Transformation of a Phosphorus-Bound Cp* Moiety in the Coordination Sphere of Manganese Carbonyl Complexes

The reaction of Cp*PCl2 with K­[Mn­(CO)5] yields
two novel anionic products, [Mn2(CO)8(μ-PC11H14O)]− (1–) and [Mn2(CO)8(μ-PHCp*)]− (2–),
instead of the expected phosphinidene complexes with a delocalized
Mn–P–Mn bond system. By treating these anionic complexes
with [Ph3PAuCl], they are converted into corresponding
neutral derivatives [Mn2(CO)8(μ-PC11H14O)­(μ-AuPPh3)] (3) and [Mn2(CO)8(μ-PHCp*)­(μ-AuPPh3)] (4). NMR investigations and X-ray structural
analyses for 1–, 3, and 4 show that the compounds 1– and 3 as well as 2– and 4 reveal similar molecular structures
in which the neutral complexes 3 and 4 contain
AuPPh3 units bridging Mn–Mn bonds. In comparison
to 1 and 3, complexes 2 and 4 possess one additional H atom bound at the P atom. The structures
of 1 and 3 include a novel bicyclic unit
consisting of a C5 ring of the former Cp* moiety conjugated
to a CCC­(O)P four-membered ring. The latter is built by a CO group
of a former Mn carbonyl fragment connecting a P and a C atom of the
cycle. One of the methyl groups of the Cp* ligand became a CH2 unit, resulting in two isomers containing an exocyclic CH2 moiety in the positions three or five of the C5 ring. Both isomers were found in the reaction mixture, with one
as the major isomer. The proposed reaction pathway is based on XRD,
NMR, and MS data and includes the reduction of a transient [Cp*P­(Mn­(CO)5)2] complex by [Mn­(CO)5]−, a proton transfer from the neutral to the reduced complex, and
a successive reduction of the protonated species. The neutral bicyclic
compound 3, containing a 2-phosphacyclobutanone ring,
is light sensitive and decomposes to tetramethylfulvene, possibly
by a radical decarbonylation mechanism via a transient phosphacyclobutane
derivative, [C5(CH3)4(CH2)­P­(Mn­(CO)4)2AuPPh3] (5), detected by NMR spectroscopy.