Koushik Saha

Sourav Gayen

Urminder Kaur

Thierry Roisnel

Sundargopal

Sundargopal Ghosh

Stabilization of dichalcogenide ligands in the coordination sphere of ruthenium system

published or not. The documents may come

Stabilization of dichalcogenide ligands in the coordination sphere of a ruthenium system

Introduction

A continuous interest in developing the transition-metal dichalcogenide complexes is observed due to their use as suitable precursors for generating binary metal chalcogenide quantum dots and thin films. 1 The transition-metal dichalcogenide complexes are comparatively less explored than the transition metal chalcogenide complexes and the field of dichalcogenide complexes was developed rapidly in the last two decades. 2 The well-established method to prepare these species is the oxidative transfer of the chalcogen source to metal complexes. For example, complex [{(OC)3Fe}4(μ-S2)(μ-SPh)2] 3a was isolated via the SO2Cl2 oxidation of [{(OC)3Fe}2(μ-S)(μ-SPh)]Li (Chart 1). 3 Whereas, thermolysis of the chromium sulfide complex [(CpCr)2(μ-S t Bu)2(μ-S)2(NO)2(Cl)2] led to the formation of [(CpCr)4(NO)3(Cl)(μ-S t Bu)2(μ-S2)], 3c . Other transition metal dichalcogenide complexes featuring {M2E-EM2} core, listed in Chart 1, were also synthesized using different synthetic strategies. 3 It was observed that the ancillary ligand/s (such as Cp, Cp* etc.) stabilize the metal-dichalcogenide complexes. Thus, the metal centers bearing ancillary ligands have been used to stabilize the dichalcogenide complexes. On the other hand, it becomes apparent that the sterically bulky and electronegative ancillary ligands, such as Cp*, are not mandatory to stabilize the lanthanide chalcogenide complexes having Ln-E bonds. 4 Therefore, inspection on the reactivity of other ancillary ligand/s (such as p-cymene, cod (η 4 -C4H8), etc.) containing transition metal complexes become of particular interest to synthesize transition metal dichalcogenides. A detailed investigation would be advantageous to comprehend the prerequisite conditions desired to stabilize the dichalcogenide ligands in the coordination sphere of the transition metal centers.

In an attempt to isolate the chalcogen-rich transition metal species, the reactivity of chalcogenated ligands Li[BH2E3] 5 (E = S, Se and Te), were investigated. 6 In our earlier report, we have effectively substantiated the utility of Li[BH2E3] (E = S or Se) to isolate metal chalcogenide complexes of molybdenum, [(OC)2MoCp*(η 2 -SC(Me)S)] and [(OC)2MoCp*(η 1 -SeEt)]. 6a Further, the reaction of [Cp*MCl4] (M = Nb and Ta) with Li[BH2E3] (E = S, Se, or Te) yielded various metal chalcogenide complexes along with chalcogen rich trimetallic cubane type complexes. 7 Therefore over the years, the interest have grown to investigate the reactivity of Li[BH2E3] with heavier metal precursors. 8 To investigate the effect of the ancillary ligand as well as chalcogenide ligand on the stability of metal-chalcogenide complexes of ruthenium, we chose [(p-cymene)Ru(μ-Cl)Cl]2 as a suitable metal precursor. Herein, we have studied the syntheses and structural characterizations of dichalcogenide complexes and bis/tris-homocubane analogues of ruthenium.

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Results and Discussion

Synthesis of tetrametallic dichalcogenide complexes, [{Ru2Cl2(p-cymene)2}2(μ4,η 2 -E2)] (E = S or Se)

After the isolation of metal-chalcogenide complexes of early transition metals using Li[BH2E3] (E = S or Se) ligand, we applied the same approach with late transition metals. As a result, the room temperature reaction of [ClRu(pcymene)(μ-Cl)]2 with Li[BH2S3] was performed. The color of the reaction mixture turned brown from red during the progress of the reaction. After the chromatographic separation, a red solid, [{Ru2Cl2(p-cymene)2}2(μ4,η 2 -S2)] 1 was obtained in 6% yield. Note that, some air and moisture sensitive compounds were also formed in this reaction which could not be isolated due to the lower yields and unstable nature. Due to complexity of the reaction, presently we don't have clear idea about the main product/s and we were only able to isolate complex 1. In case of selenium, the reaction of [ClRu(μ-Cl)(p-cymene)]2 and Li[BH2Se3] under similar reaction conditions permitted us to isolate red [{Ru2Cl2(p-cymene)2}2(μ4,η 2 -Se2)], 2 and orange [Ru(p-cymene){Se7(BH)3}], 3 in 5 and 38% yields respectively (Scheme 1). The reaction of [ClRu(p-cymene)(μ-Cl)]2 and Li[BH2Te3] led to decomposition of the starting materials. The 1 H and 13 C{ 1 H} NMR chemical shifts confirmed the presence of p-cymene units in 1 and 2.

The solid-state structure of 1 revealed the identity as a tetraruthenium disulfide complex where a disulfide unit bridges two {(p-cymene)Ru(Cl)}2 moieties (Fig. 1). Complex 1 resides in a general position in the monoclinic space group P21/c as similar to that in [{Fe2(CO)6(μ4-E)Re2(CO)8(μ-PCy2)}2(μ4,η 2 -S2)] 9 and the two asymmetric moieties i.e. {(p-cymene)Ru(Cl)}2S are arranged in a triangular geometry. All the Ru atoms in 1 are in a distorted octahedral environment. Two {(p-cymene)Ru(Cl)} fragments in 1 are held together through a Ru-Ru single bond (2.959(2) Å). The metal-metal bond distance in 1 is similar to that of arachno-[(Cp*Ru)2B3H8(CS2H)] 10 (2.9738(3) Å) but longer compared to the bridging boryl borylene complex of ruthenium 11 (2.4608(10) Å). The {Ru2SCl} units in 1 are planar and similar to the {Cr2S2Cl} unit in [Cl(CpCr)4(NO)3(μ-S t Bu)2(μ4,η 2 -S2)] 3c . Complex 1 contains two planar {Ru2S} units, which are linked by a disulfide bond and with a dihedral angle of 16.83° between the two {Ru2S} planes. The bridging disulfide ligand in 1 is neither vertical (as that in [Fe3(CO)8(Ph2PCS)}2(μ4,η 2 -S2)] 3b ) nor parallel as in case of [{(OC)6Fe2(μ-SEt)}2(μ4,η 2 -S2)] 12 ) to the perpendicular axes of the two {Ru2S} subunits. The S-S bond distance (2.1089(8) Å) in 1 is slightly elongated as compared to the S-S bonds in S8 molecule (2.037(5) Å). However, the S-S bond distance in 1 is comparable to [{Fe2(CO)6(μ-SC6H5)}2(μ4,η 2 -S2)] 3a (2.106(3) Å) and slightly shorter than [{Fe2(CO)6(μ-S)}2(μ4,η 2 -S2)][K(benzo-15-crown-5)2]2 3f (2.1657(17) Å). Two acute ∠Ru-S-Ru angles (78.30(2) and 78.52(2)°) confirm that the S2 2-ligand are positioned with respect to the metal centers in almost orthogonal direction. The ruthenium-sulfur bond lengths in 1 (avg. 2.325 Å) fall in the region of single bond lengths and comparable with that in [(Cp*Ru)2(μ,η 3 -CHS)2] 13a (2.3733 Å) and [{Ru(p-cymene) Interestingly, similar types of metal complexes with E-E bond are known in the literature, and some of them are listed in Table 1 . 3a,3g,3h,12,14 Having a good number of complexes, similar to that of 1, allowed us to compare the structural parameters of 1 with relevant species. As shown in Table 1, the E-E bond distances are within the range of 2.059(3)-2.2573( 12) Å (for sulfur), 2.3803(7)-2.416( 4) Å (for selenium) and 2.8319( 14) Å (for tellurium) corresponding to the single E-E bond distance. It is interesting to note that, in [{Fe2Se(CO)6}2-(μ4,η 2 -Se2)(PPh4)2] 14a , the Se-Se separation of 2.847(5) Å is merely an interaction rather than a single bond. As a result, the E-E distance of these species provide a preliminary indication of the true nature of the E-E interaction. As presented in Table 1, it is apparent that the substitution at metal center directly affects the E-E bond distance that generally depends on the electronic nature of the substituents associated with the metal center. On the other hand, the M-M bond distance of these complexes directly depends both on the substituents at metal centers and strength of the chelation between dichalcogenide unit/s and metal centers. Limited availability of the examples of M4E2 complexes containing an E-E bond made complexes 1, 2 a prominent entry in the metal dichalcogenide complexes. Although, many examples of the tetrametallic complexes featuring an E-E bond are known, lack of theoretical investigations casted uncertainty on the bonding of these complexes. Therefore, the density functional theory (DFT) calculations were carried out to analyze the electronic structures of the tetrametallic dichalcogenide complexes 1, 2 and 2' (Te analogue of 1) using B3LYP functional and SDD/6-31G basis set. The symmetrical X-ray structure of 1 has slightly been distorted after the optimization and as a result, the Ru3-Ru4 bond distance was lengthened to 3.074 Å. However, the Ru1-Ru2 bond distance remained almost same (2.940 Å). Although, a bond critical point between the Ru-Ru bond is not observed, the HOMO and LUMO of the complexes 1, 2 and 2' showed bonding interactions between the ruthenium atoms of the {(p-cymene)2Ru2(Cl)2E} units (Figs. 2a, 2c and S16). The NBO calculations revealed the WBI (Wiberg bond indices) values in the range of 0.413-0.293 for these Ru-Ru bonds which is typical for the formal single bonds between transition metals (0.2-0.3) [START_REF] Lupan | Ch 1. Metal-Metal Multiple Bonding in Dimetallaboranes in Handbook of Boron Science[END_REF] and similar to that diruthenium complexes [(RuCp*)2{μ-C(Me)C6H4B-κ 2 C:κ 2 B}(μ-HBCH2S2-κ 2 S:κ 2 B)] 11 (0.3753) and [(Cp*-Ru)2(μ,η 2 -C2 t Bu2)(μ,η 2 -P2)] [START_REF] Rödl | [END_REF] (0.26). Based on the bond distances and WBI values of Ru-Ru separation, we believe the presence of a Ru-Ru single bond in 1, 2 and 2'. The MO analyses of these complexes suggest that the HOMO-LUMO gaps follow the order of 1 < 2 < 2' (0.073 (1), 0.072 (2) and 0.069 eV (2'), Fig. S16) and are considerably smaller as compared to the reported disulfide species [(Cp*Ru)2(μ,η 2 :η 2 -S2)(μ,η 1 :η 1 -S2)] 11 (3.115 eV).

[(µ-SCH2SHB-κ 2 B:κ 2 S)(RuCp*)2{µ-C(Me)(C6H4)B- κ 2 B:κ 2 C}]
}2Ru- (μ3,η 2 :η 2 -S2)(μ,η 1 :η 1 -S2)(μ-S2C2B10H10)2] 13b (2.3733 Å).
The HOMO-13 of 1, 2 and 2' showed the overlap between the p orbitals of the corresponding chalcogen atoms (Figs. 2b andS17) that indirectly verifies the presence of a strong E-E single bond. Further, the NBO analysis also indicate a strong E-E bonding interaction that indirectly verifies the presence of a strong E-E bond (Fig. S18). The topological analyses for 1 revealed the existence of bond critical point (BCP) between S1 and S2 (ρ(0.110 amu), ∇ 2 ρ(-0.042 amu)) atoms (Fig. 2d). A decreasing trend is observed in the WBI values of the E-E bonds for 1, 2 and 2' (1: 0.8096, 2: 0.7832 and 2': 0.7476). In addition, a series of strong bonding interactions between the Ru and E atoms have been observed that indirectly demonstrate the stabilization of the dichalcogenide ligands by the tetrametallic system (Fig. S18). The DFT calculations also elaborate the fact that the dichalcogenide ligand acts as a dianionic E2 2-species instead of neutral E2. As shown in Tables S2 andS3, a charge transfer from chalcogen centre to ruthenium centre has been observed. Thus, chalcogen atoms in 1-2' possess positive natural charges and Ru atoms possess negative natural charges. Thus, one would expect a positive increase in the value of natural charges on the chalcogen atoms of 1-2' as compared to the free E2 ligand. As a result, we have compared the natural charges on the sulfur atoms in 1 (qE = 0.290), free neutral disulfur (S2) (qE = 0.00) and the dianionic disulfide (S2 2-) (qE = -1.00) species. We have observed that the positive increase of natural charges on sulfur atoms is more prominent from S2 2-to 1 as compared to S2 to 1. A similar observation has also been found in case of 2 and 2' (Table S2). The electron donation from the dichalcogenide ligand to the metal centre has also been supported by the decrease of natural valance population of the E atom (pE) and increase in Ru atom simultaneously (Table S2). The extent of ligand to metal charge transfer follows a gradually increasing trend from 1-2' (Table S3). The WBI values of the E-E bonds for 1, 2 and 2' (1: 0.8096, 2: 0.7832 and 2': 0.7476) are closer to free E2 2-than neutral E2 species and follows a decreasing trend of 1 ˃ 2 ˃ 2' (Table S2). This fact indirectly suggests the presence of an E-E single bond. We believe this could be one of the main reasons for not able to isolate complex 2' from the reaction of [ClRu(p-cymene)(μ-Cl)]2 and Li[BH2Te3]. The detailed theoretical investigation on 1-2' shed light on the true nature of the E-E bond.

In order to know the nature of Ru2S triangular ring in 1, the investigation of aromaticity has been carried out by nucleus independent chemical shift (NICS) method. The Ru2S ring in 1 showed negative NICS values both for NICS(0) and NICS (1). The NICS(0) value is -58.22 which is quite higher as compared to the aromatic cyclic hydrocarbons. On the other hand, the NICS(1) value of -18.824. Thus, the Ru2S ring shows both the σ-and π-aromaticity.

Tris-homocubane analogue, 3

Along with the formation of 2, compound 3 was also isolated as an orange solid in 38% yield. This compound is characterized by multinuclear spectroscopy and single crystal X-ray crystallography. The 11 B{ 1 H} chemical shift that appeared at the downfield region (δ -7.2 ppm) indicate the presence of tetracoordinated boron atom. The 1 H NMR spectrum displayed a septet at δ 2.04 ppm for the methine proton of the p-cymene unit suggesting the possibility of one chemically distinct metal environment. The signal at δ 4.28 ppm is attributed to the BHt protons. The molecular ion [M+CH3CN] + peak at m/z 864.6 corresponds to the molecular formula of C12H20B3RuNSe7. This indicates that the three boron atoms may reside in similar positions and the molecule possesses a symmetry.

Further, the X-ray diffraction analysis was performed on a single crystal to elucidate the structure of 3. The solidstate X-ray structure of 3 revealed a monometallic tris-homocubane alike core formulated as [Ru(pcymene){Se7(BH)3}] (Fig. 3). Compound 3 is isostructural and isoelectronic with another tris-homocubane analogue [Cp*Co{Se7(BH)3}] 17 . Three selenium atoms in 3 are bridged between the edge-shared boron and selenium atoms of the tris-homocubane core. As a result, the bond separations between the boron and selenium are elongated (for e.g., B1 and Se5). All the boron atoms in 3 are in symmetrical tetrahedral environment and surrounded by three Se and one H atom/s. The average B-Se bond length in 3 (2.043 Å) is notably longer as compared to that of [(Cp*Mo)2B4Se2H4] 18 (1.923 Å). The average ∠Se-B-Se in 3 (108.3°) is slightly shorter than the tetrahedral angle (109.3°) with a maximum deviation of 3.6°. This confirms the tetrahedral nature of the boron atoms. On the other hand, the average B-E and E-E bond distances are in the range of corresponding single bond distance. Compound 3 can be considered as a tris-homocubane analogue of the pentacyclo-[6.3.0.02,6.03, 10.05.9]-undecane and possesses D3 symmetry. Note that, the known example of smallest rigid organic compound having D3 symmetry is the D3-tris-homocubane. On the other hand, compound 3 having a p-cymene unit as an ancillary ligand is prone to the arene displacement due to the labile nature of the η 6 -coordinated ligand and susceptible towards the ligand displacement reactions. 19 The cubanes and their derivatives are electron-precise molecules and generally they obey cluster valence electron (cve) count. The cve count for tris-homocubane C11H14 consisting of eight CH vertices and three CH2 bridges is 58 [8(CH)×5 + 3(CH2)×6 = 58]. Therefore, the tris-homocubane analogue comprising one transition metal should have the electron count of 68. Indeed, compound 3 has 68 cve [1{(p-cymene)Ru}×14 + 7(Se)×6 + 3(BH)×4 = 68].

Syntheses of diruthenium bis/tris-homocubane analogues, [(Cp*Ru)2{μ-Sn(BH)2}] (n = 7 and 6)

Recently, we have reported the synthesis of ruthenium disulfide 5)) with an ~21% combined yield along with [(RuCp*)2(μ,η 2 :η 2 -S2)(μ,η 1 :η 1 -S2)] and [(RuCp*)2(μ-SBHS-κ 1 B:κ 2 S:κ 2 S)(μ,η 1 :η 1 -S2)] (Scheme 2). These compounds were characterized by multinuclear spectroscopy and single crystal X-ray crystallography. The 1 H chemical shifts at δ 1.94 and 1.78 ppm for 4 and 5, respectively indicated the presence of two Cp* units and this was further confirmed by the 13 C{ 1 H} NMR spectrum.

[(RuCp*)2(μ,η 2 :η 2 -S2)(μ,η 1 :η 1 -S2)] and diruthenium boryl [(RuCp*)2(μ-SBHS-κ 1 B:κ 2 S:κ 2 S)(μ,η 1 :η 1 -S2)]
Similarly, the presence of three boron environments at δ -5.4, -12.2 and -12.7 ppm were observed in the 11 B{ 1 H} NMR spectrum. The mass spectra of them revealed molecular ion peaks at m/z 743.8735 ([M+Na] + ) and 719.9303 ([M+CH3OH] + ) corresponding to the molecular formulae C20H23B2S7Ru2Na and C21H36B2OS6Ru2. The spectrometric and spectroscopic data were not adequate enough to predict the structures of 4 and 5. Therefore, X-ray quality crystals were grown from the solution of hexane/CH2Cl2 (70:30). 10), Ru1-Se5 2.4767( 10), Ru1-Se7 2.4787( 11), Se1-Se2 2.3628( 12), Se4-Se7 2.3600( 12), Se5-Se6 2.3542( 12), Se1-B1 2.076(8), Se3-B1 2.065(8), Se6-B1 2.020(8), Se2-B2 2.013(8), Se3-B2 2.065(8), Se7-B2 2.073(8), Se3-B3 2.054( 9), Se4-B3 2.014( 9), Se5-B3 2.099(8); B2-Se2-Se1 89.6( 2), B3-Se4-Se7 90.2(2), B1-Se6-Se5 90.4(2).

Table 1. Structural parameters of transition metal dichalcogenide complexes containing {M2E-EM2} core.

Dichalcogenide complexes d(E-E) a d(M-M) a

∠M-E-M b Ref.

[{Fe2(CO)6(μ-SEt)}2(μ4,η As shown in the Fig. 4a, the single-crystal X-ray structures of 4 and 5 revealed a tris-homocubane and bishomocubane alike cores, respectively. The occupancy ratio of molecules 4 and 5 in the structure is 0.62:0.38. The structures of 4 and 5 match well with the spectroscopic and mass spectrometric data that displayed the presence of two trithioborate (HBS3) units. The presence of both types of sulfur atoms (two-coordinated sulfur and threecoordinated sulfur atoms) are observed and the two-coordinated sulfur atoms bridged over the three longer B-S edges in compound 4. The average S-S bond length of 2.059 Å in 4 is smaller as compared to that of 1 (2.1089(8) Å). The ruthenium atoms draw the sulfur atoms towards them more closely in 1 as compared to 4 and resulting the elongation of disulfide bond. The average ∠B-S-S in 4 is 106.93° and is consistent with the obtuse sulfur bridge. The square Ru1-S3-Ru2-S4 face is slightly deviated from the plane with a dihedral angle of 9.60°. The Ru-Ru bond distance of 2.7804( 5) Å is longer as compared to the ruthenium boryl complex [(Cp*Ru)2(μ,η 2 :η 2 -BHS)(μ-SCH2SBH)] 13b (2.6388(10) Å) but smaller than the ruthenaborane complex [(RuCp*)2{μ-Se6(BH)2}] 20 (2.877(4) Å). On the other hand, compound 5 resembled the 1,3-isomer of [(RuCp*)2{μ-Se6(BH)2}] 20 , in which six sulfur atoms are two and three coordinated. The average S-S bond distance of 2.0275 Å of the disulfide units is shorter than S8 (2.037(5) Å). Two boron centers in 5 are tetrahedrally bonded by three sulfur and one hydrogen atoms. The Ru1-Ru2 bond distance of 2.7804(5) Å in 5 is shorter as compared to that of another bis-homocubane analogue [(RuCp*)2{μ-Se6(BH)2}] 20 (2.877(4) Å). The average B-S distance in 5 (1.9518 Å) is shorter than the boryl complex [(RuCp*)2(μ-SBHS-κ 1 B:κ 2 S:κ 2 S)(μ,η 1 :η 1 -S2)] 11 (1.8235 Å). Unlike 1,3-bis-homocubane isomer core, the core of 1,4isomer comprising ruthenium, boron and chalcogen atoms are exceptional and therefore 5 is a distinguished entry to the family of bis-homocubane derivatives. 11), B1-S7 1.868( 6), B2-S2 1.866(13), B2-S3 2.095( 12), B2-S6 1.866( 14), S1-S2 2.013(2), S5-S6 2.052( 14), S4-S7 2.1130( 15), Ru1-S1 2.3637 (11), Ru1-S3 2.2976 (11), Ru1-S4 2.2927 (10), Ru2-S3 2.3049 (11), Ru2-S4 2.2875(10), Ru2-S5 2.356( 12), Ru1-Ru2 2.7804( 5), S1-B1-S5 108.8( 5), S1-B1-S7 106.6(3), S5-B1-S7 104.8 (5). 5: B1-S1' 1.932( 5), B1-S5' 1.91( 2), B2'-S1' 2.165( 16), B2'-S5' 2.02(3), B2'-S8' 1.816 (17), S3-S8' 1.942( 5), Ru1-S1' 2.3637 (11), Ru2-S5' 2.39(2); S1'-B2'-S5' 92.5(8), S5'-B2'-S8' 114.0 (10). In order to understand the bonding situation, the DFT based theoretical calculations were performed for 3, 4' and 5' (4' and 5' are the Cp analogues of 4 and 5, respectively). The experimental data were compared with the theoretical ones of 3, 4' and 5' that revealed similar bond lengths. The molecular orbital (MO) analysis revealed that the nonbonding d orbitals of metal and p orbitals of two-coordinated Se atoms were localized in HOMO and HOMO-3 of compound 3 (Fig. 5a). The HOMO of 4' displayed the presence of non-bonding metal d orbitals along with two types of interactions. For example, (i) σ-bonding interaction between the p orbitals of two-coordinated S and B atoms and (ii) the π-antibonding interaction between the two-coordinated S and three-coordinated S atoms (Fig. 5b). In addition, the σ-antibonding overlaps along the three disulfide bonds were observed in LUMO+6 of 4' (Fig. 5c). In case of 5', the HOMO-1 revealed the presence of nonbonding orbitals on each sulfur atoms where the size of the lobe of two connected atoms were bigger than three-coordinated S atoms. Considering all these interactions, we believe that the two-coordinated E atoms in 3, 4' and 5' are more potential to react with electrophiles as compared to the three-coordinated E atoms. Along with the localized d orbitals of ruthenium centers, delocalization of the electron density over the non-planar S-B-S-B square face was observed in HOMO of compound 5'. Likewise, the LUMO of 5' also comprised an extrapolated π-antibonding interaction among the d and p orbitals of the metal and sulfur atoms, respectively throughout the Ru-S-Ru-S square face (Fig. 5d). These interactions play a prominent role when any kind of bond making or bond breaking has been taken place due to further inclusion of the ligands.

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The theoretical calculations suggest that 3 can react through its labile S-S bonds, whereas 4 and 5 can react both through its square faces and reactive disulfide bonds. In order to compare the structural properties of the bis/tris-homocubane analogues and find out the influence of chalcogen atoms on the stability of these species, we have listed the spectroscopic and structural parameters of some selected bis-and tris-homocubane analogues in Table 2. 17,20,21 Among the bis-homocubanes analogues, the 1,3-isomer of bis-homocubanes containing metal, chalcogen and boron atoms is generally known. However, compound 5 is a unique example of bis-homocubane analogue which was isolated as a 1,4-isomer. On the other hand, the tris-homocubane analogues (3 and 4) are observed as 1,3,3-and 1,2,3-isomers of the tris-homocubane analogues. The organic bis-homocubane (C10H12) and tris-homocubane (C11H14) are electron-precise compounds with the valence electron counts of 52 and 58, respectively. Generally, the main group element and the transition metal follow the octet and 18-electron rule, respectively for these cubane derivatives. 22 Most of the compounds listed in the Table 2 possess The dichalcogenide complexes 1 and 2 show intense red color which can be explained by UV-Vis absorption spectroscopy. The UV-Vis absorption spectra for 1-5 were recorded in CH2Cl2 that show three absorption bands each (Fig. 6). An intense high energy absorption band around 291 nm is observed for both 1 and 2, which is assigned for the spin-allowed π-π* ligand-centered transitions localized in the p-cymene ligands. 23 On the other hand, the low energy absorption bands in the range of 315-484 nm for 1 and 2 correspond to the spin-allowed metal to ligand charge transfer (MLCT) transitions. 24 The ligands in 1 and 2 might have allowed an extended delocalization of the acceptor orbital of MLCT excited states. The intense lower energy peak at 529 nm for 1 can be useful for optical limiting applications. 25 The energy of the HOMO associated with the metal centers followed a decreasing trend from 1 to 2 and that is reflected in the blue-shifted UV-Vis spectrum of 2. Moreover, the high-energy absorption bands around 230-270 nm correspond to the π-π* transition for the p-cymene and the Cp* ligands for 3 and 4/5, respectively. Fig. 6 displayed an intense absorption around 500 nm for 1-2 and this adsorption due to the interaction between the Ru2 centres through the bridging dichalcogenide ligand. The low-energy bands around 341 and 436 nm were also observed for 3 and 4/5, respectively, that may be assigned to the charge transfer bands.

Conclusions

In this article, we have synthesized different ruthenium dichalcogenide species using different synthetic methods.

We have efficiently demonstrated the reactivity of Li[BH2E3] (E = S or Se) with [(p-cymene)Ru(μ-Cl)Cl]2 that generated tetrametallic dichalcogenide complexes. In these complexes, the dichalcogenide (E2 2-) unit is stabilized by {(p-cymene)Ru}4 unit. The detailed theoretical investigation on 1-2' explicitly revealed the bonding and shed light on the true nature of the E-E bond. The theoretical studies revealed the dianionic nature of the E-E ligand in these tetrametallic complexes and also confirmed the presence of subtle metal-metal bonding interactions in these complexes which indicated lability of the Ru-Ru bonds. The intrinsic electronic properties of these species were demonstrated by UV-Vis spectroscopy that reveals a low-energy absorbance due to MLCT transitions. Investigations to evaluate the scope of synthesis of dichalcogenide complexes concerning early transition metals are underway.

Experimental Details

General procedures and instrumentation

All the syntheses were performed under argon atmosphere using standard Schlenk line and glove box techniques and all the solvents were purified by distillation method in presence of proper drying agents. CDCl3 was degassed through three freeze-pump-thaw cycles. The starting materials [(p-cymene)RuCl2]2, 26 [Cp*RuCl2]2, 27 and Li[BH2E3] 5 were synthesized following the corresponding literature. All the syntheses reported here are reproducible. These reactions were performed multiple times which gave same product distribution. Thin-layer chromatography was performed using 250 mm aluminium supported silica gel TLC plates. Chemical shifts are referenced to residual solvent signals ( 1 H/ 13 C{ 1 H}; CDCl3: δ = 7.26/77.16 ppm). For the 11 B{ 1 H} NMR, [Bu4N(B3H8)] 28 prepared in d6-benzene was taken in a sealed tube (δB, ppm, -30.07) and used as an external reference. The 6545 Qtof LC/MS instrument was used for recording mass spectra. The infrared spectra and UV-vis spectra of all compounds were recorded on a Jasco FTIR-4100 spectrometer and a Thermo Scientific Evolution UV-vis spectrometer, respectively.

Synthesis of 1

A freshly prepared solution of four equivalents [LiBH2S3] was added dropwise in the toluene (20 mL) solution of [(pcymene)Ru(μ-Cl)Cl]2, (0.05g, 0.082 mmol), taken in a flame dried Schlenk tube, over 15 min at -78 °C. The reaction mixture was stirred vigorously and kept in room temperature for 6 hours. Then, the solvent was dried using vacuum and the solid deposit was extracted using the hexane/THF (95:5 v/v) solution mixture and ran through a G3 sintered frit partially filled with celite. Then, the volatiles were removed under vacuum and the chromatographic workup was carried out on a silica gel TLC plates using the solid residue. The hexane was used as an eluent and that allowed us to isolate red 1 (0.006 g, 6%). 3, 108.3, 107.7, 94.9 (s; Cp-cym), 93.3, 89.4, 88.7, 88.4, 82.3, 81.4, 80.4, 78.3 (s; HCp-cym), 32.9, 31.7 (s; HCiPr), 25.1, 25.0 (s; H3CiPr), 20.4, 19.6 ppm (s; H3C); UV-Vis (CH2Cl2, λ, nm): 291, 315, 463, 529.

Synthesis of 2 and 3

A freshly prepared solution of four equivalent [LiBH2Se3] was added dropwise in the toluene (20 mL) solution of [(pcymene)Ru(μ-Cl)Cl]2, (0.05g, 0.082 mmol), taken in a flame dried Schlenk tube, over 15 min at -78 °C. The reaction mixture was stirred vigorously and kept in room temperature for 6 hours. Then, the solvent was dried using vacuum and the solid deposit was extracted using the hexane/thf (95:5 v/v) solution mixture and ran through a G3 sintered frit partially filled with celite. Then, the volatiles were removed under vacuum and the chromatographic workup was carried out on a silica gel TLC plates using the solid residue. The hexane was used as an eluent and that allowed us to isolate red 2 (0.005 g, 5%) and orange 3 (0.026g, 38%).

Syntheses of 4 and 5

A freshly prepared solution of four equivalents [LiBH2S3] was added dropwise in the toluene (20 mL) solution of [Cp*Ru(μ-Cl)Cl]2, (0.05g, 0.081 mmol), taken in a flame dried Schlenk tube, over 15 min at -78 °C. The reaction mixture was stirred vigorously and kept in room temperature for 24 hours. Then, the solvent was dried using vacuum and the solid deposit was extracted using the hexane/CH2Cl2 (70:30 v/v) solution mixture and ran through a G3 sintered frit partially filled with celite. Then, the volatiles were removed under vacuum and the chromatographic workup was carried out on a silica gel TLC plates using the solid residue.

Computational Details

All molecules were fully optimized with the Gaussian 16 program 29 using the gradient-corrected B3LYP functional in conjunction with the 6-31g(d)-SDD basis set from EMSL Basis Set Exchange Library. The model compounds were fully optimized in gaseous state (no solvent effect) starting from the X-ray crystallographic coordinates. Frequency calculations were performed at the same level of theory to verify the nature of the stationary state and the absence of any imaginary frequency confirmed that all structures represent minima on the potential energy hypersurface. Further, gauge including atomic orbital (GIAO) 30 method has been employed to compute the 11 B chemical shifts. The NMR chemical shifts were calculated using the hybrid Becke-Lee-Yang-Parr (B3LYP) functional 31 and 6-31g(d)-SDD basis set on the B3LYP/6-31g(d)-SDD optimized geometries. The 11 B NMR chemical shifts were calculated relative to B2H6 (B3LYP B shielding constant 80.05 ppm) and converted to the usual [BF3.OEt2] scale using the experimental δ( 11 B) value of B2H6, 16.6 ppm. 32 Natural bonding analyses were performed with the natural bond orbital (NBO) partitioning scheme as implemented in the Gaussian 16 suite of programs. 33 Wiberg bond indexes (WBI) were obtained on natural bond orbital analysis. In order to understand the nature of bonding in the synthesised molecules in greater detail, the topological properties of the resultant electron density, ρ, obtained from the wave functions of all the optimized structures were analyzed with the quantum theory of atoms in molecules (QTAIM). [START_REF] Bader | Atoms in Molecules: a Quantum Theory[END_REF] The QTAIM analysis were carried out utilizing Multiwfn V.3.6 package 35 whereas the wave functions were generated with Gaussian09 at the same level of theory as was used for geometry optimization. All the optimized structures and orbital graphics were generated using the Gaussview 36 and Chemcraft 37 visualization programs.

ACCEPTED MANUSCRIPT

X-ray Analysis Details

Suitable crystals were grown through slow diffusion process of respective hexane-CH2Cl2 solutions of 1, 3 and 4, 5.

D8 VENTURE Bruker AXS diffractometer with graphite monochromated Mo-Kα (λ = 0.71073 Å) radiation was used for collection and integration of crystal data of 1 at 150(2) K. Bruker AXS Kappa APEX2 CCD diffractometer with graphite monochromated Mo-Kα (λ = 0.71073 Å) radiation was used for collection and integration of crystal data of 3 at 296(2) K. Bruker APEX2 CCD diffractometer with graphite monochromated Mo-Kα (λ = 0.71073 Å) radiation was used for collection and integration of crystal data of 4 and 5 at 150(2) K. The structure of 1 was solved by heavy atom methods using SHELXT-2015 and refined using SHELXL-2018/3. [START_REF] Sheldrick | [END_REF] The structure of 3 was solved by heavy atom methods using SIR92 39 and refined using SHELXL-2014/7. The structures of 4 and 5 were solved by heavy atom methods using SHELXS-97 and refined using SHELXL-2018/3. Olex 2 software was used for drawing the molecular structures. 40 The non-hydrogen atoms were refined with anisotropic displacement parameters. All the hydrogen atoms were fixed geometrically at chemically meaningful positions and were allowed to refine by riding model restraint. The C-H distances used for fixing the hydrogen atoms of methine, aromatic and methyl groups are 1.0, 0.95, 0.98 Å respectively. The thermal parameters of methine and aromatic hydrogen atoms were assigned 1.2 times the Ueq of the respective carbon atoms while the methyl hydrogen atoms were assigned 1.5 times Ueq of the parent carbon. The distances B2-H2 and B2'-H2' of cocrystal 4 and 5 are fixed at 1.15(0.02) Å while all other B-H hydrogen atoms were allowed to refine with their thermal parameters restrained to be equal to the Ueq of the respective boron atoms within the limits of standard deviations.

Chart 1 .

 1 Selected examples of transition metal dichalcogenide complexes featuring an E-E bond (E = S, Se or Te). All the bond distances mentioned are the E-E bond distances.

Fig. 1

 1 Fig. 1 Molecular structure of 1. Selected bond lengths (Å) and angles (°) are: S1-S2 2.1089(8), Ru1-Ru2 2.9392(3), Ru3-Ru4 2.9395(4), Ru1-S1 2.3202(6), Ru2-S1 2.3350(6), Ru3-S2 2.3298(6), Ru4-S2 2.3150(6); Ru1-S1-Ru2 78.30(2), Ru4-S2-Ru3 78.52(2), S1-S2-Ru3 110.03(3), S1-S2-Ru4 115.03(3).

Fig. 2

 2 Fig. 2 (a) HOMO, (b) HOMO-13 and (c) LUMO of 1. (d) Contour line diagram of Laplacian of electron density of 1 along the Ru-S1-S2 plane. Solid red lines specify areas of charge concentration (∇ 2 ρ(r) < 0) and dashed black lines display areas of charge depletion ((∇ 2 ρ(r) > 0). Orange dots show bond critical points and solid brown lines display bond paths.

  species from the reaction of [Cp*Ru(μ-Cl)Cl]2 with Li[BH2S3]. In a stark contrast, the reaction of [ClRu(μ-Cl)(p-cymene)]2 with Li[BH2S3] yielded tetrametallic disulfide complex 1. As a result, we reexamined the chemistry of Cp*Ru-based systems and performed the reaction of [Cp*Ru(μ-Cl)Cl]2 with Li[BH2S3] under various reaction conditions. Although we did not able to isolate the Cp* analogue of 1, i.e. [(Cp*Ru)2Cl2]2(μ4,η 2 -E2), the reaction yielded two inseparable ruthenium bis/tris-homocubane analogues, [(Cp*Ru)2{μ-Sn(BH)2}] (n = 7 (4) and 6 (

Fig. 3

 3 Fig. 3 Molecular structure of 3. Selected bond lengths (Å) and angles (°): Ru1-Se1 2.4730(10), Ru1-Se5 2.4767(10), Ru1-Se7 2.4787(11), Se1-Se2 2.3628(12), Se4-Se7 2.3600(12), Se5-Se6 2.3542(12), Se1-B1 2.076(8), Se3-B1 2.065(8), Se6-B1 2.020(8), Se2-B2 2.013(8), Se3-B2 2.065(8), Se7-B2 2.073(8), Se3-B3 2.054(9), Se4-B3 2.014(9), Se5-B3 2.099(8); B2-Se2-Se1 89.6(2), B3-Se4-Se7 90.2(2), B1-Se6-Se5 90.4(2).

Scheme 2

 2 Scheme 2 Reaction of [Cp*Ru(μ-Cl)Cl]2 with Li[BH2S3].

Fig. 4 .

 4 Fig. 4. Molecular structures of (a) 4 and (b) 5. Selected bond lengths (Å) and bond angles (°). 4: B1-S1 1.932(5), B1-S5 1.946(11), B1-S7 1.868(6), B2-S2 1.866(13), B2-S3 2.095(12), B2-S6 1.866(14), S1-S2 2.013(2), S5-S6 2.052(14), S4-S7 2.1130(15), Ru1-S1 2.3637(11), Ru1-S3 2.2976(11), Ru1-S4 2.2927(10), Ru2-S3 2.3049(11), Ru2-S4 2.2875(10), Ru2-S5 2.356(12), Ru1-Ru2 2.7804(5), S1-B1-S5 108.8(5), S1-B1-S7 106.6(3), S5-B1-S7 104.8(5). 5: B1-S1' 1.932(5), B1-S5' 1.91(2), B2'-S1' 2.165(16), B2'-S5' 2.02(3), B2'-S8' 1.816(17), S3-S8' 1.942(5), Ru1-S1' 2.3637(11), Ru2-S5' 2.39(2); S1'-B2'-S5' 92.5(8), S5'-B2'-S8' 114.0(10).

Fig. 5

 5 Fig. 5 Selected frontier molecular orbitals of tris-homocubane analogues 3 (a) and 4' (b, c) and bis-homocubane analogue 5' (d).

Fig. 6

 6 Fig. 6 Combined UV-vis spectra of 1, 2, 3, 4 and 5 in CH2Cl2

  the expected electron counts. However, 4 and 5 possess 76 and 70 electrons, two lesser than 78 and 72, respectively (4: 2(Cp*Ru)×13 + 2(BH)×4 + 7(S)×6 = 76; 5: 2(Cp*Ru)×13 + 2(BH)×4 + 6(S)×6 = 70). The loss of two electrons is balanced due to the presence of one Ru-Ru bond. The 11 B chemical shifts of these compounds lie within the range of δ -12.7 to 8.8 ppm that is typical for the trichalcogenato borate unit/s. The average B-E and M-E distances in these bis-and tris-homocubane analogues lie within the single bond distances.

1: 1 H

 1 NMR (500 MHz, CDCl3, 22 °C): δ = 7.11 (d, 3 JHH = 4.5 Hz, 2×1H; HAr(p-cym)), 6.91 (d, 3 JHH = 6.4 Hz, 2×1H; HAr(p-cym)), 6.12 (d, 3 JHH = 5.3 Hz, 2×1H; HAr(p-cym)), 5.63 (d, 3 JHH = 4.5 Hz, 2×1H; HAr(p-cym)), 5.46 (d, 3 JHH = 6.4 Hz, 2×1H; HAr(p-cym)), 5.23 (d, 3 JHH = 5.8 Hz, 2×1H; HAr(p-cym)), 4.53 (d, 3 JHH = 5.8 Hz, 2×1H; HAr(p-cym)), 4.31 (d, 3 JHH = 5.3 Hz, 2×1H; HAr(p-cym)), 3.30 (sept, 3 JHH = 6.8 Hz, 2×1H; HCiPr), 2.93 (sept, 3 JHH = 6.7 Hz, 2×1H; HCiPr), 2.72 (s, 2×3H; H3C), 1.68 (s, 2×3H; H3C), 1.65(d, 3 JHH = 6.6 Hz, 3H; H3CiPr), 1.61 (d, 3 JHH = 6.6 Hz, 3H; H3CiPr), 1.46 (d, 3 JHH = 6.6 Hz, 3H; H3CiPr), 1.36 ppm (d,3 JHH = 6.7 Hz, 3H; H3CiPr); 13 C{ 1 H} NMR (125 MHz, CDCl3, 22 °C): δ = 111.

Table 2

 2 Selected spectroscopic and structural parameters of bis-and tris-homocubane analogues.

	Bis/tris-	Compound	CVE	11 B NMR (in ppm) davgB-E (in Å) a	davgM-E (in Å) a	Ref.
	homocubane core
		H	B					E		E
		E	E	Rh E			Rh	E B	H E = S , Se	72 (S) 72 (Se)	5.4, -4.9 (S) 4.1, -6.8 (Se)	1.968 (S) 2.0777 (Se)	2.3515 (S) 2.4656 (Se)	20
							1,3 isomer
		B Ru E E H E	E	E Ru	E B	H S , Se E =	70 (S) 70 (S)	1.7, -10.2 (Se) 8.8, -4.0 (S)	2.0912 (Se) -	2.4353 (Se) -	20
							1,3 isomer
	bis
		B S H		S		S	B	H
			S				Ru		S	70	-5.4, -12.2, -12.7 b	1.9568	2.3153	This Work
				Ru				S	5
							1,4 isomer
		H		B						E
		E		E	E Co		E Co	B E	H S , Se E =	72 (Se) 72 (S)	-4.7, -10.1 (Se) 3.5, 0.9 (S)	-1.9488 (S)	-2.2427 (S)	21
								1,3 isomer
				Co			E
				E E	B E 1,3,3 isomer B E B E E H H H S , Se E =	68 (Se) 68 (S)	0.2 (Se) 6.3 (S)	-1.8991 (S)	-2.246 (S)	17
				Ru Se	Se	Se B Se Se H B	H 3	68	8.3	2.0531	2.4763	This Work
				B 1,3,3 isomer Se Se H
		Se H B	Se	Se Se	B	H
	tris		Se				Rh	78	-2.0, -4.3	-	-	21
						Rh 1,3,3 isomer Se Se
		H	B					S	S
		S	S		S		Ir	B	H S	78	4.3, 2.2	1.902	2.3375	21
						Ir				S
						1,2,4 isomer
		H S	B S		S	S Ru S	B S	H	76	-5.4, -12.2, -12.7 b	1.9287	2.3148	This Work
					Ru				S	4
						1,2,3 isomer

a E= chalcogen, CVE = cluster valence electron count, avg = average, -= data is not available, b = combined spectrum of 4 and 5.

  ESI-MS calcd for C12H20B3RuNSe7 + [M+CH3CN] + m/z 864.5, found 864.6; 11 B{ 1 H} NMR (160 MHz, CDCl3, 22 °C): δ = -7.2 (br, 3B); 1 H NMR (400 MHz, CDCl3, 22 °C): δ = 7.73 (d, 3 JHH = 5.0 Hz, 1H; HAr(p-cym)), 7.72 (d, 3 JHH = 5.7 Hz, 1H; HAr(pcym)), 7.54 (d, 3 JHH = 5.1 Hz, 1H; HAr(p-cym)), 7.53 (d, 3 JHH = 5.5 Hz, 1H; HAr(p-cym)), 4.28 (br, 3H; BHt), 2.04 (sept, 3 JHH = 6.8

	1.98 (s, 2×3H; H3C), 1.78 (s, 2×3H; H3C), 1.70 (d, 3 JHH = 5.4 Hz, 3H; H3CiPr), 1.62 (d, 3 JHH = 5.9 Hz, 3H; H3CiPr), 1.08 (d,
	3 JHH = 6.9 Hz, 3H; H3CiPr), 1.05 ppm (d, 3 JHH = 6.4 Hz, 3H; H3CiPr); 13 C{ 1 H} NMR (100 MHz, C6D6, 22 °C): δ = 88.6 (s; Cp-
	cym), 88.3 (s; HCp-cym), 37.6 (s; HCiPr), 32.4 (s; H3CiPr), 27.6 ppm (s; H3C); UV-Vis (CH2Cl2, λ, nm): 291, 396, 484.
	Hz, 1H; HCiPr), 0.99 ppm (d, 3 JHH = 6.7 Hz, 3×3H; H3C and H3CiPr); 13 C{ 1 H} NMR (100 MHz, CDCl3, 22 °C): δ = 167.9,
	132.6 (s; Cp-cym), 131.1, 129.0, 77.4, 71.9 (s; HCp-cym), 27.9 (s; HCiPr), 23.0 (s; H3CiPr), 19.3 ppm (s; H3C); UV-Vis (CH2Cl2,
	λ, nm): 240, 253, 341; IR (CH2Cl2, 𝜈𝜈 � , cm -1 ): 2437 (BHt).
	2: MALDI-TOF calcd for C40H58Ru4Se2Cl4 2+ [M+2H] 2+ m/z 1245.8, found 1245.8; 1 H NMR (400 MHz, C6D6, 22 °C): δ =
	7.75 (d, 3 JHH = 7.9 Hz, 2×1H; HAr(p-cym)), 7.71 (d, 3 JHH = 7.4 Hz, 2×1H; HAr(p-cym)), 5.01 (d, 3 JHH = 5.7 Hz, 2×1H; HAr(p-cym)),
	4.88 (d, 3 JHH = 5.4 Hz, 2×1H; HAr(p-cym)), 4.75 (d, 3 JHH = 5.4 Hz, 2×1H; HAr(p-cym)), 4.59 (d, 3 JHH = 5.7 Hz, 2×1H; HAr(p-cym)),
	4.56 (d, 3 JHH = 5.9 Hz, 2×1H; HAr(p-cym)), 4.49 (d, 3 JHH = 5.2 Hz, 2×1H; HAr(p-cym)), 2.69 (sept, 3 JHH = 6.6 Hz, 2×1H; HCiPr),

3:

Table 3

 3 Crystal data and structure refinement details for 1, 3, 4 and 5.

	Complex	1	3	4 and 5
	CCDC no.	1974958	1974959	2073890
	Empirical formula	C40H56Cl4Ru4S2	C10H17B3RuSe7	C20H32B2Ru2S6.62
	Formula weight	1147.04	823.45	708.45
	Crystal system	Monoclinic	Monoclinic	Monoclinic
	Space group	P21/c	P21/c	P21/n
	a (Å)	14.8508(15)	8.264(3)	9.7476(10)
	b (Å)	17.5180(18)	17.571(7)	14.5175(16)
	c (Å)	18.851(2)	14.898(5)	18.5731(18)
	α (°)	90	90	90
	β (°)	106.413(4)	101.254(16)	95.722(4)
	γ (°)	90	90	90
	V (Å 3 )	4704.4(9)	2121.9(13)	2615.2(5)
	Z	4	4	4
	ρcalcd (g/cm 3 )	1.620	2.578	1.799
	μ (mm -1 )	1.601	12.735	1.692
	F(000)	2288	1496	1424

Acknowledgements

This work was supported by the Indo-French Center for the Promotion of Advanced Research (IFCPAR-CEFIPRA), Grant No. 5905-1, New Delhi, India. S. G. thanks, CSIR, India, for the research fellowship.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript, which was prepared through contribution of all the authors.

Conflicts of interest

There are no conflicts to declare.

Table of Content Entry Only

Stabilization of dichalcogenide ligand/s in tetraruthenium and bis-/trishomocubane cores (see picture).


Jessica Flores González

Bertrand Lefeuvre

Bastien Degraeve

Olivier Cador

Fabrice Pointillart

Study of the nuclear spin and dilution influence over the slow relaxation in a 3d4f heterobimetallic Single-Molecule Magnet

The effect of the isotopic enrichment and magnetic dilution has been investigated in an heterobimetallic complex of formula [Zn2L2 A DyCl3]•2H2O] ( A DyZn2) (A=162 and 163) presenting slow relaxation of the magnetization. The isotopic substitution for 162 Dy (I=0) and 163 Dy (I=5/2) leads to a shift in the relaxation times depending on the suppression or favor of the hyperfine interactions. The release of the dipolar interactions through magnetic dilution in an Y(III)-based matrix, enhances the slow relaxation of the magnetization and the visibility of the nuclear spin effect. A comparison of the hysteresis loop at 0.5 K for bulk and diluted analogues of pure isotopically enriched complexes suggested a role of the nuclear spin in the interaction between the active system and the matrix.

Introduction

The single molecule magnets are in scope since 1993, with the first publication of a molecular entity displaying magnetic bistability. 1 Since, an increased interest of these kind of properties is produced due to their potential applications in the high-density data storage 2 and spintronic devices [START_REF] Thiele | [END_REF] . In this kind of complexes, the magnetic bistability results from the appearance of an anisotropic barrier, responsible of the slow relaxation at low temperature. In this sense, mononuclear complexes based on lanthanides have been extensively studied because of their large magnetic moments, high intrinsic magnetic anisotropy and a bistable ground state under appropriate conditions. In order to achieve this requisites, some strategies have been studied: on one side, the optimization of the coordination environment (symmetry and crystal field) to enhance a large splitting of the ground multiplet state. This has recently driven researchers to create molecules achieving open hysteresis up to 60 K 4 and later 80 K 5 ; on the other side, the minimization of the dipolar and hyperfine interactions which interfere in the relaxation process, enhancing some throughbarrier relaxations. This second block, includes the isolation of the paramagnetic centres (by frozen solution 6 , doping in a diamagnetic matrix 7 or even inclusion of diamagnetic divalent ions in the structure (M=Co(II), Zn(II) [8][9][10] ) to study the contribution of the environment, and the isotopic enrichment of the magnetic centre in order to have a better insight of the contribution of its own nuclear spin (I) [11][12][13][14][15] . In the lasts ten years, different groups have considered the effect of the isotopic enrichment of Ln(III) in order to tune the relaxation of the magnetization. In 2010, F. Luis et al. 11 studied the isotopic effect of the first Er-based SMM enriched with 167 Er(III) (I=7/2). Some of us reported in 2015 12a the isotopic effect in a Dy-based SMM with two different isotopes ( 161 Dy (I=5/2) and 164 Dy (I=0)) and showed that the magnetization relaxation slows down for nuclear spin silent isotopes in the quantum regime. This study was extended in 2019 with the investigations of the four main stable isotopes of the same system 12b , showing the difference between two isotopes with the same I=5/2 nuclear spin. More recent publications have argued the importance of this isotopic effect in other Dy-based systems 13,14 and for the first time in 2021 similar effect was demonstrated for the prolate Yb(III) 15 . Until now, Dy-and Yb-based systems tend to show a common magnetic behavior consisting in the slowing down of the relaxation when suppressing the nuclear spin and the acceleration with nuclear spin active analogues, in the quantum regime only. [12][13][14][15] This tendency is opposite to polyoxometallate Er-based system proposed by F. Luis et al. 11 . These opposite tendencies have not yet been elucidated and the question remains open. Heterobinuclear 3d-4f systems, resulted from the two-step complexation of a divalent diamagnetic metal ion, Zn(II), and a Lanthanide cation with a Schiff base ligand, have been reported in the literature to show good SMM performances. 9-10 These systems, enable the inclusion of a divalent diamagnetic Zn(II) ion in the second coordination sphere of the metal, polarizing the phenoxido oxygen atoms and favouring an axial distribution of the electron density, which enhances the SMM behavior. In this paper, we propose to go a step further in the magnetic study of a mononuclear lanthanide based Zn-Dy-Zn type SMM formed by the salen-type ligand H2L (H2L=N, N'-bis(3methoxysalicylidene)phenylene-1,2-diamine), previously studied by W.-B. Sun et al. in 2016 (hereafter abbreviated as DyZn2) 10 . Indeed, this system fulfils certain requirements: i) it involves a magnetically isolated Dy(III) ion preventing any intramolecular magnetic interaction able to cancel the isotopic effect, 14a ii) the maxima of the out-of-phase component of the magnetic susceptibility are centred in the experimental frequency windows (0.1-1500 Hz) and iii) the system is easy to synthetize and well-known in a magnetic point of view. Thus, the isotopic enrichment and the dilution in an isomorphous diamagnetic matrix have been coupled in order to study the different contributions to the relaxation mechanism.

Materials and methods

Single crystals of DyZn2, 162 DyZn2, 163 DyZn2, (Dy@Y)Zn2, ( 162 Dy@Y)Zn2 and ( 163 Dy@Y)Zn2 were mounted on a APEXIII D8 VENTURE Bruker-AXS diffractometer for cell measurements (MoKα radiation source, λ=0.71073 Å, T=150 K) in the "Centre de Diffractométrie X" (CDIFX), University of Rennes 1, France. X-ray diffraction (XRD) patterns were recorded at room temperature in the 2θ range 5-30° with a step size of 0.026° and a scan time per step of 600 s using a PANalytical X'Pert Pro diffractometer (Cu-L2,L3 radiation, λ=1.5418 Å, 40 kV, 40 mA, PIXcel 1D detector). Data collector and HighScore Plus softwares were used, respectively, for recording and analysis of the patterns. The Dy/Y ratio of the compounds ( 162 Dy@Y)Zn2, (Dy@Y)Zn2 and ( 163 Dy@Y)Zn2 was determined using SEM (Scanning Electron Microscopy). All observations and measurements were carried out with a JEOL JSM 6400 scanning electron microscope (JEOL Ltd, Tokyo, Japan) with an EDS (Energy Dispersive Spectrometry) analysis system (Oxford Link INCA). The voltage was kept at 9 kV, and the samples were mounted on carbon stubs and coated for 5 min with a gold/palladium alloy using a sputter coater (Jeol JFC 1100). These analyses were performed by the "Centre de Microscopie Electronique à Balayage et microAnalyse (CMEBA)" from the University of Rennes 1 (France). The ac and dc magnetic susceptibility measurements were performed on solid polycrystalline samples with a Quantum Design MPMS-XL SQUID magnetometer equipped with iQuantum He3 insert. The sample was immobilized in a pellet made with Teflon tape. These measurements were corrected for the diamagnetic contribution as calculated with Pascal's constants.

Experimental

The isotopically enriched Dy2O3 oxides are commercially available from Eurisotop Company and Innova-Chem SAS. All other reagents were purchased from Sigma-Aldrich Co. Ltd and were used without further purification. [Zn2L2 A DyCl3]•2H2O] ( A DyZn2) complexes with L=N,N'-bis(3methoxysalicylidene)phenylene-1,2-diamine and A=162 and 163, were synthesized following a modified version of an already published procedure. 10,16 Isotopically enriched A DyCl3•6H2O were formed by dissolving 50 mg of A Dy2O3•6H2O, enriched at 94.40 % for A=162 and at 92.8 % for A=163, in 130 μL of 37% HCl. After 30 minutes at 60°C with continuous stirring, the resulting solution was diluted with 1 mL of distilled water and let evaporated giving A DyCl3•6H2O which can be used without further purification. A A DyCl3•6H2O (12-13.1 mg, 0.03 mmol) solution in 10 mL of methanol was added to a suspension solution of Zn(N,N'-bis(3methoxysalicylidene)phenylene-1,2-diamine), previously synthesized following a reported procedure. 10 (27-28 mg, 0.06 mmol) in the same volume of acetonitrile. The mixture was first stirred and heated under reflux for 6 h, and then filtered after cooling it to room temperature. The resulted solution was distributed into different tubes and crystallization took place by diffusion of diethyl ether in a sealed container. Yields from isolated single crystals: DyZn2: 4 mg (11%); 162 DyZn2: 4.8 mg (13%);

Results and discussions

Structural description

The compounds 162 DyZn2, 163 DyZn2, DyZn2 and diluted ( 162 Dy@Y)Zn2, ( 163 Dy@Y)Zn2, (Dy@Y)Zn2 are isostructural (Table S1) to the previously published structure. 10 The phase purity of all the samples was checked by powder X-ray diffraction (Fig. S1). The ratio Dy/Y have been determined by EDS analyses: 162 Dy/Y:0.17/0.83, 163 Dy/Y:0.15/0.85 and Dy/Y:0.13/0.87. The crystal structure (Fig. 1) is briefly remembered hereafter. The (N2O2O2) ligand enables a first coordination to a diamagnetic Zn(II) ion in the N2O2 pocket and a second one between the eight outer oxygen from two L moieties, in which the Dy(III) is encapsulated. The ninth coordination site of the Dy(III) centre is occupied with a Cl -anion located on C2 symmetry axis. The polarization effect driven by the two Zn(II) cations 9d , enhances the difference in the electronic density between the bridging oxygens (rich electron density) and those situated in the perpendicular plane (poor electron density), thus favouring an Ising axis of the magnetization pointing in the direction of the highest electron density.

Fig. 1 Molecular structure of DyZn2, with the hard plane represented. Hydrogen atoms and water molecules are omitted for clarity. Adapted from ref. [10].

Magnetic properties

The dynamic magnetic properties of the two isotopologues 162 DyZn2 (I=0) (Fig. 2a, S2a and S3) and 163 DyZn2 (I=5/2) (Fig. 2c, S2c and S4) are determined by measuring the frequency dependence of the magnetic susceptibility from 2 to 30 K, and have been compared to the natural isotope DyZn2 (Fig. 2b, S2b and S5). The last, reported in the literature, 10 has been remeasured in the same experimental condition than the two isotopologues for a better comparison. The out-of-phase components of the magnetic susceptibility for each compound pass through maxima at frequencies that depend on the isotopologue: at 2 K, it is centred at 0.4 Hz for 162 DyZn2, at 0.66 Hz for DyZn2 and 1 Hz for 163 DyZn2. The introduction of nuclear spin free 162 Dy(III) results into a shift of the out-of-phase component to lower frequencies (Fig. 2). The relaxation times (τ) for the different isotopologues at each temperature have been extracted from the extended Debye model (see the SI, Fig. S6, Tables S2-S4). The normalized Cole-Cole plots are depicted in Fig. S7-9 respectively for 162 DyZn2, 163 DyZn2 and DyZn2 showing a typical semi-circle shape curves and demonstrating that almost the 100 % of the magnetization of the sample are slowly relaxing. The temperature dependence of the relaxation times (Fig. 3) follows an Orbach process at the highest temperatures with a visible deviation caused by the Raman process at the intermediate temperatures. Below 10 K the relaxation is dominated by a thermally-independent regime i.e. the Quantum Tunnelling of the Magnetization (QTM). 10,16 Based on the previous works, it is known that the thermally dependent Orbach and Raman processes should not be affected by the isotopic enrichment, thus the thermal variation of the relaxation time is simultaneously fitted for the three systems with the shared contribution of Orbach (∆ and τ0) and Raman (C and n), and individual QTMs (τTI): The value of n is initially 9 for Kramers ions [START_REF] Abragam | Electron Paramagnetic Resonance of Transition Ions[END_REF] but can be found between 2 and 7 18 because of the presence of acoustic and optical phonons, and in some cases even lower. 19 The best fitted parameters for Orbach, Δ=374(21) K and τ0=3(2)×10 -10 s, and Raman, C=0.039(36) s -1 K -n and n=3.0(3), are close to the previously reported 10 ; with the addition of the QTM at lower temperatures (τTI( 162 DyZn2)=0.35(3) s; τTI(DyZn2)=0.18(1) s and τTI( 163 DyZn2)=0.11(1) s). In order to have a better insight of the isotopic effect, the diluted ( A Dy@Y)Zn2 systems have been investigated. Indeed, our previous work on a mononuclear Dy(III) complex demonstrated that the isotopic effect can be partially hidden by the presence of dipolar interactions. 12b At 2 K, the out-of-phase component of the natural (Dy@Y)Zn2 led to a relaxation of the magnetization shifted out of the lowest frequency limit (Fig. S10) of the experimental window. Then, the comparison has been envisaged through the opening of the hysteresis loop at 0.5 K for the different diluted and concentrated compounds. As predicted by the ac properties, the original butterfly shaped hysteresis loop is open at zero field for both DyZn2 and 162 DyZn2 whereas it is closed for the nuclear spin active 163 DyZn2 (Fig. 4a). Moreover the observed coercive field (Hc) trend is 162 DyZn2 (Hc=329 Oe)>DyZn2 (Hc=203 Oe)> 163 DyZn2 (Hc=0 Oe) and comforts the isotopic enrichment effect observed on the magnetic relaxation in zero field for T<12 K (Fig. 3). The fast relaxation coming from dipolar interactions is minimized on dilution and the hysteresis loops continue to open (Fig. 4b). One should mention larger coercive field for diluted samples than for the bulk ones with the measured values of 436 Oe, 320 Oe and 99 Oe for ( 162 Dy@Y)Zn2, (Dy@Y)Zn2 and ( 163 Dy@Y)Zn2 respectively. Moreover, these differences in the hysteresis loops are not only visible at zero field but also in-field (i.e. the wings of the butterfly) for both dilution and enrichment (Fig. 4, 5 and S11). The pure nuclear spin active isotopologues ( 163 Dy@Y)Zn2 and 163 DyZn2 are characterized by smaller gap between upper and lower branches of the loops in-field compared with the nuclear spin free and the natural analogues, thus endorsing the reported idea of a greater probability of relaxation paths and interaction with the matrix (phonons bath) because of the greater number of sublevels coming from the splitting of the I=5/2 in eight states under the hyperfine coupling. 14c In order to prove the role of the MJ levels in this opening, and discard the diffusion of the zero field memory in field, we determined gaps (δ1, δ2 and δ3, Fig. S12) in magnetization between two isotopologues at the same magnetic field (1.25 kOe) and compared them to the same gaps at zero field. δ1, δ2 and δ3 increase respectively of 38%, 15% and 29 % from 0 Oe to 1.25 kOe (Table S5). Such differences of hysteresis loops between the different isotopologues were already observed by some of us in a Dy(III) mononuclear complex. 12a To support the hypothesis of stronger interaction with the matrix in case of the pure 163 Dy isotope (I=5/2), one could remark that for both natural and nuclear spin free isotopes, the hysteresis loops are more open for the diluted samples between 0 and 10 kOe (Fig. 5a and5b) while the reverse trend is observed for 163 Dy (Fig. 5c andS10).

Conclusions

The Single Molecule Magnet properties of the two 162 Dy(III) and 163 Dy(III) isotopes have been investigated in the DyZn2 system. The nuclear spin free isotope ( 162 DyZn2) displayed a slightly slower magnetic relaxation than the nuclear spin active 163 DyZn2 because of the suppression of the hyperfine interactions effect coming from the metal centre. Thus, demonstrating that the isotopic enrichment effect on the magnetic relaxation in a hetero-bimetallic complex followed the same trend than already observed in pure lanthanide complex. By magnetic dilution, the dipolar interactions were suppressed leading to a slowing down of the magnetic relaxation rate which could be observed by means of the hysteresis loop. Finally, the comparison of the different hysteresis for diluted samples make us point to an isotopic effect in zero field as already proven in other systems, but also that we may consider that this has some effect under applied magnetic field which might be due to the interaction between the MJ sublevels and the matrix.

  163 DyZn2: 4 mg (11%). Anal. Calcd (%) for C44H36Cl3DyN4O10Zn2: C 44.62, H 3.40, N 4.73; found: C 44.59, H 3.31, N 4.70; Anal. Calcd (%) for C44H36Cl3 162 DyN4O10Zn2: C 44.62,H 3.40, N 4.73; found: C 44.55, H 3.29, N 4.69; Anal. Calcd (%) for C44H36Cl3 163 DyN4O10Zn2: C 44.62,H 3.40, N 4.73; found: C 44.66, H 3.34, N 4.68. IR (cm -1 ): DyZn2: 3053(w), 3029(w), 2974(m), 2940(m), 2881(m), 2839(w), 1735(w), 1613(s), 1586(m), 1551(m), 1453(s), 1393(m), 1304(m), 1281(m), 1233(s), 1195(s), 1097(m), 1073(m), 1049(m), 969(m), 882(m), 846(m), 782(m), 741(s), 648(w), 631(w), 559(m), 511(m), 445(w); 162 DyZn2: 3060(w), 3029(w), 2936(w), 2841(w), 1733(w), 1613(s), 1587(m), 1552(m), 1457(s), 1392(m), 1234(s), 1194(s), 1098(m), 1073(w), 969(m), 848(m), 783(m), 742(s), 649(w), 633(w), 559(m), 512(m), 447(w); 163 DyZn2: 3058(w), 3029(w), 2974(w), 2935(w), 2839(w), 1612(s), 1587(m), 1551(m), 1453(s), 1392(m), 1301(w), 1281(m), 1235(s), 1196(s), 1097(m), 1071(m), 1044(w), 968(m), 848(m), 783(m), 742(s), 649(w), 633(w), 560(m), 510(m), 446(w). [Zn2L2( A Dy0.1Y0.9)Cl3]•2H2O, ( A Dy@Y)Zn2, have been prepared with the same protocol but starting with A DyCl3•6H2O (6 mg, 0.015 mmol) and YCl3•6H2O (43.3-43.5 mg, 0.14 mmol) dissolved in methanol (50 mL) and added to a suspension solution of Zn (N,N'-bis(3-methoxysalicylidene)phenylene-1,2-diamine) (135 mg, 0.3 mmol) in the same volume of acetonitrile. Calculated yields from isolated single crystals: (Dy@Y)Zn2: 29.5 mg (17.2%); ( 162 Dy@Y)Zn2: 10.6 mg (6%); ( 163 Dy@Y)Zn2: 30.1 mg (16.9%). IR: (Dy@Y)Zn2: 3061(w), 3028(w), 2974(m), 2937(w), 2885(w), 2840(w), 1612(s), 1586(s), 1552(s), 1453(s), 1390(s), 1303(m), 1282(s), 1235(s), 1195(s); 1098(m), 1047(s), 968(m), 880(w), 848(m), 783(m), 739(s), 649(w), 634(w), 589(w), 560(m), 511(m), 447(w); ( 162 Dy@Y)Zn2: 3053(w), 3030(w), 2973(m), 2937(m), 2889(m), 2842(w), 1614(s), 1586(s), 1552(s), 1453(s), 1392(s), 1302(m), 1282(s), 1234(s), 1194(s), 1097(m), 1073(m), 1047(m), 881(m), 847(m), 783(m), 742(s), 649(m), 632(w), 586(w), 559(m), 510(m), 446(w); ( 163 Dy@Y)Zn2: 3062(w), 3028(w), 2972(m), 2936(m), 2888(m), 2840(w), 1614(s), 1586(s), 1552(s), 1450(s), 1391(s), 1303(m), 1282(s), 1236(s), 1195(s), 1168(m), 1097(s), 1047(s), 968(m), 882(m), 848(m), 782(m), 739(s), 650(m), 632(m), 588(w), 560(m), 512(m), 446(w).

Fig. 2

 2 Fig. 2 Zoom of the frequency dependence (0.1 -10 Hz) of the out-of-phase component of the ac susceptibility measured in zero dc field at various temperatures for a) 162 DyZn2, b) DyZn2 and c) 163 DyZn2.

Fig. 3

 3 Fig. 3 Temperature dependence of the relaxation time of the magnetization for 162 DyZn2 (red), DyZn2 (black) and 163 DyZn2 (blue). Full lines are the best fitted curves, whereas the dashed lines refer to the Raman (purple), Orbach (green) and QTM (colour of the compound) contributions.

Fig. 4

 4 Fig. 4 Normalized magnetic hysteresis loops at 0.5 K and at a sweep rate of 16 Oe s -1 for (a) the condensed compounds DyZn2 (black), 162 DyZn2 (red) and 163 DyZn2 (blue); and (b) the diluted (Dy@Y)Zn2 (grey), ( 162 Dy@Y)Zn2 (orange) and ( 163 Dy@Y)Zn2 (light blue) analogues. The insets correspond to a zoomed view of the origin.

Fig. 5

 5 Fig. 5 Zoom of the positive section of the normalized magnetic hysteresis loops at 0.5 K and at a sweep rate of 16 Oe s -1 for the condensed and diluted samples of (a) 162 Dy-( 162 DyZn2 (red) and ( 162 Dy@Y)Zn2 (orange)), (b) Dy-(DyZn2 (black) and (Dy@Y)Zn2 (grey)) and (c) 163 Dy-( 163 DyZn2 (blue) and ( 163 Dy@Y)Zn2 (light blue)) based systems analogues.

Acknowledgements

This work was supported by CNRS, Université de Rennes and the European Research Council through the ERC-CoG 725184 MULTIPROSMM (project n. 725184).

Conflicts of interest

There are no conflicts to declare.

Notes and references

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