EVALUATION OF THE PI-BONDING ABILITY OF IMIDAZOLE - STRUCTURE DETERMINATION AND CHARACTERIZATION OF CATENA-(H2O)2(1-CH3IM)2MG(MU-CN)(CN)4(1-CH3IM)FEIII.H2O (1-CH3IM=1-METHYLIMIDAZOLE)

The preparation of catena-diaquabis(1-methylimidazole)magnesium(II)-mu-cyanotetracyano(1-methylimidazole)ferrate(III) monohydrate (3) is reported. Crystals of catena-[Mg(H2O)2(1-CH3im)2(mu-CN)Fe(CN)4(1-CH3im)].H2O (1-CH3im = 1-methylimidazole) are monoclinic, space group P2(1), with a = 8.471 (2) angstrom, b = 15.678 (4) angstrom, c = 9.722 (2) angstrom, V = 1190.5 (4) angstrom 3, Z = 2, and R (R(w) = 0.026 (0.029) for 2104 reflections. The structure contains (CN)5Fe(1-CH3im)2- units linked in extended chains through bridging cyanides (cis to 1-CH3im), which are coordinated to the Mg2+ counterions. Two N(CN-), two O(H2O), and two N(1-CH3im) coordinate to Mg. The chains are bent about the N of the bridging cyanides at angles of 156.3 (2) and 152.3 (3)-degrees. The bending is attributed to a combination of electronic effects due to back-bonding, electrostatic attraction between the metal centers, and hydrogen bonding. The Fe-N(1-CH3im) bond length [1.950 (2) angstrom] is shorter than the Fe-NH3 bond lengths of similar complexes. The imidazole ring coordinated to Fe is staggered with respect to the cis cyanides, but at an angle (phi = 34.5-degrees) that is less than the sterically favorable 43.6-degrees. The cyanide trans to the 1-CH3im ligand on the Fe has a shorter bond length [1.130 (4) angstrom] than the average cis cyanide bond length of 1.146 angstrom. The N3-C2 bond length is longer for the 1-CH3im coordinated to Fe than for those of 1-CH3im coordinated to Mg. The complex has a very large quadrupole splitting (from Mossbauer spectroscopy) of 2.62 +/- 0.2 mm/s at 291 K (2.78 mm/s at 77 K). Collectively, our results indicate pi bonding between the 1-CH3im and the low-spin d5 Fe(III). 1-CH3im acts as a pi director to align the electron hole in the d-pi Fe orbitals along the Fe-N(1-CH3im) axis. The electron hole primarily occupies a single orbital. The observed pi-bonding properties of imidazole have important biological implications.