MONONUCLEAR (ALLENYLIDENE)METAL COMPLEXES OF A D8 SYSTEM - SYNTHESIS AND MOLECULAR-STRUCTURE OF TRANS-[RHCL(=C=C=CRR')(PIPR3)2]

The reaction of[RhCl(PiPr3)2] (1) with 1-alkynols HC=CCRR'OH leads to the formation of either(alkyne)- or (alkynyl)hydridorhodium complexes as the first isolable products. In toluene at room temperature (R = H, Me, R' = Ph; R, R' = C12H8) or on irradiation (R = R' = Ipr), the initially formed compounds smoothly rearrange to give the isomeric (vinylidene)rhodium derivatives trans-[RhCl(=C=CHCRR'OH)(PiPr3)2] (8-11). On treatment with alumina or traces of acid, compounds 8-11 eliminate water to give the four-coordinate rhodium allenylidenes trans-[RhCl(=C=C=CRR') (PiPr3)2] (12, R = H, R' = Ph; 13, R, R' = Ci2H8) or a mixture of isomeric (allenyhdene)- and (vinylvinylidene)metal complexes. These mixtures are converted upon reaction with acidic Al2O3 to give pure samples of trans-[RhCl(=C=CHC(Ph)=CH2)-(PiPr3)2] (16) and trans-[RhCl(=C=CHC(Ipr)=CMe2)(PiPr3)2] (17) in nearly quantitative yields. From 1 and HC=CCPh(o-Tol)OH, the complete series of (alkyne)-, (alkynyl)hydrido-, (vinylidene)-, and (allenylidene)rhodium compounds 18-21 has been prepared, and the molecular structure of trans-[RhCl(=C=C=CPh(o-Tol))(PiPrs)2] (21) has been determined. Crystallographic data: orthorhombic, space group Pca2(1) (No. 29), a = 17.804(4) angstrom, b = 10.891(1) angstrom, c = 17.934(5) angstrom, V = 3477(1) angstrom3, Z = 4. The Rh-C distance in 21 is somewhat longer than in related (vinylidene)rhodium complexes but is almost identical with that in rhodium carbenes.