N-(aryl)picolinamide complexes of ruthenium: Usual coordination and strategic cyclometalation

Article abstract of DOI:10.1002/ejic.200600964

Reaction of five N-(4-R-phenyl)picolinamides (R = OCH₃, CH ₃, H, Cl, and NO₂) with [Ru(PPh₃) ₂(CO)₂Cl₂] in refluxing 2-methoxyethanol in the presence of a base (NEt₃) affords two geometrical isomers of a group of complexes (1-R and 2-R), each of which contains an amide ligand coordinated to the metal center as a monoanionic bidentate N,N donor along with two triphenylphosphanes, a carbonyl, and a hydride. Similar reaction of N-(naphthyl)picolinamide with [Ru(PPh₃)₂(CO) ₂Cl₂] affords an organometallic complex, 3, in which the amide ligand is coordinated to the metal center, by C-H activation of the naphthyl ring at the 8-position, as a dianionic tridentate N,N,C donor along with two triphenylphosphanes and one carbonyl. Structures of the 1-OCH ₃, 2-CH₃, and 3 complexes have been determined by X-ray crystallography. In all the complexes the two triphenylphosphanes are trans. In the 1-R complexes the hydride is trans to the pyridine nitrogen and in the 2-R complexes it is trans to the amide-nitrogen. All the complexes are diamagnetic, and show characteristic ¹H NMR signals and intense MLCT transitions in the visible region. Cyclic voltammetry on all the complexes shows a Ru ^II-Ru^III oxidation within 0.71-0.93 V versus SCE. An oxidation and a reduction of the coordinated amide ligand are also observed within 1.29-1.69 V versus SCE and -1.02 to -1.21 V versus SCE respectively. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200600964

FULL PAPER
DOI: 10.1002/ejic.200600964
N-(Aryl)picolinamide Complexes of Ruthenium: Usual Coordination and
Strategic Cyclometalation
Sumon Nag,[a] Ray J. Butcher,[b] and Samaresh Bhattacharya*[a]
Keywords: Picolinamide ligands / Ruthenium complexes / Cyclometalation
Reaction of five N-(4-R-phenyl)picolinamides (R = OCH3,
CH3, H, Cl, and NO2) with [Ru(PPh3)2(CO)2Cl2] in refluxing
2-methoxyethanol in the presence of a base (NEt3) affords
two geometrical isomers of a group of complexes (1-R and 2-
R), each of which contains an amide ligand coordinated to
the metal center as a monoanionic bidentate N,N donor
along with two triphenylphosphanes, a carbonyl, and a hy-
dride. Similar reaction of N-(naphthyl)picolinamide with
[Ru(PPh3)2(CO)2Cl2] affords an organometallic complex, 3, in
which the amide ligand is coordinated to the metal center,
by C–H activation of the naphthyl ring at the 8-position, as a
dianionic tridentate N,N,C donor along with two tri-
phenylphosphanes and one carbonyl. Structures of the 1-
OCH3, 2-CH3, and 3 complexes have been determined by X-
ray crystallography. In all the complexes the two tri-
phenylphosphanes are trans. In the 1-R complexes the hy-
dride is trans to the pyridine nitrogen and in the 2-R com-
plexes it is trans to the amide-nitrogen. All the complexes are
diamagnetic, and show characteristic 1H NMR signals and
intense MLCT transitions in the visible region. Cyclic voltam-
metry on all the complexes shows a RuII–RuIII oxidation
within 0.71–0.93 V versus SCE. An oxidation and a reduction
of the coordinated amide ligand are also observed within
1.29–1.69 V versus SCE and –1.02 to –1.21 V versus SCE
respectively.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
Introduction
There has been considerable current interest in the chem-
istry of ruthenium complexes, largely because of their fasci-
nating redox, photophysical, and photochemical proper-
ties.[1] As all these properties are directed primarily by the
coordination environment around the metal center, com-
plexation of ruthenium by ligands of selected types is of
significant importance. In the present study, which has orig-
inated from our continued interest in the chemistry of ru-
thenium in different coordination environments,[2] we have
selected a group of amide ligands (L1) derived from pico-
linic acid and anilines with five different para-substituents
in order to study their influence, if any, on the redox proper-
ties of the resulting ruthenium complexes.
Chemistry of the amide ligands is of particular interest
with reference to their role in biological processes.[3] For
example, the amide linkage plays a key role in the formation
and maintenance of protein architectures, which are crucial
for their performance in biological systems.[4] The selected
amide ligands (L1) are known to bind to metal ions either
as neutral N,O donors (amide form, I) or as monoanionic
N,N donors (amidate form, II) by loss of the amide pro-
[a] Department of Chemistry, Inorganic Chemistry Section, Ja-
davpur University,
Kolkata 700032, India
[b] Department of Chemistry, Howard University,
Washington, DC 20059, USA
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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S. Nag, R. J. Butcher, S. Bhattacharya
FULL PAPER
ton.[5] Chelation in the amidate mode is known to stabilize phanes, a carbonyl, and a hydride in the coordination
metal ions in their high oxidation states, while that in the sphere. In order to find the stereochemistries of these com-
amide mode is reported to favor relatively lower oxidation plexes as well as the coordination mode of the N-(4-R-
states of a metal.[5] Interconversion between the amide (I) phenyl)picolinamides in them, the structure of one repre-
and amidate (II) modes of binding has been utilized to sentative complex from each family, viz. 1-OCH3 and 2-
chemically manipulate redox properties of the metal cen- CH3, has been determined by X-ray crystallography. The
ter.[6] It may be mentioned here that though the chemistry structure of the 1-OCH3 is shown in Figure 1 and selected
of amide complexes of many transition metals has been ex- bond parameters are listed in Table 1.
tensively studied,[7] that of ruthenium appears to have re-
ceived much less attention.[2j,8] As the source of ruthenium
we have selected the [Ru(PPh3)2(CO)2Cl2] complex, because
of its demonstrated ability to accommodate bidentate li-
gands through displacement of carbonyl and chloride.[2f,2m]
Reaction of the selected amides (L1) as well as a related
ligand, viz. N-(naphthyl)picolinamide (L2), with [Ru(PPh3)2-
(CO)2Cl2] has been found to afford interesting complexes
where the amides (L1 and L2) are bound to ruthenium, as
in II and III respectively. An account of the chemistry of
all these complexes is presented in this report, with special
reference to their formation, characterization, and spectral
and electrochemical properties.
Results and Discussion
Reactions of the N-(4-R-phenyl)picolinamides (L1) with
[Ru(PPh3)2(CO)2Cl2] proceed smoothly in refluxing 2-me-
thoxyethanol in the presence of triethylamine. From each of
these reactions two complexes were obtained, viz. a golden
yellow complex and a yellow complex (1-R and 2-R). The
combined yield of these two complexes was reasonable.
Figure 1. View of the 1-OCH3 complex.
The structure shows that the N-(4-methoxyphenyl)pico-
linamide is coordinated to ruthenium, by dissociation of the
acidic proton, as a monoanionic N,N donor (II). Two tri-
phenylphosphanes, a hydride, and a carbonyl are also coor-
dinated to the metal center. Ruthenium is therefore sitting
in a C1H1N2P2 coordination environment, which is dis-
torted octahedral in nature, as reflected in all the bond pa-
rameters around ruthenium. The coordinated picolinamide,
hydride, and carbonyl constitute one equatorial plane with
the metal at the center, where the hydride is trans to the
pyridine nitrogen and the carbonyl is trans to the amide-
nitrogen. The PPh3 ligands occupy the remaining two axial
positions and hence they are mutually trans. The observed
Ru–P, Ru–C, and Ru–H distances are all quite normal,[9]
and so are the bond lengths in the Ru(amide) frag-
ment.[8b,8c] In the crystal lattice of the 1-OCH3 complex,
there are two water molecules present per molecule of the
complex. In order to find out the link between these water
molecules and the complex molecule, the packing pattern
in the lattice has been scrutinized (Figure 2), which shows
that a layer of water molecules lies in between two layers of
complex molecules and thereby holds them together. A
closer inspection into the network reveals that the methoxy-
oxygen in each complex molecule is hydrogen-bonded to a
phenyl hydrogen of a coordinated PPh3 of an adjacent com-
plex molecule and two such hydrogen bonds result in the
formation of a dimeric unit. Four water molecules, which
Preliminary (microanalytical, spectroscopic, magnetic, are interlinked by hydrogen bonds, bridge two such dimeric
etc.) characterizations on these complexes (vide infra) indi- units by hydrogen bond formation involving the amide oxy-
cate the presence of an amide ligand, two triphenylphos- gen and a phenyl hydrogen of a PPh3. These hydrogen-
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N-(Aryl)picolinamide Complexes of Ruthenium
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Table 1. Selected bond lengths and bond angles for 1-OCH3, 2-
CH3, and 3.
1-OCH3
Bond lengths [Å]
Ru–C
1.839(2)
C–O
1.164(2)
1.329(3)
1.358(2)
1.253(2)
Ru–N(2)
Ru–N(1)
Ru–H
2.1567(16)
2.1792(17)
1.5475
N(2)–C(6)
C(5)–N(1)
C(6)–O(1)
Ru–P(1)
Ru–P(2)
2.3636(5)
2.3689(5)
Bond angles [°]
P(1)–Ru–P(2)
C–Ru–N(2)
N(1)–Ru–H
178.495(17)
173.76(8)
168.3
N(2)–Ru–N(1) 76.50(6)
Ru–C–O
173.63(19)
2-CH3
Bond lengths [Å]
Ru–C
1.844(3)
2.203(2)
2.152(2)
1.58(3)
C–O
1.162(3)
1.334(3)
1.352(3)
1.249(3)
Ru–N(2)
Ru–N(1)
Ru–H
N(2)–C(7)
C(6)–N(1)
C(7)–O(1)
Ru–P(1)
Ru–P(2)
2.3687(7)
2.3569(7)
Bond angles [°]
P(1)–Ru–P(2)
C–Ru–N(1)
N(2)–Ru–H
172.61(2)
175.20(9)
170.2(9)
N(2)–Ru–N(1) 75.46(8)
Ru–C–O
175.1(2)
3
Bond lengths [Å]
Ru–C
1.853(2)
C–O
1.157(3)
1.340(2)
1.351(2)
1.238(2)
Ru–N(1)
Ru–N(2)
Ru–C(15)
Ru–P(1)
Ru–P(2)
2.1588(13)
2.0882(18)
2.0883(16)
2.3691(5)
2.3825(5)
N(2)–C(6)
C(5)–N(1)
C(6)–O(1)
Bond angles [°]
Figure 2. (a) Hydrogen bonding and (b) packing diagram (down
the b axis) of the 1-OCH3 complex.
P(1)–Ru–P(2)
N(2)–Ru–C
174.40(2)
176.48(8)
N(1)–Ru–N(2) 76.97(5)
N(2)–Ru–C(15) 80.31(6)
N(1)–Ru–C(15) 157.04(6)
Ru–C–O
176.57(17)
bonding interactions in the crystal lattice appear to contrib-
ute a lot to its stability. As all the 1-R complexes have been
synthesized similarly and they show similar properties (vide
infra), the other four 1-R (R ϶ OCH3) complexes are as-
sumed to have a similar structure to the 1-OCH3 complex.
The structure of the 2-CH3 complex (Figure 3) shows
that it is very similar to the 1-OCH3 complex; the only dif-
ferences are in the disposition of coordinated carbonyl and
hydride with respect to the coordinated picolinamide. The
hydride is trans to the amide-nitrogen and the carbonyl is
trans to the pyridine nitrogen. All the bond parameters in
this 2-CH3 complex compare well with those observed in
the 1-OCH3 complex (Table 1). In the crystal lattice of 2-
CH3 there is one molecule of methanol per molecule of the
complex. The packing pattern in the lattice shows (Figure
S1, supporting information) that this methanol molecule is
involved in three hydrogen-bonding interactions, two with
the amide oxygen and a phenyl hydrogen of a PPh3 of one Figure 3. View of the 2-CH3 complex.
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S. Nag, R. J. Butcher, S. Bhattacharya
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complex molecule, and the third with the phenyl hydrogen
of a PPh3 of a second complex molecule. The methanol
molecules thus bridge the individual complex molecules
through hydrogen bonding. Based on the similarity in their
synthesis and properties (vide infra), the other four 2-R (R
϶ CH3) complexes are assumed to have similar structures
to 2-CH3. Each 2-R complex is therefore a geometrical iso-
mer of the corresponding 1-R complex. The observed ele-
mental (C, H, N) analytical data of the 1-R and 2-R com-
plexes agree well with their compositions.
Formation of the two stereoisomers (1-R and 2-R) from
the same reaction may be rationalized in terms of Scheme 1.
Upon reaction with [Ru(PPh3)2(CO)2Cl2] the amide ligands
(L1) bind to the metal center, through dissociation of the
acidic N-H proton, as monoanionic N,N donors with simul-
taneous dissociation of a CO and a chloride from the ruthe-
nium starting complex to generate two stereoisomers (A
and B) of an intermediate. These two stereoisomers, which
are believed to remain in equilibrium, are probably formed
by different kinetic routes. The Ru–Cl bond in these inter-
mediates is then converted into a Ru–H bond under the
prevailing reaction condition affording the 1-R and 2-R
complexes respectively. Such conversion of a Ru–Cl bond
into a Ru–H bond in alcoholic solvent in the presence of
base is well documented in the literature.[10]
Disposition of the Ru–H bond with respect to the pen-
dent phenyl ring in 1-R points to the possibility of a C–H
activation of the aryl ring. Such an activation could not
take place in the 1-R complexes, because in them the phenyl
ring is not close enough to the metal center, and had the
phenyl ring undergone an orthometalation it would have
resulted in the formation of a sterically unstable four-mem-
bered metallacycle. In order to undergo C–H activation the
aryl fragment should be so chosen that one of its C–H
bonds should come in close proximity to the Ru–H bond.
With this simple strategy in mind, N-(naphthyl)picolin-
amide (L2) has been chosen as the target ligand for the C–
H activation. Reaction of N-(naphthyl)picolinamide with
[Ru(PPh3)2(CO)2Cl2] has been carried out similarly as be-
fore, which has afforded an orange complex (3) in good
yield. Preliminary characterizations on this complex indi-
cate the presence of an amide ligand, two triphenylphos-
phanes, and a carbonyl in the coordination sphere, as well
as the absence of any hydride. The identity of complex 3
Scheme 1.
has been revealed by its structure determination by X-ray longing to two adjacent complex molecules are found to be
crystallography. The structure (Figure 4) shows that N- hydrogen bonded to the π-cloud of two other phenyl rings
(naphthyl)picolinamide is indeed coordinated to ruthenium of the same two PPh3 ligands. However, this hydrogen-
in the expected N,N,C fashion (III). The Ru–C(naphthyl) bonding interaction is directed to an edge of the phenyl
bond is found to be normal[2m] and the other bond param- ring in an η2 fashion. The amide oxygen of a third complex
eters compare well with those observed in the earlier struc- molecule is also hydrogen-bonded to two phenyl hydrogen
tures (Table 1). The absence of any solvent of crystallization atoms of two PPh3 ligands of the first two complex mole-
in the crystal lattice of complex 3 indicates the possible exis- cules. These C–H···O and C–H···π interactions are extended
tence of some noncovalent interaction(s) between the indi- throughout the entire lattice of 3. The noncovalent interac-
vidual complex molecules. A careful inspection of the pack- tions, observed in all three crystals, play a key role in the
ing pattern in the lattice shows that noncovalent interac- packing of the molecules in the crystals, and it may be rel-
tions of two different types, viz. C–H···O and C–H···π inter- evant to note here that such interactions are of significant
actions, are active in the lattice (Figure S2, Supporting In- importance in molecular recognition processes as well as in
formation). Two phenyl protons of two PPh3 ligands be- crystal engineering.[11]
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N-(Aryl)picolinamide Complexes of Ruthenium
FULL PAPER
Figure 4. View of complex 3.
Probable sequences behind formation of the cyclomet-
alated complex 3 are illustrated in Scheme 2. In the early
stage of the synthetic reaction an equilibrium mixture of
two stereoisomers (C and D) of a hydride intermediate (as
shown in Scheme 1) are believed to be formed, in both of
which ligand L2 is coordinated as a N,N donor. Isomer C
of the intermediate then undergoes the C–H activation at
the 8-position of the naphthyl ring affording the cyclomet-
alated species 3 through elimination of molecular hydro-
gen.[12] As isomer C is irreversibly transformed into the cy-
clometalated species 3, the equilibrium between the two iso-
mers gradually shifts towards C, and thus only the cyclo-
metalated species 3 is obtained as the sole product from this
reaction and none of the two intermediates can be isolated.
Magnetic susceptibility measurements show that the 1-R,
2-R, and 3 complexes are diamagnetic, which corresponds
Scheme 2.
1
to the +2 oxidation state of ruthenium (low-spin d6, S = 0) small shifts in the signal positions, the rest of the H NMR
1
in these complexes. H NMR spectra of the 1-R complexes spectrum of each 2-R complex is qualitatively similar to
show broad signals within 7.1–7.7 ppm for the coordinated that of the corresponding 1-R complex. It may be men-
PPh3 ligands. The hydride signal is observed as a distinct tioned here that in CDCl3 solution, each 1-R (or 2-R) com-
triplet, because of coupling with two magnetically equiva- plex is slowly and partially converted into the correspond-
lent phosphorus nuclei, near δ = –10.6 ppm. Most of the ing 2-R (or 1-R) isomer, which is easily recognized by the
aromatic proton signals from the coordinated amide ligand appearance of the characteristic hydride signal of the gener-
are clearly observed in the expected region, while a few ated isomer. It may also be noted that conversion of 1-R to
could not be detected because of their overlap with other 2-R is relatively faster (takes about 6 d to reach equilibrium,
signals in the same region. Signals for the methoxy and where concentration ratio of 1-R and 2-R becomes approxi-
methyl groups in the 1-OCH3 and 1-CH3 complexes are mately 2:1), while that of 2-R to 1-R is much slower (takes
1
observed respectively at δ = 3.67 and 2.11 ppm. In the H about 8 d to reach a concentration ratio of 1-R and 2-R of
NMR spectra of the 2-R complexes, the hydride signal approximately 1:8). Except for the absence of the hydride
(triplet) is observed at around δ = –12.1 ppm, and this sig- signal and the presence of a few additional signals in the
1
nificant shift in position, compared to the 1-R complexes, aromatic region, H NMR spectral features of complex 3
has been useful as a diagnostic property in distinguishing are similar to those of the 1-H complex. 31P NMR spectra
this group of 2-R complexes from their 1-R isomers. Besides of all the 1-R and 2-R complexes show a single resonance
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S. Nag, R. J. Butcher, S. Bhattacharya
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near 41.7 and 44.0 ppm respectively. In complex 3, the 31P (aryl)picolinamide (LUMO). The other intense absorptions
signal is observed at δ = 31.42 ppm. in the visible region may be assigned to charge-transfer
The infrared spectrum of each 1-R complex shows many transitions from the ruthenium t2 orbitals to the higher-
bands of different intensities in the 400–4000 cm–1 region. energy vacant orbitals. The nature of the molecular orbitals
Assignment of each individual band to a specific vibration in the 2-R complexes is very similar to that in the 1-R com-
has not been attempted. However, comparison with the plexes, and the electronic spectral features of these two
spectra of the corresponding uncoordinated ligands shows groups of complexes are also very similar. In complex 3,
that the N–H stretch, observed near 3140 cm–1 in the unco- however, the HOMO, HOMO-1, and HOMO-2 are found
ordinated ligands, is absent in the complexes. The amide to have large contributions from the N-(naphthyl)picolin-
C=O stretch, observed at around 1680 cm–1 in the uncoor- amide besides having significant contributions from the ru-
dinated ligands, is also found to be shifted to around thenium dxy, dyz, and dxz orbitals. The LUMO of complex
1578 cm–1 in the complexes. A strong band observed near 3 is concentrated largely on the pyridine ring and the car-
1900 cm–1 in all the 1-R complexes is due to the coordinated bonamide functionality. The next two vacant orbitals
carbon monoxide, and three strong bands have been ob- (LUMO+1 and LUMO+2) are close in energy; LUMO+1
served near 517, 692, and 746 cm–1 in 1-R complexes, indi- is primarily centered on the pyridine ring, while LUMO+2
cating the presence of the coordinated PPh3 ligands.[2m] is delocalized over both the naphthyl and pyridine ring of
Sharp bands are also observed near 1092, 1433, 1479, and the N-(naphthyl)picolinamide. The absorption at 472 nm
1557 cm–1 in the 1-R complexes, which are found to be ab- may therefore be assigned to a HOMO-to-LUMO charge-
sent in the spectrum of [Ru(PPh3)2(CO)2Cl2], and hence transfer transition and the other absorptions in the visible
these are attributed to the coordinated N-(aryl)picolin- region to charge-transfer transitions from the filled orbitals
amide. Infrared spectra of the 2-R and 3 complexes are sim- to the higher-energy vacant orbitals.
1
ilar to those of the 1-R complexes. The H NMR and IR
spectroscopic data of the 1-R, 2-R, and 3 complexes are
Table 2. Electronic spectral and cyclic voltammetric data.
Electronic spectroscopic data[a] Cyclic voltammetric data[b]
therefore consistent with their compositions.
The 1-R, 2-R, and 3 complexes are soluble in acetone,
dichloromethane, chloroform, and so forth, producing
bright yellow solutions. Electronic spectra of the complexes
have been recorded in dichloromethane solution. All the
complexes show several intense absorptions in the visible
and ultraviolet regions (Table 2). The absorptions in the ul-
traviolet region are attributable to transitions within the li-
gand orbitals and those in the visible region are probably
due to charge-transfer transitions. To have a better insight
into the nature of the absorptions in the visible region,
qualitative EHMO calculations have been performed[13] on
computer-generated models of the complexes where phenyl
rings of the triphenylphosphanes have been replaced by hy-
drogen. Partial MO diagrams of a selected 1-R complex are
shown in Figure 5 and those of a representative 2-R and 3
complexes are deposited as supporting information (Figures
S3 and S4). Compositions of selected molecular orbitals are
given in Table S1 (supporting information). In the 1-R com-
plexes, the highest occupied molecular orbital (HOMO)
and the next two filled orbitals (HOMO-1 and HOMO-2)
have major contributions from the ruthenium dxy, dyz, and
dxz orbitals.[14] These three occupied orbitals may therefore
be regarded as the ruthenium t2 orbitals. The lowest unoc-
cupied molecular orbital (LUMO) has more than 90% con-
tribution from the N-(aryl)picolinamide and is concentrated
largely on the pyridine ring and the amido fragment.
Among the next few vacant orbitals (LUMO+1, LUMO+2
λmax [nm] (ε [–1 cm–1])
Epa [V]
Epc [V]
–1.19
1-OCH3 230 (55300), 254[c] (32400),
0.71, 1.29
348 (7800), 400 (3800)
1-CH3
1-H
230 (34900), 254[c] (21600),
338 (5300), 396 (1800)
0.77, 1.31
0.80, 1.47
0.83, 1.56
0.93, 1.69
–1.21
–1.19
–1.13
–1.12
230 (35500), 254[c] (25300),
336 (6800), 392 (2500)
1-Cl
230 (32700), 256[c] (22800),
340 (7400), 394 (4100)
1-NO2
230 (37800), 254[c] (25400),
322 (9400), 420 (1500)
2-OCH3 230 (50300), 272[c] (19700),
0.54, 1.26
0.55, 1.28
0.60, 1.32
0.64, 1.35
0.69, 1.42
–1.05
–1.03
–1.11
–1.10
–1.09
354 (5600), 430 (2250)
2-CH3
2-H
230 (95900), 272[c] (31250),
362 (7580), 432 (3000)
230 (47900), 274[c] (15100),
368 (3600), 424 (1700)
230 (36000), 274[c] (13480),
356 (3800), 440 (980)
2-Cl
2-NO2
230 (15500), 278[c] (4800),
348 (3000), 448 (2600)
3
246 (28900), 348 (11800),
472 (2000)
0.49, 1.34
–1.02
[a] In dichloromethane. [b] Solvent: dichloromethane/acetonitrile,
1:9; supporting electrolyte, TBAP; reference electrode, SCE; Epa
and Epc are anodic and cathodic peak potentials; scan rate,
50 mVs–1. [c] Shoulder.
Electrochemical properties of the 1-R, 2-R, and 3 com-
etc.), LUMO+1 is primarily centered on the pyridine ring, plexes have been studied by cyclic voltammetry in 1:9
while LUMO+2 is delocalized over both the pyridine ring dichloromethane/acetonitrile solution (0.1  TBAP).[15]
and the pendent phenyl ring of the N-(aryl)picolinamide. Each complex shows two anodic waves at positive poten-
The lowest energy absorption in the 392–420 nm region tials of the SCE reference electrode and a cathodic one at
may therefore be assigned to the charge-transfer transition negative potential (Table 2). All the responses are irrevers-
taking place from the highest filled ruthenium t2 orbital ible in nature. In view of the composition of the HOMO,
(HOMO) to the vacant orbital delocalized over the N- the first oxidation is assigned to RuII–RuIII oxidation. Simi-
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N-(Aryl)picolinamide Complexes of Ruthenium
FULL PAPER
ing a carbonyl and a chloride, as well as turning a Ru–Cl
bond into a Ru–H bond. This study also demonstrates that
the Ru–H bond can be utilized for inducing C–H activation
of selective amides, such as N-(naphthyl)picolinamide. Utili-
zation of the Ru–H bond for other useful reactions is cur-
rently under exploration.
Experimental Section
General Procedure: Commercial ruthenium trichloride was pur-
chased from Arora Matthey, Kolkata, India and was converted to
[Ru(PPh3)2(CO)2Cl2] by following a reported procedure.[18] The N-
(aryl)picolinamides were prepared by condensing picolinic acid
with para-substituted anilines or α-naphthylamine.[8a] Purification
of dichloromethane and acetonitrile, and preparation of tetrabu-
tylammonium perchlorate (TBAP) for electrochemical work were
performed as reported in the literature.[19] All other chemicals and
solvents were reagent grade commercial materials and were used as
received. Microanalyses (C, H, N) were performed using a Heraeus
Carlo Erba 1108 elemental analyzer. Magnetic susceptibilities were
measured using a PAR 155 vibrating sample magnetometer fitted
1
with a Walker scientific L75FBAL magnet. H NMR spectra were
Figure 5. Partial molecular orbital diagram of the 1-H complex.
recorded in CDCl3 solution with a Bruker Avance DPX 300 NMR
spectrometer using TMS as the internal standard. IR spectra were
obtained with a Shimadzu FTIR-8300 spectrometer with samples
prepared as KBr pellets. Electronic spectra were recorded with a
JASCO V-570 spectrophotometer. Electrochemical measurements
were made using a CH Instruments model 600A electrochemical
larly based on composition of the LUMO, the reduction is
assigned to reduction of the coordinated amide ligand. The
second oxidation is attributed tentatively to oxidation of the
coordinated amide ligand. Comparison of the potential of
the RuII–RuIII couple in the 1-R and 2-R complexes shows analyzer. A platinum disc working electrode, a platinum wire auxil-
iary electrode, and an aqueous saturated calomel reference elec-
trode (SCE) were used in the cyclic voltammetry experiments. All
electrochemical experiments were performed under dinitrogen. All
electrochemical data were collected at 298 K and are uncorrected
for junction potentials.
that the bivalent state of ruthenium is much more stabilized
in the former group of complexes, which may be attributed
to the presence of carbonyl ligand cis to the pyridine nitro-
gen in the coordination sphere of ruthenium in the 1-R
complexes, favoring the back donation from the metal cen-
ter to these two π-acidic ligands, rendering the rutheni-
um(II) state much more stable than the 2-R complexes. In
these 1-R and 2-R complexes, the potential of the RuII–
RuIII oxidation has been observed to be sensitive to the na-
ture of the substituent R in the N-(aryl)picolinamide. The
potential (Epa) increases with increasing electron-with-
drawing character of the substituent R. The plots of Epa
versus σ [σ = Hammett constant of R;[16] for OCH3: –0.27,
CH3: –0.17, H: 0.00, Cl: 0.23, NO2: 0.78] are linear for both
the 1-R and 2-R complexes (see Figures S5 and S6 in the
supporting information) with slopes (ρ) of 0.19 and 0.15 V
(ρ = reaction constant of this oxidation[17]) respectively. For
both 1-R and 2-R complexes, the potential of the second
oxidative response also shows linear correlation with the
electron-withdrawing nature of substituent R (Figures S7
and S8, supporting information), with slopes (ρ) of 0.39 and
0.15 V, respectively. Potentials of the irreversible reductive
response do not show any systematic variation with the na-
ture of the substituent R.
Preparations of Complexes
The 1-R and 2-R complexes were obtained by following a general
procedure. Specific details are given below for a particular pair of
complexes.
1-OCH3 and 2-OCH3: N-(4-Methoxyphenyl)picolinamide (L1, R =
OCH3) (30 mg, 0.13 mmol) was dissolved in warm 2-methoxy-
ethanol (50 mL) and triethylamine (13 mg, 0.13 mmol) was added
to it followed by [Ru(PPh3)2(CO)2Cl2] (100 mg, 0.13 mmol). The
mixture was refluxed for 24 h, whereby a yellow solution was ob-
tained. Evaporation of this solution afforded a dark yellow solid,
which was subjected to purification by thin layer chromatography
on a silica plate. With 1:3 acetonitrile/benzene as the eluent, a dis-
tinct golden yellow band separated first followed by an orangish-
yellow band, both of which were extracted separately with 1:3
dichloromethane/acetonitrile. Evaporation of these extracts respec-
tively gave 1-OCH3 as a crystalline golden yellow solid and 2-
OCH3 as a yellow crystalline solid.
1-OCH3: Yield: 35 mg, 30%. C50H42N2O3P2Ru (881): calcd. C
68.10, H 4.76, N 3.17; found C 67.47, H 4.80, N 3.19. 1H NMR:[20]
δ = –10.64 (t, J = 19.5 Hz, 1 H); 3.67 (OCH3); 6.20 (d, J = 9.0 Hz,
2 H); 6.65 (t, J = 5.7 Hz, 1 H); 6.84 (d, J = 9.0 Hz, 2 H); 7.14–7.72
(2PPh3+d)*; 7.92 (d, J = 5.0 Hz, 1 H) ppm. 31P NMR: δ = 41.72
ppm. 1-CH3: Yield: 38 mg, 33%. C50H42N2O2P2Ru (865): calcd. C
Conclusions
1
69.36, H 4.85, N 3.23; found C 69.07, H 4.88, N 3.25. H NMR:
The present study shows that upon reaction with
δ = –10.59 (t, J = 19.4 Hz, 1 H); 2.11 (CH3); 6.41 (d, J = 8.2 Hz,
2 H); 6.65 (t, J = 6.2 Hz, 1 H); 6.73 (d, J = 8.2 Hz, 2 H); 7.12–7.46
[Ru(PPh3)2(CO)2Cl2] the N-(aryl)picolinamides (L1) can
bind to ruthenium as monoanionic N,N-donors by displac- (2PPh3); 7.69 (d, J = 7.8 Hz, 1 H); 7.93 (d, J = 5.0 Hz, 1 H) ppm.
Eur. J. Inorg. Chem. 2007, 1251–1260
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1257

S. Nag, R. J. Butcher, S. Bhattacharya
FULL PAPER
31P NMR: δ = 41.74 ppm. 1-H: Yield: 34 mg, 30%. C49H40N2O2- 7.11–7.36 (2PPh3); 7.51 (d, J = 8.1 Hz, 1 H); 7.66 (d, J = 8.3 Hz,
P2Ru (851): calcd. C 69.09, H 4.93, N 3.29; found C 68.67, H 4.92,
N 3.26. H NMR: δ = –10.54 (t, J = 19.4 Hz, 1 H); 6.60–6.68 (t +
1 H); 7.70 (d, J = 9.3 Hz, 2 H) ppm. 31P NMR: δ = 44.14 ppm.
1
3: N-(1-Naphthyl)picolinamide (L2) (32 mg, 0.13 mmol) was dis-
solved in warm 2-methoxyethanol (50 mL) and triethylamine
(13 mg, 0.13 mmol) was added to it followed by [Ru(PPh3)2-
(CO)2Cl2] (100 mg, 0.13 mmol). The mixture was refluxed for 24 h,
whereby a yellow solution was obtained. Evaporation of this solu-
tion afforded an orange solid, which was subjected to purification
by thin layer chromatography on a silica plate. With 1:3 acetoni-
trile/benzene as the eluent, a distinct orange band separated, which
was extracted with 1:3 dichloromethane/acetonitrile. Evaporation
of this extract gave 3 as a crystalline orange solid. Yield: 84 mg,
70%. C53H40N2O2P2Ru (899): calcd. C 71.54, H 4.49, N 3.14;
t + t, 3 H)*; 6.92 (d, J = 6.1 Hz, 2 H); 7.13–7.49 (2PPh3); 7.73 (d,
J = 7.8 Hz, 1 H); 7.92 (d, J = 5.0 Hz, 1 H) ppm. 31P NMR: δ =
41.71 ppm. 1-Cl: Yield: 38 mg, 32%. C49H39ClN2O2P2Ru (885.5):
1
calcd. C 66.40, H 4.40, N 3.16; found C 66.07, H 4.46, N 3.14. H
NMR: δ = –10.64 (t, J = 19.4 Hz, 1 H); 6.54 (d, J = 8.9 Hz, 2 H);
6.64 (t, J = 6.1 Hz, 1 H); 6.87 (d, J = 8.8 Hz, 2 H); 7.13–7.43
(2PPh3); 7.731 (d, J = 7.8 Hz, 1 H); 7.90 (d, J = 5.0 Hz, 1 H) ppm.
31P NMR: δ = 41.72 ppm. 1-NO2: Yield: 40 mg, 34%. C49H39N3O4-
P2Ru (896): calcd. C 65.62, H 4.35, N 3.12; found C 65.30, H 4.30,
N 3.14. 1H NMR: δ = –10.64 (t, J = 19.4 Hz, 1 H); 6.72 (t, J =
6.3 Hz, 1 H); 7.13–7.43 (2PPh3); 7.48 (d, J = 6.0 Hz, 2 H); 7.79 (d,
J = 7.8 Hz, 1 H), 7.95 (d, J = 5.1 Hz, 1 H) ppm. 31P NMR: δ =
41.73 ppm.
1
found C 72.23, H 4.46, N 3.11. H NMR: δ = 6.62 (t, J = 6.2 Hz,
1 H); 6.76 (t, J = 7.5 Hz, 1 H); 6.82–6.90 (t + 2d, 3 H)*, 6.99–7.15
(2PPh3); 7.36 (t, J = 7.8 Hz, 1 H); 7.48 (d, J = 6.3 Hz, 1 H); 7.69
(d, J = 7.9 Hz, 1 H), 8.05–8.07 (d + d, 2 H)* ppm. 31P NMR: δ =
31.42 ppm.
2-OCH3: Yield: 40 mg, 35%. C50H42N2O3P2Ru (881): calcd. C
1
68.10, H 4.76, N 3.17; found C 67.80, H 4.80, N 3.21. H NMR:
δ = –12.16 (t, J = 20.5 Hz, 1 H); 3.77 (OCH3); 6.20 (t, J = 6.4 Hz,
1 H); 6.46 (d, J = 8.9 Hz, 2 H); 6.82 (d, J = 8.9 Hz, 2 H); 7.10–
7.40 (2PPh3); 7.47 (d, J = 5.3 Hz, 1 H); 7.62 (d, J = 7.6 Hz, 1
H) ppm. 31P NMR: δ = 44.09 ppm. 2-CH3: Yield: 37 mg, 32%.
C50H42N2O2P2Ru (865): calcd. C 69.36, H 4.85, N 3.23; found C
X-ray Crystallography: Single crystals of the 1-OCH3 and 3 com-
plexes were obtained by slow evaporation of acetonitrile solutions
of the respective complexes. Single crystals of the 2-CH3 complex
were obtained by slow evaporation of a solution of the complex in
a 1:1 mixture of methanol and dichloromethane. Selected crystal
data and data collection parameters are given in Table 3. Data were
collected on a Bruker CCD diffractometer using graphite-mono-
chromated Mo-Kα radiation. X-ray data reduction, and structure
solution and refinement were done using SHELXS-97 and
SHELXL-97 programs.[21] The structures were solved by the direct
methods.
1
69.24, H 4.82, N 3.19. H NMR: δ = –12.05 (t, J = 20.7 Hz, 1 H);
2.24 (CH3); 6.21 (t, J = 6.3 Hz, 1 H); 6.69 (d, J = 8.1 Hz, 2 H);
6.78 (d, J = 8.1 Hz, 2 H); 7.11–7.48 (2PPh3); 7.48–7.70 (t + d + d,
3 H)* ppm. 31P NMR: δ = 44.04 ppm. 2-H: Yield: 37 mg, 33%.
C49H40N2O2P2Ru (851): calcd. C 69.09, H 4.93, N 3.29; found C
1
68.87, H 4.92, N 3.26. H NMR: δ = –12.09 (t, J = 20.7 Hz, 1 H);
6.21 (t, J = 6.2 Hz, 1 H); 6.85–6.89 (2d + 2t, 4 H)*; 7.11–7.48
(2PPh3); 7.53–7.60 (t + d, 2 H)*; 7.69 (d, J = 7.0 Hz, 1 H) ppm.
31P NMR: δ = 44.06 ppm. 2-Cl: Yield: 40 mg, 34%. C49H39ClN2O2-
P2Ru (885.5): calcd. C 66.40, H 4.40, N 3.16; found C 66.17, H
CCDC-623582, -623583, and -623584 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.
1
4.35, N 3.12. H NMR: δ = –12.16 (t, J = 20.5 Hz, 1 H); 6.20 (t,
J = 6.3 Hz, 1 H); 6.52–6.80 (d + d, 4 H)*; 7.11–7.63 (2PPh3); 7.82
Supporting Information (see also the footnote on the first page of
(d, J = 5.1 Hz, 1 H) ppm. 31P NMR: δ = 44.10 ppm. 2-NO2: Yield: this article): Packing diagram showing hydrogen bonding in com-
40 mg, 34%. C49H39N3O4P2Ru (896): calcd. C 65.62, H 4.35, N
3.12; found C 65.38, H 4.31, N 3.11. H NMR: δ = –12.36 (t, J =
20.1 Hz, 1 H); 6.24 (t, J = 6.2 Hz, 1 H); 6.93 (d, J = 9.0 Hz, 2 H);
plexes 2-CH3 (Figure S1) and 3 (Figure S2), partial molecular or-
bital diagrams of complexes 2-H (Figure S3) and 3 (Figure S4),
least-squares plots of Epa values of RuII–RuIII couple versus Ham-
1
Table 3. Crystallographic data for 1-OCH3, 2-CH3 and 3.
1-OCH3·2H2O
2-CH3·CH3OH
3
Empirical formula
Formula mass
Space group
a [Å]
b [Å]
c [Å]
C50H46N2O5P2Ru
917.90
C51H46N2O3P2Ru
897.91
monoclinic, P21/c
19.958(3)
10.8273(18)
19.795(3)
90
103.234(3)
90
4164.1(12)
4
C53H40N2O2P2Ru
899.88
monoclinic, Cc
11.6130(7)
22.0267(14)
16.7471(11)
90
109.2660(10)
90
4043.9(4)
4
triclinic, P1
10.1885(10)
13.7277(13)
15.0590(14)
87.675(2)
89.174(2)
82.557(2)
2086.7(3)
2
α [°]
β [°]
γ [°]
V [Å3]
Z
λ [Å]
0.71073
0.66ϫ0.45ϫ0.15
293(2)
0.505
0.0337
0.0847
1.029
0.71073
0.44ϫ0.35ϫ0.15
293(2)
0.501
0.0372
0.0880
1.022
0.71073
0.17ϫ0.36ϫ0.67
103(2)
0.514
0.0211
0.0517
1.039
Crystal size [mm]
T [K]
µ [mm–1]
R1[a]
wR2[b]
Gof[c]
[a] R1 = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = [Σ{w(Fo – Fc ) }/Σ{w(Fo2)}]1/2. [c] Gof = {Σ[w(Fo – Fc ) ]/(M – N)}1/2, where M is the number of
reflections and N is the number of parameters refined.
2
2 2
2
2 2
1258
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 1251–1260

N-(Aryl)picolinamide Complexes of Ruthenium
FULL PAPER
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Acknowledgments
Financial assistance received from the Department of Science and
Technology [Grant No. SR/S1/IC-15/2004] is gratefully acknowl-
edged. S. N. thanks the Council of Scientific and Industrial Re-
search, New Delhi, for his fellowship [Grant No. 9/96(401)2003-
EMR-I].
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FULL PAPER
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Received: October 13, 2006
Published Online: February 14, 2007
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