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

The dinuclear platina-beta-diketone [Pt-2{(COMe)(2)H}(2)(mu-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 center dot center dot center dot 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 center dot center dot center dot 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.