_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

8.2.2 Cyclobutadiene complexes
Diene complexes
For many years chemists, including the famous German scientist, Richard
Willstătter, who was the first to obtain cyclooctatetraene, strove to isolate
cyclobutadiene, but it proved to be very elusive. In 1956 Longuet-Higgins and
Orgel predicted that its complexes with transition elements should be stable, on
account of the favourable symmetry properties of its n-orbitals (p. 200). In 1959
this was confirmed by the isolation of [(IJ 4 -C 4 Me 4 )NiC1 2 L by Criegee and Schroder
and of (1J 4 -C 4 Ph 4 )Fe(C0) 1 by Hiibel and his group. The first complex of the
unsubstituted ligand, (IJ 4 -C 4 H 4 )Fe(C0) 3 was prepared in 1965. Since then the
parent hydrocarbon has been generated by irradiation of photo-0(-pyrone in a
frozen noble gas matrix at 8-20 K and its infrared spectrum recorded. It has a
rectangular D 2 h symmetry in the singlet ground state, rather than the square D 4 h
geometry. This is predicted on the hasis of the Jahn-Teller theorem. The reason
that free cyclobutadiene cannot be isolated under normal conditions is that it
rapidly undergoes Diels-Alder condensation with itself. This can be slowed down
by introduction of bulky substituents so that tri-tert-butyl-cyclobutadiene
survives at room temperature for severa! hours.
The first cyclobutadiene complexes were obtained either from alkynes (p. 246)
or by reaction of cis-dichlorocyclobutenes with metal carbonyls. Many more
derivatives were then prepared by transfer ofthe cyclobutadiene ligand from one
transition metal to another, especially from palladium in [(Ph 4 C 4 )PdCl 2 ] 2 • In their
complexes cyclobutadienes adopt a planar square structure.
R R R R R Br R
)d··.CL ~ R~ N~i~Br'NiiSLL:\
Fe2(CO)g
pentane R " ,- J U R
'··Cl R Br Br
R R R R
Fe
(R=H,Me) (C0) 3
Cyclobutadienetricarbonyliron can also be prepared by irradiation of photo-0(­
pyrone with Fe(CO),. It is a pale yellow solid, m.p. 26°C.
Ct - ITl
hll
hu
:x-pyrone
Fe(C0)5
• photo- :x -pyrone'
Lfl
Fe
(C0)3
The coordinated cyclobutadiene readily undergoes electrophilic substitution.
It can be acylated under Friedel-Crafts conditions, sulphonated and mercurated.
Like ferrocene (p. 284) it performs the Vilsmeier reaction with
PhN(Me)CHO/POCl 3 giving the aldehyde (1J 4 -C 4 H 3 CHO)Fe(C0) 3 and the Mannich
condensation with CH 2 0/Me 2 NH giving (1J 4 -C 4 H 3 CH 2 NMe 2 )Fe(C0) 1 , which occur
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