Octahedral niobium cluster-based solid state halides and oxyhalides: effects of the cluster condensation via an oxygen ligand on electronic and magnetic properties

Abstract

International audienceThe influences of an oxygen ligand on the structural, magnetic and electronic properties of octahedral niobium cluster-based oxides and oxychlorides are reported. The Nb6 metal cluster is edge-bridged by twelve inner ligands and additionally bonded to six apical ligands to form Nb6Li12La6 units (L = Cl, O) wherein oxygen and chlorine are perfectly ordered. Oxygen favours the interconnection of clusters via double Oi-a/Oa-i bridges in a similar way to the double Si-a/Sa-i bridges found in Chevrel phases based on face capped Mo6Li8La6 units. Periodic density functional theory (DFT) calculations confirm that increasing the number of inner oxygen ligands at the expense of chlorine atoms favours the 14 metal-electron (ME) count per octahedral cluster unit. It is also shown that weak interactions occur between neighbouring clusters. Indeed, magnetic measurements performed on AxNb6Cl12O2 (A = Rb, x = 0.816(8); A = Cs, x = 1) series containing 15-ME species evidence antiferromagnetic interactions at low temperatures. Broken-symmetry DFT calculations of exchange parameters within spin dimer analysis confirm the experimental results

Air‐Stable, Well‐Soluble AI2[Nb6Cl18] Cluster Compounds (AI = Organic Cation): A New Route for Preparation, Single‐Crystal Structures, Properties, and ESI‐Mass Spectra

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] If compounds from high temperature cluster syntheses are used directly as precursors for subsequent chemical reactions, quite often difficulties are observed in performing stoichiometric reactions, since the compounds from the solid state chemical reactions are accompanied by side products and/or unreacted starting materials.…”

Unprecedented Electron‐Poor Octahedral Ta6 Clusters in a Solid‐State Compound: Synthesis, Characterisations and Theoretical Investigations of Cs2BaTa6Br15O3

“…Previous theoretical studies have shown that replacing one halogen inner ligand by an oxygen atom hardly modify the electronic structure of the cluster unit, whereas a larger number of bridging oxygen ligands destabilises the a 2u level making the ME count of fourteen optimal 21. 29 It has also been shown that the [(M 6 ${{\rm Cl}{{{\rm i}\hfill \atop 9\hfill}}}$ ${{\rm O}{{{\rm i}\hfill \atop 3\hfill}}}$ )${{\rm Cl}{{{\rm a}\hfill \atop 6\hfill}}}$ ] 5− (M=Nb, Ta) isomer of D 3 symmetry with alternate inner oxygen ligands is more stable than the arrangement of C 2 symmetry where the inner oxygen ligands are adjacent 21. The geometry of both isomers of [(Ta 6 ${{\rm Br}{{{\rm i}\hfill \atop 9\hfill}}}$ ${{\rm O}{{{\rm i}\hfill \atop 3\hfill}}}$ )${{\rm Br}{{{\rm a}\hfill \atop 6\hfill}}}$ ] 5− with 14‐MEs have been optimised.…”

“…DFT calculations have shown that the inner ligands play a crucial role in the propagation of magnetic interactions in such compounds. Weaker intercluster antiferromagnetic interactions also exist in A x Nb 6 Cl 12 O 2 (A=Rb, Cs) compounds that contain some 15‐ME hexanuclear species 29. In these latter, neighbouring clusters are connected through O i‐a /O a‐i double bridges in two directions 29.…”

“…Some species contain 15‐MEs, clusters with one unpaired electron on the highest occupied molecular orbital (HOMO) level. If singly occupied MOs (SOMOs) of neighbouring 15‐ME clusters overlap, antiferromagnetic interactions can occur at low temperature, as found for instance in the halides Nb 6 F 15 , LuNb 6 Cl 18 or in the oxyhalide CsNb 6 Cl 12 O 2 29…”

“…If singly occupied MOs (SOMOs) of neighbouring 15-ME clusters overlap, antiferromagnetic interactions can occur at low temperature, as found for instance in the halides Nb 6 F 15 , LuNb 6 Cl 18 or in the oxyhalide CsNb 6 Cl 12 O 2 . [29] Electron counts lower than fourteen are disfavoured because some strongly MÀM bonding MOs should be depopulated. Hitherto, KNb 8 O 14 , based on [(Nb 6 O i 12 )O a 6 ] nÀ cluster units is the single example of a solid-state compound containing thirteen ME clusters but the semiconducting behaviour of this material conflicts with its reported magnetic properties.…”