Five-Coordinate Trispyrazolylborate Dihydridosilyl Platinum(IV) Complexes

Abstract

Reductive elimination of C-H and C-C bonds from octahedral platinum(IV) complexes has been shown to occur from a fivecoordinate intermediate generated by ligand dissociation. 1 Given the importance of such intermediates in bond activation at platinum, we wish to communicate isolation of well-defined fivecoordinate platinum(IV) silyldihydride complexes. 2 The chemistry of TpPt(R)(R′)H [Tp ) hydridotris(pyrazolyl)borate; Tp′ ) hydridotris(3,5-dimethylpyrazolyl)borate; R,R′ ) combinations of alkyl, aryl, silyl and hydride ligands] has blossomed since 1995. 3 The stability of these complexes stems from canonical tridentate coordination of the tris(pyrazolyl)borate ligand. 4 Other stable platinum(IV) alkyl hydride complexes also contain chelating ligands in the platinum coordination sphere. 5 Protonation under mild conditions induces reductive elimination of methane from Tp′PtMe 2 H and Tp′PtMe(H) 2 via five-coordinate intermediates, leading to isolable cationic platinum(II) complexes after addition of a trapping ligand. 3g,6 Given the ability of silyl and hydride ligands to stabilize highoxidation state metals, 3f,7 and noting the elongated Pt-N bond (2.30 Å) trans to the triethylsilyl ligand in Tp′Pt(SiEt 3 )(H) 2 (1), 3f it seemed plausible that we could access a stable five-coordinate platinum(IV) complex 2 simply by protonation of 1 (eq 1).

Hydrocarbon C—H Bond Activation with Tp'Pt Complexes

“…Isolation of related fivecoordinate Pt(IV) complexes resembling the proposed intermediates discussed in previous sections was reported in 2001 (71,72). Prior attempts to prepare fivecoordinate complexes resulted either in weakly coordinating anions occupying the open coordination site or in immediate reductive elimination of alkane (64,73).…”

“…Complexes 8 -10 can also be synthesized directly by protonation of the methyldihydride complex 3 to generate the cationic platinum(II) solvent complex, [(HTpOPtiHXsolvHBAr^] [solv = CEfeCk or Et 2 0] (11), which can then be trapped by added silane (eq 5) (72). Immediate oxidative addition of the silane Si-Η bond occurs to give the five-coordinate complex; no evidence for a σ-silane adduct was observed.…”

“…In our Tp'Pt system, acid induced reductive elimination of benzene from the phenyl dihydride complex 12 afforded a surprisingly stable Pt(II)-r| 2 -benzene adduct [^-(HTpOPtiC^^^CeHôîiH^fBAry (17) which was crystallized and examined by X-ray crystallography (83). These Pt(II) rf-arene adducts were probed with NMR techniques, and for the benzene hydride adduct 17 (83) and the p-xylene adduct 19 (84) single crystal X-ray structural analysis was accomplished. !…”

Oxidatively induced reactions of a diimine platinum(IV) tetramethyl complex

“…On the other hand, in acetonitrile solvent, ligand reassociation occurs much faster, and this inhibits the scrambling process. (Recent isolated five-coordinate Pt(IV) trialkyl species have a near square pyramidal structure with distinct axial and equatorial positions in the solid state; these sites undergo rapid exchange in solution [22][23][24][25]). The involvement of acetonitrile dissociation was demonstrated by the addition of 10 lL of acetonitrile to a sample of 3 AE OTf in chloroform-d. A new resonance arising from the uncoordinated acetonitrile (d 2.06 at À45°C) appeared.…”

Reactions of diphenylpyridylphosphine with H2Os3(CO)10 and H4Ru4(CO)12, P–C bond splitting in the coordinated ligand and isolation of the oxidative addition products

“…In all these cases the reductive elimination is so kinetically and thermodynamically favorable that it was impossible to observe the intermediate hydride cluster. On the contrary, a series of mono-and binuclear transition metal complexes containing hydride and alkyl (aryl) ligands were isolated and studied in detail because of their importance for understanding the mechanisms of catalytic activation of the hydrocarbon C-H bond, see for example [21][22][23][24][25][26][27][28][29][30][31][32]. Some particularly stable cis ''hydride-aryl'' complexes of ruthenium [33], iridium [34][35][36], platinumtungsten [37] dimers, [Me 2 Si]-ansa-bridged permethylmetallocene systems [22,24,38] allowed X-ray crystallographic study.…”