Controlled Ring Opening of a Tetracyclic Tetraphosphane with Twofold Metallocene Bridging

A direct route to a doubly ferrocene bridged tetracyclic tetraphosphane 1 was developed via reductive coupling of Fe(CpPCl₂)₂ (2 a), where a chlorine terminated linear P₄-compound 3 could be identified as an intermediate. Selective P-P bond activation was further achieved by reacting 1 with elemental selenium or [Cp*Al]₄, where regioselective insertion of Se or Al atoms resulted in ferrocenylene bridged [P₄Se] (4) or [P₄Al] (7) moieties. Compound 7 can be transformed to a hydrogen terminated linear P₄ species, 8, with protic solvents. Methylation of compound 1 with MeOTf, proceeds via intermediate formation of monomethylated species 5, which gradually produced Me₂-terminated dicationic 6, again containing a linear P₄-unit. Besides spectroscopic characterization, the structural details of compounds 1, 4, 6, and 8 could be determined by SC-XRD. Moreover, DFT calculations were used to rationalize the reactivity of 1, derived compounds and intermediates. As a key feature, 1 undergoes ring opening polymerization to a linear polyphosphane 9.