Structural and theoretical insights into the perturbation of uranium-rhenium bonds by dative Lewis base ancillary ligands

Article abstract of DOI:10.1039/c0cc01387k

Amine-elimination gave the two uranium-rhenium complexes [(Ts ^Xy)(THF)_nURe(η⁵-C₅H ₅)₂] [Ts^Xy = HC(SiMe₂N-3,5-Me ₂C₆H₃)₃; n

Full text of DOI:10.1039/c0cc01387k

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Structural and theoretical insights into the perturbation
of uranium–rhenium bonds by dative Lewis base ancillary ligandswz
Dipti Patel, David M. King, Benedict M. Gardner, Jonathan McMaster, William Lewis,
Alexander J. Blake and Stephen T. Liddle*
Received 13th May 2010, Accepted 10th June 2010
DOI: 10.1039/c0cc01387k
Amine-elimination gave the two uranium–rhenium complexes
[(TsXy)(THF)nURe(g5-C5H5)2] [TsXy
HC(SiMe2N-3,5-
Me2C6H3)3; n = 0 or 1]; structural and theoretical analyses,
and comparison to [(TrenTMS)URe(g5-C5H5)2] [TrenTMS
N(CH2CH2NSiMe3)3], reveal an increasing r-component to
the U–Re bond upon removal of dative ancillary ligands from
uranium with the p-component remaining essentially invariant.
Ligand design is of paramount importance to designing well
defined uranium complexes. We have therefore sought to
employ a range of tripodal ligands to support U–M bonds.
We recently reported tris(N-arylamidodimethylsilyl)methane
uranium amide and halide complexes as precursors to U–M
bonds.12 Here, we extend this chemistry with two new U–Re
bonds, and comparison with 2 presents the first opportunity to
systematically examine the impact on the U–Re bond that
results from modification of the type and number of dative
ancillary ligands at uranium.
We prepared 2 using salt elimination;11 however, some
TM-anions react sluggishly using this route so we explored
amine-elimination13 as a general entry to U–TM bonds,
Scheme 1. Treatment of [(TsXy)(THF)U(NCy2)] (3)12
[TsXy = HC(SiMe2NAr)3; Ar = 3,5-Me2C6H3; Cy = cyclohexyl]
with one equivalent of [(Z5-C5H5)2ReH]14 in THF gave, after
work-up and recrystallisation from hexane, the red complex
[(TsXy)(THF)URe(Z5-C5H5)2] (4) in 68% crystalline yield.z
The characterisation data support this formulation; the 1H NMR
spectrum of 4 spans the range +34 to ꢀ71 ppm and the
room temperature solution magnetic moment of 3.02 mB is
characteristic of a U(IV) complex.15
=
=
Understanding the nature and reactivity of metal–metal bonds
is fundamentally important.1 Metal–metal bonds involving
d- and p-block elements are widespread and well understood,
and even examples from the s-block are known.2 However,
despite the tendency of metal–metal complexes to display novel
bonding and reactivity, complexes containing uranium–metal
bonds are scarce.3–5 Structurally characterised uranium–metal
bonds were previously limited to the p-block derivatives
[{(Ar)(But)N}3USi(SiMe3)3] (Ar = 3,5-Me2C6H3),6 [(Z5-C5H5)3-
USnPh3],7 and [(Z5-C5H4SiMe3)3UAl(Z5-C5Me5)].8
The molecular structure of 4 is illustrated in Fig. 1a with
pertinent metrical data.y The salient feature of 4 is the
We have been investigating the chemistry of tripodal triamido
uranium complexes,9 and have employed Tren-based ligands to
isolate the U–Ga complex [(TrenTMS)(THF)UGa(NAr0CH)2]
[1, Ar0 = 2,6-Pri2C6H3; TrenTMS = N(CH2CH2NSiMe3)3];10
and the U–Re complex [(TrenTMS)URe(Z5-C5H5)2] (2).11 Both of
these complexes are notable because they exhibit s- and p-
components within the U–M bonds. Complex 2 was the first,
and remains the only, structurally characterised uranium–transition
metal (U–TM) bond.4 Little is known of the strength
and nature of such bonds, but this information is vital to
exploit any intrinsic reactivity patterns associated with
uranium–metal bonds.
School of Chemistry, University of Nottingham, University Park,
Nottingham, NG7 2RD, UK.
E-mail: stephen.liddle@nottingham.ac.uk;
Fax: +44 (0)115-951-3563; Tel: +44 (0)115-846-7167
w This article is part of the ‘Emerging Investigators’ themed issue for
ChemComm.
z Electronic supplementary information (ESI) available: Full
synthetic, spectroscopic, crystallographic, and computational details
for 4, 6, and 8. CCDC 777012–777014. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c0cc01387k
Scheme 1 Synthesis of 4, 6, and 8.
ꢁc
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Fig. 1 Molecular structuresy of 4 (a: values for second molecule in asymmetric unit in []), 6 (b), and 8 (c) with displacement ellipsoids at 40% and
selective atom labelling. Uꢂ ꢂ ꢂC contacts and hydrogen atoms are omitted for clarity. Selected bond lengths (A) and angles (1) for 4: U(1)–Re(1)
3.0021(8) [2.9541(8)], U(1)–N(1) 2.261(12) [2.279(11)], U(1)–N(2) 2.256(12) [2.297(12)], U(1)–N(3) 2.288(13) [2.308(11)], U(1)–O(1) 2.531(9)
[2.573(9)], and Ct–Re(1)–Ct 166.3(4) [167.1(15)], for 6: U(1)–N(1) 2.253(2), U(1)–N(2) 2.212(2), U(1)–N(3) 2.242(2), and U(1)–N(4) 2.257(2), for 8:
U(1)–Re(1) 2.9307(8), U(1)–N(1) 2.239(12), U(1)–N(2) 2.207(12), U(1)–N(3) 2.232(12), and Ct–Re(1)–Ct 166.9(6).
unsupported U–Re bond and retention of a THF molecule
coordinated to uranium, which adopts a distorted trigonal
bipyramidal geometry. This contrasts to 2 which is THF-free
but contains a coordinated trialkyl amine.11 Two molecules of
4 crystallise in the asymmetric unit, but bond lengths and
angles are similar and both U–Re bonds are longer than in 8.
In order to prepare a solvent-free U–Re complex we designed
and prepared a new uranium amide precursor.
Reaction of pro-ligand 5 with trivalent [U{N(SiMe3)2}3]16 in
toluene afforded red-brown, tetravalent [(TsXy)UN(SiMe3)2]
(6), with concomitant formation of finely divided uranium
from disproportionation, in 42% crystalline yield after work-up.
Alternatively, reaction of [(TsXy)(THF)UCl] (7)12 with one
equivalent of [LiN(SiMe3)2] in toluene afforded 6 in 40%
crystalline yield following work-up.z The characterisation data
for 6 support the formulation; the room temperature solution
magnetic moment of 3.09 mB for 6 is similar to that for 3, and
the 1H NMR spectrum occupies the range of +4 to ꢀ30 ppm.
Fig. 1b shows the molecular structurey of 6 with significant
metrical parameters. In contrast to 3, 6 is THF-free and the
pseudo-tetrahedral uranium centre is supplemented by a contact
to a xylyl ipso-carbon [U(1)ꢂ ꢂ ꢂC(4) = 3.001(3) A] to ameliorate
its electron deficiency. Since 6 was found to be free of ancillary
Lewis base ligands it was judged to be an excellent precursor to
solvent-free uranium–metal bonds.
and the uranium centre adopts a distorted tetrahedral geometry.
Abatement of the electron deficient nature of uranium
in 8 is suggested by agostic contacts to the closest two
cyclopentadienyl C–H bonds [U(1)ꢂ ꢂ ꢂC(32) = 2.894(12);
U(1)ꢂ ꢂ ꢂC(37) = 2.952(12) A].
In order to gain further insight into the nature of the U–Re
bonds in 4 and 8, and to make comparisons with 2, we carried
out DFT calculations on 4 and 8. Calculated data are listed in
Table 1 and compare well to the experimentally determined
structures; we thus conclude that the DFT calculations
provide a qualitative description of the electronic structures.
With combined experimental and theoretical analyses in hand,
several trends can be identified even though the nature of the
tripodal ligand changes from 2 to 4 and 8.
The U–Re bond length contracts from 3.0475(4) A in 2 to
3.0021(8) [2.9541(8)] A in 4 following exchange of a trialkyl-
amine for THF, which corresponds to a reduction of B0.04
[B0.09] A. Following the loss of THF, the U–Re bond
contracts by 0.07 [0.02] A to 2.9307(8) A in 8.17 Overall, this
corresponds to a U–Re bond distance decrease of 0.11 A from
2 to 8. This trend is qualitatively matched by the DFT
calculations, Table 1. The contracted U–Re bond distances
in the calculated structures are matched by increased
Nalewajski–Mrozek bond indices,18 which were computed to
be 1.15 (2), 1.18 (4), and 1.30 (8), respectively. The average
value of the U–Namide bond indices (B1.35) suggests some
p-bonding to uranium and they vary only a modest amount in
2, 4, and 8. The suggested agostic Uꢂ ꢂ ꢂCHCp contacts from the
experimentally determined structure of 7 are supported by
computed bond indices of 0.24 and 0.23 from U(1) to C(32)
and C(37), respectively. A significant change is observed in
the Mulliken charge at uranium following removal of the
trialkylamine donor from 2 to 4, which is followed by a more
modest change following removal of the THF to give 8; a
modest decrease is noted for rhenium and almost no change is
observed for the amide nitrogen atoms. The Mulliken
Accordingly, treatment of
6 with one equivalent of
[(Z5-C5H5)2ReH]14 in toluene afforded [(TsXy)URe(Z5-C5H5)2]
(8) as red blocks in 35% crystalline yield following work-up
and recrystallisation from hexane.z The room temperature
solution magnetic moment of 3.06 mB for 8 is intermediate to
those of 4 and 6. The 1H NMR spectrum is similar to that
observed for 4, spanning +15 to ꢀ68 ppm, except THF
resonances are absent.
To confirm the structure of 8 we carried out an X-ray
diffraction study and the molecular structure is depicted in
Fig. 1c with selected metrical data.y Complex 8 is solvent-free,
ꢁc
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296 | Chem. Commun., 2011, 47, 295–297

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Table 1 Selected computed DFT data for 2, 4, and 8
Orbital contributions to s
(HOMO ꢀ4) and p
U–Re bond
interaction
energy
Calculated U–Re and Nalewajski–
Complex
Atomic spin
densities
(averaged for N)
(HOMO ꢀ3) components
of the uranium–rhenium bond
(U/Re% contributions)
U–Namide (av.) bond Mrozek
lengths (A)
Atomic charges
bond indices (averaged for N)
(ref. 11 for
complex 2)
(kJ molꢀ1
)
U–Re
U–Namide U–Re U–N U Re
N
U
Re
N
s
Total
p
Total
2
4
8
3.0514 2.3032
3.0258 2.2965
3.0028 2.2632
1.15 1.34
1.18 1.34
1.30 1.37
+1.77 +0.36 ꢀ1.40 +2.28 ꢀ0.09 ꢀ0.05 11.3/54.4 65.7 10.4/58.8 69.2 ꢀ561.31
+2.15 +0.35 ꢀ1.38 +2.31 ꢀ0.09 ꢀ0.06 15.0/56.7 71.7 11.8/57.4 69.2 ꢀ599.53
+2.10 +0.30 ꢀ1.40 +2.37 ꢀ0.13 ꢀ0.07 17.5/57.3 74.8 13.1/56.6 69.7 ꢀ657.51
population analyses show that the spin densities at uranium
3
M = 943.40, monoclinic, a = 16.1847(4), b = 11.3466(2), c =
23.8954(5) A, b = 94.371(2)1, U = 4375.42(16) A3, T = 90 K, space
group P21/c, Z = 4, 44 442 reflections measured, 7716 unique reflections
(Rint = 0.027), R (F2 > 2s) = 0.0205, Rw (F2, all data) = 0.0532.
CCDC 777013. For 8: C41H56N3ReSi3U, M = 1099.39, monoclinic,
a = 15.9356(4), b = 15.1651(5), c = 33.4591(10) A, b = 91.052(3)1,
U = 8084.5(4) A3, T = 90 K, space group C2/c, Z = 8, 25 376
are consistently greater than would be expected for a H4 f2
uranium(IV) centre and were computed to be +2.28 (2), +2.31 (4),
and +2.37 (8). These values show a net transfer of electron
density to uranium, and that this increases from 2 to 4 to 8.
In line with this observation, the rhenium and nitrogen centres
exhibit small decreases in their computed spin densities.
Inspection of the Kohn–Sham orbitals of 4 and 8z shows
that, like 2, similar s- and p-components are present in the
U–Re bonds and they are localised to HOMOs ꢀ4 (principally
B54% Re 5dz2 and B15% U 6dz2 and 5fz3) and ꢀ3 (mainly
B56% Re 5dyz and B12% U 5dyz and 5fz2y) in each complex,
respectively. Analysis of the frontier orbitals involved in the
s-components (Table 1) shows that as Lewis base ancillaries
are removed the contributions to HOMO ꢀ4 from uranium-
and rhenium-based orbitals increase so the total contribution
to these molecular orbitals grows. In contrast, the total
contribution from uranium- and rhenium-based orbitals to the
HOMO ꢀ3 p-molecular orbital stays almost constant for 2, 4,
and 8 (Table 1). Thus, as uranium becomes more electron
deficient on moving from 2 to 4 to 8, the principal mechanism
by which this is ameliorated appears to be through the U–Re
s-bond. We examined the U–Re bonds in 2, 4, and 8 using energy
decomposition analyses and values are given in Table 1. This gave
calculated U–Re interaction energies of ꢀ561.31, ꢀ599.53, and
ꢀ657.51 kJ molꢀ1, respectively, for 2, 4, and 8, which follows the
anticipated trend as revealed by the U–Re bond distances.
To summarise, we have prepared two new U–Re complexes
by amine-elimination with systematic variation of the nature
and number of dative Lewis base ancillary ligands at uranium.
Combined experimental and theoretical analyses have given
insights into the perturbation of U–Re bonds by dative Lewis
base ancillary ligands. The data suggest that increased electron
deficiency at uranium is ameliorated principally through the
s-component of the U–Re bond, whilst the p-component
remains essentially invariant.
reflections measured, 7278 unique reflections (Rint
= 0.085),
R (F2 > 2s) = 0.0764, Rw (F2, all data) = 0.1824. CCDC 777014.
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elimination has afforded yttrium– and ytterbium–rhenium bonds,
see: M. V. Butovskii, O. L. Tok, F. R. Wagner and R. Kempe,
Angew. Chem., Int. Ed., 2008, 47, 6469.
We thank the Royal Society, the Engineering and Physical
Sciences Research Council, the European Research Council,
the University of Nottingham, and the UK National Nuclear
Laboratory for generously supporting this work.
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Notes and references
y Crystal data for 4: C45H64N3OReSi3Uꢂ0.5C6H14, M = 1214.58,
triclinic, a = 17.0448(4), b = 18.6938(4), c = 19.3062(5) A,
a = 63.838(3)1, b = 89.402(2)1, g = 63.080(2)1, U = 4783.5(2) A3,
17 The sum of uranium and rhenium covalent radii is calculated to be
3.01 A, see: P. Pyykko and M. Atsumi, Chem.–Eur. J., 2009, 15, 186.
ꢀ
T = 90 K, space group P1, Z = 2, 67 044 reflections measured,
¨
17 172 unique reflections (Rint = 0.100), R (F2 > 2s) = 0.0680,
18 A. Michalak, R. L. DeKock and T. Ziegler, J. Phys. Chem. A,
2008, 112, 7256.
Rw (F2, all data) = 0.1680. CCDC 777012. For 6: C37H64N4Si5U,
ꢁc
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 295–297 | 297