Published on Web 11/27/2003
Evidence for Alkane Coordination to an Electron-Rich Uranium Center
Ingrid Castro-Rodriguez, Hidetaka Nakai, Peter Gantzel, Lev N. Zakharov, Arnold L. Rheingold, and
Karsten Meyer*
Department of Chemistry and Biochemistry, UniVersity of California, San Diego, 9500 Gilman DriVe,
MC 0358, La Jolla, California 92093-0358
Received August 14, 2003; E-mail: kmeyer@ucsd.edu
Scheme 1
Metal-alkane complexes are believed to be key intermediates
in C-H activation processes. The C-H σ bond of saturated
hydrocarbons is strong and notoriously unreactive, and thus,
selective intermolecular carbon-hydrogen bond activation has been
identified as a fundamental and practical challenge to synthetic
chemists.1 Although theoretical chemists have made significant
progress to elucidate the fundamental nature of metal-alkane
interactions, detailed structural information for metal-alkane
adducts is exceedingly rare. Most known examples of transition
metal-alkane complexes to date have been detected in gas phases,
matrices, and solutions in situ.2 In virtually all reported cases, the
The X-ray diffraction analysis of both complexes clearly revealed
metal-alkane adducts were identified spectroscopically as fleeting
atom positions and connectivities of one molecule of cycloalkane
intermediates at cryogenic temperatures.
in the coordination sphere of the uranium(III) center and a second
Noteworthy exceptions were recently reported by George et al.3
molecule of cycloalkane cocrystallized in the lattice. The quality
and Geftakis and Ball.4 The latter group generated a cyclopentane
of the X-ray data, however, did not allow for discussion of metric
adduct, [(Cp)Re(CO)2(C5H10)], via photolysis of [(Cp)Re(CO)3] that
parameters. As a result, the solid-state structure of 1a and 1b could
was detected NMR-spectroscopically as an intermediate in neat
only be refined isotropically with the cycloalkane modeled as a
cyclopentane solution at -93 °C. On the basis of a comparison of
rigid body.8 In contrast, single crystals obtained from n-pentane
1
the experimentally determined 13C and H coupling constants and
solutions with methyl cyclohexane (1c), methyl cyclopentane (1d),
chemical shifts with those of structurally closely related, â-agostic
and neohexane (1e) produced good diffraction data. Molecules 1a-e
bonded C-H moieties, an η2-H,C metal-alkane interaction was
are isostructural and isomorphous and crystallize in the monoclinic
proposed (see below).
space group P21/n. All complexes contain an additional solvent
molecule in the crystal lattice that is distant from the uranium atom
and often disordered. The X-ray analysis of 1c and 1e resulted in
high-angle diffraction data, revealing rotationally twinned structures
along the a axis. Crystals of [((ArO)3tacn)U(neo-C6)]‚(cy-C5) (1e)
were obtained from a competition experiment, in which an
n-pentane solution of 1 was treated with cyclopentane and neo-
In 1997, Reed et al. reported the only example of an X-ray
diffraction analysis of a simple alkane in the coordination sphere
of a metal complex.5 In this iron porphyrin complex, (dap)Fe‚(n-
heptane), the hydrophobic pocket of a double A-framed porphyrin
supported the heptane-iron adduct through a host/guest effect.
We report here the X-ray diffraction analysis of a series of alkane
adducts of the low-valent, coordinatively unsaturated, tris-aryl oxide
uranium(III) complex [((ArO)3tacn)U] (1, Scheme 1).6,7 These
species exhibit evidence for bonding interactions between the
uranium ion as well as the macrocyclic ligand and the axial alkane
and, thus, raise the question whether the axial alkane is held in
place through metal-alkane coordination, a host-guest effect, or
a combination of both.
Recrystallization of highly reactive 1 from neat n-pentane,
n-hexane, benzene, and/or toluene, or mixtures thereof, did not yield
single crystals suitable for X-ray diffraction analysis. We found,
however, that cube-shaped, red-brown crystals could be obtained
from an n-pentane solution if trace amounts of cyclohexane were
present in the glovebox atmosphere. If a solution of 1 in n-pentane
is treated with 50 equiv of cyclohexane or cyclopentane, cube-
shaped crystals of [((ArO)3tacn)U(cy-C6)]‚(cy-C6) (1a) and
[((ArO)3tacn)U(cy-C5)]‚(cy-C5) (1b) can be isolated reproducibly.
hexane (1:1). Interestingly, the solid-state structure revealed that
the two solvent molecules show a high site preference.8 While the
neohexane molecule predominantly occupies the apical position of
the uranium center, the cyclopentane molecule cocrystallizes in the
lattice. Compared to all other carbon atoms of the apical solvent
molecules in 1c, 1d, and 1e, carbon C1S in closest proximity to
the uranium ion exhibits significantly smaller thermal ellipsoids.8
This likely can be attributed to decreased thermal activity resulting
from coordination to the uranium ion. Thus, crystallographic data
for 1c-e (Table 1) allow for a more detailed discussion of the
complexes’ core and especially the U-C1S metric parameters.
Figure 1 shows the molecular structure of 1c representative of the
series of uranium-alkane complexes reported herein. The structure
of the ((ArO)3tacn)U core fragment of all alkane adducts is similar
to that found for the previously reported seven-coordinate complex
[((ArO)3tacn)U(NCCH3)] (2).9 The average U-N(tacn) and U-O(A-
rO) bond distances in 1c, for instance, were determined to be 2.676-
(4) and 2.244(3) Å, which are comparable to 2.699(6) and 2.265(5)
Å found in 2.
A significant difference between the solid-state molecular
structures of 1c-e and 2 is the out-of-plane shift, d(Uo-o-p), of
the uranium center from an idealized trigonal plane formed by the
9
15734
J. AM. CHEM. SOC. 2003, 125, 15734-15735
10.1021/ja0379316 CCC: $25.00 © 2003 American Chemical Society
Castro-Rodriguez, Ingrid
