One‐Electron Oxidation of [M(PtBu3)2] (M=Pd, Pt): Isolation of Monomeric [Pd(PtBu3)2]+ and Redox‐Promoted C−H Bond Cyclometalation

Abstract Oxidation of zero‐valent phosphine complexes [M(PtBu3)2] (M=Pd, Pt) has been investigated in 1,2‐difluorobenzene solution using cyclic voltammetry and subsequently using the ferrocenium cation as a chemical redox agent. In the case of palladium, a mononuclear paramagnetic PdI derivative was readily isolated from solution and fully characterized (EPR, X‐ray crystallography). While in situ electrochemical measurements are consistent with initial one‐electron oxidation, the heavier congener undergoes C−H bond cyclometalation and ultimately affords the 14 valence‐electron PtII complex [Pt(κ 2 PC‐PtBu2CMe2CH2)(PtBu3)]+ with concomitant formation of [Pt(PtBu3)2H]+.

1 Synthesis of new compounds 1

.1 General methods
All manipulations were performed under an atmosphere of argon, using Schlenk and glove box techniques.
Glassware was oven dried at 150ºC overnight and flamed under vacuum prior to use. Anhydrous CH 2 Cl 2 , THF and pentane (<0.005% H 2 O) were purchased from ACROS or Sigma-Aldrich and freeze-pump-thaw degassed three times before being placed under argon. CD 2 Cl 2 was dried over CaH 2 , vacuum distilled, and freeze-pump-thaw degassed three times before being placed under argon. 1,2-Difluorobenzene (DiFB) was stirred over neutral aluminum oxide, filtered, dried over CaH 2 , vacuum distilled, and freeze-pump-thaw degassed three times before being placed under argon over 3 Å molecular sieves. [Pd(P t Bu 3 2 Cl 2 (15 mL) was stirred at room temperature for 3 h. The solution was filtered and layered with pentane to afford deep blue needles that were subsequently isolated by filtration and washed with pentane (2 × 10 mL). Yield = 278 mg (75%). 19 F{ 1 H} NMR (282 MHz, CD 2 Cl 2 ): δ -63.6 (s, Ar F ).
All other solvents and reagents are commercial products and were used as received.
NMR spectra were recorded on Bruker DPX-400, AV-400, AV-500, AVIIIHD-500 and AVIII-600 spectrometers at 298 K unless otherwise stated. 1 H NMR spectra recorded in DiFB were referenced using the highest intensity peak of the highest (δ 6.87) frequency fluoroarene multiplet. An internal sealed capillary of 0.25 M OP(OMe) 3 in C 6 D 6 was used to lock and shim samples for acquisition of NMR data, and additionally act as an internal reference for 1 H and 31 P{ 1 H} NMR data. Chemical shirts are quoted in ppm and coupling constants in Hz. EPR spectra were acquired on a Bruker EMX spectrometer using a TM 110 cylindrical mode resonator (ER 4103TM). Samples were cooled by nitrogen gas flow through a standard quartz insert from a nitrogen evaporator with a B-VT 2000 temperature control unit. To limit the dielectric loss arising from the solvent all samples were contained in 2.2 mm i.d. quartz tubes (Wilmad 705-SQ), and the quartz insert was removed for room temperature operation. The reported g-factor is referenced to a DPPH standard (g = 2.0036(3), ref.
3) and all EPR spectra are background subtracted unless otherwise noted. The background was recorded for a sample of [Fc][BAr F 4 ] in DiFB under identical conditions giving a featureless spectrum attributed to cavity background (see Figure S18).
ESI-HRMS analyses were recorded on Bruker Maxis Impact instrument.
Microanalyses were performed by Stephen Boyer at London Metropolitan University. ESI-3

Preparation of 2,6-bis(decyl)pyridine
To a cooled and stirred solution of lutidine (2.0 mL, 17.3 mmol) in dry THF (60 mL, -78 o C) was added n BuLi (1.6 M in hexanes, 25.6 mL, 41.0 mmol) dropwise. Upon addition, the colourless solution turned bright orange and then red. After 30 minutes stirring at -78˚C, 1-bromononane (7.47 mL, 39 mmol) was added and solution slowly warmed to room temperature over 16 h. The mixture was carefully quenched with water (10 mL) and the organic phase extracted with hexane (3 x 40 mL), washed with water (3 x 10 mL) and dried over MgSO 4 . Volatiles were removed under vacuum and the residue was purified by silica column chromatography (hexane/ethyl acetate 20:1) to afford the product as a colourless liquid. Yield = 4.10 g (66%).

General conditions
Reactions were carried in 5 mm J. Young's valve NMR tubes using 0.015 mmol complex (i.e. 7.7 mg 1a, 9.0 mg 1b, 9.9 mg 2a, 11.2 mg 3b) in DiFB (0.50 mL) solvent and an internal capillary containing 60 μL of a 0.25 M solution of trimethylphosphate in C 6 D 6 . Reactions were monitored by 1 H and 31 P NMR spectroscopy.

General methods
Cyclic voltammetry (CV) experiments were carried out in an inert atmosphere glovebox under argon out using a CHI 760 C potentiostat (CH Instruments, Inc.) in a typical 3-electrode set-up where a glassy carbon substrate, platinum mesh and silver wire were used as the working (WE), counter (CE) and reference electrode (RE), respectively. All potentials are calibrated to the ferrocene/ferrocenium (Fc/[Fc] + ) redox couple which was used as an internal standard.
The half-wave potentials, E 1/2 were determined from: where E p red and E p ox are the reduction and oxidation peak potentials, respectively.

ESI-12
The peak-to-peak potential separation, ΔE p is 108 mV at ν = 100 mV s -1 which deviates from the expected value of 60 mV (reversible) for the Fc/[Fc] + redox couple. 4 This is reasonably attributed to the high internal resistance of the solution arising from incomplete ionic dissociation resulting in ohmic resistance of ~ 1 KΩ.
Furthermore, i p red / i p ox ~ 0.99 is characteristic of a chemically reversible process.

Additional details
Diffusion coefficients, D were determined using the Randle-Sevcik 4 equation: i p = 2.69×10 5 n 3/2 ACD 1/2 ν 1/2 where n is the number of electrons transferred per redox event, A is the electrode area and C is the concentration.

Crystallography
Full crystallographic details including solution, refinement and disorder modelling procedures are document in CIF format and have been deposited with the Cambridge Crystallographic Data Centre under CCDC 1440602 (2a), 1440603 (3b) 1440604 (6, C 2 /c) and 1440605 (6, P2 1 /c). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Notably two different solid-state structures for 6 were obtained. Both sets of data were collected from crystals grown from the same solvent, but the samples crystallised in different space groups. In the P2 1 /c structure the cation and anion are extensively disordered over two sites ( Figure S18, left), however, only a small degree of disorder is observed in the C 2 /c structure ( Figure S18, right). In the latter case, only disorder of the (heavy) platinum atom was modelled due to the very low occupancy of the minor component (5%).