Paramagnetic Planar Complexes of Ni^II: Influence of R and R′ on the Formation, Coordination Geometry and Magnetic Properties of Phosphinic Amidato Bischelates [R₂P(O)NR′]₂Ni

Abstract

(0)NR']-the donor quality necessary for bischelation as well as the singular field strength required for the formation of planar paramagnetic species. As the combination of paramagnetism and planarity is bound to the solid state, lattice energy effects may be finally decisive for this unique constellation of tetracoordinated Ni" complexes.Some time ago we have reported on paramagnetic phosphinic amidato bischelates [tBu2P(O)NR'I2Ni (la: R' = iPr[l]; lb: R' = Cy, olive-green and ink-blue isomer; lc: R' = tBu[']). Crystal structure determination showed l a and l b (green) to be planar and l b (blue) to be tetrahedral. As the electronic spectrum of l c (reflectance spectrum) was very similar to the solid-state spectra of l a and l b (green) it was concluded that l c was also planar.All the compounds 1 mentioned above show a broad absorption between 6500-8000 cn-', 3 , , ,being shifted from 7300-7500 cm-' in the case of the planar species to 6600 cm-' with the tetrahedral complex l b (blue). However, their solution spectra proved to be almost superimposable and nearly identical with the powder spectrum of tetrahedral lb. So it was assumed that these compounds are predominantly tetrahedral in solution.While l a and l b (green and blue) show pen values of 3.3-3.4 pB (solid state, room temperature) and Curie-Weiss behavior between 280-90 K (solid state) the magnetic moment of solid l c ranges from 1.7 to 0.6 pB (280-80 K).Thus, diamagnetism and lack of ligand-field bands <10000 ~m -' [~] turned out to be no longer essential features of planar Nil' complexes.By extended angular overlap model calculations it was possible to explain the ligand-field spectra of the planar complexes l a and l b (green) and to interpret the triplet ground states of both as well as a singlet ground state with l c that is followed by an electronic level of slightly higher energy accessible by thermal energy kTf41. Now we investigated the influence of R and R' on the formation, coordination geometry and magnetic properties of chelate complexes of general type 1 more systematically. For this purpose we prepared a series of new phosphinic amides R2P(0)NR'H with very different substituents R and R'C51 that may be regarded in deprotonated form as potential ligands in complexes of type 1. Some of the results are now reported.Bischelate Complexes [tBu2P(0)NR'l2Ni Id-g Preparation la-c were obtained by a metathesis reaction between tBu2P(0)NR'Li and Ni(PPh3)2C12. However, it proved to be somewhat troublesome to separate the PPh3 set free in this reaction from the complex by fractional crystallization because of similar solubilities. In later experiments we found that this problem can be avoided when Ni(dme)Br2 is used as starting compound. In this case the reaction proceeds smoothly and 1,2-dimethoxyethane (m.p. -58 "C) does not affect the crystallization of the complex. In a typical experiment an equimolar amount of nBuLi in hexane isChem.