Reactivity of a Dinuclear PdI Complex [Pd2(μ-PPh2)(μ2-OAc)(PPh3)2] with PPh3: Implications for Cross-Coupling Catalysis Using the Ubiquitous Pd(OAc)2/nPPh3 Catalyst System

[PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] is the reduction product of PdII(OAc)2(PPh3)2, generated by reaction of ‘Pd(OAc)2’ with two equivalents of PPh3. Here, we report that the reaction of [PdI2(μ-PPh2)(μ2-OAc)(PPh3)2] with PPh3 results in a nuanced disproportionation reaction, forming [Pd0(PPh3)3] and a phosphinito-bridged PdI-dinuclear complex, namely [PdI2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2]. The latter complex is proposed to form by abstraction of an oxygen atom from an acetate ligand at Pd. A mechanism for the formal reduction of a putative PdII disproportionation species to the observed PdI complex is postulated. Upon reaction of the mixture of [Pd0(PPh)3] and [PdI2(μ-PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2] with 2-bromopyridine, the former Pd0 complex undergoes a fast oxidative addition reaction, while the latter dinuclear PdI complex converts slowly to a tripalladium cluster, of the type [Pd3(μ-X)(μ-PPh2)2(PPh3)3]X, with an overall 4/3 oxidation state per Pd. Our findings reveal complexity associated with the precatalyst activation step for the ubiquitous ‘Pd(OAc)2’/nPPh3 catalyst system, with implications for cross-coupling catalysis.

and dried under high vacuum, before being stored over P2O5 for ca. 7 days. 2-Bromopyridine (Merck) was degassed by the freeze-pump-thaw method and stored under N2. THF was dried by refluxing over sodium metal pieces (2 × 8 hours) before being distilled, transferred to an ampoule and subsequently deoxygenated by bubbling with argon for ca. 30 min. THF-d8 was dried over freshly sliced K metal pieces for 2 days, at room temperature before freeze-pumpthaw degassing and distilling into an ampoule and stored under Ar. Hexane and pentane were dried by refluxing over freshly sliced sodium metal pieces, before distilling and subsequent storage in an ampoule under N2.
The Pd I -dinuclear complex; [Pd2(µ-PPh2)(µ-OAc)(PPh3)2] 1 and authentic [Pd3(μ-Br)(μ-PPh2)(PPh3)3]Br were synthesised using previously published literature procedures. 1 [Pd 0 (PPh3)4] was prepared using a literature procedure, published by Coulson and stored in an Ar-filled glovebox at -30 °C. 2 All reactions were carried out either using an Ar-atmosphere glovebox or using Schlenk techniques (high vacuum, liquid nitrogen trap on a standard in-house built dual line manifold {vacuum and N2}), to eliminate atmospheric air or moisture from the reaction systems. NMRbased experiments were carried out in J. Youngs NMR tubes. Unless otherwise stated, all operations were carried out at room temperature, for which 21-23 °C was recorded. S3
This was practically carried out by inserting a sealed, vacuum dried capillary tube containing 85% H3PO4 in H2O (w/w) into an NMR tube containing the sample of interest, collecting a 31 P NMR spectrum and setting the H3PO4 resonance to 0 ppm. HRMS ESI-MS spectra were measured using a Bruker Daltronics micrOTOF MS, Agilent series 1200LC with electrospray ionisation (ESI) or on a Thermo LCQ using electrospray ionisation, with <5 ppm error recorded for all HRMS samples. LIFDI mass spectrometry was carried out using an JEOL AccuTOF GCx-plus instrument (JMS-T200GC), fitted with a probe produced by Linden CMS. The probe was equipped with 13 µm emitters on an AccuTOF.
Alternatively, LIFDI-MS was carried out using a Waters GCT Premier MS Agilent 7890A GC instrument. Mass spectral data is quoted as the m/z ratio along with the relative peak height in brackets (base peak = 100). Mass to charge ratios (m/z) are reported in Daltons. High resolution mass spectra (HRMS) are reported with <5 ppm error (ESI and LIFDI). For clarity, LIFDI data are reported for 106 Pd, the most abundant natural isotope of Pd: the 'exact mass' is given for this isotope.
X-ray crystallography: Diffraction data were collected at 110 K on an Oxford Diffraction SuperNova diffractometer with Cu-K radiation ( = 1.54184 Å using a EOS CCD camera.
The crystal was cooled with an Oxford Instruments Cryojet. Diffractometer control, data collection, initial unit cell determination, frame integration and unit-cell refinement were carried out with CrysAlisPro. a Face-indexed absorption corrections were applied using spherical harmonics, implemented in SCALE3 ABSPACK b scaling algorithm within CrysAlisPro.
OLEX2 c was used for overall structure solution, refinement and preparation of computer graphics and publication data. Within OLEX2, the algorithm used for structure solution was ShelXT dual-spaced. d Refinement by full-matrix least-squares used the SHELXL e algorithm S4 within OLEX2. c All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed using a "riding model" and included in the refinement at calculated positions. For the single crystal X-ray structures shown in the main paper, CrystalMarker® X (version 10.4.6) was used and images output as appropriate graphics files.   Youngs NMR tube. A colour change from dark red to bright red-orange was immediately observed upon shaking the NMR tube at room temperature. The NMR sample was swiftly introduced to an NMR spectrometer for 31 P spectroscopic analysis, where data was collected. Bright red crystals, isolated by filtration, from the same batch of crystals from which the X-ray structure of 2 was obtained, were picked, combined manually (using tweezers) under air.

Procedure: reaction between complex 1 and two equivalents of PPh3
These selected crystals were confirmed by XRD to be compound 2 (selected and collected as detailed above). The crystals were subsequently taken into a glovebox and dissolved in THF-d8 under an atmosphere of argon. A 31 P NMR spectrum is shown in Figure S1 (see overleaf).

PPh2){κ2-P,O-μ-P(O)Ph2}(κ-PPh3)2] (2), as verified by X-ray diffraction analysis.
In the above case, with the handpicked crystals ( Figure S1), there was a small amount of contamination with other 31 P-containing specieswith one species identified as O=PPh3(δP 24.2 ppm), a likely breakdown product of the system on short exposure to air.

2.1.2.3.
Evidence for the formation of Ac2O accompanying complex 2 1 H and 13 C NMR spectroscopic evidence was gained for the formation of acetic anhydride (Ac2O) form the reaction of 1 with two equivalents of PPh3 ( Figure S3). Ac2O was identified by 1 H NMR analysis as a singlet at δH 2.15 ppm (externally referenced to residual 1 H signal of THF-d8 {3.57 ppm}) and by 13

Further 1 H NMR analysis showing 1:1 ratio of complex 2 with acetic anhydride
Due to overlap of peaks of 2 and [Pd 0 (PPh3)3] it was challenging to ascertain the comparative stoichiometry with acetic anhydride (Ac2O) from the 1 H NMR spectrum of the crude reaction mixture. The peak at δH 6.92 ppm has been assigned as PCCH of the bridging diphenylphosphido ligand of 2, due to 1 H chemical shift similarities with the bridging diphenylphosphido ligand of 1. 1 If the Ac2O peak is set as 6H, an integration of 6.44 H is measured for the peak at δH 6.92 ppm, which is higher than the 4H expected. This is a result of an overlap with phenyl resonances derived from [Pd 0 (PPh3)3]. What can be reasoned from this observation is that the amount of Ac2O is, to a close approximation, 1:1. See Figure S4.
The solution was transferred to a J. Youngs NMR tube. A colour change from dark red to bright red-orange was again immediately observed upon shaking the NMR tube at room temperature. The reaction was analysed then analysed by 31 P NMR spectroscopy ( Figure S5).

Reaction of complex 2 and [Pd 0 (PPh3)3] and 2-bromopyridine
The reaction between complex 1 and two equivalents of PPh3 was carried out as above. In an Ar-filled glovebox, the solution was treated with bromopyridine (
Integrated NMR spectra for the reaction mixture in Figure 8 (main paper) The 31 P NMR spectra of the reaction product of 2 with 2-bromopyridine are shown in figures S8 and S9, below.

Analysis of background reaction of [Pd 0 (PPh3)4] with 2-bromopyridine.
A background reaction enabled confirmation and NMR identification of products of the

S15
The mixture was transferred into a J. Youngs NMR tube before being sealed and introduced to an NMR spectrometer for in operando 31 P spectroscopic analysis (25 o C) ( Figure S10). . This conversion may be due to increased stability of the dimeric which forms a 6-membered ring, known to adopt a boat conformation. 7 There also may be an entropic contribution to this stability due to the increased solution disorder because of two molecules of monomer complex affording one dinuclear complex releasing two equivalents of PPh3 for each dimerization reaction. It is notable that the more thermodynamically stable dimeric complex features substitution of dative bonds from the pyridine donor at expense of the PPh3, the latter of which are known to bond more strongly with Pd II than pyridines.

Refinement Special Details
The All the phenyl rings were constrained to be regular hexagons with a C-C bond-length of 1.39 angstroms. S18

Refinement Special Details
The THF of crystallization was disordered about the center of inversion with 0.5 molecules per asymmetric unit. C-C distances were restrained to be 1.50 angstroms and the C-O distances to 1.42 angstroms. The ADP of C61 and C64 were restrained to be approximately isotropic.

Computational studies: collated energies for complex 2
Density functional theory (DFT) methods were used to probe the structure of complex 2. All calculations were performed at the DFT level using the B3LYP functional [8][9][10] in the Gaussian16(revision A.03) suite of programs. 11 The def2-SVP basis set was used for all atoms, 12