Unexpected mild C-N bond cleavage mediated by guanidine coordination to a niobium iminocarbamoyl complex

Article abstract of DOI:10.1039/c3cc44952a

The complex [Nb(NMe₂)₂{(NMe₂)CN ^tBu}{N(2,6-ⁱPr₂C₆H₃)}] reacts with trialkylguanidines and undergoes a room temperature C-N bond cleavage of the iminocarbamoyl moiety. This reaction affords the guanidinate complexes [Nb(NMe₂)₂{N(2,6-ⁱPr ₂C₆H₃)}{(NⁱPr)₂C(NH ⁱPr)}] or [Nb(NMe₂)₂{N(2,6-ⁱPr ₂C₆H₃)}{(NⁱPr)₂C(NH ⁿBu)}] and free isocyanide. The first crystal structure of a niobium iminocarbamoyl complex is reported.

Full text of DOI:10.1039/c3cc44952a

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Unexpected mild C–N bond cleavage mediated by
guanidine coordination to a niobium iminocarbamoyl
complex†
Cite this: Chem. Commun., 2013,
49, 8701
Received 2nd July 2013,
Accepted 26th July 2013
David Elorriaga, Fernando Carrillo-Hermosilla,* Antonio Antin˜olo,*
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Isabel Lopez-Solera, Rafael Fernandez-Galan and Elena Villasen˜or
DOI: 10.1039/c3cc44952a
www.rsc.org/chemcomm
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The complex [Nb(NMe2)2{(NMe2)C N Bu}{N(2,6- Pr2C6H3)}] reacts
with trialkylguanidines and undergoes a room temperature C–N bond
cleavage of the iminocarbamoyl moiety. This reaction affords the
guanidinate complexes [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHiPr)}]
or [Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHnBu)}] and free isocyanide.
The first crystal structure of a niobium iminocarbamoyl complex is
reported.
Guanidines are an emerging class of nitrogen donor poly-
dentate ligands and they are used to stabilize different metal
centres in high oxidation states.1 The presence of this kind of
ligand has an interesting influence on the reactivity of the metal
centre. It has been stated that the guanidinate ligands present in the
coordination sphere of alkyl titanium, zirconium or niobium com-
plexes influence the final product of the migratory insertion reaction
of isocyanides. In fact, iminoacyl, enediamido, vinylamido or imido
ligands can be obtained depending on the nature of the guanidinate
ligand.2 As part of our continuing study into the chemistry of
imido and guanidinate niobium complexes,3 we discovered that
Scheme 1 Guanidinate and iminocarbamoyl derivatives from 1.
the reaction of the complex [Nb(NMe2)3(N-2,6-iPr2C6H3)] (1)4 with insertion of isocyanides, have been reported.6 In this context,
1,2,3-trisisopropylguanidine yielded the guanidinate complex attempts to obtain the migratory insertion products of 2
[Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHiPr)}] (2) (Scheme 1).
using tert-butylisocyanide resulted in the recovery of starting
The NMR spectra of this new compound indicate a square materials, even upon heating at 70 1C for 16 hours.‡ However,
pyramidal coordination around the metal centre. Insertion the reaction of 1 with tert-butylisocyanide results in the migratory
reactions of unsaturated substrates, such as CO or isocyanides, insertion to give the corresponding iminocarbamoyl compound
into metal–carbon bonds have been reported to give a wide [Nb(NMe2)2{(NMe2)CQNtBu}{N(2,6-iPr2C6H3)}] (3) in good yield.
variety of acyl and iminoacyl derivatives.5 These studies have The molecular structure was determined using X-ray diffrac-
led to the expansion of the synthesis of transition metal–amido tion (Fig. 1). Selected bonds and angles are listed in the legend
complexes with the aim of obtaining related products due to of Fig. 1.
the insertion into metal–nitrogen bonds. Due to the lack of
Very few examples of group 5 iminocarbamoyl derivatives
reactivity of many metal–amido compounds toward insertion have been structurally characterized6f,i,7 and, to the best of our
few examples of metal-iminocarbamoyl derivatives, formed by knowledge, this is the first example of an iminocarbamoyl
niobium complex to be described. The complex is chiral and
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Departamento de Quımica Inorganica, Organica y Bioquımica, Facultad de Ciencias
both enantiomers are present in the crystals. The imido ligand
is quasi-linear with an Nb1–N1–C1 angle of 177.3(2)1 and an
Nb1–N1 bond distance of 1.783(2) Å.3 The N2–C13 and the
N3–C13 distances of 1.292(3) Å and 1.341(3) Å, respectively,
reveal p delocalization in the iminocarbamoyl moiety sup-
ported by a partial donation of the lone pair on NMe2 to the
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y Tecnologıas Quımicas, Universidad de Castilla-La Mancha, Campus Universitario
de Ciudad Real, 13071-Ciudad Real, Spain. E-mail: Fernando.Carrillo@uclm.es,
Antonio.Antinolo@uclm.es; Fax: +34 926 295 318
† Electronic supplementary information (ESI) available: Full experimental pro-
cedures, selected spectra and crystal data. CCDC 948135. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/c3cc44952a
c
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Fig. 2 Evolution of the reaction of 3 with trisisopropylguanidine monitored by
1H NMR. Selected peaks are shown.
Fig. 1 Thermal ellipsoids plot of 3 at 30% probability. Selected bonds (Å) and
angles (1): Nb1–N1 1.783(2); Nb1–N2 2.128(2); Nb1–N4 2.005(2); N2–C13 1.292(3);
N3–C13 1.341(3); N1–Nb1–N4 104.4(1); N4–Nb1–N5 104.7(1); Nb1–N1–C1
177.3(2).
to methyl groups of an amido ligand bonded to metal and an
amino group of an iminocarbamoyl moiety, respectively, in an
intermediate complex. These peaks increase in intensity from the
beginning of the reaction and start to disappear after approxi-
mately 5 hours with the simultaneous appearance of peaks
corresponding to complex 2 and free isocyanide (Scheme 2).
The long life time of this intermediate allows partial charac-
terization by NMR spectroscopy. The 13C{1H} NMR spectrum
shows the characteristic peak of a chelating guanidinate ligand
at 164.9 ppm.13 NOESY-1D experiments reveal a pseudocta-
hedral disposition around the niobium atom in this intermediate.
The effect of the coordination of a strong donor guanidinate
ligand was proven by the reaction of complex 3 with 2-butyl-1,3-
diisopropylguanidine. In this case, the guanidinate complex
[Nb(NMe2)2{N(2,6-iPr2C6H3)}{(NiPr)2C(NHiPr)}] (4), as a mixture
of symmetrically and asymmetrically coordinated guanidinate
isomers,3c and free isocyanide were obtained. Complex 4 can
also be obtained by the direct reaction of 1 with 2-butyl-1,3-
diisopropylguanidine (Scheme 1).
resonance contribution.6f If one considers that the imino-
carbamoyl moiety occupies one coordination position, the niobium
atom adopts a pseudotetrahedral coordination, making the two
amido ligands inequivalent. In contrast, in solution, even at
very low temperatures, both groups appear to be equivalent in
the NMR spectra. This finding can be explained by a rapid
formal rotation of the iminocarbamoyl ligand.8
Surprisingly, the reaction of 3 with 1,2,3-trisisopropylguanidine,
at room temperature for 24 h, gave a mixture of the guanidinate
complex 2 and free isocyanide. This represents an example of
facile C–N single bond activation. Reactions that involve the
metal-mediated rupture of C–N bonds are limited to activated
substrates or the use of highly reactive metal complexes.9
Recently, a room temperature C–N bond cleavage of an anionic
guanidinate ligand promoted by rare-earth complexes has been
reported.9j In the reaction of 3 and guanidine, as a result of the
C–N bond cleavage, deinsertion of isocyanide takes place.
Migratory insertion reactions have been extensively studied as
fundamental mechanisms and as key steps in many important
catalytic reactions.10 According to the principle of microscopic
reversibility, deinsertion of CO from acyl ligands is a widely
reported process.5a,11 The migratory insertion reaction of iso-
cyanides is usually an irreversible process. Isocyanide deinser-
tion reactions of iminoacyl or iminocarbamoyl ligands are less
well known and, in most cases, involve thermal treatments
to give iminoacyl–iminocarbamoyl exchanges or mixtures of
equilibrium products.6g,12 In an effort to gain an insight into
In summary, we report here the first example of a charac-
terized niobium iminocarbamoyl complex that undergoes a facile
C–N bond cleavage, presumably driven by the coordination of a
1
the reaction, the process was monitored by H NMR in deuter-
ated benzene (Fig. 2). It can be observed from Fig. 2 that
dimethylamine (doublet close to 2.20 ppm) is present from
the outset of the reaction and is produced by a protonolysis
reaction by the acidic protons of guanidine.
We also observed the disappearance of the peaks corre-
sponding to the starting complex 3 and the appearance, amongst
Scheme 2 Proposed mechanism for the deinsertion of isocyanide, mediated by
others, of two peaks at 3.72 and 2.91 ppm, which were assigned guanidine coordination.
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8702 Chem. Commun., 2013, 49, 8701--8703
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We gratefully acknowledge financial support from the
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‡ All manipulations were carried out under dry nitrogen using standard
Schlenk and glovebox techniques. Solvents were purified by passing
through a column of activated alumina (Innovative Technologies)
and degassed under nitrogen before use. Microanalyses were carried
out using a Perkin-Elmer 2400 CHN analyzer. NMR spectra were
recorded on a Varian FT-400 spectrometer using standard VARIAN-FT
software for NOESY-1D, COSY, g-HSQC, and g-HMBC. The compounds
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