Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials

Muhammad I (2014)
Bielefeld: Universität Bielefeld.

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Bielefeld Dissertation | English
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The proposed chemistry in this project was aimed for the fundamental exploration of the preparative accessibility and properties of a broad range of boron centred soft ligands and their metal complexes particularly with bismuth(III) ions, investigation of their structures in the solid and solution state followed by preliminary photo-physical measurements and optimization of the emitter quality after a feed-back from such measurements. Consequently three main types of soft ligands were synthesized. (1) Tri-substituted boron-centred soft ligands (Janus scorpionate ligands), (2) di-substituted boron-centred soft ligands (3) heterocyclic precursors as soft ligands. Two novel Janus scorpionate ligands [TrMe]– and [TttMe]– were produced in-situ by the reaction of MBH4 and respective heterocycle (3-mercapto-4-methyl-1,2,4-triazole and 5-mercapto-1-methyltetrazole) and were isolated in the form of alkali metal complexes [MTrMe] (1 & 2) and [MTttMe] (3 & 4) (M = K, Na), respectively. Later on, two mixed complexes such as [NaK(TrMe)2] (5) and [NaK(TttMe)2] (6) were also synthesized with the intention to observe Janus type coordination behaviour and orientation of Na and K towards hard and soft sites of these ligands. These alkali metal complexes exhibit polymeric and sheet-like structures in solid state. K[TrMe] (2), Na[TttMe] (3) and Na[HB(mtdaMe)3] (11)[65] (11 is a reported Janus scorpionate ligand) were reacted with BiX3 (X = Cl–, I–, NO3– and CH3COO–) to afford bismuth complexes [Bi(TrMe)(Cl)(µ-Cl)2]2 (12) and [Bi(TrMe)(Cl)2(µ-Cl)]n (13), [Bi(TttMe)2(CH3COO)] (14), [{HB(mtdaMe)3}2BiCl] (15), [{HB(mtdaMe)3}2Bi(NO3)]n (16) and Na[{HB(mtdaMe)3}2BiI2] (17). The resulting bismuth complexes are monomeric (14, 15 and 17), dimeric (12) and polymeric (13, 16) in nature. Except 17, all of them have distorted geometries with stereochemically active lone pairs on the bismuth atoms. In contrast to Trofimenko’s protocol for 1 – 6, alkali metal complexes [NaBb] (7), [NaBtMe] (8), [NaBttPh] (9) and [KBttMe] (10) of di-substituted boron-centred soft ligands were prepared using THF/toluene solutions. [NaBb] (7), [NaBtMe] (8), [NaBttPh] (9) were reacted with BiX3 (X = Cl–, CH3COO–) to yield bismuth complexes [BiBb2Cl] (18), [BiBbCl(µ Cl)2]2 (19), [BiBtMe3]•CH2Cl2 (20), [BiBtMe3]•CHCl3 (21), [Bi(L2)2]2•THF (22) and [Bi(L2)2]2•2CH3CN (23). Complex 18 exhibits a (B)H...Bi interaction at 2.58 Å which is unprecedented in bismuth chemistry. Complexes 22 and 23 have Bi2+ instead of Bi3+ as central ions which is because of a reduction reaction which occurred during their synthesis. The third category of heterocyclic precursors as soft ligands (3-mercapto-4-methyl-1,2,4-triazole (L1H), 2-mercapto-benzimidazole (L2H), 2-mercapto-4-methylthiazole (L3H) and 2-mercapto-4-phenylthiazole (L4H)) upon reaction with BiCl3 resulted in bismuth complexes [Bi(L1H)4(Cl)2]Cl (24), [Bi(L1H)4Cl2][Bi(L1H)2Cl4] (25), [Bi(L2H)2Cl2(µ-Cl)]2 (26) and [Bi(L4)3] (27). These complexes possess relatively regular coordination geometries when compared with above two types of bismuth complexes 12 – 17 and 18 – 23. Generally the Bi–S bond lengths in 24 – 27 are shorter revealing strong coordination compared to 12 – 23. Finally three mixed-ligand bismuth complexes [{HB(mtdaMe)3}Bi(phen)Cl2] (29), [{HB(mtdaMe)3}Bi(bipy)Cl2] (30) and [BttMeBi(phen)Cl2] (31) were prepared by using boron-centred soft ligands as primary ligands. 29 and 30 feature interestingly (B)H...Bi interactions at 2.76(3) Å for 29 and 2.71(2) Å for 30; these are weaker compared to 2.58 Å found in 18. Lead(II) complexes were also synthesized by the reaction of a tri-, a di-substituted boron-centred soft ligand and eight small heterocyclic soft ligands with Pb(NO3)2. The resulting complexes are [Pb(L1)2(L7H)2Lʹ] 33 (where Lʹ is 1-methyl-2-pyrrolidinone), [PbL22] 34 (where L2 = dihydrobis(thiazolyl)borate(BtMe)), [PbL32(NO3)2] 35, [PbL43(μ-L4)(NO3)2]2 36, [Pb(L5)(L5H)2(NO3)(H2O)]n 37, [PbL64(NO3)2] 38, [Pb(L7)2(L7H)]n 39a, [Pb(L7)2(L7H)(CH3OH)]n 39b, [PbL82]n 40, [PbL92]n 41, [PbL102]n 42. The coordination numbers of these complexes vary from 4 to 8 and the majority of them are polymeric in nature with hemidirected environments around the lead ions. Preliminary photo-physical studies were carried out on a few selected bismuth complexes such as 12, 20, 24, 25 etc. Generally, it has been observed that upon complexation of the Bi(III) ion by heterocyclic thione units, the π-π* absorption band are bathochromically shifted. MC sp transitions were also observable. Some of the complexes are not emissive in solution, however, at 77 K in ethanol glasss show emission bands (e.g 25 exhibit this band at 537 nm). 25 also exhibits thermochromic behaviour, its crystalline sample is orange at room temperature and changes its colour to yellow when cooled with liquid nitrogen. Conclusively this work provided a successful approach to study the versatile coordination behavior (such as κ3-S,S,S, κ2-S,S, κ1-S, κ3-H,S,S, κ4-H,S,S,S; κ2-H,S, κ3-H,H,S, κ3-H,N,S, κ3-N,N,N, κ2-N,N and κ1-N) of tri- and di-substituted boron-centred soft ligands towards Na, K, Bi(III) and Pb(II) ions. Additionally for bismuth complexes, the use of different anions or conjugated neutral systems as co-ligands influenced the stereochemically activity of lone pair resulting interesting bonding situations e.g unprecedented B–H...Bi interactions. These unprecedented interactions in bismuth chemistry can be further explored to investigate bismuth-hydride activation or Bi→B dative bond formation (metalloboranes). Bismuth complexes based on smaller soft heterocyclic ligands were found to be better luminescent compared to those based on tri- and di-substituted boron-centred soft ligands, as the former lead to a stronger binding of the ligand (as revealed from M–S bond lengths) and to an enhanced interaction between metal-centred and ligand-centred molecular orbitals thus enhancing the desired spin-orbit coupling effects. From these preliminary luminescence studies, it can be suggested that if such soft systems are used and the lewis-acidity of the bismuth atom is further increased by some electron withdrawing groups like perfluoro alky/aryl substituents etc., the desire of strong spin-orbit coupling effects as well as the stability of the complexes will be enhanced and this approach will decisively contribute to further improvement of luminescence activity for their potential applications in displays and area light sources.
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Muhammad I. Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials. Bielefeld: Universität Bielefeld; 2014.
Muhammad, I. (2014). Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials. Bielefeld: Universität Bielefeld.
Muhammad, I. (2014). Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials. Bielefeld: Universität Bielefeld.
Muhammad, I., 2014. Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials, Bielefeld: Universität Bielefeld.
I. Muhammad, Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials, Bielefeld: Universität Bielefeld, 2014.
Muhammad, I.: Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials. Universität Bielefeld, Bielefeld (2014).
Muhammad, Imran. Boron-centred soft ligands and their complexes with Na, K, Bi(III) and Pb(II) ions : an approach towards inexpensive luminescent materials. Bielefeld: Universität Bielefeld, 2014.
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