Catalytic addition of amines to carbodIImides by bis(β-diketiminate)lanthanide(II) complexes and mechanistic studies

Article abstract of DOI:10.1039/c5dt03674g

Reduction reactions of bis(β-diketiminate)lanthanide(iii) chlorides formed in situ by reactions of anhydrous LnCl₃ with 2 equiv. of sodium salt of the β-diketiminate ligand in THF with a Na/K alloy afforded a series of bis(β-diketiminate)lanthanide(ii) complexes LnL₂(THF)_n (L = L²6-Me₂ = [N(2,6-Me₂C₆H₃)C(Me)]₂CH^-, n = 1, Ln = Eu (1); L = L²,4,6-Me₃ = [N(2,4,6-Me₃C₆H₂)C(Me)]₂CH^-, n = 1, Ln = Eu (2); L = L²,6-iPr₂ = [N(2,6-ⁱPr₂C₆H₃)C(Me)]₂CH^-, n = 0, Ln = Eu (3), Sm (4); L = L²,6-ipr_2Ph = [(2,6-ⁱPr₂C₆H₃)NC(Me)CHC(Me)N(C₆H₅)]^-, n = 0, Ln = Eu (5), Yb (6); L = L^2-Me = [N(2-MeC₆H₄)C(Me)]₂CH^-, n = 1, Ln = Yb (7)) in high yields. All the complexes, especially the complexes of Sm^II (4) and Eu^II (5), were found to be excellent pre-catalysts for catalytic addition of amines to carbodiimides to multi-substituted guanidines with a wide scope of substrates. The activity depends both on the central metals and the ligands with the active sequence of Yb^II < Eu^II and Eu^II < Sm^II and L²,6-Me₂ < L²,4,6-Me₃ ～ L²,6-iPr₂ < L²,6-ipr_2Ph for the ligands. The mechanistic study by the isolation of guanidinate species and their reactivity revealed that Eu^II monoguanidinate complexes Eu(L²,6-Me₂)[(C₆H₅N)C(NHCy)(NCy)](DME) (8) and Eu(L²,6-ipr_2Ph)[(C₆H₅N)C(NHCy)(NCy)](THF)₂ (9) should be the key active intermediates for the systems with Eu^II complexes and a Yb^III bis(guanidinate) complex Yb(L^2-Me)[(C₆H₅N)C(NHCy)(NCy)]₂ (11) for the system using a Yb^II complex.

Full text of DOI:10.1039/c5dt03674g

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Proofs to: Prof. Mingqiang Xue
College of Chemistry, Chemical
Engineering and Materials Science
Dushu Lake Campus
Soochow University
Suzhou, 215123, P. R. China
Fax. No.: (86)512ꢀ65880305
Tel. No.: (86)512ꢀ65880329
Eꢀmail: xuemingqiang@suda.edu.cn
Catalytic addition of amines to carbodiimides by
bis(βꢀdiketiminate)lanthanide(II) complexes and mechanistic studies
Mingqiang Xue,* Yu Zheng, Yubiao Hong, Yingming Yao, Fan Xu,* Yong Zhang,
and Qi Shen
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical
Engineering and Materials Science, Dushu Lake Campus, Soochow University, Suzhou 215123,
People’s Republic of China.
Eꢀmail: xuemingqiang@suda.edu.cn, xufan@suda.edu.cn; Fax: +86ꢀ512ꢀ65880305; Tel:
+86ꢀ512ꢀ65880329.
Electronic supplementary information (ESI) available.
CCDC 1409284 (for 4•toluene), 1409285 (for 7•THF), 1409286 (for 8), 1409282 (for 9), 1409281
(for 10), 1409283 (for 11). For crystallographic data in CIF or other electronic format see DOI:
1

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Catalytic addition of amines to carbodiimides by
bis(βꢀdiketiminate)lanthanide(II) complexes and mechanistic studies
Mingqiang Xue,* Yu Zheng, Yubiao Hong, Yingming Yao, Fan Xu,* Yong Zhang,
and Qi Shen
Reduction reactions of bis(βꢀdiketiminate)lanthanide(III) chlorides formed inꢀsitu by
reactions of anhydrous LnCl3 with 2 equiv. of sodium salt of the βꢀdiketiminate ligand
in THF with Na/K alloy afforded a series of bis(βꢀdiketiminate)lanthanide(II)
complexes LnL2(THF)n (L = L2,6ꢀMe2 = [N(2,6ꢀMe2C6H3)C(Me)]2CH−, n = 1, Ln = Eu
(1); L = L2,4,6ꢀMe3 = [N(2,4,6ꢀMe3C6H2)C(Me)]2CH−, n = 1, Ln = Eu (2); L = L2,6ꢀiPr2
=
=
=
[N(2,6ꢀiPr2C6H3)C(Me)]2CH−, n = 0, Ln = Eu (3), Sm (4); L = L2,6ꢀipr2
Ph
[(2,6ꢀiPr2C6H3)NC(Me)CHC(Me)N(C6H5)]−, n = 0, Ln = Eu (5), Yb (6); L = L2ꢀMe
[N(2ꢀMeC6H4)C(Me)]2CH−, n = 1, Ln = Yb (7)) in high yields. All the complexes,
especially the complexes of SmII (4) and EuII (5), were found to be excellent
preꢀcatalysts for catalytic addition of amines to carbodiimides to multiꢀsubstituted
guanidines with a wide scope of substrates. The activity depends both on the central
metals and the ligands with the active sequence of YbII < EuII and EuII < SmII and
L2,6ꢀMe2 < L2,4,6ꢀMe3 ~ L2,6ꢀiPr2 < L2,6ꢀipr2Ph for the ligands. The mechanistic study by the
isolation of guanidinate species and their reactivity revealed that EuII
monoguanidinate complexes Eu(L2,6ꢀMe2)[(C6H5N)C(NHCy)(NCy)](DME) (8) and
Eu(L2,6ꢀipr2Ph)[(C6H5N)C(NHCy)(NCy)](THF)2 (9) should be the key active
2

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intermediates for the systems with EuII complexes and YbIII bis(guanidinate)
complex Yb(L2ꢀMe)[(C6H5N)C(NHCy)(NCy)]2 (11) for the system using a YbII
complex.
Introduction
Guanidines are an important class of organic compounds, which can serve as synthons
for the syntheses of natural products and pharmaceuticals.1 They can also be applied
as ancillary ligands in organometallic chemistry of main, early transition metals and
lanthanide metals and as catalysts in organic synthesis.1a,e,2 Catalytic addition of an
amine to a carbodiimide provides a convenient and atomꢀeconomical approach for the
preparation of multiꢀsubstituted guanidines.3 However, this addition reaction without
a catalyst requires forcing conditions.4 In 2003, Richeson et al first reported the
catalytic addition reaction of primary aromatic amines to unꢀactivated carbodiimides
induced by “Ti=N” imido species using titanium imido complexes.5 Then, Hou et al.
revealed a halfꢀsandwich Ln alkyl complex could serve as an excellent preꢀcatalyst for
the catalytic addition of both primary and secondary amines with carbodiimides and
the reaction was promoted by “LnꢀN” amido species.6 Since then, metalꢀcatalyzed
addition of amines to carbodiimides has became one of the interest topics in
organometallic chemistry and organic chemistry.1a,7 Lanthanide metals are particularly
attractive, due to their unique character of high coordination numbers and varied size
of the metal atom from La to Lu, which are promising for optimizing steric factors in
catalysis by varying the size of the metal atom and the number of the ligands. To date,
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vast number of trivalent lanthanide metal (LnIII) complexes have been explored to be
efficient catalysts and/or perꢀcatalysts for the catalytic addition reaction, including the
complexes containing LnꢀC,6,8 LnꢀN,9 LnꢀO,10 Lnꢀguanido bonds and lanthanide
triflates as well.11 The catalytic mechanism has also been studied in detail.
Divalent lanthanide (LnII) complexes have been widely used as one electronic
reduction agents or preꢀcatalysts in various transformations including organic
synthesis12 and polymerization reactions13 because of their pronounced reductive
properties. However, the application of LnII complexes in catalytic addition of amines
to carbodiimides remains poorly explored. In 2008, we reported the first example of
the catalytic addition reaction of amines to carbodiimides using LnII complexes
LnL2(sol)x, where L = N(TMS)2, sol = THF, x = 3, Ln = Sm, Eu, Yb; L = MeC5H4, sol
= THF, x = 2, Ln = Sm; L = O[2,6ꢀ(tBu)2ꢀ4ꢀMeC6H2)], sol = THF, x = 2, Ln = Sm, and
the proposed reaction pathway including the formation of a trivalent bimetallic
bis(amidinate) SmIII species through the reductionꢀcoupling reaction of carbodiimide
by the SmII complex.12b
βꢀDiketiminate anions are wellꢀknown to be the useful ligands in stabilizing
various organolanthanide complexes including recently developed monoamide14 and
monoborohydride15 LnIII complexes supported by two βꢀdiketiminate ligands. Besides,
the βꢀdiketiminate ligand in a complex itself can also serve as an active group
participating various transformations under certain conditions, such as
deprotonation,16 oxidationꢀcoupling,17 catalytic polymerization of lactones and
catalytic addition of amines to carbodiimides9h,18 as well as reducing a EuIII ion to a
4

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EuII ion as a reducing agent.17 These interesting properties stimulated us to choose
bis(βꢀdiketiminate) LnII complexes for our expanding research on exploring novel and
efficient LnII catalysts/preꢀcatalysts for catalytic addition reaction of amines to
carbodiimides and the detailed reaction mechanism. Here we report the syntheses and
catalytic activity of a series of bis(βꢀdiketiminate) LnII complexes with the three
metals of SmII, EuII and YbII, and the possible catalytic mechanism: a divalent active
species of (βꢀdiketiminate)EuII(guanidinate) for the system with EuII complexes and a
trivalent active species of (βꢀdiketiminate)YbIIIbis(guanidinate) for the system with a
YbII complex.
Results and discussion
Synthesis and characterization of bis(βꢀdiketiminate) LnII complexes
The bis(βꢀdiketiminate) EuII complexes EuL2(THF)n (1: L = L2,6ꢀMe2
[N(2,6ꢀMe2C6H3)C(Me)]2CH− L2,4,6ꢀMe3
1; 2:
=
=
,
n
=
L
=
[N(2,4,6ꢀMe3C6H2)C(Me)]2CH−, n = 1; 3: L = L2,6ꢀiPr2 = [N(2,6ꢀiPr2C6H3)C(Me)]2CH−,
n = 0; 5: L = L2,6ꢀipr2 = [(2,6ꢀiPr2C6H3)NC(Me)CH(Me)N(C6H5)]−, n = 0) have been
Ph
reported previously, and they were synthesized by the reaction of EuCl3 with 3 equiv.
of sodium salt of the ligand via the reduction of EuIII by a βꢀdiketiminate ion.17 The
same complexes were prepared here in good yields by the reaction of EuCl3 with 2
equiv. of sodium salt of the ligand, followed by treatment with Na/K alloy in one pot
synthesis (Scheme 1). The fine crystals were facilely obtained upon crystallization
from a mixture of THF and hexane for complexes 1 and 2, and a mixture of toluene
and hexane for complexes 3 and 5. The identity of complexes 1ꢀ3 and 5 synthesized
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here was unequivocally established by the satisfactory elemental analyses and the
same data of the unit cell of the crystals measured by Xꢀray crystal analyses as those
reported previously.17 The analogous SmII and YbII complexes LnL2(THF)n (4: L =
L
2,6ꢀiPr2, Ln = Sm, n = 0; 6: L = L2,6ꢀipr2Ph, Ln = Yb, n = 0; 7: L = L2ꢀMe
=
[N(2ꢀMeC6H4)C(Me)]2CH−, Ln = Yb, n = 1) were synthesized similarly in high yields
by the same procedure (Scheme 1). However, attempts to synthesize and characterize
the YbII complex bearing L2,6ꢀiPr2 ligand and the SmII complex with L2,6ꢀipr2 ligand
Ph
were unsuccessful.
These novel complexes were fully characterized by elemental analyses, 1H NMR
1
and Xꢀray crystal structural analysis for complexes 4 and 7. The H NMR spectra of
complexes 4, 6, and 7 in C6D6 clearly showed the expected set of signals
corresponding to the βꢀdiketiminate, indicating that they are diamagnetic SmII and
YbII complexes. The detailed crystal data and the selected bond distances and angles
of complexes 4 and 7 were collected in Tables 1 and 2, respectively. Complex 4 is a
fourꢀcoordinated homoleptic one in which the central metal SmII is coordinated by
four nitrogen atoms of the two ligands in a distorted tetrahedron (Figure 1). The
molecular structure is quite similar to those of complexes 3 and 5 reported17a and the
known homoleptic bis(βꢀdiketiminate) YbII complexes.19 Complex 7 is a
THFꢀsolvated five coordinated one, due to the smaller βꢀdiketiminate ligand used in
complex 7 compared to that in complex 4. The coordination polyhedron around the
YbII ion in complex 7 can be described as a distorted trigonal bipyramid (Figure 2),
which is similar to those found in complexes 1 and 2 reported.17 The two nitrogen
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atoms from two ligands respectively occupy the axial sites in an distorted setup to
form a N(1)−Ln(1)−N(3) angle of 176.10(19)°. The average Ln−N bond distance in
complex 7 is 2.430(6) Å, which is comparable with those found in the reported
analogue.17
Ar
N
Na
2
N
Ar
N
N
Na/K
Ar'
Ar'
LnCl3
Ln
(THF)n
THF
Ar'
Ar
N
N
Ar = Ar' = 2,6-Me2C6H3, n = 1, Ln = Eu (1);
Ar = Ar' = 2,4,6-Me3C6H2, n = 1, Ln = Eu (2);
Ar = Ar' = 2,6-iPr2C6H3, n = 0, Ln = Eu (3), Sm (4);
Ar = 2,6-iPr2C6H3, Ar' = C6H5, n = 0, Ln = Eu (5), Yb (6);
Ar = Ar' = 2-MeC6H4, n = 1, Ln = Yb (7).
Scheme 1 Synthesis of bis(βꢀdiketiminate) LnII complexes.
Catalytic addition of amines to carbodiimides
i
The reaction of PhNH2 with PrNCNiPr (diisopropylcarbodiimide) as mode reaction
was assessed under various conditions (Table 3). Without a catalyst, the reaction did
o
not occur to any detectable degree, even raised reaction temperature to 100 C (Table
3, entry 1). Rapid addition was observed by an addition of 0.5 mol% of complex 4 to
o
give the guanidine 12 in a 97% yield at 60 C in 8 min (Table 3, entry 2). Other
complexes 1ꢀ3 and 5ꢀ7 were also effective. The substituents on the nitrogen atom of
the βꢀdiketiminate have a great influence on the reactivity with the active sequence of
L2,6ꢀMe2 < L2,4,6ꢀMe3 ~ L2,6ꢀiPr2 < L2,6ꢀipr2Ph (Table 3, entries 3ꢀ6). The influence of central
metal on activity was also observed in the present catalytic addition reaction. The
increasing order in activities is EuII < SmII (Table 3, entries 5 and 2) and YbII < EuII
(Table 3, entries 7 and 6), which is consistent with that reported previously.12b It was
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noticed that the EuII complex 5 with an asymmetric ligand exhibits very high activity.
The addition reaction with 0.5 mol% of complex 5 afforded 12 in 99% yield (Table 3,
entry 6). The yield of 12 can still be 71% after 30 min even the catalyst amount
decreasing to 0.25 mol% (Table 3, entry 9). The reactions in THF or toluene gave 12
in lower yields under the same conditions: 71% for THF and 59% for toluene (Table 3,
entries 10 and 11).
Thus, complex 5 was used as a catalyst to evaluate the scope of substrates. A
variety of aromatic amines and secondary amines and two carbodiimides of
iPrNCNiPr and CyNCNCy (dicyclohexylcarbodiimide) were examined. The results
are given in Table 4. All the reactions with primary aromatic amines could take place
o
at 60 C with use of only 0.5 mol% of complex 5 to yield the corresponding
N,N′,N′′ꢀtrisubstituted guanidines 12ꢀ30 in excellent yields (Table 4, entries 1ꢀ19). The
reaction was not influenced by either electronꢀwithdrawing (Fꢀ, Clꢀ, and Brꢀ) or
electronꢀdonating substituents (Meꢀ and MeOꢀ) at the phenyl group, showing good
functional group tolerance. However, a great influence of steric effects on the catalytic
reaction was observed as reported previously.9a,j,h,10a,b,11b For example, the reaction of
iPrNCNiPr with an aromatic amine bearing two bulky substituents on both
orthoꢀpositions at the phenyl ring required prolong reaction time to 12 h to afford the
corresponding guanidine 26 in good yield (Table 4, entry 15). The reactions with
secondary aliphatic amines also went smoothly. In the presence of 0.5 mol% of
complex 5, cyclic secondary amines, pyrolidine and piperidine, could react with
o
iPrNCNiPr at 60 C to give the corresponding guanidines 27 and 28 in good yields
8

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after 24 h (Table 4, entries 16 and 17). These results indicated that the EuII complexes
are highly active and compatible with a wide range of substrates.
Mechanistic studies
In our previous work the reaction mechanism for the catalytic addition using a SmII
complex catalyst was proposed to include the formation of a trivalent bimetallic
bis(amidinate) SmIII species, as the first step, via the reductionꢀcoupling of
carbodiimides by a SmII complex.12b Thus, the active species was a SmIII amide or a
SmIII guanidinate complex formed by amine protonolysis for the former or the
addition of an amido species to a carbodiimide for the latter. The mechanism was
supposed on the basis of a color change of the reaction solution indicative of the
transfer of a SmII ion to a SmIII ion and the reactivity of SmII complexes with
carbodiimides reported previously.20 To see whether the proposed mechanism is
available in the systems with YbII and EuII complexes, the detailed reaction
mechanism was investigated by the stoichiometric reactions of LnII complexes (1, 5
and 7) with amine, carbodiimide and a mixture of amine and carbodiimide,
respectively.
Reaction mechanism catalyzed by EuII complexes
Reaction of complex 1 with CyNCNCy
The reaction of complex 1 with CyNCNCy was first examined. The reaction between
o
complex 1 and CyNCNCy in a 1:1 molar ratio was conducted in THF at 60 C for 24
h. Concentration of the reaction solution led to a yellow tar. Unfortunately, efforts to
isolate the product were unsuccessful due to its high solubility in various solvents
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even in hexane.
Reaction of complex 1 with PhNH2
The reaction of complex 1 with PhNH2 in a 1:1 molar ratio was then carried out in
THF at 60 oC for 24 h. After work up, only complex 1 was recovered completely from
the reaction solution, indicating complex 1 did not react with PhNH2 under the
reaction conditions.
Reactions of EuII complexes (1, 5) with a mixture of PhNH2 and CyNCNCy
The reaction of complex 1 with PhNH2 and CyNCNCy in a 1:1:1 molar ratio was
conducted at 60 oC in THF in an attempt to gain information on the real active species.
Stirring for 24 h gave a light red solution, from which red crystals were isolated in
76% yield upon crystallization from a mixture of heptane and DME (dimethoxyethane)
at room temperature. Elemental analysis of the crystals is consistent with the formula
of a monoguanidinate EuII complex supported by a βꢀdiketiminate ligand
Eu(L2,6ꢀMe2)[(C6H5N)C(NHCy)(NCy)](DME) (8) (Scheme 2: a). IR spectra exhibited
strong absorptions near 1550 and 1510 cmꢀ1, indicating the partial C=N bond
character of the βꢀdiketiminate ligands21 and a C=N stretch approximate 1640 cmꢀ1 for
the guanidinate ligand.22 The identity of complex 8 was unequivocally established by
an Xꢀray structure determination. Molecular structure of complex 8 is shown in figure
3 with the selected bond distances and angles. Complex 8 is a sixꢀcoordinated
monomer, in which the central metal EuII ion is bound to one βꢀdiketiminate and one
guanidinate both in a chelating mode and one DME molecule in a distorted octahedral
geometry. As expected, the coordinated guanidinate group forms essentially a planar
10

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fourꢀmember ring with the metal atom [N(3)ꢀC(22)ꢀN(4)ꢀEu(1)] within experimental
errors (the sum of bond angles is 360.04o). The bond angles around C(22) (118.0(5),
122.4(5) and 119.6(4)o, respectively) are consistent with sp2 hybridization. The
C(22)ꢀN(3) bond distance (1.312(6) Å) is shorter than that of the C(22)ꢀN(4) bond
distance (1.346(6) Å) and both are significantly shorter than the C(22)ꢀN(5)
singleꢀbond distance (1.395(6) Å), indicating that the electrons are delocalized over
the N(3)ꢀC(22)ꢀN(4) unit (Figure 3).
N
N
1) CyNCNCy + PhNH2/THF
N
N
N
THF
Eu
H
a:
Eu
N
N
2) DME + heptane
N
N
N
DME
1
8
N
1) CyNCNCy + PhNH2
2) 60 oC, 24 h, THF
N
b:
THF
H
Eu
N
N
Eu
THF
N
N
N
N
5
9
Scheme 2 Reaction of complex 1 (5) with PhNH2 and CyNCNCy.
The oxidation state of a central metal in a complex can be estimated by Bond
Valence Sum (BVS).23 Indeed, the BVS calculation for Eu in complex 8 gave the
value of 2.20, indicating Eu being in the 2+ oxidation state.
The formation of complex 8 could be realized by two possible roads. One is the
nucleophilic addition of the amido species, which could be formed directly by the
reaction between an amine and complex 1, to a carbodiimide. The other way is the
protonation of a βꢀdiketiminate ligand by a guanidine molecule formed via an
attacking of an amine to a carbodiimide coordinated to complex 1, as amine does not
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react with a carbodiimide without a catalyst. The formation of the amido species
directly from complex 1 is not possible, because complex 1 does not react directly
with amine as mentioned above. Thus, complex 8 was most probably synthesized by
the second road. Although the efforts to isolate the carbodiimideꢀcoordinated EuII
complex were unsuccessful, the similar reaction pathway was reported previously in
the catalytic system with
carbodiimideꢀcoordinated
a
tris(phenolate)LnIII complex, where both
tris(phenolate)LnIII
complex and
a
a
bis(phenolat)LnIII(guaninate) complex via the protonation of a phenolate ligand by a
guanidine were successfully isolated.10c
To see whether the formation of a monoguanidinate EuII complex 8 from the
system with complex 1 is not a special case, the similar reaction of complex 5 with
PhNH2 and CyNCNCy was then carried out under the same conditions. After work up,
an analogous EuII complex Eu(L2,6ꢀipr2Ph)[(C6H5N)C(NHCy)(NCy)](THF)2 (9) with
two coordinated THF molecules, instead of a DME molecule in complex 8, was
obtained in 83% yield (Scheme 2: b). Complex 9 was fully characterized by elemental
analysis, IR spectrum and Xꢀray crystal structural determination. The molecular
structure of complex 9, which is quite similar to that of complex 8, is shown in Figure
4 with selected bond distances and angles. The bond distances and angles are
comparable to those found in complex 8. The C(24)ꢀN(3) bond distance (1.316(3) Å)
is much shorter than that of the C(24)ꢀN(4) bond distance (1.360(3) Å) in the
guanidinate unit, and the latter is close to the C(24)ꢀN(5) bond distance (1.388(3) Å),
suggesting that the C=N double bond of the guanidinate unit in complex 9 is highly
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localized between the C(24) and N(3) atoms. The average EuꢀN (guanidinate N)
distance of 2.6032 Å is longer than 2.578 Å in 8. This may be attributed to the steric
demand resulting from the bulky substituents on the arene ring of βꢀdiketiminate
ligand in 9.
The Bond Valence Sum (BVS) for complex 9 was also calculated. The value of
2.09 was obtained indicating Eu in complex 9 being in the 2+ oxidation state. Thus, a
monoguanidinate EuII complex might be the real active species for a EuII catalyst.
Catalytic activities of complexes 8 and 9.
The catalytic activities of complexes 8 and 9 were further assessed in the addition
i
reaction of PhNH2 to PrNCNiPr. For comparison the same addition reaction with
complexes 1 and 5 was also conducted under the same conditions. A rapid addition of
i
o
PhNH2 to PrNCNiPr was observed at 60 C under solvent free condition when 0.5
mol% of complex 8 or complex 9 was added. The reaction using complex 8 within 8
min gave the guanidine 12 in 96% yield, while a 77% yield was achieved by use of
complex 1 (Table 5, entries 1 and 2). The higher yield for complex 8, compared to that
for complex 1, is reasonable because the formation of an active species from its
precursor generally needs some time, unless the transformation is fast enough.
Complex 9 exhibited the same high level as complex 5 does. Both reactions gave 12
in 83% yields after 5 min (Table 5, entries 3 and 4), indicating that the formation of
the active species 9 from complex 5 is very fast.
Suggested mechanism for the catalytic addition by EuII complexes.
According to the above results, a possible reaction mechanism for the catalytic
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addition of amines to carbodiimides by EuII complexes could be proposed as shown in
Scheme 3. The reaction of a EuII complex with a carbodiimide would yield the species
A. The addition of an amine to the carbodiimide, followed by protonation of a
βꢀdiketiminate ligand by the formed guanidine afforded the active species B.
Protonation of B by an amine would give the amido species C and release the
guanidine D. Rearrangement of D and then 1,3ꢀhydrogen shift led to the more stable
isomer E. Nucleophilic addition of the amido species C to a carbodiimide would
regenerate the guanidinate species B. The isolation and the reactivity of
monoguanidinate EuII complexes 8 and 9 support the proposed mechanism. Thus, an
oneꢀelectron reduction reaction by a EuII ion does not occur here, thereby, the
oxidation state of a EuII ion is not changed during the catalytic addition reaction.
R
N
L2Eu
C
R
N
C
N R
EuL2(THF)n
A
B
N
R
ArNH2
LH
R
N
H
LEu
C
N
N
R
Ar
Ar-NH2
R
R
N H
N H
N
1,3-H shift
Ar
Ar
R
N
C
N R
N
N
H
N H
R
R
LEu NHAr
D
E
C
Scheme 3 A possible mechanism for the catalytic addition of amines to
carbodiimides by EuII complexes.
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Reaction mechanism catalyzed by YbII complexes
Reaction of vomplex 7 with CyNCNCy
To gain information on the reaction mechanism catalyzed by a YbII complex, the
reaction of complex 7 with CyNCNCy was investigated, although the reactivity of
LnII complexes to carbodiimides has been studied in details.24 An addition of a
CyNCNCy solution in THF into an equiv. of complex 7 in THF led to a color change
of the reaction mixture gently from dark green to light yellow, indicative of the
oxidation of YbII to YbIII. After work up, the bis(βꢀdiketiminate)YbIII(monoamidinate)
complex Yb(L2ꢀMe)2[(NHCy)C(C3H6OCH)(NCy)] (10) was obtained as yellow
crystals in moderate yield (Scheme 4). Complex 10 is the only isolable product. The
identity of complex 10 was confirmed by an Xꢀray structure determination. The
molecular structure is depicted in Figure 5 with selected bond distances and angles.
The Yb ion is coordinated by two βꢀdiketiminate and one amidinate ligands, all of
which being bound in a κ2ꢀfashion. The coordination geometry around the Yb ion can
be described as highly distorted octahedral geometry. Four nitrogen atoms, N(2), N(3),
N(5), and N(6), can be considered to occupy equatorial positions within the
octahedron about the Yb ion (the sum of these bond angles is 360.01(9)o), and the
other two nitrogen atoms, N(1) and N(4), occupy axial positions for the angle of
N(1)ꢀYb(1)ꢀN(4) being 164.10(9)o. The CꢀN bond distances in the chelating NꢀCꢀN
unit are nearly equal (1.320(4) and 1.330(4) Å), indicating the delocalization of the π
bond in the NꢀCꢀN unit.
The present reaction is believed to involve a oneꢀelectron reduction of
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•−
CyN=C=NCy by a YbII ion to give the corresponding “C(NCy)2 ” radical anionic
species , then attacking a THF molecule by the radical anion species to yield complex
10 (Scheme 4). The similar reaction mode was also found in the reactions with SmII
complexes reported previously.24a
To see whether the formation of an amidinate YbIII complex was included in the
catalytic cycle, the catalytic activity of complex 10 for the addition of PhNH2 to
o
iPrNCNiPr was then examined at 60 C under solvent free condition using a catalyst
amount of 0.5 mol%. The reaction occurred to afford guanidine 12 in a 15% yield
within 18 min. However, the yield is much lower than that obtained with complex 7
(Table 5, entries 5 and 6). The much lower reactivity shown by complex 10 indicated
that the reaction between complex 7 and a carbodiimide could not be included in the
catalytic cycle.
Cy
L
N
L
L
O
CyN=C=NCy
II
Yb
III
THF
Yb
60 oC, 7 days, THF
L
N
L = L2-Me
7
Cy
10
Cy
N
L
L
Solvent THF
III
Yb
N
Cy
Scheme 4 Formation of 10 by reaction of 7 with CyN=C=NCy.
Reaction of complex 7 with a mixture of PhNH2 and CyNCNCy
The reaction of complex 7 with PhNH2 and CyNCNCy in a molar ratio of 1:1:1 was
then
conducted
in
THF
at
60
oC.
Unexpectedly,
a
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mono(βꢀdiketiminate)YbIIIbis(guanidinate)
complex
Yb(L2ꢀMe)[(C6H5N)C(NHCy)(NCy)]2 (11) was isolated in rather low yield (40%)
(Scheme 5). Complex 11 was confirmed by elemental analysis, IR and Xꢀray crystal
structure analysis. The solidꢀstate structure of complex 11 is depicted in Figure 6 with
selected bond distances and angles. The Yb ion is coordinated by one βꢀdiketiminate
ligand and two guanidinate anions all in a κ2ꢀfashion. The coordination geometry
around the Yb ion can be described as a highly distorted octahedron. The bond
parameters of both Ybꢀguanidinate and Ybꢀβꢀdiketiminate units can be compared to
those found in the related guanidinateꢀ and βꢀdiketiminateꢀYb complexes
reported.9a,10b,c
NH
N
N
N
N
DCC+ PhNH2
N
N
N
O
Yb
H
+
N
60 oC, 5 days, THF
N
N
N
N
NH
7
11
Scheme 5 Formation of 11 by reaction of 7 with PhNH2 and CyN=C=NCy.
The synthesis of complex 11 can be considered to proceed through either a
disproportion reaction of a bis(βꢀdiketiminate)YbIII(guanidinate) intermediate F, or
protonation of a βꢀdiketiminate anion by another molecule of guanidine formed by the
reaction of F with an amine and a carbodiimide, due to F being too crowded to be
stable. The intermediate F might be formed via a oneꢀelectron reduction of a
•‒
CyN=C=NCy by a YbII ion to give the radical anionic species “C(NCy)2 ” G (as
mentioned above), followed by attacking an amine molecule (Scheme 6).
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Cy
L
L
L
N
CyN=C=NCy
-THF
II
Yb
THF
III
Yb
L
N
Cy
L = L2-Me
G
PhNH2
Cy
N
L
L
N
NH
Cy
H
Y
b
N
Yb III
N
III
N
disproportion
L
N
N
NH
11
F
PhNH2 + CyN=C=NCy
- LH
Scheme 6 Formation of 11 by the reaction of 7 with carbodiimide and aniline.
The catalytic activity of complex 11 for the addition reaction of PhNH2 to
iPrNCNiPr was further examined. The reaction proceeded at 60 oC using 0.5 mol% of
complex 11 under solvent free condition to afford guanidine 12 in 60% yield within
18 min. The yield is somewhat higher than that obtained with its precursor complex 7
(Table 5, entries 5 and 7). Thus, complex 11 could be considered to be the real active
species for the catalytic system with complex 7.
Proposed mechanism for the catalytic addition by YbII complexes.
On the basis of the above results, a possible catalytic cycle for the addition reaction of
amines to carbodiimides by YbII complexes could be proposed in Scheme 7. Reaction
of a YbII complex with a carbodiimide should afford the radical anion intermediate G′.
Attacking to an amine by G′ should give the intermediate F′. Reaction of F′ with an
another amine and carbodimide, followed by protonation of a βꢀdiketiminate ligand
by the guanidine formed should yield the guanidinate species H. Protonation of H by
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another molecule of amine would generate the amido complex I and release the
guanidine D. Rearrangement of D and then 1,3ꢀhydrogen shift would afford E.
Nucleophilic addition of an amido species to a carbodiimide would regenerate the
guanidinate complex H (Scheme 7).
R
N
R
N
L
L
L
L
R
H
ArNH2
RN=C=NR
-THF
II
YbIII
III
Yb
N
Yb THF
L
N
L
N
R
L = L2-Me
Ar
G'
F'
ArNH2 + RN=C=NR
LH
Ar
R
L
N
NH
III
N
Ar
R
L
N
R
ArNH2
N
R
R
NH
R
R
H
N
N
H
N
N
Ar
Ar
1,3-H shift
N
N
H
H
H
RN=C=NR
N
III
Yb
R
R
R
N
H
N
E
D
HN
Ar
Ar
I
Scheme 7 A proposed mechanism for addition of amines to carbodiimides catalyzed
by YbII complexes.
Conclusion
LnII complexes bearing two βꢀdiketiminate ligands 1ꢀ7, which were synthesized by
reduction reaction of inꢀsitu formed bis(βꢀdiketiminate) LnIII chlorides, can serve as
efficient catalyst precursors for the catalytic addition of amines to carbodiimides to
multiꢀsubstituted guanidines with a wide range of substrates. The study on the
isolation of monoguanidinate EuII complexes 8 and 9, and bis(guanidinate) YbIII
complex 11, and their reactivity indicated the present catalytic reactions proceed by
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diverse mechanisms depending on the bis(βꢀdiketiminate)LnII complexes used. The
reaction using a EuII complex proceeds through the formation of monoguanidinate
EuII species by the reaction of a EuII complex with an amine and a carbodiimide,
protonation of the guanidinate species by an amine, and nucleophilic addition of the
resultant amido species to a carbodiimide. The oxidation state of the central metal
remains unchanged during the catalytic reaction. The catalytic cycle for the addition
reaction using a YbII complex contains the following steps: formation of a
bis(guanidinate) YbIII species formed through one electron reduction of carbodiimide
by the YbII complex and the reaction with amines, then protonation of a
βꢀdiketiminate ligand by a guanidine; protonation of the guanidinate species by an
amine; nucleophilic addition of the resulted amido species to a carbodiimide.
Experimental Section
General procedures
All preparations and manipulations involving airꢀ and moistureꢀsensitive complexes
were carried out under an inert atmosphere of purified argon using standard Schlenk
techniques. The solvents of THF, DME, toluene, nꢀheptane and nꢀhexane were dried
and distilled from sodium/benzophenone ketal prior to use. [D6]Benzene was dried
over fresh Na chips in a glovebox for NMR reactions. Carbodiimides and amines
1
13
were purchased from TCI and were used as supplied. H NMR and C NMR spectra
were run on a Unity Inovaꢀ400 spectrometer. The uncorrected melting points of
crystalline samples in sealed capillaries (under argon) are reported as ranges.
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Lanthanide analyses were performed by EDTA titration with a xylenol orange
indicator and a hexamine buffer. Elemental analyses were performed by direct
combustion using a CarloꢀErba EA 1110 instrument. The Infrared spectra were
recorded on a MagnaꢀIR 550 spectrometer as KBr pellets.
Synthesis of (L2ꢀMeH, L2,6ꢀMe2H, L2,4,6ꢀMe3H, L2,6ꢀipr2H and L2,6ꢀipr2PhH). The
preligands L2ꢀMeH, L2,6ꢀMe2H, L2,4,6ꢀMe3H and L2,6ꢀipr2H were prepared by the published
method.17a,25 The asymmetrical preligand L2,6ꢀipr2PhH was synthesized by a similar
procedure for the synthesis of asymmetrically substituted diimines.26
Synthesis of complexes (1‒11)
EuII(L2,6ꢀMe2)2(THF) (1). A THF solution of NaL2,6ꢀMe2 (14.30 mL, 0.731 M), which
was formed by reaction of L2,6ꢀMe2H with NaH in THF, was added to a slurry of
anhydrous EuCl3 (1.35 g, 5.23 mmol) in THF (about 20 mL) at room temperature.
The reaction mixture was stirred at room temperature for 24 h. And then a solution of
Na/K alloy (0.166 g, 6.79 mmol) in toluene (10 mL) was added. The reaction mixture
was stirred for 48 h at room temperature. After the precipitate was removed by
centrifugation, the black solution was concentrated to dryness and then about 6 mL
THF and 4 mL hexane was added. Crystallization at room temperature afforded red
crystals 1 (3.28 g, 72%). Anal. Calcd for C48H62N4OEu (863.01): C, 66.80; H, 7.24; N,
6.49; Eu, 17.61. Found: C, 66.27; H, 7.45; N, 6.64; Eu, 17.81.
EuII(L2,4,6ꢀMe3)2(THF) (2). By the procedure described for 1, reaction of EuCl3 (1.23 g,
4.76 mmol) with NaL2,4,6ꢀMe3 (11.45 mL, 0.832 M) in THF and then a solution of
Na/K alloy (0.152 g, 6.19 mmol) in toluene (10 mL) was added gave 2 as red crystals
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(3.22 g, 76%). Anal. Calcd for C50H66N4OEu (891.03): C, 67.40; H, 7.47; N, 6.29; Eu,
17.05. Found: C, 66.72; H, 7.61; N, 6.47; Eu, 17.41.
EuII(L2,6ꢀipr2)2 (3). A THF solution of NaL2,6ꢀipr2 (17.68 mL, 0.578 M) in THF was
added to a slurry of anhydrous EuCl3 (1.32 g, 5.11 mmol) in THF (about 20 mL) at
room temperature. And then a solution of Na/K alloy (0.163 g, 6.64 mmol) in toluene
(10 mL) was added. Crystallization at room temperature afforded red crystals 3 (3.64
g, 66%). Anal. Calcd for C58H82N4Eu (987.28): C, 70.56; H, 8.37; N, 5.67; Eu, 15.39.
Found: C, 70.35; H, 8.44; N, 5.68; Eu, 15.53.
SmII(L2,6ꢀipr2)2 (4). By the procedure described for 3, reaction of SmCl3 (1.31 g, 5.10
mmol) with NaL2,6ꢀipr2 (17.66 mL, 0.578 M) in THF and then a solution of Na/K alloy
(0.163 g, 6.63 mmol) in toluene (10 mL) was added gave 4 as black crystals (4.02 g,
80%). Mp: 131−132 oC. 1H NMR (400 MHz, C6D6): δ 7.18−7.07 (m, 12H, ArH), 4.73
(s, 2H, CNCHCN), 3.30−3.23 (m, 8H, CH(CH3)2), 1.82 (m, 12H, CH3CN), 1.25−1.23
(d, 24H, ꢀ(CH3)2), 1.14−1.12 (d, 24H, ꢀ(CH3)2) ppm. 13C NMR (101 MHz, C6D6): δ
23.82, 23.85, 24.49, 27.96, 90.89, 122.06, 123.33, 139.94, 150.68, 162.86 ppm. IR
(KBr, cmꢀ1): 3437 (m), 2961 (w), 2928 (w), 2869 (w), 1622 (w), 1550 (s), 1463 (w),
1439 (w), 1383 (w),1362 (w), 1323 (w), 1277 (w), 1252 (w), 1221 (w), 1171 (w),
1157 (m), 1054 (w), 933 (w), 793 (w), 758 (w), 506 (w). Anal. Calcd for C58H82N4Sm
(985.66): C, 70.68; H, 8.39; N, 5.68; Sm, 15.25. Found: C, 70.41; H, 8.47; N, 5.85;
Sm, 15.33.
EuII(L2,6ꢀipr2Ph)2 (5). By the procedure described for 3, reaction of EuCl3 (1.04 g, 4.03
mmol) with NaL2,6ꢀipr2 (11.06 mL, 0.728 M) in THF and then a solution of Na/K
Ph
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alloy (0.129 g, 5.24 mmol) in toluene (10 mL) was added gave 5 as red crystals (2.71
g, 82%). Anal. Calcd for C46H58N4Eu (818.92): C, 67.46; H, 7.14; N, 6.84; Eu, 18.56.
Found: C, 67.15; H, 7.18; N, 6.55; Eu, 18.63.
YbII(L2,6ꢀipr2Ph)2 (6). By the procedure described for 1, reaction of YbCl3 (1.11 g, 3.97
mmol) with NaL2,6ꢀipr2 (10.91 mL, 0.728 M) in THF and then a solution of Na/K
Ph
alloy (0.127 g, 5.16 mmol) in toluene (10 mL) was added gave 6 as black crystals
o
1
(2.27 g, 68%). Mp: 163−165 C. H NMR (400 MHz, C6D6): δ 7.23 (s, 4H, ArH),
7.08−6.96 (m, 12H, ArH), 4.83 (s, 2H, CNCHCN), 2.98−2.73 (d, 4H, CH(CH3)2),
1.93 (m, 6H, CH3CN), 1.53 (m, 6H, CH3CN), 1.06−1.01 (m, 18H, ꢀ(CH3)2), 0.71 (s,
6H, ꢀ(CH3)2) ppm. 13C NMR (101 MHz, C6D6): δ 23.78, 23.92, 24.24, 24.65, 25.56,
28.40, 28.56, 96.47, 122.39, 123.85, 124.65, 129.50, 141.13, 141.55, 144.89, 150.77,
162.30, 165.82 ppm. IR (KBr, cmꢀ1): 3441 (m), 2962 (w), 2927 (w), 2868 (w), 1629
(m), 1556 (s), 1541 (w), 1522 (w), 1507 (w),1489 (w), 1457 (w), 1430 (w), 1419 (w),
1369 (w), 1232 (s), 1160 (s), 1027 (w), 753 (w), 665 (w), 506 (m), 424 (w). Anal.
Calcd for C46H58N4Yb (840.03): C, 65.77; H, 6.96; N, 6.67; Yb, 20.06. Found: C,
65.55; H, 6.97; N, 6.58; Yb, 20.11.
YbII(L2ꢀMe)2(THF) (7). A THF solution of NaL2ꢀMe (13.64 mL, 0.803 M) which was
formed by reaction of L2ꢀMeH with NaH in THF was added to a slurry of anhydrous
YbCl3 (1.53 g, 5.48 mmol) in THF (about 20 mL) at room temperature. And then a
solution of Na/K alloy (0.162 g, 5.48 mmol) in toluene (10 mL) was added. After the
undissolved portion was removed by centrifugation, the black solution was
concentrated to dryness then about 7 mL THF and 1 mL hexane was added.
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Crystallization at room temperature afforded black crystals 7 (4.06 g, 85%). Mp:
o
1
141−143 C. H NMR (300 MHz, C6D6): δ 7.28 (s, 2H, ArH), 7.18−7.13 (m, 6H,
3
ArH), 7.02−6.97 (m, 4H, ArH), 6.59−6.57 (d, JHH = 8.0 Hz, 4H, ArH), 5.01 (s, 2H,
CH3CNCH), 2.88 (s, 4H, THF), 2.25 (s, 12H, CH3−Ar), 1.76 (s, 12H, CH3CN ), 0.98
(s, 4H, THF) ppm. 13C NMR (75 MHz, C6D6): δ 17.61, 23.87, 24.50, 67.04, 94.63,
122.44, 124.22, 126.38, 130.01, 130.76, 151.53, 162.96 ppm. IR (KBr, cmꢀ1): 3437
(m), 3062 (w), 2855 (w), 1628 (m), 1555 (m), 1504 (w), 1482 (m), 1438 (w), 1383
(m), 1234 (s), 1158 (s), 1112 (m), 1027 (m), 988 (w), 930 (w), 816 (w), 790 (m), 743
(s), 637 (m), 555 (m), 504 (s), 445 (w). Anal. Calcd for C42H50N4OYb (799.93): C,
63.06; H, 6.30; N, 7.00; Yb, 21.63. Found: C, 62.97; H, 6.41; N, 6.89; Yb, 21.73.
EuII(L2,6ꢀMe2)[(C6H5N)C(NHCy)(NCy)](DME) (8). A certain amount of aniline (0.20
ml, 10.96 M, 2.2 mmol) was added to a solution of 1 (1.91 g, 2.2 mmol) in THF (20
mL). Then CyNCNCy in THF (2.31 ml, 0.95 M, 2.2 mmol) was added to the mixture.
o
The mixture was stirred at 60 C for 24 h. After the solvent was removed under
reduced pressure, the residue was washed by hexane three times then a certain amount
of DME was added to the solution. Then the red solution was concentrated to dryness
and 8 mL nꢀheptane was added. Crystallization at room temperature afforded red
crystals 8 (1.34 g, 72%). Mp: 134−135 oC. IR (KBr, cmꢀ1): 3441 (m), 2932 (m), 2853
(m), 1626 (m), 1588 (w), 1552 (m), 1506 (w), 1398 (w), 1214 (s), 1155 (s), 764 (w),
692 (w), 668 (w), 638 (m), 555 (m), 504 (s), 457 (w). Anal. Calcd for C44H63N5O2Eu
(845.96): C, 62.47; H, 7.51; N, 8.28; Eu, 17.96. Found: C, 62.67; H, 7.53; N, 8.43; Eu,
17.81.
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EuII(L2,6ꢀipr2Ph)[(C6H5N)C(NHCy)(NCy)](THF)2 (9). Reaction of 5 (1.45 g, 1.77
mmol) with CyNCNCy (1.87 mL, 0.95 M in THF) and aniline (0.16 mL, 1.77 mmol)
o
in THF (20 mL) was stirred at 60 C for 24 h. After the solvent was removed under
reduced pressure, the residue was washed by hexane three times then about 0.5 mL
THF and 6 mL nꢀhexane were added. Crystallization at room temperature afforded
o
red crystals 9 (1.36 g, 83%). Mp: 152−153 C. IR (KBr, cmꢀ1): 3454 (m), 3243 (w),
3062 (w), 2932 (m), 2853 (m), 1633 (s), 1588 (m), 1557 (s), 1506 (m), 1488 (w),
1384 (m), 1364 (w), 1277 (w), 1254 (w), 1235 (m), 1158 (m), 1054 (w), 1027 (w),
894 (w), 836 (w), 785 (w), 753 (m), 695 (m), 677 (w), 504 (m). Anal. Calcd for
C50H73N5O2Eu (928.09): C, 64.71; H, 7.93; N, 7.55; Eu, 16.37. Found: C, 64.64; H,
8.05; N, 7.43; Eu, 16.71.
YbIII(L2ꢀMe)2[(NCy)C(CHOC3H6)(NCy)] (10). Reaction of 7 (0.49 g, 0.56 mmol)
with CyNCNCy (0.39 mL, 0.95 M in THF) in THF (20 mL) was stirred at 60 oC for 7
days. After the solvent was removed under reduced pressure, the residue was washed
by hexane three times then about 2.5 mL toluene and 2 mL nꢀhexane were added.
Crystallization at room temperature afforded yellow crystals 10 (0.39 g, 70%). Mp:
163−165 oC. IR (KBr, cmꢀ1): 3441 (m), 3063 (w), 2928 (m), 2853 (m), 1628 (s), 1557
(s), 1506 (w), 1482 (w), 1383 (m), 1363 (m), 1281 (m), 1236 (s), 1157 (s), 1112 (m),
1065 (w), 1027 (w), 930 (w), 861 (w), 818 (w), 790 (w), 741 (s), 719 (w), 669 (w),
637 (w), 555 (w), 504 (m), 445 (w). Anal. Calcd for C55H71N6OYb (1005.22): C,
65.71; H, 7.12; N, 8.36; Yb, 17.22. Found: C, 65.59; H, 7.18; N, 8.31; Yb, 17.40.
YbIII(L2ꢀMe)[(C6H5N)C(NHCy)(NCy)]2 (11). Reaction of 7 (1.57 g, 1.95 mmol) with
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CyNCNCy (2.05 mL, 0.95 M in THF) and aniline (0.18 mL, 1.95 mmol) in THF (20
o
mL) was stirred at 60 C for 5 days. After the solvent was removed under reduced
pressure, the residue was washed by hexane three times then about 2 mL toluene and
6 mL nꢀhexane were added. Crystallization at room temperature afforded green
crystals 11 (0.82 g, 40%). Mp: 179−180 oC. IR (KBr, cmꢀ1): 3290 (w), 3244 (w), 3066
(w), 2932 (s), 2852 (m), 2759 (w), 1628 (s), 1587 (s), 1557 (s), 1506 (s), 1447 (w),
1384 (m), 1363 (w), 1349 (w), 1275 (m), 1256 (w), 1179 (w), 1167 (w), 1112 (w),
1084 (w), 1027 (w), 894 (w), 836 (w), 789 (w), 754 (m), 695 (m), 668 (w), 510 (w),
452 (w), 425 (w). Anal. Calcd for C57H77N8Yb (1047.31): C, 65.37; H, 7.41; N, 10.70;
Yb, 16.52. Found: C, 65.41; H, 7.37; N, 10.60; Yb, 16.53.
General Procedure for the Addition Reaction of Amines to Carbodiimides. A 10
mL Schlenk tube under dried argon was charged with 5 (0.0082 g, 0.01 mmol). To the
flask were added the aniline (PhNH2) (0.182 mL, 10.96 M, 2.00 mmol),
N,N’ꢀdiisopropylcarbodiimide (iPrNCNiPr) (0.312 mL, 6.418 M, 2.00 mmol). The
resulting mixture was stirred at 60 oC for 8 min. After the reaction was completed, the
reaction mixture was hydrolyzed by water (2 mL), extracted with dichloromethane
(3×10 mL), dried over anhydrous Na2SO4, and filtered. Then the solvent was removed
under reduced pressure, and the final products were further purified by crystallization
from nꢀhexane to give the colorless solid 12 (0.4343 g, 99% yield).
Xꢀray Crystallography. Crystals suitable for Xꢀray diffraction of complexes 1−11
were sealed, respectively, in a thinꢀwalled glass capillary filled with argon for
structural analysis. Only the data of the unit cell of the crystals were measured for
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complexes 1ꢀ3 and 5 (see supporting information Table S1). Crystal structure of
complex 6 was not determined for its deterioration in the crystal quality. Diffraction
data were collected on a Rigaku Saturn CCD area detector in the ω scan mode using
Mo Kα radiation (λ = 0.71075 Å) for complexes 4 and 9 and using Mo Kα radiation (λ
= 0.71073 Å) for complexes 7 and 8, and on an Agilent Xcalibur CCD area detector
in the ω scan mode using Cu Kα radiation (λ = 1.54184 Å) for complexe 10 and using
Mo Kα radiation (λ = 0.71073 Å) for complexe 11. The diffracted intensities were
corrected for Lorentzpolarization effects and empirical absorption corrections. Details
of the intensity data collection and crystal data are given in Table 1. The structures
were solved by direct methods and refined by fullꢀmatrix leastꢀsquares procedures
based on |F|2. All of the nonꢀhydrogen atoms were refined anisotropically. The
hydrogen atoms in these complexes were all generated geometrically, assigned
appropriate isotropic thermal parameters, and allowed to ride on their parent carbon
atoms. All of the hydrogen atoms were held stationary and included in the structure
factor calculations in the final stage of fullꢀmatrix leastꢀsquares refinement. The
structures were refined using SHELXLꢀ97 programs.27
Acknowledgments
We are grateful to the National Natural Science Foundation of China (Grants No.
21272168 and 21372171), a project funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions for financial support.
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