Dinuclear palladium(ii) compounds with bridging cyclometalated phosphines. Synthesis, crystal structure and electrochemical study

Abstract

The structural characterization of bis-cyclometalated palladium(II) compounds of formula Pd2[(micro-(C6X4)PPh2]2(micro-O2CR)2 [X = H, R = CH3 (3), CF3 (4), C(CH3)3 (5) and C6F5 (6); X = F, R = CH3 (7) and CF3 (8)], has confirmed its paddle wheel structure with two palladium atoms bridged by two acetates and two metalated phosphines in a head-to-tail arrangement. The Pd...Pd distances are in the range 2.6779(16)-2.7229(8) A. Under cyclic voltammetric conditions, compounds 3-6, in CH2Cl2 solution, were found to undergo a reversible oxidation peak in the range of potential values 0.84-1.25 V. A second partially-reversible oxidation is observed at more positive potentials (1.37-1.55 V). For compounds 3-5 in the presence of chlorides, the first oxidation becomes a two-electron process presumably leading to a neutral [Pd(III)-Pd(III)] species with a metal-metal bond.

Multifaceted Palladium Catalysts Towards the Tandem Diboration–Arylation Reactions of Alkenes

“…General: PhI·Cl 2 [18] and the palladium complexes 1 a-d [9] and 4 a-b [14] were prepared by following reported methods. Commercially available reagents were purchased from Sigma-Aldrich.…”

“…[7] It has been postulated that the base plays a role in the cleavage of diboron to afford metal-boryl intermediates in the catalytic cycle, instead of the unfavourable palladium-diboryl species expected by the oxidative addition of B À B to Pd. [8] We have reported the synthesis and characterization of the paddlewheel bis-cyclometalated palladium(II) compounds, cis-[Pd 2 A C H T U N G T R E N N U N G (C 6 H 4 PPh 2 ) 2 A C H T U N G T R E N N U N G (O 2 CR) 2 ] (1), [9] with a Pd 2 4 + core and no metal-metal bond. Cyclic voltammetry shows that some of these compounds have two reversible oxidation peaks, so they are potential starting products for the synthesis of dinuclear palladium compounds in higher oxidation states.…”

Cyclopalladated complexes containing 2-C6R4PPh2 ligands (R = H, F): one-electron electrochemical reduction leading to metal–carbon σ-bond cleavage via palladium(i)

“…After the addition of a catalytic amount of [n-Bu 4 N]Br, a new low intensity peak appeared at δ 23.9, the chemical shift of which is similar to that observed in the crude electro-reduced solution of 7, reinforcing our suggestion that the species responsible for this resonance is cis-[PdBr 2 {PPh 2 (2-C 6 F 4 H)} 2 ]. Additional confirmation that the resonance at δ 17.7 in the 31 P NMR spectrum of the electro-reduced solution was due to a chloride-containing species was obtained by mixing CH 2 Cl 2 -C 6 . The UV-Visible spectrum of 14 showed a broad, barely resolved absorption in the region of 400-450 nm in addition to two more intense bands at 312 and 362 nm, so the absorption clearly is not diagnostic of a Pd-Pd bond, at least in the present series of compounds.…”

“…The supporting electrolyte in all cases was 0.1 M [n-Bu 4 N][PF 6 ]. In cyclic voltammetry, peak potential and peak current data provided in the text below for the irreversible reduction processes refer to a scan rate of 100 mV s −1 , unless otherwise stated.…”

Electronic structures of cyclometalated palladium complexes in the higher oxidation states

“…If this is the case, it was speculated that the two-electron transition observed at the lower potential could be the result of two successive 1-electron oxidations, due to M⋯M interactions. 28,30 To pursue this further, the electrochemistry of complex 1 was revisited by duplicating the CV measurements under the reported conditions: using a glassy carbon (GC) electrode with [Bu 4 N][PF 6 ] as an electrolyte in acetonitrile. At a scan rate of 1 V s −1 , we were indeed able to resolve an additional, quasi-reversible transition at +0.28 V, in addition to the previously reported redox transition at ∼+0.5 V (vs. Fc/Fc + ) (Fig.…”