om961002o_si_003.cif (63.77 kB)
Synthesis, Structure, and Protonation Studies of Cp*MH3(dppe) (M = Mo, W). Pseudo-Trigonal-Prismatic vs Pseudo-Octahedral Structures for Half-Sandwich Group 6 M(IV) Derivatives
dataset
posted on 1997-04-15, 00:00 authored by Brett Pleune, Rinaldo Poli, James C. FettingerThe compounds Cp*MH3(dppe) (M = Mo,
1; W, 2) are accessible in good yields from
reacting
the corresponding compound Cp*MCl4 and LiAlH4
in toluene/Et2O followed by methanolysis.
The X-ray structure of 1 shows a
pseudo-trigonal-prismatic geometry which is unprecedented
for half-sandwich CpML5-type compounds. Protonation
with HBF4·Et2O in Et2O at
low
temperature affords
[Cp*MH4(dppe)]+BF4-
salts (M = Mo, 3; W, 4). While
3 spontaneously
decomposes, even at low temperatures, in coordinating solvents and
CH2Cl2, 4 is stable
at
room temperature in MeCN. An X-ray structure of 4 is
consistent with a classical
tetrahydrido species with a distorted pseudo-pentagonal-bipyramidal
structure. The low-temperature NMR properties, JHD ≤ 1 Hz for
4-d3, and the
T1(min) value for the hydride
resonance are also consistent with this structural assignment.
Decomposition of 3 in MeCN
at room temperature selectively affords
[Cp*MoH2(MeCN)(dppe)]+BF4-,
5. The NMR
properties of this complex indicate a fluxional structure with
inequivalent H and P nuclei
and are consistent with a pseudo-trigonal-prismatic structure analogous
to that of the
precursor 1. Further protonation of 5 in
MeCN or direct protonation of 1 with excess
acid
in MeCN affords two isomers of complex
[Cp*MoH(MeCN)2(dppe)](BF4)2,
6 and 7. Thermal
treatment in MeCN slowly converts 7 into 6
initially, but both isomers further transform
into a third isomer, 8, upon prolonged heating. The
structure of 6 has been elucidated by
X-ray crystallography and consists of a highly distorted
pseudo-octahedral geometry with
relative trans MeCN ligands. The structures of
7 and 8 and mechanisms of the
interconversions between the various isomeric structures are proposed on the
basis of the solution
NMR studies.