Mono- and dimetalation of a tridentate bisimidazole-phosphine ligand

Article abstract of DOI:10.1021/om500592u

A bisimidazole-phosphine ligand, PhP[(CH₂)₂Im^ArMe2]₂, has been prepared and has been metalated using [CpRu(μ³-Cl)]₄, generating a bis(N-heterocyclic carbene)-phosphine Ru(II) species throug

Full text of DOI:10.1021/om500592u

Communication
pubs.acs.org/Organometallics
Mono- and Dimetalation of a Tridentate Bisimidazole-Phosphine
Ligand
Sarah E. Flowers and Brandi M. Cossairt*
Department of Chemistry, University of Washington, Box 351700, Bagley Hall, Seattle, Washington 98195-1700, United States
S
* Supporting Information
ABSTRACT: A bisimidazole-phosphine ligand, PhP-
[(CH2)2ImArMe2]2, has been prepared and has been metalated
using [Cp*Ru(μ3-Cl)]4, generating a bis(N-heterocyclic
carbene)-phosphine Ru(II) species through ligand coordina-
tion followed by tautomerization. This is the first example of
an NHC−donor−NHC complex with two unsubstituted N−
H wingtips. Furthermore, a second metal center was installed
by deprotonation of both N−H wingtips and subsequent salt
metathesis to generate Ru(II)−Fe(II) and Ru(II)−Co(II)
bimetallic complexes.
supports this type of binding motif, as well as the mono- and
dimetalation of this ligand.
-heterocyclic carbenes (NHCs) and their complexes with
Ntransition metals enable a variety of important trans-
formations, such as the activation of white phosphorus,1 olefin
metathesis,2 and substrate oxidation.3 Often compared with
electron-rich organophosphines,4,5 NHCs are considered
strong σ donors with weak back-bonding character.5 The
NHC framework can be functionalized at both the C−C
backbone and at the nitrogen positions (wingtips), allowing the
steric and electronic properties of these ligands to be easily
tuned. Although transition-metal NHC complexes containing
one metal center have been thoroughly studied, there are
limited reports of NHCs acting as bridging ligands in
heterometallic complexes, with a geometry where one metal
is bound at the C1 position and the second metal is bound at
one of the wingtip nitrogen atoms.6−12 With this in mind, we
envision using a tridentate ligand bearing two NHCs to create a
macrocyclic framework supporting two different metal centers,
as shown in Figure 1. Our approach builds upon the flourishing
field of “protic” NHC complexes.11,13−17 With an appropriate
choice of ancillary ligands, such structures could be poised to
carry out a variety of unique transformations through
cooperation between the two metal centers. This report details
our preparation of a new bisimidazole-phosphine ligand that
The bisimidazole-phosphine ligand PhP[(CH2)2ImArMe2
]
2
was prepared using a three-step synthesis from commercially
available 5,6-dimethylbenzimidazole using methods derived
from the literature.11,18 In the final step, vinyl-substituted
benzimidazole (2 equiv) was added to a −35 °C solution of
phenylphosphine (1 equiv) and KOtBu (0.8 equiv) in THF
(Scheme 1).19 Monitoring the reaction by 31P{1H} NMR
spectroscopy showed conversion to PhP[(CH2)2ImArMe2]2, with
a characteristic singlet at −35 ppm, a downfield shift from the
starting phenylphosphine singlet at −120 ppm.
Addition of [Cp*Ru(μ3-Cl)]4 (0.25 equiv) to a −35 °C
solution of PhP[(CH2)2ImArMe2]2 in CH2Cl2 rapidly produces
the orange homobimetallic complex 1 (Scheme 1), as seen by
1
in situ monitoring by 31P{1H} NMR and H NMR spectros-
copy. The appearance of a 31P{1H} NMR resonance at 22 ppm,
along with the concomitant disappearance of the 31P{1H} NMR
signal for the starting ligand at −35 ppm, suggests complete
1
conversion to 1. The H NMR spectrum of 1 displays four
methyl singlets as well as six singlets in the aromatic region that
correspond to two electronically different imidazole groups
within the complex (Figure S5, Supporting Information). The
X-ray structure (Figure 2) reveals that 1 is a dimeric species
related in symmetry by a crystallographically imposed C2
proper rotation axis. Complex 1 has an inner-sphere chloride
with a Ru−Cl interatomic distance of 2.464(2) Å. A related
example of a Cp*Ru dimer species with bridging phosphine-N-
bound ligands has been reported in the literature.20
Figure 1. Motivating structural motif for a bimetallic tridentate
biscarbene-phosphine complex.
Received: June 3, 2014
Published: August 22, 2014
© 2014 American Chemical Society
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halide dissociation is required in this case. The 31P{1H} NMR
spectrum of 2 displays a characteristic singlet at 38 ppm in
THF-d8. The presence of a broad N−H singlet in the 1H NMR
spectrum of 2 at 14.5 ppm along with a doublet centered at
Scheme 1. Synthesis and Metalation of a Tridentate
Bisimidazole-Phosphine Ligand
2
198.5 ppm with a 17.5 Hz JCP value in the 13C{1H} NMR
spectrum are consistent with a biscarbene ruthenium complex
where the NHC moieties are magnetically equivalent.
High-quality, yellow crystals of 2 were obtained via vapor
diffusion of Et2O into a saturated THF solution of 2 at 22 °C.
The X-ray crystal structure shows that 2 contains a pseudo
mirror plane through P1 and Ru1, as well as an outer-sphere
chloride with hydrogen-bonding interactions to the N−H
imidazole wingtips (Figure 3). This structure is the first
Figure 3. Single-crystal X-ray diffraction structure of 2 with thermal
ellipsoids shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.
reported tridentate ligand with an NHC−phosphine−NHC
backbone (CPC ligand) bound to a metal center, though
related PCPC tetradentate macrocycles and bidentate CP
complexes have been reported.21−24 Importantly, complex 2
contains an unprecedented NHC−donor−NHC motif with
two N−H wingtips poised for further functionalization.
The N−H wingtips of 2 are cleanly deprotonated with 2
equiv of n-BuLi in THF at −78 °C to form 3 (Scheme 2).
Complex 3 was isolated as a goldenrod yellow powder in 63%
yield. A downfield shift in the 31P{1H} NMR spectrum, with
respect to 2, in THF-d8 from 38 to 46 ppm indicated the
formation of a new species. A doublet at 195.8 ppm (2JCP = 20
Hz) in the 13C{1H} NMR spectrum and the absence of any N−
1
H peaks in the H NMR spectrum are consistent with the
ruthenium−carbene bond remaining intact and complete
Figure 2. Single-crystal X-ray diffraction structure of 1 with thermal
ellipsoids shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.
deprotonation of the N−H wingtips.
X-ray-quality crystals of 3 were grown via slow evaporation of
THF at 22 °C. The X-ray crystal structure of 3 reveals Li ion
coordination between the deprotonated imidazole wingtips
with a N1−Li1 distance of 1.925(17) Å and a N3−Li1 distance
of 1.916(18) Å (Figure 4), along with the coordination of one
THF molecule to the lithium ion. The structure and synthesis
of 3 are related to recently reported transition-metal and rare-
earth-metal complexes linked to lithium through a bridging
imidazole or through a “protic” carbene ligand.11,12
Complex 3 is a useful precursor for the synthesis of
heterobimetallic complexes 4-Fe and 4-Co (Scheme 2). The
addition of a metal dichloride salt, either FeCl2 or CoCl2, to an
equimolar amount of 3 in acetonitrile at room temperature
When 1 is heated at 100 °C for 30 h in DMSO,
tautomerization of the imidazole units and complete conversion
to the monometallic ruthenium biscarbene complex 2 is
observed in situ (Scheme 1) by 1H and 31P{1H} NMR
spectroscopy. Complex 2 was isolated in 71% yield as a pale
yellow powder. It should be noted that tautomerization of the
proton at C1 also occurs in CH2Cl2 at room temperature over
1
the course of 1 week, as observed by H and 31P{1H} NMR
studies. This observation suggests that 2 is more thermody-
namically stable than 1 and that base assistance is not strictly
required for tautomerization.10 However, we postulate that
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Communication
Scheme 2. Insertion of a Second Metal Center into the
Biscarbene Ligand Framework
Figure 5. Single-crystal X-ray diffraction structure of 4-Fe with thermal
ellipsoids shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.
direction as the Cp* ring, and both Fe and Co centers are
bound to one molecule of acetonitrile. The M−N1 and M−N3
bond lengths of 1.9601(16) and 1.9697(16) Å (M = Co) and
2.0017(17) and 2.0006(17) Å (M = Fe) are consistent with a
symmetric electronic structure with both nitrogen atoms
assigned as anionic.25 There is no significant variation in the
Ru−C1 or Ru−C12 interatomic distances from 2 (2.029(8)
and 2.029(10) Å) to 3 (2.073(9) and 2.024(9) Å) to 4-Fe
(2.0355(19) and 2.0407(19) Å) and 4-Co (2.029(19) and
2.0348(19) Å). Both 4-Fe and 4-Co have a Ru−M interatomic
distance of roughly 3.6 Å.
In this paper, we have reported the synthesis of
heterobimetallic complexes 4-Fe and 4-Co supported by a
new CPC tridentate ligand. These complexes have been
prepared by initial coordination of the bisimidazole-phosphine
ligand to [Cp*Ru(μ3-Cl)]4 to form 1, followed by tautomeriza-
tion in DMSO to form 2. Deprotonation of the N−H wingtips
of 2 leads to the formation of 3, which can then be converted to
4-Fe and 4-Co by salt metathesis with the respective metal
chlorides. We hypothesize that this strategy for the synthesis of
bimetallic complexes supported by this type of bisimidazole-
phosphine ligand could be generalized to many other metal
centers. We are currently targeting other bimetallic complexes
Figure 4. Single-crystal X-ray diffraction structure of 3 with thermal
ellipsoids shown at the 50% probability level. Hydrogen atoms are
omitted for clarity.
supported by PhP[(CH2)2ImArMe2
]
that will allow for
2
cooperative reactivity between the two adjacent metal centers
to study the influence of metal identity on the electronic
structure and reactivity of these complexes.
immediately evokes a color change, from light yellow to
emerald green in the case of CoCl2 and from light yellow to a
ASSOCIATED CONTENT
* Supporting Information
1
■
darker yellow in the case of FeCl2. The H NMR spectra of
S
both 4-Fe and 4-Co display broad, contact-shifted resonances
that are consistent with paramagnetic complexes (Figures S8
and S9, Supporting Information). This observation, combined
with an understanding of the electron localization in 3 and
Evans method analysis of 4-Fe and 4-Co, is consistent with a
preliminary assignment of Ru(II)−Fe(II) and Ru(II)−Co(II),
respectively. High-quality crystals suitable for X-ray diffraction
precipitated out of the NMR solutions after 24 h in sealed J.
Young tubes. Complexes 4-Fe (Figure 5) and 4-Co (Figure S2,
Supporting Information) are isostructural. Both complexes have
M−Cl bonds with the chloride ligand pointing in the same
Text, tables, figures, and CIF files giving experimental details,
crystallographic data for 1−3, 4-Fe, and 4-Co, and additional
characterization data. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for B.M.C.: cossairt@chem.washington.edu.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This research was supported by startup funds from the
University of Washington and the ACS Petroleum Research
Fund Doctoral New Investigator grant PRF# 54226-DNI3. We
gratefully acknowledge the University of Washington for
funding this research. We also acknowledge Werner Kaminsky
for invaluable help in collecting and solving the single-crystal X-
ray diffraction data presented in this paper.
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