Mixed Chloride Phosphine Complexes of Dirhenium Cores. 1. New Reactions and Unprecedented Structures Involving Trimethylphosphine

The unusual 1,2,7-isomer of
Re2Cl5(PMe3)3
(1) has been used as a starting material to prepare
previously
unknown dirhenium complexes. One-electron reduction of 1
by cobaltocene followed by nonredox substitution of
the resulting anionic species with PMe3 led to the
formation of a triply bonded
1,2,7,8-Re2Cl4(PMe3)4
(2). In the
crystal structure of 2, phosphine ligands on both metal
centers exhibit a cis arrangement with a P−Re−P angle
of
93.3° in contrast to the well-known type of 1,3,6,8-isomers with a
trans arrangement of the monodentate phosphines
connected to each metal atom. Complex 2 represents the
first example of an isomer of this type in the large
M2X4(PR3)4 class of compounds (M = Re, Tc, W, Mo;
X = Cl, Br, I; PR3 = monodentate phosphine). The
one-electron
oxidation product of 1 cocrystallized with one molecule of
tetrabutylammonium chloride afforded
Re2Cl6(PMe3)2
(3), for which all previous synthetic attempts had failed.
This quadruply bonded complex exhibits an unusual
3-fold
disorder of the Re2 unit with equal populations for all
three orientations. We also report that when the
“classic”
reaction of octachlorodirhenate anion,
Re2Cl82-, with
trimethylphosphine is carried out in benzene at room
temperature
the reduction processes do not occur and the product is a novel
paramagnetic complex,
Re2Cl6(PMe3)4
(4), which
does not have a metal−metal bond (the Re−Re separation is
3.8476(4) Å). Another interesting feature of
compound
4 is that the PMe3 ligands have a cis disposition at
each rhenium center and are located in the same plane as
the
metal atoms and bridging chlorine ligands. For such a ligand
arrangement the molecule of 4 is the only example
of
a nonmetal−metal-bonded dinuclear compound with monodentate phosphine
ligands.