Relieving Steric Strain at Octahedral Platinum(IV): Isomerization and Reductive Coupling of Alkyl and Aryl Chlorides

Reversible C-C bond formation at a triply cyclometallated platinum(iv) centre

The oxidation of the tribenzylphosphine derivative of the doubly cylcometallated platinum(ii) complex of diphenylpyridine, 1, with PhICl2 led, as a first step, to the formation of a highly electrophilic metal centre which attacked the benzyl phosphine to give a triply cyclometallated species as the arenium ion. The highly acidic arenium ion protonated unreacted starting 1, a reaction that could be supressed by the addition of water, and gave the neutral species 2(t). Octahedral complex 2(t) was induced to reductively couple, with two five-membered rings coupling to give square planar complex 5 containing a nine-membered ring. The crystal structure of 5 showed the nine-membered ring to span trans across the square planar metal accompanied by considerable distortion: the P-Pt-N bond angle is 155.48(5)°. Oxidation of 5 with PhICl2 resulted in the addition of two chlorides and a change of the nine-membered ring ligand coordination to cis at an octahedral centre, still with considerable distortions: the P-Pt-N bond angle in the crystal structure of 6 is 99.46(5)°. Treatment of 2(t) with AgBF4 also induced a coupling to give a nine-membered ring, and the fluxional three coordinate complex 7. A mono-methylated version of 1, Me-1, was prepared and similar reactions were observed. The presence of the methyl group allowed us to observe selectivity in the coupling reaction to give the nine-membered ring, with two products (a-Me-7 and b-Me7) being initially formed in the ratio 7 : 1. The concentrations of two products changed with time giving a final ratio of 1 : 8 at room temperature (half-life 48 hours), the equilibration being made possible by a reversible C-C bond forming reaction. Reaction of complexes 7 with CO or hydrogen left the nine-membered ring intact, though oxidative degradation resulted in decomplexation of the phosphine donor, accompanied by formation of a P[double bond, length as m-dash]O group.

Direct Formation of Carbon(sp3)-Heteroatom Bonds from RhIII To Produce Methyl Iodide, Thioethers, and Alkylamines

Thermolysis of the Rh^III-Me complex (DPEphos)RhMeI₂ (1) results in reductive elimination of MeI. Mechanistic studies are consistent with S_N2 attack by I^- at the Rh^III-Me group via two separate competing paths. Addition of sulfur and nitrogen nucleophiles allows effective competition and formation of C(sp³)-S and C(sp³)-N coupled products in high yields. C(sp³)-N bond formation is second-order in amine, consistent with amine substitution of iodide at the metal followed by nucleophilic attack at carbon by a second amine.

Selective C-C coupling at a Pt(iv) centre: 100% preference for sp2-sp3 over sp3-sp3

The oxidative addition of three different organic halides RX to the non-symmetric platinum(ii) mer coordinated dicyclometallated C^N^C complex 1 yielded short-lived six-coordinate platinum(iv) complexes 2(R) (R = Me, allyl, Bn), with the incoming groups trans across the platinum centre. A spontaneous reductive coupling reaction then occurred with, in each case, a completely chemoselective sp²-sp³ coupling, and exclusively gave R-3, with the newly introduced R group bonded to the previously cyclometallated aryl ring. Following a recyclometallation reaction, the oxidative addition/reductive elimination cycle was repeated and gave the same selectivity. A one-pot route to doubly alkylating the aryl ring was developed. The observed selectivity might have been predicted on the normal basis of a steric barrier associated with non-flat sp³ hybridised groups, but we suggest that it arises from the stereochemistry at the metal, and the orientation of the ligands.

Intramolecular transcyclometallation: the exchange of an aryl-Pt bond for an alkyl-Pt bond via an agostic intermediate

Oxidation of a square-planar platinum complex leads to a five coordinate cationic intermediate that can be stabilized and trapped out via an agostic interaction with the alkyl chain of a ligand. Subsequent reaction of this species leads to the formation of an alkyl-Pt bond at the expense of an aryl-Pt bond: an intramolecular transcyclometallation.

Rollover-assisted C(sp2)-C(sp3) bond formation

Rollover cyclometalation involves bidentate heterocyclic donors, unusually acting as cyclometalated ligands. The resulting products, possessing a free donor atom, react differently from the classical cyclometalated complexes. Taking advantage of a "rollover"/"retro-rollover" reaction sequence, a succession of oxidative addition and reductive elimination in a series of platinum(II) complexes [Pt(N,C)(Me)(PR3)] resulted in a rare C(sp(2))-C(sp(3)) bond formation to give the bidentate nitrogen ligands 3-methyl-2,2'-bipyridine, 3,6-dimethyl-2,2'-bipyridine, and 3-methyl-2-(2'-pyridyl)-quinoline, which were isolated and characterized. The nature of the phosphane PR3 is essential to the outcome of the reaction. This route constitutes a new method for the activation and functionalization of C-H bond in the C(3) position of bidentate heterocyclic compounds, a position usually difficult to functionalize.