Nickel, Ruthenium, and Rhodium NCN-Pincer Complexes Featuring a Six-Membered N-Heterocyclic Carbene Central Moiety and Pyridyl Pendant Arms

Article abstract of DOI:10.1021/acs.organomet.8b00022

NCN pincer ligand precursor 1·HX (X = Cl, PF₆), having a six-membered N-heterocyclic carbene central moiety and pyridyl pendant arms, was prepared via N-C cross-coupling of 2-bromo-6-tert-butylpyridine with propane-1,3-diamine, followed by ring closure with triethylorthoformate. The free ligand 1 was not accessible via deprotonation, yet its copper(I) complex 2 was prepared by reacting 1·HCl with CuI and potassium hexamethyldisilazide. It was used further as a transmetalation agent to prepare nickel(II) and ruthenium(II) dihalide NCN-pincer complexes 3a, 4a, and 4b, featuring a distorted square-pyramidal geometry at the metal, with the carbene ligand in the axial position. The rhodium(III) NCN-pincer complex 6b was obtained from 2 and [Rh(cod)Cl]₂ via transmetalation followed by oxidation with Cu(I) in THF. The rhodium(III) NCC-pincer complex 9b was prepared under similar conditions in acetonitrile, where a spontaneous rollover cyclometalation occurred. Complex 9b has a distorted octahedral geometry, with the acetonitrile ligand trans to the carbene. Very short Ru-C_NHC and Rh-C_NHC bonds were measured in complexes 4a, 4b, and 9b. Halogen exchange was observed by ¹H and ¹³C NMR spectroscopy in both ruthenium and rhodium systems during the transmetalation stage, and the purification was accomplished by pushing the respective equilibrium with excess halide.

Full text of DOI:10.1021/acs.organomet.8b00022

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Nickel, Ruthenium, and Rhodium NCN-Pincer Complexes Featuring a
Six-Membered N‑Heterocyclic Carbene Central Moiety and Pyridyl
Pendant Arms
Yanmin Jiang, Chris Gendy, and Roland Roesler*
Department of Chemistry, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4 Canada
*
S Supporting Information
ABSTRACT: NCN pincer ligand precursor 1·HX (X = Cl,
PF ), having a six-membered N-heterocyclic carbene central
6
moiety and pyridyl pendant arms, was prepared via N−C cross-
coupling of 2-bromo-6-tert-butylpyridine with propane-1,3-
diamine, followed by ring closure with triethylorthoformate.
The free ligand 1 was not accessible via deprotonation, yet its
copper(I) complex 2 was prepared by reacting 1·HCl with CuI
and potassium hexamethyldisilazide. It was used further as a
transmetalation agent to prepare nickel(II) and ruthenium(II)
dihalide NCN-pincer complexes 3a, 4a, and 4b, featuring a
distorted square-pyramidal geometry at the metal, with the
carbene ligand in the axial position. The rhodium(III) NCN-
pincer complex 6b was obtained from 2 and [Rh(cod)Cl] via
2
transmetalation followed by oxidation with Cu(I) in THF. The rhodium(III) NCC-pincer complex 9b was prepared under
similar conditions in acetonitrile, where a spontaneous rollover cyclometalation occurred. Complex 9b has a distorted octahedral
geometry, with the acetonitrile ligand trans to the carbene. Very short Ru−C
and Rh−C
bonds were measured in
NHC
NHC
1
13
complexes 4a, 4b, and 9b. Halogen exchange was observed by H and C NMR spectroscopy in both ruthenium and rhodium
systems during the transmetalation stage, and the purification was accomplished by pushing the respective equilibrium with
excess halide.
INTRODUCTION
derivatives D have shown catalytic activity toward Heck
11
■
coupling. Pincer complexes E, (M = Ni, Rh, Ir) incorporating
a polycyclic, chiral backbone derived from (1R)-camphor have
been investigated as catalysts for hydrogenation and transfer
The robust and easily tunable 2,2′:6′,2″-terpyridine NNN-
pincer ligands (terpy or tpy) have found numerous applications
1
in coordination and materials chemistry, as well as catalysis.2
12
hydrogenation reactions. Only two types of PCP pincer
ligands incorporating six-membered NHCs in the central
positions have been reported to date. Complexes F (M = Ru,
Os, Rh, Ir) were obtained from the corresponding N CH
Notable are recent advances in the activation of dinitrogen,
3
labilization of the NH bonds in ammonia, carbon dioxide
4
5
reduction, and especially water splitting. The replacement of
pyridine moieties with N-heterocyclic carbenes (NHCs) in the
terpyridine scaffold is expected to increase the electron density
2
2
precursors either via a double C−H bond activation or via a
1
3
6
single C−H bond activation followed by hydride abstraction.
at the coordinated metal, although the precise impact of this
exchange on the reactivity at the metal center is difficult to
predict.
The methodology was expanded to the synthesis of dinuclear
rhodium complexes G with perylene-based PCP ligands.
C -symmetric NCN-ligands featuring pyridyl donors bonded
14
2v
Six-membered NHCs were serendipitously incorporated in
directly to the more common 5-membered NHCs in the central
position have been shown to coordinate exclusively in a
bidentate fashion as in H, due to the excessively large NN bite
the central position of potential NCN-pincer architectures A
7
(
(
M = Pd, Rh) well before they could be isolated as free ligands
Chart 1), although the NCN coordination mode has not been
documented in these derivatives. Complexes B on the other
15
8
imposed by the narrower carbene angle (Chart 2). Longer
pendant arms allow for pincer coordination and a number of
hand were the product of targeted, modular synthesis (M = Cr,
16
9
a,b
complexes I (M = Ru, Pd, Pt) have been described. Their
benzimidazole analogues J are also well-known (M = Ru, Ni,
Fe).
Iron derivatives in particular were extensively
investigated, and the strongly electron-donating ligand proved
to be capable to stabilize complexes having the metal in formal
oxidation states 0, I, II, and III.
17
Pd, Pt). The nonpincer coordination chemistry of Group 11
9d,e
and 12 metals with the ligands shown in complexes I has been
Some of these iron
complexes were shown to be proficient at promoting lactide
9
c
polymerization. The photophysical properties of derivatives C
Received: January 12, 2018
10
(
M = Ru, Pd, Pt) have been discussed, while palladium
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00022
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Article
Chart 1. Pincer Complexes with Six-Membered NHCs in the
Central Position
Scheme 1. Synthesis of Ligand Precursors 1·HX and
Copper(I) Complex 2
Chart 2. Complexes Featuring C -Symmetric NCN-Pincer
Ligands with Five-Membered NHCs in the Central Position
and Pyridyl Pendant Arms
2
v
RESULTS AND DISCUSSION
■
Ligand Synthesis. The desired N,N′-dipyridylpropane-1,3-
diamine was obtained by reaction of two equivalents 2-bromo-
20
6
-tert-butylpyridine with propane-1,3-diamine (Scheme 1) in
toluene under Buchwald−Hartwig amination conditions: 0.5
mol % tris(dibenzylideneacetone)dipalladium (Pd (dba) ), 1
2
3
mol % rac-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene
(
rac-BINAP), and tBuONa. The product was obtained as a
thick red oil that was used in the subsequent synthetic step
without further purification. Depending on the presence of
minor impurities that affect its rate of exchange in CDCl , the
3
1
H NMR spectrum of this precursor may or may not feature a
resonance corresponding to the NH proton. The rate of
exchange also has an impact on the coupling pattern of the
extensively investigated, and these moieties have also been
incorporated in tetradentate cyclophane ligands. Of note is that
all NCN-pincer ligands featuring central NHC moieties and
pyridyl pendant arms that have been reported to date lack any
substitution of the pyridyl ring, precluding steric control of the
chemistry at the metal center.
3
methylene bridge adjacent to this nitrogen (triplet, J = 6.7
HH
3
3
Hz, or triplet of doublets, J = 6.7 Hz, J = 6.1 Hz). The
HH
HH
method could not be expanded to the synthesis of the mesityl-
and 2,4,6-triisopropylphenyl-substituted pyridyl analogues: the
C−N cross-coupling to the propane-1,3-diamine backbone was
unsuccessful for these substrates. The ring-closing reaction to
give ligand precursor 1·HCl or 1·HPF6 (Scheme 1) was
completed in a classical fashion using excess triethylorthofor-
We recently reported the reversible, ligand-assisted activation
of ammonia on a nickel complex of type B (ML = Ni·KCl, R =
n
1
8
Dipp, R′ = Ph). The carbene assumed a highly uncommon,
noninnocent role in this reaction, with the highly nucleophilic
carbene carbon binding the NH proton while the nickel center
connected to the amide functionality, through a mechanism
that was not yet elucidated. Aiming to improve on the stability
of the ligands in B and further expand the chemistry of NHC-
mate and the respective dry ammonium salt. The N CH proton
2
in 1·HCl resonates at 10.72 ppm, with the corresponding
carbon resonance at 148.7 ppm. The N CH bridge turned out
2
to be extremely sensitive toward hydrolytic cleavage, with all
synthesis and handling requiring scrupulous exclusion of
moisture. A silver complex to be used in transmetalation
could not be synthesized because the water byproduct of the
1
9
based pincer systems, we explored the synthesis and
properties of ligands 1, formally obtained by adorning the
ligands in complexes of type C with tert-butyl groups (Scheme
reaction between 1·HCl and Ag O induced hydrolytic ring
2
opening of the ligand. The hydrolysis of N,N′-disubstituted-
1
,4,5,6-tetrahydropyrimidinium ions under basic conditions to
21
1). The replacement of the imine pendant arms with pyridyl
give acyclic aminoformamides is well-known. Interestingly,
closely related derivatives C were prepared in protic solvents,
including water and methanol, and hydrolytic opening of the
functionalities was expected to substantially improve the
robustness of the metal systems.
B
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Article
1
0
central ring was not reported as being a problem. As observed
with analogues B, the free carbene 1 was not stable and could
not be accessed via deprotonation even at low temperatures.
Copper(I) Iodide Complex 2. Copper carbene complexes
have been found to be excellent transmetallating reagents,
employable when more common avenues to carbene derivatives
very long, yet not unprecedented for tetracoordinated pyridyl
complexes of copper(I).
Nickel(II) Complex 3a. Reaction of 2 with NiBr (dme) in
2
THF at room temperature gave nickel(II) dibromide complex
3a as a green, paramagnetic solid in 66% yield (Scheme 2).
9a,22
are not successful.
1·HPF showed no reactivity toward
Stryker’s reagent, ([(PPh )CuH] ), but this reagent led to
deprotonation and subsequent copper-carbene bond formation
Scheme 2. Synthesis of Nickel(II) and Ruthenium(II)
Complexes 3 and 4
6
23
3
6
with 1·HCl. However, the resulting copper complex did not
undergo transmetalation with [Rh(cod)Cl] (see Supporting
2
Information). Fortunately, the more suitable transmetallating
agent 2 could be prepared by adding a THF suspension of the
ligand precursor 1·HCl to a THF suspension of CuI containing
potassium hexamethyldisilazide (KHMDS) at low temperature.
The resulting copper complex was isolated in 37% yield as a
pale orange solid (Scheme 1) that could be recrystallized from
1
toluene. The broad H NMR signals suggested that at room
temperature complex equilibria could be present in CD Cl , as
2
2
Single crystals were grown by slow evaporation of a saturated
dichloromethane solution. The complex adopts a distorted
trigonal bipyramidal geometry in the solid state (Figure 2), with
previously described for copper(I) complexes with nitrogen-
2
4
based pincer ligands. A crystallographic determination
revealed an ionic structure, [Cu(1) ] [ICu(μ-I) CuI], in the
2
2
2
solid state. The planar, doubly charged [ICu(μ-I) CuI] anion
2
featuring bridging and terminal iodide ligands has been
2
5
observed before. In the [Cu(1) ] cation, the copper center
2
is tetracoordinated by two chelating ligands in a coordination
geometry that could be described as strongly distorted
26
(flattened) tetrahedral, or distorted disphenoidal (Figure 1).
Figure 2. Solid-state structure of 3a with thermal ellipsoids drawn at
5
0% probability level. All hydrogen atoms and the dichloromethane
crystallization solvent were omitted for clarity. Selected bond lengths
(
2
1
Å) and angles (deg): Ni1−C1 = 1.887(5), Ni1-N = 2.218(4),
.229(4), Ni1−Br = 2.4752(9), 2.4836(9); N3−Ni1−N4 =
57.83(16), Br1−Ni1−Br2 = 164.81(3), N1−C1−N2 = 120.5(5).
the two nitrogen atoms in the axial position forming a fairly
acute N3−Ni1−N4 angle of 157.83(16)°. The Br1−Ni1−Br2
angle is in turn remarkably wide, at 164.81(3)°, so that the
description of the structure as distorted square pyramidal with
the carbene carbon in the axial position is equally accurate.
Such geometry is fairly unusual in pincer complexes of nickel,
and when observed, it is associated with steric blocking of the
sixth coordination site at the metal. Only one (terpy)NiBr2
compound is available for comparison, and its metric
parameters are similar, except for a much narrower Br−Ni−
Figure 1. Solid-state structure of the cation in 2 with thermal ellipsoid−s
drawn at 50% probability level. All hydrogen atoms and the Cu2I42
counterion were omitted for clarity. Selected bond lengths (Å) and
angles (deg): Cu1−C1 = 1.930(4), Cu1−C23 = 1.930(4), Cu1−N1 =
2
.374(3), Cu1−N5 = 2.396(3); C1−Cu1−C23 = 151.87(16), N1−
Cu1−N5 = 120.85(12).
Both noncoordinating pyridine groups are rotated by ca. 180°,
with the nitrogen atoms oriented away from the metal. The
carbene ligands form a C1−Cu1−C23 angle of 151.87(16)°,
which could be seen as the axial arrangement of the
disphenoidal geometry, while the pyridyl ligands form a more
acute N1−Cu1−N5 angle of 120.85(12)° in the equatorial
plane. The two copper−carbene bonds Cu1−C1 and Cu1−
C23 are equivalent (1.930(4) Å) and fall in the typical range for
NHC complexes. The two copper-pyridyl bonds Cu1−N1 and
Cu1−N5 are similar in length (2.374(3) and 2.396(3) Å) and
27
Br angle of 129.51(6)°. A comparison of the pincer ligand
binding reveals that the C−Ni bond in 3a is 0.1 Å shorter than
the central N−Ni bond in its terpy analogue, while the N−Ni
bonds in 3a are 0.1 Å longer than their counterparts in the
terpy complex. The effective magnetic moment of 3a,
28
determined using Evans method, had a value of 3.51. The
elemental analysis, crystal structure, and mass spectrum of 3a
indicate that no significant bromide−iodide exchange leading to
3c and 3b has taken place during the synthesis.
C
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Ruthenium(II) Complexes 4a and 4b. Reaction of 2 with
[
RuCl (p-cymene)] in THF at room temperature (Scheme 2)
2 2
gave a product that was isolated as a burgundy colored,
diamagnetic powder. Multinuclear NMR characterization in
CD Cl revealed the presence of a major constituent, as well as
2
2
closely related impurities (Figure 3). The major product,
Figure 4. Solid-state structure of 4a with thermal ellipsoids drawn at
5
0% probability level. All hydrogen atoms and the THF crystallization
solvent were omitted for clarity. Selected bond lengths (Å) and angles
(
2
1
deg): Ru1−C1 = 1.820(6), Ru1-N = 2.117(5), 2.132(6), Ru1−Cl =
.446(2), 2.441(2); N3−Ru1−N4 = 160.5(2), Cl1−Ru1−Cl2 =
68.87(6).
Figure 3. Progress of the halogen exchange reaction of crude 4a with
1
LiI in CD Cl , monitored by H NMR.
2
2
identified as [Ru(1)Cl ], 4a, exhibited a resonance correspond-
2
1
ing to the tert-butyl signal at 2.05 ppm in the H NMR
spectrum and one corresponding to the carbene carbon at
1
3
2
40.9 ppm in the C NMR spectrum. This represents a
substantial downfield shift from 148.7 ppm in the ligand
1
3
precursor 1·HCl, and compares well with the carbene
C
0
1
chemical shift in ruthenium analogues C (235−237 ppm).
1
13
The signals at 2.02 ppm ( H NMR) and 238.0 ppm ( C
NMR) were assigned to the [Ru(1)I ] impurity, 4b, while the
2
1
remaining resonance at 2.04 ppm in the H NMR spectrum was
Figure 5. Solid-state structure of 4b with thermal ellipsoids drawn at
0% probability level. All hydrogen were omitted for clarity. Selected
bond length (Å) and angles (deg): Ru1−C1 = 1.815(2), Ru1-N =
attributed to [Ru(1)ClI], 4c. The halogen exchange equili-
brium could be pushed both ways with AgCl or LiI (Figure 3),
allowing for the isolation of pure 4a and 4b.
5
2
.0956(18), 2.1227(18), Ru1−I = 2.7213(2), 2.7043(2); N1−Ru1−
Single crystals of 4a were grown by slow evaporation of a
saturated THF solution. The complex adopts a square
pyramidal geometry with the carbene carbon in the axial
position in the solid state, very similar to that of 3a (Figure 4).
Only few pincer complexes of pentacoordinated ruthenium(II)
N4 = 161.15(7), I1−Ru1−I2 = 177.978(8).
with 177.978(8)°, even wider than its Cl−Ru−Cl counterpart
in 4a. This is a result of the steric interaction of the larger
halogen with the tert-butyl groups that block the coordination
site in trans to the carbene carbon, and brings the geometry at
the metal very close to an idealized square pyramid. At
1.820(6) and 1.815(2) Å, the Ru−C bonds in complexes 4a
29
dihalide (LRuX ) are known, and neither of them features a
2
terpy ligand. Most frequently, the sixth coordination site is
occupied by carbon monoxide or a phosphine, and in some
cases where the sixth coordination site is available, agostic and
anagostic interactions involving the ligand substituents have
and 4b are the shortest Ru−C
bonds reported to date. They
NHC
are substantially shorter than the closest reported values of
1.885(7) and 1.892(3) Å, measured in pincer and pentadentate
2
9b,d,f
been identified at this position.
The two Ru−Cl bonds are
30
almost colinear, with the Cl1−Ru−Cl2 angle measuring
complexes featuring pyridyl pendant arms. The average Ru−
CNHC bond measures 2.05(5) Å over 1279 structures (vs
1
68.87(6)°. The N3−Ru1−N4 angle is, at 160.5(2)°, fairly
31
typical of pincer geometry. The Ru−C bond in 4a is 0.15 Å
2.07(5) Å over 792 structures for Ru−Caryl). To this
remarkably short bond contribute, among other influences,
the lack of a trans substituent, the σ-donor ability of six-
membered carbenes, which is slightly superior to that of their
five-membered counterparts, and the rigid geometry of the
pincer ligand, with the pendant arms holding the metal close to
the central donor. In turn, the Ru−N bonds in 4a and 4b are,
with 2.096(2)−2.132(6) Å, somewhat longer than the average
shorter than the corresponding distance in the octahedral,
10
homoleptic compound C, reflecting the lack of a trans
substituent in the former.
Single crystals of 4b were grown from CH Cl2 and a
2
crystallographic determination revealed a structure similar to
that observed for 4a (Figure 5). Aside from the length of the
ruthenium-halogen bonds, the metric parameters of the
structure are effectively identical to those measured in 4a,
with one notable exception. The I−Ru−I bond angle in 4b is,
reported Ru−N
bond in 2,2′:6′,2″-terpyridine complexes
pyridyl
3
1
(2.07(2) Å). The steric strain between the tert-butyl
D
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substituents is alleviated by a twisting of the planes of pyridyl
rings, which form dihedral angles of 18.6° (3a), 18.1° (4a), and
of Rh(III) rollover cyclometalation product 9b, which was
isolated as an orange solid (Scheme 3). In THF, the reaction
did not proceed past 6b, and after isolation, pure 6b did not
convert to 9b in acetonitrile, in the presence or absence of CuI.
Rollover cyclometalation has been observed on a few occasions
in complexes with N-pyridyl- and N-(1,8-naphthyridyl)-NHC
ligands, exclusively on iridium and rhodium(I), and in all cases
a sterically demanding substituent was present in the 6 position
of the pyridyl ring. It appears therefore that the steric
hindrance of the tert-butyl groups on the pendant arms has a
non-negligible contribution to the driving force behind the
formation of 9b.
1
4.1° (4b) with each other.
Rhodium(III) Complexes 6b and 9b. Reaction of 2 with
[
Rh(cod)Cl] in THF at room temperature gave a diamagnetic
2
compound initially believed to be Rh(I) complex 5 (Scheme 3).
Scheme 3. Synthesis of Rhodium Complexes 6a−c and 9b
32
Mechanistically, precursor 5b could undergo rollover cyclo-
metalation to generate C -symmetric Rh(III)-pincer complex
s
7b. Reductive elimination of HI in 7b, potentially under the
influence of the intramolecular free pyridine site generated in
the previous step, could lead to 8b. The oxidation of the latter
to 9b could be accomplished with the copper iodide byproduct
present in the reaction mixture, as postulated above for the
transformation of 5 to 6. It is highly unlikely that Rh(III)
complex 6b is an intermediate in the formation of 9b. It is
generally accepted that rollover cyclometalation requires a
It was isolated as a brown solid and shown to have time-
averaged C2v symmetry by H NMR, and limited solubility in
1
33
Rh(I) precursor for C−H oxidative addition, and there are no
obvious paths for the conversion of 6 to a Rh(I) intermediate
organic solvents. Halogen exchange with AgCl in CH CN-d ,
3
3
1
13
however, was monitored by H and C NMR spectroscopy
Figure 6) and indicated that the isolated product was the ionic
such as 5. Reductive elimination of I is thermodynamically
2
(
unfavorable and the reduction of Rh(III) to Rh(I) in the
presence of CuI is incompatible with the postulated role of CuI
as an oxidant in the formation of 6. It is rather likely that the
higher polarity and coordinating ability of acetonitrile lowers
the activation barrier for the formation of 7b from the common
intermediate 5b. The critical role of donor solvents in
promoting efficient rollover cyclometalation is well-
33,34
known.
No spectroscopic evidence for the existence of
intermediates 5, 7, and 8 could be obtained.
Single crystals of 9b were grown by slow evaporation of a
saturated acetonitrile solution and the solid-state structure
confirmed the proposed geometry of the compound (Figure 7).
Figure 6. Progress of the halogen exchange reaction of crude 6b with
1
AgCl in CH CN-d , monitored by H NMR. * corresponds to
3
3
CH CN-d .
3
2
Rh(III) iodide 6b, structurally very similar to Ru(II) complex
b described above. In the presence of excess AgCl, diiodide
complex 6b led to the formation of dichloride complex 6a, via
the chloroiodide intermediate 6c. The existence of the latter is
incompatible with a monohalide complex such as 5. The
needle-shaped crystals of 6b were too thin for a diffraction
experiment and its identity could not be confirmed crystallo-
Figure 7. Solid-state structure of 9b with thermal ellipsoids drawn at
50% probability level. All hydrogen atoms and the acetonitrile
crystallization solvent were omitted for clarity. Selected bond length
4
(
Å) and angles (deg): Rh1−C10 = 1.903(3), Rh1−C8 = 1.994(3),
Rh1−N4 = 2.346(3), Rh1−N5 = 2.130(3), Rh1−I = 2.6535(3),
2
.6687(3); C8−Rh1−N4 = 158.78(11), C10−Rh1−N5 = 172.85(12),
I1−Rh1−I2 = 174.332(11).
graphically, but the ESI-HRMS spectrum featured the expected
+
[
6b-I] ion. The transmetalation of 2 with [Rh(cod)Cl] is
Rollover cyclometalation led to the formation of a monoanionic
NCC-pincer ligand on a pseudooctahedral Rh(III) center. The
pyridyl-NHC-aryl pincer ligand architecture has not been
described before; however, its analogous 6-phenyl-2,2′-
bipyridyl (pyridyl−pyridyl-aryl) is ubiquitous in coordination
chemistry. A handful of examples have been obtained via
rollover cyclometalation in 2,2′:6′,2″-terpyridine on platinum
2
expected to yield in the first step C -symmetric Rh(I)-pincer
2
v
complex 5, which could be oxidized to 6 by the copper iodide
byproduct. This proposal is supported by the observation of a
copper mirror on the walls of the reaction vessel.
The reaction of 2 with [Rh(cod)Cl] in acetonitrile took an
entirely different path than in THF, leading to clean formation
2
E
DOI: 10.1021/acs.organomet.8b00022
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3
5
and ruthenium. With 1.903(3) Å, the Rh−C10 bond in 9b is
CNHC bonds were measured in 4a, 4b, and 9b, the shortest on
record for the respective metal-carbene complexes, attesting to
the superior binding ability of the NCN and NCC-pincer
ligands developed herein. The coordination of molecules small
enough to fit into the coordination pocket in trans to the
extremely short, on par with the shortest reported Rh−C
bonds, which were measured in SCS- and PCP-pincer systems
1.892(12) - 1.905(6) Å). For comparison, the average Rh−
CNHC bond measures 2.03(4) Å over 829 structures (vs 2.01(5)
NHC
36
(
31
Å over 722 structures for Rh−C ). The substantial steric
carbene moiety of square pyramidal complexes 1MX , as well as
aryl
2
influence of the tert-butyl substituents onto the coordination
site in trans to the carbene ligand is reflected by the bond angles
involving the acetonitrile ligand: N4−Rh1−N5 = 109.74(10)°
and C8−Rh1−N5 = 91.48(11)°. The dihedral angle formed by
the best planes of the two pyridyl rings, which was discussed
above as an indicator of the steric strain between the tert-butyl
substituents, is only 3.7° in 9b, reflecting the alleviating effect
rollover cyclometalation had on the steric strain in this
molecule.
the chemistry of NHC-based pincer ligands obtained via
rollover cyclometalation, are currently being investigated.
EXPERIMENTAL SECTION
■
General Considerations. Unless otherwise specified, all oper-
ations were carried out with careful exclusion of air and moisture using
standard Schlenk and glovebox techniques. Solvents used in the
preparation of air- and/or moisture-sensitive compounds were dried
by using an MBraun Solvent Purification System fitted with alumina
columns (dichloromethane, toluene, and pentane), or dried by
refluxing over potassium and subsequent distillation (THF, benzene
and hexanes) under a positive pressure of argon. Starting materials
were purchased from commercial suppliers, or prepared according to
literature procedures. Deuterated solvents were vacuum-distilled over
potassium (THF-d , benzene-d and toluene-d ) or CaH (CD Cl
In spite of the marked differences in the σ-donor and π-
acceptor properties of NHCs vs the phenyl anion,
3
7
a
comparison of the M−C
bonds in 4a,b and 9b with all
NHC
M−C bonds reported in the literature may be warranted. It
aryl
reveals that the shortest Ru−C bonds measure 1.901(9)−
aryl
8
6
8
2
2
2
1
C
.939(3) Å and are found in NCN-pincer complexes (vs Ru−
38
and CH CN-d ) and then degassed using three freeze−pump−thaw
3 3
cycles. All NMR spectra were acquired on Bruker Avance III 400 or
Avance I 400 spectrometers and chemical shifts are reported in δ units
= 1.820(6) and 1.815(2) Å in 4a and 4b, respectively).
NHC
The shortest reported Rh−Caryl bonds are also found in NCN-
pincer complexes and measure 1.860(12)−1.878(5) Å and (vs
(
ppm) using the solvent as an internal reference: CH Cl -d (5.32
2 2 1
39
1
13
1
Rh−C
= 1.903(3) Å in 9b). Although the Ni−C
bond
ppm, H) and CH Cl -d (54.00 ppm, C); CHCl (7.24 ppm, H)
and CDCl (77.23 ppm, C); acetone-d (2.05 ppm, H) and acetone-
3 5
d (29.92 ppm, C); CH CN-d (1.94 ppm, H) and CH CN-d (1.39
NHC
NHC
2 2 2 3
13
1
length in 3a does not stand out (1.887(5) Å vs the shortest
Ni−C bonds reported in the literature: 1.742(2)−1.795(7)
1
3
1
NHC
6 3 2 3 3
13
1
ppm, C); benzene-d (7.16 ppm, H) and benzene-d (128.39 ppm,
Å and the average Ni−C
bond: 1.90(5) Å over 896
5
6
NHC
13
1
8,40
C).
structures),
it can be noted that the shortest reported Ni−
2
0
2
-Bromo-6-tert-butylpyridine.
In a two-neck flask, 2,6-
Caryl bonds measure 1.788(10)−1.819(4) Å and are found, with
one exception, in NCN-pincer complexes (vs the average Ni−
dibromopyridine (12.30 g, 51.92 mmol) and copper iodide (2.48 g,
13.0 mmol, 25 mol %) were stirred in THF (50 mL) at −78 °C. A
solution of tert-butylmagnesium chloride (78.2 mL, 1.0 M in THF,
41
C
aryl
bond of 1.90(3) Å over 880 structures). From this
comparison, it can be concluded that, for the three examined
metals at least, the rigid structure and ligand metric parameters
in NCN-pincer complexes are conducive to the formation of
very short metal−carbon bonds involving the central carbene or
7
8.2 mmol) was added dropwise, and the reaction mixture was stirred
for 24 h while it warmed up from −78 °C to room temperature. The
reaction was quenched by addition of saturated aqueous ammonium
chloride solution (30 was extracted by dichloromethane (3 × 20 mL).
Once the organic phase was dried with magnesium sulfate and the
solvent was removed at reduced pressure, the residue was distilled (bp
aryl moiety. The observation that the M−C bonds compiled
aryl
above are somewhat shorter than the M−C
bonds for M =
NHC
∼
80 °C/3.3 Torr), affording 8.06 g (72%) of product as a clear liquid.
Rh, somewhat longer for M = Ni, and noticeably longer for M
Ru emphasizes the complexity of the factors that influence
1
H NMR (400 MHz, 298 K, acetone-d ): δ = 1.33 (s, 9H, C(CH ) ),
6
3 3
=
3
4
3
7
7
.38 (dd, 1H, J = 7.8 Hz, J = 0.7 Hz, CH), 7.44 (dd, 1H, J
.7 Hz, J = 0.7 Hz, CH), 7.65 (t, 1H, J = 7.8 Hz, CH).
=
HH
HH
HH
this bonding parameter.
4
3
HH
HH
N,N′-Bis-[6-tert-butyl-2]pyridylpropane-1,3-diamine. 1,3-Dia-
CONCLUSIONS
■
minopropane (1.921 g, 25.92 mmol), 2-bromo-6-tert-butylpyridine
(12.220 g, 57.076 mmol), sodium tert-butoxide (6.231 g, 64.83 mmol),
The work presented herein proves that NCN-pincer ligands
with a central NHC moiety and pyridyl pendant arms can be
conveniently adorned with sterically demanding substituents in
the 6,6’ position of the pyridyl rings, allowing for an effective
protection of the coordination site in trans to the carbene
Pd
(dba) (0.122 g, 0.133 mmol, 0.5 mol %), and rac-BINAP (0.161 g,
2
3
0
.258 mmol, 1 mol %) were loaded into a one-neck flask. Toluene (50
mL) was condensed, and the resulting suspension was refluxed for 12
h. After the suspension was allowed to cool to room temperature, the
solids were removed by filtration, and all volatiles from the filtrate were
removed in vacuo. The crude product was left behind in a quantitative
yield as a deep red gel of satisfactory purity and was used for
moiety. Specifically, ligand 1 allowed for the isolation of pincer
+
2
complexes 1MX (MX = NiBr , 3a; RuCl , 4a; RuI , 4b; RhI ,
2
2
2
2
2
6
b) displaying a square pyramidal geometry that is unchar-
1
subsequent steps without further purification. H NMR (400 MHz,
3
acteristic for these metals and reflects the steric impact of the
pendant arms onto the sixth coordination site of the metal. The
formal replacement of the imine pendant arms in complexes B
by pyridyl groups in 1 leads to a stabilization of the carbene
structure, which was exclusively observed in all metal
complexes, to the detriment of the diaminoalkyl moiety
observed in B. This is easily explained through the reduced
competitive delocalization of the NHC-nitrogen lone pairs of
electrons onto the aromatic pyridyl ring, vs the imine groups.
Rollover cyclometalation in Rh(I) precursor 5 eventually led to
the isolation of Rh(III) complex 9b, featuring a novel
monoanionic pincer ligand. Very short Ru−CNHC and Rh−
298 K, CDCl ): δ = 1.29 (s, 18H, C(CH ) ), 1.92 (pent, 2H, J
=
3
3
3
HH
3
6.7 Hz, (CH ) CH ), 3.41 (t, 4H, J = 6.7 Hz, CH NH), 6.15 (dd,
2H, JHH = 8.2 Hz, JHH = 0.4 Hz, C H), 6.57 (dd, 2H, JHH = 7.5 Hz,
JHH = 0.5 Hz, C H), 7.31 (t, 2H, J = 7.7 Hz, C H). C{ H} NMR
2
2
2
HH
2
3
4
3
3
4
5
3
4
13
1
HH
(
(
(
DEPT-Q, 100 MHz, 298 K, CDCl ): δ = 30.0 ((CH ) CH ), 30.3
3 2 2 2
3
C(CH ) ), 37.3 (C(CH ) ), 39.9 (NHCH ), 103.8 (C H), 107.8
3
3
3
3
2
5
4
2
6
C H), 137.6 (C H), 158.0 (N C ), 168.2 (tBuC ).
2
Ligand Precursor 1·HCl. Dry ammonium chloride (2.005 g, 37.48
mmol) and N,N′-bis(6-tert-butyl-2-pyridyl)propane-1,3-diamine
(
8.820 g, 25.90 mmol) were stirred in triethylorthoformate (25 mL)
for 12 h at 120 °C. After the mixture was allowed to cool to room
temperature, all volatiles were removed in vacuo, and the residue was
washed once with diethyl ether (30 mL) and then pentane (30 mL),
F
DOI: 10.1021/acs.organomet.8b00022
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
(N2Ccarbene). HRMS (ESI+), m/z (%): [ C H3035ClN4102Ru] , calcd
12
+
leaving behind 6.904 g (69%) product as a beige solid. H NMR (400
22
MHz, 298 K, CD Cl ): δ = 1.40 (s, 18H, C(CH ) ), 2.60 (pent, 2H,
487.1202, found 487.1198.
2
2
3 3
3
3
JHH = 5.8 Hz, (CH ) CH ), 4.51 (t, 4H, J = 5.8 Hz, CH N), 7.43
Ruthenium(II) Diiodide Complex 4b. THF (30 mL) was added
at room temperature to a mixture of copper complex 2 (394 mg, 0.364
mmol) and (p-cymene)ruthenium dichloride dimer (223 mg, 0.364
mmol). The mixture was stirred for 12 h, resulting in the formation of
a burgundy solution and a brown precipitate. The solids were removed
by filtration, and excess LiI (127 mg, 0.949 mmol) was added to the
burgundy filtrate. After the mixture was sonicated for 12 h, the liquid
phase was filtered and concentrated down to a volume of 5 mL. An
equal volume of toluene was condensed onto the solution, leading to
the precipitation of red-brown solids. The liquid part was decanted off,
2
2
2
HH
2
3
3
(
2
d, 2H, J = 7.8 Hz, CH), 7.47 (d, 2H, J = 8.0 Hz, CH), 7.91 (t,
HH HH
3
4
13
1
H, J = 8.0 Hz, C H), 10.72 (s, 1H, N CH). C{ H} NMR
H
H
2
(
(
(
DEPT-Q, 100 MHz, 298 K, CD Cl ): δ = 19.6 ((CH ) CH ), 30.3
2 2 2 2 2
3
C(CH ) ), 38.2 (C(CH ) ), 44.3 (NCH ), 110.2 (C H), 119.6
3
3
3
3
2
5
4
2
6
C H), 140.9 (C H), 148.7 (N CH), 150.0 (N C ), 169.9 (tBuC ).
2
2
12
+
HRMS (ESI+), m/z (%): [ C H N ] , calcd 351.2549, found
22
31
4
3
51.2552. Anal. Calcd for C H ClN : C 68.29; H 8.08; N 14.48.
22 31 4
Found: C 67.36; H 8.07; N 14.42.
Ligand Precursor 1·HPF . Dry ammonium hexafluorophosphate
6
and the solids were washed with hexanes (10 mL), yielding 280 mg
(
577 mg, 3.54 mmol) and N,N′-bis(6-tert-butyl-2-pyridyl)propane-1,3-
1
(
(
(
54%) brown powder. H NMR (400 MHz, 298 K, CD Cl ): δ = 2.02
diamine (804 mg, 2.36 mmol) were stirred in triethylorthoformate (5
mL) for 12 h at 120 °C. After the mixture was allowed to cool to room
temperature, all volatiles were removed in vacuo, and the residue was
extracted with THF (15 mL). THF was removed in vacuo, and the
residue was subsequently washed with diethyl ether (5 mL) and then
2
2
3
s, 18H, C(CH ) ), 2.67 (pent, 2H, J = 6.0 Hz, (CH ) CH ), 4.15
t, 4H, J = 6.0 Hz, CH N), 6.94 (dd, 2H, J = 8.2 Hz, J = 0.9
Hz, C H), 7.28 (dd, 2H, J = 8.1 Hz, J = 1.0 Hz C H), 7.62 (t,
H, J = 8.1 Hz, C H). C{ H} NMR (DEPT-Q, 100 MHz, 298 K,
3
3
HH
2
2
2
3
3
4
HH
2
HH
HH
3
3
4
5
HH
13 1
HH
3
4
2
HH
CD Cl ): δ = 22.1 ((CH ) CH ), 31.3 (C(CH ) ), 39.6 (C(CH ) ),
2
2
2
2
2
3
3
3 3
pentane (10 mL), leaving behind 860 mg (73%) product as a yellow
3
5
4
2
1
43.6 (NCH ), 106.6 (C H), 118.9 (C H), 137.1 (C H), 160.7 (N C ),
174.5 (tBuC ), 238.0 (N Ccarbene). HRMS (ESI+), m/z (%):
2
[ C22
for C22
N 7.74.
2
2
powder. H NMR (400 MHz, 298 K, CD Cl ): δ = 1.42 (s, 18H,
2
2
6
3
C(CH ) ), 2.58 (pent, 2H, J = 5.9 Hz, (CH ) CH ), 4.20 (t, 4H,
= 5.9 Hz, CH N), 7.25 (dd, 2H, J = 8.2 Hz, J = 0.4 Hz,
C H), 7.48 (dd, 2H, J = 7.7 Hz, J = 0.4 Hz, C H), 7.93 (t, 2H,
3
3
HH
2
2
2
12
127
102
+
3
3
4
H Ru] , calcd 579.0559, found 579.0554. Anal. Calcd
H I N Ru: C 37.46; H 4.29; N 7.94. Found: C 37.09; H 4.36;
30 2 4
30 IN
4
J
HH
3
2 HH HH
3
4
5
HH
HH
3
4
JHH = 8.0 Hz, C H), 10.77 (s, 1H, N CH).
2
Rhodium(III) Iodide Complex 6b. THF (50 mL) was condensed
onto a mixture of copper complex 2 (309 mg, 0.285 mmol) and
Copper(I) Iodide Complex 2. Potassium hexamethyldisilazide
0.670 g, 3.36 mmol) and copper iodide (0.640 g, 3.36 mmol) were
(
[
Rh(cod)Cl] (140 mg, 0.284 mmol). The mixture was stirred at room
2
mixed in THF (30 mL) at −40 °C. After stirring for 1 h, the resulting
suspension was added via cannula transfer to a suspension of 1·HCl
temperature for 12 h, forming an orange solution and a brown
precipitate. The precipitate was filtered off, and all volatiles from the
filtrate were removed in vacuo. The solid residue was washed with
(
1.302 g, 3.365 mmol) in THF (20 mL) at −78 °C. The reaction
mixture was stirred and allowed to warm up to room temperature over
4 h. All volatiles were removed in vacuo, and the residue was
toluene (2 × 10 mL), yielding the product as a brown powder (150
2
1
mg, 31%). H NMR (400 MHz, 298 K, CD CN): δ = 1.80 (s, 18H,
3
extracted with toluene (60 mL). The solvent was removed in vacuo,
3
C(CH ) ), 2.63 (pent, 2H, J = 6.0 Hz, (CH ) CH ), 4.36 (t, 4H,
3
3
HH
2
2
2
and the remaining product was washed twice with hexanes (2 × 30
3
3
4
J
= 6.0 Hz, CH N), 7.47 (dd, 2H, J = 8.3 Hz, J = 1.0 Hz,
C H), 7.68 (dd, 2H, J = 8.2 Hz, J = 1.1 Hz, C H), 8.12 (t, 2H,
JHH = 8.3 Hz, C H). C{ H} NMR (DEPT-Q, 100 MHz, 298 K,
HH
3
2 HH HH
1
mL), yielding the product as a pale orange solid (0.680 g, 37%). H
3
4
5
HH
13
HH
NMR (400 MHz, 298 K, CD Cl ): δ = 1.38 (br, 18H, C(CH ) ), 2.31
3
4
1
2
2
3 3
(
br, 2H, (CH ) CH ), 3.97 (br, 4H, CH N), 7.13−7.90 (br, 6H, CH).
2
2
2
2
CD CN): δ = 20.4 ((CH ) CH ), 30.5 (C(CH ) ), 39.9 (C(CH ) ),
3
2
2
2
3
3
3 3
Anal. Calcd for C H I N Cu : C 48.85; H 5.59; N 10.36. Found: C
3
5
4
2
44
60 2
8
2
4
1
6.3 (NCH ), 112.9 (C H), 123.6 (C H), 143.2 (C H), 158.6 (N C ),
72.8 (tBuC ). HRMS (ESI+), m/z (%): [ C H
30
2
2
4
9.55; H 6.23; N 10.72.
Nickel(II) Bromide Complex 3a. THF (10 mL) was condensed
6
12
127
103
+
I2N4 Rh] ,
2
2
calcd 706.9609, found 706.9596.
Rhodium(III) Iodide Complex 9b. Acetonitrile (30 mL) was
added at room temperature to a mixture of copper complex 2 (475
mg, 0.438 mmol) and [Rh(cod)Cl] (216 mg, 0.438 mmol). The
mixture was stirred for 12 h, resulting in the formation of an orange
solution and a red precipitate. The solids were removed by filtration,
and all volatiles from the filtrate were removed in vacuo. The residue
was washed with toluene (2 × 10 mL), yielding 320 mg (49%)
product as an orange powder. H NMR (400 MHz, 298 K, CD CN):
onto a mixture of copper complex 2 (200 mg, 0.185 mmol) and
NiBr (dme) (114 mg, 0.369 mmol). The mixture was stirred at room
2
temperature for 12 h, resulting in the formation of a dark brown
solution and a brown-green precipitate. The liquid phase was decanted
off, and the brown-green solids were extracted with dichloromethane
2
(
20 mL). The solids were removed by filtration, and the solvent was
removed in vacuo. The solid residue was washed with pentane (20
mL), affording 138 mg (66%) of product as deep green crystals.
1
3
12
79
58
+
HRMS (ESI+), m/z (%): [ C H BrN4 Ni] , calcd 487.1007,
22
30
δ = 1.36 (s, 9H, C(CH ) ), 1.69 (s, 9H, C(CH ) ), 2.37 (pent, 2H,
3
3
3 3
found 487.1005. Anal. Calcd for C H Br N Ni·CH Cl : C 42.24; H
3
3
22
30
2
4
2
2
JHH = 5.9 Hz, (CH ) CH ), 4.12 (t, 4H, J = 5.9 Hz, CH N), 6.92
2 2 2 HH 2
4.93; N 8.57. Found: C 42.79; H 5.21; N 8.74.
3
3
4
(
d, 1H, J = 7.8 Hz, CH), 7.28 (dd, 1H, J = 8.5 Hz, J = 0.9
HH HH HH
Ruthenium(II) Dichloride Complex 4a. THF (30 mL) was
3
4
Hz, CH), 7.44 (dd, 1H, J = 8.0 Hz, J = 0.9 Hz, CH), 7.68 (dd,
HH
HH
added at room temperature to a mixture of copper complex 2 (318
mg, 0.294 mmol) and (p-cymene)ruthenium dichloride dimer (180
mg, 0.294 mmol). The mixture was stirred for 12 h, resulting in the
formation of a burgundy solution and a brown precipitate. The solids
were removed by filtration, and excess AgCl (86 mg, 0.600 mmol) was
added to the burgundy filtrate. After sonicating the mixture for 12 h,
the liquid phase was filtered and concentrated down to a volume of 5
mL. An equal volume of toluene was condensed onto the solution,
leading to the precipitation of burgundy solids. The liquid part was
decanted off, and the solids were washed with hexanes (10 mL),
3
3
3
1
H, J = 7.8 Hz, J
= 1.1 Hz, CH), 7.89 (t, 1H, J = 8.2 Hz,
HH
HRh
HH
103
12
127
+
CH). HRMS (ESI+), m/z (%): [ C H
IN5 Rh] , calcd
2
4
32
6
20.0757, found 620.0748.
ASSOCIATED CONTENT
■
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00022.
1
yielding 190 mg (62%) burgundy microcrystalline product. H NMR
Complete NMR spectra for all new diamagnetic
compounds, as well as the relevant crystallography tables
(PDF)
(
400 MHz, 298 K, CD Cl ): δ = 2.04 (s, 18H, C(CH ) ), 2.69 (pent,
2 2 3 3
3
3
2
6
8
H, J = 6.0 Hz, (CH ) CH ), 4.18 (t, 4H, J = 6.0 Hz, CH N),
HH
2
2
2
HH
2
3
4
3
3
.95 (dd, 2H, J = 8.2 Hz, J = 0.9 Hz, C H), 7.29 (dd, 2H, J
.0 Hz, J = 1.0 Hz C H), 7.64 (t, 2H, J = 8.1 Hz, C H).
C{ H} NMR (DEPT-Q, 100 MHz, 298 K, CD Cl ): δ = 22.4
=
HH
HH
HH
4
4
5
3
HH
HH
1
3
1
Accession Codes
2
2
(
(
(CH ) CH ), 32.1 (C(CH ) ), 39.8 (C(CH ) ), 43.5 (NCH ), 106.5
CCDC 1569177−1569181 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
2
2
2
3
3
3
3
2
3
5
4
2
6
C H), 118.2 (C H), 137.5 (C H), 161.2 (N C ), 174.3 (tBuC ), 240.9
2
G
DOI: 10.1021/acs.organomet.8b00022
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
(8) Alder, R. W.; Blake, M. E.; Bortolotti, C.; Bufali, S.; Butts, C. P.;
Linehan, E.; Oliva, J. M.; Orpen, A. G.; Quayle, M. J. Complexation of
stable carbenes with alkali metals. Chem. Commun. 1999, 241−242.
(9) (a) Al Thagfi, J.; Lavoie, G. G. Preparation and Reactivity Study
of Chromium(III), Iron(II), and Cobalt(II) Complexes of 1,3-
Bis(imino)benzimidazol-2-ylidene and 1,3-Bis(imino)pyrimidin-2-yli-
dene. Organometallics 2012, 31, 7351−7358. (b) Kaplan, H. Z.; Li, B.;
Byers, J. A. Synthesis and Characterization of a Bis(imino)-N-
heterocyclic Carbene Analogue to Bis(imino)pyridine Iron Com-
plexes. Organometallics 2012, 31, 7343−7350. (c) Manna, C. M.;
Kaplan, H. Z.; Li, B.; Byers, J. A. High molecular weight poly(lactic
acid) produced by an efficient iron catalyst bearing a bis(amidinato)-
N-heterocyclic carbene ligand. Polyhedron 2014, 84, 160−167.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: roesler@ucalgary.ca.
ORCID
Roland Roesler: 0000-0002-1235-3976
Notes
The authors declare no competing financial interest.
(d) Drake, J. L.; Kaplan, H. Z.; Wilding, M. J. T.; Li, B.; Byers, J. A.
Spin transitions in bis(amidinato)-N-heterocyclic carbene iron(II) and
iron(III) complexes. Dalton Trans. 2015, 44, 16703−16707.
(e) Kaplan, H. Z.; Mako, T. L.; Wilding, M. J. T.; Li, B.; Byers, J. A.
Electron-donating capabilities and evidence for redox activity in low
oxidation state iron complexes bearing bis(amidine)pyrimidylidene
ligands. J. Coord. Chem. 2016, 69, 2047−2058.
ACKNOWLEDGMENTS
■
Financial support for this work was provided by the Natural
Sciences and Engineering Research Council of Canada in the
form of a Discovery Grant #262037 to R.R.
(
10) (a) Friese, V.; Nag, S.; Wang, J.; Santoni, M.-P.; Rodrigue-
II
Witchel, A.; Hanan, G. S.; Schaper, F. Red Phosphorescence in Ru
Complexes of a Tridentate N-Heterocyclic Carbene Ligand Incorpo-
rating Tetrahydropyrimidine. Eur. J. Inorg. Chem. 2011, 2011, 39−44.
b) Moussa, J.; Haddouche, K.; Chamoreau, L.-M.; Amouri, H.;
Williams, J. A. G. New N C N-coordinated Pd(II) and Pt(II)
complexes of a tridentate N-heterocyclic carbene ligand featuring a
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Organometallics XXXX, XXX, XXX−XXX