Synthesis, Structure, and Bonding of Mononuclear Aryloxide Derivatives of Niobium in Oxidation States +5, +3, +2, and +1

Article abstract of DOI:10.1021/ja00195a030

The reduction of a series of niobium(V) aryloxides in the presence of the chalating phosphine dmpe (dmpe = Me2PCH2CH2PMe2) has been investigated.Treatment of hydrocarbon solutions of either Nb2(OAr-4Me)10 (1a) or Nb2(OAr-3,5Me2)10 (1b) (OAr-4Me = 4-methylphenoxide; OAr-3,5Me2 = 3,5-dimethylphenoxide) with sodium/amalgam (3 equiv) in the presence of dmpe (2 equiv) leads to the deep purple species Nb(OAr-4Me)2(dmpe)2 (2a) and Nb(OAr-3,5Me2)2(dmpe)2 (2b), respectively.Spectroscopic and magnetic data on these compounds is consistent with them containing low spin d³-configurations, while a solid-state structure of 2a shows a pseudooctahedral environment about the niobium atom with mutually trans-aryloxide ligands.Treatment of 2b with HOAr-3,5Me2 (2 equiv) is found to lead to oxidation and the formation of a mixed-valence salt of composition (1+)(1-) (3b).Treatment of Nb2(OAr-3,5Me2)10 with Na/Hg (1 equiv) in the presence of dmpe was also found to yield 3b.The (1)H NMR of 3b clearly show the noncontact shifted spectra for the low spin d²-cation and d⁰-anion.A solid-state study of 3b confirmed the formulation and showed the monocation to contain trans-aryloxides as in the neutral 2a.However, differences in the orientations of the aryloxide aryl rings within 2a and 3b were noted.The treatment of 2b with CO (1 atm) was found to lead to the 18-electron dicarbonyl Nb(OAr-3,5Me2)(CO)2(dmpe)2 (4b).A monocapped, trigonal-prismatic structure for 4b was found in the solid state with the aryloxide ligand being the capping group.A small angle of only 66.7(2) deg was also found to exist between the two carbonyl groups in 4b.The structural parameters in all of these molecules and in particular the Nb-OAr bond lengths and angles have been evaluated and discussed in terms of the importance of oxygen-p to metal-d ?-bonding.In this context the solid-state (trigonal-bipyramidal) structure of the monomeric niobium(V) aryloxide Nb(OAr-2,6Me2)5 (1c) has also been determined for comparison.The summary of the crystal data is as follows: for Nb(OAr-2,6Me2)5 (1c) at 24 deg C, a = 11.854(6) Angstroem, b = 16.305(4) Angstroem, c = 18.140(5) Angstroem, β = 99,44(3) deg, Z = 4, d_calcd = 1.312 g cm^-3 in space group P2₁/n; for Nb(OAr-4Me)2(dmpe)2 (2a) at -158 deg C, a = 12.646(4) Angstroem, b = 14.016(5) Angstroem, c = 9.707(3) Angstroem, α = 110.87(2) deg, β = 105.83(2) deg, γ = 76.21(2) deg, Z = 2, d_calcd = 1.321 g cm^-3 in space group PI; for (3b) at -156 deg C, a = 27.96(2) Angstroem, b = 23.49(2) Angstroem, c = 13.32(1) Angstroem, β = 119.72(3) deg, Z = 4, d_calcd = 1.272 g cm^-3 in space group P2₁/a; for Nb(OAr-3,5Me2)(CO)2(dmpe)2 (4b) at -160 deg C, a = 16.751(3) Angstroem, b = 12.133(2) Angstroem, c = 27.331(7) Angstroem, Z = 8, d_calcd = 1.364 g cm^-3 in space group Pcab.