Synthesis of ROMP Monomers Containing Metal–Metal Bonds

Abstract

Methods for the synthesis of cyclic monomers that have both metal–metal bonds and carbon–carbon double bonds are reported. Ring opening metathesis polymerization (ROMP) of these monomers would yield polymers that are photochemically degradable. The first method investigated involved substitution of Cp₂Fe₂(CO)₄ by the bidentate phosphine ligand DPPEN (Ph₂P ${-}$CH=CH–PPh₂). Cp₂Fe₂(CO)₂( $\upmu$-DPPEN) was synthesized and the X-ray crystal structure is reported but the molecule could not be polymerized by a ROMP method using Grubbs’s catalyst. The inability of this monomer to polymerize (or copolymerize with cyclooctatetraene) was attributed to the bulky phenyl rings being in close proximity to the C=C in the DPPEN ligand, which prevents coordination of the monomer to the catalyst. To decrease the steric interactions, the DPPBN ligand was synthesized (DPPBN=Ph $_{2}{-}$CH $_{2}{-}$CH=CH ${-}$CH $_{2}{-}$PPh₂). However, the reaction of DPPBN with Cp₂Fe₂(CO)₄ yielded the product Cp₂Fe₂(CO)₂( $\upmu$-1,2,4-triphos), where the 1,2,4-triphos ligand is a tridentate ligand formed by the formal additional of Ph₂PH to DPPBN (1,2,4-triphos=Ph $_{2}{-}$CH $_{2}{-}$CH(PPh₂) ${-}$CH $_{2}{-}$CH $_{2}{-}$PPh₂). An X-ray structure of the Cp₂Fe₂(CO)₂( $\upmu$-1,2,4-triphos) complex revealed that the 1,2,4-triphos ligand chelates exclusively through the two phosphorus atoms that are bridged by two carbon atoms. It is suggested that this structural feature may simply reflect the increased stability of the 6-membered ring over the 7- and 8-membered rings. The reactions of the Cp₂Mo₂(CO)₆ and Cp₂Mo₂(CO)₄ dimers with DPPBN were investigated next. Reactions of Cp₂Mo₂(CO)₆ and Cp₂Mo₂(CO)₄ with the DPPEN and DPPBN ligands resulted in the disproportionation of the dimers. The X-ray crystal structure of [CpMo(CO)₂(DPPEN)][CpMo(CO)₃] was determined and is reported. The CpMo(CO)(DPPEN)Cl complex was formed when these same reactions were carried out in the presence of CH₂Cl₂. The X-ray crystal structure of this molecule is also reported.