Coordination and Coupling of OH-Functionalized C2 Units at a Single Metal Center: The Synthesis of Alkynyl(Vinylidene), Alkynyl(Enyne), Bis(Alkynyl)Hydrido, Enynyl, and Hexapentaene Rhodium Complexes from Propargylic Alcohols as Precursors

The reaction of [Rh(η3‐C3H5)‐(PiPr3)2] (1) with HCCCH(Ph)OH yields the alkynyl(vinylidene) complex trans‐[Rh{CCCH(Ph)OH}{CCH–CH(Ph)OH}(PiPr3)2] (2), while from 1 and HCCCPh2OH the alkynyl‐(enyne)metal derivative trans‐[Rh(CCCPh2OH){n2‐(E)‐Ph2(OH)CCCCHCH–CPh2OH} (PiPr3)2] (3) is obtained. On treatment with 1‐alkyn‐3‐ols HCCCR2OH (R = Me, Ph, iPr), the highly reactive π‐benzyl compound [Rh(n3‐CH2Ph)(PiPr3)2] (4) yields the five‐coordinate complexes [RhH(CCCR2OH)2(PiPr3)2] (5–7) of which those with R = Me and Ph can be converted to the alkynyl(vinylidene)metal isomers trans‐[Rh(CCCR2OH)(CCH–CR2OH)‐(PiPr3)2] (8, 9). Compounds 8 and 9 react with L′ = CO and isocyanides by migration of the alkynyl ligand to the vinylidene unit to give the enynylrhodium(I) complexes trans‐[Rh{n1‐(Z)‐C(CCCR2OH)CH–CR2OH}(L′)(PiPr3)2] (10, 11: L′ = CO; 12–15: L′ = CNR′). Cleavage of the Rh–C s̀‐bond of 10 with CF3‐CO2H affords trans‐[Rh(n1‐O2CCF3)‐(CO)(PiPr3)2] (16) and the enyne (E)‐Me2(OH)CCCCHCH–CMe2OH (17). Compounds 5–7 react with L′ = CO and isocyanides to give the octahedral 1:1 adducts [RhH(CCCR2OH)2(L′)(P‐iPr3)2] (18–20 and 24–27), of which the CO derivatives 18–20 readily eliminate HCCCR2OH to yield trans‐[Rh‐(CCCR2OH)(CO)(PiPr3)2] (21–23). On treatment of 6 or 9 (R = Ph) with Al2O3 in the presence of chloride ions, besides trans‐[RhCl(CCCPh2)(P‐iPr3)2] (28) the hexapentaenerhodium(I) complex trans‐[RhCl(n2‐Ph2CCCCCCPh2)(PiPr3)2] (29) is formed. The kinetically preferred isomer trans‐[RhCl(n2‐Ph2CCCCCCPh2)(P‐iPr3)2] (33) has been prepared from [RhCl(PiPr3)2]2 and Ph2CCCCCCPh2; it rearranges smoothly at room temperature to the thermodynamically more stable isomer 29. The molecular structures of 7 and 29 have been determined. The unprecedented coupling of two C3 units in the coordination sphere of a metal to form the hexapentaene Ph2CCCCCCPh2 is reported in this paper exploring the synthesis and reactivity of OH‐functionalized alkynyl(enyne), bis(alkynyl)hydrido, and alkynyl(vinylidene) rhodium complexes. Compound 1 reacts with Lewis bases L′ (L′ = CO, CNR) by migratory insertion to yield enynyl complexes 2, whereas under dehydrating conditions the hexapentaene derivative 3 is formed (L = PiPr3).