Solid-state thermolysis of a fac-rhenium(I) carbonyl complex with a redox non-innocent pincer ligand

Article abstract of DOI:10.1002/chem.201203045

The development of rhenium(I) chemistry has been restricted by the limited structural and electronic variability of the common pseudo-octahedral products fac-[ReX(CO)₃L₂] (L₂=α-diimine). We address this constraint by first preparing the bidentate bis(imino)pyridine complexes [(2,6-{2,6-Me₂C₆H₃N=CPh} ₂C₅H₃N)Re(CO)₃X] (X=Cl 2, Br 3), which were characterized by spectroscopic and X-ray crystallographic means, and then converting these species into tridentate pincer ligand compounds, [(2,6-{2,6-Me₂C₆H₃N=CPh}₂C ₅H₃N)Re(CO)₂X] (X=Cl 4, Br 5). This transformation was performed in the solid-state by controlled heating of 2 or 3 above 200 °C in a tube furnace under a flow of nitrogen gas, giving excellent yields (≤ 95 %). Compounds 4 and 5 define a new coordination environment for rhenium(I) carbonyl chemistry where the metal center is supported by a planar, tridentate pincer-coordinated bis(imino)pyridine ligand. The basic photophysical features of these compounds show significant elaboration in both number and intensity of the d-π* transitions observed in the UV/Vis spec tra relative to the bidentate starting materials, and these spectra were analyzed using time-dependent DFT computations. The redox nature of the bis(imino)pyridine ligand in compounds 2 and 4 was examined by electrochemical analysis, which showed two ligand reduction events and demonstrated that the ligand reduction shifts to a more positive potential when going from bidentate 2 to tridentate 4 (+160 mV for the first reduction step and +90 mV for the second). These observations indicate an increase in electrostatic stabilization of the reduced ligand in the tridentate conformation. Elaboration on this synthetic methodology documented its generality through the preparation of the pseudo-octahedral rhenium(I) triflate complex [(2,6-{2,6-Me₂C ₆H₃N=CPh}₂C₅H₃N)Re(CO) ₂OTf] (7, 93 % yield). Copyright

Full text of DOI:10.1002/chem.201203045

DOI: 10.1002/chem.201203045
Solid-State Thermolysis of a fac-Rhenium(I) Carbonyl Complex with a
Redox Non-Innocent Pincer Ligand
Titel Jurca,[a] Wen-Ching Chen,[b] Sheila Michel,[a] Ilia Korobkov,[a] Tiow-Gan Ong,[b] and
Darrin S. Richeson*[a]
Abstract: The development of rhen
ACHTUNGTRENNUNGum(I) chemistry has been restricted by
the limited structural and electronic
variability of the common pseudo-octa-
i-
(ꢀ95%). Compounds 4 and 5 define a
new coordination environment for rhe-
nium(I) carbonyl chemistry where the
metal center is supported by a planar,
and 4 was examined by electrochemical
analysis, which showed two ligand re-
duction events and demonstrated that
the ligand reduction shifts to a more
positive potential when going from bi-
dentate 2 to tridentate 4 (+160 mV for
the first reduction step and +90 mV
for the second). These observations in-
dicate an increase in electrostatic stabi-
lization of the reduced ligand in the tri-
dentate conformation. Elaboration on
this synthetic methodology document-
ed its generality through the prepara-
tion of the pseudo-octahedral rheni-
hedral products fac-
N
tridentate
pincer-coordinated
bis-
(L2 =a-diimine). We address this con-
straint by first preparing the bidentate
ACHTUNGTRENN(UNG imino)pyridine ligand. The basic pho-
tophysical features of these compounds
show significant elaboration in both
number and intensity of the d–p* tran-
sitions observed in the UV/Vis spec
tra relative to the bidentate starting
materials, and these spectra were ana-
lyzed using time-dependent DFT com-
putations. The redox nature of the bis-
bis
{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)3-
X] (X=Cl 2, Br 3), which were charac-
ACHTUNGTRENNUNG(imino)pyridine complexes [(2,6-
ACHTUNGTRENNUNG
terized by spectroscopic and X-ray
crystallographic means, and then con-
verting these species into tridentate
pincer ligand compounds, [(2,6-{2,6-
Me2C6H3N=CPh}2C5H3N)Re(CO)2X]
(X=Cl 4, Br 5). This transformation
was performed in the solid-state by
controlled heating of 2 or 3 above
2008C in a tube furnace under a flow
of nitrogen gas, giving excellent yields
(imino)pyridine ligand in compounds 2
ACHTUNGTRNEuNUNG m(I) triflate complex [(2,6-{2,6-
Me2C6H3N=CPh}2C5H3N)Re(CO)2OTf]
(7, 93% yield).
Keywords: carbonyl ligands · non-
innocent ligands · rhenium · solid-
state reactions · tridentate ligands
Introduction
with the quantitative replacement of two cis carbonyl lig-
AHCTUNGTRENNUNG
ands in the ReI starting material.[5] It is both interesting and
Although the element was discovered less than ninety years
ago, the expanse of rhenium chemistry is impressive, with
impact in the fields of catalysis,[1] fundamental photophysical
properties,[2] and radiopharmaceuticals.[3] For example, the
attractive photochemical and photophysical properties of a-
diimine ReI have attracted a great deal of attention since
the mid-1970s, with pseudo-octahedral fac-[ReX(CO)3L2]
and fac-[Re(CO)3L2(L’)]+ complexes being the dominant
species.[4] This large family of compounds is readily prepared
by the addition of chelating s-donor ligands to [ReX(CO)5]
significant that the formation of bidentate coordination to
facial tricarbonyl isomers are the only reported products
even when a potentially tridentate s-donor is employed in
the reaction (for example, A and B).[6,7] These robust species
[a] Dr. T. Jurca, S. Michel, Dr. I. Korobkov, Dr. D. S. Richeson
Centre for Catalysis Research and Innovation and
Department of Chemistry, University of Ottawa
Ottawa, Ontario, K1N 6N5 (Canada)
Fax: (+1)613-562-5170
E-mail: darrin@uottawa.ca
have been implicated for applications in organic light-emit-
ting diodes (OLEDs),[8] chemosensors and biotechnology
probes,[9] fluorescence microscopy imaging of cells,[10] and
the photochemical reduction of CO2 to CO.[11] Furthermore,
the compact and robust rhenium tricarbonyl core is also a
[b] Dr. W.-C. Chen, Dr. T.-G. Ong
Institute of Chemistry Academia Sinica
Taipei, 115 (Taiwan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201203045.
4278
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Chem. Eur. J. 2013, 19, 4278 – 4286

FULL PAPER
promising organometallic frag-
ment for the labeling of biomo-
lecules and in the development
of diagnostic and therapeutic
radiopharmaceuticals. Among
the key photophysical features
of the a-diimine ReI com-
pounds is the electron transfer
capability of this system and
the interplay between the Re
center and the well-known non-
innocent redox-activity of the
ligands.
The
bis
G
pyridine
family of ligands, pioneered by
Brookhart and co-workers[12]
and Gibson and co-workers[13]
in base-metal olefin polymeri-
zation catalysis, present an in-
teresting redox non-innocent
ligand scaffold. While common-
ly employed as a neutral spe-
Scheme 1. Synthesis of tridentate pincer complexes 4 and 5.
cies, this ligand is stable in several chemically accessible re-
duced states represented by mono-, di-, or trianionic forms.
The mono- and trianions are odd-electron, p radical species,
in contrast to the dianionic state, which can be in a singlet
or a triplet, unpaired ground state.[14]
Clearly the full potential of this versatile ligand scaffold
cannot be exploited with the current, mature state of the
chemistry of the fac-[ReX(CO)3L2] owing to the limits im-
posed by the bidentate coordination. Furthermore, it would
appear that, on the basis of the tridentate ligands that have
been investigated, the concerted efforts to produce the tri-
dentate species has been unsuccessful. Attracted by this
challenge we sought to synthesize, crystallographically au-
thenticate, and instigate investigation of the first low-valent
rhenium pincer complex displaying an N,N’,N-chelated bis-
Table 1. Summary of data collection and crystallographic parameters for
2 and 3.
Compound
2
3
empirical formula
[C38H31ReClN3O3]
[C4H8O]
871.41
[C38H31ReBrN3O3]
[C4H8O]0.5
879.82
formula weight
T [K]
200(2)
296(2)
l [ꢁ]
0.71073
0.71073
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
triclinic
P1
triclinic
P1
¯
¯
8.4994(3)
13.6344(4)
16.6194(5)
102.8350(10)
97.3350(10)
93.202(2)
1855.36(10)
2
9.645(3)
13.366(4)
15.955(5)
107.958(13)
99.607(15)
90.663(14)
1924.7(11)
2
a [8]
b [8]
g [8]
V [ꢁ3]
Z
ACHTUNGTRENNUNG(imino)pyridine array. This report for the unconventional
1 (calcd) [Mgmꢁ3
]
1.560
3.393
1.518
4.235
m [mmꢁ1
]
but accessible synthesis of tridentate pincer complexes
promises to enhance the versatile chemistry of ReI and yield
new venues for exploration.
asbsorption correction:
final R indices [I>2s(I)]
R1[a]
semi-empirical from equivalents
0.0667
0.1285
0.0201
0.0513
wR2[b]
ꢀ
ꢁ
P
P
P
P
1=2
2
2
[a] R1¼ kFoj ꢁ jFck= jFoj. [b] wR2¼
wðjFoj ꢁ jFcjÞ = wjFoj
.
Results and Discussion
In a sealed reaction flask under N2 atmosphere, the bis-
A
CPh}2C5H3N)Re(CO)3Br] (3) exhibited analogous ligand
dispositions with the ReI center in an octahedral geometry,
as represented in Figure 1 and Figure 2. Selected bond dis-
tances and angles for 2 and 3 are presented in Table 2. As
expected, the ligand coordinates in a bidentate mode
through the pyridine nitrogen (N2) and one of the imine ni-
trogen atoms (N1). The second imine N-center (N3) is not
coordinated to Re. The three carbonyl groups are facially
oriented, a feature consistent with the observation of three
IR bands for the CO stretches.[3b,4,15] The coordination ge-
lent of [Re(CO)5X] (X=Cl or Br) as the reaction mixture
was heated to 1008C for 24 h (Scheme 1). The resulting
bright red/orange powders were isolated in yields of 85%
and 97%, respectively. While microanalysis provided the
formulae of these two compounds and 1H and 13C NMR
spectroscopy suggested their structural features, we were
also successful in obtaining the detailed metrical parameters
from single-crystal X-ray analyses for both products
(Table 1). The two products, [(2,6-{2,6-Me2C6H3N=
CPh}2C5H3N)Re(CO)3Cl] (2) and [(2,6-{2,6-Me2C6H3N=
ACHUTNGRENoNUG mAHCTUNGTRNENeUGN tries of these two compounds is completed by the re-
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4279

D. S. Richeson et al.
Table 2. Selected bond lengths [ꢁ] and angles [8] for compounds 2 and 3.
2
3
bond lengths:
C1–C2
C6–C7
C1–N1
C7–N3
1.481(3)
1.511(3)
1.297(2)
1.276(3)
1.368(2)
1.339(3)
2.1655(18)
2.2200(15)
2.4778(6)
1.930(2)
1.905(2)
1.900(2)
1.142(3)
1.152(3)
1.156(3)
C1–C2
C6–C7
C1–N1
C7–N3
C2–N2
C6–N2
Re1–N1
Re1–N2
Re1–Br1
Re1–C36
Re1–C37
Re1–C38
C36–O1
C37–O2
C38–O3
1.46(2)
1.49(2)
1.303(18)
1.285(18)
1.376(18)
1.350(18)
2.171(12)
2.249(12)
2.623(2)
1.93(2)
1.907(17)
1.91(2)
1.127(18)
1.152(17)
1.135(18)
C2–N2
C6–N2
Re1–N1
Re1–N2
Re1–Cl1
Re1–C36
Re1–C37
Re1–C38
C36–O1
C37–O2
C38–O3
Figure 1. X-ray structure of compound 2. Hydrogen atoms and co-crystal-
lized THF omitted for clarity; ellipsoids set at 50% probability.
angles:
N3-C7-C6
C7-C6-N2
C6-N2-Re1
N1-Re1-N2
N1-C1-C2
112.93(19)
121.67(16)
128.01(13)
74.69(6)
116.07(17)
116.43(15)
117.76(16)
118.37(14)
113.43(12)
105.41(8)
167.22(8)
96.45(8)
80.48(5)
174.41(9)
92.63(8)
97.89(8)
87.45(5)
N3-C7-C6
C7-C6-N2
C6-N2-Re1
N1-Re1-N2
N1-C1-C2
110.6(14)
121.1(14)
127.8(10)
74.9(5)
116.4(13)
117.3(13)
118.0(12)
117.5(11)
110.5(9)
103.2(6)
167.4(6)
97.3(6)
81.4(3)
177.1(7)
94.2(6)
95.1(6)
90.4(3)
C1-C2-N2
C6-N2-C2
C1-C2-N2
C6-N2-C2
Re1-N1-C1
Re1-N2-C2
N2-Re1-C36
N2-Re1-C38
N2-Re1-C37
N2-Re1-Cl1
N1-Re1-C36
N1-Re1-C38
N1-Re1-C37
N1-Re1-Cl1
Re1-N1-C1
Re1-N2-C2
N2-Re1-C36
N2-Re1-C37
N2-Re1-C38
N2-Re1-Br1
N1-Re1-C36
N1-Re1-C37
N1-Re1-C38
N1-Re1-Br1
Figure 2. X-ray structure of compound 3. Hydrogen atoms and co-crystal-
lized THF omitted for clarity; ellipsoids set at 50% probability.
spective chloro or bromo groups that are trans to a carbonyl
ligand and on an axis perpendicular to the plane of the coor-
dinated nitrogen ligand.
Having compounds 2 and 3 in hand, our goal was to trans-
form these species, through release of CO and coordination
of the pendant imine group, into tridentate pincer ligand
compounds and thus define a new coordination environment
for ReI carbonyl chemistry. With this in mind a computa-
tional analysis of the loss of CO and subsequent rearrange-
ment of the pendant imine group to form a pincer type ge-
ometry, as shown in Scheme 1, was performed with DFT cal-
culations using the Gaussian09 suite of programs.[16] Fre-
quency analysis on the optimized structures, using the
B3LYP functional and mixed TZVP/LanL2DZ basis set,
confirmed that these structures were minima and provided
the data for the calculation of free energy (DG) and enthal-
py (DH) of the reaction. Both of these values were positive
(DG=11.5 kcalmolꢁ1 and DH=21.5 kcalmolꢁ1). Combining
this information with the fact that the entropy is a favorable,
positive value, we successfully exploited this entropy-driven
reaction by heating powders of 2 or 3 in a tube furnace
under a dynamic flow of nitrogen. With this method and
heating to 2008C, compound 2 was transformed into a dark
brown powder corresponding to a 97% yield of 4. The sym-
definitive structural features of 4 were obtained by single-
crystal X-ray analysis (Table 3). Along with the structural
representation in Figure 3, selected bond distances and
angles for 4 are presented in Table 4.
The ReI center in the octahedral complex [(2,6-{2,6-
Me2C6H3N=CPh}2C5H3N)Re(CO)2Cl] 4 is supported by a
planar, pincer-coordinated bisACTHNUTRGNEUGN(imino)pyridine ligand defined
by the pyridine center (N2) and the imine nitrogen atoms
1
metrical H and 13C NMR spectra, the appearance of only
two CO stretches in the IR spectrum, and a satisfactory mi-
croanalysis confirmed the identity of this new species, and
Figure 3. X-ray structure of compound 4. Hydrogen atoms and THF
omitted for clarity; ellipsoids set at 50% probability.
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Rhenium(I) Carbonyl Pincer Complexes
FULL PAPER
Table 3. Summary of data collection and crystallographic parameters for
4.
2.0964(15) ꢁ and 2.1089(16) ꢁ; these bond lengths are sig-
nificantly shorter than those observed for bidentate complex
2, indicating a stronger ligand–metal interaction. The re-
maining metal ligand distances are similar to those observed
in 2.
Compound
4
empirical formula
formula weight
T [K]
[C37H31ReClN3O2] [C4H8O]1.5
879.45
296(2)
To achieve a complete transformation of bidentate 3 to
the pincer complex 5, an increased temperature of 2408C
and a reaction time of 1 hour were required. Once again
this procedure led to the isolation of dark brown powder of
l [ꢁ]
0.71073
crystal system
space group
a [ꢁ]
monoclinic
P21/n
9.2686(3)
1
5 in 95% yield. The H and 13C NMR, IR spectroscopic sim-
b [ꢁ]
18.1949(6)
c [ꢁ]
22.0310(7)
ilarities to 4, and also the microanalysis of this compound
confirmed its identity as the octahedral ReI complex [(2,6-
{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)2Br] 5. However, crys-
tals suitable for X-ray crystallography could not be ob-
tained.
a [8]
90.00
b [8]
98.035(2)
g [8]
90.00
3678.9(2)
V [ꢁ3]
Z
4
1 (calcd) [Mgmꢁ3
]
1.588
3.422
m [mmꢁ1
]
UV/Vis and electrochemical characterization: The deeper
color of 4 compared to 2 is evident in the UV/Vis spectra of
these species and is presented in Figure 4. These spectra
were obtained in acetonitrile with identical concentrations
absorption correction:
final R indices [I>2s(I)]
R1[a]
semi-empirical from equivalents
0.0170
0.0407
wR2[b]
ꢀ
ꢁ
P
P
P
P
1=2
2
2
[a] R1¼ kFoj ꢁ jFck= jFoj. [b] wR2¼
wðjFoj ꢁ jFcjÞ = wjFoj
.
Table 4. Selected bond lengths [ꢁ] and angles [8] for compound 4.
4
bond lengths:
C1–C2
C6–C7
C1–N1
C7–N3
C2–N2
C6–N2
1.474(3)
1.477(3)
1.315(3)
1.309(3)
1.355(2)
1.353(2)
Re1–N1
Re1–N2
Re1–N3
Re1–Cl1
Re1–C36
Re1–C37
C36–O1
C37–O2
2.0964(15)
2.0672(16)
2.1089(16)
2.4751(5)
1.897(2)
1.914(2)
1.160(3)
1.158(3)
Figure 4. UV/Vis spectra of compound 2 (bidentate) and 4 (tridentate) in
acetonitrile (concentration=0.08 mm).
of 0.08 mm. The most obvious differences between the two
species involve the d–p* transitions in the 350–550 nm
region of the spectrum. The higher-energy bands are ligand
based p–p* transitions. Bidentate ligated 2 has a broad
signal at 419 nm, while tridentate species 4 has more intense
signals at 367 and 485 nm, which are likely to be responsible
for the color change observed (see inset in Figure 4).
These UV/Vis spectra were modeled in acetonitrile solu-
tion using time-dependent DFT computations with Gaus-
AHCTUNGTREGsNNUN ian09 software, the B3LYP functional, mixed TZVP/
angles:
N1-C1-C2
N3-C7-C6
C1-C2-N2
C7-C6-N2
C2-N2-C6
C2-N2-Re1
C6-N2-Re1
N1-Re1-N2
N3-Re1-N2
N1-Re1-N3
N2-Re1-C37
N2-Re1-C36
N2-Re1-Cl1
115.30(16)
115.88(17)
112.38(16)
112.56(17)
121.51(16)
118.18(13)
117.92(12)
75.79(6)
75.95(6)
151.72(6)
170.88(7)
105.01(7)
79.67(4)
LanL2DZ basis set, and the PCM solvent model.[17] The re-
sulting computed spectra were excellent matches to the ex-
perimental spectra, and are shown in Supporting Informa-
tion, Figures S1 and S2. In the case of compound 2, the ob-
served 419 nm (23900 cmꢁ1) absorbance consists of two
equal intensity electronic transitions centered at 440 nm
(22700 cmꢁ1) and 392 nm (22500 cmꢁ1). The computations
reveal that the 440 nm contribution is a transition from the
HOMOꢁ2, rhenium-centered d level to the LUMO that is a
ligand-based p* orbital (d–p*), while the 395 nm component
(N1, and N3). One of the carbonyl groups lies in this plane
and trans to the pyridyl group. The remaining CO (C36) and
chloro ligands are perpendicular to this plane and define the
axial sites. The Re N pyridine distance of 2.0672(16) ꢁ is
slightly shorter than the Re N imine distances of
ꢁ
ꢁ
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D. S. Richeson et al.
is a ligand-based p–p* transition. In contrast, the two dis-
tinctive transitions at 485 nm (20600 cmꢁ1) and 367 nm
(27250 cmꢁ1) for tridentate species 4 are both d–p* transi-
tions. The computational model provided a large absorbance
transition at 458 nm (21800 cmꢁ1) that was made up of
equal contributions from three d–p* transitions that involve
electronic transitions from three different rhenium d-orbital
levels to ligand p* orbitals. The computations also provided
a band at 368 nm (27200 cmꢁ1) that corresponds to a d–p*
transition.
ꢁ2.04 V (E1/2 =ꢁ1.98 V, DEp =130 mV). We attribute the
quasi-reversible nature of the first reduction wave to a
chemical transformation of this first reduction product, [2]ꢁ
or [4]ꢁ.[18] The smaller current observed for the second re-
duction wave was ascribed to this transformation and con-
comitant decrease in concentration of these species. The
first reduction wave for both compounds have similar values
to those reported for coordinated bisACTHNUTRGNEUNG(imino)pyridine ligands
and the potential separation between the two redox process-
es (680 mV for 2 and 750 mV for 4) is a typical value for ar-
omatic N-donor ligands bound to rhenium(I).[18] The drastic
difference in potentials between the first reduction in 2 and
4 (ꢁ1.45 and ꢁ1.29 V respectively), and that of free ligand 1
(ꢁ2.52 V, E1/2 =ꢁ2.36 V) illustrates the electrostatic stabili-
zation of the reduced forms of the ligand by the rhenium(I)
cation. Furthermore, the shift to less-negative potential
when going from bidentate 2 to tridentate 4 (+160 mV for
the first reduction step and +90 mV for the second) indi-
cates an increase in electrostatic stabilization of the reduced
ligand in the tridentate conformation. This is not surprising
as ligand–metal contact has increased from two to three N-
donor interactions.
The redox-non-innocent nature of the bisACTHNUGRTENUNG(imino)pyridine
ligation was examined using cyclic voltammetry on the free
ligand and of both the bi- and tridentate species. Cyclic vol-
tammograms (CV) of 1, 2, and 4 reveal quasi-reversible re-
duction waves (Figure 5). The CV of 1 reveals only one
ꢁ
Synthesis of a triflate analogue: Activation of the Re X
moiety is an initial fundamental feature for exploration of
the reactivity of these new species, and exchange of chloride
by triflate, which is known to be a more labile anion, was
chosen to begin this examination. Reaction of 4 with a slight
excess of AgOTf resulted in the isolation of a mixture of
dark brown and purple powder. Unfortunately, despite our
efforts, we have so far been unable to isolate significant
quantities of these products to carry out further analysis. As
direct halide abstraction from 4 did not lead to clean prod-
uct formation, an alternative route to formation of the tar-
geted triflate complex was applied. As outlined in Scheme 2,
the direct synthesis of the triflate complex 6 was targeted
with subsequent solid-state conversion to the desired pincer
geometry.
Figure 5. Cyclic voltammograms in acetonitrile of free bisACTHNUTRGNE(NUG imino)pyridine
ligand 1, and complexes 2 and 4 referenced to the ferrocene/ferrocenium
cation couple.
quasi-reversible reduction within the window afforded by
acetonitrile. This signal corresponds to the established for-
mation of the bisACHTUNGTRENNUNG(imino)pyridine monoanion. The quasi-re-
versiblility of all of these reductions is supported by the fact
that they exhibit a DEp greater than that of the reversible
one-electron ferrocene/ferrocenium couple (DEp for Fc/Fc+
=80 mV) used as a reference, as well as cathodic currents
that are smaller than the anodic reduction currents. For 2
and 4, the waves are assigned as successive one-electron re-
Reaction of 2 with a slight excess of AgOTf resulted in
the isolation of bright yellow/orange powder in a moderate
yield. While the spectroscopic features were consistent with
the proposed formulation, X-ray analysis (Table 5) con-
firmed the product to be the octahedral ReI complex [(2,6-
ductions of the bis
tive rhenium complex. Formation of ML2ꢁ species is not un-
common for bis(imino)pyridines, which have been shown to
facilitate the formation of mono-, di-, and trianionic com-
plexes.[14] The reduction behavior of compound 2 is similar
to the electrochemical behavior that has been noted previ-
ously for related aromatic N-donor ligand systems (diimine-
and pyridine-based).[14h,18,19] No oxidation waves were ob-
served within the solvent window.
(imino)pyridine ligand within the respec-
{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)3ACTHUNGTERN(UNG OTf)] 7 (Figure 6).
Selected bond lengths and angles for 7 are presented in
Table 6. Complex 7 is the analogue of compound 2 and 3
with halide replaced by a triflate anion.
Compound 6 could be converted cleanly into the triden-
tate species 7 by heating a powder sample to 1908C under a
flow of nitrogen for 25 min. The resulting dark brown
powder was confirmed to be [(2,6-{2,6-Me2C6H3N=
ACHTUNGTRENNUNG
CPh}2C5H3N)Re(CO)2ACTHNUGTRNE(NUG OTf)] 7 by spectroscopic characteri-
Bidentate complex 2 has two quasi-reversible reduction
zation, microanalysis, and X-ray structural analysis.
waves with cathodic maxima at ꢁ1.45 V and ꢁ2.13 V (E1/2
=
Figure 7 shows the results of the structural examination
(Table 5), with selected bond distances and angles for pre-
sented in Table 6. These data confirm this complex to be the
octahedral ReI complex [(2,6-{2,6-Me2C6H3N=CPh}2C5H3N)-
ꢁ2.07 V, DEp =140 mV) versus the Fc/Fc+ couple. Triden-
tate complex 4 presented a similar pattern, with cathodic
maxima at ꢁ1.29 V (E1/2 =ꢁ1.25 V, DEp =106 mV) and
4282
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 4278 – 4286

Rhenium(I) Carbonyl Pincer Complexes
FULL PAPER
those observed in 4 and 5. The
preparative route to 7 demon-
strates the generality of the
thermal synthesis approach.
Conclusion
We have reported the first crys-
tallographically authenticated
low-valent rhenium pincer com-
plexes and have thus addressed
a major deficiency in the chem-
istry of imine-coordinated ReI
carbonyl chemistry. This family
of compounds was accessed
through an unconventional but
highly
thermoACHTNUGTRENNUNG
they possess
efficient
lysis synthetic route and
redox active
solid-state
Scheme 2. Synthesis of the tridentate triflate complex 7.
a
non-innocent tridentate bis-
Table 5. Summary of data collection and crystallographic parameters for
6 and 7.
Compound
6
7
empirical
formula
formula weight
T [K]
ACHTUNGTRENNUNG[C38H31ReCAHTUNTGNERGUN(CF3SO3)N3O3] ACHUTNGERT[NGNUN C37H31ReCATHNUGTERN(NUGN CF3SO3)N3O3]
[C4H8O]
985.03
[CHCl3]
1004.29
200(2)
200(2)
l [ꢁ]
0.71073
triclinic
P1
0.71073
triclinic
P1
crystal system
space group
a [ꢁ]
b [ꢁ]
c [ꢁ]
¯
¯
8.6739(2)
13.8597(11)
15.6045(11)
18.8333(14)
81.639(4)
84.582(4)
78.114(4)
3934.7(5)
4
12.3727(3)
19.2872(4)
87.8370(10)
83.6820(10)
76.1410(10)
1997.29(8)
2
a [8]
b [8]
g [8]
V [ꢁ3]
Z
1 (calcd) [Mgmꢁ3
]
1.638
3.163
1.695
3.407
m [mmꢁ1
]
Figure 6. X-ray structure of compound 6. Hydrogen atoms and THF
omitted for clarity; ellipsoids set at 50% probability.
absorption
correction:
final R indices
[I>2s(I)]
R1[a]
semi-empirical from equivalents
0.0298
0.0814
0.1921
wR2[b]
0.0602
ꢀ
ꢁ
P
P
P
P
1=2
2
2
[a] R1¼ kFoj ꢁ jFck= jFoj. [b] wR2¼
wðjFoj ꢁ jFcjÞ = wjFoj
.
ACHTUNGTRENNUNGRe(CO)2ACHTUNGTRENNUNG
(OTf)] 7.[20] This transformation involved loss of
CO trans to the coordinated imine moiety with rearrange-
ment of the non-coordinated imine side-arm to afford the
desired pincer ligand geometry that was isolated in 93%
yield with no further workup. The geometry around the ReI
center is similar to that of compound 4 with an OTfꢁ anion
replacing the chloro ligand and a plane defined by the pyri-
dine center (N2), imine nitrogen atoms (N1 and N3), and a
CO carbon atom (C37). The axial sites are defined by a CO
carbon (C36) and a triflate oxygen (O3). IR stretches for
the carbonyl group at 1915 and 1855 cmꢁ1 are similar to
Figure 7. X-ray structure of compound 7. Hydrogen atoms, CHCl3, and
THF omitted for clarity; ellipsoids set at 50% probability.
Chem. Eur. J. 2013, 19, 4278 – 4286
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4283

D. S. Richeson et al.
Table 6. Selected bond lengths [ꢁ] and angles [8] for compounds 6 and 7.
Experimental Section
6
7
General methods: Reactions were performed in a glovebox with a nitro-
gen atmosphere. Solvents were sparged with nitrogen and then dried by
passage through a column of activated alumina using an apparatus pur-
chased from Anhydrous Engineering. Deuterated chloroform was dried
using activated molecular sieves. Rhenium starting materials were pur-
chased from Strem Chemicals and used as received. All other chemicals
were purchased from Aldrich and used without further purification.
bond lengths:
C1–C2
C6–C7
C1–N1
C7–N3
1.489(4)
1.505(4)
1.288(4)
1.275(4)
1.364(4)
1.348(4)
2.154(2)
2.228(2)
2.200(2)
1.908(3)
1.882(3)
1.923(3)
1.150(4)
1.162(4)
1.146(4)
C1–C2
C6–C7
C1–N1
C7–N3
C2–N2
C6–N2
Re1–N1
Re1–N2
Re1–N3
Re1–O3
Re1–C36
Re1–C37
C36–O1
C37–O2
1.54(3)
1.47(3)
1.37(3)
1.30(3)
1.27(3)
1.44(3)
2.073(16)
2.028(17)
2.136(17)
2.198(13)
1.86(2)
1.87(3)
1.18(3)
1.21(3)
C2–N2
C6–N2
NMR spectra were run on Bruker Avance 300 and 500 MHz spec
ACHTUNGTRENNUNGtromACHTUNGTRENNUNGe-
Re1–N1
Re1–N2
Re1–O1
Re1–C36
Re1–C37
Re1–C38
C36–O4
C37–O5
C38–O6
A
ACHTUNGTRENNUNG
ligand (1) was synthesized according to literature procedure.[21] Elemental
analyses for 2–7 were performed by Midwest Microlab LLC, Indianapolis
IN. Solid state reactions were carried out in a Lindberg Blue M Mini-
Mite Tube Furnace (model TF55035A-1). Infrared spectra were collected
using a Varian 640 FTIR spectrometer using an ATR attachment.
Electrochemical measurements: Electrochemical measurements were
performed with a Princeton Applied Research (PAR) 2273 potentiostat/
galvanostat running Power Suite 2.58 electrochemical software using a
one-compartment three-electrode cell. Platinum wire working and count-
angles:
N3-C7-C6
114.2(3)
N1-C1-C2
112(2)
AHCTUNGTERGeNNUN r electrodes, a silver wire pseudo-reference electrode, and 0.1m tetrabu-
C7-C6-N2
121.4(3)
128.3(2)
74.23(9)
115.6(3)
114.5(3)
117.2(3)
117.38(19)
109.69(18)
101.32(11)
168.19(11)
103.58(12)
76.06(8)
175.43(11)
99.01(11)
93.94(12)
82.23(8)
N3-C7-C6
C1-C2-N2
C7-C6-N2
C2-N2-C6
C2-N2-Re1
C6-N2-Re1
N1-Re1-N2
N3-Re1-N2
N1-Re1-N3
N2-Re1-C37
N2-Re1-C36
N2-Re1-O3
116(2)
109(2)
113(2)
117(2)
125.8(17)
116.7(14)
74.2(7)
77.4(7)
151.6(7)
170.2(8)
103.1(9)
81.8(6)
tylammonium bromide (Sigma Aldrich, 99.0%) in acetonitrile (Fisher
Scientific, 99.9%) as supporting electrolyte were employed; ferrocene
was added as an internal standard at the end of each experiment. The
electrochemical cell was degassed with nitrogen prior to measurement.
Cyclic voltammograms of 5 mm 1, 3, and 4 were recorded at 50 mVsꢁ1. A
nitrogen environment was maintained in the cell throughout all of the
measurements.
C6-N2-Re1
N1-Re1-N2
N1-C1-C2
C1-C2-N2
C6-N2-C2
Re1-N1-C1
Re1-N2-C2
N2-Re1-C38
N2-Re1-C36
N2-Re1-C37
N2-Re1-O1
N1-Re1-C38
N1-Re1-C36
N1-Re1-C37
N1-Re1-O1
ACHTNUTGERG[NUNN (2,6-{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)3Cl] (2): [Re(CO)5Cl] powder
(68 mg, 0.190 mmol) was added to a clear yellow solution of 1 (100 mg,
0.203 mmol) in toluene (8 mL), in which [Re(CO)5Cl] remained insolu-
ble. The reaction mixture was placed in a Teflon-sealed flask and allowed
to stir at 1008C for 24 h, over which time a bright orange precipitate
formed. The solution was filtered, the precipitate was washed with of
hexanes (5ꢂ2 mL), and allowed to dry under vacuum. A bright red/
orange powder was isolated in 97% yield. Large red plate-like crystals
suitable for X-ray analysis were grown from a saturated solution of THF
with hexanes, and storing at ꢁ208C for several days. n˜(CO)=2013 (s),
1918 (s), 1896 cmꢁ1 (s). 1H NMR (CDCl3, 500 MHz): d=8.21 (brd, 1H,
py, aromatic), 8.09 (brt, 1H, py, aromatic), 7.59 (brd, 1H, aromatic),
7.45–6.83 (brm, 16H, aromatic), 6.80 (brd, 1H, aromatic), 2.60 (brs, 3H,
Me), 2.41 (brs, 3H, Me), 1.99 (brs, 3H, Me), 1.71 ppm (brs, 3H, Me).
13C NMR (CDCl3, 125 MHz). d=196.3 (CO), 195.1 (CO), 186.0 (CO),
178.1 (C=N imine), 163.9 (py, C=N imine), 162.8 (o-C=N), 157.7 (o-C=
N), 148.3 (Ar, i-C), 147.2 (Ar, i-C), 139.2 (Ar-CH), 133.4 (Ar, i-C), 133.1
(Ar, C-CH3), 131.4 (Ar-CH), 131.1 (Ar-CH), 131.0 (Ar, C-CH3), 130.9
(Ar-CH), 130.5 (Ar-CH), 130.0 (Ar-CH), 128.8 (Ar-CH), 128.7 (Ar-CH),
128.6 (Ar-CH), 128.4 (Ar-CH), 128.2 (Ar-CH), 128.1 (Ar-CH), 127.7 (Ar-
CH), 126.9 (Ar-CH), 126.2 (Ar, C-CH3), 123.9 (Ar-CH), 122.8 (Ar, C-
CH3), 20.3 (Ar-Me, CH3), 18.7 (br, Ar-Me, CH3), 18.4 ppm (Ar-Me,
CH3). Elemental analysis calcd(%) for [C38H31ReClN3O3]: C 57.10,
H 3.91, N 5.26; found: C 57.48, H 3.92, N 5.38.
ACHTUNGTRENNUNG
(imino)pyridine chelate. While known a-diimine ReI com-
pounds display very interesting and useful photophysical
and redox features, the newly reported contributions to this
family, reported herein, expand on these characteristics. Spe-
cifically, these new pincer complexes display enhanced
metal-to-ligand d–p* electronic transitions both in number
and intensity. DFT analysis allowed a detailed understanding
for these transitions. Furthermore, there are some clear
modifications to the redox chemistry provided by this newly
obtained tridentate ligation. The shift of the reduction po-
tentials to less negative values for the tridentate system indi-
cated an increase in electrostatic stabilization of the reduced
ligand in the tridentate conformation. Furthermore, an effi-
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(150 mg, 0.305 mmol) in toluene (8 mL), in which [Re(CO)5Br] remained
insoluble. The reaction mixture was placed in a Teflon-sealed flask and
allowed to stir at 1008C for 24 h, over which time a bright red/orange
precipitate formed. The solution was filtered, the precipitate was washed
with hexanes (5ꢂ2 mL), and allowed to dry under vacuum. A bright red/
orange powder was isolated in 85% yield. Large red plate-like crystals
suitable for X-ray analysis were grown from a saturated solution of THF
with hexanes, and storing at ꢁ208C for several days. n˜(CO)=2024 (s),
1932 (s), 1897 cmꢁ1 (s). 1H NMR (CDCl3, 500 MHz): d=8.24 (br d, 1H,
py, aromatic), 8.09 (br t, 1H, py, aromatic), 7.62 (brd, 1H, aromatic),
7.47–6.88 (brm, 15H, aromatic) 6.84 (brd, 1H, aromatic), 6.80 (brd, 1H,
aromatic), 2.65 (brs, 3H, Me), 2.44 (brs, 3H, Me), 1.96 (brs, 3H, Me),
1.66 ppm (brs, 3H, Me). 13C NMR (CDCl3, 125 MHz). d=195.7 (CO),
194.9 (CO), 185.3 (CO), 177.9 (C=N imine), 163.9 (py, C=N imine), 163.0
cient route to generate bisACHTNUGRTNEUNG
(imino)pyridine ReI pincer com-
plexes bearing the weakly bound triflate anion should facili-
tate future reactivity studies and potential catalytic applica-
tions. The substantially enhanced features of these new spe-
cies as well as the clear future avenues for steric and elec-
tronic modifications promise to amplify the versatile
chemistry of ReI and yield new avenues for exploration.
4284
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Chem. Eur. J. 2013, 19, 4278 – 4286

Rhenium(I) Carbonyl Pincer Complexes
FULL PAPER
(o-C=N), 157.9 (o-C=N), 148.5 (Ar, i-C), 147.1 (Ar, i-C), 139.1 (Ar-CH),
133.3 (Ar, i-C), 133.2 (Ar, C-CH3), 131.3 (Ar-CH), 131.1 (Ar-CH), 131.0
(Ar-CH), 130.9 (Ar, C-CH3), 130.5 (Ar-CH), 129.2 (Ar-CH), 128.7 (Ar-
CH), 128.7 (Ar-CH), 128.6 (Ar-CH), 128.5 (Ar-CH), 128.4 (Ar-CH),
128.3 (Ar-CH), 128.2 (Ar-CH), 128.1 (Ar-CH), 127.8 (Ar-CH), 126.1 (Ar,
C-CH3), 123.9 (Ar-CH), 122.5 (Ar, C-CH3), 21.1 (Ar-Me, CH3), 18.7 (Ar-
Me, CH3), 18.6 (Ar-Me, CH3), 18.4 ppm (Ar-Me, CH3). Elemental analy-
sis calcd (%) for [C38H31ReBrN3O3]: C 53.90, H 4.05, N 4.96; found:
C 54.01, H 3.93, N 5.05.
CH), 130.5 (Ar, C-CH3), 130.1 (Ar, C-CH3), 129.5 (Ar-CH), 128.9 (Ar-
CH), 128.8 (Ar-CH), 128.7 (Ar-CH), 128.4 (Ar-CH), 128.0 (Ar-CH),
127.7 (Ar-CH), 127.2 (Ar-CH), 126.2 (Ar, C-CH3), 124.2 (Ar-CH), 18.8
(Ar-Me, CH3), 18.4 (Ar-Me, CH3), 18.3 (Ar-Me, CH3), 18.2 ppm (Ar-Me,
CH3). A sample for elemental analysis was obtained by recrystallization
in CHCl3, resulting in
a
1:1 CHCl3 adduct of
7 calcd(%) for
[C39H31F3N3O6ReS]
H 3.22, N 4.22.
ACHTUNGRTENUN[NG CHCl3]: C 46.54, H 3.12, N 4.07; found: C 46.85,
ACHUTNETGRNUNN[G (2,6-{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)2AHCTUNGRTEG(NNUN CF3SO3)] (7): A powder
sample of 6 (50 mg, 0.055 mmol) was placed in a ceramic boat, inside a
quartz tube under flow of N2 and placed inside a furnace. The reaction
was allowed to run for 25 min at 1908C, after which the boat was re-
moved and allowed to cool. A dark brown/red powder was isolated in
93% yield. Small orange plate-like crystals suitable for X-ray analysis
were grown in a saturated solution of CHCl3 with hexanes, and storing at
ꢁ208C for several days. n˜(CO)=1915 (s), 1855 cmꢁ1 (s). 1H NMR
(CDCl3, 500 MHz): d=7.59 (brd, 2H, py, m-CH), 7.49–6.99 (brm, 13H,
aromatic), 6.81 (brt, 2H, aromatic), 6.71 (brd, 2H, aromatic), 2.66 (brs,
6H, Me), 2.19 ppm (brs, 6H, Me). 13C NMR (CDCl3, 125 MHz). d=
220.4 (CO), 178.9 (C=N imine), 169.1 (CO), 163.9(py, o-C=N), 150.4 (Ar,
i-C), 140.7 (Ar-CH), 131.7 (Ar, i-C), 131.6 (Ar, i-C), 131.3 (Ar-CH),
129.4 (Ar-CH), 129.3 (Ar-CH), 128.9 (Ar-CH), 128.6 (Ar-CH), 128.3 (Ar-
CH), 128.2 (Ar-CH), 127.3 (Ar-CH), 126.7 (Ar, i-C), 126.6 (Ar-CH),
126.4 (Ar-CH), 19.4 (Ar-Me, CH3), 19.0 ppm (Ar-Me, CH3). Elemental
analysis calcd(%) for [C38H31F3N3O5ReS]: C 51.58, H 3.53, N 4.75; found:
C 51.31, H 3.44, N 4.80.
ACHTUNGTRENNUNG[(2,6-{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)2Cl] (4): A powder sample of
2 (120 mg, 0.149 mmol) was placed in a ceramic boat inside a quartz tube
under flow of N2 and placed inside a furnace. The reaction was allowed
to run for 1 hour at 2008C, after which the boat was removed and al-
lowed to cool. A dark brown/red powder was isolated in 97% yield.
Small dark brown prism-like crystals suitable for X-ray analysis were
grown from a saturated solution of THF with hexanes, and after storing
at ꢁ208C for several days. n˜(CO)=1919 (s), 1849 cmꢁ1 (s). 1H NMR
(CDCl3, 500 MHz): d=7.66 (brd, 2H, py, m-CH), 7.42 (brt, 2H, aromat-
ic), 7.33 (brt, 2H, aromatic), 7.16 (brt, 2H, aromatic), 7.07 (brm, 3H, ar-
omatic), 7.01 (brd, 2H, aromatic), 6.96 (brd, 2H, aromatic), 6.81 (brt,
2H, aromatic), 6.70 (brd, 2H, aromatic), 2.76 (brs, 6H, Me), 2.12 ppm
(brs, 6H, Me). 13C NMR (CDCl3, 125 MHz). d=221.8 (CO), 174.6 (C=N
imine), 166.2 (CO), 161.8 (py, o-C=N), 151.2 (Ar, i-C), 138.1 (Ar-CH),
132.5 (Ar, i-C), 132.1 (Ar, i-C), 130.9 (Ar-CH), 130.0 (Ar-CH), 128.8 (Ar-
CH), 128.6 (Ar-CH), 128.5 (Ar-CH), 128.3 (Ar-CH), 128.2 (Ar-CH),
127.9 (Ar-CH), 127.8 (Ar-CH), 126.8 (Ar, i-C), 126.2 (Ar-CH), 124.4 (Ar-
CH), 20.2 (Ar-Me, CH3), 19.1 ppm (Ar-Me, CH3). Elemental analysis
calcd(%) for [C37H31ReClN3O3]: C 57.61, H 4.05, N 5.45: found: C 57.43,
H 4.03, N 5.44.
ACHTUNGTRENNUNG[(2,6-{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)2Br] (5): A powder sample of
Acknowledgements
3 (60 mg, 0.071 mmol) was placed in a ceramic boat inside a quartz tube
under flow of N2 and placed inside a furnace. The reaction was allowed
to run for 1 hour at 2408C, after which the boat was removed and al-
lowed to cool. A dark brown/red powder was isolated in 95% yield.
Small dark prism-like crystals were grown from a variety of solvents
(ether, THF, chloroform) but were consistently too small and unsuitable
for X-ray analysis. n˜(CO)=1906 (s), 1837 cmꢁ1 (s). 1H NMR (CDCl3,
500 MHz): d=7.66 (brm, 2H, py, aromatic), 7.49–6.88 (brm, 13H, aro-
matic), 6.83 (brt, 2H, aromatic), 6.72 (brd, 2H, aromatic), 2.82 (brs, 6H,
Me), 2.14 ppm (brs, 6H, Me). 13C NMR (CDCl3, 125 MHz). d=221.6
(vbr CO), 174.9 (C=N imine), 167.1 (CO), 161.9 (py, o-C=N), 151.8 (Ar,
i-C), 138.4 (Ar-CH), 133.1 (Ar, i-C), 132.3 (Ar, i-C), 131.3 (Ar-CH),
130.4 (Ar-CH), 129.2 (Ar-CH), 128.9 (Ar-CH), 128.6 (Ar-CH), 128.4 (Ar-
CH), 128.2 (Ar-CH), 127.1 (Ar, i-C), 126.6 (Ar-CH), 124.6 (Ar-CH), 21.7
(Ar-Me, CH3), 19.6 ppm (Ar-Me, CH3). Elemental analysis calcd(%) for
[C37H31ReBrN3O3]: C 54.48, H 3.83, N 5.15; found: C 53.94, H 3.78,
N 5.09.
We thank the Natural Sciences and Engineering Research Council
(NSERC) of Canada for funding.
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ACHTUNGTRENNUNG[(2,6-{2,6-Me2C6H3N=CPh}2C5H3N)Re(CO)3AHCTUNGRTEG(NNUN CF3SO3)] (6): A powder
sample of 2 (150 mg, 0.187 mmol) and slight excess of AgOTf (51 mg,
0.200 mmol) were added to toluene (8 mL) in a darkened reaction flask.
The reaction mixture was allowed to stir for 18 h, over which time a
bright yellow precipitate formed. The solution was filtered, the solid was
washed with hexanes (5ꢂ2 mL) then dissolved in dichloromethane and
filtered through a plug of Celite. This solution was then concentrated
under vacuum. A bright yellow powder was isolated in 55% yield.
Yellow plate-like crystals suitable for X-ray analysis were grown from a
saturated solution of THF with hexanes, and storing at ꢁ208C for several
days. n˜(CO)=2030 (s), 1931 (s), 1912 cmꢁ1 (s). 1H NMR (CDCl3,
500 MHz): d=8.21 (brd, 1H, py, aromatic), 8.42 (brd, 1H, py, aromatic),
8.19 (brt, 1H, aromatic), 7.65 (brd, 1H, aromatic), 7.45 (brt, 1H, aro-
matic), 7.37 (brm, 2H, aromatic), 7.32 (brm, 2H, aromatic), 7.28–7.11
(brm, 4H, aromatic), 7.03 (brm, 3H, aromatic), 6.93 (brt, 2H, aromatic),
6.84 (brd, 1H, aromatic), 6.77 (brd, 1H, aromatic), 2.49 (brs, 3H, Me),
2.38 (brs, 3H, Me), 2.02 (brs, 3H, Me), 1.52 ppm (brs, 3H, Me).
13C NMR (CDCl3, 125 MHz). d=195.9 (CO), 194.6 (CO), 187.4 (CO),
179.6 (C=N imine), 163.7 (py, C=N imine), 163.3 (o-C=N), 157.5 (o-C=
N), 147.4 (Ar, i-C), 147.2 (Ar, i-C), 140.3 (Ar-CH), 133.4 (Ar, i-C), 132.4
(Ar, C-CH3), 131.9 (Ar-CH), 131.4 (Ar-CH), 131.3 (Ar-CH), 130.7 (Ar-
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[20] Higher temperatures or prolonged reaction results in decomposition.
The powder turns black and there is a multitude of unassignable
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Received: August 28, 2012
Revised: November 15, 2012
Published online: February 1, 2013
4286
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Chem. Eur. J. 2013, 19, 4278 – 4286