Probing the steric and electronic characteristics of a new bis-pyrrolide pincer ligand

Article abstract of DOI:10.1021/ic402120r

A new pincer ligand is synthesized to be dianionic, with the potential to be redox active. It has pyrrrole rings attached to both ortho sites of a pyridine, as the linking element. This H₂L can be doubly deprotonated and then used to replace two chloride ligands in MCl₂(NCPh) ₂, to form LM(NCPh) for M = Pd, Pt. The acid form H₂L reacts with ZnEt₂ with elimination of only 1 mol of ethane to yield (HL)ZnEt, a three-coordinate species with one pendant pyrrole NH functionality. This molecule binds the Lewis base p-dimethylaminopyridine (DMAP) to give first a simple 1:1 adduct that eliminates ethane on heating to form four-coordinate LZn(DMAP), which has an unusual structure due to the strong preference of the pincer ligand to bind in a mer (planar) geometry. A molecule with two HL ^- ligands each bonded in a bidentate manner to FeCl₂ is synthesized and shown to contain four-coordinate iron with a flattened-tetrahedral structure. The electrochemistry of LM(NCPh) and (L)Zn(DMAP) shows three oxidation processes, which is interpreted to involve at least two oxidations of the pyrrolide arms.

Full text of DOI:10.1021/ic402120r

Article
pubs.acs.org/IC
Probing the Steric and Electronic Characteristics of a New Bis-
Pyrrolide Pincer Ligand
Nobuyuki Komine, Rene W. Buell, Chun-Hsing Chen, Alice K. Hui, Maren Pink,
́
and Kenneth G. Caulton*
Department of Chemistry, Indiana University, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States
S
* Supporting Information
ABSTRACT: A new pincer ligand is synthesized to be dianionic, with the
potential to be redox active. It has pyrrrole rings attached to both ortho sites of
a pyridine, as the linking element. This H2L can be doubly deprotonated and
then used to replace two chloride ligands in MCl2(NCPh)2, to form
LM(NCPh) for M = Pd, Pt. The acid form H2L reacts with ZnEt2 with
elimination of only 1 mol of ethane to yield (HL)ZnEt, a three-coordinate
species with one pendant pyrrole NH functionality. This molecule binds the
Lewis base p-dimethylaminopyridine (DMAP) to give first a simple 1:1 adduct
that eliminates ethane on heating to form four-coordinate LZn(DMAP), which
has an unusual structure due to the strong preference of the pincer ligand to
bind in a mer (planar) geometry. A molecule with two HL− ligands each bonded
in a bidentate manner to FeCl2 is synthesized and shown to contain four-coordinate iron with a flattened-tetrahedral structure.
The electrochemistry of LM(NCPh) and (L)Zn(DMAP) shows three oxidation processes, which is interpreted to involve at least
two oxidations of the pyrrolide arms.
donating substituent by its Hammett parameters and is certainly
also a π donor to a metal due to its amide character. This latter
feature will certainly be diminished, in comparison to a
dialkylamide, by virtue of involvement of the amide lone pair in
the aromaticity of the pyrrole (smaller resonance energy in
comparison to that of benzene).11 Pyrroles themselves are
powerful ancillaries to increasing the reducing power of metals,
promoting reduction of N2, CO2, and other poor ligands.14−18
By coupling the pyrrolide ligand to a reducible ligand, pyridyl,
(whose π* orbitals permit reduction), we create a push/pull
ligand of enhanced potential redox character in comparison to
that of neutral bipyridyl. The push/pull character makes the
pyridylpyrrolide capable of being reduced or of being oxidized,
hence a useful ancillary to metal complexes under either
reducing or oxidizing conditions. We report here the chemistry
of a pincer ligand, H2L in Scheme 1, which includes these
functionalities. Since the two pyrroles employed here carry two
tBu groups, this new ligand should be especially electron rich
and subject to ready oxidation.
INTRODUCTION
■
As a class, pincer ligands have seen explosive growth,1 and their
advantages are increasingly understood.2−10 Their mer stereo-
chemistry leaves coplanar coordination sites; this is in contrast
to tris(pyrazolyl)borates or cyclopentadienyl ligands with their
obligatory fac geometry and allows pincer-induced stereo-
chemical control, including imposed chirality. As working
functionalities, pincers are modular and carry a number of sites
for NMR evaluation, for solubilizing the complex (even in
noninteracting solvents), and for enhancing their capacity to be
analyzed in the gas phase (mass spectrometry). Their tridentate
character suppresses degradative ligand loss and even
discourages “arm off” mechanistic steps (a “self-repairing” κ2
→ κ3 arm reattachment is entropically favored by ∼11 kcal/mol
at room temperature).
In constructing a pincer ligand which is potentially redox
active, we have chosen to include a pyrrolide.11−13 As a
substituent on an aryl (pyridine) ring, pyrrolide gives (1 in
Scheme 1) amide nitrogen character to the pyridine partner, via
conjugation between the two rings. This ring is an electron-
Characterization of 2,6-bis(R)pyridine pincer complexes (R
= indolyl, azaindolyl) of several divalent metals have been
reported, for their optical properties, and all have conventional
κ3 pincer connectivity.19,20
Scheme 1
We illustrate different methods for attaching this pincer
ligand to metals, the redox activity of the resulting complexes,
and the ability to break the 2-fold symmetry of this pincer by
synthesis of complexes with inequivalent arms. We find a
Received: August 19, 2013
Published: January 10, 2014
© 2014 American Chemical Society
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Scheme 2
surprising ability of one acidic pyrrole NH functionality to
coexist with a metal alkyl, and we attempt to utilize the general
phenomenon of our pendant pyrrole NH functionality to
participate actively by interaction with substrate.
RESULTS
■
Ligand Synthesis and Its d8 Complexes. The ligand was
synthesized according to the route in Scheme 2, first installing
the amino substituents on the pyridyl ortho positioned CH2
arms and then ring closing twice via the procedure of
McNeill.21 Spectroscopic characteristics are unexceptional.
The ligand free base is soluble not only in the polar solvents
acetone and methanol but also in benzene and pentane. Single
crystals, grown by slow evaporation from acetonitrile, were
shown (Figure 1) by X-ray diffraction to adopt a conformation
Figure 2. ORTEP view (50% probability) of the non-hydrogen atoms
of LPt(NCPh), showing selected atom labeling. Unlabeled atoms are
carbon. Selected structural parameters (distances in Å and angles in
deg): Pt1−N1, 2.061(2); Pt1−N2, 2.053(2); Pt1−N3, 1.950(2); Pt1−
N4, 1.982(3); N4−C30, 1.143(4); N1−Pt1−N2, 161.08(10); N1−
Pt1−N3, 80.78(10); N2−Pt1−N3, 80.34(10); N1−Pt1−N4,
98.57(10); N2−Pt1−N4, 100.34(10); N3−Pt1−N4, 78.04(10);
Pt1−N4−C30, 178.1(3).
N1−Pt−N2 at 161.08(10)°. The dihedral angles NCCN
involving pyrrolide and pyridyl in LPt(NCPh) are 2.1 and
4.1°, indicating that the rings are conventionally eclipsed and
t
coplanar. The steric impact of the Bu groups is seen in the
benzonitrile conformation, which is perpendicular to the
t
coordination plane to maximize distances to the Bu groups.
Figure 1. ORTEP drawing of the non-hydrogen atoms of H2L
showing selected atom labeling. Unlabeled atoms are carbons. The
acetonitrile guest molecule in the asymmetric unit is not shown but is
responsible for the outward orientation of the N1−H1n bond.
The two LM(NCPh) complexes show rich electron transfer
reactivity. Cyclic voltammograms were run in o-difluoroben-
zene with 0.3 M [N(n-Bu)4][PF6] at 100 mV s−1, and all are
referenced to Fc/Fc+ as 0.0 V. Both molecules show (Figure 3)
three oxidations, with the first (very low potential of about
+0.13 V) and third being reversible (about +1.10 V). The
potentials for these are metal independent to within 0.05 V,
which leads to assigning these to ligand-based oxidations, and
are primarily from the electron-rich pyrroles.22 Since, for a
given metal, these first and third potentials are very different
(by ∼1 V), this means that the two pyrrolides communicate
strongly: the single oxidation occurs delocalized over both
pyrrolides. The second oxidation is reversible only for Pt. Down
to −2.5 V, the Pt compound is not electroactive, but the Pd
example shows a reversible reduction with E1/2 = −1.48 V; this
different reductive behavior is consistent with the generally
easier reduction of 4d than 5d metal, hence supporting an
assignment of the reduction as metal based.
with the pyrroles not directed exclusively inward, but one
pyrrole NH bond is directed outward (NCCN dihedral angle
137.5°) in order to hydrogen bond to a lattice guest acetonitrile
nitrogen (N−N = 3.124(4) Å). The second pyrrole has an
NCCN dihedral angle of 26.1°; both of these pyrrole ring
t
rotations diminish steric interference between the close Bu
group and the pyridyl ring.
This free base H2L is readily deprotonated with KH in THF
to yield a free-flowing solid K2L salt, for use in installing the
ligand on transition-metal halides. Reaction of K2L with
MCl2(NCPh)2 for M = Pd, Pt gives products, following
filtration and vacuum removal of volatiles, whose NMR spectra
are consistent with C2v symmetry, with intensities consistent
with the formula LM(NCPh). A single-crystal X-ray structure
determination of LPt(NCPh) (Figure 2) shows the molecule to
have a planar structure with benzonitrile nitrogen trans to the
pyridyl nitrogen. Nitrile nitrogen has a shorter bond length to
Pt than the pyrrolides, with the pyridyl nitrogen shortest of all.
The pincer bite angle constraint leaves the interpyrrolide angle
For comparison, CV of H2L itself under identical conditions
shows no reduction and shows three irreversible oxidations, at
pa = 0.55, 1.07, and 1.33 V (see the Supporting Information).
Carbonylation. We were interested in obtaining a CO
stretching frequency to compare the donor power of bis-
E
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Figure 3. Cyclic voltammograms of LPd(NCPh) (top) and LPt(NCPh) (bottom) in o-C6H4F2.
pyrrolylpyridine pincer to that of other ligand sets. Reaction of
LPd(NCPh) with 1 atm of CO in CH2Cl2 yields LPd(CO).
The CO stretching frequency of this product is 2197 cm−1.
Curiously, this is significantly higher than the value 2135 cm−1
for PtCl2(pyridine)(CO),23 which would place pyrrolide as a
weaker donor than chloride. While the generally poor π donor
ability of Pt(II) leaves all its carbonyl complexes at relatively
high frequency, this ranking still shows that the Nπ lone pair of
pyrrolide is significantly tied up in pyrrole aromaticity.
ligand are inequivalent, as judged both by ring hydrogens and
by observation of four tBu chemical shifts. In addition, there is,
at stoichiometric intensity, an A2X3 spin system evident
indicative of one ethyl group per pincer ligand. The
protonolysis of the ethyl groups thus proceeded only to the
first step, leaving one pyrrole NH proton, evident at 6.94 ppm
in a product of formula (HL)ZnEt. It is interesting that the
NMR spectrum shows that there is no NH proton transfer (fast
1
on the H NMR time scale) to the deprotonated pyrrolide
Zinc. Zinc coordination chemistry defies generalization.24−29
Coordination numbers range from 3 to 6, although any
coordination number higher than 4 violates the 18-electron
rule. For example, there is firm structural confirmation that the
water complex of zinc contains six water molecules coordinated
in an octahedral structure. While conventional thinking
indicates that four-coordinate, 18-valence-electron d10 divalent
zinc should have a tetrahedral structure, there are violations.
There are a huge number of zinc porphyrin complexes,25,26 all
of which have four-coordinate planar zinc with no axial ligation.
Three-coordinate examples28 generally require steric bulk to
achieve such a level of unsaturation, and the resulting species
are planar. However, a C2-symmetric tridentate ligand (i.e., a
pincer) is incompatible with the steric requirements of four-
coordinate Zn(II).
nitrogen, since that would make the two pincer arms time-
averaged equivalent.
Single crystals grown by slow evaporation of a pentane
solution were shown by single-crystal X-ray diffraction (Figure
4) to be of formula (HL)ZnEt, containing a monomeric species
with three-coordinate zinc and singly deprotonated pincer
ligand bound in a bidentate fashion to the metal. The three-
coordinate geometry around zinc is approximately planar; the
ethyl group is disordered over two sites so that N(pyrrolide)−
Zn−C angles range from 146.6 to 151.89° while the
N(pyridyl)−Zn−C angles range from 132.6 to 124.2°. The
coordinated rings are nearly coplanar, with a NCCN dihedral
angle of 6.6°. The NH-bearing pyrrole ring has a NCCN
dihedral angle of 115.9°, and thus this ring presents its face to
zinc or to the Zn−C bond. The NH bond is directed outward,
away from zinc. The Zn−N(pyrrolide) distance is significantly
shorter than the Zn−N(pyridyl) distance, 1.946 vs 2.074 Å, with
the Zn−C distance intermediate at 1.970 Å; this is the opposite
distance trend in comparison to LPt(NCPh). In spite of the
Reaction of H2L with Zn(C2H5)2 (1:1 mole ratio) in benzene
occurs to completion within 30 min at 25 °C to form a single
yellow product which is soluble in benzene, pentane, and THF.
The 1H NMR spectrum shows that the two pyrrole arms of the
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5) by single-crystal X-ray diffraction to be four-coordinate
monomers with a tridentate pincer ligand and DMAP
Figure 4. ORTEP view (50% probabilities) of the non-hydrogen
atoms in the structure of (LH)Zn(C2H5), showing selected atom
labeling; unlabeled atoms are carbons. Hydrogen on the pyrrole
nitrogen is shown, for clarity. Selected structural parameters (distances
in Å and angles in deg): Zn1−N1, 1.9462(14); Zn1−C30A, 1.969(8);
Zn1−N2, 2.0740(15); N1−Zn1−C30A, 151.9(3); N1−Zn1−N2,
80.80(6); C30A−Zn1−N2, 124.1(3); C31A−C30A−Zn1, 119.6(13).
Figure 5. ORTEP view (50% probabilities) of the non-hydrogen
atoms in the structure of (L)Zn(DMAP), showing selected atom
labeling; unlabeled atoms are carbons. Selected structural parameters
(distances in Å and angles in deg): N1−Zn1, 2.0036(15); N2−Zn1,
1.9886(14); N3−Zn1, 1.9883(15); N4−Zn1, 2.0162(15); N3−Zn1−
N2, 81.85(6); N3−Zn1−N1, 146.80(6); N2−Zn1−N1 81.26(6);
N3−Zn1−N4, 108.71(6); N2−Zn1−N4, 110.05(6); N1−Zn1−N4
103.89(6).
distortion of the ZnN2C coordination geometry away from Y-
shaped and toward T-shaped, there are no short inter- or
intramolecular contacts with zinc shorter than van der Waals;
the shortest are 2.5 Å to H and 3.0 Å to carbon.
Reactivity of (HL)ZnEt. Given our three-coordinate zinc
ethyl complex (HL)ZnEt, we felt this might offer access to a T-
shaped complex of zinc which could test the coordination
geometry preferences and limits, as well as reactivity of such a
Lewis acid. Efforts were therefore made to effect ethane
elimination to form LZn. Heating (HL)ZnEt in benzene at 60
°C for up to 8 h showed no change, with recovery of reagent
complex. We find this to be a remarkable coexistence of a metal
alkyl with a nearby acidic proton. However, zinc and even
aluminum alkyls are known to decrease dramatically the
carbanionic character of the residual alkyl following proteolysis
of the first alkyl,30−35 although some examples of the reaction
of HL with ZnMe2 effect the complete loss of methyl groups,
forming ZnL2.36
coordinated through the pyridine nitrogen. The molecule has
idealized mirror symmetry, with equivalent pincer arms, in
agreement with the 1H NMR evidence. All four Zn−N
distances fall in the narrow range 1.99−2.02 Å, but the
constraint of the pincer ligand distorts the geometry so that
zinc lies only slightly above the plane defined by the three
pincer nitrogens and the DMAP nitrogen lies approximately
perpendicular to that plane (the angle N2−Zn1−N4 is
110.05(6)°); the geometry is much closer to distorted
tetrahedral than it is to planar, although a “see-saw” or cis-
divacant octahedral description is the most valid. The main
distortion from tetrahedral angles is the angle between the
pyrrolides: N3−Zn1−N1 = 146.80(6)°. Symptomatic of the
mismatch of pincer and zinc structural preferences, the pyrrole
rings are not coplanar with the pyridine plane: the dihedral
angles NCCN are 16.7 and 19.8°. Even more telling is the
misdirection of the pyrrolyl nitrogen σ lone pair downward,
away from the DMAP side of the pincer plane, and the pincer
pyridyl nitrogen lone pair is oppositely misdirected (the
C(para)−N−Zn angle is 172.5°). There is no evidence of
We next attempted to probe the Lewis acidity of (HL)ZnEt,
because of its low coordination number. This reacts in the time
of mixing with p-dimethylaminopyridine (DMAP) to give a 1:1
1
adduct, established by integration of its H NMR spectrum.
This retains an NH proton and inequivalent pyrrole rings, as
1
established by H NMR. This (HL)ZnEt(DMAP) shows rapid
exchange between free and coordinated DMAP in the presence
of <1 equiv of additional DMAP, indicating rapid exchange of
this Lewis base. Such exchange, by a dissociative mechanism, is
thus the reason the diastereotopic inequivalence of the ethyl
t
steric clash between the Bu groups adjacent to pyrrolyl
1
CH2 protons is not resolved in the H NMR spectrum of
nitrogen (Figure 6), but that region of the coordination sphere
is too crowded to permit DMAP to coordinate there, trans to
pincer pyridyl nitrogen. Nitrogen in the Me2N substituent is
planar, enhancing the electron richness of the DMAP pyridyl
ring.
Cyclic Voltammetry of (L)Zn(DMAP). No reductions
occurred out to −1.50 V. Three oxidation events are apparent
(Figure 7) between 0 and 2 V. The first wave is quasi-reversible
with Epa at 0.018 V vs Fc/Fc+. A control experiment shows that
free DMAP in o-difluorobenzene has one irreversible oxidation
at 0.568 V, suggesting that one oxidative process in Zn(L)-
(DMAP) may be attributed to coordinated DMAP. The third
wave is quasi-reversible with Epa at 1.04 V vs Fc/Fc+. This
shows that the bis(pyrrolide)pyridyl ligand can be oxidized by
up to two electrons below 1 V. The two pyrrolide arms
communicate with each other, making the two oxidations
(HL)ZnEt(DMAP); the adduct is in rapid equilibrium with
traces of (HL)ZnEt and free DMAP.
Synthesis of this DMAP adduct has a beneficial effect on
ethane elimination. Simply heating a benzene solution of
(HL)ZnEt(DMAP) to 50 °C gives conversion to (L)Zn-
(DMAP). Proton NMR shows that this molecule has 2-fold
symmetry of the two pyrrolide arms, shows no NH or ethyl
proton signals, and retains the 1:1 stoichiometry of DMAP vs L.
Of special interest is whether this is a monomer and whether
such a monomer is planar or nonplanar. Attempts to see less
1
than 2-fold symmetry of the DMAP in (L)Zn(DMAP) by H
NMR at −40 °C showed no decrease in symmetry of the two
ortho or of the two meta DMAP protons.
Structure of (L)Zn(DMAP). Yellow crystals grown by slow
evaporation of a dichloromethane solution were shown (Figure
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Figure 6. Space-filling models of (L)Zn(DMAP) (left) viewed trans to the DMAP, showing the open side of the coordination sphere, and (right)
viewed trans to the pincer pyridyl nitrogen, to show close approach of the nearby tBu groups, which forces the DMAP (vertical) out of the Zn/pincer
plane.
1
group, as judged by H NMR spectroscopy. However, after
volatiles (including HN(SiMe3)2) were removed by vacuum,
1
the ethyl group was no longer present, as judged by the H
NMR spectrum (benzene solvent). Vacuum causes a change in
product. The ethyl group could then be lost either as butane or
as a 1:1 mixture of ethane and ethylene. The pyrrolide arms in
this product have become chemically equivalent (on the NMR
time scale), since only two tBu peaks and three ligand backbone
peaks are present. The ability of a base to form the new
complex (which does not contain bound ethyl or trimethylsilyl
amide/amine) suggests a product with the formula [LiZn(L)]n.
Attempts to crystallize the product have thus far been
unsuccessful.
A key question is this: why does the pyrrole hydrogen not
attack the zinc−ethyl bond in (LH)ZnEt? There is also a
corollary question: why does binding Lewis base B to zinc
facilitate that hydrogen transfer? The former question may
hinge on the product, three-coordinate and T-shaped LZn
being too high in energy to be reached in monomeric form.
Figure 7. CV of (L)Zn(DMAP) in o-difluorobenzene. The scan rate is
The latter question may require consideration of a non-least-
motion mechanism for proton transfer. Instead of the reaction
happening within an (LH)ZnEt(B) adduct, it may be (Scheme
3) that Lewis base B simply deprotonates the pyrrole nitrogen,
and the resulting BH+ then transfers its proton to the Zn−C
bond in the LZnEt− anion, which may already have κ3 pincer
ligand connectivity and thus have a more polar Zn−C bond,
hence being more acid sensitive; if proton transfer is concurrent
with B/Zn binding, the transition state energy is kept low.
In summary, the importance of the Lewis base is not to lower
the barrier to intramolecular proton transfer but instead to
shuttle the proton from pyrrole nitrogen to the zinc−carbon
bond. This clearly demands a Lewis base whose Brønsted
basicity meets a narrow pKa window: able to deprotonate
100 mV/s, and the supporting electrolyte is 1.0 M [TBA]PF6.
separate events at two different potentials. It is significant that
three waves are also seen for the Pd and Pt complexes reported
here (Figure 3) but that the oxidation potential of the zinc
complex is less positive. This may be due to π/π repulsions37
between the two pyrrolides and d10 zinc.
Mobilizing the NH Proton? In order to form a complex
with zinc containing a dianionic κ3-bis(pyrrolyl)pyridyl ligand,
lithium bis(trimethylsilyl)amide was added to a benzene
solution of (HL)Zn(Et) at room temperature in order to
deprotonate the pyrrole nitrogen to form [Zn(L)(Et)]Li and
bis(trimethylsilyl)amine. This reaction proceeds in the time of
mixing, to give a new species which still contains an ethyl
Scheme 3. Proposed Mechanism for Base-Assisted Elimination of Ethane from the Reaction Zn(HL)(Et) + Base
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pyrrole nitrogen but still willing to surrender that proton to the
zinc−carbon bond.
We investigated the hypothesis that it is not the adduct alone
which facilitates protonolysis of the Zn−C bond by study of
THF as the Lewis base; this has very poor Brønsted basicity.
Adding 3 equiv of THF to a C6D6 solution of (LH)ZnEt shows
adduct formation by a shift of all proton chemical shifts,
including that of the pyrrole NH group. At 25 °C, there is no
1
ethane release. The H NMR spectrum also establishes that
there is rapid exchange between free and coordinated THF,
since the signals of both are coalesced. Heating of this solution
1
to 60 °C for 24 h shows no change in the H NMR spectrum,
indicating that no proton transfer has occurred. Excess THF
was employed in this experiment on the basis of the hypothesis
that it is free base which transfers the proton from the pyrrole
to the ethyl ligand. The basicity of THF is not strong enough to
deprotonate the pyrrole nitrogen, and so this base is unsuitable
for promoting loss of the ethyl moiety for produce Zn(L)-
(base), in contrast to the case for DMAP. Also, the hypothesis
that the presence of a base on Zn promotes ethane loss by
sterically forcing the NH and ethyl groups closer together is
contradicted by this THF adduct forming experiment.
Figure 8. ORTEP view (50% probabilities) of the non-hydrogen
atoms of Fe(HL)2 viewed with the approximate C2 axis vertical.
Unlabeled atoms are carbon, and the two pyrrole NH hydrogens are
shown. This shows the flattened-tetrahedral structure around iron and
the inward direction of the NH hydrogens. Selected structural
parameters (distances in Å and angles in deg): Fe1−N4, 2.011(2);
Fe1−N1, 2.012(2); Fe1−N5, 2.055(2); Fe1−N2, 2.077(2); N4−Fe1−
N1, 115.25(9); N4−Fe1−N5, 82.11(9); N1−Fe1−N2, 83.44(9); N5−
Fe1−N2, 133.80(9).
1
Oxidation of (HL)Zn(Et) by 1 equiv of AgBF4 or /2 equiv
of I2 in benzene at room temperature results in degradation of
the starting material into free ligand and several minor
products. Removal of the pyrrole N−H proton in (HL)Zn(Et)
by addition of KH was unsuccessful. H2L itself undergoes no
reaction with oxidants AgBF4 and I2 in THF at room
temperature. Hence, attachment of the ring system to Zn(II)
increases its tendency to oxidation (although this does lead to
production of H2L). This may originate from the anionic
charge of the complexed ligand but may also be due to filled/
filled repulsion37 between the pyrrolide π system and the filled
d orbitals.
Iron Complex with Two HL− Ligands. Given the
persistence of an HL− ligand on zinc, we were interested in
the generality of this form. Since we have characterized an iron
complex with two monopyrrolylpyridine ligands,38 we chose to
explore iron with our new pincer. Reaction of FeCl2 with a 1:1
mixture of K2L and H2L in THF forms a single product, which
we assign as Fe(HL)2. This pentane-soluble molecule is most
easily identified by detection of inequivalent arms in the ligand,
Figure 9. ORTEP view (50% probabilities) of Fe(HL)2 viewed down
the approximate C2 axis, showing the face to face alignment of the two
pyrroles bearing N3 and N6. Note also the misalignment of the N1
lone pair with respect to Fe1; Fe1 is not in the pyrrolide plane,
involving N1. H on N3 is obscured by that nitrogen; H on N6 is
visible.
t
particularly four strong 18H Bu chemical shifts in benzene.
The two ligands are thus symmetry related. Since the ring arms
1
within a given pincer are inequivalent at 25 °C by H NMR,
there is no rapid proton transfer in this molecule, in spite of the
close proximity of NH and pyrrolide N.
The product was established by single-crystal X-ray
diffraction (Figure 8) as Fe(HL)2, a complex with two singly
deprotonated pincer ligands. The structure shows near-C2
symmetry, with a flattened-tetrahedral four-coordinate structure
around iron and a larger angle (133.80(9)°) between the two
pyridyls, in contrast to the structure of Fe(L0)2 (where the
corresponding angle is 106.5(2)°).38 In Fe(HL)2, the angle
between the pyrrolides is small, 115.25(9)°, so that the
(pyridyl)2Fe angle can open and make room for the pendant
pyrrole groups. These pyrroles direct their NH bonds inward,
pyridyl and pyrrolide have corresponding dihedral angles of 19
and 6°. Although the Fe−N(pyrrolide) distances are statistically
identical, one of the pyrrolides (that with the 19° dihedral
angle, containing N1) is significantly misdirected, not pointing
its lone pair directly toward iron (Figure 9). This lack of bond
lengthening with misdirected σ donation is unusual and,
together with the large variations between angles in Fe(HL)2
and Fe(L0)2, shows the coordination sphere of Fe(II) here to
be highly deformable, or plastic. The distance between N1 and
the proton on N6 is 2.613 Å, while that from the proton on N3
to N4 is 2.709 Å. Judging by the more reliably located
nitrogens, the N6−N1 distance is 3.30 Å while the N3−N4
distance is 3.45 Å. The N6−H−N1 bond angle is 150° and so is
in an acceptable range for hydrogen bonding. The N1 lone pair
misdirection is also evident in the fact that the sum of angles
around N1 is only 351.8° (i.e., nonplanar) while that around
t
apparently to direct the bulky Bu groups outward, toward the
less crowded molecular surface. These two pendant rings have
their planes approximately parallel (Figure 9), to minimize
crowding, but the interatomic distances are all longer than 4.8
Å. These two pyrroles have dihedral angles toward their
attached pyridyls of 41 and 57°, while the κ2-coordinated
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purified either using an Innovative Technology SPS-400 PureSolv
solvent system or distilling from conventional drying agents and
degassing by the freeze−pump−thaw method twice prior to use or by
activated alumina and Q-5 deoxygenation columns. Glassware was
oven-dried at 150 °C overnight. NMR spectra were recorded in C6D6,
THF-d8, and toluene-d8 at 25 °C or on a Varian Inova-400
spectrometer (1H, 400.11 MHz; 13C, 100.61 MHz; 19F, 376.48
MHz) or on a 300 MHz spectrometer. Proton and carbon chemical
shifts are reported in ppm versus Me4Si. Electrochemical studies were
carried out with an Autolab Model PGSTAT30 potentiostat (Eco
Chemie). A three-electrode configuration consisting of a working
electrode (platinum-button electrode), a Ag/AgNO3 (0.01 M in
MeCN with 0.1 M n-Bu4NPF6) reference electrode, and a platinum-
coil counter electrode was used. All electrochemical potentials were
referenced with respect to the Cp2Fe/Cp2Fe+ redox couple, added
internally with the sample at the end of a study.
N4 is 359.1°. This asymmetry of these hydrogen bonds
degrades the C2 symmetry slightly, but the Fe−N1 distance is
not significantly longer than Fe−N4. This flexibility in the
coordination sphere should translate into ready binding of a
fifth ligand.
We felt that Fe(HL)2, with its two NH functionalities, might
have some potential for substrate transformation that exploited
those hydrogens.39−42 It is necessary to think more generally
than merely H+ transfer, and H atom transfer, perhaps as part
of proton-coupled electron transfer, was anticipated. Another
aspect of such reactivity would be that the nitrogen, once
stripped of its hydrogen, is likely to coordinate to the metal,
yielding an κ3 pincer; such behavior might even promote
release of the transformed substrate.
Synthesis of 2,6-Bis(phthalimidomethyl)pyridine. The syn-
thesis was carried out according to a literature method46 with
modifications. A mixture of 2,6-bis(hydroxymethyl)pyridine (4.9381 g,
35.49 mmol) and triphenylphosphine (18.7797 g, 71.60 mmol) was
dissolved in THF (300 mL) and cooled to 0 °C. Phthalimide (10.8280
g, 73.60 mmol) was added, and diethyl azodicarboxylate ester (14.0
mL, 77.17 mmol) was subsequently added dropwise. The cooling bath
was removed and the mixture was stirred for 1 day at room
temperature. The solvent was evaporated, and the residue was washed
with CH2Cl2 and dried under vacuum to give 2,6-bis-
(phthalimidomethyl)pyridine (12.0225 g, 30.25 mmol, 85%) as a
white powder. Without further purification, the product was used in
the next step. 1H NMR (400 MHz, CDCl3): δ 4.89 (s, 4H) 7.10 (d, J =
7.6 Hz, 2H), 7.57 (t, J = 7.8 Hz, 1H), 7.66 (m, 4H), 7.73 (m, 4H).
Synthesis of 2,6-Bis(aminomethyl)pyridine. The synthesis was
carried out according to some modification of a literature method.46
Hydrazine hydrate (6 mL) was added to a solution of 2,6-
bis(phthalimidomethyl)pyridine (4.9922 g, 12.56 mmol) in ethanol/
water (120 mL/6 mL). The mixture was refluxed for about 30 min to
form a white solid. After filtration and disposal of the solid, the solvent
was evaporated from the filtrate on a rotary evaporator. After
dissolution of the residue into CHCl3 and filtration, the solvent was
removed to give 2,6-bis(aminomethyl)pyridine (2.1242 g, 12.20 mmol,
97%) as a yellow oil. 1H NMR (400 MHz, DMSO-d6): δ 3.80 (s, 4H),
7.26 (d, J = 8.0 Hz, 2H), 7.70 (t, J = 7.8 Hz, 1H). 13C NMR (101
MHz, DMSO-d6): δ 52.57, 123.68, 141.92, 167.16.
DISCUSSION AND CONCLUSIONS
■
At one point, we worried that the conformation of the free H2L
suggested that this ligand might favor κ2 or (κ1) binding to
t
minimize Bu clash with pyridyl ring atoms. Using complexes
now in hand, distances between pyrrolide and pyridyl nitrogens
can be used to evaluate the bite of this pincer. Of special
t
interest is whether the Bu groups ortho to the pyridyl ring
create steric interference with the pyridyl ring hydrogens or
whether that repulsion is beneficial by redirecting the pyrrolide
nitrogens more inward, thus favoring smaller 3d transition
metals. In fact, those N(pyridyl)−N(pyrrolide) nonbonded
distances are 2.59−2.60 Å for both Pt(L)PhCN and (L)Zn-
(DMAP). The similarity of the values for Pt and Zn suggests
that the ligand does not strongly disfavor smaller metals. In
t
every case, the ortho Bu groups adopt a conformation which
t
nests the tooth of the pyridyl meta CH in the gear of two Bu
methyls.
The size of the metal seems to influence the geometry of
M(L)(Lewis base). The N(pyrrolide)−N(pyrrolide) distances
are as follows: Pt, 4.058 Å; Zn, 3.825 Å. The smaller distance
t
for Zn puts the Bu groups closer to each other, eventually
choking out any possible coordination of a base trans to the
pyridyl ring and accounting for the unusual geometry of
(L)Zn(DMAP) (see also Figure 6). This is equally evident
from the distance between the two quaternary carbons on these
tBu groups: 6.83 Å for Pt and 6.22 Å for Zn.
The M−N(pyridyl) distance is shorter than the M−N(pyrr)
distances by 0.11 Å in the Pt complex. The differences between
M−N distances in the Fe and Zn complexes are less significant,
and thus there are no generalizations possible about relative
M−N distances involving these two ring types.
Two compounds characterized here, Fe(HL)2 and (LH)-
ZnEt, show that pendant monoprotonated pyrrole still permits
the pincer ligand to be bidentate. This pendant pyrrole provides
Brønsted acid functionality, or even two of these, nearby for
useful subsequent reactivity.43−45 Given the cyclization
approach from β-diketones, the ligand is clearly modular:
different substituents on the pyrrolide arms will open up a
range of electronic/steric variants for ML complexes. In
addition, the flexibility of the ligand in terms of mode of
binding, bi- or tridentate and carrying a proton as HL− or
accepting a proton when it is L2−, show the potential of this
ligand class.
Synthesis of 2,6-Bis(3,5-di-tert-butylpyrrol-2-yl)pyridine. Bis-
(aminomethyl)pyridine (1.0 g, 7.3 mmol), 2,2,6,6-tetramethyl-3,5-
heptadione (3.2 mL, 15.3 mmol,) and tosic acid hydrate (0.80 g, 4.7
mmol) were added to 250 mL of mesitylene in a three-neck flask
equipped with a condenser and a Dean−Stark trap under nitrogen.
The mixture was refluxed under N2 for 4 days. After it was cooled, the
crude material was passed through a column of silica to give a yellow
solution. Upon removal of solvent, 1.29 g (41% yield) of 2,6-bis(3,5-
di-tert-butylpyrrol-2-yl)pyridine was collected as a pale yellow solid. 1H
NMR (400 MHz, CDCl3): δ 1.31 (s, 9H), 1.43 (s, 9H), 5.97 (d, J =
3.2 Hz, 2H), 7.35 (d, J = 8.0 Hz, 2H), 7.59 (t, J = 7.8 Hz, 1H), 8.86
(brs, 2H, NH). 13C{1H} NMR (101 MHz, CDCl3): δ 30.5, 31.4, 31.5,
31.8, 104.5, 118.7, 124.9, 133.0, 135.6, 141.1, 151.9. GC-MS (EI): m/z
433 (M+), 418 (M+ − CH3). If this reaction is done with a larger
amount of tosic acid catalyst, the doubly amine protonated 2,6-
bis(aminomethyl)pyridine ditosylate precipitates as a colorless salt
1
(identified by H NMR spectroscopy) and reduces the yield of the
desired product.
Preparation of Potassium Salt of 2,6-Bis(3,5-di-tert-butyl-
pyrrol-2-yl)pyridine. A 50.5 mg portion of 2,6-bis(3,5-di-tert-
butylpyrrol-2-yl)pyridine (0.116 mmol) in 4 mL of THF was slowly
added to a stirred mixture of 10.0 mg of KH (2.15 equiv, 0.249 mmol)
in 4 mL of THF. After 2 h, the solution was filtered and used without
1
further purification (removal of solvent yields a pale yellow solid). H
EXPERIMENTAL SECTION
General Procedures. All manipulations were carried out under an
atmosphere of purified nitrogen using standard Schlenk techniques or
in a glovebox. Solvents were purchased from commercial sources,
NMR (400 MHz, THF-d8): δ 1.30 (s, 9H), 1.42 (s, 9H), 5.88 (s, 2H),
7.03 (d, J = 7.2 Hz, 2H), 7.21 (t, J = 7.8 Hz, 1H).
Synthesis of LPt(PhCN). Deprotonation of H2L (90.6 mg, 0.209
mmol) with KH (25.3 mg, 0.631 mmol) in 13 mL of THF was carried
■
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out with stirring for 2 h. After filtration through a Celite pad (Celite
pad was then washed with 10 mL of THF), the solution was added to
109.5 mg (0.232 mmol) of PtCl2(NCPh)2 in 10 mL of THF at room
temperature. The reaction mixture was stirred at room temperature for
1 h. After removal of solvent under vacuum, the residue was extracted
with benzene. After filtration through Celite, the solvent was
evaporated and the crude compound obtained. NMR analysis showed
that the crude product was contaminated with free ligand.
Recrystallization for toluene/pentane in −40 °C gave the pure
platinum complex as a red powder. Yield: 28.5 mg (0.0390 mmol,
19%). 1H NMR (400 MHz, C6D6): δ 1.20 (s, 9H), 1.68 (s, 9H), 6.28
(s, 2H), 6.61 (t, J = 7.3, 2H, Ph), 6.77 (t, J = 7.2, 1H, Ph), 6.85 (t, J =
7.8, 1H, py), 6.92 (d, J = 7.2, 2H, Ph), 6.96 (d, J = 7.8, 2H, py).
Synthesis of LPd(PhCN). Stirring 15.8 mg (0.394 mmol) of KH
with 47.5 mg (0.110 mmol) of H2L in 7 mL of THF yielded K2L
within 2 h. After filtration with a Celite pad (the Celite pad was
washed with 5 mL of THF), the solution was added to 42.4 mg (0.111
mmol) of PdCl2(NCPh)2 in 3 mL of THF at room temperature. The
reaction mixture was stirred at room temperature for 1 h. After
removal of solvent under vacuum, the residue was extracted with
benzene. The solid residue was recrystallized from toluene/hexane at
−40 °C to give a red powder, yield 20.6 mg (0.321 mmol, 29%),
washed with benzene and pentane, and removal of solvent by vacuum
gave solid Zn(L)(DMAP) in 86% yield (53 mg, 0.086 mmol). H
1
NMR (400 MHz, CD2Cl2): δ 1.39 (s, 18H, tBu), 1.45 (s, 18H, tBu),
2.95 (s, 6H, Me), 6.15 (s, 2H, pyrrolide-H), 6.37 (d, J = 6.8 Hz, 2H,
DMAP), 6.99 (d, J = 6.8 Hz, 2H, DMAP), 7.17 (d, J = 8 Hz, 2H, py),
7.45 (t, J = 8 Hz, 1H, py). No decoalescence of peaks was observed
down to −40 °C. All CV scans were done with Pt as the working
electrode, Pt as the counter electrode, and Ag/AgCl wire as the
reference electrode. TBAPF6 (0.1 M) was employed as a supporting
electrolyte. The solvent was o-difluorobenzene. A 10.1 mg portion of
Zn complex was used. All CVs were referenced to internal Fc/Fc+ as
the standard, which appears at E1/2 = +0.7115 V vs the reference
electrode. The open circuit potential of Zn(L)(DMAP) was −0.460 V
vs Fc/Fc+.
Reaction of (HL)Zn(Et) with LiN(SiMe3)2. A solution of lithium
bis(trimethylsilyl)amide (17.0 mg, 0.102 mmol) in C6D6 was slowly
added to a stirred solution of (HL)Zn(Et) (50 mg, 0.095 mmol) in
C6D6 at 25 °C. Upon mixing the solution became pale orange-yellow
with formation of NH(SiMe3)2. The product solution contained
unreacted (HL)Zn(Et) as well as a new pincer complex, assigned as
1
[Zn(L)Et]Li. H NMR (400 MHz, C6D6): δ 0.29 (s, NH(SiMe3)2),
0.37 (q, J = 10 Hz, 2H, CH2CH3), 0.97 (t, J = 10 Hz, 3H, CH2CH3)
1.23 (s, 18H, tBu), 1.52 (s, 18H, tBu), 6.23 (s, 2H, pyrrolide-H), 6.97
(t, overlaid by (HL)Zn(Et) peak, 1H, py), 7.29 (d, J = 7.6 Hz, 2H, py).
After vacuum removal of volatiles, the ethyl peak disappeared, as well
as most of the bis(trimethylsilyl)amine. What remained was a product
containing a pincer ligand with equivalent pyrrolide arms, possibly
1
following crystallization from toluene/hexane. H NMR (400 MHz,
C6D6): δ 1.33 (s, 9H), 1.56 (s, 9H), 6.46 (s, 2H), 6.63 (t, J = 7.3, 2H,
Ph), 6.78 (t, J = 7.2, 1H, Ph), 6.87 (t, J = 7.8, 1H, py), 6.97 (d, J = 7.2,
2H, Ph), 7.08 (d, J = 7.8, 2H, py).
Carbonylation of LPd(NCPh). CD2Cl2 (0.5 mL) was vacuum-
transferred in to an NMR tube (5 mm i.d. × 180 mm length)
containing LPd(NCPh) (3.7 mg, 0.0058 mmol). The solution was
degassed by freeze−pump−thaw cycles, and CO (0.1 MPa) was
introduced. After the reaction mixture became dark, the sample tube
was placed in an NMR probe and the 1H NMR spectrum was
1
[ZnL]Li or a dimer thereof. H NMR (400 MHz, C6D6): δ 1.23 (s,
18H, tBu), 1.52 (s, 18H, tBu), 6.23 (s, 2H, pyrrolide-H), 6.97 (t, J = 8
Hz, 1H, py), 7.29 (d, J = 8 Hz, 2H, py).
Synthesis of Fe(HL)2. KH (4.6 mg, 0.11 mmol) was added to a
stirred solution of H2L (50 mg, 0.11 mmol) in 10 mL of THF. After 30
min, the reaction was complete to form a 1:1 mixture of H2L and K2L
1
measured. The H NMR spectrum indicates formation of free PhCN
1
1
in solution. A H NMR spectrum in THF showed the persistence of
and the palladium complex LPd(CO). H NMR (CD2Cl2): δ 1.40 (s,
H2L in solution, as well as peaks which corresponded to K2L, and
these two species were present in a 1:1 ratio. No additional species
were found in solution, suggesting that KHL is not a product of H2L
reacting with 1 equiv of KH. Solid FeCl2 (7.3 mg, 0.058 mmol) was
added to the stirred mixture and reacted for 2 h, forming a dark red
solution. After removal of solvent under reduced pressure, the dark red
product was extracted into pentane and KCl was filtered away. Upon
removal of pentane Fe(HL)2 (38 mg, 72% yield) remained as a dark
18H, tBu), 1.49 (s, 18H, tBu), 6.00 (s, 2H, pyrr), 7.06 (d, 2H, J = 8.0
Hz, py), 7.43 (t, 1H, J = 8.0 Hz, py). The IR spectrum in CH2Cl2
indicates formation of a carbonyl complex. IR (cm−1, CH2Cl2): 2197
(νCO).
Synthesis of (HL)Zn(Et). Diethylzinc (0.1 mL of 1.0 M ZnEt2 in
diethyl ether, 0.1 mmol of ZnEt2) was slowly added to a stirred
solution of 34.3 mg of H2L (0.079 mmol) in benzene at 25 °C. Upon
mixing the solution turned bright yellow, and the reaction was
complete after 30 min. Removal of solvent by vacuum gave a yellow
powder: yield 36 mg (0.068 mmol, 86%). Crystals were grown by slow
evaporation of a pentane solution. 1H NMR (400 MHz, C6D6): δ 0.15
(q, J = 10.8 Hz, 2H, −CH2CH3), 1.15 (s, 9H), 1.23 (s, 9H), 1.31 (t, J
= 10.8 Hz, 3H, −CH2CH3), 1.55 (s, 9H), 1.63 (s, 9H), 6.03 (d, 4JH−NH
= 3.2 Hz, 1H, pyrrole), 6.51 (s, 1H, pyrrolide), 6.59 (d, J = 7.6 Hz, 1H,
py), 6.95 (dd, J = 8.4, 7.6 Hz, 1H, py), 6.96 (br, 1H, NH), 7.81 (d, J =
8.4 Hz, 1H, py).
1
red solid. H NMR (400 MHz, C6D6): δ −56.44 (2H), −6.56 (bs,
18H, tBu), −1.74 (bs, 18H, tBu), 8.94 (bs, 18H, tBu), 13.56 (bs, 18H,
tBu), 71.13 (2H), 78.70 (2H), 79.83 (2H). The two remaining proton
resonances for the ligand backbone could not be located and are
apparently hidden under other stronger resonances. Paramagnetism
makes it difficult to use traditional features (line width or chemical
shift) to assign which is the signal of the NH protons.
Synthesis of (HL)Zn(DMAP)(Et). A solution of p-dimethylami-
nopyridine (4.0 mg, 0.033 mmol) in C6D6 was slowly added to a
stirred solution of (HL)Zn(Et) (17.9 mg, 0.034 mmol) in C6D6 at 25
°C. Upon mixing the solution became yellow green to form
(HL)Zn(DMAP)(Et). Removal of solvent by vacuum gave a yellow-
green powder. 1H NMR (400 MHz, C6D6): δ 0.51 (q, J = 8.0 Hz, 2H,
−CH2CH3), 1.24 (s, 9H), 1.25 (s, 9H), 1.60 (s, 9H), 1.69 (t, J = 8.0
Hz, 3H, −CH2CH3), 1.78 (s, 9H), 1.93 (s, 6H, Me), 5.68 (d, J = 5.6
Hz, 2H, DMAP), 6.15 (d, J = 3.2 Hz, 1H, pyrrole-H), 6.65 (s, 1H,
pyrrolide-H), 6.74 (d, J = 7.6 Hz, 1H, py), 7.03 (apparent t, J = 8.0 Hz,
1H, py), 7.49 (br, 1H, NH), 7.75 (d, J = 5.6 Hz, 2H, DMAP), 7.99 (d,
J = 8.4 Hz, 1H). In the presence of of a modest excess of DMAP, the
DMAP lines broadened and those of the HL ligand changed chemical
shift, consistent with the rapid exchange of free and coordinated
DMAP on the NMR time scale.
ASSOCIATED CONTENT
■
S
* Supporting Information
Text and CIF files giving full crystallographic details and figures
giving NMR spectra and CV scans. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*K.G.C.: e-mail, caulton@indiana.edu; tel, 812-855-4798.
Notes
The authors declare no competing financial interest.
Synthesis of Zn(L)(DMAP). (HL)Zn(DMAP)(Et) (65 mg, 0.10
mmol) was dissolved in 10 mL of C6H6 and gently heated to 50 °C for
20 min with stirring. When this solution was cooled to room
temperature, the product Zn(L)(DMAP) precipitated as pale yellow
needles. The product was filtered from the benzene solution and
ACKNOWLEDGMENTS
■
We thank Indiana University Bloomington (FRSP) for financial
support of this research. N.K. acknowledges support from JSPS
Institutional Program for Young Researcher Overseas Visits.
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K.; Morgan, D. G.; Mindiola, D. J.; Caulton, K. G. Inorg. Chem. 2013,
52, 5611.
We thank Atanu K. Das for crystal growth skills and Keith
Searles for CV measurements.
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(40) Fong, H.; Moret, M.-E.; Lee, Y.; Peters, J. C. Organometallics
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