Comparative Studies on Ether−Phosphine-Assisted Exchange Processes in the Rhodium(III) and Iridium(III) Complexes ClMH₂(P⌒O)(P∼O)

Abstract

The rhodium and iridium complexes ClRhH2(P⌒O)(P∼O) (2a,b) and ClIrH2(P⌒O)(P∼O) (3a) (P⌒O = η2(P,O) coordination, P∼O = η1(P) coordination of Cy2PCH2CH2OCH3 (a) and Cy2PCH2C4H7O (b); C4H7O = tetrahydrofuranyl) were subjected to dynamic NMR studies. They exhibit a fast fluxional behavior both of the two ether oxygen donors and of the two cis hydrides at room temperature. The exchange of the hydrides is assisted by the fact that one of the ether moieties simulates an external solvent. The complexes 2a,b show the same behavior as is suggested for the coordinatively unsaturated species ClRhH2(PPh3)2, which is generally postulated to be an active intermediate in the catalytic hydrogenation of olefins. In the course of the formation of 3a from its ortho-metalated (4a; = η2(P,CH2) coordination of Cy2PCH2CH2OCH2) and dihydrogen, one C−H bond of the methoxy group and an additional Ir−H bond were formed. This was confirmed by the corresponding reaction of 4a with D2 to give the analogous dideuterium complex 3a‘. The opposite behavior was observed when 4a was treated with DCl: one C−D and one Ir−Cl bond were formed in the complex Cl2IrH(P⌒O)(P∼O) (5a; one Cy2PCH2CH2OCH2D ligand) while the hydride remained at the metal. The 103Rh chemical shift has been determined for compound 2b.

Iridium and Rhodium Complexes with the Hemilabile Ligand [2‐(1,3‐Dioxolane‐2‐yl)phenyl]diphenylphosphane – Behaviour in Solution and Structural Characterization

“…The NMR spectra of 9 in CDCl 3 shows broad resonances at room temperature, which suggests dynamic behaviour in solution. This is in accordance with the lower oxophilicity of rhodium(III) compared to iridium(III) 31. A related rhodium complex containing a phosphane–aldehyde chelate is also highly fluxional 32.…”

“…The acyl group and the other oxygen atom are also mutually trans. [31] A related rhodium complex containing a phosphane-aldehyde chelate is also highly fluxional. The Ir-P1 bond [2.281(2) Å] is shorter than the Ir-P2 bond [2.330(2) Å], and this may be due to P1 forming a five-membered chelate and P2 being in a nonplanar sixmembered chelate.…”

“…At room temperature, the NMR spectra in CDCl 3 show sharp signals. [31,39] The 31 P{ 1 H} and 1 H NMR resonances of 12 broaden when the temperature is lowered, which suggests that it is an octahedral species of the [IrCl(H) 2 (P~O)(P ∧ O)] type that exchanges phosphane and also hydride ligands. Accordingly, a pentacoordinate structure could be envisaged with two monodentate phosphane ligands mutually trans and two hydride ligands and a chloride ligand in the equatorial plane of a trigonal bipyramid, which resembles some analogues with phosphanes [37] or pincer ligands.…”

“…In the 1 H NMR spectrum, only one triplet at -26.64 ppm (J P,H = 16.8 Hz) owing to hydride ligands can be seen, and the 31 P{ 1 H} NMR spectrum displays a singlet at δ = 14.1 ppm. The most accepted mechanism for this behaviour is the so called "migratory mechanism", [31,40] which involves O-dissociation of the coordinated ether-phosphane ligand to afford a trigonal bipyramidal intermediate and O-coordination of the other ether-phosphane ligand. [38] However, rhodium or iridium dihydrides containing phosphane-ether ligands of the [MCl(H) 2 -(P~O)(P ∧ O)] type have been reported to show fluxional behaviour with very low energy barriers.…”

Structural and Theoretical Studies ofortho-Substituted Triphenylphosphane Ligands and Their Rhodium(I) Complexes

“…[6] With the possible hemilabile character of these ligands, a more weakly coordinated donor site may appear between the coordinated/uncoordinated equilibrium, allowing an interesting setup for catalytic reactions. Hemilabile character has earlier been reported for several mixed heteroatom-phosphorus ligands, including ether-phosphanes, [7] hydroxyphenyl phosphanes, [8] phosphonate-phosphanes, [9] and amido-diphosphanes. [10] The fluxional behavior of this type of ligand around the metal center is considered to activate CO and H 2 molecules, thus improving the oxo-reactions such as carbonylation and hydroformylation.…”