Synthesis and characterization of a new ditopic bipyridine-terpyridine bridging ligand using a Suzuki cross-coupling reaction

Synthesis of a new bridging ligand 4 ′ -{4-[(2,2 ′ -bipyridin)-4-yl]-phenyl}-2,2 ′ :6 ′ -2 ′′ -terpyridine ( I ) was reported. A Suzuki cross-coupling reaction was conducted for the preparation of such ligand in two different routes either between 4 ′ -(4-bromophenyl)- 2,2′:6′ - 2′′ -terpyridine and 2,2 ′ -bipyridyl-4-boronic acid or 4′ -(4-boronatophenyl)- 2,2′:6′,2′′ -terpyridine and 4-bromo-2,2 ′ -bipyridine


Introduction
As part of our studies, we have been working on a research programme with the aim of synthesis of heterodinuclear complexes for the purpose of exploring the possibility of achieving light-induced ligand release. 1,2In these studies, we are focussing on the preparation and use of ditopic bridging ligands where the two metal ion binding sites are differentiated either by the configuration of the binding sites or by the number of donor atoms in each site.
This binding site differentiation allows us to use the different coordination properties of the binding sites to prepare Ru(II)-Co(III) systems to ensure that the correct metal ion is incorporated at the correct binding site in the ligand, which provide a suitable environment during the reactions of complexes formation to decrease the number of isomers that might occur.
In this context, polypyridyl types of ligands have been selected as candidates.We were interested in the preparation of such bridging ligands in which the numbers of donor atoms in the two binding sites were different.This kind of approach has already been used and a number of heterodinuclear complexes have been developed using ligands with different binding sites.For example, Constable et al. 3 and Li et al. 4 in two different studies have prepared heteroditopic ligands involving a bipyridine bidentate metal ion binding site at one end and a tridentate terpyridine metal ion binding site at the other end.They investigated the electronic properties of heterodinuclear complexes prepared using such ligands.Das et al. 5 developed a polypyridylimidazole system where the number of the donor sites on the ligand as well as the types of the binding sites are differentiated for the purpose of multichannel anion and cation sensing studies.][23] In our series of researches development of bridging ligands with bipyridine types at one end and terpyridine types of binding sites at the other end appealed.

AUTHOR(S)
C-C bond formation using Suzuki-Myiaura reaction which was employed by S. Bonnet et al., 24 was adopted for the preparation of ligand (I).For this purpose, two different routes were considered (Scheme 1(C)).The coupling reaction of 4′-(4-bromophenyl)-2,2′:6′,2′′-terpyridine (8) 31 and 1.5 equivalent of 2,2′-bipyridine-4boronic acid (5) 29 or the coupling reaction of 4-bromo-2,2′-bipyridine (4) 28 and 1.5 equivalent of 4′-(4boronatophenyl)-2,2′:6′,2′′-terpyridine (9) 22,29 yielded in the formation of the desired ligand (I) in relatively high yield.The product which was obtained was then analyzed by NMR (Figure 3 and 4), MS (Figure 5) and elemental analysis techniques.COSY and HMQC experiments allowed the assignment of the spectra of the ligand.As it is shown in Figure 3, 1 H NMR spectrum indicated 2 characteristic singlets assigned to single protons associated with the pyridine rings of bipyridyl and terpyridyl domains which are bound to the phenyl ring in the middle of the ligand (I) appearing at δ 8.65 and 8.60, respectively.The signals associated with the protons of the phenyl group appeared as iso(AB) spin system with the chemical shift at δ 7.68-7.53(7.68 (2 H, d, JAB = 8 Hz, Hphenyl), 7.54 (2 H, d, JAB = 8 Hz, Hphenyl)) which show that the phenyl group is symmetrical in the NMR time scale.Table 1 show the comparison study of the 1 H NMR data of the ligand and the bridging ligands which were reported by Constable et al. 3 and Li et al.As it is shown in Table 1, in all compounds, the pyridine rings in the terpyridyl domains exhibit symmetrical pattern in the NMR time scale and additionally, in all spectra significant overlapping of signals were observed which were arising from the protons associated with the bipyridyl rings. 13C NMR spectrum of the ligand (I) (Figure 4) exhibits seven characteristics peaks corresponding to the C atoms bound to H atoms belong to the phenyl ring and the pyridine rings of the terpyridyl site which supports the structure of the ligand (I).
Product could also be identified from its MS by the facile loss of a pyridine ring from the parent ion.The parent ion mass/charge ratio signal of the product in LC-MS determination was 464.3000 ([M + H] + ), while the mass/charge ratio signal of 388.7000 can be assigned as ([M -C5H4N + H] + ) (Figure 5) which was in consistent with the results reported previously for the ligand synthesized by Constable et al.Overall, since the two synthetic methods of the target ligand (I) are different in their last steps of the reactions, considering the yields of the bromo and boronic acid derivatives (i.e.61%, 73%, 79%, and 73% for compounds (4), ( 5), (8), and (9), respectively, see Experimental Section), compound (4) is the one which can form the smallest amount of the product considered and can be counted as a limiting compound for the synthesis in both routes and using compound (4) directly can influence the overall yield of the product.In this prospect, based on the yields obtained, the route two (i.e. the one involving the reaction of 4′-(4boronatophenyl)-2,2′:6′,2′′-terpyridine (9) and 4-bromo-2,2′-bipyridine (4)) is preferred.

Table 1 .
1H NMR spectral comparison of the ligand (I) with the literature values of analogs