A dianionic C3-symmetric scorpionate: Synthesis and coordination chemistry

Article abstract of DOI:10.1039/d0dt02601h

Introducing charges into ligand systems fine-tunes their electronic properties and influences the solubility of their metal complexes. Herein, we present a synthesis of a dianionic, C3-symmetric ligand combining three anionic N-donors tethered to a positively charged phosphonium center. The tris-skatylmethylphosphonium (TSMP) ligand, isolated in the form of its dipotassium salt TSMPK2, is the first dianionic homoscorpionate capable of metal exchange. The potassium cations in TSMPK2 are exchangeable for other metals, which results in rich coordination chemistry. Thus, the ligand displays a bridging μ2:κ2:κ1 coordination mode with trigonal planar Cu(i) centers in the tetrameric complex [(TSMP)Cu]44-. The κ3 mode is accessed upon addition of 1 equiv. of P(OEt)3 per Cu(i) to yield the tetrahedral monomeric complex [(TSMP)CuP(OEt)3]-. Both Fe(ii) and Ni(ii) in pyridine give octahedral high-spin κ3 complexes with composition (TSMP)M(Py)3 (M = Fe, Ni). Displacement of three pyridine ligands in (TSMP)Fe(Py)3 for a second equivalent of TSMP gives a high-spin pseudotetrahedral 2?:?1 complex [(TSMP)2Fe]2- with the ligands in κ2 coordination mode. The reduction in coordination number is likely due to electrostatic repulsion of the negatively-charged indolides as well as their weaker π-accepting character as compared to pyridine. This journal is

Full text of DOI:10.1039/d0dt02601h

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Douglas W. Stephen et al.
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DOI: 10.1039/D0DT02601H
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A Dianionic C -Symmetric Scorpionate: Synthesis and
3
Coordination Chemistry†
a
a
b
a
Serhii Tretiakov, Johannes A. M. Damen, Martin Lutz, Marc-Etienne Moret*
Received 00th January 20xx,
Accepted 00th January 20xx
Introducing charges into ligand systems fine-tunes their electronic properties and influences the solubility of their metal
complexes. Herein, we present a synthesis of a dianionic, C -symmetric ligand combining three anionic N-donors tethered
to a positively charged phosphonium center. The tris-skatylmethylphosphonium (TSMP) ligand, isolated in the form of its
DOI: 10.1039/x0xx00000x
3
dipotassium salt TSMPK
2
, is the first dianionic homoscorpionate capable of metal exchange. The potassium cations in TSMPK
2
2
2
1
are exchangeable for other metals, which results in rich coordination chemistry. Thus, the ligand displays a bridging μ :κ :κ
4
-
3
4
coordination mode with trigonal planar Cu(I) centers in the tetrameric complex [(TSMP)Cu] . The κ mode is accessed upon
addition of 1 equiv. of P(OEt)
3
per Cu(I) to yield the tetrahedral monomeric complex [(TSMP)CuP(OEt)
3
]-. Both Fe(II) and Ni(II)
3
in pyridine give octahedral high-spin κ complexes with composition (TSMP)M(Py)
3
(M=Fe, Ni). Displacement of three
3
pyridine ligands in (TSMP)Fe(Py) for a second equivalent of TSMP gives a high-spin pseudotetrahedral 2:1 complex
Fe]2- with the ligands in κ coordination mode. The reduction in coordination number is likely due to electrostatic
2
[
(TSMP)
2
repulsion of the negatively-charged indolides as well as their weaker π-accepting character as compared to pyridine.
4
5
6
applications in biomimetics, catalysis, material science and
7
Introduction
production of radiopharmaceuticals. Following this success,
the definition of scorpionates has been extended to tripodal
tridentate systems with other donor groups and bridging atoms.
1
The term “scorpionate” was introduced by Trofimenko and
refers to tripodal tridentate ligand systems that are able to
coordinate to a metal with two identical donor moieties,
similarly to the pincers of a scorpion. The third donor moiety
rotates forward akin to a scorpion stinger to attack the metal in
a fac manner. If it is identical to the first two, a C -symmetric
3
homoscorpionate complex forms; otherwise, coordination
results into a heteroscorpionate complex.
8
9
Among the employed donors are imidazole, pyridine,
1
0
11
12
13
triazole, indole, methimazole, oxazoline, N-heterocyclic
1
4
15
carbenes and others, and even acyclic donor groups.
Variation of the bridging atom allowed to further tune
electronics and charge of the scorpionate ligands, thus
influencing coordination behavior and solubility of their
9
16
complexes. Reported systems include CH/COH, CH
3
- 20
/C
6
H
5
-Si,
Most scorpionates are six-electron donors, which makes them
isoelectronic to another common ligand, cyclopentadienyl (Cp).
A considerable amount of research was done to compare these
two systems.1 It is important to point out, however, that while
9
a
8,9,17
+ 18
9b,19
9a
9b
-
N, P,
[R-P] , P=O,
As, As=O, [CH
3
-Al] , [CH
3
-Ga]
2
1
- 22
23
-
- 24
- 20
,
C , Si-, Ge , Sn , Pb .
,2
3
they are isoelectronic, they are not isolobal, therefore the
extent of such a comparison is limited. Additionally, in some
2
situations, scorpionates are capable of displaying a κ
coordination mode freeing the third arm for binding to another
1
metal center, which sets them apart from Cp ligands.
The first generation of homoscorpionates involved
1
trispyrazolylborates and proved to be highly versatile spectator
ligands. By modifying the nature, number and position of
substituents of the pyrazolyl rings, a wide range of ligands was Chart 1. C -symmetric scorpionates with distinct formal charges.
3
prepared enabling both electronic and steric properties of a
coordinated metal to be fine-tuned. Such systems have found A more general way to classify the existing C -symmetric
3
scorpionates is by formal charge, which is a combined property
of the donor moieties and the bridging atom. Thus, one can
a. Utrecht University, Organic Chemistry
&
Catalysis, Debye Institute for
differentiate
neutral,
mono-,
di-
and
trianionic
Nanomaterials Science, Faculty of Science, 3584 CG Utrecht, The Netherlands.
b.
Utrecht University, Crystal and Structural Chemistry, Bijvoet Centre for homoscorpionates (Chart 1). Whereas there is a plethora of
Biomolecular Research, Faculty of Science, 3584 CH Utrecht, The Netherlands.
8,9,14,16,17
1,10,12,13,20-22,24
known neutral
and monoanionic
†Electronic Supplementary Information (ESI) available: Synthetic and
1
1
homoscorpionates as well as some trianionic examples,
characterization data for all new compounds, additional figures. CCDC 2010902-
010907. See DOI: 10.1039/x0xx00000x
2
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1
8
dianionic ligands are rare with only one reported precedent (A ether. X-ray crystal structure determination confirmed the
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in Chart 2). The latter was isolated as a LAlCl complex, and its expected molecular structure with a K:P rDaOtIi:o10o.f1023:91/D(F0iDgTu0r2e6101).H
LAlMe complex could be methylated at phosphorus to afford a
methylphosphonium derivative. However strong Al–O bonds
would likely preclude the possibility of a subsequent metal
exchange and make broader exploration of its coordination
chemistry difficult.
2
Figure 1. Asymmetric unit of the TSMPK (1) crystal. Displacement ellipsoids are drawn
at the 30% probability level. Hydrogen atoms and acetonitrile solvent molecules are
omitted for clarity. Potassium atoms are shown in a wireframe style.
3
Chart 2. Dianionic C -symmetric scorpionates.
+
The K cations are involved in an extensive network of cation-N
3
Herein, we present the synthesis of a C -symmetric dianionic
1
6
and cation-π interactions (η to η ). The K–N distances vary
between 2.723(4) and 3.425(4) Å, and the K–C distances
between 2.995(4) and 3.510(4) Å. With such a large variation of
distances it is not possible to derive a clearly defined
ligand B (Chart 2), isolated as its dipotassium salt 1, which fills
the aforementioned gap in the assortment of charged
scorpionates available for complexation. Taking advantage of
the fact that potassium cations are easily exchangeable for
other metal ions, we further delve into its coordination
chemistry with Cu(I), Fe(II) and Ni(II) salts.
+
coordination number. Overall, K coordination leads to the
formation of one-dimensional chains in the [110] direction,
which are stacked upon each other in c-direction. This stacking
is interrupted by the inclusion of acetonitrile solvent molecules
between every second layer (see Supporting Information
Section S3).
Results and discussion
Ligand synthesis
The dipotassium salt of tris-(2-skatyl)methylphosphonium (1),
further abbreviated as TSMPK
Scheme 1. In the first step, skatole (2) was treated with di-tert-
butyl dicarbonate (Boc O) in the presence of 4-
2
, was synthesized as depicted in
2
dimethylaminopyridine (DMAP) to yield the corresponding N-
Boc derivative (3). The second position of the indole ring was
subsequently lithiated with lithium diisopropylamide (LDA)
using N-Boc as a directing group. Without isolation, the lithium
3
salt was quenched with a third of an equivalent of PCl to yield
tris-2-(N-Boc-skatyl)phosphine (4). The phosphine was then
methylated by treatment with an excess (2.0 equiv.) of methyl
iodide to form the corresponding methylphosphonium iodide
(5). Attempted methylation with an equimolar amount of
methyl iodide instead resulted in the incomplete methylation
and partial deprotection of the N-Boc-skatole subunits. It is
worth
pointing
out
that
tris-2-(N-Boc-
solution
skatyl)methylphosphonium iodide (5) exists in DCM-d
2
as a ca. 1.00:0.15 mixture of interexchanging tris-exo and bis-
exo-mono-endo rotamers (see Supporting Information Section
S4.1).
Complete deprotection of the isolated methylphosphonium
iodide 5 can be achieved in a 1:1 mixture of dichloromethane
and trifluoroacetic acid (TFA). The product, 6, was subjected to
ion exchange using the chloride form of the anion exchange
resin Amberlite® IRA-400, affording a white crystalline solid of
tris-(2-skatyl)methylphosphonium chloride (7).
Deprotonation of tris-indole 7 with either KHMDS or KH gives
the TSMPK
2
salt (1). A highly pure material (>99% according to
NMR) was isolated by crystallization from acetonitrile/diethyl
2
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DOI: 10.1039/D0DT02601H
Scheme 1. Synthesis of TSMPK
2
salt 1. Unless otherwise stated, parentheses underneath reaction arrows indicate isolated yields.
Scheme 2. Coordination behavior of the TSMP ligand platform.
2
The existence of a free alkaline trianionic-monocationic Reacting equimolar amounts of TSMPK salt 1 with cuprous
phosphonium salt is, to our knowledge, unprecedented. chloride in pyridine led to an orange-red solution, from which
Quaternary phosphonium salts are generally incompatible with yellow crystalline compound 8 was isolated upon freeing from
2
6
basic counterions and convert either into ylides,
solvent in vacuo and extraction with THF. NMR spectroscopy in
phosphoranes2 or form charge-transfer complexes followed acetonitrile-d
by complex decomposition manifolds. The exceptions, oligomer. To elaborate, the P NMR spectrum (Figure 2) shows
however, feature electronically stabilized bulky anions (e.g. a single quartet consistent with JP,H coupling with a methyl
We group. In the H NMR spectrum (Figure 2), there are two singlets
speculate that the stability of 1 is due to the above reasons as in a 2:1 ratio corresponding to aromatic methyl groups CH
well: the anionic indolide nitrogen atoms lose a lot of their and CH
7,28
reveals one predominant species, albeit an
3
2
9
31
2
diphenylamide30
and
2,4,6-trimethylphenolate ).
31
1
Ar
3
A
Ar
3
B
. Moreover, multiplets in the aromatic region are
nucleophilicity due to being involved in an aromatic π-system, present in the same ratio and, according to magnitude gCOSY
and the phosphonium center is too encumbered to (see Supporting Information Section S4.2), constitute two
accommodate a fourth indolide in its vicinity. Additionally, the distinct spin systems. This implies 2:1 inequivalence of indolide
latter would experience electrostatic repulsion from other units in the complex. A NOESY spectrum (see Supporting
anionic indolides already present in a molecule.
Information Section S4.2) shows a correlation between the
P
Ar
phosphonium methyl group CH
(mapped out in Figure 2) but not CH
3
and two aromatic CH
3
A
Ar
Coordination chemistry of TSMP
3
B
, from which it follows
Ar
that CH
3
B
is pointing away from the phosphonium methyl
In order to study ligation behaviour of the novel TSMP platform,
we undertook a series of metal exchange reactions with salt 1.
It shows rich coordination chemistry with a number of first-row
transition metal salts (Scheme 2).
4
7
group. Lastly, H
intensities with CH
A
and H
A
have comparable correlation
Ar
3 A
. Since NOE rapidly decays with distance,
7
Ar
4
and H
A
is clearly further away from CH
3
A
than H
A
, it can only
be the case if there is an equivalent methyl group in the vicinity
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of H7
reasoning, in Figure 2 we suggest an oligomeric structure of the multiplets, a methylphosphonium doublDeOt Ia: 1n0d.10a3n9/eDt0hDyTl0g2r6o0u1Hp
complex 8 in solution. of triethylphosphite (see Supporting Information Section S4.3),
A
, i.e. if the complex is oligomerized. Based on this simplifies to a single aromatic methyl peak, foViuewr AartricolemOanltinice
Figure 2. H (400 MHz) and 31P NMR (162 MHz) spectra of 8 in acetonitrile-d
1
. Lilac arrows in the structural formula indicate
3
P–CH
Ar
correlations is shown.
diagnostic NOESY correlations discussed in the text. Only one of two equivalent CH
3
3
A
It is worth mentioning that the NOESY spectrum also features signifying the formation of the C
multiple exchange peaks between indolide groups A and B, [(TSMP)CuP(OEt) ]K (9a). Furthermore, NOE spectra (see
which means that these positions exchange on the mixing Supporting Information Section S4.4) do not show a correlation
3
-symmetric structure
3
4
7
A A
timescale. This could either be due to reversible dissociation of analogous to H –H in 8 (Figures 2 and 3), which points at a
the oligomer to smaller units or to an intramolecular exchange monomeric structure of 9a in solution. This assignment is, again,
process. Furthermore, the asterisk-labeled aromatic peaks in consistent with an X-ray crystal structure determination of the
the spectrum show exchange with some of the assigned signals, 18-crown-6
adduct
9b
crystallized
from
which suggests that they may belong to a minor unassigned acetonitrile/benzene/ether (Figure 3). With the exception of
form of 8 in solution. However, the number and intensity of the 18-crown-6 peaks, solution NMR spectra of 9a and 9b are
peaks do not allow to deduce molecular connectivity.
While the degree of oligomerization in solution is unclear, our center (angle variance of 244.58 deg ) with N^Cu^N angles
NMR assignment is consistent with the X-ray structure of a varying from 92.38(8) to 95.06(7) . The Cu–P bond length is
3
identical. The complex features a distorted tethahedral CuN P
3
3
2
o
3
4
crystal grown from pyridine/hexane. Complex 8 crystallizes as a 2.1201(2) Å, as expected for Cu(I), and Cu–N distances lie
cyclic {[(TSMP)Cu]K} tetramer (Figure 3) with distorted trigonal within 2.0622(19)-2.0759(19) Å (see Supporting Information
planar CuN
4
3
centers and CuN distances ranging within Section S3), similarly to those in tetranuclear 8. The
-
1
.926(3)-2.007(3)Å (see Supporting Information Section S3). [(TSMP)CuP(OEt)
3
] anion has approximate C
3
symmetry with
2
2
1
3
TSMP ligand adopts a μ :κ :κ coordination mode, which has the P(OEt) ligand in an approximately staggered conformation
been previously observed for other Cu(I) complexes with with respect to the scorpionate [O^P^Cu^N 41.79(11)°].The
scorpionate ligands.32 The unit cell contains two independent geometry of 9b closely resembles that of the neutral
tetrameric molecules, both being located on exact, tris(pyrazolyl)methanide
analogue
{[C(3,5-
+
crystallographic inversion centers. K ions in the proximity of Me
2
pz)
3 3
]CuP(OMe) }, which features Cu–N bonds in the range
3
4b
indolide moieties show clear cation-π interactions. Their 2.047(2)-2.102(2) Å and a Cu–P bond length of 2.122(2) Å
environment is saturated by coordinated pyridine molecules. Reactions of equimolar amounts of TSMPK (1) with either FeCl
·dme adduct in pyridine give, correspondingly, bright-
.
2
2
The two independent {[(TSMP)Cu]K}
number of K -coordinated pyridines: 14 pyridine molecules for yellow or olive-brown solutions. Subsequent evaporation of
4
molecules differ in the or NiCl
2
+
the first tetramer, and 12 for the second. There are also solvent and extraction with THF yield yellow (TSMP)Fe(Py)
3
(10)
(11) complexes. All attempts to form
under the same conditions led to
significant differences between the two independent molecules and green (TSMP)Ni(Py)
in the coordination mode of K to the indolide moieties (see related complexes with CoCl
3
+
2
Supporting Information Section S3). The content of the intractable mixtures of products.
crystallographic unit cell is completed by six non-coordinated
pyridine molecules.
4
The oligomeric structure of {[(TSMP)Cu]K} (8) in solution can be
broken down upon addition of one equivalent of P(OEt)
equivalent of copper. The H NMR spectrum immediately
3
per
1
4
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DOI: 10.1039/D0DT02601H
Figure 3. Molecular structure of 8, 9b, 10, 11 and 12b according to X-ray crystal structure determination. Displacement ellipsoids are drawn at the 30% probability level. Hydrogen
atoms, some counterions and non-coordinated solvent molecules are omitted for clarity. The asymmetric unit of 8 contains two independent molecular fragments that make up
two independent molecules; only one full molecule is shown. Symmetry code: ii: 1-x, 1-y, 1-z. The asymmetric units of 10 and 11 contain two independent molecules, only one of
which is shown. Selected bond distances and angles are provided in the Supporting Information Section S3.
1
According to H NMR of 10 and 11 in pyridine-d
5
, these the TSMP nitrogen atoms are within 2.115(7)-2.148(7) Å,
compounds are paramagnetic, and the number of lines with whereas the corresponding values for pyridine nitrogens are
their integral intensity correspond to three-fold symmetric 2.163(8)-2.241(8) Å (see Supporting Information Section S3).
metallabicyclo[2.2.2]octane topology. Remarkably, in both The long metal-ligand bond lengths in 10 and 11 indicate high-
3
6
spectra, the spacing between the pyridine peaks deviates from spin electronic states. This assignment is also supported by the
the normal values by up to 0.3 ppm with α-pyridine hydrogens effective solution magnetic moments measured by Evans
being most affected. This difference is likely due to a hyperfine method in pyridine-d : 5.19 μ for Fe(II) complex 10 and 2.82 μ
5 B B
shift induced in pyridine hydrogens upon labile coordination to for Ni(II) complex 11, whereas the spin-only expectation values
the metal centers. X-ray structure determination of crystals for S=2 and S=1 metal centers, respectively, are 4.90 and
grown from pyridine/hexane reveals isostructural octahedral 2.82 μ
complexes that feature one TSMP and three pyridine ligands (10 Interestingly, replacement of three pyridine ligands in 10 with
and 11, respectively, in Figure 3). Importantly, the FeN core in another equivalent of TSMP gives a highly air-sensitive bright-
0 has much shorter bonds with TSMP nitrogens than pyridine yellow tetracoordinate complex [(TSMP) Fe]K , 12a (Scheme 2).
nitrogens, viz. 2.158(6)-2.201(6) Å vs. 2.261(7)-2.351(6) Å (see The complex can be crystallized from
Supporting Information Section S3). In fact, the latter are even acetonitrile/toluene/ether as the tetrakis(benzo-15-crown-5)
B
.
6
1
2
2
2
+
longer on average than Fe-N distances in the [FePy
.22(3)-2.29(3) Å.35 This is consistent with rather weak bonding solution are identical with the exception of benzo-15-crown-5
and the observed lability of pyridine ligands in solution. The peaks. Compound 12b in the crystal features a pseudo-
6
] solvate: adduct 12b (Figure 3). The NMR spectra of 12a and 12b in
2
NiN
6
core in 11 shows a similar situation: the Ni–N distances for tetrahedral FeN
4
core where each TSMP ligand coordinates with
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two arms while the third remains uncoordinated (κ mode). The
Acknowledgements
This project has received funding from the NoNoMeCat Marie
Skłodowska-Curie training network funded by the European
Union under the Horizon2020 Program (675020-MSCA-ITN-
View Article Online
DOI: 10.1039/D0DT02601H
N^Fe^N angles vary from 96.35(13)o to 119.68(13)o (see
Supporting Information Section S3). The angle variance of
11.29 deg is, consequently, rather large. The distortion is
mainly caused by the chelate effect: the dihedral angle between
the N11-Fe1-N21 and the N12-Fe1-N22 planes is 85.2(2) and
deviates only slightly from perfect 90 . The Fe–N distances of
.017(3)-2.028(3) Å indicate a high-spin electronic state of the
Fe(II) center. A comparison with structurally related high-spin
complexes shows similar bonding distances of >2.0Å. Solution
effective magnetic moment measurement by Evans method in
, which is close to the spin-only
for an S=2 metal center. The
tetrahedral geometry of 12a contrasts with the common
3
3
2
1
2
015-ETN). Support with NMR spectroscopic analysis by Dr.
o
J.T.B.H. Jastrzebski is gratefully acknowledged. The X-ray
diffractometer has been financed by the Netherlands
Organization for Scientific Research (NWO).
o
2
3
7
Notes and references
pyridine-d
5
gives 5.17 μ
B
1
The following review and references therein: S. Trofimenko,
Chem. Rev., 1993, 93, 943–980.
expectation value of 4.90 μ
B
2
(a) S. Trofimenko, Scorpionates; Imperial College Press:
London, 1999; pp 1–25. (b) C. Pettinari and C. Santini,
Comprehensive Coordination Chemistry II: From Biology to
Nanotechnology. Volume 1: Fundamentals: Ligands,
Complexes, Synthesis, Purification, and Structure; Lever, A. B.
P., McCleverty, J. A., Meyer, T. J., Eds.; Elsevier Pergamon:
Amsterdam; Boston, 2004; pp 159–210. (с) D. M. Tellers, S. J.
Skoog, R. G. Bergman, T. B. Gunnoe and W. D. Harman,
Organometallics, 2000, 19, 2428–2432.
octahedral geometry of neutral and dicationic pyrazolate-based
3
8
bis(scorpionate) Fe(II) complexes and of the tris-pyridine
complex 11. We speculate that the reduction in coordination
number is due to electrostatic repulsion of the negatively-
charged indolides as well as their weaker π-accepting properties
as compared to pyridine or pyrazolate ligands.
3
4
C. Janiak, Coord. Chem. Rev. 1997, 163, 107–216.
The following reviews and references therein: (a) C. Santini,
M. Pellei, G. G. Lobbia and G. Papini, Mini-Rev. Org. Chem,
2010, 7, 84–124. (b) M. Pellei and C. Santini, Biomimetic Based
Applications; George, A., Ed.; InTech, 2011.
Conclusions
A
dianionic
TSMP) ligand platform can be synthesized in the form of
dipotassium salt TSMPK (1). This system is the first dianionic
3
C -symmetric tris-skatylmethylphosphonium
(
5
(a) The following review and references therein: C. Slugovc, R.
Schmid and K. Kirchner, Coord. Chem. Rev., 1999, 185–186,
2
homoscorpionate capable of metal exchange and fills a gap in
the assortment of charged scorpionates. Despite a possibility of
recombination between negatively charged indolides and a
positively charged phosphonium atom, salt 1 is stable both in
the solid state and solution, which is likely due to a combination
of electronic and steric factors.
The potassium cations in TSMPK (1) are exchangeable for other
2
metals, demonstrating the versatility of TSMP as a ligand for
transition metals. The expected scorpionate κ binding mode is
observed in octahedral, high spin complexes (TSMP)M(Py)
1
09–126. (b) M. M., Díaz-Requejo; T. R. Belderraín, S.
Trofimenko and P. J. Pérez, J. Am. Chem. Soc., 2001, 123,
3167–3168. (c) M. M. Díaz-Requejo, T. R. Belderraín and P. J.
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the tetrameric complex {[(TSMP)Cu]K}
2
7
8
2
-
2
complex [(TSMP) Fe] , which displays a high-spin ground state.
The rich coordination chemistry of the dianionic
homoscorpionate ligand invites further investigations. Amongst
other, it has a potential to electronically stabilize high-valent
metal states due to its electron-rich character, but additional
derivatization might be required for kinetic stabilization. Studies
in these directions are currently ongoing in our laboratories.
Conflicts of interest
There are no conflicts to declare.
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Dalton Transactions
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View Article Online
DOI: 10.1039/D0DT02601H
Investigations into the synthesis and coordination chemistry of a tridentate C3-symmetric dianionic
scorpionate ligand