Synthetic, Spectroscopic, and Electrochemical Studies of the Isomerically-Rich [M(CO)(2)(P(2)P')X](+/0) (M = Mn, Re; X = Cl, Br; P(2)P' = eta(3)-Ph(2)P(CH(2))(2)P(Ph)(CH(2))(2)PPh(2)) System: Structural Characterization of a Novel Pair of Diastereoisomers of cis,mer-Re(CO)(2)(P(2)P')Cl.

Abstract

Reaction of Mn(CO)(5)X (X = Cl, Br) with Ph(2)P(CH(2))(2)P(Ph)(CH(2))(2)PPh(2) (P(2)P') in refluxing xylene led to the formation of isomerically pure cis,mer-Mn(CO)(2)(eta(3)-P(2)P')X. Cyclic voltammograms in dichloromethane (0.1 M Bu(4)NPF(6)) show a reversible one-electron oxidation (process 1, E(1/2) = 0.142 V) to give cis,mer-[Mn(CO)(2)(P(2)P')X](+). However, in acetone (0.1 M Bu(4)NPF(6)) at room temperature, process 1 is not reversible and an additional redox process 4 (E(1/2) = 0.048 V) is observed. Process 4 is not observed at low temperatures, and at higher temperatures in acetone it merges with process 1 and also a new reversible redox couple (process 5, E(1/2) = -0.411 V) appears. A combination of electrolyses, chemical oxidation, and subsequent reduction, coupled with IR and (31)P NMR spectroscopies and electrospray mass spectrometry (ESMS), is used to show that processes 1, 4, and 5 are all associated with redox and interconversion reactions of different isomers of Mn(CO)(2)(P(2)P')X and [Mn(CO)(2)(P(2)P')X](+). Other irreversible processes due to oxidation of the different isomers of [Mn(CO)(2)(P(2)P')X](+) are observed at very positive potentials. Reaction between Re(CO)(5)X and P(2)P' in refluxing mesitylene gives a soluble product and a small amount of precipitate. The major soluble product was identified as cis,mer-Re(CO)(2)(P(2)P')X, and the oxidative chemistry is similar to that of the manganese analogues. The precipitate consists of five compounds, one of which was cis,mer-Re(CO)(2)(P(2)P')X. The new compounds A-D were identified as follows: A is cis,mer-{Re(CO)(2)(P(2)P')X}(2), a dimeric species with bridging P(2)P' ligands. The spectroscopic data for B indicated that it was a form of cis,mer-Re(CO)(2)(P(2)P')X, but not the same as the major product. Compound C is cis,fac-Re(CO)(2)(P(2)P')X, and compound D was shown to be fac-[Re(CO)(3)(P(2)P')]X. The crystal structures of cis,mer-Re(CO)(2)(P(2)P')Cl(I) and cis,mer-Re(CO)(2)(P(2)P')Cl(II) show the compounds to be diastereoisomers with the same mer geometry of the P(2)P' ligand, which of necessity generates a cavity formed by three phenyl rings on one side of the rhenium atom. The coordination geometry of the two compounds differ only by the interchange of the mutually trans chloro and carbonyl ligands. In (I) the carbonyl ligand is within the cavity and in (II) the chloro ligand is within the cavity.