Crystal structure of a Pd4 carbonyl triphenylphosphane cluster [Pd4(CO)5(PPh3)4]·2C4H8O, comparing solvates

The reported homonuclear Pd4 cluster is presented in relation to structural analogues. Gradual evaporation of the trapped solvent molecules results in a unilateral contraction of the unit cell, transforming it into the solvent-free structure.


Chemical context
Heterometallic compounds are ideal precursors for mixed oxides or mixed-metal nanoparticles, especially when the two considered metals are difficult to alloy. In the case of the Pd-Au combination, a tremendous amount of work has been carried out recently in heterogeneous catalysis to prepare supported bimetallic catalysts with a fine control over composition and size of the supported heterometal nanoparticles (Paalanen et al., 2013). These materials find, for example, application in the direct synthesis of hydrogen peroxide from hydrogen and oxygen (Edwards et al., 2015), or liquid-phase oxidation of alcohols and aldehydes (Villa et al., 2015;Hermans & Devillers, 2005;Hermans et al., 2010Hermans et al., , 2011. However, synthesizing molecular compounds presenting a hetero metal-metal bond is challenging. Several strategies have been described, such as reactions of metal salts in the presence of a reducing agent or reactions under irradiation (favoring formation of metal-metal bonds). In the present work, we explore the reactivity of Au and Pd compounds in a CO atmosphere, with the hope of providing the reducing agent and additional ligands through dissolved carbon monoxide. The direct synthesis of Au-Pd heterometallic complexes has already been achieved using similar strategies, for example starting from [Pd(PPh 3 ) 2 Cl 2 ] and [Au(PPh 3 )NO 3 ] in the presence of NaBH 4 (Ito et al., 1991;Quintilio et al., 1994). One major drawback of this type of strategy is that the product formed is unpredictable, with easy cluster formation by aggregation and homometal bond formation. We have devised in parallel a more predictable synthesis method for Au-Pd compounds by adding a cationic Au fragment to a reduced Pd species (Willocq et al., 2011). Here we describe a homometallic Pd 4 cluster formed by reductive carbonylation and coalescence of a Pd complex in presence of an Au phosphine compound. The reported cluster is closely related to known Pd 4 cluster structures (Willocq et al., 2011;Mednikov et al., 1987;Feltham et al., 1985).

Structural commentary
The structure of the Pd cluster ( Fig. 1) shows internal symmetry and is located on a twofold rotation axis, passing through the central carbonyl, giving four complex molecules in the unit cell (Z 0 = 0.5). Crystallized from a THF/MeOH mixture, the reported structure is a THF solvate, revealing eight tetrahydrofuran molecules in the unit cell. Around the inversion centres, 60 Å cavities are located which were treated by the PLATON (Spek, 2009) SQUEEZE (Spek, 2015) algorithm, accounting to 15 electrons. A single peak, on the special position, was visible in this cavity, which is believed to be the oxygen atom of a partially occupied and disordered MeOH molecule. Partial evaporation of the solvent molecules probably explains the limited resolution of the collected data. Reflection data up to 0.94 Å were used during refinement, this being the best diffracting crystal amongst several tested.
The central unit of the complex consists of four Pd atoms at the corners of a distorted tetrahedron. Of the six edges, five are occupied by bridging carbonyl ligands, the remaining one has a non-bonding PdÁ Á ÁPd distance of 3.170 (1) Å . The bonding Pd-Pd distances are in the range 2.7381 (8)-2.8006 (12) Å ( Table 1). The same compound had been crystallized earlier by our group (Willocq et al., 2011) as a CH 2 Cl 2 solvate in the triclinic space group P1. The molecular geometry of both structures is quite different, the most pronounced difference being the lack of internal symmetry in the P1 structure, which can be extended to the symmetry of the Pd core. The Pd-Pd distances opposite the non-bonding Pd-Pd are very similar, 2.801 (1) and 2.805 (1) Å (P1). Although the average of the four remaining Pd-Pd bond lengths in the two structures is quite similar (2.741 Å for the current structure and 2.746 Å for the triclinic structure), the bond-length distribution is quite different, showing equal bond lengths for the current structure and two shorter [2.678 (1) and 2.720 (1) Å ] and two longer ones [2.797 (1) and 2.790 (1) Å ] for the triclinic structure.
No classical hydrogen-bond interactions are observed, but a weak C-H Á Á ÁO interaction (Table 2) can be considered to the oxygen atom of the THF molecule.

Database survey
A survey of the Cambridge Structural Database (Groom & Allen, 2014) revealed two more occurrences of the title compound, both crystallized in the C2/c space group. In the paper by Mednikov et al. (1987) the homonuclear Pd cluster is reported as a co-former, together with a trinuclear Pd cluster [Pd 3 (CO) 3 (PPh 3 ) 4 ], here as well the Pd cluster is found onto a twofold rotation axis and superposition of both molecular entities reveals similar features, right up to similar orientations of the triphenylphosphines.
The second occurrence is however much more interesting as the structure of Feltham et al. (1985) shows remarkable similarities with the reported structure, not only with respect to the molecular conformation -the r.m.s. deviation between the two structures is 0.757 Å for all atoms, and 0.356 Å when omitting the phenyl rings -but also with respect to the overall crystal packing. Closer inspection of the unit-cell parameters, listed   below, reveals that for both structures only the a axis differs significantly by more than 2 Å (2.297 Å ): a = 27.254 (9) (Feltham et al., 1985).
While the Feltham et al. (1985) structure contains a total of 400 Å 3 of voids distributed over six sites (pore sizes from 5-32 Å 3 ), none of these is big enough to host even small solvent molecules, characterizing this structure as solvent-free. Gradual loss of solvent molecules is believed to provoke a transformation from the solvent-rich title compound to the desolvated structure reported by Feltham et al. (1985). The reported problems during crystal harvesting of the latter structure (see section 4) tends to support this hypothesis. The transformation itself appears to occur in a sequential process where two types of solvent cavities gradually lose their solvent molecules, leading to a contraction of the a axis. The first affected cavities are the 61 Å 3 voids treated by SQUEEZE (Spek, 2015) in the current structure, followed by the cavity hosting the loosely trapped THF molecule (289 Å 3 ). After correcting for the interstitial voids observed in the contracted structure, the volume loss during the transformation is in complete agreement with the solvent loss in both cavities.  Feltham et al. (1985) (all Pd atoms within one unit cell were considered); evaporation of the THF molecules and small rearrangements of the homonuclear cluster allows the transformation of the solvated structure into the solvent-free analogue to be completed. This transformation only involves one dimension and a projection along the a axis of the superimposed unit cells reveals practically fully overlapped molecules, even when considering the orientation of the phenyl rings.

Synthesis and crystallization
The synthesis of the title compound was an attempt to obtain mixed Au-Pd complexes in a one-step reaction. Through a THF solution of [Pd(P t Bu 3 ) 2 ] and [Au(PPh 3 )Cl] carbon monoxide gas was passed and the solid material left after evaporation of the solvent was characterized by NMR and IR spectroscopy. One intense IR band at 1870 cm À1 indicated the formation of a complex with CO ligands. 31 P NMR showed two signals at 28.1 and 97.2 p.p.m. with a 4:1 ratio, which indicate the presence of two types of phosphines, while the 1 H NMR indicated the presence of both triphenylphosphine and tri-tert-butylphosphine. Dissolution of the solid in a THF/ MeOH mixture yielded red crystals which were suitable for X-ray diffraction. Rather than a mixed Au-Pd species, the crystals contained a homonuclear Pd complex.
Previously the synthesis of the title compound was reported as the reduction of an oxygen-free CH 2 Cl 2 solution of 122  Table 2 Hydrogen-bond geometry (Å , ).

Figure 2
Packing overlay of the title compound measured at 120 K and the solvent-free structure of Feltham et al. (1985) measured at room temperature, obtained by pairwise fitting of all Pd atoms of the four molecules in the unit cell. The projection along the b axis reveals that, upon evaporation of the solvent molecules, the unit cell contracts, while keeping the global packing arrangement. Phenyl rings have been omitted for clarity.
[Pd(NO 2 ) 2 (PPh 3 ) 3 ] under CO. Crystals were formed upon cooling after addition of CO-saturated hexane and were reported to decompose rapidly and could finally be measured at room temperature in a CO-filled sealed capillary (Feltham et al., 1985). The homonuclear Pd 4 cluster can also be synthesized by the reaction of [Pd 2 (dba) 3 ] (dba is dibenzylideneacetone) and three equivalents of PPh 3 under CO (Willocq et al., 2011).

Refinement
Crystal data and structure refinement details are summarized in Table 3. Data were collected on a MAR345 image plate, using Mo K radiation generated on a Rigaku UltraX 18S generator (Zr filter). Diffaction data were not corrected for absorption, but the data collection mode with high redundancy, partially takes the absorption phenomena into account.
(111 images, ÁÈ = 2 , 21617 reflections measured for 4740 independent reflections). H atoms were placed at calculated positions with isotropic temperature factors fixed at 1.2U eq of the parent atom.