Encabezados de planillas

Single-crystal XRD studies show three Et2O molecules367,368 _{could be successfully refined in the} lattice of 16 and 17. However, these samples are air sensitive and over time lose their crystallinity. Compounds 15-18 were further characterised by EA (chapter eleven), TGA (Appendix B, S3.5) and ESI-MS (Appendix B, S3.1-3.4).

Compounds 15 - 18 crystallize in the monoclinic space group P21/c and are isostructural, thus only compound 16 is discussed further. Four CoII _{and two Gd}III _{cations form a twisted “boat-like”} core with CoII _{ions occupying the four central body positions and the two Gd}III _{ions occupying} positions in the “bow” and “aft” (Figure 3.2), this configuration will be assigned the notation (1Ln: 4Co: 1Ln).

The hexanuclear core is supported by two μ3-OH groups between the GdIII _{ion and the two nearest} CoII _{ions. Each of the four organic ligands adopts the same coordination mode (Figure 3.3) and} each is bonded to one GdIII _{and two Co}II _{ions. Each ligand is chelated to a Co}II _{centre through one} imino and two phenoxido oxygen atoms, forming a “metalloligand” which is further chelated to a GdIII _{through a phenoxido and methoxido oxygen atom and another Co}II _{via the phenoxido} oxygen atom of the aminophenol moiety. Each hydroxyl group bridges two CoII _{and one Gd}III centres and the angles are within the range 102.16(12) ° - 106.05(13) °.

The coordination environment of Co2 and Co4 is fulfilled by a [Cl]- _{ion. All Co centres adopt an} essentially distorted octahedral geometry, where the Bond Valence Sum (BVS) analysis is indicative of oxidation state II (2.082, 1.900, 2.040 and 1.913 for Co1, Co2, Co3 and Co4, respectively).

Figure 3.2. The molecular structure of compound 16 (upper). The core of compound 16 (middle)

Co1 - Co4 shown in the body positions. Gd1 shown occupying the “bow” and Gd2 the “aft”. Representation of 2,3,4M6-1 core of compound 16 (lower). Colour code: GdIII_{, light blue; Co}II_, pink; C, yellow; N, pale blue; O, red; Cl, green Hydrogens omitted for clarity

Figure 3.3. The coordination mode of H2L1 found in 15 - 18.

The coordination number of each GdIII _{ion is nine. Using SHAPE software}339 _{the geometry of} both GdIII _{ions can be described as capped square antiprism with an S(P) agreement factor of}

1.349. There are four GdIII_···CoII _{distances within the range 3.4745(7) - 3.5837(7) Å and four} CoII_···CoII _{distances in the range 3.148 (7) -3.366(7) Å. The distance from the “bow” and “aft”} between GdIII _{ions is 7.680 Å. There is no hydrogen bonding between adjacent entities, with the} spacing being well defined.

3.2.2 Topological aspects

According to this NDK-m nomenclature, the core of 15 - 19 can be enumerated as 2,3,4M6-1 (Figure 3.3). A literature survey indicates that the first example of this topology was found in a heterometallic Co2Na4 compound.369 _{The first 3d-4f PCCs of this topology were a family of} MnIII_4LnIII _{2 PCCs (2Mn : 2Ln :2Mn) supported by H3RL3.1, which were reported in 2008 by} Oshi et al.348 _{Since then, few organic ligands (Figure 3.4) have been used for the synthesis of a} several of 3d-LnIII _{PCCs with this topology.}

Ligands H3RL3.2, H3RL3.3 and H3RL3.4 gave access to MnII2LnIII4 (1Mn: 4Ln: 1Mn)116 and MnIII_2MnII_2LnIII_{2 (1Ln: 1Mn}III_{: 2Mn}II_{: 1Mn}III_{: 1Ln).}350 _{In 2009, N-butyl-diethanolamine} (H2RL3.5) was used to isolate a MnIV_2CeIV_{4 (1Mn: 4Ce: 1Mn)}349 _{PCC, while recently its use has} resulted in the formation of a FeIII_4DyIII_{2 (2Fe : 2Dy : 2Fe) PCC.}82 _{Ligands H3RL3.8 and H}

2L1 afforded NiII_4LnIII_{2 (1Ln : 4Ni : 1Ln)}343 _{and (2Ni : 2Dy : 2Ni)}55 _{PCCs respectively.}

More recently, four series of CoII/III_-LnIII _{PCCs that possess the 2,3,4M6-1 topology have been} described. The first example is a CoIII_2DyIII_{4 PCC based on the (E)-2-((3-} hydroxypropylimino)methyl)phenol ligand (H2RL3.7, Figure 3.4) which displays a slow

magnetization relaxation.351

The second example is a family of eight CoIII_2Ln4III _{PCCs constructed from HRL3.8 (Figure 3.4).} The DyIII _{analogue exhibits slow relaxation of magnetization with a Ueff = 3.8 K and τ0 = 4.8 x} 10-6 _s.352 _{The third example is a series of four Co}II_2LnIII_{4 PCCs constructed from ligands H2RL3.9} and HRL3.10 (Figure 3.4) reported by Du et al. in 2014,115 _{this example demonstrates the} importance of Ln – O – Ln bond angles on the magnetic coupling between the centres. The fourth example is a family of CoII_2LnIII_{4 PCCs derived from 6,6′-{(2-(dimethylamino)ethyl} azanediyl)bis(methylene)}bis(2-methoxy-4-methylphenol) (H2RL3.11, Figure 3.4), which

exhibits a slow magnetic relaxation behaviour for the DyIII _analogue.204

In all four reported families the CoII_-LnIII _{ratio is 2/4 and all four Ln}III _{are close together, unlike} the compounds reported in this work (15 – 19), (1Ln: 4Co: 1Ln)), where the “bow” and “aft” positions are filled by CoII _{cations (1Co: 4Ln: 1Co). Compounds 15 – 19 are therefore the first} examples of the CoII_4LnIII_{2 core configuration.}

Figure 3.4. The protonated form of the organic ligands used to for the synthesis of 3d-4f PCCs

possessing 2,3,4M6-1 topology.

It is worth noting that despite H2L1 offering a similar coordination environment to H2R3.9, its employment in CoII_-LnIII _{chemistry resulted in a hexanuclear PCC with 2/4 Co/Ln ratio. This} difference may be attributed to the in situ synthesis of ligand H2RL3.9.115

To further confirm the structural stability of the CoII_4LnIII_{2 PCCs reported herein, the organic} ligand H2L5 366 _{was applied under similar reaction conditions towards the synthesis of a Co}II_-DyIII PCC. Ligand H2L5 offers similar coordination environment to H2L1. The reaction resulted in compound 19 (Figure 3.5) which is isoskeletal to 15 – 18.

3.2.3 ESI-MS studies

To confirm the identity of the reported compounds in solution, they were identified by ESI-MS. For 16, four peaks in the MS (positive-ion mode) were observed at m/z 553.6409, 810.9421, 819.4479 and at m/z 828.8667 which correspond to the fragments, [CoII_4LnIII_2(µ3- OH)2(L1)4(NO3)2(MeOH)2]3+_{, [Co}II_2GdIII_{2(µ3-OH)2(L1)4Cl2+2H]}2+_{, [Co}II_2GdIII_2(µ3- OH)2(L1)4Cl2+2H+H2O]2+ _{and [Co}II_2GdIII_{2(µ3-OH)2(L1)4Cl2+2H+2H2O]}2+_{, respectively} (Appendix B, S3.1-S3.4).

Figure 3.5. The molecular structure of compound 19. Colour code: GdIII_{, light blue; Co}II_{, pink;} C, white; N, pale blue; O, red; Cl, green; Br, brown. Hydrogen atoms omitted for clarity.

3.2.4 TGA

TGAs of compounds 16 and 18 (Appendix B, S3.5) confirm their solvent sensitivity (Et2O and MeOH) and degradation of the core begins at 300 o_{C and 260} o_{C, respectively. The final residue} at 1000 o_{C perfectly corresponds to a mixture of metal CoO/Gd2O3 (34.10% expected - 34.22%} found) and CoO/Tb4O7 (33.31% expected - 33.22% found) oxides.

3.2.5 Magnetic studies

Variable-temperature dc magnetic susceptibility data was collected for compounds 15 - 18 in the temperature range 1.8 – 300 K in an applied field of 0.1 T. The data is shown as χMT vs. T plots

in Figure 3.6. The χMT product of compound CoII_4YIII_{2 (15) has a room temperature value of 11.13} cm3 _mol-1 _{K which is typical for four uncoupled high spin Co}II _{ions, but larger than that for four} free S = 3/2 spins (χT = 7.50cm3 _{K mol}-1 _{and g = 2.0) as a result of the presence of a significant} spin-orbital contribution in the susceptibility of octahedral high-spin CoII_.15,356,370–373 _Upon lowering the temperature the value of χMT gradually decreases and reaches a value of 2.17 cm3 _K mol-1 _{at 1.8 K (Figure 3.6).}

In the case of PCCs containing octahedral CoII _{ions, the decrease of the χMT at low temperature} can be attributed to the important spin-orbital contribution and dominant antiferromagnetic interaction between the four paramagnetic centres. The value of χMT at low temperature is low

interaction between the four paramagnetic centres. The antiferromagnetic interaction is further confirmed by magnetization measurements at 2 - 5 K where the magnetization does not saturate at the high-field limit (5 T) and the experimental value of the magnetization at 2.0 K and 5 T, is 6.12 Nβ, which is low for four non coupled CoII _{ions (Appendix A, S3.1 and S3.2).}372

The room temperature χMT value for 16 is 27.94 cm3 _{K mol}−1 _{which is slightly higher than the} expected value for two non-interacting GdIII _{(free ion; S = 7/2; g = 2)}373 _{and four Co}II _{centres as} in compound 16. The difference between the χMT value of 16 and 15 at 300 K is 17.51 cm3 _K mol−1 which is higher of the theoretical expected for two GdIII _{ions (15.75 cm}3 _{K mol}−1_{). This can} be justified by the presence of ferromagnetic interaction (in conjunction with antiferromagnetic coupling in CoII_{4 core) as well as the small variation of g factor for octahedral Co}II_{. By decreasing} the temperature, the χMT product decreases steadily up to a value of 27.15 cm3 _mol-1 _{K at 30 K.} Upon lowering to 30 K, a rapid increase of the χMT value is observed indicating the presence of

ferromagnetic interaction in 16.

The temperature dependence difference between the χMT value of 16 and χMT value of 15 (see black stars in Figure 3.6) suggest that the ferromagnetic interaction is operated between the Gd ions and Co4 core. The field dependence of magnetization of compound 16 confirms the presence of important (non-zero) spin state with a significant component of anisotropy.

The room temperature χMT values for 17 (Co4Dy2) and 18 (Co4Tb2) (39.62 and 36.46 cm3 _{K mol}−1_, respectively) are slightly higher than the expected values for two non-interacting DyIII ₍6_{H15/2 free} ion; S = 5/2; L = 5; J = 15/2; gJ = 4/3), Tb (7_{F6 free ion; S = 3; L = 3; J = 6; gJ = 3/2) and four Co}II centres.149,163 _{Similar to the Gd analogue, the temperature dependence of χMT values have a} minimum at 25 K and 7 K for 17 and 18, respectively. Upon a further decrease in temperature, the χMT products increase to reach the values of 44.07 and 33.61 cm3 _{K mol}−1 _{for 17 and 18,} respectively. This behaviour suggests the presence of some ferromagnetic interaction between CoII_{4 core and lanthanides ions in 17 and 18.}

Figure 3.6. Temperature dependence of the χMT product at Hdc = 0.1 T for compounds 15 – 18.

The dynamic properties of 15 - 18 have been investigated using ac susceptibility measurements as a function of temperature at varying frequencies and at different temperatures as a function of frequency at 4.0 Oe oscillating field between 1 and 1500 Hz.

Compounds 15, 16 and 18 do not show an out-of-phase signal at zero dc magnetic field. No modifications in out-of-phase susceptibility after applying the dc magnetic field (0 - 3.0 T) are detected for 15, 16 and 18. In the case of compound 17, at Hdc = 0, the magnetic field ac measurement clearly shows the presence of an out-of-phase signal χ’’ in ac susceptibility with strong temperature and frequency dependence (Figure 3.7).

Figure 3.7. Temperature dependence of the in-phase (’) (upper) and out-of-phase (”) (lower)

ac susceptibility for 17 at indicated frequencies at zero applied dc field.

Figure 3.8. Arrhenius plot Ueff (17) = 13.4 K (τ0 = 8.5×10-7 _{s) at zero applied dc field.}

0.0 5.0 10.0 15.0 20.0 25.0 30.0 1.0 3.0 5.0 7.0 9.0 11.0 χ' (cm 3m ol -1) T (K) Hdc=0 Oe 1 Hz 10 Hz 100 Hz 200 Hz 400 Hz 800 Hz 1000 Hz 1200 Hz 1500 Hz 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 1.0 3.0 5.0 7.0 9.0 11.0 χ' '(cm 3m ol -1) T (K) Hdc=0 Oe 1 Hz 10 Hz 100 Hz 200 Hz 400 Hz 800 Hz 1000 Hz 1200 Hz 1500 Hz -9.5 -8.5 -7.5 -6.5 0.3 0.4 0.5 0.6 ln (τ ) 1/T (T-1₎ ln(1/τ) Linéaire (ln(1/τ))

The effective barrier of slow magnetic relaxation, Ueff ,is obtained according to the Arrhenius law from temperature (Figure 3.8) measurements (Ueff (17) = 13.4 K τ0 = 8.5×10-7 _{s).The ac} susceptibility follows the generalized Debye model.153,181 _{Simultaneous fitting of χ'(ν), χ''(ν) and} Cole-Cole plot are shown in the Appendix A (Figure S3.3). The relatively large value of α (0.015 - 0.388) indicates that more than one relaxation process generated by QTM might be operational at this condition.

3.3 Conclusion

The synthetic strategy of using the dianionic ligand H2L1 in CoII_-LnIII _{chemistry resulted in a} family of hexametallic CoII_4LnII_{2 (15 – 18) PCCs possessing a 2,3,4M6-1 topology. After an} extensive review, compounds 15 – 18 were determined to be the first CoII_4LnIII_{2 examples of this} topology. Magnetic studies of 15 – 18 reveal the presence of ferromagnetic and antiferromagnetic interactions and interestingly, compound 17 shows SMM behaviour.

The use of H2L5, which provides the same coordination pockets as H2L1, with a similar synthetic strategy that afforded 15 – 18, results in the formation of compound 19 which is isoskeletal to CoII_4LnIII_{2 series. In contrast when H2RL3.9, which provides similar coordination modes similar} to those of H2L1, was employed in CoII_-LnIII _{chemistry a PCC with the 2,3,4M6-1 topology but} with a different core configuration is observed.

Future studies will be focused in two directions:

a) To perform further systematic synthetic studies which employ organic ligands with similar coordination pockets to H2L1, H2L5 and H2RL3.4 in CoII-LnIII chemistry, to afford hexanuclear PCCs bearing 2,3,4M6-1 topology and determine how ligand structure determines the core configuration.

b) To further develop the topological approach353 _{by including valuable information for the} synthesis of a specific topology.