Article Cite This: Organometallics XXXX, XXX, XXX−XXX pubs.acs.org/Organometallics 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 Inorganica,́ Organicá y Bioquímica-Centro de Innovacioń en Química Avanzada (ORFEO−CINQA), Universidad de Castilla-La Mancha, Campus Universitario, E-13071 Ciudad Real, Spain ‡ Departamento de Química Inorganica,́ Instituto de Síntesis Química y Catalisiś Homogeneá (ISQCH), Centro de Innovacioneń Química Avanzada (ORFEO−CINQA), Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain *S Supporting Information ABSTRACT: The discovery of a reaction which allows preparation of boroxide complexes of platinum group metals and study of their behavior under CO atmosphere is described. κ3 The trihydride-osmium(IV)-hydroxo complex OsH3(OH){ - i i P,O,P-[xant(P Pr2 ) 2 ]} (1,xant(PPr2 ) 2 =4,5-bis- (diisopropylphosphino)xanthene) reacts with the amino-bor- i i anes Pr(H)NBCy2 and Pr(H)NBBN to give the osmium(IV)- κ3 i boroxide derivatives OsH3(OBR2){ -P,O,P-[xant(P Pr2)2]} (BR2 = BCy2 (2), BBN (3); BBN = 9-borabicyclo[3.5.1]- i − − 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 ff − amino-boranes. At room temperature under CO atmosphere, complexes 2 and 3 eliminate H2 to a ord the osmium(II) κ2 i κ2 boroxide compounds OsH(OBR2)(CO)2{ -P,P-[xant(P Pr2)2]} (BR2 = BCy2 (4), BBN (5)) bearing a -P,P-coordinated ether-diphosphine. The subsequent reductive elimination of the borinic acids R2BOH needs heating and a long duration and κ2 i leads to the tricarbonyl-osmium(0) derivative Os(CO)3{ -P,P-[xant(P Pr2)2]} (6) with the phosphorus atoms of the Organometallics diphosphine lying in the equatorial plane of a pentagonal bypyramid of donor atoms around the metal center. In contrast to 2 κ3 and 3, under CO atmosphere, precursor 1 eliminates water to initially give the trans-dihydride OsH2(CO){ -P,O,P- i κ2 [xant(P Pr2)2]} (7), which subsequently evolves to the cis-dihydride-cis-dicarbonyl derivative OsH2(CO)2{ -P,P-[xant- i fi (P Pr2)2]} (8) and nally into the tricarbonyl 6. ■ INTRODUCTION uncommon species,7 it contains the POP group 4,5-bis- 1 i 8 Hydroxide complexes of platinum group metals are a rare and (diisopropylphosphino)xanthene (xant(P Pr2)2). particularly fascinating group of weak hydroxoacids, which The previously mentioned duality of the hydroxide have an oxo derivative as a conjugate base, and are at the same complexes is expressed in the respective nucleophilicity and time the strong conjugate base of an aquo-complex (Scheme electrophilicity of the oxygen and hydrogen atoms of the 1). The exciting nature of these species does not correspond to 9 Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 11/22/18. For personal use only. hydroxo group in the scarcely performed reactions, as the addition of aldehydes to [OsH(OH)(CPh)(IPr)(PiPr )]OTf Scheme 1. Brønsted−Lowry Equilibria for a Metal 3 Hydroxoacid (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene, OTf = ff CF3SO3)whichaords the corresponding carboxylate derivatives and molecular hydrogen (Scheme 2).10 Scheme 2. Reactions of an Osmium-Hydroxo Complex with a rich chemistry, which is by contrast underdeveloped, in Aldehydes particular for osmium. In addition to a few dimers,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 because 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 recently was isolated a hydride-osmium(IV)-hydroxo complex.5f In agree- Received: October 8, 2018 ment with the marked ability of pincer ligands to stabilize © XXXX American Chemical Society A DOI: 10.1021/acs.organomet.8b00727 Organometallics XXXX, XXX, XXX−XXX Organometallics Article Boroxide groups have been proposed as promising Scheme 4. Formation of Boroxide Complexes 2 and 3 alternative to the alkoxide ligands to better modulate the electronic and steric properties of catalyst precursors of interesting reactions of organic synthesis.11 The empty p- orbital on the boron allows electron donation from the oxygen lone−lone pairs, which gives rise to a poorer electron donating oxygen 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 to be sterically less 12 demanding. Boroxide compounds of metal of s and p respectively. The coordination geometry around the metal 13 11,14 blocks and complexes of transition elements of all groups center can be rationalized as a distorted pentagonal bipyramid have been reported. Nevertheless, one could think that there is with the boroxide group and the hydride ligands lying in the a “boroxide wall” around the platinum group metals because perpendicular plane to the P−Os−P direction along with the the species of these elements are unknown until now. Two oxygen atom of the diphosphine, which is situated between the main procedures have been employed to generate this type of boroxide O1 atom and the hydride H01 (O1−Os−O2 = compound: reactions of a metal halide with a lithium boroxide 81.89(10)°). The osmium−boroxide distance of 2.089(3) Å is and protonolysis of metal−carbon, −nitrogen, or −oxygen long. It compares well with the higher part of the range of the bonds with a borinic acid12 (a and b in Scheme 3). reported osmium(IV)−alkoxide bond lengths (1.90−2.10 Å),16 whereas the Os−O1−B1 angle of 137.9(3)° significantly Scheme 3. Synthetic Procedures to Prepare Boroxide deviates from the expected value for sp2 hybridized oxygen. Compounds This angle is about 20° larger than the Os−S−B angle in the η2 i borothiolate compounds OsH(SBR2)( -H2)(CO)(P Pr3)2 (113−119°).17 This is consistent with that observed by Braunschweig and co-workers for the M−chalcogene−B t angle, in the borachalcogene complexes Cp(CO)2Mn[EB Bu- (IMe)] (E = S, Se, Te), which increases 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 chalcogene−boron single bond.19 The DFT optimized structure (Figure 1b) confirms the trihydride character of the OsH unit, although the calculated ffi 3 Boron shows higher a nity for oxygen than for nitrogen, separations between H01 and H02 and between H02 and H03 which is consistent with the higher dissociation energy of the of 1.57 and 1.64 Å, respectively, suggest the presence of weak − − 20 B O bonds with regard to the B N bonds in tricoordinate nonclassical interactions between the hydride ligands. boron species.15 This, along with the oxygen nucleophilicity 1 The H NMR spectra of 2 and 3 in toluene-d8 also support and the hydrogen electrophilicity of the hydroxo ligand in the presence of weak nonclassical interactions between the hydroxide complexes, prompted us to try the formation of hydride ligands. At room temperature, they display only one boroxide species of a platinum group metal by means of the δ ≈− resonance ( 1H 13), indicating that they are involved in displacement of the amino group of amino-boranes by the two thermally activated site exchange processes. At temper- conjugate base of a hydroxoacid (c in Scheme 3), in reactions atures lower than 223 K, three signals are however observed that resemble that shown in Scheme 2. Here, we report a novel (δ −11.60, −11.71, −13.49 (2); −11.63, −11.75, −13.61 fi 1H procedure which allowed us to isolate the rst boroxide (3)) in agreement with the structures shown in Figure 1. Two complexes of a platinum group metal. of them form an AB spin system with a large cis JAB coupling constant of 46.9 Hz for 2 and 98.5 Hz for 3 at 193 K, which ■ RESULTS AND DISCUSSION slightly decreases as the temperature increases, as expected for Osmium(IV)−Boroxide Derivatives: Formation and quantum−mechanical exchange coupling between the involved 21 i 31 1 Characterization. Metal ions in high oxidation state form hydrides. According to equivalent P Pr2 groups, the P{ H} strong bonds with oxygen, increasing the electrophilicity of the NMR spectra contain a singlet at 44.2 ppm for 2 and 45.6 ppm hydrogen atom of the hydroxo ligand. This fact led us to select for 3, which is temperature invariant. In the 11B spectra, the κ3 the osmium(IV) complex OsH3(OH){ -P,O,P-[xant- boroxide ligands give rise to a broad resonance centered at i (P Pr2)2]} (1) as starting point to achieve our goal. Treatment 48.3 ppm for 2 and at 50.6 ppm for 3. of toluene solutions of 1 with 1.0 equiv of the amino-boranes Osmium(II)−Boroxide Derivatives: Reactions with i i Pr(H)NBCy2 and Pr(H)NBBN (BBN = 9-borabicyclo[3.3.1]- CO. Complexes 2 and 3 are not only novel examples of nonane) at room temperature for 30 min produces the release boroxide derivatives but also notable polyhydrides bearing a of isopropylamine and the formation of the desired osmium- monodentate monoanionic oxygen-donor ligand.20 The κ3 − − (IV)-boroxide derivatives OsH3(OBR2){ -P,O,P-[xant- stability of the H Os OBR2 unit is certainly noticeable.