Synthesis, structures and spectroscopic properties of palladium(II) complexes with tridentate piperidyl-containing pincer ligands

Article abstract of DOI:10.1016/j.ica.2005.07.035

A new series of square planar palladium(II) complexes with pincer ligands, pip₂NCN^- (pip₂NCNH = 1,3-bis(piperidylmethyl) benzene) and pip₂NNN (2,6-bis(piperidylmethyl)pyridine), has been prepared: Pd(pip₂NCN)X (X = Cl, Br, I), [Pd(pip₂NCN)(L)] (BF₄) (L = pyridine, 4-phenylpyridine), and [Pd(pip ₂NNN)Cl]Cl. The X-ray crystal structures of Pd(pip₂NCN)Br, [Pd(pip₂NCN)(L)]BF₄, and [Pd(pip₂NNN)Cl]Cl confirm the tridentate coordination geometries of the pincer ligands. For the pip₂NCN^- complexes, each piperidyl ring adopts a chair conformation with the metal center at an equatorial position on the N(piperidyl) atom. However, one of the piperidyl groups of Pd(pip₂NNN)Cl ⁺ adopts a previously unobserved coordination geometry, effectively placing the metal center at an axial position on the N(piperidyl) atom. ¹H NMR and UV-Vis absorption measurements provide additional insight into the electronic structures of these complexes. The ¹H NMR spectra of Pd(pip₂NCN)X (X = Cl, Br, I) are consistent with deshielding of the pip₂NCN^- ligand resonances along the Cl < Br < I series, in opposition to the relative halogen electronegativities. It is suggested that this trend is consistent with decreasing filled/filled repulsions between the dπ orbitals of the metal center and the lone pair orbitals of the halide ligands along this series. Electronic absorption spectra support the notion that ligand-to-metal charge-transfer states are stabilized in these palladium(II) complexes relative to their platinum(II) analogues.

Full text of DOI:10.1016/j.ica.2005.07.035

Inorganica Chimica Acta 359 (2006) 1889–1898
www.elsevier.com/locate/ica
Synthesis, structures and spectroscopic properties of
palladium(II) complexes with tridentate piperidyl-containing
pincer ligands
*
Seher Tastan, Jeanette A. Krause, William B. Connick
Department of Chemistry, University of Cincinnati, P.O. Box 210172, Cincinnati, OH 45221-0172, United States
Received 5 July 2005; accepted 22 July 2005
Available online 8 September 2005
Dedicated to Professor Gerard van Koten.
Abstract
A new series of square planar palladium(II) complexes with pincer ligands, pip2NCNꢀ (pip2NCNH = 1,3-bis(piperidylmethyl)ben-
zene) and pip2NNN (2,6-bis(piperidylmethyl)pyridine), has been prepared: Pd(pip2NCN)X (X = Cl, Br, I), [Pd(pip2NCN)(L)](BF4)
(L = pyridine, 4-phenylpyridine), and [Pd(pip2NNN)Cl]Cl. The X-ray crystal structures of Pd(pip2NCN)Br, [Pd(pip2NCN)(L)]BF4,
and [Pd(pip2NNN)Cl]Cl confirm the tridentate coordination geometries of the pincer ligands. For the pip2NCNꢀ complexes, each
piperidyl ring adopts a chair conformation with the metal center at an equatorial position on the N(piperidyl) atom. However, one
of the piperidyl groups of Pd(pip2NNN)Cl+ adopts a previously unobserved coordination geometry, effectively placing the metal cen-
ter at an axial position on the N(piperidyl) atom. 1H NMR and UV–Vis absorption measurements provide additional insight into the
electronic structures of these complexes. The 1H NMR spectra of Pd(pip2NCN)X (X = Cl, Br, I) are consistent with deshielding of the
pip2NCNꢀ ligand resonances along the Cl < Br < I series, in opposition to the relative halogen electronegativities. It is suggested that
this trend is consistent with decreasing filled/filled repulsions between the dp orbitals of the metal center and the lone pair orbitals of
the halide ligands along this series. Electronic absorption spectra support the notion that ligand-to-metal charge-transfer states are
stabilized in these palladium(II) complexes relative to their platinum(II) analogues.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Pd(II) and Pt(II) complexes; Pincer ligands; Electronic structure; Ligand-to-metal charge transfer; X-ray crystal structures; Electronic
spectroscopy
1. Introduction
[8–12]. Building on this rich chemistry, we have recently
explored the use of the cyclometallating aryldiamine
ligand pip2NCNꢀ in the preparation of outer-sphere
two-electron platinum reagents [13]. To better under-
stand the electronic structures of this system, we have
examined the UV–Vis absorption and emission spectro-
scopic properties of a series of Pt(pip2NCN)L derivatives
and discovered that variation of the monodentate ligand
(L) provides remarkable control over the electronic struc-
tures of this class of compounds. Most notably, the
luminescence originates from a lowest state having metal-
to-ligand charge-transfer (MLCT), ligand-centered or
The inspiring work of van Koten and coworkers
[1–12] has drawn attention to platinum(II) and palla-
dium(II) pincer ligand complexes and their potential util-
ity in a wide range of applications, including gas sensing
devices [1–3], catalysis [4–7], and the preparation of orga-
nometallic supramolecular structures and materials
*
Corresponding author. Tel.: +1 513 556 0148; fax: +1 513 556
9239.
E-mail address: bill.connick@uc.edu (W.B. Connick).
0020-1693/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.07.035

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S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
(ppm): 1.38–1.82 (12H, m, CH2), 3.18 (4H, dd, CH2),
3.92 (4H, dd, CH2), 4.29 (4H, s, benzylic CH2), 6.75
(2H, d, CH), 6.95 (1H, t, CH).
N
N
N
N
2.2.2. Pd(pip2NCN)Cl
Method 1: Excess NaCl was added to Pd-
(pip2NCN)Br (0.20 g, 0.44 mmol) dissolved in 20 mL
acetone, and the mixture was stirred for an hour. After
removal of the solvent, the solid was washed several
times with water and ether. An off-white solid was col-
lected. Yield: 0.08 g, 45%.
N
pip2NCN-
pip2NNN
Scheme 1.
Method 2: In the dark, AgBF4 (0.096 g, 0.5 mmol)
was added to Pd(pip2NCN)Br (0.20 g, 0.44 mmol) dis-
solved in 20 mL acetone. The mixture was stirred for
30 min and then filtered through Celite. After stirring
the filtrate with excess NaCl for 30 min, the solvent
was removed, and the resulting solid was washed with
water and ether. Yield: 0.14 g, 76%.
ligand field character, depending on the nature of L. In
this report, we extend these investigations to a series of
palladium(II) analogues, including a complex with the
novel pip2NNN ligand (Scheme 1).
2. Experimental
Method 3: The same procedure as for Pd-
(pip2NCN)Br substituting
Pd(COD)Cl2 (0.38 g,
2.1. Materials
1.34 mmol) for Pd(COD)Br2. Yield: 0.39 g, 70%. Anal.
Calc for C18H27N2PdCl Æ 0.5CH2Cl2: C, 48.70; H, 6.14;
N, 6.14. Found: C, 48.36; H, 5.99; N, 6.20%. 1H
NMR (CDCl3) d (ppm): 1.40–1.78 (12H, m, CH2),
3.28 (4H, dd, CH2), 3.94 (4H, dd, CH2), 4.26 (4H, s,
Pd(COD)Br2 (COD = 1,5-cyclooctadiene), Pd(COD)-
Cl2 [14], and pip2NCNBr (2,6-bis(piperdyl-methyl)-1-
bromobenzene)[15] were prepared according to the
literature procedures. Deuterated acetonitrile and chloro-
form (0.03% tetramethylsilane (TMS) (v/v)) were pur-
chased from Cambridge Isotope Laboratories. All other
reagents were purchased from commercial sources (Acros
and Aldrich) and used as received. Benzene and acetoni-
trile were distilled from CaH2 under argon, and tetrahy-
drofuran was distilled from Na(s) and benzophenone
under argon.
1
benzylic CH2), 6.75 (2H, d, CH), 6.95 (1H, t, CH). H
NMR (CD3CN) d (ppm): 1.38–1.82 (12H, m, CH2),
3.14 (4H, dd, CH2), 3.81 (4H, dd, CH2), 4.27 (4H, s,
benzylic CH2), 6.75 (2H, d, CH), 6.94 (1H, t, CH).
2.2.3. Pd(pip2NCN)I
Excess NaI was added to a 25 mL solution of Pd-
(pip2NCN)Br (0.20 g, 0.44 mmol) in acetone and the
mixture was stirred for an hour. After removal of the
solvent, the solid was washed with water and ether. A
light brown solid was collected. Yield: 0.19 g, 85%.
Crystallization from acetone gave pale yellow crystals.
Anal. Calc for C18H27N2PdI: C, 42.83; H, 5.39; N,
2.2. Preparation of compounds
2.2.1. Pd(pip2NCN)Br
N-butyllithium (0.92 mL, 1.6 M in hexanes) was
added to a stirred solution of pip2NCNBr (0.56 g,
1.6 mmol) in 30 mL of THF under argon at ꢀ70 °C.
After 45 min, the contents of the flask was cannula
transferred into a suspension of Pd(COD)Br2 (0.50 g,
1.34 mmol) in 50 mL THF at ꢀ70 °C. The mixture
was stirred for 1 h and subsequently allowed to warm
to room temperature. After stirring overnight, the black
residue was removed by filtration, and the filtrate was
rotoevaporated to dryness. Water was added to the so-
lid, and the product was extracted with dichlorometh-
ane. The organic layer was dried over MgSO4 and
reduced in volume to 5 mL. Addition of hexanes affor-
ded an off-white solid. Yield: 0.43 g, ꢁ70%. Anal. Calc
for C18H27N2PdBr: C, 47.23; H, 5.95; N, 6.12. Found:
C, 47.55; H, 5.99; N, 6.12%. 1H NMR (CDCl3) d
(ppm): 1.40–1.78 (12H, m, CH2), 3.27 (4H, dd, CH2),
4.02 (4H, dd, CH2), 4.26 (4H, s, benzylic CH2), 6.75
1
5.55. Found: C, 43.02; H, 5.41; N, 5.47%. H NMR
(CDCl3) d (ppm): 1.36–1.80 (12H, m, CH2), 3.30 (4H,
d, CH2), 4.23 (4H, t, CH2), 4.28 (4H, s, benzylic CH2),
1
6.75 (2H, d, CH), 7.00 (1H, t, CH). H NMR (CD3CN)
d (ppm): 1.36–1.90 (12H, m, CH2), 3.21 (4H, d, CH2),
3.97 (4H, t, CH2), 4.31 (4H, s, benzylic CH2), 6.77
(2H, d, CH), 6.99 (1H, t, CH).
2.2.4. [Pd(pip2NCN)(py)]BF4
To 25 mL of an acetone solution of Pd(pip2NCN)Br
(0.10 g, 0.22 mmol) in the dark was added AgBF4
(0.048 g, 0.25 mmol). After stirring for 30 min, the mix-
ture was filtered through Celite. The filtrate was stirred
with pyridine (py, 0.0174 g, 0.22 mmol) for 30 min, be-
fore removal of the solvent by rotary evaporation. The
residue was redissolved in a minimum amount of dichlo-
romethane. Addition of hexanes afforded an off-white so-
lid. Yield: 0.096 g, 80%. Anal. Calc for C23H32N3PdBF4:
1
(2H, d, CH), 6.94 (1H, t, CH). H NMR (CD3CN) d

S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
1891
C, 50.80; H, 5.93; N, 7.73. Found: C, 50.87; H, 5.97; N,
7.58%. 1H NMR (CD3CN) d (ppm): 1.45–1.86 (12H, m,
CH2), 3.14 (4H, m, CH2), 3.35 (4H, m, CH2), 4.29 (4H, s,
benzylic CH2), 6.80 (2H, d, CH), 7.01 (1H, t, CH), 7.38
(2H, dd, CH), 7.80 (1H, dd, CH), 8.60 (2H, d, CH).
added 5 mL of an acetonitrile solution of pip2NNN
(0.16 g, 0.6 mmol) to give a tan precipitate. The mixture
was stirred for 3 h at room temperature before filtering,
and the filtrate was reduced to dryness by rotary evapo-
ration. The yellow solid was dissolved in dichlorometh-
ane, and hexanes were added to induce precipitation.
Yield: 0.19 g, 80%.
2.2.5. [Pd(pip2NCN)(phpy)]BF4
The same procedure as for [Pd(pip2NCN)(py)]BF4,
except 4-phenylpyridine (phpy, 0.040 g, 0.22 mmol)
was substituted for pyridine. Yield: 0.13 g, 92%. Anal.
Calc for C29H37N3PdBF4: C, 56.10; H, 6.00; N, 6.77.
Found: C, 55.65; H, 5.90; N, 6.67%. 1H NMR (CD3CN)
d (ppm): 1.45–1.85 (12H, m, CH2), 3.14 (4H, d, CH2),
3.37 (4H, t, CH2), 4.29 (4H, s, benzylic CH2), 6.81
(2H, d, CH), 7.01 (1H, t, CH), 7.53 (3H, m, CH), 7.63
(2H, d, CH), 7.74 (2H, d, CH), 8.63 (2H, d, CH).
Method 2: To a solution of PdCl2 (0.18 g, 1 mmol) in
15 mL of acetonitrile at 40 °C was added a solution of
pip2NNN (0.032 g, 1.1 mmol) in 5 mL of acetonitrile,
and the mixture was stirred for 30 min at room temper-
ature. The tan solid was removed by filtration, and the
filtrate was evaporated to dryness. The resulting yellow
solid was dissolved in dichloromethane and precipitated
with hexanes. Yield: 0.32 g, 72%. Anal. Calc for
C17H27N3PdCl2 Æ 1.5H2O: C, 42.74; H, 6.33; N, 8.79.
1
Found: C, 42.71; H, 6.65; N, 8.78%. H NMR (CDCl3)
2.2.6. pip2NNN
d (ppm): 1.52–1.95 (12H, m, CH2), 3.38 (4H, m, CH2),
Piperidine (11.3 mL, 0.11 mmol) was added to 20 mL
of benzene under argon. After cannula transfer of a ben-
zene solution of 2,6-bis(chloromethyl)pyridine (2.0 g,
0.0114 mmol), the mixture was stirred for 2 h at room
temperature. The white suspension was filtered, and
the yellow filtrate was reduced in volume to give a yel-
low oil that was used in subsequent steps. Yield: 2.8 g,
3.74 (4H, m, CH2), 4.88 (4H, s, benzylic CH2), 7.85
(2H, d, CH), 8.08 (1H, t, CH). H NMR (CD3CN) d
(ppm): 1.42–1.90 (12H, m, CH2), 3.29 (4H, m, CH2),
3.62 (4H, m, CH2), 4.64 (4H, s, benzylic CH2), 7.49
(2H, d, CH), 8.06 (1H, t, CH).
1
1
90%. H NMR(CD3CN) d (ppm): 1.46 (4H, m, CH2),
2.3. Measurements
1.60 (8H, m, CH2), 3.62 (4H, s, pyridyl-CH2), 7.28
(2H, d, CH), 7.60 (1H, t, CH).
1H NMR spectra were recorded using a Bruker AC
400 MHz Spectrometer. UV–Vis absorption spectra
were recorded using a HP8453 UV–Vis spectrometer.
Emission spectra were recorded with a SPEX Fluoro-
log-3 fluorimeter.
2.2.7. [Pd(pip2NNN)Cl]Cl
Method 1: To 20 mL of an acetonitrile solution of
Pd(COD)Cl2 (0.15 g, 0.53 mmol) under argon was
Table 1
Crystal and structure refinement data for Pd(pip2NCN)Br, [Pd(pip2NCN)(phpy)]BF4, and [Pd(pip2NNN)Cl]Cl
Pd(pip2NCN)Br
18H27N2BrPd
457.73
triclinic
[Pd(pip2NCN)(phpy)]BF4
[Pd(pip2NCN)Cl]Cl
Formula
C
[C29H36N3Pd]BF4
619.82
orthorhombic
Pbca
[C17H27N3CIPd]C1
450.72
triclinic
Fw (g/mol)
Crystal system
Space group
ꢀ
ꢀ
P1
P1
˚
a (A)
9.8776(3)
10.2385(3)
19.1904(3)
75.369(1)
76.144(1)
71.215(1)
1751.11(9), 4
150(2)
10.8038(2)
19.5002(4)
25.7324(6)
90
90
90
5421.2(2), 8
150(2)
1.519
5.959
2544
6.23160(1)
10.8100(2)
14.3097(3)
101.549(1)
91.598(1)
106.117(1)
903.74(3), 2
150(2)
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
V (A ), Z
T (K)
Dcalc (g cmꢀ3
)
1.736
11.189
920
1.656
11.009
460
l (mmꢀ1
F(000)
)
h Range (°)
2.42–65.08
14018
5753/397
0.0307
1.033
0.0283/0.0738
0.0315/0.0760
3.43–68.02
32427
4859/352
0.0293
1.039
0.0286/0.0732
0.0295/0.0739
3.16–67.83
7609
3102/235
0.0163
1.073
0.0229/0.0603
0.0234/0.0606
Reflections collected
Independent reflections/parameters
Rint
Goodness-of-fit on F2
R1/wR2 [I > 2r(I)]
R1/wR2 (all data)

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S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
2.4. Crystal structure determination
all three structures were calculated based on geometric
criteria and treated with a riding model. Crystal and
refinement data are collected in Table 1.
Colorless intergrown plates of Pd(pip2NCN)Br were
grown by slow evaporation of an acetone-hexanes solu-
tion. Pale-yellow blocks of [Pd(pip2NCN)(phpy)]BF4
were prepared by slow evaporation of a dichlorometh-
ane-ether solution. Yellow plates of [Pd(pip2NNN)Cl]Cl
were obtained by slow evaporation of an acetonitrile-
ether solution. Diffraction data were collected at 150 K
using a standard Bruker SMART6000 CCD diffractom-
eter with graphite-monochromated Cu Ka radiation,
3. Results and discussion
3.1. Synthesis
Drawing on the work of van Koten and coworkers
[19–22], we have prepared a series of palladium(II) com-
plexes with the pip2NCNꢀ ligand (Scheme 2). The
chloro and bromo adducts, Pd(pip2NCN)X (X = Cl,
Br), were synthesized by allowing Pd(COD)X2 to react
with the lithiated tridentate ligand. The bromo deriva-
tive can be readily converted to the chloro or iodo prod-
uct by stirring with excess NaCl or NaI, respectively. In
analogy to previously reported platinum(II) complexes
[15,23], the py and phpy compounds were prepared by
reaction of the bromo adduct with one equivalent of
AgBF4, followed by treatment with the appropriate pyr-
idyl ligand. The nearly colorless products are soluble in
chloroform, dichloromethane, acetonitrile and acetone.
The coordination properties of the new tridentate
pip2NNN ligand also were investigated (Scheme 3).
˚
k = 1.54178 A. The data frames were processed using
the program SAINT [16]. The data were corrected for de-
cay, Lorentz and polarization effects. Absorption and
beam corrections based on the multi-scan technique
were applied using SADABS [17]. The structures were
solved by a combination of direct methods SHELXTL
[18] and the difference Fourier technique. The models
were refined by full-matrix least-squares on F2. For all
compounds, non-hydrogen atoms were refined with
anisotropic displacement parameters with the exception
of the disordered F atoms of [Pd(pip2NCN)(phpy)]BF4.
The BF4ꢀ counterion is disordered for atoms F2–F4; the
disorder was modeled using a three component model
with relative populations of 0.5:0.3:0.2. H atoms for
N
I
Pd
N
(iv)
(v)
+
N
N
N
Pd
N
(i)
Br
N
Pd Br
N
N
(ii)
(iii)
(vi)
F
E
+
D
C
N
N
Pd
N
B
A
I
J
H
G
Pd Cl
N
N
Scheme 2. (i) THF, BuLi, ꢀ70 °C; Pd(COD)Br2. (ii) THF, BuLi, ꢀ70 °C; Pd(COD)Cl2. (iii) Excess NaCl, acetone. (iv) Excess NaI, acetone. (v)
AgBF4, pyridine, acetone. (vi) AgBF4,4-phenylpyridine, acetone.

S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
1893
set of pyridyl resonances, indicating that ligand ex-
change is fast on the NMR timescale. This behavior
contrasts sharply with that noted for related platinum
complexes [15,21]; though partial ligand dissociation oc-
curs in CD3CN, the exchange is comparatively slow and
distinct sets of resonances for coordinated and free pyr-
idyl ligand are observed.
+
N
N
N
Pd(COD)Cl2
or PdCl2
N
N
Pd Cl
N
acetonitrile, 40˚C
It is noteworthy that, for the Pd(pip2NCN)X com-
plexes (X=Cl, Br, I), the resonances undergo a slight
downfield shift along the Cl < Br < I series. A similar
trend is observed for analogous platinum(II) complexes
[15]. The deshielding effect going from the chloro to iodo
complex is greatest (ꢁ0.16 ppm) for the furthest down-
field of the piperidyl a-proton resonances (D0). The
trend opposes the relative electronegativities of the hal-
ogen groups, as well as patterns in 195Pt NMR experi-
mental and computational results [25]. However, this
behavior has been noted for related compounds [26]
and is consistent with structural, spectroscopic and reac-
tivity patterns of many transition metal complexes [27].
Antipin, Grushin and coworkers [26] have argued that
similar trends in crystallographic and NMR data for
trans-Pd(PPh3)2(Ph)X (X = F, Cl, Br, I) can be under-
stood in terms of filled/filled repulsions between the dp
orbitals of the metal center and the lone pair orbitals
of the halide ligands. Infrared studies of five-coordinate
RuHX(CO)(Pt(CMe3)2Me)2 (X = F, Cl, Br, I) have
established that the carbonyl stretching frequency in-
creases along the F < Cl < Br < I series, indicating that
filled/filled repulsions decrease along this series [28].
The deshielding of the pip2NCNꢀ ligand resonances
along the Cl < Br < I series is likewise consistent with
decreasing filled/filled repulsions and electron releasing
properties of the Pd–X unit.
Scheme 3.
Stirring palladium dichloride or Pd(COD)Cl2 with one
equivalent of pip2NNN yields [Pd(pip2NNN)Cl]Cl.
The yellow product is soluble in polar solvents, includ-
ing acetonitrile, ethanol and water. A minor product
with limited solubility in acetonitrile also was obtained.
1
3.2. H NMR spectroscopy
A general labeling scheme for nonequivalent protons
is shown in Scheme 2 for Pd(pip2NCN)(phpy)+. The 1H
NMR spectra of the pip2NCNꢀ complexes exhibit pat-
terns consistent with C2 symmetry and are qualitatively
similar to those of their platinum analogues [15,23,24].
A triplet and a doublet due to the para and meta protons
of the phenyl ring (A, B) occur between 6.7 and 7.0 ppm
(Fig. 1). The benzylic proton resonance (C) appears as a
singlet near 4.2–4.3 ppm. Two multiplets between 3.14
and 3.99 ppm are attributable to the diastereotopic a-
protons (D0,D00) of piperidyl rings; the gap between
these resonances (0.2–0.9 ppm) decreases along the ser-
ies I > Br > Cl >phpy ꢁ py and is ꢁ0.1 ppm smaller
than observed for analogous Pt(II) complexes. The
remaining aliphatic proton resonances (E, F) appear as
complex multiplets further upfield (1.4–1.9 ppm). For
the two py and phpy complexes, the pyridyl resonances
are shifted downfield of the corresponding free ligand
resonances, confirming interaction with the palladium
center. However, addition of a 3-fold excess of pyridine
to a solution of Pd(pip2NCN)(py)+ resulted in a single
The pattern of resonances in the 1H NMR spectrum of
Pd(pip2NNN)Cl+ is qualitatively similar to that of
Pd(pip2NCN)Cl, though the pip2NNN resonances are
generally shifted downfield, in keeping with observations
for complexes with Me4NNN (2,6-bis(dimethylaminom-
ethyl)pyridine) and Me4NCNꢀ ligands (Me4NCNH =
1,3-bis(dimethylaminomethyl)benzene) [29–31]. For
Fig. 1. 1H NMR spectra of: (a) Pd(pip2NCN)Cl; (b) Pd(pip2NCN)Br; (c) Pd(pip2NCN)I; (d) [Pd(pip2NNN)Cl]Cl in CD3CN and (e)
[Pd(pip2NNN)Cl]Cl in CDCl3. indicates solvent (CHCl3).
*

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S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
example, in CD3CN the phenyl resonances associated
with protons A and B are shifted downfield by 1.1 and
0.7 ppm, respectively, from those of the corresponding
pip2NCNꢀ complex. The shifts are smaller for the ben-
zylic protons (C, 0.4 ppm) and the furthest upfield of the
diastereotopic piperidyl a-protons (D00, 0.01 ppm). In
contrast the remaining a-proton resonance (D0) is shifted
upfield, resulting in a considerably smaller gap (0.3 ppm)
between the a-proton resonances than observed for
Pd(pip2NCN)Cl (0.7 ppm).
In CD3CN, the difference (0.6 ppm) between the
chemical shifts (Dd = dpara ꢀ dmeta) of the para and meta
protons of the pip2NNN pyridyl ring is significantly lar-
ger than found for complexes with the pip2NCNꢀ pincer
ligand (ꢁ0.2 ppm). Interestingly, Dd is substantially re-
duced in CDCl3 (0.2 ppm), mostly as a consequence of
a 0.36 ppm downfield shift of dmeta. In contrast, values
of dpara and dmeta for the pip2NCNꢀ complexes are only
weakly dependent on solvent. Chemical shift correla-
tions suggest that dpara is more strongly influenced by
resonance effects than dmeta [32–36]. Therefore, the
solvent sensitivity of the 1H NMR spectrum of Pd-
(pip2NNN)Cl+ is consistent with unequal perturbation
of the resonance and field effects by specific solvent-
solute interactions. It is reasonable to anticipate that
Pd-solvent interactions are enhanced for the more acidic
metal center of Pd(pip2NNN)Cl+, as compared to
Pd(pip2NCN)Cl.
3.3. X-ray crystallography
In order to investigate the coordination geometries of
these complexes, the structures of Pd(pip2NCN)Br,
[Pd(pip2NCN)(phpy)]BF4 and [Pd(pip2NNN)Cl]Cl were
determined by X-ray crystallography. ORTEP diagrams
are shown in Fig. 2, and relevant data are summarized in
Tables 1–3. For the two salts, the anions and cations
pack as discrete units, and for all three structures, there
are no unusually short intermolecular contacts.
Fig. 2. ORTEP diagrams for: (a) Pd(pip2NCN)Br; (b)
[Pd(pip2NCN)(phpy)]BF4; and (c) [Pd(pip2NCN)Cl]Cl with 50%
probability ellipsoids. Anion and H-atoms omitted for clarity.
For each complex, the piperidyl ligand (pip2NCNꢀ or
pip2NNN) is tridentate, bonded to an approximately
square planar palladium center with the central phenyl
or pyridyl ring positioned trans to the monodentate
ligand (Brꢀ, phpy or Clꢀ). The geometries of the
pip2NCNꢀ ligands are similar to those found for plati-
num(II) complexes. Each piperidyl ring adopts a chair
conformation with the metal center at an equatorial po-
sition on the N(piperidyl) atom. As expected for an
equatorial substituent, the angle formed by the normal
to the plane defined by the a- and b-carbon atoms of
a piperidyl group (C8, C9, C11, C12; C14, C15, C17,
C18) and the corresponding Pd–N(piperidyl) vector ap-
proaches 90° (Brꢀ, 86.2°, 80.3°, 81.6°, 83.1°; phpy, 79.7°,
82.8°), whereas the normal is nearly parallel to the
C(benzylic)–N(piperidyl) vector (Brꢀ, 13.7°, 8.7°,
10.3°, 11.0°; phpy, 7.8°, 10.7°). One of the piperidyl
groups of Pd(pip2NNN)Cl+ adopts a similar orientation
with the plane normal forming an 85.2° angle with the
Pd–N2 vector. Surprisingly, the remaining piperidyl
group is flipped toward the Clꢀ ligand, placing the pal-
ladium center at an axial position on N3. The normal to
the plane defined by the a- and b-carbon atoms of the
piperidyl group (C14, C15, C17, C18) forms an 11.8° an-
gle with the Pd–N3 vector and a 77.3° angle with the
C13–N3 vector. This unusual coordination geometry
has not previously been encountered for palladium
and platinum complexes with pip2NNN or related li-
gands [15,23,24,37].
The bond lengths and angles for the three compounds
are in good agreement with those observed for palla-
dium(II) complexes with NCNꢀ and NNN ligands.
For example, though the Pd–C(phenyl) distances (Br,

S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
1895
˚
Table 2
[Pd(Me4NNN)Me](CF3SO3) (1.996(8) A) [46,47]. The
corresponding Pd–N2 bond for [Pd(pip2NNN)-
Cl]Cl(2.109(2)) is slightly shorter, as expected [37,46–
48]. Interestingly, the Pd–N3 distance for the flipped
˚
Selected distances (A) and angles (°) for Pd(pip2NCN)Br and
[Pd(pip2NCN)(phpy)]BF4
Pd(pip2NCN)Br
[Pd(pip2NCN)(phpy)]BF4
˚
Pd(1)–C(1)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–La
N(1)–C(7)
N(2)–C(13)
C(2)–C(7)
C(6)–C(13)
1.927(3), 1.922(3)
2.135(2), 2.137(2)
2.128(2), 2.126(3)
2.5468(4), 2.5612(4)
1.513(4), 1.513(4)
1.517(4), 1.511(4)
1.501(4), 1.503(4)
1.508(4), 1.499(4)
1.922(2)
piperidyl group is comparatively long (2.133(2) A),
2.1206(18)
2.1274(18)
2.1650(19)
1.514(3)
1.514(3)
1.499(3)
reflecting the steric consequences of positioning the me-
tal center at an axial site of the N3 atom. For the
monodentate ligands of the pip2NCNꢀ complexes, the
metal ligand bonds (Pd–Br; Pd–N(phpy)) are long, as
a result of the strong trans influence of the phenyl ligand
[12,20,39,41,49,50]. Similarly, the Pd–Cl distance for
1.503(3)
Pd(pip2NNN)Cl+ (2.3059(6) A) is consistent with bond
˚
distances reported for related palladium compounds
with bidentate and tridentate pyridyl donor ligands
[38,46,47].
The N(piperidyl)–Pt–N(piperidyl) bond angles are
somewhat less than 180° (Brꢀ, 163.02(10)°, 163.20(10)°;
phpy, 163.46(8)°), falling within the 161–165° range ob-
served for related platinum(II) complexes [15,23,24,37].
In the case of Me4NCNꢀ ligands, this deviation has been
ascribed to the geometric preferences of the two five-mem-
bered chelate rings [15,23,51,52]. These rings are slightly
puckered, resulting in displacement of the benzylic car-
bons above and below (Brꢀ, C7/C13, 0.54/0.49; 0.43/
C(1)–Pd(1)–N(1)
C(1)–Pd(1)–N(2)
C(1)–Pd(1)–La
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–La
N(2)–Pd(1)–La
C(7)–N(1)–Pd(1)
C(13)–N(2)–Pd(1) 108.44(17), 107.90(18) 107.89(13)
C(2)–C(7)–N(1)
C(6)–C(13)–N(2)
81.56(11), 81.63(11)
81.57(11), 81.57(11)
178.54(9), 176.28(9)
163.02(10), 163.20(10) 163.46(8)
97.82(7), 96.64(7)
99.10(7), 100.11(7)
81.59(9)
82.06(8)
178.36(9)
99.46(7)
96.94(7)
107.51(18), 108.68(18) 108.03(14)
109.6(2), 109.6(2)
109.6(2), 109.8(2)
109.36(18)
109.84(18)
a
L = Br for Pd(pip2NCN)Br and N3 of 4-phpy for
[Pd(pip2NCN)(phpy)]BF4.
Table 3
˚
˚
0.55 A; phpy, 0.51/0.51) the metal coordination plane,
Selected distances (A) and angles (°) for [Pd(pip2NNN)Cl]Cl
as defined by the metal and the four directly bonded
atoms. For Pd(pip2NNN)Cl+, the displacements are on
the same side of the plane and somewhat smaller (0.28/
[Pd(pip2NNN)Cl]Cl
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–N(3)
Pd(1)–Cl(1)
N(2)–C(7)
N(3)–C(13)
C(2)–C(7)
1.956(2)
2.109(2)
2.133(2)
2.3059(6)
1.505(3)
1.497(3)
1.498(4)
1.498(4)
˚
0.42 A). To accommodate this puckering, the planar phe-
nyl and pyridyl rings of the tridentate ligands are slightly
rotated about the Pd–C(phenyl) or Pd–N(pyridyl) bonds,
forming small dihedral angles with the metal coor-
dination plane (Pd(pip2NCN)Br, 8.43(6)°, 9.19(10)°;
C(6)–C(13)
Pd(pip2NCN)(phpy)+,
8.5(1)°;
Pd(pip2NNN)Cl+,
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–N(3)
N(1)–Pd(1)–Cl
N(2)–Pd(1)–N(3)
N(2)–Pd(1)–Cl(1)
N(3)–Pd(1)–Cl(1)
C(7)–N(2)–Pd(1)
C(13)–N(3)–Pd(1)
C(2)–C(7)–N(2)
C(6)–C(13)–N(3)
82.31(8)
81.44(8)
175.04(6)
157.70(8)
94.55(6)
102.58(6)
106.83(14)
104.83(14)
111.7(2)
9.6(2)°). These angles fall within the rather broad range
(6–16°) observed for other palladium(II) NCN and
NNN complexes [12,20,22,37,39–44,46–48]. On the other
hand, the pyridyl group of Pd(pip2NCN)(phpy)+ is nearly
perpendicular to the coordination plane, forming a dihe-
dral angle of 77.9(1)°. As noted for platinum(II) ana-
logues, this orientation is consistent with steric, as well
as electronic effects, since the metal-pyridine p-interac-
tions do not strongly compete with the metal-phenyl p-
interactions [23,53,54].
111.6(2)
ꢀ
˚
1.927(3), 1.922(3); phpy, 1.922(2) A) for the pip2NCN
complexes are significantly shorter than found for com-
plexes with a phenyl group positioned trans to a halide
3.4. Electronic spectroscopy
˚
ligand (e.g., trans-Pd(Ph)(PPh3)2Cl, 2.005(3) A) [38],
they fall within the 1.90–1.94 A range typically observed
The pip2NCNꢀ complexes dissolve to give nearly col-
orless solutions, whereas [Pd(pip2NNN)Cl]Cl dissolves
to give yellow solutions. To better understand the elec-
tronic structures of these complexes, their absorption
spectra were recorded in acetonitrile solution (Table 4;
Fig. 3).
˚
for complexes with NCNꢀ ligands [12,20–22,39–44]. The
Pd–N(piperidyl) distances for the Brꢀ and phpy com-
˚
plexes (2.12–2.14 A) also are similar to those reported
for related compounds [12,20,22,40–45]. The Pd–N(pyr-
idyl) bond length for Pd(pip2NNN)Cl+ (Pd–N1,
˚
1.956(2) A) is in reasonable agreement with distances re-
The UV spectra of the Pd(pip2NCN)X (X = Cl, Br, I)
are dominated by an intense absorption band near
ported for complexes with similar pincer ligands, such as

1896
S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
pip2NCNꢀ ligand. The apparent blue shift of these
bands in the spectra of these palladium(II) complexes
is in accord with this assignment. It is noteworthy that,
as observed for the platinum(II) analogue [23],
Pd(pip2NCN)(phpy)+ absorbs strongly from 250 to
283 nm, exhibiting a very broad absorption manifold
with e > 13000 cmꢀ1 Mꢀ1. Free 4-phpy exhibits a broad
p–p* absorption near 255 nm (16600 Mꢀ1 cmꢀ1) that
shifts to 285 nm (ꢁ17000 Mꢀ1 cmꢀ1) on protonation
[61]. Similar ligand centered transitions have been iden-
tified in the spectra of Pt(pip2NCN)(phpy)+ (280 nm,
32100 cmꢀ1 Mꢀ1) [23], Ru(NH3)5(4-phpy)2+ (289 nm,
Table 4
UV–Vis absorption data for palladium pip2NCNꢀ and pip2NNN
complexes
Compound
k )
max, nm (e, cmꢀ1 Mꢀ1
Pd(pip2NCN)Cl
247sh (8800), 279 (2600), 286sh (2300),
304 (1650)
Pd(pip2NCN)Br
Pd(pip2NCN)I
205 (27100), 216sh (24700), 239 (15800),
274 (2300), 286sh (2150), 308 (1600)
205 (24900), 216sh (23100), 243 (20700),
274sh (3300), 281sh (3000), 300sh (1900)
201(49000), 243 (9450), 262sh (3500),
273sh (2250), 281sh (2000), 300sh (1050)
202(66200), 271 (14200), 295sh (11600)
209 (34200), 252 (13150), 277sh (4300),
362 (1000)
Pd(pip2NCN)(py)+
Pd(pip2NCN)(phpy)+
Pd(pip2NNN)Cl+
18600 Mꢀ1 cmꢀ1
)
[62],
fac-Re(CO)3(Cl)(4-phpy)2
(268 nm, 36500 Mꢀ1 cmꢀ1) [58] and Cu4Cl4(4-phpy)4
(286 nm) [63], and a significant fraction of the intensity
observed for Pd(pip2NCN)(phpy)+ in this region is rea-
sonably attributed to phpy-centered transitions.
a
40
20
Pd(pip2NCN)X (X = Cl, Br, I) and Pd(pip2NCN)-
(py)+ also exhibit a broad band between 300 and
310 nm (1000–2000 Mꢀ1 cmꢀ1). The intensity is some-
what greater than expected for a d–d transition (<500
x10
ꢀ1 cmꢀ1) [64–66], and Pd(II)-centered d ! p charge-
0
M
b
transfer transitions are expected to occur at consider-
ably shorter wavelengths (<200 nm) [67]. Similarly, it
seems unlikely that this band has substantial MLCT
character, since it is clearly shifted to longer wavelengths
than any comparably intense bands in the spectra of
platinum(II) analogues [15,23,24]. The accumulated
data support the notion that this transition has signifi-
cant ligand-to-metal charge-transfer (LMCT) character
involving the pip2NCNꢀ ligand. In keeping with this
interpretation, the band is relatively insensitive to the
nature of the monodentate ligand. At longer wave-
lengths, the spectra of the pip2NCNꢀ complexes exhibit
long tailing absorption profiles that are attributable to
ligand field transitions.
40
20
x10
200
250
300
350
400
450
Wavelength (nm)
Fig. 3. UV–Vis absorption spectra in acetonitrile: (a) [Pd(NNN)Cl]Cl
(—) and [Pd(NCN)py]BF4 (Æ-Æ-); (b) Pd(NCN)Cl (—), Pd(NCN)Br (ꢂ ꢂ ꢂ)
and Pd(NCN)I (----).
240 nm (9000–21000cmꢀ1 Mꢀ1), tentatively assigned as
having MLCT character. The transition is likely shifted
to shorter wavelengths in the spectrum of Pd(pip2-
NCN)(py)+, since the metal center is expected to be
comparatively less electron rich. However, this complex
also exhibits an absorption maximum in this region
(243 nm, 9450 cmꢀ1 Mꢀ1), in part, because of a pyridyl-
centered p–p* transition that also appears in the spectra
of free pyridine (252 nm, 2000 cmꢀ1 Mꢀ1) and proton-
ated pyridine (256 nm, 5360 cmꢀ1 Mꢀ1) [55], as well as
other pyridyl complexes (e.g., Ru(NH3)5(py)2+, 244 nm,
4600 cmꢀ1 Mꢀ1) [56–60]. At longer wavelengths, the
absorption intensity for the halide and py derivatives
is considerably weaker. Notably, in the region from
265 to 285 nm, the molar absorptivity is <3000 cmꢀ1
Mꢀ1 for the Clꢀ, Brꢀ and py derivatives and <4000
cmꢀ1 Mꢀ1 for the iodo complex. In contrast, the spectra
of the corresponding platinum(II) complexes exhibit two
relatively intense bands in this region (8000–10000
cmꢀ1 Mꢀ1) [15,23]. We previously proposed that the
transitions in the spectra of the platinum(II) derivatives
have significant MLCT character involving the
The absorption spectrum of Pt(pip2NNN)Cl+ pro-
vides additional insight into the electronic structures of
these complexes. The UV spectrum is dominated by in-
tense absorptions between 250 and 300 nm (252 nm,
13150 cmꢀ1 Mꢀ1; 277sh nm, 4300 cmꢀ1 Mꢀ1). These
bands most likely have some contribution from pip2NNN
ligand-centered transitions, since the free ligand absorbs
moderately in this region (265 nm, 3800 cmꢀ1 Mꢀ1). As
for the pip2NCNꢀ complexes, the spectrum exhibits a
tailing profile at longer wavelengths (<1000 cmꢀ1 Mꢀ1
)
due to ligand field transitions. It is noteworthy that, in
contrast to our observations for the pip2NCNꢀ com-
plexes, there is no evidence of a charge-transfer transition
near 300 nm in this spectrum. On the other hand, a
broad charge-transfer feature with comparable intensity
occurs at considerably longer wavelengths (362 nm,
1000 cmꢀ1 Mꢀ1, FWHM = 4400 cmꢀ1). A comparison
to related complexes confirms that this band is unlikely
to have MLCT character. For example, the lowest spin-
allowed metal-to-ligand(pyridyl) charge-transfer band

S. Tastan et al. / Inorganica Chimica Acta 359 (2006) 1889–1898
1897
of Pt(2,6-bis(aminomethyl)pyridine)(OH)+ in aqueous
solution is shifted to the blue of 320 nm [68], and the cor-
responding transition for the palladium(II) analogue is
expected to occur at even shorter wavelengths. Similarly,
the lowest spin-allowed metal-to-ligand(bpy) charge-
transfer transition of Pd(bpy)Cl2 (bpy = 2,20-bipyridine)
occurs near 320 nm in aqueous solution [69]; the MLCT
transition of Pd(pip2NNN)Cl+ is expected to occur at
shorter wavelengths because of stabilization of the unoc-
cupied p*(bpy) level relative to the p*(pip2NNN) level. On
the other hand, there are a number of examples of palla-
dium(II) complexes that exhibit LMCT transitions in the
300–400 nm range [70–72], including trans-Pd(PPh3)2Cl2-
(CH2Cl2: 345 nm, 20135 cmꢀ1 Mꢀ1, FWHM ꢁ 3000
cmꢀ1) and Pd(TPA)Cl+ (TPA = tris(pyridylmethyl)-
amine) (DMSO: 338 nm, 485 cmꢀ1 Mꢀ1; 380 nm, 416
cmꢀ1 Mꢀ1) [73–75]. We suggest that the 362 nm band of
Pd(pip2NNN)Cl+ also has significant Xꢀ ! dx2ꢀy2 (Pd)
charge-transfer character. The corresponding band for
Pd(pip2NCN)X (X = Cl, Br, I) is evidently shifted to
shorter wavelengths, in keeping with our expectation that
the LMCT state will be destabilized by the strong sigma
donor pip2NCNꢀ ligand, which increases electron density
on the metal center and destabilizes the dx2ꢀy2 level.
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[17] SADABS v2.10 was used for the application of semi-empirical
absorption and beam corrections. G.M. Sheldrick, University of
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Acknowledgments
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of figures and tables. Neutral-atom scattering factors were used as
stored in this package. G.M. Sheldrick, University of Go¨ttingen,
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Funding for the SMART6000 CCD diffractometer
was through NSF-MRI Grant CHE-0215950. W.B.C.
thanks the National Science Foundation (CHE-
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and Mabel Beckman Foundation for a Young Investiga-
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Appendix A. Supplementary data
Complete sets of X-ray crystallographic data for
Pd(pip2NCN)Br (CCDC No. 276443) [Pd(pip2NCN)(ph-
py)]BF4 (CCDC No. 276444) and [Pd(pip2NNN)Cl]Cl
(CCDC No. 276445) are available free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html or from the
Cambridge Crystallographic Data Centre (CCDC), 12
Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223
336 033; email: deposit@ccdc.cam.ac.uk) on request,
quoting the deposition number. Supplementary data
associated with this article can be found, in the online
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