A dianionic C₃-symmetric scorpionate: synthesis and coordination chemistry

Abstract

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.

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Introduction

The term “scorpionate” was introduced by Trofimenko¹ 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 C₃-symmetric homoscorpionate complex forms; otherwise, coordination results into a heteroscorpionate complex.

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.^1,2 It is important to point out, however, that while they are isoelectronic, they are not isolobal,³ therefore the extent of such a comparison is limited. Additionally, in some situations, scorpionates are capable of displaying a κ² coordination mode freeing the third arm for binding to another metal center,¹ which sets them apart from Cp ligands.

The first generation of homoscorpionates involved trispyrazolylborates¹ 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 prepared enabling both electronic and steric properties of a coordinated metal to be fine-tuned. Such systems have found applications in biomimetics,⁴ catalysis,⁵ material science⁶ and production of radiopharmaceuticals.⁷ Following this success, the definition of scorpionates has been extended to tripodal tridentate systems with other donor groups and bridging atoms. Among the employed donors are imidazole,⁸ pyridine,⁹ triazole,¹⁰ indole,¹¹ methimazole,¹² oxazoline,¹³ N-heterocyclic carbenes¹⁴ and others, and even acyclic donor groups.¹⁵ Variation of the bridging atom allowed to further tune electronics and charge of the scorpionate ligands, thus influencing coordination behavior and solubility of their complexes. Reported systems include CH/COH,⁹ CH₃/C₆H₅-Si,¹⁶ N,^9a P,^8,9,17 [R–P]⁺,¹⁸ P=O,^9b,19 As,^9a As=O,^9b [CH₃-Al]⁻,²⁰ [CH₃-Ga]⁻,²¹ C⁻,²² Si-,²³ Ge⁻, Sn⁻,²⁴ Pb⁻.²⁰

A more general way to classify the existing C₃-symmetric scorpionates is by formal charge, which is a combined property of the donor moieties and the bridging atom. Thus, one can differentiate neutral, mono-, di- and trianionic homoscorpionates (Chart 1). Whereas there is a plethora of known neutral^8,9,14,16,17 and monoanionic^{1,10,12,13,20–22,24} homoscorpionates as well as some trianionic examples,¹¹ dianionic ligands are rare with only one reported precedent¹⁸ (A in Chart 2). The latter was isolated as a LAlCl complex, and its 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.

Chart 1 C₃-Symmetric scorpionates with distinct formal charges.

Chart 2 Dianionic C₃-symmetric scorpionates.

Herein, we present the synthesis of a C₃-symmetric dianionic 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.

Results and discussion

Ligand synthesis

The dipotassium salt of tris-(2-skatyl)methylphosphonium (1), further abbreviated as TSMPK₂, was synthesized as depicted in Scheme 1. In the first step, skatole (2) was treated with di-tert-butyl dicarbonate (Boc₂O) in the presence of 4-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 salt was quenched with a third of an equivalent of PCl₃ 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-skatyl)methylphosphonium iodide (5) exists in DCM-d₂ solution as a ca. 1.00 : 0.15 mixture of interexchanging tris-exo and bis-exo-mono-endo rotamers (see ESI section S4.1†).²⁵

Scheme 1 Synthesis of TSMPK₂ salt 1. Unless otherwise stated, parentheses underneath reaction arrows indicate isolated yields.

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 TSMPK₂ salt (1). A highly pure material (>99% according to NMR) was isolated by crystallization from acetonitrile/diethyl ether. X-ray crystal structure determination confirmed the expected molecular structure with a K : P ratio of 2 : 1 (Fig. 1).

Fig. 1 Asymmetric unit of the TSMPK₂ (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.

The K⁺ cations are involved in an extensive network of cation–N and cation–π interactions (η¹ to η⁶). 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 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 ESI section S3†).

The existence of a free alkaline trianionic-monocationic phosphonium salt is, to our knowledge, unprecedented. Quaternary phosphonium salts are generally incompatible with basic counterions and convert either into ylides,²⁶ phosphoranes^27,28 or form charge-transfer complexes followed by complex decomposition manifolds.²⁹ The exceptions, however, feature electronically stabilized bulky anions (e.g. diphenylamide³⁰ and 2,4,6-trimethylphenolate³¹). We speculate that the stability of 1 is due to the above reasons as well: the anionic indolide nitrogen atoms lose a lot of their nucleophilicity due to being involved in an aromatic π-system, and the phosphonium center is too encumbered to accommodate a fourth indolide in its vicinity. Additionally, the latter would experience electrostatic repulsion from other anionic indolides already present in a molecule.

Coordination chemistry of TSMP

In order to study the 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).

Scheme 2 Coordination behavior of the TSMP ligand platform.

Reacting equimolar amounts of TSMPK₂ salt 1 with cuprous chloride in pyridine led to an orange-red solution, from which yellow crystalline compound 8 was isolated upon freeing from solvent in vacuo and extraction with THF. NMR spectroscopy in acetonitrile-d₃ reveals one predominant species, albeit an oligomer. To elaborate, the ³¹P NMR spectrum (Fig. 2) shows a single quartet consistent with ²J_P,H coupling with a methyl group. In the ¹H NMR spectrum (Fig. 2), there are two singlets in a 2 : 1 ratio corresponding to aromatic methyl groups CH₃^Ar_A and CH₃^Ar_B. Moreover, multiplets in the aromatic region are present in the same ratio and, according to magnitude gCOSY (see ESI section S4.2†), constitute two distinct spin systems. This implies 2 : 1 inequivalence of indolide units in the complex. A NOESY spectrum (see ESI section S4.2†) shows a correlation between the phosphonium methyl group CH₃^P and two aromatic CH₃^Ar_A (mapped out in Fig. 2) but not CH₃^Ar_B, from which it follows that CH₃^Ar_B is pointing away from the phosphonium methyl group. Lastly, H⁴_A and H⁷_A have comparable correlation intensities with CH₃^Ar_A. Since NOE rapidly decays with distance, and H⁷_A is clearly further away from CH₃^Ar_A than H⁴_A, it can only be the case if there is an equivalent methyl group in the vicinity of H⁷_A, i.e. if the complex is oligomerized. Based on this reasoning, in Fig. 2 we suggest an oligomeric structure of the complex 8 in solution.

Fig. 2 ¹H (400 MHz) and ³¹P NMR (162 MHz) spectra of 8 in acetonitrile-d₃. Lilac arrows in the structural formula indicate diagnostic NOESY correlations discussed in the text. Only one of two equivalent CH₃^P–CH₃^Ar_A correlations is shown.

It is worth mentioning that the NOESY spectrum also features multiple exchange peaks between indolide groups A and B, which means that these positions exchange on the mixing timescale. This could either be due to reversible dissociation of the oligomer to smaller units or to an intramolecular exchange process. Furthermore, the asterisk-labeled aromatic peaks in the spectrum show exchange with some of the assigned signals, which suggests that they may belong to a minor unassigned form of 8 in solution. However, the number and intensity of the peaks do not allow to deduce molecular connectivity.

While the degree of oligomerization in solution is unclear, our NMR assignment is consistent with the X-ray structure of a crystal grown from pyridine/hexane. Complex 8 crystallizes as a cyclic {[(TSMP)Cu]K}₄ tetramer (Fig. 3) with distorted trigonal planar CuN₃ centers and Cu–N distances ranging within 1.926(3)–2.007(3) Å (see ESI section S3†). The TSMP ligand adopts a μ²:κ²:κ¹ coordination mode, which has been previously observed for other Cu(I) complexes with scorpionate ligands.³² The unit cell contains two independent tetrameric molecules, both being located on exact, crystallographic inversion centers. K⁺ ions in the proximity of indolide moieties show clear cation–π interactions. Their environment is saturated by coordinated pyridine molecules. The two independent {[(TSMP)Cu]K}₄ molecules differ in the number of K⁺-coordinated pyridines: 14 pyridine molecules for the first tetramer, and 12 for the second. There are also significant differences between the two independent molecules in the coordination mode of K⁺ to the indolide moieties (see ESI section S3†). The content of the crystallographic unit cell is completed by six non-coordinated pyridine molecules.

Fig. 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 ESI section S3.†

The oligomeric structure of {[(TSMP)Cu]K}₄ (8) in solution can be broken down upon addition of one equivalent of P(OEt)₃ per equivalent of copper. The ¹H NMR spectrum immediately simplifies to a single aromatic methyl peak, four aromatic multiplets, a methylphosphonium doublet and an ethyl group of triethylphosphite (see ESI section S4.3†), signifying the formation of the C₃-symmetric structure [(TSMP)CuP(OEt)₃]K (9a). Furthermore, NOE spectra (see ESI section S4.4†) do not show a correlation analogous to H⁴_A–H⁷_A in 8 (Fig. 2), which points at a monomeric structure of 9a in solution. This assignment is, again, consistent with an X-ray crystal structure determination of the 18-crown-6 adduct 9b crystallized from acetonitrile/benzene/ether (Fig. 3). With the exception of 18-crown-6 peaks, solution NMR spectra of 9a and 9b are identical. The complex features a distorted tethahedral CuN₃P center (angle variance³³ of 244.58 deg²) with N^Cu^N angles varying from 92.38(8) to 95.06(7)°. The Cu–P bond length is 2.1201(2) Å, as expected for Cu(I),³⁴ and Cu–N distances lie within 2.0622(19)–2.0759(19) Å (see ESI section S3†), similarly to those in tetranuclear 8. The [(TSMP)CuP(OEt)₃]⁻ anion has approximate C₃ symmetry with the P(OEt)₃ ligand in an approximately staggered conformation with respect to the scorpionate [O^P^Cu^N 41.79(11)°]. The geometry of 9b closely resembles that of the neutral tris(pyrazolyl)methanide analogue {[C(3,5-Me₂pz)₃]CuP(OMe)₃}, which features Cu–N bonds in the range 2.047(2)–2.102(2) Å and a Cu–P bond length of 2.122(2) Å_.^34b

Reactions of equimolar amounts of TSMPK₂ (1) with either FeCl₂ or NiCl₂·dme adduct in pyridine give, correspondingly, bright-yellow or olive-brown solutions. Subsequent evaporation of solvent and extraction with THF yield yellow (TSMP)Fe(Py)₃ (10) and green (TSMP)Ni(Py)₃ (11) complexes. All attempts to form related complexes with CoCl₂ under the same conditions led to intractable mixtures of products.

According to ¹H NMR of 10 and 11 in pyridine-d₅, these compounds are paramagnetic, and the number of lines with their integral intensity correspond to three-fold symmetric metallabicyclo[2.2.2]octane topology. Remarkably, in both spectra, the spacing between the pyridine peaks deviates from the normal values by up to 0.3 ppm with α-pyridine hydrogens being most affected. This difference is likely due to a hyperfine shift induced in pyridine hydrogens upon labile coordination to the metal centers. X-ray structure determination of crystals grown from pyridine/hexane reveals isostructural octahedral complexes that feature one TSMP and three pyridine ligands (10 and 11, respectively, in Fig. 3). Importantly, the FeN₆ core in 10 has much shorter bonds with TSMP nitrogens than pyridine nitrogens, viz. 2.158(6)–2.201(6) Å vs. 2.261(7)–2.351(6) Å (see ESI section S3†). In fact, the latter are even longer on average than Fe–N distances in the [FePy₆]²⁺ solvate: 2.22(3)–2.29(3) Å.³⁵ This is consistent with rather weak bonding and the observed lability of pyridine ligands in solution. The NiN₆ core in 11 shows a similar situation: the Ni–N distances for the TSMP nitrogen atoms are within 2.115(7)–2.148(7) Å, whereas the corresponding values for pyridine nitrogens are 2.163(8)–2.241(8) Å (see ESI section S3†). The long metal–ligand bond lengths in 10 and 11 indicate high-spin electronic states.³⁶ This assignment is also supported by the effective solution magnetic moments measured by Evans method in pyridine-d₅: 5.19μ_B for Fe(II) complex 10 and 2.82μ_B for Ni(II) complex 11, whereas the spin-only expectation values for S = 2 and S = 1 metal centers, respectively, are 4.90 and 2.82μ_B.

Interestingly, replacement of three pyridine ligands in 10 with another equivalent of TSMP gives a highly air-sensitive bright-yellow tetracoordinate complex [(TSMP)₂Fe]K₂, 12a (Scheme 2). The complex can be crystallized from acetonitrile/toluene/ether as the tetrakis(benzo-15-crown-5) adduct 12b (Fig. 3). The NMR spectra of 12a and 12b in solution are identical with the exception of benzo-15-crown-5 peaks. Compound 12b in the crystal features a pseudo-tetrahedral FeN₄ core where each TSMP ligand coordinates with two arms while the third remains uncoordinated (κ² mode). The N^Fe^N angles vary from 96.35(13)° to 119.68(13)° (see ESI section S3†). The angle variance³³ of 111.29 deg² 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 2.017(3)–2.028(3) Å indicate a high-spin electronic state of the Fe(II) center. A comparison with structurally related high-spin complexes³⁷ shows similar bonding distances of >2.0 Å. Solution effective magnetic moment measurement by Evans method in pyridine-d₅ gives 5.17μ_B, which is close to the spin-only expectation value of 4.90μ_B for an S = 2 metal center. The tetrahedral geometry of 12a contrasts with the common octahedral geometry of neutral and dicationic pyrazolate-based bis(scorpionate) Fe(II) complexes³⁸ and of the tris-pyridine complex 10. 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.

Conclusions

A dianionic C₃-symmetric tris-skatylmethylphosphonium (TSMP) ligand platform can be synthesized in the form of dipotassium salt TSMPK₂ (1). This system is the first dianionic 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 TSMPK₂ (1) are exchangeable for other metals, demonstrating the versatility of TSMP as a ligand for transition metals. The expected scorpionate κ³ binding mode is observed in octahedral, high spin complexes (TSMP)M(Py)₃ (10: M = Fe; 11: M = Ni) as well as in the tetrahedral complex [(TSMP)CuP(OEt)₃]⁻ (9). In addition, the bridging μ²:κ²:κ¹ mode is preferred with Cu(I) in the absence of a co-ligand, affording the tetrameric complex {[(TSMP)Cu]K}₄ (8). Finally, the bidentate κ² mode is observed in the tetracoordinate 2 : 1 Fe(II) complex [(TSMP)₂Fe]²⁻, 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.

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-2015-ETN). Support with NMR spectroscopic analysis by Dr J. T. B. H. Jastrzebski is gratefully acknowledged. The X-ray diffractometer has been financed by the Netherlands Organization for Scientific Research (NWO).

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Footnote

† Electronic supplementary information (ESI) available: Synthetic and characterization data for all new compounds, additional figures. CCDC 2010902–2010907. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0dt02601h

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