Reaction of (bisimido)niobium(V) complexes with organic azides: [3 + 2] cycloaddition and reversible cleavage of β-diketiminato ligands involving nitrene transfer

Article abstract of DOI:10.1021/ja413194z

We describe the unusual reactivity of a highly labile diethyl ether adduct of an asymmetric niobium(V) bis(imide) 2.OEt₂ containing the monoazabutadiene (MAD) ligand. This species undergoes clean nitrene transfer on treatment with tert-butyl- or di-isopropylphenyl azide resulting in the unprecedented reformation of nacnac ligands bound to the metal center. Corresponding reactions with trimethylsilyl- or tert-butyl azide allowed the isolation of two rare intermediates prior to N₂ loss; mechanistic studies support the involvement of two different niobium species.

Full text of DOI:10.1021/ja413194z

Communication
pubs.acs.org/JACS
Reaction of (Bisimido)niobium(V) Complexes with Organic Azides: [3
+ 2] Cycloaddition and Reversible Cleavage of β‑Diketiminato
Ligands Involving Nitrene Transfer
Andreas H. Obenhuber, Thomas L. Gianetti, Xavier Berrebi, Robert G. Bergman,* and John Arnold*
Department of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
intractable mixture of products was formed upon either slight
ABSTRACT: We describe the unusual reactivity of a
highly labile diethyl ether adduct of an asymmetric
niobium(V) bis(imide) 2.OEt2 containing the mono-
azabutadiene (MAD) ligand. This species undergoes clean
nitrene transfer on treatment with tert-butyl- or di-
isopropylphenyl azide resulting in the unprecedented
reformation of nacnac ligands bound to the metal center.
Corresponding reactions with trimethylsilyl- or tert-butyl
azide allowed the isolation of two rare intermediates prior
to N2 loss; mechanistic studies support the involvement of
two different niobium species.
heating (40 °C) or photolysis. The resulting complex
(MAD)Nb(NtBu)(NAr)(N3TMS) (3) (Scheme 1) was iso-
lated as red crystals from HMDSO (−40 °C) in 68% yield. The
IR spectrum revealed one strong stretch at 2201 cm−1 in the
solid state and three in solution (ν = 2199, 2138, and 2093
cm−1) consistent with absorbances due to NN function-
alities. Solution NMR spectroscopy suggested a low-symmetry
complex in which the monoazabutadiene and the two imido
ligands were conserved.
Crystals suitable for X-ray diffraction (XRD) were obtained
from hexanes cooled to −40 °C, clarifying coordination of the
linear azide to the metal center via the γ-nitrogen (Figure 1,
left). Complex 3 possesses a distorted trigonal-bipyramidal
geometry, similar to that of the previously described bis-imido
MAD niobium complexes (see SI). The TMSN3 ligand is
located in the apical position with a Nb−N azide bond length
(Nb−N4: 2.271(3) Å) comparable to the Nb−Npy bond length
of the MAD pyridine adduct (Nb−Npyridine: 2.271(3) Å).2b The
two short N−N bonds (N4−N5: 1.131(4) Å and N5−N6:
1.208(4) Å) further indicate the presence of an electronically
unperturbed azide ligand. This contrasts with known terminal
azide complexes of group 5 metals, which have been found to
possess formally reduced diazenylimido entities.6b,c
he β-diketiminato (or ‘nacnac’) moiety has found
T
widespread application as a supporting ligand for catalytic
transformations,1 complexes containing metal−ligand multiple
bonds,2 reactive organometallic species with low coordination
numbers and oxidation states across the periodic table.3
However, limitations in its conventional role as a spectator
ligand in early transition (and recently also in main group)
metal compounds have been reported resulting from overall
irreversible nitrene group transfer from a nacnac ligand to the
metal center.4 Reversing such intramolecular ligand degradation
would allow access to masked low valent metal centers and
make them available for further reactivity. Additionally,
intermolecular synthetic routes to metal-imido complexes
routinely comprise the use of organic azides (RN3) which
upon loss of N2 act as valuable nitrene-transfer agents.5 Despite
this long-standing synthetic application, few unfragmented
organoazidometal intermediates have been isolated and fully
characterized,6 and mechanistic information on how this
nitrene transfer occurs remains scarce.
t
Interestingly, treating 2.OEt2 with BuN3 at room temper-
ature for 15 min resulted in a different color change (from
orange to dark brown), and dark red crystals of 4 were obtained
in 65% yield by directly cooling the reaction solution to −40 °C
overnight (Scheme 1). 1D and 2D NMR spectra measured at
−22 °C revealed the presence of one compound with two
1
different NtBu moieties (1.39 and 1.26 ppm in the H NMR
spectrum), in which the MAD ligand remained unchanged. The
restricted rotation about both N−Ar bonds (4 different
methine and 8 methyl groups in 1H NMR spectrum) provided
an indirect hint that the arylimido moiety was incorporated into
a cyclic structure. XRD revealed the formation of a rare
example of an early transition metal tetraazametallacyclo-
pentene complex, formed by [3 + 2] cycloaddition to one
imido group (see Figure 1, right).6e,8 Complex 4 possesses two
Nb−N bond distances consistent with the description of the
corresponding ligands as amido groups (Nb−N2: 2.106(4) and
Nb−N4: 2.069(4) Å), along with two N−N single (N4−N5:
As part of our continuing interest in the chemistry of group 5
mono-1c,2a,b,3d,e and bis-(imido)7 complexes, we recently
reported the synthesis of (MAD)Nb(NtBu)(NAr)(L) (L =
Py, THF),2b in which both the pyridine and THF ligands were
tightly bound to the metal center, thus hampering their
displacement under mild conditions. We speculated that a more
labile Et2O adduct might pave the way to a π-loaded niobium
center displaying imido reactivity.
Complex 2.OEt2 was therefore synthesized as shown in
Scheme 1 and was found to be structurally analogous to its
congeners (see Supporting Information (SI)). Adding TMSN3
to a benzene solution of 2.OEt2 at room temperature resulted
in an immediate color change from orange to deep red. No
further reaction was observed after several hours, and an
Received: January 1, 2014
Published: February 13, 2014
© 2014 American Chemical Society
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Scheme 1
two new compounds, 5 and 6, in a ratio of 85:15 (based on 1H
NMR spectroscopy), was observed (Scheme 1). The same
product mixture was obtained by directly treating 2.L (L =
THF or Et2O); however, their respective ratios were dependent
on both the azide concentration and temperature. Complex 5
was formed as the major product when an excess of tBuN3 and
moderate heating (T = 60 °C) were employed. Crystallization
from HMDSO at −40 °C allowed isolation of the major
species, 5, in 42% yield. NMR spectroscopy and an XRD study
clearly confirmed the formation of (BDI)Nb(NtBu)2 (5)
(Figure 2, left). Complex 5 is an example of a rare four-
Figure 1. ORTEP diagrams of complexes 3 (left) and 4 (right). H
atoms have been removed for clarity. Selected bond distances are
presented in Table S.3.
1.393(6) Å and N6−N2: 1.388(6) Å) and one NN double
bond (N5−N6: 1.273(6) Å).
An equilibrium reversing the [3 + 2] cycloaddition in 4,
liberating the aryl-imido moiety, was observed via a variable
1
temperature H NMR study in toluene (Scheme 1). Starting
with 4 at −30 °C and incrementally raising the temperature to
+35 °C, we observed a continuous, yet reversible, increase in
t
the concentration of a second compound 3-N3 Bu. Comparing
1
t
the H NMR profile of 3-N3 Bu to that of (MAD)Nb(NtBu)-
Figure 2. ORTEP diagram of complexes 5 (left) and 6 (right). H
atoms have been removed for clarity. Selected bond distances are
presented in Table S.4.
(NAr)(N3TMS) 3, along with similar solution IR fingerprints
t
(see SI), prompted us to assign 3-N3 Bu as a γ-coordinated
tBuN3 species. Further evidence for the proposed equilibrium
t
1
between 4 and 3-N3 Bu was provided through a H,1H EXSY
experiment and DFT calculation (see SI). A van’t Hoff plot
(Keq = [4]/[3-N3 Bu]) allowed the determination of
coordinate bis-imido niobium complex, exhibiting a pseudote-
trahedral geometry in which both imides are structurally
equivalent (Nb−Nav: 1.81 Å; see SI).9 Interestingly, a change in
selectivity resulting in the exclusive formation of 6 was achieved
t
thermodynamic parameters ΔH° = −10.2
0.4 kcal/mol
and ΔS° = −32.5 1.5 cal/mol·K; see SI). The large negative
reaction entropy might be explained by the loss of rotational
degrees of freedom around the N−Ar bond in the tetrazene
complex 4. Interestingly, variable temperature experiments with
3 provided no evidence for a related equilibrium process. DFT
calculations reveal that a putative tetrazine complex with
TMSN3 is significantly less thermodynamically stable than the
γ-adduct and also requires a more energetically demanding
transition state (see SI).
t
when a more dilute solution of BuN3 (0.1 M) was slowly
added to the THF adduct 2 at 70 °C. The identity of 6 was
unequivocally assigned based on NMR analysis and X-ray
crystallography (72% yield). Thus both metal-bonded imido
groups are able to participate in the reformation of differently
substituted nacnac species. The anticipated highly reactive
nature of a four-coordinate bis-imido complex is reflected in the
cyclometalation of the tert-butyl imino group of the newly
formed nacnac ligand (Figure 2, right), in which C−H bond
activation is found to occur exclusively across the presumably
more basic NbNtBu bond.
Surprisingly, unlike the intractable mixture obtained with 3,
t
when the mixture (4)/(3-N3 Bu) was warmed to a temperature
higher than +35 °C, the fast, clean, and irreversible formation of
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Communication
Treating 2.THF with a slight excess of ArN3 (1.6 equiv, Ar =
2,6-diisopropylbenzene) at 55 °C in benzene led to similar but
slower (24 h) reactivity, compared with that of the alkyl azide.
Two new niobium compounds 7 and 8 were formed selectively
in a 1/1 ratio (Scheme 2). Complex 7 was crystallized from
activation. We further suggest, as the origin of the observed
selectivity, the involvement of either (a) five-coordinate
niobium(V) bis(imides), supported via first order kinetics
(vide infra), favoring 5 (Scheme 3, Path A), or (b) a 3-fold
coordinated d2 niobium(III) mono(imide), supported via
1H,1H EXSY NMR and DFT calculations, favoring 6 (Scheme
3, Path B). Finally, each path is highly dependent on azide
concentration: excess azide promotes the formation of (4)/(3-
Scheme 2
t
N3 Bu) and favors pathway A, while a substoichiometric
amount of azide promotes pathway B.
Further studies providing more details of this unusual azide
activation mechanism/nacnac reformation will be addressed in
a full account.
ASSOCIATED CONTENT
* Supporting Information
■
S
pentane at −40 °C (32% yield), and 8 was recrystallized from a
THF/hexanes mixture at −40 °C (30% yield). Both were
analyzed by NMR spectroscopy and X-ray diffraction (see
SI).10 The crystal structures of 7 and 8 provide two further
examples of four-coordinate, tetrahedral bis-imido niobium
complexes in which the resulting Nb−N imido bond distances
are comparable to those previously reported for BDI niobium
imido complexes (see SI; Nb−Nav: 1.82 Å).2a,b We expect this
reaction, demonstrated for ArN3 and tBuN3, to be generalizable
to other organic 1,3-dipolar compounds.
Experimental procedures, analytical data, NMR spectra, crystals
data, Cif files for complexes 2.OEt2, 3, 4, 5, 6, 7, and 8; and
DFT calculations method and results. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
arnold@berkeley.edu
■
Notes
Intrigued by the different reaction conditions changing the
selectivity of 5/6, we undertook a preliminary mechanistic
study of this unexpected nitrene-transfer reaction. We were able
to gain further insight by monitoring the disappearance of the
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the AFOSR (Grant Number FA9550-11-1-0008) for
financial support. A.H.O. thanks the DFG (Grant Number OB
382/1-1) for a fellowship. Drs. A. DiPasquale and Kathleen
Durkin are acknowledged for experimental assistance. Prof. R.
A. Andersen, Dr. G. Nocton, Dr. H. S. La Pierre, and B. M.
Kriegel are thanked for helpful discussions.
t
azide complexes 4 and 3-N3 Bu by means of 1H NMR
spectroscopy at 55 °C. Under these conditions, in which the
t
regioisomer 5 is favored, the disappearance of (4)/(3-N3 Bu)
follows exponential decay, and no dependence of the calculated
rate constant on the initial concentration was observed,
suggesting a first order formation of 5 and 6, consistent with
an intramolecular transformation. However, starting from
2.THF, with a substoichiometric amount of azide and at a
higher temperature (70 °C), we observed the selective
formation of 6, suggesting a different reaction pathway. A
1H,1H EXSY experiment performed on a C7D8 solution of
2.THF at 100 °C (see SI) revealed the chemical exchange of
the MAD backbone methyl groups together with broadening
and coalescence of the associated THF signals. Such a chemical
exchange suggests that a slow and reversible reformation of
nacnac ligands occurs even in the absence of azide. DFT
calculations predict an activation barrier that is rather high, yet
accessible at elevated temperature (see SI).
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