An Adaptable Chelating Diphosphine Ligand for the Stabilization of Palladium and Platinum Carbenes

Abstract

Group 10 metal carbenes are proposed in catalytic transformations; however, their isolation remains difficult without the presence of a heteroatom donor. The adaptable cis and trans coordinating ligand PterP (1,2-bis­(2-(diisopropylphosphino)­phenyl)­benzene) is key in stabilizing two-coordinate palladium and platinum(0) precursors. Reacting these precursors with di-p-tolyldiazomethane ((p-tol)2CN2) leads to the formation of the unprecedented trigonal-planar diarylcarbenes [(PterP)­MC­(p-tol)2] (M = Pd, Pt), upon transformation of the trans coordinating ligand into a wide-bite, cis-coordinating ligand. Both palladium and platinum diarylcarbenes were characterized by multinuclear NMR spectroscopy. The unusual stability of the platinum analogue allowed its characterization via X-ray crystallography. Furthermore, the reactivity of the palladium and platinum diarylcarbenes with Ph2SiH2 and CH3I was investigated.

Synthesis and Structures of Stable PtII and PtIV Alkylidenes: Evidence for π‐Bonding and Relativistic Stabilization

“…The solid-state structure shows aP t IV carbene formulation (Figure 1), with as lightly distorted octahedral geometry consistent with ad iamagneticd 6 complex and an early planar arrangemento ft he ligand aryl rings (C aryl torsion = 2.068). Notably,t he PtÀCb ond length is significantly elongated from that in 2 Pt to 1.997 (6) .T his is roughly equivalentt op reviously reported aminocarbene complexes of Pt IV , [10] but shorter than Pt IV NHC complexes, [11] which would be consistentw ith ad ecrease in the back-bonding to aF ischer-type carbene upon oxidation and increased contribution from ac arbenium resonance form, as well as an increase in coordination number.…”

“…The Pd−C carbene distance is 1.974(3) Å elongated by about 0.03 Å when compared to 2 Pt , which has a Pt−C contact of 1.946(9) Å. The Pt−C distance in 2 Pt is notably contracted when compared to 2.067(5) Å found in the PC alkyl P complex 1 Pt , and comparable to the three‐coordinate Pt–carbene reported by Iluc [1.942(3) Å], indicative of double‐bond character …”

“…The PdÀC carbene distance is 1.974(3) elongated by about 0.03 when compared to 2 Pt ,w hich has aP t ÀCc ontact of 1.946(9) .T he PtÀCd istance in 2 Pt is notably contracted when compared to 2.067 (5) found in the PC alkyl Pc omplex 1 Pt ,a nd comparable to the three-coordinate Pt-carbene reported by Iluc [1.942(3) ], indicative of double-bondc haracter. [6] The behavior of cationic alkylidene 2 Pt towards nucleophiles was probedb yr eaction with ethyldiazoacetate,4 -dimethylamino pyridine (DMAP)o rh ydrogen to observe analogous reactivity to Carmona's metallocyclic carbene complex II. [7a] All reactions failed to give well-defined, isolable products (Scheme2).…”

Synthesis and Structures of Stable Pt(II) and (IV) Alkylidenes: Evidence for π-Bonding and Relativistic Stabilization

“…The rarity of Pt alkylidenes without mesomeric stabilisation by heteroatoms is interesting in light of the several computational studies that suggest high bond strength in Pt alkylidenes due to relativistic effects. 19--24 The sole structurally characterised example of a Pt(II) alkylidene, I, 25 (Chart 1) with its unusual low coordination number and nucleophilic, Schrock--like, reactivity is unlikely to accurately represent intermediates in π--acid catalysis, which are electrophilic in nature. Pt alkylidenes II 26,27 and III, 28 which behave as electrophiles, defied isolation due to their thermal instability.…”

“…The Pt--C distance in 2 Pt is notably contracted when compared to 2.067(5) Å found in the PC alkyl P complex 1 Pt , and comparable to the three coordinate Pt--carbene reported by Iluc (1.942(3) Å), indicative of double bonding character. 25…”