Bis(6-diphenylphosphino-acenaphth-5-yl)sulfoxide: A New Ligand for Late Transition Metal Complexes

The synthesis of the new ligand bis(6-diphenylphosphinoacenaphth-5-yl)sulfoxide, [6-(Ph2P)-5-Ace-6]2-SO (1), is presented along with six transition metal complexes thereof, namely, 1·MCl (M = Rh, Cu, Ag, Au) and 1·MCl2 (M = Ni, Pd). Within these novel complexes, close metal-sulfur distances are


Introduction
Diorganosulfoxides, such as Me 2 SO (DMSO), are frequently encountered ligands in transition metal complexes. [1] For the majority of transition metals, the coordination occurs via the hard oxygen atom, whereas in a smaller number of complexes the softer sulfur atom is involved in the coordination to the transition metal. [2] In general, the harder 3d-transition metals prefer coordination by oxygen, whereas the softer 4d-and 5d-transition metals have a higher affinity to sulfur. The preference of the coordination mode was recently addressed in a DFT study on DMSO complexes of transition metals including natural bond orbital (NBO) analyses and energy decomposition analyses (EDA). [2] The preferred coordination mode in sulfoxides may be influenced by multidentate ligands that force the metal into a desired spatial arrangement. In 1996, Mok et al. reported bis[(2-diphenylphosphino)ethyl]sulfoxide (I) as well as two complexes with Pd and Pt (Scheme 1). [3] In 2012, Suess and Peters described a variation of I, namely bis(2-diphenylphospinophenyl)sulfoxide (II) and a number of complexes with Rh, Ir, Ni, Pd, and Pt (Scheme 1). [4] In all of these complexes, the flanking observed and the nature of the M-S coordination, as well as the response of the + S-Obond, are investigated in detail with a set of spectroscopic, crystallographic and real-space bonding indicators. P atoms within the ligands I and II enforce a P,S,P-coordination of the transition metal. Our interest in ligands based upon (ace-)naphthyl scaffolds [5] prompted us to prepare a similar ligand with a different bite angle in this study, namely, bis-(6-diphenylphosphinoacenaphth-5-yl)sulfoxide (1), which was used to prepare the complexes 1·MCl (M = Rh, Cu, Ag, Au) and 1·MCl 2 (M = Ni, Pd), which all show pronounced P,S,P-coordination. The nature of the M-S and M-P bonds as well as the effect of the coordination on the bipolar + S-Obond was analyzed using a set of real-space bonding indicators (RSBIs). Similar joint experimental-theoretical studies on the nature of the S-O bond have shown that the sulfur atom is never hypervalent so that the bipolar character of the bond dominates over double-bonding contributions. [6] However, a certain degree of π-bonding contributions can be present via negative hyperconjugation, but never exceeding total bond orders of 1.0 for S-O bonds. [7] (Scheme 2). Complexation with one equivalent of Wilkinson's catalyst (Ph 3 P) 3 RhCl leads to the formation of the Rh(I) complex 1·RhCl in 77 % yield. Notably, other suitable Rh-sources are (cod)RhCl (cod = cylcooctadiene) or (Ph 3 P) 2 Rh(CO)Cl leading to the formation of 1·RhCl in 81 % and 69 % yield, respectively. Coordination of the sulfoxide ligand 1 with nickel(II) chloride hexahydrate and palladium(II) chloride gave rise to the formation of the transition metal complexes 1·NiCl 2 and 1·PdCl 2 in 67 % and 61 % yield, respectively. It should be noted that the analogous reaction with various Pt sources, such as PtCl 2 , (MeCN) 2 PtCl 2 , or [(C 2 H 5 ) 2 S] 2 PtCl 2 , was not successful and only starting materials could be isolated, which is in sharp contrast to the behaviour of the known parent ligands I and II. The reaction of 1 with the coinage metal precursors copper(I) chloride, silver(I) chloride, and tetrahydrothiophene gold(I) chloride gave the complexes 1·MCl (M = Cu, Ag, Au) in yields of 46 %, 60 %, and 38 %, respectively (Scheme 2). Scheme 2. Synthesis of sulfoxide ligand 1 and complexation towards transition metal complexes 1·RhCl, 1·NiCl 2 , 1·PdCl 2 , and 1·MCl (M = Cu, Ag, Au).
All newly synthesized transition metal complexes show poor solubility in common organic solvents such as dichloromethane, tetrahydrofuran, diethyl ether, and n-hexane. The 31 P{ 1 H}-NMR signal of 1·RhCl consists of two sets of doublets of doublets centered at 18.7 ppm and 16.0 ppm, respectively, giving rise to coupling constants of 1 J( 103 Rh-31 P) = 136.9 and 137.2 Hz as well as of 2 J( 31 P-31 P) = 350.0 Hz (averaged). The Pd complex 1·PdCl 2 gives rise to two sets of doublets with a 2 J( 31 P-31 P) coupling constant of 420.9 Hz. The Cu complex 1·CuCl shows two broad singlets at -11.2 and -12.3 ppm. Notably, in the coinage metal complexes 1·MCl (M = Ag, Au), the P-atoms are chemically equivalent in solution and show only one signal in the 31 P{ 1 H}-NMR spectra with an unresolved 1 J( 107/109 Ag-31 P) coupling constant of 374.0 Hz in case of 1·AgCl.

Molecular Structures
The molecular structures derived by single-crystal X-ray diffraction measurements of 1 as well as of the complexes 1·MCl     [4] Coinage metal complexes containing a SO functionality are significantly less studied and, to the best of our knowledge, only a single study of a Cu complex coordinating via Cu-S(O) interactions is reported to date but with considerably longer Cu-S distances of 2.707(1) Å to 2.833(1) Å. [9] The short Rh-S [2.134(1) Å] and long S-O [1.485(1) Å] bond lengths in 1·RhCl agree with the respective values found by Suess and Peters for which also a strong Rh-S(O) π-backbonding interaction was proposed. [4] The more positively charged Ni and Pd metals (vide infra) give rise to rather polar M-S(O) interactions with simultaneous electron donation from oxygen to sulfur. [4] Interestingly, the M-S and S-O bond lengths for the group 10 complexes 1·NiCl 2

Real-Space Bonding Analysis
The electronic characteristics of the M-S (M = Rh, Ni, Pd, Cu, Ag, Au) interactions are investigated by a variety of topological, surface, and integrated real-space bonding indicators (RSBIs) derived from the calculated electron densities (EDs) and pair densities. To this end, density functional theory (DFT) computations were conducted starting with the crystallographic geometries to give optimized isolated-molecule geometries. Bond topological parameters of the M-S and S-O bonds obtained from the Atoms-In-Molecules (AIM) [10] space-partitioning scheme and from the Electron Localizability Indicator (ELI-D) [11] method, which provides basins of paired electrons, are collected in Table 2. The Raub-Jansen Index (RJI), [12] combining AIM and ELI-D, which is particularly useful to analyze (polar-)covalent and dative bonds, are also given in Table 2. The corresponding AIM atomic and fragmental charges are given in Table 3. In addition, the AIM bond topologies and ELI-D (iso-)surface representations of compounds 1·RhCl, 1·CuCl, and 1·AgCl are displayed in Figure 3, Figure 4, and Figure 5 together with isosurface representations according to the Non-Covalent Interactions Index (NCI) [13] approach, which uncovers (extended) For all bonds, ρ(r) bcp is the electron density at the bond critical point, ∇ 2 ρ(r) bcp is the corresponding Laplacian, G/ρ(r) bcp and H/ρ(r) bcp are the kinetic and total energy density over ρ(r) bcp   Characteristically, they form small ELI-D SO bonding basins of 1.3 to 1.4 Å 3 containing the small number of 1.38 to 1.44 e. According to the RJI, 66.2 to 67.6 % of these electrons are located within the AIM O atomic basin. These values are typical for highly polar covalent bonds, where the S-O bond was found to exhibit even some charge-shift character, underlining the importance of the + S-Oresonance form. [7a] The much longer S-C bonds are predominantly covalent with a negative Laplacian and with |H/ρ(r) bcp | > G/ρ(r) bcp , being close to homopolar as indicated by RJI values of 49.2 to 55.2 %. Compared to the short and strong S-O bonds, the M-P and M-Cl interactions are longer and weaker with slightly dominating ionic bond contributions (Tables S2 and S3). The M-S bonds, however, differ considerably within the series. Among the investigated the complexes, 1·RhCl shows the highest electron density at the M-S bcp [ρ(r) bcp = 0.91 e Å -3 ] pointing to a strong Rh-(SO) π-backbonding interaction ( Table 2). In contrast, 1·NiCl 2 ,  Figure 5b). For the M-S bonds, no NCI basin is observed in 1·RhCl and 1·PdCl 2 , whereas a ringshaped and red-colored basin is observed for 1·NiCl 2 and 1·CuCl. In contrast, a disc-shaped and blue-colored NCI basin is observed for 1·AgCl and 1·AuCl, supporting the weak noncovalent bonding character of the Ag-S, and Au-S bonds. The spatial requirements of the ELI-D M-S/LP(S) basins are displayed in Figure 3d, Figure 4d, Figure 5d with the ELI-D distribution mapped on the basin's surface. In 1·RhCl, 1·NiCl 2 , 1·PdCl 2 , and 1·CuCl, the basins are located between the M and S atoms and typically show inwardly curved surfaces in direction of the atoms forming the bond (less pronounced in 1·PdCl 2 and 1·CuCl). In contrast, the LP(S) basins in 1·AgCl and 1·AuCl are not located on the M-S axis and show significantly outwardly curved basin shapes towards the direction of the metal atom, almost not affecting the trigonal-planar P 2 MCl arrangement. The charge transfer between molecular fragments becomes visible by inspection of AIM atomic and fragmental charges of a series of related compounds (Table 3 and S4). Metallation of 1 results in charge loss of the organic acenaphthyl and phenyl ligands as well of the P and S atoms, since all MCl n fragments are negatively charged. This effect is strongest for 1·NiCl 2 and

Experimental Section
General Information: Chemicals and solvents were obtained commercially (e.g. Sigma Aldrich) and were used unchanged. Dry solvents were collected from a SPS800 mBraun solvent system. The starting material 5-bromo-6-diphenylphosphinoacenaphthene was synthesized according to the literature procedures. [14] The corresponding 1 H-, 13