Synthesis and characterisation of disubstituted di-µ-methylene-bis(η-pentamethylcyclopentadienyl)dirhodium(IV) complexes; X-ray structure of [{(C5Me5)Rh}2(µ-CH2)2(CO)2]2+

Abstract

Reaction of either cis- or trans-[{(C₅Me₅)Rh}₂(µ-CH₂)₂Me₂] with HCl in pentane gave the trans-dichloro-complex [{(C₅Me₅)Rh}₂(µ-CH₂)₂X₂](4a; X = Cl), from which a variety of other complexes (4; X = Br, I, SCN, N₃, CN, or NCO) were made by metathesis. Reaction of (4a; X = Cl) with neutral ligands (L) in the presence of non-co-ordinating anions gave first the monocations, [{(C₅Me₅)Rh}₂(µ-CH₂)₂(L)Cl]⁺, and then the dications, [{(C₅Me₅)Rh}₂(µ-CH₂)₂L₂]²⁺(5; L = MeCN, CO, or H₂O). The X-ray crystal structure of (5b; L = CO) shows the trans configuration predicted on the basis of the ¹H n.m.r. spectra. Reaction of (5b) with methanol and base gave the bis(methoxycarbonyl) complex [{(C₅Me₅)Rh}₂(µ-CH₂)₂(CO₂Me)₂]. cis Complexes [{(C₅Me₅)Rh}₂(µ-CH₂)₂X]ⁿ⁺(X = O₂CMe or O₂CCF₃, n= 1; X = pyridazine or Ph₂PCH₂PPh₂, n= 2) were isolated and identified by their ¹H n.m.r. spectra which showed the diastereotopic methylene protons as two resonances, only one of which was coupled to the rhodiums. The nitrate complex [{(C₅Me₅)Rh}₂(µ-CH₂)₂(NO₃)₂] was found to exist as two isomers, one trans and the other cis with one bridging and one ionic nitrate, as shown by the n.m.r. spectra.