Heterodinuclear uranium/molybdenum dinitrogen complexes [16]

Full text of DOI:10.1021/ja980095t

5836
J. Am. Chem. Soc. 1998, 120, 5836-5837
Scheme 1a
Heterodinuclear Uranium/Molybdenum Dinitrogen
Complexes
Aaron L. Odom, Polly L. Arnold, and
Christopher C. Cummins*
Department of Chemistry Room 2-227
Massachusetts Institute of Technology
Cambridge, Massachusetts 02139-4307
ReceiVed January 9, 1998
Binding of the prototypical π-acid CO by molecular uranium
complexes has been limited to spectroscopic observation1 and a
single recent crystal structure.2 This being the case, it is not
surprising that little is known about the interaction of uranium
complexes with dinitrogen. The most important work in the latter
area to date comprises the very recent isolation and structural
characterization of a diuranium complex with N2 as an η2 (side-
on) bridging ligand;3 the supporting ligands used were of the
triamidoamine variety.4 As reported herein, uranium can be
decorated with N-tert-butylanilide ligands and coaxed into an end-
on interaction with dinitrogen. The key complexes reported here
are the first heterodinuclear complexes with a U-N2 interaction.
Our investigation takes advantage of the versatile starting
material, U(I)3(THF)4, which has been shown to represent a
convenient entry into uranium(III) chemistry.5 The major product
resulting from interaction of Li(N[R]Ar)(OEt2)6 with U(I)3(THF)4
was found to be the yellow uranium(IV) species U(I)(N[R]Ar)3
(1). The optimum stoichiometry (Scheme 1) assumes that
reduction of one sacrificial equiv of U3+ to U0 occurs en route to
formation of 1. NMR data for 1 recorded in C6H6 at 25 °C are
indicative of a single ligand environment.
With the objective of synthesizing a homoleptic amide of
uranium(III),7 1 was treated in THF with 1% Na/Hg (Scheme 1).
The reaction resulted in high-yield formation of the black THF
adduct U(THF)(N[R]Ar)3 (2). Characterization data are in accord
with the compound’s formulation as a monomeric uranium(III)
derivative with a single -N(R)Ar ligand environment at 25 °C.
X-ray crystallography confirmed the formulation of 2 (Figure
1). A rare example of a crystallographically characterized
uranium(III) amide, the U-N bond lengths in 2 (av 2.320 Å) are
∼0.04 Å shorter than the terminal U-N bonds in [U(N[SiMe3
]2)2(µ-NHMes)]2.8 The electrophilic nature of low-coordinate
uranium(III) is reflected in the structure of 2 by close U‚‚‚Cipso
contacts of ∼2.9 Å, as expected for uranium(III)-arene π-com-
plexation.9 Interaction of N-tert-butylanilide Cipso carbons with
electrophilic metal centers is precedented.10 In addition, the
a Key: (i) C7H8, -90 to 0 °C, -3 LiI, -“U0”; (ii) 4 equiv Na/Hg (1%
w/w), THF, 20 min; (iii) Mo(N[t-Bu]Ph)3, N2 (1 atm), 25 °C, C7H8,
-THF; (iv) Mo(N[Ad]Ar)3, N2 (1 atm), 25 °C, C7H8, -THF.
2.518(8) Å U-O distance in U(THF)(N[R]Ar)3 is shorter by
∼0.03 Å than that in Cp3U(THF).11 A striking aspect of the solid-
state conformation of 2 is the location of the THF ligand in the
arene “bowl”, rather than in the tert-butyl “pocket” as observed
frequently for transition-metal analogues.12
As 2 by itself does not evince any detectable reactivity toward
N2, it seemed plausible that the compound might participate in
formation of heterodinuclear N2 complexes. Accordingly, stirring
a 1:1 mixture of 2 and Mo(N[t-Bu]Ph)313 in toluene under N2 (1
atm) led over 20 min to quantitative formation of the desired
U(µ-N2)Mo complex 3a (Scheme 1), which was isolated in 66%
yield as an orange solid.14 A plausible rationale for the observed
result is that the putative dinitrogen complex (N2)Mo(N[t-Bu]-
Ph)3 is more efficiently trapped by 2 than by Mo(N[t-Bu]Ph)3.
Reactions of Mo(N[t-Bu]Ph)3 and derivatives with dinitrogen have
been described in detail.13
(1) Brennan, J. G.; Andersen, R. A.; Robbins, J. L. J. Am. Chem. Soc.
1986, 108, 335.
(2) Parry, J.; Carmona, E.; Coles, S.; Hursthouse, M. J. Am. Chem. Soc.
1995, 117, 2649.
(3) Roussel, P.; Scott, P. J. Am. Chem. Soc. 1997, 120, 1070.
(4) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9. Verkade, J. G. Acc. Chem.
Res. 1993, 26, 483.
(5) Clark, D. L.; Sattelberger, A. P.; Bott, S. G.; Vrtis, R. N. Inorg. Chem.
1989, 28, 1771. Avens, L. R.; Bott, S. G.; Clark, D. L.; Sattelberger, A. P.;
Watkin, J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 2248. Clark, D. L.;
Sattelberger, A. P. Inorg. Synth. 1997, 31, 307-315.
Inspection of the IR spectrum of paramagnetic 3a did not reveal
an obvious band, thus providing suggestive negatiVe evidence
(6) Laplaza, C. E.; Davis, W. M.; Cummins, C. C. Organometallics 1995,
14, 577.
(7) Andersen, R. A. Inorg. Chem. 1979, 18, 1507.
(8) Stewart, J. L.; Andersen, R. A. New. J. Chem. 1995, 19, 587.
(9) Cesari, M.; Pedretti, U.; Zazzetta, A.; Lugli, G.; Marconi, W. Inorg.
Chim. Acta 1971, 5, 439. See also: Van der Sluys, W. G.; Burns, C. J.;
Huffman, J. C.; Sattelberger, A. P. J. Am. Chem. Soc. 1988, 110, 5924.
(10) Wanandi, P. W.; Davis, W. M.; Cummins, C. C.; Russell, M. A.;
Wilcox, D. E. J. Am. Chem. Soc. 1995, 117, 2110. Johnson, A. R.; Davis, W.
M.; Cummins, C. C. Organometallics 1996, 15, 3825. Ruppa, K. B. P.;
Desmangles, N.; Gambarotta, S.; Yap, G.; Rheingold, A. L. Inorg. Chem.
1997, 36, 1194.
(11) Wasserman, H. J.; Zozulin, A. J.; Moody, D. C.; Ryan, R. R.; Salazar,
K. V. J. Organomet. Chem. 1983, 254, 305.
(12) Hahn, J.; Landis, C. R.; Nasluzov, V. A.; Neyman, K. M.; Ro¨sch, N.
Inorg. Chem. 1997, 36, 3947.
(13) Laplaza, C. E.; Johnson, M. J. A.; Peters, J. C.; Odom, A. L.; Kim,
E.; Cummins, C. C.; George, G. N.; Pickering, I. J. J. Am. Chem. Soc. 1996,
118, 8623.
(14) Characterization data, synthetic protocols, and details of the X-ray
investigations are deposited as Supporting Information.
S0002-7863(98)00095-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/29/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 23, 1998 5837
Figure 1. Structural drawing of U(THF)(N[R]Ar)3 (2) with thermal
ellipsoids at the 35% probability level. Selected distances (Å) and angles
(deg): U-N(1), 2.295(10); U-N(3), 2.306(9); U-N(2), 2.361(9); U-O,
2.518(8); U-C(31), 2.886(12); U-C(11), 2.889(12); U-C(21),
2.980(12); U-C(22), 3.027(11); N(1)-U-N(3), 108.0(3); N(1)-U-N(2),
107.9(3); N(3)-U-N(2), 112.1(3); N(1)-U-O, 115.1(3); N(3)-U-O,
108.8(3); N(2)-U-O, 104.9(3); C(31)-U-C(11), 116.4(4); C(31)-U-
C(21), 123.6(4); C(11)-U-C(21), 116.1(4); O-U-N(1)-C(17), -149.05
(1.09); O-U-N(2)-C(27), -126.76 (1.20); O-U-N(3)-C(37), -151.15
(1.16).
Figure 2. Structural drawing of (Ph[t-Bu]N)3Mo(µ-N2)U(N[R]Ar)3 (3a)
with thermal ellipsoids at the 35% probability level. Selected distances
(Å) and angles (deg): U-N(7), 2.220(9); U-N(5), 2.249(8); U-N(1),
2.254(8); U-N(3), 2.267(8); U-C(51), 2.893(10); Mo-N(8), 1.773(8);
Mo-N(4), 1.978(8); Mo-N(6), 1.976(8); Mo-N(2), 1.978(8); N(7)-
N(8), 1.232(11); N(8)-N(7)-U, 173.8(7); N(7)-N(8)-Mo, 179.1(7);
N(7)-U-N(5), 104.4(3); N(7)-U-N(1), 104.5(3); N(5)-U-N(1),
111.4(3); N(7)-U-N(3), 104.2(3); N(5)-U-N(3), 116.1(3); N(1)-U-
N(3), 114.7(3); N(8)-Mo-N(4), 102.1(3); N(8)-Mo-N(6), 98.1(4);
N(4)-Mo-N(6), 117.6(3); N(8)-Mo-N(2), 103.8(4); N(4)-Mo-N(2),
114.2(3); N(6)-Mo-N(2), 116.9(3).
for a side-on bonding mode for the N2 ligand. A second derivative
3b, containing a more sterically encumbered Mo center, was
therefore prepared by mixing 2 with Mo(N[Ad]Ar)3 (Ad )
1-adamantyl) in toluene under N2, as in the synthesis of 3a. It
was postulated that the more hindered nature of the Mo center in
3b would favor an end-on, linear bridging bonding mode for the
N2 ligand. Accordingly, a prominent νNN band was observed for
3b at 1568 cm-1, the band shifting to 1527 cm-1 for 3b-15N2.
Upon subsequent synthesis of 3a-15N2 and examination of its IR
spectrum, νNN was located at 1547 cm-1, such that the lack of
observation of νNN for unlabeled 3a was finally attributed to
overlap with prominent amide aryl ring νCC modes. By way of
comparison, the previously characterized complex (µ-N2)[Mo-
requires six electrons, which could in principle be supplied by
Mo3+ in conjunction with U3+, as in the reactions leading to
complexes 3. Complexes 3 appear to be thermally stable,
however, a fact possibly reflective of the relative difficulty of
accessing uranium(VI) in the absence of oxo or fluorine ligands.
Terminal nitrides of uranium are not yet known in isolable
complexes,15 although uranium(V) and -(VI) imido complexes
are documented.16
Further reactivity studies involving 2 are underway. Among
other things, it will be of interest to assess the lability of the THF
ligand in 2 and to understand in detail the mechanism of formation
of complexes 3.
(N[R]Ar)3]2 has its νNN at 1630 cm-1 13
.
Further corroboration of the end-on bridging bonding mode
for the N2 ligand in complexes 3 comes from an X-ray structure
determination for 3a (Figure 2).14 Both metal centers in the
complex display pseudotetrahedral coordination, and they are
conformationally similar in that the N2 ligand resides inside a
“cage” consisting of the six tert-butyl groups. Only one U‚‚‚Cipso
interaction is in evidence, contrasting with the situation for 2 and
consistent with a higher formal oxidation state for the U center
in 3a. The smaller radius of uranium in 3a vis-a`-vis 2 is reflected
in shorter U-Namide distances (av 2.257 Å). Substantial reduction
of the N2 molecule is indicated by the N-N distance of
1.232(11) Å, longer by 0.13 Å than that in free dinitrogen. Some
degree of multiple bonding between N(7) and uranium is perhaps
indicated, the U-N(7) distance being nominally shorter than the
three U-Namide distances.
Note Added in Proof: Stewart and Andersen have determined
the structure of U[N(SiMe3)2]3.17
Acknowledgment. For financial support of this work C.C.C. thanks
the National Science Foundation (CAREER Award CHE-9501992),
DuPont (Young Professor Award), the Packard Foundation (Packard
Foundation Fellowship), Union Carbide (Innovation Recognition Award),
and 3M (Innovation Fund Award). C.C.C. is a Sloan Foundation Fellow
(1997-2000). P.L.A. is grateful to the Fulbright commission for a travel
scholarship.
Supporting Information Available: Synthesis and characterization
data for all new complexes and X-ray details for complexes 2, 3a, and
3b (13 pages, print/PDF). An X-ray crystallographic file, in CIF format,
is available via the Web only. See any current masthead page for ordering
information and Web access instructions.
The structure of 3b, which was also determined by X-ray
crystallography, is quite similar to that of 3a with respect to overall
conformation and metrical parameters.14
The valence-bond resonance structure depicted for compounds
3 in Scheme 1 is inferred from the crystal structure data. It
implies a formal oxidation state of +4 for both metals, with
molybdenum acting as the more effective π-donor to the com-
plexed N2 ligand. Reductive cleavage of N2 to two nitrides
JA980095T
(15) Hunt, R. D.; Yustein, J. T.; Andrews, L. J. Chem. Phys. 1993, 98,
6070.
(16) Burns, C. J.; Smith, W. H.; Huffman, J. C.; Sattelberger, A. P. J. Am.
Chem. Soc. 1990, 112, 3237.
(17) Stewart, J. L.; Andersen, R. A. Polyhedron 1998, 17, 953.