Carborane-based pincers: Synthesis and structure of SeBSe and SBS Pd(II) complexes

Article abstract of DOI:10.1021/ja902526k

(Figure Presented) We report the synthesis of several unique, boron-rich pincer complexes derived from mcarborane. The SeBSe and SBS pincer ligands can be synthesized via two independent synthetic routes and are metalated with Pd(II). These structures rep

Full text of DOI:10.1021/ja902526k

Published on Web 06/17/2009
Carborane-Based Pincers: Synthesis and Structure of SeBSe and SBS Pd(II)
Complexes
Alexander M. Spokoyny, Matthew G. Reuter, Charlotte L. Stern, Mark A. Ratner, Tamar Seideman,
and Chad A. Mirkin*
Department of Chemistry and International Institute for Nanotechnology, Northwestern UniVersity,
2145 Sheridan Road, EVanston, Illinois 60208
Received March 30, 2009; E-mail: chadnano@northwestern.edu
Pincer complexes,1 formed from tridentate ligands (the “pincer”)
and metals or metalloids, are an exciting class of structures with
utility in catalysis,2 molecular electronics,3 and medicine.4 The
pincer ligand provides tailorability with respect to (1) the anchoring
site on the base, the central moiety from which the arms extend,5a
(2) metal-binding heteroatoms, which control the electronic nature
of the complexed atom,5b and (3) the auxiliary sites on the arms,5c
which can be modified to control the chirality of the resulting
structure and the steric environment around the complexed atom.
To date, the base has been made primarily from hydrocarbons,
including aromatic groups such as benzene and heteroatom
analogues,6a aliphatic groups,6b,c and, more recently, carbene-type
moieties.6d
consists of a region of four overlapping singlet resonances (from δ
-3 to -18) and an isolated resonance at δ -0.7. Integration of the
area under these peaks shows a 9:1 ratio, respectively. The H
1
coupled 11B NMR spectrum of complex 5 exhibits a broad multiplet
in the δ -3 to -18 region of the spectrum, but the resonance at δ
-0.7 remains a singlet, consistent with its assignment as the B(2)
atom bound to Pd(II).10a This B atom is the only one not bound to
a hydrogen atom and therefore remains a singlet in the proton-
coupled 11B NMR spectrum.
Scheme 1. Syntheses of 5 and 7a
Icosahedral carboranes represent one class of structures7 that have
not been explored as base units in pincer complexes. This is
surprising considering these structures are highly tailorable,8a exhibit
extraordinary stability,8b and provide a wide range of accessible
chemical derivatization pathways.8c Indeed, Hawthorne et al. have
pioneered synthetic methods for preparing metallacarboranes,9a,b
the metallocene analogues of carboranes. These studies have led
to many important discoveries pertaining to the fundamental
chemistry of boron-containing cage structures and to their applica-
tions in nuclear waste remediation9c and as potential pharma-
ceuticals.9d
Herein we report the first carborane-based pincer ligand family
and the complexes of these ligands metalated with Pd. The structure
formed represents a new class of compounds with a previously
unobserved Pd-B σ-coordination bond pincer motif.10 Additionally,
through density functional theory (DFT) calculations we show that
the electronic structure of this novel complex, derived in part from
this new ligand, is substantially different from its hydrocarbon
analogues. The carborane-based pincer complex 5 was synthesized
in four steps starting with the commercially available m-carborane
1 (Scheme 1a) to form the desired pincer ligand 4. Pincer complex
5 is made in 76% yield by reacting ligand 4 with Pd(CH3CN)4[BF4]2
in acetonitrile followed by 2 equiv of (nBu)4NCl. Importantly,
complex 5 is stable indefinitely in air and can be chromatographed
on silica. All spectroscopic data are consistent with the proposed
structure for 5. For example, the mass spectrum of 5 exhibits an
[M + Cl]- ion at 659 m/z in negative ionization mode and [2M -
Cl]+ at 1210 m/z in positive mode as the highest intensity peaks,
respectively. Elemental analysis (C, H, Cl) confirms the elemental
ratio of the pincer complex. The 11B NMR spectra of ligand 4 and
its Pd(II) complex 5 are consistent with the proposed complexation
of Pd (Figure 1a). Complexation results in a 10 ppm downfield
shift for the 11B resonance associated with the B-Pd bond.
Metalation of ligand 4 breaks the C2V symmetry of its parent
m-carborane cage and results in a larger number of 11B resonances
as compared with the spectrum for 4. The 11B{1H} spectrum of 5
a (i) n-BuLi, ether, paraform, reflux, 85%; (ii) Br2, PPh3, benzene, reflux,
78%; (iii) PhSeSePh, NaBH4, ethanol, reflux, 72%; (iv) Pd(CH3CN)4[BF4]2,
CH3CN, reflux, then (nBu)4NCl (2 equiv), 76%; (v) MeLi, ether, then
ClCH2SPh, 66%; (vi) same as (iv), 71%.
Temperature-dependent 77Se NMR spectroscopy is also consistent
with the proposed structure for 5. At room temperature, 77Se{1H} NMR
(see Supporting Information (SI), p S12) reveals three broad reso-
nances13 at δ 505, 535, and 543 ppm, respectively, which are assigned
to the interconverting conformational forms of complex 5 in solution.
Cooling the sample to -20 °C leads to resolution of the resonances
and four distinct peaks. Two of these (δ 545.2 and 506.1) are assigned
to the syn isomers, both of which have magnetically equivalent Se
atoms. The remaining two, at δ 504.1 and 533.6, are assigned to the
anti isomer, which contains magnetically inequivalent Se atoms due to
the different faces of the carborane cage. Importantly, if one compares
the 77Se NMR resonance for ligand 4 (δ 354.1) with the resonances
assigned to the different conformations of 5, one finds a ∼δ 180
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C O M M U N I C A T I O N S
downfield shift, consistent with Pd(II) complexation. Fluxional behavior
was also confirmed via 13C NMR (see SI, p S14). 1H NMR
spectroscopy also shows a significant broadening of the CH2-arm
moiety resonances, attributed to the syn and anti isomers of 5. A
with little π-back bonding, whereas some Pd-C analogues1 have
significant multiple bond character with both σ-electron donation and
π-back bonding. Taking into consideration that the electronegativity
of palladium is higher than that of boron,13 the possibility of Pd bearing
a formal oxidation state of zero in 5 and 7 cannot be dismissed at this
point.
1
temperature dependent H NMR study enabled us to estimate the
activation energy for this process to be ∼64 kJ/mol (see SI, p S13),
based upon coalescence temperature and literature methods.11
Finally, the solid-state molecular structure of 5 was determined by
single crystal X-ray diffraction methods. The Pd(II) center exhibits a
distorted square planar coordination geometry, with two selenoether
ligands coordinated in a trans fashion (Figure 1b) and a chloride atom
ligated to Pd(II) in a trans fashion to B(2). The Pd-Cl bond length
(2.44 Å) in 5 is slightly longer than similar motifs in aryl-based pincers
(see SI, p S21, ref S5), indicating a stronger trans influence of the
2-boryl moiety on the m-carborane cage than its phenyl analogue. The
Pd-B distance is one of the shortest (1.98 Å) reported to date (see SI,
p S21, ref S4), based on a search performed in the Cambridge
Crystallographic Data Center. To our knowledge, this structure
represents the first crystallographically characterized Pd-B σ bond in
carborane systems and the first metal-boron(2) bond in m-carborane
chemistry.
Work toward probing the stoichiometric and catalytic chemistry14
of these complexes is currently underway. In particular, we are
interested in exploring the nature of the M-B bond in carboranes,
which was recently suggested to be more thermodynamically stable
than the M-C analogue.15 In addition, this is a novel type of hemilabile
ligand which can be used for preparing a wide variety of supramo-
lecular architectures, a primary focus of our current research.16
Acknowledgment. We thank Dr. Omar Farha for helpful
suggestions. C.A.M. acknowledges NSF, ARO, and NIH Director’s
Pioneer Award for generous financial support. M.G.R. acknowl-
edges the DoE Computational Science Graduate Fellowship Pro-
gram for support (Grant No. DE-FG02-97ER25308). M.A.R. and
T.S. acknowledge NSF-CHEM for partial support. We thank
IMSERC for analytical instrumentation help.
Supporting Information Available: Experimental and characteriza-
tion for 4-7. X-ray crystallographic files for 5 and 7 in CIF format.
Details on DFT studies of 5. These materials are available free of charge
via the Internet at http://pubs.acs.org.
References
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Figure 1. (a) 11B NMR based structural evidence for pincer ligand 4 and
its palladated complex 5. (b and c) Crystallographically derived molecular
structures of 5 and 7, respectively (hydrogen atoms are omitted for clarity;
the carbons are gray and the borons light coral).
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“XBX” pincer complexes, where X is a general heteroatom, have
not yet been made, and therefore, a general route to such structures is
of a fundamental interest. As a demonstration of generality in accessing
ligands with different heteroatom arm moieties we also synthesized
and characterized the thioether, “SBS”, analogue of 4, ligand 6, and
its corresponding Pd(II) complex 7. Pincer ligand 6 was synthesized
in one step, via the dilithiation of m-carborane 1 and subsequent
alkylation with commercially available R-chlorothioanisole (Scheme
1b). This ligand was palladated following the procedure used to prepare
complex 5. Single crystal X-ray diffraction (Figure 1c), mass spec-
trometry (showing a similar fragmentation pattern), and 11B NMR
spectroscopy (see SI, p S17) confirm that 7 is the thioether analogue
of 5. Interestingly, 1H and 13C NMR spectra of 7 (see SI, pp S19-20)
suggest that only the anti-conformation of the complex exists at room
temperature and below, unlike its “SeBSe” counterpart 5. This is likely
due to the larger trans effect imposed by a selenoether moiety, which
weakens the Se-Pd bond and lowers the interconversion barrier.12
Preliminary DFT calculations (see SI, pp S5-8) provide interesting
insights with respect to this unique structural motif. Based on calculated
Mulliken and Lo¨wdin charge densities, there is a net negative charge
localized on the Pd atom and a relative positive charge concentrated
on B(2), compared to other borons in the cage. These calculations
also suggest a single bond between the Pd and B. Furthermore, the
Pd-B bonds in complexes 5 and 7 exhibit strong σ-electron donation
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