Crystal structures of a manganese(I) and a rhenium(I) complex of a bipyridine ligand with a non-coordinating benzoic acid moiety

In the title MnI and ReI complexes of the ligand 2-(2,2′-bipyridin-6-yl)benzoic acid, the o-benzoic acid substituent does not coordinate to the metal. In fac-[2-(2,2′-bipyridin-6-yl)benzoic acid-κ2 N,N′]tricarbonylchloridorhenium(I) tetrahydrofuran monosolvate, the benzoic acid fragment is positioned near the axial carbonyl ligand, whereas in fac-[2-(2,2′-bipyridin-6-yl)benzoic acid-κ2 N,N′]bromidotricarbonylmanganese(I) tetrahydrofuran monosolvate, the benzoic acid fragment is disordered, such that in the major component the benzoic fragment is positioned near the bromide ligand and in the minor fragment near the axial carbonyl ligand.


Structural commentary
The molecular structures of compounds I and II are illustrated in Figs. 1 and 2, respectively. Both compounds crystallize as tetrahydrofuran (THF) monosolvates, THF having been used for the recrystallization of both compounds. The metal atoms exhibit distorted octahedral geometries and contain primary coordination spheres similar to those of other fac-[Re(-diimine)(CO) 3 Cl] and fac-[Mn(-diimine)(CO) 3 Br] complexes; including fac-tricarbonylchlorido(4,4 0 -dihydroxy-2,2 0 -bipyridine)rhenium(I) (III; Manbeck et al., 2015), fac-tricarbonyliodido(2,2 0 -bipyridine)manganese(I) (IV; Stor et al., 1995), and fac-tricarbonylbromido[2-(2,2 0 -bipyridin-6-yl-2 N,N 0 )phenol]manganese(I) (V; Agarwal et al., 2015). The metalligand bond distances are similar to those previously reported for complexes of this type, for e.g., in I the Mn-N bond A molecular structure of compound II, with atom labeling and 50% probability ellipsoids. The two minor solvent disorder components have been omitted for clarity The molecular structure of compound I, with partial atom labeling and 50% probability displacement ellipsoids. Both disorder components of the benzoic acid group are shown (the minor one with dashed lines), and H atoms have been omitted for clarity.

Figure 3
The molecular structure of compound I, with atom labeling and showing the position of the major component of the disordered benzoic acid group. Displacement ellipsoids are drawn at the 50% probability level. distances are 2.029 (2) and 2.082 (2) Å , while in V the Mn-N bond distances are 2.0347 (8) and 2.091 (1) Å .
In I, the benzoic acid fragment is disordered over two positions (Fig. 1). In the major component, the carboxylic acid group is positioned near the bromide ligand (see Fig. 3), whereas in the minor component the benzoic acid fragment is rotated such that the carboxylic acid group is positioned near the axial carbonyl ligand (see Fig. 4). In II, the benzoic acid fragment is not disordered, and the carboxylic acid group is positioned near the axial carbonyl ligand (Fig. 2).
Molecules with similar motifs, in which a benzoic acid is bound to a pyridyl ring in the ortho position (Charris-Molina et al., 2017) or to a phenyl ring in the ortho position (Dobson & Gerkin, 1998), have been structurally characterized. Compared to the torsion angles between the benzoic acid fragment and the pyridyl or phenyl rings in these structures, the benzoic acid fragment and the pyridyl ring in I and II are closer to being perpendicular to each other, with the N2-C13-C14-C19 torsion angle being À116.4 (3) in the major component of I and 100.55 (19) in II. In the minor component of I, the N2-C13-C14A-C19A torsion angle is 85.7 (8) . In contrast, for the structures reported by Charris-Molina et al. (2017) the analogous torsion angles are 52.6 (4), À40.5 (3), À51.5 (5) and 48.8 (3) . In the structure of biphenyl-2-carboxylic acid itself (Dobson & Gerkin, 1998), the analogous torsion angles of the four molecules of the asymmetric unit are À46.7 (4), À52.3 (4), 48.2 (4) and 52.3 (4) . Smaller absolute values of the torsion angles for I and II would result in closer contacts between the atoms of the benzoic acid fragment and the ancillary ligands around the metal, which may explain the more perpendicular torsion angles found in I and II.

Supramolecular features
In compound I, the THF solvate molecule is disordered over several positions and probably forms intermolecular hydrogen bonds. In the crystal, complex molecules are linked by pairs of C-HÁ Á ÁBr hydrogen bonds, forming inversion dimers (Table 1). A view of the crystal packing is given in Fig. 5 and shows the voids occupied by the disordered THF solvent molecules.
In compound II, there is hydrogen bonding between the benzoic acid group and the oxygen atom of the disordered THF molecule (Table 2). In the crystal, the complex molecules are linked by C-HÁ Á ÁCl hydrogen bonds, forming layers lying parallel to the bc plane (Table 2 and Fig. 6), which are separated by layers of THF solvent molecules.    The molecular structure of compound I, with atom labeling and showing the position of the minor component of the disordered benzoic acid group. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ) for (I). Symmetry code: (i) Àx þ 1; Ày þ 1; Àz þ 1.

Electrochemistry
In order to determine whether I and II could act as precatalysts for the reduction of CO 2 , cyclic voltammetry experiments were performed. These studies were conducted in acetonitrile containing 1 mM I or II and 0.1 M tetrabutylammonium hexafluorophosphate using a glassy carbon working electrode, a platinum wire auxiliary electrode, and an Ag/Ag + non-aqueous reference electrode. Ferrocene was used as an internal standard. In order to determine whether a catalytic current enhancement was observed in the presence of the substrate, the current response was measured under an inert gas atmosphere, after bubbling CO 2 through the solution, and after bubbling CO 2 through the solution in the presence of an external Brønsted acid (5% water by volume). In the presence of CO 2 and water, similar complexes have shown a catalytic current enhancement at the potential at which the complexes undergo a second one-electron reduction (Bourrez et al., 2011;Agarwal et al., 2015;Smieja & Kubiak, 2010). Interestingly, neither complex presented here showed electrocatalytic activity for the reduction of CO 2 at or near this potential, even in the presence of an external Brønsted acid (5% water by volume). In order to probe whether catalysis was inhibited specifically by the intramolecular nature of the benzoic acid substituent, the cyclic voltammetry of fac-[Mn(2,2 0 -bipyridyl)(CO) 3 Br] was performed in the presence of CO 2 , 5% water, and up to 50 molar equivalents of benzoic acid. Even in the presence of 50 molar equivalents of benzoic acid, the current enhancement was similar to that in the presence of only CO 2 and 5% water (Bourrez et al., 2011), indicating that it is the presence of the benzoic acid substituent in an intramolecular fashion that inhibits catalysis.

Synthesis and crystallization
Toluene, ethanol, and acetonitrile used in syntheses were degassed by sparging with N 2 . THF, hexane, and pentane were dried over molecular sieves and degassed using the freezepump-thaw method when used for recrystallization. All other reagents and solvents were purchased commercially and used as received. Metallated complexes were manipulated and stored in the dark to minimize exposure to light.
Synthesis of methyl 2-(2,2 0 0 0 -bipyridin-6-yl)benzoate: The reagents 6-bromo-2,2 0 -bipyridine (0.500 g, 2.13 mmol) and 2-methoxycarbonylphenylboronic acid, pinacol ester (0.715 g, 2.73 mmol) and the catalyst tetrakis(triphenylphosphine)palladium(0) (0.11 g, 0.095 mmol) were placed in a Kjedahlshaped Schlenk flask. The flask was then evacuated and refilled with nitrogen three times, ending with the flask under nitrogen. Toluene (26 ml), ethanol (2.6 ml), and 2 M aqueous K 2 CO 3 (2.1 ml) were added to the flask, which was then heated at 368 K under nitrogen under stirring for 41 h. The reaction mixture was cooled to room temperature, and then saturated aqueous ammonium chloride (26 ml) and deionized water (26 ml) were added to the reaction flask. The product mixture was then extracted with dichloromethane three times (42 ml, 30 ml, 25 ml). The combined organic layers were dried over magnesium sulfate and then filtered. The solvent was removed under vacuum. The product was purified by column chromatography using silica gel 60 as the solid phase and diethyl ether as the eluant (R f = 0.60). A view along the c axis of the crystal packing of compound II. The O-HÁ Á ÁO and C-HÁ Á ÁCl hydrogen bonds are shown as dashed lines (Table 2). Only the major component of the disordered THF molecule is shown and H atoms not involved in these interactions have been omitted for clarity.
hydroxide was added to methyl 2-(2,2 0 -bipyridin-6-yl)benzoate (0.175 g, 0.60 mmol). The reaction mixture was refluxed for 3 h under stirring. The reaction was then cooled to room temperature. Aqueous hydrochloric acid (2 N) was added dropwise until the pH was approximately 4. The white precipitate that appeared was collected on a Bü chner funnel by vacuum filtration. The precipitate was washed with 4 ml deionized water and then dried in vacuo (yield 0.167g, 100%). Synthesis of compound I: Bromopentacarbonylmanganese(I) (0.0525 g, 0.191 mmol) and 2-(2,2 0 -bipyridin-6yl)benzoic acid (0.0500 g, 0.181 mmol) were placed in a Schlenk flask, which was then evacuated and refilled with nitrogen three times, ending with the flask under nitrogen. Acetonitrile (9.4 ml) was added to the flask, which was then covered with aluminium foil. The reaction was heated to 333 K and stirred under a nitrogen atmosphere for 12 h. The reaction was cooled to room temperature and the solvent then removed under vacuum. The crude product was dissolved in a minimal amount of THF and recrystallized by slow diffusion of hexane into the solution. These recrystallization conditions performed on a smaller scale produced the yellow needle-like crystals used for X-ray crystallographic analysis. IR CO (KBr pellet, cm À1 ): 2019(s), 1933(s), 1912(s).
Synthesis of compound II: Pentacarbonylchlororhenium(I) (0.0273 g, 0.0755 mmol) and 2-(2,2 0 -bipyridin-6-yl)benzoic acid (0.0207 g, 0.0749 mmol) were placed in a Schlenk flask, which was then evacuated and refilled with nitrogen three times, ending with the flask under nitrogen. Acetonitrile (3.5 ml) was added to the flask, which was then covered with aluminium foil. The reaction was heated to 333 K and stirred under a nitrogen atmosphere for 12 h. The reaction was cooled to room temperature and the solvent then removed under vacuum. The crude product was dissolved in a minimal amount of THF and recrystallized by slow diffusion of pentane into the solution. These recrystallization conditions performed on a smaller scale produced the yellow block-like crystals used for X-ray crystallographic analysis. IR CO (KBr pellet, cm À1 ): 2072(s), 1977(s), 1936(s).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. For both compounds the C-bound H atoms were included in idealized positions and allowed to ride on the parent atoms: C-H = 0.95-0.99 Å with U iso (H) = 1.2U eq (C). In compound II, the carboxylic H atom was located in a difference-Fourier map and freely refined.
In compound I, the benzoic acid fragment is disordered over two positions with the major component contribution of   (3) ratio. The disordered molecules were refined with restraints and constraints (Guzei, 2014). For both structures, data collection: APEX3 (Bruker, 2015); cell refinement: SAINT-Plus (Bruker, 2015); data reduction:

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (