Photoinduced Electron Transfer in Covalently Linked Oxomolybdenum(V) Porphyrin Systems

Abstract

The pterin-containing molybdenum enzymes are comprised of three general classes which perform simple oxygen-atom transfer either to or from substrate (sulfite oxidase and DMSO reductase families) and the oxidative hydroxylation of various substrates (xanthine oxidase family). 1 Following oxygen-atom transfer, the resting state of the enzyme is regenerated by two sequential one-electron transfers. Therefore, it is of interest to investigate electron transfer events in oxomolybdenum systems which mimic specific structural features of the active site. Here we describe the results of our initial investigations on photoinduced electron transfer in {5,10,15-tri-p-tolyl-20-[2,3-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]-porphyrinato}zinc(II) (ZnTTP(2,3-Mo V O(pzb)), 1) and {5,10,15tri-p-tolyl-20-[3,4-[((hydrotris(3,5-dimethylpyrazolyl)borato)oxomolybdenio)dioxy]phenyl]porphyrinato}zinc(II) (ZnTTP-(3,4-Mo V O(pzb)), 2), where a catechol moiety is used to covalently link the donor (porphyrin) and acceptor (Mo) sites via the meso position of the porphyrin. 2 The incorporation of oxo-Mo(V) centers as electron acceptors make these systems effective first generation models of electron transfer processes occurring at the active site of pterin-containing oxomolybdenum enzymes. 1 The structural rigidity of the catechol linker is particularly attractive since the through-bond and through-space distances, and the relative orientation of the donor and acceptor, can be easily modified by changing the spacer group from a 2,3-catechol to a 3,4-catechol. This provides a basis for testing various theoretical aspects of electron transfer and allows for detailed spectral probing of the electronic effects of linkage isomers on electron transfer rate constants. Furthermore, peripherally metalated porphyrin systems also provide a general means of probing real-time electron transfer dynamics at redox active transition metal sites. 3 In this communication we report direct evidence for photoinduced electron transfer in covalently linked donor-acceptor molecules 1 and 2.Since the [Mo V O(pzb)] 2+ fragment possesses a comparatively low oscillator strength ( < 10 3 M -1 cm -1 ), the ground-state electronic absorption spectra of 1 and 2 are dominated by the intense π f π* transitions of the porphyrin. The roomtemperature electronic absorption spectra of the nonperipherally metalated {5,10,15-tri-p-tolyl-20-[2,3-dimethoxyphenyl]por-phyrinato}zinc(II) (ZnTTP(2,3-(OMe) 2 ), 3) and {5,10,15-tri-ptolyl-20-[3,4-dimethoxyphenyl]porphyrinato}zinc(II) (ZnTTP-(3,4-(OMe) 2 ), 4) and the peripherally metalated derivatives 1 and 2 in DMSO are virtually identical. This indicates no significant porphyrin excited state (S 1 or S 2 ) electronic structural differences occur upon covalent attachment of the pendant [Mo V O(pzb)] 2+ subunit. However, steady-state emission studies show that covalent attachment of the [Mo V O(pzb)] 2+ pendant to the porphyrin periphery in 1 and 2 results in complete quenching of the porphyrin fluorescence as o...

An updated overview on the development of new photosensitizers for anticancer photodynamic therapy

“…The disadvantages associated with first generation PSs have led to extensive investigation aimed at improving the efficacy of PS molecules via alteration of the peripheral functionality of the porphyrin 33 , or direct modification of the porphyrin core 34 , The following the seminal works on the first generation PS have resulted in the production of several new non-porphyrinoid PS molecules ( Fig. 2 ).…”

Light-Induced Activation of a Molybdenum Oxotransferase Model within a Ru(II)–Mo(VI) Dyad

“…Enemark and Kirk et al already demonstrated in an early study that a Mo center can be photoactivated via an antenna-mediated electron transfer process by covalently linking an oxo-Mo(V) unit to porphyrin-Fe(III) or Zn(II) complexes (Scheme 1a). 18,19 Although catalytic investigations were not reported for these dyads, the study highlighted photoactivation as a potential way of initiating intercomponent electron transfer. More recently, Heinze et al appended two redox-active ferrocene units to a cisdioxo-Mo(VI) complex to mimic the electron-transfer chain in the molybdenum enzyme sulfite oxidase (Scheme 1b).…”

“…Since Mo is chosen by nature as OAT catalyst par excellence based on its ability to redox cycle between oxidation states VI, V, and IV, we are interested in utilizing bioinspired Mo complexes as catalysts in dyads designed to facilitate substrate oxidation via a photoredox process. Enemark and Kirk et al already demonstrated in an early study that a Mo center can be photoactivated via an antenna-mediated electron transfer process by covalently linking an oxo-Mo­(V) unit to porphyrin-Fe­(III) or Zn­(II) complexes (Scheme a). , Although catalytic investigations were not reported for these dyads, the study highlighted photoactivation as a potential way of initiating intercomponent electron transfer. More recently, Heinze et al appended two redox-active ferrocene units to a cis -dioxo-Mo­(VI) complex to mimic the electron-transfer chain in the molybdenum enzyme sulfite oxidase (Scheme b) .…”

“…19 F NMR (CD 3 CN, 376.5 MHz): δ −151.6. MS-ESI: m/z 371.5775, [M [Ru(bpy) 2 (H 2 -L 2 )](BF 4 ) 2 .…”

Unveiling the origin of photo-induced enhancement of oxidation catalysis at Mo(vi) centres of Ru(ii)–Mo(vi) dyads

“…13 Enemark and Kirk et al demonstrated that oxo-molybdenum(V) can be photoactivated via an antenna-mediated eT process by covalently linking the oxo-Mo(V) unit to porphyrin-Fe (III) or Zn(II) complexes. 14,15 Recently, Duhme-Klair et al developed biomimetic molybdenum complexes 16 with appended ruthenium-based photoactive units to facilitate oxygen atom transfer (OAT) catalysis via photo-induced eT ( Fig. 1).…”