Functionalized thiosemicarbazone ligands and their complexes

The purpose of this work is the design, synthesis and characterization of thiosemicarbazone ligands bearing a phenyl boronic acid functionality. The main interest is the synthesis of organopalladium compounds with the boronic acid included within the corresponding molecule. These could lead to further modification of the properties of the organometallic species applicable to the Suzuki-Miyaura reaction. The ligands were prepared by reaction of acetylphenylboronic acid with several thiosemicarbazides. The compounds were characterized by IR, 1 H and 31 P NMR spectroscopy.


Introduction
Thiosemicarbazones are versatile ligands which are able to coordinate to metals through C, N and S atoms.When bonding to the metal is through the sulfur, iminic nitrogen and a carbon atoms cyclometallated compounds are formed.These have aroused great interest due to their wide range of applications.Their biological activity depends on the parent aldehyde or ketone [1][2]; also, palladium (II) complexes have shown to be catalytically active in the Heck and Suzuki-Miyaura cross-coupling reactions [3][4].
Previous studies have shown the tetranuclear structure of these compounds [5][6], which implies a low solubility in the common organic solvents, limiting their potential applications.For this reason we pursue tetranuclear compounds with better solution properties by using different organic precursors; in this particular case an organic precursor bearing the boronic acid functionality.The boronic acid could lead to further modification of the structure of the cyclometallated tetranuclear compound using the Suzuki cross-coupling reaction.
Herein we report the synthesis of a series of cyclometallated compounds derived from a boronic thiosemicarbazone.

Results and discussion
The synthesis of the ligands is shown in Scheme 1.

Scheme 1
The reaction of the ligands with potassium tetrachloropalladate gave the corresponding Pd(II) compounds; in which the ligands are bonded to the metal through the ortho-C of the phenyl ring and coordinated to the metal by the nitrogen and sulfur atoms.Each palladium atom is bonded to two different sulfur atoms, forming two types of bonds: Metal-S chelate and Metal-S bridging .(Scheme 2) Scheme 2 The 1 H NMR spectra of the ligands and their corresponding Pd(II) compounds confirmed the formation of the species and gave information on their structure.
In the spectra of the ligands a singlet signal ca.10.2 ppm was assigned to the hydrazinic proton and AA'BB' system ca.7.75 ppm to the phenyl protons (4H).In the spectra of the compounds, hydrazinic proton resonance is absent in accordance with coordination in the thiolic form.Metallation is evidenced by changes in the aromatic region.The IR spectra of the ligands showed bands for the C=S group and hydrazinic N-H stretch, which are absent in the spectra of the compounds, confirming the deprotonation of the thiosemicarbazone and the coordination in the thiolic form.
The shift of the ν(C=N) band implies coordination is through the nitrogen lone pair and not through the C=N double bond.

Experimental
The ligands were prepared by the condensation of acetylphenylboronic acid with different thiosemicarbazides.Acetylphenylboronic acid and hydrochloric acid (35 %) were added to a suspension of the corresponding thiosemicarbazide in water (25 cm 3 ) to give a clear solution, which was stirred at room temperature for 4 h.The white solid that precipitated was filtered off, washed with cold water and dried in air.
The palladium compounds were prepared by the addition of a solution of K 2 PdCl 4 in water to a ethanolic solution of the ligand, as appropiate.The reaction mixture was then kept at room temperature for 24 h.The precipitated yellow solids were filtered, redisolved in DCM and filtered.The solvent was eliminated to yield the compounds in 78-88% yield.

1 H
NMR spectrum of ligand L2 and the corresponding compound C2.