Bimetallic nickel complexes supported by 2,5-bis(phosphine)-1,4-hydroquinonate ligands. Structural, electrochemical and theoretical investigations

o-Hydroxyarylphosphanes: Strategies for Syntheses of Configurationally Stable, Electronically and Sterically Tunable Ambiphiles with Multiple Applications

o-Hydroxyarylphosphanes are fascinating compounds by their multiple-reactivity features, attributed to the ambident hard and soft Lewis- and also Brønstedt acid-base properties, wide tuning opportunities via backbone substituents with ±mesomeric and inductive, at P and in o-position to P and O also steric effects, and in addition, the configurational stability at three-valent phosphorus. Air sensitivity may be overcome by reversible protection with BH3 , but the easy oxidation to P(V)-compounds may also be used. Since the first reports on the title compounds ca. 50 years ago the multiple reactivity has led to versatile applications. This includes various P-E-O and P=C-O heterocycles, a multitude of O-substituted derivatives including acyl derivatives for traceless Staudinger couplings of biomolecules with labels or functional substituents, phosphane-phosphite ligands, which like the o-phosphanylphenols itself form a range of transition metal complexes and catalysts. Also main group metal complexes and (bi)arylphosphonium-organocatalysts are derived. Within this review the various strategies for the access of the starting materials are illuminated, including few hints to selected applications.

2,5-Dihydroxy-1,4-quinones appended with two phosphinyl groups: syntheses, structures, and redox properties

Low temperature reactions of 1,4-dichloro-2,5-dimethoxybenzene with two equivalents of lithium diisopropylamide (LDA) followed by quenches with chlorophosphines ClPR2 (R = Ph or iPr) yielded 1,4-bis(diphenylphosphino)-2,5-dichloro-3,6-dimethoxy-benzene (1a) and 1,4-bis(diisopropylphosphino)-2,5-dichloro-3,6-dimethoxy-benzene (1b). Reactions of 1a-b with 30% hydrogen peroxide yielded 1,4-bis(diphenylphosphinyl)-2,5-dichloro-3,6-dimethoxybenzene (2a) and 1,4-bis(diisopropylphosphinyl)-2,5-dichloro-3,6-dimethoxybenzene (2b). Subjecting compounds 2a-b to BBr3/CH3OH resulted in 2,5-bis(diphenylphosphinyl)-3,6-dichlorohydroquinone 3a and 2,5-bis(diisopropylphosphinyl)-3,6-dichlorohydroquinone 3b. Reactions of 3a-b with K2S2O8 under basic conditions followed by acidification allowed for isolation of 2,5-bis(diphenylphosphinyl)-3,6-dihydroxy-quinone (4a) and 2,5-bis(diisopropylphosphinyl)-3,6-dihydroxy-quinone (4b). Compounds 1a-b – 4a-b were fully characterized by spectroscopic methods (nuclear magnetic resonance [FT-NMR] spectroscopy, infrared [FT-IR] spectroscopy, and high resolution mass spectrometry [HRMS]); 3a-b and 4a-b were also investigated by cyclic voltammetry. Compounds 1a-b , 2a , and 4a-b were also characterized by single-crystal X-ray diffraction methods.

The binuclear dual emitter [Br(CO)3Re(PN)(NP)Re(CO)3Br] (PN): 3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine, a new bridging P,N-bidentate ligand resulting from the ring opening of tetrahydrofuran

Lithium diphenylphosphide unexpectedly provokes the ring-opening of tetrahydrofuran (THF) and by reaction with 3,6-dichloropyridazine leads to the formation of the ligand 3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine (P⋯N), which was isolated. The reaction of this ligand with the (Re(CO)₃(THF)Br)₂ dimer yields the novel complex [Br(CO)₃Re(μ-3-chloro-6-(4-diphenylphosphinyl)butoxypyridazine)₂Re(CO)₃Br] (BrRe(P⋯N)(N⋯P)ReBr), which was crystallized in the form of a chloroform solvate, (C₄₆H₄₀Br₂Cl₂N₄O₈P₂Re₂)·(CHCl₃). The monoclinic crystal (P2₁/n) displays a bimetallic cage structure with a symmetry inversion centre in the middle of the rhenium to rhenium line. The molecule shows two oxidation signals occurring at +1.50 V and +1.76 V which were assigned to the Re^I/Re^II and Re^II/Re^III metal-centered couples, respectively, while signals observed at -1.38 V and -1.68 V were assigned to ligand centered reductions. Experimental and DFT/TDDFT results indicate that the UV-Vis absorption maximum of BrRe(P⋯N)(N⋯P)ReBr occurring near 380 nm displays a metal to ligand charge transfer (MLCT) character, which is consistent with CV results. Upon excitation at this wavelength, a weak emission (Φ_em < 1 × 10^-3) is observed around 580 nm (in dichloromethane) which decays with two distinct lifetimes τ₁ and τ₂ of 24 and 4.7 ns, respectively. The prevalence of non-radiative deactivation pathways is consistent with efficient internal conversion induced by the high conformational flexibility of the P⋯N ligand's long carbon chain. Measurements in a frozen solvent at 77 K, where vibrational deactivation is hindered, show intense emission associated with the ³MLCT state. These results demonstrate that BrRe(P⋯N)(N⋯P)ReBr preserves the dual emitting nature previously reported for the mononuclear complex RePNBr, with emission associated with and states.

Coordination versatility of p-hydroquinone-functionalized dibenzobarrelene-based PC(sp3)P pincer ligands

Bifunctional hydroquinone-based PC(sp³)P pincer complexes.