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“Stapled” Bis(phthalocyaninato)niobium(IV), Pc2Nb:  X-ray Crystal Structure, Chemical and Electrochemical Behavior, and Theoretical Studies. Perspectives for the Use of Pc2Nb (Thin Films) as an “Optically Passive Electrode” in Electrochromic Devices

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posted on 20.12.2002, 00:00 by Elvira M. Bauer, Maria Pia Donzello, Claudio Ercolani, Enrico Masetti, Stefania Panero, Giampaolo Ricciardi, Angela Rosa, Angiola Chiesi-Villa, Corrado Rizzoli
Recrystallization of the previously reported monosolvated bis(phthalocyaninato)niobium(IV), [Pc2Nb]·ClNP (ClNP = 1-chloronaphthalene), has allowed isolation of a single crystal of a new solvated form, i.e. [Pc2Nb]·3.5ClNP, whose structure has been elucidated by X-ray work:  space group P21/n (No. 14); a = 16.765(3), b = 23.800(4), c = 19.421(4) Å; α = γ = 90°, β = 92.51(2)°; Z = 4. The sandwiched material is a “stapled” molecule, characterized by the presence of two intramolecular interligand C−C σ bonds and highly strained phthalocyanine units, as formerly observed by crystallographic work for its Ti(IV) analogue, [Pc2Ti], and the +1 corresponding fragment, [Pc2Nb]+, present in the species [Pc2Nb](I3)(I2)0.5·3.5ClNP. [Pc2Nb] appears to be reluctant to undergo further oxidation above the +1 oxidation state. Detailed theoretical studies by DFT and TDDFT methods have been developed on [Pc2Nb] and [Pc2Nb]+, also extended for comparison to the Ti(IV) complex [Pc2Ti], and an adequate picture of the ground-state electronic structure of these species has been achieved. Moreover, the excitation energies and oscillator strengths calculated for the closed-shell systems, [Pc2Ti] and [Pc2Nb]+, provide a satisfactory interpretation of their characteristic visible optical spectra and help to rationalize the similar features observed in the visible spectrum of the open-shell “stapled” complex, [Pc2Nb]. Thin solid films (100−250 nm) of [Pc2Nb] deposited on ITO (indium-doped tin oxide) show a reversible redox process in neutral or acidic aqueous electrolytes. The electrochemical and electrochromic properties of the sandwiched complex, combined with impedance and UV/visible spectral measurements, are presented and discussed. The achieved electrochemical information, while substantially in keeping with the observed chemical redox behavior and theoretical predictions, qualifies [Pc2Nb] as an “optically passive” electrode and a promising material for its use in electrochromic devices.