Synthesis and crystal structures of two new Cd(II) complexes with 3-(2-pyridy)pyrazole-based ligand: Influence of anions

Article abstract of DOI:10.1016/j.molstruc.2005.07.001

Two new Cd(II) complexes with a 3-(2-pyridyl)pyrazole-based ligand, [Cd(L)₂(SCN)₂] (1) and {[Cd(L)₂N ₃](ClO₄)}_n (2) (L=3-(2-pyridyl)pyrazol-1- ylmethylbenzene) were synthesized and structur

Full text of DOI:10.1016/j.molstruc.2005.07.001

Journal of Molecular Structure 754 (2005) 71–76
www.elsevier.com/locate/molstruc
Synthesis and crystal structures of two new Cd(II) complexes
with 3-(2-pyridy)pyrazole-based ligand: influence of anions
Xue-Song Shia, Chun-Sen Liua, Jian-Rong Lia, Yue Guoa,
Jiang-Ning Zhoua, Xian-He Bua,b,
*
aDepartment of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
bThe State Key Laboratory of Structural Chemistry, Fuzhou 350002, People’s Republic of China
Received 27 June 2005; revised 3 July 2005; accepted 3 July 2005
Available online 18 August 2005
Abstract
Two new Cd(II) complexes with a 3-(2-pyridyl)pyrazole-based ligand, [Cd(L)2(SCN)2] (1) and {[Cd(L)2N3](ClO4)}n (2) (LZ3-(2-
pyridyl)pyrazol-1-ylmethylbenzene) were synthesized and structurally characterized by elemental analyses, IR and single crystal X-ray
˚
diffraction analysis. Complex 1 crystallizes in the monoclinic system, space group C2/c, with aZ14.833(3), bZ13.790(3), cZ15.970(3) A,
bZ110.89(3)8 and ZZ4, while 2 crystallizes in the monoclinic system, space group P21/c, with aZ13.622(4), bZ23.286(7), cZ10.547(3)
˚
A, bZ111.084(6)8 and ZZ4. In the two complexes, the Cd(II) centers are coordinated by six nitrogen atoms, in which four from two distinct
L ligands and two from thiocyanato (1) or azido (2) anions. Complex 1 has a mononuclear structure, whereas 2 has a 1D chain structure
bridged by azido anions. In 2, the azido adopts a m-1,3-trans coordination mode, which is not common in the azide Cd(II) complexes. In
addition, in the structure of 2, the 1D chains were further assembled into a quasi-3D supramolecular network by the C–H/O hydrogen-
bonding interactions. The structural difference of the two complexes is attributable to the different anions, which have different coordination
natures.
q 2005 Published by Elsevier B.V.
Keywords: Anion effects; Cd(II) complexes; Crystal structures; 3-(2-pyridyl)pyrazole-based ligand
1. Introduction
very similar to 2,2-bipyridine and its substituted derivatives
were also reported [4]. Some complexes with 3-(2-
pyridyl)pyrazole-based ligands have been found to have
potential biological and catalytic activities [5].
The synthesis of discrete complexes or coordination
polymers is a rapidly developing field in current coordi-
nation and supramolecular chemistry [1,2]. One of the
ultimate aim of coordination chemistry is to control the
structure of the target product with tailored properties and
structures. Therefore, the selection of proper ligands as
‘building blocks’ is a key factor in manipulating the
structures of complexes. For instance, some typical chelate
ligands, such as 2,2-bipyridine (bpy) and its derivatives
have been well used in the synthesis of functional
complexes [3], and many coordination architectures with a
chelating ligand 3-(2-pyridyl)pyrazole whose structure is
In this paper, we synthesized a 3-(2-pyridyl)pyrazole-
based ligand L (LZ3-(2-pyridyl)pyrazol-1-ylmethylben-
zene) (see Chart 1), in which the benzene ring acts as a
directing group for p–p stacking. We report herein the
syntheses and structures of two Cd(II) complexes with this
ligand, [Cd(L)2(SCN)2] (1) and {[Cd(L)2N3](ClO4)}n (2),
and the anion effects have also been discussed.
2. Experimental
2.1. Materials and general methods
*
Corresponding author. Tel.: C86 22 23502809; fax: C86 22 23502458.
E-mail address: buxh@nankai.edu.cn (X.-H. Bu).
3-(2-Pyridyl)pyrazole was synthesized with reported
procedures [6]. All the other reagents for synthesis were
commercially available and employed as received or
0022-2860/$ - see front matter q 2005 Published by Elsevier B.V.
doi:10.1016/j.molstruc.2005.07.001

72
X.-S. Shi et al. / Journal of Molecular Structure 754 (2005) 71–76
15.71. Calcd for C32H26CdN8S2: C, 54.97; H, 3.75; N,
16.03. IR (cmK1): 3124w, 2359w, 2076s, 1605s, 1570m,
1499m, 1436s, 1371m, 1339s, 1231m, 1159w, 1096s,
1030w, 960w, 774s, 730s, 706m, 691m, 624m, 462w, 409w.
{[Cd(L)2N3](ClO4)}n (2) Complex 2 was obtained as
single crystals by the similar method to that for complex 1
except for using NaN3 (7 mg, 0.1 mmol) instead of
NH4SCN. Yield: 47%. Anal. Found: C, 49.32; H, 3.44; N,
17.07. Calcd for C30H26CdClN9O4: C, 49.74; H, 3.62; N,
17.40. IR (cmK1): 3138w, 2359m, 2040s, 1607m, 1570s,
1506w, 1438m, 1372w, 1245w, 1089s, 1028w, 777m,
723m, 705w, 692w, 623m.
N
N
N
L
Chart 1.
Caution: Metal perchlorate and azide compounds are
potentially explosive. Only a small amount of material
should be prepared and handled with caution. On the other
hand, cadmium compounds and their wastes are extremely
toxic and must be handled carefully.
purified by standard methods prior to use. Elemental
analyses were performed on a Perkin-Elemer 240C
analyzer and IR spectra on a Tensor 27 OPUS (Bruker)
FT-IR spectrometer with KBr pellets. 1H NMR spectra
were recorded on a Bruker AC-P500 spectrometer
(300 MHz) at 25 8C with tetramethylsilane as the internal
reference.
2.4. X-ray crystallographic studies
Single-crystal X-ray diffraction measurements for
complexes 1 and 2 were carried out on a Bruker Smart
1000 CCD diffractometer equipped with a graphite crystal
monochromator situated in the incident beam for data
collection at 293(2) K. The determinations of unit cell
parameters and data collections were performed with Mo–
2.2. Synthesis of ligand
3-(2-Pyridyl)pyrazol-1-ylmethylbenzene (L) A mixture
of benzyl chloride (1.67 g, 13.2 mmol), 3-(2-pyridyl)-
n
pyrazole (1.74 g, 12 mmol), benzene (52 mL), Bu4NOH
(0.5 cm3, 40% aqueous solution) and NaOH aqueous
solution (10 cm3, 10 M) was refluxed (ca. 80 8C) for ca.
48 h with vigorous stirring. After cooling to room
temperature, the organic phase was separated, washed
three times with H2O, and extracted with CHCl3. The
organic layer was collected and dried over anhydrous
MgSO4. 3-(2-Pyridyl)pyrazol-1-ylmethylbenzene (L) was
obtained as yellow solid after CHCl3 was removed with a
rotary evaporator and it was further purified by recrystalli-
zation from CHCl3/hexane (Yield: 60%). M.p.: 93–95 8C.
IR (cmK1): 3050w, 2935w, 1591m, 1488m, 1454m, 1399m,
1368w, 1349w, 1298w, 1285w, 1226s, 1202w, 1145m,
1111w, 1050m, 1040m, 991w, 914w, 895w, 848w, 796w,
761vs, 726s, 692w, 621w, 579w. 1H NMR (CDCl3): d 5.41
(s, 2H), 6.95 (d, 1H), 7.18–7.41 (m, 7H), 7.72 (t, 1H), 7.97
(d, 1H), 8.63 (d, 1H). Anal. Found: C, 76.11, H, 5.09, N,
17.50. Calcd for C15H13N3: C, 76.60; H, 5.53; N, 17.87.
˚
Ka radiation (lZ0.71073 A) and unit cell dimensions were
Table 1
Crystal data and structure refinement summary for complexes 1–2
1
2
Empirical formula
Formula weight
Crystal system
Space group
C32H26CdN8S2
699.13
C30H26CdClN9O4
724.45
Monoclinic
C2/c
Monoclinic
P2(1)/c
˚
Unit cell dimensions (A, 8)
a
B
c
14.833(3)
13.790(3)
15.970(3)
110.89(3)
3052(1)
4
13.622(4)
23.286(7)
10.547(3)
111.084(6)
3121(2)
b
3
˚
Volume (A )
Z
4
Dcalcd (g/cm3)
1.522
1.542
m (mmK1
)
0.889
0.836
2.3. Preparation of complexes 1 and 2
F (000)
1416
1464
Range of h, k, l
0/19, 0/17, K20/
19
K17/16, K23/29,
K13/11
18030/6429/3637
[Cd(L)2(SCN)2] (1) The reaction of L (47 mg, 0.2 mmol)
with Cd(ClO4)2$6H2O (42 mg, 0.1 mmol) and NH4SCN
(15 mg, 0.2 mmol) in ethanol (10 mL) and acetonitrile
(2 mL) for a few minutes afforded a yellow solid. Then, to
this mixture system 10 mL acetonitrile was added and
stirred for about 30 min until the solid was dissolved. The
solution was filtered, and the filtrate was kept at room
temperature. After several days single crystals suitable for
X-ray analysis were obtained by slow evaporation of the
solvent. Yield: 42%. Anal. Found: C, 54.63; H, 3.35; N,
Reflections collected/
unique/observed
3419/3419/2222
Max. & min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
0.8708 and 0.8422
3419/0/190
0.9063 and 0.8506
6429/0/407
0.952
1.061
Ra & Rw
0.0584 & 0.1469
1.190 & K0.749
0.0664 & 0.1178
0.718 & K0.736
b
Largest diff. Peak & hole
3
˚
(e/A )
a
RZSjjF0jKjFCjj=SjF0j.
b
2
2 1=2
wRZ½SwðF02 KFC2 Þ =SwðF02Þ ꢀ
.

X.-S. Shi et al. / Journal of Molecular Structure 754 (2005) 71–76
73
Fig. 1. (a) View of the molecular structure of 1 with 20% displacement ellipsoids probability (thiocyanate N atom have small atomic displacement parameters,
but the disordered mode is not successful for it or thiocyanate group) and (b) p–p stacking diagram of 1 (symmetry code A: Kx, y, KzK1/2; B: Kx, Ky,
K1Kz; BA: KxK1/2, 1/2Ky, K1Kz).
obtained with least-squares refinements. The program
SAINT [7] was used for integration of the diffraction
profiles. All structures were solved by direct methods using
the SHELXS program of the SHELXTL package and
refined with SHELXL [8]. All non-hydrogen atoms were
located in successive difference Fourier syntheses. The final
refinement was performed by full matrix least-squares
methods with anisotropic thermal parameters for non-
hydrogen atoms on F2. The hydrogen atoms were added
theoretically, riding on the concerned atoms and refined
with fixed thermal factors. Crystallographic data and
experimental details for structural analyses are summarized
in Table 1. CCDC-271942 and 271943 contain the
supplementary crystallographic data of complexes 1
and 2. These data can be obtained from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; Fax: C44 1223 336033; E-mail: deposit@
ccdc.cam.ac.uk.
3. Results and discussion
3.1. Synthesis and general characterizations
of complexes 1 and 2
Complex 1 was prepared by the reaction of Cd(ClO)4$
6H2O, 3-(2-pyridyl)pyrazol-1-ylmethylbenzene (L) and
NH4SCN in 1:2:2 molar ratio in acetonitrile/ethanol system,
while 2 was prepared by following an analogous procedure
to that of 1 except for using NaN3 instead of NH4SCN and in
1:2:1 molar ratio. The IR spectra of the two complexes show
absorption bands resulting from the skeletal vibrations of
the aromatic rings in 1600–1400 cmK1 region. In complex
1, the IR spectrum exhibits a strong and sharp band at
2076 cmK1, indicating the presence of an N-bound, terminal
thiocyanato ligand. The IR spectrum of 2 shows a very
strong band at 2040 cmK1 due to nas(N3), and this band is
very similar to some complexes containing m-1,3 bridging

74
X.-S. Shi et al. / Journal of Molecular Structure 754 (2005) 71–76
Table 2
˚
Selected bond lengths [A] and angles [8] for 1
related coordination complexes (see Table 2) [10]. In 1,
each L adopts typical N,N-bidentate chelating coordination
mode to form a five-membered Cd–N–C–C–N chelating
ring (Cd1–N1–C5–C6–N2). The thiocyanato anions adopt
N-terminal mode to coordinate to the Cd(II) center. The Cd–
N–C bond angle (C16–N4–Cd1) is significantly bent at
111.1(4)8 and the bond angle of N4–C16–S1 is 159.9(7)8.
Cd(1)–N(1)
Cd(1)–N(2)
Cd(1)–N(4)
2.367(4)
2.418(4)
2.567(4)
C(16)–N(4)
C(16)–S(1)
1.174(9)
1.597(8)
N(1A)–Cd(1)–N(1) 165.2(2)
N(1)–Cd(1)–N(2) 69.4(1)
N(1)–Cd(1)–N(2A) 100.6(1)
N(2)–Cd(1)–N(4)
N(1)–Cd(1)–N(4A)
N(2)–Cd(1)–N(4A)
160.2(1)
98.4(1)
82.7(1)
˚
The Cd1–N4 bond length [2.567(4) A] is a little longer than
˚
N(2)–Cd(1)–N(2A)
N(1)–Cd(1)–N(4)
99.0(2)
90.9(1)
N(4)–Cd(1)–N(4A) 102.3(2)
the other Cd–N bond lengths [i.e. Cd1–N1Z2.367(4) A,
˚
Cd1–N2Z2.418(4) A], which are still on the normal ranges
of distances for analogous terminal Cd–NCS complexes
Symmetry code: A Kx, y, KzK1/2.
˚
[11]. In addition, the bond lengths of C16–N4 [1.174(9) A]
˚
and C16–S1 [1.597(8) A] in the thiocyanate anions show the
azido groups [9]. In addition, the IR spectra of 2 also exhibit
the characteristic bands of perchlorate anions at 1089 cmK1
.
normal structure of the thiocyanato in complex 1 [12].
It is should be noted that, in the crystal structure of 1,
there are weak intermolecular p–p stacking interactions
(Fig. 1b) [13]. The dihedral angle between two benzene
rings of adjacent molecules is 0.038, the centroid–centroid
3.2. Description of the crystal structures
of complexes 1 and 2
[Cd(L)2(SCN)2] 1 The crystal structure of complex 1
consists of a neutral [Cd(L)2(SCN)2] molecule. The view of
the mononuclear entity in 1 with atomic labeling is given in
Fig. 1(a) and the selected bond lengths and angles are listed
in Table 2. Each Cd(II) center is six-coordinated to four N
donors from two distinct L ligands and two N donors
thiocyanato anions. The coordination geometry around the
Cd(II) center could be described as a distorted octahedron.
˚
separation is 4.0985 A and the perpendicular distance is
˚
3.805 A. The related values for two pyrazole rings are 08,
˚
3.925 and 3.360 A. Such two types of p–p stacking
interactions expend this mononuclear molecule to a 2D
supramolecular sheet.
{[Cd(L)2N3](ClO4)}n 2 The crystal structure of complex
2 consists of 1D {[Cd(L)2N3]C}n cation chains and free
ClOK4 anions. Fig. 2(a) shows the coordination environment
of Cd(II) center. The selected bond lengths and angles are
listed in Table 3. Similar to the complex 1, the coordination
˚
All the Cd–N bond lengths [from 2.367(4) to 2.567(4) A]
and the bond angles around each Cd(II) center [from
69.38(14) to 165.2(2)8] are in the normal range expected for
Fig. 2. (a) Coordination environment of Cd(II) in 2 with 20% displacement ellipsoids probability, (b) view of the 1D chains of 2, (c) view of two kinds of C–
H/O hydrogen bonds and (d) view of 3D supramolecular framework linked by C–H/O hydrogen bonds (symmetry code A: x, 3/2Ky, zK1/2; B: x, 3/2Ky,
zC1/2).

X.-S. Shi et al. / Journal of Molecular Structure 754 (2005) 71–76
75
Table 3
Selected bond lengths [A] and angles [8] for 2
M
˚
N
N
N
N
N
N
M
M
M
Cd(1)–N(1)
2.365(6)
2.385(5)
2.329(5)
70.1(2)
86.6(2)
87.7(2)
157.4(2)
108.0(2)
70.8(2)
101.8(2)
82.4(2)
Cd(1)–N(6)
2.412(5)
2.291(6)
2.301(6)
100.2(2)
163.8(2)
99.9(2)
cis
trans
Cd(1)–N(2)
Cd(1)–N(7)
Cd(1)–N(4)
Cd(1)–N(9A)
Scheme 2.
N(1)–Cd(1)–N(2)
N(1)–Cd(1)–N(6)
N(2)–Cd(1)–N(6)
N(4)–Cd(1)–N(1)
N(4)–Cd(1)–N(2)
N(4)–Cd(1)–N(6)
N(7)–Cd(1)–N(1)
N(7)–Cd(1)–N(2)
N(7)–Cd(1)–N(4)
N(7)–Cd(1)–N(6)
N(7)–Cd(1)–N(9B)
N(9B)–Cd(1)–N(1)
N(9B)–Cd(1)–N(2)
N(9B)–Cd(1)–N(4)
N(9B)–Cd(1)–N(6)
(EE or 1,3) and end-on (EO or 1,1). For the end-to-end
coordination, there are also trans or cis coordination
configurations (see Scheme 2) [16]. In 1, the SCNK adopts
terminal mode to coordinate to the CdII center, while NK3 in 2
acts as a bridge in an end-to-end (m-1,3) coordination mode,
and for each NK3 it is a trans coordination configuration.
In summary, two Cd(II) complexes with 3-(2-pyridyl)
pyrazole-based ligand, which have mononuclear and 1D
chain structures, have been synthesized. The structural
difference of the two complexes is attributed to the different
anions, which have different coordination methods.
92.3(2)
162.3(2)
89.0(2)
93.4(2)
Symmetry codes: A x, KyC3/2, zC1/2; B x, KyC3/2, zK1/2.
geometry around the Cd(II) center in 2 could also be
described as a distorted octahedron. All the Cd–N bond
˚
lengths [region from 2.291(6) to 2.412(5) A] are in the
normal range for analogous complexes [10], and the bond
angles around each Cd(II) center range from 70.1(2) to
157.38(19)8. In addition, the dihedral angles between the
pyridyl-pyrazole ring and benzene ring is 9.28 in the unit
around each Cd(II) center, and the centroid-centroid
˚
separation between them is 3.684 A, indicating the presence
of intramolecular p–p stacking interaction [13].
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 20373028).
It is interesting that in 2 the azido anion acts as a bridge to
link the Cd(II) ions in an end-to-end (m-1,3) coordination
mode to form a 1D chain (Fig. 2(b)), and the two
coordinated azido is in cis positions, but for bridging
azido it is a trans coordination mode. It is worthy to point
out that the bridging mode of azide adopting cis-M-(m-N3)-
M to coordinate to Cd(II) center is rare, although similar
bridging mode has been observed in the structures of other
metal azido complexes (i.e. [Ni(1,2-diamino-2-methyl-
propan)2(m-N3)]n(PF6)n [14], [Ni(aep)2(m-N3)]n(ClO4)n
[15]). Furthermore, the adjacent {[Cd(L)2N3]C}n chains
are linked through intermolecular C–H/O weak hydrogen-
bonding interactions into a 3D framework (Fig. 2c and d)
References
[1] For some recent reviews: S. Kitagawa, R. Kitaura, S. Noro, Angew.
Chem. Int. Ed. 43 (2004) 2334; M. Ruben, J. Rojo, F.J. Romero-
Salguero, L.H. Uppadine, J.M. Lehn, Angew. Chem. Int. Ed. 43
(2004) 3644.
[2] For examples R.G. Xiong, X.Z. You, B.F. Abrahams, Z. Xue,
C.M. Che, Angew. Chem. Int. Ed. 40 (2001) 4422; M. Pascu, F. Tuna,
E. Kolodziejczyk, G.I. Pascu, G. Clarkson, M.J. Hannon, Dalton
Trans. (2004) 1546; M.L. Tong, Y.M. Wu, J. Ru, X.M. Chen,
´
H.C. Chang, S. Kitagawa, Inorg. Chem. 41 (2002) 4846; R. Garcıa-
¨
Zarracino, H. Hopfl, J. Am. Chem. Soc. 127 (2005) 3121; D. Guo,
K.L. Pang, C.Y. Duan, C. He, Q.J. Meng, Inorg. Chem. 41 (2002)
5978.
˚
˚
[C(9A)/O(3B)Z3.3593 A, H(9AA)/O(3B)Z2.540 A,
C(9A)-H(9AA)/O(3B)Z147.18; C(8B)/O(3B)Z
[3] C. Kaes, A. Katz, M.W. Hosseini, Chem. Rev. 100 (2000) 3553;
B.H. Ye, M.L. Tong, X.M. Chen, Coord. Chem. Rev. 249 (2005)
545.
˚
˚
3.2331 A, H(8AB)/O(3B)Z2.332 A, C(8B)-H(8AB)/
O(3B)Z163.18].
Both SCNK and NK3 anions have mono-terminal and
bridging coordination modes, respectively (see Scheme 1)
[12,16]. But in more examples, the former prefers to mono-
terminal coordination, while the latter prefers to bridging
[16,17]. Furthermore, when the NK3 anion acts as bridging
ligand, there are two typical coordination modes: end-to-end
[4] J.S. Fleming, K.L.V. Mann, C.A. Carraz, E. Psillakis, J.C. Jeffery,
J.A. McCleverty, M.D. Ward, Angew. Chem. Int. Ed. 37 (1998) 1279;
Z.R. Bell, J.C. Jeffery, J.A. McCleverty, M.D. Ward, Angew. Chem.
Int. Ed. 41 (2002) 2515; D.A. McMorran, P.J. Steel, Chem. Commun.
(2002) 2120; M.H.W. Lam, D.Y.K. Lee, S.S.M. Chiu, K.W. Man,
W.T. Wong, Eur. J. Inorg. Chem. (2000) 1483.
[5] M.J. Hinner, M. Grosche, E. Herdtweck, W.R. Thiel, Z. Anorg. Allg.
Chem. 629 (2003) 2251; H. Zhang, C.S. Liu, X.H. Bu, M. Yang,
J. Inorg. Biochem. 99 (5) (2005) 1119.
[6] J.S. Fleming, K.L.V. Mann, S.M. Couchman, J.C. JeVery,
J.A. McCleverty, M.D. Ward, J. Chem. Soc., Dalton Trans. (1998)
2047.
M
M
M
M
M
M
SCN
SCN
SCN
M
M
[7] Bruker AXS, SAINT Software Reference Manual, Madison, WI,
1998.
M
M
NNN
NNN
[8] G.M. Sheldrick, SHELXTL NT Version 5.1. Program for Solution and
¨
Refinement of Crystal Structures, University of Gottingen, Germany,
Scheme 1.
1997.

76
X.-S. Shi et al. / Journal of Molecular Structure 754 (2005) 71–76
[9] M.A.S. Goher, F.A. Mautner, A.K. Hafez, M.A.M. Abu-Youssef,
C. Gspan, A.M.A. Badr, Polyhedron 22 (2003) 975.
[14] J. Ribas, M. Monfort, C. Diaz, C. Bastos, C. Mer, X. Solans, Inorg.
Chem. 34 (1995) 4986.
[15] J. Ribas, M. Monfort, B. Kumar-Gosh, R. Cortes, X. Solans, M. Font-
´
[10] A.G. Orpen, L. Brammer, F.H. Aleen, O. Kennard, D.G. Watson,
R. Taylor, J. Chem. Soc., Dalton Trans. (1989) S1.
´
Bardıa, Inorg. Chem. 35 (1996) 864.
´
[16] J. Ribas, A. Escuer, M. Monfort, R. Vicente, R. Cortes, L. Lezama,
[11] S.L. Li, Z.Z. Zhang, T.C.W. Mak, Inorg. Chim. Acta 268 (1998) 177.
[12] H. Zhang, X. Wang, K. Zhang, B.K. Teo, Coord. Chem. Rev. 183
(1999) 157.
T. Rojo, Coord. Chem. Rev. 193-195 (1999) 1027.
[17] D.L. Reger, T.D. Wright, M.D. Smith, A.L. Rheingold, S. Kassel,
T. Concolino, B. Rhagitan, Polyhedron 21 (2002) 1795.
[13] J. Christoph, J. Chem. Soc., Dalton Trans. (2000) 3885.