A new oxorhenium(V) complex with benzothiazole derived ligand: Relative stability and global chemical reactivity indices

Article abstract of DOI:10.1016/j.ica.2016.04.002

A new rhenium complex, cis-[ReO(btmp)PPh₃Cl₂] (2a) with 2-benzothiazolyl-5-methoxyphenol (Hbtmp) has been synthesized and characterized by means of FT-IR, UV-Vis, ¹H NMR and elemental analyses. The structural parameters of

Full text of DOI:10.1016/j.ica.2016.04.002

Accepted Manuscript
A new oxorhenium(V) complex with benzothiazole derived ligand: Relative
stability and Global chemical reactivity indices
Arnab Bhattacharya, Jnan Prakash Naskar, Pinki Saha, Rakesh Ganguly, Baptu
Saha, Swagata Tarat Choudhury, Shubhamoy Chowdhury
PII:
S0020-1693(16)30154-2
http://dx.doi.org/10.1016/j.ica.2016.04.002
ICA 16986
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
29 October 2015
1 April 2016
3 April 2016
Please cite this article as: A. Bhattacharya, J.P. Naskar, P. Saha, R. Ganguly, B. Saha, S.T. Choudhury, S.
Chowdhury, A new oxorhenium(V) complex with benzothiazole derived ligand: Relative stability and Global
chemical reactivity indices, Inorganica Chimica Acta (2016), doi: http://dx.doi.org/10.1016/j.ica.2016.04.002
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A new oxorhenium(V) complex with benzothiazole derived ligand:
Relative stability and Global chemical reactivity indices
Arnab Bhattacharya,[a] Jnan Prakash Naskar,[b] Pinki Saha,[a] Rakesh Ganguly,[c] Baptu Saha,[a]
Swagata Tarat Choudhury,[d] Shubhamoy Chowdhury*[a]
[a] Department of Chemistry, Tripura University, Suryamaninagar, Tripura-799 022, India.
E-mail: shubha103@yahoo.com
[b] Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Jadavpur,
Kolkata-700 032, India.
[c] Division of Chemistry & Biological Chemistry, SPMS-CBC-01-18D, Nanyang
Technological University, 21 Nanyang Link, Singapore 637371
[d] Department of Chemistry, M. B. B. College, Agartala, Tripura-799 004, India.
1

Abstract
A new rhenium complex, cis-[ReO(btmp)PPh3Cl2] (2a) with 2-benzothiazolyl-5-
methoxyphenol (Hbtmp) has been synthesized and characterized by means of FT-IR,UV-vis,
1HNMR and elemental analyses. The structural parameters of 2a as obtained from XRD studies
have been compared with the calculated [PBEPBE/STMIDI] values. The observed results are in
good agreement. The vibrational frequencies, electronic transitions of 2a have been calculated
and compared with the experimental results. The global chemical reactivity descriptors based on
conceptual density functional theory have also been calculated for a set of analogous systems
derived from 2a varying the central metal center (Re or Tc) and substituted phosphine moieties.
A good relationship between the calculated hardness (η) and Tolman’s cone angle (θ) of
phosphines is observed both for Re and Tc complexes. Similar harmony between electronic
chemical potential (µ) and Tolman’s electronic parameter (ν) of phosphines is also noted.
Keywords: Oxorhenium(V) complex
X-ray crystal structure
DFT and TD-DFT calculations
Global chemical reactivity indices
Ligand substitution energy
2

1. Introduction
Metastable technetium, 99mTc, is the most suitable radionuclide for diagnostic imaging. It
is used almost in 80% of all routine purposes of diagnosis worldwide.[1] The suitable nuclear
properties of 99mTc (t1/2 = 6.02 h, γ particle energy 140 keV) make this a reality. Rhenium
complexes also have the efficacies to be used for diagnostic and therapeutic purposes due to their
similar amenable nuclear properties (186Re: Emax = 137 KeV for γ-emission with t1/2 = 90.6 h,
Emax = 1.1 MeV for β-emission and 188Re: Emax = 155 KeV for γ-emission with t1/2 = 17 h, Emax
=
2.1 MeV for β-emission) comparable with Tc nucleus.[2] Tc and Re, a Group 7 ‘matched pair’,
foster analogous complexes with akin chemical properties so much so that rhenium complexes
have been prepared to be studied as models for technetium diagnostic imaging. One of the
common examples of oxorhenium and oxotechnetium cores suitable for such purpose is
[MVO]3+, which is readily available from reduction of [MO4]- ions.[3] Again, the 186/188Re
analogue is generally expected to exhibit a trend similar to 99mTc in terms of its bio-distribution.
Tc agents and their Re counter parts are used for the imaging and treatment of tumors, thyroid,
heart and brain.[4]
Much work have been done on Tc and Re complexes with structurally simple
benzothiazole type of ligands, especially with 2-(4'-aminophenyl)benzothiazole, to be trialed for
breast cancer radiopharmaceuticals.[5] 2-(2'-pyridyl)benzothiazole has also been reported to bind
technetium and rhenium radioisotopes to serve as preliminary models for radiopharmaceutical
applications.[6] Benzothiazoles possess the intrinsic pharmacological properties to act as anti-
tumor, anti-microbial and anti-diabetic agents. They have proven action against cancerous
growth. Thus, benzothiazoles and their derivatives have been widely sought to anchor [MVO]3+
(M = Re and Tc).[7]
3

The reactivity descriptors derived from conceptual density functional theory (DFT) have
recently been the foci of organic chemistry researchers as a tool to describe global and/or local
reactivity in molecules.[8] The descriptors are defined in terms of general chemical parameters
like ionization potential (I), electron affinity (A), electronegativity (χ), hardness (ƞ), chemical
potential (µ), electrophilicity index (ω) and local reactivity descriptors such as Fukui function
and local hardness.[9] Recently the bond strength of metal and ligand has been evaluated with the
help of reactivity descriptors.[10] Interactions of Cu(II), Zn(II), Sn(II) with carbohydrates in
aqueous solution were investigated by reactivity descriptors derived through DFT.[11] Besides,
the global chemical reactivity indices have been correlated with the bond angle.[12]
Here we wish to report the synthesis, characterization and X-ray crystal structure of an
oxorhenium(V) complex, [ReO(btmp)PPh3Cl2] (2a), generated from a pharmacologically
significant 2-benzothiazolyl-5-methoxyphenol (Hbtmp) ligand. The structural, vibrational and
electronic aspects of 2a have also been delved into through conceptual DFT calculations. The
structure of 2a has been considered as a model to undertake a systematic study on a family of
[ReO]3+ bound phosphines. For comparison, analogous Tc complexes were also studied. The
conceptual DFT-based global reactivity descriptors like hardness (ƞ), chemical potential (µ) and
electrophilicity index (ω) were also analyzed both for Re and Tc complexes.
2. Experimental
2.1. Materials and physical measurements
All chemicals and solvents were obtained from Sigma Aldrich and Spectrochem Pvt.
Ltd., and were used as received. The precursor, trans-[ReOCl3(PPh3)2] was prepared and
characterized following the method of Chatt and Rowe.[13] C, H and N microanalyses were
4

performed with a Perkin–Elmer 2400II elemental analyzer. Solid phase FT-IR spectra were
recorded as KBr pellets on a Perkin–Elmer FTIR-100 spectrophotometer. UV–vis absorption
spectra of the complexes were recorded on a Perkin–Elmer Lambda 25 spectrophotometer.
Proton-decoupled 31P NMR experiments (162 MHz) in CDCl3 solution were performed on a
Bruker DPX-400 spectrometer at room temperature with 85% H3PO4 as an external reference
(0.00 ppm).
2.2. Synthesis of 2-benzothiazolyl-5-methoxyphenol (Hbtmp)
2-aminothiophenol (0.188 g, 1.5 mM) and 2-hydroxy-4-methoxybenzaldehyde (0.228 g,
1.5 mM) were mixed in 10 mL chloroform and was heated under reflux for 3 hours. The
resulting clear orange-brown solution was left for aerial evaporation. A gummy mass was
obtained thereby. This gummy mass was triturated with petroleum ether and dried under reduced
pressure to have an orange-brown powder. Yield: 303 mg, 77%; Anal.: Calcd. for C14H11NO2S:
C, 65.35; H, 4.31; N, 5.45. Found: C, 65.58; H, 4.28; N, 5.73%. FT-IR (KBr, cm-1): (C=N) 1633
1
(vs); (C-S) 952 (s). H NMR (CDCl3, 400 MHz) (δ, ppm): 7.66 (d, 1H); 7.32 (d, 1H); 7.25 (d,
1H); 7.16 (dd, 2H); 6.57 (d, 1H); 6.53 (s, 1H); 3.88 (s, 3H). UV-Vis (CHCl3), λmax (ε, M-1 cm-1):
255 (4542); 285sh (3642); 348 (3513); 425 (599); 503 (334).
2.3. Synthesis of [ReO(btmp)(PPh3)Cl2] (2a)
To a solution of trans-[ReO(PPh3)2Cl3] (0.208 g, 0.25 mM) in chloroform (10 mL) was
added Hbtmp (0.0647 g, 0.25 mM), dissolved also in 10 mL of chloroform. The resulting
reaction mixture was heated under reflux for 3 h. During reflux, the color changed from
greenish-yellow to brown. The resulting solution was evaporated to dryness under reduced
5

pressure to have a gummy mass. The gummy mass was then thoroughly triturated with
diethylether to get the dark brown amorphous product. Yield: 150 mg, 76%; Anal.: Calcd. for
C32H25Cl2NO3PReS: C, 48.52; H, 3.18; N, 1.77. Found: C, 48.68; H, 3.23; N, 1.72%. FT-IR
1
(KBr, cm-1): (C=N) 1607 (vs); (Re=O) 975 (s); (C-S) 946 (s). H NMR (CDCl3, 400 MHz) (δ,
ppm): 8.50 (d, 1H); 7.76 (d, 1H); 7.58-7.41 (m, 15H); 7.22 (dd, 2H); 6.97 (d, 1H); 6.38 (d, 1H);
31
6.17 (s, 1H); 3.63 (s, 3H). P NMR (CDCl3, 400 MHz) (δ, ppm): 29.43. UV-Vis (CHCl3), λmax
(ε, M-1 cm-1): 255 (5807); 305sh (3068); 351 (3249); 414 (2015); 490 (541); 558 (150). ΛM
(CH3OH): 08 Ω−1cm2 M−1 (nonelectrolyte).
Single crystals of 2a, suitable for X-ray crystal structure determination, were grown by
slow aerial evaporation of a moderately concentrated solution of it in acetone at room
temperature.
2.4. Crystal structures determination
Data for 2a were collected with graphite monochromated Mo-Kα radiation (λ = 0.71073
Ǻ) using the Bruker SMART APEX II CCD area detector diffractometer at 293K. Intensity data
of 2a were reduced using SAINT.[14] Empirical absorption corrections were performed with
SADABS package.[15] The structures were solved using direct methods and refined by full-
matrix least-square methods based on |F|2 using SHELXL-97.[16] All non-hydrogen atoms were
refined with anisotropic thermal parameters. Hydrogen atoms were placed in calculated positions
and constrained to ride on their parent atoms. All the calculations were carried out using
SHELXS-97, SHELXL-97 and SHELXTL[16] programs. Crystal data and structural parameters
are provided in Table 1.
6

2.5. Computational details
The computational studies presented in this work were performed on the gas phase
geometries of 2a and its analogues derived from substituted phosphines. Full geometry
optimizations for all the species were undertaken without using any symmetry restrictions at the
level of DFT. The functional, PBEPBE[17] was used along with STMIDI basis set. The
calculations for hydrogen, carbon, nitrogen, oxygen, phosphorus, sulphur, fluorine, chlorine and
bromine atoms were performed using MIDI.[18] For Tc and Re metals, respective Stuttgart (ST)
basis set were used in which the core electrons were replaced by Stuttgart effective core pseudo-
potentials.[19] Gaussian 09, Revision C.01 program package was used for computational
purposes.[20]
3. Results and discussion
3.1. Syntheses
The ligand (Hbtmp), used for the present work, is a 1:1 condensate of 2-aminothiophenol
and 2-hydroxy-4-methoxybenzaldehyde in chloroform. The title oxorhenium(V) complex, 2a
was prepared by heating equimolar proportions of trans-[ReOCl3(PPh3)2] and Hbtmp under
reflux in chloroform. The C, H and N micro-analytical data of 2a are found to be consistent with
our proposed empirical formula. The molar electrical conductivity (ΛM = 8 Ω−1cm2 M−1) of 2a in
methanol confirms that it is a non-electrolyte. 2a is soluble in a wide range of polar and non-
polar solvents like acetone, acetonitrile, methanol, chloroform and dichloromethane. 2a is stable
for months in the solid state, but only for weeks in solution.
7

3.2.FT-IR and 1H NMR spectroscopy
The IR spectrum of free Hbtmp shows characteristic band at 1633 cm-1. It is attributed to
C=N (benzothiazole) stretching vibration.[21] This band in the IR spectrum of 2a shifts to lower
wavenumber, 1607 cm-1. This shift is expected as the binding of C=N to a metal center generally
1
lessens its stretching frequency. This is also reflected in the HNMR spectrum of 2a. The
1HNMR spectrum of 2a manifests signals due to four aromatic protons for the benzothiazole
moiety with marked shift to downfield relative to that of the free ligand. On the contrary, the
1
sharp HNMR signal at 3.63 ppm due to the methoxy group in 2a is shifted to downfield (3.88
ppm) relative to the free ligand resonance. This is most likely due to strong CH···π
interactions.[22] The IR spectrum of 2a exhibits a strong band at 975 cm-1 assignable to the Re=O
stretching vibration.[23] The experimental and calculated (non-scaled) IR frequencies of 2a are
presented in Fig. S1, as Supplementary materials. The calculated vibrational stretching
frequencies of 2a are in good agreement with their respective experimental data. The
experimental and calculated IR spectra of 2a are in good agreement in terms of their band
positions, band intensities and shapes of the bands.
3.3. Molecular structure
The oxorhenium(V) compound crystallizes as 2a.½(CH3)2CO. ORTEP of 2a along with
their atom labeling scheme is shown in Fig. 1. Selected metrical parameters are listed in Table 2.
In 2a, the rhenium atom is at the center of a distorted octahedral environment. The Re1-O1 bond
distance in 2a is 1.684(2) Å. The normal range of this Re=O bond length is 1.668-1.720 Å for
oxorhenium(V) complexes.[23, 24] The Re1-O2 and Re1-N1 bond distances are 1.943(2) and 2.163
(4) Å respectively. The chelate bite angle, ∠O2-Re1-N1 of 2a is 81.78 (10)°. The ∠O2-Re1-O1
8

angle is of 165.24(10)°, a value deviated considerably from the idealized value of 180° required
for a regular octahedral geometry. Consequently, the geometry around the rhenium center in 2a
is distorted octahedral with ‘NO2Cl2P’ donor centers. Cl1-Re1-Cl2 angle [87.68(3) Å] is close to
90°. Thus, 2a is a cis-Cl,Cl complex with the Re–Cl bonds of 2.3581(11) and 2.3961(9)Å.[25]
The crystal structure of 2a is stabilized as a 3D supramolecular assembly made up of
hydrogen bonding interactions, (C3-H3···O4, C17-H17···O1, C29-H29···O4) and C-H···π (C12-
H12a···Cg1, C12-H12c···Cg2 where Cg1 and Cg2 respectively are the centroids of the C1−C6
and C27−C32 rings.[26] These short interaction parameters are listed in Table 3. The adjacent
molecules are joined together with non-covalent edge-to-face CH···π (3.300 Å) interactions
comprising the methoxy group of one btmp with the benzyl ring of the benzothiazol moiety of
the neighboring btmp. This primarily results in a dimeric supramolecular assembly (Fig. 2). This
dimeric unit is further assembled through non-covalent interactions, CH···π (3.875 Å) between
the methoxy group of btmp with the phenyl group of PPh3 of adjacent molecule to gives rise a
supramolecular 1D network. This 1D network is shown in Fig. 2. These 1D networks are
aggregated further through intermolecular C-H···O hydrogen bonds and finally gives rise to the
observed 3D supramolecular lattice structure (Fig. S2).
The optimizations starting from the crystallographically determined molecular structure
of 2a led to a global minimum as stationary level. The calculated bond lengths and bond angles
along with their experimental values are listed in Table 2. The results are in good agreement.
The theoretical bond lengths differ less than by 0.07 Å and the angles by 4.2° or so from their
respective experimental values. The general trend as noted in the experimental data set is also
preserved in our calculations.
9

3.4. Electronic Spectra
The absorption spectrum of 2a was simulated in presence of the CPCM solvent model
employing the TD-DFT methods with the same basis set and functional as used in geometry
optimization. The calculated findings are in good agreement with the experimental data set (Fig.
S3). The calculated spin-allowed singlet to singlet electronic transitions for 2a together with their
experimental counter parts observed in chloroform are listed in Table S1. The high energy
transitions having oscillator strength values greater than 0.0100, were only considered here.
Again, the transitions only with orbital contributions more than 5% were taken into account for
2a. A schematic representation depicting the energies of MOs and contours of some selected
HOMOs and LUMOs of 2a are presented in Figs. S4, S5. For 2a, the HOMO to LUMO energy
gap is 1.24 eV. The highest occupied molecular orbital (HOMO) of 2a enjoys significant
metallic character. The calculated low energy absorption bands in solution were obtained at
634.54 and 557.05 nm for 2a. The transitions, (HOMO → LUMO+2) and (HOMO-1 → LUMO,
HOMO → LUMO+2, HOMO-2 → LUMO) are assigned as d-d along with some degree of
ligand-to-metal charge transfer (LMCT) contributions. The calculated absorption bands in
solution at 502.79 and 477.49 nm can be assigned respectively as (HOMO-1 → LUMO, HOMO-
1 → LUMO+1) and (HOMO-3 → LUMO, HOMO-1 → LUMO, HOMO-2 → LUMO)
transitions. The band at 413.78 nm may originate from HOMO-6 → LUMO, HOMO-3 →
LUMO+1 or HOMO-4 → LUMO+1 transition. Our MO analyses show that electron densities
are transferred from ligands to the metal indicating that these transitions might have ligand-to-
metal charge transfer (LMCT) character. In the electronic spectrum of 2a, the higher energy
bands at 329.29, 294.04 and 254.30 nm are respectively assigned to the transitions (HOMO-4 →
LUMO+2, HOMO-13 → LUMO, HOMO-6 → LUMO+2), (HOMO-14 → LUMO+1), (HOMO-
10

1 → LUMO+7) and (HOMO-8 → LUMO+4, HOMO-1 → LUMO+11). These may be intra-
ligand or ligand to ligand transitions as well. The experimentally observed bands at 650, 558sh,
500sh, 455sh, 410, 350, 300sh and 255 nm match well with the theoretically predicted bands at
634.54, 557.05, 502.79, 477.49, 413.78, 329.29, 294.04 and 254.30 nm in terms of the band
wavelengths (energy), intensities and molar extinction coefficients.
3.5. Stability pattern
Being physico-chemical congeners, rhenium and technetium complexes display akin
chemical attributes. Rhenium complexes have often been prepared and studied as models for
technetium radiopharmaceuticals. With this viewpoint, we have attempted to generate a set of
oxorhenium(V) and oxotechnetium(V) compounds through systematic variation of the phosphine
moieties and the central metal (Tc or Re) from the determined crystal structure of 2a. Subsequent
plan was to evaluate the various parameters leading to their stabilities and to find out their global
reactivity parameters. The general structure of our studied compounds has been shown in
Scheme 1.
O
Cl
N
Cl
M = Tc, Re
M
PZ3 =PPh3, P(4-F-C6H4)3, P(4-Cl-C6H4)3,
PF3, PCl3, P(i-Pr)3, P(t-Bu)3, P(OMe)3,
P(OEt)3, P(OPh)3
PZ3
S
O
OMe
Cis-Cl, Cl [MO(btmp)(PZ3)Cl2]
Scheme 1
11

The geometries of all the molecules under this investigation have been optimized. Some
selected calculated bond lengths are listed in Table S2. The chemical reactivity descriptors
reaction energy (∆E), chemical hardness (η), electronic chemical potential (µ), and
electrophilicity (ω) have also been calculated using DFT. The thermodynamic parameters and
the reaction energy (∆E) so obtained allow us to compare the stability order of a series of Tc and
Re complexes generated via the following substitution reaction, (1) and ∆E expression (2).
[MO(btmp)(PPh3)Cl2] + PZ3 → [MO(btmp)(PZ3)Cl2] + PPh3
(1)
(2)
∆E = {E[MO(btmp)(PZ3)Cl2] + E(PPh3)}–{E[MO(btmp)(PPh3)Cl2] + E(PZ3)}
Varying only the phosphine moiety and keeping the rest of the coordinating ligands intact
both for the Re and Tc complexes, we have studied the effect of substitutions on the ligand frame
work in the course of our calculations. For the sake of computational simplicity, we have
considered ∆E values instead of ∆G. The total number of species remains intact before and after
the ligand exchange reactions for the systems, [MO(btmp)(PZ3)Cl2] (M = Re or Tc).
Accordingly, the entropies of the reactants and products are almost the same. Thus, the
approximation, ∆G ≅ ∆E, holds good.[27] All the calculated values of energy (∆E) due to
substitution for the complexes are listed in Table 4. The ∆E values for the ligands P(4-F-C6H4)3,
P(OMe)3 and P(OEt)3 are negative both for the Tc and Re compounds. These results indicate that
the isomers are thermodynamically more stable than the corresponding PPh3-boundisomers. All
other isomers under this investigation are less stable than their corresponding PPh3-bound
isomers. Our calculated energy (in au) of ligand substitution for the Tc and Re complexes based
on Scheme 1 in the gas phase versus different categories of phosphine ligands are shown in Fig.
3. The stability orders are found to be the same both for the Tc and Re compounds. Our
determined stability order of the PR3, P(OR)3, PX3 and PAr3 type of ligands runs as P(i-Pr)3 >
12

P(t-Bu)3, P(OEt)3 > P(OMe)3 > P(OPh)3, PCl3 > PF3 and P(4-F-C6H4)3 > PPh3 > P(4-Cl-C6H4)3.
This trend is common both for Re and Tc complexes.
The coordination chemistry of rhenium is greatly influenced by the nature of the solvent
so much so that it may not even be feasible to isolate various oxorhenium(V) species through
variation of phosphorous based co-ligands like PPh3, P(OEt)3 etc. of the precursor via
substitution. Steric and electronic effects of the substituents, tethered to the central phosphorus
atoms, are also profound. These impede us to synthesize our desired compounds. However, our
theoretical results hint that the compound, [ReO(btmp)P(OEt)3Cl2] (2i), is the most stable among
all the rhenium compounds under study. This prompted us to undertake the substitution of PPh3
in 2a by triethyl phosphite [P(OEt)3] in solution and to characterize the substituted product with
the help of 31PNMR (Fig. S6). The initial CDCl3 solution of 2a showed a sharp signal of metal-
31
bound PPh3 at 29.43 ppm in its P NMR spectrum. 1:1.1 molar proportion of 2a and P(OEt)3
was added prior to subsequent scanning. Two signals at 7.26 and 138.80 ppm with almost the
same intensity emerged. Two additional small intensity signals at 29.43 and 139.90 ppm were
also noted. The two signals at 7.26 and 139.9 ppm are respectively assigned to the resonances of
free PPh3 and P(OEt)3.[28] The signal at 138.80 ppm is indicative of the phosphorous resonance
from the newly-formed 2i from 2a.
3.6. Global chemical reactivity indices
In the light of Density Functional Theory, it can be possible to define universal concepts
of molecular structural stability and reactivity through the global reactivity parameters. These
global reactivity parameters are the electronic chemical potential (µ) and hardness (η) which may
be defined by equations (1) and (2),[29, 30]
13

ꢃꢄ
ꢀ =ꢁ− ꢂ ꢆ
ꢃꢅ
(1)
(2)
ꢇ(ꢈꢉ)
ꢇ(ꢈꢉ)
ꢍ
ꢊ =ꢁꢋꢌ ꢂꢃ ꢄꢍꢆ
=ꢁꢋꢌ ꢂꢃꢎꢆ
ꢃꢅ
ꢃꢅ
ꢇ(ꢈꢉ)
Where E, N and v(ꢏꢉ) respectively are the total energy of the system, the number of electrons in
the system and the external potential of the system. For the sake of simplicity and computational
time, we have taken recourse to calculate the chemical hardness (η) and chemical potential (µ) in
the spirit of Koopmans’ theorem employing equations (3) and (4).[29]
ꢔꢄ
ꢌ
ꢊ =ꢁꢄ
ꢀ =ꢁꢄ
(3)
(4)
ꢐꢑꢒꢓ
ꢕꢓꢒꢓ
ꢖꢁꢄ
ꢌ
ꢐꢑꢒꢓ
ꢕꢓꢒꢓ
The global electrophilicity index (ω) as defined by Parr et al has been described as[31]
ꢍ
ꢗ = ꢁꢎꢌꢘ
(5)
The global electrophilicity index is a useful parameter that permits to have an insight into the
reactivity of a molecule in its ground state. It can be reckoned that the hardness is directly
connected with the stability and reactivity of a chemical system in its ground state. Again, the
hardness is the measure of the resistance imparted during the change in the electronic distribution
in a molecule.[29a, 30a, 32] Conclusively, larger the value of η for a complex, the higher is its
stability and lesser is its reactivity. The chemical potential, µ describes the change in electronic
energy with respect to the number of electrons and is usually associated with the charge transfer
ability of the system in its ground state. Alternatively, this µ is the index of escaping tendency of
electrons from an equilibrium system.
In 1977, Tolman introduced a parameter for sequencing a variety of phosphine ligands in
terms of their steric bulk and electron donating ability. The steric bulk of a phosphine ligand was
determined from simple three-dimensional space-filling models of phosphine ligands. Tolman
14

coined the term ‘cone angle’ (θ) to indicate the extent of ‘volume’ that a ligand occupies around
the metal center. [33]
The energies of the frontier molecular orbitals, HOMO and LUMO, are listed in Table
S3 for 1a-1j and 2a-2j. The chemical reactivity descriptors like the chemical hardness (η) and
electronic chemical potential (µ) were also calculated employing the HOMO and LUMO
energies. Those are listed in Table 5. The calculated hardness of Tc (1a-1j) and Re (2a-2j)
complexes are plotted against their respective Tolman’s cone angle of phosphines. The data fit,
shown in Fig. 4, follows linear correlation. Both for oxotechnetium(V) and oxorhenium(V)
compounds, hardness decreases with increase in cone angle of phosphine ligand. The ‘cone
angle’ is the measure of steric factor as well. Thus, proportional increment in steric factor around
a phosphine ligand will bring about concomitant proportional decrement in its hardness. This is
the general trend observed both for the [ReO]3+ and [TcO]3+ cores. It is also noted that the
hardnesses of Re complexes are higher than their corresponding Tc complexes. This might be
due to the greater thermodynamic stabilities of the Re complexes over their corresponding Tc
analogues.
The electron donating ability of a phosphine ligand can be determined by Tolman’s
electronic parameter (ν). A phosphine ligand with the highest electron donating ability manifests
the least value for Tolman’s electronic parameter and vice versa.[33] Plots of the calculated
chemical potential (µ) of both set (Tc and Re) of molecules against the Tolman’s electronic
parameter (ν) of phosphine ligands are shown in Fig. 5. The plot shows that the chemical
potential decreases with increase in Tolman’s electronic parameter of phosphine ligands. The
plot also predicts that the values of µ are higher for Re compounds than the corresponding Tc
15

compounds. Thus, steric factor and electronic parameter respectively controls the hardness and
chemical potential of a compound.
By definition, the global electrophilicity index (ω) measures the susceptibility of a
chemical species to accept electron. The higher is the value of ω for a chemical species; the
better is its electrophilicity. Better nucleophiles have lower values for ω.[31] The calculated ω
values for Tc and Re complexes under present study are listed in Table 5. The values of ω for Tc
complexes are found to be higher than that of the corresponding Re complexes. Consequently,
Tc complexes are more electrophilic in nature than their corresponding Re complexes. Amongst
all the Tc and Re complexes under study, PCl3 bound compound manifests the highest value
forω; while P(OMe)3 bound compound manifests the least. The electrophilicity order for Tc
complexes as determined by us runs as PCl3 > PF3 > P(4-Cl-C6H4)3 > P(4-F-C6H4)3 > P(t-Bu)3 >
P(OPh)3> PPh3 > P(i-Pr)3 > P(OEt)3 > P(OMe)3. For Re complexes, the deciphered
electrophilicity sequence is PCl3 > PF3 > P(4-Cl-C6H4)3 > P(t-Bu)3 > P(OPh)3 > P(4-F-C6H4)3 >
P(i-Pr)3 > PPh3 > P(OEt)3 > P(OMe)3.
Due to ‘lanthanide contraction’ the radius of technetium is comparable to that of rhenium.
As a result, analogous complexes of Tc and Re often show similar coordinative properties.
Physical properties like size, lipophilicity, dipole moment and formal charge make their
-
chemistry comparable in many respects. Reducing 99mTcO4 technetium complexes are simpler to
-
prepare than the generation of rhenium complexes from the reduction of ReO4 . Because of
higher standard reduction potential, stable rhenium complexes in their higher oxidation states are
easily obtainable than their technetium analogues. This observation is in tune to our theoretical
findings — chemical potentials of rhenium complexes are higher than the corresponding
technetium complexes. Again, the rhenium complexes are kinetically more inert than technetium
16

complexes in terms of substitution reactions. This is also reflected in our hardness calculations.
Our calculated hardness of rhenium complexes exceeds that of analogous technetium
complexes.[34]
4. Conclusion
Here we have synthesized a new oxorhenium(V) complex, [ReO(btmp)PPh3Cl2] (2a),
from an ancillary ligand, 2-benzothiazolyl-5-methoxyphenol (Hbtmp). The X-ray crystal
structure of 2a reveals that the rhenium(V) center is in distorted octahedral ‘O2NPCl2’
coordination environment. The two chloride donors are cis to each other. Accordingly, 2a is a
cis-Cl,Cl oxorhenium(V) complex. A conceptual DFT studies on 2a was undertaken to delve into
its electronic and structural aspects and also to correlate its chemical reactivity. The structurally
disposed configuration of 2a was subsequently considered as a model for the theoretical
calculations of a chosen set of Tc and Re compounds. The stability orders of such Re and Tc
complexes were attempted to determine. The calculated chemical hardness (η) for both the Tc
and Re analogue are found to correlate with the cone angle (steric parameter) of metal-bound
phosphine ligands. Again, the chemical potential (µ) of both the Tc and Re analogues was shown
to correlate satisfactorily with the Tolman’s electronic parameter (ν) of metal-bound phosphine
ligands. Thus, the results expounded here generalize that the hardness of a molecule is controlled
by steric factor and the chemical potential by electronic parameter.
Acknowledgements
Financial support (F. No. SR/FT/CS-75/2010) received from the SERB under Department
of Science and Technology (DST), India is gratefully acknowledged. J. P. Naskar sincerely
17

acknowledges the financial support received from the University Grants Commission (UGC),
New Delhi, India [F. No. 41-220/2012 (SR)]. P. Saha sincerely acknowledges the research
fellowship received from the UGC, India. We sincerely thank our revered reviewers for their
valuable suggestions to enrich the MS.
Appendix A. Supplementary material
CCDC 990190 contains the supplementary crystallographic data for 2a. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
18

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23

Table(s)
Table 1
Crystal data and structure refinement parameter for 2a
Parameter
Complex 2a
Formula
C32H25Cl2NO3PReS.½(C3H6O)
Formula weight
Crystal system
Space group
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
β (°)
820.695
Triclinic
P-1
10.6926(7)
11.2544(7)
14.1977(10)
90.522(1)
90.702(1)
116.936(1)
1522.86(17)
1
γ (°)
Volume (Å3)
Z
Temperature (K)
ρcalc (g/cm3)
μ (Mo-Kα) (mm-1)
F(000)
293
1.790
4.325
808
Crystal size (mm)
λ (Mo-Kα) (Å)
θ range (°)
Total reflection
Reflection independent (Rint)
h/k/l
0.24 x 0.28 x 0.30
0.71073
1.4 < θ < 32.3
18756
8246 (0.037)
-14, 15/-16,14/-18, 18
7315
Reflection observed (I > 2σ)
R1
0.0316
wR2
0.0730
Goodness of fit
1.03
2.31, -2.25
Δmax and Δmin (e Å-3)
R1 = Σ||Fo| - |Fc||/ Σ |Fo| , wR2 = [Σw(Fo -Fc2)2/Σw(Fo 2 )2]1/2, calcd. w = 1/[σ2(Fo 2 ) + (0.0340P)2]
2
where P = (Fo 2+ 2Fc2)/3

Table 2
Selected bond lengths and angles for 2a
Experimental
Optimized
Bond lengths (Å)
Re1-O1
Re1-O2
Re1-N1
Re1-P1
1.684(2)
1.943(2)
2.163(4)
2.4761(9)
2.3581(11)
2.3961(9)
1.72722
2.01107
2.16241
2.48935
2.35670
2.34955
Re1-Cl1
Re1-Cl2
Bond angles (°)
O1-Re1-O2
O1-Re1-N1
O1-Re1-P1
O1-Re1-Cl1
O1-Re1-Cl2
O2-Re1-N1
O2-Re1-P1
O2-Re1-Cl1
O2-Re1-Cl2
N1-Re1-P1
N1-Re1-Cl1
N1-Re1-Cl2
P1-Re1-Cl1
P1-Re1-Cl2
Cl1-Re1-Cl2
165.24(10)
88.78(11)
85.28(7)
98.31(8)
100.82(7)
81.78(10)
84.56(6)
92.18(7)
89.90(6)
96.48(8)
171.42(8)
86.23(8)
88.96(3)
173.40(3)
87.68(3)
162.30697
88.51010
83.02188
98.78067
105.03461
83.09740
82.53871
90.97075
89.88332
96.76349
171.58194
85.67360
88.34400
171.68028
88.35625
25

Table 3
Hydrogen bonds (Å, °) for complex 2a
D-H···A
D-H
0.93
0.93
0.93
0.96
0.96
H···
2.49
2.39
2.52
3.06
3.42
D···
3.22
3.30
3.37
3.30
3.87
∠D-
136
165
152
84
A
0
0
0
0
3
A
H-A
C3-H3···O4
6(12)
2(5)
2(11)
0(6)
5(7)
C17-H17···O1
C29-H29···O4
C12-H12a···Cg1
C12-H12c···Cg2
111
a = 1-x, 1-y, 1-z; c = 1+x, 1+y, z
Cg1 and Cg2 are the centroids of the C1−C6 and C27−C32 rings, respectively.
26

Table 4
Calculated energy of substitution (∆E) for the analogues Technetium and Rhenium
complexes with varying phosphine ligand PZ3 (values in au) in the gas phase
Complex
En
Complex
E
ergy
nergy
difference
difference
(∆E) with
1a
(∆E) with
2a
[TcO(btmp)(PPh3)Cl2]
[TcO(btmp)P(4-F-
C6H4)3Cl2] 1b
[TcO(btmp)P(4-Cl-
C6H4)3Cl2] 1c
[TcO(btmp)(PF3)Cl2] 1d
0
[ReO(btmp)(PPh3)Cl2]
[ReO(btmp)P(4-F-
C6H4)3Cl2] 2b
[ReO(btmp)P(4-Cl-
C6H4)3Cl2] 2c
[ReO(btmp)(PF3)Cl2] 2d
0
(1a)
(2a)
-
0.0007
0.0
006
0.0
-
0.0009
0.
0005
0.
109
0.0
076
0.0
016
0.0
108
-
0.0026
-
0.0043
0.0
0104
0.
0072
0.
0019
0.
0113
-
0.0041
-
0.0049
0.
[TcO(btmp)(PCl3)Cl2] 1e
[TcO(btmp)P(i-Pr)3Cl2]
[TcO(btmp)P(t-bu)3Cl2]
[TcO(btmp)P(OMe)3Cl2]
[TcO(btmp)P(OEt)3Cl2]
[TcO(btmp)P(OPh)3Cl2]
[ReO(btmp)(PCl3)Cl2] 2e
[ReO(btmp)P(i-Pr)3Cl2] 2f
[ReO(btmp)P(t-bu)3Cl2]
[ReO(btmp)P(OMe)3Cl2]
[ReO(btmp)P(OEt)3Cl2]
[ReO(btmp)P(OPh)3Cl2]
1f
1g
1h
1i
2g
2h
2i
033
0024
1j
2j
27

Table 5
The calculated values (in eV) of chemical hardness (η), electronic chemical potential (µ)
and global electrophilicity (ω) of metal (M = Re and Tc) complexes are listed.
Co
mpound
η
µ
ω
Com
pound
η
µ
ω
0
-
1
0.
-
1
2a
2b
2c
2d
2e
2f
1a
1b
1c
1d
1e
1f
.465
.460
.465
.530
.510
.460
.430
.540
.525
3.935
4.040
4.245
4.580
4.640
3.880
3.890
4.090
4.065
6.650
7.741
9.376
9.789
1.107
6.363
7.595
5.489
5.737
620
615
615
700
670
585
560
710
695
3.590
3.705
3.905
4.280
4.370
3.555
3.580
3.760
3.705
0.394
1.160
2.398
3.085
4.251
0.802
1.443
.9561
.8756
0
0
0
0
0
0
0
0
0
-
-
-
-
-
-
-
-
-
1
1
1
2
1
1
1
1
1
0.
0.
0.
0.
0.
0.
0.
0.
0.
-
-
-
-
-
-
-
-
-
1
1
1
1
1
1
9
9
1
2g
2h
1g
1h
2i
2j
1i
1j
28

.510
4.220
7.459
675
3.915
1.354
29