Cyanido-Bridged Clusters with Remote N-Oxide Groups for Branched Multimetallic Systems

Abstract

The combination of [WV(CN)8]3– anions with 3d metal cations MII in MeOH leads to the formation of pentadecanuclear spherical cyanido-bridged clusters {M­[M­(solv)3]8[M′(CN)8]6}, M 9 M′ 6 . By decorating their surface with organic ligands or/and by installation of different ions in their coordination skeleton, one could tune high spin in the ground state, slow relaxation of magnetization, or structural/spin phase transition. In this work we present the extended molecular high spin (S GS = 15/2, g eff = 3.4) clusters or chains of clusters {Co9W6(N,O-L) x } (N,O-L – pyrazine mono-N-oxide, pzmo; 4,4-bipyridine mono-N-oxide – 4,4′-bpmo) equipped with the structurally ordered remote (2–2.5 nm) N-oxide functions, as a result of deliberate combination of solvated Co9W6 supercomplexes with asymmetric N,O-donor linkers L. The systematic occurrence of such motifs in the series 1–3 is a result of preference for the Co–NL coordination over the Co–OL coordination, controlled also by strongly competing supramolecular interactions including simple hydrogen bonding {LNO···H-donor} as well as cooperative π-costacked hydrogen bonding in double cyclic synthons {Co–O–HMeOH···O–NbpmoN−}2. The observed coordination backbones are discussed in terms of the potential to bind the specific external molecular units and create the new type of branched molecular organization. The magnetic properties are confronted with structural differences along 1–3, considering coordination polyhedra, Co–N bond lengths, Co–N–C angles, and hydrogen bonds. The diversity of slow magnetic relaxation images for the known Co9W6 based phases are discussed in terms of local deformation of Co coordination polyhedra and global deformation of cyanide bridged backbones.

Structural Disorder in High-Spin {CoII9WV6} (Core)-[Pyridine N-Oxides] (Shell) Architectures

“…The application of a new type of ligands to fifteen-centered clusters allowed obtaining a completely new character of the ligand shell. In 1, pyNO ligands connected to the Co3 moieties interacts with each other with π-π interactions, which has not been seen in M 9 M' 6 L x clusters with monodentate coordination of ambidentate linkers (L = pyrazine mono-N-oxide (pzmo)) and 4,4-bipyridine mono-N-oxide (4,4'-bpdo, Figure S6, ESI †) [16]. Supramolecular arrangement is also dominated by this type of interaction, but there are also weak O3 MeOH •••N8 CN hydrogen bonds in the direction [001].…”

“…In 1, pyNO ligands connected to the Co3 moieties interacts with each other with π-π interactions, which has not been seen in M9M'6Lx clusters with monodentate coordination of ambidentate linkers (L = pyrazine mono-N-oxide (pzmo)) and 4,4bipyridine mono-N-oxide (4,4'-bpdo, Figure S6, ESI †). [16] Supramolecular arrangement is also dominated by this type of interaction, but there are also weak O3MeOH•••N8CN hydrogen bonds in the direction [001]. Each cluster is surrounded symmetrically by six other clusters, creating a densely packed 3D supramolecular architecture.…”

“…The ligand shell disorder in 1 and 2 is unprecedented along the entire family, and may be understood in terms of the degree of M-L bond rigidity and intercluster interactions inscribed in the ligand structure and M-L bonding mode. The longer linker ligands (e.g., 4,4 -bipyridine mono-N-oxide, 4,4 -bpmo; 4,4 -bipyridine di-N-oxide, 4,4 -bpdo) [16,17] form quite easily the bridging connections between clusters, and/or participate in the hydrogen bond supramolecular interaction network. On the other hand, the convergent bidentate ligand (frequently equipped with the remote substituents, e.g., 4,4 -dimetyl-2,2 -bipyridine, 4,4 -di-tert-butyl-2,2 -bipyridine, or 4,7-di-phenyl-1,10-phenantroline) [20] fill the intercluster space with the π-π synthons or van der Waals contacts.…”

“…In both cases, the degree of freedom is strongly limited, which prevents the disorder. Interestingly, the dataset for pyrazine mono-N-oxide (pzmo) ligand [16], the shorter analog of 4,4 -bpmo, fairly resembles that of pyNO in 1: (i) one of the pzmo ligands reveals severe positional disorder, and (ii) preparation of the product requires the ratio L:Co 2+ ratio of 20:1, a little less than 30:1 in 1. The occurrence of disorder in both cases may be correlated with the small volumes of both ligands, whereas the differences are definitely related to the natural preferences for intermolecular interactions.…”

“…Along this line, investigation of the crystalline phases composed of six capped body-centered cube Co 9 M' 6 clusters as molecular platforms for magnetic relaxations suggests the following image of structure-properties correlations. Twelve compounds were studied structurally and magnetically, counting the original {Co 9 M 6 (MeOH) 24 }•solv (M = Mo, W) phases [12] and those equipped with various ligands: chelating, 2,2 -bipyridine-N-oxide (2,2 -bpmo) [13], 2,2 -bipyridine-N,N'-dioxide (2,2 -bpdo) [14], and methylpyridinemethanol (mpm) [15], as well as bridging 4,4 -bipyridine-N-oxide (4,4 -bpmo) [16], 4,4 -bipyridine-N,N'-dioxide (4,4 -bpdo) [17], and pyrazine-N-oxide (pzmo) [16]. The unequivocal direct fitting of lnτ(T −1 ) point set using the Arrhenius law was possible only in the case {Co 9 W 6 (2,2 -bpdo) 6 (MeOH) 12 }{Co 9 W 6 (2,2 -bpdo) 7 (MeOH) 10 }•8H 2 O•2MeCN•27MeOH to give the best ∆E/k B of 30.0(8) K along the whole series [14].…”