Abstract
The complex (η3-C3H5)Ru(CO)3Br exists in solution in a conformational equilibrium between endo and exo isomers differing principally in the orientation of the allyl group. At room temperature, the two isomers interconvert slowly {t1/2> 10 min) and the endo isomer predominates (>95%). Near-UV photolysis of (η3-C3H6)Ru(CO)3Br in alkane solution shifts the equilibrium toward exo-(η3-C3H5)Ru(CO)3Br. Thermal equilibrium is reestablished in the dark at 298 K. An intermediate in the 298 K photochemical rearrangement is shown by IR, 1H NMR, and mass spectrometry studies to be [(η3-C3H5)Ru(CO)2(μ-Br)]2. Consistent with the intermediacy of dihalo-bridged species, irradiation of a mixture of (η3-C3H5)Ru(CO)3Br and (η3-2-methylallyl)Ru(CO)3Cl yields “crossover” products (η3-C3H5)Ru(CO)3Cl and (η3-2-methylallyl)Ru-(CO)3Br. Near-UV irradiation of (η3-C3H5)Ru(CO)3Br at 77 K in a 2-methyltetrahydrofuran matrix results in loss of CO as evidenced by the growth of an IR absorption due to free CO. At room temperature, the 16e (η3-C3H5)Ru(CO)2Br dimerizes forming the dibromo-bridged dimer [(η3-C3H6)Ru(CO)2(μ-Br)]2. The dimer subsequently reacts with photoreleased CO to generate >90% exo-(η3-C3H5)Ru(CO)3Br with <10% of the endo isomer being detected. The formation rate of the exo isomer is much faster than the rate for thermal isomerization of exo-(η3-C3H6)Ru(CO)3Br to endo-(η3-C3H5)Ru(CO)3Br. Change in the ratio of endo-and exo-(η3-C3H6)Ru(CO)3Br upon near-UV irradiation clearly indicates that [(η3-C3H5)Ru(CO)2(μ-Br)]2 and CO yield exo-(η3-C3H5)Ru(CO)3Br more rapidly than endo-(η3-C3H5)Ru(CO)3Br. Irradiation of (η3-C3H6)Ru(CO)3Br in 3-methylpentane at 298 K in the presence of excess 2e donor ligands (L = PPh3, C2H4, 13CO) results in the direct trapping of (η3-C3H6)Ru(CO)2Br to form (η3-C3H5)Ru(CO)2(L)Br with only negligible yields of [(η3-C3H5)Ru(CO)2(μ-Br)]2. Interestingly, C2H4 does not react with [(η3-C3H5)-Ru(CO)2(μ-Br)]2 to form (η3-C3H5)Ru(CO)2(C2H4)Br under conditions where photolysis of (η3-c3h5)ru-(CO)3Br does yield (η3-C3H5)Ru(CO)2(C2H4)Br. This result suggests that reaction of [(η3-C3H5)Ru-(CO)2(M-Br)]2 with L = CO or PPh3 to yield (η3-C3H5)Ru(CO)2(L)Br is initiated by attack on the dimer, not capture of (η3-C3H5)Ru(CO)2Br from the dimer.
| Original language | English |
|---|---|
| Pages (from-to) | 1839-1845 |
| Number of pages | 7 |
| Journal | Organometallics |
| Volume | 7 |
| Issue number | 8 |
| DOIs | |
| State | Published - Aug 1988 |