Synthesis and versatile reactivity of scandium phosphinophosphinidene complexes
Due to the energy mismatching between the frontier orbitals of rare-earth metal and ligand atom, the rare-earth metal-phosphorus multiple bonds are very unstable. This Behind introduces our group’s struggling trip to the first rare-earth monometallic phosphinidene complex during the past 10 years.
We reported a bis(µ-phosphinidene)dineodymium diiodide in 2008.1 We tried to synthesize the neodymium terminal phosphinidene complex from a salt metathesis of this iodide with potassium hydrotris(pyrazolyl)borate, the special sharp of hydrotris(pyrazolyl)borate ligands could probably break the bimetallic structure. But we obtained bis(µ-phosphinidene)dineodymium dihydrotris(pyrazolyl)borate and a C–H bond activation product of dihydrotris(pyrazolyl)borate.2 Shortly after that, we obtained the first scandium terminal imido complex by using a β-diketiminato-based tridentate ligand developed by ourselves. Therefore we studied the synthesis of rare-earth terminal phosphinidene complexes with the β-diketiminato-based tridentate ligands, but we only obtained several bis(µ-phosphinidene)discandium complexes and bis(µ-phosphinidene)dilutetium complexes although different β-diketiminato-based tridentate ligands and phosphinidene ligand precursors with different aryl substituents were used.3 As the pendant amino group of the tridentate ligands does not coordinate to the metal center in some complexes, we studied the situation when a bulky β-diketiminato bidentate ligand is used and obtained a discandium complex bearing both µ-phosphinidene and µ-methylidene ligands.4 We also tried to synthesize the rare-earth metal terminal phosphinidene complex by using a rigid and bulky tridentate ligand, we obtained an unexpected scandium 2,3-azaphosphametallacyclobutane complex via a C=N bond cleavage of the tridentate ligand.5
In 2017, we decided to use phosphinophosphinidene ligand, and obtained scandium phosphinophosphinidene complexes 1 and 2 in early 2018. Complexes 1 and 2 are monometallic, and bonding analysis indicates the presence of a Sc=Pα double bond and a weak Sc-Pβ donor-acceptor interaction. Complex 1 shows a peculiar reactivity (reaction at the least nucleophilic site Pβ), meanwhile complex 2 presents a normal reactivity at the most nucleophilic phosphorus site (Pα). This intriguing difference of reactivity was then rationalized using DFT calculations, which demonstrate that the abnormal reactivity of 1 is induced by the strong coordination of the THF molecule. Although the phosphinophosphinidene ligand is 2η-bonded, these results are the very important progress in the rare-earth metal-ligand multiple bonding chemistry on considering the difficulty in taming the rare-earth metal-phosphorus multiple bonds. The details of this work can be found at https://www.nature.com/articles/s41467-020-16773-w. The next step of the research is to obtain the more synthetically challenging rare-earth metal complexes containing terminal-bounded phosphinidene.
- Cui, P., Chen, Y. F., Xu, X. & Sun, J. An unprecedented lanthanide phosphinidene halide: synthesis, structure and reactivity. Chem. Commun. 5547-5549 (2008).
- Cui, P., Chen, Y. F. & Borzov, M. V. Neodymium(III) phosphinidene complexes supported by pentamethylcyclopentadienyl and hydrotris(pyrazolyl)borate ligands. Dalton Trans. 39, 6886-6890 (2010).
- Lv, Y. D. et al. Versatile reactivity of a four-coordinate scandium phosphinidene complex: reduction, addition, and CO activation reactions. J. Am. Chem. Soc. 135, 14784-14796 (2013).
- Zhou, J. L., Li, T. F., Maron, L., Leng, X. B. & Chen, Y. F. A scandium complex bearing both methylidene and phosphinidene ligands: synthesis, structure, and reactivity. Organometallics 34, 470-476 (2015).
- Zhou, J. L., Xiang, L., Guo, J. J., Leng, X. B. & Chen, Y. F. Formation and Reactivity of a C-P-N-Sc Four-Membered Ring: H2, O2, CO, Phenylsilane, and Pinacolborane Activation. Chem. Eur. J. 23, 5424-5428 (2017).