3462
Organometallics 2000, 19, 3462-3465
Nu cleop h ilic Rea ctivity of th e Mu ltip ly Bon d ed Ca r bon
Cen ter in Gr ou p 4-P in cer
Bis(im in op h osp h or a n o)m eth a n ed iid e Com p lexes
Ruppa P. Kamalesh Babu, Robert McDonald,† and Ronald G. Cavell*
Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Received February 28, 2000
Sch em e 1
Summary: The bis(iminophosphorano)methanediide (car-
bene) complexes [MCl2{C(Ph2PdNSiMe3)2-κC,κ2N,N′}]
(M ) Hf (1), Zr (2)) form Lewis acid-base adducts with
THF, nitriles, and isonitriles and undergo 1,2-addition
reactions with amines, alcohols, and alkyl iodides and
[2 + 2]-cycloaddition reactions with heteroallenes. The
halide complexes can be alkylated with lithium alkyl
reagents without attack at the carbene center. All these
reactions are consistent with high-valent metal alky-
lidene character.
recently reported a system of 12-valence-electron bis-
(iminophosphorano)methanediide complexes of the group
4 metals,7,8 which contain a metal multiply bound to
the carbon and which may be regarded as a carbene. In
addition there are no CO or Cp substituents on the
metal. The MdC unit is supported by two chelating
iminophosphorane substituents to form a “pincer” type
system with conjugation (Scheme 1) represented as the
two limiting structures: a carbene (A) or the ylide-
amide (B).
The short metal-carbon bond in these new complexes,
revealed by the structural characterization, and a
semiempirical calculation support the metal carbene
formulation.7,8 Here we report reactions of representa-
tive Zr and Hf complexes 1 and 2 with selected electro-
philes and nucleophiles which suggest alkylidene char-
acter. We observe both 1,2-addition and [2 + 2]-cycloaddi-
tion processes, with C-C bond formation (Scheme 2).
The reactivity pattern of these carbene complexes
implies that the carbon center is nucleophilic, and thus,
the complexes are similar to high-oxidation-state metal
alkylidenes.9 The nucleophilicity may arise because the
phosphorus substituents introduce behavior similar to
phosphorus ylides.10
The reactivity of a MdC(carbene) functionality is
determined by several factors, such as the substituents
on the carbon center, the ligation at the metal, and the
oxidation state of the metal. Broadly, species with a Md
C functionality can be subdivided into those in which
the carbon has nucleophilic character (Schrock alkyli-
denes), when alkyl or hydrogen substituents are present
on the carbene center, or where it is electrophilic, which
is the general case when the carbon carries a hetero-
atom (O, S, N, etc.) substituent.1 Many transition-metal
complexes in these categories have been studied because
of their relevance to organic syntheses.2 Most contain
CO or Cp substituents. Group 4 carbene complexes are
rather rare,3 being limited to a few titanium4 and
zirconium5 alkylidenes and the zirconium Fischer car-
bene.6 Heteroatom-substituted carbene complexes of
group 4 metals have received only minor attention.3 We
* To whom correspondence should be addressed. Telephone: 780-
492-5310. FAX: 780-492-8231. E-mail: Ron.Cavell@Ualberta.ca.
† X-ray Structure Determination Laboratory.
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The halide substituents of 1 or 2 are readily replaced
by alkyl groups using simple nucleophilic substitution
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10.1021/om000181d CCC: $19.00 © 2000 American Chemical Society
Publication on Web 08/02/2000