Synthesis of some dinuclear Fe and Ru complexes containing an 8e donating α-diimine ligand and μ 2-bridging alkyne; crystal structure of (σ,σ- N, N′,η 2,η 2-CN,CN′-1,4-diisopropyl-1,4-diaza-1,3-butadiene)(μ 2-methyl propynoate) tetracarbonyl di-iron

Abstract

Reaction of Fe 2(CO) 6(i-Pr-DAB) [i-Pr-DAB=i-PrNC(H)C(H)N-i-Pr] with methyl propynoate at room temperature in heptane solution, through which a stream of nitrogen is passed, yields the complex Fe 2(CO) 4(i-Pr-DAB)(μ 2-HCCC(O)OMe) among a number of other products. The α-diimine ligand is σ,σ- N, N′,η 2,η 2-CN,CN′ coordinated, thus donating eight electrons to the metal carbonyl core. This compound, of which the structure is confirmed by an X-ray crystal structure determination, is the first Fe complex with an R-DAB ligand in this coordination mode. Crystals of Fe 2C 16H 20N 2O 6·0.15C 6H 6 are monoclinic, space group A2/ a, and have cell constants a= 31.450(6), b=9.602(3), c=28.191(6) Å, β=91.606(19)° and Z=16.3017 reflections (Mo Kα, μ=13.9 cm −1) have been used in the refinement resulting in a final R value of 0.056. In the compound, which is structurally analogous to the previously reported Ru 2(CO) 4(i-Pr-DAB)(μ 2HCCH), the alkyne is in a bridging position along the metal-metal bond [Fe(11)C(15): 1.924(8); Fe(12)C(16): 2.007(7) Å]. The FeFe distance of 2.670(4) Å is normal for a single FeFe bond. The carbonyl ligands are all terminally bonded, two to either Fe atom, with normal bond lengths and angles. The Fe atoms and the non-H atoms of the coordinated alkyne lie in one plane, which is also a plane of symmetry in the molecule. The bond distances within the DAB ligand and between the ligand and the Fe atoms are as expected for the σ,σ- N, N′,η 2, η 2-CN,CN′ coordination mode. Reaction of Ru 2(CO) 6(i-Pr-DAB) with phenylacetylene in heptane solution at 90 °C, yields two isomeric compounds Ru 2(CO) 4(i-Pr-DAB)(μ 2-PhC CH), which have a molecular structure comparable to that of Fe 2(CO) 4(i-Pr-DAB)(μ 2-HCCC(O)OMe). The formation of the alkyne complexes from M 2(CO) 6(i-Pr-DAB) [M=Fe, Ru] probably proceeds via M 2(CO) 5(i-Pr-DAB) [M=Fe, Ru], through the loss of one CO with concomitant change of coordination of the i-Pr-DAB from the 6e to the 8e donating mode. Substitution of a second CO by an alkyne yields M 2(CO) 4(i-Pr-DAB)(μ 2-RCCH) [M=Fe, R= C(O)OMe; M=Ru, R=H, Ph]. This indicates that Fe 2(CO) 5(i-Pr-DAB), which until now never has been observed or isolated, is a likely intermediate in reactions of Fe 2(CO) 6(i-Pr-DAB).