Intramolecular N-H?Cl hydrogen bonds in the outer coordination sphere of a bipyridyl bisurea-based ligand stabilize a tetrahedral FeLCl 2 complex

Article abstract of DOI:10.1039/c4cc02297a

A bipyridyl-based anion receptor is utilized as a ligand in a tetrahedral FeCl₂ complex and demonstrates secondary coordination sphere influence through intramolecular hydrogen bonding to the chloride ligands as evidenced by X-ray crystallography.

Full text of DOI:10.1039/c4cc02297a

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Intramolecular N–HÁ Á ÁCl hydrogen bonds in
the outer coordination sphere of a bipyridyl
bisurea-based ligand stabilize a tetrahedral
FeLCl2 complex†
Cite this: Chem. Commun., 2014,
50, 7173
Received 28th March 2014,
Accepted 14th May 2014
Jesse V. Gavette,a Christina M. Klug,b Lev N. Zakharov,c Matthew P. Shores,*b
DOI: 10.1039/c4cc02297a
Michael M. Haley*a and Darren W. Johnson*a
www.rsc.org/chemcomm
A bipyridyl-based anion receptor is utilized as a ligand in a tetra-
hedral FeCl2 complex and demonstrates secondary coordination
sphere influence through intramolecular hydrogen bonding to the
chloride ligands as evidenced by X-ray crystallography.
Ligands capable of metal coordination while simultaneously
hydrogen bonding to anionic guests have drawn considerable
interest.1–4 Perhaps due to the ubiquity of iron and its relevance in
biological systems, numerous coordination complexes designed
Fig. 1 Structural representation of 1.
as ‘‘hosts’’ for anions feature iron in their binding site. Some of charges and bipyridyl nitrogen lone-pairs for stabilizing positive
these systems have utilized intramolecular hydrogen bonding to charges, was the source of the preference toward this diprotic
mimic the subtle secondary coordination sphere influences found oxoanion. Based on this receptor design, it was recognized that
in metalloproteins.5,6 Intermolecular hydrogen bonding systems the binding pocket should be suitable to a variety of guests
have also been used to allow secondary coordination of anions to possessing either (or both) positive and negative charges. Given
modulate the spin states of metal complexes for potential applica- the ubiquity of bipyridine as a ligand for transition metals, we
tions in data storage and sensing.7–11 In light of these emerging were interested in investigating if the hydrogen bond donor
applications and the inherent challenge in targeting coordination groups on 1 would act in tandem with the bipyridyl group to
of a metal complex rather than just a single metal ion, the design coordinate and hydrogen bond to small metal complexes. Such
of novel ligands that coordinate metal complexes through both molecular recognition of metal complexes has been termed
direct chelation and hydrogen bonding, remains an active area of ‘‘stereognostic’’ coordination chemistry.13
ion coordination chemistry.
Herein we report solid state X-ray analysis and SQUID studies of
Recently we reported anion binding studies of bipyridyl the coordination of 1 with FeCl2, indicating the formation of a
bisurea-based receptor 1 (Fig. 1).12 This receptor displayed a tetrahedral 1ÁFeCl2 metal complex. Surprisingly, this species features
particular affinity toward the dihydrogen phosphate anion, four tight intramolecular hydrogen bonding interactions between
H2PO4À, in 10% DMSO–chloroform-based solvent mixtures. The the two urea groups and the coordinated chloride ligands remaining
ditopic binding environment of this receptor, which provides two on the Fe2+ metal centre.
urea groups for convergent hydrogen bonding to stabilize negative
Slow evaporation of 1 with excess FeCl2Á4H2O in acetonitrile
resulted in yellow block-shaped crystals.14 The subsequent X-ray
structure shows an Fe2+ ion coordinated in a tetrahedral geometry
by two chlorides and the bipyridyl core; the binding is likely
reinforced by the interaction between the two urea ‘‘arms’’ of
the ligand and the metal-bound chloride ligands (Fig. 2a). The
urea N–Cl bond distances and N–HÁÁÁCl bond angles indicate the
formation of two moderate to weak hydrogen bonds15 to each of
the coordinated chloride ligands by the respective urea functional
groups (Table 1). Examples of this type of intramolecular hydrogen
bonding to metal halides in the solid state are limited, and
are typified by hydrogen bonding to a single halide ligand.16–19
a Department of Chemistry & Biochemistry and Materials Science Institute,
University of Oregon, Eugene, OR 97403-1253, USA. E-mail: dwj@uoregon.edu,
haley@uoregon.edu; Tel: +1-541-346-1253, +1-541-346-0456
b Department of Chemistry, Colorado State University, Fort Collins, CO 80523-1872,
USA. E-mail: matthew.shores@colostate.edu; Tel: +1-970-491-7235
c CAMCOR—Center for Advanced Materials Characterization in Oregon,
University of Oregon, Eugene, OR 97403-1253, USA
† Electronic supplementary information (ESI) available: Experimental and mag-
netic measurements, X-ray analysis of receptor. CCDC 978233. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4cc02297a
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In an effort to characterize the iron complex in solution,
determination of the magnetic susceptibility (wM) by 1H NMR using
Evans method26,27 was attempted. The low solubility of ligand 1
and of the resultant 1ÁFeCl2 complex in common organic solvents
dictated the use of highly coordinating solvents (e.g. DMSO-d6) to
achieve appreciable concentrations of the Fe2+ species in a 90 : 10
CDCl3 : DMSO-d6 solution. The limited solubility and temperature
range available in this solvent system lead to 1H NMR spectra of the
metal complex that suffered from problems ranging from extreme
peak broadening to spectra that were nearly identical to that of the
free ligand. This prevented the collection of reliable magnetic
susceptibility measurement values in solution. These observations
are consistent with the dissociation of the metal salt and ligand in
highly polar (competitive) solvents. The complex was also not stable
under ESI-MS conditions, and dissociated iron species and free
ligand represent the major ions observed, although small amounts
of ligated Fe-complexes were observed (see Fig. S3 and S4, ESI†).
The apparent dissociation in solution is likely perpetuated by a
frustrated coordination environment indicated by the short Fe–N
and Fe–Cl bond distances observed in the solid state.
Fig. 2 X-Ray data of 1 represented as (a) an ORTEP structure (ellipsoids at
50% probability and non-coordinating hydrogens removed for clarity) and
(b) space filling structures of both P and M helices.
Although virtually all tetrahedral Fe2+ complexes are high spin,
the short Fe–N and Fe–Cl bond distances observed in 1ÁFeCl2
imply that the compound is close to the spin crossover regime. In
the solid state, magnetic susceptibility measurements of 1ÁFeCl2
show a high spin Fe2+ (S = 2) species at 295 K with a wMT value of
3.78 emu K molÀ1 (meff = 5.5) (Fig. 3). This value is consistent with
other reported high-spin tetrahedral Fe2+ complexes,20,22,28 sup-
porting the tetrahedral structure observed by X-ray analysis. At
lower temperatures, a slight decrease in susceptibility is observed,
dropping to 3.15 emu K molÀ1 (meff = 5.0) at 15 K, with a more
drastic decrease in the wMT value at 5 K. This downturn is
consistent with zero-field splitting of the high-spin Fe2+ ion due
to the low symmetry ligand field.
Table 1 Selected bond distances (Å) and angles (1) of 1ÁFeCl2
Bond
Distance
Angle
Fe1–N1
Fe1–N2
Fe1–Cl1
Fe1–Cl2
2.030(5)
2.066(5)
2.188(3)
2.201(3)
3.407(6)
3.096(7)
3.581(6)
3.190(6)
—
—
—
—
—
—
—
—
—
N3Á Á ÁCl1
151a
N4Á Á ÁCl1
156a
N6Á Á ÁCl2
152a
N7Á Á ÁCl2
171a
N1–Fe1–N2
Cl1–Fe1–Cl2
N1–Fe1–Cl1
N2–Fe1–Cl1
N1–Fe1–Cl2
N2–Fe1–Cl2
79.6(2)
122.06(10)
112.23(17)
110.64(18)
108.94(17)
115.45(16)
Coordination of bipyridyl-based ligands with steric bulk adjacent
to the donor atom tends to allow spin crossover for octahedral
Fe2+ complexes;29 otherwise, aromatic diimines generally lead
—
a
Angle of N–HÁ Á ÁCl.
The observation that this structure allows the formation of up
to four intramolecular hydrogen bonds to the two coordinated
halides on the metal highlights the novelty of the presented
ligand design. Curiously, the average Fe–N and Fe–Cl distances
of 1ÁFeCl2 (2.048(5) Å and 2.194(8) Å, respectively) are shorter
on average than those of similarly structured biheteroaryl-
coordinated tetrahedral FeCl2 complexes (with average distances
ranging from 2.1029(65)–2.115(8) Å and 2.2209(6)–2.2303(7) Å,
respectively).20–22 This may be a result of the ligand constricting
the metal salt within the binding pocket. Additionally, the crystal
structure reveals that the convergent coordination of the urea-
based ligand appendages results in a helical twist in the binding
conformation, and both enantiomers (P and M helices, Fig. 2b)
are present in a racemic mixture in the solid state. Such guest-
induced helical chirality has received much attention recently,23–25
and represents an interesting area of further research for this
and related systems.
Fig. 3 Temperature dependence of wMT for 1ÁFeCl2, obtained under a
1000 Oe measuring field. Line added only as a guide for the eye.
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[PhB(MesIm)3Fe-NdPPh3], has a more flexible ligand set imparted
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the intramolecular hydrogen-bonding network. Thus, the complex
remains trapped in the high-spin state even though the Fe–N
bond lengths suggest that the low-spin state should be accessible.
In conclusion, we have demonstrated that a bipyridyl bisurea-
based receptor designed to ditopically coordinate protic anions
provides a suitable framework as a ligand toward metal halide
salts. Solid state investigation of the Fe2+ complex reveals the
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M = 953.65, 0.12 Â 0.07 Â 0.05 mm, T = 173(2) K, triclinic, space
%
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Dc = 1.447 Mg mÀ3, m = 0.528 mmÀ1, F(000) = 988, 2ymax = 50.001,
17 504 reflections, 7664 independent reflections [Rint = 0.0971],
R1 = 0.0838, wR2 = 0.1879 and GOF = 1.006 for 7664 reflections
(586 parameters) with I 4 2s(I), R1 = 0.1693, wR2 = 0.2368 and
GOF = 1.006 for all reflections, max/min residual electron density
+1.126/À0.610 e Å3.
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This work was supported by NIH grant R01-GM087398, which
also funded early stage intellectual property that was licensed by
SupraSensor Technologies, a company co-founded by D.W.J and
M.M.H. M.P.S. and C.M.K. thank the NSF (CHE-1058889) and
Colorado State University for support of this work.
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Chem. Commun., 2014, 50, 7173--7175 | 7175