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.