Magnetic Supramolecular Grid Structures: Intramolecular Coupling of Four Separate Spins
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ABSTRACT The magnetization of novel tetranuclear supramolecular grid structures and their mononuclear analogues was measured. In the tetranuclear complexes with Co 2+or Ni2 +ions an intramolecular antiferromagnetic coupling of the four metal centers is observed. The isotropic coupling strength was determined to be J = -8 K for the Ni 2+ grids, and J = -2 K for the Co2+ grids. INTRODUCTION Supramolecular chemistry is a unique tool to produce, by self-assembly, metallorganic complexes with a defined number of magnetic metal centers [1]. Recently, it has been shown that a novel self-assembled supramolecular grid structure with four Co2+ metal centers, the Co-[2x2] grid, represents an almost ideal model system for investigating magnetic interactions in a discrete entity [2]. The Co2+ spins exhibit an antiferromagnetic interaction which is confined to within a grid. Thus, each grid forms an antiferromagnetic domain. In this work we present recent results on the same grid structure but containing four Ni2+ ions, denoted as Ni-[2x2] grids. We estimate the coupling constants for both Ni-[2x2] grids and Co[2x2] grids and compare them with each other. In order to demonstrate the effects of the antiferromagnetic coupling, we also performed measurements of the mononuclear analogues. THEORY Within the crystal field approximation the Hamiltonian of the lowest LS-multiplett appropriate for the mononuclear complexes may be written as [3] H, =V+T+AL.S+pB(L+2S).H,
(1)
where 2 denotes the spin-orbit coupling constant, /uB the Bohr magneton, Vthe octahedral ligand field and T fields of lower symmetry. V and T may be expressed using Stevens operators 3]. In the case of Ni2 , the octahedral field V splits the 3F multiplett into 3T1 , 3T2 and 3A 2 , A2 being lowest. Applying the perturbation theory for an orbital singlet [3] to the Hamiltonian H1 , the effective spin Hamiltonian for the 3 A 2 level (S = 1) is written as H 2 = DS' + E(S2
-
S2)+ !.tg,,HS.+ fltgHSy + ,ugzHzSz.
(2)
For Co2+, the splitting of the 4F multiplet by the octahedral ligand field leads to 4T, as lowest level. Two cases will be distinguished. If the splitting of the levels due to lower symmetric fields is larger than that due to the spin-orbit coupling, i.e. T >> 2, then again the perturbation theory 841
Mat. Res. Soc. Symp. Proc. Vol. 488 ©1998 Materials Research Society
for an orbital singlet can be applied leading to the same Hamiltonian as in Eq. 2, but with S = 3/2. However, in the case T 2A the effective Hamiltonian for the 4 T1 level is obtained by replacing L -->-1, where I = 1 [4]. On restricting ourselves to tetragonal symmetry, this yields [4] H3 = g
12 _ 3t a)lS -- a'A(IS. +IS
-!JlX+2S•)H.
- a2(lS +ySy ) + JB(-fl
2
H
IU8+2S•)Hz,
+/JB(- 2
(3)
where we distinguished between the a's and f/'s in order to allow for orbital reduction. For the tetranuclear complexes, the Hamiltonian within the lowest LS-multiplett may be written as [5] 4
HxF = -J(S
1
.S 2 + S 2 -S 3 + S 3 •S 4 -S 4 .S,) .[V, =11
+1T, +a-Li"S, + IUB (Li + 2S,)" UH],
(4)
wh
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