Mixed alkyl hydrido complexes of zinc: Synthesis, structure, and reactivity

Article abstract of DOI:10.1021/om500190c

The (NNNN)-type macrocycle 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane (Me₃TACD, 1,4,7-Me₃[12]aneN₄) reacted with 1 equiv of ZnEt₂ under ethane elimination to give the mononuclear ethyl complex [(Me₃TACD)ZnEt] (1). Upon treatment of (Me₃TACD)H with 2 equiv of ZnEt₂, the dinuclear complex [(Me ₃TACD)(ZnEt)(ZnEt₂)] (2) was formed, which was converted with an additional 1 equiv of (Me₃TACD)H to 1. Reaction of 1 with PhSiH₃ led to the formation of a tetranuclear ethyl hydrido complex [{(Me₃TACD)ZnEt}₂(ZnEtH)₂] (3). Single-crystal X-ray diffraction study revealed 3 to be a centrosymmetric dimer featuring two [(Me₃TACD)ZnEt] units coordinated to a [Zn(μ-H)₂Zn] core via amido nitrogen atoms of the Me₃TACD ligands. Substitution of the two [(Me₃TACD)ZnEt] units in 3 by N-heterocyclic carbene IMes [1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] gave [(IMes)ZnEtH] ₂ (4b). The mixed alkyl hydrido complexes [(IMes)ZnRH]₂ (R = Me, 4a; Et, 4b) were alternatively synthesized in quantitative yield by reacting [(IMes)ZnR₂] (R = Me, Et) with [(IMes)ZnH₂] ₂ in 2:1 ratio. Methyl complex 4a reacted with CO₂ (p(CO₂) = 0.5 bar) under facile insertion of CO₂ into Zn-H bonds to give dinuclear formate complex [(IMes)ZnMe(O₂CH)] ₂ (5a). Treatment of 4b with CO₂ (p(CO₂) = 0.5 bar) afforded a mixture of di- and trinuclear formate complexes [(IMes)ZnEt(O₂CH)]₂ (5b) and [(IMes)₂Zn ₃Et₃(O₂CH)₃] (6) under elimination of one IMes as CO₂ adduct IMes·CO₂.

Full text of DOI:10.1021/om500190c

Article
pubs.acs.org/Organometallics
Mixed Alkyl Hydrido Complexes of Zinc: Synthesis, Structure, and
Reactivity
Arnab Rit,† Thomas P. Spaniol,† Laurent Maron,*,‡ and Jun Okuda*,†
†Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, 52056 Aachen, Germany
‡Universite
́
de Toulouse, INSA, UPS, CNRS, UMR 5215, LPCNO, 135 Avenue de Rangueil, 31077 Toulouse, France
S
* Supporting Information
ABSTRACT: The (NNNN)-type macrocycle 1,4,7-trimethyl-
1,4,7,10-tetraazacyclododecane (Me3TACD, 1,4,7-Me3[12]-
aneN4) reacted with 1 equiv of ZnEt2 under ethane elimination
to give the mononuclear ethyl complex [(Me3TACD)ZnEt]
(1). Upon treatment of (Me3TACD)H with 2 equiv of ZnEt2,
the dinuclear complex [(Me3TACD)(ZnEt)(ZnEt2)] (2) was
formed, which was converted with an additional 1 equiv of (Me3TACD)H to 1. Reaction of 1 with PhSiH3 led to the formation
of a tetranuclear ethyl hydrido complex [{(Me3TACD)ZnEt}2(ZnEtH)2] (3). Single-crystal X-ray diffraction study revealed 3 to
be a centrosymmetric dimer featuring two [(Me3TACD)ZnEt] units coordinated to a [Zn(μ-H)2Zn] core via amido nitrogen
atoms of the Me3TACD ligands. Substitution of the two [(Me3TACD)ZnEt] units in 3 by N-heterocyclic carbene IMes [1,3-
bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] gave [(IMes)ZnEtH]2 (4b). The mixed alkyl hydrido complexes [(IMes)ZnRH]2
(R = Me, 4a; Et, 4b) were alternatively synthesized in quantitative yield by reacting [(IMes)ZnR2] (R = Me, Et) with
[(IMes)ZnH2]2 in 2:1 ratio. Methyl complex 4a reacted with CO2 (p(CO2) = 0.5 bar) under facile insertion of CO2 into Zn−H
bonds to give dinuclear formate complex [(IMes)ZnMe(O2CH)]2 (5a). Treatment of 4b with CO2 (p(CO2) = 0.5 bar) afforded
a mixture of di- and trinuclear formate complexes [(IMes)ZnEt(O2CH)]2 (5b) and [(IMes)2Zn3Et3(O2CH)3] (6) under
elimination of one IMes as CO2 adduct IMes·CO2.
either decomposition (for CH3) or dismutation/redistribution
(for CH2SiMe3) to give higher clusters.8b The zinc alkyl
hydrido complexes of 1,3-bis(2-pyridylmethyl)acetamidinato
ligand decompose readily at room temperature.8c A more stable
complex A (Chart 1) was obtained by using the bulky
INTRODUCTION
■
Zinc hydrides have been mainly used as reducing agents in
organic synthesis,1 as models for zinc-containing enzymes,2 and
as molecular precursors in material science.3 More recently,
molecular zinc hydrides have received much attention as
nonprecious metal catalysts for silane coupling with protic
substrates,4a,b for hydrogenation of imines,4c and for homoge-
neous hydrosilylation of carbon dioxide,4a ketones, nitriles,4d
and activated alkenes.4e As a result of poor solubility and low
thermal stability of polymeric zinc dihydride (ZnH2)n,5 more
stable zinc hydride derivatives have been sought over the last
decades.6,7 Compounds of general formula [(LX)ZnH] were
stabilized by using chelating monoanionic ligands such as
nacnac,6a,b tris(4,4-dimethyl-2-oxazolinyl)phenylborate,6c [tris-
(2-pyridylthio)methyl],6d and phenolate diamine ligands.6e We
have recently isolated the thermally robust molecular zinc
dihydrides containing neutral NHC ligands of composition
[(L)ZnH2]2 (L = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-
ylidene, IMes; 1,3-bis(2,6-diisopropylphenyl)imidazol-2-yli-
dene, IPr).7
Well-defined zinc complexes that feature both alkyl and
hydrido ligands are rare. Most of the stable neutral derivatives
reported to date contain chelating alkyl ligands such as tris(2-
pyridylthio)methyl,6d HC(SiMe3)2(SiMe2hpp) (hpp =
1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine),8a and
methylidyne-3,3′-bis(N-tert-butylimidazol-2-ylidene).4e Alkyl
hydrido complexes with nonchelating alkyl groups (CH3,
CH2SiMe3) of the di(2-pyridylmethyl)amide ligand undergo
Chart 1. Stable Neutral Complexes of Zinc Featuring
Nonchelating Alkyl and Hydrido Ligands
CH(SiMe3)2 group.8b In 1978, De Koning et al. have reported
the formation of the only known neutral ethyl hydrido complex
of zinc of tentative composition [(py)ZnEtH]3, B (Chart 1), by
treating (ZnH2)n with ZnEt2 in the presence of pyridine in
THF or C6H6. This compound was not crystallographically
characterized.9 No further zinc alkyl hydrides of this type
became known, although the parent [ZnMeH] was charac-
Received: February 22, 2014
Published: April 11, 2014
© 2014 American Chemical Society
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terized in an argon matrix.8d We have found out that the
monoanionic macrocyclic Me3TACD ligand10 is suitable for
stabilizing several lanthanide, early transition, and main group
metal hydrides.11 As most of the zinc hydrides reported to date
are also stabilized by a monoanionic chelating ligand,6 we
became interested in using the Me3TACD ligand to support
zinc hydride species. Herein we describe the synthesis and
characterization of a zinc alkyl hydrido complex containing the
Me3TACD ligand [{(Me3TACD)ZnEt}2(ZnEtH)2]. We also
describe a straightforward synthesis and characterization of
mixed alkyl hydrido complexes [(IMes)ZnRH]2 (R = Me, Et)
stabilized by a IMes ligand, as well as their reactivity toward
carbon dioxide.
aliphatic hydrocarbons. The formation of two different
products 1 and 2 in benzene and n-pentane may be due to
the insolubility of 2 in n-pentane. When the reaction was
performed in n-pentane, 2 precipitated out of the reaction
mixture, preventing further reaction. On the other hand,
solubility of the intermediate 2 in benzene facilitates the
reaction to proceed further at higher temperature to give 1.
Both 1 and 2 were characterized by elemental analysis and
multinuclear NMR spectroscopy.
The 1H NMR spectrum of 1 in C6D6 shows one set of signals
at δ 0.41 (q, 2H) and 1.79 (t, 3H) ppm for an ethyl group, in
agreement with the formulation of a mononuclear complex of
composition [(Me3TACD)ZnEt]. The 1H NMR spectrum of 2
in C6D6 shows two distinct sets for two nonequivalent ethyl
groups at δ 0.73 (q, 4H) and 2.07 (t, 6H) ppm as well as at δ
0.24 (q, 2H) and 1.66 (t, 3H) ppm in 2:1 ratio, corresponding
to the formulation [(Me3TACD)(ZnEt)(ZnEt2)]. A similar
type of bridging by an amide N atom to two metal centers has
been observed previously for metal complexes containing a
Me3TACD ligand11f,12a or the related dianionic 1,7-dimethyl-
1,4,7,10-tetraazacyclododecane (Me2TACD) ligand.12b−d Sev-
eral sets of signals for the CH2 resonances of the Me3TACD
ligand backbone were observed for both complexes 1 and 2 in
the range of δ 1.1−3.0 ppm. The N-Me groups of the
Me3TACD ligand are recorded as two singlets at δ 1.88 and
2.14 ppm for 1 and at δ 1.69 and 1.99 ppm for 2. The DFT
geometry optimization on 1 leads to a monomeric molecule
with a zinc center coordinated by the tetradentate Me3TACD
and a terminal ethyl ligand (see Supporting Information).
Zinc Ethyl Hydrido Complex. Several metal alkyl
complexes of Me3TACD ligands reacted with aromatic silanes
or dihydrogen to form metal hydrides stabilized by the
Me3TACD ligand.11 This makes ethyl complexes 1 and 2
possible precursors for zinc hydride species. Treatment of 2
with PhSiH3 did not lead to any clean reactions. The
mono(ethyl) complex 1 reacted with PhSiH3 in C6H6/Et2O
at room temperature to give a colorless crystalline precipitate of
[{(Me3TACD)ZnEt}2(ZnEtH)2] (3) (Scheme 2). This was
isolated within 30 min in 70% yield (based on Zn) to prevent
further reaction to an unidentified yellow product mixture.
Complex 3 is highly soluble in THF, but only moderately
soluble in aromatic hydrocarbons. In THF solution, the
decomposition of 3 is probably caused by the reduction of
Zn(II) to metallic zinc together with the oxidation of hydride to
dihydrogen. 3 is stable for months as a solid at −35 °C. 3 was
characterized by elemental analysis, NMR spectroscopy, and
single-crystal X-ray diffraction.
RESULTS AND DISCUSSION
■
Zinc Ethyl Complexes. Reaction of 1 equiv of ZnEt2 with
(Me3TACD)H10 in benzene at 60 °C for 1 h afforded the
mononuclear complex [(Me3TACD)ZnEt] (1) (Scheme 1).
Scheme 1. Zinc Ethyl Complexes Supported by a Me3TACD
Ligand
Whereas the treatment of (Me3TACD)H with 1 equiv of ZnEt2
in n-pentane at room temperature give a dinuclear complex
[(Me3TACD)(ZnEt)(ZnEt2)] (2) in less than 50% yield with
0.5 equiv of (Me3TACD)H remaining unreacted, the yield
increased to 90% by using 2 equiv of ZnEt2 (Scheme 1).
Reaction of isolated 2 with an additional 1 equiv of
(Me3TACD)H in benzene at 60 °C for 1 h gave the
mononuclear complex 1 in 98% yield (Scheme 1). While 1 is
soluble in all common polar as well as nonpolar organic
solvents, 2 is soluble only in polar organic solvents but not in
1
The H NMR spectrum of 3 in THF-d8 shows two sets of
signals at δ −0.07 (q) and −0.04 (q) ppm for the CH2 moieties,
whereas resonances for the CH3 moieties of the ethyl groups
Scheme 2. Mixed Zinc Ethyl Hydrido Complex Stabilized by a Me3TACD Ligand
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appear at δ 1.21 ppm. The N-Me groups of the Me3TACD
ligand were identified by two singlets at δ 2.34 and 2.37 ppm.
The CH2 resonances of the Me3TACD ligand backbone were
observed as several sets of broad signals in the range δ 2.3−3.1
ppm. The Zn−H resonance was not reliably assigned due to
overlap with the Me3TACD ligand backbone signals. Two sets
of signals were observed for the Me3TACD ligand in the
13C{1H} NMR spectrum. The νZn−H absorption for 3 could not
be reliably assigned due to overlap with the Me3TACD ligand
stretching modes.
hydrido species from other zinc alkyl complexes, amides were
found to be more reactive toward PhSiH3 than alkyl ligands.8b,c
Mixed Zinc Alkyl Hydrido Complexes of a NHC
Ligand. Due to the higher σ-donor ability of an N-heterocyclic
carbene (NHC)14 as compared to neutral N-donors, it was
expected that the [(Me3TACD)ZnEt] units in 3 can be
replaced by an NHC ligand. 3 reacted with 2 equiv of IMes at
room temperature in C6H6 for 2 h to give the mixed alkyl
hydrido complex [(IMes)ZnEtH]2 (4b) in 66% yield (Scheme
Single crystals were obtained from C6H6/Et2O at room
temperature. As shown in Figure 1, the molecular structure
Scheme 3. Zinc Ethyl Hydrido Complex Stabilized by a NHC
Ligand
3) along with [(Me3TACD)ZnEt] (1), which was detected by
1H NMR spectroscopic analysis of the washing solution. The
sterically more crowded IPr [1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene] ligand did not substitute the [(Me3TACD)-
ZnEt] fragments in 3 under the same condition, probably due
to the steric bulk of IPr compared to that of IMes.
Figure 1. Molecular structure of 3 with displacement parameters at the
50% probability level. Hydrogen atoms except the hydride connected
to zinc are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Zn1−N1 = 2.047(5), Zn1−N2 = 2.259(7), Zn1−N3 =
2.387(6), Zn1−N4 = 2.367(6), Zn2−N1 = 2.064(5), Zn1−C1 =
2.008(9), Zn2−C3 = 2.023(7), Zn2−H1 = 1.66(7), Zn2−H1* =
1.74(7), Zn2···Zn2* = 2.5374(14); Zn2−H1−Zn2* = 97(2), H1−
Zn2−H1* = 83(3), N1−Zn2−C3 = 115(2), N1−Zn2−H1 = 109(2),
H1−Zn2−C3 = 121(2).
7
Comproportionation of the [(IMes)ZnH2]2 with [(IMes)-
ZnR2] (R = Me, Et) turned out to be a more efficient route to
synthesize the mixed alkyl hydrido complexes [(IMes)ZnRH]2
(R = Me, Et). Reaction of [(IMes)ZnH2]2 with 2 equiv of
[(IMes)ZnR2] (R = Me, Et), synthesized from IMes and a 1 M
solution of ZnR2, for 1 h at room temperature in C6H6 readily
gave the complexes [(IMes)ZnRH]2 (R = Me, 4a; R = Et, 4b)
in quantitative yield (Scheme 4).
consists of a centrosymmetric dimer with an inversion center at
the centroid of the Zn2H2 core. The Zn1 centers are
coordinated by one ethyl ligand (X-type) and one tetradentate
amido triamine Me3TACD ligand (L3X-type). The coordina-
tion geometry of the five-coordinated Zn1 centers can be
regarded as either trigonal bipyramidal (considering N2 and N4
in axial positions) or distorted square-based pyramidal. The
distorted tetrahedral Zn2 is surrounded by an amido N1 atom,
one ethyl group, and two bridging hydrido ligands. The
distances between the amido nitrogen N1 and the zinc centers
are significantly shorter (Zn1−N1 = 2.047(5), Zn2−N1 =
2.064(5) Å) than the dative bonds between N2−N4 and the
zinc center (average distance: 2.34 Å). The average Zn−Cethyl
bond length of 2.02 Å is comparable to other zinc ethyl
complexes.4e,13 The average Zn−(μ-H) bond length of 1.70 Å
is comparable to 1.766 Å found in [{HC(CMeNR)2}ZnH]2 (R
= 2,6-Me2C6H3).6a Two zinc atoms Zn2 and Zn2* are bridged
by hydrido ligands via three-center two-electron bonds. The
Zn2···Zn2* distance of 2.5374(14) Å is comparable to the value
of 2.4513(9) Å in [{HC(CMeNR)2}ZnH]2.6a
In the reaction of 1 with PhSiH3, most likely one
[Me3TACD−] unit is first replaced, and the resulting “zinc
ethyl hydrido” fragments are then stabilized via coordination to
the amido N atoms of Me3TACD ligands of another molecule
of the remaining [(Me3TACD)ZnEt] as 3. Replacement of the
[Me3TACD−] unit in 1 by PhSiH3 is indicated by the 1H NMR
spectroscopic analysis of the reaction mixture after precipitation
of 3. The resonances at δ 3.91 (Si−H), 6.98−7.11 (Ph), and
7.53−7.60 (Ph) ppm in THF-d8 correspond to a PhSiH2 group,
suggesting the formation of [(Me3TACD)SiH2Ph], which
could not be isolated in pure form. In the synthesis of alkyl
Most of the isolated stable alkyl hydrido complexes of zinc
reported to date contain chelating alkyl ligands.6d,8a,4e
A
structurally characterized stable alkyl hydrido complex featuring
nonchelating alkyl groups was achieved only by using the bulky
CH(SiMe3)2 group.8b As for [(IMes)ZnH2]2,7 NHC ligand
IMes enabled us to prepare the simplest alkyl hydrido
complexes of type [(L)ZnRH] (R = Me, Et).
Complexes 4a and 4b are highly soluble in all common polar
organic solvents but not in aliphatic hydrocarbons and are
stable at room temperature in C6D6 solution over a week.
Complexes 4a and 4b were fully characterized by elemental
analysis and multinuclear NMR spectroscopy and also by
single-crystal X-ray diffraction for 4b. Exposure of 4a and 4b in
C6D6 to D2 (1 bar) showed no significant H−D exchange at
ambient temperature over 1 week. Other zinc hydride
compounds [(L)ZnH2]2 (L = IMes, IPr) are also found to be
inert.7
The 1H NMR spectra of 4a and 4b in C6D6 show the hydride
signals at δ 2.75 (4a) and 2.78 ppm (4b) along with the typical
pattern for the IMes ligand.7 The methyl group signals of 4a are
found as two broad signals at δ −0.51 and −0.77 ppm. The
broadness of the 1H NMR resonances of 4b and the appearance
of two sets of signals for the alkyl groups in 4a and 4b at 25 °C
might be due to a monomer−dimer equilibrium. In order to
1
obtain some insight, we performed variable-temperature H
NMR studies of 4b in toluene-d8 in the temperature range of
−60 to +75 °C (see Figure S12 in the Supporting Information).
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Scheme 4. Mixed Zinc Alkyl Hydrido Complexes Stabilized by an NHC Ligand
At 75 °C, one set of sharp signals was observed, suggesting the
presence of only the monomer. Gradually cooling the sample to
−60 °C led to a new set of signals with a hydride resonance at δ
2.52 ppm, probably due to the dimer formation. Another set of
signals with a hydride resonance at δ 3.78 ppm was also
observed. Others have observed temperature-dependent
monomer−dimer equilibria for zinc hydride complexes.6b,e In
the 13C{1H} NMR spectra, the characteristic carbene carbon
resonances are found at δ 190.70 (4a) and 191.19 ppm (4b), in
agreement with other NHC compounds of zinc(II).4e,7 The
νZn−H absorptions for 4a and 4b could not be reliably assigned
due to overlap with the aromatic vibrations of the NHC
ligands.7,15
Single crystals suitable for X-ray diffraction were obtained by
slow diffusion of hexane into a saturated solution of 4b in
toluene at room temperature. The molecular structure of 4b
shows a noncrystallographic C2 axis passing through the H1−
H2 vector (Figure 2). The zinc centers are tetrahedrally
coordinated by one ethyl, one IMes, and two bridging hydrido
ligands. The average Zn−C(ethyl) bond length of 2.02 Å is
similar to that found in 3. The average Zn−(μ-H) bond length
of 1.78 Å is comparable to that in 3 (1.70 Å) and
[{HC(CMeNR)2}ZnH]2 (R = 2,6-Me2C6H3) (1.77 Å).6a The
Zn1···Zn2 distance of 2.5066(5) Å is comparable with that in 3
(2.537(14) Å); the Zn−CNHC bond lengths (Zn1−C1
2.084(3), Zn2−C24 2.081(3) Å) are similar to those in
[(IMes)ZnH2]2 (2.052(3)−2.054(3) Å).7
Figure 2. Molecular structure of 4b with displacement parameters at
the 50% probability level. All methyl groups of the mesityl moieties
and hydrogen atoms except hydrides connected to the zinc atom are
omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−
C1 = 2.084(3), Zn2−C24 = 2.081(3), Zn1−C22 = 2.009(3), Zn2−
C45 = 2.024(3), Zn1−H1 = 1.73(3), Zn1−H2 = 1.77(3), Zn2−H1 =
1.82(3), Zn2−H2 = 1.78(3), Zn1···Zn2 = 2.5066(5); Zn1−H1−Zn2 =
90(1), H2−Zn1−H1 = 91.6(12), C1−Zn1−C22 = 117.81(13), C1−
Zn1−H1 = 111.4(8), C1−Zn1−H2 = 103(8).
are reported to react with heterocumulenes and CO2 only at
elevated temperatures to give insertion products,16a−e whereas
[(NHC)ZnCp*(C6F5)] (Cp* = C5Me5) reacts with CO2 at
room temperature to give insertion products.16f Both formates
are soluble in common polar organic solvents, but insoluble in
aliphatic hydrocarbons. They were fully characterized by
elemental analysis, NMR spectroscopy, and single-crystal X-
ray diffraction.
The DFT-optimized geometry of complex 4b matches well
with the experimental one (see Supporting Information). In
particular, the Zn1−C1 and Zn2−C24 distances (2.128 and
2.124 Å computed vs 2.084(3) and 2.081(3) Å experimental)
and the Zn1−C22 and Zn2−C45 bond distances (2.022 and
2.020 Å calculated vs 2.009(3) and 2.024(3) Å experimental)
are well reproduced. The Zn2H2 core is also well represented by
the employed method with calculated Zn−H distances of 1.765
Å (Zn1−H1), 1.784 Å (Zn1−H2), 1.767 Å (Zn2−H1), and
1.786 Å (Zn2−H2) as well as a Zn1···Zn2 distance of 2.535 Å.
Reaction of 4a and 4b with CO2. The mixed alkyl hydrido
complexes 4a and 4b readily reacted with excess CO2 (p(CO2)
= 0.5 bar) at ambient temperature to give formate species 5a
The formate units in 5a are characterized by one broad
1
singlet at δ 7.59 in the H NMR spectrum and by a signal at δ
167.16 ppm in the 13C{1H} NMR spectrum in C6D6. These
chemical shifts are similar to those reported for formate ligands
7
1
in [(IMes)Zn(O2CH)2]2. The H NMR integration ratio of
formate:IMes:Me group of 1:1:1 in 5a agrees with the
formation of a diformate as a result of CO2 insertion into the
Zn−H bonds (Scheme 5).
1
and 5b·6, respectively (Scheme 5). H NMR spectroscopic
analysis showed that all hydrido ligands reacted within 30 min,
but the zinc alkyl bonds remained inert for several days under
these conditions. Zinc alkyls ZnR2 (R = Me, Et) and ZnCp*2
Single crystals of 5a were obtained from Et2O/hexane at
room temperature as colorless blocks. 5a is a dinuclear
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Scheme 5. Reaction of 4a and 4b with CO2
diformate (Figure 3). The zinc centers are unsymmetrically
bridged by one bidentate formate μ2-O2CH-κO,O′ ligand; the
other formate is bridging in the monodentate μ2-OCH(O)-
κO coordination mode in the solid state. A similar coordination
mode where two metal centers are bridged by two formate
ligands was reported before.17c The C−O bond lengths indicate
localized single and double bonds within the η1-formate ligand
(1.303(4) and 1.217(4) Å, respectively). The appearance of a
single resonance in the 1H NMR spectrum of 5a may be due to
the dissociation into a monomeric complex and/or rapid
exchange between both the coordination modes. DFT
calculations showed that the symmetrically bridged (η2−η2)
dimer would be more stable than the experimentally observed
unsymmetrical dimer with (η2−η1) bridging modes by 0.43
kcal/mol (see Supporting Information). This suggests a rapid
exchange between the two coordination modes in solution at
ambient temperature.
The formate units in the product obtained from the reaction
of 4b with CO2 give rise to a broad signal at δ 7.97 ppm in the
1H NMR spectrum and a signal at δ 168.88 ppm in the
1
13C{1H} NMR spectrum. The H NMR integration ratio of
formate:IMes:Et group and the formation of the byproduct
IMes·CO2 via dissociation of a IMes ligand in 4b indicate a
composition different from that of the dinuclear diformate 5a.
To conclusively determine the structure of the product, crystals
of composition 5b·6·C4H10O suitable for X-ray diffraction were
grown from Et2O/hexane at −35 °C. In the solid state, the
dinuclear complex 5b was found to cocrystallize with the
trinuclear complex 6 in 1:1 ratio (Figure 4). Although two
distinct sets of signals corresponding to 5b and 6 were not
Figure 3. Molecular structure of 5a·C4H10O with displacement
parameters at the 50% probability level. All methyl groups of the
mesityl moieties, hydrogen atoms except for those of the formate units,
and the cocrystallized solvent molecule Et2O are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Zn1−C5 = 2.066(3),
Zn2−C26 = 2.066(3), Zn1−C3 = 2.02(3), Zn2−C4 = 1.981(3), C1−
O1 1.303(4), C1−O2 1.217(4), Zn2···O2 2.918(3), Zn1···Zn2 =
3.6784(9); C3−Zn1−C5 = 118.54(14), C3−Zn1−O1 = 112.5(12),
C3−Zn1−O3 = 115.24(13), O1−Zn1−O3 = 98.49(9), O1−C1−O2
= 124.9(4), O3−C2−O4 = 129.4(4).
1
observed in the H NMR spectrum at 25 °C, the integration
ratio of 5:4:5 for the formate:IMes:Et group suggests the
existence of 5b and 6 in 1:1 ratio in solution as well. Cooling
the sample led to the appearance of several sets of signals in the
1H NMR spectra in toluene-d8 (see Figure S17 in the
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Information). On the other hand, the diformate 5b and
triformate 6 differ only by 1.2 kcal/mol in energy at room
temperature, which explains why both species are formed.
CONCLUSION
■
Deprotonation of the (NNNN)-type macrocycle (Me3TACD)-
H by ZnEt2 gives the mono(ethyl) complex [(Me3TACD)-
ZnEt] (1) under ethane elimination. Formation of the ZnEt2
adduct of 1, [(Me3TACD)(ZnEt)(ZnEt2)] (2), indicates that
the amido nitrogen of Me3TACD is rather basic.11f,12 The
mononuclear complex 1 reacted with PhSiH3 under redis-
tribution of Me3TACD to the dimeric ethyl hydrido complex
[{(Me3TACD)ZnEt}2(ZnEtH)2] (3), which can be regarded as
a “[ZnEt(μ-H)]2” unit trapped by the basic amido nitrogen
atoms of the Me3TACD ligands in [(Me3TACD)ZnEt]. IMes
was found to substitute both [(Me3TACD)ZnEt] units in 3 to
give [(IMes)ZnEtH]2 (4b) due to the strong σ-donor property
of NHC.14 The mixed alkyl hydrido complexes 4a and 4b can
also be synthesized in high yields by comproportionation of the
dihydride [(IMes)ZnH2]2 with the dialkyl [(IMes)ZnR2] (R =
Me, Et). 4a and 4b reacted with CO2 to give formate
complexes. The relatively common structural motif of an
unsymmetrical diformate bridge in 5a and 5b was found.7,17c
The formation of the trinuclear complex 6 is explained by the
increased lability of the IMes ligand at the zinc center of 4b
compared to 4a.
Figure 4. Molecular structure of 5b·6·C4H10O with displacement
parameters at the 50% probability level. The ethyl groups are shown
only with their methylene carbon atom. All methyl groups of the
mesityl moieties and hydrogen atoms except for those of the formate
units, as well as the cocrystallized solvent molecule Et2O, are omitted
for clarity. Selected bond lengths (Å) and angles (deg): range Zn−
CNHC = 2.028(5)−2.064(5), range Zn−C(ethyl) = 1.973(5)−2.015(6),
C2−O3 1.295(5), C2−O4 1.209(5), Zn2···O4 3.061(3), Zn1···Zn2 =
3.6498(8) Å, Zn3···Zn5 = 4.1778(11), Zn4···Zn5 = 3.6727(9); C7−
Zn1−C3 = 124.79(19), C7−Zn1−O1 = 104.52(16), C7−Zn1−O3 =
104.04(15), O1−Zn1−O3 = 97.16(13), O2−C1−O1 = 130.7(5),
O3−C2−O4 = 126.1(5).
Supporting Information). Due to similar solubility of 5b and 6,
it was not possible to separate them, but 1:1 cocrystals were
obtained reproducibly from Et2O/hexane. Two bridging
modes, μ2-OCH(O)-κO and μ2-O2CH-κO,O′, were found
for the formate ligands in 5b, whereas another type, μ3-O2CH-
κO,O′, was observed in 6 in the solid state. The first two
bridging modes have been observed previously,7,17 but the μ3-
O2CH-κO,O′ bridging mode in 6 is unprecedented for zinc
formate or acetate complexes. In 6 all zinc atoms are
tetrahedrally coordinated by two formate units, one IMes
ligand, and one ethyl group except for Zn5, which is
coordinated by three formato ligands and one ethyl group.
No bonding interaction of the η1-formate CH(O) ligand
with the zinc center was observed (Zn2···O4 3.061(3) Å);
localized single and double (C−O) bonds of 1.295(5) and
1.209(5) Å, respectively, were observed within the η1-formato
ligand.
The Zn···Zn distances in 5b and 6 are significantly longer
than found in the precursor hydride 4b (2.5066(5) Å). These
vary depending on the formate bridge; for example, Zn1···Zn2
(3.6498(8) Å) and Zn4···Zn5 (3.6727(9) Å) distances, where
zinc centers are bridged by one η1 and one η2 bridging formate,
are significantly shorter than the Zn3···Zn5 distance
(4.1778(11) Å), where zinc centers are bridged by two η2
formates. The Zn−CNHC and Zn−Cethyl bond lengths are
comparable with that in 4b. The only known zinc ethyl formate
complex is [(dmpzm)ZnEt(O2CH)] (dmpzm = bis(3,5-
dimethylpyrazolyl)methane).18
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under argon atmosphere using standard Schlenk or glovebox
techniques. All glassware was dried at 130 °C in an oven overnight.
The solvents used for the synthesis and NMR experiments were dried,
distilled, and degassed by standard methods and stored over 4 Å
molecular sieves. NMR measurements were performed on a Bruker
Avance II 400 or on a Bruker Avance III 400 spectrometer at 25 °C.
1
The chemical shifts in the H NMR spectra were referenced to the
residual proton signals of the deuterated solvents and reported relative
to tetramethylsilane. IR spectra were measured using an AVATAR 380
FT-IR spectrometer. Abbreviations for intensities in the IR spectra: w
(weak), m (medium), and s (strong). The starting materials
(Me3TACD)H,10 PhSiD3,19 IMes,20 [(IMes)ZnEt2],13 and [(IMes)-
7
ZnH2]2 were prepared according to literature procedures. All other
chemicals were purchased from commercial sources and used as
received without further purification. Elemental analyses and mass
spectral analyses were performed in the Microanalytical Laboratory of
this department.
[(Me3TACD)ZnEt] (1). To a solution of 2 (1.432 g, 3.32 mmol) in
benzene (5 mL) was added a solution of (Me3TACD)H (0.710 g, 3.32
mmol) in benzene (1 mL), and the reaction mixture was stirred at 60
°C for 1 h. Then the reaction mixture was evaporated to dryness under
vacuum and washed with a small amount of cold n-pentane; yield: 2.00
g (6.50 mmol, 98%). 1H NMR (400.1 MHz, C6D6, 298 K): δ 0.41 (q,
3
3JHH = 8.2 Hz, 2H, CH2CH3), 1.63−1.70 (m, 4H, CH2), 1.79 (t, JHH
The IR spectrum of 5a (KBr) shows two distinct intense
absorptions for νasy(CO) (HCO2) at 1605 and 1651 cm−1
for the η2 and η1 formate moieties. The IR spectrum of 5b·6
shows a broad intense band for νasy(CO) (HCO2)
absorption at 1608 cm−1, shifted to 1574 cm−1 for the
corresponding 13C isotopomer. The νsy(CO) (HCO2)
absorptions for both 5a and 5b·6 are not reliably assigned
due to overlap with the aromatic stretching modes of the NHC
ligands.
= 8.2 Hz, 3H, CH2CH3), 1.88 (s, 3H, NCH3), 1.95−2.01 (m, 2H,
CH2), 2.14 (s, 6H, NCH3), 2.23−2.31 (m, 4H, CH2), 2.85−3.03 (m,
4H, CH2), 3.45−3.49 (m, 2H, CH2) ppm. 13C{1H} NMR (100.1
MHz, C6D6, 298 K): δ −1.97 (CH2CH3), 15.51 (CH2CH3), 44.12
(NCH3), 46.53 (NCH3), 50.83 (CH2), 54.59 (CH2), 56.27 (CH2),
61.63 (CH2). Anal. Calcd for C13H30N4Zn (307.79 g/mol): C, 50.73;
H, 9.82; N, 18.21. Found: C, 50.43; H, 10.10; N, 17.92.
[(Me3TACD)(ZnEt)(ZnEt2)] (2). To a solution of (Me3TACD)H
(0.856 g, 4.00 mmol) in n-pentane (10 mL) was slowly added a 1 M
solution of ZnEt2 in hexane (8 mL, 8.00 mmol). Then the reaction
mixture was stirred for 1 h at room temperature. A white solid
precipitated during this time, which was isolated by filtration, washed
with n-pentane, and dried under vacuum; yield: 1.548 g (3.59 mmol,
The DFT calculations show that the diformate 5a is more
stable by 13.7 kcal/mol than a hypothetical triformate, related
to 6, which was not experimentally observed (see Supporting
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Organometallics
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90%). 1H NMR (400.1 MHz, C6D6, 298 K): δ 0.24 (q, 3JHH = 8.2 Hz,
2H, CH2CH3), 0.73 (q, 3JHH = 8.2 Hz, 4H, CH2CH3), 1.42−1.48 (m,
2H, CH2), 1.53−1.59 (m, 2H, CH2), 1.63−1.68 (m, 2H, CH2), 1.66
(t, 3JHH = 8.2 Hz, 3H, CH2CH3), 1.69 (s, 3H, NCH3), 1.76−1.82 (m,
2H, CH2), 1.97−2.02 (m, 2H, CH2), 1.99 (s, 6H, NCH3), 2.07 (t, 3JHH
= 8.2 Hz, 6H, CH2CH3), 2.28−2.33 (m, 4H, CH2), 3.09−3.14 (m, 2H,
CH2) ppm. 13C{1H} NMR (100.1 MHz, C6D6, 298 K): δ −0.75
(CH2CH3), 5.67 (CH2CH3), 15.21 (CH2CH3), 15.29 (CH2CH3),
44.24 (NCH3), 44.56 (NCH3), 50.82 (CH2), 53.12 (CH2), 54.16
(CH2), 57.04 (CH2). Anal. Calcd for C17H40N4Zn2 (431.29 g/mol):
C, 47.34; H, 9.35; N, 12.99. Found: C, 44.59; H, 8.50; N, 13.02; a
satisfactory value for carbon was not obtained even after several
attempts. Low values for carbon have been reported previously for
other zinc complexes.21 MS (ESI, positive ions): m/z 307.1842 (calcd
for [M − ZnEt2 + H]+ 307.1840), 277.1338 (calcd for [M − ZnEt2 −
Et]+ 277.1370).
for 3 h at room temperature. The mixture was concentrated to 1 mL,
and hexane (3 mL) was added. The precipitate was isolated by
filtration, washed with hexane, and dried in vacuo; yield: 0.026 g
(0.033 mmol, 66%). Method B: To a solution of [(IMes)ZnEt2]
(0.170 g, 0.40 mmol) in benzene (2 mL) was added a solution of
[(IMes)ZnH2]2 (0.149 g, 0.20 mmol) in benzene (2 mL). After
stirring for 1 h at ambient temperature, the reaction mixture was
evaporated to dryness in vacuo. The colorless residue was washed with
n-pentane (2 mL) and dried under vacuum; yield: 0.312 g (0.39 mmol,
1
97%). H NMR (400.1 MHz, C6D6, 298 K): δ 0.01−0.40 (br m, 4H,
CH2CH3), 1.42−1.61 (br m, 6H, CH2CH3), 2.10 (br s, 24H, o-CH3),
2.19 (br s, 12H, p-CH3), 2.78 (br s, 2H, Zn-H), 6.08 (br s, 4H, Im-H),
6.73 (br s, 8H, m-Ph). 13C{1H} NMR (100 MHz, C6D6, 298 K): δ
1.54 and 2.17 (CH2CH3), 16.58, 16.83 (CH2CH3), 18.58 (o-CH3),
21.55 and 21.60 (p-CH3), 121.77 (Im-C), 121.93 (Im-C), 129.42 (m-
Ph), 135.90 (o-Ph), 136.76 (i-Ph), 138.01 (p-Ph), 191.19 (carbene-C)
ppm. IR (KBr): The νZn‑H absorptions were not reliably assigned due
to overlap with the aromatic vibration of the ligands. Anal. Calcd for
C46H60N4Zn2 (799.94 g/mol): C, 69.08; H, 7.56; N, 7.01; Zn, 16.35.
Found: C, 68.83; H, 7.42; N, 6.95; Zn, 15.92.
[{(Me3TACD)ZnEt}2(ZnEtH)2] (3). To a solution of 1 (0.369 g,
1.20 mmol) in 10 mL of Et2O/benzene (4:1) was slowly added a
solution of PhSiH3 (0.130 g, 1.20 mmol) in Et2O (2 mL). Colorless
crystals started to precipitate. After stirring the reaction mixture for 30
min at room temperature, the precipitate was isolated, washed with
Et2O/benzene (5:1), and dried under vacuum; yield: 0.169 g (0.21
Reaction of 4a with CO2. A solution of 4a (0.046 g, 0.06 mmol)
in 2 mL of benzene was pressurized with 0.5 bar of CO2 at room
temperature for 10 min, resulting in the precipitation of a small
amount of colorless solid within 2 h. The solution was filtered, and the
filtrate was evaporated to dryness to give a colorless compound, 5a;
yield: 0.043 g (0.05 mmol, 83%). Crystals of 5a suitable for X-ray
diffraction analysis were grown by layering hexane over a saturated
1
mmol, 70% based on Zn). H NMR (400.1 MHz, THF-d8, 298 K): δ
−0.07 (q, 3JHH = 8.2 Hz, 4H, CH2CH3), −0.04 (q, 3JHH = 8.0 Hz, 4H,
CH2CH3), 1.21 (t, 3JHH = 8.1 Hz, 12H, CH2CH3), 2.32−2.39 (m, 4H,
CH2), 2.34 (s, 6H, NCH3), 2.37 (s, 12H, NCH3), 2.48−2.79 (30H,
CH2 and Zn−H, was not reliably assigned), 3.02−3.09 (m, 4H, CH2)
ppm. 13C{1H} NMR (100.1 MHz, THF-d8, 298 K): δ −1.17
(CH2CH3), 4.32 (CH2CH3), 14.88 (CH2CH3), 44.51 (br, NCH3),
44.55 (NCH3), 45.29 (br, NCH3), 45.40 (br, NCH3), 51.51 (CH2),
52.82 (br, CH2), 52.99 (CH2), 53.21 (br, CH2), 54.92 (CH2), 55.62
(CH2), 57.92 (CH2), 58.97 (br, CH2). IR (KBr): The νZn‑μ‑H
absorptions could not be reliably assigned. Anal. Calcd for
C30H72N8Zn4 (806.476 g/mol): C, 44.68; H, 9.00; N, 13.90. Found:
C, 44.08; H, 8.98; N, 13.90.
1
solution of 5a in Et2O at room temperature. H NMR (400.1 MHz,
C6D6, 298 K): δ −0.88 (s, 6H, CH3), 2.08 (s, 24H, o-CH3), 2.11 (s,
12H, p-CH3), 5.97 (s, 4H, Im-H), 6.76 (s, 8H, m-Ph), 7.59 (br s, 2H,
OCHO). 13C{1H} NMR (100 MHz, C6D6, 298 K): δ −12.86 (br,
CH3), 18.19 (o-CH3), 21.43 (p-CH3), 122.38 (Im-C), 129.70 (m-Ph),
135.95, 136.06, 139.14, 167.16 (OCHO), 182.68 (carbene-C) ppm. IR
data (KBr): 3165 (w), 3140 (w), 2954 (w), 2922 (m), 2859 (w), 2820
(w), 1651 (s, νasy(CO), HCO2), 1605 (s, νasy(CO), HCO2), 1488
(s), 1461 (w), 1401 (w), 1380 (m), 1363 (m), 1276 (w), 1259 (m),
1233 (s), 1161 (w), 1099 (m), 1035 (m), 931 (m), 852 (s), 790 (w),
754 (m), 743 (w), 734 (w), 698 (w), 641 (m), 571 (m), 509 (m)
cm−1. Anal. Calcd for C46H56N4O4Zn2 (859.708 g/mol): C, 64.26; H,
6.57; N, 6.52; Zn, 15.21. Found: C, 63.56; H, 6.78; N, 6.03; Zn, 15.41.
Reaction of 4b with CO2. A solution of 4b (0.048 g, 0.06 mmol)
in benzene (3 mL) was pressurized with 0.5 bar of CO2 at room
temperature for 10 min, which resulted in the precipitation of colorless
crystals. After 2 h, the crystals were isolated, washed with 1 mL of
benzene, and dried under reduced pressure to give IMes·CO2,
identified by comparing NMR spectroscopic data with those reported
in the literature.23 The combined filtrates were evaporated to dryness,
triturated with n-pentane, and dried under vacuum to give a colorless
compound; yield: 0.038 g (0.02 mmol, 83% based on Zn). Crystals
suitable for X-ray diffraction were grown by layering hexane over a
[{(Me3TACD)ZnEt}2(ZnEtD)2] (3-D2). This compound was pre-
pared using 1 (0.062 g, 0.20 mmol) and PhSiD3 (0.022 g, 0.20 mmol)
instead of PhSiH3 and obtained in 64% yield based on zinc (0.026 g,
0.032 mmol). IR data (KBr): The νZn‑μ‑D absorptions could not be
reliably assigned.
[(IMes)ZnMe2] (ref 22). To a solution of IMes (0.182 g, 0.60
mmol) in toluene (2 mL) was added a 1.2 M solution of ZnMe2 in
toluene (0.5 mL, 0.60 mmol). After stirring for 2 h at ambient
temperature, the reaction mixture was evaporated to dryness. The
residue was washed with cold n-pentane (1 mL) and dried in vacuo;
1
yield: 0.228 g (0.57 mmol, 95%). H NMR (400.1 MHz, C6D6, 298
K): δ −0.49 (s, 6H, CH3), 2.05 (s, 6H, p-CH3), 2.06 (s, 12H, o-CH3),
6.11 (s, 4H, Im-H), 6.70 (s, 8H, m-Ph). 13C{1H} NMR (100 MHz,
C6D6, 298 K): δ −8.0 (CH3), 17.94 (o-CH3), 21.38 (p-CH3), 121.82
(Im-C), 129.79 (m-Ph), 135.48 (o-Ph), 135.92 (i-Ph), 139.45 (p-Ph),
192.15 (carbene-C) ppm.
1
saturated solution of 5b and 6 in Et2O at −35 °C. H NMR (400.1
[(IMes)ZnMeH]2 (4a). To a solution of [(IMes)ZnMe2] (0.200 g,
0.50 mmol) in benzene (4 mL) was added a solution of
[(IMes)ZnH2]2 (0.186 g, 0.25 mmol) in benzene (4 mL). After
stirring for 1 h at ambient temperature, the reaction mixture was
evaporated to dryness in vacuo. The residue was washed with n-
pentane (3 mL) and dried under vacuum; yield: 0.376 g (0.49 mmol,
98%). 1H NMR (400.1 MHz, C6D6, 298 K): δ −0.51 (br s, 3H, CH3),
−0.77 (br s, 3H, CH3), 2.06 (s, 24H, o-CH3), 2.21 (s, 12H, p-CH3),
2.75 (br s, 2H, Zn-H), 6.07 (br s, 4H, Im-H), 6.69 (br s, 8H, m-Ph).
13C{1H} NMR (100 MHz, C6D6, 298 K): δ −12.32 (CH3), −8.03
(CH3), 18.47 (p-CH3), 21.60 (o-CH3), 121.76 (Im-C), 129.42 (m-Ph),
135.94 (o-Ph), 136.86 (i-Ph), 137.90 (p-Ph), 190.70 (carbene-C) ppm.
IR (KBr): νZn‑H absorptions were not reliably assigned due to overlap
with the aromatic stretching frequencies of the ligands. Anal. Calcd for
C44H56N4Zn2 (771.69 g/mol): C, 68.48; H, 7.31; N, 7.26; Zn 16.94.
Found: C, 68.37; H, 7.29; N, 7.20; Zn, 17.34.
MHz, C6D6, 298 K): δ 0.08−0.24 (br m, 8H, CH2CH3), 1.12 (t, 6H,
3
(CH3CH2)2O), 1.42 (br t, JHH = 7.3 Hz, 12H, CH2CH3), 2.06 (s,
48H, o-CH3), 2.09 (s, 24H, p-CH3), 3.27 (q, 4H, (CH3CH2)2O), 5.98
(s, 8H, Im-H), 6.76 (s, 16H, m-Ph), 7.97 (br s, 5H, OCHO). 13C{1H}
NMR (100 MHz, C6D6, 298 K): δ 1.29 (br, CH2CH3), 13.99
(CH2CH3), 18.01 (o-CH3), 21.39 (p-CH3), 122.56 (Im-C), 129.73
(m-Ph), 135.77 (o-Ph and i-Ph), 139.55 (p-Ph), 168.88 (OCHO),
181.59 (carbene-C) ppm. IR (KBr): 3158 (w), 3130 (w), 2922 (m),
2882 (m), 2841 (m), 2726 (w), 1608 (br s, νasy(CO), HCO2), 1488
(m), 1462 (w), 1400 (w), 1378 (m), 1364 (m), 1318 (w), 1276 (w),
1236 (m), 1162 (w), 1107 (w), 1036 (m), 1016 (w), 984 (w), 964
(w), 931 (m), 851 (s), 786 (w), 757 (m), 697 (w), 601 (m), 575 (m),
502 (m) cm−1. Anal. Calcd for C99H126O10N8Zn5 (1914.98 g/mol): C,
62.09; H, 6.63; N, 5.85; Zn, 17.07. Found: C, 62.87; H, 6.44; N, 5.79;
Zn 17.55.
Reaction of 4b with 13CO2. This compound was prepared using
4b (0.040 g, 0.05 mmol) and 13CO2 (0.5 bar) instead of CO2 and
isolated as a mixture of 5b-(13C)2 and 6-(13C)3 in 80% yield based on
Zn (0.031 g, 0.016 mmol). IR (KBr): 3158 (w), 3129 (w), 2922 (m),
[(IMes)ZnEtH]2 (4b). Method A: To a suspension of 3 (0.040 g,
0.05 mmol) in benzene (2 mL) was slowly added IMes (0.037 g, 0.12
mmol) dissolved in benzene (1 mL). The reaction mixture was stirred
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2883 (w), 2843 (m), 2732 (w), 1574 (br s, νasy(13CO), H13CO2),
1488 (m), 1462 (w), 1405 (m), 1379 (m), 1344 (m), 1296 (w), 1237
(m), 1163 (w), 1110 (w), 1082 (w), 1036 (m), 1016 (w), 982 (w),
964 (w), 931 (m), 852 (s), 753 (m), 699 (w), 602 (m), 575 (m), 507
(m) cm−1.
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ACS Catal. 2011, 1, 698−702. (c) Jochmann, P.; Stephan, D. W.
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X-ray Crystallography. Data were collected on a Bruker CCD
area-detector diffractometer with Mo Kα radiation (monolayer optics,
λ = 0.71073 Å) using ω scans.24a The SMART program package was
used for the data collection and unit cell determination; processing of
the raw frame data was performed using SAINT; absorption
corrections were applied with SADABS (3, 4b, and 5b·6) or MULABS
(5a).24b,c The structures were solved by direct methods (SIR-92)24d
and refined against F2 using all reflections with the SHELXL-97 as
implemented in the WinGX program system.24e,f Non-hydrogen atoms
were refined anisotropically. All hydrogen atoms were placed in
calculated positions and treated as riding except for the hydride atoms
H1 in 3 and H1 and H2 in 4b as well as the hydrogen atoms H1 and
H2 of the formate unit in 5a that were located in difference Fourier
maps and refined in their position. Compound 3 shows crystallo-
graphic inversion symmetry. Complexes 5a and 5b·6 crystallized with
an additional diethyl ether in the lattice. The latter was included with
split positions and isotropic displacement parameters in 5a (due to
disorder). Distance restraints were used for this molecule. A distance
restraint was also used for the refinement of the bond C1−C2 in 3.
The graphical representations were performed with the program
DIAMOND.24g
Computational Details. Zinc was treated with a Stuttgart−
Dresden pseudopotential in combination with its adapted basis set.25
The basis set has been augmented by a set of polarization functions (f
for Zn).26 All other atoms have been described with a 6-31G(d,p)
double-ζ basis set.27 Calculations were carried out at the DFT level of
theory using the hybrid functional B3PW91.28,29 Geometry
optimizations were carried out without any symmetry restrictions,
and the nature of the extrema (minimum) was verified with analytical
frequency calculations. All computations have been performed with
the Gaussian 0330 suite of programs. The bonding situation was
analyzed using the NBO technique.31
(6) (a) Hao, H.; Cui, C.; Roesky, H. W.; Bai, G.; Schmidt, H.-G.;
Noltemeyer, M. Chem. Commun. 2001, 1118−1119. (b) Spielmann, J.;
Piesik, D.; Wittkamp, B.; Jansen, G.; Harder, S. Chem. Commun. 2009,
3455−3456. (c) Mukherjee, D.; Ellern, A.; Sadow, A. D. J. Am. Chem.
Soc. 2010, 132, 7582−7583. (d) Sattler, W.; Parkin, G. J. Am. Chem.
Soc. 2011, 133, 9708−9711. (e) Brown, N. J.; Harris, J. E.; Yin, X.;
Silverwood, I.; White, A. J. P.; Kazarian, S. G.; Hellgardt, K.; Shaffer,
M. S. P.; Williams, C. K. Organometallics 2014, 33, 1112−1119.
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2013, 52, 5228−5246.
(8) (a) Coles, M. P.; El-Hamruni, S. M.; Smith, J. D.; Hitchcock, P. B.
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ASSOCIATED CONTENT
* Supporting Information
■
S
1H and 13C{1H} NMR spectra of complexes 1−6; DFT-
optimized structure of [(Me3TACD)ZnEt] (1) and Cartesian
coordinates of the calculated structures; crystallographic data
and refinement details for 3, 4b, 5a·C4H10O, and 5b·6·C4H10O;
CIF files giving crystallographic data for compounds 3, 4b, 5a·
C4H10O, and 5b·6·C4H10O. This material is available free of
charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: laurent.maron@irsamc.ups-tlse.fr.
*E-mail: jun.okuda@ac.rwth-aachen.de.
■
Notes
The authors declare no competing financial interest.
(15) (a) Zhu, Z.; Wright, R. J.; Olmstead, M.; Rivard, M. E.; Brynda,
M.; Power, P. P. Angew. Chem., Int. Ed. 2006, 45, 5807−5810. (b) Zhu,
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ACKNOWLEDGMENTS
■
We are grateful to the Deutsche Forschungsgemeinschaft for
financial support by the International Research Training Group
(GRK 1628). L.M. thanks the Alexander von Humboldt
Foundation for a fellowship.
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