Synthesis of mixed-donor amido/amino/siloxo ligands from symmetrical diamidosilylether ligands via a retro-Brook rearrangement and their chromium(II) complexes

Article abstract of DOI:10.1039/b506322a

The synthesis of mixed-donor amido/amino/siloxo ligands from dilithio salts of two chelating diamidosilylether ligands via a retro-Brook rearrangement, was analyzed. If the deprotonation is conducted, of if the ligands are stirred in THF, they undergo an anionic 1,3-silyl retro-Brook rearrangement in which one silyl group migrates from oxygen to nitrogen. To investigate the fundamental coordination chemistry of the potential tridentate ligands, chromium(II) complexes were prepared. It was observed that the potentially tridentate acts as a bidentate amido-siloxo donor with C(II). The results show that the well-known retro-Brook rearrangement can be used as a tool to readily synthesize in one step a new class of unsymmetrical mixed-donor ligands.

Full text of DOI:10.1039/b506322a

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C O M M U N I C A T I O N
Synthesis of mixed-donor amido/amino/siloxo ligands from
symmetrical diamidosilylether ligands via a retro-Brook
rearrangement and their chromium(II) complexes†
Farzad Haftbaradaran,a Garry Mund,a Raymond J. Batchelor,a James F. Brittenb and
Daniel B. Leznoff*a
a Department of Chemistry, Simon Fraser University, Burnaby, BC, Canada V5A 1S6.
E-mail: dleznoff@sfu.ca; Fax: 1-604-291-3765; Tel: 1-604-291-4887
b Department of Chemistry, McMaster University, Hamilton, ON, Canada L8S 4M1
Received 9th May 2005, Accepted 1st June 2005
First published as an Advance Article on the web 14th June 2005
The dilithio salts of two chelating diamidosilylether lig-
ands undergo a retro-Brook rearrangement from oxygen
to the amido-nitrogen, thereby directly yielding a new
mixed-donor amido/amino/siloxo ligand. When reacted
with chromium(II), siloxo-bridged dimers and a very strong
of silyl groups from oxygen to nitrogen have been reported as
this requires the breaking of a stronger Si–O bond to form an
Si–N bond; a release of steric pressure may be responsible for
enabling this reaction by these amidosilylether ligands.
In order to investigate the fundamental coordination chem-
istry of these potentially tridentate [NNꢀO] ligands, chromium(II)
complexes were prepared; analogous complexes with the sym-
metrical diamidosilylether ligands were also pursued for com-
2
g -(N,Cipso) arylamido interaction are featured.
Ligand design and synthesis is an integral part of coordination
chemistry.1 In particular, mixed-donor ligands have attracted
considerable interest due to their potential to generate electron-
ically non-symmetrical reaction sites.2 Often, the synthesis of
mixed-donor ligands demands the assembly of two or more
building blocks, each of which may also require a multi-step
synthesis in its own right. We have been investigating the chem-
istry of symmetrical diamidosilylether ligands3 and realized that
their retro-Brook rearrangement reaction could be a facile, high-
yield route to a new class of non-symmetrical mixed-donor
ligands. Although Brook-type silyl-migrations have been studied
extensively,4 the value of the anionic products of such reactions
as new ligands has, to our knowledge, not been previously
considered.5
parison. Thus, reaction of Li2[Me PhNON] with CrCl2 resulted in
3
a violet powder, {Cr[Me PhNON]}2 (5).7 The dinuclear structure9
3
(Fig. 1) has two terminal and two bridging amido ligands with
˚
average Cr–N distances of 2.018(6) and 2.096(6) A, respectively.
Completing the distorted square-planar geometry, each central
silylether donor is also bound to a metal, with an average Cr–
˚
˚
O distance of 2.126(6) A. The Cr–Cr distance is 2.384(2) A,
suggesting potential metal–metal bonding.10
Thus, the reaction of (ClSiMe2)2O with a variety of lithium
amides RNHLi yields diamidosilylether ligand precursors of the
n
form {[RNH(SiMe2)]2O}. Deprotonation with BuLi in ether
or hexanes yields the ligands {[RNLi(SiMe2)]2O} (R = 2,4,6-
Me3Ph, [Me PhNON] (1); 2,6-iPr2Ph, [iPr PhNON] (2)).6
3
2
Fig. 1 The molecular structure of 5 (ORTEP, 33% ellipsoids). Phenyl
Scheme 1
˚
groups are simplified for clarity. Selected bond lengths (A) and angles
(◦): Cr1–Cr2 2.384(2), Cr1–N1 2.104(6), Cr1–N2 2.083(7), Cr1–N3
2.016(6), Cr2–N1 2.102(6), Cr2–N2 2.095(6), Cr2–N4 2.019(6), Cr1–O1
2.119(6), Cr2–O2 2.133(6), N2–Cr1–N3 120.1(3), N1–Cr2–N2 91.3(3),
N1–Cr2–N4 122.2(3), N2–Cr2–N4 146.5(3), O1–Cr1–N2 165.3(2),
O2–Cr2–N1 161.2(2), Cr1–N2–Cr2 69.6(2).
However, if the deprotonation is conducted in THF, or
if the isolated Li2[RNON] ligands are stirred in THF, they
undergo an anionic 1,3-silyl retro-Brook rearrangement in
which one silyl group migrates from oxygen to nitrogen, as
shown above. The THF donor likely induces ion-pair sep-
aration, thereby increasing the nucleophilicity of the amido
anion; no rearrangement occurs in non-donor solvents. The
final product of the silyl-rearrangement is a new already-
deprotonated mixed-donor amido/amino/siloxo ligand of
When the equivalent reaction is performed with the re-
arranged amido/amino/siloxo ligand Li2[Me PhNNꢀO], another
3
violet powder, {Cr[Me PhNNꢀO] (THF)}2 (6), was isolated. In this
3
case, the dinuclear structure (Fig. 2) has two bridging siloxo
the form {RNLiSiMe2N(R)SiMe2OLi} (R = 2,4,6-Me3Ph,
ligands with Cr–O bond lengths of 2.0151(16) and 2.0244(15)
Me
3
2
[
PhNNꢀO] (3); 2,6-iPr2Ph, [iPr PhNNꢀO] (4)).7 In general, most
˚
A; the more basic amido-donors remain terminally bound with
silyl-migrations involve 1,2 shifts as opposed to the 1,3-shift
a Cr–N distance of 2.0498(18). The square-planar geometry
around each chromium is satisfied in this case not by the
central amino ligand, which remains unbound (Cr–N distance
occurring in this case.8 More significantly, very few migrations
˚
of 3.341 A) but by one THF molecule. This weak donor can
† Electronic supplementary information (ESI) available: Synthetic pro-
cedures and characterization data for 3–7; X-ray data for 5–7 in CIF
format. See http://dx.doi.org/10.1039/b506322a
be reversibly removed in vacuo, effecting a colour change from
˚
violet to dark green. Also, the Cr–Cr distance of 3.019 A is
T h i s j o u r n a l i s
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 5
D a l t o n T r a n s . , 2 0 0 5 , 2 3 4 3 – 2 3 4 5
2 3 4 3
©

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the shortest and most acute in the literature. Such interactions
may help to stabilize the resting state of amido-supported metal
complexes that are active in alkene polymerization.15
Differences in the Cr(II)-coordination chemistry of the [NON]
and [NNꢀO] ligands are reflected in their magnetic moments.
For 5, leff at 298 K is 2.38 B.M., consistent with signifi-
cant antiferromagnetic coupling through the bridging amido-
ligands, metal–metal bonding or both.16 In contrast, siloxo-
bridged 6 and 7 have leff of 2.88 and 3.80 B.M., respectively
(vs. 4.89 B.M. for an uncoupled system). The longer Cr–Cr
distances in the [NNꢀO] systems may be responsible for these
higher values, but it is also possible that alkoxo ligands are
poorer mediators of magnetic exchange relative to the amido-
bridges.17
In summary, we have illustrated that the well-known retro-
Brook rearrangement can be used as a tool to readily synthesize
in one step a new class of unsymmetrical mixed-donor ligands.
Further generalizations of this reaction and a more detailed
study of the intriguing coordination chemistry and reactivity of
the resulting metal complexes are in progress.
Fig. 2 The molecular structure of 6 (ORTEP, 33% ellipsoids). Phenyl
˚
groups are simplified for clarity. Selected bond lengths (A) and angles
(◦): Cr1–Cr1* 3.019, Cr1–O1 2.0151(16), Cr1–N1 2.0498(18), Cr1–O2
2.0993(17), Cr1–O1–Cr1* 96.73(6).
We are grateful to NSERC Canada (D. B. L.) and Natural
Resources Canada (F. H.) for financial support.
much longer than that observed in 5, perhaps due to the less-
basic nature of the bridging siloxo-ligand in 6 vs. the silylamido
group in 5. Note that the potentially tridentate [Me PhNNꢀO] acts
3
Notes and references
as a bidentate amido-siloxo donor with Cr(II). Thus, differences
in coordination chemistry can readily be observed between the
two related mixed-donor ligands.
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When the more sterically congested ligand 4 is reacted with
CrCl2, a THF-free siloxo-bridged dimer of {Cr[iPr PhNNꢀO]}2 (7)
2
is obtained (Fig. 3). The large steric profile of the [iPr PhNNꢀO]2−
2
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electron Cr(II) centre,11 which is presumably too hindered to
bind the THF solvent. Instead, the ancillary [NNꢀO] ligand
itself is activated by the electron-deficient chromium centre to
˚
generate a Cr–C11ipso bond of 2.287(4) A. The aromaticity in
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˚
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˚
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2
this interaction and can be compared with related g -benzyl
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˚
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3
Ti(III) g -amido complexes14 have been reported but the metrical
2
parameters for the g -(N,Cipso) arylamido bonding in 7 are by far
9 Crystal Data for 5: C44H68Cr2N4O2Si4, M = 901.37, triclinic, space
¯
˚
group P1, a = 11.349(3), b = 11.513◦(3), c = 20.346(4) A, a =
3
˚
105.80(2), b = 95.03(2), c = 103.31(2) , V = 2457.0(11) A , Z =
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6892 unique reflections, 2975 observed (Io > 2.5r(Io)). The final
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˚
a = 11.146(3), b = 27.506(7), c = 22.192(5) A, a = 90, b = 99.630(4),
◦
c = 90 , V = 6708(3) A , Z = 4, qcalcd = 1.178 g.cm−3, l(Mo–
3
˚
Ka) = 0.443 mm−1, T = 173(2) K, 23870 unique reflections, 7682
observed (Io > 2.5r(Io)). The final RF = 0.0451 and RWF = 0.1226
(observed data). Crystal Data for 7: C56H92Cr2N4O2Si4, M = 1069.69,
monoclinic, space group P21/n, a = 9.727(4), b = 15.844(6), c =
◦
3
˚
˚
19.010(7) A, a = 90, b = 100.599(6), c = 90 , V = 2879.7(18) A ,
Z = 2, qcalcd = 1.234 g.cm−3, l(Mo-Ka) = 0.503 mm−1, T = 173(2)
K, 15641 unique reflections, 4955 observed (Io > 2.5r(Io)). The final
RF = 0.0508 and RWF = 0.1113 (observed data). CCDC reference
numbers 254124–254126. See http://dx.doi.org/10.1039/b506322a
for crystallographic data in CIF or other electronic format.
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Fig. 3 The molecular structure of 7 (ORTEP, 50% ellipsoids). Isopropyl
˚
groups are simplified for clarity. Selected bond lengths (A) and angles
(◦): Cr1–Cr1* 2.9298(16), Cr1–O1 1.990(3), Cr1–N1 1.963(3), Cr1–C11
2.287(4), C11–C12 1.425(6), C12–C13 1.369(6), C13–C14 1.389(6),
Cr1–O1–Cr1* 95.68(11), Cr1–N1–C11 84.2(2).
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2 3 4 4
D a l t o n T r a n s . , 2 0 0 5 , 2 3 4 3 – 2 3 4 5

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D a l t o n T r a n s . , 2 0 0 5 , 2 3 4 3 – 2 3 4 5
2 3 4 5