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Mono- and Dinuclear Rhodium(I) and Rhodium(III) Complexes with the Bulky Phosphine 2,6-Me2C6H3CH2CH2PtBu2, Including the First Structurally Characterized Cis-Configurated Dicarbonyl Compound, cis-[RhCl(CO)2(PR3)]

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posted on 01.03.2004, 00:00 authored by Giuseppe Canepa, Carsten D. Brandt, Helmut Werner
The bulky functionalized phosphine 2,6-Me2C6H3CH2CH2PtBu2 (3) was prepared in a stepwise fashion from the Grignard reagent 2,6-Me2C6H3CH2CH2MgCl, tBuPCl2, and tBuLi. Phosphine 3 reacts with the olefin compounds [RhCl(C2H4)2]2 and [RhCl(C8H14)2]2 to give the substitution products [RhCl(olefin)(3)]2 (8, 9), of which 9 (olefin = C2H4) has been characterized crystallographically. While 9 is rather inert toward 3, it reacts with 3 under a hydrogen atmosphere to give the dihydrido complex [RhH2Cl(3)2] (11) via the dimer [RhH2Cl(3)]2 (10) as an intermediate. In the presence of ethene, 11 is reconverted to 9. The reaction of 8 (olefin = C8H14) with the phosphonium salt 3·HCl affords a mixture of [RhHCl2(3)]2 (12) and 3, which upon warming at 60 °C produces [RhHCl2(3)2] (13). The attempted separation of 12 by column chromatography on Al2O3 led unexpectedly to the formation of the novel half-sandwich-type compound [(η6-2,6-Me2C6H3CH2CH2PtBu2P)RhCl] (14). Treatment of either 8 or 14 with CO gives cis-[RhCl(CO)2(3)] (15), which represents the first structurally characterized dicarbonyl complex with the CO ligands in a cis disposition. In the absence of carbon monoxide, 15 is rather labile and readily converted to [RhCl(CO)(3)]2 (16). The reaction of 9 with HCl affords the dinuclear ethylrhodium(III) compound 18, being built up by two 14-electron [RhCl2(C2H5)(3)] units. These units are linked by two bridging chlorides which are unsymmetrically situated between the two metal centers. Compound 18 reacts with CO to give 15 and with 3 to give 13, presumably in both cases via [RhHCl2(C2H4)(3)] as an intermediate. The half-sandwich-type complexes [(η6-2,6-Me2C6H3CH2CH2PtBu2P)Rh(olefin)]PF6 (20, 21), obtained from [Rh(acetone)2(C8H14)2]PF6 as the precursor, react in acetone in the presence of H2 to give [RhH2(acetone)3(3)]PF6 (22), which upon addition of diethyl ether generates the dihydride [(η6-2,6-Me2C6H3CH2CH2PtBu2P)RhH2]PF6 (23). Compound 23 is a suitable starting material for the preparation of compounds having the general composition [(η6-2,6-Me2C6H3CH2CH2PtBu2P)Rh(L)]PF6, where L is acetone, CH2CHR (R = tBu, Ph), PhC⋮CH, and CO, respectively.