Cationic, Neutral, and Anionic PNP Pd^IIand Pt^IIComplexes: Dearomatization by Deprotonation and Double-Deprotonation of Pincer Systems

Abstract

A series of cationic, neutral, and anionic Pd(II) and Pt(II) PNP (PNP = 2,6-bis-(di-tert-butylphosphinomethyl)pyridine) complexes were synthesized. The neutral, dearomatized complexes [(PNP*)MX] (PNP* = deprotonated PNP; M = Pd, Pt; X = Cl, Me) were prepared by deprotonation of the PNP methylene group of the corresponding cationic complexes [(PNP)MX][Cl] with 1 equiv of base (KN(SiMe(3))(2) or (t)BuOK), while the anionic complexes [(PNP**)MX](-)Y(+) (PNP** = double-deprotonated PNP; Y = Li, K) were prepared by deprotonation of the two methylene groups of the corresponding cationic complexes with either 2 equiv of KN(SiMe(3))(2) or an excess of MeLi. While the reaction of [(PNP)PtCl][Cl] with an excess of MeLi led only to the anionic complex without chloride substitution, reaction of [(PNP)PdCl][Cl] with an excess of MeLi led to the methylated anionic complex [(PNP**)PdMe](-)Li(+). NMR studies, X-ray structures, and density functional theory (DFT) calculations reveal that the neutral complexes have a "broken" aromatic system with alternating single and double bonds, and the deprotonated arm is bound to the ring by an exocyclic C=C double bond. The anionic complexes are best described as a pi system comprising the ring carbons conjugated with the exocyclic double bonds of the deprotonated "arms". The neutral complexes are reversibly protonated to their cationic analogues by water or methanol. The thermodynamic parameters DeltaH, DeltaS, and DeltaG for the reversible protonation of the neutral complexes by methanol were obtained.

Electrostatic and Non‐covalent Interactions in Dicationic Imidazolium–Sulfonium Salts with Mixed Anions

“…The coordination geometry at the Pt atom of [ 6b ] 2+ is essentially square planar. The bond lengths and angles around the coordination sphere of the Pt atom are all comparable to those of the reported platinum complex bearing pyridine‐based PNP pincer ligands 12c,17. The small dihedral angle [16.0(14)°] between N ‐methylpyridinium ring and the pyridine ring on the PNP pincer core suggests an effective conjugation in metalloviologen [ 6b ] 2+ .…”

“…Reactions of PtCl 2 (PhCN) 2 with [ 1a ][BF 4 ] or [ 1b ][OTf] proceeded smoothly to give the corresponding dicationic platinum complexes [ 6a ][BF 4 ] 2 and [ 6b ][OTf] 2 in 82 % and 35 % yield, respectively (Scheme a). The 31 P{ 1 H} NMR spectrum of [ 6a ][BF 4 ] 2 [ δ 48.9 (s with satellite J Pt,P = 2389 Hz)] indicated the similar geometry to those of the reported platinum complexes bearing pyridine‐based PNP pincer ligands 12c,17. Dicationic palladium complex [ 7a ][BF 4 ] 2 was also prepared in a similar manner from the reaction of PdCl 2 (PhCN) 2 (Scheme b).…”

Anionic Nickel(II) Complexes with Doubly Deprotonated PNP Pincer-Type Ligands and Their Reactivity toward CO2

“…46 With the X-ray structure of 9-DME in hand, we can now analyze the experimental bond lengths for the C1−C7 scaffold (average C−C bond length 1.40 ± 0.02 Å) suggesting a similar delocalization of charge for the organonickel species 9, in line with DFT calculations (vide infra).…”

“…Similar results were observed for the analogous platinum(II) and palladium-(II) systems. 46 Reactivity toward Carbon Dioxide. A solution of complex 9 in THF, placed in an NMR tube and fitted with a rubber septum, rapidly reacts upon injection of 1 equiv of CO 2 (Scheme 3).…”

“…45 Our group recently reported anionic PNP-type Pd(II) and Pt(II) complexes (PNP = 2,6-bis[(di-tert-butylphosphino)methyl]pyridine) as rare examples of anionic TM complexes lacking strong π-acceptor ligands such as CO or olefins, including a DFT study, which suggested the formation of a conjugated π-system spread throughout both the deprotonated arms and the pyridine backbone (Scheme 1B). 46 However, to the best of our knowledge, no crystallographically characterized doubly deprotonated complex was reported. Along these lines, we herein report the fully structurally characterized neutral and anionic organo-nickel(II) complexes [Ni(PNP iPr *)Me] (7), [Li-(solvent) n ][Ni(PNP R **)Me] (9, R = i Pr; n = 3, solvent = 1,2-dimethoxyethane (DME); 10, R = t Bu, n = 2, solvent = Et 2 O; PNP tBu = 2,6-bis[(di-tert-butylphosphino)methyl]-pyridine, PNP iPr = 2,6-bis[(di-iso-propylphosphino)methyl]-pyridine; one asterisk indicates single deprotonation and two asterisks indicate double deprotonation).…”