Highly Reactive Scandium Phosphinoalkylidene Complex: C–H and H–H Bonds Activation

Abstract

The first scandium phosphinoalkylidene complex was synthesized and structurally characterized. The complex has the shortest Sc-C bond lengths reported to date (2.089(3) Å). DFT calculations reveal the presence of a three center π interaction in the complex. This scandium phosphinoalkylidene complex undergoes intermolecular C-H bond activation of pyridine, 4-dimethylamino pyridine and 1,3-dimethylpyrazole at room temperature. Furthermore, the complex rapidly activates H under mild conditions. DFT calculations also demonstrate that the C-H activation of 1,3-dimethylpyrazole is selective for thermodynamic reasons and the relatively slow reaction is due to the need of fully breaking the chelating effect of the phosphino group to undergo the reaction whereas this is not the case for H.

Silyl‐Phosphino‐Carbene Complexes of Uranium(IV)

“…Supporting this latter point, the P−C carbene bond orders average 1.20, reflecting the aforementioned mixing by negative hyperconjugation. So, some 3‐centre U−C−P π‐topology is found in 3 – 5 , however the U=C double bonds in 3 – 5 with U=C bond orders about 1.5 times the P−C bond orders contrast to the more delocalised 3‐centre Sc−C−P π‐bonding scenario in II where the situation is reversed with the C−P bond order about 1.6 times the Sc=C bond order 7. Thus, the bonding situation in 3 – 5 is closer to the localised one found in [Ta(CHPMe 2 )(η 5 ‐C 5 Me 5 ) 2 (PMe 3 )]8f than in II 7.…”

“…So, some 3‐centre U−C−P π‐topology is found in 3 – 5 , however the U=C double bonds in 3 – 5 with U=C bond orders about 1.5 times the P−C bond orders contrast to the more delocalised 3‐centre Sc−C−P π‐bonding scenario in II where the situation is reversed with the C−P bond order about 1.6 times the Sc=C bond order 7. Thus, the bonding situation in 3 – 5 is closer to the localised one found in [Ta(CHPMe 2 )(η 5 ‐C 5 Me 5 ) 2 (PMe 3 )]8f than in II 7. This underscores the key, dominant role of uranium stabilisation of the carbenes in 3 – 5 that is also rather different to the situation found in related free carbenes such as Me 3 SiCP(NPr i 2 ) 2 18…”

“…Experimentally, it is interesting to note that addition of DMAP to 3 only forms the DMAP adduct 5 , whereas addition of DMAP to II 7 results in rapid C−H activation of DMAP. The coordination of two DMAP molecules in 5 suggests that there are no steric barriers, and thus the lack of DMAP C−H activation by 3 experimentally supports the notion that the U=C carbene bonds in 3 – 5 are more covalent, and thus less reactive units than that in II 7.…”

The high reactive site and the unusually short Sc‐C bond of the scandium phosphinoalkylidene complex, an explanation from first‐principles calculation

“…These novel RE metal complexes show rich chemical reactivity, for example activating alkene, nitrile, isonitrile, PhCHO, and so on . Among these complexes, the RE metal alkylidene/carbene complexes rose puzzling questions concerning the chemical bonding of Sc‐C for their unusually short Sc‐C bond lengths (2.044‐2.113 Å) as determined by X‐ray diffraction method . On the other hand, the high reactivity of the first scandium phosphinoalkylidene complex [L{ScC(SiMe 3 )PPh 2 }THF] (LScPT) with a new β‐diketiminato‐based bidentate nitrogen ligand (L = [MeC(NDIPP)CHC(NDIPP)Me]) is also interesting in synthetic chemistry because it can activate pyridine, 4‐dimethylamino pyridine, 1, 3‐dimethylpyrazole, and H 2 under mild conditions.…”

“…The earlier theoretical work focused on the possible reaction paths for the C‐H and H‐H bonds activation, while left the important high reactivity of LScPT and the unusually short Sc‐C bond unexplained. In addition, a three center π interaction between Sc, C, and P atoms was proposed for the HOMO without atomic orbital resolved quantitative analyses …”