Metal carbonyl complexes of potentially ambidentate 2,1,3-benzothiadiazole and 2,1,3-benzoselenadiazole acceptors

1 Introduction

The 2,1,3-benzochalcogenadiazoles (bcd) btd and bsd (Fig. 1) have become widely used as electron acceptor components in electronically functional organic (low band-gap) polymers [1], [2], [3], [4], [5], [6], [7], [8]. Other physical properties such as optical and electrochemical activity have also been well studied [1], [2], [3], [4], [5], [6], [7], [8]. However, little is known about the interaction of such molecules with metals [9], [10], [11], although the chalcogen, the two N-donor atoms, and the conjugated π system are potential metal binding sites (Fig. 2). It was shown in previous EPR reports that the reduced forms, the radical anions btd˙⁻ or bsd˙⁻, can even serve as bridging ligands in symmetrical dinuclear complexes (μ-btd˙⁻)[ML_n]₂ and (μ-bsd˙⁻)[ML_n]₂ [12], [13], [14], [15]. EPR Spectra suggested high symmetry and thus the possibility of chalcogen coordination in certain mononuclear compounds [12].

Fig. 1:

The 2,1,3-benzochalcogenadiazole (bcd) ligands.

Fig. 2:

Different metal coordination alternatives for mononuclear metal complexes of the 2,1,3-benzochalcogenadiazoles.

In this work we describe the experimental and computational investigation of the coordination potential of btd and bsd acceptors versus the electron-rich metal carbonyl complex fragments W(CO)₅ and [Re(CO)₃(bpy)]⁺. Whereas the former has been used in earlier studies [12], [13], [14], [15], the latter proved to be an excellent binding site for weakly basic acceptor ligands such as TCNQ or N-heterocyclic cations [16], [17], [18].

2 Experimental section

3 Results and discussion

In the non-reduced neutral forms the ligands btd and bsd form only mononuclear isolable complexes with the carbonyl-tungsten and -rhenium complex fragments employed here. This was observed both for the solids (single crystal XRD) and for solutions (¹H NMR). The EPR-detected N,N′-coordinated dinuclear systems of reduced states are thus due to a strongly enhanced charge at the partially quinonoid nitrogen centers [12], [13], [14], [15], leading to an ECEC mechanism as apparent from cyclic voltammetry (charge transfer assisted polynucleation) [15].

The complexes {W(CO)₅(btd)] (1), [W(CO)₅(bsd)] (2) and [Re(CO)₃(bpy)(bsd)](BF₄) (3(BF₄)), bpy=2,2′-bipyridine, were characterized analytically and, in part, by X-ray diffraction of single crystals (Figs. 3–5 and Tables 1 and 2). Due to poor crystal and X-ray data quality the individual bond parameters of 1 will not be discussed in detail.

Fig. 3:

Molecular structure of 2 in the crystal. H atoms are omitted for clarity.

Fig. 4:

Molecular structure of [3⁺] in the crystal of [3] (BF₄)×CH₂Cl₂.

Fig. 5:

SeN/SeN Dimer formation of 2 (left) and π/π interaction of 2 in the crystal (right).

The structural studies reveal the expected [31] o-quinonediimine-type C–C and C–N bond alternancy and the metal coordination to one of the two equivalent N atoms, with bond parameters that are typical and which can be well reproduced by DFT calculations (Table 2, Fig. 6). The complex ion [Re(CO)₃(bpy)(bsd)]⁺ in 3⁺ exhibits the common [16], [17], [18] fac arrangement of the carbonyl ligands (Fig. 4). Three kinds of intermolecular interactions were observed: The tungsten compound 2 reveals intermolecular SeN/SeN interactions (Fig. 5) at 282.3 pm – a previously observed feature [32], [33]. The tungsten-containing compounds 1 and 2 show stacking through π/π interactions [34] of the btd (380 pm) or bsd (350 pm) ligands with anti arrangement (Fig. 5). The BF₄⁻ anion in the crystal of [3](BF₄)×CH₂Cl₂ exhibits F···H hydrogen bonding to the solvent and to the bsd and bpy ligands.

Fig. 6:

DFT (PBE0 in vacuo) calculated energy minima of the isomers 2a and 2b from [W(CO)₅(bsd)].

According to DFT calculations both N and Se coordination of W(CO)₅ result in local energy minima for 2 (Fig. 6), with the N-bonded isomer being more stable by 0.42 eV. This value is diminished to 0.34 eV for the reduced forms 2a⁻ and 2b⁻; the rhenium complex ion 3⁺ is calculated with an even larger preference (0.51 eV) for N vs. Se coordination.

The reversible reduction of complexes [12], [13], [14], [15], including the E_1/2=–1.25 V vs. Fc^+/0 in CH₂Cl₂/0.1 M Bu₄NPF₆, invited studies of the monoanionic forms by EPR, IR and UV/VIS spectroelectrochemistry [35].

In addition to the previously reported pentacarbonyltungsten radical complexes [12], [13], [14], [15] we have now investigated by EPR the electrochemically generated neutral rhenium compound [Re(CO)₃(bpy)(bsd)]˙ (3˙) (Fig. 7). The partially resolved spectrum (g=2.0057) reveals hyperfine coupling from ^185,187Re (0.94 mT), ¹⁴N (0.63 mT) and ¹H (0.21 mT); the ^185,187Re isotopes (37.4 and 62.6% nat. abundance) have I=5/2 and very similar nuclear magnetic moment. These results are fully compatible with previously reported data of related carbonylrhenium radical complexes [16], [17], [18], [36] and with DFT calculated spin densities as depicted in Fig. 8. The spin is confined to the bsd ligand in all cases. The contribution from the chalcogen is supported by experiment, a coupling (6.8 mT) of the unpaired electron with ⁷⁷Se (I=½, 7.6% nat. abundance) is observed in an amplified spectrum. The spin density on Re was calculated at only –0.020, and there is negligible spin density on bpy although the corresponding π* acceptor MO is only 0.3 eV higher than that of bsd.

Fig. 7:

ESR spectrum of electrogenerated 3˙ in 0.1 M CH₂Cl₂/NBu₄PF₆ (black, bottom) with simulation (red, top).

Fig. 8:

Spin distribution of 2˙⁻ (left) and 3˙ (right) (PBE0, PCM/CH₂Cl₂).

The calculated bond length changes on reduction of 2 or 3⁺ (Table 3) are rather small, in agreement with the excellent acceptor capacity of the bsd ligand.

Infrared spectroelectrochemistry in the carbonyl stretching region (Fig. 9, Table 4) exhibits only minor low-frequency shifts (10–60 cm⁻¹) on reduction, confirming the EPR-supported notion of a btd- or bsd-centered electron transfer. The larger spread of the bands in the reduced states of 1˙⁻ and 2˙⁻ agrees with the diminished π acceptor capacity of bcd˙⁻ relative to unreduced bcd. The decreased low frequency split (“E”) for 3˙ relative to 3⁺ signifies similar ligand effects of bsd˙⁻ and unreduced bpy.

Fig. 9:

IR spectroelectrochemical measurements of the reduction of 1 (left, top) and 2 (right, top) and of [3](BF₄) (bottom) in the CO stretching region in 0.1 M CH₂Cl₂/NBu₄PF₆.

The long-wavelength absorptions of the compounds 1, 2 and 3(BF₄) are due to metal-to-ligand charge transfer (MLCT) processes [12], [13], [14], [15]. In each case the LUMO is located at the btd or bsd acceptor, even in the presence of the secondary acceptor bpy for 3⁺. Reduction causes the MLCT bands at about λ_max=500 nm to disappear in 1°^/− and 2°^/− from the visible region due to single occupancy of the acceptor MO. On the other hand, a new long-wavelength band emerges at 485 nm on reduction of 3⁺ which reflects the presence of a close lying second acceptor MO π*(bpy) (Fig. 10, Table 5).

Fig. 10:

UV/Vis/NIR spectroelectrochemical response on reduction of 2 (top) and of [3](BF₄) (bottom) in 0.1 M CH₂Cl₂/NBu₄PF₆.

4 Conclusion

This study has shown that the neutral btd and bsd ligands are indeed good π acceptors but poor σ bases. Coordination in the unreduced state is thus restricted to mononuclear N-coordinated complexes even with electron rich carbonylmetal fragments. Reduction occurs exclusively on the bcd ligands with almost vanishing spin distribution to the metal or co-ligands such as bpy. It is not surprising, therefore, that the well used bcd acceptor components have not been used more in coordination chemistry despite the presence of several potential coordination sites. Less conventional metal species may be required to take advantage of this potential, extending the application of such components beyond pure organic systems [1], [2].