Nickel complexes with heterobutadiene ligands

Article abstract of DOI:10.1021/om980450o

The reactions of 4,4-bis(trifluoromethyl)-substituted hetero-1,3-dienes with Ni(cod)₂ and Ni(PMe₃)₂(cod) afford stable coordination compounds containing a trigonal planar Ni(0) center. The substitution of co-ligands by tert-butylisonitrile leads to five-membered nickelacycles by an unprecedented oxidative [4+1] cycloaddition of the hetero-1,3-diene to Ni(0) under formation of new Ni-C and Ni-O or Ni-N bonds. The nature of the resulting nickelacycloheteropentene derivatives decisively depends on the electronic and steric nature of the hetero-1,3-dienes. All compounds have been characterized by NMR, IR, or mass spectroscopy. X-ray crystal structure determinations are reported for four typical examples.

Full text of DOI:10.1021/om980450o

90
Organometallics 1999, 18, 90-98
Nick el Com p lexes w ith Heter obu ta d ien e Liga n d s
Hans H. Karsch,* Andreas W. Leithe, Manfred Reisky, and Eva Witt
Anorganisch Chemisches Institut, Technische Universita¨t Mu¨nchen, Lichtenbergstrasse 4,
D-85747 Garching, Germany
Received J une 2, 1998
The reactions of 4,4-bis(trifluoromethyl)-substituted hetero-1,3-dienes with Ni(cod)2 and
Ni(PMe3)2(cod) afford stable coordination compounds containing a trigonal planar Ni(0)
center. The substitution of co-ligands by tert-butylisonitrile leads to five-membered nick-
elacycles by an unprecedented oxidative [4+1] cycloaddition of the hetero-1,3-diene to Ni(0)
under formation of new Ni-C and Ni-O or Ni-N bonds. The nature of the resulting
nickelacycloheteropentene derivatives decisively depends on the electronic and steric nature
of the hetero-1,3-dienes. All compounds have been characterized by NMR, IR, or mass
spectroscopy. X-ray crystal structure determinations are reported for four typical examples.
In tr od u ction
These unsaturated fluoroorganics rank among the
most reactive conjugated heterodienes. A rich organic
chemistry evolved from their outstanding properties,
resulting in many types of new compounds containing
trifluoromethyl substituents.7 They are known to add
to a variety of substrates, in particular to carbenes and
carbene homologues. Reactions of 1,3-diaza-1,3-buta-
dienes 1d with in situ generated SiCl2,8,9 with GeCl2,10
or with SnCl211 were shown to give metallacyclopentene
derivatives via [4+1] cycloaddition.
The introduction of the trifluoromethyl group or the
substitution of selected hydrogen atoms by fluorine in
organic compounds gained growing interest during the
past decade.1 In particular, selectively fluorinated het-
erocyclic compounds have attracted much attention in
terms of their biological and physiological activities,
which are often dramatically raised or changed by the
introduction of fluorine in the molecules.2
The large dipole moment of the CF3 group and its
chemical stability as well as the enhanced lipophilic
behavior render it attractive as a functional group.
Considerable efforts have been made to develop effective
routes to trifluoromethyl-substituted organic com-
pounds. Trifluoromethylation,1a,3 fluorination,4 and halo-
gen-exchange reactions5 are possible means, but most
of these methods suffer from low selectivity and/or
drastic reaction conditions and are generally only suit-
able for the synthesis of trifluoromethylated aromatic
compounds.
In this report, we present the first transition metal
complexes of the heterobutadienes 1a -1d . Due to the
specific reactions occurring at a transition metal center
and particularly at a metal-carbon bond (e.g., insertion)
and, furthermore, due to the “Umpolung”10 of the hetero-
butadiene backbone by coordination to a metal center,
the scope of reactions with these heterobutadienes
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Chem. 1992, 334, 219.
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An attractive alternative approach makes use of the
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available from hexafluoroacetone.6
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10.1021/om980450o CCC: $18.00 © 1999 American Chemical Society
Publication on Web 12/12/1998

Nickel Complexes with Heterobutadiene Ligands
Organometallics, Vol. 18, No. 1, 1999 91
lacyclopentene structure II or the η4-s-trans-1,3-diene
structure IV, as for example Cp2Zr(butadiene).16 Bor-
derline cases are also known (e.g., between I and II,
which differ only slightly in their geometrical param-
eters), and interconversions and/or dynamic behavior
in solution is also often encountered.17b Regarding
heterobutadienes, extended work on the coordination of
1,4-diaza-1,3-butadienes (DAD) to several transition
metal fragments was reported by tom Dieck18 and
others,19-20 including also coordination of DAD to Ni(0)
and Ni(II) centers.19 The structural types V, VI, and VII
are dominant in the coordination chemistry of hetero-
butadienes, e.g., 1-aza-1,3-butadienes, 1-oxa-1,3-buta-
dienes, or DAD,20 but structural types I and III have
also been observed (see Table 1).20,21
F igu r e 1. Structural formula of heterobutadienes 1a -1d .
Several transition metal centers have been envisaged
during the course of this work to achieve coordination
with the heterobutadienes 1a -1d . However, it turned
out that there are only a few successful cases, due to
the high reactivity of the heterobutadienes, which is
responsible for low selectivity or unwanted side reaction.
Thus, reactions with the co-ligands at the metal center
(preferably phosphines) and/or fluorination of the metal
center with concomitant formation of imidazole deriva-
tives are often observed.
The metal center of choice turned out to be nickel(0).
It should be noted here that L2Ni(0) fragments are
isolobal to carbene homologues. The coordination chem-
istry of nickel(0) is of interest for synthetical and
catalytical reasons, and a number of reviews have
appeared,12,13 as well as reports on oxidative addition
and coupling reactions at Ni(0) centers, forming metal-
carbon σ-bonded Ni(II) complexes.28,29 In particular,
stoichiometric reactions of carbon dioxide with unsatur-
F igu r e 2. Coordination modes of butadienes to transition
metal centers.
Ta ble 1. Selected Tr a n sition Meta l Bu ta d ien e
Com p lexes
complex
type
metal center and reference
LnM(butadiene)
LnM(heterobutadiene)
L2M(butadiene)
I
I
II
Cr, Mo, W;22a-c Ni,22d Ru15b
Fe26
Ru;22 Ti, Hf, Zr;16
Nb, Ta, Ln, U, Th17
22e-g
LnM(C4F6)
LnM(butadiene)
M(PR3)2(butadiene)
LnM(heterobutadiene)
LnM(heterobutadiene)
III
IV
V
III, VI
VII
Pt;22d Fe, Rh
Zr16a-c
Pd, Pt22d
Ti, Zr;19 Si;7 Pt;24 Mo18b,25
Pd;27a Cr27b
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should increase considerably, thus resulting in new
perspectives for organic synthesis. Particularly useful
in this respect would be the formation of metallahet-
erocycles via a [4+1] cycloaddition reaction, due to the
generation of a metal-carbon σ-bond (M+-C-).
The coordination chemistry of butadienes and hetero-
butadienes toward transition metal centers is of broad
general interest, since numerous organic synthesis and
catalytic processes are based on those complexes.12,13 As
a consequence, conjugated diene complexes of metals
from all transition metal groups have been reported, and
a variety of coordination modes have been described.14
Figure 2 represents the coordination modes of mono-
nuclear butadiene and heterobutadiene complexes, and
typical examples are presented in Table 1.
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The vast majority of the middle and late transition
metal diene complexes adopt the η4-s-cis-1,3-diene struc-
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92 Organometallics, Vol. 18, No. 1, 1999
Karsch et al.
ated cosubstrates at Ni(0) centers have been investi-
gated intensively by different groups.30 These reactions
lead to five-membered cyclic nickel(II) complexes. More-
over, there are many other examples of insertion reac-
tions leading to nickelaheterocycles.31
Reactions of 1,3-butadienes and hetero-1,3-butadienes
with Ni(0) fragments have been described in the litera-
ture, but there is a complete lack of reports on formal
oxidative cycloadditions of the [4+1] type.32,33 Hetero-
butadienes 1a -1d seemed promising candidates to fill
this gap.
The reactions of hetero-1,3-butadienes 1a -d with Ni-
(cod)2 led to red crystals of the coordination compounds
2a and 2b in quantitative yields (eq 2), while 2c and
2d could not be isolated: removal of the solvent from
the resulting red solutions afforded brown oils, spec-
troscopically identified as the fluoride elimination prod-
ucts, the 5-fluoro-4-trifluoromethyloxazoles37 and -im-
idazoles,38 respectively, analogous with the tin(II) chloride
reaction.39
We now report on respective reactions and the isola-
tion and characterization of the first [4+1] cycloaddition
products of heterobutadienes with nickel(0) complexes.
Resu lts a n d Discu ssion
A. [2+1] Cycloa d d ition of Heter o-1,3-bu ta d ien es
1a -1d to Nick el(0) Cen ter s. Reactions of unsaturated
compounds with nickel(0) complexes according to eq 1
are well-known (cod ) 1,5-cyclooctadiene, L2 ) 2 PR3,
cod, bipy, tmeda).
19F NMR spectroscopic data of 2a and 2b provide
strong evidence for a π-coordination of the hetero-1,3-
butadiene via the vinyl double bond in 2a and the imine
double bond in 2b, indicated by a decreased shielding
of the 19F nuclei. In comparison to the uncoordinated
hetero-1,3-butadienes, the coordination to the Ni(cod)
fragment causes a downfield shift of the 19F resonance
of about 9 ppm, while the IR spectrum displays the
ν(CdO) band at 1643.3 cm-1, consistent with a nonco-
ordinated CdO group in 2a and 2b. The proton NMR
of 2a shows an upfield chemical shift for the olefinic
proton in the coordinated heterobutadiene 1a of ∆δ )
-2.71 ppm, the signal being observed at δ 4.27. The
olefinic protons of the coordinated 1,5-cod ligand cause
two separate signals for both complexes 2a and 2b,
indicating a rigid conformation in solution at room
temperature. Furthermore 2a and 2b were detected by
mass spectroscopy (2a , m/z ) 434, 21.2; 2b, m/z ) 416,
100). The solid-state structure of 2a confirms the
expected coordination of the hetero-1,3-butadiene (Fig-
ure 3). Crystallographic data are given in Table 6, and
selected bond distances and angles are given in
Table 2.
The stability of the resulting coordination compounds
is strongly dependent on the co-ligands L2 and the
substitution pattern of the unsaturated substrate. Some
of the complexes have been characterized by X-ray
crystallography,34c while others could only be identified
by subsequent reaction with CO2.30c,d,36a
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1984, 96, 89. (b) Peng, S.-M.; Wang, Y.; Chiang, C.-K. Acta Crystallogr.
1984, C40, 1541.
(39) Burger, K.; Hu¨bl, K.; Geith, D. Synthesis 1988, 194.

Nickel Complexes with Heterobutadiene Ligands
Organometallics, Vol. 18, No. 1, 1999 93
Ta ble 4. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p ou n d 4d
C(1)-N(1)
C(2)-N(2)
Ni(1)-N(1)
Ni(1)-C(20)
C(20)-N(3)
C(2)-C(3)
1.362(2)
1.457(2)
1.864(2)
1.879(2)
1.148(3)
1.512(3)
C(1)-N(2)
Ni(1)-C(2)
C(1)-C(14)
Ni(1)-C(25)
C(25)-N(4)
C(2)-C(4)
1.294(2)
1.978(2)
1.499(3)
1.818(2)
1.150(3)
1.520(3)
C(2)-Ni(1)-N(1)
C(20)-Ni(1)-C(25)
Ni(1)-N(1)-C(1)
C(1)-N(2)-C(2)
Ni(1)-N(1)-C(5)
N(1)-C(1)-C(14)
N(2)-C(2)-C(3)
Ni(1)-C(2)-C(3)
C(3)-C(2)-C(4)
C(25)-Ni(1)-N(1)
C(20)-N(3)-C(21)
82.7(1) N(1)-Ni(1)-C(20)
89.1(1) C(25)-Ni(1)-C(2)
113.5(1) N(1)-C(1)-N(2)
111.2(2) N(2)-C(2)-Ni(1)
124.4(1) C(5)-N(1)-C(1)
122.6(2) C(14)-C(1)-N(2)
106.2(2) N(2)-C(2)-C(4)
113.1(1) Ni(1)-C(2)-C(4)
111.2(2) Ni(1)-C(25)-N(4)
175.3(1) Ni(1)-C(29)-N(3)
94.5(1)
93.9(1)
121.4(2)
111.0(1)
121.6(2)
116.0(2)
105.5(2)
109.5(1)
175.0(2)
176.1(2)
177.9(2) C(25)-N(4)-C(26) 178.1(2)
Ta ble 5. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p ou n d 6a
F igu r e 3. Crystal structure of Ni(cod)(η2-Ph-CO-CHd
C(CF3)2) (2a ).
C(1)-O(1)
C(2)-C(3)
1.366(3)
1.521(5)
1.856(2)
1.818(3)
1.142(4)
1.817(3)
1.460(4)
1.859(2)
1.504(5)
C(1)-C(2)
1.331(4)
1.950(3)
2.672(1)
1.920(2)
1.930(2)
1.150(4)
1.456(5)
1.525(5)
1.503(5)
C(3)-Ni(1)
Ni(1)-Ni(2)
Ni(1)-O(2)
Ni(2)-O(1)
C(28)-N(2)
N(1)-C(13)
C(3)-C(4)
Ni(1)-O(1)
Ni(1)-C(12)
C(12)-N(1)
Ni(2)-C(28)
N(2)-C(29)
Ni(2)-O(2)
C(3)-C(5)
Ta ble 2. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p ou n d 2a
C(1)-O(1)
C(1)-C(6)
1.216(3)
1.510(3)
1.507(3)
1.955(2)
1.360(3)
2.087(2)
2.119(2)
C(1)-C(2)
1.471(3)
1.442(3)
1.504(3)
1.962(2)
1.360(4)
2.118(2)
2.131(2)
C(2)-C(3)
C(3)-C(4)
C(3)-C(5)
Ni(1)-C(2)
C(12)-C(13)
Ni(1)-C(12)
Ni(1)-C(16)
Ni(1)-C(3)
C(16)-C(17)
Ni(1)-C(13)
Ni(1)-C(17)
C(19)-C(20)
O(1)-C(1)-C(2)
C(2)-C(3)-Ni(1)
Ni(1)-O(1)-C(1)
C(12)-Ni(1)-O(2)
Ni(1)-C(3)-C(4)
C(2)-C(3)-C(4)
C(4)-C(3)-C(5)
C(2)-C(1)-C(6)
Ni(1)-C(12)-N(1)
C(12)-N(1)-C(13)
C(12)-Ni(1)-O(1)
C(3)-Ni(1)-O(2)
116.5(3)
104.5(2)
113.8(2)
99.6(1)
111.2(2)
108.1(3)
112.2(3)
127.3(3)
178.8(3)
177.7(5)
176.5(1)
164.0(1)
C(1)-C(2)-C(3)
C(3)-Ni(1)-O(1)
C(3)-Ni(1)-C(12)
O(2)-Ni(1)-O(1)
Ni(1)-C(3)-C(5)
C(2)-C(3)-C(5)
Ni(1)-O(1)-Ni(2)
C(6)-C(1)-O(1)
Ni(2)-C(28)-N(2)
C(28)-N(2)-C(29)
C(28)-Ni(2)-O(2)
C(19)-Ni(2)-O(1)
117.3(3)
87.8(1)
95.7(2)
O(1)-C(1)-C(6)
C(2)-C(1)-O(1)
C(2)-C(3)-C(4)
Ni(1)-C(2)-C(3)
C(3)-Ni(1)-C(2)
Ni(1)-C(3)-C(4)
C(4)-C(3)-C(5)
118.4(2) C(6)-C(1)-C(2)
123.4(2) C(1)-C(2)-C(3)
122.5(2) C(2)-C(3)-C(5)
68.7(1) C(2)-C(3)-Ni(1)
43.2(1) C(1)-C(2)-Ni(1)
113.2(2) Ni(1)-C(3)-C(5)
118.2(2)
127.6(2)
116.7(2)
68.1(1)
113.7(2)
115.7(2)
76.9(1)
111.3(3)
109.2(3)
89.7(1)
115.7(2)
177.9(3)
176.5(3)
175.4(1)
162.9(1)
112.9(2) C(16)-C(17)-C(18) 124.6(2)
C(14)-C(13)-C(12) 125.4(2) C(13)-C(12)-C(19) 125.9(2)
Ta ble 3. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for Com p ou n d 3d
accordance with the only known trigonal (cod)(olefin)-
Ni(0) complex that was structurally characterized.40
The substitution of the cod ligand by 1a , 1c, and 1d
in Ni(PMe3)2(cod) (eq 3) resulted in the coordination
compounds 3a , 3c, and 3d in quantitative yields, while
the reaction with heterobutadiene 1b did not result in
the formation of a stable Ni(0) complex.
C(1)-N(1)
1.285(3)
1.380(3)
1.939(2)
2.174(1)
1.822
C(1)-C(51)
C(2)-N(2)
1.501(3)
1.395(3)
1.859(2)
2.194(1)
1.822
C(1)-N(2)
Ni(1)-C(2)
Ni(1)-P(1)
P(1)-C(11/12/13)
Ni(1)-N(2)
Ni(1)-P(2)
P(2)-C(21/22/23)
N(1)-C(1)-C(51)
N(2)-C(1)-N(1)
N(2)-C(2)-C(3)
C(1)-N(1)-C(61)
Ni(1)-N(2)-C(2)
C(2)-Ni(1)-N(2)
Ni(1)-C(2)-C(3)
C(2)-Ni(1)-P(1)
P(2)-Ni(1)-N(2)
124.7(2)
119.9(2)
120.8(2)
122.8(2)
71.6(1)
C(51)-C(1)-N(2)
C(1)-N(2)-C(2)
N(2)-C(2)-C(4)
C(3)-C(2)-C(4)
N(2)-C(2)-Ni(1)
C(1)-N(2)-Ni(1)
Ni(1)-C(2)-C(4)
P(1)-Ni(1)-P(2)
C(3)-C(2)-C(4)
115.3(2)
126.4(2)
115.2(2)
113.6(2)
65.4(1)
129.9(2)
114.3(2)
101.6(1)
113.6(2)
43.0(1)
119.3(2)
110.4(1)
105.0(1)
bond in the coordinated heterobutadiene 1a (1.442(3)
Å). The bonded carbon atoms of the coordinated hetero-
butadiene bond define one coordination plane together
with the nickel atom, while both olefinic bonds of the
1,5-cod ligand are oriented perpendicular to this plane.
The bond length of the carbonyl function (1.216(3) Å),
the sum of the bond angles around carbon (360.0°), and
the overall geometry exclude an interaction of the
oxygen atom with the nickel atom. The coordination
geometry around the nickel center, as well as the nickel
carbon bond lengths, both to the cod ligand and to the
coordinated heterobutadiene 1a , and also the olefinic
bond length in the heterobutadiene are in excellent
In 3a , the 19F nuclei of the trifluoromethyl groups are
even more deshielded than in 2a (∆δ ) 12 ppm), which
is consistent with a stronger interaction between the
hetero-1,3-butadiene 1a and the Ni(PMe3)2 moiety. The
31P{1H} NMR spectrum displays two separate signals:
a doublet at δ -13.01, with a coupling constant of 2J P-P
) 17.2 Hz, and a multiplet (dqq) at δ -15.80 (2J P-P
)
17.2 Hz, 4J P-F ) 8.6 Hz, 4J P-F ) 8.6 Hz). Small amounts
of impurities (i.e., excess 1a ) catalyze a fast ligand
(40) Buch, H. M.; Binger, P.; Kru¨ger, C. Organometallics 1984, 3,
1504.

94 Organometallics, Vol. 18, No. 1, 1999
Ta ble 6. Cr ysta llogr a p h ic Da ta for Ni{η2-[P h -CO-CHdC(CF 3)2]}[η4-(1,5-cod )] (2a ),
Karsch et al.
Ni{η2-[P h -C(N-Mes)-NdC(CF 3)2]}(P Me3)2 (3d ), (t-Bu -NC)2Ni-N(Mes)-C(P h )dN-C(CF 3)2 (4d ), a n d
[(t-Bu -NC)Ni-O-C(P h )dCH-C(CF 3)2]2 (6a )
2a
3d
4d
6a
formula
C
19H18F6NiO
C25H34F6N2NiP2
597.191
199(2)
C29H34F6N4Ni
611.311
194(2)
C32H30F12N2Ni2O2
820.001
203(2)
fw (g mol-1
temp (K)
)
435.041
199(2)
cryst size (mm)
cryst syst
space group
a (Å)
b (Å)
c (Å)
0.1 × 0.15 × 0.5
monoclinic
P21/c
12.115(1)
9.311(1)
15.057(1)
90
91.44(1)
90
1697.9(3)
4
0.6 × 0.45 × 0.45
orthorhombic
Pca21
19.754(2)
12.066(1)
12.019(1)
90
0.21 × 0.3 × 0.6
monoclinic
P21/n
11.479(1)
18.482(1)
15.128(1)
90
108.71(1)
90
3039.9(4)
4
0.3 × 0.45 × 0.45
monoclinic
P21/c
18.994(2)
11.377(1)
18.074(2)
90
108.09(1)
90
3712.6(7)
4
R (deg)
â (deg)
90
90
γ (deg)
V (Å3)
2864.7(4)
4
Z
cryst color, habit
red block
1.702
1.211
red block
1.385
0.844
orange block
1.336
0.699
red block
1.467
1.104
d calc (g cm-3
)
µ Mo KR (mm-1
F(000)
)
888
1240
1272
1664
θ range (deg)
3.0 to 27.0
-15 e h e 15
0 e k e 11
0 e l e 19
3826
3.16 to 26.91
0 e h e 25
0 e k e 15
-15 e l e 15
6192
3.0 to 26.0
0 e h e 14
-22 e k e 0
-18 e l e 17
5934
3.26 to 25.95
-23 e h e 22
0 e k e 14
0 e l e 22
7475
limiting indices
no. of reflns coll
no. of indep reflns
max, Fmin (e Å-3
goodness of fit on F2
3682
0.679, -0.419
1.069
6192
0.271, -0.188
1.143
5934
0.663, -0.334
1.044
7229
0.729, -0.691
1.046
F
)
with the Ni atom. The C(1)-N(1) bond length (1.285(3)
Å) and the coordination geometry around the sp2 imine
carbon atom are in good accordance with this finding.
The coordinated imine bond C(2)-N(2) in 3d is signifi-
cantly elongated; in fact it is in the range for C-N single
bonds (1.395(3) Å). Accordingly, the Ni-N bond in 3d
(1.859(2) Å) is the shortest of structurally investigated
η2-imine complexes (cf.: Ni-N ) 1.915(3) Å in (PR3)2-
Ni(η2-t-Bu-NdCHPh);41 but Ni-N ) 1.82(1) Å in (PMe3)-
NiCl(σ-C,σ-N-C[CH(SiMe3)2]dN-t-Bu42), whereas the
nickel-carbon bond (1.939(2) Å) is similar to the fore-
going example (Ni-C, 1.959(4);41 but Ni-C, 1.84(1) Å42).
Ni-P bond distances (2.1742(8) and 2.1944(8) Å) range
within the upper region of documented Ni-P dis-
tances41,42 and indicate a low degree of back-donation
from the Ni center. Nevertheless there is no fast
exchange of the trimethylphosphine ligands in solution
at room temperature (see above). The sum of the in-
volved angles around N(2) in 3d amounts to the
tetrahedral value of 327.9°, which is significantly more
than usually found in three-membered azacycles (sum
of angles around 280°).43 The overall geometry of 3d
suggests that the complex is a borderline case between
nickel(0) and nickel(II).
F igu r e 4. Crystal structure of Ni(PMe3)2(η2-Ph-C(N-
Mes)-NdC(CF3)2) (3d ).
exchange in 3a , giving rise to a single 31P resonance at
δ -14.72. In 3d , the 1,3-diaza-1,3-butadiene 1d is
bonded to the (Me3P)2Ni(0) fragment in the same
manner. The imine moiety exhibits a strong interaction
with the Ni(0) center, as the two phosphine ligands
again show different 31P resonances: a doublet at δ
B. [4+1] Cycloa d d ition of th e Heter o-1,3-bu ta -
d ien es 1a -1d to Nick el(0) Cen ter s. To accomplish
oxidative [4+1] cycloadditions at Ni(0) moieties, several
ligand properties must be tuned carefully. Soft electron-
donating ligands obviously are less suitable, and sterical
hindrance in a square planar coordination sphere re-
duces the variety of suitable ligands drastically. Tri-
methylphosphine, a good but soft electron-donating
ligand of modest sterical demand, obviously does not
2
-12.32, with a coupling constant of J P-P ) 10.0 Hz,
and an unresolved multiplet at δ -14.16, arising from
an ABX6 spin system. Furthermore, 3a and 3d were
detected by mass spectroscopy (3a , m/z ) 479, 18.9; 3d ,
m/z ) 521, [3d - PMe3]+, 4.3). Crystals suitable for
X-ray analysis of 3d (Figure 4) were obtained by slowly
removing the solvent of the filtered solution. Crystal-
lographic data are given in Table 6, and selected bond
distances and angles for 3d are given in Table 3.
The coordination geometry around the nickel center
in 3d can be described as trigonal planar (sum of angles
around Ni: 359.3°). The N-mesityl-substituted imine
double bond does not participate in any interactions
(41) Gabor, B.; Kru¨ger, C.; Marczinke, B.; Mynott, R.; Wilke, G.
Angew. Chem., Int. Ed. Engl. 1991, 30, 1666.
(42) Belderrain, T. R.; Paneque, M.; Poveda, M. L.; Sernau, V.;
Carmona, E.; Gutierrez, E.; Monge, A. Polyhedron 1995, 14, 323.
(43) Dru¨ck, U.; Littke, W. Acta Crystallogr. 1979, B35, 1905.

Nickel Complexes with Heterobutadiene Ligands
Organometallics, Vol. 18, No. 1, 1999 95
1d in 4d , compared to that in 3d (C(1)-N(1), 1.285(3)
Å; C(1)-N(2), 1.380(3) Å; and C(2)-N(2), 1.395(3) Å).
The nickel bonds Ni(1)-C(2), 1.978(2), and Ni(1)-N(1),
1.864(2) Å, like all other distances and angles match
well the expectations for a nickel(II) cycloaddition
product.
Quite remarkably, the reaction of 2a with 2 equiv of
tert-butylisonitrile (eq 5) led again to a [2+1] cycload-
dition product 5a in quantitative yield, analogous with
the reaction of 2a with 2 equiv of trimethylphosphine,
which gave 3a (eq 6).
F igu r e 5. Crystal structure of Ni(-N(Mes)-C(Ph)d
N-C(CF3)2)(t-Bu-NC)2 (4d ).
allow for an oxidative [4+1] cycloaddition of the hetero-
1,3-butadienes to the Ni(0) moiety.
Substitution of both trimethylphosphine ligands in 3d
by 2 equiv of tert-butylisonitrile (eq 4) did not result in
just another Ni(0) coordination compound, but [4+1]
cycloaddition of the heterobutadiene did occur.
5a was isolated as an orange solid from toluene.
Recrystallization from benzene afforded orange crystals
in quantitative yield. According to NMR spectroscopy,
5a is subject to stereochemical nonrigidity in solution
at room temperature. The isonitrile ligands exhibit one
broad singlet in the proton NMR, while the trifluoro-
methyl groups in the 19F NMR appear as two quartets.
The coordination via the olefinic double bond is indi-
cated by a downfield 19F chemical shift, compared to the
free ligand 1a (∆δ ) 10 ppm).
Whereas the reaction of 2a with only 1 equiv of
trimethylphosphine led to 3a (eq 7), the analogous
reaction of 2a with only 1 equiv of tert-butylisonitrile
gave a dimeric compound 6a (eq 8).
Orange crystals of 4d could be isolated from the
toluene solution in quantitative yield. The compound
shows typical features for Ni(II) complexes. The IR
spectrum of 4d displays two absorption bands at ν(Ct
N) ) 2205.3 cm-1 and ν(CtN) ) 2188.1 cm-1, charac-
teristic for tert-butylisonitrile coordinated to nickel(II)
1
centers.34a The H NMR spectrum in solution at room
temperature of 4d reveals two different resonances (δ
) 0.58, s, 9 H; δ ) 0.94, s, 9 H) for the tert-butyl protons,
thus indicating two nonequivalent isonitrile ligands, due
to a rigid square planar arrangement around the nickel
center. This is in agreement with the results of an X-ray
study (Figure 5), indicating a slightly distorted square
planar coordination geometry around the nickel center.
The mean deviation from the best plane is 0.0498 Å.
Crystallographic data are given in Table 6, and selected
bond distances and angles for 4d are given in Table 4.
The bond distances and angles correspond well to
localized bonds within a metallaheterocyclopentene
moiety. The five-membered metallacycle is almost pla-
nar, with the Ni atom deviating by 0.0133 Å from the
plane. The endocyclic bond lengths for the organic part
in 4d , C(1)-N(1) (1.362(2) Å), C(1)-N(2) (1.294(2) Å),
and C(2)-N(2) (1.457(2) Å), mirror the change in the
electronic structure of the coordinated heterobutadiene
The 19F NMR spectrum of the isolated red crystals of
compound 6a exhibits one singlet at δ 19.50. This clearly
is at variance with the findings for 5a and rules out a
π-bonded coordination via the olefinic double bond. The
equivalence of both trifluoromethyl groups indicates a
[4+1] cycloaddition product with formation of a penta-
cycle, instead. The IR spectrum of 6a displays only one
absorption band at 2213.9 cm-1. This is consistent with
the CtN stretching frequency of isonitrile ligands
coordinated to Ni(II) centers.34a From signal intensities
1
in the H NMR spectrum, a ratio 1a :t-Bu-NC of 1:1 can
be inferred. A tricoordinate Ni(II) center, stabilized by
only one neutral donor ligand, should be unstable, and

96 Organometallics, Vol. 18, No. 1, 1999
Karsch et al.
described, and bond lengths and angles are in good
accordance.44,45
Con clu sion
The nickel(0) complexes 2a and 3d undergo intramo-
lecular coupling reactions of the nickel moiety and the
hetero-1,3-butadiene ligand with formation of com-
pounds 6a and 4d , containing localized, unsaturated,
five-membered nickelaheterocycles. The generation of
the nickelaheterocycles can be viewed as formal [4+1]
cycloaddition of heterobutadienes 1a -1d to a Ni(0)
moiety with formation of metal-carbon σ-bonds. This
type of reaction, analogous with the Diels-Alder addi-
tion, is unprecedented for nickel(0) centers. In addition,
these complexes provide the first examples of a coordi-
nation of fluoromethyl-substituted heterobutadienes to
a transition metal center. The concomitant “Umpolung”
of the heterobutadiene backbone should decisively ex-
tend the scope of reactions for organic synthesis.
Exp er im en ta l Section
F igu r e 6.
Crystal structure of [Ni(-O-C(Ph)d
CH-C(CF3)2)(t-Bu-NC)]2 (6a ).
Gen er a l Con sid er a tion s. All operations were performed
under an atmosphere of dry nitrogen and with thoroughly
dried glassware by use of standard high-vacuum-line tech-
niques. Pentane, diethyl ether, toluene, and THF were distilled
under nitrogen from sodium/potassium alloy. All organic
reagents were purified by conventional methods. Elemental
analyses were determined by Microanalytisches Labor der TU
Mu¨nchen. In all cases where satisfying elemental analyses
could not be obtained (compounds 2 and 3), mass spectra give
the correct composition by identification of the molecular peak
and a reasonable fragmentation pattern. High-resolution elec-
tron impact (EI) and chemical ionization (CI) mass spectral
data were obtained employing a Varian Mat 311A spectrom-
eter; peaks are reported as m/z (assignment, relative intensity).
1H, 13C{1H}, 19F, and 31P{1H} NMR spectra were recorded on
a J EOL J NM-LA400 spectrometer operating at 399.8, 100.5,
376.1, and 161.8 MHz, respectively. 1H and 13C NMR chemical
shifts (δ) are referenced to the internal residual proton or
natural abundance 13C resonances of the deuterated solvent
relative to TMS (δ ) 0). 19F and 31P NMR spectra are
referenced to external CF3COOH (δ ) 0) or 85% H3PO4 (δ )
0), respectively. All measurements were carried out at 25 °C
in C6D6. Exceptions are given separately. Chemical shifts are
reported according to the δ convention in parts per million
(ppm) and coupling constants J are in Hz. IR data were
collected on a Nicolet 5DX FT-IR spectrophotometer. Ni(cod)2
was a donation of Dr. K. Po¨rschke, MPI fu¨r Kohleforschung,
Mu¨lheim a. d. Ruhr, and was not further purified prior to use.
therefore dimerization had to be envisaged. In fact, the
postulated dimeric structure was confirmed by an X-ray
crystallographic study (Figure 6). Crystallographic data
are given in Table 6, and selected bond distances and
angles for 6a are given in Table 5. The tert-butyl groups
in 6a are disordered and could not be resolved compu-
tationally. This has no influence on the structural
findings concerning the environment of the central
metal atoms.
The oxygen of one 1-nickela-2-oxa-cyclopentene moi-
ety is acting as a bridge to the second metallaheterocycle
and thus establishes the dimeric structure. Although
the distance between both nickel atoms (2.6718(6) Å) is
in the range for Ni-Ni single bonds, a bonding interac-
tion seems not to be operating. Both nickel atoms are
surrounded almost equally, although there is no center
of symmetry in the molecule. They possess a distorted
square planar environment constituted by two oxygen
atoms, one isonitrile carbon atom, and the bistrifluo-
romethyl-substituted carbon atom of the former hetero-
butadiene. The bridging oxygen atoms deviate slightly
from this plane (mean deviation from planarity 0.0584
Å). The nickelaheterocycles are nearly planar (mean
deviation from planarity 0.0026 Å). Similar to the ring
in 4d , the bonds in the heterocycle are not delocalized,
as discrete single and double bonds are found around
the ring (Ni(1)-O(1) 1.856(2) Å, O(1)-C(1) 1.366(3) Å,
C(1)-C(2) 1.331(4) Å, C(2)-C(3) 1.521(5) Å, and C(3)-
Ni(1) 1.950(3) Å). A comparison with compound 2a
underlines the difference in the electronic structure of
both compounds (2a : O(1)-C(1) 1.216(3) Å, C(1)-C(2)
1.471(3) Å, C(2)-C(3) 1.442(3) Å). The Ni and O atoms
form a folded four-membered ring; the planes defined
by the atoms Ni(1), O(1), O(2) and Ni(2), O(2), O(1)
include an angle of 128.4°. The bridging Ni-O bonds
are only slightly longer (Ni(1)-O(2) 1.920(2) Å, Ni(2)-
O(1) 1.930(2) Å) than the Ni-O bonds within the
metallacyclopentenes. Although there is no direct pre-
cedent of 6a , dimeric organonickel alkoxides and Ni(II)
complexes with Schiff base type ligands have been
46
47
PMe3 and Ni(cod)(PMe3)2 were prepared by literature
methods. The heterobutadienes 1a ,48 1b, 1c,49 and 1d 50 were
either prepared by literature methods or provided by Prof. K.
(44) Dimeric Ni(II) complex of the Schiff base type: Ni-Oendocyclic
1.851(3), Ni-Oexocyclic 1.912(4): Lu, Z.; White, C.; Rheingold, A. L.;
Crabtree, R. H. Inorg. Chem. 1993, 32, 3991.
(45) Dimeric Ni(II) complex of the Schiff base type: Ni-Oendocyclic
1.849(6), Ni-Oexocyclic 1.884(6): Bertrand, J . A.; Kirkwood, C. E. Inorg.
Chim. Acta 1970, 4, 192.
(46) Luetkens, M. L. J r.; Sattelberger, A. P.; Murray, H. H.; Basil,
J . D.; Fackler, J . P., J r. Inorg. Synth. 1990, 28, 305.
(47) (a) Mynott, R.; Neidlein, R.; Schwager, H.; Wilke, G. Angew.
Chem. 1986, 98, 374. (b) Ca´mpora, J .; Gutie´rrez, E.; Poveda, M. L.;
Ruiz, C.; Camona, E. J . Chem. Soc., Dalton Trans. 1992, 1763.
(48) (a) Helmreich, B. Dissertation TU Mu¨nchen, 1992, 77. (b)
Burger, K.; Helmreich, B. J . Prakt. Chem. 1992, 334, 219.
(49) Steglich, W.; Burger, K.; Du¨rr, M.; Burgis, E. Chem. Ber. 1974,
107, 1488.
(50) (a) Burger, K.; Penninger, S. Synthesis 1978, 524. (b) Burger,
K.; Wassmuth, U.; Penninger, S. J . Fluorine Chem. 1982, 20, 813.

Nickel Complexes with Heterobutadiene Ligands
Organometallics, Vol. 18, No. 1, 1999 97
Burger (University of Leipzig). All other reagents were pur-
chased from Aldrich.
Calcd for C16H23F6NNiOP2: C, 40.04; H, 4.83; N, 2.92. Found:
C, 37.71; H, 5.2; N, 2.67.
Ni(P Me3)2(η2-P h -C(N-Mes)-NdC(CF 3)2) (3d ). A pentane
solution of Ni(PMe3)2(cod) was prepared, analogous with the
procedure for 3a , by using 0.74 g of Ni(cod)2 (2.69 mmol) and
0.56 mL of PMe3 (5.38 mmol). At -78 °C 0.75 mL (2.52 mmol)
of heterobutadiene 1d was added and the mixture allowed to
warm to room temperature with continuing stirring for 20 h.
The reaction mixture was filtered, and slowly removing the
solvent under reduced pressure yielded 1.40 g of orange
P r ep a r a tion s. Ni(cod )(η2-P h -CO-CHdC(CF 3)2) (2a ).
Ni(cod)2 (1.62 g, 5.89 mmol) was suspended in 50 mL of
pentane, and the suspension was cooled to -78 °C. Hetero-
butadiene 1a (1.2 mL, 6.5 mmol) was added while stirring the
suspension. The color changed from yellow to orange. After
the reaction was stirred for 3 h and allowed to warm to room
temperature, the solvent was removed under reduced pressure.
The product was dissolved in 50 mL of toluene. The resulting
deep red solution was filtered, and slowly removing the solvent
under reduced pressure gave red crystals of 2a . Yield: 2.39 g
(93.4%). Mass spectrum (CI): 434 (21.2), [M]+; 268 (42.3),
[1a ]+; 166 (43.8), [M+ - 1a ]. 1H NMR (C6D6): δ 1.31-2.03 (m,
8H, CH2), 4.27 (s, 1H, CH), 4.58 (m, 2H, CH), 5.38 (m, 2H,
CH), 7.06-7.20 (m, 3H, Ph), 7.82-7.85 (m, 2H, Ph). 19F NMR
crystals of 3d (93.0%). Mass spectrum (CI): 520 (7.0), [M+
-
PMe3]. 1H NMR (C6D6): δ 0.88 (d, 2J (H-P) ) 7.6 Hz, 9H, PMe3),
2
1.13 (d, J (H-P) ) 8.2 Hz, 9H, PMe3), 2.16 (s, 3H, p-CH3, mes),
2.32 (s, 6H, 2 o-CH3, mes), 6.80 (s, 2H, m-H, mes), 6.93-7.07
(m, 3H, Ph), 7.62-7.65 (m, 2H, Ph). 19F NMR (C6D6): δ 19.90
(s, 6F, 2CF3). 31P{1H} NMR (C6D6): δ -12.32 (d, 2J (P-P) ) 10.0
Hz, PMe3), -14.16 (m, PMe3). Anal. Calcd for C25H34F6N2-
NiP2: C, 51.85; H, 5.92; N, 4.84. Found: C, 48.35; H, 5.67; N,
4.33.
4
4
(C6D6): δ 21.50 (q, J (F-F) ) 9.2 Hz, 3F, CF3), 26.63 (q, J (F-F)
) 9.2 Hz, 3F, CF3). IR (Nujol): 1643.3 (m), ν(CdO); 1572.9
(w), ν(CdC). Anal. Calcd for C19H18F6NiO: C, 52.46; H, 4.17.
Found: C, 51.23; H, 4.20.
Ni(-N(Mes)-C(P h )dN-C(CF 3)2)(t-Bu -NC)2 (4d ). To a
solution of 0.17 g of 3d (0.28 mmol) in 40 mL of toluene was
added with stirring 0.1 mL (0.96 mmol) of tert-butylisonitrile
at -78 °C. The reaction mixture was stirred for 15 h at -78
°C and then allowed to warm to room temperature, filtered,
and concentrated to dryness in vacuo. The resulting yellow
solid was recrystallized from benzene at room temperature to
afford 0.17 g (99.3%) of orange crystals of 4d . IR (Nujol):
2205.3 (m), ν(CtN); 2188.1 (m), ν(CtN); 1557.0 (w), ν(CdN).
1H NMR (C6D6, rt): δ 0.58 (s, 9H, t-Bu), 0.94 (s, 9H, t-Bu),
1.92 (s, 3H, p-CH3, mes), 2.47 (s, 6H, 2 o-CH3, mes), 6.51 (s,
2H, m-H, mes), 6.89 (br, 3H, Ph), 7.58 (br, 2H, Ph). 19F NMR
(C6D6): δ 17.21 (s, 6F, 2CF3). Anal. Calcd for C29H34F6N4Ni:
C, 56.97; H, 5.61; N, 9.17. Found: C, 56.06; H, 5.59; N, 8.56.
Ni(t-Bu -NC)2(η2-P h -CO-CHdC(CF 3)2) (5a ). To a solu-
tion of 0.53 g of 2a (1.20 mmol) in 40 mL of toluene was added
with stirring 0.25 mL (2.41 mmol) of tert-butylisonitrile at -78
°C. The reaction mixture was stirred for 1 h at -78 °C and
then allowed to warm to room temperature, filtered, and
concentrated to dryness in vacuo. The resulting orange solid
was recrystallized at room temperature from benzene to afford
0.59 g (99.7%) of orange crystals of compound 5a . IR (Nujol):
2181.5 (m), ν(CtN); 2156.6 (m), ν(CtN); 1649.3 (w), ν(CdO).
1H NMR (C6D6): δ 0.68 (s, 18H, 2t-Bu), 4.75 (s, 1H, CH), 7.01-
7.10 (m, 3H, Ph), 8.08-8.10 (m, 2H, Ph). 19F NMR (C6D6): δ
Ni(cod )(η2-t-Bu -CO-NdC(CF 3)2) (2b). Ni(cod)2 (1.51 g,
5.50 mmol) was suspended in 50 mL of pentane, and the
suspension was cooled to -78 °C. Heterobutadiene 1b (1.1 mL,
5.7 mmol) was added while stirring the suspension. After the
addition, the reaction mixture was allowed to warm to room
temperature and stirred for 24 h. The color changed from
yellow to orange-red. The reaction mixture was filtered, and
slowly removing the solvent under reduced pressure gave an
orange microcrystalline solid of compound 2b (0.69 g). The
residue was dissolved in 30 mL of toluene and filtered, and
after slowly removing the solvent under reduced pressure red
crystals of 2b (1.48 g) were isolated. Yield: 2.17 g (94.8%).
Mass spectrum (CI): 416 (100), [M]+; 166 (43.7), [M+ - 1b].
1H NMR (C6D6): δ 1.37 (s, 9H, t-Bu), 1.46-1.54 (m, 4H, 2 CH2),
1.87-1.91 (m, 4H, 2 CH2), 5.00 (br, 2H, 2 CH), 6.02 (m, 2H, 2
CH). 19F NMR (C6D6): δ 17.97 (s, 6F, 2 CF3). Anal. Calcd for
C
16H21F6NNiO: C, 46.1; H, 5.1; N, 3.4. Found: C, 43.3; H, 4.9;
N, 3.2.
Ni(P Me3)2(η2-P h -CO-CHdC(CF 3)2) (3a ). A pentane so-
lution of Ni(PMe3)2(cod) was prepared by addition of 2 equiv
of PMe3 (0.67 mL, 6.46 mmol) to a stirred suspension of Ni-
(cod)2 (0.89 g, 3.23 mmol) in 50 mL of pentane at -78 °C. The
reaction mixture was allowed to warm to room temperature
and stirred for 15 h. After cooling the resulting green solu-
tion again to -78 °C, heterobutadiene 1a was added. The
mixture was allowed to warm to room temperature and stirred
for 20 h. The green solution turned to an orange suspension.
The solvent and volatile components were removed in vacuo,
and the residue was dissolved in 30 mL of toluene. The
resulting red solution was filtered, and by slowly removing
the solvent 1.39 g of a wine red solid (89.8%) was obtained. IR
(Nujol): 1632.0 (m), ν(CdO); 1573.7 (w), ν(CdC). Mass spec-
trum (CI): 479 (18.9), [M+]; 403 (1.5), [M+ - PMe3]; 326 (3.3),
[M+ - 2 PMe3]; 210 (63.3%), [M+ - 1a ]. 1H NMR (C6D6): δ
0.82 (d, 2J (H-P) ) 7.0 Hz, 18 H, 2 PMe3), 3.93 (s, 1H, CH), 7.09-
7.17 (m, 3H, Ph), 8.17-8.21 (m, 2H, Ph). 19F NMR (C6D6): δ
4
4
22.06 (q, J (F-F) ) 9.3 Hz, 3F, CF3), 26.95 (q, J (F-F) ) 9.3 Hz,
3F, CF3). Anal. Calcd for C21H24F6N2NiO: C, 51.15; H, 4.91;
N, 5.68; Ni, 11.90. Found: C, 50.83; H, 4.91; N, 5.68; Ni, 12.30.
[Ni(-O-C(P h )dCH-C(CF 3)2)(t-Bu -NC)]2 (6a ). To a so-
lution of 1.05 g of 2a (2.41 mmol) in 20 mL of toluene was
added with stirring 0.25 mL (2.41 mmol) of tert-butylisonitrile
at -40 °C. The reaction mixture changes its color spontane-
ously from dark red to light green but returns to red within a
few seconds. The solution was then allowed to warm to room
temperature and was stirred for 15 h. Slowly removing the
solvent gave red crystals of compound 6a . Yield: 0.98 g
(99.2%). IR (Nujol): 2213.9 (m), ν(CtN); 1645.6 (w), ν(CdC).
1H NMR (C6D6): δ 0.45 (s, 18H, 2t-Bu), 4.77 (s, 2H, CH), 6.85
(br, 6H, Ph), 7.51 (br, 4H, Ph). 19F NMR (C6D6): δ 19.50 (s,
12F, 4CF3). Anal. Calcd for C32H30F12N2Ni2O2: C, 46.87; H,
3.69; N, 3.42. Found: C, 46.93; H, 3.61; N, 3.24.
X-r a y Str u ctu r a l Solu tion a n d Refin em en t. The same
general procedures were employed to collect the X-ray diffrac-
tion data for complexes 2a , 3d , 4d , and 6a . Crystal data
collection and refinement parameters are given in Table 6,
while selected bond lengths and angles are presented in Tables
2, 3, 4, and 5. Diagrams showing the solid-state conformation
of 2a , 3d , 4d , and 6a can be found in Figures 3, 4, 5, and 6,
respectively.
4
4
23.19 (q, J (F-F) ) 9.2 Hz, 3F, CF3), 29.35 (q, J (F-F) ) 9.2 Hz,
3F, CF3). 31P{1H} NMR (C6D6): δ -14.72 (s, PMe3). Anal. Calcd
for C17H24F6NiOP2: C, 42.63; H, 5.05. Found: C, 40.66; H, 4.22.
Ni(P Me3)2(η2-P h -CO-NdC(CF 3)2) (3c). A pentane solu-
tion of Ni(PMe3)2(cod) was prepared, analogous with the
procedure for 3a , by using 0.93 g of Ni(cod)2 (3.38 mmol) and
0.70 mL of PMe3 (6.8 mmol). At -78 °C 0.65 mL (3.38 mmol)
of heterobutadiene 1c was added. The mixture was allowed
to warm to room temperature and stirred for 20 h. After
filtration, the solvent was removed slowly under reduced
pressure and 1.51 g of a red microcrystalline solid (93.1%) was
obtained. 1H NMR (C6D6): δ 0.92 (br, 18H, 2PMe3), 7.06-7.11
(m, 3H, Ph), 8.13-8.14 (m, 2H, Ph). 19F NMR (C6D6): δ 19.88
(s, 6F, 2CF3). 31P{1H} NMR (C6D6): δ -13.00 (br, PMe3). Anal.
Com p ou n d s 2a , 3d , 4d , a n d 6a . Suitable single crystals
of 2a , 3d , 4d , and 6a were sealed into glass capillaries and

98 Organometallics, Vol. 18, No. 1, 1999
Karsch et al.
used for measurement of precise cell constants and for
intensity data collection. X-ray intensity data were recorded
at -74 °C for 2a and 3d , at -79 °C for 4d , and at -70 °C for
6a on a Enraf-Nonius CAD4 diffractometer utilizing mono-
chromated Mo KR radiation (λ ) 0.710 73 Å). During data
collections, three standard reflections were measured periodi-
cally as a general check of crystal and instrument stability.
No significant changes were observed. The structures were
solved by direct methods (SHELXS-86)51 and refined by full-
matrix least-squares techniques against F2 (SHELXL-93).52
Diffraction intensities were corrected for Lp and absorption
effects. The thermal motions of all non-hydrogen atoms were
treated anisotropically. All hydrogen atoms were calculated
in idealized positions, and their isotropic thermal parameters
were tied to that of the adjacent carbon by a factor of 1.5. The
only exceptions are the olefinic protons in the bonded hetero-
butadiene in structure 6a . They were refined directly. The
structure of 2a converged for 244 refined parameters to R1
(wR2) ) 0.0314 (0.0794) for 3826 reflections with F > 4σ(F).
The structure of 3d converged for 335 refined parameters to
R1 (wR2) ) 0.0292 (0.0894) for 6192 reflections with F >
4σ(F). The structure of 4d converged for 361 refined param-
eters to R1 (wR2) ) 0.0353 (0.0880) for 5934 reflections with
F > 4σ(F). The structure of 6a converged for 451 refined
parameters to R1 (wR2) ) 0.0402 (0.0942) for 7475 reflections
with F > 4σ(F). Further information may be obtained from
the Fachinformationszentrum Karlsruhe, Gesellschaft fu¨r
wissenschaftlich-technische Information mbH, D-76344 Egg-
enstein-Leopoldshafen, on quoting the depository numbers
CSD-408605, 408604, 408603, 408606, the names of the
authors, and the full journal citation.
Ack n ow led gm en t. This research was supported by
the Volkswagenstiftung.
Su p p or tin g In for m a tion Ava ila ble: Tables of crystal
data, bond lengths, atomic coordinates, and thermal param-
eters for 2a , 3d , 4d , and 6a (21 pages). See any current
masthead page for ordering and Internet access instructions.
(51) Sheldrick, G. M. SHELXS-86; University of Go¨ttingen, 1986.
Sheldrick, G. M., Krieger, C., Goddard, R., Eds.; Crystallographic
Computing 3, Oxford University Press, 1985; pp 175-189.
(52) Sheldrick, G. M. SHELXTL PC, version 5.03; Siemens Analyti-
cal X-ray Instruments Inc.: Madison, WI, 1990. Refinement: Sheldrick,
G. M. SHELXL-93; University of Go¨ttingen, 1993.
OM980450O