Coordination and hydrogenation of 1,3-cyclohexadiene by niobium and tantalum aryl oxide compounds: Relevance to catalytic arene hydrogenation

Article abstract of DOI:10.1021/ja964073n

The sodium amalgam (2 Na per M) reduction of hydrocarbon solutions of the chloro, aryl oxide compounds [M(OC₆H₃Pr(i)₂-2,6)₂Cl₃]₂ (1) and [M(OC₆H₃Pr(i)₂-2,6)₃C1₂] (2) (a, M = Nb; b, M = Ta) in the presence of 1,3-cyclohexadiene leads to formation of the η⁴-cyclohexadiene derivatives [M(OC₆H₃Pr(i)₂-2,6)₂Cl(η⁴-C₆H₈)] (3) and [M(OC₆H₃Pr(i)₂-2,6)₃(η⁴-C₆H₈)] (4). Spectroscopic studies of compounds 3 and 4 show in all cases a strongly bound cyclohexadiene ligand which does not readily undergo displacement (NMR) with added reagents such as PMe₂Ph and cyclohexene. Single crystal X-ray diffraction analyses of 3a and the isomorphous pair 4a and 4b show in all three cases a geometry about the metal center best described as three-legged piano stool. Compound 4a will catalyze the disproportionation of 1,3-cyclohexadiene into cyclohexene and benzene as well as the hydrogenation of 1,3-cyclohexadiene and cyclohexene into cyclohexane. Mechanistic studies clearly show that cyclohexene is not released during the conversion of 1,3-cyclohexadiene to cyclohexane catalyzed by 4a. In contrast, solutions of 3a will convert 1,3-cyclohexadiene slowly to cyclohexene prior to conversion to cyclohexane. The addition of 1,3-cyclohexadiene to the trihydride compounds [Ta(OC₆H₃Cy₂-2,6)₂(H)₃(PMe₂Ph)₂] and [Ta(OC₆HPh₂-3,5-Cy₂-2,6)₂(H)₃(PMe₂Ph)₂] leads to the interesting products [Ta(OC₆H₃Cy₂-2,6)₂(η¹-C₆H₁₀-η⁴-C₆H₇)] (5) and [Ta(OC₆HPh₂-3,5-Cy₂-2,6)₂(η¹ C₆H₁₀-η⁴-C₆H₇)] (6) which, based upon structural studies of 5 contain a partially hydrogenated non-Diels-Alder dimer of 1,3-cyclohexadiene. The addition of 1,3-cyclohexadiene to the dihydride compounds [Ta(OC₆H₃Pr(i)₂2,6)₂(Cl)(H)₂(PMe₂Ph)₂] and [Ta(OC₆H₃Bu(t)₂-2,6)₂(Cl)(H)₂(PMe₂Ph)] leads to the dehydrogenation product [Ta(OC₆H₃Pr(i)-η²-CMe=CH₂)(OC₆H₃Pr(i)₂-2,6)(Cl)(PMe₂Ph)₂] (7) and the cyclohexyl compound [Ta(O₆H₃Bu(t)-CMe₂CH₂)(OC₆H₃Bu(t)₂-2,6)(Cl)(C₆H₁₁)] (8), respectively. The mechanistic implications of these stoichiometric and catalytic reactions are discussed. Crystal data for 3a at 20°C: NbClO₂C₃₀H₄₂. M = 563.03, space group P(nma) (no. 62), a = 12.237(1), b = 21.633(1), c = 10.883(2) A?, V = 2881.0(9) A?³, D(c) = 1.298 g cm^-3, Z = 4; for 4a at 20 °C: NbO₃C₄₂H₅₉. M = 704.84, space group P2₁/c (no. 14), a = 11.562(1), b = 16.117(2), e = 21.914(3) A?, β = 103.69(1)°, V = 3967(2) A?³, D(c)= 1.180 g cm^-3, Z = 4; for 4b at -57 °C: TaO₃C₄₂H₅₉. M = 792.88, space group R2₁/c (no. 14), a = 11.452(2), b = 16.175(3), c = 21.765(3) A?, β = 103.52(1)°, V = 3919(2) A?³, D(c) = 1.343 g cm^-3, Z = 4; for 5 at 20 °C: TaO₂C₄₈H₆₇. M = 857.02, space group P2₁ (no. 4), a = 10.559(9), b = 15.828(10), c = 13.266(12) A?, V = 2095(6) A?³, D(c) = 1.358 g cm^-3, Z = 2.

Full text of DOI:10.1021/ja964073n

3490
J. Am. Chem. Soc. 1997, 119, 3490-3499
Coordination and Hydrogenation of 1,3-Cyclohexadiene by
Niobium and Tantalum Aryl Oxide Compounds: Relevance to
Catalytic Arene Hydrogenation
Valerie M. Visciglio, Janet R. Clark, Mindy T. Nguyen, Douglas R. Mulford,
Phillip E. Fanwick, and Ian P. Rothwell*
Contribution from the Department of Chemistry, 1393 Brown Building, Purdue UniVersity,
West Lafayette, Indiana 47907
ReceiVed NoVember 25, 1996X
Abstract: The sodium amalgam (2 Na per M) reduction of hydrocarbon solutions of the chloro, aryl oxide compounds
[M(OC6H3Pri2-2,6)2Cl3]2 (1) and [M(OC6H3Pri2-2,6)3Cl2] (2) (a, M ) Nb; b, M ) Ta) in the presence of 1,3-
cyclohexadiene leads to formation of the η4-cyclohexadiene derivatives [M(OC6H3Pri2-2,6)2Cl(η4-C6H8)] (3) and
[M(OC6H3Pri2-2,6)3(η4-C6H8)] (4). Spectroscopic studies of compounds 3 and 4 show in all cases a strongly bound
cyclohexadiene ligand which does not readily undergo displacement (NMR) with added reagents such as PMe2Ph
and cyclohexene. Single crystal X-ray diffraction analyses of 3a and the isomorphous pair 4a and 4b show in all
three cases a geometry about the metal center best described as three-legged piano stool. Compound 4a will catalyze
the disproportionation of 1,3-cyclohexadiene into cyclohexene and benzene as well as the hydrogenation of 1,3-
cyclohexadiene and cyclohexene into cyclohexane. Mechanistic studies clearly show that cyclohexene is not released
during the conversion of 1,3-cyclohexadiene to cyclohexane catalyzed by 4a. In contrast, solutions of 3a will convert
1,3-cyclohexadiene slowly to cyclohexene prior to conversion to cyclohexane. The addition of 1,3-cyclohexadiene
to the trihydride compounds [Ta(OC6H3Cy2-2,6)2(H)3(PMe2Ph)2] and [Ta(OC6HPh2-3,5-Cy2-2,6)2(H)3(PMe2Ph)2]
leads to the interesting products [Ta(OC6H3Cy2-2,6)2(η1-C6H10-η4-C6H7)] (5) and [Ta(OC6HPh2-3,5-Cy2-2,6)2(η1-
C6H10-η4-C6H7)] (6) which, based upon structural studies of 5 contain a partially hydrogenated non-Diels-Alder
dimer of 1,3-cyclohexadiene. The addition of 1,3-cyclohexadiene to the dihydride compounds [Ta(OC6H3Pri2-
2,6)2(Cl)(H)2(PMe2Ph)2] and [Ta(OC6H3But2-2,6)2(Cl)(H)2(PMe2Ph)] leads to the dehydrogenation product [Ta(OC6H3-
Pri-η2-CMedCH2)(OC6H3Pri2-2,6)(Cl)(PMe2Ph)2] (7) and the cyclohexyl compound [Ta(OC6H3But-CMe2CH2)(OC6H3-
But2-2,6)(Cl)(C6H11)] (8), respectively. The mechanistic implications of these stoichiometric and catalytic reactions
are discussed. Crystal data for 3a at 20°C: NbClO2C30H42. M ) 563.03, space group Pnma (no. 62), a ) 12.237(1),
b ) 21.633(1), c ) 10.883(2) Å, V ) 2881.0(9) Å3, Dc ) 1.298 g cm-3, Z ) 4; for 4a at 20 °C: NbO3C42H59. M
) 704.84, space group P21/c (no. 14), a ) 11.562(1), b ) 16.117(2), c ) 21.914(3) Å, â ) 103.69(1)°, V ) 3967(2)
Å3, Dc ) 1.180 g cm-3, Z ) 4; for 4b at -57 °C: TaO3C42H59. M ) 792.88, space group P21/c (no. 14), a )
11.452(2), b ) 16.175(3), c ) 21.765(3) Å, â ) 103.52(1)°, V ) 3919(2) Å3, Dc ) 1.343 g cm-3, Z ) 4; for 5 at
20 °C: TaO2C48H67. M ) 857.02, space group P21 (no. 4), a ) 10.559(9), b ) 15.828(10), c ) 13.266(12) Å, V
) 2095(6) Å3, Dc ) 1.358 g cm-3, Z ) 2.
Introduction
aromatic hydrocarbons3 as well as aryl-phosphine ligands4 by
catalysts of this type no products containing partially hydroge-
nated aryl rings have been detected. Furthermore, in a number
of cases that have been studied a predominantly all-cis stere-
ochemistry has been established for the H(D) atoms introduced
to saturate the arene substrate.1,3a It is difficult to envisage any
reasonable mechanistic pathway for the hydrogenation of a
benzene ring that does not proceed via intermediate cyclohexa-
diene and cyclohexene fragments. The above observations,
therefore, strongly imply that metal-bound cyclohexadiene and
cyclohexene species should exhibit displacement/dissociation
rates slower than subsequent reaction with H2 under the
conditions of catalysis. In this paper we report the synthesis
and chemistry of a series of new η4-cyclohexadiene derivatives
of niobium and tantalum containing aryl oxide ligation.5 This
The demonstrated ability of various aryl oxide derivatives
of niobium and tantalum to carry out the regio- and stereose-
lective hydrogenation of aromatic substrates1,2 raises many
mechanistic questions. During the hydrogenation of simple
X Abstract published in AdVance ACS Abstracts, March 15, 1997.
(1) Rothwell, I. P. J. Chem. Soc., Chem. Commun. in press.
(2) For leading references to arene hydrogenation, see: (a) Muetterties,
E. L.; Bleeke, J. R. Acc. Chem. Res. 1979, 12, 324. (b) Stuhl, L. S.;
Rakowski DuBois, M.; Hirsekorn, F. J.; Bleeke, J. R.; Stevens, A. E.;
Muetterties, E. L. J. Am. Chem. Soc. 1978, 100, 2405. (c) Olah, G. A.;
Molnar, A. Hydrocarbon Chemistry; John Wiley: New York, 1985. (d)
Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; John Wiley:
New York, 1992. (e) James, B. R. Homogeneous Hydrogenation; John
Wiley: New York, 1973. (f) Bennett, M. CHEMTECH 1980, 444. (g)
Maitlis, P. M. Acc. Chem. Res. 1978, 100, 2405. (h) Collman, J. P.; Kosydar,
K. M.; Bressan, M.; Lamanna, W.; Garrett, T. J. Am. Chem. Soc. 1984,
106, 2569. (i) Amer, I.; Amer, H.; Ascher, R.; Blum, J.; Sasson, Y. J. Mol.
Catal. 1987, 39, 185. (j) Januszkiewicz, K. R.; Alper, H. Organometallics
1983, 2, 1055. (k) Baralt, E.; Smith, S. J.; Hurwitz, J.; Horvath, I. T.; Fish,
R. H. J. Am. Chem. Soc. 1992, 114, 5187. (l) Fish, R. H.; Baralt, E.; Smith,
S. J. Organometallics 1991, 10, 54. (m) Sanchez-Delgado, R. A.; Rondon,
D.; Andriollo, A.; Herrera, V.; Martin, G.; Chaudret, B. Organometallics
1993, 12, 4291. (n) Abley, P.; Jardine, I.; McQuillin, F. J. J. Chem. Soc.
(C) 1971, 840. (o) Taylor, P. D.; Orchin, M. J. Org. Chem. 1972, 37, 3913.
(p) Halpern, J. Pure Appl. Chem. 1987, 59, 173. (q) Linn, D. E.; Halpern,
J. J. Am. Chem. Soc. 1987, 109, 2969.
(3) (a) Yu, J. S.; Ankianiec, B. C.; Nguyen, M. T.; Rothwell, I. P. J.
Am. Chem. Soc. 1992, 114, 1927. (b) Ankianiec, B. C.; Fanwick, P. E.;
Rothwell, I. P. J. Am. Chem. Soc. 1991, 113, 4710. (c) Visciglio, V. M.;
Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc., Chem. Commun. 1992,1505.
(d) Clark, J. R.; Fanwick, P. E.; Rothwell, I. P. J. Chem. Soc., Chem.
Commun. 1995, 553.
(4) (a) Yu, J. S.; Rothwell, I. P. J. Chem. Soc., Chem. Commun. 1992,
632. (b) Potyen, M. C.; Rothwell, I. P. J. Chem. Soc., Chem. Commun.
1995, 849.
S0002-7863(96)04073-5 CCC: $14.00 © 1997 American Chemical Society

Coordination/Hydrogenation of 1,3-Cyclohexadiene
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3491
Scheme 1
Scheme 2
synthetic work is complemented by an investigation of the
reactivity of previously isolated tantalum hydride compounds6
as well as related niobium catalysts toward 1,3-cyclohexadiene
and cyclohexene. The studies reported here focus on gaining
a better insight into the overall mechanism of arene hydrogena-
tion by this specific pedigree of catalyst.
Results and Discussion
Synthesis and Spectroscopic Characterization. The hy-
drogenation of an arene substrate can be catalyzed either using
discrete tantalum hydride compounds [Ta(OAr)5-x(H)x(PR3)n]
(x ) 2, 3; n ) 1 or 2)6 or via the hydrogenolysis of niobium
alkyl precursors [Nb(OAr)2R3].1 The tantalum reagents exhibit
low reactivity and are inhibited by trace amounts of free
phosphine ligand. The niobium compounds require initial
hydrogenolysis of the Nb-alkyl bonds, typically slow at
temperatures below 80 °C, in order to generate the active
catalyst. A recent development has been the use of niobium
chloride precursors [Nb(OAr)2Cl3] and [Nb(OAr)3Cl2] “acti-
vated” by addition of BunLi in the presence of the arene substrate
prior to pressurization with H2.4b This method generates a
catalyst active even at ambient temperatures.
The initial attack of a catalyst containing at least two hydride
ligands on a benzene ring would be expected to proceed via an
intermediate η1-, η3-, or potentially η5-cyclohexadienyl, hydride
prior to formation of a cyclohexadiene complex (Scheme 1).
We have previously isolated just such a cyclohexadiene complex
formed via a stoichiometric intramolecular hydrogenation of one
of the o-phenyl substituents of a 2,6-diphenylphenoxide ligand
attached to niobium.7 We have, however, yet to detect the
intermolecular reaction of isolated hydrido, aryl oxide com-
pounds of tantalum with arene substrates; these molecules
exhibit considerable thermal stability, for instance, in benzene
solvent. We have found that an alternative synthetic route to
cyclohexadiene derivatives of these metals involves reduction
of mixed chloro, aryl oxides in the presence of 1,3-cyclohexa-
diene. Previous work has shown that low valent aryl oxide8
and related siloxide9 derivatives of Nb and Ta are highly reactive
species able to coordinate and activate a variety of unsaturated
substrates. The sodium (2 Na per M) amalgam reduction of
benzene solutions of the compounds [M(OC6H3Pri2-2,6)2Cl3]2
(1) and [M(OC6H3Pri2-2,6)3Cl2] (2) (a, M ) Nb; b, M ) Ta)10
in the presence of 1,3-cyclohexadiene leads to formation of the
η4-cyclohexadiene derivatives [M(OC6H3Pri2-2,6)2Cl(η4-C6H8)]
(3) and [M(OC6H3Pri2-2,6)3(η4-C6H8)] (4) (Scheme 2). Al-
though 3b could be observed in solution by 1H NMR spectros-
copy, it was found to convert to mixtures of 4b and other
unidentified products presumably via ligand exchange processes.
The other three compounds 3a, 4a, and 4b were found to be
stable in C6D6 solution for weeks without any noticeable change
1
in their H NMR spectra.
1
The H NMR spectra (C6D6 solution) of 3 and 4 show well
resolved resonances for the aryl oxide and cyclohexadiene
ligands (Figure 1). In 3 the lack of a plane of symmetry through
the aryl oxide ligands leads to diastereotopic CHMe2 doublets.
In all cases four separate, equal intensity multiplets can be
readily assigned to the nonequivalent protons in the η4-C6H8
ligand. In the 13C NMR spectra the three carbon atoms of this
ligand are also easily detected.
We have also investigated the reaction of various tantalum
hydride compounds toward 1,3-cyclohexadiene. The colorless
trihydride compounds [Ta(OC6H3Cy2-2,6)2(H)3(PMe2Ph)2] and
[Ta(OC6HPh2-3,5-Cy2-2,6)2(H)3(PMe2Ph)2]6 react slowly over
days with 1,3-cyclohexadiene in C6D6 solution to produce bright
yellow solutions of new organometallic compounds 5 and 6
(Scheme 3). The 1H NMR spectra of these compounds is
extremely complicated, containing a large number of multiplets
in the δ 1-7 ppm region. A single crystal X-ray analysis of 5
(Vide infra) shows it to contain, besides two terminal aryl oxide
groups, a new ligand derived by coupling and partial reduction
of 2 equiv of 1,3-cyclohexadiene. At first sight the exact regio-
and stereochemistry adopted by the organic fragment in this
molecule appears haphazard. This particular product can,
however, be rationalized by consideration of results recently
obtained by our group on the extremely rapid regio- and
stereoselective dimerization of 1,3-cyclohexadiene catalyzed by
titanium aryl oxides (Scheme 4).11 By analogy with this
chemistry we propose dimerization of 1,3-cyclohexadiene occurs
at tantalum yielding threo-5-(3-cyclohexenyl)-1,3-cyclohexa-
diene. This occurs by initial loss of H2 (observed in solution
(5) Visciglio, V. M.; Nguyen, M. T.; Clark, J. R.; Fanwick P. E.;
Rothwell, I. P. Polyhedron 1996, 15, 551.
(6) Parkin, B. C.; Clark, J. C.; Visciglio, V. M.; Fanwick, P. E.; Rothwell,
I. P. Organometallics 1995, 14, 3002.
(7) Lockwood, M. A.; Potyen, M. C.; Steffey, B. D.; Fanwick P. E.;
Rothwell, I. P. Polyhedron 1995, 14, 3293.
(8) Allen, K. D.; Bruck, M. A.; Gray, S. D.; Kingsborough, R. P.; Smith,
D. P.; Weller, K. J.; Wigley, D. E. Polyhedron 1995, 14, 3315.
(9) Wolczanski, P. T. Polyhedron 1995, 14,3335.
1
by H NMR) yielding a monohydride species. The coupling
of 2 equiv of 1,3-cyclohexadiene at the resulting d2-metal center
can produce six isomeric 9-tantalaoctahydrofluorene complexes.
The cis-anti-cis- and cis-syn-cis-3,4,5,6,1a,4a,5a,8a-octahydro
derivatives (Scheme 5) have the potential for stabilization via
(10) Clark, J. R.; Pulvirenti, A. L.; Fanwick, P. E.; Sigalas, M.; Eisenstein,
O.; Rothwell, I. P. To be submitted for publication.
(11) Waratuke, S. A.; Johnson, E. S.; Thorn, M. G.; Fanwick, P. E.;
Rothwell, I. P. J. Chem. Soc., Chem. Commun. 1996, xxxx.

3492 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Visciglio et al.
1
Figure 1. 200 MHz (C6D6) H NMR spectrum of [Ta(OC6H3Pri2-2,6)3(η4-C6H8)], 4b.
Scheme 3
Scheme 5
Scheme 4
transfer of the remaining hydride ligand to the 3-cyclohexenyl
ring (Scheme 5). The regio- and stereochemistry observed in
5 can only arise via the cis-anti-cis intermediate, the cis-syn-
cis producing an erythro stereochemistry.
The stoichiometric reaction of the tantalum dihydride com-
pound [Ta(OC6H3Pri2-2,6)2(Cl)(H)2(PMe2Ph)2] with 1,3-cyclo-
hexadiene and cyclohexene is also slow at room temperature
and leads to the dehydrogenation product [Ta(OC6H3Pri-η2-
formation of π-allyllic interactions. Direct elimination from
either of these intermediates is disallowed as both central
â-hydrogens are trans to the metal center. The intermediate
5-(3-cyclohexenyl)-1,3-cyclohexadiene can be generated by
abstraction of a hydrogen atom from the 4-position of one of
the rings. This hydrogen can be made accessible to the metal
center via a 1,3-shift. The final product is then obtained by

Coordination/Hydrogenation of 1,3-Cyclohexadiene
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3493
Table 1. Selected Spectroscopic Data on the Tantalum-Cyclohexyl
Ligand in 8
Scheme 6
δ (ppm)
1.05 m
coupling constants (Hz)
H1
Scheme 7
H2e, H2e′
H2a, H2a′
H3e, H3e′
H3a, H3a′
H4e
3.07 d, 3.42 d
2.05 m, 1.84 m
1.40-1.50 m
1.25-1.35 m
1.58 m
2J(1H2a-1H2e)
12.8
H4a
C1
C2, C2′
C3, C3′
C4
1.20 m
131.3
43.0, 41.6
33.2, 32.9
27.3
1J(13C1-1H1)
1J(13C2-1H2)
1J(13C3-1H3)
1J(13C4-1H4)
102.3
124.8, 129.1
a
131.8
a Not resolved.
8
CMedCH2)(OC6H3Pri2-2,6)(Cl)(PMe2Ph)2] (7) and cyclohexene
or cyclohexane, respectively (Scheme 6). Previous studies have
shown that 7 is formed when the seven coordinate dihydride is
reacted with styrene.3d The dihydride reacts much faster with
1,3-cyclohexadiene than with cyclohexene as monitored by 1H
and 31P NMR (Vide infra).
The reaction of the six-coordinate dihydride [Ta(OC6H3But2-
2,6)2(Cl)(H)2(PMePh2)] with 1,3-cyclohexadiene produces the
cyclohexyl compound [Ta(OC6H3But-CMe2CH2)(OC6H3But2-
2,6)(Cl)(C6H11)] (8) (Scheme 7). Compound 8 has also been
obtained by reacting the trichloride [Ta(OC6H3But2-2,6)2(Cl)3]12
with either a mixture of [(Bun)3SnH]/1,3-cyclohexadiene or [Mg-
1
(c-C6H11)]. The H and 13C NMR spectra clearly show the
presence of the cyclometalated 2,6-di-tert-butylphenoxide ligand.13
Based upon previously isolated cyclometalated compounds we
assign a pseudotrigonalbipyramidal structure to 8 with the aryl
oxide oxygen atoms mutually trans. The solution NMR
spectroscopic properties of the cyclohexyl group in 8 are of
some interest. The lack of a plane of symmetry leads to six
nonequivalent carbon atoms and 11 nonequivalent protons for
the Ta-C6H11 unit. Using COSY and HETCOR experiments
most of these resonances and certain coupling constants could
be assigned (Table 1).
Figure 2. Molecular structure of [NbCl(OC6H3Pri2-2,6)2(η4-C6H8)], 3a.
The chemical shift for the R-carbon C1 is significantly
downfield of the region typical for tantalum alkyl groups.
Figure 3. Molecular structure of [Nb(OC6H3Pri2-2,6)3(η4-C6H8)], 4a.
An identical numbering scheme is used for isomorphous [Ta(OC6H3-
Pri2-2,6)3(η4-C6H8)], 4b.
1
Furthermore, the J(13C1-1H1) coupling of 102.3 Hz is low.
We interpret these spectroscopic data as possible indications
of an agostic interaction between the R-CH bond and the
tantalum metal center.14
and a chelated alkyl group (5). Due to the complexity, a view
of the central coordination sphere of 5 is given in Figure 5. A
series of molecules [M(η4-C6H8)(PMe3)2Cl2] (M ) Zr, Hf) has
been reported by Green et. al.15 The M-O(aryl oxide) distances
in the new compounds lie in the very narrow range of 1.886-
(6)-1.916(5) Å. The most interesting structural feature of these
molecules concerns the bonding of the cyclohexadiene fragment
to the metal center. In Figure 6 the most important features
are highlighted. In compound 3a there is a crystallographically
imposed mirror-plane which indicates a totally symmetric
interaction of the metal center with both sides of the cyclo-
hexadiene ring (Figure 6). In contrast, in the other three
structures there is a pronounced asymmetry to the structural
parameters arising from a slightly closer contact of the metal
Solid State Structures. In order to gain a better understand-
ing of the interaction of 1,3-cyclohexadiene ligands with these
particular metal systems, the solid state molecular structures of
3a, the isomorphous pair 4a and 4b, and the dimerization
product 5 were determined. The molecular structures are shown
in Figures 2-4, while selected structural parameters are
contained in Tables 2-4. It can be seen that these four
compounds adopt structures that can best be described as three-
legged piano stools, the legs consisting of two aryl oxides and
a chloride (3a), three aryl oxides (4a, 4b), or two aryl oxides
(12) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J. C. Inorg. Chem.
1984, 23, 2575.
(13) Rothwell, I. P. Acc. Chem. Res. 1988, 21, 153.
(14) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem.
1988, 36, 1.
(15) Diamond, G. M.; Green, M. L. H.; Walker, N. M.; Howard, J. A.
K.; Mason, S. A. J. Chem. Soc., Dalton Trans. 1992, 2641.

3494 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Visciglio et al.
Table 4. Selected Bond Distances (Å) and Angles (deg) for
[Ta(OC6H3Cy2-2,6)2(η1-C6H10-η4-C6H7)], 5
Ta-O(10)
Ta-O(20)
Ta-C(43)
Ta-C(31)
Ta-C(32)
Ta-C(33)
Ta-C(34)
1.915(6)
1.916(5)
2.166(10)
2.218(10)
2.297(11)
2.438(10)
2.374(10)
C(31)-C(32)
C(32)-C(33)
C(33)-C(34)
C(34)-C(35)
C(35)-C(36)
C(31)-C(36)
1.44(2)
1.40(2)
1.40(2)
1.53(2)
1.56(1)
1.55(1)
O(10)-Ta-O(20)
O(10)-Ta-C(43)
O(10)-Ta-C(31)
O(10)-Ta-C(32)
O(10)-Ta-C(33)
O(10)-Ta-C(34)
O(20)-Ta-C(43)
O(20)-Ta-C(31)
O(20)-Ta-C(32)
98.5(3)
108.1(3)
98.7(3)
120.0(3)
153.9(3)
159.5(3)
108.1(3)
131.3(3)
95.7(4)
O(20)-Ta-C(33)
O(20)-Ta-C(34)
C(43)-Ta-C(31)
C(43)-Ta-C(32)
C(43)-Ta-C(33)
C(43)-Ta-C(34)
Ta-O(10)-C(11)
Ta-O(20)-C(21)
84.6(3)
101.8(3)
115.9(4)
135.7(4)
110.5(4)
77.5(4)
160.3(5)
141.6(5)
Figure 4. Molecular structure of [Ta(OC6H3Cy2-2,6)2(η1-C6H10-η4-
C6H7)], 5.
Table 2. Selected Bond Distances (Å) and Angles (deg) for
[NbCl(OC6H3Pri2-2,6)2(η4-C6H8)], 3a
Nb-O(1)
1.897(2)
2.367(1)
1.37(1)
Nb-C(10)
2.344(4)
2.326(5)
1.388(7)
1.52(2)
Nb-Cl
Nb-C(11)
C(10)-C(10)′
C(11)-C(12)
C(10)-C(11)
C(12)-C(12)′
1.496(9)
Cl-Nb-O(1)
105.24(8)
87.4(1)
115.6(2)
102.3(1)
109.2(1)
141.6(1)
151.9(2)
C(10)-Nb-C(10)
C(10)-Nb-C(11)
C(10)-Nb-C(11)
C(11)-Nb-C(11)
O(1)-Nb-C(11)
O(1)-Nb-C(11)
33.9(2)
34.6(2)
59.9(2)
66.8(3)
82.2(2)
136.3(2)
Cl-Nb-C(10)
Cl-Nb-C(11)
O(1)-Nb-O(1)′
O(1)-Nb-C(10)
O(1)-Nb-C(10)
Nb-O(1)-C(1)
Figure 5. View of the central coordination sphere of [Ta(OC6H3Cy2-
2,6)2(η1-C6H10-η4-C6H7)], 5.
Table 3. Selected Bond Distances (Å) and Angles (deg) for
[M(OC6H3Pri2-2,6)3(η4-C6H8)] (M ) Nb, 4a; Ta, 4b)
Nb
Ta
M-O(1)
1.886(6)
1.906(6)
1.898(7)
2.25(1)
2.27(1)
2.34(1)
2.40(1)
1.42(1)
1.50(1)
1.37(2)
1.38(2)
1.50(2)
1.52(2)
1.888(3)
1.889(3)
1.902(3)
2.243(5)
2.310(5)
2.366(4)
2.337(5)
1.433(7)
1.515(7)
1.375(7)
1.413(7)
1.516(7)
1.504(8)
M-O(2)
M-O(3)
M-C(41)
M-C(42)
M-C(43)
M-C(44)
C(41)-C(42)
C(41)-C(46)
C(42)-C(43)
C(43)-C(44)
C(44)-C(45)
C(45)-C(46)
O(1)-M-O(2)
O(1)-M-O(3)
O(2)-M-O(3)
O(1)-M-C(41)
O(1)-M-C(42)
O(1)-M-C(43)
O(1)-M-C(44)
O(2)-M-C(41)
O(2)-M-C(42)
O(2)-M-C(43)
O(2)-M-C(44)
O(3)-M-C(41)
O(3)-M-C(42)
O(3)-M-C(43)
O(3)-M-C(44)
M-O(1)-C(11)
M-O(2)-C(21)
M-O(3)-C(31)
103.6(3)
102.9(1)
103.0(3)
109.9(3)
91.5(3)
100.6(1)
108.3(1)
91.7(2)
Figure 6. Structural parameters (Å) for the η4-bound cyclohexadiene
ligands in 3a, 4a, 4b, and 5.
95.4(4)
95.9(2)
125.6(4)
155.1(4)
99.4(3)
132.7(4)
125.2(4)
91.8(4)
142.8(4)
107.1(4)
83.2(4)
125.6(1)
156.2(2)
103.6(1)
136.9(1)
127.7(1)
92.9(1)
142.0(1)
105.8(1)
82.9(1)
The literature now contains a large number of transition metal
compounds containing η4-bound 1,3-cyclohexadiene to which
the compounds obtained in this study can be compared. Directly
related are the even larger set of transition metal derivatives of
1,3-butadiene.16 In these acyclic species there is the potential
for formation of η4-s-trans-1,3-diene structures. In the case of
the η4-s-cis-1,3-butadiene complexes, Nakamura and Yasuda
have carried out an extensive analysis of the determined solid
state structures.16a They have focused upon three key parameters
for discussing the bonding in these molecules. The first is the
dihedral angle (θ) subtended by the C1-M-C4 and C1-C2-
89.6(4)
91.0(2)
162.5(6)
150.8(6)
149.8(6)
164.8(3)
149.5(3)
150.9(3)
center with one of the diene double bonds. This situation is
not detected in solution for 4a and 4b where the proton
multiplicities indicate either a symmetric interaction is present,
or else the molecules are fluxional.
(16) (a) Yasuda, H.; Nakamura, A. Angew. Chem., Int. Ed. Engl. 1987,
26, 723. (b) Yasuda, H.; Tatsumi, K.; Nakamura, A. Acc. Chem. Res. 1985,
18, 120.

Coordination/Hydrogenation of 1,3-Cyclohexadiene
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3495
Table 5. Structural Parameters for 1,3-Cyclohexadiene Complexes
Scheme 8
compd
∆d
θ
∆l
ref
3a
4a
4b
5
-0.018 85.1
0.020 83.7
0.018
0.030
0.048
0.020
a
a
a
a
-0.048 86.0
-0.075 89.6
[CpCr(η4-C6H8){P(OMe)3}]
[CpMo(η4-C6H8)(dppe)][PF6]
[(Cp-CH2CH2NMe2)Mo
(η4-C6H8)(CO)][PF6]
0.113 78.5 -0.003 17
0.006 85.6
0.116 79.0
0.027 18
0.027 19
[W(η4-C6H8)(CO)3(C6H10F4Si2)]
[Mn(η4-C6H8)2(CO)]
0.145 77.4 -0.093 20
0.083 80.0 0.011 21
0.080 80.3 -0.036
0.032 81.6
0.153 81.4
[Re(η4-C6H8)(η6-C6H6)(H)]
[Re(η4-C6H8)(H)3(PPh3)]
[Fe(η4-C6H8)2(CO)]
0.026 22
0.002 23
C3-C4 best fit planes. This is also a key parameter in
discussing related heteroatom containing five-membered met-
allacycles, although in these cases the “fold” angle (180-θ) is
normally quoted to highlight distortions from planarity. The
other two parameters are differences in metal-carbon and
carbon-carbon bond lengths defined as follows:
0.083 80.4 -0.008 24
0.088 80.0 -0.044
[Fe(η4-C6H8)(CO)3]
0.071 80.1
0.017 25
[Fe(η4-C6H8)(CO)2(PPh3)]
[Fe(η4-C6H8)(CO)2{P(o-tol)3}]
[Fe(η4-C6H8)(CO)2(CNEt)]
[Fe(η4-C6H8)(CO)2{C(OEt)(o-tol)}]
[Fe(η4-C6H8)(CO)2(C6H10F4Si2)]
[Fe2(η4-C6H8)(PHPh)2(CO)4]
[FeRe(η4-C6H8)(η4-C7H9)(CO)6]
[Fe3(η4-C6H8)(NPη)(CO)8]
[Fe3(η4-C6H8)(S)(CO)8]
0.052 81.1 -0.008 26
0.085 80.0 0.011 27
0.033 81.1 -0.028 28
0.165 75.9 0.102 29
∆d ) [d(M-C1) + d(M-C4)]/2 -
[d(M-C2) + d(M-C3)]/2
0.159 74.5 -0.053 30
0.111 78.6
0.073 80.9
0.027 31
0.004 32
∆l ) [l(C1-C2) + l(C3-C4)]/2 - l(C2-C3)
0.095 79.4 -0.003 33
0.077 79.5 0.006 34
0.114 79.5 -0.047 35
0.111 80.3 -0.036
For transition metal derivatives of 1,3-butadiene, there was
a distinct trend involving a larger value of θ for early d-block
metal complexes compared to later transition metals. This
correlated well with values of ∆d (the two being geometrically
tied) while there was a less pronounced trend involving ∆l.
These trends were interpreted in terms of two limiting resonance
bonding pictures; a “conventional” η4-s-cis-1,3-butadiene struc-
ture (A) adopted by later transition metals and a (σ2,π-bonded)
metallacyclopent-3-ene structure (B) found for the Group 5 and
in particular by the Group 4 metals (Scheme 8).16 By direct
analogy we can evaluate the structural parameters for 1,3-
cyclohexadiene complexes to asses the importance of η4-1,3-
cyclohexadiene and (σ2,π-bonded) metallanorbornene bonding
pictures (Scheme 8). In Table 5 are collected the values of θ,
∆d, and ∆l for the compounds obtained in this study and for
other derivatives of 1,3-cyclohexadiene contained in the Cam-
bridge Crystallographic Database.17-43 It can be seen that the
new Group 5 metal compounds have the largest values for θ
[Ru(η4-C6H8)(acac)2]
[CpRu(η4-C6H8)(CO)][BF4]
[Ru3(CO)8(η4-C6H8)(H)2(COMe)]
[Ru4(η4-C6H8)(cyclophane)(CO)9]
[Ru5(η4-C6H8)2(C)(CO)11]
0.124 79.7 -0.032 36
0.042 82.2 -0.007 37
0.070 81.1 -0.017 38
0.059 81.4
0.049 80.5
0.002 39
0.050 40
[Os3(η4-C6H8)(CO)10]
[Os4(η4-C6H8)(η6-C6H6)(H)4(CO)10]
0.006 83.2 -0.089 41
0.059 81.0 0.046
0.086 81.0 -0.028 42
0.038 82.3 -0.014 43
[Co2(η4-C6H8)2(CO)4]
[Ir(η4-C6H8)2(2-furyl)]
a This study.
(most negative value of ∆d Figure 7) implying that these
molecules may best be represented by the (σ2, π-bonded)
metallanorbornene bonding model (B). A study of related
Group 5 metal η6-arene complexes has identified a metallan-
orbonadiene bonding description as reasonable.44
Reactivity Studies. Solutions of the η4-1,3-cyclohexadiene
complexes 3a, 4a, and 4b in C6D6 solvent are stable for weeks
(17) Betz, P.; Dohring, A.; Emrich, R.; Goddard, R.; Jolly, P. W.; Kruger,
C.; Romao, C. C.; Schonfelder, K. U.; Tsay, Y.-H. Polyhedron 1993, 12,
2651.
(18) Prout, K.; Daran, J.-C. Acta Crystallogr., Sect. B 1977, 33, 2303.
(19) Wang, T.-F.; Wen, Y.-S. J. Organomet. Chem. 1992, 439, 155.
(20) Hseu, T.-H.; Lin, C.-H.; Lee, C.-Y.; Liu, C.-S. J. Chin. Chem. Soc.
(Taipei) 1989, 36, 91.
(21) Kundig, E. P.; Timms, P. L.; Kelly, B. A.; Woodward, P. J. Chem.
Soc., Dalton Trans. 1983, 901.
(22) Green, M. L. H.; O’Hare, D.; Bandy, J. A.; Prout, K. J. Chem. Soc.,
Chem. Commun. 1984, 884. Green, M. L. H.; O’Hare, D. J. Chem. Soc.,
Dalton Trans. 1987, 403.
1
at ambient temperatures when monitored by H NMR. As
discussed above, 3b slowly is converted to 4b. When these
solutions are heated at 110 °C, thermal decomposition occurs
to produce benzene, cyclohexene, and cyclohexane (typically a
50:40:10 mixture by 1H NMR and GC) along with unidentified
(33) Song, Jeong-Sup; Han, Sung-Hwan; Nguyen, S. T.; Geoffroy, G.
L.; Rheingold, A. L. Organometallics 1990, 9, 2386.
(34) Nekhaev, A. I.; Alekseeva, S. D.; Kolobkov, B. I.; Aleksandrov,
G. G.; Toshev, M. T.; Dustov, H. B. J. Organomet. Chem. 1991, 401, 75.
(35) Ernst, R. D.; Melendez, E.; Stahl, L. Organometallics 1991, 10,
3635.
(23) Baudry, D.; Ephritikhine, M.; Felkin, H.; Dromzee, Y.; Jeannin, Y.
J. Organomet. Chem. 1984, 272, 403.
(24) Kruger, C.; Tsay, Y.-H. J. Organomet. Chem. 1971, 33, 59.
(25) Tam, W.; Eaton, D. F.; Calabrese, J. C.; Williams, I. D.; Wang, Y.;
Anderson, A. G. Chem. Mater. 1989, 1, 128.
(26) Pearson, A. J.; Raithby, P. R. J. Chem. Soc., Dalton Trans. 1981,
884.
(27) Howell, J. A. S.; Palin, M. G.; Tirvengadum, M.-C.; Cunningham,
D.; McArdle, P.; Goldschmidt, Z.; Gottlieb, H. E. J. Organomet. Chem.
1991, 413, 269.
(36) Crocker, M.; Green, M.; Morton, C. E.; Nagle, K. R.; Orpen, A. G.
J. Chem. Soc., Dalton Trans. 1985, 2145.
(37) Churchill, M. R.; Beanan, L. R.; Wasserman, H. J.; Bueno, C.;
Rahman, Z. A.; Keister, J. B. Organometallics 1983, 2, 1179.
(38) Blake, A. J.; Dyson, P. J.; Ingham, S. L.; Johnson, B. F. G.; Martin,
C. M. J. Chem. Soc., Dalton Trans. 1995, 1063.
(39) Braga, D.; Sabatino, P.; Dyson, P. J.; Blake, A. J.; Johnson, B. F.
G. J. Chem. Soc., Dalton Trans. 1994, 393.
(28) Behrens, H.; Thiele, G.; Purzer, A.; Wurstl, P.; Moll, M. J.
Organomet. Chem. 1978, 160, 255.
(29) Chen, J.; Yu, Y.; Hu, L.; Jin, Z. J. Organomet. Chem. 1993, 447,
113.
(40) Blake, A. J.; Dyson, P. J.; Johnson, B. F. G.; Martin, C. M.; Nairn,
J. G. M.; Parisini, E.; Lewis, J. J. Chem. Soc., Dalton Trans. 1993, 981.
(41) Chen, H.; Johnson, B. F. G.; Lewis, J.; Braga, D.; Grepioni, F.;
Parisini, E. J. Chem. Soc., Dalton Trans. 1991, 215.
(30) Hseu, T.-H.; Lin, C.-H.; Lee, C.-Y.; Liu, C.-S. J. Chin. Chem. Soc.
(Taipei) 1989, 36, 91.
(31) De, R. L.; Vahrenkamp, H. Z. Naturforsch., Teil B 1986, 41, 273.
(32) Huffer, S.; Wieser, M.; Polborn, K.; Sunkel, K.; Beck, W. Chem.
Ber. 1994, 127, 1369.
(42) Stephens, F. S. J. Chem. Soc., Dalton Trans. 1972, 1752.
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Organometallics 1991, 10, 2319.

3496 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Visciglio et al.
Figure 7. A plot of θ vs ∆d for the η4-cyclohexadiene metal
compounds contained in Table 5.
Figure 8. Parent molecular ions of cyclohexane produced from the
reaction of cyclohexene, 1,3-cyclohexadiene, and a mixture of cyclo-
hexene and 1,3-cyclohexadiene with D2 catalyzed by 4a (see Table 6).
metal products. The niobium monochloride 3a and the tantalum
compound 4b appear to be thermally more robust than 4a.
Solutions of 4a catalyze the disproportionation of 1,3-cyclo-
hexadiene into benzene and cyclohexene.45 When a 0.085 M
solution of 4a in C6D6 in the presence of 50 equiv of
o-isopropyl substituent to generate chelated η2-vinyl groups.46
Furthermore, the stoichiometric reaction of the tantalum dihy-
dride compound [Ta(OC6H3Pri2-2,6)2(Cl)(H)2(PMe2Ph)2] with
1,3-cyclohexadiene and cyclohexene leads to the dehydroge-
nation product [Ta(OC6H3Pri-η2-CMedCH2)(OC6H3Pri2-2,6)-
(Cl)(PMe2Ph)2] (7)3d and cyclohexene or cyclohexane, respec-
tively. The formation of cyclohexyl compound 8 (Scheme 7)
presumably arises via initial formation of a cyclohexene
intermediate which then undergoes intramolecular CH bond
activation. It is hence possible that cyclometalation processes
can lead to simultaneous incorporation of excess H into the
product cyclohexane and incorporation of D label into the aryl
oxide ligands (e.g., the process outlined in Scheme 9). When
a mixture of 25 equiv each of cyclohexene and 1,3-cyclohexa-
diene is partially hydrogenated in C6D6 using D2 and catalyst
4a, the mass spectrum of the product cyclohexane has essentially
the same isotopic distribution as that obtained in the absence
of cyclohexene (Figure 8, Table 6). This experiment is of
particular mechanistic significance. We can conclude that this
catalyst preferentially hydrogenates 1,3-cyclohexadiene over
cyclohexene. During the course of hydrogenation of 1,3-
cyclohexadiene no intermediate cyclohexene is released by
catalyst precursor 4a.
1
1,3-cyclohexadiene is heated at 110 °C and monitored by H
NMR complete conversion to a 50:50 mixture of benzene and
cyclohexene occurs over 18 h with less than 3% cyclohexane
being produced.
The 1,3-cyclohexadiene complex 4a will catalyze the hydro-
genation of cyclohexene and 1,3-cyclohexadiene under condi-
tions (ambient temperatures and H2 pressures of 15-1000 psi)
which are mild compared to those required for benzene
hydrogenation. At pressures of 900 psi of H2, 4a will
hydrogenate 50 equiv of cyclohexene or 1,3-cyclohexadiene in
less than 5 h at 25 °C. When a mixture of 4a and 1,3-
cyclohexadiene in C6D6 is exposed to 1 atm of H2 in a sealed
5 mm NMR tube, slow conversion to cyclohexane is observed
The niobium monochloride species [Nb(OC6H3Pri2-2,6)2Cl-
(η4-C6H8)] (3a) will also catalyze the hydrogenation of cyclo-
hexene and 1,3-cyclohexadiene. In this case the reactivity at
25 °C is much lower than that of 4a. Furthermore, the reaction
with 1,3-cyclohexadiene is found to proceed entirely via
intermediate cyclohexene. Only upon exhaustion of all 1,3-
cyclohexadiene is cyclohexene converted by 3a to cyclohexane.
Hence, in this case intermediate cyclohexene is released into
solution by the catalyst and does not compete with 1,3-
cyclohexadiene as a substrate.
1
with no detection by H NMR of cyclohexene.
We have further probed this hydrogenation reactivity utilizing
D2 as reagent gas. The reaction of 25 equiv of cyclohexene or
25 equiv of 1,3-cyclohexadiene in C6D6 solvent with D2 (900
psi) catalyzed by 4a produced cyclohexane which was analyzed
by mass spectrometry (Figure 8). Correcting for the 13C natural
abundance gives the distribution of isotopologs (Table 6).
It can be seen that the major products are cyclohexane-d2
and cyclohexane-d4, respectively. There are, however, mol-
ecules in both samples that are enriched in D-label as well as
molecules that contain lower amounts of deuterium. The
incorporation of excess deuterium can be rationalized in terms
of scrambling processes (Vide infra). The source of the
hydrogen atoms for formation of molecules such as C6H11D
from C6H10 and C6H9D3 from C6H8 appears to be the aryl oxide
ligands. Related work has shown that during the hydrogenation
of arene substrates with D2 gas, incorporation of deuterium label
into the Pri substituents of 2,6-diisopropylphenoxide ligands
occurs.7 We have also shown that low valent niobium deriva-
tives of this ligand can undergo facile dehydrogenation of an
The preferential hydrogenation of 1,3-cyclohexadiene over
cyclohexene is also observed in the stoichiometric reactivity of
[Ta(OC6H3Pri2-2,6)2(Cl)(H)2(PMe2Ph)2]. This compound reacts
with an excess of an equimolar mixture of cyclohexene and
1,3-cyclohexadiene to produce compound 7 along with only
trace amounts of cyclohexane. When the labeled compound
[Ta(OC6H3Pri2-2,6)2(Cl)(D)2(PMe2Ph)2] is reacted with 1,3-
2
cyclohexadiene under mild conditions in C6H6 solvent, the H
NMR spectrum of the reaction mixture shows deuterium present
in equal amounts in both methylene groups of cyclohexene. This
is consistent with the reaction occurring by preferential 1,2-
(45) Fischer, M. B.; James, E. J.; McNeese, T. J.; Nyburg, S. C.; Posin,
B.; Wong-Ng, W.; Wreford, S. S. J. Am. Chem. Soc. 1980, 102, 4941.
(46) Yu, J. S.; Felter, L.; Potyen, M. C.; Clark, J. R.; Visciglio, V. M.;
Fanwick, P. E.; Rothwell, I. P. Organometallics 1996, 15, 4443.

Coordination/Hydrogenation of 1,3-Cyclohexadiene
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3497
Scheme 9
Table 6. Product Distribution from the Reaction of Cyclohexene,
1,3-Cyclohexadiene, and a Mixture of Cyclohexene and
1,3-Cyclohexadiene with D2 Catalyzed by 4aa
Scheme 10
substrates
product
cyclohexane
C6H10,b %
C6H8,b %
[C6H8 + C6H10],c %
C6H12
0
13
67
14
4
2
0
0
0
4
21
44
28
3
0
0
3
20
47
26
4
C6H11D
C6H10D2
C6H9D3
C6H8D4
C6H7D5
C6H6D6
C6H5D7
0
0
0
a 4a, 0.085 mmol; substrate(s), 2.25 mmol (25 per Nb); C6D6, 0.8
mL; D2, 900 psi; 25 °C. b 24 h, 100% conversion. c 5 h, 20%
conversion.
hydrogenation. Interestingly, when this reaction mixture is
heated further, scrambling of the deuterium label occurs to
produce a statistical distribution over all three sites within the
cyclohexene molecule (2H NMR). We propose that this
scrambling process is being catalyzed by the organometallic
product 7. Low valent early d-block metal compounds can
catalyze olefin isomerization via the formation of allyl, hydride
intermediates.47,48 Applying this process to cyclohexene-3,4-
D2 can account for the statistical distribution of D over the
molecule (Scheme 10). It should also be pointed out that this
process will ultimately lead to the loss of any stereochemistry
of the labels within the product cyclohexene.
Mechanistic Discussion. The above results can be accom-
modated into an overall reaction scheme for the hydrogenation
of benzene, 1,3-cyclohexadiene, and cyclohexene by a metal
dihydride species (Scheme 11). The η4-cyclohexadiene species
formed by transfer of the two hydrides to benzene has been
shown in this study to be a robust species which reacts readily
with hydrogen. The disproportionation of 1,3-cyclohexadiene
catalyzed by 4a cleanly produces cyclohexene and benzene. A
reasonable pathway for this disproportionation reaction involves
initial loss of benzene and formation of a dihydride which
attacks 1,3-cyclohexadiene to produce a cyclohexene complex.
(47) Akita, M.; Yasuda, H.; Nagasuna, K.; Nakamura, A. Bull. Chem.
Soc. Jpn. 1983, 56, 554.
(48) Negishi, E.; Maye, J. P.; Choueiry, D. Tetrahedron 1995, 51, 4447.

3498 J. Am. Chem. Soc., Vol. 119, No. 15, 1997
Visciglio et al.
Scheme 11
[M(OC6H3Pri2-2,6)3Cl2] (1), [M(OC6H3Pri2-2,6)2Cl3]2 (2) (a, M ) Nb;
b, M ) Ta), [Ta(OC6H3Cy2-2,6)2(H)3(PMe2Ph)2], [Ta(OC6HPh2-3,5-
Cy2-2,6)2(H)3(PMe2Ph)2], and [Ta(OC6H3But2-2,6)2(Cl)(H)2(PMe2Ph)]
were prepared by reported procedures.6,10 The spectroscopic data on
[Ta(OC6H3Pri-η2-CMe)CH2)(OC6H3Pri2-2,6)(Cl)(PMe2Ph)2] (7) has
Displacement by 1,3-cyclohexadiene regenerates 4a. In the case
of the hydrogenation of 1,3-cyclohexadiene by 3a and 4a some
of the elementary steps are identical to those in the dispropor-
tionation sequence. The differing selectivities observed for
catalysts 3a and 4a can be ascribed to the reactivity of the
cyclohexene intermediate [Nb(OC6H3Pri2-2,6)2(X)(η2-C6H10)].
In the case of X ) OC6H3Pri2-2,6 (catalyst 4a) the intermediate
undergoes reaction with H2 faster than displacement by 1,3-
cyclohexadiene and vice versa for X ) Cl (catalyst 3a). A
cohesive picture of a much faster reaction of metal-hydride
species with 1,3-cyclohexadiene over cyclohexene is also
present. Toward the end of the disproportionation reaction there
are many more equivalents of cyclohexene in solution than 1,3-
cyclohexadiene. Despite this only traces of cyclohexane are
produced, indicating that the intermediate dihydride selectively
attacks the 1,3-cyclohexadiene. The labeling experiments
involving 4a as hydrogenation catalyst (Table 6) as well as the
reactivity of tantalum dihydrides with cyclohexene and 1,3-
cyclohexadiene echo this hypothesis.
1
been reported.3d The H 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. The 2H NMR spectra were recorded
on a Varian VXR500 spectrometer. Microanalytical data was obtained
in-house at Purdue. All high pressure reactions were performed in a
Parr Model 4561, 300 mL internal volume minireactor.
[Nb(OC6H3Pri2-2,6)2Cl(η4-C6H8)] (3a). To a sodium amalgam
(0.18 g Na/5 mL Hg) was added [Nb(OC6H3Pri2-2,6)2Cl3]2 (1a) (2.0 g,
3.6 mmol) dissolved in hexane (25 mL) and 1,3-cyclohexadiene (0.58
g, 7.2 mmol) was then added to the system. The mixture was stirred
vigorously for 20 h resulting in the formation of a dark red suspension.
The red solution was filtered from the mixture and dried in Vacuo.
The crude product was dissolved in pentane, and slow cooling of the
solution gave 3a as red crystals. Yield 1.50 g (74%). Anal. Calcd
for NbClO2C30H42: C, 64.00; H, 7.52; Cl, 6.30. Found: C, 63.44; H,
7.51; Cl, 6.55. 1H NMR (C6D6, 30 °C): δ 7.04-7.17 (m, 6H,
aromatics); 5.73 (m, 2H, CH2CH-CH); 3.87 (m, 2H, CH2-CH); 2.66
Conclusions
2
(d, 2H), 2.40 (d, 2H, CH2) J (1H-1H) ) 9.2 Hz; 3.60 (septet, 4H,
Stable η4-cyclohexadiene complexes of niobium and tantalum
containing ancillary aryl oxide ligation have been isolated.
Studies show that the cyclohexadiene molecule is strongly bound
to the metal center with structural parameters consistent with a
metallanorbornene bonding description. The molecule [Nb-
(OC6H3Pri2-2,6)3(η4-C6H8)] (4a) catalyzes the disproportionation
of 1,3-cyclohexadiene into a mixture of cyclohexene and
benzene. Compound 4a also hydrogenates 1,3-cyclohexadiene
selectively over cyclohexene to produce cyclohexane with no
intermediate cyclohexene being released. The much higher
reactivity of isolated tantalum hydride compounds towards 1,3-
cyclohexadiene compared to cyclohexene indicates the possible
origin of this selectivity.
CHMe2); 1.28 (d, 12H, CHMe2); 1.25 (d, 12H, CHMe2). 13C NMR
(C6D6, 30 °C): δ 159.2 (Nb-O-C); 124.9 (CH2CH-CH); 99.1 (CH2-
CH); 33.0 (CH2); 27.8 (CHMe2); 24.5, 24.2 (CHMe2); 119.2-123.9
(aromatics).
[Ta(OC6H3Pri2-2,6)2Cl(η4-C6H8)] (3b). To a sodium amalgam (0.15
g Na/5 mL Hg) was added [Ta(OC6H3Pri2-2,6)2Cl3]2 (1b) (2.0 g, 3.1
mmol) in benzene (25 mL), and 1,3-cyclohexadiene (1.25 g, 15.5 mmol)
was then added to the system. The mixture was stirred vigorously for
20 h resulting in the formation of a dark orange suspension. The orange
solution was filtered from the mixture and dried in Vacuo. Analysis
of the crude product by NMR showed the presence of 3b contaminated
with 4b. Over time the 3b present was converted to 4b and another
unidentified tantalum component. 1H NMR (C6D6, 30 °C): δ 6.90-
7.12 (m, 6H, aromatics); 6.31 (m, 2H, CH2CH-CH); 3.33 (m, 2H, CH2-
CH); 2.91 (d, 2H), 2.65 (d, 2H, CH2) 2J(1H-1H) ) 7.8 Hz; 3.61 (septet,
4H, CHMe2); 1.26 (d, 12H, CHMe2); 1.23 (d, 12H, CHMe2).
Experimental 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. 1,3-
Cyclohexadiene was purchased from Aldrich Chemical Co. and was
dried over 3Å molecular sieves prior to use. The compounds
[Nb(OC6H3Pri2-2,6)3(η4-C6H8)] (4a). To a sodium amalgam (0.1
g Na/5 mL Hg) was added [Nb(OC6H3Pri2-2,6)3Cl2] (2a) (1 g, 1.4 mmol)
in hexane (10 mL) and 1,3-cyclohexadiene (0.23 g, 2.8 mmol) was
then added to the system. The mixture was stirred vigorously for 15
h resulting in the formation of a dark red suspension. The red solution
was filtered from the mixture and dried in Vacuo. The crude product

Coordination/Hydrogenation of 1,3-Cyclohexadiene
J. Am. Chem. Soc., Vol. 119, No. 15, 1997 3499
was then dissolved in pentane, and slow cooling of the solution gave
respectively was observed over the course of several hours. The
reaction was complete after 1 h when heated at 100 °C. Reactions
between the labeled compound [Ta(OC6H3Pri2-2,6)2(Cl)(D)2(PMe2Ph)2]
and 1,3-cyclohexadiene were carried out in C6H6 and monitored by 2H
NMR.
4a as red crystals. Yield 0.73
g (72%). Anal. Calcd for
NbO3C42H59: C, 71.57; H, 8.44. Found: C, 71.46; H, 8.48. 1H NMR
(C6D6, 30 °C): δ 6.9-7.2 (m, 9H, aromatics); 5.92 (m, 2H, CH2CH-
2
CH); 3.98 (m, 2H, CH2-CH); 2.51 (d, 2H), 2.22 (d, 2H, CH2) J(1H-
1H) ) 8.7 Hz; 3.61 (septet, 6H, CHMe2); 1.25 (d, 36H, CHMe2);. 13C
NMR (C6D6, 30 °C): δ 159.9 (Nb-O-C); 138.5-123.9 (aromatics);
122.3 (CH2CH-CH); 92.4 (CH2-CH); 30.2 (CH2); 27.6 (CHMe2); 24.2
(CHMe2).
[Ta(OC6H3But-CMe2CH2)(OC6H3But2-2,6)(Cl)(C6H11)] (8). Method
1. To a suspension of [Ta(OC6H3But2-2,6)2Cl3] (0.5 g, 0.71 mmol) in
hexane (5 mL) was added 1,3-cyclohexadiene (0.29 g, 3.62 mmol)
followed by (Bun)3SnH (0.63 g, 2.16 mmol), and the resulting mixture
was allowed to stand for 20 h. Removal of the solvent in Vacuo left
the crude product as a yellow oil contaminated with (Bun)3SnCl.
Layering the oil with hexane (5 mL) precipitated the product as a
crystalline yellow solid which was washed with hexane and dried in
Vacuo. Anal. Calcd for C34H54 ClO2Ta: C, 57.60; H, 7.40; Cl, 4.94.
Found: C, 57.14; H, 7.71; Cl, 5.43. 1H NMR (C6D6, 30 °C): δ 6.81-
[Ta(OC6H3Pri2-2,6)3(η4-C6H8)] (4b). To a sodium amalgam (0.1
g Na/5 mL Hg) was added [Ta(OC6H3Pri2-2,6)3Cl2] (2b) (1 g, 1.3 mmol)
in hexane (10 mL), and 1,3-cyclohexadiene (0.23 g, 2.8 mmol) was
then added to the system. The mixture was stirred vigorously for 15
h resulting in the formation of an orange-yellow suspension. The
orange solution was filtered from the mixture and dried in Vacuo. The
crude product was then dissolved in pentane, and slow cooling of the
solution gave 4b as orange-yellow crystals. Yield 0.72 g (71%). Anal.
Calcd for TaO3C42H59: C, 63.62; H, 7.50. Found: C, 63.39; H, 7.30.
1H NMR (C6D6, 30 °C): δ 6.9-7.2 (m, 9H, aromatics); 6.28 (m, 2H,
CH2CH-CH); 3.48 (m, 2H, CH2-CH); 2.72 (m, 4H, unresolved CH2);
3.65 (septet, 6H, CHMe2); 1.25 (d, 36H, CHMe2);. 13C NMR (C6D6,
30 °C): δ 156.8 (Ta-O-C); 138.9-124.0 (aromatics); 125.8 (CH2CH-
CH); 84.0 (CH2-CH); 31.9 (CH2); 27.3 (CHMe2); 24.3 (CHMe2).
[Ta(OC6H3Cy2-2,6)2(C12H17)] (5). To a suspension of [Ta(OC6H3-
Cy2-2,6)2(H)3(PMe2Ph)2] (0.1 g, 0.11 mmol) in benzene (1 mL) was
added 1,3-cyclohexadiene (0.04 g, 0.55 mmol). The reaction was
warmed at 60 °C for 3 h until the trihydride was completely dissolved.
The resulting orange solution was layered with hexane to produce
orange crystals of 5 which were washed with hexane and dried in Vacuo.
Anal. Calcd for TaO2C48H67: C, 67.27; H, 7.88. Found: C, 66.98;
H, 7.87. 1H NMR (C6D6, 30 °C): δ 7.58-6.56 (m); 6.43 (t); 6.13 (t);
5.22 (s, br); 4.74 (d, br); 3.42 (m); 3.21 (m); 3.15 (m); 2.96 (d); 2.8
(m); 2.27-1.11 (m).
2
7.26 (m, 11H, aromatics); 3.08 (d, 1H), 2.55 (d, 1H, Ta-CH2, J(1H-
1H) ) 15.3 Hz); 1.45 (s), 1.47 (s, CH2CMe2); 1.24 (s, CMe3). Selected
13C NMR (C6D6, 30 °C): 162.6 (Ta-O-C); 155.6 (Ta-O-C); 99.4 (Ta-
CH2CMe2).
Method 2. Addition of an excess 1,3-cyclohexadiene to a C6D6
solution of [Ta(OC6H3But2-2,6)2(Cl)(H)2(PMe2Ph)] was found upon
1
standing for several hours to produce 8 identified by H NMR.
Method 3. When a solution of [Ta(OC6H3But2-2,6)2Cl3] in C6D6 is
reacted with [Mg(c-C6H11)2], the pale yellow mixture was shown by
1H NMR to contain 8 along with other unidentified tantalum species.
Hydrogenation Reactions. Similar procedures were used for all
hydrogenation reactions. A representative example is given for the
hydrogenation of 1,3-cyclohexadiene by 4a. In the Dri-Lab a sample
of 4a (60 mg, 0.085 mmol) was dissolved in C6D6 (0.8 mL) in a 25
mL round bottom flask. To this solution was added 1,3-cyclohexadiene
(215 µL, 2.25 mmol, 25 per Nb), and the flask containing the mixture
was placed inside the pressure reactor which was sealed. After
removing from the Dri-Lab the reactor was “flushed” with 200 psi of
D2, pressurized to 900 psi, and allowed to stand for 3 days. The reactor
was depressurized and taken into the Dri-Lab. The reaction mixture
was initially analyzed by 1H NMR spectroscopy (complete conversion
to cyclohexane) and by GC/MS.
[Ta(OC6HPh2-3,5-Cy2-2,6)2(C12H17)] (6). To a suspension of
[Ta(OC6HPh2-3,5-Cy2-2,6)2(H)3(PMe2Ph)2] (0.1 g, 0.08 mmol) in
benzene (1 mL) was added 1,3-cyclohexadiene (0.04 g, 0.55 mmol).
The reaction was warmed at 60 °C for 3 h until the trihydride was
dissolved. The solvent was removed in Vacuo to yield the crude 6 as
an orange oil. 1H NMR (C6D6, 30 °C): δ 7.36-6.87 (m); 6.70 (s);
6.30 (t); 5.55 (t); 4.88 (d); 4.08 (t, br); 3.84 (t, br);3.75 (s, br); 3.38
(m); 3.37 (m); 3.27 (t, br); 3.05 (m); 2.99 (s, br); 2.17 (m); 2.07 (m);
1.95-0.85 (m).
Acknowledgment. We thank the Department of Energy for
financial support of this research.
Supporting Information Available: Description of the
experimental procedures for X-ray diffraction studies and tables
of thermal parameters, bond distances and angles, intensity data,
torsion angles, and multiplicities (87 pages). See any current
masthead page for ordering and Internet access instructions.
Reactivity of [Ta(OC6H3Pri2-2,6)2(Cl)(H)2(PMe2Ph)2]. A suspen-
sion of the dihydride compound [Ta(OC6H3Pri2-2,6)2(Cl)(H)2(PMe2Ph)2]
in C6D6 was treated with cyclohexene or 1,3-cyclohexadiene, and the
1
reaction was monitored by H and 31P NMR spectroscopy. The slow
build up of the organometallic species [Ta(OC6H3Pri-η2-CMedCH2)-
(OC6H3Pri2-2,6)(Cl)(PMe2Ph)2] (7)3d and cyclohexene or cyclohexane,
JA964073N