Activation of Acetyl Ligands through Hydrogen Bonds: A New Way to Platinum(II) Complexes Bearing Protonated Iminoacetyl Ligands

The dinuclear platina-β-diketone
[Pt2{(COMe)2H}2(μ-Cl)2] (1) reacted
with 2-pyridyl-functionalized monoximes and with dioximes in the presence
of NaOMe to yield oxime–diacetyl platinum­(II) complexes [Pt­(COMe)2(2-pyCRNOH)] (R = H, 4a; Me, 4b; Ph, 4c) and [Pt­(COMe)2(HONCR–CRNOH)]
(R/R = Me/Me, 5a; Ph/Ph, 5b; (CH2)4, 5c; NH2/NH2, 5d), respectively. The strong intramolecular O–H···O
hydrogen bonds in these complexes give rise to an activation of the
acetyl ligands for Schiff-base type reactions, thus forming with primary
amines iminoacetyl platinum complexes [Pt­(COMe)­(CMeNHR′)­(2-pyCRNO)]
(R/R′ = H/Bn, 6a; Me/Bn, 6b; Ph/Bn, 6c; H/CH2CH2Ph, 6d; H/CH2CHCH2, 6e; Bn = benzyl) and
[{Pt­(CMeNHR′)2(ONCR–CRNO)}2] (R/R = Me/Me, 7a–d; Ph/Ph, 8a–d; (CH2)4, 9a; R′ = Bn, a; CH2CH2Ph, b; CH2CHCH2, c; CH2CH2OH, d). The intramolecular
N–H···O hydrogen bonds in type 6–9 complexes make clear that protonated iminoacetyl
ligands (i.e., aminocarbene ligands) and deprotoanted oxime ligands
are present. These complexes could also be obtained in reactions of
[Pt­(COMe)2(NH2R′)2] (3) with pyridyl-functionalized monoximes and with dioximes
where type 4/5 complexes were found to be
intermediates. In solution, the bis­(iminoacetyl) complexes 7–9 were found to be present as dimers (as also 8a in the solid state) with smaller amounts of monomers. The
importance of hydrogen bonds for activation of acetyl ligands was
further evidenced by synthesis of complexes [Pt­(COMe)2(2-pyCHNOMe)]
(10) and [Pt­(COMe)2(HONCMe–CMeNOMe)]
(11) bearing O-methylated oxime ligands
and their reactivty toward amines. The hydrogen-bond activated acetyl
and iminoacetyl ligands in type 5, 7, and 8 complexes were found to undergo in CD3OD solutions
facile H/D exchange reactions resulting in complexes bearing C­(CD3)O/C­(CD3)NDR′ ligands. The
constitution of all complexes was unambiguously confirmed analytically,
spectroscopically and in part by single-crystal X-ray diffraction
analyses. Structural and NMR parameters as well as DFT calculations
gave evidence for relatively strong intramolecular hydrogen bonds.