Half-sandwich η6-benzene Ru(II) complexes of phenolate-based pyridylalkylamine/alkylamine ligands: Synthesis, structure, and stabilization of one-electron oxidized species

Article abstract of DOI:10.1016/j.jorganchem.2007.03.041

Four half-sandwich ruthenium(II) complexes [(η⁶-C₆H₆)Ru(L¹-O)][PF₆] (1), [(η⁶-C₆H₆)Ru(L²-O)][PF₆] (2), [(η⁶-C₆H₆)Ru(L³-O)][PF₆] (3), [(η⁶-C₆H₆)Ru(L⁴-O)][PF₆] (4a), and [(η⁶-C₆H₆)Ru(L⁴-O)][BPh₄] (4b) [L¹-OH, 4-nitro-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L²-OH, 2,4-di-tert-butyl-6-{[(2′-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L³-OH, 2,4-di-tert-butyl-6-{[2′-((pyridin-2-yl)benzylamino)methyl}-phenol; L⁴-OH, 2,4-di-tert-butyl-6-{[(2′-imethylaminoethyl)methylamino]methyl}-phenol (L⁴-OH)], supported by a systematically varied series of tridentate phenolate-based pyridylalkylamine and alkylamine ligands are reported. The molecular structures of 1-3, 4a, and 4b have been elucidated in solution using ¹H NMR spectroscopy and of 1, 3, and 4b in the solid state by X-ray crystallography. Notably, due to coordination by the ligands the Ru center assumes a chiral center and in turn the central amine nitrogen also becomes chiral. The ¹H NMR spectra exhibit only one set of signals, suggesting that the reaction is completely diastereoselective [1: S_Ru,S_N/R_Ru,R_N; 2: R_Ru,R_N/S_Ru,S_N; 3: S_Ru,R_N/R_Ru,S_N; 4b: S_Ru,R_N/R_Ru,S_N]. The crystal packing in 1 and 3 is stabilized by C-H^...O interactions, in 4b no meaningful secondary interactions are observed. From the standpoint of generating phenoxyl radical, as investigated by cyclic voltammetry (CV), complex 1 is redox-inactive in MeCN solution. However, 2, 3, and 4a generate a one-electron oxidized phenoxyl radical coordinated species [2]^{2+{radical dot}}, [3]^{2+{radical dot}}, and [4a]^{2+{radical dot}}, respectively. The radical species are characterized by CV, UV-Vis, and EPR spectroscopy. The stability of the radical species has been determined by measuring the decay constant (UV-Vis spectroscopy).

Full text of DOI:10.1016/j.jorganchem.2007.03.041

Journal of Organometallic Chemistry 692 (2007) 3248–3260
www.elsevier.com/locate/jorganchem
Half-sandwich g6-benzene Ru(II) complexes of phenolate-based
pyridylalkylamine/alkylamine ligands: Synthesis, structure,
and stabilization of one-electron oxidized species
*
Haritosh Mishra, Rabindranath Mukherjee
Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India
Received 2 January 2007; received in revised form 21 March 2007; accepted 21 March 2007
Available online 5 April 2007
Abstract
Four half-sandwich ruthenium(II) complexes [(g6-C6H6)Ru(L1-O)][PF6] (1), [(g6-C6H6)Ru(L2-O)][PF6] (2), [(g6-C6H6)Ru(L3-
O)][PF6] (3), [(g6-C6H6)Ru(L4-O)][PF6] (4a), and [(g6-C6H6)Ru(L4-O)][BPh4] (4b) [L1-OH, 4-nitro-6-{[(20-(pyridin-2-yl)ethyl)methyla-
mino]methyl}-phenol; L2-OH, 2,4-di-tert-butyl-6-{[(20-(pyridin-2-yl)ethyl)methylamino]methyl}-phenol; L3-OH, 2,4-di-tert-butyl-6-
{[20-((pyridin-2-yl)benzylamino)methyl}-phenol; L4-OH, 2,4-di-tert-butyl-6-{[(20-imethylaminoethyl)methylamino]methyl}-phenol (L4-
OH)], supported by a systematically varied series of tridentate phenolate-based pyridylalkylamine and alkylamine ligands are reported.
The molecular structures of 1–3, 4a, and 4b have been elucidated in solution using 1H NMR spectroscopy and of 1, 3, and 4b in the solid
state by X-ray crystallography. Notably, due to coordination by the ligands the Ru center assumes a chiral center and in turn the central
amine nitrogen also becomes chiral. The 1H NMR spectra exhibit only one set of signals, suggesting that the reaction is completely dia-
stereoselective [1: SRu,SN/RRu,RN; 2: RRu,RN/SRu,SN; 3: SRu,RN/RRu,SN; 4b: SRu,RN/RRu,SN]. The crystal packing in 1 and 3 is stabi-
lized by C–H. . .O interactions, in 4b no meaningful secondary interactions are observed. From the standpoint of generating phenoxyl
radical, as investigated by cyclic voltammetry (CV), complex 1 is redox-inactive in MeCN solution. However, 2, 3, and 4a generate a
one-electron oxidized phenoxyl radical coordinated species [2]2+Å, [3]2+Å, and [4a]2+Å, respectively. The radical species are characterized
by CV, UV–Vis, and EPR spectroscopy. The stability of the radical species has been determined by measuring the decay constant
(UV–Vis spectroscopy).
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Ruthenum(II); Benzene; Phenolate-based pyridylalkylamine/alkylamine ligands; Diastereoselective; Crystal structures; Non-covalent inter-
actions; Electrochemical oxidation; Phenoxyl radical species
1. Introduction
interest in the synthesis and structural characterization of
complexes of types [(g6-C6H6)Ru(L)]2+ [4] and [(g6-C6H6)-
Ru(L0)Cl]+ [5] (L = neutral tridentate pyridylalkylamine
ligand; L0 = neutral bidentate heterocyclic pyridyl/pyra-
zole/imidazole-hybrid N-donor ligands). The purpose of
the present study is two-fold. First, although few examples
of structurally characterized mononuclear three-legged
half-sandwich arene–ruthenium (arene = p-cymene) com-
plexes supported by phenol-based Schiff base ligands are
known [6], to our knowledge, there is only one report in
the literature of a structurally characterized complex [(g6-
Half-sandwich g6-benzene–RuII complexes belong to a
well-established family of metal–organic molecules in orga-
nometallic chemistry [1]. The impetus for the synthesis and
properties of new complexes having {(g6-C6H6)Ru}2+ unit
arises, owing to their catalytic potential in a range of
organic transformations [2] and very promising cytotoxic
properties [3]. The present work stems from our continued
p-cymene)Ru(L00)Cl] with
(L00 = bidentate uninegative ligand) [2d]. We are not aware
a
non-Schiff base ligand
*
Corresponding author. Tel.:/fax: +91 512 2597437/7436.
E-mail address: rnm@iitk.ac.in (R. Mukherjee).
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.03.041

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
3249
of any structurally characterized arene–ruthenium complex
of tridentate uninegative non-Schiff base phenol/pyr-
idylalkylamine/alkylamine-hybrid chelating ligand. From
the standpoint of synthetic chemistry we find it really chal-
lenging. To provide such examples in this study we have
utilized simple but systematically modified tridentate
ligands (L1-OH, L2-OH, L3-OH, and L4-OH; Scheme 1),
in their deprotonated form. As before [5] we wished to test
if the different electronic/steric environment provided by a
particular tridentate ligand has an impact on the relative
strength of the Ru–benzene bonding in these three-legged
‘‘piano-stool’’ {(g6-C6H6)Ru(L)}+ complexes. Secondly,
due to covalency in a bond elementary chemical reactions
in general and catalysis in particular mostly involve two-
electron changes in the metal coordination sphere.
However, one-electron processes are also of fundamental
importance in organometallic chemistry [7]. Notably, a
long-standing problem in the redox chemistry of metal–
organic molecules of ligands that are themselves potential
sites of facile electron transfer is identifying whether the
site of redox is metal-centered or ligand-centered [8]. From
this standpoint we have initiated a program to synthesize
half-sandwich complexes with redox-active ligands. Among
the ligands chosen in this work (Scheme 1), the ligands L2-
OH, L3-OH, and L4-OH contain 2,4-di-tert-butyl-pheno-
late groups and are redox-active [9]. Studies on redox
chemistry followed by spectral characterization (absorp-
tion and EPR) of one-electron oxidized (ligand-centered
oxidation) counterparts of original ligands is expected to
provide information pertinent to understanding of the sta-
bility/reactivity of such metal-coordinated radical species.
Our present endeavor in this area is set against this
background.
[{(g6-C6H6)RuCl(l-Cl)}2] [11] were prepared following lit-
erature methods.
2.2. Preparation of ligand
2.2.1. 4-Nitro-6-{[(20-(pyridin-2- yl)ethyl)methylamino]-
methyl}-phenol (L1-OH)
A
mixture of N-methyl-2-(pyridin-2-yl)ethanamine
(1.09 g, 8.00 mmol) and triethylamine (0.81 g, 8.00 mmol)
in CH3OH (20 mL) was stirred at 25 ꢁC to which was
added
a
solution of 2-(chloromethyl)-4-nitrophenol
(1.50 g, 8.00 mmol) in CH3OH (20 mL) dropwise over a
period of 30 min under nitrogen atmosphere. The resulting
mixture was refluxed for 2.5 h and then cooled to room
temperature. The solvent was evaporated under reduced
pressure and the yellow sticky solid that obtained was dis-
solved in dry THF (20 mL) and cooled to 0 ꢁC. Triethylam-
monium chloride that precipitated was filtered off and the
filtrate was evaporated to dryness under reduced pressure
to give 2 g of crude product. The yellow sticky crude oil
was subjected to column chromatography on silica gel by
using CH2Cl2 as eluent. Solvent was removed to yield yel-
low sticky oil which solidified after recrystallization from
CH2Cl2/n-hexane. Yield: 1.60 g (70%). 1H NMR (80
MHz; CDCl3): d 8.63 (d, 1H, H6 of py), 8.50–6.8 (m, 6H,
H3,4,5 of py and H3,5,6, of PhOH), 4.82 (s, 2H, –CH2– of
PhOH), 3.50–3.10 (s, 4H, –CH2CH2– of Py), 2.40 (s, 3H,
–CH3).
2.2.2. 2,4-Di-tert-butyl-6-{[(20-(pyridin-2- yl)ethyl)-
methylamino]methyl}-phenol (L2-OH)
Triethylamine (1.3 mL) was added dropwise to a stirring
mixture of N-methyl-2-(pyridin-2-yl)ethanamine (0.272 g,
2 mmol) and 2,4-di-tert-butyl-6-(chloromethyl)-phenol
(0.509 g, 2 mmol) in dry dioxane (3 mL). After stirring the
mixture for 2 h it was heated to 60 ꢁC and more triethyl-
amine (2 mL) was added dropwise over a period of 2 h.
During this addition of triethylamine, the pH of the mixture
was maintained below 10. The reaction mixture was then
cooled to room temperature, further stirred for 15 min,
and filtered. Solvent and excess of triethylamine was
removed under reduced pressure to obtain the product as
yellow sticky oil. Yield: 0.500 g, ca. 71%. 1H NMR
(80 MHz; CDCl3): d 8.33 (d, 1H, H6 of py), 7.45–6.46 (m,
5 H, H3,4,5 of py and H3,5, of PhOH), 3.66 (s, 2H, –CH2–
of PhOH), 2.90 (s, 4H, –CH2CH2– of Py), 2.15 (s, 3H, –
NCH3), 1.33 (s, 9H, 2-tert-butyl), 1.15 (s, 9H, 4-tert-butyl).
2. Experimental
2.1. Materials
Reagent or analytical grade starting materials were
obtained from commercial suppliers and used without fur-
ther purification. 2,4-di-tert-butyl-6-(hydroxymethyl)- phe-
nol [9a] and 2,4-di-tert-butyl-6-(chloromethyl)phenol [9h]
were synthesized following the literature procedures. The
ligand 2,4-di-tert-butyl-6-{[(20-dimethylaminoethyl)meth-
ylamino]methyl}-phenol (L4-OH) [10] and the dimer
NO2
2.2.3. 2,4-Di-tert-butyl-6-{[20-((pyridin-2-
yl)benzylamino)methyl}-phenol (L3-OH)
The starting material phenyl-N-(pyridin-2-ylmethyl)-
methanamine necessary for the synthesis of L3-OH was
prepared by the following procedure. A solution of pyri-
din-2-ylmethanamine (0.540 g, 5 mmol) and benzaldehyde
(0.530 g, 5 mmol) in C2H5OH (10 mL) was stirred for 1 h
at 25 ꢁC, under dinitrogen atmosphere. Solid NaBH4
(0.760 g, 10 mmol) was then added slowly in small portions
OH
OH
N
OH
OH
N
N
N
N
N
N
N
L4-OH
L3-OH
L1-OH
L2-OH
Scheme 1.

3250
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
over a period of 30 min and the resulting suspension was
further stirred at 25 ꢁC for 30 min, under dinitrogen atmo-
sphere. The reaction mixture was then refluxed for 12 h.
After cooling to 25 ꢁC, the mixture was diluted with
C2H5OH (10 mL) and excess of NaBH4 was destroyed by
adding aq. HCl (10 mL, 5 M) dropwise. The reaction mix-
ture was then made alkaline by 20% aq. NaOH solution
(pH 12). Finally, the desired product was extracted in
CH2Cl2 (4 · 10 mL) and dried over anhydrous MgSO4.
The solvent was evaporated under reduced pressure to
get yellow oil. The oil was then dried in vacuo and used
in the next step, without further purification. Yield:
8.95 (d, JHH = 5.6 Hz, 1H, H6 of py), 7.85–7.77 (m, 2H,
0
0
H4 of py and H6 of PhO), 7.67 (d, JHH = 7.0 Hz, 1H, H5
0
of PhO), 7.40 (s, 1H, H3 of PhO), 7.36 (t, JHH = 7.8 Hz,
1H, H5 of py), 6.68 (d, JHH = 9.0 Hz, 1H, H3 of py) 5.80
(s, 6H, C6H6), 3.89 (d, Jgem = 12.7 Hz, 1H, –CH2–), 3.66
(s, 3H, –NCH3), 3.20 (m, 1H, –CH2CH2–), 3.07 (m, 1H, –
CH2CH2–), 2.99 (d, Jgem = 12.7 Hz, 1H, –CH2–), 2.55 (m,
1H, –CH2–CH2–), 2.01 (m, 1H, –CH2CH2–). Molar conduc-
tance, KM(CH3CN, 298 K) = 125 Xꢁ1 cm2 molꢁ1 (expected
1:1 range: 120–160 Xꢁ1 cm2 molꢁ1). UV–Vis (in CH3CN):
k/nm (e/dm3 molꢁ1 cmꢁ1) 380 (19100), 270 sh (7500), 235
sh (15400).
1
0.750 g, ca. 75%. H NMR (80 MHz; CDCl3): d 8.30 (d,
1H, H6 of py), 7.33–6.66 (m, 8H, H3,4,5 of py and H2–6 of
Ph), 3.45 (s, 4H, –CH2–).
2.3.3. [(g6-C6H6)Ru(L2-O)][PF6] (2)
Yield: 0.170 g (63%). Anal. Calc. for C29H39N2F6OPRu:
C, 51.40; H, 5.76; N, 4.13. Found: C, 50.79; H, 5.80; N,
4.18%. IR (KBr, cmꢁ1): 2951 m(C–H of tert-butyl), 840
In the next step, phenyl-N-(pyridin-2-ylmethyl)methan-
amine (0.396 g, 2 mmol) and 2,4-di-tert-butyl-6-(chloro-
methyl)-phenol (0.509 g, 2 mmol) were coupled following
the above-mentioned method for the synthesis of ligand
L2-OH. Yield: 0.530 g, ca. 63%. 1H NMR (80 MHz;
CDCl3): d 8.40 (d, 1H, H6 of py), 7.25–6.45 (m, 10H,
H3,4,5 of py, H2–6 of Ph and H3,5, of PhOH), 3.25 (s, 2H,
–CH2– of PhOH), 3.15 (4H, s, –CH2– of Ph and –CH2–
of Py), 1.46 (s, 9H, 2-tert-butyl), 1.15 (s, 9H, 4-tert-butyl).
1
m(PF6ꢁ). H NMR (CD3CN; 400 MHz; 298 K): d 8.98 (d,
0
JHH = 4.8 Hz, 1H, H6 of py), 7.46 (t, JHH = 7.56 Hz, 1H,
0
0
H4 of py), 7.48 (d, JHH = 7.8 Hz, 1H, H3 of py), 7.36 (t,
1-4
0
JHH = 6.2 Hz, 1H, H5 of py), 7.10 (d, JHH = 2.6 Hz, 1H,
H3 of PhO), 6.56 (d, J = 2.6 Hz, 1H, H5 of PhO), 5.84
1-4
HH
(s, 6H, C6H6), 3.41 (m, 1H, –CH2CH2–), 3.31 (m, 1H, –
CH2CH2–, overlaped with –NCH3), 3.29 (s, 3H, –NCH3),
2.77 (d, Jgem = 12.7 Hz, 1 H –CH2–), 2.73 (m, 1H, –
CH2CH2–), 1.98 (d, Jgem = 12.7 Hz, 1H –CH2–) 1.53 (s,
9H, o-tert-butyl), 1.15 (s, 9H, p-tert-butyl). Molar conduc-
tance, KM(CH3CN, 298 K) = 110 Xꢁ1 cm2 molꢁ1. UV–Vis
(in CH3CN): k/nm (e/dm3 molꢁ1 cmꢁ1) 450 sh (1000), 350
sh (2450), 305 sh (8100), 254 (24500).
2.3. Preparation of complexes
2.3.1. General procedure
A mixture of the ligand (0.4 mmol) and triethylamine
(0.4 mmol) was dissolved in CH3OH (15 mL) under dini-
trogen atmosphere and to it was added solid [{(g6-
C6H6)RuCl(l-Cl)}2] (0.2 mmol). The mixture was stirred
for 8 h (complexes 1 and 3) or 12 h (complexes 2 and 4a)
at 25 ꢁC. The resulting reddish yellow (complexes 1 and
3) or reddish orange (complexes 2 and 4a) solution was fil-
tered and the volume of the filtrate was reduced (ꢀ5 mL)
and to it was added solid NH4PF6 (0.4 mmol). For com-
plex 3 the resulting mixture was heated at 65 ꢁC for
10 min and cooled to 25 ꢁC. The yellow (complex 2) or
orange (complexes 3 and 4a) microcrystalline solid that
formed was filtered, washed with cold CH3OH, and dried
in vacuo. The complex 1 was precipitated out by addition
of diethyl ether (10 mL) into the filtrate which led to isola-
tion of an orange solid. It was filtered, washed with a mix-
ture of diethyl ether and CH3OH (2:1; v/v), and dried in
vacuo. Recrystallization was achieved from CH3CN/diethyl
ether (1, 3, and 4a) or from CH3CN and CH3OH (1:1; v/v)/
diethyl ether (2). X-ray quality single-crystals were
obtained by diffusion of diethyl ether into a solution
(1 mL) of the compound in MeCN (complex 1) or in a mix-
ture (v/v; 3:1) of CH3OH and CH3CN (complex 3).
2.3.4. [(g6-C6H6)Ru(L3-O)][PF6] (3)
Yield: 0.150 g (50%). Anal. Calc. for C34H41N2F6OPRu:
C, 55.20; H, 5.54; N, 3.78. Found: C, 54.98; H, 5.53; N,
3.81%. IR (KBr, cmꢁ1): 2950 m(C–H of tert-butyl), 838
1
m(PF6ꢁ). H NMR (CD3CN; 400 MHz; 298 K): d 8.75(d,
0
JHH = 5.8 Hz, 1H, H6 of py), 7.66–7.54 (d m, 5H, phenyl),
0
7.44 (t, JHH = 8.4 Hz, 1H, H4 of py), 6.95 (t, JHH = 6.5 Hz,
0
0
1 H, H5 of py), 6.84 (d, JHH = 7.8 Hz, 1H, H3 of py), 6.69
1–4
1–4
HH
(d, J = 2.4 Hz, 1H, H3 of PhO), 6.14 (d, J = 2.4 Hz,
HH
1H, H5 of PhO), 5.94 (s, 6H, C6H6), 5.45 (dd, 2H,
00
00
Jgem = 12.9 Hz, Ha and Hb of –CH2– of pyridyl), 4.72
0
(d, Jgem = 15.3 Hz, 1H, Ha of –CH2ꢁ of PhO), 3.83 (d,
0
Jgem = 15.3 Hz, 1H, Hb of –CH2– of PhO), 3.39 (d,
Jgem = 11.2 Hz, 1H, Hb of –CH2– of Benzyl), 2.98 (d,
Jgem = 11.2 Hz, 1H, Ha of –CH2– of Benzyl), 1.37 (s, 9H,
o-tert-butyl), 0.94 (s, 9H, p-tert-butyl). Molar conductance,
KM(CH3CN, 298 K) = 120 Xꢁ1 cm2 molꢁ1. UV–Vis (in
CH3CN): k/nm (e/dm3 molꢁ1 cmꢁ1) 430 sh (1550), 325 sh
(5700), 270 sh (10200), 250 sh (17600).
2.3.5. [(g6-C6H6)Ru(L4-O)][PF6] (4a)
2.3.2. [(g6-C6H6)Ru(L1-O)][PF6] (1)
Yield: 0.165 g (64%). Anal. Calc. for C26H41N2F6OPRu:
C, 48.52; H, 6.37; N, 4.35. Found: C, 48.54; H, 6.37; N,
4.37%. IR (KBr, cmꢁ1): 2951 m(C–H of tert-butyl), 839
Yield: 0.140 g (57%). Anal. Calc. for C21H22N3F6O3PRu:
C, 41.31; H, 3.60; N, 6.88. Found: C, 41.29; H, 3.52; N,
6.91%. IR (KBr, cmꢁ1): 1518 masym(NO2), 1330 msym(NO2),
842 m (PF6ꢁ). 1H NMR (CD3CN; 400 MHz; 298 K): d
m(PF6ꢁ). H NMR (CD3CN; 400 MHz; 298 K): d 7.14 (d,
1
1–4
HH
1–4
J
= 2.6 Hz, 1 H, H3 of PhO), 6.74 (d, J = 2.6 Hz,
HH

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
3251
1H, H5 of PhO), 5.65 (s, 6H, C6H6), 4.68 (d, 1H,
Jgem = 14.9 Hz, –CH2–), 3.53 (s, 3H, -N(CH3)2), 3.48 (d,
1H, Jgem = 14.9 Hz, –CH2–), 3.41 (m, 1H, –CH2CH2–),
3.37 (s, 3H, –N(CH3)2), 2.69 (s, 3H, –NCH3), 2.57 (m,
1H, –CH2CH2–), 2.43 (m, 1H, –CH2CH2–), 2.16 (m, 1H,
–CH2CH2–, overlaped with H2O signal), 1.45 (s, 9H,
2-tert-butyl), 1.22 (s, 9H, 6-tert-butyl). Molar conductance,
KM(CH3CN, 298 K) = 120 Xꢁ1 cm2 molꢁ1. UV–Vis (in
CH3CN): k/nm (e/dm3 molꢁ1 cmꢁ1) 460 sh (700), 306
(6800), 253 (19500).
platinum-inlay working electrode, a platinum-wire auxil-
iary electrode and a saturated calomel electrode (SCE) as
reference; no corrections were made for junction potentials.
Details of cell configuration and criterion for reversibility
are as reported previously [8]. For constant potential elec-
trolysis experiments a Pt mesh was used as working
electrode. The solutions were ꢀ1 mM in complexes and
0.1 M in supporting electrolyte, TBAP. Under our
experimental conditions, in CH3CN the E1/2 value (V) for
Fc+/Fc couple was 0.40 vs. SCE [8].
2.3.6. [(g6-C6H6)Ru(L4-O)][BPh4] (4b)
2.5. Crystal structure determination
Compound 4a (0.064 g, 0.1 mmol) was dissolved in
CH3CN (2 mL) and to it was added a solution of NaBPh4
(0.040 g, 0.116 mmol) in CH3OH (4 mL) dropwise. The
resulting mixture was stirred for 10 min at 25 ꢁC and the
orange solution was filtered through celite pad. Diffusion
of diethyl ether into the filtrate yielded an orange micro-
crystalline solid, which was filtered, washed with CH3OH,
and dried in vacuo. X-ray quality single-crystals were
obtained by diffusion of diethyl ether into a solution
(1 mL) of the compound in a mixture (v/v; 3:1) of CH3OH
and CH3CN. Yield: 0.075 g (90%). Anal. Calc. for
C50H61N2BORu: C, 73.43; H, 7.46; N, 3.42. Found: C,
Diffracted intensities were collected on a Bruker
SMART APEX CCD diffractometer at 100(2) K (1), (3),
and (4b) Æ 1/2CH3OH Æ H2O 293(2) K using graphite mono-
˚
chromated Mo Ka (k = 0.710 69 A) radiation. 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 [12]. 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. For 4b Æ 1/2CH3O-
H Æ H2O, some degree of disorder was observed with H2O
molecule; two positions for O2W atom could be located
and they were refined with a site occupation factor of
0.6/0.4. A summary of the data collection and structure
refinement information is provided in Table 1. For com-
plex 4b Æ 1/2CH3OH Æ H2O, after anisotropic refinement
some unassigned electron density peaks were observed in
the final difference Fourier map. For 4b Æ 1/2CH3OH Æ H2O
1
73.44; H, 7.45; N, 3.44%. H NMR (CD3CN; 400 MHz;
298 K): d 7.26 (m, 8H, H2 ,6 of BPh4ꢁ), 7.14 (d, J1–
0
0
4
HH = 2.4 Hz, 1H, H3 of PhO), d 6.98 (t, 8H, JHH = 7.5 Hz,
H3 ,5 of BPh4ꢁ), 6.83 (t, 4H, JHH = 7.0 Hz, H4 of BPh4ꢁ),
6.73 (d, J = 2.4 Hz, 1H, H5 of PhO), 5.61 (s, 6H, C6H6),
0
0
0
1–4
HH
4.65 (d, 1H, Jgem = 14.4 Hz, –CH2–), 3.49 (s, 3H, –
N(CH3)2), 3.45 (d, 1H, Jgem = 14.4 Hz, –CH2–), 3.41 (m,
1H, –CH2CH2–), 3.33 (s, 3H, –N(CH3)2), 2.67 (s, 3H, –
NCH3), 2.55 (m, 1H, –CH2CH2–), 2.43 (m, 1H, –
CH2CH2–), 2.16 (m, 1H, –CH2CH2–, overlaped with
H2O signal), 1.44 (s, 9H, o-tert-butyl), 1.21 (s, 9H, p-tert-
butyl). Molar conductance, KM(CH3CN, 298 K) = 80
Xꢁ1 cm2 molꢁ1
.
UV–Vis (in CH3CN): k/nm (e/dm3
peaks of 5.50 e A and 4.01 e A were found near Ru10
˚
ꢁ3
ꢁ3
˚
molꢁ1 cmꢁ1) 460 sh (750), 305 (6500), 253 (21500).
and Ru1 atom at a distance of 1.310 A and 1.254 A, respec-
tively, which may be due to the poor quality of crystal cho-
sen for data collection (high Rint value). Unfortunately, we
could not grow single crystals of 4a/4b that were any better
than the one used for the present study, as they were the
best we could have. The overall crystal refinement was poor
(Table 1). A preliminary structural drawing of compound 2
has been presented in the Supplementary data for its
authenticity (the details of which will be published else-
where). Intermolecular contacts of the C–H. . .O type were
examined with the DIAMOND package [13]. C–H distances
were normalized along the same vectors to the neutron
˚
˚
2.4. Instrumentation
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 spectrophotome-
1
ter. H NMR spectral measurements were performed on
˚
a JEOL-JNM-LA-400 FT (400 MHz) NMR spectrometer.
X-band EPR, Bruker EMX 1444 spectrometer operating at
9.455 GHz. The EPR spectra were calibrated with diph-
enylpicrylhydrazyl, DPPH (g = 2.0037).
derived values of 1.083 A [14].
3. Results and discussion
The cyclic voltammetric experiments were performed at
298 K by using a CH Instruments, Electrochemical Ana-
lyzer/Workstation Model 600B Series. A standard three
electrode cell was employed with a Beckman M-39273
3.1. Synthesis and characterization of complexes
From the standpoint of bridge-cleavage reactivity of
chloro-bridged dimer [{(g6-C6H6)RuCl(l-Cl)}2] reactions

3252
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
Table 1
Data collection and structure refinement parameters for [(g6-C6H6)Ru(L1-O)][PF6] (1), [(g6-C6H6)Ru(L3-O)][PF6] (3), and [(g6-C6H6)Ru(L4-O)][BPh4] Æ 1/
2CH3OH Æ H2O (4b Æ 1/2CH3OH Æ H2O)
1
3
4b Æ 1/2CH3OH Æ H2O
Chemical formula
M
C21H22N3O3-PF6Ru
610.46
C34H41N2O-PF6Ru
504.76
C50.5H62N2O2.5-BRu
849.41
Crystal color, habit
T (K)
Orange, block
100(2)
Orange, block
293(2)
Orange, block
293(2)
Crystal system
Space group
Triclinic
Triclinic
Triclinic
ꢀ
ꢀ
ꢀ
P1 (#2)
P1 (#2)
P1 (#2)
˚
a (A)
7.7629(11)
9.8439(13)
14.995(2)
98.883(2)
98.249(2)
103.616(2)
1080.8(3)
2
9.7737(16)
11.8689(19)
15.843(3)
109.348(2)
102.390(3)
97.006(3)
1656.1(5)
2
13.431(5)
19.070(5)
19.801(5)
111.006(5)
90.162(5)
99.539(5)
4658(2)
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
4
dcalc (g cmꢁ3
)
1.876
0.883
612
1.483
0.585
760
1.211
0.377
1794
l (mmꢁ1
F(000)
)
Number of reflections collected
Number of independent reflections [R (int)]
Number of reflections used [I > 2r(I)]
Goodness-of-fit on F2
7224
5110 [0.0307]
4305
1.170
0.0574, 0.1206
0.0759, 0.1527
11047
7905 [0.0210]
6505
1.026
0.0525, 0.1276
0.0748, 0.1808
31343
22286 [0.0609]
10530
0.986
0.1152, 0.2944
0.2103, 0.3482
Final R indices [I > 2r(I)]a,b
Final R indices (all data)
P
P
a
R1 = (jFoj ꢁ jFcj)/ jFoj.
P
P
b
2
2 2
2 2
wR2 = { [w(jFoj ꢁ jFcj ) ]/ [w(jFoj ) ]}1/2
.
with L1-OH, L2-OH, L3-OH, and L4-OH (Scheme 1) in
CH3OH in presence of triethylamine followed by subse-
quent treatment with NH4PF6 led to the isolation of [(g6-
C6H6)Ru(L1-O)][PF6] (1), [(g6-C6H6)Ru(L2-O)][PF6] (2),
[(g6-C6H6)Ru(L3-O)][PF6] (3), and [(g6-C6H6)Ru(L4-
O)][PF6] (4a) as yellow to yellow–orange microcrystalline
solids. It is to be noted here that triethylamine is essential
during the course of complex formation for deprotonation
of phenolic hydroxyl groups of the chosen ligands to gen-
erate anionic ligands L1-Oꢁ, L2-Oꢁ, L3-Oꢁ, and L4-Oꢁ.
The complex [(g6-C6H6)Ru(L4-O)][BPh4] (4b) was pre-
pared by simple metathesis reaction between [(g6-
C6H6)Ru(L4-O)][PF6] (4a) and NaBPh4 in a mixture of
CH3CN and CH3OH (v/v; 1:2).
Table 2). As already mentioned, due to the poor
quality of the data set the refinement of the structure of
4b Æ 1/2CH3OH Æ H2O (its asymmetric unit contains crystal-
lographically independent two molecules) did not reach a
satisfactory level. Nevertheless, the cation seems to have
been determined well [Fig. 1c]. The cations exhibit the
expected and usual pseudo-octahedral half-sandwich
‘‘piano-stool’’ disposition around the Ru atom. The ruthe-
nium(II) ion is p bonded to the g6-C6H6 group with the
benzene ligand occupying one face of the octahedron and
the coordination of a tridentate phenolate-based ligand
on the other face [N(1) of pyridine in 1 and 3 or alkylamine
in 4b Æ 1/2CH3OH Æ H2O; N(2) of alkylamine, and O(1) of
phenolate]. The pyridyl rings in 1 and 3 and phenolate
rings of L1-Oꢁ in 1, of L3-Oꢁ in 3, and of L4-Oꢁ in
4b Æ 1/2CH3OH Æ H2O are each planar. However, the pyr-
idyl mean plane of 1 and 3 is tilted to the phenolate ring
at an angle of 81.754ꢁ and 37.895ꢁ, respectively.
Interestingly, X-ray structural analyses revealed notice-
able differences in the bonding characteristics (metric
parameters) of tridentate ligands L1-Oꢁ, L3-Oꢁ, and L4-
Oꢁ 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 the following generalizations emerge. (i) The
Ru–O(phenolate) bond is strongest in 4b Æ 1/2CH3O-
Characterization of the new compounds was accom-
plished by elemental analysis, solution electrical conductiv-
1
ity, IR, and H NMR spectroscopy. Consistent withꢁtheir
formulation, IR stretching vibration of PF6
at
ꢀ840 cmꢁ1 confirm cationic nature of complexes 1–4a
and conductivity studies revealed that compounds 1–3,
4a, and 4b are 1:1 electrolyte [15]. As proof of their color,
compounds 1–3, 4a, and 4b exhibit absorption spectral
band in the 380–460 nm region.
3.2. Molecular structures of [(g6-C6H6)Ru(L1-O)][PF6]
(1), [(g6-C6H6)Ru(L3-O)][PF6] (3), and [(g6-C6H6)-
Ru(L4-O)][BPh4] Æ 1/2CH3OH Æ H2O (4b Æ 1/2-
CH3OH Æ H2O)
˚
˚
H Æ H2O [2.041(5) A {2.040(5) A}] followed by that in 3
˚
˚
[2.054(3) A], and weakest in 1 [2.096(4) A]. It is under-
standable given the fact that in L3-Oꢁ and L4-Oꢁ, two elec-
tron-releasing tert-butyl groups are present at the ortho and
para position of the phenolate ring, whereas in L1-Oꢁ an
X-ray crystallographic analyses confirm the structure of
the compounds 1, 3, and 4b Æ 1/2CH3OH Æ H2O (Fig. 1,

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
3253
Table 2
6
1
˚
Selected bond lengths (A) and angles (ꢁ) in [(g -C6H6)Ru(L -O)][PF6] (1),
[(g6-C6H6)Ru(L3-O)][PF6] (3), and [(g6-C6H6)Ru(L4-O)][BPh4] Æ 1/
2CH3OH Æ H2O (4b Æ 1/2CH3OH Æ H2O)
1
3
4b Æ 1/2CH3OH Æ H2O
Ru(1)–N(1)
Ru(1)–N(2)
Ru(1)–O(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)
2.098(5)
2.170(5)
2.096(4)
2.195(6)
2.179(6)
2.194(6)
2.175(6)
2.212(5)
2.192(6)
1.409(9)
1.415(8)
1.416(8)
1.429(8)
1.402(9)
1.417(9)
2.104(4)
2.203(4)
2.054(3)
2.180(6)
2.176(6)
2.202(6)
2.126(6)
2.167(7)
2.142(6)
1.350(12)
1.360(12)
1.394(13)
1.381(13)
1.337(13)
1.381(13)
2.141(6) [2.147(6)]a
2.1408(8) [2.1443(12)]a
2.041(5) [2.040(5)]a
2.182(10) [2.176(11)]a
2.205(9) [2.199(11)]a
2.193(9) [2.161(9)]a
2.205(9) [2.180(9)]a
2.154(9) [2.163(10)]a
2.177(10) [2.158(11)]a
1.323(15) [1.397(19)]a
1.429(16) [1.301(18)]a
1.418(15) [1.307(16)]a
1.392(15) [1.485(17)]a
1.393(15) [1.375(17)]a
1.387(16) [1.362(19)]a
83.4(2) [82.6(2)]a
C(2)–C(3)
C(3)–C(4)
C(4)–C(5)
C(5)–C(6)
C(1)–C(6)
N(1)–Ru(1)–O(1)
N(1)–Ru(1)–N(2)
N(2)–Ru(1)–O(1)
a
83.40(17)
83.46(18)
89.65(16)
87.73(14)
78.04(14)
87.10(14)
80.35(19) [80.2(2)]a
83.32(15) [82.6(2)]a
The data are for two molecules.
Table 3
6
1
˚
Summary of relevant bond distances (A) in [(g -C6H6)Ru(L -O)][PF6] (1),
[(g6-C6H6)Ru(L3-O)][PF6] (3), and [(g6-C6H6)Ru(L4-O)][BPh4] Æ 1/
2CH3OH Æ H2O (4b Æ 1/2CH3OH Æ H2O)
1
3
4b Æ 1/2CH3OH Æ H2Oa
Av Ru–C
Ru–C6H6 (centroid)
Av C–C
2.1911(6)
1.674
1.4146(9)
2.098(5)
2.170(5)
–
2.165(6)
1.680
1.367(13)
2.104(4)
2.203(4)
–
2.186(9) [2.172(10)]
1.687 [1.686]
1.390(15) [1.372(17)]
–
2.141(6) [2.147(6)]
2.1408(8) [2.1443(12)]
2.041(5) [2.040(5)]
Ru–N(py)
Ru–NMe(amine)
Ru–NMe2(amine)
Ru–O(phenolate)
a
2.096(4)
2.054(3)
The data are for two molecules.
of methyl group(s) in 1 and 4b Æ 1/2CH3OH Æ H2O than that
of benzyl group in 3. (iv) Within the present group of com-
plexes the phenolate oxygen provides strongest binding to
RuII and the N(amine) provides least effective binding.
The Ru–N(py) bond in 1 and 3 is stronger than that of
Ru–N(amine) bond and weaker than that of Ru–O(pheno-
late) bond. As O(phenolate) carries a negative charge it
forms strong r-bond with Ru(II) ion. Pyridine being a
p-acceptor binds more effectively than the r-donating
alkylamine (Table 3). (v) In these compounds the distortion
at the coordinated benzene ligand is present, with respect
to the Ru–C bond distances. In 1 the longest Ru–C bond
Fig. 1. Thermal ellipsoid plots of (a) (SRu, SN)-[(g6-C6H6)Ru(L1-O)]+ in
1, (b) (SRu, RN)-[(g6-C6H6)Ru(L3-O)]+ in 3, and (c) (SRu, RN)-[(g6-
C6H6)Ru(L4-O)]+ in (4b) Æ 1/2CH3OH Æ H2O at 30% probability level.
Hydrogen atoms have been omitted for clarity.
electron-withdrawing nitro group is present at the para
position of the phenolate ring. (ii) The Ru–N(py) bond is
˚
˚
˚
of equal strength in 1 [2.098(5) A] and 3 [2.104(4) A]. The
slightly shorter Ru–N(py) bond distance (Table 3) in 1 than
in 3 may be attributed to the temperature difference at
which the data of the two complexes were collected. (iii)
of 2.212(5) A [the other Ru–C bonds are between
˚
2.175(6) and 2.195(6) A] is trans to pyridine N atom and
˚
the shortest Ru–C bond of 2.175(6) A is trans to phenolate
O atom of L1-Oꢁ. Thus in 1 the extent of trans influence
follows the order: pyridine > amine > phenolate. In 3 the
The Ru–N(amine) bond is strongest in 4b Æ 1/2CH3O-
˚
˚
H Æ H2O [2.141(6) {2.147(6)} A] and weakest in
3
longest Ru–C bond of 2.202(6) A [the other Ru–C bonds
˚
˚
[2.203(4) A]. This finding reflects the greater inductive effect
are between 2.142(6) and 2.180(4) A] is trans to phenolate

3254
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
˚
O atom and the shorter Ru–C bond of 2.167(7) A is trans
to amine N atom of L3-Oꢁ. Thus in 3 the extent of trans
influence follows the order: phenolate > pyridine > amine.
In 4b Æ 1/2CH3OH Æ H2O the longest Ru–C bond of
exhibit only one set of signals it can be concluded that the
reaction is completely diastereoselective. Therefore, an
attempt has been made to assign the configurations of dia-
stereomers of the complexes 1, 3, and 4b, by X-ray analysis
of single-crystals. It is worth mentioning here that for all the
structures presented in this work the space group is centro-
˚
˚
2.205(11) A [2.199(11) A] [the other Ru–C bonds are
between 2.193(9) A {2.189(9) A} and 2.154(9) A
{2.158(11)} A] is trans to phenolate O atom of L -O
˚
˚
˚
4
ꢁ
˚
ꢀ
symmetric P1. Therefore, both enantiomers of the isolated
˚
˚
and the shorter Ru–C bond of 2.177(10) A {2.158(11)} A]
is trans to NMe2 amine nitrogen of L4-Oꢁ. Thus in
4b Æ 1/2CH3OH Æ H2O the extent of trans influence follows
the order: phenolate > amine (NMe) > amine (NMe2). This
trend could be rationalized if we take into account the fact
that between p-acceptor and r-donor ligands, in general
former ligands have greater trans influence than the latter
ligands. Moreover, among the r-donating ligands greater
the r-donor strength larger the trans influence. The greater
trans influence of pyridine in 1 is thus understandable. The
O(phenolate) group carrying a negative charge is a better
r-donor, thus its greater trans influence than amine N in
1 and 4b Æ 1/2CH3OH Æ H2O is also understandable. But
in 3 the trans influence of phenolate O atom is greater than
the pyridine N atom. It might be due to the very strong
binding by phenolate oxygen in 3 because of the presence
of two electron releasing tert-butyl groups at the ortho
and para position of the phenolate ring in L3OH (vide
supra). In essence, for 1, 3, and 4b Æ 1/2CH3OH Æ H2O the
observed trend in Ru–C, Ru–N(py), Ru–N(amine), and
Ru–O(phenolate) distances (Table 2), reflecting mutual
trans influence, is a consequence of interplay between steric
and electronic factors associated with the coordinating
ability of tridentate ligands L1-Oꢁ, L3-Oꢁ, and L4-Oꢁ in
a closely similar metal coordination environment.
diastereomeric pair must be present in the unit cell in 1:1
ratio. Notably, in all the cases, structural analysis reveals
the presence of only one diastereomer (1: SRu,SN; 3:
SRu,RN; 4b: SRu,RN) in the asymmetric unit. In case of
complex 4b, there are two molecules in the asymmetric unit
having same configuration. However, in each case closer
inspection of the contents of the unit cell reveals the
presence of both the enantiomers of the isolated diastereo-
meric pair (1: SRu,SN/RRu,RN; 3: SRu,RN/RRu,SN; 4b:
SRu,RN/RRu,SN) in 1:1 ratio. The configuration of the
ruthenium center in the complexes 1, 3, and 4b is S, in accor-
dance with the ligand priority sequence [16a] (g6C6H6) >
O(PhO) > N(Py) > N(amine) or (g6C6H6) > O(PhO) >
NMe > NMe2. While the configuration of the central amine
nitrogen center in the complexes 1, 3, and 4b are S, R, and
R, respectively, in accordance with the priority order [16b]
of the four groups Ru > CH2PhO > (CH2)2Py > CH3,
Ru > CH2PhO > (CH2)2Py > CH2Ph, and Ru > CH2PhO >
(CH2)2NMe2 > CH3, respectively, for the complexes 1, 3,
and 4b.
Unfortunately, the crystal data obtained for the com-
pound 2 could not be refined properly because of the pres-
0
ꢀ
ence of 8 molecules [Z = 8, triclinic crystal system (P1)]
[17] in the asymmetric unit and severe disorder associated
with tert-butyl carbon atoms. However, all eight molecules
have been identified and refined isotropically [Fig. S1 (Sup-
plementary material)] and one of the eight molecules is
refined anisotropically [Fig. S2 (Supplementary material)].
Interestingly, all eight molecules present in the asymmetric
unit are found to have the same configuration about the
Ru(II) center and central amine nitrogen center. However,
closer inspection of the contents of the unit cell reveals the
presence of both enantiomers of the isolated diastereomeric
pair (RRu,RN/SRu,SN) in 1:1 ratio.The configuration of the
ruthenium center and central amine nitrogen center of all
the molecules in the complex 2 is R, in accordance with
the priority sequences (g6C6H6) > O(PhO) > N(Py) > N
(amine) and Ru > CH2PhO > (CH2)2Py > CH3.
The slightly shorter Ru–C6H6 centroid distance (Table 3)
˚
˚
in 1 (1.674 A) than that in 3 (1.680 A) may be attributed to
the temperature difference at which the data of the com-
plexes were collected. Between 3 and 4b Æ 1/2CH3OH Æ H2O,
˚
the Ru–C6H6 centroid distance in 3 (1.680 A) is less than
˚
˚
that in 4b Æ 1/2CH3OH Æ H2O [1.687 A {1.686 A}]. So, the
phenolate-based ligands L1-Oꢁ present in 1 and L3-Oꢁ pres-
ent in 3 provide more relative strength to {(g6-C6H6)Ru}2+
unit than that by L4-Oꢁ in 4b Æ 1/2CH3OH Æ H2O.
Average Ru–C distances in 1, 3, and 4b Æ 1/2CH3O-
H Æ H2O (Table 3) are comparable to that reported in sim-
ilar three-legged piano-stool complexes including {(g6-
C6H6)RuCl}+ moiety [4,5]. The Ru–N(py) and Ru–
N(amine) bond lengths observed in 1 and 3 compare well
with the values found in similar three-legged piano-stool
complexes with N-donor ligands [4,5]. The Ru–O distances
are comparable to that reported in the literature for half-
sandwich complexes with phenolate ligands [6].
1
3.3. H NMR spectroscopy
1H NMR data (in CD3CN) of 1–3, 4a, and 4b along with
their assignments are recorded in the Experimental section,
supporting their expected ‘‘piano-stool’’ structure [Figs.
S3–S7 (Supplementary material)]. The proton resonances
Notably, due to coordination by the ligands the Ru cen-
ter assumes a chiral center and in turn the central amine
nitrogen also becomes chiral. Therefore, the compounds
can be obtained as two diastereomeric pairs of isomers,
i.e. SS/RR and SR/RS [2f,16]. However, the two enantio-
mers of a diastereomeric pair must be in 1:1 ratio to give rise
1
were assigned based on available H NMR spectral results
for the free ligands L1-OH, L2-OH, and L3-OH (this work),
and L4-OH [10]. For complex 3, the three methylene spacers
were unambiguously assigned by the 1H NOE experiments.
The following comments regarding the spectral data are in
1
to a racemic mixture. As the H NMR spectra (see below)

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
3255
1
order. (i) The chemical shift values for coordinated benzene
in the complexes 1, 2, 3, 4a, and 4b are d 5.80, d 5.84, d 5.94,
d 5.65, and d 5.61, respectively. Maximum upfield shift
(d 5.65 and 5.61) for 4a and 4b implies the presence of more
electron density on the benzene ring than in other com-
pounds. It is understandable as ligand L4Oꢁ is providing
all three coordination (two alkyl amine N atoms and one
phenolate Oꢁ anion) with r-donor ligands and hence RuII
center is pulling less electron density from benzene ring.
Maximum downfield shift (d 5.94) for 3 implies the presence
of least electron density on the benzene ring. Among the
compounds with pyridine-containing phenolate ligands
(L1-OH, L2-OH, and L3-OH) ligand L3-Oꢁ is a better
p-acceptor due to the formation of five- membered chelate
ring. Hence in 3 the RuII center is withdrawing more elec-
tron density from benzene ring than that in other cases. It
is clearly reflected in its av Ru–C distance (Table 3), imply-
ing poor interaction of C6H6 ligand with Ru(II) (cf. X-ray
structure). (ii) An AB quartet for –CH2– protons of the
ligands confirms the presence of two diastereotopic protons,
axial and equatorial. It implies that these protons are not
interconverting on the NMR time-scale; otherwise a singlet
would have resulted. (iii) The methylene hydrogen atoms of
–CH2CH2– backbone of the ligands are non-equivalent
upon complexation and appeared as four multiplets. Some
geminal and vicinal coupling in –CH2CH2–, make a clearer
interpretation difficult. (iv) The two aromatic phenolate
hydrogen atoms (H3 and H5) of L2-Oꢁ, L3-Oꢁ, and L4-
Oꢁ in 2, 3, 4a, and 4b split on complexation and resulted
in two doublets. The formation of rigid structure on com-
plexation supports the clearer visualization of aromatic pro-
tons in all the complexes.
In essence, the H NMR results of 1–3, and 4b clearly
indicate that the solid state structures (vide supra) are
retained in solution.
3.4. Non-covalent interactions
A closer inspection of the crystal packing diagrams of 1
and 3 reveals that these organometallic molecules are
engaged in secondary interactions (see below). Relevant
bond distances, bond angles, and symmetry are summa-
rized in Table 4. The C–H. . .O hydrogen-bonding parame-
˚
ters observed in this work [2.466–2.598 A and 117.1–145.2ꢁ
˚
(1); 2.658 A and 171.5ꢁ(3)] are in good agreement with
literature tabulations (C–H. . .O: 2.045–2.399 A and
˚
90.7–176.7ꢁ) [18], literature precedents [19] including our
own findings [20]. These can be classified as intermediate
contacts (2.439–2.598 A) which are appreciably shorter
than the sum of the van der Waals radii for the H and
˚
˚
the neutral O atoms (2.72 A) [19b].
3.4.1. [(g6-C6H6)Ru(L1-O)][PF6] (1) and [(g6-C6H6)-
Ru(L3-O)][PF6] (3)
The analysis of crystal structures of 1 and 3 reveals that
these half-sandwich organometallic molecules are engaged
in secondary interactions in the form of hydrogen bonds
leading to the formation of supramolecular arrays [20]. In
both the structures dimeric units are formed via intermolec-
ular C–H. . .O hydrogen-bonding interactions. In 1 the phe-
nolate oxygen is not participating in hydrogen bonding but
the oxygen atoms of nitro group present at para position
on phenolate ring of L1-Oꢁ are involved in bifurcated
hydrogen-bonding. This is an example of self-complemen-
tary hydrogen-bonding interactions. The bifurcated
C–H. . .O hydrogen-bonding interactions involving H(14B)
of N-methyl and H(6) of Ru-coordinated benzene with
O(3) of nitro group leads to the formation of dimeric motif
[Fig. 2a]. These dimeric units in 1 are involved in another
bifurcated self-complementary C–H. . .O hydrogen-bonding
interactions involving H(1) and H(2) of Ru-coordinated
benzene with O(2) of nitro group leading to the formation
of one-dimensional hydrogen-bonded chain (Fig. 3).
1
The only one set of H NMR signals for the complexes
1–4a, and 4b suggest to believe that the complex formation
is diastereoselective. So, to ascertain the configuration of
diastereomer present in solution for complexes 1–4a, and
4b, 1H NOE experiments were carried out. No NOEs were
observed between the methylene spacer of phenolate group
and ethylene spacer of pyridine in complexes 1, 2, and 4b.
This clearly suggests that methylene and ethylene spacers
are far away from each other in these complexes. However,
for compound 3 several NOEs were observed between the
methylene protons [Fig. S5 (Supplementary material)].
The irradiation of the Ha and Hb (–CH2– of benzyl) pro-
Table 4
Hydrogen-bonding (C–H. . .O) parameters for [(g6-C6H6)Ru(L1-O)]+ in 1
and [(g6-C6H6)Ru(L3-O)]+ in 3
0
tons induces NOE enhancement of the Hb (–CH2– of
00
PhO) and Hb (–CH2– of Py) protons and vice versa,
D–H. . .
A
H. . .A (A)
D. . .A (A)
D–H. . .
A
˚
˚
0
respectively and the irradiation of Ha (–CH2– of PhO)
[(g6-C6H6)Ru(L1-O)]+ unit in 1
00
induces NOE enhancement of the Ha (–CH2– of Py) pro-
C1–H1. . .O2
C2–H2. . .O2
C6–H6. . .O3
C14–H14B. . .O3
2.598i
2.466i
2.496iii
2.481iii
3.2487(4)i
3.1929(3)i
3.1420(3)iii
3.4273(4)iii
117.8ꢁii
123.3ꢁii
117.1ꢁiii
145.2ꢁiii
ton and vice versa. The NOE results for compound 3
clearly indicate that the diastereomeric pair present in solu-
tion is in agreement with that obtained (SRu,RN/RRu,SN)
from X-ray analysis (single crystals of compound 3). The
1H NMR spectral features of the samples of 1–3, and 4b
(crushed single-crystals) are identical with those obtained
from the bulk powdered samples of the corresponding
complexes, showing the presence of a single diastereomeric
pair in solution.
[(g6-C6H6)Ru(L3-O)]+ unit in 3
C25–H25A. . .O1
2.658iv
3.6988(4)iv
171.5ꢁiv
i
1 + x, 1 + y, z.
ii
ꢁ1 + x, 1 + y, z.
iii
2 ꢁ x, 1 ꢁ y, 1 ꢁ z.
iv
2 ꢁ x, ꢁy, ꢁz.

3256
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
Fig. 2. View of the formation of dimer through C–H. . .O hydrogen
bonding in (a) [(g6-C6H6)Ru(L1-O)]+ unit in 1 and (b) [(g6-C6H6)Ru(L3-
O)]+ unit in 3. All the hydrogen atoms except those involved in hydrogen
bonding have been omitted for clarity.
Fig. 3. View of the formation of the bimolecular 1-D chain through C–
H. . .O hydrogen bonding in [(g6-C6H6)Ru(L1-O)]+ unit in 1. All the
hydrogen atoms except those involved in hydrogen bonding have been
omitted for clarity.
In 3 the C–H. . .O interaction involving C–H(H25A) of
tert-butyl group present at the para position of phenolate
ring and Ru-coordinated phenolate O(1) leads to the for-
mation of dimeric motifs, via self-complementary hydro-
gen-bonding interactions (Fig. 2b).
stable enough for their CV, absorption, and EPR spectra
to be recorded. It is, therefore, logical to assume that there
is no gross structural change during redox process. One-
electron oxidized complexes [2]2+Å, [3]2+Å, and [4a]2+Å dis-
play in CH3CN solution reversible (DEp = 70 mV for
[2]2+Å and [3]2+Å; 80 mV for [4a]2+Å) CV responses at 0.75,
0.82, and 0.77 V vs. SCE, respectively (Figs. S8–S10, Sup-
plementary material).
It is worth comparing the redox behavior of complexes
2, 3, and 4a with that of 1. In the latter case no oxidative
response could be seen. This finding is understandable
given the presence of two electron-releasing tert-butyl
groups at o- and p-positions of phenolate rings in the
ligands L2-Oꢁ, L3-Oꢁ, and L4-Oꢁ rendering the phenolate
rings electron-rich and hence facilitating the removal of an
electron, which in turn stabilizes phenoxyl radical formed
due to one-electron oxidative processes. The bulky tert-
butyl groups provide hydrophobic environment which
helps in stabilization of the generated radical species. It is
also worth noting here that the complexes 2, 3, and 4a all
3.5. Electrochemical studies
From the standpoint of investigating the potential of the
chosen ligands (Scheme 1) to generate phenoxyl radical
species, cyclic voltammetric (CV) experiments on the pres-
ent compounds were performed at 298 K in CH3CN con-
taining 0.1 M TBAP as the supporting electrolyte. It was
revealed that complex 1 is redox-inactive. As expected,
due to the presence 2,4-di-tert-butylphenolate groups, com-
pounds 2, 3, and 4a display a reversible one-electron oxida-
tive process at E1/2 values (V vs. SCE) of 0.74, 0.82, and
0.78, respectively. The redox processes correspond to the
reversible formation of phenoxyl radical species [2]2+Å
,
[3]2+Å, and [4a]2+Å, respectively. The one-electron nature
of redox processes are revealed by constant potential
(V vs. SCE) electrolysis [0.94, n (the number of electron
passed per molecule) = 0.97; 1.0, n = 1.12; 0.97, n = 0.94
for 2, 3, and 4a, respectively]. The electrogenerated blue
([2]2+Å and [3]2+Å) and greenish blue ([4a]2+Å) solutions are

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
3257
having identical phenolate moiety but still the observed
E1/2 values are different. The present trend in E1/2 values
of complexes 2, 3, and 4a can be rationalized by consider-
ing the coordinating properties of the two N-donor sites
provided by the appended arm to the phenolate moiety,
which differ in ligands L2-Oꢁ, L3-Oꢁ, and L4-Oꢁ in com-
plexes 2, 3, and 4a, respectively. Apart from benzene and
one phenolate oxygen coordination in 2 and 3 both the fifth
and sixth coordinations are provided by one pyridine nitro-
gen and one alkylamine nitrogen. But in 2 the pyridine
nitrogen and alkylamine nitrogen is forming a six-mem-
bered chelate ring and in 3 it is forming a five-membered
chelate ring. It is well-established that a five-membered che-
late-ring forming ligand provide better binding to a metal
ion than a six-membered chelate-ring forming ligand. Pyr-
idine is a p-accepting ligand and it is expected to act as a
better p-acceptor, when present as a part of a five-
membered chelate ring. Therefore, in 3 pyridine is with-
drawing more electron density from RuII center and as a
consequence of which RuII is withdrawing more electron
density from phenolate oxygen and benzene (cf. 1H
NMR and X-ray structure). This in turn makes the pheno-
late oxygen comparatively poorer in electron density in 3
than in 2. Thus, between 2 and 3 it is understandable
why the E1/2 value for 3 (0.82 V vs. SCE) is more anodic
than 2 (0.74 V vs. SCE). Notably, in 4a two coordinations
are provided by r-donating alkylamine nitrogens and they
are involved in the formation of a five-membered chelate
ring. So why the E1/2 value for 4a (0.78 V vs. SCE) is lower
than that of 3 is understandable. The higher E1/2 value for
4a than that of 2 (0.74 V vs. SCE), may be attributed to
better binding ability of pyridine nitrogen, due to p-accept-
ing property, than the alkylamine nitrogen Fig. 4.
a
0.70
0.78
3.6. Stability of phenoxyl radical coordinated RuII species
b
0.78
We subjected the one-electron oxidized solutions of
phenoxyl radical coordinated RuII species to UV–Vis and
EPR spectroscopy. Solutions of 2, 3, and 4a in CH3CN
are orange; however, the one-electron oxidized counter-
parts are blue to greenish blue in color. The electronic
spectra of electrochemically generated solutions of [2]2+Å
[Fig. 5 (dotted line)], [3]2+Å and [4]2+Å (Figs. S11 and S12,
Supplementary material) show two characteristic absorp-
tions: 390 nm (ꢀ ꢀ 3100 dm3 molꢁ1 cmꢁ1) and 680 nm
0.85
6
5
4
3
2
1
0
4
3
2
1
0
c
0.73
0.82
400
600
λ (nm)
800
1000
Fig.
4. Cyclic
voltammogram
(scan
rate:
100 mV sꢁ1
)
of
ꢀ1.0 · 10ꢁ3 mol dmꢁ3CH3CN (ꢀ0.1 mol dmꢁ3 in TBAP) solution of (a)
[(g6-C6H6)Ru(L2-O)][PF6] (2), (b) [(g6-C6H6)Ru(L3-O)][PF6] (3), and (c)
[(g6-C6H6)Ru(L4-O)][PF6] (4a) at a Pt working electrode. Indicated
potentials (in V) are vs. SCE.
Fig. 5. UV–Vis spectra (in CH3CN) of [(g6-C6H6)Ru(L2-O)][PF6] (2) (—)
and [(g6-C6H6)Ru(L2-O)]2+Å [2]2+Å (– – –), generated in solution by
coulometric oxidation of 2.

3258
H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
(ꢀ ꢀ 2650 dm3 molꢁ1 cmꢁ1) for [2]2+Å; 395 nm (ꢀ ꢀ 2200
for [3]2+Å; 395 nm (ꢀ ꢀ 3200 dm3 molꢁ1 cmꢁ1) and 720 nm
(ꢀ ꢀ 3250 dm3 molꢁ1 cmꢁ1) for [4a]2+Å. Similar absorptions
have previously been observed for phenoxyl radicals and
are attributed to p–p* transitions [9a]. All the three phe-
noxyl radical complexes are moderately stable in air and
at ambient temperature, but gradually decompose via
first-order kinetics. Complex [4a]2+Å is most stable with
dm3 molꢁ1 cmꢁ1) and 850 nm (ꢀ ꢀ 1550 dm3 molꢁ1 cmꢁ1
)
kdecay = 4.99 · 10ꢁ3 minꢁ1 followed by [2]2+Åwith kdecay
=
1.3
8.15 · 10ꢁ3 minꢁ1 and [3]2+Å with kdecay = 4.33 · 10ꢁ2
minꢁ1. A plot of decay for [2]2+Å is shown in Fig. 6 (inset)
and for [3]2+Å and [4a]2+Å in Figs. S13 and S14 (Supporting
Information). Kinetic studies reveal that the phenoxyl rad-
ical coordinated to Ru(II) complexes having ‘‘piano-stool
geometry’’ is more stable than the reported ZnII and CuII
coordinated similar phenoxyl radical species [9d]. An inter-
esting spectral change was observed during the gradual
decomposition of [2]2+Å and [3]2+Å. An intense band around
1000 nm appears. So far we could not identify the decom-
posed products. Efforts are underway to isolate and estab-
lish the identity of such end-products.
1.2
2.0
1.1
1.0
0.9
0.8
1.5
0.7
0.6
0
100
200
300
400
Time (min)
1.0
0.5
0.0
In line with expectation the X-band EPR spectra in
CH3CN/toluene at 298 K of the electrochemically oxidized
phenoxyl radical forms of 2, 3, and 4a exhibit isotropic
(giso) signals at 2.008, 2.036, and 2.008, respectively
(Fig. 7).
400
600
800
1000
(nm)
Fig. 6. Spectral change due to the formation and decomposition of the
phenoxyl radical complex [(g6-C6H6)Ru(L2-O)]2+Å [2]2+Å in CH3CN
(0.1 mol dmꢁ3 TBAP) at 298 K. Inset: first-order plot of decay.
4. Conclusions
a
Despite a few examples of structurally characterized
mononuclear three-legged half-sandwich complexes of
type [{(g6-C6H6)Ru(L)]+ [L = mononegative salicylalde-
hyde-based Schiff base ligand (only one example with
reduced Schiff base ligand)], no report is available in the
literature on systematic studies of similar complexes with
use of pyridylalkylamine/alkylamine–phenolate-based tri-
dentate non-Schiff base ligands. In this work we have pro-
vided four such examples. Notably, due to coordination
by the ligands the Ru center assumes a chiral center
and in turn the central amine nitrogen also becomes chi-
2.008
b
1
ral. The H NMR spectra exhibit only one set of signals;
thus the reaction is completely diastereoselective. Interest-
ingly, we were able to discover that two such complexes
are involved in non-covalent interactions (C–H. . .O) in
the solid state. Notably, out of four ligands chosen in this
work three carry sites suitable for one-electron ligand-
based oxidation, leading to generation of RuII-coordi-
nated phenoxyl radical-based ligands. Such species have
been characterized by cyclic voltammetry, UV–Vis, and
EPR spectroscopy. Stability of these species has been fol-
lowed by absorption spectroscopy. The present study thus
provides further information on complexes of type [{(g6-
C6H6)Ru(L)]2+Å, which are one-electron oxidized counter-
parts of [{(g6-C6H6)Ru(L)]+.
2.036
c
2.008
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.
Fig. 7. EPR spectra in a mixture of CH3CN and toluene (2:1; v/v) of (a)
[(g6-C6H6)Ru(L2-O)]2+Å [2]2+Å (b) [(g6-C6H6)Ru(L3-O)]2+Å [3]2+Å and (c)
[(g6-C6H6)Ru(L4-O)]2+Å [4a]2+Å [coulometrically generated in CH3CN].

H. Mishra, R. Mukherjee / Journal of Organometallic Chemistry 692 (2007) 3248–3260
3259
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HM gratefully acknowledges University Grants Commis-
sion (UGC), New Delhi for a Junior Research Fellowship.
We thank Atasi Mukherjee for the synthesis of 2,4-di-tert-
butyl-6-(chloromethyl)phenol fragment. We thank Himan-
shu Arora for kinetic study of the complexes. We gratefully
acknowledge Prof. Y.D. Vankar of our Department for his
valuable suggestion in determining the configuration of the
compounds. R.M. gratefully acknowledges the award of a
joint collaborative Grant with Prof. Siegfried Schindler
funded by the Volkswagen Foundation, Germany and
the donation of a HP 8453 diode-array spectrophotometer.
Comments of the reviewers at the revision stage are highly
appreciated.
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˘
ˇ
(h) P. Steˇpnicˇka, J. Ludvik, J. Canivet, G. Suss-Fink, Inorg. Chim.
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CCDC 631899, 631900, and 632188 contain the supple-
mentary crystallographic data for 1, 3, and 4b Æ 1/2CH3O-
H Æ H2O. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from
the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary
data associated with this article can be found, in the online
version, at doi:10.1016/j.jorganchem.2007.03.041.
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