Novel intramolecular CArylS bond activation by an electron rich, ring-expanded-NHC-Rh centre: A combined experimental and DFT study

Article abstract of DOI:10.1071/CH11243

The reaction of (o-MeSPh)-N-functionalized tetrahydropyrimidinium salts with KN(SiMe₃)₂ and [Rh(COD)Cl]₂ in THF leads to the formation of a novel dimeric Rh^III bis-carbene complex. The reaction involves the unex

Full text of DOI:10.1071/CH11243

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2011, 64, 1141–1147
Novel Intramolecular CAryl S Bond Activation by
]
an Electron Rich, Ring-Expanded-NHC-Rh centre:
A Combined Experimental and DFT Study
A
A C
,
B C
,
Abeer Binobaid, Kingsley J. Cavell,
Mikhail S. Nechaev,
A
and Benson M. Kariuki
A
School of Chemistry, Main Building, Cardiff University, Cardiff CF10 3AT, Wales, UK.
Department of Chemistry, M.V. Lomonosov Moscow State University,
Leninskie Gory 1/3 and A.V. Topchiev Institute of Petrochemical Synthesis,
RAS, Leninsky Prosp., 29, Moscow, 119991, Russian Federation.
B
C
Corresponding authors. Email: cavellkj@cf.ac.uk; nechaev@nmr.chem.msu.ru
The reaction of (o-MeSPh)-N-functionalized tetrahydropyrimidinium salts with KN(SiMe3)2 and [Rh(COD)Cl]2 in THF
leads to the formation of a novel dimeric RhIII bis-carbene complex. The reaction involves the unexpected cleavage/
oxidative addition of the aryl-sulfur bond to give dimeric metallated RhIII with bridging MeS, moieties. This unusual
reaction is probably a consequence of the sterically imposing structure and strong donor capacity of ring-expanded
N-heterocyclic carbenes (RE-NHCs). An X-ray structure of the [(Ph,DIPP-NHC)Rh(Cl)(SMe)2] product complex has
been obtained, and DFT studies were undertaken to gain an insight into the reaction pathway.
Manuscript received: 15 June 2011.
Manuscript accepted: 1 July 2011.
Introduction
example of the use of a NHC as the ‘spectator’ ligand in a C–S
activation process.
Ring expanded-NHCs (RE-NHCs) offer markedly changed
steric and electronic properties when compared with the more
traditional 5-membered ring NHCs.[1–11] Several investigations
into their structural properties, rich chemistry, and catalytic
applications are now appearing.[1c,d,2,3,6–20] Key features of
these large ring systems are their electronic properties, but more
particularly their substantial steric demands. There is a marked
increase in steric ‘pressure’ on the metal centre as the ring size
increases from 5 through 6 to 7 for a set of related ligands and
complexes,[5,6,7] leading to the interesting coordination chem-
istry and reactivity now noted for these systems. The large ring
NHCs are highly effective in stabilizing novel complexes and
they also promote unusual chemistry and catalytic perfor-
mance;[11–19] for example, recent studies by Whittlesey and
coworkers demonstrated that large ring NHCs can generate
either metallated NiII, or novel NiI complexes depending on
reaction conditions.[11]
Results and Discussion
Preparation of Functionalized Tetrahydropyrimidinium
Salts, and Reaction with a RhI Precursor
The salts 1a and b were prepared in high yield via a stepwise
process, as previously described for other unsymmetrically-
substituted large ring heterocyclic salts.[13]
Reaction of the thioether-functionalized six-membered ring
NHCs, 1a or 1b, with [Rh(COD)Cl]2 led to aryl-S bond cleavage
to give a RhIII methylthio-bridged dimer, 2 (Scheme 1). It would
appear that the reaction involves cleavage of the aryl-thioether
by the electron rich RhINHC centre, followed by coordination
There is a substantial body of literature on C–S bond
activation, largely related to dehydrodesulfurization of hydro-
carbon feedstocks. An excellent review on the early literature
was published in 1994.[21] More recent contributions report on a
range of C–S activation processes (which include thiophenes,
aliphatic thioethers etc. as the substrate).[22–30] To our knowl-
edge there is only one report of aryl-S activation, which appears
to be driven by chelation of the resulting product.[27] We report
here a novel intramolecular aryl-S activation in which a SMe
moiety is dissociated from the benzene ring of a methylthio-
N
N
Ar
BFϪ4
Cl
Rh
ϩ
1 KN(SiMe3)2
N
N
Ar
MeS SMe
2 [Rh(COD)Cl]2
THF
Rh
Cl
SMe
Ar
N
N
Ar ϭ Mes, 1a
DIPP 1b
Ar ϭ Mes, 2a
DIPP 2b
phenyl-substituted RE-NHC, to form
metallated-Rh dimer (Scheme 1). This also represents the first
a
SMe-bridged
Scheme 1. Synthesis of the RhIII dimer.
Ó CSIRO 2011
10.1071/CH11243
0004-9425/11/081141

RESEARCH FRONT
1142
A. Binobaid et al.
C4
C5
C6
C3
C7
C36
N1
C2
C37
C8
C1
C46
C44
C35
CI1
C9
C43
C34
C39
C42
N2
S1
Rh1
C33
C21
C20
S2
Rh2
N4
C22
C19
C15
C40
C32
C45
C23
C12
C18
C31
C12
C13
C24
C30
N3
C14
C28
C29
C25
C27
C26
Fig. 1. Solid state molecular structure of complex 2b. Hydrogen atoms have been omitted for clarity.
o
Table 1. Selected bond lengths (A) and angles ( ) for complex 2b
˚
(metallation) of the resulting benzene ring and formation of the
SMe-bridged RhIII dimer.
Angles [o]
˚
Lengths [A]
The small CNHC–N–Ar angle (a consequence of the very
large N–CNHC–N angle), which allows effective overlap with
the CAr-sp2 orbital, in combination with coordination of the
strong s-donor six-membered ring NHC (6-NHC), which
generates an electron rich RhI centre, facilitates the rapid
intramolecular attack on the MeS-phenyl moiety to form the
ortho-metallated thio-bridged RhIII dimer. Characterisation of
C10–Rh1 1.996(7)
C1–Rh1 1.971(7)
S1–Rh1 2.406(2)
S2–Rh1 2.4302(2)
Cl1–Rh1 2.362(2)
C10–N1 1.334(8)
C10–N2 1.345(8)
C32–Rh2 2.011(7)
C23–Rh2 1.970(7)
S1–Rh2 2.308(2)
S2–Rh2 2.417(2)
Cl2–Rh2 2.370(2)
C32–N3 1.358(9)
C32–N4 1.339(9)
N1–C10–N2 118.0(6)
C10–N1–C6 114.4(6)
C10–Rh1–C1 80.4(3)
C10–Rh1–Cl1 88.2(2)
C1–Rh1–S1 96.2(2)
Cl1–Rh1–S2 171.0(7)
S1–Rh1–S2 82.6(6)
N3–C32–N4 116.8(6)
C32–N3–C23 114.7(7)
C32–Rh2–C23 81.0(3)
C32–Rh2–Cl2 92.6(2)
C23–Rh2–S2 95.3(2)
Cl2–Rh2–S1 171.1(7)
S1–Rh2–S2 82.2(4)
1
these complexes was carried out by H NMR, 13C NMR and
mass spectroscopy and in one case by X-ray analysis. The
13C NMR signals for the carbene carbon appeared as a doublet
at 202.3 ppm for complex 2a and at 202.7 ppm for complex 2b,
indicating the formation of a Rh–CNHC bond. The signals are
within the range of Rh–CNHC resonances observed for related
NHC-Rh complexes. The 1H NMR signals of m-CHAr are
shifted downfield, and the hydrogen atoms in the SCH3 group
appeared as sharp singlets at 1.69 and 1.52 ppm for complex 2a
and 2b, respectively.
Crystals suitable for X-ray crystallography of complex 2b
were obtained by the diffusion of hexane into a saturated THF
solution of the complex. The crystal structure of complex 2b is
shown in Fig. 1 and selected bond distances and angles are
presented in Table 1. The structural arrangement of the complex
shows that the molecular geometry around the rhodium cation is
distorted square pyramidal with two coordination sites occupied
by a bidentate aryl-carbene, a third coordination site is occupied
by chloride (the two chloride atoms are in trans arrangement
overall), and the remaining two sites are occupied by bridging
SCH3 groups.
the CPh(sp2)–Rh bond lengths. The CNHC–Rh–CPh bite-angles
(C10–Rh1–C1) and (C32–Rh2–C23) are 80.4(3)o and 81.0(3)o.
The tilt angles, y, defined by the coordinated (Rh–Cl) atoms and
the N–CNHC–N plane are 79.388 and 86.858.
The specific reaction noted in Scheme 1 appears unique
for the o-SMe-aryl-substituted tetrahydropyrimid-2-ylidene
ligands. No such reaction occurs with the o-OMe-substituted
ligand, which reacts according to Eqn 1, Scheme 2.13 However,
the reaction of the pyridyl-functionalized salt (6-Py-Mes), with
KN(SiMe2)3 and [M(COD)Cl]2, (M ¼ Rh or Ir), leads to the
formation of bis(carbene) complexes [M(6-Py-Mes)2Cl2]
[M(COD)Cl2], (M ¼ Rh, 3 and Ir, 4; Eqn 2, Scheme 2)[13] in
which half the MI in the complex [M(COD)Cl]2 is oxidized to
˚
The Rh–CNHC bond lengths (1.996(7), 2.011(7) A) are
typical for Rh–CNHC (s-bond) complexes and imply a symmet-
ric ligand coordination mode. The two Rh–CNHC bond lengths
are the same, within experimental error, and also very similar to
M
III to give the cation, [M(6-Py-Mes)2Cl2]þ, and electrons are
transferred to chloride ions, which then form the anionic
MI species [M(COD)Cl2]-.

RESEARCH FRONT
Intramolecular CAryl–S Bond Activation: An Experimental and DFT Study
1143
BFϪ4
N
N
ϩ
Cl
1 KN(SiMe3)2
N
N
Eq 1
Rh
OMe
2 [Rh(COD)Cl]2
THF
OMe
[M(COD)Cl2]
BFϪ4
N
Cl
N
R
N
ϩ
1 KN(SiMe3)2
N
N
M
Eq 2
N
2 [M(COD)Cl]2
THF
Cl
N
N
R
N
R ϭ Mes; M ϭ Rh
M ϭ Ir
3
4
Scheme 2. Reaction of N-functionalized tetrahydropyrimidinium salts with KN(SiMe3)2 and [M(COD)Cl]2.
MeS
MeS
Dipp
SMe
Cl
N
Cl
N
N
N
N
N
N
S
N
Rh
Rh
Rh Cl
iPr
Rh
Cl
Dipp
Rh
N
S
N
Cl
Dipp
Dipp
N
Dipp
iPr
22.5
B
0.0
A
23.5
C
23.1
D
Cl
S
N
Rh
Rh
N
S
N
Cl
Dipp
Dipp
N
30.3 (7.1)
TS D-H
Cl
Rh
S
SMe
SMe
SMe
N
N
N
N
S
N
N
N
Rh
Cl
Dipp
Rh
Rh
Cl
Dipp
Rh
Cl
N
N
Cl
Dipp
Dipp
N
Dipp
35.4 (35.4)
TS A-E
25.8 (3.3)
TS B-F
38.1 (14.6)
TS C-G
12.9
H
Cl
S
N
Rh
Rh
S
N
N
Cl
Dipp
Dipp
N
16.5 (3.6)
TS H-I
Cl
Rh
N
N
N
N
SMe
SMe
S
Rh
Cl
N
SMe
Rh
Cl
Dipp
Rh
Cl
Dipp
Rh
Cl
Dipp
N
S
N
N
N
Dipp
11.4
E
10.5
F
11.5
G
-5.5
I
Scheme 3. Calculated possible reaction pathways leading to 2b (I). Numbers are Gibbs free energies Gvr in kcal molꢀ1. Numbers in brackets are relative free
energies of transition states. The structures within the dashed lines represent the lowest energy reaction path.
The detailed mechanism for the process depicted in Scheme 1
is unclear. However, in broad terms, it is thought that in situ-
generated NHC coordinates to the RhI to generate an extremely
electron rich metal centre, which then undergoes rapid intra-
molecular oxidative addition of the MeS-phenyl moiety to form
the o-metallated thio-bridged RhIII dimer. It would appear that
these RE-NHCs are extremely powerful electron donors with
structural features that promote oxidation of the metal centre
and metallation of the N-substituent. Oxidation of the metal
centre was also noted for the pyridine-functionalized NHC in
Scheme 2. To help better understand the process occurring, we
have undertaken a computational study of the reaction depicted
in Scheme 1.
Computational Studies: Rhodium Insertion into C–S Bond
DFT calculations were performed in order to gain better insight
into the mechanism of the reaction. The various possible path-
ways that have been considered for the formation of 2b are
depicted in Scheme 3.

RESEARCH FRONT
1144
A. Binobaid et al.
It was proposed that in situ-generated carbene reacts with
[Rh(COD)Cl]2 to give RhI complex A. This compound was
chosen as the energy reference. We considered several
possible RhI species. Complex B bears a COD ligand coordi-
nated to rhodium with only one double bond. The rhodium atom
coordination sphere is saturated by Z2-coordination of the
2-(MeS)-Ph- group. Its formation from A is endothermic by
polarization functions [5,1,1,1,1/5,1,1,1,1/5,1,1,1] for Rh,
[3,1,1/3,1,1/1,1] for Cl, S, N, C and [3,1,1/1] for H were used.
The innermost electrons of Rh, Cl, S, N and C atoms were
treated using ECP-SBKJC effective core potentials.[34–36]
Vibrational frequencies were calculated for all stationary points
either as minima (i ¼ 0) or first order transition states (i ¼ 1).
Vibrational and rotational movements were considered in the
estimation of the free energies Gvr ¼ Gvib þ Grot. Translational
movements were not taken into consideration since the reaction
studied was carried out in solution, in which the translations are
mostly suppressed. The IRC procedure was used for reliable
identification of the transition states.[37] All calculations were
performed using the PRIRODA program.[38,39]
22.5 kcal molꢀ1
.
The release of COD from B to give C is endothermic by
1.0 kcal molꢀ1. In C the rhodium atom is s-coordinated to SMe,
Cl and the carbene carbon, and Z2-coordinated to the bulky Dipp
group. The formation of a dimer D from a monomeric C leads to
only 0.4 kcal molꢀ1 energy gain. In D two RhI atoms are bridged
by SMe groups. All complexes B-D lie significantly higher in
energy than the initial complex A.
Conclusions
For all complexes A-D insertion of the rhodium into the C–S
bond was considered. For complexes A-C there is only one
transition state leading to the RhIII species, while for dimer D a
two-step process with intermediate formation of mixed valence
RhI/ RhIII complex H was considered.
It is now apparent from evidence presented here that these very
basic (strong donor) and sterically highly demanding large-ring
NHCs can lead to unique chemistry and the formation of
unusual/unexpected structures. Here we have reported an
unexpected C–S bond activation via oxidative addition of a
CAryl–SMe bond to a RhI centre to give a novel a SMe-bridged
RhIII complex. The new complexes have been fully character-
ized, including an X-ray crystallographic structure. Importantly,
DFT calculations indicate that the reaction follows a pathway in
which the main energy barrier is the dissociation of the COD
ligand. Based on these results and on previous studies by us and
by others, it can be expected that this novel class of RE-NHC
ligands will provide further unusual chemistry and new oppor-
tunities and applications.
Transition state TS A-E corresponds to C–S bond activation
in A to give the RhIII complex E. The resulting complex lies
11.4 kcal molꢀ1 higher in energy than the starting complex A. In
contrast to the transition from A to B (þ22.5 kcal molꢀ1) the loss
of one coordination bond to the COD ligand in E yielding F is
energetically favourable (ꢀ0.9 kcal molꢀ1). The release of COD
to give complex G is endothermic by only 1.0 kcal molꢀ1
.
Dimerization of the complex G gives the final bimetallic
complex I, which was observed experimentally.
Transition from G to I is characterized by a significant
decrease in free energy (Gvr) of ꢀ17.0 kcal molꢀ1. It is of
interest to compare this with the formation of a RhI dimer D,
for which DGvr is only ꢀ0.4 kcal molꢀ1. As a result, formation
of the RhIII dimer is a driving force for the total reaction. All
intermediates B-H lie higher in energy than the starting complex
A, and only complex D is lower in energy than A.
Insertion of rhodium into the C–S bond in A, without prior
dissociation of COD, is associated with a high energy barrier of
35.4 kcal molꢀ1 (TS A-E). We suppose that this can be
explained by high steric crowding of the rhodium coordination
sphere by the COD and NHC ligands. For other RhI species, the
energy barriers are significantly lower, from 14.6 kcal molꢀ1 for
TS C-G down to only 3.3 kcal molꢀ1 for TS B-F.
Experimental Section
General Remarks: All manipulations were performed using
standard Schlenk techniques under an argon atmosphere, except
where otherwise noted. [Rh(COD)Cl]2, was synthesized
according to literature methods. Solvents were dried using a
Braun SPS system (hexane, CH2Cl2) or a Vacuum Atmospheres
recirculating SPS system (THF). Deuterated solvents for NMR
measurements were distilled from the appropriate drying agents
under N2 immediately before use, following standard literature
methods. Air-sensitive compounds were stored and weighed in a
glovebox. All other reagents were obtained commercially and
1
used as received. H and 13C NMR spectra were obtained on
Transition state TS B-F corresponds to the total energy
barrier (25.8 kcal molꢀ1) of the lowest energy reaction pathway:
A - B - TS B-F - F - G - I. Notably, by far the major
share of this energy is due to the loss of one coordination arm of
the chelating COD ligand. Thus, if a less strongly bound ligand
was chosen, the total energy barrier would be expected to be
significantly reduced.
As a result, DFT modelling clearly shows that if the coordi-
nation sphere of the RhI complex is suitably preorganized,
insertion of RhI into the CAr–S bond can readily proceed with
a very low energy barrier. We expect that this observation can be
utilized for the design of catalytic transformations of organo-
sulfur compounds.
Bruker Advance AMX 400, 500 or Jeol Eclipse 300 spectro-
meters, and referenced to tetramethylsilane (d ¼ 0 ppm).
Coupling constants J are expressed in Hz. HRMS were
obtained on a Waters LCT Premier XE instrument and are
reported as m/z (%).
Crystallographic Data
Crystallographic data has been deposited with the Cambridge
Crystallographic Data Centre (CCDC deposition number:
830919)
Data collection was carried out at 150K on a Bruker-Nonius
Kappa CCD diffractometer equipped with an Oxford Cryosys-
tems cooling apparatus and graphite monochromated Mo Ka
˚
radiation (l ¼ 0.71073 A). The structure was solved by direct
methods and refined with SHELX-97.[40] All non-hydrogen
atoms (except solvent and disordered Cl atoms) were refined
anisotropically and the hydrogen atoms were inserted in ideal-
ized positions with Uiso set at 1.2 or 1.5 times the Ueq of the
parent atom.
Computational Procedure
Computed structures are designated by letters, (e.g. A, TS-
ABy) to differentiate them from the real complexes. Geometry
optimization was carried out using the PBE generalized gradient
functional.[31–33] Triple-x valence basis sets including TZ2P

RESEARCH FRONT
Intramolecular CAryl–S Bond Activation: An Experimental and DFT Study
1145
C50H68Cl2N4Rh2S2, 2(C4H8O), FW¼ 1154.03, t ¼ 150(2)K,
Preparation of 1-(2-methylthiophenyl)-3-
(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydro-3H-
[1,3]pyrimidinium bromide
˚
˚
Monoclinic, C2/c, a¼ 43.1080(3) A, b ¼ 14.0820(4)A,
3
˚
˚
c ¼ 18.2190(6)A, b ¼ 97.6640(10), V ¼ 10961.0(5)A , Z ¼ 8,
s(cal) ¼ 1.399 Mg/m3, Crystal size ¼ 0.10 ꢁ 0.10 ꢁ 0.06 mm3,
Total reflections ¼ 28680, Independent reflections¼ 9317,
R(int) ¼ 0.0679, R1 ¼ 0.0735, wR2 ¼ 0.1485.
N-(2-methylthiophenyl)-N0-(2,4,6-trimethylphenyl)formamidine
(2.84 g), K2CO3 (0.7 g, 5.0 mmol) and 2.03 mL of 1,3-
dibromopropane (20.0 mmol) in 250 mL acetonitrile was heated
under reflux for two weeks to yield 3.10 g (76 %) of white,
crystalline material. dH (CDCl3, 400 MHz, 298K) 8.06 (d, 3JHH
6.7, 1H, o-CHo-Mesph), 7.55 (s, 1H, NCHN), 7.41 (t, 3JHH 6.2, 1H,
m-CHo-Mesph), 7.21 (t, 3JHH 6.2, 1H, p-CHo-Mesph), 7.18 (d, 3JHH
6.7, 1H, m-CHo-Mesph), 6.88 (s, 2H, m-CHMes), 4.29 (t, 3JHH 5.2,
2H, NCH2), 4.05 (t,3JHH 5.2, 2H, NCH2), 2.61 (m, 2H,
NCH2CH2), 2.49 (s, 3H, SCH3 o-Mesph), 2.37 (s, 6H, o-CH3 Mes),
2.23 (s, 3H, p-CH3 Mes). dC (CDCl3, 100 MHz, 298K) 154.3
(s, NCHN), 140.3 (s, Ar-Co-Mesph), 137.7 (s, Ar-CMes), 136.7
(s, Ar-CMes), 135.6 (s, Ar-CMes), 134.7 (s, Ar-Co-Mesph), 131.1
(s, Ar-CHo-Mesph), 129.9 (s, Ar-CHMes), 128.9 (s, Ar-CHo-Mesph),
126.5 (s, Ar-CHo- Mesph), 125.8 (s, Ar-CHo-Meoph), 47.4
(s, NCH2), 46.8 (s, NCH2), 20.9 (s, NCH2CH2), 19.7 (s, p-CH3 Mes),
18.4 (s, o-CH3 Mes) 14.9 (s, SCH3 o-Mesph). m/z (ES) 325.1748,
Calc. for C23H31N2O: 325.1738. Anal. Calc. for C20H25N2OBr:
C 58.27, H 6.07, N 6.80. Found: C 58.09, H 6.07, N 6.99 %.
Preparation of Ethyl(2-methylthiophenyl)formamidate
2-(Methylmercapto)aniline (27.8 g, 0.20 mol), triethylortho-
formate (50 mL; excess) and two drops of 2 M HCl were charged
into a 200 mL flask. The flask was heated slowly using a heating
mantle. At approximately 1108C, ethanol began to distil. When
95 % (22 mL) of the theoretical amount of ethanol had been
collected, the flask was allowed to cool slowly. The excess of
triethylorthoformate was removed by vacuum distillation at
(60–808C, 10 hPa). Upon further heating the final product, a pale
yellow liquid distilled at (120–1508C). Yield 21 g (64 %). dH
(CDCl3, 400 MHz, 298K) 7.53 (s, 1H, NCH), 7.00 (d, 1H,
o-CH), 6.95 (t, 1H, m-CH), 6.92 (t, 1H, p-CH), 6.64 (d, 2H,
m-CH), 4.27 (m, 2H, OCH2CH3), 2.28 (s, 3H, SCH3) 1.31 (t, 3H,
OCH2CH3). dC (CDCl3, 100 MHz, 298K) 155.4 (s, NCHN),
145.5 (s, Ar-C), 133.3 (s, Ar-C), 125.4 (s, Ar-CH), 125.2
(s, Ar-CH), 124.7 (s, Ar-CH), 119.6 (s, Ar-CH), 63.1
(s, OCH2CH3), 14.9 (s, SCH3), 14.7 (s, OCH2CH3).
Preparation of 1-(2-methylthiophenyl)-3-
(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydro-3H-
[1,3]pyrimidinium tetrafluoroborate 1a
A solution of pyrimidinum bromide salt (2.42 g, 6.0 mmol) in
30 mL acetonitrile was mixed with a solution of sodium tetra-
fluoroborate (0.99 g, 9.0 mmol) in 30 mL distilled water. White
crystals formed (2.15 g, 87 %). dH (CDCl3, 400 MHz, 298K)
7.68 (d, 3JHH 6.5, 1H, o-CHo-Mesph), 7.44 (s, 1H, NCHN), 7.38
(t, 3JHH 6.4, 1H, m-CHo-Mesph), 7.21 (t, 3JHH 6.4, 1H, p-CHo-Mesph),
7.18 (d, 3JHH 6.5, 1H, m-CHo-Mesph), 6.86 (s, 2H, m-CHMes), 3.94
(t, JHH 5.4, 2H, NCH2), 3.81 (t, JHH 5.4, 2H, NCH2), 2.52
(m, 2H, NCH2CH2), 2.46 (s, 3H, SCH3 o-Mesph) 2.30 (s, 6H,
o-CH3 Mes), 2.21 (s, 3H, p-CH3 Mes). dC (CDCl3, 100 MHz,
298K) 154.1 (s, NCHN), 140.4 (s, Ar-Co-Mesph), 137.8 (s, Ar-
CMes), 136.7 (s, Ar-CMes), 135.7 (s, Ar-CMes), 134.8 (s, Ar-
Preparation of N-(2-methylthiophenyl)-N‘-(2, 4, 6-
trimethylphenyl) formamidine
A 50-mL acid-free flask was charged with ethyl (2-
methylthiophenyl)formamidate (5.85 g, 30.0 mmol) and 2,4,6-
trimethylaniline (4.05 g, 30.0 mmol). The mixture solidified
after stirring for 3 h at 508C. The residue was recrystallized from
toluene affording white crystals. Yield 7.3 g (86 %). dH (CDCl3,
400 MHz, 298k) 7.68 (d, 1H, o-CHo-Mesph), 7.58 (s, 1H, NCHN),
7.25 (t, 1H, m-CHo-Mesph), 7.07 (t, 1H, p-CHo-Mesph), 6.92 (d, 1H,
m-CHo-Mesph), 6.83 (s, 2H, m-CHMes), 2.33 (s, 3H, SCH3 o-Mesph),
2.18 (s, 3H, p-CH3 Mes), 2.04 (s, 6H, o-CH3 Mes). dC (CDCl3,
100 MHz, 298k): d 149.6 (s, NCHN), 133.8 (s, Ar-Co-Mesph),
131.0 (s, Ar-CMes), 128.9 (s, Ar-CMes), 128.2 (s, Ar-CMes), 127.7
(s, Ar-Co-Mesph), 125.3 (s, Ar-CHo-Mesph), 123.1 (s, Ar-CHMes),
119.3 (s, Ar-CHo-Mesph), 116.6(s, Ar-CHo-Mesph), 114.6
(s, Ar-CHo-Mesph),18.6 (s, p-CH3), 17.3(s, o-CH3), 14.2 (s, SCH3).
3
3
Co-Mesph), 131.1 (s, Ar-CHo-Mesph), 130.1 (s, Ar-CHMes), 128.3
(s, Ar-CHo-Mesph), 126.7 (s, Ar-CHo-Mesph), 125.9 (s, Ar-
CHo-Mesph), 46.8 (s, NCH2), 46.4 (s, NCH2), 20.9 (s, NCH2CH2),
19.4 (s, p-CH3 Mes), 17.7 (s, o-CH3 Mes) 14.9 (s, SCH3 o-Mesph).
Preparation of 1-(2-methylthiophenyl)-3-
(2,6-diisopropylphenyl)-3,4,5,6-tetrahydro-3H-
[1,3]pyrimidinium bromide
Preparation of N-(2-methylthiophenyl) -N‘-
(2,6-diisopropylphenyl) formamidine
A
50-mL acid-free flask was charged with ethyl(2-
N-(2-methylthiophenyl)-N0-(2,6-diisopropylphenyl) formami-
dine (3.26 g), K2CO3 (0.7 g, 5 mmol) and 2.03 mL of 1,3-
dibromopropane (20 mmol) in 250 mL acetonitrile was heated
under reflux for three weeks to yield 3.25 g (72 %) of white,
crystalline material. dH (CDCl3, 400 MHz, 298 K) 7.88
methylthiophenyl) formamidate (5.85 g, 30.0 mmol) and 2,6-
diisopropylaniline (5.32 g, 30.0 mmol). The mixture solidified
after stirring for 3 h at 508C. The residue was recrystallized from
toluene affording white crystals. Yield 6.2 g (63 %). dH (CDCl3,
400 MHz, 298 K) 7.93 (d, 1H, o-CHo-Mesph), 7.71 (s, 1H,
NCHN), 7.16 (t, 1H, m-CHo-Mesph), 7.12 (t, 1H, p-CHDipp), 7.05
(d, 2H, m-CHDipp), 6.92 (t, 1H, p-CHo-Mesph), 6.83 (d, 1H,
m-CHo-Mesph), 3.13 (sept., 2H, CH(CH3)2 Dipp), 2.34 (s, 3H,
SCH3 o-Mesph), 1.12 (d, 12H, CH(CH3)2 Dipp). dC (CDCl3,
100 MHz, 298k) 155.5 (s, NCHN), 142.6 (s, Ar-Co-Mesph), 133.8
(s, Ar-CDipp), 132.8 (s, Ar-CDipp), 129.4 (s, Ar-Co-Mesph), 125.5
(s, Ar-CHo-Mesph), 124.7 (s, Ar-CHDipp), 123.9 (s, Ar-CHDipp),
119.6 (s, Ar-CHo-Mesph), 118.9 (s, Ar-CHo-Mesph), 114.2
(s, Ar-CHo-Mesph, 28.4 (s, CH(CH3)2 Dipp) 24.1 (s, CH(CH3)2),
18.5 (s, CH(CH3)2), 14.9 (s, SCH3)).
3
(d, JHH 6.7, 1H, o-CHo- Meoph), 7.56 (s, 1H, NCHN), 7.37
(t, 3JHH 6.8, 1H, m-CHo-Meoph), 7.34 (t, 3JHH 7.8, 1H, p-CHDipp),
3
7.20 (d, JHH 7.8, 2H, m-CHDipp), 7.04 (t, JHH 6.8, 1H, p-
3
3
CHo-Meoph), 6.94 (d, JHH 6.7, 1H, m-CHo-Meoph), 4.38
(t, 3JHH 5.5, 2H, NCH2), 4.02 (t, 3JHH 5.5, 2H, NCH2), 3.83 (s, 3H,
OCH3 o-Meoph), 3.14 (sept., 3JHH 6.8, 2H, CH(CH3)2 Dipp), 2.59
(m, 2H, NCH2CH2), 1.27 (d, 3JHH 6.8, 6H, CH(CH3)2 Dipp), 1.17
(d, 3JHH 6.8, 6H, CH(CH3)2 Dipp). dC (CDCl3, 100 MHz, 298k)
154.3 (s, NCHN), 152.9 (s, Ar-Co-Meoph), 149.3 (s, Ar-CDipp),
136.7 (s, Ar-CDipp), 131.6 (s, Ar-Co-Meoph), 131.5 (s, Ar-
CHo-Meoph), 129.7 (s, Ar-CHDipp), 128.1 (s, Ar-CHDipp), 125.5

RESEARCH FRONT
1146
A. Binobaid et al.
(s, Ar-CHo-Meoph), 122.4 (s, Ar-CHo-Meoph), 112.5 (s, Ar-
CHo-Meoph), 56.6 (s, OCH3), 48.9 (s, NCH2), 48.4 (s, NCH2),
28.9 (s, CH(CH3)2), 25.1 (s, NCH2CH2), 24.8 (s, CH(CH3)2),
20.0 (s, CH(CH3)2). m/z (ES) 367.2216, Calc. for C23H31N2O:
367.2208. Anal. Calc. for C23H31N2OBr: C 60.81, H 6.83,
N 6.17. Found: C 60.62, H 6.81, N 6.15 %.
C46H60N4S2Rh2Cl2.0.5CH2Cl2: C 53.01, H 5.84, N 5.34. Found:
C 52.65, H 5.84, N 6.31 %.
Accessory Publication
The Accessory Publication contains the CIF file and checkCIF
file for complex 2b. Computational data (Cartesian coordinates,
energies of ground and transition states, and imaginary fre-
quencies for transition states) are also included. The Accessory
Publication is available on the Journal’s website.
Synthesis of complexes
Synthesis of complex 2a
KNSi(Me3)2 (82 mg, 0.41 mmol) and (6-o-MeSPh-Mes)BF4
salt, 1a (0.168 mg, 0.41 mmol) were placed in a Schlenk tube
followed by the addition of THF (15 mL). The solution was
stirred for 30 min and subsequently filtered into another Schlenk
tube containing a THF solution (10 mL) of [Rh(COD)Cl]2
(0.20 mmol). An immediate colour change was observed from
light to dark yellow. After the reaction mixture was stirred at
room temperature for 4 h, the volatiles were removed under
reduced pressure. The yellow solid obtained was washed with
hexane (2 ꢁ 10 mL) and dried. The yield was 0.114 g (60 %). dH
(CD2Cl2, 500 MHz, 298 K) 7.56 (t, 1H, m-CHo-MeSPh), 7.45
(d, 1H, o-CHo-MeSPh), 7.34 (d, 1H, m-CHo-MeSPh), 6.91 (t, 1H,
p-CHo-MeSPh), 6.84 (s, 1H, m-CHMes), 6.80 (s, 1H, m-CHMes),
3.83 (t, 2H, NCH2), 3.72 (t, 2H, NCH2), 2.38 (m, 2H,
NCH2CH2), 2.25 (s, 3H, o-CH3), 1.69 (s, 3H, SCH3), 1.67 (s,
3H, o-CH3), 1.61 (s, 3H, p-CH3). dC (CD2Cl2, 100 MHz, 298 K)
202.3 (d, NCRhN), 149.8 (s, Ar-Co-MeSPh), 148.0 (s, Ar-CMes),
140.3 (s, Ar-CMes), 140.1 (s, Ar-CMes), 137.3 (s, Ar-CMes), 136.4
(s, Ar-Co-MeSPh), 132.9 (s, Ar-CHo-MeSPh), 123.4 (s, Ar-CHMes),
122.2 (s, Ar-CHMes), 121.9 (s, Ar-CHo-MeSPh), 120.6 (s, Ar-
CHo-MeSPh), 108.3 (s, Ar-CHo-MeSPh), 45.4 (s, NCH2), 44.2 (s,
NCH2), 20.5 (s, NCH2CH2), 19.2 (s, p-CH3), 18.3 (s, o-CH3),
17.7 (s, o-CH3), 9.6 (s, SCH3). m/z (HR-ES) 930.1425 [M-Cl)]þ,
C40H48N4S2Rh2ClCH3CN requires 930.1384. Anal. Calc. for
C40H48N4S2Rh2Cl2: C 51.90, H 5.23, N 6.05. Found: C 52.67,
H 5.84, N 6.24 %.
Acknowledgements
We thank the Saudi Arabian Government for generously providing the PhD
stipend and research costs (for A. B.). Support from the Engineering and
Physical Sciences Research Council (EPSRC) is gratefully acknowledged.
Johnson-Matthey is also thanked for the generous supply of precious
metal salts.
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The same procedure as described for the synthesis of 2a was
employed using KNSi(Me3)2 (82 mg, 0.41 mmol), (6-o-MeSPh-
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100 MHz, 298 K) 0.202.7 (d, NCRhN), 150.7 (s, Ar-Co-MeSPh),
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(s, Ar-CHo-MeSPh), 109.5 (s, Ar-CHo-MeSPh), 50.4 (s, NCH2),
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