The synthesis and characterization of new nickel complexes with unusual coordination modes

Article abstract of DOI:10.1016/j.ica.2016.04.008

Two mononickel complexes, K₂[Ni(Hccdp)](CH₃OH)·8H₂O (1), and [Ni(H₂O)₆][Ni(H₂ccdp)(H₂O)₂]₂·12H₂O (2); a dinickel complex, K₂[Ni_2<

Full text of DOI:10.1016/j.ica.2016.04.008

Accepted Manuscript
The Synthesis and Characterization of New Nickel Complexes with Unusual
Coordination Modes
William Seals, Hadi Arman, Steven Batha, Ghezai T. Musie
PII:
S0020-1693(16)30160-8
http://dx.doi.org/10.1016/j.ica.2016.04.008
ICA 16992
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
15 February 2016
4 April 2016
5 April 2016
Please cite this article as: W. Seals, H. Arman, S. Batha, G.T. Musie, The Synthesis and Characterization of New
Nickel Complexes with Unusual Coordination Modes, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/
10.1016/j.ica.2016.04.008
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The Synthesis and Characterization of New Nickel Complexes
with Unusual Coordination Modes
William Seals, Hadi Arman, Steven Batha, Ghezai T. Musie*
Department of Chemistry, The University of Texas at San Antonio, San Antonio, TX
78249. E-mail: ghezai.musie@utsa.edu
Abstract
Two mononickel complexes, K2[Ni(Hccdp)](CH3OH).8H2O(1), and
.
[Ni(H2O)6][Ni(H2ccdp)(H2O)2]2 12H2O(2); a dinickel complex,
K2[Ni2(ccdp)(acac)(H2O)2].7H2O (3); and a tetranickel complex, Cs6[Ni4(ccdp)2(µ-η2-η2-
CO3)2].23H2O (4) (ccdp5- = N,N’-Bis[2-carboxybenzomethyl]-N,N’-Bis[carboxymethyl]-
1,3-diaminopropan-2-ol; acac- = acetylacetonato) have been synthesized and fully
characterized using different techniques including single crystal X-ray crystallography
and electrochemistry. The redox behavior of complexes 1-4 were studied using cyclic
voltammetry either in water or DMF solution systems. Whereas complex 1, 2 and 4
found to be redox active, complex 3 showed no redox activity in water.
1

1. Introduction
Microorganisms have evolved to utilize nickel ions in several different enzymes.
Examples of such enzymes include Ureases, Ni-Fe hydrogenase, CO dehydrogenase,
Methyl-coenzyme M reductase, etc.1,2,3 Coordination environment around the metal
centers in each of these enzyme’s active sites are different. Furthermore, some of these
metal centers are redox active; others are inactive and act as Lewis acid centers for
enzymatic hydrolysis. Ureases are a class of enzymes that catalyze the hydrolysis of urea
to carbon dioxide and ammonia. Urease has always generated a wide research interest
since it was the first enzyme purified and isolated in crystalline form in 1926.4
Additionally, it was the first enzyme found to utilize nickel(II) as a cofactor, which was
curious considering how other metallohydrolases typically utilize a strong Lewis acid,
Zn(II).5,6,7 The first X-ray crystal structure for urease was determined by Karplus and
group in 1995, from Klebsiella aerogenes, and since then many more have been solved
for different types of ureases with greater resolution to produce further insight into the
overall structure of the enzyme and its active site.8-14 As shown in Scheme 1, the active
site contains two nickel(II) ions bridged together by a carbamylated lysine residue and a
bridging deprotonated water molecule.14-16 The Ni(II)---Ni(II) internuclear distances in
ureases of different bacterial sources range from 3.5-3.7Å. Although the reaction
mechanism of the enzyme is still being debated, urease is considered to be one of the
most efficient catalyst found in nature with a Kcat/KM value of about 1014 times higher
than the uncatalyzed reaction.16-18
Although many dinucleating ligands have been synthesized in an effort to mimic
urease, and other metalloenzymes, most of these turn out to be poor functional models as
they lack catalytic activities. Hence, the interest for developing functional/structural
mimics is pursued to help understand the catalysis and mechanism of reactions catalyzed
by the enzyme.
In
this
study,
the
ligand
N,N’-Bis[2-carboxybenzomethyl]-N,N’-
Bis[carboxymethyl]-1,3-diaminopropan-2-ol (H5ccdp) is used to synthesize a series of
systematically altered nickel(II) complexes. While the H5ccdp ligand has two symmetric
NO3 coordination pockets suitable for binding to metal ions in close proximity, its
flexibility and reactivity has yielded various coordination products when reacted with
2

nickel(II) under different reaction conditions. Herein, we report two mononickel
complexes,
K2[Ni(Hccdp)](CH3OH).8H2O(1),
and
.
[Ni(H2O)6][Ni(H2ccdp)(H2O)2]2 12H2O(2);
a
dinickel
complex,
K2[Ni2(ccdp)(acac)(H2O)2].7H2O (3); and a tetranickel complex, Cs6[Ni4(ccdp)2(µ-η2-η2-
CO3)2].23H2O (4). Of particular interest are the binuclear and tetranuclear nickel
complexes, 3 and 4, in which the nickel centers have unsymmetrical donor and
2-
coordination environments, provided by the ligand ccdp5-, H2O, acac- and CO3 ligands.
All the complexes have been characterized by crystal structure analyses and
spectroscopic methods. Moreover, the redox properties of the complexes are studied in
detail.
2. Experimental
2.1. Materials and Methods.
All starting materials were purchased from commercial sources and were used
without further purification. UV-vis spectrometry was performed using an Agilent 8453
UV-visible Spectroscopy System in the spectrum/peak mode using a 1 cm Quartz
cuvette. The ligand H5ccdp, N,N’-Bis[2-carboxybenzomethyl]-N,N’-Bis[carboxymethyl]-
1,3-diaminopropan-2-ol, has been prepared using the reported procedure.19 Magnetic
susceptibility of complexes 3 and 4 was measured at room temperature on a Gouy
.
balance using CuSO4 5H2O as a callibrant.
2.2. Synthesis
2.2.1. Synthesis of K2Ni[Hccdp].8H2O (1)
.
A green wet methanol solution (1:1 by volume, 6mL) of Ni(CH3CO2)2 4H2O
(0.3720g, 1.495mmol) was slowly added into a stirring 6 mL methanolic solution (1:1 by
volume) of H5ccdp (0.500 g, 1.054 mmol) and KOH (0.3548g, 6.323 mmol). The reaction
mixture was then stirred for 19 hrs at room temperature. The dark green solution was
collected and set for crystallization. Dark blue X-ray-quality single crystals were grown
by slow acetone diffusion into the H2O-MeOH filtrate solution. Yield: 0.2607g (34.9%)
3

Anal. Calcd. for C23H38Ni1N2K2O17 : C, 36.75; H, 5.10; N, 3.73; Found: C, 36.17; H,
4.81; N, 3.05. UV λmax (H2O) nm (ε): 358 (23.9), 584 (14.8), 762 (7.8)
.
2.2.2. Synthesis of Ni(H2O)6[Ni(Hccdp)(H2O)2]2 12H2O (2)
.
A wet methanol solution (1:1 by volume, 6mL) of Ni(NO3)2 6H2O (0.1247g,
0.4288 mmol) was slowly added into a stirring 6 mL methanolic solution (1:1 by volume)
of H5ccdp (0.1500g, 0.3164 mmol) and (CH3)4NOH.5H2O (0.1799g, 0.9939 mmol). The
reaction mixture was then left stirring overnight at room temperature. A light blue
solution was obtained, filtered and portions of it were set for crystallization. The light
green X-ray-quality single crystals were grown by slow diethyl ether diffusion into the
light blue filtrate. Yield: 0.0942g (42.8%) Anal. Calcd for C46H88Ni3N4O40 : C, 36.49; H,
5.82; N, 3.70; Found: C, 35.75; H, 5.72; N, 3.46. UV λmax (DMSO) nm (ε): 395 (50.0),
661 (26.8)
2.2.3. Synthesis of K2[Ni2(ccdp)(C5H7O2)(H2O)2].7H2O (3)
A wet methanol suspension (1:1 by volume, 6mL) of Ni(2,4-pentanedionate)2
(0.5420g, 2.109 mmol) was slowly added into a stirring 6 mL water-methanol solution
(1:1 by volume) of H5ccdp (0.5000g, 1.054 mmol) and KOH (0.3551g, 6.329 mmol) at
room temperature. The reaction mixture was left to react while stirring for 19 hrs at room
temperature. A pale green precipitant was formed and was removed after filtration. Dark
green X-ray-quality single crystals were grown by slow diethyl ether diffusion into the
dark green H2O-MeOH filtrate solution. Yield: 0.3163g (37.6%) Anal. Calcd for
C28H43Ni2N2O18 : C, 37.72; H, 4.86; N, 3.14; Found: C, 37.64; H, 4.51; N, 2.59. UV λmax
(H2O) nm (ε): 634 (35.8)
2.2.4. Synthesis of Cs6[Ni4(ccdp)2(µ-η2-η2-CO3)2].23H2O (4)
.
A green wet methanol solution (1:1 by volume, 6mL) of Ni(CH3CO2)2 4H2O
(0.3720g, 1.495mmol) was slowly added into a stirring 6 mL methanolic solution (1:1 by
volume) of H5ccdp (0.250 g, 0.527 mmol) and Cs2CO3 (1.031g, 3.165 mmol). The
reaction mixture was then stirred for 19 hrs at room temperature. The dark green solution
4

was collected and was set for crystallization. Green X-ray-quality single crystals were
grown by slow acetone diffusion into the H2O-MeOH filtrate solution. Yield: 0.5236g
(61.4%) Anal. Calcd for C48H88Cs6Ni4N4O47 : C, 23.00; H, 3.54; N, 2.24; Found: C,
22.03; H, 2.87; N, 1.67. UV λmax (H2O) nm (ε): 388 (78.35), 644 (36.41), 659 (35.49).
Room temperature magnetic study.
2.3. X-ray Crystallography
The data were collected at 98(2) K using a Rigaku AFC12/Saturn 724 CCD fitted
with Mo Kα radiation (λ= 0.71073 Å). Data collection and unit cell refinement were
performed using Crystal Clear software.20 The data was measured in the range 2.20 < θ <
27.5° using ω scans. Data processing and absorption correction, giving minimum and
maximum transmission factors, were accomplished with Crystal Clear20 and ABSCOR21,
respectively. The structure, using SHELXL-97, was solved by direct methods and refined
(on F2) using full-matrix, and least-squares techniques.22, 23 All non-hydrogen atoms were
refined with anisotropic displacement parameters. All carbon bound hydrogen atom
positions were determined by geometry and refined by a riding model. Electron density
peaks were used to identify oxygen bound hydrogen atoms and the displacement
parameters were set to 1.5 times the displacement parameters of the bonded atoms.
2.4. Electrochemical Measurement
Electrochemical measurements were made on a BAS-CV50 electroanalyzer
controlled with a Pentium III computer and utilizing three-electrodes: glassy carbon
working electrode, platinum wire counter electrode, and a Vycor-tipped Ag/AgNO3
reference electrode. Working electrodes were polished to a mirror finish on a microcloth
of diamond or alumina (1.0 and 0.05 mm particles, respectively) and were cleaned
electrochemically. Cyclic voltammograms were obtained from 1.0 mM analyte
concentration in either water or DMF, using 0.1 M KCl or [n-Bu4N][PF6] supporting
electrolytes, respectively. Data were analyzed using the software provided with this
instrumentation. Solutions were degassed with a purge of N2 for 10 min and a blanket of
N2 was maintained over the solution while making the measurements. The iR
compensation between the working and reference electrodes was accomplished by
5

applying the positive feedback from the BAS-CV50 current follower. All potentials were
measured at room temperature and scaled to NHE using either methyl viologen (MV2+/
MV+ literature value is E1/2NHE = -0.45 V Vs NHE in water)24 or ferrocene (Cp2Fe/Cp2Fe+
literature value E1/2NHE = 0.40 V Vs NHE in DMF)25 as an internal standards.
3.
Results and discussion
3.1. Synthesis
As shown in Scheme 2, the ligand H5ccdp was synthesized in two pot reactions. The
synthesis of the precursor reduced Schiff base, H3cdp, was accomplished by the
condensation of stoichiometric amounts of 2-carboxybenzaldehyde and 1,3-diamino-
propan-2-ol in presence of NaOH in methanol under refluxing conditions for 4 hrs,
followed by the subsequent reduction using NaBH4. Acidification of the resulting
solution by addition of HCl to pH of 5 yielded a precipitation of white solid product. The
product was characterized to be a reduced Schiff base, H3cdp·H2O, by different analytical
techniques such as elemental analysis, FTIR and NMR spectroscopy. Alkylation of the
secondary amines of H3cdp with stoichiometric quantities of iodoacetic acid produced the
dinucleating ligand, H5ccdp, in good yield. The reaction of Ni(II) salt with H5ccdp in
several different molar ratio in presence of mild bases, such as (CH3)4NOH, NEt3 or
K2CO3, or strong bases such as NaOH in methanol at room temperature conditions for
several hours afforded different complexes, which were crystallized into a range of
mono-
to
tetra-nickel
complexes,
K2Ni[Hccdp].8H2O
(1),
.
Ni(H2O)6[Ni(Hccdp)(H2O)2]2 12H2O (2), K2[Ni2(ccdp)(C5H7O2)(H2O)2].7H2O (3) and
Cs6[Ni4(ccdp)2(µ-η2-η2-CO3)2].23H2O (4). Details of the synthesis are given in the
experimental section.
3.2. Crystal structures
Cell parameters and data collection summaries for all complexes are given in
Table 1. The molecular structures for complexes 1 - 4 are shown in Fig.s 1, 2, 4 and 5,
respectively; selective bond angles and bond lengths are given in Table 2.
3.2.1. K2Ni[Hccdp].8H2O (1)
6

Crystal structure of the mononuclear complex, 1 consists of one dianionic
[Ni(Hccdp)]2- species, two potassium counter cations along with eight water molecules of
crystallization. A view of the anion of 1 is depicted in Fig. 1. The crystallographic
summary and selected metric data are given in Tables 1 and 2, respectively. The
molecular structure contains a nickel(II) coordinated to the Hccdp4- ligand and two non-
coordinated potassium counter cations. The coordination geometry around the nickel ion
is best described as a distorted octahedral geometry with an N2O4 environment where
there are two aliphatic carboxylate oxygens, two aromatic carboxylate oxygen and two
tertiary amine nitrogens of the ligand. The nickel ion is coordinated to N(1) and N(2)
from the tertiary amines and to O(3) and O(6) from the aliphatic carboxylate oxygens to
form the equatorial plane. Both the amines and the carboxylate oxygens are coordinated
to the nickel center in cis- fashion of the plane. The O(1) and O(8) from the aromatic
carboxylate oxygen atoms reside trans to each other occupying the axial positions of the
octahedron. The major geometrical distortion of the originates from the large bite angle
[O(3)-Ni-O(6) = 101.68o] as well as in the small bite angles [O(3)-Ni-N(1) = 82.57o];
[O(6)-Ni-N(2) = 81.20o]; and [O(3)-Ni-O(8) = 84.00o] of the dinucleating ligand. One
interesting feature of the crystal structure of complex 1 is that, although the pendant
alcoholic part of the ligand is expected to act as a bridging alkoxo group, it is turned out
of the coordination environment. The H-bond network systematically distributed through
the crystal lattice, as shown in the packing diagram, has contributed towards the overall
stability of the crystal lattice. The extended network of H-bonding is constructed through
the water molecules, alcoholic-OH, aliphatic-COO-, and aromatic-COOH groups of the
ligands. The bond distances and bond angles for complex 1 are in well within the range
of metric values of similar nickel(II) complexes reported in the literature.26-39
.
3.2.2. Ni(H2O)6[Ni(Hccdp)(H2O)2]2 12H2O (2)
Crystal structure of complex 2, consists of two monoanionic [Ni(H2ccdp)]-
2+
species, a Ni(H2O)6 counter cation and eleven cocrystallized water molecules engaged
in stabilizing the crystal lattice. An ORTEP that shows the anion part of complex 2 is
presented in Fig. 2, and crystallographic summary and selected metric data for the
structure are listed in Tables 1 and 2, respectively. The monoanionic species of complex
7

contain a nickel(II) center coordinated to the dinucleating ligand H2ccdp3- and two water
molecules. The coordination geometry around the nickel ion is best described as a
distorted octahedral geometry formed by the alcoholic oxygen, acetate oxygen, benzoate
oxygen and tertiary amine nitrogen of the ligand in addition to the two oxygen atoms
form water molecules. Whereas the O(8) and O(6) from the respective benzoate and the
acetate carboxylates groups of the ligand are coordinated to the nickel(II) center in a
trans fashion, the O(10) and O(11) of the two water molecules are cis- to each other. The
major distortion of the resulting octahedral coordination sphere originates in the large bite
angles for the O(10)-Ni-N(2) = 100.6o and O(10)-Ni-O(8) = 97.6o as well as in the small
bite angles for the O(6)-Ni-N(2) = 76.14o, O(6)-Ni-O(10) = 83.5o and O(5)-Ni-N(2) =
83.1o of the ligand. One interesting feature of the crystal structure of complex 2 is that
the pendant alkoxo arm of the ligand and that one of the tertiary amines (N2) is
protonated and thus only one Ni(II) cation is incorporated into the coordinating pocket of
the unprotonated side of the dinucleating ligand. The stability of this structure is assumed
due to the eleven waters of crystallization which stabilize the system by forming a strong
H-bond network systematically distributed through the lattice, Fig. 3. The extended
network of the H-bonding is constructed through the coordinates and free water
molecules, alcoholic –OH, aliphatic –COO-, aromatic –COOH and ammonium groups of
the vacant site of the ligand as well as the hexa-aqua nickel cation that sits in the
crystallographic center if symmetry. Previously, we have reported literature similar
network of H-bonding that stabilized Zn(II) complexes.19 The rest of the bond distances
and bond angles observed for complex 2 are consistent with values reported in the
literature.30-41
3.2.3. K2[Ni2(ccdp)(acac)(H2O)2].7H2O (3)
The crystal structure of the dinuclear complex 3 consists of one dianionic
[Ni2(ccdp)(acac)(H2O)2]2- specie, and two potassium counter cations. A view of the
dianion portion of complex 3 is depicted in Fig. 4, and selected metric data are presented
in Table 2. The dianionic complex consists of two Ni(II) ions bridged by the alkoxo
group, the ccdp5- ligand, two water molecules coordinated to Ni(1), and exogenous
acetylacetonato ligand coordinated to the Ni(2) center in a bidentate fashion is
8

crystallized in P1 space group. Despite the fact that ccdp5- is a symmetrical dinucleating
ligand, the coordination environments around each of the Ni(II) centers are not
symmetrical. The crystal structure of the complex indicates that the coordination of the
binucleating ccdp5- ligand to the two Ni(II) centers is best described as octahedral
geometries with each metal center missing cis--sites. The missing cis--sites are, then,
taken with two water molecules and acetylacetonate ion around Ni (1) and Ni(2) center,
respectively. While the equatorial plane around Ni(1) is defined by the bridging alkoxo,
the tertiary amine and the two water molecules, the equatorial plane around the Ni(2) has
the bridging alkoxo, tertiary amine and the acetylacetonate group coordinated in a
bidentate fashion. While the acetate and the benzoate carboxylate arms on one end of the
ccdp5- ligand take the axial positions around Ni(2), the acetate and the benzoate
carboxylate on the other end of the ligand take equatorial positions on a cis—fashion to
each other around Ni(1). The two acetate and two benzoate arms of the ligand appear at
the opposite sides of the plane defined by the propyldiamine backbone and the two nickel
centers. Recently, we have reported similar configuration of the ccdp5- ligand around a
polynuclear cobalt complex.42 The degree of distortion around the Ni(1) center is
pronounced by the large bite angle of O(13)-Ni(1)-N(1) = 107.5o as well as in the small
bite angles for the O(5)-Ni(1)-N(1) = 82.0o, O(13)-Ni(1)-O(12) = 82.1o and O(3)-Ni(1)-
N(1) = 82.3o. Despite the fact that the Ni(2) is coordinated to the ccdp5- ligand in a
similar manner to the Ni(1) center, the octahedral distortions are not as severe as around
Ni(1). Slight distortions are observed in the O(6)-Ni(2)-O(11) = 93.72o and O(6)-Ni(2)-
N(2) = 79.79o bite angles. Another interesting structural feature of the complex is the
133.4o for the Ni(1)-O(5)-Ni(2) bond angle. The degree is wider than expected for a
tetrahedral oxygen but is slightly smaller to values observed for similar complexes
reported in the literature.29 The average Ni-Owater, Ni-Obenzote, Ni-Oacetate, Ni-Oalkoxo, Ni-
Oacac and Ni-Namine bond distances are 2.110, 2.076, 2.081, 2.042, 2.029 and 2.075 Å,
respectively. The observed bond lengths are comparable to values reported for similar
complexes in the literature. 19, 26-50
The internuclear Ni(1)---Ni(2) distance in complex 3 is 3.75 Å. While this
distance is at the long end range of distances observed in the active site of the urease, it is
9

comparable to distances in enzyme model dinickel complexes reported in the
literature.29,38,39,44-50 Moreover, the nickel coordinated water molecules present an
opportunity as a good leaving group for substrate binding and/or protonation of urea in
hydrolysis. The fact that one of the water molecules has an extended bond length to the
nickel is a testament to its candidacy for a good leaving group in presence of substrates.
Hence, Complex 3 is an suitable structural model for the active site of urease, which
contains two nickel(II) ions bridged together by a carbamylated lysine residue and
deprotonated water molecule, histidine and carboxylate ligands, Scheme 1.6
3.2.4. Cs6[Ni4(ccdp)2(µ-η2-η2-CO3)2].23H2O (4)
The unit cell of the crystal structure contains a tetranuclear Ni(II) complex anion,
4, consists of two trianionic [Ni2(ccdp)( µ-η2-η2-CO3)]3- fragments, and six cesium
counter cations, along with 23 water molecules of crystallization. While the ORTEP of
the molecular structure of the complex 4 anion is shown in Fig. 5, selected metric data for
the structure are summarized in Table 2. The X-ray characterization of 4 reveals that the
complex is terta-nickel cluster produced as a result of self-assembly of two [Ni2(ccdp)]-
2-
units and two bridging CO3 ions coordinated to the nickel centers in the rare anti-anti,
pseudo-oxo, µ-η2-η2−carbonato fashion. The coordination geometries around each Ni(II)
center is best described as a distorted octahedral geometry defined by NO5 coordination
environments. All the nickel centers are coordinated to the µ-alkoxo, the acetate oxygen,
the benzoate oxygen, the tertiary amine nitrogen of the ccdp5- ligand and two oxygens
from the bridging carbonato ligand. The octahedral geometry around Ni(1) is defined by
the O(10), O(11), O(5), and N(1) atoms at the equatorial positions and the O(1) and O(3)
at the axial positions. While the benzoate and the acetate arms of ccdp5- ligand take the
distorted axial positions, the amine, the alkoxo and the carbonate groups occupy the
equatorial positions of the octahedral. The bond angles for O(1)-Ni(1)-O(3), O(5)-Ni(1)-
O(10), O(10)-Ni(1)-O(11), O(10)-Ni(1)-N(1) and O(5)-Ni(1)-N(1) are 169o, 105o, 63o,
105o, 87o, respectively. Similar coordination behaviors were observed around other three
Ni(II) centers of the complex. These metric values indicate the severity of distortion of
10

the octahedral geometry around the nickel centers. Furthermore, the crystal structure
indicates displacement of the nickel centers from their respective equatorial planes by
0.08 Å and 0.11 Å for Ni(1) and Ni(2), respectively.
One other interesting feature of the structure is the 142º bond angles between the
two nickel centers bridged by the alkoxo moiety of ccdp5- ligand, Ni(1)-O(5)-Ni(2). The
rather large bond angles observed in this structure is rare among similar complexes
reported in the literature.26,46,51-61 This bond angle indicates that the alkoxo oxygens take
a curious closer to sp2 than sp3 hybridization. This hybridization on oxygen is quite
2
2
interesting, with the lone pair on the pz orbital overlapping with the partially filled dx -y
orbital from Ni(II) ion. When compared to the Ni-O-Ni bond angle observed in complex
3, it is interesting to note that the angles are larger in complex 4. The observed large
bond angle is attributed to the highly flexible nature of the ccdp5- ligand to open up the
angle upon binding to the carbonato bridges in making complex 4.
The average Ni-Obenzoate, Ni-Oacetate, Ni-Oalkoxo and Ni-Namine bond distances are
2.045, 2.116, 2.038 and 2.060 Å, respectively. These bond lengths are comparable to
values reported for similar complexes in the literature but longer than other reported
phenoxo-O bridged dinuclear Ni(II) complexes.27,28,44,45,49,50 Whereas the alkoxo bridged
average Ni---Ni internuclear distance is 3.86 Å, a longer carbonato bridged Ni---Ni
average distance of 4.28 Å is observed. While the former Ni---Ni internuclear distance is
slightly longer than observed in complex 3 and other alkoxo bridged dinuclear nickel
complexes in the literature, the latter is shorter than distances reported for similar
complexes reported in the literature.27,28,44,45 As the result of this rather rare coordination
ability the carbonato ligand, a cluster with a distorted octagonal shape cavity defined by
Ni(1)-O(5)-Ni(2)-O(12)-Ni(2a)-O(5a)-Ni(1a)-O(10) atoms is formed. The front and side
of the cavity are depicted in Fig. 6. The Ni(1)---Ni(2a) internuclear distance, 5.75 Å, can
be considered as reasonable approximation of the diametric size of the cavity. The Ni(1)-
O(10)-Ni(1A) and Ni(2)-O(12)-Ni(2A) bond angles of 173 and 170º, respectively,
constructs an interesting slightly distorted irregular hexagon out of the octagonal
geometry. The other observed bond distances and bond angles for complex 4 are
consistent with values reported in the literature.19,26-50,56,57,60,61
11

Several coordination modes of carbonato ligands with polynuclear nickel
complexes have been reported in the literature.27,28,62 However, the observed µ-η2-η2-
cabonato coordination, as shown in Fig. 6, to produce the tetranuclear nickel complex has
not been previously reported. The coordination modes described in the literature for the
carbonate ion as a bridge in polynickel complexes is summarized in Scheme 3.27,28,62
3.3. Electrochemical Study
The electrochemical behavior of complexes 1-4 were studied using cyclic
voltammetry at room temperature in water or DMF solutions containing 0.1 M KCl and
(Bu)4NPF6 supporting electrolytes, respectively. The electrochemical data are presented
in Table 3. All potentials are referenced to NHE using either ferrocene or methyl
viologen as internal standards. Except complex 3, all show some form of redox behavior
within water and DMF potential windows. Under similar experimental conditions, the
electrochemical studies of the free ligand using the cyclic voltammetry showed no
electrochemical activity within the scanned potential window. Thus, the electrochemical
waves observed in the cyclic voltammagrams of the complexes are metal based.
The metal based reduction potential data for the complexes is summarized in
Table 3. As presented in the table, complex 1 showed one quasi reversible reduction at
1.29 V (∆Ep = 100 mV). The values of ∆Ep and peak current ratio of the wave at 1.29 V
are slightly bigger than what is expected for one-electron transfer processes. This redox
event for complex 1 and its E1/2 value is constant with different scan rates (25-500 mV/s).
Due to poor water solubility, the electrochemical measurement of complex 2 was
performed in dry DMF. The complex showed a persistent irreversible reduction peak at -
0.92 V with in the potential window. Whereas complex 3 didn’t display any redox
activity in water, under the same electrochemical conditions, complex 4 showed two
irreversible waves centered at potential of 1.15 and 1.00 V. The large anionic charge of
the complex 4 is partly responsible for the considerably resistance to reduction as
indicated to by the Epc potential value of 1.15 V when compared to the rest of the
complexes in this report.
3.4. Magnetic Study
12

Whereas the dinickel complex 3 shows an effective magnetic moment, µeff, of
4.01 µB, the tetranickel complex 4 shows a room temperature magnetic moment value of
4.62 µB. These values are lower than that expected for two and four noninteracting
nickel(II) centers in an octahedral geometry. These indicate that the nickel(II) centers in
complexes 3 and 4 are antiferromagnetically coupled. The measured magnetic moments
for complex 1 and 2 are 2.96 and 2.98 µB, respectively. These values are expected for
mononickel(II) complexes in octahedral geometries as two unpaired electrons are
responsible for the overall magnetic moment.
4. Conclusion
Four new nickel complexes of the ligands H5ccdp with interesting coordination chemistry
have been synthesized and fully characterized. The nickel centers exhibited different
coordination numbers and several coordination modes with the carboxylate and carbonato
groups of the ligands. Whereas, the nickel center of complex 1 is in a N2O4 octahedral
environment involving a single ligand wrapping around Ni(II), the other single
mononickel complex, 2, has a NO5 coordination sphere utilizing a single coordination
pocket of the ligand and two waters to complete the octahedral environment. For
complex 3, an asymmetric dinickel(II) complex was realized as the two Ni(II) centers
acquire a NO5 coordination environments with Ni(1) coordinated with two water
molecules and Ni(2) with a single acetylacetonate exogenous ligands. On the other hand,
a highly symmetrical tetranickel complex 4 with four NO5 Ni(II)s with a rare µ4
carbonato bridging motif has been characterized. With the exception of complex 3, all
complexes showed fairly accessible redox activities in water. The unusually larger
anionic charge of the complex 4 is partly responsible for the considerably accessible
oxidation potential when compared to the rest of the complexes in this report. Room
temperature magnetic study of complex 3 and 4 suggested that the metal centers are
antiferromagnetically coupled. Overall, these studies indicates that H5ccdp is a highly
flexible ligand that provides a highly versatile and tunable coordination scaffold for
nickel(II).
13

Acknowledgements
We are indebted to the Robert A. Welch Foundation for their support of this work under
Grant AX-1540, and the Chemistry Department at the University of Texas at San Antonio
for the support.
Supporting Information: The cyclic voltammograms in aqueous solution or DMF of
complexes 1 - 4 in 0.1 M KCl or (Bu)4NPF6 with platinum or GC working electrodes at
scan rate ranges of 100 mV/s are reported in Figs 1S-5S.
14

Scheme 1
Lys217
His246
N
N
His134
O
O
N
N
N
Ni
N
Ni
H2O
N
His136
N
N
O
H
OH2
O
O
Asp320
His272
Scheme 1
Schematic depiction of the structure of the active site of Klebsiella
aerogenes urease.6
15

Scheme 2
O
+
H2N
NH2
2
OH
OH
O
1. NaOH / MeOH
2. NaBH4 / MeOH
3. HCl / H2O
N
H
N
H
.
H2O
OH
OH
HO
O
O
H3cdp.H2O
OH
I
1.
O
2. HCl / H2O
HO
OH
O
O
N
N
.
2HCl
OH
OH
HO
O
O
H5ccdp.2HCl
16

Ni
O
O
O
Ni
O
O
O
Ni
O
Ni
Ni
Ni
O
O
O
Ni
Ni
Ni
Ni
O
O
O
O
O
4
1
2
3
Ni
O
O
O
Ni
Ni
Ni
O
O
Ni
O
O
Ni
Ni
O
O
Ni
O
O
Ni
O
O
O
O
Ni
Ni
Ni
Ni
5
6
7
8
Scheme 3 Cast of carbonato coordination modes in polynuclear nickel complexes
reported in the literature.
17

Scheme 4. Schematic representation of the synthesis of complexes 1-4, shown are only the anioinc complexes.
18

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21

Fig 1. Thermal ellipsoid plot (50% probability) of the core dianionic complex ion,
[Ni(Hccdp)]2- (1).
1

Fig 2. Thermal ellipsoid plot (50% probability) of the core monoanionic complex ion,
[Ni(H2ccdp)]-, (2).
2

Fig 3. Thermal ellipsoid plot (50% probability) showing the extensive H-bonding
network among the three monoanionic complex ions, [Ni(H2ccdp)]- (2), twelve
molecules of water of crystallizations and the [Ni(H2O)6]2+ ion.
3

Fig 4. Thermal ellipsoid plot (50% probability) of the core dianionic complex ion of
[Ni2(ccdp)(acac)(H2O)2]2-, (3).
4

Fig 5. Thermal ellipsoid plot (50% probability) of the anionic complex ion of
[Ni4(ccdp)2(μ-η2-η2-CO3)2]6-, (4).
5

(a)
(b)
Fig 6. (a) Thermal ellipsoid plot (50% probability) of the core anionic complex of 4,
[Ni4(ccdp)2(μ-η2-η2-CO3)2]6-, demonstrating the 4-cabonato and the octagonal cavity defined
by Ni(1)-O(5)-Ni(2)-O(12)-Ni(2a)-O(5a)-Ni(1a)-O(10). (b) Side view of the cavity.
6

Table 1 Crystal data and structure refinement for complexes 1, 2, 3 and 4
Complex
1
2
3
4
C48H40Cs5.3
N4Ni4O34.2
2159.30
Monoclinic
P 21/m
13.597(3)
22.209(4)
14.446(3)
90
Empirical formula C48.40H61.20K4N4 C46H84N4Ni3O40 C28H45K2
Ni2O27
N2Ni2O19
908.6
Triclinic
Formula weight
Crystal system
Space group
a(Ǻ)
b(Ǻ)
c(Ǻ)
1404.83
Triclinic
P-1
1509.30
Triclinic
P-1
P-1
9.1881(16)
12.795(2)
13.942(2)
108.373(3)
99.105(2)
106.5194(13)
1434.9(5)
2,1
10.3408(9)
12.6088(16)
14.1077(17)
88.854(8)
71.459(7)
66.063(5)
1581.3(5)
2,1
12.063(2)
12.876(2)
13.704(2)
63.940(7)
81.347(7)
89.942(9)
1885.2(5)
2,1
α(°)
β(°)
γ(°)
109.327(3)
90
4116.5(15)
2,1
Volume(Ǻ3)
Z, Z'
ρ(calc.)
λ
Temp.(K)
F(000)
1.623
0.71073
98(2)
1.585
0.71073
98(2)
1.513
0.71073
98(2)
1.742
0.71073
98(2)
728
792
880
2052
μ(mm-1)
Tmin, Tmax
2θrange(°)
Reflections
Collected
Independent
reflections
1.038
0.991
1.291
0.8641, 1.000
2.59 to 27.50° 2.37 to 26.50°
14754
8620
3.119
0.5763, 1.000
0.6458, 1.000
2.43 to 27.50
10313
0.8142, 1.000
2.20 to 26.50
11622
19833
8555
6528
6492
[R(int) = 0.0217] [R(int)=0.0302] [R(int)=0.0245] [R(int)=0.0245]
Data / restraints / 6528 / 0 / 415
parameters
6492 / 0 / 490
8620 / 0 / 505
8555 / 0 / 496
2
0.0791
0.0995
0.1550
0.1788
wR(F all data)
R(F obsd data)
0.0396
1.008
0.0464
1.017
0.0616
1.04
0.0783
1.074
2
GOOF on F
Observed data
6210
6019
7796
7447
[I] > 2σ(I)]
Largest and mean 0.001/ 0.000
shift / s.u.
Largest diff. peak 1.69/ -0.66
0.006/ 0.000
1.01/ -0.63
0.000/ 0.000
2.53/ -0.76
0.001/ 0.000
4.46/ -2.45
3
and hole, e/Å
wR2 = {  [w(F - F ) ] /  [w(F ) ] } ; R1 =  ||F | - |F || /  |F |
2
2 2
2 2 1/2
o
c
o
o
c
o

Table 2. Selected bond lengths [Å] and angles [o] for complexes 1, 2, 3 and 4
1
2
3
4
Bond Length [Å]
Ni-O1
Ni-O3
Ni-O6
Ni-O8
Ni-N1
Ni-N2
2.056(2)
2.060(2)
2.058(1)
2.067(2)
2.093(2)
2.103(2)
Ni-O5
Ni-O6
Ni-O8
Ni-O10
Ni-O11
Ni-N2
2.106(2)
Ni1…Ni2
Ni1-N1
Ni1-O1
Ni1-O3
Ni1-O5
Ni1-O12
Ni1-O13
Ni2-N2
Ni2-O5
Ni2-O6
Ni2-O9
Ni2-O10
Ni2-O11
3.750(1)
2.071(3)
2.067(5)
2.063(4)
2.016(3)
2.059(3)
2.161(4)
2.079(3)
2.067(3)
2.098(3)
2.085(4)
2.033(5)
2.024(3)
Ni1A…Ni2A
Ni1A…Ni1
Ni1A…Ni2
Ni1-O5
3.857(2)
4.279(2)
5.750(2)
2.039(6)
2.052(6)
2.1459(13)
2.131(7)
2.115(6)
2.055(7)
2.038(6)
2.1339(14)
2.043(6)
2.111(7)
2.066(7)
2.117(6)
2.071(2)
2.050(2)
2.083(2)
2.066(3)
2.088(3)
Ni1-O1
Ni1-O10
Ni1-O11A
Ni1-O3
Ni1-N1
Ni2-O5
Ni2-O12
Ni2-O9
Ni2-O13A
Ni2-N2
Ni2-O6
Bond Angle [o]
O3-Ni-O6 101.68(7) O10-Ni1-N2 100.61(9) Ni1-O5-Ni2
133.4(1)
Ni2-O12-Ni2A
169.9(5)
142.1(3)
171.3(5)
92.2(3)
104.9(3)
95.4(3)
87.0(3)
95.2(3)
89.1(3)
92.8(3)
62.5(3)
90.7(3)
85.3(3)
105.3(3)
79.5(3)
105.4(3)
93.3(3)
93.7(3)
87.4(3)
94.3(3)
63.1(3)
91.1(3)
90.2(3)
93.4(3)
84.4(3)
79.4(3)
O3-Ni-N1
O6-Ni-N2
O3-Ni-O8
82.57(7)
81.20(7)
84.00(7)
O6-Ni1-N2
O6-Ni1-O10 83.47(9)
O5-Ni1-N2 83.07(9)
76.14(9) O13-Ni1-O12 82.14(12) Ni1A-O5-Ni2A
O13-Ni1-N1
O5-Ni1-N1
O12-Ni1-O1 92.78(13)
O3-Ni1-N1 82.28(15)
107.5(2) Ni1A-O10-Ni1A
81.96(13)
O5-Ni1-O1
O5-Ni1-O10
O5-Ni1-O3
O3-Ni-N2 174.17(7) O8-Ni1-O10 97.58(9)
O1-Ni-O8 174.63(7) O11-Ni1-O6 95.31(11)
N1-Ni-O8
O8-Ni-N2
O6-Ni-O1
O1-Ni-O3
89.78(7)
90.80(7)
85.75(7)
91.02(7)
O5-Ni1-O13 88.19(12)
O11-Ni2-O5 90.68(12)
O5-Ni1-N1
O1-Ni1-O10
O1-Ni1-O11A
O1-Ni1-N1
O10-Ni1-O11
O10-Ni1-O3
O11A-Ni1-O3
O11A-Ni1-N1
O3-Ni1-N1
O9-Ni2-N2
93.30(14)
O6-Ni2-O10 88.81(12)
O6-Ni2-O11 93.72(13)
O6-Ni2-N2
79.79(13)
O5-Ni2-O12
O5-Ni2-O9
O5-Ni2-O6
O5-Ni2-N2
O12-Ni2-O9
O12-Ni2-O13A
O12-Ni2-O6
O9-Ni2-O13A
O9-Ni2-N2
O6-Ni2-O13A
O6-Ni2-N2