The chemistry of a palladium­(II) complex containing both an alkyl– and an aryl–palladium bond is reported. The reaction of [Pd­(CH2CMe2C6H4)­(MesNCHCHNMes)] with bromine or iodine leads to reductive elimination of 1,1-dimethylcyclobutabenzene with formation of [PdX2(MesNCHCHNMes)] (X = Br, I). However, the reaction with hydrogen peroxide gives [Pd­(CH2CMe2C6H4O)­(MesNCHCHNMes)] by overall oxygen atom insertion into the aryl–palladium rather than the alkyl–palladium bond. This complex [Pd­(CH2CMe2C6H4O)­(MesNCHCHNMes)] reacts with bromine, iodine, or hydrogen peroxide to give 3,3-dimethyl-2,3-dihydrobenzofuran and the corresponding complex [PdX2(MesNCHCHNMes)]. The mechanisms of reaction and basis for selectivity are discussed. The results support the view that oxygen atom insertion is a mechanistically viable pathway for selective catalytic oxidation of hydrocarbons by the green oxidant hydrogen peroxide.

A bidentate nitrogen-donor ligand with an appended phenol group, CHNCH[double bond, length as m-dash]N-2-CHOH, H(L1) was treated with a palladium cycloneophyl complex [Pd(CHCMeCH)(COD)], with both Pd-aryl and Pd-alkyl bonds, to give a Pd-alkyl complex, [Pd(CHCMeCH)(κ-N,N',O-OCHN[double bond, length as m-dash]CH(2-CHN))], 1. The cleavage of the Pd-aryl bond and the deprotonation of the ligand phenol to afford a bound aryloxide, indicates facile Pd-aryl bond protonolysis. Deuterium labelling experiments confirmed that the ligand phenol promotes protonolysis and that the reverse, aryl C-H activation, occurs under very mild reaction conditions (within 10 min at room temperature). An unusual isomerization of the Pd-alkyl complex 1 to a Pd-aryl complex, [Pd(CH(2-t-Bu))(κ-N,N',O-OCHN[double bond, length as m-dash]CH(2-CHN))], 2, was observed to give an equilibrium with [2]/[1] = 9 after 5 days in methanol. The isomerization requires that both aryl C-H activation and Pd-alkyl protonolysis steps occur. The very large KIE value (k/k = ca. 40) for isomerization of 1 to 2, suggests a concerted S2-type mechanism for the Pd-alkyl protonolysis step.

The ligand N­(CH2-2-C5H4N)2(CH2CH2CH2OH), L1, reacted with [Pd­(CH2CMe2C6H4)­(COD)] to give a new fluxional “cycloneophyl” organopalladium complex [Pd­(CH2CMe2C6H4)­(κ2-L1)], 1, which on attempted recrystallization from THF gave the monodentate carbonate complex [Pd­(CO3)­(κ3-L1)], 2. Complex 2 was prepared in designed syntheses by reaction of [PdCl­(κ3-L1)]+ with silver carbonate or by reaction of [Pd­(OH)­(κ3-L1)]+ with CO2. Complex 1 reacted with aqueous CO2 to give the cationic neophylpalladium complex [Pd­(CH2CMe2C6H5)­(κ3-L1)]+(HCO3)−, 6. Complex 6 reacts with hydrogen peroxide to give complex 2 with release of a mixture of organic products, the major one being 2-phenyl-2-butanol, PB. The formation of PB involves a neophyl rearrangement with the unprecedented preference for methyl over phenyl migration. A mechanistic basis for this unexpected reaction is proposed, involving β-carbon elimination at a palladium­(IV) center.

A new route to cycloneophylplatinum­(II) complexes is reported and the selectivity of protonolysis of the platinum–aryl and −alkyl bonds has been determined. Reaction of [PtCl2(SMe2)2] with neophylmagnesium chloride gives the binuclear cycloneophylplatinum­(II) complex [Pt2(CH2CMe2C6H4)2(μ-SMe2)2], 1, which is shown to exist as a mixture of syn and anti isomers. Complex 1 reacts reversibly with SMe2 to give [Pt­(CH2CMe2C6H4)­(SMe2)2], 2, and irreversibly with bidentate ligands NN = 3,4,7,8-tetramethyl-1,10-phenanthroline (phen*) or 4,4′-di-t-butyl-2,2’bipyridine (bubipy) to give the corresponding complexes [Pt­(CH2CMe2C6H4)­(phen*)], 3, and [Pt­(CH2CMe2C6H4)­(bubipy)], 4, respectively. Complex 2 reacts with HCl initially by cleavage of the aryl–platinum bond to give mostly trans-[PtCl­(CH2CMe2Ph)­(SMe2)2], which then rearranges to an equilibrium mixture with trans-[PtCl­(C6H4-2-t-Bu)­(SMe2)2], while 3 and 4 react to give [PtCl­(CH2CMe2Ph)­(phen*)] and [PtCl­(CH2CMe2Ph)­(bubipy)], which do not undergo the isomerization reaction. The protonolysis reactions occur by way of a platinum­(IV) hydride complex in each case, and the unusual reactivity of complex 2 is attributed to the ease of dissociation of the Me2S ligands.

The combination of a palladium(ii) precursor with a diimine-phenol ligand and an oxidant (HO or O) under different conditions has, serendipitously, given both a molecular square and a molecular tetrahedron by self-assembly of building blocks comprising palladium(ii) centres coordinated to the oxidised forms of the ligand.