Zinc complexes of bipyrrolidine-based diamine-diphenolato and diamine-diolato ligands: Predetermination of helical chirality around tetrahedral centres

Article abstract of DOI:10.1002/ejic.201300151

The coordination chemistry of chiral tetradentate dianionic ligands of the diamine-diphenolato and diamine-diolato families having the (R,R)-2,2′- bipyrrolidine core around Zn^II was investigated. Reactions with diethylzinc led to complexes of the type [{ONNO}Zn] for most ligands and to bridging dinuclear complexes of the type [μ-Lig⁴(ZnEt) ₂] for one of the diamine-diolato ligands. Reactions with bis(hexamethyldisilazide)zinc led to complete conversions. Most complexes were obtained as mononuclear complexes. The least bulky Salan ligand, Lig ³, led to an equilibrium between mononuclear and dinuclear complexes in noncoordinating solvents. All ligands were found to wrap around the tetrahedral zinc with very high diastereoselectivities supporting predetermined chiral induction from the bipyrrolidine core to the helical ligand wrapping. Molecular structures determined by single-crystal X-ray diffraction for two complexes of the type [{ONNO}Zn] substantiated the predicted Δ wrapping of the (R,R)-based ligands. In contrast, representative Salan and diamine-diolato ligands assembled around the trans-1,2-diaminocyclohexane core led to diastereomer mixtures of the corresponding complexes. Chiral tetradentate dianionic ligands of the diamine-diphenolato and diamine-diolato families based on the (R,R)-2,2′-bipyrrolidine core led to chiral-at-metal complexes of the type [{ONNO}Zn] as single diastereomers supporting a predetermined chiral induction from the bipyrrolidine core to the helical ligand wrapping. Copyright

Full text of DOI:10.1002/ejic.201300151

FULL PAPER
DOI:10.1002/ejic.201300151
Zinc Complexes of Bipyrrolidine-Based Diamine-
Diphenolato and Diamine-Diolato Ligands:
Predetermination of Helical Chirality Around Tetrahedral
Centres
Ekaterina Sergeeva,[a] Konstantin Press,[a] Israel Goldberg,[a] and
Moshe Kol*[a]
Keywords: Zinc / Chirality / Tetradentate dianionic ligands / {ONNO} ligands / Salan ligands
The coordination chemistry of chiral tetradentate dianionic
ligands of the diamine-diphenolato and diamine-diolato fam-
ilies having the (R,R)-2,2Ј-bipyrrolidine core around ZnII was
investigated. Reactions with diethylzinc led to complexes of
the type [{ONNO}Zn] for most ligands and to bridging dinu-
clear complexes of the type [μ-Lig4(ZnEt)2] for one of the di-
amine-diolato ligands. Reactions with bis(hexamethyldisilaz-
ide)zinc led to complete conversions. Most complexes were
obtained as mononuclear complexes. The least bulky Salan
ligand, Lig3, led to an equilibrium between mononuclear and
dinuclear complexes in noncoordinating solvents. All ligands
were found to wrap around the tetrahedral zinc with very
high diastereoselectivities supporting predetermined chiral
induction from the bipyrrolidine core to the helical ligand
wrapping. Molecular structures determined by single-crystal
X-ray diffraction for two complexes of the type [{ONNO}Zn]
substantiated the predicted Δ wrapping of the (R,R)-based
ligands. In contrast, representative Salan and diamine-diol-
ato ligands assembled around the trans-1,2-diaminocyclo-
hexane core led to diastereomer mixtures of the correspond-
ing complexes.
demonstrate for the first time that tetradentate dianionic
{ONNO}-type ligands assembled around the 2,2Ј-bipyrroli-
dine core wrap in a highly diastereoselective helical manner
around tetrahedral centres, zinc being a representative
metal, to give enantiomerically pure chiral-at-metal com-
plexes.
Introduction
The synthesis of enantiomerically pure “chiral-at-metal”
complexes is a topic of major current interest with implica-
tions in the fields of asymmetric catalysis, materials science
and pharmacology.[1–5] For relatively labile metals, the most
general approach for attaining chirality at the metal relies
on the helical wrapping of a chelating ligand (or chelating
ligands). Helically wrapping achiral ligands lead to racemic
chiral complexes which need to be resolved should they be
required in their enantiomerically pure form.[6–8] A more
efficient method relies on chiral chelating ligands that are
able to wrap around the metal in a highly diastereoselective
manner and which should be available in their enantiomer-
ically pure form.[9–12] However, such high chiral induction
from ligand backbone to the helical sense of ligand wrap-
ping depends on the design of selective chiral cores and
is still scarce. For example, trans-1,2-diaminocyclohexane,
which is a successful chiral core for nonhelically wrapping
ligands like the {ONNO}-type Salens,[13] has led to unpre-
dictable and diverse diastereoselectivities in helically wrap-
ping ligands like the {ONNO}-type Salans.[14] Herein, we
Chiral 2,2Ј-bipyrrolidine and its derivatives have been in-
vestigated in organocatalysis[15] and as chiral cores for neu-
tral ligands[16–18] and yet the unique ability of 2,2Ј-bipyr-
rolidine to direct a diastereoselective helical wrapping of
multidentate ligands has been somewhat overlooked. A sim-
ple “ball-and-stick” model shows that converging of the
electron pairs of the two nitrogen atoms (namely, chelating
to a metal) requires that they emerge from specific faces of
the pyrrolidine rings. Arms that lead to the peripheral do-
nors of the multidentate ligands should emerge from the
opposite faces of the pyrrolidine rings, thus favouring a spe-
cific helical wrapping around a metal. Recently, we intro-
duced the first two families of tetradentate dianionic
{ONNO}-type ligands that feature the chiral bipyrrolidine
backbone: the diamine-diphenolato (Salans)[19] and the di-
amine-diolato families.[20] When such ligands chelate to an
octahedral metal centre in a fac-fac helical mode,[21] the bi-
pyrrolidine motif exhibits a dramatic preference for forma-
tion of one of the possible diastereomers: an (R,R) tetra-
dentate ligand wraps in a Δ manner whereas an (S,S) ligand
wraps in a Λ manner as can be readily predicted by the
[a] School of Chemistry, Raymond and Beverly Sackler Faculty of
Exact Sciences, Tel Aviv University,
Ramat Aviv, Tel Aviv 69978, Israel
Fax: +972-3-6409293
E-mail: moshekol@post.tau.ac.il
Homepage: http://tau.ac.il/chemistry/kol/
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molecular models and supported by molecular structures
determined by X-ray diffraction. In the current work, we
set out to explore the coordination chemistry of these two
ligand families around zinc(II). Specifically, we wished to
explore whether the same high degree of (predictable) dia-
stereoselectivity in helical ligand wrapping found for octa-
hedral complexes would also be found for tetrahedral com-
plexes of the [{ONNO}Zn]-type, despite the difference in
helical pitch in these two geometries.
The coordination chemistry of tetradentate dianionic
{ONNO}-type ligands around zinc is well-developed and
revolves mostly around Salen ligands. Salen zinc complexes
have found various applications including CO2 fixation,[22]
asymmetric catalysis,[23] supramolecular assemblies[24] and
fluorescence-based detection.[25] The tendency of the (two-
carbon bridge) Salen ligands to wrap in a planar form con-
flicts with the tendency of the closed shell ZnII to adopt
a tetrahedral geometry. Therefore, Salen-Zn complexes are
often found to increase their coordination number by either
O-bridging or binding of an additional ligand.[26] Salan zinc
complexes have been explored to a lesser extent.[27] Non-
bulky Salans tend to adopt a bridging dinuclear structure[28]
but adding a bulky substituent at the ortho-position of the
phenolato group leads to mononuclear tetrahedral com-
plexes.[27a,29] Relevant to the present work, enantioselective
aryl transfer to aldehydes with catalytic bipyrrolidine-based
Salans[19] in the presence of a large excess of diethylzinc was
recently reported.[30] However, no description of a presum-
ably formed zinc complex was given.
Scheme 1. Bipyrrolidine-based {ONNO}-type diamine-diphenolato
and diamine-diolato ligand precursors described in this work, and
their respective syntheses.
room temp. resulted in the formation of undefined prod-
ucts, according to 1H NMR spectroscopy. However, ex-
tending the reaction time to 24 h resulted in selective forma-
tion of complexes of the form [{ONNO}Zn] for the bulky
1
ligands Lig1 and Lig2. The H NMR spectra supported the
formation of single diastereomers of C2 symmetry: two nar-
row doublets in the aromatic region and a single AB system
at δ ≈ 3.8 and ≈ 2.7 ppm for the enantiotopic protons of the
methylene groups of these two complexes. This is consistent
with perfect chiral induction from ligand to metal. Single
crystals of the complex [Lig1Zn] were obtained from diethyl
ether at –35 °C and the molecular structure was determined
by X-ray diffraction. The ORTEP representation and se-
lected bond lengths and angles are outlined in Figure 1. The
structure reveals a tetracoordinate mononuclear zinc com-
plex having crystallographic C2 symmetry. The (R,R)-con-
figuration of the bipyrrolidine core was established and the
helical screw-sense of the {ONNO} ligand around the zinc
centre was found to be Δ. This is equivalent to the screw
sense that was found for wrapping the bipyrrolidine-based
Salan ligands around octahedral group 4 metals [(S,S)-bi-
pyrrolidine based Salans were found to wrap in a Λ helical
sense in that report].[19] Hence, the formation of a chelate
Results and Discussion
The six chiral {ONNO}H2 ligand precursors employed
in this study are outlined in Scheme 1. All ligands are based
on the (R,R)-2,2Ј-bipyrrolidine backbone and are C2-sym-
metric having identical anionic phenolato or alcoholato
arms. Lig2H2 is described herein for the first time while the
other ligands have been described previously in coordina-
tion chemistry of group 4 metals.[19,20] Ligand precursors
Lig1–3H2 are diamine-diphenolato(Salan) type ligands
which may form 6–5-6 chelate arrays if all donor atoms
bind to a given metal atom. Lig1H2 and Lig2H2 bear bulky
alkyl substituents ortho to the phenolato oxygen. Lig3H2
includes small electron-withdrawing chloro substituents in
the ortho positions. Ligand precursors Lig4–6H2 are di-
amine-diolato type ligands, featuring two phenyl, methyl or
trifluoromethyl groups α to the oxygen donors, respectively,
thus exhibiting different steric and electronic properties.
They may form 5–5-5 chelate arrays when all donor atoms
bind to a given metal atom. The two families of ligands
were synthesised by SN-type reactions from (R,R)-bipyrroli-
dine and the corresponding substituted bromomethyl
phenols or substituted oxiranes, respectively.
The coordination chemistry of the chiral diamine-di-
phenolato ligands around zinc was first explored by treating
the ligand precursors (R,R)-Lig1–3H2 with diethylzinc as
Scheme 2. Coordination tendencies of bipyrrolidine-based Salan li-
outlined in Scheme 2. Conducting these reactions for 2 h at gands around zinc.
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ring that includes the two nitrogen donors of the bipyrroli- dinuclear with one of the phenolato oxygens of each Salan
dine motif around a given metal atom determines the chi- bridging between two zinc atoms.[28] The low steric bulk of
rality at the metal irrespective of the coordination number. Lig3 may lead to formation of dinuclear complexes having
a similar structure.[29,31]
To see whether similar diastereoselectivity in ligand
wrapping could also be achieved with chiral Salans as-
sembled around the common trans-1,2-diaminocyclohexane
core, we prepared the Salan ligand featuring that core and
ortho-tert-butyl
para-chloro
phenolato
substituents
(Scheme 3). Its reaction with diethylzinc led to a mixture of
products but its reaction with Zn(HMDS)2 led to a product
of the form [{ONNO}Zn] as was found for the bipyrrol-
idine based Salans. However, this complex was obtained as
a 1:0.4 mixture of C2 symmetric diastereomers indicating a
very low induction from the diaminocyclohexane backbone
to the helicity in ligand wrapping, as previously found for
octahedral titanium complexes.[14a,14b] This further sup-
ports the uniqueness of the bipyrrolidine core in inducing
a specific screw-sense of helical ligand wrapping.
Figure 1. ORTEP representation of complex [Lig1Zn] with 50%
probability ellipsoids and hydrogen atoms omitted for clarity. Se-
lected bond lengths [Å] and angles [°]: Zn–O2, 1.879(2); Zn–N3,
2.076(2); O2–Zn–O2i, 118.0(1); N3–Zn–N3i, 84.97(9).
Scheme 3. Low diastereoselectivity in wrapping around zinc for
trans-diaminocyclohexane based Salan ligands.
Employing the same reaction conditions for Lig3H2 re-
sulted in the formation of a mixture of two or more com-
plexes that could not be identified by 1H NMR spec-
Different reactivity was observed during the attempted
troscopy and extending the reaction time to several days complexation of the diamine-diolato ligand precursor
did not lead to a well-defined complex (Scheme 2). To try Lig4H2 with Et2Zn. Using diethyl ether as a solvent and
to carry the reaction with zinc to completion, we treated stirring the reaction for 2, 24 and even 48 h did not show
the ligand precursor Lig3H2 with Zn(HMDS)2 which in- formation of any single well-defined product according to
cludes labile amido groups. Indeed, the amine elimination 1H NMR spectroscopy. However, following storage of an
reaction proceeded more smoothly to give a complex of the NMR-tube with the reaction product dissolved in C6D6 at
type [Lig3Zn] as evident from the disappearance of the tri- –18 °C for 24 h, the formation of a defined complex was
1
methylsilylamide peaks in the 1H NMR spectrum. The evident by H NMR spectroscopy (taken at r.t.). While the
sharp peaks observed for [Lig1Zn] and [Lig2Zn] were not exact structure could not be deciphered, high field peaks
1
observed in the H NMR spectrum of [Lig3Zn] which con- supported the presence of CH3CH2-Zn groups signifying an
sisted of broad humps in the aliphatic region. VT-NMR incomplete reaction and possible formation of a dinuclear
experiments in [D8]toluene revealed that heating the sample complex. This complex was crystallised from diethyl ether
up to 358 K led to a spectrum with sharp peaks consistent at –35 °C and its molecular structure was determined by X-
with a practically single diastereomer of [Lig3Zn] (ac- ray diffraction. The asymmetric unit was found to contain
companied by some impurities) while cooling the sample to two independent molecules each having the same consti-
218 K led to sharpening and doubling of the number of tution and connectivity and very similar bond lengths and
peaks. We propose that [Lig3Zn] is in equilibrium between angles. The ORTEP representation of one of these mole-
a C2-symmetric mononuclear complex (dominant at high cules is outlined in Figure 2. The structure reveals a di-
temperature) and a C1-symmetric dinuclear Zn complex nuclear complex of the form [μ-Lig4(ZnEt)2(OEt2)] in which
(dominant at low temperature). Zn complexes of achiral each zinc atom still carries an ethyl group and resides in
and nonbulky Salan ligands were reported by Atwood and a different environment (Scheme 4, top). Zn1 (internal) is
coworkers, and were found by X-ray crystallography to be pentacoordinate, binding to all nitrogen and oxygen donors
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and to an ethyl group. Its geometry is best described as tion but this time employing noncoordinating toluene as
square-pyramidal with the ethyl ligand occupying the apical solvent. Suitable crystals were obtained at –35 °C but the
position. Atypically, the diamine-diolato ligand does not molecular structure was of lower quality and, except for the
wrap around the metal in a helical manner in this complex. zinc atoms, could only be refined isotropically. Two mole-
This is evident in opposite configurations of the two tetra- cules were found in the asymmetric unit and the ORTEP
hedral N-donors. Zn2 (external) binds to the two O-donors representation of one of them is outlined in Figure 3. In
of the ligand and to a second ethyl group and its tetrahedral line with the previous structure, the current complex was
coordination sphere is completed by an ether molecule. The also found to be dinuclear. However, the diamine-diolato
Zn–O and Zn–C bonds around Zn2 are shorter than the ligand was now bound to the zinc atoms symmetrically cre-
corresponding bonds around Zn1 in accordance with the ating two tetracoordinate metal centres: each zinc atom
lower coordination number. The Zn···Zn distance is binds to a different N-donor, to the two (bridging) O-do-
short[32] (3.061 and 3.065 Å in the two molecules) and may nors, and to an ethyl group (Scheme 4, middle). An even
indicate a metal-metal interaction. A possible explanation tighter Zn···Zn distance of 2.93 Å is consistent with a bind-
for the incomplete reaction in this diamine-diolato ligand is ing interaction. Since each metal atom binds to only three
the lower acidity of the OH groups relative to that of the donors of the {ONNO} ligand, the sense of chiral helicity
diamine-diphenolato ligands.
is irrelevant but the stereogenic nature of the two N-donors
is still valid. These were found to be of identical configura-
tion, equivalent to that found in [Lig1Zn]. We repeated the
reaction using toluene as a solvent. The 1H NMR spectrum
taken after 48 h in toluene was consistent with that ob-
tained when using diethyl ether, indicating the formation of
an undefined product at room temperature that converted
to a single well defined product after cooling the solution.
Based on the 1H NMR spectra and molecular structures we
assume that these two dinuclear complexes may intercon-
vert and the preference for the former may result from the
presence of the coordinating ether.
Figure 2. ORTEP representation of [μ-Lig4(ZnEt)2(OEt2)] with
50% probability ellipsoids and hydrogen atoms omitted for clarity.
Selected bond lengths [Å] and angles [°]: Zn1···Zn2, 3.061; Zn1–
O3, 2.079(6); Zn1–O4, 2.148(6); Zn1–N6, 2.379(8); Zn1–N7,
2.257(8); Zn2–O3, 1.967(6); Zn2–O4, 1.989(6); O3–Zn1–O4,
78.5(3); N6–Zn1–N7, 78.2(3); O4–Zn1–O3, 77.1(3); Zn1–O3–Zn2,
98.3(3); Zn1–O4–Zn2, 95.4(3); O3–Zn2–O4, 85.1(3).
Figure 3. ORTEP representation of [μ-Lig4(ZnEt)2] in which only
the Zn atoms were refined anisotropically. Zn1···Zn2, 2.927 Å.
Both complexes of the type [μ-Lig4(ZnEt)2] represent an
incomplete reaction of Et2Zn. Attempting to drive the reac-
tion to completion by prolonging it at elevated temperature
Scheme 4. Coordination tendencies of bipyrrolidine-based diamine-
diolato ligands around zinc.
did not lead to
a
mononuclear complex of the
To investigate the possible influence of the coordinating [{ONNO}Zn] type. However, starting from Zn(HMDS)2
1
solvent on product formation, we repeated the crystallisa- did yield a mononuclear complex of the [Lig4Zn]form. H
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NMR spectroscopic analysis indicated the formation of a wrapping of the {ONNO} ligand around the zinc. However,
single diastereomer of C2 symmetry, consistent with a spe- the smaller 5-5-5 chelate array leads to a shallower helix
cific helical ligand wrapping.[33] We propose that this (R,R)- relative to the 6-5-6 helix observed for [Lig1Zn].
diamine-diolato ligand wraps in the Δ form around Zn as
previously found for the Salan ligands Lig1–3. Reaction be-
tween Lig5H2 and diethyl zinc for 24 h resulted in formation
of an undefined product. However, leaving the compound
in toluene solution for a week resulted in formation of a
single well-defined complex having the empirical formula of
[Lig5Zn], the NMR spectrum of which was consistent with
a mononuclear complex of C2 symmetry. Based on the pre-
dictable wrapping of these bipyrrolidine-based {ONNO} li-
gands, we propose that the sense of helical chirality around
the zinc metal in Lig5Zn is identical to that found for
[(R,R)-Lig2Zn], i.e. Δ.
Conclusions
The chiral tetradentate bipyrrolidine-based {ONNO} li-
gands of the diamine-diphenolato and diamine-diolato fam-
ilies featuring different steric bulk and electronic character
were found to exhibit rich coordination chemistry around
zinc, depending on the zinc precursor. The reactions of all
ligand precursors with diethylzinc were sluggish. Longer re-
action times were typically required for the diamine-diolato
ligands to form the final product of the [{ONNO}Zn]-type,
possibly due to their reduced acidity. For Lig4, this product
could not be formed and “arrested” dinuclear complexes
amenable to connectivity manipulation by coordinating sol-
vents were the end-products. Zn(HMDS)2 reacted more
readily with the ligand precursors and so [{ONNO}Zn]-
type complexes could be obtained for all ligands. Except for
[Lig3Zn] which includes nonbulky phenol arms and tends
to form dinuclear complexes in noncoordinating solvents at
room temp., all complexes were found to form as single C2
symmetric chiral-at-metal diastereomers attesting to a very
high induction from bipyrrolidine backbone chirality to he-
lical-sense wrapping around the tetrahedral zinc centre.
Simple “ball-and-stick” molecular models predict a Δ heli-
cal chirality for these (R,R)-bipyrrolidine ligands and in the
two cases where crystallographic data was available
([Lig1Zn] and [Lig6Zn]), this prediction was validated.
These results stand in sharp contrast to those obtained with
the {ONNO} ligands based on the common chiral trans-
1,2-diaminocyclohexane core that led to mixtures of dia-
stereomers. We are currently broadening the scope of pyr-
rolidine-based chiral ligands and exploring the reactivity of
their resultant complexes.
1
When Lig6H2 was treated with Et2Zn, the H NMR and
19F NMR spectra showed the formation of several prod-
ucts. Following cooling of a C6D6 solution of this mixture
to –18 °C, a single product of empirical formula Lig6Zn was
formed according to 1H and 19F NMR spectra taken at
room temp. Single crystals suitable for X-ray diffraction
analysis were grown from diethyl ether at –35 °C and the
structure was solved confirming the formation of a mono-
nuclear complex of tetracoordinate zinc. The ORTEP repre-
sentation and selected bond lengths and angles for [Lig6Zn]
are outlined in Figure 4. The Zn-N and Zn–O bond lengths
and N–Zn–N and O–Zn–O angles are similar to those
found for [Lig1Zn]. Again, the (R,R)-chirality of the bipyr-
rolidine skeleton was translated to a Δ helical screw-sense
Experimental Section
General: All experiments employing zinc complexes were per-
formed under an atmosphere of dry nitrogen in a nitrogen-filled
glovebox. Ether was purified by reflux and distillation under a dry
argon atmosphere from Na/benzophenone. Pentane was washed
with HNO3/H2SO4 prior to distillation from Na/benzophenone/tet-
raglyme. Toluene was heated to reflux over Na and distilled. (R,R)-
bipyrrolidine was purchased from OBITER. The ligand precursors
Lig2,3H2 were prepared according to previously published pro-
cedures from (R,R)-bipyrrolidine and the corresponding disubsti-
tuted bromomethylphenols.[19] The ligand precursors Lig4–6H2 were
prepared according to previously published procedures from (R,R)-
bipyrrolidine and the corresponding disubstituted oxiranes.[20]
NMR spectroscopic data were recorded on Bruker Avance 400 and
Avance 500 spectrometers. CDCl3 was used as NMR solvent for
the ligands (chemical shift of TMS at δ = 0.00, and 13C chemical
shift of the solvent at δ = 77.16 were used as reference) and C6D6
was used as an NMR solvent for the metal complexes (referenced
to proton impurities at δ = 7.15, and to the 13C chemical shift at δ
= 128.7 ppm of benzene). Diffraction measurements were per-
formed on a Nonius Kappa CCD diffractometer system using Mo-
Figure 4. ORTEP representation of complex [Lig6Zn] with 50%
probability ellipsoids and hydrogen atoms omitted for clarity. Se-
lected bond lengths [Å] and angles [°]: Zn–O2, 1.866(3); Zn–O3,
1.875(4); Zn–N4, 2.077(4); Zn–N5, 2.090(4); O2–Zn–O3, 129.9(2);
N4–Zn–N5, 87.0(2).
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Kα (λ = 0.7107 Å) radiation. The analysed crystals grown from di-
ethyl ether or toluene solutions at –35 °C were embedded within a
drop of viscous oil and freeze-cooled to ca. 110 K. The structures
were solved by a combination of direct methods and Fourier tech-
niques using SIR-97[34] software and were refined by full-matrix
least-squares methods with SHELXL-97.[35] Elemental analyses
were performed in the microanalytical laboratory in the Hebrew
University of Jerusalem.
(R,R)-Lig2H2: Et3N (1.2 mL, 2 equiv.) was added dropwise to a
solution of (R,R)-bipyrrolidine (0.60 g, 4.27 mmol) and 2-ada-
mantyl-4-methyl-6-bromomethylphenol[36] (2.86 g, 8.54 mmol) in
THF (50 mL). The mixture was stirred for 24 h at room temp. pro-
ducing a white precipitate of Et3N·HBr which was filtered off and
extracted with cold THF. The filtrate was washed twice with water
and dried under vacuum. The resultant solid was recrystallised
from a mixture of pentane and methanol, yielding 1.9 g (69%) of
(R,R)-Lig2H2. MS (DCI): m/z = [MH+] 647.5. 1H NMR (400 MHz,
CDCl3): δ = 6.92 (d, J = 1.6 Hz, 2 H), 6.62 (d, J = 1.6 Hz, 2 H),
3.92 (d, J = 13.6 Hz, 2 H, AB system), 3.38 (d, J = 13.6 Hz, 2 H,
AB system), 3.01 (m, 2 H), 2.78 (m, 2 H), 2.24 (s, 3 H), 2.14 (br. s, 6
H), 2.06 (br. s, 3 H), 1.78 (br. s, 6 H) ppm. 13C NMR (100.66 MHz,
CDCl3): δ = 155.1 (C), 137.1 (C), 127.8 (C), 127.1 (CH), 127.0
(CH), 123.6 (C), 66.3 (CH2), 59.7 (CH2), 55.6 (CH), 41.1 (CH),
37.9 (CH2), 29.9 (CH3), 26.3 (C), 24.5 (CH2), 21.5 (CH2) ppm.
C44H60N2O2 (648.97): calcd. C 81.43, H 9.32, N 4.32; found C
80.73, H 9.02, N 4.14.
Complex [Lig2Zn]: (R,R)-Lig2H2 (37.5 mg, 0.058 mmol) was dis-
solved in toluene (ca. 2 mL) and cooled to –35 °C. A solution of
Et2Zn in hexane (1.0 m, 0.06 mL) was added dropwise. The mixture
was warmed to room temp. and stirred for 24 h. The solvent was
removed under reduced pressure to give the product as a white
1
solid (40.3 mg, 96%). H NMR (400 MHz, CDCl3): δ = 7.25 (d, J
= 2.4 Hz, 2 H), 6.44 (d, J = 2.4 Hz, 2 H), 3.80 (d, J = 12.2 Hz, 2
H, AB system), 2.73 (d, J = 12.2 Hz, 2 H, AB system), 2.64 (m, 6
H), 2.51 (m, 2 H), 2.37 (m, 2 H), 2.32 (s, 3 H), 2.19 (s, 3 H), 2.96
(m, 6 H), 1.82 (m, 6 H), 1.24 (m, 2 H), 0.79 (m, 2 H), 0.53 (m, 2
H) ppm. 13C NMR (100.66 MHz, C6D6): δ = 165.2 (C), 140.5 (C),
130.2 (CH), 129.6 (CH), 122.9 (C), 122.8 (C), 64.1 (CH), 60.5
(CH2), 52.9 (CH2), 41.5 (CH2), 38.2 (CH2), 30.5 (CH2), 24.4 (CH3),
21.9 (CH2), 21.5 (CH2) ppm. C44H58N2O2Zn (712.33): calcd. C
74.19, H 8.21, N 3.93; found C 74.50, H 8.52, N 2.86.
Complex [Lig3Zn]: Lig3H2 (117 mg, 0.24 mmol) was dissolved in
toluene (ca. 1 mL) and added dropwise to a solution of Zn-
(HMDS)2 (92 mg, 0.24 mmol) in toluene (ca. 1 mL). The mixture
was stirred for 10 h and the solvent removed under reduced pres-
sure to give the product as a white solid which was washed with
cold pentane to remove traces of HMDS-containing compounds
1
(118 mg, 89%). H NMR ([D8]toluene, 500 MHz, 358 K): δ = 7.30
(s, 2 H, ArH), 6.56 (s, 2 H, ArH), 3.53 (d, J = 15.0 Hz, 1 H, AB
system), 3.25 (d, J = 15.0 Hz, 1 H, AB system), 2.59–2.54 (m, 2 H,
CH), 1.42–1.35 (m, 3 H, CH), 1.10 (m, 1 H, CH), 0.82 (m, 1 H,
1
CH) ppm. H NMR ([D8]toluene, 500 MHz, 218 K): δ = 7.51 (s, 2
(R,R)-Lig7H2: A mixture of 2-tert-butyl-4-chlorophenol (1.1 g,
6.0 mmol), rac-trans-N,NЈ-dimethylcyclohexane-1,2-diamine (0.4 g,
3.0 mmol) and paraformaldehyde (0.3 g, 8.3 mmol) in methanol
(1 mL) was heated in a pressure glass vessel to 90–95 °C overnight.
The solution was cooled to room temp. and the white solid that
had formed was collected by filtration and washed several times
with cold methanol yielding 1.1 g of rac-Lig7H2 (40% yield). 1H
NMR (CDCl3, 400 MHz): δ = 10.70 (br. s, 2 H, ArH), 7.13 (d, J
= 2.5 Hz, 2 H, ArH), 6.81 (d, J = 2.5 Hz, 2 H, ArH), 3.73 (s, 4 H,
CH2), 2.64 (m, 2 H, CH), 2.16 (s, 6 H, CH3), 1.95 (m, 2 H, CH),
1.82 (m, 2 H, CH), 1.33 [s, 18 H, (CH3)3], 1.16 (m, 4 H, CH) ppm.
13C NMR (CDCl3, 100.67 MHz): δ = 155.8 (C), 138.7 (C), 126.3
(CH), 123.7 (C), 122.9 (C), 61.2 (CH), 58.0 (CH2), 35.0 (CH3),
29.4 (CH3), 25.3 (CH2), 22.1 (CH2) ppm. C30H44Cl2N2O2 (535.60):
calcd. C 67.28, H 8.28, N 5.23; found C 67.58, H 8.33, N 5.23.
H, ArH), 7.46 (s, 2 H, ArH), 6.62 (s, 2 H, ArH), 6.49 (s, 2 H, ArH),
5.33 (d, J = 18.0 Hz, 1 H, AB system), 4.81 (d, J = 12.5 Hz, 1 H,
AB system), 4.53 (m, 1 H, CH), 3.69 (d, J = 12.5 Hz, 1 H, AB
system), 3.58 (m, 1 H, CH), 3.39 (m, 1 H, CH), 3.26 (m, 1 H, CH),
2.86 (d, J = 18.0 Hz, 1 H, AB system), 1.97 (m, 1 H, CH), 1.88 (m,
1 H, CH), 1.71 (m, 3 H, CH), 1.40–1.01 (m, 3 H, CH), 0.76 (m, 2
1
H, CH) ppm. H NMR ([D8]THF, 400 MHz, 298 K): δ = 7.18 (d,
J = 2.4 Hz, 1 H), 6.81 (d, J = 2.4 Hz, 1 H), 4.05 (d, J = 11.2 Hz,
1 H, AB system), 3.41 (d, J = 11.2 Hz, 1 H, AB system), 2.79–2.74
(m, 2 H, CH), 2.01–1.98 (m, 3 H, CH), 1.56 (m, 1 H, CH), 1.47
(m, 1 H, CH) ppm. C22H22Cl4N2O2Zn (553.62): calcd. C 47.73, H
4.01, N 5.06; found C 48.29, H 4.53, N 4.03.
Complex
[μ-Lig4(ZnEt)2(OEt2)]:
(R,R)-Lig4H2
(36.2 mg,
0.068 mmol) was dissolved in ether (ca. 2 mL) and cooled to
–35 °C. A solution of Et2Zn in hexane (1.0 m, 0.07 mL) was added
dropwise. The mixture was warmed to room temp. stirred for 48 h
and then returned to –35 °C for 24 h. The solvent was removed
under reduced pressure to give the product as a white solid
Complex [Lig1Zn]: (R,R)-Lig1H2 (14.2 mg, 0.025 mmol) was dis-
solved in ether (ca. 2 mL) and cooled to –35 °C. A solution of
Et2Zn in hexane (1.0 m, 0.02 mL) was added dropwise. The mixture
was warmed to room temp. and stirred for 24 h. The solvent was
1
removed under reduced pressure to give the product as a white
(47.8 mg). H NMR (400 MHz, C6D6): δ = 7.61 (d, J = 8.4 Hz, 4
1
solid (12.5 mg, 79%). H NMR (400 MHz, C6D6): δ = 7.58 (d, J H), 7.58 (d, J = 8.4 Hz, 4 H), 7.25 (m, 4 H), 7.11 (m, 4 H), 7.04
= 2.5 Hz, 2 H), 6.71 (d, J = 2.5 Hz, 2 H), 3.80 (d, J = 9.4 Hz, 2 H,
AB system), 2.78 (d, J = 9.4 Hz, 2 H, AB system), 2.53 (m, 2 H),
2.25 (m, 4 H), 1.82 (s, 18 H), 1.26 (s, 18 H), 1.24 (m, 4 H), 0.82
(m, 2 H), 0.58 (m, 2 H) ppm. 13C NMR (100.66 MHz, C6D6): δ =
(m, 2 H), 6.99 (m, 2 H), 3.31 (d, J = 12.5 Hz, 2 H, AB system),
2.96 (d, J = 12.5 Hz, 2 H, AB system), 2.24 (m, 2 H), 1.49 (t, J =
7.9 Hz, 6 H), 1.21 (m, 4 H), 0.79 (m, 4 H), 0.54 (m, 4 H), 0.18 (m,
4 H) ppm. 13C NMR (100.66 MHz, C6D6): δ = 153.3 (C), 153.2
164.9 (C), 139.6 (C), 129.6 (C), 126.4 (CH), 125.2 (CH), 122.4 (C), (C), 129.6 (CH), 128.9 (CH), 127.2 (CH), 126.9 (CH), 126.4 (CH),
64.2 (CH), 61.1 (CH2), 52.9 (CH2), 36.3 [C(CH3)3], 34.5 126.0 (CH), 80.4 (C), 78.4 (CH), 70.3 (CH2), 69.2 (CH2), 55.9
[C(CH3)3], 32.5 [C(CH3)3], 30.6 [C(CH3)3], 24.6 (CH2), 22.1
(CH2) ppm. Single crystals of the complex were grown from cold
diethyl ether. Crystal data for C38H58ZnN2O2; M = 640.23; ortho-
rhombic; space group P21212; a = 11.2070(4), b = 21.8553(10), c =
(CH2), 26.0 (CH2), 23.6 (CH2), 21.7 (CH3) ppm. Single crystals of
the complex were grown from cold diethyl ether. Crystal data for
C44H58Zn2N2O3; M = 793.66; triclinic; space group P1; a =
9.7241(3), b = 10.5708(4), c = 19.2159(8) Å; α = 94.7256(14), β
8.4109(3) Å; V = 2060.1 Å3; Z = 2; Dc = 1.032 gcm–3; μ(Mo–Kα) = 94.2445(16), γ = 100.360(3)º; V = 1928.46(12) Å3; Z = 2; Dc =
= 0.625 mm–1; T = 110(2) K; No. of data collected 4636; R1
0.0471 and wR2 = 0.1059 for 3130 reflections with I Ͼ 2σ(I); R1
0.0846 and wR2 0.1178 for all 4636 reflections. with I Ͼ 2σ (I); R1 = 0.1128 and wR2 = 0.1568 for all 12512
=
1.367 gcm–3; μ(Mo–Kα) = 1.286 mm–1; T = 110(2) K; No. of data
collected 12512; R1 = 0.0619 and wR2 = 0.1304 for 8224 reflections
=
=
C42H68N2O3Zn·Et2O (714.39): calcd. C 70.61, H 9.59, N 3.92;
found C 70.06, H 9.64, N 3.77.
reflections. C44H58N2O3Zn2 (793.71): calcd. C 66.58, H 7.37, N
3.53; found C 65.14, H 6.50, N 3.63.
Eur. J. Inorg. Chem. 2013, 3362–3369
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FULL PAPER
Complex [μ-Lig4(ZnEt)2]: This complex was prepared in an analo-
gous way as [μ-Lig4(ZnEt)2(OEt2)] in 89% yield, using toluene as
a solvent. The NMR measurements in C6D6 showed only broad
humps. Single crystals of the complex were grown from cold tolu-
ene. Crystal data for C43H46Zn2N2O2; M = 753.56; monoclinic;
space group P21; a = 13.1330(2), b = 15.4570(3), c = 20.3670(4) Å;
β = 108.7000(8)º; V = 3916.18(12) Å3; Z = 4; Dc = 1.278 gcm–3;
μ(Mo–Kα) = 1.261 mm–1; T = 110(2) K; No. of data collected
15937; R1 = 0.1325 and wR2 = 0.3384 for 13440 reflections with I
Ͼ 2σ (I); R1 = 0.1483 and wR2 = 0.3384 for all 15937 reflections.
C47H56N2O2Zn2·C7H8: C 69.54, H 6.95, N 3.45; found C 70.02, H
6.97; N 3.37.
Complex [Lig7Zn]: rac-Lig7H2 (39 mg, 0.07 mmol) was dissolved in
toluene (ca. 1 mL) and added dropwise to a solution of Zn-
(HMDS)2 (28 mg, 0.07 mmol) in toluene (ca. 1 mL). The mixture
was stirred for 10 h and the solvent removed under reduced pres-
sure to give the product as a mixture of two diastereomers of C2
symmetry in a ratio of 1:0.4 and quantitative yield. Major dia-
1
stereomer: H NMR (400 MHz, C6D6): δ = 7.51 (d, J = 2.4 Hz, 2
H, ArH), 6.71 (d, J = 2.4 Hz, 2 H, ArH), 3.45 (d, J = 12.4 Hz, 2
H, AB system), 2.59 (d, J = 12.4 Hz, 2 H, AB system), 2.01 (m, 2
H, CH), 1.57 [s, 18 H, (CH3)3], 1.40 (s, 6 H, NCH3), 1.23–1.18 (m,
1
4 H, CH), 1.03–0.95 (m, 4 H, CH) ppm. Minor diastereomer: H
NMR (400 MHz, C6D6): δ = 7.55 (d, J = 2.8 Hz, 2 H, ArH), 6.78
(d, J = 2.8 Hz, 2 H, ArH), 3.14 (d, J = 12.4 Hz, 2 H, AB system),
2.39 (d, J = 12.4 Hz, 2 H, AB system), 1.73 (s, 6 H, NCH3), 1.59
[s, 18 H, (CH3)3], 1.23–1.01 (m, 10 H, CH) ppm. C33H42Cl2N2O2Zn
(634.99): calcd. C 60.16, H 7.07, N 4.68; found C 61.47, H 6.92, N
4.10.
Complex Lig4Zn: (R,R)-Lig4H2 (35 mg, 0.06 mmol) was dissolved
in toluene (ca. 1 mL) and added dropwise to a solution of
Zn(HMDS)2 (25 mg, 0.06 mmol) in toluene (ca. 1 mL). The mix-
ture was stirred for 10 h and the solvent was removed under re-
duced pressure to give the product as a white solid (36 mg, 92%).
1H NMR (C6D6, 400 MHz): δ = 8.02 (d, J = 6.0 Hz, 2 H, ArH),
7.85 (d, J = 6.0 Hz, 2 H, ArH), 7.26 (t, J = 6.0 Hz, 2 H, ArH),
7.12 (t, J = 6.0 Hz, 2 H, ArH), 7.07 (t, J = 6.0 Hz, 2 H, ArH), 6.98
(t, J = 6.0 Hz, 2 H, ArH), 3.25 (d, J = 10.0 Hz, 1 H, AB system),
2.81 (d, J = 10.0 Hz, 1 H, AB system), 2.26 (m, 1 H, CH), 2.10 (m,
1 H, CH), 1.82 (m, 1 H, CH), 1.21 (m, 1 H, CH), 1.12 (m, 1 H,
CH), 0.82 (m, 1 H, CH), 0.55 (m, 1 H, CH) ppm. 13C NMR (C6D6,
100.67 MHz): δ = 153.1 (C), 152.9 (C), 126.9 (CH), 126.6 (CH),
126.2 (CH), 126.1 (CH), 78.3 (C), 70.0 (CH2), 68.8 (CH2), 55.6
(CH), 25.6 (CH2), 23.3 (CH2) ppm. C41H50N2O2Zn·C5H12: C
73.69, H 7.54, N 4.19; found C 73.81, H 6.69, N 4.10.
CCDC-920515 (for [Lig1Zn]), -920516 {for [μ-Lig4(ZnEt)2-
(OEt2)]}, -920517 (for [μ-Lig4(ZnEt)2]) and -920518 (for [Lig6Zn])
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Center via www.ccdc.cam.ac.uk/data_
request/cif.
Acknowledgments
The authors thank the Israel Science Foundation for support.
Complex [Lig5Zn]: (R,R)-Lig5H2 (24.5 mg, 0.086 mmol) was dis-
solved in toluene (ca. 2 mL) and cooled to –35 °C and a solution
of Et2Zn in hexane (1.0 m, 0.086 mL) was then added dropwise.
The mixture was warmed to room temp. stirred for 4 h and then
returned to –35 °C for 2 weeks. The solvent was removed under
reduced pressure to give the product as a white solid (28.7 mg,
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96%). H NMR (500 MHz, C7D8): δ = 3.11 (m, 2 H), 2.70 (m, 2
H), 2.52 (d, J = 13.2 Hz, 2 H, AB system), 2.27 (m, 6 H), 2.20 (d,
J = 13.2 Hz, 2 H, AB system), 1.65 (m, 4 H), 1.52 (m, 4 H), 1.30
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70.8 (CH), 70.5 (CH2), 69.5 (CH2), 57.6 (C), 32.1 (CH3), 30.5
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Complex [Lig6Zn]: (R,R)-Lig6H2 (29.0 mg, 0.058 mmol) was dis-
solved in toluene (ca. 2 mL) and cooled to –35 °C and a solution
of Et2Zn in hexane (1.0 m, 0.06 mL) was then added dropwise. The
mixture was warmed to room temp. stirred for 4 h and then re-
turned to –35 °C for 2 weeks. The solvent was removed under re-
duced pressure to give the product as a white solid (30.4 mg, 93%).
1H NMR (400 MHz, C6D6): δ = 2.58 (m, 2 H), 3.57 (d, J = Hz, 2
H, AB system), 2.35 (d, J = 14.9 Hz, 2 H, AB system), 2.19 (m, 2
H), 2.01 (m, 2 H), 1.55 (m, 2 H), 1.16 (m, 2 H), 0.52 (m, 2 H) ppm.
13C NMR (100.66 MHz, C6D6): δ = 125.8 (CF3), 123.5 (CF3), 74.1
(CH), 68.3 [C(CF3)2], 59.8 (CH2), 55.6 (C), 25.5 (CH2), 22.9
(CH2) ppm. 19F NMR (188.16 MHz, C6D6): δ = –78.5 [s, (CF3)2],
–78.8 [s, (CF3)2] ppm. Single crystals of the complex were grown
from diethyl ether at –35 °C. Crystal data for C16H18F12ZnN2O2;
M = 563.39; orthorhombic; space group P212121; a = 7.9713(2), b
= 11.9870(4), c = 21.2891(8) Å; V = 2034.22(11) Å3; Z = 4; Dc =
1.841 gcm–3; μ(Mo–Kα) = 1.333 mm–1; T = 110(2) K; No. of data
collected 3993; R1 = 0.0516 and wR2 = 0.0998 for 3101 reflections
with I Ͼ 2σ (I); R1 = 0.0774 and wR2 = 0.1114 for all 3993 reflec-
tions. C16H18F12N2O2Zn (563.69): calcd. C 34.09, H 3.22, N 4.97;
found C 33.57, H 3.51, N 4.09.
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Published Online: May 21, 2013
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© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim