Iridium tertiary phosphine

Although trialkyl- and triarylbismuthines are much weaker donors than the corresponding phosphoms, arsenic, and antimony compounds, they have nevertheless been employed to a considerable extent as ligands in transition metal complexes. The metals coordinated to the bismuth in these complexes include chromium (72—77), cobalt (78,79), iridium (80), iron (77,81,82), manganese (83,84), molybdenum (72,75—77,85—89), nickel (75,79,90,91), niobium (92), rhodium (93,94), silver (95—97), tungsten (72,75—77,87,89), uranium (98), and vanadium (99). The coordination compounds formed from tertiary bismuthines are less stable than those formed from tertiary phosphines, arsines, or stibines. 

The iridium(II) complexes are rarer that those of rhodium(II). Iridium does not seem to form carboxylates Ir2(02CR)4 with the lantern structure complexes analogous to trans-RhX2 (PR3 )2 are not formed with bulky tertiary phosphines, probably because the greater strength of Ir-H bonds leads to IrHX2(PR3)2. 

A considerable number of the tertiary phosphine and arsine complexes of iridium(III) have been synthesized [4, 8] they generally contain 6-coordinate iridium and are conventionally prepared by refluxing Na2IrCl6 with the phosphine in ethanol or 2-methoxyethanol [154]... 

In an alternative sequence suggested by Eisenberg et al. (80), X in Eq. (19) is hydride. C02 production then proceeds via /3-elimination from the oxygen OH in 18, giving a rhodium(III) dihydrido species which can then reductively eliminate H2. Some support for this latter suggestion is provided by the observation that iridium(lll) species of type 19, formed by oxidative addition of formic acid to /ra/ s-[IrCI(CO)L2] (L = tertiary phosphine), rapidly lose C02 to give the dihydrido species 20 (81). 

Chemistry of Tertiary Phosphine Complexes of Rhodium, Iridium, and Platinum... 

The products of oxidative addition of acyl chlorides and alkyl halides to various tertiary phosphine complexes of rhodium(I) and iridium(I) are discussed. Features of interest include (1) an equilibrium between a five-coordinate acetylrhodium(III) cation and its six-coordinate methyl(carbonyl) isomer which is established at an intermediate rate on the NMR time scale at room temperature, and (2) a solvent-dependent secondary- to normal-alkyl-group isomerization in octahedral al-kyliridium(III) complexes. The chemistry of monomeric, tertiary phosphine-stabilized hydroxoplatinum(II) complexes is reviewed, with emphasis on their conversion into hydrido -alkyl or -aryl complexes. Evidence for an electronic cis-PtP bond-weakening influence is presented. 

We conclude that in octahedral alkyliridium(III) complexes the presence of tertiary phosphines favors exclusively the n -alkyl over the corresponding secondary alkyl, irrespective of the size or basicity of the phosphine. This preference is probably largely electronic in origin, but steric factors cannot be ruled out. A key step that generates a vacant coordination site for both alkyl-group migration and isomerization in octahedral tertiary phosphine complexes of rhodium(III) and iridium(III) is dissociation of halide ion. 

Several systematic experimental and computational studies have compared the sigma-donating abilities of NHCs and tertiary phosphines for a variety of transition-metal complexes [8-17]. As illustrative examples, analyses of the nickel-carbonyl complex 1 and iridium carbonyl complex 2 (Fig. 1) re-... 

It will also be noticed that all the above catalysts contain second transition series metals. Generally, the slower reactions of the third transition series elements are not normally conducive to catalytic efficiency, although some very active iridium catalysts are now known. First transition series metals seldom form stable, lower oxidation state tertiary phosphine complexes. 

An NMR study (597) of ligand exchange in the system (diene)MCl(L) (diene = norbornadiene or 1,5-cyclooctadiene M = Rh or Ir L = tertiary phosphine, arsine, or stibene) shows a first-order dependence of the rate upon both L and the olefin complex in the temperature range from —70° to —10°C. The exchange involves an 8 2 mechanism with the five-coordinate complex (diene)MCl(L)2 as intermediate. The intermediate iridium complexes (l,5-CgHi2)IrCl(L)2 can be isolated from ethanolic solution. The activation energy for the process ranges from 4 to 10 kcal/mole (597). 

Similarly, there are examples of rhodium(I) and iridium(I) tertiary phosphine complexes that form isolable dihydrides, which with separate treatment with external base yield monohydrides, equation (k) . Hydrogenations catalyzed by rran5-RhCl(CO)(PPh3)2 " may involve rrani-RhH(CO)(PPh3)2 formed according to equation (1) via an undetected dihydride intermediate. In some aminophosphine analogues, a coordinated N atom may act as proton acceptor s. 

The rhodium complexes are more resistant towards oxidative addition than their iridium counterparts and this is believed to be linked to steric crowding, especially when employing bulky tertiary phosphine ligands. Work by Wilkinson explored several aspects of the steric and electronic properties of the rhodium(i) analogues, but definite crystal structural confirmation of the reaction... 

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