Journal of Molecular Structure 1040 (2013) 221–225

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Journal of Molec ular Stru cture

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Spectroscopic, and electrochemical studies of [MCl 2(g2-N,N-dpksc)] (M = Zn, Cd,Hg and dpksc = di-2-pyridylketone semicarbazone)

Mohammed Bakir ⇑, Covel McDermot, Toni Johnson Department of Chemistry, The University of the West Indies–Mona Campus, Kingston 7, Jamaica, West Indies

h i g h l i g h t s

" [MCl2(g2-N,N-dpksc)] (M = Zn, Cd or Hg; and dpksc = di-2-pyridylket one semicarbazone) were prepared from the direct reactions between MCl 2 anddpksc in CH 3CN.

" The amine protons are more sensitive to temperature variations compared to the amide proton." Reversible inter-conversion between [MCl 2(g2-N,N-dpksc)] and its conjugate base was established." [MCl2(g2-N,N-dpksc)] and dpksc are not stable to redox transformat ions.

a r t i c l e i n f o

Article history:Received 14 December 2012 Received in revised form 25 February 2013 Accepted 25 February 2013 Available online 5 March 2013

Keywords:Di-2-pyridylketone semicarbazone Group 12 metal chlorides SpectroscopyElectrochemistry

0022-2860/$ - see front matter � 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.molstruc.2013.02.027

⇑ Corresponding author. Tel.: +1 876 935 8164; faxE-mail address: mohammed.bakir@uwimona.edu.j

a b s t r a c t

A series of group 12 metal chlorides of dpksc of the type [MCl 2(g2-N,N-dpksc)] (M = Zn, Cd and Hg) were synthesized from the reactions between MCl 2 and dpksc in refluxing CH 3CN. Spectroscopic measure- ments performed on protophilic solutions of [MCl 2(g2-N,N-dpksc)] divulg ed sensitivity of [MCl 2(g2-N,N-dpksc)] to chan ges in their surroundings. Variable temperature 1H NMR studies revealed the amine protons to be more sensitive to temperat ure variations compared to the amide and aromatic protons.Electronic absorption spectral measurements disclosed acid–base inter-conversion between [MCl 2(g2-N,N-dpksc)] and its conjugate base [MCl 2(g2-N,N-dpksc-H)]�. In dmf, the acidity of [MCl 2(g2-N,N-dpksc)]increases in the following order: [ZnCl 2(g2-N,N-dpksc)] > [CdCl 2(g2-N,N-dpksc)] > [HgCl 2(g2-N,N-dpksc)]. The electrochemical properties of [MCl 2(g2-N,N-dpksc)] are dominated by a series of irreversible redox transformat ions pointing to electrochemical decomposition of [MCl 2(g2-N,N-dpksc)] upon elec- tronic transfers .

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Di-2-pyridy lketone [(C5H4N)2C@O] (dpk) and a variety of its derivatives that include oxime, hydrazonic ligands (see Scheme 1)and their metal compound s continue to attract research interest because of their reactivity patterns, physical propertie s and chem- ical and biological applications [1–35]. Di-2-pyridy lketone deriva- tives bind to metal moieties utilizing a range of binding modes to form a wide range of complexes [1–35]. The excellent chelating properties of di-2-pyidylket one derivatives allow for their use as potent and specific anti-cancer agents [20–24]. A number of di-2- pyridylketo ne derivatives have been utilized as sensitive analytical reagents for the detection and determinati on of trace amounts of metal ions in environmental and pharmaceuti cal samples [13,14].

We have been intereste d in the chemistry of di-2-pyridy lketone derivatives and reported on the synthesis of a series of metal com-

ll rights reserved.

: +1 876 977 1835.m (M. Bakir).

pounds containing di-2-pyridylketon e derivatives [26–34]. In our efforts to explore the coordinatio n chemistry and molecular sens- ing behavior of di-2-pyridy lketone derivatives, in this report we describe the spectroscopi c, and electrochemical properties of di- 2-pyridy lketone semicarbazo ne (dpksc) and its group 12 metal chlorides of the type [MCl 2(g2-N,N-dpksc] (see Scheme 2).

The solution chemistry of dpksc is scarce. Only two reports ap- peared on the synthesis and solid state structures of Pb(II), Mn(II),Co(II), Ni(II) and Cu(II) nitrates of O,N,N-tridenta te dpksc [35,36].

2. Experimen tal

2.1. Reagents and reaction procedures

All reagents were obtained from commercial sources and used without further purification. Following a procedure similar to that reported in the literature, dpksc (1) melting point 210–212 �C, was prepared by refluxing a mixture of semicarbazide, dpk and

Scheme 2. [dpksc] and its [MCl 2(g2-N,N-dpksc)].

Fig. 1. 1H NMR spectra of dpksc, and [MCl 2g2-N,N-dpksc)] (M = Zn, Cd and Hg)measured at 308 K in d6-dmso.

Scheme 1. Selected di-2-pyridylketone hydrazonic ligands.

222 M. Bakir et al. / Journal of Molecular Structure 1040 (2013) 221–225

methanol for 3 h (see Supplement data for synthetic details, ele- mental analysis and spectroscopi c data) [35].

2.2. Synthesis of [MCl 2(g2-N,N-dpks c)] (M = Zn, Cd or Hg)

In a typical synthesis a mixture of equi-molar amounts (�0.10 mmol) of MCl 2 and dpksc was refluxed in CH 3CN for several hours. The resulting reaction mixture was allowed to cool to room temperature . Slow evaporati on of the mixture gave yellow powder that was filtered off, washed with diethyl ether and dried (see Sup-plemental data for synthetic details, elemental analyses and spec- troscopic data).

2.3. Physical measuremen ts

The elemental microanalys es were performed using a Perkin–Elmer Series II CHNS/O Analyzer. Electronic absorption spectra were recorded on a Hewlett Packard 8453 spectrophot ometer.Baseline corrections on blank solvents were recorded prior to mea- surements. 1H NMR and 13C NMR spectra were recorded on a Bru- ker Avance 500 DRX spectromete r and referenced to the residual protons in the incompletely deuteriat ed solvent. Infrared spectra of the neat compounds were recorded with a Bruker-Tens or 37 spectromete r, OPUS 6.5. Electrochem ical measureme nts were per- formed with the use of a Princeton Applied Research (PAR) Model 173 potentiostat/ga lvanostat and model 276 interface in conjunc- tion with a 286 PC. Measurements were performed in solutions that were 0.1 M in Tetrabutyl ammonia hexafluorophosphate (TBAH). The Ep,a, Ep,c and E1/2 = (Ep,a + Ep,c)/2 values were referenced to the quasi-revers ible silver electrode (QRSE) at room tempera- ture and are uncorrected for junction potential. Electrochemical cells were of conventional design, based on scintillat ion vials or H cells. A glassy carbon disk was used as the working electrode,and a Pt wire as the counter electrode.

3. Results and discussion

The reactions between dpksc and MCl 2 (M = Zn, Cd or Hg) in refluxing CH 3CN gave [MCl 2(g2-N,N-dpksc)] (M = Zn, Cd and Hg].The infrared spectra of [MCl 2(g2-N,N-dpksc)] (see Supplement al data) revealed peaks due to m(NH), m(NH2), m(CH), m(C@O), and

the combined m(C@C) and m(C@N) of the pyridine rings and other peaks consisten t with the coordination of dpksc. The amide m(NH) stretching frequency, observed at 3442 cm �1 in free dpksc,shifts to lower wavenumber , while the m(C@O) stretching fre- quency, observed at 1658 cm �1, and the combined m(C@C) and m(C@N) of the pyridine rings, observed between 1600 and 1400 cm �1 in free dpksc, shift to higher wavenumber s upon coor- dination to MCl 2. The appearance of mC@O) stretching vibration in the spectra of [MCl 2(g2-N,N-dpksc)] hints to the absence of coordi- nation between the carbonyl group (C@O) and the metal center.The increase in the combined pyridine m(C@C) and m(C@N) stretch- ing frequenc ies to higher wavenum bers upon coordinatio n to MCl 2is consisten t with the N,N-coordin ation of the pyridine rings to the metal center.

The 1H NMR spectra of dpksc and [MCl 2(g2-N,N-dpksc)] mea- sured at 308 K in d6-dmso are shown in Fig. 1. In the spectra of dpksc, eight resonances appeared in the aromatic region, a single resonance appeared in the amide (NH) region, and a single reso- nance appeared in the amine (NH2) region. The 13C NMR spectra of dpksc revealed 12 resonances with C15 and C25 observed at 151.92 and 156.92 ppm, respectively and C11 and C21 observed at 126.86 and 123.56, respectively. The observed resonances re- vealed that the pyridine rings are not equivalent. A series of NMR measure ments confirmed the assigned resonances (see Supple-mental data ). Upon coordinatio n to MCl 2, changes appeared in the chemical shifts of coordinated dpksc in [MCl 2(g2-N,N-dpksc)]compare d to the chemical shifts of free dpksc. The aromatic proton H11 of the free ligand, observed at 7.47 ppm, shifts down-field to 7.65, 7.54 and 7.54 ppm whereas the aromatic proton H21 of the free ligand, observed at 8.17 ppm, shifts up-field to 7.95, 7.98 and 7.95 ppm upon coordination to ZnCl 2, CdCl 2 and HgCl 2, respec- tively. These results signify structural changes in dpksc when coor- dinated to MCl 2 and hints to N,N-coordinati on of dpksc through the nitrogen atoms of the pyridine rings. This is due to the close

ig. 2. Variable temperature 1H NMR spectra of [MCl 2g2-N,N-dpksc)] measured in 6-dmso.

Table 1Slope of 1/ T � 103 versus d of NH and NH 2 of dpksc, [MCl 2(g2-N,N-dpksc)] (M = Zn, Cd and Hg) in d6-dmso.

Compound Amide (NH) Amine (NH2) H2O (d6-dmso)

Dpksc +0.11 +0.59 +0.58 [ZnCl 2(g2-N,N-dpksc)] +0.10 +0.70 +0.56 [CdCl 2(g2-N,N-dpksc)] +0.14 +0.58 +0.52 [HgCl 2(g2-N,N-dpksc)] �0.14 +0.67 +0.48

Fig. 3. Electronic absorption spectra of dpksc, and [MCl 2g2-N,N-dpksc)] (M = Zn, Cd and Hg) measured in dmf (see experimental section for concentrations).

M. Bakir et al. / Journal of Molecular Structure 1040 (2013) 221–225 223

proximity H11 and H21 to the metal center. The other aromatic protons of [MCl 2(g2-N,N-dpk sc)] shift down-field compared to free dpksc. The chemical shift of C11 of the free ligand, observed at 126.86 ppm, shifts to 124.25, 123.57 and 123.78 ppm, while C21 of the free ligand, observed at 123.56 shifts to 126.65, 126.16 and 126.10 ppm upon coordina tion to ZnCl 2, CdCl 2 and HgCl 2,respectively . These observations confirm the pyridyl N,N-coord ina- tion of dpksc. The amine protons of the free ligand, observed at 6.83 ppm, shift to 7.10, 6.90 and 6.84 upon coordinatio n to ZnCl 2,CdCl2 and HgCl 2, respectivel y. This is due to the delocalization of electron density from the amine to the metal center and hints a de- crease in electron density about the amine protons. The amide (NH) proton of free dpksc, observed at 11.86 ppm, shifts to 11.78,11.69 and 11.67 ppm upon coordination to ZnCl 2, CdCl 2, and HgCl 2,respectively . Integration of the amide (NH), H14 and amine (NH2)protons revealed the absence of proton/deuter ium exchange be- tween the amide proton and solvent deuterium atoms while slight proton/deuter ium exchange between the amine protons of [MCl 2(g2-N,N-dpk sc)] (M = Zn or Cd) and solvent deuterium. In the 13CNMR spectra of dpksc, the chemical shift of the carbonyl carbon (C@O) atom (C02) and the hydrazonic carbon atom (C01) observed at 156.49 and 140.90 ppm, respectivel y, insignificantly shift (�±0.1 ppm) upon coordinatio n to MCl 2 (see Supplement al data ).This confirm the absence of coordination between the metal center and the semicarbazone (C@NANHACOANH2) moiety.

Variable temperature 1H NMR measurements performed on d6-dmso solutions of dpksc, and [MCl 2(g2-N,N-dpksc)] (M = Zn, Cd,Hg) disclosed temperature dependent and temperat ure indepen- dent protons. Temperatur e dependent protons point to the shielding of the protons due to solvent-c ompound interaction or intra-molecul ar hydrogen bonds. Temperatur e independent pro- tons point to the exposure of the protons to their surroundings.The 1H NMR spectra of [CdCl 2(g2-N,N-dpksc)] in the amide, aro- matic and amine regions measured at different temperat ures are shown in Fig. 2. These spectra show the amine protons to be more sensitive to temperat ure variations compare d to the amide and aromatic protons. As the temperature increased, the amine protons shift up-field pointing to an increase in their electron density. This may be due to an increase in solvent-compo und interaction and electron donation from the solvent to the amine protons. Similar results were obtained for [MCl 2(g2-N,N-dpk sc)] (M = Zn and Hg).Plots of 1/ T � 103 K versus d (chemical shift) of the amine, amide and water protons of d6-dmso of [dpksc] and [MCl 2(g2-N,N-dpksc)] (M = Zn, Cd or Hg) gave straight lines with slopes as shown in Table 1. The slope is directly proportional to the sensitivit y of the proton to temperature changes. These results disclosed that the amine protons are more sensitive to temperature variations than the amide proton. The water protons show significant tem- perature dependence comparable to that of the amine protons and suggest a possible amine-water interaction.

The electroni c absorption spectra of dpksc, and [MCl 2(g2-N,N-dpksc)] (M = Zn, Cd and Hg) measured in dmf are shown in Fig. 3. A single electronic transition appeared in the spectra of dpksc and [HgCl 2(g2-N,N-dpksc)] and two electronic transitions appeared in the spectra of [MCl 2(g2-N,N-dpk sc)] (M = Zn and Cd].In dmso, a single electronic transition appeared in the spectra of dpksc and two electronic transitions appeared in the spectra of [MCl2(g2-N,N-dpksc)] (M = Zn, Cd or Hg) (see Supplement al data ).The electronic transitions of dpksc are intra-ligand charge transfer (ILCT) assigned to amine n ? p� electronic transition followed by semicarbazo ne to dpk charge transfer. In the case of [MCl 2(g2-N,N-dpksc)], the electronic transitions are ILCT transitions of coor- dinated [dpksc] mixed with metal to ligand charge transfer (MLCT) transitions of MCl 2. The appearance of the low energy electronic transition in the spectra of [MCl 2(g2-N,N-dpksc)](M = Zn or Cd) in dmf points to acid–base inter-con version

Fd

Scheme 3. Acid–base inter-conversion between [MCl 2(g2-N,N-dpksc] and its conjugate base [MCl 2(g2-N,N-dpksc)]�.

224 M. Bakir et al. / Journal of Molecular Structure 1040 (2013) 221–225

between [MCl 2(g2-N,N-dpksc)] (M = Zn or Cd), observed at �310 nm, and its conjugat e base [MCl 2(g2-N,N-dpksc-H)]�

(M = Zn or Cd), observed at 395 nm (see Scheme 3). The proposed acid–base inter-con version between [MCl 2(g2-N,N-dpksc)] and [MCl2(g2-N,N-dpksc-H)]� is analogous to that reported for the acid–base inter-conver sion di-2-pyridylketon e hydrazones and their conjugate bases [29–33]. In the presence of excess acid (ben-zoic acid) the intensity of the electronic transition due to [MCl 2(g2-N,N-dpksc-H )]� disappea red and a slight increase in the intensity of the electronic transition of [MCl 2(g2-N,N-dpksc-H)] was ob- served. The reverse was observed when excess base (NaBH4) was used in place of benzoic acid (see Fig 4). These results point to the strong absorptivity of [MCl 2(g2-N,N-dpksc)]� in protophilic solvents and divulged that the acidity of [MCl 2(g2-N,N-dpksc)] in- crease in the following order: [ZnCl 2(g2-N,N-dpksc)] > [CdCl 2(g2-N,N-dpksc)] > [HgCl 2(g2-N,N-dpk sc)].

Fig. 4. Electronic absorption spectra of [ZnCl 2g2-N,N-dpksc)] (5.00 � 10�5 M)measured in dmf in the presence and absence of excess benzoic acid and excess NaBH 4.

The electrochemical properties of dpksc and [MCl 2(g2-N,N-dpksc)] measured in dmf were investigated using voltammetr ic techniqu es. Cyclic voltammogr ams of dpksc and [MCl 2(g2-N,N-dpksc)] are shown in Fig. 5. These voltammogram s show irrevers- ible redox processes signifying electrochemical decomposition upon electronic transfer. In the voltammogr am of [dpksc], on areductive ly initiated scan, an irreversible reduction appeared at Ep,c = �1.57 V and irreversible oxidation appeared at Ep,a = +1.62 V,along with an electrochemical ly generated product wave at Ep,a = +0.36 V. On an oxidativel y initiated scan, the reductively gen- erated product wave disappea red. In the voltammogr ams of [MCl2(g2-N,N-dpk sc)] (M = Zn, Cd and Hg), irreversible redox transformat ions appeared due to coordinated MCl 2 moiety along with a series of irreversibl e redox processes due to dpksc.In the voltammogr am of [ZnCl 2(g2-N,N-dpksc)], two irreversible

Fig. 5. Cyclic voltammograms of dpksc, and [MCl 2g2-N,N-dpksc)] (M = Zn, Cd and Hg) measured in dmf solutions 0.1 M in TBAH at a glassy carbon working electrode at a scan rate of 400 mV s�1.

M. Bakir et al. / Journal of Molecular Structure 1040 (2013) 221–225 225

reductions appeared at Ep,c = �1.04 and �1.25 V due to coordinated ZnCl2 and irreversibl e process due to dpksc observed Ep,c = �1.80 Vand Ep,a = +1.58 and 1.80 V. An irreversibl e redox process appeared at Ep,a = +1.10 V due to the oxidation of chloride to chlorine (2Cl� toCl2). In the voltammogr am of [CdCl 2(g2-N,N-dpksc)], an irrevers- ible reduction appeared at Ep,c = �0.90 V due to coordinated CdCl 2and irreversibl e redox processes appeared at Ep,c = �1.72 and Ep,a = +1.46 and +1.82 V due to coordinate-dpk sc. The irreducible electrochemi cally generated product wave observed at Ep,a = +1.0 Vis due to 2Cl �? Cl2 oxidation. In the voltammogram of [HgCl 2(g2-N,N-dpksc)], irreversible redox processes due to coordinated HgCl 2appeared at Ep,c = �0.17, and �0.49 and Ep,a = �0.46, �0.03, +0.34,+0.50 V. The oxidation of the electrochemical ly generated chloride appeared at Ep,a = +1.10 V. An irreversible redox process due to the ligand appeared at Ep,a = +1.4 V. The assigned MCl 2 redox processes are similar to those reported previously for other di-2-pyridylke- tone derivatives containing MCl 2 [32–34]. These results show that dpksc and its [MCl 2(g2-N,N-dpksc)] compounds act as electronic sinks. The redox potentials of [MCl 2(g2-N,N-dpksc] show the acid- ity of coordina ted [dpksc] in [MCl 2(g2-N,N-dpksc] to increase in the following order [ZnCl 2(g2-N,N-dpksc] > [CdCl 2(g2-N,N-dpksc] > [HgCl 2(g2-N,N-dpk sc]. Similar trend was observed in the electronic spectral propertie s of these compounds.

4. Conclusion

dpksc coordina tes to MCl 2 (M = Zn, Cd or Hg) via the nitrogen atoms of its pyridine rings to form compounds of the type [MCl 2(-g2-N,N-dpk sc)]. Spectroscop ic measure ments performed on proto- philic solutions of [MCl 2(g2-N,N-dpksc)] disclosed sensitivity of [MCl2(g2-N,N-dpksc)] to changes in their surroundi ngs. Electro- chemical measure ments performed on dmf solutions of dpksc and [MCl 2(g2-N,N-dpksc)] revealed irreversible redox processes pointing to instability of these compounds to electroni c transfer(s).

Acknowled gment

The authors acknowledge the Universit y of the West Indies for financial support.

Appendix A. Supplemen tary material

Supplement ary data associated with this article can be found, in the online version, at http://dx.doi .org/10.1016/j.molstruc.2013.02.027.

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