Rhodium allenylidenes, which represent a class of ‘flat’
metallabutatrienes, are useful tools for the generation of metal-bound
species that are hardly accessible (sometimes not accessible at all) by
other synthetic routes. In contrast to various octahedral
allenylidene metal complexes, e.g. of Cr
0
,
Mn
I
or Ru
II
, the square-planar rhodium compounds
trans-
[RhX(CCCRR′)(PPr
i
3
)
2
] are attacked by both nucleophiles and electrophiles.
Moreover, non-polar substrates such as H
2
or Cl
2
react cleanly with the title complexes to give products in which the
allenylidene unit is preserved as part of a newly formed ligand.
Attempts
to prepare a square-planar cationic metallaheptahexaene,
trans-[Rh(CCCRR′)
2
(PP
r
i
3
)
2
]
+
, unexpectedly led to
the formation of two isomeric hexapentaenerhodium(I)
complexes
of which the isomer having the polyene coordinated in an unsymmetrical
fashion is the thermodynamically more stable. C–C coupling
reactions
also occur on treatment of
trans-[RhCl(CCCPh
2
)(PPr
i
3
)
2
] with vinyl Grignard reagents or
phenylacetylene leading to highly unsaturated RhC
5
and
RhC
5
P frameworks from which vinylallenes and
phosphacumuleneylides are obtained.
铑烯丙基
铑是一种“平面”
金属
三烯丙基
铑,是生成
金属结合物种的有用工具,这些物种很难通过其他合成途径获得(有时根本无法获得)。与各种八面体烯丙基
金属配合物(例如Cr0、MnI或Ru II)相比,反式[RhX(CCCRR′)(PPr i3)2]的方形平面
铑化合物可同时被亲核试剂和亲电试剂攻击。此外,非极性底物(如H2或Cl2)与标题化合物反应后,烯丙基单元作为新形成的
配体的一部分得以保留。尝试制备方形平面阳离子
金属庚烯时,反式[Rh(CCCRR′)2(PP r3)2]意外地形成了两种异构体六
戊烯铑(I)配合物,其中以不对称方式配位的多烯异构体在热力学上更稳定。在反式[RhCl(CCCPh2)(PPr i3)2]与
乙烯基格氏试剂或
苯乙炔反应时,也会发生C-C偶联反应,从而形成高度不饱和的RhC5和RhC5P骨架,从中可得到
乙烯基烯丙基和
磷杂茂烯基。