Intramolecular dehydrogenation of alkyl groups at niobium aryloxide centers: Bonding and reactivity of the ensuing niobacyclopropane ring

Article abstract of DOI:10.1021/om9604839

The niobium dichloride [Nb(OC₆H₃Prⁱ₂-2,6) ₃Cl₂] (OC₆H₃Prⁱ₂-2,6= 2,6-diisopropylphenoxide) undergoes reduction (2 Na per Nb)) in tetrahydrofuran (thf) solvent to produce the dark green complex [Nb(OC₆H₃Prⁱ-η²-CMe=CH ₂)(OC₆H₃Prⁱ₂-2,6) ₂(thf)] (1). The solid-state structure of 1 shows the α-methylvinyl group to be strongly η²-bound to the niobium metal center. The dehydrogenation of the ortho-isopropyl group of an aryloxide group to generate 1 is argued to proceed via CH bond activation within a d²-Nb(III) aryloxide followed by β-hydrogen abstraction/elimination of H₂. The thf ligand in 1 can be substituted by pyridine (py) and Bu^tNC to yield the adducts [Nb(OC₆H₃Prⁱ-η²-CMe=CH ₂)(OC₆H₃Prⁱ₂-2,6) ₂(L)₂] (L = py, 2; L = Bu^tNC, 3). The solid-state structure of 3 shows both isocyanide ligands to be terminally bound with no interaction with the η²-olefin fragment. The spectroscopic properties of these three adducts give insight into the degree of π-back-bonding between the metal and the chelated olefin fragment. The metallacyclopropane ring in 1 will undergo ring expansion (coupling of the olefin) with a variety of unsaturated substrates. The addition of Ph₂C=O leads to a 2-oxaniobacyclopentane ring in 4 while PhC≡CPh produces a niobacyclopent-3-ene derivative, [Nb(OC₆H3Prⁱ-CMeCH₂CPh=CPh)(OC ₆H₃Prⁱ₂-2,6)₂] (5), which was structurally characterized. In the case of addition of PhC≡CH to 1 the initial, kinetic product 6a, possibly containing a 2-phenylniobacyclopent-2-ene ring, is thermally isomerized to the 3-phenylniobacyclopent-2-ene compound 6b. With the protic reagents HOC₆H₃Prⁱ₂-2,6 and PhNH₂ the compounds [Nb(OC₆H₃PrⁱCMe₂)(OC ₆H₃Prⁱ₂-2,6)₃] (8) and [Nb(OC₆H₃PrⁱCMe₂)(OC ₆H₃-Prⁱ₂-2,6)₂(NHPh)] (9) are produced. The solid-state structure of 8 confirmed the presence of the cyclometalated aryloxide formed via protonation of the methylene group in 1.

Full text of DOI:10.1021/om9604839

Organometallics 1996, 15, 4443-4449
4443
In tr a m olecu la r Deh yd r ogen a tion of Alk yl Gr ou p s a t
Niobiu m Ar yloxid e Cen ter s: Bon d in g a n d Rea ctivity of
th e En su in g Nioba cyclop r op a n e Rin g
J oyce S. Yu, Laura Felter, Mark C. Potyen, J anet R. Clark, Valerie M. Visciglio,
Phillip E. Fanwick, and Ian P. Rothwell*
Department of Chemistry, 1393 Brown Building, Purdue University,
West Lafayette, Indiana 47907-1393
Received J une 17, 1996X
The niobium dichloride [Nb(OC6H3Pri2-2,6)3Cl2] (OC6H3Pri2-2,6 ) 2,6-diisopropylphenoxide)
undergoes reduction (2 Na per Nb)) in tetrahydrofuran (thf) solvent to produce the dark
green complex [Nb(OC6H3Pri-η2-CMedCH2)(OC6H3Pri -2,6)2(thf)] (1). The solid-state struc-
2
ture of 1 shows the R-methylvinyl group to be strongly η2-bound to the niobium metal center.
The dehydrogenation of the ortho-isopropyl group of an aryloxide group to generate 1 is
argued to proceed via CH bond activation within a d2-Nb(III) aryloxide followed by â-hydrogen
abstraction/elimination of H2. The thf ligand in 1 can be substituted by pyridine (py) and
ButNC to yield the adducts [Nb(OC6H3Pri-η2-CMedCH2)(OC6H3Pri -2,6)2(L)2] (L ) py, 2; L
2
) ButNC, 3). The solid-state structure of 3 shows both isocyanide ligands to be terminally
bound with no interaction with the η2-olefin fragment. The spectroscopic properties of these
three adducts give insight into the degree of π-back-bonding between the metal and the
chelated olefin fragment. The metallacyclopropane ring in 1 will undergo ring expansion
(coupling of the olefin) with a variety of unsaturated substrates. The addition of Ph2CdO
leads to a 2-oxaniobacyclopentane ring in 4 while PhCtCPh produces a niobacyclopent-3-
ene derivative, [Nb(OC6H3Pri-CMeCH2CPhdCPh)(OC6H3Pri -2,6)2] (5), which was structur-
2
ally characterized. In the case of addition of PhCtCH to 1 the initial, kinetic product 6a ,
possibly containing a 2-phenylniobacyclopent-2-ene ring, is thermally isomerized to the
3-phenylniobacyclopent-2-ene compound 6b. With the protic reagents HOC6H3Pri -2,6 and
2
PhNH2 the compounds [Nb(OC6H3PriCMe2)(OC6H3Pri2-2,6)3] (8) and [Nb(OC6H3PriCMe2)(OC6H3-
Pri -2,6)2(NHPh)] (9) are produced. The solid-state structure of 8 confirmed the presence of
2
the cyclometalated aryloxide formed via protonation of the methylene group in 1.
In tr od u ction
Resu lts a n d Discu ssion
Syn th esis a n d Sp ectr oscop ic P r op er ties. The
niobium and tantalum chloride compounds [M(OC6H3-
Pri -2,6)2Cl3]2 and [M(OC6H3Pri -2,6)3Cl2] (M ) Nb, Ta;
The early transition metal chemistry supported by
sterically demanding aryloxide ligation continues to
grow.1 One continuing complexity in the development
of the organometallic chemistry of these ligands is the
sometimes facile intramolecular activation (cyclo-
metalation) of the CH bonds attached to the aryloxide
ligand.1,2 We wish to report here on the facile dehy-
drogenation of the ortho-substituent of a 2,6-diiso-
propylphenoxide, a group that has typically been con-
sidered as less susceptible to intramolecular CH bond
activation. Despite their somewhat exotic nature, the
resulting species do allow valuable insight into the
bonding and reactivity of olefin ligands coordinated to
this transition metal center. Some aspects of this work
have been communicated previously.3
2
2
OC6H3Pri -2,6 ) 2,6-diisopropylphenoxide)4 are useful
2
starting materials for studying the organometallic
chemistry of these metals. With neutral donor ligands
such as pyridine,5 quinoline,5 PMe2Ph,4 tetrahydro-
furan,6 and diethyl ether7 adducts of the type cis-mer-
[M(OAr)2Cl3(L)] and trans-mer-[M(OAr)3Cl2(L)] are
formed and the structures of a number of these adducts
have been obtained and analyzed. The trichloride
compounds undergo facile reduction leading to a series
of d1-derivatives, all-trans-[M(OC6H3Pri2-2,6)2Cl2(L)2].5c,8
(4) Clark, J . R.; Pulverenti, A. L.; Fanwick, P. E.; Eisenstein, O.;
Rothwell, I. P. Results to be published.
(5) (a) Gray, S. D.; Smith, D. P.; Bruck, M. A.; Wigley, D. E. J . Am.
Chem. Soc. 1992, 114, 5462. (b) Gray, S. D.; Fox, P. A.; Kingsborough,
R. P.; Bruck, M. A.; Wigley, D. E. ACS Prepr. Div. Petrol. Chem. 1993,
39, 706. (c) 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.
(6) (a) Kanehisa, N.; Kai, N.; Yasuda, H.; Nakayama, Y.; Takei, K.;
Nakamura, A. Chem. Lett. 1990, 2167. (b) Yasuda, H.; Nakayama, Y.;
Takei, K.; Nakamura, A.; Kai, N.; Kanehisa, N. J . Organomet. Chem.
1994, 473, 105.
(7) (a) Bruck, M. A.; Copenhaver, A. S.; Wigley, D. E. J . Am. Chem.
Soc. 1987, 109, 6525. (b) Strickler, J . R.; Bruck, M. A.; Wexler, P. A.;
Wigley, D. E. Organometallics 1990, 9, 266. (c) Arney, D. J .; Wexler,
P. A.; Wigley, D. E. Organometallics 1990, 9, 1282.
X Abstract published in Advance ACS Abstracts, September 15, 1996.
(1) (a) Chisholm, M. H.; Rothwell, I. P. Comprehensive Coordination
Chemistry Wilkinson, G., Gillard, R., McCleverty, J . A., Eds; Pergamon
Press: Oxford, U.K., 1987; Vol. 2, Chapter 15.3. (b) Alkoxides and
Related O-Donor Ligands in Organometallic Chemistry; Chisholm, M.
H., Rothwell, I. P., Eds. Polyhedron 1995, 14, 3175-3366.
(2) (a) Rothwell, I. P. Acc. Chem. Res. 1988, 21, 153. (b) Rabinovich,
D.; Zelman, R.; Parkin G. J . Am. Chem. Soc. 1990, 112, 9632. (c)
Rabinovich, D.; Zelman, R.; Parkin G. J . Am. Chem. Soc. 1992, 114,
4611.
(3) Yu, J . S.; Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. Soc. 1990,
112, 8171.
(8) Wigley, D. E.; Rothwell, I. P. Results to be published.
S0276-7333(96)00483-9 CCC: $12.00 © 1996 American Chemical Society

4444 Organometallics, Vol. 15, No. 21, 1996
Yu et al.
Sch em e 1
Sch em e 2
Sch em e 3
In the case of tantalum more forcing conditions have
been shown by Wigley et al. to give access to derivatives
such as [{η2-(N,C)-quin}Ta(OC6H3Pri -2,6)3] and [{η2-
2
(N,C)-6-m et h ylqu in olin e}Ta (OC6H 3P r i2-2,6)2Cl(O-
Et2)].5 We have observed that the niobium dichloride
[Nb(OC6H3Pri -2,6)3Cl2] is reduced in tetrahydrofuran
2
solvent by sodium amalgam (2 Na per Nb) to produce
intense green solutions of the complex [Nb(OC6H3Pri-
η2-CMedCH2)(OC6H3Pri2-2,6)2(thf)] (1) (Scheme 1). Com-
pound 1 can be isolated in good yield by simple filtration
followed by vacuum removal of thf and addition of
hexane solvent. It was noted, however, that the yields
of 1 were reduced if the reduction was carried out for
periods exceeding 12 h prior to workup. Following
isolation, the reactivity of 1 was examined in hydro-
carbon solvents, typically benzene. The addition of
pyridine or tert-butyl isocyanide to solutions of 1 rapidly
produced deep red and orange solutions of the bis-ligand
complexes [Nb(OC6H3Pri-η2-CMedCH2)(OC6H3Pri2-2,6)2-
(L)2] (2, L ) py; 3, L ) CNBut) along with free thf
(Scheme 1). It was found possible to generate 2 by
addition of an aryloxide CH bond followed by a second
cyclometallation (σ-bond metathesis) leading to elimina-
tion of H2. This proposal has support in the work of
Wolczanski et al. showing the conversion of the tanta-
lum siloxide [Ta(OSiBut )3] into the compound
3
[Ta(OSiBut CMe2CH2)(H)(OSiBut ) ].11 There are, how-
2
3 2
ever, two possible pathways for the dehydrogenation of
the diisopropylphenoxide ligand leading to compounds
such as 1. Both pathways proceed via intermediate
monohydride compounds which undergo â-hydrogen
elimination (Scheme 4). The uncertainty lies in whether
the initial cyclometallation occurs via activation of the
methyl or methine CH bonds leading to six- and five-
membered metallacycles, respectively. The formation
of the six-membered metallacycle is favored statistically,
although protonation of 1 leads to compounds containing
the five-membered ring (vide infra). The literature
contains many examples of structurally characterized
4-,12 5-,13 and 6-membered10,14 metallacycles derived
from aryloxide ligation.
direct reduction of [Nb(OC6H3Pri -2,6)3Cl2] in the pres-
2
ence of pyridine in diethyl ether solvent. Another
example of the dehydrogenation of a 2,6-diisopropyl-
phenoxide ligand involves the reaction of styrene with
a tantalum dihydride substrate yielding [Ta(OC6H3Pri-
η2-CMedCH2)(OC6H3Pri -2,6)(Cl)(PMe2Ph)2] (Scheme
2
2).9
Previous work has shown that reduction of the
chloro-aryloxides [Ta(OC6H3But -2,6)2Cl3] and [M(OC6-
2
H3Ph2-2,6)3Cl2] (M ) Nb, Ta) with 2 equiv of sodium
leads to bis-cyclometalated derivatives (Scheme 3).10
These reactions are believed to proceed via Nb(III) and
Ta(III) intermediates which undergo intramolecular
(11) LaPointe, R. E.; Wolczanski, P. T.; Mitchell, J . F. J . Am. Chem.
Soc. 1986, 108, 6382.
(12) (a) Bag, N.; Choudhury, S. B.; Lahiri, G. K.; Chakravorty A. J .
Chem. Soc., Chem. Commun. 1990, 1626. (b) Bag, N.; Choudhury, S.
B.; Pramanik, A.; Lahiri, G. K.; Chakravorty A. Inorg. Chem. 1990,
29, 5013.
(9) Clark, J . R.; Fanwick, P. E.; Rothwell, I. P. J . Chem. Soc., Chem.
Commun. 1995, 553.
(10) Steffey, B. D.; Chamberlain, L. R.; Chesnut, R. W.; Chebi, D.
E.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1989, 8, 1419.
(13) (a) Gutierrez, A.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse,
M. B. Polyhedron 1990, 9, 2081. (b) Chisholm, M. H.; Parkin, I. P.;
Folting, K.; Lubkovsky, E. B.; Streib, W. E. J . Chem. Soc., Chem.
Comm. 1991, 1673.

Dehydrogenation at Niobium Aryloxide Centers
Organometallics, Vol. 15, No. 21, 1996 4445
Sch em e 4
the vinyl group as judged by these two parameters. This
effect becomes dramatically apparent in complex 3
(Table 3) which contains two strongly π-accepting iso-
cyanide ligands which can compete with the chelated
π-vinyl group for the electron density on the metal
center. The structural changes associated with this
bonding argument are discussed in a separate section
later in this paper.
In common with many related early transition metal
olefin complexes the niobacyclopropane ring in 1 un-
dergoes ring expansion (coupling) with a variety of
unsaturated substrates. Addition of benzophenone to
benzene solutions of 1 led to the rapid (as judged by
the color change) formation of a bright orange solution
of the 2-oxaniobacyclopentane complex [Nb(OC6H3Pri-
CMeCH2CPh2O)(OAr)2] (4) along with free thf (Scheme
5). Besides the complex signals due to the aryloxide
ligands, the 1H NMR spectrum of 4 shows an AB pattern
(14 Hz coupling) for the methylene protons within the
five-membered metallacycle ring. In the 13C NMR
spectrum of 4 a signal at δ 96.8 ppm can be assigned to
the Nb-CMeCH2 carbon resonance while a peak at δ
99.6 ppm is due to the Nb-OCPh2 carbon. This latter
assignment was confirmed by synthesis of [Nb(OC6H3-
Pri-CMeCH213CPh2O)(OAr)2] (*4) utilizing labeled
Ph213CdO as substrate. Interestingly only one of the
diastereotopic methylene protons was found to be
coupled (5 Hz) with the labeled carbon atom.16
The solution spectroscopic properties of compounds
1-3 are of interest. Due to the lack of any symmetry
elements in these molecules the aliphatic regions of the
1H and 13C NMR spectra are complex. The presence of
diastereotopic CHMe2 groups from the aryloxide ligands
leads to six overlapping methyl doublets (ratio 2:2:2:2:
Upon addition of PhCtCPh to dark green benzene
solutions of 1, formation of orange solutions of the
niobacyclopent-2-ene compound 5 occurs over minutes
(Scheme 6). The metal-mediated coupling of olefins and
alkynes at transition metal centers has excellent pre-
cedence and has its greatest synthetic utility in the ene-
yne cyclization reaction.17 The spectroscopic character-
istics of 5 are entirely consistent with its formulation
and observed solid-state molecular structure (vide infra).
In the 13C NMR spectrum the Nb-CMe and Nb-
CPhdCPh carbon atoms are found to resonate at δ 96.4
and 200.5 ppm, respectively. Two doublets at δ 3.91
1
1:1) in the H NMR spectrum along with three methine
septets in the ratio of 2:2:1. The metal bound R-methyl-
vinyl group appears as a singlet (CMedCH2) and a well-
1
resolved pair of doublets (CMedCH2) in the H NMR
while the CMedCH2 carbons appear as a broad singlet
and triplet in the proton-coupled 13C NMR spectra. As
with nearly all organometallic derivatives of niobium,
the quadrupolar broadening of Nb-C resonances neces-
sitates much longer acquisition times in order to detect
these signals. Of particular interest are the values of
the coupling constants 2J (1H-1H) and 1J (13C-1H) for
the methylene group CMedCH2. These data are col-
lected in Table 1.
1
and 4.15 ppm in the H NMR spectrum can be assigned
to the CMeCH2CPhdCPh protons within the metalla-
cycle ring.
It can be seen that on moving from 1 to 3 the coupling
between the methylene protons drops while the coupling
between these protons and the carbon atom increases.
We argue that these parameters correlate with the
strength of the bonding between the chelated vinyl
group and the niobium metal center. In compound 1
there is significant π-back-bonding from the metal
center (metallacyclopropane in the Dewar-Chatt-Dun-
The reaction of 1 with PhCtCH in hydrocarbon
solvents proved to be more complicated (Scheme 6).
Monitoring the reaction in C6D6 solvent by 1H NMR
showed the initial rapid formation of a new organo-
1
metallic product 6a along with free thf. The H NMR
spectrum of 6a showed a pattern for the aryloxide
ligands very similar to that observed for 5, and the
CMeCH2 methyl group can be seen as a sharp singlet
at δ 1.88 ppm. A broad singlet at δ 3.36 ppm appears
to be due to the CMeCH2 protons. Slowly over hours
at room temperature and over minutes at 100 °C, 6a is
converted completely into a new compound 6b whose
NMR spectra are entirely consistent with its formula-
tion as the 3-phenylniobacyclopent-2-ene compound
2
canson model) resulting in a large J (1H-1H) coupling
constant approaching that typical of diastereotopic sp3-
1
methylene protons (12-15 Hz) and a reduced J (13C-
1H) coupling constant compared to that found in free
olefins (Table 1). Replacement of the thf (a straight-
forward σ-donor) by two pyridine ligands (potential
π-acceptors)15 can be seen to decrease the degree of
π-back-bonding between the niobium metal center and
[Nb(OC6H3Pri -2,6)2(OC6H3Pri-CMeCH2CPhdCH)]. A
2
1
particularly significant peak in the H NMR spectrum
is a sharp singlet at δ 9.18 ppm assigned to the R-CH
proton of the metallacycle ring. One possible formula-
tion for intermediate 6a is as a 2-phenylniobacyclopent-
2-ene compound [Nb(OC6H3Pri2-2,6)2(OC6H3Pri-CMeCH2-
(14) (a) Chamberlain, L. R.; Keddington, J .; Rothwell, I. P.; Huffman,
J . C. Organometallics 1982, 1, 1538. (b) Chamberlain, L. R.; Kerschner,
J . L.; Rothwell, A. P.; Rothwell, I. P.; Huffman, J . C. J . Am. Chem.
Soc. 1987, 109, 6471. (c) Latesky, S. L.; McMullen, A. K.; Rothwell, I.
P.; Huffman, J . C. J . Am. Chem. Soc. 1985, 107, 5981. (d) Baley, A.-
S.; Chauvin, Y.; Commereuc, D.; Hitchcock, P. B. New J . Chem. 1991,
15, 609. (e) Koo, K.; Hillhouse, G. L.; Rheingold, A. L. Organometallics
1995, 14, 456.
(16) Breitmaier, E.; Voelter, W. Carbon-13 NMR Spectroscopy;
VCH: New York, 1990.
(15) Kriley, C. E.; Fanwick, P. E.; Rothwell, I. P. J . Am. Chem. Soc.
1994, 116, 5225.
(17) Negishi, E. Comprehensive Organic Synthesis; Trost, B. M.,
Fleming, I. Eds.; Pergamon: Oxford, U.K., 1991; Vol. 5, Chapter 9.5.

4446 Organometallics, Vol. 15, No. 21, 1996
Ta ble 1. Cou p lin g Con sta n ts Obta in ed for th e CMedCH2 Gr ou p in 1-3
Yu et al.
compd
2J (1H-1H)/Hz
1J (13C-1H)/Hz
[Nb(OC6H3Pri-η2-CMedCH2)(OAr)2(thf)] (1)
[Nb(OC6H3Pri-η2-CMedCH2)(OAr)2(py)2] (2)
[Nb(OC6H3Pri-η2-CMedCH2)(OAr)2(CNBut)2] (3)
HOC6H5-CH2CHdCH2-2
9.8
5.5
4.2
1.4
140.8
146.8
153.4
156.0
Sch em e 5
Sch em e 7
Sch em e 6
Upon addition of 2,6-diisopropylphenol or aniline to
solutions of 1 the initial green color faded over several
hours to produce dark orange solutions of the corre-
sponding aryloxide and phenylamido derivatives 8 and
9 (Scheme 7). Both of these molecules contain a five-
membered metallacycle formed by protonation of the η2-
CMedCH2 methylene carbon atom of the metallacyclo-
propane ring within 1. In the 13C NMR spectrum the
remaining Nb-CMe2 carbon resonates at δ 90.2 and
92.4 ppm for 8 and 9, respectively.
Solid -Sta te Str u ctu r es. The molecular structures
of niobacyclopropane compounds 1 and 3, niobacyclo-
pent-2-ene compound 5, and the cyclometalation deriva-
tive 8 are shown in Figures 1-4, and selected structural
parameters are listed in Tables 2-5. The geometry
about the metal center in 1 is best described as trigonal
bipyramidal with the η2-vinyl group and the thf occupy-
ing axial sites. The three aryloxide oxygen atoms form
the equatorial plane. In pseudo-octahedral 3 the two
isocyanide ligands are mutually trans and the three
aryloxide oxygen atoms are meridianal. The molecular
geometry of 5, containing two five-membered metalla-
cycles, is a highly distorted trigonal bipyramid with the
axial groups bound through O(10) and C(125) being bent
toward equatorial C(122) to which they are chelated. A
much more regular trigonal-bipyramidal geometry is
present for 8, with the metalation carbon occupying an
equatorial site. All four compounds contain one aryl-
oxide which is part of a five-membered metallacycle
ring. The angle at oxygen in these systems lies in the
narrow range of 120-123°. This compares with much
larger M-O-Ar angles typically found for terminal
aryloxides.20 The Nb-O distances and Nb-O-Ar angles
for the aryloxide ligands are presented graphically in
Figure 5. The most remarkable structural parameter
is the unusually long distance found for the ligand
bound through O(1) in the isocyanide complex 3. This
ligand is situated directly trans to the η2-vinyl group
as well as cis to both isocyanide ligands.
CHdCPh)]. The broad resonance at δ 3.36 ppm would
then have to be assigned to isochronous CMeCH2-
CHdCPh protons. The isomerization of metallacycles
formed by coupling of alkynes or olefins has prece-
dence.18 Assuming the formulation of 6a is correct, then
it would represent the kinetic product of the coupling
of PhCtCH with the olefinic group in 1, and the
subsequent isomerization to the thermodynamic isomer
6b would presumably require fragmentation back into
an alkyne complex. Another possible formulation of 6a
is in fact as an alkyne adduct intermediate 7 in which
the vinyl substituent on the aryloxide ligand is un-
coupled. Stable alkyne adducts of tantalum aryloxides
have been isolated and studied by Wigley et al.19 Due
to its thermal instability we have been unable to obtain
good-quality 13C NMR spectra on 6b (vide supra),
hindering our attempts to identify the exact chemical
nature of this intermediate.
The structural parameters for the η2-vinyl groups in
1 and 3 are consistent with the conclusions drawn from
(18) Balaich, G. J .; Hill, J . E.; Waratuke, S. A.; Fanwick, P. E.;
Rothwell, I. P. Organometallics 1995, 14, 656.
(19) Wexler, P. A.; Wigley, D. E.; Koerner, J . B.; Albright, T. A.
Organometallics 1991, 10, 2319.
(20) Steffey, B. D.; Fanwick, P. E.; Rothwell, I. P. Polyhedron 1990,
9, 963.

Dehydrogenation at Niobium Aryloxide Centers
Organometallics, Vol. 15, No. 21, 1996 4447
F igu r e 1. Molecular structure of [Nb(OC6H3Pri-η2-
CMedCH2)(OC6H3Pri -2,6)2(thf)] (1).
2
F igu r e 4. Molecular structure of [Nb(OC6H3Pri-
CMe2)(OC6H3Pri -2,6)3] (8).
2
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Nb(OC6H3P r i-η2-
CMedCH2)(OC6H3P r i2-2,6)2(th f)] (1)
Nb-O(10)
1.875(2)
1.949(2)
1.967(2)
1.453(4)
Nb-O(41)
Nb-C(261)
Nb-C(262)
2.293(2)
2.167(3)
2.208(2)
Nb-O(20)
Nb-O(30)
C(261)-C(262)
O(10)-Nb-O(20)
O(10)-Nb-O(30)
O(10)-Nb-O(41)
136.40(7) O(30)-Nb-O(41)
116.40(7) O(30)-Nb-C(261)
84.18(7) O(30)-Nb-C(262)
81.24(7)
87.22(9)
123.04(8)
168.47(9)
151.68(8)
38.8(1)
O(10)-Nb-C(261) 101.1(1)
O(41)-Nb-C(261)
O(10)-Nb-C(262)
O(20)-Nb-O(30)
O(20)-Nb-O(41)
O(20)-Nb-C(262)
95.50(8) O(41)-Nb-C(262)
101.84(7) C(261)-Nb-C(262)
81.62(6) Nb-O(10)-C(11)
79.25(8) Nb-O(30)-C(31)
157.6(2)
137.7(2)
F igu r e 2. Molecular structure of [Nb(OC6H3Pri-η2-
CMedCH2)(OC6H3Pri -2,6)2(CNBut)2] (3).
2
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Nb(OC6H3P r i-η2-
CMedCH2)(OC6H3P r i2-2,6)2(CNBu t)2] (3)
Nb-O(1)
2.057(5)
1.973(5)
1.900(4)
2.230(8)
1.157(9)
Nb-C(51)
2.260(9)
2.249(8)
2.341(7)
1.42(1)
Nb-O(2)
Nb-C(221)
Nb-C(222)
C(221)-C(222)
C(51)-N(52)
Nb-O(3)
Nb-C(41)
C(41)-N(42)
1.150(9)
O(1)-Nb-O(2)
O(1)-Nb-O(3)
O(1)-Nb-C(41)
O(1)-Nb-C(51)
O(1)-Nb-C(221)
O(1)-Nb-C(222)
O(2)-Nb-O(3)
O(2)-Nb-C(41)
O(2)-Nb-C(51)
O(2)-Nb-C(221)
O(2)-Nb-C(222)
O(3)-Nb-C(41)
O(3)-Nb-C(51)
90.9(2)
99.5(2)
83.4(2)
87.3(2)
159.3(3)
158.9(2)
167.9(2)
91.7(2)
79.9(2)
82.1(3)
75.8(2)
95.7(2)
94.2(2)
O(3)-Nb-C(221)
O(3)-Nb-C(222)
C(41)-Nb-C(51)
C(41)-Nb-C(221)
C(41)-Nb-C(222)
C(51)-Nb-C(221)
C(51)-Nb-C(222)
Nb-O(1)-C(11)
Nb-O(2)-C(21)
Nb-O(3)-C(31)
Nb-C(41)-N(42)
Nb-C(51)-N(52)
90.2(3)
92.5(2)
167.4(3)
77.4(3)
112.8(3)
74.4(3)
35.9(3)
136.5(4)
122.9(4)
171.6(4)
177.4(7)
165.8(7)
F igu r e 3. Molecular structure of [Nb(OC6H3Pri-CMeCH2-
The Nb-C(isocyanide) distances of 2.230(8) and
2.260(9) Å in compound 3 are among the longest so far
reported for RNC adducts of either Nb or Ta.23 They
are comparable to the distances of 2.25-2.32 Å found
to the terminal isocyanides in the di- and trinuclear
compounds [Nb3Cl8(CNBut)5],23a,b [Nb2Cl6(CNBut)6], and
[Ta2Cl6(CNPri)6].23c All of these bond distances are
dramatically longer than the distances of 2.017(7) and
2.022(7) Å found to the two isocyanide ligands in [Ta-
(CNMe)2(Cl)(dmpe)2].23i
CPhdCPh)(OC6H3Pri -2,6)2] (5).
2
the spectroscopic properties. The Nb-C distances
within the niobacyclopropane ring within 1 are 2.167(3)
and 2.208(2) Å. These compare favorably with the
distance of 2.216(3) Å found for the Nb-C(alkyl) bond
in 8. In contrast the distances in 3 are longer at 2.260(9)
and 2.230(8) Å, implying significantly reduced π-back-
bonding. Less striking are the C-C distances of 1.453(4)
and 1.42(1) Å in 1 and 3, respectively. These are shorter
than the distance of 1.477 Å found in [Cp*Ta(η2-C2H4)-
(dCHCMe3)(PMe3)]21 but longer than the value of 1.407
Å reported for [CpNb(η2-C2H4)(C2H5)].22
(21) Schultz, A. J .; Brown, R. K.; Williams, J . M.; Schrock, R. R. J .
Am. Chem. Soc. 1981, 103, 169.
(22) Guggenberger, L. J .; Meakin, P.; Tebbe, F. N. J . Am. Chem.
Soc. 1974, 96, 5420.

4448 Organometallics, Vol. 15, No. 21, 1996
Yu et al.
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Nb(OC6H3P r i-
or methyl CH bond. The isolation of stable five-
membered metallacycles by protonation of the chelated
η2-CMedCH2 methylene carbon supports but does not
prove that metallation of the methine CH bond occurs
first.
CMeCH2CP h dCP h )(OC6H3P r i2-2,6)2] (5)
Nb-O(10)
Nb-O(20)
Nb-O(30)
Nb-C(122)
1.943(4)
1.873(4)
1.882(4)
2.261(6)
Nb-C(125)
2.125(6)
1.532(8)
1.501(9)
1.349(8)
C(122)-C(123)
C(123)-C(124)
C(124)-C(125)
Exp er im en ta l Section
O(10)-Nb-O(20)
O(10)-Nb-O(30)
O(10)-Nb-C(122)
102.0(2) O(30)-Nb-C(125)
93.0(2) C(122)-Nb-C(125)
75.1(2) Nb-O(10)-C(11)
91.2(2)
72.3(2)
123.0(4)
164.2(4)
164.3(4)
All operations were carried out under a dry N2 atmosphere
or in vacuo in a Vacuum Atmospheres Dri-Lab or by standard
Schlenk techniques. Hydrocarbon solvents and tetrahydro-
furan (thf) were dried by distillation from sodium/benzophe-
none and stored under dry N2. NbCl5 was used as supplied
O(10)-Nb-C(125) 145.2(2) Nb-O(20)-C(21)
O(20)-Nb-O(30) 135.1(2) Nb-O(30)-C(31)
O(20)-Nb-C(122) 107.7(2) Nb-C(122)-C(123) 114.0(4)
O(20)-Nb-C(125)
99.1(2) Nb-C(122)-C(12)
106.1(4)
by the Alfa Chemical Co. The chemicals HOC6H3Pri -2,6,
2
O(30)-Nb-C(122) 117.1(2) Nb-C(125)-C(124) 125.1(5)
PhCtCPh, HCtCPh, and Ph2CdO were purchased from
Aldrich Chemical Co. and were used without further purifica-
tion. Pyridine and aniline were supplied by Aldrich Chemical
Co. and were dried over KOH pellets prior to use. 1H and 13C
NMR spectra were recorded on a Varian Associates Gemini
200 spectrometer using the protio impurities of commercial
benzene-d6 or toluene-d8 as internal standards. The infrared
spectra were obtained by using a Perkin-Elmer 1800 Fourier
transform infrared spectrometer. Mass spectra were recorded
on a Finnigan 4000 spectrometer using perfluorobutylamine
for calibration. Microanalysis data were obtained in-house at
Purdue.
Ta ble 5. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [Nb(OC6H3P r i-CMe2)(OC6H3P r i2-2,6)3] (8)
Nb-O(10)
Nb-O(20)
Nb-O(30)
1.952(2)
1.893(2)
1.897(2)
Nb-O(40)
Nb-C(122)
1.874(2)
2.216(3)
O(10)-Nb-O(20)
O(10)-Nb-O(30)
O(10)-Nb-O(40)
O(10)-Nb-C(122)
O(20)-Nb-O(30)
O(20)-Nb-O(40)
O(20)-Nb-C(122)
O(30)-Nb-O(40)
166.03(9) O(30)-Nb-C(122) 117.1(1)
91.37(9) O(40)-Nb-C(122) 111.3(1)
89.90(9) Nb-O(10)-C(11)
76.3(1)
121.1(2)
176.2(20)
143.4(2)
144.1(2)
Nb-O(20)-C(21)
94.77(9) Nb-O(30)-C(31)
95.59(9) Nb-O(40)-C(41)
89.7(1)
Nb-C(122)-C(12) 106.1(2)
P r ep a r a t ion of [Nb (OC6H 3P r i2-2,6)2(OC6H 3P r i-η2-
CMedCH2)(th f)] (1). To a sodium amalgam (0.42 g Na/10
130.4(1)
mL of Hg) solution was added [Nb(OC6H3Pri -2,6)3Cl2] (5.0 g,
2
7.2 mmol) in tetrahydrofuran (100 mL). The mixture was
stirred vigorously for 10 h at room temperature resulting in
the formation of a dark green suspension. The clear deep
green solution was separated from the mercury pool and
filtered, and the filtrate was dried under vacuum. The crude
product was then dissolved in a minimum quantity of hexane
and the solution allowed to stand, whereupon deep-green
crystals of product slowly formed. Additional product was
obtained by slow evaporation of the supernatant. Yield ) 3.72
g (74%). Anal. Calcd for NbC40H57O4: C, 69.15; H, 8.27.
Found: C, 68.90; H, 8.78. 1H NMR (C6D6, 30 °C): δ 1.2-1.5
(doublets, CHMe2), 2.15 (s, CMeCH2), 2.48 (d) and 2.81 (d,
9.8Hz, AB pattern, CMeCH2), 3.2-3.5 (br multiplets, CHMe2),
1.08 (m) and 4.15 (m, CH2 on thf), 6.8-7.2 (m, aromatics). 13
C
NMR (C6D6, 30 °C): δ 90.1 (CMeCH2, 1J (13C-1H) ) 140.8 Hz),
95.9 (CMeCH2), 28.7, 27.0, and 24.8 (CHMe2), 23.9, 23.7, and
21.7 (CHMe2), 28.5 (CMeCH2), 162.7, and 161.7 and 158.0
(Nb-O-C), 144.4, 138.6, 137.3, and 134.2 (ortho-C’s at central
phenoxy rings).
F igu r e 5. Plot showing Nb-O distances and Nb-O-Ar
angles for the aryloxide ligands in molecules 1, 3, 5, and
8.
P r ep a r a t ion of [Nb (OC6H 3P r i2-2,6)2(OC6H 3P r i-η2-
CMedCH2)(p y)2] (2). To a sodium amalgam (0.17 g Na/5 mL
of Hg) was added [Nb(OC6H3Pri -2,6)3Cl2] (2.0 g, 2.88 mmol)
2
Con clu sion s
and pyridine (py) (1.0 mL, 12.4 mmol) dissolved in diethyl
ether (30 mL). The mixture was stirred vigorously overnight
at room temperature resulting in the formation of a brownish
red suspension. The clear red solution was filtered from the
mixture and dried under vacuum. The crude product was
dissolved in a minimum quantity of hexane and the solution
allowed to stand, whereupon large red crystals slowly formed.
Yield ) 1.27 g (57%). Anal. Calcd for NbC46H59N2O3: C,
The chemistry outlined in this paper demonstrates
that the 2,6-diisopropylphenoxide ligand can undergo
the sometimes facile dehydrogenation of a substituent
isopropyl group at niobium metal centers. The resulting
ligand gives some insight into the bonding and reactivity
of olefin ligands attached to niobium. The pathway for
dehydrogenation is believed to proceed via cyclometal-
lation of the 2,6-diisopropylphenoxide ligand although
it is unclear whether initial attack occurs on a methine
1
70.75; H, 7.62; N, 3.59. Found: C, 70.83; H, 7.57; N, 3.49. H
NMR (C7D8, -20 °C): δ 0.9-1.3 (overlapping doublets, CHMe2),
2.26 (s, CMeCH2), 2.62 (d) and 3.23 (d, 5.5 Hz, AB pattern,
CMeCH2), 3.04, 3.40, and 3.82 (septets, CHMe2), 6.4-7.4 (m,
aromatics), 8.88 (d) and 9.67 (d, ortho-H’s on pyridines). 13C
NMR (C6D6, 30 °C): δ 87.0 (CMeCH2, 1J (13C-1H) ) 146.8 Hz),
88.1 (CMeCH2), 20.0-30.0 (CHMe2), 118.0-141.0 (aromatics),
150.1 and 151.9 (ortho-carbons of py’s), 159.5, 158.1, and 155.7
(Nb-O-C).
(23) (a) Cotton, F. A.; Roth, W. J . J . Am. Chem. Soc. 1983, 105, 3734.
(b) Cotton, F. A.; Roth, W. J . Inorg. Chim. Acta 1987, 126, 161. (c)
Cotton, F. A.; Duraj, S. A.; Roth, W. J . J . Am. Chem. Soc. 1984, 106,
6987. (d) Aspinall, H. C.; Roberts, M. M.; Lippard, S. J . Inorg. Chem.
1984, 23, 1782. (e) Cotton, F. A.; Duraj, S. A.; Roth, W. J . J . Am. Chem.
Soc. 1984, 106, 6987. (f) Koschmieder, S. U.; Hussain-Bates, B.;
Hursthouse, M. B.; Wilkinson, G. J . Chem. Soc., Dalton Trans. 1991,
2785. (g) Alcalde, M. I.; de la Mata, J .; Gomez, M.; Royo, P.; Pellinghelli,
M. A.; Tiripicchio, A. Organometallics 1994, 13, 462. (h) Carnahan, E.
M.; Rardin, R. L.; Bott, S. G.; Lippard, S. J . Inorg. Chem. 1992, 31,
5193. (i) Aharonian, G.; Hubert-Pfalzgraf, L. G.; Zaki, A.; Le Borgne,
G. Inorg. Chem. 1991, 30, 3105.
P r ep a r a t ion of [Nb (OC6H 3P r i2-2,6)2(OC6H 3P r i-η2-
CMedCH2)(CNBu t)2] (3). To a solution of [Nb(OC6H3Pri -
2
2,6)2(OC6H3Pri-η2-CMedCH2)(thf)] (1) (0.2 g, 0.29 mmol) in
hexane (5 mL) was added ButNC (0.1 mL, large excess) by
syringe. Slow cooling of the resulting dark mixture yielded

Dehydrogenation at Niobium Aryloxide Centers
Organometallics, Vol. 15, No. 21, 1996 4449
the product as greenish-orange chunks. 1H NMR (C6D6, 30
°C): δ 0.87 (br, CNCMe3), 1.1-1.5 (overlapping doublets,
CHMe2), 3.41, 3.59, 3.94 (septets, CHMe2), 2.34 (s, CMeCH2),
2.31 (d), 3.42 (d, 4.2 Hz, AB pattern, CMeCH2), 6.6-7.4 (m,
aromatics). 13C NMR (C6D6, 30° C): δ 68.2 (CMeCH2, 1J (13C-
1H) ) 153.4 Hz), 81.9 (CMeCH2), 23.5, 23.7, 23.8, 24.5, 24.9,
25.0, 25.4, 26.0, 26.4, 29.7, 30.0 (CHMe2 and CMe3), 166.3,
158.2, 155.6, 153.7, 138.3, 137.1, 133.0 (Nb-CNCMe3, Nb-
O-C, ortho-C’s at central phenoxy rings). IR (Nujol mull):
mmol). The mixture changed color from green to orange in
several seconds and was stirred for 24 h. The orange crystal-
line product was isolated upon slow cooling of the concentrated
hexane solution. Additional crystalline product was obtained
by slow evaporation of the supernatant. Yield ) 0.27 g (74%).
Anal. Calcd for NbC44H55O3: C, 72.91; H, 7.65. Found: C,
72.93; H, 7.58. 1H NMR (C6D6, 30 °C): δ 0.9-1.4 (doublets,
CHMe2), 1.90 (s, CMeCH2CPhdCH), 3.17, 3.40, and 3.94
(septets, CHMe2), 3.89 (d) and 4.18 (d, AB pattern, CMeCH2-
CPhdCH), 6.8-7.2 (m, aromatics), 7.5 (d, ortho-H’s on CMeCH2-
CPhdCH), 9.18 (s, CMeCH2CPhdCH). 13C NMR (C6D6, 30
°C): δ 55.4 (CMeCH2CPhdCH), 95.8 (Nb-CMeCH2), 168.1
(CMeCH2CPhdCH), 185.0 (Nb-C), 168.0 (NbCHCPhCH2-
CMe), 163.6, 160.8, and 159.1 (Nb-O-C), 149.9 (ipso-C’s on
CMeCH2CPh)CH), 138.8, 137.9, 137.7 and 133.7 (ortho-C’s
at central phenoxy rings), 123.0-130.0 (aromatics), 21.0-30.0
(diasterotopic CHMe2).
2194 (sh), 2172 cm-1
.
P r ep a r a t ion of [Nb (OC6H 3P r i2-2,6)2(OC6H 3P r i-CMe-
CH2CP h 2O)] (4). To a solution of [Nb(OC6H3Pri2-2,6)2(OC6H3-
Pri-η2-CMedCH2)(thf)] (1) (0.4 g, 0.58 mmol) in hexane (10 mL)
was added Ph2CdO (0.11 g, 0.63 mmol). The mixture was
stirred for 2 h to afford a yellow solution. The yellow
crystalline product was obtained upon slow evaporation of the
hexane solution. Additional crystals were obtained by con-
centration of the supernatant. Yield ) 0.33 g (71%). Anal.
Calcd. for NbC49H59O4: C, 73.12; H, 7.39. Found: C, 72.86;
H, 7.35. 1H NMR (C6D6, 30 °C): δ 0.7-1.4 (overlapping
doublets, CHMe2), 1.85 (s, CMeCH2CPh2O), 2.8-3.1 and 4.05
(septets, CHMe2), 3.85 (d) and 4.52 (d, 14 Hz, AB pattern,
CMeCH2CPh2O), 6.7-7.3 (m, aromatics), 7.49 (d) and 7.78 (d,
ortho-H’s on CMeCH2CPh2O). 13C NMR (C6D6, 30 °C): δ 60.5
(CMeCH2CPh2O, 1J (13C-1H) ) 127.2 Hz), 96.8 (CMeCH2-
CPh2O), 99.6 (CMeCH2CPh2O), 162.6, 160.6, and 158.4 (Nb-
O-C), 148.7 and 148.6 (ipso-C’s on NbOCPh2), 151.0, 138.4,
137.0, and 134.0 (ortho-C’s at central phenoxy rings), 121.0-
132.0 (aromatics), 22.0-30.0 (diastereotopic CHMe2). The
P r ep a r a t ion of [Nb (OC6H3P r i2-2,6)3(OC6H3P r i-CMe2)]
(8). To a solution of [Nb(OAr-2,6Pri2)2(OC6H3Pri-η2-CMedCH2)-
(thf)] (1) (3 g, 4.3 mmol) in benzene (25 mL) was added
HOC6H3Pri -2,6 (0.8 g, 4.5 mmol). The mixture was stirred
2
for 2 h to afford an orange solution. The crystalline product
was isolated from the supernatant after slow evaporation of
the solvent. Yield
) 2.35 g (68%). Anal. Calcd for
1
NbC48H67O4: C, 71.98; H, 8.43. Found: C, 70.99; H, 9.10. H
NMR (C6D6, 30 °C): δ 0.93 (d) and 1.12 (d, CHMe2), 2.51 (s,
Nb-CMe2), 2.88 and 3.65 (septets, CHMe2), 6.6-7.1 (m,
aromatics). 13C NMR (C6D6, 30 °C): δ 90.2 (Nb-CMe2), 159.2
and 151.8 (Nb-O-C), 138.9 (ortho-C’s at nonmetalated central
phenoxy rings), 134.7 and 138.5 (ortho-C’s at metalated central
phenoxy ring), 122.0-126.0 (aromatics), 22.0-29.0 (CHMe2
and Nb-CMe2). Mass spectrometric analysis: +CI, 70 eV
(source temp ) 250 °C; probe temp ) 140 °C) gave the highest
mass peak at m/e 801 identified as [M + H]+, the base peak
compound [Nb(OC6H3Pri -2,6)2(OC6H3Pri-CMeCH213CPh2O)]
2
was obtained by addition of Ph213CdO to a C6D6 solution of 1.
The compound was only characterized in the reaction mixture
1
by NMR spectroscopy. Selected H NMR (C6D6, 30 °C): δ 3.84
(dd, 14 Hz, 5 Hz) and 4.53 (d, 14 Hz, CMeCH213CPh2O). 13C
NMR (C6D6, 30 °C): δ 99.6 (CMeCH213CPh2O).
at m/e 623 identified as [Nb(OAr-2,6Pri )2(OC6H3Pri-η2-
2
P r ep a r a t ion of [Nb (OC6H 3P r i2-2,6)2(OC6H 3P r i-CMe-
CMedCH2]. -CI, 70 eV (source temp ) 250 °C, probe temp
) 120 °C) gave the highest mass peak and base peak both at
m/e 800 identified as [M-].
CH 2CP h dCP h )] (5). To a solution of [Nb(OC6H3Pri -
2
2,6)2(OC6H3Pri-η2-CMedCH2)(thf)] (1) (0.42 g, 0.61 mmol) in
hexane (10 mL) was added diphenylacetylene (0.12 g, 0.67
mmol). The initial green solution rapidly changed its color to
orange and was stirred for 1 h. The bright orange crystalline
product was obtained after slow cooling of the concentrated
hexane solution. Yield ) 0.37 g (76%). Anal. Calcd for
P r ep a r a tion of [Nb(OC6H3P r i2-2,6)2(OC6H3P r i-CMe2)-
(NHP h )] (9). To a solution of [Nb(OAr-2,6Pri )2(OC6H3Pri-
2
η2-CMedCH2)(thf)] (1) (0.5 g, 0.72 mmol) in hexane (10 mL)
was added aniline (0.1 g, 1.08 mmol). The mixture was stirred
for 4 h to afford an orange solution. The orange crystalline
product was obtained after slow evaporation of the hexane
solution. Concentration of the supernatant gave additional
1
NbC50H59O3: C, 74.98; H, 7.42. Found: C, 75.06; H, 7.56. H
NMR (C6D6, 30 °C): δ 0.75-1.45 (doublets, CHMe2), 2.13 (s,
CMeCH2), 2.91, 3.35, and 3.75 (septets, CHMe2), 3.91 (d) and
4.15 (d, AB pattern CMeCH2), 6.7-7.3 (m, aromatics). 13C
NMR (C6D6, 30 °C): δ 59.2 (CMeCH2, 1J (13C-1H) ) 125.1 Hz),
96.4 (Nb-CMeCH2), 160.8 (CMeCH2CPhdCPh), 200.5 (Nb-
C), 163.9, 161.4, and 158.9 (Nb-O-C), 150.6 and 139.7 (ipso-
C’s on CMeCH2CPhdCPh), 143.4, 139.7, 138.3, and 133.4
(ortho-C’s at central phenoxy rings), 22.0-30.0 (diastereotopic
CHMe2), 122.0-130.0 (aromatics).
crystals. Yield ) 0.34 g (66%). Anal. Calcd for NbC42H56
-
NO3: C, 70.47; H, 7.89; N, 1.96. Found: C, 70.21; H, 8.23; N,
2.04. 1H NMR (C6D6, 30 °C): δ 0.9-1.2 (doublets, CHMe2),
1.91 (s, CMe2), 3.27 and 3.53 (septets, CHMe2), 6.6-7.1 (m,
aromatics), 8.49 (br s, NHPh). 13C NMR (C6D6, 30 °C): δ 92.4
(Nb-CMe2), 152.3, 157.9 and 150.1 (Nb-O-C), 161.8 (ipso-C
at NHPh), 118.0-140.0 (aromatics), 22.0-28.0 (diastereotopic
CHMe2). Infrared (Nujol mull): ν(N-H) ) 3288 cm-1
.
Sp ectr oscop ic Ch a r a cter iza tion of 6a . To a solution of
[Nb(OC6H3Pri2-2,6)2(OC6H3Pri2)2(OC6H3Pri-η2-CMedCH2)-
(thf)] (1) (0.05 g) in C6D6 (1.0 mL) was added phenylacetylene
(HCtCPh) (0.01 g). The initial deep green solution rapidly
changed its color to orange, and the initially formed organo-
metallic product was characterized in solution. 1H NMR (C6D6,
30 °C): δ 0.8-1.4 (doublets, CHMe2), 1.88 (s, CMeCH2-
CH)CPh), 2.96, 3.28, and 3.78 (septets, CHMe2), 3.36 (br,
CMeCH2CHdCPh), 6.7-7.2 (m, aromatics), 7.37 (d, ortho-H’s
on CMeCH2CHdCPh).
Ack n ow led gm en t. I.P.R. thanks the National Sci-
ence Foundation (Grant CHE-9321906) for research
support.
Su p p or tin g In for m a tion Ava ila ble: Text describing
X-ray procedures and tables of experimental details associated
with data collection and crystal structure refinement, final
atomic coordinates, thermal parameters, and complete bond
distances and angles for complexes 1, 3, 5, and 8 (84 pages).
Ordering information is given on any current masthead page.
P r ep a r a t ion of [Nb (OC6H 3P r i2-2,6)2(OC6H 3P r i-CMe-
CH 2CP h dCH )] (6b ). To a solution of [Nb(OC6H3Pri -
2
2,6)2(OC6H3Pri-η2-CMe)CH2)(thf)] (1) (0.35 g, 0.5 mmol) in
hexane (10 mL) was added phenylacetylene (0.06 g, 0.59
OM9604839