Reversible CC coupling reactions between alkynes and CO ligands on di-iron α-diimine complexes

Abstract

Fe 2CO) 6(L) [L=i-Pr-DAB, i-Pr-Pyca] ( 1) and Fe 2(CO) 7(2,2′-bipyridine) ( 2) react with RCCC- (O)OMe [R=H, C(O)OMe] to give the complexes Fe 2[μ 2-R′CC(R″)C(O)](CO) 5(σ,σ-N,N′-L) [L= i-Pr-DAB: R′=H, R″=C(O)OMe ( 5a); L=i-Pr-Pyca: R′=H, R″=C(O)OMe ( 5b); R′=C(O)OMe, R″= H ( 5c); R′=R″=C(O)OMe ( 5d); L=2,2′-bipyridine (bipy): R′=R″=C(O)OMe ( 5e)] in yields varying from<5% for L=i-Pr-DAB to almost quantitative for L=bipy. The X-ray single crystal structure of Fe 2[μ 2-MeOC(O)CCHC(O)](CO) 5(i-Pr-Pyca) ( 5c) has been determined. The purple crystals of Fe 2C 19H 16N 2O 8, M= 512.04, are triclinic, space group P 1 , a=10.1428- (11), b=10.7084(16), c=10.0367(10) Å, α= 94.368(9)°, β=100.927(13)°, γ=93.552(13)°, Z= 2. A total of 2507 absorption corrected reflections (Cu Kα, μ=114.18 cm −1) have been used in the refinement resulting in a final R value of 0.036 ( R w= 0.054). The molecular structure of 5c consists of an Fe(CO) 3 and an Fe(CO) 2(σ,σ-N,N′-i-Pr-Pyca) unit linked by a single FeFe bond [Fe(1)Fe(2): 2.7191(9) Å], that is bridged by an organic MeOC- (O)CC(H)C(O) fragment, resulting from the CC coupling of a CO ligand and a methyl propynoate molecule. This fragment is σ-bonded via the C(C(O)- OMe) atom to the Fe(CO) 2(Pyca) center and via the C(O) atom to the Fe(CO) 3 center to which also the olefinic bond is π-coordinated. The complexes 5, which are most likely formed via an intermediate with a chelating α-diimine and an η 2-CC bonded alkyne, react with a second equivalent of alkyne to give the ferracyclopentadienyl complexes Fe 2[RCCRCRCR](CO) 4(L). These reactions probably proceed via the fission of the CC bond between the CO and the alkyne in 5, followed by the loss of a CO ligand. The resulting intermediate with the alkyne in the μ 2, η 2-perpendicular bonding mode reacts with the second alkyne molecule. Heating of 5b, c yields the ferracyclopentadienyl complexes Fe 2[MeOC(O)CCHC(C(O)OMe)CH](CO) 4− (i-Pr-Pyca(4e)). When R′=R″=C(O)OMe ferracyclopentadienyl complexes are formed with the α-diimine bonded to the Fe in the ferracycle, whereas in the case of R′=H/C(O)OMe and R″=C(O)OMe/H the formation of ferracyclopentadienyl complexes with the α-diimine bonded to the second Fe center is observed. This product distribution is probably the result of the steric properties of the alkynes.