PNP-Pincer-Type Phosphaalkene Complexes of Late Transition Metals

Palladium(II) complexes of bis(diphosphene) with different coordination behaviors

Organophosphorus compounds possessing a P-P double-bond character are intriguing materials in coordination chemistry because it is possible to form a variety of coordination modes from the π-bond in addition to the lone pairs. We report herein the complexation of a new bidentate ligand, ethylene-tethered bis(binaphthyldiphosphene) (S,S)-2, with palladium(II) species. The reaction of (S,S)-2 with [Pd(π-allyl)(cod)](SbF₆) and PdCl₂(cod) afforded η¹/η¹-bis(diphosphene) complex 7 and η¹-diphosphene/η²-phosphanylphosphide complex 8, respectively. The latter was characterized by a chloride migration from the palladium atom to a phosphorus atom due to the high electron-accepting character of the PP moiety. Theoretical calculations revealed the migration process and nature of complex 8.

Cu(i) complex bearing a PNP-pincer-type phosphaalkene ligand with a bulky fused-ring Eind group: properties and applications to FLP-type bond activation and catalytic CO2 reduction

Herein, we report the synthesis of [Cu(Eind2-BPEP)][PF6] (2) (Eind2-BPEP = 2,6-bis(2-Eind-2-phosphaethenyl)pyridine, Eind = 1,1,3,3,5,5,7,7-octaethyl-1,2,3,5,6,7-hexahydro-s-indacen-4-yl), a three-coordinated Cu(i) complex bearing a PNP-pincer-type phosphaalkene ligand with bulky fused-ring Eind groups. The Gutmann-Beckett test revealed that complex 2 is highly Lewis acidic and comparable in strength to B(C6F5)3, which is a relatively strong Lewis acid. In addition, 2 is more Lewis acidic than [Cu(Mes*2-BPEP)][PF6] (3), the analogous complex with less-bulky Mes* instead of Eind groups. DFT calculations using model compounds revealed that the higher Lewis acidity of 2 compared to 3 is not due to the electronic effects of the ligand, but due to a reduction in the LUMO energy caused by the steric effect of the bulky Eind groups. When combined with a tertiary amine, the highly Lewis acidic and bulky 2 exhibits the reactivity of a frustrated Lewis pair (FLP) and can activate hydrogen and phenylacetylene. Complexes 2 and 3 were found to catalyze the hydrogenation and hydrosilylation of CO2 in the presence of DBU under relatively mild conditions.

E-Selective Synthesis and Coordination Chemistry of Pyridine-Phosphaalkenes: Five Ligands Produce Four Distinct Types of Ru(II) Complexes

Pyridine-phosphaalkene (PN) ligands 2a-e were prepared in an E-selective fashion using phospha-Wittig methodology. Treatment of these five ligands, varying only in their 6-substituent with RuCl₂(PPh₃)₃, produced four distinct types of coordination complexes: pyridine-phosphaalkene-derived 3b,d, cyclized 4e, and six-coordinate 5a and 6c. Prolonged heating of 3b,d in THF resulted in C-H activation of the Mes* group and cyclization to give 4b,d featuring a bidentate pyridine-phospholane ligand bound to the metal center. Complex 5a, also possessing a newly formed phospholane ring, contained a different spatial arrangement of donors to Ru(II) with an agostic Ru-H-C interaction serving as the sixth donor to the transition metal center. Ligands 2b,d,e and Ru(II) complexes 3b, 4b,e and 5a were all characterized by X-ray crystallography. Six-coordinate 6c featured a structure similar to 4b,d,e, but with the CF₃ substituent acting as a weakly bound sixth ligand to the Ru(II) center, as observed by ³¹P{¹H} and¹⁹F NMR spectroscopy. The calculated structure of 6c established that the closest Ru- - -F contact was at 2.978 Å.

Organocopper cross-coupling reaction for C-C bond formation on highly sterically hindered structures

We describe a powerful, broadly applicable cross-coupling protocol that enables carbon-carbon bond formation at highly sterically hindered carbon centers (both sp2 and sp3) by employing organocopper reagents under palladium catalysis. Experimental studies and theoretical calculations indicated that the key to the unique reactivity of copper is the relatively low activation energy of the compact transmetalation transition state, due to Cu(i)-Pd(ii) interaction, which is associated with small values of deformation energy of the reactants. This reaction is applicable to a variety of bulky substrates, including compounds inert to previous cross-coupling chemistry and has high functional group tolerance.

On the Geometrical Stability of Square-Planar Platinum(0) Complexes That Bear a PNP-Pincer-Type Phosphaalkene Ligand (Eind2 -BPEP)

The four-coordinate Pt⁰ complex [Pt(PPh₃ )(Eind₂ -BPEP)] (Eind=1,1,3,3,5,5,7,7-octaethyl-1,2,3,5,6,7-hexahydro-s-indacen-4-yl; BPEP=2,6-bis(1-phenyl-2-phosphaethenyl)pyridine), which bears a PNP-pincer-type phosphaalkene ligand (Eind₂ -BPEP; PNP=N,N-bis(diphenylphosphine)-2,6-diaminopyridine), were found to adopt a square-planar configuration around the Pt center (τ₄ =0.11). This coordination geometry is very uncommon for formal d¹⁰ complexes. In this study, a series of ligands with different electronic properties (i.e., DMAP, 2,6-lutidine, PMe₃ , tBuNC, and CO) were introduced in place of PPh₃ , and their effects on the coordination geometry were examined. X-ray diffraction analysis revealed that all complexes adopted a square-planar configuration (τ₄ =0.20-0.27). In contrast, DFT calculations indicated that the geometrical stability towards distortion around Pt varied with the ligand. The complexes with pyridine-based ligands had rigid planar structures, whereas those with π-accepting ligands, such as CO, were relatively flexible towards distortion. The electronic effects of the ligands were reflected in the spectroscopic properties of the complexes, which showed a large color change in the near-infrared region.

A Square-Planar Complex of Platinum(0)

The Pt⁰ complex [Pt(PPh₃ )(Eind₂ -BPEP)] with a pyridine-based PNP-pincer-type phosphaalkene ligand (Eind₂ -BPEP) has a highly planar geometry around Pt with ∑(Pt)=358.6°. This coordination geometry is very uncommon for formal d¹⁰ complexes, and the Pd and Ni homologues with the same ligands adopt distorted tetrahedral geometries. DFT calculations reveal that both the Pt and Pd complexes are M⁰ species with nearly ten valence electrons on the metals whereas their atomic orbital occupancies are evidently different from one another. The Pt complex has a higher occupancy of the atomic 6s orbital because of strong s-d hybridization due to relativistic effects, thereby adopting a highly planar geometry reflecting the shape and orientation of the partially unoccupied dx2-y2 orbital.