Quest for lithium amidinates containing adjacent amino donor group at the central carbon atom

Article abstract of DOI:10.1016/j.jorganchem.2013.07.067

Lithium amidinate complexes bearing different peripheral substituents on both nitrogen atoms (iPr and Dipp (2,6-di(isopropyl)phenyl-) and pendant, potentially coordinating group originated from the 2-(N,N-dimethylaminomethyl) phenyl moiety, at the central

Full text of DOI:10.1016/j.jorganchem.2013.07.067

Journal of Organometallic Chemistry 745-746 (2013) 186e189
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Note
Quest for lithium amidinates containing adjacent amino donor group
at the central carbon atom
*
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ꢀ ꢁꢀ ꢀ
Jana Nevoralová, Tomás Chlupatý, Zdenka Padelková, Ales Ruzicka
Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 573, CZ-532 10 Pardubice,
Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 July 2013
Accepted 26 July 2013
Lithium amidinate complexes bearing different peripheral substituents on both nitrogen atoms (iPr and
Dipp (2,6-di(isopropyl)phenyl-) and pendant, potentially coordinating group originated from the 2-(N,N-
dimethylaminomethyl)phenyl moiety, at the central carbon atom were prepared by addition of the 2-
(N,N-dimethylaminomethyl)phenyllithium to the appropriate carbodiimides. The structure of the iso-
propyl group substituted lithium amidinates is dimeric with bidentate amidinate unit and four-
coordinated lithium atom. On contrary the Dipp group substituted lithium amidinate is monomeric
with trigonal planar geometry of the lithium atom, monodentately bound amidinate unit and coordi-
nated pendant arm.
Keywords:
Amidinate
C,N-chelate
Lithium
NMR
Ó 2013 Elsevier B.V. All rights reserved.
XRD
1. Introduction
arrangement would probably lead to a hemilabile connection of an
extra donor to the metal centre, and creation of so called metal
Metal complexes containing amidinate ligands, or its variations,
are frequently tested as catalysts of various organic chemistry
transformations in the present time. [1,2] Moreover the application
of these complexes as precursors of new materials [3] together with
stabilization of unusual [4], generally lower, oxidation states of
metals in its complexes opened new areas of chemistry. The
essential step of synthesis of metal amidinates is the preparation of
lithium amidinate precursors, which in contrary to the target
compounds are rarely described in the literature [5]. Lithium ami-
dinates reveal usually three different types of structures [6]
(Scheme 1) monomeric isobidentate with extra coordination of
two donor atoms from such as THF or TMEDA molecule(s) (Scheme
1A), dimeric isobidentate with one donor on both of lithium atoms
(Scheme 1B), or higher aggregates (Scheme 1C).
One of the goals in the metal amidinate chemistry is the modi-
fication of the amidinate skeleton and a fine tuning of the electronic
and steric properties of the ligand. Except the usual transformations
or modifications of nitrogen atom substituents, a substituent
with an adjacent neutral flexible donor group could be attached to
the central carbon atom of the amidinate ligand. This ligand
heteroscorpionate complex. Only preliminary attempts to prepare
this type of complexes were performed, and therefore these are still
rare (except of guanidinates and phosphaguanidinates) in the
literature. Chronologically, the first attempt is the use of the ami-
dinate containing the pyrazole heterocycle [7] in the Li, Ti and Zr
complexes (Scheme 2A), and then the incorporation of 2-pyridyl-
[8] or 2-furyl- [6b] groups to Li, Y or rare earth metal complexes
(Scheme 2B and C). To the best of our knowledge the most recent
examples of the real monoanionic system came from work pub-
lished by Braunstein et al. (Scheme 2D) [9] where the amidinato-
methylene bridged eNHC complexes of K, Ag and Cr are reported.
An o-carboranyl substituted amidine ligand [10] which reveals a H-
atom transfer from the carborane to the amidinate part (Scheme
2E), and related Ir complexes were published during last two
years. The last example is the dianionic ligand reported by Trifonov
and Carpentier [11] which combines a rare earth metal complexa-
tion by a bulky phenoxide substituted amidinate (Scheme 2F).
On the basis of the fact that none of above mentioned ligands
has a monoanionic tridentate bonding fashion to the metal centre,
we started to investigate the synthesis, as well as the structure, of
amidinates, which could fulfil such a demand, prepared from 2-
(N,N-dimethylaminomethyl)phenyllithium [12] and isopropyl- and
2,6-di(propan-2-yl)phenyl- (Dipp) disubstituted carbodiimides,
respectively, in solution and in the solid state.
* Corresponding author. Fax: þ420 466037068.
ꢁꢀ ꢀ
E-mail address: ales.ruzicka@upce.cz (A. Ruzicka).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.07.067

J. Nevoralová et al. / Journal of Organometallic Chemistry 745-746 (2013) 186e189
187
Scheme 1.
2. Results and discussion
2.1. Synthesis
All lithium compounds were prepared by the same way from the
lithium precursor e C,N-chelated organolithium compound 1 and
appropriate carbodiimide in high yield (Fig. 1). A smaller isopropyl
group containing carbodiimide compounds 2 and 4 in diethylether
and THF, respectively. Compound 5 can be obtained by a hydrolysis
of 4 by water or air moisture. The use of bigger carbodiimide sub-
stituent e 2,6-di(isopropyl)phenyl- instead of the isopropyl one,
has nearly no influence to the yield of the target molecule 3.
Fig. 1. Reaction routes to compounds 2e5 e reagents and conditions.
for CH2 group in 13C NMR spectra: from 72.6 ppm (for 1) to 62.2 (for
2 in THF-d8), 63.5 (for 3 in C6D6) and 64.9 ppm (for 3 in THF-d8); and
in 1H NMR spectra: 4.52, 2.86 ppm (AX spin system for 1 in C6D6)
and 3.48 ppm (for 1 in THF-d8) to 3.44 ppm (for 2 in THF-d8) and
4.70, 2.85 ppm (AX spin system for 3 in C6D6) and 4.68, 2.97 ppm
(AX spin system for 3 in THF-d8). Analogous change of chemical
shift values is found for the ipso carbon bound to the lithium atom
in 1 (164.3 ppm) transformed to the ipso carbon (w140 ppm in 2
and 3, respectively) bound to the central carbon atom of the ami-
dinate NCN skeleton in 13C NMR spectra. Similar upfield shift trend
is seen also when chemical shifts in 7Li NMR spectra for 1 (3.6 and
2.6 ppm) and 2 and 3 (1.6, ꢀ0.3 for 2, 0.9 and 0.2 ppm for 3 in
different solvents e see Supporting information) are compared. On
the other hand, the opposite but solvent independent trend is
found for the signal of the central carbodiimide carbon atom when
2.2. NMR spectroscopic studies in solution
The purity as well as structure of prepared compounds was
studied by the multinuclear NMR spectroscopic approach (1H, 7Li,
and 13C) in solutions of both non-coordinating and coordinating
deuterated solvents (C6D6 and THF-d8).
There are some similarities to the recently described lithium n-
butylamidinates [5a] but the general comparison of these groups of
compounds is not made because of the presence of an adjacent
donor group and complexity of some spectral patterns in 2e4. All
spectra can be compared to the same type of spectra of precursors
(carbodiimides [5a] and 1) and a hydrolysis product (5). When 1 is
reacted with carbodiimide, the significant upfield shift is detected
Scheme 2.

188
J. Nevoralová et al. / Journal of Organometallic Chemistry 745-746 (2013) 186e189
a carbodiimide (w 140 ppm) is reacted with 1 (167.4 ppm for 2 and
w 159 ppm for 3). Upfield shifted signal attributed to the same
carbon atom is detected when 2 is treated with water giving ami-
dine 5 (155.1 ppm). This change is already seen in 1H NMR spec-
trum of 2, where the methylene group resonates as a sharp singlet
but for the same group of 5 an AX spin pattern is detected indi-
cating a free rotability of this group in 2. All these changes are
connected to the formation of lithium amidinates from starting
compounds or a further hydrolysis.
Significant differences were found also within the group of
lithium amidinates containing additional donor group (2 and 3).
While very broad signals were detected in the 1H NMR spectrum of
2 in benzene solution and one set of well resolved signals is
measured in each 1H and 13C NMR spectrum in THF-d8, all spectra of
3 in both solvents are complex revealing AX spin patterns for
methylene groups and two signals for aminomethyl groups in 13C
NMR spectrum in benzene. Good indicia for the structural diversity
between these two structures are the chemical shift values for the
central carbon atom of the NCN skeleton of 2 and 3. For 2 the value
of 167.4 ppm was detected which is slightly lower (w 171 ppm)
than was found for dimers of lithium n-butylamidinates [5a] co-
ordinated by Et2O and THF molecules, respectively, which is most
likely caused by a change of the aliphatic chain for aromatic ring
substituent of the central carbon atom. Tremendous shift is
observed going from 2 to 3 being 159.3 ppm. Moreover the value of
this parameter in 3 is nearly the same in both solvents used which
is a further proof of structural similarities.
Fig. 2. Molecular structures of {LCN-iPrLi.Et2O}2 (2) and {LCN-iPrLi.THF}2 (4) (ORTEP view,
40% probability level). Hydrogen atoms and methylene groups from Et2O molecules are
omitted for clarity. Selected interatomic distances [A] and angles [ ] for {LCN-iPrLi.Et2O}2
(2) and {LCN-iPrLi.THF}2 (4, appropriate values are given in italics in parenthesis): C1ae
Li1 2.373(5) (2.369(4)); N1eC2 1.468(3) (1.467(3)); N1aeLi1 2.134(5) (2.162(4)); N1eLi1
2.080(4) (2.079(4)); N2aeLi1 2.017(5) (1.994(4)); Li1eO1 1.974(4) (1.942(5)); C1eN2
1.321(3) (1.323(3)); C1eN1 1.337(3) (1.342(3)); C8aeC1aeN2a 121.04(18) (121.23(7));
C8aeC1aeN1a 121.67(18) (121.52(17)); N1eC1eN2 117.29(18) (117.29(18)); Li1eN1ae
Li1a 72.26(16) (70.45(15)); N1aeLi1aeO1a 114.3(2) (113.0(2)); N2aeLi1eO1 120.62(19)
(118.76(19)); C8eC1eN2 121.04(18) (121.18(18)); C8eC1eN1 121.67(18) (121.52(17));
N1eC1eN2 117.29(18) (117.29(18)); C2eN1eLi1 111.03(17) (111.12(17)); C5eN2eLi1a
147.12(19) (145.2(2)); Li1eN1eLi1a 72.26(16) (70.45(15)); N2eLi1a-O1a 120.62(19)
(118.76(19)); N1eLi1eO1 114.3(2) (113.0(2)); N1aeLi1eN1 107.74(18) (109.55(18)).
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In contrary to the 1H and 13C NMR spectra which are quite
indicative for the evaluation of structural changes and concentra-
tion independent, the 7Li NMR spectra parameters reveal chaotic
changes of values even in cases of a preparation of more diluted
sample of the same compound (see Supporting information).
Except the evaluation of the reaction of 1 with carbodiimides no
additional relevant information can be obtained from this
parameter.
ꢁ
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planes by 0.797(2) A for 2 and 0.837(3) A for 4, respectively, which
is a bit over the values found for similar compounds in the literature
[6j]. Parallel planes defined by lithium and nitrogen atoms of the
amidinate (N1eLi1eN2) ligand are perpendicularly oriented to the
planes defined by the phenyl ring, Li and oxygen atoms.
In contrary to the dimeric 2 and 4 (Fig. 2 and Fig. S1), 3 (Fig. 3)
reveals monomeric structure with very short distance between the
nitrogen from dimethylaminomethyl group and the lithium atom
which is similar as the sum of the covalent radii of both atoms. A
high affinity of the pendant donor amino group to coordinate the
lithium atom is further demonstrated in rather huge torsion angle
2.3. Solid state study
The structures of centrosymmetric dimers of 2 and 4 were
determined by X-ray diffraction techniques. Although different
donor solvent molecule is present in both compounds, they reveal
very close structures reflected in unit cell parameters, interatomic
distances and angles one to each other (Fig. 2 and Fig. S1). The
coordination geometries of the lithium atoms in 2 and 3 are dis-
torted tetrahedral, with the largest deviations from ideal shape in
N1eLi1eN2 angles which are only about 66ꢁ. The LieLi distance
ꢁ
(w2.45 A) is in line with previously described dimeric lithium
amidinates [5a]. Although the dimethylamino group is a very good
donor, the formation of the dimer is preferred over its coordination
in 2 and 4. A high degree of conjugation within the amidinate
moiety is detected for these compounds (see Fig. 2 and Fig. S1
captions). The LieN distances are nearly equivalent with separa-
tions only slightly longer than the sum of the covalent radii of both
ꢁ
atoms (2.04 A) [13] and the closest contact of each lithium atom to
the outer non-bridging N2 atom. The NCN amidinate planes are
mutually parallel in both molecules with interplanar distances of
ꢁ
ꢁ
0.309(3) A for 2 and 0.102(3) A for 4, respectively, which are the
closest in cases of dimeric lithium amidinates and are dependent on
the steric repulsion of the substituents and a donor ability of the
Fig. 3. Molecular structure of LCN-DippLi.Et2O (3) (ORTEP view, 30% probability level).
ꢁ
Hydrogen atoms are omitted for clarity. Selected interatomic distances [A] and angles
ꢁ
ꢁ
[ꢁ] for LCN-DippLi.Et2O (3) which is consist of two independent molecules (appropriate
values of second unit are given in italics in parenthesis): C2eC1 1.523(7) (1.508(7));
C1eN2 1.359(7) (1.356(6)); C1eN3 1.290(7) (1.295(6)); N2eLi1 1.930(10) (1.944(11));
Li1eO11.992(10) (1.953(11)); Li1eN1 2.073(11) (2.056(11)); C23eN3 1.411(6) (1.412(6));
C11eN2 1.425(6) (1.430(6)); C2eC1eN3 122.6(5) (123.3(5)); C2eC1eN2 113.8(4)
(114.0(4)); N2eC1eN3 123.2(5) (122.5(5)); C11eN2eLi1 126.3(4) (125.3(4)); O1eLi1e
N1 112.5(5) (114.7(5)); N2eLi1eN1 110.1(5) (108.4(5)); C1eN3eC23 126.8(5) (129.0(4)).
solvent (0.244 A for [n-BuC(N-iPr)2Li(THF)]2 [5a], 0.471 A for [n-
BuC(NCy)2Li(THF)]2 [6i], 0.652(3) A for [n-BuC(N-iPr)2Li(Et2O)]2
[5a] and 1.028 A for a lithium triphenyl amidinate adduct with
ꢁ
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HMPA [6g]), while this parameter is always bigger when the Et2O is
coordinated to the lithium atom of the same compound than the
THF molecule. Lithium atoms are located out of the NCN amidinate

J. Nevoralová et al. / Journal of Organometallic Chemistry 745-746 (2013) 186e189
189
(C2eC3eC8eN1) of 91.8(2)ꢁ which define the deflection of the
Acknowledgements
nitrogen atom from the plane of the parent aromatic ring. Only one
nitrogen atom of the amidinate skeleton is coordinated to the
lithium atom with slightly shorter distance than is found for N2eLi
in 2 and 4. The bonding fashion is further reflected in different
interatomic distances within the amidinate moiety (C1eN2
1.359(7) (1.356(6)); C1eN3 1.290(7) (1.295(6))) where the C1eN3
distance can be seriously attributed to a double bond and the angle
N2eC1eN3 is slightly wider than in 2 and 4. The coordination
geometry of the Li atom is trigonal planar with the orientation of
the plane 61.6(2)ꢁ from the plane defined by NCN amidinate
moiety.
The structure of the amidine 5 (Fig. S2) has been determined in
order to compare it with prepared lithium complexes where the
amidine NCN unit is very similar to the same fragment in 3. Sur-
prisingly, no H-bridge promoted connectivity of the amidine NeH
and dimethylaminomethyl group is detected.
The authors would like to thank the Science Foundation of the
Czech Republic (grant no. P207/12/0223) for financial support.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.07.067.
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({LCN-RLi(solvent)}x) (2e4)
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To a suspension of 2-[(dimethylamino)methyl]phenyllithium
(1) in Et2O cooled to 0 ꢁC, one equivalent of appropriate N,N0-
disubstituted carbodiimide (a solution in Et2O) was added. Het-
erogeneous reaction mixture was allowed to warm to the room
temperature and stirred until all the solid particles of 1 dis-
appeared. Colourless solution was filtered and Et2O was evaporated
under vacuo to give white solids. Crude products of 2e4 were
washed with hexane and dried under vacuo to give pure white
crystalline of lithium N,N0-disubstituted [1-(N,N-dimethylamino-
methyl)-phen-2-yl]amidinates 2e4.
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