We report the preparation and the full characterization of a novel mononuclear trigonal bipyramidal CoII complex [Co(NS3iPr)Br](BPh4) ( 1 ) with the tetradentate sulfur‐containing ligand NS3iPr (N(CH2CH2SCH(CH3)2)3). The comparison of its magnetic behaviour with those of two previously reported compounds [Co(NS3iPr)Cl](BPh4) ( 2 ) and [Co(NS3tBu)Br](ClO4) ( 3 ) (NS3tBu=N(CH2CH2SC(CH3)3)3) with similar structures shows that 1 displays a single‐molecule magnet behaviour with the longest magnetic relaxation time (0.051 s) at T=1.8 K, which is almost thirty times larger than that of 3 (0.0019 s) and more than three times larger than that of 2 (0.015 s), though its effective energy barrier (26 cm?1) is smaller. Compound 1 , which contains two crystallographically independent molecules, presents smaller rhombic parameters (E=1.45 and 0.59 cm?1) than 2 (E=2.05 and 1.02 cm?1) and 3 (E=2.00 and 0.80 cm?1) obtained from theoretical calculations. Compounds 2 and 3 have almost the same axial (D) and rhombic (E) parameter values, but present a large difference of their effective energy barrier and magnetic relaxation which may be attributed to the larger volume of BPh4? than ClO4? leading to larger diamagnetic dilution (weaker magnetic dipolar interaction) for 2 than for 3 . The combination of these factors leads to a much slower magnetic relaxation for 1 than for the two other compounds. 相似文献
A POM to remember : Hexanuclear FeIII polyoxometalate (POM) single‐molecule magnets (see structure) can be noncovalently assembled on the surface of single‐wall carbon nanotubes. Complementary characterization techniques (see TEM image and magnetic hysteresis loops) demonstrate the integrity and bistability of the individual molecules, which could be used to construct single‐molecule memory devices.
We report the synthesis of the novel heterometallic complex [Fe3Cr(L)2(dpm)6]?Et2O ( Fe3CrPh ) (Hdpm=dipivaloylmethane, H3L=2‐hydroxymethyl‐2‐phenylpropane‐1,3‐diol), obtained by replacing the central iron(III) atom by a chromium(III) ion in an Fe4 propeller‐like single‐molecule magnet (SMM). Structural and analytical data, high‐frequency EPR (HF‐EPR) and magnetic studies indicate that the compound is a solid solution of chromium‐centred Fe3Cr (S=6) and Fe4 (S=5) species in an 84:16 ratio. Although SMM behaviour is retained, the |D| parameter is considerably reduced as compared with the corresponding tetra‐iron(III) propeller (D=?0.179 vs. ?0.418 cm?1), and results in a lower energy barrier for magnetisation reversal (Ueff/kB=7.0 vs. 15.6 K). The origin of magnetic anisotropy in Fe3CrPh has been fully elucidated by preparing its Cr‐ and Fe‐doped Ga4 analogues, which contain chromium(III) in the central position (c) and iron(III) in two magnetically distinct peripheral sites (p1 and p2). According to HF‐EPR spectra, the Cr and Fe dopants have hard‐axis anisotropies with Dc=0.470(5) cm?1, Ec=0.029(1) cm?1, Dp1=0.710(5) cm?1, Ep1=0.077(3) cm?1, Dp2=0.602(5) cm?1, and Ep2=0.101(3) cm?1. Inspection of projection coefficients shows that contributions from dipolar interactions and from the central chromium(III) ion cancel out almost exactly. As a consequence, the easy‐axis anisotropy of Fe3CrPh is entirely due to the peripheral, hard‐axis‐type iron(III) ions, the anisotropy tensors of which are necessarily orthogonal to the threefold molecular axis. A similar contribution from peripheral ions is expected to rule the magnetic anisotropy in the tetra‐iron(III) complexes currently under investigation in the field of molecular spintronics. 相似文献
The selective replacement of the central iron(III) ion with vanadium(III) in a tetrairon(III) propeller‐shaped single‐molecule magnet has allowed us to increase the ground spin state from S=5 to S=13/2. As a consequence of the pronounced anisotropy of vanadium(III), the blocking temperature for the magnetization has doubled. Moreover, a significant remnant magnetization, practically absent in the parent homometallic molecule, has been achieved owing to the suppression of zero‐field tunneling of the magnetization for the half‐integer molecular spin. Interestingly, the contribution of vanadium(III) to the magnetic anisotropy barrier occurs through the anisotropic exchange interaction with iron(III) spins and not through single ion anisotropy as in most single‐molecule magnets. 相似文献
Efficient modulation of single‐molecule magnet (SMM) behavior was realized by deliberate structural modification of the Dy2 cores of [Dy2( a ′ povh )2(OAc)2(DMF)2] ( 1 ) and [Zn2Dy2( a′povh )2(OAc)6] ? 4 H2O ( 2 ; H2 a ′ povh =N′‐[amino(pyrimidin‐2‐yl)methylene]‐o‐vanilloyl hydrazine). Compound 1 having fourfold linkage between the two dysprosium ions shows high‐performance SMM behavior with a thermal energy barrier of 322.1 K, whereas only slow relaxation is observed for compound 2 with only twofold connection between the dysprosium ions. This remarkable discrepancy is mainly because of strong axiality in 1 due to one pronounced covalent bond, as revealed by experimental and theoretical investigations. The significant antiferromagnetic interaction derived from bis(μ2‐O) and two acetate bridging groups was found to be crucial in leading to a nonmagnetic ground state in 1 , by suppressing zero‐field quantum tunneling of magnetization. 相似文献
One of the main challenges in the field of molecular materials is the design of molecular ferromagnets. General design strategy includes two steps, that is molecular magnetic engineering and crystal magnetic engineering. The first step is the synthesis of ferromagnetically coupled polymetallic systems. The second step is the assembly of polymetallic systems with muti‐dimensional structure and exhibiting a ferromagnetic transition. This paper summarized the strategies of molecular design and crystal engineering allowed to obtain such systems and our efforts in the fields of molecular magnetism and molecular‐based magnets. 相似文献