Tetranuclear Complexes from the Reaction of Tetraphosphorus Triselenide with Cyclopentadienylchromium Tricarbonyl Dimer. Synthesis, Thermal Degradation, and X-ray Crystal Structures of Cp₄Cr₄(CO)₉(P₄Se₃)·¹/₂C₆H₆ and Cp₄Cr₄(CO)₈(P₂Se₂)¹

The reaction of [CpCr(CO) 3 ] 2 with P 4 Se 3 has been investigated. Under mild conditions, Cp 4 Cr 4 (CO) 9 (P 4 Se 3 ) ( 4 ) was the main product (48% yield), alongside low yields of CpCr(CO) 3 H ( 5 ), CpCr(CO) 2 P 3 ( 6 ), [CpCr(CO) 2 ] 2 P 2 ( 7 ), and Cp 4 Cr 4 (CO) 8 (P 2 Se 2 ) ( 8 ). Product isolation under various reaction conditions, together with 1 H NMR spectral monitoring of the reaction and thermolytic studies of the complexes 4 and 8, indicates that 4 and 5 were the primary products of the reaction. Thermolysis of 4 led to 8, which in turn degraded to give 6, 7, and Cp 4 Cr 4 Se 4 ( 9 ). The complexes 4 and 8 were also obtained from the reaction of [CpCr(CO) 2 ] 2 Se ( 3 ) with P 4 Se 3 or P 4 S 3; the reaction with the latter also gave [CpCr(CO) 2 ] 2 S ( 10 ), the sulfur analogue of 3, as an additional product. Mechanistic pathways via the monomeric CpCr(CO) 3 species and the Cr 2 Se complex 3 are proposed. The cothermolysis of [CpCr(CO) 2 ] 2 (Cr⋮Cr) ( 2 ) with P 4 Se 3 led to the isolation of 5, 6, 7, and 9 . All the complexes have been characterized spectroscopically, while 4 and 8 have also been studied by X-ray diffraction analysis. The complex 8 possesses an unusual open-book framework with a P−P bond as its backbone and a 2-fold symmetry axis passing through its midpoint.