PHOSPHAPENTADIENYL-IRIDIUM-PHOSPHINE CHEMISTRY - SYNTHESIS, STRUCTURE, AND SPECTROSCOPY OF PHOSPHAPENTADIENYL-BRIDGED IRIDIUM DIMERS

The reactions of new lithium phosphapentadienide reagents with ClIr(PR (3))(3) have been investigated. Treatment of ClIr(PEt(3))(3) with lith ium phosphapentadienide produced the phosphapentadienyl-bridged dimer [mu-eta(1)-phosphapentadienyl)Ir(PEt(3))(2)](2) (1) as a mixture of tr ans (1a) and cis (1b) isomers. The analogous reaction involving ClIr(P Et(2)Ph)3 produced (mu-)eta(1)-phosphapentadienyl)Ir(PEt(2)Ph)(2)](2) (2). Again, a 1.4:1 mixture of trans (2a) and cis (2b) isomers was obt ained. The mechanism of trans --> cis isomer conversion was probed by means of a ''crossover'' experiment: pure samples of trans isomers 19 and 2a were combined and their conversion to the equilibrium mixture o f trans and cis isomers was monitored. The absence of ''mixed-dimer'' products (i.e., those containing both Ir(PEt(3))(2) and Ir(PEt(2)Ph)(2 ) moieties) in these reactions ruled out a trans --> cis isomerization mechanism involving monomeric (phosphapentadienyl)Ir(PR(3))(2) units and supported a mechanism involving dissociation of one iridium-phosph ido bond, inversion at the resulting terminal phosphido center, and re -formation of the iridium-phosphido bond. Treatment of ClIr(PEt(3))(3) with lithium 2,4-dimethylphosphapentadienide generated )-2,4-dimethyl phosphapentadienyl)Ir(PEt(3))(2)](2) (3). Again, an equilibrium mixtur e of trans (3a) and cis (3b) isomers was obtained, but the presence of the bulky dimethylphosphapentadienyl ligand led to a higher fraction of trans isomer (3a:3b = 8:1). The structure of 3a was confirmed by X- ray crystallography (triclinic, P ($) over bar 1, a = 11.756(4)Angstro m, b = 12.011(3)Angstrom, c = 17.716(7)Angstrom, alpha = 93.50(3)degre es, beta = 92.89(3)degrees, gamma = 108.67(3)degrees, V = 2359.0(15) A ngstrom(3), Z = 2, R = 0.0277, R(W) = 0.0345).