Synthesis, structure and DFT study of asymmetrical NHC complexes of cymantrene derivatives and their application in the dehydrogenative dimerization reaction of thiols

Article abstract of DOI:10.1016/j.jorganchem.2017.03.047

Asymmetrical NHC complexes of cymantrene and methylcymantrene have been synthesised through the photochemical substitution of carbonyl ligands in the presence of the imidazol-2-ylidene salts. The carbene substituents have been varied between compact (L1) and bulky (L2 and L3) substituents to produce an array of differently sized carbene ligands. The solid state crystal structures of three of the complexes confirmed the bonding pattern of the ligand towards available metal centres and an in-depth DFT study provided insight into electronic and steric aspects. Application of the Extended Transition State coupled with Natural Orbitals for Chemical Valence (ETS-NOCV) energy decomposition technique indicated various NOCV channels for each Mn–L bond (L?=?3-ethyl-1-methylimidazolylidene carbene, triphenylphosphine or acetonitrile), describing the density and energy changes of specific (σ and π) attributes of each bond. N-heterocyclic carbenes of cymantrene derivatives and their triphenylphosphine-substituted analogue are equivalent σ-donors, with 53% and 56%, respectively, of the total binding energy originating from σ-donation. However, NHC complexes show considerably less π character in the metal-carbene bond. Hydrogen interactions in the NHC complexes were also identified in the ETS-NOCV calculations and provided quantification of the hydrogen interactions witnessed in the solid state structures. Calculated Wiberg bond indices, bond dissociation enthalpies, percentage buried volumes and percentage sigma and pi-bonding characteristics quantified the bonding and electronic aspects of the ligand-metal interactions within the complex.

Full text of DOI:10.1016/j.jorganchem.2017.03.047

Journal of Organometallic Chemistry 840 (2017) 11e22
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, structure and DFT study of asymmetrical NHC complexes of
cymantrene derivatives and their application in the dehydrogenative
dimerization reaction of thiols
*
ꢀ
Roan Fraser, Petrus H. van Rooyen, Jurgens de Lange, Ignacy Cukrowski, Marile Landman
Department of Chemistry, University of Pretoria, Private Bag X20, Hatfield, 0028, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Asymmetrical NHC complexes of cymantrene and methylcymantrene have been synthesised through the
photochemical substitution of carbonyl ligands in the presence of the imidazol-2-ylidene salts. The
carbene substituents have been varied between compact (L1) and bulky (L2 and L3) substituents to
produce an array of differently sized carbene ligands. The solid state crystal structures of three of the
complexes confirmed the bonding pattern of the ligand towards available metal centres and an in-depth
DFT study provided insight into electronic and steric aspects. Application of the Extended Transition State
coupled with Natural Orbitals for Chemical Valence (ETS-NOCV) energy decomposition technique indi-
cated various NOCV channels for each MneL bond (L ¼ 3-ethyl-1-methylimidazolylidene carbene, tri-
Received 11 January 2017
Received in revised form
15 March 2017
Accepted 25 March 2017
Available online 28 March 2017
Keywords:
N-heterocyclic carbene
Cymantrene derivatives
DFT
Steric parameters
Bond order
phenylphosphine or acetonitrile), describing the density and energy changes of specific (
attributes of each bond. N-heterocyclic carbenes of cymantrene derivatives and their
triphenylphosphine-substituted analogue are equivalent -donors, with 53% and 56%, respectively, of the
total binding energy originating from -donation. However, NHC complexes show considerably less
s and p)
s
s
p
character in the metal-carbene bond. Hydrogen interactions in the NHC complexes were also identified
in the ETS-NOCV calculations and provided quantification of the hydrogen interactions witnessed in the
solid state structures. Calculated Wiberg bond indices, bond dissociation enthalpies, percentage buried
volumes and percentage sigma and pi-bonding characteristics quantified the bonding and electronic
aspects of the ligand-metal interactions within the complex.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
subsequent isomerisation of ruthenium NHC complexes have been
reported and indicate the ability of photochemical processes to
Since the isolation of the first stable, free N-heterocyclic carbene
(NHC) by Arduengo in 1991, NHC's have transformed the field of
organometallic chemistry [1]. The adjustability of the steric and
electronic properties of these ligands has resulted in their persis-
tent use for the synthesis and stabilization of metal complexes
capable of cleaving very inert bonds [2]. The majority of synthetic
pathways in the synthesis of transition metal (TM) NHC complexes
have been thermally initiated [3] whereas photochemical induced
pathways have, however, been mostly ignored. This aspect is sur-
prising, especially if taken into consideration the similarities be-
tween tertiary phosphines and NHC ligands and the applicability of
photochemical reactions in producing highly reactive, coor-
dinatively unsaturated TM-PR3 species [3]. The photolysis and the
produce isomeric variations of target complexes (Fig. 1) [3].
Methylcyclopentadienyl manganese tricarbonyl (MMT), and to a
far lesser extent, cymantrene have been successfully applied in the
fuel industry as supplements in unleaded gasoline. MMT not only
increases octane ratings but are particularly useful as anti-knocking
agents, improving combustion of fuels [4]. Studies on carbene
complexes of MMT and cymantrene complexes are limited in
literature and mainly focus on the synthesis of novel, symmetrical
N-heterocyclic carbene (NHC's) complexes of MMT and cymantrene
(Fig. 2) [4e7]. The synthesis of symmetrical NHC complexes of MMT
has been reported as early as 1977 when Lappert [8] prepared
complexes of manganese from the Wanzlick dimeric NHC ligand
under high thermal conditions. The complexes were stable above
180 ꢀC and displayed exceptional inertness against thermal
decomposition. A small variety of symmetrical NHC cymantrene
derived carbene complexes have been synthesised by means of
photochemical processes and have even found application in the
* Corresponding author.
E-mail address: marile.landman@up.ac.za (M. Landman).
http://dx.doi.org/10.1016/j.jorganchem.2017.03.047
0022-328X/© 2017 Elsevier B.V. All rights reserved.

12
R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
Fig. 1. Isomerisation of Ru-NHC complexes afforded through photochemical processes [3].
Fig. 2. NHC complexes of cymantrene and MMT (a) and agostic interaction of the cymantrene NHC complex (b).
hydrosilylation of aldehydes and ketones [4]. The study found that
the incorporation of at least one mesityl group leads to the most
selective and active systems. DFT calculations also indicated the
presence of an agostic stabilization when a second carbonyl ligand
is removed by photolytic irradiation [5].
Since agostic interactions are also witnessed in the triphenyl-
phosphine analogue [5] and both Cp'Mn(CO)PPh3 and Cp'Mn(CO)
NHC stabilize carbonyl expulsion through agostic interactions
rather than with solvent ligation [6], both analogues can be seen as
quite similar in chemical nature. Although the synthetic method-
ology and catalytic potential of symmetric NHC complexes of
cymantrene and MMT have been determined for a variety of
different reactions and conditions, very limited understanding ex-
ists for the structural and electronic effects of different unsym-
metrical NHC ligands towards the metal centre. For this purpose,
asymmetrical NHC complexes with increasing steric bulk have been
synthesised (Fig. 4) and studied with theoretical calculations. This
study reports the synthesis of the six novel asymmetrical NHC
complexes of cymantrene and MMT, the synthesis of the acetoni-
trile and triphenylphosphine analogues of the NHC complexes, X-
ray crystal structures of three of the complexes and a DFT study
(Fig. 3). The triphenylphosphine and acetonitrile analogues were
synthesised to form a baseline to which the percentage sigma and
pi-bonding characteristics of the NHC complexes could be
compared.
2. Experimental
2.1. General
All reactions, unless noted otherwise, were performed under
inert nitrogen or argon atmospheres using typical Schlenk tech-
niques [9]. All solvents used, were freshly distilled, dried and
collected under inert conditions. Column chromatography was
carried out under inert nitrogen and argon atmospheres using silica
gel (particle size 0.063e0.200 mm) as the stationary phase. Per-
centage yields were calculated relative to the limiting reactant.
Crystallization was done using hexane:DCM or hexane:diethyl
ether diffusion methods. The reagents CpMn(CO)3, MeCpMn(CO)3,
n-butyl lithium (1.6 M solution in hexane) and other commercial
reagents were used as purchased. Complexes B1 [10] and C1 [11]
were synthesised according to literature procedures. NMR spectra
were recorded on a Bruker ARX-300. NMR spectra were recorded in
Fig. 3. Cymantrene derived NHC complexes of this study.

R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
13
Fig. 4. NHC ligands used in this study.
CD3CN using the deuterated solvent peak as internal reference. The
1H and 13C NMR spectra were measured at 300.1 and 75.5 MHz,
respectively. IR spectra were recorded on a Perkin Elmer Spectrum
RXI FT-IR spectrophotometer as KBr pellets or in hexane and only
the vibration bands in the carbonyl-stretching region (ca. 1500-
2200 cmꢁ1) are reported.
ylidene L2 (0.481 g, 3.0 mmol) was added to photo-irradiated
MeCpMn(CO)3 (0.654 g, 3.0 mmol). A single yellow solid product
of complex 4 was isolated (yield: 0.255 g; 27%).1H NMR, ppm
(CD3CN):
d
6.85 (s, 1H, NCH¼CHN), 6.83 (s, 1H, NCH¼CHN), 3.96 (s,
2H, Cp), 3.86 (s, 3H, NCH3), 3.85 (s, 2H, Cp), 3.76 (s br, 2H,
NCH(CH3)2), 2.89 (s, 3H, Cp-CH3), 1.33 (d, 6H, JHH ¼ 6.3, NCH(CH3)2).
13C{1H} NMR, ppm:
d
235.1 Mn(CO)2, 195.0 NCN, 124.9 NC¼CN,
2.2. Synthesis of complexes 1-6
124.1 NC¼CN, 34.7 Cp-CH3, 102.8 Cp, 81.9 Cp, 80.4 Cp, 60.5 NCH3,
52.4 NCH(CH3)2, 13.7 NCH(CH3)2. IR (cmꢁ1): 1943
HRESIþ-MS, m/z: 317.1081 (calcd. 317.1062).
n(CO), 1852 n(CO).
2.2.1. Synthesis of 1, CpMn(CO)2L1
All complexes were synthesised according to a similar meth-
odology as reported by Lugan [4]. CpMn(CO)3 (0.612 g, 3.0 mmol)
was dissolved in 40 ml of dry THF and irradiated for 60 min at room
temperature. Imidazol-2-ylidene L1 (0.714 g, 3.0 mmol) was added
to the reaction mixture via cannula and the reaction allowed to stir
for 90 min. The reaction colour turned to deep brown and the
mixture was subsequently filtered through a small aluminium ox-
ide column. The solvent was evaporated, the product redissolved
and purified with column chromatography with hexane:ether
eluent to recover 1 as an orange oil. Yield (0.267 g (31%) %) 1H
2.2.5. Synthesis of 5, CpMn(CO)2L3
A similar methodology as for 1 was followed. Imidazol-2-
ylidene L1 (0.801 g, 3.0 mmol) was added to photo-irradiated
MeCpMn(CO)3 (0.612 g, 3.0 mmol). A single yellow solid product
of complex 3 was isolated (yield: 0.381 g; 35%). 1H NMR, ppm
(CD3CN):
NHC¼CHN), 4.44 (m, 2H, NCH2Bn); 4.38 (s, 5H, Cp), 3.75 (s, 3H,
NCH3), 3.03 (m, 2H, NCH2CH2Ph). 13C{1H} NMR, ppm:
235.2
Mn(CO)2, 195.1 NCN, 138.9 Ph(ipso), 129.3 Ph(meta), 129.0 Ph(ortho)
d
7.45 (s, 1H, NHC¼CHN), 7.34 (s br, 5H, Ph), 7.26 (s, 1H,
d
,
NMR(CD3CN):
d
7.33 (s, 1H, NCH¼CHN), 7.30 (s, 1H, NHC¼CHN),
127.0 Ph(para), 124.5 NC¼CN, 122.1 NC¼CN, 82.7Cp, 64.4 NCH3, 52.0
4.44 (s, 5H, Cp), 4.27 (q, 2H, JHH ¼ 7.3, NCH2CH3), 3.75 (s, 3H, NCH3),
NCH2Bn, 40.8 NCH2CH2Ph,. IR (cmꢁ1): 1921
HRESIþ-MS, m/z: 365.1033 (calcd. 365.1062).
n(CO), 1855 n(CO).
1.30 (t, 3H,, JHH ¼ 7.3, NCH2CH3) 13C{1H} NMR:
d 235.22 Mn(CO)2,
195.7 NCN, 124.6 NC¼CN, 121.5 NC¼CN, 82.6 Cp, 65.4 NCH3, 45.5
NCH2CH3, 16.7 NCH2CH3. IR (cmꢁ1): 1922
HRESIþ-MS, m/z: 287.0513 (calcd. 287.0592).
n
(CO), 1856
n
(CO).
2.2.6. Synthesis of 6, MeCpMn(CO)2L3
A similar methodology as for 1 was followed. Imidazol-2-
ylidene L1 (0.801 g, 3.0 mmol) was added to photo-irradiated
MeCpMn(CO)3 (0.654 g, 3.0 mmol). A single yellow solid product
of complex 6 was isolated (yield: 0.374 g; 33%). 1H NMR, ppm
2.2.2. Synthesis of 2, MeCpMn(CO)2L1
A similar methodology as for 1 was followed. Imidazol-2-
ylidene L1 (0.714 g, 3.0 mmol) was added to photo-irradiated
MeCpMn(CO)3 (0.654 g, 3.0 mmol). A single brown-orange oily
product of complex 2 was isolated. Yield: 0.271 g (30%) 1H NMR,
(CD3CN):
d
7.37 (s br, 4H, Ph), 7.46 (s, 1H NHC¼CHN), 7.25 (s, 1H
NCH¼CHN), 4.46 (m, 2H, NCH2Bn), 4.26 (s br, 5H, Cp), 3.75 (s, 3H,
NCH3) 3.05 (m, 2H, NCH2CH2Ph), 2.51 (s, 3H, Cp-CH3). 13C{1H} NMR,
ppm (CD3CN):
d
7.20 (s, 1H, NHC¼CHN), 7.15 (s, 1H, NHC¼CHN, 4.34
ppm:
d
Mn(CO) 235.3, 196.5 NCN, 124.6 NC¼CN, 122.2 NC¼CN,
(s br, 2H, NCH2CH3), 4.09 (s, 2H, Cp), 4.02 (s, 2H, Cp)), 3.36 (s, 3H,
138.9 Ph(ipso), 129.3 Ph(meta), 129.0 Ph(ortho), 127.0 Ph(para), 103.2 Cp,
NCH3), 2.86 (s, 3H, Cp-CH3), 1.48 (s br 3H, NCH2CH3). 13C{1H} NMR,
82.1 Cp, 80.4 Cp, 64.4 NCH3, 52.1 NCH2Bn, 40.8 NCH2CH2Ph,.35.6
ppm:
d
234.7 Mn(CO)2, 200.7 NCN, 129.0NC ¼ CN, 128.6 NC¼CN, Cp
Cp-CH3. IR (cmꢁ1): 1917 (CO). HRESIþ-MS, m/z:
n(CO), 1852 n
101.4, Cp 81.5, Cp 79.8, 65.0 NCH3; NCH2CH3 52.27, Cp-CH3 37.5,
379.1248 (calcd. 379.1218).
NCH2CH3 13.7. IR (cmꢁ1): 1918 (CO). HRESIþ-MS, m/z:
n(CO), 1852 n
303.0939 (calcd. 303.0905).
2.3. X-ray crystallography
2.2.3. Synthesis of 3, CpMn(CO)2L2
Data for complexes 3 and 6 and C1 were collected at 150 K on a
Bruker D8 Venture kappa geometry diffractometer, with duo I
sources, a Photon 100 CMOS detector and APEX II control software
using Quazar multi-layer optics, monochromated Mo-K radiation
and by means of a combination of and scans. Data reduction
A similar methodology as for 1 was followed. Imidazol-2-
ylidene L2 (0.0481 g, 3.0 mmol) was added to photo-irradiated
MeCpMn(CO)3 (0.612 g, 3.0 mmol). A single yellow solid product
of complex 3 was isolated (yield: 0.271 g; 30%).1H NMR, ppm
ms
a
f
u
(CD3CN):
5H, Cp), 3.76 (s, 3H, NCH3), 3.60 (m, 2H, NCH(CH3)2), 1.32 (s, 6H,
NCH(CH3)2). 13C{1H} NMR, ppm:
235.1 Mn(CO)2, 193.5 NCN,
d
7.42 (s, 1H, NHC¼CHN), 7.32 (s, 1H, NCH¼CHN), 4.45 (s,
was performed using SAINTþ and the intensities were corrected for
absorption using SADABS [12]. The structures were solved by
intrinsic phasing using SHELXTS and refined by full-matrix least
squares using SHELXTL and SHELXL-2013 [13]. In the structure
refinement, all hydrogen atoms were added in calculated positions
and treated as riding on the atom to which they are attached. All
nonhydrogen atoms were refined with anisotropic displacement
parameters, all isotropic displacement parameters for hydrogen
atoms were calculated as X ꢂ Ueq of the atom to which they are
d
124.90 NC¼CN, 118.7 NC¼CN, Cp 82.5, 67.5 NCH3, 51.9 NCH(CH3)2,
25.6 NCH(CH3)2. IR (cmꢁ1): 1922
z: 301.0789 (calcd. 301.0748).
n(CO), 1856 n
(CO). HRESIþ-MS, m/
2.2.4. Synthesis of 4, MeCpMn(CO)2L2
A similar methodology as for 1 was followed. Imidazol-2-

14
R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
attached, X ¼ 1.5 for the methyl hydrogens and 1.2 for all other
hydrogens. Crystallographic data and refinement parameters are
given in Table 1. Ortep drawings [14] of the three structures are
included in Fig. 5, Fig. 6 and Fig. 9. The crystal structures (cif) have
been deposited at the Cambridge Crystallographic Data Centre and
allocated the deposition numbers: CCDC 1517726e1517728. Data
collection, structure solution and refinement details are available in
each cif.
2.4. Molecular modelling
The calculations reported in this paper were obtained using the
Gaussian 09 [15] suite of programs. Calculations were carried out in
the singlet spin state using the hybrid functional B3LYP [16,17].
Geometries of the neutral complexes were optimized in gas phase
with the triple-z basis set 6-311G* on all atoms except for the metal
atoms. Stuttgart/Dresden (SDD) pseudopotential was used to
describe the metal electronic core, while the valence electrons were
described using def2-TZVPP [18]. No symmetry constraints were
applied and only the default convergence criteria were used during
the geometric optimizations. Vibrational frequencies [19] were
calculated for the optimized geometries and no imaginary fre-
quencies were observed, confirming true minima. Donor-acceptor
interactions have been computed using the natural bond order
(NBO) method [20]. For the ETS-NOVC analysis, complexes 1, B1
and C1 were optimized in ADF 2014.01 [21], using BP86 with a
Fig. 5. Perspective view of 3 with thermal ellipsoids drawn at the 50% probability
level.
3. Results and discussion
3.1. Synthesis and characterization
triple-z basis set and with up to 2p frozen cores on metal atoms, in
the gas phase. Optimized structures were then investigated using
the Extended Transition State coupled with Natural Orbitals for
Chemical Valence (ETS-NOCV) [22] energy decomposition tech-
nique. MneL bonds were broken and the corresponding fragments
were used as promolecules, without any adjustment of fragment
occupations.
Complexes 1e6 were prepared in reasonable yields by photo-
chemical substitution of a carbonyl ligand with the free NHC ligand,
generated in situ from the reaction between an imidazolium salt
and BuOK (Scheme 1). The imidazolium salts (L1-L3) were pre-
pared according to literature procedures [24].
The infrared (IR) spectrum of the complexes have been recorded
using hexane as the solvent, and indicated the presence of two
stretching frequencies. Both stretching frequencies are attributed to
the carbonyl ligands of the Mn(CO)2-moiety. The observations of
these stretching frequencies are supported by literature [4,5]. The
structural predictions were also confirmed with X-ray diffraction
studies of three complexes and will be presented in the next
sections.
t
2.5. General procedure for the dimerization of thiols
On the basis of literature [23] and for purposes of the compar-
ison of computed percentage buried volume and solid angle pre-
dictions to experimental obtainable results, 3 mol% of catalyst, in
the presence of the ethylthiol and cyclohexane were irradiated in
an inert argon atmosphere for the duration of 2.5 h. The resulting
solution was analysed directly by GC/MS using hexadecane as in-
ternal standard upon which the dimerization yields were based. All
reported yields are based on the average of three runs.
3.2. Spectroscopic characterization
Using NMR spectroscopy, with CD3CN as solvent, the structural
aspects of the NHC complexes could be investigated. The 1H NMR
spectroscopic peaks for both the imidazolium salts and the com-
plexated ligands had similarity in their spectral pattern and
chemical shifts. The imidazolium ligand salts had a characteristic
Table 1
Selected bond lengths and angles.
3
6a
carbene carbon proton peak around
d
¼ 9 ppm, which disappeared
Bond length (Å)
Mn-COx (x ¼ 1e2)
Mn-C6
N1-C6
N2-C6
Bond angle (ꢀ)
C1-Mn-C2
N1-C6-N2
N1-C6-Mn
N2-C6-Mn
Torsion angle (ꢀ)
Mn-C6-N1-C7
Mn-C6-N2-C8
Plane angle (ꢀ)
Carbene/Cp
instantly upon deprotonation and coordination to the metal cen-
tres, indicating the formation of the desired carbene complexes.
The protons peaks on the C4 and C5 ring positions of the imida-
zolylidene ligand of 1e6 were also clearly visible at chemical shifts
1.7565 (19)
1.9904 (17)
1.363 (2)
1.761 (4)
1.990 (3)
1.373 (4)
1.377 (5)
1.367 (2)
between
d
¼ 6.83 and
d
¼ 7.46 ppm. The 13C NMR of complexes 1e6
87.96 (9)
89.44 (15)
102.6 (3)
128.6 (2)
128.6 (2)
indicated the presence of a single distinct CO peak at ca. 235 ppm, a
down-field shift of 10 ppm from the MMT and cymantrene starting
synthons. The distinct carbene carbon peak can be witnessed at ca.
195 ppm. The NMR spectroscopy data is in accordance with liter-
ature [4,5]. The IR spectra of 1e6 were measured in hexane and
indicated two stretching frequencies attributed to the CpMn(CO)2
moiety carbonyl stretching frequencies. The stretching frequencies
for the two carbonyls ligands were observed at ca. 1850 (symmetric
103.08 (14)
128.79 (12)
128.12 (12)
ꢁ179.94 (12)
ꢁ179.97 (12)
ꢁ0.5 (4)
0.6 (4)
31.77
30.40
a
Bond lengths and angles are average values calculated due to the presence of
two units per cell.
A0) and 2020 (antisymmetric A00) cmꢁ1
.

R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
15
Fig. 6. Perspective view of 6 (molecule A) with thermal ellipsoids drawn at the 50% probability level.
Fig. 9. Perspective view of C1 with thermal ellipsoids drawn at the 50% probability
level.
Fig. 7. The angle (ꢀ) between the Cp and carbene planes of 3.
3.3. Single crystal X-ray diffraction studies
Selected structural parameters are summarized in Table 1. The
conformation of the carbene moiety, as represented by the C7-N1-
C6-Mn dihedral angles, deviate from planarity by less than 1ꢀ (3,
see Fig. 5) and 7ꢀ (6, see Fig. 6). The carbene-metal bond lengths
were determined to be 1.9904 (17) Å and 1.990 (3) Å respectively
and seem to be insensitive to the size of the carbene substituents.
The carbene-metal bond lengths are slightly shorter compared to
published literature on symmetrical NHC cymantrene derived
complexes [5]. The angles between the carbene plane (Mn, C6, N1
and N2) and the Cp ring were measured as 31.77ꢀ for 3 and 30.40ꢀ
for 6 (Fig. 7). Complex 6 crystallized with two molecular units
(molecule A and molecule B) in the asymmetric unit and displays
intramolecular hydrogen interactions between the carbene sub-
stituents and the carbonyl groups.
The incidence of hydrogen interactions can also be witnessed in
both 3 and in 6 between the methyl group on the N substituent and
the carbonyl ligands of the manganese metal atom. The presence of
these hydrogen interactions will be quantified using the (ETS-
NOCV) [22] energy decomposition technique in the DFT study
section (Fig. 8).
Fig. 8. Hydrogen interactions (in Å) witnessed in 3.

16
R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
Scheme 1. Synthesis of group VII NHC complexes.
During the synthesis of complex C1 a single crystal, suitable for
substituents on the N atoms of the NHC ligands (26% for L1, 25% for
L2 and ca. 26% for L3). The %Vbur was slightly higher (28.5%) for A1
and significantly higher (33%) for A2 [4]. Therefore the %Vbur values
are elevated only when both N-substituents are bulky. Due to size
and rotation possibilities, steric repulsion is minimized by rotating
the bulky substituents away from the coordination sphere of the
manganese metal centre. The steric contour map of complex 3
(Fig. 11(b)), generated with SambVca2 [27b], indicates that the NHC
ligand is not seen to be sterically crowding the metal centre.
Complexes 1 and 5 show similar steric contour maps (see SI).
Although groups are increasingly bulkier, only hydrogen atoms
are allowed to penetrate the coordination centre, while the alkyl
and aryl groups are accommodated away from the coordination
sphere, minimizing the steric requirements of the substituents.
Since the %Vbur of all the complexes are similar in value, and the
NHC ligands occupy only a quarter of the total available space
around the manganese atom, the deduction can be made that the
steric demands of the NHC ligands have little consequence on
available catalytic space.
X-ray diffraction, was also obtained. C1 crystallized in a P21/c space
group and displays a linearly coordinated nitrile functionality
(Fig. 9). The Mn-N-C3 displays a bond angle of nearly 180ꢀ (175.6ꢀ)
and a C1-M-C2 bond angle of 91.37ꢀ which is bigger compared to
the NHC ligated complexes 3 and 6.
3.4. Theoretical study
Theoretical aspects of manganese carbene complexes are well
described in literature [4,25,26]. Computational studies typically
focus on the interactions of the frontier orbitals and the predictions
that can be made from these interactions on the oxidation and
reduction potential trends witnessed for metal centres [26].
Theoretical studies by Whittlesey [5] focused on aspects of agostic
interactions witnessed in MMT NHC complexes. These studies have,
however, been confined to electronic parameters and limited data
have been provided on the steric aspects of NHC-substituted
complexes of cymantrene and MMT. A DFT study has thus been
considered to describe not only the electronic influences that the
carbene moiety imparts on the coordination sphere of metal cen-
tres but also structural and steric influences by these ligands on the
metal moiety. Complexes 1e6, with varying steric bulk on one of
the nitrogen substituents of the carbene ligand, have been studied.
The calculated properties of the novel complexes 1e6 and C1 were
compared with those of literature complexes A2 [4] and B1 [10] as
well as theoretical complex A1 (Fig. 10).
Solid angle measurements of ligand steric parameters [30] have
been attempted to establish a correlation between the steric bulk of
a ligand and chemical and physical characteristics of inorganic and
organometallic systems [29,31,32]. Shielded regions would severely
limit metal-substrate interactions and affect the catalytic potential
of the pre-catalyst. The solid angle parameters are provided in
Table 2.
The GM of 1, 3, 5 and the cymantrene starting synthon have been
visualized and presented in Fig. 12.
The solid angles of 1e6 indicate the size of the ligand increases
3.4.1. Ligand steric parameters
in the order of L1 z L2 < L3. The GM and G(g of the cymantrene
)
In this study the steric characteristics of NHC ligands have been
determined using two different methods, namely the percentage
buried volume (%Vbur) and the solid angle measurement of the li-
gands [27a,28,29]. Increasing the bulkiness of the substituents on
the N atoms of the ligand should result in a greater %Vbur in relation
to the metal-ligand bond distance. The %Vbur values were found to
be similar irrespective of the identity or bulkiness of R-group
synthon has also been calculated and indicates a small decrease in
both parameters after the coordination of the NHC moiety. As ex-
pected, the GM and G(g) increase when two ethyl groups occupy the
carbene nitrogen substituents positions compared to the ethyl
methyl substituted ligand Fig. 13. The G(M) predictions for 1e6 have
similar values as also seen in the %Vbur. Although the solid angles
Fig. 10. NHC complexes 1e6, A1, A2, phosphine complex B1 and acetonitrile complex C1 of the theoretical study.

R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
17
Fig. 11. (a) Graphical representation of the sphere defining the %Vbur of metal complexes 1e6; (b) steric contour map of 3; and (c) representation of the solid angle and GM of a
complex.
Table 2
well with dimeric Mn NHC complexes reported recently [29]. The
bonding interaction of the NHC ligands was determined to be
Solid angle parameters for 1e6.
Ligand
Complex
Solid Angle Steradiansa
GM(Complex) %
G(g %
)
similar to that of phosphine ligands (M-L bond order of 0.77) and
stronger in comparison to the nitrile ligand analogues such as C1
(BO ¼ 0.66). These values are, however, in line with reported
literature values [35].
The bond dissociation energies [29,37] for 1e6 are listed in
Table S1 in SI. From the data presented, NHC ligands bound more
tightly towards the manganese centre bearing cyclopentadienyl
ligands compared to the methylcyclopentadienyl analogues. NHC
ligands with ethyl phenyl substituents had greater BDEs followed
by isopropyl and finally ethyl groups respectively. The dissociation
energies thus decreases in the following trend: CpMn-
1
1
2
2
3
3
1
2
3
4
5
6
A1
A2
3.11
3.20
3.06
3.18
3.34
3.34
4.05
4.92
e
96.88
96.91
96.70
96.93
96.35
96.55
98.04
98.60
97.07
95.41
12.89
13.19
13.87
14.14
15.50
16.03
18.79
25.69
16.66
12.08
CpMn(CO)3
MeCpMn(CO)3
e
a
Solid angle of the NHC ligand.
L3
> MeCpMn-L3 > CpMn-L2 > CpMn-L1 > MeCpMn-
L2 > MeCpMn-L1.
indicate a similar size of L1 and L2, the increasing G(g values is
)
The average bond dissociation energy for the NHC ligands were
significantly greater compared to both phosphine and nitrile li-
gands despite the similarity in Wiberg bond orders. The heightened
BDEs are ascribed to the superior coordination interactions offered
by NHC ligands. Complexes with larger substituents on the nitrogen
backbone of the ligands tend to display greater BDE compared to
smaller substituents. This finding is supported by literature [38]
and the steric effects of the heteroatomic substituents are well
described. Increasing the bulkiness of the nitrogen substituents
leads to increases in the stability of the NHC ligands and greater
donation to the ligated metal centre.
indicative of increasing steric bulk of the ligands in the order of
L1 < L2 < L3 < LA1 < LA2
.
3.4.2. Wiberg bond indices and bond dissociation energies (BDEs)
Literature exists for the determination and applicability of
Wiberg bond orders in the endeavour to study NHC ligands [33,34]
and the coordination to metal centres [35,36]. The Wiberg bond
indices quantified single or double bond character and can be uti-
lized to determine bond strength orders (BSOs). From our calcu-
lated results, the Wiberg bond indices (see Table S1 in SI) indicate a
relatively strong sigma-donation from the NHC ligand towards the
metal centre (BO ¼ 0.73e0.78). The indices are similar in all the
NHC complexes and show no variation despite steric difference
between the carbene nitrogen substituents. These values correlate
3.4.3. Extended Transition State coupled with Natural Orbitals for
Chemical Valence (ETS-NOCV)
ETS-NOCV is a powerful computational tool with which the
Fig. 12. The GM(Complex) of 1 (a), 3 (b) and 5 (c) and CpMn(CO)3 (d).

18
R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
Fig. 13. Solid angle parameters of the (a) MeCpMn(CO)3, (b) diethyl substituted NHC and (c) complex 2.
formation of an MeC bond can be investigated and analysed in
terms of the absorbed or released interaction energy, EInt, as well
as the change in electron density, commonly known as the defor-
NOCV approach allow qualitative as well as quantitative compari-
son of specific attributes of different MeC bonds in various systems
[21,22].
D
mation density, Dr, associated with bond formation [21,22]. Both
In this section the bond characteristics of three different ligand
systems coordinated to a CpMn(CO)2 moiety were probed. The
formation of various MneL bonds in 1, B1 and C1 was investigated
theoretically using ETS-NOCV. Of primary interest is the various
NOCV channels of each MneL bond, describing the density and
DEInt and Dr can be decomposed into different components to aid
interpretation of a bond formation process. The interaction energy
can be decomposed into three terms,
DEInt
¼
DEElstat
þ
D
EPauli
þ
D
EOrb
þ
D
EDisp
(1)
energy changes of specific (
primary NOCV channels for the NHC (structure 1) are shown in
s and p) attributes of each bond. The
where
D
EElstat gives the change in electrostatic energy upon
Fig. 14. Drs describes the clear s-donation from the ligand to the
bringing unperturbed promolecules from an infinite separation to
the final bonding distance, EPauli gives the change in energy
metal with an associated energy change of ꢁ146.86 kJ,molꢁ1
,
D
whereas Drp1 and Drp2 describe the p-back donation from metal
required to orthogonalize (ensuring antisymmetry) the promo-
lecular (ensuring antisymmetry) wavefunctions and is commonly
to ligand for two different axes, with a combined energy contri-
bution of ꢁ59.50 kJ,molꢁ1. Finally, Dr4 shows the formation of two
hydrogen CH,,,C interactions, contributing ꢁ5.69 kJ,molꢁ1 to the
total interaction energy. The NOCV channels for the remaining
structures B1 and C1 are qualitatively very similar, except for the
lack of CH,,,C interactions in C1. The presence of hydrogen in-
teractions was also witnessed in the solid-state crystal structures of
3 and 6.
known as Pauli repulsion,
to the minimization of the energy of the final structure and is
commonly known as the orbital interaction term and EDisp is the
DEOrb gives the change in energy related
D
dispersion energy, usually calculated through empirically-derived
functionals. The electrostatic and Pauli energy changes are often
combined into a single value,
DESteric, to give an indication of the
steric (electrostatic þ electronic) interaction between promole-
cules. Similarly, the deformation density can be decomposed into
two terms,
In Table 3, decomposition of the total interaction energy into
relevant components for each structure is shown; for a complete
ETS-NOCV decomposition. Comparing first the total interaction
energies,
DEInt, 1 forms the strongest MeL bond, with the phos-
Dr
¼
DrPauli
þ
DrOrb
(2)
phine B1 and the acetonitrile C1 weaker by þ29.12
and þ 125.10 kJ,molꢁ1, respectively. The biggest contribution to the
where each terms gives the change in density associated with the
energetic term of Eq. (1) (no density change is observed with the
change in electrostatic energy) [21,22]. The various decomposition
terms of Eq. (1) can be used to compare the properties associated
with the formation of different MeC bonds, and the associated
density changes can be used for visual inspection [21,22]. Of more
strength of
1
is
a
very large electrostatic contribution
(ꢁ485.26 kJ,molꢁ1 as opposed to ꢁ348.61 and ꢁ227.61 kJ,molꢁ1 in
B1 and C1, respectively, as seen in Table 3). Electronically, however,
1 and B1 is equivalent, with a total orbital interaction of ꢁ233.47
and ꢁ237.61 kJ,molꢁ1, respectively. Interestingly, the
s-donation of
both 1 and B1 is very similar (differing by ꢁ7.41 kJ,molꢁ1 in favour
interest to us, however, is the further decomposition of the
D
EOrb
of 1) but 1 experiences considerably weaker p-back bonding
and DrOrb terms into contributions from natural orbitals, as given
by the NOCV addition to ETS. Specifically, the total DrOrb term can
be decomposed into i natural orbitals,
(differing by ꢁ16.78 kJ,molꢁ1 in favour of B1). On the other hand,
B1 and C1 show equivalent
p contributions, but C1 forms a
considerably weaker
s bond than the other structures. Therefore,
the combination of large electrostatic attractions coupled with
comparatively small electronic steric repulsions result in 1 forming
a stronger MeL bond than B1, despite comparative orbital and
dispersion interactions.
X
DrOrb
¼
Dri
(3)
i
The ratio of the
structure is shown in Table 3. This metric gives us a direct indication
of the relative -donation over -back bonding capability of each
system. Clearly, 1 shows by far the most character (2.47 times
more -bonding than -bonding), whereas the phosphine B1 is
slightly more prevalent for -bonding (1.83) and the C1 is more or
less equivalent for both and (1.11). However, since the total
s contributions over p contributions for each
and each NOCV channel is associated with a specific change in
energy,
EiOrb. The highest occupied NOCV channels can usually be
associated with specific components of molecular orbital in-
teractions between promolecules, such as and bonding, giving
associated deformation density channels and energy changes for
these components, i.e. Drs and describing a -bonding
Es
component of an MeC bond [21,22]. These features of the ETS-
D
s
p
s
s
p
s
p
s
D
s
Orb
s
p

R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
19
Fig. 14. Primary NOCV channels and associated orbital energy changes for 1. Blue and red regions indicate an accumulation and depletion of electron density, respectively. Iso-
surfaces for Drs Drp Drp2 and Dr4 are 0.002 au, 0.001 au, 0.001 au and 0.00015 au, respectively. (For interpretation of the references to colour in this figure legend, the reader is
,
,
1
referred to the web version of this article.)
Table 3
energetically weaker electronic interactions (such as weak non-
covalent interactions and various polarizations), as well as stearic
effects (including electrostatic interactions and Pauli repulsion).
The stronger M-L bond in 1 illustrates the superior ability of the
NHC ligand to stabilize the metal centres compared to phosphine
and acetonitrile ligands. The ETS-NOCV results also mirror findings
of the bond dissociation energies were NHC-M complexes display
strong M-L bonds.
Selected ETS-NOCV energy contributions.
a
a
a
a
a
a
D
EInt
D
EElstat
D
EPauli
D
ESteric
D
EOrb
DEDisp
1
B1
C1
ꢁ276.14
ꢁ247.02
ꢁ151.04
ꢁ485.26
ꢁ348.61
ꢁ227.61
þ493.96
þ405.68
þ265.27
þ8.70
ꢁ233.47
ꢁ237.61
ꢁ173.64
ꢁ51.35
ꢁ66.46
ꢁ15.21
þ57.07
þ37.66
sc
%
pd
a
a
D
EOsrb
D
EOprb
s
/
pb
%
1
B1
C1
ꢁ35.10
ꢁ33.33
ꢁ20.40
ꢁ14.22
ꢁ18.23
ꢁ18.40
2.47
1.83
1.11
53%
56%
13%
22%
31%
12%
3.5. Catalytic study
a
All values in kJ,molꢁ1
Ratio of
Percentage of
Percentage of
.
b
c
D
Es
over
D
Ep
.
Orb
Catalytic applications of cymantrene [39] include dehydrocou-
pling reactions of amine-borane adducts [40], dehydrogenative
silane alcoholysis [41] and the photoconversion of thiols into
disulfides [23]. Of the three reactions mentioned, the final process
was chosen to evaluate the catalytic activity of our complexes.
Organosulfur chemistry has always been of particular interest
both in the field of chemistry and biology [42]. The transformation
of sulfur-containing compounds has attracted much research in-
terest and specifically the oxidation of thiols to disulfides has
remained an area of importance [43]. Reactions focussing explicitly
on the dimerization of sulfides into disulfides normally require a
basic catalyst [44], stoichiometric amounts of oxidants [45] or toxic
reagent and long reaction times. Apart from the high demand of
these reaction conditions, many synthetic methodologies generate
elevated levels of unwanted side-products. The side-products are
commonly produced as a result from over oxidation, producing
Orb
s
p
contribution to
contribution to
D
D
EInt
.
d
EInt
.
interaction energies of each system differ, the final columns of
Table 3 present each attribute of bonding as a percentage of the
total interaction energy, allowing direct comparison between
different ligands.
Complexes 1 and B1 are equivalent
of the total binding energy originating from
tively. However,1 shows considerably less character than B1, with
22% -back donation for 1 and 31% for B1. The acetonitrile C1 shows
equivalent and character (13% and 12%, respectively), and
s
-donors, with 53% and 56%
s
-donation, respec-
p
p
s
p
overall lower contributions from electronic interactions. The
remaining contributions to the total interaction energy includes

20
R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
sulfoxides, sulfones, thiosulfinates and thiosulfonates [46e48].
Literature examples have been reported for the employment of
rhodium(I) transition-metal complexes that illustrated the ability
to selectively oxidize thiols into disulfides as an attractive alter-
native to the use of hazardous oxidants [23], but such processes
have been fairly unexploited. The UV photolysis of cymantrene with
thiols at room temperature offered a viable, greener alternative to
classic published method of dimerization of thiols and produced H2
as the only side product [23]. Pre-catalysts 1e6 have been inves-
tigated for application in the dimerization of thiols into disulfides
[23]. Based on literature [23] and for purposes of the simple com-
parison of %Vbur and solid angle predictions to experimental
obtainable results, 3 mol% of catalyst, in the presence of the sub-
strate and cyclohexane were photo-irradiated in an inert argon
atmosphere for the duration of 2.5 h to test for catalytic activity.
All six complexes displayed catalytic activity in the dimerization
of thiols into disulfides (Table 4). Although lower yields were ob-
tained for complexes 3 and 4, complexes 5 and 6 displayed superior
activity with yields of ca. 47% and turn over frequencies (TOF) of 49
Fig. 15. Possible carbonyl substitution positions COa and COb available on 1e6.
The ligands thus afford greater stability while not crowding the
metal atom. Complexes 5 and 6 contain the most steric ligand L3,
according to the steric parameters calculated, which then implies
that the most stable complexes were found to have the best cata-
lytic activity. A feasible catalytic cycle is proposed in Fig. 16,
describing the catalytic conversion of the thiol substrates into
disulfides. The cycle is based on experimental data obtained from
the similar study on the dimerization of thiols using cymantrene
only as catalyst [23,49]. In the article by Zhang et al. [49], an in-
depth DFT study on the key intermediates and two possible path-
ways for this conversion, is described. Their results correlate with
the experimental observations of Tan et al. [23]. The cycle is initi-
ated with the photo-induced substitution of a carbonyl ligand for a
thiol substrate and terminates with the liberation of disulfides and
the regeneration of the active catalyst species. The process is
dependent on the availability and lability of carbonyl ligands of the
pre-catalyst species and thus might explain the lower activity of our
pre-catalysts in comparison with the cymantrene analogue.
h
ꢁ1. The obtained yields were lower in comparison to literature [23]
(ca. 99% for alkyl thiols) for the CpMn(CO)3-photoconversion of
thiols. Complexes displayed diminished activity over the 150 min
reaction period, which might indicate the decomposition of the
catalysis under the harsh photo-irradiation conditions. The higher
catalytic activity of 5 and 6 could thus be explained by the greater
stability of these complexes where decomposition is concerned,
displaying higher BDE of the metal-carbene bond and slightly
larger HOMO-LUMO band gaps (see SI) compared to complexes
1e4. The dimerization occurs through the substitution of a carbonyl
ligand for an ethylthiol group. The ligand exchange can occur at two
different carbonyl ligand positions in reference to the substituents
on the NHC ligand. Bond dissociation energies of the metal-
carbonyl bonds indicate similar dissociation energies for carbonyl
ligands COa and COb (Fig. 15). It can thus be hypothesized that
ligand exchange could occur at either carbonyl ligand to initiate the
catalytic cycle.
The ligand steric parameter results indicate that the steric trend
increases in the order L1 < L2 < L3 and literature associates the
steric influences of NHC substituents with the greater stability
these ligands afford to the metal moiety. The solid angle and %Vbur
parameters thus correlate with the stability associated with the
pre-catalysts and therefore might suggest a probable explanation
for the catalytic activity witnessed. Although the ligands increase in
bulkiness, the steric groups do not directly influence the coordi-
nation space of the metal moiety (as indicated by the solid angle).
4. Conclusion
NHC complexes 1e6 were synthesised in satisfactory yields
from MMT and cymantrene synthons. The molecular structures of
the novel complexes were confirmed with NMR and IR spectros-
copy, and single crystal X-ray diffraction studies of 3 and 6. Through
DFTcalculations, it was possible to describe the steric influences the
NHC ligands impose on the metal synthon. Wiberg bond indices
indicated orders of ca. 0.7 for the NHC complexes, which is similar
to bonding aspects of triphenylphosphine. Both ligands thus
display negligible
p-back donation from the metal centre. The
frontier orbital analysis indicated increases in the band gaps upon
Table 4
Dimerization of ethylthiol by manganese(I) NHC complexes 1e6.
,c
Catalysta
Complex
R
R0
Cat. (mol%)
Conversionb (%)
1
2
3
4
5
6
Et
H
Me
H
Me
H
Me
3
3
3
3
3
3
28
22
16
6
47
48
Et
iPr
iPr
CH2CH2Ph
CH2CH2Ph
a
General reaction conditions: Ethylthiol (2.0 mmol), Catalyst (0.06 mmol), Cyclohexane (10 ml), Room temperature, 150 min.
Conversion based on internal standard.
TOF, as cited in text, was calculated after the first 10 min of the catalytic reaction.
b
c

R. Fraser et al. / Journal of Organometallic Chemistry 840 (2017) 11e22
21
Fig. 16. Catalytic cycle associated with the dimerization of thiol substrates.
the substitution of NHC ligands with larger molecular masses (see
References
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band gaps may indicate higher stability of complexes 5 and 6 or
greater inertness towards decomposition pathways. Finally, BDE
energies indicated a linear relationship between the associated
energies and the molar mass of the NHC moiety. The BDE of the
NHC-metal bonds were higher in cymantrenecomplexes compared
to MMT analogues.
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