Alcoholate and Mixed Alcoholate/Iodide Supported Hexanuclear Niobium Cluster Compounds with a Mixed Face-Bridged/Edge-Bridged Cluster Pair Example

Abstract

Chemical reactions of ball-mill activated [Nb6I11] with alcohol solutions of Mg alcoholates result in the formation of the three new hexanuclear Nb cluster compounds [Nb6I8(HOCH3)6][Nb6(OCH3)18]·2CH3OH (1), (PPh4)2[Nb6(OC2H5)12I6]·C2H5OH (2) and (PPN)2[Nb6(OC2H5)12I6] (3). They contain cluster anions, which have the edges of the Nb6 octahedra, i.e. the inner coordination sites, μ2-bridged by the O atoms of methanolato or ethanolato ligands. Thereby, they represent further characterized members of the so far very small group of hexanuclear cluster alcoholates. The terminal metal sites are bonded to further methanolato (1) or iodido ligands (2 and 3). Sterically demanding organic cations (PPh4+, PPN+, 2 and 3) or a cationic Nb6 cluster complex with terminal methanol ligands [Nb6I8(HOCH3)6]2+ in 1 compensate the negative charges of the cluster anions. Compound 1 is the first example of a M6 cluster ion pair of both the text-book known edge-bridged [M6X12] and face-bridged [M6X8] cluster units. The structures of the three title compounds were determined by means of single-crystal X-ray structure analysis. Graphical Abstract

Hexanuclear Niobium Cluster Alcoholates – First Representatives with Isopropanolato Ligands

“…The title compounds, which are mixed niobium halide‐alkoxides, are accessible by ligand exchange reactions starting from (Ph 4 P) 2 [Nb 6 Cl 12 I 6 ] [39] . The substitution of halide (Cl and I) by alcoholate anions requires strong Brønsted basic conditions, which are achieved by Mg(O R ) 2 ( R = CH 3 , C 2 H 5 ) for 1 and 2 or Li(OC 3 H 7 ) solutions for 3 and 4 [26–27] . Under these conditions Nb 6 cluster units are easily oxidized and decompose such that strict anaerobic conditions are required.…”

“…The cluster anion in 2 has all inner sites occupied by ethanolato ligands, as decribed by the formula [Nb 6 (OC 2 H 5 ) i 12 Cl a 6 ] 2− . Comparable anions are [Nb 6 (OC 2 H 5 ) i 12 I a 6 ] 2− and the Ta analogue [Ta 6 (OC 2 H 5 ) i 12 I a 6 ] 2− [26–27] . The average M − M atom ditances in these three compounds are 2.861 Å ( 2 ), 2.856 Å ([Nb 6 (OC 2 H 5 ) i 12 I a 6 ] 2− ), and 2.834 Å ([Ta 6 (OC 2 H 5 ) i 12 I a 6 ] 2− ).…”

“…[39] The substitution of halide (Cl and I) by alcoholate anions requires strong Brønsted basic conditions, which are achieved by Mg(OR) 2 (R = CH 3 , C 2 H 5 ) for 1 and 2 or Li(OC 3 H 7 ) solutions for 3 and 4. [26][27] Under these conditions Nb 6 cluster units are easily oxidized and decompose such that strict anaerobic conditions are required. Nevertheless, the yields do not exceed ~59 %.…”

“…All compounds but 1 contain 14 CBE cluster cores, as do all the other so far known Nb 6 cluster compounds with O-donor functions on inner coordination sites. [25][26][27]35] The stability of the higher oxidation state of the cluster core likely is related to the higher Pearson hardness of the O donor ligand compared to Cl, Br, or I. [36] The cluster anion in 2 has all inner sites occupied by ethanolato ligands, as decribed by the formula [Nb .…”

“…Further examples of Nb 6 alcoholates are [Nb 6 I 8 (HOCH 3 ) 6 ][Nb 6 (OCH 3 ) 18 ] • 2CH 3 OH and A 2 [Nb 6 (OC 2 H 5 ) 12 I 6 ] (A = Ph 4 P, PPN). [27] In this publication four new compounds are described, which carry alcoholato ligands around their Nb 6 metal atom core: They have the formulas [Mg(HOCH…”