The reaction between equimolar amounts of ligand H4LCH2NH
2 and NiCl26H2O in acetonitrile in the presence of triethylamine and air provided complex 10 (Scheme 3.4).
Interestingly, under the same reaction condition, ligand H3L1 and H4L2 also provided complex 10.
Scheme 3.4: Synthetic route for complex 10.
IR spectrum of complex 10 showed a small intense sharp band at 3346 cm–1 due to asymmetric (N–H) stretching. The asymmetric, symmetric, and bending overtone bands for
(C–H) stretches of tert–butyl groups were found at 2945, 2860, and 2900 cm–1, while the corresponding bending vibrational band arose at 1476 cm–1. Two bands appeared at 1617, and 1599 cm–1 for two different (C=N) stretching units.15 The phenolate (C–O) band
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appeared at 1264 cm–1,15 while the band at 1154 cm–1 was appeared due to the (C–N) stretch.16
Figure 3.7: ESI–mass spectrum of 10 with experimental and simulated isotopic distribution pattern (inset).
Electrospray ionization mass spectrum (ESI–MS) of complex 10 was examined in acetonitrile solution in positive mode. In the spectrum a 100% molecular ion peak appeared at m/z = 484.20 that corresponded to [M + H]+; where M = molecular mass (Figure 3.7). The composition of the observed mass and hence the complex was C28H31N3NiO as confirmed by isotope pattern distribution examination.
Figure 3.8: 1H NMR spectrum of 10.
1H NMR spectrum of 10 was measured in CDCl3 solvent and is shown in Figure 3.8.
Complex showed the normal isomer shifting value at 1.35 (s, 9H), 1.49 (s, 9H), 5.97 (s, 1H), 6.51 (t, J = 7.2 Hz, 1H), 7.03 (d, J = 8.4 Hz, 1H), 7.15 (t, J = 6.8 Hz, 1H), 7.18 (s, 1H), 7.40 (t, J = 7.6 Hz, 2H), 7.46 (s, 1H), 7.55 (t, J = 7.8 Hz, 1H), 7.69 (d, J = 8.0 Hz, 1H), 7.82 TH-1360_11612213
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(d, J = 7.6 Hz, 1H), 8.56 (s, 1H), 8.73 (s, 1H) ppm and strongly suggested its diamagnetic nature.
Figure 3.9: ORTEP representation 10. Thermal ellipsoids were drawn at 50% probability level. H atoms (except the one attached to N3 atom) were omitted for clarity.
The molecular structure of complex 10 was analyzed by X–ray single crystal diffraction measurement at 298 K. The complex crystallized in the monoclinic system, C12/c1 space group. The molecular structure is shown in Figure 3.9 and the selected bond distances and bond angles are given in Table 3.2.
In the neutral complex 10 the central Ni1 atom was four–coordinate with a slightly distorted square planar geometry ( = 0.11).17 The dihedral angle between N2–N1–N3 and N1–Ni1–O1 planes was 8.3 and indicated a small twist around the Ni atom in the molecule.
An asymmetric environment around Ni1 atom was reflected by the Ni1–O1 = 1.858(2), Ni1–
N1 = 1.867(3), Ni1–N2 = 1.876(3), and Ni1–N3 = 1.830(3) Å bond distances. The difference in the bond distances was due to the different nature (hybridization) of the coordinating atoms. The C–C bond distances of the tert–butyl groups containing C6 phenyl ring were almost all within 1.390.02 Å range and were in accord with the phenyl C=C bond distances.18 Furthermore, both C1–N1 = 1.417(5), O1–C2 = 1.321(4) Å bond distances implied their single bond character. These bond distances along with the double bond characterizing N1–C7 = 1.301(4) Å bond confirmed an iminophenolate form of the tert–butyl TH-1360_11612213
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groups containing phenyl ring where the phenolate1– charge was located on phenolate oxygen atom. The C–C bond distances in the C6 ring comprised of C15–to–C20 atoms showed a substantial elongation in C15–C16 = 1.422(6), C20–C15 = 1.426(5), and C19–C20
= 1.424(6) Å bond distances compared to the C16–C17 = 1.357(6), C17–C18 = 1.398(6), C18–C19 = 1.365(6) Ǻ and indicated a quinoid–type distortion in the C6 ring. This distortion was due to the delocalization of amide1- charge over the iminosalicylidene moiety as evidenced by short N3–C20 = 1.327(5), and C14–C15 = 1.408(5) Å bond distances compared to their corresponding single band characterizing values19 followed by an elongated C14–N2
= 1.324(5) Å bond distance compared to its double bond characterizing value (1.28 Å).20 This type of alternative shortening and elongation is common in salen complexes where the ligand is found to be in its fully reduced form.19 Hence, from the X–ray structural analysis it was evidenced that the neutral asymmetric complex 10 was Ni(II) salen–type with two imine N1 and N2 atoms, one amide N3 atom, and one phenolate O1 atom in its coordination environment.
Table 3.2: Selected bond distances (Å), and bond angels () for complex 10.
C1–C2 1.402(5) C16–C17 1.357(6)
C2–C3 1.412(5) C17–C18 1.398(6)
C3–C4 1.373(6) C18–C19 1.365(6)
C4–C5 1.421(6) C19–C20 1.424(6)
C5–C6 1.372(5) C20–C15 1.426(5)
C6–C1 1.392(6) C1–N1 1.417(5)
C7–C8 1.418(5) N1–C7 1.301(4)
C8–C9 1.408(5) C13–N2 1.434(5)
C9–C10 1.355(6) N2–C14 1.324(5)
C10–C11 1.382(7) C20–N3 1.327(5)
C11–C12 1.389(6) C2–O1 1.321(4)
C12–C13 1.387(5) Ni1–O1 1.858(2)
C13–C8 1.403(6) Ni1–N1 1.867(3)
C14–C15 1.408(5) Ni1–N2 1.876(3)
C15–C16 1.422(6) Ni1–N3 1.830(3)
O1–Ni1–N1 85.76(12) C13–N2–C14 115.1(3)
N1–Ni1–N2 95.67(13) N2–C14–C15 127.9(4)
N2–Ni1–N3 93.54(14) C14–C15–C20 122.0(4)
N3–Ni1–O1 85.46(13) C15–C20–N3 119.6(4)
O1–Ni1–N2 176.61(13) N1–Ni–N3 168.20(14)
O1–C2–C1 117.1(4) Ni1–O1–C2 112.9(2)
C2–C1–N1 111.7(4) Ni1–N1–C1 111.3(2)
C1–N1–C7 121.8(3) Ni1–N1–C7 126.8(3)
N1–C7–C8 124.3(4) Ni1–N2–C13 122.3(3)
C7–C8–C13 124.8(3) Ni1–N2–C14 122.6(3)
C8–C13–N2 121.1(4) Ni1–N3–C20 131.2(3)
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Figure 3.10: UV–vis/NIR spectrum of 10 measured at room temperature in dichloromethane solution in 3001000 nm range.
The electronic absorption spectrum of complex 10 is shown in Figure 3.10. All the observed bands appeared in the UV–vis region and bands were high in intensity. This indicated that the bands were appearing because of charge transfer. The absorption maxima at
max = 518 nm ( = 25100 M–1cm–1) appeared due to metal–to–ligand charge transfer (MLCT),21 while, the band at max = 326 nm ( = 42050 M–1cm–1) was due to the ligand–to–
metal charge transfer (LMCT).22 A ligand–to–ligand charge transfer (LLCT) band owing to
→ transition was appeared at max = 390 nm ( = 40200 M–1cm–1).21b
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A plausible reaction mechanism for the formation of complex 10 from the initially used ligand H4LCH2NH
2 via the in situ generation of ligand H3L1 and H4L2 and their corresponding Ni(II) complexes is presented in Scheme 3.5.
Scheme 3.5: Showing mechanistic proposals for the formation of H3L3 from ligand H4L2, and ligand H4LCH2NH
2.
Ligand H4LCH2NH
2 initially reacted with NiCl26H2O in the presence of triethylamine and provided a diradical–coordinated square planar Ni(II) complex, [NiII(H2LCH2NH
2
)2].
Mass spectrometric analysis (Figure 3.11) of the reaction solution (CH3CN) that obtained after immediate mixing of the reactants (H4LCH2NH2, NiCl26H2O, and Et3N) under air showed a mass peak at m/z = 706.39, which correspond to [NiII(H2LCH2NH2)2] species and supported its formation. The N atom from free amine group of benzylamine then attacked at TH-1360_11612213
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the imine carbon atom present in [NiII(H2LCH2NH2)2] species. This resulted an aryl migrated diradical–coordinated octahedral Ni(II) complex [NiII(H2L2)2] formation and a complete modification of the initially used ligand backbone. [NiII(H2L2)2] underwent further two–
electron aerial oxidation and provided two equivalents of organic ligand H3L1, which upon coordination with Ni(II) ion provided [NiIIH2L1] species. This species then succumbed to ligand imine hydrolysis and provided 2–aminobenzaldehyde (A) and 4,6–di–tert–butyl–2–
aminophenol (B). The coordination of imine N atom to Lewis acidic Ni(II) center activated the imine carbon atom by increasing its electrophilicity and hence, the hydrolysis was occurred favorably.
To consolidate the formation of new ligands H3L1 and H4L2, and its corresponding Ni(II) complex as intermediate to the formation of complex 10, both ligand H3L1 and H4L2 were synthesized (Experimental section). In accordance with the proposal, complex 10 was isolated by employing both ligands under the same reaction condition. Hence, their formation was confirmed.
Figure 3.11: ESI–mass spectrum of [NiII(H2LCH2NH 2
)2] with experimental and simulated isotopic distribution pattern (inset).
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