A chromium tricarbonyl complex featuring the 4,6-bis(diphenylphosphinomethyl)dibenzothiophene (PSPPh) ligand

Abstract The new PSP pincer ligand 4,6-bis(diphenylphosphinomethyl)dibenzothiophene (PSPPh) was prepared in 89 % yield. With this ligand, a solvothermal synthesis of a Cr complex of the type [Cr(κ3P,S,P-PSP)(CO)3] is described. The X-ray structure of this compound is presented. We demonstrate that the solvothermal synthesis technique provides a powerful, simple, and practical synthetic method resulting in a high isolated yield in a short reaction time. Graphical abstract Electronic supplementary material The online version of this article (doi:10.1007/s00706-016-1707-9) contains supplementary material, which is available to authorized users.


Introduction
Among the many ligand systems that can be found in the chemical literature pincer ligands play an important role and their complexes have attracted tremendous interest due to their high stability, activity, and variability [1][2][3][4][5]. Pincer ligands are often planar scaffolds consisting of an anionic or neutral central aromatic backbone tethered to two, mostly bulky, two-electron donor groups by different spacers where steric, electronic, and stereochemical parameters can be manipulated by modifications of the substituents at the donor sites and/or the spacers. Phosphine-based PCP and PNP type ligands having central C and N donors have received the most attention. Accordingly, many applications of mostly precious second and third row transition metal pincer complexes in the fields of catalysis, molecular recognition, and supramolecular chemistry were discovered turning this area into an intensively investigated subject in organometallic chemistry.
In the present contribution we report on the synthesis and characterization of a new PSP pincer ligand based on dibenzothiophene, and describe a simple solvothermal synthesis of a chromium tricarbonyl complex bearing this ligand. It has to be noted that tridentate bis-phosphine ligands with a central S donor (PSP ligands) are extremely rare [6].
In order to obtain group 6 tricarbonyl complexes with the new PSP pincer ligand 3, a simple and fast solvothermal approach with no need for a microwave equipment was chosen, which we developed recently for the synthesis of zero valent Cr, Mo, and W complexes [M(PNP)(CO) 3 ] with PNP pincer ligands based on the 2,6-diaminopyridine scaffold [10]. Accordingly, a suspension of hexacarbonyl complexes [M(CO) 6 ] and 3 in CH 3 CN were placed in a sealed microwave glass tube and stirred for 6 h at 140°C. From these three precursors only [Cr(CO) 6 ] underwent the desired reaction, while in the case of molybdenum and tungsten surprisingly only intractable materials were recovered. After workup, the analytically pure complex [Cr(j 3 P,S,P-PSP Ph )(CO) 3 ] (4) was obtained in 89 % isolated yield (Scheme 2). This complex is air sensitive both in solution and in the solid state.
Complex 4 was fully characterized by a combination of 1 H and 31 P{ 1 H} NMR spectroscopy, IR spectroscopy, ESI MS, and elemental analysis. Due to the poor solubility of this complex a useful 13 C{ 1 H} NMR spectrum could not be obtained. The 31 P{ 1 H} NMR spectra exhibit singlet resonances at 79.3 ppm (cf. -17.3 ppm in the free ligand 3). The IR spectrum exhibits two strong bands at 1820 and 1850 cm -1 assignable to the symmetric and the two superimposed strong asymmetric m CO stretching modes. In addition the molecular structure of 4 was determined by X-ray crystallography. A structural view is depicted in Fig. 1 with selected bond distances and angles given in the caption. The coordination geometry around the chromium center corresponds to a slightly distorted octahedron with the PSP ligand coordinated in the typical meridional j 3 P,S,P bonding mode. The P1-Cr1-P2, S1-Cr1-C2, and C1-Cr1-C3 angles deviate from 180°being 174.82(1)°, 176.52(5)°, and 176.79(6)°, respectively. As expected, the Cr-C distances of the CO ligands trans to one another are slightly longer (1.891(1) and 1.888(1) Å ) than the one trans to the thiophene moiety (1.834(1) Å ) due the strong trans influence of the CO ligand.
Since ESI-MS enables not only the detection and the study of reaction substrates and products but also shortlived reaction intermediates and decomposition products as they are present in solution,  [M] ?Á is compared with the theoretical pattern, which turned out to correlate quite well. In sum, the first benzothiophene PSP pincer ligand was prepared. The Cr(0) tricarbonyl complex [Cr(j 3 P,S,P-PSP Ph )(CO) 3 ] was synthesized via a solvothermal reaction and was fully characterized by a combination of 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy, IR spectroscopy, ESI MS, and elemental analysis.

Experimental
All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques or in a MBraun inertgas glovebox. 4,6-Bis(hydroxymethyl)dibenzothiophene (1) was prepared according to the literature [11,12]. The solvents were purified according to standard procedures [13]. The deuterated solvents were purchased from Aldrich and dried over 4 Å molecular sieves. 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectra were recorded on a Bruker AVANCE-250 spectrometer operating at 250.13, 62.86, and 101.26 MHz. 1 H and 13 C{ 1 H} NMR spectra were referenced internally to residual protio-solvent, and solvent resonances, respectively, and are reported relative to tetramethylsilane (d = 0 ppm). 31 P{ 1 H} NMR spectra were referenced externally to H 3 PO 4 (85 %) (d = 0 ppm). As reaction vessel 20 cm 3 microwave vials from Biotage or VWR with an aluminium septum cap were used.

X-ray structure determination
Crystals of 4 of good optical quality were pre-selected, embedded in perfluorinated polyether and mounted on MiTeGen MicroLoops (CCDC 1446055). X-ray diffraction data were measured using xand u-scans at T = 100 K on a Bruker APEX-II diffractometer with Mo-Ka radiation. The collection strategy for the measurement was optimized with the APEX-2 software [14] to result in a data set of the complete reciprocal sphere up to high angles and with high completeness. After integration of the data with the program SAINT [14], an absorption correction based on the semi-empirical ''multi-scan'' approach was performed with the SADABS program [14]. The crystal structure was solved by direct methods and was refined using the SHELXTL program package [15]. All H atoms were placed geometrically and refined in the riding model approximation, with C-H = 0.95 Å for aromatic H atoms (C-H = 0.99 Å for methylene H atoms) and with U iso (-H) = 1.2 U eq (C). The crystal contained acetonitrile solvent molecules disordered around an inversion centre. In the final model, the occupancy of each atom of the acetonitrile solvent molecule was constrained to 0.5. All non-hydrogen atoms were refined anisotropically. The methyl H atoms of the solvent molecule were not modelled but are included in the formula of the compound. Molecular graphics were generated with the program MERCURY [16].
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