Half-Sandwich Arene-Osmium(II) Complexes with Phosphinite Ligands Half-Sandwich Arene-Osmium(II) Complexes with Phosphinite Ligands

: The synthesis of a series of arene-osmium(II) complexes containing phosphinite-type ligands, namely, [OsCl 2 ( η 6 - p -cymene){R 2 PO(CH 2 ) n Ph}] (R = Ph, n = 1 ( 4a ), 2 ( 4b ), 3 ( 4c ); R = i Pr, n = 1 ( 5a ), 2 ( 5b ), 3 ( 5c )) and [OsCl 2 ( η 6 -benzene){ i Pr 2 PO(CH 2 ) 2 Ph}] ( 7 ), is presented. All these compounds were characterized by elemental analysis and multinuclear NMR spectroscopy ( 31 P{ 1 H}, 1 H and 13 C{ 1 H}), and the structure of [OsCl 2 ( η 6 - p -cymene){Ph 2 PO(CH 2 ) 3 Ph}] ( 4c ) unequivocally conﬁrmed through a single-crystal X-ray di ﬀ raction study. Attempts to generate the tethered species [OsCl 2 { η 6 : κ 1 ( P )-C 6 H 5 (CH 2 ) n OPR 2 }] by intramolecular exchange of the coordinated arene in 4-5a-c or 7 , upon thermal or MW heating, failed. Abstract: The synthesis of a series of arene-osmium(II) complexes containing phosphinite-type ligands, namely, [OsCl 2 ( η 6 - p -cymene){R 2 PO(CH 2 ) n Ph}] (R = Ph, n = 1 ( 4a ), 2 ( 4b ), 3 ( 4c ); R = i Pr, n = 1 ( 5a ), 2 ( 5b ), 3 ( 5c )) and [OsCl 2 ( η 6 -benzene){ i Pr 2 PO(CH 2 ) 2 Ph}] ( 7 ), is presented. All these compounds were characterized by elemental analysis and multinuclear NMR spectroscopy ( 31 P{ 1 H}, 1 H and 13 C{ 1 H}), and the structure of [OsCl 2 ( η 6 - p -cymene){Ph 2 PO(CH 2 ) 3 Ph}] ( 4c ) unequivocally confirmed through a single-crystal X-ray diffraction study. Attempts to generate the tethered species [OsCl 2 { η 6 : κ 1 ( P )-C 6 H 5 (CH 2 ) n OPR 2 }] by intramolecular exchange of the coordinated arene in 4-5a-c or 7 , upon thermal or MW heating, failed.


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These compounds are of interest as they represent rare examples of tethered arene-ruthenium(II) complexes incorporating pendant phosphinite donors [2][3][4], a field largely dominated by the use of arene ligands featuring classical phosphines as the pendant donor group [1,5]. Moreover, we also demonstrated their utility as catalysts for the cross dehydrogenative coupling of hydrosilanes with alcohols [1], a process of relevance since it allows a simple access to useful alkoxysilane reagents [6,7], and with potential application in the field of hydrogen storage and production [8]. As shown in Scheme 1, the synthesis of complexes [RuCl 2 {η 6 :κ 1 (P)-C 6 H 5 (CH 2 ) n OPR 2 }] was accomplished in two steps involving the initial cleavage of the chloride bridges of [{RuCl(µ-Cl)(η 6 -arene)} 2 ] by the phosphinites, to generate the corresponding mononuclear adducts [RuCl 2 (η 6 -arene){R 2 PO(CH 2 ) n Ph}], and a subsequent intramolecular exchange of the coordinated arene under thermal conditions (120 • C) [1].
On the other hand, tethered osmium complexes are uncommon, and most of the examples currently known involve η 5 -cyclopentadienyl-type ligands connected to P- [9,10], N- [10], C- [11], or Sn-donor [12] groups. In fact, the only tethered η 6 -arene-osmium(II) derivatives described to date in the literature are compounds B, synthesized by Xia and co-workers from the reactions of the hydride-alkenylcarbyne complex A with allenoates (Scheme 2) [13].
Once characterized, compounds 4-5a-c were next evaluated as potential precursors of the corresponding tethered species [OsCl 2 {η 6 :κ 1 (P)-C 6 H 5 (CH 2 ) n OPR 2 }]. Against our wishes, heating solutions of 4-5a-c in 1,2-dichloethane (DCE) or toluene at 120 • C did not lead to the intramolecular exchange of the coordinated arene, even after long reaction periods (24 h). In all the cases, complexes 4-5a-c were recovered unchanged. Application of microwaves irradiation (MW), instead of conventional oil-bath thermal heating, was also unsuccessful. In view of this, we decided to explore the possibility of using related osmium(II) precursors containing benzene instead of p-cymene, as, in our previous work with ruthenium, the displacement of the η 6 -coordinated benzene ligand proceeded in general much faster than that of the p-cymene one (Scheme 1) [1]. To this end, complex [OsCl 2 (η 6 -benzene){ i Pr 2 PO(CH 2 ) 2 Ph}] (7) was synthesized by reacting dimer [{OsCl(µ-Cl)(η 6 -benzene)} 2 ] (6) with i Pr 2 PO(CH 2 ) 2 Ph (2b) (see Scheme 4). Due to the poor solubility of 6, the bridge-splitting reaction required in this case harsher conditions (refluxing in toluene for 24 h), and 7 could only be isolated in moderate yield (synthetic details and characterization data are included in the Materials and Methods section). Unfortunately, all attempts to generate [OsCl 2 {η 6 :κ 1 (P)-C 6 H 5 (CH 2 ) 2 OP i Pr 2 }] from 7, upon thermal or MW heating, also failed.  7)).
Once characterized, compounds 4-5a-c were next evaluated as potential precursors of the corresponding tethered species [OsCl2{η 6 :κ 1 (P)-C6H5(CH2)nOPR2}]. Against our wishes, heating solutions of 4-5a-c in 1,2-dichloethane (DCE) or toluene at 120 °C did not lead to the intramolecular exchange of the coordinated arene, even after long reaction periods (24 h). In all the cases, complexes 4-5a-c were recovered unchanged. Application of microwaves irradiation (MW), instead of conventional oil-bath thermal heating, was also unsuccessful. In view of this, we decided to explore the possibility of using related osmium(II) precursors containing benzene instead of p-cymene, as, in our previous work with ruthenium, the displacement of the η 6 -coordinated benzene ligand proceeded in general much faster than that of the p-cymene one (Scheme 1) [1]. To this end, complex [OsCl2(η 6 -benzene){ i Pr2PO(CH2)2Ph}] (7) was synthesized by reacting dimer [{OsCl(μ-Cl)(η 6 -benzene)}2] (6) with i Pr2PO(CH2)2Ph (2b) (see Scheme 4). Due to the poor solubility of 6, the bridge-splitting reaction required in this case harsher conditions (refluxing in toluene for 24 h), and 7 could only be isolated in moderate yield (synthetic details and characterization data are included in the Materials and Methods section). Unfortunately, all attempts to generate [OsCl2{η 6 :κ 1 (P)-C6H5(CH2)2OP i Pr2}] from 7, upon thermal or MW heating, also failed. The marked differences in reactivity found between the osmium complexes herein synthesized, i.e., 4-5a-c and 7, and their ruthenium counterparts can be ascribed to the higher activation energies associated to ligand substitution processes in osmium vs. ruthenium complexes, a fact well documented in the literature [16][17][18].

Materials and Methods
All manipulations were carried out under an inert atmosphere of dry argon employing  The marked differences in reactivity found between the osmium complexes herein synthesized, i.e., 4-5a-c and 7, and their ruthenium counterparts can be ascribed to the higher activation energies associated to ligand substitution processes in osmium vs. ruthenium complexes, a fact well documented in the literature [16][17][18].

Materials and Methods
All manipulations were carried out under an inert atmosphere of dry argon employing vacuum-line and Schlenk techniques. Solvents were dried and purified before use, according to standard procedures [19]. The phosphinite ligands 1-2a-c [20,21] and the osmium(II) dimers 3 [22] and 6 [23] were synthesized as described in the literature. A Bruker DPX-300 instrument (Billerica, MA, USA) was employed for NMR measurements (all the spectra were recorded at room temperature). For the 13 C{ 1 H} and 1 H NMR chemical shifts, the residual signal of deuterated solvent was employed as reference, while for the 31 P{ 1 H} NMR ones 85% H 3 PO 4 was used as external standard. DEPT experiments were carried out for all the compounds synthesized. Elemental analyses were provided by the Analytical Service of the Instituto de Investigaciones Químicas (IIQ-CSIC) of Seville using a LECO TruSpec CHN analyzer (St. Joseph, MI, USA).

X-ray Crystal Structure Determination of Compound 4c
Crystals of 4c suitable for X-ray diffraction analysis were obtained by slow diffusion of hexane into a saturated solution of the complex in dichloromethane. The most relevant crystal and refinement data are collected in Table 2. Diffraction data were recorded on an Oxford Diffraction Xcalibur Nova single-crystal diffractometer using Cu-Kα radiation (λ = 1.5418 Å). Images were collected at a fixed crystal-to-detector distance of 62 mm using the oscillation method with 1.10 • oscillation and 1.25-2.5 s variable exposure time per image. Data collection strategy was calculated with the program CrysAlis Pro CCD [24]. Data reduction and cell refinement were performed with the program CrysAlis Pro RED [24], and an empirical absorption correction was applied by means of a SCALE3 ABSPACK algorithm as implemented in the program CrysAlis Pro RED [24]. The software package WINGX was used for space group determination, structure solution, and refinement [25]. The structure was solved by Paterson interpretation and phase expansion using DIRDIF2008 [26]. Isotropic least-squares refinement on F 2 using SHELXL97 was performed [27].
During the final stages of the refinements, all the positional parameters and the anisotropic temperature factors of all non-H atoms were refined except C(8), C(9A), C(9B), C(10A), and C(10B), because the isopropyl group of the p-cymene ligand was found in two disordered positions (69% for the major position and 31% for the minor one). The distances from C(9A), C(9B), C(10A), and C(10B) to C (8) were fixed at 1.50 Å (by comparison with other structures previously described [14,15]). All H atoms were geometrically located and their coordinates were refined riding on their parent atoms. The function from counting statistics and P = (Max (F o 2 ,0) + 2F c 2 )/3. Atomic scattering factors were taken from the International Tables for X-ray Crystallography [28]. Geometrical calculations related to the centroid C* were made with PARST [29]. CCDC-1972510 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk).