Bis-glycinato complexes of palladium(II): Synthesis, structural determination, and hydrogen bonding interactions

Article abstract of DOI:10.1016/j.ica.2014.07.024

Palladium(II) acetate and palladium(II) chloride react with glycine and glycine derivatives in acetone/water to yield square planar bis-chelated palladium amino acid complexes. Glycine, N-methylglycine and N,N-dimethylglycine all adopt the trans configuration when prepared from palladium(II) acetate, whereas glycine adopts a cis configuration when prepared from palladium(II) chloride. The crystal structures of all four complexes were obtained. The extended lattice structures arise from N-H··O or O··(HOH)··O hydrogen bonding. Nuclear magnetic resonance, infrared, high resolution mass spectroscopy, and UV-Vis spectroscopic data are evaluated.

Full text of DOI:10.1016/j.ica.2014.07.024

Inorganica Chimica Acta 423 (2014) 21–30
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Bis-glycinato complexes of palladium(II): Synthesis, structural
determination, and hydrogen bonding interactions
⇑
David B. Hobart Jr., Michael A.G. Berg, Joseph S. Merola
Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2014
Received in revised form 5 July 2014
Accepted 7 July 2014
Available online 24 July 2014
Palladium(II) acetate and palladium(II) chloride react with glycine and glycine derivatives in acetone/
water to yield square planar bis-chelated palladium amino acid complexes. Glycine, N-methylglycine
and N,N-dimethylglycine all adopt the trans configuration when prepared from palladium(II) acetate,
whereas glycine adopts a cis configuration when prepared from palladium(II) chloride. The crystal
structures of all four complexes were obtained. The extended lattice structures arise from N–HÁÁO or
OÁÁ(HOH)ÁÁO hydrogen bonding. Nuclear magnetic resonance, infrared, high resolution mass spectroscopy,
and UV–Vis spectroscopic data are evaluated.
Keywords:
Palladium
Amino acid
Glycine
Ó 2014 Elsevier B.V. All rights reserved.
Hydrogen bonding
Trans effect
15
N NMR
1
. Introduction
electron-transfer transitions when irradiated with ultraviolet radi-
ation at 254 nm yet suffered no decomposition when irradiated at
Palladium(II)-amino acid chelates are known although only a
313 nm, which corresponds to the d ? d transition seen in the UV–
few of the possible amino acids have been used and their proper-
ties have not been studied in great depth [1]. The simplest amino
acid, glycine, affords some advantages in terms of analytical sim-
plicity for a beginning of a broader, more detailed study of these
complexes. As we began a study of the catalytic and biological
activity of transition metal amino acid complexes [2], we realized
that there was still much to be learned about the bis-glycine palla-
dium(II) complexes and we report on our findings here.
Pinkard reported the first syntheses of cis and trans palla-
dium(II)-glycine complexes [1]. Ley [3] and Grünberg [4] had
earlier reported analogous platinum complexes. Assignment of cis
and trans isomers was based on differing crystal habits and on dif-
fering reaction products with aqueous thiourea. Saraceno [5] and
Lane [6] made detailed assignments of infrared bands for the trans
and cis isomers, respectively. Further infrared spectral analysis was
reported by Condrate [7], and far-infrared peak assignments of
these complexes were made by Walter [8]. The electronic spectra
and photochemical behavior of palladium(II)-amino acid
complexes were investigated by Balzani [9]. Balzani reported
Vis spectrum. Zhang reported on the photoacoustic spectrum [10].
Isomerization of trans-Pd(gly) to cis-Pd(gly) was reported by Coe
2 2
along with the first-order rate constant for the isomerization [11].
Farooq studied the formation of palladium(II)-glycine complexes
+
by titrating the [H ] liberated during the reaction of glycine and
2 4
Na PdCl and calculated their stability constants [12]. The X-ray
crystal structures of the cis and trans palladium(II)-glycine com-
plexes were reported by Baidina [13,14]. Nuclear magnetic reso-
1
5
nance data on N labeled glycinate complexes of palladium(II)
were reported by Appleton [15]. Two resonances were observed
at d 55.7 and 47.4 ppm indicating a mixture of cis and trans isomers
1
5
13
of Pd( N-gly)
2
. The C NMR spectral data also shows a slight
upfield shift of the methylene carbon resonance as compared to
1
3
the free ligand. C NMR data reported by Krylova [16] is in
1
excellent agreement with that reported by Appleton; H NMR
data is reported as well with two distinct resonances reported
for –NH
Pd(gly)
2
and –CH
2
– moieties. Electrochemical properties of
2
complexes have been reported by Chornenka [17] and
Kublanovsky [18].
that trans-Pd(gly)
2
undergoes decomposition resulting from
In this paper we describe our work with palladium(II)-amino
acid chelates where glycine, N-methyl glycine, and N,N-dimethyl
glycine were used as the chelating ligands. We have begun an
extensive investigation into the syntheses and properties of
⇑
Corresponding author.
E-mail address: jmerola@vt.edu (J.S. Merola).
http://dx.doi.org/10.1016/j.ica.2014.07.024
020-1693/Ó 2014 Elsevier B.V. All rights reserved.
0

2
2
D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
these bis-amino acid chelates of palladium with an eye toward
ultimately using these complexes for catalysis. Before investigating
the chiral amino acids, we realized that the palladium chemistry of
the basic amino acid, glycine, was not completely studied and
there were some gaps in the literature that warranted a new, more
complete examination of glycine complexes of palladium. In this
paper, we present our results on the syntheses, structural and
spectroscopic characterization of palladium glycine complexes
that clarify some earlier studies and add new information on struc-
tures, both solid state and in solution. We also look at the series of
N-methylated glycines and the effect of methyl substitution on
structures and isomer formation.
vacuum. The combined yield of single crystals and precipitate
was 33.2 mg of product (91% yield). The same procedure was uti-
1
5
13
lized to prepare the N-enriched (compound 5) and C-enriched
1
5
13
(compound 6) variants, using N and C labeled glycine, respec-
tively. Trans-Pd(C NO (1, 5, 6) were identified on the basis
of the following data: H NMR (400 MHz, 95/5 H O/D O) d 3.55
(t, J = 6.4 Hz, 2H), 3.53 (t, J = 6.6 Hz, 2H). C NMR (126 MHz, 95/5
O/D O) d 189.66 (d, J = 57.5 Hz), 188.04 (d, J = 57.3 Hz), 50.39
(d, J = 57.3 Hz), 48.84 (d, J = 57.5 Hz). N NMR (61 MHz, 95/5
H
2 4
2 2
)
1
2
2
1
3
H
2
2
1
5
+
+
H
Pd(C
2
O/D
2
O) d À34.89, À26.52. HRMS/ESI (m/z): [M+H] calcd for
2
H
4
NO , 254.9597; found, 254.9593. Anal. Calc. for C H N
2
)
2
4 8 2-
O
4
Pd: C, 18.80; H, 3.55; N, 11.01. Found: C, 19.09; H, 3.19; N,
À1
1
1.06%. FTIR (solid state): 3234, 3106, 1592 cm . UV–Vis (H
2
O):
À1
À1
2
. Experimental
kmax = 194 nm,
e
= 26793 M cm
(UV);
kmax = 323 nm,
À1
À1
e
= 244 M cm (Vis).
2.1. General information
2.3. Synthesis of trans-bis-(N,N-dimethylglycinato)palladium(II) (2)
All reagents used in the preparation of the following com-
pounds were purchased from commercial suppliers and used as-
received. Palladium(II) acetate was obtained from Pressure Chem-
ical, Pittsburgh, PA 15201. Palladium(II) chloride was purchased
from Alfa Aesar, Ward Hill, MA 01835. Glycine was purchased from
Qiagen Sciences, Germantown, MD 20874. N,N-dimethylglycine
was purchased from Spectrum Chemical, Gardena, CA 90248. N-
methylglycine and reagent grade solvents (ether, acetone) were
purchased from Sigma–Aldrich, St. Louis, MO 63103. Deuterated
solvents for NMR spectroscopy were obtained from Cambridge Iso-
tope Laboratories, Tewksbury, MA 01876.
A four dram vial was fitted with a magnetic stir bar and charged
with 33.7 mg palladium(II) acetate (0.150 mmol) and 2.5 mL of 2/1
(v/v) acetone/water. The mixture was stirred until all solids had
dissolved. To this mixture was added N,N-dimethylglycine
(31.2 mg, 0.300 mmol) and stirred overnight. The reaction solution
changed from a clear red–orange to a clear pale-yellow superna-
tant with a pale yellow precipitate. The supernatant was trans-
ferred via pipette to a clean vial and allowed to evaporate to give
clear yellow needles which were used for X-ray diffraction. The
precipitate was washed with water and dried under vacuum. The
combined yield of single crystals and precipitate was 43.3 mg of
product (93% yield). Trans-Pd(C H NO ) (2) was identified on
1
H NMR spectra were collected on a Varian MR-400 NMR spec-
1
3
15
trometer. C and N NMR spectra were collected on a Bruker
Avance III 600 MHz NMR spectrometer. High Resolution Mass
Spectra (HRMS) were collected on an Agilent 6220 Accurate Mass
TOF LC–MS. Ultraviolet–Visible (UV–Vis) spectra were collected
on a Hewlett–Packard 8453 spectrophotometer with quartz cuv-
ettes. Solid-state Fourier Transform Infrared (FTIR) spectra were
collected on a Midac M2000 FTIR spectrometer equipped with a
DuraScope diamond ATR accessory. X-ray crystallographic data
were collected at 100 K on an Oxford Diffraction Gemini diffrac-
4
8
2 2
1
the basis of the following data: H NMR (400 MHz, Deuterium
+
+
Oxide) d 3.41 (s, 2H), 2.46 (s, 6H). HRMS/ESI (m/z): [M+H] calcd
for Pd(C H NO ) , 311.0223; found, 311.0216. Anal. Calc. for Pd(C
4
8
2 2
4-
H NO ) : C, 30.93; H, 5.19; N, 9.02. Found: C, 31.05; H, 5.23; N,
8
2 2
À1
9.03%. FTIR (solid state):
kmax = 205 nm,
= 285 M cm (Vis).
m
C@O
À1
1636 cm
.
UV–Vis (H O):
2
À1
e
= 22899 M cm
(UV);
kmax = 316 nm,
À1
À1
e
tometer with an EOS CCD detector and Mo K
a
radiation. Data col-
2.4. Synthesis of trans-bis-(N-methylglycinato)palladium(II) (3)
lection and data reduction were performed using Agilent’s
CrysAlisPro software [19]. Structure solution and refinement were
performed with SHELXL [20], and OLEX2 was used for graphical repre-
sentation of the data [21]. Powder X-ray diffraction data were col-
lected on a Rigaku MiniFlex 600 powder X-ray diffractometer.
All molecular modeling calculations were performed using
GAUSSIAN 09 [22] on the Virginia Tech Chemistry Department Clus-
ter, ‘‘Cerebro’’, using the WebMO interface. Full geometry optimi-
zations and single-point energy calculations of all structures in
water were performed via density functional theory (DFT) with
the Becke-3-parameter exchange functional [23] and the Lee–
Yang–Parr correlation functional [24,25]. Because palladium is
not covered in the cc-pVDZ basis set used, computations involving
Pd employed Stuttgart/Dresden quasi-relativistic pseudopotentials
A four dram vial was fitted with a magnetic stir bar and charged
with 35.2 mg palladium(II) acetate (0.157 mmol) and 3.0 mL of 50/
50 (v/v) acetone/water. The mixture was stirred until all solids had
dissolved. To this mixture was added 31.3 mg N-methylglycine
(0.351 mmol) and stirred overnight. The mixture changed from a
clear red–orange solution to a clear pale-yellow supernatant with
a pale yellow precipitate. The supernatant was transferred via pip-
ette to a clean vial and allowed to evaporate to give clear yellow
needles. The precipitate was washed with water and dried under
vacuum. The combined yield of single crystals and precipitate
was 41.3 mg of product (93% yield). Trans-Pd(C H NO ) (3) was
3
6
2 2
1
identified on the basis of the following data: H NMR (400 MHz,
D O) 3f/3g: d 3.70 (dd, J = 17.1, 8.7 Hz, 1H), 3.45 (dd, J = 16.8,
2
[
26].
7.9 Hz, 1H), 2.47 (s, 3H); 3b/3c: d 3.91 (d, J = 16.8, 1H), 3.33 (d,
J = 16.8, 1H), 2.60 (s, 3H); 3d/3e: d 3.93 (d, J = 16.7, 1H), 3.35 (d,
+
+
2.2. Synthesis of trans-bis-(glycinato)palladium(II) (1, 5, 6)
J = 16.6, 1H), 2.65 (s, 3H). HRMS/ESI (m/z): [M+H] calcd for Pd(C3-
NO , 282.9910; found, 282.9930. Anal. Calc. for Pd(C NO
H
6
2
)
2
H
3 6
2 2
) :
A four dram vial was fitted with a magnetic stir bar and charged
with 32.2 mg palladium(II) acetate (0.143 mmol) and 3.0 mL of 50/
0 (v/v) acetone/water. The mixture was stirred until all solids had
C, 25.50; H, 4.28; N, 9.91. Found: C, 25.56; H, 4.34; N, 9.91%. FTIR
À1
(solid state): 3107, 1625 cm . UV–Vis (H
2
O): kmax = 202 nm,
À1
À1
À1
À1
5
e = 37,605 M cm (UV); kmax = 321 nm, e = 373 M cm (Vis).
dissolved. To this was added 27.1 mg glycine (0.361 mmol) and
stirred overnight. The reaction solution turned from a clear red–
orange to a clear pale-yellow supernatant with a pale-yellow pre-
cipitate. The supernatant was transferred via pipette to a clean vial
and allowed to evaporate to give clear yellow needles. The
pale-yellow precipitate was washed with water and dried under
2.5. Synthesis of cis-bis-(glycinato)palladium(II) (4, 7, 8)
A 10-mL Erlenmeyer flask was fitted with a magnetic stir bar,
glycine (0.5573 g, 7.42 mmol) and water (3.0 mL). The mixture
was stirred until solids had completely dissolved then heated in

D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
23
a 70 °C water bath. To this mixture was added palladium(II) chlo-
ride (0.3061 g, 1.73 mmol). Stirring was continued for 30 min. The
reaction solution changed from a red-brown suspension to a clear,
pale orange-red solution. The reaction was cooled in an ice water
bath (0 °C) and then placed in a refrigerator at 4 °C overnight to
give clear yellow needles with an orange-red supernatant. The
supernatant was decanted and the crystals were washed with cold
water (2 Â 1 mL), cold ethanol (2 Â 1 mL), and followed by diethyl
ether (2 Â 1 mL) and dried under vacuum to yield the product
Fig. 2. General reaction scheme for the synthesis of cis-palladium(II)-glycinate
complexes.
(
0.3911 g, 89% yield). The same procedure was utilized to prepare
1
5
13
the N-enriched (compound 7) and C-enriched (compound 8)
1
5
13
variants, using N and C labeled glycine, respectively. Cis-Pd(C2-
NO (4, 7, 8) was identified on the basis of the following data:
H NMR (400 MHz, 95/5 H O/D O) d 3.55 (t, J = 6.4 Hz, 2H), 3.52 (t,
J = 6.5 Hz, 2H). C NMR (126 MHz, 95/5 H O/D O) d 189.65 (d,
J = 57.5 Hz), 188.04 (d, J = 57.4 Hz), 50.39 (d, J = 57.2 Hz), 48.85 (d,
H
4
2 2
)
1
2
2
1
3
2
2
1
5
J = 57.9 Hz). N NMR (61 MHz, 95/5 H
2
O/D
2
O) d À34.85, À26.50.
Pd, 254.9597; found,
4 8 2 4
H N O Pd: C, 18.80; H, 3.55; N, 11.01.
+
+
HRMS/ESI (m/z): [M+H] calcd for C
54.9592. Anal. Calc. for C
4 8 2 4
H N O
2
Found: C, 19.09; H, 3.19; N, 11.06%. FTIR (solid state): 3221,
À1
À1
3
118, 1625 cm . UV–Vis (H
2
O): kmax < 190 nm,
e
> 21,108 M
-
À1
À1
À1
cm (UV); kmax = 324 nm,
e = 182 M cm (Vis).
Fig. 3. Thermal ellipsoid plot of the molecular structure of crystalline trans-
bis(glycinato)palladium(II), 1. Thermal ellipsoids are shown at the 50% probability
level.
3
. Results and discussion
In the following discussion, the trans bis amino acid complexes
, 2, 3, 5 and 6 were synthesized via the following general reaction
1
scheme (Fig. 1).
The cis complexes 4, 7 and 8 were synthesized in a slightly dif-
ferent manner, as shown in Fig. 2, below.
The simplest Pd(II)-amino acid complexes are those of the bis-
glycinato chelates. Glycine, being achiral, is limited to the forma-
tion of only the cis or the trans isomers of the complex. Compounds
1
, 5, and 6 were prepared as the trans isomer via the scheme shown
in Fig. 1 and confirmed by X-ray crystal structure analysis (Fig. 3).
The complex crystallizes in the P2 /n space group. Pd–N and
1
Pd–O bond lengths are 2.033 and 1.997 Å, respectively. N–Pd–O
bond angles are 83.959 degrees for each chelate ring and 96.041
degrees between the chelate rings. All bond lengths and angles
are within the ranges reported for similar d8 metal chelates, and
our 100 K data offers significant improvement in precision over
that reported by Baidina at room temperature [14]. In addition,
the current structure was refined in P2
has the smaller oblique value of b. Molecular modeling calculations
Gaussian, B3LYP with cc-pVDZ and SDD basis sets) place the trans
1
/n in which the unit cell
(
isomer 0.6 kJ/mol lower in energy than the cis isomer [22]. Within
error, then, cis and trans isomers of the bis-glycinate are the same.
Intermolecular hydrogen bonding is observed between the amine
protons and the non-coordinated carboxyl oxygen atoms (Fig. 4).
1
The H NMR spectrum in 95/5 H
2
O/D
2
O shows two triplets for
the methylene protons at d 3.55 and 3.53 ppm, which are shifted
upfield from the non-coordinated ligand at 3.57 ppm, indicating
that these protons are slightly more shielded upon coordination.
Fig. 4. Crystal packing diagram of (1) as viewed along [100] showing the
intermolecular hydrogen bonding motif.
Interestingly both resonances collapse to singlets in D
2
O, indicting
that proton exchange does occur at the coordinated nitrogen atom.
Fig. 1. General reaction scheme for the synthesis of trans-palladium(II)-glycinate complexes.

2
4
D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
Krylova reported resonances for both –NH
singlets) and –CH – (3.09 ppm, multiplet) groups in a mixture of
the cis and trans glycinate isomers in DMSO-d [16]. The amine res-
onances disappeared on addition of D O, confirming what we
2
(d 4.85 and 4.51 ppm,
water, the cis complex is less stable by 17.3 kJ/mol but in the
2
absence of water, the difference increases greatly to 58.8 kJ/mol.
In the crystal lattice no intermolecular complex to complex hydro-
gen bonding is present; however, hydrogen bonding between
water molecules in the crystal lattice and the carboxylate oxygen
6
2
observed with respect to amine proton exchange. They report that
the methylene protons resolved into two singlets at d 3.14 and
atom was observed (Fig. 6).
1
3
2
.04 ppm on addition of D O and use this as evidence of the two
The H NMR spectrum in D
2
O shows a singlet for the methylene
isomers, providing a proposed pathway for cis–trans isomerization
through a DMSO-coordinated trigonal-bipyramidal intermediate.
Our data show no evidence of isomerization in H O or D O, which
2 2
protons at d 3.56 ppm, which is shifted upfield from the non-coor-
dinated ligand at 3.71 ppm. The methyl groups resonate at d
2.60 ppm versus 2.91 ppm in the free ligand indicating that these
protons are more shielded on coordination to the metal center.
Again, the typical isotopic splitting pattern for palladium is
observed in the HRMS spectrum with peaks at 309.0217,
310.0231, 313.0219, and 315.0230 amu.
In an effort to more fully understand the effects of N-methyla-
tion on hydrogen bonding, on molecular geometry and on crystal
packing, trans-bis-(N-methylglycinato) palladium(II) was synthe-
sized (compound 3, Fig. 7) using N-methylglycine as the ligand.
N-methylglycine allows a degree of hydrogen bonding and ste-
ric control in between that of glycine and N,N-dimethylglycine.
There are seven different isomeric forms possible with this com-
plex (Fig. 8). The complexes can be cis or trans. The cis isomer
can have three possible stereoisomers: the meso compound (3a)
and the enantiomeric pair cis-(S,S) (3b) and cis-(R,R) (3c). The trans
isomer has four possible stereoisomers: trans-(R,R) (3d), trans-(S,S)
(3e), trans-(S,R) (3f), and trans-(R,S) (3g).
is expected given that free water is not likely to coordinate strongly
to a chelated square planar palladium(II) center and displace a che-
1
15
late. H NMR spectra of the N-enriched trans complex 5 were also
obtained and showed the same triplets for the methylene protons.
The amine protons were not observed, however two-bond J15NCH
coupling of 0.9 Hz was observed for the trans glycine complex.
1
3
C NMR spectra show two peaks each for the carbonyl (189.66
and 188.04 ppm) and methylene carbons (50.39 and 48.84 ppm),
1
15
and H– N HSQC NMR spectroscopy shows two resonances for
the amine nitrogens at À34.89 and À26.52 ppm. The typical isoto-
pic splitting pattern for palladium was observed in the HRMS spec-
trum with peaks at 252.960, 253.961, 256.962, and 258.962 amu.
An analogous complex (compound 2) in which the amine pro-
tons were both replaced with methyl groups was synthesized in
an effort to understand the effects of N-alkylation on the formation
of Pd(II)-AA complexes. When N,N-dimethylglycine was used as
the ligand, trans bis-(N,N-dimethylglycinato) palladium(II) is
formed (Fig. 5).
X-ray crystallography indicates that compound 3 crystallizes as
the trans-(R,S) isomer, 3f. The complex crystallizes in the P2 /n
1
X-ray crystallographic analysis confirmed formation of the trans
isomer, which would be expected based on steric considerations.
Placing the amine nitrogens adjacent to one another results in such
a degree of steric crowding between the N-methyl groups that for-
mation of the cis complex is not favorable. The trans complex, 2,
space group with Pd–N and Pd–O bond lengths of 2.0341 and
1.9921 Å, respectively. N–Pd–O bond angles are 84.639 degrees
in the chelate ring and N–Pd–O bond angles between the chelate
rings are 95.361 degrees. All bond lengths and angles are within
8
the ranges reported for similar d metal chelates. DFT calculations
crystallizes in the P2
are 2.0481 and 2.0010 Å, respectively. N–Pd–O bond angles are
3.474 degrees in the chelate ring and N–Pd–O bond angles
1
/c space group. Pd–N and Pd–O bond lengths
(WebMO/Gaussian, B3LYP with cc-pVDZ and SDD basis sets) car-
ried out on a water solution show that the trans-(R,S)/(S,R) isomers
have the lowest calculated steric energy of all the isomers with the
next lowest energy isomer set, the trans-(S,S)/(R/R) being +0.52 kJ/
mol higher in energy; space filling models suggest that the cis-
meso isomers may be unfavorable, however the calculated energy
values are not unreasonable (Table 1) [22]. The isomers are ranked
in the following order based on calculated molecular energy: trans-
(R,S)/(S/R) < trans-(R,R)/(S,S) < cis-(R,R)/(S,S) < cis-meso. However,
one cannot make much of a 0.5 kJ/mol difference – within error,
these values are the same.
8
between the chelate rings are 96.531 degrees. All bond lengths
8
and angles are within the ranges reported for similar d metal che-
lates. Molecular modeling calculations (Gaussian, B3LYP with
cc-pVDZ and SDD basis sets) identify the trans isomer as the most
stable isomer, the exact value depending upon the use of water
solvent or no solvent in the calculations [22]. In the presence of
The amine proton forms an intermolecular hydrogen bond with
the carboxylate oxygen of an adjacent molecule (Fig. 9).
1
The H NMR spectrum of a single crystal of 3 in D
2
O shows a
complex mixture of products. Three singlets at d 2.47, 2.60 and
.65 ppm correspond to the amine methyl groups and suggest that
2
there are three isomers present in solution. The ratio of these peaks
is approximately 9:2:3, which would correspond to a 65/15/20 per-
cent isomeric distribution. The methylene protons in the chelate
backbone give rise to a complicated set of resonances from 3.3 to
4
.0 d. In order to resolve the methylene resonances a 1D NOESY
experiment was performed. Irradiation of the methyl resonance
at d 2.47 shows a corresponding response in the peaks at d 3.45
and 3.70 ppm, which correspond to a pair of doublets of doublets.
Integration of these resonances yields peaks areas of 3:1:1. Irradi-
ation at d 2.60 ppm yields a methylene response at d 3.33 and
3
.91 ppm corresponding to a pair of doublets. Integration again
shows a ratio of 3:1:1. Finally, irradiation at d 2.65 ppm shows that
the resonances at 3.35 and 3.93 correspond to another pair of dou-
blets with area ratios of 3:1:1. A peak at 282.9930 amu in the
+
HRMS corresponds to the [M+H] parent ion, and the typical isoto-
pic splitting pattern for palladium is observed with peaks at
280.9977, 281.9975, 284.9935, 286.9947.
Fig. 5. Thermal ellipsoid plot of the molecular structure of crystalline trans-Bis-
(N,N-dimethylglycinato) palladium(II) hydrate, 2. Thermal ellipsoids are shown at
the 50% probability level.

D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
25
Fig. 6. Crystal packing diagram of (2) as viewed along [101] showing the intermolecular hydrogen bonding motif.
amine nitrogen to a position trans to N1. The carboxylate oxygen
atoms coordinate to form the chelate ring resulting in the trans
complex.
X-ray crystallography confirms formation of the cis isomer from
2
PdCl starting material. The complex crystallizes in the C2/c space
group. Pd–N and Pd–O bond lengths are 2.006 and 2.017 Å, respec-
tively. N–Pd–O bond angles are 84.21 degrees in the chelate ring.
The N–Pd–N bond angles between the chelate rings are 96.156
degrees and the O–Pd–O bond angles between the chelate rings
are 95.418 degrees. There are three water molecules in the lattice;
two are intermolecularly hydrogen bonded between the amine
protons and non-coordinated carboxylate oxygen atoms in adja-
cent molecules, and one is intramolecularly hydrogen bonded to
the coordinated oxygen atoms of a single molecule (Fig. 11). As
before with the trans complex 1, our 100 K data provide greater
precision when compared with the room temperature data
reported by Baidina [13]. One key difference is that we report
hydrogen atom positions for the structure whereas Baidina did not.
Fig. 7. Thermal ellipsoid plot of the molecular structure of crystalline trans-bis-(N-
methylglycinato) palladium(II), 3. Thermal ellipsoids are shown at the 50%
probability level.
To complete the picture, the cis glycine complexes (compounds
1
4
, 7, 8, Fig. 10) were prepared following the procedures set forth by
Chornenka et al. [17]. In this case palladium(II) chloride was used.
Trans-effect theory predicts that a PdCl precursor would be
As in the case for the trans complex, the H NMR spectrum in
95/5 H
2
O/D
2
O shows two triplets for the methylene protons at d
O.
2
3.55 and 3.52 ppm. Both resonances appear as singlets in D
2
1
15
favored to yield the cis complex. If palladium has a higher affinity
for nitrogen than oxygen, based on hard/soft acid–base theory,
then the two chlorine ligands direct the incoming amine nitrogen
atoms to positions trans to the chlorine atoms. This places the
two nitrogen atoms cis to each other. Dissociation of the chloride
ligand then opens up the adjacent coordination site, and formation
of the chelate ring occurs with coordination of the carboxylate oxy-
gen. Indeed, molecular modeling calculations (Gaussian, B3LYP
with cc-pVDZ and SDD basis sets) confirm that the cis-(N)-mono-
dentate glycine dichloride complex is favored by approximately
The H NMR spectrum of the N-enriched cis complex 7 was also
obtained and showed the same triplets for the methylene protons.
The amine protons were again not observed; two-bond J15NCH cou-
pling of 0.7 Hz was observed for the cis glycine complex. The range
of J15NCH couplings reported in the literature span from 0.6 to 1.4 Hz
[27]. The difference observed of 0.2 Hz between cis and trans iso-
mers may therefore lend itself as another tool for the determina-
1
3
tion of chelate geometry in solution. C NMR spectra again show
two peaks each for the carbonyl (189.65 and 188.04 ppm) and
methylene carbons (50.39 and 48.85 ppm), and the 1H– N HSQC
NMR spectrum shows two resonances for the amine nitrogens at
À34.85 and À26.50 ppm. Taken as a whole, the NMR data of the
bis-glycine complexes strongly suggest the formation of an aquo
complex in solution.
15
2
0.6 kJ/mol over the trans isomer [22]. As discussed above for com-
pound 1, when utilizing palladium(II) acetate as the starting mate-
rial we observe formation of the trans isomer. In this case
coordination of the first amine nitrogen (N1) directs the second

2
6
D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
Fig. 8. The seven possible isomers of bis-(N-methylglycinato) palladium(II). Clockwise from upper left: cis-meso, cis-(S,S), cis-(R,R), trans-(R,S) = trans-(S,R), trans-(S,S), and
trans-(R,R).
Table 1
DFT calculated energies for Pd(NMG)
2
isomers.
Pd-(NMG)
isomer (Fig. 8)
2
D
E from lowest energy
DE from lowest energy
isomer, kJ/mol no solvent
isomer, kJ/mol in water
3
3
3
3
b, 3c
a
d, 3e
f
+13.7
+6.5
+0.52
0.00
+55.2
+49.9
+0.38
0.0
Fig. 10. Thermal ellipsoid plot of the molecular structure of crystalline cis-
bis(glycinato)palladium(II) trihydrate, 4. Thermal ellipsoids are shown at the 50%
probability level.
confirm that the second species present is an aquo complex and
1
3
not due to isomerization in solution. C NMR data were collected
at 25, 50, and 75 °C, and again at 25 °C. The integrated peak ratios
are summarized in Table 2.
The data clearly show that isomerization is not occurring and
that generation of the aquo complex is favored at higher tempera-
tures. Molecular modeling calculations (Gaussian, B3LYP with cc-
pVDZ and SDD basis sets) were performed in an attempt to model
the various aquo complexes; those data are summarized in Table 3.
The calculated energy differences show that dissociation of the
carboxylate group to generate the aquo complex is plausible. When
the carboxylate dissociates the carbonyl carbon moves further away
from the metal center, effectively deshielding it more than the coor-
dinated carbonyl carbon. This is borne out in the NMR data by a
downfield shift in the carbonyl resonance for the minor species. Sim-
ilar shifts are observed for the amine nitrogen and methylene carbon
resonances. The ability of these complexes to form aquo complexes
may hold significant importance to their catalytic activity.
Fig. 9. Crystal packing diagram of (3) as viewed along [100] showing the
intermolecular hydrogen bonding motif.
A variable-temperature, NOE-suppressed 13C NMR experiment
was performed on the cis-bis-(glycinato) complex 8 in order to

D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
27
Fig. 11. Crystal packing diagram of (4) as viewed along [0 2 À ] showing the intermolecular hydrogen bonding motif.
1
3
3
Table 2
Table 4
1
3C VT NMR C@O peak ratios.
Pd–N and Pd–O bond lengths for compounds 1–4.
Temperature, °C
Major:minor peak ratio
Compound
Pd–N, Å
Pd–O, Å
25
50
75
25
4.2:1
3.4:1
2.6:1
4.0:1
1
2
3
4
2.033(3)
2.048(1)
2.033(1)
2.006(3)
1.997(3)
2.0009(9)
1.991(1)
2.017(3)
Enantiomeric selectivity was not determined, since enantioselec-
tivity is not expected due to the lack of a chiral center on the
ligand. Catalytic asymmetric coupling is a key focus of our research
group and future papers will expand and expound upon these
interesting results.
Several observations can be made pertaining to the effects of N-
alkylation on the ligand properties of glycines. From the crystallo-
graphic data, we note for the trans complexes a lengthening of the
Pd–N bonds as alkylation of the nitrogen atom progresses
Table 3
Calculated energy values for cis-glycinate aquo species.
Complex
RB3LYP energy,
Hartrees
D
E from bis-chelate
Bis-chelate
Mono-aquo
Bis-aquo
À695.726905303
À695.682220716
À695.629601043
0
117 kJ/mol
255 kJ/mol; 138 kJ/mol from mono-aquo
(Table 4).
The addition of methyl groups onto the coordinated nitrogen
atom results in sufficient steric bulk such that the nitrogen atoms
are prevented from being drawn closer into the metal center. The
2
cis-Pd(GLY) isomer has considerably shorter Pd–N bonds and
longer Pd–O bonds than any of the trans complexes. In the case
of the trans isomers, so long as there is at least one amine hydrogen
atom remaining then no water molecules are incorporated into the
crystal lattice. All of the trans isomers contain a 2 screw axis in the
1
unit cell.
UV–Vis spectrometric data shows two major electronic transi-
Fig. 12. Model oxidative coupling reaction.
tions for each complex. An ultraviolet absorption corresponding
⁄
to a
p
?
p
transition from the carbonyl group of the amino acid
A simple oxidative coupling reaction between phenylboronic
acid and methyl tiglate (Fig. 12, below) was carried out to deter-
mine the suitability of these complexes as catalysts.
Interestingly, the cis-bis-(glycinato)palladium(II) isomer shows
catalytic activity for the formation of the coupled products,
whereas the trans-bis-(glycinato)palladium(II) isomer does not.
ligand occurs at approximately 190–205 nm, with the kmax shifting
to higher wavelengths with increasing alkylation of the amine.
Molar absorptivities (e) for this transition are in the range of
À1
À1
21000–38000 M cm . A visible absorption corresponding to a
d ? d transition is observed at 316–324 nm, with the kmax shifting
to lower wavelengths with increasing amine alkylation. Molar

2
8
D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
Table 5
Selected single crystal X-ray crystallographic data for compounds 1–4.
1
2
3
4
Space group
a (Å)
b (Å)
P2
1
/n
P2
1
/n
P2
1
/c
C2/c
5.6671
8.9795
6.9077
90
105.158
90
5.21929
10.14699
8.91248
90
104.336
90
5.87120
8.68080
12.7612
90
94.8310
90
18.6605
8.0056
6.7424
90
110.415
90
c (Å)
a
(°)
b (°)
(°)
c
3
V (Å )
339.287
2
457.308
2
648.086
2
943.973
4
Z
CCDC #
969582
969583
969584
969585
Fig. 13. Calculated vs. observed powder X-ray pattern for cis-bis-(glycinato)palladium(II) (4).
absorptivities (
e
) for this transition are in the range of 180–
is shortened. The N–H stretches for compound 1 absorb at 3234
À1
À1
À1
À1
3
75 M cm
.
This d ? d transition can be assigned to
a
and 3106 cm ; for compound 3 at 3106 cm ; and for compound
2 the N–H stretches are no longer observed. A summary of
selected single crystal X-ray diffraction data is shown in Table 5.
The issue of how well a single crystal structure reflects the
bulk constitution of a material is a perennial problem. For the
cis and trans glycinate complexes, 1 and 4, the calculated energy
difference between them was small enough, approximately 0.6 kJ/
mol, that it seemed plausible that isomerization in solution might
occur and that single crystal structures do not necessarily repre-
sent the bulk. To address that issue powder X-ray diffraction
spectra were collected on compounds 1, 3, and 4 and compared
to the powder patterns calculated from the single-crystal X-ray
diffraction data. If the bulk consisted of both cis and trans iso-
mers, we would expect to see evidence of that mixture in the
powder pattern. Comparison of the calculated versus observed
powder X-ray diffraction data shows extremely good correlation
HOMO ? LUMO transition from the dz
on molecular modeling calculations.
Fourier transform infrared spectra of these chelates show the
expected absorption bands for carboxylate and amine stretching,
and in the case of the cis and trans glycine isomers agree with
previously published data. The cis isomer (compound 4) shows
2
to the dx
2Ày2
orbitals based
both symmetric and asymmetric stretching carbonyl bands at
À1
1
624 and 1576 cm
.
The symmetric and asymmetric N–H
À1
stretches are evident at 3219 and 3118 cm . Compound 1 shows
a carbonyl stretch at 1612 cm with a shoulder at 1592 cm
Compound 3’s carbonyl stretch is also centered at 1624 cm
with a shoulder at 1587 cm . Compound 2 displays a carbonyl
stretch at 1636 cm with a shoulder at 1607 cm . Evident from
this progression is the fact that, for the trans isomers, the car-
bonyl stretch is shifting to lower wavelengths as the Pd–O bond
À1
À1
À1
.
,
À1
À1
À1

D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
29
Fig. 14. Calculated vs. observed powder X-ray pattern for trans-bis-(glycinato)palladium(II) (1).
Fig. 15. Calculated vs. observed powder X-ray pattern for trans-bis-(N-methylglycinato)palladium(II) (3).
in each case (Figs. 13 and 14), indicating that the isolated product
4. Conclusions
is in fact a single isomer.
For the N-methylglycinate complex 3 the range of calculated
molecular energies is also small, and the NMR data does suggest that
Alkylation of the coordinated nitrogen atom in palladium(II)
bis-glycinates leads to the formation of only trans isomers. There
is a lengthening of the Pd–N and shortening of the Pd–O bonds
as alkylation proceeds from n = 0 to n = 2, where n is the number
of methyl groups on the nitrogen. The electronic spectra show a
shift of the UV absorption to longer wavelength with increasing
alkylation, with a corresponding shift of the visible absorption to
a
mixture of products exists in solution. The calculated
powder pattern (Fig. 15) is again in extremely good agreement with
the observed powder pattern. This seems to indicate that the crystal-
line product is a single isomer but that generation of aquo complexes
in solution is possible.

3
0
D.B. Hobart Jr. et al. / Inorganica Chimica Acta 423 (2014) 21–30
shorter wavelengths. Compelling evidence of the formation of a
palladium–amino acid–aquo complex has been presented and cat-
alytic activity confirmed for the cis complex. We have reported
new X-ray crystal structures and NMR data on the N-methyl and
N,N-dimethyl bis-glycinate complexes. Additionally, improved X-
ray crystal structures and NMR data on the cis and trans bis-glycin-
ate complexes of palladium(II) are reported, as well as confirma-
tion of the previously reported infrared and electronic absorption
spectra. Powder X-ray diffraction has been used to verify formation
of a single isomer in the crystalline phase for the glycinate and N-
methylglycinate complexes. Future papers from our group will
describe the synthesis, characterization, and catalytic activity of
palladium(II)-amino acid complexes possessed of chiral R-groups
on the amino acid ligand.
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The authors would like to acknowledge the Virginia Tech
Department of Chemistry and the Virginia Tech Crystallography
Lab for their generous support and the Hamilton Syringe Company
for their grant of syringes. We further acknowledge Mr. Jeffrey W.
Merola for his assistance in carrying out some of the studies and
Dr. Narasimha Murthy, Ph.D. for his expertise and assistance with
1
15
variable-temperature and H– N HSQC NMR spectroscopy.
Appendix A. Supplementary material
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c.uk/data_request/cif. Supplementary data associated with this
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1
0.1016/j.ica.2014.07.024. These data include MOL files and InChi-
Keys of the most important compounds described in this article.