Direct formation of an organonitrogen compound from a molybdenum nitrido species

Article abstract of DOI:10.1039/b305774g

The molybdenum nitrido complex ¹⁵NMo[N(R)Ar]₃ (where R = C(CD₃)₂CH₃, Ar = 3,S-C₆H ₃Me₂) reacted with the anhydride of trifluoroacetic acid at room temperature to afford the correspondent organonitrogen compound in almost quantitative yield without the necessity of using additional reagents to achieve the C-N coupling.

Full text of DOI:10.1039/b305774g

Direct formation of an organonitrogen compound from a molybdenum
nitrido species
Huub Henderickx, Gerard Kwakkenbos, Alexander Peters,* Jan van der Spoel and Koen de Vries
DSM Research, P.O. Box 18, 6160MD Geleen, The Netherlands. E-mail: alexander.peters@dsm.com
Received (in Cambridge, UK) 23rd May 2003, Accepted 25th June 2003
First published as an Advance Article on the web 9th July 2003
The molybdenum nitrido complex 15NMo[N(R)Ar]3 (where
R = C(CD3)2CH3, Ar = 3,5-C6H3Me2) reacted with the
anhydride of trifluoroacetic acid at room temperature to
afford the correspondent organonitrogen compound in
almost quantitative yield without the necessity of using
additional reagents to achieve the C–N coupling.
was observed at the position mentioned, in accordance with a
15NH2-group (1J (H,N) = 85 Hz). Subsequently the reaction
was monitored by GC-MS, applying a Varian CP-Sil 43 CB (25
m 3 0.25 mm, df 0.2 mm) capillary column to separate 3 from
the DMF solvent, allowing the conclusion that the 15N nitrogen
in the starting material has been transferred quantitatively into
product 3.
Industrially, the synthesis of organonitrogen compounds de-
pends essentially on ammonia, produced by activation of
nitrogen by the Haber–Bosch process.1 This “key intermediate”
ammonia is the precursor for a wide range of commercially very
useful products, including amines, amides, ammonium and
alkylammonium salts, ureas, carbamates, isocyanates, nitriles
and amino acids.2 Whilst the synthesis of simple molecules as
ureas or carbamates is relatively straightforward, the synthesis
of other species, e.g. caprolactam, involves a cascade of
reaction steps.3 In the case of the classic caprolactam route,
molecular nitrogen (0) undergoes several oxidation states via
ammonia (23), nitric acid (+5), hydroxylamine (21), ending
with the lactam (23) formed after the Beckmann rearrangement
of cyclohexanone oxime (21).4 Thus, a synthetic route to
caprolactam in an industrial process involves five reaction steps
and three redox changes to reach the target.
Another, totally different, approach would be to avoid
ammonia as an intermediate. This can be envisioned when it is
possible to add a carbon source directly to an activated N–N
triple bond, or, even better, to a well-defined nitrido transition
complex to obtain the desired C–N bond.
With this aim, we here report the formation of a C–N bond by
reaction of a nitrido ligand, attached to a transition metal
complex, with a highly reactive carbonyl compound under very
mild conditions. Contrary to recent developments on nitrogen
functionalization chemistry,5 no additional reagents were
necessary to achieve a C–N coupling,6 and the resultant
organonitrogen species is ready to use without the necessity of
splitting an additional M–N bond located at the C–N product7 or
a single N–N bond in a hydrazine derivative obtained from
activated dinitrogen.8
As the reaction was conducted under dry conditions, one final
question remains: how can a primary amine group be formed
without offering a direct proton source to the reactants? To
answer this question the reaction has been repeated by changing
the reaction medium from DMF to pure 2. After storage of the
reaction solution over a period of two days at 5 °C this
manipulation affords orange crystals of the molybdenum
2
species [N(R)Ar](NAr)Mo(h -CF3CO2)(O2CF3)2 (4; where R
= C(CD3)2CH3, Ar = 3,5-C6H3Me2) as confirmed by X-ray
diffraction.† The most relevant feature of the crystal structure,
shown in Fig. 1, is the substitution of the labelled nitrido and
one alkyl-aryl carrying amide ligand by three trifluoroacetate
groups to afford a distorted octahedral coordination sphere
around the central metal.
Whilst one of the two remaining amide ligands is still intact,
the second one is degraded to an imide species carrying only the
3,5-dimethylphenyl group at the nitrogen atom (the bond length
of 1.71 Å between Mo and N2 in the MoVI-imido species 4 is
consistent with earlier reported values).10 The latter ligand lost
its tert-butyl group under the formation of one mole of
2-methylpropene (6) and one mole of trifluoroacetic acid (7).
The observation of 6 and 8 by GC analysis supports the here
outlined reaction pathway via the intermediate 5 (see Scheme
2).
In summary, the starting material 1 itself acts as a proton
source, via the formation of 5, and finally by dissociation of the
acid into a proton and trifluoroacetate, which acts as mono-
dentate or bidentate ligand. At the present it is not clear whether
Scheme 1 shows the reaction of 15NMo[N(R)Ar]3 (1; where
R = C(CD3)2CH3, Ar = 3,5-C6H3Me2)9 with trifluoroacetic
acid anhydride (2), which proceeds at room temperature in
dimethylformamide within several minutes to form
CF3C(O)15NH2 (3) in almost quantitative yield (based on the
consumed 15N material).
The 15N-NMR spectrum of the reaction mixture shows the
disappearance of the satellite nitrido signal of 1 at d = 840 ppm
and the exclusive appearance of a new signal at d = 100.7 ppm,
which is in agreement with the formation of 3. When the
measurement was repeated without proton decoupling a triplet
Fig. 1 Structure of 4. One set of the disordered fluorine atoms (F4–F6) has
been omitted for clarity. Selected bond lengths [Å] and angles [°]: Mo1–N1
1.897(1), Mo1–N2 1.712(1), N1–C11 1.460(2), N2–C19 1.394(2); Mo1–
N2–C19 163.9(19).
Scheme 1
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Scheme 2
3 J. Ritz, H. Fuchs, H. Kieczka and W. C. Moran, “Caprolactam,”
the hydrogen attack at the methyl group or the C–N bond
coupling occurs first. Whilst a concerted mechanism can also be
envisioned, a hydrogen attack seems to be likely, due to steric
hindrance around the nitrido ligand on one hand and thermody-
namic considerations on the other (favouring the C–H bond
activation in comparison to a Mo–N triple bond).11
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Wiley-VCH, Weinheim, 2001, electronic release; K.-L. Ring, “Capro-
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4 K. Weissermel and H.-J. Arpe, Industrielle Organische Chemie, 5.
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The new insights into the functionalization of activated
nitrogen indicate an approach to obtain isolated organonitrogen
compounds without making use of additional reagents. Further
experiments in our laboratories are currently under way to study
the detailed mechanism of this new C–N bond formation
reaction type by making use of other substrates for example,
carbon acids, aldehydes, acid chlorides or acid esters.
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Notes and references
† Crystal structure determination of 4: C26H21D6F9MoN2O6, M = 738.11,
monoclinic P21/c, a = 10.093(1), b = 19.122(1), c = 15.853(1) Å, b =
100.230(1)°, V = 3011.0(3) Å3, Z = 4, m = 0.53 mm21. 10079 independent
reflections (8103 observed) were measured (Bruker AXS area detector, Mo
Ka-radiation, w-scan, T = 190 K). Structure refinement based on F2
(SHELXTL V5.1,12 non-hydrogen atoms anisotropic, hydrogen atoms
located and refined isotropically, one CF3-group is disordered, R1 = 0.029
(observed reflections), wR2 = 0.080 (all reflections). CCDC 207154. See
http://www.rsc.org/suppdata/cc/b3/b305774g/ for crystallographic data in
.cif format.
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12 SHELXTL V5.1, Bruker AXS, Madison, WI, 1998.
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