Ruthenium(II) complexes containing a phosphine-functionalized thiosemicarbazone ligand: Synthesis, structures and catalytic C-N bond formation reactions via N-alkylation

Article abstract of DOI:10.1039/c4ra14797a

A series of ruthenium(II) complexes incorporating a thiosemicarbazone chelate tethered with a diphenylphosphine pendant have been studied. Thus, [(PNS-Et)RuCl(CO)(PPh₃)] (1), [N,S-(PNS-Et)RuH(CO)(PPh₃)₂] (2) and [(PNS-Et)RuCl(PPh₃)] (3) were synthesized by reactions of various Ru^II precursors with 2-(2-(diphenylphosphino)benzylidene)-N-ethylthiosemicarbazone (PNS-Et). However, complexation of PNS-Et with an equimolar amount of [RuCl₂(dmso)₄] resulted in two different entities [(PNS-Et)RuCl(dmso)₂] (4) and [(PNS-Et)₂Ru] (5) with different structural features in a single reaction. All the Ru^II complexes have been characterized by analytical and various spectroscopic techniques. Compounds 1-5 were recrystallized, and the X-ray crystal structures have been reported for 1, 2 and 5. In the complexes 1 and 3-5 the ligand coordinated in a tridentate monobasic fashion by forming PNS five- and six-membered rings, whereas in 2, the ligand coordinated in a bidentate monobasic fashion by forming a strained NS four-membered ring. Furthermore, compounds 1-5 showed catalytic activity in N-alkylation of heteroaromatic amines. Notably, complexes 1-3 were found to be very efficient catalysts toward N-alkylation of a wide range of heterocyclic amines with alcohols. In the presence of a catalytic amount of 2 with 50 mol% of KOH, N₁,C₅-dialkylation of 4-phenylthiazol-2-amine has been investigated. Reaction of in situ generated aldehyde with amine yields the N₁,C₅-dialkylated products through the hydride ion transformation from alcohol. Complexes 1-3 also catalyzed a variety of coupling reactions of benzyl alcohols and sulfonamides, which were realized often with 99% isolated yields. Advantageously, only one equivalent of the primary alcohol was consumed in the process.

Full text of DOI:10.1039/c4ra14797a

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Ruthenium(II)
Complexes
Containing
PhosphineꢀFunctionalized
Thiosemicarbazone Ligand: Synthesis, Structures and Catalytic CꢀN Bond
Formation Reactions via Nꢀalkylation
Rangasamy Ramachandran,a Govindan Prakash,a Sellappan Selvamurugan,a Periasamy
Viswanathamurthi,*a Jan Grzegorz Malecki,b Wolfgang Linert,c Alexey Gusevd
aDepartment of Chemistry, Periyar University, Salemꢀ636 011, India.
bDepartment of Crystallography, Silesian University, Szkolna 9, 40ꢀ006 Katowice, Poland
cInstitute of Applied Synthetic Chemistry, Vienna University of Technology, Vienna, Austria
dGeneral Chemistry Department,V.I.Vernadsky Taurida National University, Ukraine
*To whom correspondence should be made, eꢀmail: viswanathamurthi72@gmail.com;
Fax: +91 427 2345124
1

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Abstract
A series of ruthenium(II) complexes incorporating thiosemicarbazone chelate tethered with a
diphenylphosphine pendant have been studied. Thus, [(PNSꢀEt)RuCl(CO)(PPh3)] (1), [N,Sꢀ
(PNSꢀEt)RuH(CO)(PPh3)2] (2) and [(PNSꢀEt)RuCl(PPh3)] (3) were synthesized by reactions of
various RuII precursors with 2ꢀ(2ꢀ(diphenylphosphino)benzylidene)ꢀNꢀethylthiosemicarbazone
(PNSꢀEt). However, complexation of PNSꢀEt with an equimolar amount of [RuCl2(dmso)4]
resulted two different entities [(PNSꢀEt)RuCl(dmso)2] (4) and [(PNSꢀEt)2Ru] (5) with different
structural features in a single reaction. All the RuII complexes have been characterized by
analytical and various spectroscopic techniques. Compounds 1ꢀ5 were recrystallized, and the Xꢀ
ray crystal structures have been reported for 1, 2 and 5. In the complexes 1 and 3ꢀ5 the ligand
coordinated in a tridentate monobasic fashion by forming PNS fiveꢀ and sixꢀmembered rings,
whereas in 2, the ligand coordinated in a bidentate monobasic fashion by forming strained NS
fourꢀmembered ring. Furthermore, compounds 1ꢀ5 showed catalytic activity in Nꢀalkylation of
hetero aromatic amines. Notably, complexes 1ꢀ3 were found to be very efficient catalysts toward
Nꢀalkylation of a wide range of heterocyclic amines with alcohols. In the presence of a catalytic
amount of 2 with 50 mol% of KOH, N1,C5ꢀdialkylation of 4ꢀphenylthiazolꢀ2ꢀamine has been
investigated. Reaction of in situ generated aldehyde with amine yields the N1,C5ꢀdialkylated
products through the hydride ion transformation from alcohol. Complexes 1ꢀ3 also catalyzed a
variety of coupling reactions of benzyl alcohols and sulfonamides, which were realized, often
with 99% isolated yields. Advantageously, only one equivalent of the primary alcohol was
consumed in the process.
Key words: Coordination behavior, Phosphinoꢀthiosemicarbazone, Ruthenium(II)
complexes, Nꢀalkylation, N,N’ꢀdialkylation, N1,C5ꢀdialkylation.
2

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Introduction
The catalytic construction of C−N bonds via “borrowing hydrogen methodology’’ (Scheme 1),1
also called “hydrogen autotransfer process’’,2 has recently received much attention since
nitrogen functionalities occur in various compounds of synthetic and pharmaceutical significance
as well as in important biologically active molecules.3 In contrast with other transitionꢀmetalꢀ
catalyzed methodologies, for e.g., amination of arylhalides,4 reductive amination of carbonyl
compounds5 or the hydroamination,6 and hydroaminoꢀmethylation7 of C−C multiple bonds, etc.,
the Nꢀalkylation of amines/amides with alcohols may serve as a relatively green and
environmentally benign alternative since water is the sole byproduct.8,9 Moreover the use of
alcohols as the alkylating agent is direct and simple as the alcohols are readily available, highly
stable, low in toxicity, easily stored and handled, low in cost, and relatively high in atom
efficiency.
Some of the first homogeneous catalysts for Nꢀalkylation reactions were reported in 1981ꢀ
1985 by Grigg10 and Watanabe11, but more recent developments have led to more active catalysts
and milder conditions. Beller’s group used a dimeric Shvo’s ruthenium catalyst system12 and
realized the alkylation of indoles using alcohols, while Williams and coꢀworkers have used
ruthenium dimer catalyst13 to achieve good results. Koten, Milstein and coꢀworkers have
obtained good yields in the Nꢀalkylation of aromatic amines with diols using 0.1ꢀ1 mol % of the
o
PNP pincer type ruthenium catalyst14,15 at high temperature (130ꢀ180 C). Also, Yamaguchi and
coꢀworkers reported an elegant synthesis of Nꢀalkyl substituted amines using readily available
16
o
[Cp*IrCl2]2 (1 mol %) at 110 C. Kemp also reported that small loadings of [IrCl(PN)COD]
afford excellent results in the Nꢀalkylation of amines, forming selective monoꢀ and dialkylated
products.17 Two very recently reported compounds of Ru(II)CNN18 and Ir PNP19 pincer type
catalyst from Matute, Ozawa and coꢀworkers, are the first selective CꢀN bond formation reaction
catalyst for the synthesis of Nꢀalkylated amines with excellent results. Our own group has also
successful in applying the ruthenium carbonyl complexes20 to couple a wide range of amines,
diamines and alcohols together. Among the aforementioned cases, most of the reported complex
catalysts bear ligands containing phosphine coordinating arms usually exhibit much higher
catalytic activity due to the steric (bulkiness) and electronic effects (basicity) of the ligand.
Unfortunately, such highly active complex catalysts have less been investigated.
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Thiosemicarbazones are versatile ligands of considerable attention with respect to their
variable coordination behavior and promising biological21 and catalytic properties.22
Modifications of the thiosemicarbazone framework, to find new compounds with higher activity
and/or to tune their catalytic activity,22e,f,23 have been extensively studied and relationships
between the catalytic activity and chelate formation are evident in a number of cases.24 Among
the most widely studied thiosemicarbazone ligands, a prominent position is occupied by
phosphinoꢀthiosemicarbazones,25 including their terminal alkyl or aryl derivatives.26ꢀ29 With the
simultaneous presence of soft and hard donors, the phosphinoꢀthiosemicarbazone ligands exhibit
various coordination modes: PNSꢀtridentate,25,27 PꢀS bidentate28 and PꢀS bridge in binuclear
compounds as well as in oligomers29 and clusters.26,29 This makes structural studies more
interesting, which are the rational base for structureꢀactivity relationships.
In continuation of our ongoing research in the utility of ruthenium thiosemicarbazone
complexes for various organic transformations.20a,30 In this paper, we report the coordination
flexibility of 2ꢀ(2ꢀ(diphenylphosphino)benzylidene)ꢀNꢀethylthiosemicarbazone in ruthenium(II)
complexes, together with their catalytic properties with regard to Nꢀalkylation of amines,
diamines and sulphonamides using KOH as the base. The attractive features of these reactions
include the use of low toxicity organic materials, excellent atom economy, water is the only byꢀ
product, and high selectivity towards the products.
Results and discussion
Synthesis and Characterization of Ruthenium(II) Complexes
The synthetic route to the targeted ligand 2ꢀ(2ꢀ(diphenylphosphino)benzylidene)ꢀNꢀ
ethylthiosemicarbazone (PNSꢀEt) are displayed in Scheme 2. First, following the classical
methodologies to prepare 2ꢀ(diphenylphosphino)benzaldehyde31 AꢀD, combinations of the 4ꢀ
ethylꢀ3ꢀthiosemicarbazide and the accurate 2ꢀ(diphenylphosphino)benzaldehyde (D) precursor in
the presence of acetic acid led to the formation of PNSꢀEt in 96% yield.29 A series of
ruthenium(II) complexes containing PNSꢀEt were synthesized as depicted in Scheme 3. The
reactions of the isolated PNSꢀEt with equimolar amounts of [RuHCl(CO)(PPh3)3],
[RuH2(CO)(PPh3)3] and [RuCl2(PPh3)3] yielded the substituted products 1ꢀ3 respectively.
However, instead of the expected complex 2a, ESIꢀmass spectrometry as well as single crystal
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Xꢀray diffraction study (see below) revealed that an unexpected complex 2 had been formed
(Scheme 3). Treatment of PNSꢀEt with an equimolar amount of [RuCl2(dmso)4] resulted in two
different entities 4 and 5 with different structural features in a single reaction (see Scheme 3 for
further details). The new complexes are soluble in common organic solvents such as
dichloromethane, chloroform, benzene, acetonitrile, ethanol, methanol, dimethylsulfoxide and
dimethylformamide. The analytical data of the complexes agreed well with the proposed
molecular formulae. The appearance of the [MꢀCl]+ peak in the ESIꢀMS+ spectra of complexes
1,3 and 4 confirms the proposed stoichiometries. However, the molecular ion peak [M+H]+ and
[M]+ were identified for complexes 2 and 5.
Spectroscopic Studies
The IR spectra of the new complexes have been compared with that of the ligand to further
elucidate the mode of the ligand coordination. A strong vibration observed at 1583 cmꢀ1 in the
ligand corresponding to νC=N was shifted to 1556−1581 cmꢀ1 in the complexes 1 and 3ꢀ5
indicating the participation of azomethine nitrogen in bonding.29 A sharp band observed at 744
cmꢀ1, ascribed to νC=S in the ligand, has completely disappeared in the spectra of all the new Ru
complexes and the appearance of a new band at 741−749 cmꢀ1 due to νC−S indicated the
coordination of the sulfur atom after enolisation followed by deprotonation.32 The complexes 1, 2
display a medium to strong band in the region 1946−1944 cmꢀ1, which is attributed to the
terminally coordinated carbonyl group (C≡O) and is observed at a slightly higher frequency than
in the precursor complexes.33 The Ru–H stretch for 2 could not be observed as an isolated signal,
as it falls in the same region of the ν(C≡O) stretch, but it appears as a shoulder (near 1867 cm–1) on
the intense ν(C≡O) band. Complex 4 displayed a band at 1092 cmꢀ1 characteristic of ν(SO)(Sꢀ
bonded DMSO) that substantiates the replacement of one Oꢀbonded DMSO and one Sꢀbonded
DMSO from the precursor cisꢀ[RuCl2(DMSO)4] by the ligand, which is in accord with the earlier
works.34 Moreover, the characteristic absorption bands due to PPh3 were also present in the
expected region. The electronic spectra of the complexes (1ꢀ5) have been recorded in
dichloromethane and they displayed four bands in the region around 240−459 nm. The highꢀ
energy absorption shoulder in the region 240−328 nm have been assigned to ligandꢀcentered
(LC) transitions,35 the shoulder observed in the region 340−392 nm have been attributed to the
ligand to metal charge transfer (LMCT) transitions, and the bands 435−459 nm has been
assigned to a forbidden (d→d) transition.
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1
The H NMR spectra of the complexes show the signals in the expected regions. The
singlets that appeared for the N–NH–C=S proton of the free ligand at 11.53 ppm is absent in the
complexes, supporting the enolization and coordination of the thiolate sulfur to the Ru(II) ion
(Fig. S4−S6, ESI†). The doublet due to azomethine proton (8.85−8.06 ppm) in the complexes is
slightly downfield compared to the free ligand, suggesting deshielding upon coordination to
Ru(II) ion. Further, the spectra of all the complexes showed a series of signals for aromatic
protons at 7.88−6.55 ppm. The DMSO methyl resonances appear as four singlets between 2.84
and 3.45 ppm, a pattern typical for DMSO coordinated to ruthenium in a cis fashion.36 In
addition, a group of peaks appeared around 2.82−0.71 ppm for complexes 1ꢀ5 corresponding to
the terminal ethyl group protons. In the complex 2, the hydride signal is clearly observed as a
triplet due to coupling with the two magnetically equivalent phosphorus nuclei near –6.75 ppm.
37
The 13C NMR spectra show the expected signals in the appropriate regions. For the
uncoordinated ligand, the C=N and C=S signals appear in the regions around 140.25 ppm and
176.62 ppm. Upon coordination and formation of the new Ru complexes, a downfield shift is
observed for the signals of the C=N (around 2 ppm), while the C=S carbon atom signals are
appeared in the upfield region between 171.80 and 169.78 ppm. This is consistent with the P, N,
S coordination and thioenolization of the C=S of thiosemicarbazone moieties. In complexes
(1−5), aromatic carbon atoms of the phenyl group observed around 138.92−125.26 ppm are
comparable to the literature values.29 The C≡O carbon resonating at 206.56−201.12 ppm is
comparable with earlier observations.38 In addition, a couple of signal appeared around
36.27−14.13 ppm for complexes corresponding to the terminal ethyl group carbon.
The presence of a residual PPh3 coordinated to Ru(II) is confirmed by 31P NMR (Fig.
S7−S12, ESI†), where two doublets are observed, respectively at 30.08 ppm (Jpp = 22.3 Hz,
PPh3) and 28.31 ppm (Jpp = 20.4 Hz, PPh2) for the complex 1. These values for coupling
constants suggest a cis disposition between the phosphorus nuclei, as already reported for other
Ru(II) phosphine complexes.39 The singlet observed at 48.23 ppm in complex 2, suggested the
presence of two magnetically equivalent triphenylphosphines trans to each other.40 The 31P NMR
spectrum of 3 shows a single resonance at 38.06 ppm, which is shifted by ca. 10 ppm compared
31
to that of 2. The P resonance signals for 4 and 5 are shifted downfield compared to the free
ligand, which is typical for the coordination of phosphinoꢀthiosemicarbazone ligand.
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Molecular Structures
The structures of complexes 1, 2 and 5 have been established by single crystal Xꢀray analysis.
The details concerning the data collection and structure refinement of the complexes are
summarized in Table 1. The molecular structures of 1, 2 and 5 are displayed in Figures 1ꢀ3. The
differences in the unit cell packing arrangement of the complexes are shown in Figures S1ꢀS3
(ESI†). An orange crystal of the complex 1 with approximate dimensions 0.50 x 0.26 x 0.18 mm
was isolated and the single crystal Xꢀray diffraction experiments were carried out at 296(2) K.
From the unit cell dimensions, it is clear that the crystal is monoclinic belonging to the P21/c
space group. The coordination geometry around the Ru(II) ion is a slightly distorted octahedron,
where the basal plane is constructed of phosphorus atom, the imine nitrogen and the thiolate
sulfur atom of the ligand in its mononegative tridentate PNS fashion, and a triphenylphosphine.
The remaining apical coordination sites are filled up by a chlorine atom and carbonyl group. The
chloride and carbonyl ligand of the complex is disordered in two orientations, only the major
disorder component is shown in Figure 1.
The tridentate PNSꢀEt ligand coordinated equatorially to the metal ion with the formation of
one sixꢀmembered ring and another five membered ring with the bite angles of 90.67(7)° [N(1)
−Ru(1)−P(1)] and 81.48(7)° [N(1)−Ru(1)−S(1)]. This results in a significant distortion of the
{RuP2NS(CO)Cl} core from the ideal octahedral geometry, which is reflected in the twelve cis
and three trans angles. As expected, the PPh3 ligand occupies mutually cis position to PPh2 head
in the thiosemicarbazone chain for better π interaction.41 The large deviation of the
[P(1)−Ru(1)−P(2)] angle [99.33(3)°] from 90 may be ascribed to the steric repulsion between the
two adjacent bulky phosphine moieties. The equatorial bond lengths are 2.141(3) [Ru(1)−N(1)],
2.338(8) [Ru(1)−P(1)], 2.383(8) [Ru(1)−S(1)] and 2.389(8) Å [Ru(1)−P(2)]. For complexes
bearing P, N−iminophosphine ligands, the higher trans influence of the phosphorus atom in
comparison to that of the imine donor functionality leads to longer distances for bonds trans to
the phosphorus.42 On the other hand, in the presence of tridentate P,N,S ligands (with N being an
iminic nitrogen) the reverse situation is usually observed,29 the related bond distances in [(PNSꢀ
Et)RuCl(CO)(PPh3)] (namely, Ru(1)−N(1) = 2.141(3) Å, Ru(1)−P(2) = 2.389(8) Å) follow this
rule. The other two axial sites are occupied by a carbonyl group and one chlorine ligand with
Ru(1)−C(41A) and Ru(1)−Cl(1A) distance of 1.855(9) and 2.424(2) Å. The CO group occupies
the site trans to the Cl [C(41A)− Ru(1)−Cl(1A), 171.5(3)°]. This may be a consequence of
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strong RuII→CO back donation as indicated by the short Ru−C [1.855(9) Å] bond and low CO
stretching frequency (1944 cmꢀ1), which prefers σ or π weak donor groups occupying the site
opposite to CO to favor the d−π back donation.
In complex 2, the ligand [PNSꢀEt] is coordinated to ruthenium as a monoanionic bidentate
N,Sꢀdonor ligand forming a more strained fourꢀmembered chelate ring with a bite angle
N(1)−Ru(1)− S(1) of 64.32(8)° (Fig: 2). The formation of such a fourꢀmembered chelate ring by
the PNSꢀEt and similar ligands is quite normal.43 The PNSꢀEt ligand, ruthenium, carbonyl and
hydride constitute one equatorial plane of the octahedron with the metal at the center and the two
triphenylphosphine ligands take up the remaining two axial positions; hence, they are mutually
trans. The carbonyl is trans to the coordinated nitrogen atom of the thiosemicarbazone and the
hydride is trans to the sulfur atom. In complexes of ruthenium(II) containing the Ru(PPh3)2
fragment, the arrangement of PPh3 ligands ensures optimum π interaction,41 but in the complexes
2 they are mutually trans, probably due to steric reasons as well as the presence of the stronger π
acid CO. The Ru(1)–H, Ru(1)–C(1), Ru(1)−P(1) and Ru(1)−P(2) distances are 1.88(4), 1.839(4),
2.3665(10) and 2.3546(11) Å. The observed bond distances are comparable with those found in
other reported ruthenium complexes containing PPh3.44 Within the Ru(PNSꢀEt) fragment the
Ru–N length is comparable to that found in similar fourꢀmembered chelates,43b,c whereas the Ru–
S distance is a little longer than usually observed. The elongation of the Ru–S bond, which is
trans to the Ru–H bond, may be attributed to the trans effect of the hydride ligand. Comparison
of the bond lengths in the coordinated thiosemicarbazone ligand with those in the uncoordinated
ligand29 shows that upon coordination the C–S bond has undergone elongation, whereas the
adjacent C–N bond has undergone contraction. These changes in bond lengths are consistent
with the iminothiolate form of the thiosemicarbazone ligand that appears to be stabilized upon
coordination to the metal through loss of the hydrazinic proton. The cis angles P(1)−Ru(1)−S(1)
= 88.92(4)°, P(2)−Ru(1)−H = 87.6(12)°, N(1)−Ru(1)−P(1) = 88.64(8)°, N(1)−Ru(1)−P(2) =
88.17(8)° and C(1)−Ru(1)−H = 81.6(13)° are acute, whereas the other cis angles P(1)−Ru(1)−H
90.1(12)°, P(2)−Ru(1)−S(1) = 92.42(4)°, N(1)−Ru(1)−H = 101.4(13)°, C(1)−Ru(1)−S(1) =
112.66(12)°, C(1)−Ru(1)−P(1) = 91.14(12)° and C(1)−Ru(1)−P(2) = 92.21(12)° are obtuse. The
trans angles C(9)−Ru(1)−N(2) = 166.4(2)°, P(2)−Ru−P(1) = 178.48(5)° and S(1)−Ru(1)−Cl(1) =
164.23(5)° deviate from linearity. The variations in bond lengths and angles lead to a significant
distortion from an ideal octahedral geometry for the complex.
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A dark red crystal of the complex 5 with approximate dimensions 0.34 х 0.20 x 0.17 mm was
isolated and the single crystal Xꢀray diffraction experiments were carried out at 295 K. From the
unit cell dimensions, it is clear that the crystal is orthorhombic belonging to the C2221 space
group. The two PNSꢀEt ligands around the metal centre in 5 are arranged in a mer–mer
configuration with the two nitrogen atoms trans to one another and the two phosphorus and two
sulfur atoms in a cis configuration (Fig: 3). The average Ru–P, Ru–N and Ru–S bond lengths in
5 are 2.30, 2.06 and 2.38 Å, respectively, which compare well to those observed in related RuII
complexes.20,41 The average cis angles are S(1)−Ru(1)−S(1) = 85.11(8)°, P(1)−Ru(1)−S(1) =
88.66(5)°, P(1)−Ru(1)−P(1), 97.95(5)°, N(1)−Ru(1)−S(1) = 82.10(9)° and N(1)−Ru(1)−P(1) =
93.34(9)° respectively. The trans angles P(1)−Ru(1)−S(1) = 172.06(5)° and N(1)−Ru(1)−N(1) =
173.40(17)° deviate from linearity. The variations in bond lengths and angles lead to a
significant distortion from an ideal octahedral geometry for the complex.
Checking the Factors Involved in the Coordination Behavior of Phosphinoꢀ
Thiosemicarbazone Ruthenium(II) Complexes
Thiosemicarbazones are versatile ligands and adopt various binding modes with ruthenium
ions.21ꢀ35 Based on the previous works those analyzing the factors involved in the coordination
behavior of ruthenium with thiosemicarbazones, we remarked on some aspects involving in the
formation of these types of compounds. At this time, in the light of the new structures presented
in this work, some of the factors previously established for the versatile coordination behavior of
ruthenium thiosemicarbazone aggregates must be revised. One of these aspects is related with the
deprotonation degree of the thiosemicarbazone ligand, because all of the previous examples
clearly indicated that the thiosemicarbazone ligand units involved in the coordination must be
deprotonated. It is noteworthy that this basic principle is obeyed by the new structures herein
reported (1ꢀ5). In addition the reported examples indicated that the formation of a fiveꢀmembered
ring is an unusual mode of binding and a fourꢀmembered ring is possible. They supported the
formation of a fourꢀmembered ring as in 2. A number of reasons have been offered as
responsible for their versatility in coordination, such as bulky coligands, bulky
diphenylphosphino pendant in thiosemicarbazone and restrict rotation about the NꢀN bond.
These trends have been broken by compounds 1, 3 and 4 as they were formed using PNS donor
atoms of ligand strand. It is concluded that the above mentioned factors are not the only
responsible factors in determining the coordination behavior of thiosemicarbazones and there
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may be some other factors and/ or their collective influence in directing them. There are a
number of complexes which reported trans configuration of merꢀmer arrangement. These trends
have been broken by complex 5, and in which cis configuration was observed. Another new
factor to highlight is that the substitution of ethyl group in the thioamide nitrogen atom of the
thiosemicarbazone does not modify the structures formed, but it notably affects the
microarchitecture of the crystal structures. Thus, the structures reported herein constitute unique
examples of diphenylphosphinoꢀthiosemicarbazone ruthenium(II) complexes, as they exhibit
unprecedented arrangements.
Catalysis
Optimization of reaction conditions
Transitionꢀmetal catalyzed Nꢀalkylation using alcohol as an alkyl source has become an efficient
method in organic synthesis as illustrated by several useful applications reported in recent
years.45 The reaction conditions for this important process are relatively mild and environment
friendly. The ruthenium(II) complexes 1–5 catalyzed the alkylation of heteroaromatic amines to
the corresponding Nꢀalkylated products via hydrogen autotransfer processes with KOH as the
promoter. At the start of our studies, we investigated the Nꢀalkylation of 2ꢀaminobenzothiazole
with 4ꢀmethoxybenzyl alcohol in the presence of various bases, and the results are summarized
in Table 2. In the absence of base, Nꢀalkylation of amine was not observed. The results show the
screening of bases as initiators of the catalytic reaction. Weak bases such as Na2CO3, K2CO3 and
Cs2CO3 were not effective (Table 2, entries 1−3). However, the use of strong bases such as
NaOH, NaO(tꢀBu), KO(tꢀBu) and KOH lead to high yields of the desired product (Table 2,
entries 4−11). When the reaction was carried out in the presence of KOH, benzothiozolꢀ2ꢀylꢀ
benzylamine was formed in an excellent yield (up to 96%), which we considered to be the choice
of the base (Table 2, entry 6). In addition, to obtain almost quantitative yields and avoid the
presence of the imine as a secondary product, from lack of hydrogenation of the condensation
product, at least 50 mol % of base is needed. The reaction conditions were further optimized
through different solvents, the results are given in Table 3. Toluene was found as the best solvent
for the Nꢀalkylation reaction (Table 3, entry 4). No reaction occurred in the case of water or THF
as solvent (Table 3, entries 5, 7), while, 1,4ꢀdioxane, DMF, oꢀxylene, benzene and ethanol
resulted in much lower yields (Table 3, entries 1−3, 6 and 8).
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We continued the Nꢀalkylation reaction optimization process after finding the need for a
strong base to activate the ruthenium complex 1. The following step was done to study the
influence of the catalyst loadings on the catalytic activity. The results are summarized in Table 4.
Catalyst screening in the model reaction revealed that all the ruthenium(II) complexes triggered
the reaction, except 5. Owing to the bischelation of the PNSꢀEt ligands around the metal center
in 5, the lack of a vacant coordination site hinders its catalytic activity. The results also indicate
that lower catalyst loadings lead to moderate yields, higher catalyst loadings led to higher yields
and higher amine content in the product distribution (Table 4, entries 1−4). Furthermore,
considering the results when 0.5 mol % of catalyst was used, it is clear that ruthenium complexes
containing hydride species (Table 4, entry 3) lead to higher yields than those containing chlorine
as coꢀligand (Table 4, entries 1, 2, 4 and 6).
The optimization process led us toward the determination of the best reaction conditions to
evaluate the substrate scope. Catalysts 1ꢀ3 proved to be the most efficient complexes for the Nꢀ
alkylation of 2ꢀaminobenzothiazole in terms of yield and selectivity and 0.5 mol % catalyst
loading was chosen, given the high yields and shorter reaction times needed to complete the
process.
Substrate Scope and Limitation
To expand the scope of the present homogeneous catalyst system, the Nꢀalkylation reaction has
been extended to the copious benzyl alcohol consisting of diverse functional groups. Table 5
summarizes the catalytic activity of 1ꢀ3 under the toluene/KOH recipe for this coupling reaction.
When an equimolar solutions of pꢀmethoxybenzyl alcohol and 2ꢀaminobenzothiazole with 0.5
mol % of 1ꢀ3, the reactions went smoothly to afford the products in 91−98% isolated yield
(Table 5, entry 1). The reaction of 2ꢀaminobenzothiazole was carried out with benzyl alcohol to
obtain benzothiozolꢀ2ꢀylꢀbenzylamine in 81−97% yields (Table 5, entries 2). The formation of
1
13
benzothiozolꢀ2ꢀylꢀbenzylamine was confirmed by H and CꢀNMR spectra, which are given in
Figure 4 and 5 as a representative alkylated product. Assignment of signals was further
confirmed by DEPTꢀ135 and HSQCꢀNMR studies (Figure 6 and 7). Benzyl alcohol bearing
methyl substituent at paraꢀposition, still gave coupling products in good yields 76−94% (Table
5, entries 3). The Nꢀalkylation with benzyl alcohols bearing a halogen atom at para position (Cl
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or Br) proceeded to give the corresponding products with excellent yields 89−99% (Table 5,
entries 4 and 5).
Further, we have extended the present transformation to other heteroaromatic amines such as
2ꢀaminopyrimidine and 2ꢀaminopyridine. These heterocyclic amines also afford good to
excellent yields of Nꢀalkylated products with alcohols under the optimized conditions.
Unsubstituted benzyl alcohol with 2ꢀaminopyrimidine provided the desired products in excellent
yield (Table 5, entry 6). Nꢀalkylation of benzyl alcohols bearing electronꢀdonating substituents,
such as methyl and methoxy, gave the desired products in 84−92% yields (Table 5, entries 7 and
8). Benzyl alcohols bearing a halogen atom, such as chloro and bromo, were proven to be
suitable substrates and reactions gave the desired products in 87−97% yields, respectively (Table
5, entries 9 and 10). Similar to the case of 2ꢀaminopyrimidine, the Nꢀalkylation of 2ꢀ
aminopyridine with benzyl alcohol and pꢀMe, pꢀOMe, pꢀCl, pꢀBr substituted benzyl alcohols,
gave the corresponding products (Table 5, entries 11−15) with 74−96 % yields, respectively. The
present protocol also performed for Nꢀalkylation of aromatic amines such as aminobenzene, 4ꢀ
methyl, 4ꢀchloro and 4ꢀbromoaniline with ferrocenemethanol in high yield 59−91 % (Table 5,
entries 15−18).
One of the outstanding properties of the present catalyst is its high selectivity for
monoalkylation of heteroaromatic amines. Hence, it was of interest to determine whether this
selectivity for the monoalkylation of primary aromatic functions could be used for the N,Nꢀ
dialkylation of diamines. First 2,6ꢀdiaminopyridine was reacted with 4ꢀmethoxy benzyl alcohol,
which afforded N,N’ꢀdialkylated product with 70−90 % (Table 5, entry 19). The N,N’ꢀ
dialkylation with benzyl alcohols bearing a halogen atom (Cl or Br) proceeded to give the
corresponding products with excellent yields 70−94 % (Table 5, entries 20 and 21). We also
tested the alkylation of aliphatic alcohol such as isoamyl alcohol, nꢀheptanol and cinnamyl
alcohol under the optimized reaction conditions. However, these substrates afforded the products
in trace amounts. Neither the use of amines that cannot be oxidized to imines gave the desired
product (see Supporting Information, Table S1).
The catalytic amine alkylation with ruthenium(II) complexes follows the ‘borrowed
hydrogen’ pathway, extensively studied by Williams, Fujita, Yamaguchi and us.9ꢀ20 The alcohol
is catalytically dehydrogenated to the corresponding aldehyde (inꢀsitu oxidation) in the first step.
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Then, the aldehyde condenses with the amine to give an intermediate imine, which is
subsequently hydrogenated (reduction) by the catalyst. Thus, there is no net H2 consumption in
this process, however, the reaction benefits from being run in a closed system preventing
irreversible H2 loss.
N1, C5ꢀDialkylation
Having developed a synthetic method for Nꢀalkylation of heterocyclic amines, we explored the
current concise transformation for dialkylation of 4ꢀphenylthiazolꢀ2ꢀamine (Scheme 4). The
earlier optimized conditions were applied to 4ꢀphenylthiazolꢀ2ꢀamine with 4ꢀmethoxy benzyl
alcohol as alkylating agent, and a mixture of diꢀ and monoalkylated products A and B was
obtained. Under these conditions, the formation of dialkylated product A was quite interesting,
and no such reports exist in the ruthenium catalysis. Hence, we became interested in
investigating the conditions for the selective formation of a single dialkylated product A. The
identified conditions for this transformation were 4 equiv of 4ꢀmethoxy benzyl alcohol, 2 equiv
of 4ꢀphenylthiazolꢀ2ꢀamine at 120 °C in toluene, and 24 h of reaction time. These revised
optimum conditions were applied to explore the synthesis of N1,C5ꢀdialkylꢀ4ꢀphenylthiazolꢀ2ꢀ
amines (scheme 5).
The reaction of 4ꢀphenylthiazolꢀ2ꢀamine (1 mmol) with benzyl alcohol (4 mmol) was carried
out for 24 h, affording the corresponding N1,C5 ꢀdialkylated products with 88% yield. It should
be noted that apart from the desired N1, C5ꢀdialkylated products, no isomer (the Nꢀendo
substituted products) and overꢀalkylated products were observed in all cases. In addition, 1 mol%
catalyst was required in all the cases to obtain the products with high yields. These revised
optimum conditions were applied to benzyl alcohol bearing pꢀOMe, pꢀCl group, yields the
desired products (82% and 91%). Similarly, reactions of 2ꢀaminoꢀ4ꢀ(4ꢀchlorophenyl)thiazole
with benzyl alcohol afforded the desired product in 93% yield (Scheme 5). The dialkylation of
benzyl alcohol bearing an electronꢀdonating methoxy substituent gave the corresponding product
89% yield. The benzyl alcohol bearing an electronꢀwithdrawing substituent (−Cl) was also
converted into the corresponding product in 96% yield.
The formation of dialkylated product may be due to the electronꢀrich nature of fiveꢀmembered
thiazole compared to six membered pyrimidine and pyridine systems. The substitution of phenyl
ring at C4 may further increase the electron density on C5 of 4ꢀphenylthiazolꢀ2ꢀamine. To
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confirm this, few experiments were carried out. In the absence of amine, oxidation of 4ꢀ
methoxybenzyl alcohol to 4ꢀmethoxy benzaldehyde was observed under present reaction
conditions confirming the dehydrogenation step (Scheme 6a). When the reaction was carried out
o
at low temperature (70 C) or stopped inꢀbetween, imine was observed as major product which
confirms the condensation step. We performed the reaction starting from imine (Scheme 6b),
under the same reaction conditions, and 76% of dialkylated product was isolated after workꢀup.
Based on the above experimental evidence and previous known hydrogen autotransfer
methodologies,9ꢀ20 we propose a possible mechanism for N1,C5ꢀDialkylation of 4ꢀphenylthiazolꢀ
2ꢀamine (Scheme 7). Accompanied by the catalytic cycle of ruthenium, the alcohol is first
partially dehydrogenated to form the aldehyde I with the generation of ruthenium hydride
species in the presence of bases, followed by the condensation of the resulting aldehyde with 4ꢀ
phenylthiazolꢀ2ꢀamine II to afford an imine intermediate III, which is hydrogenated
simultaneously to afford the Nꢀalkylated product and the ruthenium hydride species are also
consumed. In the presence of ruthenium catalyst, the dehydrogenation of alcohol with
simultaneous hydrogenation of imine leads to aldehyde IA and Nꢀalkylated product IV,
respectively. Nucleophilic addition of Nꢀalkylated product IV to aldehyde may form imino
alcohol V, and its dehydration gives another intermediate VI. Subsequent hydrogenation of VI
provides N1,C5ꢀdialkylation product VII and the ruthenium hydride species are also consumed to
complete the catalytic cycle.
NꢀAlkylation of sulfonamides
Nꢀalkylated sulfonamides constitute an important class of compounds because the sulfonamide
moiety is found in a large number of agrochemicals and pharmaceuticals.46 The most commonly
used methods for the synthesis of sulfonamides include the condensation of amines with sulfonyl
chlorides,47 reductive amination of sulfonamides with alkyl halides48 and the reaction of
activated sulfonate esters with amines.49 Most of these methods are associated with the
limitations such as poor selectivity, side reactions, tedious workꢀup procedures and the
generation of unwanted inorganic salts. Alternatively, few reports are also available on the
synthesis of Nꢀalkylated sulfonamides directly from alcohols. Recently, Ru/diphosphine,50
51
Ru/Fe3O4 and Cu52 have been applied for the synthesis of Nꢀalkylated sulfonamides directly
from benzyl alcohols.
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Inspired by the above interesting result, we envisioned that ruthenium based catalysts may be
also suitable for the synthesis of amides through the dehydrogenationꢀcondensationꢀ
hydrogenation sequence (so called borrowing hydrogen methodology). Although, the reaction
conditions were similar as for the Nꢀalkylation of heterocyclic amines, however, reaction took at
slightly higher temperature. In order to explore the scope of the present method, the reactions of
copious sulfonamides were successfully performed with a variety of alcohols. As shown in Table
6, substituted groups on the aromatic ring had no negative influence on the alkylation reactions
and the corresponding secondary sulfonamides were produced in 82–99 % isolated yields (Table
6, entries 1–5). By comparing the Nꢀalkylation reactions of pꢀtoluenesulfonamide, pꢀ
chlorobenzenesulfonamide and benzenesulfonamide, it is clear that electronꢀrich as well as
electron poor groups were well tolerated. In addition, the coupling reaction of pꢀ
toluenesulfonamide with cyclohexyl alcohol gave the corresponding product in moderate yield
(62–74 %, Table 6, entry 6), however, low yield (21–54 %) were obtained by the reactions of
methanesulfonamide with benzyl alcohol (Table 6, entry 7).
Conclusion
We have reported the synthesis and characterization of a series of ruthenium complexes bearing
2ꢀ(2ꢀ(diphenylphosphino)benzylidene)ꢀNꢀethylthiosemicarbazone (PNSꢀEt) ligands. Typically,
[(PNSꢀEt)RuCl(CO)(PPh3)]
(1),
[N,Sꢀ(PNSꢀEt)RuH(CO)(PPh3)2]
(2)
and
[(PNSꢀ
Et)RuCl(PPh3)2] (3) were synthesized by reactions of various RuII precursors with deprotonated
PNSꢀEt. Complexation of PNSꢀEt with an equimolar amount of [RuCl2(dmso)4] resulted two
different entities [(PNSꢀEt)RuCl(dmso)2] (4) and [(PNSꢀEt)2Ru] (5) with different structural
features in a single reaction. The structures of these complexes were unambiguously determined
by spectroscopic methods and Xꢀray single crystal analysis (1, 2 and 5). It is interesting to note
that one of the complexes obtained (2) has thiosemicarbazone coordinated as NS through the
hydrazinic nitrogen and thiolate sulfur by forming fourꢀmembered ring, whereas in other
complexes, the same ligand coordinated as PNS monobasic tridentate donor. The catalytic study
of complexes 1−5 towards amine Nꢀalkylation reactions was completed, showing that all
catalysts are active toward catalytic transformations. In the Nꢀalkylation process, the complexes
1−3 have been proven to be efficient catalysts under mild conditions in comparison to its
analogues and other ruthenium and iridium complexes.53 Also, 1−3 have shown high tolerance to
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functional groups in the hetero (aromatic) amine and benzyl alcohol moieties, however, some
limitations for aliphatic alcohols. Furthermore, selective N,N’ꢀdialkylation occurred when hetero
aromatic diamines were subjected to the reaction conditions. Reaction of 4ꢀphenylthiazolꢀ2ꢀ
amines with benzyl alcohols gave N1,C5ꢀdibenzylꢀ4ꢀphenylthiazolꢀ2ꢀamines in good yields in the
presence of KOH. On the other hand, 1−3 have been proven to be very efficient catalysts in the
Nꢀalkylation of a wide variety of sulfonamides and alcohols. From synthetic point of view, the
protocol is highly attractive because of easily available starting materials, high atom efficiency,
broad substrate scope, water as only byproduct and environmental friendliness. Further studies to
explore new potential of alcohols as electrophiles are under way.
Experimental section
General Procedures
Unless otherwise noted, all reactions were performed under an atmosphere of air. Thinꢀlayer
chromatography (TLC) was carried out on Merck 1.055 aluminum sheets precoated with silica
gel 60 F254 and the spots were visualized with UV light at 254 nm or under iodine. Column
chromatography purifications were performed by Merck silica gel 60 (0.063−0.200 mm). The
1H, 13C, and 31PꢀNMR spectra were measured on a Bruker AV400 instrument with chemical
shifts relative to tetramethylsilane (1H, 13C) and oꢀphosphoric acid (31P) at 400, 100, and 162
MHz, respectively. The C, H, N and S analyses were carried out with a Vario EL III Elemental
analyzer. Infrared spectra of the ligands and the metal complexes were recorded as KBr discs in
the range of 4000−400 cm−1 using a Nicolet Avatar model FTꢀIR spectrophotometer. The
electronic spectra of the complexes were performed on a Shimadzu UV−1650 PC
spectrophotometer using dichloromethane as the solvent. Mass spectra were measured on a
LC−MS Q−ToF Micro Analyzer (Shimadzu), using electrospray ionization (ESI) mode. The
melting points were checked with a Lab India melting point apparatus. All solvents were dried
and distilled before use by standard procedures. The common reagents and chemicals available
commercially within India were used. Ruthenium(III) trichloride hydrate, triphenylphosphine
and 4ꢀethylꢀ3ꢀthiosemicarbazide were procured from SigmaꢀAldrich and used as received. The
reported methods were used for the synthesis of 2ꢀ(diphenylphosphino)benzaldehyde,31
[RuHCl(CO)(PPh3)3],54 [RuH2(CO)(PPh3)3],55 [RuCl2(PPh3)3]56 and [RuCl2(dmso)4].57
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2ꢀ(2ꢀ(diphenylphosphino)benzylidene)ꢀNꢀethylthiosemicarbazone (PNSꢀEt):
To a solution of 2ꢀ(diphenylphosphino)benzaldehyde (1 g, 3.4 mmol) and 4ꢀethylꢀ3ꢀ
thiosemicarbazide (0.42 g, 3.4 mmol) in ethanol (30 mL) were added 2−3 drops of glacial acetic
acid. The pale yellow solution was heated under reflux over a 2 h period, and then concentrated
to dryness. The resulting yellow oil was treated with diethyl ether (2 х 10 mL). The pale yellow
precipitate was filtered off, washed with diethyl ether (10 mL) and dried under vacuo. Yield:
96% (1.62 g). Mp: 198 °C. Anal. Calcd for C22H22N3PS: C, 67.50; H, 5.66; N, 10.73; S, 8.19%.
Found: C, 67.72; H, 5.62; N, 10.74; S, 8.26%. IR (KBr disks, cm−1): 3326, 3120 (m, νNH); 1584
+ 1478 (s, νC=N + νC−N); 744 (s, νC=S). UVꢀVis (CH2Cl2), λmax (nm): 297, 332, 353. 1H NMR (400
MHz, DMSOꢀd6): δ 1.07 (t, 3H, J = 7.2 Hz, −CH3), 3.46−3.53 (m, 2H, −CH2), 6.75−6.78 (m, 1H,
Ar H), 7.16−7.21 (m, 4H, Ar H), 7.31 (t, 1H, J = 7.2 Hz, Ar H), 7.39−7.45 (m, 6H, Ar H),
7.51−7.72 (m, 2H, Ar H), 8.14 (q, 1H, −NHterminal), 8.64 (d, IH, J = 4.8 Hz, −CH=N ), 11.53 (s,
13
IH, −NHhydrazinic). C NMR (100 MHz, DMSOꢀd6): δ 14.47 (−CH3), 38.14 (−CH2), 127.24 (Ar
C), 127.28 (Ar C), 128.64 (Ar C), 128.79 (Ar C), 128.86 (Ar C), 128.99 (Ar C), 129.09 (Ar C),
129.59 (Ar C), 131.25 (Ar C), 131.35 (Ar C), 131.75 (Ar C), 131.92 (Ar C), 133.03 (Ar C),
133.35 (Ar C), 133.55 (Ar C), 133.59 (Ar C), 133.86 (Ar C), 135.65 (Ar C), 135.83 (Ar C),
135.94 (Ar C), 137.75 (Ar C), 140.25 (−CH=N), 176.62 (C=S). 31P NMR (162 MHz, DMSOꢀd6):
δ −10.81. ESIꢀMS: m/z = 391.01 [M+H]+.
[(PNSꢀEt)RuCl(CO)(PPh3)] (1):
A suspension of [RuHCl(CO)(PPh3)3] (0.5 g, 0.52 mmol) in ethanol (40 mL) was treated with
PNSꢀEt (0.2 g, 0.52 mmol) and the mixture was gently refluxed for 8h. During this time the
color changed to orange. The solvent was reduced to half of the volume on a rotary evaporator,
and the suspension was filtered, washed thoroughly with cold ethanol (10 mL) and diethyl ether
(2 х 20 mL). The product was finally dried under vacuum, affording an orange solid in 79%
(0.33 g) yield. Crystals suitable for Xꢀray diffraction were obtained by slow evaporation of a
concentrated solution of 1 in CH2Cl2/C2H5OH. Mp: 258 °C. Anal. Calcd for C41H36ClN3OP2RuS:
C, 60.25; H, 4.44; N, 5.14; S, 3.92%. Found: C, 60.49; H, 4.14; N, 5.36; S, 4.02%. IR (KBr
disks, cm−1): 3412, 3050 (m, νNH); 1944 (s, νC≡≡O); 1582 + 1480 (s, νC=N + νC−N); 747 (s, νC−S);
1
1432, 1091, 695 (s, for PPh3). UVꢀVis (CH2Cl2), λmax (nm): 258, 283, 358, 440. H NMR (400
MHz, DMSOꢀd6): δ 1.12 (t, 3H, J = 7 Hz, −CH3), 2.46−2.53 (m, 2H, −CH2), 6.58 (td, 2H, J = 7,
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1.6 Hz, Ar H), 6.70 (t, 1H, J = 8.8 Hz, Ar H), 6.89 (t, 2H, J = 7.2 Hz, Ar H), 7.12 (td, 2H, J =
6.0, 2.0 Hz, Ar H), 7.20−7.28 (m, 3H, Ar H), 7.35−7.42 (m, 3H, Ar H), 7.44 (td, 3H, J = 8.0, 1.6
Hz, Ar H), 7.49−7.63 (m, 8H, Ar H), 7.74 (d, 4H, J = 8.0 Hz, Ar H), 7.85−7.88 (m, 1H, Ar H),
13
8.42 (s, 1H, −NHterminal), 8.85 (d, IH, J = 4.8 Hz, −CH=N ). C NMR (100 MHz, DMSOꢀd6): δ
14.24 (−CH3), 24.5 (−CH2), 127.56 (Ar C), 127.65 (Ar C), 128.04 (Ar C), 128.64 (Ar C), 128.69
(Ar C), 128.76 (Ar C), 128.94 (Ar C), 129.39 (Ar C), 129.80 (Ar C), 129.98 (Ar C), 131.38 (Ar
C), 131.48 (Ar C), 131.96 (Ar C), 131.99 (Ar C), 132.18 (Ar C), 133.10 (Ar C), 133.29 (Ar C),
133.47 (Ar C), 133.56 (Ar C), 133.82 (Ar C), 139.96 (−CH=N), 170.14 (C=S), 206.56 (C≡O).
31P NMR (162 MHz, DMSOꢀd6): δ 30.08 (d, J = 22.3 Hz, PPh3), 28.31 (d, J = 20.4 Hz, PPh2).
ESIꢀMS: m/z = 781.57 [M–Cl]+.
[N,S-(PNSꢀEt)RuH(CO)(PPh3)2] (2)
A suspension of [RuH2(CO)(PPh3)3] (0.2 g, 0.22 mmol) in CH2Cl2 (20 mL) was treated with
PNSꢀEt (0.086 g, 0.22 mmol) in ethanol (20 mL) and the mixture was gently refluxed for 12 h.
During this time the color changed to green. The solvent was reduced to half of the volume on a
rotary evaporator, and the suspension was filtered, washed thoroughly with cold ethanol (10 mL)
and diethyl ether (2 х 20 mL). The product was finally dried under vacuum, affording a green
solid in 75 % (0.13 g) yield. Crystals suitable for Xꢀray diffraction were obtained by slow
evaporation of a concentrated solution of 2 in CH2Cl2/C2H5OH. Mp: 234 °C. Anal. Calcd for
C59H52N3OP3RuS: C, 67.80; H, 5.02; N, 4.02; S, 3.07%. Found: C, 67.95; H, 5.12; N, 4.14; S,
3.16%. IR (KBr disks, cm−1): 3414, 3051 (m, νNH); 1946 (s, νC≡≡O); 1867 (w, νRuꢀH) 1583 + 1498
(s, νC=N + νC−N); 741 (s, νC−S); 1432, 1074, 693 (s, for PPh3). UVꢀVis (CH2Cl2), λmax (nm): 261,
1
293, 392, 435. H NMR (400 MHz, CDCl3): δ −6.75 (t, 1H, J = 19.1 Hz, RuH), 0.71 (t, 3H, J =
7.2 Hz, −CH3), 2.70−2.77 (m, 2H, −CH2), 6.78 (t, 1H, J = 4 Hz, Ar H), 6.99−7.05 (m, 3H, Ar H),
7.08 (td, 2H, J = 5.9, 1.4 Hz, Ar H), 7.14 (td, 2H, J = 5.0 Hz, Ar H), 7.18−7.39 (m, 21H, Ar H),
7.44 (t, 2H, J = 7.5 Hz, Ar H), 7.58−7.62 (m, 13H, Ar H), 7.90 (s, 1H, −NHterminal), 8.06 (d, IH, J
13
= 3.6 Hz, −CH=N ). NMR (100 MHz, CDCl3): δ 14.39 (−CH3), 36.27 (−CH2), 127.32 (Ar C),
127.98 (Ar C), 128.40 (Ar C), 128.45 (Ar C), 128.57 (Ar C), 128.91 (Ar C), 129.34 (Ar C),
129.85 (Ar C), 130.64 (Ar C), 131.41 (Ar C), 131.93 (Ar C), 131.95 (Ar C), 132.01 (Ar C),
132.10 (Ar C), 132.15 (Ar C), 132.63 (Ar C), 133.62 (Ar C), 133.82 (Ar C), 134.56 (Ar C),
135.03 (Ar C), 135.18 (Ar C), 135.39 (Ar C), 135.59 (Ar C), 138.29 (Ar C),138.43 (Ar C),
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31
138.92 (Ar C), 140.29 (−CH=N), 170.21 (C=S), 201.12 (C≡O). P NMR (162 MHz, CDCl3): δ
30.67 (PPh2), 48.23 (s, PPh3). ESIꢀMS: m/z = 1046.21 [M+H]+.
[(PNSꢀEt)RuCl(PPh3)] (3)
A suspension of [RuCl2(PPh3)3] (0.2 g, 0.207 mmol) in CH2Cl2 (40 mL) was treated with PNSꢀ
Et (0.081 g, 0.207 mmol) in ethanol and the mixture was gently refluxed for 8 h. During this
time the color changed to red. The solvent was reduced to half of the volume on a rotary
evaporator, and the suspension was filtered, washed thoroughly with cold ethanol (10 mL) and
diethyl ether (2 х 20 mL). The product was finally dried under vacuum, affording a red solid in
82 % (0.18 g) yield. Mp: 245 °C. Anal. Calcd for C40H36ClN3P2RuS: C, 60.87; H, 4.60; N, 5.32;
S, 4.06%. Found: C, 60.61; H, 4.16; N, 5.14; S, 4.26%. IR (KBr disks, cm−1): 3401, 3051 (m,
νNH); 1556 + 1522, 1481 (s, νC=N + νC−N); 746 (s, νC−S); 1433, 1089, 694 (s, for PPh3). UVꢀVis
1
(CH2Cl2), λmax (nm): 247, 340, 459. H NMR (400 MHz, DMSOꢀd6): δ 1.12 (t, 3H, J = 7 Hz,
−CH3), 2.72−2.82 (m, 2H, −CH2), 6.70 (t, 1H, J = 8.8 Hz, Ar H), 7.04 (t, 1H, J = 8.1 Hz, Ar H),
7.09 (t, 2H, J = 8.9 Hz, Ar H), 7.15 (t, 4H, J = 7.2 Hz, Ar H), 7.19−7.25 (m, 5H, Ar H), 7.33 (td,
2H, J = 6.0, 1.7 Hz, Ar H), 7.38−7.41 (m, 1H, Ar H), 7.47 (td, 3H, J = 8.3, 1.9 Hz, Ar H),
7.73−7.52 (m, 8H, Ar H), 7.80−7.88 (m, 4H, Ar H), 8.11 (s, 1H, −NHterminal), 8.52 (d, IH, J = 8.1
13
Hz, −CH=N ). C NMR (100 MHz, DMSOꢀd6): δ 14.14 (−CH3), 36.12 (−CH2), 127.41 (Ar C),
128.31 (Ar C), 128.66 (Ar C), 128.70 (Ar C), 128.77 (Ar C), 128.96 (Ar C), 131.39 (Ar C),
131.49 (Ar C), 131.99 (Ar C), 132.02 (Ar C), 133.10 (Ar C), 133.30 (Ar C), 133.74 (Ar C),
31
133.82 (Ar C), 136.42 (Ar C), 136.64 (Ar C), 140.04 (−CH=N), 169.78 (CꢀS). P NMR (162
MHz, DMSOꢀd6): δ 26.23 (s, PPh2), 38.06 (s, PPh3). ESIꢀMS: m/z = 753.82 [M–Cl]+.
[(PNSꢀEt)RuCl(dmso)2] (4) and [(PNSꢀEt)2Ru] (5)
A suspension of [RuCl2(dmso)4] (0.2 g, 0.44 mmol) in CH2Cl2 (40 mL) was treated with PNSꢀEt
(0.172 g, 0.44 mmol) in ethanol and the mixture was gently refluxed for 12 h. During this time
the color changed to dark red. The solvent was removed on a rotary evaporator. The residue was
then chromatographed on alumina oxide with ethyl acetate/ diethyl ether. A light brown band
was eluted with 2:8 ethyl acetate/diethyl ether mixtures and it was identified as 4 (yield 0.132 g,
43%). A brownish yellow band was eluted with 6:4 ethyl acetate/diethyl ether mixture and it was
identified as 5 (yield 0.105 g, 27%). Single crystals of 5 were obtained by slow diffusion of
diethyl ether into a saturated solution of the complex in ethyl acetate. Complex 4: Mp: 273 °C.
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Anal. Calcd for C26H33ClN3OP2RuS3: C, 45.70; H, 4.87; N, 6.15; S, 14.08%. Found: C, 45.92; H,
4.78; N, 6.05; S, 14.26%. IR (KBr disks, cm−1): 3430, 320 (m, νNH); 1572 + 1482 (s, νC=N
+
νC−N); 749 (s, νC−S); 1092 (s, for Sꢀbonded dmso). UVꢀVis (CH2Cl2), λmax (nm): 254, 309, 356,
427. 1H NMR (400 MHz, DMSOꢀd6): δ 1.10 (t, 3H, J = 7 Hz, −CH3), 2.46−2.74 (m, 2H, −CH2),
2.84−3.45 (s, 12H, dmso), 6.55 (t, 2H, J = 8.0 Hz, Ar H), 6.55 (t, 2H, J = 8.0 Hz, Ar H), 6.72 (t,
1H, J = 7.6 Hz, Ar H), 6.89−6.86 (m, 1H, Ar H), 7.63−7.10 (m, 8H, Ar H), 7.91 (s, 1H,
NHterminal), 8.41 (d, 1H, J = 8.1 Hz, −CH=N). 13C NMR (100 MHz, DMSOꢀd6): δ 14.18 (−CH3),
24.21 (dmso), 34.12 (−CH2), 125.26 (Ar C), 125.55 (Ar C), 127.44 (Ar C), 128.01 (Ar C),
128.78 (Ar C), 128.85 (Ar C), 129.01 (Ar C), 129.12 (Ar C), 129.87 (Ar C), 132.94 (Ar C),
133.36 (Ar C), 133.56 (Ar C), 135.60 (Ar C), 135.70 (Ar C), 136.05 (Ar C), 136.24 (Ar C),
137.24 (Ar C), 137.43 (Ar C), 138.82 (Ar C), 138.91 (Ar C), 140.13 (−CH=N), 170.12 (C−S).
31P NMR (162 MHz, DMSOꢀd6): δ 28.78 (s, PPh2). ESIꢀMS: m/z = 662.57 [M–Cl]+. Complex 5:
Mp: 285 °C. Anal. Calcd for C44H42N6P2RuS2: C, 59.92; H, 4.80; N, 9.53; S, 7.27%. Found: C,
60.25; H, 4.56; N, 9.24; S, 7.38%. IR (KBr disks, cm−1): 3432, 3208 (m, νNH); 1579 + 1481 (s,
1
νC=N + νC−N); 749 (s, νC−S). UVꢀVis (CH2Cl2), λmax (nm): 240, 328, 435. H NMR (400 MHz,
DMSOꢀd6): δ 1.05 (t, 6H, J = 7 Hz, −CH3), 2.42−2.63 (m, 4H, −CH2), 6.57 (m, 4H, J = 7.2 Hz,
Ar H), 6.70 (t, 2H, J = 8.8 Hz, Ar H), 6.89 (t, 6H, J = 7.2 Hz, Ar H), 7.75−7.02 (m, 14H, Ar H),
13
7.86 (t, 2H, J = 7.2 Hz, Ar H), 8.11 (s, 2H, −NHterminal), 8.46 (d, 2H, J = 8.3 Hz, −CH=N). C
NMR (100 MHz, DMSOꢀd6): δ 14.13 (−CH3), 30.60 (−CH2), 128.00 (Ar C), 128.10 (Ar C),
128.41 (Ar C), 128.64 (Ar C), 129.06 (Ar C), 129.89 (Ar C), 130.32 (Ar C), 130.48 (Ar C),
131.03 (Ar C), 131.29 (Ar C), 131.38 (Ar C), 131.68 (Ar C), 131.79 (Ar C), 132.80 (Ar C),
132.91 (Ar C), 133.12 (Ar C), 133.54 (Ar C), 133.79 (Ar C), 133.90 (Ar C), 134.00 (Ar C),
138.91 (C=N), 171.80 (C−S). 31P NMR (162 MHz, DMSOꢀd6): δ 29.12 (s, PPh2). ESIꢀMS: m/z =
881.84 [M]+.
Catalysis
Typical procedure for Nꢀalkylation of (hetero)aromatic amines with alcohols
In a 25 mL round bottomed flask were placed 0.5 mol % of ruthenium(II) catalyst, 2 mmol of
alcohol, 2 mmol of amine, 50 mol % of KOH and 2 mL of toluene. The reaction flask was heated
at 100 °C for 12 h in an oil bath. Upon completion (as monitored by TLC), the reaction mixture
was cooled at ambient temperature, H2O (3 mL) was added and the organic layer was extracted
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with CH2Cl2 (3 х 10 mL). The combined organic layers were dried with magnesium sulfate and
concentrated. The crude product was purified by column chromatography (ethyl acetate/hexane).
Reported isolated yields are an average of two runs.
1
Representative spectral data for NꢀBenzylbenzo[d]thiazolꢀ2ꢀamine: H NMR (400MHz,
CDCl3): δ = 4.59 (s, 2H, ꢀCH2ꢀ), 7.07 (t, J = 7.5 Hz, 1H, ArH), 7.21−7.28 (m, 2H, ArH),
13
7.33−7.40 (m, 5H, ArH), 7.89 (d, J = 7.9, 1H, ArH), 8.51 (t, J = 5.6 Hz, 1H, ꢀNH). C NMR
(100 MHz, CDCl3): δ = 49.49, 118.97, 120.81, 121.63, 126.26, 128.96, 127.95, 127.72, 137.71,
152.81, 167.79. Assignment of signals was further confirmed by DEPTꢀ135 and HSQCꢀNMR
studies.
Typical procedure for N1, C5-Dialkylation of amines with alcohols
In a 25 mL round bottomed flask were placed 1 mol % of ruthenium(II) catalyst, 4 mmol of
benzyl alcohol, 2 mmol of 4ꢀphenylthiazolꢀ2ꢀamine, 50 mol % of KOH and 2.0 mL of toluene.
The reaction flask was heated at 120 °C for 24 h in an oil bath. Upon completion (as monitored
by TLC), the reaction mixture was cooled at ambient temperature, H2O (3 mL) was added and
the organic layer was extracted with CH2Cl2 (3 х 10 mL). The combined organic layers were
dried with magnesium sulfate and concentrated. The crude product was purified by column
chromatography (ethyl acetate/dichloromethane). Reported isolated yields are an average of two
runs.
Typical procedure for N-alkylation of sulfonamides with alcohols
In a 25 mL round bottomed flask were placed 0.5 mol % of ruthenium(II) catalyst, 2 mmol of
benzyl alcohol, 2 mmol of sulfonamide, 50 mol % of KOH and 2 mL of toluene. The reaction
flask was heated at 120 °C for 12 h in an oil bath. Upon completion (as monitored by TLC), the
reaction mixture was cooled at ambient temperature, H2O (3 mL) was added and the organic
layer was extracted with CH2Cl2 (3 х 10 mL). The combined organic layers were dried with
magnesium sulfate and concentrated. The crude product was purified by column chromatography
(ethyl acetate/hexane). Reported isolated yields are an average of two runs.
The catalytic reactions given in Tables 2−6 were similarly conducted. The resulting amines and
1
amides were identified by comparison of the H and 13C NMR data with those previously
reported (S16−S28, ESI†).
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Xꢀray Crystallographic Study
Crystals of 1, 2 and 5 were mounted on glass fibers and used for data collection. Crystal data
were collected at 296(2) K(1) and 295(2) K (2 and 5) using a Gemini A Ultra Oxford Diffraction
automatic diffractometer. Graphite monochromated MoꢀKα radiation (λ = 0.71073 Å) was used
throughout. The absorption corrections were performed by the multiꢀscan method. Corrections
were made for Lorentz and polarization effects. The structures were solved by direct methods
using the program SHELXS.58 Refinement and all further calculations were carried out using
SHELXL.58 The H atoms were included in calculated positions and treated as riding atoms using
the SHELXL default parameters. The nonꢀhydrogen atoms were refined anisotropically, using
weighted fullꢀmatrix least squares on F2. Atomic scattering factors were incorporated in the
computer programs. In the solid state of complex 1, a disorder is observed within the two
different enantiomeric forms in such a manner that only the CO group and the chlorine atom
share the ligand positions mutually. Despite several attempts to get better crystals of complex 5
and a better data set, only poorꢀquality data were obtained.
Acknowledgment
The authors are grateful to Indian Institute of Technology, Chennai, Indian Institute of Science,
Bangalore and Punjab University, Chandigarh, for providing instrumental facilities.
Electronic supplementary information (ESI) available:
The unit cell packing diagram for complexes 1, 2 and 5 (Fig. S1ꢀS3), representative NMR (1H
and 31P) spectra of ligand and complexes, detailed experimental procedures and spectral data for
Nꢀalkylated products. CCDC reference numbers 1021256, 1021259 and 1021258 for complexes
1, 2 and 5. For ESI and crystallographic data in CIF or other electronic format see DOI:
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R2
C
R2
C
H
H
R1
R3
OH
N
H
R1
[M]
[MH2]
R2
C
R2
C
R1
R3
R1
O
N
R3NH2 H2O
Scheme 1 Borrowing hydrogen strategy for C-N bond formation reaction
30