Lecren Lollita, Roubeau Olivier, Coulon Claude, Li Yang-Guang, Le Goff Xavier F, Wernsdorfer Wolfgang, Miyasaka Hitoshi, Clérac Rodolphe
Centre de Recherche Paul Pascal, CNRS UPR-8641, 115 av. Albert Schweitzer, 33600 Pessac, France.
J Am Chem Soc. 2005 Dec 14;127(49):17353-63. doi: 10.1021/ja0551685.
Four discrete Mn(III)/Mn(II) tetranuclear complexes with a double-cuboidal core, Mn(4)(hmp)(6)(CH(3)CN)(2)(H(2)O)(4)(4).2CH(3)CN (1), Mn(4)(hmp)(6)(H(2)O)(4)(4).2H(2)O (2), Mn(4)(hmp)(6)(H(2)O)(2)(NO(3))(2)(2).4H(2)O (3), and Mn(4)(hmp)(6)(Hhmp)(2)(4).2CH(3)CN (4), were synthesized by reaction of Hhmp (2-hydroxymethylpyridine) with Mn(ClO(4))(2).6H(2)O in the presence of tetraethylammonium hydroxide and subsequent addition of NaNO(3) (3) or an excess of Hhmp (4). Direct current (dc) magnetic measurements show that both Mn(2+)-Mn(3+) and Mn(3+)-Mn(3+) magnetic interactions are ferromagnetic in 1-3 leading to an S(T) = 9 ground state for the Mn(4) unit. Furthermore, these complexes are single-molecule magnets (SMMs) clearly showing both thermally activated and ground-state tunneling regimes. Slight changes in the [Mn(4)] core geometry result in an S(T) = 1 ground state in 4. A one-dimensional assembly of [Mn(4)] units, catena-{Mn(4)(hmp)(6)(N(3))(2)(2)} (5), was obtained in the same synthetic conditions with the subsequent addition of NaN(3). Double chairlike N(3)(-) bridges connect identical [Mn(4)] units into a chain arrangement. This material behaves as an Ising assembly of S(T) = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with Delta(tau)/k(B) = 47 K and tau(0) = 7 x 10(-)(11) s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the noncompensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of polydisperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expressions of the characteristic energy gaps Delta(xi) and Delta(tau) were estimated and successfully compared to the experimental values.
合成了四种具有双立方烷核心的离散型Mn(III)/Mn(II)四核配合物，即Mn(4)(hmp)(6)(CH(3)CN)(2)(H(2)O)(4)(4).2CH(3)CN (1)、Mn(4)(hmp)(6)(H(2)O)(4)(4).2H(2)O (2)、Mn(4)(hmp)(6)(H(2)O)(2)(NO(3))(2)(2).4H(2)O (3)和Mn(4)(hmp)(6)(Hhmp)(2)(4).2CH(3)CN (4)，它们是通过2-羟甲基吡啶（Hhmp）与Mn(ClO(4))(2).6H(2)O在氢氧化四乙铵存在下反应，随后加入NaNO(3)（3）或过量的Hhmp（4）合成的。直流（dc）磁性测量表明，在1 - 3中，Mn(2 +)-Mn(3 +)和Mn(3 +)-Mn(3 +)磁性相互作用都是铁磁性的，导致Mn(4)单元的基态为S(T)= 9。此外，这些配合物是单分子磁体（SMMs），清楚地显示出热激活和基态隧穿机制。[Mn(4)]核心几何结构的微小变化导致4中的基态为S(T)= 1。在相同的合成条件下，通过随后加入NaN(3)，得到了[Mn(4)]单元的一维组装体，链状-{Mn(4)(hmp)(6)(N(3))(2)(2)} (5)。双椅状N(3)(-)桥将相同的[Mn(4)]单元连接成链状排列。这种材料表现为S(T)= 9的四聚体的伊辛组装体，弱反铁磁耦合。在反铁磁链中首次在低温下观察到磁化强度的缓慢弛豫，遵循激活行为，Δ(tau)/k(B)= 47 K，tau(0)= 7×10(-)(11) s。这种原始热激活弛豫过程的观察是由有限尺寸效应引起的，特别是由奇数单元段中自旋的非补偿引起的。推广了关于反铁磁耦合伊辛自旋的多分散有限段动态性质的已知理论，估计了特征能隙Δ(xi)和Δ(tau)的理论表达式，并成功地与实验值进行了比较。
