Synthesis and Reactivity of a Cyclooctatetraene-Like Polyphosphorus Ligand Complex [Cyclo-P 8 ].

The thermolysis of Cp'''Ta(CO) 4 with white phosphorus (P 4 ) gives access to [{Cp'''Ta} 2 (μ,η 2 : 2 : 2 : 2 : 1 : 1 -P 8 )] (A), representing the first complex containing a cyclooctatetraene-like (COT) cyclo-P 8 ligand. While ring sizes of n >6 have remained elusive for cyclo-P n structural motifs, the choice of the transition metal, co-ligand and reaction conditions allowed the isolation of A. Reactivity investigations reveal its versatile coordination behaviour as well as its redox properties. Oxidation leads to dimerization to afford [{Cp'''Ta} 4 (μ 4 ,η 2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1 -P 16 )][TEF] 2 (4, TEF=[Al(OC{CF 3 } 3 ) 4 ] - ). Reduction, however, leads to the fission of one P-P bond in A followed by rapid dimerization to form [K@[2.2.2]cryptand] 2 [{Cp'''Ta} 4 (μ 4 ,η 2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1 -P 16 )] (5), which features an unprecedented chain-type P 16 ligand. Lastly, A serves as a P 2 synthon, via ring contraction to the triple-decker complex [{Cp'''Ta} 2 (μ,η 6 : 6 -P 6 )] (B).