Ring-expanded iridium and rhodium N-heterocyclic carbene complexes: a comparative DFT study of heterocycle ring size and metal center diversity

Article abstract of DOI:10.1080/00958972.2017.1287906

A new series of ring-expanded six- and seven-membered N-heterocyclic carbene precursors (re-NHCs) and their transition metal complexes were synthesized. The basic properties of the synthesized materials were investigated by density functional theory (DFT)

Full text of DOI:10.1080/00958972.2017.1287906

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
Ring-expanded iridium and rhodium N-
heterocyclic carbene complexes: a comparative
DFT study of heterocycle ring size and metal
center diversity
E. Ö. Karaca, M. Akkoç, E. Öz, S. Altin, V. Dorcet, T. Roisnel, N. Gürbüz, Ö.
Çelik, A. Bayri, C. Bruneau, S. Yaşar & I. Özdemir
To cite this article: E. Ö. Karaca, M. Akkoç, E. Öz, S. Altin, V. Dorcet, T. Roisnel, N. Gürbüz, Ö.
Çelik, A. Bayri, C. Bruneau, S. Yaşar & I. Özdemir (2017): Ring-expanded iridium and rhodium
N-heterocyclic carbene complexes: a comparative DFT study of heterocycle ring size and metal
center diversity, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2017.1287906
To link to this article: http://dx.doi.org/10.1080/00958972.2017.1287906
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Date: 21 February 2017, At: 01:17

Journal of Coordination Chemistry, 2017
http://dx.doi.org/10.1080/00958972.2017.1287906
Ring-expanded iridium and rhodium N-heterocyclic carbene
complexes: a comparative DFT study of heterocycle ring size
and metal center diversity
E. Ö. Karacaa, M. Akkoça, E. Özb, S. Altinb, V. Dorcetc, T. Roisnelc, N. Gürbüza, Ö. Çelikd,
A. Bayrib, C. Bruneauc, S. Yaşara and I. Özdemira
adepartment of Chemistry, İnönü university, malatya, turkey; bdepartment of Physics, İnönü university,
c
malatya, turkey; université de rennes, umr 6226: Cnrs sciences Chimiques de rennes, rennes, france;
ddepartment of Physics, dicle university, diyarbakır, turkey
ABSTRACT
ARTICLE HISTORY
received 11 september 2016
accepted 29 december 2016
A new series of ring-expanded six- and seven-membered N-
heterocyclic carbene precursors (re-NHCs) and their transition metal
complexes were synthesized. The basic properties of the synthesized
materials were investigated by density functional theory (DFT). The
six- and seven-membered re-NHCs were synthesized in good yield
via reaction of the corresponding alkyldibromides or alkyldiiodides
with N,N′-bis-(2-phenylbenzene)formamidine in the presence of
K2CO3 under aerobic conditions. Complexes, represented by the
formula [ML1,2(COD)Cl] (where M = Ir or Rh and L is a ring-expanded N-
heterocyclic carbene ligand), were synthesized in the presence of the
corresponding free carbene and iridium or rhodium metal precursors
in tetrahydrofuran. All new re-NHC complexes were characterized by
different analytical techniques, including NMR spectroscopy, X-ray
diffraction, UV spectroscopy and elemental analysis. According
to molecular electrostatic potential calculations, the electrophilic
properties of the complexes were aligned, from highest to lowest,
as Ir-6-DiPh, Rh-6-DiPh and Ir-7-DiPh. The HOMO, LUMO and energy
gaps of the complexes were calculated by DFT. On the basis of the
DFT analysis, it can be predicted that Rh-6-DiPh is the most stable
complex and Ir-7-DiPh is more reactive than Ir-6-DiPh.
KEYWORDS
iridium; rhodium; N-
heterocyclic carbene; dft
calculation; X-ray
CONTACT s. yaşar
syasar44@gmail.com, sedat.yasar@inonu.edu.tr
supplemental data for this article can be accessed http://doi.dx.org/10.1080/00958972.2017.1287906.
© 2017 informa uK limited, trading as taylor & francis Group

2
E. Ö. KARACA ET AL.
1. Introduction
Ligands are extremely important for metal complexes. N-heterocyclic carbenes (NHCs) are among
the most important ligands in organometallic chemistry and catalysis in the last few decades
due to their unique tunable steric and electronic parameters and strong metal-carbon bonds
[1]. It is well known that NHCs can be easily tuned by changing the groups on the nitrogen atom
and the ring size [2]. Until now, the majority of studies have focused on imidazole-, imidazolidine-
and benzimidazole-based NHCs. Insufficiently explored expanded-ring NHCs (re-NHCs) have
drawn much attention recently because they offer significantly different steric and electronic
properties. The expanded ring on NHCs leads to a significant increase in the donor ability and
steric hindrance to the metal center.Therefore, the unique tunable steric and strong σ-donation
properties of expanded-ring NHCs make them desirable ligands for transition metals [1–3].
The preliminary work on re-NHCs was carried out by the groups of Stahl and Cavell. They
reported the synthesis and catalytic application of six-, seven- and eight-membered NHCs
and their Pd, Ir, Rh, Ag and Au complexes [1, 3c, 3d, 4]. According to their findings, large rings
and N-CNHC−N bond angles offer steric and electronic advantages compared to conventional
five-membered analog derivatives in catalytic applications. The key points of these ligands
are the basic carbon and the steric effect on the metal center [5]. As a result of these features,
Re-NHCs have demonstrated unique activity in C–C coupling [3c], hydrosilylation [6], hydro-
genation [7] and transfer hydrogenation [8].
To better understand the nature and behavior of this class of ligands, herein we report the
synthesis and characterization of the more sterically demanding 6-DiPh.HBr- and 7-DiPh.HBr-
membered NHC precursors, which bear a diphenyl group, and their iridium(I) and rhodium(I)
complexes. Density Functional Theory (DFT) calculations of their properties are also presented.
2. Results and discussion
As seen in Figure 1, 6-DiPh.HBr and 7-DiPh.HI were synthesized in good yields (80–90%)
by known methods starting from commercially available reagents [3a, 3c, 4, 6]. The

JOURNAL OF COORDINATION CHEMISTRY
3
Figure 1. synthesis of 1 (6-diPh.hBr) and 2 (7-diPh.hi) ring expanded n-heterocyclic carbene precursors.
1
nonhygroscopic re-NHC precursors (Figure 1) were fully characterized by H and 13C NMR
and LCMS-MS spectroscopies. The 1H NMR spectra of the re-NHC salts supported the assigned
structures and the resonances for acidic C(2)–H were observed as sharp singlet signals at
7.77 and 8.56 ppm for 1 and 2, respectively. The 13C NMR chemical shifts were consistent
with the proposed structure and the imino carbon appeared as a typical singlet in the
1H-decoupled mode at 154.7 and 160.3 ppm for 1 and 2, respectively.
The ring-expanded iridium(I) and rhodium(I)-carbene complexes 3–5 with the general
formula [M(NHC)(COD)Cl] were obtained through direct addition of in situ generated free
carbene to the corresponding dimeric [IrCl(COD)]2 and [RhCl(COD)]2 precursors in tetrahy-
drofuran (THF) under inert conditions. This afforded the desired complexes 3–5 as air stable
yellow or orange solids in yields ranging from 40 to 80% (Figure 2) [5a, 9, 10]. Synthesis of
the iridium and rhodium complexes was attempted using a transmetalation method from
silver(I) to iridium and rhodium metals. Unfortunately, we could not obtain any isolated
rhodium or iridium complexes from in situ transmetalation from silver(I).
Our observation contrasts with that of Nechaev [4a], but consistent with the observations
of Herrmann [11], Cavell [4c, 4d, 5a] and Buchmeiser [12, 13]. According to Nechaev, trans-
metalation is reversible with bulky six- and seven-membered NHCs [4a]. This approach seems
to be compatible with our results.
The 13C{1H} NMR shift of the carbenic carbons of 3–5 were observed at 205.9, 212.3 and
216.7 ppm, respectively, and a downfield shift in resonance with increasing heterocycle-ring
size was observed. This result is consistent with previous observations [11–13].
Complex 3 was investigated by X-ray crystal structure analysis. A single crystal suitable
for X-ray diffraction (XRD) was obtained by slow vapor diffusion of diethylether into a con-
centrated dichloromethane solution of the complex in question [14]. Single crystal XRD was
carried out at 293(2) K using a Bruker APEX-II CCD diffractometer. The intensity data were
collected using graphite monochromated MoKα (λ = 0.71073 Å) radiation. The structure was
solved by direct method SHELXS-97 software [14] and refined by full-matrix least-squares
on F2 using SHELXL-97 with refinement of F2 against all reflections. Reflections collected/
uniquely used 7000/4557 [Rint = 0.022]/4557, 181 parameters refined, [Δρ]max/[Δρ]min = 3.691/-
2.931 eÅ−1, R1/wR2 (for all data) = 0.054/0.081. Hydrogen atoms were constrained by differ-
ence maps and refined isotropically. Other atoms were refined anisotropically. For absorption

4
E. Ö. KARACA ET AL.
Figure 2. Preparation of iridium and rhodium re-nhC complexes.
correction, a multi-scan Bruker (2005), SADABS Bruker AXS Inc., Madison, Wisconsin, USA
was applied. The molecular structure plots were prepared using PLATON and ORTEPIII [15].
The crystal data, experimental parameters, anisotropic thermal parameters, structure factors
(observed and calculated) and a full list of bond distances, bond angles and torsion angles
are given in the Supplementary Material.
A perspective view of the molecular structure of 3–5 with the atom numbering scheme
is given in Figure 3. The bond lengths (Å) and angles (°) for 3 are given in Table 1. The Ir atom
lies on a crystallographic center of symmetry and is coordinated to five C and Cl atoms. The
Ir–C and Ir–Cl average bond lengths are 2.103(3) Å and 2.3989(17) Å, respectively. These
values are in agreement with the literature [16, 17]. As expected, 3–5 have a distorted
square-planar geometry. Each iridium and rhodium center were coordinated to a re-N-
heterocyclic carbene ligand, a chloro atom and η4-coordinated 15-cyclooctadiene (COD),
giving rise to four-coordinate environments. The dihedral angles between P1-P2, P1-P3 and
P2-P3 (P1:(Ir/C1/C2/C3/C4), P2:(C8/C9/C10/C11/C12/C13) and P3:(C14/C15/C16/C17/C18/
C19)) are 38.25°, 89.95° and 53.16°, respectively. Pyrimidine rings are in a cis-configuration.
The C5/N1/C6/C7/C6–1/N1–1 ring has a boat-like configuration [18]. The torsion angles of
C9–C8–N1–C5and C9–C8–N1–C6 are 63.1° and 108.6°, respectively. A summary of the crys-
tallographic data for 3 is provided in Table 1. For 4 and 5, see supplementary file. In addition,
the structures of 3–5 were investigated by DFT calculations. It can be seen in Table 2a–c that
the experimental data and theoretical calculations are in agreement for 3–5. The numerical
estimation of crystallographic data is in accord with experimental results.
The hydrogen bond molecular geometry values and hydrogen-bonding geometries
between some atoms and C for the ring center of gravity (Å) for 3 are given in Table 3. The
molecular geometry was affected by the two C−H … Cl and C–H … N intramolecular hydro-
gen-bond interactions. Otherwise, there are two C–H … Cg interactions between the atom
and the ring center of gravity. The ring center of gravities are Cg(1):C8/C9/C10/C11/C12/C13
and Cg(2):C14/C15/C16/C17/C18/C19.
With different symmetry codes, such as −x,−y,1-z, −x,−y,1-z and 1-x,−y,1-z, Cg(1) and Cg(2),
given in Table 4, π–π interactions and their view without H atoms are given in Figure 4. The

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 3. orteP ellipsoid plot at 50% probability of the molecular structure of 3, 4 and 5, respectively.
hydrogens have been omitted for clarity.
Table 1. selected bond lengths (Å), angles (°) and torsion angles (°) of 3.
Bond
Distance (Å)
Torsion angle
(°)
ir–C5
ir–C1
ir–C4
ir–Cl
n1–C5
n1–C8
n1–C6
Bond angle
C5–ir–C1
C5–ir–C4
C1–ir–C4
C5–ir–Cl
C1–ir–Cl
C4–ir–Cl
C3–C4–ir
C2–C1–ir
2.054(3)
2.094(3)
2.162(4)
2.3989(17)
1.351(3)
1.439(3)
1.475(4)
(°)
94.96(14)
161.69(15)
81.32(19)
90.31(10)
159.59(12)
87.50(17)
114.5(4)
111.2(4)
C5–ir–C4–C3
C1–ir–C4–C3
Cl–ir–C4–C3
C8–n1–C5–ir
C6–n1–C5–ir
C4–ir–C5–n1
C1–ir–C5–n1
C4–ir–C5–n1
Cl–ir–C5–n1
C3–C2–C1–ir
C5–ir–C1–C2
C1–ir–C1–C2
C4–ir–C1–C2
Cl–ir–C1–C2
ir–C4–C3–C2
C5–n1–C6–C7
−75.9(8)
3.5(5)
−159.3(5)
4.5(4)
−166.4(3)
−5.8(7)ii
−71.5(3)
5.8(7)
88.8(3)
22.9(7)
148.3(3)ii
−120.0(3)ii
−13.7(3)
43.9(5)
8.0(8)
18.9(7)
package view of 3 (without H atoms and along the a-axis) is also given (Supplementary
Material, figure S1).
2.1. Computational methods
DFT computations of 3–5 were performed using the GAUSSIAN 09 W software package [19]
with the B3LYP (Becke, 3-parameter, Lee-Yang-Parr)/LanL2DZ (Los Alamos National Laboratory
2 Double Zeta) basis set.
Firstly, the geometries of the complex structures were optimized to obtain the global
minimum energy geometries. Afterward, UV–vis calculations performed with the same basis
set were used for the optimized molecule structures. The energy of the excited states
together with the ground-states for the optimized molecules were computed using DFT and
visualized by the GAUSSVIEW 5 software package [20].
2.2. Molecular electrostatic potential
Molecular electrostatic potential (MEP) analysis is a technique that shows the mapping of
the electrostatic potential surface of a complex using constant electron density which can

6
E. Ö. KARACA ET AL.
Table 2a. analogy of experimental and theoretical computation bond length and angle results of 3.
Ir-6–DiPh
(Exp.)
Ir-6–DiPh
(DFT)
Bond
Length (Å)
Length (Å)
ir–C5
ir–C1
ir–C1 *
ir–C4
ir–C4 *
ir––Cl
n1–C5
2.054(3)
2.094(3)
2.07818
2.15006
2.13750
2.22762
2.25420
2.56906
1.37799
2.094(3)ii
2.162(4)
2.162(4)ii
2.3989(17)
1.351(3)
n1–C8
n1–C6
1.439(3)
1.475(4)
1.45534
1.49925
angle
(°)
(°)
96.171
93.35947
39.5402
C5–ir–C1
C5–ir–C1 *
C1–ir–C1
C5–ir–C4
C1–ir–C4
C1–ir–C4 *
C5–ir–C4
C1–ir–C4
C1–ir–C4
C4–ir–C4
C5–ir––Cl
C1–ir––Cl
C1–ir–Cl *
C4–ir–Cl
C4–ir–Cl *
C4–C4–ir
C3–C4–ir
n1–C5–ir
n1–C5–ir
C1–C1–ir
C2–C1–ir
94.96(14)
94.96(14)ii
39.3(2)ii
161.69(15)
81.32(19)
93.38(19)ii
161.69(15)ii
93.38(19)ii
81.32(19)ii
36.3(3)ii
159.72787
80.60365
88.29325
159.72787
88.29325
80.60365
36.84753
90.43165
161.47182
157.44696
87.20786
89.85563
70.55637
109.61261
122.09478
121.88927
70.69689
114.83943
90.31(10)
159.59(12)
159.59(12)ii
87.50(17)
87.50(17)ii
71.83(15)ii
114.5(4)
122.09(15)ii
122.09(15)ii
70.34(12)
111.2(4)
be obtained using the molecular size, shape and electrostatic potential value. The electro-
static potential of compounds is due to the potential related to dipole moments, electron-
egativity, partial charges and the site of chemical reactivity of the molecule [21]. The positive
electrostatic potential corresponds to repulsion of the protons by the nuclei and the negative
electrostatic potential is due to the attraction of electrons by nuclei [21]. Furthermore, the
electrostatic potential around the molecule can be defined as the energy between the charge
distribution of the molecule and a unit positive test charge, as shown below:
ꢀ(r1)
ꢁr1 − rꢁ
ꢀ
ZA
V(r) =
−
dr1
ꢀ
ꢁRA − rꢁ
A
where ZA is the charge of nucleus A in RA, ρ(r1) is the density of the charge at r1 and r2 is the
dummy integration variable [22, 23]. The MEP of the complexes can also give information
about electrophilic and nucleophilic attack in the molecules. The MEP results for the com-
plexes are given (figure S2a–c). While the green bump regions on the complexes correspond
to positive electrostatic region, the red sink regions represent the negative electrostatic
potential regions (figure S2a–c) for 3–5. The value of the positive charged region is low when
compared with the negative charged region (figure S2a–c). Thus, it can be predicted that
the complexes have nucleophilic character and are sensitive against electrophilic attack. It

JOURNAL OF COORDINATION CHEMISTRY
7
Table 2b. analogy of experimental and theoretical computation bond length and angle results of 4.
Rh-6–DiPh
(Exp.)
Rh-6–DiPh
(DFT)
Bond
Length (Å)
Length (Å)
rh–C1
rh–C21
rh–C21a
rh–C24
rh–C24a
rh–C24b
n–C10
n–C21
2.061
2.119
2.119
2.192
2.192
2.192
1.342
1.444
1.481
(°)
2.08086
2.17221
2.15428
2.27365
2.30597
2.20583
1.37387
1.45551
1.50042
n–C12
angle
(°)
C1–rh–C21
C1–rh–C21a
C21–rh–C21a
C1–rh–C24
C21rh C24b
C21–rh–C24b
C1–rh–C24
C21–rh–C24
C21–rh–C24
C24–rh–C24
C1–rh–Cl
C21–rh–Cl
C21–rhCl
C24–rh–Cl
C24b–rh–Cl
C24–C24–rh
C22–C21–rh
n1–C1–rh
n1a–C1–rh
C24–C24–rh
C23b–C22b–rh
94.1
94.1
39.8
162.01
81.6
93.6
162.01
81.6
93.6
35.8
90.44
159.54
159.54
88.04
88.04
72.8
106.3
121.6
121.6
72.08
115.6
96.55341
93.19030
38.60115
160.93880
80.66932
87.90624
160.93880
80.66932
80.39730
35.50085
90.35779
162.46872
157.27202
87.53717
89.98380
70.98638
107.42468
121.70071
121.69516
71.37705
113.84766
is seen that the electrophilic region of the complexes have small values, meaning a weak
attractive potential for negative complexes, but the red region has a good electrophilic
effect, especially in the region of Cl ions. From the time dependent DFT calculations, the
electrophilic properties of the complexes align from highest to lowest as 3, 4 and 5 by the
intensity of the red region.
2.3. HOMO and LUMO analysis
The B3LYP/LanL2DZ basis set was used for the calculation of the energy of the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).
The HOMO and LUMO orbitals of 3–5 are given in Figure 5. It is well known that the HOMO
and LUMO orbitals and the energy gap, which can be calculated from the energy of the
HOMO and LUMO, are correlated for the chemical activity of molecules.
While the HOMO acts as an electron donor to donate an electron, the LUMO acts as an
acceptor to accept an electron. The obtained values from DFT are given inTable 5. The highest
HOMO and LUMO energies are obtained for 5 and the lowest HOMO and LUMO energies
are observed for 4. Therefore, when the energy gaps of the complexes are ranked in order
from highest to lowest, they can be written as 4, 3 and 5.

8
E. Ö. KARACA ET AL.
Table 2c. analogy of experimental and theoretical computation bond length and angle results of 5.
Ir-7–DiPh
(Exp.)
Ir-7–DiPh
(DFT)
Bond
Length (Å)
Length (Å)
ir–C10
ir–C4
ir–C4*
ir–C1
ir–C1*
ir–Cl
n–C10
n–C21
2.053
2.111
2.111
2.164
2.164
2.407
1.353
1.449
1.481
(°)
2.05877
2.15105
2.13775
2.23902
2.27080
2.56484
1.37688
1.45926
1.50508
n–C12
angle
(°)
C10–ir–C4
C10–ir–C4a
C4–ir–C4*
C10–ir–C1
C4–ir–C1
C1–ir–C4
C10–ir–C1
C4–ir–C1a
C4–ir–C1
C4–ir–C4
C10–ir–Cl
C4–ir–Cl
C4a–ir–Cl
C1–ir–Cl
C1a–ir–Cl
C1–C1–ir
C2–C1–ir
n11–C10–ir
n11a–C10–ir
C4–C4–ir
C3–C4–ir
95.8
95.8
38.8
161.7
81.1
89.06
161.7
93.04
93.0
98.47423
94.88054
39.53973
160.07156
80.31240
88.42896
162.92951
88.42896
96.48108
36.53316
88.45510
162.63069
156.14242
87.74727
89.42668
70.51058
109.46238
121.60599
123.37961
70.72443
115.08804
38.2
89.06
160.0
160.0
88.2
88.2
71.9
109.5
121.2
121.2
70.6
112.8
Table 3. hydrogen-bond interactions for 3.
D–H … Cg
D–H (Å)
H … Cg (Å)
D … Cg (Å)
D–H … Cg (°)
C4–h4 ... Cl
C9–h9 ... Cl
C19–h19 ... n1
C12–h12 … Cg2ii
C15–h15 … Cg1iii
0.980(5)
0.930(3)
0.930(4)
0.930(4)
0.930(5)
2.955(2)
2.552(1)
2.884(3)
3.044(2)
3.098(2)
3.159(7)
3.356(3)
3.183(5)
3.681(5)
3.768(5)
92.83(35)
144.90(19)
100.17(30)
127.19(14)
130.38 (18)
Cg1:C8/C9/C10/C11/C12/C13, Cg(2):C14/C15/C16/C17/C18/C19.
symmetry codes: [ii] 1–x,–y,1–z; [iii] –x,–y,1–z.
Table 4. π–π interactions for 3.
Cg–Cg
Cg–Cg (Å)
Dihedral angle (°)
Symmetry code
Cg(1)–Cg(1)
Cg(1)–Cg(2)
Cg(2)–Cg(1)
5.225(4)
4.907(4)
4.797(4)
26.69
24.52
64.11
–x,–y,1–z
–x,–y,1–z
1–x,–y,1–z
From the DFT calculations, it can be predicted that 4 is the most stable complex and that
5 is more reactive than 3. This observation is consistent with the work of Whittlesey and
Cavell [24]. They carried out a comparative study of ring-expanded NHCs with the focus on

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 4. π–π interactions for 3 (without h atoms) obtained by single crystal X-ray diffraction of
experimentally obtained crystal.
ring size in transition metal-catalyzed reactions. An upward trend in activity of the re-NHC
complexes was observed with increasing heterocycle ring size [24].
2.4. Photophysical properties of 3–5
The molecular orbitals of the complexes are related to the electronic transition process that
has strong correlation photophysical properties, such as UV–vis spectra, which are shown
in Table 6. Also, theoretical and experimental UV–vis spectra of complexes are compared in
Table 6; the results were found to be compatible.
The complexes in this study have a transition metal in a distorted square-planar environ-
ment, as given in the structural properties section. The fivefold degenerate d-orbitals on a
transition metal in the pure square-planar environment (D4h) generally split to four different
level orderings of b1g (x2 – y2) > b2g (xy) > eg (xz,yz) > a1g (z2) and the splitting energy for dif-
ferent coordination of d-orbitals can be in a range of visible color regions. The colors of the
square planar complexes are due to some d–d transitions together with charge transfer from
either the ligand-to-metal or metal-to-ligand [25].
It is already quite well known that for square-planar complexes there are three spin-al-
1
1
1
lowed d–d transitions corresponding to A1g−1A2g, A1g−1Eg and A1g−1B1g. Since parity is
important for the Oh and D4h systems, these three transitions are parity forbidden for electric
dipole transitions, the bands of the UV–vis spectra of d-d transitions would be expected to
have lower intensities. The UV–vis spectra of 3–5 are presented in Figure 6. It is seen that the
bands due to d–d transitions which correspond to a pure square geometry are not detected
properly. This is probably due to the distortion from the square-planar geometry on the
complexes. In this case, the energy of d electrons would be different from the pure square-pla-
nar environments. So the molecular orbital diagram of the complexes cannot be as pictured
as shown in figure S3, which is for the pure square-planar geometry. It can be noted that
ordering of d electrons may change in the complexes due to level of the distortion. Hence,
the term symbols mentioned in figure S3 would be different than the labeled ones for the
square-planar geometry.
The extinction coefficient (ε) of each complex was calculated according to Lambert–Beer
law. The calculated values of ε for complexes inserted in the UV–vis spectra, which is shown
in Figure 6. It was mentioned that the bands in the UV–vis spectra of the complexes have

10
E. Ö. KARACA ET AL.
Figure 5. homo (left) and lumo (right) of (a) 4, (b) 3 and (c) 5 by dft calculation.
Table 5. homo, lumo and eg of the complexes.
Complex
HOMO (eV)
LUMO (eV)
Eg (eV)
4
3
5
−5.1614
−5.1160
−5.0958
−1.2590
−1.2694
−1.4524
3.9024
3.8466
3.6364
more intense than the complex which has pure square-planar environment since the parity
forbidden transitions would disappear. In the UV–vis spectra of 3, the first band of highly
intense absorption at around 300 nm (ε ≈ 2692 M−1 cm−1) is mainly due to spin-allowed tran-
sitions since this shows high peak intensities in UV spectra. The band at around 360 nm
(ε ≈ 7092 M−1 cm−1) is due to ligand-metal charge-transfer transitions, and it is well known

JOURNAL OF COORDINATION CHEMISTRY
11
Table 6. Comparison of calculated and experimentally obtained uV–vis results of the complexes.
Excitation wavelength (nm)
3
5
4
Exp.
Calc.
Exp.
Calc.
Exp.
Calc.
364
–
–
352.32
388.5
410.63
–
–
440
359.42
387.62
451.46
–
408
–
358.46
388.76
440.69
Figure 6. experimental uV–vis spectra of 3, 4 and 5 in dmso (50 ppm).
that intra-ligand transitions of the organic ligands have lower intensities with similar results
being obtained for Ir(I) complexes [25].
Assuming there is no intra-ligand π orbital, the allowed charge transfer transitions for
square-planar complexes may be 1A1g−1A2u and 1A1g−1Eu [25, 26]. As should be anticipated,
these transitions are from molecular orbitals mainly localized on the ligands to molecular
orbitals mainly localized on metals. Both these transitions for 4 have moderate intensities
and are located at around 408 nm (ε ≈ 3096 M−1 cm−1) [26, 27]. Although the UV–vis spectrum
of 4 showed a similar absorption to Ir(I) complexes, the UV–vis spectrum of 5 shows the peak
at 440 nm (ε ≈ 325 M-1 cm-1). It can be noted that the band at around 400 nm for 4 is shifted
towards the high wavelength. This is mainly due to the fact that the influence of 4d electrons
from the distortion and ligand-field strength are greater than that of the 5d electrons.
When the calculated and experimentally obtained bands are compared, it is seen that
the experimentally obtained data of 3 are in agreement with the computational calculations.
However, the experimentally obtained bands of 4 and 5 do not correspond to the calculated
data and it should be pointed out that the calculations do not take into account π electrons.
The experimental peaks do, however, have broad halos around the peaks, which includes
the calculated data (Table 6).
Since Ir(I) has a d8 electronic configuration, the highest level of filled d-orbitals is b2g(xy),
such that the electronic ground state is 1A1g (as expected for all 1A1g ground-state electronic
configurations, our complexes are diamagnetic also). It must be pointed out here that this

12
E. Ö. KARACA ET AL.
labeling for the term symbols is named for pure square-planar complexes. Since our com-
′
plexes are distorted square planar, one may use 1A1 instead of 1A1g which may indicate the
distorted geometry. The energy level splitting of d orbitals in a square-planar environment
is pictured in figure S3.
3. Experimental
3.1. Materials and methods
Unless stated otherwise, all manipulations were performed using standard Schlenk tech-
niques under an argon or nitrogen atmosphere. Chemicals and solvents were purchased
from Sigma Aldrich Co. (Dorset, UK) and used directly without further purifications. Solvents
of analytical grade were freshly distilled using an MBraun SPS-800 solvent purification system.
Mass spectra were recorded by ESI (electron spray ionization) method. 1H NMR and 13C NMR
spectra were recorded using a Bruker Avance AMX 300 or 600. The chemical shifts are given
as dimensionless δ values and are frequency-referenced relative to the peak for TMS for 1H
and 13C. The NMR studies were carried out in high-quality 5 mm NMR tubes. Coupling con-
stants (J values) are given in Hz. NMR multiplicities are abbreviated as follows: s = singlet,
d = doublet, t = triplet, m = multiplet, bs = broad singlet.
3.1.1. General procedure for synthesis of N-aryl functionalized Re-NHC.HX
Compounds 1 and 2 were synthesized via reaction of 13-dibromopropane (1) or 14-diiodobu-
tane (2) (10 mmol) with N,N′-bis(2-phenylbenzen)formamidine (10 mmol) in the presence
of K2CO3 (5 mmol) in acetonitrile under aerobic conditions heated to reflux for 24 h (Figure 1).
Solution was left to cool at room temperature and the solvent was removed in vacuo. Residue
was solved in DCM (20 mL), filtered to separate all potassium salts and recrystallized via
addition of diethylether to get white crystals.
3.1.2. 6-DiPh.HBr, (1)
Yield: (3.42 g, 73%). mp. 256–258 °C (from dichloromethane). 1H NMR (400 MHz, CDCl3), δ/
ppm: 1.17 [t, J = 8 Hz, 2H, NCH2CH2CH2 N], 3.72 [t, J = 8 Hz, 4 H, NCH2CH2CH2 N], 7.06–8.30 [m,
18H, C6H4(C6H5)-2], 7.77 [s, 1 H, NCHN]. 13C NMR (100 MHz, CDCl3), δ/ppm: 19.3, 47.5, 128.4,
128.6, 128.8, 129.2, 129.9, 130.1, 131.2, 137.3, 137.5, 154.7. C28H25N2Br (MW = 469.41):
LCMS-MS (ESI): m/z, (%) 389.20 (100) [M – Br]+.
3.1.3. 7-DiPh.HI, (2)
Yield: (3.04 g, 75%). mp. 218–220 °C (from dichloromethane). 1H NMR (600 MHz; DMSO-d6),
δ/ppm: 1.48 [br s, 4H, NCH2CH2CH2CH2 N], 3.81 [bs, 4H, NCH2CH2CH2CH2 N] 7.42–7.61 [m,
18H, C6H4(C6H5)-2], 8.56 [s, 1H, NCHN]. 13C NMR (150 MHz, DMSO-d6), δ/ppm: 23.5, 53.8, 127.1,
128.3, 128.9, 129.1, 129.7, 131.3, 137.4, 137.5, 141.5, 160.3. C29H27N2I (MW = 530.44), LCMS-MS
(ESI): m/z, (%) 404.20 (100) [M + H]+.
3.2. General procedure for synthesis of re-Ir-NHC and Re-Rh-NHC complexes
The general synthetic procedure for 3–5 is as follows: The desired Re-NHC.HBr (1 or 2)
(1 mmol) was loaded into a flame-dried Schlenk and dried under high vacuum for 1 h
before suspended in dry degassed THF (30 mL) and KHMDS [au: The abbreviation should

JOURNAL OF COORDINATION CHEMISTRY
13
be given] (1 mmol) added in glove box. The solution was stirred for 45 min at room tem-
perature and transferred via an oven dried filter to a stirred THF solution of corresponding
[M(COD)Cl]2 (0.05 mmol) metal precursors. After the solution was stirred overnight at
room temperature, solvents were removed under vacuum and residue yellow (in case of
rhodium) or red solid (in case of iridium) was washed with cold hexane. For more purifi-
cation of 3–5 were passed a short column of silica with DCM to get as microcrystalline
solids.
3.2.1. [IrCl(6-DiPh)(COD)] (3)
Yield: (0.5 g, 75%). mp: 185–186 °C (from dichloromethane). UV–vis (DMSO), λmax/nm: 440. 1H
NMR (300 MHz, CDCl3), δ/ppm: 1.0–1.97 [m, 10H, CHCH2 and NCH2CH2CH2 N], 2.78 [t,
J = 2.4 Hz, 2H, CHCH2], 3.02 [m, 2H, NCH2], 3.35 [m, 2H, NCH2], 4.23 [t, J = 2.4 Hz, 2H, CHCH2],
7.28–7.52 [m, 16H, C6H4(C6H5)-2], 8.90 [d, J = 6.6 Hz, 2H, C6H4(C6H5)-2]. 13C NMR (75 MHz,
CDCl3), δ/ppm: 21.3, 28.3, 32.7, 49.5, 50.0, 80.1, 127.3, 127.6, 128.6, 128.8, 130.6, 133.5, 137.0,
140.1, 144.0, 205.9. Anal. Calcd for C36H36N2ClIr (%): C, 59.69; N, 3.87; H, 5.01. Found: C, 59.71;
N, 4.01; H, 5.15.
3.2.2. [RhCl(6-DiPh)(COD)] (4)
Yield: 0.5 g (80%). mp: 168–169 °C (from dichloromethane). UV–vis (DMSO), λmax/nm: 411. 1H
NMR (300 MHz, CDCl3), δ/ppm: 1.33–1.89 [m, 10H, CHCH2 and NCH2CH2CH2 N], 2.90 [m, 2H,
CHCH2], 3.07 [m, 2H, NCH2], 3.24 [m, 2H, NCH2], 4.69 [m, 2H, CHCH2], 7.28–7.65 [m, 16H,
C6H4(C6H5)-2], 9.34 [d, J = 9.0 Hz, 2H, C6H4(C6H5)-2]. 13C NMR (75 MHz, CDCl3), δ/ppm: 21.1,
27.7, 31.8 49.4, 65.5, 65.8, 94.9, 95.0, 127.3, 127.7, 128.0, 128.6, 128.7, 130.8, 132.2, 140.1,
140.7, 212.3, 212.7. Anal. Calcd for C36H36N2ClRh (%): C, 68.09; N, 4.41; H, 5.71. Found: C, 68.13;
N, 4.52; H, 5.82.
3.2.3. [IrCl(7-DiPh)(COD)] (5)
Yield: (0.29 g, 40%). mp: 220–222 °C (from dichloromethane). UV–vis (DMSO), λmax/nm: 361,
430. 1H NMR (300 MHz, CDCl3), δ/ppm: 0.85–1.61 [m, 12H, CHCH2 and NCH2CH2CH2CH2 N],
2.61 [m, 2H, CHCH2], 3.28 [m, 2H, NCH2], 3.69 [m, 2H, NCH2], 4.14 [m, 2H, CHCH2], 7.21–7.40
[m, 16H, C6H4(C6H5)-2], 8.77 [d, J = 3.9 Hz, 2H, C6H4(C6H5)-2]. 13C NMR (75 MHz, CDCl3), δ/ppm:
23.6, 28.1, 32.7, 49.3, 56.5, 81.0, 127.2, 127.3, 128.5, 128.9, 129.3, 130.5, 134.7, 137.2, 140.1,
144.5, 216.7. Anal. Calcd for C37H38N2ClIr (%): C, 60.19; N, 3.79; H, 5.19. Found: C, 60.32; N,
3.93; H, 5.36.
4. Conclusion
Synthesis and characterization of ring-expanded Ir and Rh-NHC complexes have been per-
formed and the structure of complexes was determined by XRD, NMR and other spectro-
scopic techniques. DFT study of complexes on account heterocycle ring size and metal center
diversity has been comparatively documented with experimental observations. According
to the observations, it appears that the experimental and theoretical results obtained are
highly compatible with each other. We calculated and compared the stability of the Ir and
Rh re-NHC complexes by DFT calculations. The results represent that the seven-membered
iridium complex, 5, displays a higher chemical activity due to lower energy gap than the
six-membered iridium, 3, and rhodium complex, 4.

14
E. Ö. KARACA ET AL.
Supplementary material
LCMS-MS spectra for 1 and 2, NMR spectra for 1–5, X-ray crystallographic data and refinement param-
eters and CIF for 3 (CCDC 1454752), 4 (CCDC 1511413) and 5 (CCDC 1511414) are supplied as supple-
mentary file.
Acknowledgements
Authors give special thanks to Dr F. Işık who performed the Gaussian calculations in his lab.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was financially supported by the Scientific and Technological Research Council of Turkey
(TÜBİTAK Project no 113Z285).
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