Importance of the C−H···N Weak Hydrogen Bonding on the Coordination Structures of Manganese(III) Porphyrin Complexes

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Abstract
The reactions between Mn(Por)Cl and Bu4N+CN- have been examined in various solvents by UV−vis and 1H NMR spectroscopy, where Por's are dianions of meso-tetraisopropylporphyrin (TiPrP), meso-tetraphenylporphyrin (TPP), meso-tetrakis(p-(trifluoromethyl)phenyl)porphyrin (p-CF3-TPP), meso-tetramesitylporphyrin (TMP), and meso-tetrakis(2,6-dichlorophenyl)porphyrin (2,6-Cl2-TPP). Population ratios of the reaction products, Mn(Por)(CN) and [Mn(Por)(CN)2]-, have been sensitively affected by the solvents used. In the case of Mn(TiPrP)Cl, the following results are obtained: (i) The bis-adduct is preferentially formed in dipolar aprotic solvents such as DMSO, DMF, and acetonitrile. (ii) Both the mono- and bis-adduct are formed in the less polar solvents such as CH2Cl2 and benzene though the complete conversion to the bis-adduct is achieved with much smaller amount of the ligand in benzene solution. (iii) Only the mono-adduct is formed in CHCl3 solution even in the presence of a large excess of cyanide. (iv) Neither the mono- nor the bis-adduct is obtained in methanol solution. The results mentioned above have been explained in terms of the C−H···N and O−H···N hydrogen bonding in chloroform and methanol solutions, respectively, between the solvent molecules and cyanide ligand; hydrogen bonding weakens the coordination ability of cyanide and reduces the population of the bis-adduct. The importance of the C−H···N weak hydrogen bonding is most explicitly shown in the following fact: while the starting complex is completely converted to the bis-adduct in CH2Cl2 solution, the conversion from the mono- to the bis-adduct is not observed even in the presence of 7000 equiv of Bu4N+CN- in CHCl3 solution. The effective magnetic moments of the bis-adduct has been determined by the Evans method to be 3.2 μB at 25 °C, suggesting that the complex adopts the usual (dxy)2(dxz, dyz)2 electron configuration despite the highly ruffled porphyrin core expected for [Mn(TiPrP)(CN)2]-. The spin densities of [Mn(TiPrP)(CN)2]- centered on the π MO have been determined on the basis of the 1H and 13C NMR chemical shifts. Estimated spin densities are as follows: meso-carbon, −0.0014; α-pyrrole carbon, −0.0011; β-pyrrole carbon, +0.0066; pyrrole nitrogen, −0.022. The spin densities at the pyrrole carbon and meso nitrogen atoms are much smaller than those of the corresponding [Mn(TPP)(CN)2]-, which is ascribed to the nonplanar porphyrin ring of [Mn(TiPrP)(CN)2]-. This study has reveled that the C−H···N weak hydrogen bonding is playing an important role in determining the stability of the manganese(III) complexes.

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