EPR study of intramolecular dynamics in o-semiquinonic nickel complexes with PCP-pincer ligand in solution

Abstract

A number of square pyramidal o-semiquinonic nickel complexes with 2,6-bis(diphenylphosphinomethyl)phenyl were investigated by EPR spectroscopy in solution. The complexes have flexible coordination spheres and can exist in solution as two isomers rapidly (on the EPR timescale) interconverting one to another at room temperature. The equilibrium is entropy driven and does not significantly depend on the substituents in the semiquinone ring. The spin density distribution depends on mesomeric properties and bulkiness of substituents in ortho-position to oxygen atoms.

Synthesis and Reactivity of (N2P2)Ni Complexes Stabilized by a Diphosphonite Pyridinophane Ligand

“…Similarly, large phosphorus coupling ranging from 18−31 G has been reported for nickel complexes stabilized by PCP pincer ligand, where Kozhanov et al speculates that the phosphorus coupling reflects the amount of contribution from a group metal−pincer orbital to the orbital containing the unpaired electron. 51,52 Therefore, the strong accepting t-BuNC ligand leads to a greater superhyperfine coupling constant versus the OTf or MeCN ligands in 5. The stronger interaction with the P atoms could also lead Table 1.…”

o-Semiquinonato and o-iminosemiquinonato rhodium complexes. EPR study of the reactions in coordination sphere of rhodium

“…One of the mostly interesting our papers was dedicated to carbonyl-osemiquinonato complexes of rhodium [2]. In some cases the nature of o-semiquinone used as spin label has some significance [4]. Analogously to previously reported [2] we have used the number of quinones -the derivatives of 3,6-di-tert-butyl-o-benzoquinone and o-iminoquinone for synthesis of new dicarbonyl rhodium(I) complexes.…”

“…Using of previous experience [1][2][3][4] The EPR spectrum of reaction mixture of complex 5 and dppbiphen is doublet of triplets of multiplets: g i = 2.0012, a i (P) = 7.53 G, a i (P/Rh) ffi 1.8/1.65 G, a i (4H) ffi 0.59 G. Only two coupling constants can be unambiguously attributed: a i (P) = 7.53 G and a i (4H) ffi 0.59 G. Coupling constants 1.8 and 1.65 G are close one to another and it is impossible to attribute each of them to rhodium or to the second phosphorus. Different coupling constants on the phosphorous atoms indicate their different positions: one occupies planar position and the other one -axial.…”

“…o-Semiquinones and o-iminosemiquinones were successfully used as spin labels for investigation of reactions [1,2] and dynamic processes [3,4] occurring in the coordination sphere of metals. One of the mostly interesting our papers was dedicated to carbonyl-osemiquinonato complexes of rhodium [2].…”

Synthesis and Reactivity of (N2P2)Ni Complexes Stabilized by a Novel Diphosphonite Pyridinophane Ligand

“…speculates that the phosphorous coupling reflects the amount of contribution from a group metal-pincer orbital to the orbital containing the unpaired electron. 51,52 Therefore, the strong accepting tBuNC ligand leads to a greater superhyperfine coupling constant vs. the OTf or MeCN ligands in 5. The stronger interaction with the P atoms could also lead to a preferred trigonal bipyramidal geometry for this Ni I species.…”