Metathesis of nitrogen atoms within triple bonds involving carbon, tungsten, and molybdenum

Article abstract of DOI:10.1021/ic8003917

(Bu^tO)₃Mo≡N and W₂(OBu ^t)₆(M≡M) react in hydrocarbons to form Mo ₂(OBu^t)₆(M≡M) and (Bu^tO) ₃W≡N via the reactive intermediate MoW(OBu^t) ₆(M≡M). (Bu^tO)₃W≡N and CH ₃C≡N¹⁵ react in tetrahydrofuran (THF) at room temperature to give an equilibrium mixture involving (Bu^tO) ₃W≡N¹⁵ and CH₃C≡N. The (Bu ^tO)₃W≡N compound is similarly shown to act as a catalyst for N¹⁵-atom scrambling between MeC¹³≡ N¹⁵ and PhC≡N to give a mixture of MeC¹³≡N and PhC≡N¹⁵. From studies of degenerate scrambling of N atoms involving (Bu^tO)₃W≡N and MeC¹³≡N in THF-d₈ by ¹³C{¹H} NMR spectroscopy, the reaction was found to be first order in acetonitrile and the activation parameters were estimated to be ΔH^? = 13.4(7) kcal/mol and ΔS^? = -32(2) eu. A similar reaction is observed for (Bu^tO)₃Mo≡N and CH₃C≡N¹⁵ upon heating in THF-d₈. The reaction is suppressed in pyridine solutions and not observed for the dimeric [(Bu^tMe ₂SiO)₃W≡N]₂. The reaction pathway has been investigated by calculations employing density functional theory on the model compounds (MeO)₃M≡N and CH₃C≡N where M = Mo and W. The transition state was found to involve a product of the 2 + 2 cycloaddition of M≡N and C≡N, a planar metalladiazacyclobutadiene. This resembles the pathway calculated for alkyne metathesis involving (MeO) ₃W≡CMe, which modeled the metathesis of (Bu^tO) ₃W≡CBu^t. The calculations also predict that the energy of the transition state is notably higher for M = Mo relative to M = W.

Full text of DOI:10.1021/ic8003917

Inorg. Chem. 2008, 47, 5377-5385
Metathesis of Nitrogen Atoms within Triple Bonds Involving Carbon,
Tungsten, and Molybdenum
Beth A. Burroughs, Bruce E. Bursten,* Shentan Chen, Malcolm H. Chisholm,* and Andy R. Kidwell
Department of Chemistry, The Ohio State UniVersity, 100 West 18th AVenue,
Columbus, Ohio 43210
Received February 29, 2008
t
(OBut)
(OBut)
t
(
BuO)
reactive intermediate MoW(OBut)
temperature to give an equilibrium mixture involving (Bu O)
3
MotN and W
2
6
(MtM) react in hydrocarbons to form Mo
2
6
(MtM) and (BuO)
3
WtN via the
t
15
6
(MtM). (BuO) WtN and CH CtN react in tetrahydrofuran (THF) at room
3 3
t
15
t
3
WtN and CH
3
CtN. The (BuO)
3
WtN compound is
15
13
15
similarly shown to act as a catalyst for N -atom scrambling between MeC tN and PhCtN to give a mixture
13
15
t
13
of MeC tN and PhCtN . From studies of degenerate scrambling of N atoms involving (BuO)
3
WtN and MeC tN
13
1
in THF-d
8
by C{ H} NMR spectroscopy, the reaction was found to be first order in acetonitrile and the activation
q
q
parameters were estimated to be ∆H ) 13.4(7) kcal/mol and ∆S ) -32(2) eu. A similar reaction is observed
t
15
for (BuO)
3
MotN and CH
3
8
CtN upon heating in THF-d . The reaction is suppressed in pyridine solutions and not
t
observed for the dimeric [(BuMe
2
SiO)
3
WtN]
2
. The reaction pathway has been investigated by calculations employing
MtN and CH CtN where M ) Mo and W. The transition
state was found to involve a product of the 2 + 2 cycloaddition of MtN and CtN, a planar metalladiazacyclob-
utadiene. This resembles the pathway calculated for alkyne metathesis involving (MeO) WtCMe, which modeled
WtCBu. The calculations also predict that the energy of the transition state is notably
higher for M ) Mo relative to M ) W.
density functional theory on the model compounds (MeO)
3
3
3
t
t
the metathesis of (BuO)
3
4
Introduction
and co-workers involving the reactions between tungsten
trialkoxyalkylidynes and -alkynes. This reaction too has
entered into the tool kit of organic reagents. We describe
The evolution of olefin metathesis involving transition-
metal alkylidenes is one of the most fascinating and
preparatively useful stories in chemistry. Not long after the
5
here our findings of the metathesis reaction involving M-N
and C-N triple bonds where the metals are either W or Mo.
1
initial industrial discovery of olefin metathesis, Pennella et
6
A preliminary report on this work appeared in 2003, and
2
al. disclosed a seemingly even more miraculous tale of
subsequently related work was reported by Johnson in
triple-bond metathesis involving alkynes. In 1975, Katz and
7,8
2
005.
3
McGinnis addressed the mechanism of olefin metathesis
involving metal carbene intermediates and proposed that
alkyne metathesis proceeded via a similar mechanism
involving the 2 + 2 cycloaddition of CtC and MtC bonds
to give a planar metallacyclobutadiene intermediate. This was
soon thereafter demonstrated by the elegant work of Schrock
Results and Discussion
MotN + WtW f MotW + WtN. The reaction
t
9
t
t
10
between (Bu O)
3
MotN and (Bu O)
3
WtW(OBu )
3
pro-
ceeds quite rapidly in hydrocarbon solutions to give the
(
4) Wengrovius, J. H.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1981,
*
To whom correspondence should be addressed. E-mail: chisholm@
103, 3932.
chemistry.ohio-state.edu.
(5) Schrock, R. R. Chem. ReV. 2002, 102, 145.
(
1) (a) Grubbs, R. H. AdV. Synth. Catal. 2007, 349, 34. (b) Bielawski,
C. W.; Grubbs, R. H. Prog. Polym. Sci. 2007, 32, 1. (c) Grubbs, R. H.
Prix Nobel 2006, 194. (d) Schrock, R. R.; Czekelius, C. AdV. Synth.
Catal. 2007, 349, 55. (e) Schrock, R. R. AdV. Synth. Catal. 2007, 349,
(6) Chisholm, M. H.; Delbridge, E. E.; Kidwell, A. R.; Quinlan, K. R.
Chem. Commun. 2003, 126.
(7) Gdula, R. L.; Johnson, M. J. A.; Ockwig, N. W. Inorg. Chem. 2005,
44, 9140.
4
1. (f) Schrock, R. R. Prix Nobel 2006, 216.
(8) Gdula, R. L.; Johnson, M. J. A. J. Am. Chem. Soc. 2006, 128, 9614.
(9) Chan, D. M.-T.; Chisholm, M. H.; Folting, K.; Huffman, J. C.;
Marchant, N. S. Inorg. Chem. 1986, 25, 4170.
(
(
2) Pennella, F.; Banks, R. L.; Bailey, G. C. Chem. Commun. 1968, 1548.
3) Katz, T. J.; McGinnis, J. J. Am. Chem. Soc. 1975, 97, 1592.
1
0.1021/ic8003917 CCC: $40.75  2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 12, 2008 5377
Published on Web 05/06/2008

Burroughs et al.
t
mixed-metal MoW triply bonded complex MoW(OBu )
6
with
carbon solvent, the tungsten nitride is formed as a finely
divided microcrystalline product. The compound (Bu O) -
3
WtN is only sparingly soluble in solvents such as toluene
t
11
17
t
the formation of (Bu O)
3
WtN. The compound Mo-
can readily be identified in the mass spectrum by
t
W(OBu )
6
the presence of a molecular ion that shows the distinct
isotopic distribution predicted for a MoW-containing species.
It is also seen in the H NMR spectrum where two Bu
and in the solid state exists as an infinite polymeric chain
1
1
involving connectivity (f WtN f WtN f). Because
of poor solubility in hydrocarbons, our studies of N-atom
exchange were carried out in the more polar and coordinating
1
t
singlets are observed, as is expected for each of the three
alkoxide ligands bound to each metal. However, it has not
been possible to isolate this heteronuclear MM′-bonded
compound because this compound reacts further with
solvent tetrahydrofuran (THF). The molybdenum analogue
t
(
Bu O)
3
MotN adopts an analogous structure in the solid
state but is somewhat more soluble in hydrocarbon solvents
because of the weaker donor strength of the less polar MotN
bond.
t
t
(
Bu O)
3 2 6
MotN with the ultimate formation of Mo (OBu )
t
t
and (Bu O)
W(OBu )
3
WtN. The compounds Mo
2
(OBu )
6
, Mo-
t
t
6
, and W
2
(OBu )
6
show similar solubilities and
15
t
The reaction between MeCt N and (Bu O)
3
WtN in
almost certainly cocrystallize. The driving force for this
reaction rests with the favorable redox reaction involving
15
THF-d
8
was followed by N NMR. The signal at δ ∼ 248
ppm decreased in intensity as the signal due to
VI
III
III
VI
the two metals, Mo + W f Mo + W , and almost
certainly proceeds by way of an activated trinuclear cluster
involving a triply bridging nitride. Structural precedent for
t
15
(
Bu O)
3
Wt N grew in intensity at δ ∼ 732 ppm. A similar
t
reaction was carried out employing (Bu O)
3
MotN in THF-
8
d , but no reaction was observed at room temperature.
this exists in the form of the tetranuclear cluster Mo
4
(µ
(µ
3
-
-
However, heating the solution to 70 °C did bring about a
i
12
13
N)
2
(OPr )12 and in related M
3
(µ
3
-O)(OR)10
,
MoW
2
3
t
15
3
similar reaction with the formation of (Bu O) Mot N.
1
3
14
O)(OR)10
,
and W
(µ -P)(OR) clusters.
3 3 9
The tungsten nitride complex was then used as a catalyst
for the N-atom exchange as shown in reaction 4. The
AlkylidyneandNitrideLigandTransferbetweenMoly-
bdenum and Tungsten. Although the triple bonds between
the metals Mo and W and between C and N are estimated
1
5
reactions were again monitored by N NMR spectroscopy.
cat.
1
5,16
1
5
15
to be ∼120-155 kcal/mol,
they are kinetically labile
MeCt N + RCtN y z MeCtN + RCt N
(4)
and readily transferable between metal centers as shown in
reactions 1–3, where L ) Bu O.
t
A broad range of nitriles was chosen both to test the
13
generality of the N-atom exchange and to test the tolerance
L Wt CMe + L MotCPh h L WtCPh +
3
3
3
t
of (Bu O)
3
WtN toward various functional groups. For the
F, -C Cl, (CH Cl,
C, and -CHdCHCN scram-
1
3
L Mot CMe (1)
3
groups where R ) ortho and p-C
Me C, Ph, PhCH , Ph CH, Ph
H
6 4
H
6 4
2 4
)
13
13
L Wt CMe + L MotN h L WtN + L Mot CMe
3
2
2
3
3
3
3
3
bling of the label was observed. The reaction is quite tolerant
to steric factors and carbon-halogen as well as CdC bonds.
Rather interestingly, the o-fluorophenyl nitrile showed
(2)
15
15
L Wt N + L MotN h L WtN + L Mot N (3)
3
3
3
3
15
19
N- F coupling, with its signal at δ ) 271.7 ppm appearing
These reactions can be monitored by NMR spectroscopy in
as a doublet (see Figure 1). At this point, it is worth noting
that Johnson has recently demonstrated that alkynes will react
with fluorinated alkoxide molybdenum and tungsten nitrides
1
83
1
solvents such as toluene-d
8
2
. The presence of W, I ) / ,
which occurs in ∼15% natural abundance aids in the
1
3
15
1
detection of C and N nuclei by the presence of J183
W
8
to yield the respective alkylidynes and nitriles.
satellites.
In order to establish that in reaction 4 the metathesis
involved solely N-atom exchange and not a cyano group
These observations led us to interrogate the metathesis
reactions involving MtN and CtN described below.
13
exchange, we also examined the reaction between Me CdN,
WtN* + RCtN h WtN + RCtN*
15
MeCt N, and PhCtN. As shown in Figure 2, the exchange
t
15
t
The compound (Bu O)
reaction between W
3
Wt N is readily prepared from the
catalyzed by (Bu O)
3
WtN involved exclusively WtN and
t
(OBu )
6
and labeled acetonitrile
CtN metathesis.
2
1
5
CH
3
Ct N, and if this reaction is carried out in a hydro-
The exchange reactions noted above do not occur in
pyridine-d , which is a good donor to tungsten and presum-
ably blocks coordination of the weaker nitrile donor. Also
5
(
10) (a) Chisholm, M. H.; Extine, M. W. J. Am. Chem. Soc. 1975, 97,
625. (b) Chisholm, M. H.; Gallucci, J. C.; Hollandsworth, C. B.
5
t
2 3
the siloxide-bridged dimeric compound [(Bu Me SiO) -
Polyhedron 2006, 25, 827.
1
8
15
(
11) Chisholm, M. H.; Hoffman, D. M.; Huffman, J. C. Inorg. Chem. 1983,
WtN]
does not react with MeCt N.
2
2
2, 2903.
(
12) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Leonelli, J.; Marchant,
N. S.; Smith, C. A.; Taylor, L. C. E. J. Am. Chem. Soc. 1985, 107,
(16) Cherry, J.-P. F.; Johnson, A. R.; Baraldo, L. M.; Tsai, Y.-C.; Cummins,
C. C.; Kryatov, S. V.; Rybak-Akimova, E. V.; Capps, K. B.; Hoff,
C. D.; Harr, C. M.; Nolan, S. P. J. Am. Chem. Soc. 2001, 123, 7271.
(17) Weingrovius, J. H.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc.
1981, 103, 3932.
3
722.
(
(
(
13) Chisholm, M. H.; Folting, K.; Huffman, J. C.; Kober, E. M. Inorg.
Chem. 1985, 24, 241.
14) Chisholm, M. H.; Folting, K.; Pasterczyk, J. W. Inorg. Chem. 1988,
2
7, 3057.
(18) Chisholm, M. H.; Folting, K.; Lynn, M. L.; Tiedtke, D. B.; Lemiogno,
F.; Eisenstein, O. Chem.sEur. J. 1999, 5, 2318.
15) Chisholm, M. H.; Davidson, E. R.; Quinlan, K. B. J. Am. Chem. Soc.
002, 124, 15351.
2
(19) Parkin, I. P.; Folting, K. J. Chem. Soc., Dalton Trans. 1992, 2343.
5378 Inorganic Chemistry, Vol. 47, No. 12, 2008

Metathesis of Nitrogen Atoms within Triple Bonds
Eyring plot shown in Figure 3. Based on this, an estimation
q
of the activation parameters can be made: ∆H ) +13.4(7)
q
kcal/mol and ∆S ) -32(2) eu.
Although the alkyne metathesis reaction has been well
explored in terms of its synthetic utility and reaction pathway,
to our knowledge, the kinetics and activation parameters have
only been determined for a related reaction involving the
exchange of alkyne units and the metallacyclobutadiene
n
complex, (DIPP)
)
3
W(C
3
R
3
), where R ) Et and Pr and DIPP
2
0
2,6-diisopropyl phenoxide, with 3-hexyne-d10 (see
n
reaction 6, where R ) Et and Pr and R′ ) CD
2 3
CD ).
(
DIPP) W(C R ) + R′CtCR′ f (DIPP) W(C RR′ ) +
3
3
3
3
3
2
RCtCR (6)
Figure 1. 15N NMR spectra of Cl(CH2)4Ct N (B), p-Cl(C6H4)Ct N
15
15
1
5
For R ) Et, a variable-temperature study led to the
(
(
C), and o-F(C6H4)Ct N (D), byproducts of a scrambling reaction with
t
15
q
q
BuO)3WtN and MeCt N (A).
determination of ∆H ) +26.1(4) kcal/mol and ∆S )
q
n
The chiral menthoxide complexes, both [(+)-men-
+15.2(15) eu, while for R ) Pr , ∆H ) +25.4(5) kcal/mol
q
20
1
9
19
O]
3
WtN and [(-)-men-O]
3
WtN , were prepared and
and ∆S ) +16.3(16) eu. The reaction was also shown to
be first order in tungsten and independent of 3-hexyne-d10
in the concentration range studied. Thus, the activation
parameters reported by Schrock and co-workers pertain to a
rate-limiting elimination of RCtCR from the metallacy-
clobutadiene, whereas in our study, the parameters most
likely reflect an associative 2 + 2 reaction. Qualitatively,
we can see that this is reflected in the positive entropy of
1
5
shown to catalyze exchange of N between MeCt N and
the bulky enantiomerically pure (S)-2-(6-methoxy-2-napth-
yl)propionitrile (see reaction 5). This raises the possibility
of enantioselective N-atom exchange, but we have not
investigated this further at this time.
q
activation for reaction 6 and the negative ∆S value for the
N-atom metathesis reaction.
Theoretical and Computational Considerations. Fol-
2
1
lowing the initial theoretical considerations of Bursten in
983, considerable attention has been given to the relative
1
stabilities and bonding in metallacyclobutadiene and metal-
latetrahedrane complexes and their role in the alkyne
2
2–24
metathesis reaction.
This work has led up to the most
2
5
recent 2006 publication dealing with the total reaction
pathway for alkyne metathesis by molybdenum and tungsten
(
3
RO) MtCMe complexes as determined by density func-
Estimation of Activation Parameters. The atom ex-
change reaction proceeds at convenient rates under ambient
conditions to allow its study by a variety of conventional
techniques. However, the specifics of the system proved
tional theory employing B3LYP and the effective core
potentials of Hay and Walt with a double-ꢀ valence basis
set including one f polarization function for Mo and W
atoms.
We have employed density functional theory using ex-
change and gradient-corrected correlation functional PW91
to investigate the reaction pathway leading to the N-atom
metathesis reactions described above. Two kinds of basis
set systems, BS-I (LanL2DZ basis sets augmented by an f
polarization function for metal atoms and the 6-31G* basis
sets for nonmetal atoms) and BS-II (SDD basis sets for metal
atoms and cc-pVTZ basis sets for nonmetal atoms), were
used to study the potential energy surfaces of the metathesis
problematic. For example, the limited solubility and the high
t
air sensitivity of (Bu O)
3
WtN made following the course
of the reaction by react-IR unreliable. Similarly, mass
+
spectrometry was complicated by relative intensities of RCN
and RCNH ions due to experimental conditions, and
+
15
N
NMR required too long acquisition times. We finally resorted
1
3
to the use of C NMR spectroscopy to follow the reaction
1
3
t
15
13
between Me CtN and (Bu O)
3
Wt N. The C NMR signal
1
3
for Me CtN appears as a singlet at δ ) 116.96 ppm and
upon N-atom exchange coupling to N, I ) / , leads to a
2
1
5
15
1
reactions between (MeO) MtN (where M ) W, Mo) and
MeCtN. The reaction pathways found by employing a larger
basis set system, BS-II, are similar to those found by using
3
doublet at δ ) 117.01 and 116.77 ppm. The line-shape
analysis program in XWIN-NMR makes it possible to
1
3
extrapolate relative concentrations of Me CtN and
1
3
15
Me Ct N with time and, hence, obtain data for kinetics.
t
15
(20) Chirchill, M. R.; Ziller, J. W.; Freudenberger, J. H.; Schrock, R. R.
3
By keeping the concentration of (Bu O) Wt N constant and
Organometallics 1984, 3, 1554.
varying the concentration of nitrile, it was determined that
the reaction is first order in nitrile (see the Experimental
Section). A variable-temperature study (20-50 °C) following
(
(
(
(
21) Bursten, B. E. J. Am. Chem. Soc. 1983, 105, 121.
22) Woo, J.; Folga, E.; Ziegler, T. Organometallics 1993, 12, 1289.
23) Lin, Z.; Hall, M. B. Organometallics 1994, 12, 2878.
24) Sheng, Y.-H.; Wu, Y.-D. J. Am. Chem. Soc. 2001, 123, 6662.
1
3
the decay of Me CtN allowed for the production of the
(25) Guochen, J. Z.; Lin, Z. Organometallics 2006, 25, 1812.
Inorganic Chemistry, Vol. 47, No. 12, 2008 5379

Burroughs et al.
Figure 2. 15N NMR spectra of the reaction between Me CtN, MeCt N, and PhCtN in the presence of a trace of (Bu O)3WtN recorded in THF-d8 at
13
15
t
1
5
15
1
13
2
98 K, 50.6 MHz. The PhCt N signal shows enhancement due to N-atom exchange and appears as a singlet due to lack of coupling to H or C, whereas
the MeCtN N signal shows coupling to H, J H- N ) 1.7 Hz, and for Me Ct N coupling to C, J C- N ) 17 Hz. The signal thus appears as
a central 1:3:3:1 quartet flanked by C satellites. The unsymmetrical nature of the C satellites arises from C/ C isotopic chemical shift perturbation.
1
5
1
3
1
15
13
15
13
1
13
15
1
3
13
12 13
2
q
Figure 3. Eyring plot of ln(kobs/T) vs 1/T where y ) -6727x + 7.7142 and R ) 0.9937. The enthalpy parameters were calculated to be ∆H ) +13.4(7)
q
15
t
15
13
kcal/mol and ∆S ) -32(2) eu for the N isotope exchange reaction between ( BuO)3W N and Me CN.
BS-I. For both basis set systems, the same number of
stationary points with similar structures on the reaction
pathway was found for each reaction. The slightly higher
activation free energies obtained by using BS-II than by using
BS-I are mainly due to the lack of an f polarization function
for metal atoms in BS-II. This is evident by comparing the
activation free energies for the reaction between
(MeO) WtN and MeCtN obtained from BS-I, BS-II, BS-
III (LanL2DZ for metal atoms and 6-31G* for nonmetal
atoms), and BS-IV (SDD for metal atoms and 6-31G* for
3
5380 Inorganic Chemistry, Vol. 47, No. 12, 2008

Metathesis of Nitrogen Atoms within Triple Bonds
Figure 4. Free energy (at 298 K) profiles for the N-atom exchange reaction between (MeO)3WtN and MeCtN and structures of transition states and
intermediates with selected calculated structural parameters (Å).
nonmetal atoms). The activation free energy is defined by
the difference of free energies between the high-lying
transition structure (TS1A) and the starting materials. The
calculated activation free energies from BS-I to BS-IV are
by using BS-I is closest to the experimental value. On
the basis of the above facts, we presented our results
calculated by using BS-I.
The calculated free energy profiles for the N-atom
25.9, 31.7, 29.4, and 29.3 kcal/mol, respectively. The almost
3
exchange reaction between (MeO) MtN and MeCtN and
same values based on BS-III and BS-IV indicate that the
different effective core potentials seem to have no effect
on the activation free energy for this calculation. Employ-
ment of an the f function brings down the energy by about
structures of transition states and intermediates with selected
calculated structural parameters (Å) for W and Mo using
BS-I are shown in Figures 4 and 5, respectively.
3
The overall reaction between (MeO) WtN and MeCtN
3
.5 kcal/mol, while employment of the triple basis sets
involves N-atom exchange and a conformation change. The
N-atom exchange is endothermic (∆G ) 4.6 kcal/mol, which
is principally caused by the presence of the different
cc-pVTZ for nonmetal atoms slightly increases the activa-
tion free energy but does not have a significant effect on
the reaction pathway. The calculated activation free energy
3
conformations of (MeO) WtN in RA and PA′). In this step,
Inorganic Chemistry, Vol. 47, No. 12, 2008 5381

Burroughs et al.
Figure 5. Free energy (at 298 K) profiles for the N-atom exchange reaction between (MeO)3MotN and MeCtN and structures of transition states and
intermediates with selected calculated structural parameters (Å).
two transition states, Ts1A and Ts2A, are located. The
higher-lying transition state Ts1A connects minima Int1A
and Int2A. The intermediate Int2A is transformed into
another intermediate Int3A through transition state Ts2A.
However, we shall not overemphasize the existence of
intermediate Int2A and Ts2A because the barrier for
transformation from Int2A to Int3A is only about 0.6 kcal/
mol and the analogous intermediate and transition state were
not found in the reaction pathway involving the molybdenum
analogue. Dissociation of Int1A and Int3A, leading to the
formation of separate tungsten nitride and acetonitrile, can
occur without barriers. Int1A has a free energy of 3.3 kcal/
mol, which is essentially caused by the loss of entropy
associated with the interaction between (MeO) WtN and
3
MeCtN. The higher free energy of Int3A than that of PA′
is also caused by such a loss of entropy. As can be seen in
3
Figure 4, different conformations of (MeO) WtN are present
in the product of the N-atom exchange step, PA′, and the
reactant, RA. The conformer A′ in PA′ is calculated to be
higher in free energy than conformer A in RA by 4.6 kcal/
mol, which accounts for the difference of the free energies
between RA and PA′. A′ can be readily transformed into
5382 Inorganic Chemistry, Vol. 47, No. 12, 2008

Metathesis of Nitrogen Atoms within Triple Bonds
t
30
the more stable conformer A by crossing an energy barrier
of about 3 kcal/mol. A similar metathesis pathway was
achieved. Interestingly, the compound ( BuO)
3
CrN does not
appear to entertain this type of chemistry, though in reactions
t
calculated for the reaction between (MeO)
3
MotN and
with M
2 6
(O Bu) , redox chemistry is observed with the
t
31
MeCtN except that neither the analogue of Int2A nor the
analogue of TS2A was found in this pathway. The transition
state Ts1B connects minima Int1B and Int2B and further
formation of the blue volatile compound Cr(O Bu)
4
along
3
MN as identified by their
t
with the compounds ( BuO)
molecular ions in the mass spectrometer.
3
dissociates to form (MeO) MotN and MeCtN. Similar
conformation transfer from B′ to B was also observed.
The transition state Ts1A was calculated to be higher in
free energy than the starting materials by 25.9 kcal/mol
Experimental Section
General Procedures. All operations were carried out under an
inert atmosphere of argon with dry and deoxygenated solvents using
standard Schlenk-line and drybox procedures. All chemicals were
purchased from commercial sources and were used as received
unless duly noted. Aromatic and aliphatic hydrocarbon solvents
were dried over either sodium or potassium, and THF and diethyl
(
about 12 kcal/mol in enthalpy). This calculated free-energy
barrier for the N-atom exchange reaction between
MeO) WtN and MeCtN is in good agreement with the
experimental observations, while for the reaction involving
MeO) MotN and MeCtN, the transition state Ts1B was
(
3
ether were distilled from sodium benzophenone. CH
3
CN was dried
(
3
over CaH and distilled prior to use. All distilled solvents were
2
calculated to be higher in free energy than the starting
materials by 39.2 kcal/mol, which is about 13 kcal/mol higher
than that for the tungsten analogue. This result is also
stored over 4 Å molecular sieves. The nitriles employed in the
N-atom exchange reactions were stored over 4 Å sieves and were
subjected to three freeze-thaw-degas cycles prior to use. Deu-
consistent with the experimental observation that N-atom
13 15
terated solvents and C/ N-labeled isotopically enriched com-
t
exchange between (Bu O)
at elevated temperatures.
3
MotN and MeCtN only occurs
pounds were obtained from Cambridge Isotope Laboratories, Inc.,
and were similarly dried and degassed prior to use.
2 6
Syntheses. The compounds W (OR)
, where R ) OBut, (+)-
10
The calculated pathways leading to N-atom metathesis and
alkyne metathesis bear the similarity that reactions proceed
through 2 + 2 cycloaddition but that differences exist
between them. In both types of metathesis reactions, the ring-
closing step is the rate-determining step. However, no metal
alkyne adduct can be located along the pathway for alkyne
1
9
19
menthyl, and (-)-menthyl were prepared by alcoholysis reac-
3
2
t
30
t
9
tions involving W
2
(NMe
2 3 3
), (Bu O) CrN, (Bu O) MoN, and
t
33
3
(Bu O) MoCPh, were prepared according to literature procedures.
1
19
13
1
15
Spectroscopic Methods. H, F, C{ H}, and N NMR spectra
were recorded on Bruker Avance 400 and 500 MHz spectrometers.
IR spectra were recorded on a Perkin-Elmer GX FT-IR spectro-
photometer as KBr pellets. Mass spectra were recorded on a Kratos
MS25 RFA double-focusing magnetic sector mass spectrometer as
positive ions.
0
metathesis, and this was attributed to the fact that the d metal
center cannot stabilize an alkyne complex by back-donation.
Metal nitrile complexes (i.e., Int1A and Int1B) were found
along the reaction coordinates for the N-atom exchange
reactions, and the formation of metal nitrile complexes is
probably due to the dipole-dipole interactions and weak van
der Waals interactions between the metal nitride fragment
and nitrile because the distance between the two fragments
is fairly large and both fragments are almost unperturbed.
Density functional theory may have a problem handling such
interactions, and a higher level of theoretical considerations
might be needed to study these weakly associated complexes.
Computational Methods. All calculations were performed using
density functional theory as implemented in the Gaussian 03 suite
3
4
35
of programs. The PW91PW91 [Perdew and Wang’s 1991
exchange and gradient-corrected correlation functional] density
functional theory was used for all calculations. Two types of basis
set systems, BS-I and BS-II were used for potential energy surface
3
6
calculations. In BS-I, the LanL2DZ basis sets augmented by an
f polarization function [ꢀ(f) ) 0.823, W; 1.043, Mo] were used
for transition-metal atoms and the 6-31G* basis sets were used
for all other nonmetal atoms in the model compounds. In BS-II,
3
7
3
8
(
30) (a) Chiu, H.-T.; Chen, Y.-P.; Chuang, S.-H.; Jen, J.-S.; Lee, G.-H.;
Peng, S.-M. Chem. Commun. 1996, 139. (b) Fickes, M. G.; Davis,
E. M.; Cummins, C. C. J. Am. Chem. Soc. 1995, 117, 6384.
Concluding Remarks
The facility of N-atom metathesis described herein together
(31) Alyea, E. C.; Basi, J. S.; Bradley, D. C. J. Chem. Soc. A 1971, 772.
(32) Chisholm, M. H.; Cotton, F. A.; Extine, M. W.; Stultz, B. R. J. Am.
Chem. Soc. 1976, 98, 4477.
(33) Strutz, H.; Schrock, R. R. Organometallics 1984, 3, 1600.
(
7
,8
with the recent report by Johnson et al. on the metathesis
involving MotN bonds and alkynes to form molybdenum
alkylidynes and organic nitriles shows the kinetic lability of
these strong MtN bonds. If these reactions can be coupled
34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T., Jr.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth,
G. A.; Salvador, P.; Dannenberg, J. J.; Zakrewski, V. G.; Dapprich,
S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck,
A. D.; Raghavachari, K.; Fresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul,
A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko,
A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.;
Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challocombe, M.;
Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian 03, revision C.02;Gaussian, Inc.: Wallingford,
CT, 2004.
2
6–29
to Cummins and co-workers’
NtN by Mo(NR compounds, then a relatively simple and
inexpensive route to N-labeled organic compounds will be
reductive cleavage of
2 3
)
15
(
(
26) Cummins, C. C. Chem. Commun. 1998, 1778.
27) Laplaza, C. E.; Johnson, M. J. A.; Peters, J.; Odom, A. L.; Kim, E.;
Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc.
1
996, 118, 8623.
28) Laplaza, C. E.; Johnson, A. R.; Cummins, C. C. J. Am. Chem. Soc.
996, 118, 709.
29) Laplaza, C. E.; Cummins, C. C. Science (Washington, D.C.) 1995,
68, 861.
(
(
1
2
Inorganic Chemistry, Vol. 47, No. 12, 2008 5383

Burroughs et al.
3
9
d, 8H), 1.0 (m, 1H). 13C NMR (500 MHz, C
the SDD basis sets were used for transition-metal atoms and the
6 6
D ): δ 88.41, 51.59,
4
0
cc-pVTZ basis sets were used for all other nonmetal atoms. Two
other basis set systems, BS-III (LanL2DZ for metal atoms and
45.49, 35.12, 32.28, 25.87, 23.23, 23.13, 21.59, 16.45. IR (KBr
plates, Nujol mull): 1023 cm- [ν(WN)].
1
6
6
-31G* for nonmetal atoms) and BS-IV (SDD for metal atoms and
-31G* for nonmetal atoms), were also used for activation energy
Synthesis of [(-)-Men-O]
3
3
WtN. [(-)-Men-O] WtCEt (750
mg, 1.09 mmol) was dissolved in hexane (150 mL) with rapid
stirring. Benzonitrile (1 mL) was added via a gastight syringe to
the stirring hexane solution. The vessel was placed under vacuum,
sealed, and stirred for 24 h. After stirring, the volume was reduced
to 2 mL of hexane. Acetonitrile (5 mL) was added, and a brown
jellylike solid was formed. The solid was titurated (15 min) until
a brown powder formed. The brown solid was filtered, washed with
calculations to assess the influences of different effective core
potential and the f polarization functions. All of the structures were
fully optimized. Frequency calculations were also performed to
confirm that all of the stationary points were minima or transition
states (no imaginary frequency for the minimum and one imaginary
frequency for the transition state). Intrinsic reaction coordinate
calculations were carried out on transition states to confirm that
these structures are indeed connecting two minima. The discussed
4
1
1
acetonitrile five times, and dried in vacuo (512 mg, 72% yield). H
NMR (500 MHz, C D ): δ 4.77 (m, 1H), 2.91 (d, 1H), 2.54 (m,
6
6
energies are relative Gibbs free energies (∆G298
enthalpies (∆H298 ) are also provided for reference. All of the
relative energies were defined with respect to the starting materials.
K
). The relative
1H), 1.82 (d, 2H), 1.58 (qt, 2H), 1.34 (d, 3H), 1.19 and 1.15 (d of
1
3
K
6 6
d, 8H), 1.0 (m, 1H). C NMR (500 MHz, C D ): δ 88.41, 51.59,
45.49, 35.12, 32.28, 25.87, 23.23, 23.13, 21.59, 16.45. IR (KBr
t
15
t
-1
Synthesis of (O Bu)
3
Wt N. W
2
( BuO)
6
(1.00 g, 1.24 mmol)
plates, Nujol mull): 1021 cm [ν(WN)].
t
t
t
was dissolved in hexane (250 mL) with rapid stirring. A hexane
Reaction of ( BuO) MotN with W (O Bu) . W (O Bu) (0.25
3
2
6
2
6
1
5
solution (10 mL) containing MeC N (0.4 g, 9.5 mmol) was added
to the dark-red solution with rapid stirring. Immediately after the
addition of acetonitrile, the color changed to dark amber and a
brown precipitate was formed. The brown precipitate
g) and hexanes (10 mL) were added to a Schlenk flask and allowed
to stir. Separately ( BuO) MotN (0.20 g) and hexanes (10 mL)
were added to a second Schlenk flask and allowed to stir. The
BuO) MotN solution was added to the W (O Bu) solution via a
t
3
t
t
(
3
2
6
t
15
[
( BuO)
3
Wt N] was collected over an air-sensitive filter frit,
cannula. The mixture was left to stir at room temperature overnight.
A white precipitate formed, and the solution was filtered. The white
washed with three 10 mL portions of hexane, and dried in vacuo
1
(
500 mg, 96% yield). H NMR (500 MHz, THF-d
8
): δ 1.49 (s, 9H,
t
solid was identified as (Bu O)
3
WN by NMR and mass spectrometry.
15
183
15
3 3 8
OC(CH ) ). N NMR (500 MHz, THF-d ): δ 732 [(s), J( W- N)
The filtrate was concentrated and yielded a red microcrystalline
-1
15
)
54 Hz]. IR (KBr plates, Nujol mull): 934 cm [ν(W N)]; the
t +
solid. By mass spectrometry, the ions Mo
2
(OBu )
6
and Mo-
-
1
1
020 cm band attributed to ν(WN) for the unlabeled derivative
t +
W(OBu )
6
were identified.
was absent.
t
t
t
Reaction of ( BuO)
3
CrtN with Mo
2
(O Bu)
6
. ( BuO) CrtN
3
Synthesis of [(+)-Men-O]
3 3
WtN. [(+)-Men-O] WtCEt (500
(
0.057 g, 0.2 mmol) was added with hexanes (10 mL) to a Schlenk
flask and allowed to stir. Separately, Mo
mmol) was added with hexanes (10 mL) to a Schlenk flask and
allowed to stir. Both flasks were placed in a dry ice/acetone bath
mg, 0.72 mmol) was dissolved in hexane (150 mL) with rapid
stirring. Benzonitrile (1 mL) was added via a gastight syringe to
the stirring hexane solution. The vessel was placed under vacuum,
sealed, and stirred for 24 h. After stirring, the volume was reduced
to ca. 2 mL of hexane. Acetonitrile (5 mL) was added, and a brown
jellylike solid was formed. The solid was titurated (15 min) until
a brown powder formed. The brown solid was filtered, washed with
t
2
(O Bu)
6
(0.126 g, 0.2
and were allowed to stir for 1 h until the solution had cooled to
t
-
78 °C. The ( BuO)
3
CrtN solution was slowly (15 min) added
t
via a cannula to the Mo
2
(O Bu)
6
solution. The mixture was kept at
-
78 °C and was allowed to stir for 2 h. The solvent was removed
in vacuo. As the flask slowly warmed, a bluish-green liquid formed
1
acetonitrile five times, and dried in vacuo (389 mg, 81% yield). H
NMR (500 MHz, C
6 6
D ): δ 4.77 (m, 1H), 2.91 (d, 1H), 2.54 (m,
around the stopper of the flask. The bluish-green volatile compound
1
H), 1.82 (d, 2H), 1.58 (qt, 2H), 1.34 (d, 3H), 1.19 and 1.15 (d of
t
was identified as Cr(O Bu)
4
1
by comparison of the IR spectrum with
t
that of known Cr(O Bu)
4
. H NMR of the reaction mixture (C
6
D
6
(
35) (a) Burke, K.; Perdew, J. P.; Wang, Y. In Electronic Density Functional
Theory: Recent Progress and New Directions; Dobson,J. F., Vignale,
G., Das, M. P., Eds.; Plenum: New York, 1998; pp 81-111. (b)
Perdew, J. P. In Electronic Structure of Solids ’91; Ziesche, P., Eschrig,
H., Eds.; Akademie Verlag: Berlin, 1991; p 11. (c) Perdew, J. P.;
Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Sing,
D. J.; Fiolhais, C. Phys. ReV. B: Condens. Matter 1992, 46, 6671. (d)
Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson,
M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B: Condens. Matter 1993,
solvent, C
6
D
5
H reference, ppm): δ 1.59 (bs), 1.57 (s), 1.55 (s), 1.48
t
(s, ( BuO)
3
MotN). The mass spectrum of the dried reaction product
t
MotN)+ ion.
showed the ( BuO)
Reaction of ( BuO)
3
t
t
t
3
CrtN with W
2
(O Bu)
6
. ( BuO)
3
CrtN
(0.063 g, 0.22 mmol) was added with hexanes (10 mL) to a Schlenk
flask and allowed to stir. Separately, W
mmol) was added with hexanes (10 mL) to a Schlenk flask and
allowed to stir. Both flasks were placed in a dry ice/acetone bath
t
2
(O Bu)
6
(0.177 g, 0.22
4
8, 4978. (e) Perdew, J. P.; Burke, K.; Wang, Y. Phys. ReV. B:
Condens. Matter 1996, 54, 1653.
(
(
36) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt,
W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt,
W. R. J. Chem. Phys. 1985, 82, 299.
37) (a) Huzinaga, S. Gaussian Basis Sets for Molecular Calculations;
Elsevier Science Publishing Co.: Amsterdam, The Netherlands, 1984.
and were allowed to stir for 1 h until the solutions had cooled to
t
-
78 °C. The ( BuO)
3
CrtN solution was slowly (15 min) added
t
2 6
via a cannula to the W (O Bu) solution. The mixture was kept at
-78 °C and was allowed to stir for 2 h. The solvent was removed
in vacuo. As the flask slowly warmed, a bluish-green liquid formed
(
b) Ehlers, A. W.; Bohme, M.; Dapprich, S.; Gobbi, A.; Hollwarth,
A.; Jonas, B.; Kohler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking,
G. Chem. Phys. Lett. 1993, 208, 111.
around the stopper of the flask. The bluish-green volatile compound
t
(
38) (a) Ditchfield, R.; Hehre, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
was identified as Cr(O Bu)
4
by comparison of the IR spectrum with
H NMR of the reaction mixture (C
t
31
1
7
24. (b) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972,
that of known Cr(O Bu)
4
.
7 8
D
5
6, 2257. (c) Hariharan, P. C.; Pople, J. A. Mol. Phys. 1974, 27, 209.
solvent, C
7
D
7
H reference, ppm): δ 3.42 (s), 2.1 (s), 1.59 (s,
(
d) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (e) Hariharan,
t
(
3
BuO) WtN), 1.48 (s), 1.45 (s). The mass spectrum of the dried
reaction product has the correct peak range for ( BuO) WtN).
3
P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213.
39) Andrae, D.; Hauessermann, U.; Dolg, M.; Preuss, H. Theor. Chim.
Acta 1990, 77, 123.
t
(
t
15
t
t
Reaction of ( BuO)
3
Wt N with ( BuO)
3
MotN. ( BuO)
MotN (0.0165 g, 0.05
mmol) were added to a NMR tube and allowed to react in THF-d
3
Wt
(
40) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007.
41) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem.
Res. 1981, 14, 363.
1
5
t
N (0.0209 g, 0.05 mmol) and ( BuO)
3
(
8
5384 Inorganic Chemistry, Vol. 47, No. 12, 2008

Metathesis of Nitrogen Atoms within Triple Bonds
1
at room temperature for 1 week. The H NMR spectrum of the
spectra were collected. In this instance, the catalyst and nitrile
concentrations remained constant throughout the temperature range
(50, 40, 35, and 21 °C). In each case, the sample was heated in the
probe, and once the overnight run finished, the sample was removed
and placed in its respective temperature oil bath for the time. In
the case of the determination of the reaction order in nitrile, the
amount of catalyst remained the same, while the amount of stock
solution was changed according to the desired ratio (cat:nitrile )
1:1, 1:0.5, 1:2).
reaction mixture shows no change from the starting materials. The
N NMR spectrum of the reaction mixture (THF-d solvent, THF-
8
15
t
15
t
d
7
reference, ppm): δ 828.8 (s, ( BuO)
3
Mot N), 731.8 ( BuO)
3
-
1
5
Wt N). Mass spectral analysis of the reaction products verifies
t
15
3
the presence of ( BuO) Mot N.
(tBuO)
(tBuO)
3
MotN.
Reaction of
(tBuO)
3
WtCMe with
WtCMe (0.0215 g, 0.05 mmol) was made in an NMR tube
according to a previously reported method. ( BuO)
g, 0.05 mmol) was added to the tube and allowed to react at room
3
t
3
MotN (0.0165
1
3
The nitrile cleavage reaction involving Me CN was monitored
H} NMR spectroscopy. The parameters used were as
1
by 13C{
1
6 6
temperature for 1 week. H NMR of the reaction mixture (C D
solvent, C
6
D
5
H reference, ppm): δ 3.57 (s, 3H, JHW ) 7.2 Hz,
follows: number of scans (NS ) 15), number of data points (TD
t
t
) 65 536), receiver gain (RG ) 2896), and relaxation time (D1 )
(
(
1
3
3
BuO)
BuO)
.42 (s). C{ H} NMR (C
3 3
WtCMe), 2.58 (s, 3H, ( BuO) MotCMe), 1.48 (s, 27H,
t
t
75 s). The deconvolution of 13C NMR spectra was done using
3
MotN), 1.46 (s), 1.44 (s, 27H, ( BuO)
3
WtCMe), 1.44 (s),
1
3
1
XWIN-NMR version 3.5 PL6. The T relaxation times for aceto-
6
D
6
solvent, C
6
D
5
H reference, ppm): δ
1
t
nitrile (15.2 s) 13C NMR signals were measured at 298 K. The
33.1 (s), 279.6 (s, 1H, ( BuO) MotCMe), 254.1 (s, 1H, JCW
3
)
t
Ernst equation (1) (where t ) T relaxation time, pw 90 ) 90°
06.5 Hz, ( BuO)
Reaction of (tBuO)
(tBuO)
3
WtCMe).
1
1
WtCMe with (tBuO)
pulse width, at ) acquistion time, and d1 ) delay time between
pulses). NMR probe temperatures were calibrated with a sample
of ethylene glycol, and van Greet’s equation was used to determine
the optimal pulse widths for the delay time used in NMR
experiments.
3
3
MotCPh.
3
WtCMe (0.0054 g, 0.0125 mmol) was made in an NMR
t
tube according to a previously reported method. ( BuO)
3
MotCPh
(
0.0051 g, 0.0125 mmol) was added to the tube and allowed to
1
react at room temperature for 1 week. H NMR of the reaction
mixture (C solvent, C H reference, ppm): δ 7.49 (d, 2H,
MotCPh), 7.33 (d, 2H, ( BuO)
Calculation of the Kinetics and Activation Parameters. The
absolute concentration of the catalyst in solution was determined
6
D
6
6 5
D
t
t
(
(
(
BuO)
BuO)
BuO)
3
3
3
3
WtCPh), 7.23 (t, 2H,
3
MotCPh), 6.87 (t, 1H,
t
t
t
by a proton integration of the O Bu resonance (s, 1.49 ppm) versus
WtCPh), 7.09 (t, 2H, ( BuO)
MotCPh), 6.82 (t, 1H, ( BuO)
13
t
t
the Me CtN methyl resonance (d, 1.95 J) 10.7 Hz). The cleavage
3
WtCPh), 3.57 (s, 3H, JHW
t
t
and atom exchange was measured by following the disappearance
)
7.2 Hz, ( BuO)
3
WtCMe), 2.58 (s, 3H, ( BuO)
3
t
MotCMe), 1.49
1
3
t
of a singlet at 115.9 ppm (Me CtN) and the appearance of a
(s, 27H, ( BuO)
3
t
MotCPh), 1.48 (s), 1.44 (s, 27H, ( BuO)
3
WtCMe),
WtCPh). C{ H} NMR of reaction mixture
H reference, ppm): δ 333.1 (s), 307.8 (s), 279.8
MotCPh), 254.1 (s, 1H, JCW ) 306.5 Hz,
WtCMe).
Reaction of
1
3
15
13
1
doublet at 115.95 and 115.82 ppm (Me Ct N). The Lorentzian
1
.42 (s, 27H, ( BuO)
solvent, C
3
1
3
deconvolution of C NMR spectra was performed using XWIN-
NMR 3.5 PL6. All of the cleavage and atom exchange reactions
were of the type A + B S C + D, where the rate law was
(
(
(
C
D
6 6
6 5
D
t
s, 1H, ( BuO)
t
3
BuO)
3
(tBuO)
i
(tBuO)
3
MotN.
3
MotN (0.0165
determined (eq 2) and where ∆ ) [A] - [A]eq
When ln[(∆/(∆(1 - 1/K) + [A] + [B] + (1/K)([C]
is plotted versus time, the slope is inserted into eq 2 to solve for
obs. The enthalpy parameters were calculated by plotting ln(kobs/
.
3
WtC Pr with
(tBuO)
i
t
e
e
e
e
+ [D] ))]
3
WtC Pr (0.0229 g, 0.05 mmol) and ( BuO)
g, 0.05 mmol) were added to a NMR tube and allowed to react at
room temperature for 1 week. H NMR of reaction mixture (C
1
k
6 6
D
q
q
T) versus 1/T, where the slope ) -∆H /R and the intercept ) ∆S /R
+ 23.8.
solvent, C H reference, ppm): δ 4.05 (sept, 1H, JHW ) 6.85 Hz,
6
D
5
t
i
t
i
(
BuO)
3
WtC Pr), 3.11 (sept, 1H, ( BuO)
MotN), 1.47 (s, 27H, ( BuO)
3
MotC Pr), 1.48 (s, 27H,
t
t
i
(
BuO)
3
3
MotC Pr), 1.46 (s, 27H,
Acknowledgment. We thank the National Science Foun-
dation for financial support and gratefully acknowledge the
Ohio Supercomputer Center for computational resources.
t
i
t
(
t
3 3
BuO) WtC Pr), 1.45 (s, 27H, ( BuO) WtN), 1.24 (d, 6H,
i
t
i
(
BuO)
Preparation of Samples for the Determination of Kinetics.
All samples were prepared in an NMR tube equipped with a J.
3 3
WtC Pr), 1.16 (d, 6, ( BuO) MotC Pr).
Supporting Information Available: Free energy (at 298 K)
1
3
Young adapter. For all runs, a stock solution of Me CN in THF-
was prepared by weighing 48.5 mg in a 5 mL volumetric flask
to produce a 0.231 M solution. For determination of the enthalpy
parameters, 890 µL of THF-d was added to 11.3 mg of
W N via a gastight syringe and transferred to the NMR
profiles for a N-atom exchange reaction between (MeO)
3
MtN
d
8
(where M ) W, Mo) and MeCtN and structures of transition states
and intermediates with selected calculated structural parameters
calculated based on BS-II (Figures S1 and S2). This material is
available free of charge via the Internet at http://pubs.acs.org.
8
t
15
(
BuO)
3
tube. Addition of 110 µL of the stock solution with a gastight
syringe followed. Preparation occurred immediately before NMR
IC8003917
Inorganic Chemistry, Vol. 47, No. 12, 2008 5385