A modular approach to neutral P,N-ligands: synthesis and coordination chemistry

We report the modular synthesis of three different types of neutral κ2-P,N-ligands comprising an imine and a phosphine binding site. These ligands were reacted with rhodium, iridium and palladium metal precursors and the structures of the resulting complexes were elucidated by means of X-ray crystallography. We observed that subtle changes of the ligand backbone have a significant influence on the binding geometry und coordination properties of these bidentate P,N-donors.


General Information
All manipulations, except those indicated, were carried out under exclusion of air and moisture using standard Schlenk and glove box techniques. As inert gas, Argon 5.0, purchased from Messer Group GmbH, was used after drying over Granusic © phosphorus pentoxide granulate. Solvents were dried over activated alumina columns using a solvent puri cation system (M. Braun SPS 800) or according to standard literature-known methods and stored in glass ampules under an argon atmosphere [1]. Toluene was distilled from sodium, n-pentane from sodium/potassium alloy, tetrahydrofuran, benzene and n-hexane from potassium, and dichloromethane and chloroform from calcium hydride. The same procedures were used to dry the deuterated solvents. Degassed solvents were obtained by three successive freezepump-thaw-cycles. NMR spectra were recorded on Bruker Avance (400 MHz, 600 MHz) instruments. Chemical shifts (δ ) are reported in parts per million (ppm) and are referenced to residual proton solvent signals or carbon resonances [2,3]. H 3 PO 4 ( 31 P) and CCl 3 F ( 19 F) were used as external standards. The following abbreviations were used: s (singlet), d (doublet), dd (doublet of doublets), t (triplet), q (quartet), sept (septet), m (multiplet), br s (broad signal). Highresolution mass spectra were acquired on Bruker ApexQe hybrid 9.4 T FT-ICR (ESI, DART) and JEOL JMS-700 magnetic sector (FAB, EI, LIFDI) spectrometers at the mass spectrometry facility of the Institute of Organic Chemistry, of the University of Heidelberg. Elemental analyses were carried out in the Microanalysis Laboratory of the Heidelberg Chemistry Department on a vario MICRO cube (Elementar). All chemicals were obtained from commercial suppliers and were used without further puri cation. The formamidines 1a-c were prepared according to literature procedures [4][5][6][7][8][9]. Isobutyraldehyde 2,4,6-trimethylphenylimine 4 was synthesized following a standard condensation protocol [10,11]. 2-(Diphenylphosphino)benzaldehyde 6 was obtained commercially from Sigma Aldrich (CAS 50777-76-9). Alternatively, 6 can be synthesized starting from commercially available 2-(2-bromophenyl)-1,3-dioxolane and chlorodiphenylphosphine through a lithiation, nucleophilic substitution, and deprotection sequence [12][13][14].
2 Synthetic Procedures and Analytical Data

Synthesis of Ligands 2a-c and 3a-c
Compounds 2a-c, 3a-c were synthesized following a general procedure. Preparation of Chiral Chlorophosphines: A procedure adapted from Cramer et al. was used [15]. A mixture of (S)-BINOL (10.0 mmol, 1.0 equiv.), freshly distilled PCl 3 (10 mL), and 3 drops of NMP was heated to re ux in 100 mL toluene for 10 min. The reaction mixture was concentrated in vacuo and the residue was distilled twice azeotropically with toluene to give the chiral chlorophosphine in quantitative yield. The product was used in GP 1 without further puri cation.

Synthesis of Ligand 5
This compound was synthesized according to an adapted literature procedure [11]: -78 °C to r.t.

5
Isobutyraldehyde 2,4,6-trimethylphenylimine (2.01 g, 10.6 mmol, 1.0 equiv.) was dissolved in 50 mL of THF and cooled to −78°C. A solution of t-butyl lithium (1.7 M in hexanes, 6.24 mL, 10.6 mmol, 1.0 equiv.) was added dropwise, the mixture was allowed to warm to r.t., and stirred for 1 h at this temperature. A solution of chlorodiphenylphosphine (2.34 g, 1.90 mL, 10.6 mmol, 1.0 equiv.) in 50 mL of THF was cooled to −78°C and the lithiated imine was added dropwise. The mixture was stirred over night and the volatiles were removed under reduced pressure. The residue was dried thoroughly, extracted with toluene and ltered through a plug of Celite © . The clear yellow ltrate was concentrated under reduced pressure, yielding an orange-brown oil (3.17 g, 8.48 mmol, 80 %).

Synthesis of Ligand 7
This compound was synthesized according to a modi ed literature procedure [16][17][18]: 11  in vacuo yielded the product as a yellow solid, which was used in following syntheses without further puri cation (2.60 g, 6.38 mmol, 89 %). General Procedure 2 (GP 2): A solution of the ligand (100 µmol, 1.0 equiv.) in 5 mL of DCM was added to the metal precursor [M]-X (100 µmol, 1.0 equiv.) and the mixture was stirred for 30 minutes. At this point, the product was either isolated by layering with toluene and pentane yielding the desired neutral product or AgBF 4 (100 µmol, 1.0 equiv.) was added to produce the cationic derivative. The suspension was then stirred in the dark for another 30 minutes, the solid residue was ltered o and the ltrate was layered with toluene and pentane, and stored at −40°C. This procedure yielded a powder or in several cases single crystals suitable for X-ray di raction. The solid was then washed with pentane and dried under high vacuum for several days to remove residual solvent.

Synthesis of Metal Complexes
General Procedure 3 (GP 3): A solution of the ligand (100 µmol, 1.0 equiv.) in 5 mL DCM was added to the metal precursor [M]-BF 4 (100 µmol, 1.0 equiv.). The mixture was stirred for 30 minutes, ltered, layered with toluene and pentane and stored at −40°C. This procedure yielded a powder or in several cases single crystals suitable for X-ray di raction. The solid was then washed with pentane and dried under high vacuum for several days to remove residual solvent.

VT-NMR Studies
Compound [2a-PdCl] 2 (BF 4 ) 2 is dimeric in the solid state, with two chlorides bridging the cationic palladium centers. Its 31 P-NMR spectrum in CD 2 Cl 2 at room temperature features a single, broad resonance, whereas in CDCl 3 solution, three broad signals were found. To clarify these ndings, a variable-temperature NMR study of this compound in dichloromethane was conducted, revealing that at low temperatures three species can be distinguished in solution (Figure 1). This is in line with a solvent-dependent equilibrium between dimeric and monomeric solvated T-shaped stereoisomers, although an additional stabilizing coordination of the BF 4 anion is also possible [20].

X-Ray Crystal Structure Determinations
Crystal data and details of the structure determinations are compiled in Tables 1-4. Full shells of intensity data were collected at low temperature with Agilent Technologies Supernova E (Mo-or Cu-K α radiation, microfocus X-ray tube, multilayer mirror optics) or Bruker AXS Smart 1000 (Mo-K α radiation, sealed X-ray tube, graphite monochromator) CCD di ractometers. Data were corrected for air and detector absorption, Lorentz and polarization e ects [21][22][23]; absorption by the crystal was treated with a semiempirical multiscan method (data collected with the Bruker instrument) [24][25][26] or numerically (data collected with the Agilent instrument, Gaussian grid) [21,22,27]. For datasets collected with the microfocus tube(s) an illumination correction was performed [28,29]. The structures were solved by intrinsic phasing (for [2b-Ir(cod)]BF 4 · 0.5 CH 2 Cl 2 · C 7 H 8 ) [30][31][32], by direct methods with dual-space recycling (for [2a-Ir(cod)]BF 4 · CH 2 Cl 2 ) [33,34], by the heavy atom method combined with structure expansion by direct methods applied to di erence structure factors (for [7-Rh(cod)]BF 4 · CH 2 Cl 2 ) [35,36], or by the charge ip procedure (all other structures) [37,38]. Re nement was carried out by full-matrix least squares methods based on F 2 against all unique re ections [39][40][41]. All non-hydrogen atoms were given anisotropic displacement parameters. Hydrogen atoms were generally input at calculated positions and re ned with a riding model. When justi ed by the quality of the data, the positions of some hydrogen atoms were taken from di erence Fourier synthesis and re ned. When found necessary, disordered groups and/or solvent molecules were subjected to suitable geometry and adp restraints. The two independent complex cations in the structures of [2b-M(cod)]BF 4 · 0.5 CH 2 Cl 2 · 0.5 C 7 H 8 (M = Rh, Ir) are related by a pseudosymmetry translation. The symmetry is however broken by the toluene solvent molecule. Due to severe disorder and/or fractional occupancy, electron density attributed to solvent of crystallization was removed from the structures of [2a-Cp*IrI]BF 4 · 1.5 CH 2 Cl 2 and [7-Rh(cod)]BF 4 · 1.x CH 2 Cl 2 with the BYPASS procedure [42,43], as implemented in PLATON (SQUEEZE) [44,45]. Partial structure factors from the solvent masks were included in the re nement as separate contributions to F obs .