Oxidative ring expansion of a low-coordinate palladacycle: Synthesis of a robust T-shaped alkylpalladium(II) complex

Abstract The synthesis of an unusual T-shaped alkylpalladium(II) complex featuring a cyclometalated tri-tert-butylphosphineoxide ligand by oxidation of the corresponding cyclometalated tri-tert-butylphosphine complex with PhIO is reported. We speculate that this reaction proceeds by formation of a transient palladium oxo intermediate and there are structural similarities with a late transition metal exemplar: Milstein’s seminal pincer ligated Pt(IV) oxo (Nature 2008, 455, 1093–1096).

The formation of 2 can be reconciled by idealised mechanisms involving (a) ring strain promoted hemi-labile coordination of the tethered phosphine or (b) intermediate formation of a palladium oxo derivative (Scheme 2). In order to probe the former, 1·BAr F 4 was reacted with 2,6-(tBu 2 PO) 2 C 5 H 3 N (PONOP) at RT in DFB leading to the formation of 3·BAr F 4 by substitution of PtBu 3 , dissociation of the tethered phosphine donor, and the (unusual) partial chelation of the pincer ligand; as evidenced by singlet 31 P resonances at δ 182.6, 180.0, and −12.0 (PtBu 2 ) in an integral 1:1:1 ratio, and a doublet of doublets metal alkyl 13 C resonance at δ 24.2 ( 2 J PC = 70, 34 Hz). Whilst the X-ray structure of isolated 3 indicates the palladacycle is retained in the solid state ( Fig. 2), the solution-phase behaviour suggests outer-sphere phosphine oxidation is a viable option. As a gauge for the timescales associated with such a mechanism, the oxidation of PtBu 3 with PhIO (t > 9 h) was studied, but the corresponding rate is incongruent with the formation of 2 under equivalent conditions (t < 1 h). Correspondingly, we favour an inner-sphere explanation, with the formation of a discrete metal oxo at one extreme and concerted O-atom transfer into the Pd-P bond at the other [10]. Whilst the formation of terminal oxo complexes beyond the "oxo wall" between groups 8 and 9 is rare [11,12], a directly pertinent platinum example supported by an anionic PCN pincer ligand has been reported and, moreover, its onward reactivity involves intramolecular O-atom transfer (B → C, Scheme 2) [10]. On this basis, whilst we currently cannot definitively distinguish between the two possibilities, we postulate a discrete terminal oxo derivative is involved.
Complex 2 is remarkably stable in solution, with no reaction evident upon exposure to air for 2 months. Moreover, whilst complete decomposition of 1 to [Pd(PtBu 3 ) 2 ], PtBu 3 and palladium black was observed on placing under H 2 in DFB at RT (t < 2 days), 2 persists for > 3 days under the same conditions and only upon heating to 50 °C was any evidence of a reaction evident.

Scheme 2.
Possible reaction mechanisms and associated evidence/precedents.

Conclusions
In summary, we report the synthesis of an unusual T-shaped alkylpalladium(II) complex featuring a cyclometalated tri-tert-butylphosphineoxide ligand by oxidation of the corresponding cyclometalated tri-tert-butylphosphine complex with PhIO. We speculate that this reaction may include transient formation of a palladium oxo intermediate, however, further work is needed to substantiate this claim.

General experimental methods
All manipulations were performed under an inert atmosphere of argon using Schlenk and glovebox techniques unless otherwise stated. Glassware was oven dried at 150 °C overnight and flame-dried under vacuum prior to use. Molecular sieves were activated by heating at 300 °C in vacuo overnight. CD 2 Cl 2 was dried over activated molecular sieves (3 Å) and stored under an argon atmosphere. 1,2-F 2 C 6 H 4 (DFB) was pre-dried over Al 2 O 3 , distilled from calcium hydride and dried over two successive batches of 3 Å molecular sieves under argon. t-Butyl methyl ether (MTBE) was sparged with argon prior to use. All other anhydrous solvents were purchased from Aldrich or Acros, freeze-pumpthaw degassed, and stored over 3 Å molecular sieves under argon. [Pd(PtBu 3 ) 2 ][PF 6 ] [4a], Pd(κ 2 P,C -PtBu 2 CMe 2 CH 2 ) (PtBu 3 )(OAc)]·HOAc [7], Na [BAr F 4 ] [13], PhIO [14], 2,6-(tBu 2 PO) 2 C 5 H 3 N (PONOP) [15] were prepared using literature procedures. All other reagents are commercially available and were used as received. NMR spectra were recorded on Bruker spectrometers at 298 K. Chemical shifts are quoted in ppm and coupling constants in Hz. NMR spectra in DFB were recorded using an internal capillary of C 6 D 6 . 31 P NMR spectra are referenced to a solution of O]P(OMe) 3 in C 6 D 6 (0.025 mmol L −1 , δ 3.80 relative to 85% H 3 PO 4 ). High resolution (HR) ESI-MS were recorded on a Bruker Maxis Plus spectrometer. Microanalyses were performed at the London Metropolitan University by Stephen Boyer.

Preparation of [Pd(κ 2 P,C -PtBu 2 CMe 2 CH 2 )(PtBu 3 )][BAr F 4 ] 1·BAr F 4
To a solution of [Pd(κ 2 P,C -PtBu 2 CMe 2 CH 2 )(PtBu 3 )(OAc)]·HOAc (56.6 mg, 90.1 μmol) in MTBE (5 mL) was added a solution of PtBu 3 in pentane (0.12 mL of a 0.78 M solution, 94 µmol) and the resulting solution was stirred at room temperature for 5 min before being transferred onto a 5 mL degassed aqueous suspension of Na[BAr F 4 ] (79.9 mg, 90.2 μmol). The biphasic mixture was stirred vigorously for 5 min and the organic phase transferred dropwise into excess hexane, affording a yellow precipitate that was isolated by filtration and dried in vacuo. Yield: 63.2 mg (51%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a DFB solution at room temperature.  To a solution of [Pd(κ 2 P,C -PtBu 2 CMe 2 CH 2 )(PtBu 3 )(OAc)]·HOAc (130.6 mg, 207.9 μmol) in MTBE (5 mL) was added a solution of PtBu 3 in pentane (0.27 mL of a 0.78 M solution, 210 µmol) and the resulting solution was stirred for 5 min at room temperature, before being transferred onto a 5 mL degassed aqueous suspension of Na[PF 6 ] (35.4 mg, 211 μmol). The biphasic mixture was stirred vigorously for 5 mins and hexane (5 mL) was added. The yellow precipitate was isolated by filtration and washed with hexane (3 × 5 mL). The product was then extracted into DFB, precipitated by addition of excess hexane, isolated by filtration and dried in vacuo. Yield: 78.4 mg (58%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a DFB solution at room temperature.

NMR scale reactions of 1·BAr F 4 and PtBu 3 with PhIO
A suspension of PhIO (22.1 mg, 100 μmol) in a solution of 1·BAr F 4 (13.9 mg, 10.0 μmol) in DFB (0.5 mL) within a J. Young's valve NMR tube was monitored by 31 P NMR spectroscopy, with constant mixing at room temperature when not in the spectrometer. Quantitative conversion to 2·BAr F 4 was observed within 1 h. A suspension of PhIO (22.1 mg, 100 μmol) in a solution of PtBu 3 (15 μL of a 0.67 M solution in hexane, 10 μmol) in DFB (0.5 mL) within a J. Young's valve NMR tube was monitored by 31 P NMR spectroscopy, with constant mixing at room temperature when not in the spectrometer. Quantitative conversion to O]PtBu 3 (δ 64.4 ppm) [16] was observed within 24 h.  PhIO (119.6 mg,543.4 μmol) in DFB (5 mL) was stirred for 30 min at room temperature. The solution was filtered into hexane (20 mL) affording a yellow precipitate that was isolated by filtration, washed with hexane (3 × 5 mL) and dried in vacuo. Yield: 40.7 mg (55%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a DFB solution at room temperature. (d, 2 (5 mL) was stirred vigorously for 15 min at room temperature. The solution was filtered into hexane (20 mL), affording a yellow precipitate which was isolated by filtration, washed with hexane (3 × 5 mL) and dried in vacuo. Analytically pure material was obtained by recrystallisation from dichloromethane/hexane. Yield: 6.5 mg (10%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a DFB solution at room temperature.

NMR scale reaction of 1·BAr F 4 with PONOP
A solution of 1·BAr F 4 (13.8 mg, 10.1 μmol) and PONOP (3.9 mg, 11 μmol; δ 156.0) in DFB (0.5 mL) was prepared in a J. Young's valve NMR tube containing an internal sealed capillary of O]P(OMe) 3 in C 6 D 6 . Analysis by 31 P NMR spectroscopy after 30 min at room temperature indicated complete conversion to [Pd(PONOP) (PtBu 2 CMe 2 CH 2 )][BAr F 4 ] (δ 182.6, 180.0 and −12.0) and PtBu 3 (δ 62.8).  (15.1 mg, 44.0 μmol) in DFB (2 mL) was stirred for 30 min at room temperature. The solution was concentrated in vacuo and transferred dropwise into excess hexane, affording a pale blue precipitate that was isolated by filtration and dried in vacuo. Yield: 33.1 mg (53%). Single crystals suitable for X-ray diffraction were obtained by slow diffusion of hexane into a DFB solution at room temperature.