Bis-NHC chelate complexes of nickel(0) and platinum(0)

Article abstract of DOI:10.1002/anie.201401024

For a long time d¹⁰-ML₂ fragments have been known for their potential to activate unreactive bonds by oxidative addition. In the development of more active species, two approaches have proven successful: the use of strong σ-donating

Full text of DOI:10.1002/anie.201401024

Angewandte
Chemie
DOI: 10.1002/anie.201401024
NHC Complexes
Bis-NHC Chelate Complexes of Nickel(0) and Platinum(0)**
Matthias Brendel, Carolin Braun, Frank Rominger, and Peter Hofmann*
Dedicated to the MPI fꢀr Kohlenforschung on the occasion of its centenary
Abstract: For a long time d10-ML2 fragments have been known
for their potential to activate unreactive bonds by oxidative
addition. In the development of more active species, two
approaches have proven successful: the use of strong s-
donating ligands leading to electron-rich metal centers and the
employment of chelating ligands resulting in a bent coordina-
tion geometry. Combining these two strategies, we synthesized
bis-NHC chelate complexes of nickel(0) and platinum(0).
Bis(1,5-cyclooctadiene)nickel(0) and -platinum(0) react with
bisimidazolium salts, deprotonated in situ at room temper-
ature, to yield tetrahedral or trigonal-planar bis-NHC chelate
olefin complexes. The synthesis and characterization of these
Scheme 1. Bisphosphine platinum(0) systems active in bond-activation
reactions (R=Me, Et; Cy=cyclohexyl).
À
complexes as well as a first example of C C bond activation
hence be explained by the fact that the steric bulk of the
phosphines in the fragment limits the accessible degree of
deviation from linearity during the oxidative addition step.[3]
Complexes with chelating bisphosphine ligands enforcing
a bent coordination geometry show enhanced reactivity: The
platinum(0) fragment B with bis(dicyclohexylphosphino)-
ethane as the ligand, formed by reductive elimination of
neopentane from the corresponding neopentylhydrido-
with these systems are reported. Due to the enforced cis
arrangement of two NHCs, these compounds should open
interesting perspectives for bond-activation chemistry and
catalysis.
T
ransition-metal-mediated activation and functionalization
À
À
À
of C H, C C, and strong C X bonds is a vital area of research
in homogeneous catalysis. For low-valent, electron-rich com-
plexes of late transition metals bond activation is mainly
achieved by oxidative addition to the metal center.[1] Metal
fragments isolobal to singlet methylene exhibit the ideal
energetic and geometric characteristics crucial for the design
of highly reactive systems.
In the 1970s Stone et al. observed completely different
behavior for two nearly identical linear bisphosphine plati-
num(0) complexes A formed from appropriate precursor
complexes (Scheme 1).[2] An MO analysis revealed that the
frontier orbitals of d10-ML2 fragments substantially depend in
terms of energy and hybridization on the L-M-L angle. In the
case of linear complexes like A, the different reactivity can
platinum(II) complex (P-Pt-P angle of 87.28), activates
[4]
À
a large variety of C H bonds. With an even smaller bite
angle of 74.78 the analogous bis(di-tert-butylphosphino)-
methaneplatinum neopentyl hydride showed very unusual
reactivity patterns, presumably via intermediate C.[5] In the
À
À
reaction with tetramethylsilane both C H and C Si bonds are
activated,[5a] and epoxides undergo unprecedented C C bond
À
activation forming 3-platinaoxetanes.[5c]
In the last two decades N-heterocyclic carbenes (NHCs)
have emerged as one of the most important classes of
spectator ligands in organometallic chemistry.[6] Due to their
stronger s-donor character compared to tertiary phosphines,
the employment of NHCs can create more electron-rich metal
centers. Thus, the activity of d10-ML2 fragments bearing
NHCs is expected to exceed that of related phosphine
complexes. NHCs are superior to phosphines in catalytic
processes in which bond activation is followed by oxidation as
the latter are prone to oxidation.[7]
The investigation of platinum(0) complexes bearing two
NHCs is scarce.[8] The only example for bond-activation
chemistry was reported by Nolan et al. for Pt(IMes)2 (IMes =
1,3-dimesitylimidazol-2-ylidene), who observed intramolecu-
lar C–H activation of the ligand.[8d] Nickel(0) bis-NHC
complexes are more established; they were found to be
active in stoichiometric bond activations and as catalysts for
cross-coupling reactions.[8a,b,9] Comprehensive studies were
performed by Radius et al. using [Ni2(IiPr)4(cod)] (cod = 1,5-
cyclooctadiene; IiPr= 1,3-diisopropylimidazol-2-ylidene) as
the precursor for various bond activations.[10]
[*] M. Brendel, C. Braun, Dr. F. Rominger, Prof. Dr. P. Hofmann
Institute of Organic Chemistry
Ruprecht-Karls-University Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: ph@oci.uni-heidelberg.de
Prof. Dr. P. Hofmann
Catalysis Research Laboratory (CaRLa)
Im Neuenheimer Feld 584, 69120 Heidelberg (Germany)
[**] This work was funded by the Deutsche Forschungsgemeinschaft
(SFB 623 “Molecular Catalysts: Structure and Functional Design”).
P.H. works at CaRLa of Heidelberg University, which is cofinanced
by the University of Heidelberg, the State of Baden-Wuerttemberg,
and BASF SE. We gratefully acknowledge generous support from
these institutions. NHC=N-heterocyclic carbene.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201401024.
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Based on the current state of research with respect to
reactive d10-ML2 systems, it seemed challenging to synthesize
compounds by employing chelating bis-NHC ligands that
enforce a cis configuration at the metal center.[11] Already in
the first application of NHC complexes in homogeneous
catalysis, bis-NHC chelate complexes of palladium(II) were
studied.[12a] Complexes bearing this type of ligand are now
established for most late transition metals in all prevalent
oxidation states.[12b] Bis-NHC chelate complexes are known
for platinum(II) and platinum(IV) and were first studied by
Strassner et al.[13] Nickel(II) complexes of these ligands were
already investigated by the groups of Herrmann and Green
fifteen years ago.[14] However, to the best of our knowledge,
compounds of this type are unprecedented for platinum(0)
and only three examples have been reported for
nickel(0).[15,20]
and secures complete separation of the lithium bromide. The
addition of water prior to toluene affects the yield adversely.
Separation of the phases and washing of the product with
hexane leads to the desired pure complex [LDippNi(h4-cod)]
(1) in 50% yield.
The orange compound 1 is extremely oxygen-sensitive in
solution and as a solid. In dichloromethane or chloroform the
complex is not stable. The 1H NMR spectrum of 1 shows
a singlet for the methylene bridge of the bis-NHC ligand,
which means that boat-to-boat inversion of the six-membered
nickelacycle occurs rapidly at room temperature. The olefinic
protons of the cod ligand give rise to one singlet with an
integral of four protons at d = 3.66 ppm, indicating h4-
coordination. Red single crystals suitable for X-ray diffraction
analysis were grown by layering a THF solution of 1 with
pentane. The solid-state structure has a distorted tetrahedral
geometry with a bite angle of the bis-NHC ligand of 93.88
(Figure 1). This is significantly smaller than that in the Radius
system, in which the two mono-NHCs span an angle of
118.18.[10a]
In our study we employed the aryl-substituted bis-NHC
LDipp (Dipp = 2,6-diisopropylphenyl) and the alkyl-substi-
tuted LtBu. The bisimidazolium salts used as precursors are
easily accessible by microwave-assisted SN2 reactions of the
corresponding imidazoles with dibromomethane (Scheme 2).
Scheme 2. Microwave-assisted synthesis of bisimidazolium salts used
as precursors for the bis-NHC ligands employed in this study.
R=Dipp: xylenes, 1908C, 2 h, 79%; R=tBu: THF, 1308C, 5 h, 74%.
As the isolated biscarbenes are very sensitive towards air and
not stable at room temperature, we chose to deprotonate the
bisimidazolium salts in situ. When lithium hexamethyldisila-
zide (LiHMDS) serves as a base, lithium adducts are formed.
The signals for their carbene carbon atoms in 13C NMR
spectra are shifted to higher field by Dd = 8.2 ppm (LDipp) and
2.8 ppm (LtBu) compared to the free biscarbenes synthesized
by deprotonation with KHMDS.
Figure 1. Solid-state structure of 1.[16a] Ellipsoids at the 50% probability
level; hydrogen atoms are omitted for clarity. Selected bond lengths [ꢀ]
and angles [8]: Ni–C1 1.938(3), Ni–C6 1.953(3), Ni–C12 2.136(3), Ni–
C13 2.114(3), Ni–C16 2.065(3), Ni–C17 2.075(3), C12–C13 1.383(5),
C16–C17 1.355(5); C1-Ni-C6 93.83(13).
In an analogous manner the alkyl-substituted bisimidazo-
lium salt LtBuH2Br2 can be used to synthesize the nickel(0)
complex 2 (Scheme 4, see SI). The synthesis and purification
of 2 are similar to that of 1 but using diethyl ether instead of
toluene. Complex 2 is isolated in 34% yield as an orange
solid. The 1H NMR spectrum differs significantly from that of
1. Because the ligandꢀs methylene bridge shows two doublets,
boat-to-boat inversion of the nickelacycle does not occur at
Addition of [Ni(cod)2] to deprotonated LDippH2Br2 in THF
at room temperature immediately leads to a deep red solution
(Scheme 3, see the Supporting Information (SI)). After
removal of the volatiles in vacuo, a mixture of water and
toluene is added to the crude product. It is important to add
these solvents at the same time, as the product reacts with
toluene in the presence of small amounts of remaining base to
give a yet unidentified product. Water hydrolyzes the base
Scheme 3. Synthesis of [LDippNi(h4-cod)] (1).
Scheme 4. Synthesis of [LtBuNi(h2-cod)] (2).
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room temperature due to the steric bulk of the tert-butyl
groups. Surprisingly, the chemical shifts for the two double
bonds of the olefin ligand differ distinctly. One multiplet at
d = 2.92 ppm corresponds to a coordinated double bond. The
second signal appears in the range of a noncoordinated olefin
at d = 6.08 ppm. This indicates h2-coordination, which is
unusual for this diene.[17]
When a solution of 2 in hexane was cooled to À308C,
orange single crystals were obtained. The solid-state structure
of 2 confirms the h2-coordination of cod, which adopts a chair
conformation (Figure 2). Several attempts at locating poten-
ature. The platinum atom couples with one of these protons
(4J(Pt,H) = 14.6 Hz), which is identified by NOESYas the one
oriented towards the metal center. As in the nickel complex,
two multiplets at d = 2.08 and 5.60 ppm for the olefinic double
bonds suggest h2-coordination of the cod ligand.
Slow evaporation of a solution of 3 in diethyl ether
solution provided yellow single crystals (Figure 3). The unit
cell contains two independent molecules with h2-coordinated
Figure 2. Solid-state structure of 2.[16b] Ellipsoids at the 50% probability
level; hydrogen atoms of the tert-butyl groups are omitted for clarity.
Selected bond lengths [ꢀ] and angles [8]: Ni–C1 1.905(3), Ni–C6
1.921(4), Ni–C12 1.951(4), Ni–C13 1.955(3), C12–C13 1.426(5);
C1-Ni-C6 91.00(16).
Figure 3. Solid-state structures of 3.[16c] Ellipsoids at the 50% proba-
bility level; hydrogen atoms of the tert-butyl groups are omitted for
clarity. The unit cell contains two independent molecules with different
conformations of the cod ligand. Selected bond lengths [ꢀ] and angles
[8]: twist (top): Pt–C1 2.049(6), Pt–C6 2.052(6), Pt–C12 2.077(6), Pt–
C13 2.091(6), C12–C13 1.468(8); C1-Pt-C6 84.6(2); chair (bottom): Pt–
C1 2.065(7), Pt–C6 2.052(6), Pt–C12 2.072(6), Pt–C13 2.082(7), C12–
C13 1.461(11); C1-Pt-C6 83.7(2).
tial energy minima with DFT calculations for a tetrahedral
complex with h4-coordination were unsuccessful and led to h2-
coordination instead. The steric demand of the two bis-NHC
ligands differs greatly. The area close to the metal center is far
more crowded in the complex bearing LtBu due to the tert-
butyl group, and the usual chelating coordination of the cod
ligand in 2 is impeded. The bite angle of the chelate ligand is
91.08 and therefore smaller than in 1.
The approach is not limited to nickel compounds.
Addition of [Pt(cod)2] to deprotonated LtBuH2Br2 leads to
the platinum(0) complex 3 (Scheme 5, see SI). In a procedure
analogous to that used for the nickel complexes the pale
yellow compound is isolated in a yield of 56%. The 1H NMR
spectrum is similar to that of 2. Recognizable by two doublets
for the protons of the methylene bridge, boat-to-boat
inversion of the platinacycle does not occur at room temper-
cod in a twist and a chair conformation, respectively. The bite
angles of the chelate ligand are 84.68 and 83.78, just between
the bite angles in the bisphosphine systems B and C described
above.
First studies testing the reactivity of these new complexes
look promising. The cod ligand is readily replaced by
electron-deficient olefins (see SI). To corroborate that the
compounds are precursors for reactive 14 valence electron
fragments, nickel complex 2 is reacted with benzonitrile
(Scheme 6, see SI). The nitrile replaces the weakly coordi-
nated olefin rapidly at room temperature forming the h2-
nitrile complex 4. The side-on coordination results in a large
shift of the singlet for the nitrile carbon atom to lower field in
the 13C NMR spectrum compared to the free nitrile (d =
173 ppm vs. 119 ppm) and a lower stretching frequency nCꢀ
N
in the IR spectrum (n˜ = 1713 cmÀ1 vs. 2235 cmÀ1). At room
temperature the substrate is slowly activated via oxidative
addition. The resulting nickel(II) complex 5 has a stretching
frequency of n˜ = 2101 cmÀ1 for the cyano ligand. After five
hours at 658C in [D8]THF the conversion is complete.
Due to the alkyl substitution, complex 2 is more electron-
À
rich than 1 and therefore undergoes C C activation signifi-
cantly faster. In contrast to the bis(diisopropylphosphino)-
Scheme 5. Synthesis of [LtBuPt(h2-cod)] (3).
ethane nickel(0) fragment with a similar bite angle,[18] the
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In summary, we have developed a short and efficient
approach to new bis-NHC chelate complexes of nickel(0) and
platinum(0). These types of compounds complete the range of
accessible complexes in the search for active d10-ML2 systems
for stoichiometric bond activations and new homogeneous
catalysts for, for example, cross-coupling reactions, hydro-
cyanation, and selective functionalizations of unreactive
molecules.
Received: January 30, 2014
Revised: March 17, 2014
Published online: && &&, &&&&
Keywords: carbene ligands · chelates · nickel · platinum
.
À
Scheme 6. C C bond activation of benzonitrile.
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4
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16.040(1) ꢈ, b = 115.206(2)8, V= 3376.8(4) ꢈ3, Z = 4, 1 =
1.250 gcmÀ3, qmax = 25.748, mean redundancy 5.58, 36656 reflec-
tions measured, 6434 unique (Rint = 0.0609), m = 0.61 mmÀ1
min = 0.85, Tmax = 0.94, 421 parameters refined, olefinic hydro-
,
T
gen atoms were refined isotropically, R1(F) = 0.047, wR(F2) =
0.111 for observed reflections, residual electron density À0.39 to
0.66 eꢈÀ3; b) CCDC 991443 contains the supplementary crys-
tallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif,
0.24 ꢉ 0.23 ꢉ 0.17 mm3,
orthorhombic, Fdd2, a = 53.344(2), b = 11.8460(5), c =
14.7010(6) ꢈ, a = 90, b = 90, g = 908, V= 9289.7(7) ꢈ3, Z = 16,
1 = 1.222 gcmÀ3, qmax = 26.768, mean redundancy 9.06, 23913
reflections measured, 4324 unique (Rint = 0.0422), m = 0.85 mmÀ1
,
Tmin = 0.82, Tmax = 0.89, 267 parameters refined, olefinic hydro-
gen atoms H12 and H13 were refined isotropically, R1(F) =
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(Rint = 0.0355), m = 6.08 mmÀ1
min = 0.54, max = 0.62, 517
c = 19.4204(7) ꢈ,
b = 100.7324(9)8,
V=
,
qmax = 25.0488, mean
,
T
T
parameters refined, R1(F) = 0.034, wR(F2) = 0.079 for observed
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Pritzkow, W. Siebert, D. Steiner, A. Berndt, Angew. Chem. Int.
Ed. Engl. 1995, 34, 1111 – 1113; Angew. Chem. 1995, 107, 1194 –
1196; b) M. E. Tauchert, T. R. Kaiser, A. P. V. Gçthlich, F.
Rominger, D. C. M. Warth, P. Hofmann, ChemCatChem 2010, 2,
674 – 682.
[18] J. J. Garcia, W. D. Jones, Organometallics 2000, 19, 5544 – 5545.
[19] M. L. Lejkowski, R. Lindner, T. Kageyama, G. ꢊ. Bꢂdizs, P. N.
Plessow, I. B. Mꢁller, A. Schꢃfer, F. Rominger, P. Hofmann, C.
Futter, S. A. Schunk, M. Limbach, Chem. Eur. J. 2012, 18,
14017 – 14025.
[16] Intensity data were collected on a Bruker APEX II diffractom-
eter at 200(2) K, Mo Ka irradiation, l = 0.71073 ꢈ, 0.58 w-scans
covered the complete reciprocal space, empirical absorption
correction with SADABS,[16d] structures refined against F2 with
a full-matrix least-squares algorithm using SHELXL,[16e] hydro-
gen atoms were treated using appropriate riding models except
noted otherwise. a) CCDC 983372 contains the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif, 0.13 ꢉ 0.12 ꢉ
0.12 mm3, monoclinic, P21/n, a = 14.881(1), b = 15.636(1), c =
[20] Note added in proof: For the first fully characterized bis-NHC
nickel(0) chelate complex, see N. D. Harrold, G. L. Hillhouse,
Chem. Sci. 2013, 4, 4011 – 4015.
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
NHC Complexes
M. Brendel, C. Braun, F. Rominger,
P. Hofmann*
&&&& — &&&&
Bis-NHC Chelate Complexes of Nickel(0)
and Platinum(0)
Adjusting the bite: d10-ML2 fragments are
well known to activate unreactive bonds
by oxidative addition. Starting from lith-
ium carbene adducts formed in situ,
nickel and platinum olefin complexes of
such fragments bearing chelating bis-
NHCs were synthesized. Combining
these strong electron donors with the
bent coordination geometry enforced by
chelation opens interesting perspectives
for bond-activation chemistry and catal-
ysis.
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!