On the origin of selective nitrous oxide N-N bond cleavage by three-coordinate molybdenum(III) complexes

Article abstract of DOI:10.1021/ja0031063

Reaction of Mo(N[R]Ar)₃ (R = ^tBu or C(CD₃)₂CH₃) with N₂O gives rise exclusively to a 1:1 mixture of nitride NMo(N[R]Ar)₃ and nitrosyl ONMo(N[R]Ar)₃, rather than the known oxo complex OMo(N[R]Ar)₃ and dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)₃ with N₂O and, independently, the heat of reaction of Mo(N[R]Ar)₃ with NO. Derived from the latter measurements is an estimate (155.3 ± 3.3 kcal·mol^-1) of the molybdenum-nitrogen bond dissociation enthalpy for the terminal nitrido complex, NMo(N[R]Ar)₃. Comparison of the new calorimetry data with those obtained previously for oxo transfer to Mo(N[R]Ar)₃ shows that the nitrous oxide N-N bond cleavage reaction is under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both Mo(N[R]Ar)₃ and N₂O, consistent with a mechanism featuring post-rate-determining dinuclear N-N bond scission, but also consistent with cleavage of the N-N bond at a single metal center in a mechanism requiring the intermediacy of nitric oxide. The new 2-adamantyl-substituted molybdenum complex Mo(N[2-Ad]Ar)₃ was synthesized and found also to split N₂O, resulting in a 1:1 mixture of nitrosyl and nitride products; the reaction exhibited first-order kinetics and was found to be ca. 6 times slower than that for the tert-butylsubstituted derivative. Discussed in conjunction with studies of the 2-adamantyl derivative Mo(N[2-Ad]Ar)₃ is the role of ligand-imposed steric constraints on small-molecule, e.g. N₂ and N₂O, activation reactivity. Bradley's chromium complex Cr(NⁱPr₂)₃ was found to be competitive with Mo(N[R]Ar)₃ for NO binding, while on its own exhibiting no reaction with N₂O. Competition experiments permitted determination of ratios of second-order rate constants for NO binding by the two molybdenum complexes and the chromium complex. Analysis of the product mixtures resulting from carrying out the N₂O cleavage reactions with Cr(Nⁱpr₂)₃ present as an in situ NO scavenger rules out as dominant any mechanism involving the intermediacy of NO. Simplest and consistent with all the available data is a post-rate-determining bimetallic N-N scission process. Kinetic funneling of the reaction as indicated is taken to be governed by the properties of nitrous oxide as a ligand, coupled with the azophilic nature of three-coordinate molybdenum(III) complexes.

Full text of DOI:10.1021/ja0031063

J. Am. Chem. Soc. 2001, 123, 7271-7286
7271
On the Origin of Selective Nitrous Oxide N-N Bond Cleavage by
Three-Coordinate Molybdenum(III) Complexes
John-Paul F. Cherry,† Adam R. Johnson,† Luis M. Baraldo,† Yi-Chou Tsai,†
Christopher C. Cummins,*,† Sergey V. Kryatov,‡ Elena V. Rybak-Akimova,*,‡
Kenneth B. Capps,§ Carl D. Hoff,*,§ Christopher M. Haar, and Steven P. Nolan*,
Contribution from the Departments of Chemistry, Room 2-227, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139-4307, Tufts UniVersity, Medford, Massachusetts 02155, UniVersity of
Miami, Coral Gables, Florida 33124, and UniVersity of New Orleans, New Orleans, Louisiana 70148
ReceiVed August 21, 2000. ReVised Manuscript ReceiVed May 4, 2001
Abstract: Reaction of Mo(N[R]Ar)3 (R ) tBu or C(CD3)2CH3) with N2O gives rise exclusively to a 1:1 mixture
of nitride NMo(N[R]Ar)3 and nitrosyl ONMo(N[R]Ar)3, rather than the known oxo complex OMo(N[R]Ar)3
and dinitrogen. Solution calorimetry measurements were used to determine the heat of reaction of Mo(N[R]Ar)3
with N2O and, independently, the heat of reaction of Mo(N[R]Ar)3 with NO. Derived from the latter
measurements is an estimate (155.3 ( 3.3 kcal‚mol-1) of the molybdenum-nitrogen bond dissociation enthalpy
for the terminal nitrido complex, NMo(N[R]Ar)3. Comparison of the new calorimetry data with those obtained
previously for oxo transfer to Mo(N[R]Ar)3 shows that the nitrous oxide N-N bond cleavage reaction is
under kinetic control. Stopped-flow kinetic measurements revealed the reaction to be first order in both
Mo(N[R]Ar)3 and N2O, consistent with a mechanism featuring post-rate-determining dinuclear N-N bond
scission, but also consistent with cleavage of the N-N bond at a single metal center in a mechanism requiring
the intermediacy of nitric oxide. The new 2-adamantyl-substituted molybdenum complex Mo(N[2-Ad]Ar)3
was synthesized and found also to split N2O, resulting in a 1:1 mixture of nitrosyl and nitride products; the
reaction exhibited first-order kinetics and was found to be ca. 6 times slower than that for the tert-butyl-
substituted derivative. Discussed in conjunction with studies of the 2-adamantyl derivative Mo(N[2-Ad]Ar)3
is the role of ligand-imposed steric constraints on small-molecule, e.g. N2 and N2O, activation reactivity.
Bradley’s chromium complex Cr(NiPr2)3 was found to be competitive with Mo(N[R]Ar)3 for NO binding,
while on its own exhibiting no reaction with N2O. Competition experiments permitted determination of ratios
of second-order rate constants for NO binding by the two molybdenum complexes and the chromium complex.
Analysis of the product mixtures resulting from carrying out the N2O cleavage reactions with Cr(NiPr2)3 present
as an in situ NO scavenger rules out as dominant any mechanism involving the intermediacy of NO. Simplest
and consistent with all the available data is a post-rate-determining bimetallic N-N scission process. Kinetic
funneling of the reaction as indicated is taken to be governed by the properties of nitrous oxide as a ligand,
coupled with the azophilic nature of three-coordinate molybdenum(III) complexes.
1. Introduction
diazenes (azo compounds),18-21 and molecular nitrogen itself.22-47
The latter substrate continues to present a dramatic and seductive
challenge to chemistry, namely the development of new
Activation of N-N Bonds. Reactions involving the reductive
cleavage of N-N bonds by metal atoms1-14 or metal complexes
constitute an intriguing and expanding facet of chemical science.
Symmetrical substrates of interest include hydrazines,15-17
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† Massachusetts Institute of Technology.
‡ Tufts University.
§ University of Miami.
University of New Orleans.
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10.1021/ja0031063 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/07/2001

7272 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
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Nitrous Oxide N-N Bond CleaVage by Mo(III) Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7273
and by Diamantis and Sparrow,88-92 salts of the [Ru(NH3)5N2O]2+
ion have been isolated as microcrystalline solids, but structural
information, i.e., a single-crystal X-ray diffraction study, is not
available. Much now is known about the vibrational spectra for
bound nitrous oxide in complexes, the latter advances being
largely to the credit of Bottomley,93-96 who also devised a
synthesis of [Ru(NH3)5N2O]2+ salts involving reaction of the
nitrosyl derivative with hydroxylamine.95 Access to the ion
previously had involved reaction of the aquo derivative with
N2O gas, a method used also by Armor and Taube in the context
of their study of linkage isomerism in 15N2-labeled ruthenium(II)
pentaammine complexes.86
To this day it appears that [Ru(NH3)5N2O]2+ remains the only
well-characterized N2O coordination complex, although transient
infrared spectroscopy97 and matrix isolation98 studies have been
carried out for the transient tungsten carbonyl adduct W(CO)5-
(N2O). In contrast, the coordination chemistry of dinitrogen has
undergone a dramatic evolution since its inception in the
1960s.99 Perhaps the origin of this dichotomy rests in the
inherent reactivity of bound nitrous oxide with respect to oxo
transfer, it being the case oftentimes that bound dinitrogen is
inert to further transformations. Much work has been conducted
in which N2O serves as an oxo source in transition metal and
other systems,69-76 but the observation of intermediate N2O
complexes typically is not documented.72,73
to the Mo center without â-H elimination becoming manifest.41
In contrast to this is the chemistry observed when iPr substituents
are employed on the amido nitrogens: the complex of formula
“Mo(N[iPr]Ar)3” has been shown to exist as its metallaziridine
hydride tautomer.41 In the present work the role of complex 2
is to provide a contrast with complex 1 in terms of reactivity
with N2O.
The first three-coordinate complexes of a trivalent group 6
element were the tris-diisopropylamide and the tris-hexa-
methyldisilazide complexes of chromium.105-108 Bradley’s
Cr(NiPr2)3 is complex 3 of this work, significant because in
108
contrast to the Mo complexes it is shown to exhibit no reaction
with N2O. Coupled with its propensity to bind NO,107,109,110 the
lack of reaction of complex 3 with N2O has rendered the
complex invaluable as a mechanistic probe in the present study.
2. Results and Discussion
Kinetics with Toluene as Solvent. Bubbling of nitrous oxide
through a sealed cuvette containing 1 (6 × 10-4 M in toluene)
elicited a color change from orange to pale yellow over several
minutes, corresponding to the conversion of 2 equiv of 1 to an
equimolar mixture of nitride 1-N and nitrosyl 1-NO as indicated
pictorially in Figure 1.
As displayed in Figure 2, spectrophotometric measurements
revealed two wavelength regions suitable for stopped-flow
experiments: 350-380 nm, where the optical absorbance was
observed to increase, and 395-500 nm, where the optical
absorbance was observed to decrease. An isosbestic point was
located at 390 nm (ꢀ ) 2500 M-1 cm-1).
Kinetic data corresponding to the reaction between 1 and a
large excess of N2O (200-1300 equiv) was studied in detail in
toluene solution over the temperature range from +5 to +25
°C. The spectral changes observed in stopped-flow experiments
correspond well to those observed in the conventional spectro-
photometric study (Figure 2). The observed pseudo-first-order
rate constants measured in the regions both of increase and of
decrease in optical absorbance agree to within 5%. Thus, the
spectral changes observed in stopped-flow experiments may be
taken to be indicative of the reaction of 1 with N2O, as intended.
It is worth noting that complex 1 is exceedingly air and water
sensitive,45,80 such that the stopped-flow measurements had to
be carried out with rigorous exclusion of air and water from
the system. As a control, the decomposition of complex 1 upon
exposure to air was studied spectrophotometrically (spectrum
c of Figure 5), with the result that air exposure causes a decrease
in optical absorbance across the entire spectral window. The
latter profile distinguishes the reaction of 1 with air as opposed
to N2O.
Three-Coordinate Complexes of Mo and Cr. Nitrous oxide
N-N bond cleavage was one of the first reactions described
for the unique three-coordinate molybdenum(III) complex,
Mo(N[R]Ar)3 (R ) tBu, 1; R ) C(CD3)2CH3, 1-d18; Ar ) 3,5-
C6H3Me2).79,80 Early characterization data showed 1 to be a
monomeric species possessed of a quartet ground state, red in
color, and readily soluble in hydrocarbon solvents. Particularly
noteworthy vis-a`-vis the class of well-characterized molyb-
100,101
denum(III) dimers exemplified by Mo2(NMe2)6
is the
monomeric nature of 1, a characteristic enforced by the sterically
encumbering nature of its amido nitrogen substituents.44,102-104
In view of the importance of steric considerations in fostering
the monomeric nature of 1,44,102-104 variants have been sought
that incorporate even larger substituents on their amido nitro-
gens.42 Such variants include structurally characterized three-
coordinate Mo(N[1-Ad]Ar)342 (where 1-Ad is the 1-adamantyl
substituent), which like 1 engages in mononuclear N2 chemistry,
but which unlike 1 is prevented sterically from engaging in the
dinuclear N2 chemistry required for dinitrogen reductive
cleavage.42,45-47 Another adamantyl-substituted derivative is
Mo(N[2-Ad]Ar)3, complex 2 of this work, a complex noteworthy
in that its secondary adamantyl substituents present â-hydrogens
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The kinetic curves, representing absorbance change as a
function of time, at different wavelengths were fitted to a single-
exponential equation, giving mean standard deviations within
2% over a period of five half-lives. No systematic changes in
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Commun. 1970, 1177.
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1970, 219.
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1971, 411.
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J. Chem. Soc., Chem. Commun. 1972, 567.
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7274 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
Figure 1. Reaction of 1 with N2O to give equimolar amounts of nitrosyl 1-NO and nitrido 1-N. Mechanistic possibilities to be considered are path
1, a monometallic pathway with free NO as an intermediate, and path 2, a pathway involving the unobserved bimetallic intermediate 1-NNO-1.
Common to both pathways is the unobserved N2O adduct, 1-NNO. The second-order rate constant for reaction of 1 with N2O is defined as kNNO1
,
while the second-order rate constant for the binding of NO by 1 is defined as kNO1
.
Figure 3. Dependence of the observed pseudo-first-order rate constant
on N2O concentration for the reaction between Mo(N[R]Ar)3 (1) and
N2O in toluene at 25 °C. In each case the initial concentration of 1
was 0.3 mM. A least-squares fit of the data gives an intercept of 0.007
s-1 and a slope (corresponding to a second-order rate constant, see text)
of 3.4 M-1 s-1, with R2 ) 0.999.
The N2O dependence of the reaction rate and the zero
intercept representing no reaction in the absence of N2O (Figure
3) together constitute a further confirmation that the process
under scrutiny in the stopped-flow experiments is indeed the
reaction of 1 with N2O, as opposed to some undesired
decomposition reaction. It should be borne in mind, however,
that the observed second-order kinetic behavior is documented
here under conditions involving a great excess of N2O vis-a`-
vis complex 1. One should refrain, therefore, from drawing
conclusions as to what behavior to expect under conditions, for
example, involving a great excess of complex 1. Unfortunately,
technical difficulties obviate kinetic study of the 1/N2O system
under conditions involving an excess of complex 1. At low N2O
concentrations the reaction becomes prohibitively slow, while
instrumental limitations do not allow the use of inordinately
high concentrations of chromophore 1.
Figure 2. Visible spectra of Mo(N[R]Ar)3 (1) in toluene before (a)
and after (b) mixing with N2O. Inset: kinetic traces of the reaction in
typical stopped-flow shots at 25 °C. Initial concentration of 1, 0.31
mM; initial concentration of N2O, 66.5 mM (corresponding to a half-
diluted saturated solution of the gas at 25 °C).
concentration of 1 in the range from 0.1 to 0.6 mM, using a
saturated solution of N2O in toluene ([N2O]0 ) 66.5 mM) at
25 °C. Thus, the reaction clearly is first order in Mo(N[R]Ar)3
(1).
A plot of the pseudo-first-order rate constant kobs versus the
initial concentration of N2O is a straight line with practically
zero intercept, as displayed in Figure 3. Such data were obtained
both by using solutions prepared by saturation with calibrated
N2O/Ar gas mixtures, and with the use of the multimixing
stopped-flow mode, the latter allowing the saturated N2O
solution to be diluted twice directly in the SF apparatus. This
result shows that the reaction is first order in N2O. The slope
of the line gives the value of the second-order reaction rate
constant kNNO1 ) 3.4 ( 0.3 M-1 s-1 for the reaction between
1 and N2O at 25 °C.
Activation Parameters. The reaction of complex 1 with N2O
in toluene was studied as a function of temperature (Figure 4),
from +5 to +25 °C. Activation parameters obtained from fitting
the data to the Eyring equation111 are as follows: ∆Hq ) 9.7 (
0.5 kcal‚mol-1, and ∆Sq ) -23.6 eu. The observed activation

Nitrous Oxide N-N Bond CleaVage by Mo(III) Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7275
the Lewis acidity of the complex is so weak that it fails to bind
even strong donors such as pyridine. Furthermore, the visible
spectrum of 1 is essentially identical in toluene to that in diethyl
ether (Figure 2). Although it is possible that the broad features
of the visible spectrum of 1 are insensitive to coordination of
an ethereal solvent molecule, a more satisfying explanation of
the reaction rate attenuation in ether stems from differential
solvation of the putative polar intermediate nitrous oxide
complex (ONN)Mo(N[R]Ar)3 (1-NNO, see Figure 1). Such an
explanation is, in effect, an argument based on dielectric strength
considerations.111 Undue attention need not be devoted to the
relatively small observed difference in reaction rate in ether
versus toluene.
Kinetics for Reaction of 1 with NO. One potentially
important pathway (path 1, Figure 1) for the N2O cleavage
reaction involves fast trapping of NO after the N2O binding
and activation. Therefore, we investigated the kinetic parameters
associated with NO binding by complex 1.
Figure 4. Plot of the temperature dependence of the second-order rate
constant for the reaction of Mo(N[R]Ar)3 (1) with N2O in toluene. The
fit is to the linear form of the Eyring equation, giving activation
parameters of ∆Hq ) 9.7 kcal‚mol-1, ∆Sq ) -24 eu.
Bubbling of nitric oxide through a sealed cuvette containing
a 6 × 10-4 M concentration of complex 1 in toluene elicited
an extremely rapid color change from orange to pale yellow,
corresponding to the rapid binding of NO by 1 to form 1-NO.
This reaction has been shown to supply in quantitative fashion
the diamagnetic nitrosyl 1-NO.79
enthalpy is very close to values reported for other reactions of
N2O with transition metal complexes,112,113 reactions which,
however, give rise to oxygen atom transfer and dinitrogen
formation rather than N-N bond scission. A negative activation
entropy is typical for associative reactions, such as dioxygen
binding to metal complexes.114-116
Below +5 °C the reaction between complex 1 and N2O
becomes too slow for reliable determination of its rate under
our experimental conditions. In stopped-flow experiments at
-40 °C, mixing the solutions of 1 and N2O gave rise to
approximately the same kinetic traces as diluting the solution
of complex 1 with pure toluene. In both cases there were minor
changes in absorbance on a relatively long time scale (g100
s), which may be attributed to side reactions. Upon bubbling
gaseous N2O through a 6 × 10-4 M toluene solution of complex
1 at -78 °C in a Schlenk vessel, there was a slow fading of the
orange color to pale yellow over 20-40 min, attributable to
reaction with N2O. In this way it was confirmed that no colored
intermediates accumulate in any significant concentration when
the reaction is carried out at low temperature.
Kinetics with Ether as Solvent. The pseudo-first-order rate
constant for the reaction of 1 with saturated solutions of N2O
in diethyl ether at 25 °C was found to be kobs ) 0.085 ( 0.005
s-1. Under the assumption of second-order kinetics, this
corresponds to a second-order rate constant, k2, of 0.57 ( 0.05
M-1 s-1 for this, the case of diethyl ether as solvent. Thus, the
nitrous oxide splitting by 1 is seen to be approximately 6 times
slower in ether than in toluene. Weak coordination of a diethyl
ether molecule to the three-coordinate Mo center in 1 is an
unlikely explanation for the observed slower reactivity in this
solvent, it being the case that Mo(N[R]Ar)3 (1) crystallizes from
ethereal solvents including THF without retaining solvent,45 and
Spectrophotometric measurements revealed three wavelength
regions suitable for stopped-flow experiments: below 360 nm,
where an increase in optical absorbance was noted, 365-390
nm (increase), and 395 nm (decrease), divided by isosbestic
points at 360 nm (ꢀ ) 3600 M-1 cm-1) and 395 nm (ꢀ ) 2550
M-1 cm-1).
The final spectral changes observed in stopped-flow experi-
ments using complex 1 at 6 × 10-4 M and NO at 1.2 × 10-2
M in toluene correspond to those discerned upon bubbling NO
through a solution of 1 in the conventional spectroscopic study.
According to the absorbance changes, the reaction was at least
90% complete in the “blind” mixing time of the stopped-flow
apparatus (2 ms) at temperatures +25, -55, and -78 °C. In all
cases (see Figure 5), the stopped-flow traces were horizontal
straight lines with the absorption corresponding to the product,
nitrosyl 1-NO. No absorbance changes attributable to NO
binding were seen even in the earliest period of stable detection.
For the purposes of estimating the rate of nitric oxide binding
by 1, we will assume the binding to be first order in complex
1, first order in NO, and irreversible. The pseudo-first-order
rate constant k′ can be evaluated as >103 s-1. The second-
obs
order rate constant kNO1 then can be estimated at >105 M-1
s-1. The observed fast reaction between 1 and nitric oxide is
not surprising in view of the published data on the rates of NO
binding by other transition metal complexes.117-123
The most detailed studies on the rate of NO binding by metal
complexes have been carried out for iron(II) and iron(III) heme
proteins and related phthalocyanine and porphyrin com-
plexes.117-123 Other systems having received scrutiny in this
(111) Carpenter, B. Determination of Organic Reaction Mechanisms;
Wiley: New York, 1984.
(112) Vaughan, G. A.; Rupert, P. B.; Hillhouse, G. L. J. Am. Chem.
Soc. 1987, 109, 5538.
(113) Tezuka, M.; Iwasaki, M. Chem. Lett. 1998, 1017.
(114) Rybak-Akimova, E. V.; Marek, K.; Masarwa, M.; Busch, D. H.
Inorg. Chim. Acta 1998, 270, 151.
(115) Rybak-Akimova, E. V.; Otto, W.; Deardorf, P.; Roesner, R.; Busch,
D. H. Inorg. Chem. 1997, 36, 2746.
(117) Laverman, L. E.; Hoshino, M.; Ford, P. C. J. Am. Chem. Soc. 1997,
119, 12663.
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(119) Petrich, J. W.; Lambry, J. C.; Kuczera, K.; Karplus, M.; Poyart,
C.; Martin, J. L. Biochemistry 1991, 30, 3975.
(120) Hoshino, M.; Kogure, M. J. Phys. Chem. 1989, 93, 5478.
(121) Jongeward, K. A.; Magde, D.; Taube, D. J.; Marsters, J. C.; Traylor,
T. G.; Sharma, V. S. J. Am. Chem. Soc. 1988, 110, 380.
(122) Sharma, V. S.; Traylor, T. G.; Gardiner, R.; Mizukami, H.
Biochemistry 1987, 26, 3837.
(123) Ascenzi, P.; Brunori, M.; Pennesi, G.; Ercolani, C.; Monacelli, F.
J. Chem. Soc., Dalton Trans. 1987, 369.
(116) Kolchinski, A. G.; Korybut-Daszkiewicz, B.; Rybak-Akimova, E.
V.; Busch, D. H.; Alcock, N. W.; Clase, H. J. J. Am. Chem. Soc. 1997,
119, 4160.

7276 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
mol-1 130-133
.
In a study on the reactions of nickel cluster ions,
dioxygen was compared with nitrous oxide as an oxygen atom
source, the conclusion being that N2O is the more efficient O
donor because it possesses a good leaving group, namely
dinitrogen.134 The DN-O value for nitric oxide has been given
as 151 kcal‚mol-1 132
.
Ab initio computational methods have been used to assess
the binding energy between NO and first-row transition metal
cations.135
All-electron ab initio calculations have been performed for
the diatomic metal nitrides YN,136 MoN,137 and RhN,138 said
work serving to identify the ground states for these systems
and serving also to provide a bonding description, spectroscopic
constants, and bond dissociation energies. The calculated bond
dissociation energies for these systems are 106, 119, and 40
kcal‚mol-1, respectively. This partial series of diatomic nitrides
of second row transition metals illustrates the propensity of
molybdenum to form a relatively strong triple bond to nitrogen.
In addition, although the molybdenum and rhodium diatomic
nitrides enjoy a description as triply bonded entities, the yttrium
nitride molecule is indicated to have double bond character,
consisting solely of two π bonds, with no σ component to the
Y-N interaction.
Figure 5. (a) Spectrum of Mo(N[R]Ar)3 (1) in toluene solution. (b)
Spectrum of 1 in toluene after exposure to NO. (c) Spectrum of a
toluene solution of 1 after brief exposure to air. (d) Stopped-flow kinetic
trace obtained on mixing a toluene solution of 1 with the pure solvent.
(e) Stopped-flow kinetic trace obtained on mixing a toluene solution
of 1 with a solution of NO in toluene. Initial concentrations (after
mixing) for the latter experiment: [1], 0.33 mM; NO, 6 mM.
Interestingly, what little information is available suggests that
the uranium-nitride bond energy is similar to that for molyb-
denum: DU-N for [OUN]+ is ≈120 kcal‚mol-1, while DU-N
for [UN]+ is ≈116 kcal‚mol-1 139
.
The latter information may
be useful in assessing the potential for dinitrogen splitting in
heterodinuclear molybdenum/uranium dinitrogen complexes,43
or in related diuranium systems.140-143
regard include the aqueous divalent ions of chromium124 and
iron,125 a number of iron(II) complexonates,126-128 and the
ruthenium-edta system.129 Association of NO with vacant
coordination sites as in ferroheme proteins, and as in metallo-
porphyrins in nondonor solvents, has been identified with rate
constants on the order of 109 M-1 s-1 at ambient temperature,
close to the diffusion-controlled limit.117-123 Such fast processes
have been studied by flash photolysis techniques, due to the
photoreversibility of NO coordination in the systems in question.
Thermochemistry. Solution calorimetry studies were carried
out in order to assess enthalpic factors relevant to the selective
splitting of the nitrous oxide N-N bond by complex 1. The
reactions in question, namely the N2O splitting reaction and the
NO binding reaction, both were particularly amenable to analysis
by solution calorimetry because of their rapid and essentially
quantitative nature.
The stopped-flow technique in some cases can be applied to
the study of near-diffusion-controlled reactions, by using very
dilute reaction solutions at cryogenic temperatures.114-116
Unfortunately, this approach is not applicable to the study of
the reaction of 1 with NO, which most likely is nearly a
diffusion-controlled process. Decreasing the reagent concentra-
tions by several orders of magnitude will lead to the predominant
registration of side reactions, due to the extreme air and water
sensitivity of complex 1. Moreover, comparing the low-
temperature kinetic parameters with those obtained for the
nitrous oxide reaction in the range from +5 to +25 °C is an
endeavor of dubious merit. Since the major focus of the current
study is the reaction of 1 with N2O, further investigations into
the kinetics of NO binding were not pursued. In any event, it is
clear that the binding of NO by complex 1 is at least 5 orders
of magnitude faster than its reaction with N2O.
Figure 6 displays the NO binding reaction as C, the enthalpy
of which is found to be -82.5 ( 2.5 kcal‚mol-1, while the N2O
splitting reaction is indicated as D, the corresponding enthalpy
change being -123.8 ( 0.8 kcal‚mol-1. As an aside, the latter
two values in combination with A, the 114 kcal‚mol-1 literature
value for the N-NO bond dissociation enthalpy,130-133 yield
the value B for the MotN BDE of 155.3 ( 3.3 kcal‚mol-1
.
Striking is the agreement of this value for B with values
predicted using relativistic density functional theory calculations
(130) Hintz, P. A.; Sowa, M. B.; Ruatta, S. A.; Anderson, S. L. J. Chem.
Phys. 1991, 94, 6446.
(131) Resat, M. S.; Smolanoff, J. N.; Goldman, I. B.; Anderson, S. L. J.
Chem. Phys. 1994, 8784.
(132) Sanderson, R. Chemical Bonds and Bond Energy, 2nd ed.;
Academic: New York, 1976.
(133) Okabe, H. Photochemistry of Small Molecules; Wiley: New York,
1978.
(134) Hintz, P. A.; Ervin, K. M. J. Chem. Phys. 1995, 103, 7897.
(135) Thomas, J. L. C.; Bauschlicher, C. W.; Hall, M. B. J. Phys. Chem.
A 1997, 101, 8530.
(136) Shim, I.; Gingerich, K. A. Int. J. Quantum Chem. 1993, 46, 145.
(137) Shim, I.; Gingerich, K. A. Theochem (J. Mol. Struct.) 1999, 460,
123.
(138) Shim, I.; Mandix, K.; Gingerich, K. A. Theochem (J. Mol. Struct.)
1997, 393, 127.
(139) Heinemann, C.; Schwarz, H. Chem.-Eur. J. 1995, 1, 7.
(140) Kaltsoyannis, N.; Scott, P. Chem. Commun. 1998, 1665.
(141) Roussel, P.; Tinker, N. D.; Scott, P. J. Alloys Compd. 1998, 271,
150.
Thermochemistry of N2O and NO: Literature Data. Bond
dissociation enthalpies (BDEs) for N2O have been given as
follows: DN-NO ) 114 kcal‚mol-1 and DN -O ) 40 kcal‚
2
(124) Melton, J. D.; Bakac, A.; Espenson, J. H. Inorg. Chem. 1986, 25,
3360.
(125) Kustin, K.; Taub, I.; Weinstock, E. Inorg. Chem. 1966, 5, 1079.
(126) Zang, V.; van Eldik, R. Inorg. Chem. 1990, 29, 4462.
(127) Zang, V.; Kotowski, M.; van Eldik, R. Inorg. Chem. 1988, 27,
3279.
(128) Chang, S. G.; Littlejohn, D.; Lynn, S. EnViron. Sci. Technol. 1983,
17, 649.
(129) Davies, N. A.; Wilson, M. T.; Slade, E.; Fricker, S. P.; Murrer, B.
A.; Powell, N. A.; Henderson, G. R. Chem. Commun. 1997, 47.
(142) Roussel, P.; Scott, P. J. Am. Chem. Soc. 1998, 120, 1070-1071.
(143) Roussel, P.; Hitchcock, P. B.; Tinker, N.; Scott, P. Chem. Commun.
1996, 2053.

Nitrous Oxide N-N Bond CleaVage by Mo(III) Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7277
quantity equal to G, namely 231.2 ( 3.2 kcal‚mol-1. Since the
latter value is in excess of the value of D by 107.4 ( 4.0
kcal‚mol-1, one sees that oxidation of complex 1 by nitrous
oxide would have been vastly preferable to N-N bond splitting,
the pathway which prevails evidently for reasons that are entirely
kinetic in origin.
Note that in Figure 6, the left branch considering the
hypothetical oxidation of complex 1 requires 2 equiv of N2O
for comparison with the right branch, because in the right branch
each molecule of N2O effects conversion of 2 equiv of complex
1. In other words, from the standpoint of bonds formed, one
must compare the formation of two Mo-O bonds against the
formation of one MotN and one Mo-NO bond.
Synthetic and Structural Considerations for Mo(N[2-Ad]-
Ar)3 (2). The required aniline HN(2-Ad)Ar was synthesized
according to the literature procedure for HN(2-Ad)Ph.147 Simply
heating 2-adamantanone with 2 equiv each of formic acid and
3,5-dimethylaniline using Lueckart conditions148 resulted in the
formation of the desired aniline in essentially quantitative yield,
after accounting for the recovery of unreacted starting materials.
Carrying out the synthesis using the published procedure with
a 5 h reflux resulted in smooth product formation, but a
substantial increase in conversion was realized when the reflux
was extended to 24 h. Crystallization from pentane serves as
an efficient purification for the aniline, HN(2-Ad)Ar. The
signature 1H NMR signal for HN(2-Ad)Ar in C6D6 is a
resonance at 3.50 ppm assigned to the C-H moiety adjacent to
nitrogen.
Deprotonation of HN(2-Ad)Ar using n-BuLi in a hydrocarbon
solvent was found to result in clean, quantitative formation of
Li(N[2-Ad]Ar), which precipitates directly from the reaction
mixture. The lithium reagent is essentially insoluble in hydro-
carbon solvents but has good solubility in ether and THF. The
1H NMR spectrum of Li(N[2-Ad]Ar) in THF-d8 features a signal
at 3.31 ppm for the C-H moiety adjacent to nitrogen.
Complex 2 is synthesized straightforwardly. Treatment of
MoCl3(THF)3149 with a deficiency of Li(N[2-Ad]Ar) according
to standard procedures45 provided the desired three-coordinate
molybdenum(III) complex in 64% recrystallized yield. The
solubility properties of complex 2 are different enough from
those for highly lipophilic 1 that crystallization was initiated
upon storage of a concentrated solution at room temperature.
The solubility properties of complex 2 are similar to those of
the previously described N-1-adamantyl-substituted molybde-
num(III) derivative.43
The 1H NMR spectrum of complex 2 consists of several broad
resonances, the most prominent of which is the signature
resonance for the 18 benzylic hydrogens C6H3(CH3)2 appearing
at -3.00 ppm. An Evans method150,151 magnetic susceptibility
measurement is consistent with assignment of complex 2 as
having a quartet ground state (µeff ) 3.88 µB), as expected for
three-coordinate molybdenum(III) complexes of this type.45,144-146
The increased steric bulk of the 2-adamantyl substituent relative
to the tert-butyl substituent of 1 manifests itself during recrys-
tallization of N-2-adamantyl derivative 2. Storage of the purified
material at -35 °C under N2 (1 atm) does not result in the
formation of a bridging dinitrogen complex, which in the case
of 1 is deep purple in color,42,45,47 and the diamagnetic nitrido
complex NMo(N[2-Ad]Ar)3 (2-N) is not produced under these
Figure 6. Thermodynamic parameters for the reaction between
complex 1, here represented as {Mo}, and N2O. Note that 2 equiv of
1 and either 2 equiv (left branch) or 1 equiv (right branch) of N2O are
taken as the zero of energy. Quantities measured directly by solution
calorimetry in this work correspond to C and D, respectively -82.5 (
2.5 kcal‚mol-1 (for 1 + NO) and -123.8 ( 0.8 kcal‚mol-1 (for 2 equiv
of 1 + N2O). Using the literature value of 114 kcal‚mol-1 for A in
combination with C and D gives 155.3 ( 3.3 kcal‚mol-1 for B, the
Mo-N triple bond energy in 1-N. Oxo transfer is calculated for 2 equiv
of 1 to be downhill by twice the previously measured Mo-O BDE of
155.6 ( 1.6 kcal‚mol-1 (F) offset by E, twice the literature value (40
kcal‚mol-1) for the nitrous oxide N-O BDE, giving G ) -230
kcal‚mol-1
.
(154 kcal‚mol-1 for NtMo(NH2)3 and 157 kcal‚mol-1 for
NtMo(NMe2)3).144 It appears that the calorimetrically deter-
mined MotN BDE for nitride 1-N is the first such value to be
so obtained, and it is of interest further in that it indicates that
the splitting of N2 by 2 equiv of complex 1, giving ultimately
2 equiv of 1-N, is favored enthalpically by ca. 86 kcal‚mol-1
.
As has been surmised previously, therefore, the formation of
two very strong MotN triple bonds serves as the driving force
for N2 cleavage by complex 1.47,45,144-146
Whereas reducing metal complexes typically effect oxo
abstraction from N2O,68-76 with liberation of N2 as a stable
byproduct, the reaction of complex 1 with N2O is completely
selective for N-N bond cleavage.79,80 It is of interest to
determine if the latter observed outcome is in accord with
thermodynamic considerations. It has been reported that the
Mo-O BDE for the terminal molybdenum(V) oxo complex 1-O
is 155.6 ( 1.6 kcal‚mol-1, a value equal to F/2 in Figure 6.
This fact, taken in conjunction with the knowledge that the
NN-O BDE value is 40 kcal‚mol-1 130-133
permits the conclu-
,
sion that the conversion of 2 equiv of complex 1 to 2 equiv of
1-O (if effected by 2 equiv of N2O) would be downhill by a
(144) Neyman, K. M.; Nasluzov, V. A.; Hahn, J.; Landis, C. R.; Ro¨sch,
N. Organometallics 1997, 16, 995-1000.
(147) Lavrova, L. N.; Klimova, N. V.; Shmar’yan, M. I.; Ul’yanova, O.
V.; Vikhlyaev, Y. I.; Skoldinov, A. P. Zh. Org. Khim. 1974, 10, 761.
(148) Leuckart, R. Chem. Ber. 1885, 18, 2341.
(149) Dilworth, J. R.; Zubieta, J. Inorg. Synth. 1986, 24, 193.
(150) Evans, D. F. J. Chem. Soc. 1959, 2003.
(145) Hahn, J.; Landis, C. R.; Nasluzov, V. A.; Neyman, K. M.; Ro¨sch,
N. Inorg. Chem. 1997, 36, 3947-3951.
(146) Cui, Q.; Musaev, D. G.; Svensson, M.; Sieber, S.; Morokuma, K.
J. Am. Chem. Soc. 1995, 117, 12366-12367.
(151) Sur, S. K. J. Magn. Reson. 1989, 82, 169.

7278 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
whether the reaction proceeds by [NNN] radical transfer155,156
followed by N2 loss, or by N atom abstraction followed by fast
decomposition of the diazenyl radical [NNAr′] (Ar′ ) mesityl
or p-tolyl).157,158
The organoazide N atom abstraction reaction serves also as
a convenient independent synthesis of nitride derivative NMo(N[2-
Ad]Ar)3 (2-N), the complex being obtained in 34% yield after
an experiment performed in ether. Multiple recrystallizations
from ether were required to provide 2-N in pure form,
accounting for the low isolated yield. In its proton NMR
spectrum a characteristic resonance at 4.29 ppm (C6D6) is
associated with the three amido â-hydrogens of diamagnetic
2-N.
To characterize complex 2 chemically, its reaction with white
phosphorus was investigated. A solution of complex 2 was
treated with a large excess of P4 to generate the diamagnetic
phosphide complex, PMo(N[2-Ad]Ar)3 (2-P), as a gold-brown
solid, which was isolated in 54% yield (not optimized).
Figure 7. Structural diagram of Mo(N[2-Ad]Ar)3 (2) with thermal
ellipsoids drawn at the 30% probability level. Selected distances (Å):
Mo-N1, 2.001(3); Mo-N2, 1.990(3); Mo-N3, 1.981(3); N1-C111,
1.473(4); N1-C11, 1.426(4); N2-C211, 1.482(4); N2-C21, 1.430(4);
N3-C311, 1.479(4); N3-C31, 1.429(4). Selected angles (°): N1-
Mo-N2, 121.24(11); N1-Mo-N3, 119.92(11); N2-Mo-N3,
118.79(11); Mo-N1-C111, 128.9(2); Mo-N1-C11, 112.2(2);
C11-N1-C111, 116.6(3); Mo-N2-C211, 127.4(2); Mo-N2-C21,
111.2(2); C21-N2-C211, 117.8(3); Mo-N3-C311, 132.6(2);
Mo-N3-C31, 108.0(2); C31-N3-C311, 116.1(3).
1
Phosphide 2-P exhibits a H NMR resonance at 4.70 ppm for
its three â-hydrogens, and in its 31P NMR spectrum a single
signal is located at 1215 ppm, the extremely large downfield
shift being characteristic of terminal molybdenum(VI) and
tungsten(VI) phosphide derivatives.159-171 The significance of
the reaction of 2 with P4 rests in its indication that despite
substantial crowding imposed by the 2-adamantyl substituents
(Figure 7), sufficient space still is available at the molybdenum
center to permit access to substrates at least as large as the
tetrahedral P4 molecule.
conditions. As has been expounded on in the case of the
1-adamantyl derivative,41 Mo(N[1-Ad]Ar)3, the sterically en-
cumbering adamantyl substituents serve to inhibit formation of
a dinuclear dinitrogen complex, thereby shutting down the
dinitrogen cleavage reaction.41
A crystal suitable for an X-ray diffraction study of phosphide
2-P was obtained from a chilled ether solution. The Mo-P
vector of the molecule (Figure 8) is coincident with a crystal-
lographic three-fold axis of symmetry, and the dMo-P value of
2.107(3) Å is in the expected range for molybdenum(VI)
terminal phosphide complexes.160-162 The dMo-N value in the
complex is 1.984(5) Å, not appreciably smaller than in the
molybdenum(III) precursor, complex 2 (Figure 7). Phosphide
2-P displays a pseudo-tetrahedral MoPN3 core, with the
Crystals of complex 2 suitable for an X-ray diffraction study
were obtained from a chilled ether solution. A representative
d
Mo-N value is 1.990(3) Å for the pseudo-three-fold-symmetric
complex, while a representative N-Mo-N value is 119.92(11)°
(Figure 7). The sum of the three values is 359.95°,
N-Mo-N
reflecting the trigonal planar coordination environment of the
molybdenum center. A conformational feature of some interest
involves the three C-H moieties adjacent to the nitrogen atoms.
The three C-H vectors corresponding to these three â-hydro-
gens are roughly normal to the molecular pseudo-three-fold axis,
and they also lie in a plane approximately coparallel with the
MoN3 plane. With the substituents disposed in such a manner,
the conformational restrictions on the motions of the 2-ada-
mantyl groups may be responsible for the absence of â-hydrogen
elimination reactivity in the chemistry of 2. By way of contrast,
it should be recalled that the analogue of formula “Mo(N[iPr]-
Ar)3” exists as its metallaziridine hydride tautomer and has a
doublet rather than a quartet ground state.41
value equal to 102.76(14)° and the
equal to 115.27(10)°. The observed compression of
value
N-Mo-P
N-Mo-P
N-Mo-N
(155) Johnson, A. R.; Davis, W. M.; Cummins, C. C. Organometallics
1996, 15, 3825-3835.
(156) Osborne, J.; Rheingold, A.; Trogler, W. J. Am. Chem. Soc. 1985,
107, 7945.
(157) Suehiro, T.; Masuda, S.; Tashiro, T.; Nakausa, R.; Taguchi, M.;
Koike, A.; Rieker, A. Bull. Chem. Soc. Jpn. 1986, 59, 1877.
(158) Suehiro, T.; Masuda, S.; Nakausa, R.; Taguchi, M.; Mori, A.;
Koike, A.; Date, M. Bull. Chem. Soc. Jpn. 1987, 60, 3321.
(159) Wu, G.; Rovnyak, D.; Johnson, M. J. A.; Zanetti, N. C.; Musaev,
D. G.; Morokuma, K.; Schrock, R. R.; Griffin, R. G.; Cummins, C. C. J.
Am. Chem. Soc. 1996, 118, 10654-10655.
(160) Laplaza, C. E.; Davis, W. M.; Cummins, C. C. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2042-2044.
(161) Zanetti, N. C.; Schrock, R. R.; Davis, W. M. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2044-2046.
(162) Johnson, M. J. A.; Lee, P. M.; Odom, A. L.; Davis, W. M.;
Cummins, C. C. Angew. Chem., Int. Ed. Engl. 1997, 36, 87-90.
(163) Scheer, M.; Kramkowski, P.; Schiffer, M.; Muller, J. Phosphorus,
Sulfur, Silicon Relat. Elem. 1999, 146, 717.
(164) Kramkowski, P.; Baum, G.; Radius, U.; Kaupp, M.; Scheer, M.
Chem.-Eur. J. 1999, 5, 2890.
Insights from Reaction Chemistry of Complex 2. In the
original report on complex 1, it was shown that a terminal nitrido
derivative, namely NMo(N[R]Ar)3 (1-N), could be prepared by
N atom abstraction from mesityl azide (MesN3).79 This reaction
was unusual in that, normally, reducing metal complexes react
with organic azides to give organoimido species with loss of
dinitrogen.58,152-154 Via a labeling experiment employing 15NNN-
p-tolyl as the source of the nitrido nitrogen atom, it was shown
that the abstracted nitrogen originates from the terminal posi-
tion.79 From a mechanistic point of view, it is not known yet
(165) Scheer, M.; Kramkowski, P.; Schuster, K. Organometallics 1999,
18, 2874.
(166) Chisholm, M.; Folting, K.; Scheer, M. Polyhedron 1998, 17, 2931.
(167) Scheer, M.; Leiner, E.; Kramkowski, P.; Schiffer, M.; Baum, G.
Chem.-Eur. J. 1998, 4, 1917.
(168) Scheer, M.; Muller, J.; Baum, G.; Haser, M. Chem. Commun. 1998,
1051.
(169) Scheer, M. Coord. Chem. ReV. 1997, 163, 271.
(170) Scheer, M.; Muller, J.; Haser, M. Angew. Chem., Int. Ed. Engl.
1996, 35, 2492.
(152) Proulx, G.; Bergman, R. G. J. Am. Chem. Soc. 1995, 117, 6382-
6383.
(153) Proulx, G.; Bergman, R. Organometallics 1996, 15, 684.
(154) Fickes, M. G.; Davis, W. M.; Cummins, C. C. J. Am. Chem. Soc.
1995, 117, 6384-6385.
(171) Scheer, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1997.

Nitrous Oxide N-N Bond CleaVage by Mo(III) Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7279
through by the constant concentration of nitrous oxide [N2O]
yields a corresponding second-order rate constant kNNO2 ) 0.47
( 0.03 M-1 s-1. Thus it is seen that the splitting of N2O by the
sec-adamantyl-substituted derivative complex 2 is slower by
approximately an order of magnitude than for tert-butyl-
substituted complex 1.
That the reaction in question leads to a 1:1 mixture of nitride
2-N and nitrosyl 2-NO was verified by proton NMR analysis
of the crude mixtures stemming from the kinetic studies, and
also by multiple independent experiments.
Competitive NO Binding by Complexes 1, 2, and 3. The
binding of NO by three-coordinate metal complexes was
investigated first by Bradley and co-workers in the context of
the chromium derivative Cr(NiPr2)3, complex 3.107,109,110
Obtained smoothly upon treatment of complex 3 with 1 equiv
of NO is the terminal nitrosyl derivative 3-NO, a linear nitrosyl
characterized by a low νNO of 1651 cm-1, and serving as a
chromium analogue of four-coordinate 1-NO. Pseudo-tetrahedral
chromium nitrosyls such as 3-NO have been employed recently
as intermediates in the synthesis of chromium(VI) terminal
nitrido derivatives.110,174 It was hoped in the present study that
chromium complex 3 would be competitive with molybdenum
complex 1 with respect to rate of NO binding, because control
experiments showed 3 to be inert to N2O. Subject to the criteria
articulated below, complex 3 is to be used as an NO scavenger
for the purpose of mechanistic discrimination. Also, 3-NO
showed no reactivity (i.e., NO or O transfer) with complexes 1
or 2.
In the case of the 2-adamantyl-substituted three-coordinate
complex 2, the reaction with NO also proceeds smoothly, and,
in a preparative experiment, nitrosyl (ON)Mo(N[2-Ad]Ar)3 (2-
NO) was obtained in 70% yield after recrystallization from THF.
The proton NMR resonances of diamagnetic 2-NO are similar
to those for nitride 2-N, but the diagnostic signal for the three
â hydrogens of 2-NO appears at 4.58 ppm (C6D6).
Prerequisite to the inclusion of Cr derivative 3 in the nitrous
oxide reaction with 1, information on the relative rates of NO
binding by 3 and 1 was required. Accordingly, experiments were
carried out in which a 1:1 mixture of 3 and 1 was treated with
1 equiv of NO, conditions under which the latter constitutes
the limiting reagent. Thus, the product mixture is expected to
contain nonzero concentrations of four components: Mo
complex 1, Cr complex 3, and the two corresponding nitrosyls
1-NO and 3-NO. The experiment is designated schematically
in Figure 9, the quantity of interest being the fraction H
measured at t∞.
Because of the complexity of the proton NMR spectra of such
mixtures, 1H NMR spectroscopy was used solely for verification
of the presence of the four components, and to ascertain that
the transformation had proceeded without side reactions.
Quantification of the fraction H (Figure 9) was accomplished
using 15N NMR spectroscopy, to which end all experiments were
carried out using the isotopomer 15NO. 15N NMR chemical shifts
for 1-15NO and 3-15NO were determined to be 420.0 and 436.7
ppm, respectively, with relaxation times (T1) measured at 3.38
( 0.16 and 5.87 ( 0.26 s, respectively, as determined for the
pure components. Analysis of the product mixture by 15N NMR
spectroscopy therefore is satisfactory since T1 values in the
presence of paramagnetic complexes generally are smaller than
those for pure diamagnetic substances, and, in any event, the
latter was shown by in situ measurement to be the case for the
mixtures in question. Spectra for integration were acquired with
Figure 8. Structural diagram of PMo(N[2-Ad]Ar)3 (2-P) with thermal
ellipsoids drawn at the 30% probability level. Selected distances (Å):
Mo-P, 2.107(3); Mo-N, 1.984(5); N-C11, 1.447(8); N-C111,
1.504(8). Selected angles (°): N-Mo-N′, 115.27(10); P-Mo-N,
102.76(14); Mo-N-C11, 110.4(4); Mo-N-C111, 135.8(4); C11-
N-C111, 111.0(5).
relative to the tetrahedral angle allows for maximum orthogo-
nality of the Mo-P π-bonds (axial) relative to the Mo-N π
bonds (equatorial).
That the introduction of a phosphorus atom into the space
bounded by the three 2-adamantyl groups requires substantial
flexing of the molecular framework is indicated by the ca. 7°
increase of the
(to the 2-adamantyl group) vis-a`-vis
Mo-N-C
precursor 2. A complementary indication of this flexing to
accommodate the phosphide phosphorus atom is a compressed
value of
relative to precursor 2, the decrease in this
C-N-C
metric parameter being ca. 6°. Inspection of a space-filling
model of phosphide 2-P suggests that the metal atom is
completely encapsulated by this complement of four ligands,
such that the complex is expected not to undergo any reactions
requiring coordination of a substrate to the metal center. A
propellor-like arrangement of the three aryl paddles, an arrange-
ment allowing for three edge-to-face interactions172,173 which
align one ortho hydrogen per ring with a proximal π cloud, is
observed for both complexes 2 and 2-P. The latter conforma-
tional attribute now is recognized to be a structural calling card
for three-fold-symmetric molecules possessing an M(N[hydro-
carbyl]Ar)3 moiety.45
In view of the rapid reaction of complex 1 with N2O, the
reaction of 2-adamantyl derivative 2 with N2O is noteworthy
for being rather more sluggish. Accordingly, treatment of 2 in
toluene solution with an excess of N2O, added as a saturated
toluene solution, resulted in consumption of complex 2 accord-
ing to a smooth pseudo-first-order kinetic profile. Kinetic data
for the reaction, carried out at 26 °C, were collected multiple
times by conventional diode array spectrophotometric analysis
at both of two initial concentrations of complex 2. Suitable
wavelengths for monitoring the decay of complex 2 were
determined to be 427 or 500 nm, depending on the initial
concentration of 2 employed. Simultaneous monitoring of the
appearance of nitrosyl and nitrido products unfortunately was
not possible due to spectral overlap. The pseudo-first-order rate
constant kobs obtained for the process was 0.031 ( 0.002 s-1
,
for initial concentrations [2]0 of 0.6 and 2.4 mM. Dividing
(172) Engkvist, O.; Hobza, P.; Selzle, H.; Schlag, E. J. Chem. Phys.
1999, 110, 5758.
(173) Li, Z.; Ohno, K.; Kawazoe, Y.; Mikami, M.; Masuda, Y. Surf.
ReV. Lett. 1996, 3, 359.
(174) Chiu, H. T.; Chen, Y. P.; Chuang, S. H.; Jen, J. S.; Lee, G. H.;
Peng, S. M. Chem. Commun. 1996, 139.

7280 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
considered here are in the order 1 > 3 > 2. It is consistent with
chemical intuition that the most hindered system, namely the
2-Ad-substituted complex 2, should be less competitive in its
t
ability to scavenge NO than its Bu-substituted congener,
complex 1. Less obvious is that the chromium(III) complex 3
should be intermediate in reactivity, but this outcome is dramatic
in its illustration of the ability of ligands to attenuate reactiv-
ity.176,177
Internal consistency in the above-determined numbers was
obtained by carrying out a third competition experiment, namely
the treatment with 15NO (1 equiv) of a 1:1 mixture of the two
molybdenum complexes, 1 and 2. Measured in this case was a
ratio of nitrosyls J ) 2.29, a value in very good agreement with
that obtained indirectly by dividing the results of the two
foregoing experiments that involve chromium (i.e., 1.65/0.73
) 2.26). Since J ) 2.29, the corresponding ratio of second-
order rate constants kNO1/kNO2 ) 3.28 is also in good agreement
with dividing the respective independent calculations (i.e., 2.06/
0.64 ) 3.22).
Figure 9. Schematic representation of three separate competition
experiments in which 1:1 mixtures of 1 and 3, 2 and 3, and 1 and 2
were treated with NO (1 equiv). Experiments carried out with 15NO
were analyzed by 15N NMR spectroscopy to determine the three ratios
of product nitrosyls, the quantities H, I, and J as indicated, at a time t
corresponding to complete reaction. An internal check is present since
H/I ) J.
Competition Experiments Involving N2O. Knowing the
relative rate constants for NO binding by the three-coordinate
molybdenum(III) and chromium(III) complexes, it is possible
to generate expectation values for the ratio of nitrosyls to be
produced in experiments involving nitrous oxide splitting (Figure
12), assuming a mechanism that involves molecular NO as a
post-rate-determining intermediate (e.g., path 1, Figures 1 and
11).
Accordingly, kinetic simulations for the treatment of a 1:1
mixture of molybdenum complex 1 and chromium complex 3
with an excess (10 equiv) of 15N2-labeled nitrous oxide led to
a predicted value for the ratio of nitrosyls K ) 0.89, whereas
the observed value (15N NMR analysis) after complete con-
sumption of complex 1, the limiting reagent, was 27.7. This
striking result implies that the capture of molecular NO by the
metal complexes is not the dominant mechanism of metal
nitrosyl formation in the nitrous oxide N-N bond cleavage
reaction. Given the system proposed in Figure 11, one can
account for the formation of small amounts of 3-15NO by either
the heterobimetallic pathway or by some linear combination of
path 2 and path 1. Either way, the conclusion may be drawn
firmly that path 1, the pathway involving free NO in a post-
rate-determining step, is not dominant.
Figure 10. 15N NMR data for three competition experiments in which
1:1 mixtures of 1 and 3, 2 and 3, and 1 and 2 were treated with 15NO
(1 equiv). The experiments serve to provide the quantities H, I, and J,
as defined schematically in Figure 9.
delay times of at least 10 sec (45° pulse width), and measured
values for the fraction H were found to be reproducible. Similar
considerations apply for the quantities I and J, as discussed
below.
Introduction of chromium complex 3 into the relatively slow
N2O splitting reaction system involving the bulkier 2-Ad-
substituted Mo complex 2 again led to the observation of far
smaller amounts of the chromium nitrosyl 3-15NO than expected
if molecular NO were important as an intermediate. Accordingly,
treatment of a 1:1 mixture of 2 and 3 with 10 equiv of 15N2-
labeled nitrous oxide led after complete reaction to the ratio of
nitrosyls L ) 1.24, whereas the expected value, assuming NO
capture as the sole mechanism of nitrosyl formation, was 0.34.
The observation indeed revealed a greater percentage of 2-15NO
formation than expected if NO capture were the dominant
nitrosylation mechanism. Furthermore, if the dominant nitro-
sylating agent in the present case is the putative adduct 2-NNO,
then the observed value of L indicates that Cr complex 3 is
competitive with Mo complex 2 for interception of this
intermediate. Consideration of steric factors indicates that this
should be so, inasmuch as it ought to be relatively difficult to
pack two molecules of 2-adamantyl-substituted 2 around a single
N2O molecule. In line with such reasoning is the observed value
of K, which suggests complex 3 to be less efficient at scavenging
putative 1-NNO than is complex 1 itself.
The observed value for H after nitrosylation of a 1:1 mixture
of 1 and 3 was 1.65, a value corresponding to the following
ratio of second-order rate constants for NO binding: kNO1/kNO3
) 2.06. Numerical integration of the differential equations
describing this kinetic system was employed to obtain the value
of kNO1/kNO3, given the observed product ratio H and the stated
initial conditions. Importantly, the same value within error for
H was obtained regardless of whether the NO was predissolved
and introduced as a solution, or added in gaseous form to the
headspace in the reaction vessel.175
It was possible also to gauge the efficiency of NO binding
by Cr complex 3 relative to the more sterically encumbered
three-coordinate molybdenum(III) complex 2. Accordingly,
treatment of a 1:1 mixture of 2 and 3 with 1 equiv of 15NO led
to an observed ratio of nitrosyls I ) 0.73, corresponding to a
ratio of second-order rate constants kNO2/kNO3 ) 0.64. Thus it
is seen that the NO binding efficiencies for the three complexes
(176) Schrock, R. R. Pure Appl. Chem. 1994, 66, 1447.
(177) Schrock, R. Acc. Chem. Res. 1990, 23, 158.
(175) McNeill, K.; Bergman, R. J. Am. Chem. Soc. 1999, 121, 8260.

Nitrous Oxide N-N Bond CleaVage by Mo(III) Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7281
Figure 11. Reaction of 1 with N2O in the presence of 3 (Cr(NiPr2)3, 1 equiv) to give a mixture of Mo and Cr nitrosyls 1-NO and 3-NO, along with
the Mo nitrido 1-N. Known Cr nitrido 3-N is not observed among the products. Formation of 3-NO may occur by NO capture with second-order
rate constant kNO3 if path 1 is the dominant N2O cleavage pathway, or via an unobserved heterobimetallic adduct 1-NNO-3 if path 1 is not operative.
the absence of observable intermediates. Only very recently has
the reverse reaction been observed, namely the formation of
nitrous oxide upon treatment of a nitrido-metal complex with
NO.81
The present work has shed light on the thermodynamic
considerations involved in the cleavage reaction, it being most
striking that the observed reaction is enthalpically far less
favorable than a plausible alternative scenario that would involve
nitrous oxide deoxygenation. A thermodynamic quantity derived
from the present work is a value for the molybdenum-nitrogen
triple bond dissociation energy for nitrido derivative 1-N, this
quantity (155.3 ( 3.3 kcal‚mol-1) being relevant also to another
Figure 12. Definition of the quantity K as the ratio of nitrosyls
important reaction of complex 1, namely the splitting of
produced upon treatment of a 1:1 mixture of complexes 1 and 3 with
dinitrogen via a well-defined bimetallic intermediate.40-47
10 equiv of N2O. Implicit in this experiment is that for every nitrosyl
Computational studies have devoted effort to prediction of the
MotN bond energy,144-146 and from the present work an
experimental value now is available for comparison.
formed, also formed is an equivalent of the molybdenum nitride 1-N.
If the mechanism of nitrosyl formation were NO capture (path 1 of
Figure 11), then the value of K would be predictable based on the
independently determined quantity H; see Figures 9 and 10 and the
text. Similar considerations apply to the quantities L and M.
Given that the nitrous oxide N-N bond cleavage reaction is
not under thermodynamic control, kinetic parameters were
investigated. The reaction of complex 1 with an excess of N2O
was studied by stopped-flow techniques, while the reaction of
the more hindered three-coordinate Mo(III) derivative complex
2 with N2O, which proceeds similarly to give a 1:1 mixture of
nitrosyl and nitride products, was amenable to monitoring by
conventional diode array spectrophotometric techniques. In both
cases it was determined that the reaction is first order in the
starting molybdenum complex, mandating that if the N-N bond
cleavage process is bimetallic in nature, then the relevant
bimetallic intermediate is post-rate-determining in nature. That
N2O binding is the slow step may reflect attendant structural
changes, e.g., the bending of N2O upon complexation, along
with a change in spin state from quartet to doublet.178-183
Complexation of N2O by the molybdenum complexes of
Also examined was the competition experiment involving
addition of 0.5 equiv of 15N2O to a solution containing a 1:1
mixture of the two molybdenum complexes 1 and 2 (Figure
12). Determination of the ratio M of nitrosyls produced after
complete reaction was effected by 15N NMR spectroscopy, the
value obtained being too large to measure accurately because
of the extremely small amount of 2-NO produced. Using the
values determined above for kNO1/kNO2 and kNNO1/kNNO2, it was
predicted via simulation that the value for M would have been
1.75 if NO capture were the sole means of nitrosyl formation.
This final competition experiment serves to underscore further
the inconsistency of path 1 (Figure 11), the monometallic bond
cleavage pathway, with the collective competition experiments.
(178) Smith, K.; Poli, R.; Harvey, J. New J. Chem. 2000, 24, 77.
(179) Cacelli, I.; Poli, R.; Quadrelli, E.; Rizzo, A.; Smith, K. Inorg. Chem.
2000, 39, 517.
(180) Poli, R.; Smith, K. Eur. J. Inorg. Chem. 1999, 2343.
(181) Poli, R.; Smith, K. Eur. J. Inorg. Chem. 1999, 877.
(182) Smith, K.; Poli, R.; Legzdins, P. Chem. Commun. 1998, 1903.
(183) Poli, R. Acc. Chem. Res. 1997, 30, 494.
3. Concluding Remarks
First reported in 1995,79 and elaborated upon in 1998,80 the
intriguing and counterintuitive observation of nitrous oxide N-N
bond cleavage by a three-coordinate molybdenum(III) com-
plex44,102 has been difficult mechanistically to unravel due to

7282 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
Stopped-Flow Kinetic Measurements. Nitrous oxide (99.99%,
Electronic Gases, The BOC Group, Inc.), nitric oxide (99.9%, Airgas),
and argon (ultra-high-purity grade, Airgas) were used as received.
Solutions of Mo(N[R]Ar)3 in toluene or diethyl ether (0.3-1.2 mM)
were prepared in a Vacuum Atmospheres glovebox filled with argon.
Solutions of nitric oxide were prepared at 25 °C by bubbling the gas
through the degassed dry solvent in a Hamilton gastight 10 mL syringe
furnished with a Kontes three-way stopcock in a gas-bag filled with
argon. Solutions of nitrous oxide of various known concentrations were
prepared in the same way from N2O and Ar gas mixtures generated
using Bel-Art Products flowmeters calibrated for these gases. The
solubilities of the gases at 25 °C are 133 ( 4 mM N2O in toluene, 30
( 1 mM N2O in diethyl ether, and 12 ( 1 mM NO in toluene (see
Supporting Information for details).
relevance here is expected to be a reductive process akin to the
binding of organic azides at their nitrogen terminus by group 5
metal complexes.152-154 Activation parameters were determined
for the reaction system featuring N2O cleavage by complex 1,
said parameters being indicative of the energetics for N2O
reductive complexation.
One mechanistic scenario consistent with the information
summarized in the preceding paragraphs implicates free nitric
oxide as a post-rate-determining intermediate. To investigate
this possibility, Bradley’s three-coordinate chromium(III) com-
plex107,109,110 was used as an in situ NO scavenger in the nitrous
oxide N-N bond cleavage reactions. Appropriate control and
competition experiments revealed the formation of Bradley’s
nitrosyl in quantities far less than predicted by post-rate-
determining NO capture. The simplest mechanism consistent
with all the data is a post-rate-determining bimetallic scission
of the nitrous oxide molecule. This conclusion is particularly
intriguing when juxtaposed with the contrary results of a recent
computational study, a study suggesting that a bimetallic
mechanism ought to lead to deoxygenation rather than N-N
bond cleavage.184
Perhaps most telling vis-a`-vis the kinetically driven selectiVity
of the nitrous oxide N-N bond cleavage reaction is the
observation that the N2O binding event represents the rate-
determining step. Corresponding to this key binding event is a
transition state that appears thus far to have defied description
via computational methods.184 Evidently the putative adduct,
e.g. 1-NNO, is a potent nitrosylating agent. It is predicted that
if steric considerations can be so arranged as to preclude the
formation of a post-rate-determining bimetallic intermediate,
then other chemistries of the mononuclear N2O adduct might
be realized.
Caution! The solutions containing N2O must be bubbled through
with an inert gas before disposal! A nonchemical explosion happened
during the storage of a tightly closed bottle with such wastes, probably
due to the slow reaction of N2O with organic substances to form much
less soluble N2.
Visible spectra were recorded on a Hitachi U-2000 UV-vis
spectrophotometer. Kinetic measurements were performed at temper-
atures from -78 to +25 °C using a Hi-Tech Scientific (Salisbury,
Wiltshire, UK) SF-43 Multi-Mixing CryoStopped-Flow instrument
equipped with stainless steel plumbing, a stainless steel mixing cell
with sapphire windows, and an anaerobic gas-flushing kit. The
instrument was connected to an IBM computer with IS-2 Rapid Kinetics
software by Hi-Tech Scientific. The temperature in the mixing cell was
maintained to (0.1 K, and the mixing time was 2 ms. The driving
syringe compartment and the cooling bath filled with hexanes (Fisher)
were flushed with argon before and during the experiments, using
anaerobic kit flush lines. All flow lines of the SF-43 instrument were
extensively washed with degassed anhydrous solvent before charging
the driving syringes with reactant solutions. Most of the experiments
were performed in a single-mixing mode of the instrument, with a
solution of Mo(N[R]Ar)3 being directly mixed with a N2O solution in
1:1 ratio. A multimixing mode was used in some experiments to dilute
N2O stock solution 2 times with the solvent directly in the stopped-
flow apparatus, before the Mo(N[R]Ar)3 was added. These measure-
ments were designed in order to avoid any possible contact of N2O
solutions with air upon dilution. In all kinetic experiments, series of
8-12 shots gave standard deviations within 5%, with overall reproduc-
ibility (established by repeated preparation and mixing of starting
solutions) within 10%.
Although N2O adducts have not in this work been directly
observed, it seems nevertheless that the activation pathway
followed ultimately is dictated by the properties of N2O as a
ligand,82,83 in conjunction with the unique N-atom-accepting
propensity demonstrated for tris-amido molybdenum(III) com-
plexes.44
4. Experimental Procedures
Synthesis of HN(2-Ad)Ar. 3,5-Dimethylaniline (33.4 mL, 268
mmol, 2.01 equiv), formic acid (10.4 mL of a 95-98% solution, ca. 2
equiv), and 2-adamantanone (20.06 g, 133.5 mmol) were heated at 95-
100 °C for 24 h. Concentrated HCl (70 mL) was then added, and the
solution was heated at 95 °C for 1 h. A copious amount of white solid
appeared. The reaction mixture was diluted with water (100 mL), and
the white solid was collected on a frit. The filtrate contained the
hydrochloride of 3,5-dimethylaniline. The filter cake was washed with
water (100 mL), and 3,5-dimethylaniline was recovered from the filtrate
by adding concentrated NaOH solution (ca. 5 M) until the pH was ca.
14, at which point a yellow oil appeared. The aqueous layer was washed
with ether (2 × 150 mL), and the solvent was removed in vacuo to
yield a yellow oil (22 mL, 176 mmol). The filter cake was then washed
with ether (2 × 100 mL), removing unreacted 2-adamantanone, which
was recovered by removing the ether in vacuo (9.82 g, 73.5 mmol).
The filter cake was slurried in water (200 mL) and ether (200 mL),
and NaOH (12 g) was added to neutralize the hydrochloride. The
organic layer was separated, and the aqueous layer was washed with
ether (2 × 200 mL). The combined organics were washed with saturated
aqueous sodium chloride (200 mL) and saturated aqueous sodium
bicarbonate (200 mL). The combined aqueous layers were then extracted
with ether (200 mL). The combined organics were evaporated, resulting
in a white solid (14.9 g, 58.3 mmol, 43.7%). The solid was recrystallized
from pentane at -35 °C in five crops (13.67 g, 53.54 mmol, 40%).
Mp: 57-58 °C. 1H NMR (500 MHz, C6D6): δ ) 6.412 (s, 1H, para),
6.252 (s, 2H, ortho), 3.636 (s, 1H, NH), 3.502 (s, 1H), 2.222 (s, 6H,
ArMe), 1.948 (s, 2H), 1.614-1.802 (m, 10H), 1.4 (d, 2H). 13C NMR
(76 MHz, CDCl3): δ ) 147.41 (s), 138.73 (q), 118.63 (d), 110.82 (d),
General Considerations. Solvents were dried and deoxygenated
according to procedures detailed elsewhere.80 Diethyl ether was dried
according to a literature procedure.185 White phosphorus (Monsanto)
149
was purified by recrystallization from toluene. MoCl3(THF)3 and
43
Mo(N[tBu]Ar)3 were prepared via published procedures. Other
chemicals were purified via standard procedures or used as received.
Magnetic susceptibilities were measured by NMR spectroscopy.150,151
NMR spectra were recorded on Varian XL-300, Varian Unity-300,
Varian Mercury-300, or Varian VXR-500 spectrometers. H and 13C
1
NMR chemical shifts were reported with reference to solvent resonances
(C6D6, 7.15, 128.0; CDCl3, 7.24, 77.0; C4D8O, 3.58 and 1.73, 67.6 and
25.4). 15N NMR spectra were referenced as reported previously.110 CHN
analyses were performed by Oneida Research Services (Whitesboro,
NY), Microlytics (South Deerfield, MA), or Mikroanalytisches Labor-
torium (Mu¨lheim, Germany). Melting points were obtained in sealed
glass capillaries and are uncorrected. 15NO, 15N2O, and N15NO gas as
well as C6D6, CDCl3, and C4D8O were purchased from Cambridge
Isotope Laboratory and used as is. Infrared spectra were recorded on a
Bio-Rad 135 Series FTIR spectrometer. UV-visible spectra were
recorded on a Hewlett-Packard 8453 diode array spectrophotometer.
X-ray diffraction data were collected on a Siemens Platform goniometer
with a charge-coupled device (CCD) detector.
(184) Khoroshun, D.; Musaev, D.; Morokuma, K. Organometallics 1999,
18, 5653.
(185) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.

Nitrous Oxide N-N Bond CleaVage by Mo(III) Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7283
56.60 (d), 37.68 (t), 37.37 (t), 31.65 (d), 31.54 (t), 27.43 (d), 27.30
(d), 21.43 (q). MS (70 eV): m/z (relative intensity) 255 (100) [M+].
Anal. Calcd for C18H25N: C, 84.65; H, 9.87; N, 5.48. Found: C, 84.94;
H, 10.03; N, 5.52.
1604 cm-1. Anal. Calcd for C54H72MoN4O: C, 72.95; H, 8.16; N,
6.30. Found: C, 72.63; H, 8.43; N, 5.80.
Synthesis of PMo(N[2-Ad]Ar)3 (2-P). Complex 2 (0.5212 g, 0.6067
mmol) was dissolved in toluene (10 mL), and P4 (0.0861 g, 0.695 mmol,
4.58 equiv) was added. The reaction mixture was stirred for 1 h, at
which point the solvent was removed in vacuo. The solids obtained
were dissolved in ether (20 mL) and filtered, removing a dark brown-
orange solid (ca. 90 mg). The filtrate was evaporated to dryness and
dissolved in methylene chloride (5 mL). Acetonitrile (10 mL) was
added, causing precipitation of a brown solid, which was collected on
a frit. This process was repeated with the filtrate, resulting in a second
crop of material (0.2896 g, 0.3254 mmol, 54%). Mp: 223-224 °C
Synthesis of LiN(2-Ad)Ar. HN(2-Ad)Ar (10.30 g, 40.33 mmol) was
dissolved in pentane (50 mL) and cooled to -35 °C. n-Butyllithium
(28 mL of a 1.6 M solution, 44.8 mmol, 1.1 equiv) was added via
syringe over about a 5 min period, causing the precipitation of a fine
white powder. The solid was collected on a frit and dried in vacuo to
1
a constant weight (10.19 g, 38.99 mmol, 97%). H NMR (500 MHz,
C4D8O): δ ) 5.590 (s, 2H, ortho), 5.349 (s, 1H, para), 3.306 (s, 1H),
2.150 (m, 2H), 1.962 (s, 6H, ArMe), 1.87 (s, 2H), 1.83 (m, 4H), 1.716
(s, 3H), 1.38 (d, 2H), 1.28 (m, 1H). 13C NMR (126 MHz, C4D8O): δ
) 161.87 (s, aryl ipso), 137.09 (s, meta), 110.63 (br d, ortho), 107.74
(d, para), 62.32 (d), 39.84 (t), 39.23 (t), 34.02 (t), 33.97 (d), 29.72 (d),
22.56 (q).
1
(dec). H NMR (500 MHz, C6D6): δ ) 6.654 (s, 3H, para), 5.987 (s,
6H, ortho), 4.700 (s, 3H), 3.129 (s, 6H), 2.086 (s, 18H, ArMe), 1.937
(m, 12H), 1.800 (br m, 6H), 1.651 (s, 12H), 1.413 (d, 6H). 13C NMR
(126 MHz, CDCl3): δ ) 149.94 (s), 137.06 (q), 128.33 (d), 127.15
(d), 79.40 (d), 38.68 (t), 38.21 (t), 35.32 (d), 30.83 (t), 28.11 (d), 27.83
(d), 21.57 (q). 31P NMR (203 MHz, CDCl3): δ(∆ν1/2) ) 1215.46(86).
MS (70 eV): m/z (relative intensity) 891.8 (0.72) [M+]. HRMS (EI,
70 eV): calcd mass, 891.451793; found mass, 892.451(2). Anal. Calcd
for C54H72MoN3P: C, 72.87; H, 8.15; N, 4.72. Found: C, 73.64; H,
7.92; N, 4.34.
Synthesis of Mo(N[2-Ad]Ar)3 (2). LiN(2-Ad)Ar (3.8001 g, 14.541
mmol, 2.02 equiv) was slurried in ether (100 mL), and the slurry was
then chilled until frozen. The frozen slurry was allowed to thaw, and
when magnetic stirring was just possible, MoCl3(THF)3 (3.0200 g,
7.2145 mmol) was added. The reaction mixture was stirred vigorously
for 4.5 h, during which time the solution turned dark brown. The
reaction mixture was filtered through Celite, and the filter cake was
washed with ether (2 × 20 mL) until the washings were colorless. The
filtrate was evaporated in vacuo, redissolved in ether (35 mL), and
allowed to stand at room temperature overnight. A dark precipitate was
removed by filtration, and the filtrate was reduced in volume and stored
at room temperature until crystallization initiated. The saturated solution
was then stored overnight at -35 °C. The solid powdery material was
collected on a frit and washed with cold pentane (3 × 5 mL), yielding
a dark orange material. A second crop was obtained by a similar
crystallization from pentane (15 mL) (2.279 g, 2.653 mmol, 64%).
Mp: 211-212 °C. 1H NMR (300 MHz, C6D6): δ(∆ν1/2) ) 12.818(71),
6H; 9.288(148), 6H; 5.299(ca. 300); 3.534(109); 1.962(ca. 300);
-2.995(124), 18H, ArMe; -47.64(193), 3H. MS (70 eV): m/z (relative
intensity): 861.1 (6.6) [M+]. µeff (300 MHz, C6D6): 3.875 µB. Anal.
Calcd for C54H72MoN3: C, 75.49; H, 8.45; N, 4.89. Found: C, 74.96;
H, 8.63; N, 4.70.
Exposure of Mo(N[2-Ad]Ar)3 (2) to N2. Several hundred milligrams
of complex 2 was dissolved in ether (ca. 10 mL), and the solution was
stored at -35 °C overnight. No color change was observed. A 1H NMR
spectrum of the sample indicated that complex 2 was pure and had
undergone no net reaction.
15N NMR General Considerations. For all experiments involving
15NO, 15N2O, or N15NO, the amount of gas employed was measured
using a gastight syringe. Manipulations involving the transfer of labeled
gases were carried out inside an N2-filled glovebox as follows. A septum
was fitted over the open end of the commercially obtained glass vessel
containing the appropriate 15N-labeled gas (100 mL, 1 atm). The
headspace between the septum and the break-seal was evacuated by
using a needle connected to a source of vacuum. An amount of mercury
equivalent in volume to the just-evacuated headspace was then
introduced. Next, the break-seal was broken. To maintain a constant
pressure of 1 atm inside the vessel containing the labeled gas, each
time a desired quantity of gas was withdrawn, an equal volume of
mercury was simultaneously added.
15N NMR Chemical Shifts of Relevant Species. The nitrido and
nitrosyl derivatives considered in the present study were found to have
the following 15N NMR chemical shifts (C6D6, shifts given in ppm
downfield of 15NH3): 1-15N, 837.9; 1-15NO, 420.0; 2-15N, 839.8;
2-15NO, 418.4; 3-15NO, 436.7. The 15N NMR chemical shift for the
chromium nitrido derivative 3-15N has been reported previously as 929
ppm.110
15N NMR Integration Standards. The T1 value for each 15N-labeled
nitrosyl complex was determined prior to employing 15N NMR
integration as the method for determining relative concentrations. Said
T1 values are as follows: 1-15NO, 3.38 ( 0.16 s; 2-15NO, 3.34 ( 0.29
s; 3-15NO, 5.87 ( 0.26 s. In view of the latter values and taking into
account the acquisition time (0.810 s) and delay time (10.000 s) used
for the collection of the 15N NMR spectra, integration of the NMR
spectrum is expected to be a reliable measure of concentration. This
was done through the use of integration standards.
Synthesis of NMo(N[2-Ad]Ar)3 (2-N). Complex 2 (0.4747 g, 0.5525
mmol) was dissolved in ether (10 mL) and chilled to -35 °C. Mesityl
azide (90 µL, 0.6186 mmol, 1.1 equiv) was dissolved in ether (15 mL)
and chilled to -35 °C. The solution of 2 was added to the azide solution
over 15 min. The reaction mixture turned purple and then changed to
brown over 5 min. After 1 h of stirring, the solvent was removed in
vacuo. Two crops of tan-brown powder were obtained from a
concentrated ether solution at -35 °C (0.1648 g, 0.1887 mmol, 34%).
1
Mp: 235-240 °C (dec). H NMR (300 MHz, CDCl3): δ ) 6.730 (s,
3H, para), 6.708 (s, 6H, ortho), 3.898 (s, 3H), 2.490 (s, 6H), 2.082 (s,
18H, ArMe), 2.000 (d, 6H), 1.84 (d, 12 H), 1.72 (m, 6H), 1.36 (d,
6H), 1.273 (6H). 1H NMR (300 MHz, C6D6): δ ) 6.886 (s, 3H, para),
6.031 (s, 6H, ortho), 4.29 (s, 3H), 2.96 (s), 2.35 (m), 2.097 (s, 18H,
ArMe), 1.851 (s), 1.682 (s), 1.52 (m). 13C NMR (75 MHz, CDCl3): δ
) 151.951, 137.731, 127.268, 127.125, 72.733, 38.414, 38.215, 32.327,
30.921, 28.540, 27.677, 21.632. Anal. Calcd for C54H72MoN4: C, 74.28;
H, 8.31; N, 6.42. Found: C, 74.21; H, 8.38; N, 6.49.
Synthesis of (ON)Mo(N[2-Ad]Ar)3 (2-NO). Complex 2 (0.6658 g,
0.7750 mmol) was dissolved in ether (10 mL), and NO (21 mL, 0.8594
mmol, 1.1 equiv) was then added via syringe. The reaction mixture
was stirred for 20 min, and the solution turned light brown-orange in
color. The solvent was removed in vacuo, and the remaining solid was
dissolved in ca. 10 mL of THF with stirring. Two crops of a powdery
solid were collected (0.4800 g, 0.5399 mmol, 70%). Mp: 280-285
°C (dec). 1H NMR (300 MHz, CDCl3): δ ) 6.770 (s, 3H, para), 5.779
(s, 6H, ortho), 4.190 (s, 3H), 2.162 (s, 6H), 2.100 (s, 18H, ArMe),
During the NO competition experiments, paramagnetic material was
present; therefore, the T1 characteristics obtained for this and other
competition reactions would have a lower magnitude of T1. The values
determined by 15N NMR were as follows: 1-15NO, 2.44 ( 0.16 s;
2-15NO, 1.23 ( 0.12 s; 3-15NO, 4.05 ( 0.26 s.
To a 100 mL Schlenk flask were added complexes 1 (24 mg, 3.89
× 10-5 mol, 1.0 equiv) and 2 (33 mg, 3.89 × 10-5 mol, 1.0 equiv),
along with 5 mL of toluene solvent. Also added to the flask was a
magnetic, Teflon-coated stir bar, and then the flask was fitted with a
septum. The mixture was degassed and placed under partial vacuum
while stirring, following which the gas 15NO (2.2 mL, 8.56 × 10-5
mol, 2.2 equiv) was added via syringe. After the reaction mixture was
stirred for 20 min at 25 °C, the solvent and all volatile material were
removed in vacuo. The red-orange solid residue was dissolved in C6D6
(750 µL), and the solution was transferred into an NMR tube. Proton
1
1.902-1.697 (m, 30 H), 1.40 (d, 6H). H NMR (300 MHz, C6D6): δ
) 6.884 (s, 3H, para), 6.092 (s, 6H, ortho), 4.579 (s, 3H), 2.638 (s,
6H), 2.10 (m, 9H), 2.080 (s, 18H, ArMe), 1.866 (s, 12H), 1.7-1.5 (m,
15H). 13C NMR (75 MHz, CDCl3): δ ) 148.943 (s), 137.929 (q),
128.119 (d), 127.515 (d), 75.536 (d), 38.361 (t), 38.194 (t), 33.835
(d), 31.019 (d), 28.260 (d), 27.897 (d), 21.714 (q). IR (benzene, 2 cm-1):

7284 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
NMR spectra indicated that the starting materials had been consumed
completely and that 1-15NO and 2-15NO had been produced in
essentially quantitative fashion. Integration of the 15N NMR spectrum
of the mixture indicated the ratio [1-15NO]/[2-15NO] to be 0.94.
Control Experiment: Exposure of Complex 3 to N2O. A 100 mL
Schlenk flask was charged with complex 3 (20 mg, 5.56 × 10-5 mol),
5 mL of toluene solvent, and a Teflon-coated magnetic stir bar. The
flask was fitted with a septum and attached to a vacuum line, and the
mixture was degassed. Gaseous N2O (95 mL, 3.89 mmol, 70 equiv)
then was introduced. The mixture was stirred at 25 °C for 90 min, at
which point the solvent was removed in vacuo. The solid residue was
dissolved in C6D6 (750 µL) for analysis. Proton NMR spectroscopy
verified that complex 3 had undergone no net reaction, since peaks
attributable to it (very broad peak at 30 ppm) were the only peaks
observed in the spectrum.
Control Experiment: Exposure of Complex 3-NO to 1. To a 20
mL vial were added complex 3-NO (24 mg, 6.16 × 10-5 mol, 1.0
equiv), 1 (41 mg, 6.16 × 10-5 mol, 1.0 equiv), 10 mL of ether, and a
Teflon-coated magnetic stir bar. The mixture was stirred at 25 °C for
45 min, at which point the solvent was removed in vacuo. The solid
residue was dissolved in C6D6 (750 µL) for analysis. Proton NMR
spectroscopy verified that complex 3-NO and 1 had undergone no
reaction.
15N2O Reaction with a Mixture of Complexes 2 and 3. A 100 mL
Schlenk flask was charged with complex 2 (33 mg, 3.89 × 10-5 mol,
1.0 equiv) and complex 3 (14 mg, 3.89 × 10-5 mol, 1.0 equiv), 5 mL
of toluene solvent, and a magnetic Teflon-coated stir bar. The flask
then was fitted with a septum and attached to a vacuum line, and the
mixture was outgassed. Gaseous 15N2O (10.0 mL, 3.89 × 10-4 mol,
10.0 equiv) was then introduced via syringe, and the mixture was stirred
vigorously for 90 min, at which point the solvent was removed in vacuo.
The brown-orange residual solid was dissolved in C6D6 (750 µL), and
transferred into an NMR tube for analysis. Proton NMR spectra revealed
the presence of starting complex 3 in addition to product nitrosyls
2-15NO and 3-15NO, along with the product nitrido derivative 2-15N.
The chromium nitrido species 3-15N was not observed to be present
among the products. The 15N NMR resonances were integrated and
revealed the [2-15NO]/[3-15NO] ratio to be 1.25. Also, the integrated
intensity for the 2-15N signal was equal to the sum of integrated
intensities for the 2-15NO and 3-15NO signals. Repeating this experiment
produced similar results, with a value for [2-15NO]/[3-15NO] of 1.22
being obtained.
A standard integration sample for a mixture of 1-15NO and 3-15NO
was prepared similarly as follows. A 100 mL Schlenk flask was charged
with complexes 1 (24 mg, 3.89 × 10-5 mol, 1.0 equiv) and 3 (14 mg,
3.89 × 10-5 mol, 1.0 equiv), along with 5 mL of toluene solvent and
a Teflon-coated magnetic stir bar. The experiment was conducted as
1
in the preceding paragraph. Upon workup, H NMR spectra verified
that the starting materials had been consumed quantitatively and that
1-15NO and 3-15NO had been produced in essentially quantitative
fashion. Integration of the 15N NMR spectrum of the mixture indicated
the ratio [1-15NO]/[3-15NO] to be 0.98.
A third integration standard for the mixture of complexes 2-15NO
and 3-15NO was prepared similarly as follows. A 100 mL Schlenk flask
was charged with complexes 2 (33 mg, 3.89 × 10-5 mol, 1.0 equiv)
and 3 (14 mg, 3.89 × 10-5 mol, 1.0 equiv), along with 5 mL of toluene
solvent and a Teflon-coated magnetic stir bar. The experiment involving
the addition of 15NO was conducted as described above. Upon workup,
1H NMR spectra indicated that the starting materials had been converted
completely to the corresponding nitrosyls 2-15NO and 3-15NO. Integra-
tion of the 15N NMR spectrum of the mixture indicated the ratio
[2-15NO]/[3-15NO] to be 1.03.
Competitive 15NO Binding by 1 and 2. A 100 mL Schlenk flask
was charged with complex 1 (24 mg, 3.89 × 10-5 mol, 1.0 equiv),
complex 2 (33 mg, 3.89 × 10-5 mol, 1.0 equiv), 5 mL of toluene
solvent, and a Teflon-coated magnetic stir bar. The flask then was fitted
with a septum, attached to a Schlenk line, and the mixture degassed.
With stirring was introduced 15NO (1.0 mL, 3.89 × 10-5 mol, 1.0 equiv)
via syringe. After 20 min all volatile material including the toluene
solvent was removed under reduced pressure. The residual red-orange
solid was dissolved in C6D6 (750 µL), and the solution was transferred
into an NMR tube for analysis. Proton NMR spectra revealed the
presence of precursor complexes (1 and 2), along with the product
nitrosyl complexes (1-15NO and 2-15NO). Integration of the 15N NMR
spectrum showed the [1-15NO]/[2-15NO] ratio to be 2.22. Repetition
of this experiment gave a value of 2.38 for the quantity of interest.
15N2O Reaction with a Mixture of Complexes 1 and 3. To a 100
mL Schlenk flask was added complex 1 (24 mg, 3.89 × 10-5 mol, 1.0
equiv), complex 3 (14 mg, 3.89 × 10-5 mol, 1.0 equiv), 5 mL of toluene
solvent, and a Teflon-coated magnetic stir bar. The flask was fitted
with a septum and attached to a vacuum line, and the mixture was
outgassed. 15N2O (10.0 mL, 3.89 × 10-4 mol, 10.0 equiv) was added
to the vessel via syringe, and the mixture was stirred vigorously for 90
min, at which point the solvent was removed in vacuo. The brown-
orange residual solid was dissolved in C6D6 (750 µL), and the solution
was transferred into an NMR tube for analysis. Proton NMR spectra
revealed the presence of starting complex 3, along with product nitrosyls
1-15NO and 3-15NO and product nitrido derivative 1-15N. Integration
of the 15N NMR resonances showed the [1-15NO]/[3-15NO] ratio to be
26.1. Also, the integrated intensity for the signal corresponding to
complex 1-15N was equal to the sum of those corresponding to nitrosyls
1-15NO and 3-15NO. Repetition of this experiment was carried out
similarly and yielded a value for [1-15NO]/[3-15NO] of 29.3.
15N2O Reaction with a Mixture of Complexes 1 and 2. To a 100
mL Schlenk flask were added complex 1 (24 mg, 3.89 × 10-5 mol,
1.0 equiv) and complex 2 (33 mg, 3.89 × 10-5 mol, 1.0 equiv), along
with 5 mL of toluene solvent and a magnetic Teflon-coated stir bar.
The flask was fitted with a septum and attached to a vacuum line, and
the mixture then was outgassed. Gaseous 15N2O (1.0 mL, 3.89 × 10-5
mol, 0.5 equiv) was then introduced via syringe, and the mixture was
stirred vigorously for 90 min, at which point the solvent was removed
in vacuo. The brown-orange solid residue was dissolved in C6D6 (750
µL), and the solution was transferred into an NMR tube for analysis.
Proton NMR spectra revealed the presence of starting complex 2, in
addition to product nitrosyl 1-15NO and nitride 1-15N and a trace amount
of nitrosyl 2-15NO. The 15N NMR resonances were integrated, but the
[1-15NO]/[2-15NO] ratio was deemed too large for accurate determi-
nation. No nitride 2-15N was observed in the 15N NMR spectrum of
the mixture.
Competitive 15NO Binding by 1 and 3. A 100 mL Schlenk flask
was charged with complex 1 (24 mg, 3.89 × 10-5 mol, 1.0 equiv) and
complex 3 (14 mg, 3.89 × 10-5 mol, 1.0 equiv), 5 mL of toluene
solvent, and a Teflon-coated magnetic stir bar. The experiment was
conducted as described above, using 1.0 equiv of 15NO. Upon workup,
1H NMR analysis indicated the presence of starting complexes 1 and
3 in addition to the product nitrosyls, complexes 1-15NO and 3-15NO.
Integration of the mixture’s 15N NMR spectrum revealed the [1-15NO]/
[3-15NO] ratio to be 1.82. Repetition of this experiment gave a value
of 1.80 for the quantity of interest.
Competition between 1 and 3 with 15NO Toluene Solution. In a
20 mL vial, 1 (24 mg, 3.89 × 10-5 mol, 1.0 equiv) and 3 (14 mg, 3.89
× 10-5 mol, 1.0 equiv) were dissolved in toluene (3 mL). The vial
was equipped with a Teflon-coated magnetic stir bar and a septum. A
saturated 15NO toluene solution (3.3 mL of solution containing 1.0 mL
of 15NO, 3.89 × 10-5 mol, 1.0 equiv) was added via a syringe over a
period of 5 min. The solution was rapidly stirred for 30 min. The solvent
then was removed in vacuo. A second reaction was performed following
the same protocol. For both samples, 1H NMR spectra verified a mixture
of 1, 3, 1-15NO, and 3-15NO. The 15N NMR spectrum was integrated
and revealed the 1-15NO:3-15NO ratio to be 1.64 and 2.12, respectively
for the two runs.
Competitive 15NO Binding by 2 and 3. A 100 mL Schlenk flask
was charged with complex 2 (33 mg, 3.89 × 10-5 mol, 1.0 equiv) and
complex 3 (14 mg, 3.89 × 10-5 mol, 1.0 equiv), 5 mL of toluene
solvent, and a Teflon-coated magnetic stir bar. The experiment was
1
conducted as above with 1.0 equiv of 15NO. Upon workup, H NMR
analysis indicated the presence of starting complexes 2 and 3), in
addition to product nitrosyls 2-15NO and 3-15NO. Integration of the
mixture’s 15N NMR spectrum revealed the [2-15NO]/[3-15NO] ratio to
be 0.82. Repetition of this experiment gave a value of 0.65 for the
quantity of interest.

Nitrous Oxide N-N Bond CleaVage by Mo(III) Complexes
J. Am. Chem. Soc., Vol. 123, No. 30, 2001 7285
N15NO Reaction with Complex 1. Complex 1 (24 mg, 3.89 × 10-5
mol, 1 equiv) was dissolved in 5 mL of toluene solvent in a 100 mL
Schlenk flask. The flask was then equipped with a magnetic Teflon-
coated stir bar and a septum. After attachment of the flask to a vacuum
line, the mixture was outgassed. Gaseous N15NO (5.0 mL, 1.95 × 10-5
mol, 5.0 equiv) was then introduced via syringe. The mixture was stirred
for 2 h, at which point the solvent was removed in vacuo. The brown-
orange solid residue was dissolved in C6D6 (750 µL), and the solution
was transferred into an NMR tube for analysis. Proton NMR spectra
showed the presence of only two products, nitrosyl 1-15NO and nitride
1-N, in an approximate equimolar ratio. The 15N NMR spectrum of
the mixture was found to consist solely of the signal for 1-15NO,
showing that the nitrous oxide N-O bond remains unbroken during
the course of N-N bond cleavage.
Kinetic Data for Reaction of Complex 2 with N2O. Kinetic
experiments were carried out in a vessel having a 5 mL quartz cell
connected via a graded seal and 5 mm diameter glass tube to a 50 mL
glass Schlenk reaction vessel. The cell and associated reaction vessel
were equipped with Teflon stopcocks such that they could be mutually
isolated and independently addressed. Both compartments were equipped
with a Teflon-coated magnetic stir bar.
structure was solved by direct methods (SHELXTL V5.0, G. M.
Sheldrick and Siemens Industrial Automation, Inc., 1995) in conjunction
with standard difference Fourier techniques. Least-squares refinement
based upon F2 converged with residuals of R1 ) 0.0448, wR2 ) 0.1178,
and GOF ) 1.255 based upon I > 2σ(I). All non-hydrogen atoms were
refined anisotropically, and hydrogen atoms were placed in calculated
(dC-H ) 0.96 Å) positions except for solvent molecules. The largest
residual peak and hole electron density was 0.463 and -0.270 e‚Å-3
.
Crystal data: monoclinic, a ) 11.761(3) Å, b ) 19.737(5) Å, c )
22.761(5) Å, V ) 5170(2) Å3, â ) 101.89(2)°, space group P21/n, Z
) 4, µ ) 0.294 mm-1, Mr ) 923.13 for C58H72MoN3O, F(calcd) )
1.186 g‚cm-3, and F(000) ) 1964.
X-ray Structural Determination for PMo(N[2-Ad]Ar)3 (2-P). A
yellow parallelepiped of approximate dimensions 0.31 × 0.25 × 0.22
mm was obtained from a chilled ether solution. The crystal was mounted
on a glass fiber. Data collection was carried out on a Siemens Platform
goniometer with a CCD detector at 183 K using Mo KR radiation (λ
) 0.71073 Å). The total data collected were 14 332 reflections (-37
e h e 36, -37 e k e 36, -37 e l e 37), of which 1697 were unique
(Rint ) 0.0352). Corrections applied: semiempirical from Ψ-scans. The
structure was solved by direct methods (SHELXTL V5.0, G. M.
Sheldrick and Siemens Industrial Automation, Inc., 1995) in conjunction
with standard difference Fourier techniques. The unit cell contains 1/3
of a disordered C6H6 molecule; the disorder was not modeled, the C6H6
carbons were refined isotropically and hydrogens were not included.
Least-squares refinement based upon F2 converged with residuals of
R1 ) 0.0661, wR2 ) 0.1830, and GOF ) 1.307 based upon I > 2σ(I).
All non-hydrogen atoms were refined anisotropically, and hydrogen
atoms were placed in calculated (dC-H ) 0.96 Å) positions. The residual
peak and hole electron density was 1.053 and -0.510 e‚Å-3. Crystal
data: rhombohedral, a ) b ) c ) 38.3067(6) Å, V ) 5033.71(14) Å3,
Toluene solvent (50 mL) was added to complex 2 (50.8 mg, 59.1
mmol) to provide a 1.2 mM stock solution. The solution of complex 2
was freeze-pump-thawed and stored under an argon atmosphere at
-35 °C. The stock solution was twice the desired concentrated for the
kinetic measurements, since the addition of the N2O solution leads to
dilution, giving an initial concentration of complex 2 of 0.6 mM at the
start of data collection.
A second stock solution containing complex 2 (41.3 mg, 48.0 mmol)
in 10 mL of toluene solvent also was made following the above
protocol. This 4.8 mM solution was employed to achieve an initial
concentration of 2.4 mM in complex 2 at the start of data collection.
A typical kinetic run was carried out by addition of 2.50 mL of
either stock solution to the quartz cell, under an argon atmosphere.
The cell was then attached to a vacuum system and outgassed. Toluene
(2.50 mL) was then added to the 50 mL glass compartment and was
subjected to three freeze-pump-thaw cycles. Into the 50 mL glass
compartment was then introduced 1 atm of N2O. After 90 min of
stirring, it was assumed that a saturated solution of N2O solution had
been produced. The solution of complex 2 was stirred at 1000 rpm
while in the spectrometer, and the solution was allowed to equilibrate
at the desired temperature for 5 min prior to the start of each kinetic
run. Runs were started by opening the stopcocks, allowing the two
compartments to communicate, and permitting the solutions of complex
2 and N2O to be mixed. The reaction mixture was quickly transferred
to the quartz cell for observation. After 1 h of data collection, the
reaction mixture was transferred to the glass compartment, and the
solvent removal in vacuo was effected. Proton NMR spectroscopy was
used to examine the reaction mixture, confirming that a 1:1 ratio of
2-N and 2-NO indeed had been produced.
Kinetic data were collected on a Hewlett-Packard 8453 diode array
spectrophotometer equipped with a built-in magnetic stirrer. The
reaction temperature for all runs was set to 26.4 ( 0.5 °C using an HP
89090A Peltier temperature control accessory. The decay of 2 was
monitored at 427 nm (λmax) and at 500 nm. In a typical run, 40 data
points were collected at 90 s intervals. Plots of ln[(A - A∞)/A0] vs
time were linear through three half-lives. Rate constants were deter-
mined using the Kaleidagraph least-squares curve-fitting routine with
the equation At ) A∞ + A0 e-kt. At least three runs were carried out at
each initial concentration of complex 2, 0.6 and 2.4 mM. The data
from these runs were determined to be 0.031, 0.028, and 0.034 s-1 for
the 0.6 mM experiments and 0.034, 0.029, and 0.030 s-1 for the 2.4
mM experiments.
R ) â ) γ ) 18.6720(10)°, space group R3hc, Z ) 3, µ ) 0.334 mm-1
,
Mr ) 1282.82 for C80H96Mo1.33N4P1.33, F(calcd) ) 1.270 g‚cm-3, and
F(000) ) 2040.
Thermochemistry: General Considerations. All manipulations
involving organomolybdenum complexes were performed under an inert
atmosphere of argon in a Vacuum Atmospheres glovebox containing
less than 1 ppm oxygen and water. Nitric oxide was obtained from
Matheson Gas and handled in a stainless steel manifold following
passage through a coiled copper tubing trap at -78 °C. Toluene was
distilled from sodium benzophenone ketyl, stored over Na/K alloy, and
vacuum transferred prior to calorimetric use. Only materials of high
purity as indicated by NMR spectroscopy were used in the calorimetric
experiments. Calorimetric measurements were performed using a Calvet
calorimeter (Setaram C-80) which was periodically calibrated using
the TRIS reaction186 or the enthalpy of solution of KCl in water.187
The experimentally determined enthalpies for these two standard
calibration reactions were the same within experimental error as
literature values. The calorimeter has been described previously,188 and
typical procedures are described below. Experimental enthalpy data
are reported with 95% confidence limits.
1H NMR Titrations. Prior to the calorimetric experiments, an
accurately weighed amount (ca. 0.1 mg) of the molybdenum complex
was placed in an NMR tube, and toluene was subsequently added. The
solution was titrated with a solution of the ligand of interest. The
reactions were monitored by 1H NMR spectroscopy, and the reactions
were found to be rapid, clean, and quantitative under the experimental
calorimetric conditions. These conditions are necessary for accurate
and meaningful calorimetric results and were satisfied for the organo-
molybdenum reactions investigated.
Calorimetric Measurement for Reaction of Mo(N[R]Ar)3 (1) with
N2O. The mixing vessels of the Setaram C-80 were cleaned, dried in
an oven maintained at 120 °C, and then taken into the glovebox. A
20-30 mg sample of 1 was accurately weighed into the lower vessel,
which was then closed and sealed with 1.5 mL of mercury. Four
milliliters of a N2O-saturated stock toluene solution was added, and
the remainder of the cell was assembled, removed from the glovebox,
X-ray Structural Determination for Mo(N[2-Ad]Ar)3 (2). A dark
orange needle of approximate dimensions 0.57 × 0.37 × 0.19 mm
was obtained from a chilled ether solution. The crystal was mounted
on a glass fiber. Data collection was carried out on a Siemens Platform
goniometer with a CCD detector at 183 K using Mo KR radiation (λ
) 0.71073 Å). The total data collected were 20 644 reflections (-13
e h e 13, -21 e k e 11, -24 e l e 25), of which 7395 were unique
(Rint ) 0.0300). Corrections applied: semiempirical from Ψ-scans. The
(186) Ojelund, G.; Wads, I. Acta Chem. Scand. 1968, 22, 1691-1699.
(187) Kilday, M. V. J. Res. Natl. Bur. Stand. (U.S.) 1980, 85, 467-481.
(188) Nolan, S.; Hoff, C. D. J. Organomet. Chem. 1985, 282, 357-
362.

7286 J. Am. Chem. Soc., Vol. 123, No. 30, 2001
Cherry et al.
and inserted in the calorimeter. The reference vessel was loaded in an
identical fashion with the exception that complex 1 was not added to
the lower vessel. After the calorimeter had reached thermal equilibrium
at 30.0 °C (about 1.5 h), the calorimeter was inverted, thereby allowing
the reactants to mix. After reaching thermal equilibrium, the vessels
were removed from the calorimeter, taken into the glovebox, opened,
and analyzed using NMR spectroscopy. Conversion to nitride 1-N and
nitrosyl 1-NO in a 1:1 ratio was found to be quantitative under these
Caution! Nitric oxide, and in particular its oxidation product nitrogen
dioxide, are hazardous materials and should be handled with care.
Two ampules containing a total of approximately 0.4 g of complex
1 were broken into the solution to remove any residual water contained
in the calorimeter or solution. Before and after breaking an ampule, a
series of electrical calibrations were performed. Ampules were broken
by depressing a Pyrex rod fitted with a Teflon holder such that only
glass and Teflon were in contact with the calorimetry solution. Ampules
were changed under a flow of argon from a port above the calorimeter
in such a way that the initially saturated solution did not lose any
significant amount of NO. The value of -73.6 ( 2.3 kcal‚mol-1 is the
average of four measurements. Analysis of the FTIR of a small-scale
reaction in C6D6 done under analogous conditions shows a peak at 1602
cm-1, in agreement with the literature value of 1604 cm-1, and also
with that obtained from the reaction with N2O.
reaction conditions. The enthalpy of reaction, 53.0 ( 0.3 kcal‚mol-1
,
of 1 represents the average of five individual calorimetric determina-
tions. The enthalpy of solution of 1 was then added to this value to
obtain a value of -61.9 ( 0.4 kcal‚mol-1 for all species in solution.
Calorimetric Measurement of Enthalpy of Solution of Mo(N[R]-
Ar)3 (1). To consider all species in solution, the enthalpies of solution
of 1 had to be directly measured. This was performed by using a
procedure similar to the one described above with the exception that
no ligand was added to the reaction cell. This enthalpy of solution
represents the average of three individual determinations and is 8.9 (
0.2 kcal‚mol-1 in toluene.
Acknowledgment. The National Science Foundation (CHE-
9988806) and the National Science Board (1998 Alan T.
Waterman to C.C.C.) are gratefully acknowledged for financial
support. S.P.N. gratefully acknowledges partial support of this
work by the National Science Foundation. C.C.C. thanks D. B.
Collum for suggesting the use of the secondary adamantyl
substituent in metal amide chemistry, and D. G. Nocera, G. L.
Hillhouse, M. L. Chisholm, G. Parkin, P. T. Wolczanski, and
J. M. Mayer for helpful discussions. J.-P.F.C. appreciates
technical assistance from J. Simpson and M. Wall while
conducting the 15N NMR experiments.
General Considerations for the Calorimetry with Nitric Oxide.
Deuteriobenzene and toluene were obtained from Aldrich Chemical
Co. and were purified by distillation from sodium benzophenone ketyl
under an argon atmosphere. Nitric oxide was obtained from Matheson
Gas and handled in a stainless steel manifold following passage through
a coiled copper tubing trap at -78 °C. Nitrous oxide (99.9995%) and
argon (99.999%) were obtained from Praxair and used as received.
Calorimetric measurements were made in either a Setaram C-80 or a
Guild isoperibol calorimeter. FTIR spectra were obtained on a Perkin-
Elmer system 2000 using solution cells obtained from Harrick Scientific
1
using CaF2 windows. H NMR spectra were measured in C6D6 using
a Bruker 300 MHz NMR spectrometer.
Enthalpy of Reaction of Mo(N[R]Ar)3 (1) with NO in the Guild
Calorimeter. Glass ampules containing between 0.12 and 0.23 g of
solid 1 were loaded in the glovebox and sealed under an inert
atmosphere. The solution calorimeter of approximate volume 500 mL
was evacuated for 2 h prior to loading with 400 mL of freshly distilled
toluene under an argon atmosphere. The atmosphere was switched to
nitric oxide, which was bubbled through the stirred calorimeter for a
sufficient time to ensure displacement of the argon atmosphere by one
of nitric oxide. Excess nitric oxide was vented through a bubbler and
to an exhaust hood.
Supporting Information Available: Differential equations
used for kinetic simulations, evaluation of the solubility of N2O
and NO in organic solvents, plots of simulated kinetic traces
for interpretation of competition experiments, raw UV-visible
data for reactions of molybdenum complexes with excess N2O,
and crystal structure data and tables (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0031063