The formation of dinuclear trichloro-bridged and mononuclear ruthenium complexes from the reactions of dichlorotris(p-tolylphosphine)ruthenium(II) with diazabutadiene ligands

Article abstract of DOI:10.1007/s11243-018-00293-0

Ru(II) complexes with diazabutadiene (R-DAB) ligands have been prepared. The reaction of RuCl₃·nH₂O with P(p-tolyl)₃ gave a [RuCl₂{P(p-tolyl)₃}] precursor, whose reactions with R-DAB in toluene gave d

Full text of DOI:10.1007/s11243-018-00293-0

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-00293-0
The formation of dinuclear trichloro‑bridged
and mononuclear ruthenium complexes from the reactions
of dichlorotris(p‑tolylphosphine)ruthenium(II) with diazabutadiene
ligands
Meng Guan Tay1 · Ying Ying Chia1 · Suzie Hui Chin Kuan1 · Tze Pei Phan1
Received: 18 September 2018 / Accepted: 24 November 2018
© Springer Nature Switzerland AG 2018
Abstract
Ru(II) complexes with diazabutadiene (R-DAB) ligands have been prepared. The reaction of RuCl3·nH2O with P(p-tolyl)3
gave a [RuCl2{P(p-tolyl)3}] precursor, whose reactions with R-DAB in toluene gave dinuclear trichloro-bridged Ru(II)
complexes [Ru2Cl3(P(p-tolyl)3)2(R-DAB)2](BF4) which have been characterized by spectroscopic methods. In addition,
one of the complexes was characterized using X-ray crystallography. Meanwhile, two mononuclear Ru(II) complexes
[RuCl2(P(p-tolyl)3)2(R-DAB)] were obtained from the reactions of the [RuCl2{P(p-tolyl)3}] precursor with R-DAB ligands
in THF. The two trans-mononuclear complexes were characterized by X-ray crystallography and solid-state 31P NMR. A
temperature-dependent 31P NMR study was carried out to monitor the formation of dinuclear and mononuclear complexes.
Introduction
[4, 13–16], mononuclear ruthenium R-DAB complexes are
more rare. In 1980, Chaudret and Poilblanc [13] reported the
formation of dinuclear ruthenium R-DAB complexes with
three proposed structures; but they could not identify the
exact structures of these complexes. In 2014, Ghosh and
co-workers reported mononuclear ruthenium R-DAB com-
plexes obtained from the reaction between the precursor
complex dichlorotris[tri(p-{tolyl})phosphine] ruthenium(II),
[RuCl2{P(p-tolyl)3}3 and R-DAB ligands [17]. Surprisingly,
although we used similar methodology to that reported by
Ghosh et al. in our own subsequent studies, the ruthenium
R-DAB complexes obtained were dinuclear analogues rather
than the mononuclear molecules. This has drawn our inter-
est to further investigate the formation of dinuclear as well
as mononuclear ruthenium R-DAB complexes by using the
R-DAB ligands as shown in Scheme 1. In addition, a 31P
NMR temperature-dependent study has been carried out in
order to gain insight into the formation of dinuclear versus
mononuclear complexes. The outcome of these experiments
is presented herein.
A number of derivatives of 1,4-diaza-1,3-butadiene, also
known as diazabutadiene (R-DAB), and their complexes
have been reported in the past decades [1–6]. Coordina-
tion between the R-DAB ligand and metal centre generally
occurs through the two nitrogen atoms of the imine groups,
giving complexes with fve-membered chelate rings [1, 7].
Such fve-membered chelate ring ruthenium complexes have
drawn signifcant attention in view of the remarkable photo-
physical properties of ruthenium bipyridine (bpy) complexes
[8–12]. However, compared with ruthenium bpy complexes,
the synthesis of ruthenium R-DAB complexes is much less
documented. This can in part be attributed to the forma-
tion of dinuclear ruthenium complexes instead of the mono-
nuclear analogues. Although several dinuclear ruthenium
R-DAB complexes were reported in the 1980s and 1990s,
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-00293-0) contains
supplementary material, which is available to authorized users.
* Meng Guan Tay
mgtay@unimas.my
1
Faculty of Resource Science and Technology, Universiti
Malaysia Sarawak, 94300 Kota Samarahan, Sarawak,
Malaysia
Vol.:(0123456789)
1 3

Transition Metal Chemistry
R
R
Crystal structure determinations
N
N
N
N
The crystal data were collected on a Bruker X8-APEX II
difractometer with a CCD area detector and multi-layer
mirror/graphite-monochromated MoKα radiation. The
structures were solved using intrinsic phasing methods
[19] refned with the ShelXL program [20] and expanded
using Fourier techniques. All non-hydrogen atoms were
refned anisotropically. Hydrogen atoms were included
in structure factor calculations. All hydrogen atoms were
assigned to idealized geometric positions.
R
R
R-DAB
R-DAB-Me
Scheme 1 Diazabutadiene compounds used in this study, R=OMe or
Me
Experimental
Materials and methods
Preparation of the R‑DAB compounds
All of the complexes were synthesized using standard
Schlenk techniques. RuCl3·nH2O was purchased from Pre-
cious Metals Online and used without further purifcation.
All other starting materials were purchased from com-
mercial sources such as Sigma-Aldrich and Merck. The
chemicals were generally used without further purifcation
except for p-tolyl phosphine which was recrystallized from
hot ethanol prior to use. All the solvents were dried by
standard procedures and degassed. [RuCl2{P(p-tolyl)3}3]
was synthesized according to the method reported by Linn
[18]. Toluene was purifed with an Innovative Technology
Inc. Pure-Solv 400 Solvent Purifcation System. THF-d8
was dried by refux over potassium for a day and then
distilled. The NMR spectra were recorded using either
Bruker Avance 500 MHz, Bruker Avance 300 MHz or
JEOL 500 MHz NMR spectrometers. Mass spectra were
obtained using an Agilent 7890A/5975C Inert GC/MSD
or Varian 320 MS-GC/MS systems operating in EI mode.
Mass spectrometric analysis was performed on complexes
1–4 using the MALDI technique on an Autofex Bruker
Daltonic instrument. The matrix used was 2-[(2E)-3-(4-
tert-butylphenyl)-2-methylprop-2-enylidene] malononitrile
(DCTB) 1:3 in THF. The high-resolution mass spectra,
MS–ESI, were recorded on a Thermo Scientifc Exactive™
Plus Orbitrap Mass Spectrometer. ESI measurements were
taken using a HESI Source with an Aux-Gas temperature
of 50 °C. ASAP/APCI measurements were taken using
an APCI Source with Corona Needle and Aux-Gas tem-
perature of 400 °C. Electronic spectra were recorded on
a PerkinElmer Lambda-40-UV–Vis spectrometer with
1-cm quartz cuvettes in the range of 200–800 nm. Elemen-
tal analyses were obtained on a Firma Elementar Vario
MICRO Cube analyser. IR spectra were recorded using a
Thermo Scientifc Fourier transform infrared spectrometer
(FTIR) model—Nicolet iS10 on KBr discs in the range of
4000–400 cm−1.
The R-DAB compounds (where R=OMe or Me) were synthe-
sized according to literature procedures with slight modifca-
tions [21] as follows.
Preparation of 1,4‑di(4‑methoxyphenyl)‑1,4‑di‑
aza‑1,3‑butadiene (OMe‑DAB)
Glyoxal (1.27 g, 8.76 mmol) and p-methoxyaniline (2.155 g,
17.5 mmol) were stirred in distilled ethanol (20 mL) at room
temperature for 4 h. The yellow precipitate was collected and
recrystallized by vapour difusion of hexane into a saturated
dichloromethane solution. Yield: 1.91 g, 81%. Anal. Calcd.
for C16H16N2O2: C, 71.6; H, 6.0; N, 10.5; Found: C, 71.6; H,
6.0; N, 10.4%. 1H NMR (500 MHz, DMSO-d6, ppm): 3.79 (s,
6H, OCH3), 7.00 (d, 4H, J=9 Hz, HAr), 7.42 (d, 4H, J=9 Hz,
HAr), 8.46 (s, 2H, HC=N). 13C NMR (125 MHz, DMSO-
d6, ppm): 159.37, 157.58, 142.55, 123.32, 114.68, 55.48. IR
(KBr, cm−1): 2964(w); 1607(s); 1499(s); 1285(m); 1250(m).
UV–Vis (CH2Cl2 λmax/nm): 238, 297 and 375.
Preparation of 1,4‑di(4‑methylphenyl)‑1,4‑di‑
aza‑1,3‑butadiene (Me‑DAB)
Glyoxal (1.27 g, 8.76 mmol) and p-toluidine (1.881 g,
17.5 mmol) were stirred in distilled ethanol (20 mL) at room
temperature for 4 h. Work-up as above gave Me-DAB. Yield:
1.53 g, 74%. Anal. Calcd. for C16H16N2: C, 81.3; H, 6.8; N,
11.9; Found: C, 80.6; H, 6.7; N, 11.8%. 1H NMR (500 MHz,
DMSO-d6, ppm): 2.34 (s, 6H, CH3), 7.25 (d, 8H, J=8 Hz,
HAr), 8.44 (s, 2H, HC=N). 13C NMR (125 MHz, DMSO-d6,
ppm): 159.12, 147.20, 137.62, 129.94, 121.49, 20.72. IR (KBr,
cm−1): 2912(w); 1607(s); 1500(s); 1304(m). UV–VIS (CH2Cl2
λmax/nm): 237, 289 and 349.
1 3

Transition Metal Chemistry
was then added through a syringe, and the mixture was
refuxed for 2 h. The solvent was removed by concentra-
tion to 5 mL. Hexane was added to precipitate the product,
which was isolated by fltration. The dark brown powder
was washed with hexane and dried under vacuum. Yield:
0.20 g, 67%. 31P{1H} NMR (202 MHz, CDCl3, ppm):
Preparation of 1,4‑di(4‑methoxyphenyl)‑1,4‑di‑
aza‑2,3‑methyl‑1,3‑butadiene (OMe‑DAB‑Me)
A mixture of 2,3-butanedione (0.491 g, 5.7 mmol) and
p-methoxyaniline (1.404 g, 11.4 mmol) was stirred in
methanol (15 mL) with a few drops of formic acid at room
temperature. After 4 h, the precipitate was collected and
washed with cold methanol. The product was recrystallized
by vapour difusion of hexane into a dichloromethane solu-
tion. Yield: 1.05 g, 62%. Anal. Calcd. for C18H20N2O2: C,
1
39.14 (s, 2P, [Ru2Cl3(P(p-tolyl)3)2(OMe-DAB)2]+). H
NMR (500 MHz, CDCl3, ppm): 8.31 (s, 2H, –HC=N),
6.87 (d, J = 8 Hz, 8H, Ar–H), 6.75 (d, J = 8 Hz, 6H,
Ar–H), 6.64 (dd, J = 8, 18 Hz, 14H, Ar–H), 3.92 (s, 8H,
–OCH3), 2.33 (s, 9H, –CH3). MS-MALDI (m/z): 1453.23,
[Ru2Cl3(P(p-tolyl)3)2(OMe-DAB)2]+. IR (KBr, cm−1):1601,
1
73.0; H, 6.8; N, 9.5; Found: C, 73.0; H, 6.6; N, 9.3%. H
NMR (500 MHz, DMSO-d6, ppm): 2.08 (s, 6H, H3C-C=N),
3.76 (s, 6H, OCH3), 6.79 (d, 4H, J = 8 Hz, HAr), 6.95 (d,
4H, J=8 Hz, HAr). 13C NMR (125 MHz, DMSO-d6, ppm):
167.65, 155.98, 143.40, 120.66, 114.23, 55.11, 15.17. IR
(KBr, cm−1): 2961(w); 1632(s); 1500(s); 1240(s). UV–Vis
(CH2Cl2 λmax/nm): 292 and 350.
1502, 1456, 1252, 831, 526, 445. UV–Vis (CH2Cl2 λmax
/
nm): 248, 406 and 545.
Preparation of complex 2
A Schlenk tube was charged with Me-DAB (0.0472 g,
0.2 mmol) and the ruthenium precursor (0.2214 g,
0.204 mmol) to prepare complex 2 by a method simi-
lar to that used for complex 1. Yield: 0.18 g, 63%.
31P{1H} NMR (202 MHz, CDCl3, ppm): 38.50 (s, 2P,
Preparation of 1,4‑di(4‑methylphenyl)‑1,4‑di‑
aza‑2,3‑methyl‑1,3‑butadiene (Me‑DAB‑Me)
A mixture of 2,3-butanedione (0.981 g, 11.4 mmol) and
p-toluidine (2.443 g, 22.8 mmol) was stirred in methanol
(20 mL) with a few drops of formic acid at room tempera-
ture. Work-up as described above gave Me-DAB-Me. Yield:
1.68 g, 56%. Anal. Calcd for C18H20N2: C, 81.8; H, 7.6; N,
10.6; Found: C, 81.5; H, 7.5; N, 10.7%. 1H NMR (500 MHz,
DMSO-d6, ppm): 2.06 (s, 6H, H3C-C=N), 2.30 (s, 6H, CH3),
1
[Ru2Cl3(P(p-tolyl)3)2(Me-DAB)2]+). H NMR (500 MHz,
CDCl3, ppm): 8.30 (s, 2H, –HC=N), 6.90 (d, J=8 Hz, 4H,
Ar–H), 6.84 (d, J=8 Hz, 6H, Ar–H), 6.66 (d, J=7 Hz, 4H,
Ar–H), 6.56 (dd, J = 8 Hz, J = 8 Hz, 6H, Ar–H), 2.48 (s,
6H, –CH3), 2.32 (s, 9H, –CH3). MS-MALDI (m/z): 1391.25,
[Ru2Cl3(P(p-tolyl)3)2(Me-DAB)2]+. IR (KBr) (cm−1): 1599,
1501, 1463, 810, 510, 444. UV–Vis (CH2Cl2 λmax/nm): 235,
378 and 533.
6.71 (d, 4H, J=8 Hz, HAr), 7.18 (d, 4H, J=8 Hz, HAr). 13
C
NMR (125 MHz, DMSO-d6, ppm): 167.60, 148.00, 132.80,
129.48, 118.88, 20.48, 15.11. IR (KBr, cm−1): 2916(w);
1627 (s); 1500(m); 1361(m). UV–Vis (CH2Cl2 λmax/nm):
336.
Preparation of complex 3
A Schlenk tube was charged with OMe-DAB-Me
(0.0622 g, 0.2 mmol) and the ruthenium precur-
sor (0.2214 g, 0.204 mmol) to prepare complex 3 by a
method similar to that used for complex 1. Yield: 0.16 g,
52%. 31P{1H} NMR (202 MHz, CDCl3, ppm): 38.7(s,
Preparation of dichlorotris(tri(p‑tolyl)phosphine)
ruthenium(II)
A mixture of RuCl3·nH2O (1.70 g, 6.46 mmol) and excess
tri(p-tolyl)phosphine (7.90 g, 11.20 mmol) in dried ethanol
(70 mL) was refuxed for 3 h under argon. Schlenk fltration
yielded a brown precipitate which was washed three times
with hot ethanol and then twice with Et2O. Yield: 2.8 g,
85%. 31P{1H} NMR (202 MHz, THF-d8, ppm): − 7.56 (s,
1
2P, [Ru2Cl3(P(p-tolyl)3)2(OMe-DAB-Me)2]+). H NMR
(500 MHz, CDCl3, ppm): 7.05 (s, 2H, Ar–H), 6.87 (d,
J = 7 Hz, 4H, Ar–H), 6.84 (d, J = 7 Hz, 4H, Ar–H), 6.43
(broad doublet, J=6 Hz, 10 H, Ar–H), 3.92 (s, 6H, –OCH3),
2.34 (s, 5H, –CH3), 2.16 (s, 2H, –CH3). MS-MALDI (m/z):
1511.38, [Ru2Cl3(P(p-tolyl)3)2(OMe-DAB-Me)2]+. IR (KBr,
cm−1): 1604, 1503, 1246, 807, 526, 445. UV–Vis (CH2Cl2
λmax/nm): 241, 380 and 507.
1
P{p-tolyl}3) 23.58 (s, [RuCl2{P(p-tolyl)3}3]). H NMR
(500 MHz, THF-d8, ppm): 7.22 (d, 6H, J = 8 Hz, Ar–H),
6.75 (d, 6H, J=8 Hz, Ar–H), 2.24 (s, 9H, –CH3). IR (KBr,
cm−1): 3020, 1599, 1499, 805, 523, 461.
Preparation complex 4
Preparation of complex 1
A mixture of Me-DAB-Me (0.0528 g, 0.2 mmol) and the
ruthenium precursor (0.2214 g, 0.204 mmol) was used to
prepare complex 4 according to the procedure used for
complex 1. A light brown precipitate was collected. Yield:
To a 100-mL Schlenk tube containing the ruthenium pre-
cursor (0.2214 g, 0.204 mmol), solid OMe-DAB (0.0536 g,
0.2 mmol) was added under argon. Dried toluene (20 mL)
1 3

Transition Metal Chemistry
0.19 g, 64%. 31P{1H} NMR (202 MHz, CDCl3, ppm): 38.5
Cl
P(p-tolyl)3
P(p-tolyl)3
Ru
1
(s, 2P, [Ru2Cl3(P(p-tolyl)3)2(Me-DAB-Me)2]+). H NMR
RuCl3.nH2O
(p-tolyl)3P
EtOH,
reflux
P(p-tolyl)3
Cl
(500 MHz, CDCl3, ppm): 7.36 (d, 2H, J=7 Hz, Ar–H), 7.18
(d, 2H, J=8 Hz, Ar–H), 7.04 (d, 6H, J=6 Hz, Ar–H), 6.68
(s, 6H, Ar–H), 6.47 (m, 4H, Ar–H), 2.39 (s, 10H, –CH3 of
(p-tolyl)phosphine), 2.23 (s, 3H, –CH3 of diimine R group),
1.99 (s, 2H, –CH3 of diimine backbone). MS-MALDI (m/z):
1447.39, [Ru2Cl3(P(p-tolyl)3)2(Me-DAB-Me)2]+. IR (KBr,
cm−1): 1599, 1502, 1259, 796, 526, 445. UV–VIS (CH2Cl2
λmax/nm): 235, 258 and 504.
R-DAB,
dry toluene,
reflux, 2 h
+
Cl
N
N
Cl
Ru
Ru
N
+
4 P(p-tolyl)3
N
l
C
(lylot-p)3P
P(p-tolyl)3
Preparation of complex 5
In a glove box, the ruthenium precursor (0.010 g,
9.216×10−6 mol) was dissolved in dried THF (1 mL), and
OMe-DAB (0.005 g, 1.843 × 10−5 mol) was dissolved in
diethyl ether. The clear yellow solution of OMe-DAB was
layered onto the brown solution of ruthenium precursor. The
sealed vial was kept at −35 °C. After several days, reddish
single crystals of complex 5 were isolated, mounted in oil
and subjected to X-ray crystallography analysis. Further
characterization could not be carried out due to the small
amount of compound obtained.
R'
R
N
N
=
N
N
R
R'
R = CH3, R' = OCH3 (3)
R = CH3, R' = CH3 (4)
R = H, R' = OCH3 (1)
R = H, R' = CH3 (2)
Scheme 2 Preparation of dinuclear ruthenium R-DAB complexes
from the [RuCl2(P{p-tolyl}3)] precursor
Preparation of complex 6
the ruthenium metal centres are connected by three chloro-
bridging ligands, such that each metal centre is bound to one
R-DAB and one P(p-tolyl)3 ligand.
The synthesis of complex 6 was carried out by the same
method as for complex 5, but replacing OMe-DAB with Me-
DAB (0.006 g, 2.765 × 10−5 mol) which was dissolved in
hexane. The purple precipitate was collected, washed with
hexane and dried. Recrystallization by layering hexane onto
a THF solution aforded single crystals. 31P{1H} NMR (solid
According to the literature [13–16], the formation
of trichloro-bridged dinuclear ruthenium complexes
is very common due to the stability of the resulting cat-
ion. For example, this type of complex was observed by
Poilblanc and co-workers, who reported the dinuclear
[Ru2Cl4(PPh3)3(DAB)]+ and [Ru2Cl3(PPh3)2(DAB)2]Cl
obtained from the reactions of [RuCl2(PPh3)3] with DAB
ligands [13]. The formation of dinuclear complexes has also
been reported by Cotton and Torralba, who synthesized fve
face-sharing bioctahedral diruthenium complexes [14]. In
addition, Fogg and James have explored the reactions of
a dimer precursor and two-electron donor ligands to form
[RuCl(dppb)(µ-Cl)3Ru(dppb)(L)] complexes [15].
2
state): 16.76, 14.59, −7.32, −9.48 (d, J(P,P) = 330 Hz).
MS–ESI (m/z): 981.28 [Ru(Me-DAB){P(p-tolyl)3}2Cl]+,
677.14 [Ru(Me-DAB)Cl]+.
Results and discussion
Synthesis of dinuclear ruthenium complexes
The MALDI analysis (see Supporting Informa-
tion) for complex 1 showed the parent ion peak
at 1453.23, in agreement with the formula of the
[Ru2Cl3{P(p-tolyl)3}2(OMe-DAB)2]+ cation. Similar
results were obtained for dinuclear complexes 2, 3 and
4, where the parent ion peaks at 1391.25, 1511.28 and
1447.39, respectively, agreement with the molecular
masses of the cationic dimers, excluding the chloride ion.
Meanwhile, dinuclear complexes 1, 2 and 3 were resyn-
thesized using BF4¯ as the anion in order to confirm the
presence of chloride in the primary valence sphere. This
was done by dissolving the respective R-DAB ligands in
a small amount of toluene, then adding the ruthenium
The ruthenium precursor, [RuCl2{P(p-tolyl)3}3], was synthe-
sized according to the literature procedure [18]. However,
tri(p-tolyl)phosphine, P(p-tolyl)3, was chosen as a substitute
for triphenylphosphine in order to simplify the aromatic sig-
nals in the 1H NMR spectra.
The synthesis of the ruthenium complexes was similar
to that described by Ghosh et al. [17], where the R-DAB
ligand and the precursor complex were refuxed in dry tol-
uene for around 2 h (Scheme 2). We initially expected to
obtain mononuclear complexes, but surprisingly, the ruthe-
nium complexes that were isolated proved to be the dinu-
clear analogues. Thus, the crystallographic data showed that
1 3

Transition Metal Chemistry
Table 1 Analytical data for dinuclear complexes 1, 2 and 3 with BF4¯
as the anion
Complex Formula
Obs. (calc.)
C%
H%
N%
1
Ru2Cl3C74H-
58.9 (57.7) 5.1 (4.8) 3.9 (3.6)
74N4O4P2BF4
2
3
Ru2Cl3C74H74N4P2BF4
59.9 (60.1) 5.0 (5.1) 3.9 (3.8)
57.6 (58.7) 5.4 (5.2) 3.4 (3.5)
Ru2Cl3C78H-
82N4O4P2BF4
precursor [RuCl2{P(p-tolyl)3}3] and stirring at room
temperature overnight. After removing the solvent, the
red–brown residue was washed with ether. The precipi-
tate was dissolved in methanol, and a solution of NaBF4
in methanol was added dropwise. The resulting crystals
were collected by filtration. The elemental analyses data
as given in Table 1 confirmed that ruthenium complexes
1, 2 and 3 retain the dimeric structure.
Fig. −1 Molecular structure of dinuclear ruthenium complex 2 with
BF4 anion. Thermal ellipsoids are drawn at 50% probability. The
hydrogen atoms are omitted for clarity
The UV spectrum of free OMe-DAB shows three
absorption bands at 238, 297 and 375 nm. The first
two bands can be attributed to the primary and second-
ary π → π* transitions of the benzene fragment, whilst
the peak at 375 nm is assigned to n → π* transitions of
the C=N chromophore [22–24]. The band at 238 nm
was shifted to the lower energy region at 248 nm upon
complexation, and similarly, the band at 375 nm was
bathochromically shifted to 406 nm in complex 1. An
additional band was observed at 545 nm in the spectrum
of dinuclear ruthenium(II) complex 1, which might arise
from the MLCT transitions. For complexes 2, 3 and 4,
possible MLCT bands were observed at 533, 507 and
504 nm, respectively. The presence of methyl groups
in the backbone of the R-DAB-Me ligands (Me-C=N)
in complexes 3 and 4 conferred hypsochromic shifts on
the MLCT absorption bands (as compared to those of
complexes 1 and 2). This result suggests that the methyl
groups increase the band gap between the π d-orbitals of
ruthenium and π antibonding orbitals of the ligand.
In the IR spectra of the free R-DAB compounds,
the C=N stretching band is located in the region of
1632–1607 cm−1. The presence of methyl groups in
the backbone of the diazadiene moiety (Me-C=N) gave
the C=N stretching at a higher frequency, specifically
1632 cm−1 for OMe-DAB-Me and 1627 cm−1 for Me-
DAB-Me. After complexation, the C=N band was shifted
X‑ray crystal structure of complex 2
A single crystal of the dinuclear ruthenium complex 2 with
−
BF4 as the anion was collected from methanol solution
after addition of NaBF4 solution. The molecular structure
of complex 2 is shown in Fig. 1. The Ru–N bond lengths
are around 2.013–2.024 Å, slightly shorter than the value of
2.056 Å observed for [Ru(bpy)3]2+ [26], because the imine
bond in bpy has stronger trans-infuence than the chloro-
bridging ligands. Meanwhile, the bond lengths within the
Ru–Cl bridge are in the range of 2.405–2.496 Å, which is
similar to the average reported Ru–Cl-bridging bond length
of 2.428 Å [27].
Synthesis of mononuclear ruthenium complexes
Compared with the dinuclear complexes, the synthe-
sis of mononuclear ruthenium complexes was rather
more difcult. In an argon-flled glove box, the reaction
of [RuCl2{P(p-tolyl)3}3] with R-DAB compounds was
attempted by dissolving [RuCl2{P(p-tolyl)3}3] in THF and
layering with a solution of the R-DAB compound in hexane.
A rapid colour change from brown to purple was observed.
The vial was then immediately transferred to a freezer at
− 35 °C. After 1–2 days, a fne precipitate was observed;
this was fltered of and washed. Solid-state 31P{1H} NMR
analysis was carried out, and the spectrum in Fig. 2 shows
two doublets with a coupling constant of 437 Hz for the cis
isomer, which might be the reaction intermediate.
to lower frequencies, in the range of 1604–1599 cm−1
.
This indicated the presence of backbonding from the Ru
metal centre to the π*orbitals of the diazadiene moiety. In
addition, a Ru–N band was observed in the 445–450 cm−1
region in the spectra of the complexes [25].
1 3

Transition Metal Chemistry
Fig. 2 Solid-state 31P{1H}
NMR spectrum of mononuclear
complex 6
Fig. 3 Solid-state 31P{1H}
NMR spectrum for the larger-
scale synthesis of complex 6
The reaction was repeated on a large scale by reacting
150 mg of [RuCl2{P(p-tolyl)3}3] with 90 mg of Me-DAB-
Me. The solid-state 31P{1H} NMR spectrum showed a mix-
ture (Fig. 3). Apart from the cis-intermediate which gives
rise to the two doublets at δ = − 8.16 and + 15.75 ppm,
an intense new singlet was observed at 1.5 ppm, which is
believed to be the trans-mononuclear ruthenium complex.
We suspected that the mixture also contained some unre-
acted precursors, by comparing to the solid-state 31P{1H}
NMR spectrum of [RuCl2{P(p-tolyl)3}3]. The precipitate
was washed with hexane and then recrystallized by vapour
difusion of hexane into a THF solution. A single crystal was
isolated thereafter, and this product exhibited the mononu-
clear geometry.
X‑ray crystal structures of complexes 5 and 6
Single crystals of mononuclear complexes 5 and 6 were
obtained by layering hexane onto THF solution at −35 °C.
Unlike the dinuclear complexes, the tri(p-tolyl)phosphine
ligands in these mononuclear complexes are trans to each
other, with bond angles of 177.268°–170.207°. As shown in
Fig. 4, the structure of complex 5 includes two independent
molecules, both of which are distorted octahedral structures.
For the ruthenium complex on the left of Fig. 4a, there are
two disordered diazadiene segments and two p-tolyl rings
overlapping each other. The structure in Fig. 4 is plotted
to the one with a higher site occupancy of 0.588(4). It is
worth noting that the Ru–N, Ru–P, Ru–Cl, N–C and C–C
bond lengths are all similar in both Ghosh’s complex and our
complex 5, except for Ru2–N3A (2.024 Å) and Ru2–N4A
(2.099 Å), which are shorter in our complex than the one
reported by Ghosh et al. (2.0519 Å). The Ru–N bond
lengths for the structure in Fig. 4a are diferent (2.0997 and
Meanwhile, the high-resolution mass spectrum in ESI
mode (ESI–MS) showed a fragment at 981.27 g/mol corre-
sponding to [RuCl2{P(p-tolyl)3}2(Me-DAB-Me)]. Notwith-
standing, the trichloro-bridged dimer peak was also present
at 1391.25 g/mol (see Supporting Information).
1 3

Transition Metal Chemistry
Fig. 4 Molecular structure of 5(a) and (b), with thermal ellipsoids drawn at 50% probability. The hydrogen atoms connected to carbon and the
THF solvate molecules are omitted for clarity
2.0249 Å), and this might be due to packing efects in the
crystal [28]. However, the N–C and C–C bond lengths are
indistinguishable in this structure.
The mononuclear complex 6 (Fig. 5) bearing a p-CH3
diazadiene ligand was crystallized in an orthorhombic
Pna21 space group. The bond lengths and angles in complex
6 are similar to those in complex 5. Comparing the bond
lengths of the free diimines and the coordinated ligands,
the –C=N–bond length increased from 1.28 to 1.31 Å
upon coordination to the Ru metal centre. Conversely, the
=CH–CH= bond length became shorter compared with the
free diimines, from 1.45 to 1.40 Å. The free diimine showed
a typical carbon–nitrogen double-bond length (1.28 Å) and
carbon–carbon single bond (1.45 Å) in a Csp2–Csp2 con-
jugated system. The deformed diazadiene, showing longer
–C=N– and shorter =CH–CH= bond lengths after coordi-
nation, is an indicator of strong back donation [17]. Upon
coordination to the ruthenium centre, the angles C2–C1–N1
and C1–N1–C1_2 showed a slight 3° decrease from the
free diazadiene (~119°) as the two sides of the –C=N– are
brought together in order to coordinate to the metal centre.
Temperature‑dependent 31P{1H} NMR study
Fig. 5 Molecular structure of complex 6, with thermal ellipsoids
drawn at 50% probability. The hydrogen atoms connected to carbon
A temperature-dependent 31P{1H} NMR experiment was
and two molecules of THF solvate are omitted for clarity
carried out to determine the efect of this parameter on
1 3

Transition Metal Chemistry
Scheme 3 Equilibrium of
dinuclear and mononuclear
ruthenium(II) complexes
+
Cl
Cl
P(p-tolyl)3
N
N
N
N
Cl
Cl
Ru
Cl-
2
Ru
Ru
N
N
P(p-tolyl)3
l
C
(lylot-p)3P
P(p-tolyl)3
Me
N
N
=
N
N
Me
Me-DAB
Table 2 Relative intensities of dinuclear and mononuclear complex
signals with free P(p-tolyl)3 at diferent temperatures
synthesis of the dinuclear and mononuclear ruthenium com-
plexes, in which the dinuclear complexes were obtained at
higher temperature and vice versa. However, for the dinu-
clear complexes, the reaction solvent was toluene, whereas
THF was used for the mononuclear complexes. It is probable
that the diferent polarities of these two solvents are respon-
sible for the formation of dinuclear or mononuclear com-
plexes. In a paper published by Scholz et al. [29] on R-DAB
complexes, the authors stated that dinuclear complexes
dissociated in THF. It was also observed that the dinuclear
complexes have poor solubility in non-polar solvents such
as toluene because of their monocationic nature.
Temp (°C)
ppm
+31
+1
−7
+25
+20
+15
+10
+5
–
47.76
52.51
47.77
40.69
27.09
6.03
52.24
47.48
46.73
47.56
52.58
60.90
66.76
69.60
Trace
5.49
11.75
20.31
33.06
32.02
29.31
0
−20
−38.2
1.21
1.08
Signal identifcations: +31 ppm dinuclear complex; +1 ppm mono-
nuclear complex; −7 ppm free p(tolyl)3
Conclusions
The syntheses of dinuclear and mononuclear ruthenium
complexes using R-DAB ligands with [RuCl2{P(p-tolyl)3}]
precursor have been demonstrated. The structures of both
types of complexes have been confrmed by X-ray crys-
tallography, NMR and MALDI. Temperature-dependent
31P{1H} NMR experiments showed that the dinuclear
complex was more favourable at low temperature, whereas
the mononuclear complex was more preferable at a higher
temperature.
the formation of dinuclear and mononuclear complexes
(Scheme 3). A mixture of [RuCl2{P(p-tolyl)3}3] and Me-
DAB was placed in an NMR tube in the glove box. It was
then transferred to a Schlenk line and dried THF-d8 was
added. The NMR tube was immediately placed in an iso-
propanol cold bath (− 40 °C), and the reaction was moni-
tored at diferent temperatures: − 38.2, − 20, 0, + 5, + 10,
+15, +20 and +25 °C using 31P{1H} NMR spectroscopy.
The analyses were performed in triplicate at each tempera-
ture before the temperature was increased. In the resulting
spectra, three peaks were observed at +31 ppm (dinuclear),
+1 ppm (mononuclear) and −7 ppm [free P(p-tolyl)3]. The
relative integrations of the peaks are given in Table 2. It can
be seen that at a low temperature, the peak at 31 ppm, which
is attributed to the dinuclear complex, has a higher intensity
compared with the monomer at around 1 ppm. When the
temperature was increased, the proportion of the monomer
also increased, while the dimer peak gradually disappeared.
The results from the temperature-dependent 31P{1H}
NMR study contradicted the observations made in the
Supplementary data
CCDC 1871392, 1871390, 1871393 and 1871391 contain
the supplementary crystallographic data of ruthenium(II)
precursor, [RuCl2{P(p-tolyl)3}3], dinuclear ruthenium(II)
complex 2 and mononuclear ruthenium(II) complexes 5 and
6, respectively. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
1 3

Transition Metal Chemistry
10. Lytle FE, Hercules DM (1969) J Am Chem Soc 91:253–257
11. Klassen DM, Crosby GA (1968) J Chem Phys 48:1853–1857
12. Bensasson R, Salet C, Balzani V (1976) J Am Chem Soc
98:3722–3724
Acknowledgements The authors are thankful to Professor Todd B.
Marder for providing us the facilities in his research laboratory. We
would like to thank Dr. Martin Hähnel and Dr. Daniel Sieh for their
opinion on the synthetic methods as well as sharing their knowledge
on complexation. In addition to that, we are grateful to Dr. Rüdiger
Bertermann for his help with the solid-state NMR analysis and Dr.
Alexandra Friedrich for her help with the X-ray crystallography analy-
sis. We would also like to thank Malaysian Ministry of Education for
the fnancial support to the research through Exploratory Research
Grant Scheme [ERGS/STG01(01)/1021/2013(01)].
13. Chaudret B, Poilblanc R (1980) Dalton Trans (3):539–544
14. Cotton FA, Torralba RC (1991) Inorg Chem 30:2196–2207
15. James BR, Fogg DE (1997) Inorg Chem 36:1961–1966
16. Cruz SS, Amenta DS, Gilge JW, Yap GPA (2016) Polyhedron
114:179–183
17. Patra SC, Roy AS, Manivannan V, Weyhermüller T, Ghosh P
(2014) Dalton Trans 43:13731–13741
18. Linn DE Jr (1999) J Chem Educ 76:70–72
19. Sheldrick GM (2015) Acta Crystallogr C Struct Chem 71:3–8
20. Sheldrick GM (2008) Acta Crystallogr A Found Crystallogr
64:112–122
References
21. Zhong HA, Labinger JA, Bercaw JE (2002) J Am Chem Soc
124:1378–1399
1. Keijsper J, Poel HVD, Polm LH, Koten GV, Vrieze K (1983)
Polyhedron 2:1111–1116
22. Sathiyaraj S, Ayyannan G, Jayabalakrishnan C (2014) J Serb
Chem Soc 79:151–165
2. Koten GV, Jastrzebski JTBH, Vrieze K (1983) J Organomet Chem
250:49–61
23. Kliegman JM, Barnes RK (1970) J Org Chem 35:3140–3143
24. Bhowon MG, Li HKW, Narain R (1999) Polyhedron 18:341–345
25. Nagajothi A, Kiruthika A, Chitra S, Parameswari K (2013) Res J
Chem Sci 3:35–43
3. Bildstein B, Malaun M, Kopacka H, Fontani M, Zanello P (2000)
Inorg Chim Acta 300–302:16–22
4. Chaudret B, Köster H, Poilblanc H (1981) JCS Chem Commun
(6):266–268
26. Biner M, Bürgi HB, Ludi A, Röhr C (1992) J Am Chem Soc
114:5197–5203
5. Rosenberger V, Dieck HT (1990) Chem Ber 123:83–85
6. Grupp A, Bubrin M, Ehret F, Zeng Q, Hartl F, Kvapilová H, Záliš
S, Kaim W (2014) Eur J Inorg Chem 2014:110–119
7. Ceder RM, Muller G, Ordinas M, Font-Bardia M, Solans X (2003)
Dalton Trans (15):3052–3059
27. Orpen AG, Brammer L, Allen FH, Kennard O, Watson DG, Taylor
R (1989) J Chem Soc Dalton Trans 12:1–83
28. Lucaciu R, Ionescu C, Wildervanck A, Caira MR (2008) Anal Sci
24:87–88
8. Cannizzo A, Mourik FV, Gawelda W, Zgrablic G, Bressler C,
Chergui M (2006) Angew Chem Int Ed 45:3174–3176
9. Juris A, Balzani V, Barigelletti F, Campagna S, Belser P, Zelewsky
AV (1988) Coord Chem Rev 84:85–277
29. Scholz J, Hadi GA, Thiele K-H, Görls H, Weimann R, Schumann
H, Sieler J (2001) J Org Chem 626:243–259
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