The reactivity of benzamidinate-stabilized yttrium complexes
[PhC(NSiMe3)2]2YR (R
=
CH2Ph·THF,
CH(SiMe3)2) and
{[PhC(NSiMe3)2]2Y(μ-H)}2
has been investigated. The
complexes are thermally stable showing no sign of decomposition, ligand
or solvent
metalation, or H/D exchange after hours at 100 °C in
cyclohexane-d
12 or
benzene-d
6. The
alkyls are also stable in ethereal solvents. However,
{[PhC(NSiMe3)2]2Y(μ-H)}2
induces C−O
cleavage in THF solutions. The complexes
[PhC(NSiMe3)2]2YCH2Ph·THF
and {[PhC(NSiMe3)2]2Y(μ-H)}2
are modestly active in ethylene polymerization but are inactive
toward
propylene and 1-hexene. Terminal alkynes react stoichiometrically
with
[PhC(NSiMe3)2]2YCH(SiMe3)2 and
{[PhC(NSiMe3)2]2Y(μ-H)}2
to give μ-acetylide dimers,
{[PhC(NSiMe3)2]2Y(μ-C⋮CR)}2 (1, R = H; 2, R =
Me; 3, R = n-Pr; 4, R =
SiMe3; 5, R = Ph; 6, R =
CMe3).
Treatment with THF leads to cleavage of these dimers, yielding
[PhC(NSiMe3)2]2YC⋮CR·THF (7, R = H; 8, R = CMe3).
[PhC(NSiMe3)2]2Y(μ-Me)2Li·TMEDA
reacts with HC⋮CCMe3
to afford
[PhC(NSiMe3)2]2Y(μ-C⋮CCMe3)2Li·TMEDA.
[PhC(NSiMe3)2]2YCH(SiMe3)2
catalyzes
the regioselective dimerization of bulky 1-alkynes. With small
1-alkynes, HC⋮CR (R = H,
Me, n-Pr), no dimerization was observed and the reaction
stops with the formation of the
alkynyl dimers
{PhC(NSiMe3)2]2Y(μ-C⋮CR)}2
(1−3). Treatment of
[PhC(NSiMe3)2]2YR
with
acetonitrile gives either C−H bond activation or insertion. For
R = CH(SiMe3)2, C−H bond
activation occurs, yielding
{[PhC(NSiMe3)2]2Y(μ-(N,N ‘)-N(H)C(Me)C(H)C⋮N)}2
(10). For
R = CH2Ph·THF a mixture of C−H bond
activation (10, 10%) and insertion products,
{[PhC(NSiMe3)2]2Y(μ-NC(Me)CH2Ph)}2
(11a, 50%) and
{[PhC(NSiMe3)2]2Y(μ-N(H)C(Me)C(H)Ph)}2 (11b, 40%), was obtained. The
hydride
{[PhC(NSiMe3)2]2Y(μ-H)}2
exclusively gives
insertion of acetonitrile, affording
{[PhC(NSiMe3)2]2Y(μ-NC(H)Me)}2
(12). With pyridine,
[PhC(NSiMe3)2]2YCH(SiMe3)2
gives C−H bond activation, whereas
[PhC(NSiMe3)2]2YCH2Ph·THF and
{[PhC(NSiMe3)2]2Y(μ-H)}2
undergo insertion yielding
[PhC(NSiMe3)2]2Y(NC5H5R) (13, R = H; 14,
R = CH2Ph). In contrast, with α-picoline,
[PhC(NSiMe3)2]2YR
(R
= CH(SiMe3)2,
CH2Ph·THF) and
{[PhC(NSiMe3)2]2Y(μ-H)}2
afford the α-picolyl derivative,
[PhC(NSiMe3)2]2Y(η2-(C,N)-2-CH2NC5H4
(15). The difference in reactivity of the
bis(benzamidinate)-stabilized complexes compared to the corresponding
Cp*2YR systems, e.g.
the low tendency to catalyze C−C bond formation, the reduced or even
the absence of C−H/C−O activation properties, and the enhanced nucleophilic and Brønsted
base reactivities,
is interpreted in terms of the high ionicity of the
benzamidinate-stabilized yttrium complexes.
The contracted yttrium orbitals are less exposed than in the
dicyclopentadienyl derivatives
and therefore not suited to establish the first initiating interaction
with substrate molecules.