Pincer and Macrocyclic Pyridylidene Amide (PYA) Au^III Complexes

Abstract

Gold-based homogeneous catalysis is dominated by redox neutral Au I systems. Redox-active gold-based catalysts are less common, principally because of redox cycles between Au I and Au III being hampered by unfavorable potentials. We report gold(III) complexes containing pincer-based, donor-flexible pyridylidene amide (PYA) ligands to address these issues. These complexes act as electron reservoirs through two limiting resonance structures consisting of either soft, imine coordination sites or harder, zwitterionic amide donors. We further alter the donor properties by using the ortho-, meta-, and para-pyridylidene amide variants of the PYA pincer arms. These bis-PYA pincer ligands exhibited a high contribution of amide coordination in the solid-state of the gold(III) complexes; however, the solution data suggests a high contribution from the neutral L-type resonance forms. This Ltype contribution, primarily shown through cyclic voltammetry studies, prevents reversible gold(III) reduction and also disfavors abstraction of the ancillary chloride ligand. Furthermore, a novel macrocyclic-PYA ligand is introduced, which shows secondary metal−ligand interactions.

Pincer Platinum(II) Hydrides: High Stability Imparted by Donor-Flexible Pyridylidene Amide Ligands and Evidence for Adduct Formation before Protonation

“…The crystallographic analysis of [Pt] metaMe –H revealed a distinctly planar structure, especially when compared to that of other meta -PYA structures (θ = ∼50°), ,, with the PYA heterocycles rotated only by 4 and 14° out of the backbone pyridine plane (Figure b). Unlike in [Pt] para –H , this essentially coplanar arrangement does not result in a short C–N distance with double bond character, and instead, a distinct C–N single bond character is deduced from the 1.411(5) Å bond length.…”

“…We utilized the previously reported N , N , N -ligand H 2 ( x- PYA) PF6 ( x = para- or meta- ) for complexation to platinum­(II) (Scheme ). Constructed from a central pyridine unit with two amide bridges with appropriately substituted pyridinium arms, these ligands have both been employed toward metal-based oxidation catalysis , and toward stabilizing gold­(III) organometallic complexes . For investigating potential intermediates, soft platinum­(II) was chosen as the metal center to further aid kinetic stabilizations.…”

“…Similar dihedral angles were observed in complexes of the same ligand in other complexes, e.g., 39 and 46°for Ru II and Au III complexes, respectively. 15,18 Those ligand structures were all defined as primarily X-type in nature. The N1−Cγ bond is elongated (∼1.40 Å) and indicates an increased C−N single bond character, and the difference between the average Cγ−Cβ and Cβ−Cα distances is reduced to <0.04 Å.…”

“…A planar structure was identified, however (θ = ∼4°), suggestive of a Ltype coordination mode. To observe the bonding metrics in greater quality, we switched to a previously reported variant of the meta-ligand with an additional methyl group in para- The crystallographic analysis of [Pt] metaMe −H revealed a distinctly planar structure, especially when compared to that of other meta-PYA structures (θ = ∼50°), 14,15,18 with the PYA heterocycles rotated only by 4 and 14°out of the backbone pyridine plane (Figure 5b). Unlike in [Pt] para −H , this essentially coplanar arrangement does not result in a short C−N distance with double bond character, and instead, a distinct C−N single bond character is deduced from the 1.411(5) Å bond length.…”

Pyridylidene Amines and Amides: Donor-Flexible Ligands for Catalysis

“…Complementary DFT calculations of ν CO in [CpIr­( L )­(CO)] complexes also suggest PYE ligands to be stronger donors (ν CO = 2120 cm –1 for p iPr PYE ) than PMe 3 (2156 cm –1 ) and IMes (ν CO = 2152 cm –1 ), even though natural resonance theory calculations predict predominantly imine character . In agreement with these studies, electrochemical analysis of Au­(III) complexes, [AuCl­( bis‑py PYA )] ( 4 ), reveals that the PYA ligands are weaker donors than similar yet dianionic κ 3 -N,N,N bis-amidate ligands with 2,6-diisopropylphenyl aromatic groups instead of PYA heterocycles . These data suggest that the PYA ligand is a poorer electron donor than purely X-type amido donors, hence implying a significant contribution of the L-type form.…”