Synthesis, crystal structures and catalytic activity of tetrakis(acetato)dirhodium(II) complexes with axial picoline ligands

Article abstract of DOI:10.1016/j.ica.2015.05.017

Three complexes were synthesized in high yields by reaction of Rh₂(O₂CCH₃)₄ with 2-picoline (1), 3-picoline (2) and 4-picoline (3), respectively, and characterized by elemental analysis, ESI⁺-MS, FT-I

Full text of DOI:10.1016/j.ica.2015.05.017

Inorganica Chimica Acta 434 (2015) 113–120
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, crystal structures and catalytic activity of
tetrakis(acetato)dirhodium(II) complexes with axial picoline ligands
a,b
c
d,
⇑
b
b
b
b
b
Qing-Song Yea,b,, Xiao-Nian Li , Yi Jin , Juan Yu , Qiao-Wen Chang , Jing Jiang , Cai-Xian Yan , Jie Li ,
⇑
Wei-Ping Liu
a
School of Material Science and Engineering, Kunming University of Science and Technology, Kunming 650093, China
State Key Lab of Advanced Technologies for PGM, Kunming Institute of Precious Metals, Kunming 650106, China
Analysis and Test Center, State Key Laboratory of Phytochemistry and Plant Resources in West China, Kunming Institute of Botany, Chinese Academy of Sciences, Kunming 650201,
b
c
China
d
Key Laboratory of Medicinal Chemistry for Natural Resource, Ministry of Education, School of Chemical Science and Technology, Yunnan University, Kunming 650091, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three complexes were synthesized in high yields by reaction of Rh (O CCH ) with 2-picoline (1), 3-pi-
coline (2) and 4-picoline (3), respectively, and characterized by elemental analysis, ESI -MS, FT-IR and H
NMR along with single-crystal X-ray structural analysis. All picoline ligands coordinate to the axial sites
2
2
3 4
Received 27 December 2014
Received in revised form 9 May 2015
Accepted 15 May 2015
+
1
of Rh
assisted by two intramolecular C–Hꢀ ꢀ ꢀO hydrogen bonds formed between the methyl of 2-picoline and
the oxygen atoms of Rh (O CCH
2 2 3 4
(O CCH ) via the pyridine nitrogen atoms, and interestingly, the coordination of 2-picoline in 1 is
2
2
3
)
4
. Moreover, the intermolecular C–Hꢀ ꢀ ꢀO interactions play the main role
Keywords:
Rhodium
Acetate
Picoline
Catalytic activity
C–Hꢀ ꢀ ꢀO hydrogen bond
Axial ligand
in the structural stacking of 1–3. Their catalytic activity was evaluated in the C–H insertion reactions for
the preparation of 4-nitrobenzyl-(4R,5R,6S)-6-((R)-1-hydroxyethyl)-4-methyl-3,7-dioxo-1-azabicyclo
[3.2.0] heptane-2-carboxylate, a key intermediate of Meropenem. The isolated yields for 1, 2 and 3 are
4
4%, 16% and 22%, respectively, significantly lower than the value of Rh (O CCH (73%), indicating that
2
2
3 4
)
the axial ligands have negative but different influence on the catalytic activity. The activities of 1–3 are
related to the displacement rate of the axial ligands, and essentially related to the Rh–N bond lengths
which strong affect the displacement rate. Therefore, it is possible to tune the catalytic activity of
2 2 3 4
Rh (O CCH ) by changing its axial ligands.
Ó 2015 Elsevier B.V. All rights reserved.
1
. Introduction
Dirhodium(II) tetracarboxylates of type Rh
resulted in the isolation of several dirhodium(II) tetracarboxylates
and large amounts of axial adducts [16–18]. Among these adducts,
the axial ligands are mostly nitrogenous heterocyclic compounds.
Typical examples of this type of ligands are pyridine and its deriva-
tives. The adducts, formed by reaction of pyridine and its deriva-
tives with dirhodium(II) tetracarboxylates, have been extensively
investigated since the discovery the paddlewheel structure of
dirhodium(II) tetraacetate [17,18]. However, it is surprising that
the picoline adducts of dirhodium(II) tetracarboxylates have not
been systemically explored up to now, although picoline is an obvi-
ous derivative of pyridine. In this context, we report the character-
ization, crystal structures and catalytic activity of three picoline
adducts of dirhodium(II) tetraacetate.
2
2 4 2
(O CR) L , in which
the four carboxylates (the equatorial ligands) bridge the two rho-
dium atoms and L represents a Lewis base (the axial ligands)
bound to the Rh–Rh axis, have attracted considerable attention in
recent years due to their potential applications as catalysts [1–6],
antitumor agents [7–13] and building blocks for supramolecular
arrays [14–16]. Dirhodium(II) tetracarboxylates are quite stable
and can be used to construct various new derivatives via equatorial
ligand substitution or axial ligand exchange [16,17]. The equatorial
ligand substitution usually takes place at reflux in high boiling-
point solvents, while the axial ligand exchange reaction can
quickly occur at room temperature. This different reactivity
2
. Experimental
⇑
Corresponding authors at: School of Material Science and Engineering, Kunming
University of Science and Technology, Kunming 650093, China (W.-P. Liu). Tel./fax:
2.1. General
+
86 871 6832 9899.
2 2 3 4
Tetrakis(acetato)dirhodium(II) (Rh (O CCH ) ) was prepared
E-mail addresses: jinyi@ynu.edu.cn (Y. Jin), liuweiping0917@126.com (W.-P.
Liu).
using literature procedure [19]. (3S,4R)-3-[(1R)-1-Hydroxyethyl]-
http://dx.doi.org/10.1016/j.ica.2015.05.017
0
020-1693/Ó 2015 Elsevier B.V. All rights reserved.

1
14
Q.-S. Ye et al. / Inorganica Chimica Acta 434 (2015) 113–120
Table 1
Crystal data and structural refinements for 1–3.
1
2
3
Empirical formula
C
20
H
26
N
2
O
8
Rh
2
C
20
H
26
N
2
O
8
Rh
2
C
20 26 2 8 2
H N O Rh
M
T (K)
k (Å)
r
628.25
100(2)
0.71073
628.25
100(2)
0.71073
628.25
100(2)
0.71073
Crystal system
Space group
a (Å)
b (Å)
c (Å)
monoclinic
P2 /c
7.6476(12)
19.930(3)
8.2011(13)
90
115.963(2)
90
1123.8(3)
2
triclinic
P1ꢀ
7.7224(8)
8.2418(9)
10.6156(11)
77.661(1)
71.673(1)
62.427(1)
566.66(10)
1
monoclinic
P2 /c
10.4429(13)
12.9252(16)
8.8736(11)
90
101.897(2)
90
1172.0(3)
2
1
1
a
(°)
b (°)
(°)
c
3
V (Å )
Z
D
l
calc (g cmꢁ3)
1.857
1.517
1.841
1.505
1.780
1.455
ꢁ
1
(mm
)
F(000)
Crystal size (mm )
h range (°)
628
314
628
3
0.28 ꢂ 0.09 ꢂ 0.04
0.23 ꢂ 0.20 ꢂ 0.04
0.13 ꢂ 0.13 ꢂ 0.06
2.04–27.99
2.03–27.99
1.99–28.00
Limiting indices
ꢁ10 6 h 6 10,
ꢁ10 6 h 6 10,
ꢁ10 6 k 6 10,
ꢁ14 6 l 6 13
7351
ꢁ13 6 h 6 13,
ꢁ17 6 k 6 17,
ꢁ11 6 l 6 11
11,286
ꢁ
ꢁ
26 6 k 6 25,
10 6 l 6 10
Reflection collected
10,657
Independent reflection (Rint
)
2704 (0.0410)
0.9418 and 0.6760
2704/0/146
1.048
2707 (0.0243)
0.9423 and 0.7235
2707/0/233
1.071
2816 (0.0494)
0.9178 and 0.8334
2816/0/148
1.014
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r(I)]
R
R
1
= 0.0312, wR
= 0.0449, wR
2
= 0.0650
= 0.0692
R
R
1
= 0.0243, wR
= 0.0284, wR
2
= 0.0598
= 0.0621
R
R
1
= 0.0300, wR
= 0.0504, wR
2
= 0.0644
= 0.0709
1
2
1
2
1
2
ꢁ
3
D
qmax/min (e Å
)
1.008/ꢁ0.838
0.586/ꢁ1.270
0.762/ꢁ0.650
4
-[(1R)-1-methyl-3-diazo-3-(p-nitrobenzyloxycarbonyl)-2-oxo-
(w), 1593 (vs), 1486 (m), 1434 (vs), 1345(m), 1308(m), 772 (m),
1
propyl]azetidin-2-one (4) was purchased from Sigma–Aldrich Co.
Ltd. All chemicals and solvents were used as received without fur-
ther purification, unless otherwise stated. Elemental analyses for C,
699 (s). H NMR (400 MHz, DMSO-d
7.71 (br s, 2H, Py-H), 7.24 (br, 4H, Py-H), 2.49 (s, Py-CH
lapped with the methyl of DMSO), 1.79 (s, 12H, 4CH COO).
6
) d 8.49 (br s, 2H, Py-H),
3
, over-
3
H
and N were performed with a Carlo-Erba Instrument.
Electrospray ionization mass spectra (ESI-MS) studies were carried
out on an Agilent G6230 Spectrometer. FT-IR spectra were
2
.2.2. Rh
Yield: 40 mg (91%). Anal. Calc. for C20
.17; N, 4.46. Found: C, 38.2; H, 4.1; N, 4.3%. ESI -MS (in MeOH)
2 2 3 4 3 5 4 2
(O CCH ) (3-CH -C H N) (2)
26 2 8 2
H N O Rh : C, 38.24; H,
ꢁ1
recorded in the 4000–400 cm
region on a Bruker Tensor 27
+
4
Spectrometer with KBr pellets. 1H and C NMR spectra were
obtained on a Bruker Avance 400 spectrometer with TMS as an
internal standard. Melting points were determined on a Thomas
Hoover capillary melting point apparatus and were uncorrected.
13
+
+
m/z: 465, [M ꢁ 2Picoline + Na] ; 506, [M ꢁ 2Picoline + 2CH
3
OH] ;
+
+
5
58, [M ꢁ Picoline + Na] ; 907, [2 M ꢁ 4Picoline + Na] ; 1000,
+
[
1
2 M ꢁ 3Picoline + Na] . IR (KBr): 3017 (w), 2983 (w), 2928 (w),
591 (vs), 1481 (m), 1433 (vs), 1345 (m), 792 (m), 706 (s). 1
H
Optical rotations were measured with
a Perkin–Elmer 240
NMR (400 MHz, DMSO-d
H), 7.63 (br, 2H, Py-H), 7.32 (br, 2H, Py-H), 2.32 (s, Py-CH
overlapped with the methyl of DMSO), 1.77 (s, 12H, 4CH
6
) d 8.94 (br, 2H, Py-H), 8.42 (br, 2H, Py-
polarimeter. UV–Vis spectra were recorded on a Varian Cary 50
spectrophotometer equipped with a PCB-150 water circulator.
Column chromatography was performed on silica gel (200–300
mesh).
3
, partially
3
COO).
2.2.3. Rh
2 2 3 4 3 5 4 2
(O CCH ) (4-CH -C H N) (3)
Yield: 41 mg (93%). Anal. Calc. for C20
26 2 8 2
H N O Rh : C, 38.24; H,
+
2
.2. Synthesis
4.17; N, 4.46. Found: C, 37.9 H, 4.1; N, 4.4%. ESI -MS (in MeOH)
+
+
m/z: 465, [M ꢁ 2Picoline + Na] ; 506, [M ꢁ 2Picoline + 2CH
3
OH] ;
+
+
Complexes 1–3 were prepared by a similar procedure. In gen-
558, [M ꢁ Picoline + Na] ; 599, [M ꢁ Picoline + 2CH OH] ; 907,
3
+
+
eral, an aqueous solution (30 mL) of Rh
0
solution (1:1 v/v, 10 mL) was carefully layered on the previous
solution as a middle layer, an ethanol solution (10 mL) of picoline
2
(O
2
CCH
3
)
4
(0.031 g,
[2 M ꢁ 4Picoline + Na] ; 1000, [2 M ꢁ 3Picoline + Na] ; 1093,
+
.07 mmol) was placed in a Schlenk tube. After an ethanol aqueous
[2 M ꢁ 2Picoline + Na] . IR (KBr): 3013 (w), 2985 (w), 2921 (w),
1592 (vs), 1498 (m), 1432 (vs), 1348 (m), 818 (m), 697 (s). 1
H
NMR (400 MHz, DMSO-d ) d 8.44 (br, 4H, Py-H), 7.21 (br s, 4H,
6
(
0.052 g, 0.56 mmol) was put onto the layer. The tube was left
Py-H), 2.32 (s, Py-CH , partially overlapped with the methyl of
3
undisturbed for one week to yield pink single crystals. The crystals
were collected on a frit, washed with water and ethanol, and dried
in vacuo.
DMSO), 1.77 (s, 12H, 4CH COO).
3
2.3. Preparation the sample solutions for UV–Vis analysis
2
.2.1. Rh
Yield: 41 mg (93%). Anal. Calc. for C20
.17; N, 4.46. Found: C, 38.0; H, 4.1; N, 4.4%. ESI -MS (in MeOH)
2
(O
2
CCH
3
)
4
(2-CH
3
-C
5
H
4
N)
2
(1)
UV–Vis signals of 1–3 in the range 350–700 nm were recorded
in three solvents, dimethyl sulfoxide (DMSO), methanol and ethyl
acetate. For the DMSO solutions, each complex (10 mg, for 1, 2, 3
H
26
N
2
O
8
Rh
2
: C, 38.24; H,
+
4
+
+
m/z: 465, [M ꢁ 2Picoline + Na] ; 506, [M ꢁ 2Picoline + 2CH
3
OH] ;
2 2 3 4
or Rh (O CCH ) ) was suspended in 10 mL of DMSO, then the
+
9
07, [2 M ꢁ 4Picoline + Na] . IR (KBr): 3026 (w), 2988 (w), 2935
resulting mixtures were stirred at room temperature. Complex 1

Q.-S. Ye et al. / Inorganica Chimica Acta 434 (2015) 113–120
115
Fig. 1. UV–Vis analysis of complexes 1–3 in different solutes: (a) in DMSO; (b) in methanol; (c) in ethyl acetate; UV–Vis analysis of complexes 1–3 in DMSO from 20 to 50 °C:
d) complex 1; (e) complex 2; (f) complex 3.
(
and Rh
2
(O
2
CCH
3
)
4
were quickly dissolved within 30 min, but 2 and
as below. Firstly, four different catalysts of complexes 1, 2, 3 and
Rh (O CCH (1 mg, 5 mg or 10 mg, respectively) were added to
3
were dissolved within 5 h. For the methanol and ethyl acetate
2
2
3 4
)
solutions, each complex (5 mg, for 1, 2 or 3) was added to two dif-
ferent solvents (15 ml), methanol or ethyl acetate, respectively.
Then these mixtures were stirred for 24 h at room temperature.
Complex 1 was completely dissolved in the both solvents, and
the color of solutions was changed to light blue, different from
the purple color of free complex 1. Complexes 2 and 3, however,
were only partially dissolved in the both solvents, and the colors
of solutions were light purple same as the purple color of free com-
plexes 2 or 3. All solution samples have been filtered by filtration
10 ml of ethyl acetate, and stirred for 1 h at room temperature.
Because of the low solubilities of complexes 2 or 3 in ethyl acetate,
which are only partially dissolved, all mixed solutions were filtered
by filtration membranes (PTFE, / = 0.45 lm). Then, these filtrates
were immediately added to the solution of compound 4 (500 mg,
1.28 mmol) and 10 ml of ethyl acetate in a 25-ml round-bottomed
flask, respectively. The result solutions were heated to reflux, gas
evolution occurred. After refluxing for 15 min, the mixture was
evaporated in vacuo to give crude product as foams and purified
by flash column chromatography (Petroleum ether: Ethyl acet-
ate = 1: 1) to yield compound 5. Each sample was measured in par-
allel for three trials.
membranes (PTFE, / = 0.45 lm) prior to test for UV–Vis analysis.
Meanwhile, the solutions of complexes 1–3 in DMSO were ana-
lyzed by UV–Vis at different temperature from 20 to 50 °C.
2.4. Single crystal X-ray data collection and structure determination
2.5.1. (3S,4R)-3-[(1R)-1-Hydroxyethyl]-4-[(1R)-1-methyl-3-diazo-3-
(
p-nitrobenzyloxycarbonyl)-2-oxopropyl]azetidin-2-one (4)
Melting Point = 115–117 °C; H NMR (400 MHz, CDCl
d, J = 6.8 Hz, 3H, 1-b-methyl), 1.24 (d, J = 6.4 Hz, 3H, CH
3
Single crystals of 1–3 were obtained as described above.
Intensity data for single crystals of each complex were collected
on a BRUKER SMART APEX II CCD detector with graphite-
1
3
): d 1.17
CHOH),
.85 (d, J = 4.8 Hz, 1H, H3), 3.16 (s, 1H, OH), 3.71 (q, J = 6.4 Hz,
H, CHCH ), 3.82 (t, J = 4.4 Hz, 1H, H4), 4.08 (t, J = 5.6 Hz, 1H,
CH ), 6.65 (s, 1H, NH), 7.51 and 8.22 (d,
J = 8.4 Hz, 2H, aromatic protons); C NMR (125 MHz, CDCl
(
2
1
monochromatized Mo K
a radiation (k = 0.071073 nm). The struc-
3
tures were solved by direct method using the program SHELXS-97
CHOH), 5.34 (s, 2H, CO
2
2
and subsequent Fourier difference techniques, and refined
13
3
): d
CHOH), 45.02 (C6), 53.21 (C4),
1.95 (C3), 65.48 (CHOH), 65.68 (CO CH ), 124.02 (aromatic car-
2
anisotropically by full matrix least-squares on F using SHELXL-97
1
6
3.41 (1-b-methyl), 21.03 (CH
3
[
20]. Crystal data and structural refinements of 1–3 are shown in
Table 1.
2
2
bons), 128.78 (aromatic carbons), 141.94 (2C, aromatic carbon),
47.98 (C8), 160.43 (CO), 168.271 (CO), 194.78 (CO); IR (KI, film)
1
ꢁ
1
21
2
.5. Evaluation of catalytic activity
2140, 1750, 1720 and 1650 cm ; [
ESI -MS (in MeOH) m/z: [M + Na] Calcd. for C17
413.1073; Found 413.1067. The H NMR of compound 4 is consis-
tent with the reported values in the literature [21]. ( H NMR
(CDCl
3H, CH
a
]
D
= ꢁ50.6° (c = 2.5, CH
2
Cl
2
);
Na
+
+
H
18
N
4
O
7
1
Complexes 1–3 and dirhodium(II) tetraacetate were used as
catalysts for the preparation of 4-nitrobenzyl-(4R,5R,6S)-6-((R)-1-
hydroxyethyl)-4-methyl-3,7-dioxo-1-azabicyclo [3.2.0] heptane-
1
3
): d 1.22 (d, J = 6.0 Hz, 3H, 1-b-methyl), 1.32 (d, J = 6.0 Hz,
CHOH), 2.38 (d, J = 3.2 Hz, 1H, OH), 2.92 (dd, J = 2.4 and
2
-carboxylate (5). The general preparation method was described
3

1
16
Q.-S. Ye et al. / Inorganica Chimica Acta 434 (2015) 113–120
Table 2
Selected bond lengths (Å) and angles (°) for 1.
Rh1–O1
Rh1–O3
Rh1–N1
2.051(2)
2.042(2)
2.317(3)
Rh1–O2
Rh1–O4A
Rh1–Rh1A
2.039(2)
2.045(2)
2.4121(5)
O1–Rh1–O3
O2–Rh1–O3
O1–Rh1–O2
N1–Rh1–Rh1A
90.58(9)
88.78(9)
175.49(9)
176.42(7)
O1–Rh1–O4A
O2–Rh1–O4A
O3–Rh1–O4A
90.14(9)
90.14(9)
175.38(9)
Symmetry code: A: ꢁx + 1, ꢁy + 1, ꢁz + 2.
Table 3
Selected bond lengths (Å) and angles (°) for 2.
Rh1–O1
Rh1–O3A
Rh1–N1
2.027(3)
2.038(2)
2.260(5)
2.3837(4)
Rh1–O2A
Rh1–O4
Rh1–N1
2.018(3)
2.048(2)
2.252(5)
Fig. 2. A view of the molecule structure of 1 with the atomic labeling scheme.
Hydrogen atoms were omitted for clarity.
0
Rh1–Rh1A
O1–Rh1–O3A
O2A–Rh1–O3A
O1–Rh1–O2A
N1–Rh1–Rh1A
90.18(9)
89.80(9)
176.15(13)
170.30(15)
O1–Rh1–O4
O2–Rh1–O4
O3A–Rh1–O4A
89.60(9)
90.14(9)
175.90(6)
168.87(15)
yields (91–93%). The complexes are insoluble in common solvents,
but slightly soluble in strong donor solvents such as dimethyl sul-
foxide (DMSO) or pyridine.
0
N1 –Rh1–Rh1A
Symmetry code: A: ꢁx + 2, ꢁy + 2, ꢁz + 1.
All complexes were characterized by elemental analysis, FT-IR,
+
1
ESI -MS and H NMR. The elemental analysis data are in good
agreement with the calculated values. In all three complexes, the
symmetric and asymmetric stretching vibration bands of the acet-
Table 4
Selected bond lengths (Å) and angles (°) for 3.
ꢁ1
ate ligands, observed as strong intensity bands at 1432–1434 cm
ꢁ
1
Rh1–O1
Rh1–O3
Rh1–N1
2.033(2)
2.031(2)
2.247(3)
Rh1–O2
Rh1–O4
Rh1–Rh1A
2.045(2)
2.043(2)
2.4010(5)
and 1591–1593 cm , respectively, are essentially equal. The val-
ues of the difference between
expected, are less than 200 cm
masym(COO) and msym(COO), as
(Dm = 158–160 cm ), consistent
ꢁ1
ꢁ1
O1–Rh1–O3
O2–Rh1–O3
O1–Rh1–O2
N1–Rh1–Rh1A
89.96(9)
O1–Rh1–O4
O2–Rh1–O4
O3–Rh1–O4
89.95(10)
90.02(10)
175.82(9)
with a bidentate bridging mode for the acetates [23]. These IR data
agree well with those observed in dimeric rhodium complexes of
other alkyl carboxylates, in which the carboxylate acts as a bridg-
ing bidentate ligand [18].
89.76(10)
175.84(8)
178.08(7)
Symmetry code: A: ꢁx, ꢁy, ꢁz + 2.
Complex 1 shows two peaks at m/z 465 and 506 which can be
+
assigned
to
the
fragment
[M ꢁ 2Picoline + Na]
and
+
7
6
1
.6 Hz, 1H, H3), 3.77 (q, J = 6.0 Hz, 1H, CHCH
.0 Hz, 1H, H4), 4.15 (m, 1H, CHOH), 5.38 (s, 2H, CO
H, NH), 7.57 and 8.30 (d, J = 6.6 Hz, 2H, aromatic protons)).
3
), 3.86 (dd, J = 2.4 and
CH ), 5.90 (s,
[M ꢁ 2Picoline + 2CH OH] , respectively. Interestingly, 1 develops
3
2
2
a peak at m/z 907, due to the loss of four picoline residues from
two molecules of 1 accompanied by the gain of one sodium cation.
Besides the three peaks observed in 1, 2 shows two more peaks at
+
2.5.2. 4-nitrobenzyl-(4R,5R,6S)-6-((R)-1-hydroxyethyl)-4-methyl-3,7-
m/z 558 and 1000, corresponding to [M ꢁ Picoline + Na] and
+
dioxo-1-azabicyclo [3.2.0] heptane-2-carboxylate (5)
[2 M ꢁ 3Picoline + Na] , respectively. The former was attributed
1
Melting Point = 125–127 °C; H NMR (400 MHz, CDCl
3
): d 1.19
CHOH),
), 3.27 (d, J = 6.8 Hz, 1H, H6), 4.23 (d,
J = 6.8 Hz, 1H, H5), 4.31 (m, 1H, CHOH), 4.47 (s, 1H, H2), 5.33–
.22 (dd, J = 13.6 Hz, 2H, CO CH ), 8.20 and 7.51 (d, J = 8.4 Hz, 2H,
aromatic protons); C NMR (125 MHz, CDCl ): d 12.62 (1-b-
methyl), 21.92 (CH CHOH), 41.96 (C4), 55.57 (C5), 61.69 (C6),
2.05 (C2), 65.64 (CHOH), 66.27 (CO CH ), 123.90 (aromatic car-
bons), 128.34 (aromatic carbons), 141.94 (aromatic carbon),
47.85 (aromatic carbon), 164.95 (CO), 172.70 (CO), 208.88 (CO);
to a loss of one picoline molecule accompanied by the gain of
+
(
d, J = 6.8 Hz, 3H, 1-b-methyl), 1.34 (d, J = 6.4 Hz, 3H, CH
3
one Na , while the latter was formed by the same procedure as
2
.82 (m, 1H, CHCH
3
the peak at m/z 907 except for the loss of three picoline residues.
As compared to 2, two more peaks, at m/z 599 and 1093, are
+
5
2
2
observed in 3, and they are assigned to [M ꢁ Picoline + 2CH OH]
3
1
3
+
3
and [2 M ꢁ 2Picoline + Na] , respectively. It is noteworthy that no
+
3
complexes show M peaks, presumably due to the low stability
6
2
2
of their axial ligands under electron spray ionization, which are
1
Table 5
ꢁ1
21
D
IR (KI, film) 1780, 1745 and 1605 cm ; [a] = +43.3° (c = 2.5,
Selected bond lengths in the Rh
dmp, aamp, 4-mpy and pym.
2
(O
2
CCH
3
)
L
4 2
adducts, where L = py, 2-mpy, ampy,
+
+
a
CH
C H N O
17 18 2 7
2
Cl
2
); ESI -MS (in MeOH) m/z: [M + Na]
Calcd. for
Na 385.1012; Found 385.1006. The H NMR of com-
pound 5 is consistent with the reported values in the literature
1
L
Rh–Rh (Å)
Rh–N (Å)
Ref.
1
py
2.3963(2)
2.4121(5)
2.417(3)
2.227(3)
2.317(3)
2.36(1)
[
22]. ( H NMR (CDCl
J = 6.1 Hz, 3H, CH ), 2.96 (m, 1H, CH), 3.12 (m, 1H, CH), 3.98 (m,
H, CH), 4.00 (m, 1H, CH), 4.56 (s, 1H, CH), 4.86–5.02 (m, 2H,
CH2), 7.69 (d, J = 8.7 Hz, Ar–H), 8.25 (d, J = 8.6 Hz, 2H, Ar–H).
3 3
): d 1.18 (d, J = 6.2 Hz, 3H, CH ), 1.28 (d,
2-mpy
This work
b
3
ampy
1
dmpy
aampy
2.4137(5)
2.4112(6)
2.4010(5)
2.4030(7)
2.403(4)
2.439(4)
2.247(3)
2.243(3)
4-mpy
This work
pym
3
. Results and discussion
a
py, 2-mpy, ampy, dmpy, aampy, 4-mpy and pym denote pyridine, 2-picoline, 2-
amino-6-methylpyridine, 2,6-dimethylpyridine, 2-acetylamino-6-methylpyridine,
-picoline, 4-pyridinemethanol.
Complexes 1–3 were synthesized by the direct reaction of
4
Rh
2
(O
2
3
CCH )
4
with an excessive amounts of the picoline ligands.
b
The values are the complex where the two axial sites of the dirhodium unit are
The reactions were essentially quantitative to afford high isolated
occupied with pyridine.

Q.-S. Ye et al. / Inorganica Chimica Acta 434 (2015) 113–120
117
Fig. 3. (a) A portion of the sheet in 1 formed by C–Hꢀ ꢀ ꢀO hydrogen bonds of C3ꢀ ꢀ ꢀO3 (3.448(5) Å), C10ꢀ ꢀ ꢀO2 (3.378(5)ꢀ ꢀ ꢀÅ) and C10ꢀ ꢀ ꢀO4 (3.500(6) Å). The axial 2-picoline
ligands were omitted for clarity. (b) The crystal packing of molecules 1.
completely turned to be Rh
Complexes 2 and 3 are partially stabilization, and they could be
solvated to be the Rh (O CCH (methanol)(picoline) or mixture
of Rh (O CCH (methanol) and Rh (O CCH (methanol)(picol-
2 2 3 4 2
(O CCH ) (methanol) in methanol.
2
2
3 4
)
2
2
3
)
4
2
2
2
3 4
)
ine). These are consistent with the ESI-MS results, for 1, there are
+
fragment [M ꢁ 2Picoline + 2Na (or 2Solvent)] , but not found frag-
+
ment [M ꢁ Picoline + Na (or Solvent)] . However, for 2 and 3, frag-
+
ment [M ꢁ Picoline + Na (or Solvent)] can be found. In ethyl
acetate, similar absorption spectra to methanol were observed
except that the maximum absorptions were blue-shifted by about
2
0 nm (Fig. 1c).
Complexes 1–3 have a sharp singlet for the signals of the acet-
1
ate protons in H NMR spectra. All signals of pyridine protons in 1–
3, however, are broad singlet, possibly owing to the fast exchange
of axial ligands with DMSO. The hydrogens of the picoline methyl
of 1 are fully obscured by the methyl of DMSO solvent (d
2.49 ppm). In the case of 2 and 3, the methyl of DMSO was
observed at d 2.32 ppm and is partially overlapped with the picol-
ine methyl. The C NMR spectra of 1–3, unfortunately, could not
be detected because of the poor solubility in any solvents.
The structures of all three complexes were unambiguously
determined by X-ray crystallography. Quality single crystals of
Fig. 4. A view of the molecule structure of 2 with the atomic labeling scheme.
Hydrogen atoms were omitted for clarity.
1
3
consistent with our previous observation for bis(pyridine) adducts
of dirhodium tetraoctanoate [24].
The UV–Vis spectra of 1–3 in three different solvents are
showed in Fig. 1(a–c). The max absorption peaks of 1–3 in DMSO
1
–3, suitable for X-ray analysis, were obtained by slow diffusion
of an ethanol solution of picoline into an aqueous solution of
Rh (O CCH . Details of data collection and structure solution of
(
Fig 1a) are almost the same (498.2, 503.1 505.5 nm for 1, 2, 3,
respectively) and they are also same as Rh (O CCH (DMSO)
max = 498.0 nm) which is directly prepared by dissolving
Rh (O CCH in DMSO. This result shows that 1–3 are unstable
and may be converted to Rh (O CCH (DMSO) in DMSO solution,
2
2
3
)
4
2
3 4
)
2
2
(k
these complexes are summarized in Table 1. The selected bond
lengths and angles are listed in Tables 2–4.
2
2
3 4
)
2
2
3
)
4
2
The structure of 1 contains paddlewheel dirhodium units, as
shown in Fig. 2, joined equatorially by four bidentate acetate
ligands and two monodentate 2-picoline axial ligands bonded to
the rhodium atoms by the nitrogen atom. Thus, each rhodium
shows a slightly distorted octahedral environment having four
equatorial positions occupied by the oxygen atoms of the acetate
ligands; the axial sites are occupied by one nitrogen atom of 2-pi-
coline and by the other Rh atom of the dirhodium unit. There is a
further evidence for which is provided by that the absorption
curves of 1–3 in DMSO are almost no change from 20 to 50 °C
(Fig 1d–f). Compared to the almost same absorption curves of 1–
3
in DMSO, the kmax of 1 in methanol (584.9 nm) is very different
from the values of 2 and 3 (549.0 nm and 547.0 nm for 2 and 3,
respectively, Fig. 1b). This result may be explained by their differ-
ent stability in methanol. Complex 1 is also unstable and maybe
Fig. 5. The crystal packing of molecules 2.

1
18
Q.-S. Ye et al. / Inorganica Chimica Acta 434 (2015) 113–120
but is less affected (Table 5). The coordination of 2-picoline is
assisted by two intramolecular C–Hꢀ ꢀ ꢀO hydrogen bonds:
C5ꢀ ꢀ ꢀO4 = 3.044(5) Å and C5ꢀ ꢀ ꢀO1 = 3.530(6) Å. The range of Rh–O
distances extends from 2.039(2) to 2.051(2) Å, in agreement with
those other dirhodium carboxylate compounds described in the lit-
erature [18].
The intermolecular C–Hꢀ ꢀ ꢀO interactions play an important role
in the structural stacking of 1 since stronger hydrogen bonding is
absent [28]. As shown in Fig. 3a, a sheet is formed by the inter-
molecular C–Hꢀ ꢀ ꢀO hydrogen bonds of C3ꢀ ꢀ ꢀO3 (3.448(5) Å),
C10ꢀ ꢀ ꢀO2 (3.378(5) Å) and C10ꢀ ꢀ ꢀO4 (3.500(6) Å). The sheets are
further packed to afford three dimensions network (Fig. 3b).
The crystal structure of 2, shown in Fig. 4, consists of cen-
trosymmetric dinuclear dirhodium core with an inversion center
located at the center of the Rh–Rh bond. The Rh–Rh distance
Fig. 6. A view of the molecule structure of 3 with the atomic labeling scheme.
Hydrogen atoms were omitted for clarity.
(
2.3837(4) Å) is 0.013 Å and 0.028 Å shorter than the values of
Rh (O CCH (pyridine) [25] and 1, respectively. Nevertheless,
2
2
3
)
4
2
the Rh–N bond lengths (2.260(5), 2.252(5) Å) are significantly
crystallographic inversion center at the center of the Rh–Rh bond.
The axial N–Rh–Rh–N chain is nearly linear with N–Rh–Rh angles
of 176.42(7)°. The two groups of four equatorial oxygen atoms
are almost perfectly eclipsed with respect to each other with a
maximum O–Rh–Rh–O torsion angle of 1°. The ring atoms of the
picoline are coplanar with a maximum deviation from the least-
squares plane of 0.011 Å, and the methyl carbon (C5) lies
shorter than the value of 1 but slightly longer than that of
Rh (O CCH ) (pyridine) (2.227(3) Å) [25]. The Rh–O bond lengths
2 2 3 4 2
are similar to those of 1 and other dirhodium carboxylates [17,18].
The axial 3-picoline ligand and two carbon atoms (C7 and C8) of an
acetate group are disordered, even though the data was collected
at low temperature (100 K). The pyridine plane of 3-picoline
0
0
almost parallels to the O1–O2–O1 –O2 plane of the bridging acet-
0
.075(5) Å from this plane. The pyridine plane approximately
ates with a dihedral angle of 13.4(1)°.
bisects the acetate groups; it forms dihedral angles of 59.72(9)°
and 30.33(9)° with the O1–O2–O1 –O2 and O3–O4–O3 –O4
planes of the bridging acetates, respectively.
As illustrated in Fig. 5, the molecules 2 are linked by p–p inter-
0
0
0
0
actions between the neighboring pyridine rings, with which are
parallel to each other at a distance of 3.585 Å and a centroids dis-
tance of 3.760 Å. Moreover, the intermolecular C–Hꢀ ꢀ ꢀO interac-
The Rh–Rh bond length, 2.4121(5) Å, is slightly longer than that
of Rh (O CCH (pyridine) (2.3963(2) Å) [25], whereas the Rh–N
bond length (2.317(3) Å) shows a significantly longer than the
value of Rh (O CCH (pyridine) (2.227(3) Å). The longer Rh–N
0
)
4
2
tions, formed between C10 and O3, C8 and O3, C8 and O2 with
2
2
3
d(Cꢀ ꢀ ꢀO) of 3.452(4), 3.305(8) and 3.541(8) Å, respectively, also
)
4
2
play an important role in the crystal packing of 2.
2
2
3
bond found in 1 is obviously due to the steric repulsion between
the 2-methyl groups and acetate oxygens, which have been
The crystal structure of 3 is very similar to those of 1 and 2
except that the axial sites are occupied by 4-picoline instead of
2-picoline and 3-picoline for 1 and 2, respectively (Fig. 6). The
Rh–Rh distance is 2.4010(5) Å with the axial 4-picoline N atoms
coordinates Rh at a distance of 2.247(3) Å. The Rh–N bond length
is shorter than the value of 1 due to lack of steric repulsion adja-
cent to the 4-picoline nitrogen donor, but it still increase by
reported for Rh
or 6-methyl groups, Rh
2.403(4) Å) [26], Rh (O CCH
2 2 3 4
(O CCH ) with axial pyridine derivatives having
2
(
(
-
2
(O
2
CCH
3
)
4
(2,6-dimethylpyridine)
ethylyridine)
2
2
2
3
4
) (2-amino-6-
2
2.36(1) Å [27], bound to Rh atoms by the pyridine nitrogens)
(O CCH (2-acetylamino-6-methylpyridine) (2.439(4) Å,
and Rh
2
2
3
)
4
2
bound to Rh atoms by the pyridine nitrogens). The Rh–N bond
length increases in the following sequence of the ligand: pyri-
dine < 2-picoline < 2-amino-6-methylpyridine < 2,6-dimethylpyri-
dine < 2-acetylamino-6-methylpyridine. This is consistent with the
steric effect caused by the substituents adjacent to the nitrogen
donor. The Rh–Rh bond length also increases in the same order
2 2 3 4 2
0.02 Å as compared to the value of Rh (O CCH ) (pyridine)
(2.227(3) Å) [25]. It is interesting to note that both the Rh–Rh
and Rh–N bond lengths of 3 are almost completely equal to the val-
ues of dirhodium tetraacetate 4-pyridinemethanol adduct
(2.4030(7) and 2.243(3) Å for the Rh–Rh and Rh–N bond lengths,
respectively) [29], even though the axial ligands vary from 4-
i
Fig. 7. A portion of the chain in 3 formed by C–Hꢀ ꢀ ꢀ
p
and C–Hꢀ ꢀ ꢀO hydrogen (C10ꢀ ꢀ ꢀCg = 3.345 Å, Cg is the centroid of pyridine ring at (ꢁx, ꢁy, 1 ꢁ z); C2ꢀ ꢀ ꢀO3 = 3.601(5) Å and
ii
C2ꢀ ꢀ ꢀO3 = 3.604(5) Å; symmetry code: (i) x, ꢁ0.5 ꢁ y, 0.5 + z; (ii) ꢁx, ꢁ0.5 + y, 1.5 ꢁ z).

Q.-S. Ye et al. / Inorganica Chimica Acta 434 (2015) 113–120
119
Table 6
O3 at a distance of 3.784(5) Å. These chains form the same three
Catalytic activity of 1–3 and dirhodium(II) tetraacetate.
dimensions network as that of 1, assisted by two intermolecular
i
C–Hꢀ ꢀ ꢀO
hydrogen
bonds
(C2ꢀ ꢀ ꢀO3 = 3.601(5) Å
and
ii
OH
C2ꢀ ꢀ ꢀO3 = 3.604(5) Å; symmetry code: (i) x, ꢁ0.5 ꢁ y, 0.5 + z; (ii)
OH
CH3 N2
CH
3
H
H
(R)
H
H
(R)
ꢁx, ꢁ0.5 + y, 1.5 ꢁ z).
OPNB
(R)
(S)
(
R)
(R)
Cat.
(
S)
(R)
O
Dirhodium(II) complexes are exceptionally active catalysts for
the decomposition of diazo compounds to form rhodium car-
benoids, which undergo a number of highly selective reactions
such as cyclopropanation, carbon-hydrogen insertion, ylide gener-
ation, and aromatic cycloaddition [1,2,30–32]. Inspiring by their
unique catalysis, much attentions has focused on tuning their cat-
alyst properties by modification of the bridge ligand structure. The
axial ligand, however, had long been considered to have a less
important or negative role in catalysis, since it was observed that
dirhodium(II) tetracarboxylates were more efficient in solvents
with poor coordinative capabilities than in medium to strong coor-
dinative solvents. In other words, medium to strong donor ligands
can partially or totally inhibit the catalysis. Therefore, less work
was done about the catalysis of dirhodium(II) tetracarboxylates
adducts over a long time [1,2]. This concept is changing, and sev-
eral studies have proven that the axial ligand modification also is
a valuable and simple strategy to prepare new catalysts of
dirhodium(II) tetracarboxylates [33]. In this context, we evaluated
the catalytic activity of 1–3 for the preparation of a Meropenem
intermediate (5, Table 6). Compounds 4 and 5 were characterized
by NMR (Fig. 8), specific rotation and ESI-MS, being in good agree-
ment with the assigned structures.
N
NH
O
O
O
O
CO PNB
2
5
4
Catalyst
Axial
ligands
Amount
(mg)
Rh–Rh
(Å)
Rh–N
(Å)
Isolated
yield
No catalyst
–
–
–
1
1
1
1
5
5
5
5
10
–
–
–
N.R.b
70 ± 2%
a
Rh
1
2
3
Rh
1
2
3
Rh
1
2
(O
2
(O
2
(O
2
CCH
2
CCH
2
CCH
3
3
3
)
4
)
4
)
4
2.386(1)
2-picoline
3-picoline
4-picoline
–
2-picoline
3-picoline
4-picoline
–
2.4121(5) 2.317(3) 40 ± 3%
2.3837(4) 2.260(5) 15 ± 3%
2.4010(5) 2.247(3) 20 ± 3%
a
2.386(1)
–
72 ± 3%
2.4121(5) 2.317(3) 45 ± 4%
2.3837(4) 2.260(5) 17 ± 3%
2.4010(5) 2.247(3) 22 ± 3%
a
2.386(1)
–
77 ± 3%
2-picoline 10
3-picoline 10
4-picoline 10
2.4121(5) 2.317(3) 48 ± 2%
2.3837(4) 2.260(5) 16 ± 4%
2.4010(5) 2.247(3) 25 ± 2%
2
3
a
The value for Rh
N.R. means no reaction.
2 2 3 4 2 2
(O CCH ) (H O) .
b
As can be seen from Table 6 (line 1), without a catalyst, the reac-
tion is not going to happen. However, under different amount of
the catalyst, there is litter difference in isolated yields, such as
for complex 1, the yields are 40% (using 1 mg), 45% (using 5 mg)
and 48% (using 10 mg), respectively. For complex 2, the yields
are 15% (using 1 mg), 17% (using 5 mg) and 16% (using 10 mg),
respectively. And for complex 3, the yields are 20% (using 1 mg),
picoline to 4-pyridinemethanol, as seen in Table 5. The pyridine
rings in 4-picoline are planar with deviations not greater than
0
0
0
.01 Å, which nearly parallels to the O1–O2–O1 –O2 plane of the
bridging acetates with a dihedral angle of 7.7(1)°.
The crystal structure of 3 contains an intermolecular C–Hꢀ ꢀ ꢀ
p
interactions in which atom C10 interact with the nearest pyridine
rings of 4-picoline at a distance of 3.345 Å from the centroid of the
pyridine rings. This interaction links the molecules 3 to form an
infinite chain, as seen in Fig. 7. The chain is further stabilized by
a weak C–Hꢀ ꢀ ꢀO hydrogen bond formed between atoms C10 and
2
2% (using 5 mg) and 25% (using 10 mg), respectively. The possible
reason is that the poor solubilities of 1–3 have made them satura-
tion in the reaction system under different amount of the catalyst.
It is noted that all the isolated yields of the evaluation reaction for
1–3 are significantly lower than the value of dirhodium(II)
Fig. 8. The 1H NMR and 13C NMR spectrums of compounds 4 and 5.

1
20
Q.-S. Ye et al. / Inorganica Chimica Acta 434 (2015) 113–120
tetraacetate (Table 6). The main reason may be that the axial sites
of dirhodium(II) tetraacetate are occupied by the strong donor
ligands picoline, which makes it difficult for the axial ligands to
be displaced from the catalysts, and then limits the chance of the
rhodium atom contact with the reaction substrate. In additional,
it is worthy to note that the most efficient complex 1 has the long-
est Rh–N bond distance in the three adducts (Table 6). This can be
explained by that the longer Rh–N bond distance of 1 makes its
axial ligands leave readily and then more better catalytic activity
is observed, which is also supported by the UV–Vis analysis that
Data
Centre
via
www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.ica.2015.05.017.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
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Appendix A. Supplementary data
1
143.
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CCDC 1022719, 1022720 and 1022720 contains the supplemen-
tary crystallographic data for Complexes 1–3. These data can be
obtained free of charge from The Cambridge Crystallographic
(