A catalyst with two-coordinate nickel: Theoretical and catalytic studies

Article abstract of DOI:10.1002/ejic.201301598

The bisadduct (cAAC)₂Ni^IICl₂ [1; cAAC = cyclic (alkyl)(amino)carbene] was directly synthesized by treating cAAC with NiCl₂. Compound 1 was reduced to (cAAC)₂Ni (2) by using lithium diisopropylamide or KC₈. Crystals of 2 were stable under an inert gas for several months and decomposed upon heating above 165°C. On the basis of the calculated natural bond orbital charge values of the nickel atom in 2, the oxidation state of nickel was determined to be between Ni^I and Ni (+0.34). Theoretical calculations suggested a closed-shell singlet electronic configuration of 2 with little biradical character. Ab initio multiconfigurational C(R)ASSCF/CASPT2 calculations predicted a closed-shell singlet electronic configuration (Ni), whereas excited spin states possessed Ni^I character with unpaired electrons on neighboring carbon atoms. The catalytic activity of complex 2 was investigated for the homocoupling of various unactivated aryl chlorides/fluorides. The biaryls were obtained in good yields at moderate temperature. Theoretical studies showed that an intermediate containing Ni^III was more favored than one with Ni^IV. The synthesis of (cAAC)₂Ni^IICl₂ (1) and its conversion into (cAAC)₂Ni (2) through reduction is reported [cAAC = cyclic (alkyl)(amino)carbene]. Efficient reductive homocoupling of unactivated aryl chlorides and fluorides is catalyzed by 2 through C-Cl and C-F bond activation. Detailed high-level theoretical investigations are performed to understand the electronic and catalytic cycle of 2. Copyright

Full text of DOI:10.1002/ejic.201301598

SHORT COMMUNICATION
DOI:10.1002/ejic.201301598
A Catalyst with Two-Coordinate Nickel: Theoretical and
Catalytic Studies
Kartik Chandra Mondal,[a] Prinson P. Samuel,[a] Yan Li,[a]
Herbert W. Roesky,*[a] Sudipta Roy,[b] Lutz Ackermann,*[b]
Navdeep S. Sidhu,[a] George M. Sheldrick,[a] Elena Carl,[a]
Serhiy Demeshko,[a] Susmita De,[c] Pattiyil Parameswaran,[c]
Liviu Ungur,[d] Liviu F. Chibotaru,[d] and Diego M. Andrada[e]
Dedicated to Professor Marius Andruh on the occasion of his 60th birthday
Keywords: π-Accepting carbene / Nickel / Homocoupling / C–Cl activation / C–F activation
The bisadduct (cAAC)2NiIICl2 [1; cAAC = cyclic (alkyl)-
(amino)carbene] was directly synthesized by treating cAAC
with NiCl2. Compound 1 was reduced to (cAAC)2Ni0 (2) by
using lithium diisopropylamide or KC8. Crystals of 2 were
stable under an inert gas for several months and decomposed
upon heating above 165 °C. On the basis of the calculated
natural bond orbital charge values of the nickel atom in 2,
the oxidation state of nickel was determined to be between
NiI and Ni0 (+0.34). Theoretical calculations suggested a
closed-shell singlet electronic configuration of 2 with little
biradical
character.
Ab
initio
multiconfigurational
C(R)ASSCF/CASPT2 calculations predicted a closed-shell
singlet electronic configuration (Ni0), whereas excited spin
states possessed NiI character with unpaired electrons on
neighboring carbon atoms. The catalytic activity of complex
2 was investigated for the homocoupling of various unacti-
vated aryl chlorides/fluorides. The biaryls were obtained in
good yields at moderate temperature. Theoretical studies
showed that an intermediate containing NiIII was more fa-
vored than one with NiIV.
metal (M) complexes (M = Ni0, Pd0, CuI, CoI). However,
they are not yet known to form stable mono- or bisad-
ducts[2] upon reaction with NiX2 (X = Cl, Br, I). [(NHC)-
Ni(μ-Cl)Cl]2, rather (NHC)2NiX2, have been prepared by
different synthetic routes.[3] The bisadducts of NiX2 are ef-
fective catalysts for various C–C cross-coupling reactions
(Suzuki, Heck, Kumada, Negishi, and reductive Ullmann
coupling).[3b] Thus, palladium compounds that also catalyze
such reactions can alternatively be replaced by nickel–carb-
ene complexes. Compounds such as (NHC)NiII=N–R,[4a]
(NHC)NiI(NH–R),[4b] and (NHC)2Ni0[5] with two-coordi-
nate nickel atoms are known. (NHC)3Ni0 and [(NHC)Ni0]2
with three- and seven-coordinate nickel atoms, respectively,
were also reported.[6] A few computational studies have
been accomplished for the better understanding of the elec-
tronics of such species containing zero-valent nickel
atoms.[5] Radius and Bickelhaupt[5e] performed a detailed
study of the bonding situation of (NHC)2Ni0 derivatives by
using energy decomposition analysis. They showed that the
π interaction is about 43–32% of the total orbital interac-
tion between Ni0 and the NHC ligands.[5e] (NHC)2Ni0 com-
pounds might be promising catalysts for unusual organic
transformations.[7] However, none of the (NHC)2Ni0[5] com-
pounds were prepared by utilizing (NHC)2NiIIX2 as a pre-
Introduction
Compounds containing low-coordinate transition metal
atoms have been the focus of great interest in the field of
catalysis in recent times as a result of the presence of unsat-
urated coordination sites.[1] Such species can act as active
catalysts.[1a] Recently, stable d1–d9 open-shell complexes[1b]
with two-coordinate transition-metal ions were summa-
rized, and their reactivities were explored.[1] N-Heterocyclic
carbenes (NHCs) are also known to form two-coordinate
[a] Institut für Anorganische Chemie, Universität Göttingen,
Tammannstrasse 4, 37077 Göttingen, Germany
E-mail: hroesky@gwdg.de
http://www.uni-goettingen.de/en/46240.html
[b] Institut für Organische und Biomolekulare Chemie Georg-
August-Universität
Tammannstrasse 2, 37077 Göttingen, Germany
E-mail: Lutz.Ackermann@chemie.uni-goettingen.de
http://www.ackermann.chemie.uni-goettingen.de
[c] Dept. of Chemistry, NIT Calicut,
Campus P. O., Kozhikhode 673601, Kerala, India
[d] DQPC and INPAC – Institute of Nanoscale Physics and
Chemistry, Katholieke Universiteit Leuven
Celestijnenlaan 200F, 3001 Heverlee, Belgium
[e] Institut für Physikalische Chemie, Georg-August-Universität,
Tammannstrasse 6, 37077 Göttingen, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201301598.
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SHORT COMMUNICATION
cursor. Instead, a reverse approach was applied for the reducing agent.[10] The mass spectrum of this solution re-
preparation of (NHC)2NiIIX2, (NHC)2NiIIXPh, and vealed the formation of Me2C=NiPr, which suggests that
(NHC)2NiIX.[3,8]
the dehalogenation of (cAAC)2NiIICl2 (1) occurred through
The carbene carbon atom in the NHC is bound to two α-hydride transfer[10] from LDA to 1 by elimination of LiCl
σ-withdrawing and π-donating N atoms. However, in the to form a biscarbene–NiII chlorohydride intermediate, and
case of a cyclic (alkyl)(amino)carbene (cAAC), one of the this intermediate then reductively eliminated HCl (Support-
σ-withdrawing and π-donating N atoms is replaced by one ing Information) to give 2. The latter is consumed by an-
σ-donating quaternary C atom. Thus, the cAAC becomes other equivalent of LDA to form HNiPr2 and LiCl. The
more nucleophilic but also more electrophilic than the formation of HNiPr2 was confirmed by mass spectrometry.
NHC.[9] Hence, the use of a cAAC might be advantageous The calculated complexation energy for the formation of
over an NHC for certain reactions. Herein, we report a di- (cAAC)2NiIICl2 (1) from the cAAC and NiCl2 is highly exo-
rect synthesis of the bisadduct (cAAC)2NiIICl2 (1) and its thermic [ΔE1 = –125.1 kcalmol–1; see Equation (1)] at the
reduction to a singlet (cAAC)2Ni (2) with two-coordinate M06/def2-TZVPP//BP86/def2-SVP level of theory.[11,12] The
nickel(0) atom (Scheme 1). Theoretical investigations sug- reductive dehalogenation of 1 by 2 equiv. of LDA to
gest that the first low-lying excited state of 2 possesses NiI (cAAC)2Ni (2) is also exothermic [ΔE2 = –12.8 kcalmol–1;
character having a delocalized radical electron over two see Equation (2)]. Both of these reactions indicate the
carbene carbon atoms. Magnetic susceptibility measure- thermodynamic preference for the formation of complex 1
ments and detailed theoretical calculations of 2 were per- and its reductive dehalogenation by LDA to complex 2.
formed to support the electronic scenario. Compound 2
2 cAAC + NiCl2 Ǟ (cAAC)2NiIICl2 (1); ΔE1 = –125.1 kcalmol–1
acts as an efficient catalyst for the reductive homocoupling
of aryl chlorides and fluorides through C–Cl and C–F bond
(1)
activation at 50 °C.
(cAAC)2NiIICl2 (1) + 2 LiNiPr2 Ǟ (cAAC)2Ni (2) + 2 LiCl +
Me2CNiPr + HNiPr2; ΔE2 = –12.8 kcalmol–1
(2)
Reaction of 1 with KN(SiMe3)2 does not yield 2 because
of the absence of an α-hydrogen atom. A plausible mecha-
nism for the formation of 2 is given in the Supporting Infor-
mation. Compound 2 is also obtained upon treating
(cAAC)2NiIICl2 (1) with KC8 in a 1:2 molar ratio in THF.
According to our synthetic strategy, reduction of 1 to 2 with
LDA is superior to that with KC8 because of the solubility
of LDA in THF and its commercial availability.
Compound 2 has purple color in solution, whereas it ap-
pears as dark black shiny plates in the solid state. Com-
pound 2 decomposes at 161 °C, while precursor 1 decom-
poses above 185 °C. Compound 2 is stable under an inert
gas, although 1 is air-stable for 4 months. The black shiny
plates of 2 are stable for 45 min upon exposure to air, but
they then slowly lose their color and finally decompose into
an unknown colorless solid. The UV/Vis spectrum of 2 in
n-hexane shows absorption bands at 533, 611, and 807 nm
(see the Supporting Information). The 13C NMR spectrum
of 2 exhibits a resonance at δ = 242.4 ppm (Ccarbene), which
Scheme 1. Synthetic strategy for compounds 1 and 2.
Results and Discussion
The bisadduct (cAAC)2NiIICl2 (1) was formed upon di- is upfield-shifted relative to the resonance of the free cAAC
rect reaction of anhydrous NiCl2 with the cAAC in a 1:2 (δ = 304.2 ppm).[9]
molar ratio in THF. First, the formation of a yellow suspen-
Compound 2 crystallizes from toluene as 2·toluene in the
¯
sion was observed, which was heated to 70 °C for 3–4 h to triclinic space group P1. The X-ray crystal structure of
obtain an orange-yellow slurry. The resultant solution was 2·toluene shows two five-membered rings attached to one
stirred at room temperature for an additional 2 d to obtain nickel atom. The nickel center is bound to each of the carb-
a clear red solution of 1. For comparison reasons, a similar ene carbon atoms and adopts an acyclic, bent, two-coordi-
reaction was applied for a suspension of NHC {NHC = nate geometry (Figure 1).
C[N(2,4-diisopropylphenyl)(CH)]2} and NiCl2. However,
The Ni–C and C–N bond lengths of 2 are 1.8448(14)/
the latter reaction failed to produce an NHC analogue of 1.8419(13) and 1.3381(16)/1.3420(16) Å, respectively, which
1. Compound 1 was crystallized from toluene as air-stable are comparable to those previously reported for
red rods in 85% yield. Furthermore, 1 was treated with lith- (NHC)2Ni0.[5]
CNHCǞNi bond lengths are found in the
ium diisopropylamide (LDA) in a 1:2 molar ratio to form range from 1.82 to 1.96 Å,[3–8] whereas those of (NHC)-
an immediate dark purple solution of (cAAC)2Ni (2). LDA NiII=N–R[4a] and (NHC)NiI(NHR)[4b] with two-coordinate
generally serves as a strong base, but it can also act as a nickel atoms are 1.917(3) and 1.879(2) Å, respectively. The
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SHORT COMMUNICATION
nickel atom, which in two cases has a small overlap with
the p orbitals of the carbene carbon atoms on each ligand
(HOMO–3 and HOMO–4). Interestingly, the LUMO is a
π-type orbital on the Ccarbene–Ni–Ccarbene bridge, which has
a large contribution to the Ccarbene atoms and a small con-
tribution to the Ni atom (Figure 2).
Figure 2. (a) HOMO and (b) LUMO of compound 2.
Natural bond orbital (NBO)[13–16] analysis was per-
Figure 1. Molecular structure of compound 2 with thermal ellip-
soids drawn at the 50% probability level. H atoms are omitted for
clarity. Selected [calculated at M06/def2-SVP singlet, triplet] bond
lengths [Å] and angles [°]: Ni1–C1 1.8448(14) [1.846, 1.898], Ni1–
C1_1 1.8419(13) [1.846, 1.898], C1–N1 1.3381(16) [1.340, 1.354],
C1–N1_1 1.3420(16) [1.341, 1.352]; C1–Ni–C1_1 166.42(5) [165.6,
formed to evaluate the natural charges over the Ccarbene
–
Ni–Ccarbene bridge of 2. The computed natural population
analysis (NPA) charges on the carbene carbon atoms are
0.03 a.u., whereas the charge on the Ni atom is 0.34 a.u.,
168.3], C2–C1–N1 106.49 (10) [106.4, 107.0], C2_1–C1_1–N1_1 which suggests the nature of this metal atom is between NiI
and Ni0. To confirm the contribution from the NiI charac-
106.40 (10) [106.4, 107.0].
ter of (cAAC)2Ni (2), it was treated with the TEMPO
Ni–C single bond lengths are 1.9150(14) and 1.9297(15) Å (2,2,6,6-tetramethylpiperidine N-oxide) radical. (cAAC)-
in the (cAAC)2NiIICl2 (1) precursor. Shortening of the Ni– NiI–TEMPO was obtained as the product, which was char-
C bond and lengthening of the C–N bond (Supporting In- acterized by ESI-MS [m/z (%) = 499.3(100)] (Supporting
formation) in 2 is due to electronic backbonding from the Information). For the sake of comparison between (NHC)2-
nickel center to the carbene carbon atom. The C–Ni–C Ni0 and (cAAC)2Ni (2), we also optimized the structures of
bond angle of 2 is 166.42°, which is 13° less than that of (NHC)2Ni0 (Supporting Information). The theoretical
(NHC)2Ni0, also with two-coordinate nickel atom.[5] The
C
NHCǞNi bond lengths[5e] are 1.843 Å, which is in good
angle between the two N–Ccarbene–CMe2 planes of 2 is agreement with the reported values.[5] For (NHC)2Ni0, NPA
60.68°. The distance between the two carbene carbon atoms charges are 0.137 and –0.070 a.u. for the Ccarbene and Ni
in 2 is 3.66 Å. The N–C–C–N torsion angle is 78° for 2, atoms, respectively, which is clearly suggestive of the Ni0
whereas it is 180° for precursor 1. The nickel atom of 2 also nature. In contrast, the Wiberg bond order[17] of the Ni–
weakly interacts with the hydrogen atoms (2.90–3.00 Å) of Ccarbene bonds, for both complexes, is 0.74, as expected.
the methyl groups of the carbene.
To obtain a more detailed picture, natural resonance
To obtain deeper insight into the electronic structure of theory (NRT)[18–20] analysis on 2 as a closed-shell species
2, theoretical calculations were performed within the DFT was performed. Particular focus on delocalization over
framework. Geometry optimizations were performed for
Ccarbene–Ni–Ccarbene was placed in our analysis. The re-
the singlet closed-shell and triplet states at the M06/def2- sulting resonance structures (Scheme S5) indicate that there
SVP[11,12] level of theory. Selected bond lengths and angles is a strong delocalization involving the N–Ccarbene–Ni–
are provided in the caption of Figure 1 (see Figures S7 and
Ccarbene–N atoms, as a consequence of the π backdonation
S8 for the superposition plots). The theoretical geometries from the Ni d occupied orbitals to the empty Ccarbene
p
indicate that the singlet state is in better agreement with orbitals. Indeed, application of second-order perturbation
the X-ray structure than the triplet species; the root-mean- theory within the NBO method revealed the occurrence of
square deviation values relative to the crystal structures are stabilizing two-electron delocalization from the sp2 lone
0.127 Å for the former and 0.200 Å for the latter. We also pairs of electrons of the carbene [LP(Ccarbene)] to the s or-
checked the optimization of the open-shell species, but all bital in nickel [LP*(Ni)] and from the d orbital of nickel
attempts led to the singlet closed-shell species. The tempera- [LP(Ni)] to the formally empty p orbitals on the carbene
ture dependence of the magnetic susceptibility (Supporting moiety [LP*(Ccarbene)]. The computed associated energies
Information) confirms the singlet spin ground state of 2.
[ΔE(2)] are 135.0 and 59.8 kcalmol–1 for LP(Ccarbene)Ǟ
Theoretical calculations at different levels predicted that LP*(Ni) and LP(Ni)ǞLP*(Ccarbene), respectively. NBO also
the singlet closed-shell state lies below the triplet state by indicated a significant charge transfer from the occupied
18.7–34.9 kcalmol–1 (Supporting Information).
Ni d orbitals to the antibonding σ*(C–H) orbital on the
The frontier molecular orbitals for the singlet closed- isopropyl substituents (see structure description), which is
shell species are displayed in Figure S10. Clearly, the occu- a common interaction in complexes with low-valent metal
pied frontier molecular orbitals are the d orbitals of the oxidation states.[21–23]
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All ab initio calculations predicted the spin singlet to be uct under the same reaction conditions (Table S34, En-
the lowest in energy (Supporting Information).[24] We found try 5). For comparison, (NHC)2Ni {NHC=C[N(2,4-diiso-
that the Ni0 electronic configuration is dominant in the propylphenyl)(CH)]2} was also employed as a catalyst in-
ground singlet state. This result implies that compound 2 is stead of (cAAC)2Ni (2). The yield of the reaction dropped
a closed-shell singlet. Interestingly, the lowest and first ex- to 10–12%, which suggests that 2 is a superior catalyst than
cited spin triplets have one unpaired electron on Ni and the NHC analogue.
another electron spanning one 2p orbital of the closest car-
bon atom (C1_1 in the structure file). Similar calculation
Table 1. Catalytic studies of compound 2.[a]
on (cAAC)2Mn revealed that the ground state possesses
MnI with a delocalized electron over two carbene carbon
atoms.[25] To quantify the biradical character, we made use
of the singlet biradical index proposed by Neese and co-
workers (see the Supporting Information).[26]
Next, we intended to explore the catalytic reactivity of 2.
The homocoupling of aryl halides is one of the most power-
ful methods to form C–C bonds to give rise to an important
class of compounds known as biaryls[27–29] and their het-
eroaromatic analogs. A stoichiometric amount of metallic
Zn is used as the reducing agent in almost all Ni-catalyzed
homocoupling reactions mainly involving aryl bromides or
iodides as the self-coupling reagents. The use of readily
available and inexpensive aryl chlorides is still scarce proba-
bly because of the high energy of the C–Cl bond. Catalytic
activation of unactivated C–F bonds[1a] also remains a chal-
lenge till to date. In this context, nickel-catalyzed cross-cou-
pling between unactivated aryl chlorides/fluorides and aryl
Grignard reagents in the presence of different stabilizing
ligands is noteworthy.[30,31]
Our initial investigations started with the use of 10 mol-
% of 2 as the catalyst for the homocoupling of para-chloro-
[a] Reaction conditions: A mixture of 3 (2 mmol), catalyst 2
toluene (3c) in the presence of LDA as the reducing agent
(0.1 mmol), and LDA (2 mmol) in dry THF (2 mL) was stirred
under N2 at 50 °C for 4–6 h. [b] Yield of isolated product after col-
umn chromatography.
(Table S34). We obtained only a trace amount of the desired
4,4Ј-dimethylbiphenyl (4c) product after 24 h upon treating
3c with 20 mol-% of LDA in THF at 65 °C. Increasing the
amount of LDA to 1 equiv. (Table S34, Entry 2) also im-
A plausible reaction pathway is proposed, as shown in
proved the product yield to 42%. Employing 2 equiv. of Scheme 2, on the basis of the energetics of the various steps
LDA successfully produced the desired product 4c after 6 h involved and earlier experimental reports.[8] The first step
in a good yield of 85% (Table S34, Entry 3). We next turned of reaction can be considered as the activation of the Ph–
our attention to performing the reaction at ambient tem- X bond by oxidative addition to model catalyst 5. The cor-
perature, but unfortunately, we did not observe any reaction responding reaction energy (ΔE1) for the formation of 6 by
under this condition. Finally, we found 50 °C as the optimal Ph–X (1 equiv.) is highly exothermic. This was also experi-
temperature for our system, which gave rise to desired prod- mentally observed. Upon adding Ph–X, (cAAC)2Ni reacted
uct 4c in 84% yield after 6 h (Table S34, Entry 4). Reducing vigorously, and heat was generated. This reaction process is
the catalyst loading to 5 and 2 mol-% led to a large decrease more favorable for Ph–Cl [ΔE1(Cl) = –50.8 kcalmol–1] than
in the product yield (29% and 3%, respectively). For better for Ph–F [ΔE1(F) = –31.8 kcalmol–1], which can be corre-
functional-group tolerance, we screened weaker reducing lated to the strength of the C–Cl and C–F bonds. The sec-
agents, for example, potassium tert-butoxide (Table S34, ond step can be either dehalogenation of bisadduct 6 by
Entry 6), but we obtained the biaryl in a much lower yield LDA to give tricoordinate NiI complex 7 or further oxidat-
of 35% under the optimized reaction conditions. We also ive addition of a second molecule of Ph–X to give hexaco-
confirmed that no background reaction occurs in the ab- ordinate NiIV complex 10 (Scheme S1). The latter process
sence of 2 (Table S34, Entry 7). Having the optimized reac- is highly endothermic (ΔECl = 50.5 kcalmol–1 and ΔEF
=
tion conditions in hand, we next explored the generality of 44.7 kcalmol–1 for 1 equiv. of 6) and hence can be excluded.
the reaction; to our delight, we observed that not only the However, the formation of tricoordinate complex 7 is only
C–Cl bond but also the more unreactive C–F bond could marginally endothermic by ΔE2(Cl) = 14.0 kcalmol–1 and
activated by 2 to produce the desired biaryls in very good ΔE2(F) = 3.2 kcalmol–1 for 1 equiv. of 6. The formation of
yields under relatively mild reaction conditions (Table 1). the Li–F bond, which is stronger than the Li–Cl bond, is
Utilization of CyMgCl (Cy = cyclohexyl) instead of LDA reflected in the lower endothermicity for the defluorination
produced only a trace amount of the homocoupling prod- process relative to that of the dechlorination reaction. No-
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SHORT COMMUNICATION
tably, the formation of tetracoordinate (NHC)2NiII(Ar)(Br) nickel(0) compound starting from nickel(II) dichloride and
and tricoordinate (NHC)2NiI(Br) in the reaction of a carbene. On the basis of the calculated NBO charge val-
(NHC)2Ni0 and with an aryl bromide (ArBr) has been re- ues of the nickel atom for the (NHC)2Ni and (cAAC)2Ni
ported.[8] To form Ph–Ph, two phenyl groups should be in (2) complexes, the oxidation state of nickel was found to be
close proximity, and as a result, the oxidative addition of Ni0 in (NHC)2Ni (–0.07) and between NiI and Ni0 in 2
the second molecule of Ph–X to tricoordinate NiI interme- (+0.34). CASSCF(2,2)/def2-SVP//M06/def2-SVP calcula-
diate 7 is proposed. The formation of corresponding penta- tions suggested a closed-shell singlet electronic configura-
coordinate intermediate 8 by the oxidative addition of Ph– tion for 2 with little biradical character. Ab initio multicon-
Cl is exothermic [ΔE3(Cl) = –9.95 kcalmol–1], whereas that figurational C(R)ASSCF/CASPT2 calculations predicted a
of Ph–F is slightly endothermic [ΔE3(F) = 3.35 kcalmol–1] closed-shell singlet electronic configuration (Ni0), whereas
for 1 equiv. of 7. Product formation through homocoupling the excited spin states were predicted to possess NiI charac-
of the two Ph groups is a highly favorable process [ΔE4(Cl) ter with unpaired electrons on neighboring carbon atoms.
= –42.25 kcalmol–1 if X = Cl and ΔE4(F) = –35.7 kcalmol–1 The singlet ground state of 2 was confirmed by magnetic
if X = F for 1 equiv. of Ph–Ph].
susceptibility measurements. The catalytic activity of com-
plex 2 was investigated for the homocoupling of various
unactivated aryl chlorides/fluorides. The biaryls were ob-
tained in good yields at moderate temperature. A plausible
reaction mechanism was proposed and supported by the
favorable exothermic reaction energetics (Scheme 2).
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details.
Acknowledgments
H. W. R. thanks the Deutsche Forschungsgemeinschaft (DFG)
(RO 224/60-I) for financial support. The authors thank Prof. Dr.
R. Mata for his kind support. L. U. thanks the Fonds Wetenschap-
pelijk Onderzoek-Vlaanderen (FWO). S. D. and P. P. thank the De-
partment of Science and Technology (DST) for the financial sup-
port. We thank N. Bruckner for recording the UV/Vis spectrum.
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are for X = F.
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Received: December 19, 2013
Published Online: January 23, 2014
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