Formation of cyanates in low-valent uranium chemistry: A synergistic experimental/theoretical study

Article abstract of DOI:10.1039/c4dt00618f

Computational studies on the reductive activation of a mixture of CO and NO by the U(iii) complex [U(η-C₈H₆{SiⁱPr ₃-1,4}₂)(η-Cp*)], which affords a mixture of [U(η-C₈H₆{SiⁱPr₃-1,4} ₂)(η-Cp*)]₂(μ-OCN)₂1 and [U(η-C₈H₆{SiⁱPr₃-1,4} ₂)(η-Cp*)]₂(μ-O) 2, show that the reaction proceeds via an initial attack of CO on a μ-η²: η² coordinated NO, side-on bridged between two uranium centres. This leads to the formation of the bridging oxo complex 2 and the cyanate radical; coordination of the latter to the starting complex and dimerisation affords 1. The DFT studies also predict the existence of the monocyanate-bridged, mixed valence species [U(η-C₈H ₆{SiⁱPr₃-1,4}₂)(η-Cp*)] ₂(μ-OCN) 3, which has now been experimentally observed. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00618f

Dalton
Transactions
PAPER
Formation of cyanates in low-valent uranium
chemistry: a synergistic experimental/theoretical
study†
Cite this: Dalton Trans., 2014, 43,
11202
Christos E. Kefalidis,a Alistair S. P. Frey,b S. Mark Roe,b F. Geoffrey N. Cloke*b and
Laurent Maron*a
Computational studies on the reductive activation of a mixture of CO and NO by the U(III) complex
[U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)], which affords a mixture of [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2(μ-OCN)2 1 and
[U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2(μ-O) 2, show that the reaction proceeds via an initial attack of CO on a
μ–η2:η2 coordinated NO, side-on bridged between two uranium centres. This leads to the formation of
the bridging oxo complex 2 and the cyanate radical; coordination of the latter to the starting complex
and dimerisation affords 1. The DFT studies also predict the existence of the monocyanate-bridged,
mixed valence species [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2(μ-OCN) 3, which has now been experimentally
observed.
Received 28th February 2014,
Accepted 3rd April 2014
DOI: 10.1039/c4dt00618f
www.rsc.org/dalton
of a monocyanate-bridged mixed valence species, which has
now been experimentally verified.
Introduction
Low-valent uranium complexes, especially those of U(III), have
attracted much attention in recent years in the field of small
molecule activation requiring highly reducing metal com-
plexes. Reductive chemistry of N2, CO, and CO2 has all featured
prominently in this area, which has recently been comprehen-
sively reviewed.1 DFT computational studies have proved
crucial in understanding and rationalising the reactivity and
structure, for example in the reduction of N2,2 reductive cycli-
sation of CO,3 and reductive disproportionation of CO2.4 We
have recently described the reductive activation of a mixture
of CO and NO by the mixed sandwich U(III) complex
[U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)], which affords a mixture of the
bridging oxo complex [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2(μ-O) and
the doubly cyanate bridged complex [U(η-C8H6{SiiPr3-1,4}2)-
(η-Cp*)]2(μ-OCN)2.5 Herein we report a joint experimental/
theoretical study on defining the reaction mechanism of this
novel transformation. The synergy between theory and exper-
iment allows one to demonstrate that the mechanism involves
a bimetallic species with an unusual μ–η2:η2 coordination of
NO, similar to that reported in the case of CO by Mckay et al.3
In addition, DFT calculations suggested the potential existence
Computational details
All the structures reported in this study were fully optimized
with the Becke’s 3-parameter hybrid functional6 combined
with the non-local correlation functional provided by Perdew/
Wang7 (denoted as B3PW91). For the estimation of the redox
step of the NO reduction by the uranium(III) system a Hess–
Born–Haber cycle method is used, as proposed by Castro
et al.4a For this step, the relativistic energy-consistent small-
core pseudopotential obtained from the Stuttgart-Köln ECP
library has been used in combination with its adapted seg-
mented basis set.8 For the reaction steps involving intermedi-
ates in which the uranium’s oxidation state is fixed to +IV, the
corresponding 5f-in-large-core ECP (augmented by a f polari-
zation function, α = 1.0) was used.9 The aforementioned com-
putational scheme was found previously by our group to give
coherent results.4b,10 For the silicon atoms the quasi-relativistic
energy-adjusted ab initio pseudopotential was used, along with
its corresponding energy-optimized valence basis set,11 aug-
mented by a d polarization function.12 For the remaining
atoms the 6-31G(d,p) basis set was used.13,14 In all compu-
tations no constraints were imposed on the geometry. All
stationary points have been identified as minima (number
of imaginary frequencies Nimag = 0) or transition states
(Nimag = 1). Intrinsic reaction paths (IRPs)15 were traced from
the various transition structures to make sure that no further
aUniversité de Toulouse et CNRS, INSA, UPS, UMR 5215, LPCNO, 135 Avenue de
Rangueil, F-31077 Toulouse, France. E-mail: laurent.maron@irsamc.ups-tlse.fr
bDepartment of Chemistry, School of Life Sciences, University of Sussex, Brighton,
BN1 9QJ, UK
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4dt00618f
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Paper
intermediates exist. The vibrational modes and the corres- ture resulting in a change in colour to red-brown. Standing at
ponding frequencies are based on a harmonic force field. ambient temperature for 5 days, −50 °C for 3 days and 4 °C for
Enthalpy energies were obtained at T = 298.15 K within the 3 days gave the product as black crystals. The product was
i
harmonic approximation. The Pr substituents of the silicon rinsed with pentane at 0 °C (significant amount of product
atoms of the side arms of the experimentally used COT ligand was lost to dissolution during rinsing) and dried in vacuo to
have been replaced by hydrogens. This simplification is not afford the title compound as black prisms suitable for
changing the results as it has been shown in a previous study single crystal X-ray diffraction (10 mg, 3% based on starting
in a related ligand environment.3 The GAUSSIAN09 program [U(h-C8H6{SiiPr3-1,4}2)(h-Cp*)](THF)).
suite was used in all calculations.16 Finally, for the 3D rep-
1H NMR (C6D5CD3, 303 K): δ ppm 67.1 (s, 2H, COT ring-
resentation of the structures the Cylview17 graphical program CH), 34.6 (s, 2H, COT ring-CH), 6.43 (s, 15H, Cp*–CH3), −1.33
was used, and Chemcraft18 was used for the visualization of (6H, s, iPr–CH), −3.82 (18H, s, iPr–CH3), −7.01 (18H, s,
the molecular orbitals.
iPr–CH3), −8.29 (18H, s, br, iPr–CH3), −8.63 (24H, s, br,
i
iPr–CH3 and Pr–CH), −14.6 (s, br, 15H, Cp*–CH3), −73.7 (s,
2H, COT ring-CH), −83.0 (s, 2H, COT ring-CH), −99.4 (s, 2H,
COT ring-CH), −114 (s, 2H, COT ring-CH). 13C NMR (C6D5CD3,
303 K, selected data): δ ppm 223.5. MS (EI): m/z = 832 (65%,
[(η-C8H6{SiiPr3-1,4}2)(η-Cp*)U(O13CN)]+).The crystal structure of
Experimental section
General methods
Unless specified otherwise all manipulations were carried out 3 has been deposited at the Cambridge Crystallographic Data
using conventional high vacuum and Schlenk line techniques, Centre and allocated the deposition number 988885. For
under an atmosphere of dry argon, or under a dinitrogen details of data collection and refinement see ESI.†
atmosphere in an MBraun glove box. Solvents were refluxed
over suitable drying agents under a dinitrogen atmosphere,
and distilled and degassed prior to use: toluene was refluxed
over sodium; THF was refluxed over potassium; pentane was
Results and discussion
refluxed over sodium/potassium alloy. d8-Toluene was dried by
The previously reported, overall experimental result is shown
refluxing over sodium and then vacuum transferred into an
in Scheme 1.5
1
ampoule and stored under dinitrogen prior to use. H and 13C
The initial coordination of the highly reactive nitric oxide
NMR spectra were recorded on a Varian VNMRS 400 spectro-
(in preference to carbon monoxide) substrate to the uranium(III)
meter at room temperature, unless otherwise stated. Chemical
complex precursor can be achieved in two different ways:
in a terminal fashion, through nitrogen atoms or in a side-on
fashion, involving the πNO. This is the first plausible step in
shifts reported in ppm (δ) are relative to the residual proton
chemical shift of the deuterated solvent (1H) or the carbon
chemical shift of the deuterated solvent (13C). Mass spectra
the reaction sequence which, upon an electrophilic attack of
were recorded on a VG autospec Fisons instrument (electron
the CO, ultimately yields the doubly bridged cyanate bis-
impact ionisation at 70 eV). Gas transfer was via a Toepler
uranium complex 1 along with the bridging oxo-species 2
pump with calibrated (by transfer of xenon into a receiving
(Scheme 1). Due to the presence of unpaired electrons in the
vessel of accurately known volume, and differential weighing)
uranium(III) fragment as well as in the NO substrate, all possi-
ble spin state combinations with the two different bonding
modes of the NO in the initially formed adducts have to be
delivery pressure. Control of the stoichiometry of the gas reac-
tions was achieved by the use of standard 5 mm (for the NMR
studies) or shortened 10 mm (for preparative scale) Young’s
NMR tubes of accurately known volume containing measured
volumes (microlitre syringe) of solvent (and therefore of
known headspace). [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)THF] was pre-
pared as previously described.19 High purity NO was prepared
by treatment of nitrosyl sulphuric acid with mercury,20 and
purified by transferring through a column of KOH pellets into
a 1 litre storage bulb on the Toepler pump line. 13CO (99%
enriched, Cambridge Isotopes) was used as supplied.
Synthesis of [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2(μ-OCN) 3. A
sample of black, crystalline [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]-
(THF) (400 mg, 4.64 × 10−4 mol) was placed in an ampoule
(high-vacuum PTFE stopcock, 50 mL volume) and heated
under vacuum (110 °C/1 × 10−5 mbar) for 1 h. The desolvated
material was dissolved in d8-toluene (0.5 mL). The solution
was cooled to −78 °C, the headspace evacuated and 13CO (0.07
bar, 1.55 × 10−4 mol) was admitted followed by NO (0.07 bar,
Scheme 1 Reaction of [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)] with NO and CO
1.55 × 10−4 mol). The flask was warmed to ambient tempera- in toluene.
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Scheme 2 Structures of different coordination modes of NO on mono- and bimetallic uranium(IV) complexes. All the given distances are in
angstroms (Å).
Fig. 1 Selected frontier occupied molecular orbitals for intermediate III.
considered. The results are depicted in Scheme S1.† The most fixed in the oxidation state IV for the uranium, the side-on
stable adduct is found to be terminally bonded NO in its coordination is found to be more stable by 5.1 kcal mol−1 com-
quintet spin state (4 unpaired electrons), which implies a pared to the terminal nitrosyl analogue. It should be noted
partial oxidation of the uranium centre. In particular, its elec- that in the η2-bound NO, the N–O bond length of 1.24 Å is
*1.5
NO
tronic configuration (EC) is 5f2.6
π
, based on the Mulliken 0.04 Å greater than that in the terminal NO complex, and is
atomic spin densities. The next most stable complex (1.3 kcal hence more reminiscent of a mono-reduced substrate (dNO
=
mol−1 higher in energy) has the same type of bonding mode 1.27 Å for the free (NO•)− molecule, see ESI†). This behaviour
for the NO substrate but in its triplet state (2 unpaired elec- is probably due to the better overlap of the d orbital of the
trons). For the latter, based on its Mulliken atomic spin den- uranium centre with that of the π bond of the NO. This transi-
sities, the uranium has an EC of 5f2.2 with the NO fragment ent species readily coordinates a second uranium(III) fragment
forming a pair of electrons and developing a consequently in a μ–η2:η2 fashion (III in Scheme 2), thus inducing a second
stronger bond with the uranium atom. The latter is also single electron transfer to the already mono-reduced nitrosyl
reflected in a decrease of the U–N bond distance with respect substrate. This coordination results in an extra energetic stabili-
to that in the quintet state (by 0.17 Å). Finally, the singlet spin zation, in line with the stability of related bimetallic species
state (0 unpaired electrons) of the terminally bound NO already reported in the literature for other substrates.3,4,10c
complex is 10.5 kcal mol−1 higher in energy than the most Overall, the exothermicity of the double reduction of the NO
stable (quintet) spin state, making its existence unlikely. The by the two uranium centers is found to be −60.5 kcal mol−1
singlet spin state describes an electronic situation where there Close inspection of some important molecular orbitals of
.
are no unpaired electrons on either the uranium atom or the the “key intermediate” III clearly reveals the bonding situation.
nitric oxide ligand. Identical electronic configurations have In particular, SOMOa and SOMOa−1 correspond to the anti-
been found for the respective η2-bound complexes, although bonding lone pairs of the bridged NO (Fig. 1). These two mole-
they are slightly higher in energy than the most stable linear cular orbitals are perpendicular to each other and their nodal
complex by 3.7, 2.8, and 5.5 kcal mol−1 for the singlet, triplet, planes do not share the same plane with the two uranium
and quintet spin states, respectively. As these energy differ- atoms, being slightly rotated. This is consistent with the di-
ences are within the precision of the method,21 the NO coordi- anionic character of the NO bridged moiety, which accumulates
nation can easily induce oxidation of the uranium centre upon most of the negative charge resulting from the two consecutive
η2 coordination as well. Indeed, using the 5f-in-core RECPs one electron oxidations of each uranium centre. This makes
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Fig. 2 Energy reaction profile for the electrophilic attack of CO molecule on intermediate III leading to complex V.
complex III highly nucleophilic, and able to react with sub-
strates like CO. The molecular orbital (SOMOa−9) in inter-
mediate III explains the bonding situation between the
uranium centres and the μ–η2:η2 bridging mode of the NO
(Fig. 1). In particular, the latter involves d orbitals of the two
uranium atoms interacting with a vacant π* orbital of the NO.
Mechanism
As the coordination of the NO in intermediate III is a highly
exothermic process (−60.5 kcal mol−1 with respect to the reac-
tants, vide supra), the reaction profile will be described from
complex III (Fig. 2). It should be noted that other possible
mechanistic scenarios have been considered computationally:
for example, the cleavage of the NO bond instead of the elec-
trophilic attack by CO, which has been excluded since it
Fig. 3 Frontier molecular orbitals of TSIII–IV
.
resulted in the formation of a highly unstable mixed valence
U(V)/U(VI) species.
The reaction can proceed then through the TSIII–IV, namely
an electrophilic attack of CO on the doubly reduced NO. In
TSIII–IV, the distance between the carbon of the CO group and
the nitrogen of NO remains long (2.05 Å) allowing an
extra interaction between the carbon of CO and the U1 centre
(dU1–C = 2.58 Å). The N–O1 bond distance remains almost
unchanged with respect to the intermediate III (1.37 and
1.38 Å respectively). On the other hand, the bonding distance
between the U1 and the ancillary ligands (Cp* and COT) is
increased significantly with respect to III, by 0.5 Å. The latter
is essential for the electrophilic attack to occur. The electrophi-
lic nature of this reaction can be seen by inspection of the
molecular orbitals of the transition state TSIII–IV (see Fig. 3):
the SOMOa orbital of TSIII–IV results from the overlap between
the SOMOa of III, which corresponds to the lone pair of the
NO, with the LUMO of the CO. It is also interesting to note
that the LUMO of the TSIII–IV is a combination of a non-
bonding available d orbital of the U1 atom with the second
degenerate π* orbital of CO, which probably accounts for the
low-energy transition state (Fig. 3).
In terms of kinetics, this step involves an activation barrier
of only 7.7 kcal mol−1, and thermodynamically corresponds to
a process which is exothermic by 8.8 kcal mol−1. In the result-
ing intermediate IV, the O1–N–C–O2 moiety displays a zig-zag
configuration with the C and O1 having a κ2 coordination
mode with the U1 centre, whereas the U2 atom is ligated in a
η2-fashion to the N–O1 bond. The N–O1 bond in IV is
elongated by 0.08 Å compared to that in the intermediate III,
and is further activated by passing through TSIV–V, requiring
again a small activation barrier (5.8 kcal mol−1). At the tran-
sition state, TSIV–V, the interatomic distance between N and O1
is 1.69 Å, corresponding to an increase of 0.24 Å compared to
the corresponding intermediate IV. The product of this step is
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Fig. 4 Pathways for the trapping of the NCO radical by precursor complex I.
Scheme 3 Possible spin states for the monomeric structure of binding cyanates.
the oxo-intermediate V. It is noteworthy that the calculated pair, while the third one forms a bonding electron pair with
geometry is in good agreement with the experimentally the cyanate radical. The triplet state corresponds to an electron
derived structure, with daverage(U–O) = 2.17 Å, compared to the configuration where the uranium atom possesses two unpaired
experimental value of 2.11 Å. The bond angle U1–O1–U2 is electrons and a bonding pair with the NCO group. Finally, the
found to be 161.5, 7° smaller than the experimental value, a quintet spin state corresponds to a loosely bonded cyanate
difference which is mainly due to the absence of the steric adduct, having all the electrons unpaired (three on uranium
i
effects induced by the Pr groups which are absent in our sim- and one on NCO). It is worth noting that the two former spin
plified system. Finally, this step is found to be exothermic by states correspond to a uranium(IV) system whereas the latter
22.8 kcal mol−1, even though the concomitant release of the corresponds to a U(III) complex. The calculations predict that
CNO radical occurs. The latter highly reactive radical can then the spin triplet state is the most stable compared to the other
bind to the precursor complex I to form a very stable complex two, by more than 27 kcal mol−1. The U–N bond lengths in the
VI, in which the NCO• is bound through its nitrogen singlet and the triplet spin states are shorter by only 0.05 Å
atom (Fig. 4), and which results in a large stabilization of compared to the quintet, while all the other significant bond
91.9 kcal mol−1
.
lengths, such as NvC and CvO, are essentially the same as in
The electronic configuration of this highly stable cyanate the free NCO radical (Scheme 3 and Scheme S1†). In the triplet
intermediate VI requires some analysis. To that effect, spin state, the spin density of the uranium is 2.36|e|. Some
different spin states for this intermediate (singlet, triplet and residual negative charge is located at the carbon atoms of the
quintet) were computed. Ideally, in the closed-shell singlet COT ligand and at the nitrogen atom of the NCO fragment.
spin state, two electrons of the uranium atom form a lone Examination of the Mulliken spin densities of the quintet spin
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state reveals that the uranium atom has a spin density of 3.17,
0.17|e| higher than the ideal, without any sizeable spin
density at the NCO ligand. This extra density is found to be
accumulated on some carbon atoms of the COT ligand, result-
ing in distortion of the COT ligand, and the consequent esti-
mated energy increase. The latter is due to the inability of the
NCO fragment to receive more density from the metal.
Despite its high energetic stability, intermediate VI can
anchor another uranium(III) species to form the bimetallic
complex VII (Fig. 4). The formation of the latter species is an
exothermic process (10.5 kcal mol−1 more stable than inter-
mediate VI). This intermediate is interesting as it corresponds
to a mix-valence complex with U(IV) and U(III) metal centres. In
terms of geometrical features, the U1–N and CvO bond dis-
tances are slightly elongated by 0.07 and 0.01 Å respectively,
while the NvC bond decreased by only 0.02 Å with respect to
intermediate VI. These small differences are the result of the
newly formed bond between the second uranium centre and
the oxygen centre (2.67 Å). Moreover, the bond angle U1–N–C
decreases by almost 5 degrees, with respect to the intermediate
VI, while the bond angle U2–O–C decreases to 153.0°. Alterna-
tively, intermediate VI may dimerise to form complex VIII, i.e.
the experimentally observed product 1 in Scheme 1, as pre-
viously reported.5 This process is predicted to be exothermic
by 17.8 kcal mol−1, being however only 7.3 kcal mol−1 more
stable than the mixed valence complex VII. Thus, one can con-
clude that both complexes VII and VIII may exist. Indeed, by a
slight change in the stoichiometry of the reaction (see Experi-
mental section), we have now found that the mixed valence
compound VII (3 in Scheme 4 below) maybe isolated in small
amounts together with the bridging oxo compound 2.
Fig. 5 The X-ray structure of 3 (thermal ellipsoids at 30%, hydrogens
and isopropyl groups are removed for clarity).
The structure of 3, as determined by X-ray crystallography,
is shown in Fig. 5. The structure includes a single bridging
cyanate moiety, but the cyanate is disordered, both linearly, i.e.
N–C–O over O–C–N, and radially. This required that the
cyanate moiety be heavily restrained to expected values and, as
such, no meaningful inferences can be made about the bond
lengths and angles in and around the bridging cyanate group.
Several models can be constructed that refine equally well, but
crucially there is no evidence for a second bridging cyanate.
Similar disorder issues were encountered in the X-ray structure
of the doubly bridged cyanate complex 1.5
Conclusions
1
DFT computational studies on the reductive activation of a
mixture of CO and NO by the U(III) complex [U(η-C8H6{SiiPr3-
1,4}2)(η-Cp*)], which affords a mixture of [U(η-C8H6{SiiPr3-
1,4}2)(η-Cp*)]2(μ-OCN)2 1 and [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2-
(μ-O) 2, show that the reaction proceeds via an initial attack of
CO on a dimeric intermediate containing an µ–η2:η2 co-
ordinated NO. This leads to the formation of the bridging oxo
complex 2 and the cyanate radical, which is trapped by the
starting complex [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)] to afford [U(η-
C8H6{SiiPr3-1,4}2)(η-Cp*)(NCO)]. Theory suggests that the latter
can either dimerise to afford the previously experimentally
observed, doubly cyanate-bridged dimer [U(η-C8H6{SiiPr3-1,4}2)-
(η-Cp*)]2(μ-OCN)2 1 or coordinate a second molecule of the
starting complex [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)] to yield the
mixed valence species [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)]2(μ-OCN).
This mixed valence species 3 has now been isolated from the
reaction mixture for the first time and crystallographically
characterised, thus confirming the outcome predicted by the
computational studies.
Complex 3 displays an H NMR spectrum in which the ligand
resonances associated with the two uranium centres are clearly
different, consistent with the non-centrosymmetric structure
imposed by the single cyanate bridge. A resonance at δ ppm
223.5 in the 13C{1H} NMR spectrum of 3 is assigned to the
single bridging cyanate carbon, and maybe compared to that
at δ ppm 249.3 for the doubly bridging cyanate ligands in 1.
Acknowledgements
We thank the European Research Council for support. CINES
and CalMip are acknowledged for a generous grant of comput-
ing time. LM is also grateful to the Humboldt Foundation.
Scheme 4 Reaction of [U(η-C8H6{SiiPr3-1,4}2)(η-Cp*)] with NO and CO
in toluene, using a modified stoichiometry.
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13 (a) W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys.,
1972, 56, 2257–2261; (b) P. C. Hariharan and J. A. Pople,
Theor. Chim. Acta, 1973, 28, 213–222.
References
1 (a) P. Arnold, Chem. Commun., 2011, 47, 9005–9010;
(b) B. M. Gardner and S. T. Liddle, Eur. J. Inorg. Chem., 14 J. S. Binkley, J. A. Pople and W. J. Hehre, J. Am. Chem. Soc.,
2013, 3753–3770; (c) O. P. Lam and K. Meyer, Polyhedron, 1980, 102, 939–947.
2012, 32, 1–96; (d) H. S. La Pierre and K. Meyer, Prog. Inorg. 15 (a) C. Gonzalez and H. B. Schlegel, J. Chem. Phys., 1989, 90,
Chem., 2014, 58, in press.
2 F. G. N. Cloke, J. C. Green and N. Kaltsoyannis, Organo-
metallics, 2004, 23, 832–835.
2154–2161; (b) C. Gonzalez and H. B. Schlegel, J. Phys.
Chem., 1990, 94, 5523–5527.
16 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand,
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene,
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador,
J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
GAUSSIAN 09, (Revision A.02), Gaussian, Inc., Wallingford,
CT, 2009.
3 D. McKay, A. Frey, J. C. Green, F. G. N. Cloke and L. Maron,
Chem. Commun., 2012, 48, 4118–4120.
4 (a) L. Castro, O. P. Lam, S. C. Bart, K. Meyer and L. Maron,
Organometallics, 2010, 29, 5504–5510; (b) V. Mougel,
C. Camp, J. Pécaut, C. Copéret, L. Maron, C. E. Kefalidis
and M. Mazzanti, Angew. Chem., Int. Ed., 2012, 51, 12280–
12284.
5 A. S. P. Frey, F. G. N. Cloke, M. P. Coles and
P. B. Hitchcock, Chem. – Eur. J., 2010, 16, 9446–9448.
6 A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652.
7 J. P. Perdew and Y. Wang, Phys. Rev. B: Condens. Matter,
1992, 45, 13244–13249.
8 (a) W. Küchle, M. Dolg, H. Stoll and H. Preuß, J. Chem.
Phys., 1994, 100, 7535–7543; (b) X. Y. Cao, M. Dolg and
H. Stoll, J. Chem. Phys., 2003, 118, 487–497; (c) X. Cao and
M. Dolg, J. Mol. Struct. (THEOCHEM), 2004, 673, 203–209.
9 A. Moritz, X. Y. Cao and M. Dolg, Theor. Chem. Acc., 2007,
118, 845–854.
10 (a) L. Castro, A. Yahia and L. Maron, ChemPhysChem, 2010, 17 CYLview, 1.0b; Legault, C. Y, Université de Sherbrooke, 2009
11, 990–994; (b) L. Castro, A. Yahia and L. Maron, Dalton (http://www.cylview.org).
Trans., 2010, 39, 6682–6692; (c) B. Kosog, C. E. Kefalidis, 18 G. A. Zhurko, ChemCraft Home Page: a set of graphical
F. W. Heinemann, L. Maron and K. Meyer, J. Am. Chem.
Soc., 2012, 134, 12792–12797.
tools for facilitating working with quantum chemistry
computations (http://www.chemcraftprog.com).
11 A. Bergner, M. Dolg, W. Küchle, H. Stoll and H. Preuß, Mol. 19 O. T. Summerscales, F. G. N. Cloke, P. B. Hitchcock,
Phys., 1993, 80, 1431–1441. J. C. Green and N. Hazari, Science, 2006, 311, 829–831.
12 A. W. Ehlers, M. Böhme, S. Dapprich, A. Gobbi, 20 Handbook of Preparative Inorganic Chemistry, ed. G. Brauer,
A. Höllwarth, V. Jonas, K. F. Köhler, R. Stegmann, Academic Press, 2nd edn, 1965, vol. 1, p. 485.
A. Veldkamp and G. Frenking, Chem. Phys. Lett., 1993, 208, 21 Y. Zhao and D. G. Truhlar, J. Chem. Phys., 2006, 125,
111–114.
194101–194118.
11208 | Dalton Trans., 2014, 43, 11202–11208
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