Half-sandwich η6-benzene Ru(II) complexes of pyridylpyrazole and pyridylimidazole ligands: Synthesis, spectra, and structure

Article abstract of DOI:10.1016/j.jorganchem.2006.04.037

New series of half-sandwich ruthenium(II) complexes supported by a group of bidentate pyridylpyrazole and pyridylimidazole ligands [(η⁶-C₆H₆)Ru(L²)Cl][PF₆] (1), [(η⁶-C₆H₆

Full text of DOI:10.1016/j.jorganchem.2006.04.037

Journal of Organometallic Chemistry 691 (2006) 3545–3555
www.elsevier.com/locate/jorganchem
Half-sandwich g6-benzene Ru(II) complexes of pyridylpyrazole and
pyridylimidazole ligands: Synthesis, spectra, and structure
*
Haritosh Mishra, Rabindranath Mukherjee
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
Received 10 March 2006; received in revised form 25 April 2006; accepted 27 April 2006
Available online 5 May 2006
Abstract
New series of half-sandwich ruthenium(II) complexes supported by a group of bidentate pyridylpyrazole and pyridylimidazole ligands
[(g6-C6H6)Ru(L2)Cl][PF6] (1), [(g6-C6H6)Ru(HL3)Cl][PF6] (2), [(g6-C6H6)Ru(L4)Cl][PF6] (3), and [(g6-C6H6)Ru(HL5)Cl][PF6] (4) [L2,
2-[3-(4-chlorophenyl)pyrazol-1-ylmethyl]pyridine; HL3, 3-(2-pyridyl)pyrazole; L4, 1-benzyl-[3-(20-pyridyl)]pyrazole; HL5, 2-(1-imidazol-2-
yl)pyridine] are reported. The molecular structures of 1–4 both in the solid state by X-ray crystallography and in solution using 1H NMR
spectroscopy have been elucidated. Further, the crystal packing in the complexes is stabilized by C–Hꢀ ꢀ ꢀX (X = Cl and p), N–Hꢀ ꢀ ꢀCl,
and p–p interactions.
Ó 2006 Elsevier B.V. All rights reserved.
Keywords: Ruthenium(II); Benzene; Pyridyl/pyrazole/imidazole ligands; Crystal structures; Non-covalent interactions
1. Introduction
donor ligands are known [6], there is no report in the liter-
ature, to our knowledge, of structurally characterized com-
plexes of pyridyl/pyrazole/imidazole-hybrid chelating
ligand systems containing two different heterocyclic N-
donor sites in a given bidentate ligand. From the stand-
point of synthetic chemistry we find it really challenging
and in this report we provide examples of such structural
motifs. In this study we have selected a uniformly varied
group of bidentate heterocyclic N-donor ligands. They
include: (i) a nonplanar pyridyl and a pyrazole unit of L2
separated by a CH2 spacer with a p-chlorophenyl ring
directly attached to the pyrazole moiety; (ii) a planar
directly attached pyridyl and pyrazole unit of HL3; (iii) a
planar directly attached pyridyl and pyrazole unit with
the pyrazole moiety having an appended benzyl group of
L4; and (iv) a planar directly attached pyridyl and imid-
azole unit of HL5. Notably, the two heterocyclic rings of
the ligands HL3, L4, and HL5 can communicate electroni-
cally; however, in the ligand L2 such a situation does not
exist.
Arene ruthenium compounds belong to a well-estab-
lished family of robust metal-organic molecules that played
an important role in the development of organometallic
chemistry [1]. Continued interest in such systems arises
due to their catalytic potential in a wide range of organic
reactions [2,3] and very promising anticancer activity of
[(g6-arene)Ru(en)Cl]+ (en = 1,2-diaminoethane) class of
complexes [4,5]. These findings provide the impetus for
the synthesis and properties of new {(g6-C6H6)Ru(L)-
Cl}+(L = bidentate N-donor ligand) complexes. Properties
of the complexes are determined in large measure by the
nature of ligands bound to the metal ion. For the com-
plexes of pertinence to this study the role of a chelating
ligand is to fine-tune the Ru–benzene bonding interaction.
Although several examples of structurally characterized
mononuclear three-legged half-sandwich organometallic
complexes having {(g6-C6H6)Ru}2+ unit supported by N-
*
The purpose of the present work is two-fold. First, dur-
ing our investigations into the chemistry of three-legged
Corresponding author. Tel.:/fax: +91 512 2597437/7436.
E-mail address: rnm@iitk.ac.in (R. Mukherjee).
0022-328X/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.04.037

3546
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
‘‘piano-stool’’ complexes [7] we prepared the complexes
[(g6-C6H6)Ru(L1)Cl][PF6] [L1 = 2-(pyrazol-1-ylmethyl)
2.2. Preparation of ligand
pyridine and its 3,5-dimethylpyrazole derivatives] [7a]. In
an attempt to widen this class of half-sandwich ruthe-
nium(II) complexes of ‘‘piano-stool geometry’’ we have
investigated the reactions of the dimer [{(g6-C6H6)-
RuCl(l-Cl)}2] with L2, HL3, L4, and HL5 and character-
ized the complexes [(g6-C6H6)Ru(L2)Cl][PF6] (1),
[(g6-C6H6)Ru(HL3)Cl][PF6] (2), [(g6-C6H6)Ru(L4)Cl][PF6]
(3), and [(g6-C6H6)Ru(HL5)Cl][PF6] (4). We wished to test
if the different electronic/steric environment provided by a
particular bidentate ligand has an impact on the relative
strength of Ru–benzene bonding.
2.2.1. 2-[3-(4-Chlorophenyl)pyrazol-1-ylmethyl]pyridine
(L2)
The starting material 3-(4-chlorophenyl)-1-pyrazole nec-
essary for the synthesis of L2 was prepared following a pro-
cedure similar to that used for the synthesis of HL3 [15]. A
solution of 1-(4-chlorophenyl)ethanone (2.0 g, 0.013 mol)
in N,N-dimethylformamide dimethylacetal (5 mL; 4.7 g,
0.039 mol) was refluxed for 10 h. After cooling to 25 °C,
the excess of solvent was removed under reduced pressure.
The resulting sticky solid was dried in vacuo and used in
next step without further purification. Yield: 1.64 g, ca.
70%. A mixture of resulting compound 1-(4-chlorophe-
nyl)but-2-en-1-one (1.64 g, 0.009 mol) and hydrazine
hydrate (4 mL) in EtOH (3 mL) was stirred at 60 °C for
30 min. After cooling to 25 °C, the reaction mixture was
poured into 20 g of ice which afforded a white precipitate.
It was filtered, washed with cold water several times and
dried in air. Recrystallization from chloroform/n-hexane
mixture afforded a white crystalline solid. Yield: 1.04 g,
NR
R
HN
N
N
N
N
N
N
N
1
ca. 65%. H NMR (80 MHz; CDCl3): 6.32 (1H, d, pyra-
zole H4), 7.30–7.58 (4H, m, benzene), 7.65 (1H, d, pyrazole
H5).
R = H (L1)
R = H (HL3)
HL5
= -C6H4-p-Cl (L2)
= -CH2-C6H5 (L4)
A mixture of 2-(chloromethyl)pyridine hydrochloride
(0.92 g, 5.62 mmol), 3-(4-chlorophenyl)-1-pyrazole (1.0 g,
5.62 mmol), benzene (60 mL), 40% aqueous NaOH
(8 mL), and 40% aqueous tetra-n-butylammonium hydrox-
ide (8 drops) was refluxed with stirring for 8 h and then
stirred at 25 °C for 12 h. The organic layer was then sepa-
rated, washed twice with brine water (100 mL), dried over
anhydrous MgSO4, and filtered. Solvent removal afforded
a thick yellowish white solid. Yield: 1.06 g, ca. 70%. The
ligand was further purified by recrystallization from chlo-
roform/n-hexane. 1H NMR (80 MHz; CDCl3): d 5.42
(2H, s, CH2), 6.54 (1H, d, pyrazole H4), 7.00–7.68 (7H,
m, pyridine H3,4,5 and phenyl H2,3,5,6), 7.80 (1H, d, pyra-
zole H5), 8.62 (1H, d, pyridine H6).
Secondly, existence of non-covalent interactions [8,9]
and particularly C–Hꢀ ꢀ ꢀCl interactions [10–13] have been
identified in organometallic molecules, including half-
sandwich complexes [(g6-C6H6)Ru(L)Cl]+ (L = two
monodentate or a bidentate neutral N-donor ligand)
[5a,6k], [(g6-C6H6)Ru(Me2HPz)Cl2] [6k], and [(g6-C6H6)-
Ru(L)(PPh3)Cl]+ (L = 1-(4-cyanophenyl)imidazole) [11d].
From this background and our own activity in this field
[14] we wished to identify primarily C–Hꢀ ꢀ ꢀCl hydrogen-
bonding interactions in the present group of half-sand-
wich ‘‘piano-stool’’ organometallic molecules. In this
report we investigate the use of the half-sandwich orga-
nometallic units {(g6-C6H6)Ru(L)Cl}+ present in 1–4 to
demonstrate a rich variety of non-covalent (C–Hꢀ ꢀ ꢀCl–
Ru, N–Hꢀ ꢀ ꢀCl–Ru hydrogen-bonding; C–Hꢀ ꢀ ꢀp and
p–p) interactions.
2.3. Preparation of complexes
2.3.1. General procedure
The ligand (0.4 mmol) was dissolved in MeOH (15 mL)
and to it was added solid [{(g6-C6H6)RuCl(l-Cl)}2]
(0.2 mmol). The mixture was stirred for 12 h (complexes 1,
2, and 3) or 6 h (complex 4) at 25 °C. The resulting yellow
(complexes 1, 2, and 4) or orange (complex 3) solution was
filtered and the volume of the filtrate was reduced (7 mL),
and to it was added solid NH4PF6 (0.4 mmol). The yellow
or orange microcrystalline solid that formed was filtered,
washed with cold MeOH, and dried in vacuo. Recrystalliza-
tion was achieved from hot MeOH solutions. X-ray quality
single crystals were obtained by diffusion of diethyl ether into
a solution (1 mL) of the compound in a mixture of MeOH
and MeCN (v/v, 1:4 for complex 1; v/v, 1:2 for complex 3;
v/v, 1:1 for complex 4) or in MeCN (complex 2).
2. Experimental
2.1. Materials
Reagent or analytical grade starting materials were
obtained from commercial suppliers and used without
further purification. The ligands 3-(2-pyridyl)pyrazole
(HL3) [15], 1-benzyl-[3-(20-pyridyl)pyrazole (L4) [16], and
2-(1-imidazol-2-yl)pyridine (HL5) [17] were synthesized
following the literature procedures. The dimer [{(g6-C6H6)-
RuCl(l-Cl)}2] was prepared following a reported procedure
[18].

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
3547
2.3.2. [(g6-C6H6)Ru(L2)Cl][PF6] (1)
2.4. Instrumentation
Yield: 0.125 g, 49.7%. Anal. Calc. C21H18Cl2F6N3PRu:
C, 40.06; H, 2.86; N, 6.67. Found: C, 40.14; H, 2.84; N,
6.69%. H NMR (CD3CN; 400 MHz; 25 °C): d 9.06 (d,
JHH = 5.8 Hz, 1H, H6 of py), 8.05 (d, JHH = 8.3 Hz,
Elemental analyses were obtained using Thermo Quest
EA 1110 CHNS-O, Italy. Conductivity measurements
were done with an Elico type CM-82T conductivity
bridge (Hyderabad, India). Spectroscopic measurements
were made using the following instruments: IR (KBr,
4000–600 cmꢁ1), Bruker Vector 22; electronic, Perkin–
Elmer Lambda 2 and Agilent 8453 diode-array spectro-
photometer. 1H NMR spectral measurements were
1
0
1H, H5 of pz), 7.99 (t, JHH = 8.0 Hz, 1H, H4 of py),
00
00
7.72 (m, 3H, H5 of py, H3 , H4 of phenyl), 7.56 (m,
00
00
3H, H3 of py and H2 , H5 of phenyl), 6.56 (d,
0
JHH = 7.1 Hz, 1H, H4 of pz), 5.72 (s, 6H, C6H6), 5.72
(1H, CH2–, overlaps with C6H6 resonance), 5.44 (d,
Jgem = 9.8 Hz, 1H, CH2–). Molar conductance, KM
(MeCN, 25 °C) = 145 Xꢁ1 cm2 molꢁ1 (expected 1:1 range:
120–160 Xꢁ1 cm2 molꢁ1) [19]. UV–VIS (in MeCN): k/nm
(e/dm3 molꢁ1 cmꢁ1) 230 (24900), 412 (450). IR (KBr,
cmꢁ1): 838 mðPFꢁ6 Þ.
performed on
a JEOL-JNM-LA-400 FT (400 MHz)
NMR spectrometer.
2.5. Crystal structure determination
Diffraction intensities were collected on a Bruker
SMART APEX CCD diffractometer at 100(2) K using
2.3.3. [(g6-C6H6)Ru(HL3)Cl][PF6] (2)
˚
graphite-monochromated Mo Ka (k = 0.710 69 A) radia-
Yield: 0.135 g, 66.9%. Anal. Calc. C14H13ClF6N3PRu:
C, 33.29; H, 2.58; N, 8.32. Found: C, 33.43; H, 2.62;
N, 8.46%. IR (KBr, cmꢁ1): 838 mðPFꢁ6 Þ. Molar conduc-
tion. Intensity data were corrected for Lorentz polarization
effects. Empirical absorption correction (SADABS) was
applied. The structures were solved by SIR-97, expanded
by Fourier-difference syntheses and refined with SHELXL-
97, incorporated in WINGX 1.64 crystallographic collective
package [20]. Hydrogen atoms were placed in idealized
positions, and treated using riding model approximation
with displacement parameters derived from those of the
atoms to which they were bonded. All non-hydrogen atoms
were refined with anisotropic thermal parameters by full-
matrix least-squares procedures on F2. A summary of the
data collection and structure refinement information is
provided in Table 1. Intermolecular contacts of the C–
Hꢀ ꢀ ꢀCl, N–Hꢀ ꢀ ꢀCl, p–p types and an intramolecular con-
tact of the C–Hꢀ ꢀ ꢀp type were examined with the DIAMOND
package [21]. C–H and N–H distances were normalized
along the same vectors to the neutron derived values of
tance, KM (MeCN, 25 °C) = 140 Xꢁ1 cm2 molꢁ1
.
1H
NMR (CD3CN; 400 MHz; 298 K): d 9.32 (d, JHH
=
5.6 Hz, 1H, H6 of py), 8.09–7.98 (m, 3H, H3 and H4 of
1
2
0
py and H5 of pz), 7.53 (dd, JHH ¼ 7:1 Hz, JHH ¼ 6:1
0
Hz, 1H, H5 of py), 7.02 (d, JHH = 2.7 Hz, 1H, H4 of
pz), 6.02 (s, 6H, C6H6). UV–VIS (in MeCN): k/nm
(e/dm3 molꢁ1 cmꢁ1) 260 sh (8350), 292 (10000), 320 sh
(4400), 407 (600).
2.3.4. [(g6-C6H6)Ru(L4)Cl][PF6] (3)
Yield: 0.100 g, 56.1%. Anal. Calc. C21H19ClF6N3PRu:
C, 42.42; H, 3.20; N, 7.07. Found: C, 42.45; H, 3.18; N,
7.12%. IR (KBr, cmꢁ1): 841 mðPFꢁ6 Þ. Molar conductance,
.
KM (MeCN, 25 °C) = 115 Xꢁ1 cm2 molꢁ1 1H NMR
(CD3CN; 400 MHz; 25 °C): d 9.28 (d, JHH = 5.6 Hz, 1H,
H6 of py), 8.09 (t, JHH = 7.8 Hz, 1H, H4 of py), 7.95 (d,
˚
˚
1.083 A and 1.009 A, respectively [12c].
0
JHH = 7.8 Hz, 1H, H5 of pz), 7.70 (d, JHH = 2.9 Hz, 1H,
3. Results and discussion
H3 of py), 7.57 (t, JHH = 5.8 Hz, 1H, H5 of py), 7.38–
00
00
7.50 (5H, H2 ꢁ6 of benzyl), 7.03 (d, JHH = 2.7 Hz, 1H,
3.1. Synthesis and characterization of complexes
0
H4 of pz), 6.01 (s, 6H, C6H6), 5.87 (d, Jgem = 15.9 Hz,
Reactions of chloro-bridged dimer [{(g6-C6H6)RuCl-
(l-Cl)}2] with L2, HL3, L4, and HL5 in MeOH followed
by subsequent treatment with NH4PF6 resulted in the iso-
lation of [(g6-C6H6)Ru(L2)Cl][PF6] (1), [(g6-C6H6)Ru-
(HL3)Cl][PF6] (2), [(g6-C6H6)Ru(L4)Cl][PF6] (3), and
[(g6-C6H6)Ru(HL5)Cl][PF6] (4), as microcrystalline solid.
The synthesis of compounds 1–4 passes through chloro
bridge-cleavage reactions of [{(g6-C6H6)RuCl(l-Cl)}2].
Characterization of the new compounds was accom-
plished by elemental analysis, solution electrical conductiv-
1H, NCH2–), 5.74 (d, Jgem = 15.2 Hz, 1H, NCH2–). UV–
VIS (in MeCN): k/nm (e/dm3 molꢁ1 cmꢁ1) 260 (12150),
300 (13450), 415 (550).
2.3.5. [(g6-C6H6)Ru(HL5)Cl][PF6] (4)
Yield: 0.135 g, 66.9%. Anal. Calc. C14H13ClF6N3PRu:
C, 33.29; H, 2.58; N, 8.32. Found: C, 33.43; H, 2.62; N,
8.46%. IR (KBr, cmꢁ1): 838 mðPFꢁ6 Þ. Molar conductance,
KM (MeCN, 298 K) = 120 Xꢁ1 cm2 molꢁ1
.
1H NMR
(CD3CN; 400 MHz; 25 °C): d 9.32 (d, JHH = 4.4 Hz, 1H,
1
0
H6 of py), 8.94 (d, JHH = 6.6 Hz, 1H, H4 of HIm), 7.93
ity, IR, and H NMR spectroscopy. Consistent with their
0
(d, JHH = 7.1 Hz, 1H, H5 of HIm), 7.67 (t, JHH = 2.5 Hz,
formulation, conductivity studies revealed that compounds
1–4 are 1:1 electrolyte [19]. As proof of their yellow to yel-
low orange color, compounds 1–4 exhibit absorption spec-
tral band in the 400–415 nm region, due to Clꢁ ! Ru(II)
charge-transfer transition.
1H, H4 of py), 7.54 (t, JHH = 2.6 Hz, 1H, H5 of py), 7.45
(t, JHH = 4.2 Hz, 1H, H3 of py), 5.94 (s, 6H, C6H6). UV–
VIS (in MeCN): k/nm (e/dm3 molꢁ1 cmꢁ1
) 270 sh
(11200), 304 (13450), 400 (650).

3548
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
Table 1
Data collection and structure refinement parameters for [(g6-C6H6)Ru(L2)Cl][PF6] (1), [(g6-C6H6)Ru(HL3)Cl][PF6] (2), [(g6-C6H6)Ru(L4)Cl][PF6] (3), and
[(g6-C6H6)Ru(HL5)Cl][PF6] (4)
1
2
3
4
Chemical formula
M
C21H18N3Cl2PF6Ru
629.32
C14H13N3ClPF6Ru
504.76
C21H19N3ClPF6Ru
594.88
C14H13N3ClPF6Ru
504.76
Crystal colour, habit
T (K)
Yellow, block
100(2)
Orange, block
100(2)
Orange, block
100(2)
Orange, block
100(2)
Crystal system
Space group
Triclinic
Monoclinic
P21/c (#14)
7.206(5)
21.640(5)
10.803(5)
90.0
102.110(5)
90.0
1647.1(14)
4
Triclinic
Monoclinic
P21/n (#14)
7.785(5)
8.385(5)
25.934(5)
90.0
92.326(5)
90.0
1691.5(15)
4
ꢀ
ꢀ
P1 ð#2Þ
P1 ð#2Þ
˚
a (A)
12.693(5)
12.784(5)
15.506(5)
79.616(5)
72.429(5)
69.404(5)
2238.0(14)
4
9.329(5)
10.208(5)
11.917(5)
108.051(5)
94.686(5)
93.945(5)
1070.1(9)
2
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A)
Z
dcalc (g cmꢁ3
l/mmꢁ1
)
1.868
1.077
2.036
1.279
1.846
1.000
1.982
1.245
F(000)
1248
992
592
992
Number of reflections collected
Number of independent reflections [Rint
Number of reflections used [I > 2r(I)]
GOF on F2
Final R indices [I > 2r(I)]a,b
Final R indices (all data)
14924
10495 [0.0192]
8924
1.119
0.0450, 0.1060
0.0549, 0.1154
10996
4069 [0.0239]
3838
1.074
0.0241, 0.0583
0.0262, 0.0595
7082
5014 [0.0155]
4590
1.272
0.0351, 0.0756
0.0466, 0.1086
10977
4186 [0.0485]
3487
1.142
0.0688, 0.1436
0.0855, 0.1540
]
P
P
a
R1 = (jFoj ꢁ jFcj)/ jFoj.
P
P
b
2
2 2
2 2
wR2 = { [w(jFoj ꢁ jFcj ) ]/ [w(jFoj ) ]}1/2
.
3.2. Molecular structures of [(g6-C6H6)Ru(L2)Cl][PF6]
(1), [(g6-C6H6)Ru(HL3)Cl][PF6] (2), [(g6-C6H6)-
Ru(L4)Cl][PF6] (3), and [(g6-C6H6)Ru(HL5)Cl]-
[PF6] (4)
were undertaken. Structural analysis of 1 revealed that
the asymmetric unit contains two crystallographically inde-
pendent molecules. Both molecules have essentially identi-
cal coordination geometry, but the corresponding bond
lengths and bond angles are slightly different. The data
for the molecule, which is not shown, are in brackets (Table
2). X-ray crystallographic analyses confirm the structures
In order to confirm the identity of the chloride-bound
structures of the cations in 1–4 crystal structure analyses
Table 2
6
2
6
3
6
4
6
˚
Selected bond lengths (A) and angles (°) in [(g -C6H6)Ru(L )Cl][PF6] (1), [(g -C6H6)Ru(HL )Cl][PF6] (2), [(g -C6H6)Ru(L )Cl][PF6] (3), and [(g -
C6H6)Ru(HL5)Cl]-[PF6] (4)
1a
2
3
4
Ru(1)–N(1)
Ru(1)–N(3)
Ru(1)–Cl(1)
Ru(1)–C(1)
Ru(1)–C(2)
Ru(1)–C(3)
Ru(1)–C(4)
Ru(1)–C(5)
Ru(1)–C(6)
C(1)–C(2)
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(1)–C(6)
2.117(4) [2.121(4)]
2.1095(17)
2.0511(17)
2.4170(13)
2.200(2)
2.186(2)
2.168(2)
2.175(2)
2.165(2)
2.213(2)
1.415(3)
1.410(3)
1.422(3)
1.409(3)
1.427(3)
1.396(3)
2.118(4)
2.107(3)
2.4155(13)
2.191(4)
2.184(4)
2.200(4)
2.190(4)
2.192(4)
2.188(4)
1.392(6)
1.432(6)
1.394(6)
1.422(6)
1.406(6)
1.421(6)
2.123(5)
2.072(5)
2.4249(16)
2.195(8)
2.180(7)
2.167(6)
2.176(6)
2.151(7)
2.193(7)
1.429(11)
1.403(11)
1.415(11)
1.401(11)
1.403(13)
1.385(12)
2.111(4) [2.109(4)]
2.4000(12) [2.3968(12)]
2.182(4) [2.177(4)]
2.195(4) [2.215(4)]
2.190(4) [2.176(4)]
2.199(4) [2.200(4)]
2.194(4) [2.185(5)]
2.194(4) [2.191(4)]
1.399(6) [1.408(6)]
1.424(6) [1.427(6)]
1.400(6) [1.407(7)]
1.424(6) [1.406(7)]
1.396(6) [1.400(6)]
1.427(6) [1.424(6)]
N(1)–Ru–N(3)
N(1)–Ru–Cl(1)
N(3)–Ru–Cl(1)
84.70(16) [83.85(15)]
82.66(11) [84.71(11)]
88.44(11) [86.49(11)]
75.62(7)
85.51(6)
83.90(6)
75.83(14)
86.44(11)
84.39(9)
76.3(2)
85.87(14)
85.31(14)
a
The data are for two molecules.

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
3549
of the compounds 1–4 (Figs. 1 and 2, Table 2). The cations
exhibit the expected and usual pseudo-octahedral half-
sandwich ‘‘piano-stool’’ disposition around the Ru atom
[6,7a], with the benzene ligand occupying one face of the
octahedron (the ruthenium atom is p bonded to the g6-
C6H6 group) and the coordination of a bidentate heterocy-
clic N-donor ligand [N(1) of pyridyl ring and N(3) of other
heterocyclic ring of pyrazole/imidazole] and a chloride ion
on the other face. The N–Ru–N angles have values of
84.70(16)° [83.85(15)°] (1), 75.62(7)° (2), 75.83(14)° (3),
and 76.3(2)° (4), deviated from 90° as per demand of the
bite of the ligand. The bite angle is ꢂ76° when two hetero-
cyclic units, a six-membered and a five-membered, are
directly attached and the angle is in the range 83–85°, when
the two rings are separated by a methylene spacer.
In the case of 1 the pyridyl and pyrazole rings of L2 are
each planar; however, the pyridyl mean plane is tilted to
adjacent pyrazole ring at an angle of 51.62(7)° [61.68(7)°],
attesting its nonplanarity [22,23]. In addition, the pyrazole
ring and p-chlorophenyl group in L2 make an angle of
76.86(12)° [63.93(12)°]. The pyridyl and pyrazole/imidazole
Fig. 2. Thermal ellipsoid plots of: (a) [(g6-C6H6)Ru(L4)Cl]+ in 3 and (b)
[(g6-C6H6)Ru(HL5)Cl]+ in 4 at 30% probability level. Hydrogen atoms
have been omitted for clarity.
rings of HL3 in 2 or of L4 in 3 or of HL5 in 4 are each pla-
nar and also planar to each other [the angle between the
two heterocyclic rings: 3.60(11)° (2); 10.61(15)° (3) (the pyr-
azole ring and phenyl group make an angle of 89.47(15)°);
1.64(6)° for (4)].
For complexes 1–4 the observed trend in Ru–C, Ru–
N(py), Ru–N(pz), Ru–N(im) or Ru–Cl distances, reflecting
mutual trans influence, is a consequence of interplay
between steric and electronic factors associated with the
coordinating ability of bidentate ligands L2, HL3, L4 or
HL5, in a closely similar metal coordination environment.
Interestingly, X-ray structural analyses revealed notice-
able differences in the bonding characteristics (metric
parameters) of bidentate ligands L2, HL3, L4 and HL5 as
well as the characteristics of p-bonded benzene rings in
these cations (Tables 2 and 3). From a careful look at the
metric parameters of Table 3, which lists pertinent bonding
parameters for the complexes 1–4, the following generaliza-
tions emerge. (i) Within the complexes 1–3, the pyrazole
binds stronger than pyridine and the pyrazole ring directly
attached to pyridine (HL3 and L4) binds stronger than the
pyrazole ring attached to pyridine with a methylene spacer
(L2). (ii) Between the complexes 2 and 3, the presence of
Fig. 1. Thermal ellipsoid plots of: (a) [(g6-C6H6)Ru(L2)Cl]+ in 2 and (b)
[(g6-C6H6)Ru(HL3)Cl]+ in 3 at 30% probability level. Hydrogen atoms
have been omitted for clarity.

3550
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
Table 3
6
2
6
3
6
4
6
˚
Summary of relevant bond distances (A) in [(g -C6H6)Ru(L )Cl][PF6] (1), [(g -C6H6)Ru(HL )Cl][PF6] (2), [(g -C6H6)Ru(L )Cl][PF6] (3), and [(g -
C6H6)Ru(HL5)Cl]-[PF6] (4)
1a
2
3
4
Av Ru–C
Ru–C6H6 centroid
Av C–C
Ru–N(py)
Ru–N(Im)
Ru–N(pz)
Ru–Cl
2.192(4) [2.190 (4)]a
1.677 (1.675)a
2.184(2)
1.666
1.413(3)
2.1095(17)
–
2.190(4)
1.676
1.411(6)
2.118(4)
–
2.177(7)
1.662
1.406(12)
2.123(5)
2.072(5)
–
1.411(7) [1.412(7)]a
2.117(4) [2.121(4)]a
–
2.111(4) [2.109(4)]a
2.4000(12) [2.3968(12)]a
2.0511(17)
2.4170(13)
2.107(3)
2.415(13)
2.4249(16)
a
The data are for two molecules.
steric effect is revealed in L4 over HL3. (iii) Between the
complexes 2 and 4, pyrazole (HL3) binds stronger than
imidazole (HL5). (iv) Within the present group of com-
plexes 1–4, both pyridine and pyrazole in the ligand HL3
bind most effectively in 2 and the least effective binding
of pyridine and pyrazole are observed by the ligands L4
and L2, respectively.
tate ligand L1, L2, HL3, L4 or HL5. The proton resonances
were assigned based on the available H NMR spectral
1
results for the free ligands L2 (this work), HL3 [15], L4
[16], and HL5 [17] and those for closely similar compounds
[7a]. The following comments regarding the spectral data
are in order. (i) The chemical shift values for coordinated
benzene in chloro complexes 1, 2, 3, and 4 are d 5.72, d
6.02, d 6.01 and d 5.94, respectively. Maximum upfield shift
(d 5.72) for 1 implies the presence of more electron density
on the benzene ring. It is clearly reflected in its av Ru-C dis-
tance (Table 3), implying poor interaction of C6H6 ligand
with Ru(II) (cf. X-ray structure). In the case of [(g6-
C6H6)Ru(Me2Hpz)Cl2] (Me2Hpz = 3,5-dimethylpyrazole)
a similar high field (d 5.76) shift of coordinated benzene
was reported [6k]. For [(g6-C6H6)Ru(L1)Cl][PF6] the chem-
ical shift value of d 5.94 [7a], corroborates better coordina-
tion of C6H6 ligand than that in 1. An AB quartet for CH2
protons of L2 in 1 confirms the presence of two diastereo-
topic protons, axial and equatorial. It implies that these
protons are not interconverting on the NMR time-scale,
otherwise a singlet would have resulted.
The Ru–N(py) and Ru–N(pz) distances in 2 are uni-
˚
˚
formly shorter than those in 1 by ꢂ0.007 A and ꢂ0.06 A,
respectively (Table 3). Thus the pyrazole ring of HL3 binds
strongly to ruthenium(II) in 2 compared to that of pyrazole
ring of L2 in 1. This trend must be associated with the pla-
narity of HL3 for a better metal-ligand orbital overlap than
that in the case with nonplanar L2. A general trend of pyr-
azole coordinating better than pyridine, as observed in 1–3
was observed before in trans-[Co(L1)2Cl2] [23], [Zn(L1)Cl2]
[14a], [Fe2(HL3)4(l-O)(OSO3)2] Æ 2MeOH Æ 3H2O [24],
[Cu(L4)(MeCN)2][ClO4] [16], and [{Cu(L4)(dmf)(l-O2C-
Me)}2][ClO4]2 Æ dmf Æ 0.5MeCO2H [16]. In 4 the observed
trend of imidazole binding Ru(II) stronger than pyridine
follows a similar trend. The shortest Ru–C6H6 centroid dis-
tance in 2 clearly reveals that amongst pyridylpyrazole
ligands present in 1–3 the ligand HL3 provides maximum
relative strength of {(g6-C6H6)Ru}2+ unit.
1
In essence, the H NMR results of 1–4 clearly indicates
that the solid state structures (vide supra) are retained in
solution.
Average Ru–C distances in 1–4 (Table 3) are compara-
ble to that reported in similar three-legged piano-stool
complexes including {(g6-C6H6)RuCl}+ moiety [6,7a].
The Ru–N(py) bond lengths observed in 1–4 and Ru–
N(pz) bond lengths observed in 1–3 compare well with
the values found in similar three-legged piano-stool com-
plexes including {(g6-C6H6)RuCl}+ moiety, with N-donor
ligands [6a,6i,6k,7a]. The Ru-Cl distances are comparable
to that reported in the literature for closely similar com-
plexes [6f,6i,6k,6l,25].
3.4. Non-covalent interactions
A closer inspection of the crystal packing diagrams of 1–
4 reveals that these organometallic molecules are engaged
in a number of secondary interactions (see below). Rele-
vant bond distances, bond angles, and symmetry are sum-
marized in Table 4. The C–Hꢀ ꢀ ꢀCl hydrogen-bonding
˚
parameters observed in this work [2.588–2.712 A and
˚
˚
137.2–152.8° (1); 2.715 A and 127.9° (2); 2.718–2.731 A
˚
and 147.7–162.2° (3); 2.414 A and 169.2°(4)] are in good
1
3.3. H NMR spectroscopy
agreement with literature tabulations (C–Hꢀ ꢀ ꢀCl: 2.569–
˚
2.944 A and 119.3–169.2°) [10a], literature precedents
1H NMR spectroscopy provides easy means of charac-
terization of these half-sandwich compounds. The data
(in CD3CN) along with their assignments are recorded in
Section 2, supporting their expected ‘‘piano-stool’’ struc-
ture. The spectral feature of compounds 1–4 displayed in
Figs. S1–S4 (Supplementary material) is consistent with
the presence of a coordinated benzene ligand and a biden-
[5a,6k,10–13] including our own findings [14]. These can
˚
be classified as intermediate contacts (2.41–2.73 A), which
are appreciably shorter than the sum of the van der Waals
radii for the H and the neutral Cl atoms (2.95 A) [10b,12c].
˚
˚
The observed short C–Hꢀ ꢀ ꢀCl distance in 4 (2.414 A)
implies a strong imidazole C(4)–Hꢀ ꢀ ꢀCl hydrogen-bonding
interaction.

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
3551
Table 4
gen-bonding interaction (Fig. 3) [14a]. The dimeric units
in turn are involved in C–Hꢀ ꢀ ꢀCl contacts with molecules
in the same layer via C–H of pyridyl and Ru-coordinated
Clꢁ ion, resulting in the formation of a one-dimensional
hydrogen-bonded chain (Fig. 3). Non-covalent interac-
tions involving metal-bound HL3 has been reported
recently [27].
Hydrogen-bonding (C/N–Hꢀ ꢀ ꢀCl/N) parameters for [(g6-C6H6)Ru(L2)-
Cl]+ in (1), [(g6-C6H6)Ru(HL3)Cl]+ in (2), [(g6-C6H6)Ru(L4)Cl]+ in (3),
[(g6-C6H6)Ru(HL5)Cl]+ in (4)
˚
˚
D–Hꢀ ꢀ ꢀA
Hꢀ ꢀ ꢀA (A)
Dꢀ ꢀ ꢀA (A)
D–Hꢀ ꢀ ꢀA
[(g6-C6H6)Ru(L2)Cl]+ unit in 1
C4–H4ꢀ ꢀ ꢀCl1
2.588
2.712
3.4572(10)
3.7030(9)
137.2°i
152.8°i
C6–H6ꢀ ꢀ ꢀCl2
[(g6-C6H6)Ru(HL3)Cl]+ unit in 2
3.4.2. [(g6-C6H6)Ru(L4)Cl][PF6] (3)
N2–H1Nꢀ ꢀ ꢀCl
2.231ii
2.715iii
3.1747(8)ii
3.4881(7)iii
155.0°ii
127.9°iii
Intermolecular C–Hꢀ ꢀ ꢀCl contacts between Ru-coordi-
nated Clꢁ ion and C–H of both pyridine and pyrazole
rings are present. The C–Hꢀ ꢀ ꢀCl interaction involving
C–H of pyrazole and Ru-coordinated Clꢁ ion leads to
the formation of dimeric motifs, via self-complementary
hydrogen-bonding interactions. Such dimeric units are
involved in additional C–Hꢀ ꢀ ꢀCl contacts with molecules
in the same layer via C–H of pyridyl and the Clꢁ ion,
affording tetrameric units (Fig. 4(a)). In essence, Ru-coor-
dinated Clꢁ ion is engaged in bifurcated hydrogen-bond-
ing interactions [12b,13a,14], leading to the formation of
a one-dimensional hydrogen-bonded chain (Fig. 4(a)).
These one-dimensional chains are involved in two types
of p–p interactions [28]. One such interaction is involving
coordinated C6H6 rings of neighbouring molecules of
adjacent chains (Fig. 4(b)). The centroid–centroid dis-
C9–H9ꢀ ꢀ ꢀCl
[(g6-C6H6)Ru(L4)Cl]+ unit in 3
C7–H7ꢀ ꢀ ꢀCl
2.731iv
2.718v
3.7773(13)iv
3.6805(13)v
162.2°iv
147.7°v
C14–H14ꢀ ꢀ ꢀCl
[(g6-C6H6)Ru(HL5)Cl]+ unit in 4
N2–H1Nꢀ ꢀ ꢀCl
2.244vi
2.414vii
3.2053(7)vi
3.4534(19)vii
158.5°vi
C13–H13ꢀ ꢀ ꢀCl
169.2°viii
i
2 ꢁ x, 1 ꢁ y, ꢁz.
ii
2 ꢁ x, ꢁy, 1 ꢁ z.
iii
x, 0.5 ꢁ y, 0.
iv
1 ꢁ x, ꢁy, 2 ꢁ z.
v
1 ꢁ x, ꢁy, 1 ꢁ z.
vi
ꢁx, ꢁy, 2 ꢁ z.
vii
1 + x, y, z.
viii
ꢁ1 + x, y, z.
3.4.1. [(g6-C6H6)Ru(L2)Cl][PF6] (1) and [(g6-C6H6)-
Ru(HL3)Cl][PF6] (2)
tance between the two benzene rings is 3.732 A, with a
˚
˚
perpendicular distance between the rings of 3.289 A and
In 1, two intermolecular C–Hꢀ ꢀ ꢀCl hydrogen bonding
interactions [5a,6k,10–14] linking two neighboring mole-
cules: (i) the Ru-coordinated Clꢁ ion Cl(1) and C–H(4)
of C6H6 ring; and (ii) the organic chlorine atom Cl(2) pres-
ent in p-chlorophenyl group of L2 and C–H(6) of C6H6 ring
are present. Such an interaction leads to the formation of a
discrete dimeric unit (Fig. S5, Supplementary material).
In 2, intermolecular N–Hꢀ ꢀ ꢀCl contacts [26] between
Ru-coordinated Clꢁ ion and N–H of pyrazole are pres-
ent. This is an example of self-complementary hydro-
displacement angle (measured by the angle between the
benzene ring normal and the centroid-centroid vector),
the dihedral angle between the planes is 0.0° (dihedral
angle for ideal stacked geometry = 0°), b = 28.21°, indi-
cating strong parallel displaced p–p stacking interaction
between the rings [28–30]. The other one is involving phe-
nyl rings of benzyl groups of neighboring molecules of
adjacent chains (Fig. 4(b)). The centroid–centroid dis-
˚
tance between the two phenyl rings is 3.685 A, with a
˚
perpendicular distance between the rings of 3.230 A, the
Fig. 3. View of the formation of the dimer and 1-D chain through C–Hꢀ ꢀ ꢀCl hydrogen bonding in [(g6-C6H6)Ru(HL3)Cl]+ unit in 2. All the hydrogen
atoms except those involved in hydrogen bonding have been omitted for clarity.

3552
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
Fig. 4. (a) A perspective view of the formation of a tetramer through C–Hꢀ ꢀ ꢀCl hydrogen bonding of [(g6-C6H6)Ru(L4)(Cl)]+ unit in 3. (b) A view of the
dimerization of tetramer through p–p stacking of: (i) phenyl (L4)-phenyl (L4); and (ii) (g6-C6H6)Ru–(g6-C6H6)Ru. (c) A view of C–Hꢀ ꢀ ꢀp interaction
involving C6H6 ring with the dangling phenyl arm of L4. All the hydrogen atoms except those involved in hydrogen bonding have been omitted for clarity.

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
3553
dihedral angle between the planes is 0.0° and displacement
angle, b = 28.78°, indicating strong parallel displaced p–p
stacking interaction between the rings [28–30]. These p–p
stacking interactions eventually lead to the formation
of a two-dimensional network (Fig. 5). In addition to
aforementioned intermolecular interactions, 3 is engaged
in intramolecular C–Hꢀ ꢀ ꢀp interactions [8a,29,30] involving
C–H of C6H6 with the dangling phenyl ring of benzyl
group of L4 [C2–H2ꢀ ꢀ ꢀCt (phenyl ring centroid) distance:
3.4.4. Rationalization of observed non-covalent interactions
In the present group of organometallic molecules, there
are four distinct types of C–Hꢀ ꢀ ꢀCl interactions between
the metal-bound chloride ion (acceptors) and the C–H
(donors) on the same or an adjacent molecule (vide supra).
The identified donors are the C–H of coordinated C6H6,
the H4 or H6 atom of a pyridine ring, the H5 atom of a pyr-
azole ring, and the NH groups of non-coordinated nitrogens
of pyrazole and imidazole. Careful analysis of the non-cova-
lent interactions/supramolecular architectures noticed here
(Figs. 3, 4, 6, and Fig. S5), along with the above observations,
led us topresent the followinghypotheses. (1) From the point
of view of charge density on the coordinated benzene rings,
which is tuned by the extent of donation of electron density
to the Ru(II) ion, the rings are deactivated. This will cause
the C–H groups ideally suited to take part in C–Hꢀ ꢀ ꢀCl
hydrogen bonding interactions. This has been observed in
this work. (2) Considering charge distribution in the pyridine
ring(s) present in ligands L2, HL3, L4, and HL5, which is
tuned by the withdrawal of electrons from the ring-carbon
atoms towards the nitrogen atom, and donation of electron
density to the metal ion, the rings are deactivated. This will
cause the positions 4 and 6 to be electron-deficient, and the
C–H groups would thereby be ideally suited to take part in
C–Hꢀ ꢀ ꢀCl hydrogen bonding interactions [14a]. This has
been identified in this work. 3) For pyrazole groups present
in L2, HL3, and L4 the electron-deficient sites are 3- and
˚
˚
2.798 A; C2ꢀ ꢀ ꢀCt distance: 3.641 A; C–Hꢀ ꢀ ꢀCt angle,
h = 134° (Fig. 4(c)) [8a,28–30].
3.4.3. [(g6-C6H6)Ru(HL5)Cl][PF6] (4)
The packing diagram of 4 reveals intermolecular C–
Hꢀ ꢀ ꢀCl contacts between Ru-coordinated Clꢁ ion and C–
H of imidazole ring and additional N–Hꢀ ꢀ ꢀCl contacts
[26] with a neighboring dimer in the same layer generating
tetrameric units (Fig. 6(a)). To our knowledge, this is the
first time that such tetrameric units have been identified,
solely via a combination of C–Hꢀ ꢀ ꢀCl and N–Hꢀ ꢀ ꢀCl
hydrogen-bonding interactions. (ii) Parallel displaced p–p
stacking interaction between the pyridyl/imidazolyl rings
of HL5 (Fig. 6(b)) (centroid–centroid distance: 3.876 A;
˚
perpendicular distance between two pyridyl-imidazolyl
˚
planes: 3.243 A; the dihedral angle between the planes is
0.0°; displacement angle, b = 33.19°) have also been real-
ized [28–30].
Fig. 5. Three horizontal chains of [(g6-C6H6)Ru(L4)(Cl)]+ unit in 3 formed by C–Hꢀ ꢀ ꢀCl hydrogen-bonding interactions; three vertical chains are formed
by p–p stacking of phenyl groups of L4.

3554
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 691 (2006) 3545–3555
4. Conclusions
Despite several examples of structurally characterized
mononuclear three-legged half-sandwich complexes of type
{(g6-C6H6)Ru(L)Cl}+ (L = neutral bidentate heterocyclic
N-donor hybrid ligands), few systematic studies have been
made to design this class of half-sandwich complexes with
the critical roles that the ancillary ligands play in fine-tun-
ing structure-bonding properties of these organometallic
molecules. In this work particular attention has been
placed to bidentate N-donor ligands with two different het-
erocyclic rings. The ligands used in this work carry sites
suitable for linking molecules preferentially through non-
covalent interactions. In the course of the studies on these
piano-stool complexes we were able to discover that Ru-
coordinated benzene rings and the bidentate ligands chosen
here had a pronounced tendency to participate in non-
covalent interactions (C–H ꢀ ꢀ ꢀ Cl, N–Hꢀ ꢀ ꢀCl, C–Hꢀ ꢀ ꢀp
and p–p) in the solid state. The present study thus provides
further information pertinent to a better understanding of
the non-covalent interactions in the structure directing
organometallic units [(g6-C6H6)Ru(L)Cl]+.
Acknowledgements
This work is supported by grants from the Department
of Science and Technology, Government of India and the
Council of Scientific and Industrial Research, New Delhi.
H.M. gratefully acknowledges University Grants Commis-
sion (UGC), New Delhi for a Junior Research Fellowship.
We thank Vibha Mishra for the synthesis of HL5. R.M.
gratefully acknowledges the award of a joint collaborative
grant with Prof. Siegfried Schindler funded by the Volks-
wagen Foundation, Germany and the donation of a HP
8453 diode-array spectrophotometer. Comments of the
reviewers at the revision stage are highly appreciated.
Fig. 6. (a) View of the tetramer formed via C–Hꢀ ꢀ ꢀCl and N–Hꢀ ꢀ ꢀCl
hydrogen-bonding of [(g6-C6H6)Ru(HL5)Cl]+ unit in 4. (b) View of the
dimer of [(g6-C6H6)Ru(HL5)Cl]+ unit in 4, pyridyl-imidazole/p–p stack-
ing. All the hydrogen atoms except those involved in hydrogen bonding
have been omitted for clarity.
5-positions [14a]. Therefore, involvement of pyrazole 5H
proton in C–Hꢀ ꢀ ꢀCl hydrogen bonding interaction observed
here justifies our hypothesis. (4) The pyrazole NH and imid-
azole NH protons in coordinated HL3 and HL5 are expected
to be acidic and therefore be ideally suited to participate in
N–Hꢀ ꢀ ꢀCl hydrogen bonding interactions. In fact, such an
expectation has been realized here. (5) We believe that the
C–Hꢀ ꢀ ꢀp and p–p stacking interactions observed here are
due to the flexible benzyl group present in the ligand L4.
The only structurally characterized closely similar
complex from the corresponding 2,20-bipyridine (bipy)
family is [(g6-C6H6)Ru(bipy)(MeCN)][CF3SO3]2 [6l].
Understandably, this complex is not capable of showing
C–Hꢀ ꢀ ꢀCl interactions. We examined the X-ray structure
of [(g6-C6H6)Ru(bipy)(CH3CN)][CF3SO3]2 [6l] but no
other intermolecular non-covalent interactions are found.
Examination of the X-ray structure of [(g6 ꢁ p-MeC6H4-
CHMe2)Ru(pyz)2Cl[PF6] (pyz = pyrazine) [25a] however
Appendix A. Supplementary material
Crystallographic data for the structures included in this
paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as Supplementary Publication No.
CCDC reference numbers 290119 (1), 290120 (2), 290121
(3), and 290122 (4). Copies of the data can be obtained
free of charge on application to CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK, fax: +44 1223 336 033, e-mail:
deposit@ccdc.cam.ac.uk, www: http://www.ccdc.cam.ac.uk.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.
2006.04.037.
˚
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We are inclined to believe that the presence of both a
five-membered and a six-membered hetrocyclic rings in
the chosen ligands has contributed to the observation of
noteworthy non-covalent interactions.
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