Unusual Equilibria Involving Group 4 Amides, Silyl Complexes, and Silyl Anions via Ligand Exchange Reactions

Article abstract of DOI:10.1021/ja031899y

Silyl anion SiBu^tPh₂^- (2) was found to substitute an amide ligand in Zr(NMe₂)₄ (3) to give the disilyl complex Zr(NMe₂)₃(SiBu^tPh₂)₂^- (

Full text of DOI:10.1021/ja031899y

Published on Web 03/19/2004
Unusual Equilibria Involving Group 4 Amides, Silyl Complexes, and Silyl
Anions via Ligand Exchange Reactions
Xianghua Yu,† Hu Cai,† Ilia A. Guzei,‡ and Ziling Xue*,†
Department of Chemistry, The UniVersity of Tennessee, KnoxVille, Tennessee 37996-1600, and
Department of Chemistry, The UniVersity of Wisconsin, Madison, Wisconsin 53706-1396
Received December 22, 2003; E-mail: xue@utk.edu
Nucleophilic substitution reactions have been one of the primary
methods to prepare transition-metal silyl complexes.1 In many such
reactions, replacement of halide (X-) ligands by silyl anions with
the precipitation of MX (M+ ) alkali metal) is used to yield metal-
silyl bonds, and the reactions are usually not reversible.1 Cleavage of
metal-silyl bonds by the attack of nucleophiles has been reported
+
in, e.g., the reactions of (OC)4Co-SiPh3 with LiAlH4, MeLi, and
RMgBr, affording HSiPh3, “LiSiPh3”, and “BrMg-SiPh3”, respective-
ly.2 “LiSiPh3” and “BrMg-SiPh3” were believed to be the interme-
diates, and they react with H2O to give HSiPh3. ReVersible ex-
changes of ligands3,4 in transition-metal complexes are known, and
equilibria of some reactions have been studied. There are, however,
few reported reVersible exchange reactions and equilibria involving
silyl ligands in transition-metal complexes.1,3,5 In our studies of
metal silyl complexes,6 we observed substitutions of amide ligands
Figure 1. A molecular drawing of 1b-Li2 shown with 30% probability
ellipsoids. Selected bond lengths (Å) and angles (deg): Zr(2)-N(4) 2.243-
(2), N(4)-Zr(2)-N(4A) 172.54(11).10 The structure of the anion (1a)10 is
similar to that reported earlier.6c
When crystals of 1 were dissolved in THF-d8, the reverse reaction
1
in eq 1 was observed in H and 13C NMR spectra of the solution,
yielding 3 and SiButPh2- (2).12 In fact, Zr(NMe2)3(SiButPh2)2- (1a),
Zr(NMe2)5- (1b), 3, and 2 in the solution of 1 were found to be in
an unusual equilibrium in eq 1. This equilibrium was also observed
when 2 and 3 were mixed in THF-d8.10 In 1H EXSY spectrum10 of
a mixture of 2 and 3 at 32 °C, strong cross-peaks were observed
among three -NMe2 peaks and between two -SiButPh2 peaks,
indicating an exchange process for the complexes in eq 1. NMR
studies of 4-Li2 by Chisholm and co-workers suggest that LiNMe2
dissociates from 4-Li2.8a Dissociation of Zr(NMe2)3(SiButPh2)2- (1a)
to give 2 and Zr(NMe2)3(SiButPh2)13 is also known.6c Such
7,8
in M(NMe2)4 and M(NMe2)3[N(SiMe3)2] (M ) Zr, Hf) by silyl
anions to give disilyl and silyl complexes, and M(NMe2)5- and N-
(SiMe3)2-, respectively. These reactions are reversible, and nucleophi-
lic amides attack the M-SiR3 bonds in the reverse reactions, leading
to ligand exchange equilibria such as that in eq 1. Such substitutions
of amide ligands by silyl anions and the exchange equilibria, to
our knowledge, have not been reported. These exchange reactions
and our thermodynamic studies of the equilibria are reported here.
-
dissociations may be present in the reactions in eq 1. NMe2
dissociated from Zr(NMe2)5- (1b) subsequently attacks the Zr-Si
bonds in 1a or in Zr(NMe2)3(SiButPh2) to give Zr(NMe2)4 (3).
Variable-temperature H NMR spectroscopy between 243 and
1
308 K was used to study this equilibrium. The equilibrium constants
K
eq range from 242(18) at 243 K to 6.8(0.5) at 308 K,10 indicating
that the forward reaction to give disilyl 1a and pentaamide 1b is
favored, and that decreasing the temperature shifts the equilibrium
toward them. A plot of ln Keq vs 1/T by the van’t Hoff equation14
(Figure 2a) gives the thermodynamic parameters of the equilibri-
um: ∆H° ) -8.3(0.2) kcal/mol, ∆S° ) -23.3(0.9) eu, and ∆G°298K
) -1.4(0.5) kcal/mol at 298 K.10 The forward reaction is
exothermic, and the entropy change ∆S° ) -23.3(0.9) eu reflects
the fact that, in the forward reaction, 2 equiv of 3 and 2 each yield
1 equiv of 1a and 1b. The enthalpy change outweighs the entropy
change in the forward reaction to give ∆G°298K ) -1.4(0.5) kcal/
mol in favor of 1a and 1b.
Zr(NMe2)4 (3) reacts with Li(THF)2SiButPh2 (2-Li)9 in toluene,
affording an ionic compound [Zr(NMe2)5Li2(THF)4]+ [Zr(NMe2)3-
(SiButPh2)2]- (1) as a solid precipitate (eq 2).10 In this reaction,
substitution of an amide ligand in 3 by silyl anion 2 leads to the
formation of disilyl anion 1a. The amide anion that is replaced
apparently reacts with 3 to give the pentaamide cation Zr(NMe2)5-
+
Li2(THF)4 (1b-Li2+, Figure 1). Chisholm and co-workers have
reported the formation of hexaamide Zr(NMe2)6Li2(THF)2 (4-Li2)
from the addition of LiNMe2 to 3.8a The substitution of an amide
ligand by a silyl ligand in this reaction was unexpected. Substitution
of amide ligands by alkoxide ligands has been reported.11 To our
knowledge, this is the first known case of a replacement of an amide
ligand by a silyl anion.1 Similar to the structure of [Zr(NMe2)4]2,8a
1b-Li2+ exhibits a distorted trigonal bipyramidal configuration about
Zr(2) atom with N(4) and N(4A) atoms in the axial positions.10
Freezing point depression studies of 1 in benzene suggest that
dissolving
1
equiv of [Zr(NMe2)5Li2(THF)4]+ [Zr(NMe2)3-
(SiButPh2)2]- (1) in benzene generates ca. 3 equiv of neutral and/
or ionic species.10,15 This is consistent with the fact that, in solution,
1 is involved in an equilibrium that gives several additional species
including 1a, 1b, 3, Li(THF)n+, 2, and 4. The reaction between
Hf(NMe2)4 (5) and 2-Li gives an equilibrium similar to that in eq
1
1. Overlaps of the H NMR resonances of the -NMe2 ligands in
both THF-d8 and toluene-d8 over a large temperature range prevent
quantitative studies of the equilibrium.
Zr(NMe2)3[N(SiMe3)2] (6) was prepared from the reaction of Zr-
(NMe2)3Cl with LiN(SiMe3)2 (7-Li).10 When LiSiButPh2 (2-Li) was
† The University of Tennessee.
‡ The University of Wisconsin.
9
4472
J. AM. CHEM. SOC. 2004, 126, 4472-4473
10.1021/ja031899y CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
added to Zr(NMe2)3[N(SiMe3)2] (6) in THF-d8, the equilibrium in
eq 3 was observed. In the forward reaction in this equilibrium, the
silyl anion 2 selectiVely replaces the -N(SiMe3)2 ligand in 6,
Cooling the equilibrium mixtures in eqs 4 and 5 to -36 °C,
however, gives crystals of M(NMe2)3[Si(SiMe3)3] (M ) Zr, 10;
Hf, 11), thus shifting the equilibria to the right side.
Studies are underway to probe the scope and kinetics of such
reversible amide-silyl substitutions.
Acknowledgment. Acknowledgment is made to the National
Science Foundation (CHE-0212137), Camille Dreyfus Teacher-
Scholar program, and the Ziegler Research Fund for support of
the research. We thank Prof. Zhenyang Lin’s research group for
helpful discussions.
-
affording exclusively Zr(NMe2)3(SiButPh2)2- (1a) and N(SiMe3)2
(7). No substitution of the -NMe2 ligand in Zr(NMe2)3[N(SiMe3)2]
-
(6) was observed. In the reverse reaction, amide anion N(SiMe3)2
replaces the silyl ligands in 1a or Zr(NMe2)3(SiButPh2), leading to
the equilibrium in eq 3. It is interesting to note that, in the reverse
-
reaction in eq 3, the amide anion N(SiMe3)2 (7) in LiN(SiMe3)2
did not replace the -NMe2 ligand in Zr(NMe2)3(SiButPh2)2- (1a).
Supporting Information Available: Details of the experiments and
calculations of ∆H° and ∆S°, a list of Keq, EXSY spectra, and
crystallographic data for 1 (PDF, CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
It is not clear why the substitutions in the forward and reverse
1
reactions are selective. In H EXSY spectrum10 of a mixture of 2
and 6 at 32 °C, cross-peaks were observed between -NMe2, -SiBut-
Ph2, and -N(SiMe3)2 peaks, respectively, indicating an exchange
process for the complexes in eq 3. Thermodynamic studies of this
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10
ducted. The equilibrium constants Keq range from 10.0(0.2) at
223 K to 3.77(0.09) at 303 K. As in eq 1, the forward reaction in
eq 3, the selective substitution of the amide -N(SiMe3)2 ligand in
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) -2.6(0.5) eu, and ∆G°298K ) -0.8(0.3) kcal/mol. The forward
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-
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Equilibria were also observed in the reactions of M(NMe2)4 (M
) Zr, 3; Hf, 5) and M(NMe2)3[N(SiMe3)2] (M ) Zr, 6; Hf, 8) with
Li(THF)2Si(SiMe3)3 (9-Li),16 yielding the monosilyl complexes
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(10) See Supporting Information for details.
Figure 2. ln Keq vs 1/T plots of the equilibria: (a) eq 1 and (b) eq 3.
the -Si(SiMe3)3 ligand perhaps prevents the formation of disilyl
complexes. In contrast to eq 1, the reverse reactions in eq 4 yielding
-
amides M(NMe2)4 and silyl anion Si(SiMe3)3 (9) dominate the
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equilibria. For example, in THF-d8, Keq ) 0.0045(0.0009) and
∆G°298K ) 3.20(0.13) kcal/mol for the equilibrium involving 3 in
eq 4, and Keq ) 0.0111(0.0009) and ∆G°298K ) 2.66(0.06) kcal/
mol for the equilibrium in eq 5 involving 6. These data indicate
that the formation of silyl complexes M(NMe2)3[Si(SiMe3)3] by
eqs 4 and 5 is not thermodynamically favored.
+
(12) In these THF-d8 solutions, Li+ cations in 1b-Li2 probably exist as free
+
10
Li(THF-d8)4 ions. In addition, our studies of [Zr(NMe2)4]2 in THF-d8
suggest that it exists as the monomeric adduct Zr(NMe2)4(THF-d8)2.10
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