Tetra-2,3-pyrazinoporphyrazines with externally appended pyridine rings. 8. Central (Zn(II), Cu(II), Mg(II)(H2O), Cd(II)) and exocyclic (Pd(II)) metal ion binding in heteropentametallic complexes from tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine

Abstract

A series of heteropentametallic porphyrazine macrocycles, represented as [(PdCl(2))(4)LM], where L = dianion of tetrakis-2,3-[5,6-di(2-pyridyl)pyrazino]porphyrazine and M = Zn(II), Cu(II) Mg(II)(H(2)O) or Cd(II), were prepared by reaction of the corresponding mononuclear [LM] species, and their behavior was examined by UV-visible and NMR spectroscopy, electrochemistry, and thin layer spectroelectrochemistry in nonaqueous media. The PdCl(2) units in [(PdCl(2))(4)LM] are coordinated at the pyridine N atoms of the external dipyridinopyrazine fragments ("py-py" coordination) and are displaced out of the plane of the central pyrazinoporphyrazine macrocycle as verified by (1)H and (13)C NMR data on [(PdCl(2))(4)LZn]. The same arrangement is also strongly suggested by similar NMR data on the Mg(II) and Cd(II) analogues. The predominant component in the synthesized materials among the four predictable macrocyclic isomers has the four exocyclic N(2(py))PdCl(2) square planar coordination sites on the same side of the central macrocyclic framework (4:0 isomer, C(4v) symmetry), and this is accompanied by a minor isomeric component (2:2 cis or trans), in line with previous findings on the pentapalladated species [(PdCl(2))(4)LPd]. IR, UV-visible, and NMR spectral data also provide evidence for transmetalation reactions of the type [(PdCl(2))(4)LMg(H(2)O)] --> [(PdCl(2))(4)LPd] and [(PdCl(2))(4)LCd] --> [(PdCl(2))(4)LPd], with the amount of [(PdCl(2))(4)LPd] formed varying from batch to batch. Dissociation of the four exocyclic PdCl(2) units from [(PdCl(2))(4)LM] occurs in pyridine, but the compounds are stable in N,N-dimethylformamide (DMF) or dimethylsulfoxide (DMSO) and can be stepwise reduced via two one-electron reversible or quasi-reversible processes, prior to an irreversible electroreduction of the bound PdCl(2) group at more negative potentials. This metal-centered reduction leads to a [LM](2-) product which is then further reduced to [LM](3-) and [LM](4-) at the electrode surface. The first two reductions of the heteropentametallic compounds are easier than those of the monometallic [LM] species but generally more difficult than reduction of the related octacationic [L'M](8+) derivatives (L' = the octamethylated free-base dianion) whose redox properties were previously reported. The Cd(II) octacation [L'Cd](8+), isolated as an iodide salt, was also synthesized for the first time in the current study, and its spectroscopic and electrochemical properties are compared to that of the previously examined analogues.