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 1H 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 ∇2ρ­(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.