178325-33-2Relevant academic research and scientific papers
Stereochemical and electronic control of functionalized tripodal phosphines. Reactivity of the adamantane-type Ir(tripod) (CO)Cl (tripod = cis,cis-1,3,5-(PPh2)3-1,3,5-X3C 6H6; X = H, COOMe, CN) complexes toward H+, H2, CO, and C2H4
St?ssel, Philipp,Heins, Wolfgang,Mayer, Hermann A.,Fawzi, Riad,Steimann, Manfred
, p. 3393 - 3403 (2008/10/08)
Treatment of the tripodal phosphine ligand cis,cis-1,3,5-tris(diphenylphosphino)-1,3,5-tris-(methoxycarbonyl)cyclohexane (tdppcyme) (2) with Ir(PPh3)2(CO)Cl gives the carbonyl chloro complex Ir(tdppcyme)(CO)Cl (2a). NMR spectroscopic investigations prove a dynamic equilibrium between a square-planar (2a′) and a trigonal-bipyramidal (2a) complex with two and three phosphine groups coordinated to iridium, respectively. This is in contrast to the comparable complexes Ir(tripod)(CO)Cl (1a, 3a) [tripod = cis,czs-1,3,5-tris(diphenylphosphino)cyclohexane (tdppcy) (1), cis,Cis-1,3,5-tricyano-cis,cis-1,3,5-tris(diphenylphosphino)-cyclohexane (tdppcycn) (3)] which only exist in the trigonal-bipyramidal form. The three compounds Ir(tripod)(CO)Cl (1a-3a), which only differ in the functionalization of the ligand backbone, display altered basicity and thus different reaction patterns. The carbonyl chloro complexes 1a-3a are readily protonated by the strong acid HBF4 to give the octahedral cations 1b-3b. The weaker acid NH4PF6 is only able to protonate the most basic complex Ir(tdppcy)(CO)Cl (1a). While Ir(tdppcycn)(CO)Cl (3a) shows no reaction in the presence of NH4PF6, 2a is orthometalated to complex 2f′. The compounds Ir(tdppcy)(CO)Cl (1a) and Ir(tdppcycn)(CO)Cl (3a) oxidatively add H2 with dissociation of Cl- to yield the octahedral dihydrides [Ir(tripod)(CO)(H)2]Cl (1c′, 3c′). Under the same conditions a phosphine arm dissociates from 2a to form the dihydride Ir(η2-tdppcyme)(CO)(H)2Cl (2c′). In a CO atmosphere the Ir(tripod)(CO)Cl (1a-3a) complexes give the dicarbonyl derivatives [Ir-(tripod)(CO)2]Cl (1d′-3d′). The dicarbonyls 2d′ and 3d′ are in an equilibrium with their starting materials. Replacement of the Cl- by BPh4- allows the isolation of all three dicarbonyl complexes. Treatment of 1a-3a with ethylene in the presence Of NaBPh4 gives the ethylene complexes [Ir(tripod)(CO)(C2H4)BPh4 (1e-3e). Variable-temperature NMR studies show that there is an exchange process which equilibrates the equatorial with the apical phosphorus nuclei while the CO and ethylene ligands remain rigid. In the cases of 2e and 3e a second dynamic process with a low-energy barrier exists. Above 45 and 35 °C, respectively, 1e and 2e lose ethylene and are orthometalated to 1f and 2f. The cyclometalated products 1f and 2f can also be obtained by the direct reaction of 1a and 2a with NaBPh4. For the dicarbonyl complex 3a and the orthometalated product 2f X-ray structure determinations have been performed.
