| Abstract
| - Treatment of trans-[PtCl4(RCN)2] (R = Me, Et) with R‘OH (R‘ = Me, Et, n-Pr, i-Pr, n-Bu) at 45 °C in all casesallowed the isolation of the trans-[PtCl4{(E)-NHC(R)OR‘}2] imino ester complexes, while the reaction betweencis-[PtCl4(RCN)2] and the least sterically hindered alcohols (methanol and ethanol) results in the formation of cis-[PtCl4{(E)-NHC(R)OR‘}2] (R/R‘ = Me/Me) or trans-[PtCl4{(E)-NHC(Et)OR‘}2] (R‘ = Me, Et), the latter beingformed via thermal isomerization (ROH, reflux, 3 h) of the initially formed corresponding cis isomers. The reactionbetween alcohols R‘OH and cis-[PtCl4(RCN)2] (R = Me, R‘ = Et, n-Pr, i-Pr, n-Bu; R = Et; R‘ = n-Pr, i-Pr, n-Bu),exhibiting greater R/R‘ steric congestion, allowed the isolation of cis-[PtCl4{(E)-NHC(R)OR‘}{(Z)-NHC(R)OR‘}] as the major products. The alcoholysis reactions of poorly soluble [PtCl4(RCN)2] (R = CH2Ph, Ph) performedunder heterogeneous conditions, directly in the appropriate alcohol and for a prolonged time and, for R = Ph, withheating led to trans-[PtCl4{(E)-NHC(R)OR‘}2] (R = CH2Ph, R‘ = Me, Et, n-Pr, i-Pr; R = Ph, R‘ = Me) isolatedin moderate yields. In all of the cases, in contrast to platinum(II) systems, addition of R‘OH to the organonitrileplatinum(IV) complexes occurs under mild conditions and does not require a base as a catalyst. The formedisomerically pure (imino ester)Pt(IV) complexes can be reduced selectively, by Ph3PCHCO2Me, to the correspondingisomers of (imino ester)Pt(II) species, exhibiting antitumor activity, without change in configuration of the iminoester ligands. Furthemore, the imino esters NHC(R)OR‘ can be liberated from both platinum(IV) and platinum(II)complexes [PtCln{NHC(R)OR‘}2] (n = 2, 4) by reaction with 1,2-bis(diphenylphosphino)ethane and pyridine,respectively. All of the prepared compounds were characterized by elemental analyses (C, H, N), FAB massspectrometry, IR, and 1H, 13C{1H}, and 195Pt (metal complexes) NMR spectroscopies; the E and Z configurationsof the imino ester ligands in solution were determined by observation of the nuclear Overhauser effect. X-raystructure determinations were performed for trans-[PtCl4{(E)-NHC(Me)OEt}2] (2), trans-[PtCl4{(E)-NHC(Et)OEt}2] (10), trans-[PtCl4{(E)-NHC(Et)OPr-i}2] (11), trans-[PtCl4{(E)-NHC(Et)OPr-n}2] (12), and cis-[PtCl4{(E)-NHC(Et)OMe}2] (14). Ab initio calculations have shown that the EE isomers are the most stable ones for bothplatinum(II) and platinum(IV) complexes, whereas the most stable configurations for the ZZ isomers are less stablethan the respective EZ isomers, indicating an increase of the stability on moving from the ZZ to the EE configurationswhich is more pronounced for the Pt(IV) complexes than for the Pt(II) species.
- Metal-mediated addition of alcohols to nitriles in (nitrile)Pt(IV) complexes, giving coordinated E and Z imino esters, occurs under mild conditions and does not require OH- as activator.
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