662 DOI: 10.17344/acsi.2014.1299_Acta Chim. Slov. 2015, 62, 662-671
Scientific paper
Experimental and DFT Study on the Compounds [PdCl2L2] (L = 4-methylpyrazole, 4-iodopyrazole)
Carolina V. Barra,1 Oswaldo Treu-Filho,1 Fillipe V. Rocha,1* Thales R. Moura,1 Adelino V. G. Netto,1 Antonio E. Mauro,1 José C. Pinheiro2 and Rogério T. Kondo3
1 Instituto de Quimica, UNESP, CP 335, 14800-900 Araraquara, SP, Brazil.
2 Laboratòrio de Quimica Teòrica e Computational, Departamento de Quimica, Centro de Ciencias Exatas e Naturais, Universidade Federal do Parà, CP 101101, 66075-110 Belém, PA, Amazonia, Brazil.
3 Segäo de Suporte, Centro de Informàtica de Säo Carlos, Universidade de Säo Paulo, 13560-970
Säo Carlos, SP, Brazil.
* Corresponding author: E-mail: fillipevrocha@gmail.com
Received: 15-12-2014
Abstract
Theoretical molecular structures of the complexes [PdCl2(HmPz)2] (1) and [PdCl2(HIPz)2] (2) (HmPz = 4-methylpyrazo-le; HIPz = 4-iodopyrazole) were studied using B3LYP/DFT method. The new complex 2 and the complex 1 were synthesized and characterized by elemental analysis and IR spectroscopy. The calculated bond distances and angles showed that both compounds exhibited a slightly distorted square planar coordination environment around the palladium center. The theoretical IR spectra of Cs symmetry (electronic state 1A') of the complexes agree well with the experimental data.
Keywords: DFT calculation • Infrared Spectroscopy • Palladium(II) complexes • Pyrazoles
1. Introduction
N-unsubstituted pyrazolyl-type ligands have been widely employed in coordination and metallosupramole-cular chemistry since they possess two chemically distinct nitrogen atoms, one pyrrolic and one pyridinic N atoms, which are very useful for coordination and hydrogen bonding interactions, respectively.1,2 Under the crystal engineering point of view, ligands of this type are of interest because their mononuclear metal-based derivatives can be used as molecular building blocks for the rational design of new hydrogen bonded metallosupramolecular species.3
Furthermore, palladium complexes attract attention due to the biological proprieties.4-7 A great variety of palladium complexes already exhibit significant cytotoxi-city, in most cases surpasses the activity of platinum ana-logs.8 The palladium(II) compounds containing pyrazole as ligands have been studied by Keter and it has been confirmed in vitro that these compounds cause apoptosis of cancer cells of human and ovarian fibroma of hamster.9 In addition, our previous studies have been showed the capacity of palladium complexes to bind to DNA. These
compounds affect the DNA structure and cause a low electrophoretic mobility.10
Previous investigations in our laboratory have been devoted to the study of the molecular self-assembly via hydrogen bonds of pyrazolyl metal complexes of general formulae [MX2(HPz)4] (M = Co(II), Ni(II); X = N3, SCN; HPz = pyrazole),11 [PdX2(HdmPz)2] (X = N3, SCN; HdmPz = 3,5-dimethylpyrazole),12 and [Cd(|>X)2(HPz)2]n (X = Cl, SCN).13 Particularly, we are interested in acquiring more information about the building blocks [PdCl2L2] (L = 4-substi-tuted pyrazoles) since the reaction of these compounds with azide ion gives rise to unusual coordination polymers instead of hydrogen-bonded supramolecular arrays.14,15
As a part of our ongoing studies involving DFT calculations for metal-based compounds,16 we describe herein the synthesis and spectroscopic characterization of the two molecular building blocks dichlorobis(4-methylp-yrazole)palladium(II) (1) and dichlorobis(4-iodopyrazo-le)palladium(II) (2) together with the results of DFT calculations via B3LYP method. The geometry of the complexes 1 and 2 were optimized and their theoretical IR spectra were discussed based on the experimental data.
2. Experimental
2. 1. Materials and Methods
The reagents were employed without further purification and the solvents were dried by usual procedures. Elemental analysis was performed on an analyzer CE Instrument model EA 1110-CHNS-O. The IR data were recorded on a Nicolet FT-400 spectrophotometer in the range from 4000 to 400 cm-1 at a resolution of 4 cm-1 in KBr pellets.
The synthesis of dichloro(4-methylpyrazole)palla-dium(II) (1) was realized as described in the literature.17 Synthesis of dichloro(4-iodopyrazole)palladium(II) (2): to a deep orange solution of [PdCl2(MeCN)2] (50 mg, 0.19 mmol) in 10 mL of a MeOH, 4-iodopyrazole (75 mg, 0.38 mmol) in 1 mL of MeOH was added dropwise. The mixture was stirred at room temperature for 1 h. The light yellow suspension was filtered off and the yellow solid obtained was washed with methanol and dried under vacuum. ( Yield 82%) Anal. Calcd. for C6N4H6Cl2I2Pd (%): C, 12.75; N, 9.91; H, 1.07. Found: C, 12.39; N, 9.43; H, 1.50.
2. 2. Computational Strategy
In this work, the employed quantum chemical approach to determining the molecular structures was Becke three-parameter hybrid method18 using the Lee-Yang-Par (LYP) correlation functional19 and the basis sets used for
calculations were: [4s] for H (2S),19 [5s4p] for C (3P) and N (4S),20 [11s7p] for Cl (2P),21 [16s9p5d] for I (2P),22 and [12s8p5d] for Pd (1S).20 In order to better describe the properties of 1 and 2 in the implementation of the calculations, it was necessary to include polarization functions for all atoms of the compounds. The strategy to choice of the polarization functions was previously described.20 The polarization functions are: ap = 0.33353749 for H (2S), ad = 0.72760279 and ad = 0.35416230 for C (3P) and N (4S), respectively, and af = 0.14057699 for Pd (1S) atoms.20 For Cl atom (2P), the polarization function to the basis set previously reported19 is ad = 0.47236655. The polarization function for I atom (2P) reported in this work is af = 0.51068618.22 For the complexes 1 and 2 a constraint of Cs symmetry (electronic state 1A') was added to the geometry optimization as condition.
The performed molecular calculations in this work were done using the Gaussian 03 routine.23 The computational strategy used in this work has already been used successfully in previous studies on coordination compound and organic molecules.16,20,21
3. Results and Discussion
Compounds of general formulae [PdX2L2] (X = ha-lides and pseudohalides; L = pyrazolyl type ligands) have
Figure 1. Computed structure of [PdCl2(HmPz)2] (1) and [PdCl2(HIPz)2] (2).
Table 1. Selected calculated bond lengths (À), bond and torsion angles (°) for [PdCl2(HmPz)2] (1), [PdCl2(HIPz)2] (2) and they analogue trans-[Pd-
Cl2(Hpz)2].25
Bond lengths (A)
1	2	trans- [PdCl2(Hpz)2]25
Pd-N1	2.08	Pd-N1	2.07	Pd-N	2.028(6)
Pd-N3	2.08	Pd-N3	2.07	-	-
Pd-Cl1	2.31	Pd-Cl1	2.30	Pd-Cl	2.3079(6)
Pd-Cl2	2.31	Pd-Cl2	2.30	-	-
		Bond angles (°)			
	1		2	trans-[PdCl2(Hpz)2]	
Cl1-Pd-Cl2	179.97	Cl1-Pd-Cl2	180.00	Cl1-Pd-Cl2	180
Cl1-Pd-N1	90.51	Cl1-Pd-N1	90.51	Cl1-Pd-N1	90.08
Cl1-Pd-N3	89.48	Cl1-Pd-N3	89.49	Cl1-Pd-N3	89.92
Cl2-Pd-N3	90.49	Cl2-Pd-N3	90.51	Cl2-Pd-N3	90.08
Cl2-Pd-N1	89.52	Cl2-Pd-N1	89.49	Cl2-Pd-N1	89.92
N1-Pd-N3	179.99	N1-Pd-N3	180.00	N1-Pd-N3	180
attracted considerable interest since they are able to self-assembly in interesting hydrogen-bonded supramolecular species during the crystallization. Since single crystals suitable for crystallographic studies of 1 and 2 could not be grown, we undertook a computed geometry optimization using the optimized algorithm of Berny.24 The calculated molecular structures of 1 and 2 are illustrated in Figure 1.
According to our calculations, in the calculated molecular structures of 1 and 2, the palladium atom lies in a square planar coordination environment made by two chloro ligand and two nitrogen atoms from the pyrazolyl ligand (N1 and N3), in a trans relationship. The four donor atoms in 1 and 2 are coplanar. A selection of calculated bond lengths and angles is shown in Table 1.
The calculated structures for 1 and 2 show a satisfactory agreement with the available crystal structure data of similar compounds of the type [PdCl2L2] (L = pyra-zolyl ligands), mainly in terms of bond angles. The calcu-
Figure 2. Experimental (a) and theoretical (b) IR spectra of the HmPz.
lated Pd-N bond lengths in 1 and 2 are ca. 0.05 À longer than those found for analogous compounds.12,26
To achieve a more accurate assignment of the vibra-tional spectra obtained for the palladium(II) compounds 1 and 2, we have initially undertaken the study of the experimental and computed vibrational spectra FT-IR of the li-gands 4-methylpyrazole (HmPz) and 4-iodopyrazole (HIPz) (Figures 2-3). Despite the fact that experimental IR and Raman spectra of HmPz and HIPz have been tentatively assigned by Vos and Groeneveld,27,28 there are no reports for these ligands dealing with the IR spectra calculated from DFT study. Theoretical infrared spectra of HmPz and HIPz were calculated using a harmonic field29 based on C1 symmetry (electronic state 1A). Table 2 and 3 list some selected experimental FT-IR and computed IR frequencies for 4-methylpyrazole and 4-iodopyrazole, respectively, together with relative intensities, assignments and description of vibrational modes. The theoretical frequencies values are not scaled.
Wave numbers (cm'')	Wave numbers (cm'')
a)
1.0
o,e
s
о
(Л
■О 0.4 <
0.2
0.0 4000
HIPz
vNII
vC2C3->
асзн«
Ч\;
vONI
U
v.
3500 3000
-1-
2500
-1-
1500
—I
500
b)
Wave numbers (cm'l
Figure 3. Experimental (a) and theoretical (b) IR spectra of the HIPz.
1 fi-
■e 0.5
о
S <
o.o
HIPz

\ LN2
vCICS »CICI
V
Ш
4000 3500 3000 2500 2000 1500 1000 500
Wave numbers (cm' )
Table 2. Experimental and theoretical frequencies of 4-methylpyrazole (HmPz).
Frequencies (cm 1)
Vibrational Mode	Experimental	Theoretical	Relative Intensities	Percentual Error (%)	Assignment
1	3400	3389	0.36	-0.32	vNH
2	3184	3114	0.01	-2.20	v (C1H + C2H)
3	3067	3101	0.29	+1.11	v (C1H+C2H)
4	2953	2951	0.57	-0.07	vCH3 vCH3 vCH3
5	-	2927	0.60	-	
6	-	2887	1.00	-	
7	1578	1575	0.01	-0.19	v (C2-C3)
8	1497	1486	0.07	-0.93	5CH3 . . 3 scissoring 8CH 3 scissoring 5NH, , g in plane 5CH3 umbrella + V (C3-N1)
9	1460	1474	0.03	+0.96	
10	-	1436	0.07	-	
11	1387	1404	0.10	+1.22	
12	1381	1379	0.10	-0.15	v (C2C1-C2C3)
13	1342	1293	0.08	-3.65	v (N1C3-N2C1) + v C2-CH3
14	1225	1245	0.08	+1.63	5(NH + C1H)m plane
15	1138	1137	0.01	-0.09	5C3H, l + §NH. l in plane in plane v (C1-N2)
16	1051	1075	0.12	+2.28	
17	-	1038	0.02	-	0СНл i. „г
18	1001	984	0.07	-1.70	3 out of plane Ring breathing + §CH3 out of plane 3 out of plane vNN
19	-	980	0.03	-	
20	949	939	0.35	-1.05	
21	-	875	0.09	-	5ring in plane
22	858	842	0.58	-1.86	5C3H u f l out of plane 5C1H t f p, out of plane 5NH t f l out of plane vC2-CH3 5C2H t f l out of plane 3 out of plane 5NH tfl + 5CH3 t f l out of plane 3 out of plane
23	804	766	0.61	-4.73	
24	650	644	0.30	-0.92	
25	613	630	0.04	+2.77	
26	-	592	0.03	-	
27	-	298	0.01	-	
28	-	289	0.38	-	
29	-	220	0.14	-	5NH t f p out of plane 3 rocking
30	-	51	0.01	-	
* Broad and intense band
a)
5
1,0-
o.e-
0,4-
► vN.w (PdCUHmPZ)j
ÖCII.,*» + 6NIL.H-,
vNNi.ri»ki n^, + öC'ILpiu«
+ vNHi_
|(CH>
, "НЧ


Щ|1
tooo
3500
3000
2600
2000
T" 1500
1000
T
500
b)
1,0
^„»W Pdas(HmPzy
vCHj li

ÖCHJ
VCNuh4
\
(örinp>)i>
■ÄhJ MÀI
(Srinqfc
vpaa
J
3500
3000
2600
Wave numbers (cm"') Figure 4. Experimental (a) and theoretical (b) IR spectra of the [PdCl2(HmPz)2].
2000 1SOO Waven umbers (cm'1!
1000
—i—
500
The experimental and theoretical vibrational spectra FT-IR for the compounds 1 and 2 are showed in Figures 4 and 5, and in Tables 4 and 5, respectively. The theoretical
infrared spectra were also calculated using a harmonic field29 based on Cs symmetry (electronic state 1A'). Frequency values (not scaled), intensities, assignments and
Table 3. Experimenta! and theoretica! frequencies of 4-iodopyrazo!e (HIPz).
Frequencies (cm 1)
Vibrational Mode	Experimental	Theoretical	Relative Intensities	Percentual Error (%)	Assignment
1	-	3396	0.74	-	vNH
2	3137	3145	0.04	+0.25	vCH
3	3114	3135	0.09	+0.67	vCH
4	1533	1508	0.03	-1.63	vC2C1
5	1423	1406	0.03	-1.19	vC1N2
6	1363	1357	0.18	-0.44	vC2C3
7	1263	1256	0.13	-0.55	vC3N1
8	1140	1190	0.30	+4.39	vC1N2
9	1117	1109	0.01	-0.72	SC3H. , in p,ane vC1N2
10	1032	1073	0.22	+3.97	
11	955	933	0.47	-2.30	vNN
12	935	899	0.66	-3.85	vC-I
13 14	870 808	859 827	0.45 1.00	-1.26 +2.35	S(C1N2N1). . , v 7 ring in p,ane SC3H t f, out of p,ane SC1H t f, out of p,ane SNH t f , out of p,ane SC2H t f, out of p,ane vC-I
15	779	748	1.00	-3.98	
16	607	649	0.36	+6.92	
17	534	527	0.01	-1.31	
18	*	308	0.01	-	
19	322	294	0.64	-8.69	SNH t f , out of p,ane SC2H t f , +SN2H t f , out of p,ane out of p,ane SC2-I. , in p,ane
20 21	-	233 199	0.01 0.01	-	
* not detected
a)
■,<'[!,.„,, » vNlW
[PdCyHtPzy

«[[.*., i SMI.л
v(lU„+vNU
b)
4000 3500 3000 2500 2000 1500
1000
500
-о <
1.0-
0,8-
0,6-
0.2-
0,0
vflH^i + vNHii
[PdCI3(HIPz)J
vnn,^,™., + асн»*-
vCTW+SCHbH- \ „ sou,
vCC \ I	'
Ail ■ . i L.
1000 3500 3000 2500 2000 1500 1000
500
Wavenumbers (cm )
Wave numbers (cm )
Figure 5. Experimenta, (a) and theoretica, (b) IR spectra of the [Pdd2(HIPz)2].
description of vibrationd modes are presented. The captations of vibrationa, frequencies were ako emptoyed to determine whether optimized geometry constitutes minimum or saddte points. The principa, infrared-active fundamenta, modes assignments and descriptions were done by the GaussView W 4.1.2 graphics routine.30
3. 1. N-H and C-H Vibrations
The two experimenta, spectra of the HmPz and HIP-z Hgands (Figs. 2(a) and 3(a), respective^) show a very
broadened and intense NH—N association bands over the 3600-2500 cm-1 range. The theoretica, catenations predict that the N-H stretches shouM occur as singte peaks at 3389 and 3396 cm-1 (Figs. 2(b) and 3(b), respective^). Two C-H stretching modes are expected for HmPz (3114 and 3101 cm-1) and HIPz (3145 and 3135 cm-1). Nevert-hetess, the broad range of the NH-N bands covers the C-H stretching absorptions, preventing the comparison between the experimenta, and theoretica, vatees.
In the catenated IR spectrum of HmPz, the bands at 1245 and 1137 cm-1 are assigned to an insane N-H ben-
Table 4. Experimental and theoretical frequencies of of [PdCl2(HmPz)2] (1).
Frequencies (cm 1)
Vibrational Mode	Experimental	Theoretical	Relative Intensities	Percentual Error (%)	Assignment
1	3325	3186	1.00	+4.18	VCH(„ngS) + VNH(„ngS)
2	3134	3128	0.01	-0.19	VCH(„ngS) + VNH(„ngS)
3	2956	2960	0.06	+0.10	VCH3(left ring)
		2959	0.04		VCH3(right ring)
4	2925	2942	0.09	+0.58	VCH3
5	2869	2895	0.25	+0.91	VCH33
6	1483	1490	0.02	+0.47	(SCH3) . . v ^scissoring
7	1450	1443	0.07	-0.48	VCN(rings)
8	1394	1407	0.05	+0.93	(SCH3) • 3 wagging
9	1355	1386	0.03	+2.29	vCC(. ) + (SCH3) • • (rings) 3 scissoring
10	1292	1322	0.06	+2.32	VCN(rings)+ (SCH3)wagging
11	1255	1259	0.03	+0.32	SCHin plane + SNHin plane
12	1128	1111	0.28	-1.51	SCHin plane + SNHin plane
13	1076	1026	0.06	-4.65	VNN(right ring) + SCHin plane
		1025	0.02		VNN(left ring) + SCHin plane
14	1001	974	0.07	-2.70	(Sring)in plane
15	862	843	0.10	-2.20	SCHout of plane + ^NHout of plane
16	829	762	0.12	-8.08	SCHout of plane
17	682	644	0.03	-5.57	SNHout of plane
18	576	580	0.02	+0.69	(Sring)out of plane
19	331	365	0.06	-9.31	SNHout of plane
20	302	307	0.05	+1.65	VPdCl
21	286	266	0.02	-7.52	VPdN
22	-	202	0.03	-	SPdNin plane
23	-	187	0.03	-	SPdCl- ,
					in plane
Table 5. Experimental and theoretical frequencies of [PdCl2(HIPz)2] (2).
Frequencies (cm 1) Experimental Theoretical
Vibrational Mode
Relative Intensities
Percentual Error (%)
Assignment
1 2
3
4
5
6
7
8
9
10 11 12
13
14
15
16
17
18 19
3315
3136 1512 1462 1381 1344 1246 1194 1124 1061 941 849
675 579 368 288
3176 3162 3139 1489 1441 1373 1311 1193 1159 1111 1017 903 890 889 707 592 366 276 248 184
0.10 0.31 1.00 0.03 0.05 0.04 0.07 0.03 0.05 0.24 0.12 0.13 0.08 0.11 0.07 0.02 0.03 0.04 0.04 0.02
-4.19
+0.09 -1.52 -1.44 -0.58 -2.46 -4.25 -2.93 -1.16 -4.15 -4.04 +4.83 +4.71 +4.74 +2.24 -0.54 -4.17
VCH(nngS) + VNH(nngS) VCH + VNH(nngs) VCH(nngS) VNH(nngs) VCN(nngs) + VCC(„ngs)
vCN(n„gS)
VCC(nngS)
VCN(n„gS) + SCHi„
plane
SCHin plane + SNHin plane SCHin plane + SNHin plane SCHin plane + SNHin plane VNN(nngS) + SCHin plane VC-I(nngS)
SCHin plane + SNHin plane SCHout of plane+ SNHout of plane SCHout of plane (Sring)out of plane
VPd-Cl
SPdNin plane + VPdCl
VPdN + SPdCl
SPdCl
ding with contribution from in-plane C-H bending vibrational modes. This is in disagreement with Vos and Groe-
neveld,28 who assigned the experimental bands at 1225 and 1140 cm-1 to C-CH3 stretching and ring vibrations.
For HIPz molecule, the experimental band at 1117 cm-1 is assigned to the in-plane C-H bending mode. The 850-750 cm-1 region is where the most intense bands arising from out-of-plane C-H bending vibrational modes (5opC-H) are found. The two theoretical frequencies for 5opC-H modes for HmPz (842 and 766 cm-1) and HIPz (827 and 748 cm-1) are in good agreement with those experimentally found (Tables 2 and 3). The out-of-plane N-H bending mode appears at 644 cm-1 and 220 cm-1 in the calculated IR spectrum of HmPz. Vos and Groeneveld28 attributed the band at 650 cm-1, observed in the experimental spectrum of HmPz, to C-CH3 stretching and ring vibrations. The 5op N-H vibration gives a band at 607 cm-1 in the measured IR spectrum of HIPz which agrees well with that observed in its calculated spectrum at 649 cm-1.
The N-H and C-H vibrations of compounds 1 and 2 correlate well with the IR data of transition metal complexes bearing pyrazole derivatives literature data.31 In the experimental IR spectra of the complexes 1 and 2 (Figs. 4(a) and 5(a), respectively), it is observed two peaks of different intensities in the spectral range of 3325-3134 cm-1, which belong to the C-H stretching mode with considerable vN-H content. The bands appearing in the 1246-1124 cm-1 range in both experimental spectra are due to in-plane C-H bending (5ipC-H), with contributions from in-plane N-H bend, 5; p N-H. The major contributions to the bands 862 (1) and 849 cm-1 (2) are out-of-plane C-H bending, 5opC-H, together with out-of-plane N-H bending, 5opN-H. The intense experimental band at 682 cm-1 in the IR spectrum of 1 is assigned to 5opN-H mode only.
3. 2. CH3 Vibrations
Three fundamental vCH3 absorptions related to one symmetric and two asymmetric vibrations are usually observed in the region between 2950 and 2990 cm-1.32 Our calculations predict these modes for HmPz at 2951 (vasCH3), 2887 (vasCH3) and 2927 cm-1 (vsCH3). However, the presence of a broad and intense band in the 3600-2700 cm-1 region of the experimental IR spectrum of HmPz has prevented us to make some comparisons with the experimental values. For compound 1, the calculated modes 2960 (vasCH3), 2895 (vasCH3) and 2942 cm-1 (vsCH3) agree very well with the experimental values observed at 2956, 2869 and 2925 cm-1, respectively.
The bending vibrational modes of the methyl groups are expected to appear between at 1410 and 1550 cm-1.32 The two bands located at 1497 and 1460 cm-1 in the experimental IR spectrum of HmPz involve the vibrations arising from methyl H-C-H bends, 5asCH3. This is in agreement with the earlier study of Vos and Groeneveld.33 The peak at 1387 cm-1, which was previously assigned to ring vibrations,28 is a mixture of the 5sCH3 (umbrella) and vCN modes. In the experimental IR spectrum of complex
1, the bands associated with the bending modes of methyl groups appear at 1483 and 1394 cm-1, which are in accordance with the calculated values at 1490 and 1407 cm-1, respectively.
3. 3. C-I Vibrations
According to the data available in the literature,34,35 there appears to be no pure C-I stretching vibration band for iodo-aryl compounds due to its coupling with ring vibration. The position of C-I band is strongly dependent on many factors, such as the type of the ring, the mass and relative position of their substituents. Therefore, the assignment and comparison of C-I stretching vibrations in the IR spectra of aromatic compounds remains a difficult task since only a restricted number of iodo-aromatic compounds have been investigated. In addition, the description on the vCI frequencies remains at least contradictory in some reports. For instance, in the spectra of p-substitu-ted iodobenzenes (R = OH, Me, COOMe),36 the vCI frequencies occur within the spectral range of 1060-1055 cm-1 whereas the vCI appears at 454 cm-1 for p-iodonitro-benzene.37 PM3 calculations on 5-iodo-2'-deoxyuridine performed by Bailey et al.38 indicated that C-I stretching vibration found at 554 cm-1 agrees well with the experimental value. In the FT-IR spectrum of 2-amino-5-io-dopyridine, Sundaraganesan et al.39 assigned the strong absorption at 514 cm-1 to C-I stretching vibration. However, their theoretical calculations predicted the vCI vibration at 228 cm-1. Vos and Groeneveld27 assigned the C-I stretching vibration in the IR spectrum of 4-iodopyrazole to the band at 321 cm-1. Our calculations predict this mode at 899 cm-1 which shows a good agreement with the experimental value of 935 cm-1. It is also expected an absorption of low intensity at 308 cm-1 which could not be detected in the experimental IR spectrum of HIPz. For compound 2, the calculated frequency of 903 cm-1 agrees very well with the experimental values observed at 941 cm-1.
3. 4. CC, CN and ring Vibrations
According to literature, pyrazole nucleus possesses several absorptions of variable intensities in the range of 1530-1013 cm-1 due to CC, CN and ring stretching motions.40 The identification of vCN and vCC stretching frequencies is a very difficult task, since the mixing of bands is very common in this region.
In the experimental IR spectrum of HmPz [Fig. 2(a)], the band located at 1578 cm-1 involves the vibrations arising from C2-C3 stretching. The absorption associated with the asymmetric stretching mode of the C2-C1 and C2-C3 bonds is predicted to occur at 1379 cm-1, which agrees well with the experimental value of 1381 cm-1. The major contributions to the experimental band at 1342 cm-1 are symmetric stretching mode of the C3-N1
and C1-N2 bonds together with C-CH3 stretching motion. The peak at 1051 cm-1 in the experimental IR spectrum of HmPz, which was previously assigned to in-plane C-H and methyl bending modes,28 is due to the C1-N2 stretching vibration.
The band at 1450 cm-1 observed in the FT-IR spectrum of [PdCl2(HmPz)2] (1) arises from C-N stretches and compares well with the calculated value of 1443 cm-1. In addition, we find that the experimental band at 1355 cm-1 is a combination of vCC and methyl bends, with its neighbor at 1292 cm-1 arising from C-N stretching motion with contributions from methyl bends.
With regard to the FT-IR spectrum of HIPz (Fig. 3), the C-C stretching bands appear at 1533 and 1363 cm-1 whereas the absorptions at 1423, 1263, 1140, and 1032 cm-1 arise from C-N stretches only. These experimental IR bands show good agreement with those predicted by our calculations (Table 3).
In the experimental IR spectrum of [PdCl2(HIPz)2] (2) [Fig. 5(a)], a mixture of C-C and C-N stretching motions contributes to the band at 1512 cm-1 whereas the absorptions at 1462 and 1381 cm-1 arises from C-N and C-C stretching vibrational modes, respectively. The next band, at 1344 cm-1, is a combination of vCN and §,p.C-H.
Our calculations predict the N-N stretching vibrational mode for HmPz and HIPz at 939 and 933 cm-1, respectively. These calculated frequencies compare well with those observed in their FTIR spectra (HmPz, 949 cm-1; HIPz, 935 cm-1). Our theoretical vNN frequencies are significantly lower than that found in the calculated IR spectrum of 3,5-dimethylpyrazole (1125 cm-1).41 Regarding the FT-IR spectra of [PdCl2(HmPz)2] (1) and [PdCl2(HIPz)2] (2), a combination of vNN and 8ip.C-H motions contributes for the band at 1076 cm-1 (1) and 1061 cm-1 (2). Taking into account both experimental and theoretical spectra it can be concluded that positions of the peaks representing the vNN mode occurring within the pyrazolyl-type ligand differ significantly from these for a free molecule and for molecules involved in the complex. The vNN band shifts up to 127 cm-1 towards greater wave numbers upon complexation. The blue-shift of the vNN frequencies found in the spectra of 1 and 2 may be attributed to the increase of the NN bond force constant and bond order in the complex relatively to the free pyrazolyl molecules.
The ring stretching motions frequencies in HmPz, HIPz, 1 and 2 are predicted to occur below 1000 cm-1. The experimental IR absorption positioned at 1001 cm-1 in the spectrum of HmPz involve the vibrations arising from ring breathing with a contribution from methyl H-C-H bends whereas the peak at 870 cm-1, observed in the experimental IR spectrum of HIPz, is associated with in-plane ring stretching vibration. Such IR bands are localized at wavenumbers comparable to those predicted by our calculations at 984 cm-1 (HmPz) and 859 cm-1
(HIPz). For complex 1, the experimental bands at 974 and 576 cm-1 are due to in-plane ring stretching and out-of-plane ring bending vibrations, respectively. The peak associated with the out-of-plane ring bending vibration in the experimental IR spectrum of complex 2 appear at 579 cm-1, which is in accordance with the calculated value at 592 cm-1.
3. 5. Pd-N and Pd-Cl Vibrations
According to Nakamoto,42 terminal M-Cl stretching bands are expected to occur in the region of 300-200 cm-1. Particularly, the vPd-Cl absorption observed in the experimental IR spectra of square-planar Pd(II) compounds falls between 360 and 300 cm-1. Theoretical studies on K2PdCl4 performed by Zhang et al.43 indicated that Pd-Cl stretching vibration found at 292 cm-1 agrees well with the experimental value (336 cm-1). Our calculations predict the Pd-Cl stretching vibrational mode for 1 and 2 at 307 and 368 cm-1, respectively. These calculated frequencies compare well with those observed in their FT-IR spectra (1, 302 cm-1; 2, 366 cm-1), which is depicted in Figure 6. These experimental values are similar to those found in trans-[PdCl2L2] where L = diethyl-sulphide (358 cm-1),41 triethylphosphine (355 cm-1),42 pyridine (350 cm-1),44 2-thiophenecarboxylic hydrazide (341 cm-1).33
The Pd-N stretching and in-plane bending vibrational modes for compound 1 are predicted to occur at 266 and 202 cm-1, respectively. For complex 2, a combination of 5ipPd-N and vPd-Cl motions contributes for the theoretical band at 276 cm-1 whereas the theoretically predicted absorption at 248 cm-1 is a mixture vPd-N and 5ip Pd-Cl vibrations.
400	300 400	300
Wavertumbers (cm1)
Figure 6. Far FT-IR spectra of the compounds HmPz, HIPz, 1 and 2.
4. Conclusions
The optimized molecular structures and vibrational frequencies of 4-methylpyrazole, 4-iodopyrazole, [Pd-Cl2(HmPz)2] (1) and [PdCl2(HIPz)2] (2) were calculated using DFT (B3LYP) theory. The theoretical bond angles of 1 and 2 reveal a slightly distorted square-planar geometry around the metallic center. According to the vibratio-nal frequencies analysis of B3LYP results for HmPz and HIPz (C1 symmetry, electronic estate 1A), and for the metal-based compounds 1 and 2 (Cs symmetry, electronic state 1A'), the calculated and experimental FT-IR spectra showed a satisfactory agreement with each other. Our theoretical methodology used in this work represents a good choice for future calculations on vibrational frequencies of organic and metal-based compounds.
5. Acknowledgments
The authors are very grateful for the financial support by CAPES, CNPq, and FAPESP (Brazilian Agencies). The computational facilities were employed at CE-NAPAD-UNICAMP. We thank Prof. Stanlei I. Klein for use of the software GaussView W 4.1.2.
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Povzetek
Izračunane molekulske strukture kompleksov [PdCl2(HmPz)2] (1) in [PdCl2(HIPz)2] (2) (HmPz = 4-metilpirazol; HIPz = 4-jodopirazol) so bile proučevane z B3LYP/DFT metodo. Sintetizirana novi kompleks 2 in kompleks 1 sta bila okarakterizirana s pomočjo elementne analize in IR spektroskopije. Izračunane vrednosti razdalj in kotov kažejo, da imata obe spojini rahlo popačeno kvadratno-planarno razporeditev ligandov okoli paladijevega centralnega atoma. Izračunani IR spektri za strukturi s Cs simetrijo (elektronsko stanje 1A') se dobro ujemajo z eksperimentalnimi podatki.