Reversible binding and reduction of dinitrogen by a uranium(III) pentalene complex

Article abstract of DOI:10.1021/ja027000e

The U(III) mixed-sandwich compound [U(η-Cp*)(η-C₈H₄{SiⁱPr₃-1,4}₂)] 1 may be prepared by sequential reaction of UI₃ with KCp* followed by K₂[C₈H₄{SiⁱPr₃-1,4}₂], and has been crystallographically characterized. 1 reacts reversibly with dinitrogen to afford dimeric [{U(η-Cp*)(η-C₈H₄{SiⁱPr₃-1,4}₂)}₂(μ-η²:η²-N₂)] 2 , whose X-ray crystal structure reveals a sideways-bound, bridging diazenido (N₂^2-) ligand. Copyright

Full text of DOI:10.1021/ja027000e

Published on Web 07/19/2002
Reversible Binding and Reduction of Dinitrogen by a Uranium(III) Pentalene
Complex
F. Geoffrey N. Cloke* and Peter B. Hitchcock
The Chemistry Laboratory, School of Chemistry, Physics, and EnVironmental Science,
UniVersity of Sussex, Brighton BN1 9QJ, UK
Received May 22, 2002
The binding and activation of dinitrogen by well-defined
molecular complexes is an area of considerable current interest.1
While N2 coordination, reduction, and cleavage are all well-
established for transition-metal systems, examples of such behavior
for the actinide elements is extremely uncommon. Scott has reported
a binuclear, U(III) triamidoamine complex containing a side-on
bridging N2 ligand in which the N-N bond length is essentially
unperturbed from that in free N2.2 The only other example is the
heterobimetallic U-Mo compound of Cummins, also supported by
2-
amide-type ligands, which contains a bridging, linear N2 (diaz-
enido) ligand.3 We have recently developed the synthesis of the
2-
silylated pentalene dianion C8H4{SiiPr3-1,4}2 as a ligand for
Figure 1. Molecular structure of 1 (thermal ellipsoids at 50%).
organo-f-element chemistry,4 and given the ability of tris(cyclo-
pentadienyl)uranium(III) complexes to bind CO,5 we were encour-
aged to investigate the synthesis and reactivity of uranium(III)
pentalene derivatives. In this work we report the preparation of
the mixed-sandwich U(III) complex [U(η-Cp*)(η-C8H4{SiiPr3-
1,4}2)] and its ability to reversibly bind and reduce dinitrogen to
afford a binuclear U(IV) complex, which contains a bridging,
about the latter bond is 26°, compared with that of 24° in the more
sterically congested Th(IV) complex [Th(η-C8H4{SiiPr3-1,4}2)2].10
The U-C1 (2.733(7) Å), U-C3 (2.721(7) Å), U-C6 (2.683(7)
Å), and U-C8 (2.722(7) Å) bond distances (which constitute the
major, δ-symmetry bonding interaction in actinide pentalene
complexes11) in 1 are correspondingly slightly shorter than those
in [Th(η-C8H4{SiiPr3-1,4}2)2] (2.797(11), 2.748(10), 2.748(10), and
2.797(11) Å). The U-Cp* ring carbon distances lie in the range
2.734(8)-2.766(7) Å, comparable to those found in [UCp*(COT)-
(THF)] (average 2.752 Å).12
2-
sideways-bound N2 ligand.
[UI3(THF)4] is a useful starting material for U(III) chemistry;6
however, since we wished to prepare U(III) pentalene complexes
in the absence of strong donor ligands for subsequent reactions
with small molecules, base-free UI3 was employed in the present
work. The latter may be conveniently prepared by reaction of U
turnings with 1.5 equiv of HgI2 in a sealed tube at 320 °C, in a
modification of the method of Corbett for the synthesis of lanthanide
triodides.7 The reaction of UI3 with 1 equiv of KCp* in diethyl
ether affords a dark-green material which we assumed to be
{UCp*I2}n, or an etherate thereof. This was not isolated but reacted
directly with K2[C8H4{SiiPr3-1,4}2] in toluene under argon to afford
purple-black, crystalline [U(η-Cp*)(η-C8H4{SiiPr3-1,4}2)] 1 in
moderate (40%) yield after workup (see eq 1).8
Both the 1H and 29Si NMR spectra of 1 in argon-saturated
d6-benzene exhibit the expected resonances for a pseudo-C2-sym-
metric molecule, and the 29Si spectrum in d8-toluene at -70 °C
gave no evidence for a static agostic structure in solution on the
NMR time scale. However, exposure of the sample to an atmo-
spheric pressure of dinitrogen generated an additional set of 11
resonances in the 1H spectrum and two new resonances in the 29Si
spectrum. Freeze-pump thawing (×3) of the sample led to com-
plete disappearance of signals due to the new species 2.13 Green-
black, X-ray-quality crystals of 2 were grown by fractional crystal-
lization from a pentane solution of 1 under a 5 psi overpressure of
N2 at -20 °C, and the molecular structure is shown in Figure 2.14
2 has a binuclear structure, in which two units of 1 are bridged
by a sideways-bound dinitrogen unit. The key structural feature of
the latter is the N1-N2 bond length of 1.232(10) Å, consistent
with an N-N double bond and comparable to that found in
[(TmCp*)2(µ-η2:η2-N2)] (1.259(4) Å),15 but significantly longer than
the N-N triple bond in [(U{NN′3})2(µ-η2:η2-N2)] (1.109(7) Å).2
It has been suggested that the inability of [U(NN′3)] to reduce N2
is due to steric repulsion between the ligands in [(U{NN′})2(µ-η2:
η2-N2)] and consequent poor orbital overlap between the uranium
centers and the µ-η2:η2-N2 fragment.2 However, the U-N distances
in 2 (2.401(8)-2.423(8) Å) are essentially identical (within esds)
to those in [(U{NN′})2(µ-η2:η2-N2)],2 so the difference in N-N
bond order is surprising, but may be a consequence of different
frontier orbital geometries in the two ligand environments. The U2N2
Crystals of 1 suitable for X-ray diffraction studies were grown
from hexane at -20 °C under argon, and the molecular structure
is shown in Figure 1.9
1 adopts a slightly bent sandwich structure, in which the
M1-U-M2 angle is 170.1° where M1 and M2 are the centroid of
the Cp* ring and the midpoint of the pentalene (bridgehead)
C4-C5 bond, respectively. The fold angle of the pentalene ring
* To whom correspondence should be addressed. E-mail: f.g.cloke@sussex.ac.uk.
9
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J. AM. CHEM. SOC. 2002, 124, 9352-9353
10.1021/ja027000e CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
However, even under 50 psi of N2 the reaction only proceeds to
ca. 75% completion in an NMR tube, and 2 loses dinitrogen
extremely easily both in solution and the solid state.16 The instability
of 2 with respect to loss of N2 and reformation of 1 is likely a
reflection of the consequent relief of steric crowding and regenera-
tion of the essentially parallel sandwich structure; this is in contrast
to [U(NN′3)], which is clearly pre-organized toward N2 binding.
Acknowledgment. We thank EPSRC for financial support.
Supporting Information Available: X-ray data for 1 and 2 (PDF).
X-ray crystallographic files (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
References
Figure 2. Molecular structure of 2 (isopropyl groups removed for clarity,
thermal ellipsoids at 50%).
(1) Fryzuk, M. D.; Johnson, A. J. Coord. Chem. ReV. 2000, 200, 379.
(2) Roussel, P.; Scott, P., J. Am. Chem. Soc. 1998, 120, 1070; Roussel, P.;
Errington, W.; Kaltsoyannis, N.; Scott P. J. Organomet. Chem. 2001, 635,
69.
unit in the core of 2 is folded from planarity away from the pen-
talene ligands (presumably for steric reasons) by 5° about the
N1-N2 bond, and relevant angles within the core are: 74.3(5)°
(N2-N1-U1), 74.3(5)° (N2-N1-U2), 148.1(3)° (U1-N1-U2),
76.2(5)° (N1-N2-U1), 76.2(5)° (N1-N2-U2), and 151.9(3)°
(U1-N2-U2). The two pentalene ligands are differentiated by
slightly different fold angles (26° about C40-C41 and 22.5° about
C4-C5), and the significant U-ring C interactions (to C37, C39,
C42, C44, and to C1, C3, C6, C8), which range from 2.688(11) to
2.775(9), are essentially identical to those in 1. Similarly, the
U-Cp* centroid distances in 2 (U2-M2, 2.524 Å; U1-M1, 2.505
Å) are the same as that in 1 (2.486 Å) within esds. Hence the change
in formal oxidation state from U(III) in 1 to U(IV) in 2 is not
reflected in the structural parameters, but this is almost certainly
due to the steric congestion in 2.
(3) Odom, A. L.; Arnold, P. L.; Cummins, C. C. J. Am. Chem. Soc. 1998,
120, 5836.
(4) Cloke, F. G. N. Pure and Appl. Chem. 2001, 73, 233.
(5) Conejo, M. D.; Parry, J. S.; Carmona, E.; Schultz, M.; Brennann, J. G.;
Beshouri, S. M.; Andersen, R. A.; Rogers R. D.; Coles, S.; Hursthouse
M. Chem. Eur. J. 1999, 5, 3000.
(6) Avens, L. R.; Burns, C. J.; Butcher, R. J.; Clark, D. L.; Gordon, J. C.;
Schake, A. R.; Scott, B. L.; Watkin, J. G.; Zwick, W. D. Organometallics
2000, 19, 451.
(7) Corbett, J. D. Inorg. Synth. 1983, 22, 31.
(8) Synthesis of 1 (under Ar). To a suspension of UI3 (0.619 gm, 1 mmol) in
Et2O (75 mL) was added a suspension of KCp* (0.174 gm, 1 mmol) in
Et2O (25 mL) and the mixture stirred for 24 h. The resultant green solution
was filtered from precipitated KI, stripped to dryness and final traces of
Et2O removed at 60 °C under vacuum. The residue was taken up in toluene
(75 mL), and to this solution was added a solution of K2[C8H4{SiiPr3-1,
4}2] (0.492 g, 1 mmol) in toluene (25 mL) dropwise with stirring, and
the mixture stirred for 6 h. The brown suspension was stripped to dryness,
extracted with pentane (2 × 50 mL), and the pentane extracts were filtered
through a pad of Celite on a frit. The resultant deep brown solution was
concentrated to ca. 15 mL, and slow cooling to -45 °C afforded purple-
black crystals of 1 which were isolated, washed with cold pentane (3 ×
Assuming that the low-temperature X-ray structure, in which the
pentalene ligands are differentiated by different fold angles, is not
maintained in solution, 2 has C2 symmetry. The C2 axis renders
the two pentalene groups equivalent, but the two five-membered
rings of the individual pentalene ligands are no longer symmetry-
related, in agreement with the solution NMR data. The NdN stretch
is predicted to be IR active in this point group; however, no
absorptions assignable to this stretch were observed in the ap-
propriate region of the IR spectrum in solution or the solid state
for either 2-14N or 2-15N. This absorption may well be very weak,
or obscured by the strong ligand vibrations in the region 1350-
1
5 mL), and dried in vacuo. Yield 0.31 g, 40%. H NMR (C6D6, 293 K):
i
i
δ ppm -10.5 (br s, Pr-CH3, 18H), -13.4 (br s, Pr-CH, 6H), -17.2 (br
s, Cp*-CH3, 15H), -20.1 (br s, pentalene ring-CH, 2H), -22.0 (br s,
iPr-CH3, 18H), -42.2 (br s, pentalene ring-CH, 2H). 29Si{1H} NMR (C6D6,
298 K): δ ppm -176. 29Si{1H} NMR (C7D8, 201K): δ ppm -295. MS
(EI): m/z 787 (M+). Anal. Calcd for C36H61Si2U: C, 54.87; H, 7.80.
Found: C, 54.78; 8.29.
(9) Crystal data for 1: Monoclinic, FW 788.06, in the space group P21/n
(No. 14); a ) 16.5194(4) Å, b ) 11.6816(3) Å, c ) 18.7404(5) Å, R )
90°, â ) 90.504(1)°, γ ) 90°, Z ) 4. Final residual wR2(all data) )
0.107 (R ) 0.062, with goodness of fit 1.093 on F2).
(10) Cloke, F. G. N.; Hitchcock P. B. J. Am. Chem. Soc. 1997, 119, 7899.
(11) Cloke, F. G. N.; Green, J. C.; Jardine C. N. Organometallics 1999, 18,
1080.
1500 cm-1
.
(12) Schake, A. R.; Avens, L. R.; Burns, C. J.; Clark, D. L.; Sattelberger, A.
P.; Smith, W. H. Organometallics 1993, 12, 1497.
The reversible formation of 2 (eq 2) involves a formal oxidation
(13) NMR data for 2 (C6D6, 293 K). 1H: δ ppm 57.1 (br s, pentalene ring-CH,
1H), 53.4 (br s, pentalene ring-CH, 1H), 9.2 (br s, pentalene ring-CH,
of two U(III) centers to U(IV) with concomitant reduction of N2
i
1H), -2.4 (br s, Pr-CH, 3H), -3.7 (br s, Cp*-CH3, 15H), -3.75 (br s,
2-
to N2
.
i
i
iPr-CH3, 9H), -3.8 (br s, Pr-CH3, 9H), -4.1 (br s, Pr-CH3, 9H), -9.4
i
(br s, Pr-CH3, 9H), -12.4 (br s, pentalene ring-CH, 1H), -15.4 (br s,
iPr-CH, 3H). 29Si{1H}: δ ppm -108, -185.
(14) Crystal data for 2: Monoclinic, FW 1604.14, in the space group P21/n
(No. 14); a ) 13.1165(3) Å, b ) 29.7960(5) Å, c ) 19.8885(10) Å, R )
90°, â ) 109.203(8)°, γ ) 90°, Z ) 4. Final residual wR2(all data) )
0.158 (R ) 0.071, with goodness of fit 1.133 on F2).
(15) Evans, W. J.; Allen, N. T.; Ziller, J. W. J. Am. Chem. Soc. 2001, 123,
7927.
(16) The facile loss of N2 from 2 preluded meaningful microanalysis.
JA027000E
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