Designing a Dy₂ Single-Molecule Magnet with Two Well-Differentiated Relaxation Processes by Using a Nonsymmetric Bis-bidentate Bipyrimidine-N-Oxide Ligand: A Comparison with Mononuclear Counterparts

Abstract

Herein we report a dinuclear [(μ-mbpymNO){(tmh)Dy}] (1) single-molecule magnet (SMM) showing two nonequivalent Dy centers, which was rationally prepared from the reaction of Dy(tmh) moieties (tmh = 2,2,6,6-tetramethyl-3,5-heptanedionate) and the asymmetric bis-bidentate bridging ligand 4-methylbipyrimidine (mbpymNO). Depending on whether the Dy ions coordinate to the N^O or N^N bidentate donor sets, the Dy sites present a NO ( D geometry) or NO ( D) coordination sphere. As a consequence, two different thermally activated magnetic relaxation processes are observed with anisotropy barriers of 47.8 and 54.7 K. Ab initio calculations confirm the existence of two different relaxation phenomena and allow one to assign the 47.8 and 54.7 K energy barriers to the Dy(NO) and Dy(NO) sites, respectively. Two mononuclear complexes, [Dy(tta)(mbpymNO)] (2) and [Dy(tmh)(phenNO)] (3), have also been prepared for comparative purposes. In both cases, the Dy center shows a NO coordination sphere and SMM behavior is observed with U values of 71.5 K (2) and 120.7 K (3). In all three cases, ab initio calculations indicate that relaxation of the magnetization takes place mainly via the first excited-state Kramers doublet through Orbach, Raman, and thermally assisted quantum-tunnelling mechanisms. Pulse magnetization measurements reveal that the dinuclear and mononuclear complexes exhibit hysteresis loops with double- and single-step structures, respectively, thus supporting their SMM behavior.

A Chiral Bipyrimidine-Bridged Dy2 SMM: A Comparative Experimental and Theoretical Study of the Correlation Between the Distortion of the DyO6N2 Coordination Sphere and the Anisotropy Barrier

“…The inherent chirality of the ligand may introduce additional functions to coordination compounds with interesting magnetic properties such as magnetochiral dichroism (MChD) effect (Rikken and Raupach, 1997 , 2000 ; Train et al, 2008 , 2011 ), second harmonic generation (SHG) (Bogani et al, 2006 ; Train et al, 2009 ), and ferroelectric properties (Wang et al, 2012 ; Li et al, 2017 ). It has been recently shown that mononuclear tris(β-diketonate) Dy III complexes containing a N,N-bidentate chelate aromatic ligand, such as 2,2′-bipyrimidine and 1,10-phenanthroline derivatives, as well as Dy 2 dinuclear complexes containing bis(didentate) bridging ligands connecting two tris(β-diketonate) Dy III moieties, such as 2,2'-bipyrimidine and 2,2'-bipyrimidine-N-oxide, exhibit Single-Molecule Magnet (SMM) behavior at zero field with significant thermal energy barriers ( U eff ) (Chen et al, 2011 , 2012 ; Wang et al, 2013 ; Tong et al, 2015 ; Sun et al, 2016 ; Yu et al, 2016 ; Cen et al, 2017 ; Díaz-Ortega et al, 2018 ). SMMs are nanomagnets that, in addition to the classical properties of a magnet, such as freezing of magnetization and magnetic hysteresis below the so called blocking temperature (T B ), exhibit interesting quantum properties, such as quantum tunneling of the magnetization (QTM) and quantum phase interference (Aromí and Brechin, 2006 ; Gatteschi et al, 2006 ; Andruh et al, 2009 ; Bagai and Christou, 2009 ; Sessoli and Powell, 2009 ; Brechin, 2010 ; Guo et al, 2011 ; Rinehart and Long, 2011 ; Sorace et al, 2011 ; Clemente-Juan et al, 2012 ; Luzon and Sessoli, 2012 ; Wang and Gao, 2012 ; Habib and Murugesu, 2013 ; Woodruff et al, 2013 ; Zhang et al, 2013 ; Bartolomé et al, 2014 ; Layfield, 2014 ; Sharples and Collison, 2014 ; Craig and Murrie, 2015 ; Layfield and Murugesu, 2015 ; Liddle and van Slageren, 2015 ; Rosado Piquer and Sañudo, 2015 ; Tang and Zhang, 2015 ; Frost et al, 2016 ; Harriman and Murugesu, 2016 ).…”

Field-Induced Single-Molecule Magnets of Dysprosium Involving Quinone Derivatives

“…The compounds 1 and 2 displayed a magnetic relaxation mainly through a pure Raman process (Figure 4, Figure 6 and Figure S9), which could be justified by the applied magnetic field (cancelling of the QTM), the moderate value of the field (the direct process is not the fastest magnetic relaxation process) and the low temperature range at which the relaxation times are extracted (below 10 K), leading to a negligible Orbach contribution. Recently, several Dy III -based SMMs displaying pure Raman magnetic relaxation under moderate applied field in the low temperature range have been reported [43][44][45][46][47][48][49]. For some of the reported example in Table 1, an Orbach process can be involved in the magnetic relaxation in the high temperature range, in which an out-of-phase contribution of the magnetic susceptibility was measured.…”

“…[Dy(tta) 3 (phenNO)] (phenNO = 1,10-phenantroline-1-oxide) Square antiprism geometry H = 1000 Oe U eff = 159.1 K τ 0 = 1.9 × 10 −10 s C = 5 × 10 −3 s −1 K −n n = 5.2 [46] [Dy(tmhd) 3 (dppz)] (tmhd = 2,2,6,6-tetramethyl-3,5-heptanedione) (dppz = dipyrido [3,2-…”

Asymmetric Dinuclear Lanthanide(III) Complexes from the Use of a Ligand Derived from 2-Acetylpyridine and Picolinoylhydrazide: Synthetic, Structural and Magnetic Studies

“…However, the preparation of asymmetric Ln III 2 complexes is by no means easy, because nature often favors symmetric molecules. Asymmetric dinuclear Ln(III) complexes are also valuable, because they allow detailed studies on the two possible thermally activated relaxation processes of molecular origin that are often observed in Ln(III) SMMs [43][44][45].…”

“…For the above-mentioned reasons, it is evident that the synthesis of asymmetric Ln III 2 complexes is desirable [42][43][44][45][46]. The design principle behind the implementation of this goal is the appropriate choice of the bridging organic ligand, which should be ditopic.…”