Formation of novel T-shaped NNN ligands via rare-earth metal-mediated Si-H activation

Article abstract of DOI:10.1021/ic402941z

Reactions of silylamides [Ln{N(SiHMe₂)₂} ₃(thf)₂] with sterically crowded terphenylamine DmpNH ₂ (Dmp = 2,6-Mes₂C₆H₃ with Mes = 2,4,6-Me₃C₆H₂) afforded via a template reaction the formation of a new tridentate ligand, and derived complexes of composition [LnN{SiMe₂N(Dmp)}₂] (Ln = Ce, Pr) were obtained. Usage of the even more bulky amine ArNH₂ (Ar* = 2,6-Trip ₂C₆H₃ with Trip = 2,4,6-iPr₃C ₆H₂) yielded the free protonated ligand NH{SiMe ₂NH(Ar)}₂.

Full text of DOI:10.1021/ic402941z

Communication
pubs.acs.org/IC
Formation of Novel T‑Shaped NNN Ligands via Rare-Earth Metal-
Mediated Si−H Activation
Denis Bubrin† and Mark Niemeyer*,‡
†Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany
̈
̈
̈
‡Institut fur Anorganische und Analytische Chemie, Johannes Gutenberg-Universitat Mainz, Duesbergweg 10−14, D-55128 Mainz,
̈
Germany
S
* Supporting Information
much better yield by the reaction of 2 equiv of the amine
DmpNH2 with the corresponding lanthanide silylamides Ln{N-
(SiHMe2)2}3(thf)2 [Ln = Ce (1a) or Pr (1b)].12 The starting
materials were melted together at 100 °C for 2 days and
afterward treated with n-heptane. The obtained precipitates were
extracted into tetrahydrofuran (THF/thf), and the THF extracts
were stored at −20 °C to afford complexes [LnN{SiMe2N-
(Dmp)}2] [Ln = Ce (2a), Pr (2b)] as orange (2a) or pale-yellow
(2b) crystalline materials in 74% and 62% yield, respectively.
A plausible mechanism for the formation of 2a and 2b is
depicted in Scheme 1. It first involves dissociation of one
ABSTRACT: Reactions of silylamides [Ln{N-
(SiHMe2)2}3(thf)2] with sterically crowded terphenyl-
amine DmpNH2 (Dmp = 2,6-Mes2C6H3 with Mes =
2,4,6-Me3C6H2) afforded via a template reaction the
formation of a new tridentate ligand, and derived
complexes of composition [LnN{SiMe2N(Dmp)}2] (Ln
= Ce, Pr) were obtained. Usage of the even more bulky
amine Ar*NH2 (Ar* = 2,6-Trip2C6H3 with Trip = 2,4,6-
iPr3C6H2) yielded the free protonated ligand NH-
{SiMe2NH(Ar*)}2.
Scheme 1. Proposed Reaction Path for the Formation of 2a
and 2b
he formation of macrocycles and chelating ligands
Tpromoted by lanthanide(III) cations is well established
and, in particular, investigated for Schiff-base-type condensa-
tions.1 Although other metals, in particular alkaline-earth and
transition metals, are also qualified to promote template
reactions,2,3 the use of lanthanides partly benefits from a number
of advantages such as low cost and toxicity. Moreover, because of
their different ionic radii, fine control of the reaction outcome is
possible because the character and size of the formed ligand
essentially depends on the size of the cation.4 Lanthanide
complexes with multidentate ligands exhibit some very
interesting properties and possible applications, such as
polymerization catalysts,5 contrast enhancing agents in magnetic
resonance imaging,6 luminescent biolabels and materials,7 ionic
liquid crystals,8 and extracting agents for lanthanide actinide
separation.9 In the following, we report the synthesis of novel
tripodal nitrogen donor ligands via Si−H activation promoted by
rare-earth metal ions.10
coordinated THF molecule from the starting material 1a. An
additional agostic Ln···Si−H interaction may be formed that
provides some electronic compensation for loss of the electron
density close to the Lewis acidic metal center. Similar
interactions have been observed before in a number of
structurally characterized lanthanide disilazanides.13,14 The
weakened Si−H bond is activated and reacts with the amine
DmpNH2 under elimination of H2. In a further step,
tetramethyldisilazane is eliminated, and the whole sequence is
repeated once more.
As part of a project on the preparation of well-defined
cerium(III) complexes, which might be used for cerium-
catalyzed α-hydroxylation of β-diketo compounds with molec-
ular oxygen,11 we have examined reactions between the
silylamide [Ce{N(SiHMe2)2}3(thf)2] (1a) and sterically
crowded 4-arylpent-3-en-2-ones. Therefore, the solvent-free
reaction of 1a with ketimine DmpNHC(Me)C(H)C(Me)O
(Dmp = 2,6-Mes2C6H3 with Mes, 2,4,6-Me3C6H2) afforded as a
byproduct in very small yield the novel complex [CeN{SiMe2N-
(Dmp)}2] (2a) as air-sensitive orange crystals. The formation of
2a can be explained either by the presence of DmpNH2
impurities in the ketimine or generation of the latter by
unintentional hydrolysis during the reaction. Compound 2a
and its homologous praseodymium complex [PrN{SiMe2N-
(Dmp)}2] (2b) can be synthesized in a more rational way and in
While metal-mediated activation of both SiH groups and
generation of a trianionic tris(amido) ligand are novel features in
Received: November 27, 2013
Published: January 23, 2014
© 2014 American Chemical Society
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Inorganic Chemistry
Communication
tetramethyldisilazanide chemistry, formation of the dianionic
bis(amido) ligand [SiMe2(NSiHMe2)2] via elimination of
H2SiMe2 has been observed previously by different groups.15
In one of these papers, Buffet and Okuda15b reported
functionalization of the cyclic polyamine 1,4,7-trimethyl-
1,4,7,10-tetraazacyclododecane [(Me3TACD)H] with a pendant
SiMe2N(SiHMe2) group via H2 elimination from KN(SiHMe2)2
and (Me3TACD)H. Similar attempts to prepare derivatives of
the protonated ligand HN{SiMe2N(Dmp)H}2 in a different way
either via silylation of the amine with a catalytic amount of
potassium or sodium hydride as previously described for a
number of compounds with Si−N bonds16 or by reaction of the
aluminum silylamide Al{N(SiHMe2)2}317 with the amine under
the same reaction conditions as those described above were not
successful and afforded the unreacted starting materials only.
In further experiments, the reaction of the bulkier amine
Ar*NH2 (Ar* = 2,6-Trip2C6H3 with Trip, 2,4,6-iPr3C6H2) with
lanthanide silylamides Ln{N(SiHMe2)2}3(thf)2 was examined.
Instead of the expected metal derivative, formation of the free
protonated ligand NH{SiMe2NH(Ar*)}2 (3) was observed. The
propensity of very bulky terphenyl-substituted ligand systems for
unexpected demetalation reactions has been noted before.18
In the solid-state structure of the tris(amine) 3 (Figure 1), the
Si1−N2−Si2 angle at the central nitrogen atom is widened to
units of THF, whereas 2a was isolated from either n-heptane or
THF with 1 and 1.5 formula units of solvent, respectively.
Because of their very similar features, only the solid-state
structure of the cerium complex 2a·1.5THF will be discussed in
detail in the following. In all three structures, there are no
significant interactions between the cocrystallized solvent
molecules and metal atoms. The later show an exceptional
anchor-like coordination by three nitrogen atoms of the chelating
amido ligand (Figure 2). Steric and electronic saturation of the
Figure 2. Molecular structure of 2a·1.5THF with thermal ellipsoids set
to 30% probability. Hydrogen atoms and cocrystallized solvent
molecules have been omitted for clarity. Selected bond lengths (Å)
and angles (deg) for 2a·1.5THF and 2b·3THF (in brackets): M−N1
2.456(2) {2.4240(18)}, M−N2 2.292(2) {2.280(2)}, M−N3 2.422(2)
{2.4260(18)}, M···C21 2.953(3) {2.956(2)}, M···C22 3.035(3)
{3.159(3)}, M···C23 3.189(3) {3.307(3)}, M···C24 3.325(3)
{3.287(3)}, M···C25 3.211(3) {3.069(3)}, M···C26 3.034(3)
{2.942(2)}, M···C41 2.945(3) {2.939(2)}, M···C42 2.980(3)
{2.961(2)}, M···C43 3.137(3) {3.075(3)}, M···C44 3.361(3)
{3.227(2)}, M···C45 3.332(3) {3.218(2)}, M···C46 3.148(3)
{3.083(2)}, M···X5 2.764 {2.763}, M···X4 2.781 {2.727}; Nl−M−N2
65.82(8) {66.41(6)}, Nl−M−N3 131.57(7) {132.72(7)}, N2−M−N3
65.85(8) {66.32(7)}, N1−Si1−N2 97.92(12) {97.26(10)}, N2−Si2−
N3 96.77(12) {97.28(10)}, Si1−N2−Si2 156.74(16) {157.75(14)},
X4···M···X5 127.9 {127.3}. X5 and X4 define the centroids of the
coordinated C21, C22, C23, C25, and C26 (for 2a), C21, C22, C24,
C25, and C26 (for 2b), and C41, C42, C43, and C46 (for 2a and 2b)
arene carbon atoms.
metal atom is achieved by additional Ce···π interactions to two
pending mesityl rings of the terphenyl groups. On the basis of the
observed shortest metal···centroid distances, a η5/η4-π coordi-
nation is formally assigned involving the carbon atoms (C21 →
C23, C25, C26)/ (C41 → C43, C46) with Ce···C and Ce···
centroid distances in the ranges of 2.945(3)−3.211(3) and
2.764/2.781 Å, respectively.
Figure 1. Molecular structure of 3·1.5THF with thermal ellipsoids set to
30% probability. Most hydrogen atoms and cocrystallized solvent
molecules have been omitted, and Ar* substituents are shown as lines
for clarity. Selected bond lengths (Å) and angles (deg): N1−C11
1.404(2), Si1−N1 1.7477(18), N1−H1 0.74(2), Si2−N2 1.715(2),
Si1−N2 1.7216(19), N2−H2 0.76(2), N3−C31 1.407(2), Si2−N3
1.7477(18), N3−H3 0.78(2), Si1−C1 1.847(2), Si1−C2 1.849(2),
Si2−C3 1.847(2), Si2−C4 1.848(2); Si1−N2−Si2 136.76(15), C11−
N1−Si1 132.90(15), C31−N3−Si2 133.25(15), N1−Si1−N2
104.78(9), N2−Si2−N3 104.57(9).
Taking into account the additional π contacts, a formal
coordination number (cn) of 9 is calculated. However, if the
observed average Ce−N distance of 2.390(2) Å is compared with
the corresponding distances in other cerium amides {cn = 3,
[Ce{N(SiMe3)2}3]19a = 2.320(3) Å and [Ce(Tmp)3] (Tmp =
2,2,6,6-tetramethylpiperidinate)19b = 2.323(7) Å; cn = 4,
136.76(15)°. While N2 shows a strictly planar environment, as
reflected by the angle sum of 359.9°, there is slight
pyramidalization at the outer nitrogen atoms [Σ°(N1) =
348.6° and Σ°(N3) = 351.1°].
Complexes 2a and 2b cocrystallize with different solvent
molecules located in cavities of the structure. 2b contains three
[Ce(Tmp)3(thf)]19c = 2.361(2) Å and [Ce(NCy2)3(thf)]19d
=
2.317(2) Å; cn = 5, [Ce{N(SiHMe2)2}3(thf)2] = 2.388(3) Å, see
the Supporting Information (SI); cn = 6, [Ce{N(SitBuMe2)(2-
C5H3N-6-Me)}3]19e = 2.437 Å}, a lower effective cn of
approximately 5 may be estimated for complex 2a (and 2b),
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Communication
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ASSOCIATED CONTENT
* Supporting Information
■
S
122, 3080. (c) Klimpel, M. G.; Gorlitzer, H. W.; Tafipolsky, M.; Spiegler,
̈
M.; Scherer, W.; Anwander, R. J. Organomet. Chem. 2002, 647, 236.
Details of syntheses and characterization for compounds 2a, 2b,
and 3, including X-ray data in CIF format for 1a, 1b, 2a·1.5THF,
2b·3THF, 2a·n-heptane, and 3, and computational details. This
material is available free of charge via the Internet at http://pubs.
acs.org.
(d) Yan, K.; Upton, B. M.; Ellern, A.; Sadow, A. D. J. Am. Chem. Soc.
2009, 131, 15110. (e) Crozier, A. R.; Bienfait, A. M.; Maichle-Mossmer,
̈
C.; Tornroos, K. W.; Anwander, R. Chem. Commun. 2013, 49, 87.
̈
(14) The bis(THF) starting materials 1a and 1b show no significant
Ln···Si−H interactions however (see the SI).
(15) (a) Yuen, H. F.; Marks, T. J. Organometallics 2009, 28, 2423.
(b) Buffet, J.-C.; Okuda, J. Dalton Trans. 2011, 40, 7748. (c) Chen, F.;
Fan, S.; Wang, Y.; Chen, J.; Luo, Y. Organometallics 2012, 31, 3730.
AUTHOR INFORMATION
Corresponding Author
*E-mail: niemeyer@uni-mainz.de.
■
(d) Jende, L. N.; Maichle-Mossmer, C.; Schadle, C.; Anwander, R. J.
̈
̈
Organomet. Chem. 2013, 31, 744.
Author Contributions
The manuscript was written through contributions of all authors.
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Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was performed within the Collaborative Research
Centre (SFB 706; Selective Catalytic Oxidations Using
Molecular Oxygen; Stuttgart) and funded by the German
Research Foundation.
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