Synthesis, crystal structure and hydrolytic activity of a trispyrazolyl borato cadmium hydroxo complex

Article abstract of DOI:10.1016/j.inoche.2015.02.006

A dinuclear tris(3,5-diisopropyl-pyrazolyl borate) (Tpy ^iPr2) based cadmium hydroxo complex, [(Tpy^iPr2)₂Cd₂(μ-OH)(μ-Pz^iPr2)](DCM)₅, has been synthesized and characterized by the single crys

Full text of DOI:10.1016/j.inoche.2015.02.006

Inorganic Chemistry Communications 54 (2015) 31–33
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, crystal structure and hydrolytic activity of a trispyrazolyl
borato cadmium hydroxo complex
Kapil Tomar
Department of Chemistry, Indian Institute of Technology, Roorkee, Roorkee 247667, Uttarakhand, India
a r t i c l e i n f o
a b s t r a c t
A dinuclear tris(3,5-diisopropyl-pyrazolyl borate) (Tpy iPr2) based cadmium hydroxo complex, [(TpyiPr2)2Cd2(μ-
OH)(μ-PziPr2)](DCM)5, has been synthesized and characterized by the single crystal X-ray diffraction technique.
Structural analysis shows that the two Cd-TpyiPr2 moieties are bridged by one hydroxide and one pyrazolate
molecule. Hydrolytic activity of the complex was checked using 4-nitro phenylacetate as the substrate. The
pseudo-first order rate constant calculated on the basis of kinetic study was 1.535 × 10−4 s−1 while the second
Article history:
Received 20 October 2014
Received in revised form 15 December 2014
Accepted 2 February 2015
Available online 4 February 2015
order rate constant was 0.226 M−1 s−1
.
Keywords:
Cadmium
© 2015 Published by Elsevier B.V.
Hydroxide
Ester hydrolysis
Kinetic study
Crystal structure
The lightest element of group 12, zinc, occurs as an essential constit-
uent in numerous proteins whereas cadmium and mercury are consid-
ered to be toxic. Cadmium along with toxicity and carcinogenicity
has some positive biological effects, i.e., in the growth of fungus spe-
cies [1]. The recently published first cadmium containing carbonic
anhydrase, CDCA1, again suggests the role of cadmium in the biological
system [2]. Like the major nutrients, nitrate and phosphate, cadmium is
depleted from surface waters, increases with depth and reached a fairly
constant concentration in deep waters [3]. Many studies have been
done on the biological role of cadmium metal where other metals in
the biological species were replaced by cadmium [4–6].
In this regard, hydrotris(pyrazolyl)borate (Tpy) ligands have been
studied extensively in modeling the active site of various metal contain-
ing proteins [7]. But, till now there is no report of cadmium based
hydroxo complex with the Tpy family of ligands. Although, Allred
et al. [8–10] reported cadmium hydroxo complexes with a different li-
gand but still there is a dearth of cadmium hydroxo complexes in the lit-
erature. The present paper deals with the synthesis, characterization
and hydrolytic activity of a novel dinuclear cadmium hydroxo complex
with hydrotris(3,5-diisopropyl-pyrazolyl)borate ligand, TpyiPr2. To the
best of our knowledge, this result is the first example of the study of
hydrolytic efficiency of a cadmium hydroxo complex with Tpy family
of ligands.
were not successful. This probably happens because Cd(II) favors high
coordination number as compared to zinc which can stabilize low
coordination number as well. Due to this reason, there is no terminal
cadmium hydroxo complex reported till now. However, in our present
work, when Cd(II) ion and KTpiPr2 were reacted in the presence of Pz-
iPr2H and sodium hydroxide, a dinuclear bridged hydroxo complex,
[(TpiPr2)2Cd2(μ-OH)(μ-PziPr2)](DCM)5, (1) (Scheme 1) was successfully
obtained [13]. This complex showed the characteristic small but sharp
peak of μ-OH at 3578 cm−1 in the IR spectrum. Single crystals were
obtained from a mixture of dichloromethane and acetonitrile at −10 °C.
Single crystal X-ray structural analysis reveals that [(TpiPr2)2Cd2(μ-
OH)(μ-PziPr2)](DCM)5 (1) crystallized in the monoclinic system with
P21/c space group. The asymmetric unit has one neutral [(TpiPr2)2Cd2(μ-
OH)(μ-PziPr2)] unit with five dichloromethane molecules in the lattice
out of which one is on special position and is severely disordered
(Fig. 1). The two Cd(II) ions are charge balanced by two TpiPr2 mole-
cules, one charge from bridged pyrazole and one charge from the
hydroxide ion. Both the metal centers are five coordinated with coordi-
nation from three nitrogens of TpiPr2, one nitrogen of Pzipr2H and one
oxygen of the hydroxide ion giving a CdN4O environment. The calculat-
ed τ values for the two Cd (II) centers were 0.3 and 0.33 which show
that the geometry can best be described as distorted square pyramidal.
The Cd–N bond distances for the two Cd(II) ions are in the range of
2.242 (3) Å to 2.417 (4) Å, and Cd–O distances are 2.180(4) Å and
2.163(7) Å. The Cd–μOH distances are lower than the reported cadmi-
um hydroxo complex and the Cd⋯Cd distance (3.727 (5) Å) is little
larger [8]. The Cd–O–Cd angle is 118.19° while the average intraligand
N–Cd–N angles for the two pairs are 83.09° and 83.72°. The Cd2(μ-
Attempts to prepare mononuclear cadmium hydroxo like the
analogue Zn-Tpy hydroxo complex [11] using Cd(II) ion and KTpiPr2
E-mail address: stereochemistry.1@gmail.com.
http://dx.doi.org/10.1016/j.inoche.2015.02.006
1387-7003/© 2015 Published by Elsevier B.V.

32
K. Tomar / Inorganic Chemistry Communications 54 (2015) 31–33
Fig. 2. C–H⋯π interactions between pyrazole ring and dichloromethane molecule.
The kobs of complex 1 has a comparable value with some of the
reported Zn-OH complexes. The value is even higher than some of the
reported complexes [14,15] which further supports the fact that com-
plex 1 is a strong nucleophile which is capable of hydrolyzing 4-nitro
phenylacetate like the Zn–OH complexes.
Scheme 1. Synthetic scheme for the preparation of complex 1.
OH)(μ-PziPr2) core is not planar and exhibited a hinge-type distortaion
(Fig. S2) similar to the reported Cd-OH complex [9] where the Cd(II)
ion acquires a distorted octahedral geometry.
The two dichloromethane molecules present in the lattice make C–
H⋯π interactions with the pyrazole rings of the TpyiPr2 moiety (C–H⋯π
distances 2.905(6) Å, 2.721(7) Å, 2.625(4) Å, and 2.767(8) Å) (Fig. 2).
The three dimensional packing diagram (Fig. S3) shows the formation
of cavities for dichloromethane molecules.
In summary, we have successfully synthesized a novel tris(3,5-
diisopropyl-pyrazolyl borate) based cadmium hydroxo complex,
[(TpyiPr2)2Cd2(μ-OH)(μ-PziPr2)](DCM)5. Structural analysis shows
that it is a bridged hydroxo and bridged pyrazolato complex. Ester
hydrolysis was studied using 4-nitro phenylacetate. The first order
rate constant was found out to be 1.535 × 10−4 s−1 while the second
order rate constant is 0.226 M−1 s−1
.
The nucleophilicity of the bridged cadmium hydroxide complex 1
was proved by the cleavage of 4-nitro phenylacetate. The kinetic mea-
surement of the ester hydrolysis was monitored spectrophotometrically
in the acetonitrile solvent at 25 °C. The cleavage of 4-nitro phenyl car-
boxylate ester generates 4-nitro phenolate anion, which shows strong
absorption peak at 360 nm. This characteristic peak has been used to
study the hydrolytic activity of the ester by complex 1. The absorbance
(At) at 360 nm wasa recorded at 10 min intervals over 250 min. The
plots of log (Aint − At) versus time were linear and provided the esti-
mates of the pseudo-first order rate constant kobs (s−1). Rate constant
was calculated by using the literature method [12] which comes out
to be 1.535 × 10−4 s−1. The result indicated that complex 1 is kinetically
active species, i.e., Cd–OH–Cd specie is indeed a nucleophile. Plots of kobs
vs five different concentrations of 1 were linear confirming the first
order dependence on 1 (Fig. S4). The second order rate constant obtain-
ed according to formula, kobs = k [1], is 0.226 M−1 s−1. Some of the re-
ported second order rate constants (kobs) for Zn hydroxo complexes are
0.10 [14], 0.047 [15], 0.60 [16], and 0.93 [17] (Fig. 3).
Acknowledgment
KT is highly acknowledged for the financial support from the DST,
New Delhi, India. KT is thankful to IIT Roorkee for the Single Crystal
XRD facility.
Appendix A. Supplementary material
The experimental details, bond distance and angles, and X-ray
crystallographic data in CIF format. Supplementary data to this arti-
cle can be found online at http://dx.doi.org/10.1016/j.inoche.2015.
02.006.
Fig. 1. Molecular view of complex 1 (hydrogen atoms have been omitted for clarity).
Fig. 3. Plot showing the release of 4-nitro phenolate at 360 nm as a function of time.

K. Tomar / Inorganic Chemistry Communications 54 (2015) 31–33
33
[12] R. Jairam, M.L. Lau, J. Adornate, P.G. Potvin, Ester hydrolysis with 2,6-di(pyrazol-3-
yl)pyridines and their CoII complexes in homogeneous and micellar media, J.
Inorg. Biochem. 84 (2001) 113–118.
[13] Crystallographic data for 1 (CCDC 1027654): C68H115B2Cd2Cl10N14O (Mr =
1745.69); monoclinic; P 21/c; a = 22.253(3) Å; b = 13.090(14) Å; c = 30.163(4);
α = 90; β = 108.649(6); γ = 90; V = 8325.0(18) Å3; z = 4; μ = 0.880; D =
1.393 g cm−3; θ range = 2.67–27.78°; GOF on F2 = 1.229; R1(int) = 0.0781; wR2
[I N 2σ(I)] = 0.2.
[14] T. Koike, M. Takamura, E. Kimura, Role of zinc(II) in β-lactamase II: a model study
with a zinc(II)-macrocyclic tetraamine (1,4,7,10-tetraazacyclododecane, cyclen)
Complex, J. Am. Chem. Soc. 116 (1994) 8443–8449.
[15] T. Koike, S. Kajitani, I. Nakamura, E. Kimura, M. Shiro, The catalytic carboxyester
hydrolysis by a new zinc(II) complex with an alcohol-pendant cyclen (1-(2-
hydroxyethyl)-1,4,7,10-tetraazacyclododecane): a novel model for indirect activa-
tion of the serine nucleophile by zinc(II) in zinc enzymes, J. Am. Chem. Soc. 117
(1995) 1210–1219.
[16] C. Bazzicalupi, A. Bencini, A. Bianchi, V. Fusi, C. Giorgi, P. Paoletti, B. Valtancoli, D.
Zanchi, Carboxy and phosphate esters cleavage with mono- and dinuclear zinc(II)
macrocyclic complexes in aqueous solution. Crystal structure of [Zn2L1(μ-
PP)2(MeOH)2](ClO4)2 (L1) = [30]aneN6O4, PP− = diphenyl phosphate), Inorg.
Chem. 36 (1997) 2784–2790.
[17] X.-C. Su, H.-W. Sun, Z.-F. Zhou, H.-K. Lin, L. Chen, S.-R. Zhu, Y.-T. Chen, Kinetics and
mechanism of carboxyester hydrolyses using Zn(II) complexes with functionalized
phenanthroline complexes, Polyhedron 20 (2001) 91–95.
References
[1] H.-U. Meisch, A.-R. Scholl, J.A. Schmitt, Cadmium as a growth factor for the
mushroom Agaricus abruptibulbus (Peck) Kauffmann, Z. Naturforsch. C 36 (1981)
765–771 (In German).
[2] T.W. Lane, M.A. Saito, G.N. George, I.J. Pickering, R.C. Prince, F.M.M. Morel, A
cadmium enzyme from a marine diatom, Nature 435 (2005) 42.
[3] E.A. Boyle, F. Sclater, J.M. Edmond, On the marine geochemistry of cadmium, Nature
263 (1976) 42–44.
[4] N.M. Price, F.M.M. Morel, Cadmium and cobalt substitution for zinc in a marine
diatom, Nature 344 (1990) 658–660.
[5] J.G. Lee, F.M.M. Morel, Replacement of zinc by cadmium in marine phytoplankton,
Mar. Ecol. Prog. Ser. 127 (1995) 305.
[6] J.G. Lee, S.B. Roberts, F.M.M. Morel, Cadmium nutrient for the marine diatom
Thalassiosira weiss-ogii, Limnol. Oceanogr. 40 (1995) 1056.
[7] S. Trofimenko, Coordination chemistry of pyrazole-derived ligands, Chem. Rev. 72
(1972) 497.
[8] R.A. Allred, A.M. Arif, L.M. Berreau, A binuclear cadmium(II) hydroxide complex and
its CO2 reaction product, J. Chem. Soc., Dalton Trans. 3 (2002) 300–301.
[9] R.A. Allred, L.H. McAlexander, A.M. Arif, L.M. Berreau, Chemistry of a binuclear
cadmium(II) hydroxide complex: formation from water, CO2 reactivity and compar-
ison to a zinc analogue, Inorg. Chem. 41 (2002) 6790–6801.
[10] R.A. Allred, S.A. Huefner, K. Rudzka, A.M. Arif, L.M. Berreau, A cadmium hydroxide
complex of a N3S-donor ligand containing two hydrogen bond donors: synthesis,
characterization and CO2 reactivity, Dalton Trans. 3 (2007) 351–357.
[11] M. Rombach, C. Maurer, K. Weis, E. Killer, H. Vahrenkamp, Evidence for a trajectory
of hydrolytic reactions brought about by [L3Zn–OH] species, Chem. Eur. J. 5 (1999)
1013–1027.