Date of Award

9-27-2018

Publication Type

Doctoral Thesis

Degree Name

Ph.D.

Department

Chemistry and Biochemistry

Keywords

1, 2, 4-benzothiadiazines, coordination chemistry, N-donor ligands, polynuclear metal complexes

Supervisor

Rawson, Jeremy

Rights

info:eu-repo/semantics/openAccess

Abstract

Chapter 1 provides an introduction to coordination chemistry and polydentate N-donor ligands along with a synopsis of heterocycles with N and S/N donor atoms. An overview of the chemistry and applications of 1,2,4-benzothiadiazines as well as their previously reported coordination chemistry follows. Chapter 2 describes the synthesis of the ligand 3-(2ʹ-pyridyl)benzothiadiazine (pybtdaH). Computational studies on the ligand determining the most stable conformation and tautomer, as well as the energy barrier of the pyridyl ring rotation. The coordination chemistry of pybtdaH with more basic counterions (hfac– and OAc–) or Lewis acidic metal (FeIII) results to oxidation of pybtdaH forming either pybtdaHox or pybtdaox–. The ligand acts as terminal in the mononuclear complexes with formulas FeCl3(pybtdaHox)(CH3OH), M(hfac)2(pybtdaHox) (M = Mn, Co, Ni) and Co(pybtdaox)3. The pybtdaox– offers a N,Nʹ-pocket and an additional O-donor via the S=O group leading to polynuclear metal complexes with formulas Fe4Cl4(OCH3)6(pybtdaox)2, Cu2(OAc)2(pybtdaox)2(H2O)2, Zn2(OAc)2(pybtdaox)2, and the polymer [Cu(hfac)(pybtdaox)]n. The addition of base (Et3N) in the reaction schemes favoured aggregation resulting to the polynuclear complexes Ni3(hfac)(pybtdaox)5(H2O), Cu4(OH)4(pybtdaox)4 and Cu14(OH)12(CO3)2(pybtdaox)12(H2O)2. The synthesis of the novel redox active ligand 3-(2',6'-pyrimidine)-benzo-1,2,4-thiadiazine (pmbtdaH) is reported in Chapter 3. The radical pmbtda• can be prepared by in situ 1e– oxidation and its radical character confirmed by EPR spectroscopy and DFT calculations. Reaction of pmbtdaH with MCl2·2H2O (M = Mn, Fe, Co, Ni) affords a series of mononuclear complexes with formula MCl2(pmbtdaH)2 and the dinuclear complex Zn2Cl4(pmbtdaH)2 in which the ligand coordinates in a simple N,Nʹ-chelate fashion. The reactions of pmbtdaH with M(hfac)2 (M = Mn, Co, Ni, Cu, Zn) rapidly afforded the mononuclear complexes M(hfac)2(pmbtdaH). The hfac– ligand appears sufficiently basic to promote slow aerial oxidation of the pmbtdaH ligand and a series of complexes were isolated on extended storage incorporating either pmbtdaHox or pmbtdaox–. These include the mononuclear complexes M(hfac)2(pmbtdaHox) (M = Co, Ni) and the dimer Mn2(hfac)2(tfa)2(pmbtdaHox)2. The deprotonated and oxidized form of the ligand bridges three metal centres via two N,Nʹ-chelate coordination pockets and the S-O group resulting in the tetranuclear complexes Cu4(hfac)4(tfa)2(pmbtdaox)2 and Zn4(hfac)6(pmbtdaox)2. The complexes were characterized by X-ray diffraction, elemental analysis, IR and UV-Vis spectroscopies, as well as 1H NMR spectroscopy for the diamagnetic complex Zn(hfac)2(pmbtdaH). Chapter 4 describes the S-functionalization of the pybtdaH ligand to afford the 1-methyl-3-(2ʹ-pyridinyl)benzothiadiazine (pybtdaSMe). The reaction of the ligand with MCl2·2H2O (M = Mn, Ni, Zn) gives two mononuclear complexes with general formula MCl2(pybtdaSMe)2 and the 1:1 adduct ZnCl2(pybtdaSMe). The reaction of CuCl2 with pybtdaSMe is sensitive to the solvent resulting in two polymorphs, Cu2Cl4(pybtdaSMe)2 and Cu2Cl4(pybtdaSMe)2·2CuCl2(pybtdaSMe). Preliminary results examining the reactivity of this ligand with M(hfac)2 (M = Co, Ni) afforded the corresponding mononuclear complexes M(hfac)2(pybtdaSMe) (M = Co, Ni). In Chapter 5, the synthesis and characterization of the novel terpy-like ligand 3,3'-pyridine-2,6-diylbis(benzothiadiazine) (bisbtdaH2) is reported. Electrochemical studies on the free ligand bisbtdaH2 reveal a single well-defined 2e– oxidation process and EPR studies of the in situ chemical oxidation on bisbtdaH2. The coordination chemistry of the ligand with a range of divalent transition metal salts MX2 (M = Mn, Fe, Zn, X = CF3SO3; Fe, X = BF4; Co, Ni, X = Cl) afforded mononuclear complexes of general formula [M(bisbtdaH2)2][X]2, whereas reaction of Cu(NO3)2 with bisbtdaH2 afforded the 1:1 complex Cu(bisbtdaH2)(NO3)2. In all cases the bisbtdaH2 ligand binds in a tridentate N,Nʹ,Nʺ-chelate fashion via both NBTDA and Npy atoms. The low-spin FeII complexes were implemented for NMR and UV-Vis solution studies of ligand reactivity, as well as cyclic voltammetry which reveals the oxidation process occurs via two single e– oxidation waves. The metal complexes reveal a range of 6-coordinate geometries between octahedral and trigonal prismatic with the greatest deviation from octahedral symmetry apparent for ions with no crystal field stabilization energy. Chapter 6 provides a brief overview of the results presented in this dissertation, the insight provided within this research area and the potential for future exploitation.

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