Closing the Gap: Preparation and Characterization of the First Half-Open and Open Manganocene Complexes

The first preparations
of half-open and open manganocenes were
accomplished. Treatment of KPdl′ (Pdl′ = 2,4-(Me3C)2C5H5)) with [{(η5-Cp”)­Mn­(thf)­(μ-I)}2] (Cp″ =
1,2,4-(Me3C)3C5H2) and
MnI2(thf)2 resulted in the formation of [(η5-Cp″)­Mn­(Pdl′)] (2) and [(Pdl′)2Mn] (4), respectively. Both compounds adopt a
high-spin (S = 5/2) ground state. Maximum spin states
are rather unusual for pentadienyl complexes, since these ligands
generally stabilize transition metal complexes in their low-spin state.
In addition, the electronic structure of 2 was compared
to its closed analogue [(η5-Cp″)­Mn­(η5-Cp′)] (Cp′ = 1,3-(Me3C)2C5H3), which also adopts the high-spin configuration
because of steric hindrance destabilizing the electronically more
favorable low-spin state. Reaction of KPdl′ with [(C5H5)2Mn] yields the trimetallic compound [{(η5-Pdl′)­Mn­(η5-C5H5)]}2Mn] (5) concomitant with the formation
of 2,4,7,9-tetra-tert-butyl-1,3,7,9-decatetraene
(Pdl′2), indicating reduction of two Mn atoms. Solid-state
magnetic susceptibility studies and density functional theory computations
suggest that the electronic structure in 5 is best described
as two diamagnetic (S = 0) [(η5-Pdl′)­Mn­(η5-C5H5)]− Mn­(I) anions,
each being coordinated to a central Mn­(II) cation with a high-spin
(S = 5/2) configuration.