SYNTHESIS, CHARACTERIZATIONS AND REACTIONS OF LOW VALENT COMPLEXES OF MOLYBDENUM CONTAINING [BIS(2-DIPHENYLPHOSPHINOETHYL) PHENYL-PHOSPHINE] AND A NITROGEN FIXATION APPROACH

Hassan H. HAMMUD

 High energy electron rich molybdenum phosphine complexes were synthesised by the reduction of [Mo(TRI)C1₃] in THF with sodium amalgam in the presence of various ligands: arenes, cyclopentadiene, indene, dinitrogen, phosphines and phosphites; (TRI is [Bis(2 ­diphenylphosphinoethyl)phenylphosphine]. The products are: i, [Mo(,η₆ ‑arene) (TRI)], (arene = C₆H₆, C₆H₅OCH₃, C₆H₅CH₃, C₁₀H₈ or PPh₃); ii, [Mo(η⁵‑C₅H₅)(TRI)H]; iii, [Mo(η⁵‑C₉H₇)(TRI)H];iv,fac- [Mo(N₂)(TRI)(PMe₃) ₂]; v, [Mo(N₂)( η³‑TRI)( η ²‑TRI)]; vi, mer-­[Mo(TRI)(P(OR)₃)₃], (R = Me or Et). When THF solvent was used and in the abscence of added ligands, the products are [Mo(TRI)(THF)3] under argon and { Mo(N₂) (TRI) (THF₃) }x under dinitrogen.

 Heating fac‑[Mo(N₂)(TRI)(PPhMe₂)₂] in THF with ethylene produced trans‑[Mo(CH₂ CH₂) ₂(TRI) (PPhMe₂. Heating trans­- [Mo(N₂)₂(TRI)(PPh₃ in THF formed [Mo(η⁶‑PhPPh₂ (TRI)] by rearrangement of σ- to η⁶‑bonded PPh₃. Reaction of [Mo(η^6-­arene)(TRI)] with strong acid formed [Mo(η⁶‑C₆H₆)(TRI)H]+ and [Mo(η⁶‑arene)(TRI)(H)₂]²⁺.

The hydride complex [Mo(η⁵‑C₅H₅)(TRI)H] was also prepared by heating fac‑[Mo(N₂)(TRI)(PMe₃)₂] with cyclopentadiene in heptane. The hydride complex reacted with CDC1₃ forming [Mo(η⁵‑C₅H₅) ‑(TRI)C1] and [Mo(η⁵‑C₅H₅)(TRI)(CC1₃)l (Mass spectra: m+l/z = 732 and 818 respectively).

Mer‑[Mo(TRI)(P(OR)₃)₃] isomerizes to fac‑[Mo(TRI)(P(OR)₃)₃] both thermally and photochemically.

Both Mer-[Mo(TRI)(P(OR)₃)₃] and [Mo(η⁶‑arene)(TRI)] exhibit two oxidation waves in the cyclic voltammograms in THF. The first oxidation is reversible but the second oxidation is pseudoreversible.

The reaction of HBr with trans‑[Mo(N₂)₂(dppe)(PPh₂Me)₂] in THF formed  [Mo(NNH₂) (THF) (dppe) (PPh₂Me)Br]Br, [Mo(NNH₂)(dppe)-­(PPh₂Me)₂ Br]Br and [Mo(NNH₂)     (dppe)(PPh₂Me)Br₂]. The reaction of [Mo(NNH₂) (dppe) (PPh₂Me) (Br)₂]with pyridine formed the diamagnetic [Mo(NNH₂)(dppe)(PPh₂Me)(C₅H₅N)Br]Br. In contrast, the reactions of [Mo(NNH₂)(dppe)(PPh₂Me)Br₂] or [Mo(NNH₂)(dppe)-­(PPh₂Me)₂Br]Br with pyrazine formed the purple paramagnetic [Mo(NNH₂)(dppe)(PPh₂Me)(C₄H₄N₂)Br]Br, (UV, λ = 572 nm). A mechanism for ammonia and hydrazine production was proposed. It involves a series of electronation and protonation steps.

Finally, the reactions of fac‑[Mo(N₂)(TRI)(PMe₃)₂] and fac- [Mo(N₂) (TRI) (dppm) ]       produced the corresponding carbon dioxide complexes, IR (KBr), v(C0₂) = 1758.80 and 1779.98 cm‑¹ respectively.