Mechanism of ammonia formation by reaction of trans-[Mo(NH)X(Ph2PCH2CH2PPh2)2]+(X = F, Cl, Br, or I) with base in methanol

Abstract

At ambient temperatures in methanol the complexes trans-[Mo(NH)X(dppe)₂]⁺[where X = F, Cl, Br, or I and dppe = 1,2-bis (diphenylphosphino)ethane] are inert to acid, but give ammonia in high yield on treatment with methoxide. An initial rapid deprotonation of the substrate renders the trans halide labile and when X = Cl, Br, or I the halide is liberated rapidly (k > 300 s^–1) to yield a common species which on spectrophotometric evidence is an equilibrium mixture of the contact and solvent-separated ion pairs of [MoN(dppe)₂]⁺ and X^–. This cation reacts with methoxide ion (k= 1.7 ± 0.4 × 10⁶ dm³ mol^–1s^–1), to yield [MoN(OCH₃)(dppe)₂] which rapidly abstracts a proton from solvent to give [Mo(NH)(OCH₃)(dppe)₂]⁺. When X = F, the same sequence of reactions occurs but dissociation of fluoride from the intermediate [Mo(N)F(dppe)₂] is rate-limiting (k= 1.78 s^–1). The pK_a of trans-[Mo (NH)X(dppe)₂]⁺ is very sensitive to the nature of the trans ligand and π-bonding influences dominate. [Mo(NH)(OCH₃)(dppe)₂]⁺ reacts (k= 1.57 × 10^–4 s^–1 at 25 °C, ΔH‡= 50 kJ mol^–1, ΔS‡=–143 J K^–1 mol^–1) in basic methanol with rate-limiting phosphine-chelate ring opening to give a product which gave a positive indophenol test for ammonia (70% yield). [Mo(NH)(OCH₃)(dppe)₂]⁺ reacts with HX in methanol to form trans-[Mo(NH)X(dppe)₂]⁺ with rate-limiting dissociation of methanol (k₅ > 3.9 × 10² s^–1) formed by rapid protonation of the co-ordinated methoxy-group (pK_a < 1.8).