Phosphido-Bridged Di- and Trinuclear Palladium Complexes from Electron-Poor Phosphanes R2PH (R = C2F5, C6F5, (CF3)2C6H3)

Electron-withdrawing substituents R in complexes [Ln M(PR2)] influence the P–M bond length due to a decreased σ-donation and enhanced π-back-bonding, leading to an increased Lewis acidity of the metal ion and therefore strengthening the M–L bond to electron-rich ligands L. This influences the Lewis acidity and the redox behavior of corresponding transition-metal complexes, which is important for the design of optimized catalytic systems. To investigate this effect, the electronpoor phosphanes R2PH with R = C2F5, C6F5, 2,4-(CF3)2C6H3 were treated with Pd(F6acac)2 (F6acac = hexafluoroacetylacetonato)

The increased electron-withdrawing character of perfluoroorganyl groups was successfully employed by our group for the development of highly active catalysts for the Suzuki coupling. These complexes were synthesized via the treatment of phosphinous acids R 2 POH with different palladium precursors. [18] To access a broader range of finely tunable palladium complexes, we investigated the reaction of secondary phosphanes R 2 PH with palladium acetylacetonato derivatives.

31
P NMR spectroscopic monitoring of the reaction of (CF 3 ) 2 PH with Pd(F 6 acac) 2 (F 6 acac = hexafluoroacetylacetonato) discloses a broadening of the phosphane resonance. A second resonance at -8.6 ppm is assigned to the diphosphane (CF 3 ) 2 PP(CF 3 ) 2 [21] as the result of a reductive elimination.
The reaction of the heavier homologue (C 2 F 5 ) 2 PH with Pd(F 6 acac) 2, however, selectively gives rise to the formation of [{(F 6 acac)Pd{μ-[P(C 2 F 5 ) 2 ]}} 2 ], 1 (Scheme 1). NMR spectroscopy, usually a highly valuable tool for compounds featuring a perfluoroalkyl-or -aryl phosphorus unit, failed for a satisfactory characterization of the complexes described in this paper. The molecular structures could not be derived from NMR experiments alone and had to be elucidated by an X-ray crystal structure analysis (see below). In the 31 P NMR spectrum of 1 the phosphorus atom gives rise to a multiplet at δ( 31 P) = -98.8 ppm which is observed as a sharp singlet upon 19 F decoupling. This resonance is markedly shielded in comparison to the one of (C 2 F 5 ) 2 PH (δ( 31 P) = -50.8 ppm). [15] The 19 F NMR spectrum exhib-its one set of signals for the CF 3 and CF 2 units. The resonance of the CF 3 units is observed as a singlet at -80.2 ppm, while the CF 2 units gave a multiplet of higher order at -97.6 ppm with a 2 J(PF) coupling constant of about 35 Hz, which is comparatively small for bis(pentafluoroethyl)phosphane derivatives. 31 P decoupling again leads to a sharp singlet. Additionally, the resonance for the CF 3 groups of the F 6 acac ligand is observed at -75.1 ppm in the typical range for one F 6 acac ligand chelating a palladium atom. Scheme 1. Synthesis of palladium complexes 1, 2 and 3.
The reaction of (C 6 F 5 ) 2 PH with Pd(F 6 acac) 2 proceeds analogously to 1, with a selective formation of [{(F 6 acac)Pd-{μ-[P(C 6 F 5 ) 2 ]}} 2 ], 2 (Scheme 1). 2 is only poorly soluble which impeded meaningful 13 C NMR spectra. The 31 P NMR spectrum shows a multiplet at -175.8 ppm which, upon 19 F decoupling, turns into a sharp singlet. The resonances for the F 6 acac ligand and the C 6 F 5 rings in the 19 F NMR spectrum are observed in the expected range and their integrals are consistent with the proposed structure.
In contrast to many functionalized bis[2,4-bis(trifluoromethyl)phenyl]phosphane derivatives, bis[2,4-bis(trifluoromethyl)phenyl]phosphane, [(CF 3 ) 2 C 6 H 3 ] 2 PH, has not been described in the literature before. The corresponding aminophosphane [(CF 3 ) 2 C 6 H 3 ] 2 PNEt 2 [22] was chosen as a conducive precursor which upon treatment with two equivalents of gaseous HBr selectively afforded the corresponding bromophosphane as a colorless solid in an 84 % yield. Its NMR data agree with the ones reported by Dillon et al. [23] Similar to the synthesis of (C 6 F 5 ) 2 PH described by Schmutzler et al., [24] the bromophosphane was treated with a 1 M solution of LiAlH 4 in diethyl ether. The mixture was subsequently quenched with aqueous HCl. After the removal of all volatile compounds in vacuo and recrystallization of the residue from n-pentane, [(CF 3 ) 2 C 6 H 3 ] 2 PH was obtained as a colorless solid in a 67 % yield (Scheme 2). Scheme 2. Synthesis of bis[2,4-bis(trifluoromethyl)phenyl]phosphane. Its 31 P NMR spectrum exhibits a doublet of septets at -49.8 ppm with a 1 J(PH) coupling constant of 232 Hz which is comparable to the C 2 F 5 derivative ( 1 J(PH)=230 Hz) [15] as well as the C 6 F 5 ( 1 J(PH)=218 Hz) [24] and Ph derivative ( 1 J(PH)= 214 Hz). [25] The 4 J(PF) coupling constant of 38 Hz is rather small compared to other bis[2,4-bis(trifluoromethyl)phenyl]phosphane derivatives which usually are found in the range of 55-65 Hz. [22,23,26,27] The reaction of the phosphane [(CF 3 ) 2 C 6 H 3 ] 2 PH with Pd-(F 6 acac) 2 , analogously, selectively furnished the dinuclear palladium complex [{(F 6 acac)Pd{μ-{P[C 6 H 3 (CF 3 ) 2 ] 2 }}} 2 ], 3 (Scheme 1). The 31 P NMR resonance is shifted about 30 ppm to higher field and is observed at -80.6 ppm as a broad multiplet. The 19 F NMR spectrum displays two broad signals for the ortho-CF 3 groups in a ratio of 1.3:1. This is probably due to a hindered rotation of one ortho-CF 3 group per P[C 6 H 3 (CF 3 ) 2 ] 2 unit, as in the solidstate structure of 3 an F···P contact was observed (see below). The 1 H NMR spectrum exhibits resonances for the aromatic protons that also point at a hindered rotation.
Treating (CF 3 ) 2 PH with the non-fluorinated palladium precursor Pd(acac) 2 (acac = acetylacetonato) again results in a broadening of the resonance in the 31 P NMR spectrum without any significant shifts in the 31 P or 19 F NMR spectrum, as well as in the formation of the diphosphane (F 3 C) 2 PP(CF 3 ) 2 .
The resonance in the 31 P NMR spectrum is detected as a multiplet of higher order at δ( 31 P) = -88.6 ppm. 19 F decoupling leads to a sharp singlet. The 19 F NMR spectrum is similar to that of the F 6 acac complex, with a singlet at -80.2 ppm for the CF 3 units and a higher-order multiplet at -98.8 for the CF 2 units with a 2 J(PF) coupling constant of about 30 Hz. The 1 H NMR spectrum displays signals for the acetylacetonato ligand at 5.4 and 2.2 ppm with corresponding signals in the 13 C NMR spectrum at 26.5 for the CH 3 groups, 99.1 for the CH unit and 185.8 for the oxygen-bound carbon atom. The latter resonance as well as the resonance for the CH 3 groups are split into triplets with coupling constants of 3 J(PC) = 2 and 4 J(PC) = 6 Hz, respectively. After removal of all volatile compounds, the compound remained as a red powder.
At a first glance, the reaction of (C 6 F 5 ) 2 PH with Pd(acac) 2 seems to proceed analogously to that with Pd(F 6 acac) 2 . The 31 P{ 19 F} NMR spectrum reveals a sharp singlet at -177.0 ppm. But contrary to the complexes discussed above, complex 5 (Scheme 4) is obtained as a trinuclear palladium complex with four bridging bis(pentafluorophenyl)phosphido units and two chelating acac ligands, as confirmed by an X-ray analysis (see below).
The resonances of the C 6 F 5 rings in the 19 F NMR spectrum are comparable to those of 2.
The 31 P NMR spectrum is characterized by a broad multiplet of higher order at -27.8 ppm. Proton-decoupling shows a slightly decreased linewidth, while fluorine-decoupling leads to a broad singlet with shoulders. The 19 F NMR spectrum exhibits two signals: the resonance of the para CF 3 groups is observed as a singlet at -63.5 ppm, while the ortho CF 3 groups give rise to a multiplet (formally an [[A 3 ] 2 X] 2 spin system; A = 19 F, X = 31 P) at -57.6 ppm which on 31 P decoupling is observed as a singlet.

X-ray Structural Investigation
Compound 1 crystallizes in the monoclinic space group P2 1 with two molecules per unit cell ( Figure 2); two of the six C 2 F 5 groups are disordered. The overall structure is quite similar to that of [{(F 6 acac)Pd[μ-(PPh 2 )]} 2 ] described by Röschenthaler et al. [12] The only striking difference concerns the averaged Pd-O distance of 204.8 pm which is about 6 pm shorter than in the Ph derivative [{(F 6 acac)Pd[μ-(PPh 2 )]} 2 ]. [12] The P-Pd bond lengths of 1, however, are with d av = 223.9 pm comparable to those of the Ph derivative (d av = 223.6 pm). While the π-backbonding from the metal in 1 clearly compensates for the re-  duced σ-basicity which results in comparable Pd-P bond lengths of the C 2 F 5 and Ph derivative, the increased Lewis acidity at the metal atom in 1 leads to shortened Pd-O bond lengths.
The Pd-P-Pd bond angles are 104.45(6) and 104.67(6)°and the P-Pd-P bond angles amount to only 75.20 (5) and 75.17(5)°. This results in a Pd1-Pd2 distance of 354.23(6) pm and a short P1-P2 distance of 273.2(2) pm, which is about 100 pm shorter than the sum of the van-der-Waals radii. These structural features are also observed in many neutral Pd II complexes with a Pd(μ-P) 2 Pd four-membered ring, although especially the P-P distance of 1 is rather short compared to the average P-P distance of about 280 pm. [9,12,28] The C-P-C angles amount to 101.6(3) and 106.6(5)°.
2 crystallizes in the triclinic space group P1 with two molecules in the unit cell ( Figure 3). The distances and angles are largely comparable to 1 and are summarized in Table 1.  4 crystallizes in the triclinic space group P1 at a center of inversion ( Figure 5) with disordered C 2 F 5 groups. Due to a decomposition of the crystal at low temperatures, the measurement was performed at 250 K which led to large thermal ellipsoids. The bond lengths and angles are generally comparable to its F 6 acac counterpart 1, with a slightly longer P-P distance of 278.36(2) pm. ( Figure 6). The two four-membered rings (Pd(μ-P) 2 Pd) deviate significantly from planarity with fold angles of 23.55(2)°resp. 39.59(2)°along the P-P line and 29.11(2)°resp. 47.02(2)°along Pd-Pd. This results in considerably shortened Pd-Pd distances of 354.65(1) pm (Pd1-Pd2) and 341.07(1) pm (Pd2-Pd3). The mean P-Pd-P angle of the (acac)Pd(μ-P) 2 unit (77.91°) is slightly widened compared to its counterpart in the Pd(μ-P) 4 unit (74.17°). These units also exhibit significantly differing Pd-P bond lengths: The mean Pd-P bond length in the (acac)Pd-(μ-P) 2 units of 225.2 pm is comparable with those obtained in complexes 1-4, while the average Pd2-P bond length of the central Pd(μ-P) 4 unit of 234.8 pm is significantly longer. A similar observation has been made by Mathey and Le Floch for their trinuclear bis(diphosphaferrocene) palladium complex in which the Pd-P bonds of 243.05(7) and 251.60(7) pm of the central Pd(μ-P) 4 unit are between 20 and 30 pm longer than those in the outer L 2 Pd(μ-P) 2 units. [29] Figure 6. Molecular structure of [Pd{{μ-[P(C 6 F 5 ) 2 ] 2 }Pd(acac)} 2 ] (5)·2PhCl. Thermal ellipsoids are shown at the 50 % probability level. For clarity, acac ligands are displayed in a wires/sticks model and solvent molecules were omitted. 6 crystallizes in the monoclinic space group C2/c at a twofold axis with four formula units per unit cell ( Figure 7) and heavily disordered solvent molecules. The P-P′ distance of 223.15(5) pm fits well into the range of P-P bonds in diphosphanes R 2 P-PR 2 , for example 221.7 for R = Ph, [30] 224.6 for R = CF 3 , [31] 224.8 for R = C 6 F 5 [32] and 226.0 for R = Mes. [33] The C-P-C angle of 101.12(4)°is well comparable to the ones observed in solid-state structures of other bis[2,4-bis(trifluoromethyl)phenyl)phosphane derivatives. [22,23,26] Like these examples, 6 also exhibits weak P···F contacts between ortho-CF 3 fluorine atoms and the phosphorus atom in a range of 307-313 pm.

Experimental Section
(C 2 F 5 ) 2 PH, [15] (C 6 F 5 ) 2 PH [24] and [2,4-(CF 3 ) 2 C 6 H 3 ] 2 PNEt 2 [22] were synthesized following literature procedures. All other chemicals were obtained from commercial sources and used without further purification. Standard high-vacuum techniques were employed for all preparative procedures. Non-volatile compounds were handled in a dry N 2 atmosphere using Schlenk techniques. NMR spectra were recorded with a Bruker Avance III 300 ( H, 13 C)]. IR spectra were recorded on an ALPHA-FT-IR spectrometer (Bruker Daltonik GmbH, Bremen, Germany) using an ATR unit with a diamond crystal for liquids and solids. Melting and visible decomposition points were determined using a Mettler Toledo MP70-Melting Point System. Elemental analyses were carried out with a HEKAtech Euro EA 3000. Crystal data were collected with a Rigaku Supernova diffractometer with Mo Kα (λ=71.073 pm) radiation at 100.0 K except for 4 which was measured at 250 K. Using Olex2, [34] the structures were solved with the ShelXS [35] structure solution program using Direct Methods and refined with the ShelXL [36 refinement package using Least Squares minimization. Crystals of 6 contained heavily disordered diethyl ether molecules that could not be refined reasonably, so a solvent mask was applied. Details of the X-ray investigation are given in Table 2.