Evidence for the Existence of Terminal Scandium Imidos: Mechanistic Studies Involving Imido–Scandium Bond Formation and C–H Activation Reactions

The anilide–methyl complex (PNP)­Sc­(NH­[DIPP])­(CH3) (1) [PNP– = bis­(2-diisopropylphosphino-4-tolyl)­amide,
DIPP = 2,6-diisopropylphenyl] eliminates methane (kavg = 5.13 × 10–4 M–1s–1 at 50 °C) in the presence of pyridine
to generate the transient scandium imido (PNP)­ScN­[DIPP]­(NC5H5) (A-py), which rapidly activates
the C–H bond of pyridine in 1,2-addition fashion to form the
stable pyridyl complex (PNP)­Sc­(NH­[DIPP])­(η2-NC5H4) (2). Mechanistic studies suggest
the C–H activation process to be second order overall: first
order in scandium and first order in substrate (pyridine). Pyridine
binding precedes elimination of methane, and α-hydrogen abstraction
is overall-rate-determining [the kinetic isotope effect (KIE) for 1-d1 conversion to 2 was 5.37(6) at 35 °C and 4.9(14) at 50 °C] with activation
parameters ΔH⧧ = 17.9(9)
kcal/mol and ΔS⧧ = −18(3)
cal/(mol K), consistent with an associative-type mechanism. No KIE
or exchange with the anilide proton was observed when 1-d3 was treated with pyridine or thermolyzed
at 35 or 50 °C. The post-rate-determining step, C–H bond
activation of pyridine, revealed a primary KIE of 1.1(2) at 35 °C
for the intermolecular C–H activation reaction in pyridine
versus pyridine-d5. Complex 2 equilibrated back to the imide A-py slowly, as the
isotopomer (PNP)­Sc­(ND­[DIPP])­(η2-NC5H4) (2-d1) converted
to (PNP)­Sc­(NH­[DIPP])­(η2-NC5H3D) over 9 days at 60 °C. Molecular orbital analysis of A-py suggested that this species possesses a fairly linear
scandium imido motif (169.7°) with a very short Sc–N distance
of 1.84 Å. Substituted pyridines can also be activated, with
the rates of C–H activation depending on both the steric and
electronic properties of the substrate.