•Cationic Mn(III), Co(III) and Ni(II) porphyrins show characteristic fluorescence.•Their long-lived triplet excited states play key roles in photocatalytic systems.•Anionic Ln(III) mono- and ...bisporphyrins exhibit similar photoinduced properties.•Their photolysis at the Soret-band produces a radical type intermediate.•However, during their irradiation at the Q-bands, a stable photoproduct appears.
Metal ions can form normal (in-plane) metalloporphyrins, fitting into the central hole of the porphyrin ring, or several of them are located out of the ligand plane, resulting in sitting-atop (SAT) complexes. Kinetically inert water-soluble complexes of Mn(III), Co(III), and Ni(II) with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin display a weak, short-lived fluorescence. This can be affected by elongation of the alkyl substituent and using micellar environment in the case of Mn(III) porphyrins. In the presence of a suitable electron donor (triethanolamine, TEOA) and acceptor (methylviologen, MV2+), these metalloporphyrins proved to be efficient photocatalysts transferring electrons between the ground-state donor and acceptor via outer-sphere mechanism. In these systems triplet excited-state Mn(III) and Co(III) porphyrins are dynamically quenched with TEOA. The Mn(II) and Co(II) complexes formed in this way need also photoexcitation for the transfer of electron to the ground-state acceptor. However, the triplet excited state of Ni(II)TMPyP4+ cannot be dynamically quenched with TEOA. Instead, this electron donor forms an associate with Ni(II)TMPyP4+ in a ground-state equilibrium. The lifetime of the triplet excited state of this species is much longer than that of the nickel(II) porphyrin alone, and it can undergo an efficient dynamic oxidative quenching with MV2+. Thus, a one-step electron transfer can be realized between the electron donor and acceptor, generating MV•+, which can be utilized for hydrogen generation from water.
Lanthanide(III) porphyrins are of typical SAT complexes, the photophysical and—chemical features of which can be tuned by the size of the metal center. Anionic, early lanthanide(III) mono- and bisporphyrin complexes exhibit very similar photoinduced properties as a consequence of a special type of aggregation, through the peripheral substituents. The rather slow formation of complexes and transformation between the mono- and bisporphyrins can be accelerated by the irradiation of the system. These by-processes play considerable roles beside the photoredox degradation and demetalation. Depending on the wavelength of irradiation, two types of photoproducts can appear: during the photolysis at the Soret-band, a radical type intermediate can be observed, which disappears in dark. However, irradiation at the Q-bands, generates the formation of a new, stable photoproduct.
Thioether-phosphite ligands (SP's) with axially chiral (S)-H
0
- or (S)-H
8
-binaphthyl moiety and their Rh(COD)(SP)BF
4
complexes have been synthesized to study their coordination chemistry and ...catalytic features and to compare them to those of the structurally analogous phosphine-phosphites (PP's). NMR exchange studies on Rh(COD)(SP)BF
4
complexes showed selective 1,5-cyclooctadiene dynamics. Firm evidence has been found that this fluxional process involves dissociation of the Rh-sulfur bond. Based on in situ NMR studies, SP ligands can form two types of bis-ligated species at ligand-to-metal ratio higher than 1. Unlike bis-ligated phosphine-phosphite complexes, bis-ligated thioether-phosphite Rh-complexes can efficiently catalyze asymmetric hydrogenation reactions due to their distinct coordination chemistry.
Novel alkane-diyl based heterobidentate P,N and S,N ligands with the general formula R1R2NCH(R3)(CH2)nCH(R4)Q (R1 = Me or iPr; R2 = H or Me; R3, R4 = H or Me; n = 0, 2; Q = PPh2 or SPh) have been ...prepared starting from cyclic sulfate esters or naturally occurring compounds with C1 symmetry. The length of the ligands’ backbone and the reaction conditions applied strongly affected the stereochemical outcome of the synthesis when using cyclic sulfates as starting materials. Palladium(II)-complexes of the new ligands were characterized by 1D and 2D NMR spectroscopy in solution and in several cases by X-ray crystallography in the solid phase. The structural versatility of the ligands enabled the straightforward comparison of the stereoselectivity of their coordination as a function of their tether length, backbone substitution pattern, donor sets and relative carbon atom configuration in their backbone. The catalytic features of the novel compounds were investigated in asymmetric allylic alkylation reactions where the tether length proved to be a crucial factor in determining enantioselectivity.
Display omitted
Kinetically inert cationic Ni(II)TMPyP4+ (H2TMPyP4+=5,10,15,20-tetrakis(methylpyridinium-4-yl)porphyrin) displayed a characteristic fluorescence (τ=1.2–1.4ns, Φ=2.0×10−3), which was quenched with ...triethanolamine (TEOA) in a static way. This complex proved to be an efficient photocatalyst in the system containing TEOA and methylviologen (MV2+) as electron donor and acceptor, respectively. Interestingly, however, deviating from the behavior of the analogous Co(III) and Mn(III) complexes in such a system, TEOA did not dynamically quench the triplet excited state of Ni(II)TMPyP4+ (τ=6.31μs), hence no reduction of the metal center occured upon irradiation. Instead, in the presence of this electron donor (at 1×10−3M) the excited-state lifetime dramatically increased (to τ=36.6μs), indicating the formation of a Ni(II)TMPyP4+-TEOA associate. This longer-lived triplet was efficiently quenched by MV2+ (kq=9.9×106M−1s−1), leading to the formation of MV•+. The overall quantum yield of this one-step photoassisted electron transfer is considerably high (Φ=0.011–0.013 at Soret-band irradiation). Hence, this system, combined with a suitable co-catalyst, may be applicable for visible light-driven hydrogen generation from water.
Water-soluble Ni(II)TMPyP4+ displayed characteristic fluorescence and proved to be an efficient photocatalyst in a visible light-driven one-step electron transfer between triethanolamine and methylviologen. Display omitted
•Characteristic fluorescence of Ni(II)TMPyP4+ was statically quenched with TEOA.•Besides, TEOA increased the lifetime of the triplet excited state of this complex.•This phenomenon indicated the formation of an Ni(II)TMPyP4+-TEOA associate.•Its triplet excited state was quenched with methylviologen (MV2+), producing MV•+.•This photoassisted one-step ET between TEOA and MV2+ may be used for H2 generation.
•A highly modular approach has been developed for the synthesis of chiral PN,S-ligands.•The ligands were used in Pd-catalyzed asymmetric allylic etherification.•High yields (up to 95%) could be ...obtained at unprecedentedly low catalyst loadings (0.2 mol%).•The excess of the reagents were significantly decreased compared to literature examples.•The main factors affecting activity and enantioselectivity have been investigated.
Six novel thioether-aminophosphine type ligands with a general formula (Ar1)2PN(R1)CHR2(CH2)nCH(R3)SAr2 has been synthesized. The modular structure of the ligands and the new methodologies developed for their preparation enabled the systematic variation of their bridge length (n = 0 or 1), the substitution pattern of the backbone (R2, R3 = H or Me) as well as the P-, N- and S-substituents (Ar1 = Ph or 3,5-Me2C6H3, R1 = Et or iPr and Ar2 = Ph, 4-MeC6H4, or 4-MeOC6H4, respectively). The ligands proved to be effective in Pd-catalyzed asymmetric allylic etherification reactions providing the products in high yields (up to 95%) and with good enantioselectivities (up to 86%) using unprecedentedly low (0.2 mol%) loadings of the chiral Pd-catalyst. Based on these findings, a new scalable protocol has been developed for the preparation of chiral allylic ethers. Furthermore, the Pd(II) coordination chemistry of the ligands was thoroughly investigated by 1D and 2D NMR methods as well as by X-ray crystallography with special attention to the conformation of the chelate ring and the stereoselectivity of the sulfur coordination. Based on these studies, the main factors determining activity and selectivity of the catalytic system have been identified.
Display omitted
PDF
