Three-Coordinate Zinc Methyl Complexes with Sterically Demanding Formazanate Ligands

Article abstract of DOI:10.1021/acs.organomet.0c00720

A series of heteroleptic three-coordinate mono(formazanate)zinc methyl complexes were synthesized, and the influence of the ligand on the structure as well as redox and optical properties of these complexes was investigated. The heteroleptic mono(formazan

Full text of DOI:10.1021/acs.organomet.0c00720

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Three-Coordinate Zinc Methyl Complexes with Sterically
Demanding Formazanate Ligands
Folkert de Vries, Raquel Travieso-Puente, Peter Roewen, and Edwin Otten*
Cite This: Organometallics 2021, 40, 63−71
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sı Supporting Information
ABSTRACT: A series of heteroleptic three-coordinate mono-
formazanate)zinc methyl complexes were synthesized, and the
(
influence of the ligand on the structure as well as redox and optical
properties of these complexes was investigated. The heteroleptic
mono(formazanate)zinc methyl complexes were found to show
ligand redistribution in solution, reminiscent of the Schlenk
equilibrium, to generate an equilibrium mixture containing the
corresponding homoleptic complexes as well. Monitoring the
approach to equilibrium by NMR spectroscopy in benzene-d6
allowed determination of the forward and backward rate constants.
A correlation was found between the steric environment around
the zinc center and equilibrium concentration of (formazanate)-
zinc methyl compounds, whereas the kinetics for approach to
equilibrium are also dependent on the electronic properties.
INTRODUCTION
Chart 1. Representative “Bulky” Bidentate N-Donor
Ligands
■
Ligand design is key to the development of new structures and
reactivity in organometallic chemistry. In particular, sterically
demanding ligands have been instrumental in providing access
to highly reactive, low-coordinate complexes that are relevant
for catalysis. An illustration of this is provided by the 2013−
2
014 Organometallics Roundtable, where (bulky) ligand
synthesis was identified as a major challenge for the field. β-
1
Diketiminate (BDI) ligands are a case in point. Despite being
2
used as ligands since the 1960s, their popularity in
organometallic chemistry was boosted during the mid-1990s
deleterious reactions (ligand redistribution and other decom-
position pathways). Sterically demanding dipyrrinato ligands
with the discovery of sterically demanding versions of the β-
(
B, Chart 1) were introduced in the past decade by Betley and
3
diketiminate ligand. This has made it possible to obtain
co-workers to access low-coordinate complexes with first-row
monomeric, low-coordinate organometallic complexes, ena-
bling the study of highly reactive species such as monomeric
transition metals, resulting in unusual terminal imidos that are
12
active in C−H bond amination. Moreover, these ligands
4
5
metal hydrides and unusual low-valent Mg compounds.
were recently also used by Harder and co-workers to obtain
Moreover, these ligands have provided well-defined species
13
monomeric zinc hydride complexes.
6
that can be applied in homogeneous catalysis, such as
Our group has in recent years developed the coordination
chemistry of formazanate ligands (C, Chart 1) as analogues of
β-diketiminates. Although the two ligand structures are similar,
the exchange of two carbon atoms in the β-diketiminate
NCCCN ligand backbone for nitrogen (i.e., NNCNN) makes
7
8
9
hydroamination, hydrophosphination, and hydrosilylation,
among others. In particular, three-coordinate zinc complexes
bearing BDI ligands have shown great promise in a variety of
polymerization reactions, such as ring-opening polymerization
10
(
ROP) of lactide or the copolymerization of CO with
2
11
epoxides.
Received: November 12, 2020
Published: December 23, 2020
The popularity of β-diketiminate ligands in part originates
from their modular synthesis, which provides facile access to a
large variety of different substitution patterns. This includes
ligands with sterically demanding N-substituents exemplified
by the often-used 2,6-diisopropylphenyl group (DIPP, A,
Chart 1), which effectively protects the metal complex from
©
2020 American Chemical Society
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6
3
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Scheme 1. General Synthetic Routes toward Triaryl Formazans
that formazanate ligands have low-lying π*-orbitals and thus
described above, the attempted preparation of a formazan with
1
4
1
5
are redox-active. Following our initial report on homoleptic
bis(formazanate)zinc complexes, which were isolated in three
R = R = DIPP, a direct analogue of the symmetrical DIPP-
substituted BDI ligand A (Chart 1), via route B was met with
limited success.. Although it proved possible to obtain the
desired product in low yield (see the Supporting Information
for NMR spectral data), the purification was tedious and
poorly reproducible in our hands.
15
distinct redox-states arising from ligand-based reduction, we
and others have started a systematic investigation of
formazanate coordination compounds with both main group
16
elements and transition metals. Despite significant advances
in this field in the past decade, it has proven challenging to
prepare triaryl formazanate ligands with a steric profile
sufficient to stabilize low-coordinate formazanate complexes
and study their (catalytic) reactivity. Indeed, attempts to
prepare low-coordinate mono(formazanate) metal complexes
with simple, unhindered ligands results instead in formation of
bis(formazanate) complexes as the thermodynamic products
Regardless, the synthesis of formazans via route B is a useful
alternative to the commonly used diazonium coupling
chemistry and may be extended to ligands for which diazonium
precursors are not available (e.g., formazans with N-alkyl
substituents). Finally, L6H was synthesized via the direct
coupling of a diazonium compound to phenylpyruvic acid
21
following a literature procedure.
17
via ligand redistribution.
Complex Synthesis and Characterization. Mono-
(formazanate)zinc methyl complexes were obtained by treat-
ment of the free ligands (L1H−L6H) with 1 equiv of
dimethylzinc in toluene solution at room temperature (Scheme
2). Elongated reaction times are required for the more
In this work we present a series of heteroleptic three-
coordinate (formazanate)zinc methyl complexes and their
ligand redistribution reactions to form the corresponding
homoleptic compounds. Monitoring the approach to equili-
brium allows determination of the thermodynamic stability of
the heteroleptic complexes, which is dependent on the size of
the N−Ar substituents. In addition, the rates of ligand
exchange are sensitive to the electronic properties of the
ligand (electron-donating ability).
Scheme 2. Synthesis of Mono(formazanate)zinc Methyl
Complexes
RESULTS AND DISCUSSION
■
18
Ligand Synthesis. Formazan ligands L1H−L3H were
synthesized by using literature procedures, involving the
condensation of a hydrazine with an aldehyde to yield a
hydrazone. The next step is the coupling of a diazonium salt to
the hydrazone, which affords the formazans in yields of 11−
sterically hindered ligands (overnight), whereas L1H is fully
converted in 30 min. Removal of the volatiles afforded the
mono(formazanate)zinc complexes 1−6 in good yields.
Crystals of 1−6 suitable for X-ray diffraction were obtained
by cooling concentrated solutions of the mono(formazanate)
zinc methyl complexes in toluene or hexane. For complexes
that crystallized less readily (i.e., 4 and 5), prolonged standing
of the solutions resulted ultimately in the isolation of crystals of
the corresponding bis(formazanate)zinc complexes (see the
Supporting Information). To prevent ligand redistribution,
crystallization was repeated in the presence of an additional 0.2
4
8% (Scheme 1, route A). A major disadvantage of this route is
the limited stability of aryldiazonium salts, especially when
sterically demanding 2,6-substituents are present. In search for
a method to synthesize formazan ligands with more sterically
demanding N−Ar substituents, we explored the coupling of
hydrazines with hydrazonoyl chlorides (Scheme 1, route B).
This method for the synthesis of formazan was already
19
described by Pechmann in 1894 but has thereafter been used
20
only sporadically. The synthesis of ligands L4H and L5H,
which we were unable to obtain using route A, was successfully
accomplished with route B. Formazan L4H was prepared from
the reaction of the corresponding N-phenylhydrazonoyl
chloride with 2,6-diisopropylphenylhydrazine. Carrying out
the reaction in EtOH under air resulted in in situ oxidation of
the intermediate benzohydrazonohydrazide to form the desired
formazan L4H in 34% yield after crystallization. Similarly, the
symmetrical 2,6-dichlorophenyl-substituted formazan L5H was
obtained via route B, albeit that the final step was relatively
low-yielding (17%). In contrast to the formazan syntheses
equiv of ZnMe present in solution, and this allowed the
2
isolation of 4 and 5 in pure, crystalline form. The X-ray data
show that complexes 1−6 are present in the solid state as
monomeric compounds with a three-coordinate Zn center
surrounded by the bidentate formazanate ligand and a methyl
group in an approximately trigonal-planar arrangement
(Figures 1 and 2; pertinent metrical parameters in Table 1).
Compound 1 crystallizes with three independent, virtually
planar molecules in the unit cell which show stacking in an
alternating (“ABA”) arrangement. The molecular planes
6
4
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Figure 1. Molecular structures of 1−5 showing 50% probability ellipsoids. One of three (for 1) or of two (for 2 and 3) independent molecules is
shown. Hydrogen atoms are omitted for clarity. The angles α and β are defined as the dihedral angle between the N−N vector and the least-squares
plane of the attached aromatic ring.
molecules with a small interplanar separation of 3.27 Å for
the ZnN C planes, with interactions between the methyl
4
ZnCH group and the chlorines of an adjacent molecule
3
(
Figure S31).
In all complexes, there is full charge delocalization over the
NNCNN ligand backbone as indicated by the equivalent
intraligand C−N and N−N bond lengths. Furthermore, with
increasing size of the N−Ar groups these aryl rings rotate out
of the ligand NNCNN plane to minimize steric interactions:
whereas both N-Ph rings in 1 are essentially coplanar with the
ligand backbone (dihedral angles α/β of 6.24° and 12.12°), the
larger N-Mes group in 2 is rotated out of the plane (58.49°). A
similar effect is seen in complex 3 with two N-Mes
substituents, which are both oriented away from the ligand
backbone with dihedral angles between the mesityl ring and
the NNCNN plane of ∼62°. The out-of-plane rotation leads to
loss of conjugation between the aromatic ring and the ligand
backbone. Consequently, the nitrogen becomes a better donor,
which is reflected in the shorter (average) Zn−N bond
distances (1.996 vs 1.989 vs 1.972 Å, in 1, 2, and 3,
respectively). In contrast, the Zn−N bond length in compound
5 is somewhat longer (2.013 Å), suggesting it is a weaker
Figure 2. Molecular structure of 6 showing 50% probability ellipsoids.
The toluene solvent molecule and hydrogen atoms are omitted for
clarity.
(
defined by the ZnN C ring) are coplanar with interplanar
4
angles of 3.83° and 1.45° between adjacent molecules and
distances between the Zn atoms and the neighboring
molecular planes of <3.2 Å. In contrast, the presence of two
sterically encumbering N-Mes substituents in compound 3
makes it distinctly nonplanar, and no intermolecular stacking is
observed in the solid state. The solid-state structure of
compound 5 shows a steplike stacking in neighboring
Table 1. Selected Bond Distances (Å) and Angles (deg) for Compounds 1−6
a
a
a
b
1
2
3
4
5
6
Zn1−C1
Zn1−N1
Zn1−N4
N1−N2
N3−N4
N2−C2
N3−C2
α
1.956(3)
2.003(2)
1.989(2)
1.299(3)
1.310(3)
1.346(2)
1.337(3)
12.12
1.949(2)
1.986(2)
1.992(2)
1.303(2)
1.300(2)
1.349(3)
1.353(3)
6.16
1.945(5)
1.968(3)
1.976(3)
1.306(4)
1.309(4)
1.348(4)
1.345(4)
62.80
1.945(2)
1.972(2)
2.001(2)
1.311(3)
1.297(3)
1.342(3)
1.364(3)
4.70
1.943(3)
2.013(2)
1.968(4)
2.047(3)
2.033(3)
1.302(4)
1.322(4)
1.348(4)
1.350(4)
9.93
1.300(2)
1.351(2)
56.60
β
6.24
58.49
62.82
78.64
15.36
a
b
Only one of the independent molecules is discussed. The asymmetric unit of 5 contains half the molecule; the other half is generated by
reflection in the mirror plane.
6
5
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Article
donor despite having a similar out-of-plane orientation of the
N−Ar ring as in 3, which is attributed to the electron-
withdrawing Cl substituents. The Zn−C bond in these
complexes is in the range 1.935−1.968 Å, which is comparable
to the bond lengths in β-diketiminate zinc methyl complexes
Table 2. Reduction Potentials and UV/Vis Absorption
Maxima for Compounds 1−6
compound
E1/2 (V vs Fc)
λmax (nm)
1
2
3
4
5
6
−1.61
−1.81
−1.91
−1.78
−1.52
−1.69
567
503
462
487
480
606
22
reported by Parkin et al.
In compound 6 the OMe substituents on the N-Ar rings are
weakly coordinated to Zn. The geometry around the zinc
center is best described as pseudo-five-coordinate, with strong
bonding interactions with both nitrogens of the formazanate
ligand and the carbon of the methyl group, in a distorted
trigonal arrangement, and additional weak interactions with the
OMe groups (Zn−O1, 2.329 Å, and Zn−O2, 2.511 Å).
Although the interaction between the Zn center and the OMe
group in 6 appears to be relatively weak, the presence of the
two additional donor sites can potentially stabilize 6 toward
ligand redistribution reactions. At the same time, the solid-state
structure of 6 shows that the Zn atom is significantly displaced
out of the formazanate coordination plane (0.957 Å), making it
still relatively accessible (Figure 2). An analogous copper
complex of 1,5-bis(o-methoxyphenyl)-3-cyanoformazan was
reported by Hicks, which also showed a pseudo-five-coordinate
geometry with the Cu center raised 0.487 Å above the NNOO
−
1.61 V and a second, less reversible reduction at −2.37 V (all
0/+
potentials vs Fc ; Figure S34). The first reduction at −1.61 V
•
−
likely forms the radical anion 1 , in which one electron is
deposited in the formazanate ligand. Upon sweeping further to
a more negative potential, a second reduction forms the two-
2−
electron-reduced complex 1 , which appears to have limited
stability under these conditions, indicated by I /I < 1.
p,a p,c
The cyclic voltammograms of compounds 1−3 show that
0/−
the LZnMe
redox couple occurs at progressively more
negative potentials (cathodic shifts of 200 and 300 mV for 2
and 3, respectively). This can be attributed to the decreased
conjugation between the formazanate NNCNN core and the
N−Ar substituents (reflected in larger values for dihedral
angles α/β in Table 1) that results from the steric pressure of
the 2,6-substituents of the N-Ar groups.
2
3
plane.
In contrast to the coordination behavior observed with the
tetradentate formazanate ligand L6, the related β-diketiminate
ligand (2-OMe-C H NC(Me)CHC(Me)N(2-OMeC H ),
In agreement with this notion is the observation that the
reduction potential of 4 is very similar to that of 2, despite the
presence of more electron-donating isopropyl substituents. On
the other hand, the 2,6-dichlorophenyl-substituted compound
6
4
6
4
BDI-OMe) does not show coordination of the OMe groups
to the metal center in the corresponding zinc amide or alkoxide
2
4
0
/+
complexes. Similarly, a zinc ethyl complex with the BDI
5
shows a reduction potential of −1.52 V vs Fc , which is
i
ligand 2-OMe-C H NC(Me)CHC(Me)N(2,6- PrC H ) con-
6
4
6
3
shifted anodically by 90 mV in comparison to 1. It appears in
this case that the Cl substituent is sufficiently electron-
withdrawing that it more than compensates for the cathodic
shift that is expected based on the out-of-plane orientation of
the N−Ar rings in 5 (α = β = 56.60°).
taining a single OMe donor also showed no propensity for
25
Zn···OMe coordination. Thus, it appears that formazanate
anions are overall relatively poor donors, making the zinc
center in these formazanate compounds more Lewis acidic
than in the BDI analogues.
UV−Vis Spectroscopy. The optical properties of the
mono(formazanate)zinc methyl complexes were measured by
UV−vis absorption spectroscopy of the compounds in toluene
solution. All compounds show intense absorption bands in the
range 460−600 nm (Figure 4 and Table 2), similar to other
formazanate complexes, which can be assigned to the π−π*
transition of the conjugated system in the NNCNN back-
Cyclic Voltammetry. The redox chemistry of the mono-
formazanate)zinc methyl complexes was evaluated by using
(
cyclic voltammetry (Figure 3; redox potentials tabulated in
Table 2). Compound 1 shows a quasi-reversible redox wave at
17a
bone. When comparing λmax of compounds 1 (567 nm), 2
503 nm), and 3 (462 nm), a significant blue-shift in the
(
absorption maximum is observed. We assign this blue-shift to
the out-of-conjugation, perpendicular orientation of the aryl
Figure 3. Cyclic voltammograms of 1−3 (1.5 mM solution in THF,
0
.1 M [Bu N][PF ]) recorded at 50 mV/s.
Figure 4. Absorption spectra of compounds 1−6 in toluene solution.
4
6
6
6
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Figure 5. Kinetic traces of the approach to equilibrium of compounds 1−5 in C D at 80 °C (black squares (1), magenta circles (2), purple
6
6
diamonds (3), blue down triangle (4), and red up triangle (5)). Filled markers indicate the trace for the conversion of mono(formazanate)zinc
methyl, and open markers signify the formation of bis(formazanate)zinc. Ph = phenyl, Mes = mesityl, DIPP = 2,6-diisopropylphenyl, and 2,6-Cl2-
C H = 2,6-di(chloro)phenyl.
6
3
1
5
groups R and R with respect to the ligand core (indicated by
large values for α/β in the solid-state structures; vide supra).
This orientation is likely retained in solution and precludes π-
conjugation between the NNCNN backbone and the N-Ar
groups.
conversion to the homoleptic complexes is observed for the
(formazanate)zinc methyl complex 1, which has the least
sterically demanding substituents (N-Ph). Increasing the size
of the N-Ar substituents results in an equilibrium composition
that is shifted more to the mono(formazanate) zinc complexes,
such that for the most hindered complex 3 (with two N-Mes
groups) the equilibrium constant is <10 . Even after several
days at 80 °C, no further conversion of 3 is observed,
indicating that sterically demanding ligands effectively shut
down ligand redistribution in these three-coordinate complexes
by destabilizing the bis(formazanate) zinc products. This is
reflected in the Gibbs free energy change (ΔG) at 80 °C,
which increases from 5.2 to 21.1 kJ/mol on going from 1 to 3.
Based on these measurements, the asymmetric ligand in 4 (N-
Ph/N-DIPP substituents) is more prone to ligand redistrib-
ution than the symmetric complex 3 (N-Mes), with a
difference in equilibrium thermodynamics (ΔΔG) of ca. 5.9
kJ/mol. Monitoring the ligand redistribution for the 2,6-
dichlorophenyl-substituted complex 5 shows that it has the
highest reaction rate within the series and reaches an
equilibrium that favors the homoleptic species the most (K =
Compounds 4 and 5 show absorption bands that are very
similar to those of 2, as anticipated based on the structural data
discussed above. Compound 6, on the other hand, shows a
significant red-shift and has an absorption maximum at 606
nm. This is in agreement with the planar geometry of the
NNCNN backbone and its N-aryl substituents due to
coordination of the o-OMe groups. Additionally, the oxygen
lone pairs or the OMe group can be delocalized into the π-
system to further increase planarity and lead to donor−
acceptor character of the electronic transition as observed in
the corresponding BF complexes.
Kinetics of Ligand Redistribution. To study the stability
of compounds 1−5 toward ligand redistribution in solution, we
monitored C D solutions by H NMR spectroscopy. An NMR
sample of complex 1 left at room temperature was found to
form a new formazanate-containing species and Me Zn over
the course of several hours. Upon closer inspection, the new
species was identified as the corresponding bis(formazanate)
−
3
26
2
1
6
6
2
0
.20). Thus, in addition to steric effects there is a substantial
contribution from electronic perturbations due to the electron-
withdrawing ortho-chloro substituents in 5. With regard to the
electron-donating ortho-methoxy groups in complex 6, it was
hypothesized that OMe groups would stabilize the mono-
1
5
zinc complex. Monitoring the NMR spectral changes at 80
C in C D solution showed that an equilibrium is established
between 1 and the homoleptic analogues, with K = 0.17 based
on H NMR integration (Figure 5). Fitting the data to an
approach to equilibrium for opposing bimolecular reactions
°
6
6
(
formazanate)zinc methyl species by coordination to the Zn
center. Indeed, when an NMR sample of 6 was heated to 80
C, the formation of the corresponding bis(formazanate)zinc
1
27
°
−
1
−1
afforded the rate constants k = 58.9 M min and k = 345.4
f
r
complex was not observed. Moreover, compound 6 was found
to be stable even in the presence of excess ligand L6H,
indicating that the additional MeO···Zn interaction makes 6
substantially more stable than the three-coordinate zinc methyl
complexes. It should be noted that, in contrast to 6, the
corresponding diketiminate complex [BDI-OMe]ZnEt was
−
1
−1
M
min . The ligand redistribution reactions for the other
compounds were measured in the same manner, allowing a
comparison of the reaction rates and equilibrium constants
within the series (Table 3). As expected, the largest extent of
Table 3. Equilibrium and Rate Constants for the Approach
to Equilibrium
reported to slowly disproportionate into [BDI-OMe] Zn and
2
24b
ZnEt2.
This difference may be attributed to the lower
propensity of MeO···Zn interactions in the (BDI)ZnR
complexes; the Zn center in BDI complexes is apparently
less Lewis acidic due to the stronger donor ability of
diketiminate ligands in comparison to formazanates.
0
ΔG
kf
kr
(M− min−1
1
)
(M−1 min−1
)
compound
Keq
0.17
4.0 × 10
0.071
7.6 × 10
5.9 × 10
0.20
(kJ/mol)
1
5.2
16.2
7.8
21.1
15.1
4.7
58.9
24.2
19.6
1.21
3.04
150
345
6075
277
1590
514
−
3
1
2
3
4
5
(THF)
When the ligand redistribution of complex 1 was monitored
1
by H NMR in the more polar solvent THF-d , the formation
8
−4
3
of the homoleptic species was suppressed significantly.
−
Although k is not affected much by the change in solvent, kr
f
755
for complex 1 is ca. 20 time larger in THF-d than in C D ,
8
6
6
6
7
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resulting in a much smaller equilibrium constant (K = 4.0 ×
the NNOO plane (vide supra) effectively shield access to the
zinc center from one side, which results in a high Vbur of 57.0%.
eq
−
3
1
0 ). This may be attributed to the formation of a more stable
four-coordinate THF adduct in solution.
Steric Maps and Buried Volume. By calculation of the
CONCLUSIONS
■
buried volume (V ) based on the solid-state structures of
bur
The influence of the steric environment around the zinc center,
introduced by the formazanate ligand, on the stability of
mono(formazanate)zinc methyl complexes was investigated.
The equilibrium between the mono(formazanate)zinc methyl
complexes and the corresponding homoleptic species can be
related to the steric effects of the aromatic groups. More
sterically demanding N-Ar substituents shift the equilibrium to
favor the mono(formazanate)zinc methyl species and retard
the forward reaction. However, it appears that electronic
factors are also of importance for the rate of approach to
equilibrium, with electron-withdrawing groups exhibiting an
increased reaction rate. Examination of the solid-state
structures of complexes 1−6 revealed that the presence of
substituents on the ortho-position of the aromatic ring induces
an out-of-plane twist of the aromatic group. This rotation of
the N-Ar groups out of the NNCNN plane leads to a loss of
conjugation of the Ar substituents with the NNCNN
backbone. The effect of this loss of conjugation can be
observed in the electronic and optical properties, where a blue-
shift was observed in the absorption spectra. A similar trend is
observed for the redox properties, with more cathodic
potentials required for the first reduction upon going from 1
to 3. However, electronic properties can override this effect as
the first reduction of complex 5 is shifted anodically by 90 mV
in comparison to 1, which is attributed to the electron
withdrawing effect of the chloride substituents. The Gibbs free
energy change of the ligand redistribution reaction was found
to be correlated to the “buried volume” (a measure of the
ligand steric properties), showing that the ligand environment
around the zinc center has a significant influence on the
kinetics and thermodynamics of the ligand redistribution. In
addition to stabilizing three-coordinate (formazanate)zinc
methyl complexes via steric effects, the introduction of
additional OMe donor groups as substituents in complex 6 is
shown to completely suppress ligand exchange.
complexes 1−6, the steric pressure exerted by the ligands was
2
8
quantified. The effect of the N-Ar substituents on the steric
environment can be visualized by steric maps shown in Figure
6
, where the introduction of one or two mesityl groups (2 and
Figure 6. Steric maps and buried volumes of the ligands in complexes
−6. Showing a 3.5 Å sphere around the Zn atom, blue is to the rear
and red to the front of the metal.
1
3
, respectively) results in an increased occupancy of the first
coordination sphere around the zinc center with %Vbur
increasing from 43.9 (1) to 47.9 (2) to 51.5 (3). This increase
in Vbur is correlated to the increased stability of the
mono(formazanate)zinc methyl complexes, represented by a
higher ΔG for the equilibrium reaction (Figure 7). Compound
4
also follows this trend, as the V of 49.7% is in between that
bur
of 2 and 3, congruent with a ΔG of 15.1 kJ/mol.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations, except for ligand
synthesis, were performed under a nitrogen atmosphere by using
glovebox, Schlenk, and vacuum-line techniques. Glassware was dried
before use at 150 °C. The reagents used for the synthesis of ligands
L1H, L2H, L3H, L4H, L5H, and L6H were used as received; 2,6-
diisopropyl-1-bromobenzene (Fluorochem & ABCR), Mg turnings
(
Acros), di-tert-butyl azodicarboxylate (Fluorochem & Sigma-
Aldrich), CCl4 (Acros), PPh3 (Merck), Et N (Sigma-Aldrich),
3
dimethyl sulfide (Sigma-Aldrich), N-chlorosuccinimide (Sigma-
Aldrich), p-tolualdehyde (Sigma-Aldrich), p-toluoyl chloride (Sigma-
Aldrich), phenylhydrazine (Sigma-Aldrich), and 2,6-dichlorophenyl-
18
18
hydrazine hydrochloride (Fluorochem). The ligands L1H, L2H, ,
Figure 7. Correlation between Vbur and the Gibbs free energy change
18
21
L3H, and L6H were synthesized via literature procedures.
Toluene and hexane (Sigma-Aldrich, anhydrous, 99.8%) were passed
over columns of Al O (Fluka), BASF R3-11-supported Cu oxygen
(ΔG) at 80 °C.
2
3
scavenger, and molecular sieves (Sigma-Aldrich, 4 Å). THF (Sigma-
Aldrich, anhydrous, 99.8%) was dried by percolation over columns of
Al O (Fluka). Deuterated solvents were vacuum transferred from the
A similar analysis for 5 shows that it has the highest Vbur in
2
3
the series (52.9%). However, as observed in the character-
ization data discussed above, the electronic effect of the
chlorine distinguishes it from the others, and this is also
reflected in 5 breaking the trend between Vbur and ΔG (Figure
). For compound 6, the steric map clearly shows that the
coordination of the OMe and the displacement of Zn out of
Na/K alloy (C D , toluene-d , THF-d , Euriso-top) and stored under
6
6
8
8
nitrogen. Dimethylzinc (Sigma-Aldrich, 2.0 M in toluene or ABCR, 10
wt % in hexane) was used as received.
NMR spectra were recorded on Agilent 400 MR, Varian Mercury
Plus 400, Varian Inova 500, or Bruker Avance NEO 600
7
1
13
spectrometers. The H and C NMR spectra were referenced
6
8
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Organometallics 2021, 40, 63−71

Organometallics
pubs.acs.org/Organometallics
Article
internally by using the residual solvent resonances and reported in
ppm relative to TMS (0 ppm). Electrochemical measurements were
General Procedure for the Synthesis of Mono-
(formazanate)zinc Methyl Complexes. Formazan was dissolved
in toluene, and 1.2 equiv of dimethylzinc (as a 2.0 M solution in
toluene) was added at room temperature with stirring. After the
appropriate reaction time (given for each compound below), the
volatiles were removed under reduced pressure. The solid product
was isolated and analyzed by NMR spectroscopy, which indicated full
conversion to the desired product. The bulk purity of the solids was
established by elemental analysis. Crystals suitable for X-ray
diffraction were subsequently obtained as indicated below.
performed under an inert N atmosphere in a glovebox by using an
2
Autolab PGSTAT 204 computer-controlled potentiostat. Data were
recorded with an Autolab NOVA software (ver. 2.1.4). Cyclic
voltammetry (CV) was performed by using a three-electrode
configuration comprised of a Pt wire counter electrode, a Ag wire
pseudoreference electrode, and a Pt disk working electrode (CHI102,
CH Instruments, diameter = 2 mm). The Pt working electrode was
polished before experiment using alumina slurry (0.05 μm), rinsed
with distilled water, and subjected to brief ultrasonication to remove
any adhered alumina microparticles. The electrodes were then dried
in an oven at 75 °C overnight to remove any residual traces of water.
The CV data were calibrated by adding ferrocene to the THF solution
at the end of experiments. In all cases, there is no indication that
addition of ferrocene influences the electrochemical behavior of
products. All electrochemical measurements were performed at
ambient temperatures under an inert N2 atmosphere in THF
containing 0.1 M [Bu N][PF ] as the supporting electrolyte. UV/
[PhNNC(p-tolyl)NNPh]ZnMe [1]. PhNNC(p-tolyl)NNHPh (715
mg, 2.27 mmol) was reacted in 20 mL of toluene with Me Zn for 30
2
min. A color change was observed from dark red to dark violet
concurrent with gas evolution. Removal of the volatiles under reduced
pressure gave 705 mg of a dark blue powder (1.79 mmol, 89% yield).
Crystals suitable for X-ray diffraction were obtained by cooling a
1
concentrated toluene solution to −30 °C. H NMR (400 MHz, C
D
,
6
6
25 °C): δ 8.34 (d, 2H, p-tol o-CH), 7.83 (d, 4H, Ph o-CH), 7.31 (d,
2H, p-tol m-CH), 7.20 (t, 4H, Ph m-CH), 7.04 (t, 2H, Ph p-CH),
4
6
−
5
13
vis spectra were recorded in a toluene solution (≈ 10 M) by using
an Avantes AvaSpec-2048 UV/vis spectrophotometer.
2.27 (s, 3H, p-tol p-CH
(126 MHz, C
3
), −0.13 (s, 3H, Zn-CH
3
) ppm. C NMR
D
, 25 °C): δ 154.01, 144.11, 137.93, 136.92, 129.68,
6
6
Synthesis of 1-(Phenyl)-5-(2,6-diisopropylphenyl)-3-(p-
tolyl)formazan [L4H]. N′-(Phenyl)-4-methylbenzohydrazonoyl
chloride (1.018 g, 4.16 mmol) was dissolved in 50 mL of EtOH
129.44, 127.71, 126.20, 121.12, 21.29, −9.02 ppm. Anal. Calcd for
C H N Zn: C 64.05, H 5.12, N 14.23; found: C 64.44, H 5.16, N
2
1
20
4
13.93.
[PhNNC(p-tolyl)NNMes]ZnMe [2]. PhNNC(p-tolyl)NNHMes
(70 mg, 0.20 mmol) was reacted in 3 mL of toluene with Me Zn
(100%), and triethylamine (0.993 g, 9.81 mmol) was added to the
mixture. The solution was stirred, and (2,6-diisopropylphenyl)-
hydrazine HCl was added in portions (1.001 g in total, 4.38
mmol). The reaction was stirred overnight, and the volatiles were
removed under reduced pressure. The resulting dark red oil was
dissolved in ethyl acetate, and the suspension that formed was filtered.
The filtrate was concentrated under reduced pressure to a sticky oil, to
which 3 mL of DCM was added, which was layered with methanol
and put in a freezer at −30 °C. After 3 days the layers had mixed and
crystals had formed, which were filtered off, washed with cold
2
for 3 h. A color change was observed from deep red to violet
concurrent with gas evolution. Removal of the volatiles under reduced
pressure afforded 57 mg of 2 (0.13 mmol, 66% yield). Crystals were
1
obtained by cooling a concentrated toluene solution to −30 °C. H
NMR (400 MHz, C D , 25 °C): δ 8.30 (d, 2H, p-tol o-CH), 7.95 (d,
6 6
2H, Ph o-CH), 7.24−7.18 (m, 4H, p-tol m-CH and Ph m-CH), 7.03
(t, 1H, Ph p-CH), 6.74 (s, 2H, Mes m-CH), 2.21 (s, 3H, p-tol p-
CH
3H, Zn−CH
), 2.10 (s, 3H, Mes p-CH
), 2.05 (6H, Mes o-CH
) ppm. C NMR (101 MHz, C , 25 °C): δ 154.20,
), −0.28 (br s,
3
3
3
1
3
methanol, and dried in vacuo. The product was obtained as dark red
3
D
6 6
1
crystals (571 mg, 1.43 mmol, 34.4%). H NMR (400 MHz, CDCl , 25
149.06, 145.15, 137.27, 137.04, 136.70, 130.62, 130.10, 129.71,
3
°
C): δ 14.94 (s, 1H), 7.99 (d, 2H), 7.60 (d, 2H), 7.43 (t, 2H), 7.31
129.69, 127.51, 125.81, 120.80, 21.23, 20.92, 18.64, −12.98 ppm.
(
s, 3H), 7.25−7.19 (m, 3H), 3.30 (sept, 2H), 2.41 (s, 3H) 1.28 (d,
Anal. Calcd for C24
H N Zn: C 66.13, H 6.01, N 12.85; found: C
26 4
1
1
1
2H) ppm. 13C NMR (151 MHz, CDCl , 25 °C): δ 146.75, 144.62,
65.99, H 6.01, N 12.71.
[MesNNC(p-tolyl)NNMes]ZnMe [3]. MesNNC(p-tolyl)-
NNHMes (50 mg, 0.125 mmol) was reacted in 3 mL of toluene
3
38.17, 137.36, 131.29, 130.74, 129.57, 129.30, 125.84, 125.54,
17.28, 21.41, 21.22, 21.11 ppm. MS (ESI, negative mode): exact
−
mass calculated for [C H N ] : 397.2398; exact mass found:
3
with Me Zn for 18 h, during which the solution took on a light orange
2
2
6
29
4
97.2403; difference: + 1.3 ppm.
Synthesis of 1,5-Bis(2,6-dichlorophenyl)-3-(p-tolyl)-
color. Removal of the volatiles afforded 40 mg of 3 (0.084 mmol, 67%
yield). Crystals suitable for X-ray analysis were obtained by slow
1
formazan [L5H]. (2,6-Diisopropylphenyl)hydrazine HCl (200 mg,
0
evaporation of C D . H NMR (600 MHz, C D , 25 °C): δ 8.23 (d,
6
6
6
6
.94 mmol) was added to 10 mL of EtOH (100%) to give a
2H, p-tol o-CH), 7.13 (d, 2H, p-tol m-CH), 6.78 (s, 4H, Mes CH),
2.17 (s, 12H, Mes o-CH ), 2.14 (s, 3H, p-tol CH ), 2.12 (6H, Mes p-
suspension; addition of triethylamine (127 mg, 1.26 mmol) dissolved
all solids and afforded a clear solution. The solution was stirred, and
N′-(2,6-dichlorophenyl)-4-methylbenzohydrazonoyl chloride (220
mg, 0.70 mmol) was added, after which a very gradual color change
was noticeable. The reaction was stirred overnight, and the next day
the reaction mixture had turned deep red. The volatiles were removed
under reduced pressure, resulting in a dark red oil which was dissolved
in minimal DCM, layered with methanol, and placed in a freezer at
3
3
1
3
CH ), −0.38 (br. s, 3H, Zn−CH ) ppm. C NMR (151 MHz, C D ,
3
3
6
6
25 °C): δ 149.08, 145.78, 136.97, 136.93, 136.56, 130.71, 130.22,
129.70, 125.35, 21.15, 20.93, 18.75, −15.61 ppm. Anal. Calcd for
C H N Zn: C 67.85, H 6.75, N 11.72; found: C 67.87, H 6.87, N
2
7
32
4
11.61.
[DiPPNNC(p-tolyl)NNPh]ZnMe [4]. PhNNC(p-tolyl)NNHDiPP
(126 mg, 0.32 mmol) was reacted in 6 mL of toluene with Me Zn for
2
−
30 °C. After 4 days the layers had diffused into each other, and
6 h, leading to a magenta solution, which upon removal of the
volatiles afforded 116 mg of 4 (0.24 mmol, 76% yield). Crystallization
was achieved by slow diffusion of hexane into a concentrated toluene
solution containing the product and a drop of dimethylzinc to prevent
crystals had formed. Dark red crystals were collected, washed with
methanol, and dried in vacuo to give 38 mg of L5H. The filtrate was
concentrated under reduced pressure to a sticky oil, to which 3 mL of
DCM was added, which was layered with methanol and put in a
freezer at −30 °C. After 3 days the layers had mixed, and crystals had
formed, which were isolated, washed with cold methanol, and dried in
1
the formation of the bis(formazanate)zinc complex. H NMR (400
MHz, C D , 25 °C): δ 8.29 (d, 2H, p-tol o-CH), 7.98 (d, 2H, Ph o-
6
6
CH), 7.22−7.17 (m, 5H*, p-tol m-CH and Ph m-CH and DIPP p-
CH), 7.12 (d, 2H, DIPP m-CH), 7.01 (t, 1H, Ph p-CH), 2.88 (sept,
2H, DIPP CH(CH ) ), 2.19 (s, 3H, p-tol p-CH ), 1.09 (d, 12H, DIPP
vacuo to give another 32 mg of product. The total yield of L5H was
1
7
1
2
1
0 mg (0.155 mmol, 17%). H NMR (400 MHz, CDCl , 25 °C): δ
3
3
2
3
3.99 (s, 1H), 7.99 (d, 2H), 7.42 (d, 4H), 7.24 (d, 2H), 7.13 (t, 2H),
CH(CH ) ), −0.27 (br s, 3H, Zn−CH ) ppm *(overlapping with
3
2
3
1
3
13
.40 (s, 3H) ppm. C NMR (101 MHz, CDCl , 25 °C): δ 143.65,
41.99, 138.25, 133.43, 129.73, 129.35, 127.58, 127.43, 126.37, 21.47
solvent). C NMR (101 MHz, C D , 25 °C): δ 154.12, 148.42,
3
6 6
145.27, 141.82, 137.12, 137.02, 129.78, 129.73, 127.58, 125.79,
123.99, 120.71, 28.71, 24.66, 23.18, 21.21, −14.19 ppm. Anal. Calcd
for C H N Zn: C 67.85, H 6.75, N 11.72; found: C 67.90, H 6.79,
ppm. MS (ESI, negative mode): exact mass calculated for
−
[
C H N Cl ] : 450.9870; exact mass found: 450.9872; difference:
20 13 4 4
27 32
4
+
0.4 ppm.
N 11.70.
6
9
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Organometallics 2021, 40, 63−71

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Article
[
2,6-DiCl-PhNNC(p-tolyl)NN-2,6-DiCl-Ph]ZnMe [5]. 2,6-DiCl-
ACKNOWLEDGMENTS
■
PhNNC(p-tolyl)NN-2,6-diCl-Ph (24 mg, 0.053 mmol) was reacted in
mL of toluene with Me Zn for 6 h, which gave a deep red solution.
Subsequent removal of the volatiles afforded 18 mg of 5 (0.034 mmol,
4% yield). Crystals suitable for X-ray analysis were grown by cooling
Financial support from The Netherlands Organisation of
Scientific Research (NWO) is gratefully acknowledged.
1
2
6
a concentrated hexane solution, to which a drop of ZnMe solution
2
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AUTHOR INFORMATION
Corresponding Author
Edwin Otten − Stratingh Institute for Chemistry, University of
Groningen, 9747 AG Groningen, The Netherlands;
orcid.org/0000-0002-5905-5108; Email: edwin.otten@
rug.nl
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Folkert de Vries − Stratingh Institute for Chemistry,
University of Groningen, 9747 AG Groningen, The
Netherlands
Raquel Travieso-Puente − Stratingh Institute for Chemistry,
University of Groningen, 9747 AG Groningen, The
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Peter Roewen − Stratingh Institute for Chemistry, University
of Groningen, 9747 AG Groningen, The Netherlands
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00720
of Epoxides and CO Using β-Diiminate Zinc Catalysts: Evidence for
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Notes
The authors declare no competing financial interest.
7
0
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Organometallics 2021, 40, 63−71