A new twist on pincer ligands and complexes

Article abstract of DOI:10.1021/om060256z

A new class of palladium pincer complexes bearing m-terphenyl scaffolds have been synthesized and structurally characterized. As a result of integrating phenyl rings into the chelating arms of m-xylyl type pincer ligands, highly twisted and rigid structures have been achieved.

Full text of DOI:10.1021/om060256z

Organometallics 2006, 25, 3301-3304
3301
A New Twist on Pincer Ligands and Complexes
Liqing Ma,† Robert A. Woloszynek,† Weizhong Chen,‡ Tong Ren,‡ and
John D. Protasiewicz*,†
Departments of Chemistry, Case Western ReserVe UniVersity, CleVeland, Ohio 44106, and UniVersity of
Miami, Coral Gables, Florida 33146
ReceiVed March 21, 2006
Scheme 1. Typical “Pincer” Binding and Atropisomers
Summary: A new class of palladium pincer complexes bearing
m-terphenyl scaffolds haVe been synthesized and structurally
characterized. As a result of integrating phenyl rings into the
chelating arms of m-xylyl type pincer ligands, highly twisted
and rigid structures haVe been achieVed.
Pincer ligands1,2 are an exciting class of ligands that are
receiving increasing attention for applications ranging from
catalysis and sensors to materials science.3 The name bestowed
upon these ligands3f reflects their tenacious and terdentate
binding nature. If one considers the most well-known pincer
ligand platform, the m-xylyl framework [2,6-(ECH2)2C6H3]- (an
ECE pincer, E ) donor atoms or groups such as NR2, PR2,
OR, etc.; Scheme 1, top), one can envision how the presence
of a formal negative charge and two exemplary five-membered
chelate rings can impart great stability to such complexes. More
recently, related pincer ligands have been constructed featuring
N-heterocyclic carbenes as C-donors (ECE and CEC types) that
vastly increase the diversity of this ligand class. Many of these
systems are also designed such that the central, “anchoring”
donor atom is often part of a planar group (such as a phenyl,
pyridine, or N-heterocyclic carbene group). It has been recog-
nized that if this ring is not in the plane containing the metal
and the two outer donor atoms, then a twisted conformation is
realized. The C2-symmetric and, hence, chiral nature of these
complexes offers the potential of resolution and use in perform-
ing catalytic enantioselective transformations. Interconversion
between these two atropisomers (Scheme 1, a and a′), however,
prevents isolation of individual enantiomers, resulting in an
“averaged” planar structure (Scheme 1, a′′). Highly twisted
structures have been produced,4 and configurational stability for
a twisted 2,6-lutidinyl-bis(carbene) complex was maintained
up to 80 °C.4f It should be mentioned that chiral pincer ligands
have been reported that include modifications to the methylene
carbons or the use of stereogenic centers at the donor atoms of
this versatile motif.5 In this report we demonstrate not only the
attainment of the highest twist angles to date but also systems
having a very high degree of nonfluxionality and versatility.
Our strategy was inspired by our past success in employing
m-terphenyls to stabilize various materials having low-coordinate
phosphorus atoms.6 Specifically, we sought to reposition
phosphorus atoms on a terphenyl framework, leading to a new
class of pincer ligands (Scheme 2). Our initial efforts, however,
were somewhat disappointing in that ligands of the form [2,6-
(2-R2PCH2C6H4)2C6H4] did not undergo cyclometalation and
pincer complex formation upon reaction with Pd(II) salts (unlike
the case for [(2,6-(R2PCH2)2C6H4] ligands) but, instead, yielded
complexes bearing trans-spanning diphosphines (Scheme 2,
left).7 Using a common workaround, installation of a more
reactive halogen atom in place of a hydrogen atom at the critical
(4) (a) Grundemann, S.; Albrecht, M.; Loch, J. A.; Faller, J. W.; Crabtree,
R. H. Organometallics 2001, 20, 5485. (b) Diez-Barra, E.; Guerra, J.; Lopez-
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Hursthouse, M. B. Dalton Trans. 2003, 1009.
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R. G.; Tilley, T. D. Organometallics 2003, 22, 47.
* To whom correspondence should be addressed. E-mail: protasiewicz@
case.edu. Fax: (+1) 216-368-3006.
† Case Western Reserve University.
‡ University of Miami.
(1) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020.
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10.1021/om060256z CCC: $33.50 © 2006 American Chemical Society
Publication on Web 06/01/2006

3302 Organometallics, Vol. 25, No. 14, 2006
Communications
Scheme 2. Relationship between Previous Trans-Spanning
(Left) and New Pincer Complexes (Right)
Scheme 3. Pincer Ligand Synthesis
Figure 1. ORTEP drawing (30% probability ellipsoids) of the
molecular structure of 3a. Hydrogen atoms are omitted for clarity.
Select bond lengths (Å) and angles (deg): Pd-P(1), 2.3071(7);
Pd-P(1A), 2.3071(7); Pd-C(1), 2.067(4); Pd-Br, 2.5024(5);
P(1)-Pd-P(1A), 169.48(4); C(1)-Pd-Br, 180.0.
analogues [2,6-(2-R2PCH2C6H4)2C6H4] had to be modified.
Specifically, the key intermediate [2,6-(2-BrCH2C6H4)2C6H3-
Br] (v) could not be cleanly prepared by direct bis-monobro-
mination of the tolyl groups of i, due to the formation of
mixtures of mono-, bis-, tri-, and tetrahalogenated products that
were difficult to separate.8 Instead, synthesis of the readily
purified ii allowed facile synthesis of iii and iv, which could
then be used to access v from i in 56% overall yield. From
precursor v the three new diphosphines 1a-c were isolated in
good yields. Each material is readily characterized by standard
1H and 31P NMR spectroscopy.9 Notably, the diphosphine 1a
shows two 31P NMR signals in its spectrum, consistent with
the presence of two isomers (syn and anti) in about a 50:50
ratio. 31P NMR spectra of diphosphines 1b,c indicate a much
smaller amount (ca. 10% or less) of a second isomer. The
increased steric bulk of the benzyl groups in 1b,c presumably
disfavors the syn isomers, as the precursors i-v all show syn
1
and anti isomers in about a 50:50 ratio (by H NMR spectros-
copy). Nevertheless, diphosphines 1a-c react with Pd2(dba)3
to quantitatively yield pincer complexes 3a-c (Scheme 2, right),
as indicated by new signals ca. 25-35 ppm downfield from
signals for the corresponding free ligands in the 31P NMR
spectra. Likewise, reaction of 1a with Ni(COD)2 yields the
analogous nickel pincer complex 4a. While reaction of 1a with
Pd2(dba)3 is rapid and complete in 1 h at room temperature to
yield 3a, reaction of the more hindered 1c with Pd2(dba)3
requires 12 h or more for completion. These new pincer
complexes are robust, display great air stability, and can be
purified by flash column chromatography on silica gel under
ambient conditions. Full characterization of 3a-c and 4a was
possible by a combination of both 1-D and 2-D (HCOSY,
HMQC and HMBC) NMR spectroscopy.10
C-1 position of the central ring afforded ligand precursors that
readily reacted with low-valent complexes to provide the new
C2-symmetric pincer complexes 3 and 4 (Scheme 2, right).
The synthesis of the ligand precursors [2,6-(2-R2PCH2C6H4)2-
C6H3Br] (1a-c) is outlined in Scheme 3. It was discovered that
the shorter synthesis used to prepare the nonhalogenated
(7) (a) Smith, R. C.; Protasiewicz, J. D. Organometallics 2004, 23, 4215.
(b) Smith, R. C.; Bodner, C. R.; Earl, M. J.; Sears, N. C.; Hill, N. E.; Bishop,
L. M.; Sizemore, N.; Hehemann, D. T.; Bohn, J. J.; Protasiewicz, J. D. J.
Organomet. Chem. 2005, 690, 477.
(8) Vinod, T.; Hart, H. J. Org. Chem. 1990, 55, 881.
(9) Selected data for 1a-c are as follows. 1a: 31P NMR (CDCl3, 162
MHz) δ -10.3, -10.2 (∼1:1). 1b: 31P NMR (CDCl3, 162 MHz) δ 1.3. 1c:
31P NMR (CDCl3, 162 MHz) δ 30.4, 32.4 (∼10:1).

Communications
Organometallics, Vol. 25, No. 14, 2006 3303
1
Figure 2. Temperature-dependent H NMR spectra (toluene-d8, 600 MHz) and a calculated (SPARTAN PM3) space-filling molecular
model of 3c (as viewed down the Br-Pd bond).
Complex 3a was further characterized by single-crystal X-ray
diffraction analysis (Figure 1).11a The crystal structure of 3a
reveals a four-coordinate planar palladium(II) center, as well
as a crystallographically imposed C2 axis passing through the
bromine, palladium, and C(1) atoms. Although individual
molecules of 3a are chiral, the crystal is comprised of both
enantiomers, which are related by a crystallographic inversion
center. The bond lengths of Pd-P and Pd-C are in reasonable
agreement with published values for PCP pincer complexes.12,13
In comparison to the known closely related PCP pincer complex
[(C6H3(CH2PPh2)2)PdBr)] (5a),13 having P-Pd-C bond angles
of 80.6 and 81.8°, complex 3a has C(1)-Pd-P bond angles of
84.74°. These values are actually closer to those of the nonpincer
analogue trans-[(PPh3)2Pd(C6H5)Br]14 (86.3 and 88.8°), which
does not possess any ring constraints. The most striking
difference between the solid-state structures of 3a and 5a lies
in the distortion of P atoms away from the plane containing the
anchor atom C(1), its ring, and the Pd and Br atoms. One can
assign the “twist” angle Φ (see Scheme 1), defined as the angle
between the plane of the anchoring ring and square plane
containing the metal and its four directly attached atoms, to
assess this effect for comparative purposes. A Φ value of 76.0°
for 3a greatly surpasses the Φ ) 18.4° found for 5a.13 The
twist angle for 3a is also 34° greater than that found for CEC
pincer complexes (C ) N-heterocyclic carbene) having the
largest Φ values reported to date (up to 41.8°).4
The structural rigidity of this pincer platform is maintained
up to 130 °C, as shown by variable-temperature 1H NMR studies
of 3a in CDCl2CDCl2 (see the Supporting Information for
details). Analysis of spectra for the tert-butyl derivative 3c
(Figure 2) revealed hindered rotation about the C(tert-butyl)-P
bonds, as well as an equally rigid pincer backbone. At low
temperature, the two types of tert-butyl groups are inequivalent,
and remain so, up to 105 °C in toluene-d8. One of the signals,
however, is resolved into three independent methyl resonances
at low temperature. This observation suggests that the tert-butyl
(10) Selected data for 3a-c and 4a are as follows. 3a: 1H NMR (CDCl3,
600 MHz) δ 2.91 (doublet of virtual triplets, JHH ) 13 Hz, 2H), 3.21 (doublet
of virtual triplets, JHH ) 13 Hz; 2H), 6.38 (m, 2H), 6.65 (d, JHH ) 7 Hz,
2H), 6.73 (d, JHH ) 7 Hz, 2H), 7.02 (t, JHH ) 7 Hz, 1H), 7.10-7.15 (m,
8H), 7.20 (t, JHH ) 7 Hz, 4H), 7.28 (t, JHH ) 7 Hz, 2H), 7.45 (t, JHH ) 7
Hz, 4H), 7.57 (t, JHH ) 7 Hz, 2H), 7.58-7.61 (m, 4H); 31P NMR (CDCl3,
162 MHz) δ 26.3 (s). 3b: 1H NMR (CDCl3, 400 MHz) δ 1.08-1.82 (m,
38 H), 1.97 (d, JHH ) 12 Hz, 2H), 2.13 (s, 2H), 2.79 (m, 2H), 2.53 (doublet
of virtual triplets, JHH ) 13 Hz, 2H), 2.84 (doublet of virtual triplets, JHH
) 13 Hz, 2H), 6.75 (d, JHH ) 8 Hz, 2H), 7.04 (t, JHH ) 8 Hz, 1H), 7.22-
7.25 (m, 4H), 7.27-7.31 (m, 2H), 7.34 (d, JHH ) 7 Hz, 2H); 31P NMR
(CDCl3, 162 MHz) δ 33.7 (s). 3c: 1H NMR (CDCl3, 400 MHz) δ 1.29
(virtual triplet, 18H), 1.32 (broad, 6H), 1.59 (broad, 6H), 1.84 (broad, 6H),
2.69 (doublet of virtual triplets, JHH ) 13 Hz, 2H), 2.93 (doublet of virtual
triplets, JHH ) 13 Hz, 2H), 6.58 (d, JHH ) 8 Hz), 6.94 (t, JHH ) 8 Hz),
7.22 (m, 2H), 7.32 (m, 4H), 7.37 (m, 2H); 31P NMR (CDCl3, 162 MHz) δ
55.1 (s). 4a: 1H NMR (CDCl3, 400 MHz) δ 2.71 (doublet of virtual triplets,
JHH ) 12 Hz, 2H), 3.04 (doublet of virtual triplets, JHH ) 12 Hz, 2H), 6.26
(m, 2H), 6.58 (d, JHH ) 7 Hz, 2H), 6.68 (d, JHH ) 7 Hz, 2H), 6.94 (t, JHH
) 7 Hz, 1H), 7.07-7.18 (m, 12H), 7.25 (t, JHH ) 7 Hz, 2H), 7.46 (t, JHH
) 7 Hz, 4H), 7.57-7.65 (m, 6H); 31P NMR (CDCl3, 162 MHz) δ 27.0 (s).
(11) (a) Crystallographic data for 3a: yellow blocks, crystal dimensions
0.45 × 0.30 × 0.29 mm3, space group C2/c, a ) 16.1720(7) Å, b )
14.2258(7) Å, c ) 16.1442(8) Å, â ) 104.3920(10)°, V ) 3597.6(3) Å3,
Z ) 4, Fcalcd ) 1.499 g cm-3, µ(Mo KR) ) 1.746 mm-1, data measured on
a Bruker SMART 1000 CCD-based diffractometer (Mo KR radiation, λ )
0.710 73 Å) at 300(1) K, structure solved by direct methods, 9361 reflections
collected, 3160 unique reflections (Rint ) 0.0266), data/restraints/parameters
316/0/219, final R indices (I > 2σ(I)) R1 ) 0.030 and wR2 ) 0.083, R
indices (all data) R1 ) 0.038 and wR2 ) 0.086, largest difference peak
and hole +0.384 and -1.277 e Å-3. (b) Crystallographic data for 7a: pale
brown block, crystal dimensions 0.12 × 0.20 × 0.26 mm3, space group
Pbcn, a ) 14.9444(2) Å, b ) 11.6289(2) Å, c ) 19.7745(3) Å, V )
3436.55(9) Å3, Z ) 4, Fcalcd ) 1.577 g cm-3, µ(Mo KR) ) 1.833 mm-1
,
data measured on a Bruker Kappa APEX II CCD-based diffractometer (Mo
(12) (a) Gorla, F.; Venanzi, L. M.; Albinati, A. Organometallics 1994,
13, 43. (b) Kraatz, H. B.; Milstein, D. J. Organomet. Chem. 1995, 488,
223. (c) Cotter, W. D.; Barbour, L.; McNamara, K. L.; Hechter, R.;
Lachicotte, R. J. J. Am. Chem. Soc. 1998, 120, 11016. (d) Longmire, J. M.;
Zhang, X. M.; Shang, M. Y. Organometallics 1998, 17, 4374.
(13) Bachechi, F. Struct. Chem. 2003, 14, 263.
KR radiation, λ ) 0.710 73 Å) at 100(2) K, structure solved by direct
methods, 11 657 reflections collected, 6006 unique reflections (Rint
)
0.0306), data/restraints/parameters 6006/0/235, final R indices (I > 2σ(I))
R1 ) 0.036 and wR2 ) 0.077, R indices (all data) R1 ) 0.068 and wR2
) 0.089, largest difference peak and hole +0.679 and -0.727 e Å-3. These
data can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(14) Flemming, J. P.; Pilon, M. C.; Borbulevitch, O. Y.; Antipin, M.
Y.; Grushin, V. V. Inorg. Chim. Acta 1998, 280, 87.

3304 Organometallics, Vol. 25, No. 14, 2006
Communications
Scheme 4. Synthesis of Diphosphinite Complex 7a
developed for use in stabilizing lanthanide complexes. While
twist angles of up to 41° have been characterized, the rigidity
of these systems is unknown.17
In summary, we have prepared new pincer ligands constructed
using m-terphenyl scaffolds and showed their efficacy in
yielding novel palladium and nickel pincer complexes. A
structural analysis of 3a shows that despite a chelate ring size
larger than that of earlier conventional pincer complexes
(compare 7 to 5), the new pincer complexes better match the
metrical parameters found for related unconstrained nonchelate
structures. In addition, these systems have the greatest twist
angles determined to date for pincer complexes and also display
a very high degree of nonfluxionality (up to 130 °C). These
air- and heat-stable materials thus hold much potential for
resolution and use in catalysis. Efforts are now ongoing to
resolving the enantiomers of 3a-c and to examine their catalytic
behavior relative to that of conventional pincer complexes.
groups most proximal to the bromine atom face steric clashes
with the bromine atom and cause rotation about the C-CMe3
bond, thus resulting in three inequivalent methyl groups
(portrayed as A, A′, and A′′ in Figure 2, right). If the
atropisomerization process depicted in Scheme 1 were occurring,
the two types of tert-butyl groups would undergo exchange and
lead to NMR broadening. In contrast, the pincer PCP complexes
[(C6H3(CH2PtBu2)2)PdCl)]1 and [(C6H3(CH2PtBu2)2)Pd(THF-
d8)]BF415 only show a single 1H NMR resonance for their freely
rotating and equivalent tert-butyl groups.
This new type of pincer architecture is expected to be very
general and adaptable for a broad variety of donor atoms and
for many transition metals. For example, starting with 2,6-(2-
HOC6H4)2C6H3Br, the new diphosphinite ligand 6a can be
prepared. This material reacts with Pd2(dba)3 to yield the
analogous pincer complex 7a (Scheme 4). The overall structure
Acknowledgment. This work was supported by the National
Science Foundation (Grant No. CHE 0202040). We thank Dr.
Charles Campana (Bruker AXS, Inc.) for assistance with the
crystallographic determination of compound 7a.
Supporting Information Available: Text and figures giving
experimental procedures, NMR spectra, and analysis results and
CIF files giving X-ray crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
is quite similar to that determined for 3a (Φ ) 73.8°, dPd-P
)
2.2824(5) Å, dPd-C1 ) 2.063(3) Å).11b The other intermediates
described in Scheme 3 lend themselves naturally for easy access
to other types of high-twist-angle pincer ligands and com-
plexes.16 These pincer ligands can be contrasted to the related
binding mode of the 2,6-(2-MeOC6H4)2C6H3 terphenyl (Danip)
OM060256Z
(16) A related diimine pincer complex (2,6-(2-(PhNdC(H))2C6H4)2C6H3-
PdI) has been structurally characterized with Φ ) 63.6°: Ma, L.;
Protasiewicz, J. D. Manuscript in preparation.
(17) (a) Rabe, G. W.; Zhang-Presse, M.; Riederer, F. A.; Yap, G. P. A.
Inorg. Chem. 2003, 42, 3527. (b) Rabe, G. W.; Berube, C. D.; Yap, G. P.
A. Inorg. Chem. 2001, 40, 4780.
(15) Kimmich, B. F. M.; Marshall, W. J.; Fagan, P. J.; Hauptman, E.;
Bullock, R. M. Inorg. Chim. Acta 2002, 330, 52.