Reactivity of 1-alkynes at dimanganese carbonyl centers. Synthesis and structural characterization of alkenyl-, alkenylidene-, alkynyl-, and 5-oxo-2-furanylidene-bridged dimanganese compounds

Article abstract of DOI:10.1021/om00037a062

The new unsaturated hydride [Mn₂(μ-H)₂(CO)₆(μ-dppm)] (1) is prepared by treatment of [Mn₂(μ-Cl)₂(CO)₆(μ-dppm)] with 4 equiv of Li[HBEt₃] in tetrahydrofuran at room temperature. 1 reacts with 1-alkynes (R = H, Ph, ^tBu, COOMe) at room temperature to give either hydrido-alkenyl complexes [Mn₂(μ-H)(μ₂,η¹,η ²-CR=CH₂)(CO)₆(μ-dppm)] (R = H (3a), Ph (3b)) or alkenylidene species [Mn₂-(μ,η¹η²-C=CH ^tBu)(CO)₆(μ-dppm)] (4c) and [(CO)₃Mn(μ-dppm)]{μ-C=CHC(O)OMe}Mn(CO)₃] (6). The photochemical reaction of [Mn₂(CO)₈(μ-dppm)] (2) with 1-alkynes (R = H, Ph, ^tBu, CH₃OCH₂) at -20°C results in the formation of vinylidene [Mn₂(μ,η¹,η²-C=CHR)(CO) ₆(μ-dppm)] (R = Ph (4b), ^tBu (4c), CH₃OCH₂ (4d)), hydrido-alkynyl [Mn₂(μ-H)(μ,η¹,η²-C ₂R)(CO)₆(μ-dppm)] (R = H (5a), Ph (5b), ^tBu (5c), CH₃OCH₂ (5d), or 5-oxo-2-furanylidene complexes [(CO)₄Mn(η,-dppm){μ,η¹,η ⁴-C-CH=C(R)C(O)O)Mn-(CO)₂] (R = H (7a), CH₃OCH₂ (7d)) and [Mn₂(μ-CO){μ₂,η¹,η ¹-C-CH=C(R)C(O)O}(CO)₆(μ-dppm)] (R = H (8a), CH₃OCH₂ (8d)). The relative amounts of the above species depend strongly on the nature of the alkyne and the reaction solvent. The 5-oxo-2-furanylidene ligands in 7 bind the dimetal moiety in a novel μ₂η¹,η⁴-fashion acting as a 6-electron donor. The alkenylidene complexes 4b and 4c can also be obtained through UV irradiation of the alkenyl complex 3a in the presence of an excess of 1-alkyne (R = Ph, ^tBu). Unexpectedly, these complexes isomerize spontaneously in solution at room temperature to yield the corresponding hydrido-alkynyls 5b and 5c. The latter regenerate their vinylidene precursors 4b and 4c by treatment with Li[AlH₄] followed by addition of HBF₄ to the resulting mixture. Complexes 3a, 5a-d, and 7 are stereochemically nonrigid in solution. In the case of the alkynyl species 5 the fluxional process appears to be especially fast, as even evidence for incipient coalescence is not observed down to -90°C. The structures of 4c, 5b, 6, 7d, and 8d have been fully elucidated by X-ray diffraction studies.