Dipyrromethene and β-Diketiminate Zinc Hydride Complexes: Resemblances and Differences

Article abstract of DOI:10.1021/acs.organomet.9b00334

A new dipyrromethene (DPM) ligand with bulky DIPP-substituents is introduced (DIPP = 2,6-diisopropylphenyl). The ligand, abbreviated as ^DIPPDPM, was deprotonated with ZnEt₂ to give (^DIPPDPM)ZnEt, which reacted with I₂ to form (^DIPPDPM)ZnI. Reaction of the latter with K[N(ⁱPr)HBH₃] afforded a labile Zn amidoborane complex which, after β-hydride elimination, formed (^DIPPDPM)ZnH. Crystal structures of (^DIPPDPM)ZnX (X = Et, I, H) revealed their monomeric nature. The Zn-N bond distances are somewhat longer than those in the corresponding monomeric β-diketiminate complexes (^DIPPBDI)ZnX (^DIPPBDI = CH[C(Me)N-DIPP]₂). This is in agreement with calculated NPA charges, which are lower on the N atoms of DPM compared to those on BDI. Reaction of (^DIPPDPM)ZnH with CO₂ gave (^DIPPDPM)Zn(O₂CH), which crystallized as a monomer with a symmetrically bound η²-formate ligand. In contrast, the β-diketiminate complex crystallizes as a dimer [(^DIPPBDI)Zn(O₂CH)]₂ with bridging formate ligands. Reaction of (^DIPPDPM)Zn(O₂CH) with various silanes regenerated the hydride complex (^DIPPDPM)ZnH. Catalytic CO₂ hydrosilylation with (EtO)₃SiH using (^DIPPDPM)ZnH as a catalyst gave full reduction to [Si]-OMe species, whereas the catalyst (^DIPPBDI)ZnH only partially reduced CO₂ to [Si]-OC(O)H. The advantage of the ^DIPPDPM ligand is the arrangement of the DIPP-substituents, which form a pocket around the Zn-X unit, preventing dimerization and influencing its reactivity. In addition, in contrast to the negatively charged central backbone carbon in the ^DIPPBDI ligand, that in ^DIPPDPM is neutral. This makes it less nucleophilic and Br?nsted basic, as expected for a true spectator ligand.

Full text of DOI:10.1021/acs.organomet.9b00334

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Dipyrromethene and β‑Diketiminate Zinc Hydride Complexes:
Resemblances and Differences
Gerd Ballmann, Samuel Grams, Holger Elsen, and Sjoerd Harder*
̈
̈
Inorganic and Organometallic Chemistry, Friedrich-Alexander-Universitat Erlangen-Nurnberg, Egerlandstraße 1, 91058 Erlangen,
Germany
S
* Supporting Information
ABSTRACT: A new dipyrromethene (DPM) ligand with
bulky DIPP-substituents is introduced (DIPP = 2,6-
diisopropylphenyl). The ligand, abbreviated as DIPPDPM, was
deprotonated with ZnEt2 to give (DIPPDPM)ZnEt, which
reacted with I2 to form (DIPPDPM)ZnI. Reaction of the latter
with K[N(iPr)HBH3] afforded a labile Zn amidoborane
complex which, after β-hydride elimination, formed
(
DIPPDPM)ZnH. Crystal structures of (DIPPDPM)ZnX (X =
Et, I, H) revealed their monomeric nature. The Zn−N bond
distances are somewhat longer than those in the correspond-
ing monomeric β-diketiminate complexes (DIPPBDI)ZnX
(
DIPPBDI = CH[C(Me)N-DIPP]2). This is in agreement with calculated NPA charges, which are lower on the N atoms of
DPM compared to those on BDI. Reaction of (DIPPDPM)ZnH with CO2 gave (DIPPDPM)Zn(O2CH), which crystallized as a
monomer with a symmetrically bound η2-formate ligand. In contrast, the β-diketiminate complex crystallizes as a dimer
[(DIPPBDI)Zn(O2CH)]2 with bridging formate ligands. Reaction of (DIPPDPM)Zn(O2CH) with various silanes regenerated the
hydride complex (DIPPDPM)ZnH. Catalytic CO2 hydrosilylation with (EtO)3SiH using (DIPPDPM)ZnH as a catalyst gave full
reduction to [Si]−OMe species, whereas the catalyst (DIPPBDI)ZnH only partially reduced CO2 to [Si]−OC(O)H. The
advantage of the DIPPDPM ligand is the arrangement of the DIPP-substituents, which form a pocket around the Zn−X unit,
preventing dimerization and influencing its reactivity. In addition, in contrast to the negatively charged central backbone carbon
in the DIPPBDI ligand, that in DIPPDPM is neutral. This makes it less nucleophilic and Brønsted basic, as expected for a true
spectator ligand.
high thermal stability of (MgH2)∞ (H2 desorption > 300 °C)
compared to that of (ZnH2)∞ (slow H2 desorption at 20 °C).
Using a bulky tridentate scorpionate ligand, the Parkin group
reported the first monomeric zinc hydride complex with a
terminal Zn−H bond (I).11 We introduced a monomeric β-
diketiminate zinc hydride complex (DIPPBDI)ZnH (II)
INTRODUCTION
■
The first detailed report on the synthesis of zinc dihydride
appeared in 1951.1 Schlesinger noted that zinc dihydride is
intermediate in chemical character between aluminum hydride
and the s-block metal hydrides. Together with LiAlH4 and its
closely related hydridozincates, these metal hydrides are
powerful reducing agents.2,3 However, (ZnH2)∞ is a white
powder that at room temperature turns gray due to formation
of metallic Zn and H2. It never found widespread utilization
due to its limited thermal stability and insolubility in organic
solvents.1
This prompted interest in more suitable molecular
heteroleptic organozinc hydrides.4,5 Simple alkyl or aryl zinc
hydrides also show a limited thermal stability but can be
stabilized by coordination of additional Lewis bases, as has
been reported for RZnH·(pyridine) (R = Et, Ph).6 Moreover,
sterically demanding ligands that prevent aggregation were
found to stabilize zinc hydride species.7,8 In contrast to Mg
hydride complexes, for which it has been shown that the H2
desorption temperature increases with increasing aggregation,9
Zn hydride complexes tend to become less stable with
increasing cluster size.10 This is in agreement with the very
(
DIPPBDI = CH[C(Me)N-DIPP]2, DIPP = 2,6-diisopropyl-
phenyl)12 in which the metal features an unusually low
coordination number of three. In the solid state, this complex
releases 0.5 equiv of H2 at 150 °C.10 Complex III, in which
two (BDI)ZnH units have been coupled, crystallized as a
tetramer and, when heated at 80 °C as a solid, desorbs two
equivalents of H2. Depending on the solvent, in solution H2
evolution is observed between 20 and 50 °C.10 Circumventing
formation of H−Zn−H bridges seems to be the key to high
thermal stability. Therefore, well-defined monomeric Zn
hydride complexes with terminal Zn−H bonds are desirable
entities for reactivity studies.13 They are attractive model
systems for zinc enzymes11,14−16 or can be used as precursors
in material chemistry.17 Due to their high reactivity, molecular
Received: May 20, 2019
© XXXX American Chemical Society
A
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zinc hydrides also found application in catalysis4 varying from
hydrosilylation of unsaturated substrates8,18−25 to dehydrogen-
ative coupling of alcohols and silanes.8,20,23,26 Most recently,
Chen and coworkers introduced a simple β-diketiminate zinc
hydride complex in which mononuclearity is enforced by a
combination of sterics and a pendant amine donor arm (IV).24
This tetracoordinate zinc hydride complex exhibited unprece-
dentedly high activities in CO2 hydrosilylation catalysis. In
contrast, the zinc hydride complex II, which is monomeric only
on behalf of steric pressure, was hardly active. This is likely due
to catalyst decomposition during one of the stages in the
catalytic cycle.24
and its performance in the catalytic CO2 hydrosilylation. At all
stages, we directly compare the chemistry of (DIPPDPM)ZnH
with that of the closely related complex (DIPPBDI)ZnH (II).
RESULTS AND DISCUSSION
■
Syntheses. The synthesis of the new ligand DIPPDPM-H is
shown in Scheme 1. 2-(2,6-Di-iso-propylphenyl)-1H-pyrrole
(1) was prepared by slightly modifying Sadighi’s procedure35
and was obtained as a yellow powder in 78% yield. Acid-
catalyzed condensation (cat. PPTS = pyridinium p-toluenesul-
fonate) of this building block with mesitaldehyde dimethyl
acetal gave 2 in a yield of 96%. This intermediate was
subsequently oxidized by DDQ (2,3-dichloro-5,6-dicyano-1,4-
benzoquinone), affording dipyrromethene DIPPDPM-H (3).
The latter was isolated in the form of large orange crystals in a
yield of 62%.
For the synthesis of its Zn hydride complex, we slightly
modified our earlier procedure for (DIPPBDI)ZnH (II);12 see
Scheme 1. Whereas (DIPPBDI)ZnEt was obtained by heating
the β-diketiminate ligand overnight with ZnEt2 at 80 °C in
toluene,36 the deprotonation of DIPPDPM-H by ZnEt2
proceeded at room temperature (Scheme 1), indicating that
this ligand is more acidic than the corresponding β-diketimine.
The alkylzinc complex (DIPPDPM)ZnEt (4) was isolated in
86% yield in the form of orange crystals that are, as is well-
known for dipyrromethene metal complexes,37 highly fluo-
rescent in daylight. A solution of complex (DIPPDPM)ZnEt in a
benzene/n-heptane mixture reacted at room temperature
smoothly with I2 and gave after removal of all volatiles the
corresponding iodide complex (DIPPDPM)ZnI (5) in quanti-
tative yield. Subsequent reaction with K[N(iPr)HBH3] gave
the target Zn hydride complex (DIPPDPM)ZnH (6), which is
most likely formed via β-hydride elimination in a labile Zn
amidoborane intermediate.12 After removal of various
oligomeric BN side products by filtration, 6 crystallized from
cold toluene as orange blocks in a yield of 86%.
Crystal Structures. The crystal structure of the ligand is
shown in the Supporting Information (Figure S1), and those of
the Zn complexes (4−6) are displayed in Figure 1. All
DIPPDPM complexes are monomeric entities featuring three-
coordinate Zn atoms with a perfectly planar trigonal
coordination geometry (the sum of the valence angles at Zn
lies in all cases between 359.86(13)° and 360.00(8)°. There is
not much variation in Zn−N bond distances: 1.940(2)−
1.992(2) Å (see Figure 2a). For comparison, selected distances
and angles for the analogue DIPPBDI complexes are also given.
The Zn−X (X = Et, I, H) bond distances in the BDI and DPM
complexes are within standard deviation equal to each other.
The largest difference between the two sets of complexes can
be found in the Zn−N distances which are consistently on
average always 0.023−0.028 Å longer for the DPM complexes.
Related to this, the N−Zn−N′ bite angles in the DPM
complexes are systematically always 2.7−3.7° smaller. The
largest difference between BDI and DPM complexes can be
found in the orientation of the DIPP-substituents with respect
to each other. While the least-squares planes for the aryl rings
in the BDI complexes make a rather large angle of 126.36(4)−
138.44(15)° with each other, interplanar angles in the range of
44.33(6)−47.23(9)° have been found for the DPM complexes
(Figure 2b). The Zn atom in the DPM complexes is positioned
in a cavity spanned by the two DIPP-substituents. The ligand
bulk in the Zn hydride complexes was quantified by analysis of
its buried volume (Vbur).38 For the DIPPDPM ligand, a Vbur of
The current work aims at the synthesis of a highly reactive
three-coordinate zinc hydride complex which is monomeric on
account of remote steric hindrance, thus leaving the
coordination sphere largely open for reactivity. To this end,
we replaced the BDI ligand in II for a dipyrromethene system
(DPM). This ligand, which is a partial unit of porphyrin, has
been known since 1924.27 It received major attention due to
applications in metal organic frameworks, light-harvesting
arrays, coordination polymers, C−H activation, and function-
alization chemistry.28 In addition, there is a vast body of
literature involving dipyrromethenes in relation to borondi-
fluoride complexes (BODIPY), which find application as
highly efficient fluorophores.28,29 Following Betley’s pioneering
work on dipyrromethene Fe and Co complexes,30−32 Chisholm
and coworkers introduced the mesityl-substituted ligand
MesDPM to Mg chemistry (V)33 and at a later stage reported
a similar Zn alkoxide catalyst for lactide polymerization (VI).34
Herein, we present the synthesis of a bulkier DIPPDPM ligand
with DIPP-substituents in the pole positions and discuss
synthesis, structure, and a charge analysis of (DIPPDPM)ZnH.
In addition, we report on its stoichiometric reactivity with CO2
B
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Scheme 1. Synthesis of the Dipyrromethene Ligand DIPPDPM-H (3) and Corresponding Dipyrromethene Zinc Complexes (4−
7)
Figure 1. Crystal structures of (a) (DIPPDPM)ZnEt (4), (b) (DIPPDPM)ZnI (5), and (c) (DIPPDPM)ZnH (6). Selected distances are schematically
shown in Figure 2a; ORTEP figures can be found in the Supporting Information.
64.7% was calculated, a value that is considerably larger than
the Vbur of 61.7% for the DIPPBDI ligand. More important, the
steric maps clearly show a pronounced cavity around the metal
in (DIPPDPM)ZnH (Figure 2c). The closest Zn-ring distances
lie in the range of 3.325(2)−3.419(2) Å and are too long for a
bonding interaction. A space filling model of (DIPPDPM)ZnH
clearly shows that, despite flanking bulky DIPP-substituents,
there is still ample space around the metal for reactivity with
substrates (Figure 2d).
ppm in comparison to that in (DIPPBDI)ZnH (4.39 ppm). The
chemical shift for the hydride in the latter BDI complex was
found to be strongly dependent on the temperature, which was
explained by a monomer−dimer equilibrium in toluene
solution.12 Variable temperature studies on the DPM analogue,
however, hardly showed a temperature dependence of the Zn−
H chemical shift (Figure S13). Diffusion-ordered spectroscopy
(DOSY) of (DIPPDPM)ZnH in C6D6 clearly demonstrates its
monomeric nature in solution (MWexp = 631, MWcalcd = 648);
see Supporting Information. This is in agreement with
Chisholm’s observation that the MesDPM ligand in V is more
sterically demanding than the comparable MesBDI ligand, and
therefore, V is more stable toward ligand scrambling than the
corresponding MesBDI complex.33
DFT Calculations. Optimization at the B3PW91/6-311+
+G** level reproduced the crystal structures of (DIPPDPM)-
ZnH and (DIPPBDI)ZnH rather well (Figure S21, see
Supporting Information) and frequency calculations confirmed
these to be true minima on the potential energy surface. At the
same level of theory, the NPA charges were calculated (Figure
3).
NMR Analyses. The fluorescent Zn complexes 4−6 all
1
dissolve well in aromatic solvents, and their H NMR spectra
in C6D6 are in agreement with the approximate C2v symmetry
of their crystal structures. Each DIPP substituent shows one
heptet for the CH(Me)2 proton and two doublets for the
CH(Me)2 groups (one for the Me groups pointing toward the
Zn and one for those at the side of the ligand backbone). The
most striking feature of DPM complexes is the considerable
high-field shift of NMR signals for ligands that are bound in
the cleft of the DPM ligand. Being sandwiched between two
rings, there is a strong aromatic solvent induced shift (ASIS)
effect resulting in an upfield shift for the Zn-Et signals from
0.89/0.24 ppm in (DIPPBDI)ZnEt to 0.46/−0.31 ppm in
The charges on Zn (DPM +1.06/BDI +1.10) and the
hydride (DPM −0.44/BDI −0.47) in both systems are in a
similar range but the Zn−H bond in the BDI complex is
(
(
DIPPDPM)ZnEt. Also, the hydride 1H NMR signal for
DIPPDPM)ZnH (3.78 ppm) is high-field shifted by 0.61
C
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Figure 2. (a) Selected distances in the monomeric complexes 4−6 compared to those in the corresponding monomeric DIPPBDI complexes. (b)
Illustration of the different arrangement of DIPP-substituents in DPM and BDI ligands. (c) Steric maps for the buried volume in (DIPPDPM)ZnH
(Vbur = 64.7%) compared to that in (DIPPBDI)ZnH (Vbur = 61.7%); 3.5 Å sphere around Zn, blue is to the rear and red to the front of the metal. (d)
Space-filling models for (DIPPDPM)ZnH and (DIPPBDI)ZnH.
slightly more polar. The most important difference between
the two complexes lies in the charge distribution within the
N,N-chelating ligands. The β-diketiminate ligand shows an
alternating charge distribution Nδ−−Cδ+−Cδ‑−Cδ+−Nδ− that is
in agreement with its resonance structures. Most of the
negative charge is located on the N atoms (−0.76), but there is
still a substantial electron density on the central backbone C
atom (−0.41). For the dipyrromethene ligand, the negative
charge is spread over both pyrrole rings, and therefore,
significantly lower charges on N (−0.71) and a neutral central
backbone C (−0.01) were calculated. The difference in
electron density on the backbone carbons is partially due to
different substituents (C−H versus C−Mes), but group
charges on C−H (−0.20) in BDI are also clearly more
negative than those on C−Mes in DPM (+0.02). The low
electron density on C in the DPM ligand can be explained by
D
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Figure 3. NPA charges for (DIPPDPM)ZnH and (DIPPBDI)ZnH calculated at the B3PW91/6-311++G** level (atoms are color coded: red =
positive, blue = negative) and resonance structures for the DPM and BDI anions.
the mesomeric forms for the dipyrromethene anion: there is no
resonance structure with a formal negative charge on the
backbone C (Figure 3). This most striking difference suggests
that the backbone C in the dipyrromethene scaffold is much
less nucleophilic than that in the β-diketiminate ligand.
Whereas β-diketiminate complexes are often plagued by
undesired side reactions that involve nucleophilic attack of
the ligand’s backbone,39 the reactivity of the dipyrromethene
complex (DIPPDPM)ZnH is expected to be centered at the
Znδ+−Hδ− unit. The protection of the central backbone C by
the mesityl substituent likely also suppresses electrophilic
attack at this weak spot.
bonds of 2.0516(12) and 2.0699(15) Å (Figure 4). Equal C−
O bonds of 1.254(3) and 1.256(2) Å verify complete
delocalization of negative charge. It should be noted that this
is a unique bonding mode for Zn-formate. The corresponding
β-diketiminate complex crystallizes as a dimer with μ2-(O2CH)
ligands bridging between two Zn centers. All monomeric Zn
formate complexes display η1-bonding of the formate ligand
with at most weak intramolecular coordination of the CO
bond.14,25,40 This is due to the fact that these complexes
generally contain bulky three-coordinate ligands which
consequently do not leave sufficient room at the metal for
η2-bonding of the formate ligand. Also, Zn(O2CH)2 complexes
stabilized by neutral bidentate N,N-ligands show η1-coordina-
tion of both formate ligands.41 The dipyrromethene framework
is unique in the sense that it is a bidentate ligand which leaves
ample space at the metal for η2-bonding of the formate ligand
but at the same time blocks the formation of dimeric
complexes.
Stoichiometric and Catalytic Reactivity. Similar to
(
DIPPBDI)ZnH,24 a solution of (DIPPDPM)ZnH in toluene-d8
readily reacted at room temperature with 1 bar of CO2, and full
1
conversion was obtained within 20 min (Scheme 1). The H
NMR signal for Zn−H (3.78 ppm) disappeared, and a new
singlet assigned to the formate unit Zn(O2CH) emerged at
7.19 ppm. The latter signal is considerably high-field shifted in
comparison with the formate resonances in the analogue β-
diketiminate complex (DIPPBDI)Zn(O2CH): dimer 8.53 ppm,
monomer 7.91 ppm.24 The high-field shift for the formate
anion in (DIPPDPM)Zn(O2CH) is typical for a ligand that is
bound in the cleft spanned by the DIPP-substituents. The
DOSY measurements of (DIPPDPM)Zn(O2CH) in C6D6
clearly demonstrate that it is also monomeric in solution
(MWexp = 591, MWcalcd = 691); see Supporting Information.
This contrasts with the corresponding β-diketiminate complex:
a solution of the dimer [(DIPPBDI)Zn(O2CH)]2 in C6D6 gave
at room temperature two sets of sharp signals for species
which, based on DOSY measurements, have different
molecular weights.24 This was interpreted as a slow
monomer−dimer equilibrium which is largely at the side of
the dimer.
i
appearance of a single heptet and two doublets for the Pr
groups signify that the product is C2v symmetric.
The crystal structure of (DIPPDPM)Zn(O2CH) reveals a
highly symmetric monomeric complex in which the formate
ligand is bound in a symmetric η2-fashion with similar Zn−O
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Scheme 2. Proposed Catalytic Cycle and Possible Products
for the Zn Hydride Catalyzed Hydrosilylation of CO2 with
(EtO)3SiH
Figure 4. Crystal structure of the Zn formate complex (DIPPDPM)-
ZnO2CH (a) compared with the structure for the dimer [(DIPPBDI)-
ZnO2CH]2 (b); ORTEP figures can be found in the Supporting
Information.
CONCLUSIONS
■
The new dipyrromethene ligand with bulky DIPP-substituents,
DIPPDPM, was prepared by a synthetic route that was
previously established for this ligand class. The dipyrrome-
thene ligand was converted to the mononuclear Zn hydride
complex (DIPPDPM)ZnH following a pathway that involves β-
hydride elimination from an intermediate Zn-amidoborane
species. The crystal structures of (DIPPDPM)ZnEt, (DIPPDPM)-
ZnI, and (DIPPDPM)ZnH show slightly longer Zn−N bond
distances and more acute N−Zn−N′ bite angles than those
observed in the corresponding BDI complexes. The somewhat
longer and weaker ligand−metal bonding observed in the
dipyrromethene complexes is supported by calculation of the
NPA charges in (DIPPDPM)ZnH and (DIPPBDI)ZnH. The
electron densities on the N atoms in DIPPDPM are ca. 7% lower
when compared to those in DIPPBDI. Most striking, the charge
on the central backbone C atom is neutral, which makes this
ligand relatively inert toward electrophiles or Brønsted acids.
The main difference between dipyrromethene and β-
diketiminate ligands is the arrangement of the bulky DIPP-
substituents that in case of DIPPDPM clearly form a pocket
encapsulating the metal. This causes high-field shifts for the
NMR signals of ligands bound in the cleft on account of ring
current effects (ASIS). The different ligand architectures also
lead to different aggregation. The Zn formate complex,
The truly monomeric dipyrromethene complex (DIPPDPM)-
Zn(O2CH) reacted in C6D6 at 60−90 °C with hydrosilanes
such as (RO)3SiH (R = Me, Et, Ph) or PhSiH3, regenerating
the hydride complex (DIPPDPM)ZnH. These stoichiometric
reactions indicate that (DIPPDPM)ZnH could be an efficient
catalyst for CO2 hydrosilylation. The hydrosilylation of CO2
with (EtO)3SiH and the catalyst (DIPPDPM)ZnH has been
investigated and was compared to that with the β-diketiminate
catalyst (DIPPBDI)ZnH; the latter was previously shown to be
poorly active.24
The proposed catalytic cycle is shown in Scheme 2, and the
following conditions were used: C6D6, catalyst/silane ratio =
1/40, 10 bar CO2, 90 °C. After a reaction time of 5 h, the
reactor was cooled and depressurized, and the conversion was
evaluated by 1H NMR and quantitative 13C NMR as well as by
GC-MS (see Supporting Information). As has been found
previously, the conversion of (EtO)3SiH is unselective due to
competitive disproportionation reactions giving side products
among which are (EtO)4Si and (EtO)2SiH2. The β-
diketiminate catalyst gave silane conversion of ca. 52% and
only produced silylformate species, [Si]−OC(O)H, in a yield
of approximately 15% (based on silane). The poor catalytic
performance of (DIPPBDI)ZnH was already described earlier
and has been attributed to catalyst decomposition.24 The latter
may be explained by facile reaction of the backbone C in the
DIPPBDI ligand with CO2. In contrast to these results, the
dipyrromethene catalyst gave ca. 77% silane conversion and
full reduction to [Si]−OCH3 products in ca. 37% yield. NMR
analysis or GC-MS measurements did not give any indication
for the presence of intermediate [Si]−OC(O)H species. These
results show that there is a clear difference in activity and
selectivity between β-diketiminate and dipyrromethene Zn
hydride catalysts.
(
DIPPDPM)Zn(O2CH), obtained by reaction of (DIPPDPM)-
ZnH with CO2, is monomeric, whereas the corresponding β-
diketiminate ligand forms the dimer [(DIPPBDI)Zn-
(O2CH)]2.24 Whereas monomeric Zn formate complexes
always show η1-formate bonding that may be assisted by a
weaker CO···Zn interaction, the dipyrromethene complex
(
DIPPDPM)Zn(O2CH) displays symmetric η2-formate bonding
with full delocalization of the negative charge over both O
atoms. This higher denticity is enabled by the low coordination
number at Zn and bonding of the formate ligand in a cavity
spanned by the DIPP-substituents. Stoichiometric reaction of
(
DIPPDPM)Zn(O2CH) with a variety of silanes regenerated the
Zn hydride complex. Both (DIPPDPM)ZnH and (DIPPBDI)ZnH
F
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1H, pyrrole-γ-CH), 6.09 (m, 1H), 2.72 (sept, J = 6.9 Hz, 2H,
CH(CH3)2), 1.13 (d, J = 6.9 Hz, 12H, CH(CH3)2) ppm. 13C{1H}
NMR (101 MHz, CDCl3, 25 °C): δ 150.0 (Carom), 131.3 (Cpyrrole),
129.0 (Carom), 129.0 (Carom), 122.6 (Carom), 116.8 (Cpyrrole), 109.1
(Cpyrrole), 108.5 (Cpyrrole), 30.7 (Caliph), 24.7 (Caliph) ppm. Elemental
analysis: Calculated values for C16H21N (227.17 g/mol): C 84.53, H
9.31, N 6.16; Found: C 84.11, H 9.24, N 5.89. Although the C value is
outside the range viewed as establishing analytical purity, it is
provided to illustrate the best values obtained to date.
Synthesis of 1,9-Di-iso-propylphenyl-5-mesityldipyrromethane
(2). A 250 mL three-neck flask was dried carefully and charged with 2-
(2,6-di-iso-propylphenyl)-1H-pyrrole (1, 3.7 g, 16.3 mmol) and 2-
(dimethoxy-methyl)-1,3,5-trimethylbenzene (1.58 g, 8.14 mmol). The
solids were dissolved in 100 mL of dichloromethane, and pyridinium
p-toluenesulfonate (0.409 g, 1.63 mmol) was added. The light orange
suspension was heated to 50 °C for 5 days, resulting in a color change
to red-orange. The reaction mixture was passed through a plug of
silica gel, and the eluent was removed under reduced pressure,
yielding a yellow foam. Trituration with n-hexane (3 × 20 mL) gave a
yellow powder. Yield: 4.56 g (7.80 mmol; 96%).
were investigated in catalytic hydrosilylation of CO2 with
(EtO)3SiH. There are not only clear differences in activity but
also in selectivity: the more active dipyrromethene catalyst
gave full reduction to [Si]−OMe species, whereas the β-
diketiminate catalyst only produced [Si]−OC(O)H com-
pounds.
These results demonstrate that the new dipyrromethene
ligand DIPPDPM with its sterically demanding DIPP-substitu-
ents is an attractive bulky capping ligand. The large DIPP-
substituents span a pocket that encapsulates the metal and,
although the ligand is weaker bound than the corresponding β-
diketiminate ligand, its geometry inhibits dimer formation and
possibly also suppresses ligand exchange reactions. Another
important feature of the dipyrromethene ligand is the low
electron density at the ligand’s central backbone C, which
prevents undesired reactivity, making it a true spectator ligand.
We will continue to explore this ligand in related group 2 metal
chemistry.
1H NMR (400 MHz, CDCl3, 25 °C): δ 7.72 (s, 2H, pyrrole-NH),
7.34 (t, J = 7.7 Hz, 2H, Dipp-aryl-CH), 7.17 (d, J = 7.7 Hz, 4H, Dipp-
aryl-CH), 6.82 (s, 2H, Mes-aryl-CH), 6.18−6.14 (m, 2H, pyrrole-
CH), 6.08 (s, 1H, bridge-CH), 6.05 (d, J = 2.9 Hz, 2H, pyrrole-CH),
2.83 (hept, J = 6.9 Hz, 4H, CH(CH3)2), 2.24 (s, 3H, C6H2(CH3)3),
2.12 (s, 6H, C6H2(CH3)3), 1.12 (dd, J = 10.9, 5.8 Hz, 24H,
CH(CH3)2) ppm. 13C{1H} NMR (101 MHz, CDCl3, 25 °C): δ 150.0
(Carom), 136.8 (Carom), 134.7 (Carom), 131.6 (Carom), 129.9 (Carom),
129.1 (Carom), 128.9 (Carom), 126.7 (Carom), 122.7 (Carom), 116.8
(Carom), 109.7 (Carom), 109.1 (Carom), 108.5 (Carom), 106.3 (Carom),
38.2 (Caliph), 30.7 (Caliph), 24.6 (Caliph), 20.9 (Caliph) ppm. Elemental
analysis: Calculated values for C42H52N2 (584.41 g/mol): C 86.25, H
8.96, N 4.79; Found: C 85.10, H 8.97, N 4.88. Although the C value is
outside the range viewed as establishing analytical purity, it is
provided to illustrate the best values obtained to date.
EXPERIMENTAL SECTION
■
General Considerations. All experiments were carried out in dry
glassware under N2 using standard Schlenk techniques, glovebox
techniques (MBraun, Labmaster SP), and freshly dried and degassed
solvents. All solvents were dried over a column (Solvent Purification
System: Pure Solv 400-4-MD, Innovative Technology) except for
THF, which was dried over Na and redistilled. The following
compounds were synthesized according to literature procedures: 2-
(dimethoxy-methyl)-1,3,5-trimethylbenzene,42a mesitaldehyde di-
methyl acetal,42b and 1-bromo-2,6-diisopropylbenzene.42c The follow-
ing reagents were obtained commercially: iodine (99%, ABCR),
pyridinium p-toluenesulfonate (98%, ABCR), 2,3-dichloro-5,6-dicya-
no-1,4-benzoquinone (98%, ABCR), ZnCl2 (anhydrous 98%, ABCR),
pyrrole (>99.0%, TCI), JohnPhos (97%, Sigma-Aldrich), tris-
(dibenzylidenacetone)dipalladium(0) (97%, Sigma-Aldrich), and
CO2 (99.9995%, AirLiquide). NMR spectra were measured on
Bruker Avance III HD 400 MHz and Bruker Avance III HD 600 MHz
spectrometers. The spectra were referenced with respect to residual
signals of the deuterated solvents. Elemental analysis was performed
with a Hekatech Eurovector EA3000 analyzer. IR spectra were
measured on a Bruker Alpha II Platinum ATR. All crystal structures
were measured on a SuperNova (Rigaku) diffractometer with dual Cu
and Mo microfocus sources and an Atlas S2 detector. Crystallographic
data were deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 1917102 for
DIPPDPM-H, 1917103 for (DIPPDPM)ZnEt, 1917104 for (DIPPDPM)-
ZnI, 1917105 for (DIPPDPM)ZnH, and 1917106 for (DIPPDPM)Zn-
(O2CH) (see Accession Codes).
Synthesis of 1,9-Di-iso-propylphenyl-5-mesityldipyrromethene,
DIPPDPM-H (3). A Schlenk flask was charged with 1,9-di-iso-
propylphenyl-5-mesityldipyrromethane (2, 19.1 g, 32.7 mmol) and
dissolved in 300 mL of dichloromethane, resulting in an orange
solution. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (8.17 g, 36.9
mmol, 1.1 equiv) was added, and the reaction mixture immediately
turned dark red. The reaction mixture was stirred at room
temperature overnight. After removal of small amount of solids by
filtration through a plug of silica eluting with dichloromethane, the
crude product was purified by column chromatography on silica gel
using an eluent ratio of 1:1 n-hexane to toluene to give a yellow solid.
Recrystallization from toluene at −30 °C yielded orange crystalline
blocks. Yield: 11.8 g (20.2 mmol; 62%).
1H NMR (400 MHz, C6D6, 25 °C): δ 13.03 (s, 1H, amine-NH),
7.23−7.18 (m, 2H, Dipp-aryl-CH), 7.08 (d, J = 7.7 Hz, 4H, Dipp-aryl-
CH), 6.82 (s, 2H, Mes-aryl-CH), 6.60 (d, J = 4.0 Hz, 2H, pyrrole-
CH), 6.26 (d, J = 4.0 Hz, 2H, pyrrole-CH), 3.14 (sept, J = 6.8 Hz, 4H,
CH(CH3)2), 2.26 (s, 6H, C6H2(CH3)3), 2.19 (s, 3H, C6H2(CH3)3),
1.14 (d, J = 6.9 Hz, 24H, CH(CH3)2). 13C{1H} NMR (101 MHz,
C6D6, 25 °C): δ 155.2 (Carom), 148.2 (Carom), 141.2 (Carom), 139.3
(Carom), 137.6 (Carom), 137.1 (Carom), 134.3 (Carom), 133.1 (Carom),
129.4 (Carom), 128.2 (Carom), 127.3 (Carom), 123.0 (Carom), 120.4
(Carom), 31.2 (Caliph), 24.5 (Caliph), 21.2 (Caliph), 20.2 (Caliph) ppm.
Elemental analysis: Calculated values for C42H50N2 (582.40 g/mol):
C 86.55, H 8.65, N 4.81; Found: C 86.41, H 8.61, N 4.69.
Synthesis of 2-(2,6-Di-iso-propylphenyl)-1H-pyrrole (1). A
mixture of sodium pyrrol-1-ide (17.2 g, 193 mmol) and ZnCl2
(26.8 g, 193 mmol) was dissolved in 500 mL of THF. The reaction
mixture was allowed to cool to room temperature, and JohnPhos
(tBu2P(2-naphtyl), 93.1 mg, 0.5 mol %), Pd2(dba)3 (142 mg, 0.25
mol %), and 2,6-diisopropylphenyl bromide (15.0 g, 62.2 mmol) were
added. The resulting dark suspension was heated to reflux for 7 days.
After cooling to room temperature, the reaction mixture was
quenched with 500 mL of water and 500 mL of diethyl ether. The
phases were separated, and the aqueous phase was extracted 4 times
with 200 mL of diethyl ether. The combined organic phases were
washed 3 times with 100 mL of an aqueous solution of NaHCO3. The
organic phase was dried over MgSO4, and the solvent was removed
under reduced pressure, yielding an orange powder. The crude
product was purified by column chromatography on silica gel using an
eluent ratio of 95:5 n-hexane to ethyl acetate. Removal of all volatiles
yielded a yellowish powder. Yield: 11.0 g (48.4 mmol; 78%).
Synthesis of (DIPPDPM)ZnEt (4). In a 50 mL Schlenk flask,
DIPPDPM-H (3) (306 mg, 0.525 mmol) was dissolved in benzene (5
mL). Neat ZnEt2 (36.3 mg, 0.294 mmol, 0.0301 mL) was added to
the stirred orange solution, resulting in an illumination to bright
orange. The reaction mixture was stirred at room temperature
overnight, and the solvent was removed in vacuo, yielding an orange
powder. Recrystallization from toluene at −20 °C yielded orange
crystalline blocks. Yield: 304 mg (0.449 mmol; 86%).
1H NMR (400 MHz, CDCl3, 25 °C): δ 7.92 (s, 1H, pyrrole-NH),
7.37 (t, J = 7.7 Hz, 1H, aryl-CH), 7.21 (s, 1H, aryl-CH), 7.19 (s, 1H,
aryl-CH), 6.86 (q, J = 2.6 Hz, 1H, pyrrole-α-CH), 6.31 (q, J = 2.8 Hz,
1H NMR (600 MHz, C6D6, 25 °C): δ 7.24−7.27 (m, 2H, Dipp-
aryl-CH), 7.12 (d, J = 7.8 Hz, 4H, Dipp-aryl-CH), 6.82 (s, 2H, Mes-
G
DOI: 10.1021/acs.organomet.9b00334
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
aryl-CH), 6.74 (d, J = 4.0 Hz, 2H, pyrrole-CH), 6.34 (d, J = 4.0 Hz,
2H, pyrrole-CH), 3.00 (sept, J = 6.9 Hz, 4H, CH(CH3)2), 2.24 (s,
6H, C6H2(CH3)3), 2.22 (s, 3H, C6H2(CH3)3), 1.23 (d, J = 6.9 Hz,
12H, CH(CH3)2), 1.11 (d, J = 6.9 Hz, 12H, CH(CH3)2), 0.46 (t, J =
8.1 Hz, 3H, ZnCH2CH3), −0.31 (q, J = 8.1 Hz, 2H, ZnCH2CH3)
ppm. 13C{1H} NMR (151 MHz, C6D6, 25 °C): δ 162.4 (Carom), 148.0
(Carom), 146.5 (Carom), 139.9 (Carom), 137.5 (Carom), 136.8 (Carom),
135.8 (Carom), 134.1 (Carom), 132.1 (Carom), 129.7 (Carom), 128.6
(Carom), 123.1 (Carom), 120.6 (Carom), 31.3 (Caliph), 25.0 (Caliph), 24.1
(Caliph), 21.2 (Caliph), 20.2 (Caliph), 11.3 (Caliph), 0.8 (Caliph) ppm.
Elemental analysis: Calculated values for C44H54N2Zn (674.36 g/
mol): C 78.14, H 8.05, N 4.14; Found: C 77.81, H 8.01, N 3.91.
Synthesis of (DIPPDPM)ZnI (5). A 50 mL Schlenk flask was charged
with (DIPPDPM)ZnEt (4) (1.35 g, 2.00 mmol) and dissolved in
benzene (15 mL). A solution of elemental iodine (558 mg, 2.20
mmol, 1.1 equiv) in n-heptane (15 mL) was added slowly at room
temperature. The reaction mixture was stirred at room temperature
overnight, and the solvent and excess of iodine were removed in
vacuo, yielding an orange powder (1.47 g, 1.90 mmol; 95%).
Trituration with pentane (3 × 4 mL) and recrystallization in toluene
at −20 °C afforded the title compound as crystalline orange blocks.
Yield: 738 mg (0.956 mmol; 48%).
solution was stirred at room temperature for 20 min until full
conversion was indicated by 1H NMR spectroscopy. Removal of
solvent in vacuo gave the crude product as an orange solid.
Crystallization from a concentrated toluene solution at −20 °C
yielded crystalline orange blocks. Yield: 36 mg (0.052 mmol, 62%).
1H NMR (600 MHz, C6D6, 25 °C): δ 7.18−7.24 (m, 3H, Dipp-
aryl-CH signal overlap with ZnO2CH), 7.09−7.07 (m, 4H, Dipp-aryl-
CH), 6.79 (s, 2H, Mes-aryl-CH), 6.74 (d, J = 4.0 Hz, 2H, pyrrole-
CH), 6.37 (d, J = 4.0 Hz, 2H, pyrrole-CH), 2.95 (sept, J = 6.9 Hz, 4H,
CH(CH3)2), 2.19 (s, 9H, C6H2(CH3)3), 1.34 (d, J = 6.8 Hz, 12H,
CH(CH3)2), 1.09 (d, J = 6.9 Hz, 12H, CH(CH3)2). 13C{1H} NMR
(151 MHz, C6D6, 25 °C): δ 162.7 (Carom), 148.0 (Carom), 146.7
(Carom), 139.9 (Carom), 137.6 (Carom), 136.8 (Carom), 135.4 (Carom),
133.5 (Carom), 132.2 (Carom), 129.6 (Carom), 128.4 (Carom), 122.9
(Carom), 120.5 (Carom), 31.3 (Caliph), 24.8 (Caliph), 24.2 (Caliph), 21.2
(Caliph), 20.1 (Caliph) ppm. Elemental analysis: Calculated values for
C43H50N2O2Zn (690.32 g/mol): C 74.61, H 7.28, N 4.05; Found: C
74.31, H 7.20, N 3.75. IR (pure, cm−1): ν 2960 (m), 2925 (w), 2866
(w), 1840 (w, νCO), 1536 (s), 1438 (w), 1379 (w), 1332 (w), 1234
(s), 1209 (m), 1046 (m), 1005 (s), 865 (m), 840 (s), 803 (m), 791
(s), 752 (m), 718 (m), 620 (w), 493 (w), 459 (w).
1H NMR (400 MHz, C6D6, 25 °C): δ 7.29−7.33 (m, 2H, Dipp-
aryl-CH), 7.14−7.16 (m, 4H, Dipp-aryl-CH), 6.79 (s, 2H, Mes-aryl-
CH), 6.68 (d, J = 4.0 Hz, 2H, pyrrole-CH), 6.32 (d, J = 4.1 Hz, 2H,
pyrrole-CH), 2.90 (sept, J = 6.9 Hz, 4H, CH(CH3)2), 2.21 (s, 3H,
C6H2(CH3)3), 2.17 (s, 6H, C6H2(CH3)3), 1.33 (d, J = 6.9 Hz, 12H,
CH(CH3)2), 1.10 (d, J = 6.8 Hz, 12H, CH(CH3)2) ppm. 13C{1H}
NMR (101 MHz, C6D6, 25 °C): δ 163.3 (Carom), 147.7 (Carom), 146.5
(Carom), 140.0 (Carom), 137.9 (Carom), 136.8 (Carom), 135.1 (Carom),
132.9 (Carom), 132.3 (Carom), 130.2 (Carom), 128.3 (Carom), 123.4
(Carom), 121.5 (Carom), 31.6 (Caliph), 25.3 (Caliph), 24.4 (Caliph), 21.2
(Caliph), 20.0 (Caliph) ppm. Elemental analysis: Calculated values for
C42H49N2ZnI (772.22 g/mol): C 65.16, H 6.38, N 3.62; Found: C
64.61, H 6.18, N 3.41. Although the C value is outside the range
viewed as establishing analytical purity, it is provided to illustrate the
best values obtained to date.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00334.
■
S
Crystallographic details, including ORTEP plots, infra-
red spectra, GC-MS data, selected 1H, 13C, HSQC,
HMBC, DOSY, and temperature dependent NMR
spectra (PDF)
(XYZ)
Accession Codes
CCDC 1917102−1917106 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Synthesis of (DIPPDPM)ZnH (6). A 50 mL Schlenk flask was charged
with (DIPPDPM)ZnI (5) (600 mg, 0.777 mmol) dissolved in toluene
(20 mL), and K[N(iPr)HBH3] (104 mg, 0.932 mmol, 1.2 equiv) was
added. The milky orange suspension was stirred at room temperature
overnight and subsequently filtered through a syringe filter to give an
orange solution. Removal of solvent in vacuo gave the crude product
as orange solid in quantitative yield. Crystallization from a
concentrated pentane solution at −20 °C yielded crystalline orange
blocks. Yield: 434 mg (0.671 mmol; 86%).
AUTHOR INFORMATION
Corresponding Author
*E-mail: sjoerd.harder@fau.de.
ORCID
Sjoerd Harder: 0000-0002-3997-1440
Notes
The authors declare no competing financial interest.
■
1H NMR (600 MHz, C6D6, 25 °C): δ 7.26−7.29 (m, 2H, Dipp-
aryl-CH), 7.13−7.11 (m, 4H, Dipp-aryl-CH), 6.80 (s, 2H, Mes-aryl-
CH), 6.73 (d, J = 4.0 Hz, 2H, pyrrole-CH), 6.34 (d, J = 4.0 Hz, 2H,
pyrrole-CH), 3.78 (s, 1H, ZnH), 2.97 (sept, J = 6.8 Hz, 4H,
CH(CH3)2), 2.22 (s, 6H, C6H2(CH3)3), 2.20 (s, 3H, C6H2(CH3)3),
1.19 (d, J = 6.8 Hz, 12H, CH(CH3)2), 1.11 (d, J = 6.9 Hz, 12H,
CH(CH3)2) ppm. 13C{1H} NMR (151 MHz, C6D6, 25 °C): δ 162.7
(Carom), 148.0 (Carom), 146.7 (Carom), 139.9 (Carom), 137.6 (Carom),
136.8 (Carom), 135.4 (Carom), 133.5 (Carom), 132.2 (Carom), 129.6
(Carom), 128.4 (Carom), 122.9 (Carom), 120.5 (Carom), 31.3 (Caliph), 24.8
(Caliph), 24.2 (Caliph), 21.2 (Caliph), 20.1 (Caliph) ppm. Elemental
analysis: Calculated values for C42H50N2Zn (646.33 g/mol): C 77.82,
H 7.77, N 4.32; Found: C 77.34, H 7.79, N 4.06. Although the C
value is outside the range viewed as establishing analytical purity, it is
provided to illustrate the best values obtained to date. IR (pure,
cm−1): ν 2956 (w), 2924 (w), 2864 (w), 1536 (s), 1456 (w), 1381
(w), 1334 (w), 1246 (s), 1043 (m), 1008 (s), 865 (w), 841 (m), 787
(w), 755 (m), 736 (w), 722 (w). The signal for the Zn−H stretching
vibration at around 1700 cm-1 could not be observed.
ACKNOWLEDGMENTS
We acknowledge Mrs. C. Wronna (University of Erlangen-
Nurnberg) for numerous CHN analyses and Dr. A. Zahl, J.
■
̈
̈
̈
Schmidt and Dr. C. Farber (University of Erlangen-Nurnberg)
for assistance with the NMR analyses.
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DOI: 10.1021/acs.organomet.9b00334
Organometallics XXXX, XXX, XXX−XXX

Organometallics
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