Amine - phenolate ligands in niobium chemistry: π-interactions probed by an ancillary alkyne ligand

Article abstract of DOI:10.1021/om801139m

A series of alkyne niobium complexes based on tetrapodal amine bis(phenolato) ligands NbCl[ON-NO]_R(MeCCMe) ([0NN0]_R = N(CH₂CH₂NMe₂)₂(CH₂C ₆H₂-3,5-R₂

Full text of DOI:10.1021/om801139m

2188
Organometallics 2009, 28, 2188–2194
Amine-Phenolate Ligands in Niobium Chemistry: π-Interactions
Probed by an Ancillary Alkyne Ligand
Emmanuelle Despagnet-Ayoub,* Sophie Schigand, Laure Vendier, and Michel Etienne*
CNRS, LCC (Laboratoire de Chimie de Coordination), 205, route de Narbonne, F-31077 Toulouse,
France, and UniVersite´ de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
ReceiVed NoVember 30, 2008
A series of alkyne niobium complexes based on tetrapodal amine bis(phenolato) ligands NbCl[ON-
NO]R(MeCCMe) ([ONNO]R ) N(CH2CH2NMe2)2(CH2C6H2-3,5-R2-2-O), R ) H, Me) (4), and tripodal
amine phenolato ligands NbCl2[ONN]Me(MeCCMe) ([ONN]Me ) NMe(CH2CH2NMe2)(CH2C6H2-3,5-
Me2-2-O)) (5) and NbCl2[ONOO]R(MeCCMe) ([ONOO]R ) N(CH2CH2OMe)2(CH2C6H2-3,5-R2-2-O))
(R ) tBu, Me) (6), have been synthesized and characterized by spectroscopic and X-ray studies. For
complexes 4 and 5, two isomers differing by the alkyne position have been obtained, separated, and
fully characterized. The alkyne ligand is either trans to the sidearm NMe2 group or to the tripodal nitrogen
atom. The methyl derivatives of 4 and 5, NbMe[ONNO]H(MeCCMe) (7) and NbMe(Cl)[ONN]Me(MeC-
CMe) (8), respectively, have been synthesized. Throughout the series, structural and 13C NMR data testify
to the competition between alkyne and phenolato orbitals for available d orbitals on the niobium, a rarely
observed picture for group 5 metal complexes.
Introduction
Results
Synthesis and Characterization of NbCl[ONNO]R-
(MeCCMe)(4a,b). Reacting NbCl3(DME)(MeCCMe)7 with
[ONNO]RH2 (1a,b) in tetrahydrofuran in the presence of
triethylamine affords after workup the desired niobium complex
NbCl[ONNO]R(MeCCMe) (4a,b) as a yellow powder in 73-75%
yield (Scheme 1).
The room-temperature 1H NMR spectrum of 4a,b reveals the
presence of two isomers in each case. In both of them, the
phenolate rings are equivalents assuming Cs symmetry for
the octahedral complex with the two phenoxy groups in trans
configuration. The Ar-CH2-N methylene protons are diaste-
reotopic exhibiting an AB system, and due to the plane of
symmetry they are pairwise equivalent. From these spectroscopic
data, we suggest that the two isomers possess the alkyne ligand
The design of non-Cp-type ligands has been explored to
control the stability and activity of early transition metal
catalysts, particularly in the field of olefin polymerization. Amine
mono(phenolate) [ONX]1 or bis(phenolate) ligands [ONXO]2
whose steric and electronic properties can easily be tuned are
one of these new generation ligands. Such ability has particularly
been studied in the polymerization of 1-hexene showing that
the presence of a sidearm donor is important to obtain a highly
active zirconium catalyst.3 Phenoxy-type ligands have been
successful in early transition metal complexes due to the
oxophilic behavior of these metals.4 Vanadium5 and tantalum6
complexes of amine mono- or bis(phenolate) ligands have been
described, but to the best of our knowledge, the coordination
of such ligands to a niobium center was surprisingly unknown
before this work. Most probably this can be attributed to the
general behavior of less stable and more labile niobium
derivatives.
(3) Tshuva, E. Y.; Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt,
Z. Organometallics 2002, 21, 662.
(4) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Inorg.
Chem. 2001, 40, 4263. (b) Michalczyk, L.; de Gala, S.; Bruno, J. W.
Organometallics 2001, 20, 5547. (c) Groysman, S.; Goldberg, I.; Kol, M.;
Goldschmidt, Z. Organometallics 2003, 22, 3793. (d) Groysman, S.; Tshuva,
E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z.; Shuster, M. Organometallics
2004, 23, 5291. (e) Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.;
Goldschmidt, Z. AdV. Synth. Catal. 2005, 347, 409. (f) Redshaw, C.; Rowan,
M. A.; Homden, D. M.; Dale, S. H.; Elsegood, M. R. J.; Matsui, S.;
Matsuura, S. Chem. Commun. 2006, 3329. (g) Chomitz, W.; Minasian, S.;
Sutton, A.; Arnold, J. Inorg. Chem. 2007, 46, 7199. (h) Groysman, S.;
Goldberg, I.; Goldschmidt, Z.; Kol, M. Inorg. Chem. 2005, 44, 5073. (i)
Groysman, S.; Sergeeva, E.; Goldberg, I.; Kol, M. Inorg. Chem. 2005, 44,
8188.
In this Article, we describe the synthesis and characteriza-
tion of new amine mono- or bis(phenolate) alkyne niobium
complexes. Our study focuses on a dianionic [ONNO] ligand
bearing a sidearm nitrogen donor (1), and monoanionic
[ONN] (2) and [ONOO] (3) ligands with amino and methoxy
sidearms, respectively (Chart 1). Simple methyl complexes
have also been obtained. The electron reservoir properties
of the alkyne ligands will be highlighted thanks to NMR and
structural studies.
(5) (a) Wolff, F.; Lorber, C.; Choukroun, R.; Donnadieu, B. Inorg. Chem.
2003, 42, 7839. (b) Wolff, F.; Lorber, C.; Choukroun, R.; Donnadieu, B.
Eur. J. Inorg. Chem. 2004, 2861. (c) Lorber, C.; Wolff, F.; Choukroun, R.;
Vendier, L. Eur. J. Inorg. Chem. 2005, 2850. (d) Maity, D.; Marek, J.;
Sheldrick, W. S.; Mayer-Figge, H.; Ali, M. J. Mol. Catal. A: Chem. 2007,
270, 153. (e) Maity, D.; Mijanuddin, Md.; Drew, M. G. B.; Marek, J.;
Mondal, P. C.; Pahari, B.; Ali, M. Polyhedron 2007, 26, 4494.
(6) Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.; Goldschmidt, Z.
Organometallics 2004, 23, 1880.
* To whom correspondence should be addressed. E-mail:
emmanuelle.despagnet-ayoub@lcc-toulouse.fr (E.D.-A.); michel.etienne@
lcc-toulouse.fr.
(1) Marinescu, S.; Agapie, T.; Day, M.; Bercaw, J. Organometallics
2007, 26, 1178.
(2) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organome-
tallics 2001, 20, 3017.
(7) Hartung, J. B., Jr.; Pedersen, S. F. Organometallics 1990, 9, 1414.
10.1021/om801139m CCC: $40.75
2009 American Chemical Society
Publication on Web 03/11/2009

Amine-Phenolate Ligands in Niobium Chemistry
Organometallics, Vol. 28, No. 7, 2009 2189
Chart 1
Scheme 1. Synthesis of NbCl[ONNO]R(MeCCMe) (4)
either trans to the tripodal nitrogen (4a1 or 4b1) or trans to the
alkyne as a (4e)-donor (both π systems are involved) and
including donation from the tripodal nitrogen make com-
plexes 4 formally 16e species. Adding one phenolate π
interaction would make them 18e species (see below).
sidearm NMe2 group (4a2 or 4b2) (Scheme 1). For each
isomer, only one broad singlet is observed for the methyl
groups of the alkyne ligand, indicating easy rotation about
the Nb-alkyne bond. Addition of pyridine to 4b does not
Crystals of 4a were obtained from a toluene/pentane mixture
of 4 at -25 °C. An ORTEP drawing of complex 4a is shown
in Figure 1. Selective crystallization is observed because only
the isomer with the alkyne ligand trans to the tripodal nitrogen
atom crystallizes (4a1). The complex adopts a distorted octa-
hedral geometry with the alkyne occupying a single coordination
site. The equatorial plane containing the NMe2 sidearm, the
chloro ligand, and the phenolate groups fold down toward the
tripodal nitrogen (∼80°), and the alkyne and the trans tripodal
nitrogen are not strictly aligned (176°). The Nb(1)-O lengths
(1.929(4) and 1.925(4) Å) and the Nb(1)-Cl(1) (2.5057(16)
Å) distances are in a classical range for such a bond. The tripodal
N(1)-Nb(1) bond distance (2.440(5) Å) is significantly longer
than the sidearm N(2)-Nb(1) length (2.365(6) Å) probably due
toastrongtransinfluenceofthealkyneligand.TheNb(1)-O-C(Ph)
angle of ca. 144° is in the classical range for phenolate d0 or d2
Nb complexes, although this suggests π donation from the
aryloxy oxygen to the metal is mainly significant for the oxygen
1
affect the H NMR spectrum after 5 h at room temperature
or 1 h at 50 °C, indicating that the sidearm NMe2 group is
strongly coordinated to the metal. Moreover, no intercon-
version between the two isomers is observed by variable-
temperature studies in the range of 193-353 K and by EXSY
experiments at 298 and 353 K. Thus, it is remarkable that
for both 4a,b the relative amount of the two isomers only
depends on the exact synthetic experimental conditions, and
that they do not readily interconvert in solution. NMR spectra
at low temperature (193 K) were carried out for complexes
4. In the slow exchange limit, the phenolate rings and the
ArCH2N methyl diastereotopic pairs for a given isomer are
no longer equivalent, indicating that C1 symmetry is adopted
at low temperature. All complexes 4 lose their Cs symmetry,
indicating some degree of flexibility of the dianionic [ON-
NO]R ligand. At 193 K, the alkyne methyl groups appear as
two singlets for each isomer, implying slow rotation on the
NMR time scale. In the 13C NMR spectra, the chemical shifts
of the coordinated alkyne carbons are in the region δ
181-177. The 13C chemical shifts of the metal-bound alkyne
carbons have been correlated to the electron donation ability
of the alkyne.8,9a Typically, δ > 200 is characteristic of “four-
electron” (4e)-donor alkynes, 185 > δ > 165 describes
formally (3e)-donor alkynes, whereas conventional (2e)-donor
alkynes are characterized by δ ≈ 120. It must be emphasized
that the formal (3e)-donor description strictly applies to
bis(alkyne) complexes where there is no metal orbital of
appropriate symmetry to interact with the symmetric linear
combination of alkynes π orbitals.9 Nonetheless, the chemi-
cal shifts for all isomers of 4a,b indicate competition between
alkyne and phenoxy π-type orbitals for an available Nb d
orbital, the phenoxy being a good π-donor. Counting the
Figure 1. Molecular structure of complex 4a1. Selected bond lengths
(Å) and angles (deg): Nb(1)-O(2) 1.929(4), Nb(1)-O(3) 1.925(4),
Nb(1)-Cl(1) 2.5057(16), Nb(1)-N(1) 2.440(5), Nb(1)-N(2)
2.365(6), Nb(1)-C(1) 2.092(7), Nb(1)-C(2) 2.123(7), C(1)-
C(2) 1.274(10), Nb(1)-O(2)-C(24) 144.7(4), Nb(1)-O(3)-C(21)
144.3(5).
(8) Templeton, J. L.; Ward, B. C. J. Am. Chem. Soc. 1980, 102, 3288.
(9) (a) Templeton, J. L. AdV. Organomet. Chem. 1989, 29, 1. (b) Etienne,
M.; Carfagna, C.; Lorente, P.; Mathieu, R.; de Montauzon, D. Organome-
tallics 1999, 18, 3075. (c) Felten, C.; Rehder, D.; Pampaloni, G.; Calderazzo,
F. Inorg. Chim. Acta 1992, 202, 121.

2190 Organometallics, Vol. 28, No. 7, 2009
Despagnet-Ayoub et al.
alkyne trans influence, an alternative situation as compared to
4a1. The bond lengths of Nb(1)-C(1)/Nb(1)-C(2) (2.098(5)
and 2.140(5) Å, respectively) and C(1)-C(2) (1.282(8) Å) are
in the same range as the values observed for 4a1, indicating
similar electronic and orbital interactions as discussed below.
Synthesis and Characterization of NbCl2[ONN]Me
-
(MeCCMe) (5) and NbCl2[ONOO]R(MeCCMe) (6). The
complex NbCl2[ONN]Me(MeCCMe) (5) is obtained as an orange
powder in 69% yield following addition of a mixture of
triethylamine and aminophenol [ONN]MeH 2 in THF to a
solution of NbCl3(DME)(MeCCMe) (Scheme 2). Complex 5 is
fully characterized by elemental analysis, 1H and 13C NMR, as
well as by determination of an X-ray crystal structure.
As for the dianionic [ONNO]R complexes 4a, two noninter-
1
converting isomers are observed by H NMR spectroscopy,
Figure 2. Molecular structure of complex 4b2. Selected bond lengths
(Å) and angles (deg): Nb(1)-O(1) 1.943(4), Nb(1)-O(2) 1.922(4),
Nb(1)-Cl(1) 2.4654(14), Nb(1)-N(1) 2.373(4), Nb(1)-N(2)
2.467(4), Nb(1)-C(1) 2.098(5), Nb(1)-C(2) 2.140(5), C(1)-
C(2) 1.282(8), Nb(1)-O(1)-C(5) 143.2(4), Nb(1)-O(3)-C(13)
144.9(3).
probably one with the alkyne trans to the tripodal nitrogen atom
(51) and another one with the alkyne ligand trans to the sidearm
NMe2 (52). Depending on crystallization conditions, complete
isomer separation can be achieved, whereas this has never been
realized for the other complexes in this study. The 13C NMR
spectrum displays a signal in the region of δ 207 for the
coordinated carbons of the alkyne, a value characteristic of a
(4e)-donor alkyne.
p orbital lying in the O2ClN plane (see Discussion).3,4c-e,5a-c,6,10
The alkyne ligand approximately lies in the pseudo mirror plane
of the molecule, which bisects the O-Nb-O angle and contains
the tripodal nitrogen atom and the sidearm NMe2 group (the
torsion angle Cl(1)-Nb(1)-C(2)-C(1) is 14°). This perpen-
dicular alkyne conformation with respect to the π-donor aryloxy
groups can be rationalized on simple orbital grounds (see below).
The Nb(1)-C(1) and Nb(1)-C(2) bond distances (2.092(7) and
2.123(7) Å, respectively) are relatively long for a (4e)-donor
alkyne in niobium complexes (typical range [2.00-2.08]
Å).11-15 The C(1)-C(2) bond length (1.274(10) Å) is between
that of double and triple C-C bonds. Again, this characterizes
competition between π-donors for available orbital space on
the metal.
An X-ray diffraction study was also carried out on complex
4b. Two independent molecules are observed in the cell with
very similar structural parameters, but only one of them is shown
in Figure 2 and described herein. In contrast to the case of 4a,
selective crystallization surprisingly affords isomer 4b2 in which
the alkyne ligand is trans to the sidearm NMe2 group (and
consequently the chloro ligand is now trans to the tripodal
nitrogen). The crystal structures of complexes 4a,b confirm the
two possible types of isomers suggested by the spectroscopic
data. Beyond this difference, the coordination spheres in 4a1
and 4b2 are fully comparable. In 4b2, the Nb(1)-N(2) (sidearm
nitrogen atom) bond length (2.467(4) Å) is longer than the
Nb(1)-N(1) (tripodal nitrogen atom) (2.373(4) Å) due to the
An X-ray diffraction study of complex 5 was carried out
(Figure 3). A single isomer of the six coordinated 51 is observed
with the alkyne trans to the tripodal nitrogen atom N(1) and cis
to the two chloro ligands. The alkyne is roughly aligned with
the Nb(1)-Cl(1) bond, and perpendicular to O(1)-Nb(1)-Cl(2).
Nb(1)-Cl(1) is significantly longer (>0.06 Å) than Nb(1)-Cl(2).
Both structural features again translate competing π effects on
niobium. The Nb-Ophenoxy bond is the shortest in this study.
Attempts at discussing net Nb-O bond order here and for the
other complexes in this study are hampered by the presence of
several competing π-type ligands (Cl, phenoxy-O, alkyne)
whose number and relative stereochemistry are varied. It is
noteworthy that the niobium-alkyne carbon bonds in 51 are
shorter than those in 4a1, whereas the coordinated alkyne CC
bonds are (slightly) longer in 51 than in 4a1. These data are in
agreement with the 13C NMR data, indicating that the alkyne
behaves as a “genuine” (4e)-donor.
The synthesis of a complex bearing a methoxy sidearm
instead of an amino group was also investigated. In addition, a
second dangling methoxy sidearm has been added. This potential
extra ligand may reversibly coordinate to a cationic metal center
once an X ligand (i.e., Cl) is removed. Yellow NbCl2-
[ONOO]tBu(MeCCMe) (6a) is obtained by addition of a triethy-
lamine and [ONOO]tBu mixture in THF to a solution of
1
NbCl3(DME)(MeCCMe) (Scheme 3). As ascertained by H
NMR, a single isomer is present: in this case, this is probably
due to the strong field alkyne ligand preferring the trans weaker
methoxy sidearm (see X-ray structure below). The coordinated
alkyne carbons are detected (13C NMR) at δ 209, indicating
(4e) behavior. The complex NbCl2[ONOO]tBu(MeCCMe) (6b)
obtained from the less hindered [ONOO]Me ligand bearing Me
groups in place of tBu groups has also been synthesized in the
form of a single isomer according to its 1H NMR spectrum. In
contrast to 6a, 6b readily reacts at room temperature in solution
with adventitious proton sources to give the zwitterionic
NbCl4[ONOO]Me[ON(H)OO]Me (6H), which has been identified
by its X-ray crystal structure (see Supporting Information).
(10) Gendler, S.; Segal, S.; Goldberg, I.; Goldschmidt, Z.; Kol, M. Inorg.
Chem. 2006, 45, 4783.
(11) (a) Cotton, F. A.; Shang, M. Inorg. Chem. 1990, 29, 508. (b) Oshiki,
T.; Tanaka, K.; Yamada, J.; Ishiyama, T.; Kataoka, Y.; Mashima, K.; Tani,
K.; Takai, K. Organometallics 2003, 22, 464.
(12) (a) Boulho, C.; Keys, T.; Coppel, Y.; Vendier, L.; Etienne, M.;
Locati, A.; Bessac, F.; Maseras, F.; Pantazis, D. A.; McGrady, J. E.
Organometallics 2009, 28, 940. (b) Jaffart, J.; Cole, M. L.; Etienne, M.;
Reinhold, M.; McGrady, J. E.; Maseras, F. Dalton Trans. 2003, 4057. (c)
Jaffart, J.; Etienne, M.; Reinhold, M.; McGrady, J. E.; Maseras, F. Chem.
Commun. 2003, 876.
(13) (a) Etienne, M.; Ze´line, P.; Templeton, J. L.; White, P. S. New
J. Chem. 1993, 17, 515. (b) Etienne, M.; White, P. S.; Templeton, J. L.
Organometallics 1993, 12, 4010.
(14) Hinshaw, C. J.; Peng, G.; Singh, R.; Spence, J. T.; Enemark, J. H.;
Bruck, M.; Kristofzski, J.; Merbs, S. L.; Ortega, R. B.; Wexler, P. A. Inorg.
Chem. 1989, 28, 4483.
(15) Groysman, S.; Sergeeva, E.; Goldberg, I.; Kol, M. Eur. J. Inorg.
Chem. 2006, 2739.
Crystals of 6a suitable for an X-ray structure determination
were obtained from a pentane solution at room temperature.
Two independent molecules with very similar structural pa-
rameters are observed in the asymmetric unit, and only one of

Amine-Phenolate Ligands in Niobium Chemistry
Organometallics, Vol. 28, No. 7, 2009 2191
Scheme 2. Synthesis of NbCl2[ONN]Me(MeCCMe) (5)
them is shown in Figure 4. The alkyne ligand is trans to the
methoxy sidearm. The niobium is bound to two cis chloro
ligands, and the cis alkyne bisects the Cl-Nb-Ophenoxy and
Cl-Nb-Ntripodal angles. In contrast to the other structures
presented in this work, the alkyne is not perpendicular to the
niobium-π-donor bond. Shorter Nb(2)-Calkyne bond lengths
(2.059(5) and 2.081(5) Å) and a longer Calkyne-Calkyne bond
group affects the alkyne behavior. The methyl niobium com-
plexes display a low field alkyne signal (δ 191 and 213 for 7
and 8, respectively) as compared to the chloro analogue (δ 181
and 208 for 4b and 5, respectively), indicating slightly increased
donation to the metal. Despite repeated attempts to synthesize
so-called R-CC agostic complexes,12 no reaction was observed
between either 4b or 5 and cyclopropyl magnesium or lithium
salts.
distance (1.302(6) Å) as compared to complex 5 (Nb-Calkyne
:
2.109(5) and 2.085(4) Å and Calkyne-Calkyne 1.281(7) Å), which
also contains two cis choro ligands and a tripodal nitrogen,
indicate that the (4e) character is stronger in 6 than in 5. Recall
that the barrier to alkyne rotation is relatively low because single
alkyne signals are observed at room temperature.
Discussion
Alkyne ligands possess tunable π-donation ability and behave
as electron buffers.8 This flexibility is due to the presence of a
set of two orthogonal π systems (π|/π|* or π /π *). Alkynes
behave as formal (2e)-donors when only the π|/π|* system is
involved in the coordination. Adding interaction of the filled
π orbital with the symmetry appropriate vacant metal orbital
will implement formal electron donation up to 4e. Beyond this
formal electron count, the alkyne nicely responds to electron
demand on the metal. In this context, 13C NMR spectroscopy
has been shown to be a very sensitive probe of the true electron
flow between the metal and the alkyne. M-Calkyne and coordi-
nated CC bond of the alkyne also respond to the variation of
electron density with shorter M-C bonds and longer CC bonds
as the donation of the alkyne increases. This overall picture is
well documented for group 6 metal alkyne complexes,9a but
less so for group 5 complexes. We briefly discuss these effects
for the new complexes described in the Results section.
Aryloxo ligands can formally be considered as up to 6e
donors, with one σ and two π-type interactions. Depending on
molecular symmetry and taking into account the electronega-
tivity of O, one of these two π-type orbitals will interact more
One goal of this work was to synthesize alkyl complexes on
the basis of these new ligands. Treatment of 4b or 5 with
methyllithium or methylgrignard reagent, respectively, yielded
NbMe[ONNO]H(MeCCMe) (7) and NbMeCl[ONN]Me
(MeCCMe) (8) complexes, which were characterized by 1H and
13C NMR. As expected, two isomers are present in each case.
In the 1H and 13C NMR spectra, the methyl signals are observed
around δ 1.6 and 40-45, respectively, for both complexes. The
substitution of a π-donor chloro ligand by a σ-donor methyl
Figure 3. Molecular structure of complex 51. Selected bond lengths
(Å) and angles (deg): Nb(1)-O(1) 1.890(3), Nb(1)-Cl(1) 2.4779(14),
Nb(1)-Cl(2) 2.4112(12), Nb(1)-N(1) 2.496(4), Nb(1)-N(2)
2.360(4), Nb(1)-C(1) 2.109(5), Nb(1)-C(2) 2.085(4), C(1)-C(2)
1.281(7), Nb(1)-O(1)-C(17) 149.1(3).
Scheme 3. Synthesis of NbCl2[ONOO]R(MeCCMe) (6)
Figure 4. Molecular structure of complex 6a. Selected bond lengths
(Å) and angles (deg): Nb(2)-O(5) 1.915(3), Nb(2)-O(4) 2.380(3),
Nb(2)-Cl(3) 2.4176(11), Nb(2)-Cl(4) 2.4436(13), Nb(2)-N(2)
2.388(3), Nb(2)-C(30) 2.059(5), Nb(2)-C(31) 2.081(5), C(30)-
C(31) 1.302(6), Nb(2)-O(5)-C(43) 144.4(2).

2192 Organometallics, Vol. 28, No. 7, 2009
Despagnet-Ayoub et al.
Scheme 4. Qualitative π-type Orbital Splitting in Phenoxo Complexes of Niobium: (Left) Fragment Analysis without the Alkyne
Ligand, and (Right) with the Alkyne Ligand
strongly. A qualitative orbital diagram depicted in Scheme 4
summarizes potential interactions, indicating that there is an
orbital preference for the alkyne to sit perpendicular to either
one or two π-donors. By doing so, the alkyne satisfies its need
for one filled and one vacant dπ orbital on Nb. With
Nb-O-C(aryl)bond angles much below 180° as is the case
for 4 and 5 (ca. 144°), the involvement of the oxygen pz can be
minimized, and dxz is then almost a bystander. The number of
π-donors is relatively unimportant because this qualitative
picture applies both for the dialkoxo 4a and 4b and for the
monoalkoxo 5. When two trans alkoxo π-donors are present,
they additionally compete with each other. Even if their role
can be qualitively neglected as significant π-donors, we have
seen above that the chloro ligands also responded to this
competition.
Remarkably, the two possible coordination sites have been
characterized in 4a1 and 4b2. A high barrier prevents intercon-
vertion between these isomers, and as already mentionned the
isomer ratios vary from one synthesis to the other. In each case,
competition between alkoxo and alkyne orbitals for orbital space
on the metal is testified by variations of key structural and NMR
parameters. For the dialkoxo, the upfield shift of the alkyne
carbon resonances in the 13C NMR spectrum is accompanied
by longer Nb-Calkyne bonds and shorter alkyne CC bonds.13
Interestingly, the competition is such that the alkyne chemical
shift approaches that commonly observed for so-called (3e)-
donor alkynes in bis(alkyne) complexes. Similarly in 5, the
Nb-Cl bond is longer to the chloro cis to the π-donor alkoxo,
and fully similar to that observed in 4a, 4b where Cl is
necessarily cis. Competition between the trans alkoxo ligands
in 4a, 4b can also be seen from the longer Nb-O bonds (ca.
1.93 Å) as compared to the shorter one in 5 (ca. 1.89 Å). Given
the number of possible competing effects and their effect on
the structural parameters (and most prominently metal-ligand
bond lengths), a detailed discussion based on tentative Nb-O
bond orders does not seem as appealing as might be thought.
Using space-filling models to examine the structures, it can be
noted that there are short contacts between one alkyne Me and
either of the Me groups of the NMe2 sidearm (4a1, 5), or the H
atoms from the benzylic CH2 (4b2), so the orbital preference
seems sufficient to overcome small potential steric interactions.
Recall, however, that barriers to alkyne rotation remain low in
1
all cases studied here because single H or 13C NMR alkyne
signals are observed at room temperature.
The case of complex 6 remains puzzling in the sense that
the alkyne ligand does not occupy the supposedly preferred
conformation perpendicular to the alkoxo π-donor. The absence
of competition between the alkyne and the alkoxo orbitals for
Nb orbitals is nicely indicated by shorter Nb-Calkyne bonds,
longer alkyne CC bonds, and deshielded alkyne carbons, all of
which point to a “true” (4e)-alkyne behavior. Considering space-
filling models of the X-ray structure of 6, we have been unable
to find any intramolecular interaction that would have hampered
alkyne alignment with the cis Nb-O bond. Once again, orbital
preferences remain modest in these pseudooctahedral systems,
and any small unidentified effect, most probably from electronic
origin, would be significant enough to overcome this preference.
Summary
This work represents the first syntheses and characterizations
of niobium alkyne complexes based on popular amine phenolate
ligands. The alkyne responds to the π-interactions imposed by
the coordination of the phenoxo ligands. In one case, both
possible alkyne coordination isomers have been characterized
by X-ray diffraction. Methyl derivatives have been obtained,
and this suggests that chemistry related to alkene polymerization
is at hand within this new series.
Experimental Section
All experiments requiring a dry atmosphere were performed using
a conventional vacuum line and Schlenk tube techniques. THF and

Amine-Phenolate Ligands in Niobium Chemistry
Organometallics, Vol. 28, No. 7, 2009 2193
toluene were dried and distilled by refluxing over sodium under
argon. Triethylamine was refluxed and dried over NaOH under an
atmosphere of argon, collected, and stored over molecular sieves.
Starting materials for ligand precursor synthesis were purchased
from Aldrich Inc. and used as received. The ligands [ONNO]MeH2,4a
[ONNO]HH2,14 [ONOO]MeH,15 [ONOO]tBuH,1 and the complex
NbCl3(DME)(MeCCMe)7 were synthesized according to published
procedures. NMR solvents were stored over molecular sieves under
argon. NMR data were recorded using ARX-250, DPX-300, or AV-
500 MHz Bruker spectrometers. Elemental analyses were performed
in the Analytical Service of our laboratory.
Synthesis of [ONN]MeH. A methanol solution (10 mL) of 2,4-
dimethylphenol (2 mL, 16.55 mmol), N,N,N′-trimethylethylenedi-
amine (2.1 mL, 16.55 mmol), and formaldehyde (5.6 mL of a 36%
aqueous solution, 66.20 mmol) was refluxed overnight. After
volatile materials were removed, an aqueous solution of HCl (7
mL of a 37% solution) was added to the reaction mixture. The
aqueous layer was washed with petroleum ether (3 × 50 mL). The
aqueous layer was neutralized with KOH solution and extracted
with ether (3 × 50 mL). The organic phase was dried over
anhydrous MgSO4. After evaporation, the desired product was
yielded as a colorless oil (2.6 g, 67%). Anal. Calcd for C14H24N2O:
C, 71.14; H, 10.23; N, 11.85. Found: C, 70.97; H, 10.10; N, 11.78.
1H NMR (250.13 MHz, 298 K, C6D6): 7.01 (s, 1H, Haryl), 6.76 (s,
1H, Haryl), 3.44 (s, 2H, ArCH2), 2.58 (s, 3H, CH3), 2.34 (s, 3H,
CH3), 2.33-2.23 (m, 4H, NCH2CH2N), 2.13 (s, 6H, N(CH3)2), 2.09
(s, 3H, CH3). 13C NMR (62.89 MHz, 298 K, C6D6): δ 154.3 (Caryl),
130.8 (CarylH), 127.3 (CarylH), 126.6 (Caryl), 124.8 (Caryl), 121.9
(Caryl), 59.7 (CH2), 56.5 (CH2), 54.2 (CH2), 44.9 (NCH3), 41.6
(NCH3), 20.5 (CH3), 16.2 (CH3).
1.90 (s, 3H, CH3), 1.85 (m, 1H, NCH2). 13C NMR (125.80 MHz,
193 K, CD2Cl2): Isomer 1, δ 185.3 (MeCt ), 176.4 (MeCt ), 155.0
(Caryl), 153.9 (Caryl), 131.5 (CarylH), 130.9 (CarylH), 130.0 (Caryl),
129.8 (Caryl), 128.6 (CarylH), 128.2 (CarylH), 125.9 (Caryl), 125.7
(Caryl), 125.4 (Caryl), 124.3 (Caryl), 63.3 (ArCH2), 62.4 (ArCH2), 61.9
(NCH2), 53.2 (NCH3), 51.2 (NCH2), 47.3 (NCH3), 20.7 (CH3), 20.6
(CH3), 18.0 (CH3Ct ), 16.2 (CH3Ct ), 16.0 (CH3), 15.8 (CH3).
Isomer 2, δ 178.9 (MeCt ), 177.2 (MeCt ), 155.6 (Caryl), 153.9
(Caryl), 131.7 (Caryl), 131.2 (Caryl), 128.9 (Caryl), 128.2 (Caryl),
128.0 (Caryl), 124.7 (Caryl), 124.8 (Caryl), 123.6 (Caryl), 122.9 (Caryl),
121.5 (Caryl), 66.3 (ArCH2), 65.5 (ArCH2), 59.2 (NCH2), 54.4
(NCH2), 49.2 (NCH3), 45.5 (NCH3), 17.7 (CH3), 17.2 (CH3), 17.1
(CH3), 16.8 (CH3), 16.2 (CH3Ct ), 15.7 (CH3), 15.8 (CH3Ct ).
NbCl[ONNO]H(MeCCMe) (4b). Yellow powder (520 mg, 1.08
mmol, 75% yield). Crystals were obtained from a toluene/pentane
mixture at -25 °C. Anal. Calcd for C22H28ClN2NbO2: C, 54.95;
1
H, 5.87; N, 5.83. Found: C, 54.51; H, 5.60; N, 5.65. H NMR
(250.13 MHz, 298 K, C6D6): Isomer 1, δ 6.98-6.65 (m, 6H, Haryl),
2
2
6.47 (d, JHH ) 8.0 Hz, 2H, Haryl), 5.55 (d, JHH ) 13.5 Hz, 2H,
2
ArCH2N), 3.17 (br. s, 3H, CH3alk), 2.61 (d, JHH ) 13.5 Hz, 2H,
ArCH2N), 2.19 (s, 6H, N(CH3)2), 2.17 (m, 2H, N-CH2), 1.70 (m,
2
2H, N-CH2). Isomer 2, δ 6.98-6.65 (m, 6H, Haryl), 6.51 (d, JHH
) 8.2 Hz, 2H, Haryl), 4.85 (d, 2JHH ) 13.6 Hz, 2H, ArCH2N), 3.04
(d, 2JHH ) 13.6 Hz, 2H, ArCH2N), 2.57 (br.s, 3H, CH3alk), 2.27 (s,
1
6H, N(CH3)2), 2.17 (m, 2H, N-CH2), 1.70 (m, 2H, N-CH2). H
NMR (500.33 MHz, 193 K, CD2Cl2): Isomer 1, δ 7.25-7.12 (m,
3
4H, Haryl), 6.90-6.82 (m, 2H, Haryl), 6.47 (d, JHH ) 7.9 Hz, 2H,
Haryl), 5.35 (d, 2JHH ) 13.7 Hz, 1H, ArCH2N), 5.18 (d, 2JHH ) 13.9
2
Hz, 1H, ArCH2N), 3.37 (d, JHH ) 13.9 Hz, 1H, ArCH2N), 3.33
(d, JHH ) 13.7 Hz, 1H, ArCH2N), 2.98 (m, 1H, NCH2), 2.75 (s,
2
3H, CH3alk), 2.62 (s, 3H, CH3alk), 2.53 (m, 1H, NCH2), 2.50 (s, 3H,
NCH3), 2.46 (m, 1H, NCH2), 2.32 (s, 3H, NCH3), 2.02 (m, 1H,
NCH2). Isomer 2, δ 7.25-7.12 (m, 4H, Haryl), 6.90-6.82 (m, 2H,
Haryl), 6.50 (dd, 3JHH ) 7.9 Hz, 4JHH ) 3.1 Hz, 2H, Haryl), 5.18 (d,
Synthesis of NbCl[ONNO]R(MeCCMe) (4). All compounds
were prepared according to the same general procedure described
here in detail for 4a. A solution of [ONNO]MeH2 (517 mg, 1.45
mmol) and triethylamine (0.47 mL, 3.34 mmol) in THF (20 mL)
was added to a solution of NbCl3(DME)(MeCCMe) (500 mg, 1.45
mmol) in THF (20 mL), and the mixture was stirred at room
temperature for 3 h. After the volatiles were removed, the residue
was dissolved in toluene and filtered through a pad of celite,
affording a yellow solution. The solvent was removed under
vacuum, and NbCl[ONNO]Me(MeCCMe) 4a was obtained.
NbCl[ONNO]Me(MeCCMe) (4a). Yellow powder (585 mg, 1.09
mmol, 75% yield). Crystals were obtained from a toluene/pentane
mixture at -25 °C. Anal. Calcd for C26H36ClN2NbO2: C, 58.16;
2
2JHH ) 13.9 Hz, 1H, ArCH2N), 5.07 (d, JHH ) 13.6 Hz, 1H,
2
2
ArCH2), 3.91 (d, JHH ) 13.9 Hz, 1H, ArCH2), 3.84 (d, JHH
)
13.6 Hz, 1H, ArCH2), 3.10 (m, 1H, NCH2), 2.83 (s, 3H, CH3alk),
2.69 (s, 3H, CH3alk), 2.42 (m, 1H, NCH2), 2.43 (s, 3H, NCH3),
2.28 (m, 1H, NCH2), 2.12 (s, 3H, NCH3), 1.92 (m, 1H, NCH2).
13C{1H} NMR (125.80 MHz, 193 K, CD2Cl2): Isomer 1, δ 185.8
(MeCt ), 176.9 (MeCt ), 158.7 (Caryl), 157.4 (Caryl), 130.7 (Caryl),
130.1 (Caryl), 130.0 (Caryl), 129.2 (Caryl), 126.8 (Caryl), 126.2 (Caryl),
121.4 (Caryl), 121.2 (Caryl), 117.7 (Caryl), 116.6 (Caryl), 63.2 (ArCH2),
62.4 (ArCH2), 61.8 (NCH2), 53.1 (NCH3), 51.6 (NCH2), 48.4
(NCH3), 18.2 (CH3Ct ), 16.2 (CH3Ct ). Isomer 2, δ 181.0 (MeCt ),
180.1 (MeCt ), 159.5 (Caryl), 157.8 (Caryl), 130.6 (Caryl), 130.4 (Caryl),
130.0 (Caryl), 129.9 (Caryl), 124.3 (Caryl), 123.8 (Caryl), 120.2 (Caryl),
119.9 (Caryl), 117.1 (Caryl), 116.0 (Caryl), 66.2 (ArCH2), 65.5 (ArCH2),
59.1 (NCH2), 54.5 (NCH2), 49.8 (NCH3), 46.2 (NCH3), 17.8
(CH3Ct ), 15.7 (CH3Ct ). DCI/NH3-MS: m/z: 481.3 - complex
+ H+.
1
H, 6.76; N, 5.22. Found: C, 58.04; H, 6.53; N, 5.44. H NMR
(250.13 MHz, 298 K, C6D6): Isomer 1, δ 6.87 (s, 2H, Haryl), 6.66
(s, 2H, Haryl), 5.75 (d, 2JHH ) 13.5 Hz, 2H, ArCH2), 3.33 (br.s, 6H,
2
CH3alk), 2.85 (d, JHH ) 13.5 Hz, 2H, ArCH2), 2.35 (s, 6H, CH3),
2.27 (s, 6H, CH3), 2.26 (m, 2H, N-CH2), 2.26 (s, 6H, CH3), 1.82
(m, 2H, N-CH2). Isomer 2, δ 6.81 (s, 2H, Haryl), 6.71 (s, 2H, Haryl),
2
2
5.03 (d, JHH ) 13.5 Hz, 2H, ArCH2), 3.24 (d, JHH ) 13.5 Hz,
2H, ArCH2), 2.65 (br. s, 6H, CH3alk), 2.26 (m, 2H, N-CH2), 2.24
(s, 6H, CH3), 2.18 (s, 6H, CH3), 2.02 (s, 6H, CH3), 1.82 (m, 2H,
Synthesis of NbCl2[ONX]R(MeCCMe) (5, 6a,b): General
Procedure. A solution of [ONX]H (1.30 mmol) and triethylamine
(0.27 mL, 1.95 mmol) in THF (20 mL) was added to a solution of
NbCl3(DME)(MeCCMe) (620 mg, 1.30 mmol) in THF (20 mL),
and the mixture was stirred at room temperature for 3 h. After the
volatiles were removed, the residue was dissolved in toluene and
filtered through a pad of celite. The solvent was removed under
vacuum, and the desired complex was obtained.
1
N-CH2). H NMR (500.33 MHz, 193 K, CD2Cl2): Isomer 1, δ
6.86 (s, 1H, Haryl), 6.83 (s, 1H, Haryl), 6.79 (s, 1H, Haryl), 6.75 (s,
1H, Haryl), 5.23 (d, JHH ) 13.5 Hz, 1H, ArCH2), 5.08 (d, JHH
13.5 Hz, 1H, ArCH2), 3.23 (d, JHH ) 13.5 Hz, 1H, ArCH2), 3.19
(d, 2JHH ) 13.5 Hz, 1H, ArCH2), 2.93 (m, 1H, NCH2), 2.74 (s, 3H,
CH3alk), 2.72 (s, 3H, NCH3), 2.62 (s, 3H, CH3alk), 2.49 (m, 1H,
NCH2), 2.42 (m, 1H, NCH2), 2.37 (s, 3H, NCH3), 2.19 (s, 6H, CH3),
1.98 (m, 1H, NCH2), 1.89 (s, 3H, CH3), 1.85 (s, 3H, CH3). Isomer
2, δ 6.87 (s, 1H, Haryl), 6.83 (s, 1H, Haryl), 6.80 (s, 1H, Haryl), 6.71
2
2
)
2
NbCl2[ONN]Me(MeCCMe) (5). Orange powder (406 mg, 0.94
mmol, 72% yield). Crystals were obtained in a toluene solution at
-25 °C. Anal. Calcd for C18H29Cl2N2NbO: C, 47.70; H, 6.45; N,
2
2
(s, 1H, Haryl), 5.09 (d, JHH ) 13.5 Hz, 1H, ArCH2), 4.97 (d, JHH
2
1
) 13.1 Hz, 1H, ArCH2), 3.77 (d, JHH ) 13.5 Hz, 1H, ArCH2),
6.18. Found: C, 47.76; H, 6.54, N, 6.00. H NMR (300.13 MHz,
2
3.71 (d, JHH ) 13.1 Hz, 1H, ArCH2), 3.09 (m, 1H, NCH2), 2.73
298 K, C6D6): Isomer 1, δ 6.67 (s, 1H, Haryl), 6.40 (s, 1H, Haryl),
(s, 3H, CH3alk), 2.69 (s, 3H, CH3alk), 2.43 (m, 1H, NCH2), 2.44 (s,
3H, NCH3), 2.41 (s, 3H, CH3), 2.30 (m, 1H, NCH2), 2.20 (s, 3H,
CH3), 2.09 (s, 3H, NCH3), 2.11 (s, 3H, CH3), 1.97 (s, 3H, CH3),
5.57 (d, 2JHH ) 14.0 Hz, 1H, ArCH2), 3.06-2.56 (m, 2H, NCH2),
2
2.93 (s, 6H, CH3alk), 2.89 (s, 3H, CH3), 2.74 (d, JHH ) 14.0 Hz,
1H, ArCH2), 2.69 (s, 3H, CH3), 2.14 (s, 3H, CH3), 2.04 (s, 3H,

2194 Organometallics, Vol. 28, No. 7, 2009
Despagnet-Ayoub et al.
CH3), 1.84 (s, 3H, CH3), 1.54-1.36 (m, 2H, NCH2). Isomer 2, δ
6.69 (s, 1H, Haryl), 6.38 (s, 1H, Haryl), 4.15 (d, 2JHH ) 14.5 Hz, 1H,
ArCH2), 3.10-2.88 (m, 2H, NCH2), 2.95 (m, 1H, ArCH2), 2.90 (s,
3H, CH3), 2.78 (s, 6H, CH3alk), 2.73 (s, 3H, CH3), 2.19 (s, 3H,
CH3), 2.17 (s, 3H, CH3), 2.15 (s, 3H, CH3), 1.54-1.36 (m, 2H,
NCH2). 1H NMR (500.33 MHz, 193 K, CD2Cl2): Isomer 1, δ 6.84
residue was dissolved in toluene and filtered through a pad of celite.
The solvent was removed under vacuum, and the desired complex
7 was obtained as a brown powder (60 mg, 0.13 mmol, 62% yield).
Anal. Calcd for C23H31N2NbO2: C, 60.00; H, 6.79; N, 6.08. Found:
C, 60.51; H, 7.08, N, 5.60. 1H NMR (500.33 MHz, 323 K, C7D8):
δ Isomer 1, δ 6.86-7.00 (m, 4H, Haryl), 6.68 (pt d, JHH ) 7.4 Hz,
JHH ) 1.1 Hz, 2H, Haryl), 6.39 (dd, JHH ) 8.1 Hz, JHH ) 1.0 Hz,
2
(s, 1H, Haryl), 6.72 (s, 1H, Haryl), 4.35 (d, JHH ) 15.2 Hz, 1H,
ArCH2), 3.64 (d, 2JHH ) 15.2 Hz, 1H, ArCH2), 3.30 (m, 1H, NCH2),
3.21 (m, 1H, NCH2), 3.06 (s, 3H, NCH3), 2.76 (s, 3H, NCH3), 2.72
(br. s, 3H, CH3alk), 2.60 (br.s, 3H, CH3alk), 2.35 (s, 3H, NCH3),
2.32 (m, 1H, NCH2), 2.30 (m, 1H, NCH2), 2.17 (s, 3H, CH3), 1.96
(s, 3H, CH3). Isomer 2, δ 6.83 (s, 1H, Haryl), 6.73 (s, 1H, Haryl),
2
2
2H, Haryl), 4.92 (d, JHH ) 13.6 Hz, 2H, ArCH2), 2.87 (d, JHH
)
13.6 Hz, 2H, ArCH2), 2.68 (br. s, 6H, CH3alk), 2.32 (m, 2H, NCH2),
2.22 (s, 6H, NCH3), 1.71 (m, 2H, NCH2), 1.54 (s, 3H, CH3). Isomer
2, δ 6.86-7.00 (m, 4H, Haryl), 6.66 (pt d, JHH ) 7.4 Hz, JHH ) 1.2
Hz, 2H, Haryl), 6.37 (dd, JHH ) 8.1 Hz, JHH ) 1.0 Hz, 2H, Haryl),
2
5.20 (d, JHH ) 14.0 Hz, 1H, ArCH2), 3.44 (m, 1H, NCH2), 3.19
2
2
4.90 (d, JHH ) 13.5 Hz, 2H, ArCH2), 3.13 (d, JHH ) 13.5 Hz,
2H, ArCH2), 2.32 (m, 2H, NCH2), 2.80 (br. s, 6H, CH3alk), 2.15 (s,
6H, NCH3), 1.82 (m, 2H, NCH2), 1.66 (s, 3H, CH3). 13C NMR
(125.80 MHz, 324.5 K, C7D8): Isomer 1, δ 189.4 (MeCt ), 159.9
(Caryl), 129.6 (Caryl), 129.4 (Caryl), 125.8 (Caryl), 119.5 (CHaryl), 117.97
(CHaryl), 63.6 (ArCH2), 60.1 (NCH2), 53.2 (NCH3), 48.4 (NMe2),
44.3 (br, NbCH3), 15.8 (br, CH3C′). Isomer 2, δ 192.5 (MeCt ),
160.7 (Caryl), 129.9 (CHaryl), 129.6 (CHaryl), 123.8 (Caryl), 118.2
(CHaryl), 117.3 (CHaryl), 65.2 (ArCH2), 60.8 (NCH2), 52.3 (NCH3),
47.3 (NMe2), 45.0 (NbCH3), 16.8 (CH3Ct ).
(d, 2JHH ) 14.0 Hz, 1H, ArCH2), 3.04 (s, 3H, NCH3), 2.94 (m, 1H,
NCH2), 2.86 (s, 3H, CH3alk), 2.80 (s, 3H, NCH3), 2.74 (s, 3H,
CH3alk), 2.55 (s, 3H, NCH3), 2.38 (m, 1H, NCH2), 2.27 (m, 1H,
NCH2), 2.16 (s, 3H, CH3), 1.82 (s, 3H, CH3). 13C{1H} NMR (125.80
MHz, 193 K, CD2Cl2): Isomer 1, δ 212.1 (MeCt ), 206.5 (MeCt ),
154.2 (Caryl), 130.7 (Caryl), 130.0 (Caryl), 126.9 (Caryl), 124.7 (Caryl),
120.2 (Caryl), 64.8 (ArCH2), 56.7 (NCH2), 55.4 (NCH2), 52.2
(NCH3), 50.7 (NCH3), 46.1 (NCH3), 20.6 (CH3Cd), 20.6 (CH3),
18.7 (CH3Cd), 16.0 (CH3). Isomer 2, δ 210.8 (MeCt ), 199.7
(MeCt ), 153.7 (Caryl), 131.9 (Caryl), 131.0 (Caryl), 128.5 (Caryl), 126.1
(Caryl), 125.5 (Caryl), 62.6 (ArCH2), 60.8 (NCH2), 54.2 (NCH3), 52.0
(NCH2), 49.0 (NCH3), 47.5 (NCH3), 19.6 (CH3Cd), 18.3 (CH3Cd),
17.1 (CH3), 16.0 (CH3).
Synthesis of NbMeCl[ONN]Me(MeCCMe) (8). A solution of
methylmagnesium chloride in tretrahydrofuran (0.24 mL, 0.73
mmol) was added to a solution of NbCl[ONN]Me(MeCCMe) (5)
(300 mg, 0.66 mmol) in THF (10 mL) at -20 °C. After 6 h at -20
°C, the volatile materials were removed. The residue obtained was
dissolved in toluene and filtered through a pad of celite. After
evaporation, complex 8 was obtained as a brown powder (150 mg,
0.35 mmol, 53% yield). Anal. Calcd for C19H32ClN2NbO: C, 52.72;
NbCl2[ONOO]tBu(MeCCMe) (6a). 6a was readily protonated,
and no elemental analysis or yield was obtained. Yellow powder.
1H NMR (300 MHz, 298 K, C6D6): δ 7.43 (d, 4JHH ) 2.3 Hz, 1H,
Haryl), 7.00 (d, 4JHH ) 2.3 Hz, 1H, Haryl), 5.37 (d, 1H, 2JHH ) 13.9
Hz, ArCH2N), 4.21, 3.89, 2.99, 2.44 (m, 4H, NCH2CH2O), 4.19,
3.72, 3.39, 3.17 (m, 4H, NCH2CH2O), 3.82 (s, 3H, OCH3), 3.58
1
H, 7.45; N, 6.47. Found: C, 53.19; H, 7.85, N, 6.11. H NMR
2
(d, 1H, JHH ) 13.9 Hz, ArCH2N), 3.01 (s, 3H, OCH3), 2.82 (s,
(300.13 MHz, 298 K, C6D6): Two isomers, δ 6.72, 6.66, 6.47, 6.42
6H, ’C-CH3), 1.63 (s, 3H, tBuCH3), 1.37 (s, 3H, tBuCH3). 13C
NMR (75 MHz, 298 K, C6D6): 209.8 (MeCt ), 156.4 (Caryl), 143.1
(Caryl), 137.3 (Caryl), 128.1 (Caryl), 124.2 (CHaryl), 121.6 (CHaryl),
70.7, 67.5, 61.1, 57.7, 51.2 (NCH2, CH2O), 61.6 (OCH3), 58.5
(OCH3), 35.0 (CCH3), 30.0 (CCH3), 31.7 (CCH3), 29.6 (CCH3),
19.1 (CH3Ct ).
2
2
(s, Haryl), 4.62 (d, JHH ) 14.6 Hz, 1H, ArCH2), 3.92 (d, JHH
)
14.6 Hz, 1H, ArCH2), 3.11-3.05, 2.89-2.81, 2.53-2.44, 1.53-1.38
2
(m, 8H, NCH2), 3.00 (d, JHH ) 14.6 Hz, 1H, ArCH2), 2.91 (d,
2JHH ) 13.7 Hz, 1H, ArCH2), 2.85 (s, 6H, CH3alk), 2.81 (s, 3H,
NCH3), 2.76 (s, 3H, NCH3), 2.72 (s, 6H, CH3alk), 2.65 (s, 3H,
NCH3), 2.55 (s, 3H, NCH3), 2.13 (s, 3H, ArCH3), 2.12 (s,
3H, ArCH3), 2.03 (s, 3H, NCH3), 1.99 (s, 3H, ArCH3), 1.81
(s, 3H, ArCH3), 1.86 (s, 3H, NCH3), 1.64 (s, 3H, NbCH3), 1.55 (s,
3H, NbCH3). 13C NMR (75 MHz, 298 K, C6D6): Two isomers: δ
214.4, 211.9 (MeCt ), 155.4, 154.8, 127.8, 127.2 (CHaryl), 137.5,
130.8, 130.6, 129.1, 126.1, 124.2, 123.6, 120.5 (Caryl), 64.1, 62.7,
58.2, 58.0, 53.5, 53.2 (NCH2), 51.7, 50.1, 49.8, 49.0, 45.7, 45.5
(NCH3), 42.1, 39.8 (br. s, NbCH3), 20.3, 20.2, 15.6, 15.5 (ArCH3),
18.5, 18.1 (CH3Ct ).
NbCl2[ONOO]Me(CH3C′CCH3) (6b). 6b was even more readily
protonated than 6a, and no elemental analysis or yield was obtained.
Similarly, overlapping of the 13C NMR signals of 6a and 6H (see
text) prevented definitive assignments, except for the alkyne
1
carbons. Yellow powder. H NMR (300 MHz, 298 K, C6D6): δ
6.72 (s, 1H, Haryl), 6.49 (s, 1H, Haryl), 5.24 (d, 2JHH ) 13.7 Hz, 1H,
ArCH2N), 4.00-1.72 (m, 9H, NCH2, ArCH2N), 3.75 (s, 3H, OCH3),
3.09 (s, 3H, OCH3), 2.90 (s, 6H, CH3alk), 2.34 (s, 3H, CH3), 2.23
(s, 3H, CH3). 13C NMR (75 MHz, 298 K, C6D6): δ 215 (MeCt ).
Synthesis of NbMe[ONNO]H(MeCCMe) (7). A solution of
methyl lithium in diethyl ether (0.13 mL, 0.21 mmol) was added
to a solution of NbCl[ONNO]H(MeCCMe) (4b) (100 mg, 0.21
mmol) in THF (5 mL). The reaction mixture was stirred at room
temperature for 3 h. After the volatile materials were removed, the
Supporting Information Available: Tables of crystallographic
data and CIF files for all complexes including 6H. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM801139M