Square planar Cu(II) complexes [CuLMixed(H)]; (17), and [CuLMixed(tBu)]; (18) of the mixed ligands were synthesized by the reaction of equimolar amounts of CuCl22H2O and H3LMixed(H) or by the addition of 1 equivalent amount of CuCl22H2O to the in situ generated ligand H3LMixed(tBu) in the presence of triethylamine under air. Interestingly, 18 under air in solution converted to [CuLMixed(tBu), oxidized
]; (18a). The rate of reaction was very slow and hence, it took about 7 days for 20% conversion of 18 to 18a.
Scheme 4.6: Synthetic route of Cu(II) complexes [17, 18, and 18a] with Mixed ligands.
Infrared (IR) spectra of 17, 18, and 18a showed no band corresponds to (O–H), and
(N–H) stretches. The band corresponds to asymmetric, symmetric, and bending overtone mode of (C–H) stretches were appeared at 2960–2862 cm–1, region. The (C=N) band appeared at 1626, 1619, and 1621 cm–1 for 17, 18, and 18a, respectively. While, 18a showed a sharp band at 1657 cm–1, due to (C=O) stretch. The band at 1450 cm–1 for 17, 1438 cm–1 for 18, and 18a was arose due to (C…O) stretch.25
Electrospray ionization mass spectra (ESI–MS) of 17, 18, and 18a in CH3CN in positive mode showed a 100% molecular ion peak (corresponded to M+, M = molecular mass) at m/z = 490.36, for 17; m/z = 602.53, for 18, and m/z = 616.54, for 18a, respectively
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(Figure 4.17). Isotope distribution pattern examination revealed the compositions C28H31CuN2O2 for 17, C36H47CuN2O2 for 18, and C36H45CuN2O3 for 18a, respectively.
Figure 4.17: ESI–mass spectra of [A] CuLMixed(H); (17), [B] CuLMixed(tBu); (18), and [C] CuLMixed(tBu), oxidized
; (18a); experimental and calculated isotope distribution pattern (inset).
[A]
[B]
[C]
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Single crystal X–ray molecular structure of complexes 18 and 18a are shown in Figure 4.18. Complex 18 crystalized in the monoclinic space group C12/c1, while, complex 18a crystalized in the orthorhombic space group P212121.
Figure 4.18: ORTEP representation of [A] CuLMixed(tBu); (18), and [B] CuLMixed(tBu), oxidized
; (18a) were drawn at 50% probability thermal ellipsoid.
From the molecular structure of complex 18 and complex 18a, it was found that the central Cu1 atom was coordinated with N2O2 donor set form the mixed ligand. Complex 18 and complex 18a were differed from each other only at the C13 position where a benzyl
[B]
[A]
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group in 18 was oxidized to carbonyl group in 18a. Cu1–O1 and Cu1–O2 bond distances were 1.9265(15) [18]; 1.928(4) [18a] and 1.8577(14) [18]; 1.877(4) [18a] Å, respectively.
While, the Cu1–N1 and Cu1–N2 bond distances were in the range of 1.9250.005 Å.
Notably, the Cu1–O2 bond distances were shorter compared to the Cu1–O1 bond distances, because of having higher covalent character that appeared due to presence of two tert–butyl groups at the 3, 5–positions. However, the above mentioned metal–ligand bond distances were in accord with the +II oxidation state of the center Cu atom.26 The angle between N1–
Cu1–O2 and N2–Cu1–O1 were 165.02(7) [18]; 169.60(18) [18a] and 163.85(8) [18];
169.97(18) [18a] (parenthesis represent the complex), respectively. Structural distortion parameter, 4, for the complex 18 and 18a were 0.22 and 0.14, respectively. Therefore, the structural distortion from the planar geometry to the non–planar geometry was more pronounced in 18 compared to 18a.
Interestingly, all the C–C bond distances in the C6 aryl rings were not same and were in the range of 1.390.01 Å. Alternating short and long CC bond distances i.e. a quinoid–
type distortion was observed in the tert–butyl groups–containing amidophenolate units. The bond distances at C1–C2, C2–C3, C3–C4, C4–C5, C5–C6, and C6–C1 were 1.443(3), 1.438(3), 1.360(3), 1.423(3), 1.363(3), and 1.419(3) Å for complex 18, while, those were 1.421(8), 1.440(7), 1.368(8), 1.402(8), 1.372(8), 1.423(8) Å for complex 18a, respectively.
Furthermore, C2–O1 and C1–N1 bond length were 1.289(3) [18]; 1.288(7) [18a] Å and 1.354(5) [18]; 1.367(7) [18a] Å, respectively. These bond distances were neither commensurating with their respective single bond character nor double bond character. i.e.
the coordinating amidophenolate units were present in their one–electron oxidized iminosemiquinonate (ISQ1–) form. To note, an alternate short and long bond distances were also found in the salicylidene unit in both complexes, because of delocalization of phenolate1–
charge over phenolate to imine unit.14c–f This type of distortion was found more pronounced in the complex 18a compared to 18 because of higher conjugation of the phenolate1 charge up to newly formed C=O bond at C13 position. This type of distortion in the salen unit is common in the metal complexes, where the salicylidene unit exist in the fully reduced form.
Thus, the deprotonated mixed ligands were bearing dinegative charges and it provided the neutral Cu(II) complexes.
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Table 4.6: Selected bond distances (Å), and bond angels () for 18, and 18a.
18 18a
Cu1–N1 1.9312(17) 1.934(4)
Cu1–N2 1.9205(17) 1.935(5)
Cu1–O1 1.9265(15) 1.928(4)
Cu1–O2 1.8577(14) 1.877(4)
C2–O1 1.289(2) 1.299(6)
C1–N1 1.354(2) 1.350(7)
C1–C2 1.443(3) 1.421(8)
C2–C3 1.438(3) 1.440(7)
C3–C4 1.360(3) 1.368(8)
C4–C5 1.423(3) 1.402(8)
C5–C6 1.363(3) 1.372(8)
C6–C1 1.419(3) 1.423(8)
N1–C7 1.404(3) 1.384(7)
C7–C8 1.399(3) 1.401(8)
C8–C9 1.378(3) 1.348(8)
C9–C10 1.380(3) 1.388(9)
C10–C11 1.386(3) 1.357(9)
C11–C12 1.385(3) 1.395(8)
C12–C7 1.404(3) 1.436(8)
C12–C13 1.510(3) 1.466(9)
C13–O3 1.212(7)
C13–N2 1.475(2) 1.425(7)
C20–O2 1.312(2) 1.281(6)
C14–N2 1.290(2) 1.307(7)
C14–C15 1.446(3) 1.414(8)
C15–C16 1.407(3) 1.422(8)
C16–C17 1.372(3) 1.373(9)
C17–C18 1.409(3) 1.400(8)
C18–C19 1.385(3) 1.358(8)
C19–C20 1.423(3) 1.454(8)
C20–C15 1.425(3) 1.426(8)
O1–Cu1–N1 83.25(7) 83.20(18)
N1–Cu1–N2 95.36(7) 95.3(2)
N2–Cu1–O2 94.21(7) 94.75(19)
O2–Cu1–O1 90.68(6) 87.17(17)
O1–Cu1–N2 163.85(7) 170.3(2)
N1–Cu1–O2 165.01(7) 169.72(18)
Cu1–O1–C2 113.17(14) 111.6(4)
Cu1–N1–C1 112.87(13) 110.6(4)
Cu1–N1–C7 122.87(14) 125.3(4)
Cu1–N2–C13 115.89(13) 125.0(4)
Cu1–N2–C14 124.63(15) 120.4(4)
Cu1–O2–C20 127.76(13) 128.9(4)
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Table 4.7: A comparative study of solid state structure of 18, and 18a:
Plane A: C1–C2–C3–C4–C5–C6 Plane B: N1–C1–C2–O1
Plane C: Cu1–O1–N1 Plane D: Cu1–N2–O2 Plane E: N1–Cu1–N2
Plane F: N2–C14–C15–C20–O2 Plane G: Cu1–N2–C14–C15–C20–O2
Dihedral angel between Complex 18 Complex 18a
Plane A and Plane D 20.15 23.24
Plane B and Plane C 6.32 16.46
Plane D and Plane E 11.55 1.95
Plane C and Plane D 20.65 9.99
Plane A and Plane F 19.10 34.93
Plane A and Plane G 19.73 28.26
[A]
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Figure 4.19: [A] µeff vs T plots for complex 18; [B] µeff vs T plots for complex 18a.
Variable–temperature magnetic susceptibility measurement of 18 and 18a in solid state were performed in the temperature range 2–300 K at an external magnetic field 0.1 T using a SQUID magnetometer. Figure 4.19[A] represents the measured as well as the simulated µeff vs T plots for complex 18, on the other hand Figure 4.19[B] represents the µeff vs T plots for complex 18a. Complex 18 showed µeff = 2.52 µB, at 300 K, which was close to non–interacting a Cu(II) and a –radical system. Upon cooling, µeff value increases to µeff = 2.58 µB at 20 K and then decreased. Increase in µeff with decrease in temperature indicated a ferromagnetic coupling between the two spins.27 Further decrease in µeff was due to intermolecular antiferromagnetic coupling. The experimental result was simulated using the following parameters; gCu(II) = 2.06, gR = 2.00, J = +9.57 cm–1, and = 2.23 K. On the other hand, complex 18a showed a diamgentic ground state (S = 0) where two spin were strongly antiferromagnetically coupled to each other. The eff value at 300 K was below 1.00 B and at 2 K was closed to zero.
Planar Cu(II) semiquinone complexes show a ferromagnetic ground state because of orthogonalty of two interacting spin systems, where a the unpaired electron of Cu(II) resides on dx2
–y2 magnetic orbital and the –radical of the semiquinone system locates on pz magnetic orbital.28 The distortion from the planar geometry means the loss of orthogonalty between two spin states and that leads to the antiferromagnetic interaction between same, and consequently, stabilizes a diamagnetic ground state (S = 0). Although, it is reported and theoretically proved that a distorted square planar Cu(II) semiquinone complex shows a
[B]
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ferromagnetic ground state below the twist angel ~ 20 (twist angle refers the angel between the two biting planes around the Cu(II) center). 27a However, above the twist angle value the complex shows an antiferromagnetically coupled ground state (S = 0).27a The twist angel in complexes 18, and 18a were ~ 20.65 and ~ 9.99, respectively. Therefore, both complexes, according to the report, should show a ferromagnetic ground state (S = 1). In fact, complex 18 showed a ferromagnetic ground state, while, complex 18a showed an antiferromagnetic ground state. Hence, to interpret this discriminating results the dihedral angle between the Cu(II) plane comprised of N2–to–O2 atoms (plane F) and the plane passing through the C6– aryl ring (plane A) where the –radical was residing need to be considered (Table 4.7). In 18 the dihedral angle was ~ 19, while in 18a the angle was ~ 35. Because of the higher deviation an antiferromagnetic coupling was observed.
Figure 4.20: UV–vis/NIR spectra for 17, 18, and 18a were measured in dichloromethane solution at ambient temperature in 300–1500 nm range.
The electronic absorption spectra for complexes 17, 18, and 18a are shown in Figure 4.20. Complex 17, and 18 showed a broad absorption band at max = 970 nm ( = 1800 M–1cm–1) due to the intervalence ligand(phenolate)–to–ligand(iminosemiquinone) charge transfer (IVCT). While, in 18a, that band appeared around max = 1000 nm ( = 800 M1cm1). The intra–ligand charge transfer due to presence of –radical appeared at max = 845 nm ( = 2900 M–1cm–1) for both 17 and 18.21a,26a Absorption at max = 540 nm ( = 3400 M–1cm–1) for 17 and max = 540 nm ( = 3050 M–1cm–1) for 18 was due to TH-1360_11612213
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ligand(phenolate)tometal[Cu(II)] charge transfers.5f,26c Complex 18a showed absorption maxima at max = 800 nm ( = 1950 M–1cm–1) due to ligand center –radical (intra ligand charge transfer), while, the band at max = 460 nm ( = 6800 M–1cm–1) appeared due to charge transfer transition for amide N–C=O unit.22
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