Macrocyclic Complexes of Niobium(III):  Synthesis, Structure, and Magnetic Behavior of Mononuclear and Dinuclear Species That Incorporate the [P₂N₂] System

Abstract

The coordination chemistry of the two mixed donor macrocyclic ligands R [P 2 N 2 ] (where R [P 2 N 2 ] ) RP(CH 2 SiMe 2 NSiMe 2 CH 2 ) 2 PR, R ) cyclohexyl (Cy) or phenyl (Ph)) with niobium-(III) is presented. The reaction of the dilithio precursors R [P 2 N 2 ]Li 2 (S) (R ) Cy, S ) THF; R ) Ph, S ) 1,4-dioxane) with NbCl 3 (DME) (DME ) 1,2-dimethoxyethane) generates the complexes R [P 2 N 2 ]NbCl (R ) Cy, 1; R ) Ph, 2). For R ) Cy, single-crystal X-ray diffraction studies and variable-temperature magnetic susceptibility measurements indicate that 1 is mononuclear in the solid state; however, analogous variable-temperature magnetic data suggest that 2 is dinuclear in the solid state due to the observation of antiferromagnetic exchange. In solution, 2 is apparently monomeric similar to 1. Adduct formation between these mononuclear complexes is also evident; reaction of 2 with neutral donors and coordinating solvents produces the mononuclear derivatives Ph [P 2 N 2 ]NbCl(L) (L ) py, CO, PMe 3 , THF, MeCN), of which the pyridine adduct, 3e, has been characterized crystallographically. Subsequent replacement of the chlorides can be achieved to generate the paramagnetic alkyl complexes R [P 2 N 2 ]NbR′ (R ) Cy, Ph; R′ ) CH 2 SiMe 3 , CH(SiMe 3 ) 2 ). The representative compounds Cy [P 2 N 2 ]NbCH 2 SiMe 3 (4a) and Ph [P 2 N 2 ]NbCH(SiMe 3 ) 2 (5b) have been characterized by X-ray crystallography.

Dinitrogen Activation by Fryzuk’s [Nb(P2N2)] Complex and Comparison with the Laplaza–Cummins [Mo{N(R)Ar}3] and Schrock [Mo(N3N)] Systems

“…The structure possesses a pseudo-trigonal-bipyramidal geometry similar to that observed experimentally for the full ligand system, in which the two phosphine donors are located in the axial positions. The calculated bond lengths are in reasonable agreement with the values obtained from X-ray crystallography [35] given the differences between the real and model systems. …”

“…The real situation will lie somewhere between these two limits, but much closer to the C 2v structures. The experimental structure for [ClNbA C H T U N G T R E N N U N G (P 2 N 2 )] [35] is very close to ideal C 2v symmetry, except that the R groups are tilted slightly. Apart from the evident computational efficiencies, the model system has greater flexibility than the real system, allowing us to investigate the effects of the ligand orientation and donor atom on the reaction energetics, which would not be possible for the full ligand system.…”

“…To correct for this, the system was initially constrained to C 2v symmetry to reflect the geometry of the [ClNbA C H T U N G T R E N N U N G (P 2 N 2 )] complex. [35] Following this, calculations were repeated on the model system, but without symmetry constraints. The multidentate P 2 N 2 ligand does exhibit flexibility in the ligand backbone, but the degree of distortion away from an idealised C 2v structure depends on which metal complex is considered.…”

“…2.596 2.036 exptl 2.5973 2.089 [35] [ [6] [a] Note this is not a true minimum but represents an upper bound to the energy.…”

Dinitrogen activation by group 4 and group 5 metal complexes supported by phosphine-amido containing ligand manifolds

“…Although access to low-valent tantalum starting materials prevented facile synthesis of reduced P 2 N 2 derived complexes, the ready availability of the niobium(III) reagent, NbCl 3 (DME) (DME = 1,2-dimethoxyethane) [60], allowed us to explore dinitrogen activation with this second row group 5 metal. The reaction of syn-[P 2 N 2 ]Li 2 with NbCl 3 (DME) results in the formation of [P 2 N 2 ]NbCl (9.1) [60].…”