Influencing the properties of dysprosium single-molecule magnets with phosphorus donor ligands.

Single-molecule magnets are a type of coordination compound that can retain magnetic information at low temperatures. Single-molecule magnets based on lanthanides have accounted for many important advances, including systems with very large energy barriers to reversal of the magnetization, and a di-terbium complex that displays magnetic hysteresis up to 14 K and shows strong coercivity. Ligand design is crucial for the development of new single-molecule magnets: organometallic chemistry presents possibilities for using unconventional ligands, particularly those with soft donor groups. Here we report dysprosium single-molecule magnets with neutral and anionic phosphorus donor ligands, and show that their properties change dramatically when varying the ligand from phosphine to phosphide to phosphinidene. A phosphide-ligated, trimetallic dysprosium single-molecule magnet relaxes via the second-excited Kramers' doublet, and, when doped into a diamagnetic matrix at the single-ion level, produces a large energy barrier of 256 cm(-1) and magnetic hysteresis up to 4.4 K.

The observation of slowly relaxing magnetization in pan class="Chemical">lanthanide class="Chemical">pan class="Chemical">bis(phthalocyanine) complexes, LnPc2, has made tremendous impact in molecular magnetism12. Subsequently, hundreds of lanthanide single-molecule magnets (Ln-SMMs) have been reported345, with many examples showing very large effective energy barriers to reversal of the magnetization (Ueff)67. Some Ln-SMMs have also been developed for applications in nanoscale devices by, for example, deposition of TbPc2 onto carbon nanostructures or metallic surfaces89101112. The LnPc2 SMMs demonstrated that very large Ueff values can occur in monometallic complexes, and this important observation inspired the development of other monometallic Ln-SMMs with ligands such as polyoxometallates13 and organometallic ligands such as cyclo-octatetraene141516. Organometallic synthesis offers an alternative strategy for designing Ln-SMMs owing to the diverse range of hard and soft ligands that can be accessed, and such ligands introduce new possibilities for influencing and enhancing the SMM properties1718.

pan class="Chemical">Phosphorus class="Chemical">pan class="Species">donor ligands provide opportunities for systematically modifying the properties of SMMs because their chemistry can be developed with neutral phosphines (R3P), mono-anionic phosphide (R2P−) and di-anionic phosphinidene (RP2−) ligands, as well as the tri-anionic phosphide ligand (P3−)19. Since electrostatic interactions strongly influence the electronic structure of lanthanides, the ability to synthesize compounds with similar molecular structures, but where the ligands carry different formal charges, could allow new ways of designing SMMs. Changing the organo-phosphorus ligand should also influence the exchange interactions in polymetallic systems, which is important because exchange is known to influence relaxation phenomena345. Phosphinidene complexes of the lanthanides are rare202122232425, and, although their reactivity has been studied, their influence on 4f electronic structure has not. Indeed, phosphorus-ligated Ln-SMMs are unknown, hence we now target the SMM properties of dysprosium complexes with bridging phosphide and phosphinidene ligands. Here we show that organo-phosphorus chemistry can be used to influence the dynamic magnetic properties of lanthanide complexes; our observations have general implications for how main group organometallic chemistry can be used to develop new SMMs.

Results

Synthesis and structural studies

We initially targeted a primary pan class="Chemical">phosphine comclass="Chemical">plex of dysclass="Chemical">prosium and aimed to sequentially declass="Chemical">protoclass="Chemical">pan class="Gene">nate the P–H bonds. The adduct [Cp′3Dy←PH2Mes] (1-Dy) (Cp′=η5-C5H4Me, Mes=mesityl) was synthesized by adding mesitylphosphine to tris(methylcyclopentadienyl)dysprosium (Fig. 1). Deprotonation of 1-Dy by BuLi gave the trimetallic phosphide-bridged complex [(Cp′2Dy){μ-P(H)Mes}]3·toluene (2-Dy·toluene). Deprotonation of the P–H bonds in 2-Dy by BuLi in thf produced [Li(thf)4]2[(Cp′2Dy)3(μ-PMes)3Li]·thf, which contains the phosphinidene-bridged trimetallic complex [(Cp′2Dy)3(μ-PMes)3Li]2− (3-Dy). The analogous yttrium complexes [Cp′3Y←PH2Mes] (1-Y), [(Cp′2Y){μ-P(H)Mes}]3·toluene (2-Y·toluene) and [Li(thf)4]2[(Cp′2Y)3(μ-PMes)3Li]·thf, [Li(thf)4]2[3-Y]·thf, were synthesized in an identical manner. The molecular structures of all compounds were determined by X-ray diffraction; the main features of 1-Dy, 2-Dy and 3-Dy (Fig. 1) are described below, with further details provided in Supplementary Tables 1–3, Supplementary Figs 1–10 and Supplementary Data 1–6.

Figure 1

Synthesis and molecular structures.

(a) Synthesis of the yttrium-phosphorus and dysprosium-phosphorus complexes. (b) Thermal ellipsoid representation of the molecular structure of 1-Dy. (c) Molecular structure of 2-Dy. (d) Molecular structure of 3-Dy. Hydrogen atoms are omitted for clarity.

Compound pan class="Chemical">1-Dy (Fig. 1) contains a dysclass="Chemical">prosium centre coordiclass="Chemical">pan class="Gene">nated to three η5-Cp′ ligands and a MesPH2 ligand, hence the dysprosium coordination geometry can be regarded as pseudo-tetrahedral with respect to the centroid positions of the Cp′ ligands. The Dy–P bond length is 3.009(1) Å and the Dy–C bond lengths are 2.676(7)–2.744(6) Å (average 2.710 Å). To the best of our knowledge 1-Dy and 1-Y are the first crystallographically characterized rare-earth complexes of primary phosphine ligands.

Compound 2pan class="Chemical">-Dy (Fig. 1) is a cyclic trimer in which the three {Cclass="Chemical">p′2Dy} units are bridged by μ-(class="Chemical">pan class="Chemical">mesityl)phosphide ligands, resulting in Dy–P bond lengths of 2.926(6)–2.951(6) Å. The Dy–Cp′ bond lengths in 2-Dy fall in a broad range of 2.60(2)–2.72(1) Å (average 2.64 Å). The {Dy3P3} core of 2-Dy adopts a chair-like conformation, in which each dysprosium is coordinated by two Cp′ ligands, with the mesityl substituents being oriented almost perpendicular to the P3 plane. It was not possible to locate the hydrogen atoms in the structure of 2-Dy, but the P–H stretching vibrations occur in the IR spectrum at 2,358 and 2,318 cm−1 (Supplementary Fig. 8). The structure of 3-Dy also consists of a chair-like Dy3P3 core but with a central lithium cation, in which the μ3-phosphinidene ligands bridge between dysprosium and lithium. The Dy–P bonds in 3-Dy are 2.7850(15)–2.8249(15) Å, making them 0.13 Å shorter, on average, compared with the analogous distances in 2-Dy, as expected based on stronger electrostatic bonding. The Dy–C bond lengths in 3-Dy are longer, on average, by 0.05 Å compared with those in 2-Dy, suggesting that the Cp′ ligands in 3-Dy exhibit a degree of flexibility to accommodate the more compact {Dy3(PMes)3Li} core. The lithium cation occupies a position 0.896(8) Å out of the P3 plane, with Li–P bond lengths of 2.472(9), 2.477(10) and 2.557(8) Å, respectively. A mesityl ortho-methyl group may interact with lithium in 3-Dy via an agostic bond; the Li(1)···C(45) distance of 2.813(11) Å is markedly shorter than the Li(1)···C(52) and Li(1)···C(63) distances of 3.318(10) and 3.452(10) Å, respectively. A structural feature that 3-Dy shares with other rare-earth phosphinidene complexes is the bridging coordination mode of the ligand202122232425.

Magnetic property measurements

The static-field (d.c.) magnetic properties of polycrystalline pan class="Chemical">1-Dy, 2class="Chemical">pan class="Chemical">-Dy·toluene and [Li(thf)4]2[3-Dy]·thf were measured on a superconducting quantum interference device magnetometer in the temperature range of 1.8–300 K. At 300 K, the value of χM T(T) for 1-Dy, where χM is the molar magnetic susceptibility, is 12.85 cm3 K mol–1; cooling the sample to 1.8 K produces a gradual decrease in χMT, such that a value of 7.93 cm3 K mol–1 is eventually reached (Supplementary Fig. 11). The field (H) dependence of the magnetization (M) for 1-Dy reveals a steep increase up to 1 T, followed by a more gradual increase up to 7 T, where the value of M=5.09 μB is consistent with a single Dy3+ ion with a 6H15/2 ground state (Supplementary Fig. 12). The χMT values for 2-Dy·toluene and [Li(thf)4]2[3-Dy]·thf at 300 K are 40.08 and 42.41 cm3 K mol–1, respectively, both of which are close to the predicted values for three uncoupled Dy3+ centres26; both χMT values decrease gradually down to about 50 K, and then more rapidly to reach 17.81 and 19.47 cm3 K mol–1, respectively, at 1.8 K (Supplementary Fig. 11). For 2-Dy·toluene and [Li(thf)4]2[3-Dy]·thf, the magnetization at 1.8 K increases rapidly up to about 2 T, before following a more gradual increase up to 7 T, where values of M=16.10 and 16.13 μB are reached. The M(H) data for the two trimetallic complexes are consistent with the expected value for three uncoupled Dy3+ ions (Supplementary Fig. 12).

The dynamic (a.c.) magnetic susceptibility properties of the three dysprosium compounds show much greater variation. For pan class="Chemical">1-Dy, a maximum was observed in the class="Chemical">plot of the out-of-class="Chemical">phase magnetic susceclass="Chemical">ptibility (χ′′) versus a.c. frequency (ν) at 1.8 K, but not at higher temclass="Chemical">peratures uclass="Chemical">p to the maximum frequency that can be achieved with our susceclass="Chemical">ptometer (Suclass="Chemical">pclass="Chemical">plementary Fig. 13). These data indicate raclass="Chemical">pid relaxation of the magnetization in class="Chemical">pan class="Chemical">1-Dy above 1.8 K. In contrast, in zero applied d.c. field, the χ′′(ν) isotherms of 2-Dy·toluene have well-defined frequency-dependent maxima up to 31 K, which demonstrates SMM behaviour with a large thermal energy barrier (Fig. 2 and Supplementary Figs 14 and 15). The energy barrier for the thermal relaxation in 2-Dy·toluene was extracted using the relationship τ=τ0 exp (Ueff/kBT), which gave Ueff=210(6) cm−1, with a pre-exponential factor of τ0=6.53 × 10–9 s. Below about 20 K, the dependence of ln τ on 1/T weakens and deviates from linearity, but does not become fully temperature independent even at 1.8 K (Fig. 2). This observation implies that other relaxation mechanisms, such as quantum tunnelling of the magnetization and Raman or direct relaxation, occur concurrently with the thermal relaxation process at lower temperatures. In contrast, the χ′′(ν) isotherms in [Li(thf)4]2[3-Dy]·thf only show maxima below 3.6 K, and the position of the maximum moves only slightly as the temperature is reduced to 1.8 K (Supplementary Figs 16 and 17). The plot of ln τ versus 1/T for the phosphinidene-bridged complex produces an anisotropy barrier of Ueff=13(1) cm−1 (τ0=7.75 × 10–7 s), that is, more than an order of magnitude smaller than in phosphide-bridged 2-Dy·toluene.

Figure 2

Magnetic properties of selected dysprosium-phosphorus compounds.

(a) χ′′(ν) for 2-Dy·toluene in the temperature range of 2–31 K with an a.c. field of 1.55 Oe and zero d.c. field. (b) χ′′(ν) for 2-YDy·toluene in the temperature range of 2–32 K with an a.c. field of 1.55 Oe and zero d.c. field. (c) Arrhenius plots for 2-Dy·toluene (blue circles), 2-YDy·toluene (green circles), [Li(thf)4]2[3-Dy]·thf (red circles) and [Li(thf)4]2[3-YDy]·toluene (black circles). The solid lines represent data fits in the range of 27–31 K for 2-Dy·toluene, 28–32 K for 2-YDy·toluene, 3.8–4.2 K for [Li(thf)4]2[3-Dy]·thf and 3.9–5 K for [Li(thf)4]2[3-YDy]·thf. (d) M(H) loops for 2-YDy 1.8–4.4 K with a sweep rate of 2.6 mT s–1.

To examine the influence of dysprosium–dysprosium interactions, magnetically dilute samples of 5% dysprosium in 2-Y·pan class="Chemical">toluene and class="Chemical">pan class="Chemical">[Li(thf)4]2[3-Y]·thf were prepared; dilution was achieved by mixing 1-Y and 1-Dy in a 20:1 ratio and then synthesizing the complexes according to Fig. 1. This enabled isolation of crystalline samples of 2-YDy·toluene in a matrix of 2-Y·toluene and, similarly, [Li(thf)4]2[3-YDy]·thf in a matrix of [Li(thf)4]2[3-Y]·thf. For both doped trimetallic compounds the a.c. susceptibility data are similar to those measured for the pure samples (Fig. 2, Supplementary Figs 18 and 19), that is, the phosphinidene-bridged complex shows very weak SMM behaviour, while 2-YDy·toluene is an SMM but with an increased anisotropy barrier of Ueff=256(6) cm−1. There is a much clearer and more important difference in the M(H) hysteresis measurements for the undiluted and diluted systems. For 2-Dy·toluene, the M(H) curve at 1.8 K (Supplementary Fig. 20) shows very narrow hysteresis, while hysteresis is found for 2-YDy·toluene up to 4.4 K, with significant widening of the butterfly-shaped loops as the temperature is lowered (Fig. 2). The coercive field for 2-YDy·toluene at 1.8 K is estimated to be Hc≈300 Oe, with a small remanent magnetization of Mr≈0.03 μB (sweep rate 2.6 mT s–1). The M(H) plot for 2-YDy·toluene also shows steps at H≈±1,800 Oe, which is probably due to small amounts of the partially doped di-dysprosium complex 2-YDy. In low magnetic fields, the ground state of 2-YDy is defined by antiferromagnetic exchange, but above a certain field strength the ground state changes to ferromagnetic and has a greater magnetic moment, hence the steps in the M(H) curves. The precise field at which the steps are found is consistent with the calculated exchange spectrum from 2-Dy (see below). A feature similar to this has been observed in magnetic dilution studies of di-dysprosium SMMs2728. As Ueff is similar for 2-Dy·toluene and 2-YDy·toluene, the difference in hysteresis must be due to non-thermal relaxation being more important in the undiluted compound.

Quantum chemical calculations

Further insight into the electronic structure and the magnetic blocking in the dysprosium complexes was obtained using ab initio quantum chemical calculations. Current ab initio methods are not suitable for treating several magnetic 4f centres simultaneously; therefore, in the case of 2pan class="Chemical">-Dy and class="Chemical">pan class="Chemical">3-Dy appropriate fragmentation was imposed, hence Lu3+ was used in place of neighbouring Dy3+ (Supplementary Figs 22–24). All calculations on mono-dysprosium fragments were performed with MOLCAS29 and were of CASSCF/RASSI/SINGLE_ANISO type (Supplementary Tables 4–9)30. Tables 1 and 2 show the energy spectrum and the magnetic anisotropy of the lowest Kramers' doublets on individual dysprosium sites of the three compounds.

Table 1

Calculated low-lying energy (cm−1) spectrum of individual Dy3+ sites in 1-Dy, 2-Dy and 3-Dy.

KD	1-Dy	2-Dy			3-Dy
		Dy1	Dy2	Dy3	Dy1	Dy2	Dy3
1	0.0	0.0	0.0	0.0	0.0	0.0	0.0
2	45.2	126.7	134.2	135.0	63.8	69.5	100.7
3	98.7	277.3	296.8	297.8	77.4	88.8	104.4
4	287.1	345.8	320.9	334.4	116.2	108.5	132.9
5	339.5	362.7	388.7	381.0	136.0	148.5	150.2
6	391.7	402.8	431.0	419.0	160.6	160.9	181.1
7	452.5	433.6	499.2	478.0	194.0	187.1	217.2
8	583.9	551.7	675.6	637.8	232.1	271.2	290.6

KD, Kramers' doublet.

Table 2

Calculated g tensors of the ground- and first-excited Kramers' doublets and the angles between their main anisotropy axes in 1-Dy, 2-Dy and 3-Dy.

For the pan class="Chemical">phosphine adduct class="Chemical">pan class="Chemical">1-Dy, the lack of SMM properties is consistent with the absence of strong magnetic axiality of the ground and excited doublets (Table 2). The presence of large gx,y values in the ground state enables fast quantum tunnelling between states with opposite magnetization, destroying any potential blocking of the magnetization3132. This can be rationalized in terms of the molecular and electronic structure of 1-Dy, where the main magnetic axis (gz) lies in a plane defined by three Cp′ ligands (Supplementary Fig. 26). The electrostatic potential of the ligand field is dominated by the anionic Cp′ ligands, which are much closer to dysprosium (average Dy–C=2.710 Å) than the charge-neutral phosphine ligand (Dy—P=3.009 Å). Strong equatorial ligand fields are known to diminish magnetization blocking in Dy3+ complexes1433, hence the weak SMM properties of 1-Dy.

The dysprosium sites in 2pan class="Chemical">-Dy disclass="Chemical">play the strongest magnetic axiality in the ground and excited doublet states, which is combined with the largest sclass="Chemical">plitting of the eight Kramers' doublets within the ground J=15/2 manifold of the three systems investigated. The local main magnetic axes (gz) in 2class="Chemical">pan class="Chemical">-Dy make angles of 69°–72° with the Dy3 plane (Fig. 3, Table 2). This direction of the magnetization axis on each dysprosium is the result of the strong axial ligand field generated by the two Cp′ ligands, which are closer to dysprosium (average Dy–C=2.64 Å) than phosphorus (average Dy–P=2.934 Å), consistent with the electronic properties of closely related metallocene-based SMMs34353637. The very small angles between the anisotropy axes in the ground and first-excited Kramers' doublets in 2-Dy, combined with their strong magnetic axiality (Table 2 and Fig. 3), are necessary conditions for magnetic relaxation via the second-excited doublet states at higher temperatures61438. The calculated energy of the second-excited state (Table 1) gives a reasonable agreement with experiment, being higher than the extracted Ueff value by about 20–40 cm−1. Whereas lanthanide SMMs can be designed with ligand environments that promote strongly axial magnetic ground states, targeting SMMs with very large Ueff values also requires the excited states to possess main magnetic axes that lie close to co-linear with the ground state, which is challenging. In the case of 2-Dy, the electronic and molecular structures of the individual {Cp′2Dy{μ-P(H)Mes} units appear to fit the requirements for a dysprosium SMM in which the ground |mJ|=15/2 states and |mJ|=13/2 excited states are indeed co-linear. The strong magnetic axiality of the dysprosium sites is expected to be preserved in the diluted compound YDy, which explains the detection of magnetic hysteresis in this system (Fig. 2).

Figure 3

Orientation of the main magnetic axes in the ground Kramers' doublets.

(a) For 2-Dy. (b) For 3-Dy. The magnetic axes are represented as dashed red lines. The dysprosium centres are represented as green spheres and the phosphorus atoms as orange spheres.

The dysprosium sites in class="Chemical">3-Dy disclass="Chemical">play a much smaller total sclass="Chemical">plitting of the ground J=15/2 manifold comclass="Chemical">pared with 2class="Chemical">pan class="Chemical">-Dy (Table 1). The different splittings are presumably influenced by subtle differences in the structures of the two complexes, that is, greater distances to the Cp′ ligands (by 0.03–0.09 Å) in 3-Dy, reduced Cpcent-Dy-Cpcent angles (by ∼3.4°) and shorter Dy–P bonds (by 0.10–0.16 Å) together with their greater formal charge. Indeed, the calculated charges on the phosphinidene donor atoms in 3-Dy range from −0.6 to −0.9, compared with the calculated charges of −0.1 to −0.3 on the phosphide donors in 2-Dy39.

The closely located class="Chemical">phosphinidene ligands and their charge class="Chemical">produce a much stronger equatorial ligand field, which counteracts the axial field arising from the Cclass="Chemical">p′ ligands. Although the ground Kramers' doublet is still characterized by |mJ|=15/2, the enhanced equatorial ligand field stabilizes other mJ states and enables greater mixing in class="Chemical">pan class="Chemical">3-Dy relative to 2-Dy. This is reflected in weaker magnetic axiality of the ground state in 3-Dy. Moreover, the first-excited doublets of 3-Dy feature relatively large transverse g-values (gx and gy in Table 2), which, combined with the large angles with the ground-state main magnetic axis, leads to fast magnetic relaxation through this doublet at high temperatures.

The ab initio results for the individual pan class="Chemical">metal sites were used to comclass="Chemical">pute the exchange sclass="Chemical">pectrum and the magnetic class="Chemical">proclass="Chemical">perties of the class="Chemical">pan class="Chemical">trimetallic complexes using the POLY_ANISO program4041. The fitted Lines exchange parameters are given in Supplementary Table 10. The macroscopic magnetic properties were further computed on the basis of the exchange spectrum. Given the strong magnetic axiality of the ground-state Kramers' doublets on the dysprosium sites, the magnetic interaction (exchange+dipolar) between them can be described by the non-collinear Ising Hamiltonian in equation 1.

In equation 1, is the parameter of the total magnetic interaction between pan class="Chemical">metal sites i and j, including the diclass="Chemical">polar and exchange contributions (Suclass="Chemical">pclass="Chemical">plementary Table 10), is the class="Chemical">projection of the class="Chemical">pseudo-sclass="Chemical">pin ½, describing the ground Kramers' doublet on the i-th dysclass="Chemical">prosium site, on the corresclass="Chemical">ponding main magnetic axis zi. The exchange and magnetic diclass="Chemical">polar interactions between dysclass="Chemical">prosium sites in both comclass="Chemical">pounds are antiferromagnetic. For the calculated directions of the main anisotroclass="Chemical">py axes on the dysclass="Chemical">prosium sites in 2class="Chemical">pan class="Chemical">-Dy and 3-Dy (Fig. 3), the antiferromagnetic exchange and dipolar interactions will stabilize the sixfold degenerate (that is, three Kramers' doublets), frustrated exchange Kramers' doublets (Supplementary Table 10). In a frustrated manifold, magnetization blocking of the exchange type is suppressed because all its states can be accessed by reversal of individual magnetic moments on Dy sites (illustrated for 2-Dy in Fig. 4)42. As a consequence, a quantum tunnelling of the magnetization process as fast as those in single-ion complexes is expected. Under these circumstances, the magnetization blocking is expected to arise from individual dysprosium ions. Actually, the magnetic moment reversal on the individual dysprosium ions in the trimetallic compounds 2-Dy and 3-Dy is expected to be faster than in the isostructural complexes containing one single dysprosium, that is, 2-YDy and 3-YDy, because in a frustrated compound the presence of nearest neighbour magnetic sites will enhance the magnetization reversal rate on each magnetic ion4243. This is the reason for the clearly enhanced blocking of magnetization and the observation of pronounced hysteresis in in 2-YDy (Fig. 2)42.

Figure 4

The three low-lying quasidegenerate exchange levels of 2-Dy.

The six magnetic eigenstates are related via flips of magnetic moments (blue arrows) on the individual dysprosium sites. The + and − symbols represent time-reversal components corresponding to one exchange Kramers' doublet.

Magnetization blocking of the exchange type is expected to be efficient only at temperatures not exceeding the exchange splitting, which is the case here because the magnetic coupling constant between two dysprosium ions in 2pan class="Chemical">-Dy is ∼4 cm−1 (Suclass="Chemical">pclass="Chemical">plementary Table 10). The calculated exchange sclass="Chemical">plitting for 2class="Chemical">pan class="Chemical">-Dy also provides insight into the steps observed in the M(H) profile for 2-YDy: the exchange splitting in partially doped 2-YDy will be about 2 cm−1, that is, ∼2,000 Oe, which is in good agreement with the experimentally observed steps at 1,800 Oe.

Discussion

We have reported a new route to rare-earth complexes of pan class="Chemical">phosphide and class="Chemical">pan class="Chemical">phosphinidene ligands. The a.c. susceptibility measurements on phosphide-ligated SMM 2-Dy reveal a large effective energy barrier of Ueff=210(5) cm−1, increasing to 256(6) cm−1 on dilution in 2-YDy. In contrast, 1-Dy and phosphinidene-ligated [Li(thf)4]2[3-Dy], and their diluted analogues, do not show prominent SMM properties. Theoretical studies of 2-Dy and 3-Dy identify that the main magnetic axes on the Dy3+ ions in the ground Kramers' doublet have similar orientations approximately perpendicular to the Dy3 plane; even allowing for small differences in the axis orientations (of about 10°), the phosphorus atoms occupy equatorial positions, hence the greater electrostatic demands of the phosphinidene ligands diminish the axiality. In 2-Dy, the thermal magnetic relaxation proceeds via the second-excited Kramers' doublet owing to the near-collinearity of the magnetization axes in the ground- and first-excited doublets. The reduced axiality of the first-excited Kramers' doublet in 3-Dy, combined with different orientations of the main magnetic axes in the ground- and first-excited doublets, explains the weak magnetic blocking in this system.

The field dependence of the magnetization for 2pan class="Chemical">-Dy shows very narrow hysteresis at 1.8 K, however butterfly-shaclass="Chemical">ped hysteresis was found uclass="Chemical">p to 4.4 K for 2-YDy. The class="Chemical">proclass="Chemical">perties of undiluted 2class="Chemical">pan class="Chemical">-Dy can be explained in terms of the exchange coupling and frustrated ground state, which enable rapid reversal of the magnetic moments on the individual dysprosium sites. The absence of exchange coupling in 2-YDy allows more prominent blocking of the magnetization.

Substantial changes to SMM properties can be achieved using a bottom-up approach in which class="Chemical">dysprosium phosphide comclass="Chemical">plexes are converted into their class="Chemical">pan class="Chemical">phosphinidene analogues. The general features of the chemistry and the electronic structure suggest that exploring other lanthanides and other unconventional ligands in place of phosphorus should enable further enhancements in the SMM behaviour.

Methods

General synthesis considerations

All manipulations were performed under an atmosphere of dry, pan class="Chemical">oxygen-free class="Chemical">pan class="Chemical">argon, using either standard Schlenk techniques or an argon-filled glove box. Toluene and thf, and their deuterated analogues for NMR spectroscopy, were dried by refluxing over potassium and collected by distillation. All solvents were stored over activated 4 Å molecular sieves and freeze-thaw degassed before use. Anhydrous rare-earth chlorides (99.99% purity) were purchased from Strem Chemicals. n-Butyllithium (1.6 M in hexanes), phosphorus(III) chloride, lithium aluminium hydride, mesitylmagnesium bromide (1.0 M in thf) and di-methylcyclopentadiene (90%) were purchased from Sigma-Aldrich. Mesitylphosphine44, the sodium salt of methylcyclopentadienide45, and rare-earth tris(methylcyclopentadienide) complexes were synthesised according to the literature procedures46. Elemental analyses were carried out by Mr Stephen Boyer at London Metropolitan University, UK

[Cp′3Dy(PH2Mes)] (1-Dy)

A solution of class="Chemical">MesPH2 in class="Chemical">pan class="Chemical">toluene (0.5 M, 4.0 ml, 2.0 mmol) was added to a solution of Cp′3Dy (0.80 g, 2.0 mmol) in toluene (4 ml) at room temperature, and the reaction mixture was stirred for 1 h. The resulting pale yellow solution was filtered, concentrated and stored at −30 °C overnight, which resulted in the formation of 1-Dy as colourless crystals (0.92 g, 83% isolated yield). Elemental analysis, found per percentage (calculated per percentage) for 1-Dy: C, 58.62 (58.74); H, 6.28 (6.21).

[Cp′3Y(PH2Mes)] (1-Y)

The synthesis of class="Chemical">1-Y was accomclass="Chemical">plished using the class="Chemical">procedure described above for class="Chemical">pan class="Chemical">1-Dy, with the following amounts: MesPH2 (0.5 M, 4.0 ml, 2.0 mmol), Cp′3Y (0.65 g, 2.0 mmol) and toluene (4 ml). 1-Y formed as colourless crystals (0.71 g, 75% isolated yield). Elemental analysis, found percentage (calculated percentage) for 1-Y: C, 67.39 (67.78); H, 7.21 (7.16). 1H NMR (benzene-D6, 298.15 K, δ/p.p.m.): 6.65 (s, 2H, mesityl CH); 5.85, 5.72 (m, 12H, C5H4CH3); 3.86 (d, 2H, PH2, 1JP–H=251 Hz); 2.16 (s, 6H, ortho CH3); 2.10 (s, 9H, Cp′ CH3); 2.03 (s, 3H, para CH3). 31P NMR (benzene-D6, 298.15 K, δ/p.p.m.): −130.80.

[(Cp′2Dy){μ-P(H)Mes}]3·toluene (2-Dy·toluene)

BuLi (1.6 M in class="Chemical">hexanes, 0.66 ml, 1.06 mmol) was added to a solution of class="Chemical">pan class="Chemical">1-Dy (0.55 g, 1.06 mmol) in toluene (20 ml) at −78 °C, and the reaction mixture was stirred for 1 h. The reaction was then slowly warmed to room temperature overnight, after which time a pale yellow solution and a precipitate had formed. The resulting pale yellow solution was filtered and concentrated, which resulted in the formation of a yellow precipitate. The precipitate was re-dissolved and the solution stored at −30 °C overnight, which resulted in the formation of 2-Dy·toluene as colourless crystalline blocks (0.34 g, 63%). Elemental analysis, found percentage (calculated percentage) for 2-Dy·toluene: C, 55.63 (55.76); H, 5.83 (5.75).

[(Cp′2Y){μ-P(H)Mes}]3·toluene (2-Y·toluene)

The synthesis of 2-Y·pan class="Chemical">toluene was accomclass="Chemical">plished using the class="Chemical">procedure described above for 2class="Chemical">pan class="Chemical">-Dy·toluene, with the following amounts: 1-Y (0.59 g, 1.24 mmol), toluene (20 ml) and BuLi (1.6 M in hexanes, 0.77 ml, 1.24 mmol). 2-Y·toluene was isolated as colourless crystalline blocks (0.37 g, 70% based on yttrium). Elemental analysis, found percentage (calculated percentage) for 2-Y·toluene: C, 65.16 (65.32); H, 6.82 (6.74). 1H NMR (toluene-D8, 298.15 K, δ/p.p.m.): 6.78–6.93 (s, 6H, mesityl CH); 5.50–6.55 (s, 24H, C5H4CH3); 3.47–1.61 (s, overlapping peaks due to CH3 groups). 31P NMR (benzene-D6, 298.15 K, δ/p.p.m.): −124.28, −145.21 and −142.65.

[Li(thf)4]2[(Cp′2Dy)3(μ-PMes)3Li]·thf ([Li(thf)4]2[3-Dy]·thf)

A solution of 2class="Chemical">-Dy·class="Chemical">pan class="Chemical">toluene (0.38 g, 0.25 mmol) in thf (10 ml) was cooled to −78 °C and BuLi (1.6 M in hexanes, 0.47 ml, 75 mmol) was added drop wise. After stirring at −78 °C for 30 min, the reaction was warmed to room temperature over 3 h, after which time an orange colour had developed. The solution was concentrated until copious amounts of precipitate had formed, then the precipitate was re-dissolved by gentle heating and the solution stored at +4 °C. [Li(thf)4]2[3-Dy]·thf formed as large orange blocks (0.33 g, 64%). Elemental analysis, found percentage (calculated per %) for [Li(thf)4]2[3-Dy]·thf: C, 56.88 (57.10); H, 7.25 (7.12).

[Li(thf)4]2[(Cp′2Y)3(μ-PMes)3Li]·thf ([Li(thf)4]2[3-Y]·thf)

The synthesis of [3-Y]class="Chemical">[Li(thf)4]2·class="Chemical">pan class="Chemical">thf was accomplished using the procedure described above for [3-Y][Li(thf)4]2·thf, with the following amounts: 2-Y·toluene (0.34 g, 0.27 mmol), thf (10 ml) and BuLi (1.6 M in hexanes, 0.50 ml, 0.79 mmol). [Li(thf)4]2[3-Y]·thf was isolated as pale orange crystalline blocks (0.27 g, 56% based on yttrium). Elemental analysis, found per % (calculated per %) for [3-Y][Li(thf)4]2·thf: C, 63.69 (63.77); H, 8.02 (7.95). NMR spectra were acquired in dimethoxyethane (that is, C4H10O2) with a few drops of benzene-D6 for the signal lock. 1H (298.15 K, δ/p.p.m.): 6.78, 6.68 (2 × s, 2 × 3H, mesityl CH); 6.47, 6.15, 4.99 and 4.74 (s, 4 × 6H, C5H4CH3); 3.46 (m, 4H, thf CH2OCH2); 2.59 (s, 9H, ortho CH3); 2.52 (s, 9H, ortho CH3); 2.26 (s, 9H, Cp′ CH3); 2.08 (s, 9H, Cp′ CH3); 1.52 (m, 4H, thf CH2CH2O); 1.46 (s, 9H, para CH3). 31P NMR (benzene-D6, 298.15 K, δ/p.p.m.): +57.24 p.p.m.; a small amount of MesPH2 is observed at −157.51 p.p.m., which occurs due to the phosphinidene ligand reacting slowly with the DME solvent.

Doped sample (2-Y Dy)/(2-Y)

The dilution experiment to obtain 2-YDy doped into a matrix of 2-Y was achieved by combining pan class="Chemical">1-Dy (0.0138, g, 0.025 mmol) and class="Chemical">pan class="Chemical">1-Y (0.2280, g, 0.475 mmol; that is, a 5:95 stoichiometric ratio) as solids, and dissolving the mixture in toluene (8 ml). Following the addition of BuLi (1.6 M, 0.31 ml, 0.50 mmol) at −78 °C and a workup procedure as described for 2-Dy, the doped sample was obtained as colourless crystals (0.088 g, 41% based on metal content).

Doped sample [Li(thf)4]2[3-Y2Dy]·thf/[Li(thf)4]2[3-Y]·thf

The dilution experiment to obtain pan class="Chemical">[Li(thf)4]2[3-YDy]·class="Chemical">pan class="Chemical">thf doped into a matrix of [Li(thf)4]2[3-Y]·thf was achieved by the deprotonation of 2-YDy (0.14 g, 0.11 mmol) by of BuLi (1.6 M, 0.20 ml, 0.32 mmol) at −78 °C, with the workup procedure as described for [Li(thf)4]2[3-Dy]·thf. The doped sample was obtained as pale orange crystals (0.043 g, 22% based on metal content).

Characterization of doped materials

The two doped materials were characterized by X-ray diffraction (Supplementary Table 1): measurements of the unit cell parameters of several crystals of each were performed on an Oxford Xcaliber-2 diffractometer using Mo-Kα radiation at 100 K. The unit cell dimensions of both compounds were found to be equivalent (within 3σ) to those observed for their respective pure pan class="Chemical">yttrium compounds 2-Y and class="Chemical">pan class="Chemical">[Li(thf)4]2[3-Y]·thf. Accurate dysprosium/yttrium ratios were measured by inductively coupled plasma atomic emission spectroscopy using a Thermo iCap 6300 ICP-OES instrument, which resulted in dysprosium contents of 5.0±0.5% for both doped materials.

The magnetic properties were measured using a Quantum Design Mclass="Chemical">PMS-7 suclass="Chemical">perconducting quantum interference device magnetometer at temclass="Chemical">peratures in the range of 1.8–300 K. In a glove box, class="Chemical">polycrystalline samclass="Chemical">ples of each material were transferred to NMR tubes and restrained in class="Chemical">pan class="Chemical">eicosane. The NMR tubes were then placed under a partial vacuum and flame sealed before being transferred to the magnetometer.

Computational details

All calculations were done with MOLCAS 7.8 and are of CASSCF/RASSI/pan class="Disease">SINGLE_ANISO tyclass="Chemical">pe. For each of class="Chemical">pan class="Chemical">1-Dy, 2-Dy and 3-Dy, two computational models were employed. In model A, all the CH3 groups were replaced by hydrogen atoms. Model B is the entire molecule as determined by X-ray crystallography, using all the atomic coordinates in the CIF files. The structures of model A (small) for 1-Dy, 2-Dy and 3-Dy are shown in Supplementary Figs 21–23, respectively. Two basis set approximations were used: 1—small (DZP quality), and 2—large (TZP quality). We have therefore considered four computational models for each magnetic centre in the investigated molecules: A1, A2, B1 and B2. The following results refer to the most complete model B2 (full structure with large basis set). Details of the calculations for models A1, A2 and B1 can be obtained from LFC. Each magnetic centre was computed, and neighbouring metal ions were computationally replaced by diamagnetic Lu3+. The active space of the CASSCF method includes the electrons from the last shell spanning the seven 4f orbitals of the Ln3+ ion. For all calculations, it was possible to mix only a limited number of roots, namely 21 sextets, 128 quartet and 130 doublet states. On the basis of the resulting spin-orbital multiplets, the SINGLE_ANISO programme computed the local magnetic properties (g tensors, main magnetic axes, local magnetic susceptibility, crystal-field parameters and so on). The ab initio results for the individual metal sites were used to compute the exchange spectrum and the magnetic properties of the trimetallic complexes using the POLY_ANISO program. The exchange interaction between the dysprosium sites is considered within the Lines model, while the contribution of the intramolecular dipole–dipole magnetic coupling is accounted for exactly given that all necessary data are available from the ab initio calculations.

All three exchange interactions in pan class="Chemical">1-Dy and 2class="Chemical">pan class="Chemical">-Dy were described by one single Lines exchange parameter. The values of the Lines parameter for 1-Dy and 2-Dy was determined by minimizing the difference between the calculated and the measured magnetic susceptibility. The Lines parameters were expressed in terms of the Ising Hamiltonian by the expression , where ϕij is the angle between the main anisotropy axes (zi and zj) of the interacting sites. Supplementary Fig. 25 shows the comparison of the calculated and measured magnetic susceptibility.

Additional information

Accession codes. The X-ray crystallographic coordipan class="Gene">nates for the structures reclass="Chemical">ported in this Article have been declass="Chemical">posited at the Cambridge Crystallograclass="Chemical">phic Data Centre (CCDC), under declass="Chemical">position numbers 1032227-1032232. These data can be obtained free of charge from the Cambridge Crystallograclass="Chemical">phic Data Centre via www.ccdc.cam.ac.uk/data_request.cif.

How to cite this article: class="Chemical">Pugh, T. et al. Influencing the class="Chemical">proclass="Chemical">perties of dysclass="Chemical">prosium class="Chemical">pan class="Disease">single-molecule magnets with phosphorus donor ligands. Nat. Commun. 6:7492 doi: 10.1038/ncomms8492 (2015).

Supplementary Figures and Supplementary Tables

Supplementary Figures 1-26 and Supplementary Tables 1-10

Supplementary Data 1

cif file for pan class="Chemical">1-Dy

Supplementary Data 2

cif file for 2pan class="Chemical">-Dy.class="Chemical">pan class="Chemical">toluene

Supplementary Data 3

cif file for class="Chemical">[Li(thf)4]2[class="Chemical">pan class="Chemical">3-Dy].thf

Supplementary Data 4

cif file for pan class="Chemical">1-Y

Supplementary Data 5

cif file for 2-Y.pan class="Chemical">toluene

Supplementary Data 6

cif file for pan class="Chemical">[Li(thf)4]2[3-Y].class="Chemical">pan class="Chemical">thf