Diethynylrhodacyclopentadienes: a new class of luminescent organometallics

Abstract

A series of mer,cis-[tris(trimethylphosphine)-X-2,5-bis(4-R-phenylethynyl)-3,4- bis(4-R-phenyl)rhodacyclopenta-2,4-diene] compounds have been synthesised by the regiospecific reductive coupling of 1,4-bis(4-R-phenyl)buta-1,3-diynes with КҺ(РМe(_3))(_4)Х (where X = -C≡С –TMS, -С≡С(_6)Н(_4)-4-ММe(_2), -С≡С-С≡С-С(_6)Н(_4)-4-МРһ(_2), and R = H, Me, ОМе, SMe, СF(_3), CN, СО(_2)Ме, NMe(_2), NO(_2), C≡C-TMS). The compounds absorb strongly in the range 453-544 nm, and are photoluminescent, in solution at room temperature, emitting in the range 496-631 nm with singlet lifetimes and quantum yields of 3-18 %. The photophysical properties are strongly influenced by the nature of R, but appear to be almost independent of the acetylide used; the latter is related to unfavourable steric interactions. Acceptor R groups red-shift both absorption and emission much more significantly than donor groups. A number of the structures have been solved by single crystal X-ray diffraction and the structures are compared and contrasted. Analogous compounds with X = Me and СІ have also been prepared. The absorbance properties are similar to the acetylide-substituted derivatives, although absorption maxima are slightly shifted to lower energy. Also correlated with this are the values of J(_Rh-P) for the sets of compounds which surprisingly suggest better donating ability of both Me and СІ over the acetylides. The СІ compound is luminescent, its emission maximum again shifted to lower energy from that of the acetylide examples. In stark contrast to the acetylide and СІ rhodacycle derivatives, the Me compounds are not emissive at room temperature in solution. The rhodacycles react slowly, with additional diyne only under forcing conditions, to form highly substituted benzenes. It is shown that the rhodacycles can catalyse the regiospecific cyclotrimerisation of 1,4-bis(4-R-phenyl)buta-1,3-diynes to give l,2,4-tris(4- R-phenylethynyl)-2,5,6-tri(4-R-phenyl)benzenes exclusively. The activity of the rhodacycles as catalysts for trimerisation are low, but due to the 2,5- selectivity of the rhodacycle formation, the reaction is regiospecific. The cyclotrimers are luminescent and the photophysical properties have been studied. The single-crystal X-ray structure for the parent cyclotrimer, l,2,4-tris(phenylethynyl)-2,5,6-tri(phenyl)benzene, has been obtained and is discussed. The mechanism for rhodacycle formation has been probed. A number of Rh-diyne π-complexes, in which one of the diyne C≡C bonds is coordinated to Rh, which were observed as intermediates during rhodacycle syntheses, have been isolated and characterised. 1H, (^19) F and (^31) P NMR spectroscopy has been used to propose a general structure, and several related Rh- π -complexes of symmetrical and unsymmetrical tolans have been prepared to support this, some of which have been characterised by X-ray diffraction and are discussed. The crystal structures of two solvates of the unusual centrosymmetric bimetallic complex, [Rh(PMe(_3))(_3)(Cl)](μ-( 1 ,2-ƞ(^2)):(3,4- ƞ(^2))-4-F (_3)C-C(_6)H(_4)-C≡C-C≡C-C(_6)H(_4)-4-CF(_3)) have also been obtained.