Structural diversity in metal complexes with a dinucleating ligand containing carboxyamidopyridyl groups

Article abstract of DOI:10.1021/ic200881t

The synthesis of a (carboxyamido)pyridinepyrazolate (H₅bppap) dinucleating ligand is described. Bimetallic iron and cobalt complexes of H ₅bppap ([M^II₂H₂bppap]⁺) showed structural differenc

Full text of DOI:10.1021/ic200881t

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pubs.acs.org/IC
Structural Diversity in Metal Complexes with a Dinucleating Ligand
Containing Carboxyamidopyridyl Groups
Gary K.-Y. Ng, Joseph W. Ziller, and A. S. Borovik*
Department of Chemistry, University of California—Irvine, 1102 Natural Science II, Irvine, California 92697, United States
S Supporting Information
b
ABSTRACT: The synthesis of a (carboxyamido)pyridi-
nepyrazolate (H5bppap) dinucleating ligand is described. Bime-
tallic ironandcobaltcomplexes ofH5bppap([MII H2bppap]+)
2
showed structural differences in both their primary and
secondary coordination spheres. The binding of small
molecules into the preorganized ligand cavity is verified by
Figure 1. Possible binding modes of the carboxyamido unit.
the hydration of [FeII H2bppap]+ and [CoII H2bppap]+,
2
2
leading to the formation of complexes [{CoII(OH)}-
CoIIH3bppap]+ and [{FeII(OH)}FeIIH3bppap]+, in which
one of the metal centers has a terminal hydroxo ligand.
Scheme 1. Synthesis of H5bppapa
ydrogen bonds (H-bonds) within the secondary coordi-
H
nation sphere of metalloprotein active sites are effective at
regulating metal-mediated processes.1 In many cases, these intra-
molecular H-bonds are formed between amino residues and
external ligands, which are often derived from water molecules. A
key feature of biological H-bonding networks is their ability to
change structure during a chemical transformation to aid in the
transfer of protons and electrons or in the orientation of substrates.2
There have been numerous efforts to incorporate these proper-
ties in synthetic systems, and notable advancements have been
made.3 One approach is to place basic functional groups proximal
to metal centers that upon protonation can serve as H-bond donors.4
Following this concept, we have developed a new dinucleating
ligand, 3,5-bis[bis(N-6-pivalamido-2-pyridylmethyl)aminomethyl]-
1H-pyrazole (H5bppap), that when bonded to metal ions, adopted
a variety of structures depending on its protonation state.5 This
report illustrates that H5bppap can produce dinuclear species, in
which each metal ion can have a different primary coordination
sphere. Furthermore, we show that the ligand can aid in forming
varied intramolecular H-bonding networks.
a Conditions: (a) Na2CO3, MeCN, Δ, 24 h; then HCl(aq)/EtOH, rt,
21 h, 80%.
Alternatively, the protonated form of the carboxyamido group
assumes a conformation such that it is an intramolecular H-bond
donor (Figure 1B). We have found that coupling these units to
a pyrazolate ring led to H5bppap, a dinucleating ligand whose
complexes can have metal centers with differing primary and sec-
ondary coordination spheres.
H5bppap was synthesized in multigram quantities via a con-
vergent synthetic route, as outlined in Scheme 1. The protected
bridging unit, 3,5-bis(chloromethyl)tetrahydropyran-2-yl-1H-
pyrazole,5,11a,12 and bis[N-(6-pivalamido-2-pyridylmethyl)]-
amine1 were coupled to afford a protected form of the ligand,
which was subsequently deprotected to afford H5bppap as a white
powder in good yield (84%).
To evaluate the structural properties of metal complexes with
Nearly all of the binucleating ligands that have appeared in the
literature provide symmetrical primary coordination spheres
around each metal ion.6 However, there are several situations
in which differing metal coordination geometries would be ben-
eficial for function.6b,7 For example, the reduced form of the
respiratory protein hemerthyrin contains one six-coordinate and
one five-coordinate ferrous center: dioxygen binding occurs at
the coordinately unsaturated site.8 Our group9 and others10 have
been utilizing (carboxyamido)pyridyl units in the design of multi-
dentate ligands because they potentially coordinate to metal ions
in two distinct modes. In one case, the carboxyamido group is
deprotonated and functions as a bidentate ligand through coordi-
nation of the pyridyl nitrogen and carbonyl oxygen atoms (Figure 1A).
H5bppap, [FeII H2bppap]+ and [CoII H2bppap]+ were prepared
2
2
(Scheme 2) and crystallized.12 Treating H5bppap with 3 equiv of
KH in anhydrous tetrahydrofuran gave the [H2bppap]3À anion,
which was allowed to react with 2 equiv of Fe(OTf)2. The salt
[FeII H2bppap](OTf) was isolated in almost quantitative yields
2
via separation of the product from insoluble KCl. [CoII H2bppap]-
2
(BPh4) was prepared by treating a methanolic solution of H5bppap
with NaOMe, Co(OAc)2, and NaBPh4; the salt was isolated as a
Received: April 27, 2011
Published: July 27, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ic200881t Inorg. Chem. 2011, 50, 7922–7924
|

Inorganic Chemistry
COMMUNICATION
Scheme 2. Synthesis of Bimetallic Complexes of H5bppapa
a Conditions: (a) 3 equiv of KH; (b) 2 equiv of Fe(OTf)2; (c) 3 equiv of
NaOMe; (d) 2 equiv of Co(OAc)2; (e) 1 equiv of NaBPh4.
lime green solid in high yield (86%). Both complexes were stable
in a dry, anaerobic environment for several weeks.
The molecular structures for [FeII H2bppap]+ and [CoII H2-
2
2
bppap]+ were determined by X-ray diffraction methods (Figure 2)
and illustrate the versatility of the [H2bppap]3À ligand in reg-
ulating the primary and secondary coordination spheres. The
structure [CoII H2bppap]+ shows that both CoII centers have
2
nearly identical distorted trigonal-bipyramidal coordination geo-
metries (τCo1 = 0.74; τCo2 = 0.76).13 The pyridyl and pyrazolate
nitrogen atoms define the trigonal planes: for pyrazolate nitrogen
atoms, the Co1ÀN4 and Co2ÀN10 bond distances are equiva-
lent within experimental error. However, each cobalt center
has two distinct CoÀNpy bond lengths: for example, the Co1À
N2 and Co1ÀN3 bond distances are 2.028(2) and 2.135(2) Å,
respectively. This difference in bond lengths reflects the fact that
N2 is contained within two chelating rings, while N3 is only
involved in a single chelating ring. The apical nitrogen atoms
N1 and N9 bind to one of the axial coordination sites on each
metal ion [avg. CoÀNaxial = 2.171(2) Å]. The other axial posi-
tions around each cobalt center are occupied by atoms O1 and
O3 from the deprotonated carboxyamido groups, which co-
ordinate in an identical manner (Table S2 in the Supporting
Information). The remaining carboxyamido groups are protonated
and form intramolecular H-bonds with O1 and O3, in which
N6 O1 and N12 O3 distances of less than 3 Å are
Figure 2. Thermal ellipsoid plots and schematic drawings for
[CoII2H2bppap]+ (A) and [FeII2H2bppap]+ (B). Only carboxyamido
hydrogen atoms are shown for clarity. Selected metrical parameters are
given in the Supporting Information.12
Scheme 3. Synthesis of Dimetallic [{MII(OH)}MIIH3bppap]+
Complexes12
different conformations that may be useful in controlling the
binding of exogenous ligands. To examine this possibility and eval-
uate the accessibility of the metal centers, we studied the binding
3 3 3
3 3 3
observed. A Co1 Co2 separation of 4.513 (1) Å was observed
3 3 3
in [CoII H2bppap]+.
2
The structural analysis of [FeII H2bppap]+ revealed that each
of water molecules to [MII H2bppap]+ complexes (Scheme 3).
2
2
iron center has different primary and secondary coordination
spheres (Figure 2B). Fe1 has a distorted trigonal-bipyramidal
geometry (τFe1 = 0.68), containing an N4O donor set that is
Treating either complex with 1 equiv of water produced the
monohydroxide complexes [{MII(OH)}MIIH3bppap]+.12,14 Our
analytical and spectroscopic studies suggested that water addi-
tion caused protonation of the dinucleating ligand with concom-
itant binding of a hydroxide ion to one of the metal centers. Note
that additional amounts of water, up to 40 equiv, afford only the
monohydroxo complexes.
similar to what was observed for the cobalt centers in [CoII H2-
2
bppap]+. In contrast, Fe2 is six-coordinate, having an N4O2
arrangement of donor atoms, in which both oxygen atoms of the
carboxyamido groups are bonded to the iron center. The
significant difference in the Fe2ÀO3 and Fe2ÀO4 bond lengths
of 1.957(2) and 2.267(2) Å reflects the protonation states of the
carboxyamido groups. The shorter Fe2ÀO3 bond length agrees
with the carboxyamido group containing O3 being deprotonated
and serving as an anionic ligand. The carboxyamido group that
includes O4 is protonated and thus is a neutral donor. Differ-
ences in the ligand NÀC(O) bond lengths support this assign-
ment: for the neutral group, N12ÀC45 is 1.358(3) Å, whereas
for the anionic group, N11ÀC34 is 1.307(4) Å, indicating
increased double-bond character. Furthermore, this type of
coordination prevents the formation of intramolecular H-bonds
around Fe2. It is not clear why there is a difference in the coor-
X-ray diffraction studies revealed that the cobalt ions in
[{CoII(OH)}CoIIH3bppap]+ (Figure 3A) have dissimilar trigonal-
bipyramidal coordination geometries (τCo1 = 0.88; τCo2 = 0.74).
For Co2, the metrical parameters and overall structures of the
primary and secondary coordination spheres are nearly identical
with those found for the cobalt centers in [CoII H2bppap]+. For
2
Co1, the hydroxo ligand is terminally bonded at one of the axial
sites with a Co1ÀO5 bond length of 1.931(2) Å and a N1ÀCo1À
O5 bond angle of 176.154°. This binding caused a major struc-
tural change in the dinucleating ligand: O1 is not bonded to Co1
(as in [CoII H2bppap]+) but is positioned outside the cavity sur-
2
rounding the metal ions. Moreover, the appended carboxyamido
group containing O1 is now protonated, with the N5ÀH5B vector
placed within the cavity. The structural rearrangement resulted in
two intramolecular H-bonds forming between the hydroxo ligand
and the carboxyamido groups (N5 O5, 2.773 Å; N3 O5,
dination spheres for the two iron centers in [FeII H2bppap]+.
2
The synthetic and structural results with the [MII H2bppap]+
2
(MII = Fe, Co) complexes suggested that the appended carboxy-
amido groups of [H2bppap]3À are flexible and could readily adopt
3 3 3
3 3 3
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Inorganic Chemistry
COMMUNICATION
’ ASSOCIATED CONTENT
S
Supporting Information. Details for all experiments,
b
spectra, and crystallographic details (CIF). This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: aborovik@uci.edu.
’ ACKNOWLEDGMENT
Acknowledgment is made to the NIH (Grant GM50781 to A.
S.B.) for financial support.
’ REFERENCES
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Figure 3. Thermal ellipsoid plots and schematic drawings for
[{CoII(OH)}CoIIH3bppap]+ (A) and [{FeII(OH)}FeIIH3bppap]+
(B). Only carboxyamido hydrogen atoms are shown for clarity. Selected
metrical parameters are given in the Supporting Information.12
2.773 Å). The Fourier transform IR spectrum supports this premise
by having a single ν(OÀH) peak at 3613 cmÀ1 and several broad
ν(NÀH) signals between 3400 and 3200 cmÀ1, consistent with
H-bonds involving the carboxyamido units.14e
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The structural analysis on [{FeII(OH)}FeIIH3bppap]+ showed
that again one five-coordinate and one six-coordinate iron
center exist, yet now an endogenous hydroxo ligand is termin-
ally bonded to one of the iron sites. The hydroxo ligand is
bonded to the five-coordinate iron site (τFe1 = 0.68), with a
Fe1ÀO5 bond length of 1.938(2) Å and a N1ÀFe1ÀO5 bond
angle of 172.62(9)° (Figure 3B). Both carboxyamido groups
are protonated around Fe1 and form intramolecular H-bonds
to the Fe1ÀO5 unit in a manner similar to that observed in
[{CoII(OH)}CoIIH3bppap]+. The Fe2 site remained six-coor-
dinate, with both O3 and O4 of the appended carboxyamido
groups bonded to the iron center.
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(12) See the Supporting Information for details.
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In summary, we have developed a new dinucleating ligand that
utilizes a pyrazolate bridge and (carboxyamido)pyridyl groups.
The structures of [MII H2bppap]+ and their hydrated products
2
[{MII(OH)}MIIH3bppap]+ showed the versatility of this design
to produce complexes with varied coordination chemistry. The
formation of a single metal ion site with a terminal hydroxo ligand
is rare because of the propensity of this ligand to bridge between
metal ions. The formation of only one MÀOH site in each com-
plex is attributed to several factors, including the spacing pro-
vided by the pyrazolate unit,11b the intramolecular H-bonds, and
the bidentate binding mode of the carboxyamido groups. Our
findings illustrate the utility of this design in preparing new
classes of unsymmetrical dinuclear complexes.
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