A comparison of the structure, spectroscopy, and oxygen atom-transfer reactivity of cofacial bisporphyrins anchored by xanthene (DPX) and dibenzofuran (DPD) pillars is presented. The synthesis and characterization of dicopper(II) and dinickel(II) complexes of DPD completes a homologous series of homobimetallic zinc(II), copper(II), and nickel(II) complexes for both cofacial platforms. X-ray crystallographic analysis of the parent free-base porphyrins H(4)DPX (1) and H(4)DPD (5) confirms the face-to-face arrangement of the two porphyrin macrocycles with a large available range of vertical pocket sizes: 1 (C(80)H(92)Cl(2)N(8)O), triclinic, space group P1 macro, a = 13.5167(12) A, b = 21.7008(18) A, c = 23.808(2) A, alpha = 80.116(2) degrees, beta = 76.832(2) degrees, gamma = 80.4070(10) degrees, Z = 4; 5 (C(80)H(83)N(8)O(2)), monoclinic, space group C2/c, a = 22.666(2) A, b = 13.6749(14) A, c = 42.084(4) A, beta = 94.554(2) degrees, Z = 8. EPR spectroscopy of dicopper(II) derivatives Cu(2)DPX (3) and Cu(2)DPD (7) complements the crystallographic studies by probing intramolecular metal-metal arrangements in frozen solution. Exciton interactions between the porphyrin subunits in fluid solution are revealed by steady-state and time-resolved electronic absorption and emission spectroscopy. The resulting compilation of structural and spectroscopic data provides a benchmark for the use of these and related platforms for the activation of small-molecule substrates. A structure-function relation is developed for the photoinduced oxygen atom-transfer reactions of bisiron(III) mu-oxo derivatives of DPX and DPD. The efficiency of the photochemical process is markedly dependent (approximately 10(4)-fold) on the vertical flexibility of cofacial architecture provided by the spacer.
The first catalytic oxidation reactions with cofacial bisporphyrins using molecular oxygen as the terminal oxidant are presented. The photocatalytic reactions proceed under mild conditions (ambient temperature and pressure) without the need for a co-reductant. A systematic reactivity study of the photooxidation of dimethyl sulfide (DMS) with a homologous series of bisiron(III) mu-oxo porphyrins containing dibenzofuran, xanthene, or no bridge reveals that the facility of these processes is markedly dependent on the vertical flexibility of the catalyst to provide a phototriggered molecular spring.
The synthesis and oxygen atom transfer (OAT) photoreactivity of a diiron(III) mu-oxo meso-tripentafluorophenyl bisporphyrin appended to a dibenzofuran spacer are presented. Reaction of 4,6-diformyldibenzofuran under standard Lindsey conditions furnishes the parent cofacial porphyrin architecture in a single step. These cofacial porphyrins photocatalyze the oxidation of sulfides and olefins using visible light and molecular oxygen as the terminal oxidant. High turnover numbers reflect the enhanced stability of the electron-deficient diiron(III) mu-oxo bisporphyrin core appended to a dibenzofuran spacer under aerobic conditions.
Picosecond transient absorption spectroscopy of diiron(III) mu-oxo bisporphyrins appended to xanthene, (DPX)Fe2O and (DPXM)Fe2O, and dibenzofuran (DPD)Fe2O have been investigated in order to decipher the effect of a spring-loaded cleft on their photophysics and attendant oxidation photocatalysis. The tension of the cofacial pocket is systematically tuned with the bridge span and meso-substitution opposite to the bridge; the distances of the relaxed cofacial pockets and clamped Fe-O-Fe pockets are known from X-ray crystallography (Deltad(M-M)(relaxed-clamped)=4.271 A (DPD), 2.424 A (DPXM), 0.208 A (DPX)). The photophysical and chemical properties of these cofacial platforms are compared to the unbridged diiron(III) mu-oxo analogue, (Etio)2Fe2O. Photon absorption by the diiron(III) mu-oxo chromophore prompts Fe-O-Fe photocleavage to release the spring and present a PFeIVO/PFeII pair (P=porphyrin subunit); net photooxidation is observed when oxygen atom transfer to substrate occurs before the spring can reclamp to form the mu-oxo species. The inherent lifetimes of the PFeIVO/PFeII pairs for the four compounds are surprisingly similar (tau[(DPD)Fe2O]=1.36(3) ns, tau[(DPX)Fe2O]=1.26(5) ns, tau[(DPXM)Fe2O]=1.27(9) ns, and tau[(Etio)2Fe2O]=0.97(3) ns), considering the structural differences arising from tensely clamped (DPD and DPXM), relaxed (DPX), and unbridged (Etio) cofacial architectures. However, the rates of net oxygen atom transfer for (DPD)Fe2O and (Etio)2Fe2O are found to be 4 orders of magnitude greater than that of (DPX)Fe2O and 2 orders of magnitude greater than that of (DPXM)Fe2O. These results show that the spring action of the cleft, known as the Pacman effect, does little to impede reclamping to form the mu-oxo species but rather is manifest to opening the cofacial cleft to allow substrate access to the photogenerated oxidant. Consistent with this finding, photooxidation efficiencies decrease as the steric demand of substrates increase.
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