Reactions of Alkynes and Olefins with Tantalum Hydrides Containing Aryloxide Ancillary Ligation: Relevance to Catalytic Hydrogenation

Article abstract of DOI:10.1021/om990664l

The reactivity of the three hydride compounds [Ta(OC₆H₃Ph₂2,6)₂(H) ₂Cl(PMe₃)₂] (1), [Ta(OC₆H₃Prⁱ₂-2,6) ₂(H)₂Cl(PMe₂Ph)₂] (2), and [Ta(OC₆H₃Bu^t2-2,6)₂(H)2Cl(PMePh ₂)] (3) toward olefins and alkynes has been investigated. The reactivity observed is highly dependent on the nature of the ancillary aryloxide ligands. The 2,6-diphenylphenoxide 1 reacts with styrene to produce 1 equiv of ethylbenzene and the styrene adduct [Ta(OC₆H₃Ph₂-2,6)₂(η ²CH₂=CHPh)Cl(PMe₃)] (5). In contrast, 1 reacts with 3-hexyne to eliminate H₂ along with formation of the analogous alkyne complex 6. Structural studies of 5 and 6 show a squarepyramidal geometry with an axial olefin (alkyne) unit lying along the Cl-Ta-P axis. Structural parameters support a tantalacyclopropane (tantalacyclopropene) bonding picture for these molecules. Compound 5 is converted back into 1 under H₂ along with formation of PhEt. The dihydride 2 reacts with styrene to form 1 equiv of PhEt, H₂, and the dehydrogenation product [Ta(OC₆H₃Prⁱ-η²-CMe=CH ₂)(OC₆H₃Peⁱi ₂-2,6)Cl(PMe₂Ph)₂] (7). The related adduct [Ta(OC₆H₃Prⁱη-²-CMe=CH ₂)(OC₆H₃Prⁱ ₂-2,6)Cl(PEt₃)₂] (9) was isolated by treatment of [Ta(OC₆H₃Prⁱ₂-2,6) ₂Cl₃] with PEt₃/Bu₃SnH and was structurally characterized. Labeling studies show that the H₂ generated comes exclusively from the aryloxide o-Prⁱ group which was dehydrogenated. Both hydrides initially attached to the metal are transferred to the olefin substrate. In the case of the 2,6-di-tert-butylphenoxide compound 3, reaction with styrene generates the mono-cyclometalated compound [Ta(OC₆H₃Bu^tCMe₂CH ₂)(OC₆H₃Bu^t₂2,6)(CH ₂CH₂Ph)Cl] (9). Structural studies of 9 confirm the presence of a phenethyl group. The related trans-phenylvinyl compound 10 is produced when 3 is reacted with phenylacetylene. Addition of 2,6-dimethylphenyl isocyanide (xyNC) to 10 produces the bis(iminoacyl) derivative 11, in which xyNC has inserted into the cyclometalated carbon as well as the Ta-CH=CHPh bond in 10. Structural studies of 11 confirmed the trans arrangement of the hydrogen atoms in the phenylvinyl group. Mechanistic studies of the formation of 10 and 11 show the presence of two competing pathways. The first involves direct elimination of H₂ from the dihydride and formation of an intermediate olefin/alkyne adduct. The product then arises by CH bond activation of the aryloxide with the hydrogen transferring to a carbon atom of the tantalacyclopropane (tantalacyclopropene) ring. The second pathway involves insertion of olefin/alkyne into a Ta-H bond followed by CH bond activation by the remaining hydride.

Full text of DOI:10.1021/om990664l

4448
Organometallics 1999, 18, 4448-4458
Rea ction s of Alk yn es a n d Olefin s w ith Ta n ta lu m
Hyd r id es Con ta in in g Ar yloxid e An cilla r y Liga tion :
Releva n ce to Ca ta lytic Hyd r ogen a tion †
Douglas R. Mulford, J anet R. Clark, Scott W. Schweiger, Phillip E. Fanwick, and
Ian P. Rothwell*
Department of Chemistry, 1393 Brown Building, Purdue University,
West Lafayette, Indiana 47907
Received August 17, 1999
The reactivity of the three hydride compounds [Ta(OC6H3Ph2-2,6)2(H)2Cl(PMe3)2] (1), [Ta-
(OC6H3Pri -2,6)2(H)2Cl(PMe2Ph)2] (2), and [Ta(OC6H3But -2,6)2(H)2Cl(PMePh2)] (3) toward
2
2
olefins and alkynes has been investigated. The reactivity observed is highly dependent on
the nature of the ancillary aryloxide ligands. The 2,6-diphenylphenoxide 1 reacts with styrene
to produce 1 equiv of ethylbenzene and the styrene adduct [Ta(OC6H3Ph2-2,6)2(η2-
CH2dCHPh)Cl(PMe3)] (5). In contrast, 1 reacts with 3-hexyne to eliminate H2 along with
formation of the analogous alkyne complex 6. Structural studies of 5 and 6 show a square-
pyramidal geometry with an axial olefin (alkyne) unit lying along the Cl-Ta-P axis.
Structural parameters support a tantalacyclopropane (tantalacyclopropene) bonding picture
for these molecules. Compound 5 is converted back into 1 under H2 along with formation of
PhEt. The dihydride 2 reacts with styrene to form 1 equiv of PhEt, H2, and the
dehydrogenation product [Ta(OC6H3Pri-η2-CMedCH2)(OC6H3Pri -2,6)Cl(PMe2Ph)2] (7). The
2
related adduct [Ta(OC6H3Pri-η2-CMedCH2)(OC6H3Pri -2,6)Cl(PEt3)2] (9) was isolated by
2
treatment of [Ta(OC6H3Pri -2,6)2Cl3] with PEt3/Bu3SnH and was structurally characterized.
2
Labeling studies show that the H2 generated comes exclusively from the aryloxide o-Pri group
which was dehydrogenated. Both hydrides initially attached to the metal are transferred to
the olefin substrate. In the case of the 2,6-di-tert-butylphenoxide compound 3, reaction with
styrene generates the mono-cyclometalated compound [Ta(OC6H3ButCMe2CH2)(OC6H3But -
2
2,6)(CH2CH2Ph)Cl] (9). Structural studies of 9 confirm the presence of a phenethyl group.
The related trans-phenylvinyl compound 10 is produced when 3 is reacted with pheny-
lacetylene. Addition of 2,6-dimethylphenyl isocyanide (xyNC) to 10 produces the bis-
(iminoacyl) derivative 11, in which xyNC has inserted into the cyclometalated carbon as
well as the Ta-CHdCHPh bond in 10. Structural studies of 11 confirmed the trans
arrangement of the hydrogen atoms in the phenylvinyl group. Mechanistic studies of the
formation of 10 and 11 show the presence of two competing pathways. The first involves
direct elimination of H2 from the dihydride and formation of an intermediate olefin/alkyne
adduct. The product then arises by CH bond activation of the aryloxide with the hydrogen
transferring to a carbon atom of the tantalacyclopropane (tantalacyclopropene) ring. The
second pathway involves insertion of olefin/alkyne into a Ta-H bond followed by CH bond
activation by the remaining hydride.
In tr od u ction
pounds of niobium and tantalum have been isolated and
implicated as key intermediates in arene hydrogenation.
As an extension of this work, we have investigated the
reactivity of isolated hydride aryloxides of tantalum
with olefins and alkynes.3 In this paper we report the
results of this study, hinting at the plethora of reaction
pathways that are available to these systems.
It has been demonstrated that hydride derivatives of
niobium and tantalum containing aryloxide ligation
possess the ability to catalyze the hydrogenation of a
variety of arene substrates.1 This discovery has led us
to pursue studies that may give insights into the key
steps within the catalytic cycle. Previous work has
shown that cyclohexene and 1,3-cyclohexadiene are
hydrogenated many orders of magnitude faster than
arenes.2 Furthermore, stable 1,3-cyclohexadiene com-
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of Com p ou n d s.
The reactivity of the previously synthesized4 hydrides
† Dedicated to Professor Richard A. Walton on the occasion of his
60th birthday.
(1) Rothwell, I. P. J . Chem. Soc., Chem. Commun. 1997, 1331
(Feature Article).
(2) Visciglio, V. M.; Clark, J . R.; Nguyen, M. T.; Mulford, D. R.;
Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. Soc. 1997, 119, 3490.
(3) Clark, J . R.; Fanwick, P. E.; Rothwell, I. P. J . Chem. Soc. Chem.
Commun. 1995, 553.
(4) Parkin, B. C.; Clark, J . C.; Visciglio, V. M.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1995, 14, 3002.
10.1021/om990664l CCC: $18.00 © 1999 American Chemical Society
Publication on Web 09/21/1999

Reactions of Alkynes and Olefins with Ta Hydrides
Organometallics, Vol. 18, No. 21, 1999 4449
Sch em e 1
F igu r e 1. Molecular structure of [Ta(OC6H3Ph-η1-
C6H4)(OC6H3Ph2-2,6)(H)Cl(PMe3)2] (4).
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Ta (OC6H3P h -η1-C6H4)(OC6H3P h 2-2,6)-
(H)Cl(P Me3)2] (4)
Ta-Cl
2.500(1)
2.729(2)
2.603(1)
1.947(3)
1.910(3)
Ta-C(122)
Ta-H
O(1)-C(11)
O(2)-C(21)
2.220(5)
1.7(4)
1.347(6)
1.376(6)
Ta-P(3)
Ta-P(4)
Ta-O(1)
Ta-O(2)
Cl-Ta-P(3)
Cl-Ta-P(4)
Cl-Ta-O(1)
Cl-Ta-O(2)
Cl-Ta-C(122)
Cl-Ta-H
P(3)-Ta-P(4)
P(3)-Ta-O(1)
P(3)-Ta-O(2)
P(3)-Ta-C(122)
P(3)-Ta-H
75.45(5)
P(4)-Ta-O(1)
P(4)-Ta-O(2)
P(4)-Ta-C(122)
P(4)-Ta-H
O(1)-Ta-O(2)
O(1)-Ta-C(122)
O(1)-Ta-H
O(2)-Ta-C(122)
O(2)-Ta-H
C(122)-Ta-H
85.8(1)
91.8(1)
130.6(1)
66(1)
168.43(2)
83.4(2)
79(1)
89.7(2)
90(1)
65(1)
[Ta(OC6H3Ph2-2,6)2(H)2Cl(PMe3)2] (1), [Ta(OC6H3Pri -
2
77.47(5)
95.8(1)
94.8(1)
151.5(1)
143(1)
148.62(5)
81.5(1)
105.9(1)
76.3(1)
138(1)
2,6)2(H)2Cl(PMe2Ph)2] (2), and [Ta(OC6H3But -2,6)2(H)2-
2
Cl(PMePh2)] (3) are the focus of this paper. Seven-
coordinate 1 and 2 have been shown to adopt pentagonal-
bipyramidal structures both in solution and the solid
state with trans, axial aryloxide ligands. In the case of
six-coordinate 3 a dramatic distortion from octahedral
geometry is observed and the underlying electronic
reasons for the distortion have been investigated.5
During the course of this study we discovered that
compound 1 is thermally unstable. After 24 h a C6D6
solution of 1 is converted into a 50/50 mixture of 1 and
a new hydride product. The conversion is complete
within minutes at 100 °C, yielding the mono-cyclo-
metalated derivative 4 (Scheme 1), which shows a sharp
Ta-H resonance as a doublet of doublets at δ 18.68 ppm
coupling constants obtained for 4 are totally consistent
with the previous conclusions. Hence, the H,P and H,P′
coupling constants of 83 and 8 Hz as well as the P,P′
coupling of 108 Hz are comparable in magnitude to
those calculated for dihydrides such as 1 and 2.4 The
pathway of cyclometalation of the aryloxide ligand in 1
is relevant to the other reactivity in this paper and is
discussed in more detail in the mechanistic section
below.
1
in the H NMR spectrum and nonequivalent phosphine
ligands in the 31P NMR spectrum. The solid-state
structure of 4 (Figure 1, Table 1) shows pentagonal-
bipyramidal geometry with trans, axial oxygen atoms.
The structure is closely related to that of the precursor
1, except that the Ta-C bond of a cyclometalated 2,6-
diphenylphenoxide ligand has replaced one of the
original Ta-H groups. Detailed analysis of the 1H NMR
spectra of 1 and 2 indicates that the molecules are
stereochemically rigid.4 This results in an AA′XX′
spectrum for the hydride ligands coupled not only to
each other but the magnetically nonequivalent phos-
phine ligands. Simulation of the pattern yields all four
(H,H′, H,P, H,P′, and P,P′) coupling constants. The
The addition of excess styrene to C6D6 solutions of 1
leads to the slow formation of the new organometallic
product 5 as monitored by H NMR spectroscopy. The
1
solution was also found to contain uncoordinated PMe3
(31P NMR) and 1 equiv of ethylbenzene per tantalum
(Scheme 1). Compound 5 was structurally characterized
and can be seen to contain 1 equiv of styrene π-bound
to the metal center (Figure 2, Table 2). The C-C
distance of 1.452(7) Å for the bound styrene in 5 is
significantly elongated over the distance of 1.339 Å
found for ethylene but is comparable to the value of
1.477 Å found in [Cp*Ta(η2-C2H4)(dCHCMe3)(PMe3)].6
(5) Clark, J . R.; Pulvirenti, A. L.; Fanwick, P. E.; Sigalis, M.;
Eisenstein, O.; Rothwell, I. P. Inorg. Chem. 1997, 36, 3623.
(6) Schultz, A. J .; Brown, R. K.; Williams, J . M.; Schrock, R. R. J .
Am. Chem. Soc. 1981, 103, 169.

4450 Organometallics, Vol. 18, No. 21, 1999
Mulford et al.
F igu r e 3. Molecular structure of [Ta(OC6H3Ph2-2,6)2(η2-
EtCtCEt)Cl(PMe3)] (6).
F igu r e 2. Molecular structure of [Ta(OC6H3Ph2-2,6)2(η2-
CH2dCHPh)Cl(PMe3)] (5).
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Ta (OC6H3P h 2-2,6)2(η2-EtCtCEt)Cl(P Me3)]
(6)
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Ta (OC6H3P h 2-2,6)2(η2-CH2dCHP h )Cl-
(P Me3)] (5)
Ta-Cl
1.913(2)
2.633(1)
1.926(2)
1.913(2)
2.045(3)
2.092(4)
C(1)-C(2)
C(2)-C(3)
C(3)-C(4)
C(4)-C(5)
C(5)-C(6)
1.405(4)
1.500(6)
1.291(6)
1.504(7)
1.352(9)
Ta-P(2)
Ta-O(3)
Ta-O(4)
Ta-C(3)
Ta-C(4)
Ta-Cl
2.442(1)
1.895(3)
1.915(3)
2.609(1)
Ta-C(30)
Ta-C(31)
C(30)-C(31)
2.131(5)
2.242(5)
1.452(7)
Ta-O(10)
Ta-O(20)
Ta-P
O(4)-Ta-O(3)
O(3)-Ta-Cl
O(4)-Ta-Cl
O(3)-Ta-P(2)
O(4)-Ta-P(2)
Cl-Ta-P(2)
148.2(1)
89.45(7)
88.28(8)
82.38(7)
84.37(8)
150.98(4)
C(3)-Ta-C(4)
C(3)-C(4)-Ta
O(3)-Ta-C(3)
O(3)-Ta-C(4)
O(4)-Ta-C(3)
O(4)-Ta-C(4)
36.3(2)
73.5(2)
105.2(1)
106.5(1)
106.5(1)
100.8(1)
Cl-Ta-O(10)
Cl-Ta-O(20)
Cl-Ta-P
Cl-Ta-C(30)
Cl-Ta-C(31)
P-Ta-O(10)
P-Ta-O(20)
C(30)-Ta-C(31)
88.6(1)
P-Ta-C(30)
117.7(1)
79.1(1)
158.9(1)
98.4(2)
101.5(2)
102.6(2)
96.0(2)
89.1(1)
150.88(5)
91.3(1)
129.7(1)
89.6(1)
P-Ta-C(31)
O(10)-Ta-O(20)
O(10)-Ta-C(30)
O(10)-Ta-C(31)
O(20)-Ta-C(30)
O(20)-Ta-C(31)
82.3(1)
38.7(2)
NMR) along with 1 equiv of ethylbenzene (Scheme 1).
In the presence of an excess of styrene, 5 acts as a
hydrogenation catalyst precursor, yielding ethylbenzene
at rates varying from 1 equiv/day (room temperature,
20 psi H2) to 1 equiv/h (room temperature, 1200 psi H2).
Exhaustive hydrogenation of 20 equiv of styrene by 5
can occur in the presence of 1 equiv of PMe3 over 24 h
(room temperature, 1200 psi H2) with re-formation of 1
as determined by 31P NMR analysis of the final reaction
mixture.
Compound 1 reacts readily with 3-hexyne to generate
the corresponding alkyne adduct 6 (Scheme 1). How-
ever, in this case no hexene is produced. Instead, H2 is
detected in solution (1H NMR), and in fact bubbles of
gas are observed upon addition of 3-hexyne to a con-
centrated C6D6 solution of 1. Compound 6 adopts a
structure in the solid state similar to that observed for
5 (Figure 3, Table 3). The axially, π-bound hexyne unit
lies along the P-Ta-Cl plane with trans-basal arylox-
ides. The C-C distance of 1.291(6) Å is elongated over
a typical alkyne distance but is still 0.1 Å shorter than
that found in olefins. However, as with related early
d-block metal derivatives of alkynes, a tantalacyclopro-
pene description is justified. The solution spectroscopic
properties of 6 are consistent with the molecule adopting
an structure identical with that found in the solid state.
Furthermore, the presence of two sharp sets of CEt
signals shows that the alkyne unit is not undergoing
rotation on the NMR time scale. In the 1H NMR
spectrum the nonequivalent hexyne protons appear as
This parameter along with the Ta-C distances supports
a tantalacyclopropane description of the bonding in 5.7
The molecular structure of 5 is best described as square
pyramidal with trans-basal aryloxides. The π-bound
styrene occupies the axial site, and its C-C vector lies
along the P-Ta-Cl plane. The phenyl substituent is
proximal to the phosphine ligand. The Ta-CHPh dis-
tance of 2.242(5) Å is slightly longer than the Ta-CH2
distance of 2.131(5) Å, possibly for steric reasons. In the
13C NMR spectrum of 5 the Ta-CH2 carbon appears as
a doublet (2J coupling to 31P), while the Ta-CHPh
carbon is a singlet. This is consistent with the same
structure being adopted in solution as is observed in the
solid state. Studies of the olefin complexes [(ArO)2Ti-
(η2-olefin)(PR3)] show stronger 31P coupling to the distal
carbon of the olefin (larger P-Ti-C angle).8 The PMe3
protons in 5 appear upfield as a sharp doublet at δ
-0.01 ppm. This upfield shifting is due to the diamag-
netic anisotropy of the adjacent 2,6-diphenylphenoxide
ligands.
When a solution of 5 in the presence of an extra 1
equiv of PMe3 is exposed to H2 at ambient temperatures,
regeneration of dihydride 1 is observed (1H and 31P
(7) Yu, J . S.; Felter, L.; Potyen, M. C.; Clark, J . R.; Visciglio, V. M.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 1996, 15, 4443.
(8) Thorn, M. G.; Hill, J . E.; Waratuke, S. A.; J ohnson, E. S.;
Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. Soc. 1997, 119, 8630.

Reactions of Alkynes and Olefins with Ta Hydrides
Organometallics, Vol. 18, No. 21, 1999 4451
Sch em e 2
two sets of quartets (nondiastereotopic CH2) and triplets
(CH3). Hence, a virtual plane runs through the unit with
each end being nonequivalent. The Ta(η2-EtCtCEt)
carbon atoms give rise to a doublet at δ 188.5 ppm (2J
coupling to 31P) and a singlet at δ 195.0 ppm.
The addition of 3-hexyne to a C6D6 solution of the
styrene complex 5 was found to slowly (1H and 31P
NMR) form the alkyne adduct 6 and free styrene
(Scheme 1). However, even in the presence of a large
excess of styrene the alkyne adduct 6 failed to form any
detectable amounts of 5. This clearly demonstrates a
much stronger affinity of the metal and this particular
ligand set for 3-hexyne over styrene.
Figu r e 4. Molecular structure of [Ta(OC6H3Pri2-2,6)(OC6H3-
Pri-η2-CMedCH2)Cl(PEt3)2] (8).
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Ta (OC6H3P r i2-2,6)(OC6H3P r i-η2-
CMedCH2)Cl(P Et3)2] (8)
Ta-Cl
Ta-O(1)
Ta-O(2)
2.484(2)
1.981(5)
1.922(5)
Ta-C(121)
Ta-C(122)
Ta-P(3)
2.193(7)
2.295(7)
2.651(2)
O(2)-Ta-O(1)
O(1)-Ta-C(121)
O(1)-Ta-C(122)
Cl-Ta-O(2)
Cl-Ta-C(122)
Ta-O(2)-C(21)
178.1(2)
O(2)-Ta-C(121)
O(2)-Ta-C(122)
Cl-Ta-O(1)
Cl-Ta-C(121)
Ta-O(1)-C(11)
96.4(3)
102.4(3)
85.0(2)
153.1(2)
123.2(5)
82.8(3)
75.9(2)
96.4(2)
156.0(2)
177.6(5)
The addition of styrene to the di-isopropylphenoxide
2 generates both 1 equiv of ethylbenzene and H2
(Scheme 2). The organometallic product 7 was identified
spectroscopically as containing an aryloxide chelated to
the metal via an R-methylvinyl group generated by
dehydrogenation of an original isopropyl substituent.
Previous studies have shown that dehydrogenation of
the ortho substituents of 2,6-diisopropylphenoxide can
occur at d2 niobium metal centers.7 The proposed
also contains a phenethyl group attached to the metal
center (Scheme 3). No ethylbenzene is detected in the
1H NMR spectrum of the reaction mixture. However, a
singlet at δ 4.48 ppm (C6D6) can be assigned to H2
generated during the reaction. Uncoordinated PMePh2
is also observed. The solid-state structure of 9 (Figure
5, Table 5) shows a trigonal-bipyramidal geometry about
the metal center with the oxygen atom of the cyclom-
etalated aryloxide and the chloro group in axial posi-
tions. The phenethyl ligand can be clearly seen to
contain the phenyl substituent attached to the â-carbon.
The presence of the cyclometalated aryloxide is indi-
cated by the presence of an AB pattern and a pair of
structure for [Ta(OC6H3Pri-η2-CMedCH2)(OC6H3Pri -
2
2,6)Cl(PMe2Ph)2] (7) is given strong support in the
structurally characterized complex [Ta(OC6H3Pri -
2
2,6)(OC6H3Pri-η2-CMedCH2)Cl(PEt3)2] (8), which was
obtained during the reaction of the trichloride
[Ta(OC6H3Pri -2,6)Cl3] with PEt3/Bu3SnH. Compound
1
2
methyl singlets in the H NMR spectrum assignable to
8 adopts a structure (Figure 4, Table 4) that is very
similar to that of the previously reported dcpm deriva-
tive.9 Furthermore, complex 7 is also generated when
2 is added to either 1,3-cyclohexadiene or cyclohexene,
yielding cyclohexene or cyclohexane, respectively.2 In
the proton-decoupled 13C NMR spectra of 7 the η2-
CMedCH2 carbon atoms appear as two doublets (2J
coupling to 31P) at δ 71.9 and 68.2 ppm, respectively.
the Ta-CH2CMe2 protons.10 The protons of the tert-
butyl groups attached to the nonmetalated aryloxide
appear as two broad resonances due to restricted
rotation about the Ta-O-Ar bonds within the crowded
coordination sphere. The Ta-CH2CH2Ph methylene
protons are all diastereotopic and appear as two muli-
tiplets centered at δ 3.75 (â) and 2.40 ppm (R). The
results of labeling studies are discussed in the mecha-
1
In the H NMR spectrum the methyl peak is a sharp
1
nistic section. The J (1H-13C) coupling constant of 119
singlet, while the methylene protons appear as overlap-
ping multiplets due to their similar chemical shifts. The
possible mechanism of formation of 7 is discussed below.
The addition of styrene to the dihydride 3 results in
formation of the mono-cyclometalated product 9, which
Hz measured for the R-CH2CHPh carbon in the 13C
NMR spectrum appears normal.
Treatment of 3 with phenylacetylene leads to the
related cyclometalated species 10 (Scheme 3). Besides
(10) (a) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J . C. J . Am.
Chem. Soc. 1986, 108, 1502. (b) Latesky, S. L.; McMullen, A. K.;
Rothwell, I. P.; Huffman, J . C. J . Am. Chem. Soc. 1985, 107, 5981.
(9) Riley, P. N.; Clark, J . R.; Fanwick, P. E.; Rothwell, I. P. Inorg.
Chim. Acta 1999, 288, 35.

4452 Organometallics, Vol. 18, No. 21, 1999
Mulford et al.
F igu r e 6. Molecular structure of [Ta(OC6H3ButCMe2-
CH2)(OC6H3But -2,6)(CHdCHPh)Cl] (10).
2
Ta ble 5. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Ta (OC6H3Bu tCMe2CH2)(OC6H3Bu t2-2,6)-
(CH2CH2P h )Cl] (9)
F igu r e 5. Molecular structure of [Ta(OC6H3ButCMe2-
Ta-Cl
2.383(2)
1.873(4)
1.863(4)
Ta-C(10)
Ta-C(222)
2.145(7)
2.171(7)
CH2)(OC6H3But -2,6)(CH2CH2Ph)Cl] (9).
2
Ta-O(2)
Ta-O(3)
Sch em e 3
Cl-Ta-O(2)
Cl-Ta-O(3)
Cl-Ta-C(10)
Cl-Ta-C(222)
O(2)-Ta-O(3)
166.4(1)
93.4(1)
87.2(2)
85.5(2)
96.8(2)
O(2)-Ta-C(10)
O(2)-Ta-C(222)
O(3)-Ta-C(10)
O(3)-Ta-C(222)
C(10)-Ta-C(222)
94.5(2)
81.6(2)
124.0(2)
121.5(2)
114.4(3)
Ta ble 6. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Ta (OC6H3Bu tCMe2CH2)(OC6H3Bu t2-2,6)-
(CHdCHP h )Cl] (10)
Ta-Cl
Ta-O(2)
Ta-C(222)
2.369(1)
1.871(3)
2.173(5)
Ta-C(1)
Ta-O(3)
C(1)-C(2)
2.124(5)
1.863(3)
1.333(7)
O(3)-Ta-O(2)
O(3)-Ta-C(1)
O(3)-Ta-Cl
98.8(1)
O(2)-Ta-C(1)
92.84(18)
164.83(10)
126.2(4)
121.38(17) O(2)-Ta-Cl
93.56(10) C(2)-C(1)-Ta
C(221)-C(222)-Ta 119.9(4)
no agostic interaction with the metal is occurring for
the R-CH bond.11
Both 9 and 10 can be synthesized on a larger scale
by treating a mixture of the trichloride [Ta(OC6H3But -
2
2,6)2Cl3] and styrene or phenylacetylene with Bun3SnH
in benzene solvent. Previous work has shown that only
two of the Ta-Cl bonds in the trichloride react with the
hydride reagent to form Ta-H bonds.4 It is therefore
possible that the dihydride [Ta(OC6H3But -2,6)2(H)2Cl]
2
is first generated in the reaction mixtures prior to
reaction with olefin or alkyne. Treatment of 10 with
excess 2,6-dimethylphenyl isocyanide (xyNC) led to the
new product 11, in which the isocyanide has inserted
into both the vinyl and metalated Ta-C bonds (Scheme
3). The presence of two η2-iminoacyl groups is indicated
by the appearance of two resonances at δ 243.0 and
228.2 ppm in the 13C NMR spectrum.12 The solid-state
structure of 11 (Figure 7, Table 7) confirms the molecu-
larity and also confirms the trans orientation of the
original phenyl-vinyl group (Figure 8). If one considers
the presence of the aryloxide ligand signals, a sharp pair
of doublets are observed in the H NMR spectrum of 10
1
at δ 9.48 and 8.48 ppm. These are assigned as being
due to the R- and â-protons of a â-phenylvinyl ligand.
3
The J coupling of 18 Hz between these protons indi-
cates they are mutually trans, as shown in Scheme 3
and supported by the solid-state structure (Figure 6,
Table 6). The R-vinyl carbon Ta-CHdCHPh of this
group resonates at δ 203.5 ppm. This is the region
typical for vinyl groups attached to early d-block metals,
(11) Zwettler, R.; Hyla-Kryspin, I.; Gleiter, R.; Kruger, C.; Erker,
G. Organometallics 1990, 9, 517.
(12) Rothwell, I. P.; Durfee, L. D. Chem. Rev. 1988, 88, 1059.
1
and the J (1H-13C) coupling constant of 133 Hz implies

Reactions of Alkynes and Olefins with Ta Hydrides
Organometallics, Vol. 18, No. 21, 1999 4453
Sch em e 4
Figu r e 7. Molecular structure of[Ta(OC6H3But -2,6)(OC6H3-
2
ButCMe2CH2-η2-CdNxy)(PhCHdCH-η2-CdNxy)Cl] (11).
metathesis at a d0 metal center or by addition of the
CH bond to a d2 metal intermediate.14 This mechanistic
quandary exists for the conversion of 1 to 4. It is highly
unlikely that σ-bond metathesis15 could occur directly
within seven-coordinate 1. Besides being sterically
crowded, all of the equatorial orbitals within the pen-
tagonal-bipyramidal structure are involved in bonding,
while the formally unoccupied dxz and dyz orbitals will
be involved in π-bonding to the axial aryloxides. Hence,
for σ-bond metathesis to occur, initial dissociation of
phosphine would have to occur, yielding an unsaturated
intermediate. Alternatively, direct elimination of H2 is
feasible, yielding a five-coordinate d2 Ta(III) intermedi-
ate which can oxidatively add (insert into) the aryloxide
CH bond (Scheme 4). To rule out one of these pathways,
two C6D6 solutions of 1 were monitored over time by
1H NMR spectroscopy. To one of the solutions was
initially added PMe3 (5 equiv by integration). After 24
h the solution of 1 with no added PMe3 had converted
to a 60/40 mixture of 4/1, as judged by integration of
the hydride signals. Only a trace amount of 4 was
observed in the solution of 1 to which the PMe3 had been
added. Hence, the dramatic inhibition of CH bond
activation by added phosphine argues strongly for the
reaction proceeding via initial phosphine dissociation
followed by σ-bond metathesis.
All of the reactions of the hydrides 1-3 with olefins
and alkynes are inhibited by the presence of excess
phosphines. Hence, in all cases it appears that initial
dissociation of phosphine has to occur in order to allow
the various substrates to coordinate to the metal center.
In the case of 1 addition of styrene leads to 1 equiv of
ethylbenzene and the complex 5 (Scheme 1). However,
3-hexyne leads to the hexyne adduct 6 with elimination
of H2. It seems reasonable to assume that both proceed
via olefin/alkyne intermediates (Scheme 5). Clearly,
insertion of styrene must proceed to generate a phen-
F igu r e 8. Central coordination sphere of [Ta(OC6H3But -
2
2,6)(OC6H3ButCMe2CH2-η2-CdNxy)(PhCHdCH-η2-CdNxy)-
Cl] (11).
Ta ble 7. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Ta (OC6H3Bu t2-2,6)(OC6H3Bu tCMe2CH2-η2-
CdNxy)(P h CHdCH-η2-CdNxy)Cl] (11)
Ta-Cl
2.424(3)
1.936(7)
1.960(8)
2.123(9)
2.20(1)
Ta-C(40)
2.14(1)
2.11(1)
1.48(2)
1.35(2)
1.45(2)
1.27(1)
Ta-O(10)
Ta-O(20)
Ta-N(30)
Ta-N(50)
N(30)-C(40)
Ta-C(50)
C(40)-C(41)
C(41)-C(42)
C(42)-C(43)
N(50)-C(50)
1.29(1)
C(40)-C(41)-C(42)
N(30)-Ta-C(40)
Ta-O(10)-C(11)
121(1)
C(41)-C(42)-C(43) 127(1)
34.3(4)
148.6(7)
35.1(4) N(50)-Ta-C(50)
153.4(7) Ta-O(20)-C(12)
the iminoacyl groups to occupy single sites then the
structure of 11 is best described as trigonal bipyramidal
about the metal center with axial oxygen atoms. The
two η2-iminoacyl groups and the chloride ligand occupy
equatorial sites (Figure 8). An alternative description
is as a pentagonal-bipyramidal geometry. The η2-imi-
noacyl groups lie within the equatorial plane of the
molecule with the carbon atoms mutually cis. An
identical geometry is observed for the related molecule
[Ta(OC6H3Me2-2,6)2(η2-xyNCMe)2(Me)].13 The structural
parameters for the η2-iminoacyl groups are similar to
those reported for related molecules.13
Mech a n ism . We have carried out a number of
experiments to try and gain mechanistic insight into the
reactions outlined in Schemes 1-3. A number of the
reactions involve CH bond activation of aryloxide ligands.
It is now well-established that such reactions can occur
for niobium and tantalum aryloxides either by σ-bond
(14) (a) Rothwell, I. P. Acc. Chem. Res. 1988, 21, 153. (b) Steffey, B.
D.; Chamberlain, L. R.; Chesnut, R. W.; Chebi, D. E.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1989, 8, 1419.
(15) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J .;
Nolan, M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J . E. J . Am.
Chem. Soc. 1987, 109, 203.
(13) Chamberlain, L. R.; Durfee, L. D.; Fanwick, P. E.; Kobriger, L.
M.; Latesky, S. L.; McMullen, A. K.; Rothwell, I. P.; Folting, K.;
Huffman, J . C.; Streib, W. E.; Wang, R. J . Am. Chem. Soc. 1987, 109,
390.

4454 Organometallics, Vol. 18, No. 21, 1999
Mulford et al.
Sch em e 5
ethyl group. Reductive elimination of ethylbenzene could
then generate an unsaturated intermediate that can
coordinate styrene. There is precedence for this pathway
in the reactivity of 2 below. An alternative pathway is
via a bis(phenethyl) intermediate formed by insertion
of 2 equiv of styrene. The final styrene complex 5 is then
formed via â-hydrogen abstraction from this bis(alkyl)
intermediate. As a possible test of this hypothesis the
trichloride [Ta(OC6H3Ph2-2,6)2Cl3] was reacted with
PhCH2CH2MgCl (thf solution, 2 equiv) in benzene in the
presence of excess PMe3. Analysis of the reaction
mixture by 31P NMR did show trace amounts of 5 with
other unidentified products. Hence, this result indicates
that under these conditions a bis(phenethyl) species
does not lead to 5 in high yield.
No hexene is detected during the formation of the
alkyne adduct 6. However, there are still two possible
pathways for the elimination of H2. The first involves
reductive elimination of H2 from the d0 alkyne adduct,
while the second proceeds via an extra step in which
CH bond activation occurs from a vinyl hydride inter-
mediate (Scheme 5). We have also shown that 6 can be
formed in moderate yield by carrying out the sodium
amalgam reduction (2 Na per Ta) of [Ta(OC6H3Ph2-2,6)2-
Cl3] in the presence of PMe3 and 3-hexyne.
PhCDdCD2 in C6D6 solvent showed no incorporation of
H into the ethylbenzene and the formation of H2 (no
detectable HD). These results are consistent with a
reaction pathway involving initial insertion of styrene
followed by clean elimination of ethylbenzene to gener-
ate a d2 Ta(III) fragment (Scheme 6). No CH bond
activation occurs within the intermediate phenethyl
hydride. The low-valent metal center then induces the
dehydrogenation of an o-Pri group via CH bond activa-
tion and â-hydrogen abstraction and elimination of H2.
Some support for this pathway is given by the fact that
reduction of [Ta(OC6H3Pri -2,6)2Cl3] with sodium amal-
2
gam (2 Na per Ta) in the presence of dcpm leads to an
analogue of 7.9 Hence, in the proposed mechanism
(Scheme 6) the first equivalent of styrene acts in effect
as a reducing agent toward the dihydride to generate
the d2 metal intermediate. These results also imply that
in this case the low-valent species is not trapped by
styrene to generate an analogue of 5.
Similar labeling studies of the reaction of 3 toward
styrene and phenylacetylene show a much more complex
situation. This reagent only reacts with 1 equiv of
substrate to produce mono-cyclometalated 9 and 10 and
1 equiv of H2 (Scheme 3). The addition of the dideuteride
[Ta(OC6H3But -2,6)2(D)2Cl(PMePh2)]4 to PhCDdCD2 in
2
A great deal of mechanistic insight into the reactivity
of 2 with styrene has been gained using labeling
experiments (Scheme 6). This reaction produces the
dehydrogenation product 7 along with 1 equiv of H2 and
ethylbenzene. Treatment of the dideuteride [Ta(OC6H3-
C6D6 produces detectable amounts of HD, as evidenced
by a 1:1:1 triplet at δ 4.46 ppm (δ 4.48 for H2 under
1
these conditions) in the H NMR spectrum. The phen-
ethyl group within the compound 9 generated in this
reaction also contained significant amounts of protio
label exclusively within the â-methylene group (Scheme
7). Two equal-intensity broad singlets were present at
δ 3.68 and 3.73 ppm, assigned to protio label at the two
(diastereotopic) Ta-CD2CHDPh positions. No protio
label was observed (1H NMR) in the R-methylene group.
Integration of the phenethyl proton signal versus the
Ta-CH2CMe2 AB pattern (metalated aryloxide) indi-
Pri -2,6)2(D)2Cl(PMe2Ph)2] with protiostyrene in C6D6
2
solvent was observed to produce H2 with no detectable
amounts of HD (1H NMR). When the reaction was
2
carried out in C6H6, the H NMR spectrum showed the
formation of PhCHDCH2D with equal amounts of
deuterium label in the methylene and methyl positions.
Addition of [Ta(OC6H3Pri -2,6)2(D)2Cl(PMe2Ph)2] to
2

Reactions of Alkynes and Olefins with Ta Hydrides
Organometallics, Vol. 18, No. 21, 1999 4455
Sch em e 6
1
cated the presence of 32% TaCD2CHDPh groups, with
the remainder being TaCD2CD2Ph. The addition of
protio 3 to PhCDdCD2 in C6D6 was found to produce
exclusively Ta-CD2CHDPh groups along with H2
(Scheme 7).
These results imply that two competing pathways are
operating for the formation of 9 (Scheme 8). The first
pathway involves initial insertion of styrene into one
of the Ta-H(D) bonds. The final product then arises via
cyclometalation within the phenethyl hydride interme-
diate. The competing pathway involves initial loss of H2
(D2) and formation of a styrene complex. The CH bond
activation then involves opening of the tantalacyclopro-
pane ring to generate a phenethyl group. This step,
which constitutes about one-third of the reactivity,
generates the Ta-CD2CHDPh group in the reaction of
the dideuteride with styrene-d8.
groups (Scheme 9). In the H NMR spectrum a pair of
doublets are present due to the formation of
Ta-CHdCHPh groups. However, also present is a
broad singlet at δ 9.47 ppm, showing the presence of
Ta-CHdCDPh groups. Integration of these signals
versus the Ta-CH2CMe2 AB pattern showed no deute-
rium label in the R-CH position of the vinyl group. The
ratio of Ta-CHdCHPh/Ta-CHdCDPh groups is mea-
sured as 58/42. Similarly, the addition of [Ta(OC6H3-
But -2,6)2(D)2Cl(PMePh2)2] to PhCtCD in C6D6
2
also produces two types of â-phenylvinyl groups
(Scheme 9). A singlet at δ 8.44 ppm can be assigned to
Ta-CDdCHPh (43%) with the remaining 57% of mol-
ecules containing Ta-CDdCDPh groups. As with the
styrene reaction, these results imply two competing
pathways are operating for the formation of 10. The first
pathway is via elimination of H2 (D2) and formation of
an alkyne complex which undergoes ring opening via
CH bond activation. The second pathway involves
insertion followed by elimination of H2 by CH bond
activation by the remaining Ta-H(D) bond. The results
A similar situation is found for the reaction of 3
toward phenylacetylene. The addition of the labeled
compound [Ta(OC6H3But -2,6)2(D)2Cl(PMePh2)2] to
2
PhCtCH in C6D6 produces two types of â-phenylvinyl

4456 Organometallics, Vol. 18, No. 21, 1999
Mulford et al.
Sch em e 7
can take place to regenerate the initial dihydride. The
elimination path generates a d2 metal species that can
either bind with olefin or add H2. The actual path
followed will clearly be influenced by the concentration
of H2, and it appears further insight into the mechanism
can only be obtained by kinetic studies.
Exp er im en ta l Section
All operations were carried out under
a dry nitrogen
atmosphere or in vacuo either in a Vacuum Atmosphere Dri-
Lab or by standard Schlenk techniques. Hydrocarbon solvents
were dried by distillation from sodium/benzophenone and
stored under dry nitrogen. Trimethylphosphine was purchased
from Strem Chemical Co., and 1,3-cyclohexadiene and styrene
were purchased from Aldrich Chemical Co. All reagents were
dried over 3 Å molecular sieves prior to use. The 1H and 13C
NMR spectra were recorded on a Varian Associates Gemini
200 and a General Electric QE-300 spectrometer and were
referenced using protio impurities of commercial benzene-d6
as an internal standard. Microanalytical data were obtained
in house at Purdue. All high-pressure reactions were per-
formed in a Parr Model 4561 300 mL internal volume minire-
actor.
The X-ray diffraction studies were completed in house at
Purdue University. Crystal data and data collection param-
eters are given in Table 8. The crystals were examined under
deoxygenated Nujol and mounted in an appropriate size glass
capillary surrounded by epoxy resin. They were aligned and
indexed, and data were collected in an Enraf-Nonius CAD4
diffractometer. General operating procedures have been
reviewed.16
P r ep a r a tion of [Ta (OC6H3P h -η1-C6H4)(OC6H3P h 2-2,6)-
(H)Cl(P Me3)2] (4). A suspension of [Ta(OC6H3Ph2-2,6)2(H)2-
Cl(PMe3)2] in C6D6 in an NMR tube was heated at 100 °C for
4 min. Crystals grew upon cooling, which were washed with
hexane and dried in vacuo. 1H NMR (C6D6, 30 °C): δ 18.68
(dd, 1H, Ta-H; 2J (31P-1H) ) 83 and 8 Hz); 6.60-7.95 (m, 25H,
aromatics); 0.58 (d, 18H, P-Me3); 2J (31P-1H) ) 9.3 Hz. 31P
NMR (C6D6, 30 °C): δ -0.873 (d), -21.81 (d), 2J (31P-31P) )
108.1 Hz.
P r ep a r a tion of [Ta (OC6H3P h 2-2,6)2(η2-CH2dCHP h )Cl-
(P Me3)] (5). To a concentrated solution of [Ta(OC6H3Ph2-2,6)2-
Cl(H)2(PMe3)2] (0.2 g, 0.23 mmol) in benzene-d6 (1 mL) was
added styrene (0.12 g, 1.2 mmol). The reaction mixture was
allowed to stand for 20 h, and the resulting red solution was
layered with hexane (1 mL), yielding the product as red
crystals which were washed with hexane and dried in vacuo.
Anal. Calcd for C47H43PClO2Ta: C, 63.34; H, 4.89; Cl, 3.95; P,
3.50. Found: C, 63.26; H, 4.83; Cl, 4.24; P, 3.62. 1H NMR (C6D6,
30 °C): δ 2.01 (m, 1H), 1.55 (m, 1H), 0.62 (m, 1H, η2-CH2d
CHPh); -0.01 (d, 9H, P-Me3); 2J (31P-1H) ) 7.8 Hz. 13C NMR
(C6D6, 30 °C): δ 63.1 (d, 2J (13C-31P) ) 8.0 Hz), 60.1 (s,
TaCH2CHPh). 31P NMR (C6D6, 30 °C): δ 1.46.
of the labeling studies in this case show (within error)
an approximately equal probability for the two path-
ways.
P r ep a r a t ion of [Ta (OC6H 3P h 2-2,6)2(η2-E t CtCE t )Cl-
(P Me3)] (6). Meth od 1. To a suspension of [Ta(OC6H3Ph2-
2,6)2(H)2Cl(PMe3)2] (0.41 g, 0.48 mmol) in benzene was added
3-hexyne (0.117 g, 1.43 mmol, 3.0 equiv), and the solution was
allowed to sit overnight. The resulting yellow crystalline solid
was dried in vacuo to yield 0.27 g (66%) of 6. Anal. Calcd for
Con clu sion s
All three hydrides 1-3 act as catalysts (precursors)
for the hydrogenation of olefins and arenes. This work
has identified a number of reactions that have to be
considered as potential steps within the catalytic cycle
(Scheme 10). All of the steps shown in Scheme 10, which
begin with a d0 metal dihydride, have been observed.
Hence, although initial insertion of olefin into a metal-
hydride bond can occur, the elimination of H2 and
formation of a metallacyclopropane species must also
be considered. Two reaction paths are also possible for
the intermediate alkyl hydride. Direct elimination of
alkane can occur, or hydrogenolysis of the metal alkyl
C
45H45TaClPO2: C, 62.47; H, 5.24; Cl, 4.10; P, 3.58. Found:
1
C, 61.66; H, 5.10; Cl, 4.39; P, 3.05. H NMR (C6D6, 30 °C): δ
6.86-7.50 (m, 26H, aromatics); 2.94 (quartet, 2H, TaCCH2-
CH3); 1.86 (quartet, 2H, TaCCH2CH3); 1.46 (t, 3H, TaCCH2CH3);
0.67 (t, 3H, TaCCH2CH3); 0.23 (d, 9H, P-Me3); J (31P-1H) )
2
7.98 Hz. 13C NMR (CDCl3, 30 °C): δ 195.00 (s, TaCCH2CH3);
188.54 (d, TaCCH2CH3, 2J (31P-13C) ) 15.64 Hz); 156.87 (s,
TaOC); 121.34-139.62 (singlets, TaOC6H3Ph2-2,6); 30.20 (s,
(16) Fanwick, P. E.; Ogilvy, A. E.; Rothwell, I. P. Organometallics
1987, 6, 73.

Reactions of Alkynes and Olefins with Ta Hydrides
Organometallics, Vol. 18, No. 21, 1999 4457
Sch em e 8
Ta ble 8. Cr ysta l Da ta a n d Da ta Collection P a r a m eter s
4
5
6
8
9
10
11
formula
TaClP2O2-
TaClPO2C47H43
TaClPO2C45H45
TaClP2O2C36H62
TaClO2C36H50
TaClO2C36H47
TaClO2N2-
C
42H44
C59H78
fw
859.17
887.24
865.23
805.24
731.20
728.18
1063.69
space group
(No.)
a, Å
b, Å
c, Å
P21/c (14)
P21/n (14)
P1h (2)
P21/c (14)
P21/c (14)
P21/c (14)
P21/n (14)
10.959(3)
18.965(5)
19.067(3)
90
105.528(16)
90
3818(3)
4
1.502
9.810(1)
17.858(3)
22.981(84)
90
100.57(1)
90
3957(2)
4
1.489
9.099(2)
9.9130(11)
22.2217(14)
84.344(7)
83.152(11)
80.687(13)
1957.3(9)
2
11.092(2)
30.216(5)
11.589(2)
90
101.697(11)
90
3803.2(2)
4
1.406
10.0577(5)
18.590(2)
18.648(1)
90
90.3(3)
90
3486.8(8)
4
1.393
16.282(3)
11.548(3)
18.798(4)
90
107.252(16)
90
3375.3(3)
4
1.433
10.499(3)
18.676(5)
27.583(8)
90
98.29(3)
90
5351(5)
4
1.320
293
R, deg
â, deg
γ, deg
V, Å3
Z
F
calcd, g cm-3
1.468
296
temp, K
radiation
(λ, Å)
R
296
293
193
293
193
Mo KR (0.710 73)
0.031
0.036
0.029
0.036
0.024
0.060
0.053
0.147
0.033
0.039
0.042
0.090
0.047
0.054
Rw
2
TaCCH2CH3); 28.21 (s, TaCCH2CH3); 12.78 (d, PMe3, J (31P-
13C) ) 19.81 Hz). 31P NMR (C6D6, 30 °C): δ -6.43 (s).
Meth od 2. To a sodium amalgam (0.12 g of Na/5 mL of Hg)
was added [Ta(OC6H3Ph2-2,6)2Cl3] (2.0 g, 2.57 mmol) in
benzene (25 mL) and an excess of 3-hexyne. Several equiva-
lents of trimethylphosphine was added via condensation
through a vacuum line. This solution was stirred for 3 days,
resulting in a yellow-brown solution which was dried in vacuo
to give the desired product as a yellow solid, which was washed
with hexane to yield 0.70 g (32%).
NMR (C6D6, 30 °C): δ 6.78-7.34 (m, 16H, aromatics); 3.72
(septet, 3H, CHMe); 3.07 (m, 2H, Ta-CH2); 2.40 (s, 3H,
Ta-CMe); 1.34 (d, 6H, PMe, 2J (31P-1H) ) 7.63 Hz) 1.24 (d,
2
6H, PMe, J (31P-1H) ) 6.90 Hz); 1.04-1.23 (m, 18H, CHMe).
13C NMR (C6D6, 30 °C): δ 156.3 (TaOC); 155.0 (TaOC); 121.8-
2
140.1 (aromatics); 71.95 (d, Ta-CMe), J (31P-13C) ) 7.1 Hz;
68.2 (d, Ta-CH2), 2J (31P-13C) ) 7.1 Hz; 11.4-26.9 (aliphatics).
2
31P NMR (C6D6, 30 °C): δ -0.67 (d), -4.73 (d), J (31P-31P) )
145 Hz.
P r ep a r a t ion of [Ta (OC6H 3Bu t CMe2CH 2)(OC6H 3Bu t
-
2
P r ep a r a t ion of [Ta (OC6H 3P r i-η2-CMedCH 2)(OC6H 3-
P r i2-2,6)Cl(P Me2P h )2] (7). To a concentrated solution of
2,6)(CH2CH2P h )Cl] (9). To a suspension of [Ta(OC6H3But -
2
2,6)2Cl3] (0.5 g, 0.71 mmol) in hexane (5 mL) was added styrene
(0.37 g, 3.58 mmol) followed by tri-n-butyltin hydride (0.63 g,
2.16 mmol), and the resulting mixture was allowed to stand
for 18 h. Removal of the solvent in vacuo left the crude product
as a yellow oil. Layering the oil with pentane (3 mL) yielded
9 as yellow crystals that were washed with pentane and dried
[Ta(OC6H3Pri -2,6)2Cl(H)2(PMe2Ph)2] (0.2 g, 0.24 mmol) in
2
benzene-d6 (1 mL) was added styrene (0.12 g, 1.2 mmol). The
reaction was allowed to stand for 20 h, and a red solution was
observed. Removal of the solvent in vacuo left the product as
1
a red oil which could not be purified by recrystallization. H

4458 Organometallics, Vol. 18, No. 21, 1999
Mulford et al.
Sch em e 9
Sch em e 10
TaCHdCHPh), 2J (1H-1H) ) 18.0 Hz; 6.81-7.35 (m, 11H,
2
aromatics); 3.29 (d, 1H), 2.51 (d, 1H, Ta-CH2), J (1H-1H) )
14.1 Hz; 1.62 (br, 9H), 1.39 (br, 9H, CMe3); 1.49 (s, 3H), 1.54
(s, 3H, CMe2); 1.44 (s, 9H, CMe3). 13C NMR (C6D6, 30 °C): δ
203.5 (TaCHdCHPh); 163.5 (TaOC); 156.2 (TaOC); 101.8
(Ta-CH2CMe2).010
P r ep a r a tion of [Ta (OC6H3Bu t2-2,6)(OC6H3Bu tCMe2CH2-
η2-CdNxy)(P h CHdCH-η2-CdNxy)Cl] (11). To a concen-
trated solution of [Ta(OC6H3But -2,6)(OC6H3ButCMe2CH2)-
2
(CHdCHPh)(Cl)] (10; 0.1 g, 0.14 mmol) in benzene-d6 (1 mL)
was added 2,6-dimethylphenyl isocyanide (0.05 g, 0.34 mmol).
The reaction mixture was allowed to stand for 20 h, and then
the resulting red solution was layered with pentane (1 mL),
yielding the product as orange crystals of the pentane solvate,
which were washed with pentane and dried in vacuo. Anal.
Calcd for C54H66ClN2O2Ta‚C5H12: C, 66.62; H, 7.39; Cl, 3.33;
N, 2.63. Found: C, 66.26; H, 7.00; Cl, 3.33; N, 2.82. Selected
spectroscopic data are as follows. 1H NMR (C6D6, 30 °C): δ
in vacuo. Anal. Calcd for C36H50ClO2Ta: C, 59.14; H, 6.89; Cl,
4.85. Found: C, 59.34; H, 6.91; Cl, 4.24. 1H NMR (C6D6, 30
°C): δ 6.83-7.26 (m, 11H, aromatics); 3.73 (m), 3.77 (m, 1H,
2
8.05 (d, 1H, CHdCHPh), 7.80 (d, 1H, CHdCHPh), J (1H-1H)
2
PhCH2CH2); 2.82 (d, 1H), 2.64 (d, 1H, Ta-CH2, J (1H-1H) )
2
) 16.3 Hz; 3.63 (d, 1H), 3.45 (d, 1H, CH2CMe2), J (1H-1H) )
15.2 Hz); 2.40 (m, 2H, PhCH2CH2); 1.62 (br, 9H), 1.40 (br, 9H,
11.4 Hz. 13C NMR (C6D6, 30 °C): δ 243.0 (Ta-C{CH2}dN);
228.2 (Ta-C{CH}dN); 50.0 (CHdCHPh).
CMe3); 1.42 (s, 3H), 1.44 (s, 3H, CMe2); 1.34 (s, 9H, CMe3). 13
C
NMR (C6D6, 30 °C): δ 162.8 (TaOC); 155.8 (TaOC), 105.6
(Ta-CH2CMe2-); 97.8 (Ta-CH2CH2Ph); 39.5 (TaCH2CH2Ph);
40.1(Ta-CH2CMe2-); 35.0, 35.2 (Ta-CH2CMe2-); 32.0, 30.9
(CMe3); 36.5, 41.0 (CMe3).
Ack n ow led gm en t. We thank the Department of
Energy, Office of Basic Energy Sciences, for financial
support of this research.
P r ep a r a t ion of [Ta (OC6H 3Bu t CMe2CH 2)(OC6H 3Bu t
-
2
2,6)(CHdCHP h )Cl] (10). To a suspension of [Ta(OC6H3But -
2
2,6)2Cl3] (0.5 g, 0.71 mmol) in hexane (5 mL) was added
phenylacetylene (0.37 g, 3.62 mmol) followed by tri-n-butyltin
hydride (0.63 g, 2.16 mmol), and the resulting mixture was
allowed to stand for 20 h. upon which the crude product
precipitated as a red solid which was washed with hexane and
Su p p or tin g In for m a tion Ava ila ble: Text giving descrip-
tions of the experimental procedures for X-ray diffraction
studies and tables of thermal parameters, bond distances and
angles, intensity data, torsion angles, and mutiplicities for 4-6
and 8-11. This material is available free of charge via the
Internet at http://pubs.acs.org.
dried in vacuo. Yield: 0.39 g (75%). Anal. Calcd for C36H47
-
ClO2Ta: C, 59.38; H, 6.51. Found: C, 59.88; H, 6.82. 1H NMR
(C6D6, 30 °C): δ 9.48 (d, 1H, TaCHdCHPh), 8.48 (d, 1H,
OM990664L