An NMR study of some reactions of an arachno-molybdapentaborane: Formation of an unstable nido-molybdapentaborane

Photolysis of the molybdaborane [(η5-C5H5)(η5:η1-C5H4)-arachno-2-MoB4H7] (1) in benzene-d6 gives ca. 60% conversion to the compound [(η5-C5H5)(η5:η1-C5H4)-nido-2-MoB4H5] (2). Compound 2 could not be isolated as a solid and is thermally unstable at 20 °C in solution with a half-life of 3–4 h. Repeated photolysis and thermolysis of 1 in the presence of BH3 · thf gives a low yield of the known metallacarbaborane [(η5-C5H5)(η2:η3-C3H3)-closo-1-MoC2B9H9] (3) suggesting that 3 is formed from 1 via 2. Reaction of 1 with PEt3 gives initially [(η5-C5H5)(η5:η1-C5H4)-arachno-2-MoHB4H4PEt3] (4). Longer reaction times (>10 min, 20 °C) give in addition [(η5-C5H5)(η5:η1-C5H4)-arachno-1-MoHB3H3PEt3] (5). Both 4 and 5 are unstable in solution or the solid state decomposing to the molybdacarbaborane [(η5-C5H5)(η3:η2- C3H3)-nido-1-MoC2B3H5] (6), [Mo(η-C5H5)2H2] and BH3 · PEt3. Compound 1 is deprotonated cleanly by KH in thf at the Mo–H–B bridging proton to give [ ( η 5 - C 5 H 5 ) ( η 5 : η 1 - C 5 H 4 ) - arachno - 2 - MoB 4 H 6 - ] [ K + ] (7).