One‐Electron Oxidation of [M(P^tBu₃)₂] (M=Pd, Pt): Isolation of Monomeric [Pd(P^tBu₃)₂]⁺ and Redox‐Promoted C−H Bond Cyclometalation

Abstract

Oxidation of zero-valent phosphine complexes [M(P t Bu 3 ) 2 ]( M = Pd, Pt) has been investigated in 1,2-difluorobenzene solution using cyclic voltammetry and subsequently using the ferrocenium cation as ac hemical redox agent. In the case of palladium, am ononuclear paramagnetic Pd I derivative was readily isolated from solution and fully characterized (EPR, X-ray crystallography Over the past few decades ar ich variety of chemistry has emerged based on the reactions of palladium and platinum complexes in the 0a nd + II formal oxidation states,e pitomized by the omnipresence of palladium catalyzed crosscoupling reactions in contemporary organic chemistry. [1,2] In contrast, the organometallic chemistry of well-defined complexes of these elements bearing formal + Ioxidation states is much less established and examples are largely limited to unstable or dinuclear species with distinct metal-metal bonds. [3,4] Halogen bridged palladium complexes of the type [Pd(m-X)(P t Bu 3 )] 2 (X = Br,I )a re notable examples and are believed to act as reservoirs for reactive {Pd 0 (P t Bu 3 )} fragments in catalytic transformations.[5] In other systems,Pd I and Pt I species have been postulated as intermediates,b ut with little supporting evidence.[6] With av iew to isolating welldefined mononuclear complexes in the + Io xidation state relevant to catalysis,wereport herein our work involving oneelectron oxidation of widely used and commercially available palladium(0) and platinum (0) ).

Cationic Two-Coordinate Complexes of Pd(I) and Pt(I) Have Longer Metal-Ligand Bonds Than Their Neutral Counterparts

“…Chaplin et al also examined oxidation of the analogous Pt complex ( t Bu 3 P) 2 Pt (2) but were not successful in isolating or observing the elusive Pt(I) cation; instead, they detected only cyclometallated Pt(II) products. 30 In the present article, we disclose our results concerning closely analogous chemistry of the oxidation of 1 and 2. Our work utilized [Ph 3 C] [HCB 11 Cl 11 ] 37 as a one-electron oxidant, thus resulting in cations paired with a very weakly coordinating [HCB 11 Cl 11 ] À .…”

“…13 However, authentic monomeric Pd(I) complexes had not been observed or characterized as isolable species until a very recent report by the Chaplin group. 30 This is in contrast to the lighter metal of group 10. For nickel, complexes in the +1 oxidation state, including some two-coordinate complexes, 31,32 are relatively common, [33][34][35] and there is mounting evidence for the participation of Ni(I) intermediates in some Ni-catalyzed coupling reactions.…”

“…46 The Pd-P distance in 3 in the present work is indistinguishable from the Pd-P distance (2.3470(6) Å ) in Chaplin's [( t Bu 3 P) 2 Pd][PF 6 ]. 30 Elongation of the Ni-C bond in a closely related oxidation of (NHC) 2 Ni 0 to [(NHC) 2 Ni] + has been reported. 31 X-ray diffraction analysis of crystals of 11 revealed the structure of a three-coordinate acetonitrile adduct of complex 3 (Figure 1).…”

“…36 Chaplin et al reported that complex ( t Bu 3 P) 2 Pd (1) can be cleanly oxidized by [Cp 2 Fe][PF 6 ] to yield an isolable, monomeric Pd(I) complex, [( t Bu 3 P) 2 Pd][PF 6 ]. 30 This compound was fully examined experimentally and displayed a surprising longer Pd-P bond than the structure of the neutral compound 1. Chaplin et al also examined oxidation of the analogous Pt complex ( t Bu 3 P) 2 Pt (2) but were not successful in isolating or observing the elusive Pt(I) cation; instead, they detected only cyclometallated Pt(II) products.…”

“…43 The cyclometalation reactions we observed here were closely analogous to those reported (and described in greater detail) by Chaplin et al upon oxidation of 2 with [Cp 2 Fe][PF 6 ]. 30 Complexes 3 and 4 showed divergent reactivity with acetonitrile (Scheme 1). Dissolution of 4 in CD 3 CN resulted in the immediate formation of 9 and 10, the acetonitrile adducts of 5 and 6, respectively.…”