Palladium(II) complexes of a bridging amine bis(phenolate) ligand featuring κ2 and κ3 coordination modes

The crystal structure of a palladium(II) coordination compound features the same ligand bound to the metal center in both a κ2 and κ3 fashion.


Chemical context
The activity of early transition-metal and rare-earth complexes of amine bis(phenolate) ligands for olefin (Tshuva et al., 2000) and cyclic ester polymerization (Carpentier, 2015) has been well documented. Several studies (Tshuva et al., 2001;Qian et al., 2011) demonstrated that the coordination mode and donor identity play a significant role in the activity of complexes derived from amine bis(phenolate) and related ligands. Amine bis(phenolate) complexes of iron have been employed as catalysts for cross-coupling (Chowdhury et al., 2008), polymerization (Allan et al., 2014) and CO 2 conversion (Andrea et al., 2018) and as functional models for various nonheme metalloenzymes (Karimpour et al., 2013;Strautmann et al., 2011). While a relatively limited number of late transitionmetal amine bis(phenolate) complexes have been employed as catalysts, nearly all have been observed to bind through both amine and both phenolate donor atoms to form 4 complexes. Related complexes featuring 2 or 3 coordination modes may offer unique insight into catalyst identity for species that may not be directly observed.
Several related complexes feature ligands similar to these amine bis(phenolate) species bound in a 3 fashion. Notably, Zn phenoxy diamine complexes are highly active catalysts for ISSN 2056-9890 the polymerization of lactide. (Williams et al., 2003;Labourdette et al., 2009) Related modification to the amine bis(phenolate) framework generated 'claw-type' 3 Zn (Song et al., 2012;Wang et al., 2010) and Ti (Zhao et al., 2014) complexes that serve as competent polymerization catalysts. To our knowledge, only one report describes Pd complexes with amine bis(phenolate) ligands bound in a 2 or 3 coordination mode, in which both amine donors remain bound, and one phenolate donor may bind to the Pd center (Graziano et al., 2019). These species exhibit coordination behavior that varies with the steric parameters of the phenolate ortho and para substituents, with larger cumyl substituents favoring the formation of 2 complexes. In this work, we describe diffraction data for a related Pd complex featuring the ligand {6,6 0 -[(ethane-1,2-diylbis(methylazanediyl)]bis(methylene)}bis(2,4di-tert-butylphenol) bound in both 2 or 3 coordination modes within a single unit cell. The presence of palladium(II) complexes displaying both 2 and 3 coordination modes arising from the same solution suggests a dynamic process in which phenol donors may coordinate or de-coordinate based on the electronic demands at the metal center.

Structural commentary
The asymmetric unit of the structure (Fig. 1) consists of two distinct palladium(II) complexes of the amine bis(phenolate) {6,6 0 -[(ethane-1,2-diylbis(methylazanediyl)]bis(methylene)}bis(2,4-di-tert-butylphenol) and fractional quantities of methanol and water crystallization solvents. Both metal centers adopt similar distorted square-planar geometric arrangements, in which both nitrogen atoms of the ligand are bound to the Pd center to form a five-membered ring, and either one or two chlorine atoms are present to complete the coordination sphere depending on the coordination mode of the ligand. In both complexes, the N-Pd-N bond angle is similar to those observed for related amine bis(phenolate) Pd complexes (Graziano et al., 2019), as described in Table 1.
Deprotonation and coordination of O1, presumably in the presence of water during crystallization, gives rise to the N,O)PdCl] complex. This complex is only slightly distorted from ideal square-planar geometry ( 4 parameter = 0.0823; Yang et al., 2007), and Pd1 lies 0.073 Å above the plane defined by O1/N1/N2/Cl1. The phenol ring containing O2 is disordered by rotation about the C20-C21 bond, such that in the minor component a close O2B-H2CÁ Á ÁCl1 interaction of $2.065 Å is observed (see Refinement section for details of the disorder). An additional close contact of $2.460 Å is observed between Cl1 and the O3-H bond of an unbound phenol from the neighboring [( 2 -N,N)PdCl 2 ] complex.
Solution NMR data (see Synthesis and crystallization section) suggest that the conformations observed in the solid state are retained on the NMR timescale. Signals attributed to both the 2 and 3 complexes are observed, including signals attributed to the protonated phenol moieties. Signals attributed to the ligand methylene groups are rendered diastereotopic upon coordination of the distal donor atoms to the Pd center, while methylene units for unbound donors remain magnetically equivalent. The asymmetric unit of the title compound with only the major components of disorder shown. Displacement ellipsoids are drawn at the 50% probability level. H atoms bonded to C atoms are omitted for clarity and hydrogen bonds are shown as dotted lines.

Supramolecular features
Hydrogen bonding (Table 2) is observed between phenol O2-H2, co-crystallized methanol solvent O5-H5, and O1 of a neighboring complex, forming a two-dimensional network in the bc plane between [( 3 -N,N,O)PdCl] subunits. Details of this interaction are illustrated in Fig. 2, which depicts the interaction between neighboring 3 species, viewed along the a axis. Additional O-HÁ Á ÁCl interactions are observed between O3-H3 and Cl1, and O6-H6 and Cl3, though neither of these interactions forms an extended network. The interaction between O3-H3 and Cl1 is of interest as it is the only observed close contact between the 2 and 3 complexes within the asymmetric unit. This feature is absent in the minor component, in which the [( 2 -N,N)PdCl 2 ] phenol hydroxy moiety O3B-H3D exhibits an intramolecular close contact with Cl3. Within the minor component, a related intramolecular close contact is observed between the remaining [( 2 -N,N)PdCl 2 ] phenol hydroxy group O4B-H4D and Cl2.

Figure 2
Part of the crystal structure viewed along the a axis, showing hydrogen bonding (in pink) between neighboring 3 amine bis(phenolate) Pd II complexes.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Water H atoms were restrained to have O-H bond lengths of 0.84 (2) Å , and 1.36 (2) Å HÁ Á ÁH distances (DFIX, esd = 0.02 Å ). All H atoms attached to carbon atoms as well as phenol and methanol hydroxyl hydrogens were positioned geometrically and constrained to ride on their parent atoms. C-H bond distances were constrained to 0.95 Å for aromatic C-H moieties, and to 0.99 and 0.98 Å for aliphatic CH 2 and CH 3 moieties, respectively. Phenol and methanol O-H distances were constrained to 0.84 Å . Methyl CH 3 and hydroxyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. U iso (H) values were set to a multiple of U eq (C/O) with 1.5 for CH 3 , OH and water, and 1.2 for C-H, CH 2 , units, respectively.
Three of the four phenol hydroxyl groups are positionally disordered by rotation of the aromatic ring. For two of the three minor moieties, the O-C distance and the 1,3 O to C distances of the minor and major moieties were restrained to be similar (SADI command of SHELX, esd = 0.02 Å ). Minor O atom O2B was constrained to have the same ADP as the C atom to which it is bonded. Two phenol H-atom positions were positionally restrained based on hydrogen-bonding considerations and to avoid close contacts to C-bound H atoms. Subject to these conditions, the occupancy rates of the major moieties refined to 0.917 (3), 0.857 (4) and 0.899 (4).
A tert-butyl group was refined as rotationally disordered. The two moieties were restrained to have similar geometries, the central C atoms to share one ADP, and the U ij components of ADPs were restrained to be similar (SIMU command of SHELX, esd = 0.01 Å 2 ). Subject to these conditions the occupancy ratio refined to 0.716 (8):0.284 (8).