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Transcript
Metallocene Organoactinide Complexes
11
Fig. 4 Crystal structure of a (η5 -C5 Me5 )3 U(CO) [95] and b (η5 -C5 Me5 )3 U(N2 ) [96].
Reprinted with permission from [95, 96]; © (2003) American Chemical Society
(15)
Sterically hindered metal centers for the class of compounds of the type
(η5 -C5 Me5 )3 M have unusually long metal ligand bonds. Although long metal
ligand distances are known in f -element complexes containing agostic interactions [90], they generally involve only one or two interactions and the rest
of the bonds are normal and predictable based on ionic radii [91â93]. In contrast, in the molecule (η5 -C5 Me5 )3 U all the metalâligand bonds were longer
than the conventional distance [94]. This phenomenon further induced researchers to investigate the reaction chemistry of the (η5 -C5 Me5 )3 U moiety.
The reaction of (η5 -C5 Me5 )3 U with CO produced the sterically more crowded
U(III) complex Cpâ 3 U(CO) [95]. With N2 [96] it afforded the monometallic
complex Cpâ 3 U(η1 -N2 ) (Fig. 4), demonstrating that end-on nitrogen coordination is possible for f -elements (Scheme 3)(vide supra). In fact, this is the
ï¬rst monometallic f -element complex of N2 of any kind because it is most
commonly found as M2 (µ-η2 :η2 -N2 ) moieties involving (N2 )â2 [97â103]. The
binding of N2 in Cpâ 3 U(η1 -N2 ) was found to be reversible, i.e., it releases
N2 when the pressure was lowered to 1 atm in a solution of C6 D6 , quantitatively regenerating the parent complex (Scheme 3). In contrast, a solution of
Cpâ 3 U(CO) was found to be stable for hours under Ar or vacuum.
The discovery of the SIR phenomenon for this kind of complex opened the
window for the multi-electron reduction system. The complex Cpâ 3 U shows
a reductive coupling of acetylene [104] and reductive cleavage of azobenzene [105] (Scheme 4), two- and four-electron processes, respectively.
These U(IV) and U(VI) complexes can also be obtained from
[(η5 -C5 Me5 )2 U][(µ-Ph)2 BPh2 ], where [BPh4 ]â acts as a one-electron reductant (Scheme 4) [104]. On the other hand the reaction of (η5 -C5 Me5 )3 U
and cyclooctatetraene giving [{U(η5 -C5 Me5 )(C8 H8 )}2 (µ-C8 H8 )] is accomplished through one U(III)/U(IV) and two (C5 Me5 )â /C5 Me5 couples [89].
