A new way to scorpionate niobium complexes: Terminal alkyne, imido, and oxo complexes and the rearrangement of α-agostic ethyl complexes

Article abstract of DOI:10.1021/om050423f

Synthesis of new hydrotris(pyrazolyl)-borate(Tp′) niobium terminal alkyne complexes and some reactions of these complexes was investigated. THF and diethyl ether were obtained after refluxing purple solutions of Na/benzophenone under dinitrogen. Deuterated nuclear magnetic resonance (NMR) solvents were dried over molecular sieves, degassed by freeze-pump-thaw cycles, and stored under dinitrogen. The thermoanalysis suggested that reversible migratory insertion yield hydride complexes, although the stability of these complexes has precluded a detailed investigation of the reaction course.

Full text of DOI:10.1021/om050423f

4306
Organometallics 2005, 24, 4306-4314
A New Way to Scorpionate Niobium Complexes:
Terminal Alkyne, Imido, and Oxo Complexes and the
Rearrangement of r-Agostic Ethyl Complexes
Pascal Oulie´, Nicolas Bre´fuel, Laure Vendier, Carine Duhayon, and
Michel Etienne*,†
Laboratoire de Chimie de Coordination du CNRS, UPR 8241, 205 Route de Narbonne,
31077 Toulouse Cedex 04, France
Received May 27, 2005
This paper describes an efficient, straightforward synthesis of new hydrotris(pyrazolyl)-
borate (Tp′) niobium terminal alkyne complexes and some reactions of these complexes.
Reduction of the niobium(IV) complexes Tp′NbCl3 (Tp′ ) TpMe2 (1), TpMe2,4Cl (1-Cl)) with
zinc in the presence of a terminal alkyne HCtCR gives good yields (ca. 75%) of the new
alkyne complexes Tp′NbCl2(HCtCR) (2-R) (R ) H, Ph, C6H4-4-Me, CH2Ph, CF3) and 2-Cl-
Ph. The strategy is significant since the previously reported synthesis of Tp′NbCl2(RCt
CR′) was possible only with internal alkynes. NMR data indicate that the four-electron donor
alkyne sits in the molecular mirror plane. Restricted alkyne rotation is observed by NMR
in solution. The more abundant rotamer has the large substituent located distal to the Tp′
ligand, thereby alleviating steric interactions. Only this rotamer is observed in the crystal
structures of 2-R (R ) Ph, C6H4-4-Me, CH2Ph). In the crystal structure of 2-CF3, however,
three independent molecules are observed with both alkyne orientations. The one that has
the CF3 group proximal to TpMe2 exhibits H‚‚‚F bridges between the alkyne CF3 group and
the methyl group at the 3-position of the TpMe2 ligand. Oxo and imidoniobium complexes
are also available via the same synthetic procedure. R-C-H agostic ethyl complexes TpMe2
-
NbCl(µ-H-CHCH3)(HCtCR) (4-R) (R ) Ph, C6H4-4-Me, CH2Ph) have been synthesized. Their
thermolyses have been studied with the goal of observing the Nb-CH2CH3/H-CtCR
exchange leading to the putative TpMe2NbCl(H)(CH3CH2CtCR). This reaction mainly gives
decomposition products, although spectrocopic data suggest that it occurs. This reaction is
significant in the context of comparing kinetic and thermodynamic data for M-H versus
M-C stability and reactivity.
Introduction
chemistry developed so far has relied upon the prior
introduction of the desired functional group followed by
subsequent addition of the Tp′ ligand. In this vein,
TpMe2TaMe3Cl,4 TpMe2MCl2(RCtCR′) (M ) Nb, Ta),5,6
The lack of convenient, simple, easily accessible
starting material has been a recurrent problem that has
hampered a broad development of the scorpionate or
hydro(trispyrazolyl)borate (Tp′) chemistry of the heavier
group 5 elements Nb and Ta.1 The synthesis of simple
chloro niobium(V) complexes such as TpNbCl4 and
TpMe2NbCl4 is highly dependent on experimental condi-
tions, and byproducts formation has precluded the
utilization of these complexes as useful starting mater-
8
TpMe2Ta(dCH-t-Bu)Cl2,7 and TpMe2M(dNR)Cl2 have
been obtained from TaMe3Cl2, MCl3(dme)(RCtCR′)
(dme ) 1,2-dimethoxyethane), Ta(dCH-t-Bu)Cl2(thf)2,
and M(dNR)Cl3L2 (L ) thf, pyridine), respectively.
Some of these reactions suffer from occasional draw-
backs however. For example, our one-pot synthesis of
TpMe2NbCl2(RCtCR′) is possible only in the case of
internal alkynes since it uses [NbCl3(dme)]n as the
niobium source, a compound that catalytically trimer-
izes terminal alkynes.9 More recently,10 the synthesis
ial.2 For tantalum, we recently suggested that TpMe2
TaCl2(tht) (tht ) tetrahydrothiophene) might be a good
candidate, but the unsymmetrical TadTa dimer (TpMe2
-
-
Ta)(TaCl3)(µ-Cl)2(µ-tht) that formed in the first step of
a proposed reaction sequence was found to be inert
toward a second substitution of TpMe2 3
. Instead, the
(4) Reger, D. L.; Swift, C. A.; Lebioda, L. Inorg. Chem. 1984, 23,
349-354.
(5) Etienne, M.; Biasotto, F.; Mathieu, R.; Templeton, J. L. Orga-
nometallics 1996, 15, 1106-1112.
† E-mail: etienne@lcc-toulouse.fr.
(1) (a) Scorpionates - The Coordination Chemistry of Polypyra-
zolylborate Ligands; Trofimenko, S., Ed.; Imperial College Press:
London, 1999. (b) Etienne, M. Coord. Chem. Rev. 1996, 156, 201-236.
(c) We will use thoughout the nomenclature proposed by S. Trofimenko;
see ref 1a.
(6) Hierso, J.-C.; Etienne, M. Eur. J. Inorg. Chem. 2000, 839-842.
(7) Boncella, J. M.; Cajigal, M. L.; Abboud, K. A. Organometallics
1996, 15, 1905-1912.
(8) (a) Sundermeyer, J.; Putterlik, J.; Foth, M.; Field, J. S.; Ramesar,
N. Chem. Ber. 1994, 127, 1201-1212. (b) Boncella, J. M.; Cajigal, M.
L.; Gamble, A. S.; Abboud, K. A. Polyhedron 1996, 15, 2071-2078 (c)
Weber, K.; Klorn, K.; Schorm, A.; Kipke, J.; Lemke, M.; Khvorost, A.;
Harms, K.; Sundermeyer, J. Z. Anorg. Allg. Chem. 2003, 629, 744-
754 (d) Schorm, A.; Sundermeyer, J. Eur. J. Inorg. Chem. 2001, 2947-
2955.
(2) (a) Hubert-Pflazgraf, L.; Tsunoda, M. Polyhedron 1983, 2, 203-
210. (b) Bradley, D. C.; Hursthouse, M. B.; Newton, J.; Walker, N. P.
C. J. Chem. Soc., Chem. Commun. 1984, 188-190.
(3) Etienne, M.; Hierso, J.-C.; Daff, P. J.; Donnadieu, B.; Dahan, F.
Polyhedron 2004, 23, 379-383.
10.1021/om050423f CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/12/2005

New Way to Scorpionate Niobium Complexes
Organometallics, Vol. 24, No. 17, 2005 4307
Scheme 1
of the niobium(IV) complex TpMe2NbCl3 (1) from NbCl4-
(thf)2 and TpMe2SnClBu2 has been described. Both the
synthesis of the tin reagent11 and that of 1 seemed
straightforward, and 1 thus appeared to be the long
sought simple starting material in Tp′Nb chemistry.
Indeed we report herein that this compound can be
readily reduced to give (i) known internal alkyne
complexes Tp′NbCl2(RCtCR′) (Tp′ ) TpMe2, TpMe2,4Cl),
(ii) more significantly, previously unknown terminal
alkyne complexes Tp′NbCl2(HCtCR), and (iii) oxo and
imido complexes based on Tp′Nb. Some reactivity of
Tp′NbCl2(HCtCR), aimed at probing the migratory
aptitudes of alkyl versus hydride to a coordinated
alkyne, is also described.
Figure 1. Plot of the molecular structure of TpMe2
NbCl2(HCtCCH2Ph), 2-CH2Ph.
-
Scheme 2
Results and Discussion
New Tp′ Complexes Based on Tp′NbCl3. Reaction
of the tin compounds TpMe2SnClBu2 and TpMe2,4Cl
-
SnClBu2 with NbCl4(thf)2 results in the formation of
TpMe2NbCl3 (1) and TpMe2,4ClNbCl3 (1-Cl), respectively
(Scheme 1).12 In the initial report,10a the yield of 1 was
limited without obvious reasons to 41% and scale-up
was desirable. We have found that carrying out the
reaction between either of the tin complexes with NbCl4-
(thf)2 first in toluene (as a slurry) for 4 days in the dark
and then, after evaporating the toluene, for 2 days in
dichloromethane yielded 1 or the new 1-Cl in 78% yield
on a 10 mmol scale or more. 1-Cl is obtained as bright
golden yellow microcystals indefinitely stable under
dinitrogen. Both compounds exhibit similar X-band ESR
parameters (for 1, dichloromethane, 293 K, g ) 1.93,
Aiso ) 170 G (10 lines); dichloromethane/toluene, 100
K, g1 ) 1.91, g2 ) 1.94, g3 ) 1.92, A1 ) 258, A2 ) 123,
A3 ) 126 G) in agreement with an almost perfect C3v
symmetry.
Table 1. Selected Bond Lengths and Angles for
TpMe2NbCl2(HCtCCH2Ph) (2-CH2Ph)
Bond Lengths (Å)
Nb(1)-Cl(1)
Nb(1)-Cl(2)
Nb(1)-C(1)
Nb(1)-C(2)
C(1)-C(2)
2.4039(5)
2.4079(5)
2.0797(19)
2.0686(18)
1.297(3)
Bond Angles (deg)
Cl(1)-Nb(1)-Cl(2)
C(1)-C(2)-C(3)
C(1)-Nb(1)-C(2)
105.07(2)
147.60(19)
36.43(8)
good yields according to Scheme 2. No trace of cyclo-
As a first probe of the potential use of 1 and 1-Cl in
synthesis, we checked that a representative complex in
the series, namely, TpMe2NbCl2(PhCtCMe),5 could be
obtained. Reduction of 1 with excess zinc in thf in the
1
trimerization products was observed in the H NMR
spectra of crude reaction mixtures. In the case of
acetylene itself, some [TpMe2Nb(O)Cl](µ-O)13 was formed
along with 2-H. The formation of this oxo dimer is
presumably due to failure to completely remove traces
of dioxygen/water present during the generation of
acetylene from NaCtCH and H2O. All complexes are
purple microcrystalline solids that have been fully
presence of PhCtCMe indeed yields the known TpMe2
-
NbCl2(PhCtCMe) in more than 75% yield after workup.
Following the same procedure, reaction of 1 or 1-Cl with
terminal alkynes HCtCR with a variety of substituents
(R ) H, Ph, C6H4-4-Me, CH2Ph, CF3) proceeds smoothly
to give Tp′NbCl2(HCtCR) 2-R and 2-Cl-R in equally
1
characterized by elemental analysis, H and 13C NMR
spectroscopy, and, in two cases, X-ray diffraction.
Prominent metric parameters and an ORTEP plot for
TpMe2NbCl2(HCtCCH2Ph) 2-CH2Ph are collected in
Table 1 and Figure 1, respectively. The alkyne sits in
the molecular symmetry plane, and the terminal C-H
is proximal to the TpMe2. This conformation directs the
benzyl moiety away from the pendant Me groups of
(9) (a) Roskamp, E. J.; Pedersen, S. F. J. Am. Chem. Soc. 1987, 109,
3170-3172. (b) Hartung, J. B.; Pedersen, S. F. Organometallics 1990,
9, 1414-1417.
(10) (a) Mashima, K.; Oshiki, T.; Tani, K. Organometallics 1997,
16, 2760-2762. (b) Oshiki, T.; Mashima, K.; Kawamura, S.; Tani, K.;
Kitaura, K. Bull. Chem. Soc. Jpn. 2000, 73, 1735-1748.
(11) Lobbia, G. F.; Bonati, F.; Cecchi, P.; Lorenzotti, A.; Pettinari,
C. J. Organomet. Chem. 1991, 403, 317-323.
(12) TpMe2,4ClSnClBu2 is a new compound obtained and characterized
in the same way as related complexes; see: Lobbia, G. F.; Cecchi, P.;
Calogero, S.; Valle, G.; Chiarini, M.; Stievano, L. J. Organomet. Chem.
1995, 503, 297-305.
(13) Antinolo, A.; Carrillo-Hermosilla, F.; Fernandez-Baeza, J.;
Lanfranchi, M.; Lara-Sanchez, A.; Otero, A.; Palomares, E.; Pelling-
helli, M. A.; Rodriguez, A. M. Organometallics 1998, 17, 3015-3019.

4308 Organometallics, Vol. 24, No. 17, 2005
Oulie´ et al.
Table 2. Selected Bond Lengths and Angles for
TpMe2NbCl2(HCtCCF3) (2-CF3)a
Bond Lengths (Å)
Nb(1)-Cl(1)
Nb(1)-Cl(2)
Nb(1)-C(1)
Nb(1)-C(2)
C(1)-C(2)
2.3775(12)
2.3760(15)
2.101(5)
2.073(5)
1.303(8)
Bond Angles (deg)
Cl(1)-Nb(1)-Cl(2)
C(1)-Nb(1)-C(2)
C(1)-C(2)-C(3)
104.19(5)
36.4(2)
140.5(5)
a Data for one of the three independent complexes, with the
same alkyne orientation as that in 2-CH2Ph; see Figure 2. Metric
parameters for the two others do not differ significantly; see text.
Figure 2. Plot of one independent molecule of TpMe2NbCl2-
(HCtCCF3), 2-CF3.
TpMe2. Short Nb-C bonds (average 2.073 Å) between
the Nb center and the coordinated alkyne, as well as a
bent back angle C(1)-C(2)-C(3) of 147.6(2)°, are indica-
tive of the four-electron donor behavior of the alkyne
ligand.15 This situation is akin to that of the phenyl-
propyne complex TpMe2NbCl2(PhCtCMe):14 the whole
alkyne including the phenyl ring lies in the molecular
symmetry plane, and the phenyl group is proximal to
the TpMe2 ligand. In the X-ray crystal structures of 2-Ph
and 2-C6H4-4-Me the same overall geometry is ob-
served: the whole alkyne with its phenyl group is in
the molecular mirror plane, the phenyl group being
distal to TpMe2 as in 2-CH2Ph. Disorder problems have
precluded a detailed discussion of the bonding param-
eters for these complexes. Accordingly, the bonding
picture in these formally niobium(III) alkyne complexes
is also consistent with a reduced alkyne and an oxidized
niobium(V) center, i.e., a metallacyclopropene descrip-
tion. Related cyclopentadienyl16 and six-coordinate9b,17
niobium complexes exhibit similar bonding features.
Perhaps more surprising is the X-ray crystal structure
of TpMe2NbCl2(HCtCCF3) (2-CF3). The asymmetric unit
now contains three independent molecules. Interest-
ingly, they exhibit different alkyne orientations. For two
of them, the overall alkyne orientation is similar to that
for 2-CH2Ph. The trifluromethyl group is oriented
toward the two cis chloro ligands, directing the CH
between the two cis pyrazolyl rings. One of the mol-
ecules is highly symmetrical (Figure 2, Table 2). The
whole alkyne including one fluorine of the trifluromethyl
group lies virtually perfectly in the molecular symmetry
plane. In the other one, there are slight differences in
the dihedral angles defining the precise alkyne orienta-
tion with respect to the TpMe2NbCl2 moiety, and the
pyrazolyl ring trans to the alkyne is slightly distorted.
Also the conformation of the trifluromethyl group is
different. There are some significant differences in the
Nb-alkyne metric parameters. One of the two molecules
exhibits a more symmetrical Nb-alkyne bonding;
Figure 3. Plot of one independent molecule of TpMe2NbCl2-
(HCtCCF3), 2-CF3, with a different alkyne orientation.
namely, Nb-CH distances are 2.101(5) and 2.171(5) Å,
and Nb-CCF3 are 2.073(5) and 2.058(6) Å, respectively,
whereas corresponding alkyne C-C distances are 1.303-
(8) and 1.331(8) Å. There are also small variations of
the angles defining the geometry of the overall Nb(HCt
CCF3) bonding.
For the third independent molecule (Figure 3), the
alkyne is rotated approximately by 180° so that the
trifluromethyl group is now directed toward the cis
pyrazolyl groups. Again, metric parameters character-
istic of (4e)-alkyne bonding are observed. They are
virtually identical to that for the molecule showing the
more symmetrical Nb-alkyne bonding in the aforemen-
tioned orientation. This highly original situation where
two rotamers of an alkyne are observed in the same
crystal is the result of competing effects of comparable
magnitudes. The most ubiquitous effects are restricted
alkyne rotation arising from steric and orbital effects
of similar energies for the two rotamers (see below for
a study in solution) and crystal packing, as for 2-CH2Ph
and related complexes. In the case of 2-CF3 in addition,
there are short intramolecular H‚‚‚F contacts (between
2.3 and 2.8 Å, when the corresponding C‚‚‚F contacts
are in the 3.2-3.8 Å range)18 between the trifluoro-
methyl fluorines and some of the pyrazole 3-CH3
hydrogens (the sum of van der Waals radii for H and F
is 2.67 Å).19 These distances have been obtained from
calculated hydrogen positions,although some hydrogens
were located by Fourier differences, in which cases the
(14) Etienne, M.; White, P. S.; Templeton, J. L. Organometallics
1991, 10, 3801-3803.
(15) Templeton, J. L. Adv. Organomet. Chem. 1989, 29, 1-100.
(16) (a) Galindo, A.; Gomez, M.; Gomez-Sal, P.; Martin, A.; del Rio,
D.; Sanchez, F. Organometallics 2002, 21, 293-304. (b) Curtis, M. D.;
Real, J.; Kwon, D. Organometallics 1989, 8, 1644-1651.
(17) Cotton, F. A.; Shang, M. Inorg. Chem. 1990, 29, 508-514.

New Way to Scorpionate Niobium Complexes
Organometallics, Vol. 24, No. 17, 2005 4309
Table 3. Thermodynamic and Kinetic Parameters
for the Alkyne Rotation in TpMe2NbCl2(alkyne)
Scheme 3
alkyne
K° a
∆Gq (Tcoal/K)/kJ‚mol-1 b
ref
HCtCH
HCtCCH2Ph
HCtCCF3
HCtCPh
HCtCC6H4-4-Me
MeCtCMe
PhCtCMe
PhCtCCH2Me
PhCtCCH2CH2Me 0.067
36 (200)
this work
47
62, 53 (293)c
52 (293)
this work
3.5
3
1.6
this work
63 (353)
this work
66 (353)
this work
68 (358)
d
d
d
d
0.17
0.11
>80 (>373)
>80 (>373)
>80 (>373)
a Toluene-d8, determined by integration in the 1H NMR spectra
in the slowest exchange limit; see Scheme 3 for details. b Deter-
mined at Tcoal of alkyne CH or CCH3 NMR signals. c Shanan-Atidi,
H.; Bar-Eli, K. H. J. Phys. Chem. 1970, 74, 961-963. d See ref 5.
distances are of similar magnitude. Even if such inter-
actions are so-called weak interactions or hydrogen
bridges,18 and are therefore not genuine C-F‚‚‚H-C
hydrogen bonds, their numbers undoubtedly contribute
to the overall alkyne orientation in the case of the
somewhat small, unhindered CF3 group. Several inter-
molecular H‚‚‚F contacts contribute to similar interac-
tions within the asymmetric unit but also to an intricate
network within the whole crystal. Recall that in the
crystal of TpMe2NbCl2(PhCtCMe) and related com-
plexes,14 the phenyl is proximal to TpMe2 and lies in the
molecular symmetry plane, suggesting that π-π or
CH-π interactions with the pyrazole rings might be
present in addition to steric effects.20 There are thus
numerous interactions at play in these systems, but the
occurrence of H‚‚‚F interactions leading to a mixture of
alkyne rotamers in the solid state of 2-CF3 was unex-
pected and is clearly the highlight of this solid state
structure.
equilibrium constant is related to the orientation of the
phenyl (or aryl) group with respect to Tp′ according to
Scheme 3. For PhCtCR (R * H) complexes, the more
populated alkyne rotamer is that with the phenyl ring
oriented proximal to TpMe2, and as the size of R
decreases from CH2CH2Me to CH2Me then to Me, the
difference in population decreases. Moving to the small-
est subtituent, i.e., H in HCtCPh or HCtCC6H4-4-Me,
the same trend is observed, but now the more stable
alkyne orientation is observed for the phenyl group
sitting away from TpMe2. However, for terminal alkyne
complexes, the more abundant rotamer is that with the
terminal HCt oriented proximal to TpMe2, a situation
also found in the X-ray crystal structure where only this
rotamer is observed for 2-CH2Ph. For both 2-CH2Ph
and 2-CF3, NOE experiments have confirmed this
orientation for the major rotamer. The same situation
prevails also in the case of aryl alkyne complexes 2-Ph
and 2-C6H4-4-Me. These equilibria between rotamers
are a delicate balance between electronic and steric
effects. In the PhCtCR complexes, at the lowest tem-
peratures attainable in toluene-d8 or dichloromethane-
d2, the phenyl protons always exhibit a 1:2:2 intensity
pattern. In the X-ray of TpMe2NbCl2(PhCtCMe) and
related complexes, the phenyl proximal to TpMe2 lies in
the molecular symmetry plane, and this conformation
is that adopted in the most abundant rotamer.
It is not obvious to reconcile solid state and solution
data. For the new terminal alkyne complexes, K°
increases in the order 2-C6H4-4-Me < 2-Ph ≈ 2-CF3 ,
2-CH2Ph. This trend does not directly follow the steric
hindrance of the substituents, which may be in the order
CF3 < Ph ≈ 4-C6H4Me < CH2Ph. Then if attractive H‚
‚‚F interactions are operative in solution for 2-CF3, they
should contribute to a decrease in K°. Note that we have
no spectroscopic evidence such as H‚‚‚F or C‚‚‚F through-
space couplings, confirming the weakness of these
H‚‚‚F interactions.18d One reason for this discrepancy
might be that phenyl groups stay in the symmetry plane
of the complex as observed in the X-ray of TpMe2NbCl2-
(PhCtCMe) and for 2-C6H4-4-Me and 2-Ph, thereby
alleviating unfavorable steric interactions. Also, steric
1
In the H NMR spectra of 2-R and 2-Cl-R, 2:1 and
6:3 intensity patterns for the 4-H (in the 2-R series) and
3- and 5-Me pyrazole protons, respectively, indicate that
the alkyne sits in the molecular mirror plane. Except
for the ethyne complex 2-H, two sets of such patterns
are observed depending on the alkyne orientation with
respect to the Tp′ ligand, hence on the alkyne rotation
barrier (see below). The terminal tC-H are character-
ized by low-field resonances in the range δ 11-14. In
the 13C NMR spectra, the alkyne carbons bound to Nb
also resonate at low field, δ 215-250. The methine
carbons appear as doublets with 1JCH ≈ 200 Hz. These
chemical shifts and coupling constants are characteristic
of (4e)-alkyne coordination.15 Restricted alkyne rotation
1
is apparent from the H NMR spectra of all of these
complexes. In most cases, coalescence of the alkyne CH
occurs around room temperature except for 2-H itself,
where coalescence is observed at 200 K (∆δ ) 1.7 ppm
at 173 K, 400 MHz, ∆Gq ) 36 kJ‚mol-1). Thermody-
200
namic and kinetic data for terminal alkyne rotation are
summarized in Table 3. They are compared with those
for PhCtCR (R ) Me, CH2Me, CH2CH2Me).14 The
(18) (a) Hyla-Kryspin, I.; Haufe, G.; Grimme, S. Chem. Eur. J. 2004,
10, 3411-3422 (b) Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565-573
(c) Dunitz, J. D.; Taylor, R. Chem. Eur. J. 1997, 3, 89-98 (d) Mele, A.;
Vergani, B.; Viani, F.; Meille, S. V.; Farina, A.; Bravo, P. Eur. J. Org.
Chem. 1999, 187, 7-196.
(19) Bondi, A. J. Phys. Chem. 1964, 68, 441-451.
(20) (a) Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J. J.
Chem. Soc., Perkin Trans. 2 2001, 651-669. (b) Janiak, C. J. Chem.
Soc., Dalton Trans. 2000, 3885-3896.

4310 Organometallics, Vol. 24, No. 17, 2005
Oulie´ et al.
Scheme 4
Scheme 5
interactions contribute to the barrier to alkyne rotation,
but for 2-CF3 the barrier is slightly smaller than for
the other terminal alkyne complexes. It could be argued
that attractive H‚‚‚F interactions should increase the
barrier by lowering the ground state. Overall, the
barriers for terminal alkynes are similar to that for the
2-butyne complex but notably higher than that for
acetylene itself.
The same reducing conditions used for the syntheses
of the terminal alkyne complexes are also efficient for
the synthesis of imido complexes. Reaction of 1 with N3-
P(N-iPr2)2 and Zn gives the phosphinoimido complex
TpMe2NbCl2[dN-P(N-iPr2)2] 3 in 30% yield (Scheme 4).
The molecular symmetry plane in 3 is evidenced by the
2:1 patterns for the pyrazolyl proton and carbon reso-
nances in its 1H and 13C NMR spectra, respectively. The
dN-P(N-iPr2)2 moiety exhibits a phosphorus resonance
at δ 129.7 similar to that (δ 118.7) of the X-ray-
characterized21 vinyl complex TpMe2NbCl(CPhdCEt2)-
[dN-P(N-iPr2)2]. We have also attempted the synthesis
of the known8 oxo complex TpMe2NbCl2(O) starting from
1, Zn, and propene oxide. However, the only oxo
compound that we have been able to isolate was the
known dimer [TpMe2Nb(O)Cl](µ-O).13 These results show
that a reductive strategy leading to a Tp′NbCl2(L)
complex from a niobium(IV) precursor Tp′NbCl3 is
particularly efficient when L is an alkyne or an imido
group and that a more intricate reaction course leads
only to a dimer when an oxo complex was sought.
r-C-H Agostic Ethyl Complexes and Their At-
tempted Rearrangement. Among the interesting
properties of TpMe2 in TpMe2NbCl(alkyl)(alkyne) com-
plexes are its directing properties that drive a bulky
substituent on the alkyl in a wedge between two
pyrazolyl rings. These directing effects lead to the
discovery of unique examples of equilibria between R-
and â-CH agostic rotamers in secondary alkyl com-
plexes.22 C-C bond metathesis occurring via a revers-
ible migratory insertion of the coordinated alkyne into
the Nb-C was observed when an R-CH agostic linear
alkyl was present. Only tC-C, not tC-Ph, was met-
athesized.21 This is summarized in Scheme 5. The Nb-
CH3 bond dissociation energy (BDE) is sufficiently high
to drive the metathesis reaction all the way to the
methyl complex. A Nb-n-propyl bond equilibrates with
a Nb-ethyl bond since the tC-C BDE is not expected
to vary to a large extent between tC-CH2CH3 and
tC-CH2CH2CH3. That the phenyl does not migrate
indicates either an R-CH agostic interaction that sta-
Scheme 6
bilizes the Nb-ethyl bond with respect to a Nb-Ph bond
and/or a high barrier for the migration of the phenyl
ring to the niobium. In the case of terminal alkyne
complexes TpMe2NbCl(alkyl)(HCtCR), it seemed of in-
terest to probe the migratory aptitudes of H versus R.
Kinetically, it is well known that migratory aptitudes
of H are higher than those of R.23 Thermodynamically,
transition metal alkyl (M-C) BDEs are weaker than
M-H BDEs, although this trend is not pronounced for
early transition metal d0 systems.24 We thus sought to
compare the bonding energetics of the systems consist-
ing of a terminal alkyne bound to an R-agostic ethyl
niobium TpMe2Nb(µ-H-CHCH3)Cl moiety as described in
Scheme 6. This would offer the opportunity to further
test the reactivity of well-defined R-C-H-agostic ethyl
complexes and intramolecularly compare relative M-H,
M-C and C-H, C-R BDEs. Depending on the relative
BDEs, migration of the Nb(µ-H-CHCH3) group to the
(23) (a) Brookhart, M.; Hauptman, E.; Lincoln, D. M. J. Am. Chem.
Soc. 1992, 114, 10394-10401. (b) Burger, B. J.; Santarsiero, B. D.;
Trimmer, M. S.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 3134-
3146. (c) Jordan, R. F. Adv. Organomet. Chem. 1991, 31, 325-387.
(24) (a) Bonding energetics in organometallic compounds; Marks,
T. J., Ed.; ACS Symposium Series 428; American Chemical Society:
Whasington DC, 1990 (b) Thompson, M. E.; Baxter, S. M.; Bulls, A.
R.; Berger, B. J.; Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.;
Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 203-219. (c) Schaller, C.
P.; Cummins, C. C.; Wolczanski, P. T. J. Am. Chem. Soc. 1996, 118,
591-611 (d) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tan, W.;
Bercaw, J. E. J. Am. Chem. Soc. 1987, 109, 1444-1456 (e) Jones, W.
D.; Hessell, E. T. J. Am. Chem. Soc. 1993, 115, 554-562 (f) Clot, E.;
Besora, M.; Maseras, F.; Me´gret, C.; Eisenstein, O.; Oelckers, B.;
Perutz, R. N. Chem. Commun. 2003, 490-491.
(21) Etienne, M.; Mathieu, R.; Donnadieu, B. J. Am. Chem. Soc.
1997, 119, 3218-3228.
(22) Jaffart, J.; Etienne, M.; Maseras, F.; McGrady, J. E.; Eisenstein,
O. J. Am. Chem. Soc. 2001, 123, 6000-6013.

New Way to Scorpionate Niobium Complexes
Organometallics, Vol. 24, No. 17, 2005 4311
alkyne followed by H migration to Nb might indeed be
favorable (Scheme 6). Assuming an sp2-type hybridiza-
tion for the Nb-bound carbons of the alkyne, the reaction
enthalpy for the reaction in Scheme 6 is then
appearing as a deshielded doublet of doublets with one
reduced and one augmented 1JCH (for 4-CH2Ph, δ 88.3,
1JCH 108, 132 Hz). These data are similar to those of
other Nb and Ta ethyl complexes6,21 and to those of
related Mo and W R-C-H agostic alkyl complexes.28
Thermolyses of complexes 4-R were conducted in ben-
zene-d6 at 323 K and followed by 1H and 13C NMR
∆rH°(6) ) [D(Csp2-H) + D(Nb-CH2CH3)] -
[D(Csp2-CH2CH3) + D(Nb-H)]
spectroscopy. In complete contrast to the case of TpMe2
-
NbCl(µ-H-CHCH3)(PhCtCR),21 disappearance of 4-R
was deceptively accompanied by large amounts of ill-
defined decomposition products. The formation of hy-
dride complexes with rearranged alkynes according to
Scheme 6 could not be unambiguously established.
However, the following observations suggest that these
rearrangements might have taken place. Significant
changes in the 1H NMR spectra include (i) the appear-
ance of slightly broad singlets in the δ 16.5-17.2 region,
generally two of unequal intensity, and (ii) the appear-
ance of signals in the δ 4.4 and 4.0 regions in the form
of quartets (q, J 7.5 Hz) and doublets of quartets (qd, J
7.5, 12.0 Hz), respectively. For (ii), the signals are
characteristic of a coordinated (4e)-alkyne RCtCCH2-
CH3 within a symmetrical complex5 or within an asym-
metric complex,21 respectively. In some cases, especially
that of 4-C6H4-4-Me, the symmetrical complex is by far
the most abundant species. Associated with the meth-
ylene quartet are its accompanying triplet (δ 1.15), an
AB type spectrum attributed to the aryl protons of C6H4-
4-Me at δ 6.73 and 6.69 (J 6 Hz), and typical TpMe2
methine (1:2 intensity pattern) and methyl (3:3:6:6
intensity pattern) signals. 13C NMR spectra confirm the
disappearance of 3-R and the formation of quaternary
(4e)-alkyne carbons in the δ 200-260 region. These data
clearly suggest that TpMe2NbCl2(CH3CH2CtCR) has
been formed from 4-R. The formation of this coordinated
alkyne clearly indicates that the rearrangement has
occurred; that is, at some stage the migratory insertion
of HCtCR into the Nb(CH2CH3) bond has taken place,
followed by formation of an unstable NbH species with
a coordinated CH3CH2CtCR.21 This is also supported
by the presence of an unsymmetrical species that
For an estimation of D(Csp2-CH2CH3) and D(Csp2-
H) in these complexes, one can rely on D(Csp2-CH2-
CH3) for 1-butene and compare it with D(Csp2-H) for
ethene, and it thus uses the reaction
CH2dCH2 + CH3-CH3 f CH2dCH-CH2CH3 + H2
for which ∆rH° ) ∆fH°(CH2CH-CH2CH3) - ∆fH°(C2H6)
- ∆fH°(C2H4) ) D(C2H5-H) + D(C2H3-H) - D(H2) -
D(CH2CH-CH2CH3).
With values taken from literature,25 D(CH2CH-CH2-
CH3) ) -∆fH°(CH2CH-CH2CH3) + ∆fH°(C2H6) + ∆fH°-
(C2H4) + D(C2H5-H) + D(C2H3-H) - D(H2) ) 0.13 +
84.7 - 52.3 + 419 + 444 - 436 ) 460 kJ‚mol-1. This
value is higher than D(C2H3-H) for ethene and D(CH2-
CH-CH2CH3) - D(C2H3-H) ≈ 14 kJ‚mol-1
.
As a guideline, transition metal M-H bonds are
generally stronger than M-R bonds. They are of com-
parable magnitude at d0 metal centers when R ) CH3,
which yields the strongest M-n-alkyl bond. M-CH2-
CH3 bonds are usually ca. 15 kJ‚mol-1 weaker than
M-CH3 bonds.26 For a formally d2 Nb(III) center in Cp2-
Nb(CO)X (X ) H, CH3), photoelectron spectra yielded
D(Nb-H) - D(Nb-CH3) g 50 kJ‚mol-1, which trans-
lates to D(Nb-H) - D(Nb-CH2CH3) g 65 kJ‚mol-1 27
.
Considering that the strength of the R-C-H agostic
interaction in TpMe2Nb(µ-H-CHCH3)Cl(alkyne) is not
known and that the bonding in these complexes can be
described with a strong Nb(V), d0, contribution, the
D(Nb-H) - D(Nb-CH2CH3) ) 65 kJ‚mol-1difference
is certainly an upper limit. However, it is unlikely that
it shrinks to ca. 10-15 kJ‚mol-1, the difference observed
for D(Zr-H) - D(Zr-CH2CH3) in tris(amido)Zr(IV)
complexes.24c Even so, the overall analysis suggests that
the reaction in Scheme 6 would be exothermic (∆rH°(6)
< 0) and thus favorable since entropy effects are likely
to cancel out. Note however that despite repeated
attempts we have never been able to synthesize hydride
complexes based on TpMe2NbCl2(alkyne).
contains this rearranged alkyne ((ii) above), i.e., TpMe2
-
NbCl(X)(CH3CH2CtCR). Importantly, the observation
of deshielded quaternary carbon resonances indicate
that no putative niobium ethylidene complex of the type
TpMe2Nb(dCHCH3)(L)(RCtCR′) has been formed via an
R-elimination pathway, a species that could have shown
the 1H NMR signals in the δ 16 region ((i) above). Now,
would it be possible that these deshielded signals are
those of NbH species? Clearly, these signals are not
pyrazole-NH protons that might have been formed by
decompositon of TpMe2 since these protons exhibit much
broader signals in the δ 11 region in benzene-d6.
Formally Nb(III), d2 complexes with an 18e count
exhibit hydride chemical shifts with various chemical
shifts, usually negative (Cp2NbH(C2H3Ph),23b δ -2.40,
-2.89; Cp2NbH(PhCtCPh),29 δ -0.23). For true d0
complexes, the hydride is much more deshielded. In a
series of imidotantalum(V) complexes that are similar
R-C-H agostic ethyl complexes TpMe2Nb(µ-H-CHCH3)-
Cl(HCtCR) (4-R) (R ) Ph, C6H4-4-Me, CH2Ph) were
prepared from 2-R according to our previously described
procedure.21 Key NMR features for the R-C-H agostic
ethyl interaction include a shielded R-CHH proton (for
4-CH2Ph, δ 0.29, dq) and its deshielded R-CHH dias-
tereotopic counterpart (δ 4.01, dq) with a large chemical
shift difference, and in the 13C NMR spectra a NbCR
(25) (a) Physical chemistry, 6th ed.; Atkins, P. W., Ed.; Oxford
University Press: Oxford, 1998. (b) CRC Handbook of Chemistry and
Physics; Weast, R. C., Astle, M. J., Beyer, W. H., Eds.; CRC Press
Inc.: Boca Raton, FL, 1988.
(26) Dias, A. R.; Diogo, H. P.; Griller, D.; Minas de Piedade, M. E.;
Martinho Simoes, J. A. In Bonding energetics in organometallic
compounds; Marks, T. J., Ed.; ACS Symposium Series 428; American
Chemical Society: Washington DC, 1990; pp 205-217, and references
therein.
(28) (a) Bau, R.; Mason, S. A.; Patrick, B. O.; Adams, C. S.; Sharp,
W. B.; Legzdins, P. Organometallics 2001, 20, 4492-4501. (b) Wada,
K.; Pamplin, C. B.; Legzdins, P.; Patrick, B. O.; Tsyba, I.; Bau, R. J.
Am. Chem. Soc. 2003, 125, 7035-7048.
(29) Yasuda, H.; Yamamoto, H.; Arai, T.; Nakamura, A.; Chen, J.;
Kai, Y.; Kasai, N. Organometallics 1991, 10, 4058-4066.
(27) Lichtenberger, D. L.; Darsey, G. P.; Kellog, G. E.; Sanner, R.
D.; Young, V. G., Jr.; Clark, J. R. J. Am. Chem. Soc. 1989, 111, 5019-
5028.

4312 Organometallics, Vol. 24, No. 17, 2005
Oulie´ et al.
2.97; N, 13.78. ESR (ν ) 9.4797 GHz, 293 K, CH2Cl2): g )
1.94; A ) 170 G (10 lines); (CH2Cl2/toluene glass, 100 K): g1
) 1.909; A1 ) 260 G; g2 ) 1.940; A2 )124 G; g3 ) 1.926; A3 )
126 G. IR (KBr, cm-1): ν(B-H) 2567, ν(C-Cl) 860, ν(CdN)
1527.
to our Nb complexes, i.e., (C5Me5)TaH(X)dNAr (Ar )
2,6-(2,4,6-C6H2Me3)2C6H3 and related aryl groups), chemi-
cal shifts as high as δ 15.28 (X ) Cl) and δ 15.20 (X )
OTf) have been observed.30a In (C5Me5)TaH(X)[PhSiH2-
NPhSiMe3]NSiMe3, the hydride resonates at δ 17.15 (X
) Me) and δ 20.44 (X ) Cl).30b Overall, although we
have as yet no definitive proof of hydride complexes,
NMR data strongly suggest that the sought rearrange-
ment in Scheme 6 has taken place. However no reliable
thermodynamic and kinetic data can be extracted.
Summary. In this paper we have disclosed a straight-
forward, efficient synthesis of terminal alkyne com-
plexes of the type Tp′NbCl2(HCtCR) that uses Zn
reduction of Tp′NbCl3 in the presence of the desired
alkyne. The overall alkyne orientation with respect to
Tp′NbCl2 has been discussed in terms of steric interac-
tions. For trifluoropropyne, intramolecular H‚‚‚F bridges
contribute to the cocrystallization of different alkyne
rotamers. R-C-H alkyl complexes are also available.
Their thermolysis has suggested that reversible migra-
tory insertion might yield hydride complexes, although
the instablity of these complexes has precluded a
detailed investigation of the reaction course.
TpMe2NbCl2(PhCtCH) (2-Ph). All Tp′NbCl2(HCtCR) com-
plexes were prepared following the same general procedure
that is described here in detail for TpMe2NbCl2(PhCtCH) (2-
Ph). PhCtCH (0.12 g, 1.2 mmol) was added at room temper-
ature to a stirred suspension of TpMe2NbCl3 (0.500 g, 1.00
mmol) in THF (50 mL) and granular zinc (30 mesh, 0.13 g, 2
mmol). The color of the solution gradually changed from orange
to green and finally after about 1 h to dark purple. Stirring
was maintained overnight. The volatiles were then removed
under vacuum. Toluene (20 mL) was added and then stripped
off. This process was repeated two times to remove traces of
THF. The residue was extracted in toluene or dichloromethane
(ca. 5 mL). Addition of pentane or hexane (10 mL) was followed
by filtration through a pad of Celite. The extraction step was
carried out several times. The clear dark red solution was then
evaporated to dryness to obtain a red-purple microcrystalline
powder, which was found pure enough for further chemistry
(0.434 g, 0.77 mmol, 77%). Recrystallization from dichlo-
romethane/pentane mixtures yielded an analytically pure
sample of 2-Ph. Anal. Calcd for C23H28BCl2N6Nb: C, 49.06;
H, 5.01; N, 14.92. Found: C, 49.54; H, 5.40; N, 14.02. 1H NMR
(300 MHz, chloroform-d1, 233 K): major rotamer δ 11.99 (s,
1H, HCtC), 8.25, 7.75-7.64 (m, 5H, C6H5), 6.04, 5.98 (both s,
1:2H, TpMe2CH), 2.75, 2.50, 2.47, 1.97 (all s, 3:3:6:6H, TpMe2CH3);
minor rotamer δ 13.78 (s, 1H, HCtC), 7.35 (m, 5H, C6H5), 5.98,
5.83 (both s, 1:2H, TpMe2CH), 2.67, 2.54, 2.51, 1.70 (all s, 3: 6:
3: 6H, TpMe2CH3). 13C NMR (75.5 MHz, chloroform-d1, 233
Experimental Section
General Procedures. All experiments were carried out
under a dry dinitrogen atmosphere using either Schlenck tube
or glovebox techniques. THF and diethyl ether were obtained
after refluxing purple solutions of Na/benzophenone under
dinitrogen. Toluene, n-hexane, pentane, 1,4-dioxane, and
dichloromethane were dried by refluxing over CaH2 under
dinitrogen. Deuterated NMR solvents were dried over molec-
ular sieves, degassed by freeze-pump-thaw cycles, and stored
1
K): major rotamer δ 249.9, 218.7 (PhCtCH, JCH 199 Hz),
153.9, 153.1, 145.5, 145.4 (TpMe2CCH3), 135.9, 134.7, 132.2,
129.5 (C6H5), 108.9, 108.6 (TpMe2CH), 16.4, 13.9 (TpMe2CH3);
1
minor rotamer δ 250.0, 221.0 (PhCtC, HCtC, JCH 205 Hz),
154.2, 153.5, 145.5, 145.2 (TpMe2CCH3), 135.9, 131.3, 129.7,
128.9 (C6H5), 109.1, 108.8 (TpMe2CH), 16.6, 14.1 (TpMe2CH3);
isomer ratio 3:1.
1
under dinitrogen. H and 13C NMR spectra were obtained on
Bruker AM 250, DPX 300, AMX 400, or DRX 500 spectrom-
eters, and 19F spectra (external reference CF3CO2H, chemical
shifts versus CFCl3 with δ(CF3CO2H) -77.0) were obtained
TpMe2NbCl2(CF3CtCH) (2-CF3). 1H NMR (200 MHz,
benzene-d6, 293 K): δ 11 (vbr, W1/2 100 Hz, 1H, HCtC), 5.55,
5.34 (both s, 1:2H, TpMe2CH), 2.81, 2.04, 1.93, 1.90 (all s, 3: 3:
6: 6H, TpMe2CH3). 19F{1H} NMR (189 MHz, benzene-d6, 293
K): δ 17.93. 1H NMR (500 MHz, toluene-d8, 193 K): major
rotamer δ 9.80 (s, 1H, HCtC), 5.42, 5.19 (both s, 1:2H,
TpMe2CH), 2.86, 2.13, 1.84, 1.78 (all s, 3:3:6:6H, respectively,
TpMe2CH3); minor rotamer δ 12.24 (s, 1H, HCtC), 5.39, 5.25
(both s, 1:2H, TpMe2CH), 2.97, 2.09, 1.96, 1.69 (all s, 3:6:3:6H,
TpMe2CH3). 13C NMR (125 MHz, toluene-d8, 193 K): major
rotamer δ 225.5 (q, 2JCH 44 Hz, CF3CtC), 209.2 (HCtC), 153.5,
152.25, 144.5, 144.2 (TpMe2CCH3), 108.2, 107.8 (TpMe2CH), 15.4,
12.4, 12.3 (TpMe2CH3); minor rotamer δ 243.0 (HCtC), 196.1
1
on a Bruker AC 200. Only pertinent JCH are quoted in the
13C spectra. Due to the sometimes extreme complexity of the
spectra and/or overlaps, several signals might not be observed.
Elemental analyses were performed in the Analytical Service
of our Laboratory. NbCl4(thf)2,31 TpMe2NbCl3 (1),10 TpMe2
-
SnClBu2, and TpMe2,4ClSnClBu211,12 were prepared according to
published procedures. An improved synthesis of TpMe2NbCl3
is described below with that of TpMe2,4ClNbCl3 (1-Cl). Acetylene
was generated by controlled hydrolysis of commercial sodium
acetylide in xylene, passed through two cold traps (-25 °C,
ethanol/liquid nitrogen baths) before being admitted in the
Schlenk tube containg the appropriate reactants.
2
(q, JCH 43 Hz, CF3CtC), 152.9, 144.2 (TpMe2CCH3), 127.1 (q,
TpMe2,4ClNbCl3 (1-Cl). A solution of TpMe2,4ClSnBu2Cl (4.70
g, 8.32 × 10-3 mol) in 100 mL of toluene is added to a
suspension of NbCl4(THF)2 (3.15 g, 8.32.mmol) in 150 mL of
toluene, at room temperature. The resulting brownish slurry
is stirred for 4 days in the dark. The solvent is evaporated to
dryness, and 250 mL of dichloromethane is added. This
mixture is stirred for 2 days to yield an orange powder, which
is collected by filtration. The dark green solution is evaporated
to half, and 100 mL of hexane is added to induce a new
crystallization, which continues at -25 °C. The two solids are
washed with two portions of 15 mL of diethyl ether and dried
under vaccum (3.22 g, 6.49 mmol, 78%). Anal. Calcd for C15H19-
BCl6N6Nb: C, 30.04; H, 3.19; N, 14.01. Found: C, 30.43; H,
JCF 271 Hz, CF3), 107.7 (TpMe2CH), 16.5, 15.9 (TpMe2CH3);
isomer ratio 3.5:1.
TpMe2,4ClNbCl2(PhCtCH) (2-Cl-Ph). Anal. Calcd for C23H25-
BCl5N6Nb: C, 41.45; H, 3.78; N, 12.61. Found: C, 41.80; H,
4.07; N, 12.23. 1H NMR (300 MHz, toluene-d8, 233 K): major
rotamer δ 11.20 (s, 1H, HCtC), 8.29, 7.34-7.20 (m, 5H, C6H5),
3.05, 2.15, 1.96, 1.77 (all s, 3:3:6:6H, Tp*CH3); minor rotamer
δ 13.42 (s, 1H, HCtC), δ 7.09 (m, 5H, C6H5). 13C NMR (75.5
MHz, toluene-d8, 233 K): major rotamer δ 249.0, 218.6 (d, 1JCH
196 Hz, PhCtCH), 150.1, 149.3, 141.4, 141.3 (TpMe2,4ClCCH3),
134.9, 129.5, 128.8, 128.4 (C6H5), 111.1, 110.8 (TpMe2,4ClCH),
14.30, 13.72, 10.75, 10.67 (TpMe2,4ClCH3); minor rotamer δ
250.0, 219.2 (PhCtCH); isomer ratio 5:1.
TpMe2NbCl2(HCtCC6H4-4-Me) (2-C6H4-4-Me). Anal. Cal-
cd for C24H30BCl2N6Nb: C, 49.94; H, 5.24; N, 14.56. Found:
C, 49.43; H, 5.25; N, 14.13. 1H NMR (250 MHz, toluene-d8,
193 K): major rotamer δ 11.30 (s, 1H, HCtC), δ 8.35, 7.16
(both d, J 7.7 Hz, 2H each, C6H4), 5.55, 5.29 (both s, 1:2H,
(30) (a) Gavenonis, J.; Tilley, T. D. Organometallics 2004, 23, 31-
43. (b) Gountchev, T. I.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119,
12831-12841.
(31) Pedersen, S. F.; Hartung, J. B.; Roskamp, E. J.; Dragovitch, P.
S. Inorg. Synth. 1992, 29, 119-123.

New Way to Scorpionate Niobium Complexes
Organometallics, Vol. 24, No. 17, 2005 4313
TpMe2CH), 3.15, 2.02, 1.94, 1.83 (all s, 3:3:6:6H, TpMe2CH3);
minor rotamer δ 13.69 (s, 1H, HCtC), 6.82, 6.41, 5.96 (br s,
1H each, C6H4), 5.32, 5.23 (both s, 1:2H, respectively, TpMe2CH),
3.15, 2.07, 2.86, 1.70 (all s, 3:6:6:3H, TpMe2CH3). 13C NMR (62.5
MHz, chloroform-d1, 253 K): major rotamer δ 249.0, 217.0
CH3), 2.92, 2.30, 2.27, 2.20, 2.18, 1.82 (all s, 3H each,
TpMe2CH3), 2.18 (s, 3H, CH3-Ph), 1.20 (dd, J 7.4, 6.4 Hz,
NbCH2CH3), 0.63 (dq, J 12.6, 6.3 Hz, 1H, NbCH2CH3); minor
rotamer δ 13.04 (s, 1H, HCtC), 11.49 (s, 1H, HCtC), 7.00,
6.86 (both d, J 7.8 Hz, 2H each, C6H4), 5.86, 5.62, 5.53 (all s,
1H each, TpMe2CH), 4.34 (dq, J 12.8, 7.7 Hz, 1H, NbCH2CH3),
2.96, 2.34, 2.32, 2.01, 2.00, 1.67 (all s, 3 H each, TpMe2CH3),
2.19 (s, 3H, CH3C6H4), 1.30 (dd, J 7.4, 6.4 Hz, 3H, NbCH2CH3),
0.72 (pseudo sextet, J 12.6, 6.4 Hz, 1H, NbCH2CH3). 13C NMR
(100.6 MHz, benzene-d6, 293 K): major rotamer δ 238.0, 213.9
1
(PhCtC, HCtC, JCH 203 Hz), 153.1, 152.8, 152.4, 144.5
(TpMe2CCH3), 134.0, 132.6, 133.0, 129.3 (C6H4), 108.1, 107.8
(TpMe2CH), 21.9 (CH3-C6H4), 15.9, 15.6, 13.0, 12.8 (TpMe2CH3);
1
minor rotamer δ 249.0, 221.0 (PhCtC, HCtC, JCH 199 Hz),
144.3, 142.1, 140.9 (TpMe2CCH3), 133.0, 131.4 (C6H5), 108.3,
107.9 (TpMe2CH), 21.6 (CH3C6H4), 15.7, 15.2, 13.2, 12.9
(TpMe2CH3); isomer ratio 1.6:1.
1
(PhCtCH, JCH 191 Hz), 153.2, 152.9, 149.7, 144.2, 144.2,
143.6 (TpMe2CCH3), 140.3, 135.1, 133.7, 130.1 (C6H4), 108.5,
1
107.8, 107.6 (TpMe2CH), 91.2 (dd, JCH 105, 128 Hz, NbCH2),
TpMe2NbCl2(HCtCH) (2-H). Anal. Calcd for C17H24BCl2N6-
Nb: C, 41.92; H, 4.97; N, 17.26. Found: C, 41.75; H, 4.90; N,
17.16. 1H NMR (400 MHz, dichloromethane-d2, 293 K): δ 12.23
(s, 2H, HCtC), 6.02, 5.92 (both s, 1:2H, respectively, TpMe2CH),
δ 2.54, 2.50, 2.44, 1.96 (all s, 3:3:6:6H, respectively, TpMe2CH3);
at 183 K δ 13.12, 11.42 (s, 1H each, HCtC), 6.02, 5.92 (both
s, 1:2H, TpMe2CH), 2.40, 2.34, 2.28, 1.83 (all s, 3:3:6:6H,
TpMe2CH3). 13C NMR (100.6 MHz, dichloromethane-d2, 183
K): δ 246.1, 214.3 (both d, 1JCH 208 Hz, 199 Hz, HCtC), 153.7,
153.3, 146.6, 146.7 (TpMe2CCH3), 108.6, 108.5 (TpMe2CH), 16.2,
16.1, 14.0, 13.8 (TpMe2CH3).
21.7 (CH3Ph), 17.9 (NbCH2CH3), 16.2, 15.8, 14.7, 13.1, 13.0,
12.6 (TpMe2CH3); minor rotamer δ 233.5, 218.4 (PhCtCH, 1JCH
191 Hz), 153.7, 153.1, 150.1, 144.3, 144.0, 143.8 (TpMe2CCH3),
139.6, 135.6, 131.8, 129.7 (C6H4), 108.0, 107.5, 106.9 (TpMe2CH),
85.8 (dd, 1JCH 103, 128 Hz, NbCH2CH3), 21.5 (CH3C6H4), 18.1
(CH3CH2), 15.3, 14.6, 13.3, 12.8, 12.7 (TpMe2CH3).
TpMe2NbCl(CH2CH3)(PhCH2CtCH) (4-CH2Ph). Anal. Cal-
cd for C26H35BClN6Nb: C, 54.71; H, 6.18; N, 14.72. Found: C,
1
54.43; H, 5.74; N, 14.28. H NMR (400 MHz, toluene-d8, 213
K): major rotamer δ 10.74 (s, 1H, HCtC), 7.50-7.05 (m, 5H,
C6H5), 5.69, 5.44, 5.36 (all s, 1H each, TpMe2CH), 5.10, 4.88
(both d, J 19.2 Hz, 1H, PhCH2), 4.01 (dq, J 11.8, 7.4 Hz, 1H,
NbCH2CH3), 2.85, 2.22, 2.11, 1.98, 1.94, 1.69 (all s, 3H each,
TpMe2CH3), 1.25 (dd, J 7.0, 6.6 Hz, 3H, CH2CH3), 0.29 (dq, J
11.7, 6.7 Hz, 1H, NbCH2CH3); minor rotamer δ 12.97 (s, 1H,
HCtC), 5.65, 5.64, 5.51 (both s, 1H eah, TpMe2CH), 2.83, 2.22,
2.21, 2.02, 1.71 (all s, 3:3:3:6:3H, TpMe2CH3). 13C NMR (100.6
MHz, toluene-d8, 213 K): major rotamer δ 248.2, 210.0 (PhCt
TpMe2NbCl2(PhCH2CtCH) (2-CH2Ph). Anal. Calcd for
C24H30BCl2N6Nb: C, 49.94; H, 5.24; N, 14.56. Found: C, 49.83;
H, 4.88; N, 13.98. 1H NMR (400 MHz, toluene-d8, 233 K):
major rotamer δ 10.83 (s, 1H, HCtC), 7.36-7.11 (m, 5H,
C6H5), 5.59, 5.38 (both s, 1:2H, TpMe2CH), 5.31 (s, 2H, CH2),
3.00, 2.08, 1.95, 1.89 (all s, 3:3:6:6H, TpMe2CH3); minor rotamer
δ 13.07 (s, 1H, HCtC). 13C NMR (100.6 MHz, toluene-d8, 233
1
K): major rotamer δ 260.6, 212.8 (PhCtCH, JCH 200 Hz),
153.6, 152.4, 144.1, 142.6 (TpMe2CCH3), 138.7 (ipso-C6H4,),
108.4, 108.0 (TpMe2CH), 45.8 (CH2), 16.4, 15.9, 12.8, (TpMe2CH3);
isomer ratio 47:1.
1
C, HCtC, JCH 190 Hz), 152.7, 152.6, 149.3, 144.3, 143.6
(TpMe2CCH3), 143.4 (C6H4), 108.5, 107.7, 107.5 (TpMe2CH), 88.3
1
(dd, JCH 108, 132 Hz, NbCH2CH3), 44.3 (PhCH2), 16.4, 15.9,
TpMe2NbCl2[dN-P(N-iPr2)2] (3). This compound was ob-
tained following the same procedure as that for Tp′NbCl2(HCt
CR) except that N3P(N-iPr2)2 was used instead of HCtCR.
Anal. Calcd for C27H50BCl2N9NbP: C, 45.91; H, 7.13; N, 17.85.
12.8, (TpMe2CH3); isomer ratio 20:1.
Thermal Rearrangement of 4-R. Under argon, a weighted
amount (ca. 40 mg) of the appropriate 4-R complex was placed
in a screw-cap NMR tube, dissolved in benzene-d6 (0.4 mL),
and placed in a oil bath at 323 K in the dark. Reaction progress
1
Found: C, 45.29; H, 6.80; N, 17.60. H{31P} NMR (400 MHz,
benzene-d6, 293 K): δ 5.58, 5.57 (both s, 1:2H, TpMe2CH), 3.64
(septet, J 6.7 Hz, 1H, CH(CH3)2), 3.01, 2.95, 2.18, 2.08 (all s,
3:3:6:6H, respectively, TpMe2CH3), 1.51, 1.29 (d, J 6.7 Hz, 12H
each, CH(CH3)2). 13C NMR (100.6 MHz, benzene-d6, 293 K):
δ 154.1, 153.6, 145.5, 143.1 (TpMe2CCH3), 107.7, 107.5 (TpMe2CH),
1
was monitored regularly by H and 13C NMR.
X-ray Analysis. X-ray analyses are summarized in Table
4. Data were collected at low temperature (180 K) on a
Xcalibur Oxford diffraction diffractometer (2-CH2Ph) or a
IPDS STOE (2-CF3) using graphite-monochromated Mo KR
radiation (λ ) 0.71073 Å), both equipped with an Oxford
Cryosystems cryostream cooler device. The final unit cell
parameters have been obtained by means of a least-squares
refinement performed on a set of 4500 (2-CH2Ph) or 8000 (2-
CF3) well-measured reflections. The structures have been
solved by direct methods using SIR9232 and refined by means
of least-squares procedures on a F2 (2-CH2Ph) with the aid of
the program SHELXL9733 included in the softwares package
WinGX version 1.63,34 or on F (2-CF3) using the programs of
the PC version of CRYSTALS.35 The atomic scattering factors
were taken from International Tables for X-Ray Crystal-
lography.36 For 2-CH2Ph, all hydrogens atoms were geo-
metrically placed and refined by using a riding model. All non-
hydrogens atoms were anisotropically refined, and in the last
2
3
47.4 (d, JCP 11.5 Hz, CHCH3), 25.0, 24.4 (d, JCP 8.5 Hz,
CHCH3), 17.8, 15.5, 13.0, 12.1 (s, s, d, s, JCP 8.5 Hz, TpMe2
-
CH3). 31P{1H} NMR (162.0 MHz, benzene-d6, 293 K): δ 129.7
(s, [dN-P(N-iPr2)2]).
TpMe2NbCl(CH2CH3)(PhCtCH) (4-Ph). All ethyl com-
plexes were prepared from the appropriate TpMe2NbCl2(RCt
CH) and commercial (CH3CH2)MgCl in ether according to a
procedure previously reported for TpMe2NbCl(CH2CH3)(PhCt
CCH3) and related complexes.21 1H NMR (250 MHz, benzene-
d6, 297 K): major rotamer δ 11.33 (s, 1H, HCtC), 8.20, 7.41-
7.20 (m, 5H, C6H5), 5.72, 5.52, 5.49 (all s, 1H each, TpMe2CH),
4.11 (dq, J 12.2, 7.2 Hz, 1H, CH2CH3), 2.78, 2.19, 2.07, 2.05,
1.69, 1.40 (all s, 3H each, TpMe2CH3), 1.00 (dd, J 6.5, 7.2 Hz,
3H, CH2CH3), 0.60 (dq, J 12.2, 6.5 Hz 1H, NbCH2CH3); minor
rotamer δ 12.90 (s, 1H, HCtC). 13C NMR (62,5 MHz, benzene-
d6, 297 K): major rotamer δ 237.6, 213.7 (PhCtCH, 1JCH 190
Hz), 152.9, 152.8, 152.7, 144.0, 143.5, 143.4 (TpMe2CCH3),
137.9, 128.5, 128.4, 128.0 (C6H5), 108.4, 107.5, 107.2 (TpMe2CH),
91.3 (dd, 1JCH 109, 127 Hz, CH2CH3), 15.5, 15.0, 14.5, 12.9, 12.8,
12.5 (TpMe2CH3); minor rotamer δ 233.4, 217.5 (PhCtCH, 1JCH
189 Hz).
(32) SIR92-A program for crystal structure solution. Altomare, A.;
Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr.
1993, 26, 343-350.
(33) Sheldrick, G. M. SHELX97 [Includes SHELXS97, SHELXL97,
CIFTAB]-Programs for Crystal Structure Analysis (Release 97-2);
Institu¨t fu¨r Anorganische Chemie der Universita¨t: Tammanstrasse
4, D-3400 Go¨ttingen, Germany, 1998.
(34) WINGX-1.63 Integrated System of Windows Programs for the
Solution, Refinement and Analysis of Single-Crystal X-Ray Diffraction
Data. Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837-838.
(35) CRYSTALS. Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.;
Prout, K.; Watkin, D. J. J. Appl. Crystallogr. 2003, 36, 1487.
(36) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, England, 1974; Vol. IV.
TpMe2NbCl(CH2CH3)(HCtCC6H4-4-Me) (4-C6H4-4-Me).
Anal. Calcd for C26H35BClN6Nb: C, 54.71; H, 6.18; N, 14.72.
Found: C, 54.43; H, 5.74; N, 14.28. 1H NMR (400 MHz,
benzene-d6, 293 K): major rotamer δ 11.49 (s, 1H, HCtC),
8.28, 7.37 (both d, J 7.8 Hz, 2H each, C6H4), 5.83, 5.64, 5.61
(all s, 1H each, TpMe2CH), 4.18 (dq, J 12.6, 7.7 Hz, 1H, NbCH2-

4314 Organometallics, Vol. 24, No. 17, 2005
Table 4. X-ray Diffraction Analysis of Compounds 2-CH2Ph and 2-CF3
Oulie´ et al.
2-CH2Ph
2-CF3
Crystal Data
chemical formula
cryst syst
space group
a (Å)
C
40H30BCl2N6Nb
C18H23BCl2F3N6Nb,(H2O)1/3
triclinic
P1h
triclinic
P1h
10.4566(8)
11.1476(7)
11.1476(7)
74.971(5)
85.341(5)
84.280(6)
1338.72(15)
2
13.909(5)
16.844(5)
17.769(5)
72.442(5)
71.083(5
70.172(5)
3618(2)
6
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
µ (cm-1
)
6.73
7.63
Data Collection
Oxford Excalibur
Mo KR
0.71073 (graphite monoch.)
180(2)
3.47-28.22
-13 e h e 10
-14 e k e 14
-15 e l e 15
11 779
diffractometer
radiation type
wavelength (Å)
temperature (K)
θ range (deg)
h k l range
STOE IPDS
Mo KR
0.71073 (graphite monoch.)
180(2)
1.99-26.18
-17 e h e 17
-20 e k e 20
-22 e l e 22
36 257
7257
0.050
no. of measd reflns
no. of indep reflns
merging R value
6578
0.0294
Refinement
empirical (DIFABS)
0.92/0.85
absorption corr
max. and min. transmn
refinement
semiempirical (multiscan)
0.95/0.90
F (full-matrix least-squares)
7257 [I>2σ(I)]
847
0.0437/0.0488
F2 (full-matrix least-squares)
6578 [I>2σ(I)]
416
no of reflns used
no of params used
final Ra/wR2b
0.0299/0.0774
0.0337/0.0798
1.061
final Ra/wR2b (all data)
GOF (S)c
1.094
a R ) ∑(||Fo| - ||Fc|)/∑|Fo|. b Rw ) [∑w(||Fo| - ||Fc|)2/∑(|Fo|)2]1/2
.
c Goodness of fit ) [∑(|Fo - Fc|)2/(Nobs - Nparameters)]1/2
.
cycles of refinement a weighting scheme was used, where
which X ) Fc/Fc(max) was used. Drawings of molecules were
performed with the program ORTEP3237 with 30% probability
displacement ellipsoids for non-hydrogen atoms.
weights are calculated from the following formula: w ) 1/[σ2-
(Fo ) + (aP)2 + bP], where P ) (Fo + 2Fc2)/3. For 2-CF3, the
asymmetric unit contained three H23BCl2F3N6Nb molecules
and one disordered water molecule. All non-hydrogen atoms
excluding the water molecules were refined anisotropically.
Hydrogen atoms were introduced in calculated positions in the
last refinements and were allocated an overall refinable
isotropic thermal parameter. A weighting scheme of the form
w ) w′[1 - ((||Fo| - |Fc||)/6σ(Fo))2]2 with w′ ) 1/∑rArTr(X) with
coefficients 3.15, -1.04, and 2.62 for a Chebychev series for
2
2
Supporting Information Available: Cif files for the
X-ray characterization of 2-CH2Ph and 2-CF3. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM050423F
(37) ORTEP3 for Windows. Farrugia, L. J. J. Appl. Crystallogr.
1997, 30, 565.