Geometrical and Substitutional Influences on the Electronic Structure and the Reactivity of First--Row Transition Metal

Samir Ghorai (Roll number under my supervision in the Department of Chemistry, Indian Institute of Technology Guwahati, India. Frontiers in Chemical Sciences (FICS)–2012, 2nd – 3rd December, Department of Chemistry, Indian Institute of Technology Guwahati. This work was carried out between July 2011 and August 2014 at the Indian Institute of Technology Guwahati, Guwahati, Assam, 781039, India.

Papers published

Doctoral Committee

General Introduction and Motivation

A tridentate ligand combined from a non-innocent core 2-aminophenol and a substrate core Benzylamine and the corresponding Co(III), Ni(II) and Cu(II) complexes: synthesis, characterization and reactivity. 2Anilino4,6ditertbutylphenol is a noninnocent ligand and yields a diradicalcontaining Cu(II) complex. 2 behaved as a noninnocent ligand and stabilized the corresponding Co(III) complex in diradical-coordinated octahedral geometry.

CONTENTS Chapter I

• b: Effect of –ortho Substituent to the Formation of the Triradical–Containing
• c: Synthesis and Characterization of a Tetraradical–Containing Octanuclear
• 4H 2 O as a Catalyst to the Ligand H 2 L CN : 51
• Synthesis and Characterization of the Square Pyramidal Co(III) Complex
• Synthesis and Characterization of Co(III) Complex (9) Formed With Ligand H 4 L CH 2 NH
• Synthesis and Characterization of the Ni(II) Complex (10) Formed with
• Synthesis and Characterization of the Cu(II) Complex (11) Formed with
• Ligand H 3 L1 and Its Selective Reactivity with Salicylaldehyde in the Presence of Zn(OAc) 2 2H 2 O: A Fluorescent Active Dinuclear Zn(II)
• a: Square Planar Ni(II) Complexes Synthesized by Using Mixed Ligands;
• Synthesis and Characterization of Monoradical–Containing Distorted
• Synthesis and Characterization of Co(III) complexes with H 2 L X
• Synthesis and Characterization of Co(III) complexes with H 2 L X
• Reactivities Toward Solvent, Benzyl Bromide, and 2–nitrobenzyl Bromide: 208
• General Introduction and Motivation
• NH 2 + H 2 O + O 2 RCHO + NH 3 + H 2 O 2

Currently, 2anilino4,6ditertbutylphenol (H2APR; R = H, Scheme 1.7) behaves as a non-innocent ligand when coordinated to copper(II), nickel(II), palladium(II) , cobalt (III), iron (III) and manganese (IV) metal ions and provides the corresponding metal-radical complexes.13i,14 On the contrary, the ligand does not show any change in its oxidation state to the corresponding vanadium. (V) complex. For example, Cu(II) complexes derived from 2[3,5disubstituted(R)anilino]4,6ditertbutylphenol (H2APR) ligands (Scheme 1.8) show a clear tendency at the first oxidation potential depending on the nature of the substituents. In addition, to study the effect of ligand-centered substituent on geometry and subsequently, geometry-dependent reactivity, H3LMixed(H) and H3LMixed(tBu) ligands are synthesized (Scheme 1.12) by placing two tertbutyl at ,5 positions of the salicylidene unit.

Introduction

Redox-noninnocent behavior of ligands depends not only on substituents attached to the ligand backbone, it strongly depends on the nature of the encapsulating transition metal ion in the complex form. However, the non-innocent nature of the dithiolene ligand in any complex is strongly dependent on the nature of the transition metal ion present in the complex. 2-Anilino-4,6-di-tert-butylphenol [H2APH] is a non-innocent ligand and stabilizes various radical-containing transition metal complexes, e.g.

Examination of the isotope distribution pattern of the observed mass peak indicated the C21H26N2O composition of the H2LCN ligand (Figure 2.3). The synthesized ligand can behave as innocent and therefore, it can exist in different oxidation states in the presence of metal ions (possibly transition metals) and molecular oxygen.

The V4 atom shared one oxygen atom (O1, O3, and O5) from each of the three moieties of 3,5-di-tert-butylcatecholate as its coordination sites. However, the presence of three catecholate moieties containing  radicals, the nearly equal vanadyl bond distances, and the mononegative character of the V4 cluster result in a +IV oxidation state for all V atoms (see below). Therefore, the monoanionic portion of the complex is best described as a tetranuclear oxo-V(IV) cluster containing a triradical.

Containing Octanuclear Vanadium Cluster

To understand the formation of complex 1 and complex 2 via ligand-centered C–N bond cleavage and C–O bond formation, ESI-MS and 1H NMR analysis of the reaction solution were performed. ESI–MS of the reaction solution after 2 h of reflux and 15 min of air stirring showed an ion peak corresponding to a vanadyl moiety ligated to a deprotonated ligand [LX]n– (X = –OMe, – CN) and a SO42– . The substituent can then undergo a weak interaction with the vanadium center of another molecule due to their ambidentate (–.CN)/bridging (–OMe) character and consequently favors clustering via C–N bond cleavage and C–O bond formation.

Therefore, the process of breaking the ligand-centered C–N bond and consequently the formation of C–O bonds during the reaction of VOSO45H2O and H2LOMe in the presence of triethylamine was monitored in a CD3OD solution (Scheme 2.9 ). Breaking the ligand-centered C–N bond would result in o-anisidine and formation of the C–O bond would yield vanadium-coordinated 3,5-di-tert-butyl-1,2-semiquinone units. To investigate the formation of o–anisidine and vanadium-coordinated 3,5–di–tert–butyl–.

The comparative studies of the spectra with the 1H NMR spectrum of the free ligand (SG_535_1_1H) and the 1H NMR spectrum of o–anisidine (o–anisidine), indicated the formation of o–anisidine (phenolic protons, SG_535_4_1H and o –anisidine) and the concentration of this species increases with time. Unfortunately, the methyl protons belong to the –OMe group of o–anisidine and were not observed in the 1H NMR spectrum of the reaction solution (SG_535_4_1H) at 3.82 ppm. The appearance of the peak at 4.11 ppm was due to the coordination of the ligand center OMe group with the metal ion.

The peak at 3.47 ppm could be due to methyl protons of the bridging –OMe groups present in complex 2.

• a: Synthesis and Characterization of a Phenoxazine Derivative Using MnCl 2 4H 2 O as a Catalyst to the Ligand H 2 L CN
• b: The Mechanistic Study
• c: Application of 4 as Biosensors
• Synthesis and Characterization of the Square Pyramidal Co(III) Complex [Co(L CN 2 )Cl]; (6) Formed with H 2 L CN Ligand
• Synthesis and Characterization of the Square Planar Ni(II) Complex [NiL CN 2 ]; (7) Formed with Ligand H 2 L CN
• Synthesis and Characterization of the Square Planar Cu(II) Complex, [CuL CN 2 ]; (8) Formed with Ligand H 2 L CN
• Conclusions
• The ligand provided square pyramidal diradical–containing Co(III) complex and square planar Ni(II) and Cu(II) complexes. In all cases a strong antiferromagnetic coupling
• Introduction
• Characterization of Tridentate Ligands
• Synthesis and Characterization of Co(III) Complex (9) Formed With Ligand H 4 L CH 2 NH 2

By adding 4 mol % of MnCl2·4H2O to an acetonitrile solution of the ligand in the presence of Et3N under air, the two-electron oxidized product, the compound iminobenzoquinone (LCNQ) was produced (Scheme 2.12). In the IR spectrum of the complex, no characteristic stretching bands were found for the –OH and –NH functional groups. The IR spectrum of the complex showed no absorption bands for the (O-H) and (N-H) bands, in addition, the presence of tert-butyl and -CN stretching frequencies at and 2225 cm-1, respectively, indicated that the ligand bound to the ion Ni where O-H and N-H were deprotonated.

A green needle-like crystalline solid was obtained by slow evaporation of the CH2Cl2–CH3CN (3:1) solvent mixture of the complex. These showed the coordination of the units in their deprotonated O– and N– forms in the corresponding copper complex. A single crystal of the X-ray quality complex was grown from a CH2Cl2:CH3CN (3:1) solvent mixture using a slow solvent evaporation technique.

X-band EPR spectrum and simulation spectrum of the experimental results of complex are shown in Figure 2.37. Herein, it was found that in the presence of excess vanadium salt and base triethylamine, one of the semiquinone moieties oxidized to its two-electron oxidized quinone form. Organic moieties containing at least one  radical and coordinated to a metal ion, especially transition metal ion, have gained great importance for the synthesis of catalysts for metal complex-catalyzed organic small molecule oxidation reactions, for example oxidation of primary alcohols to aldehydes, catechol to quinone, primary amines to aldehydes etc.1 The advantage of having –radical in a molecule is the easy acceptance of electron from substrates.

The active site of the primary amine oxidase contains a Cu(II) ion surrounded by three histidine imidazole units and a closely located organic cofactor of the top quinone.4 The top quinone part accepts two electrons from the amine substrate and oxidizes the amine to an aldehyde in the presence of water. Inclusion of the –CH2NH2 group at the – ortho position to the aniline part of the ligand would provide a new harmless H4LCH2NH ligand. X-ray examination of the single crystal complex showed that the complex is crystallized in the monoclinic space group P121/c1.

• Synthesis and Characterization of the Ni(II) Complex (10) Formed with in situ Generated Salen – Type Ligand
• Synthesis and Characterization of the Cu(II) Complex (11) Formed with in situ Generated Salen–Type Ligand
• Ligand H 3 L1 and Its Selective Reactivity with Salicylaldehyde in the Presence of Zn(OAc) 2 2H 2 O: A
• Conclusions
• The ligand H 4 L CH 2 NH
• The tridentate ligand H 4 L CH 2 NH
• The tridentate ligand H 3 L1 reacts with salicylaldehyde selectively, in the presence of Zn(OAc) 2 2H 2 O and triethylamine, produced a fluorescence active phenolate bridged
• Introduction
• Synthesis and Characterization of Mixed Ligands
• Synthesis and Characterization of Square Pyramidal Fe(III) Complexes Formed with Mixed Ligands
• a: Square Planar Ni(II) Complexes Synthesized by Using Mixed Ligands; Synthesis, Characterization, and Reactivity

The difference in bond distances was due to the different nature (hybridization) of the coordinating atoms. 2 first reacted with NiCl2·6H2O in the presence of triethylamine and provided a diradically coordinated square planar Ni(II) complex, [NiII(H2LCH2NH. The 1616 cm–1 vibrational band appeared due to the presence of the (C= N). on the backbone of the ligand of 11.

The retro-synthetic route to H3L3 implied the formation of A and B in the reaction medium (Scheme 3.7) and suggested the possible formation of a species in the reaction medium. To rationalize the formation of H3L3 and then 11 via in situ generation of A and B from ligand H4L2 and H3L1 in the presence of Cu(II) ion, Et3N and air, ESIMS measurement of the reaction solution was performed. It is important to note that the reaction in the presence of Et3N under air instantly produced a large amount of ammonia gas.

G] experimental mass spectrum of the reaction mixture containing H4L2 and CuCl2·2H2O in the absence of triethylamine; [H] experimental ESI–mass spectrum of a reaction mixture containing H4L2 and CuCl2·2H2O in the presence of triethylamine. Complex 12 was prepared by treating 1 equivalent of salicylaldehyde to the H3L1 ligand in the presence of 1 equivalent of Zn(OAc)2·2H2O and triethylamine in acetonitrile under air (Scheme 3.10). The methyl groups present in the tert-butyl groups and the hydrogen atoms have been omitted for clarity.

Complex [FeLMixed(H)Cl]; (13) was synthesized from ligand H3LMixed(H) by reacting with FeCl3 in the presence of Et3N under air. Reaction between H3LMixed(H) and NiCl2 6H2O in methanol under air in the presence of triethylamine gave complex NiLMixed(H), (15). Whereas complex 16 was synthesized from the in situ generated mixed ligand H3LMixed(tBu) by adding 1 equivalent of NiCl2 6H2O in the presence of triethylamine under air.

• b: Reactivity and kinetic Studies
• Synthesis and Characterization of MonoradicalContaining Distorted Square Planar Cu(II) Complexes with Mixed Ligands
• Conclusions
• A ferromagnetic coupling was observed in complex 18, while, an antiferromagnetic coupling between the paramagnetic Cu(II) and the radical center was found in

Complex 15 and complex 16 crystallized in the monoclinic space group P21/a and C21/c1, respectively, while complex 16a crystallized in the orthorhombic space group P212121. In the complexes, all the C-C bond distances of the tert-butyl containing C6 aryl rings (amidophenolate unit) did not fall within 1.3901 Å, rather a quinoid-type distortion was observed (Table 4.4). This type of shortening and lengthening is common in salen complexes where the salicylidene unit is in its fully reduced form.14c–f In the complexes it was therefore found that the amidophenolate part of the ligand is in its one-electron oxidized ISQ 1–. form and no oxidation in the salen unit was obsd.

A gradual and regular increase in the absorption band (dashed lines) at  652 nm was observed until 70 min. Ni(II)- or Ni(III)-superoxo-, -peroxo-, -hydroperoxo-, -oxide and -hydroxo species do not give rise to a strong absorption band at  652 nm.23 Furthermore, addition of mCPBA (1 eq ) to the CH2Cl2 solution of 16 gave rise to similar changes in the UV-vis/NIR spectral properties (Figure 4.15) as did air. Square planar Cu(II) complexes [CuLMixed(H)]; (17) and [CuLMixed(tBu)]; (18) of the mixed ligands were synthesized by reacting equimolar amounts of CuCl2·2H2O and H3LMixed(H) or by adding 1 equiv of CuCl2·2H2O to the in situ generated ligand H3LMixed(tBu) in the presence of triethylamine under air.

Interestingly, all the C–C bond distances in the C6 aryl rings were not the same and were in the range of 1.390.01 Å. It should be noted that alternative short and long bond distances were also found in the salicylidene unit in both complexes due to the delocalization of phenolate 1–. This type of distortion in the salen unit is common in metal complexes where the salicylidene unit exists in a fully reduced form.

A subtle change in the dihedral angle in the ligand backbone was found to affect the docking mode.