Synthesis, Structure, and Optical Properties of Pt(II) and Pd(II) Complexes with Oxazolyl- and Pyridyl-Functionalized DPPM-Type Ligands: A Combined Experimental and Theoretical Study

New square-planar complexes [Pt­(<b>1</b>_–H)₂] (<b>2a</b>) [<b>1</b>_–H = (oxazolin-2-yl)­bis­(diphenylphosphino)­methanide] and [Pd­(<b>1</b>_–H)₂] (<b>2b</b>), of general formula [M­{(Ph₂P)₂C<u>---</u>C<u>---</u>NCH₂CH₂O}₂] (M = Pt, <b>2a</b>; M = Pd, <b>2b</b>), result from deprotonation of 2-{bis­(diphenylphosphino)­methyl}­oxazoline (<b>1</b>) at the PCHP site. The new, functionalized dppm-type ligand 4-{bis­(diphenylphosphino)­methyl}­pyridine, (Ph₂P)₂CH­(4-C₅H₄N) (<b>4</b>), was prepared by double lithiation and phosphorylation of 4-picoline. In the presence of NEt₃, the reactions of 2 equiv of <b>4</b> with [PtCl₂(NCPh)₂] and [Pd­(acac)₂] (acac = acetylacetonate) afforded [Pt­(<b>4</b>_–H)₂] (<b>5a</b>) [<b>4</b>_–H = bis­(diphenylphosphino)­(pyridin-4-yl)­methanide] and [Pd­(<b>4</b>_–H)₂] (<b>5b</b>), of general formula [M­{(Ph₂P)₂C­(4-C₅H₄N)}₂] (M = Pt, <b>5a</b>; M = Pd, <b>5b</b>), respectively. In the absence of base, the reactions of 2 equiv of <b>4</b> with [PtCl₂(NCPh)₂] and [PdCl₂(NCPh)₂] afforded (<b>5a</b>·2HCl) (<b>6a</b>) and (<b>5b</b>·2HCl) (<b>6b</b>), respectively, in which the PCHP proton of <b>4</b> has migrated from carbon to nitrogen to give a pyridinium derivative of general formula [M­{(Ph₂P)₂C­(4-C₅H₄NH)}₂]­Cl₂ (M = Pt, <b>6a</b>; M = Pd, <b>6b</b>). The complexes <b>3a</b>, <b>5a</b>·2MeOH, and <b>6b</b>·4CH₂Cl₂ have been structurally characterized by X-ray diffraction. The absorption/emission properties of the Pt­(II) complexes <b>2a</b> and <b>5a</b> and the Pd­(II) complexes <b>2b</b> and <b>5b</b> have been investigated by UV–vis spectroscopy and theoretical analysis based on density functional theory. The UV–vis absorption spectra of the neutral complexes recorded in dilute <i>N</i>,<i>N</i>′-dimethylformamide solutions are dominated by intense spin-allowed intraligand transitions in the region below 350 nm. The complexes exhibit charge-transfer bands between 350 and 500 nm. The experimental and theoretical absorption spectra agree qualitatively and point to two low-lying ligand-to-metal charge transfer states that contribute to the bands observed between 350 and 500 nm. The complexes are emissive in frozen solutions at 77 K, in the pure solid state, and when doped into films of poly­(methyl methacrylate) but are nonemissive in solution. A red shift is observed when Pt­(II) is replaced by Pd­(II)