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

Abstract

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 RuCl 2 (PPh 3 ) 3 , produced four distinct types of coordination complexes: pyridine-phosphaalkenederived 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 CF 3 substituent acting as a weakly bound sixth ligand to the Ru(II) center, as observed by 31 P{ 1 H} and 19 F NMR spectroscopy. The calculated structure of 6c established that the closest Ru---F contact was at 2.978 Å.

Transition Metal-Induced Cyclization of 1-Phosphabutadienes: Selective Formation of 1-Phosphet-2-ene or P═C-Substituted Phosphacyclohexene Ligands

“…Despite the established π-accepting character of PC ligands, the PC bond length of the E - 1a ligand does not change upon coordination to Au I . This has been attributed to a rehybridization of the P-center whereupon the P–C σ-bonds have increased s -character in the complex, thereby giving a shorter σ-bond which cancels out the lengthening due to π-acceptance. − This rehybridization appears to be supported by the observed increase in the Mes*PC angle in [Au­( E - 1a )­Cl] [108.23(14)°] compared to E - 1a [103.4(2)°] and a shortening of the Mes*–P bond {[Au­( E - 1a )­Cl]: 1.822(3); E - 1a 1.860(4) Å}. The P(1)–Au(1) bond length [2.2228(8) Å] is in the range expected for related phosphaalkene-Au I complexes [2.20(5) to 2.242(11) Å] …”

Synthesis, Coordination Chemistry, and Mechanistic Studies of P,N-Type Phosphaalkene-Based Rh(I) Complexes

“…According to 1 H NMR spectroscopy, the phosphaalkene moieties adopt an E -configuration with the PCH proton showing the expected doublet at 8.25 or 8.60 ppm, respectively, and a 2 J P–H coupling constant of approximately 25 Hz (cf. E -configured P,N-phosphaalkene ligands 2 J P–H = ca. 25 Hz).…”

“…Apart from Geoffroy’s studies of a pyridine-based P,N-phosphaalkene Cu complex, a series of Ru P,N-phosphaalkene complexes were reported by Cain and co-workers just recently. The latter complexes showed intramolecular C–H bond activation at the Mes* groups to produce phosphaindane structures (Scheme (bottom left)) . Similarly, Ozawa and co-workers showed that using the related Mes*-substituted P,N,P-type ligand, twofold cyclization occurred to give the corresponding bis-phospholanyl RhCl-complex (Scheme (bottom right)) .…”

“…The latter complexes showed intramolecular C−H bond activation at the Mes* groups to produce phosphaindane structures (Scheme 2 (bottom left)). 30 Similarly, Ozawa and co-workers showed that using the related Mes*-substituted P,N,P-type ligand, twofold cyclization occurred to give the corresponding bis-phospholanyl RhCl-complex (Scheme 2 (bottom right)). 31 C−H bond activation is a common deactivation pathway for Mes*containing low-valent P-species.…”