Porphyrins metallacycles

It has been shown that these carbenes can insert into Fe-Npor bonds to form vinylidine (A(-alkyl-iron porphyrin metallacycle (N-C-C-Fe)) complexes, " which can then go on to form c/s-bridged vinylidines of the porphyrin, with loss of iron. Carbene equivalents of iodosylbenzene can also react with iron porphyrins to give five-membered Fe-O-C-C-N metallocycles. Iron porphyrin carbenes can also react with amines to produce the isocyanide complexes (equation 58) ... 

By using a ruthenium-complex connected bis-porphyrin metallacycle and a trans-DPyP as a bridge, relatively stable mixed zinc(II)-free base porphyrin sandwiches were obtained in solution by Alessio (36, Fig. 29c, see also the chapter by E. Alessio in this volume for a modified version of the X-ray structure) [36,82]. As judged by the relative broadness of the NMR resonances of the porphyrin-ligand, the complex is in slow to moderate exchange with the dissociated species, hence leading to an equilibrium mixture at room temperature. At temperatures below - 20 °C, the complexes become more robust and could be characterized using 2D NMR techniques. The discrete supra-... 

Fig. 15 Schematic representation of the slipped-cofacial porphyrin metallacycle [trans,cis, cis-RuCl2(CO)2(3 -cisDPyP)]2 (10) and of the corresponding fnUy- and semi-zincated derivatives...

Fan, J.,Whitehold, J.A., Olenyuk, B., Levin, M.D., Stang, P.J. and Fleischer, E.B. (1999) Self assembly of porphyrin arrays via coordination to transition metal bisphosphine complexes and the unique spectral properties of the product metallacyclic ensembles. J. Am. Chem. Soc., 121 (12), 2741-2752. 

Figure 6. X-ray crystal structure of the iron(II)-bis-N-alkylporphyrin complex incorporating twofive-membered metallacycles to stabilize the distorted porphyrin macrocycle (40).

Bidimensional metaUacycles of porphyrins, also called molecular squares, were first obtained by treatment of meso-bis(4 pyridyl)porphyrins (i.e. 4 -czsDPyP and 4 -fransDPyP) with the appropriate linear or 90°-angular metal fragments. Reaction of 4 -cisDPyP with a linear connector such as trans-[PdCl2(PhCN)2] led to the 4 + 4 metallacycle in which the tra s-[PdCl2] imits are the sides and the porphyrins are the corners of the square (Fig. 8) [49,59]. 

Fig. 8 Example of a 4 + 4 metallacycle (molecular square) of porphyrins obtained by reaction of the angular 4 -cisDPyP unit with the linear connector tra s-[PdCl2] (see [49] and [59])...

We found that the reaction of the cis bis-coordinating ruthenium precursors trans-[RuCl2(dmso-S)4] (1) and fra s,ds,ds-[RuCl2(dmso-0)2(CO)2] (2) with an equimolar amount of the angular porphyrin building block 4 -cisDPyP leads to the formation of the neutral 2-1-2 metallacycles of formula [dans,cis,cis-RuCl2(X)2(4 -cf5DPyP)]2 (X = dmso-S, 8 X = CO, 9, Scheme 2) [68,72,73] ... 

The NMR spectra of 8 and 9 unambiguously established the metallacycUc nature and high symmetry of these species. A recent X-ray structural determination performed on the zincated derivative of 9, 9Zn, showed that in the solid state the metallacycle (C2 symmetry, with the two-fold axis passing through the Ru atoms) is flat, with an almost perfect co-planar arrangement of the two porphyrins (Fig. 14). The Ru Ru and the Zn Zn distances are... 

As a minor product of the reaction between 2 and 4 -cisDPyP we also isolated in pure form, after chromatographic workup, the trinuclear metallacycle [RuCl2(CO)2(4 -cisDPyP)]3, in which the two porphyrins coordinated to the same tran cts-RuCl2(CO)2 fragment are not coplanar [73]. 

Fig. 17 Solid state structure of the cationic hetero-bimetallic 2 + 2 metallacycle of porphyrins [Pd(dppp) fra s,cis,cis-RuCl2(CO)2(4 -cisDPyP)2 ] (12, adapted from [72,73])...

As unambiguously evidenced by NMR spectroscopy, titration of 9Zn in CDCI3 solution with one equivalent of a linear ditopic N-ligand leads rapidly to the quantitative assembling of sandwich-like 2 2 supramolecular adducts of formula [(9Zn)2(Ar-L)2j (L = 4,4 -bipy, 13 L = 4 -fransDPyP, 14 L = 4 -tra sDPyP-npm, 15), formed by two parallel metallacycles connected by two bridging ligands which are axially bound to the zinc-porphyrins (Scheme 3) [68,72,73]. 

Hupp and co-workers reported a formation constant of ca. lO M" for a similar sandwich system formed by two rhenium-mediated metallacycles with electron deficient zinc-porphyrins and two 4,4 -bipy axial ligands (see [57]). 

The photophysics of these six metallacycles has been studied in chloroform [71]. As expected for weakly interacting systems, the absorption spectra of the homonuclear species 9,10 and 9Zn, lOZn are very similar to those of the parent free-base and zinc-porphyrin chromophores in the Q-band region (Fig. 22), except for minor spectral shifts. A prominent difference between the planar and the slipped cofacial macrocycles is found in the Soret band region, in which a clear exciton splitting (of ca. 500 cm ) is present only for the latter compoimds (10 and lOZn). This result is as expected on the basis of the relative center-to-center distance in the two types of metallacycles (10.1 A in the slipped cofacial geometry as compared to 14.1 A in the planar one). The photophysics of the homo-dimers is very similar to that of the corresponding monomeric species. In particular, 9 and 10 exhibit the typical fluorescence of the free-base or zinc-porphyrin units (9 A. ,ax = 655, 716 nm, T = 5.7 ns 10 Amax = 656, 716 nm) and 9Zn and lOZn that of Zn-porphyrins (9Zn Amax = 608, 651 nm, t = 1.1 ns lOZn Amax = 600, 651 nm). The fife-times (9 and 10, 5.5 ns 9Zn and lOZn, 1.04 ns) are somewhat shortened (by 30-40%) with respect to the porphyrin components, as a consequence of the heavy-atom effect of the external ruthenium centers (see above for a detailed account of this phenomenon). 

An A-confused porphyrin undergoes cycloplatination upon reaction with PtCl2 to form complex 522, whose Pt center is bonded to one porphyrin via a metallacycle and to the other porphyrin via a Pt-N single bond." Two nickelaporphyrins having a methyl group at the carbon bonded to Ni coordinate to the Pt center of 523 with a Pt-N <7-bond and 6, A-chelation. ... 

Collman and his coworkers proposed a metallacycle intermediate in the olefin epoxidation by metalloporphyrin/hypochlorite systems due to saturation kinetics [206-209]. According to Nolte et al. these saturation kinetics were explained by a dimmer formation [210-212]. Bruice et al. also rather suggested involvement of comproportiona-tion process of higher valent 0x0 complex [213]. In addition, Bruice et al. prepared sterically hindered Fe (TDBPP)Cl, TDBPP tetrakis(2,6-dibromophenyl)porphyrin, and used as the catalyst for epoxidation of nine structurally different alkenes [214]. Fe (TDBPP) was found to be an extremely efficient catalyst for epoxidation and nearly quantitative yields of epoxide formation were obtained in all cases. A computer graphics docking study (Polygen programs Quanta and CHARMm) questioned possible intermediacy of metallacycle. A recent report by Collman indicated a direct reaction of OCl and olefins could also explain the saturation kinetics [215]. 

Though the detailed mechanism of olefin epoxidation is still controversial, Scheme 8 depicts possible intermediates, metallacycle (a), K-cation radical (b), carbocation (c), carbon radical (d), and concerted oxygen insertion (e) [2, 216, 217]. As discussed above, the intermediacy of metallacycle has been questioned. One of the most attractive mechanism shown in Scheme 8 is the involvement of one electron transfer process to form the olefin 7C-cation radicals (b). Observation of rearranged products of alkenes, known to form through the intermediacy of the alkene cation radicals, in the course of oxidation catalyzed by iron porphyrin complexes is consistent with this mechanism [218, 219]. A -alkylation during the epoxidation of terminal olefins is also well explained by the transient formation of olefin cation radical [220]. A Hammett p value of -0.93 was reported in the epoxidation of substitute styrene by Fe (TPP)Cl/PhIO system, suggesting a polar transition state required for cation radical formation [221] Very recently, Mirafzal et al. have applied cation radical probes as shown in Scheme 9 to... 

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