Preparation of zirconium guanidinate complexes from the direct insertion of a carbodiimine and aminolysis using a guanidine. Comparison of the reactions

Article abstract of DOI:10.1021/om300097a

Direct insertion of 1 equiv of CyNCNCy (1; Cy = cyclohexyl) into the Zr-NMe₂ bonds in (Me₂N)₃Zr[N(SiMe ₃)₂] (2) and (Me₂N)₃Zr[Si(SiMe ₃)₃] (3) gave exclusively [CyNC(NMe₂)NCy] Zr(NMe₂)₂[N(SiMe₃)₂] (5) and [CyNC(NMe₂)NCy]Zr(NMe₂)₂[Si(SiMe ₃)₃] (6), respectively. The reaction between 2 and guanidine CyNHC(NMe₂)NCy (9) gave 5 and HNMe₂ through the preferred cleavage of a Zr-NMe₂ bond in 2. The reaction between 3 and 9 led to the preferred cleavage of the Zr-Si(SiMe₃)₃ bond in 3, yielding [CyNC(NMe₂)NCy]Zr(NMe₂)₃ (7) and HSi(SiMe₃)₃ and, upon cleavage of another Zr-NMe ₂ bond, forming [CyNC(NMe₂)NCy]₂Zr(NMe ₂)₂ (8). The aminolysis of Zr(NMe₂)₄ (4) by 9 first afforded 7 and then 8. The structures of 5, 6, and 9 have been determined by X-ray diffraction.

Full text of DOI:10.1021/om300097a

Note
pubs.acs.org/Organometallics
Preparation of Zirconium Guanidinate Complexes from the Direct
Insertion of a Carbodiimine and Aminolysis Using a Guanidine.
Comparison of the Reactions
Shu-Jian Chen,† Brenda A. Dougan,† Xue-Tai Chen,‡ and Zi-Ling Xue*,†
†Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996, United States
‡State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing
210093, People's Republic of China
S
* Supporting Information
ABSTRACT: Direct insertion of 1 equiv of CyNCNCy
(1; Cy = cyclohexyl) into the Zr−NMe2 bonds in (Me2N)3Zr-
[N(SiMe3)2] (2) and (Me2N)3Zr[Si(SiMe3)3] (3) gave
exclusively [CyNC(NMe2)NCy]Zr(NMe2)2[N(SiMe3)2] (5)
and [CyNC(NMe2)NCy]Zr(NMe2)2[Si(SiMe3)3] (6), respec-
tively. The reaction between 2 and guanidine CyNHC(NMe2)NCy (9) gave 5 and HNMe2 through the preferred cleavage of
a Zr−NMe2 bond in 2. The reaction between 3 and 9 led to the preferred cleavage of the Zr−Si(SiMe3)3 bond in 3, yielding
[CyNC(NMe2)NCy]Zr(NMe2)3 (7) and HSi(SiMe3)3 and, upon cleavage of another Zr−NMe2 bond, forming [CyNC-
(NMe2)NCy]2Zr(NMe2)2 (8). The aminolysis of Zr(NMe2)4 (4) by 9 first afforded 7 and then 8. The structures of 5, 6, and 9
have been determined by X-ray diffraction.
ransition-metal guanidinate complexes have been actively
studied in recent years for their unique chemistry and
EXPERIMENTAL SECTION
■
T
Manipulations were performed under dry N2. Solvents were purified
applications.1−3 Complexes containing ancillary guanidinate
ligands are catalysts or precatalysts for a variety of reactions.3 In
chemical vapor deposition (CVD) and atomic layer deposition
(ALD) processes, early-transition-metal amides such as 4 have
been used as precursors to give metal-based materials.4 The
amide complexes are usually oxygen and moisture sensitive.5,6
The introduction of ancillary ligands such as guanidinates
ligands often makes the complexes more stable.2 As shown in
Scheme 1, bidentate anionic guanidinate ligands provide four
by distillation from K/benzophenone ketyl.9
Formation of 5 by Insertion. Hexanes were added to a mixture
of 2 (1.191 g, 3.102 mmol) and 1 (0.640 g, 3.102 mmol). After it was
stirred for 15 h, the solution was filtered, and cooling at −32 °C
overnight gave colorless crystals of 5 (1.556 g, 2.636 mmol, 85%
1
yield). H NMR (benzene-d6, 399.79 MHz, 23 °C): δ 3.20−3.10 (br,
2H, CH), 3.09 (s, 12H, NMe2), 2.40 (s, 6H, CNMe2), 1.85−1.15 (m,
20H, CH2), 0.43 (s, 18H, SiMe3). 13C NMR (benzene-d6, 100.53 MHz,
23 °C): δ 172.89 (CNMe2), 57.04 (CH), 45.10 (NMe2), 40.06
(CNMe2), 35.48, 26.79, 26.28 (CH2), 5.49 (SiMe3). Anal. Calcd for
C25H58N6Si2Zr: C, 50.88; H, 9.91; N, 14.24. Found: C, 50.77; H, 9.91;
N, 14.16.
Scheme 1
Formation of 6 by Insertion. To 3 (1.130 g, 2.400 mmol) in
hexanes was added 1 (0.495 g, 2.400 mmol) in hexanes. After it was
stirred for 18 h, the yellow solution was filtered, and crystallization at
−20 °C gave pale yellow crystals of 6 (1.021 g, 1.507 mmol, 63%
1
yield). H NMR (benzene-d6, 399.79 MHz, 23 °C): δ 3.20−3.10 (br,
2H, CH), 2.97 (s, 12H, NMe2), 2.34 (s, 6H, CNMe2), 1.80−1.10 (m,
20H, CH2), 0.51 (s, 27H, SiMe3). 13C NMR (benzene-d6, 100.54 MHz,
23 °C): δ 172.18 (CNMe2), 57.80 (CH), 40.69 (CNMe2), 40.47
(NMe2), 35.26, 26.44, 26.01 (CH2), 6.07 (SiMe3). Anal. Calcd for
C28H67N5Si4Zr: C, 49.64; H, 9.97; N, 10.34. Found: C, 49.76; H,
10.10; N, 10.17.
Formation of a Mixture of 7 and 8 by Insertion. In an NMR
tube, 4 (0.0269 g, 0.100 mmol) and 1 (0.0206 g, 0.100 mmol) were
dissolved in benzene-d6. After 10 min, 7 and 8 in a molar ratio of ca.
0.6:1 were found along with a white precipitate of 7.10 1H NMR of 7
(toluene-d8, 400.25 MHz, 23 °C): δ 3.13 (s, 18H, NMe2), 3.16−3.08
(br, 2H, CH), 2.48 (s, 6H, CNMe2), 1.79−1.21 (m, 20H, CH2). 13C
electrons to the metal centers. Their zwitterionic resonance
structures7 also contribute to the stability of the complexes.
We reported recently that in the reaction of Ta(NMe2)4[N-
(SiMe3)2] with 1, the complex undergoes elimination of
Me3SiNMe2 to give the imide “Ta(NMe2)3(NSiMe3),”
observed as its dimer [Ta(NMe2)3(μ-NSiMe3)]2.1t 1 captures
“Ta(NMe2)3(NSiMe3)” to give guanidinates Ta-
(NMe2)3−n(NSiMe3)[CyNC(NMe2)NCy]n (n = 1, 2). We
report here the preparation of guanidinates by insertion
reactions between 1 and 2,8a 3,8b or 48c,d and compare the
insertions with aminolysis of 2, 3, or 4 by 9 in order to
understand the nature of the formation of the complexes
containing silyl and amide ligands.
Received: February 6, 2012
Published: April 4, 2012
© 2012 American Chemical Society
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Organometallics
Note
NMR (toluene-d8, 100.65 MHz, 23 °C): δ 172.05 (CNMe2), 56.64
(CH), 43.43 (NMe2), 39.83 (CNMe2), 35.34, 26.82, 26.60 (CH2).
Anal. Calcd for C21H46N6Zr: C, 53.23; H, 9.78; N, 17.74. Found: C,
53.17; H, 9.66; N, 17.42. NMR data of 8 at 35 and 23 °C are given in
ref 1e and the Supporting Information, respectively.
other words, insertion into the Zr−NMe2 bonds in 2 and 3 is
exclusive. The bulkiness of the N(SiMe3)2 and Si(SiMe3)3
ligands perhaps makes the insertion into their bonds with the
Zr atom difficult. 5−7 are thermally stable at 23 °C.
1
The H and 13C{1H} NMR spectra of 5 are consistent with
Preparation of 9. 1 (8.212 g, 0.0398 mmol) in Et2O was added to
freshly prepared LiNMe2 (2.030 g, 0.0398 mmol) suspended in Et2O
with stirring. After 4 h, degassed H2O (0.72 mL, 0.040 mmol) was
slowly added by a syringe to the solution at 0 °C. Stirring at 23 °C for
16 h gave a colorless solution. Et2O was removed in vacuo to give an
oily residue that was extracted by hexanes. The extract was dried by
anhydrous Na2SO4, and hexanes were removed by distillation in a
Kugelrohr apparatus at 85 °C and 6 Torr. Cooling the resulting liquid
to 23 °C gave a few colorless crystals. A white solid formed at −20 °C
overnight. The solid−liquid mixture was pumped at 23 °C for 6 h to
give more crystals. The total weight of the solid and crystals was 8.110
g (0.0391 mmol, 98% yield). 1H NMR (CDCl3, 400.25 MHz, 23 °C):
δ 2.94−2.89 (m, 1H, CH), 2.82−2.77 (m, 1H, CH), 2.63 (s, 6H,
CNMe2), 1.84−0.93 (m, 21H, N-H, CH2). 13C NMR (CDCl3, 100.65
MHz, 23 °C): δ 155.60 (CNMe2), 56.58, 53.18 (CH), 39.08
(CNMe2), 35.39, 34.48, 25.76, 25.60, 25.43, 25.34 (CH2). NMR
data of 9 in benzene-d6 are given in the Supporting Information.
Formation of 5 by Aminolysis. In an NMR tube, 2 (15.3 mg,
0.0398 mmol), 9 (10.0 mg, 0.0398 mmol), and SiMePh3 (NMR
standard) were dissolved in toluene-d8. In 10 min, 5 was obtained in
ca. 100% yield.
Formation of 7 from 3 by Aminolysis. In an NMR tube, 3 (18.8
mg, 0.0398 mmol), 9 (10.0 mg, 0.0398 mmol), and SiMePh3 (NMR
standard) were dissolved in toluene-d8. In 20 min, 7 was obtained in
ca. 100% yield.
Formation of 7 from 4 by Aminolysis. NMR Tube. 9 (10.0
mg, 0.0398 mmol) was added to a solution of 4 (12.9 mg,
0.0482 mmol) and SiMePh3 (NMR standard) in toluene-d8.
The formation of 7 was complete in 5 min (yield of ca. 100%
based on 9).9
its structure.12 Both NMe2 (3.09 ppm) and N(SiMe3)2 (0.43
ppm) resonances are shifted downfield from those of 2 (2.90
and 0.30 ppm).8a In the 13C{1H} NMR spectrum, a peak at
172.89 ppm is assigned to the central C atom in the
CyNC(NMe2)NCy ligand. The 13C{1H}-APT NMR spectrum
shows the −CH− atom in the Cy ring at 57.04 ppm.
In the 1H and 13C{1H} NMR spectra of 6, the resonances of
both NMe2 and Si(SiMe3)2 are shifted downfield from those of
3 (to 2.97 from 2.40 ppm for NMe2; to 0.51 from 0.39 ppm for
Si(SiMe3)2).8b The central C atom in the CyNC(NMe2)NCy
ligand was observed at 172.18 ppm. The 57.80 ppm peak is
assigned to the −CH− atom in the Cy rings, as confirmed by a
13C{1H}-APT NMR spectrum.
NMR spectra of 7 are typical of amide guanidinates. In
comparison to Zr(NMe2)2 (4), its NMe2 peaks are less shifted
than those of the guanidinate 8.9
Synthesis of Guanidine CyNH-C(NMe2)NCy (9) and
Its Aminolysis Reactions Forming 5, 7, and 8. Guanidines
have been usually prepared by the reaction of an amine with an
electrophilic guanylating reagent13 and by catalytic hydro-
amination of carbodiimides3j,k,14 and transamination.3k,15 In the
current work, 9 was prepared through the reaction of H2O with
Li[CyNC(NMe2)NCy] which had been made by the
reaction of 1 with LiNMe2, using a modified procedure.16 In
Et2O, crystals of 9 were obtained in 98% yield. Another major
difference is that 9 was reported to be a liquid,16 and we
obtained it in the pure crystalline state, allowing it to be
characterized by single-crystal diffraction discussed below. We
did not observe the reported N−H peak at 7.40 ppm in our 1H
NMR spectrum.9 The N−H peak may overlap with multiplets
of the Cy groups. The crystals of 9 are sensitive to moisture,
probably because of the hydrophilicity of its N−H bond. When
the crystals were exposed to the air, they became a liquid.
In the aminolysis between 2 and 9, a NMe2 ligand in 2
captures a proton in 9 to give HNMe2 and 5 in nearly 100%
yield (Scheme 3). The aminolysis gives the same product (5) as
Schlenk Flask. 9 (0.999 g, 3.973 mmol) in Et2O was added
dropwise to 4 (1.063 g, 3.973 mmol) in Et2O. After the mixture was
stirred for 5 h, volatiles were removed in vacuo to give an off-white
solid of 7 (1.603 g, 3.383 mmol, 85.1% yield).
RESULTS AND DISCUSSION
■
Formation of 5−7 from Insertion Reactions. 5−7 were
prepared by the insertion of 1 into the Zr−NMe2 bonds in 2−
4, respectively (Scheme 2). In fact, the reaction of 1 equiv of 1
Scheme 2. Preparation of 5−8 through Insertion
Scheme 3. Preparation of 5, 7, and 8 through Aminolysis
that from the insertion of 1 into 2. However, the N(SiMe3)2
ligand in 2 did not undergo aminolysis, indicating that it is
more inert (and perhaps less basic) than the NMe2 ligands in 2.
Unexpectedly, the aminolysis of 3 by 9 gives 7, wherein the
Si(SiMe3)3 ligand in 3 is removed by a proton in 9. In
comparison, the insertion reaction of 3 with 1 gives 6, in which
the Si(SiMe3)3 ligand is retained. The current work suggests
with 4 gave a mixture of 7 and 8 with unreacted 4. Another 1
equiv of 1 inserts into another Zr−NMe2 bond in 7 to give 8. 8
was reported by Lappert and co-workers through a di-insertion
reaction between 1 and 4.10 It is interesting to note that only 1
equiv of 1 inserts into a Zr−NMe2 bond in 2 and 3 to form 5
and 6, respectively. There is no insertion into the Zr−
N(SiMe3)2 bond in 2 or the Zr−Si(SiMe3)3 bond in 3, although
the {CyNC[N(SiMe3)2]NCy}¯ ligand is known1p,q,2a,11 and
insertion into M−N(SiMe3)2 bonds has been reported.11f,h In
that the ligands here may have the following relative strength as
−
bases: [N(SiMe3)2]− < [CyNC(NMe2)NCy]− < NMe2
<
−
Si(SiMe3)3 .
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Organometallics
Note
It is interesting that the insertion reaction between 4 and 1
equiv of 1 unavoidably gives a mixture of 7 and 8. However, the
aminolysis of 4 by 1 equiv of 9 yields only 7 (and HNMe2).
The reaction of another 1 equiv of 9 with 7 gives 8. Thus, the
aminolysis here demonstrates an advantage over the insertion:
it gives a pure product rather than a mixture.
the guanidinate ligand (2.2610(19)−2.269(2) Å) as a result of a
dative bond in the latter.
The crystal structure of 9 (Figure 2) reveals that the NMe2
and one Cy group [C(1)] are trans to each other with respect
X-ray Structures of 5, 6, and 9. The Zr center in 5
(Figure 1) is a distorted square pyramid (SP) with an axial
Figure 2. trans structure in 9 (left) and its crystal structure (right).
Selected bond lengths (Å) and angles (°): C(1)−N(1) = 1.4615(14),
C(7)−N(1) = 1.2850(15), C(7)−N(2) = 1.3799(14), C(7)−N(3) =
1.3997(15); N(1)−C(7)−N(2) = 120.17(10), N(1)−C(7)−N(3) =
127.09(10), N(2)−C(7)−N(3) = 112.73(10).
Figure 1. Structures of 5 (left) and 6 (right). Selected bond lengths
(Å) and angles (°) are as follows. 5: Zr(1)−N(1) = 2.2530(19),
Zr(1)−N(2) = 2.3176(19), Zr(1)−N(4) = 2.143(2), Zr(1)−N(5) =
2.0664(19), Zr(1)−N(6) = 2.0456(19), C(7)−N(1) = 1.346(3),
C(7)−N(2) = 1.338(3), C(7)−N(3) = 1.384(3); N(1)−Zr(1)−N(2)
= 58.63(6), N(4)−Zr(1)−N(1) = 97.45(7), N(4)−Zr(1)−N(2) =
141.36(7), N(5)−Zr(1)−N(1) = 145.19(7), N(5)−Zr(1)−N(2) =
91.69(7), N(5)−Zr(1)−N(4) = 96.16(7), N(6)−Zr(1)−N(1) =
100.11(8), N(6)−Zr(1)−N(2) = 102.40(7), N(6)−Zr(1)−N(4) =
112.09(7), N(6)−Zr(1)−N(5) = 104.14(8). 6: Zr(1)−N(1) =
2.025(2), Zr(1)−N(2) = 2.0147(19), Zr(1)−N(3) = 2.2610(19),
Zr(1)−N(5) = 2.269(2), Zr(1)−Si(1) = 2.9240(19), N(3)−C(20) =
1.337(3), N(4)−C(20) = 1.388(2), N(5)−C(20) = 1.348(3); N(1)−
Zr(1)−Si(1) = 96.12(7), N(2)−Zr(1)−N(1) = 115.24(7), N(1)−
Zr(1)−N(3) = 93.80(8), N(1)−Zr(1)−N(5) = 124.16(7), N(2)−
Zr(1)−N(3) = 98.02(7), N(2)−Zr(1)−N(5) = 116.26(8), N(2)−
Zr(1)−Si(1) = 97.10(6), N(3)−Zr(1)−N(5) = 59.23(6), N(3)−
Zr(1)−Si(1) = 156.30(5), N(5)−Zr(1)−Si(1) = 97.61(5).
to the NC imine bond. In comparison, they are cis to each
other in the guanidinate ligands in 5 and 6. The lone pair
electrons on N(1) is trans to the N(3)(Cy)−H group (and
point away from the N(3)−H bond). The average distance
between N(1) and the H atom (on the N(3) atom) of 2.53(4)
Å and the N(1)−H- - -N(3) angle of 71(3)° suggest there is
little hydrogen-bonding interaction between N(1) and the H
atom.18
The current work shows that, in complexes with NMe2,
N(SiMe3)2, and Si(SiMe3)3 ligands, carbodiimide 1 inserts
exclusively into the Zr−NMe2 bonds. In the reactions of
guanidine CyNHC(NMe2)NCy (9) with the complexes, the
silyl ligand is preferentially removed as HSi(SiMe3)3 by the
proton in 9, followed by the removal of the NMe2 ligand as
HNMe2. The N(SiMe3)2 ligand seems to be most inert of the
three studied in the current work. The results suggest the
aforementioned relative strength of the ligands as bases.
N(6) atom, as indicated by the Addison τ value of 0.06. The τ
parameter17 has been used to describe the degree of distortion
of pentacoordinated structures (τ = 0 for a perfect SP; τ = 1 for
a perfect trigonal bipyramid (TBP)). The N(2)−C(7)−N(1)
angle of 113.05(18)° for the guanidinate ligand is significantly
smaller than 127.09(10)° in the free guanidine 9 discussed
below. This angle is, however, similar to those in other
guanidinate ligands, such as 113.00(16)° in 6. The small angle
in the guanidinate ligand is a result of a rigid four-membered
ring, making room for other large angles around the metal
atom. The bond lengths of Zr(1)−N(1) (2.2530(19) Å) and
Zr(1)−N(2) (2.3176(19) Å) are larger than the other three
Zr−N bond lengths (2.0456(19)−2.143(2) Å), reflecting the
fact that there is a significant contribution of a dative bond
between the guanidinate ligand and the metal atom. The bulky
ligand N(SiMe3)2 and less d−p π bonding between this ligand
and the Zr atom may lead to the Zr(1)−N(4) bond (2.143(2)
Å) being longer than the two Zr−NMe2 bonds (2.0664(19),
2.0456(19) Å).
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and CIF files giving additional experimental
procedures and crystallographic data. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: xue@utk.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the U.S. National Science Foundation (No. CHE-
1012173 to Z.-L.X.) and the Natural Science Grant of China
(No. 21071078 to X.-T.C.) for financial support.
In 6 (Figure 1), the Zr center is better described as a
distorted TBP, as indicated by the Addison τ value of 0.54.17
N(1)−Zr(1)−Si(1) (96.12(7)°), N(2)−Zr(1)−Si(1)
(97.10(6)°), and N(5)−Zr(1)−Si(1) (97.61(5)°) as well as
N(1)−Zr(1)−N(3) (93.80(8)°) and N(2)−Zr(1)−N(3)
(98.02(7)°) angles are close to 90° expected between an axial
and equatorial ligands. As in 5, the two Zr−NMe2 bonds
(2.0147(19)−2.025(2) Å) are shorter than the Zr−N bonds in
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