Various alkynes (EtC2Et,
MeO2CC2CO2Me, CF3C2CF3,
PhC2Ph, C6F5C2C6F5,
PhC2Me, PhC2C6-F5, PhC2CO2Me)
react with (η-C5Me5)2Rh2(μ-CO)2
in acetone at room temperature; no reaction was observed with ButC2But.
The σ-bridging alkyne complex, trans-(η-C5Me5)2Rh2(CO)2(μ-η1:η1C6F5C2C6F5),
was obtained from the reaction with C6F5C2C6F5.
With the other alkynes, dimetallaeneone complexes,
(η-C5Me5)2Rh2(μ-CO){μ-η2:η2-C(O)C2RR′},
are formed. Some of these 'eneone' complexes co-exist with (η- C5Me5)2Rh2(CO)2(μ-η1:η1-RC2R�)
in the solid state (RC2R� = PhC2C6F5)
and/or in solution (EtC2Et, CF3C2CF3,
PhC2C6F5); others (MeO2CC2CO2Me,
PhC2Ph, PhC2Me, PhC2CO2Me) exist
exclusively as (η-C5Me5)2Rh2(μ-CO){μ-η2:η2-
C(O)C2RR′} in both the solid and solution states. The geometry of
the bridging group in (η-C5Me5)2Rh2(μ-CO){μ-η2:η2-C(O)C2(CF3)2}
has been determined from an X-ray diffraction study. The compound crystallizes with
four molecules in the monoclinic space group P21/n in a unit cell of
dimensions a 9.451(4), b 15.287(5), c 18.821(8)Ǻ, β 98.66(5)°. The
structure was solved by conventional heavy atom methods and refined to R 0.066
based on 4356 observed reflections above background. The structure contains a metalla-eneone ring, Rh -C(=O)-C(CF3)=C(CF3),
with the alkene bond η2-attached to the
second rhodium atom. Variable temperature N.M.R .
measurements establish that the complexes (η- C5Me5)2Rh2(μ-CO){μ-η2:η2-C(O)C2R2}(R
= CF3 or CO2Me) are fluxional in solution. Facile cleavage
of a C(O)-C(R) bond enables the metalla-eneone ring to shift rapidly from one rhodium atom to the
other.
Other products formed in the reactions between (η-C5Me5)2Rh2(μ-CO)2
and RC2R′ include. (η-C5Me5) Rh {C4(CF3)4CO}, (η-C5Me5) Rh (η4-C6R6) (R = CF3
or CO2Me), (η-C5Me5)2Rh2-(C4R2R′2)
(R = R′ = CO2Me; or R, R′ = Ph, C6F5), (η-
C5Me5)2Rh2(CO2C2Ph2),
(η-C5Me5)- Rh (C4Ph4CO2),
(η-C5Me5)2Rh2(CO)2{C4(CF3)4},
(η-C5Me5)2Rh2(CO)(μ-CO){COC4(C6F5)4}
and C6R3R′3 (R = R′ = Ph or CO2Me;
R = Ph and R′ = Me). Reactions between (η-C5Me5)2-Co2(μ-CO)2
and alkynes at room temperature or above yield mononuclear cyclopentadienone
complexes (η-C5Me5)Co{C4R4CO}(R
= Me, CF3 or C6F5), and the mononuclear arene complex (η-C5Me5)Co{C6(CF3)6}.
Thermotropic Liquid Crystals Based on Extended 2,5-Disubstituted-1,3,4-Oxadiazoles: Structure–Property Relationships, Variable-Temperature Powder X-ray Diffraction, and Small-Angle X-ray Scattering Studies