Pd pincer complex as a probe to index the coordination ability of various ligands

Article abstract of DOI:10.1002/ejic.200700037

An NCN pincer palladium complex was developed as a probe molecule to index the coordination ability of various monodentate ligands. Coordination constants of 27 monodentate ligands (L) with a pincer palladium complex to form complexes of the type ArPd(L)₂Cl were determined by ¹H NMR spectroscopy. This allowed the coordination ability of a wide variety of ligands, including P-coordinating ligands (phosphane, phosphite), as well as As-coordinating AsPh₃, and N-coordinating pyridine, to be indexed. The differential between the highest/lowest coordination constants was 10 ¹². The relative coordination ability is described in a logarithmic manner, log(K_eq(L)/K_eq(PPh3)), with respect to the value of PPh₃ to exhibit high linearity in the Hammett plot. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700037

SHORT COMMUNICATION
DOI: 10.1002/ejic.200700037
Pd Pincer Complex as a Probe To Index the Coordination Ability of Various
Ligands
Maki Minakawa,[a] Kazuhiro Takenaka,[a] and Yasuhiro Uozumi*[a]
Keywords: Pincer complexes / Coordination / Palladium / Phosphane ligands / Mondentate ligands
An NCN pincer palladium complex was developed as a
probe molecule to index the coordination ability of various
monodentate ligands. Coordination constants of 27 mono-
dentate ligands (L) with a pincer palladium complex to form
complexes of the type ArPd(L)2Cl were determined by 1H
NMR spectroscopy. This allowed the coordination ability of
a wide variety of ligands, including P-coordinating ligands
(phosphane, phosphite), as well as As-coordinating AsPh3,
and N-coordinating pyridine, to be indexed. The differential
between the highest/lowest coordination constants was 1012
The relative coordination ability is described in a logarithmic
manner, log(Keq(L)/Keq ), with respect to the value of PPh3
.
(PPh3)
to exhibit high linearity in the Hammett plot.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
a scale of the coordination ability of various ligands would
be achieved. This would provide fundamental and useful
information for catalytic as well as organic, inorganic, orga-
nometallic, and materials research fields. We would like to
propose pincer palladium complex 1 as a novel probe to
titrate the coordination ability of various nonchelating
igands to palladium to form palladium complexes
ArPd(L)2X. Many of these complexes are recognized as key
intermediates in various palladium-catalyzed processes (e.g.
cross-couplings, the Heck reaction, carbonylation, etc.).
Introduction
Transition-metal–ligand complexes have emerged as ver-
satile and functional molecules in the domain of chemistry.
Vast arrays of functional metal complexes have been devel-
oped in which ligands undergo drastic change and/or fine-
tuning of their chemical and physical properties. Transition-
metal-catalyzed organic transformations are the most com-
monly employed reactions where the choice of ligands is
essential in controlling the catalytic reaction pathway. Coor-
dination ability is one of the most fundamental and impor-
tant properties of ligands and, therefore, quite a few studies
have appeared that index the ligand coordination ability.
However, to the best of our knowledge, no direct or precise
method has appeared that allows the determination of the
relative strength/weakness of the coordination ability of
PPh3 to palladium, for example, versus AsPh3, pyridine,
PCy3, etc. Clearly, although pioneering strides have been
made, additional studies on the ordering of the coordina-
tion abilities of ligands are warranted.
Because the coordination ability is controlled by both the
electronic and steric properties of the ligand, this renders
quantitative description difficult. Indeed, although the two
controlling factors (electronic and steric) have been de-
fined,[1,2] all of the well-developed research on the determi-
nation of the coordination ability of various ligands has
been limited to isolated reports.[3] If a practical probe for
the direct and exact observation of the coordination–disso-
ciation equilibria of a wide range of ligands were developed,
Results and Discussion
We have previously prepared various NCN pincer palla-
dium complexes[4] by the ligand introduction route
[Scheme 1, Equation (1)].[5] From our detailed studies on
the pincer complexation process, we found that intermedi-
ate complex 2 was in equilibrium with pincer complex 1.
The coordination–dissociation behavior of the PPh3 ligand
of intermediate complex 2 was observed by 1H and 31P{1H}
NMR spectroscopy, and the coordination constant
1
Keq
was unambiguously determined by the H NMR
(PPh3)
spectroscopic measurement of the intramolecular
CH=NCH2Ph ligand groups [Ha and/or Hb indicated in
Scheme 1, Equation (2)]. These observations led us to the
conclusion that complex 1 would be a good probe to index
the coordination abilities of various ligands. By using probe
complex 1, a wide range of coordination constants (the dif-
ference between the highest and lowest constants is 1012)
[a] Institute for Molecular Science (IMS)
Myodaiji, Okazaki, Aichi 444-8787, Japan
Fax: +81-564-59-5574
1
were determined with high accuracy by H NMR spectro-
E-mail: uo@ims.ac.jp
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
scopic measurements, which allowed the relative coordina-
tion ability of 27 different ligands to be indexed.
Eur. J. Inorg. Chem. 2007, 1629–1631
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1629

M. Minakawa, K. Takenaka, Y. Uozumi
Table 1. Coordination constants of various ligands.[a]
SHORT COMMUNICATION
Entry
Ligand
Keq(L) [–2][b] RCA[c]
1
2
3
4
5
6
7
8
P(nBu)3
N/A
Ͼ3
2.06
1.89
1.64
1.35
1.03
0.703
0.597
0.00
Ph2P(C6H4-4-NMe2)
P(C6H4-4-OMe)3
PCy3
PhP(C6H4-4-OMe)2
P(C6H4-4-Me)3
Ph2P(C6H4-4-Me)
P(C6H4-3-Me)3
PPh3
P(CH2CH2CN)3
PBn3
P(C6H3-3,5-Me2)3
P(C6H4-4-F)3
Ph2PCy
2.38ϫ107
1.61ϫ107
9.45ϫ106
4.58ϫ106
2.20ϫ106
1.04ϫ106
8.15ϫ105
2.06ϫ105
1.52ϫ105
1.27ϫ105
1.16ϫ105
1.10ϫ105
9.35ϫ104
3.59ϫ104
1.63ϫ104
1.39ϫ104
2.07ϫ103
1.30ϫ103
1.73ϫ102
1.28ϫ102
5.93
9
10
11
12
13
14
15
16
17
18
19
20
21
22[d]
23[d]
24[d]
25[d]
26[d]
27[d]
28[d]
29
30
31
32
33
34
35
36
–0.132
–0.210
–0.249
–0.272
–0.343
–0.759
–1.10
–1.17
–2.00
–2.20
–3.08
–3.21
–4.54
–5.89
–6.23
–6.37
–7.00
–9.29
–9.92
Ͻ–10
Ͻ–10
Ͻ–10
Ͻ–10
Ͻ–10
Ͻ–10
Ͻ–10
Ͻ–10
P(C6H4-3-OMe)3
PhPCy2
Scheme 1. Preparation and substitution equilibrium of the pincer
complex: (1) ligand introduction route to pincer complexation; (2)
ligand substitution equilibrium.
P(C6H4-4-Cl)3
P(C6H4-3-Cl)3
P(C6H4-3-F)3
Ph2P(C6H4-2-OMe)
P(C6H4-4-CF3)3
Ph2P(C6H4-2-Me)
Ph2P(C6F5)
P[C6H3-3,5-(CF3)2]3
P(2-furyl)3
P(OPh)3
Pyridine
AsPh3
P(C6H4-2-Me)3
P(tBu)3
CyP(tBu)2
A typical procedure for the equilibria study by NMR
spectroscopy for PPh3 is as follows: Complex 1 was dis-
solved in CD2Cl2 (12.7 m) at 298 K and PPh3 (2
mol equiv. to 1) was added under a nitrogen atmosphere.
The resulting mixture was analyzed by 1H NMR spec-
troscopy, where the resonances of Ha for complexes 2
(phosphane complex) and 1 (pincer complex) were observed
0.267
0.120
8.80ϫ10–2
2.09ϫ10–2
1.06ϫ10–4
2.49ϫ10–5
N/A
at δ = 4.50 and 5.01 ppm, respectively, to provide Keq
(PPh3)
N/A
N/A
N/A
N/A
N/A
N/A
N/A
= 2.06ϫ105 –2 in CD2Cl2 (298 K). The coordination con-
stants of 27 ligands obtained by similar NMR spectroscopic
experiments are shown in Table 1. Ligands, whose coordi-
nation constants could not be determined by the present
method because their coordination ability was either too
strong or too weak, are also listed in Table 1 to demonstrate
the scope and limitation of this system. The coordination
constants of Ph2P(C6H4-2-Me), Ph2P(C6F5), P[C6H3-3,5-
(CF3)2]3, P(2-furyl)3, P(OPh)3, pyridine, and AsPh3 were so
small that the VT-NMR spectroscopic studies were per-
formed at lower temperatures to observe both peaks of 1
and 2 (Table 1, Entries 22–28), and their Keq(L) values at
Cy2P(tBu)
PhP(C6H4-2-OMe)2
P(C6F5)3
PhP(C6F5)2
Cy2P(2-biphenyl)
1
[a] All titrations were performed by H NMR spectroscopic mea-
surements at 298 K. [b] Average of two or three measurements. [c]
RCA = log(Keq(L)/Keq
). [d] Keq value was estimated by the
van’t Hoff plot obtained by VT-NMR spectroscopic experiments
(PPh3)
(see Supporting Information).
Figure 1 is the Hammett plot for the RCA values of vari-
298 K were estimated by a van’t Hoff plot (see Supporting ous triarylphosphanes to quantify the electronic effect of
Information). The relative coordination ability (RCA) is de- the P substituents. Triarylphosphanes with para and meta
scribed in a logarithmic manner, log(Keq(L)/Keq(PPh )), with monosubstituted aromatic groups having cone angles of
3
respect to the value of PPh3.
145 ° show excellent proportionality for the σ values[7] of
Several representative and/or typical features obtained by the aromatic groups. Thus, the RCA value decreased pro-
this indexation include: (1) The coordination ability of li- portionally as the σ value of the aromatics increased and
gands having the same cone angle was strictly determined the RCA–σ relationship exhibits high linearity (Figure 1,
by their electronic character (vide infra). (2) The coordina- black solid line: R2 = 0.9925), which demonstrates the high
tion constant of AsPh3 is 1010 smaller than that of PPh3. accuracy of the RCA values measured by the present
(3) P(OPh)3 exhibits a 107 smaller value than that of its method. This observation should make the prediction of
carbon analogue P(CH2Ph)3. (4) The order of the coordina- the RCA value of various triarylphosphane ligands having
tion ability of phosphane ligands bearing six-membered para or meta monosubstitution possible by inter-/extrapola-
ring substituents is PCy3 Ͼ PPh3 Ͼ PPh2Cy Ͼ PPhCy2.[6] tion (dotted line), even if the RCA value is out of the mea-
(5) The coordination ability is significantly affected by the surement range with highly electron-rich or -deficient sub-
steric bulkiness of the ligand [e.g. PPh3 versus PPh2Tol-o stituents. The RCA values of triarylphosphane ligands of
(Table 1, Entries 9 and 22)]. These features would be diffi- the 147 ° cone angle were slightly smaller than those of the
cult to predict without direct observation of the Keq values. plot for the ligands of the 145 ° cone angle. Thus, P(C6H3-
1630
www.eurjic.org
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2007, 1629–1631

Pd Pincer Complex as a Probe To Index the Coordination Ability of Ligands
3,5-Me2)3 and P[C6H3-3,5-(CF3)2]3 exhibited RCA values of
SHORT COMMUNICATION
Acknowledgments
–0.249 and –6.23, respectively (Table 1, Entries 12 and 24).
More sterically demanding ligands gave lower RCA values
(Table 1, Entries 20, 22, and 23).
This work was supported by the CREST program, sponsored by
the JST. We also thank the JSPS and the MEXT for partial finan-
cial support of this work.
[1] For some recent experimental and theoretical work on the elec-
tronic effects of phosphane ligands, see: a) J. Huang, C. M.
Haar, S. P. Nolan, W. J. Marshall, K. G. Moloy, J. Am. Chem.
Soc. 1998, 120, 7806–7815; b) S. Serron, S. P. Nolan, Y. A. Ab-
ramov, L. Brammer, Organometallics 1998, 17, 104–110; c)
C. M. Haar, S. P. Nolan, Inorg. Chim. Acta 1999, 291, 32–38;
d) C. Li, S. Serron, S. P. Nolan, Organometallics 1996, 15,
4020–4029; e) J. Huang, S. Serron, S. P. Nolan, Organometallics
1998, 17, 4004–4008; f) S. T. N. Freeman, F. R. Lemke, C. M.
Haar, S. P. Nolan, J. L. Petersen, Organometallics 2000, 19,
4828–4833; g) E. J. M. Meier, W. Kozminski, A. Linden, P. Lus-
tenberger, W. Philipsborn, Organometallics 1996, 15, 2469–
2477; h) N. Mathew, B. R. Jagirdar, R. S. Gopalan, G. U. Kulk-
arni, Organometallics 2000, 19, 4506–4517; i) C. R. Landis, S.
Feldgus, J. Uddin, C. E. Wozniak, K. G. Moloy, Organometal-
lics 2000, 19, 4878–4886; j) C. H. Suresh, N. Koga, Inorg.
Chem. 2002, 41, 1573–1578; k) A. Streitwieser, A. E.
McKeown, F. Hasanayn, N. R. Davis, Org. Lett. 2005, 7, 1259–
1262.
Figure 1. Hammett plot for the RCA values: log(Keq(L)/Keq
)
versus Taft’s σ value of aromatic substituents of the triary(lpPPhho3s)-
phane ligands. The numbers in parentheses correspond to the entry
numbers in Table 1.
[2] a) C. A. Tolman, Chem. Rev. 1977, 77, 313–348; b) T. L.
Brown, K. J. Lee, Coord. Chem. Rev. 1993, 128, 89–116; c) T. L.
Brown, Inorg. Chem. 1992, 31, 1286–1294.
Conclusions
[3] a) R. J. Angelici, C. M. Ingemanson, Inorg. Chem. 1969, 8, 83–
86; ligand exchange process with [W(CO)5(PhNH2)]: b) W. Par-
tenheimer, Inorg. Chem. 1972, 11, 743–746; measurement of
∆H value of the Pd-ligand complexation: c) L. E. Manzer,
C. A. Tolman, J. Am. Chem. Soc. 1975, 97, 1955–1956; ∆H
value of the Pt-ligand complexation: d) R. P. Pinnell, C. A. Me-
gerle, S. L. Manett, P. A. Kroon, J. Am. Chem. Soc. 1973, 95,
977–978; based on the 31P NMR spectroscopic coupling con-
stants of P–Se; five examples: e) W.-Y. Jeong, R. A. Holwerda,
Inorg. Chem. 1989, 28, 2674–2679; based on the 31P NMR
spectroscopic ∆δ values between a complex and the free phos-
phane, seven examples: f) L. Perrin, E. Clot, O. Eisenstein, J.
Loch, R. H. Crabtree, Inorg. Chem. 2001, 40, 5806–5811.
[4] a) K. Takenaka, M. Minakawa, Y. Uozumi, J. Am. Chem. Soc.
2005, 127, 12273–12281; b) K. Takenaka, Y. Uozumi, Org.
Lett. 2004, 6, 1833–1835.
We have developed pincer palladium complex 1 as a
novel probe for the determination of the coordination abil-
ity of various ligands to form ArPd(L)2Cl, with the relative
coordination abilities of 27 ligands being determined easily
by 1H NMR spectroscopic experiments. A two-dimensional
analysis using probe complexes of different steric demands
(with the use of various NR groups), as well as the applica-
tion of the indexation protocol to ligands bearing other co-
ordination sites, e.g. carbene, S, Sb, etc., are currently un-
derway and will be reported in due course.
Experimental Section
[5] For other applications of the ligand introduction route, see: a)
K. Takenaka, Y. Uozumi, Adv. Synth. Catal. 2004, 346, 1693–
1696; b) T. Kimura, Y. Uozumi, Organometallics 2006, 25,
4883–4887.
[6] The orders of steric and electronic factors of these phosphane
ligands are reverse to each other [steric (small to large): PPh3
General Equilibria Studies by NMR Spectroscopy: Complex 1 and
a ligand (2 equiv.) were dissolved in [D2]dichloroethane. The equi-
librium constant, Keq, was determined by the integration of
the resonances: the resonance integration ratio 1/2 = x:y; Keq
=
2
(1/4)[{y(x + y)2}/x3][1]0 (–2). All NMR spectroscopic data
(charts, chemical shifts, integration data, etc) are provided in the
Supporting Information.
Ͼ PPh2Cy Ͼ PPhCy2 Ͼ PCy3; electronic (basicity): PCy3
Ͼ
PPhCy2 Ͼ PPh2Cy Ͼ PPh3] and, therefore, the order of their
coordination ability is difficult to predict. The spectroscopic
direct observation of the coordination ability should be re-
quired to determine the order of their coordination ability.
[7] a) C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165–
195; b) M. T. Tribble, J. G. Traynham, J. Am. Chem. Soc. 1969,
91, 379–388, and references cited therein.
Supporting Information (see footnote on the first page of this arti-
cle): 1H NMR spectroscopic data with their measurement condi-
tions for Table 1, Entries 2–28.
Received: December 11, 2006
Published Online: March 14, 2007
Eur. J. Inorg. Chem. 2007, 1629–1631
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1631