Scheme 2. 2 Proposed direct nucleophilic attack and solvotic pathways of associative substitution reaction of square planar complexes
3.3 Results
3.3.2 X-ray Crystal Determination of the Complexes
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systems as supported by the lowest ΔE value (Table 3.2) than the rest of the complexes. The more delocalization of electrons in PdL1’s ligand system than rest of the complexes results into a favourable overlap of the metal’s dπ orbitals and π* orbitals of the ligands.
The data in Table 3.2 illustrates that the complexes adopts slightly distorted square planar geometry with N5–Pd–N1 deviating from the ideal 180º by angles between 5.5º – 14.5º. On the other hand, the optimized planarity structures of PdL1, PdL2 and PdL3 shows the ligand structures are in plane with the metal centre, while in PdL4, the pyrazole ligand fragments are twisted out of plane at 56.53º away from N3–Pd–Cl main axis. This is due to the methylene spacer group causing flexibility within the ligand system hence steric hinderance which agrees with the X-ray crystal structure (Figure 3.3).
The trend in the HOMO-LUMO energy differences of the complexes increases in the order PdL1< PdL2 < PdL3< PdL4 which agrees with the reactivity trend. In addition, the successive increase in the HOMO energy level going from PdL1 to PdL4 is an indication that the electron donation density around the metal increases, while the successive increase on the LUMO energy in the same manner shows a reduction in π-acceptability of the ligand system in the complexes.81,
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parameters for PdL2, PdL3 and PdL4 are summarized in Table 3.3, while the molecular structures are shown in Figure 3.2 for PdL2 and PdL3 and Figure 3.3 for PdL4.
Table 3. 3 Crystal data and structure refinement for the complexes
PdL2 PdL3 PdL4
Molecular formula C11H9N5PdCl2.2H2O C15H17N5PdCl2. CH3OH
C17H21N5PdClBF4. CH2Cl2
Mr 424.56 476.68 608.97
Crystal size/ mm3 0.210×0.190× 0.140 0.180×0.140×0.110 0.36×0.19×0.11
T/K 100(2) 100(2) 100(2)
λ/ Å 0.71073 0.71073 0.71073
Crystal system Monoclinic Monoclinic Triclinic
Space group P21/c P 21/c P-1
a/Å 11.2092(7) 10.2667(4) 7.7353(3)
b/Å 6.7184(4) 11.0524(5) 12.3436(9)
c/Å 20.5656(12) 16.0338(7) 13.4839(9)
α/º 90 90 115.036(3)
β/º 105.340(3) 95.028(2) 90.896(3)
γ/º 90 90 92.142(4)
V/ Å3 1493.57(16) 1812.38(13) 1164.98(14)
Z 4 4 2
Dc /Mg m-3 1.888 1.747 1.736
µ/ mm-1 1.611 1.334 1.189
F(000) 840 960 608
θ range/º 1.884 - 27.168 2.241 - 28.263 1.668 – 27.075
Reflections collected (independent)
12629 (3292) 16424 (4460) 18239(4962)
Rint 0.0252 0.0279 0.0587
No. of parameters (restraints)
205 (6) 232 (0) 330(unknown)
Rindices (all data) R1=0.0386, wR2=0.0772
R1=0.0269, wR2=0.0562
R1=0.0807, wR2=0.1518
Goodness-of-fit on F2 1.054 1.072 1.034
Max, Min Δρ/e Å-3 1.742, - 0.898 0.846, - 0.606 1.564, -2.183
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Figure 3. 2 Molecular structures of a) PdL2 and b) PdL3 with atom numbering scheme. The displacement ellipsoids of atoms are shown at the 50% probability level. The counter anion, chloride, is hydrogen bonded to the solvent molecule as shown by the blue dash.
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Figure 3. 3 Molecular structure of PdL4 with atom numbering scheme. The displacement ellipsoids of atoms are shown at the 50% probability level. The counter ion and the solvent for crystallization is omitted for clarity.
The complexes crystallise with chloride counter ion, for both PdL2 and PdL3 and tetrafluoroborate for PdL4. In addition, the crystallization proceeds with two water, methanol, and dichloromethane solvent molecules for PdL2, PdL3 and PdL4, respectively. Tridentate ligands coordinate to the Pd-centre by the nitrogen of the trans-positioned pyridyl ring and the nitrogen of the cis-positioned pyrazolyl rings while the fourth position is covalently bonded to chloride ligand in all the complexes (Figure 3.2 and 3.3). The other non-coordinated chloride atom for PdL2 and PdL3 and tetrafluoroborate for PdL4 are held by unconventional hydrogen bonding contacts within the unit cell and are (indicated by the blue dash in Figure 3.2) between the hydrogen of one water molecule and methanol molecule for PdL2 and PdL3, respectively.
Selected bond lengths and angles of the X-ray structures of PdL1, PdL2 and PdL4 are tabulated in Table 3.4.
93 Table 3. 4 Selected bond distances (Å) and angles (º)
PdL2 PdL3 PdL4 Bond lengths (Å)
Pd1 – N1 2.001(3) 2.027(1) 2.019(4) Pd1 – N3 1.938(2) 1.941(1) 2.204(4) Pd1– N5 2.001(3) 2.027(1) 2.008(4) Pd1 – Cl1 2.273(1) 2.297 (1) 2.311(1) Angles (º)
N1 – Pd1 – Cl1 98.46(8) 99.32(5) 93.7(1) N3 – Pd1 – Cl1 178.74(8) 179.17(5) 177.3(1) N5 – Pd1 – Cl1 100.77(8) 100.35(4) 92.3(1) N1 – Pd1 – N3 80.34(11) 80.28(6) 87.5(2) N1 – Pd1 – N5 160.77(11) 160.33(6) 174.1(2) N3 – Pd1 – N5 80.43(11) 80.05(6) 86.3(2)
The complexes adopt a distorted square planar coordination geometry around the metal centre given that the the angles N1–Pd1–N3, N5–Pd–N3, N3–Pd–Cl1 and N5–Pd–Cl1 deviaviates approximately by angles between 2˚ – 10˚ from the ideal square planar angle 90˚. This is slight distortion is further confirmed by the bite angle N5–Pd1–N1 deviates significantly from linearity (180º) to 160.77(11)º 160.33(6)º and 174.1(2)º for PdL2 PdL3 and PdL4, respectively. In addition, N3–Pd1–Cl1 bite angles 178.74(8) º, 179.17(5) º and 177.3(1) º for PdL2, PdL3, and PdL4 respectively, also shows slight deviation from the linearity. All these bite angles are comparable to the DFT calculated values (Table 3.2). The bond angle N3–Pd1–Cl1 for PdL2 is in agreement with that of a X-ray related structure of analogous Pt(II) (178.91(13)˚)72 complex, while that of PdL4 (177.3(1)º) is almost equal to that of a similar X-ray structure (177.63(12)º).73 The bond distances N3 – Pd1 (PdL2 1.938(2) Å PdL3 1.9406(15) Å) are well below 2.0 Å and interestingly shorter than Pd1–N1 and Pd1–N5 distances while that of PdL4 2.204(4) Å is slightly above 2.0 Å. These bond lengths are within the range of values reported for other X-ray related structures. 72, 73, 83 The Pd1–N3 bond distances are noticeably shorter than
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the Pd1–Cl1 (2.273(1) Å, 2.297(1) Å and 2.311(1) Å for PdL2, PdL3, and PdL4, respectively) due to strong interactions between the trans-N-pyridyl ring and the Pd(II) metal centre.
The crystal packing for the complexes evidences an inversion dimer fashion (Figures 3.4 and 3.5). The Pd – Pd distances between the dimers (PdL2, 3.3622(3) Å and PdL3, 3.4697(3) Å) being less than 4 Å clearly indicates that there is an intermolecular metal – metal interactions as was observed in similar X-ray structure of Pt(II) complex.72 However, there is no metal-metal interactions in PdL4 (10.1507(9) Å) since Pd–Pd distances ˃ 4 Å. In PdL2, there is C6–H···Cl1 bonding linkage indicating intermolecular interaction as shown in Figure 3.4, which is not the case with PdL3 and PdL4 complexes.
Figure 3. 4 A portion of the crystal packing of PdL2 showing an inversion dimer, Pd–Pd interactions and Cl1···H interaction between the molecules (shown in blue dashed line). The chloride counter ion and the solvent water molecules are omitted for clarity.
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The dihydrate crystals of PdL2 and the solvent molecule of methanol of PdL3 results in hydrogen bonding system between the water, methanol subunits and the chlorine anion. This bonding type is inferred because of the short inter-molecular interaction distances between the water O atoms and the chloride anion (Cl2), Cl2 – O1 (3.182(4) Å) for PdL2 and the methanol O, Cl2 – O1 (3.044 (2) Å) for PdL3.
Figure 3. 5 Crystal packing of PdL3 showing an inversion dimer having Pd-Pd interactions. The chloride counter ion and methanol solvent molecule omitted for clarity.