Syntheses, characterization, structure and redox properties of new Rh(III) cyclometallates incorporating azoimine ligands

Article abstract of DOI:10.1016/j.jorganchem.2005.07.076

The reaction of 2-(arylazo) aniline, 1 (HL-NH₂) [where HL-NH₂ is 2-(ArNN)C₆H₄-NH₂; Ar = C₆H₅ (for HL¹-NH₂), p-MeC ₆H₄(for HL²-NH₂) and p-ClC ₆H₄(for HL³-NH₂). H of HL stands for dissociable proton of Ar due to orthometallation], with Rh(PPh ₃)₃Cl (Wilkinson's catalyst) furnished the cyclometallated azoimine complexes of composition (L-NH)Rh(PPh₃)₂Cl (2). All the complexes were characterized by spectroscopic technique and confirmed by X-ray structure determination of (L³-NH)Rh(PPh₃) ₂Cl as a representative one. The azoimine chelate ring size of 2 is five membered where the phenyl ring adjacent to the chelate ring is distorted with four longer and two shorter bonds. This structural feature has been attributed to the delocalization of negative charge of the anionic ligands that formed in situ from the precursor (HL-NH₂) due to dissociation of an amino proton. The (L-NH)Rh(PPh₃)₂Cl complexes display an one electron oxidative response in the range of 0.331 to 0.436 V vs. SCE.

Full text of DOI:10.1016/j.jorganchem.2005.07.076

Journal of Organometallic Chemistry 690 (2005) 4816–4821
www.elsevier.com/locate/jorganchem
Syntheses, characterization, structure and redox properties
of new Rh(III) cyclometallates incorporating azoimine ligands
a
b
a,
*
Jahar lal Pratihar , Debprasad Patra , Surajit Chattopadhyay
a
Department of Chemistry, University of Kalyani, Kalyani, West Bengal 741235, India
Department of Chemistry, East Calcutta Girlsꢀ College, Lake Town, Kolkata, India
b
Received 7 June 2005; received in revised form 15 July 2005; accepted 22 July 2005
Available online 2 September 2005
Abstract
The reaction of 2-(arylazo) aniline, 1 (HL-NH2) [where HL-NH2 is 2-(ArN@N)C6H4–NH2; Ar = C6H5 (for HL1-NH2),
p-MeC6H4(for HL2-NH2) and p-ClC6H4(for HL3-NH2). H of HL stands for dissociable proton of Ar due to orthometallation], with
Rh(PPh3)3Cl (Wilkinsonꢀs catalyst) furnished the cyclometallated azoimine complexes of composition (L-NH)Rh(PPh3)2Cl (2). All
the complexes were characterized by spectroscopic technique and confirmed by X-ray structure determination of (L3-NH)-
Rh(PPh3)2Cl as a representative one. The azoimine chelate ring size of 2 is five membered where the phenyl ring adjacent to the
chelate ring is distorted with four longer and two shorter bonds. This structural feature has been attributed to the delocalization
of negative charge of the anionic ligands that formed in situ from the precursor (HL-NH2) due to dissociation of an amino proton.
The (L-NH)Rh(PPh3)2Cl complexes display an one electron oxidative response in the range of 0.331 to 0.436 V vs. SCE.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Cyclometallation; 2-(Arylazo) aniline; Wilkinsonꢀs catalyst; Rh(III); Redox
1. Introduction
strategy of oxidative addition of polydentate azo ligand
into the tri-coordinated species RhI(PPh3)2Cl which is
obtainable upon dissociation of a PPh3 ligand of Wilkin-
sonꢀs catalyst RhI(PPh3)3Cl in solution [6]. Recently, it
has been demonstrated that pre-coordination of sp2
nitrogen donor leads the metal to undergo orthometalla-
tion at pendent phenyl ring [7] and thus azobenzene is an
appropriate choice. A third anionic donor within the
azobenzene moiety is prerequisite, apart from (N,C)
coordination (of azobenzene), to satisfy the hexacoordi-
nation and residual positive charge of Rh(III) after oxi-
dative addition. The 2-(arylazo) aniline, 1, ligands
furnished azoimine (N, N) chelates of Pd(II), Pt(II),
and Au(III) in its anionic form (HL-NH)ꢀ, dissociating
an amino proton [8–10]. Therefore, we planned to carry
out reaction of Wilkinsonꢀs catalyst with 1, which
indeed, turn out to be successful since we could isolate
the cyclometallated products of Rh(III) of the type 2.
Metal assisted C–H bond activation is of much inter-
est in current chemical research. Metal–carbon bond
formation reactions of transition metals are important
in terms of C–H bond activation [1–3]. The metal med-
iated catalytic and stoichiometric organic transforma-
tions often involve transient organometallic species in
the intermediate steps [4,5]. Orthometallation is one of
the important routes to the synthesis of metal–carbon
bond cleaving the C–H bond. Although chemistry of
cyclometallated molecules is well developed, substantial
literature lacks examples of cyclometallated complexes
of Rh(III) incorporating azo ligands. We adopted the
*
Corresponding author. Tel.: +91 33 24149725; fax: +91 33
25828282.
E-mail address: scha8@rediffmail.com (S. Chattopadhyay).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.07.076

J. lal Pratihar et al. / Journal of Organometallic Chemistry 690 (2005) 4816–4821
4817
The diamagnetic (L-NH)Rh(PPh3)2Cl complexes were
characterized on the basis of analytical (C, H, N) and
spectral data and the structure was confirmed by X-
ray structure determination of 2c (see below).
R
N
2.2. Crystal and molecular structure of
(L3-NH)Rh(PPh3)2Cl
N
Suitable crystals of (L3-NH)Rh(PPh3)2Cl were grown
by slow diffusion of hexane into a dichloromethane solu-
tion. A perspective view of the molecule has been shown
in Fig. 1 and selected bond distances and angles are col-
lected in Table 1. The Rh(CNN)(P)2Cl coordination
sphere has distorted octahedral geometry. The dianionic
(L3-NH)2ꢀ ligand coordinates the Rh(III) in a triden-
tate(C, N, N) fashion along with two mutually trans
PPh3 and a chloride ligand. There is a CH2Cl2 solvent
molecule per asymmetric unit in the crystal lattice. The
Rh–C, Rh–Cl and Rh–P bond lengths of 2c are within
the normal range [6]. Rh–N(3) length is shorter than
Rh–N(1) because of stronger dp–pp back bonding with
the azo nitrogen, while the stronger trans influence of aryl
carbon may be attributed to the longer Rh–N(1) distance
[11]. Only one hydrogen [H(4a)] atom on N(1) could be
located from difference Fourier mapping, signifying dis-
sociation of the other amino proton consistent with the
1H NMR data (see below). The C(12)–N(1) bond (1.336
NH2
1
H
N
(
)
PPh3
)Rh PPh3 2 Cl
(L NH
1
R
Cl
(
)
NH)Rh PPh3 2 Cl
(L
2a
H
Rh
PPh3
2
(
)
)Rh PPh3 2 Cl
(L NH
3
b
Me
Cl
2
N
(
)
NH)Rh PPh3 2 Cl
(L
2c
N
R
(
)
]
[
)
Rh
(L NH
PPh 2Cl
2
3
Herein, we describe the reaction of 2-(arylazo) aniline
with Rh(PPh3)3Cl and formation of cyclometallated azoi-
mine chelates ofRh(III). Thecomplex formationshave been
authenticated on the basis of X-ray structural data and 1H
NMR studies. The redox properties of the new Rh(III)
cyclometallates have been studied electrochemically.
˚
˚
A) is shorter than C(6)–N(2) single bond (1.399 A) in
the same molecule and similar to a imine (˜C@N–) length
as a result of delocalization [8–10]. The rhodium center is
˚
closer to azo nitrogen N(3) (1.969 (1) A) and the N(2)–
˚
N(3) distance (1.2875 (14) A) is longer than the azo
˚
(–N@N–) distance (ꢁ1.25 A) in free azo molecules [12].
2. Results and discussion
2.1. Complex formation
The reaction of Rh(PPh3)3Cl with 2-(arylazo) aniline
(HL-NH2) in refluxing toluene in dinitrogen atmosphere
afforded green complexes of composition [(L-NH)Rh-
(PPh3)2Cl] (2). These are a new family of orthometallated
azoiminato (C, N, N) complexes of Rh(III) incorpora-
ting the dianionic (L-NH)2ꢀ that formed in situ from
the ligand precursor 2-(arylazo) aniline (HL-NH2). It is
believed that the reactions of Rh(PPh3)3Cl and (HL-
NH2) ligands proceeds according to Eq. (1), i.e., initial
dissociation of PPh3 from Rh(PPh3)3Cl followed by the
P(2)
N(2)
C(6)
C(7)
N(3)
Cl(2)
C(1)
Cl(1)
C(12)
Rh
N(1)
P(1)
oxidative addition of (L-NH)2ꢀ
.
PPh3
Rh (
)
)
Cl
PPh3
Rh ( PPh3
Cl
3
2
+
PPh3
H
N
NH2
N
ð1Þ
PPh3
III
Cl
N
R
Rh
N
PPh3
Fig. 1. ORTEP plot of 2c with atom-numbering scheme. Hydrogen
Toluene
N
R
atoms, except that on N1, are omitted for clarity.

4818
J. lal Pratihar et al. / Journal of Organometallic Chemistry 690 (2005) 4816–4821
Table 2. The spectrum of [(L3-NH)Rh(PPh3)2Cl] has
been shown in Fig. 2. One of the important features in
the UV–Vis spectra of the complexes is the high intensity
absorption near considerably lower energy (ꢁ780 nm).
The IR spectra of [(L-NH)Rh(PPh3)2Cl] in solid KBr
pellet displayed mNH as sharp single band in the range
Table 1
˚
Selected bond distances (A) and angles (°) for 2c
˚
Distances (A)
Rh–N(1)
Rh–N(3)
Rh–Cl(1)
Rh–P(1)
Rh–P(2)
2.1489(14)
1.9695(13)
2.4029(4)
2.3583(4)
2.3709(4)
2.0307(16)
1.2875(19)
1.337(2)
1.399(2)
1.383(2)
1.426(2)
1.410(2)
3345–3360 cmꢀ1, in contrast to the ligands where mNH
2
Rh–C(1)
of NH2 function appeared as split band in the range
3360–3458 cmꢀ1 [8–10], indicating the proton dissocia-
tion from –NH2 function of (HL-NH2) upon coordina-
tion. The mN@N of (HL-NH2) (1458–1474 cmꢀ1) were red
shifted to 1359–1371 cmꢀ1 in (L-NH)Rh(PPh3)2Cl signi-
fying the coordination of azo nitrogen [8–10]. A new
band, near 1600 cmꢀ1, in the IR spectra of the com-
plexes unlike the ligands, has been attributed to imine
formation [8–10]. The relevant IR data are collected in
Table 2. The IR spectra of 2 are given in Supplementary
materials (Figs. S1–S3).
N(2)–N(3)
N(1)–C(12)
N(2)–C(6)
N(3)–C(7)
C(7)–C(12)
C(7)–C(8)
C(9)–C(10)
C(10)–C(11)
C(11)–C(12)
C(8)–C(9)
1.412(3)
1.368(3)
1.428(3)
1.362(3)
Angles (°)
Cl(1)–Rh–P(1)
Cl(1)–Rh–P(2)
Cl(1)–Rh–N(3)
Cl(1)–Rh–N(1)
Cl(1)–Rh–C(1)
P(1)–Rh–P(2)
P(1)–Rh–N(3)
P(1)–Rh–C(1)
P(2)–Rh–N(3)
P(2)–Rh–N(1)
P(2)–Rh–C(1)
N(3)–Rh–N(1)
N(3)–Rh–C(1)
C(1)–Rh–C(2)
C(2)–C(1)–C(6)
88.41(1)
89.76(1)
179.52(4)
100.90(4)
100.41(5)
176.04(1)
91.96(4)
88.93(5)
89.85(4)
93.73(4)
87.95(5)
79.40(5)
79.30(6)
133.36(12)
117.54(15)
1
The well resolved H NMR spectra of (L-NH)Rh-
(PPh3)2Cl complexes are consistent with their struc-
tures. The aromatic proton resonances could be assigned
on the basis of shielding and proton–proton interac-
tions. The 1H NMR spectra of the complexes have been
compared and correlated with the corresponding spectra
of the free ligands, HL-NH2, a representative case for
(L3-NH)Rh(PPh3)2Cl and HL3-NH2 has been shown
in Fig. 3 along with the numbering scheme in the inset.
The significant features that have been taken into con-
sideration to elucidate the structures in solution are as
follows:
(i) The NH resonance of [(L-NH)Rh(PPh3)2Cl]
appeared as a singlet in the range of d 4.5–4.7
for one equivalent of proton, in contrast to two
equivalent protons for the –NH2 of free ligand,
confirming the dissociation of one –NH2 proton
of the ligands upon complexation;
(ii) the total count of aromatic protons matched well
with the orthometallation (data are given in Sec-
tion 3);
These observations are in support of backbone conjuga-
tion within the coordinated ligand [12]. The quinonoid
distortion in the phenyl ring (C7, C8, C9, C10, C11,
C12) adjacent to the five membered chelate with two
shorter (av. ꢁ 1.365 A) and four longer (av. ꢁ 1.415 A)
bonds is also in accordance with the delocalization and
imine formation [8–10]. Thus, the formation of five-mem-
bered azoimine chelate has been inferred.
˚
˚
(iii) the nature of spectra for 2a, 2b and 2c (Figs. S4–
S6) are similar in the up field segment (d 5.4–6.4)
of the aromatic region and were assigned to the
four protons of C8, C9, C10, C11 in ring B
(Fig. 3), while the resonances in the downfield
2.3. Spectral characterization and solution structure
The green complexes [(L-NH)Rh(PPh3)2Cl] displayed
characteristic UV–Vis spectra. Data are collected in
Table 2
UV–Visa, IRb and cyclic voltammetric data
Compound
k
max/nm (e/Mꢀ1 cmꢀ1
)
m/cmꢀ1
E1/2/Vc (DEp/mV)
mN–H
mC@N
mN@N
2a
2b
2c
a
780 (5350), 720 (6200), 290 (35900) 230 (54400)
780 (5750), 720 (6500), 290 (37550) 230 (58100)
780 (11500), 720 (13400), 290 (70800) 230 (103350)
3360
3344
3345
1599
1599
1601
1371
1359
1364
0.373(60)
0.331(65)
0.436(70)
In dichloromethane.
In KBr disc.
vs. SCE in dichloromethane solution using Bu4NClO4 supporting electrolyte.
b
c

J. lal Pratihar et al. / Journal of Organometallic Chemistry 690 (2005) 4816–4821
4819
2.4. Electrochemistry
(L-NH)Rh(PPh3)2Cl complexes exhibit one electron
reversible oxidative cyclic voltammetric response in the
range of 0.331–0.436 V vs. SCE in dichloromethane
solution. Data are collected in Table 3 and a representa-
tive cyclic voltammogram for 2c has been shown in
Fig. 4.
The oxidation has been assigned according to the
couple of Eq. (2) where [(L-NH)Rh(PPh3)2Cl]+ was
believed to be the Rh(IV) analogue of [(L-NH)Rh-
(PPh3)2Cl]. The coulomb count, during
½ðL-NHÞRhðPPh3Þ Clꢂþ þ e
! ½ðL-NHÞRhðPPh3Þ2Clꢂ
2
ð2Þ
Fig. 2. UV–Vis spectra of 2c.
coulometric oxidation of [(L-NH)Rh(PPh3)2Cl] at
ꢁ+0.6 V, matched with one electron transfer process
(Section 3). The cyclic voltammogram of the electrogen-
erated [(L-NH)Rh(PPh3)2Cl]+ did not display any
voltammetric response identical to that of the original
complex, indicating a transformation upon oxidation.
Therefore, the oxidative response of [(L-NH)Rh-
(PPh3)2Cl] has been assigned by comparing the assign-
ment of analogous azophenolato complexes of Rh(III)
[6]. The oxidation potential (E1/2) of [(L-NH)Rh-
(PPh3)2Cl] was found to be sensitive to the nature of
the substituent (R) present in the (HL-NH)ꢀ ligand
increasing linearly (Fig. 4) with increasing electron-with-
drawing character of the substituent. The plot of E1/2
sector of aromatic region (d 6.4–7.1) have been
assigned to the aromatic protons of C2–C5 of ring
A, since the spectral nature, in this region, varied
significantly depending on R;
(iv) a singlet near d 7.0 for the 2b and 2c (where
R = CH3 or Cl) is for none other than C3 proton,
diagnostic to orthometallation at C2 of ring A;
(v) two multiplets (ꢁd 7.16 and ꢁ7.5) of 12 protons
each and another multiplet (d ꢁ7.26) for six pro-
tons are in agreement with the two trans PPh3
ligands.
Fig. 3. A correlation of aromatic proton resonances of free (a) HL3-NH2 and (b) 2c. The numbering scheme is shown in the inset.

4820
J. lal Pratihar et al. / Journal of Organometallic Chemistry 690 (2005) 4816–4821
Table 3
Crystallographic data for 2c
reported procedure [8–10]. o-Phenylenediamine, nitro-
benzene, p-methyl and p-chloro nitrobenzene, triphenyl-
phosphine were purchased from Loba, Kolkata, India.
Rhodium trichloride was purchased from Arora Mat-
they, India. Rh(PPh3)3Cl were synthesized following a
reported procedure [13]. The ligand 2-(phenylazo) ani-
line, 2-(p-tolylazo) aniline and 2-(p-chlorophenylazo)
aniline were prepared following the reported procedure
[8–10].
Chemical formula
Formula weight
Space group
C48H38Cl2N3P2Rh Æ CH2Cl2
977.49
Monoclinic P21/n
13.641(3)
17.793(4)
17.962(4)
94.533(1)
0.71073
4346.37(17)
4
293(2)
1.494
0.752
0.0277
˚
a (A)
˚
b (A)
˚
c (A)
b (°)
˚
k (A)
3
˚
V (A )
Z
3.2. Physical measurements
Temperature (K)
q (Mg/mꢀ3
)
l (mmꢀ1
)
Microanalysis (C, H, N) was performed using a Per-
kin–Elmer 240C elemental analyzer. Infrared spectra
were recorded on a Perkin–Elmer L120-00A FT-IR
spectrometer with the samples prepared with KBr pel-
lets. Electronic spectra were recorded on a Shimadzu
UV-2401 PC spectrophotometer. 1H NMR spectra were
obtained on Bruker RPX 500 NMR spectrometers in
CDCl3 using TMS as the internal standard. Electro-
chemical measurements were made under dinitrogen
atmosphere using a PAR model VERSASTAT-II poten-
tiostat. A platinum disc working electrode, a platinum
wire auxiliary electrode and an aqueous saturated calo-
mel reference electrode (SCE) were used in a three-elec-
trode configuration. All electrochemical data were
collected at 298 K and are uncorrected for junction
potentials.
Ra (all data)
b
wR2 [I > 2r(I)]
0.0706
1.08
GOFc
a
R = RiFo| ꢀ |Foi/R|Fo|.
2
2
1=2
2
b
wR2 ¼ ½R½wðF 2o ꢀ F c2Þ ꢂ=R½wðF o2Þ ꢂ , where w ¼ 1=r2ðF 2oÞ þ ðaPÞ þ
bP; P ¼ ðF 2o þ 2F 2c Þ=3.
2
1=2
c
GOF ¼ ½R½wðF 2o ꢀ F 2c Þ ꢂ=ðn ꢀ pÞꢂ
.
3.3. Synthesis of complexes
3.3.1. (L1-NH)Rh(PPh3)2Cl
Rh(PPh3)3Cl (200 mg, 0.22 mmol) was added to a
solution of HL1-NH2 (44 mg, 0.22 mmol) in toluene.
The mixture was then refluxed under a dinitrogen atmo-
sphere for 4 h, when a green solution was obtained.
Evaporation of this solution afforded a dark solid which
was purified by thin layer chromatography on silica
plate with toluene as the eluent. A green band separated
and the complex was extracted from it with acetonitrile.
The green solid, obtained upon evaporation of the sol-
vent, was recrystallized from dichloromethane–hexane
to afford (L1-NH)Rh(PPh3)2Cl as a crystalline green so-
lid. Yield: 126 mg, (67%). Anal. Found: C, 67.16; H,
4.56; N, 4. 91. Calc. for RhC48H39ClN3P2: C, 67.13; H,
Fig. 4. Cyclic voltammogram of 2c in dichloromethane solution (0.1
M NBu4ClO4) scan rate 50 mV sꢀ1 at 298 K. A least-squares plot of
E1/2 values of the rhodium (III)–rhodium (IV) couple vs. r (Hammett
substituent constant) is shown in the inset.
1
values vs. Hammett substituent constant is linear as
shown in inset in Fig. 4.
4.54; N 4.89%. H NMR (CDCl3): d = 7.54 (d, PPh3,
12H), 7.26 (t, PPh3, 6H), 7.16 (t, PPh3, C3-H, 13H),
6.93 (d, C2-H, 1H), 6.70 (t, C3-H, 1H), 6.42 (t, C4-H,
1H), 6.31 (t, C9-H, 1H), 5.85 (d, C8-H, 1H), 5.78 (d,
C11-H, 1H), 5.46 (t, C5-H, 1H), 4.68 (s, NH, 1H).
3. Experimental
3.1. Materials
3.3.2. (L2-NH)Rh(PPh3)2Cl
This complex was prepared and purified following a
similar procedure as described for (L1-NH)Rh(PPh3)2Cl
using ligand HL2-NH2 in place of HL1-NH2. Yield:
108 mg, (60%). Anal. Found: C, 67.47; H, 4.67; N, 4.
All starting materials were used as received from
commercial sources; the solvents were purchased from
E. Merck, Kolkata, India, and purified and dried by

J. lal Pratihar et al. / Journal of Organometallic Chemistry 690 (2005) 4816–4821
4821
85. Calc. for RhC49H41ClN3P2: C, 67.43; H, 4.70; N,
Acknowledgments
4.81%. 1H NMR (CDCl3): d = 7.54 (m, PPh3, 12H),
7.26 (t, PPh3, 6H), 7.16 (t, PPh3, 12H), 6.90 (d, C2-H,
C4-H, 2H), 6.52 (d, C5-H, 1H), 6.31 (t, C9-H, 1H),
5.87–5.76 (m, C8-H, C11-H, 2H), 5.45 (t, C10-H, 1H),
4.60 (s, NH, 1H), 1.94 (s, p-CH3, 3H).
We are thankful to the DST (New Delhi) for funding
and fellowship to JP under DST-SERC Project (SR/S1/
IC-09/2003). The necessary laboratory and infrastruc-
tural facility are provided by the Department of Chem-
istry, University of Kalyani. The support of DST under
FIST Program to the Department of Chemistry, Univer-
sity of Kalyani is acknowledged. We acknowledge Prof.
Jui-Hsien Huang, National Changhua University of
Education, Changhua, Taiwan for X-ray studies.
3.3.3. (L3-NH)Rh(PPh3)2Cl
This complex was prepared and purified following a
similar procedure as described above using ligand
HL3-NH2. Yield: 112 mg, (60%). Anal. Found: C,
64.58; H, 4.30; N, 4.65. Calc. for RhC48H38Cl2N3P2:
C, 64.53; H, 4.25; N, 4.70%. 1H NMR (CDCl3):
d = 7.54 (d, PPh3, 12H), 7.26 (t, PPh3, 6H), 7.17 (t,
PPh3, 12H), 7.04 (s, C2-H, 1H), 6.86 (d, C4-H, 1H),
6.64 (d, C5-H, 1H), 6.31 (t, C9-H, 1H), 5.86 (d, C8-H,
2H), 5.80 (d, C11-H, 1H), 5.47 (t, C10-H, 1H), 4.75 (s,
NH, 1H).
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.
jorganchem.2005.07.076.
3.4. Electrochemical measurement
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using Bu4NClO4 supporting electrolyte at +0.6 V vs.
SCE, respectively. The coulomb counts 2.39 C matched
with the calculated values for 1 equiv. electron transfer.
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(2003) 3109–3113.
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5367–5375.
4. Crystallography
Crystal data were collected by the x-scan technique
on a Bruker SMART CCD diffractometer using Mo
Ka monochromator (k = 0.71043). The structure
solution was done by direct method with SHELXS-97 pro-
gram. Full-matrix least-square refinements were per-
formed using the SHELX-97 program (PC version). All
non-hydrogen atoms were refined anisotropically using
reflections I > 2r(I). Hydrogen atoms excepting H(4a)
were included at the calculated positions. The crystal
data and data collection parameters are listed in Table
3, Figs. S1–S6 supplementary materials and CCDC
269666 containing the crystallographic details for
complex 2c in CIF format are available free of charge
from The Cambridge Crystallographic Data Centre via
e-mail: www.ccdc.cam.ac.uk/data_request/cif.
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