Selective synthesis and crystal structures of manganese(I) complexes with a bi- or tridentate terpyridine ligand

The structural comparison of two Mn carbonyl complexes comprising a terpyridine derivative engaged in bidentate or tridentate coordination of the central MnI atom is reported.


Chemical context
Carbonylmanganese(I) complexes with polypyridyl ligands are of particular interest as novel active molecules that are able to release CO in response to photoirradiation (Carrington et al., 2013;Chakraborty et al., 2014;Jimenez et al., 2015) or as electrocatalysts of CO 2 reduction (Grills et al., 2018;Stanbury et al., 2017). Among these compounds, studies have concentrated mainly on tricarbonyl complexes comprising bidentate polypyridyl supporting ligands; by contrast, only few reports exist on dicarbonyl complexes bearing tridentate ligands (Compain et al., 2015;Machan & Kubiak, 2016). In fact, even though the typically tridentate ligands 2,2 0 :6 0 ,2 00 -terpyridine and derivatives thereof coordinate to an Mn I ion, the majority of them bind the metal ion in a bidentate manner (Compain et al., 2014;Moya et al., 2001).
As indicated by the results of studies focusing on the comparison between carbonylmanganese complexes containing bidentate and tridentate terpyridines (Compain et al., 2015;Machan & Kubiak, 2016), investigating the relationship between reactivity and molecular structure is a key research objective. However, comparing these two systems experimentally is difficult, particularly considering that available structural data on complexes comprising tridentate terpyridine ligands are quite scarce.

Structural commentary
The molecular structures of compounds I and II are displayed in Figs. 1 and 2, respectively. Although I was prepared by Moya et al. (2001), its structure has not previously been determined. In I and II, the manganese(I) atoms exhibit distorted octahedral coordination environments, similar to those reported for other structurally related complexes (Compain et al., 2014(Compain et al., , 2015. In I, the fac configuration of the three CO ligands around the central manganese(I) atom is in agreement with the IR data of the complex and similar to those previously reported for complexes of this type (Compain et al., 2014(Compain et al., , 2015. As can be evinced from Fig. 1, the terpyridine ligand exhibits a bidentate coordination with respect to the central Mn I atom, so that one of the outer pyridyl rings remains outside the coordination sphere. The corresponding non-coordinating N atom, N3, is positioned on the side opposite to the Br atom. As a result, the torsion angle between the coordinating and non-coordinating pyridyl rings in I (N2-C13-C14-N3) is much smaller [47.9 (3) ] than those reported for related Mn I complexes with bidentate terpyridine derivatives (Compain et al., 2014(Compain et al., , 2015. The noncoordinating N atom is positioned in proximity of the equatorial carbonyl ligand (C2 O2), with a short value for the interatomic distance between C2 and N3 [2.900 (4) Å ]. Since this distance is considerably shorter than the sum of the two atoms' van der Waals radii (3.25 Å ; Bondi, 1964), evidence suggests that an interaction exists between the free pyridine and the adjacent CO ligand. This interaction may explain the observation that the Mn1-C2 distance [1.840 (3) Å ] is longer than the other two corresponding distances in I [Mn1-C1 = 1.805 (3) and Mn1-C3 = 1.796 (3) Å ].
The crystal structures of Mn I dicarbonyl complexes with tridentate terpyridines have very rarely been reported (Compain et al., 2015), because of the instability in solution of compounds of this type. In II, the carbonyl ligands are in cis configuration, again in accordance with IR data. Differently from I, in II the Mn I ion is coordinated by a tridentate terpyridyl ligand, as well as two CO ligands and a Br À ion. Only the central Mn-N2 bond is slightly shortened (by $0.05 Å ) as a result of geometric constraints. In contrast to I, where no disorder is observed, in II one of the CO ligands (C2 O2) and the Br À ligand are mutually disordered over two positions. The dihedral angle between the phenyl pendant and the central pyridyl ring in II is slightly larger than the corresponding angle in I. Specifically, the C10-C11-C19-C20 torsion angle has a value of À19.3 (5) in II and À9.9 (4) in I, but both values indicate an essential quasi-coplanarity. Notably, the extended conjugation made possible by the mentioned quasi-planarity may contribute to an increased stability of these compounds.

Supramolecular features
In the crystal structure of I, complex molecules display three kinds of C-HÁ Á ÁBr hydrogen bonds (i.e., between the Br À ligand and the C-H groups in the coordinating pyridyl ring, the free pyridyl ring, and the phenyl pendant), forming a three-dimensional supramolecular structure (Table 1 and Fig. 3).

Synthesis and crystallization
All the manganese(I) complexes were handled and stored in the dark to minimize exposure to light. Compound I was synthesized as described by Moya et al. (2001). The compound thus obtained proved to be analytically and spectroscopically pure (as determined by microanalysis, IR, UV-vis, and 1 H NMR data). Crystals suitable for use in X-ray diffraction experiments were grown by vapor diffusion of diethyl ether into an acetone solution of I.

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 Å ) and refined using a riding model with U iso (H) = 1.2U eq (C). In compound II, the CO group and the Br atom trans to it were refined as being disordered over two sets of sites, (Br1/C2 O2) and (Br2/ C3 O3), respectively, with an occupancy ratio of 0.807 (2): 0.193 (2).

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