Half-sandwich complexes of niobium and tantalum bearing o-xylylene, anthracene, or cyclooctatetraene: Crystal structures of (η5-C5Me5)Nb{o-(CH2) 2C6H4}Cl2, (η5-C5Me5)Ta(η 4-anthracene)(CH2Ph)2

Article abstract of DOI:10.1016/S0022-328X(97)00730-4

Half-sandwich complexes of niobium and tantalum having an extended conjugated 1,3-diene ligand such as o-xylylene, anthracene, and cyclooctatetraene have been synthesized and characterized. The molecular structures of Cp*Nb{o-(CH₂)₂C₆H₄}Cl ₂ (1) (Cp=η⁵-pentamethylcyclopentadienyl), Cp*Ta(η⁴-C₁₄H₁₀)(CH ₂Ph)₂ (14), and Cp*Nb(η⁴-C₄H₆)(η ³-C₈H₈) (16) were determined by X-ray crystallographic studies. Complexes 1 and 14 adopted a four-legged piano stool geometry and their o-xylylene and anthracene ligands coordinated to the metal center in the η⁴-coordination mode. X-ray analysis together with their NMR spectral data revealed that the o-xylylene complex 1 has a large contribution of the 2σ-1π canonical form, but otherwise the anthracene complexes have an increased contribution of 2π-η⁴-diene canonical form compared with the butadiene complexes. Thus, the electronic structures of η⁴-o-xylylene and η⁴-anthracene ligands are deviated from that of η⁴-butadiene into the opposite direction. The ¹H NMR singlet signal of the cyclooctatetraene ligand in 16 indicated the presence of the dynamic fluxionality in solution, while it coordinated to the metal in η³-fashion in the crystal.

Full text of DOI:10.1016/S0022-328X(97)00730-4

Journal of Organometallic Chemistry 557 (1998) 3–12
Half-sandwich complexes of niobium and tantalum bearing o-xylylene,
anthracene, or cyclooctatetraene: crystal structures of
(p5-C5Me5)Nb{o-(CH2)2C6H4}Cl2,
(p5-C5Me5)Ta(p4-anthracene)(CH2Ph)2, and
(p5-C5Me5)Nb(p4-butadiene)(p3-cyclooctatetraene)
Kazushi Mashima a,*, Yuushou Nakayama b, Michitaka Kaidzu b, Naoko Ikushima b,
Akira Nakamura b
a Department of Chemistry, Graduate School of Engineering Science, Osaka Uni6ersity, Toyonaka, Osaka, 560, Japan
b Department of Macromolecular Science, Graduate School of Science, Osaka Uni6ersity, Toyonaka, Osaka, 560, Japan
Received 17 June 1997; received in revised form 25 August 1997
Abstract
Half-sandwich complexes of niobium and tantalum having an extended conjugated 1,3-diene ligand such as o-xylylene,
anthracene, and cyclooctatetraene have been synthesized and characterized. The molecular structures of Cp*Nb{o-
(CH2)2C6H4}Cl2 (1) (Cp*=p5-pentamethylcyclopentadienyl), Cp*Ta(p4-C14H10)(CH2Ph)2 (14), and Cp*Nb(p4-C4H6)(p3-C8H8)
(16) were determined by X-ray crystallographic studies. Complexes 1 and 14 adopted a four-legged piano stool geometry and their
o-xylylene and anthracene ligands coordinated to the metal center in the p4-coordination mode. X-ray analysis together with their
NMR spectral data revealed that the o-xylylene complex 1 has a large contribution of the 2|–1y canonical form, but otherwise
the anthracene complexes have an increased contribution of 2y–p4-diene canonical form compared with the butadiene complexes.
Thus, the electronic structures of p4-o-xylylene and p4-anthracene ligands are deviated from that of p4-butadiene into the opposite
1
direction. The H NMR singlet signal of the cyclooctatetraene ligand in 16 indicated the presence of the dynamic fluxionality in
solution, while it coordinated to the metal in p3-fashion in the crystal. © 1998 Elsevier Science S.A. All rights reserved.
Keywords: Tantalum; Niobium; o-xylylene; Anthracene; Cyclooctatetraene; Crystal structure
1. Introduction
Nb, Ta; X=Cl, alkyl, aryl, etc.) [11,13], in which the
diene ligands are best described as having both contri-
butions of neutral 2y–p4-diene and formally dianionic
2|–1y canonical forms. Our recent interest in diene
complexes of group 5 metals stems from their unique
catalytic ability similar to group 4 metallocene com-
plexes for the polymerization of ethylene and norbor-
nene [14–20]. Thus, it would be our target to
investigate the electronic and steric effects of the diene
ligands on these Group 5 transition metals. In this
connection, we chose o-xylylene (o-quinodimethane),
anthracene, and cyclooctatetraene as alternative p4-1,3-
diene ligands having the extended py-conjugation
Conjugated dienes have been used among the most
useful and attractive ligands in organometallic chem-
istry [1–6]. We and colleagues have been interested in
chemistry of conjugated 1,3-diene complexes of early
transition metals, e.g. Cp2M(diene) (M=Ti, Zr, Hf;
Cp=cyclopentadienyl derivatives) [7,8], CpM(diene)X
(M=Ti, Zr, Hf; X=Cl, Br, I, alkyl) [7,9,10], CpM(di-
ene)2 (M=Nb, Ta) [11–13], and CpM(diene)X2 (M=
* Corresponding author. Tel.: +81
8506296; e-mail: mashima@chem.es.osaka-u.ac.jp
6 8506247; fax: +81 6
0022-328X/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved.
PII S0022-328X(97)00730-4

4
K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
Table 1
A comparison of bonding features indicated by NMR and X-ray studies
c
d
n (spn hybridization)a
q (°)b
Dd (A)
Dl (A)
ref.
˚
˚
Complex
C1,4
C2,3
Cp*Nb(xylylene)Cl2 (1)
{C5H4(SiMe3)}2Nb(xylylene) (6)
Cp2M(xylylene) (M=Ti, Zr, Hf, Nb)
M=Ti (7)
M=Zr (8)
M=Hf (9)
Cp*Ti(xylylene)Cl (10)
{Cp*Ti(xylylene)}2(v-xylylene) (11)
Terminal
Bridging
W(xylylene)3 (6)
Cp*Ta(anthracene)Cl2 (12)
Cp*Ta(anthracene)(CH2Ph)2 (14)
Cp*Ta(C4H6)Cl2 (13)
CpTa(C4H6)Cl2 (21)
Cp*Ta(C4H6)(CH2Ph)2 (15)
2.45
—
—
—
101.9
136
−0.312
−0.79
0.041
0.095
this work
[28]
[26,28]
—
—
—
2.51
—
—
—
—
139
127
127
—
−0.71
−0.56
−0.63
—
0.081
0.053
0.09
—
[49]
[27]
2.60
3.09
—
2.33
—
2.35
2.47
2.38
2.41
—
—
—
1.95
—
2.04
2.04
2.05
2.14
109.5
—
—
—
85.9
−0.35
—
−0.275
—
0.045
0.055
0.04
—
[29]
this work
this work
[13]
[13]
[15]
−0.041
0.083
94.9
93.3
101.9
−0.160
−0.127
−0.120
0.081
0.050
0.037
Cp*Nb(C4H6)(cot) (16)
this work
¸¹¹¹¹¹¹¹¹¹º
Cp*(C4H6)Nb[CH2CH(C5H8)CH] (3)
2.44
2.35
2.15
2.16
92.6
84.0
−0.12
0.049
−0.005
0.024
[14]
[22]
[11]
CpNb(C4H6)(p2-MeCꢀCPh)(PMe3) (5)
CpNb(2,3-dimethylbutadiene)2 (4)
Supine
Prone
2.49
2.38
—
—
101.1
98.7
−0.23
−0.14
0.073
0.019
1
a Estimated by using Newton’s empirical law [21] from the JCH coupling constant.
b q denotes the dihedral angle between M–C(1)–C(4) and C(1)–C(2)–C(3)–C(4) planes.
c Dd=[d(M–C(1))+d(M–C(4))]/2-[d(M–C(2))+d(M–C(3))]/2.
d Dl=[d(C(1)–C(2))+d(C(3)–C(4))]/2-d(C(2)–C(3)).
constant of 147 Hz. Thus the extent of spn hybridiza-
tion of the h-carbon of the o-xylylene ligand in 1 was
estimated by using Newton’s empirical law [21] to be
n=2.45, showing relatively higher p-character com-
pared to those of the butadiene ligand in the typical
niobium monokis(butadiene) complexes reported so far
(see Table 1) [14,22]. In the reaction course, the corre-
sponding bis(xylylene) complex such as Cp*Nb{o-
(CH2)2C6H4}2 was not detected at all; being in contrast
to a general trend that preparation of monokis(di-
ene)complexes [11] was often accompanied by the for-
mation of small amounts of the corresponding
bis(diene)complexes.
Scheme 1.
(Scheme 1). Herein we report the syntheses and charac-
terization of half-sandwich complexes of niobium and
tantalum bearing a conjugated-diene ligand such as
o-xylylene, anthracene, and cyclooctatetraene.
2. Results and discussion
2.1. Preparation and characterization of a niobium
o-xylylene complex, Cp*Nb{o-(CH2)2C6H4}Cl2 (1)
Reaction of Cp*NbCl4 (Cp*=p5-pentamethylcy-
clopentadienyl) with o-C6H4(CH2MgCl)2 gave an o-xy-
lylene complex of niobium, Cp*Nb{o-(CH2)2C6H4}Cl2
(1), in 16% yield as green crystals (Eq. (1)); its constitu-
tion was supported by elemental analysis as well as by
NMR spectroscopy. The h-protons of the o-xylylene
1
ligand in the H NMR spectrum of 1 were observed as
The molecular structure of 1 determined by X-ray
crystallography is shown in Fig. 1. The selected bond
an AB-type quartet at l 0.75 and 1.95; a resonance of
1
h-carbons appeared at l 71.3 with a JCH coupling

K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
5
Scheme 2.
Hf and Nb have been reported to have a metallacy-
clopentene structure (type A in Scheme 2) [25,26], while
a half-metallocene of titanium adopted an p4-o-xylylene
type B in Scheme 2 [27]. Complex 1 also has the type B
structure and adopts a four legged piano stool geome-
try comprised of a capping Cp* ligand together with
two carbon atoms of the o-xylylene ligand and two
Fig. 1. ORTEP drawing of Cp*Nb{o-(CH2)2C6H4}Cl2 (1) with the
numbering scheme.
˚
chlorine atoms as four legs. The Nb–C(1) (2.240(5) A)
distances and angles are listed in Table 2. The o-xy-
lylene ligand has been reported to coordinate to the
transition metal in a variety of fashions as shown in
Scheme 2 [23,24]. Metallocene-type complexes of Ti, Zr,
˚
and Nb–C(4) (2.236(5) A) distances of 1 are slightly
shorter than those of the butadiene ligands in Cp*(p4-
¸¹¹¹¹¹¹¹¹¹¹¹¹º
˚
C4H6)Nb[CH2CH(C5H8)CH] (2) (2.26(1) and 2.28(1) A)
[14], CpNb(p4-C4H6)(MeCꢀCPh)(PMe3) (3) (2.406(1)
4
˚
and 2.391(1) A) [22] and CpNb(p -2,3-dimethylbutadi-
Table 2
˚
˚
˚
Selected bond distances (A) and angles (°) of 1
ene)2 (4) (supine, 2.286(8) A; prone 2.324(8) A) [11], and
also shorter than that found for an o-xylylene complex
of niobocene, {C5H4(SiMe3)}2Nb(xylylene) (5) (av.
˚
Bond distances (A)
Nb–Cl(1)
Nb–C(1)
Nb–C(3)
Nb–C(11)
Nb–C(13)
Nb–C(15)
C(2)–C(3)
C(2)–C(5)
C(6)–C(7)
C(3)–C(8)
2.409(1)
2.240(5)
2.556(4)
2.434(4)
2.421(4)
2.420(4)
1.415(6)
1.412(7)
1.396(8)
1.407(7)
Nb–Cl(2)
Nb–C(2)
Nb–C(4)
Nb–C(12)
Nb–C(14)
C(1)–C(2)
C(3)–C(4)
C(5)–C(6)
C(7)–C(8)
2.419(1)
2.544(4)
2.236(5)
2.445(5)
2.416(4)
1.457(7)
1.455(7)
1.368(7)
1.358(8)
˚
˚
2.286 A) [28]. The Nb–C(2) (2.544(4) A) and Nb–C(3)
˚
(2.556(4) A) distances of 1 are significantly longer than
those of the butadiene ligands in 2 (2.40(1) and 2.38(1)
˚
˚
˚
A), 3 (2.354(1) and 2.345(1) A) and 4 (supine, 2.519(7)
˚
A; prone 2.467(7) A), but far shorter than that of 5 (av.
˚
3.079 A) in which there are no bonding interaction
between the niobium atom and the Ci carbons, C(2)
˚
and C(3). The C(1)–C(2) (1.457(7) A), C(2)–C(3)
˚
˚
(1.415(6) A), and C(3)–C(4) (1.455(7) A) bond dis-
tances show long-short-long bond alternation, though
the difference is smaller than that in the butadiene
complexes. The long-short-long-short-long bond alter-
nation within aromatic ring system, i.e. C(2)–C(5)
Bond angles (°)
Cl(1)–Nb–Cl(2)
Cl(1)–Nb–C(4)
Cl(2)–Nb–C(4)
Nb–C(1)–C(2)
C(1)–C(2)–C(3)
92.58(5)
138.9(2)
84.2(2)
84.1(3)
115.4(5)
Cl(1)–Nb–C(1)
Cl(2)–Nb–C(1)
C(1)–Nb–C(4)
Nb–C(4)–C(3)
C(2)–C(3)–C(4)
83.8(2)
137.9(1)
72.2(2)
84.9(3)
114.2(5)
˚
˚
(1.412(7) A), C(5)–C(6) (1.368(7) A), C(6)–C(7)
Fold angles of the best planes (°)
˚
˚
˚
(1.396(8) A), C(7)–C(8) (1.358(8) A) and C(8)–C(3)
b
q(1)a
q(3)c
101.87
157.79
q(2)
99.31
(1.407(7) A), indicates the electron localization in the
aromatic moiety of the xylylene ligand; similar bond
alternation has been observed in W(xylylene)3 (6) [29].
The large dihedral angle (101.87°) between Nb–C(1)–
C(4) and C(1)–C(2)–C(3)–C(4) planes (q), of 1 is
larger by ca. 10° than that of typical niobium mono(bu-
tadiene) complexes [14,22]. This large fold angle is
a q(1): dihedral angle between the C(1)–Nb–C(4) plane and the plane
of C(1)–C(2)–C(3)–C(4).
b q(2): dihedral angle between the C(1)–Nb–C(4) plane and the plane
of the aromatic ring.
c q(3): dihedral angle between the C(1)–Nb–C(4) plane and the plane
of C(11)–C(15).

6
K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
consistent with the increased p-character of the 1,4-car-
bons detected by the 13C NMR, thereby suggesting a
larger contribution of the 2|–1y canonical form com-
pared to the corresponding butadiene complexes.
lents of benzyl Grignard reagent readily gave a dibenzyl
complex, Cp*Ta(p4-C14H10)(CH2Ph)2 (14), in 57% yield
1
(Eq. (3)). The signals of benzyl protons in the H NMR
spectrum of 14 appeared at l 1.46 and 0.13 as an AB
quartet and the signals of anthracene protons in 14 are
quite similar to those in 12.
2.2. Preparation and characterization of tantalum
anthracene complexes
Anthracene ligand can coordinate to transition
metals in p4-coordination modes as shown in Scheme 3,
where four possible canonical structures (H–K) are
given. Here we prepared p4-anthracene complexes of
tantalum with mode I in Scheme 3.
Treatment of Cp*TaCl4 with Mg(anthracene)(thf)3
[30–36] gave Cp*Ta(p4-C14H10)Cl2 (12) in 34% yield as
1
green crystals (Eq. (1)). The H NMR spectrum of 12
confirmed that the anthracene coordinated to tantalum
via 1–4-carbons. Thus the 1,4-proton signals of the
anthracene ligand in 12 were observed at significantly
lower field (l 2.74) due to the aromatic ring system of
the anthracene compared to the corresponding butadi-
ene complex, Cp*Ta(p4-C4H6)Cl2 (13) (l 0.85 and −
0.09) [13], the chemical shift value of 2,3-protons (l
7.36) being comparable to that of 13 (l 7.09), along
with the observation of a singlet signal due to 9,10-pro-
tons at normal aromatic region. The extents of spn
hybridization of 1,4-carbons (l 78.6) and 2,3-carbons (l
125.3) of the anthracene ligand in 12 were calculated by
using Newton’s empirical law [21] to be n=2.33 and
1.95 (Table 1), respectively.
X-ray quality crystals of 14 were obtained from the
toluene solution and utilized for structure analysis. An
ORTEP drawing of the resulting molecular structure is
shown in Fig. 2. The structural data of 14 are summa-
rized in Table 3. Anthracene complexes of iron [37–39]
and chromium [40,41] were reported to be p6-an-
thracene complexes, while rhodium [42], and zirconium
[43] complexes were reported to have p4-anthracene. In
the rhodium complex, anthracene coordinates to the
metal at its 1,2,3,4-carbons (type H in Scheme 3), while
the zirconocene complex was reported to be bonded at
9,10,11,12-carbons (type J or K in Scheme 3). Complex
14 has type I mode rather than type H in Scheme 3
since the anthracene ligand of 14 is bent at C(1) and
C(4), the dihedral angle between the plane defined by
C(1)–C(2)–C(3)–C(4) and that defined by C(1)–C(4)–
˚
C(11)–C(12) is 133.9°. The C(1)–C(2) (1.444(6) A),
˚
˚
C(2)–C(3) (1.366(7) A), and C(3)–C(4) (1.453(6) A)
distances of 14 show a long-short-long bond alterna-
tion, indicating the stronger contribution of a 2|–1y
canonical form than a 2y canonical structure similar to
the conjugated diene complexes of tantalum. In the
structurally characterized rhodium complex, (p5-
C5H5)Rh(p4-C14H10), the long-short-long bond alterna-
tion was also observed but the difference between the
C(2)–C(3) distance and the C(1)–C(2) or C(3)–C(4)
distances is much smaller compared to that in 14. The
All attempts failed to obtain a single crystal of 12.
We therefore prepared a benzyl derivative of 12 that
would be expected to give better single crystals owing
its higher solubility. Reaction of 12 with two equiva-
˚
Ta–C(1) and Ta–C(4) distances (av. 2.324 A) of 14 are
˚
longer by 0.06 A than those of the corresponding
butadiene complex, Cp*Ta(p4-C4H6)(CH2Ph)2 (15). (av.
˚
2.264 A) [15], while the Ta–C(2) and Ta–C(3) distances
˚
˚
(av. 2.365 A) of 14 are shorter by 0.03 A than those of
˚
15 (av. 2.391 A). These findings are attributed to the
steric hindrance between the pentamethylcyclopentadi-
enyl and the anthracene ligands. The dihedral angle
(85.9°) between the C(1)–Ta–C(2) and C(1)–C(2)–
Scheme 3.

K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
7
Fig. 2. ORTEP drawing of Cp*Ta(p4-C14H10)(CH2Ph)2 (14) with the numbering scheme.
lower p-content in its spn hybridization, a narrower q
angle, and a smaller negative value of Dd; being consis-
tent with the larger contribution of the neutral 2y–p4-
diene canonical form than those of the butadiene
complexes as well as the o-xylylene complexes. It is
notable that the p4-coordination modes of the o-xy-
lylene and anthracene ligands have the opposite elec-
tronic properties.
C(3)–C(4) planes in 14 is smaller than that of 15
(93.3°). The coordination of the pentamethylcyclopen-
tadienyl group is quite normal. The geometry of the
˚
two benzyl groups in 14 (Ta–C(21) 2.240(5) A, Ta–
˚
C(31) 2.274(5) A, Ta–C(21)–C(22) 130.7(3)°, Ta–
C(31)–C(32) 125.0(3)°) is also quite similar to that in
˚
˚
15 (Ta–C(21)=2.249(4) A, Ta–C(31)=2.277(4) A,
Ta–C(21)–C(22)=130.4(3)°, and Ta–C(31)–C(32)=
126.8(3)°) [15].
2.4. Preparation and characterization of niobium
cyclooctatetraene complexes
2.3. Bonding features of o-xylylene and anthracene
complexes by NMR and x-ray analysis
Recently, Herberich et al. reported half-sandwich
cyclooctatetraene complexes of niobium, (p5-
Some examples of o-xylylene, anthracene, and buta-
diene complexes of niobium and tantalum along with
some early transition metals are summarized in Table 1,
which includes the hybridization degree of the corre-
sponding carbons, dihedral angles between M–C(1)–
C(4) and C(1)–C(2)–C(3)–C(4) planes (q), the
difference in M–C distances (Dd) defined by Dd=
{d(M–C(1))+d(M–C(4))}/2−{d(M–C(2))+d(M–C
(3))}/2, and the difference in C–C distances (Dl)
defined by Dl={d(C(1)–C(2))+d(C(3)–C(4))}/2−
d(C(2)–C(3)).
The o-xylylene complexes generally have higher p-
content in their spn hybridization; in accord with a
slightly large q angle, and a larger negative value of Dd.
These indicate a larger contribution of the 2|–1y
canonical form compared with the butadiene com-
plexes; nevertheless the Dl values of the o-xylylene
complexes are smaller than that of the butadiene com-
plexes because of the delocalized double bond character
of C2–C3 bond owing its aromaticity. In contrast, the
bonding of the anthracene ligand is characterized by
C5H4Me)Nb(p4-C4H6)(C8H8)
(18)
C5H4Me)Nb(C8H8)2 (19), which were prepared by the
ligand exchange reaction of niobium bis(di-
and
(p5-
a
ene)complex, (p5-C5H4Me)Nb(C4H6)2, with cyclooc-
tatetraene [44]. Schrock and coworkers prepared a
series of bis(cyclooctatetraene) complexes, M(C8H8)2R
(M=Nb, Ta; R=Ph, Me) [45] and tris(cyclooctate-
traene) complexes, [M%][M(C8H8)3] (M=Nb, Ta; M%=
K, Li(thf)4, As(C6H5)4) by the reduction of metal
halides with the cyclooctatetraene dianion [46]. Our
synthetic approach was based on the reductive elimina-
tion of a dimethyl complex Cp*Nb(p4-C4H6)Me2 (17)
[14] to form a nascent Cp*Nb(p4-C4H6) that was then
trapped by cyclooctatetraene. Treatment of 17 with one
equivalent of cyclooctatetraene in THF at 50°C resulted
in the formation of Cp*Nb(p4-C4H6)(C8H8) (16) as
purple crystals (Eq. (4)). It is notable that the reaction
of 17 with cyclic olefins such as norbornene and ace-
naphthylene afforded metallacyclobutane derivatives
via h-hydrogen elimination of the methyl group bound
to the niobium atom [14]. The complex 16 was also

8
K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
obtained by reaction of Cp*Nb(p4-C4H6)Cl2 with the
cyclooctatetraene dianion.
1
The H and 13C resonances due to the COT ligand in
16 are observed as singlet peaks (lH 5.16, lC 103.6).
The 13C chemical shift is at the higher field than that
found for uncoordinated COT (lC 132.7), but almost
comparable with those in (p5-C5H4Me)Nb(p4-
C4H6)(C8H8) (18) (lC 101.5) [44], (p5-C5H4Me)Nb(p4-
C4H6)(C8H8) (19) (lC 108.3–104.9) [44], and
(Pri2PC2H4PPri2)Ni(p2-C8H8) (lC 103.4) [47], being inter-
mediate between that of uncoordinated COT and that
of K2(cot) (lC 89.9). Additionally, the coupling con-
stant JCH=154 Hz of 16 corresponds to that in the free
COT (154.5 Hz). All of the p3-, p4-, and p8-cyclooctate-
traene ligands in early transition metal complexes gen-
Fig. 3. ORTEP drawing of Cp*Nb(p4-C4H6)(p4-C8H8) (16) with the
numbering scheme.
erally display one singlet resonance in the 1H NMR
spectra, which obviously represents the time average of
a dynamic structure since a p3-C8H8 coordination mode
has been determined for 16 in the crystal (see below).
Whereas a number of transition metal complexes
with non planar COT ligands and aromatic planar
COT ligands have been reported so far, two complexes
Table 3
˚
Selected bond distances (A) and angles (°) of 14
˚
Bond distances (A)
Ta–C(1)
Ta–C(3)
2.335(4) Ta–C(2)
2.358(4) Ta–C(4)
2.240(5) Ta–C(31)
2.468(4) Ta–C(42)
2.441(4) Ta–C(44)
2.447(4) C(1)–C(2)
1.366(7) C(3)–C(4)
1.487(6) C(4)–C(12)
1.373(6) C(5)–C(13)
1.353(7) C(6)–C(13)
1.404(8) C(8)–C(9)
1.421(7) C(10)–C(11)
1.432(6) C(11)–C(12)
1.419(6)
2.371(4)
2.313(5)
2.274(5)
2.463(4)
2.458(4)
1.444(6)
1.453(6)
1.475(6)
1.422(6)
1.414(6)
1.359(7)
1.367(6)
1.406(6)
Ta–C(21)
Ta–C(41)
Ta–C(43)
Ta–C(45)
C(2)–C(3)
C(1)–C(11)
C(5)–C(12)
C(6)–C(7)
C(7)–C(8)
C(9)–C(14)
C(10)–C(14)
C(13)–C(14)
bearing
a
semiaromatic COT ligand, Cp2Ta(p-
C3H7)(p2-C8H8) and (R2PC2H4PR2)Ni(p2-C8H8) (R=
Pri, But) [47], have been described, where a term
‘semiaromatic’ means a planar ring system with bond
localization; delocalization corresponding to fully aro-
matic COT2−. The structure of 16 in crystal has been
determined by a single crystal X-ray analysis. Fig. 3
shows a drawing of 16, which has a p3-cyclooctate-
traene ligand in the solid state. Selected bond distances
and angles are summarized in Table 4. It has been
reported that the bis(cyclooctatetraene) complex 19 has
one p4-cyclooctatetraene and one p3-cyclooctatetraene
in the solid state. The bonding nature of 16 is quite
similar to that found for the p3-cyclooctatetraene lig-
and of 19 [44]. The eight C–C bonds of the p3-cyclooc-
tatetraene ligand in 16 can be classified into two
groups; longer C–C bonds including C(11)–(12)
Bond angles (°)
C(1)–Ta–C(2)
C(1)–Ta–C(4)
C(2)–Ta–C(4)
Ta–C(1)–C(2)
Ta–C(1)–C(11)
Ta–C(3)–C(4)
C(1)–C(2)–C(3)
C(2)–C(1)–C(11)
Ta–C(21)–C(22)
C(21)–Ta–C(31)
35.7(2)
67.0(2)
60.5(2)
73.5(3)
111.1(3)
70.2(2)
115.3(4)
114.6(4)
130.7(3)
94.1(2)
C(1)–Ta–C(3)
C(2)–Ta–C(3)
C(3)–Ta–C(4)
Ta–C(2)–C(1)
Ta–C(4)–C(3)
Ta–C(4)–C(12)
C(2)–C(3)–C(4) 113.6(4)
C(3)–C(4)–C(12) 114.8(4)
Ta–C(31)–C(32) 125.0(3)
60.8(2)
33.6(2)
36.2(2)
70.8(2)
73.6(3)
112.8(3)
˚
˚
(1.411(6) A), C(11)–(18) (1.415(6) A), C(12)–C(13)
Fold angles of the best planes (°)
˚
˚
(1.411(6) A) and C(17)–C(18) (1.406(6) A), and shorter
C–C bonds including C(13)–C(14) (1.385(7) A),
C(14)–C(15) (1.384(7) A), C(15)–C(16) (1.376(7) A),
C(16)–C(17) (1.392(6) A). The former group of C–C
q(1)a
q(3)c
85.86
133.86
q(2)b
140.27
˚
˚
˚
a q(1): dihedral angle between the C(1)–M–C(4) plane and the plane
of C(1)–C(2)–C(3)–C(4).
˚
bonds contain carbons connected to niobium like a
p3-allyl ligand and are lengthened by coordination to
the metal. The latter group of short C–C bonds com-
posed of the noncoordinated carbons indicate some
b q(2): dihedral angle between the C(1)–M–C(4) plane and the plane
of C(1)–C(4)–C(11)–C(12).
c q(3): dihedral angle between the C(1)–C(2)–C(3)–C(4) plane and
the plane of C(1)–C(4)–C(11)–C(12).

K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
9
cyclooctatetraene coordinated to the niobium center in
a dynamic coordination mode in solution, while the
ligand was found in p3-fashion in the crystal.
contribution of a formal free pentadienyl monoanion.
Thus, the niobium metal center should have a formal
+1 charge to neutralize the formal pentadienyl anion
in C8H8 forming a formally 16 electron complex.
Since complex 16 has two different kinds of poly-
olefinic ligands, i.e. butadiene and cyclooctatetraene,
the competitive ligand exchange reaction of 16 with
diphenylacetylene was investigated. The reaction actu-
ally gave Cp*Nb(C8H8)(PhCꢀCPh) (20), which has a
p4-cyclooctatetraenyl ligand. The liberation of the buta-
diene ligand suggests the stronger coordination ability
of cyclooctatetraene.
4. Experimental Section
4.1. General Procedures
All manipulations involving air- and moisture-sensi-
tive organometallic compounds were carried out by use
of the standard Schlenk technique under argon atmo-
sphere. Cp*NbCl2(p4-buta-1,3-diene) was prepared ac-
cording to the literature [11]. Hexane, THF, and
toluene were dried and deoxygenated by distillation
over sodium benzophenone ketyl under argon. Ben-
zene-d6 was distilled from Na/K alloy and thoroughly
degassed by trap-to-trap distillation before use. Cy-
clooctatetraene purchased from Strem was used as
received.
3. Conclusion
We have demonstrated that the o-xylylene and an-
thracene ligands can be used as p4-ligand alternatives to
1,3-diene ligands for Group 5 transition metals. The
newly prepared o-xylylene and anthracene complexes
were revealed to have structures similar to the butadi-
ene complex by X-ray crystallography. NMR and
structural data of the both o-xylylene and anthracene
complexes indicated that the contribution of the 2|–1y
canonical form is higher than the 2y canonical form but
the extent of the contribution of the 2|–1y canonical
form is higher in the o-xylylene complexes and lower in
the anthracene complexes compared to the correspond-
ing butadiene complexes. NMR spectroscopic measure-
ment of the cyclooctatetraene complexes showed that
1
The H (500, 400, and 270 MHz) and 13C (125, 100,
and 68 MHz) NMR spectra in C6D6 were measured on
a JEOL JNM-GX500, a JEOL JNM-GSX400, and a
JEOL JNM-EX270 spectrometer. When C6D6 was used
as the solvent, the spectra were referenced to the resid-
1
ual solvent protons at l 7.20 in the H NMR spectra
and to the solvent carbons at l 128.0 (triplet for C6D6)
1
in the 13C NMR spectra. Assignments for H and 13C
NMR peaks for some of the complexes were aided by
2D 1H-1H NOESY and 2D 1H-13C COSY spectra,
respectively. Elemental analyses were performed at Ele-
mental Analysis Center, Faculty of Science, Osaka Uni-
versity. All melting points of the complexes were
measured in sealed tubes under an argon atmosphere
and were not corrected.
Table 4
˚
Selected bond distances (A) and angles (°) of 16
˚
Bond distances (A)
Nb–C(1)
Nb–C(3)
2.307(4) Nb–C(2)
2.393(4) Nb–C(4)
2.255(4) Nb–C(12)
2.424(4) Nb–C(21)
2.484(4) Nb–C(23)
2.436(4) Nb–C(25)
1.415(6) C(2)–C(3)
1.423(6) C(11)–C(12)
1.415(6) C(12)–C(13)
1.385(7) C(14)–C(15)
1.376(7) C(16)–C(17)
1.406(6)
2.424(4)
2.270(4)
2.452(4)
2.441(4)
2.471(4)
2.420(4)
1.382(6)
1.411(6)
1.411(6)
1.384(7)
1.392(6)
4.2. Preparation of Cp*Nb[o-(CH2)2C6H4]Cl2 (1)
Nb–C(11)
Nb–C(18)
Nb–C(22)
Nb–C(24)
C(1)–C(2)
C(3)–C(4)
C(11)–C(18)
C(13)–C(14)
C(15)–C(16)
C(17)–C(18)
To a solution of Cp*NbCl4 (0.508 g, 1.37 mmol) in
THF (40 ml) cooled at −78°C was added a suspension
of o-C6H4(CH2MgCl)2 (0.95 equiv, 1.30 mmol) in THF
(0.21 M, 6.20 ml) via syringe. The reaction mixture was
allowed to warm to room temperature, stirred
overnight and evaporated to dryness. The product was
extracted with hot hexane (240 ml) at 60°C. Recrystal-
lization from toluene (3.0 ml) at −20°C afforded 1 as
green crystals in 16% yield, mp 234.5–237.0°C (dec.).
1H NMR (270 MHz, C6D6, 30°C): l 0.75 and 1.95 (4H,
AB quartet, 2JHH=6.1 Hz, –CH2), 1.80 (15H, s,
C5Me5), 7.55 (4H, m, C6H4). 13C NMR (100 MHz,
C6D6, 30°C): l 12.2 (q, 1JCH=128 Hz, C5Me5), 71.3 (br
Bond angles (°)
Cl(1)–Nb–Cl(2)
Cl(1)–Nb–C(4)
Cl(2)–Nb–C(4)
Nb–C(1)–C(2)
C(1)–C(2)–C(3)
92.58(5)
138.9(2)
84.2(2)
84.1(3)
115.4(5)
Cl(1)–Nb–C(1)
Cl(2)–Nb–C(1)
C(1)–Nb–C(4)
Nb–C(4)–C(3)
83.8(2)
137.9(1)
72.2(2)
84.9(3)
C(2)–C(3)–C(4) 114.2(5)
Fold angles of the best planes (°)
1
t, JCH=147 Hz, –CH2), 125.8 (s, C5Me5), 131.2 and
a
q(1)
101.87
1
137.9 (d, JCH=161 and 165 Hz respectively, C6H4),
a q(1): dihedral angle between the C(1)–M–C(4) plane and the plane
127.3 (s, ipso-C6H4). Anal. Calcd for C18H23Cl2Nb: C,
of C(1)–C(2)–C(3)–C(4).
53.62; H, 5.75. Found: C, 53.76; H, 5.73.

10
K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
4.3. Preparation of Cp*TaCl2(p4-C14H10) (12)
solution changing from light green to dark green. While
the reaction mixture was stirred for 10 h at 50°C, the
color of the solution changed from dark green to brown
purple. All volatiles were removed under reduced pres-
sure to give a residue, from which the product was
extracted with hexane (170 ml). Recrystallization from
toluene (4.0 ml) at −20°C afforded 16 as black purple
A suspension of anthracenemagnesium (1.08 mmol)
in THF (4.5 ml) was added dropwise over a 15 min
period to Cp*TaCl4 (0.551 g, 1.20 mmol) dissolved in
THF (50 ml) at −78°C. The color of the solution
changed from orange to dark red. The reaction mixture
was stirred for 2 h and then allowed to warm to room
temperature. The color of the solution changed from
dark red to dark-green. After being evaporated to
dryness, the product was extracted with toluene (10 ml)
and hexane (500 ml). Recrystallization from toluene (8
ml) at −20°C afforded 2 as dark-green crystals in 34%
yield, m.p. 188–190°C (dec). 1H NMR (270 MHz,
C6D6, 30°C) l 7.63(m, 2H, 5,8-H of anthracene),
7.36(m, 2H, 2,3-H of anthracene), 7.27(m, 2H, 6,7-H of
anthracene), 6.88(s, 2H, 9,10-H of anthracene), 2.74(m,
2H, 1,4-H of anthracene), 1.71(s, 15H, C5Me5): 13C
1
crystals in 42% yield; mp 162–163°C (dec). H NMR
(400 MHz, C6D6, 30°C): l −0.36 (2H, m, ꢁCH2 anti),
1.48 (15H, s, C5Me5), 3.05 (2H, m, ꢁCH2 syn), 4.51
(2H, br, ꢁCH–), 5.16 (8H, s, COT). 13C NMR (100
1
MHz, C6D6, 30°C): l 10.8 (q, JCH=127 Hz, C5Me5),
1
1
53.6 (t, JCH=149 Hz, ꢁCH2), 103.6 (d, JCH=154 Hz,
COT), 110.2 (s, C5Me5), 126.4 (d, 1JCH=163 Hz,
ꢁCH–). Anal. Calcd for C22H29Nb: C, 68.39; H, 7.57.
Found: C, 67.71; H, 7.54.
4.6. Preparation of Nb(PhCꢀCPh)Cp*(cot) (20)
1
NMR(100 MHz, C6D6, 30°C) l 127.1(d, JC–H=158
Hz, 5,8-H of anthracene), 125.3 (d, 1JC–H=175 Hz,
Complex 16 (2.9 mg, 0.008 mmol) was dissolved in
0.3 ml of C6D6 in a 5 mm NMR tube. To the dark-pur-
ple solution was added PhCꢀCPh (1.4 mg, 0.008 mmol)
in 0.3 ml of C6D6 at 25°C. After the NMR tube was
sealed and placed for 5 h in an oil bath heated at 50°C,
1
2,3-H of anthracene), 125.3 (s, C5Me5), 124.7 (d, JC–
H=159 Hz, 6,7-H of anthracene), 121.8(d, 1JC–H=158
Hz, 9,10-H of anthracene), 78.6(d, 1JC–H=153 Hz,
1
1,4-H of anthracene), 11.2 (q, JC–H=128 Hz, C5Me5):
1
Anal. Calcd for C24H25Cl2Ta: C, 50.99; H, 4.46. Found:
C, 50.91; H, 4.41.
the H NMR was measured. Peaks arising from free
butadiene (l 6.3 and 5.1, m, C4H6) were observed
1
together with ones of 20. H NMR (270 MHz, C6D6,
4.4. Preparation of Cp*Ta(p4-C14H10)(CH2Ph)2 (14)
30°C): l 1.43 (15H, s, C5Me5), 4.75 (8H, s, COT), 7.11
(2H, overlapped by phenyl signals of free PhCꢀCPh
3
A solution of PhCH2MgCl (0.68 mmol) in THF (1.0
ml) was added dropwise over a 20 min period to
Cp*TaCl2(p4-C14H10) (12) (0.167 g, 0.30 mmol) dis-
solved in THF (40 ml) at −78°C. The reaction mixture
was stirred for 30 min and then allowed to warm to
room temperature. After being evaporated to dryness,
the product was extracted with hexane (180 ml). Re-
crystallization from toluene (2 ml) and hexane (4 ml) at
0°C afforded 14 as dark-green crystals in 57% yield,
peaks), 7.41 (4H, t, JHH=7.6 Hz, m-Ph), 7.97 (4H, d,
3JHH=6.9 Hz, o-Ph).
4.7. Crystallographic Data Collections and Structure
Determination of 1, 14, and 16
The crystals of 14 suitable for X-ray diffraction
sealed in glass capillaries under argon atmosphere, were
mounted on a Rigaku AFC-7R four-circle diffractome-
ter for data collection using Mo–Kh radiation. Three
standard reflections were chosen and monitored every
150 reflections. Measured non-equivalent reflections
with I\3.0|(I) were used for the structure determina-
tion. Empirical absorption correction was carried out
based on an azimuthal scan.
The systematic absences of h0l (l odd) and 0k0 (k
odd) indicate the space group to be P21/c. The struc-
ture was solved by the direct method (SHELXS86) [48]
and expanded using standard Fourier maps. In the final
refinement cycle (full-matrix), hydrogen atom coordi-
nates were included at idealized positions, and the
hydrogen atoms were not refined but were given the
same temperature factor as that of the carbon atom to
which they were bonded. All calculations were per-
formed using the TEXSAN crystallographic software
package. For 428 variable parameters and 5210 ob-
served reflections with I\3.0|(I), R and Rw values
1
m.p. 140–144°C. H NMR (270 MHz, C6D6, 30°C) l
7.57 (m, 2H, 5,8-H of anthracene), 7.34 (t, 4H, m-H of
Ph), 7.23 (m, 2H, 2,3-H of anthracene), 6.97 (t, 2H,
p-H of Ph), 6.90 (m, 2H, 6,7-H of anthracene), 6.77(d,
4H, o-H of Ph), 6.72 (s, 2H, 9,10-H of anthracene),
2.12(m, 2H, 1,4-H of anthracene), 1.59 (s, 15H, C5Me5),
2
1.46 and 0.13 (AB quartet, JH–H=11.1 Hz, 4H, CH2):
Anal. Calcd for C38H39Ta: C, 67.45; H, 5.81. Found: C,
67.36; H, 5.73.
4.5. Preparation of Cp*Nb(p4-buta-1,3-diene)(cot) (16)
To a solution of NbCl2Cp*(p4-buta-1,3-diene) (0.271
g, 0.768 mmol) in THF (50 ml) cooled at −78°C was
added cyclooctatetraene (1.2 equiv, 0.88 mmol) in THF
(0.40 M, 2.20 ml) and CH3MgI (2.3 equiv, 1.74 mmol)
in ether (0.62 M, 2.80 ml) via syringe. The reaction
mixture was allowed to warm to 20°C, the color of the

K. Mashima et al. / Journal of Organometallic Chemistry 557 (1998) 3–12
11
Table 5
Crystal data and data collection parameters
Complex
1
14
16
Formula
C18H23Cl2Nb
403.19
C38H39Ta
676.67
Monoclinic
P21/c (c14)
12.611(4)
16.521(3)
14.621(3)
101.65(2)
2983(1)
4
C22H29Nb
386.38
Monoclinic
P21/c (c14)
12.196(4)
11.113(2)
14.436(2)
112.40(1)
1808.8(6)
4
Formula weight
Crystal system
Space group
Monoclinic
P21/a (c14)
15.119(6)
8.253(3)
15.502(3)
115.79(2)
1741.6(10)
4
˚
a (A)
˚
b (A)
˚
c (A)
i (°)
V (A )
3
˚
Z
Dcalcd
1.538
1.506
1.419
Radiation
Mo-Kh
0.2×0.2×0.1
0.989
Mo-Kh
0.4×0.2×0.2
37.03
Mo-Kh
0.2×0.2×0.2
6.64
Crystal size (mm)
Abs. coeff (cm−1
Scan mode
)
ꢀ-2q
ꢀ-2q
ꢀ-2q
Temperature (°C)
Scan speed (° min−1
Scan width (°)
2qmax (°)
23
16
23
16
23
8
)
1.05+0.35 tan q
55
0.89+0.35 tan q
55
1.42+0.30 tan q
55
Unique data [I\3|(I)]
2519
5210
2835
No. of variables
218
428
264
R
0.034
0.029
0.032
Rw
0.035
0.032
0.036
GOF
D (eA
1.327
0.37, −0.40
1.28
0.40, −1.38
1.50
0.40, −0.50
−3
˚
)
[6] G. Erker, C. Kru¨ger, G. Mu¨ller, Adv. Organomet. Chem. 24
(1985) 1–39.
[7] H. Yamamoto, H. Yasuda, K. Tatsumi, K. Lee, A. Nakamura,
J. Chen, Y. Kai, N. Kasai, Organometallics 8 (1989) 105–119.
[8] H. Yasuda, Y. Kajiwara, K. Mashima, K. Nagasuna, K. Lee, A.
Nakamura, Organometallics 1 (1982) 388–396.
reached to 0.029 and 0.032, respectively. The pertinent
detail of data collection and the final cell dimensions
for 14 are given in Table 5.
The molecular structures of 1 and 16 were solved in
an essentially similar manner to that of 14, and only the
final parameters were included in Table 5.
[9] Y. Wielstra, S. Gambarotta, A.L. Spek, Organometallics 9
(1990) 572.
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Acknowledgements
[11] T. Okamoto, H. Yasuda, A. Nakamura, Y. Kai, N. Kanehisa,
N. Kasai, J. Am. Chem. Soc. 110 (1988) 5008–5017.
[12] T. Okamoto, H. Yasuda, A. Nakamura, Y. Kai, N. Kanehisa,
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[14] K. Mashima, M. Kaidzu, Y. Nakayama, A. Nakamura,
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[15] K. Mashima, Y. Tanaka, M. Kaidzu, A. Nakamura,
Organometallics 15 (1996) 2431–2433.
K.M. acknowledges the support by the Grant-in-Aid
for Scientific Research on Priority Area (No. 09238103)
from the Ministry of Education, Science, Sports and
Culture, Japan and by the Asahi Glass Foundation.
A.N. is grateful for the financial support of the Min-
istry of Education, Science, Sports and Culture of
Japan (Specially Promoted Research No. 06101004).
[16] K. Mashima, Y. Tanaka, A. Nakamura, Organometallics 14
(1995) 5642–5651.
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