Synthesis and spectro-electrochemical aspects of [RuII(trpy) (pdt)(X)]n+ (trpy = 2,2′:6′,2″-terpyridine, pdt = 5,6-diphenyl-3-pyridyl-as-triazine, X = Cl-, CH3CN, NO2-, NO+, NO.) - electrophilicity of {RuII-NO+} and photolability of {RuII- NO.}

Article abstract of DOI:10.1002/ejic.200700143

Nitrosylruthenium derivatives having NO⁺ and NO^. states have been synthesized in a stepwise manner starting from [Ru ^II(trpy)(pdt)(Cl)](ClO₄) {[1](ClO₄)} → [Ru^II(trpy)(pdt)-(CH₃CN)](ClO₄)₂ {[2](ClO₄)₂] → [Ru^II(trpy)(pdt)(NO ₂)](C1O₄) ([3](ClO₄)} → [Ru ^II(trpy)(pdt)(NO⁺)](ClO₄)₃ {[4](ClO₄)₃} → [Ru^II(trpy)(pdt)(NO ^.)](CLO₄)₂ {[4](ClO₄)₂} [trpy = 2,2′:6′,2″-terpyridine, pdt = 5,6-diphenyl-3-pyridyl- as-triazine]. The molecular identity of [1](ClO₄) and [2](ClO ₄)₂ and the subsequent stereoretentive transformation of [1](ClO₄) → [2]-(ClO₄)₂ have been authenticated by single-crystal X-ray structures. Electrochemical and spectral features are investigated as a function of the monodentate ligands (Cl ^-, CH₃CN, NO₂^-, NO⁺, NO^.). The kinetic and thermodynamic aspects of the reaction of the moderately strong electrophilic {Ru^II-NO⁺} center [ν(NO): 1944 cm^-1] in [4]³⁺ with a nucleophile such as OH^- have been studied. The nitrosyl species [Ru^II(trpy) (pdt)-(NO⁺)]³⁺ ([4]³⁺) can be selectively reduced to [Ru^II(trpy)(pdt)-(NO^.)]²⁺ ([4] ²⁺) electrochemically as well as chemically by hydrazine hydrate. On exposure to light an acetonitrile solution of [Ru^II(trpy)(pdt) (NO^.)]²⁺ ([4]²⁺) undergoes slow photocleavage of the Ru^II-NO bond over a period of 4 h, resulting in the corresponding solvated species [Ru^II(trpy)(pdt)(CH ₃CN)]²⁺ ([2]²⁺). The rate of photolability of the Ru^II-NO bond in [4]²⁺ has been monitored spectrophotometrically. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700143

FULL PAPER
DOI: 10.1002/ejic.200700143
Synthesis and Spectro-electrochemical Aspects of [RuII(trpy)(pdt)(X)]n+ (trpy =
2,2Ј:6Ј,2ЈЈ-Terpyridine, pdt = 5,6-Diphenyl-3-pyridyl-as-triazine, X = Cl–,
CH3CN, NO2 , NO+, NO·) – Electrophilicity of {RuII–NO+} and Photolability
–
of {RuII–NO·}
Somnath Maji,[a] Chandrani Chatterjee,[a] Shaikh M. Mobin,[a] and
Goutam Kumar Lahiri*[a]
Keywords: Ruthenium / Nitrosyl complexes / Spectroscopy / Electrochemistry / Reactivity
Nitrosylruthenium derivatives having NO+ and NO· states
have been synthesized in a stepwise manner starting from
the reaction of the moderately strong electrophilic {RuII–NO+}
center [ν(NO): 1944 cm–1] in [4]3+ with a nucleophile such as
OH– have been studied. The nitrosyl species [RuII(trpy)(pdt)-
(NO+)]3+ ([4]3+) can be selectively reduced to [RuII(trpy)(pdt)-
(NO·)]2+ ([4]2+) electrochemically as well as chemically by hy-
drazine hydrate. On exposure to light an acetonitrile solution
of [RuII(trpy)(pdt)(NO·)]2+ ([4]2+) undergoes slow photocleav-
age of the RuII–NO bond over a period of 4 h, resulting in the
corresponding solvated species [RuII(trpy)(pdt)(CH3CN)]2+
([2]2+). The rate of photolability of the RuII–NO bond in [4]2+
has been monitored spectrophotometrically.
[RuII(trpy)(pdt)(Cl)](ClO4) {[1](ClO4)}
Ǟ
[RuII(trpy)(pdt)-
(CH3CN)](ClO4)2 {[2](ClO4)2} Ǟ [RuII(trpy)(pdt)(NO2)](ClO4)
{[3](ClO4)} Ǟ [RuII(trpy)(pdt)(NO+)](ClO4)3 {[4](ClO4)3} Ǟ
[RuII(trpy)(pdt)(NO·)](ClO4)2 {[4](ClO4)2} [trpy = 2,2Ј:6Ј,2ЈЈ-
terpyridine, pdt = 5,6-diphenyl-3-pyridyl-as-triazine]. The
molecular identity of [1](ClO4) and [2](ClO4)2 and the subse-
quent stereoretentive transformation of [1](ClO4) Ǟ [2]-
(ClO4)2 have been authenticated by single-crystal X-ray
structures. Electrochemical and spectral features are investi-
gated as a function of the monodentate ligands (Cl–, CH3CN,
NO2–, NO+, NO·). The kinetic and thermodynamic aspects of
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
NO center in {(AL)(trpy)RuII–NO} (trpy = 2,2Ј:6Ј,2ЈЈ-ter-
pyridine) can also be nicely tuned primarily depending on
the electronic aspects of AL. Thus, the νNO+ frequency as
well as the E°(NO+/NO·) values systematically vary with
AL = 2-phenylpyridine (pp),[9] acetylacetone (acac),[10] N-
methyl-2-(2-pyridyl)benzimidazole (pNMebi),[11] (2-pyrid-
yl)benzimidazole (pbi),[11] 2,2Ј-dipyridylamine (dpa),[12]
bis(2-pyridyl) ketone (dpk),[13] 2,2Ј-bipyridine (bpy),[14] 2-
(2-pyridyl)benzoxazole (pbo),[11] and 2-(phenylazo)pyridine
(pap),[15] as shown in Table 1.
The observed distinct role of AL in the complex frame-
work of {(AL)(trpy)RuII–NO+} (Table 1) has initiated the
selective introduction of 5,6-diphenyl-3-pyridyl-as-triazine
(pdt) as AL to explore its specific effect towards the stability
of coordinated NO in accessible redox states. The ligand
pdt has been of interest in the field of coordination chemis-
try as well as organometallic chemistry since early 1964.[16]
However, only a limited number of ruthenium complexes of
pdt and its derivatives have been reported so far from dif-
ferent perspectives.[17]
Introduction
The biological[1] and environmental[2] relevance of the ni-
trosyl molecule has led to a resurgence of interest in the
metal–NO interaction. This is primarily due to fundamen-
tal issues such as (i) bonding and redox features,[3] (ii) reac-
tivity profile of metal-bound NO function with a variety
of nucleophiles,[4] (iii) uses of nitrosylmetal derivatives as
antitumor and antiseptic agents,[5] and (iv) photolytic cleav-
age of the M–NO bond and subsequent transfer of photo-
released “NO” to the biological targets such as myoglobin
(Mb) or cytochrome c oxidase.[6]
The stability of possible redox states of the nitrosyl mole-
cule, NO+, NO·, and NO–, in the complex framework of
(AL)M–NO depends on a variety of factors, including the
nature of the metal ions and the ancillary ligands (AL). For
example, in reduced vitamin B12, the cobalt center formally
binds with the NO in the form of CoIII–NO–,[7] while in
metmyoglobin the iron center binds with the NO formally
as FeII–NO+.[8] Similarly, electrophilicity of the coordinated
The present article thus describes the stepwise synthetic
[a] Department of Chemistry, Indian Institute of Technology-Bom-
bay,
aspects of the nitrosyl complexes starting from [RuII-
[RuII(trpy)(pdt)-
Powai, Mumbai 400076, India
(trpy)(pdt)(Cl)](ClO4) {[1](ClO4)}
Ǟ
E-mail: lahiri@chem.iitb.ac.in
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
(CH3CN)](ClO4)2 {[2](ClO4)2} Ǟ [RuII(trpy)(pdt)(NO2)]-
(ClO4) {[3](ClO4)}
Ǟ
[RuII(trpy)(pdt)(NO+)](ClO4)3
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

S. Maji, C. Chatterjee, S. M. Mobin, G. K. Lahiri
Table 1. ν(NO+) frequencies and reduction potentials of {RuII(NO+)} Ǟ {RuII(NO·)} in [RuII(trpy)(AL)(NO+)]n+
FULL PAPER
.
Ancillary ligand (AL)
ν(NO+)/cm–1[a]
E°298/V[b]
Ref.
[15]
[11]
[14]
[13]
[12]
pap
pbo
bpy
dpk
dpa
pdt
pbi
pNMebi
acac
pp
1960
1957
1952
1949
1945
1944
1940
1932
1914
1858
0.72
0.45
0.45
0.36
0.34
0.40
0.33
0.31
0.02
–0.28
this work
[11]
[11]
[10]
[9]
[a] In KBr disk. [b] CH3CN/0.1  Et4NClO4/SCE.
{[4](ClO4)3} Ǟ [RuII(trpy)(pdt)(NO·)]2+ {[4](ClO4)2}. Crys- tro derivative [3](ClO4) by using concentrated nitric acid
tal structures of [1](ClO4) and [2](ClO4)2 and spectroelectro- followed by concentrated perchloric acid at 273 K
chemical features of all four complexes are reported. The (Scheme 1).
reactivity of the ruthenium-bound electrophilic NO+ in
[4](ClO4)3 in the presence of a nucleophile such as OH– and
the aspects of photolytic cleavage of the RuII–NO· bond in
[4]2+ are also investigated.
Results and Discussion
Synthesis and Characterization
The precursor chlorido complex [RuII(trpy)(pdt)(Cl)]+
[1]+ [trpy = 2,2Ј:6Ј,2ЈЈ-terpyridine, pdt = 5,6-diphenyl-3-
Scheme 1.
pyridyl-as-triazine] has been synthesized from [Ru(trpy)Cl3]
and pdt in the presence of excess Et3N and LiCl in refluxing
The direct synthesis of [4](ClO4)3 either from the
EtOH. The crude product was subsequently purified by col- chlorido {[1](ClO4)} or solvated {[2](ClO4)2} derivative
umn chromatography on alumina. The product was isolated using NO gas or NOBF4 was not successful. Therefore, the
as its perchlorate salt [1](ClO4). The solvated species stepwise synthetic route as shown in Scheme 1 was fol-
[2](ClO4)2 and the nitro derivative [3](ClO4) were prepared lowed.
from [1](ClO4) and [2](ClO4)2, respectively, as shown in
The diamagnetic complexes [1](ClO4), [2](ClO4)2,
Scheme 1. The desired nitroso complex [RuII(trpy)- [3](ClO4), and [4](ClO4)3 exhibit expected molar conductivi-
(pdt)(NO+)](ClO4)3 {[4](ClO4)3} was prepared from the ni- ties and microanalytical data (see Experimental Section).
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Spectrochemistry of Nitrosylruthenium Complexes
FULL PAPER
Figure 1. ESI mass spectra of (a) [1](ClO4), (b) [2](ClO4)2, (c) [3](ClO4), and (d) [4](ClO4)3 in CH3CN.
The positive-ion electrospray mass spectra of [1](ClO4), and
[3](ClO4) show molecular ion peaks centered at m/z =
680.29 and 690.99, respectively, corresponding to [1]+ (cal-
culated molecular mass: 680.09) and [3]+ (calculated molec-
ular mass: 690.11). Molecular ion peaks for [2](ClO4)2 and
[4](ClO4)3 appear at m/z = 743.93 and 743.81 corresponding
to {[2](ClO4)2 – ClO4 – CH3CN}+ (calculated molecular
mass: 744.07) and {[4](ClO4)3 – 2 ClO4 – NO}+ (calculated
molecular mass: 744.15), respectively (Figure 1).
Orientation of the pyridyl or triazine ring of the unsym-
metric pdt with respect to the monodentate ligand (X = Cl–
–
or CH3CN or NO2 or NO) in the complex frameworks
gives rise to two possible isomeric structures, A and B.
Figure 2. Crystal structure of the cation of [1](ClO4).
The trpy ligand in the complexes is coordinated to the
ruthenium ion in the expected meridional fashion with the
bidentate pdt ligand in cis position. The intraligand trans
angles, N1–Ru–N3 of 159.1(2) and 158.5(2)°, in [1](ClO4)
and [2](ClO4)2, respectively, reflect the usual geometrical
constraints due to the meridional binding mode of triden-
tate trpy.[18] The other two interligand trans angles, N2–Ru–
N4 and N5–Ru–X, remain close to 180° for both the com-
plexes. As in other Ru–trpy derivatives,[18,19] the central
RuII–N2(trpy) bonds {1.954(6) and 1.977(5) Å in [1](ClO4)
and [2](ClO4)2, respectively} are significantly shorter than
the corresponding terminal RuII–N(trpy) distances, Ru–N1
[2.058(6) and 2.078(5) Å] and RuII–N3 [2.057(6) and
However, in practice, only one of the structural forms (A
or B) has been systematically isolated by column
chromatography for all the complexes. The preferential
crystallization of isomer A where the pyridyl nitrogen atom
is trans to “X” has been established by crystal structure
analyses of [1](ClO4) and [2](ClO4)2.
Crystal Structures of [1](ClO4) and [2](ClO4)2·3H2O
The crystal structures of [1](ClO4) and [2](ClO4)2·3H2O 2.073(5) Å]. The pdt ligand forms a five-membered chelate
are shown in Figures 2 and 3, respectively. Selected crystal- ring to the metal ion through pyridyl and triazine nitrogen
lographic and structural parameters are listed in Tables 2 atoms. The coordinated pdt ligand is slightly nonplanar; the
and 4, respectively.
dihedral angles between the chelated pyridine and triazine
Eur. J. Inorg. Chem. 2007, 3425–3434
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S. Maji, C. Chatterjee, S. M. Mobin, G. K. Lahiri
FULL PAPER
52.13(0.26), 49.12(0.30) and 44.84(0.21), 55.09(0.25)° in
[1](ClO4) and [2](ClO4)2, respectively. The RuII–Cl and
RuII–N(pdt/trpy) bond lengths and the associated angles
are in good agreement with the reported data on related
molecules.[17a,18,19] The coordinated acetonitrile molecule in
[2](ClO4)2 is in its usual nearly linear mode with an N8–
C36–C37 angle of 177.8(9)°.[13] The packing diagram of
[2](ClO4)2·3H2O is shown in Figure S1 (Supporting Infor-
mation).
1H NMR, IR, UV/Vis and Emission Spectral Aspects
1
The H NMR spectra of [1](ClO4), [2](ClO4)2, [3](ClO4),
and [4](ClO4)3 in (CD3)2SO are shown in Figure S2 (Sup-
porting Information and Experimental Section). The com-
plexes display 25 severely overlapping signals in the aro-
matic region between δ = 7.0 and 9.0 ppm. This matches
well with the calculated number of aromatic signals, 11 and
14 resonances from the trpy and pdt ligands, respectively.
The ClO4– vibrations are observed at 1096/623, 1089/625,
1097/623, and 1087/626 cm–1 for [1](ClO4), [2](ClO4)2,
[3](ClO4), and [4](ClO4)3, respectively. RuII–NO2 stretching
frequencies of [3](ClO4) fall at 1366 cm–1 (asym.) and
1276 cm–1 (sym.). The νNO+ stretching frequency of
1944 cm–1 for [4](ClO4)3 (Figure 4) is comparable to that
reported for analogous complexes with AL = pbi, dpa or
dpk (Table 1). However, it is much lower than that reported
for AL = strongly π-acidic pap, pbo or bpy and much
higher with respect to AL = strongly σ-donating pp, acac
or pNMebi (Table 1). Thus, the observed reasonably high
νNO stretching frequency of [4](ClO4)3 is suggestive of its
moderate electrophilic character.
Figure 3. Crystal structure of the cation of [2](ClO4)2. Water mole-
cules of crystallization have been removed for clarity.
Table 2. Selected bond lengths [Å] and bond angles [°] for [1](ClO4)
and [2](ClO4)2·3H2O.
[1](ClO4) (X = Cl) [2](ClO4)2 (X = CH3CN)
Ru–X
Ru–N(1)
Ru–N(2)
Ru–N(3)
Ru–N(4)
Ru–N(5)
N(4)–N(6)
N(4)–C(21)
2.392(2)
2.058(6)
1.954(6)
2.057(6)
2.042(6)
2.034(6)
1.340(9)
1.367(10)
1.462(13)
1.349(9)
1.371(10)
–
2.037(5)
2.078(5)
1.977(5)
2.073(5)
2.035(5)
2.047(4)
1.355(6)
1.348(7)
1.471(9)
1.347(7)
1.356(7)
1.121(7)
79.2(2)
158.5(2)
100.8(2)
94.84(19)
79.3(2)
175.24(17)
96.7(2)
C(16)–C(21)
N(5)–C(16)
N(5)–C(20)
C(36)–N(8)
N(1)–Ru–N(2)
N(1)–Ru–N(3)
N(1)–Ru–N(4)
N(1)–Ru–N(5)
N(2)–Ru–N(3)
N(2)–Ru–N(4)
N(2)–Ru–N(5)
N(3)–Ru–N(4)
N(3)–Ru–N(5)
N(4)–Ru–N(5)
N(1)–Ru–X
N(2)–Ru–X
N(3)–Ru–X
N(4)–Ru–X
N(5)–Ru–X
N(6)–N(4)–C(21)
N(4)–C(21)–C(16)
C(21)–C(16)–N(5)
C(16)–N(5)–C(20)
N(8)–C(36)–C(37)
79.7(2)
159.1(2)
100.6(2)
87.0(2)
79.4(2)
175.5(3)
96.9(2)
100.3(2)
96.6(2)
78.6(2)
91.66(17)
87.91(18)
86.48(17)
96.55(17)
174.68(18)
119.2(6)
114.8(6)
114.0(7)
116.1(7)
–
100.7(2)
88.00(18)
78.6(2)
Figure 4. IR spectra (KBr disk) of (a) [4](ClO4)3 (------) and (b)
[4](ClO4)2 (––) in the region of 2200–1600 cm–1.
87.4(2)
92.11(19)
93.00(19)
92.6(2)
171.2(2)
118.1(5)
114.6(6)
114.1(5)
116.8(5)
177.8(9)
The complexes exhibit multiple transitions in the UV/Vis
region in CH3CN (Figure 5, Table 3). Intense ligand-based
transitions (π Ǟ π* and n Ǟ π*) are observed in the UV
region. The ruthenium(II)-based moderately intense metal-
to-ligand charge-transfer (MLCT) transitions appear near
500 nm. The Gaussian analysis of the visible region of the
spectra reveals the presence of two close bands in each of
[1]+, [2]2+, and [3]+ (Table 3 and inset of Figure 5).
rings in [1](ClO4) and [2](ClO4)2 are 2.80(0.17) and
These are tentatively assigned to dπ(RuII) Ǟ π*(pdt)-
4.81(0.15)°, respectively. The dihedral angles between the and dπ(RuII)
Ǟ
π*(trpy)-based transitions.[13,17] The
pendant phenyl rings linked to the triazine ring are MLCT band energy in acetonitrile solution follows the or-
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Spectrochemistry of Nitrosylruthenium Complexes
FULL PAPER
Table 3. Redox potentials[a,b] and electronic spectroscopic data.
Compound
Couple
λ/nm
I
II
III
IV
(ε/–1 cm–1)[d]
[1]ClO4
0.89(90)
–1.07(120)
–1.54(190)
–
520 (15073), [522 (4668),
506 (13100)[e]], 312 (38078),
279 (38762), 236 (33057),
204 (65481)
[2](ClO4)2
[3]ClO4
1.33(80)
1.09(90)
0.40(110)
–1.04(100)
–1.05(80)
–0.33[c]
–1.50(90)
–1.54(110)
–0.90(50)
–
470 (13942), [471 (6518),
448 (9499)[e]], 327 (23247),[f]
297 (38484), 202 (67952)
482 (11273), [479 (6952),
465 (4192)[e]], 297 (31685),
205(63910)
467 (15528), [470 (4728),
451 (10036)[e]], 335 (27408),[f]
300 (40440), 207 (70624)
494 (15536), [505 (5350),
484 (13214)[e]], 307 (37634),
281 (36672), 272 (36284)
–
[4](ClO4)3
[4](ClO4)2
–1.11(120)
[a] Potentials E°298/V (∆E/mV) vs. SCE. [b] In CH3CN/0.1  Et4NClO4. [c] Epc is considered due to the irreversible nature of the wave.
[d] In acetonitrile. [e] Bands obtained by Gaussian analysis of the lowest energy transition. [f] Shoulder.
–
der Cl– (520 nm) Ͻ NO2 (482 nm) Ͻ CH3CN (470 nm) Ͻ Rates and Equilibrium Constant for the Conversion of
NO+ (467 nm) depending on the relative stabilization of the [RuII(trpy)(pdt)(NO+)]3+ [4]3+ Ǟ [RuII(trpy)(pdt)(NO2)]+
dπ(RuII) level.[20] The tripositive charge as well as pro- [3]+
nounced π-acceptor characteristics of NO+ in [4]3+ are in-
Unlike earlier reported analogous nitrosyl com-
deed reflected in the relatively higher MLCT band energy
plexes,[11–13,15] [4]3+ is found to be quite stable both in the
and the corresponding redox processes (see later). The 15-
solid state as well as in CH3CN or even in poorly soluble
nm (666 cm–1) blueshift in the MLCT band energy while
water medium, in spite of the reasonably high ν(NO+)
–
moving from RuII–NO2 [3](ClO4) (482 nm) to RuII–NO+
stretching frequency of 1944 cm–1. However, in the presence
[4](ClO4)3 (467 nm) is much less than that observed for
of a strong nucleophile such as OH–, it undergoes slow
analogous complexes having π-acidic ancillary ligands (dpk,
transformation to the corresponding nitro derivative ([3]+)
bpy, pap).[13–15] This implies relatively weaker dπRuII
Ǟ
[Equation (1)] (Figure 6). On the other hand, for the afore-
said analogous complexes, a weak nucleophile such as H2O
was found to be effective to perform the reaction stated in
Equation (1).
πɆ(NO+) backbonding in [4](ClO4)3, as is also been evi-
denced in the ν(NO+) stretching frequency (Table 1).
Figure 5. Electronic spectra in acetonitrile of [1]+, [2]2+, [3]+, [4]3+
,
and [4]2+. Inset shows the Gaussian analysis (------) of the experi-
mental spectrum (––) of [2]2+ in the visible region.
Figure 6. Time evolution of the electronic spectra (5-min time inter-
changing solution of [RuII(trpy)(pdt)(NO+)]3+
Though excitations of [1]+, [3]+, and [4]3+ at the lowest
energy MLCT bands failed to show emissions even in
EtOH/MeOH (4:1) glass (77 K), [2]2+ exhibits weak emis-
sion at 640 nm (Figure S3, Supporting Information). The
val) of
a
(0.4ϫ10–4 ) + OH– (0.1ϫ10–2 ) Ǟ [RuII(trpy)(pdt)(NO2)]+
+
H+ in CH3CN at 318 K. Arrows indicate increase or decrease in
absorbances with the progression of the reaction.
quantum yield, &πηι; = 0.026, was calculated with reference
The rate of conversion of the nitroso derivative ([4]3+
,
2+
to Ru(bpy)3 (&πηι; = 0.34).[21] The observed emission of 0.4ϫ10–4 ) to the nitro counterpart ([3]+) was monitored
[2]2+ can be attributed to the triplet MLCT state.[22]
spectrophotometrically in the temperature range of 298–
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S. Maji, C. Chatterjee, S. M. Mobin, G. K. Lahiri
FULL PAPER
318 K in CH3CN solvent using a calculated optimum con-
centration of aqueous NaOH solution (0.1ϫ10–2 ).
CH3CN was chosen as a solvent because of the poor solu-
bility of [4]3+ in H2O and thus the transformation process
was monitored without any pH control. The well-defined
isosbestic points (Figure 6) imply the involvement of ni-
trosyl and nitro species as the primary components in the
conversion process. The pseudo-first-order rate constant
(k/s), activation parameters (∆H#/∆S#), and equilibrium
constant (K) values are calculated to be: 4.13ϫ10–4
(298 K), 7.30ϫ10–4 (308 K), 1.29ϫ10–3 (318 K);
42.26 kJmol–1/–110.48 JK–1 mol–1; 6.6, respectively. The
observed appreciably large negative ∆S# value confirms
that the nucleophilic attack of the electrophilic NO+ site in
[4]3+ by OH– occurs through an associatively activated pro-
cess. The equilibrium constant value of 6.6 indicates the
presence of an appreciable concentration of the initial ni-
trosyl species [RuII(trpy)(pdt)(NO+)]3+ ([4]3+) in the equili-
brated situation even under strongly alkaline condition.
Redox and Spectroelectrochemical Aspects
The RuIII/RuII potentials of [1]+, [2]2+, and [3]+ (couple
I, Figure 7a–c; Table 3) vary depending on the electron-do-
nating/-withdrawing nature of the monodentate ligand (X)
as well as the electrostatic factor because of the overall
charge of the complex moieties. They follow the expected
Figure 7. Cyclic voltammograms of (a) [1]+, (b) [2]2+, (c) [3]+, (d)
[4]3+, (e) free ligand, pdt in CH3CN (scan rate: 50 mVs–1), and (f)
least-squares plot of E/V (RuII–NO+–RuII–NO·) vs. ν(RuII–NO+)
frequency of [RuII(trpy)(AL)(NO+)]3+ complexes where AL = pap,
bpy, pbo, pdt, dpk, dpa, pbi, pNMebi, acac, and pp as listed in
Table 1.
–
trend of X = Cl– (0.89) Ͻ NO2 (1.09) ϽϽ CH3CN (1.33
vs. SCE). The potentials are comparable to those of the
corresponding [RuII(trpy)(AL)(X)]n+, where AL = dpk,[13]
having a π-conjugated electron-withdrawing C=O function
–
(X = Cl–, 0.88; NO2 , 1.02 V; CH3CN, 1.37 V vs. SCE).
However, the potentials are much higher than those of the Therefore, couples I and II are logically considered as the
analogous AL = dpa[12] derivatives incorporating a π-donor expected successive reductions of the redox facile coordi-
–
amine group (X = Cl–, 0.64; NO2 , 0.88 vs. SCE). The two nated NO+, that is, {RuII–NO+} Ǟ {RuII–NO·} and {RuII–
ligand-based reductions are observed in the range of –1.0 NO·} Ǟ {RuII–NO–}, respectively. The difference in poten-
to –1.5 V (couples II and III, Figure 7a–c, Table 3). The tial of 0.73 V (couple II/couple I) between the successive
first reduction wave near –1.0 V appears to be less than that NO-centered reductions leads to
a
large compro-
typical for the trpy-based reductions (–1.30 and –1.80 V) portionation constant (Kc) value of 1012.4 (RTlnKc = ∆E/
observed in numerous Ru–trpy complexes.[18–20,23] On the 0.059)[24] for the intermediate {RuII–NO·} radical species
other hand, the free pdt ligand reduces reversibly, E°298/V [4]2+. The RuII–NO+ Ǟ RuII–NO· reduction potentials and
(∆E/mV) at –1.49 (120) versus SCE in CH3CN (Figure 7e). the corresponding ν(NO+) values for the analogous
Thus, couples II and III are assigned as the reduction pro- [RuII(trpy)(AL)(NO+)]3+ complexes with AL = pap,[15]
cesses involving pdt and trpy ligands, respectively.
bpy,[14] pbo,[11] dpk,[13] dpa,[12] pbi,[11] pNMebi,[11] acac,[10]
The nitrosyl complex [4]3+ exhibits four successive re- and pp[9] (Table 1) follow a general trend. Consequently, the
duction processes (couples I–IV, Figure 7d, Table 3). The plot of E1/2 of the RuII–NO+/RuII–NO· couple versus the
effects of neutrality as well as low basicity of both the pdt ν(NO+) frequency for the known [RuII(trpy)(AL)(NO+)]3+
and trpy ligands and the highly charged feature of [4]3+ fa- derivatives yields a reasonably linear relationship (Fig-
cilitate the reversible reduction at the rather positive poten- ure 7f).
tial of 0.40 V versus SCE in CH3CN (couple 1, Figure 7d).
Nitrosyl species [4]3+ can be quantitatively reduced to
The same effect has also been extended even for the second [4]2+ either coulometrically or by using hydrazine hydrate
one-electron reduction of [4]3+ (couple II, Figure 7d) at a as a chemical reducing agent. The relevant microanalytical,
reasonably less negative potential of –0.33 V, though conductivity, and mass spectrometric data are given in the
irreversible at the cyclic voltammetric timescale. The poten- Experimental Section. On reduction of [4]3+ to [4]2+, the
tials of couples I and II are much more positive relative to ν(NO) frequency is shifted from 1944 to 1633 cm–1 (Fig-
the reductions involving the coordinated pdt and trpy li- ure 4) The large ∆ν value of 311 cm–1 on moving from [4]3+
gands, –0.90 and –1.11 V (couples III and IV, Figure 7d). to [4]2+ confirms[25] the reduction of coordinated NO+ to
3430
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Eur. J. Inorg. Chem. 2007, 3425–3434

Spectrochemistry of Nitrosylruthenium Complexes
FULL PAPER
NO·. Consequently, the reduced species [4]2+ exhibits a radi- ([1]+), CH3CN ([2]2+), NO2 ([3]+), NO+ ([4]3+)}. The ni-
cal-type EPR spectrum (g = 2.003) with partially resolved trosyl function in [4]3+ undergoes facile two-step one-
nitrogen hyperfine splittings.[11–13,26] On reduction to the ni- electron reductions: {RuII–NO+} to {RuII–NO·} to {RuII–
trosyl radical complex [4]2+, the visible-energy charge-trans- NO–}. Unlike earlier reported analogous complexes
fer band has been reasonably redshifted from 467 to 494 nm {[RuII(trpy)(AL)(NO+)]n+} with AL = pap, pbo, bpy, dpk,
(Figure 5). Moreover, two distinct bands involving the sin- dpa, pbi, pNMebi, [4]3+ is found to be sufficiently stable
gly occupied MO appear at 307 and 281 nm[13] (Figure 5, both in the solid and solution (CH3CN or poorly in water
Table 3). However, the second reduced species [4]+ with the medium) states in spite of the moderately strong electro-
NO– state was found to be unstable even on the coulometric philic character of the nitrosyl function [ν(RuII–NO+) =
–
timescale at 298 K.
1944 cm–1]. [4]3+ transforms slowly and partially to the cor-
responding nitro derivative [3]+ only in the presence a of
strongly alkaline medium. The NO+ function in
[RuII(trpy)(pdt)(NO+)]3+ ([4]3+) can be quantitatively re-
duced to the corresponding NO· state in [RuII(trpy)-
(pdt)(NO·)]2+ ([4]2+) either electrochemically or chemically
by using hydrazine hydrate. Though [4]2+ is stable under
ambient light conditions, the exposure of an acetonitrile
solution of [4]2+ to light (Xenon 350 W lamp) for a period
of 4 h results in the solvated species [2]2+ by the slow photo-
release of NO with kNO of 4.4ϫ10–3 min–1 (t1/2 ≈ 157 min).
Photocleavage of the RuII–NO Bond in [RuII(trpy)(pdt)-
(NO·)]2+ ([4]2+
)
Though [4]2+ is quite stable in CH3CN under ambient
light conditions, exposure of an acetonitrile solution of
[4]2+ to light (Xenon 350 W lamp) for a period of about 4 h
results in quantitative transformation to the solvated species
[2]2+ by the selective cleavage of the RuII–NO· bond. The
slow but steady progression of photocleavage of the RuII–
NO· bond of [4]2+ was monitored spectrophotometrically in
CH3CN (Figure 8) and the presence of isosbesticities sug-
gests the clean conversion of [RuII(trpy)(pdt)(NO·)]2+
([4]2+) to [RuII(trpy)(pdt)(CH3CN)]2+ ([2]2+). The rate of
first-order photolytic release of NO (kNO) has been deter-
mined to be 4.4ϫ10–3 min–1 (t1/2 ≈ 157 min). This sort of
photolytic cleavage of the M–NO bond maintaining the
overall integrity of the rest of the molecule is known to have
biological relevance.[6] However, because of the insolubility
of [4]2+ in aqueous medium, it has not been possible so far
to explore the feasibility of transferring the photoliberated
NO to biomolecules such as myoglobin or cytochrome c ox-
idase. Thus, further work in the direction of developing
Experimental Section
Materials and Instrumentation: The precursor complexes RuIII
(trpy)Cl3
-
[27]
and 5,6-diphenyl-3-pyridyl-as-triazine (pdt)[28] were
prepared according to reported procedures. 2,2Ј:6Ј,2ЈЈ-Terpyridine,
2-cyanopyridine, and silver nitrate were purchased from Aldrich.
Other chemicals and solvents were reagent-grade and used as re-
ceived. For spectroscopic studies, HPLC-grade solvents were used.
Solution electrical conductivity was checked using a Systronic con-
ductivity bridge 305. Infrared spectra were taken with a Nicolet
spectrophotometer with samples prepared as KBr pellets. 1H NMR
spectra were recorded with a 300-MHz Varian FT spectrometer
newer classes of water-soluble nitrosyl derivatives is in pro- with (CD3)2SO used as solvent. Cyclic voltammetric and coulomet-
ric measurements were carried out using a PAR model 273A elec-
trochemistry system. A platinum wire working electrode, a plati-
num wire auxiliary electrode, and a saturated calomel reference
electrode (SCE) were used in a standard three-electrode configura-
tion. Tetraethylammonium perchlorate (TEAP) was used as the
supporting electrolyte and the solution concentration was about
10–3 ; the scan rate used was 50 mVs–1. A platinum gauze working
electrode was used in the coulometric experiments. All electrochem-
ical experiments were carried out under dinitrogen. UV/Vis spectral
studies were performed with a Perkin–Elmer Lambda 950 spectro-
photometer. The elemental analyses were carried out with a Per-
kin–Elmer 240C elemental analyzer. Electrospray mass spectra
were recorded with a Micromass Q-ToF mass spectrometer. Steady-
state emission experiments were performed using a Perkin–Elmer
LS 55 luminescence spectrometer fitted with a cryostat.
gress.
Figure 8. Time evolution of the electronic spectra (20-min time in-
terval) of a changing solution of [RuII(trpy)(pdt)(NO·)]2+ (42+
,
0.4ϫ10–4 ) in CH3CN exposed to light (Xe lamp 350 W). Arrows
indicate increase or decrease in absorbances with the progression
of the reaction. The inset shows the difference spectra of 42+ ex-
posed to light (Xe lamp 350 W) at different time intervals with
respect to unexposed [4]2+ in acetonitrile as the reference solution.
Synthesis of [RuII(trpy)(pdt)(Cl)]ClO4 {[1](ClO4)}: [RuIII(trpy)Cl3]
(100 mg, 0.23 mmol) was dissolved in ethanol (20 mL) and 5,6-di-
phenyl-3-pyridyl-as-triazine (pdt) (41.8 mg, 0.227 mmol), excess
LiCl (54 mg, 1.3 mmol), and NEt3 (0.4 mL) were added to it. The
mixture was heated at reflux under dinitrogen for 5 h. The initial
dark brown solution gradually changed to deep purple. The dry
mass thus obtained on removal of solvent under reduced pressure
was dissolved in a minimum volume of acetonitrile, and excess satu-
rated aqueous solution of NaClO4 was added to it. The resulted
solid precipitate was filtered off and washed thoroughly with cold
Conclusions
The present work demonstrates the substitutional and
electron-transfer aspects of a series of four compounds
based on the framework of [RuII(trpy)(pdt)X]n+ {X = Cl– water. The product was dried in vacuo over P4O10. It was then
Eur. J. Inorg. Chem. 2007, 3425–3434
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3431

S. Maji, C. Chatterjee, S. M. Mobin, G. K. Lahiri
FULL PAPER
purified on a neutral alumina column. The complex [1](ClO4) was
eluted with CH2Cl2/CH3CN (3:1). Evaporation of the solvents un-
der reduced pressure afforded pure complex [1](ClO4). Yield:
113.2 mg (64%). C35H25Cl2N7O4Ru (779.04): calcd. C 53.91, H
3.23, N 12.58; found C 54.27, H 3.08, N 12.27. Molar conductivity
CH3CN and excess hydrazine hydrate was added whilst stirring
over a period of 5 min. The stirring was continued for a further
1 h. The initial yellow solution of [4](ClO4)3 changed to red-brown.
The solvent was then removed under reduced pressure and the solid
mass thus obtained was washed with a little ice-cold water and then
dried in vacuo over P4O10. Yield: 67.4 mg (75%).
C35H25Cl2N8O9Ru (873.02): calcd. C 48.11, H 2.89, N 12.83; found
1
in acetonitrile: ΛM = 127 Ω–1 cm2 –1. H NMR [300 MHz, (CD3)
2SO, 298 K]: δ = 8.87 (d, J = 8.1 Hz, 1 H), 8.75 (d, J = 8.1 Hz, 2
H), 8.65 (d, J = 8.1 Hz, 1 H), 8.45 (d, J = 8.1 Hz, 1 H), 8.27 (m, 2
C 47.92, H 2.86, N 12.98. Molar conductivity (CH3CN): ΛM
=
H), 8.03 (t, J = 8.1, 6.9 Hz, 3 H), 7.77 (d, J = 4.8 Hz, 2 H), 7.59 225 Ω–1 cm–2 –1. Magnetic moment (298 K): µB = 1.87. MS: m/z =
(m, 3 H), 7.42 (m, 7 H), 7.23 (t, J = 7.8, 7.5 Hz, 1 H), 7.05 (1 H), calcd. for {[4](ClO4)2 – ClO4 – NO} 744.15, found 744.31.
6.94 (d, J = 7.2 Hz, 1 H) ppm.
Synthesis of [RuII(trpy)(pdt)(CH3CN)](ClO4)2 {[2](ClO4)2}: The
CAUTION! Perchlorate salts of metal complexes with organic li-
gands are potentially explosive. Heating of dried samples must be
chlorido complex [1](ClO4) (100 mg, 0.128 mmol) was dissolved in
CH3CN (5 mL), water (20 mL) was added and then the mixture
was heated at reflux for 10 min. An excess of AgNO3 (217 mg,
1.28 mmol) was added to the above hot solution and the heating
was continued for a further 2 h. The solution was then cooled and
the precipitated AgCl was separated by filtration through a sintered
glass crucible (G-4). On concentration of the filtrate, the desired
solid complex [2](ClO4)2 separated. It was filtered and washed with
ice-cold water and subsequently dried in vacuo over P4O10. Yield:
90.8 mg (80%). C37H28Cl2N8O8Ru (884.04): calcd. C 50.22, H 3.19,
N 12.67; found C 49.99, H 3.13, N 12.45. Molar conductivity
avoided and handling of small amounts has to proceed with great
caution, using protection.
Kinetic Experiments: For the determination of k of the conversion
process of [RuII(trpy)(pdt)(NO+)]3+ ([4]3+) Ǟ [RuII(trpy)(pdt)-
(NO2)]+ ([3]+) in acetonitrile in the presence of aqueous sodium
hydroxide solution, the increase in absorbance (At) at 482 nm corre-
sponding to λmax of the nitro complex was recorded as a function
of time (t). Aα was measured when the intensity changes leveled
off. Values of pseudo-first-order rate constants, k, were obtained
from the slopes of linear least-squares plots of –ln(Aα – At) against
t. The activation parameters ∆H# and ∆S# were determined from
the Eyring plot.[29] The first-order rate constant for the photo re-
lease of NO from [4]2+ was determined from the single-exponential
fitting of the plot of ∆(OD) against time, where ∆(OD) is the
differential absorbance at 494 nm of [4]2+ in acetonitrile, exposed to
light, with respect to unexposed [4]2+ in acetonitrile as the reference
solution.
1
(CH3CN): ΛM = 270 Ω–1 cm2 –1. H NMR [300 MHz, (CD3)2SO,
298 K]: δ = 8.99 (d, J = 8.1 Hz, 2 H), 8.83 (t, J = 6.6, 7.8 Hz, 3
H), 8.72 (d, J = 7.8 Hz, 1 H), 8.55 (t, J = 7.8, 8.1 Hz, 1 H), 8.27
(d, J = 4.8 Hz, 1 H), 8.16 (t, J = 7.5, 7.8 Hz, 2 H), 8.07 (t, J = 7.5,
6.6 Hz, 1 H), 7.91 (m, 2 H), 7.79 (d, J = 2.1 Hz,1 H), 7.53 (m, 9
H), 7.25 (t, J = 8.1, 7.5 Hz, 1 H), 6.92 (d, J = 7.2 Hz, 1 H) ppm.
Synthesis of [RuII(trpy)(pdt)(NO2)](ClO4) [3](ClO4): The complex
[2](ClO4)2 (100 mg, 0.116 mmol) was dissolved in CH3CN (5 mL)
and then water (20 mL) was added followed by an excess of NaNO2
(200 mg, 2.90 mmol). The mixture was heated at reflux for 2.5 h.
The deep red solution of [2](ClO4)2 changed to deep orange during
the course of the reaction. The pure crystalline nitro complex pre-
cipitated when the hot solution cooled to room temperature. The
solid [3](ClO4) thus obtained was filtered off, washed with ice-cold
water, and dried in vacuo over P4O10. Yield: 73.3 mg (82%).
C35H25ClN8O6Ru (790.06): calcd. C 53.20, H 3.19, N 14.18; found
C 53.37, H 3.26, N 13.94. Molar conductivity in acetonitrile: ΛM
= 120 Ω–1 cm2 –1. 1H NMR [300 MHz, (CD3)2SO, 298 K]: δ = 8.99
(d, J = 8.1 Hz, 1 H), 8.85 (m, 2 H), 8.75 (m, 2 H), 8.27 (d, J =
5.1 Hz,1 H), 8.17 (t, J = 6.3, 8.7 Hz, 1 H), 8.05 (m, 2 H), 7.90 (m,
2 H), 7.75 (m, 2 H), 7.49 (m, 10 H), 7.24 (t, J = 7.5, 7.8 Hz, 1 H),
6.95 (d, J = 7.5 Hz, 1 H) ppm.
Synthesis of [RuII(trpy)(pdt)(NO+)](ClO4)3 {[4](ClO4)3}: Dropwise
addition of concentrated HNO3 (2 mL) to the powdered nitro com-
plex [3](ClO4) (100 mg, 0.126 mmol) at 273 K whilst stirring re-
sulted in a pasty mass. The pasty mass thus formed was added
to ice-cold concentrated HClO4 (6 mL) dropwise with continuous
stirring using a glass rod. On further addition of saturated aqueous
NaClO4 solution, a yellow solid product formed. The precipitate
was filtered off immediately, washed with a little ice-cold water and
then dried in vacuo over P4O10. Yield: 98.3 mg (80%).
C35H25Cl3N8O13Ru (971.96): calcd. C 43.20, H 2.59, N 11.52;
found C 43.02, H 2.46, N 11.74. Molar conductivity (CH3CN): ΛM
= 310 Ω–1 cm2 –1. 1H NMR [300 MHz, (CD3)2SO, 298 K]: δ = 9.20
(m, 1 H), 9.08 (d, J = 3.9 Hz, 1 H), 8.96 (m, 1 H), 8.83 (t, J = 7.5,
7.8 Hz, 1 H), 8.57 (m, 3 H), 8.27 (d, J = 5.1 Hz, 1 H), 8.19 (m, 2
H), 8.05 (m, 1 H), 7.88 (m, 3 H), 7.51 (m, 9 H), 6.97 (t, J = 7.1,
6.9 Hz, 1 H), 6.85 (d, J = 7.5 Hz, 1 H) ppm.
X-ray Crystallography: Single crystals of [1](ClO4) or [2](ClO4)2
were grown by slow diffusion of an acetonitrile solution of the com-
plex into toluene, followed by slow concentration. X-ray data were
collected using an Oxford Xcalibur-S CCD single-crystal X-ray dif-
fractometer using Mo-Kα radiation. Crystal data and data collec-
tion parameters are listed in Table 4. The structures were solved
and refined by full-matrix least squares on F2 using SHELX-97
(SHELXTL).[30] Absorption corrections for [1](ClO4) and [2]-
(ClO4)2 were performed by multiscan. All the data were corrected
Table 4. Crystallographic data for [1](ClO4) and [2](ClO4)2·3H2O.
[1](ClO4)
[2](ClO4)2
Empirical formula
C35H25Cl2N7O4Ru C37H28Cl2N8O11Ru
Formula mass
779.59
932.64
Crystal symmetry
Space group
a/Å
b/Å
c/Å
monoclinic
P21
8.485(3)
14.4596(8)
13.368(3)
90.0
monoclinic
P21/a
19.612(5)
8.6085(10)
25.267(6)
90.0
α/°
β/°
102.89(3)
90.0
1598.7(7)
2
0.711
100(2)
108.21(2)
90.0
4052.2(14)
4
0.588
293(2)
γ/°
V/Å3
Z
µ/mm–1
T/K
Dcalcd./gcm–3
F (000)
2θ range/°
1.602
788
1.529
1888
5.92–50.0
5164 (0.0438)
0.0505, 0.1152
0.0629, 0.1193
1.042
6.32–57.20
8003 (0.0605)
0.0687, 0.1426
0.1657, 0.1670
0.885
e data (Rint
)
R1, wR2 [I Ͼ 2σ(I)]
R1, wR2 (all data)
Gof
Synthesis of [RuII(trpy)(pdt)(NO·)](ClO4)2 {[4](ClO4)2}: The nitroso
complex [4](ClO4)3 (100 mg, 0.102 mmol) was dissolved in 5 mL of
Largest diff. peak/hole/eÅ–3 2.354/–0.486
0.607/–0.596
3432
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© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 3425–3434

Spectrochemistry of Nitrosylruthenium Complexes
FULL PAPER
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for Lorentz and polarization effects, and the non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were included in the
refinement process as per the riding model. X-ray analysis revealed
the presence of three water molecules as solvent of crystallization
in [2](ClO4)2. CCDC-635850 {for [1](ClO4)} and -635851 {for
[2](ClO4)2} contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
[5]
Supporting Information (see footnote on the first page of this arti-
cle): Packing diagram of [2](ClO4)2·3H2O along the b axis (Fig-
ure S1), 1H NMR spectra of (a) [1]+, (b) [2]2+, (c) [3]+, and (d) [4]3+
in (CD3)2SO (Figure S2), and emission spectrum of [2]2+ in EtOH/
MeOH (4:1) glass at 77 K (Figure S3).
Acknowledgments
Financial support received from the Department of Science and
Technology and Council of Scientific and Industrial Research, New
Delhi, India is gratefully acknowledged. X-ray structural studies
for [1](ClO4) and [2](ClO4)2 were carried out at the National Single
Crystal Diffractometer Facility, Indian Institute of Technology,
Bombay. Special acknowledgment is made to the Sophisticated An-
alytical Instrument Facility (SAIF) for providing the NMR and
EPR facilities.
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