Luminescent, Redox-Active Dithiolatobis(imine)platinum(II) Divergent Complexes with Exchangeable Imine Ligands: An Experimental/Computational Study Versus Their Diiminedithiolatoplatinum(II) Convergent Congeners

Abstract

This article provides a third manifestation of a new tradition by which the editors of Comments on Inorganic Chemistry wish to lead by example, whereby we start publishing original research content that, nonetheless, preserves the journal’s identity as a niche for “critical discussion” of contemporary literature in inorganic chemistry. (For the first and second manifestations, see: (1) Otten, B. M.; Melancon, K. M.; Omary, M. A. “All that Glitters is Not Gold: A Computational Study of Covalent vs Metallophilic Bonding in Bimetallic Complexes of d10 Metal Centers: A Tribute to Al Cotton on the 10th Anniversary of His Passing,” Comments Inorg. Chem. 2018, 38, 1–35; (2) Yaseen, W. K.; Sanders, S. F.; Almotawa, R. M.; Otten, B. M.; Bhat, S.; Alamo, D. C.; Marpu, S. B.; Golden, T. D.; Omary, M. A. “Are Metal Complexes ‘Organic’, ‘Inorganic’, ‘Organometallic’, or ‘Metal-Organic’ Materials? A Case Study for the Use of Trinuclear Coinage Metal Complexes as ‘Metal-Organic Coatings’ for Corrosion Suppression on Aluminum Substrates,” Comments Inorg. Chem. 2019, 39, 1–26.) Herein, we contrast the electronic structure of two categories of Pt(II) complexes with mixed imine/thiolato ligands: a new class of [Pt(N^N)2(S^S)] bis(κ1-diimine)dithiolatoplatinum(II) divergent complexes—whereby only one N atom from the “back-to-back” diimine ligand actually coordinates to a Pt(II) center in a non-bridging monodentate manner—vis-à-vis the hitherto well-studied [Pt(N^N)(S^S)] (κ2-diimine)dithiolatoplatinum(II) convergent congeners—whereby both the diimine and dithiolate coordinate as κ2-chelating bidentate. Thus, in pursuit of using luminescent building blocks to generate light-harvesting supramolecular coordination compounds, we report the synthesis, characterization, and reactivity of Pt(pyz)2(mnt), 1, in high yield (pyz = pyrazine, mnt = maleonitriledithiolate). The complex exhibits solvent-dependent exchange of the pyrazine ligands, which can be utilized in the formation of larger complexes containing platinum dithiolate moieties. As proof of concept, 1 has been converted into the more inert Pt(4,4ʹ-bpy)2(mnt), 2, and Pt(4-ap)2(mnt), 3, (4-ap = 4-aminopyridine) complexes. These complexes exhibit redox properties, are strong absorbers of ultraviolet and visible light, and exhibit bright-luminescence at 77 K. Single-crystal X-ray diffraction analysis for 1 and 3 confirms the monotopic coordination of the ligands with intramolecular Pt-S and Pt-N bond lengths being comparable to other complexes of type (κ2-diimine)(dithiolate)Pt(II), whereas significant torsion is exhibited by the two heterocyclic rings due to the lack of tethering to one another. Solvent-dependent stability is uncovered for 1 crystals, whereas the crystal structure of 3 reveals an interesting supramolecular quadrangle resulting from hydrogen bonding between the amine groups of two complexes and two interstitial water molecules. A commentary is presented to contrast the electronic structure of the divergent complexes herein with that of a diimine-dithiolate analogue. Experimentally, the luminescence is believed to be sensitized in the new class of divergent complexes, given that their lack of conjugation of two aromatic rings renders them less-efficient π-acceptors compared to analogous diimines such as 2,2ʹ-bipyridine (2,2ʹ-bpy), hence blue-shifting the absorption toward the blue/near-UV and assisting in keeping the emission well within the visible region, whereby energy gap law considerations would suppress quenching effects in the red/near-infrared regions, especially under experimental conditions conducive for phosphorescence and aggregation effects in the solid state and higher-concentration fluid and/or frozen solutions. Computational studies for monomeric models of the three divergent complexes plus the diimine-dithiolate congener, Pt(2,2ʹ-bpy)(mnt), attain reasonable agreement with experimental structural, spectroscopic, and redox properties and provide excellent insights for the comparison between the two categories of complexes upon which we focus the commentary section. The computed photophysical properties reveal phosphorescence due to higher-lying triplet states, as the T1 is found to luminesce in the near-IR region and entail a ligand-field (dd) transition origin, whereas higher-lying states are shown to be in the visible region, close to experimental phosphorescence energies, and are consistent with the expected charge transfer transition to imine/diimine lowest-unoccupied molecular orbitals (LUMOs) from admixed dithiolate/platinum highest-occupied molecular orbitals (HOMOs). The T1 state is likely the culprit for the vanishing emission intensity at ambient temperature for both classes of complexes. We also find that both the T1 and D1 states for the neutral exciton and radical polaron (anion or cation) species, respectively, are severely distorted in 1–3 vs Pt(2,2ʹ-bpy)(mnt) models. Near-perfect orthogonality of the two heterocyclic rings can be attained for such excited and redox models of Pt(κ1-imine)2(dithiolate), whereas this distortion is hindered in Pt(κ2-diimine)(dithiolate) models due to the steric constraint in their CCNNPt metallacycle.