Adrian
Hauser
a,
Luca
Münzfeld
a,
Sören
Schlittenhardt
b,
Ralf
Köppe
a,
Cedric
Uhlmann
a,
Ulf-Christian
Rauska
a,
Mario
Ruben
bcd and
Peter W.
Roesky
*a
aInstitute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstraße 15, D-76131 Karlsruhe, Germany. E-mail: roesky@kit.edu
bInstitute of Nanotechnology, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany
cCentre Européen de Science Quantique (CESQ), Institut de Science et d'Ingénierie Supramoléculaires (ISIS, UMR 7006), CNRS-Université de Strasbourg, 8 allée Gaspard Monge BP 70028, 67083 Strasbourg Cedex, France
dInstitute of Quantum Materials and Technologies (IQMT), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
First published on 3rd February 2023
Synthesis of new organo-lanthanide polyphosphides with an aromatic cyclo-[P4]2− moiety and a cyclo-[P3]3− moiety is presented. For this purpose, the divalent LnII-complexes [(NON)LnII(thf)2] (Ln = Sm, Yb) ((NON)2− = 4,5-bis(2,6-diisopropylphenyl-amino)-2,7-di-tert-butyl-9,9-dimethylxanthene) and trivalent LnIII-complexes [(NON)LnIIIBH4(thf)2] (Ln = Y, Sm, Dy) were used as precursors in the reduction process of white phosphorus. While using [(NON)LnII(thf)2] as a one-electron reducing agent the formation of organo-lanthanide polyphosphides with a cyclo-[P4]2− Zintl anion was observed. For comparison, we investigated a multi-electron reduction of P4 by a one-pot reaction of [(NON)LnIIIBH4(thf)2] with elemental potassium. As products molecular polyphosphides with a cyclo-[P3]3− moiety were isolated. The same compound could also be obtained by reducing the cyclo-[P4]2− Zintl anion within the coordination sphere of SmIII in [{(NON)SmIII(thf)2}2(μ-η4:η4-P4)]. Reduction of a polyphosphide within the coordination sphere of a lanthanide complex is unprecedented. Additionally, the magnetic properties of the dinuclear DyIII-compound bearing a bridging cyclo-[P3]3− moiety were investigated.
Fig. 1 Selection of various rare earth polyphosphides with different [Px]n− Zintl anions.15,30,36 |
Recently, organo-lanthanide compounds bridged by a [Bi6]6− Zintl anion, which promoted strong ferromagnetic interactions between lanthanides, were reported.40
Herein, we compare SET reduction of white phosphorus induced by classical divalent LnII-compounds with a multi-electron reduction induced by the in situ reaction of trivalent lanthanide complexes with potassium. As a result, new anti-bimetallic complexes of organo-lanthanide polyphosphides, which were synthesized by direct conversion of white phosphorus using di- and trivalent lanthanide compounds, were obtained. For this purpose, we stabilized the complexes by using the bulky NON ligand (NON = 4,5-bis(2,6-diisopropylphenyl-amino)-2,7-di-tert-butyl-9,9-dimethylxanthene)), which was recently employed for stabilizing metal centres in low oxidation states.41–43 Within our studies, we showcase the reduction of a polyphosphide within the coordination sphere of a lanthanide complex, which so far is unprecedented.
Scheme 1 Synthesis of [(NON)LnII(thf)2] (1-Ln) and [(NON)LnIII(BH4)(thf)2] (3-Ln) as precursors for P4 activation to synthesize the anti-bimetallic cyclo-P4 (2-Ln) and cyclo-P3 complexes (4-Ln). |
Due to the smaller ionic radius of YbII in compound 1-Yb, the Yb–N and Yb–O distances are shortened as expected (Yb–N1 2.344(5) Å, Yb–N2 2.315(5) Å, Yb–O1 2.402(4) Å). This effect is also reflected in the N1–Ln–N2 angles, being 122.8° for 1-Sm and widened for 1-Yb to 132.0°. The NMR spectra of the compound 1-Yb (Fig S1 and S2, ESI†) in C6D6 at room temperature indicate that no coordinated THF remains on the molecule after drying the material in vacuo for several hours. All resonances can be assigned to the xanthene backbone and Dipp-groups (Dipp = diisopropylphenyl) of the ligand, respectively. Due to the paramagnetic behaviour of 1-Sm, no meaningful NMR spectra could be obtained. Elemental analysis of 1-Sm indicates that one THF molecule remains coordinated to the SmII-cation after drying the compound in vacuo.
After the successful isolation of compounds 1-Ln we investigated the reductive conversion of white phosphorus via SET with these compounds (Scheme 1, top). For this purpose, the divalent complexes 1-Ln were stirred together with P4 in toluene at room temperature. Thereby deep red solutions were obtained. After filtration of insoluble residues the anti-bimetallic complexes [{(NON)LnIII(thf)2}2(μ-η4:η4-P4)] 2-Ln could be isolated as dark red (2-Sm) and dark orange (2-Yb) crystalline compounds. It should be mentioned here that the crystallization of 2-Sm was carried out from a saturated toluene solution at −10 °C, whereas the solvent had to be changed to THF for crystallization of 2-Yb. This supports the assumption that the coordinating THF remains in 1-Sm after drying in vacuo, which is required for the crystallization of 2-Sm, while no coordinating THF molecules persist in 1-Yb. The isostructural compounds crystallize in the orthorhombic space group Pban. Similar to the previously reported [{(DippForm)2SmIII}2(μ-η4:η4-P4)] (DippForm = {(2,6-iPr2C6H3)NC(H) = N(2,6-iPr2C6H3)}−) the cyclo-[P4]2− Zintl anion coordinates in a bridging η4:η4 mode between two LnIII centres and exhibits an almost perfect square shape with internal angles of 88.62(9)° and 91.38(9)° for 2-Sm and 88.73(8)° and 91.27(8)° for 2-Yb, respectively (Fig. 3). Thereby the P–P distance of 2.15 Å in 2-Ln as well as the Sm–P distances of 3.0727(8) Å and 3.0908(10) Å are in the range of reported aromatic [P4]2− Zintl anions in the coordination sphere of two SmIII-moieties.30 Corresponding Yb–P distances are slightly shorter (Yb–P1 3.0192(8) Å, Yb–P2 3.0021(8) Å) and in the range of the reported values.32 Compared to the divalent precursors 1-Ln, the Ln–N distances (Sm–N 2.360(4) Å, Yb–N 2.290(3) Å) are moderately shorter and decrease with the ionic radius of the LnIII cation as well. To illustrate the staggered arrangement of the xanthene backbones and THF ligands in 2-Ln, the top view of 2-Yb is displayed on the right side of Fig. 3. For both compounds the torsion angle of the xanthene backbones is around 53° (see Fig. S27 and S29, ESI†). Compound 2-Sm is only the second example for P4 activation which led to the formation of a 6-π-aromatic [P4]2− ring in the coordination sphere of two SmIII cations and was entirely unknown for YbIII compounds so far. NMR spectra of 2-Ln were recorded in C6D6 at room temperature. Due to the paramagnetic behaviour of complexes 2-Ln no meaningful 1H and 13C NMR spectra were obtained. In the 31P{1H} NMR spectra, however, one singlet was found in each case. The singlet at δ = 479.5 ppm for 2-Sm could be assigned to the four chemically equivalent phosphorus atoms of the cyclo-P4 moiety. Thus, the chemical shift is in a similar range to that of the previously reported [{(DippForm)2Sm}2(μ-η4:η4-P4)].30
For compound 2-Yb, the 31P{1H} NMR signal of the four phosphorus atoms is slightly shifted to higher resonances and can be detected as a singlet at δ = 382.4 ppm.
Motivated by these results, we aimed to synthesize further LnIII polyphosphides beyond Sm and Yb in order to investigate different reducing processes and to have access to a broader range of rare-earth elements for studying their physical properties. In particular, we focused on the synthesis of a dinuclear polyphosphide-bridged DyIII compound, which could potentially exhibit SMM behaviour. Although [DyI2(dme)3] is known,48 it has proven too reactive for being a suitable precursor in organometallic chemistry. Therefore, a synthetic route starting from a robust and isolable LnIII species was chosen, which subsequently should be reduced in the presence of P4 to form new molecular LnIII polyphosphides. The first step was achieved by a salt elimination reaction between [K2(NON)] and the respective [LnIII(BH4)3(thf)3] (Ln = Y, Sm, Dy) (Scheme 1, bottom). The corresponding compounds [(NON)LnIIIBH4(thf)2] 3-Ln could be isolated by recrystallization of crude materials from a hot n-heptane solution as colourless (3-Y), red (3-Sm) and light green (3-Dy) crystals in high yields. The isostructural compounds crystallize in the monoclinic space group P21/c with one molecule in the asymmetric unit.
The molecular structures in the solid state of [(NON)LnIIIBH4(thf)2] 3-Ln are shown in Fig. 4. As expected, the central LnIII cation is coordinated by two amido functions and the oxygen atom of the NON ligand. Additionally, two molecules THF saturate the coordination sphere of the metal centre. Solely the coordination mode of the [BH4]− moiety, which was determined by free refinement of the hydrogen atoms, differs in 3-Ln. While the [BH4]− ligand in 3-Y and 3-Dy coordinates in an η2-H2BH2 mode, an η3-H3BH coordination mode is observed for 3-Sm. The dependence of the coordination mode of borohydrides on the ionic radii and steric demand of the applied ligands is well known in lanthanide chemistry.49,50 In the case of 3-Ln, the hapticity of [BH4]− decreases with the ionic radius of the LnIII cation with a constant steric demand of the ligand. The respective Ln–B distances are between 2.671(3) Å in 3-Sm and 2.717(3) Å in 3-Y. In the FT-IR spectra of compounds 3-Ln, different absorption patterns between 2400 cm−1 and 2100 cm−1 are observed for the different coordination modes of the [BH4]− moiety.
In the 1H{11B} NMR spectrum of 3-Y the singlet at δ = 1.13 ppm can be assigned to the four hydridic hydrogens of the [BH4]− moiety whereas no resonances for the hydrogen atoms of the borohydride can be found in the 1H NMR spectrum. All other resonances can be assigned to the NON ligand and one remaining coordinated THF molecule. The 11B NMR spectra show one broad signal at δ = −24.2 ppm for 3-Y and at δ = −38.5 ppm for 3-Sm, while remaining silent for 3-Dy due to the strong paramagnetic behaviour.
To reach the aim for new Ln polyphosphides beyond Sm and Yb, we applied the trivalent compounds 3-Ln in one-pot reactions with an excess of potassium and P4 at elevated temperatures in toluene (Scheme 1, bottom). After filtration, [K{(NON)Ln(thf)}2(μ3-η3:η3:η2-P3)] 4-Ln could be isolated as colourless (4-Y), red (4-Sm) and yellow (4-Dy) crystals at −10 °C from a saturated toluene solution. X-ray diffraction analysis revealed a bridging cyclo-[P3]3− unit in a μ-η3:η3 coordination mode as the central building block, which displays an almost perfect triangular shape with internal angles between 58.97(4)° and 62.04(5)° for P1–P2–P1′ and P1–P1′–P2, respectively (Fig. 5). The formation of 4-Ln shows that the multi-electron reduction applied here results in a different product. The P–P bond lengths within 4-Ln range from 2.160(2) Å to 2.238(2) Å, consistent with literature values of bridging cyclo-[P3]3− polyphosphides.34,36 Here, the P1–P1′ bond is always slightly elongated compared to the P1–P2 bond due to the coordination of P1 to the potassium ion (K–P1 ∼ 3.18 Å). The Ln–CtP3 bond distances (CtP3 = centroid of the cyclo-[P3] moiety) in 4-Ln decrease with the radius of the respective rare earth ion. This in turn affects the K–Ctphenyl distances, which thus also follow this trend (4-Sm: K–Ctphenyl 2.8112(6) Å to 4-Dy: K–Ctphenyl 2.7727(4) Å). The coordination of the potassium ion by the two phenyl rings of the Dipp-moieties further leads to the folding of the originally planar xanthene backbone along the Ln–O1–C7 axis by about 35° (see Fig. S34, S36 and S38, ESI†). Using the specific example of 4-Sm, the Sm–CtP3 distance of 2.6030(7) Å can be directly compared with the Sm–CtP4 distance 2-Sm. The latter is slightly longer with 2.6787(7) Å, which is due to the higher charge density in the cyclo-[P3]3− compared to the cyclo-[P4]2− polyanion in an otherwise similar chemical environment. The resulting stronger electrostatic interactions lead to a shorter Sm–CtP3 distance.
The 31P{1H} NMR spectra of compounds 4-Dy and 4-Sm show one singlet each for the cyclo-[P3]3− moiety at δ = −245.5 ppm (4-Dy) and δ = −336.7 ppm (4-Sm) (Fig. 6). The resonance of the cyclo-[P3]3− compound 4-Sm is thus considerably shifted to higher field frequencies compared to the corresponding cyclo-[P4]2− compound 2-Sm. For compound 4-Y a triplet is seen in the 31P{1H} NMR spectrum at δ = −240.3 ppm with a coupling constant of 1JYP = 20.2 Hz (Fig. 6), which is slightly higher than that of the reported complex K[{(L)Y(thf)}2(μ3-η3:η3:η2-P3)] (1JYP = 16.3 Hz).36
To get further insights in the formation of the [P3]3− Zintl anion in 4-Ln, we first investigated the reduction of 2-Sm with potassium in the NMR scale (Scheme 1, right). After a few hours at 65 °C, the 31P NMR resonance at 479.5 ppm in C6D6, which is assigned to the [P4]2− unit of 2-Sm (ESI, Fig. S3†), vanished and a new resonance at −320.8 ppm appeared. After removing C6D6in vacuo and resolving the residue in thf-d8, the resonance was detected at −336.7 ppm, which is consistent with the chemical shift of the isolated compound 4-Sm from the one pot reaction of 3-Sm with potassium and P4 (Fig. 6). This was further confirmed by the reaction of 2-Sm with potassium in a preparative scale with subsequent crystallisation of 4-Sm. In the 31P{1H} NMR spectrum no further resonances, which could be assigned to possible by-products, were detected. Hence, the reactivity of 2-Sm with potassium clearly shows that the [P4]2− can be reduced to a [P3]3− Zintl anion in the coordination sphere of two SmIII cations. Therefore, we anticipate that during the formation of 4-Ln, P4 is reduced to a [P4]2− moiety first and subsequently reduced to a [P3]3− Zintl anion. However, isolation of the [P4]2− species from the one-pot reaction of compounds 3-Ln was not possible even with variation of stoichiometry and temperature.
Our first attempt at calculating the molecular properties was performed following a common approach, replacing one DyIII with a diamagnetic YIII and keeping the molecular structure as it is obtained from XRD methods. For 4-Dy, however, the CASSCF procedure did not converge and we therefore opted to carry out the calculations based on a fictional Dy–Y compound (4-Dy*) in which we replaced the peripheral methyl and tert-butyl groups on the NON ligands with hydrogen atoms, as well as the iso-propyl groups on the DIPP groups with methyl (Fig. 7, left). The results of the single ion calculation showed a strongly axial ground state characterised by gx = 0.01, gy = 0.02 and gz = 19.64 with the wave function being mainly composed of mJ = 15/2, but showing some admixing of mJ = 11/2 (Table S12, ESI†). This mixing suggests that quantum tunnelling within the ground state is possible, which is also confirmed by the relatively high transition probability of the ground state (Fig. 7, middle). The first excited Kramers doublet is found at 151.0 cm−1 (217.25 K) with g-values gx = 0.36, gy = 0.67 and gz = 15.76. The main magnetic axis is only tilted 1.79° in respect to that of the ground state (Fig. 7, right), which can be an indicator that magnetic relaxation might occur via higher excited states. However, the state's wave function shows strong admixing (19%) of mJ = 9/2, which lets us assume that relaxation will mainly occur through this state (Table S13, ESI†).
The static magnetic behaviour of 4-Dy was tested on a sample in a flame sealed NMR tube upon cooling from 300 K to 2 K under an applied field of 0.1 T and through measurements of the molar magnetisation up to 7 T at temperatures 2, 3, 4 and 5 K. The behaviour of the susceptibility product χmolT against temperature shows a slow decrease followed by a strong drop around 10 K (Fig. 8), suggesting antiferromagnetic coupling of the two DyIII ions. Note that the absolute values of χmolT and the magnetisation given in Fig. 8 have been scaled up in order to allow simulations, vide infra. The observed room temperature value of χmolT was 20.16 cm3 K mol−1, which is only about 71% of the expected value for two uncoupled DyIII ions of 28.34 cm3 K mol−1. We believe that this error is introduced through sample preparation, as we have experienced it before and the handling of very small amounts of sample inside a glove box can lead to big percentage errors very quickly.64 However, in further discussion of dynamic behaviour and simulation of the data, introducing a correction factor as we did does not influence the results. Employing the crystal field parameters obtained from the single ion CASSCF calculation (Table S13†) rotated onto a common reference frame, we performed a simultaneous fit of χmolT(T) and M(H) using a single isotropic exchange. An almost perfect fit for χmolT(T) and good agreement around lower fields for M(H) was obtained with Jiso = −2.96 × 10−2 cm−1 (employing a -2J formalism), confirming our initial assumption of antiferromagnetic interactions.
Fig. 8 (Top left) Temperature product of the molar susceptibility vs. temperature and molar magnetisation vs. field of 4-Dy, solid lines are best fits obtained with PHI, experimental data points are scaled to allow for simulation.63 (Bottom left) Arrhenius plot; (top and bottom right) in-phase and out-of-phase susceptibilities vs. frequency of 4-Dy, solid lines are the results of a simultaneous fit to a generalised Debye model. |
The dynamic behaviour of 4-Dy was tested at low temperatures under application of an oscillating magnetic field (3.5 Oe). A peak in the frequency-dependent out-of-phase susceptibility was observed around 200 Hz at 2 K (Fig. 8, right). Upon the application of additional DC fields, the signal vanished quickly. Simultaneous fits of the in-phase and out-of-phase susceptibilities to a generalised Debye model (all fit parameters can be found in Table S14, ESI†) and subsequent Arrhenius analysis of the relaxation times gave an effective energy barrier Ueff = 36.8 cm−1 and relaxation times τ0 = 5.80 × 10−6 s, τQTM = 1.01 × 10−3 s. Note that we performed the Arrhenius fit employing eqn (1), leaving out the commonly employed CTn Raman-term, as we already obtained a very good fit employing only Orbach and QTM processes.
τ−1 = τ0−1exp(−Ueff/kBT) + τQTM−1 | (1) |
We estimated the blocking temperature TB as the temperature at which τ = 100 s and found TB = 3.17 K. This is in good agreement with hysteresis loops that we recorded, in which the hysteresis is slightly open at 2 K but essentially closed already at 3 K (Fig. S42, ESI†). The experimental energy barrier of 36.8 cm−1 seems extremely low, given the expected relaxation pathway through the first excited state at 151 cm−1. However, it has been shown before that single ion CASSCF calculations often fall short when predicting the relaxation barrier of polynuclear compounds. Dey and Rajaraman have proposed an empirical model to estimate the barrier height of Dy2 SMMs (2):65
(2) |
The most impressive relaxation barriers to this day have been achieved in DyIII complexes bearing aromatic Cp derived ligands due to their axial ligand fields being beneficial in stabilising the magnetic moment.66–68 The [P4]2− and [P3]3− Zintl ions might give similar results in the future when being paired with other ligands. We believe the rather poor SMM behavior is a consequence of the NON-ligand producing an unfavorable ligand field for DyIII as well as causing misalignment of the easy axes. As this is the first report of a polyphosphide containing SMM we are excited to see what other possibilities for SMMs these structural motifs will bring in the future.
Footnote |
† Electronic supplementary information (ESI) available: Full experimental procedures, spectra, and analytical data. CCDC 2224069–2224077 and 2235392. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2sc06730g |
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