Synthesis and structure of a new bulky bis(alkoxide) ligand on a terphenyl platform

A new potentially chelating bulky bis(alkoxide) exhibits anti conformation of the O-atom donors in the solid state and does not form transition-metal complexes.


Chemical context
Bulky alkoxides are becoming increasingly used as ancillary ligands in group-transfer chemistry and catalysis (Brazeau & Doerrer, 2019;Chua & Duong, 2014;Hannigan et al., 2017;Jayasundara et al., 2018;Wannipurage et al., 2020). As a result of their stereoelectronic properties, profoundly weak-field bulky alkoxides enable formation of reactive low-coordinate high-spin middle and late transition-metal centers (Bellow et al., 2016b;Grass et al., 2019b). We have previously reported bulky monodentate alkoxides that led to reactive chromium and iron nitrene-transfer catalysts, (Bellow et al., 2015;Wannipurage et al., 2021;Yousif et al., 2015Yousif et al., , 2018 and a series of low-coordinate cobalt carbene complexes capable of carbene transfer to isocyanides (Bellow et al., 2016a;Grass et al., 2019aGrass et al., , 2020. However, the lability of monodentate alkoxides affected catalyst stability and the substrate scope. To remediate the problem of lability of monodentate alkoxides, we have designed and synthesized a new chelating bis-(alkoxide) ligand [1,1 0 :4 0 ,1 00 -terphenyl]-2,2 00 -diylbis(diphenylmethanol) (H 2 [OO] Ph ) ( Fig. 1) (Kurup et al. 2019). The H 2 [OO] Ph ligand employs a 1,1 0 :4 0 ,1 00 -terphenyl platform, which increases the bite angle between the alkoxide donors to form approximately seesaw transition-metal centers. While the isolated ligand precursor H 2 [OO] Ph exhibits an anti conformation of the [CPh 2 (OH)] fragments relative to the central phenyl in the solid state (crystals obtained at 238 K), the hydroxyl groups point towards the central phenyl, exhibiting overall an anti-syn conformation ( Fig. 1) (Kurup et al., 2019). Furthermore, while two different isomers were observed by 1 H NMR spectroscopy at low temperatures, a single species was observed at room temperature, suggesting ISSN 2056-9890 facile equilibration of anti and syn conformers. As a result, H 2 [OO] Ph led to the formation of the desired bis(alkoxide) complexes with iron and chromium ( Fig. 1) (Kurup et al., 2019(Kurup et al., , 2020. The resulting iron complex exhibited broader range of nitrene transfer reactivity, forming a variety of symmetric azoarenes. The success of this strategy led us to design a new, even bulkier ligand (H 2 [OO] tBu ). The ligand was synthesized in a two-step procedure as described in Fig. 2. Previously reported 2,2 00 -dibromo-1,1 0 :4 0 ,1 00 -terphenyl was synthesized through a Suzuki-Miyaura coupling reaction between 2-bromophenylboronic acid and 1,4-diiodobenzene following a literature procedure (Velian et al., 2010). Next, 2,2 00 -dibromo-1,1 0 :4 0 ,1 00terphenyl was treated with t BuLi followed by hexamethylacetone. The formation of the desired product H 2 [OO] tBu (35% isolated yield) was accompanied by the formation of significant amounts of p-terphenyl by-product (38% isolated yield). H 2 [OO] tBu was characterized by 1 H and 13 C NMR spectroscopy, high-resolution mass spectrometry, and X-ray crystallography. 1 H NMR spectroscopy demonstrates the presence of two isomers at room temperature in an approximately 2:1 ratio, as manifested by two tert-butyl resonances (1.05 and 1.03 ppm) and two OH resonances (2.09 and 2.07 ppm). This observation suggests that, in contrast to H 2 [OO] Ph , various isomers of H 2 [OO] tBu do not readily interconvert at room temperature, possibly due to the more significant steric hindrance of the tert-butyl groups. An X-ray crystallography study (see below) suggests that in at least one of these isomers the hydroxyl groups are pointing away from each other; such an isomer is unlikely to coordinate a single metal in a chelating fashion. Accordingly, the reaction of H 2 [OO] tBu with several representative transition-metal amides (M = Cr, Mn, Fe) failed to produce isolable complexes.

Structural commentary
The crystals of H 2 [OO] tBu were obtained from dichloromethane at 238 K. The structure crystallized in space group P1 and is presented in Fig. 3. Selected bond distances and angles are given in Table 1. H 2 [OO] tBu exhibits a crystallographic inversion center, with only half of the molecule occupying the asymmetric unit. In addition to H 2 [OO] tBu , the structure contains one solvent molecule (CH 2 Cl 2 ) disordered by symmetry over two positions. Selected bond distances, angles, and torsion angles appear in Table 1. The lateral phenyls of the terphenyl unit are approximately perpendicular to the central phenyl ring, as indicated by the corresponding torsion angles close to 90 (see Table 1  Synthesis of H 2 [OO] tBu , its schematic structure, and the lack of welldefined reactivity with transition-metal amide precursors.

Figure 3
The structure of H 2 [OO] tBu (50% probability ellipsoids) is shown with the co-crystallized dichloromethane solvent molecule. The dichloromethane carbon atom was found to be disordered about an inversion center; only one orientation is shown, which is not the one belonging to the asymmetric unit. Hydroxyl H atoms are disordered over two positions, both positions are shown above.

Figure 1
Schematic representation of the 'anti-syn' structure of the previously synthesized H 2 [OO] Ph ligand and its reactivity with transition-metal precursors.
In contrast to the structure of H 2 [OO] Ph , the hydroxyls point away from each other in the structure of H 2 [OO] tBu , leading to an overall anti-anti conformation. This disposition results in the placement of the tert-butyl groups above and below the central phenyl ring. The presence of bulky groups on both sides of the central phenyl is likely responsible for the distortion of the terphenyl fragment, which is indicated by the C10-C15-C16 angle of 130.70 (15) and the C14-C15-C16 angle of 110.28 (15) . Same distortion is likely responsible for the slight variation in (lateral) phenyl bond distances (Table 1).

Supramolecular features
H 2 [OO] tBu forms one-dimensional polymer chains held together by hydrogen bonding between two neighboring molecules (Table 2). One polymer chain is shown in Fig. 4. This chain-like structure results from the anti-anti conformation of H 2 [OO] tBu in which both hydroxyl groups are pointing outward and thus can hydrogen bond with neighboring molecules. The hydrogen-bond distance (indicated by the lightblue dashed lines in Fig. 4) is 2.13 (3) Å . It is also noted that, due to the inversion center present within the molecule, the hydroxyl hydrogen atoms are disordered over two positions. As the diffraction data was of adequate quality, we were able to locate both hydrogen positions in the difference map. The corresponding O-H bonds are very similar, 0.93 (2) and 0.94 (2) Å . Only one of these hydrogen atoms participates in the hydrogen-bonding network (alternating conformations for consecutive molecules). The solvent molecules are positioned above and below the chains.

Figure 4
Chain of H 2 [OO] tBu molecules, bridged by hydrogen bonds (indicated in light blue).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Data were acquired at 100 K with an Oxford 800 Cryostream low-temperature apparatus. Hydrogen atoms were placed in calculated positions using a standard riding model and refined isotropically (with the exception of hydroxyl hydrogens); all other atoms were refined anisotropically. The hydroxyl hydrogens were found to be disordered (due to the inversion center located at the hydrogen bond to the adjacent H 2 [OO] tBu ) over two positions. Two alternating positions were identified from the difference-Fourier maps and refined to 50% occupancy. The CH 2 Cl 2 solvent was also disordered by symmetry over two positions and refined with 50% occupancy. Data collection: APEX2 (Bruker, 2016); cell refinement: SAINT (Bruker, 2016); data reduction: SAINT (Bruker, 2016); program(s) used to solve structure: SHELXT2018/2 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

3,3′-([1,1′:4′,1′′-Terphenyl]-2,2′′-diyl)bis(2,2,4,4-tetramethylpentan-3-ol) dichloromethane monosolvate
Crystal data 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. Used Part -1 on the dichloromethane (DCM) and hydroxyl hydrogens because they were disordered by inverison symmetry (sof=0.5). In addition, RIGU/DFIX/SIMU were employed to model the disorder of the DCM solvent.