# The First Metal Complexes of 4,6-diamino-1-hydro-

Authors: Sahar I. Mostafa; Constantina Papatriantafyllopoulou; Spyros P. Perlepes; Nick Hadjiliadis
Journal: Bioinorganic Chemistry and Applications
 (2008)

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## Abstract

The new complexes[M2O5L2(H2O)2]⋅H2O(M=Mo,1;M=W,2), [RuL2(H2O)2]⋅H2O  (3), [ML3]⋅xH2O(M=Rh,x=2,4;M=Ir,x=1,5), [RhL2(PPh3)2](ClO4)⋅2H2O  (6), [PdL2]⋅2H2O  (7), [PdL(phen)]Cl⋅H2O  (8), [ReOL2(PPh3)]Cl  (9) and [UO2L2]  (10) are reported, where LH is 4,6-diamino-1-hydro-5-hydroxy-pyrimidine-2-thione.
The complexes were characterized by elemental analyses, physical techniques
(molar conductivity, room-temperature magnetic susceptibility), and spectroscopic
(IR, Raman, UV/VIS/ligand field, NMR, mass) methods. The ligand L− is in its thione form and behaves as a bidentate chelate with the deprotonated
(hydroxyl) oxygen and the nitrogen of one amino group as donor atoms.
Oxobridged dinuclear (1, 2) and various mononuclear (3–10) structures are assigned for the complexes in the solid state. The metal ion coordination geometries are octahedral (1–6, 9, 10) or square planar (7, 8). The free ligand LH and complexes 1, 4, 7, and 8 were assayed in vitro for antimicrobial activity against two bacterial and two fungal cultures.


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## Body

## 1. Introduction

2-Mercaptopyrimidine
nucleotides have been detected inEscherichia
Coli sRNA and yeast tRNA; it has been found that they inhibit the synthesis
of tRNA, thus acting as antitumour and antithyroid agents [1]. A similar
inhibitory effect has been observed for pyrimidine-2-thione (I in Scheme 1) and its derivatives,
which also show pronounced in vitro bacteriostatic activity [1]. Metal
complexes of pyrimidine-2-thione or its pyrimidine-2-thiol tautomeric form
[1, 2] and its amino [2, 3] or hydroxy [4–6] derivatives have been prepared and
studied (for representative ligands see Scheme 1). Such complexes exhibit rich
structural chemistry, and interesting thermal, magnetic, sorptive, and
biological properties. However, the coordination chemistry of ligands based on
the 2-mercaptopyrimidine moiety and containing both hydroxy and amino substituents on the pyrimidine ring is
completely unkown.Scheme 1
Structural formulae of pyrimidine-2-thione (I), and some of its amino (II, III) and hydroxy (IV, V) derivatives shown in one of their tautomeric forms.We now describe here the preparation and characterization of thefirst metal complexes of
4,6-diamino-5-hydroxy-2-mercaptopyrimidine (LH, Scheme 2). We also report the
antimicrobial activity of the free ligand and four representative complexes
against two bacteria and two fungi. This work can be considered as a
continuation of our interest on the coordination chemistry of derivatized
pyrimidines [7].Scheme 2
4,6-diamino-5-hydroxy-2-mercaptopyrimidine (LH).


## 2. Experiments

All reagents
were purchased from Merck and Alfa/Aesar.Na2[IrCl6] is
commercially available. [PdCl2(phen)] was prepared by the reaction
of K2[PdCl4] and 1,10-phenanthroline in H2O/EtOH.
[ReOCl3(PPh3)2] was synthesized  as previously reported [8]. DMSO used in
conductivity measurements was dried over molecular sieves. The DMSO-d6 protons (NMR) were referenced using TMS. Warning: perchlorate salts are potentially explosive; such compounds should be used in
small quantities and treated with utmost care at all times. Elemental analyses
(C, H, N, S) were performed by the University of Ioannina (Greece) Microanalytical Unit with
an EA 1108 Carlo-Erba analyzer. The water content of the complexes was also
confirmed by TG/DTG measurements performed on a Shimadzu Thermogravimetric
Analyzer TGA-50. IR spectra were recorded on a Matson 5000 FT-IR spectrometer
with samples prepared as KBr pellets. Far-IR spectra were recorded on a Bruker
IFS 113 v FT spectrometer with samples prepared as polyethylene pellets. FT
Raman data were collected on a Bruker IFS 66 v interferometer with an FRA 106
Raman module, a CW Nd: YAG laser source, and a liquid nitrogen-cooled Ge
detector. Solution electronic spectra were recorded using a Unicam UV2−100 spectrophotometer. Solid-state (diffuse reflectance, DRS) electronic spectra in
the 300–800 nm range were
recorded on a Varian Cary 3 spectrometer equipped with an integration sphere. H1
NMR studies were performed on a Varian Gemini WM-200 spectrometer. P{H1}31
NMR spectra were recorded with a Varian Mercury equipment [ref. 85%H3PO4 (ext.)]. Mass spectra were recorded on a Matson 5988 MS spectrometer. Conductivity
measurements were carried out at room temperature on a YSI, model 32
conductivity bridge using 10−3M solutions. Room temperature
magnetic susceptibility measurements were performed using a Johnson Matthey
magnetic balance standardized with HgCo(NCS)4; diamagnetic
corrections were estimated using Pascal’s constants.


### 2.1. Preparation of the Complexes

[Mo2O5L2(H2O)2]⋅H2O(1)
An aqueous
solution (5cm3) of (NH4)2[MoO4]
(0.24 g, 1.0 mmol) was added to a solution of LH (0.16 g, 1.0 mmol) in EtOH (25 cm3). The obtained slurry was heated and the resulting orange solution
was refluxed for 4 hours, during which time an orange precipitate is formed. The solid was collected by
filtration, washed with ethanol (2 cm3) and diethyl ether (2 × 5 cm3)
and dried in vacuo. The yield
was 35% (based on the metal). Elemental
analytical calculation for
C8H16N8O10S2Mo: C,
15.00; H, 2.50; N, 17.50; S, 10.00% found that C, 14.98; H, 2.82; N, 17.51; S, 9.87%; ΛM(DMSO): 3 S cm2mol−1.[W2O5L2(H2O)2]⋅H2O(2)
Using(NH4)2[WO4]
and following exactly the same procedure as that described for complex 1, a bright yellow material was
isolated. The yield was 50% (based on the metal). Elemental analytical calculation for C8H16N8O10S2W:
C, 11.77; H, 1.96; N, 13.73; S, 7.85% found that C, 11.62; H, 1.90; N, 13.77; S, 7.95%; ΛM(DMSO): 2 S cm2mol−1.[RuL2(H2O)2]⋅H2O(3)
SolidRuCl3⋅3H2O
(0.12 g, 0.46 mmol) was added to a solution of NaO2CMe (0.62 g, 7.5 mmol) in water (30 cm3). Solid LH (0.24 g, 1.5 mmol) was then added
and the resultant reaction mixture was refluxed for 12 hours. The deep brown
solid formed was collected by filtration while the reaction mixture was hot,
washed with hot water, and dried in vacuo. The yield was 30% (based on the
metal). Elemental analytical calculation for C8H16N8O5S2Ru:
C, 20.46; H, 3.41;N, 23.88; S,13.64% found that C, 20.32; H,
3.05; N, 23.57; S, 13.21%; ΛM(DMSO):
1 S cm2mol−1.[RhL3]⋅2H2O(4)
UsingRhCl3⋅3H2O
and following the same procedure as that described for complex 3, a reddish brown material was
isolated. The yield was 60% (based on the metal). Elemental analytical calculation for C12H19N12O5S3Rh:
C, 23.61; H, 3.12; N, 27.55; S, 15.74% found that C, 23.73; H, 3.11; N, 26.36; S, 14.98%;
ΛM(DMSO): 6 S cm2mol−1.[IrL3]⋅H2O(5)
UsingNa2[IrCl6] and
following the same procedure as that described for complex 3, a yellow solid was isolated. The yield was 25% (based on the
metal). Elemental analytical calculation for C12H17N12O4S3Ir:
C, 21.14; H, 2.50; N, 24.66; S, 14.09% found that C, 21.33; H, 2.64; N, 24.75; S, 13.85%; ΛM(DMSO): 5 S cm2mol−1.[RhL2(PPh3)2](ClO4)⋅2H2O(6)
A hot ethanolic
solution (20cm3) of LH (0.25 g, 1.6 mmol) was added to a solution
of RhCl3⋅3H2O
(0.21 g, 0.8 mmol) in 6 M HClO4 (15 cm3). The resultant
orange solution was refluxed for 4 hours and to this was added a solution of
PPh3 (0.43 g, 1.6 mmol) in hot ethanol (15 cm3). The new
solution was refluxed for a further 3 hours and filtered, and its volume decreased in vacuo to give a red-brown solid.
The solid was collected by filtration, washed with hot water (2 x 2 mL) and hot
ethanol (2 x 3 cm3), and dried in vacuo. The yield was 25% (based on the metal). Elemental analytical calculation for C44CIH44N8O8S2P2Rh:
C, 49.05; H, 4.09; N, 10.41; S, 3.30% found that C, 48.79; H, 4.09; N, 10.44;
S, 3.46%; ΛM(DMSO): 48 S cm2mol−1.[PdL2]⋅2H2O(7)
To a stirred
slurry of LH (0.16 g, 1.0 mmol) in methanol (15cm3) was added an
aqueous solution (15 cm3) of K2[PdCl4] (0.16 g, 0.5 mmol). The resulting suspension was stirred at 40°C for 60 hours
and the brown solid formed was collected by filtration, washed with water (5 ×
3 cm3) and cold methanol (2 × 5 cm3), and dried in air. The
yield (based on the metal) was 50%. Elemental analytical
calculation for C8H14N8O4S2Pd:
C,21.03; H, 3.07; N, 24.54; S, 14.02% found that C, 21.23; H, 3.22; N, 24.85;
S, 14.21%;  ΛM(DMSO): 9 S cm2mol−1.[PdL(phen)]Cl⋅H2O(8)
To a stirred
yellow slurry of [PdCl2(phen)] (0.18 g, 0.5 mmol) in a methanol/benzene
solvent mixture (15 cm3, 3:2 v/v) was added a solution of KOH (0.055 g, 1.0 mmol) in methanol (15 cm3). Solid LH (0.08 g, 0.5 mmol) was
added to the reaction mixture which soon dissolved. The solution was filtered
and stirred for 48 hours at room temperature. During this time, a brown
precipitate formed which was collected by filtration, washed with water (1 cm3)
and methanol (2×3cm3), and dried in air. The yield was 40% (based
on the ligand). Elemental analytical calculation for C8H14N8O4S2Pd:
C, 21.03; H, 3.07; N, 24.54; S, 14.02% found that C, 21.23; H, 3.22; N, 24.85;
S, 14.21%;  ΛM(DMSO): 77 S cm2mol−1.[ReOL2(PPh3)]CL(9)
To a stirred slurry of [ReOCl3(PPh3)2]
(0.25 g, 0.2 mmol) in ethanol (30 cm3) was added solid LH (0.057 g,
0.4 mmol). The solid soon dissolved and stirred at 40°C for 5 hours.
The brown solution deposited a brown microcrystalline solid which was collected
by filtration, washed with ethanol (3×3cm3), and dried in vacuo. The yield was 65% (based on
the metal). Elemental analytical calculation for C26CIH25N8O3S2Re:
C, 38.34; H, 3.07; N, 13.76; S, 7.86% found that C, 38.37; H, 3.11; N, 13.87;
S, 7.98%;  ΛM(DMSO):
46 S cm2mol−1.[UO2L2](10)
Solid LH (0.08 g, 0.5 mmol) was added to a
stirred solution of[UO2(NO3)2]⋅6H2O
(0.25 g, 0.5 mmol) in methanol (10 cm3). The solid soon dissolved.
The resultant yellow solution was filtered and refluxed for 4 hours, during
which time a red microcrystalline solid was precipitated. The product was
collected by filtration, washed with methanol (5 cm3) and diethyl
ether (2 × 5 cm3), and dried in
vacuo. The yield was 55% (based on the metal). Elemental analytical calculation for C8H10N8O4S2U:
C,16.44; H, 1.71; N, 19.18; S, 10.96% found that C, 16.35; H, 2.02; N, 18.98;
S, 10.86%; ΛM(DMSO): 11 S cm2mol−1.


### 2.2. Antimicrobial Activity

The bacterial
strains (S. aureus and P. aeruginosa) were grown in Nutrient
agar slants and the fungal strains (A. niger and C. albicans) were grown in Sabouraud
dextrose agar slants. The viable bacterial cells were swabbed onto Nutrient
agar plates, while the fungal spores onto Sabouraud dextrose agar plates. The
free ligand and complexes 1, 4, 7 were dissolved in DMSO, while complex 8 was dissolved in H2O with 10, 20, 50, and 100 mg/mL concentrations. The
blank was DMSO in saline buffer. The bacterial and fungal plates were incubated
for 36 and 72 hours, respectively, and the activity of the compounds was
estimated by measuring the diameter of the inhibition zone (the affected zone
by the compounds) around the respective zone (the normal place in the agar).
The incubation temperature was 27±0.5°C.


## 2.1. Preparation of the Complexes

[Mo2O5L2(H2O)2]⋅H2O(1)
An aqueous
solution (5cm3) of (NH4)2[MoO4]
(0.24 g, 1.0 mmol) was added to a solution of LH (0.16 g, 1.0 mmol) in EtOH (25 cm3). The obtained slurry was heated and the resulting orange solution
was refluxed for 4 hours, during which time an orange precipitate is formed. The solid was collected by
filtration, washed with ethanol (2 cm3) and diethyl ether (2 × 5 cm3)
and dried in vacuo. The yield
was 35% (based on the metal). Elemental
analytical calculation for
C8H16N8O10S2Mo: C,
15.00; H, 2.50; N, 17.50; S, 10.00% found that C, 14.98; H, 2.82; N, 17.51; S, 9.87%; ΛM(DMSO): 3 S cm2mol−1.[W2O5L2(H2O)2]⋅H2O(2)
Using(NH4)2[WO4]
and following exactly the same procedure as that described for complex 1, a bright yellow material was
isolated. The yield was 50% (based on the metal). Elemental analytical calculation for C8H16N8O10S2W:
C, 11.77; H, 1.96; N, 13.73; S, 7.85% found that C, 11.62; H, 1.90; N, 13.77; S, 7.95%; ΛM(DMSO): 2 S cm2mol−1.[RuL2(H2O)2]⋅H2O(3)
SolidRuCl3⋅3H2O
(0.12 g, 0.46 mmol) was added to a solution of NaO2CMe (0.62 g, 7.5 mmol) in water (30 cm3). Solid LH (0.24 g, 1.5 mmol) was then added
and the resultant reaction mixture was refluxed for 12 hours. The deep brown
solid formed was collected by filtration while the reaction mixture was hot,
washed with hot water, and dried in vacuo. The yield was 30% (based on the
metal). Elemental analytical calculation for C8H16N8O5S2Ru:
C, 20.46; H, 3.41;N, 23.88; S,13.64% found that C, 20.32; H,
3.05; N, 23.57; S, 13.21%; ΛM(DMSO):
1 S cm2mol−1.[RhL3]⋅2H2O(4)
UsingRhCl3⋅3H2O
and following the same procedure as that described for complex 3, a reddish brown material was
isolated. The yield was 60% (based on the metal). Elemental analytical calculation for C12H19N12O5S3Rh:
C, 23.61; H, 3.12; N, 27.55; S, 15.74% found that C, 23.73; H, 3.11; N, 26.36; S, 14.98%;
ΛM(DMSO): 6 S cm2mol−1.[IrL3]⋅H2O(5)
UsingNa2[IrCl6] and
following the same procedure as that described for complex 3, a yellow solid was isolated. The yield was 25% (based on the
metal). Elemental analytical calculation for C12H17N12O4S3Ir:
C, 21.14; H, 2.50; N, 24.66; S, 14.09% found that C, 21.33; H, 2.64; N, 24.75; S, 13.85%; ΛM(DMSO): 5 S cm2mol−1.[RhL2(PPh3)2](ClO4)⋅2H2O(6)
A hot ethanolic
solution (20cm3) of LH (0.25 g, 1.6 mmol) was added to a solution
of RhCl3⋅3H2O
(0.21 g, 0.8 mmol) in 6 M HClO4 (15 cm3). The resultant
orange solution was refluxed for 4 hours and to this was added a solution of
PPh3 (0.43 g, 1.6 mmol) in hot ethanol (15 cm3). The new
solution was refluxed for a further 3 hours and filtered, and its volume decreased in vacuo to give a red-brown solid.
The solid was collected by filtration, washed with hot water (2 x 2 mL) and hot
ethanol (2 x 3 cm3), and dried in vacuo. The yield was 25% (based on the metal). Elemental analytical calculation for C44CIH44N8O8S2P2Rh:
C, 49.05; H, 4.09; N, 10.41; S, 3.30% found that C, 48.79; H, 4.09; N, 10.44;
S, 3.46%; ΛM(DMSO): 48 S cm2mol−1.[PdL2]⋅2H2O(7)
To a stirred
slurry of LH (0.16 g, 1.0 mmol) in methanol (15cm3) was added an
aqueous solution (15 cm3) of K2[PdCl4] (0.16 g, 0.5 mmol). The resulting suspension was stirred at 40°C for 60 hours
and the brown solid formed was collected by filtration, washed with water (5 ×
3 cm3) and cold methanol (2 × 5 cm3), and dried in air. The
yield (based on the metal) was 50%. Elemental analytical
calculation for C8H14N8O4S2Pd:
C,21.03; H, 3.07; N, 24.54; S, 14.02% found that C, 21.23; H, 3.22; N, 24.85;
S, 14.21%;  ΛM(DMSO): 9 S cm2mol−1.[PdL(phen)]Cl⋅H2O(8)
To a stirred
yellow slurry of [PdCl2(phen)] (0.18 g, 0.5 mmol) in a methanol/benzene
solvent mixture (15 cm3, 3:2 v/v) was added a solution of KOH (0.055 g, 1.0 mmol) in methanol (15 cm3). Solid LH (0.08 g, 0.5 mmol) was
added to the reaction mixture which soon dissolved. The solution was filtered
and stirred for 48 hours at room temperature. During this time, a brown
precipitate formed which was collected by filtration, washed with water (1 cm3)
and methanol (2×3cm3), and dried in air. The yield was 40% (based
on the ligand). Elemental analytical calculation for C8H14N8O4S2Pd:
C, 21.03; H, 3.07; N, 24.54; S, 14.02% found that C, 21.23; H, 3.22; N, 24.85;
S, 14.21%;  ΛM(DMSO): 77 S cm2mol−1.[ReOL2(PPh3)]CL(9)
To a stirred slurry of [ReOCl3(PPh3)2]
(0.25 g, 0.2 mmol) in ethanol (30 cm3) was added solid LH (0.057 g,
0.4 mmol). The solid soon dissolved and stirred at 40°C for 5 hours.
The brown solution deposited a brown microcrystalline solid which was collected
by filtration, washed with ethanol (3×3cm3), and dried in vacuo. The yield was 65% (based on
the metal). Elemental analytical calculation for C26CIH25N8O3S2Re:
C, 38.34; H, 3.07; N, 13.76; S, 7.86% found that C, 38.37; H, 3.11; N, 13.87;
S, 7.98%;  ΛM(DMSO):
46 S cm2mol−1.[UO2L2](10)
Solid LH (0.08 g, 0.5 mmol) was added to a
stirred solution of[UO2(NO3)2]⋅6H2O
(0.25 g, 0.5 mmol) in methanol (10 cm3). The solid soon dissolved.
The resultant yellow solution was filtered and refluxed for 4 hours, during
which time a red microcrystalline solid was precipitated. The product was
collected by filtration, washed with methanol (5 cm3) and diethyl
ether (2 × 5 cm3), and dried in
vacuo. The yield was 55% (based on the metal). Elemental analytical calculation for C8H10N8O4S2U:
C,16.44; H, 1.71; N, 19.18; S, 10.96% found that C, 16.35; H, 2.02; N, 18.98;
S, 10.86%; ΛM(DMSO): 11 S cm2mol−1.


## 2.2. Antimicrobial Activity

The bacterial
strains (S. aureus and P. aeruginosa) were grown in Nutrient
agar slants and the fungal strains (A. niger and C. albicans) were grown in Sabouraud
dextrose agar slants. The viable bacterial cells were swabbed onto Nutrient
agar plates, while the fungal spores onto Sabouraud dextrose agar plates. The
free ligand and complexes 1, 4, 7 were dissolved in DMSO, while complex 8 was dissolved in H2O with 10, 20, 50, and 100 mg/mL concentrations. The
blank was DMSO in saline buffer. The bacterial and fungal plates were incubated
for 36 and 72 hours, respectively, and the activity of the compounds was
estimated by measuring the diameter of the inhibition zone (the affected zone
by the compounds) around the respective zone (the normal place in the agar).
The incubation temperature was 27±0.5°C.


## 3. Results and Discussion

### 3.1. Synthetic Comments and Physical Characterization

The preparative
reactions for selected complexes can be represented by the stoichiometric
equations (1)–(7); no attempts
were made to optimize the yields,(1)2(NH4)2[MO4]+2LH→TEtOH/H2O[M2O5L2(H2O)2]M = Mo(1),  W(2),+4NH3+H2O(2)RhCl3⋅3H2O+3LH+3NaO2CMe→TH2O[RhL3](4)+3NaCl+3MeCO2H+3H2O,(3)RhCl3⋅3H2O+2LH+2PPh2+HCIO4→TEtOH/H2O[RhL2(PPh3)2](ClO4)(6)+3HCl+3H2O,(4)K2[PdCl4]+2LH→MeOH/H2O[PdL2](7)+2KCl+2HCl,(5)[PdCl2(phen)]+LH+KOH→MeOH/benzene[PdL(phen)]Cl(8)+KCl+H2O,(6)[ReOCl3(PPh3)2]+2LH→EtOH[ReOl2(PPh3)2]Cl(9)+2HCl+PPh3,(7)[UO2(NO3)2]⋅6H2O+2LH→TMeOH[UO2L2](10)+2HNO3+6H2O.The metal is reduced(RuIII→RuII,IrIV→IrIII) during the preparation of complexes 3 and 5 although the
reactions are performed in air. The redox reaction may be facilitated by the
reducing character of LH, the products from the oxidation of the ligand
remaining in the solution. Thus, LH possibly plays two roles in the reactions, that
is, the role of the ligand and that of the reducing agent. It is well known
that Ru(III) can undergo reduction reactions and that the [IrIVCl6]2− ion is a convenient one-electron oxidant [9]. The use of a base (KOH) in the
preparation of 8 is necessary to
obtain the complex in pure form; otherwise, the produced aqueous HCl decomposes
the compound.Complexes1–5, 7 and 10 are nonelectrolytes in DMSO
[10]. Complexes 7 and 10 exhibit slightly increased molar
conductivity values in DMSO. Since DMSO is a good donor solvent, this may be
due to the partial displacement of one L− ligand by two DMSO
molecules. Assuming an equilibrium between the neutral and the resulting
cationic complex, this displacement changes the electrolyte type of the
compound explaining the increased ΛM value [10].  From the molar
conductivities in DMSO (complexes 6 and 9) and DMF (complex 8), it is concluded that compounds 6, 8, and 9 behave as 1:1 electrolytes, supporting their ionic formulation
[10]. All the complexes are diamagnetic, as expected [9]. It should be
mentioned at this point that the π
bonding in the {ReV=O}3+ unit of 9 causes sufficient splitting of the t2g (in Oh)
set (5dxz,5dyz≫5dxy) that diamagnetism
occurs through the configuration (5dxy)2.Complexes1–10 are microcrystalline or powder-like,
stable in the normal laboratory atmosphere, and soluble only in DMF and DMSO.
We had hoped to structurally characterized one or two complexes by
single-crystal X-ray crystallography (working mainly with DMF or DMF/MeCN), but
were thwarted on numerous occasions by twinning problems or lack of single
crystals. Thus, the characterization of the complexes is based on spectroscopic
methods.


### 3.2. Electronic Spectra

The band at 335 nm in the DRS spectrum of1 is
assigned to an O2−→MoVI p–d LMCT transition and is
characteristic of the {MoO2}2+ moiety [11] in octahedral
complexes. The transition appears at 337 nm as a shoulder in solution (DMSO).
The DRS spectrum of 3 is indicative of its low-spin octahedral structure. The ground term
is A1g1 and the two spin-allowed
transitions to T1g1 and T2g1 are observed at 565 and 420 nm, respectively [12]; the
corresponding bands in DMSO are at 560 and 430 nm. The DRS spectra of the
Rh(III) complexes 4 and 6 both exhibit bands at ~470 and ~380 nm; the spectra resemble those of other six-coordinate Rh(III) compounds and
the bands are assigned as transitions from the A1g1 ground state to the T1g1 and T2g1 upper states
in octahedral symmetry in decreasing order of wavelength [12]. The lower
wavelength band may also have a charge-transfer character. Both complexes
exhibit an additional band in the blue region of the spectrum (~520 nm) which
is responsible for their red-brown colors; a possible origin of this band is
the singlet-triplet, spin-forbidden transition A1g1→T2g3 [12]. The spectrum of the Ir(III) complex 5 shows two bands at 380 and 335 nm, which
have a similar interpretation; the A1g1→T2g3 transition is not observed. A weak shoulder in the spectrum
of 9 is assigned to the T1g3(F)→T2g3 transition in a d2 octahedral environment, while
an intense band at 375 nm most probably has an LMCT origin [12]. The
ligand-field spectra of 7 and 8 are typical of a square planar
environment around pdII with a mixed N,O-ligation; the bands at 480,
375, and 330 nm are assigned [12] to the A1g1→A2g1,A1g1→Eg1, and A1g1→B1g1 transitions, respectively, under D4h symmetry.
The spectra in DMSO exhibit only two bands at 480 and 330 nm.


### 3.3. NMR Studies

DiagnosticH1
NMR assignments (in DMSO-d6) for representative complexes are
presented in Table 1. The study was based on comparison with the data obtained
for diamagnetic complexes with similar ligands [7, 13, 14]. In all the spectra
studied, the integration ratio of the signals is consistent with the
assignments.Table 1
DiagnosticH1
NMR (δ,ppm)a spectral data for LH and the representative complexes 1, 3, 4, and 6–8 in DMSO-d6.
Compound–N(1)H––N(4)H2/–N(6)H2–O(5)HLH7.436.07, 6.189.1317.576.18, 6.55—37.496.14, 6.47—47.446.20, 6.49—6b6.11, 6.47—77.476.17, 6.48—87.396.08, 6.46—Tahe spectra were run
10–15 min after dissolution.Obbscured by the
signals of the aromatic protons.The spectrum of LH exhibits two singlets atδ 6.07 and 6.18 assigned to the –N(4)H2/–N(6)H2 (for the numbering scheme see Scheme 2) amino hydrogens, respectively, and two
relatively broad singlets at δ
7.43 and 9.13 due to the amide and hydroxyl protons –N(1)H– and –O(5)H,
respectively. The appearance of these four peaks is consistent with the
exclusive presence of the thione form of LH (Scheme 2) in solution. The proton
of the hydroxyl group is not observed in the spectra of the complexes confirming
its deprotonation and coordination to the metal ions. In the spectra of 1, 3, 4, and 6–8, the –N(1)H– signal
undergoes a marginal shift to indicate the noninvolvement of this group in
coordination; a relatively large downfield shift would be expected if
coordination had occurred. In the same spectra, two signals appear for –NH2 protons, as expected. The most pronounced variation in chemical shift is the
downfield shift of one signal. Since more specific assignments of these two
signals seem impossible, it is difficult to conclude which amino nitrogen is coordinated.
NMR evidence for the presence of thione –thiol tautomerism
in the metal complexes in solution was not found.TheH1 NMR spectrum of 4 confirms that the three N,O-bidentate (vide
infra) ligands are equivalent (C3 symmetry), and, therefore, the complex has the fac stereochemistry [15].The spectrum of8 is
indicative of the presence of one solution species containing coordinated phen,
consisting of four resonances [16]. Assignments are as follows (the numbers in
parentheses are the positions of the protons in the classical numbering scheme
of 1,10-phenanthroline; s=singlet, dd=doublet of doublets): 9.15 dd(2H; 2,9),
8.53 dd(2H; 4,7), 8.00s(2H; 5,6), and 7.81q(2H; 3,8). Considerable downfield
coordination shifts, Δδ(Hl)[Δδ(Hl)=δ(Hl)complex−δ(Hl)freephen], are observed for all resonances, their values being 0.16, 0.27,
0.31, and 0.19 for the protons of the positions (2,9), (4,7), (5,6), and (3,8),
respectively. These shifts are characteristic of coordinated phen [16].TheP31{H1} NMR spectrum of the Re(V) complex 9 in DMSO-d6 consists of a
sharp singlet at δ−16.89, a
value which is typical for PPh3-containing oxorhenium(V) species
[17].


### 3.4. Vibrational Spectra

Tentative assignments
of selected IR ligand bands for complexes1–10 and free LH are listed in Table 2.
The assignments have been given by studying literature reports [3, 13, 14],
comparing the spectrum of LH with the spectra of the complexes and by
performing deuterium isotopic substitution experiments in few cases. As a
general remark, we must emphasize that some stretching and deformation modes
are coupled, so that the proposed assignments should be regarded as approximate
descriptions of the vibrations.Table 2
Diagnostic ligand IR bands
(cm−1) for LH and complexes 1–10.
Compoundvas(NH2)avs(NH2)v(N–H)v(C=N), v(C=C)v(C2–N1), v(C2–N3)δ(NH)v(C=S)LH339031852970165216521455117713377, 33303180, 31563005163115531460115523388, 33563188, 31603000163315561465117033399, 33433180, 30793010163215581464115743400, 33203160, 31053005164215601460117053387, 33203166, 31103010163515561455117763380, 33573170, 3095301516411552b1443b117673395, 33573164, 30983010164615501446117083397, 33603165, 31003015164015531450117693400, 33553155, 3105301216391555b1460b1170103405, 33243180, 310030101641155814621164Oaverlapping with the v(OH)water band in the
spectra of 1–8.Obverlapping with phenyl
stretching vibrations of the coordinated PPh3 ligands.In thev(OH)water region, the
spectra of complexes 1–3 show one medium-intensity band at
~3420 cm−1 attributed to the presence of coordinated water [13]. The
same spectra exhibit, in addition to the relatively sharp band of coordinated
water, a weaker broad continuous absorption covering the 3400–3200 cm−1 region; this is apparently due to the simultaneous presence of crystal and
coordinated water in these complexes [14]. In the spectra of 4–8, a medium broad absorption indicates
the presence of exclusively crystal (lattice) water.The absence of an IR or Raman band at ~2600cm−1 in the
spectrum of free LH suggests that the ligand exists in its thione form (see
Scheme 2) [18]. This is corroborated by the appearance of the medium v(C=S)
band at 1177 cm−1 (this vibration appears as a strong peak at 1160 cm−1 in the Raman spectrum) and the strong IR v(N–H) band at
2970 cm−1 (this vibration appears as a medium peak at ~3000 cm−1 in the Raman spectrum); the broadness and low frequency of the latter IR band
are both indicative of the involvement of the –NH– group in
strong hydrogen bonding.The medium IR
band at 3305cm−1 in the spectrum of free LH is assigned to the v(OH)
vibration. This band does not appear in the spectra of the complexes indicating
deprotonation of the –OH group and
suggesting coordination of the resulting, negatively charged oxygen atom. The
absence of large systematic shifts of the δ(N–H), δ(NH), v(C=N), v(C2–N1)/v(C2–N3),
and v (C=S) bands in the spectra of the complexes implies that there
is no interaction between the ring nitrogen atoms or the exocyclic sulfur atom
and the metal ions. The vas(NH2) and vs(NH2)
bands are doubled in the spectra of the complexes. One band for each mode
appears at almost the same wavenumber compared with the corresponding band in
the spectrum of free LH, whereas the other band of each pair is significantly
shifted to lower wavenumbers. This fact is a strong evidence for the presence
of one coordinated and one “free” (i.e.,
uncoordinated) amino group per L− in the complexes [7].The presence of coordinated PPh3 groups in 6 and 9 is
manifested  by the strong IR bands at
~1100 and ~750 cm−1, attributed to the v(P–C) and δ(CCH)
vibrations, respectively [17]; the former band overlaps with the ClO4− stetching vibration in the spectrum of the Rh(III) complex 6. In the spectrum of 8,
the bands at 1627, 1591, 1510, 1485, and 1423 cm−1 are due to the
phen stretching vibrations [16]; these bands are at higher wavenumbers compared
with the free phen indicating chelation. The bands at 854, 841, 743, and 725 cm−1 are assigned to the γ(CH) vibrations of the coordinated
phen [16].The vibrational spectra of the inorganic “parts” of complexes1, 2, 6, 9, and 10 are also
diagnostic. The IR spectrum of 6 exhibits a strong band at ~1100 and a medium band at 624 cm−1 due to
the v3(F2)
and v4(F2)
modes of the uncoordinated TdClO4− ion [19], respectively, the former having also v(P–C)
character [17]. In the 1000–750 cm−1 region, the spectra of 1 and 2 show bands characteristic of the cis-MO22+ units
and the {O2M–O–MO2}2+ core (M=Mo, W) [20, 21].
The IR bands at 930 and 912 cm−1 in 1 are assigned to the vs(MoO2)
and vas(MoO2)
modes, respectively [19, 20]; the corresponding Raman bands appear at 910 and
896 cm−1. As expected [19], the symmetric mode is weak in the IR
spectrum and strong in the Raman spectrum, while the opposite applies for the
asymmetric mode. The appearance of two stretching bands is indicative of the cis configuration [19]. The strong IR
band at 745 cm−1 is assigned to the
vas(Mo–O–Mo)
mode [20], indicating the presence of a μ-O2− group. The vs(WO2), vas(WO2),
and vas(W–O–W) bands appear at 945, 922, and 755 cm−1,
respectively, in the IR spectrum of complex 2 [19, 21]; the vs(WO2)
and vas(WO2) Raman bands are at 940 and 917 cm−1,
respectively. The vs(WO2)
and vas(WO2)
modes are at higher wavenumbers when compared to those of the analogous Mo(VI)
complex 1, suggesting [21] that the cis-WO22+ group
has some “triple” bond character [21]. In the spectra of 9, the band attributed to ν (Re=O)
appears at 956 (IR) and 968 (Raman) cm−1 [17, 19]. The IR spectrum
of the uranyl complex 10 exhibits
only one U=O stretching band, that is, vas(UO2), at 940 cm−1 (not
observed in the Raman spectrum) indicating its linear transdioxo configuration [19]. The vs(UO2)
mode appears as a strong Raman peak at 905 cm−1, and, as expected,
the corresponding IR band is very weak. The bands at 345 and 298 cm−1 in the far-IR spectrum of 7 are
assigned to the v(Pd–NH2) and v(Pd-O)
vibrations, respectively. The
appearance of one band for each mode (B3u and B2u under D2h) is consistent with a trans structure [19].


### 3.5. Antimicrobial Activity Studies

The free ligand
LH and its complexes1, 4, 7, and 8 were assayed in vitro
for antimicrobial activity against two bacterial (S. aureus and P. aeruginosa) and two fungal (A. niger and C. albicans) cultures. The hot plate diffusion method was adopted
for the activity measurements [22]. Results are listed in Tables 3 and 4.Table 3
Diameters (mm) of growth inhibitions zones for the antibacterial activity of LH
and complexes1, 4, 7, and 8.
CompoundS. aureus(mg/cm3)aP. aeruginosa(mg/cm3)a102050100102050100LH511285841030558132247991017418371017428691439774813294971330441101943807133261mga/cm3 represents the concentration of the reagent in the gel.Table 4
Diameters (mm) of growth inhibition zones for the antifungal activity of LH and
complexes1, 4, 7, and 8.
CompoundA. niger(mg/cm3)aC. albicans(mg/cm3)a102050100102050100LH5122344693455815214391131944897101740799163868400006112137161531637163368Nystatinb1719366520264279mga/cm3 represents the concentration of the reagent in the gel.Ab
currently prescribed antifungal drug.In general, the Pd(II) complexes7 and 8 were found to have higher
efficacy than 1, 4, and LH at the measured concentrations. The water-soluble complex 8 is the most active against the
pathogens studied. It is remarkable that the antifungal activity of 8 is comparable with, or even better
than, the activity of the antifungal drug nystatin, and this may be due to the
simultaneous presence of phen and L− in the complex. The activity of
the Pd(II) complexes 7 and 8 is tentatively attributed to their
inhibition of the DNA replication (by interacting with enzyme prosthetic groups
and altering the microbial metabolism) and their ability to form hydrogen bonds
with the cell wall and cell constituents [23]. The weaker activity of 4 is noteworthy; the reason for this is
not clear.


## 3.1. Synthetic Comments and Physical Characterization

The preparative
reactions for selected complexes can be represented by the stoichiometric
equations (1)–(7); no attempts
were made to optimize the yields,(1)2(NH4)2[MO4]+2LH→TEtOH/H2O[M2O5L2(H2O)2]M = Mo(1),  W(2),+4NH3+H2O(2)RhCl3⋅3H2O+3LH+3NaO2CMe→TH2O[RhL3](4)+3NaCl+3MeCO2H+3H2O,(3)RhCl3⋅3H2O+2LH+2PPh2+HCIO4→TEtOH/H2O[RhL2(PPh3)2](ClO4)(6)+3HCl+3H2O,(4)K2[PdCl4]+2LH→MeOH/H2O[PdL2](7)+2KCl+2HCl,(5)[PdCl2(phen)]+LH+KOH→MeOH/benzene[PdL(phen)]Cl(8)+KCl+H2O,(6)[ReOCl3(PPh3)2]+2LH→EtOH[ReOl2(PPh3)2]Cl(9)+2HCl+PPh3,(7)[UO2(NO3)2]⋅6H2O+2LH→TMeOH[UO2L2](10)+2HNO3+6H2O.The metal is reduced(RuIII→RuII,IrIV→IrIII) during the preparation of complexes 3 and 5 although the
reactions are performed in air. The redox reaction may be facilitated by the
reducing character of LH, the products from the oxidation of the ligand
remaining in the solution. Thus, LH possibly plays two roles in the reactions, that
is, the role of the ligand and that of the reducing agent. It is well known
that Ru(III) can undergo reduction reactions and that the [IrIVCl6]2− ion is a convenient one-electron oxidant [9]. The use of a base (KOH) in the
preparation of 8 is necessary to
obtain the complex in pure form; otherwise, the produced aqueous HCl decomposes
the compound.Complexes1–5, 7 and 10 are nonelectrolytes in DMSO
[10]. Complexes 7 and 10 exhibit slightly increased molar
conductivity values in DMSO. Since DMSO is a good donor solvent, this may be
due to the partial displacement of one L− ligand by two DMSO
molecules. Assuming an equilibrium between the neutral and the resulting
cationic complex, this displacement changes the electrolyte type of the
compound explaining the increased ΛM value [10].  From the molar
conductivities in DMSO (complexes 6 and 9) and DMF (complex 8), it is concluded that compounds 6, 8, and 9 behave as 1:1 electrolytes, supporting their ionic formulation
[10]. All the complexes are diamagnetic, as expected [9]. It should be
mentioned at this point that the π
bonding in the {ReV=O}3+ unit of 9 causes sufficient splitting of the t2g (in Oh)
set (5dxz,5dyz≫5dxy) that diamagnetism
occurs through the configuration (5dxy)2.Complexes1–10 are microcrystalline or powder-like,
stable in the normal laboratory atmosphere, and soluble only in DMF and DMSO.
We had hoped to structurally characterized one or two complexes by
single-crystal X-ray crystallography (working mainly with DMF or DMF/MeCN), but
were thwarted on numerous occasions by twinning problems or lack of single
crystals. Thus, the characterization of the complexes is based on spectroscopic
methods.


## 3.2. Electronic Spectra

The band at 335 nm in the DRS spectrum of1 is
assigned to an O2−→MoVI p–d LMCT transition and is
characteristic of the {MoO2}2+ moiety [11] in octahedral
complexes. The transition appears at 337 nm as a shoulder in solution (DMSO).
The DRS spectrum of 3 is indicative of its low-spin octahedral structure. The ground term
is A1g1 and the two spin-allowed
transitions to T1g1 and T2g1 are observed at 565 and 420 nm, respectively [12]; the
corresponding bands in DMSO are at 560 and 430 nm. The DRS spectra of the
Rh(III) complexes 4 and 6 both exhibit bands at ~470 and ~380 nm; the spectra resemble those of other six-coordinate Rh(III) compounds and
the bands are assigned as transitions from the A1g1 ground state to the T1g1 and T2g1 upper states
in octahedral symmetry in decreasing order of wavelength [12]. The lower
wavelength band may also have a charge-transfer character. Both complexes
exhibit an additional band in the blue region of the spectrum (~520 nm) which
is responsible for their red-brown colors; a possible origin of this band is
the singlet-triplet, spin-forbidden transition A1g1→T2g3 [12]. The spectrum of the Ir(III) complex 5 shows two bands at 380 and 335 nm, which
have a similar interpretation; the A1g1→T2g3 transition is not observed. A weak shoulder in the spectrum
of 9 is assigned to the T1g3(F)→T2g3 transition in a d2 octahedral environment, while
an intense band at 375 nm most probably has an LMCT origin [12]. The
ligand-field spectra of 7 and 8 are typical of a square planar
environment around pdII with a mixed N,O-ligation; the bands at 480,
375, and 330 nm are assigned [12] to the A1g1→A2g1,A1g1→Eg1, and A1g1→B1g1 transitions, respectively, under D4h symmetry.
The spectra in DMSO exhibit only two bands at 480 and 330 nm.


## 3.3. NMR Studies

DiagnosticH1
NMR assignments (in DMSO-d6) for representative complexes are
presented in Table 1. The study was based on comparison with the data obtained
for diamagnetic complexes with similar ligands [7, 13, 14]. In all the spectra
studied, the integration ratio of the signals is consistent with the
assignments.Table 1
DiagnosticH1
NMR (δ,ppm)a spectral data for LH and the representative complexes 1, 3, 4, and 6–8 in DMSO-d6.
Compound–N(1)H––N(4)H2/–N(6)H2–O(5)HLH7.436.07, 6.189.1317.576.18, 6.55—37.496.14, 6.47—47.446.20, 6.49—6b6.11, 6.47—77.476.17, 6.48—87.396.08, 6.46—Tahe spectra were run
10–15 min after dissolution.Obbscured by the
signals of the aromatic protons.The spectrum of LH exhibits two singlets atδ 6.07 and 6.18 assigned to the –N(4)H2/–N(6)H2 (for the numbering scheme see Scheme 2) amino hydrogens, respectively, and two
relatively broad singlets at δ
7.43 and 9.13 due to the amide and hydroxyl protons –N(1)H– and –O(5)H,
respectively. The appearance of these four peaks is consistent with the
exclusive presence of the thione form of LH (Scheme 2) in solution. The proton
of the hydroxyl group is not observed in the spectra of the complexes confirming
its deprotonation and coordination to the metal ions. In the spectra of 1, 3, 4, and 6–8, the –N(1)H– signal
undergoes a marginal shift to indicate the noninvolvement of this group in
coordination; a relatively large downfield shift would be expected if
coordination had occurred. In the same spectra, two signals appear for –NH2 protons, as expected. The most pronounced variation in chemical shift is the
downfield shift of one signal. Since more specific assignments of these two
signals seem impossible, it is difficult to conclude which amino nitrogen is coordinated.
NMR evidence for the presence of thione –thiol tautomerism
in the metal complexes in solution was not found.TheH1 NMR spectrum of 4 confirms that the three N,O-bidentate (vide
infra) ligands are equivalent (C3 symmetry), and, therefore, the complex has the fac stereochemistry [15].The spectrum of8 is
indicative of the presence of one solution species containing coordinated phen,
consisting of four resonances [16]. Assignments are as follows (the numbers in
parentheses are the positions of the protons in the classical numbering scheme
of 1,10-phenanthroline; s=singlet, dd=doublet of doublets): 9.15 dd(2H; 2,9),
8.53 dd(2H; 4,7), 8.00s(2H; 5,6), and 7.81q(2H; 3,8). Considerable downfield
coordination shifts, Δδ(Hl)[Δδ(Hl)=δ(Hl)complex−δ(Hl)freephen], are observed for all resonances, their values being 0.16, 0.27,
0.31, and 0.19 for the protons of the positions (2,9), (4,7), (5,6), and (3,8),
respectively. These shifts are characteristic of coordinated phen [16].TheP31{H1} NMR spectrum of the Re(V) complex 9 in DMSO-d6 consists of a
sharp singlet at δ−16.89, a
value which is typical for PPh3-containing oxorhenium(V) species
[17].


## 3.4. Vibrational Spectra

Tentative assignments
of selected IR ligand bands for complexes1–10 and free LH are listed in Table 2.
The assignments have been given by studying literature reports [3, 13, 14],
comparing the spectrum of LH with the spectra of the complexes and by
performing deuterium isotopic substitution experiments in few cases. As a
general remark, we must emphasize that some stretching and deformation modes
are coupled, so that the proposed assignments should be regarded as approximate
descriptions of the vibrations.Table 2
Diagnostic ligand IR bands
(cm−1) for LH and complexes 1–10.
Compoundvas(NH2)avs(NH2)v(N–H)v(C=N), v(C=C)v(C2–N1), v(C2–N3)δ(NH)v(C=S)LH339031852970165216521455117713377, 33303180, 31563005163115531460115523388, 33563188, 31603000163315561465117033399, 33433180, 30793010163215581464115743400, 33203160, 31053005164215601460117053387, 33203166, 31103010163515561455117763380, 33573170, 3095301516411552b1443b117673395, 33573164, 30983010164615501446117083397, 33603165, 31003015164015531450117693400, 33553155, 3105301216391555b1460b1170103405, 33243180, 310030101641155814621164Oaverlapping with the v(OH)water band in the
spectra of 1–8.Obverlapping with phenyl
stretching vibrations of the coordinated PPh3 ligands.In thev(OH)water region, the
spectra of complexes 1–3 show one medium-intensity band at
~3420 cm−1 attributed to the presence of coordinated water [13]. The
same spectra exhibit, in addition to the relatively sharp band of coordinated
water, a weaker broad continuous absorption covering the 3400–3200 cm−1 region; this is apparently due to the simultaneous presence of crystal and
coordinated water in these complexes [14]. In the spectra of 4–8, a medium broad absorption indicates
the presence of exclusively crystal (lattice) water.The absence of an IR or Raman band at ~2600cm−1 in the
spectrum of free LH suggests that the ligand exists in its thione form (see
Scheme 2) [18]. This is corroborated by the appearance of the medium v(C=S)
band at 1177 cm−1 (this vibration appears as a strong peak at 1160 cm−1 in the Raman spectrum) and the strong IR v(N–H) band at
2970 cm−1 (this vibration appears as a medium peak at ~3000 cm−1 in the Raman spectrum); the broadness and low frequency of the latter IR band
are both indicative of the involvement of the –NH– group in
strong hydrogen bonding.The medium IR
band at 3305cm−1 in the spectrum of free LH is assigned to the v(OH)
vibration. This band does not appear in the spectra of the complexes indicating
deprotonation of the –OH group and
suggesting coordination of the resulting, negatively charged oxygen atom. The
absence of large systematic shifts of the δ(N–H), δ(NH), v(C=N), v(C2–N1)/v(C2–N3),
and v (C=S) bands in the spectra of the complexes implies that there
is no interaction between the ring nitrogen atoms or the exocyclic sulfur atom
and the metal ions. The vas(NH2) and vs(NH2)
bands are doubled in the spectra of the complexes. One band for each mode
appears at almost the same wavenumber compared with the corresponding band in
the spectrum of free LH, whereas the other band of each pair is significantly
shifted to lower wavenumbers. This fact is a strong evidence for the presence
of one coordinated and one “free” (i.e.,
uncoordinated) amino group per L− in the complexes [7].The presence of coordinated PPh3 groups in 6 and 9 is
manifested  by the strong IR bands at
~1100 and ~750 cm−1, attributed to the v(P–C) and δ(CCH)
vibrations, respectively [17]; the former band overlaps with the ClO4− stetching vibration in the spectrum of the Rh(III) complex 6. In the spectrum of 8,
the bands at 1627, 1591, 1510, 1485, and 1423 cm−1 are due to the
phen stretching vibrations [16]; these bands are at higher wavenumbers compared
with the free phen indicating chelation. The bands at 854, 841, 743, and 725 cm−1 are assigned to the γ(CH) vibrations of the coordinated
phen [16].The vibrational spectra of the inorganic “parts” of complexes1, 2, 6, 9, and 10 are also
diagnostic. The IR spectrum of 6 exhibits a strong band at ~1100 and a medium band at 624 cm−1 due to
the v3(F2)
and v4(F2)
modes of the uncoordinated TdClO4− ion [19], respectively, the former having also v(P–C)
character [17]. In the 1000–750 cm−1 region, the spectra of 1 and 2 show bands characteristic of the cis-MO22+ units
and the {O2M–O–MO2}2+ core (M=Mo, W) [20, 21].
The IR bands at 930 and 912 cm−1 in 1 are assigned to the vs(MoO2)
and vas(MoO2)
modes, respectively [19, 20]; the corresponding Raman bands appear at 910 and
896 cm−1. As expected [19], the symmetric mode is weak in the IR
spectrum and strong in the Raman spectrum, while the opposite applies for the
asymmetric mode. The appearance of two stretching bands is indicative of the cis configuration [19]. The strong IR
band at 745 cm−1 is assigned to the
vas(Mo–O–Mo)
mode [20], indicating the presence of a μ-O2− group. The vs(WO2), vas(WO2),
and vas(W–O–W) bands appear at 945, 922, and 755 cm−1,
respectively, in the IR spectrum of complex 2 [19, 21]; the vs(WO2)
and vas(WO2) Raman bands are at 940 and 917 cm−1,
respectively. The vs(WO2)
and vas(WO2)
modes are at higher wavenumbers when compared to those of the analogous Mo(VI)
complex 1, suggesting [21] that the cis-WO22+ group
has some “triple” bond character [21]. In the spectra of 9, the band attributed to ν (Re=O)
appears at 956 (IR) and 968 (Raman) cm−1 [17, 19]. The IR spectrum
of the uranyl complex 10 exhibits
only one U=O stretching band, that is, vas(UO2), at 940 cm−1 (not
observed in the Raman spectrum) indicating its linear transdioxo configuration [19]. The vs(UO2)
mode appears as a strong Raman peak at 905 cm−1, and, as expected,
the corresponding IR band is very weak. The bands at 345 and 298 cm−1 in the far-IR spectrum of 7 are
assigned to the v(Pd–NH2) and v(Pd-O)
vibrations, respectively. The
appearance of one band for each mode (B3u and B2u under D2h) is consistent with a trans structure [19].


## 3.5. Antimicrobial Activity Studies

The free ligand
LH and its complexes1, 4, 7, and 8 were assayed in vitro
for antimicrobial activity against two bacterial (S. aureus and P. aeruginosa) and two fungal (A. niger and C. albicans) cultures. The hot plate diffusion method was adopted
for the activity measurements [22]. Results are listed in Tables 3 and 4.Table 3
Diameters (mm) of growth inhibitions zones for the antibacterial activity of LH
and complexes1, 4, 7, and 8.
CompoundS. aureus(mg/cm3)aP. aeruginosa(mg/cm3)a102050100102050100LH511285841030558132247991017418371017428691439774813294971330441101943807133261mga/cm3 represents the concentration of the reagent in the gel.Table 4
Diameters (mm) of growth inhibition zones for the antifungal activity of LH and
complexes1, 4, 7, and 8.
CompoundA. niger(mg/cm3)aC. albicans(mg/cm3)a102050100102050100LH5122344693455815214391131944897101740799163868400006112137161531637163368Nystatinb1719366520264279mga/cm3 represents the concentration of the reagent in the gel.Ab
currently prescribed antifungal drug.In general, the Pd(II) complexes7 and 8 were found to have higher
efficacy than 1, 4, and LH at the measured concentrations. The water-soluble complex 8 is the most active against the
pathogens studied. It is remarkable that the antifungal activity of 8 is comparable with, or even better
than, the activity of the antifungal drug nystatin, and this may be due to the
simultaneous presence of phen and L− in the complex. The activity of
the Pd(II) complexes 7 and 8 is tentatively attributed to their
inhibition of the DNA replication (by interacting with enzyme prosthetic groups
and altering the microbial metabolism) and their ability to form hydrogen bonds
with the cell wall and cell constituents [23]. The weaker activity of 4 is noteworthy; the reason for this is
not clear.


## 4. Conclusions

The M/LH general
reaction system fulfilled its promise as a source of interesting complexes.
From the overall evidence presented before, it seems that the ligandL− behaves as a bidentate chelate in all the prepared complexes with the deprotonated
oxygen and most probably the amino nitrogen of the position 6 of the pyrimidine
ring being the donor atoms, see Scheme 3. However, the participation of the
amino nitrogen of the position 4 of the ring cannot be ruled out. The
nonparticipation of the sulfur atom in coordination in complexes 7 and 8 may be seen as unusual given the soft character of Pd(II) in the
context of the HSAB concept.Scheme 3
The
proposed coordination mode of the anionic ligandL− in complexes 1–10; M=metal ion.The chelate
effect (a stable chelating ring with the participation of the sulfur atom
cannot be formed due to the geometry ofL−) seems to govern the
thermodynamic stability of these complexes. The proposed gross schematic
structures for 1–10 are shown in Figure 1. Due to the
fact that single-crystal, X-ray crystallographic studies are not available, few
structural features (e.g., the
symmetric structures of 1–3, 6, 7, and 10) are
tentative. The metal ions adopt octahedral (1–6, 9, 10) or square planar (7, 8)
stereochemistries.Figure 1
Schematic structures proposed for the neutral complexes1–5, 7, 10 and for the cations of complexes 6, 8, 9. Lattice H2O molecules
and counterions have been omitted for clarity. However, NO⌢ and N′N′⌢ represent the ligands L− and phen, respectively.
The N and O donor atoms of L− are the amino nitrogen of the position
6 (most probably) and the deprotonated oxygen of the position 5 of the
pyrimidine ring; M=Mo, W; M′=Rh, Ir.Finally,
complexes1, 4, 7, and 8 are new welcome additions in the growing family of metal
complexes with antimicrobial activity.The results described in this report represent the initial study of the
coordination chemistry of LH and the biological activity of its complexes.
Further studies with 3d-metal ions are in progress.



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*Source: 647873-2009-03-23.xml*