Synthesis, Structure, and Magnetic Properties of (6−9)-Nuclear Ni(II) Trimethylacetates and Their Heterospin Complexes with Nitroxides

Abstract

New polynuclear nickel trimethylacetates [Ni6(OH)4(C5H9O2)8(C5H10O2)4] (6), [Ni7(OH)7(C5H9O2)7(C5H10O2)6(H2O)] x 0.5 C6H14 x 0.5 H2O (7), [Ni8(OH)4(H2O)2(C5H9O2)12] (8), and [Ni9(OH)6(C5H9O2)12(C5H10O2)4] x C5H10O2 x 3 H2O (9), where C5H9O2 is trimethylacetate and C5H10O2 is trimethylacetic acid, have been found. Their structures were determined by X-ray crystallography. Because of their high solubility in low-polarity organic solvents, compounds 6-9 reacted with stable organic radicals to form the first heterospin compounds based on polynuclear Ni(II) trimethylacetate and nitronyl nitroxides containing pyrazole (L(1)-L(3)), methyl (L(4)), or imidazole (L(5)) substituent groups, respectively, in side chain [Ni7(OH)5(C5H9O2)9(C5H10O2)2(L(1))2(H2O)] x 0.5 C6H14 x H2O (6+1a), [Ni7(OH)5(C5H9O2)9(C5H10O2)2(L2)2(H2O)] x H2O (6+1b), [Ni7(OH)5(C5H9O2)9(C5H10O2)2(L(3))2(H2O)] x H2O (6+1c), [Ni6(OH)3(C5H9O2)9(C5H10O2)4(L(4))] x 1.5 C6H14 (6''), and [Ni4OH)3(C5H9O2)5(C5H10O2)4(L(5))] x 1.5 C7H8 (4). Their structures were also determined by X-ray crystallography. Although Ni(II) trimethylacetates may have varying nuclearity and can change their nuclearity during recrystallization or interactions with nitroxides, this family of compounds is easy to study because of its topological relationship. For any of these complexes, the polynuclear framework may be derived from the [Ni6] polynuclear fragment {Ni6(mu4-OH)2(mu3-OH)2(mu2-C5H9O2-O,O')6(mu2-C5H9O2-O,O)(mu4-C5H9O2-O,O,O',O')(C5H10O2)4}, which is shaped like an open book. On the basis of this fragment, the structure of 7-nuclear compounds (7 and 6+1a-c) is conveniently represented as the result of symmetric addition of other mononuclear fragments to the four Ni(II) ions lying at the vertexes of the [Ni6] open book. The 9-nuclear complex is formed by the addition of trinuclear fragments to two Ni(II) ions lying on one of the lateral edges of the [Ni6] open book. This wing of the 9-nuclear complex preserves its structure in another type of 6-nuclear complex (6'') with the boat configuration. If, however, two edge-sharing Ni(II) ions are removed from [Ni6] (one of these lies at a vertex of the open book and the other, on the book-cover line), we obtain a 4-nuclear fragment recorded in the molecular structure of 4. Twinning of this 4-nuclear fragment forms highly symmetric molecule 8, which is a new chemical version of cubane.

Metal–Nitroxide Complexes: Synthesis and Magnetostructural Correlations

“…175 This situation (the tendency toward the transformation of polynuclear frameworks) was encountered in developing the synthesis of heterospin systems based on polynuclear nickel(II) pivalates with nitronyl nitroxides. 193 Formally "nickel(II) pivalate" can be isolated as compounds of different nuclearities, namely, [Ni 6 (OH) 4 Piv 8 (HPiv) 4 ], [Ni 7 (OH) 7 Piv 7 (HPiv) 6 retains its structure in another type of hexanuclear complex, shaped like a bathtub. If, however, two edgesharing nickel(II) ions, one of which lies at a vertex of the "open book" and the other on the binding line of the "book," are eliminated from the starting hexanuclear fragment, a tetranuclear fragment fixed in the structure of [Ni 4 (OH) 3 Piv 5 (HPiv) 4 (L 42 )] is obtained.…”

“…The understanding of the topological scheme of mutual transformation of a certain group of polynuclear fragments makes it possible to control the synthesis of these complex heterospin complexes with nitroxides, reveal the relationship between their structures, and justifiably divide the polynuclear fragment into smaller units in describing the magnetic properties of the compound. 193 In constructing a topological scheme of this kind, the greatest difficulty is to isolate the "key" compound of the topological series, that is, the hexanuclear [Ni 6 (OH) 4 Piv 8 (HPiv) 4 ] complex in this case, because it is most susceptible to various transformations which depend on the conditions of synthesis. These transformations often hinder the isolation of the "key" compound as a single crystal, which can sometimes appear (and subsequently vanish with time) as an impurity in addition to the kinetically more stable solid crystals.…”

Heterospin complexes of polynuclear NiII compounds containing hexafluoroacetylacetonate and pivalate ligands with nitroxides

“…Finally, the starting polynuclear fragment should contain easily replaceable terminal ligands. The recently described 6 mixed ligand hexanuclear complex [Ni 6 (OH) 4 Piv 4 (hfac) 4 (Me 2 CO) 4 ], meets theses require ments. Thus the pivalate anions (Piv) that are present in this complex provide the solubility in organic solvents, the hexafluoroacetylacetonate anions (hfac) enhance the elec tron withdrawing properties of the Ni II ions, and acetone molecules serve as easily replaceable ligands.…”

“…[1][2][3][4][5] In solid phases, some of these compounds exhibit bulk magnetization. 1, 4 However, what often complicates implementation of this synthetic ap proach is poor solubility of the starting polynuclear com plexes in organic solvents, due to which they cannot be introduced into reactions with nitroxides. In addi tion, the metal ions of the polynuclear fragment should possess high electron withdrawing ability to coordi nate weak donors such as the O atoms of the nitrox ide groups.…”

Heterospin mixed-ligand heterotrinuclear CoII, GdIII, CoII complex with nitronyl nitroxide

“…1, 4 In spite of a high synthetic potential of this approach, it often presents difficulties when the starting polynuclear complexes are virtually insoluble in organic solvents and, consequently, they cannot be involved in the reactions with nitroxides. A high catalytic activity of the polynuclear core can cause the simultaneous transformation of both the polynuclear core and the nitroxide.…”