CARBONYL-COMPLEXES OF IRON(II), RUTHENIUM(II), AND OSMIUM(II) 5,10,15,20-TETRAPHENYLPORPHYRINATES - A COMPARATIVE INVESTIGATION BY X-RAY CRYSTALLOGRAPHY, SOLID-STATE NMR-SPECTROSCOPY, AND DENSITY-FUNCTIONAL THEORY

We have synthesized and characterized via single-crystal X-ray diffrac tion methods iron(II), ruthenium(II), and osmium(II) carbonyl derivati ves of (1-methylimidazole)(5, 10, 15, 20-tetraphenylporphyrinate) [(5, 10,15,20-tetraphenylporphyrinate = TPP)], Fe(TPP)(CO)(1-MeIm).toluene; Ru(TPP)(CO)(1-MeIm).chloroform, and Os(TPP)(CO)(1-MeIm).chloroform, t ogether with the osmium(II) pyridine adduct Os(TPP)(CO)(py) 2benzene. The crystallographic results permit a detailed structural comparison b etween all of the six carbonyl metalloporphyrins which can be prepared from TPP, Fe, Ru, Os, and the two axial bases 1-methylimidazole and p yridine. The structures of all three (Fe, Ru, Os) 1-methylimidazole co mplexes display major saddle distortions, with the extent of the disto rtions being Fe > Ru similar to Os. For the pyridine complexes, deviat ions from planarity of the porphyrin ring are about an order of magnit ude smaller than those for the 1-methylimidazole species. The M-C-O bo nd angles in all complexes are in the range 176.8-179.3 degrees. We al so determined the C-13 and O-17 NMR isotropic chemical shifts, the C-1 3 NMR chemical shift tensor elements, and, for the three 1-MeIm adduct s, the O-17 nuclear quadrupole coupling constants. We then used densit y functional theory (DFT) to relate the experimental spectroscopic res ults to the experimental structures. For the C-13 and O-17 isotropic s hifts, there are excellent correlations between theory and experiment (C-13, R-2 value = similar to 0.99; O-17, R-2 value = similar to 0.99) , although the slopes (C-13, similar to-0.97; O-17, similar to-1.27) d eviate somewhat from the ideal values. For the O-17 nuclear quadrupole coupling constant, our results indicate an rms error between theory a nd experiment of 0.20 MHz, for experimental values ranging from (+)1.0 to (-)0.40 MHz, where the; signs are deduced from the calculations. T he ability to Predict spectroscopic observables in metalloporphyrin sy stems having relatively well characterized structures by using density functional theory provides additional confidence in the application o f these theoretical methods to systems where structures are much less certain, such as heme proteins.