Substituent Effects on Exchange Coupling and Magnetic Relaxation in 2,2′-Bipyrimidine Radical-Bridged Dilanthanide Complexes

Abstract

Systematic analysis of related compounds is crucial to the design of single-molecule magnets with improved properties, yet such studies on multinuclear lanthanide complexes with strong magnetic coupling remain rare. Herein, we present the synthesis and magnetic characterization of the series of radical-bridged dilanthanide complex salts [(Cp*2Ln)2(μ-5,5′-R2bpym)](BPh4) (Ln = Gd, Dy; R = NMe2 (1), OEt (2), Me (3), F (4); bpym = 2,2′-bipyrimidine). Modification of the substituent on the bridging 5,5'-R2bpym radical anion allows the magnetic exchange coupling constant, JGd-rad, for the gadolinium compounds in this series to be tuned over a range from -2.7 cm -1 (1) to -11.1 cm -1 (4), with electron-withdrawing or -donating substituents increasing or decreasing the strength of exchange coupling, respectively. Modulation of the exchange coupling interaction has a significant impact on the magnetic relaxation dynamics of the single-molecule magnets 1-Dy through 4-Dy, where stronger JGd-rad for the corresponding Gd 3+ compounds is associated with larger thermal barriers to magnetic relaxation (Ueff), open magnetic hysteresis at higher temperatures, and slower magnetic relaxation rates for through-barrier processes. Further, we derive an empirical linear correlation between the experimental Ueff values for 1-Dy through 4-Dy and the magnitude of JGd-rad for the corresponding gadolinium derivatives that provides insight into the electronic structure of these complexes. This simple model applies to other organic radical-bridged dysprosium complexes in the literature, and it establishes clear design criteria for increasing magnetic operating temperatures in radical-bridged molecules.Synthesis and Structural Characterization. The molecule 2,2′-bipyrimidine (bpym) serves as an ubiquitous bridging ligand in coordination chemistry, however, symmetric 5,5′-R2bpym variants have only yet been reported for alkyl, aryl, ether, and bromine substituents. [35][36][37][38] Bipyrimidine derivatives are typically synthesized via Cu-or Ni-mediated coupling reactions, and therefore the synthesis of 5,5′-R2bpym was first attempted via a Ni-catalyzed homocoupling of the corresponding 5-R-2-chloropyrimidine. [39][40][41] This approach furnished 5,5′-R2bpym with electrondonating substituents R = Me, OEt, and NMe2 in isolated yields between 33 and 37% but did not yield electron-deficient derivatives with R = F or CF3. These results are consistent with reports of the synthesis of 5,5′-R2bpy (bpy = 2,2′-bipyridine), which found lower yields for the Ni-catalyzed homocoupling of 5-R-2-chloropyridine substrates bearing electron-withdrawing substituents. [42][43][44] Attempts to synthesize derivatives with R = F or CF3 via Cu-mediated homocoupling reactions were also unsuccessful. Instead, a Stille reaction 45,46 was used to couple 5-fluoro-2tributylstannylpyrimidine and 5-fluoro-2-chloropyrimidine, affording 5,5′-F2bpym in 57% isolated yield. Although it was not possible in our hands to isolate 5,5′-(CF3)2bpym by an analogous route, the Pd-catalyzed ...

Radical‐Bridged Ln4 Metallocene Complexes with Strong Magnetic Coupling and a Large Coercive Field

“…Them olecular structure (Figure 1a nd Figure S2) consists of four [Cp* 2 Dy III ] + moieties linked via four m-tzC À ligands to form ad iamond-like core.T he centrosymmetric structure consists of two crystallographically independent Dy III centers.T he average Dy-C Cp* bond distance is 2.397(2) ,w hile the average Cp* cent -Dy-Cp* cent angle is 137.86(4)8 8.T hese values are similar to those reported for other radical bridged SMMs containing [Cp* 2 Dy] + moieties. [22,23,25,27] Ty pically,these complexes display high single-ion anisotropy due to the strong axial ligand field imposed by the Cp* ligands.H owever,t he Dy-Cp* cent distances in 1 (average Dy-Cp* cent :2.379(2) )are larger when compared to the current [(Cp iPr5 )Dy(Cp*)] + benchmark [12] 6) ) [22,25] and, as expected, larger compared to N 2 C 3À systems (2.234(1) ). [20,21] When compared to the neutral free ligand (1.327(6) ), [41] the N=Nb onds of the reduced tetrazine rings are significantly elongated (1.351(2) and 1.341(3) ), supporting its radical nature,w hile the average C À Nb ond distance remains relatively the same ( % 1.349-(2) ).…”

“…Attempts to incorporate terms that would account for QTM and/or Raman relaxation process failed to yield ab etter fit, further supporting that the Orbach relaxation mechanism is the primary relaxation for 1.A lthough these U eff values are smaller than the N 2 -radical bridged systems, [20,21] they are among the highest values reported for organic radical bridged Dy III complexes. [22][23][24][25][26][27] Thelower energy barriers are likely due to tzC À radicals lying in the equatorial plane,s ubsequently competing with the axiality imposed by the Cp* ligands.…”

“…To overcome the lack of strong exchange interaction between 4f ions,i ncorporating the radical bridge as ad irect exchange pathway is apromising avenue.Long,Evans and coworkers elegantly demonstrated that N 2 C 3À bridged Ln 2 complexes exhibit remarkably high-blocking temperatures (T B )a nd large coercive fields indicative of magnetic hardness. [20,21] This is ascribed to strong coupling mediated through the radical N 2 C 3À bridge.Several other complexes using radical bridges were reported, [22][23][24][25][26][27] but most of them pale in comparison to the highly delocalized and compact N 2 C À system. Although N 2 C 3À is remarkable in the performance aspect, synthetically speaking, it is not trivial to rationally incorporate N 2 C 3À into polynuclear complexes and offers no room for modification.…”

A study of cation-dependent inverse hydrogen bonds and magnetic exchange-couplings in lanthanacarborane complexes

“…From Table 2 we can see all the dimeric complexes (2Dy, 4Dy, and 6Dy) possess much higher energy barriers, U eff = 1089(6) K for 2Dy, 1106(5) K for 4Dy, and 704(10) K for 6Dy, if compared to their corresponding monomers, U eff = 830(7) K for 1Dy, 940(5) K for 3Dy and none for 5Dy@5Y (Table 2). It's worth noting that the U eff values of 2Dy and 4Dy above 1000 K are very high for polynuclear SMMs (Wang, et al, 2021;Han, et al, 2020;Krylov, et al, 2017;Gould, et al, 2020;Meng, et al, 2020). The only higher value is reported in the compound Dy 2 ScN@C 80 -I h with U eff = 1735 G 21 K (Krylov, et al, 2017).…”

A Linear "Head-to-Tail" Chlorido-Bridged Didysprosium Single-Molecule Magnet Exhibiting 100-s Blocking Temperature near 8 K

“…where generates an exchange bias field to restrain the QTM under zero filed. [35][36][37][38] Introducing magnetic interactions mediated via radical bridges has been the most effective method towards stronger coupling reported until now (Tables 1 and S1). 39,40 For example, the µ−N2 • bridged {Tb2} complexes [{(Me3Si)2N)2Tb(THF)}2(μ-N2 • )] with Ueff = 154 K and TB 100s of 14 K and [(CpMe 4H 2Tb)2(µ−N2 • )] − with Ueff = 395 K and TB 100s of 20 K; while interestingly, their analogue {Dy2} complexes show both lower Ueff ~ 170 K and TB 100s ~ 8 K. 40,41 More recently, these numbers have been broken by the single-electron metalmetal bonded endohedral dimetallofullerenes via physically arc-discharge, Ln2@C80(CH2Ph).…”