Synthesis, characterization, and computational study of complexes containing Pt···H hydrogen bonding interactions.

Complexes [Pt(C6F5)(bzq)L] (bzq = 7,8-benzoquinolinate; L = 8-hydroxyquinoline, hqH (1); 2-methyl-8-hydroxyquinoline, hqH' (2)) have been prepared by replacing the labile acetone ligand in the starting material [Pt(C6F5)(bzq)(Me2CO)]. The (1)H NMR spectra of 1 and 2 show that the signals attributable to the hydroxyl proton of the hqH or hqH' ligands are displaced downfield 2.64 ppm for 1 and 2.74 ppm for 2 with respect to the respective free ligands. Moreover, in both complexes the signals present platinum satellites with J(Pt,H) coupling constant of 67.0 Hz for 1 and 80.6 Hz for 2. All these features are indicative of the existence of Pt···H-O hydrogen bonds in solution for these complexes. The structures of complexes 1 and 2 have been established by an X-ray diffraction study and allow us to confirm the existence of these interactions in the solid state too. Thus, in both cases the hydroxyl hydrogen atom is pointing toward the metal center, and the measured geometric parameters involving this hydrogen are Pt-H = 2.09(4) Å, O-H = 0.94(4) Å, Pt-H-O 162(4)°, for 1, and Pt-H = 2.10(4) Å, O-H = 0.91(4) Å, Pt-H-O 162(4)°, for 2, all of which are fully compatible with a hydrogen bond system. Complexes 1 and 2 and the analogues Pt(C6F5)3(hqH) (A) and Pt(C6F5)3(hqH') (B), prepared some time ago in our laboratory and also showing Pt···H-O hydrogen bonds, have been the object of theoretical calculations to obtain better insight into the Pt···H interactions. Their density functional theory (DFT) calculated structures show excellent agreement with the X-ray determined ones (1, 2, and B). Topological analyses of the electron density function (ρ(r)) have been performed on the four complexes according to Bader's Atoms In Molecules theory. These analyses reveal a bond path that relates the platinum atom and the hydroxyl hydrogen atom, as well as the corresponding bond critical points. The values of the Laplacian ∇(2)ρ(r) and local energy density H(r) indicate that these are closed shell, electrostatic interactions, but with partial covalence. The deprotonation of the OH fragment in 1 and 2 with BuLi leads to the formation of the unexpected trinuclear complexes (NBu4)[Li{Pt(C6F5)(bzq)(L)}2] (L = hq (3), hq' (4)). The X-ray structures of these have shown a change in the coordination of the deprotonated hq and hq', which are now bonded to the Pt atoms through their O atoms, and which are bridging the Pt and Li metal atoms.