Supramolecular Polymer and Sheet and a Double Cubane Structure in Platinum(IV) Iodide Chemistry: Solution of a Longstanding Puzzle

Supramolecular organometallic chemistry: the platinum(IV) paradigm

Supramolecular chemistry and the chemistry of alkyl derivatives of the transition metals are both topics of considerable current interest, but the combination of the two fields is still underdeveloped. The challenges are, in large part, experimental in nature. For example, the self-assembly of molecules in supramolecular chemistry often relies on intermolecular hydrogen bonding, but most alkyl-transition metal bonds are cleaved by the protic groups used in hydrogen bond formation. Alkyl-platinum(IV) bonds are inert to protonolysis or attack by other electrophiles under mild conditions, and this has allowed an extensive supramolecular chemistry of organoplatinum(IV) complexes to be developed, as outlined in this perspective review. Highlights include a zeolitic structure, a polyrotaxane, a double helix, a nanotube structure and an example of spontaneous resolution to form a chiral sheet structure.

Metal-Organic Cubane Cage with Trimethylplatinum(IV) Vertices

Herein we describe the synthesis and characterization of the first platinum(IV) metal-organic cage [(Me₃Pt^IV)₈(byp)₁₂](OTf)₈ (2), in which the organometallic moieties trimethylplatinum(IV) (PtMe₃) occupied the corners of a cubane structure and 4,4'-bipyridine ligands used as linkers. The first-principles density functional theory calculations showed that the highest occupied molecular orbitals were localized on the PtMe₃ moieties, while the lowest unoccupied molecular orbitals were distributed on the organic linkers.

Mechanisms of rearrangements in platinacyclobutane chemistry

The mechanisms of formation, decomposition, and isomerisation of platinacyclobutane complexes [PtCl2(C3H6)L2] (L is typically pyridine) are discussed on the basis of density functional theory (DFT). The isomerisation and decomposition reactions occur through 5-coordinate intermediates [PtCl2(C3H6)L] that cannot be directly detected. These 5-coordinate complexes are predicted to have distorted square pyramidal structures, but a pinched trigonal bipyramidal (PTBP) stereochemistry is easily accessible. Both α- and β-elimination from these complexes to give hydride complexes [PtHCl2(C3H5)L] are predicted to have high activation energies. The isomerisation of [PtCl2(C3H6)L2] to the ylide complex [PtCl2{CH(L)CH2CH3)L] is instead predicted to occur after cleavage of a Pt–C bond to give an intermediate that can be considered as a corner platinated cyclopropane, leading to 1,3-hydrogen transfer without a hydridoplatinum intermediate. The reaction of [PtCl2{CH(L)CH2CH3}L] to give the alkene complex [PtCl2(CH2 = CHCH3)L] involves dissociation of the C–L bond followed by a 2,1-hydrogen transfer. The selectivity of related reactions from phenylcyclopropane follows naturally from these mechanisms.

Ligand-Mediated C-Br Oxidative Addition to Cycloplatinated(II) Complexes and Benzyl-Me C-C Bond Reductive Elimination from a Cycloplatinated(IV) Complex

Reaction of the Pt(II) complexes [PtMe₂(pbt)], 1a, (pbt = 2-(2-pyridyl)benzothiazole) and [PtMe(C^N)(PPh₂Me)] [C^N = deprotonated 2-phenylpyridine (ppy), 1b, or deprotonated benzo[h]quinoline (bhq), 1c] with benzyl bromide, PhCH₂Br, is studied. The reaction of 1a with PhCH₂Br gave the Pt(IV) product complex [PtBr(CH₂Ph)Me₂(pbt)]. The major trans isomer is formed in a trans oxidative addition (2a), while the minor cis products (2a' and 2a″) resulted from an isomerization process. A solution of Pt(II) complex 1a in the presence of benzyl bromide in toluene at 70 °C after 7 days gradually gave the dibromo Pt(IV) complex [Pt(Br)₂Me₂(pbt)], 4a, as determined by NMR spectroscopy and single-crystal XRD. The reaction of complexes 1b and 1c with PhCH₂Br gave the Pt(IV) complexes [PtMeBr(CH₂Ph)(C^N)(PPh₂Me)] (C^N = ppy; 2b; C^N = bhq, 2c), in which the phosphine and benzyl ligands are trans. Multinuclear NMR spectroscopy ruled out other isomers. Attempts to grow crystals of the cycloplatinated(IV) complex 2b yielded a previously reported Pt(II) complex [PtBr(ppy)(PPh₂Me)], 3b, presumably from reductive elimination of ethylbenzene. UV-vis spectroscopy was used to study the kinetics of reaction of Pt(II) complexes 1a-1c with benzyl bromide. The data are consistent with a second-order S_N2 mechanism and the first order in both the Pt complex and PhCH₂Br. The rate of reaction decreases along the series 1a ≫ 1c > 1b. Density functional theory calculations were carried out to support experimental findings and understand the formation of isomers.

Models for Cooperative Catalysis: Oxidative Addition Reactions of Dimethylplatinum(II) Complexes with Ligands Having Both NH and OH Functionality

The role of NH and OH groups in the oxidative addition reactions of the complexes [PtMe₂(κ²-N,N'-L)], L = 2-C₅H₄NCH₂NH-x-C₆H₄OH [3, x = 2, L = L1; 4, x = 3, L = L2; 5, x = 4, L = L3], has been investigated. Complex 3 is the most reactive. It reacts with CH₂Cl₂ to give a mixture of isomers of [PtMe₂(CH₂Cl)(κ³-N,N',O-(L1-H)], 6, and decomposes in acetone to give [PtMe₃(κ³-N,N',O-(L1-H)], 7, both of which contain the fac tridentate deprotonated ligand. Complex 3 reacts with MeI to give complex 7, whereas 4 and 5 react to give [PtIMe₃(κ²-N,N'-L2))], 8, or [PtIMe₃(κ²-N,N'-L3)], 9, respectively. Each complex 3, 4, or 5 reacts with either dioxygen or hydrogen peroxide to give the corresponding complex [Pt(OH)₂Me₂(κ²-N,N'-L)], 10, L = L1; 11, L = L2; 12, L = L3. The ligand L3 in complexes 9 and 12 is easily oxidized to the corresponding imine ligand 2-C₅H₄NCH=N-4-C₆H₄OH, L4, in forming the complexes [PtIMe₃(κ²-N,N'-L4)], 13, and [Pt(OH)₂Me₂(κ²-N,N'-L4)], 14, respectively. The NH and OH groups play a significant role in supramolecular polymer or sheet structures of the complexes, formed through intermolecular hydrogen bonding, and these structures indicate how either intramolecular or intermolecular hydrogen bonding may assist some oxidative addition reactions.