Abstract
The reaction between W(CO) 3 (CH 3 CN) 3 and 1-bromopenta-2,4-diene produces (η 3 -C 5 H 7 )W(CO) 2 (CH 3 CN) 2 Br (1a). Addition of bis(diphenylphosphino)ethane (dppe) and bis(dimethylphosphino)ethane (dmpe) to 1a in CH 2 Cl 2 gives W(P'down curve sign'P)(CO) 2 (syn-η 3 -pentadienyl)Br (P'down curve sign'P = dppe (1b), dmpe (1c)) in high yields. Stirring of 1b and Ic each with 1 equiv of AgPF 6 in CH 2 Cl 2 yields the cations [(η 5 -C 5 H 7 )W(CO) 2 (P'down curve sign'P)]PF 6 (P'down curve sign'P = dppe (2a), dmpe (2b)). In CH 3 CN, 2a and 2b each undergoes rearrangement to produce the corresponding cations [(η 3 -C 5 H 7 )W(CO) 2 (P'down curve sign'P)(CH 3 CN)]PF 6 (P'down curve sign'P = dppe (3a), dmpe (3b)). In CH 2 Cl 2 and CH 3 COCH 3 , the η 3 -cations 3a and 3b readily lose CH 3 CN to re-form 2a and 2b and reach equilibrium. The equilibrium constants K eq = [η 5 ][CH 3 CN]/[η 3 ] for this η 3 ⇄ η 5 reversible process were determined from 31 P NMR spectroscopy in CD 2 Cl 2 and CD 3 COCD 3 . Measurement of K eq from -10 to + 20°C in CD 2 Cl 2 gives ΔH = 10.6 ± 0.2 kcal mol -1 and ΔS = 29.0 ± 1.2 eu (eu = entropy unit) for 3a ⇄ 2a + CH 3 CN and ΔH = 8.2 ± 0.3 kcal mol -1 and ΔS = 29.4 ± 1.3 eu for 3b ⇄ 2b + CH 3 CN. The molecular structures of 1c and 2a have been determined from an X-ray diffraction study. Crystallographic data: 1c space group Pbca, a = 12.400 (5) Å, 6 = 13.226 (4) Å, c = 22.100 (6) Å, Z = 8, R = 0.0326, and R w = 0.0333; 2a space group P1, a = 11.047 (4) Å, b = 11.457 (4) Å, c = 14.311 (5) Å, α = 86.08 (3)°, β = 80.53 (3)°, γ = 66.19 (3)°, Z = 2, R = 0.0268, and R w = 0.0274. © 1991 American Chemical Society.