Asymmetric niobium guanidinates as intermediates in the catalytic guanylation of amines

Article abstract of DOI:10.1039/c3dt50477h

The molecular structure of the guanidinate complex {NbBz₂(N ^tBu)[(4-BrC₆H₄)NC(NⁱPr)(NH ⁱPr)]}, previously obtained by reaction of [NbBz₃(N ^tBu)] and the corresponding

Full text of DOI:10.1039/c3dt50477h

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Asymmetric niobium guanidinates as intermediates in
the catalytic guanylation of amines†
Cite this: Dalton Trans., 2013, 42, 8223
David Elorriaga,a Fernando Carrillo-Hermosilla,*a Antonio Antiñolo,*a
Francisco J. Suárez,b Isabel López-Solera,a Rafael Fernández-Galána and
Elena Villaseñora
The molecular structure of the guanidinate complex {NbBz2(NtBu)[(4-BrC6H4)NvC(NiPr)(NHiPr)]}, pre-
viously obtained by reaction of [NbBz3(NtBu)] and the corresponding guanidine proligand, has been
established by X-ray diffraction. The series of complexes {NbBz2(NtBu)[(Ar)NvC(NiPr)(NHiPr)]} (Ar =
4-BrC6H4, 4-tBuC6H4, 4-MeOC6H4) and {[NbBz2(NtBu)]2[(C6H4)(NvC(NiPr)(NHiPr))2]} show a preferred
asymmetric coordination of the guanidinate ligand by means of one alkylamino nitrogen and the aryl-
imino nitrogen atom. Computational studies confirm this preference and the results suggest that elec-
tronic factors prevail over steric factors. In addition, reaction of complex [NbBz3(NtBu)] with {2-(nbutyl)-
1,3-diisopropylguanidine} did not give rise to the regioselective asymmetrical guanidinate. Instead, the
complex {NbBz2(NtBu)[(nBu)NvC(NiPr)(NHiPr)]} was obtained as a mixture of three isomers with sym-
metrical and asymmetrical coordination modes. Finally, the complex [NbBz3(NtBu)] was shown to be a
suitable precatalyst for the guanylation reaction of a wide range of amines under mild conditions.
Guanidinates are proposed as intermediates in the mechanism of this reaction. The molecular structure
of the biguanidine {2,2’-(1,4-phenylene)bis(2’,3-diisopropylguanidine)} was also established by X-ray
diffraction studies.
Received 21st February 2013,
Accepted 28th March 2013
DOI: 10.1039/c3dt50477h
www.rsc.org/dalton
with a carbodiimide; or (c) the deprotonation of a neutral gua-
nidine by means of an existent basic ligand.2 In most cases, a
Introduction
N,N′,N″-Trisubstituted guanidinate ligands (RNH)2CvNR (R = symmetrical distribution of the guanidinate ligand is obtained
alkyl or aryl) have proven to be an interesting alternative to (Scheme 1).
classical ancillary ligands, such as cyclopentadienyl, as they
It is worth noting that, in the few reported cases where one
have the advantages of tuneable electronic and steric effects by of the nitrogen atoms is substituted by an aromatic group, a
variation of the substituents on the nitrogen atoms.1 The preferential asymmetric coordination was obtained, including
preparation of monoanionic guanidinate complexes usually the rearrangement of a plausible symmetric intermediate.3 We
involves: (a) the insertion of a carbodiimide into a metal– recently extended the limited chemistry of niobium guanidi-
amido bond; (b) the reaction of a halide complex with a nate complexes by reporting the synthesis of new dialkyl
lithium guanidinate, formed by treatment of a lithium amide niobium complexes with aromatic-substituted guanidinate
ligands {NbBz2(NtBu)[(Ar)NvC(NiPr)(NHiPr)]} (Ar = 4-BrC6H4
1a, 4-tBuC6H4 1b, 4-MeOC6H4 1c) and {[NbBz2(NtBu)]2[(C6H4)-
(NvC(NiPr)(NHiPr))2]} 1d (Scheme 2), which are arranged in
aDepartamento de Química Inorgánica, Orgánica y Bioquímica, Facultad de Ciencias
an asymmetrical manner.4
y Tecnologías Químicas, Universidad de Castilla-La Mancha, Campus Universitario
de Ciudad Real, 13071-Ciudad Real, Spain. E-mail: Fernando.Carrillo@uclm.es,
Antonio.Antinolo@uclm.es; Fax: +34 926 295 318
bLaboratorio de Compuestos Organometálicos y Catálisis (Unidad Asociada al CSIC),
Departamento de Química Orgánica e Inorgánica, Instituto Universitario de Química
Organometálica “Enrique Moles”, Facultad de Química, Universidad de Oviedo,
Julián Clavería 8, 33006-Oviedo, Spain
†Electronic supplementary information (ESI) available: Crystallographic data in
CIF and refinement details (Table S1). Optimized geometries and Cartesian co-
ordinates for 1a–d, 1′a–d, 3a–d, 3′a–d, 4a–c, 5 and 5′. Experimental details of the
guanylation reactions. NMR spectra of the 4a–c mixture. CCDC 924356 and
924357. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c3dt50477h
Scheme 1 Symmetrical (a) and asymmetrical (b) coordination of monoanionic
guanidinate ligands.
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Scheme 2 Synthesis of dibenzyl niobium guanidinates.
As a continuation of our research in this field,4,5 we
describe here the structural characterization of the previously
reported complex 1a and comparative theoretical and reactivity
studies concerning the preferred coordination of the guanidi-
nate ligands in these dialkyl complexes. Catalytic guanylation
of amines, as a waste-free process to obtain substituted guani-
dines, has been a topic of interest in recent years. Lanthanoid,
main group and early transition metal complexes have been
described as catalytic systems.6 Despite the use of vanadium
complexes as catalyst precursors,6c,d,e niobium complexes have
not been reported in this context until now. As guanidinate
complexes were widely proposed as intermediates in the cata-
lytic guanylation of amines, we took the opportunity to study
the use of the trialkyl parent compound [NbBz3(NtBu)] 27 as a
new catalyst precursor.
Fig. 1 ORTEP drawing of compound 1a, with thermal ellipsoid at 30%
probability.
Table 1 Selected bond lengths (Å) and angles (°) for 1a
Bond lengths
Bond angles
Nb1–N1
Nb1–N2
Nb1–N4
Nb1–C12
Nb1–C21
N2–C5
N3–C5
N3–C9
N4–C5
1.762(2)
2.102(2)
2.367(2)
2.211(3)
2.220(3)
1.350(3)
1.356(3)
1.478(3)
1.330(3)
N1–Nb1–N2
N1–Nb1–N4
N2–Nb1–N4
Nb1–N1–C1
N2–C5–N3
N2–C5–N4
N3–C5–N4
103.5(1)
162.4(1)
59.4(1)
178.1(2)
122.7(2)
112.3(2)
125.0(2)
87.8(2)
Results and discussion
Nb1–C12–C13
Nb1–C21–C22
103.4(2)
The direct reaction between the guanidine proligands and the
trialkyl complex 2 gave rise, through a protonolysis pathway, to
the corresponding guanidinate complexes {NbBz2(NtBu)[(Ar)-
influence of the imido group and the axial coordination of the
N4 atom. The planarity of the ‘CN3’ core is evidenced by the
sum of the bond angles around C5 (360.0°).
NvC(NiPr)(NHiPr)]} (Ar
=
4-BrC6H4 1a, 4-tBuC6H4 1b,
4-MeOC6H4 1c) and {[NbBz2(NtBu)]2[(C6H4)(NvC(NiPr)(NHi-
Pr))2]} 1d. The molecular structures of complexes 1b–d were
reported previously.4 Complex 1a was crystallized and its mole-
cular structure was determined by X-ray diffraction (Fig. 1).
The crystal data are summarized in the Experimental section.
Selected bond lengths and angles for 1a are given in Table 1.
Complex 1a crystallizes in the P21/c space group. The distri-
bution of the atoms around the niobium centre shows a dis-
torted trigonal bipyramidal geometry, with an asymmetrical
disposition of the guanidinate ligand, as observed for the ana-
logous complexes 1b–d. The imido group is quasilinear with
an Nb1–N1–C1 angle of 178.1(2)° and an Nb1–N1 bond dis-
tance of 1.762(2) Å, both of which are close to those described
for other imido complexes7,8,2f and are consistent with the
existence of a metal–nitrogen fragment with a triple bond and
an sp hybridized nitrogen atom. The Nb1–N4 bond trans to the
imido group is elongated compared to the Nb1–N2 bond
(ΔNb–N ∼0.26 Å), as one would expect due to the trans
Some charge delocalization within the guanidinate ligand
is proposed from the C–N bond lengths, which average 1.34 Å
and are intermediate between the values expected for C–N
single and CvN double bonds. While the N2 atom is very
close to being planar (the sum of angles 359.2°), N4 deviates
significantly (351.2°) from planarity. The Nb1–C12–C13 angle
of 87.8(2)° and the Nb1–C13 distance of 2.603(2) Å both indi-
cate that one benzyl group shows η2-hapticity. In contrast, the
Nb1–C21–C22 angle of 103.4(2)° and the Nb1–C22 distance of
2.935(3) Å indicate that the second benzyl group shows η1-hap-
ticity. On considering the crystal packing, the hydrogen atom
H33 on the aromatic group of the guanidinate ligand almost
faces the π-electrons of the aromatic benzyl group of a neigh-
bouring molecule, with a distance of 2.668 Å. This suggests an
aromatic CH–π interaction,9 which in turn leads to the for-
mation of “dimeric” aggregates in the solid state (Fig. 2). Both
aromatic rings are in an almost ideal orthogonal arrangement
8224 | Dalton Trans., 2013, 42, 8223–8230
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Table 2 Calculated total (Hartree) and relative (kcal mol−1
niobium complexes 1a–d and 1’a–d and the guanidinate anionsa
) energies for
Total (relative)
Total (relative)
Ligand energy (kcal mol−1
Complex energy (kcal mol−1
)
)
1a
1′a
1b
1′b
1c
1′c
1d
1′d
4a
−4053.39277511 (0.0)
3a
−3242.50051774 (0.0)
−3242.48091175 (12.3)
−828.939055043 (0.0)
−828.922596635 (10.3)
−786.216464799 (0.0)
−786.187257744 (18.3)
−1111.04289212 (0.0)
−1110.99855609 (27.8)
−597.856972993 (0.0)
−597.857904302 (−0.6)
−4053.37407477 (11.7) 3′a
−1639.84103061 (0.0)
−1639.82963265 (7.2)
−1597.10861888 (0.0)
−1597.09678503 (7.4)
−2732.93378629 (0.0)
3b
3′b
3c
3′c
3d
−2732.89762279 (22.7) 3′d
−1408.78473948 (0.4)
−1408.77852231 (4.3)
−1408.78533689 (0.0)
5
5′
4b
4c
a B3LYP/6-31G(d)+LANL2DZ-optimized geometries.
1a–d are more stable than the symmetrical ones 1′a–d by
Fig. 2 The CH–π interaction scheme for complex 1a in the solid state. Hydro-
gen atoms, except those on C33, have been omitted for clarity.
7.2–22.7 kcal mol−1
. The largest energy difference was
observed for the 1d/1′d pair. This finding could be explained
by some degree of electronic delocalization between the two
metallic centres through the aromatic ring (the dihedral plane
formed by the metal-guanidinate moiety and the central
phenylene group is 61.6°). As far as the free guanidinate
anions are concerned, the unsymmetrical anions proved to be
much more stable than the symmetrical ones, with energy
differences ranging from 10.3 to 27.8 kcal mol−1. As for
complex 1d, the largest difference was found for ligand 3d. All
of these theoretical predictions suggest that electronic factors
prevail over steric factors to rationalize the observed experi-
mental asymmetric structures. The stabilization associated
with the electronic conjugation of the delocalized π-electrons
of the CN3 core with the aromatic ring may be evoked to
explain the higher thermodynamic stability of asymmetric
compounds. With these results in mind, we proceeded to react
the parent compound 2 with a trialkyl guanidine, {2-(nbutyl)-
1,3-diisopropylguanidine} 5 (see the Experimental section).
The same reaction with N-aromatic-substituted guanidines
3a–d was 100% regioselective to the asymmetrical coordinated
compounds 1a–d with the NAr moiety trans to the imido
group. However, in this case, three different isomers, 4a–c,
were obtained in a 1.3 : 1 : 0.4 ratio (Scheme 3).
(92.5°), with a distance between centroids of 4.88 Å. This inter-
action could be the cause of the deviation of the N4 atom from
planarity.
Guanidinate rearrangement from the asymmetrical (com-
plexes 1a–d) to the less sterically demanding symmetrical
coordination (complexes 1′a–d), through a formal 1,3-hydrogen
shift, was not observed in solution, even on warming.
In order to account for the preferred asymmetric coordi-
nation of the guanidinate anions, the relative stability of com-
plexes 1a–d vs. 1′a–d was studied by means of DFT calculations
at the B3LYP/6-31G(d)+LANL2DZ level of theory. For compara-
tive purposes, the free anions 3a–d and 3′a–d were also
investigated.
As an example, optimized structures of 1a and 1′a are
shown in Fig. 3. The optimized structures of 1b–d, 1′b–d, 3a–d
and 3′a–d, along with their most relevant geometrical para-
meters, are given in the ESI.† All of these structures were
characterized as minima on the potential energy surface and
their absolute and relative energies are listed in Table 2.
According to our calculations, the unsymmetrical complexes
Although the 1H and 13C NMR spectra of this mixture of
isomers were quite complex and a complete peak assignment
was not possible, three peaks were clearly assigned to tBu
groups. In addition, three pairs of coupled doublets (2JHH
1
values from 7.9 to 9.1 Hz) were observed in the H NMR spec-
trum for the diastereotopic methylene protons of the two
equivalent benzyl ligands in each isomer. Finally, three peaks
were observed close to δ 166 ppm in the 13C NMR spectrum
due to the central carbon of the κ2-monoanionic chelating gua-
nidinates.2a NOESY-1D experiments provided evidence for the
presence of three plausible relative orientations of the guanidi-
nate ligand, giving rise to a mixture of symmetrical and asym-
metrical coordination – the latter with the presence of the
two possible orientations for the coordinated nitrogen atoms.
Fig. 3 Optimized structures of 1a and 1’a.
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Scheme 4 Proposed mechanism for the catalytic guanylation of amines.
Scheme 3 Synthesis of the complexes 4a–c.
Table 3 Catalytic addition of amines to N,N’-diisopropylcarbodiimide with 2
DFT calculations were extended in order to obtain information
about the relative stability of isomers 4a–c and the free
asymmetric and symmetric anions 5 and 5′. In this case,
and in comparison with the NAr-substituted guanidinates,
both asymmetrically coordinated complexes, 4a and 4c,
had similar relative stabilities and were found to be more
stable than the symmetric one, 4b, by only 4.3 kcal mol−1. Con-
cerning the free guanidinate anions, it is worth noting that
energy differences were not found between asymmetrical and
symmetrical species. This finding provides further evidence of
the influence of an aryl-substituent through an electronic
effect.
As mentioned above, guanidinate complexes were proposed
as clear intermediates in most processes for the guanylation of
amines with carbodiimides.6 In the mechanism of this reac-
tion, an amido intermediate undergoes carbodiimide in-
sertion, giving rise to the aforementioned guanidinate.
Protonolysis by an amine molecule gives the final guanidine
product (Scheme 4).
Entry
1
Amine
Temperature (°C)
50
Conversione (%)
0
2
3
4
120
50
0
80a
50
>99b
5
6
7
8
50
50
50
50
>99c
>99b
>99b
>99b
9
50
50
50
>99b
>99b
>99b
10
11
The catalytic activity of compound 2 was investigated
(Scheme 4, Table 3). The conversion values were determined
1
by H NMR spectroscopy. We first used the reaction of p-tert-
12
50
71b
butylaniline with N,N′-diisopropylcarbodiimide as a model to
establish the appropriate amount of precatalyst. In the
absence of the complex, the reaction at 50 or 120 °C in deuter-
ated toluene did not occur (Table 3, entries 1 and 2). In con-
trast, the addition of a small amount (1–3 mol% with respect
to the carbodiimide) of 2 under mild conditions (50 °C) led to
conversions of up to 99% to the N,N′,N″-trisubstituted guani-
dine in 24 h (Table 3, entries 3–5). As a descriptive example,
the reaction was performed on a preparative scale, yielding
95% of the white crystalline guanidine.
13
14
15
50
50
50
70b
45b
>99b,d
Conditions: amine (1 mmol); N,N′-diisopropylcarbodiimide (1 mmol).
Time: 24 h. a 1 mol% catalyst. b 2 mol% catalyst. c 3 mol% catalyst.
d Conditions: amine (1 mmol); N,N′-diisopropylcarbodiimide
(2 mmol). Time: 24 h. e Determined by 1H NMR.
8226 | Dalton Trans., 2013, 42, 8223–8230
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These results were in contrast to the high temperature
needed for the imido vanadium complexes used in the same
class of reaction (105 °C).6c,d,e We then proceeded to explore
the scope of 2 in the catalytic addition of a variety of aromatic
amines bearing electron-donating and electron-withdrawing
substituents, heterocyclic and secondary cyclic amines to the
N,N′-diisopropylcarbodiimide (Table 3, entries 6–14). As can be
seen for the results in Table 3, the Nb-catalyzed guanylation
displayed remarkable functional group tolerance. The func-
tionalities present, such as halogens (F, Cl, Br), OMe, and Me,
were unaffected under the reaction conditions used (Table 3,
entries 7–11). 1,4-Phenylenediamine was also converted to the
corresponding biguanidine (Table 3, entry 15). The conversion
levels in these cases were found to be very high. The reaction
also occurred with heterocyclic or secondary cyclic amines
(Table 3, entries 12–14) in moderate yields. Finally, compound
1a was tested as a catalyst for the guanylation reaction of p-tert-
butylaniline under the same conditions. This compound
showed very similar activity to the parent compound 2, and
this provides plausible evidence that this kind of species plays
a principal role in the mechanism proposed for the guanyl-
ation reaction.
Table 4 Selected bond lengths (Å) and angles (°) for 3dH2
Bond lengths
Bond angles
C1–N1
C1–N2
C1–N3
1.358(3)
1.376(3)
1.296(3)
N1–C1–N2
N1–C1–N3
N2–C1–N3
113.8(2)
119.6(2)
126.6(2)
Conclusions
In summary, a new example of the scarce niobium(V) guanidi-
nate complexes, {NbBz2(NtBu)[(4-BrC6H4)NvC(NiPr)(NHiPr)]},
was structurally characterized by single-crystal X-ray diffraction
techniques. Experimental and computational studies demon-
strate that N-aromatic-substituted guanidinates feature a selec-
tive preference for an asymmetric coordination to the metal
centre, and suggest that electronic factors prevail over steric
factors. In addition, we demonstrated that the alkylimido
complex [NbBz3(NtBu)] is an efficient catalyst for the guanyla-
tion reaction of a variety of amines under mild conditions,
thus extending the limited catalytic applications of niobium
compounds in homogeneous catalysis.
The molecular structure of the previously described
{2,2′-(1,4-phenylene)bis(2′,3-diisopropylguanidine)} 3dH2 6o was
established by X-ray diffraction (Fig. 4).
Experimental section
This compound presents two ‘CN3’ cores and there are two
different types of distances for the carbon–nitrogen bonds.
The distances C1–N1 and C1–N2 are 1.358(3) and 1.376(3) Å,
respectively, and these are consistent with the distances of a
single carbon–nitrogen bond, whereas the distance C1–N3 of
1.296(3) Å is closer to a double carbon–nitrogen bond. This
information agrees with that found for other similar com-
pounds described in the literature.6g,h,n,s The angles N(1)–C(1)–
N(2), N1–C1–N3 and N2–C1–N3 are close to 120°, which indi-
cates that the core CN3 is practically planar. Looking at the
crystal packing, H1 and H2 show intermolecular hydrogen
bonding with the nitrogen atom N(3) of a neighbouring mole-
cule, with distances of 2.322 and 2.529 Å, respectively, and
angles of 173.3 and 156.6°, respectively. Selected bond dis-
tances and angles are shown in Table 4.
General procedures
All reactions were performed using standard Schlenk and
glove-box techniques under an atmosphere of dry nitrogen.
Solvents were purified by passage through a column of acti-
vated alumina (Innovative Tech.) and degassed under nitrogen
before use. Microanalyses were carried out with a Perkin-Elmer
2400 CHN analyzer. NMR spectra were recorded on a Varian
FT-400 spectrometer using standard VARIAN-FT software.
{NbBz2(NtBu)[(Ar)NvC(NiPr)(NHiPr)]} (Ar
= 4-BrC6H4 1a,
4-tBuC6H4 1b, 4-MeOC6H4 1c), {[NbBz2(NtBu)]2[(C6H4)(NvC-
(NiPr)(NHiPr))2]} 1d, [NbBz3(NtBu)] 2 and {2-(nbutyl)-1,3-di-
isopropylguanidine} 3e were prepared according to literature
procedures.4,7,6s
Synthesis of {NbBz2(NtBu)[(nBu)NvC(NiPr)(NHiPr)]} 4
2-Butyl-1,3-diisopropylguanidine (0.06 g, 0.46 mmol) in
toluene (20 mL) was added to a solution of [NbBz3(NtBu)]
(0.20 g, 0.46 mmol) in toluene (10 mL). The reaction mixture
was stirred for 1 h at room temperature. The resulting yellow
solution was concentrated, cooled to −30 °C for 2 h, filtered
and evaporated to dryness in vacuo to afford 4 as a yellow oily
material. Yield: 0.22 g (89%). 1H NMR (400 MHz, C6D6): δ =
(mixture of three isomers) 0.79–0.85 (m, 3 × 6H, CH(CH3)2),
0.86–0.94 (m, 3 × 3H, CH3), 1.13–1.20 (m, 3 × 6H, CH(CH3)2),
1.23–1.33 (m, 3 × 2H, CH2), 1.35, 1.37. 1.38 (3 × s, 3 × 9H, tBu),
1.44–1.53 (m, 3 × 2H, CH2), 2.20 (d, 2H, J = 7.9 Hz, CH2Ph),
2.35 (d, 2H, J = 8.2 Hz, CH2Ph), 2.55 (d, 2H, J = 9.1 Hz, CH2Ph),
2.76 (d, 2H, J = 7.9 Hz, CH2Ph), 2.85 (d, 2H, J = 8.2 Hz, CH2Ph),
2.99 (d, 2H, J = 9.1 Hz, CH2Ph), 3.16–3.28 (m, 3 × 2H, CH2),
3.26–3.56 (m, 3 × 2H, CH(CH3)2), 6.85–7.23 (m, 3 × 10H, C6H5).
Fig. 4 ORTEP drawing of compound 3dH2, with thermal ellipsoids at 30%
probability.
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13C{1H} NMR (100.6 MHz, C6D6): δ = 166.22 166.63, 166.73
(CN3). C29H48N4Nb (545.62): C, 63.84; H, 8.87. Found: C, 64.16;
H, 8.98.
Acknowledgements
We gratefully acknowledge financial support from the Minis-
terio de Ciencia e Innovación, Spain (grant nos. Consolider-
Ingenio 2010 ORFEOCSD2007-00006 and CTQ2009-09214) and
the Junta de Comunidades de Castilla-La Mancha, Spain
(grant no. PCI08-0032). Thanks are due to the Consejo
Superior de Investigaciones Científicas (CSIC) of Spain for the
award of a license for the use of the Cambridge Crystallo-
graphic Database (CSD).
Computational details
All theoretical calculations were performed with the program
package Gaussian03,10 at density functional theory (DFT) level
by means of the hybrid B3LYP functional.11 The molecular
geometries were optimized, without any molecular symmetry
constraint, using Pople’s 6-31G(d) split valence basis set for C,
H, N, O, and Br elements,12 and LANL2DZ for Nb, which com-
bines quasi-relativistic effective core potentials with a valence
double-basis set.13 Frequency calculations were performed to
determine whether the optimized geometries were minima
on the potential energy surface. Optimized geometries and
Cartesian coordinates for all the compounds studied can be
found in the ESI.†
Notes and references
1 (a) P. C. Junk and M. L. Cole, Chem. Commun., 2007, 1579;
(b) J. Barker and M. Kilner, Coord. Chem. Rev., 1994, 133,
219; (c) F. T. Edelmann, Coord. Chem. Rev., 1994, 137, 403;
(d) M. P. Coles, Dalton Trans., 2006, 985; (e) P. J. Bailey and
S. Pace, Coord. Chem. Rev., 2001, 214, 91;
(f) F. T. Edelmann, Adv. Organomet. Chem., 2008, 57, 183.
2 Some examples of insertion reactions: (a) M. K. T. Tin,
G. P. A. Yap and D. S. Richeson, Inorg. Chem., 1999, 38,
998; (b) A. Baunemann, D. Rische, A. Milanov, Y. Kim,
M. Winter, C. Gamel and R. A. Fischer, Dalton Trans., 2005,
3051; (c) A. Milanov, R. Bhakta, A. Baunemann,
H.-W. Becker, R. Thomas, P. Ehrhart, M. Winter and
A. Devi, Inorg. Chem., 2006, 45, 11008; (d) A. Baunemann,
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X-ray structure determination for compounds 1a and 3dH2
Crystals of compound 1a were mounted at low temperature in
inert oil on a glass fiber and crystals of compound 3dH2 were
mounted at room temperature. Data for compound 1a were
collected on a Bruker X8 APPEX II CCD-based diffractometer,
and for 3dH2 on a Bruker X8 APPEX. Both diffractometers were
equipped with a graphite monochromated MoKα (radiation
source λ = 0.71073 Å).
The crystal data, data collection, structural solution, and
refinement parameters for the complexes are summarized in
Table S1.† Data were integrated using SAINT14 and an absorp-
tion correction was performed with the program SADABS.15
The structures were solved by direct methods using
SHELXTL,16 and refined by full-matrix least-squares methods
based on F2. All non-hydrogen atoms were refined with aniso-
tropic thermal parameters. All H atoms were computed, except
H3 in compound 1a and H1 and H2 in compound 3dH2,
which were located in the electronic difference map, and then
were refined with an overall isotropic temperature factor using
a riding model.
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8230 | Dalton Trans., 2013, 42, 8223–8230
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