All-metal aromatic cationic palladium triangles can mimic aromatic donor ligands with Lewis acidic cations

Replacing aromatic donor ligands with cationic Pd3 + complexes.


General Remarks
Disulfide, phosphines and Pd(dba) 2 were purchased from commercial sources and used as received. Solvents were degassed by bubbling argon for at least 30 minutes prior to use.
Reactions and filtrations were carried out under argon using standard Schlenk technique. 1 H NMR, 13 C NMR, and 31 P NMR spectra were recorded in acetone-d 6 at 300 K on a Bruker 500 AVANCE spectrometer fitted with a BBFO probe head at 500, 125, and 202 MHz respectively, using the solvent acetone-d 6 as internal standard (2.05 ppm for 1 H NMR and 29.84 ppm for 13 C NMR, respectively). For 31 P, H 3 PO 4 was used as external standards. 19 F NMR spectra were recorded in acetone-d 6 at 300 K on a Bruker 300 AVANCE spectrometer fitted with a BBFO probe head at 282 MHz. Reported assignments are based on COSY, decoupling, HMBC, HSQC, NOESY and ROESY correlation experiments. The terms m, s, d, t, and q represent multiplet, singlet, doublet, triplet and quadruplet respectively, and the term br means a broad signal. Exact masses were recorded on an Agilent Q-TOF 6540 spectrometer (electrospray source). IR spectra were recorded with a Bruker Tensor 27 ATR diamant PIKE spectrometer and UV-visible spectra were recorded on a Shimadzu UV-2101 spectrophotometer.
The structure was solved with the ShelXT 2a structure solution program using Direct Methods and refined with the ShelXL 2b refinement package using Least Squares minimisation. All non-hydrogen atoms were refined with anisotropic displacement parameters and H atoms have been added geometrically and treated as riding on their parent atoms. Due to a disorder on triflate anion CF 3  in compound (1), its position was refined over two orientations using FVAR variable (occupancy factor: 0.657(4)/0.343 (4)). "Idealized Molecular Geometry Library" 3 was used for modelling this triflate anion, inserted with FRAG command in ShelXL. Rigid body restrains were applied along the entire connectivity set of complexes (2-SbF 6 ) and (2-BF 4 ) leading to more reasonable anisotropic displacement parameters, using standard deviation values: sigma for 1-2 distances of 0.004 and sigma for 1-3 distances of 0.004. Some large electron peaks due to solvent CHCl 3 molecules were found during refinement of complex (2-BF 4 ). As we failed to model them properly, the rest of the molecule was refined without the effect of the solvent molecule(s) by the PLATON SQUEEZE technique. More details including comprehensive table of crystallographic information and summary of X-ray diffraction analysis are presented in the dedicated part of this document.
Calculations were performed with Gaussian 09 at DFT level. 4 The geometries of all complexes reported herein were optimized without any symmetry constraints at the generalized gradient approximation using the Minnesota family of hybrid functionals described by Zhao and Truhlar. 5 Optimizations were carried out using Def2-svp 6  LACVP(d), 7 and RSC97 8 basis sets were also tested; they provided the same results describing delocalized molecular orbitals among metal centers but a slightly lower correlation with solid state structures found by X-ray. Single point calculations were perfomerd at the MP2 level 9 and using double hybrid B2PLYP functional 10 to exclude that different Hartree-Fock contribution could provide meaningful differences in calculated molecular orbitals.
Harmonic frequencies were calculated at the M06/Def2-svp level to characterize optimized Angstoms). Free optimization of tetranuclear complexes using Def2-TZVP basis set for metal atoms (Li, Pd, Ag and/or Au) provided minimal differences in structures (within 0.004 of lone pairs (for Pd, Ag and Au) did not change significantly compared to double-z functionals. This is consistent with previous results from our group of all-metal aromatic M 3 + complexes (see references 11 of the main article) and is likely due to the spatial contraction of d-type atomic orbitals of late transition metals compared to early ones, which often prevents the population of multiple metal-metal bonds. 11 Gaussian09 was used to obtain both canonical molecular orbitals and natural ones (NBO). The latter were used for AdNDP analysis through its dedicated software. 12 obtained by vapor diffusion using CHCl 3 /hexane. Crystals of 2-BF 4 were analyzed by 1 H, 13 C, 31 P and 19 F NMR, UV-vis., IR and ESI + -Tof HRMS.
Reaction of 1-CF 3 CO 2 with AgCF 3 CO 2 CF 3 CO 2 Ag (0.0444 mmol, 4 equiv.) was added to a solution of compound 1-CF 3 CO 2 (0.0111 mmol, 1 equiv.) in 5 mL of CHCl 3 under Ar. The deep red solution was put in the dark.
Stirring was maintained for 1 hour and the mixture was then filtered through a short pad of Celite under Ar. The solvent was removed under vacuum to leave a deep red solid. Then the compound was purified by recrystallization by vapor diffusion using THF/hexane. Crystals were then analyzed by by 1 H, 13 C, 31 P and 19 F NMR, UV-vis., IR and ESI + -Tof HRMS.

Reaction of 1-SbF 6 -PPh 3 with PPh 3 AuCl
PPh 3 AuCl (0.0667 mmol, 1 equiv.) and AgSbF 6 (0.0667 mmol, 1 equiv.) were added to freshly degassed CHCl 3 (10 mL) and the mixture stirred for 1 hour in the dark. After filtration through Celite to remove AgCl, the solvent was evaporated to dryness under vacuum, the residue was dissolved in THF (10 mL) and the resulting solution was canulated to a solution of 1-SbF 6 -PPh 3 (0.0667 mmol, 1 equiv.) in 10 mL of THF under Ar. The resulting solution was put in the dark. Stirring was maintained for 1 hour and the mixture was then filtered through a short pad of Celite under Ar. Evaporation of solvents under vacuum afforded an orange solid. The compound was purified by recrystallization by vapor diffusion using THF/hexane, although the quality of crystals did not allow to perform RX. Crystals were then analyzed by 1 H, 13 C, 31 P and 19 F NMR, UV-vis., IR and ESI + -Tof HRMS.

Reaction of 1-OTf with (CF 3 SO 3 Cu) 2 •PhCH 3
(CF 3 SO 3 Cu) 2 •PhCH 3 (0.0222 mmol, 4 equiv.) was added to a solution of compound 1-OTf (0.0111 mmol, 1 equiv.) in 5 mL of CHCl 3 under Ar. The deep red solution was put in the dark. Stirring was maintained for 1 hour and the mixture was then filtered through a short pad of Celite under Ar. The solvent was removed under vacuum to leave a deep red solid. The compound was purified by recrystallization by vapor diffusion using THF/hexane, although the quality of crystals did not allow to perform RX. Crystals were then analyzed by 1 H, 13 C, 31 P and 19 F NMR, UV-vis., IR and ESI + -Tof HRMS.   Cluster 2-SbF 6 13

Computational analyses Li + complexations
Initial structure of the adduct between the all-metal aromatic Pt 3 cation (fragment optimized at the M06/Def2-svp) and Li + (below) and optimized structure (next page). Li + has been initially put 6 A above the trimetallic core and approaches it center throughout the process. The graph plots the calculated energy during the optimization process, presenting the stabilizing contribution of Li + binding by the aromatic Pt 3 cluster (a dummy atom has been put in the center of the triangle to show clearly the distance).
Energy decreases while Li + approaches the trimetallic core during optimization (without any constrain).

Top and side view of the HOMO of the optimized [Pd 3 -Li] 2+ adduct.
Compared to the HOMO of cation 1, this MO is elongated towards the alkali metal.

Top and side view of the HOMO of the optimized [Pd 2 Pt-Li] 2+ adduct.
Compared to the HOMO of cation 1 with 1 Pt and 2 Pd nuclei, this MO is elongated towards the alkali metal.

Top and side view of the HOMO of the optimized [PdPt 2 -Li] 2+ adduct.
Compared to the HOMO of cation 1 with 2 Pt and 1 Pd nuclei, this MO is elongated towards the alkali metal.

Top and side view of the HOMO of the optimized [Pt 3 -Li] 2+ adduct.
Compared to the HOMO of cation 1 with 3 Pt nuclei, this MO is elongated towards the alkali metal.
In each case, the delocalized sigmoid MO that makes these clusters d-orbital aromatic elongates towards Li + to complex the alkali cation. This looks as the same type of bonding interaction observed modeling a regular aromatics as benzene instead of all-metal aromatic and heteroaromatic frameworks.
Optimized structure of the Li +benzene complex (Li + -C 6 H 6 = 1.97292 Angstrom, the red dot is a dummy atom put in the center of the aromatic ring).

Table of NICS values obtained from GIAO magnetic shielding tensors
Negative values confirm the presence of cyclic electron delocalization in all cases. Those of the Pd 3 face of Pd 3 Li ++ parallel that of bare Pd 3 complex (first two columns). Values calculated from a Pd 2 Li face are significantly different. They are still negative and decrease linearly at higher distances from the pyramid (third column). The trend is opposite regarding Au 4 ++ complex (last two columns). Each face provides identical absolute values (last two columns). Furthermore, the trend presents two flex points in the 0 to 5 Angstrom region, in striking contrast with values of Pd 3 M ++ complexes. This combines with experimental results (from NMR and X-ray) and other modeling techniques to strengthen the rationale for two distinct bonding mode among these tetranuclear species. Calculation with double-and triple-z basis sits gave comparable results and identical trends.

1-OTf (CCDC 1410442)
Ortep of triangular cluster (top) highlighting Pd core and CF 3 SO 3 anion. In the lower caption, -C 6 H 4 F groups on phosphorous atoms were omitted for clarity (down). Hydrogen atoms omitted for clarity. Thermal ellipsoids are shown at 50% probability.