Pyridine degradation intermediates as models for hydrodenitrogenation catalysis: Preparation and properties of a metallapyridine complex

Article abstract of DOI:10.1021/om970749r

We report the preparation, structure, and reactions of a stable metallapyridine complex of tantalum prepared in the course of model studies of hydrodenitrogenation (HDN) reactions. Monomeric Ta(=NC^tBu=CHC^tBu=CH)(OAr)₂(THF) (5·THF) is isolated upon thermolyzing the η²(N,C)-pyridine complex [η²(N,C)-2,4,6-NC₅^tBu₃H ₂]Ta(OAr)₂Me (2) in the presence of THF, while the metallapyridine dimer [Ta(μ-NC^tBu=CHC^tBu=CH)(OAr)₂]₂ (6) is isolated when this reaction is carried out in benzene. Complete NMR characterization of 5·THF is described, along with its conversion into 6. The bis(pyridine) adduct Ta(=NC^tBu=CHC-^tBu=CH)(OAr)₂(Py)₂ (5·py) is also described. Ta(=NC^tBu=CHC^tBu=CH)(OAr)₂(THF) (5·THF) is shown to react with ^tBuNCO and ⁱPrNCNⁱPr to afford the σ(η¹) and π (η³) insertion products respectively Ta[=NC^tBu=CHC^tBu=CHC(=N^tBu)O](OAr)₂ (7) and Ta[=NC^tBu=CHC^tBu=CH-(η³-C(=N ⁱPr)₂)](OAr)₂ (8). The molecular structure of the metallapyridine Ta(=NC^tBu=CHC-^tBu=CH)(OAr)₂(THF) (5·THF) was detemined by X-ray crystallography and shown to adopt a trigonal-bipyramidal configuration with aryl oxide oxygens and the metallacyclic carbon occupying equatorial positions. The TaNC₄ metallacycle is very nearly planar, and discrete single and double bonds are evident around the ring. This π localization clearly favors the imido form T(=NC^tBu=CHC^tBu=CH)(OAr)₂(THF) rather than a carbene structure. The relevance of these compounds to hydrodenitrogenation catalysis is described.

Full text of DOI:10.1021/om970749r

322
Organometallics 1998, 17, 322-329
P yr id in e Degr a d a tion In ter m ed ia tes a s Mod els for
Hyd r od en itr ogen a tion Ca ta lysis: P r ep a r a tion a n d
P r op er ties of a Meta lla p yr id in e Com p lex
Keith J . Weller,† Igor Filippov,‡ Paula M. Briggs, and David E. Wigley*
Carl S. Marvel Laboratories of Chemistry, Department of Chemistry, University of Arizona,
Tucson, Arizona 85721-0041
Received August 21, 1997X
We report the preparation, structure, and reactions of a stable metallapyridine complex
of tantalum prepared in the course of model studies of hydrodenitrogenation (HDN) reactions.
Monomeric Ta(dNCtBudCHCtBudCH)(OAr)2(THF) (5‚THF) is isolated upon thermolyzing
t
the η2(N,C)-pyridine complex [η2(N,C)-2,4,6-NC5 Bu3H2]Ta(OAr)2Me (2) in the presence of
THF, while the metallapyridine dimer [Ta(µ-NCtBudCHCtBudCH)(OAr)2]2 (6) is isolated
when this reaction is carried out in benzene. Complete NMR characterization of 5‚THF is
described, along with its conversion into 6. The bis(pyridine) adduct Ta(dNCtBudCHC-
tBudCH)(OAr)2(py)2 (5‚py) is also described. Ta(dNCtBudCHCtBudCH)(OAr)2(THF) (5‚THF)
t
is shown to react with BuNCO and iPrNCNiPr to afford the σ (η1) and π (η3) insertion products
respectively Ta[dNCtBudCHCtBudCHC(dNtBu)O](OAr)2 (7) and Ta[dNCtBudCHCtBudCH-
(η3-C(dNiPr)2)](OAr)2 (8). The molecular structure of the metallapyridine Ta(dNCtBudCHC-
tBudCH)(OAr)2(THF) (5‚THF) was detemined by X-ray crystallography and shown to adopt
a trigonal-bipyramidal configuration with aryl oxide oxygens and the metallacyclic carbon
occupying equatorial positions. The TaNC4 metallacycle is very nearly planar, and discrete
single and double bonds are evident around the ring. This π localization clearly favors the
imido form Ta(dNCtBudCHCtBudCH)(OAr)2(THF) rather than a carbene structure. The
relevance of these compounds to hydrodenitrogenation catalysis is described.
In tr od u ction
led to a cascade of subsequent heterocycle rearrange-
ment and degradation reactions.7-10 In this report, we
describe the isolation and structure of an elusive
intermediate in this pyridine degradation sequence, viz.
a monomeric metallapyridine complex. Various reac-
tions of the metallapyridine are also described, including
its conversion to the thermodynamic degradation prod-
uct, a metallapyridine dimer. A portion of these results
have been communicated.10
One major goal of petroleum hydrotreating is the
catalytic removal of sulfur-, nitrogen-, oxygen-, and
metal-containing impurities from assorted feedstocks.1,2
Hydrotreating is typically carried out over sulfided
CoMo/γ-Al2O3 or NiMo/γ-Al2O3 catalysts and is normally
optimized for hydrodesulfurization (HDS), since sulfur
is present in higher concentrations than nitrogen or
oxygen.3,4 However, declining crude oil quality is as-
sociated, in part, with increasing concentrations of
organonitrogen contaminants, lending a new urgency
to improving hydrodenitrogenation (HDN) technolo-
gies.5,6 We recently described HDN model studies in
which a pyridine C-N bond scission was achieved and
Resu lts a n d Discu ssion
Syn th esis a n d NMR Sp ectr oscop y of a Meta l-
la p yr id in e Com p lex. The η2(N,C)-pyridine complex
[η2(N,C)-2,4,6-NC5 Bu3H2]Ta(OAr)2Cl (1, Ar ) 2,6-C6H3-
t
iPr2) reacts with MeMgCl to ultimately afford a stable
† Present address: Witco Corp., OrganoSilicones Group, 777 Old
Saw Mill River Rd., Tarrytown, NY 10591-6728.
‡ Link Foundation Energy Fellow, 1997-1998; Carl S. Marvel
Fellow, 1997-1998.
(5) Weller, K. J .; Fox, P. A.; Gray, S. D.; Wigley, D. E. Polyhedron
1997, 16, 3139.
(6) Ho, T. C. Catal. Rev.-Sci. Eng. 1988, 30, 117.
(7) Gray, S. D.; Smith, D. P.; Bruck, M. A.; Wigley, D. E. J . Am.
Chem. Soc. 1992, 114, 5462.
X Abstract published in Advance ACS Abstracts, December 15, 1997.
(1) Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice,
2nd ed.; McGraw-Hill, Inc.: New York, 1991.
(2) Gary, J . H.; Handwerk, G. E. Petroleum Refining: Technology
and Economics, 3rd ed.; Marcel Dekker, Inc.: New York, 1993.
(3) Ledoux, M. J . Catalysis; The Chemical Society: London, 1988;
Vol. 7, pp 125-148.
(4) Angelici, R. J . In Encyclopedia of Inorganic Chemistry; King, R.
B., Ed.; J ohn Wiley and Sons: New York, 1994; Vol. 3, pp 1433-1443.
(8) Gray, S. D.; Weller, K. J .; Bruck, M. A.; Briggs, P. M.; Wigley,
D. E. J . Am. Chem. Soc. 1995, 117, 10678.
(9) Weller, K. J .; Gray, S. D.; Briggs, P. M.; Wigley, D. E. Organo-
metallics 1995, 14, 5588.
(10) Weller, K. J .; Filippov, I.; Briggs, P. M.; Wigley, D. E. J .
Organomet. Chem. 1997, 528, 225.
S0276-7333(97)00749-8 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 01/16/1998

Preparation and Properties of a Metallapyridine Complex
Organometallics, Vol. 17, No. 3, 1998 323
Sch em e 1
The monomer can also be isolated as a bis(pyridine)
adduct (5‚py) by recrystallizing 5‚THF from concen-
trated pentane solutions with the addition of a small
amount of pyridine.
metallapyridine dimer, [Ta(µ-NCtBudCHCtBudCH)-
(OAr)2]2 (6), Scheme 1.8 The initial kinetic product in
this sequence, [η2(N,C)-2,4,6-NC5 Bu3H2]Ta(OAr)2Me
t
(2), has been shown to undergo an intramolecular metal-
There are two isomers of 5‚THF, both Cs-symmetric,
that are consistent with the 1H and 13C NMR data. Both
structures 5a ,b are trigonal bipyramids with the coor-
dinated THF assuming an apical position and the aryl
oxide ligands occupying equatorial sites. The metal-
lapyridine ring itself can be oriented such that the imido
nitrogen is either trans to the THF ligand (as in 5a ) or
cis to the THF (as in 5b). Structure 5a appears to be
preferred since LfM π donation is maximized in this
arrangement. If the z-axis lies along the Ta-N bond
and the x-axis is coincident with the Ta-C bond in 5a ,
then the aryl oxide oxygens interact with the dxy orbital,
while the imido nitrogen π donates into dxz and dyz. In
5b, however, both aryl oxide oxygens and the imido
nitrogen compete for donation into the dxy orbital,
suggesting that 5b is less favorable.
to-ligand alkyl migration9 that induces pyridine ring
opening and forms the metallacyclic imido complex Ta-
(dNCtBudCHCtBudCHCtBuMe)(OAr)2 (3). Further
studies of metallacycle 3 revealed its instability with
respect to “ring expansion” to afford Ta(dNCtBudCHC-
tBudCHCtBuHCH2)(OAr)2 (4) that further decomposes
to give the metallapyridine dimer [Ta(µ-NCtBudCHC-
t
tBudCH)(OAr)2]2 (6) and BuCHdCH2, when the ther-
molysis is effected in benzene solution, Scheme 1.8
With the exception of 5, all of the complexes in
Scheme 1 have been observed via 1H and 13C NMR
spectroscopy, 1-3 and 6 have been isolated as the
complexes shown, and 4 has been isolated as its MeCtN
adduct.8 We believed that complex 6 is formed from the
dimerization of monomeric metallapyridine 5, which is
sufficiently short-lived in solution that it is not observed.
We have now succeeded in isolating an adduct of 5 and
describe its structure, characterization, and reactivity.
Thermolyzing toluene/THF solutions of [η2(N,C)-2,4,6-
t
NC5 Bu3H2]Ta(OAr)2Me (2) affords a new, cherry-red
species that can be isolated in moderate yield from
concentrated pentane solutions. The 1H NMR spectrum
of this red complex reveals two sharp singlets at δ 7.41
and 5.97 for the ring protons as compared to a broad
singlet at δ 5.63 for the pyridine protons of 2 that are
equilibrating by a “ring-rocking” process (all in C6D6).8,11
This new compound is also characterized by only two
tBu resonances in its 1H NMR spectrum, along with
In order to determine which structure 5‚THF adopts
in solution, as well as to make complete NMR assign-
ments, a NOESY experiment was carried out. The
numbering scheme for 5‚THF carbon nuclei used in
1
specifying H and 13C NMR assignments is shown in
Figure 1. NOE cross-peaks were observed between the
t
resonance at δ 5.97 and both Bu groups (at δ 1.34 and
1
0.95), establishing the δ 5.97 resonance as C3H. NOE
cross-peaks also appeared between the δ 7.41 ring
signals for a coordinated THF. Based upon its H and
13C NMR spectra, as well as its elemental analysis, this
new complex is formulated as the metallapyridine
t
proton and only one Bu resonance (δ 1.34), as well as
the THF CRH resonance at δ 3.89. Based upon these
observations, the structure of 5‚THF can be assigned
as that of 5a , with the C1H and C3H resonances
assigned at δ 7.41 and 5.97, respectively. The tBu
groups can also be assigned, with C6H and C8H signals
assigned at δ 1.34 and 0.97, respectively. A summary
monomer Ta(NCtBuCHCtBuCH)(OAr)2(THF), 5‚THF.
(11) At -90 °C in toluene-d8, the “ring-rocking” process that
equilibrates the ring protons in [η2(N,C)-2,4,6-NC5tBu3H2]Ta(OAr)2-
Me (2) is slowed, and their resonances in a static pyridine ligand appear
at δ 5.73 and 5.50.

324 Organometallics, Vol. 17, No. 3, 1998
Weller et al.
1
Ta ble 1. Su m m a r y of H NMR Da ta for Com p lexes Ta (dNCtBu dCHCtBu dCH)(OAr )2(THF ) (5‚THF ),
Ta (dNCtBu dCHCtBu dCH)(OAr )2(p y)2 (5‚p y), Ta [dNCtBu dCHCtBu dCHC(dNtBu )O](OAr )2 (7), a n d
Ta [dNCtBu dCHCtBu dCH(η3-C(dNiP r )2)](OAr )2 (8)a
compound
compound
assignment
C1H
C3H
C6H
C8H
5‚THF
5‚py
assignment
7
8
4
4
7.41
5.97
1.34
0.97
7.46
5.99
1.28
1.03
C2H
C4H
C9H
C11H
C6H
C7H
CHMe2
CHMe2
6.61 d, J HH ) 1.3 Hz
6.71 d, J HH ) 1.3 Hz
5.61 d, J HH ) 0.7 Hz
5.24 d, J HH ) 0.7 Hz
4
4
1.07
1.36
1.09
1.32
4.12 spt
1.16 d, 1.06 d
3.96 spt
1.36 d, 1.29 d
1.51 (CMe3)
3.67 spt
1.26 d, 1.18 d
CHMe2
CHMe2
3.74 spt
3.79 spt
1.36 d, 1.34 d
1.32 d, 1.29 d
L (THF or py)
aryl region
3.89 m, 1.19 m (THF)
7.20 A2B d, 6.99 A2B t
8.71, 6.83 t, 6.52 t (py)
7.23 A2B d, 7.02 A2B t
aryl region
7.10 A2B d, 6.95 A2B t
7.14 A2B d, 6.96 A2B t
a
Reported in δ in C6D6 at probe temperature.
Ta ble 2. Su m m a r y of 13C NMR Da ta for Com p lexes Ta (dNCtBu dCHCtBu dCH)(OAr )2(THF ) (5‚THF ),
Ta (dNCtBu dCHCtBu dCH)(OAr )2(p y)2 (5‚p y), Ta [dNCtBu dCHCtBu dCHC(dNtBu )O](OAr )2 (7), a n d
Ta [dNCtBu dCHCtBu dCH(η3-C(dNiP r )2)](OAr )2 (8)a
compound
compound
assignment
C1
C2
C3
5‚THF
178.5
172.4
121.4
5‚py
assignment
7
8
C1
C2
C3
C4
163.6
104.0
164.9
107.8
167.5
165.3
181.6
106.4
150.2
113.4
167.9
171.2
121.5
167.5
C4
167.8
C5
C5, C7
C6, C8
CHMe2 OAr
CHMe2 OAr
L (THF or py)
Cipso OAr
37.9, 36.7
31.4, 29.1
24.1, 23.9
27.2
71.6, 25.4 (THF)
160.0
136.7
123.3
121.6
37.9, 36.7
31.4, 29.1
24.16, 24.0
27.2
C6, C8, C10
C7, C9, C11
CHMe2 OAr
CHMe2 OAr
55.7, 37.6, 35.3
30.9, 30.3, 30.2
24.0, 23.9
50.4, 40.2, 38.8
24.9,c 22.9,c 30.1d
24.0, 23.7
27.1
27.1
149.9, 124.1 (py)b
160.1
Cipso OAr
Cortho OAr
Cmeta OAr
Cpara OAr
157.2
137.8
123.7
122.8
157.2
138.2
123.5
122.6
C
ortho OAr
136.9
123.4
121.7
Cmeta OAr
Cpara OAr
a
b
Reported in δ in C6D6 at probe temperature. One resonance for pyridine could not be found and is presumably coincident with an
aryl resonance. c Resonances for C7 and C7′. C9 and C11 are coincident at 30.1 ppm.
d
Ta ble 3. Su m m a r y of Obser ved NMR
Con n ectivities for
Ta (dNCtBu dCHCtBu dCH)(OAr )2(THF ) (5‚THF )
w ith NOESY, HETCOR, a n d HMBC Meth od s
1H signal NOESY (1H) HETCOR (13C)
HMBC (13C)
C1H
C3H
C6H
C8H
C9H
C10H
C9H, C6H
C6H, C8H
C1H, C3H
C3H
C1H, C10H
C9H
C1
C3
C6
C8
C9
C10
C2, C3, C5
C1, C2, C4, C5, C7
C2, C5, C6
C4, C7, C8
F igu r e 1. Numbering scheme for Ta(dNCtBudCHC-
tBudCH)(OAr)2(THF) (5‚THF) carbon nuclei used in speci-
appears farthest downfield at δ 178.5, C2 and C4 appear
at δ 172.5 and 167.8, respectively, and C3 occurs at δ
121.4 (C6D6). A summary of 13C NMR data for 5‚THF
and other compounds prepared in this study are pre-
sented in Table 2, and selected NMR correlation data
for 5‚THF are provided in Table 3 based upon the
numbering scheme in Figure 1.
1
fying H and 13C NMR assignments.
of 1H NMR data for 5‚THF and other complexes
reported here are presented in Table 1.
With the exception of C2 and C4, the 13C NMR
spectrum is also completely assigned based upon these
NOESY data and a HETCOR experiment. In the
interest of making a complete spectral assignment for
comparisons (vide infra), a heteronuclear multiple-bond
correlation (HMBC) experiment, which can provide
assignment information for nonprotonated carbons via
Str u ctu r al Stu dy of Ta(NCtBu CHCtBu CH)(OAr )2-
(THF ) (5‚THF ). One significant structural question
concerning metallapyridine 5‚THF is the extent to
which the metallacycle is delocalized.13 We have pre-
3
2J CH and J CH couplings, was also carried out.12 Based
(12) Summers, M. F.; Marzilli, L. G.; Box, A. J . Am. Chem. Soc. 1986,
108, 4285.
(13) Bleeke, J . R. Acc. Chem. Res. 1991, 24, 271.
upon the data from all of these experiments, the ring
carbons can now be conclusively assigned. Thus, C1

Preparation and Properties of a Metallapyridine Complex
Organometallics, Vol. 17, No. 3, 1998 325
Ta ble 4. Deta ils of th e X-r a y Diffr a ction Stu d ies
Ta ble 5. Selected Bon d Dista n ces (Å) a n d Bon d
An gles (d eg) in Molecu le 1 of
for Ta (dNCtBu dCHCtBu dCH)(OAr )2(THF ) (5‚THF )
Ta (dNCtBu dCHCtBu dCH)(OAr )2(THF ) (5‚THF )a
Crystal Parameters
mol formula
mol wt
F(000)
cryst color
space group
unit cell volume, Å3
a, Å
C
40H62NO3Ta
Bond Distances
1.786(9) C(2)-C(3)
2.15(1) C(3)-C(4)
1.924(8) C(4)-N
1.934(8) O(10)-C(11)
2.249(7) O(20)-C(21)
1.35(2)
785.89
1624
orange
Ta-N
1.45(2)
1.36(2)
1.34(1)
1.38(1)
1.35(1)
Ta-C(1)
Ta-O(10)
Ta-O(20)
Ta-O(30)
C(1)-C(2)
triclinic P1 (No. 2)
4013.5(9)
13.397(2)
16.925(2)
19.531(1)
108.06(1)
103.11(1)
96.86(1)
4
b, Å
c, Å
Bond Angles
Ta-N-C(4)
Ta-C(1)-C(2)
148.0(8)
127.7(9)
Ta-O(10)-C(11) 140.2(7)
Ta-O(20)-C(21) 132.7(7)
C(1)-Ta-O(10)
112.8(4)
115.8(4)
R, deg
C(1)-Ta-O(20)
â, deg
O(10)-Ta-O(20) 128.1(3)
γ, deg
C(1)-C(2)-C(3)
C(2)-C(3)-C(4)
N-C(4)-C(3)
123(1)
124(1)
117(1)
Z
N-Ta-C(1)
N-Ta-O(10)
N-Ta-O(20)
80.6(4)
104.5(4)
100.1(4)
D(calcd), g cm-3
cryst dimens, mm
ω width, deg
abs coeff, cm-1
data collection temp, °C
1.30
0.33 × 0.33 × 0.08
0.40
a
27.4
-70 ( 1
Numbers in parentheses are estimated standard deviations
in the least significant digits.
Data Collection
diffractometer
monochromator
attenuator
Mo KR radiation, λ, Å
2θ range, deg
Enraf-Nonius CAD4
graphite crystal, incident beam
Zr foil, factor 13.5
0.71073
2-50
octants collected
scan type
+h, (k, (l
ω - 2θ
scan speed, deg min-1
scan width, deg
total no. of reflns measd
corrections
0-20 (in ω)
0.8 + 0.340 tan θ
14739 (14075 unique)
Lorentz polarization
reflection averaging
(agreement on I ) 9.8%)
ψ-scan absorption
(min 0.589, max 0.999, av 0.872)
Solution and Refinement
solution
refinement
Patterson heavy-atom method
full-matrix least-squares
∑w(|Fo| - |Fc|)2
minimization function
no. of reflns used in
refinement, I > 3σ(I)
no. of params refined
R(∑||Fo| - |Fc||/∑|Fo|)
Rw([∑w(|Fo| - |Fc|)2/
7928
806
0.052
0.056
F igu r e 2. ORTEP drawing of Ta(dNCtBudCHCtBudCH)-
(OAr)2(THF) (5‚THF), with atoms shown as 50% probability
ellipsoids.
∑w(Fo)2]1/2
)
esd of obsd of unit weight
convergence, largest shift
∆/σ(max), e-1/Å3
1.29
0.28σ
3.02(17)
-0.22(17)
VAX
and metallacyclic C1 occupying equatorial positions, in
agreement with the solution structure; the THF ligand
and metallacyclic nitrogen occupy axial positions. The
aryl oxide ligands are folded upward toward the ring
with one isopropyl substituent from each ligand posi-
tioned over opposite faces of the metallacycle ring to
provide a protective cleft for the ring and the metal-
lapyridine nitrogen. The TaNC4 ring itself is very
nearly planar (mean deviation from planarity ) 0.02
Å), but it is not delocalized, as discrete single and double
bonds are evident around the ring. This π electron
∆/σ(min), e-1/Å3
computer hardware
computer software
MolEN, 1990
sented the structure of 5‚THF as an imido complex in
Figure 1, but a carbene structure of 5 (shown in Scheme
1) or a delocalized form is also a possible alternative.
The assignment and structure of 5‚THF as a metal-
lapyridine monomer, Ta(NCtBuCHCtBuCH)(OAr)2(THF),
was confirmed by an X-ray crystallographic study.
Dark-orange, almost red, single crystals of 5‚THF
suitable for a structural study were grown from a
concentrated pentane solution at -35 °C. 5‚THF crys-
tallizes in the triclinic space group P1 with the asym-
metric unit containing two crystallographically inde-
pendent, but virtually identical, molecules. Tables 4
and 5 present details of the crystallographic study and
important metrical parameters of molecule 1 of 5‚THF,
a full ORTEP drawing of molecule 1 is shown in Figure
2, and a core structure is presented in Figure 3.
Complex 5‚THF adopts a slightly distorted trigonal-
bipyramidal configuration with the aryl oxide oxygens
localization clearly favors the imido form Ta(dNC-
tBudCHCtBudCH)(OAr)2(THF) rather than a carbene
structure, which is especially significant since the metal
can dictate that either arrangement be adopted. Al-
though the imido ligand in 5‚THF is strongly bent
[TadN-C(4) ) 148.0(8)°], it is only weakly basic as
attempts to alkylate the imido nitrogen with electro-
philes were unsuccessful (vide infra).
We believe the stability of 5‚THF with respect to
dimerization arises from the protection of the imido
nitrogen afforded by the aryl oxide ligands in a five-

326 Organometallics, Vol. 17, No. 3, 1998
Weller et al.
added (C6D6, 300 MHz). One possible solid-state struc-
ture of 5‚py is presented in Scheme 2.
Although base-free 5 is not observed by 1H NMR
spectroscopy during the thermal conversion of 2 to
dimer 6 (Scheme 1), its intermediacy is supported by
the fact that Ta(dNCtBudCHCtBudCH)(OAr)2(THF)
(5‚THF) can be cleanly converted into 6. Heating a
cherry-red solution of 5‚THF with Me3SiI results in the
decolorization of the solution and the formation of
insoluble red crystals, Scheme 2. The only product
observed in solution by 1H NMR is Me3SiOCH2CH2CH2-
CH2I, which arises from ring cleavage of the THF ligand
of 5‚THF.14 The insoluble red crystals were identified
as dimer 6 by elemental analysis and by comparison of
one crystal’s unit cell parameters to those of an authen-
tic sample of 6.8
Complex 5‚THF also reacts at room temperature with
electrophiles such as isocyanates and carbodiimides to
afford insertion products. Thus, reacting 5‚THF with
tBuNCO (C6H6 solution) provides a metallacyclic inser-
F igu r e 3. ORTEP drawing of the core structure of Ta-
(dNCtBudCHCtBudCH)(OAr)2(THF) (5‚THF), with atoms
tion product formulated as Ta[dNCtBudCHCtBudCHC-
shown as 50% probability ellipsoids.
(dNtBu)O](OAr)2 (7), shown in Scheme 2. This σ (η1)
insertion complex contrasts with the proposed π (η3)
insertion species formed upon reacting 5‚THF with
iPrNCNiPr, viz. Ta[dNCtBudCHCtBudCH(η3-C(dNi-
Pr)2)](OAr)2 (8), Scheme 2. This structural formulation
Sch em e 2
i
of 8 is proposed on the basis of the equivalent Pr groups
i
of the inserted PrNCNiPr moiety, indicating a complex
with a molecular plane of symmetry. Although this
observation is also consistent with a rapid equilibrium
between two σ(η1) insertion isomers (eq 1), this process
does not appear to be operative as judged by low-
1
temperature H NMR studies of 8 (toluene, -90 °C) in
which this equivalence is maintained.
coordinate structure. Generating base-free 5 in the
absence of THF provides a less-congested, four-coordi-
nate structure that cannot be isolated, but which rapidly
dimerizes to form 6.
The 1H and 13C NMR data of 7 and 8 are summarized
in Tables 1 and 2. Their formulations were assisted by
HETCOR and HMBC experiments, and the observed
connectivities from these methods are presented in
Table 6, based upon the numbering schemes shown in
Figure 4. The spectrum of 7 is very similar to that of
5‚THF without the THF resonances; the ring protons
in 7 appear at δ 6.71 and 6.61 as doublets (4J HH ) 1.3
Rea ctivity of Meta lla p yr id in e Ta (dNCtBu dCHC-
tBu dCH)(OAr )2(THF ) (5‚THF ). When a concentrated
pentane/pyridine solution of 5‚THF is cooled to -35 °C
for several hours, red samples of the bis(pyridine)
adduct Ta(dNCtBudCHCtBudCH)(OAr)2(py)2 (5‚py)
can be isolated in good yield, Scheme 2. The 1H and
13C NMR spectra of 5‚py reveal equivalent pyridine
ligands on the NMR time scale, suggesting a rapid
equilibrium involving the dissociation of one pyridine
t
Hz, C6D6), and there are now three Bu resonances for
the metallacycle. The HMBC experiment proved to be
invaluable for the conclusive assignment of 7 as an
eight-membered ring complex, Table 6. The HETCOR
experiment gave assignments for the ring protons C2H
ligand, i.e., Ta(dNCtBudCHCtBudCH)(OAr)2(py)2
a
t
and C4H as well as for the Bu groups and provided a
convenient starting point for the assignment of the
HMBC data.
Ta(dNCtBudCHCtBudCH)(OAr)2(py) + py. Note that
a static, Cs-symmetric, six-coordinate solution structure
with trans-pyridine ligands is ruled out on the basis of
Based upon the HMBC data, the resonance at δ 6.61
1
in the H NMR spectrum of 7 can be identified as C2H,
1
the equivalent aryl oxide ligands observed by H and
since it is coupled to C1, C4, and C8. Similarly, the
13C NMR. Pyridine exchange is confirmed by the single
pyridine environment observed in the 1H NMR spec-
trum of solutions of 5‚py in which free pyridine has been
(14) van der Heijden, H.; Schaverien, C. J .; Orpen, A. G. Organo-
metallics 1989, 8, 255.

Preparation and Properties of a Metallapyridine Complex
Organometallics, Vol. 17, No. 3, 1998 327
Ta ble 6. Su m m a r y of Obser ved NMR
Con n ectivities for
Sch em e 3
Ta [dNCtBu dCHCtBu dCHC(dNtBu )O](OAr )2 (7)
a n d Ta [dNCtBu dCHCtBu dCH(η3-C(dNiP r )2)](OAr )2
(8) w ith HETCOR a n d HMBC Meth od s
1H signal
HETCOR (13C)
HMBC (13C)
Ta[dNCtBudCHCtBudCHC(dNtBu)O](OAr)2 (7)
C2H
C4H
C7H
C9H
C11H
C2
C4
C7
C9
C11
C1, C4, C8a
C2, C5, C8, C10a
C6
C3, C8
C5, C10
Ta[dNCtBudCHCtBudCH(η3-C(dNiPr)2)](OAr)2 (8)
C2H
C4H
C6H
C7H
C9H
C11H
C2
C4
C6
C7
C9
C11
C1, C3, C4, C8
C2, C8, C10b
C1, C7
C6
C3, C8
alkylate the imido nitrogen with electrophiles such as
MeI were unsuccessful. Reacting 5‚THF with carbon
dioxide failed to provide any isolable products. At-
tempts to perform insertion reactions of 5‚THF with
2-butyne (C6D6, 80 °C, 10 min) afforded no isolable
organometallic species; however, the principal product
identified from this reaction (1H NMR and GC/mass
spectrometry) is 1,5-di-tert-butyl-2,3-dimethylbenzene,
Scheme 3. Similarly, the reactions of 5‚THF with
C5, C10
a
b
Cross-peak was not observed to C3. Cross-peaks were not
observed to C3 or C5.
t
tBuCtCH and BuCtN (C6D6, 80 °C, 1 h) afforded no
isolable organometallics, but 1,3,5-tri-tert-butylbenzene
and 2,4,6-tri-tert-butylpyridine, respectively, were major
products, Scheme 3. These intriguing results suggest
a Diels-Alder addition into the diene portion of Ta-
F igu r e 4. Numbering scheme for Ta[dNCtBudCHC-
(dNCtBudCHCtBudCH)(OAr)2(THF) (5‚THF) to gener-
ate these organic products. The metal-containing byprod-
uct in both of these reactions is nominally the nitride
species [Ta(N)(OAr)2]n; however, we have been unable
to trap such a species when these reactions are carried
out in the presence of Lewis bases.
tBudCHC(dNtBu)O](OAr)2 (7) and Ta[dNCtBudCHC-
tBudCH(η3-C(dNiPr)2)](OAr)2 (8) carbon nuclei used in
1
specifying H and 13C NMR assignments.
resonance at δ 6.71 can be identified as C4H by its
coupling with C2, C5, C8, and C10. Although no
coupling from either C2H or C4H to C3 is observed, both
C2H and C4H are coupled through C3 to C8 and C9H
is coupled to C3. With the central tBu group of the ring
Discu ssion
Although metallabenzenes such as (Et3P)3Ir-
t
identified, the other two Bu groups can now be as-
signed, since C11H is coupled to both C10 and C5 while
C7H is only coupled to C6, being attached to the
nitrogen derived from the isocyanate. Finally, we note
that these data do not distinguish between the Ta-O-
bonded η1 insertion product indicated in Scheme 2 and
(CHCMeCHCMeCH) have been prepared and structur-
ally characterized as delocalized,13,16 metallapyridine
5‚THF is significant in favoring the localized imido form
Ta(dNCtBudCHCtBudCH)(OAr)2(THF) rather than a
delocalized or carbene structure, even though the metal-
a Ta-N-bonded insertion species of the form Ta-
1
labenzene ring is planar. In addition, the H and 13C
NMR chemical shifts of 5‚THF and its Diels-Alder
[dNCtBudCHCtBudCHC(dO)NtBu](OAr)2. However,
the greater steric restrictions of the Ta-N σ insertion
derivative argues for the structure indicated in Scheme
2. Similar NMR assignments for complex 8 were made
on the basis of HETCOR and HMBC experiments.
We note that although metal imido functionalities are
well-known to engage in cycloaddition reactions with
isocyanates and carbodiimides,15 these reagents both
insert at the Ta-C bond rather than at the Ta-N imido
linkage. These insertions into a d0 metal-carbon bond
are analogous to the reverse of the olefin elimination
reaction in metallacycle 4, providing further support for
the intermediacy of 5 as shown in Scheme 1.
reactivity are indicative of a localized diene structure,
again in contrast to Bleeke’s iridabenzene (Et3P)3Ir-
(CHCMeCHCMeCH).13,16
We have demonstrated the formation of a monomeric
metallapyridine complex (5‚THF) from thermolysis of
the η2(N,C)-pyridine complex [η2(N,C)-2,4,6-NC5-
tBu3H2]Ta(OAr)2Me (2) in the presence of THF. Nascent
Ta(NCtBuCHCtBuCH)(OAr)2 (5) is generated by re-
moval of a THF ligand from 5‚THF, and its conversion
into dimeric [Ta(µ-NCtBudCHCtBudCH)(OAr)2]2 (6)
has been established. These observations complete the
Although the imido moiety in 5‚THF is strongly bent,
it appears to be only very weakly basic as attempts to
(16) Bleeke, J . R.; Xie, Y.-F.; Peng, W.-J .; Chiang, M. J . Am. Chem.
Soc. 1989, 111, 4118.
(15) Wigley, D. E. Prog. Inorg. Chem. 1994, 42, 239.

328 Organometallics, Vol. 17, No. 3, 1998
Weller et al.
systems indicated in Figures 1 and 4. All 2D spectra were
acquired at 25 °C without spinning the sample. NOESY and
natural abundance {13C-1H} heteronuclear multiple-bond
correlation (HMBC) spectra12 were acquired in the phase-
sensitive mode using time-proportional phase incrementation
(TPPI). The NOESY spectrum resulted from a 1024 × 4096
data matrix with 8 scans per t1 value. The delay time between
scans was 1 s, and the total measurement time was 9.2 h. The
HMBC spectra resulted from 512 × 2048 data matrices with
16 scans per t1 value and a delay time between scans of 1 s.
The delay to allow long-range heteronuclear antiphase mag-
netization to develop for multiple-bond correlations was 60 ms.
The total acquisition time was 4.1 h/spectrum. The HETCOR
spectrum resulted from a 512 × 4096 data matrix with 16
scans per t1 value. The delay time between scans was 1 s,
and the total measurement time was 3.2 h. Infrared spectra
were recorded on a Nicolet 510P FTIR spectrometer in C6H6
solutions in a sealed CsI cell and were used as fingerprints.
Electron ionization mass spectra (70 eV) were recorded to m/ z
) 999 on a Hewlett Packard 5970 mass selective detector and
RTE-6/VM data system. For GC/mass spectra, the sample was
introduced into the mass spectrometer by a Hewlett Packard
model 5890 gas chromatograph equipped with an HP-5
column. Microanalytical samples were handled under nitrogen
and were combusted with WO3 (Desert Analytics, Tucson, AZ).
reaction sequence in Scheme 1, delineate one process
by which heterocyclic C-N bonds are cleaved, and offer
additional insight into how nitrogen heterocycles may
be further degraded after C-N bond cleavage. Thus,
in cases where a highly substituted metallacycle arises
from pyridine ring opening, subsequent heterocycle
degradation pathways exist. This information may be
relevant to catalytic HDN since under normal HDN
conditions ethane, ethylene, propane, and propylene are
the principal products of pyridine HDN with only a
minor fraction of C5 products being generated.17 We
note that Wolczanski and co-workers have uncovered
very different mechanisms by which C-N bonds may
be cleaved in nitrogen heterocycles18 and anilines.19
In the metallaaziridine description of complex 2,20,21
the C-N scission reaction in Scheme 1 transforms a
formal amido nitrogen in the η2(N,C)-pyridine to a
formal imido nitrogen in the ring-opened structure. This
reaction appears to be driven largely by the formation
of a strong metal-ligand multiple bond in 3. It now
seems likely that this strong TadN bond is maintained
through compounds 4 and 5 (neither 4 nor base-free 5
is structurally characterized) until dimerization to form
6 occurs. In this system, tantalum nitrido species may
constitute the ultimate fate of the pyridine nitrogen.
Studies of these processes are continuing.
P r ep a r a tion s. Ta (dNCtBu dCHCtBu dCH)(OAr )2(THF )
(5‚THF ). An ampule (Teflon stopcock) was charged with
[η2(N,C)-2,4,6-NC5 Bu3H2]Ta(OAr)2Me (450 mg, 0.564 mmol),
t
8 mL of toluene, and 450 µL of THF. The stopcock was sealed,
and the resulting orange solution was heated in an oil bath
maintained at 110 °C for 4 days, over which time the solution
developed a cherry-red color. The solution was allowed to cool,
the reaction volatiles were removed in vacuo, and the resulting
red solid was crystallized from a minimal amount of pentane
Exp er im en ta l Section
Gen er a l Deta ils. All manipulations were performed under
a nitrogen atmosphere either by standard Schlenk techniques22
or in a Vacuum Atmospheres HE-493 drybox at room temper-
ature (unless otherwise indicated). Solvents were distilled
under N2 from an appropriate drying agent23 and were
transferred to the drybox without exposure to air. NMR
solvents were passed down a short (5-6 cm) column of
activated alumina prior to use. Thermolyses were typically
conducted in sealed NMR tubes in an oil bath maintained at
the specified temperature. In all preparations Ar ) 2,6-C6H3-
iPr2.
at -35 °C to afford 247 mg (0.314 mmol, 56%) of Ta-
(dNCtBudCHCtBudCH)(OAr)2(THF) as red crystals. Anal.
Calcd for C40H62NO3Ta: C, 61.13; H, 7.95; N, 1.78. Found:
C, 60.89; H, 7.96; N, 1.78.
Ta (dNCtBu dCHCtBu dCH)(OAr )2(p y)2 (5‚p y). The bis-
(pyridine) adduct Ta(dNCtBudCHCtBudCH)(OAr)2(py)2 (5‚
py) is prepared in a manner analogous to that described above
Sta r tin g Ma ter ia ls. [η2(N,C)-2,4,6-NC5 Bu3H2]Ta(OAr)2-
t
Me (2) was prepared from [η2(N,C)-2,4,6-NC5 Bu3H2]Ta(OAr)2-
t
Cl (1) as previously described.8 Trimethylsilyl iodide, methyl
for Ta(dNCtBudCHCtBudCH)(OAr)2(THF) (5‚THF), except
the recrystallization is carried out in pentane with the addition
of several drops of pyridine. This procedure affords 5‚py in
74% yield. Anal. Calcd for C46H64N3O2Ta: C, 63.36; H, 7.40;
N, 4.82. Found: C, 62.98; H, 7.63; N, 4.54.
iodide, and diisopropylcarbodiimide were obtained from Ald-
t
rich and used as received. Isocyanate BuNCO was obtained
from Aldrich and distilled prior to use. Carbon dioxide was
purified by passage through a column of activated 4-Å molec-
ular sieves and activated Ridox catalyst (supported Cu) prior
to use.
[Ta (µ-NCtBu dCHCtBu dCH)(OAr )2]2 (6). Ta(dNCtBud
P h ysica l Mea su r em en ts. 1H (250 and 300 MHz) and 13C
(62.9 and 75.4 MHz) NMR spectra were recorded at probe
temperature (unless otherwise specified) on a Bruker AM-250
or Varian Unity 300 spectrometer in C6D6 solvent. Chemical
shifts are referenced to protio impurities (δ 7.15) or solvent
13C resonances (δ 128.0) and are reported downfield of Me4Si.
Spectral assignments are based upon the ring numbering
CHCtBudCH)(OAr)2(THF) (5‚THF) (32 mg, 0.041 mmol) was
dissolved in 0.5 mL of C6D6, trimethylsilyl iodide (5.8 µL, 0.041
mmol) was added neat, and the red solution was sealed in a
5-mm NMR tube. The solution was heated at 80 °C for 12 h,
over which time large, well-formed, insoluble red crystals
formed at the bottom of the tube. The solution was cooled to
room temperature, and 1H NMR examination of the nearly
decolorized solution showed the complete disappearance of
(17) Choi, J .-G.; Brenner, J . R.; Colling, C. W.; Demczyk, B. G.;
Dunning, J . L.; Thompson, L. T. Catal. Today 1992, 15, 201.
(18) Kleckley, T. S.; Bennett, J . L.; Wolczanski, P. T.; Lobkovsky,
E. B. J . Am. Chem. Soc. 1997, 119, 247.
(19) Bonanno, J . B.; Henry, T. P.; Neithamer, D. R.; Wolczanski, P.
T.; Lobkovsky, E. B. J . Am. Chem. Soc. 1996, 118, 5132.
(20) Durfee, L. D.; Fanwick, P. E.; Rothwell, I. P.; Folting, K.;
Huffman, J . C. J . Am. Chem. Soc. 1987, 109, 4720.
(21) Mayer, J . M.; Curtis, C. J .; Bercaw, J . E. J . Am. Chem. Soc.
1983, 105, 2651.
(22) Shriver, D. F.; Drezdzon, M. A. The Manipulation of Air-
Sensitive Compounds, 2nd ed.; J ohn Wiley and Sons: New York, 1986.
(23) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.
resonances attributable to Ta(dNCtBudCHCtBudCH)(OAr)2-
(THF) (5‚THF) and the appearance of resonances due to the
formation of Me3SiOCH2CH2CH2CH2I.14 The tube was broken
open, and the crystals were collected by filtration and dried
in vacuo. The identity of these crystals as [Ta(µ-NCtBud
CHCtBudCH)(OAr)2]2 (6) was confirmed by a comparison of
one crystal’s unit cell parameters to those of an authentic
sample of 6.8 Anal. Calcd for C72H108N2O4Ta2: C, 60.58; H,
7.63; N, 1.96. Found: C, 60.81; H, 7.82; N, 1.94.

Preparation and Properties of a Metallapyridine Complex
Organometallics, Vol. 17, No. 3, 1998 329
crystal of C40H62NO3Ta was crystallized from concentrated
pentane solution (-35 °C) and was mounted on a glass fiber
in a random orientation. There were no systematic absences;
the space group was determined to be P1 (No. 2). Hydrogen
atoms were not located but were added at calculated positions;
subsequently they were included in the structure factor
calculations, but their positions were not refined. Scattering
factors were taken from Cromer and Waber.24 Anomalous
dispersion effects were included in Fc.25 The values for ∆f′ and
∆f′′ were those of Cromer.26 All calculations were performed
on a VAX computer using MolEN.27 Details of the structural
determination and refinement are reported in Table 4.
Ta [dNCtBu dCHCtBu dCHC(dNtBu )O](OAr )2 (7). Ta-
(dNCtBudCHCtBudCH)(OAr)2(THF) (5‚THF) (200 mg, 0.255
t
mmol) was dissolved in 10 mL of C6H6, and neat BuNCO (30
µL, 0.262 mmol) was added at room temperature. The solution
color immediately changed from red to yellow. This mixture
was stirred for 10 min, the reaction volatiles were removed in
vacuo, and the resulting yellow solid was crystallized from
toluene/pentane at -35 °C to afford 138 mg (0.170 mmol, 67%)
of Ta[dNCtBudCHCtBudCHC(dNtBu)O](OAr)2 as yellow crys-
tals. IR (C6H6 solution, cm-1): 3092 s, 3072 s, 3038 s, 2964
m, 2324 w, 2210 w, 1961 s, 1817 s, 1601 m, 1529 w, 1479 s,
1435 m, 1395 w, 1329 w, 1257 w, 1199 w, 1037 m, 908 w, 750
w, 675 s. Anal. Calcd for C41H63N2O3Ta: C, 60.58; H, 7.81;
N, 3.45. Found: C, 60.56; H, 8.00; N, 3.20.
Ack n ow led gm en t. Thanks is given to the Division
of Chemical Sciences, Office of Basic Energy Sciences,
Office of Energy Research, U.S. Department of Energy
(DE-FG03-93ER14349), for support of this research.
Ta [dNCt Bu dCH Ct Bu dCH (η3-C(dNiP r )2)](OAr )2 (8).
Ta(dNCtBudCHCtBudCH)(OAr)2(THF) (5‚THF) (200 mg, 0.255
i
mmol) was dissolved in 10 mL of C6H6, and neat PrNCNiPr
Su p p or tin g In for m a tion Ava ila ble: Complete crystal-
lographic details, including tables of atomic positional and
thermal parameters, bond distances and angles, least-squares
(40 µL, 0.255 mmol) was added at room temperature. The
solution color immediately changed from red to yellow. This
mixture was stirred for 5 min; the reaction volatiles were
removed in vacuo to afford a yellow oil. This oil was dissolved
in pentane and the solvent removed in vacuo to form a yellow
solid that was crystallized from pentane at -35 °C to provide
130 mg (0.155 mmol, 61%) of Ta[dNCtBudCHCtBudCH(η3-
C(dNiPr)2)](OAr)2. IR (C6H6 solution, cm-1): 3080 s, 3071 s,
3036 s, 2963 w, 2359 m, 2324 m, 1961 s, 1817 s, 1529 w, 1479
s, 1435 w, 1394 w, 1332 w, 1259 w, 1207 w, 1037 s, 671 s.
Anal. Calcd for C43H68N3O2Ta: C, 61.49; H, 8.16; N, 5.00.
Found: C, 61.84; H, 8.46; N, 4.76.
planes, dihedral angles, and ORTEP figures for Ta(dNC-
tBudCHCtBudCH)(OAr)2(THF) (5‚THF) (27 pages). Ordering
information is given on any current masthead page.
OM970749R
(24) Cromer, D. T.; Waber, J . T. International Tables for X-Ray
Crystallography; The Kynoch Press: Birmingham, England, 1974; Vol.
IV, Table 2.2B.
(25) Ibers, J . A.; Hamilton, W. C. Acta Crystallogr. 1964, 7, 781.
(26) Cromer, D. T. International Tables for X-Ray Crystallography;
The Kynoch Press: Birmingham, England, 1974; Vol. IV, Table 2.3.1.
(27) MolEN: An Interactive Structure Solution Procedure;
Enraf-Nonius: Delft, The Netherlands, 1990.
X-r a y Str u ctu r a l Deter m in a tion of Ta (dNCtBu dCHC-
tBu dCH)(OAr )2(THF ) (5‚THF ). An orange, thin, plate