Unusual phenomenon in the chemistry of orthometalated ruthenium (II) complexes

Yellow cyclometalatated ruthenium (II) complexes [Ru(o-X-2-py)(MeCN)4]PF6 (1, X = C6H4 (a) or 4-MeC6H3 (b)) react readily with 1,10-phenanthroline (LL) in MeCN to give brownish-red species cis-[Ru(o-X-2-py)(LL)(MeCN)2]PF6 in high yields. The same reaction of the same complexes under the same conditions with 2,2′-bipyridine results in a significant color change from yellow to brownish-orange suggesting a formation of new species. Surprisingly, X-ray structural studies of these two complexes showed that they are structurally indistinguishable from the starting complexes 1. Referred to as complexes 4a,b, the new compounds are slightly more stable in the air though their spectral characteristics in solution are similar to 1a,b. The diffuse reflectance spectroscopy is so far the only technique that indicated differences between 1 and 4.

Graphical abstractThese are pictures of two cyclometalated ruthenium (II) compounds of similar composition [Ru(o-C6H4-2-py)(MeCN)4]PF6 derived from 2-phenylpyridine (see color graph in the main text). Can you believe they have indistinguishable X-ray crystal structures? We could not either! But they do!Figure optionsDownload full-size imageDownload as PowerPoint slide