Protonation reactions of the oxo complex cis-[Mo2(η5-C5H5)2(O)(μ-PPh2)2(CO)]. Hydroxo and tetrafluoroborate derivatives

Reactions of the Unsaturated Hydroxo Complex [W2Cp2(OH)(μ-PPh2)2(CO)]BF4 with Mono- and Bidentate Ligands Having E–H bonds (E = O, S, N)

“…This suggests that the reaction involves the initial protonation of the hydroxo ligand to give the 30-electron aquo complex [W 2 Cp 2 (μ-PPh 2 ) 2 (CO)(OH 2 )](BF 4 ) 2 , as proposed to occur in the formation of the tetrafluoroborate complex [Mo 2 Cp 2 (κ 2 -F 2 BF 2 )(μ-PPh 2 ) 2 (CO)]BF 4 . 4 The tetracarbonyl complex 4a could be isolated in pure form after recrystallization of the reaction mixture from acetone/ dichloromethane. The cation in 4a displays symmetry-related PPh 2 and Cp ligands (Table 1 and Experimental Section) and highly energetic C−O stretching bands in the IR spectrum (2085 and 2047 cm −1 ), with frequencies and relative intensities comparable to those reported for the crystallographically characterized thiolato-bridged complex [Mo 2 Cp 2 (μ-S t Bu) 2 (CO) 4 ](BF 4 ) 2 (2095 and 2055 cm −1 ).…”

“…The formation of compounds 4a and 4b only in the presence of large amounts of a strong acid as HBF 4 indicates that the protonation of the hydroxo ligand in 2 takes place only to a very small extent even when using strong acids. This leads us to propose two different pathways for the reactions under consideration, depending on the Bronsted acidity of the added molecules (Scheme 2).…”

“…3 This unsaturated complex turned out to be highly reactive and, in fact, at temperatures above about 263 K it spontaneously rearranges via an unusual O−H oxidative addition to give the coordinatively saturated oxohydride isomer [W 2 Cp 2 (μ-H)(O)-(μ-PPh 2 ) 2 (CO)]BF 4 (3). At the time we also found that 2 could react under mild conditions with acetylacetone to yield the acetonato complex [W 2 Cp 2 (O,O′-acac)(μ-PPh 2 ) 2 (CO)]-BF 4 .…”

“…3 This unsaturated complex turned out to be highly reactive and, in fact, at temperatures above about 263 K it spontaneously rearranges via an unusual O−H oxidative addition to give the coordinatively saturated oxohydride isomer [W 2 Cp 2 (μ-H)(O)-(μ-PPh 2 ) 2 (CO)]BF 4 (3). At the time we also found that 2 could react under mild conditions with acetylacetone to yield the acetonato complex [W 2 Cp 2 (O,O′-acac)(μ-PPh 2 ) 2 (CO)]-BF 4 . 3 The latter reaction proved that the hydroxo ligand in 2 might act as a good leaving group in the presence of molecules of moderate Bronsted acidity, an unusual behavior for an early transition metal hydroxo complex, thus turning complex 2 into a potential precursor of the highly unsaturated 28-electron cation [W 2 Cp 2 (μ-PPh 2 ) 2 (CO)] +2 , with an effective unsaturation equivalent to three vacant positions and six electrons.…”

“…4 Interestingly, the BF 4 − ligand in the latter complex was easily displaced by a great variety of molecules, this including simple donors as well as mono-and bidentate ligands having weakly acidic donor groups common in many biological molecules (OH, SH, NH 2 ), 5 so the complex effectively acts as a synthetic precursor of the corresponding 28-electron cation [Mo 2 Cp 2 (μ-PPh 2 ) 2 (CO)] +2 . From these studies we concluded that the structure of the final product was strongly dependent on the number (one or two) and nature (O, S, N) of the donor centers present in the incoming molecule.…”