Nucleophilic Attack at Pentaarsaferrocene [Cp*Fe(η5 -As5 )]-The Way to Larger Polyarsenide Ligands.

By the reaction of [Cp*Fe(η5 -As5 )] (I) (Cp*=C5 Me5 ) with main group nucleophiles, unique functionalized products with η4 -coordinated polyarsenide (Asn ) units (n=5, 6, 20) are obtained. With carbon-based nucleophiles such as MeLi or KBn (Bn=CH2 Ph), the anionic organo-substituted polyarsenide complexes, [Li(2.2.2-cryptand)][Cp*Fe(η4 -As5 Me)] (1 a) and [K(2.2.2-cryptand)][Cp*Fe{η4 -As5 (CH2 Ph)}] (1 b), are accessible. The use of KAsPh2 leads to a selective and controlled extension of the As5 unit and the formation of the monoanionic compound [K(2.2.2-cryptand][Cp*Fe(η4 -As6 Ph2 )] (2). When I is reacted with [M]As(SiMe3 )2 (M=Li ⋅ THF; K), the formation of the largest known anionic polyarsenide unit in [M'(2.2.2-cryptand)]2 [(Cp*Fe)4 {µ5 -η4 :η4 :η3 :η3 :η1 :η1 -As20 }] (3) occurred (M'=Li (3 a), K (3 b)).

Open Chain Polyarsenides of the Lanthanides.

The reaction of [(Cp'''Co)2 (µ,η2:2 -As2 )2 ] with the decamethylsamarocenes [Cp*2 Sm(THF)2 ] or [Cp*2 Sm], or the bis(tetramethyl-n-propyl)samarocene [(C5 Me4 (n-propyl))2 Sm] resulted in the mixed d/f polyarsenides [(Cp'''Co)2 As4 Sm(η5 -C5 Me4 R)2 ] (R=Me, n-propyl). They are the first structural representatives of open chain-like polyarsenides as ligands in the coordination sphere of lanthanides. Their formation can be explained by an intramolecular As-As coupling within the cobalt polyarsenide complex after reduction by the divalent samarium complex. Density functional theory calculations give insight into the structural change of the (As2 )2 unit in [{Cp'''Co(µ,η2:2 -As2 )}2 ] upon reduction.

Molecular Polyarsenides of the Rare-Earth Elements.

Reduction of [Cp*Fe(η(5)-As5)] with [Cp''2Sm(thf)] (Cp''=η(5)-1,3-(tBu)2C5H3) under various conditions led to [(Cp''2Sm)(µ,η(4):η(4)-As4)(Cp*Fe)] and [(Cp''2Sm)2As7(Cp*Fe)]. Both compounds are the first polyarsenides of the rare-earth metals. [(Cp''2Sm)(µ,η(4):η(4)-As4)(Cp*Fe)] is also the first d/f-triple decker sandwich complex with a purely inorganic planar middle deck. The central As4(2-) unit is isolobal with the 6π-aromatic cyclobutadiene dianion (CH)4(2-). [(Cp''2Sm)2As7(Cp*Fe)] contains an As7(3-) cage, which has a norbornadiene-like structure with two short As-As bonds in the scaffold. DFT calculations confirm all the structural observations. The As-As bond order inside the cyclo As4 ligand in [(Cp''2Sm)(µ,η(4):η(4)-As4)(Cp*Fe)] was estimated to be in between an As-As single bond and a formally aromatic As4(2-) system.