Well-defined vanadium organoazide complexes and their conversion to terminal vanadium imides: Structural snapshots and evidence for a nitrene capture mechanism

Article abstract of DOI:10.1021/ic301673g

We report the synthesis and structural characterization of a family of well-defined organoazide complexes supported by a three-fold-symmetric pyrrolide scaffold and their conversion to the corresponding terminal imido congeners. Kinetic measurements on a

Full text of DOI:10.1021/ic301673g

Article
pubs.acs.org/IC
Well-Defined Vanadium Organoazide Complexes and Their
Conversion to Terminal Vanadium Imides: Structural Snapshots and
Evidence for a Nitrene Capture Mechanism
W. Hill Harman,†,‡ Michael F. Lichterman,† Nicholas A. Piro,†,‡ and Christopher J. Chang*,†,‡,§
†Department of Chemistry and §The Howard Hughes Medical Institute, University of California, Berkeley, California 94720, United
States
‡Chemical Sciences Division, Lawrence Berkeley National Laboratory (LBNL), Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: We report the synthesis and structural characterization of a family of well-defined organoazide complexes
supported by a three-fold-symmetric pyrrolide scaffold and their conversion to the corresponding terminal imido congeners.
Kinetic measurements on a series of structurally homologous but electronically distinct vanadium organoazide complexes reveal
that the azido-to-imido transformations proceed via a process that is first-order in the metal complex and has a positive entropy
of activation. Further studies suggest that these reactions may involve metal-mediated generation and capture of nitrene
fragments.
followed by trapping of the incipient nitrene fragment by the
reduced metal complex.12 Isolated nickel azide adducts studied
by Waterman and Hillhouse10 appear to form imides by the
classical mechanism elucidated by Proulx and Bergman.3
Finally, a ruthenium system recently reported by Peters et al.
catalyzes the formation of azoarenes from organoazides via
postulated nitrene release from a low-valent ruthenium
organoazide adduct, with some production of the correspond-
ing imide complexes depending on the organoazide sub-
strate.11b
To contribute to this area, we now report a new family of
well-defined, structurally characterized metal organoazide
complexes featuring a three-fold-symmetric vanadium pyrrolide
platform and their conversion to the corresponding terminal
imido species. Although these molecules find their closest
analogues in those reported by Proulx and Bergman3 and
Cummins et al.,4 mechanistic studies reveal patent differences
with both of these landmark systems. In particular, Eyring
analysis shows that the organoazide-to-imido transformation on
this vanadium pyrrolide platform proceeds with a positive
entropy of activation, which is unique in comparison to other
previously studied monometallic systems.3,10 Moreover, the
Hammett relationship established for this pyrrolide system is
INTRODUCTION
■
The formation and study of metal imido complexes is of
fundamental interest because of their intermediacy in a wide
array of catalytic chemical and biological processes ranging
from C−N bond-forming reactions to nitrogen fixation.1 Such
species are often formed through the oxidative installation of a
neutral nitrene fragment (“NR”) onto a reducing metal
precursor. Organic azides are the most common nitrene source
in this context, wherein reaction of the azide with a reduced
metal center results in dinitrogen extrusion and concomitant
formation of a metal−nitrogen multiple bond. Despite the fact
that metal organoazide species have been proposed as
intermediates in these reactions,2 the isolation and character-
ization of such species remain rare.3−11 Interestingly, kinetic
studies have revealed surprising mechanistic diversity for this
basic transformation. A seminal study by Proulx and Bergman
on the conversion of mononuclear tantalum azide adducts into
tantalum imido complexes identified a unimolecular mechanism
involving a four-membered triazametallacyclic species that
undergoes dinitrogen loss to give the imide product.3 Parallel
work by Cummins et al. on a related vanadium trianilide system
provided evidence for an imide formation pathway that is
second order in an azide adduct.4 Alternately, reactions of
manganese and chromium corrole complexes with organoazides
to form imido species have been shown by Abu-Omar et al. to
proceed via thermal decomposition of the free organoazide
Received: July 30, 2012
Published: August 24, 2012
© 2012 American Chemical Society
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Article
(tpaMes)V[η1-N3(p-Tol)]·0.5Toluene (2c·0.5Toluene). In the
glovebox, a 20 mL scintillation vial was charged with 1 (95.1 mg,
0.131 mmol), which was dissolved in toluene (5 mL). To this solution
was added a solution of N3(p-Tol) (23.5 mg, 0.177 mmol) in toluene
with constant stirring. A rapid color change to dark orange-black was
observed. The reaction was stirred for 45 min before the solvent was
concentrated to ∼1 mL in vacuo. The resulting solution was layered
with pentane and placed in a freezer at −35 °C overnight, providing 62
mg (57%) of black crystals. Crystals suitable for X-ray diffraction were
grown in this fashion. Because if the thermal instability of this
compound, satisfactory elemental analysis could not be obtained.
NMR characterization was carried out at low temperature because of
the high degree of fluxionality at room temperature. 1H NMR (263 K,
500 MHz, CDCl3): δ 7.12 (d, J = 7.8 Hz, 2H, aryl), 6.70 (d, J = 7.6 Hz,
2H, aryl), 6.53 (s, 3H, mesityl), 6.37 (s, 3H, mesityl), 6.00 (s, 3H,
pyrrole), 5.75 (s, 3H, pyrrole), 4.58 (d, J = 13.6 Hz, 3H, methylene),
4.28 (d, J = 12.5 Hz, 3H, methylene), 2.44 (s, 3H, aryl p-CH3), 2.08 (s,
9H, mesityl methyl), 1.89 (s, 9H, mesityl methyl), 1.31 (s, 9H, mesityl
methyl). 13C NMR (126 MHz, CDCl3): δ 141.57, 138.92, 138.14,
137.07, 136.18, 134.36, 128.98, 128.10, 127.84, 118.85, 108.45, 103.86,
54.91, 22.04, 21.13, 20.68, 20.03. IR (cm−1): 2103 (wk), 1445, 1332,
1225.
opposite to that elucidated in Proulx and Bergman’s system,
although both systems are first-order in an azide adduct. Finally,
further experiments provide evidence to suggest the partic-
ipation of metal-mediated generation and capture of nitrene
fragments in an azide-to-imido conversion process.
EXPERIMENTAL SECTION
■
General Procedures. Unless otherwise noted, all manipulations
were carried out at room temperature under an atmosphere of
dinitrogen in a VAC Atmospheres glovebox or using Schlenk
techniques. Pentane and toluene were dried using a VAC Atmospheres
solvent purification system prior to use. Benzene was distilled from
purple sodium benzophenone ketyl. Benzene-d6 was purchased from
Cambridge Isotopes Laboratory, brought into the glovebox, degassed,
and stored over sieves. Potassium hydride was purchased as a
suspension in mineral oil, washed with pentane, and used as a dry solid
inside the glovebox. Literature procedures were used for the
preparation of (tpaMes)V(THF) (1)13 and the aryl azides, including
NN15NPh.14 All other reagents and solvents were purchased from
chemical suppliers and used as received. NMR spectra were recorded
on Bruker spectrometers operating at 500 MHz unless otherwise
noted. Chemical shifts are reported in ppm and referenced to a
residual protiated solvent; coupling constants are reported in hertz.
Mass spectra and elemental analyses were performed at the Mass
Spectrometry and Microanalytical Facilities at the University of
California, Berkeley.
(tpaMes)V[N3(η1-p-C6H4NMe2)·0.5Toluene (2d·0.5Toluene). In
the glovebox, a 20 mL scintillation vial was charged with tpaMesV-
(THF) (87.7 mg, 0.120 mmol), which was dissolved in toluene (5
mL). To this solution was added a solution of N3PhNMe2 (19.5 mg,
0.120 mmol) in toluene with constant stirring. A rapid color change to
teal-black was observed. The reaction was stirred for 20 min before the
solvent was concentrated to ∼1 mL in vacuo. The resulting solution
was layered with pentane and placed in a freezer at −35 °C overnight,
providing a first crop of 80.0 mg (77%) of black crystals; further
recrystallization of the mother liquor provideed yields of ca. 90%.
Crystals suitable for X-ray diffraction were grown from a saturated
solution of benzene placed in a pentane vapor chamber. Because of the
thermal instability of this compound, satisfactory elemental analysis
could not be obtained. 1H NMR (500 MHz, CDCl3): δ 6.72 (d, J = 8.3
Hz, 2H, aryl), 6.56 (d, J = 9.0Hz, 2H, aryl), 6.53 (s, 3H, mesityl), 6.39
(s, 3H, mesityl), 6.00 (s, 3H, pyrrole), 5.76 (d, J = 2.8Hz, 3H, pyrrole),
4.52 (d, J = 13.9 Hz, 3H, methylene), 4.16 (d, J = 13.9 Hz, 3H,
methylene), 3.18 (s, 6H, p-N(CH3)2), 2.10 (s, 9H, mesityl methyl),
1.95 (s, 9H mesityl methyl), 1.41 (s, 9H, mesityl methyl). 13C NMR
(126 MHz, CDCl3): δ 153.58, 141.97, 139.09, 138.86, 137.00, 135.74,
135.04, 128.04, 127.81, 111.10, 108.59, 103.69, 54.81, 40.48, 21.24,
20.77, 19.99.
(tpaMes)V[η1-N3(p-C6H4OMe)]·0.5Toluene (2a·0.5Toluene).
In the glovebox, a 20 mL scintillation vial was charged with 1 (46.4
mg, 0.0637 mmol), which was dissolved in toluene (5 mL). To this
solution was added a solution of N3(p-C6H4OMe) (9.3 mg, 0.064
mmol) in toluene with constant stirring. A rapid color change to a red-
black was observed. The reaction was stirred for 20 min before the
solvent was concentrated to ∼1 mL in vacuo. The resulting solution
was layered with pentane and placed in a freezer at −35 °C overnight,
providing a first crop of 42.7 mg (79%) of black crystals. Further
recrystallizations from the mother liquor provide a combined yield of
∼90%. Crystals suitable for X-ray diffraction were grown via this
method. Because of the thermal instability of this compound,
1
satisfactory elemental analysis could not be obtained. H NMR (500
MHz, CDCl3): δ 6.84 (br s, 2H), 6.77 (s, 2H), 6.52 (s, 3H), 6.36 (s,
3H), 6.03 (s, 2H), 5.73 (d, J = 2.1 Hz, 3H), 4.57 (br s, 3H,
methylene), 4.21 (br s, 3H, methylene), 3.92 (s, 3H, p-OCH3), 2.08 (s,
9H, mesityl methyl), 1.92 (s, 9H, mesityl methyl), 1.38 (s, 9H, mesityl
methyl). 13C NMR (126 MHz, CDCl3): δ 142.06, 139.22, 138.13,
135.96, 134.73, 128.13, 127.96, 125.16, 120.21, 115.35, 113.48, 108.62,
103.92, 55.87, 55.06, 21.15, 20.74, 20.12. IR (cm−1): 2103 (wk), 1593,
1331, 1223.
(tpaMes)V[N(p-C6H4OMe) (3a). A Schlenk tube was charged with
2a (35 mg, 0.041 mmol) and dissolved in benzene (10 mL). The
solution was degassed and heated to reflux for 1 h, during which time a
color change to orange-black was observed. The solvent was reduced
to ∼1 mL in vacuo and pentane added (9 mL). The resulting mixture
was filtered and the solvent removed from the filtrate in vacuo. The
solid residue was rinsed with pentane and dried under vacuum to yield
20 mg (63%) of a brown powder. Crystals suitable for X-ray diffraction
were grown over the course of multiple days by the slow diffusion of
pentane into a concentrated benzene solution. HRESIMS. Calcd for
(tpaMes)V(η1-N3Ph)·0.5Toluene (2b·0.5Toluene). In the glove-
box, a 20 mL scintillation vial was charged with 1 (88.5 mg, 0.122
mmol), which was dissolved in toluene (5 mL). To this solution was
added a solution of N3Ph (15.1 mg, 0.127 mmol) in toluene with
constant stirring. A rapid color change to dark orange-black was
observed. The reaction was stirred for 1 h before the solvent was
concentrated to ∼1 mL in vacuo. The resulting solution was layered
with pentane and placed in a freezer at −35 °C overnight, providing 72
mg (72%) of black crystals. Because of the thermal instability of this
compound, satisfactory elemental analysis could not be obtained.
NMR characterization was carried out at low temperature because of
the high degree of fluxionality at room temperature. 1H NMR (235 K,
500 MHz, CDCl3): δ 7.34 (s, 3H), 6.81 (s, 2H), 6.55 (s, 3H), 6.39 (s,
3H), 6.02 (s, 3H), 5.76 (s, 3H), 4.60 (d, J = 13.0 Hz, 3H), 4.32 (d, J =
13.3 Hz, 3H), 2.10 (s, 9H), 1.88 (s, 9H), 1.35 (s, 9H). 13C NMR (126
MHz, CDCl3): δ 147.86, 141.38, 138.79, 137.88, 136.98, 136.15,
134.15, 132.30, 129.09, 128.29, 128.10, 127.98, 127.72, 122.53, 108.33,
103.78, 54.78, 20.98, 20.55, 19.93. IR (cm−1): 2127 (wk), 1445, 1332,
1225. The isotopomer (tpaMes)V(η1-NN15NPh) was prepared in an
analogous manner utilizing NN15NPh. IR (cm−1): 2113 (wk), 1445,
1332, 1225.
1
C49H53N5OV ([M + H]+): m/z 778.3648. Found: m/z 778.3663. H
NMR (300 MHz, C6D6): δ 6.59 (s, 3H), 6.47 (s, 3H), 6.06 (d, J = 3.0
Hz, 3H), 6.01 (d, J = 3.0 Hz), 5.86 (d, J = 9.0 Hz, 2H, aryl), 5.20 (d, J
= 9.0 Hz, 2H, aryl), 4.20 (d, J = 14.0 Hz, 3H, methylene), 4.03 (d, J =
14.0 Hz, 3H, methylene), 3.65 (s, 3H, p-OCH3), 2.15 (s, 9H, mesityl
methyl), 2.04 (s, 9H, mesityl methyl), 1.46 (s, 9H, mesityl methyl).
13C NMR (126 MHz, CDCl3): δ 144.08, 141.96, 139.28, 137.96,
137.82, 136.46, 135.52, 132.13, 128.14, 110.07, 109.82, 103.93, 55.30,
53.83, 21.17, 21.12, 20.21.
(tpaMes)V(NPh) (3b). A Schlenk tube was charged with 2b (72 mg,
0.088 mmol) and benzene (10 mL). The solution was degassed and
heated to reflux for 3 h, during which time a color change to orange-
black was observed. The solvent was reduced to ∼1 mL in vacuo and
pentane added (9 mL). The resulting mixture was filtered through
celite, and the volatiles were removed from the filtrate in vacuo. The
solid residue was rinsed with pentane and dried in vacuo to yield 52
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Figure 1. Synthetic scheme for the formation of vanadium(V) diazenylimido and vanadium(V) imido species supported by the tripyrrolide ligand
tpaMes; R = OMe (a); H (b); OMe (c), NMe2 (d). ORTEP crystal structures for 2a and 3a with thermal ellipsoids drawn at 50% probability, with
solvent molecules and hydrogen atoms omitted for clarity, are also shown.
mg (79%) of a brown powder. Crystals suitable for X-ray diffraction
were grown over the course of multiple days by the slow diffusion of
pentane into a concentrated benzene solution. HRESIMS. Calcd for
C48H50N5V ([M + H]+): m/z747.3497. Found: m/z 747.3497. 1H
NMR (500 MHz, CDCl3): δ 6.49 (m, 4H, mesityl and p-aryl), 6.39 (s,
3H, mesityl), 6.21 (t, J = 7.5 Hz, 2H, m-aryl), 6.00 (s, 3H, pyrrole),
5.79 (s, 3H, pyrrole), 4.94 (d, J = 7.8 Hz, 2H, o-aryl), 4.45 (d, J = 13.9
Hz, 3H, methylene), 4.08 (d, J = 14.0 Hz, 3H, methylene), 2.15 (s, 9H,
mesityl methyl), 2.03 (s, 9H, mesityl methyl), 1.46 (s, 9H, mesityl
methyl). 13C NMR (126 MHz, CDCl3): δ 142.04, 139.18, 137.99,
137.82, 136.59, 135.27, 129.71, 128.58, 128.54, 128.15, 125.08, 124.99,
109.75, 104.02, 53.85, 21.13, 21.08, 20.14.
spectrum was collected every 1 min for 12 h. No diazene formation or
azide decomposition was observed.
Stability of 3a in the Presence of Excess N3 (p-C6H4OMe) at
75 °C. Both 3a (5 mg) and N3 (p-C6H4OMe, 25 mg) were dissolved
in C6D6, and the resulting solution was transferred to an NMR tube
fitted with a Teflon valve. The tube was heated to 75 °C and
monitored by 1H NMR for 12 h. No diazene formation, azide
decomposition, or imido (3a) decomposition was observed.
Catalytic N2 [(p-C6H4OMe)2] Formation from N3 (p-C6H4OMe)
in the Presence of 2a. Both 2a (5 mg) and N3 (p-C6H4OMe, 25
mg) were dissolved in C6D6, and the resulting solution was transferred
to an NMR tube fitted with a Teflon valve. The tube was inserted into
an NMR probe preheated to 55 °C, and a single-pulse 1H NMR
spectrum was collected every 1 min for 12 h. The results are shown in
Figure S2 in the Supporting Information and indicate azoarene
formation corresponding to ∼9 turnovers.
(tpaMes)V[N(p-Tol)] (3c). A Schlenk tube was charged with 2c (70
mg, 0.084 mmol) and benzene (10 mL). The solution was degassed
and then heated to reflux for 3 h. The solvent was reduced to ∼1 mL
in vacuo and pentane added (9 mL). After filtration through celite, the
volatiles were removed in vacuo. The solid residue was rinsed with
pentane and dried in vacuo to yield 60 mg (79%) of a green-black
powder. Crystals suitable for X-ray diffraction were grown by the slow
diffusion of pentane vapor into a concentrated benzene solution of the
product. HREIMS. Calcd for C49H52N5V ([M]+): m/z 761.3662.
Representative Kinetic Experiment for the Conversion of 2a
to 3a. Crystals of 2a and ferrocene were dissolved in C6D6 to a
concentration of ca. 5 mM in 2a and ca. 1 mM in ferrocene, and the
resulting dark solution was transferred to an NMR tube fitted with a
Teflon valve. The tube was inserted into an NMR probe preheated to
the temperature of interest, as determined by an ethylene glycol
1
Found: m/z 761.3674. H NMR (500 MHz, CDCl3): δ 6.49 (s, 3H,
1
standard calibrated thermocouple. A single-pulse H NMR spectrum
mesityl), 6.39 (s, 3H, mesityl), 5.98 (m, 5H, pyrrole and aryl), 5.77 (d,
J = 2.5 Hz, 3H, pyrrole), 4.82 (d, J = 8.1 Hz, 2H, aryl), 4.42 (d, J =
13.9 Hz, 3H, methylene), 4.04 (d, J = 14.0 Hz, 3H, methylene), 2.15
(s, 9H, mesityl methyl), 2.12 (s, 3H, aryl p-CH3), 2.03 (s, 9H, mesityl
methyl), 1.45 (s, 9H, mesityl methyl). 13C NMR (126 MHz, CDCl3):
δ 142.05, 139.13, 9 137.95, 137.80, 136.60, 135.51, 135.38, 129.80,
128.48, 128.06, 127.83, 125.60, 109.75, 103.94, 53.83, 21.46, 21.16,
21.09, 20.16.
was then recorded every 1 min until the reaction was complete. In
higher temperature measurements where peak broadening resulted in
overlap with the ferrocene standard, a residual solvent was used as an
internal standard. Integration data from a well-isolated resonance
corresponding to 2a were plotted as a function of time fit to an
exponential function of the form f(t) = ae−kt + c, giving the rate
constant k. Data from the first few minutes of the runs were excluded
from the fits in order to remove errors due to temperature
equilibration.
(tpaMes)V[N(p-C6H4NMe2) (3d). A Schlenk tube was charged with
2d (37 mg, 0.043 mmol) and benzene (10 mL). The solution was
degassed and heated to reflux for 30 min, during which time a color
change to green-black was observed. The solvent was reduced to ∼1
mL in vacuo and pentane added (9 mL). After filtration through celite,
the volatiles were removed from the filtrate in vacuo. The solid residue
was rinsed with pentane and dried in vacuo to yield 30 mg (88%) of a
green-black powder. Crystals suitable for X-ray diffraction were grown
by the slow diffusion of pentane vapor into a concentrated benzene
solution of the product. HRESIMS. Calcd for C50H56N6V ([M + H]+):
m/z 791.4001. Found: m/z 791.3985. 1H NMR (500 MHz, CDCl3): δ
6.50 (s, 3H, mesityl), 6.42 (s, 3H, mesityl), 5.98 (d, J = 1.9 Hz, 3H,
pyrrole), 5.80 (d, J = 2.8 Hz, 3H, pyrrole), 5.38 (d, J = 9.0 Hz, 2H,
aryl), 4.84 (d, J = 9.0 Hz, 2H, aryl), 4.36 (d, J = 13.9 Hz, 3H,
methylene), 3.98 (d, J = 14.0 Hz, 3H, methylene), 2.89 (s, 6H, p-
NC2H6), 2.17 (s, 9H, mesityl methyl), 2.04 (s, 9H, mesityl methyl),
1.49 (s, 9H, mesityl methyl). 13C NMR (126 MHz, CDCl3): δ 147.07,
141.86, 139.28, 137.86, 137.78, 136.04, 135.74, 132.40, 128.61, 128.05,
127.84, 109.73, 108.06, 103.69, 53.70, 40.13, 21.23, 21.16, 20.26.
Stability of N3 (p-C6H4OMe) in C6D6 at 75 °C. A solution of N3
(p-C6H4OMe) in C6D6 (ca. 30 mM) in an NMR tube was inserted
Error Analysis. Errors for the Hammett and Eyring analyses were
determined by setting the confidence interval to be mean ts/n1/2
,
where t is the Student’s t (12.706 for 2 runs, 4.303 for 3 runs, etc.), s is
the standard deviation of the values, and n is the number of runs for a
given compound at a given temperature (2 or 3 runs). Error bars for
Eyring analysis are set to the 95% confidence interval as the magnitude
of the 95% confidence interval over the mean, whereas error bars for
Hammett analysis are set to 0.434 times the magnitude of the 95%
confidence interval over the mean.
RESULTS AND DISCUSSION
■
The recently reported vanadium(III) complex (tpaMes)V(THF)
(1; THF = tetrahydrofuran) provides a readily accessible source
of the [(tpaMes)V] fragment.13 The treatment of 1 with p-
methoxyphenyl azide (N3ArOMe) in toluene at room temper-
ature results in an immediate color change from bright orange
to dark red/black. Layering of the reaction mixture under
pentane and subsequent storage at −35 °C overnight affords
dark, block-shaped crystals revealed by X-ray diffraction to be
1
into a NMR probe preheated to 75 °C, and a single-pulse H NMR
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the η1-azide adduct (tpaMes)V(η1-N3ArOMe) (2a). The 1H NMR
spectrum of this complex features peaks exclusively in the
diamagnetic region of the spectrum consistent with a formal
two-electron transfer from the vanadium center to the bound
ligand to give a complex best described as a
diazenylimidovanadium(V) species.4 Structurally, the bent
geometry at the β-nitrogen of the aryl azide [N5−N6−N7 =
118.2(4)°] of 2a supports this formulation. Furthermore, the
observed short V−Nazide bond distance [V1−N5 = 1.693(4) Å]
and the near-linear geometry at the imido nitrogen [V1−N5−
N6 = 170.5(3)°] are consistent with a V−N pseudo triple
bond.1a Isolable transition-metal organozide adducts of any
type are rare, and 2a represents one of only a few well-defined
terminal diazenylimido complexes.3−5
proceed with a negative entropy of activation, consistent with
dinitrogen extrusion from a highly ordered four-membered
cyclic species in which both the α- and γ-nitrogen atoms of the
organic azide fragment are bound to the transition-metal center.
The positive entropy of activation for thermolysis of 2a is
inconsistent with rate-limiting metallacycle formation. Although
we cannot rule out potential rate-limiting isomerization
pathways giving rise to the small, positive entropy of activation,
we speculate that the sterically demanding nature of [tpaMes
]
may impede the formation of such a species in the present
system, a phenomenon noted in Floriani’s tungsten calixarene
systems.5 Interestingly, small quantities of azoanisole were
formed in these reactions (<3 mol %), suggesting that a
different mechanism for decomposition of the organoazide and
formation of the imido may be operative.
Given that organic azide adducts of transition-metal
fragments are frequently invoked as intermediates in the
formation of imido complexes, we investigated the reactivity of
2a in this context. Indeed, thermolysis of 2a in benzene results
To further probe the mechanism of dinitrogen loss from 2a,
we synthesized a structurally homologous series of organoazide
complexes of the type (tpaMes)V(η1-N3ArR), where the
electronic properties of these adducts were systematically
tuned by varying the para substituents of the aryl ring [R = H
(2b), Me (2c), NMe2 (2d)]. As was observed for the complex
2a, thermolysis reactions of all four vanadium organoazide
complexes proceed with clean first-order kinetics (see the
Supporting Information). We then moved on to determine the
first-order rate constants for the thermolysis of each complex at
the same temperature (55 °C) and subjected the resulting data
to Hammett analysis (Figure 3). Surprisingly, in further
contrast to Proulx and Bergman’s Cp2TaMe(N3Ar) system,3
the only other terminal metal organoazide system for which a
detailed Hammett analysis has been performed, the conversions
of 2a−2d to 3a−3d are accelerated by electron-donating
substituents with ρ = −1.5. As in the case of 2a, small amounts
1
in a smooth conversion to a new diamagnetic product with H
NMR spectroscopic features consistent with the formation of
the corresponding vanadium(V) terminal imido complex. X-ray
diffraction studies performed on a single crystal of this product
confirmed its identity as (tpaMes)V(NArOMe) (3a; Figure 1).
The short V−N distance [V1−N5 = 1.687(2) Å] and nearly
linear arrangement of the imido ligand [V1−N5−C43 =
177.08(8)°] indicate a V−N triple bond.1
With a pair of well-defined, structurally characterized
terminal organoazide and imide complexes of vanadium and
data suggesting a clean organoazide-to-imide conversion in
hand, we proceeded to examine the kinetics of this process.
Although relatively few mechanistic investigations of the
thermolysis of metal organoazide complexes to the correspond-
ing metal imidos have been reported,3,4,6,10 these studies have
uncovered a surprisingly mechanistic diversity. For the present
case, thermolysis of 2a in C6D6 proceeds with a first-order
decay with kobs = 5.5(4) × 10−5 s−1 at 55 °C as monitored by
1H NMR. Eyring analysis (Figure 2) of the first-order rate
Figure 2. Eyring plot for thermolysis of 2a between 50 and 78.5 °C
with 95% confidence error bars; R2 = 0.9984.
constants determined for the decay of 2a over a temperature
range of 50−78.5 °C yields activation parameters of ΔH⧧ =
28.4(4) kcal mol−1 and ΔS⧧ = 7.6(1.1) eu. Interestingly, the
positive entropy of activation obtained for this vanadium
pyrrolide system is distinct compared to what was observed in
other metal organoazide platforms studied by the groups of
Bergman3 and Hillhouse.10 In these previously reported
systems, thermolysis of the organoazide adduct is found to
Figure 3. (a) Plots of ln [2]/[20] versus time for 2a−2d at 55 °C. (b)
Hammett correlation plot.
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of the corresponding diazenes were formed in these thermolysis
reactions. Furthermore, a double-label experiment in which
equal amounts of (tpaMes-D6)V(η1-N3Ph) and (tpaMes)V(η1-
NN15NPh) were thermolyzed together showed complete label
crossover. However, thermolysis of (tpaMes)V(η1-NN15NPh)
alone provides only the labeled imido (tpaMes)V(15NPh).
In contrast to the stoichiometric thermolysis reactions with
an isolated and structurally characterized vanadium organoazide
complex alone, when a C6D6 solution of 2a and 25 equiv of
N3ArOMe is heated to 75 °C for 8 h, azoanisole is generated
catalytically (TON ≈ 9) and formation of the imido 3a is
markedly impeded. Control experiments in which either the
azide alone or the azide and imido 3a were subjected to
analogous conditions did not yield detectable amounts of the
Scheme 1. Proposed Mechanism for the Formation of 3
through Nitrene Capture and Nitrene Dimerization To Give
Diazene
1
diazene product by H NMR. An experiment that directly
demonstrates the trapping of free nitrene by an external
substrate under these conditions would provide compelling
evidence for this process; however, our efforts to date using
singlet nitrene traps (e.g., cyclooctene) have been inconclusive.
We note that triplet nitrene, the likely form of transient nitrene
in this system, is difficult to trap in these conditions given that
the most effective traps either are themselves organoazides or
react with the complexes under study (e.g., nitroso com-
pounds).14
positive entropy of activation and is first-order in metal, and
further studies suggest the involvement of metal-generated
nitrene intermediates, which can lead to diazene formation.
This latter insight is significant in that the present system
demonstrates that reducing metal fragments capable of facile
reaction with organoazides may, nonetheless, generate free
nitrene species on their way to metal imido formation and adds
to the rich mechanistic diversity of transition-metal-mediated
nitrene chemistry. Further studies to utilize this information to
develop reactive yet controlled nitrogen-based catalytic
reactions are underway.
Taken together, these experiments suggest the possibility of
metal-mediated generation and trapping of nitrene fragments in
the formation of imido products 3a−3d, a proposal recently
advanced by Peters et al. for a ruthenium phosphine system.11b
Because solutions of N3ArOMe do not appreciably decompose
when subjected to the conditions employed for the formation
of 3a, simple trapping of the nitrene fragment NArOMe
generated by free azide thermolysis is inconsistent with the
observed kinetic data in our system. The kinetic experiments
were carried out in a dark NMR probe, thereby ruling out
photochemical pathways. These results complement related
work in different systems showing that manganese and
chromium imido complexes can be formed through thermal
liberation of nitrene fragments from free azides without
involvement of the metal.12 Likewise, catalytic diazene
formation from organoazides has been demonstrated for a
trigonal-bipyramidal iron system in which N−N coupling takes
place between two imidoiron(III) species,11a but the apparent
stability of 3a in solution rules out this mechanism in the
present case. We favor a mechanism in which nitrene loss
occurs directly from 2a−2d (or an isomer thereof) to give an
incipient low-valent vanadium complex that, in the absence of
excess organoazide, traps the nitrene fragment to yield 3a−3d.
The more electron-rich organoazides decompose more quickly
as they facilitate the two-electron reduction of the metal center
required for nitrene release, consistent with the results from the
Hammett analysis (vide supra). In the presence of excess
organoazide, binding of a second equivalent of organoazide to
reform 2a−2d occurs competitively with nitrene capture. The
nitrene fragments thus generated can either dimerize or react
with a second equivalent of free azide,15 leading to the observed
catalytic diazene formation (Scheme 1).
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, experimental details,
supporting figures, and X-ray structure information. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chrischang@berkeley.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by DOE/LBNL Grant 403801. C.J.C.
is an investigator with the Howard Hughes Medical Institute.
We thank Arkema (W.H.H.) and the Miller Institute for Basic
Research (N.A.P.) for fellowship support and Prof. Bob
Bergman and Dr. Ayumi Takaoka for helpful discussions.
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