Peripheral metalation

Litde is known about the stability of these porphyrins in O2 reduction, how this peripheral substitution affects O2 affinity of the metalloporphyrin, how the peripheral metal complexes perturb the energetics of various intermediates, and/or the kinetics of various steps or the mechanisms of O2 reduction by these porphyrins. At present, it remains to be seen if the strategy of coordinating metal complexes on the periphery of a metalloporphyrin can be exploited in the rational design of new ORR catalysts. 

The preparation of oxygen substituted porphyrazines as analogues to the thiol appended porphyrazines proved to be a formidable challenge. Unlike the sulfur appended porphyrazines for which Na2(mnt) was a readily available precursor, no simple dinitrile precursor could be prepared for the analogous oxygen systems. In 1997, this hurdle was overcome through the preparation of a chiral dispiroketal appended pz (11), which could be further deprotected to form the diol and then either peripherally metalated or converted to the pyridazine (10). These oxygen appended porphyrazines are described in Section VI. 

Scheme 4. Preparation of octaethylporphyrazines and subsequent peripheral metalation with iron. [Adapted from (52) and (53).]...

M[pz(A4)] A = S2ML2. The octakis(.V-R)porphyra/,ines reported by Schramm and Hoffman (2), M[pz(S-R)8 (M = Ni, Cu), (60), can be converted to the octathiolate M[pz(S )g] (Scheme 11) via reductive cleavage of the sulfur-carbon bond when R = benzyl (Bn), and this tetra-bis(dithiolate) can then be peripherally capped with metal-ligand systems to yield peripherally tetrametalated star porphyrazines. The benzyl dinitrile 57 can be macrocyclized around magnesium butoxide to form [Mg[pz(S-Bn)8] (58) (35-40%), which can then be demetalated with trifluoroacetic acid to form 59 (90%), which is subsequently remetalated with nickel or copper acetate to form 60a (95%) and 60b (70%) (Scheme 11) (3, 23, 24). Deprotection of 60a or 60b with sodium in ammonia yields the Ni or Cu tetra-enedithiolates, 61a or 61b to which addition of di-ferf-butyl or n-butyl tin dinitrate produces the peripherally metalated star porphyrazines 62a (37%), 62b (80%), and 62c (41%). 

Scheme 12. Unsymmetrical dithiol appended porphyrazines and their peripherally metalated analogues. [Adapted from (4), (21).]...

Optical Spectra. The optical spectra of the unsymmetrical porphyrazines 65 and 66a-66d exhibit Soret bands between 330 and 375 nm and a split Q band, due to reduced symmetry between 650 and 700 nm. The peripherally metalated species 67d-67h do not have a split Q band, reflecting the extension of the pz n system into the five-membered chelate ring mediated by the bridging dithiolene. Table XVII compares the Q-band region of the optical spectra for 65, 66a-66c, 67b-67d to the symmetrical octa. V-bcnzyl porphyrazines 58, 59, 60a, 60b, 61a, and 61b to the analogous phthalocyanines (4). 

Electrochemistry-EPR. The redox potentials of the porphyrazines, 69a, 69b, 70a, and 70b were measured using cyclic voltammetry (Table XX). The redox potentials of the molybdocene appended porphyrazines 70a and 70b showed marked changes from that observed for the parent ligands 69a or 69b suggesting that the peripheral metalation by molybdocene profoundly alters the physiochemical properties of the macrocycle by more than just the sum of the two parts (6). 

Peripheral Metalation. Metal ions can be peripherally coordinated to the dimethylamino units. The peripheral metal binding is evident in the UV-vis spectrum by the disappearance of the n—tz absorbance and a sharpening of the g band, thus the metal-binding capabilities of aminoporphyrazines can be monitored by UV-vis titrations. Reaction of Ni[pz(NMe2)g] (102) with PdCl2 results in the formation of the peripherally metalated star-pz (142, 70%) (Scheme 26) (137). Porphyrazines 118,119, and 125 chelate the heavy metal ions Co(II), Cu(II), Zn(II), Ag(I), Cd(II), and Hg(II) (136). 

The pz-diol with M = Ni was peripherally metalated with CpiTiCh, Cp ZrC, and CpjHfC (Cp = r]5-pentamethylcyclopentadienyl) to formporphyrazines 205 (20%), 206 (45%), and 207 (21%) (Scheme 41) (10). The optical spectra of the metalocene appended catecholato pz exhibits a loss of the n-n peak (450 nm), which indicates that the lone pairs of the peripheral oxygen atoms are unavailable for charge transfer into the pz upon peripheral metalation. 

Scheme 41. Deprotection and peripheral metalation of dispiroketal porphyrazines. [Adapted from (10).]...

M). stand for central, intermediate, and peripheral metal, respectively, and BL stand for inner and outer bridging ligand, respectively. 

Compound 23 shows the expected 12-electron oxidation wave at Eq = +1.52 V, arising from the 12 equivalent peripheral metals which are considered to be weakly interacting, but the reductive waves were overlapping and could not be assigned. 

Ni Clusters. Two new nickel compounds, [Nin Nin(MeOH)3 8(p.-CN)3o Mv(CN)3 6], where Mv is either Mov or Wv, have been studied54 by HFEPR at 230 GHz. Both compounds form neutral cluster molecules, corresponding to NiM-centred polyhedrons spanned by 14 peripheral metal ions. 

At the end of our trip through the realm of noble metal porphyrins, a glance at some biochemically or biophysically relevant new derivatives may be appropriate. Most of the complexes belong to a class of porphyrins which was not a subject of systematic examination by the authors peripheral metal-loporphyrins, in which the metal is not bound to the four central nitrogen atoms of the pyrrole rings, but rather to the side chains of macrocycles. 

The term metalloid cluster is used to describe a multinuclear molecular species in which the metal atoms exhibit closest packing (and hence delocalized inter-metallic interactions) like that in bulk metal, and the metal-metal contacts outnumber the peripheral metal-ligand contacts. Most examples are found in the field of precious-metal cluster chemistry. In recent years, an increasing number of cluster species of group 13 elements have been synthesized with cores... 

Yarasir MN, Kandaz M, Koca A, Salih B (2006) Functional alcohol-soluble double-decker phthalocyanines synthesis, characterization, electrochemistry and peripheral metal ion binding. J Porphyrins Phthalocyanines 10(8) 1022-1033... 

Another example of a trinuclear complex with one labile ligand bound to each peripheral metal ion is [Ni en)2(H20)]2[Mo (CN)g] (103). In the structure of this neutral compound, each Ni(II) center is protected by two bidentate en ligands, and the coordinated water molecule is cis to the bridging CN ligand (Fig. 17). 

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