Encapsulating ruthenium and osmium with tris(2-aminoethyl)amine based tripodal ligands

Article abstract of DOI:10.1016/j.poly.2011.09.010

The reaction of a series of tripodal ligands, H₃L^1,2 and L^3-6, with [M(PPh₃)₂Cl₂] (M = Ru, Os) affords a family of coordination cage compounds of the type [M ^IIIL^1,2]

Full text of DOI:10.1016/j.poly.2011.09.010

Polyhedron 31 (2012) 167–175
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Polyhedron
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Encapsulating ruthenium and osmium with tris(2-aminoethyl)amine based
tripodal ligands
Soumik Mandal a, Dipravath K. Seth b, Parna Gupta a,
⇑
a Department of Chemical Sciences, Indian Institute of Science Education and Research, Kolkata, BCKV Main Post Office, Mohanpur 741252, West Bengal, India
b Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
The reaction of a series of tripodal ligands, H3L1,2 and L3–6, with [M(PPh3)2Cl2] (M = Ru, Os) affords a fam-
ily of coordination cage compounds of the type [MIIIL1,2] (1–4) or [MIIL3–6](BPh4)2 (5–12). The Schiff base
ligands (H3L1, L3, L5) have been synthesized by condensation of tris(2-aminoethyl)amine with salicylalde-
hyde, pyridine-2-aldehyde and 1-methyl-2-imidazolecarboxaldehyde. These ligands were further
reduced and subsequently methylated to form the new ligands (H3L2, L4, L6). Single crystal X-ray diffrac-
tion studies of 1 and 2 show that the tripodal ligand wraps around the metal center as a hexadentate
ligand to form a cage. All the synthesized compounds have been thoroughly characterized by ESI-MS,
FT-IR, UV–Vis and NMR spectroscopic methods. To the best of our knowledge, this is the first ever report
of osmium complexes with tris(2-aminoethyl)amine based tripodal ligands. DFT calculations were per-
formed to obtain geometry optimized structures of all the other complexes (3–12).
Article history:
Received 21 July 2011
Accepted 7 September 2011
Available online 17 September 2011
Keywords:
Ru/Os-complex
Tris(2-aminoethyl)amine Schiff base
Coordination cage
Fluorescence
DFT calculation
Ó 2011 Elsevier Ltd. All rights reserved.
Geometry optimization
1. Introduction
and emission patterns [35–37]. The interesting photophysical
properties help these classes of complexes to find application in
The importance of tripodal tetraamine ligands in coordination
chemistry has been amply demonstrated over many years [1–4].
A large number of such ligands, also known as podands, have been
prepared, and metal complexes containing these ligands have been
shown to exhibit a wide variety of physical and chemical proper-
ties [5,6]. The binding sites in these podands are hard (mostly N
and O), and therefore stabilize transition metal centers very well
[7–10]. They have drawn much attention in recent years, mainly
due to their possession of spheroidal cavities which enable them
to efficiently sequester metal ions. They also have demonstrated
their potential use in the synthesis of polynuclear structures
[11–16].
Tris(2-aminoethyl)amine is one of the most rigorously used tri-
podal tetramine frameworks, due to its variable coordination
modes leading to variety of structural assembles [17–21]. It was
first synthesized long ago, dating back to 1896 [22], and it is still
highly relevant, due to its versatility as a ligand, and also for the
interesting applications of its complexes in the synthesis of novel
materials [23–25] and medicine [26–28]. One of the ways in which
the basic ligand skeleton of a tripodal tetraamine ligand can be
altered is the formation of Schiff bases [29–33]. The metal com-
plexes of Schiff bases based on tris(2-aminoethyl)amine are very
interesting as they show good anion receptive properties [34]
molecular recognition [38,39].
The tris(2-aminoethyl)amine molecule has a flexible tripodal
structure with three aminoethyl groups showing multidentate
coordination modes. Interestingly, the coordination chemistry of
the tris(2-aminoethyl)amine Schiff base based tripodal ligand has
remained largely unexplored. A literature survey shows that the
chemistry of this type of ligand with the 4d series of metals, except
ruthenium, is not so well known [40,41]. In the present study,
tris(2-aminoethyl)amine is allowed to react with salicylaldehyde,
pyridine-2-aldehyde and 1-methyl-2-imidazolecarboxaldehyde to
form Schiff bases, then these ligands were further reduced and
subsequently the imine(–CH@N–) nitrogen atoms were methyl-
ated. All six ligands (Fig. 1) were allowed to react with ruthenium
and osmium precursors to form coordination cage [42,43]
complexes through N,O or N,N chelating atoms. This study reports
the first synthesis and complete characterization of osmium
complexes with tris(2-aminoethyl)amine based ligands. All the
complexes are expected to be chiral because of the spiral coordina-
tion arrangement of the achiral ligands around the metal. The CD
spectra of [RuL1] (1), [OsL1] (2) and [RuL2] (3) indicate the presence
of a racemic mixture for each in solution, as expected. Interest-
ingly, there are considerable changes in the emission pattern as
well as the wavelength of emission between these ligands and
their corresponding complexes. DFT calculations were performed
to obtain geometry optimized structures of the ruthenium
([RuL2] (3), [RuL3(BPh4)2] (5), [RuL4(BPh4)2] (7), [RuL5(BPh4)2] (9)
⇑
Corresponding author. Tel.: +91 3473279130; fax: +91 3473279131.
E-mail address: parna.gupta@gmail.com (P. Gupta).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.09.010

168
S. Mandal et al. / Polyhedron 31 (2012) 167–175
N
N
HO
N
OH
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
HO
H3L1
L5
L3
HO
N
N
OH
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
HO
L6
H3L2
L4
Fig. 1. Schematic diagram of the ligands H3L1,2, L3–L6.
and [RuL6(BPh4)2] (11)) and osmium ([OsL2] (4), [OsL3(BPh4)2] (6),
[OsL4(BPh4)2] (8), [OsL5(BPh4)2] (10) and [OsL6(BPh4)2] (12)) com-
plexes and to establish the nature of the orbitals involved in the
transition processes and to correlate the structural parameters
with the spectroscopic properties of the complexes. An account
of the chemistry of the complexes of the tris(2-aminoethyl)amine
based tripodal ligands is presented here, with special reference to
their formation, structure and spectral and electrochemical
properties.
2.2.2. L3
To a solution of pyridine-2-aldehyde (1 g, 9.34 mmol) in ethanol
(20 mL) was added tris(2-aminoethyl)amine (0.46 g, 3.11 mmol) in
dry toluene (50 mL). The mixture was refluxed and stirred over-
night, and the resulting liquid evaporated to dryness, affording
an oily substance. The oily mass upon washing with cold ethanol
gives the pure ligand.
Yield: 0.64 g (50%); Anal. Calc. for C24H27N7: C, 69.71; H, 6.58; N,
23.71. Found: C, 69.45; H, 6.55; N, 23.92%; ESI-MS m/z: 414.83
(M)+, 147.94 (NCH2CH2N@CHPy)+. 1H NMR (400 MHz, CDCl3, d,
ppm): 10.05 (–CH, 3H), 8.57 (3H, d, aromatic ring), 7.6–8.35 (9H,
aromatic ring), 3.76 (6H, t, NCH2CH2), 2.96 (6H, t, NCH2CH2); 13C
NMR (500 MHz, CDCl3, d, ppm): 162.64 (N@C), 154.38 (aromatic
ring), 149.24 (aromatic ring), 136.44 (aromatic ring), 124.52 (aro-
matic ring), 121.18 (aromatic ring), 59.73 (NCH2CH2), 55.22
2. Experimental
2.1. Materials and methods
The starting materials RuCl3.3H2O, (NH4)2[OsCl6], tris(2-amino-
ethyl)amine, salicylaldehyde, pyiridine-2-aldehyde, 1-methyl-2-
imidazolecarboxaldehyde and triphenylphosphine were purchased
from Sigma–Aldrich and were used without purification. All the
solvents were dried by the usual methods prior to use.
[Ru(PPh3)3Cl2], [Ru(CO)2(PPh3)2Cl2] and [Os(PPh3)3Cl2] were pre-
pared according to the reported procedures [44–46].
(NCH2CH2). IR (cmÀ1, KBr pellet): 3435–2850 (
C@N), 1587, 1469, 1436, 774.
m-H), 2850, 1651
(m
2.2.3. L5
To a solution of 1-methyl-2-imidazolecarboxaldehyde (1 g,
9.08 mmol) in dry methanol (50 mL) was added tris(2-amino-
ethyl)amine (0.44 g, 3.01 mmol) in dry toluene (50 mL). The mix-
ture was refluxed and stirred for overnight, and then the
resulting liquid was purified by column chromatography. Yield
0.96 g (76%); Anal. Calc. for C21H30N10: C, 59.69; H, 7.16; N, 33.15.
Found: C, 60.01; H, 7.25; N, 33.08%; ESI-MS m/z: 422.89 (M)+; 1H
NMR (400 MHz, CDCl3, d, ppm): 2.88–2.92 (br, 6H, NCH2CH2),
3.64–3.67 (br, 6H, NCH2CH2), 3.89 (s, 9H, N-CH3), 6.86 (d, 3H, imid-
azole ring), 7.04 (d, 3H, imidazole ring), 8.25 (s, 3H, CH); 13C NMR
(500 MHz, CDCl3, d, ppm): 153.77 (N@C), 142.96 (imidazole ring),
128.92 (imidazole ring), 124.63 (imidazole ring), 60.25 (NCH2CH2),
55.29 (NCH2CH2), 35.15 (N–CH3); IR (cmÀ1, KBr pellet): 3400–2352
2.2. Synthesis of the ligands
2.2.1. H3L1
To a solution of salicylaldehyde (1 g, 8.20 mmol) in ethanol
(20 mL) was added tris(2-aminoethyl)amine (0.40 g, 2.73 mmol)
in absolute ethanol (20 mL). A yellow precipitate was formed
immediately. The mixture was refluxed and stirred for 2 h, the
resulting solid was filtered off, washed with diethyl ether, and
dried in air to obtain the desired compound.
(m-H), 1651 (mC@N), 1479, 1439, 1288, 764.
Yield 1.13 g (90%); Anal. Calc. for C27H40N4O3: C, 69.20; H, 8.60;
N, 11.96. Found: C, 69.10; H, 8.80; N, 11.90%; ESI-MS m/z: 458.88
(M+), 311.87 (M–CH2CH2N@CHPhOH)+, 165.99 (M+–{CH2CH2-
N@CHPhOH}2)+; 1H NMR (400 MHz, CDCl3, d, ppm): 2.75 (m, 6H),
3.45 (m, 6H), 5.98 (d, 3H), 6.50 (t, 3H), 6.84 (t, 3H), 7.18 (t, 3H),
7.72 (s, 3H); 13C NMR (500 MHz, CDCl3, d, ppm): 166.09 (N@C),
116.68 (aromatic ring), 118.48 (aromatic ring), 131.75 (aromatic
ring), 161.05 (aromatic ring), 57.89 (NCH2CH2), 55.80 (NCH2CH2);
2.2.4. Synthesis for H3L2, L4 and L6
H3L2, L4 and L6 were prepared by similar procedures [47]. A de-
tailed method is given for one representative case.
To a solution of H3L1 (0.50 g, 1.09 mmol) in dry methanol,
NaBH4 (0.22 g, 6 mmol) was added slowly at low-temperature
(ꢀ5 °C). The mixture was stirred overnight at room temperature.
The solvent was evaporated and 20 mL water was added. The aque-
ous solution was treated with dilute hydrochloric acid until pH 7–8
was reached. The solution was extracted with dichloromethane
IR (cmÀ1, KBr pellet): 3436 (b,
mO–H, typical for intramolecular
hydrogen bonded O–H), 3000–2800 (
mC–H), 1632 (mC@N), 1610,
1582, 1498, 1459, 1430, 1337, 756.

S. Mandal et al. / Polyhedron 31 (2012) 167–175
169
and dried over anhydrous Na2SO4. The reduced amine was ob-
IR (cmÀ1, KBr pellet): 3055–2857 (
m
C–H), 1600–1531 (
m
C@N), 1438,
tained as a colorless solid (orange and pale yellow liquids from
L3 and L5, respectively). To a solution of the amine in dichlorometh-
ane, formaldehyde (0.18 g, 6 mmol) was added with stirring. So-
dium acetoxyborohydride (2.12 g, 10 mmol) was added to this
mixture, which was subsequently stirred for 24 h. The solution
was neutralized with K2CO3 solution. The organic layer was
washed with water and separated with a separating funnel and
dried over fused Na2SO4. Removal of the solvent afforded pure
H3L2 (or L4 and L6 from L3 and L5, respectively).
1242, 1120, 722.
2.3.2. [OsL1] (2)
To a hot solution of H3L1 (0.046 g, 0.10 mmol) and triethylamine
(0.30 g, 0.30 mmol) in warm 2-methoxyethanol (40 mL) was added
[Os(PPh3)2Cl2] (0.10 g, 0.10 mmol). The mixture was heated at re-
flux for 24 h to produce a dark brownish solution. Stripping of
the solvent from the solution under vacuum gave a brownish solid,
which was subjected to thin-layer chromatography on a silica
plate. With acetonitrile:toluene (1:4) as the eluant, a brown band
separated out. The brown band was extracted with acetonitrile,
and slow evaporation of the acetonitrile extract gave the brown
crystalline [OsL1]. Yield: 0.074 g, 50%. Anal. Calc. for C27H27N4O3Os:
C, 50.22; H, 4.21; N, 8.68. Found: C, 50.28; H, 4.30; N, 8.79%; ESI-MS
m/z: 647.75 (M)+; 1H NMR: 7.52–7.56 (broad), 7.10 (broad), 6.73
(broad), 3.16–3.52 (broad), 2.61–2.67 (broad); IR(cmÀ1, KBr pellet):
3401–2680, 1599, 1456, 1109, 1037, 758.
2.2.5. H3L2
Yield: 0.342 g, 61%; Anal. Calc. for C29H40N4O3: C, 70.70; H, 8.18;
N, 11.37. Found: C, 70.85; H, 8.12; N, 11.27%; ESI-MS m/z: 506.77
(M+), 400.87 [M–(CH2PhOH)]+; 1H NMR (400 MHz, CDCl3, d,
ppm): 2.22 (s, 9N–CH3), 2.52–2.63 (complex, 12H), 3.66 (s, 6CH2),
6.74–7.18 (12H, aromatic); 13C NMR (500 MHz, CDCl3, d, ppm):
157.75 (salicylaldehyde C–OH), 128.73 (aromatic ring), 128.52
(aromatic ring), 121.92 (aromatic ring), 119.0 (aromatic ring),
116.01 (aromatic ring), 61.12 (N–CH2–aromatic ring), 54.30 (N–
CH2CH2), 52.13 (N–CH2CH2), 41.87 (N–CH3); IR (cmÀ1, KBr pellet):
2.3.3. [RuL2] (3)
To a hot solution of H3L2 (0.051 g, 0.10 mmol) and triethylamine
(0.30 g, 0.30 mmol) in ethanol (40 ml), was added [Ru(PPh3)2Cl2]
(0.096 g, 0.10 mmol). The mixture was heated at reflux for 24 h
to produce a deep purple solution. Stripping of the solvent from
the solution under vacuum gave a greenish solid. Using TLC with
acetonitrile:toluene (1:4) as the eluant, a purple band (Rf = 0.6)
separated out. The purple band was extracted with acetonitrile,
and slow evaporation of the acetonitrile extract gave a purple crys-
talline solid of the composition [RuL2]. Yield: 0.060 g, 50%. Anal.
Calc. for C30H39N4O3Ru: C, 59.58; H, 6.50; N, 9.26. Found: C,
60.03; H, 6.30; N, 9.15%; ESI-MS m/z: 607.7 (M+); 1H NMR
(400 MHz, CDCl3, d, ppm): 3.35–4.5 (complex), 6.0–8.0 (complex),
10.08. IR (cmÀ1, KBr pellet): 3792–2920, 1614, 1478, 1265, 1093,
755.
3436 (b, mO–H, typical for intramolecular hydrogen bonded O–H),
3000–2817 (mC–H), 1589, 1488, 1256 (mN–C), 754.
2.2.6. L4
Yield: 0.317 g, 57%; Anal. Calc. for C27H39N7: C, 70.25; H, 8.52; N,
21.24. Found: C, 70.21; H, 8.46; N, 21.22%; ESI-MS m/z: 461.97
(M)+; 1H NMR (400 MHz, CDCl3, d, ppm): 2.16 (s, 9N–CH3), 2.47
(complex, 6H), 2.59 (complex, 6H), 3.58 (s, 6CH2), 8.46 (d, 3H),
7.56 (t, 3H), 7.33 (d, 3H), 7.08 (t, 3H); 13C NMR (500 MHz, CDCl3,
d, ppm): 159.01 (aromatic ring adjacent to pyridine N), 148.86
(aromatic ring adjacent to pyridine N), 136.31 (aromatic ring),
123.03 (aromatic ring), 121.86 (aromatic ring), 64.02 (N–CH2–aro-
matic ring), 55.29 (N–CH2CH2), 52.60 (N–CH2CH2), 42.76 (N–CH3);
IR (cmÀ1, KBr pellet): 3389–2352 (
1297 ( N–C), 1034, 760.
mN–H), 1591, 1471, 1434, 1361,
m
2.3.4. [OsL2] (4)
To a hot solution of H3L2 (50.6 mg, 0.10 mmol) and triethyl-
amine (0.30 g, 0.30-mmol) in 2-methoxyethanol (40 mL), was
added [Os(PPh3)2Cl2] (0.10 g, 0.10 mmol). The mixture was heated
at reflux for 24 h to produce a light brownish solution. Stripping of
the solvent from the solution under vacuum gave a brownish solid,
which was subjected to thin-layer chromatography on a silica
2.2.7. L6
Yield: 0.294 g, 52%; Anal. Calc. for C24H42N10: C, 61.25; H, 8.99;
N, 29.76. Found: C, 61.23; H, 9.05; N, 29.89%; ESI-MS m/z: 470.96
(M+), 456.97 [M+–CH3]+, 331.89 [M+–(CH2N(Me)CH2(imidazole))]+;
1H NMR (400 MHz, CDCl3, d, ppm): 6.75–6.87 (imidazole ring, com-
plex), 4.66 (s, 3CH2), 3.63 (s, 9CH3), 2.39–2.46 (12H, NCH2CH2),
2.14 (s, 9CH3); 13C NMR (500 MHz, CDCl3, d, ppm): 145.13 (imidaz-
ole ring), 126.67 (imidazole ring), 121.45 (imidazole ring), 54.54
(N–CH2–aromatic ring), 54.14 (NCH2CH2), 52.04 (NCH2CH2),
42.06 (N–CH3), 32.86 (N–CH3, aromatic ring); IR (cmÀ1, KBr pellet):
plate. With acetonitrile as the eluant,
a light brown band
(Rf = 0.3) separated out. The light brown band was extracted with
acetonitrile, and slow evaporation of the acetonitrile extract gave
the brown crystalline [OsL2]. Yield: 0.057 g, 42%; Anal. Calc. for
C
30H39N4O3Os: C, 51.93; H, 5.67; N, 8.07. Found: C, 52.87; H,
3401–2360 (mN–H), 1568, 1501, 1455, 1284, 1025, 801, 755.
5.74; N, 8.12%; ESI-MS m/z: 744.59 (M+K)+; 1H NMR (400 MHz,
CDCl3, d, ppm): 2.09, 2.23, 3.2–3.54 (broad), 3.70, 6.7–6.8 (broad),
7.07–7.17 (broad), 7.51–7.68 (complex, broad); IR (cmÀ1, KBr pel-
let): 3908–2360, 1637, 1245, 1094, 723.
2.3. Synthesis of the complexes
2.3.1. [RuL1] (1)
To a hot solution of H3L1 (0.046 mg, 0.10 mmol) and triethyl-
amine (0.30 g, 0.3 mmol) in warm ethanol (40 mL) was added
[Ru(PPh3)2Cl2] (0.096 g, 0.10 mmol). The mixture was heated at re-
flux for 24 h to produce a dark brownish-orange solution. Stripping
of the solvent from the solution under vacuum gave a brownish-or-
ange solid, which was subjected to thin-layer chromatography on a
silica plate. With acetonitrile:toluene (1:4) as the eluant, a green
band separated out. The green band was extracted with acetoni-
trile, and slow evaporation of the acetonitrile extract gave a green
crystalline solid of the composition [RuL1]. Yield: 0.073 g, 60%.
Anal. Calc. for C27H27N4O3Ru: C, 58.26; H, 4.89; N, 10.07. Found:
C, 58.22; H, 4.77; N, 10.02%; ESI-MS m/z: 557.92 (M+), 413.12
[(M+–CH2CH2N@CHPhOH)]+; 1H NMR (400 MHz, CDCl3, d, ppm):
À25.73, À20.06, À9.17, 7.61, 11.20, 13.89, 17.04 (see Section 3);
2.3.5. [RuL3](BPh4)2 (5)
To a hot solution of L3 (0.041 g, 0.10 mmol) in ethanol (40 mL),
[Ru(PPh3)2Cl2] (0.096 g, 0.10 mmol) was added. The mixture was
heated to reflux for 24 h, resulting in a dark reddish-brown solu-
tion. Evaporation of this solution under vacuum gave a reddish-
brown solid, which was dissolved in methanol and NaBPh4 was
added. The mixture was again stirred for 30 min. The resulting
solution was filtered, and the residue was washed with a little
methanol to yield the dark brown colored crystalline complex 5.
Yield: 0.181 g, 65%. Anal. Calc. for C72H67B2N7Ru: C, 75.00; H,
5.86; N, 8.50. Found: C, 74.80; H, 5.94; N, 8.58%; ESI-MS m/z:
833.61 (M+), 514.70 (M–BPh4)+; 1H NMR (400 MHz, DMSO-d6, d,
ppm): 10.09 (–CH, 3H), 6.79–8.81 (aromatic ring, complex), 3.49

170
S. Mandal et al. / Polyhedron 31 (2012) 167–175
(t, 6H, NCH2CH2), 2.17 (t, 6H, NCH2CH2); IR (cmÀ1, KBr pellet):
3401–2360, 1630, 1587, 1469, 1435, 1151, 736.
2.3.10. [Os(L5)](BPh4)2 (10)
To a hot solution of L5 (0.042 g, 0.10 mmol) in ethanol (40 mL),
[Os(PPh3)2Cl2] (0.10 g, 0.10 mmol) was added. The mixture was
heated to reflux for 24 h, resulting in a dark reddish-brown solu-
tion. Evaporation of this solution under vacuum gave a reddish-
brown solid, which was dissolved in methanol and NaBPh4 was
added. The mixture was again stirred for 30 min. The resulting
solution is filtered, and the residue was washed with little metha-
nol to yield a dark maroon color crystalline complex 10. Yield:
0.166 g, 56%; Anal. Calc. for C69H70B2N10Os: C, 66.23; H, 5.64; N,
11.19. Found: C, 66.27; H, 5.69; N, 11.34%; ESI-MS m/z: 932.61
(M+); 1H NMR(400 MHz, DMSO-d6, d, ppm): 2.33 (6H, NCH2CH2),
2.66 (br, 6H, NCH2CH2), 3.92 (s, 9H, N-CH3), 6.75–7.20 (imidazole
ring, BPh4), 8.90(s, 3H, CH); IR(cmÀ1, KBr pellet): 3435–2358,
1478, 1427, 1267, 1103, 847, 733, 708.
2.3.6. [Os(L3)](BPh4)2 (6)
To a hot solution of L3 (0.041 g, 0.10 mmol) in toluene (40 mL),
was added [Os(PPh3)2Cl2] (0.10 g, 0.10 mmol). The mixture was
heated to reflux for 24 h, resulting in a dark reddish-brown solu-
tion. Evaporation of this solution under vacuum gave a reddish-
brown solid, which was dissolved in methanol and NaBPh4 was
added. The mixture was again stirred for 30 min. The resulting
solution was filtered, and the residue was washed with little meth-
anol to yield the dark maroon colored crystalline complex 6. Yield:
0.13 g, 43%; Anal. Calc. for C72H67B2N7Os: C, 69.62; H, 5.44; N, 7.89.
Found: C, 69.67; H, 5.49; N, 7.93%; ESI-MS m/z: 922.56 (M+). 1H
NMR (400 MHz, DMSO-d6, d, ppm): 2.08 (t, 6H, NCH2CH2), 3.63
(6H, NCH2CH2), 6.74–7.8 (aromatic ring, complex, BPh4), 8.45
(–CH, 3H). IR (cmÀ1, KBr pellet): 3368–2359, 1658, 1588, 1477,
1427, 1308, 1152, 845, 734, 706.
2.3.11. [Ru(L6)](BPh4)2(11)
To a hot solution of L6 (0.047 g, 0.10 mmol) in ethanol (40 mL)
[Ru(PPh3)2Cl2] (0.096 g, 0.10 mmol) was added. The mixture was
heated to reflux for 24 h resulting dark reddish-brown solution.
Evaporation of this solution under vacuum gave a reddish-brown
solid, which was dissolved in methanol and NaBPh4 was added.
The mixture was again stirred for 30 min. The resulting solution
was filtered, and the residue was washed with a little methanol
to yield the dark brown colored crystalline complex 11. Yield:
0.157 g, 62%; Anal. Calc. for C72H82B2N10Ru: C, 71.46; H, 6.83; N,
11.57. Found: C, 71.29; H, 6.63; N, 11.79%; ESI-MS m/z: 890.8
(M+); 1H NMR (400 MHz, DMSO-d6, d, ppm): 2.87, 2.96, 3.45–3.54
(complex), 6.75–7.45 (complex, BPh4); IR (cmÀ1, KBr pellet):
3436–2360, 1579, 1479, 1427, 1279, 1089, 734, 706.
2.3.7. [Ru(L4)](BPh4)2 (7)
To a hot solution of L4 (0.046 g, 0.10 mmol) in ethanol (40 mL),
[Ru(PPh3)2Cl2] (0.096 g, 0.10 mmol) was added. The mixture was
heated to reflux for 24 h, resulting in a dark reddish-brown solu-
tion. Evaporation of this solution under vacuum gave a reddish-
brown solid, which was dissolved in methanol and NaBPh4 was
added. The mixture was again stirred for 30 min. The resulting
solution was filtered, and the residue was washed with little meth-
anol to yield the dark brown colored crystalline complex 7. Yield:
0.122 g, 47%; Anal. Calc. for C75H79B2N7Os: C, 74.99; H, 6.63; N,
8.16. Found: C, 75.01; H, 6.70; N, 8.24%; ESI-MS m/z: 881.72
(M+); 1H NMR (400 MHz, DMSO-d6, d, ppm): 2.15 (s, 9N–CH3),
2.50 (complex, 6H), 2.61 (complex, 6H), 3.59 (s, 6CH2), 6.74–8.50
(complex), 9.05 (3H, –CH). IR (cmÀ1, KBr pellet): 3436–2360,
1579, 1477, 1092, 734, 706.
2.3.12. [Os(L6)](BPh4)2 (12)
To a hot solution of L6 (0.047 g, 0.10 mmol) in ethanol (40 mL),
[Os(PPh3)2Cl2] (0.10 g, 0.10 mmol) was added. The mixture was
heated to reflux for 24 h, resulting in a dark reddish-brown solu-
tion. Evaporation of this solution under vacuum gave a reddish-
brown solid, which was dissolved in methanol and NaBPh4 was
added. The mixture was again stirred for 30 min. The resulting
solution was filtered, and the residue was washed with a little
methanol to yield the maroon colored crystalline complex 12.
Yield: 0.117 g, 43%; Anal. Calc. for C72H82B2N10Os Calc: C, 66.55;
H, 6.36; N, 10.78. Found: C, 66.58; H, 6.31; N, 10.68%; ESI-MS m/
z: 1008.53 (M+K)+; 1H NMR (400 MHz, DMSO-d6, d, ppm): 2.10
(s, 9N–CH3), 2.50 (complex, 6H), 3.53 (s, 9CH3), 3.69 (s, 9CH3),
3.80 (s, 3CH2), 6.7–7.6 (complex, BPh4); IR (cmÀ1, KBr pellet):
3434–2362, 1648, 1479, 1285, 1090, 734, 706.
2.3.8. [Os(L4)](BPh4)2 (8)
To a hot solution of L4 (0.046 g, 0.10 mmol) in ethanol (40 mL),
[Os(PPh3)2Cl2] (0.10 g, 0.10 mmol) was added. The mixture was
heated to reflux for 24 h, resulting in a dark reddish-brown solu-
tion. Evaporation of this solution under vacuum gave a reddish-
brown solid, which was dissolved in methanol and NaBPh4 was
added. The mixture was again stirred for 30 min. The resulting
solution was filtered, and the residue was washed with a little
methanol to yield the dark maroon colored crystalline complex 8.
Yield: 0.131 g, 47%; Anal. Calc. for C75H79B2N7Os: C, 69.81; H,
6.17; N, 7.60. Found: C, 70.03; H, 6.11; N, 7.49%; ESI-MS m/z:
972.75 (M+); 1H NMR (400 MHz, DMSO-d6, d, ppm): 2.15 (s, 9N–
CH3), 2.50 (complex, 6H), 2.68 (broad, 6H), 3.38 (broad, 6H), 3.63
(s, 6CH2), 6.76–8.45 (aromatic ring, complex). IR (cmÀ1, KBr pellet):
3436–2373, 1593, 1469, 1102, 733, 704.
2.4. Physical measurements
IR spectra were obtained on a Perkin–Elmer Spectrum RXI spec-
trophotometer with samples prepared as KBr pellets. Elemental
analyses were performed on a Perkin–Elmer 2400 series II CHN
series. Electronic spectra were recorded on a U-4100, HITACHI
spectrometer. 1H NMR spectra were obtained on a Brucker Avance
III-500 NMR spectrometer using TMS as the internal standard.
Electrochemical measurements were made using a PAR model
273 potentiostat. A platinum disk working electrode, a platinum
wire auxiliary electrode and an aqueous saturated calomel refer-
ence electrode (SCE) were used in a three electrode configuration.
Electrochemical measurements were made under a dinitrogen
atmosphere. All electrochemical data were collected at 298 K and
are uncorrected for junction potentials. Fluorescence spectra were
taken on a HORIBA JOBINYVON spectrofluorimeter. Mass spectra
were recorded on a Q-Tof Micromass spectrometer by positive-
ion mode electrospray ionization. Optimization of the ground-state
structures and energy calculations for all the complexes were car-
2.3.9. [Ru(L5)](BPh4)2 (9)
To a hot solution of L5 (0.042 g, 0.10 mmol) in ethanol (40 mL),
[Ru(PPh3)2Cl2] (0.096 g, 0.10 mmol) was added. The mixture was
heated to reflux for 24 h, resulting in a dark reddish-brown solu-
tion. Evaporation of this solution under vacuum gave a reddish-
brown solid, which was dissolved in methanol and NaBPh4 was
added. The mixture was again stirred for 30 min. The resulting
solution was filtered, and the residue was washed with a little
methanol to yield the dark brown colored crystalline complex 9.
Yield: 0.145 g, 53%; Anal. Calc. for C69H70B2N10Ru: C, 71.32; H,
6.07; N, 12.05. Found: C, 71.34; H, 6.11; N, 12.12%; ESI-MS m/z:
842.56 (M+); 1H NMR (400 MHz, DMSO-d6, d, ppm): 2.32 (6H,
NCH2CH2), 2.67 (6H, NCH2CH2), 3.99 (s, 9H, N–CH3), 6.75–7.2
(complex, imidazole ring, BPh4), 8.85 (s, 3H, CH); IR (cmÀ1, KBr pel-
let): 3436–2356, 1634, 1579, 1480, 1288, 1128, 844, 734, 706.

S. Mandal et al. / Polyhedron 31 (2012) 167–175
171
Table 1
Crystal data for 1 and 2.
1
2
Empirical formula
Formula weight
C
27H27N4O3Ru
C27H27N4O3Os
645.76
556.6
Space group
a (Å)
b (Å)
orthorhombic, pbca
13.4886(7)
16.5664(8)
24.2155(12)
5411.1(5)
4
monoclinic, P2(1)/c
12.2154(4)
14.3003(5)
16.1433(5)
2817.42(16)
2
c (Å)
V (Å3)
Z
k (Å)
0.7107
0.7107
Crystal size (mm3)
0.25 Â 0.15 Â 0.10
0.09 Â 0.05 Â 0.04
T (K)
100
100
l
(mmÀ1
)
0.622
0.0399
0.074
4.849
0.0307
0.0809
R[F2 > 2
wR(F2)
r
(F2)]
Goodness-of-fit (GOF)
w
1.126
1.173
1/[
r
2(Fo2) + (0.0046P)2 + 12.7148P] where P = (Fo2 + 2Fc2)/3
1/[r
2(Fo2) + (0.0413P)2 + 12.2131P] where P = (Fo2 + 2Fc2)/3
ried out with the density functional theory (DFT) method using the
GAUSSIAN 03 package [48], restricted and unrestricted (for [RuL1],
[OsL1], [RuL2] and [OsL2]) spin had been considered and where
B3LYP [49] was chosen as the basis function, the 631g(d,p) basis
set was taken for H, C, N and O, and the SDD basis set for ruthe-
nium and osmium. Optimization was carried out until global min-
ima were achieved.
N
N
N
O
N
O
Ru
O
RuL1
2.5. Crystallography of 1 and 2
(BPh4)2
(BPh4)2
N
N
N
N
N
Single crystals of 1 and 2 were obtained by slow diffusion of
dichloromethane into hexane solutions of the complexes. Selected
crystal data and data collection parameters are given in Table 1.
Crystal Data were collected on a Bruker SMART APEXII CCD area-
N
N
N
N
N
N
N
Ru
N
N
Ru
N
N
N
detector diffractometer using graphite monochromated MoK
a
radiation (k = 0.71073 Å). For both the crystals, X-ray data reduc-
tion was carried out using the Bruker SAINT program. The structures
were solved by direct methods using the SHELXL-97 program [50].
The unit cell dimensions were determined by a least squares fit
of 7901 machine centered reflections (2 < h < 25°) for 1 and 9981
machine centered reflections (0 < h < 38°) for 2. Thirty-six standard
reflections were used to check the crystal stability towards X-ray
exposure, and these showed no significant intensity reduction over
the course of the data collection. X-ray data reduction, structure
solution and refinement were done using the SHELXL-97 program
package. Four final cycles of refinement converged with discrep-
RuL3(BPh4)2
RuL5(BPh4)2
Fig. 2. Predicted coordination environment around the metal.
infrared spectra, 1H NMR and ESI-MS. Moreover, The X-ray struc-
tures for two of the complexes were determined. The potentially
heptadentate ligands H3L1,2 are coordinated as a trianionic hexa-
dentate ligand, whereas L3–L6 are coordinated as a neutral hexa-
dentate ligand (Fig. 2). The tertiary amine nitrogen atom remains
uncoordinated in all the complexes. The complexes 1–12 are stable
in air. They are soluble in a wide variety of solvents, like methanol,
dichloromethane, chloroform DMF, DMSO, acetonitrile and
acetone.
ancy indices R[F2 > 2
r
(F2)] = 0.0428 and wR(F2) = 0.1311 for
[RuL1] and R[F2 > 2 (F2)] = 0.0663 and wR(F2) = 0.2246 for [OsL1].
r
3. Results and discussion
All the complexes have been prepared by the direct reaction of
3.1. Description of the crystal structure
tris(2-aminoethyl)amine based heptadentate ligands (H3L1,2
,
L3–L6), and ruthenium or osmium precursors at refluxing
temperature. The ruthenium and osmium precursors
Single crystals of complexes 1 and 2, suitable for X-ray diffrac-
tion studies, were grown by crystallization from dichloromethane–
hexane solution. The molecular structures and atom numbering
schemes of 1 and 2 are presented in Fig. 3. The crystallographic
data and structure analysis for complexes 1 and 2 are summarized
in Table 1 and selected bond distances and bond angles are given in
Table 2. The X-ray results show that the heptadentate H3L1 is coor-
dinated via three imino nitrogen [N2, N3, N4] and three oxygen
atoms [O1, O2, O3, formed through deprotonation of the OH
groups] to the metal, which resides in a distorted octahedral envi-
ronment. The nitrogen atom N1 (sp3 hybridized), which sits on the
top, does not have any bonding interaction with ruthenium or os-
mium. The crystal structure of 1 has been solved previously, but
[Ru(PPh3)3Cl2], [Ru(CO)2(PPh3)2Cl2], RuCl3ÁH2O, (NH4)2[OsCl6] and
[Os(PPh3)3Cl2] were chosen for the present study. Interestingly,
the final products are of the same stoichiometry [ML] (M = Ru,
Os; L = H3L1, H3L2) or [ML]2+ (M = Ru, Os; L = L3–L6) in every case.
The oxidation state of ruthenium and osmium is +3 in [ML]
(L = H3L1, H3L2) and +2 in [ML]2+ (L = L3–L6), irrespective of the
ruthenium or osmium precursors chosen, and the ligands were
able to displace phosphine, carbonyl and halogens efficiently. The
[ML]2+ (L = L3–L6) complexes have been precipitated from solution
as the tetraphenylborate salt. The ligands and the complexes were
characterized by the usual techniques: elemental analyses,

172
S. Mandal et al. / Polyhedron 31 (2012) 167–175
Fig. 4. Geometry optimized structure of complex 5.
Fig. 5.
D-(clockwise) and K-(anticlockwise) enantiomorphs due to the screw
arrangement of the tripod type ligand around the Ru(II) and Os(II) ions.
Fig. 3. Molecular view of 1 and 2.
86.28(11)° and 87.88(10)°, indicating distortion from a regular
octahedron. For the [Os(III)L1] complex, the deviation is significant.
Here, the N3–Os1–N4, N2–Os1–N4, N2–Os1–N3 angles have
opened up to 96.83(16)°, 94.51(14)°, and 94.96(16)° and all other
angles around Os1 have significantly reduced, of which the O1–
Os1–O3 angle of 84.95(13)° is the smallest. The Os–O and Os–N
bond lengths are quite normal and comparable with similar type
osmium(III) complexes [53,54]. Both the structures have disorders
in the solvent molecule.
To get an idea about the trapped ruthenium and osmium cen-
ters in the tripodal ligands H3L2 and L3–L6, geometry optimization
calculations were carried out using GAUSSIAN 03. The metal ion is
definitely surrounded by three Schiff base (imine) nitrogen and
three pyridine/imidazole nitrogen atoms in an octahedral fashion,
and the tertiary amine nitrogen atom is uncoordinated. A literature
survey shows there is not a single report on ruthenium or osmium
complexes with the ligands L3–L6. However, the crystal structure of
[FeL3](ClO4)2 has been reported by Brewer et al. [54], and our cal-
culated bond parameters are in very good agreement with the re-
ported figures, with a bit lengthier Os–N bond. The geometry
optimized structure of complex 5 is given in Fig. 4. The obtained
bond parameters around the metal center are found to be quiet
comparable with the structure reported by Yamaguchi et al. [40]
for complexes 9–12. The change of the sp2 nitrogen(imine) in 9
to the sp3 nitrogen(reduced imine) in 11 is reflected by the change
in the average Ru–N(imine) bond length of 2.274 to 2.291 Å. Bond
parameters (Tables S1–S3) and optimized figures (Figs. S1–S9) are
deposited in Supporting information.
Table 2
Selected bond lengths (Å) and bond angles (°) of 1 and 2.
Bond lengths
Bond angles
Bond angles
1
Ru1–O2 1.999(2)
Ru1–O3 2.018(2)
Ru1–O1 2.027(2)
Ru1–N4 2.039(3)
Ru1–N3 2.058(3)
Ru1–N2 2.070(3)
O2–Ru1–O3 85.03(9)
O2–Ru1–O1 87.88(10)
O3–Ru1–O1 88.22(10)
O2–Ru1–N4 89.03(10)
O3–Ru1–N4 92.93(11)
O1–Ru1–N4 176.60(11)
O2–Ru1–N3 89.86(10)
O3–Ru1–N3 171.55(11)
O1–Ru1–N3 84.87(10)
N4–Ru1–N3 93.70(11)
O2–Ru1–N2 171.17(10)
O3–Ru1–N2 86.28(11)
O1–Ru1–N2 90.29(11)
N4–Ru1–N2 92.97(11)
N3–Ru1–N2 98.59(11)
2
Os1–O1 2.022(4)
Os1–O3 2.028(3)
Os1–O2 2.035(3)
Os1–N2 2.043(4)
Os1–N3 2.049(4)
Os1–N4 2.054(4)
O1–Os1–O3 84.95(14)
O1–Os1–O2 86.03(13)
O3–Os1–O2 86.34(14)
O1–Os1–N2 92.06(15)
O3–Os1-N2 86.86(15)
O2–Os1–N2 173.07(14)
O1–Os1–N3 86.65(16)
O3–Os1–N3 171.47(15)
O2–Os1–N3 91.59(15)
N2–Os1–N3 94.96(16)
O1–Os1–N4 172.26(15)
O3–Os1–N4 91.33(16)
O2–Os1–N4 86.96(14)
N2–Os1–N4 94.51(15)
N3–Os1–N4 96.83(17)
with different a solvent [41]. A literature survey shows that the
average Ru–N(O) bond length relevant to salen corresponds to
2.01 Å, and this is a little higher (2.034 Å) in our case [51,52].
The angles around the ruthenium center deviate significantly from
90°. The N2–Ru1–N3, N3–Ru1–N4, O3–Ru1–N4 and N2–Ru1–N4
angles have opened up to 98.59(11)°, 93.70(11)°, 92.93(11)° and
92.97(11)° and the O2–Ru1–O3, O1–Ru1–N3, O3–Ru1–N2 and
O1–Ru1–O2 angles have reduced to 85.03(10)°, 84.87(10)°,
3.2. Electrospray ionization mass spectrometry (ESI-MS)
Elemental analysis and ESI-MS confirmed the formation of the
ligands (H3L1,2, L3–L6) and of complexes 1–12 with definite stoichi-

S. Mandal et al. / Polyhedron 31 (2012) 167–175
173
Table 3
Electrochemical and spectral data of the ligands and complexes.
a
Complex Electronic spectral data kmax (nm) (€ Â 10À4, L molÀ1 cmÀ1
)
Cyclic voltammetric data,b E, V vs. SCE Fluorescencea
Excited at (nm) Emission at (nm)
1
2
626(1.26), 402(6.36), 327(11.66), 264(53)
410(5.67), 330(10.01)
264
396
258
406
363 (vibrational fine structure)
427, 453
298
1.0844d,
À0.6743e, À1.0225e
no peak
c
3
4
326(5.09), 358(3.32), 546(2.40)
846(.022), 345(0.18), 272(0.70)
À0.886(E1/2
)
270
546
264
303, 398
no peak
308
1.029d, À1.1152e
416 (weak)
301, 431
534 (broad peak)
430
337, 396
515
344
no peak
307
5
6
7
8
464(2.45), 380(3.16), 264(13.49)
476(5.47), 440(4.53), 336(5.83)
362(11.03)
1.1441d
260
476
260
220
270
292
450
270
410
380
224
224
1.1018d, À0.6516e
0.8629, À1.1867 (E1/2
1.045d
c
)
478(2.68), 338(2.85)
9
442(4.40), 292(4.79)
270(8.67), 428(0.69)
382(2.92)
1.1032d, À0.7589e
1.1097d
10
11
12
485
no peak
338
0.9736d
417(0.42), 358(0.96), 321(1.30)
0.835d, À0.682e
337, 401
a
b
c
Electronic spectral data of the ligands in acetonitrile.
Supporting electrolyte, TBAP; reference electrode, SCE; scan rate, 50 mV sÀ1
E1/2 = 0.5(Epa + Epc), where Epa and Epc are the anodic and cathodic peak potentials.
.
d
e
Epa
Epc
.
.
ometry. The presence of a peak (m/z) at 458.88, 414.83 and 422.89
for H3L1, L3 and L5 confirm the formation of the ligand through con-
densation of tris(2-aminoethyl)amine and the corresponding alde-
hyde in a 1:3 M ratio. Simple reduction by NaBH4 and subsequent
N-methylation give rise to the ligands H3L2, L4 and L6, marked by
the peaks (m/z) at 506.77, 461.97 and 470.96. The analysis of the
higher mass region of the spectra for the complexes formed by
the synthesized ligands show a signal attributable to the presence
of [ML] (M = Ru, Os; L = H3L1, H3L2) or [ML]2+ (M = Ru, Os; L = L3–L6)
in the sample. The metal/organic ligand ratio observed is 1:1 for all
the complexes. The molecular ion peaks are present for all the
complexes at the respective m/z values, with the expected isotope
distribution patterns calculated for the ruthenium and osmium
complexes respectively. Generally, [ML]2+ type complexes show a
molecular ion peak at ML(BPh4)+. In addition, some [ML(BPh4)2]
complexes also show peaks corresponding to the composition
[ML]+ or at [ML+cation] [cation = Na+ or K+]. Two representative
mass spectra for 7 and 12 are shown in Fig. S10.
tion 2). The spectra exhibited by the [ML] (M = Ru, Os) (L = H3L1,
H3L2)} complexes are not well resolved, and are broad and ill de-
fined as expected for Ru(III) and Os(III) complexes. The spectrum
of 1 is shown in Fig. S3 (Supporting information). For the com-
plexes [ML](BPh4)2 (M = Ru, Os; L = L3–L6), having tetraphenyl bo-
rate anion, the assignment of signals could not be made with
confidence due to the overlapping of peaks in the aromatic region.
3.5. Infrared spectra
The infrared (IR) spectra for the series of ligands and com-
pounds were measured at ambient temperature as KBr pellets.
The IR spectra of the ligand H3L1 and H3L2 are characterized by
an intense band attributable to the O–H stretching at 3436 cmÀ1
(broad) [55], typical for intramolecular hydrogen bonded O–H. This
broad feature is absent in the complexes 1–4. For, H3L1,2 and L3–L6
characteristic bands attributable to C–H stretching at 3400–
2352 cmÀ1 and for H3L1, L3and L5 bands at1632–1651 cmÀ1
(mC@N)
are present. These bands are absent in the corresponding N-meth-
ylated ligands.
3.3. CD spectra
The CD spectra of 1–3 clearly indicate the presence of a racemic
mixture for each complex in solution. As resolution does not occur
spontaneously during the course of crystallization, an optically
active auxiliary agent for resolution is required. Each complex con-
3.6. Electronic spectra
The absorption and emission spectra of all the ligands and
complexes were performed in acetonitrile solution (Table 3).
The ligands absorb mainly in the ultra-violet region. The com-
plexes show bands in the visible region as well as in the ultravi-
olet region. The characteristics emission spectra of the ligands and
complexes are shown in Fig. S4 (Fig. 6 [56]).1 All the complexes
have essentially started with the tris-(2aminoethyl)amine moiety,
which emits in the region between 525 and 625 nm. The ligands
L1 and L3 show strong a emission which is blue shifted to the
400–500 nm region, while the emission intensity is quite high in
the case of L3. It can be seen that the reduction of –C@N and sub-
sequent N-methylation changes the emission pattern completely.
tains mixture of
D (clockwise) and K (anticlockwise) enant-
iomorphs, depending on the screw arrangement of the tripod-
type ligand around the metal ion. Optical resolution for the com-
plexes is in progress. However, the particular crystal of 1 is a
D-
enantiomorphs, and for complex 2, it is an -isomer. The screw
K
arrangements of the ligands around the metal have been shown
in Fig. 5.
3.4. NMR spectra
The 1H NMR spectra clearly establish the diamagnetism of all
the complexes, except [ML] (M = Ru, Os; L = H3L1, H3L2) (see Sec-
1
Fig. 6 represents the emission spectra of the ligand L3, complex 5 and complex 6.

174
S. Mandal et al. / Polyhedron 31 (2012) 167–175
Fig. 6. Emission pattern of ligand L3 and complexes 5 and 6.
So, it is obvious that the change in the p–
p⁄ transition is attributed
to the change in the fluorescence pattern [57]. The ruthenium com-
plexes fluoresce more strongly than the corresponding osmium
complexes, except for complex 9 in our case. For the complexes,
emissions arising from metal centered excited states of MLCT char-
acter are expected since ruthenium and osmium are easy to oxidize
or reduce [58]. Quenching of fluorescence by transition metal ions
during complexation is a common phenomenon which is explained
by processes such as magnetic perturbation, redox activity, elec-
tronic energy transfer, etc. [59,60]. The complex [OsL6] shows a
complete quenching of fluorescence. For complexes 5 and 7 a
new peak arises in the blue-shifted region. This may be attributed
to the presence of an additional chromophoric group, >C@N, of
the pyridine [61].
Fig. 7. Partial molecular orbital diagram of complex 2.
Table 4
Composition of selected molecular orbitals of the ruthenium and osmium complexes.
Complex
Contributing fragments
% Contribution of fragments to
HOMO
LUMO
1
Ru
L1
62.67
37.33
68.05
31.95
23.98
76.02
32.59
67.41
38.82
61.78
43.59
56.41
18.03
81.97
23.03
76.97
17.75
82.25
20.67
79.33
19.67
80.33
24.98
75.02
20.22
79.78
23.24
76.76
6.07
93.93
9.64
90.36
8.35
91.65
11.46
88.54
11.58
88.42
14.61
85.39
10.98
89.02
13.54
86.46
8.71
2
Os
L1
To get the idea about the intense lowest-energy absorption for
all the twelve complexes, DFT calculations have been performed.
From the results, it was found that in [RuL1] and its osmium ana-
logue, the highest occupied molecular orbital (HOMO) is mostly
distributed over the metal center and the lowest unoccupied
molecular orbital (LUMO) has predominant ligand character.
Hence the lowest-energy absorption is assignable to a metal-to-li-
gand charge transfer (MLCT) transition, taking place from the filled
d-orbital of the metal to the vacant p⁄-orbital of the imine ligand.
For the ruthenium and osmium complexes of ligands L2 and L3,
both the HOMO and LUMO contain major contributions from the
ligand, but still the metal has a significant contribution as well as
the ligand. So the transition may be attributed to an intra ligand
transition. For the complexes of ligands L4–6, both the HOMO and
LUMO have predominant ligand character. Hence the lowest en-
ergy transition is assignable to a ligand to ligand charge transfer
(LLCT) transition. Polypyridyl complexes of Ru and Os are excellent
fluorophores owing to their strong absorption in the visible region
and emission at low energy, originating from the spin-forbidden
metal-to-ligand charge transfer (MLCT) transitions [62,63]. It is
likely that the emission originates from the lowest energy metal
to ligand charge transfer (MLCT) state, probably derived from the
3
Ru
L2
4
Os
L2
5
Ru
L3
6
Os
L3
7
Ru
L4
8
Os
L4
9
Ru
L
10
11
12
Os
L5
Ru
L6
91.29
12.37
87.63
Os
L6
3.7. Cyclic voltammetry
excitation involving a d
p
(Ru) ? p⁄(imine) MLCT transition [64].
Cyclic voltammograms (CV) of all the complexes were recorded
in dichloromethane–acetonitrile (1:9) (0.1 M TBAP), and the data
are summarized in Table 3. The cyclic voltammograms of 1–4 are
quite similar. All of these complexes exhibit one metal-based irre-
versible oxidation in the range 0.95–1.08 V. In view of the compo-
sition of the HOMO in these complexes, the first oxidative response
is assigned to Ru(III)/Ru(IV) oxidation. Complexes 5–12 show a
common irreversible oxidation peak in the range 0.86–1.14 V.
Again, based on the composition of HOMO, these peaks are attrib-
The electron distribution in the highest occupied molecular orbital
(HOMO) and the lowest unoccupied molecular orbital (LUMO) for
complex 2 is shown in Fig. 7. The composition of these molecular
orbitals for all the complexes is given in Table 4. It is interesting
to note that the osmium complexes have a comparatively better
metal contribution in the HOMO. Probably the existence of more
metal character is responsible for the lower fluorescence of the os-
mium species.

S. Mandal et al. / Polyhedron 31 (2012) 167–175
175
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4. Conclusion
In summary, we have shown that the reaction of tris(2-amino-
ethyl)amine based ligands with ruthenium and osmium precursors
result in the formation of mononuclear complexes of the general
formula [ML] or [ML]2+, which have been characterized by different
spectroscopic techniques and cyclic voltammetry. The X-ray struc-
tures of 1 and 2 have been reported. This is the first crystallograph-
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based ligands. The ligands, as well as the complexes, have very
good emissive properties. The electronic spectra, TD-DFT method
and the transitions’ characters were discussed in connection with
the structure of the molecular orbitals of the complexes. Optical
resolutions for the complexes are in progress.
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PG would like to thank the Department of Science and Technol-
ogy, India, research Grant SR/FT/CS-057/2009. S.M. and D.K.S. (9/
096(0511)/2006-EMR-I) thank the Council of Scientific and Indus-
trial Research, New Delhi, India for their PhD fellowships. The
authors would like to thank Dr. Tapan Kanti Paine and Prof. Sou-
men Basak and for their help and Mr. G. Ramakrishna for the data
collection.
Appendix A. Supplementary data
CCDC 795997 and 795084 contain the supplementary crystallo-
graphic data for compound 1 and 2. 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: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this article
can be found, in the online version, at doi:10.1016/j.poly.2011.
09.010.
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