A nitridoniobium(V) reagent that effects acid chloride to organic nitrile conversion: Synthesis via heterodinuclear (Nb/Mo) dinitrogen cleavage, mechanistic insights, and recycling

Article abstract of DOI:10.1021/ja056408j

The transformation of acid chlorides (RC(O)Cl) to organic nitriles (RC≡N) by the terminal niobium nitride anion [N≡Nb(N[Np]Ar) ₃]^- ([1a-N]^-, where Np = neopentyl and Ar = 3,5-Me₂C₆H₃) via i

Full text of DOI:10.1021/ja056408j

Published on Web 12/31/2005
A Nitridoniobium(V) Reagent That Effects Acid Chloride to
Organic Nitrile Conversion: Synthesis via Heterodinuclear
(Nb/Mo) Dinitrogen Cleavage, Mechanistic Insights, and
Recycling
Joshua S. Figueroa, Nicholas A. Piro, Christopher R. Clough, and
Christopher C. Cummins*
Contribution from the Massachusetts Institute of Technology, Room 2-227,
77 Massachusetts AVenue, Cambridge Massachusetts 02139
Received September 17, 2005; E-mail: ccummins@mit.edu
Abstract: The transformation of acid chlorides (RC(O)Cl) to organic nitriles (RCtN) by the terminal niobium
nitride anion [NtNb(N[Np]Ar)3]- ([1a-N]-, where Np ) neopentyl and Ar ) 3,5-Me2C6H3) via isovalent N
for O(Cl) metathetical exchange is presented. Nitrido anion [1a-N]- is obtained in a heterodinuclear N2
scission reaction employing the molybdenum trisamide system, Mo(N[R]Ar)3 (R ) t-Bu, 2a; R ) Np, 2b),
as a reaction partner. Reductive scission of the heterodinuclear bridging N2 complexes, (Ar[R]N)3Mo-
(µ-N2)Nb(N[Np]Ar)3 (R ) t-Bu, 3b; R ) Np, 3c) with sodium amalgam provides 1 equiv each of the salt
Na[1a-N] and neutral NtMo(N[R]Ar)3 (R ) t-Bu, 2a-N; R ) Np, 2b-N). Separation of 2-N from Na[1a-N] is
readily achieved. Treatment of salt Na[1a-N] with acid chloride substrates in tetrahydrofuran (THF) furnishes
the corresponding organic nitriles concomitant with the formation of NaCl and the oxo niobium complex
OtNb(N[Np]Ar)3 (1a-O). Utilization of 15N-labeled 15N2 gas in this chemistry affords a series of 15N-labeled
organic nitriles establishing the utility of anion [1a-N]- as a reagent for the 15N-labeling of organic molecules.
Synthetic and computational studies on model niobium systems provide evidence for the intermediacy of
both a linear acylimido and niobacyclobutene species along the pathway to organic nitrile formation. High-
yield recycling of oxo 1a-O to a niobium triflate complex appropriate for heterodinuclear N2 scission has
been developed. Specifically, addition of triflic anhydride (Tf2O, where Tf ) SO2CF3) to an Et2O solution of
1a-O provides the bistriflate complex, Nb(OTf)2(N[Np]Ar)3 (1a-(OTf)2), in near quantitative yield. One-electron
reduction of 1a-(OTf)2 with either cobaltocene (Cp2Co) or Mg(THF)3(anthracene) provided the monotriflato
complex, Nb(OTf)(N[Np]Ar)3 (1a-(OTf)), which efficiently regenerates complexes 3b and 3c when treated
with the molybdenum dinitrogen anions [N2Mo(N[t-Bu]Ar)3]- ([2a-N2]-) or [N2Mo(N[Np]Ar)3]- ([2b-N2]-),
respectively.
studies.14-19 In one instance a heterodinuclear system (Mo/Nb,
Introduction
Scheme 1) has been reported to deliver smooth dinitrogen
Strong chemical bonds introduce an energy barrier to the
utilization in synthesis of certain abundant small molecules; the
triple NN bond in the dinitrogen molecule, the CO double bonds
in carbon dioxide, and the single CH bonds in methane being
spotlight examples. This is why “reagents and pathways for
breaking into chemical bonds previously considered to be inert,”
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10.1021/ja056408j CCC: $33.50 © 2006 American Chemical Society

Synthesis of a Nitridoniobium(V) Reagent
A R T I C L E S
Scheme 1
C6H3Me2), which is not yet available from N2. Now we show
1-
that the anionic nitridoniobium(V) reagent NNb(N[Np]Ar)3
,
employed as its sodium salt, is both readily synthesized with
dinitrogen as its nitrido nitrogen source and serves to smoothly
transform acid chlorides into corresponding organic nitriles.
Because dinitrogen can be incorporated stoichiometrically
with the chemistry described herein, efficient transfer of an 15
N
isotopic label into the organic nitrile product is possible. That
the combination of dinitrogen cleavage chemistries with 15N-
isotope labeling strategies is an attractive prospect has been
pointed out in the context of a nitridotungsten(VI)-catalyzed
15N-atom exchange between acetonitrile and benzonitrile.28
Thus, one of the goals of the present work is to provide a reagent
for the RC(O)Cl f RC15N transformation, wherein the isotopic
label derives from the economical elemental source, 15N2.
While much current research is devoted to the discovery of
mild, metal-catalyzed methods for synthesizing ammonia from
dinitrogen,29-35 there is impetus also for discovery of NH3-
independent methods of N2-derived nitrogen atom incorporation
into organic molecules.36 It is the latter pathway to which the
chemistry described herein contributes, being concerned with
the interchange of NtN, NtM, and finally NtC triple bonds.
In this respect the cleaVage of organonitrile CN triple bonds
by ditungsten systems,37 a key to the synthesis of tungsten(VI)
alkylidyne complexes that perform beautifully as alkyne meta-
thesis catalysts,38 is relevant background material.
Despite the relative simplicity of the complexes promulgated
herein (monodentate secondary amine-derived supporting ligands),
a desirable feature would be the ability to reuse them for
dinitrogen cleavage following an N-atom transfer operation (e.g.,
nitrile synthesis). This paper shows accordingly how the
oxoniobium(V) byproduct of acid chloride transformation can
be recycled in high yield by conversion into its corresponding
bistriflate. This is an important and not an intuitive point given
the great stability of the oxoniobium(V) functional group.
Also treated herein are mechanistic issues surrounding the
acid chloride to nitrile transformation. Acylimido com-
plexes,27,39-42 produced by terminal nitride acylation, are impli-
cated as the key intermediates. Stability of the intermediate acyl-
imido complexes is found to be quite sensitive to the choice of
ancillary ligand steric attributes. Density functional theory (DFT)
computational methods have been brought to bear on the profile
of the overall reaction coordinate for acid chloride to organic
nitrile transformation by anionic nitridoniobium(V) complexes.
cleavage.20 This affords in 1:1 stoichiometry a neutral terminal
nitridomolybdenum(VI) complex and an isoelectronic anionic
terminal nitridoniobium(V) reagent, as its alkali metal salt
(Scheme 1). “Reagent” is used as a descriptor in the preceding
sentence with reference to the title RC(O)Cl f RCN transfor-
mation, as will become clear shortly.
Organic nitriles (RCN, where R ) variable substituent) are
attractive organic building blocks for which metal-mediated
syntheses are not numerous. Most common are Pd- or Cu-
catalyzed aromatic nitrile syntheses that require a source of
cyanide ion.21-25 Similarly, beginning with the CN bond already
intact, a zirconium-based method focusing on carboxamide
dehydration (RC(O)NH2 f RCN) has been reported recently.26
In contrast, metal-mediated methods for CN triple bond genera-
tion represent a new vista and one that is especially inviting
for establishing the connection with dinitrogen cleavage chem-
istries. A first example of organic nitrile synthesis from corre-
sponding acid chlorides via an N for O(Cl) isovalent exchange
process was reported recently;27 it employed the four-coordinate
nitridotungsten(VI) complex NW(N[i-Pr]Ar)3 (Ar ) 3,5-
(28) Chisholm, M. H.; Delbridge, E. E.; Kidwell, A. R.; Quinlan, K. B. Chem.
Commun. 2003, 126-127.
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Organometallics 2000, 19, 1622-1624.
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A R T I C L E S
Figueroa et al.
solution was stored, under vacuum, at -35 °C for 1-2 days, whereupon
purple-brown crystals of 2b-H formed and were collected. The mother
liquor was reduced to approximately half the original volume and chilled
again to produce a second crop of crystals. Yield 7.14 g, 10.7 mmol,
45% (avg) based on multiple syntheses. 1H NMR (500 MHz, C6D6, 20
°C): δ ) 2.00 (br) and 1.50 (br) ppm. µeff (Evans method) ) 1.64 µB.
Anal. Calcd. for C39H60N3Mo: C, 70.24; H, 9.07; N, 6.30. Found: C,
71.01; H, 9.48; N, 6.01.
Experimental Section
General Considerations. Unless otherwise stated, all manipulations
were carried out at room temperature, under an atmosphere of purified
dinitrogen by use of a Vacuum Atmospheres glovebox or Schlenk tech-
niques. All solvents were obtained anhydrous and oxygen-free according
to standard purification procedures. Tetrahydrofuran-d8 (THF-d8) was
vacuum-transferred from Na metal and stored in the glovebox over 4
Å molecular sieves for at least 3 days prior to use. Benzene-d6 (C6D6)
was degassed and stored similarly. Celite 435 (EM Science) and 4 Å
molecular sieves (Aldrich) were dried under vacuum at 250 °C
overnight and stored under dinitrogen. Gaseous 15N2 (99.9% 15N) was
purchased from Cambridge Isotope Laboratories in 100 mL break-seal
containers. Solid NaOMe was slurried in anhydrous THF under an N2
atmosphere for 24 h before being dried at ca. 180 °C under vacuum
for an additional 24 h. All other reagents were purchased from
commercial sources and purified according to standard procedures or
prepared as specified in the literature. All glassware was oven-dried at
a temperature of 230 °C prior to use.
Synthesis of Na[(N2)Mo(N[Np]Ar)3] (Na[2b-N2]). In a dinitrogen-
atmosphere glovebox, freshly prepared 1% sodium amalgam (Na; 0.350
g, approximately 10 equiv) was added to a THF solution of Mo(H)-
(η2-t-Bu(H)CdNAr)(N[Np]Ar)2 (2b-H, 1.00 g, 1.50 mmol, 50 mL).
The reaction mixture was vigorously stirred for 20 h at room
temperature while it was exposed to the N2 atmosphere through an
open gas-adapter port. The color of the solution was observed to
gradually change from purple-brown to dark green. The mother liquor
was decanted from the amalgam and evaporated to dryness under
vacuum, resulting in a dark green residue. Addition of n-pentane (40
mL) resulted in a red-orange solution, which was filtered though Celite
and dried under vacuum. Addition of n-pentane (15 mL) to the resulting
red-orange residue, followed by cooling to -35 °C for 30 min,
precipitated essentially pure Na[2b-N2] (1H NMR, C6D6). Yield: 1.933
g, 2.70 mmol, 60% (avg) based on three separate syntheses. Analytically
pure, cherry-red single crystals of Na(Et2O)[2b-N2] were obtained from
a saturated, Et2O solution stored at -35 °C for 2 days. The 15N-labeled
variant, Na[2b-15N2], was prepared identically under an atmosphere of
99.9% 15N2. 1H NMR (500 MHz, C6D6, 20 °C): δ ) 6.61 (br s,
6H, o-Ar), 6.49 (s, 3H, p-Ar), 4.32 (br s, 6H, N-CH2), 2.19 (s, 18H,
Ar-CH3), and 1.00 (br s, 27H, t-Bu) ppm. 13C{1H} NMR (125.8 MHz,
C6D6, 20 °C): δ ) 159.3.8 (ipso-Ar), 138.9 (m-Ar), 122.4 (p-Ar), 119.9
(o-Ar), 84.9 (N-CH2), 36.0 (C(CH3)3), 29.1 (C(CH3)3), and 22.1
(Ar-CH3) ppm. FTIR (KBr windows, C6D6 solution): (νNN) 1730 cm-1
also 2949, 2873, 1584, 1473, 1069, and 681 cm-1. For Na[2b-15N2],
(νNN) 1784 and 1674 cm-1. Anal. Calcd. for C43H68N5OMoNa (ether-
ate): C, 65.38; H, 8.68; N, 8.87. Found: C, 65.24; H, 8.80; N, 8.52.
Solution 1H and 13C NMR spectra were recorded on Varian
XL-300 MHz and Varian INOVA 500 MHz spectrometers. 1H and 13
C
chemical shifts are reported with reference to residual solvent resonances
of 7.16 ppm (1H) and 128.3 ppm (13C) for benzene-d6 or 1.73 ppm
(1H) and 67.57 ppm (13C) for THF-d8. Solution 15N NMR spectra were
recorded on a Bruker Advance-600 spectrometer operating at a
resonance frequency of 60.84 MHz. 15N NMR chemical shifts are
reported with reference to external formamide-15N (H215NC(O)H,
55/45 v/v in dimethyl sulfoxide, DMSO); 113.0 ppm relative to liquid
NH3 (0.0 ppm). Solution IR spectra were recorded on a Perkin-Elmer
1600 series Fourier transform infrared (FTIR) spectrometer. All spectra
were recorded in C6D6 by use of a solution cell equipped with KBr
windows. Solvent peaks were digitally subtracted from all spectra by
comparison with an authentic spectrum obtained immediately prior to
that of the sample. Combustion analyses were carried out by H. Kolbe
Microanalytisches Laboratorium, Mu¨lheim an der Ruhr, Germany.
Desilylation of (Me3SiN2)Mo(N[t-Bu]Ar)3 (2a-N2SiMe3) with
Sodium Methoxide: Optimized Synthesis of Na[(N2)Mo(N[t-Bu]-
Ar)3] (Na[2a-N2]). To a THF solution of 2a-N2SiMe3 (3.00 g, 4.13
mmol, 20 mL),6 was added anhydrous NaOMe (1.11 g, 20.6 mmol,
5.0 equiv). The reaction mixture was allowed to stir for 24 h while it
gradually changed color from orange to magenta. All volatile materials
were then removed under reduced pressure and the excess NaOMe was
removed by extraction of the resulting purple residue with n-pentane
(50 mL), followed by filtration through Celite. Concentration of the
filtrate to 10 mL and cooling to -35 °C for 1-2 days provided
Na[2a-N2] as orange crystals in 70-80% yield. Spectroscopic data
for Na[2a-N2] prepared by this method were identical to those re-
ported previously.6 For the 15N-labeled variant, Na[2a-15N2], NaOMe
and 2a-15N2SiMe3 were allowed to react under static vacuum in order
to avoid 14N2/15N2 exchange.
Synthesis of Nb(I)(N[Np]Ar)3 (1a-I). To a thawing THF solution
of 1a-(I)2 (4.500 g, 4.90 mmol, 75 mL)44 was added solid Mg(THF)3-
(anthracene) (1.128 g, 2.690 mmol, 0.55 equiv). The reaction mixture
was allowed to warm to room temperature and was stirred for 1.5 h,
during which time it was observed to gradually change in color from
orange to purple. All volatile materials were removed in vacuo and
the resulting purple-brown residue was extracted with n-pentane (40
mL) and filtered through Celite to remove MgI2 and anthracene. The
purple filtrate was then reduced to a volume of 20 mL and frozen.
Upon thawing, the solution was quickly cold-filtered through Celite to
remove residual anthracene and unreacted 1a-(I)2. The resulting purple
filtrate was then reduced to a volume of 15 mL and stored at -35 °C
for 2 days, whereupon a purple crystalline solid was obtained and
1
collected. Yield: 1.270 g, 1.61 mmol, 33.0% in two crops. H NMR
(500 MHz, C6D6, 20 °C): δ ) 2.00 (br s) ppm. µeff (Evans method) )
1.81µB. Anal. Calcd. for C39H60N3INb: C, 59.24; H, 7.65; N, 5.31.
Found: C, 59.03; H, 7.60; N, 5.18. Unreacted, orange 1a-(I)2
(contaminated with approximately 25% anthracene) can be recovered
by washing the Celite pad with Et2O (25-30 mL) followed by drying
of the filtrate. The resulting orange residue was then slurried in cold
n-pentane and filtered. Yield of recovered 1a-I2: 1.050 g, 1.14 mmol,
23%.
Synthesis of Mo(H)(η2-t-Bu(H)CdNAr)(N[Np]Ar)2 (2b-H). To a
43
thawing suspension of MoCl3(THF)3 (10.0 g, 23.8 mmol) in Et2O
(150 mL) was added solid (Et2O)Li(N[Np]Ar)44 (13.6 g, 50.1 mmol).
The headspace of the reaction flask was evacuated and the reaction
was allowed to stir for 6 h. During this time the color of the mixture
gradually changed to purple-brown. The mixture was then filtered
through Celite to remove LiCl and unreacted MoCl3(THF)3; the Celite
was washed with additional Et2O (30 mL). The filtrate was dried under
vacuum and the resulting purple-brown residue was extracted with
n-pentane (100 mL) and filtered through Celite, upon which an
intractable purple solid remained. The filtrate was concentrated to
dryness and the residue was dissolved in Et2O (75-100 mL). This
Optimized Synthesis of ONb(N[Np]Ar)3 (1a-O) and Nb(OTf)2-
(N[Np]Ar)3 (1a-(OTf)2). To a thawing Et2O solution of ONbCl3(THF)2
(15.0 g, 41.7 mmol, 75 mL) was added solid (Et2O)Li(N(Np)Ar) (33.9
g, 125.1 mmol, 3 equiv). The reaction mixture was allowed to warm
to room temperature and stir for 2.5 h, during which time it gradually
became dark brown in color. Removal of the solvent in vacuo followed
by extraction with pentane, filtration, and evaporation of the brown
filtrate to dryness, provided 1a-O as a dark brown residue, which was
redissolved in Et2O (30 mL). Pure 1a-O can be obtained as golden-
(43) Stoffelbach, F.; Saurenz, D.; Poli, R. Eur. J. Inorg. Chem. 2001, 2699-
2703.
(44) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2003, 125, 4020-
4021.
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Synthesis of a Nitridoniobium(V) Reagent
A R T I C L E S
yellow blocks in 55% yield from storage of the Et2O solution at -35
°C for 2 days. However, addition of triflic anhydride (Tf2O, 17.6 g,
62.5 mmol, 1.5 equiv) directly to the chilled, brown filtrateswithout
isolation of 1a-Oseffected the precipitation of bright-yellow 1a-(OTf)2,
which was collected by filtration (yield 26.87 g, 27.9 mmol, 67% from
ONbCl3(THF)2). The spectroscopic properties of 1a-O and 1a-(OTf)2
were identical to those reported previously.45
Synthesis of Nb(O3SCF3)(N[Np]Ar)3 (1a-OTf). The synthesis of
1a-OTf was performed analogously to that of 1a-I, employing 1.00 g
(1.04 mmol) of 1a-(OTf)2 and 0.239 g (0.571 mmol, 0.55 equiv) of
Mg(THF)3(anthracene). Storage of an n-pentane solution at -35 °C
for 1 day provided purple-green crystals of 1a-OTf. Yield: 0.338 g,
0.416 mmol, 40%. µeff (Evans method) ) 2.1 µB. 1H NMR (500 MHz,
C6D6, 20 °C): δ ) 1.789 (br s) and 2.20 (br s) ppm. Anal. Calcd. for
C40H60N3O3F3SNb: C, 59.10; H, 7.44; N, 5.17. Found: C, 59.12; H,
7.51; N, 5.07.
Cobaltocene (Cp2Co) Reduction of Nb(O3SCF3)2(N[Np]Ar)3 (1a-
(OTf)2): Alternative Synthesis of 1a-OTf. Separately, a 3 mL THF
solution of 1a-(OTf)2 (1.1 g, 1.14 mmol) and a 2 mL THF solution of
Cp2Co (0.227 g, 1.20 mmol, 1.05 equiv) were frozen in a glovebox
cold well (liquid N2). Upon removal from the cold well, the thawing
solution containing Cp2Co was added dropwise over 3 min to the
thawing solution of 1a-(OTf)2, eliciting a color change from bright
orange to brown-purple. The reaction mixture was allowed to stir at
room temperature for 45 min, whereupon all volatile materials were
removed in vacuo. The resulting brown residue was extracted with
n-pentane (3 mL) and filtered through Celilte to remove the orange
[Cp2Co][O3SCF3] byproduct. Evaporation and redissolution of the
filtrate in n-pentane provided purple-green crystals of 1a-OTf when
stored at -35 °C. Yield: 0.575 g, 0.707 mmol, 62% in three crops.
Synthesis of Heterodinuclear N2 Complexes (Ar[Np]N)3Nb(µ-N2)-
Mo(N[t-Bu]Ar)3 (3b) and (Ar[Np]N)3Nb(µ-N2)Mo(N[Np]Ar)3 (3c).
Separately, a 10 mL THF solution of 1a-I (2.50 g, 3.16 mmol) and a
10 mL THF solution of the corresponding molybdenum dinitrogen anion
salt (Na[2a-N2] for 3b, 2.13 g, 3.16 mmol; Na[2b-N2] for 3c, 2.25 g,
3.16 mmol) were frozen in a glovebox cold well cooled externally with
liquid nitrogen. Upon removal from the cold well, approximately 4
mL of the thawing solution containing 1a-I was added dropwise over
1 min to the thawing solution of the molybdenum salt, eliciting a color
change from red-orange to green. The reaction mixture was allowed to
stir for an additional 3 min, whereupon both solutions were placed
back into the cold well. This procedure was repeated two more times
until complete addition of 1a-I was achieved. The reaction mixture
was then allowed to warm to room temperature and stirred for an
additional 30 min before being evaporated to dryness. The residue
obtained was extracted with n-pentane (10 mL) and filtered through
Celite to remove the NaI byproduct, and the resulting filtrate was
evaporated to dryness. The resulting green powders obtained were
sufficiently pure for subsequent synthetic applications. However,
crystalline material could be obtained by storing Et2O solutions of either
3b or 3c at -35 °C for several days. The monotriflato-Nb(IV) complex,
1a-OTf, can be readily substituted for 1a-I in this procedure, resulting
in comparable yields of 3b and 3c.
an excess of freshly prepared 1% sodium amalgam (Na: 0.135 g, 5.86
mmol, 4 equiv/Nb). The reaction mixture was allowed to stir at room
temperature for 2 h, during which time it was observed gradually to
change in color from green to orange-brown. The solution was then
decanted from the amalgam and evaporated to dryness under reduced
pressure. The resulting orange-brown residue was then extracted with
n-pentane and filtered through Celite. The filtrate was then concentrated
to dryness under reduced pressure. The resulting residue was then
exposed to two cycles of n-pentane (∼3 mL) addition and evaporation
in order to completely separate THF from Na[1a-N]. Dissolving the
residue so obtained in n-pentane (4 mL), followed by cooling to -35
°C for 30 min, followed by filtration, provided pure Na[1a-N] as an
off-white powder. Evaporation of the filtrate followed by dissolution
of the resulting residue in Et2O and cooling to -35 °C produced orange
single crystals of 2b-N after 2 days (72%). Complex 3b can be
substituted for 3c in this procedure, providing Na[1a-N] in 80% yield
(0.824 g, 1.17 mmol) after precipitation from n-pentane and the nitrido
molybdenum complex 2a-N3,4,6,8,36,46 in 75% yield (0.704 g, 1.10 mmol)
after crystallization from Et2O.
1
Na[1a-N]. Yield 0.824 g, 1.17 mmol, 80%. H NMR (500 MHz,
THF-d8, 20 °C): δ ) 6.26 (s, 6H, o-Ar), 6.11 (s, 3H, p-Ar), 4.36
(s, 6H, N-CH2), 1.90 (s, 18H, Ar-CH3), and 0.83 (s, 27H, t-Bu) ppm.
13C{1H} NMR (125.8 MHz, THF-d8, 20 °C): δ ) 159.3 (ipso-Ar),
137.7 (m-Ar), 120.9 (p-Ar), 119.9 (o-Ar), 74.6 (N-CH2), 36.4 (C(CH3)3),
29.9 (C(CH3)3), and 21.8 (Ar-CH3) ppm. 15N NMR (60.84 MHz, THF-
d8, 20 °C): δ ) 754 (s) ppm. Anal. Calcd. for C39H60N4NbNa: C,
66.84; H, 8.63; N, 7.99. Found: C, 67.08; H, 8.94; N, 8.12. Single
crystals of yellow [Na(THF)3][1a-N] were obtained by allowing a
saturated THF solution of Na[1a-N] to stand at -35 °C for several
days.
1
2b-N. Yield 0.720 g, 1.05 mmol, 72%. H NMR (500 MHz, C6D6,
20 °C): δ ) 6.50 (s, 3H, p-Ar), 6.26 (s, 6H, o-Ar), 4.98 (s, 6H, N-CH2),
1.94 (s, 18H, Ar-CH3), and 1.10 (s, 27H, t-Bu) ppm. 13C{1H} NMR
(125.8 MHz, C6D6, 20 °C): δ ) 155.8 (ipso-Ar), 138.8 (m-Ar), 126.7
(p-Ar), 122.7 (o-Ar), 85.5 (N-CH2), 36.0 (C(CH3)3), 29.3 (C(CH3)3),
and 21.7 (Ar-CH3) ppm. 15N NMR (60.84 MHz, THF-d8, 20 °C): δ )
870 (s) ppm. Anal. Calcd. for C39H60N4Mo: C, 68.80; H, 8.88; N, 8.23.
Found: C, 68.60; H, 8.95; N, 8.11.
General Procedure for Niobium-Mediated 15N-Labeled Nitrile
Generation from Acid Chloride Substrates. To a THF-d8 solution
of Na[15NNb(N[Np]Ar)3] (Na[1a-15N], 0.050 g, 0.071 mmol, 1.5 mL)
was added a THF-d8 solution of the appropriate acid chloride substrate
(0.95 equiv in all cases, 0.5 mL) at room temperature. A rapid color
change from pale brown to yellow orange accompanied the addition.
For acid chloride substrates resulting in a volatile organonitrile, the
volatile components of the reaction mixture were vacuum-transferred
to a flame-sealable NMR tube for analysis. The nonvolatile oxoniobium
byproduct, 1a-O, was liberated from NaCl by extraction with n-pentane,
followed by filtration of the extract, and set aside for further use. For
reactions generating a nonvolatile organonitrile, the reaction mixture
was allowed to stir for 20 min after addition of the acid chloride and
then evaporated to dryness, after which the dry residue was extracted
with n-pentane, the extract was filtered, and the filtrate was evaporated
to dryness. THF-d8 was then used to dissolve the crude residue
immediately prior to spectroscopic analysis. For the latter samples, 1a-O
was present in solution during analysis.
3b. Yield 2.24 g, 1.70 mmol, 54%. µeff (Evans method) ) 2.36µB.
1H NMR (500 MHz, C6D6, 20 °C): δ ) 4.4 (br), 2.2 (br), and 1.5 (br)
ppm. FTIR (KBr windows, C6D6 solution): (νNN) 1564 cm-1. For
3b-15N2, (νNN) 1519 cm-1. Anal. Calcd. for C75H114N8NbMo: C, 68.95;
H, 8.90; N, 8.25. Found: C, 69.15; H, 9.01; N, 8.49.
3c. Yield 2.06 g, 1.52 mmol, 48%. µeff (Evans method) ) 2.58µB.
1H NMR (500 MHz, C6D6, 20 °C): δ ) 2.1 (br) and 1.5 (br) ppm.
FTIR (KBr windows, C6D6 solution): (νNN) 1576 cm-1. For 3c-15N2,
(νNN) 1515 cm-1. Anal. Calcd. for C78H120N8NbMo: C, 68.95; H, 8.90;
N, 8.25. Found: C, 68.33; H, 9.12; N, 8.44.
Attempted Observation of Acylimido Intermediates (R(O)CNd
Nb(N[Np]Ar)3; 4a, R ) Me, t-Bu, 1-Ad). To a Teflon-capped NMR
tube (J-Young) were added sequentially a THF-d8 solution of
Na[1a-N] (0.015 g, 0.021 mmol, 0.4 mL), a 0.1 mL buffer zone of
neat THF-d8, and a 0.3 mL THF-d8 solution containing 0.95 equiv of
pivaloyl chloride (t-BuC(O)Cl). After each sequential addition, the sam-
(45) Figueroa, J. S.; Cummins, C. C. J. Am. Chem. Soc. 2004, 126, 13916-
13917.
(46) Laplaza, C. E.; Odom, A. L.; Davis, W. M.; Cummins, C. C.; Protasiewicz,
J. D. J. Am. Chem. Soc. 1995, 117, 4999-5000.
Reductive Cleavage of (Ar[Np]N)3Nb(µ-N2)Mo(N[Np]Ar)3 (3c):
Synthesis of [Na][NNb(N[Np]Ar)3)] (Na[1a-N]) and NMo(N[Np]Ar)3
(2b-N). To a THF solution of 3c (2.0 g, 1.47 mmol, 15 mL) was added
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 943

A R T I C L E S
Figueroa et al.
ple was frozen in a glovebox cold well (liquid N2) to prevent premature
mixing of the layers. The total concentration of Na[1a-N] was 2.6 mM.
The sample was transported frozen to the spectrometer, whereupon
reactants were allowed to mix upon melting of the mixture, immediately
prior to 1H NMR data acquisition. The resulting 1H NMR spectra
contained resonances characteristic exclusively of the reactants and
products. No new resonances were observed when additional spectra
were intermittently acquired during the process of sample warming to
ambient temperature (20 °C, ca. 20 min). Identical results were obtained
when 1-adamantoyl or acetyl chloride was employed in this experiment.
in the SAINT+ (v6.45) and SHELXTL (v6.14) program suites,
respectively (G. Sheldrick, Bruker AXS, Madison, WI).
Computational Details. All DFT calculations were carried out with
the Amsterdam Density Functional (ADF) program suite,51,52 version
2004.01.53 The all-electron, Slater-type orbital (STO) basis sets
employed were of triple-ú quality augmented with two polarization
functions and incorporated relativistic effects by use of the zero-order
regular approximation54,55 (ADF basis ZORA/TZ2P). The local exchange-
correlation potential of Vosko et al.56 (VWN) was augmented self-
consistently with gradient-corrected functionals for electron exchange
according to Becke57 and electron correlation according to Perdew.58,59
This nonlocal density functional is termed BP86 in the literature and
has been shown to give excellent results for the geometries and
energetics of transition metal systems.60 Crystallographically determined
atomic coordinates were used as input where appropriate. Each
optimized structure was subjected to a harmonic frequencies calculation
to validate characterization as local minima on the potential energy
surface (zero imaginary frequencies). All enthalpic values reported were
corrected for zero-point and internal energy considerations (SATP:
298.15 K, 1.0 atm).
Synthesis of t-BuC(O)NdNb(N[t-Bu]Ar)3 (1c-NC(O)-t-Bu). To a
thawing THF solution of Na[1c-N]47,48 (0.200 g, 0.30 mmol, 3 mL)
was added dropwise an equally cold THF solution of t-BuC(O)Cl (0.034
g, 0.28 mmol, 0.95 equiv, 2 mL). The resulting pale-yellow reaction
mixture was allowed to stir for 45 min before all volatile components
were removed under reduced pressure. Extraction of the residue with
n-pentane, followed by filtration through Celite and evaporation,
provided crude 1c-NC(O)-t-Bu as a pale-yellow powder. Near-colorless
crystals of 1c-NC(O)-t-Bu were obtained by Et2O recrystallization at
1
-35 °C (2 days). Yield: 0.131 g, 0.182 mmol, 60%. H NMR (500
MHz, C6D6, 20 °C): δ ) 6.71 (s, 3H, p-Ar), 6.02 (s, 6H, o-Ar), 2.12
(s, 18H, Ar-CH3), 1.59 (s, 9H, t-Bu-acyl), and 1.42 (s, 27H, t-Bu) ppm.
13C{1H} NMR (125.8 MHz, C6D6, 20 °C): δ ) 149.5 (ipso-Ar), 137.4
(m-Ar), 130.5 (o-Ar), 128.0 (p-Ar), 32.3 (C(CH3)3-acyl), 32.2 (C(CH3)3),
29.1 (C(CH3)3), 21.8 (Ar-CH3) ppm. FTIR (KBr windows, C6D6
solution): ν(CdO) 1640 cm-1. Anal. Calcd. for C41H63N4NbO: C,
68.31; H, 8.81; N, 7.77; Found: C, 68.44; H, 8.64; N, 7.93.
Results and Discussion
Molybdenum-Based Systems for Dinitrogen Uptake: Im-
proved Syntheses of Mo-N2 Anions. In our previous report of
Nb/Mo heterodinuclear N2 cleavage,20 the molybdenum dini-
trogen anion [2a-N2]- was combined with the chloroniobium-
(IV) complex Nb(Cl)(N[i-Pr]Ar)3 (1b-Cl), as shown in Scheme
1. The resulting Nb/Mo heterodinuclear N2 complex 3a was
Crystallographic Structure Determinations. The X-ray crystal-
lographic data collections were carried out on a Siemens Platform three-
circle diffractometer mounted with a charge-coupled device (CCD) or
APEX-CCD detector and outfitted with a low-temperature, nitrogen-
stream aperture. The structures were solved by direct methods, in
conjunction with standard difference Fourier techniques, and refined
by full-matrix least-squares procedures. A summary of crystallographic
data for complexes 2b-H, Na(THF)3[1a-N], 2b-N, and 1c-NC(O)t-Bu
is given as Table S3.1, and full crystallographic details for complexes
1a-I and 1a-OTf are provided as Supporting Information. Complex
Na(THF)3[1a-N] was refined in the trigonal space group P3h. The
systematic absences in the diffraction data are uniquely consistent with
the assigned monoclinic space groups for all other complexes. These
choices led to chemically sensible and computationally stable refine-
ments. An empirical absorption correction (SADABS) was applied to
the diffraction data for all structures. All non-hydrogen atoms were
refined anisotropically. In most cases, hydrogen atoms were treated as
idealized contributions and refined isotropically. The hydrido ligand
in complex 2b-H was located in the electron density difference map
and refined isotropically. Complex Na(THF)3[1a-N] cocrystallized with
several severely disordered molecules of THF, which could not be
reasonably modeled as discrete entities. To account for this disorder,
the crystallographic routine SQUEEZE49,50 was employed and found a
total of 119 electrons in a solvent-accessible void area of 713.5 Å3
(22.5% of unit cell volume), corresponding to 2.94 molecules of THF/
unit cell (∼1.5 molecules of THF/molecule of Na(THF)3[1a-N]). The
final stages of refinement for complex Na(THF)3[1a-N] were performed
against the solvent-free reflection file obtained from SQUEEZE and
resulted in significant improvements of the final residual values. The
residual peak and hole electron density for Na(THF)3[1a-N] were 0.590
and -0.173 e-‚Å-3, respectively. All software used for diffraction data
processing and crystal-structure solution and refinement are contained
61
subsequently reduced by KC8 to afford 1 equiv each of the
terminal nitrides 2a-N and [1b-N]-. Because the synthesis of
Na[2a-N2] required an exceedingly long addition time (t > 10
h) of trisanilide 2a to sodium amalgam (Na/Hg), we set out to
find a more convenient route to its formation. The corresponding
trimethylsilyldiazenido complex, (Me3SiN2)Mo(N[t-Bu]Ar)3
(2a-N2SiMe3), was chosen due to its availability in 15N-labeled
form and facile, one-pot synthesis from 2a, N2 and Me3SiCl.6
We have found that treatment of 2a-N2SiMe3 with sodium
methoxide (NaOMe, 5.0 equiv) readily provides the target salt,
Na[2a-N2], in yields of 75-80% after workup and crystallization
(Scheme 2). The yields of Na[2a-N2] obtained by this desily-
lation route are comparable to those reported previously, but
the necessity for slow addition of 2a to Na/Hg is obviated.
In the course of studying the chemistry of molybdaziridine
hydride complexes,7,62-65 we have discovered another system
(51) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra, C.; van
Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001,
22, 931-967.
(52) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391-403.
(53) Baerends et al., E. J. ADF, ADF2004.01; Theoretical Chemistry, Vrije
Universiteit, Amsterdam, The Netherlands, http://www.scm.com, 2004.
(54) van Lenthe, E.; Baerends, E. J.; Snijders, J. G. J. Chem. Phys. 1993, 99,
4597-4610.
(55) vanLenthe, E.; Snijders, J. G.; Baerends, E. J. J. Chem. Phys. 1996, 105,
6505-6516.
(56) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200-1211.
(57) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
(58) Perdew, J. P. Phys. ReV. B 1986, 33, 8822-8824.
(59) Perdew, J. P. Phys. ReV. B 1986, 34, 7406-7406.
(60) Deng, L. Q.; Schmid, R.; Ziegler, T. Organometallics 2000, 19, 3069-
3076.
(61) Schwindt, M. A.; Lejon, T.; Hegedus, L. S. Organometallics 1990, 9, 2814-
2819.
(47) Fickes, M. G.; Odom, A. L.; Cummins, C. C. Chem. Commun. 1997, 1993-
1994.
(62) Stephens, F. H.; Figueroa, J. S.; Cummins, C. C.; Kryatova, O. P.; Kryatov,
S. V.; Rybak-Akimova, E. V.; McDonough, J. E.; Hoff, C. D. Organo-
metallics 2004, 23, 3126-3138.
(48) Brask, J. K.; Fickes, M. G.; Sangtrirutnugul, P.; Dura-Vila, V.; Odom, A.
L.; Cummins, C. C. Chem. Commun. 2001, 1676-1677.
(49) van der Sluis, P.; Spek, A. L. Acta Crystallogr., Sect. A 1990, 46, 194-
201.
(63) Cherry, J. P. F.; Stephens, F. H.; Johnson, M. J. A.; Diaconescu, P. L.;
Cummins, C. C. Inorg. Chem. 2001, 40, 6860-6862.
(64) Tsai, Y. C.; Diaconescu, P. L.; Cummins, C. C. Organometallics 2000,
19, 5260-5262.
(50) Spek, A. L. Acta Crystallogr. 1990, A46, C34.
9
944 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Synthesis of a Nitridoniobium(V) Reagent
A R T I C L E S
Scheme 2
Scheme 3
from 2b-H is notable inasmuch as analogues 2a and 2c-H both
form their corresponding nitrido complexes (2a-N and (µ-N)-
[2c]2) when subjected to an identical synthetic regimen.6,8 While
the factors governing this fascinating dichotomy are presently
unclear, hydrocarbyl substituent variation has been shown
previously to greatly influence the small molecule chemistry
exhibited by early transition metal anilido complexes.6,62,66
Synthesis of Niobium(IV) Precursors for Heterodinuclear
N2 Cleavage. While the N[i-Pr]Ar-ligated niobium(IV) complex
1b-Cl67 was employed previously for heterodinuclear N2 cleav-
age,20 for the present study we sought a platform more
encouraging of metathetical exchange at the Nb center. There-
fore, we chose to pursue the triply N-neopentylanilide-ligated
(N[Np]Ar) system for this purpose due to its propensity for
5-coordination at Nb.44,45,68 Accordingly, reduction of the orange
diiodide complex, Nb(I)2(N[Np]Ar)3 (1a-(I)2),44 with 0.55 equiv
of Mg(anthracene)(THF)369 in THF solution provided the purple-
brown monoiodide Nb(I)(N[Np]Ar)3 (1a-I) in 33% yield after
crystallization from n-pentane. While monoiodide 1a-I was
suitable for reaction with both Na[2a-N2] and Na[2b-N2] (next
section), its low overall yield and the multistep nature of its
synthesis spurred us to define an alternative niobium(IV)
precursor synthesis.
that provides convenient, one-step access to a molybdenum dini-
trogen anion. The molybdaziridine hydride complex Mo(H)-
(t-Bu(H)CdNAr)(N[Np]Ar)2 (2b-H, Figure 1) is a purple-black,
ground-state paramagnet (µeff ) 1.64 µB) closely related to its
diamagnetic niobium analogue.44 Complex 2b-H is readily
synthesized on a multigram scale in ca. 45% isolated yield by
43
combination of MoCl3(THF)3 and (Et2O)Li(N[Np]Ar)44 in
Et2O solution. Unlike 2a and the N-isopropylanilide-ligated
Mo(H)(Me2CdNAr)(N[i-Pr]Ar)2 (2c-H),7 complex 2b-H has not
been observed to effect the six-electron reduction of N2 in
solution at room temperature or below.8 Instead, 2b-H remains
unchanged for days under a dinitrogen atmosphere but readily
forms the dinitrogen-containing salt, [Na][(N2)Mo(N[Np]Ar)3]
(Na[2b-N2]), when stirred with excess Na/Hg in the presence
of N2 (Scheme 3).
Diamagnetic Na[2b-N2] is obtained as a cherry-red crystalline
solid after crystallization from n-pentane and gives rise to an
intense νNN stretch at 1730 cm-1 in C6D6 solution (νNN ) 1674
cm-1 for Na[2b-15N2]). The direct formation of Na[2b-N2]
Since the niobium complex 1a-O is the ultimate product of
nitrile formation from [1a-N]- and acid chlorides, it was of
interest to develop a quick and efficient synthesis of it from
readily accessible starting materials. In this regard, the oxonio-
70
bium trichloride complex ONb(Cl)3(THF)2 was found to be
an ideal synthetic precursor to 1a-O. Treatment of ONb(Cl)3-
(THF)2 with 3.0 equiv of (Et2O)Li(N[Np]Ar)44 in thawing Et2O
solution provided a dark brown solution rich in 1a-O after 3 h.
Filtration of the reaction mixture, followed by concentration
and cooling at -35 °C, provided crystalline 1a-O in ca. 55%
yield. However, isolation of 1a-O obtained from this synthesis
is unnecessary. Addition of triflic anhydride71 (Tf2O, Tf )
SO2CF3, 1.5 equiv) directly to the LiCl-free reaction mixture
elicits the precipitation of the bright-orange bistriflate complex
Nb(OTf)2(N[Np]Ar)3 (1a-(OTf)2),45 in 67% overall yield from
ONb(Cl)3(THF)2 (Scheme 4). The formation of 1a-(OTf)2 from
1a-O and Tf2O is noteworthy in that the electrophilic potency
of the latter incorporates the normally inert oxoniobium(V)
function into a common triflate leaving group. Although they
(65) Blackwell, J. M.; Figueroa, J. S.; Stephens, F. H.; Cummins, C. C.
Organometallics 2003, 22, 3351-3353.
(66) Tsai, Y. C.; Stephens, F. H.; Meyer, K.; Mendiratta, A.; Gheorghiu, M.
D.; Cummins, C. C. Organometallics 2003, 22, 2902-2913.
(67) Mindiola, D. J.; Cummins, C. C. Organometallics 2001, 20, 3626-3628.
(68) Figueroa, J. S.; Cummins, C. C. Angew. Chem., Int. Ed. 2004, 43, 984-
988.
(69) Freeman, P. K.; Hutchinson, L. L. J. Org. Chem. 1983, 48, 879-881.
(70) Gibson, V. C.; Kee, T. P.; Shaw, A. Polyhedron 1988, 7, 2217-2219.
(71) Baraznenok, I. L.; Nenajdenko, V. G.; Balenkova, E. S. Tetrahedron 2000,
56, 3077-3119.
Figure 1. ORTEP drawing of 2b-H at the 35% probability level. Selected
bond distances (in angstroms): Mo1-H1, 1.687(2); Mo1-N1, 1.998(3);
Mo1-C17, 2.153(6); Mo1-N2, 1.950(5); N1-C17, 1.383(2); N3-C37,
1.478(5).
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 945

A R T I C L E S
Scheme 4
Figueroa et al.
Consequently, the 3/Na reaction system exemplifies the only
such heterodinuclear N2 scission process, despite the growing
number of reported heterodinuclear N2 complexes.76-84
Nitrido anion salt Na[1a-N] is readily separable from the
neutral derivatives 2-N due to their disparate solubility properties
in hydrocarbon solvents. Addition of n-pentane to the crude solid
obtained from reduction of 3a solubilizes 2a-N while precipitat-
ing the desolvated salt, Na[1a-N], as an off-white powder.
Filtration of the mixture affords essentially pure Na[1a-N] in
80-85% yield on a scale of up to 2.0 g. The same procedure
liberates Na[1a-N] from 2b-N in comparable yield. Subsequent
evaporation of the filtrate and recrystallization of the residue
from Et2O, recovers molybdenum complexes 2a-N and 2b-N
as yellow-orange crystals in 80% and 72% yields, respectively.
The molecular structures of Na[1a-N] (as its tris-THF solvate)
and 2b-N were determined by X-ray diffraction (Figures 2 and
3, respectively), by way of confirming their formulation.
Furthermore, when prepared from 15N2, both Na[1a-15N] (δ )
754 ppm) and 2b-15N (δ ) 870 ppm) exhibit large downfield
15N NMR chemical shifts, as is typical of terminal triply bound,
d0 nitrido,85 phosphido,86 and carbido87 functionalities.
are limited in number, examples of conversion of a d0 early
transition metal oxo into a good leaving group have been
reported.72-74 Subsequent reduction of 1a-(OTf)2 with either
Mg(anthracene)(THF)3 or cobaltocene (Cp2Co) affords the
purple-green, niobium(IV) monotriflato complex, Nb(OTf)-
(N[Np]Ar)3 (1a-OTf) in yields of 40% and 60%, respectively
(Scheme 4). Thus, it was gratifying to find a two-step route to
a suitable trisanilide niobium(IV) precursor for heterodinuclear
N2 cleavage from ONb(Cl)3(THF)2, rather than the four-step
procedure from NbCl3(DME)75 (DME ) 1,2-dimethoxyethane)
as required for 1a-I.
Metathetical N for O(Cl) Exchange Mediated by [1a-N]-:
Synthesis of Dinitrogen-Derived Organic Nitriles from Acid
Chloride Substrates. Nitrido anion [1a-N]- was competent for
the conversion of a variety of acid chlorides into corresponding
organic nitriles (Scheme 5, reaction iii). For example, treat-
ment of a THF-d8 solution of Na[1a-N] with pivaloyl chloride
(t-BuC(O)Cl, 0.95 equiv) at room temperature elicited a rapid
color change from pale yellow to orange. Analysis of the reac-
1
tion mixture by H NMR spectroscopy after 15 min indicated
complete consumption of Na[1a-N], concomitant with the
formation of 1a-O and pivalonitrile (t-BuCtN, δ ) 1.32 ppm,
THF-d8) in nearly quantitative yield. Similar results were
obtained for acid chlorides, RC(O)Cl, where R ) Ph, 1-Ad
(Ad ) adamantyl), Me, and HCdCH2. For the case of R )
HCdCH2, analysis of the reaction mixture by 1H NMR indicated
the formation of dichloride Nb(Cl)2(N[Np]Ar)3 in approximately
20% yield relative to 1a-O, an observation possibly indicative
of outer-sphere oxidation of [1a-N]- by the electron-withdraw-
ing acid chloride, H2CdCHC(O)Cl.
Synthesis of Dinitrogen-Bridged Mo/Nb Complexes and
Heterodinuclear Dinitrogen Cleavage. Dinitrogen anions
[2a-N2]- and [2b-N2]- form the heterodinuclear Nb/Mo N2
complexes (Ar[Np]N)3Nb(µ-N2)Mo(N[t-Bu]Ar)3 (3b) and
(Ar[Np]N)3Nb(µ-N2)Mo(N[Np]Ar)3 (3c), respectively, when
treated with 1a-OTf (Scheme 5, reaction i). Paramagnetic 3b
and 3c are obtained as green-brown crystalline solids after
recrystallization from n-pentane. Each possesses solution mag-
netic and spectroscopic properties similar to those reported for
the Nb/Mo heterodinuclear N2 complex 3a. While both 3b and
3c can be isolated in ca. 50% recrystallized yields, we have
found the crude materials obtained after removal of NaOTf to
be of sufficient purity for subsequent reactions. Accordingly,
scission of the dinitrogen units in 3b and 3c is effected by
addition of excess Na/Hg in THF solution, providing an orange
solution containing the Na(THF)x salt of the niobium nitrido
anion [1a-N]- and the nitrido molybdenum complexes 2a-N or
2b-N, respectively (Scheme 5, reaction ii). Cleavage of the N2
unit in derivatives 3 upon reduction is readily rationalized by
formation of the dinuclear anion [(Ar[Np]N)3Nb(µ-N2)Mo(N[R]-
Ar)3]-. The latter is isoelectronic to the homobimetallic N2
complex (Ar[R]N)3Mo(µ-N2)Mo(N[R]Ar)3 and similarly pos-
sesses an unstable (π0)4(π1)4(π2)2 electronic configuration across
the M(µ-N2)M core with respect to nitrido metal formation.4
The volatile organic nitriles (RCN, where R ) t-Bu, Me, and
HCdCH2) were isolated from the reaction mixture by simple
vacuum transfer, providing THF-d8 solutions of the pure
(76) Mercer, M.; Crabtree, R. H.; Richards, R. L. Chem. Commun. 1973, 808-
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(77) Mercer, M. J. Chem. Soc., Dalton Trans. 1974, 1637-1640.
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1975, 351-352.
(79) Schrock, R. R.; Kolodziej, R. M.; Liu, A. H.; Davis, W. M.; Vale, M. G.
J. Am. Chem. Soc. 1990, 112, 4338-4345.
(80) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R.; Reiff, W. M. Inorg.
Chem. 1999, 38, 243-252.
(81) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R. Inorg. Chem. 1998,
37, 5149-5158.
(82) O’Donoghue, M. B.; Zanetti, N. C.; Davis, W. M.; Schrock, R. R. J. Am.
Chem. Soc. 1997, 119, 2753-2754.
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Organometallics 2001, 20, 188-198.
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120, 5836-5837.
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4918.
(85) Sceats, E. L.; Figueroa, J. S.; Cummins, C. C.; Loening, N. M.; Van der
Wel, P.; Griffin, R. G. Polyhedron 2004, 23, 2751-2768.
(86) 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.
(73) Hanna, T. A.; Baranger, A. M.; Bergman, R. G. J. Org. Chem. 1996, 61,
4532-4541.
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M. Inorg. Chim. Acta 1998, 272, 176-187.
(87) Greco, J. B.; Peters, J. C.; Baker, T. A.; Davis, W. M.; Cummins, C. C.;
Wu, G. J. Am. Chem. Soc. 2001, 123, 5003-5013.
(75) Hartung, J. B.; Pedersen, S. F. Organometallics 1990, 9, 1414-1417.
9
946 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Synthesis of a Nitridoniobium(V) Reagent
A R T I C L E S
Scheme 5
compounds in yields of ca. 90% (NMR integration versus
Cp2Fe internal standard) with Na[1a-N] as the limiting reagent.
Heating was not needed to liberate the newly generated
benzonitrile,89,90 to the best of our knowledge chemical shifts
for 15N-pivalonitrile, 15N-1-adamantylcarbonitrile, and 15N-acrylo-
nitrile (Figure 4) have not been reported. However, as Table 1
shows, the new 15N chemical shift data reported here exhibit
the qualitative trend expected on the basis of inductive effects.
Although the 31P nucleus is more sensitive to inductive effects,
increasingly electron-releasing substituents promote a similar
upfield progression of 31P resonances in phosphaalkynes.91 In
addition, treatment of Na[1a-15N] with 13C-labeled acetyl
chloride (H3C13C(O)Cl) afforded doubly labeled (15N/13C)
acetonitrile (H3C13Ct15N), the 15N NMR chemical shift and
1
RCtN from 1a-O, and H NMR gave no indication for the
formation of RCtN f 1a-O Lewis acid/base adducts. While
the tris-N(Np)Ar platform readily permits 5-coordination at
Nb,44,45,68 evidently enough steric protection is provided to the
oxoniobium(V) unit in 1a-O to discourage binding of a nitrile
ligand. After vacuum transfer and removal of NaCl, oxo 1a-O
was recovered essentially pure as assayed by 1H NMR. Addition
of Tf2O to 1a-O effected its conversion to bistriflate 1a-(OTf)2,
which is subsequently reduced to 1a-OTf by Cp2Co to close
the synthetic cycle for Nb-mediated nitrile formation (Scheme
5, reactions iv and v).
When 15N-labeled Na[1a-15N] was treated with the same acid
chlorides, the corresponding 15N-organonitriles were smoothly
generated. Vacuum transfer of the volatile 15N-organonitriles
into sealable NMR tubes allowed for their analysis by 15N NMR
spectroscopy, whereas the 15N NMR spectra of 15N-benzonitrile
and 15N-adamantylcarbonitrile (R ) C6H5 and 1-Ad, respec-
tively) were recorded in the presence of oxo 1a-O. In all cases,
only one resonance was observed over the range 200-900 ppm,
highlighting the efficiency of 15N-atom transfer from
Na[1a-15N] to acid chloride substrates.
3
coupling constants (1JCN and JHN) of which were in excellent
agreement with literature values.
Similar treatment of Na[1a-15N] with 13C-labeled 1-adaman-
toyl chloride (1-Ad13C(O)Cl)92 produced 15N/13C-1-adamantyl-
carbonitrile (1-Ad13Ct15N, Figure 5), thereby providing another
example of a doubly labeled organonitrile from Na[1-15N].
Reaction Sequence Leading to Organonitrile Formation
from Na[1a-N] and RC(O)Cl. The reaction between RC(O)-
Cl and Na[1a-N] in THF is rapid, proceeding to products
RCtN, 1a-O and NaCl within minutes at room temperature or
below. To date, we have not obtained evidence for the presence
of intermediate species during the organonitrile formation
process. This contrasts with our related tungsten-mediated syn-
thesis of organic nitriles. In that case, treatment of the neutral
Listed in Table 1 are the experimentally determined solution
15N chemical shifts (δ) for the 15N-labeled organic nitriles
generated from Na[1a-15N] in this study. Whereas 15N chemical
shift data are available for both 15N-acetonitrile88 and 15N-
(89) Creary, X.; Sky, A. F.; Phillips, G.; Alonso, D. E. J. Am. Chem. Soc. 1993,
115, 7584-7592.
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Spectrosc. 1989, 45, 119-121.
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Soc. 1964, 86, 5564-70.
(91) Regitz, M.; Binger, P. Angew. Chem., Int. Ed. Engl. 1988, 27, 1484-1508.
(92) Molle, G.; Bauer, P.; Dubois, J. E. J. Org. Chem. 1982, 47, 4120-4128.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 947

A R T I C L E S
Figueroa et al.
Table 1. Solution 15N NMR Data for 15N-Labeled Organic Nitrilesa
(
15NtCR)
nitrile
δ
(ppm)
15N coupling constants (Hz)
1-Ad13C15Nb
t-BuC15N
244.1
244.2
247.9c
259.2d
260.0
1JCN ) 15.2
H3C13C15N
PhC15Nb
1JCN ) 17.0, 3JHN ) 1.67
H2CdC(H)C15N
3JHN ) 1.33, 4JHN(trans) ) 0.24e
a All spectra were acquired in THF-d8 at 20.1 °C on an instrument
operating at 60.84 MHz (1H ) 600.2 MHz). b Spectrum acquired in the
presence of 1a-O. c Literature value for 15Nt13C-CH3 δ ) 245.0 ppm
(neat). d Literature value for 15NtC-C6H5 ) 258.4 ppm in CH2Cl2. e The
4JHN(cis) coupling was not resolved.
Figure 2. ORTEP drawing of Na(THF)3[1a-N] at the 35% probability level.
Selected bond distances (in angstroms) and angles (in degrees): Nb1-N2,
1.718(2); Nb1-N1, 2.084(5); N2-Na1, 2.237(10); N1-Nb1-N2, 100.47-
(16).
Figure 4. Solution 15N NMR spectrum (THF-d8, 60.84 MHz) of 15N-
acrylonitrile.
oxo chloride, W(O)(Cl)(N[i-Pr]Ar)3.27 We postulated that bend-
ing of the linear acylimido moiety at nitrogen afforded an
unobserved tungstacyclobutene species susceptible to retro-[2
+ 2] fragmentation in a manner reminiscent of alkyne meta-
thesis by Schrock-type alkylidyne complexes.38 A similar
intramolecular mechanism was proposed for thiocyanate ion
([SCN]-) formation upon reaction of the nitrido vanadium anion
[NtV(N[t-Bu]Ar)3]- and CS2,93 in addition to the related
phosphaalkyne synthesis mediated by anion [1a-P]-.45 The latter
system was amenable to a kinetic study and was shown indeed
to adhere to a unimolecular reaction profile. Furthermore, Ruck
and Bergman26 have recently reported the conversion of amides
to organic nitriles where intramolecular retro-[2 + 2] fragmenta-
tion of a d0 zirconium-acylimido is proposed to be mechanisti-
cally operative.
While reaction between Na[1a-N] and RC(O)Cl presumably
affords a linear acylimido species (4a, Scheme 6), intramolecular
niobacyclobutene formation (4b, Scheme 6) and retro-[2 + 2]
fragmentation are evidently unimpeded by the tris-N[Np]Ar
ligand set. However, to model these events, it was of interest
to find a niobium platform capable of stabilizing an incipient
linear acylimido complex. We reasoned that the tris-N[t-Bu]-
Ar ligand set would provide a kinetically persistent acylimido
Figure 3. ORTEP drawing of 2b-N at the 35% probability level. Se-
lected bond distances (in angstroms) and angles (in degrees): Mo1-N4,
1.640(3); Mo1-N1, 1.987(2); Mo1-N2, 1.987(2); Mo1-N3, 1.970(2);
N4-Mo1-N1, 100.00(11).
nitrido complex, NtW(N[i-Pr]Ar)3, with RC(O)Cl provided the
linear acylimido39-42 species R(O)CNdW(Cl)(N[i-Pr]Ar)3 as
an observable intermediate prior to fragmentation to RCtN and
(93) Brask, J. K.; Dura-Vila, V.; Diaconescu, P. L.; Cummins, C. C. Chem.
Commun. 2002, 902-903.
9
948 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006

Synthesis of a Nitridoniobium(V) Reagent
A R T I C L E S
Figure 5. Solution 15N NMR spectrum (THF-d8, 60.84 MHz) of
15N/13C-adamantylcarbonitrile.
Figure 6. ORTEP diagram of 1c-NC(O)t-Bu at the 35% probability level.
Selected bond distances (in angstroms) and angles (in degrees): Nb1-N4,
1.818(6); Nb1-N1, 2.012(6); N4-C41, 1.389(10); C41-O1, 1.234(10);
Nb1-N4-C41, 167.7(6); N4-C41-O1, 120.6(7); N4-C41-C42,
120.1(7).
Scheme 6
Scheme 7
C6D6) and the niobium(V) oxo complex OtNb(N[t-Bu]Ar)3
(1c-O,94 Scheme 7) were cleanly generated. Thus, while the
linear acylimido functionality on Nb can be kinetically stabilized
by three ancillary tert-butyl substituents, subsequent organoni-
trile formation is not prohibited. Furthermore, no intermediates
were observed spectroscopically during the 1c-NC(O)t-Bu f
t-BuCtN + 1c-O conversion, indicating that formation of prod-
ucts occurs quickly upon coordination of acylimido oxygen atom
to the Nb center. Thus, we suggest the rapid metathetical ex-
change processes observed for the N[Np]Ar-ligated [1a-N]- can
be inferred to be facilitated by a relatiVely unhindered coordina-
tion environment proximal to the Nb center. This line of thinking
is further underscored by the observed formation of [SCN]-
from CS2 and [NtV(N[t-Bu]Ar)3]-,93 despite taking place on
a smaller and seemingly unthinkably congested vanadium center.
Calculated Thermodynamic Parameters for Organonitrile
Nitrile Formation. To elucidate in further detail the reaction
sequence leading to organonitrile formation, we performed DFT
calculations on a representative model system (4m). As shown
due to its greater steric demand in the vicinity of the metal center
relative to that of tris-N[Np]Ar. Accordingly, the nitrido niobium
salt, Na[NtNb(N[t-Bu]Ar)3] (Na[1c-N]),47 prepared in the
context of isocyanate decarbonylation rather than N2 cleavage,
was employed and served nicely for this purpose. Treatment of
Na[1c-N] with 0.95 equiv of pivaloyl chloride afforded the pale-
yellow complex t-Bu(O)CNdNb(N[t-Bu]Ar)3 (1c-NC(O)t-Bu)
in 77% yield. An X-ray diffraction study confirmed its identity
as a near-linear niobium acylimido molecular entity (Figure 6).
In accord with our expectations, complex 1c-NC(O)t-Bu was
stable at room temperature in C6D6 solution. However, upon
heating at 80 °C for 1 h, pivalonitrile (t-BuCtN, δ ) 0.76 ppm,
(94) Fickes, M. G. Synthesis and reactivity of vanadium and niobium complexes
containing sterically demanding amido ligands. Ph.D. Thesis, Department
of Chemistry, Massachusetts Institute of Technology, Cambridge, MA,
1998.
9
J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006 949

A R T I C L E S
Figueroa et al.
Chisholm et al.28 have identified computationally a four-
membered intermediate for NtW(O-t-Bu)3-catalyzed N-atom
exchange between organic nitriles with gross structural features
similar to those of 4m-B.
Concluding Remarks
Just as the olefination of carbonyl compounds by tantalum-
(V) alkylidenes can be deemed analogous to carbonyl olefination
by phosphorus ylides,96 given that [O] for [C(R1)(R2)] exchange
transpires in both cases, the chemistry delineated herein has an
analogue in phosphorus chemistry. What we have shown is that
the terminal nitride anion [NtNb(N[Np]Ar)3]- of niobium(V)
serves to replace the trivalent [O(Cl)] component of an acid
chloride with a nitrogen atom, thus installing the CtN triple
bond functionality of an organic nitrile. The phosphinimide
reagent Li[NdPPh3] is known similarly to be active for the acid
chloride f organic nitrile transformation,97 but in contrast with
what we have observed for niobium, the transformation requires
substantial heating for breakup of the intermediate formed upon
salt elimination.97
Figure 7. Calculated relative enthalpic values for the conversion of model
4m-A to products 4m-O and MeCtN. The niobacyclobutene species 4m-B
(inset) was found to be a stationary point on the potential energy surface.
Through modification of the ancillary ligands that protect the
niobium center, we have shown that acylimido ligands are
generated upon salt elimination when an acid chloride is used
for niobium nitride anion acylation. Acylimido complexes27,39-42
are strongly implicated, therefore, as intermediates in the acid
chloride to nitrile conversion, the ultimate microscopic step in
which is likely the breakup of a four-membered metallacycle.
The reverse of triple bond metathesis of a terminal niobium
oxo (NbtO) with the nitrile CtN triple bond accordingly
constitutes the mechanism for product (organic nitrile) release
from the metal center. Organic nitrile products appear not to
interact with the final niobium oxo product, OtNb(N[Np]Ar)3,
which is formed in essentially quantitative fashion and which
we have shown to be amenable to high-yield recycling.
The advance reported herein establishes a firm connection
between (i) dinitrogen cleavage chemistry and (ii) nitrogen atom
transfer reactivity. The clearest manifestation of this is our cyclic
synthesis of several 15N-labeled organic nitriles from corre-
sponding acid chlorides with 15N2 as the source of isotopic label.
A conspicuous remaining challenge, required to fulfill the
directive of atom economy, is the invention of reactions for
N-atom transfer from the neutral nitridomolybdenum(VI) com-
plexes described herein.
in Figure 7, the direct conversion of the linear acylimido
complex, H3C(O)CNdNb(NH2)3 (4m-A), to products OtNb-
(NH2)3 (4m-O) and H3CCtN is calculated to be favorable by
7.7 kcal/mol. Also shown is the relative enthalpic change
associated with isomerization of 4m-A to the corresponding
four-membered niobacycle, 4m-B. While the calculated struc-
tures of both 4m-A and 4m-O showed excellent agreement with
their experimental counterparts 1c-NC(O)t-Bu and 1a-O, that
without a counterpart, 4m-B, was similarly found to represent
a minimum on the potential energy surface. Interestingly, the
4m-A f 4m-B isomerization process, characterized by bending
of 4m-A at the acylimido nitrogen, is disfavored by only ca.
2.9 kcal/mol. Although not observed experimentally, these
calculations suggest that formation of an intermediate akin to
4m-B is not likely to be inhibited by an insurmountable energetic
barrier. Furthermore, retro-[2 + 2] fragmentation of 4m-B to
4m-O and MeCtN is enthalpically favored by 10.6 kcal/mol.
That such a niobacyclic species would be susceptible to facile
retro-[2 + 2] fragmentation is readily rationalized from inspec-
tion of the geometric structure of 4m-B. The optimized geometry
of 4m-B is consistent with its formulation as a niobacyclobutene
(Nb-NdC(R)O; Figure 7, inset) rather than a bent acylimido
(NbdN-C(dO)R) complex. Thus significant electronic reor-
ganization in the acylimido functionality is initiated by coor-
dination of oxygen to the Nb center. Calculated metrical
parameters in support of this claim include a shortening of the
N-C bond length in 4m-B relative to 4m-A (1.320 vs 1.386
Å) and a considerable lengthening of the corresponding C-O
bond (1.321 vs 1.227 Å). Additionally, the Nb-N bond length
of 1.983 Å in 4m-B clearly exceeds the range expected for an
imidoniobium(V) complex, falling closer to that expected for a
niobium amido44,45,68 or ketimido ligand.44,95 Coupled with the
acute niobacyclic Nb-N-C angle (93.53°), the foregoing
geometric parameters of 4m-B provide a reasonable picture for
the exchange of C-O and M-N multiple bonds for that of C-N
and M-O within the coordination sphere of niobium. Notably,
Acknowledgment. We gratefully thank the U.S.A. National
Science Foundation for financial support (CHE-0316823) and
for a predoctoral fellowship to N.A.P. J.S.F thanks Professor
Daniel G. Nocera for a generous gift of computer time and Dr.
Tony Bielecki for assistance with 15N NMR measurements.
Professor Paul J. Chirik is acknowledged for his initial contribu-
tions to the molybdaziridine hydride system 2b-H.
Supporting Information Available: 15N NMR spectra, results
of computational studies, and crystallographic structure deter-
minations (PDF and CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA056408J
(96) Schrock, R. R. J. Am. Chem. Soc. 1976, 98, 5399-5400.
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950 J. AM. CHEM. SOC. VOL. 128, NO. 3, 2006