Isolobal zwitterionic niobium and tantalum imido and zirconium monocyclopentadienyl complexes: Theoretical and methyl methacrylate polymerization studies

Article abstract of DOI:10.1021/om701068h

The trichloro derivatives [MCl₃(NR)(py)₂] reacted with LiOAr to give the imido aryloxo complexes [MCl₂(NR)(OAr)(Py) ₂] (M = Nb, R = tBu (1a), Ar (lb); M = Ta, R = tBu (2a); Ar = 2,6-iPr₂C₆H

Full text of DOI:10.1021/om701068h

Organometallics 2008, 27, 1417–1426
1417
Isolobal Zwitterionic Niobium and Tantalum Imido and Zirconium
Monocyclopentadienyl Complexes: Theoretical and Methyl
Methacrylate Polymerization Studies
Rocío Arteaga-Müller,† Javier Sánchez-Nieves,† Javier Ramos,‡ Pascual Royo,*,† and
Marta E. G. Mosquera†,§
Departamento de Química Inorgánica, UniVersidad de Alcalá, Campus UniVersitario, E-28871 Alcalá de
Henares, Madrid, Spain, and Grupo de Estructura Molecular y Propiedades de Polimeros, Instituto de
Estructura de la Materia, CSIC, Serrano 113bis, 28006, Madrid, Spain
ReceiVed October 24, 2007
The trichloro derivatives [MCl3(NR)(py)2] reacted with LiOAr to give the imido aryloxo complexes
[MCl2(NR)(OAr)(py)2] (M ) Nb, R ) tBu (1a), Ar (1b); M ) Ta, R ) tBu (2a); Ar ) 2,6-iPr2C6H3)
and were alkylated with [BzMgCl], affording the tribenzyl derivatives [NbBz3(NR)] (R ) tBu (3a), Ar
(3b)). Similar alkylations of the aryloxo derivatives 1 and 2 gave the dibenzyl complexes
[MBz2(NR)(OAr)(THF)] (M ) Nb, R ) tBu (5a); M ) Ta, R ) tBu (6a), Ar (6b)). The zwitterionic
imido complexes [MBz2(NtBu){η6-C6H5CH2E(C6F5)3}] (M ) Nb, E ) B (7a-B), Al (7a-Al); M ) Ta,
E ) Al (8a-Al)) were obtained upon addition of 1 equiv of E(C6F5)3 (E ) B, Al) to the tribenzyl derivatives
[MBz3(NtBu)] (M ) Nb (3a), Ta (4a)). The aryloxo compound 6a reacted with 2 equiv of the Lewis
acids E(C6F5)3 (E ) B, Al) to give the corresponding zwitterionic imido complexes [TaBz(NtBu)(OAr){η6-
C6H5CH2E(C6F5)3}] (E ) B (11a-B), Al (11a-Al)) and the adduct (THF) · E(C6F5)3, through two different
intermediates, the ionic [TaBz(NtBu)(OAr)(THF)][BzB(C6F5)3] (9a) for E ) B and the neutral
[TaBz2(NtBu)(OAr)] (10a) for E ) Al. The related monocyclopentadienyl complex [ZrCpBz2{η6-
C6H5CH2Al(C6F5)3}] (12-Al) was also isolated by reaction of [ZrCpBz3] with Al(C6F5)3. DFT calculations
were carried out to further understand this type of zwitterionic imido derivative in comparison with the
isolobal zirconium cyclopentadienyl compound [ZrCpBz2{η6-C6H5CH2B(C6F5)3}]. MMA polymerization
was investigated for the [MBz2X(NtBu)]/E(C6F5)3 (X ) Bz, OAr) and [ZrCpBz3]/E(C6F5)3 (E ) B, Al)
systems.
derivatives [MCp2X2] (M ) group 4 metal).3–16 The reactivities
of these two types of complexes are comparable, although the
half-sandwich complexes [MCp(NR)X2] (M ) group 5 metal)
were rather less active olefin polymerization catalysts than the
metallocene-type complexes [MCp2X2] (M ) group 4 metal).17–19
This difference has been attributed to the lower acidity of group
5 metal derivatives, which would consequently be expected to
show a greater affinity for polar olefins. Conversely, important
differences between these two types of complexes involve the
reactivity of the metal-nitrogen multiple bond.13,20–23
Introduction
The imido group is a strong π donor ligand with the widely
proven ability to stabilize high-valent metal complexes.1,2 The
isolobal relationship of the imido and the cyclopentadienyl
ligands3 is an important feature that allows the half-sandwich
imido complexes [MCp(NR)X2] (M ) group 5 metal) to be
studied as isoelectronic counterparts of dicyclopentadienyl
* To whom correspondence should be addressed. E-mail: pascual.royo@
uah.es; FAX: 00 34 91 885 4683.
Control of the stereochemistry and polydispersity of the
polymer chain and the production of well-defined block
† Universidad de Alcalá.
‡ Instituto de Estructura de la Materia, CSIC.
§ X-ray diffraction studies.
(1) Nugent, W. A.; Mayer, J. M. Metal Ligand Multiple Bonds; Wiley-
Interscience: New York, 1988.
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M. E. G. Polyhedron 2005, 24, 1274.
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10.1021/om701068h CCC: $40.75
2008 American Chemical Society
Publication on Web 03/04/2008

1418 Organometallics, Vol. 27, No. 7, 2008
Arteaga-Müller et al.
Scheme 1a
copolymers24–30 are essential features of the polymerization of
methacrylates by transition-metal complexes. Group 3 and 4
metallocene complexes have been widely explored over the past
few years and, more recently,24–36 tantalum cyclopentadienyl
based complexes have also emerged as active catalysts for MMA
polymerization.17,37,38 Different mechanisms based on group
transfer polymerization have been proposed, involving mono-
nuclear or dinuclear alkyl or enolate active species and depend-
ing on the cocatalyst employed, mainly E(C6F5)3 (E ) B, Al).
However, MMA polymerization has also been described for
group 4 imido metallocene complexes in the presence of
Al(C6F5)3, involving the formation of an oxametallacycle by
addition of MMA to the metal-nitrogen multiple bond and
further generation of a zwitterionic dinuclear enol aluminate
intermediate by rupture of the metal–oxygen bond produced by
Al(C6F5)3.39
In this work, we have extended our initial interest in half-
sandwichimidoneutral[MCp(NR)X2](M)group5metal)9–14,40–44
and cationic monocyclopentadienyl group 5 metal complexes45,46
to noncyclopentadienyl imido complexes of the type
[MBz2(NR)X] (M ) Nb, Ta; X ) Bz, OAr), with the aim of
synthesizing cationic derivatives by reaction with the Lewis
acids E(C6F5)3 (E ) B, Al) and exploring their activity as MMA
polymerization catalysts. Initial studies related to the tribenzyl
compounds (X ) Bz) have also been extended to the aryloxo
derivatives (X ) OAr) in order to modify their reactivity and
to stabilize these types of high-valent cationic compounds.
a
Legend: (i) LiOAr; (ii) BzMgCl.
Recently, Bergman et al. have reported the synthesis of the
tantalum tribenzyl derivative [Ta(NtBu)Bz3],47 which in the
presence of the Lewis acids B(C6F5)3 and [Ph3C][B(C6F5)4]
generates the corresponding zwitterionic and ion-unpaired
derivatives that were applied as catalysts for the hydroamination
of acetylenes.
Furthermore, considering the isolobal relationship between
the imido and cyclopentadienyl ligands,1 a similar relation could
be established between the non-cyclopentadienyl imido com-
plexes [MX3(NR)] (M ) group 5 metal) and monocyclopen-
tadienyl complexes [MCpX3] (M ) group 4 metal). This last
type of complex has found widespread application as an olefin
polymerization catalyst, including MMA,25,48 and thus compara-
tive chemical and DFT studies have also been carried out in
this work.
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Results and Discussion
Synthesis of Neutral Compounds. Following the procedure
previously reported by Wigley et al. for the synthesis of
[TaCl2(NAr)(OAr)(py)2] (Ar ) 2,6-iPr2C6H3)49 by reaction of
the trichloro complexes with LiOAr, we have isolated the new
imido aryloxo compounds [MCl2(NR)(OAr)(py)2] (M ) Nb, R
) tBu (1a), Ar (1b); M ) Ta, R ) tBu (2a)) in good yields,
using the respective trichloro imido derivatives [MCl3(NR)(py)2]
(M ) Nb, R ) tBu, Ar; M ) Ta, R ) tBu)49,50 as source
materials (Scheme 1). All of these new complexes retained the
original two molecules of pyridine coordinated to the metal
center, whichever imido substituent was used.
The trichloro imido compounds [NbCl3(NR)(py)2] (R ) tBu,
Ar) and the aryloxo imido compounds 1 and 2a were trans-
formed into the benzyl derivatives [NbBz3(NR)] (R ) tBu (3a),
Ar (3b)) and [MBz2(NR)(OAr)(THF)] (M ) Nb, R ) tBu (5a);
M ) Ta, R ) tBu (6a), Ar (6b)), respectively, after addition of
the corresponding molar amounts of [BzMgCl] (Scheme 1). The
synthesis of the aryloxo complexes 5a and 6a required addition
of stoichiometric amounts of [BzMgCl] to prevent formation
of the corresponding tribenzyl derivatives [MBz3(NR)], which
(26) Batis, C.; Karanikolopoulos, G.; Pitsikalis, M.; Hadjichristidis, N.
Macromolecules 2003, 36, 9763.
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1997, 36, 2647.

Zwitterionic Imido Group 5 Metal Complexes
Organometallics, Vol. 27, No. 7, 2008 1419
were observed when working with an excess resulting from the
simultaneous substitution of the aryloxo group. This side
reaction could be responsible for the lower yields obtained in
the synthesis of complexes 5a and 6 with respect to those
observed for complexes 3 and [TaBz3(NtBu)]47 (4a). The yields
of these reactions also reflected the different electronic stabi-
lizations provided by the two different types of imido groups.
When R was tBu, the yields were moderate to good, whereas
rather poor yields were obtained when R was Ar and in the
particular case of the complexes [TaCl3(NAr)(py)2] and
[NbCl2(NAr)(OAr)(THF)] (1b) none of the benzyl derivatives
could be observed.
The 1H and 13C NMR spectra of the tribenzyl imido
compounds 3a,b showed only one resonance for the three
equivalent benzyl ligands. The JC-H value of the methylene
Figure 1. ORTEP diagram of [NbBz3(NtBu)] (3a). Hydrogen atoms
have been omitted, and thermal ellipsoids are shown at the 50%
level.
1
group (ca. 133 Hz) and the chemical shift of the ipso carbon of
the phenyl ring (δ ∼138) suggested η2 coordination of the
benzyl ligand,47,51,52 which was confirmed by X-ray diffraction
studies (vide infra). The same spectroscopic parameters were
observed for the tantalum complex [TaBz3(NtBu)] (4a).47
The elemental analysis of 5a and 6a,b were consistent with
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the
Compounds [MBz3(NtBu)] (M ) Nb (3a), Ta47
)
M ) Nb
M ) Ta
M-N
1.747(4)
2.245(3)
2.679(3)
180.00(1)
89.62(17)
98.26(9)
1.737(6)
2.216(4)
2.678
M-C(9)
M-C(3)
1
the proposed chemical formulas, and their H and 13C NMR
M-N-C(1)
M-C(3)-C(9)
N-M-C(9)
C(9)-M-C(9)
180.00
90.80(3)
100.0(1)
117.06(7)
spectra demonstrated coordination of THF to a metal center
bound to two equivalent benzyl ligands, with the diastereotopic
methylene protons observed as a pair of doublets in the 1H NMR
spectrum. The 1JC-H value of the methylene group (ca. 120 Hz)
and the chemical shift of the ipso carbon (δ ∼145) of the benzyl
ligands were consistent with their η1 coordination. These
structural features are consistent with a trigonal-bipyramidal
geometry in which the strong donor imido and THF ligands
occupy the axial positions, whereas the aryloxo and two benzyl
ligands are located in the equatorial positions. This structure
was supported by the results of NOE experiments.
The metal centers of these imido complexes show different
coordination numbers. The trichloro and dichloroaryloxo deriva-
tives 1 and 2a are 18-electron pseudooctahedral compounds with
two additional pyridine ligands, considering the aryloxo and
imido ligands as 3- and 4-electron donors, respectively. How-
ever, considering the benzyl ligand as a 1-electron donor, the
dibenzylaryloxo derivatives 5a and 6 with one additional THF
molecule are 16-electron trigonal bipyramidal species and the
ligand-free neutral tribenzyl 3 and 4a are 12-electron compounds
with a pseudotetrahedral geometry.
117.97(4)
presented in Table 1. Complex 3a exhibits a C3V pseudotetra-
hedral structure, which resembles that found in typical mono-
cyclopentadienyl three-legged piano-stool derivatives, with three
narrower N-Nb-C and three more open C-Nb-C angles. The
bond length and bond angle values were very close to those
found in the analogous tantalum derivative [TaBz3(NtBu)]
(Table 1). The three Nb-Cipso distances (2.679(3) Å) and the
three Nb-C-Cipso angles (89.62(2)°) are clearly indicative of
η2-benzyl coordination. Although this η2 interaction is well
represented for electron-deficient metal-benzyl compounds,53–59
the presence of three equivalent η2-benzyl ligands is unusual,
probably as a consequence of the coordinative requirements of
the metal atom, in comparison with those of the related
zirconium cyclopentadienyl derivative [ZrCpBz3].60 Finally, the
Nb-N bond distance (1.747(4) Å) is in the range expected for
a Nb-N triple bond, in accordance with the linearity observed
for the Nb-N-C bond angle (180.00(1)°).1
Synthesis of Zwitterionic Compounds. The reactions of the
tribenzyl derivatives [MBz3(NtBu)] (M ) Nb (3a), Ta (4a))
with the Lewis acids E(C6F5)3 (E ) B, Al) afforded the
zwitterionic compounds [MBz2(NtBu){η6-C6H5CH2E(C6F5)3}]
(M ) Nb, E ) B (7a-B), Al (7a-Al); M ) Ta, E ) Al (8a-Al))
(Scheme 2). All of these new complexes were soluble in C6D6.
Formationofthehypotheticalniobiumcompounds[NbBz2(NAr){η6-
C6H5CH2E(C6F5)3}] in similar reactions with the arylimido
Surprisingly, neither the steric hindrance nor the strong σ and
π donor character of the aryloxo ligand in the imido complexes
5 and 6a prevented the coordination of one additional neutral
THF ligand, whereas the tribenzyl imido complexes 3 and 4a
did not coordinate additional ligands. Furthermore, the cis
coordination of the imido and aryloxo ligands was expected to
maximize the overlapping of the π orbitals of these ligands with
the metal center.49 Apparently, the σ donating ability and the
η2 coordination of the three benzyl groups weaken the interac-
tion of the metal center with the neutral donor ligand in
compounds 3 and 4a, facilitating the dissociation of this ligand,
which is removed after washing with hexane and drying under
vacuum for a few hours.
(53) Prashar, S.; Fajardo, M.; Garcés, A.; Dorado, I.; Antiñolo, A.; Otero,
A.; López-Solera, I.; López-Mardomingo, C. J. Organomet. Chem. 2004,
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tallics 2003, 22, 3793.
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2002, 3398.
X-ray Structure of 3a. Crystals suitable for X-ray diffraction
studies were obtained for the tribenzyl imido compound
[NbBz3(NtBu)] (3a). The molecular structure of 3a is shown
in Figure 1, and a selection of bond distances and angles is
(56) Schweiger, S. W.; Salberg, M. M.; Pulvirenti, A. L.; Freeman, E. E.;
Fanwick, P. E.; Rothwell, I. P. Dalton Trans. 2001, 2020.
(57) Murphy, V. J.; Turner, H. Organometallics 1997, 16, 2495.
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Rodríguez, G.; Royo, P. J. Organomet. Chem. 1997, 547, 287.
(59) Álvaro, L. M.; Cuenca, T.; Flores, J. C.; Royo, P.; Pellinghelli,
M. A.; Tiripicchio, A. Organometallics 1992, 11, 3301.
(60) Scholz, J.; Rehbaum, F.; Thiele, K. H.; Goddard, R.; Betz, P.;
Kruger, C. J. Organomet. Chem. 1993, 443, 93.
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2944.
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Huffman, J. C. Organometallics 1985, 4, 902.

1420 Organometallics, Vol. 27, No. 7, 2008
Arteaga-Müller et al.
Scheme 2. Reaction of Tribenzyl Imido Complexes with the
Scheme 3. Reaction of Dibenzyl Aryloxo Imido Complexes
Lewis Acids E(C6F5)3 (E ) B, Al)
with the Lewis Acids E(C6F5)3 (E ) B, Al)a
derivative 3b were unsuccessful, in accordance with the σ-donor
capacity of the arylimido ligand being lesser than that of the
related tert-butylimido ligand.
The 1H and 13C NMR spectra of complexes 7 and 8 showed
similar patterns which were also analogous to that reported for
[TaBz2(NtBu){η6-C6H5CH2B(C6F5)3}] (8a-B).47 The Cs sym-
metry of complexes 7 and 8 gave rise to the three resonances
observed in the 1H NMR spectra for the AA′BB′C spin system
of the η6-C6H5 ring and the two doublets for the diastereotopic
M-CH2 protons of the two equivalent benzyl ligands. The
1
values found for JC-H of the benzyl methylene group and for
the chemical shift of the ipso carbon atom of the phenyl ring
were very close to those found for the starting [MBz3(NtBu)]
complexes, indicating a similar η2 coordination. Formation of
the [C6H5CH2E(C6F5)3]- anion moiety was indicated by a broad
signal observed at about 3 ppm for the Bz-E groups. Coordina-
tion of the anionic benzylborate phenyl ring to the metal center
was in agreement with the upfield shift of three resonances with
respect to those expected for a normal phenyl ring, although
such displacement was not observed for the related zirconium
complexes [ZrCpBz2{η6-C6H5CH2E(C6F5)3}] (E ) B,61 Al (vide
infra)). We believe that this different behavior could be attributed
to slippage of the phenyl ring in complexes 7 and 8, as has
been confirmed by the DFT calculations discussed below.
Finally, the presence of the ion-pair interaction for the ben-
zylborate complex 7a-B was confirmed by means of the
δmeta-δpara difference of about 4 ppm observed in its 19F NMR
spectrum.
a
Legend: (i) B(C6F5)3; (ii) Al(C6F5)3.
directly into 11a-B can be obtained by addition of 2 equiv of
B(C6F5)3 to the starting compound 6a.
The first step of the similar reaction of 6a with Al(C6F5)3
was the elimination of the neutral (THF) · Al(C6F5)3 adduct with
formation of the neutral THF-free tantalum complex
[TaBz2(NR)(OAr)] (10a), which was also characterized by NMR
spectroscopy. Clearly, the higher oxophilicity of Al vs B was
responsible for this different reaction pathway. Furthermore, the
formation of the THF-free compound 10a from 6a demonstrated
that coordination of the THF ligand stabilizes the aryloxo
compounds 5 and 6 with respect to the less electron-deficient
tribenzyl derivatives 3 and 4, although it is not an essential
requirement for their isolation. This fact could suggest that the
benzyl ligand is a better σ donor ligand than the aryloxo ligand.
The 1H and 13C NMR spectroscopic data of 10a were very close
to those observed for 6a, with slight shifts of the whole set of
resonances. Addition of another 1 equiv of Al(C6F5)3 produced
the abstraction of one of the benzyl ligands to give the
corresponding zwitterionic compound [TaBz(NtBu)(OAr){η6-
In addition, the reactivity toward Lewis acids was studied
for the aryloxo imido compounds [MBz2(NR)(OAr)(THF)] (M
) Nb, R ) tBu (5a); M ) Ta, R ) tBu (6a), Ar (6b)). However,
only the tantalum derivative 6a reacted successfully with the
Lewis acids E(C6F5)3 (E ) B, Al) (Scheme 3). Addition of 1
equiv of B(C6F5)3 to a C6D6 solution of 6a gave an insoluble
reddish oil, which after being redissolved in BrC6D5 was
characterized by NMR spectroscopy as the ionic compound
C6H5CH2Al(C6F5)3}] (11a-Al), which showed a H and 13C
1
1
[TaBz(NtBu)(OAr)(THF)][BzB(C6F5)3] (9a). The H NMR of
NMR pattern similar to that observed for 11a-B. Furthermore,
the three resonances corresponding to the [η6-C6H5CH2Al-
(C6F5)3]-anion at δ -121.6 (Fortho), -149.9 (Fpara), and -159.4
(Fmeta) were observed in the 19F NMR spectrum.
9a showed one broad resonance for the methylene protons of
the benzyl-B group at about δ 3.20, two multiplets due to the
coordinated THF, and two doublets corresponding to the
methylene protons CH2-Nb of the niobium-bonded benzyl
group. In addition, three resonances due to the [BzB(C6F5)3]-
anion were observed in the 19F NMR spectrum with a value of
2.8 ppm for the δmeta-δpara difference, indicating that the ion-
pair interaction62 does not exist. Addition of another 1 equiv of
B(C6F5)3 to 9a produced the abstraction of THF, leading to
formation of the zwitterionic complex [TaBz(NtBu)(OAr){η6-
C6H5CH2B(C6F5)3}] (11a-B), with elimination of the
(THF) · B(C6F5)3 adduct and concomitant η6 coordination of the
C6H5 ring of the [BzB(C6F5)3]- anion. The same transformation
In contrast to the ionic compound 9a containing the free
borate anion, the new zwitterionic compounds 11a-B and 11a-
Al were soluble in C6D6. The interaction proposed for the [η6-
C6H5CH2E(C6F5)3]- anions (E ) B, Al) in both complexes was
confirmed by the upfield shifted C6H5 resonances observed in
their 1H NMR spectra. Furthermore, the 19F NMR spectrum of
11a-B showed a higher value of ca. 4.0 ppm for the δmeta-δpara
difference, in accordance with the presence of a strong ion-
pairing interaction,62 whereas three resonances corresponding
to the [η6-C6H5CH2Al(C6F5)3]- anion at δ -121.6 (Fortho),
-149.9 (Fpara), and -159.4 (Fmeta) were observed in the 19F
NMR spectrum of 11a-Al.
(61) Pellecchia, C.; Immirzi, A.; Grassi, A.; Zambelli, A. Organome-
tallics 1993, 12, 4473.
The reactivity described here for these group 5 metal imido
complexes resembles that reported for the zirconium derivative
(62) Horton, A. D.; de With, J.; van der Linden, A. J.; van der Weg, H.
Organometallics 1996, 15, 2672.

Zwitterionic Imido Group 5 Metal Complexes
Organometallics, Vol. 27, No. 7, 2008 1421
Scheme 4. Reaction of the Zirconium Tribenzyl Monocy-
clopentadienyl Complex with Lewis Acids E(C6F5)3 (E ) B,
Al)
[ZrCpBz3], which also yielded the analogous zwitterionic η6-
arene compound [ZrCpBz2{η6-C6H5CH2B(C6F5)3}] (12-B),
when reacting with B(C6F5)3.61 In view of these results, we
decided to complete our previous studies investigating the related
reaction of [ZrCpBz3] with Al(C6F5)3 (Scheme 4). The reaction
was carried out in C6D6 and monitored by NMR spectroscopy
at ambient temperature to give the expected zwitterionic
complex [ZrCpBz2{η6-C6H5CH2Al(C6F5)3}] (12-Al), which was
characterized by NMR spectroscopy. The 1H spectrum showed
broad signals resulting from the formation of 12-Al, and the
19F NMR spectrum presented three resonances corresponding
to the [η6-C6H5CH2Al(C6F5)3]- anion. However, the 1H NMR
spectrum obtained from a similar experiment in CD2Cl2 at -70
°C clearly showed three multiplets corresponding to the η6-
coordinated phenyl ring of the benzylborate anion and two
doublets for the diastereotopic methylene protons of the metal-
bound benzyl ligands. The most remarkable difference from the
related group 5 metal compounds is that the 1JC-H value of the
Zr-CH2 group (122 Hz) and the chemical shift of the phenyl
ipso carbon of this benzyl ligand (δ 148.5) show the typical
values expected for η1-coordinated benzyl ligands in contrast
to the η2 interaction found for the group 5 metal imido
derivatives 7 and 8, probably due to the bulkiness of the
cyclopentadienyl ligand.
Theoretical Calculations. Optimized geometries of the most
stable conformers for the complexes [NbMe2(NtBu){η6-
C6H5CH2B(C6F5)3}] (I) and [ZrMe2Cp{η6-C6H5CH2B(C6F5)3}]
(II) are shown in Figure 2, and calculated values of bond
distances and angles are collected in Table 2. These complexes
are models for the zwitterionic compounds [NbBz2(NtBu){η6-
C6H5CH2B(C6F5)3}] (7a-B) and [ZrCpBz2{η6-C6H5CH2B-
(C6F5)3}] (12-B), respectively, where the benzyl groups were
replaced by methyl ligands. The complexes show a pseudotet-
rahedral arrangement of ligands around the metal center. The
calculated Nb-N distance (1.743 Å) and Nb-N-C angle
(180.0°) are consistent with a Nbt N triple bond, and they are
essentially the same as those experimentally found for the similar
complex 3a (1.748 Å and 180.0° in Figure 1, respectively). The
spatial orientation of the arene ligand with respect to the metal
fragmentinbothcomplexescanbedefinedusingtheN-Nb-X-C1
and X2-Zr-X1-C1 dihedral angles (where X stands for the
centroid of the different rings). As can be seen in Figure 2, the
spatial positions of the arene ligand are roughly the same for
both complexes (64.8 and 64.1° for complexes I and II,
respectively). The distance between the metal atom and the arene
centroid is slightly longer for complex II (2.302 versus 2.269
Å). The Nb-Carene distances can be grouped in two sets, one
with short distances Nb-C1, Nb-C2, and Nb-C3 (2.646,
2.521, and 2.567 Å, respectively) and other set of long distances
Nb-C4, Nb-C5, and Nb-C6 (2.721, 2.814, and 2.742 Å,
respectively). However, the Zr-Carene distances are closer to
each other. Furthermore, the standard deviation of the distances
between the metal center and the carbon atoms belonging to
the arene ligand (0.111 and 0.026 Å for complexes I and II,
respectively) indicate a less symmetric coordination of the arene
Figure 2. DFT-optimized geometries of the complexes (a) [NbMe2-
(NtBu){η6-C6H5CH2B(C6F5)3}] (I) and (b) [ZrMe2Cp{η6-C6H5-
CH2B(C6F5)3}] (II).
Table 2. Calculated Values of Bond Distances (Å) and Angles (deg)
of the Complexes [NbMe2(NtBu){η6-C6H5CH2B(C6F5)3}] (I) and
[ZrMe2Cp{η6-C6H5CH2B(C6F5)3}] (II)a
M ) Nb
M ) Zr
M ) Zrb
Nb-N
Zr-X2
M-Me
1.742
2.200
2.258
2.260
2.302
2.719
2.668
2.663
2.702
2.726
2.707
1.687
1.472
114.2
133.1
2.180
2.166
2.170
2.269
2.646
2.521
2.567
2.721
2.814
2.742
1.690
1.470
113.8
M-X1
2.300
2.860
2.730
2.610
2.663
2.680
2.740
1.690
1.490
112.0
126.0
M-C1
M-C2
M-C3
M-C4
M-C5
M-C6
C-B
C1-B
C1-C-B
X1-Zr-X2
N-Nb-X
X2-Zr-X1-C1
N-Nb-X-C1
126.5
64.1
64.8
a X stands for the position of the centroid in the corresponding ring,
X1 for η6-C6H5CH2B, and X2 for η5-C5H5. b Experimental values for
[ZrBz2Cp{η6-C6H5CH2B(C6F5)3}].61
ligand to the metal fragment for complex I with respect to
complex II. In Table 2, a comparison of the geometry of

1422 Organometallics, Vol. 27, No. 7, 2008
Arteaga-Müller et al.
Figure 3. Molecular orbital diagram for the formation of complexes I and II from the corresponding fragments.
2
complex II and the structure obtained by Pellecchia et al. for
the zwitterionic complex [ZrBz2Cp{η6-C6H5CH2B(C5F5)3}] is
given.61 In general, the experimental and calculated structures
are quite similar; only differences of around 0.1 Å and 5° in
distances and angles, respectively, are observed.
Frontier Orbitals of Complexes I and II. In order to gain
further insight into the metal–ligand bonding, a fragment
molecular orbital approach is applied to establish the frontier
orbitals of complexes I and II. Thus, the [NbMe2(NtBu){η6-
C6H5CH2B(C6F5)3}] complex (I) can be seen as being formed
by two different fragments, the cationic fragment [NbMe2-
(NtBu)]+ (I.a) and the anionic fragment [C6H5CH2B(C6F5)3]-
(I.b), respectively. Similarly, [ZrMe2Cp{η6-C6H5CH2B(C6F5)3}]
complex (II) is made from the component fragments
[ZrCpMe2]+ (II.a) and [C6H5CH2B(C6F5)3]- (I.b), respectively.
The LUFO (lowest unoccupied fragment orbital) and
between the empty LUFO I.a (40.7%), which is mainly a dz
orbital, and LUFO+3 I.b (11.9%) with a small contribution of
the HOFO I.b (2.0%). The total Mülliken overlap population
for this orbital is -0.008e which indicates a slightly antibonding
character for this MO. For complex II, the corresponding
2
2
LUMO orbital rises from mixing LUFO+3 II.a (dx -y , 22.3%)
and LUFO II.a (dz , 25.3%) with the LUFO+3 I.b (17.3%)
2
orbital but without the participation of the HOFO I.b. The
Mülliken overlap population for the LUMO of complex II has
a value of 0.055e. In contrast, LUMO orbital energies in both
complexes are rather similar (-4.27 and -4.04 eV for com-
plexes I and II, respectively). Hence, a donor molecule might
yield a more stable donor–acceptor bond in complex II due to
the bonding character of the LUMO orbital.
Another important ligand-to-metal interaction between the
fragments defined above in both complexes arises from the
interaction between HOFO I.a (HOFO I.b), LUFO+1 I.a
(LUFO I.b), and HOFO I.b orbitals giving predominantly the
LUMO+3 and HOMO-6 molecular orbitals in complexes I (II).
The character of LUMO+3 is strongly antibonding for both
complexes (-0.220e and -0.516e for complexes I and II,
respectively), while for HOMO-6 a slightly antibonding char-
acter is observed (-0.016e and -0.005 e for complexes I and
II, respectively), probably due to the repulsive interaction
between HOMO orbitals of the two fragments. The interaction
between the HOFO and HOFO-8 of fragment I.b with HOFO-
0-HOFO-4 for both metal fragments gives an increase in energy
(destabilization) of the molecular orbital responsible of Nb-imido
and Zr-Cp bonds (HOMO-7–HOMO-10 set in Figure 3).
2
2
2
LUFO+1 from fragment I.a are essentially pure dz and dx -y
orbitals from the metal, respectively. Other unoccupied
LUFOs for I.a are antibonding π* orbitals arising from the
interaction between dxz, dyz, and dxy metal orbitals and
occupied π orbitals of the imido group, fundamentally px,
py, and pz orbitals from the nitrogen atom. The corresponding
occupied π orbitals for the metal-imido bond, HOFO,
HOFO+1, and HOFO+3 (highest occupied fragment orbital),
show a clear bonding character arising from the interaction
between Nb d and N p atomic orbitals. Frontier molecular
orbitals for the II.a fragment are quite similar to those in
fragment I.a. The HOFO and HOFO+1 orbitals for the I.b
fragment have π* character on the benzyl ring.
A simplified molecular orbital correlation diagram is depicted
in Figure 3. The isolobal and isoelectronic character of both
complexes is clear, although some differences are observed. The
LUMO orbital for complex I arises mainly from the interaction
Finally, the total Mülliken overlaps for complexes I and II
are positive (0.240e and 0.208 e, respectively,) pointing to a
stable interaction between fragments.

Zwitterionic Imido Group 5 Metal Complexes
Organometallics, Vol. 27, No. 7, 2008 1423
Table 3. Polymerization of MMA with Imido Complexes using E(C6F5)3 (E ) B, Al) as Cocatalysta
b
run
[M]
E
T (°C)
t (h)
yield (%)
Mw/Mn
104Mn
104 Mn(calcd)c
rr (%)d
mr (%)d
mm (%)d
1
3a
3a
3a
3a
6a
6a
Al
Al
Al
Al
B
-40
0
5
5
5
16
16
5
37
30
40
50
14
10
1.25
2.08
1.28
1.35
1.47
1.47
2.03
2.93
2.01
2.01
5.21
3.84
0.76
0.76
0.76
1.04
0.29
0.21
60
72
62
70
61
58
34
27
33
23
32
33
6
2
7
7
7
9
2e
3
0
4
5
6
25
25
40
Al
a Polymerization conditions: Complex [M] (4.8 × 10-5 mol) and E(C6F5)3 (9.6 × 10-5 mol) premixed in toluene (3 mL), then MMA (1 g;
[MMA]:[M] ) 206:1). b Determined by GPC in THF vs polystyrene standards. c Mn(calcd) values ) conv × [MMA]/[M] × 100.81 (100.81 is the
formula weight of MMA); d Determined by 1H NMR in CDCl3. e CH2Cl2.
Formation Energies for Complexes I and II. Electronic
formation energies of the complexes I and II, with respect to
the isolated fragments, are -117.0 and -113.8 kcal/mol,
reflecting the exergonic nature of the bond between them. The
stabilization energy for complex I is 3.2 kcal/mol higher than
that corresponding to complex II, indicating a more difficult
separation of the ionic pair for complex I with respect to
complex II.
The lesser donor capacity of the NAr imido ligand may
explain the lack of activity of 3b, in agreement with the easy
decomposition observed when it was reacted with E(C6F5)3. The
different activities for MMA polymerization shown by the
niobium complex 3a and the zirconium complex [ZrCpBz3]
could be related to the greater kinetic inertness of the Zr-benzyl
bond in [ZrCpBz3]. Finally, the activation of 6a by B(C6F5)3
suggests that in this case the tantalum center is responsible for
the polymerization activity.
MMA Polymerization. The niobium tribenzyl imido com-
plex [NbBz3(NtBu)] (3a) activated by addition of 2.0 equiv of
Al(C6F5)3 showed rather low catalytic activity for MMA
polymerization, whereas only traces of PMMA were observed
when the isolobal zirconium derivative [ZrCpBz3] was used for
comparison and when the related niobium imido (3b) and
tantalum imido (4) complexes were studied under similar
conditions. In contrast, no activation was observed for any of
these compounds when B(C6F5)3 was used as the Lewis acid.
The catalytic activity of complex 3a was variable at different
times and temperatures, the best result being obtained at 25 °C
for 16 h with a moderate yield of PMMA (run 4). Shorter
periods of time and lower temperatures (Table 3, runs 1 and 3)
gave lower yields, being significantly much lower when the time
was shorter than 5 h. However, periods of time longer than 16 h
did not improve the yield, and even lower yields were obtained
at temperatures higher than 25 °C, indicating the decomposition
of the active species with both time and temperature. The
polymers obtained were mainly syndiotactic, with higher
syndiotacticity under the conditions corresponding to run 3.
When CH2Cl2 instead of toluene was employed as the solvent,
the polymer obtained was also syndiotactic (over 70%) but in
a lower yield and with a higher polydispersity (run 2 vs 3),
probably as a consequence of the instability of the Nb-C and
Al-C bonds in this solvent. In contrast, the only tantalum
aryloxo benzyl derivative isolated, 6a, showed catalytic activity
with both Lewis acids (run 5 and 6), but again with rather poor
yields of syndiotactic PMMA.
Activation of 3a by Al(C6F5)3 but not by B(C6F5)3 suggests
either the formation of an enolate species by benzyl transfer to
a MMA · Al(C6F5)3 adduct37,63 or the [4 + 2] addition of MMA
to the metal-imido bond, generating a metallacycle.39 In the
case of the neutral complexes 3–6 presented in this work, none
of them reacted with MMA. Furthermore, we monitored the
reaction of MMA · Al(C6F5)3 with 3a in C6D6 by NMR
spectroscopy, obtaining a mixture of two compounds. Although
we have not been able to characterize these compounds, the
tBu 13C resonances at δ 71.8 and δ 72.4 confirmed the presence
of the imido moiety, in agreement with benzyl transfer to the
MMA · Al(C6F5)3 adduct. Unfortunately, the 19F NMR spectrum
did not allow us to determine the presence of the corresponding
enol aluminate anions, due to the number of resonances.
Conclusions
Benzylation with [BzMgCl] of the niobium trichloro imido
complexes [NbCl3(NR)(py)2] (R ) tBu, Ar; Ar ) 2,6-iPr2C6H3)
previously reported and of the new niobium and tantalum
dichloro imido derivatives [MCl2(NR)(OAr)(py)2] (M ) Nb,
Ta; R ) tBu, Ar) gave the corresponding benzyl complexes
[NbBz3(NR)] and [MBz2(NR)(OAr)(THF)], respectively. The
resulting yields depended on the donor capacity of the imido
ligand, being moderate to good for the tert-butyl but low for
the aryl derivatives. The lower σ donor ability of the aryloxo
group compared with the benzyl ligand was also responsible
for the coordination of one additional THF ligand to give the
trigonal-bipyramidal pentacoordinate aryloxo complexes.
One of the benzyl groups of the new isolated imido
compounds was abstracted by the Lewis acids E(C6F5)3 (E )
B, Al) generating the stable zwitterionic complexes [MBz2-
(NtBu){η6-C6H5CH2E(C6F5)3}] (M ) Nb, Ta; E ) B, Al) and
[TaBz(NtBu)(OAr){η6-C6H5CH2E(C6F5)3}] (E ) B, Al) con-
taining the η6-coordinated phenyl ring of the benzylborate anion.
When these reactions were carried out with imido complexes
containing O- and C-donor competing ligands (THF and benzyl,
respectively), abstraction of THF by the more oxophilic
Al(C6F5)3 Lewis acid was the preferred first step of the reaction,
whereas the benzyl ligand was first abstracted by the less
oxophilic B(C6F5)3 reagent. Formation of similar zwitterionic
compounds was studied by reactions of the zirconium derivative
[ZrCpBz3] with the same Lewis acids E(C6F5)3 (E ) B,47 Al).
DFT calculations showed that the cationic fragments
[NbMe2(NtBu)]+ and [ZrCpMe2]+ and the zwitterionic com-
pounds [NbMe2(NtBu){η6-C6H5CH2B(C6F5)3}] (I) and [Zr-
Me2Cp{η6-C6H5CH2B(C6F5)3}] (II) were isolobal and isoelec-
tronic, respectively. However, both theoretical and NMR data
showed a small difference in the coordination of the η6-benzyl
ligand to the niobium atom with respect to the zirconium
compound. The antibonding character of the LUMO orbital in
complex I shows an unfavorable reaction with donor monomers.
However, the LUMO orbital in complex II has a remarkable
bonding character, producing stable interactions with donor
ligands. Reactions which imply separation of the ionic pair
should be more difficult for complex I, due to the higher
stabilization energy compared with complex II.
Polymerization of methyl methacrylate was achieved with
moderate yield for the catalytic system [NbBz3(NtBu)]/Al(C6F5)3
(63) Rodríguez-Delgado, A.; Chen, E. Y. X. Macromolecules 2005, 38,
2587.

1424 Organometallics, Vol. 27, No. 7, 2008
Arteaga-Müller et al.
124.3 (m-py), 137.8 and 138.2 (p-py), 137.9 (o-C6H3), 151.2 and
153.3 (o-py), 160.0 (i-C6H3). Anal. Calcd for C26H36Cl2N3OTa
(658.44): C, 47.43; H, 5.51; N, 6.38. Found: C, 47.20; H, 5.46; N,
6.39.
and with low yields for the systems [TaBz(NtBu)(OAr)(THF)]/
E(C6F5)3 (E ) B, Al); meanwhile, the zirconium monocyclo-
pentadienyl derivative [ZrCpBz3] only produced traces of
PMMA. Two main reasons can be advanced for these results:
the reactivity of the metal-imido bond toward unsaturated
systems which could interfere with polymerization and the
kinetic inertness of the metal-benzyl bond with respect to the
usually more active metal-methyl bonds, although we have not
been able to isolate this last type of compound. Further studies
aimed at a better understanding of the polymerization process
are in progress.
[NbBz3(NtBu)] (3a). BzMgCl (2 M in THF, 6.99 mmol) was
added to a solution of [NbCl3(NtBu)(py)2] (1.00 g, 2.33 mmol) in
20 mL of Et2O at -78 °C. The resulting yellow mixture was stirred
for 16 h at 25 °C, after which the solution was filtered over Celite
and the volatiles were removed. The solid product was washed with
pentane and dried in vacuo, yield 0.81 g (80%). 1H NMR: 1.35 (s,
9H, CMe3), 1.68 (s, 6H, CH2Ph), 6.57 (m, 3H, C6H5), 6.98 (m,
12H, C6H5). 13C NMR: 31.8 (CMe3), 53.3 (CH2Ph), 125.4 (p-C6H5),
129.4 (m-C6H5), 130.1 (o-C6H5), 136.0 (i-C6H5), the i-CMe3
resonance was not observed. Anal. Calcd for C25H30NNb (437.42):
C, 68.65; H, 6.91; N, 3.20. Found: C, 68.50; H, 6.87; N, 3.20.
[NbBz3(NAr)] (3b; Ar ) 2,6-iPr2C6H3). The same procedure
described for 3a was followed, using [NbCl3(NAr)(py)2] (1.00 g,
1.88 mmol) in 20 mL of Et2O and BzMgCl (2 M in THF, 5.54
mmol), affording 3b as a clear yellow solid (0.22 g, 25%). 1H NMR:
1.34 (d, J ) 6.9 Hz, 12H, Me2CH), 1.90 (s, 6H, CH2Ph), 4.20 (sept,
J ) 6.9 Hz, 2H, Me2CH), 6.62 (m, 3H, C6H5), 7.00–7.20 (m, 15H,
C6H3, C6H5). 13C NMR: 24.4 (Me2CH), 26.3 (Me2CH), 28.4
(CH2Ph), 123.0, 124.5, 126.2, 128.5, 128.8, 130.3, (C6H3, C6H5),
134.9 (i-C6H5), 144.9 (i-C6H3). Anal. Calcd for C33H38NNb
(541.57): C, 73.19; H, 7.07; N, 3.59. Found: C, 73.25; H, 7.07; N,
2.53.
[NbBz2(NtBu)(OAr)(THF)] (5a; Ar ) 2,6-iPr2C6H3). BzMgCl
(2 M in THF, 1.40 mmol) was added to a stirred solution of
[NbCl2(NAr)(OAr)(py)2] (1b; 0.40 g, 0.70 mmol) in 30 mL of Et2O
at -78 °C. The reaction slurry was stirred for 16 h at 25 °C, and
then 10 mL of hexane was added. The solution was filtered, and
the volatiles were removed at reduced pressure to give 5a as a
yellow solid (0.12 g, 30%). 1H NMR: 0.92 (s, 9H, CMe3), 1.31 (d,
J ) 6.9 Hz, 12H, Me2CH), 1.36 (m, 4H, ꢀ-CH2, THF), 1.96 (d, J
) 10.0 Hz, 2H, CH2Ph), 2.94 (d, J ) 10.0, 2H, CH2Ph), 3.46 (sept,
J ) 6.9 Hz, 2H, Me2CH), 3.70 (m, 4H, R-CH2, THF), 6.98–7.20
(m, 13H, C6H3 and C6H5). 13C NMR: 23.3 (Me2CH), 25.3 (ꢀ-CH2,
THF), 27.1 (Me2CH), 31.3 (CMe3), 69.1 (R-CH2, THF), 69.2
(CH2Ph), 121.3, 123.1, 123.9, 128.9, 129.1, 135.7 (C6H3, C6H5),
144.0 (i-C6H5), 162.1 (i-C6H3); the i-CMe3 resonance was not
observed. Anal. Calcd for C34H48NNbO2 (595.66): C, 68.56; H,
8.12; N, 2.35. Found: C, 68.80; H, 8.20; N, 2.10.
Experimental Section
General Considerations. All manipulations were carried out
under an argon atmosphere, and solvents were distilled from
appropriate drying agents. NMR spectra were recorded at 400.13
(1H), 376.70 (19F), and 100.60 (13C) MHz on a Bruker AV400.
Chemical shifts (δ) are given in ppm using C6D6 as solvent, unless
otherwise stated. 1H and 13C resonances were measured relative to
solvent peaks considering TMS at 0 ppm; meanwhile 19F resonances
were measured relative to external CFCl3. Assignment of resonances
was made from HMQC and HMBC NMR experiments. Elemental
analyses were performed on a Perkin-Elmer 240C. The compounds
[TaCl2(NAr)(OAr)(py)2],49 [MCl3(NR)(py)2] (M ) Nb, Ta; R )
tBu, Ar),49,50 B(C6F5)3,64 and Al(C6F5)3 · 0.5(toluene)17 were pre-
pared by literature methods.
[NbCl2(NtBu)(OAr)(py)2] (1a; Ar ) 2,6-iPr2C6H3). LiOAr ·
THF (0.60 g, 2.33 mmol) in Et2O/THF (20 mL/10 mL) was added
to a solution of [NbCl3(NtBu)(py)2] (1.00 g, 2.33 mmol) in Et2O/
THF (15:5 mL) at 25 °C. The reaction mixture was stirred for a
further 16 h. The resulting yellow solution was separated by
filtration, and the volatiles were removed under reduced pressure.
The residue was washed with hexane (3 × 15 mL) and then dried
to give 1a as a yellow solid (1.06 g, 80%). 1H NMR: 1.24 (s, 9H,
CMe3), 1.44 (d, J ) 6.9 Hz, 12H, Me2CH), 4.46 (sept, J ) 6.9 Hz,
2H, Me2CH), 6.45 (m, 4H, m-py), 6.75 (m, 2H, p-py), 7.05 (m,
1H, p-C6H3), 7.25 (m, 2H, m-C6H3), 9.14 (m, 4H, o-py). 13C NMR:
24.3 (Me2CH), 26.4 (Me2CH), 30.3 (CMe3) 68.5 (CMe3), 121.9 (p-
C6H3), 123.6 (m-C6H3), 123.7 and 127–6 (m-py), 137.3 (o-C6H3),
137.8 and 137.9 (p-py), 151.9 and 152.8 (o-py), 162.0 (i-C6H3).
Anal. Calcd for C26H36Cl2N3NbO (569.13): C, 54.75; H, 6.36; N,
7.37. Found: C, 55.42; H, 6.20; N, 7.42.
[NbCl2(NAr)(OAr)(py)2] (1b; Ar ) 2,6-iPr2C6H3). Following
the same methodology described above, complex 1b was obtained
from the reaction of [NbCl3(NAr)(py)2] (1.00 g, 1.88 mmol) in 20
mL of Et2O/THF (2/1) with LiOAr · THF (0.48 g, 1.88 mmol) in
30 mL of Et2O/THF (2/1) as a yellow solid (1.01 g, 80%). 1H NMR:
1.25 (m, 24H, Me2CH), 4.33 (m, 4H, Me2CH), 6.28–6.50 (m, 8H,
py), 7.00 (m, 2H, p-C6H3), 7.05 (m, 4H, m-C6H3), 8.80 (m, 4H, o-
C6H3), 9.10 (m,2H, o-py). 13C NMR: 24.3, 24.8 (Me2CH), 26.1,
28.0 (Me2CH), 122.4, 122.9, 123.6, 124.3, 125.67, 137.6, 138.1,
147.2, 150.8, 152.6 (C6H3 and py), 151.3 (i-C6H3N), 160.4
(i-C6H3O). Anal. Calcd for C34H40Cl2N3NbO (670.51): C, 60.90;
H, 6.01; N, 6.27. Found: C, 60.66; H, 5.85; N, 6.42.
[TaBz2(NtBu)(OAr)(THF)] (6a; Ar ) 2,6-iPr2C6H3). The same
procedure described above for 5a was followed using
[TaCl2(NtBu)(OAr)(py)2] (0.40 g, 0.60 mmol) and BzMgCl (2 M
in THF, 1.21 mmol) to give 6a as a yellow solid (0.14 g, 35%). 1H
NMR: 0.92 (s, 9H, CMe3), 1.29 (d, J ) 6.9 Hz, 12H, Me2CH),
1.30 (m, 4H, ꢀ-CH2, THF), 1.90 (d, J ) 11.4 Hz, 2H, CH2Ph),
2.74 (d, J ) 11.4 Hz, 2H, CH2Ph), 3.44 (sept, J ) 6.9 Hz, 2H,
Me2CH), 3.66 (m, 4H, R-CH2, THF), 6.96–7.12 (m, 13H, C6H3
and C6H5). 13C NMR: 23.5 (Me2CH), 25.4 (ꢀ-CH2, THF), 27.3
(Me2CH), 32.6 (CMe3), 66.0 (CMe3), 69.5 (R-CH2, THF), 121.8,
123.2, 123.5, 129.6, 129.9, 135.9 (C6H3, C6H5), 146.0 (i-C6H5),
160.6 (i-C6H3). Anal. Calcd for C34H48NO2Ta (683.70): C, 59.73;
H, 7.08; N, 2.05. Found: C, 60.02; H, 7.42; N, 2.09.
[TaCl2(NtBu)(OAr)(py)2] (2a; Ar ) 2,6-iPr2C6H3). Following
the same methodology described above, complex 2a was obtained
from the reaction of [TaCl3(NtBu)(py)2] (1.00 g, 1.52 mmol) in 20
mL of Et2O/THF (2/1) with LiOAr · THF (0.50 g, 1.93 mmol) in
30 mL of Et2O/THF (2/1) as a yellow solid (1.01 g, 80%). 1H NMR:
1.31 (s, 9H, CMe3), 1.44 (d, J ) 6.9 Hz, 12H, Me2CH), 4.41 (sept,
J ) 6.9 Hz, 2H, Me2CH), 6.36 and 6.51 (m, 4H, m-py), 6.69 and
6.79 (m, 2H, p-py), 7.04 (m, 1H, p-C6H3), 9.11 (m, 4H, o-py), 7.28
(m, 2H, m-C6H3). 13C NMR: 24.5 (Me2CH), 26.3 (Me2CH), 32.4
(CMe3), 68.1 (CMe3), 121.7 (p-C6H3), 123.6 (m-C6H3), 123.9 and
[TaBz2(NAr)(OAr)(THF)] (6b; Ar ) 2,6-iPr2C6H3). The same
procedure described for 5a was followed using [TaCl2-
(NAr)(OAr)(py)2] (0.40 g, 0.52 mmol) and BzMgCl (2 M in THF,
1.04 mmol), giving complex 6b as an orange solid (0.080 g, 20%).
1H NMR: 1.25 (m, 24H, Me2CH), 1.30 (m, 4H, ꢀ-CH2, THF), 2.80
(m, 4H, CH2Ph), 3.57 (m, 4H, R-CH2, THF), 3.91 (sept, J ) 6.6
Hz, Me2CH), 4.10 (sept, J ) 6.5 Hz, 2H, Me2CH), 6.08–7.18 (m,
16H, C6H3 and C6H5). We were not able to obtain an adequate
elemental analysis for compound 6b.
[NbBz2(NtBu){η6-C6H5CH2B(C6F5)3}] (7a-B). In a glovebox,
C6D6 solutions of the compounds [NbBz3(NtBu)] (3a; 0.017 g,
0.034 mmol) and B(C6F5)3 (0.015 g, 0.034 mmol) were loaded into
(64) Lancaster, S. In www.syntheticpages.org, 2003, 215.

Zwitterionic Imido Group 5 Metal Complexes
Organometallics, Vol. 27, No. 7, 2008 1425
an NMR tube. The reaction was followed by NMR spectroscopy,
resulting in the immediate formation of complex 7a-B (100%). The
solution was layered with hexane to give a microcrystalline yellow
) 11.4 Hz, 2H, PhCH2), 2.61 (d, J ) 11.4 Hz, 2H, PhCH2), 3.40
(m, 6H, Me2CH and R-CH2 THF), 6.90–7.15 (m, 18H, C6H3 and
C6H5). 13C NMR: 23.5 (Me2CH), 24.9 (Me2CH), 27.1 (ꢀ-CH2,
THF), 33.8 (CMe3), 65.1 (R-CH2, THF), 67.5 (CMe3), 73.8
(CH2Ph), 122.9, 123.3, 125.3, 125.6, 128.5, 129.3, 129.5, 129.7,
136.9, 147.1 (C6H3, C6H5), 139.8 (i-C6H5), 158.4 (i-C6H3), 137.1
(C6F5), 142.0 (C6F5), 150.0 (C6F5).
1
solid (0.018 g, 50%). H NMR: 0.80 (s, 9H, CMe3), 1.02 (d, J )
9.0 Hz, 2H, PhCH2Nb), 1.74 (d, J ) 9.0 Hz, 2H, PhCH2Nb), 3.28
(m, 2H, PhCH2B), 5.44 (m, 1H, p-C6H5CH2B), 5.67 (m, 2H, m-
C6H5CH2B), 6.34 (m, 4H, o-C6H5CH2Nb), 6.49 (m, 2H,
o-C6H5CH2B), 6.84 (m, 4H, m-C6H5CH2Nb), 6.98 (m, 2H,
p-C6H5CH2Nb). 13C NMR: 30.6 (CMe3), 36.0 (CH2B), 45.7
(CH2Nb), 71.2 (CMe3) 117.2 (p-C6H5CH2B), 122.9 (o-C6H5CH2B),
126.2 (m-C6H5CH2B), 128.4 (C6H5CH2Nb), 129.6 (C6H5CH2Nb),
131.2 (i-C6H5CH2Nb), 136.5 (C6F5), 138.9 (C6F5), 148.0 (C6F5).
19F NMR: -129.5 (m, 6F, o-C6F5), -159.2 (m, 3F, p-C6F5), -163.4
(m, 6F, m-C6F5). Anal. Calcd for C43H30BF15NNb (949.40): C,
54.40; H, 3.18; N, 1.48. Found: C, 54.13; H, 3.00; N, 1.29.
[NbBz2(NtBu){η6-C6H5CH2Al(C6F5)3}] (7a-Al). The same pro-
cedure described for 7a-B was followed using [NbBz3(NtBu)] (3a;
0.017 g, 0.034 mmol) and Al(C6F5)3 (0.017 g, 0.034 mmol),
resulting in the immediate formation of complex 7a-Al (100% by
1H NMR). 1H NMR: 0.71 (s, 9H, CMe3), 0.84 (d, J ) 8.7 Hz, 2H,
PhCH2Nb), 1.83 (d, J ) 8.7 Hz, 2H, PhCH2Nb), 2.76 (m, 2H,
PhCH2Al), 5.21 (m, 1H, p-C6H5CH2Al), 5.81 (m, 2H, m-
C6H5CH2Al), 6.24 (m, 4H, o-C6H5CH2Nb), 6.30 (m, 2H,
o-C6H5CH2Al), 6.81 (m, 4H, m-C6H5CH2Nb), 6.99 (m, 2H,
p-C6H5CH2Nb). 13C NMR: 30.8 (CMe3), 33.9 (PhCH2Al), 47.7
(CH2Nb), 70.9 (CMe3), 110.4 (p-C6H5CH2Al), 120.6 (o-
C6H5CH2Al), 124.9 (m-C6H5CH2Al), 128.5 (C6H5CH2Nb), 129.3
(C6H5CH2Nb), 129.7 (C6H5CH2Nb), 130.9 (C6H5CH2Nb), 137.2
(C6F5), 141.2 (C6F5), 150.4 (C6F5), 159.6 (C6F5). 19F NMR: -119.9
(m, 6F, o-C6F5), -153.5 (m, 3F, p-C6F5), -160.8 (m, 6F, m-C6F5).
[TaBz2(NtBu){η6-C6H5CH2Al(C6F5)3}] (8a-Al). The same pro-
cedure described for 7a-B was followed using [TaBz3(NtBu)] (4a;
0.016 g, 0.028 mmol) and Al(C6F5)3 (0.015 g, 0.028 mmol),
resulting in the immediate formation of complex 8a-Al (100% by
[TaBz(NtBu)(OAr){η6-C6H5CH2E(C6F5)3}] (E ) B (11a-B),
Al (11a-Al); Ar ) 2,6-iPr2C6H3). In a glovebox, C6D6 solutions
of compounds 6a (0.015 g, 0.022 mmol) and E(C6F5)3 (E ) B, Al;
0.044 mmol), were added to a Teflon-valved NMR tube. The
reaction was monitored by NMR spectroscopy to determine the
formation of compounds 11a-E and the adducts THF · E(C6F5)3.
1
Complex 11a-B: H NMR 1.10 (m, 4H, ꢀ-CH2, THF), 1.18 (m,
21H, Me2CH and CMe3), 2.33 (d, J ) 8.7 Hz, 1H, PhCH2Ta), 3.24
(m, 6H, R-CH2 THF and PhCH2B), 3.46 (m, 2H, Me2CH), 3.62
(d, J ) 8.7 Hz, 1H, PhCH2Ta), 6.20–7.20 (m, 13H, C6H3 and C6H5);
13C NMR 22.9 (Me2CH), 23.1 (Me2CH), 27.3 (ꢀ-CH2, THF), 29.0
(PhCH2B), 32.9 (CMe3), 59.0 (R-CH2, THF), 76.0 (PhCH2Ta),
123.3, 123.5, 124.8, 125.6, 126.3, 129.3, 130.5, 131.5, 136.4, 146.3
(C6H3, C6H5), 141.6 (i-C6H5), 152.0 (i-C6H5), 137.3 (C6F5), 144.8
(C6F5), 148.5 (C6F5), the i-CMe3 resonance was not observed; 19
F
NMR -130.2 (m, 6F, o-C6F5), -154.2 (m, 3F, p-C6F5), -161.4
(m, 6F, m-C6F5). Complex 11a-Al: 1H NMR 0.98 (m, 4H, ꢀ-CH2,
THF), 1.16 (m, 12H, Me2CH), 1.19 (s, 9H, CMe3), 2.34 (d, J )
8.7 Hz, 1H, PhCH2Ta), 3.20 (m, 2H, PhCH2Al), 3.43 (m, 6H,
Me2CH and R-CH2 THF), 3.62 (d, J ) 8.7 Hz, 1H, PhCH2Ta),
6.10–7.20 (m, 18H, C6H3, C6H5). 13C NMR: 23.4 (Me2CH), 24.8
(Me2CH), 27.0 (ꢀ-CH2, THF), 29.0 (PhCH2Al), 33.0 (CMe3), 65.0
(R-CH2, THF), 67.2 (CMe3), 74.0 (PhCH2Ta), 122.8, 123.2, 125.2,
125.5, 128.4 129.1, 129.4, 129.6, 131.2, 136.2, 136.8, 137.7, 142.4,
146.9 (C6H3, C6H5), 139.6 (i-C6H5), 158.3 (i-C6H3), 137.2 (C6F5),
142.0 (C6F5), 150.5 (C6F5); 19F NMR -123.3 (m, 6F, o-C6F5),
-151.5 (m, 3F, p-C6F5), -161.1 (m, 6F, m-C6F5).
[Zr(η5-C5H5)Bz2{η6-C6H5CH2Al(C6F5)3}] (12-Al). In a glove-
box, C6D6 solutions of the compounds [ZrCpBz3] (0.018 g, 0.042
mmol) and Al(C6F5)3 (0.042 mmol) were mixed in a Teflon-valved
NMR tube. The reaction was followed by NMR spectroscopy, and
the formation of compound 12-Al was observed (100% by 1H
NMR). 1H NMR: 1.48 (d, J ) 11 Hz, 2H, PhCH2Zr), 1.59 (d, J )
11 Hz, 2H, PhCH2Zr), 2.64 (b.s., 2H, PhCH2Al), 5.21 (s, 5H, C5H5),
5.58 (b.s., 1H, p-C6H5CH2Al), 5.80 (br s, 2H, m-C6H5CH2Al), 6.16
(br s, 2H, o-C6H5CH2Al), 6.80–7.20 (m, 15H, C6H5CH2 and
C
1
1H NMR). H NMR: 0.80 (s, 9H, CMe3), 0.98 (d, J ) 10.5 Hz,
2H, PhCH2Ta), 1.69 (d, J ) 10.5 Hz, 2H, PhCH2Ta), 2.83 (m, 2H,
PhCH2Al), 5.18 (m, 1H, p-C6H5CH2Al), 5.80 (m, 2H, m-
C6H5CH2Al), 6.29 (m, 2H, o-C6H5CH2Al), 6.43 (m, 4H,
o-C6H5CH2Ta), 6.84 (m, 4H, m-C6H5CH2Ta), 7.10 (m, 2H,
p-C6H5CH2Ta). 13C NMR: 32.0 (CMe3), 34.0 (PhCH2Al), 50.7
(PhCH2Ta), 68.1 (CMe3), 109.6 (p-C6H5CH2Al), 120.4 (o-
C6H5CH2Al), 125.0 (m-C6H5CH2Al), 128.9 (C6H5CH2Ta), 129.7
(C6H5CH2Ta), 131.7 (C6H5CH2Ta), 133.7 (C6H5CH2Ta), 139.1
(C6F5), 150.1 (C6F5), 160.7 (C6F5). 19F NMR: -120.9 (m, 6F,
o-C6F5), -154.3 (m, 3F, p-C6F5), -161.7 (m, 6F, m-C6F5).
6H5Me). 19F NMR: -119.9 (m, 6F, o-C6F5), -154.5 (m, 3F,
p-C6F5), -161.4 (m, 6F, m-C6F5).
X-ray Structure Determination of 3a (C25H30NNb, Mr
)
[TaBz(NtBu)(OAr)(THF)][BzB(C6F5)3] (9a; Ar ) 2,6-iPr2-
C6H3). B(C6F5)3 (0.011 g, 0.022 mmol) was added to a solution of
compound 6a (0.015 g, 0.022 mmol) in C6D6. The precipitation of
an oil was immediately observed. This was then separated from
the C6D6 solution and dissolved in C6D5Br. The NMR spectra of
the solution at 25 °C showed the formation of complex 9a. 1H NMR
(C6D5Br): 1.08 (s, 9H, CMe3), 1.10 (d, J ) 6.7 Hz, 12H, Me2CH),
1.33 (m, 4H, ꢀ-CH2, THF), 1.50 (m, 2H, PhCH2Ta), 3.27 (m, 2H,
PhCH2-B), 3.43 (m, 2H, Me2CH), 3.83 (m, 4H, R-CH2, THF),
6.70–7.10 (m, 13H, C6H3 and C6H5). 13C NMR (C6D5Br): 23.4
(Me2CH), 25.6 (Me2CH), 26.4 (ꢀ-CH2, THF), 32.0 (PhCH2B), 33.6
(CMe3), 65.0 (R-CH2, THF), 68.7 (CMe3), 74.8 (PhCH2-Ta), 123.2,
124.6, 125.3, 127.5, 129.4, 132.5, 149.3, (C6H3, C6H5), 137.6 (i-
C6H5), 154.7 (i-C6H3), 126.7 (C6F5), 129.6 (C6F5), 131.5 (C6F5).
19F NMR (C6D5Br): -128.9 (m, 6F, o-C6F5), -162.3 (m, 3F,
p-C6F5), -165.2 (m, 6F, m-C6F5).
437.41). Single crystals of 3a suitable for the X-ray diffraction study
were grown from hexane. A yellow crystal was selected, covered
with perfluorinated ether and mounted on a Bruker-Nonius Kappa
CCD single-crystal diffractometer equipped with graphite-mono-
chromated Mo KR radiation (λ ) 0.710 73 Å). Data collection was
performed at 200(2) K: crystal dimensions 0.22 × 0.18 × 0.16
mm, trigonal crystal system, space group P31c, a ) 11.2683(13)
Å, c ) 10.2514(11) Å, V ) 1127.3(2) Å3, Z ) 2, Fcalcd ) 1.289 g
cm-3, F(000) ) 456, 9387/1724 collected/unique reflections (R(int)
) 0.0868), θ range 3.62-27.49°. Multiscan65 absorption correction
procedures were applied to the data (µ ) 0.542 mm-1, minimum/
maximum transmission 0.749/0.918). The structure was solved,
using the WINGX package,66 by direct methods (SHELXS-97) and
refined by using full-matrix least squares against F2 (SHELXL-
97).67 All non-hydrogen atoms were anisotropically refined.
Hydrogen atoms were geometrically placed and left riding on their
parent atoms. Full-matrix least-squares refinements were carried
[TaBz2(NtBu)(OAr)] (10a; Ar ) 2,6-iPr2C6H3). Al(C6F5)3
(0.012 g, 0.022 mmol) was added to a solution of compound 6a
(0.015 g, 0.022 mmol) in C6D6. The reaction was monitored by
NMR spectroscopy to determine the formation of complex 10a and
the adduct THF · Al(C6F5)3. 1H NMR: 0.98 (m, 4H, ꢀ-CH2, THF),
1.12 (s, 9H, CMe3), 1.20 (d, J ) 6.9 Hz, 12H, Me2CH), 1.87 (d, J
(65) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33.
(66) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837.
(67) Sheldrick, G. M. SHELXL-97; University of Gottingen, Gottingen,
Germany, 1998.

1426 Organometallics, Vol. 27, No. 7, 2008
Arteaga-Müller et al.
out by minimizing ∑w(Fo2 - Fc2)2 with the SHELXL-97 weighting
scheme and stopped at shift/error <0.001. The final cycle of full-
matrix least-squares refinement based on 1724 reflections and 82
parameters converged to final values of R1(F2 > 2σ(F2)) ) 0.0327,
wR2(F2 > 2σ(F2)) ) 0.0557, R1(F2) ) 0.0497, wR2(F2) ) 0.593).
Final difference Fourier maps showed no peaks higher than 0.190
e Å-3 or deeper than -0.284 e Å-3. CCDC-655227 (3a) contains
the supplementary crystallographic data for this paper. This data
can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Computational Methods. All energies and geometries were
calculated using the B3P86 hybrid DFT method as implemented
in Gaussian03.68 This method consists of two different functionals:
the Becke-3 parameter hybrid exchange functional69 and the Perdew
86 correlation functional.70 Both potentials use nonlocal exchange-
correlation corrections to the density. The current method has shown
to reproduce experimental geometries in which there are weak
coordinative bonds similar to those in the organometallic com-
pounds studied here.71 The electronic configurations of the atoms
were described by a double-ꢁ basis set on all atoms. The basis set
was augmented with a double-polarization function for all atoms
but the metal atom, in which a single (n + 1)p polarization function
was added.72,73 The core electrons were treated by the relativistic
LANL2 pseudopotential.74 The most stable arene conformers were
previously searched by scanning N-Nb-X1-C and X2-Zr-X1-C
dihedral angles for complexes I and II, respectively (see Figure 2
for nomenclature). The main difference between the complexes is
the relative position of the CH2B(C6F5)3 moiety with respect to the
metal fragment. After that, all conformers were optimized without
any constraint. Differences less than 2.0 kcal mol-1 have been found
for the different conformers, indicating an easy rotation of the arene
fragment. In the present paper, we only show the most stable
conformers for both complexes. The atomic orbital contributions
and overlap populations were calculated with AOMix software.75,76
The analysis of the MO composition in terms of occupied and
unoccupied fragment molecular orbitals (OFO and UFO, respec-
tively) and construction of molecular orbital interaction diagrams
were performed with the AOMix-CDA package.77
Polymerization of MMA. The catalyst (4.8 × 10-5 mol) and
activator E(C6F5)3 (E ) B, Al; 9.6 × 10-5 mol) were handled in
the glovebox and introduced into an ampule. The temperature
desired was adjusted, and a solution of MMA (9.35 mmol) in 3
mL of toluene was added using Schlenk-vacuum-line techniques.
The polymerization was terminated by adding MeOH/HCl. The
isolated polymer was washed first with MeOH/HCl and then with
1
MeOH/water and dried overnight in vacuo at 60 °C. A H NMR
(CDCl3) study of the polymer was carried out to determine its
tacticity. Melting temperatures of polymers were measured by
differential scanning calorimetry (DSC, Perkin-Elmer DSC6). Gel
permeation chromatography (GPC) analyses of polymer samples
were carried out in THF at 25 °C (Waters GPCV-2000).
Acknowledgment. We gratefully acknowledge Ministerio
de Educación y Ciencia (project MAT2007-60997 and
MAT2006-0400) and DGUI-Comunidad de Madrid (pro-
gramme S-0505/PPQ-0328 COMAL-CM) (Spain) for finan-
cial support. J.R. thanks CSIC for financial support through
an I3P postdoctoral tenure track and R. A. A.-M. MECD
for Fellowship. Authors are grateful to Centro Técnico de
Informática (CTI-CSIC) for allocation of CPU time on their
multiprocessor ORIGIN workstations.
(68) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J., J. A.; Vreven, T.; Kudin, K. N.;
Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.;
Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
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Revision B.05; Gaussian, Inc., Wallingford, CT, 2004.
Supporting Information Available: A CIF file giving crystal
data for 3b and figures giving relevant frontier molecular orbitals
for the fragment I.a, significant molecular orbitals for complexes I
and II, and selected NMR spectra. This material is available free
of charge via the Internet at http://pubs.acs.org.
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