Synthesis and crystal structures of manganese(I) carbonyl complexes bearing ester-substituted α-diimine ligands

The structural comparison of two MnI tricarbonyl complexes bearing ester-substituted bipyridine or biquinoline ligands is reported.


Chemical context
Similar to carbonyl complexes of precious metals, such as ruthenium and rhenium, those with less expensive manganese are attracting attention for their application in CO 2 reduction catalysts (Bourrez et al., 2011) and as CO-releasing molecules (CORMs) under external stimuli (Chakraborty et al., 2014a). For example, CORMs using manganese(I) carbonyl complexes controllably release CO by photoirradiation (Motterlini et al., 2002). Considering their application in vivo, photo-CORMs are expected to utilize light at lower energy. In general, extended -conjugation systems in organic ligands lead to redshifts of charge-transfer (CT) transition bands of manganese(I) carbonyl complexes (Chakraborty et al., 2014b). Therefore, it is essential to investigate the relationship between molecular structures including -conjugation systems and photophysical properties.
Thus, we focused on the comparison of bipyridines, which are prototypes of the -diimine ligand, and biquinolines with a more extended -conjugation system. In addition, the introduction of ester groups into these ligands allows chemical ISSN 2056-9890 adsorption with various metal oxides (Ardo & Meyer, 2009;Zhang et al., 2006). In this study, we synthesized manganese(I) tricarbonyl complexes bearing two types of -diimine compounds, which contain both an ester substituent and different -conjugation systems, viz. diethyl 2,2 0 -bipyridine-4,4 0 -dicarboxylate (debpy) and diethyl 2,2 0 -biquinoline-4,4 0didicarboxylate (debqn): fac-[MnBr(CO) 3 (debpy)] (I) and fac-[MnBr(CO) 3 (debqn)] (II). We successfully compared their crystal structures and photophysical properties. As expected, a CT band shift in the visible region was confirmed, depending on the size of the -conjugation system in -diimine ligands. This finding will provide information in the future design of suitable complexes for a variety of photoreactions (Chakraborty et al., 2014b).

Structural commentary
The molecular structures of compounds I and II are shown in Figs. 1 and 2, respectively. In both complexes, the manganese(I) atoms exhibit distorted octahedral coordination geometries and display primary coordination spheres that are similar to those reported for other structurally related complexes (Chakraborty et al., 2014a;Walsh et al., 2015). The metal-ligand bond lengths are similar to those previously reported for compounds of this type; in I, the Mn-N bond lengths are 2.046 (3) and 2.047 (2) Å , while in II, the Mn-N bond lengths are 2.063 (2) and 2.068 (2) Å . In I and II, the fac configuration of three CO ligands around the central manganese(I) atom is in agreement with their IR data. On the basis of their bond parameters, all CO ligands have typical triplebond characters.

Supramolecular features
In the crystal structure of I, complex molecules are linked by pairs of weak C-HÁ Á ÁBr hydrogen bonds (Table 1)  Side-on views of I (left) and II (right). H atoms are omitted for clarity.

Figure 1
Molecular structure of I with atom labeling and displacement ellipsoids drawn at the 50% probability level.

Figure 2
Molecular structure of II with atom labeling and displacement ellipsoids drawn at the 50% probability level.

Synthesis and crystallization
The ligands, debpy and debqn, were prepared as described by Chandrasekharam et al. (2011) andHoertz et al. (2006). The ligands were confirmed to be spectroscopically pure (by IR and 1 H NMR analyses).
Synthesis of I and II: Compounds I and II were handled and stored in the dark to minimize exposure to light. For the synthesis of I, [MnBr(CO) 5 ] (31 mg, 0.11 mmol) and debpy (33 mg, 0.11 mmol) were dissolved in CHCl 3 (10 ml). The reaction mixture was stirred at 313 K for 14 h under N 2 . After the solvent was evaporated under reduced pressure, an excess of Et 2 O (30 ml) was added to the solution; then, the solution was allowed to stand at 253 K overnight. The resultant precipitate was collected by filtration, washed with Et 2 O, and then dried under vacuum (37 mg yield, 64%). Red crystals, suitable for the X-ray diffraction experiment, were grown by diffusion of n-hexane into an acetone solution of I for one week. A similar reaction between [MnBr(CO) 5 ] (8 mg, 0.029 mmol) and debqn (10 mg, 0.026 mmol) for 20 h afforded II (11 mg yield, 66%). Purple crystals, suitable for the X-ray diffraction experiment, were grown by diffusion of n-hexane into an acetone solution of II for one week. FTIR (KBr pellet): CO /cm À1 = 2016, 1942, 1926    Crystal packing of II with C-HÁ Á ÁBr hydrogen bonds (blue) andcontacts (green) shown as dashed lines; ring centroids are shown as red spheres.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. All hydrogen atoms were placed at calculated positions (C-H = 0.95-0.99 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C).

Funding information
Funding for this research was provided by: Japan Society for the Promotion of Science (grant No. JP17K05799).

fac-Bromidotricarbonyl(diethyl 2,2′-bipyridine-4,4′-dicarboxylate-κ 2 N,N′)manganese(I) (I)
where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.05 e Å −3 Δρ min = −0.56 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.79 e Å −3 Δρ min = −1.07 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).