Reactions of an Osmium(IV)-Hydroxo Complex with Amino-Boranes: Formation of Boroxide Derivatives

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. The trihydride-osmium(IV)-hydroxo complex OsH3(OH){κ3-P,O,P-[xant(PiPr2)2]} (1, xant(PiPr2)2 = 4,5-bis(diisopropylphosphino)xanthene) reacts with the amino-boranes iPr(H)NBCy2 and iPr(H)NBBN to give the osmium(IV)-boroxide derivatives OsH3(OBR2){κ3-P,O,P-[xant(PiPr2)2]} (BR2 = BCy2 (2), BBN (3); BBN = 9-borabicyclo[3.5.1]nonane) and iPrNH2 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)–boroxide compounds OsH(OBR2)(CO)2{κ2-P,P-[xant(PiPr2)2]} (BR2 = BCy2 (4), BBN (5)) bearing a κ2-P,P-coordinated ether-diphosphine. The subsequent reductive elimination of the borinic acids R2BOH needs heating and a long duration and leads to the tricarbonyl-osmium(0) derivative Os(CO)3{κ2-P,P-[xant(PiPr2)2]} (6) with the phosphorus atoms of the diphosphine lying in the equatorial plane of a pentagonal bypyramid of donor atoms around the metal center. In contrast to 2 and 3, under CO atmosphere, precursor 1 eliminates water to initially give the trans-dihydride OsH2(CO){κ3-P,O,P-[xant(PiPr2)2]} (7), which subsequently evolves to the cis-dihydride-cis-dicarbonyl derivative OsH2(CO)2{κ2-P,P-[xant(PiPr2)2]} (8) and finally into the tricarbonyl 6.
Financial support from the MINECO of Spain (Projects CTQ2017-82935-P, CTQ2016-77614-P and Red de Excelencia Consolider CTQ2016-81797-REDC), the Diputación General de Aragón (E06_17R), FEDER, and the European Social Fund is acknowledged. A.R. acknowledges a postdoctoral contract funded by the “Plan Propio de I + D + i” of the Universidad de Castilla-La Mancha.