Cyclohexadienyl niobium complexes and arene hydrogenation catalysis

Article abstract of DOI:10.1021/om020395q

Hydrogenolysis of ^R[P₂N₂]NbCH₂SiMe₃ (where ^R[P₂N₂] = RP(CH₂SiMe₂NSiMe₂CH₂)₂PR; R = cyclohexyl, Cy, or phenyl, Ph) in benzene or toluene causes hydride addition to the aromatic solvent resulting in the formation of the π-bonded complexes ^R[P₂N₂]Nb(η⁵-C₆H ₇) (R = Cy, 1; R = Ph, 2) and ^R[P₂N₂]Nb(η⁵-C₇H ₉) (R = Cy, 3; R = Ph, 4) in benzene and toluene, respectively. Performing the hydrogenation at a higher pressure of 29 atm at room temperature causes the catalytic hydrogenation of benzene to cyclohexane as determined by NMR spectroscopy and GC-MS analysis. The hydrogenation of toluene to methylcyclohexane can also be performed, but the turnover frequency is considerably lower. Examination of the solid residues from the high-pressure hydrogenations indicates the formation of the π-bonded complexes 1 and 2. The addition of 29 atm of H₂ to these cyclohexadienyl derivatives in benzene or toluene, however, shows no hydrogenation products, indicating these species are not catalytically active.

Full text of DOI:10.1021/om020395q

Organometallics 2002, 21, 5047-5054
5047
Cycloh exa d ien yl Niobiu m Com p lexes a n d Ar en e
Hyd r ogen a tion Ca ta lysis
Michael D. Fryzuk,* Christopher M. Kozak, Michael R. Bowdridge, and
Brian O. Patrick†
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, V6T 1Z1 Canada
Received May 16, 2002
R
Hydrogenolysis of [P2N2]NbCH2SiMe3 (where R[P2N2] ) RP(CH2SiMe2NSiMe2CH2)2PR;
R ) cyclohexyl, Cy, or phenyl, Ph) in benzene or toluene causes hydride addition to the
R
aromatic solvent resulting in the formation of the π-bonded complexes [P2N2]Nb(η5-C6H7)
R
(R ) Cy, 1; R ) Ph, 2) and [P2N2]Nb(η5-C7H9) (R ) Cy, 3; R ) Ph, 4) in benzene and toluene,
respectively. Performing the hydrogenation at a higher pressure of 29 atm at room
temperature causes the catalytic hydrogenation of benzene to cyclohexane as determined
by NMR spectroscopy and GC-MS analysis. The hydrogenation of toluene to methylcyclo-
hexane can also be performed, but the turnover frequency is considerably lower. Examination
of the solid residues from the high-pressure hydrogenations indicates the formation of the
π-bonded complexes 1 and 2. The addition of 29 atm of H2 to these cyclohexadienyl derivatives
in benzene or toluene, however, shows no hydrogenation products, indicating these species
are not catalytically active.
In tr od u ction
genate aromatic substrates with good chemoselectivity
and stereoselectivity. Attempts at isolating intermedi-
ates in these group 5 systems have resulted in the
formation of phosphine-stabilized hydride derivatives.
While these isolated species demonstrate mild capability
as arene hydrogenation catalysts, phosphine-free de-
rivatives show much higher reactivity.
We have previously reported the synthesis and char-
acterization of a series of paramagnetic niobium(III)
chloride and hydrocarbyl derivatives stabilized by the
[P2N2] macrocyclic ligand.18 Here we report the at-
tempted isolation of niobium hydrides via hydrogenoly-
sis of these alkyl species. Although discrete niobium
hydride complexes were not isolated, there is indirect
evidence for their formation since the resulting species
display reactivity toward the hydrogenation of aromatic
substrates.
While there are numerous examples of soluble transi-
tion metal complexes that can catalyze the homogeneous
hydrogenation of alkenes and alkynes, there is a paucity
of systems that will hydrogenate aromatic substrates
such as benzene.1-3 Of the soluble complexes that do
show activity in arene hydrogenation, some are limited
to polycyclic aromatic hydrocarbons, and for others there
is some question as to whether they are truly homo-
geneous.3-12
A particularly impressive class of arene hydro-
genation catalysts is based on niobium and tantalum
complexes stabilized by bulky aryloxide ancillary
ligands.13-17 These systems have been shown to hydro-
* Corresponding author. Tel: 604-822-2897. Fax: 604-822-2847.
E-mail: fryzuk@chem.ubc.ca.
† UBC X-ray Structural Chemistry Laboratory.
(1) J ames, B. R. Homogeneous Hydrogenation; J ohn Wiley: New
York, 1973.
Resu lts a n d Discu ssion
(2) Rylander, P. N. Hydrogenation Methods; Academic Press: Lon-
don, 1985.
(3) Collman, J . P.; Hegedus, L. S.; Norton, J . R.; Finke, R. G.
Applications of Organotransition Metal Chemistry; University Science
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J . Am. Chem. Soc. 1978, 100, 1313.
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R.; Stevens, A. E.; Muetterties, E. L. J . Am. Chem. Soc. 1978, 100,
2405.
Syn th esis of Cycloh exa d ien yl Com p lexes of Nio-
biu m . The addition of 1 atm of H2 to deep blue benzene
solutions of R[P2N2]NbCH2SiMe3 results in the forma-
tion of olive green solutions within 1 h. Besides the
typical resonances attributed to diamagnetic complexes
1
of the [P2N2] ligand, a striking feature of the H NMR
spectrum is the presence of three multiplet resonances
(6) Blum, J .; Amer, I.; Vollhardt, K. P. C.; Schwarz, H.; Hohne, G.
J . Org. Chem. 1987, 53, 2804.
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1985, 296, 115.
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7395.
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Am. Chem. Soc. 1992, 114, 1927.
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Soc. 1991, 113, 4710.
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Commun. 1977, 427.
(10) Suss-Fink, G.; Faure, M.; Ward, T. R. Angew. Chem., Int. Ed.
2002, 41, 99.
(15) Visciglio, V. M.; Fanwick, P. E.; Rothwell, I. P. J . Chem. Soc.,
Chem. Commun. 1992, 1505.
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Commun. 1995, 553.
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(12) Lin, Z.; Halpern J . J . Organomet. Chem. 1991, 417, C24.
(17) Rothwell, I. P. Chem. Commun. 1997, 1331.
(18) Fryzuk, M. D.; Kozak, C. M.; Bowdridge, M. R.; J in, W.; Tung,
D.; Patrick, B. O.; Rettig, S. J . Organometallics 2001, 20, 3752.
10.1021/om020395q CCC: $22.00 © 2002 American Chemical Society
Publication on Web 10/11/2002

5048 Organometallics, Vol. 21, No. 23, 2002
Fryzuk et al.
Sch em e 1
F igu r e 1. 1H NMR spectrum (500 MHz) of Ph[P2N2]Nb-
(η5-C7H9) (4) in toluene-d8 (*) and assignment of methyl-
cyclohexadienyl resonances.
suggesting the conversion of isomers in the C7H9 frag-
ment is intramolecular. Reaction of the niobium alkyl
complexes with deuterium gas in benzene or toluene
leads to simplified ortho-H resonances as well as to loss
of coupling in the cyclohexadienyl CHH(Me) peak. The
endo addition of the deuterium was confirmed by loss
of the resonance at 2.10 ppm (Hd) for 3 and 4. The endo
hydrogen atoms in 1 and 2 are obscured by the meth-
ylene resonances of the [P2N2] ligand. Microanalysis and
mass spectrometry support the formulation of these
products as R[P2N2]Nb(η5-C7H9) (R ) Cy, 3; R ) Ph, 4)
(Scheme 1). The 31P{1H} NMR spectra of 3 and 4 are
analogous to those of 1 and 2; namely, the phenyl-
substituted complex 4 shows two broad resonances at
29.32 and 26.36 ppm, while cyclohexyl-substituted 3
displays a single broad resonance at 24.29 ppm.
An alternate route to generating niobium hydrides
was explored. It has been shown that in addition to
hydrogenolysis of vanadium alkyl complexes, reduction
of a vanadium(III) chloride complex by KC8 under H2
is also an effective route to vanadium hydride species.19
Reacting solutions of R[P2N2]NbCl with KC8 under H2
in benzene or toluene also results in the hydrogenation
of the respective aromatic solvent to generate complexes
1 through 4 in good yield (Scheme 1).
The reactivity of other simple arenes to form similarly
bound species was also explored. The addition of H2 to
hexanes solutions of R[P2N2]NbCH2SiMe3 and naphtha-
lene or anthracene did show the presence of coordinated
arene moieties determined spectroscopically. However,
these products could not be fully characterized as a
result of the difficulty in their isolation from paramag-
netic impurities that were consistently observed.
Solid Sta te Ch a r a cter iza tion . Evaporation of hex-
anes solutions of 1 and 2 allowed isolation of light green
crystals; the solid state molecular structure of 1 is
shown in Figure 2. Crystallographic data appear in
Table 1, and selected bond lengths and angles are shown
in Table 2. The structure unequivocally reveals that a
benzene molecule has been partially hydrogenated to
form a cyclohexadienyl moiety. The C(25)-C(26) and
C(25)-C(30) single bond lengths of 1.511(3) and
at 2.51, 4.14, and 5.71 ppm for R ) Cy (1), and at 2.45,
4.23, and 5.42 ppm for R ) Ph (2), in a 2:2:1 integration
ratio. This is consistent with a π-coordinated arene-type
ligand. Complexes 1 and 2 each display four sets of silyl
methyl resonances as well as four sets of methylene
resonances. The 31P{1H} NMR spectra display two broad
peaks at 30.08 and 26.52 ppm for 2 at room tempera-
ture, while only one broad resonance at 32.36 ppm for
1. Performing the hydrogenation in an NMR tube sealed
under H2 reveals that SiMe4 is eliminated by hydro-
genolysis as anticipated. However, rather than generat-
R
ing the expected [P2N2]NbHn (n ) 1 or 3), diamagnetic
species possessing cyclohexadienyl ligands were formed.
On the basis of NMR spectral data, elemental analysis,
and mass spectrometry, the resulting products can be
formulated as R[P2N2]Nb(η5-C6H7) (R ) Cy, 1; R ) Ph,
2) (Scheme 1).
Hydrogenation of R[P2N2]NbCH2SiMe3 in toluene
produces 1H NMR spectra resembling those of com-
plexes 1 and 2, with upfield shifted π-type resonances
indicative of a similarly bound arene-type ligand. In this
case, however, the 1H NMR spectrum of the initially
formed product shows a mixture of isomers. There
appear to be at least two sets of resonances arising from
the coordinated arene ligand, suggesting hydride addi-
tion at different sites within the toluene molecule. Over
time, these isomers apparently undergo rearrangement
that ultimately leads to one thermodynamically pre-
ferred species in solution. After several hours the 1H
NMR spectrum shows one series of π-type resonances
at 2.82, 4.04, and 5.72 ppm for R ) Cy (3) and at 2.85,
4.21, and 5.51 ppm for R ) Ph (4) (Figure 1). Addition-
ally, doublets at 0.64 ppm corresponding to a methyl
resonance coupled to a single proton are observed for
both 3 and 4. As in 1 and 2 above, 3 and 4 display four
sets of silyl methyl resonances as well as four sets of
methylene resonances. No exchange of the methylcy-
clohexadienyl ligand with C6D6 solvent is observed, thus
(19) Clancy, G. P.; Clark, H. C. S.; Clentsmith, G. K. B.; Cloke, F.
G. N.; Hitchcock, P. B. J . Chem. Soc., Dalton Trans. 1999, 3345.

Cyclohexadienyl Niobium Complexes
Organometallics, Vol. 21, No. 23, 2002 5049
Ta ble 1. Cr ysta llogr a p h ic Da ta a
Cy[P2N2]Nb(η5-C6H7) (1)
Cy[P2N2]Nb(η5-C7H9) (3)
Ph[P2N2]Nb(η5-C7H9) (4)
formula
fw
C
30H61N2Si4P2Nb
C31H63N2Si4P2Nb
731.05
C31H51N2P2Si4Nb•C3H7
762.04
717.02
color, habit
cryst size, mm
cryst syst
space group
a, Å
green, chip
0.35 × 0.20 × 0.20
triclinic
P1h (#2)
11.4451(4)
12.218(1)
14.2486(5)
89.522(2)
67.759(2)
89.131(2)
1844.0(2)
2
green, chip
0.20 × 0.20 × 0.20
triclinic
P1h (#2)
10.613(2)
11.2703(7)
17.031(1)
89.197(2)
85.949(2)
67.378(3)
1875.5(3)
2
green, chip
0.25 × 0.20 × 0.20
triclinic
P1h (#2)
11.0005(4)
12.4477(8)
15.7988(7)
74.735(2)
82.464(2)
73.092(3)
1993.3(2)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
Z
T, °C
-100 ( 1
-100 ( 1
-100 ( 1
F
calc, g/cm3
1.291
1.294
1.270
F(000)
764.00
5.65
0.8302-1.0000
57.5
16 404
7358
0.024
780.00
5.57
0.8123-1.0000
55.8
16 246
7533
0.052
806.00
5.27
0.7747-1.0000
55.9
16 647
7548
0.046
µ(Mo KR), cm-1
transmn factors
2θmax, deg
total no. of reflns
no. of unique reflns
Rmerge
no. reflns with I g 3σ(I)
no. of variables
R (F2, all data)
Rw (F2, all data)
R (F, I >3σ(I))
Rw (F, I >3σ(I))
gof
6671
352
0.049
0.105
0.030
0.053
1.82
5237
393
0.065
0.102
0.035
0.043
1.16
6069
408
0.059
0.098
0.034
0.0449
1.32
a
2
2
2
2
2
2 2
Rigaku/ADSC CCD diffractometer, R ) ∑||Fo | - |Fc ||/∑|Fo |; Rw ) (∑w(|Fo | - |Fc |)2/∑w|Fo | )1/2
.
(8) Å are comparable to those found in previously
reported [P2N2] complexes of Nb(III).40
Dark green crystals of 3 and 4 were also obtained by
slow evaporation of hexanes solutions, and their struc-
tures were characterized by single-crystal X-ray diffrac-
tion. The molecular structure of 3 is shown in Figure 3,
with selected bond lengths and angles listed in Table
3. The structure of 4 is shown in Figure 4, with selected
bond lengths and angles listed in Table 4. Crystal-
(20) Wolczanski, P. T. Polyhedron 1995, 14, 3335.
(21) Wexler, P. A.; Wigley, D. E.; Koerner, J . B.; Albright, T. A.
Organometallics 1991, 10, 2319.
(22) Gray, S. D.; Weller, K. J .; Bruck, M. A.; Briggs, P. M.; Wigley,
D. E. J . Am. Chem. Soc. 1995, 117, 10678.
F igu r e 2. Molecular structure (ORTEP) of Cy[P2N2]Nb-
(η5-C6H7) (1). Silyl methyl groups omitted for clarity.
(23) J onas, K. Angew. Chem., Int. Ed. Engl. 1985, 24, 295.
(24) J onas, K. J . Organomet. Chem. 1990, 400, 165.
(25) J onas, K. Pure Appl. Chem. 1990, 62, 1169.
(26) Grossheimann, G.; Holle, S.; J olly, P. W. J . Organomet. Chem.
1998, 568, 205.
(27) Pearson, A. J . J . Chem. Soc., Perkin Trans. 1977, 2069.
(28) J ohnson, B. F. G.; Lewis, J .; Yarrow, D. J . J . Chem. Soc., Dalton
Trans. 1972, 2084.
(29) Birch, A. J .; Cross, P. E.; Lewis, J .; White, D. A.; Wild, S. B. J .
Chem. Soc. A 1968, 332.
(30) Harman, W. D.; Schaefer, W. P.; Taube, H. J . Am. Chem. Soc.
1990, 112, 2682.
1.499(3) Å, respectively, are longer than those of the
planar fragment. The average bond distance of 1.420 Å
for C(26)-C(27), C(27)-C(28), C(28)-C(29), and C(29)-
C(30) is consistent with slightly elongated CdC double
bonds. All of these derivatives that possess η5-ligands
are diamagnetic and can be considered 16-electron
complexes ([P2N2] can be formally considered a six-
electron ligand). Assuming the cyclohexadienyl ligand
occupies a single coordination site, 1 possesses a dis-
torted trigonal bipyramidal geometry around niobium.
The phosphine-donor groups occupy axial positions
slightly bent back from 180°. The two amido-donor N
atoms and the cyclohexadienyl ligand reside in the
equatorial positions. Alternatively, as for cyclopentadi-
enyl ligands, the cyclohexadienyl ligand can be regarded
as being three-coordinate, resulting in a seven-coordi-
nate niobium center. The increased steric bulk of the
cyclohexadienyl ligand over the previously described
alkyl and halide substituents is apparent in the more
acute P(1)-Nb(1)-P(2) and N(1)-Nb(1)-N(2) bond
angles of 148.95(2)° and 99.06(6)°, respectively, found
in 1. The Nb-P bond lengths of 2.5417(8) and 2.5760-
(31) Pyke, R. D.; Ryan, W. J .; Carpenter, G. B.; Sweigart, D. A. J .
Am. Chem. Soc. 1989, 111, 8535.
(32) Fryzuk, M. D.; Kozak, C. M.; Bowdridge, M. R.; Patrick, B. O.;
Rettig, S. J . J . Am. Chem. Soc. 2002, 124, 8389.
(33) Maitlis, P. M. J . Chem. Soc., Dalton Trans. 1984, 1747.
(34) Fryzuk, M. D.; Kozak, C. M.; Mehrkhodavandi, P.; Morello, L.;
Patrick, B. O.; Rettig, S. J . J . Am. Chem. Soc. 2002, 124, 516.
(35) Fryzuk, M. D.; J ohnson, S. A.; Rettig, S. J . Organometallics
2000, 19, 3931.
(36) Fryzuk, M. D.; J ohnson, S. A. Unpublished results.
(37) Covert, K. J .; Neithamer, D. R.; Zonnevylle, M. C.; LaPointe,
R. E.; Schaller, C. P.; Wolczanski, P. T. Inorg. Chem. 1991, 30, 2494.
(38) 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.
(39) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(40) Berbreiter, D. E.; Killough, J . M. J . Am. Chem. Soc. 1978, 100,
2126.

5050 Organometallics, Vol. 21, No. 23, 2002
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles (d eg) in Cy[P 2N2]Nb(η5-C6H7) (1)
Distances
Fryzuk et al.
Nb(1)-P(1)
Nb(1)-N(1)
Nb(1)-CM
C(26)-C(27)
C(28)-C(29)
C(30)-C(25)
2.6194(5)
2.178(2)
1.926
1.430(3)
1.408(3)
1.499(3)
Nb(1)-P(2)
Nb(1)-N(2)
C(25)-C(26)
C(27)-C(28)
C(29)-C(30)
2.6231(5)
2.164(2)
1.511(3)
1.415(3)
1.426(3)
Angles
P(1)-Nb(1)-P(2)
P(1)-Nb(1)-CM
P(2)-Nb(1)-CM
P(1)-Nb(1)-N(1)
P(2)-Nb(1)-N(1)
Nb(1)-P(1)-C(13)
148.95(2)
110.48
100.57
76.19(5)
83.23(5)
133.12(7)
N(1)-Nb(1)-N(2)
N(1)-Nb(1)-CM
N(2)-Nb(1)-CM
P(1)-Nb(1)-N(2)
P(2)-Nb(1)-N(2)
Nb(1)-P(2)-C(19)
99.06(6)
130.48
130.08
84.03(5)
76.47(5)
132.12(7)
Angles
C(25)-C(30)-C(29)-C(28)
-22.3(3)
C(27)-C(28)-C(29)-C(30)
-11.7(3)
preferred orientation, as this arrangement minimizes
steric interaction of the toluene methyl group with the
phosphorus substituents. The resulting C-C and Nb-C
bond lengths of this fragment in both 3 and 4 confirm
an η5-coordination mode. In 4, the bond distances for
C(25)-C(30) and C(29)-C(30) are identical at 1.519(4)
Å, and the methyl carbon bond distance, C(30)-C(31),
is 1.535(4) Å. All are typical C-C single bond lengths.
Similarly, 3 possesses C(25)-C(26) and C(25)-C(30)
bond lengths of 1.517(5) and 1.527(5) Å, respectively,
and a methyl carbon bond distance of 1.547(5) Å for
C(25)-C(31).
Formation of complexes 1-4 can be rationalized by
the mechanism presented in Scheme 2. Oxidative ad-
dition of H2 to the niobium(III) alkyl species allows
formation of a niobium(V) dihydride. Reductive elimina-
tion of the alkyl ligand and a hydride forms the stable
tetramethylsilane molecule and the coordinatively un-
saturated niobium(III) hydride. The Lewis acidity of the
resulting d2 metal hydride allows π-coordination of the
aromatic solvent molecule.17,20-22 Endo addition of the
hydride onto the coordinated arene (as demonstrated
by deuterium labeling studies, above) results in the
formation of an anionic cyclohexadienyl moiety; this
route to cyclohexadienyl ligand formation has also been
exhibited by cobalt hydride species stabilized by phos-
phine ligands.23-26 Transition metal cyclohexadienyl
compounds are typically prepared by hydrogen abstrac-
tion from cyclohexadiene compounds27-29 or nucleophilic
addition to arene complexes.30,31 In the latter case, the
arene becomes activated toward nucleophilic attack by
virtue of its coordination to the metal, and the nucleo-
phile enters an exo position on the ring.
F igu r e 3. Molecular structure (ORTEP) of Cy[P2N2]Nb-
(η5-C7H9) (3). Silyl methyl groups omitted for clarity.
F igu r e 4. Molecular structure (ORTEP) of Ph[P2N2]Nb-
(η5-C7H9) (4). Silyl methyl groups are omitted for clarity.
Sch em e 2
The coordination of the cyclohexadienyl ligand to the
niobium center is inert to exchange with other aromatic
moieties. Cyclohexadienyl ligands on cobalt phosphine
systems, on the other hand, exhibit a much more weakly
bound arene. Whereas the cobalt cyclohexadienyl com-
plexes (Cy2P(CH2)nPCy2)Co(η5-C6H7) (n ) 1-3) show
ligand exchange with other aromatic species,23-26 the
niobium cyclohexadienyl complexes 1-4 are robust.
Va r ia ble-Tem p er a tu r e NMR Stu d ies. In the solid
state, the cyclohexadienyl and methycyclohexadienyl
fragments in 1-4 align themselves such that the
methylene CH2 or methine CHMe fragment resides over
one of the phosphorus atoms, resulting in inequivalent
lographic data for 3 and 4 appear in Table 1. It is
immediately apparent that the toluene fragment has
been hydrogenated in an endo fashion, thus forcing the
methyl group into an axial orientation, and can be best
described as a methylcyclohexadienyl moiety. As men-
tioned above, this is likely the thermodynamically
1
phosphine environments. The H NMR spectra reflect
this by the presence of four silyl methyl and four

Cyclohexadienyl Niobium Complexes
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles (d eg) in Cy[P 2N2]Nb(η5-C7H9) (3)
Distances
Organometallics, Vol. 21, No. 23, 2002 5051
Nb(1)-P(1)
Nb(1)-N(1)
Nb(1)-CM
C(26)-C(27)
C(28)-C(29)
C(30)-C(25)
2.6077(9)
2.187(2)
1.912
1.412(5)
1.404(5)
1.527(5)
Nb(1)-P(2)
Nb(1)-N(2)
C(25)-C(26)
C(27)-C(28)
C(29)-C(30)
C(25)-C(31)
2.6264(9)
2.164(3)
1.517(5)
1.407(5)
1.430(5)
1.547(5)
Angles
P(1)-Nb(1)-P(2)
P(1)-Nb(1)-CM
P(2)-Nb(1)-CM
P(1)-Nb(1)-N(1)
P(2)-Nb(1)-N(1)
Nb(1)-P(1)-C(13)
149.63(3)
101.76
108.59
82.76(7)
76.33(7)
130.0(1)
N(1)-Nb(1)-N(2)
N(1)-Nb(1)-CM
N(2)-Nb(1)-CM
P(1)-Nb(1)-N(2)
P(2)-Nb(1)-N(2)
Nb(1)-P(2)-C(19)
98.6(1)
130.94
130.17
76.57(7)
84.99(7)
134.1(1)
Angles
C(25)-C(30)-C(29)-C(28)
29.4(4)
C(29)-C(30)-C(25)-C(31)
66.2(4)
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles (d eg) in P h [P 2N2]Nb(η5-C7H9) (4)
Distances
Nb(1)-P(1)
Nb(1)-N(1)
Nb(1)-CM
C(26)-C(27)
C(28)-C(29)
C(30)-C(25)
2.5696(7)
2.179(2)
1.909
1.401(4)
1.426(4)
1.519(4)
Nb(1)-P(2)
Nb(1)-N(2)
C(25)-C(26)
C(27)-C(28)
C(29)-C(30)
C(30)-C(31)
2.5677(7)
2.190(2)
1.426(4)
1.418(4)
1.519(4)
1.535(4)
Angles
P(1)-Nb(1)-P(2)
P(1)-Nb(1)-CM
P(2)-Nb(1)-CM
P(1)-Nb(1)-N(1)
P(2)-Nb(1)-N(1)
Nb(1)-P(1)-C(13)
148.71(2)
101.59
114.45
76.17(6)
83.21(6)
126.7(1)
N(1)-Nb(1)-N(2)
N(1)-Nb(1)-CM
N(2)-Nb(1)-CM
P(1)-Nb(1)-N(2)
P(2)-Nb(1)-N(2)
Nb(1)-P(2)-C(19)
98.39(8)
131.94
129.48
83.90(6)
76.04(6)
128.34(9)
Angles
C(25)-C(26)-C(27)-C(28)
-3.9(4)
C(28)-C(29)-C(30)-C(31)
69.5(3)
being resolved at 280 K and sharpening at 240 K with
a coupling constant of 125 Hz. Lowering the tempera-
ture further causes line broadening and loss of coupling
information. This results from the effects of the qua-
drupolar niobium nucleus being enhanced at lower
temperatures. Similar effects on line width in low-
temperature 31P NMR spectroscopy were observed in
other diamagnetic [P2N2] niobium complexes.32
The rate of arene ligand rotation does not appear to
depend on the nature of the arene, that is, whether the
cyclohexadienyl or methylcyclohexadienyl species is
studied. The variable-temperature 31P{1H} NMR spec-
tra of Ph[P2N2]Nb(η5-C6H7), 2, and Ph[P2N2]Nb(η5-C7H9),
4, display the same properties. Likewise, the cyclohexa-
dienyl- and methylcyclohexadienyl-niobium complexes
supported by the Cy[P2N2] ligand show similar behavior.
The nature of the phosphorus-donor atom appears to
dictate the rate of arene rotation. For 2 and 4, where
phenyl phosphine donors were utilized, the rate of
rotation of the arene is slow relative to the cyclohexyl-
substituted ligand, and even at high temperature a
relatively sharp single 31P resonance is unobservable.
Cyclohexyl phosphine-substituted complexes 1 and 3,
however, show equivalent 31P environments at room
temperature. A depiction of the fluxional process is
shown in Figure 6. The rotation of the cyclohexadienyl
ligand proceeds through an arrangement where the
methylene CH2 group resides over one of the amide
ligands, producing equivalent phosphorus environ-
ments.
F igu r e 5. Variable-temperature 31P{1H} NMR spectra of
Ph[P2N2]Nb(η5-C7H9) (4).
methylene environments. The cyclohexadienyl ligand,
however, is fluxional and experiences a rotation about
the Nb-centroid axis. This process is most easily
detected by variable-temperature 31P{1H} NMR spec-
troscopy. The cyclohexyl-phosphine systems 1 and 3
possess a single broad resonance in the 31P{1H} spec-
trum at room temperature. As the temperature is
lowered, the resonance broadens until decoalescence at
260 K. At 200 K, a pair of doublet resonances is observed
with a coupling constant of 112 Hz. At this temperature,
the rotation of the arene ligand is slowed, resulting in
the resolution of the two phosphorus environments. The
phenyl-phosphine-substituted systems, on the other
hand, show two broad resonances at 300 K (Figure 5).
No coalescence of the peaks is observed even at 360 K.
Lowering the temperature results in a pair of doublets
The different fluxional behavior observed between the
cyclohexyl- and phenyl-substituted complexes may be

5052 Organometallics, Vol. 21, No. 23, 2002
Fryzuk et al.
center because of the geometric restrictions of the
macrocycle. Although intramolecular coordination is not
likely to occur in Ph[P2N2], a related intermolecular
phosphorus phenyl activation between two Ph[P2N2]Nb
fragments has been observed.34
The possibility that cyclohexadienyl complexes 1 and
2 may be intermediates in the catalytic hydrogenation
of benzene by these systems was examined. Benzene
solutions of 1 and 2 were exposed to 500 psi H2 at room
temperature for 20 h. No cyclohexane or partially
hydrogenated substrate was observed in the resulting
solution, indicating that 1 and 2 are not catalytically
active species. Even at 80 °C no benzene hydrogenation
was observed in solutions of the cyclohexadienyl species,
1 and 2.
F igu r e 6. Depiction of fluxional behavior in complexes 1
and 3 via cyclohexadienyl ligand rotation and resultant
phosphorus environment exchange between P1 and P2.
correlated to the degree of congestion imposed upon the
cyclohexadienyl ligand. An examination of the Nb-P
bond lengths and Nb-P-Cipso angles shows the cyclo-
hexyl group is positioned further away than the phenyl
substituent from the cyclohexadienyl moiety. For ex-
ample, the Nb-P bond lengths in 1 average 2.6213 and
2.6171 Å in 3 and 2.5687 Å in 4. The average Nb-P-
Cipso bond angles are 132.62° in 1, 132.1° in 3, and 127.5°
in 4. As a result, the more constricted cyclohexadienyl
ligand in the phenyl-substituted complex 4 experiences
a hindered rotation about the metal-centroid axis.
Ca ta lytic Ar en e Hyd r ogen a tion Stu d ies. In a
number of catalytic systems, the hydrogenation of
benzene is proposed to proceed via a cyclohexadienyl
intermediate before the eventual formation of cyclohex-
ane.23,33 Cyclohexadienyl complexes 1 and 2 were there-
fore examined for benzene hydrogenation activity.
That 1 and 2 show no hydrogenation activity may not
be surprising. As discussed above, they are coordina-
tively and electronically saturated species. The mecha-
nistic details of benzene hydrogenation by these sys-
tems, although likely related to those of the niobium
aryloxide systems, suggest that the formation of a
niobium η5-cyclohexadienyl complex is a catalytic dead-
end. Although it is highly likely that the catalytically
active species is a niobium hydride complex, we have
as yet been unable to isolate such a product. Performing
the reactions in nonaromatic solvents such as THF or
hexanes even in the presence of stabilizing phosphine
donors yields intractable products. A remarkably stable
dimeric Ta(IV)-hydride (Ph[P2N2]Ta)2(µ-H)4 has recently
been synthesized in our group.35 Exposure of this
dinuclear tantalum hydride to ethylene, carbon mon-
oxide, and even oxygen results in no reaction. A related
mononuclear tantalum(V) trihydride, Ph[P2N2]TaH3-
(PMe3),36 has also been prepared, but again this complex
does not react with arenes. A large number of transition
metal hydrides are stable in aromatic solvents. If
reactivity does occur, C-H bond activation is commonly
involved either by oxidative addition pathways or by
σ-bond metathesis with highly electrophilic d0 metal
hydrides.3
The catalytic hydrogenation of aromatic substrates by
mixed hydrido aryloxide derivatives of niobium studied
by Rothwell and co-workers requires high-pressure
hydrogenolysis of mixed alkyl/aryloxides, typically in the
presence of phosphine donors, to generate d0 niobium
hydrides. A drawback of these precursors is that hy-
drogenolysis of certain alkyl groups is slow. The result-
ing hydride complexes were found to effectively hydro-
genate benzene to cyclohexane at 1200 psi H2 and 100
°C. A suggested mechanistic pathway involves the
formation of a cyclohexadienylmetal hydride intermedi-
ate. We therefore decided to explore whether complexes
1-4 would display catalytic arene hydrogenation.
Solutions of the niobium alkyl complexes R[P2N2]-
NbCH2SiMe3 (R ) Ph, Cy) (0.025 mmol) in benzene (4
g, 51.3 mmol) were exposed to 500 psi of H2 at room
temperature for 20 h. Approximately 5 mmol of cyclo-
hexane was produced as shown by NMR (calculated
from the integration ratio of the benzene to cyclohexane
resonances) and GC-MS analysis yielding a turnover
number (TON ) mol product per mol catalyst) of 225.
No partially hydrogenated substrate (cyclohexadiene or
cyclohexene) was apparent. Examination of the solid
residues after removal of all solvents showed the cyclo-
hexadienyl complexes 1 and 2 had formed for their
respective cyclohexyl- and phenyl phosphine-substituted
niobium alkyl precursors. Hydrogenation of toluene to
methylcyclohexane can also be performed, but the
percent conversion is much poorer than for benzene,
yielding only 2% C7H14 after 20 h.
Benzene and pyridine have been shown to undergo
η2-coordination to coordinatively unsaturated Ta(III) d2
centers.20-22,37 Concentrated benzene solutions of (tBu3-
SiO)3Ta spontaneously form a bridging benzene adduct
[(tBu3SiO)3Ta]2(µ-η2:η2-C6H6), which decomposes via cy-
clometalation to generate a Ta(V) hydride species.37 This
suggests the possibility of a pathway whereby an η2-
arene adduct forms with a Nb(III) hydride. Homoge-
neous arene hydrogenation catalysts are commonly
believed to begin their catalytic cycle with an η6
π-bonded arene species (it should be noted that this
paradigm has been challenged by examples where such
coordination is unnecessary in catalytic hydrogena-
tion).10 What becomes apparent from our observations
is that an η6-arene species must be avoided in order for
hydrogenation to proceed in the niobium [P2N2] system.
It seems that an η2- or at most an η4-coordination of
the arene may be required. It has been shown that
cyclohexa-1,3-diene strongly binds to Nb(III) aryloxide
centers, and the resulting robust η4-cyclohexadiene
complex reacts with hydrogen to produce cyclohexane.38
Although an η4-benzene adduct is possible, it is likely
For Ph[P2N2]NbCH2SiMe3, no hydrogenation of the
phosphorus phenyl group was observed. Examination
of the catalyst residue by NMR spectroscopy shows the
product to be purely 2; no 1 was detected. This is likely
due to the inability of the phenyl substituent of [P2N2]
to undergo intramolecular interaction with the metal

Cyclohexadienyl Niobium Complexes
Organometallics, Vol. 21, No. 23, 2002 5053
assignments of the cyclohexadienyl ligands are made using
the following convention:
that migratory insertion into such a moiety would also
lead to the η5-cyclohexadienyl ligand and terminate the
catalytic cycle.
Con clu sion
Attempts at isolating niobium hydride derivatives by
hydrogenolysis of the alkyl complexes R[P2N2]NbCH2-
SiMe3 (R ) Cy or Ph) result in the activation of aromatic
solvent molecules. The addition of 1- 4 atm H2 to
benzene or toluene solutions of R[P2N2]NbCH2SiMe3
leads to hydride addition to the arene moiety, forming
the cyclohexadienyl or methylcyclohexadienyl complexes
R[P 2N2]Nb(η5-C6H7) (R ) Cy, 1; R ) P h , 2). Method A: A
benzene solution (60 mL) of R[P2N2]NbCH2SiMe3 (500 mg,
0.690 mmol, R ) Cy; 0.702 mmol, R ) Ph) was loaded into a
thick walled glass reactor fitted with a Teflon Kontes valve
and degassed by three freeze-pump-thaw cycles. Hydrogen
gas was added to the frozen solution at -196 °C, and the sealed
reactor was allowed to warm to room temperature. The
resulting olive-green colored solution was stirred for a further
4 h. All volatiles were removed in vacuo, leaving an olive green
solid. Extraction of the solid with minimal hexanes and slow
cooling to -40 °C allows isolation of diamagnetic R[P2N2]Nb-
(η5-C6H7) as green crystals. For 1: Yield: 400 mg (80%). For
2: Yield: 410 mg (83%).
R
R[P2N2]Nb(η5-C6H7) (R ) Cy, 1; R ) Ph, 2) and [P2N2]-
Nb(η5-C7H9) (R ) Cy, 2; R ) Ph, 4) in benzene and
toluene, respectively. Although niobium hydride species
could not be isolated, formation of complexes 1-4 likely
proceeds via migratory insertion of a hydride onto the
arene fragment. While the niobium alkyl complexes act
as precursors for the catalytic hydrogenation of benzene
under higher pressures of H2, their activity is short-
lived. Formation of the stable 16 e- cyclohexadienyl
complexes (1 and 2) terminates the catalytic cycle.
Regardless of the shortcomings of niobium(III) alkyl
complexes as good arene hydrogenation catalysts, the
formation of niobium cyclohexadienyl complexes from
partial hydrogenation of benzene and toluene is signifi-
cant to understanding related group 5 transition metal
arene hydrogenation catalysts.13-17 Although such metal-
bonded η5-C6H7 species are considered to be possible
intermediates in the catalytic hydrogenation of benzene,
complexes 1-4 are catalytically inactive. However, the
Method B: R[P2N2]NbCl (500 mg, 0.758 mmol) and KC8 (113
mg, 0.830 mmol) were loaded into a thick walled glass reactor
flask fitted with a Teflon Kontes valve and degassed by three
freeze-pump-thaw cycles. Hydrogen gas was added to the
frozen solution at -196 °C, and the sealed reactor was allowed
to warm to room temperature. The flask was gently stirred
for 24 h, during which the solution changed from green to dark
brown. The contents of the flask were then filtered through
Celite and solvents removed in vacuo, leaving a dark green-
brown solid. Extraction of the solid with minimal hexanes and
slow cooling to -40 °C allows isolation of diamagnetic R[P2N2]-
Nb(η5-C6H7) as green crystals. For 1: Yield: 350 mg (70%).
For 2: Yield: 375 mg (76%).
R
reduction of benzene to cyclohexane by [P2N2]NbCH2-
SiMe3 in 30 atm of H2 suggests that alternate interme-
diates exist for this system.
For 1: 1H NMR (C6D6, 400.13 MHz, 300 K): δ -0.08, 0.0,
0.40, 0.52 (s, 6H each, SiCH3), 0.80 to 1.90 (m’s, overlapping
cyclohexyl-CH2’s and macrocycle PCHH), 2.50 (m, 2H, arene-
H(c)) 4.21 (m, 2H, arene-H(b)), 5.70 (m, 1H, arene-H(a)). Arene-
H(e) and -H(d) obscured by methylene region. 31P{1H} (C6D6,
202.46 MHz, 300 K): δ 32.36 (s, w1/2 ) 330 Hz). MS (EI) m/z
(%): 716, (80) [M]+. Anal. Calcd for C30H61N2NbP2Si4: C, 50.25;
H, 8.58; N, 3.91. Found: C, 50.65; H, 7.74; N, 3.73. For 2: 1H
NMR (C6D6, 400.13 MHz, 300 K): δ 0.03, 0.21, 0.47, 0.55 (s,
6H each, SiCH3), 0.91 (ABX m, 2H, PCHH), 1.36 (ABX m, 4H,
PCHH), 1.36 (obscured, 1H, arene-CHH(d)) 1.51 (m, 1H, arene-
CHH(e)), 1.90 (ABX m, 2H, PCHH), 2.51 (m, 2H, arene-H(c))
4.29 (m, 2H, arene-H(b)), 5.48 (m, 1H, arene-H(a)), 7.19 (t, 2H,
p-H phenyl), 7.45 (dd, 4H, o-H phenyl), 7.74 (dd, 4H, m-H
phenyl). 31P{1H} (C6D6, 202.46 MHz, 300 K): δ 30.08 and 26.52
(w1/2 ) 330 Hz). MS (EI) m/z (%): 704, (80) [M]+. Anal. Calcd
for C30H49N2NbP2Si4: C, 51.12; H, 7.01; N, 3.97. Found: C,
51.48; H, 7.34; N, 4.23.
Exp er im en ta l Section
Unless otherwise stated, all manipulations were performed
under an atmosphere of dry oxygen-free nitrogen or argon by
means of standard Schlenk or glovebox techniques (Vacuum
Atmospheres HE-553-2 glovebox equipped with a MO-40-2H
purification system and a -40 °C freezer). Hexanes and
toluene were purchased anhydrous from Aldrich and further
dried by passage through a tower of alumina and degassed by
passage through
a tower of Q-5 catalyst under positive
pressure of nitrogen.39 Anhydrous THF was stored over sieves
and distilled from sodium benzophenone ketyl under argon.
Nitrogen and argon were dried and deoxygenated by passing
the gases through a column containing molecular sieves and
MnO. Deuterated benzene and toluene were dried by refluxing
over sodium and potassium alloy, and THF-d8 was dried over
sodium, in a sealed vessel under partial pressure, and then
trap-to-trap distilled. They were degassed under three freeze-
pump-thaw cycles. Unless otherwise stated, all NMR spectra
were recorded on a Bruker AMX-500 instrument operating at
500.132 MHz. 1H NMR spectra were referenced to residual
protons in the deuterated solvent. 31P NMR spectra were
referenced to external P(OMe)3 at 141 ppm relative to 85%
H3PO4 in D2O at 0 ppm. Elemental analyses were performed
by Mr. P. Borda of this department. Mass spectrometry was
performed on a Kratos MS 50 by Mr. M. Lapawa, also of this
department. The compounds R[P2N2]NbCl (R ) Ph, Cy),
R[P2N2]NbCH2SiMe3 (R ) Ph, Cy),18 and KC840 were prepared
according to literature procedures. KC840 was prepared ac-
cording to literature procedures. Hydrogen gas was used as
received from Praxair without further purification. NMR
R[P 2N2]Nb(η5-C7H9) (R ) Cy, 3; R ) P h , 4). Using toluene
in lieu of benzene in both of the above methods allows isolation
of R[P2N2]Nb(η5-C7H9) as green crystals grown from slow
evaporation of a saturated hexanes solution. For 3: Yield: 390
mg (78%). 1H NMR (C6D6, 400.13 MHz, 300 K): δ -0.06, 0.05,
3
0.42, 0.55 (s, 6H each, SiCH3), 0.64 (d, J HH ) 6.09 Hz, 3H,
arene-CH3(e)), 0.80 to 1.90 (m’s, overlapping cyclohexyl-CH2’s
and macrocycle PCHH), 2.82 (m, 2H, arene-H(c)) 4.04 (m, 2H,
arene-H(b)), 5.72 (m, 1H, arene-H(a)). H(d) obscured by
methylene region. 31P{1H} (C6D6, 161.98 MHz, 300 K): δ 32.82
(s, w1/2 ) 300 Hz) MS (EI) m/z, (%) 730, (80) [M]+. Anal. Calcd
for C31H63N2NbP2Si4: C C, 50.93; H, 8.69; N, 3.83. Found: C,
1
50.86; H, 8.74; N, 3.93. For 4: Yield: 380 mg (75%). H NMR
(C6D6, 400.13 MHz, 300 K): δ 0.08, 0.25, 0.51, 0.59 (s, 6H each,
SiCH3), 0.64 (d, 3J HH ) 6.09 Hz, 3H, arene-CH3(e)), 0.94 (ABX
m, 2H, PCHH), 1.37 (ABX m, 4H, PCHH), 1.94 (ABX m, 2H,

5054 Organometallics, Vol. 21, No. 23, 2002
Fryzuk et al.
PCHH), 2.10 (m, 1H, arene-H(d)), 2.85 (m, 2H, arene-H(c)), 4.21
(m, 2H, arene-H(b)), 5.51 (m, 1H, arene-H(a)), 7.41 (t, 2H, p-H
phenyl), 7.47 (dd, 4H, o-H phenyl), 7.65 (dd, 4H, m-H phenyl).
31P{1H} (C6D6, 161.98 MHz, 300 K): δ 29.32 (s, w1/2 ) 420 Hz),
26.36 (s, w1/2 ) 420 Hz). MS (EI) m/z (%): 718, (80) [M]+. Anal.
Calcd for C31H51N2NbP2Si4: C, 51.79; H, 7.15; N, 3.90. Found:
C, 52.09; H, 7.44; N, 4.23.
direct methods44 and expanded using Fourier techniques.45
Complex 4 contains one-half molecule of hexane in the asym-
metric unit. All non-hydrogen atoms were refined anisotropi-
cally. Hydrogen atoms on cyclohexadienyl and methylcyclo-
hexadienyl fragments were refined isotropically. Other hydrogen
atoms were fixed in calculated positions with C-H ) 0.98 Å.
H igh -P r essu r e H yd r ogen a t ion . R[P2N2]NbCH2SiMe3
(0.0253 mmol) was dissolved in benzene (4.0 g, 51.3 mmol) (ca.
2030 mol benzene per mol Nb) and loaded into a glass vessel
within a Parr Minireactor. The reactor was purged three times
by pressurization to 500 psi H2 and venting before final
repressurization to 500 psi H2. The solution was stirred for
20 h before depressurization. All volatile products were
vacuum transferred before analysis by NMR spectroscopy and
Ack n ow led gm en t. We thank the NSERC of Canada
for funding in the form of research grants.
Su p p or tin g In for m a tion Ava ila ble: Complete crystal-
lographic data including ORTEP diagrams, bond lengths and
angles, final atomic coordinates and equivalent isotropic
thermal parameters, anisotropic thermal parameters, inter-
molecular contacts, and least squares planes are available for
complexes 1, 3, and 4. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
GC-MS. H NMR (neat, 300.13 MHz, 300 K): δ 3.18 (s, 12H,
C6H12), 7.15 (s, 6H, C6H6). Integral ratio ) 1 C6H12:8 C6H6
corresponds to 5.7 mmol yield of cyclohexane. TON ) 225.
Nonvolatile residues were characterized as 1 and 2.
OM020395Q
X-r a y Cr ysta llogr a p h ic An a lyses of 1, 3, a n d 4. In all
cases, suitable crystals were selected and mounted on a glass
fiber using Paratone-N oil and freezing to -100 °C. All
measurements were made on a Rigaku/ADSC CCD area
detector with graphite-monochromated Mo KR radiation.
Crystallographic data appear in Table 1. In each case the data
were processed41 and corrected for Lorentz and polarization
effects and absorption. Neutral atom scattering factors for all
non-hydrogen atoms were taken from the International Tables
for X-ray Crystallography.42,43 All structures were solved by
(42) International Tables for X-Ray Crystallography; Kluwer Aca-
demic: Boston, MA, 1992; Vol. C, pp 200-206.
(43) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, U.K. (present distributer Kluwer Academic:
Boston, MA), 1974; Vol. IV, pp 99-102.
(44) Altomare, A.; Burla, M. C.; Cammali, G.; Cascarano, M.;
Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
A. SIR97: a new tool for crystal structure determination and refine-
ment. J . Appl. Crystallogr. 1999, 32, 115-119.
(45) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;
de Gelder, R.; Israel, R.; Smits, J . M. M. DIRDIF94; The DIRDIF-94
program system; Technical Report of the Crystallography Laboratory;
University of Nijmegen: The Netherlands, 1994.
(41) teXsan Crystal Structure Analysis Package; Molecular Structure
Corp.: The Woodlands, TX, 1995.