Synthesis and characterization of heterobimetallic organo rare earth complexes bearing aryloxide-N-heterocyclic carbene ligands

Article abstract of DOI:10.1016/j.jorganchem.2017.04.039

Two aryloxide-substituted N-heterocyclic carbene (NHC) proligands 3-[(2-hydroxynaphthalenyl) methyl]-1-mesityl-3,4,5,6- tetrahydropyrimidinium chloride (H₂L1) and 3-(2-hydroxybenzyl)-1-mesityl-3,4,5,6-tetrahydropyrimidinium chloride (H₂

Full text of DOI:10.1016/j.jorganchem.2017.04.039

Accepted Manuscript
Synthesis and characterization of heterobimetallic organo rare earth complexes
bearing aryloxide-N-heterocyclic carbene ligands
Jingjing Zhang, Min Zhang, Tianwen Bai, Xufeng Ni, Zhiquan Shen
PII:
S0022-328X(17)30277-2
10.1016/j.jorganchem.2017.04.039
JOM 19926
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 31 March 2017
Revised Date: 27 April 2017
Accepted Date: 30 April 2017
Please cite this article as: J. Zhang, M. Zhang, T. Bai, X. Ni, Z. Shen, Synthesis and characterization of
heterobimetallic organo rare earth complexes bearing aryloxide-N-heterocyclic carbene ligands, Journal
of Organometallic Chemistry (2017), doi: 10.1016/j.jorganchem.2017.04.039.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Four kinds of heterobimetallic organo rare earth complexes bearing aryloxide-NHC ligands were prepared, and
their structures were fully analyzed by single-crystal X-ray diffraction and DFT calculations.

ACCEPTED MANUSCRIPT
Synthesis and Characterization of Heterobimetallic Organo Rare
Earth Complexes Bearing Aryloxide-N-Heterocyclic Carbene
Ligands
Jingjing Zhang, Min Zhang, Tianwen Bai, Xufeng Ni*, Zhiquan Shen
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, PR China.
Abstract: Two aryloxide-substituted N-heterocyclic carbene (NHC) proligands 3-[(2-hydroxynaphthalenyl) methyl]-1-mesityl-3,4,5,6-
tetrahydropyrimidinium chloride (H2L1) and 3-(2-hydroxybenzyl)-1-mesityl-3,4,5,6-tetrahydropyrimidinium chloride (H2L2) were designed and
easily synthesized via three step synthetic processes in high yield. Four heterobimetallic potassium-NHC rare earth complexes L1KLnN”3 and
L2KLnN”3 (L1, Ln=Y (1a), Nd (1b); L2, Ln=Y (2a), Nd (2b), N”=N(SiMe3)2) were obtained by in-situ reaction of N-heterocyclic carbene
proligands H2L1 and H2L2 with Ln[N(SiMe3)2]3 and KN(SiMe3)2 in toluene. The K-NHC rare earth complexes were characterized by
single-crystal X-ray crystallography and Y complexes were characterized by 1H and 13C NMR spectroscopy. DFT calculations were employed
to further investigate charge distributions and interactions within these compounds.
Keywords: N-heterocyclic carbene, rare earth complexes, potassium-NHC, DFT calculation, X-ray diffraction
1. Introduction
N-heterocyclic carbene ligands (NHCs) have been highly
‘ate’ complex containing rare earth tri(alkyl), and these abnormal
complexes allowed a new way in the formation of carbon-carbon
bonds [29]. Herein, we designed and synthesized backbone
saturated aryloxide-N-heterocyclic carbene proligands in an easy
and effective method and prepared the corresponding
heterobimetallic organo rare earth complexes were prepared and
characterized. The structures of the complexes were further
studied by DFT calculations.
valued in organometallic chemistry [1-7] since the first thermally
stable NHC generated by Arduengo in 1991 [8]. In coordination
compounds, the NHCs mainly play a role as versatile ligands to
stabilize the metal center, modifying the steric and electronic
properties around the metal by combining with neutral or anionic
donors [9-10]. The pendent groups of NHCs have contributed
great importance to regulating the steric and electronic properties.
For instance, the pincer systems of amido-fuctionalized NHCs
were used to stabilize the complexes of Ti, Zr and Hf [11];
phenoxy-modified NHC ligand allowed the preparation of Ni
bromide compound [12]. Rare earth metal complexes bearing
indenyl- and fluorenyl- functionalized NHC ligands were
synthesized successfully [13-14] and exhibited high catalytic
activities toward polymerization and copolymerization reactions
[15-17]. Organo lanthanide complexes containing bis(phenolate)
NHCs with saturated or unsaturated backbone had given rise to
controlled polymerization of cyclic or vinyl monomers [18-20].
NHCs with saturated backbone have effect of increasing
donor ability, and their transition metal complexes with
6-membered expanded ring have been prepared [21-23], and
these ligands were barely applied in rare earth metal chemistry.
The strongly basic properties of NHCs also allow their use as
ligands for the synthesis of alkali-metal complexes. The first
diamino carbene-type alkali-metal complex was reported by
Alder [24], and then Arnold et al. presented the first crystal
structure of a potassium NHC complex [25]. Li, Na complexes
with NHC ligands [26-27] were synthesized and characterized in
the meantime. The resulting NHCs were anticipated to stabilize
the alkali metal ions in tuning the coordination sphere or rigidity
of the complexes. Density functional theory calculations were
utilized to design Li complexes [28] as well. Comparatively, rare
earth metal complexes containing functionalized alkali-NHC
structures have been less explored. Breakthroughs were
achieved by Arnold group, they discovered the lithium carbene
2. Results and discussion
2.1 Synthesis of aryloxide-N-heterocyclic carbene proligands
Roche et al. presented a valuable access to prepare
unsymmetrical and symmetrical 1,2-diamide and they expanded
their application to obtain the unsymmetrical imidazolidinium
proligand by reaction of N-monoarylate 1,2-diamine
dihydrobromide salt and 1-pyrenecarboxaldehyde [30]. Herein
we synthesized aryloxide-substituted NHC salts using similar
strategy. The reactions were carried out in three steps depicted
in scheme 1. The N-monoarylate diamine dihydrobromide salts
(A·2HBr) were prepared according to the literature [30]. Simple
neutralization reaction was processed, using N-monoarylate
diamine dihydrobromide salts (A·2HBr) and KOH to generate the
N-monoarylate diamine (A), which was condensed with aldehyde
Ar-CHO in the first step. NaBH4 was added to the mixture to yield
the unsymmetrical disubstituted diamine (M), and further
cyclization used underwent a classic procedure with HCl and
CH(OEt)3 [31], providing the desired unsymmetrical substituted
NHC proligands H2L1-H2L2. Finally, phenyl and naphthyl
functional groups were incorporated into the NHC proligands.
Synthetic routes were shown in scheme 1. Proligands H2L1-H2L2
were characterized by 1H, 13C NMR spectroscopy and elemental
analysis. The 1H NMR spectra showed typical singlet resonances
at δ 8.87 (H2L1), 8.68 (H2L2) ppm for the pyrimidinium proton
(NCHN), and carbon signals (NCHN) at δ 165.69 (H2L1), 156.73
(H2L2) ppm showed up in 13C NMR spectra, commensurating
with that reported for similar NHC structures [19, 21-22].
⁎Corresponding author.
E-mail address: xufengni@zju.edu.cn

ACCEPTED MANUSCRIPT
H
N
NH2
N
N
2HBr
A. 2HBr
KOH
Cl
NH HN
HO
HO
H2L1
Ar-CHO
HCl
HC(OEt)3
M1
then NaBH4
H
N
NH2
N
N
NH HN
Cl
HO
HO
A
M2
H2L2
H L1
, Ar = 2-hydroxynaphthalenyl;
H L2
, Ar = 2-hydroxybenzyl.
2
Ar:
2
Scheme 1. Syntheses of aryloxide-N-heterocyclic carbene proligands H2L1-H2L2.
Complexes 1a and 2a were characterized by 1H and 13C
2.2 Synthesis and characterization of heterobimetallic organo
rare earth complexes
NMR, and the structures of four complexes were fully confirmed
1
by the single-crystal X-ray diffraction analysis. In the H NMR
spectra, the characteristic resonance of ylidene proton was
disappeared completely and signals of methyl protons in the
bis(trimethylsilyl) amido groups showed at δ 0.64/0.10 ppm for 1a,
0.58/0.10 ppm for 2a respectively. The carbene carbon signal of
complex 1a appeared at δ 231.7 ppm, 2a at δ 227.2 ppm in 13C
NMR spectra, comparable to δ 226.7 ppm found in complex
The aryloxide N-heterocyclic carbene proligand H2L1 could
be applied to synthesize the rare earth metal complexes in two
pathways. H2L1 and equivalent Ln[N(SiMe3)2]3 were firstly
reacted to form phenolate rare earth complex, then 2 equiv. of
KN(SiMe3)2 were added to deprotonate the proton on the C2-H on
the ring the pyrimidinium proton (NCHN) to form carbene-K
structure. In the other approach, proligand H2L1 was first treated
with 2 equiv. of KN(SiMe3)2 at -20°C for deprotonation to obtain
anionic functionalized NHC and sequently adding Ln[N(SiMe3)2]3
to obtain rare earth complex. These procedures were speculated
through in-situ NMR analysis, and the proposed reaction
mechanism was depicted in scheme S1 (supporting information,
page 2). The complexes 1a and 1b were stable in dry nitrogen
atmosphere, but they were very sensitive to moisture. The
complexes were soluble in benzene, toluene and THF, and
partially soluble in n-hexane.
1,3-diisopropyl-3,4,5,6-tetrahydropyrimid-2-ylidene-K
[32],
indicating the coordination interaction of carbene carbon to
potassium in solution.
The single-crystal X-ray diffraction analyses revealed
monomeric structures with the Ln[N(SiMe3)2]3 bonding to the
aryloxide oxygen, generating a K-NHC structure with a toluene
molecule coordinated to K (figure 1-2). The selected bond
lengths and angles were shown in table 1. The coordination
sphere of rare earth metal center adapted a distorted tetrahedral
geometry, with the Ln-Namino average bond length being 2.279 Å
for 1a, 2.369 Å for 1b, 2.279 Å for 2a, 2.375 Å for 2b. These
structure data are longer than the corresponding distances of
2.223 Å for Y-Namino and 2.29 Å for Nd-Namino found in the starting
materials Y[N(SiMe3)2]3 [33] and Nd[N(SiMe3)2]3 [34] . The Y-O1
bond length in complex 1a (2.115(2) Å) is marginally elongated in
comparison with the Y-O1 linkage observed in 2a (2.107(2) Å),
and the Nd-O1 bond length in complex 1b is longer than 2b as
well. The Ln-O (Ln=Y, Nd) bond lengths in organometallic
complexes previously reported are in wide range, for Y-O bond
lengths (2.031-2.147 Å) [35-36] and Nd-O distances
(2.206-2.280 Å) [37-39]. We believe that it is the steric effect of
ligand to be the most important factor to influence the bond
distance. So the bond length will be elongated by addition of
ligands with more bulkier pendants.
Me3Si
SiMe3
O
a) [(Me3Si)2N]3Ln
b) 2 KN(SiMe3)2
Me3Si
N
Ln
N
Me3Si
N
N
N
Me3Si
SiMe3
K
Cl
HO
H2L1
N
N
a) 2 KN(SiMe3)2
b) [(Me3Si)2N]3Ln
1a
1b
, Ln=Y;
, Ln=Nd.
Scheme 2. Syntheses of heterobimetallic organo rare earth complexes 1a and
1b.
Sterically less bulky proligand H2L2 was also studied as a
reactant. The proligand H2L2 was reacted with Ln[N(SiMe3)2]3
(Ln=Y, Nd) and 2 equiv. of KN(SiMe3)2 in the same method as to
complex 1a, obtaining similar potassium-NHC rare earth metal
complexes 2a and 2b.
The potassium cation, however, features potassium-NHC
structure in which it connected to the carbene center. The
K-Ccarbene-centroidNHC angles are approaching linearity (range
178.3-179.9°), so there is no severe distortion. The bond length
of potassium-C(carbene), K1-C2 = 2.863(7) Å for 1a, 2.832(5) Å for
1b, 2.853(3) Å for 2a and 2.852(3) Å for 2b, which are shorter
than the reported potassium NHC complex (K-Ccarbene=3.048) [40]
Me3Si
SiMe3
O
Me3Si
N
Ln
N
Me3Si
Me3Si
N
SiMe3
K
N
N
N
[(Me3Si)2N]3Ln
2 KN(SiMe3)2
and
the
Ln(III)-K(I)
heterobimetallic
complexes
Cl
N
[N′′2Ln(L-)K(DME)]2 (K-Ccarbene=2.954) [41], but much longer than
the average bond length of normal Ln-Ccarbene [18-19, 42-43]. It is
worth noting that the potassium cation is coordinated augmented
by significant η-interaction with the π-system provided by the
N-substituent groups in NHC ligand and the toluene molecule,
which can be easily understood by the small shorter distance of
HO
H2L2
2a, Ln=Y; 2b, Ln=Nd.
Scheme 3. Syntheses of heterobimetallic organo rare earth complexes 2a and
2b.

ACCEPTED MANUSCRIPT
Figure 3. DFT-optimized structure of complex 1a and calculated NBO charges.
Figure 1. X-ray structure of complex 1a, with 50% probability thermal ellipsoids.
Hydrogen atoms are omitted for clarity. Complex 1b is isomorphous with 1a.
Figure 4. DFT-optimized structure of complex 2a and calculated NBO charges.
charge is mainly occupied on nitrogen atoms on the three
bis(trimethylsilyl)amid groups and aryloxide oxygen on the ligand.
Moreover, the second-order perturbation theory analysis of the
donor-acceptor linkage in the NBOs is employed to indicate that
the C-K interactions are contributed by donation of the
carbon-based and sp-hybridized lone pair into the potassium
metal 4s orbitals with a very weak interaction (K-C 8.6 kcal/mol,
1a; 9.9 kcal/mol, 2a). These interactions are evidently weaker
than the bond Li-C (23 kcal/mol), Na-C (15 kcal/mol) [45],
probably because the increasing size and atomic weight of the
alkali metal center. The functionalized anion anchor enhances
the interactions between NHC ligands and yttrium cation, with
much higher second-order perturbation energy in Y-O bond
(75.84 kcal/mol for 1a, 73.38 kcal/mol for 2a.).
Figure 2. X-ray structure of complex 2a, with 50% probability thermal ellipsoids.
Hydrogen atoms are omitted for clarity. Complex 2b is isomorphous with 2a.
potassium to the aromatic groups, for K-PlaneMes distances
range from 3.259 to 3.319 Å, K-PlaneAr distances range
2.908-2.920 Å and K-Planetoluene distances range 2.961-3.146 Å.
As a result, the potassium cation was wrapped tightly by the
aryloxide-N-heterocyclic carbene ligands, avoiding the affection
of other metals in the reaction system, rare earth metal, for
example.
2.3 Theoretical calculations
In order to provide an assessment of the binding of the
aryloxide-N-heterocyclic carbene ligands to the alkali metal and
the rare earth metals within these unusual heterobimetallic
compounds, we have undertaken density functional theory (DFT)
calculations upon the single-crystal structures 1a and 2a [44].
The DFT-optimized structures of complexes 1a and 2a are
showed in figure 3 and 4, and the comparison of selected
parameters between X-ray structures and DFT calculations were
depicted in table 1.
It can be seen that the DFT-optimized models are similar to
X-ray structures, but the calculated bond lengths are slightly
longer than the corresponding distances learned from X-ray
analysis. Natural bond orbital (NBO) analysis shows that the
positive charge is mainly distributed on the electropositive
potassium and rare earth metals (NBO charges, K +0.917, Y
+1.914 for 1a; K +0.920, Y +1.903 for 2a.), and only marginally
spreads to the carbene carbon of the NHC ligand. The negative
3. Conclusion
In summary, two aryloxide-substituted NHC proligands
H2L1-H2L2 were synthesized in an easy and effective strategy.
Reactions of proligands H2L1 and H2L2 with Ln[N(SiMe3)2]3
(Ln=Y, Nd) and KN(SiMe3)2 in toluene obtained the
heterobimetallic organo rare earth complexes 1a-b, 2a-b,. For
the first time, We were able to determine and the molecular
structures of heterobimetallic organo rare earth complexes 1a-b,
2a-b were determined in the solid state by single-crystal X-ray
diffractometry. These complexes showed a K-NHC structure with
rare earth Ln[N(SiMe3)2]3 bonding to the aryloxide oxygen. The
charge distributions and metal-ligand interactions were
investigated within 1a and 2a by density functional theoretical
(DFT) methods. Natural bond orbital (NBO) analysis showed that
the positive charge was mainly distributed on the electropositive
potassium and rare earth metals.

ACCEPTED MANUSCRIPT
Table 1. Selected bond lengths(Å) and angles(°) for complexes 1a-2b.
1a (Ln=Y)
1a (DFT)
1b (Ln=Nd)
2a (Ln=Y)
2a (DFT)
2b (Ln=Nd)
K1-C2
Ln-O1
2.863(7)
2.115(2)
2.279(3)
2.290(3)
2.270(3)
115.8(3)
149.7(2)
107.95(9)
107.96(9)
97.76(9)
2.924
2.195
2.309
2.321
2.297
116.34
148.65
107.23
108.63
96.95
2.832(5)
2.245(3)
2.403(4)
2.367(4)
2.339(4)
115.6(4)
155.1(3)
98.58(13)
106.17(13)
110.02(13)
2.853(3)
2.107(2)
2.269(3)
2.284(6)
2.285(3)
115.7(3)
152.3(2)
107.11(9)
106.17(9)
97.98(9)
2.939
2.194
2.295
2.301
2.334
116.32
149.95
107.62
106.46
99.04
2.852(3)
2.205(2)
2.357(2)
2.379(2)
2.388(2)
115.9(3)
152.9(3)
105.59(8)
104.60(8)
96.08(8)
Ln-N1
Ln-N2
Ln-N3
N4-C2-N5
Ln-O1-C7
O1-Ln-N1
O1-Ln-N2
O1-Ln-N3
4. Experimental
4.1 General procedures.
137.13, 135.03, 133.36, 130.69, 129.40, 128.78, 127.85, 127.03,
122.59, 121.74, 118.54, 109.43(Ar-C), 48.61(CH2), 44.92(CH2),
41.58(CH2), 20.55(CH3), 18.77(CH3), 17.09(CH2). Anal. Calcd.
for C24H27ClN2O: C, 72.99; H, 6.89; N, 7.09. Found: C, 72.65; H,
6.95, N, 6.99.
All operations except ligands synthesis were carried out
under dry argon using Schlenk techniques. Hexane and toluene
were distilled from sodium benzophenone ketyl prior to use.
Lanthanide chloride was prepared from lanthanide oxide
according to the literature [46]. All other reagents and solvents
were commercially available and used without further purification.
Metal analyses were performed by complexometric titration.
NMR spectra were recorded on a Bruker Avance 400 DMX (1H
NMR at 400 MHz, 13C NMR at 101 MHz) spectrometer or Agilent
Direct Drive 2 (1H NMR at 600 MHz, 13C NMR at 151 MHz).
Elemental analyses were performed by direct combustion with a
Flash EA-1112 instrument. Ln[N(SiMe3)2]3 (Ln=Y, Nd) were
prepared according to literature procedures [47].
4.3 Synthesis of H2L2.
Proligand H2L2 was prepared similarly to H2L1, except
2-Hydroxybenzaldehyde (1.7 g, 13.7 mmol) was used in the
second step to produce M2. Finally, white powder of H2L2 was
obtained by cyclization of M2. Yield: 70%. 1H NMR(400MHz,
CDCl3, 25°C): δ 8.68(s, 1H, NCHN), 7.84(d, 1H, C10H6), 7.75(t,
2H, C10H6), 7.70(d, 1H, C10H6), 7.48(t, 1H, C10H6), 7.29(t, 1H,
C10H6), 6.75(s, 2H, C6H2Me3), 5.37(s, 2H, CH2), 3.99-4.09(m, 4H,
NCH2CH2N), 2.20(s, 6H, C6H2Me3), 2.12(s, 3H, C6H2Me3).13C
NMR (101 MHz, d6-DMSO, 25°C) δ 156.73(NCHN), 154.63,
138.94, 137.07, 135.02, 130.87, 130.02, 129.35, 119.53, 118.86,
115.79(Ar-C), 54.24(CH2), 44.95(CH2), 41.80(CH2), 20.53(CH3),
18.78(CH3), 17.14(CH2). Anal. Calcd for C20H25ClN2O: C, 69.65;
H, 7.31; N, 8.12. Found: C, 69.57; H, 7.31, N, 7.97.
4.2 Synthesis of H2L1.
The proligand H2L1 was synthesized in three steps. Firstly,
N-mono-(2,4,6-trimethylphenyl)-1,3-diaminedihydro bromide salt
(A·2HBr 5 g, 14 mmol) was reacted with KOH (1.38 g, 28 mmol)
in water to afford N-mono-(2,4,6-trimethylphenyl)-1,3-diamine (A)
4.4 Synthesis of complex 1a.
in 98% yield. Then
A
(2.6 g, 13.7 mmol) and
A suspension of H2L1(0.05 g, 0.13 mmol) in toluene (10 mL)
was treated with Y[N(SiMe3)2]3 (0.07 g, 0.13 mmol) at room
temperature and stirred for about 30min. Then the toluene
solution of KN(SiMe3)2 (0.05g, 0.26 mmol) was added to the
stirring reactant. The mixture was stirred overnight at room
temperature, and then the solvent was removed under vacuum.
A hexane/toluene solvent mixture was added to extract complex
1a. Colorless block crystals were obtained at -20°C in a week.
Yield: 35%. 1H NMR (600MHz, C6D6, 25°C):δ 7.63(d, 1H, C10H6),
7.50(d, 1H, C10H6), 7.42(d, 1H,C10H6), 7.37(d, 1H,C10H6), 7.21(tri,
1H, C10H6), 6.44(s, 2H, C6H2Me3), 5.00(s, 2H, CH2), 2.86,2.34(t, t,
2H, 2H, NCH2CH2CH2N), 1.65,1.52(s, s, 3H, 6H, C6H2Me3),
1.23(m, 2H,NCH2CH2CH2N ), 0.64 (s, 36H, N[Si(CH3)3]2), 0.10 (s,
2-Hydroxy-1-naphthaldehyde (2.3 g, 13.7 mmol) were
suspended in MeOH (50 mL), the reaction mixture was heated to
40°C and stirred for another 1h resulting in the formation of an
orange suspension. Secondly, NaBH4 (1 g, 26 mmol) was added
slowly to the mixture until the solution turned yellow. This
reaction solution was stirred overnight before solvent was
evaporated to dryness with brown oil appeared. The resulting oil
was triturated with water, and filtrated to yield a yellow powder
(M1). In the last step, the obtained yellow powder M1 was
dissolved in EtOH, and then concentrated HCl (1 mL) was added
dropwise to the solution and stirred for about 1 hour. After the
solvent was evaporated to dryness, a yellow solid was obtained.
The preceding solid (3 g) was suspended in 20 mL of
triethylorthoformate, formic acid was added (one drop) and the
suspension was heated to 110 °C overnight. The white powder
H2L1 obtained was filtrated, washed with ethyl ether and dried in
vacuum. Yield: 67%. 1H NMR (400MHz, d6-DMSO, 25°C): δ
8.87(s, 1H, NCHN), 8.11(d, 1H, C10H6), 7.83(m, 2H, C10H6),
7.54(m, 2H, C10H6), 7.34(t, 1H, C10H6), 7.06(s, 2H, C6H2Me3),
5.20(s, 2H, CH2), 3.35-3.48 (m, 4H, NCH2CH2CH2N), 2.22-2.28
(d, 9H, C6H2Me3), 2.11(m, 2H, NCH2CH2CH2N). 13C NMR (101
MHz, d6-DMSO, 25°C) 165.69(NCHN), 156.21, 154.16, 138.97,
18H, N[Si(CH3)3]2). 13C NMR (151 MHz, C6D6, 25°C)
δ
231.71(Ccarbene-K), 161.50, 144.58, 135.75, 135.02, 134.75,
129.61, 129.50, 129.34, 128.74, 128.35, 127.65, 125.61, 124.42,
123.57, 121.29, 117.22(Ar-C), 52.90(CH2), 42.68(CH2),
42.01(CH2),
21.63(CH3),
20.70(CH3),
17.65(CH2),
5.94(N[Si(CH3)3]2), 2.67 (N[Si(CH3)3]2).
4.5 Synthesis of complex 1b.
A suspension of H2L1 (0.05 g, 0.13 mmol) in toluene (10 mL)
was treated with KN(SiMe3)2 (0.05 g, 0.26 mmol) at -20°C and

ACCEPTED MANUSCRIPT
another Nd[N(SiMe3)2]3(0.08 g, 0.13 mmol) was added to the
References
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under vacuum and a hexane/toluene solvent mixture was added
to extract complex 1b. Little blue block crystals were obtained at
-20°C in 2-3 days. Yield: 40%.
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4.6 Synthesis of complex 2a.
A suspension of H2L2 (0.05 g, 0.14 mmol) in toluene (10 mL)
was treated with Y[N(SiMe3)2]3 (0.08 g, 0.14 mmol) at room
temperature and stirred for about 30min. Then KN(SiMe3)2 (0.06
g, 0.26 mmol) toluene solution was added to the stirring reactant.
The mixture was stirred overnight at room temperature, and then
the solvent was removed under vacuum. A hexane/toluene
solvent mixture was added to extract complex 2a. Colorless
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1
block crystals were obtained at -20°C in a week. Yield: 38%. H
NMR (600 MHz, C6D6, 25°C) δ 7.13 (t, 1H, C6H4), 7.08 (d, 1H,
C6H4), 7.05 (dd, 1H, C6H4), 7.03 – 6.99 (m, 1H, C6H4), 6.57 (s, 2H,
C6H2Me3), 4.55 (s, 2H, CH2), 2.77 (s, 2H, NCH2CH2CH2N), 2.39
(s, 2H, NCH2CH2CH2N), 2.08 (s, 3H, C6H2Me3), 1.81 (s, 6H,
C6H2Me3), 1.49 (d, 2H, NCH2CH2CH2N), 0.58 (s, 36H,
N[Si(CH3)3]2), 0.10 (s, 18H, N[Si(CH3)3]2). 13C NMR (151 MHz,
C6D6, 25°C) δ 227.16(Ccarbene-K), 164.45, 144.41, 137.90, 136.46,
135.18, 132.43, 129.62, 129.34, 128.56, 126.85, 125.67, 123.63,
114.28(Ar-C), 55.63(CH2), 43.35(CH2), 42.15(CH2), 32.00(CH2),
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21.45(CH3),
21.21(tol),
20.85(CH3),
17.58(CH2),
6.90(N[Si(CH3)3]2), 2.67(N[Si(CH3)3]2).
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4.7 Synthesis of complex 2b.
A suspension of H2L2 (0.05 g, 0.14 mmol) in toluene (10 mL)
was treated with Nd[N(SiMe3)2]3 (0.09 g, 0.14 mmol) at room
temperature and stirred for about 30min. Then another
KN(SiMe3)2 (0.06 g, 0.26 mmol) toluene solution was added to
the stirring reactant. The mixture was stirred overnight at room
temperature, and then the solvent was removed under vacuum.
A hexane/toluene solvent mixture was added to extract complex
2b. Blue block crystals were obtained at -20°C in a week. Yield:
36%.
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Single crystals of complexes 1a-2b were sealed in liquid
paraffin oil for determination of the single-crystal structures. Data
were collected at 170 K on a CrysAlisPro, Oxford DiffractionLtd.
using graphite-monochromatic Mo Ka radiation (l ¼0.71073 A).
The data sets were corrected by empirical absorption correction
using spherical harmonics, implemented in SCALE3ABSPACK
scaling algorithm. All structures were solved by direct methods,
and refined by full-matrix least-square methods based on |F|2.
The structures were solved and refined using SHELX-97
programs. All non-hydrogen atoms were located successfully
from Fourier maps and were refined anisotropically.
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Appendix A. Supplementary material
CCDC
1519631-1519634
contain
the
supplementary
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crystallographic data for complexes 1a-2b.
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[44] All geometries of intermediates and transition states (TSs) were
optimized using B3LY91 method. Hydrogen, oxygen, nitrogen and
silicon atoms were optimized under all electron 6-31g(d,p) basis.
Potassium atom was described under 6-31+g(d,p) basis. Yttrium atom
was represented using Stuttgart-Drensden pseudopotential with
adapted basis. Frequency calculations confirmed that the intermediate
had zero imaginary frequency. Natural bonding orbital (NBO) analysis
was employed to calculate atom charges and coordinate energies. All
calculations were performed using Gaussian 09 program.
Acknowledgements
The authors are grateful for the financial support from the
National Natural Science Foundation of China (No. 21674089).
We thank Dr. Jun Ling (Zhejiang University) for helpful
discussions.

ACCEPTED MANUSCRIPT
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Kansson, Polyhedron. 13 (1994) 539-546.
5234-5241.
[46] M. D. Taylor, Chem. Rev. 62 (1962) 503-511.

ACCEPTED MANUSCRIPT
Highlights:
‹ Two aryloxide-substituted N-heterocyclic carbene proligands were designed and synthesized.
‹ Four heterobimetallic potassium-NHC rare earth complexes were prepared, and they have been
characterized by single-crystal X-ray diffraction.
‹ DFT calculations were employed to further investigate charge distributions and interactions
within these compounds.