Elsevier

Polyhedron

Volume 23, Issue 8, 6 May 2004, Pages 1427-1439
Polyhedron

Synthesis and electronic properties of mononuclear osmium(II) and rhenium(I) complexes containing ligands derived from [2,3-a:3,2-c]dipyridophenazine (ppb)

https://doi.org/10.1016/j.poly.2004.02.012Get rights and content

Abstract

An improved method for the synthesis of the polypyridyl ligand [2,3-a:3,2-c]dipyridophenazine (ppb) is reported. In addition, the syntheses of the new ppb-based ligands L (L=[2,3-a:3,2-c]dipyridophenazine, [2,3-a:3,2-c]dipyrido-6,7-dimethylphenazine, [2,3-a:3,2-c]dipyrido-6,7-dichlorophenazine, [2,3-a:3,2-c]dipyrido[1,2-e]benzophenazine, [2,3-a:3,2-c:3″,2″-e]tripyridoquinoxaline and [2,3-a:3,2-c]dipyrido-5-methylphenazine) are detailed. The complexes [Os(ppb)(bpy)2]2+ and [Os(L)(bpy)2]2+ have been synthesised and their electrochemical, electronic absorption and UV–Vis spectrochemical properties measured. Syntheses of the rhenium(I) complexes fac-[Re(L″)(CO)3Cl] (L″ = [2,3-a:3,2-c]dipyridophenazine, [2,3-a:3,2-c]dipyrido-6,7-dimethylphenazine, [2,3-a:3,2-c]dipyrido-6,7-dichlorophenazine, [2,3-a:3,2-c]dipyrido[1,2-e]benzophenazine) are also reported. The osmium(II) complexes [Os(L)(bpy)2]2+ (L = 2,3-di-(2-pyridyl)quinoxaline, 6,7-dimethyl-2,3-di-(2-pyridyl)quinoxaline, 6,7-chloro-2,3-di-(2-pyridyl)quinoxaline, 2,3-di-(2-pyridyl)benzo[g]quinoxaline, 4-methyl-di-(2-pyridyl)quinoxaline) have also been prepared and studied. The electronic spectra of the Os and Re complexes are dominated by strong MLCT transitions. The lowest energy MLCT transitions correlate linearly with the first reduction potential for the [Os(L)(bpy)2]2+ series. Electrochemical oxidation of the [Os(L)(bpy)2]2+ and [Os(L)(bpy)2]2+ complexes results in the appearance of LMCT transitions in the visible region while reduction of the [Os(L)(bpy)2]2+ complexes produces complex electronic spectra. Comparison with the reduced fac-[Re(L″)(CO)3Cl] complexes allows assignment of L″radical dot π→π* transitions in the NIR (1200–1000 nm) and red (850–700 nm) regions. The nonsymmetrical ligands [2,3-a:3,2-c:3″,2″-e]tripyridoquinoxaline and [2,3-a:3,2-c]dipyrido-5-methylphenazine both coordinate to the [Os(bpy)2]2+ unit in a stereospecific manner to give a single geometric isomer. The crystal structure of [Re(CO)3(ppb)Cl] is reported, in which the polypyridyl ligand deviates significantly from planarity.

A method for the synthesis of a series of new ligands based on [2,3-a:3,2-c]dipyridophenazine (ppb) is reported, and spectroscopic and electrochemical data for mononuclear [Os(bpy)2L]2+, [Os(phen)2L]2+ and [Re(CO)3(Cl)L] complexes of these and related ligands are detailed.

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Introduction

There has been a considerable amount of research into the use of polypyridyl ligands that can act as bridging units in the formation of supramolecular arrays and grids [1], [2], [3], [4], [5] and large multinuclear complexes [6], [7], [8], [9]. Such complexes may be used in photocatalysis [10], CO2 remediation [11], solar energy systems [12], [13], [14], molecular electronics [6], [7], [8], [15] and light-emitting diodes [16], [17], [18], [19], [20], [21], [22], [23], [24]. In this paper we report the syntheses and electronic properties of mononuclear Os(II) and Re(I) complexes containing a series of rigid ligands L based on the potential bridging ligand [2,3-a:3,2-c]dipyridophenazine (ppb, 21, Fig. 1) and these are compared to analogous complexes containing the more flexible ligands L (11-14, 16, Fig. 1) [25], [26], [27], [28], [29]. We also report a synthesis of 21 which utilises less forcing conditions than the previously reported method [30] and which was adapted to prepare the series of ligands L (22–26). The metal-containing moieties [Os(bpy)2]2+, [Os(phen)2]2+ and fac-[Re(CO)3Cl] were chosen in order to influence the electronic properties of the ligand [31]. Complexation of polypyridyl ligands to the fac-[Re(CO)3Cl] moiety simplifies the electronic absorption properties of the resulting compounds by removing the chromophores associated with the 2,2-bipyridine or 1,10-phenanthroline ancillary ligands from the coordination sphere, thus allowing easier assignment of the observed absorptions [32], [33].

Section snippets

Ligand synthesis

The ligands 2126 were synthesised in a four step process modified from that used previously for ppb [30] (Scheme 1). Acetylation of 2-methoxyphenylene-1,4-diamine (1) using pyridine/acetic anhydride gave 2 in reasonable yield. 2 was found to react less vigorously than 1 in the subsequent exothermic Skraup reaction and gave a good yield of product. Addition of acetic acid also helped to moderate this reaction.

The published synthesis of 3 requires two separate Skraup reactions, an additional

Conclusions

We have developed a method for the synthesis of rigid polypyridyl bridging ligands based on ppb. The electrochemical properties of the ppb-ligands (2126) are consistent with the electron withdrawing and donating effects of the substituents, and the electrochemical and electronic absorption properties of the complexes mirror these substituent effects.

The Os(II) complexes have strong MLCT transitions in the visible region that are singlet and triplet in nature. For these complexes the 1MLCT and 3

Experimental

Synthetic reagents were LR or AR grade and were used without further purification unless otherwise specified. All solvents used for spectroscopic and electrochemical measurements were Aldrich spectroscopic or HPLC grade and were used without further purification. [Os(bpy)2Cl2] was prepared by literature methods [29]. [Re(CO)5Cl] was prepared from [Re2(CO)10] (Aldrich) according to literature methods [46]. DMF (HPLC grade) was dried by stirring over CaH2 with subsequent vacuum distillation.

Supplementary material

C21H10ClN4O3Re, M=587.98, monoclinic, a=15.3346(2), b=9.42610(10), c=13.02640(10) Å, β=92.7560(10)°, U=1880.73(3) Å3, T=203(2) K, space group P21/c (no. 14), Z=4, μ(Mo Kα)=6.636 mm−1, 11 223 reflections measured, 4151 unique (Rint=0.0292) which were used in all calculations. The final wR(F2) was 0.0782 (all data).

Crystallographic data for fac-[Re(CO)3(21)Cl] (CCDC 226823) have been deposited with the Cambridge Crystallographic Data Centre. A copy of this information may be obtained free of

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