Web 07/15/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5163
Figure 1. Electronic comparison of amidate and ureate ligands.
Notably, mixed amidate-amido complexes display significant
multiple-bond character between the metal center and the
amido ligands, thereby increasing the formal electron count at
these electron-deficient metal centers. In contrast, there are rela-
tively few reports of early transition metal amidate complexes
with σ-bonding, two-electron donor ligands, such as halides or
alkyls.10 This has been attributed in part to the propensity of
these species to form ill-defined bridging aggregates in an effort
to increase electron donation to the metal center. Recently, we
have described a synthetic protocol to reliably access a small
group of amidate-supported dichlorides; however, the success of
this methodology is highly ligand dependent.10a In addition, the
resulting complexes are susceptible to fluxional behavior and
geometric isomerization in solution. The few reported examples
of amidate-supported alkyls have exhibited similar tenden-
cies.10b,c The first example of an amidate-supported Hf-
dibenzyl complex was reported in 2005 and was isolated as the
THF adduct.10c More recently, Scott and co-workers utilized seve-
ral biaryl-tethered amidate ligands in the preparation of Ti-
and Zr-dibenzyl compounds for hydroamination catalysis.10b
These complexes display various amidate coordination modes,
solution-phase isomerization, and formation of dinuclear spe-
cies through bridging interactions. In addition, several synthetic
attempts were reported as unsuccessful, leading to unidentifi-
able product mixtures. These difficulties highlight the impor-
tance and challenge of N,O ligand design in the pursuit of
discrete, well-behaved complexes.
In order to examine the electronic effects exerted by tight
bite-angle N,O chelates, our group has recently explored the
use of electron-rich ureate ligands in the synthesis of group 4
complexes.10a,11 Sterically analogous to amidates, ureates
possess an electron-donating amino group attached to the
central carbon atom (Figure 1). It has been envisaged that the
use of this electron-rich ligand would stabilize highly electro-
positive metal complexes, facilitating their isolation and
characterization, while promoting the same unique reactivity
exhibited by amidate derivatives. Indeed, the use of this
alternate ligand set allows for facile preparation of group 4
dichloride complexes10a and has resulted in the identification
of a highly active hydroamination catalyst.11
Figure 2. Proligands employed in this study; synthesis of 3.
compounds with neutral donors, resulting in the formation of
pyridine adducts that are thermally and photochemically stable
for months under inert atmosphere. Finally, we demonstrate
the potential of this class of compounds as organometallic
precursors for the synthesis of other ureate-supported metal
complexes, namely, the first ureate-supported zirconium imido
complexes.
Results and Discussion
Synthesis of Dialkyl Complexes. Through the work of
several research groups, including our own, several reliable
synthetic protocols have been developed for the installation
of amidate and ureate ligands.6-12 Foremost among these
routes is a direct reaction between an organic amide or urea
proligand and a metal complex containing ligands that are
susceptible to cleavage by protonation. These protonolysis
reactions generally lead to products that are easily isolated
and purified due to the absence of salt-containing by-
products. All previously reported examples of amidate-
supported dibenzyl complexes were prepared in this fashion
using M(CH2Ph)4 (M=Ti, Zr, Hf);10b,c we therefore have
chosen a similar protocol for the synthesis of analogous
bis(ureate) complexes. The urea proligands included in this
investigation are shown in Figure 2. Syntheses of compounds
1 and 2 have been previously established,10a,11 while the
preparation of 3 is disclosed here. These frameworks have
been chosen to vary the steric properties of the ureate ligands
and to compare with known amidate compounds possessing
comparable substitution patterns. Specifically, proligands 1
and 3 are structurally similar to amides used previously to
prepare highly active catalysts for the hydroamination of
alkynes and alkenes.8a-g,i
Herein, we report the preparation and characterization of the
first examples of ureate-supported zirconium dibenzyl and
bis(neopentyl) compounds. While use of a nontethered ureate
ligand gives limited success in the formation of dialkyls,
tethered bis(ureate) ligands are reliable supports for sterically
accessible, coordinatively unsaturated dialkyl derivatives. In
contrast to the aforementioned amidates reported by Scott,10b
these ureate complexes do not undergo isomerization or dimer-
ization. Furthermore, we have examined the reactivity of these
Treatment of Zr(CH2Ph)4 with two equivalents of 1 at
-78 °C in THF results in the formation of dibenzyl complex
4 in moderate recrystallized yield (Scheme 1, top pathway). 1H
NMR spectroscopy of the purified compound confirms the
formulation as L2ZrBn2: relative integration of the isopropyl
methine multiplet and the benzyl methylene singlet gives a
1:1 ratio. The NMR spectra also give an indication of the
(10) (a) Leitch, D. C.; Beard, J. D.; Thomson, R. K.; Wright, V. A.;
Patrick, B. O.; Schafer, L. L. Eur. J. Inorg. Chem. 2009, 2691–2701.
(b) Gott, A. L.; Clarke, A. J.; Clarkson, G. J.; Scott, P. Organometallics 2007,
26, 1729–1737. (c) Thomson, R. K.; Patrick, B. O.; Schafer, L. L. Can. J.
Chem. 2005, 83, 1037–1042. (d) Giesbrecht, G. R.; Shafir, A.; Arnold, J.
Inorg. Chem. 2001, 40, 6069–6072.
(12) (a) Reznichenko, A. L.; Hultzsch, K. C. Organometallics 2010,
29, 24–27. (b) Zi, G.; Zhang, F.; Xiang, L.; Chen, Y.; Fang, W.; Song, H.
Dalton Trans. 2010, 39, 4048–4061. (c) Zi, G.; Liu, X.; Xiang, L.; Song, H.
Organometallics 2009, 28, 1127–1137. (d) Kissounko, D. A.; Hoerter, J. M.;
Guzei, I. A.; Guzei, Q. C.; Gellman, S. H.; Stahl, S. S. J. Am. Chem. Soc.
2007, 129, 1776–1783. (e) Kissounko, D. A.; Guzei, I. A.; Gellman, S. H.;
Stahl, S. S. Organometallics 2005, 24, 5208–5210.
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Chem. Soc. 2009, 131, 18246–18247.

5164 Organometallics, Vol. 29, No. 21, 2010
Leitch and Schafer
Scheme 1. Synthesis of Dibenzyl Complex 4 and Unexpected
Formation of Tris(Ureate) 5
solution-phase coordination geometry. There are two inequi-
valent signals for the isopropyl methyl groups, resonating as
doublets at δ 1.11 and 1.38, while there is a single multiplet for
the methine protons at δ3.16. In addition, one singlet at δ 2.39 is
observed for the methylene protons of the benzyl ligands. The
13C NMR spectrum contains signals for seven aliphatic carbons,
including one at δ 75.4 for the carbons attached to zirconium,
and a diagnostic signal at δ 166.8 for the central carbon of a κ2-
chelating ureate. No evidence of THF coordination is observed
by NMR spectroscopy, in contrast to the aforementioned bis-
(amidate) hafnium dibenzyl complex.10c These spectral features
are indicative of a C2-symmetric coordination geometry, with
two equivalent κ2-(N,O) ureate ligands and two equivalent ben-
zyl groups. The inequivalence of the isopropyl methyl groups is
due to hindered rotation about the iPr-Ar bond, as hindered
rotation about N-Ar or inequivalent ligand environments
would similarly split the methine protons into two groups. All
of this is consistent with 4 as a six-coordinate, distorted octa-
hedral complex. The likely coordination mode has the benzyl
ligands in a cis-disposition and the ureate nitrogens oriented
trans, proposed by analogy to amidate complexes with a
sterically similar ligand.13 Unfortunately, attempts to grow
single crystals of 4 for definitive assignment by X-ray crystal-
lography have thus far been unsuccessful.
Figure 3. ORTEP representation of the molecular structure of 5
(ellipsoids plotted at 50% probability, hydrogens and all but
ipso-carbons of aromatic groups removed for clarity, disordered
pentane solvent removed with SQUEEZE routine) with selected
˚
bond lengths (A) and bond and torsion angles (deg): Zr-N1,
2.401(2); Zr-N3, 2.276(3); Zr-N5, 2.359(2); Zr-O1, 2.134(1);
Zr-O2, 2.199(2); Zr-O3, 2.136(2); Zr-C55, 2.279(3); C1-N1,
1.325(3); C1-O1, 1.309(3); C1-N2, 1.354(2); average ureate
bite angle, 58.13(8); average angle between C-Zr-C0, 109.2(9);
sum of angles about N2, N4, N6: 359.9(6), 354.4(6), 355.6(9);
N1-C1-N2-C14: 19.7(4).
˚
shorter than the Zr-N lengths (2.276(3)-2.401(2) A). This is
likely due to steric crowding between the three bulky 2,6-
diisopropylphenyl groups attached to the nitrogen atoms. As
in previously characterized ureate complexes, the disubstitu-
ted amino group attached to the chelate shows evidence of
electron donation.10a The sum of the angles about nitrogen
atoms N2, N4, and N6 is approximately 360°, indicating sp2-
hybridization. In addition, the C-N bond lengths in the
Performing an analogous reaction with Zr(CH2tBu)4 and 1
leads to a mixture of products, from which was isolated an
unexpected, tris(ureate) neopentyl complex (5) in low yield
(Scheme 1, bottom pathway). Careful control of reaction sto-
ichiometry and use of alternate solvents (toluene, hexanes) does
not improve the reaction outcome. Furthermore, increasing the
proligand-to-zirconium ratio to 3:1 also leads to a mixture
˚
[N2CO] core are all between 1.320(3) and 1.361(4) A, consis-
tent with electron delocalization and multiple-bond character.
Finally, the torsion angles between the plane about the distal
nitrogen and that of the NCO-chelate range from 0° to 30°,
with an average of 15.8(4)°, indicating a largely coplanar
arrangement. All of these metrical data are indicative of π-
electron donation by the NR2 group, resulting in an electron-
rich chelating ureate.
In order to rationalize the undesired formation of complex 5,
we postulated that disproportionation reactions through ligand
redistribution are responsible. This would explain the inability
to reliably isolate 5 even with proper stoichiometry. Previously,
we have employed tethered ureate ligands to minimize ligand
fluxionality in zirconium dichloride complexes;10a a similar
strategy was examined in this case, with the use of tethered urea
proligands 2 and 3. Equation 1 outlines the preparation of
ureate-supported benzyl (6) and neopentyl (7) derivatives from
proligand 2 and Zr(CH2R)4, in 78% and 69% yield, respec-
tively, as analytically pure solids. Unlike the situation outlined
above with proligand 1, the formation of bis(neopentyl) com-
plex 7 is accomplished with minimal contamination by side
products. A 1H NMR spectrum of the crude product mixture
in the synthesis of 7 does indicate the presence of a by-
product, which we have tentatively assigned as the homoleptic
tetrakis(ureate) complex; however, this impurity is insoluble in
1
of products and a similar low yield of 5. H NMR spectros-
copy and mass spectrometry of the reaction mixture are not
consistent with the intended bis(ureate) complex and instead
indicate its composition as L3Zr(CH2tBu) (5). Fortunately,
cooling a pentane solution of the crude product mixture to
-35 °C led to the deposition of single crystals; the solid-state
molecular structure as determined by X-ray diffraction is shown
in Figure 3.
The complex is seven-coordinate, with three κ2-ureate
ligands; however, the ligand arrangement about zirconium
does not conform to the typical pentagonal-bipyramidal or
monocapped octahedral geometries. If the ureate ligands are
instead viewed as occupying only one coordination site, the
geometry can be assigned as distorted tetrahedral: the average
angle between any two carbons, either as the central atom in
the ureate chelate or as the neopentyl methylene, through
zirconium is 109.20(9)° (average deviation: 10.49°). Each of
the three ureate ligandsadoptsa nonsymmetric binding mode,
˚
where the Zr-O lengths (2.134(1)-2.199(1) A) are markedly

Article
Organometallics, Vol. 29, No. 21, 2010 5165
pentane and therefore easily removed by filtration. Lowering the
reaction temperature (-78 °C) further reduces the amount of
this byproduct, but does not prevent its formation.
The NMR spectral characteristics of compounds 6 and 7
are very similar, differing only in the signals associated with
the alkyl ligands and minor chemical shift changes for the
1
ureate signals. The H NMR spectra contain single reso-
nances for each type of proton, including those of the
isopropyl groups attached to the distal ureate nitrogen; 13C
NMR spectroscopy reveals a similar high degree of magnetic
equivalence. This indicates not only high molecular sym-
metry but also fast rotation about the iPr2N-C bond, suggest-
ing reduced π-donation by the distal nitrogen atoms. In
order to compare this solution-phase behavior with solid-
state structural parameters, we sought to apply X-ray crys-
tallographic characterization. Repeated attempts to grow
single crystals of compound 6 resulted in microcrystalline
material that gave weak diffraction patterns; however, single
crystals of compound 7 were obtained from a cold pentane
solution. The solid-state molecular structure of 7 is shown in
Figure 4, confirming the ligand arrangement about zirco-
nium. The complex is six-coordinate, with the four donor
atoms of the bis(ureate) ligand in a planar arrangement. As
for 5, there is no immediately obvious six-coordinate geo-
metry to which this complex conforms; it is therefore best
described as distorted tetrahedral (average C-Zr-C0 angle:
109.33(6)°, average deviation: 9.71°). An examination of the
metrical parameters of complex 7 once again reveals evidence
of π-electron donation by the distal nitrogens (sp2-hybri-
dized nitrogens, electron delocalization, coplanar arrange-
ment), incontrast tothe solution-phasebehaviornoted above.
These conflicting facts suggest that while electron donation
by the distal nitrogen atoms may occur, it is not strong
enough to prevent rotation about the iPr2N-C bond in
solution. It should be noted that analogous tethered bis-
(amidate) dibenzyl complexes of zirconium and hafnium are
only isolable as the THF adducts.13
Given the challenges encountered previously in the prepara-
tion of tethered bis(amidate) zirconium benzyl complexes,10b,13
the isolation of base-free, monometallic, nonfluxional com-
plexes 6 and 7 points to a significant difference between the
stabilization afforded by amidates and ureates. In order to
further compare these ligand sets, biaryl-tethered proligand 3,
analogous to the proligands used by Scott and co-workers,10b
was used to prepare zirconium dialkyl compounds 8 and 9 in
75% and 54% recrystallized yield (eq 2). 1H NMR spectroscopy
indicates that the ligand binds in a C2-symmetric fashion, as the
methyl groups attached to the biaryl tether are equivalent. The
methylene protons of the Zr-CH2R group resonate as an AB
quartet in both cases, due to the axial chirality of the ureate
Figure 4. ORTEP representation of the molecular structure of 7
(ellipsoids plotted at 50% probability, hydrogens removed for
˚
clarity) with selected bond lengths (A) and bond and torsion angles
(deg): Zr-N1, 2.193(2); Zr-N3, 2.183(1); Zr-O1, 2.208(1); Zr-
O2, 2.240(1); Zr-C20, 2.266(2); Zr-C25, 2.259(2); C1-N1,
1.331(2); C1-O1, 1.295(2); C1-N2, 1.351(2); N1-Zr-O1,
59.32(4); N1-Zr-N3, 76.33(5); average angle between C-Zr-C0,
109.3(6); Zr-C20-C21, 123.8(1); Zr-C25-C26, 130.7(1); sum of
angles about N2, N4: 359.5(3), 359.2(3); N1-C1-N2-C5: 14.7(3).
ligand. 13C NMR spectra of 8 and 9 contain a single resonance
for the ureate carbon at ∼168 ppm, diagnostic of a κ2-chelating
binding mode. All of these spectral features are in striking
contrast to those observed previously for biaryl-tethered bis-
(amidate) benzyl compounds.
A major difference in the solution-phase behavior between
the alkyl-tethered complexes (6, 7) and the biaryl-tethered
complexes (8, 9) is in the disposition of NMR signals corres-
ponding to the diisopropylamino substituents on the ureate
chelate. As noted above, fast rotation about the iPr2N-C bond
on the NMR time scale is observed for 6 and 7, resulting in
equivalent methyl and methine protons. The 1H NMR signals
for isopropyl groups in 8 and 9, however, are inequivalent and
broad. The two sets of methine resonances are separated by
∼1 ppm. The methyl signals are similarly separated, with the
upfield resonance (centered around δ 0.75) further split into two
broad, overlapping signals. This splitting of the upfield isopropyl
methyl resonance is presumably due to the chiral environment
about the metal center, rendering these methyl groups diastereo-
topic. The isopropyl carbons appear as very broad resonances in
the 13C NMR spectra, preventing a reliable chemical shift assign-
ment in the case of 8. These observations are indicative of
(13) Thomson, R. K.; Schafer, L. L. Organometallics 2010, in press.

5166 Organometallics, Vol. 29, No. 21, 2010
Leitch and Schafer
Figure 5. ORTEP representation of the molecular structure of 8 (left) and abbreviated structure showing η2-benzyl interaction (right)
˚
(ellipsoids plotted at 50% probability, hydrogens removed for clarity) with selected bond lengths (A) and bond and torsion angles (deg):
Zr-N1, 2.195(1); Zr-N3, 2.183(1); Zr-O1, 2.221(1); Zr-O2, 2.257(1); Zr-C25, 2.265(2); Zr-C29, 2.282(2); C1-N1, 1.334(2);
C1-O1, 1.294(2); C1-N2, 1.346(2); N1-Zr-O1, 59.31(5); N1-Zr-N3, 75.34(5); average angle between C-Zr-C0, 109.8(6);
Zr-C29-C30, 109.8(1); Zr-C36-C37, 92.6(1); sum of angles about N2, N4: 359.6(3), 359.2(3); N1-C1-N2-C5: 14.3(3).
slow rotation about the iPr2N-C bond, in stark contrast to the
situation for 6 and 7. There are two possible explanations for
this difference: first, greater steric compression by the biaryl
tether relative to the alkyl tether hinders rotation about this
bond; and second, the electron-withdrawing aromatic sub-
stituents on the ureate nitrogen induce a greater degree of
π-donation by the diisopropylamino lone pair.
readily generate seven-coordinate complexes. Addition of excess
pyridine (>2 equiv) to C6D6 solutions of compounds 6-9
results in an immediate color change from colorless to bright
orange. In each case, the 1H NMR spectra contain broad signals
for the ortho-protons of the pyridine centered at δ 8.65, which is
close to that of free pyridine (δ 8.53). On the basis of this
relatively unperturbed chemical shift and the absence of clear
signals that would correspond to a coordinated pyridine mole-
cule, we propose that fast neutral ligand exchange occurs on the
NMR time scale. For biaryl-tethered ureate complexes 8 and 9,
the resonances corresponding to the isopropyl protons become
sharp and well resolved after pyridine addition, in contrast to the
signals observed for the parent complexes. In particular, the
broad signals for the isopropyl methyl groups resolve into two
sets of nearly overlapping doublets, which integrate for 12
protons each. This further splitting occurs due to the chiral
environment about zirconium, as proposed above for complexes
8 and 9. The pyridine adducts formed in this manner are surpris-
ingly thermally robust: heating a solution of 9 in the presence of
five equivalents of pyridine to 110 °C for several hours results in
no detectable decomposition.
While the abovespectroscopic experiments strongly suggest
the formation of seven-coordinate, pyridine-stabilized com-
plexes, they do not give an indication of the exact nature
of these species. Therefore, complexes 6-py and 8-py were
prepared on a larger scale by the reaction of 2 or 3 with
Zr(CH2Ph)4 in the presence of a slight excess of pyridine;
the compounds were recrystallized in 70% and 49% yield.
These pyridine-stabilized alkyls are stable to ambient heat
and light under an inert atmosphere for months. The NMR
spectra of 6-py and 8-py contain similar features to those ob-
served in the aforementioned small-scale reactions; however,
the pyridine ortho-proton signals are shifted downfield to
δ 9.38 for 6-py and δ 8.85 for 8-py. Signals for free pyr-
idine are not observed, suggesting that in the absence of
excess pyridine, ligand exchange and/or loss does not occur
or that the equilibrium heavily favors the seven-coordinate
species. Furthermore, subjecting solid 6-py and 8-py to high
vacuum for 24 hours at room temperature does not remove
the pyridine donor.
Single crystals of 8 were deposited from a cold pentane
solution; the solid-state molecular structure is shown in Figure
5 (left). The complex adopts a coordination geometry ana-
logous to that of the alkyl-tethered complex 7 described above.
Metrical evidence for π-donation from the iPr2N group is also
apparent. This further supports the electron-donation hypo-
thesis for the hindered rotation about the iPr2N-C bond ob-
served in solution. One other noteworthy aspect of the structure
of 8 is that one of the benzyl ligands adopts a formal η2-bonding
mode (Figure 5, right). The Zr-C-C angle (92.6(1)°) and
˚
Zr-Cipso distance (2.759(2) A) are in the range of other re-
ported η2-interactions;14 however, the second benzyl ligand is
clearly η1-bound. Due to the lack of solution-phase NMR
spectroscopic evidence for η2-benzyl ligands, we propose that
this interaction is not maintained in solution and that both
benzyls be considered as time-averaged η1-ligands. Single crys-
tals of complex 9 were also obtained, although the crystal
morphology (leaf) led to a weak diffraction pattern. However,
based on the similar solution-phase spectroscopic features, we
propose that 9 is structurally analogous to 8.
Formation of Pyridine Adducts. The isolation of tethered
bis(ureate) zirconium complexes 6-9 as stable, monometallic,
donor-free dialkyls is remarkable given the difficulties associated
with related tethered bis(amidate) complexes.10b,13 Other tita-
nium and zirconium complexes supported by the alkyl-tethered
bis(ureate) derived from proligand 2 have been isolated as base-
stabilized, seven-coordinate species, even when electron-rich
amido ligands are present.10a,11 We have therefore tested
whether the dialkyls described above constitute coordinatively
saturated species or if the presence of a neutral donor would
(14) Giesbrecht, G. R.; Whitener, G. D.; Arnold, J. Organometallics
2000, 19, 2809–2812.

Article
Organometallics, Vol. 29, No. 21, 2010 5167
Table 1. Crystallographic Parameters
6-py
5
7
8
8-py
10
11
formula
fw
cryst size (mm)
C59H92N6O3Zr C29H60N4O2Zr C42H54N4O2Zr C43H69N5O2Zr
1024.61
C47H59N5O2Zr C41H65N7O2Zr C62H110N10O4Zr2
588.03
0.50 ꢀ 0.50
738.11
0.80 ꢀ 0.50
779.25
0.25 ꢀ 0.20
817.21
0.20 ꢀ 0.20
779.22
0.40 ꢀ 0.40
1242.04
1.00 ꢀ 0.70
ꢀ 0.20
1.00 ꢀ 0.80
ꢀ 0.50
ꢀ 0.30
ꢀ 0.40
ꢀ 0.10
ꢀ 0.10
ꢀ 0.20
color, habit
cell setting
space group
colorless, prism colorless, prism yellow, prism
monoclinic
P21/c
orange, irregular orange, block
orange, prism
monoclinic
P21/n
13.7003(5)
20.8396(7)
15.3980(5)
90
yellow, oval
monoclinic
P21/n
trigonal
P63
monoclinic
P21/c
triclinic
P1
orthorhombic
C2221
15.151(2)
18.936(3)
15.576(3)
90
˚
a (A)
24.5106(9)
24.5106(9)
18.9897(7)
90
90
120
9879.7(2)
6
1.033
Mo KR (λ = 0.71073 A)
3312
2.08
50.04
99 073
13.7893(4)
15.7596(4)
15.9037(4)
90
107.1760(10)
90
3301.96(15)
4
1.173
˚
11.7720(8)
15.169(1)
22.643(2)
90
101.173(3)
90
3966.7(5)
4
1.236
12.697(1)
13.748(1)
14.213(1)
69.300(4)
69.631(4)
78.194(4)
2166.4(3)
2
12.8587(8)
29.109(2)
18.121(1)
90
˚
b (A)
˚
c (A)
R (deg)
β (deg)
γ (deg)
90
90
94.5140(10)
90
91.361(3)
90
3
˚
V(A )
Z
F
4468.8(12)
4
1.215
4382.6(4)
4
1.181
6780.8(8)
4
1.217
calcd (g cm-1
radiation
)
1.195
F(000)
1252
3.61
55.00
1560
3.16
60.22
45 994
11 674 (Rint
0.0343)
8888
836
2.93
55.82
31 903
9956 (Rint
0.0242)
8848
1728
2.87
70.72
16 782
7914 (Rint
0.0144)
7172
1664
2.91
49.98
24 316
7144 (Rint
0.1084)
4741
2656
3.57
54.98
59 909
15 448 (Rint
0.0273)
12 859
μ (Mo KR) (cm-1
2θmax (deg)
total no. of reflns
)
24 563
7524 (Rint
0.0315)
6040
no. of unique reflns 11 533 (Rint
0.0566)
no. of reflns
=
=
=
=
=
=
=
9273
with I = 2σ(I)
no. of variables
R1 (F2, all data)
wR2 (F2, all data)
R1 (F, I = 2σ(I))
wR2 (F, I = 2σ(I))
goodness of fit
638
357
452
462
255
474
731
0.0424
0.0792
0.0310
0.0764
0.973
0.0423
0.0666
0.0278
0.0619
1.021
0.0556
0.0890
0.0340
0.0773
1.031
0.0426
0.0989
0.0354
0.0933
1.065
0.0321
0.0697
0.0257
0.0645
1.091
0.1395
0.1319
0.0364
0.0919
1.064
0.0433
0.0768
0.0306
0.0699
1.049
The molecular structures of both 6-py and 8-py as deter-
mined by X-ray crystallography are shown in Figures 6 and 7,
respectively. In each case, the complex is seven-coordi-
nate, C2-symmetric, distorted pentagonal bipyramidal, with
the benzyl ligands in a trans-disposition about the metal
center; the equatorial plane is comprised of the ureate ligand
and the pyridine donor. The ureate has undergone little
structural reorganization from the six-coordinate complexes
characterized above, with Zr-N and Zr-O bond distances
that are basically unchanged. The zirconium-pyridine dis-
including [2þ2] cycloadditions with a variety of unsaturated
organic substrates,15 and hydrocarbon C-H activation.16
Furthermore, they have been implicated as key catalytic inter-
mediates in a variety of group 4 catalyzed organic transforma-
tions, including imine metathesis,17 carboamination,18 hydro-
amination,8b,d,e,i,15,19 and hydroaminoalkylation.20 Several
synthetic routes have been applied to the synthesis of these
(16) (a) Hoyt, H. M.; Bergman, R. G. Angew. Chem., Int. Ed. 2007,
46, 5580–5582. (b) Hoyt, H. M.; Michael, F. E.; Bergman, R. G. J. Am.
Chem. Soc. 2004, 126, 1018–1019. (c) Cundari, T. R.; Klickman, T. R.;
Wolczanski, P. T. J. Am. Chem. Soc. 2002, 124, 1481–1487. (d) Slaughter,
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2000, 122, 7953–7975. (e) Bashall, A.; Collier, P. E.; Gade, L. H.; McPartlin,
˚
tances are quite short (2.378(1) and 2.397(2) A), approaching
˚
the zirconium-benzyl lengths (2.350(1)-2.373(2) A). Com-
paring the Zr-C distances between 8 and 8-py reveals a
˚
significant lengthening (0.068-0.085 A), due to the increased
electron count of the pyridine adduct. Neither 6-py nor 8-py
€
M.; Mountford, P.; Trosch, D. J. M. Chem. Commun. 1998, 2555–2556.
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Chem. Soc. 1996, 118, 591–611. (i) Bennett, J. L.; Wolczanski, P. T. J. Am.
Chem. Soc. 1994, 116, 2179–2180. (j) Cummins, C. C.; Schaller, C. P.; Van
Duyne, G. D.; Wolczanski, P. T.; Chan, A. W. C.; Hoffmann, R. J. Am. Chem.
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shows any evidence of η2-benzyl interactions.
Use of Organometallic Precursors in the Synthesis of Imido
Complexes. Group 4 imidos are an important class of com-
pounds that exhibit remarkable stoichiometric reactivity,
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E.; Jones, C.; Mountford, P. Chem. Commun. 2008, 5101–5103.
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J. Am. Chem. Soc. 2003, 125, 7184–7185. (f) Zuckerman, R. L.; Bergman,
R. G. Organometallics 2001, 20, 1792–1807. (g) Sweeney, Z. K.; Salsman,
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Chem. Soc. 1998, 120, 13405–13414. (j) Blake, A. J.; Mountford, P.;
Nikonov, G. I.; Swallow, D. Chem. Commun. 1996, 1835–1836. (k) Meyer,
K. E.; Walsh, P. J.; Bergman, R. G. J. Am. Chem. Soc. 1995, 117, 974–985.
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1992, 114, 5459–5460.
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5168 Organometallics, Vol. 29, No. 21, 2010
Leitch and Schafer
Figure 7. ORTEP representation of the molecular structure of
8-py (ellipsoids plotted at 50% probability, hydrogens removed
˚
for clarity) with selected bond lengths (A) and bond and torsion
Figure 6. ORTEP representation of the molecular structure of 6-
py (ellipsoids plotted at 50% probability, hydrogens and pentane
˚
solvent removed for clarity) with selected bond lengths (A) and
angles (deg): Zr-N1, 2.200(1); Zr-N3, 2.378(1); Zr-O1,
2.1965(9); Zr-C8, 2.350(1); C1-N1, 1.356(2); C1-O1, 1.283(2);
C1-N2, 1.337(2); N1-Zr-O1, 60.06(4); N1-Zr-N1*, 79.13(4);
C8-Zr-C8*, 154.83(5); C8-Zr-N3, 77.42(5); Zr-C8-C9,
117.3(1); sum of angles about N2: 359.4(3); N1-C1-N2-C5:
20.4(2); C18-C23-C23*-C18*: 68.5(2).
bond and torsion angles (deg): Zr-N1, 2.177(2); Zr-N3, 2.186(1);
Zr-N5, 2.397(2); Zr-O1, 2.216(1); Zr-O2, 2.197(2); Zr-C20,
2.365(2); Zr-C27, 2.373(2); C1-N1, 1.326(2); C1-O1, 1.300(2);
C1-N2, 1.360(3); N1-Zr-O1, 59.49(6); N1-Zr-N3, 77.56(6);
C20-Zr-C27, 156.54(8); C20-Zr-N5, 80.02(7); C27-Zr-N5,
76.59(7); Zr-C20-C21, 126.7(2); Zr-C27-C28, 108.2(1); sum of
angles about N2, N4: 359.8(6), 360.0(6); N1-C1-N2-C2: 22.9(3).
zirconium startingmaterials ofthe form M(NR)Cl2py2, which
provide a convenient source of the metal-imido fragment.21
Through treatment of these starting materials with alkali-
metal salts of a variety of ancillary ligands, many group 4
imido complexes have been successfully prepared.22 Another
common route involves a two-step process, in which an amido
species of the form LnM(NHR)(R0) is generated by salt
metathesis, followed by thermally induced R-proton abstrac-
tion to give LnM(NR) and R0H.16,23
An alternate method to synthesize group 4 imido complexes
is through direct aminolysis between bis(amido) or dialkyl
compounds and one equivalent of a primary amine.25 Pre-
viously, we have used this route to prepare several amidate-
supported titanium and zirconium imides from L2M(NMe2)2
precursors.6,8d,8e These imido complexes have been examined as
models for catalytic hydroamination intermediates and are
themselves effective precatalysts for the hydroamination of both
alkynes and alkenes. Recently, we established that the tethered
species,21-25 most involving salt metathesis reactions.21-23
Mountford and co-workers have developed titanium and
(21) (a) Dubberly, S. R.; Evans, S.; Boyd, C. L.; Mountford, P.
Dalton Trans. 2005, 1448–1458. (b) Blake, A. J.; Collier, P. E.; Dunn,
S. C.; Li, W.-S.; Mountford, P.; Shishkin, O. V. J. Chem. Soc., Dalton Trans.
1997, 1549–1558. (c) Dunn, S. C.; Batsanov, A. S.; Mountford, P. J. Chem.
Soc., Chem. Commun. 1994, 2007–2008.
ꢀ
~
(22) (a) Otero, A.; Fernandez-Baeza, J.; Antinolo, A.; Tejeda, J.; Lara-
ꢀ
ꢀ
ꢀ
Sanchez, A.; Sanchez-Barba, L. F.; Sanchez-Molina, M.; Bo, C.; Urbano-
Cuadrado, M. Inorg. Chim. Acta 2009, 362, 2909–2914. (b) Lian, B.; Spaniol,
T. P.; Hultzsch, K. C.; Okuda, J. Eur. J. Inorg. Chem. 2009, 429–434. (c) Beck,
J. F.; Baiz, T. I.; Neshat, A.; Schmidt, J. A. R. Dalton Trans. 2009, 5001–5008.
(d) Li, Y.; Banerjee, S.; Odom, A. L. Organometallics 2005, 24, 3272–3278.
(e) Boyd, C. L.; Guiducci, A. E.; Dubberley, S. R.; Tyrrell, B. R.; Mountford, P.
J. Chem. Soc., Dalton Trans. 2002, 4175–4184. (f) Lawrence, S. C.; Skinner,
M. E. G.;Green, J. C.; Mountford, P. Chem. Commun. 2001, 705–706. (g) Blake,
A. J.; Collier, P. E.; Gade, L. H.; Mountford, P.; Lloyd, J.; Pugh, S. M.; Schubart,
€
M.; Skinner, M. E.; Trosch, D. J. M. Inorg. Chem. 2001, 40, 870–877. (h) Cloke,
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Commun. 1999, 661–662. (i)Blake, A. J.;Dunn, S. C.;Green, J. C.;Jones, N. M.;
Moody, A. G.; Mountford, P. Chem. Commun. 1998, 1235–1236. (j) Mountford,
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1555–1556.
bis(ureate) complex LZr(NMe2)2 HNMe2 (where L is the
3
ureate ligand derived from proligand 2) exhibits unique catalytic
reactivity for a group 4 system with respect to hydroamina-
tion.11 As part of our mechanistic investigations on this catalyst
system, we sought to synthesize and characterize zirconium
imido complexes supported by the same tethered bis(ureate)
ligand and to investigate their reactivity patterns.
ꢀ
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24, 1390–1393.
In contrast to the aforementioned synthesis of amidate-
supported imides, attempts to prepare imido species from
LZr(NMe2)2 HNMe2 led to incomplete protonolysis even at
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Epshteyn, A.; Fettinger, J. C.; Sita, L. R. Organometallics 2006, 25, 1076–
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3
elevated temperatures. In order to overcome this problem,
dialkyl compound 6 was used as an alternate starting mate-
rial. In protonolysis reactions, dialkyl compounds are
superior to bis(amido)s due to the higher basicity of the alkyl
ligands and the generation of inert hydrocarbon byproducts.
Accordingly, treatment of 6 with one equivalent of 2,6-
diisopropylaniline in the presence or absence of pyridine
leads to the formation of monometallic (10) or dimeric (11)
imido complexes (Scheme 2).

Article
Organometallics, Vol. 29, No. 21, 2010 5169
Zr-N1-C1 angle of 177.6(3)° are consistent with a zirconium-
nitrogen triple bond, characteristic of an imido linkage. The
pyridine that is trans to the imido is weakly bound (Zr-N6,
˚
2.579(3) A) due to the strong trans-influence of the -NAr
ligand. A previously reported six-coordinate amidate-supported
zirconium imido complex adopts a related pentagonal-pyramidal
coordination geometry, with the imido group also in the axial
position.8e In that case, the steric properties of the amidate
ligands enable the isolation of a six-coordinate species.
Combustion analysis of the crystalline material confirms
the empirical formula and purity of compound 10; however,
solution-phase NMR spectroscopy reveals a more complicated
situation. The 1H NMR spectrum of a solution of crystalline 10
indicates the presence of two compounds in a 1:1 ratio; in
addition, two broad ortho-Py-H signals were observed, as-
signed to exchanging bound and free pyridine. The addition of
excess pyridine (10 equivalents) to this solution increased the
ratio of components to 2:1. After heating to 65 °C for one hour
followed by cooling to room temperature, only one compound
was observed, assigned as the monometallic imido 10. The 1H
NMR spectral features of this compound after the above
treatment are sharp and well resolved. The methyl groups on
the alkyl tether are inequivalent, and the methylene protons
diastereotopic, indicating different magnetic environments
Figure 8. ORTEP representation of the molecular structure of
10 (ellipsoids plotted at 50% probability, hydrogens removed
˚
for clarity) with selected bond lengths (A) and bond and torsion
angles (deg): Zr-N1, 1.891(3); Zr-N2, 2.245(3), Zr-O1, 2.244(2);
Zr-N6, 2.579(3); Zr-N7, 2.375(3); C13-N2, 1.322(4); C13-O1,
1.290(4); C13-N4, 1.369(4); N2-Zr-O1, 58.44(9); Zr-N1-C1,
177.6(3); O1-C13-N4-C14, 20.2(5).
1
above and below the plane of the ligand. Relative H NMR
signal integrations confirm a 1:1 ratio between the ureate ligand
and the imido fragment.
Scheme 2. Synthesis of Monometallic and Dimeric Imido Com-
plexes 10 and 11 through Aminolysis of Dibenzyl Compound 6
The solution-phase behavior of compound 10 allows us to
consider that, while stable in the solid state, 10 is in equilibrium
with a base-free, dimeric imido species (11) while in solution,
even in the presence of a neutral donor. This tendency to dimerize
may come as a result of the weak bond between the zirconium
center and the pyridine trans to the imido and the sterically
accessible nature of the bis(ureate) ligand. Considering the size
of the substituent on the imido nitrogen, favorable dimer
formation is remarkable; only three examples of dimeric group
4 imidos with this bulky aryl group have been reported.21a,26 In
order to confirm this dimerization hypothesis, 11 was indepen-
dently synthesized by treating compound 6 with 2,6-diisopro-
pylaniline in the absence of a neutral donor. Analytically pure
crystals of 11 were isolated from the reaction mixture in 61%
yield. On the basis of electron-impact mass spectrometry and
X-ray crystallography (Figure 9), we have established that 11 is
1
indeed a dimeric, base-free imido complex. Thus, H NMR
spectroscopy of 11 confirms its identity as the second compo-
nent present in solution with the untreated compound 10.
Consistent with the anticipated reactivity of complex 11, the
treatment of this compound with an excess of pyridine and heat
forms complex 10 in solution phase.
Each of the previously mentioned amidate-supported imido
complexes have been stabilized by the presence of a neutral
donor, either pyridine or triphenylphosphine oxide, facilitat-
ing their isolation and characterization. A similar strategy has
therefore been employed in the preparation of complex 10.
Treatment of 6 with 2,6-diisopropylaniline and four equivalents
of pyridine results in a red suspension that can be clarified by
gentle heating. On standing overnight at room temperature, this
solution yields an orange crystalline product (10, 64% yield).
The solid-state molecular structure is shown in Figure 8, con-
firming the identity of 10 as a seven-coordinate, distorted
pentagonal bipyramidal, monometallic imido complex. As for
complex 7, the ureate ligand adopts a planar arrangement. The
two pyridine donors are cis-disposed, one in an equatorial
position and the other axial. The imido ligand is axial, trans to
The molecular structure of 11 reveals that each zirconium
center is six-coordinate, with distorted tetrahedral-type geo-
metry as described above for 7 and 8. One striking structural
feature is unique to this complex: the bis(ureate) ligand distorts
itself to accommodate the bulky aryl group on the imido
nitrogens. Rather than adopting a planar conformation, as
observed for every titanium or zirconium complex supported by
this same ligand,10a,11 the two ureate chelates bend upward,
away from the 2,6-diisopropylphenyl group, while maintaining
a tetradentate binding mode. Notably, treatment of either 10 or
11 with a variety of alkynes results in no reaction, in contrast
with many other group 4 imido complexes that undergo [2þ2]
(26) Arney, D. J.; Bruck, M. A.; Huber, S. R.; Wigley, D. E. Inorg.
Chem. 1992, 31, 3749–3755.
˚
the axial pyridine donor. The Zr-N1 distance of 1.891(3) A and

5170 Organometallics, Vol. 29, No. 21, 2010
Leitch and Schafer
or standard Schlenk techniques unless otherwise noted. Dichloro-
methane was distilled from calcium hydride. Tetrahydrofuran,
toluene, hexanes, and pentane were purified and dried by
passage through a column of activated alumina and sparged
with dinitrogen. d6-Benzene and d8-toluene were degassed by
˚
several freeze-pump-thaw cycles and dried over activated 4 A
molecular sieves for at least 24 hours before use in NMR experi-
ments. All common organic reagents were purchased from
Aldrich and either used as received (for proligand synthesis)
or distilled from calcium hydride and stored under an inert
atmosphere (for reaction with Zr complexes). ZrCl4 was pur-
chased from Strem and used as received. Proligands 1 and 2 were
synthesized as previously reported.10a The zirconium starting
materials Zr(CH2Ph)44d and Zr(CH2CMe3)44b were prepared as
described in the literature. 1H and 13C NMR spectra were
recorded on either a Bruker 300 or 400 MHz Avance spectro-
meter; chemical shifts are given relative to residual protio
solvent at 298 K unless otherwise noted. All 13C NMR spectra
were obtained as proton-decoupled. Mass spectra were recorded
on either a Kratos MS-50 spectrometer using an electron impact
(70 eV) source or a Bruker Esquire∼LC using an electrospray
ionization source. Elemental analyses were recorded on a Carlo
Erba elemental analyzer EA 1108. Single-crystal X-ray structure
determinations were performed at the Department of Chemistry,
University of British Columbia, by Dr. Brian O. Patrick and
Mr. Neal Yonson.
Synthesis of Proligand 3. 2,20-Diamino-6,60-dimethylbiphenyl
(4.20 g, 19.8 mmol) was dissolved in dichloromethane (40 mL).
The solution was cooled to 0 °C prior to the addition of pyridine
(3.91 g, 4.17 mL, 49.5 mmol), followed by the addition of phenyl
chloroformate (6.52 g, 5.24 mL, 41.6 mmol). The solution was
left to warm to room temperature with stirring overnight. The
reaction was quenched by the addition of 1 M HCl (50 mL). The
organic layer was separated and washed with a further portion of
1 M HCl (50 mL) and brine (50 mL) and dried over MgSO4. Removal
of the solvent in vacuo gave the bis(phenylcarbamate) as an off-
white solid (8.55 g, 4.78 mmol, 96% yield), which was used in the
next step without further purification. 1H NMR (CDCl3, 300
MHz): δ 2.01 (6H, s, 2 ꢀ -CH3), 6.48 (2H, s, 2 ꢀ -NH-), 7.09
(4H, d, J=7.8Hz, 4ꢀ Ph-H), 7.15-7.41 (10H, m, 6 ꢀ Ph-Hand
4 ꢀ Ar-H), 8.14 (2H, d, J = 8.17 Hz, 2 ꢀ Ar-H). 13C NMR
(CDCl3, 100 MHz): δ 20.8, 118.8, 122.7, 126.8, 127.3, 130.4, 130.7,
136.7, 138.6, 151.5, 152.8. MS(ESI): m/z 475 (Mþ þ Na). The
second step was performed without rigorous exclusion of air or
moisture. The bis(phenylcarbamate) (8.55 g, 18.9 mmol) was
dissolved in dimethylsulfoxide (50 mL). Diisopropylamine (4.02
g, 5.58 mL, 39.7 mmol) was added via syringe. The solution was
stirred overnight at room temperature, during which time a solid
product precipitated. Dichloromethane (75 mL) was added to
clarify the suspension. The organic phase was washed successively
with water (2 ꢀ 75 mL), 1 M HCl (75 mL), water (75 mL), 1 M
NaOH (75 mL), and brine (75 mL). The organic phase was dried
over MgSO4 and the solvent removed in vacuo. The crude product
was purified by recrystallization from a hexanes/ethyl acetate
mixture to yield 3 as off-white crystals (7.58 g, 16.3 mmol, 86%
yield). 1H NMR (CDCl3, 300 MHz): δ 0.92 (12H, d, J = 6.9 Hz,
2ꢀ -CH(CH3)2), 0.94 (12H, d, J=6.9Hz, 2ꢀ -CH(CH3)2), 1.93
(6H, s, 2ꢀ -CH3), 3.75 (4H, m, J=6.9Hz, 4ꢀ -CH(CH3)2), 6.01
(2H, s, -NH-), 6.95 (2H, d, J = 7.5 Hz, 2 ꢀ Ar-H), 7.24 (2H, t,
J = 7.9 Hz, 2 ꢀ Ar-Hmeta), 8.21 (2H, d, J = 8.3 Hz, 2 ꢀ Ar-H).
13C NMR (CDCl3, 75 MHz): δ 19.9, 20.9, 21.0, 44.9, 117.8, 124.0,
129.3, 137.3, 138.4, 154.3 (one quaternary carbon not observed).
MS(ESI): m/z 467 (Mþ þ H). Anal. Calcd for C28H42N4O2: C,
72.07; H, 9.07; N, 12.01. Found: C, 72.18; H, 9.00; N, 11.86.
Synthesis of 4. This reaction was performed with exclusion of
ambient light. Proligand 1 (0.300 g, 1.042 mmol) and Zr(CH2-
Ph)4 (0.237 g, 0.521 mmol) were dissolved in THF (10 mL)
at -78 °C in a foil-wrapped Schlenk tube. The solution was
warmed to room temperature with stirring over a period of three
hours. The solvent was removed in vacuo and the crude solid
Figure 9. ORTEP representation of the molecular structure of
11 (ellipsoids plotted at 50% probability, hydrogens and N(iPr)2
˚
groups removed for clarity) with selected bond lengths (A) and
angles (deg): Zr1-N1, 2.279(1); Zr1-N3, 2.243(1), Zr1-O1,
2.147(1); Zr1-O2, 2.192(1); Zr1-N9, 2.073(1); Zr2-N9, 2.125(1);
Zr1-N10, 2.080(1); Zr2-N10, 2.089(1); C1-N1, 1.312(2); C1-
O1, 1.314(2); C1-N2, 1.360(2); N1-Zr1-O1, 59.25(5); N1-
Zr1-N3, 78.46(5); C1-Zr1-C13, 105.31(5); C1-Zr1-N9,
122.89(5); C1-Zr1-N10, 107.50(5); N9-Zr1-N10, 80.36(5);
Zr1-N9-Zr2, 97.97(6).
cycloadditions with C-C unsaturations.15 This lack of reac-
tivity has implications for the mechanistic pathway of catalytic
hydroamination using tethered ureate-supported complexes;
these investigations are currently ongoing.
Summary and Outlook
The successful use of electron-rich ureates as supporting
ligands for zirconium dialkyl fragments has been outlined here
for a range of compounds. While the use of nontethered ureates
has led to limited success in the isolation of discrete dialkyl
species, tethered bis(ureate) ligands are effective supports for
several derivatives. Despite the sterically accessible zirconium
center in many of these complexes, fluxional behavior, ligand
redistribution, and bridging interactions are not observed. The
coordinatively unsaturated nature of dialkyl complexes with
tethered bis(ureate) ancillary ligands has been established
through their reaction with pyridine, resulting in adducts that
exhibit stability relative to thermal and photochemical degra-
dation. Finally, the utility of these compounds as starting
materials for further organometallic synthesis has been demon-
strated in the preparation of the first ureate-supported imido
complexes. Interestingly, the resultant imido complexes are not
viable for stoichiometric [2þ2] cycloadditions with alkynes,
despite the high catalytic activity for hydroamination exhibited
by related bis(amido) complexes. Given the unique reactivity
exhibited by many related group 4 amidate and ureate com-
plexes, we are currently exploring the use of this family of com-
pounds as catalysts for carbon-element bond forming reac-
tions. Particularly relevant in this regard are complexes derived
from chiral proligand 3, which could be applied to enantiose-
lective catalysis. The results of these and related studies will be
reported in due course.
Experimental Section
General Considerations. All reactions were performed under
an atmosphere of dry, oxygen-free dinitrogen using a glovebox

Article
Organometallics, Vol. 29, No. 21, 2010 5171
redissolved in hexanes. This solution was filtered through a bed
of Celite, and the solvent was removed. Recrystallization from
pentane at -35 °C afforded 0.265 g (60% yield) of 4 as colorless
6.94 (4H, d, J = 7.6 Hz, 4 ꢀ Ph-Hortho), 7.04 (1H, t, J = 7.6 Hz,
Py-Hpara), 7.18 (4H, t, J = 7.6 Hz, 4ꢀ Ph-Hmeta), 9.38 (2H, br m,
2 ꢀ Py-Hortho). 13CNMR(C6D6, 100 MHz, CH2 and Cdetermined
from DEPT): δ 22.1, 25.7, 36.1 (C), 46.7, 56.4 (CH2), 58.3 (CH2),
117.4, 123.7, 124.8, 127.4, 135.9, 147.9, 154.7 (C), 168.8 (C). MS-
(EI): m/z 535 (Mþ - py, CH2Ph). Anal. Calcd for C38H57N5O2Zr:
C, 64.54; H, 8.12; N, 9.90. Found: C, 64.25; H, 8.28; N, 9.82.
Synthesis of 7. This reaction was performed with exclusion of
ambient light. A foil-wrapped 20 mL vial was charged with
Zr(CH2CMe3)4 (0.211 g, 0.562 mmol) and a Teflon-coated stir
bar. A separate vial was charged with 2 (0.200 g, 0.562 mmol).
Toluene was added to both vials (5 mL each), and both solutions
were cooled to -35 °C. The proligand solution was added
dropwise to the stirring solution of Zr(CH2CMe3)4. The result-
ing mixture was allowed to warm to room temperature with
stirring overnight. The solvent was removed in vacuo and the
crude solid redissolved in pentane. The pentane solution was
filtered through Celite, concentrated, and cooled to -35 °C to
give colorless crystals of 7 (0.225 g, 69% yield). 1H NMR (C6D6,
400 MHz): δ 0.88 (6H, 2 ꢀ CH3), 1.23 (4H, s, Zr(CH2CMe3)2),
1.36 (24H, d, J = 6.7 Hz, 4 ꢀ CH(CH3)2), 1.53 (18H, s, Zr(CH2-
C(CH3)3), 3.12 (4H, s, 2 ꢀ CH2N), 3.61 (4H, sept, J = 6.7 Hz, 4 ꢀ
CH(CH3)2). 13C NMR (C6D6, 100 MHz, CH2 and C determined
from DEPT): δ 22.1, 25.0, 34.9, 35.0 (C), 36.2 (C), 46.9, 57.5
(CH2), 81.8 (CH2), 170.4. MS(EI) gave no molecular ion or
diagnostic fragments due to suspected instability to the ioniza-
tion conditions. Anal. Calcd for C29H60N4O2Zr: C, 59.23; H,
10.28; N, 9.53. Found: C, 59.61; H, 10.45; N, 9.52.
Synthesis of 8. This reaction was performed with exclusion of
ambient light. A foil-wrapped 20 mL scintillation vial was charged
with Zr(CH2Ph)4 (0.195 g, 0.429 mmol) and a Teflon-coated stir
bar. A separate vial was charged with 3 (0.200 g, 0.429 mmol).
Toluene was added to both vials (5 mL each), and both solutions
were cooled to -35 °C. The proligand solution was added dropwise
to the stirring solution of Zr(CH2Ph)4. The resulting mixture was
allowed to warm to room temperature with stirring overnight. The
solvent was removed in vacuo, and the crude solid was redissolved in
hexanes. The hexanes solution was filtered through Celite, concen-
trated, and cooled to -35 °C to give pale yellow crystals of 8 (0.237
g, 75% yield). 1HNMR(C6D6, 400 MHz): δ0.67-0.85 (12H, br m,
2 ꢀ -CH(CH3)2), 1.30-1.52 (12H, br m, 2 ꢀ -CH(CH3)2), 2.13
(6H, s, 2 ꢀ Ar-CH3), 2.41 (4H, AB q, J = 9.9 Hz, Zr(CH2Ph)2),
2.99 (2H, br m, 2 ꢀ CH(CH3)2), 3.97 (2H, br m, 2 ꢀ -CH(CH3)2),
6.65 (2H, d, J = 7.7 Hz, 2 ꢀ Ar-H), 7.00 (2H, d, J = 7.4 Hz, 2 ꢀ
Ar-H), 7.06 (4H, d, J = 7.5 Hz, 4 ꢀ Ph-Hortho), 7.21-7.34 (8H,
m, 2 ꢀ Ar-H þ 4 ꢀ Ph-Hmeta þ 2 ꢀ Ph-Hpara). 13C NMR
(C6D6, 100 MHz, CH2 and C determined from DEPT): δ 19.7, 65.3
(CH2), 119.9, 121.8, 124.6, 127.2, 128.7, 128.8, 132.1 (C), 137.0 (C),
143.9 (C), 145.3 (C), 168.3 (C), broad resonances (δ 15-50) for the
isopropyl carbons are not assigned. MS(EI) gave no molecular ion
or diagnostic fragments due to suspected instability to the ionization
conditions. Anal. Calcd for C42H54N4O2Zr: C, 68.34; H, 7.37; N,
7.59. Found: C, 68.56; H, 7.77; N, 7.36.
1
microcrystals. H NMR (C6D6, 400 MHz): δ 1.05-1.25 (12H,
m, 2 ꢀ (-CH2-)3), 1.21 (12H, d, J = 7.2 Hz, 2 ꢀ CH(CH3)2),
1.45 (12H, d, J = 7.2 Hz, 2 ꢀ -CH(CH3)2), 2.46 (4H, s, Zr(CH2-
Ph)2), 3.02 (8H, br m, 2 ꢀ (-CH2-)2N), 3.24 (4H, m, J = 6.8 Hz,
4 ꢀ CH(CH3)2), 6.96 (2H, m, Ar-H), 7.18 (6H, m, Ar-H), 7.25
(8H, m, Ar-H). 13C NMR (C6D6, 100 MHz): δ 24.7, 25.3, 25.8,
26.6, 28.4, 45.9, 75.4, 121.5, 124.6, 126.0, 128.7, 141.5, 143.9,
147.2, 166.8. MS(EI): m/z 755 (Mþ - CH2Ph); Anal. Calcd for
C50H68N4O2Zr: C, 70.79; H, 8.08; N, 6.60. Found: C, 70.42; H,
8.12; N, 6.42.
Synthesis of 5. This reaction was performed with exclusion of
ambient light. Proligand 1 (0.288 g, 1.00 mmol) and Zr(CH2-
CMe3)4 (0.188 g, 0.500 mmol) were dissolved in THF (10 mL)
at -78 °C in a foil-wrapped Schlenk tube. The solution was
warmed to room temperature with stirring over a period of three
hours. The solvent was removed in vacuo and the crude solid
redissolved in hexanes. This solution was filtered through a bed
of Celite, and the solvent was removed. Recrystallization from
pentane at -35 °C afforded a few colorless crystals of 5 (yield
1
not determined). H NMR (C6D6, 400 MHz, 338 K): δ 1.30
(18H, br m, 3(-CH2-)3), 1.39 (27H, m, 3 ꢀ -CH(CH3)2) þ
C(CH3)3), 1.50 (18H, d, J = 9.3 Hz, 3 ꢀ -CH(CH3)2), 3.12
(12H, br m, 3(-CH2-)2N), 3.82 (6H, br m, 6 ꢀ CH(CH3)2),
7.22-7.30 (9H, m, Ar-H), neopentyl methylene protons ob-
scured. 13C NMR (C6D6, 100 MHz, CH2 and C determined
from DEPT): δ 24.9 (CH2), 25.6, 26.7 (CH2), 28.4, 35.0, 36.2 (C),
46.2 (CH2), 82.2 (CH2), 124.4, 125.1, 143.3 (C), 144.1 (C), 166.4
(C). MS(EI): m/z 951 (Mþ - CH2tBu). Satisfactory elemental
analysis could not be obtained due to difficulty obtaining pure
material in sufficient quantity.
Synthesis of 6. This reaction was performed with exclusion of
ambient light. A foil-wrapped 20 mL scintillation vial was charged
with Zr(CH2Ph)4 (0.228 g, 0.500 mmol) and a Teflon-coated stir
bar. A separate vial was charged with 2 (0.178 g, 0.500 mmol).
Toluene was added to both vials (5 mL each), and both solutions
were cooled to -35 °C. The proligand solution was added dropwise
to the stirring solution of Zr(CH2Ph)4. The resulting mixture was
allowed to warm to room temperature with stirring overnight. The
solvent was removed in vacuo and the crude solid redissolved in
hexanes. The hexanes solution was filtered through Celite, concen-
trated, and cooled to -35 °C to give colorless microcrystals of 6
(0.245 g, 78% yield). 1H NMR (C6D6, 400 MHz): δ 0.85 (6H, 2 ꢀ
CH3), 1.27 (24H, d, J = 6.7 Hz, 4 ꢀ CH(CH3)2), 2.47 (4H, s,
Zr(CH2Ph)2), 2.94 (4H, s, 2ꢀ CH2), 3.49 (4H, sept, J=6.7 Hz, 4ꢀ
CH(CH3)2), 7.05 (2H, t, J = 7.2 Hz, 2 ꢀ Ph-Hpara), 7.39 (4H, t,
J = 8.0 Hz, 4 ꢀ Ph-Hmeta), 7.47 (4H, d, J = 7.2 Hz, 4 ꢀ
Ph-Hortho). 13C NMR (C6D6, 100 MHz, CH2 and C determined
from DEPT): δ 21.9, 24.7, 35.8 (C), 46.8, 57.1 (CH2), 62.0 (CH2),
121.0, 128.3, 129.0, 144.6 (C), 170.0 (C); MS(EI) m/z 535 (Mþ
CH2Ph). Anal. Calcd for C33H52N4O2Zr: C, 63.11; H, 8.35; N, 8.92.
Found: C, 62.80; H, 8.43; N, 9.00.
-
Synthesis of 8-py. This reaction was performed with exclusion
of ambient light. A foil-wrapped Schlenk tube was charged with
Zr(CH2Ph)4 (0.195 g, 0.429 mmol) and a Teflon-coated stir bar.
A separate tube was charged with 3 (0.200 g, 0.429 mmol) and
pyridine (0.039 g, 36.1 μL, 0.43 mmol). Toluene was added to
both flasks (10 mL each), and the Zr(CH2Ph)4 solution was
cooled to -78 °C. The diurea solution was cannula transferred
to the stirring solution of Zr(CH2Ph)4. This resulting mixture
was allowed to warm to room temperature with stirring over-
night. The solvent was removed in vacuo and the crude solid
redissolved in hexanes. The hexanes solution was filtered
through Celite, concentrated, and cooled to -35 °C to give
Synthesis of 6-py. This reaction was performed with exclusion
of ambient light. A foil-wrapped Schlenk tube was charged with
Zr(CH2Ph)4 (0.639 g, 1.40 mmol) and a Teflon-coated stir bar.
A separate tube was charged with 2 (0.500 g, 1.40 mmol) and
pyridine (0.111 g, 118.3 μL, 1.40 mmol). Toluene was added to both
flasks (10 mL each), and the Zr(CH2Ph)4 solution was cooled to
-78 °C. The proligand solution was cannula transferred to the
stirring solution of Zr(CH2Ph)4. The resulting mixture was allowed
to warm to room temperature with stirring overnight. The solvent
was removed in vacuo and the crude solid redissolved in hexanes.
The hexanes solution was filtered through Celite, concentrated, and
cooled to -35 °C to give orange crystals of 6-py (0.687 g, 70%
yield). 1H NMR (C6D6, 400 MHz): δ 1.08 (6H, s, C(CH3)2), 1.37
(24H, d, J = 6.8 Hz, 4 ꢀ CH(CH3)2), 2.14 (4H, s, 2 ꢀ -CH2Ph),
3.02 (4H, s, 2 ꢀ CH2), 3.73 (4H, br m, 4 ꢀ CH(CH3)2), 6.81 (2H, t,
J=7.2Hz,2ꢀ Ph-Hpara), 6.89 (2H, t, J=6.4Hz,2ꢀ Py-Hmeta),
1
orange crystals of 8-py (0.171 g, 49% yield). H NMR (C6D6,
400 MHz): δ 0.75-0.82 (12H, br m, 2 ꢀ CH(CH3)2), 1.37-1.45
(12H, br m, 2 ꢀ CH(CH3)2), 1.85 (2H, d, J = 8.6 Hz,
Zr-CH2Ph), 1.96 (2H, d, J = 8.6 Hz, Zr-CH2Ph), 2.17 (6H,
s, 2 ꢀ Ar-CH3), 3.00 (2H, br m, 2 ꢀ CH(CH3)2), 4.13 (2H, br m,

5172 Organometallics, Vol. 29, No. 21, 2010
Leitch and Schafer
2 ꢀ CH(CH3)2), 6.39 (4H, m, 4 ꢀ Ar-H), 6.67 (4H, m, 4 ꢀ
Ar-H), 6.90 (7H, m, 7 ꢀ Ar-H), 6.97 (2H, d, J = 7.3 Hz, 2 ꢀ
Ar-H), 7.06 (2H, t, J = 7.6 Hz, 2 ꢀ Ar-H), 8.85 (2H, br m, 2 ꢀ
Py-Hortho). 13C NMR (C6D6, 100 MHz, CH2 and C determined
from DEPT): δ 19.0, 20.9, 22.0, 22.5, 24.0, 46.4, 49.2, 62.9
(CH2), 119.1, 120.0, 124.7, 125.4, 125.6, 128.1, 128.2, 133.0 (C),
136.4, 137.9 (C), 147.2 (C), 169.7 (C), one pyridine carbon not
observed. MS(EI): m/z 645 (Mþ - py, CH2Ph). Anal. Calcd for
C47H59N5O2Zr: C, 69.08; H, 7.28; N, 8.57. Found: C, 69.32; H,
7.31; N, 8.58.
sept, J = 6.9 Hz, 2 ꢀ CH(CH3)2), 6.79 (4 H, m, 4 ꢀ Py-H), 6.83
(1 H, t, J = 7.4 Hz, Ar-Hpara), 7.11 (4 H, br m, 4 ꢀ Py-H), 7.26
(2 H, d, J = 7.6 Hz, 2 ꢀ Ar-Hmeta), 8.60 (4 H, br m, 4 ꢀ
Py-Hortho). 13C NMR (C6D6, 100 MHz, CH2 and C assigned
from DEPT, excess pyridine added): δ 22.4, 22.7, 25.7, 27.1,
27.4, 37.2 (C), 46.2, 57.7 (CH2), 113.7, 121.4, 123.5, 135.5, 142.1
(C), 150.1, 154.7 (C), 164.2 (C). MS(EI): m/z 620 (Mþ - 2 py).
Anal. Calcd for C41H65N7O2Zr: C, 63.20; H, 8.41; N, 12.58.
Found: C, 62.97; H, 8.23; N, 12.65.
Synthesis of 11. Complex 6 (0.100 g, 0.159 mmol) and 2,6-
diisopropylaniline (0.0282 g, 30.1 μL, 0.159 mmol) were dis-
solved in hexanes with gentle heating. The solution was left to
stand at room temperature overnight, during which time color-
less crystals of 11 formed. Yield: 0.060 g (61%). 1H NMR (C6D6,
400 MHz, 298 K): δ 0.67 (3H, s, CH3), 0.78 (3H, s, CH3),
1.05-1.45 (24H, br m, 4 ꢀ CH(CH3)2), 1.67 (12H, br m, 2 ꢀ
CH(CH3)2), 2.80 (2H, br m, CH2), 3.00 (2H, br d, J = 12 Hz,
CH2), 3.42 (4H, br m, 4 ꢀ CH(CH3)2), 4.38 (2H, br m, 2 ꢀ
CH(CH3)2), 7.13 (1H, t, J = 7.6 Hz, Ar-Hpara), 7.44 (2H, d, J =
7.6 Hz, 2 ꢀ Ar-Hmeta). 1H NMR (C6D6, 400 MHz, 343 K): δ
0.72 (6H, br s, 2 ꢀ CH3), 1.24 (24H, br m, 4 ꢀ CH(CH3)2), 1.61
(12H, d, J = 6.8 Hz, 2 ꢀ CH(CH3)2), 2.90 (4H, br m, 2 ꢀ CH2),
3.49 (4H, sept, J = 6.8 Hz, 4 ꢀ CH(CH3)2), 4.34 (2H, sept, J =
6.8 Hz, 2 ꢀ CH(CH3)2), 7.04 (1H, t, J = 7.6 Hz, Ar-Hpara), 7.37
(2H, d, J = 7.6 Hz, 2 ꢀ Ar-Hmeta). 13C NMR (C6D6, 100 MHz,
298 K, CH2 and C assigned from DEPT): δ 20.8 (br), 22.6 (br),
23.7 (br), 24.0, 26.0, 27.4 (br), 38.4 (C), 47.2 (br), 57.0 (br, CH2),
118.4, 121.9, 153.1 (C), two quaternary carbons not observed.
13C NMR (C6D6, 100 MHz, 343 K): δ 21.0-22.8 (br), 24.1, 28.5
(br), 38.3, 47.2, 57.1, 118.4, 121.9, 138.1, 153.2, 170.0. MS(EI):
m/z 1240 (Mþ), 620 (LZrNArþ). Anal. Calcd for C62H110N10-
O4Zr2: C, 59.95; H, 8.93; N, 11.28. Found: C, 60.08; H, 8.85; N,
11.22.
Synthesis of 9. This reaction was performed with exclusion of
ambient light. A foil-wrapped 20 mL scintillation vial was
charged with Zr(CH2CMe3)4 (0.161 g, 0.429 mmol) and a
Teflon-coated stir bar. A separate vial was charged with 3
(0.200 g, 0.429 mmol). Toluene was added to both vials (5 mL
each), and both solutions were cooled to -35 °C. The proligand
solution was added dropwise to the stirring solution of Zr-
(CH2CMe3)4. The resulting mixture was allowed to warm to
room temperature with stirring overnight. The solvent was
removed in vacuo and the crude solid redissolved in pentane.
The pentane solution was filtered through Celite, concentrated,
and cooled to -35 °C to give colorless leaf crystals of 9 (0.161 g,
54% yield); the crystal morphology was not conducive to X-ray
1
diffraction analysis. H NMR (C6D6, 400 MHz): δ 0.70-0.88
(12H, br m, 2 ꢀ CH(CH3)2), 1.42 (18H, s, Zr(CH2C(CH3)3)2),
1.45 (4H, AB q, J = 12.2 Hz, Zr(CH2CMe3)2), 1.57-1.68 (12H,
br m, 2 ꢀ -CH(CH3)2), 2.11 (6H, s, 2 ꢀ Ar-CH3), 3.12 (2H, br
m, 2 ꢀ CH(CH3)2), 4.06 (2H, br m, 2 ꢀ -CH(CH3)2), 6.96 (4H,
m, 4 ꢀ Ar-H), 7.09 (2H, t, J = 7.7 Hz, 2 ꢀ Ar-H). 13C NMR
(C6D6, 100 MHz): δ 18.3 (br), 19.7, 20.9 (br), 21.8 (br), 22.9 (br),
34.7, 35.1, 45.8 (br), 47.9 (br), 85.5, 119.8, 124.5, 126.9, 132.1,
137.0, 145.1, 168.0. MS(EI): m/z 625 (Mþ - CH2CMe3). Anal.
Calcd for C38H62N4O2Zr: C, 65.37; H, 8.95; N, 8.03. Found: C,
65.31; H, 9.19; N, 7.99.
Synthesis of 10. Complex 6 (0.100 g, 0.159 mmol), 2,6-
diisopropylaniline (0.0282 g, 30.1 μL, 0.159 mmol), and pyridine
(0.0497 g, 53.0 μL, 0.637 mmol) were dissolved in hexanes with
gentle heating. The solution was left to stand at room tempera-
ture overnight, during which time orange crystals of 10 formed.
Yield: 0.079 g (64%). 1H NMR (C6D6, 400 MHz, excess
pyridine added): δ 0.93 (3 H, s, CH3), 1.21 (12 H, d, J = 6.6
Hz, 2 ꢀ CH(CH3)2), 1.51 (12 H, d, J = 6.5 Hz, 2 ꢀ CH(CH3)2),
1.57 (15 H, d and obscured s, J = 6.9 Hz, 2 ꢀ CH(CH3)2 and
CH3), 3.43 (2 H, d, J = 11.6 Hz, CH2), 3.65 (2 H, d, J = 11.6 Hz,
CH2), 3.82 (4 H, sept, J = 6.7 Hz, 4 ꢀ CH(CH3)2), 4.74 (2 H,
Acknowledgment. The authors acknowledge funding
from NSERC (CRD Grant and CGSD to D.C.L.) and
Imperial Oil Ltd. in support of this work. We also thank
N. Yonson and B. O. Patrick for assistance with X-ray
crystallography.
Supporting Information Available: Representative NMR
spectra for new compounds and CIF for solid-state molecular
structures. This material is available free of charge via the
Internet at http://pubs.acs.org.