Imido-bridged homo- and heterobimetallic complexes

Article abstract of DOI:10.1021/ic201466f

Transamination reactions of primary amines with group 4 and 5 amido precursors M(NMe₂)₄ have been studied to prepare homo- and heterobimetallic complexes [(Me₂N)₂M¹(μ- NR¹)(μ-NR²)M²(NMe₂) ₂(NHMe₂)_x] (x = 0, 1) with two identical or distinct bridging imido ligands.

Full text of DOI:10.1021/ic201466f

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Imido-Bridged Homo- and Heterobimetallic Complexes
Christian Lorber*,†,‡ and Laure Vendier†,‡
†Laboratoire de Chimie de Coordination (LCC), Centre National de la Recherche Scientifique (CNRS), 205 route de Narbonne,
F-31077 Toulouse, France
‡Universitꢀe de Toulouse, UPS, INPT, LCC, F-31077 Toulouse, France
S Supporting Information
b
Scheme 1. Formation of Imido-Bridged Group 4 Complexes
ABSTRACT: Transamination reactions of primary amines
with group 4 and 5 amido precursors M(NMe2)4 have been
studied to prepare homo- and heterobimetallic complexes
[(Me2N)2M1(μ-NR1)(μ-NR2)M2(NMe2)2(NHMe2)x]
(x = 0, 1) with two identical or distinct bridging imido
ligands.
Scheme 2. Synthesis of Homo- and Heterobimetallic Imido
Dimers
ransition-metal imido complexes have been actively studied
T
in the past 2À3 decades, in great part because of their ability
to promote a variety of transformations in which the imido ligand
is directly involved in the reactivity or may act as a spectator
ligand.1,2 The imido ligand is particularly suitable for stabilization
of transition metals in their highest oxidation states because of
their ability to participate in extensive ligand-to-metal π donation.3
Furthermore, the imido group presents the advantage of pos-
sessing an organic substituent, through which the steric and
electronic properties of the complexes may be influenced in
order to tune its (stoichiometric or catalytic) reactivity.
We have previously described the one-pot synthesis [from
M(NMe2)4, RNH2, and Me3SiCl], coordination chemistry, and
catalytic applications of titanium(IV) and vanadium(IV) imido
complexes of the general formula M(dNR)Cl2(NHMe2)2
(M = Ti, V).2f,4 During these studies, we were confronted with
the mechanism of formation of the imido group.5 The initial step
in the formation of such species from M(NMe2)4 precursors
involves a transamination reaction with a primary amine, and in
some cases, imido-bridged dimeric complexes {M(μ-NR)-
(NMe2)2}2 have been isolated (Scheme 1), although their
involvement as intermediates in the reaction described above is
not ascertained.4a,5 Although imido-bridged group 4 metal com-
plexes have been known for almost 50 years by the pioneer work
of Bradley and Torrible6 and subsequent studies by Nugent
et al.,7 they are currently less studied than terminal imido
analogues (and they were often obtained while seeking com-
plexes with a terminal imido function).8 We are currently
exploring potential extensions of such dimeric complexes toward
more complex structures. In this report, we describe an investiga-
tion that has led to insight into the synthesis of unprecedented
homo- and heterobimetallic complexes with two identical or
distinct bridging imido ligands.9 Herein we report our initial
efforts toward this goal.
crystals in reasonable yield (Scheme 2). In a similar fashion, the
vanadium analogue precursor V(NMe2)4 affords a mixture of
complexes.10 Nevertheless, after suitable workup, the expected
parent compound 2 was obtained as dark-red crystals albeit in
very low yield. Both 1 and 2 were characterized to be the dimers
i
{M2(μ-NArPr )(μ-NAda)(NMe2)4} (M = Ti, V); this formula-
2
tion follows from spectroscopic data (see below) and elemental
analysis and has been confirmed unequivocally through crystal
structure determinations.
Figure 1 shows the molecular structure of 1. It clearly
demonstrates the compound to be the dimer formulated as
i
{Ti2(μ-NArPr )(μ-NAda)(NMe2)4} with two distinct imido
2
ligands bridging two (Me2N)2Ti moieties. The coordination
geometry of both titanium centers is distorted tetrahedral
(although Ti2 may be regarded as five-coordinated; see below).
The Ti2N2 core displays a considerable asymmetry in one of its
i
TiÀNÀTi bridges (namely, μ-NArPr ), both in the TiÀN
2
distances [Ti1ÀN1 = 1.8611(16) Å; Ti2ÀN1 = 2.0410
(17) Å] and in the distortion, which brings the arylimido
Cipso to within the η2-bonding distance of Ti2. The μ-[η1-
(N):η2(N,C)]-bonding mode exhibited by the μ-arylimido
ligand includes the following structural features: (i) The
Ti2ÀC1 distance is 2.6443(19) Å. (ii) The shorter TiÀNimido
Ti(NMe2)4 (2 equiv) reacts readily with 1 equiv of the
i
substituted aniline 2,6-iPr2C6H3NH2 (ArPr NH2) followed by
2
1 equiv of alkylamine 1-adamantanamine (AdaNH2), at room
temperature in toluene, to yield the corresponding homobime-
tallic complex 1. 1 crystallizes from the solution as red large
Received: July 11, 2011
Published: September 21, 2011
r
2011 American Chemical Society
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Figure 1. Thermal ellipsoid plot of 1 with partial atom labeling.
Displacement ellipsoids are drawn at the 50% level. Hydrogen atoms
bonded to carbon atoms are omitted.
Figure 2. Thermal ellipsoid plot of 5 with partial atom labeling.
Displacement ellipsoids are drawn at the 50% level. Hydrogen
atoms bonded to carbon atoms are omitted. Hydrogen atoms bonded
to nitrogen atoms are drawn as spheres of arbitrary radius.
distance of 1.8611(16) Å is longer by ca. 0.15 Å than that in
terminal imido complexes.3,4b (iii) The nearly linear
Ti1ÀN1ÀC1 bond angle of 159.37(14)° leads to an acute
Ti2ÀN1ÀC1 of 98.42(11)° and contrasts with the more
symmetric TiÀNAdaÀTi bridge, where TiÀNÀC angles are
131.40(12)° and 133.15(13)° associated with TiÀNAda bond
distances of 1.9554(16) and 1.8978(16) Å, respectively for
Ti1 and Ti2. Although unusual, this type of μ-[η1(N):η2
(N,C)]-bonding mode for a μ-arylimido ligand is not
unprecedented.11 The Ti2N2 core is almost planar (torsion
angle Ti1ÀN1ÀTi2ÀN2 = 0.43°) and is characterized by a
centers, either through the nitrogen atoms of the imido bridges
or as a result of the short VÀV distance (see above). Moreover,
the sharpness of the peaks of the well-resolved 1H NMR
spectrum of 2 indicates that the dinuclear structure is retained
in a C6D6 solution.
To go one step further in seeking heterobimetallic compounds
linked by μ-imido bridges, we reacted Ti(NMe2)4 with 2 equiv of
iPr2
Ar NH2, followed by 1 equiv of Zr(NMe2)4. After 12 h of
stirring, removal of the volatiles, and washing with pentane,
orange 3 was obtained with good yields. Spectroscopic as well as
X-ray diffraction studies indicate this compound to be the
dinuclear complex {(Me2N)2Ti(μ-NAriPr2)2Zr(NMe2)2(NHMe2)}.
short Ti
Ti distance of 2.8096(5) Å common in imido
3 3 3
dimers.4b,7aÀ7c The angles formed by bridging nitrogen atoms
with two titanium atoms [Ti1ÀN1ÀTi2 = 91.99(7)°;
Ti1ÀN2ÀTi2 = 93.62(7)°] deviate significantly from that
expected for an sp2 nitrogen atom. The (Me2N)2Ti moiety is
normal, with the expected TiÀN bond distances and angles
[TiÀNamido ranging from 1.810 to 1.926 Å].
1
In particular, the H NMR spectrum exhibits signals (a doublet
at 1.73 ppm) attributable to one NHMe2 ligand, in addition to
signals corresponding to two equivalent μ-NAriPr2 and two types
of ÀNMe2. In addition to NMR spectroscopic data, single-crystal
X-ray diffraction studies13 confirm the proposed connectivity: the
NHMe2 ligand is attached to a pentacoordinated zirconium center
(see also below for the structure of 5), while the geometry around
the titanium center is tetrahedral.
Following reaction conditions similar to those described for 3,
the heterobimetallic titanium/hafnium dimer 4 could be pre-
pared as well.14 As in 3, only one of the metal centers of 4 is
coordinated to a NHMe2 ligand, the hafnium center, while the
titanium center is tetrahedrally coordinated.15
The molecular structure of 2 is given in SI. The complex is a
dimer with two imido ligands bridging two (Me2N)2V moieties
with tetrahedrally coordinated vanadium centers. The VÀNimido
bond distances are in the range of those found in related
arylimido dimers [V1ÀN1 = 1.907(8) Å; V2ÀN1 = 1.832(8)
Å; V1ÀN2 = 1.809(9) Å; V2ÀN2 = 1.872(9) Å].1j,4a The main
difference observed in the structure of 2 versus 1 is a more
symmetric arrangement of the bridging arylimido ligand. Because
titanium and vanadium have almost identical ionic radii, the
symmetry in 2 cannot be explained by steric factors but should
result from electronic differences due to less Lewis acidic V d1
centers (vs Ti d0 centers). The V2N2 core is planar (torsion angle
Finally, in order to combine within the same molecule two dif-
ferent metals and two different bridging imido ligands, we
sequentially treated a solution of Ti(NMe2)4 with 1 equiv of
0.4°) with a short V V distance of 2.4967(9) Å. Such a short
3 3 3
iPr2
Zr(NMe2)4, 1 equiv of Ar NH2, and 1 equiv of AdaNH2. After
distance has been found in other dinuclear complexes of vana-
dium and may be indicative of a VÀV bond.12 For comparison, it
is almost identical with one of the shortest distances found in
related arylimido dimers {V(μ-NAr)(NMe2)2}2.1j,4a
suitable workup, a yellow solid was obtained and was chara-
cterized by 1H NMR spectroscopy as being the expected
heterobimetallic complex 5 in which the two different metals
are indeed held together by two distinct μ-imido groups. This
formula was subsequently confirmed by X-ray crystallography,
following isolation of suitable crystals from pentane. The thermal
ellipsoid plot is presented in Figure 2. The two distinct imido
ligands are bridging (Me2N)2Ti and Zr(NMe2)2(NHMe2) moi-
eties. The coordination geometry of titanium is distorted
tetrahedral, while that of zirconium is strongly distorted trigo-
nal bipyramidal (τ = 0.57)16 because of coordination of one
Complexes 1 and 2 could subsequently be characterized by 1H
and 13C NMR spectroscopy, IR, elemental analysis, and the
absence of a magnetic moment for 2. The structural features
(strong dissymmetry) observed in the solid state for 1 have not
been revealed in solution. In fact, the 1H and 13C NMR spectra at
room temperature show respectively a single resonance for
the ÀNMe2 protons (a singlet at 3.23 ppm) and carbons (at
45.6 ppm), suggesting equivalency of the four ÀNMe2 ligands at
this temperature. The symmetry in solution is also apparent in the
NMR spectrum of diamagnetic complex 2. This diamagnetism
indicates a strong electronic coupling between the two metal d1
dimethylamine [ZrÀNNHMe = 2.411(5) Å]. The TiN2Zr core
2
is almost planar (torsion angle = 0.75°) and is characterized
by a Ti
Zr distance of 3.0023(11) Å, two TiÀNimido bond
3 3 3
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distances of 1.844(4) and 1.922(5) Å, and two ZrÀNimido bond
distances of 2.139(4) and 2.109(4) Å.
2000, 4497–4498. (g) Arbaoui, A.; Homden, D.; Redshaw, C.; Wright,
J. A.; Dale, S. H.; Elsegood, M. R. J. Dalton Trans. 2009, 8911–8922.
(3) For selected reviews on imido complexes, see: (a) Wigley, D. E.
In Progress in Inorganic Chemistry; Karlin, K. D., Ed.; Interscience:
New York, 1994; Vol. 42, pp 239À482. (b) Nugent, W. A.; Mayer,
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1998. (c) Bolton, P. D.; Mountford, P. Adv. Synth. Catal. 2005, 347,
355–366. (d) Hazari, N.; Mountford, P. Acc. Chem. Res. 2005, 38, 839–849.
(4) (a) Lorber, C.; Choukroun, R.; Donnadieu, B. Inorg. Chem.
2002, 41, 4217–4226. (b) Lorber, C.; Choukroun, R.; Vendier, L. Eur.
J. Inorg. Chem. 2006, 4503–4518. (c) Lorber, C.; Choukroun, R.;
Vendier, L. Inorg. Chem. 2007, 46, 3192–3202. (d) Lorber, C.;
Choukroun, R.; Donnadieu, B. Inorg. Chem. 2003, 42, 673–675.
(5) Lorber C., manuscript in preparation.
In summary, the present work has established, for the first
time, the propensity of group 4 and 5 amido precursors toward
the formation of unique dimer complexes with two distinct
bridging μ-imido ligands, as well as heterobimetallic species
bridged by μ-imido ligands. It is important to note that the
selective isolation of such dimeric complexes (with either two
different imido ligands and/or two different metals) may be
regarded as unexpected considering the difficulties to obtain
homobimetallic complexes in a pure form, in particular with very
sterically demanding 2,6-diisopropylarylimido ligand.5 These
results point out a number of new avenues to explore in the
synthesis of homo- and heterodinuclear complexes.9 Studies are
underway to probe the scope of such reactions and their
applications, as well as to investigate in more detail the reasons
for the selective formation of such species.17
(6) Bradley, D. C.; Torrible, E. G. Can. J. Chem. 1963, 41, 134–138.
(7) (a) Thorn, D. L.; Nugent, W. A.; Harlow, R. L. J . Am. Chem. Soc.
1981, 103, 357–363. (b) Nugent, W. A.; Harlow, R. L. Inorg. Chem.
1979, 18, 2030–2032. (c) Nugent, W. A.; Haymore, B. L. Coord. Chem.
Rev. 1980, 31, 123–175.
(8) For selected recent examples of group 4 and 5 imido-bridged
dimers, see refs 1j and 4aÀ4c and the following: (a) Yu, X.; Chen, S.-J.;
Wang, X.; Chen, X.-T.; Xue, Z.-L. Organometallics 2009, 28, 4269–4275.
(b) Smolensky, E.; Kapon, M.; Eisen, M. S. Organometallics 2005,
23, 5495–5498. (c) Basuli, F.; Wicker, B.; Huffman, J. C.; Mindiola,
D. J. J. Organomet. Chem. 2011, 696, 235–243. (d) Szigethy, G.; Heyduk,
A. F. Inorg. Chem. 2011, 50, 125–135. (e) Chen, S.-J.; Xue, Z.-L.
Organometallics 2010, 29, 5575–5584. (f) Kaleta, K.; Arndt, P.; Beweries,
T.; Spannenberg, A.; Theilmann, O.; Rosenthal, U. Organometallics
2010, 29, 2604–2609. (g) Dubberley, S. R.; Evans, S.; Boyd, C. L.;
Mountford, P. Dalton Trans. 2005, 1448–1458.
’ ASSOCIATED CONTENT
S
Supporting Information. Complete X-ray crystallo-
b
graphic data in CIF format for compounds 1, 2, and 5 and
synthetic details for all compounds. This material is available free
of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: lorber@lcc-toulouse.fr. Phone: (+33) 5 61 33 31 44.
(9) For a recent review on multinuclear catalysts, see: Delferro, M.;
Marks, T. B. Chem. Rev. 2011, 111, 2450–2485.
(10) Conducting the reaction at 110 °C leads also to similar complex
mixtures. These complexes were shown by multinuclear NMR spec-
troscopy to be the expected complex 2, contaminated by {V(NMe2)2-
’ ACKNOWLEDGMENT
i
iPr2
(μ-NArPr )(μ-NAda)V(NMe2)(NHAr )}, and small amounts of
{V(μ-NAda)(NMe2)2}2.
2
The research was supported by the CNRS.
(11) Arney, D. J.; Bruck, M. A.; Huber, S. R.; Wigley, D. E. Inorg.
Chem. 1992, 31, 3749–3755.
(12) For vanadium imido-bridged dimers, see refs 4a and 4c and
references cited therein.
’ REFERENCES
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(13) See the SI.
(14) A zirconium/hafnium complex could be prepared as well;
however, in this case, it is obtained as a mixture of two isomers.
1
(15) As shown by H NMR spectroscopy with comparison with
i
homobimetallic-related complexes {M(μ-NArPr )(NMe2)2}2(NHMe2)x
2
complexes (M = Ti, Zr, Hf). The crystal structure of 4 could not be fully
resolved because of crystallographic problems (see ref 13).
(16) For a definition of τ, see: Addison, A. W.; Rao, T. N.; Reedijk, J.;
van Rijn, J. V. J. Chem. Soc., Dalton Trans. 1984, 1349–1356.
(17) In preliminary mechanistic studies, scrambling reactions be-
tween two homobimetallic complexes did not lead to the interchanged
i
products (e.g., scrambling {Ti(μ-NArPr )(NMe2)2}2 with {Ti(μ-NAda)-
2
(NMe2)2}2 caused no formation of 1 at room temperature). In agreement
with this observation, we presume that R1NH2 and R2NH2 react with
M1(NMe2)4 to form transient M1(NMe2)2(NHR1)(NHR2), which then is
able to act as a chelating metalladiamine and react with a further
M2(NMe2)4.
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