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Hydroxo Complex with Amino- Boranes: Formation of Boroxide Derivatives Antonio Antiñolo,†,* Miguel A

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Hydroxo Complex with Amino- Boranes: Formation of Boroxide Derivatives Antonio Antiñolo,†,* Miguel A Reactions of an Osmium(IV)-Hydroxo Complex with Amino- Boranes: Formation of Boroxide Derivatives Antonio Antiñolo,†,* Miguel A. Esteruelas,‡,* Cristina García-Yebra,‡,§ Jaime Martín,‡ Enrique Oñate,‡ and Alberto Ramos† †Departamento de Química Inorgánica, Orgánica y Bioquímica-Centro de Innovación en Química Avanzada (ORFEO- CINQA), Universidad de Castilla-La Mancha, Campus Universitario, E-13071 Ciudad Real, Spain ‡Departamento de Química Inorgánica, Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), Centro de Innovación en Química Avanzada (ORFEO-CINQA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain ABSTRACT: The discovery of a reaction, which allows to prepare boroxide complexes of platinum group metals and to study 3 their behavior under CO atmosphere, is described. The trihydride-osmium(IV)-hydroxo complex OsH3(OH){κ -P,O,P- i i i [xant(P Pr2)2]} (1, xant(P Pr2)2 = 4,5-bis(diisopropylphosphino)xanthene) reacts with the amino-boranes Pr(H)NBCy2 and i 3 i Pr(H)NBBN to give the osmium(IV)-boroxide derivatives OsH3(OBR2){κ -P,O,P-[xant(P Pr2)2]} (BR2 = BCy2 (2), BBN (3); BBN i = 9-borabicyclo[3.5.1]nonane) and PrNH2, as a consequence of the addition of the O-H bond of the hydroxo ligand of 1 to the B-N bond of the amino-boranes. At room temperature, under CO atmosphere, complexes 2 and 3 eliminate H2 to afford the osmium(II)- 2 i 2 boroxide compounds OsH(OBR2)(CO)2{κ -P,P-[xant(P Pr2)2]} (BR2 = BCy2 (4), BBN (5)) bearing a κ -P,P-coordinated ether- diphosphine. The subsequent reductive elimination of the borinic acids R2BOH needs heating and long time and leads to the tricar- 2 i bonyl-osmium(0) derivative Os(CO)3{κ -P,P-[xant(P Pr2)2]} (6), with the phosphorus atoms of the diphosphine lying in the equato- rial plane of a pentagonal bypiramid of donor atoms around the metal center. In contrast to 2 and 3, under CO atmosphere, precur- 3 i sor 1 eliminates water to initially give the trans-dihydride OsH2(CO){κ -P,O,P-[xant(P Pr2)2]} (7), which subsequently evolves to 2 i the cis-dihydride-cis-dicarbonyl derivative OsH2(CO)2{κ -P,P-[xant(P Pr2)2]} (8) and finally into the tricarbonyl 6. aldehydes to [OsH(OH)(CPh)(IPr)(PiPr )]OTf (IPr = 1,3- INTRODUCTION 3 bis(2,6-diisopropylphenyl)imidazolylidene, OTf = CF3SO3) Hydroxide complexes of platinum group metals1 are a rare which affords the corresponding carboxylate derivatives and and particularly fascinating group of weak hydroxoacids, molecular hydrogen (Scheme 2).10 which have an oxo derivative as a conjugate base and are at Scheme 2. Reactions of an Osmium-hydroxo Complex with the same time the strong conjugate base of an aquo-complex Aldehydes (Scheme 1). The exciting nature of these species is not corre- sponded with a rich chemistry, which is by contrast underde- veloped, in particular for osmium. In addition to a few di- mers,2 the mononuclear compounds of this element include: nitride complexes,3 half-sandwich species,4 and hydride- hydroxo derivatives.5 The latter are particularly challenging since the reductive elimination of water is generally favored from a thermodynamic point of view.6 Compounds of this class are generally osmium(II)-derivatives and only very re- Boroxide groups have been proposed as promising alterna- cently a hydride-osmium(IV)-hydroxo complex has been iso- tive to the alkoxide ligands, in order to better modulate the 5f lated. In agreement with the marked ability of pincer ligands electronic and steric properties of catalyst precursors of inter- 7 to stabilize uncommon species, it contains the POP group 4,5- esting reactions of organic synthesis.11 The empty p-orbital on i 8 bis(diisopropylphosphino)xanthene (xant(P Pr2)2). the boron allows electron donation from the oxygen lone-lone Scheme 1. Brønsted-Lowry Equilibria for a Metal Hy- pairs, which gives rise to a poorer electron donating oxygen droxoacid group. In addition, although the boroxide ligand bears twice as many carbon substituents as an alkoxide group, the presence of an additional space converts it into sterically less demand- ing.12 Boroxide compounds of metal of s and p blocks13 and complexes of transition elements of all groups11,14 have been The previously mentioned duality of the hydroxide com- reported. Nevertheless, one could think that there is a “borox- plexes is expressed in the respective nucleophilicity and elec- ide wall” around the platinum group metals since the species trophilicity of the oxygen and hydrogen atoms of the hydroxo of these elements are unknown until now. Two main proce- group in the scarce performed reactions,9 as the addition of dures have been employed to generate this type of compounds: reactions of a metal halide with a lithium boroxide and proto- atom of the diphosphine, which is situated between the borox- nolysis of metal-carbon, -nitrogen, or –oxygen bonds with a ide O1 atom and the hydride H01 (O1-Os-O2 = 81.89(10)º). borinic acid12 (a and b in Scheme 3). The osmium-boroxide distance of 2.089(3) Å is long. It com- Scheme 3. Synthetic procedures to prepare boroxide com- pares well with the higher part of the range of the reported 16 pounds osmium(IV)-alkoxide bond lengths (1.90-2.10 Å), whereas the Os-O1-B1 angle of 137.9(3)º significantly deviates from the expected value for sp2 hybridized oxygen. This angle is about 20º larger than the Os-S-B angle in the borothiolate 2 i 17 compounds OsH(SBR2)(η -H2)(CO)(P Pr3)2 (113-119º). This is consistent with that observed by Braunschweig and cowork- ers for the M-chalcogene-B angle, in the borachalcogene com- t plexes Cp(CO)2Mn[EB Bu(IMe)] (E = S, Se, Te), which in- creases by going up in the group; i.e., in the sequence Te < Se < S.18 However, in contrast to the Braunschweig’s compounds the O1-B bond length of 1.332(6) Å reveals a short chalco- gene-boron single bond.19 The DFT optimized structure (Fig- Boron shows higher affinity for oxygen than for nitrogen, ure 1b) confirms the trihydride character of the OsH3 unit, which is consistent with the higher dissociation energy of the although the calculated separations between H01 and H02 and B-O bonds with regard to the B-N bonds in tricoordinate bo- between H02 and H03 of 1.57 and 1.64 Å, respectively, sug- ron-species.15 This, along with the oxygen nucleophilicity and gest the presence of weak nonclassical interactions between 20 the hydrogen electrophilicity of the hydroxo ligand in hydrox- the hydride ligands. 1 ide complexes, prompted us to try the formation of boroxide The H NMR spectra of 2 and 3 in toluene-d8 also support species of a platinum group metal by means of the displace- the presence of weak nonclassical interactions between the ment of the amino group of amino-boranes by the conjugate hydride ligands. At room temperature they display only one base of a hydroxoacid (c in Scheme 3), in reactions that re- resonance (δ1H, ≈ -13), indicating that are involved in two semble that shown in scheme 2. Here, we report a novel pro- thermally activated site exchange processes. At temperatures cedure which allowed us to isolate the first boroxide complex- lower than 223 K, three signals are however observed (δ1H, - es of a platinum group metal. 11.60, -11.71, -13.49 (2); -11.63, -11.75, -13.61 (3)) in agree- ment with the structures shown in Figure 1. Two of them form RESULTS AND DISCUSSION an AB spin system with a large cis JAB coupling constant of Osmium(IV)-Boroxide Derivatives: Formation and 46.9 Hz for 2 and 98.5 Hz for 3 at 193 K, which slightly de- Characterization. Metal ions in high oxidation state form creases as the temperature increases, as expected for quantum- strong bonds with oxygen, increasing the electrophilicity of mechanical exchange coupling between the involved hy- 21 i 31 1 the hydrogen atom of the hydroxo ligand. This fact led us to drides. According to equivalent P Pr2 groups, the P{ H} 3 select the osmium(IV) complex OsH3(OH){κ -P,O,P- NMR spectra contain a singlet at 44.2 ppm for 2 and 45.6 ppm i 11 [xant(P Pr2)2]} (1) as starting point to achieve our goal. Treat- for 3, which is temperature invariant. In the B spectra, the ment of toluene solutions of 1 with 1.0 equiv of the amino- boroxide ligands give rise to a broad resonance centered at i i boranes Pr(H)NBCy2 and Pr(H)NBBN (BBN = 9- 48.3 ppm for 2 and at 50.6 ppm for 3. borabicyclo[3.3.1]nonane), at room temperature, for 30 min Osmium(II)-Boroxide Derivatives: Reactions with CO. produces the release of isopropylamine and the formation of Complexes 2 and 3 are not only novel examples of boroxide 3 the desired osmium(IV)-boroxide derivatives OsH3(OBR2){κ - derivatives but also notable polyhydrides bearing a monoden- i P,O,P-[xant(P Pr2)2]} (BR2 = BCy2 (2), BBN (3)), which were tate monoanionic oxygen-donor ligand.20 The stability of the isolated as pale yellow solids in 50-65% yield (Scheme 4). H-Os-OBR2 unit is certainly noticeable. In this context, it Scheme 4. Formation of Boroxide Complexes 2 and 3 should be pointed out that the reductive elimination of mo- lecular hydrogen is kinetically favored with regard to the re- lease of the borinic acid.
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