Zr(C5H5)(6,6-dmch)(PMe3)2, an edge-bridged half-open zirconocene - Synthesis, structure, and its reaction with C6H5C2SiMe3

Article abstract of DOI:10.1016/S0020-1693(02)00736-3

The synthesis and characterization of an edge-bridged half-open zirconocene, Zr(C₅H₅)(dmch)(PMe₃)₂ (dmch=dimethylcyclohexadienyl), are described. As expected, the Zr-C bond distances for the dmch ligand are significantly shorter than those for the C₅H₅ ligand. The compound readily incorporates 2 equiv. of alkyne, with loss of the PMe₃ ligands, leading to a zirconacyclopentadiene complex, so that the dmch ligand has remained intact.

Full text of DOI:10.1016/S0020-1693(02)00736-3

Inorganica Chimica Acta 334 (2002) 17Á24
/
www.elsevier.com/locate/ica
Zr(C5H5)(6,6-dmch)(PMe3)2, an edge-bridged half-open
zirconocene*
/
synthesis, structure, and its reaction with
C6H5C2SiMe3
Vichien Kulsomphob, Benjamin G. Harvey, Atta M. Arif, Richard D. Ernst *
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, UT 84112-0850, USA
Received 25 August 2001; accepted 9 November 2001
Abstract
The synthesis and characterization of an edge-bridged half-open zirconocene, Zr(C5H5)(dmch)(PMe3)2 (dmchꢀ
/dimethylcyclo-
hexadienyl), are described. As expected, the ZrꢀC bond distances for the dmch ligand are significantly shorter than those for the
/
C5H5 ligand. The compound readily incorporates 2 equiv. of alkyne, with loss of the PMe3 ligands, leading to a
zirconacyclopentadiene complex, so that the dmch ligand has remained intact. # 2002 Elsevier Science B.V. All rights reserved.
Keywords: Alkyne complexes; Half-open zirconocenes; Pentadienyl complexes
1. Introduction
zirconium is the largest transition metal [5], its size
should lead to more favorable bonding interactions with
the large pentadienyl ligands, and would also tend to
offset the substantial steric crowding that typically
characterizes pentadienyl complexes [6]. Additionally,
zirconium complexes should tend to be even more
reactive than their titanium analogs, especially when
Half-open titanocene complexes such as Ti(C5H5)-
(C5H7)(L) [1], Ti(C5H5)(2,4-C7H11)(L) [2], Ti(C5H5)(6,6-
dmch)(L) [3], and Ti(C5H5)(C8H11)(L) [4] (C7H11ꢀ
dimethylpentadienyl (1); dmchꢀdimethylcyclohexadie-
nyl (2); C8H11ꢀcyclooctadienyl (3); LꢀPMe3 or PEt3)
/
/
/
/
formation of MÁ
[7]. This could then lead to differing reaction products
and pathways, as well as to stronger interactions with*
and perhaps ultimately to bond activation of*CꢀC or
CꢀN sigma bonds, in cases in which agostic species are
/
heteroatom bonds would be involved1
have proven quite interesting in both bonding and
reactivity aspects. Thus, structural and theoretical data
have demonstrated that the pentadienyl ligands are
more strongly bound than the accompanying, ‘stabiliz-
ing’ cyclopentadienyl ligand [2c], while reactivity studies
indicate that the electronically open ligands are also
more reactive, readily coupling with unsaturated organic
molecules such as aldehydes, ketones, imines, nitriles,
/
/
/
/
generated. We have therefore turned our attention to the
syntheses as well as physical and chemical studies of
various types of half-open zirconocenes. Herein we
begin by reporting our initial isolation of such species
incorporating the 6,6-dimethylcyclohexadienyl or 2,6,6-
trimethylcyclohexadienyl ligand, and our observations
concerning the reactivity of the former toward alkynes.
isonitriles, and alkynes [1Á4]. Some of these reactions
were especially notable in that one could generate
multiple chiral centers [3], fused ring systems [2a], or
/
even complexes with apparent (Cꢀ
actions [4]. Several reasons suggest that extension to
zirconium should be especially interesting. First, as
/
C)0
/
Ti agostic inter-
1
* Corresponding author. Tel.: ꢁ
8433.
E-mail address: ernst@chemistry.utah.edu (R.D. Ernst).
/
1-801-581 6681; fax: ꢁ
/
1-801-581
On the other hand, the larger size of zirconium should improve
overlap with the dienyl ligand, which could thereby result in lower
reactivities, especially when bonds to heteroatoms are not formed.
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 0 7 3 6 - 3

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V. Kulsomphob et al. / Inorganica Chimica Acta 334 (2002) 17Á24
/
observed. After stirring overnight, the pale yellow solid
was filtered and collected on a coarse frit (5.7 g, 90%
yield).
2.2. Bis(trimethylphosphine)(cyclopentadienyl)(6,6-
dimethylcyclohexadienyl)zirconium, Zr(C5H5)(6,6-
dmch)(PMe3)2
To a magnetically stirred solution of 1.00 g (3.26
mmol) of Zr(C5H5)Cl2Br in 30 ml of THF under
nitrogen at ꢂ78 8C was added 0.96 ml (6.5 mmol) of
/
PMe3 yielding a pale pink solution. To the resulting
solution was added dropwise 1.40 g (9.80 mmol) of
potassium 6,6-dimethylcyclohexadienide dissolved in 30
ml of THF, after which the mixture was allowed to stir
for 30 min. The resulting red solution was allowed to
warm to r.t. and a color change to bright red was
observed. The solvent was removed in vacuo and the
crude product extracted with three portions of 20 ml
pentane and filtered on a coarse frit. The red filtrate was
concentrated to 15 ml and cooled to ꢂ90 8C, affording
/
a red crystalline solid in 56% yield. This compound is
very air- and moisture-sensitive, but it can be stored
under nitrogen indefinitely without any decomposition.
Melting point (nitrogen-filled, sealed capillary): 112Á
/
2. Experimental
113 8C (dec.).
1H NMR (benzene-d6, ambient): d 5.61(t, 1H, H-3),
5.57(s, 5H, Cp), 5.11(m, 2H, H-2, 4), 3.11(d, 2H, H-1,5,
Reactions were carried out in flame-dried glassware
under a nitrogen atmosphere. Ethereal, aromatic, and
hydrocarbon solvents were dried by distillation under
nitrogen from benzophenone ketyl, while methylene
chloride was dried by distillation from P4O10 under a
nitrogen atmosphere. Spectroscopic studies were carried
out as previously described [8]. Proton NMR spectra
were recorded in benzene-d6 and referenced accordingly
(d 7.15 and 128.0, respectively). Zr(C5H5)2Cl2 was either
purchased commercially or prepared according to
standard procedures [9], while 6,6-dimethylcyclohexa-
diene and 2,6,6-trimethylcyclohexadiene were prepared
according to published procedures [10]. These were then
converted to their potassium salts using published
procedures [11]. Trimethylphosphine was purchased
commercially. Elemental analyses were obtained from
Jꢀ7.5 Hz), 1.46(s, 3H, CH3), 1.34(s, 3H, CH3), 0.67(m,
/
18H, PMe3). EI MS (17 eV); m/z (relative intensity):
339(15), 263(79), 248(88), 247(23), 233(52), 219(26),
157(42), 142(100), 126(20), 76(50), 61(44). Anal. Calc.
for C19H34P2Zr: C, 54.91; H, 8.25. Found: C, 54.20; H,
8.16%.
2.3. Bis(trimethylphosphine)(cyclopentadienyl(6,6-
trimethylcyclohexadienyl)zirconium, Zr(C5H5)(2,6,6-
tmch)(PMe3)2
To a magnetically stirred solution of 1.00 g (3.26
mmol) of Zr(C5H5)Cl2Br in 30 ml of THF under
nitrogen at ꢂ78 8C was added 0.70 ml (6.8 mmol) of
/
Desert Analytics and Eꢁ
tory.
/
R Microanalytical Labora-
PMe3 yielding a pale pink solution. To the resulting
solution was added dropwise 1.56 g (9.75 mmol) of
potassium 2,6,6-trimethylcyclohexadienide dissolved in
30 ml of THF, after which the mixture was allowed to
stir for 30 min. The resulting red solution was allowed to
warm to r.t. and a color change to bright red was
observed. The solvent was removed in vacuo and the
crude product extracted with three portions of 20 ml
pentane and filtered on a coarse frit. The red filtrate was
2.1. (Bromo)(dichloro)(cyclopentadienyl)zirconium,
Zr(C5H5)Cl2Br
This compound was prepared according to the gen-
eral description provided earlier [12]. To a magnetically
stirred solution of 5.0 g (17 mmol) of Zr(C5H5)2Cl2 in 50
ml of CH2Cl2 under nitrogen at ꢂ
/
78 8C was added a
concentrated to 15 ml and cooled to ꢂ90 8C, affording
/
solution of 3.0 ml (58 mmol) of Br2 in 10 ml of CH2Cl2.
The resulting red solution was warmed up to room
temperature (r.t.) and a pale yellow precipitate was
a red crystalline solid in 60% yield. This compound is
very air- and moisture-sensitive, but it can be stored
under nitrogen without any decomposition. Melting

V. Kulsomphob et al. / Inorganica Chimica Acta 334 (2002) 17Á
/24
19
point (nitrogen-filled, sealed capillary): 118Á
(dec.).
1H NMR (benzene-d6, ambient): d 5.51(s, 5H, Cp),
5.15(t, 1H, H-3, Jꢀ7.2 Hz), 3.46(t, 1H, H-4, Jꢀ7.5
Hz), 2.38(d, H-5, Jꢀ7.5 Hz), 1.96(br, s, H-1), 1.87(s,
3H, CH3), 1.39(s, 3H, CH3), 0.88(s, 3H, CH3), 0.75(m,
18H, PMe3). EI MS (17 ev); m/z (relative intensity):
353(15), 277(79), 262(88), 261(23), 247(52), 171(52),
156(100), 140(20), 76(50), 61(44). Anal. Calc. for
C20H36P2Zr: C, 55.90; H, 8.44. Found: C, 55.72; H,
8.29%.
/
120 8C
Table 1
Crystal data and structure refinement parameters for 5
Empirical formula
Molecular weight
Temperature (K)
Space group
C20H36P2Zr
/
/
429.7
173
P21
/
Unit cell dimensions
˚
a (A)
˚
8.450(2)
11.256(5)
11.170(2)
91.89(2)
1.344
b (A)
˚
c (A)
b (8)
Dcalc (g cmꢂ3
)
3
˚
Volume (A )
1061.9
0.71073
2
˚
l (A)
2.4. 1-Cyclopentadienyl-1-(6?,6?-
Z
m (Mo Ka) (cmꢂ1
2u Range
)
6.56
dimethylcyclohexadienyl)-2,5-bis(trimethylsilyl)-3,4-
diphenylzirconacyclopentadiene, Zr(C5H5)(6,6-
dmch)(C6H5C2SiMe3)2
4.0Á50.0
/
Index ranges (h, k, l)
Reflections collected
Unique reflections
Independent observed
Reflections (Fo]s(Fo))
R(F)
ꢁ10, ꢁ13, 913
2109
1965
1825
To a magnetically stirred solution of 0.55 g (1.34
mmol) of Zr(C5H5)(6,6-dmch)(PMe3)2 in 50 ml of
pentane under nitrogen at ꢂ78 8C was added 0.50 g
/
0.0625
0.0752
8.8
R(wF)
No/Nv
(2.87 mmol) of 1-phenyl-2-(trimethylsilyl)acetylene. The
dark red solution was allowed to slowly warm up to r.t.
and stirred for 18 h. A gradual change in color to a pale
Goodness-of-fit
ꢂ3
2.17
1.83
˚
Dr (e A
)
redÁ
removed in vacuo, and the crude product extracted with
10Á20 ml portions of pentane until the extracts were
colorless, and filtered on a coarse frit. The orangeÁred
filtrate was concentrated to 5 ml and cooled to ꢂ60 8C,
/orange solution was observed. The solvent was
/
/
Table 2
Crystal data and structure refinement parameters for 6
/
affording a red crystalline solid in 30% yield. This
compound is very air- and moisture-sensitive, but it
can be stored under nitrogen indefinitely without any
apparent decomposition. Melting point (nitrogen-filled,
Empirical formula
Molecular weight
Temperature (K)
Space group
C35H44Si2Zr
612.1
200
P21/n
Unit cell dimensions
˚
sealed capillary): 152Á
1H NMR (benzene-d6, ambient): d 7.38Á
Ph), 5.63(s, 5H, Cp), 5.29(t, 1H, H-3, Jꢀ6.8 Hz), 2.88Á
/
155 8C.
a (A)
˚
10.8617(2)
17.8903(4)
17.2272(3)
107.0120(10)
1.270
/6.25(10H,
b (A)
/
/
˚
c (A)
2.73(2m, 4H, H-1,2,4,5), 0.81(s, 3H, CH3), 0.61(s, 3H,
CH3), 0.01(s, 18H, Si(CH3)3). Anal. Calc. for
C35H44Si2Zr: C, 68.67; H, 7.25. Found: C, 68.31; H,
7.31%.
The presence of a second isomer in a much smaller
quantity was suggested by an additional set of peaks
adopting a similar pattern to those of the major isomer
b (8)
Dcalc (g cmꢂ3
)
3
˚
Volume (A )
3201.09(11)
0.71073
4
˚
l (A)
Z
m (Mo Ka) (cmꢂ1
2u Range
)
4.40
6.9Á55.0
/
Index ranges (h, k, l)
Reflections collected
Unique reflections
Independent observed
Reflections (Fo]4s(Fo))
R(F)
913, 923, 922
12 419
7292
(e.g. Cp at d 5.43, H3 at 5.37(t, Jꢀ
peaks at 0.69 and 0.59, and two Me3Si peaks at ꢂ
and ꢂ0.16).
/
7 Hz), two CH3
/
0.03
6035
/
0.0316
0.0656
11.6
R(wF)
No/Nv
2.5. Crystallographic studies
Goodness-of-fit
ꢂ3
1.062
0.96
Crystal, data collection, and refinement parameters
for the two structural studies are provided in Tables 1
and 2. Suitable crystals of these compounds were stuck
on glass fibers using paratone oil, and then transferred
to a Nonius Kappa diffractometer for data collection.
Both structures were solved using direct methods and
difference Fourier synthesis, and thereafter subjected to
least-squares refinements. Non-hydrogen atoms were
˚
Dr (e A
)
refined anisotropically. For complex 5 the hydrogen
atoms were placed in idealized locations, while for 6 all
hydrogen atoms were located and subjected to isotropic
refinement.

20
V. Kulsomphob et al. / Inorganica Chimica Acta 334 (2002) 17Á24
/
3. Results and discussion
Earlier attempts to prepare a half-open zirconocene
from Zr(C5H5)Cl3 [13] led to a mixture of the desired
complex, Zr(C5H5)(2,4-C7H11)(PEt3) [14], contaminated
with approximately 30% of the known Zr(2,4-
C7H11)2(PEt3) [15], as judged by 1H NMR spectro-
scopy.2 At the time it was assumed that partial over-
chlorination of Zr(C5H5)2Cl2, leading to contamination
of the desired Zr(C5H5)Cl3 with ZrCl4, was responsible,
although it could also be that some replacement of the
highly resonance stabilized C5H5 anion by a pentadienyl
anion could be occurring3 [16]. Such over-chlorination
has in fact been documented and the problem circum-
vented through recrystallization of Zr(C5H5)Cl3(THF)2
[17]. Although other routes have been developed to
potential mono(cyclopentadienyl)Zr(IV) starting mate-
rials [18], it appeared to us that a similar bromination
reaction should be more selective than chlorination, and
it would have a potential advantage of leading to the
Lewis base-free Zr(C5H5)Cl2Br. Indeed, in the original
patent describing the chlorination of Zr(C5H5)2Cl2,
mention is also made of the related bromination
reaction [12]. We have found that this general procedure
(Eq. (1)) does indeed
been characterized analytically and spectroscopically.
Other half-open zirconocenes incorporating either the
2,4-C7H11 or the C8H11 ligand have been similarly
prepared [20]. In these reactions, 2 equiv. of the dienyl
anion serve as formal reductants, and in fact, when such
a reaction is carried out with only 2 equiv. of the 2,4-
C7H11 anion, one selectively isolates Zr(C5H5)-
Br(dmpe)2, analogous to the known Zr(C5H5)Cl(dmpe)2
[21].
Assuming h5 coordination for the two dienyl ligands,
4 and 5 would both be 18-electron complexes, whereas
open zirconocenes typically incorporate only one mono-
dentate phosphine ligand [15], resulting in 16-electron
complexes, although chelating diphosphines can lead to
18-electron complexes such as Zr(C5H7)2(h2-dmpe) [22].
The tendency for pentadienyl complexes to incorporate
fewer additional ligands than their cyclopentadienyl
analogues is well known, and results primarily from
the much greater steric demands of the pentadienyl
ligands [6].
In order to confirm the structural nature of these
complexes, an X-ray diffraction study was carried out
on complex 5 (Table 3 and Fig. 1). It is evident that the
complex does possess the 18-electron configuration,
with unsymmetric positioning of the PMe3 ligands, in
accord with the low symmetry of the NMR spectra of 4.
Notably, unlike most ligand adducts of either open or
half-open metallocenes, neither phosphine ligand is
positioned by the electronically open edge of the dienyl
ligand, a clear reflection of the additional steric demands
of the edge-bridge [6]. In this case, one phosphine is
located at the back end of the dienyl group, while the
other is located on a side, not surprisingly in this case,
on the side opposite to the 2-methyl substituent. As it is
uncommon for additional ligands to be situated on one
Zr(C5H5)2Cl2 ꢁ3Br20 Zr(C5H5)Cl2BrꢁC5H5Br5 (1)
lead to the desired Zr(C5H5)Cl2Br complex, apparently
free of any tetrahalides.
Subsequent reaction of Zr(C5H5)Cl2Br with 3 equiv.
of either the 6,6-dmch anion [11,19] or the 2,6,6-tmch
anion in the presence of PMe3 have led to the desired
half-open zirconocenes 4 and 5 (Eq. (2)), which have
Zr(C5H5)Cl2Brꢁ3Pdlꢂ ꢁ2PMe30 Zr(C5H5)(Pdl)
 (PMe3)2
(2)
(4; Pdlꢀ6; 6-dmch; 5; Pdlꢀ2; 6; 6-tmch)
side of a pentadienyl ligand, it is not surprising that the
˚
P2 distance of 2.725(3) A is significantly longer than
Zrꢀ
/
˚
that for P1(2.664(4) A). These Zrꢀ
/
P distances and the
PꢀZrꢀP angle of 88.38(10)8 for 5 are intermediate
/
/
between the values for Zr(C5H5)2(PMe3)2(2.644(2);
93.9(1)8) [23] and Zr(C5H7)2(dmpe) (2.724(1), 74.7(1)8)
[22], reflecting the greater crowding brought on by the
replacement of a C5H5 ligand by an electronically open
dienyl ligand. Similarly, one sees an increase in the Zrꢀ
/
2
1H NMR (C6D5H): d 6.13(s, 1H), 5.23(s, 5H), 2.19(s, 6H),
C(Cp) distance on going from Zr(C5H5)2(PMe3)2 to 5
˚
2.16(2H), 1.27(m, 6H), 0.84(m, 9H), ꢂ
/1.51(m, 2H) [14].
3
(2.486 vs. 2.529 A), and in the Zrꢀ
/
C(Pdl) distance on
We have in fact encountered such reactions, especially for earlier
transition metal complexes.
˚
going from 5 to Zr(C5H7)2(dmpe) (2.466 vs. 2.495 A).

V. Kulsomphob et al. / Inorganica Chimica Acta 334 (2002) 17Á
/24
21
Table 3
the electronically open dienyl ligand is significantly
shorter than that for the cyclopentadienyl ligand, 2.466
˚
vs. 2.529 A.
˚
Selected bond distances (A) and bond angles (8) for Zr(C5H5)(2,6,6-
tmch)(PMe3)2
In order to gain some understanding of how the
reactivity patterns of the 6,6-dmch ligand compare to
those for other dienyl ligands, the possible reaction of
complex 4 with the alkyne C6H5C2SiMe3 was studied.
Analytical, spectroscopic, and structural data (Table 4,
Fig. 2) demonstrated that 2 equiv. of the alkyne were
readily incorporated, yielding the zirconacyclopenta-
diene complex 6. The principal isomer, 6a, has under-
gone the expected mode of coupling, based upon
observations for other systems [24], in which the less-
sterically hindered phenyl-substituted alkyne ends have
coupled together. However, the presence of a small
amount of a second isomer, 6b, was evident from the
NMR spectra (see Section 2.4). Its formation can be
attributed to the sterically demanding nature of the 6,6-
dmch ligand, which could result in an increased though
still small tendency to orient at least one of the SiMe3-
substituted alkyne ends away from the 6,6-dmch ligand,
and hence toward the second alkyne.
Bond distances
ZrꢀP1
ZrꢀC1
ZrꢀC2
ZrꢀC3
ZrꢀC4
ZrꢀC5
P1ꢀC15
P1ꢀC16
P1ꢀC17
C1ꢀC2
C2ꢀC3
C3ꢀC4
C4ꢀC5
C1ꢀC6
C2ꢀC7
C5ꢀC6
2.664(4)
ZrꢀP2
2.725(3)
2.422(13)
2.473(13)
2.537(14)
2.473(12)
2.427(13)
1.856(16)
1.826(17)
1.823(15)
1.431(18)
1.417(19)
1.421(19)
1.446(18)
1.509(20)
1.501(20)
1.488(20)
ZrꢀC10
ZrꢀC11
ZrꢀC12
ZrꢀC13
ZrꢀC14
P2ꢀC18
P2ꢀC19
P2ꢀC20
C10ꢀC11
C10ꢀC14
C11ꢀC12
C12ꢀC13
C13ꢀC14
C6ꢀC8
2.502(15)
2.568(14)
2.566(11)
2.520(13)
2.491(12)
1.830(18)
1.829(18)
1.819(13)
1.410(23)
1.365(24)
1.365(19)
1.407(19)
1.407(21)
1.519(20)
1.559(21)
C6ꢀC9
Bond angles
P1ꢀZrꢀP2
88.38(10)
116.4(5)
119.4(6)
117.5(5)
121.4(6)
113.5(6)
119.0(4)
C6ꢀC1ꢀC2
C1ꢀC2ꢀC3
C1ꢀC2ꢀC7
C3ꢀC2ꢀC7
C2ꢀC3ꢀC4
C3ꢀC4ꢀC5
C4ꢀC5ꢀC6
C1ꢀC6ꢀC5
119.7(12)
117.4(12)
121.5(12)
117.4(12)
119.6(12)
119.3(12)
117.1(12)
104.4(12)
ZrꢀP1ꢀC15
ZrꢀP1ꢀC16
ZrꢀP1ꢀC17
ZrꢀP2ꢀC18
ZrꢀP2ꢀC19
ZrꢀP2ꢀC20
Quite unlike similar reactions for complexes of the
2,4-C7H11 or C8H11 ligands, the 6,6-dmch ligand
emerged unscathed, without undergoing any coupling
with the alkynes. From this result and others, it is clear
that 6,6-dmch is unique among pentadienyl ligands in its
much lower degree of reactivity, as can also be seen by
ꢁ
the fact that the 17-electron Fe(dmch)2 ion can be
Fig. 1. Structure of complex 5.
isolated at r.t. and fully characterized, unlike any other
known open ferricinium cation [25]. Additionally, 6,6-
dmch complexes electronically appear intermediate in
nature between those of cyclopentadienyl and other
pentadienyl ligands. Both features likely derive from the
much shorter C1Á Á ÁC5 separation for dmch compared to
other open dienyl ligands [25]. This should improve
Also of interest are the parameters associated with the
Zrꢀdienyl bonding. The presence of P1 near C3 appears
responsible for the ZrꢀC bonds being longest for C3,
but shortest for the 1 and 5 positions. As is typical for
early metal complexes [6], the average MꢀC distance for
/
/
/

22
V. Kulsomphob et al. / Inorganica Chimica Acta 334 (2002) 17Á24
/
Table 4
lower oxidation states, again an apparent result of
strong d backbonding [6].
The structural data for 6 provide interesting insight
˚
Selected bond distances (A) and bond angles (8) for Zr(C5H5)(6,6-
dmch)[C(SiMe3)C(C6H5)C(C6H5)C(SiMe3)]
into the above considerations. In particular, the Zrꢀ
distances are shorter for the cyclopentadienyl ligand
˚
than for 6,6-dmch, 2.521 versus 2.585 A, opposite to
/
C
Bond distances
ZrꢀC1
ZrꢀC2
ZrꢀC3
ZrꢀC4
ZrꢀC5
ZrꢀC9
ZrꢀC10
ZrꢀC11
ZrꢀC12
ZrꢀC13
ZrꢀC14
ZrꢀC17
2.665(2)
2.533(2)
2.505(2)
2.544(2)
2.680(2)
2.519(2)
2.537(2)
2.541(2)
2.501(2)
2.508(2).
2.275(2)
2.279(2)
C1ꢀC2
1.379(3)
1.409(3)
1.410(3)
1.373(3)
1.412(3)
1.396(3)
1.386(4)
1.398(3)
1.402(3)
1.366(2)
1.485(2)
1.370(2)
C2ꢀC3
C3ꢀC4
C4ꢀC5
˚
what was observed for 5 (2.529 vs. 2.466 A). It seems
reasonable to attribute the dramatic change in favor-
ability to the fact that the usually important d back-
bonding interaction for pentadienyl ligands would be
absent in a d0 complex, and even for M(IV) complexes,
in general, due to the high metal charge. This suggestion
C9ꢀC10
C9ꢀC13
C10ꢀC11
C11ꢀC12
C12ꢀC13
C14ꢀC15
C15ꢀC16
C16ꢀC17
is consistent with the fact that it is the ZrÁ
/tmch
bonding, rather than the Zrꢀ/C5H5 bonding, which
differs between 5 and 6. The bonding parameters for
the 6,6-dmch ligand itself are further revealing. One can
Bond angles
C14ꢀZrꢀC17
ZrꢀC14ꢀC15
C14ꢀC15ꢀC16
C15ꢀC16ꢀC17
C16ꢀC17ꢀZr
80.26(6)
107.8(1)
122.2(2)
121.8(2)
107.7(1)
C2ꢀC1ꢀC6
C1ꢀC2ꢀC3
C2ꢀC3ꢀC4
C3ꢀC4ꢀC5
C4ꢀC5ꢀC6
119.2(2)
120. 8(2)
117.4(2)
121.0(2)
119.3(2)
˚
C3 distance of 2.505(2) A is clearly
note that the Zrꢀ
/
shortest, while the average distances for the 2, 4 and 1, 5
positions become progressively longer, at 2.538(2) and
˚
2.672(1) A, respectively. This reflects a contribution
from resonance hybrid 7, which is also consistent with
the observed shortÁ
/
longÁ
/
longÁ
/
short pattern of Cꢀ
/
C
˚
bond distances (averages: short, 1.376(2) A; long,
metalÁ
the MÁ
/
ligand overlap, as well as lead to an increase in
Pdl plane separation, potentially reducing over-
˚
1.410(2) A). It is certainly interesting that the pattern
/
of Zrꢀ/C bonding favorability within the 6,6-dmch type
lap for the d backbonding interaction, which seems to
play a crucial role in determining the properties of
pentadienyl ligands in general [6]. For 5 and 6, the
of ligand has entirely reversed itself in 6 as compared to
5, for which the 3 position was least favored. Both steric
(e.g. the proximity of C3 in 5 to a PMe3 ligand) and
electronic (e.g. the generally greater negative charge at
the C3 position [27] leading to its preferential bonding to
Zr(IV)) factors could be playing a role in this reversal.
˚
C1Á Á ÁC5 separations are 2.368 and 2.393 A, respectively,
˚
compared to 3.094 A for Zr(2,4-C7H11)2(PEt3). The
uniqueness of 6,6-dmch and related ligands is also
apparent by the very existence of 6, which clearly is a
Zr(IV) complex, and also the existence of a Ti(IV)
dmch-like complex [26], whereas other pentadienyl
ligands exert a much greater preference for metals in
The bonding within the zirconacyclopentadiene frag-
ment appears reasonable [24]. The Zrꢀ
/
C single bond
distances average 2.277(1) A, significantly shorter than
any of the other ZrꢀC bonds. The expected short
˚
/
˚
˚
(1.368(1) A) versus long (1.485(2) A) Cꢀ
/C distances
within the five-membered ring (see 6) are evident. As
discussed earlier (vide supra), the coupling of the two
alkyne units has occurred preferentially at their phenyl-
substituted ends. Between the two phenyl groups there
are some CÁ Á ÁC distances which are similar to or
less than a van der Waals separation [28] (e.g.
˚
˚
C18Á Á ÁC24, 2.845 A; C19Á Á ÁC25, 3.364 A), and these
could have provided a favorable or directing influence
on the mode of coupling, as might also have been true in
other cases [4], although whether these CÁ Á ÁC interac-
tions remain stabilizing or not after the coupling is
unclear.
Although complex 6 has a formal electron count of
16, there would be a mechanism available to increase
Fig. 2. Structure of complex 6.

V. Kulsomphob et al. / Inorganica Chimica Acta 334 (2002) 17Á
/24
23
this count. A metallocyclopentadiene fragment may be
looked upon as two connected metal vinyl units, and
since three electrons can be donated through h2-vinyl
coordination [29], this would lead to the possibility of
References
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Am. Chem. Soc. 114 (1992) 6252;
achieving an 18-electron count. However, the respective
˚
Zrꢀ
/
C(15,16) distances of 2.990 and 2.995 A are suffi-
(b) T.E. Waldman, A.M. Wilson, A.L. Rheingold, E. Mele´ndez,
R.D. Ernst, Organometallics 11 (1992) 3201;
ciently long that the degree of h2-vinyl coordination
must be regarded as small.
(c) I. Hyla-Kryspin, T.E. Waldman, E. Mele´ndez, W. Trakarn-
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4. Summary
[3] (a) A.M. Wilson, F.G. West, A.M. Arif, R.D. Ernst, J. Am.
Chem. Soc. 117 (1995) 8490;
The use of Zr(C5H5)Cl2Br as a starting material has
allowed for the isolation of 18-electron bis(PMe3)
adducts of half-open zirconocenes involving the 6,6-
dmch and 2,6,6-tmch ligands and structural data reveal
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/
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electronically open 2,6,6-tmch as opposed to the C5H5
ligand. The 6,6-dmch complex readily reacts with
C6H5C2SiMe3, leading to incorporation of 2 equiv. of
the alkyne, and formation of a Zr(IV) metallocyclopen-
tadiene complex. This species is not only a rare example
of a stable M(IV) pentadienyl complex, but it also
represents the first example in which an alkyne is
incorporated into a pentadienyl complex of titanium
or zirconium without any coupling occurring for the
dienyl ligand. It is clear that dmch is unique among
pentadienyl ligands, although the favorability of cou-
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the cycloheptadienyl ligand. Finally, the fact that
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/
[13] G. Erker, K. Berg, C. Sarter, Organomet. Synth. 3 (1986) 29.
[14] L. Stahl, R.D. Ernst, unpublished results.
this dmch complex leaves open the possibility that
subsequent coupling of the di(alkyne) unit to dmch
ligand could still be induced. This would then allow for
the formation of new cage-like organic structural types
surrounding the metal center, and perhaps would lead to
[15] T.E. Waldman, L. Stahl, D.R. Wilson, A.M. Arif, J.P. Hutch-
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new varieties of (Cꢀ
/
C)0/M agostic interactions [4].
(b) E.C. Lund, T. Livinghouse, Organometallics 9 (1990) 2426;
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(e) K.W. Krebs, H. Engelhard, G.E. Nischk, Ger. Offen.
1,959,322, 1971; Chem. Abstr. 75 (1971) 88768.
5. Supplementary material
Tables of thermal parameters, hydrogen atom coor-
dinates, and structure factors may be obtained from the
authors.
[19] (a) P.T. DiMauro, P.T. Wolczanski, L. Pa´rka´nyi, H.H. Petach,
Organometallics 9 (1990) 1097;
(b) P.T. DiMauro, P.T. Wolczanski, Polyhedron 14 (1995) 149.
[20] (a) V. Kulsomphob, R.D. Ernst, unpublished results.;
(b) V. Kulsomphob, Thesis, University of Utah, 1999.
[21] Y. Wielstra, S. Gambarotta, J.B. Roedelof, M.Y. Chiang,
Organometallics 7 (1988) 2177.
[22] T.E. Waldman, A.L. Rheingold, R.D. Ernst, J. Organomet.
Chem. 503 (1995) 29.
Acknowledgements
[23] L.B. Kool, M.D. Rausch, H.G. Alt, M. Herberhold, B. Honold,
U. Thewalt, J. Organomet. Chem. 320 (1987) 37.
[24] (a) G. Erker, R. Zwettler, C. Kruger, I. Hyla-Kryspin, R. Gleiter,
¨
Partial support of this research from the University of
Utah and the National Science Foundation is gratefully
acknowledged.
Organometallics 9 (1990) 524;

24
V. Kulsomphob et al. / Inorganica Chimica Acta 334 (2002) 17Á24
/
(b) M. Westerhausen, M.H. Digeser, C. Guckel, H. Noth, J.
¨
¨
Knizek, W. Ponikwar, Organometallics 18 (1999) 2491.
[27] R.B. Bates, D.W. Gosselink, J.A. Kaczynski, Tetrahedron. Lett.
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[25] R. LeSuer, R. Basta, W.E. Geiger, R.D. Ernst, unpublished
results.
[28] L. Pauling, The Nature of the Chemical Bond, third ed.(Chapter
7), Cornell University Press, Ithaca, NY, 1960.
[26] S. Feng, J. Klosin, W.J. Kruper, Jr., M.H. McAdon, D.R.
Neithamer, P.N. Nickias, J.T. Patton, D.R. Wilson, K.A.
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