Treatment of Si(TPP)Cl2 (TPP = tetraphenylporphyrinato) with 2 equiv of Na/Hg in THF yields the reduced porphyrin complex, Si(TPP)(THF)2, in which the porphyrin ring system has an oxidation state of 4- and the complex is antiaromatic. Single-crystal X-ray diffraction reveals that Si(TPP)(THF)2 is highly ruffled and exhibits a unique C-C bond length alternation around its periphery. In addition, experimental 1H and 29Si NMR chemical shifts and NICS (nucleus-independent chemical shift) calculations on a model compound indicate a strong paratropic ring current in Si(TPP).

Treatment of GeCl2(dioxane) with Li2(TPP)(OEt2)2 (TPP = tetraphenylporphyrin) in THF yields Ge(TPP), the first free Ge(II) porphyrin complex. In pyridine Ge(TPP) is converted to Ge(TPP)(py)2, an antiaromatic Ge(IV) complex, whereas in benzene the reaction is reversed, and pyridine dissociates from Ge(TPP)(py)2 to form Ge(TPP). That reversible reaction represents an unusual, if not unique, example of an oxidation-state change in a metal induced by coordination of a dative ligand. UV-vis and 1H NMR spectroscopy show that Ge(TPP) is an aromatic Ge(II) porphyrin complex, while the 1H NMR spectrum of Ge(TPP)(py)2 clearly indicates the presence of a strong paratropic ring current, characteristic of an antiaromatic compound. Both Ge(TPP) and Ge(TPP)(py)2 have been crystallographically characterized, and the antiaromaticity of Ge(TPP)(py)2 leads to alternating short and long C-C bonds along the 20-carbon periphery of its porphine ring system. Coordination of pyridine to Ge(TPP) greatly increases its reducing ability: the Ge(TPP)0/2+ redox potential is about +0.2 V, while the Ge(TPP)(py)2(0/+) redox potential is -1.24 V (both vs. ferrocene). The equilibrium constant of the reaction Ge(TPP) + 2 py = Ge(TPP)(py)2 in C6D6 is 22 M-2. The germanium complex of the more electron-withdrawing tetrakis[3,5-bis(trifluoromethyl)phenyl]porphyrin, Ge(TArFP), and its pyridine adduct Ge(TArFP)(py)2 were synthesized. The equilibrium constant of the reaction Ge(TArFP) + 2 py = Ge(TArFP)(py)2 in C6F6/C6D6 is 2.3 x 10(4) M-2. Density functional theory calculations are consistent with the experimental observation that M(TPP)(py)2 formation from M(TPP) and pyridine is most favorable for M=Si, borderline for Ge, and unfavorable for Sn.

The reaction of GeCl2(dioxane) with K2Pc(DMF)4 yields germanium phthalocyanine, GePc. GePc dissolves in pyridine to form GePc(py)2. The 1H NMR spectrum of GePc(py)2 and nucleus-independent chemical shift (NICS) calculations on GePc(NMe3)2 both show the presence of a strong paratropic ring current. That ring current, along with the bond-length alternation in the crystal structure of GePc(tBuPy)2, indicates the presence of an antiaromatic pi-electron circuit in GePc(py)2. SiPc(py)2 was synthesized, and its electronic structure is similar to that of GePc(py)2.

Treatment of tetraphenylporphyrinato(aluminum) chloride, Al(TPP)Cl, with Na/Hg in THF yields the stable radical Al(TPP)(THF)2. Similarly, treatment of aluminum phthalocyanine chloride, Al(Pc)Cl, with Na/Hg in THF yields the stable radical Al(Pc)(THF)2. Solution-phase magnetic susceptibility measurements show that both are monomeric radicals in solution, and ESR spectroscopy and density functional theory (DFT) calculations indicate that in both molecules the unpaired electron is delocalized throughout the ring system. While DFT calculations indicate that both molecules should undergo a Jahn-Teller distortion, only the porphyrin complex exhibits the predicted C-C bond length alternation in its X-ray crystal structure. That distortion of the ring system has precedent only in the similar reduced porphyrin complex Si(TPP)(THF)2.

[reaction: see text] Oxidation of dilithium tetraphenylporphyrin, Li(2)(TPP), yields the doubly oxidized porphyrin complex [Li(TPP)][BF(4)]. Free TPP appears to be unstable. The crystal structure of [Li(TPP)][BF(4)] reveals that the 16-atom C-N inner ring of the porphyrin consists of alternating single and double bonds. NMR spectroscopy and nucleus-independent chemical shift (NICS) calculations, in addition to the distortion from planarity observed in the crystal structure, indicate that the 16 pi-electron inner C-N ring of the porphyrin is antiaromatic.