60255-23-4Relevant academic research and scientific papers
Speciation and kinetics related to catalytic carbonylation in the presence of cis-[Ir(CO)2I2]P(C6H5)4 under CO and H2 pressures
Churlaud, Raphael,Frey, Urban,Metz, Francois,Merbach, Andre E.
, p. 4137 - 4142 (2000)
The differences in the reactivities of the square-planar complexes cis-[Rh(CO)2I2]- (1) and cis-[Ir(CO)2I2]- (2), involved in the catalytic carbonylation of olefins, are investigated, with P(C6H5)4+ as the counterion, by ambient- and high-pressure NMR and IR spectroscopy. Under an elevated pressure of CO, 1 and 2 form the [M(CO)3I] complexes with the equilibrium constants K(Ir) ? 1.8 x 10-3 and K(Rh) ? 4 x 10-5. The ratio K(Ir)/K(Rh) close to 50 shows that, under catalytic conditions (a few megapascals), only complex 1 remains in the anionic form, while a major amount of the iridium analogue 2 is converted to a neutral species. The oxidative addition reactions of HI with 1 and 2 give two monohydrides of different geometries, mer,trans-[HRh(CO)2I3]- (3) and fac,cis-[HIr(CO)2I3]- (4), respectively. Both hydrides are unstable at ambient temperature and form, within minutes for Rh and within hours for Ir, the corresponding cis-[M(CO)2I2]- (1 or 2) and [M(CO)2I4]- (5 or 6) species and H2. When an H2 pressure of 5.5 MPa is applied to a nitromethane solution of complex 2, ca. 50% of 2 is transformed to cis-dihydride complexes. The formation of cis,cis,cis-[IrH2(CO)2I2]- (8a) is followed by intermolecular rearrangements to form cis,trans,cis-[IrH2(CO)2I2]- (8b) and cis,cis,trans-[IrH2(CO)2I2]- (8c). A small amount of a dinuclear species, [Ir2H(CO)4I4](x-) (9), is also observed. The formation rate constants for 8a and 8b at 262 K are k1/262 = (4.42 ± 0.18) x 10-4 M-1 s-1, k-1/262 = (1.49 ± 0.07) x 10-4 s-1, k2/262 = (2.81 ±0.04) x 10-5 s-1, and k-2/262 = (5.47 ± 0.16) x 10-6 s-1. The two equilibrium constants K1/262 = [8a]/([2][H2]) = 2.97 ± 0.03 M-1 and K2/262 = [8b]/[8a] = 5.13 ± 0.10 show that complex 8b is the thermodynamically stable addition product. However, no similar H2 addition products of the rhodium analogue 1 are observed. The pressurization with H2 of a solution containing 2 and 6 give the monohydride 4, the dihydrides 8a and 8b, the dinuclear complex 9, and the two new complexes [Ir(CO)2I3] (10) and [HIr(CO)2I2] (11). The reactions of the iridium complexes with H2 and HI are summarized in a single scheme.
