Isocyanide-Group 5 complexes and metal-centred C-C coupling

Article abstract of DOI:10.1016/S0022-328X(99)00240-5

The Nb^I complexes [XNb(CO)₂(CNR)₄] (X=I, Br; R=tBu, cHex, 2,6-Me₂C₆H₃) (4) have been identified as intermediates in the system [Nb(CO)₆]^-/CNR/X₂, which gene

Full text of DOI:10.1016/S0022-328X(99)00240-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 585 (1999) 294–307
Isocyanide–Group 5 complexes and metal-centred C–C coupling
Dieter Rehder *, Christian Bo¨ttcher, Ce´sar Collazo, Ronald Hedelt, Hauke Schmidt
Institut fu¨r Anorganische und Angewandte Chemie, Uni6ersita¨t Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
Received 26 February 1999; received in revised form 24 April 1999
Abstract
The NbI complexes [XNb(CO)2(CNR)4] (X=I, Br; R=tBu, cHex, 2,6-Me2C6H3) (4) have been identified as intermediates in
the system [Nb(CO)6]−/CNR/X2, which generates the NbIII complexes [I2Nb(CNR)6]I (1) and [X3Nb(CNR)3] (2) or, on addition
of tolane, [I2Nb(CNtBu)3tolane]I (3). In the presence of small amounts of water, the aminoalkyne–NbIII complexes
[X2Nb(CNR)4RHNCꢀCNHR]X (R=tBu; X=I: 5a, X=Br: 5b) are formed. The complex 4a (X=I, R=tBu) converts to 5a in
the presence of water, suggesting successive protonation and intramolecular reductive isocyanide coupling mediated by the NbI
centre. The VI complexes [XV(CO)2(CNR)2dppe] (X=I, R=tBu, cHex; X=Br, R=tBu) (6) and the VIII complex
[I2V(CNtBu)6]I (7) have been obtained from the system [V(CO)6]−/CNR/X2 in dry THF, while the coupling product
[I2V(CNtBu)4tBuHNCꢀCNHtBu]I (8) is recovered as water is added. In contrast to the Nb system, VII complexes of composition
[I2V(CNR)4] (9) and [I2V(CNtBu)2(PR3)2] (10) can also be synthesised. The complex 10b (PR3=PPhMe2) has also been prepared
from cis-[V(CO)4(PPhMe2)2]− (11). The system [Ta(CO)6]−/CNtBu/I2 yielded the complexes [ITa(CO)3(CNtBu)3] (13a),
[ITa(CO)3(CNtBu)dppe] (13b) and [I2Ta(CNtBu)6]I (14). The chloro–isocyanide complexes [Cl2V(CCPh)(CNtBu)2] (12) and
[Cl2Ta(CNcHex)4] (15) have been generated from the metal chlorides as starting products. [INb(CO)2(CNtBu)4]·2toluene
(4a·2toluene), trans-[I2V{CN(2,6-Me2C6H3}4] (9a) and cis-[Et4N][V(CO)4(PPhMe2)2] ([Et4N]-11) have been characterised by
single-crystal X-ray diffraction. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Isocyanides; Niobium; Vanadium; Tantalum; Carbon–carbon coupling
of the isocyanide-ligase activity of Mo- and V-nitroge-
nases from Acotobacter. While the main issue of these
enzymes is the fixation of nitrogen, they also accept
various other unsaturated substrates, one of which is
methylisocyanide, which is reductively protonated to
methane and methylamine and, in a side reaction, to
ethylene and methylamine [2]. The mechanisms which
have been proposed for the in vitro coupling of iso-
cyanides (and the isoelectronic carbonyls) to acetylenes
involve a h2-ketenyl intermediate [3], a carbene–car-
byne intermediate [4], or a bis(carbyne) intermediate [5].
The present state of knowledge as revealed for the Mo
systems favours the carbyne mechanism (where the first
step is the reduction of MoII to Mo0) [6], which may
also hold for the reductive homo- and hetero-coupling
of CO and CNR by external reductants in the isoelec-
tronic NbI system [3,7].
1. Introduction
Reductive coupling of C1 fragments is a generally
occurring reaction in biosynthesis and in various in
vitro reactions catalysed by metal centres. Understand-
ing the mechanism that leads to the coupling products
is an important goal in understanding and thus optimis-
ing catalytically conducted reactions with respect to
both yield and selectivity. Coupling reactions involving
isocyanides as C1 fragments and the Group 6 metals as
activating centres have been thoroughly investigated
during the last two decades, starting with Lippard’s
discovery that the complexes [XMo(CNR)6]X (X=
halide) can be reduced to the aminoalkyne complexes
[XMo(CNR)4RHNCꢀCNHR]X by zinc in the presence
of water as the proton source [1]. This particular reac-
tion has attained some importance also in the context
We have recently shown that the reaction between
hexacarbonylniobate(−I), iodine and isocyanide in
THF containing small amounts of water yields the
NbIII complex [I2Nb(CNR)4RHNCꢀCNHR]I [8a]. Al-
* Corresponding author. Tel.: +49-40-428386087; fax: +49-40-
428382893.
E-mail address: rehder@xray.chemie.uni-hamburg.de (D. Rehder)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00240-5

D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
295
though
a
hexakis(isocyanide) NbIII complex, viz.
[I2M(CNR)6]+(M=Nb: 1, M=V: 7, M=Ta: 14; with
a third iodide as the counter-ion), [I3Nb(CNR)3] (2),
[I2V(CNR)2L2] (9 and 10), or carbonyl–isocyanide
[I2Nb(CNR)6]I, has also been isolated from dry THF
[8a], this compound could not be converted to the
coupling product by zinc or other reductants in the
presence of water, and hence lacks analogy with the
MoII case. This seems to exclude a reaction mechanism
in which the two electrons necessary for the reductive
coupling of two CNR stem from an external electron
source. Instead, an intermediately formed NbI centre
may deliver two electrons intramolecularly. NbI iso-
cyanide complexes such as [INb(CO)2dppe(CNR)2]
(dppe=Ph2PCH2CH2PPh2) [8a] have been synthesised.
While this compound again resists conversion to an
alkyne complex (as do the analogous vanadium and
tantalum compounds; see below), its close analogue
[INb(CO)2(CNR)4], as shown in this work, undergoes
this conversion without additional reducing agents
added.
complexes
such
as
[INb(CO)2(CNR)4]
(4),
[IV(CO)2(CNR)2dppe] (6) and [ITa(CO)3(CNtBu)L2]
(13), if absolutely dry THF is used as a solvent. Which
one of the complexes forms mainly depends on the
metalate/isocyanide
and
metalate/iodine
stoi-
chiometries, reaction conditions and work-up of the
reaction mixture. Compound 2, which compares with
the known complex [Cl3Nb(CNtBu)3] [9], can be fur-
ther converted to [I2Nb(CNtBu)3tolane]I (3) by addi-
tion of tolane. Complexes with a mixed ligand sphere
also form with phosphines; see 6, 10 and 13b. In the
vanadium system, the anions [V(CO)4(PR3)2]− (11),
generated photochemically from the hexacarbonylmeta-
late and phosphine, can also be used, although with less
satisfactory yields, as precursor compounds in the
preparation of the complexes 10.
In the context of CC coupling, the formation of
[INb(CO)2(CNR)4] (4) by consecutive addition of iso-
cyanide and iodine to a solution of [Nb(CO)6]− in dry
THF is of special interest. Compound 4 may also be
obtained from [INb(CO)4dppe] [10,11] with a four-fold
molar excess of isocyanide. Exchanging THF for
toluene slows down the reaction. Addition of water
2. Results and discussion
2.1. General reaction schemes
The precursor compound in all of the reactions lead-
ing to isocyanide–niobium complexes, and to most of
the isocyanide–vanadium and –tantalum complexes, is
[M(CO)6]−, which, depending on the reaction condi-
tions, yields seven-coordinate complexes of MI, hexa-
coordinate complexes of MI and MIII (and MII in the
case of M=V) or octa-coordinate complexes of MIII
(M=V, Nb). In contrast to the corresponding vana-
dium chemistry [8b], isocyanide complexes of NbII have
not been observed. The oxidation of M−I to MI–III is
carried out with iodine (or bromine, in same cases). In
the absence of additional ligands, [{M(CO)4}2(m-I)3]−
forms [8c]; in the presence of isocyanide, the reaction
proceeds to the formation of (see Schemes 1 and 2 for
the overall reactions for M=Nb and V/Ta)
converts
4
to
the
alkyne
complexes
[I2Nb(CNR)4RHNCꢀCNHR] (5). The direct conver-
sion of [IM(CO)2(CNR)2dppe] (6 for M=V), i.e. ex-
change of two of the isocyanide ligands in
[IM(CO)2(CNR)4] by the bis(phosphine), inactivates the
compound with respect to CC coupling. On the other
hand, the coupling products 5 and 8 form by direct
action of iodine upon a solution of [M(CO)6]− and
CNR in THF plus a small amount of water added.
A similar reaction can, in principle, be carried out
with bromine as the primary oxidising reagent—and
has led to the characterisation of 5b. In contrast, chlo-
rine did not yield well defined isocyanide complexes or
coupling products when employing [M(CO)6]− as pre-
cursor compounds. With the chlorides of vanadium and
tantalum on the other hand, two chloro-isocyanide
complexes (12 and 15) have been prepared.
2.2. Structures
The basic structures of the seven- (4, 6, 12) and
eight-coordinate (1, 5, 7, 8) complexes derive from a
trigonal prism, with one or two of the tetragonal faces
capped by the iodo ligands (see Schemes 1 and 2). The
structures of 1a (R=tBu) and 5a (R=tBu) have been
reported recently [8a]. [INb(CO)2(CNtBu)4]·2toluene
(4a·2toluene) crystallises in the orthorhombic space
group P212121. Crystal data and data on the structure
refinement are collated in Table 1, selected bonding
parameters in Table 2. Fig. 1 shows a plot and the
Scheme 1.

296
D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
Scheme 2.
labelling scheme. In spite of the relatively high R value
(9.4%), a consequence of the unfavourable cell dimen-
sions (c is about four times as long as a and b) and high
residual electron densities close to the heavy atoms, the
molecular connectivities have been established unam-
biguously. The molecular point symmetry is C26. As in
the case of similar NbI complexes with the co-ordina-
tion number 7, the arrangement for the
Nb(CO)2(CNR)4 moiety is basically trigonal prismatic.
Iodine is above the face occupied by the four isocyanide
ligands, and opposite the edge formed by the two
carbonyls—in accord with the complexes [XM(CO)6−
npn] (p stands for a coordinated phosphorus function;
M=V, Nb, Ta; X=Br, I) [12] and [INb-
(CO)2(CNCy)2dppe] [8a], and in accord with orbital
calculations according to which the ligands with the
higher p-acceptor capability (carbonyl) occupy the edge
opposite the less powerful p-acceptors (isocyanide or
phosphine) [13]. The bond lengths and bond angles are
comparable to those of other isocyanide-niobium com-
plexes such as 1, 5 and [INb(CO)2(CNCy)2dppe] [8a],
but differ from [XNb(CO)CNR(dmpe)2] (dmpe=
Me2PCH2CH2PMe2) in that the Nb–CꢀN–R groups
are almost linear in the former, while they are strongly
bent (ÚC–N–R=122–139°) [14] in the latter. Inter-
,
atomic distances amount to ca. 2.7 A [d(C1···C3) and
d(C2···C4)] and hence are short enough to enable C–C
coupling. Hence, the trigonal prism is predesigned to
allow for reductive coupling [13].
The six-coordinate VII complexes 9 and 10 attain an
octahedral geometry, as indicated by the X-ray struc-
ture of 9a (Fig. 2, Tables 1 and 2) and thus resemble
other isocyanide complexes belonging to this family
[8b]. The two iodo ligands in 9a are in trans positions.
The local symmetry of the molecule is Ci; the 2,6-
dimethylphenyl rings are slightly bent towards each
other. Again, the V-isocyanide moieties are essentially
linear.
The octahedral anion of [Et4N]-11 (Fig. 3, Tables 1
and 2) shows the cis-configuration that has previously
been documented for [V(CO)4{(Ph2P)2C6H4}]− [15a],
where the cis configuration is imposed by the chelate
structure. Bond lengths and angles are close to those
for an ideal octahedral geometry, and bonding parame-

D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
297
ters lie within the range observed for [V(CO)4{(Ph2P)2-
C6H4}]− [15a] and [V(CO)5PPh3]− [15b].
MeCN (Table 4 and Fig. 4), where the complexes retain
their IR pattern pertinent from the solid state (Table 3):
While the \0 conductivities are similar for all four
complexes, the s coefficients (s is the slope of the lines
in Fig. 4 and is mainly a measure for the stoichiometry
of the electrolyte) indicate that 1a and 2a are less
dissociated than 1d (and 3).
The isocyanides in 1 and 2 cannot be replaced by
ligands (L) such as alkynes and/or phosphines. The
tolane complex 3, however, can be generated as shown
in Eq. (2). Compound 3 has been formulated as ionic
complex based on the conductivity data (see above).
2.3. Preparation, characteristics and reactions
Complexes of types 1 and 2 have been prepared
according to Eq. (1) under slightly different reaction
conditions. The reaction proceeds through an interme-
diately formed complex 4 (IR evidence) which, how-
ever, cannot be isolated under the conditions provided
for the direct generation of 1 or 2. The characteristic
w(CꢀN) data are summarised in Table 3. Both 1a and
2a, when dissolved in THF, give rise to a single sharp
w(CꢀN) band at 2199 cm−1, corresponding to an
unidentified isocyanide species, a plausible formulation
of which is [I2Nb(CNR)3(THF)3]I.
(2)
If the reactions represented by Eq. (1) are carried out
in damp instead of absolutely dry THF, the coupling
products 5 are obtained (idealised by Eq. (3)). The
previously reported [8a] structure for R=tBu (5a) is
related to that of 1a (see Scheme 1); although the
inter-atomic distance between the carbons of two adja-
(1)
Some insight into the solution structure has also been
obtained from conductivity measurements of the com-
plexes 1a, 2a, 1d and [I2Nb(CNtBu)3tolane]I (3) in
,
cent isocyanides in 1a, 2.6(1) A, allows for a coupling
reaction, type 1 complexes are not likely to be precur-
Table 1
Data on the crystal structure and refinement of [INb(CO)2(CNtBu)4]·2toluene (4a·2toluene), [I2V{CN(2,6-Me2C6H3)}4], (9a), and
[Et4N][V(CO)4(PPhMe2)2] ([Et4N]-11)
4a·2toluene
9a
[Et4N]-11
Empirical formula
C36H52IN4NbO2
792.63
173(2)
C40H44I2N4OV
901.53
153(2)
C28H42NO4P2V
569.51
153(2)
Formula weight (g mol−1
Temperature (K)
)
,
Wavelength (A)
1.54178
Orthorhombic
P212121
0.71073
Monoclinic
P21/c
0.71073
Monoclinic
P21/c
Crystal system
Space group
,
a (A)
9.577(2)
10.724(3)
38.907(14)
11.033(2)
18.263(4)
11.092(2)
116.04(3)
9.975(3)
11.900(5)
25.65(2)
90.05(4)
,
b (A)
,
c (A)
i (°)
Z
4
2
4
3
,
V (A )
3966(2)
1.318
8.77
1624
2008.1(7)
1.491
1.82
898
3044(2)
1.243
0.426
1208
Dcalc (g cm−3
)
Absorption coefficient (mm−1
F(000)
)
Crystal dimensions (mm)
q Range (°)
0.2×0.1×0.1
4.28–76.51
0.3×0.3×0.2
2.33–30.07
0.3×0.2×0.2
2.33–27.57
Index ranges
−12BhB0, −12BkB0,
−1BhB14, −1BkB25,
−1BhB12, −15BkB15,
−47BlB0
−15BlB14
−33BlB33
Reflections collected
Independent reflections
Rint
4295
4252
0.0264
6899
5850
0.0317
16 608
7037
0.0591
Parameters
411
245
333
Goodness-of-fit
1.046
1.015
1.010
Final R indices [I\2|(I0)]
R indices (all data)
Absolute structure parameter
R1=0.0942, wR2=0.2153
R1=0.1409, wR2=0.2858
−0.01(3)
R1=0.0501, wR2=0.1275
R1=0.0647, wR2=0.1369
R1=0.0567, wR2=0.1196
R1=0.0975, wR2=0.1357
Largest difference peak and hole (e 3.02 and −2.30
1.439 and −1.282
0.707 and −0.733
−3
,
A
)

298
D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
Table 2
,
Selected bond lengths (A) and bond angles (°) for 4a·2toluene, 9a and [Et4N]-11
[INb(CO)2(CNtBu)4]·2toluene (4a·2toluene)
[I2V{CN(2,6-Me2C6H3)}4] (9a)
[Et4N][V(CO)4(PPhMe2)2] ([Et4N]-11)
Bond lengths
Nb–I
2.945(3)
2.22(2)
2.23(2)
2.17(3)
2.27(3)
2.10(3)
2.07(2)
1.19(3)
1.12(2)
1.20(3)
1.14(3)
1.17(2)
1.19(2)
2.69
V–I
V–C1
V–C2
C1–N1
C2–N2
N1–C10
N2–C20
2.7699(5)
V–P1
V–P2
2.439(2)
Nb–C1
Nb–C2
Nb–C3
Nb–C4
Nb–C5
Nb–C6
C1–N1
C2–N2
C3–N3
C4–N4
C5–O1
C6–O2
C1···C3
C1···C4
2.128(3)
2.158(4)
1.150(5)
1.139(5)
1.403(3)
1.401(5)
2.451(1)
1.910(3)
1.936(3)
1.896(3)
1.945(3)
1.176(4)
1.167(4)
1.184(4)
1.163(4)
V–C1
V–C2
V–C3
V–C4
C1–O1
C2–O2
C3–O3
C4–O4
3.49
Bond angles
C1–Nb–I
C2–Nb–I
C3–Nb–I
C4–Nb–I
87.4(5)
80.7(5)
85.7(6)
83.2(6)
145.3(9)
145.5(8)
166.0(6)
103.8(9)
75.8(8)
115.0(9)
72.3(9)
176.1
I–V–C1
I–V–C2
88.34(9)
90.71(10)
95.21(14)
174.3(3)
171.0(3)
178.8(4)
173.5(4)
P1–V–P2
P1–V–C1
P1–V–C2
P1–V–C3
P1–V–C4
P2–V–C
P2–V–C2
P2–V–C3
P2–V–C4
C1–V–C2
C1–V–C3
C1–V–C4
C2–V–C3
C2–V–C4
V–C–O, av.
92.12(4)
170.84(9)
88.41(9)
89.26(10)
91.40(10)
195.43(10)
92.77(9)
176.34(10)
91.05(11)
86.09(13)
83.51(13)
93.62(13)
90.66(13)
176.18(14)
174.9
C1–V–C2
V–C1–N1
V–C2–N2
C1–N1–C10
C2–N2–C20
C5–Nb–I
C6–Nb–I
C1–Nb–C2
C1–Nb–C3
C1–Nb–C4
C1–Nb–C5
C1–Nb–C6
Nb–C–N, av.
Nb–C5–O1
Nb–C6–O2
C–N–R, av.
175(2)
172(3)
175.5
cyanide, 4a can be isolated in crystalline form (Eq. (4)).
As 4a is redissolved in damp toluene, it converts to 5a
with a yield of 31%. The role of water suggests that it
sors for type 5 complexes since, in contrast to the Mo
analogues, 1 cannot be converted to the coupling
product 5 with Zn/H2O and other reductants such as
Na/Hg, Na, Li, or Mg assisted by naphthalene or
cyclo-octatetraene. We therefore suggest that an inter-
mediately formed NbI complex, viz. 4, is the precursor
compound, and NbI itself delivers the electrons for the
reductive coupling reaction in an intramolecular pro-
cess. Evidence for an intermediate 4 formed in THF
solution is provided as the reaction shown in Eq. (3) is
monitored by IR, and by the isolation of the respective
complexes in dry THF (or toluene). Compound 5 is
formed in appreciable yields only if less than the six
equivalents of CNR demanded by Eq. (3) are em-
ployed, hinting towards a stabilising role (with respect
to minimised formation of Nb iodides) of the CO
ligands present in 4. Furthermore, only 1.5 (instead of
the required two) equivalents of iodine have been used
in order to prevent rapid oxidation to NbIV/V. If, in-
stead of THF, dry toluene is the solvent, and if the
hexacarbonylniobate(−I)
is
oxidised
to
[{Nb(CO)4}2(m-I)3]− prior to the addition of iso-
Fig. 1. SCHAKAL plot and numbering scheme for 4a.

D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
299
Fig. 2. SCHAKAL plot and numbering scheme for 9a.
acts as a proton donor to the exposed nitrogen of an
isocyanide ligand, thus initiating the coupling reaction.
The first (irreversible) oxidation potential for 4a
amounts to 600 mV (vs. SCE).
paramagnetic, exhibiting a veff of 3.89 (9a) and 4.07
(9b), hence close to the spin-only value of 3.87 BM. The
structurally characterised compound 9a (Fig. 2) indi-
cates a trans configuration and thus local D4h symmetry
for the molecule. Accordingly,
a single—though
broad—w(CN) is observed. For 10a and 10b (one
w(CN)), we also assume the all-trans arrangement. In
the case of 10c, the chelating dppe prevents this
configuration. Consequently, the IR shows two w(CN)
bands.
(3)
The trans configuration is also realised in the TaII
complex 15 (see Scheme 2), in analogy to the corre-
sponding vanadium complexes 9, and to [Cl2Ta(PMe3)4]
[16]. In the dichloro-VIII complex 12, which was iso-
lated as a 1:1 adduct with LiCl, the two isocyanide
ligands occupy cis positions, again based on IR evi-
dence. The formation of 12 and 15 is depicted by Eq.
(5) and Eq. (6). Eq. (6) is conducted so as to reduce
(4)
From a general point of view, the reactions carried
out in the V (and Ta) systems (Scheme 2) resemble
those described for Nb. Selected IR data are collated in
Table 5. The VIII and TaIII complexes 7 and 14 are
obtained in dry THF (14 as the 1:1 adduct with
[Et4N]I), while the coupling product 8 forms as H2O is
added to the reaction mixture. Again, less than the
stoichiometric amount of I2 for a complete conversion
of all of the V−I to VIII has to be used in order to keep
yields at a reasonable level. A corresponding coupling
product for Ta has also been obtained, evidenced by IR
(w(CꢀN)=2211, w(NH)=3210, w(CꢁN)=1544 cm−1
)
which, however, could not be isolated in pure form.
Nor was it possible to convert the phosphine complex 6
to the coupling product 8, in analogy to the respective
inactivity observed for [INb(CO)2(CNR)2dppe].
A special feature in the vanadium system, distinct
from the niobium case, is the formation of isocyanide
complexes with the metal in the +II oxidation state,
viz. complexes 9 and 10, if the molar ratio vanadate:I2
is 1:1.5, i.e. in between the conditions pertinent for the
formation of the VI complexes 6 (1:1) and the VIII
complexes 7 (1:2). As expected, the VII complexes 9 are
Fig. 3. SCHAKAL plot and numbering scheme for [Et4N]-11.

300
D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
Table 3
IR data (w(CꢀN) and w(CꢀO) region) of isocyanide-niobium complexes
a
b
Compound
w(CꢀN) (cm−1
)
Other
[I2Nb(CNR)6]I (1)
R=tBu (1a)
iPr (1b)
2-Pent (1c)
CH2Tos (1d)
CH2C(O)OMe (1e)
[I3Nb(CNR)3] (2)
R=tBu (2a)
2197sh
2203sh
2204sh
2204sh
2154vs
2160vs
2160vs
2156vs
2178
2022sh
2016sh
2020sh
2015sh
2203
2190
cHex (2b)
[Br3Nb(CNcHex)4] (2c)
[I2Nb(CNtBu)3tolane]I c (3)
[XNb(CO)2(CNR)4] (4)
X=I, R=tBu (4a)
cHex (4b)
2,6-Me2C6H3 (4c)
X=Br, R=tBu (4d)
[X2Nb(CNR)4RHNCꢀCNHR]X (5)
X=I, R=tBu (5a)
2205sh
2164vs
2202
2198s
2198s
2146vs
2204s
2155vs
2159vs
2107vs
2133vs
1874s, 1815m; w(CO)
1868s, 1814m; w(CO)
1870s, 1809s; w(CO)
1871s, 1822m; w(CO)
2201
3196; w(NH)
1557; w(CꢁN)
3189; w(NH)
1564; w(CꢁN)
X=Br, R=tBu (5b)
2194sh
2213vs
a Abbreviations: tBu, tertbutyl; iPr, isopropyl; 2-Pent, 2-pentyl; Tos, tosyl; cHex, cyclohexyl.
b In KBr. Compare the w(CꢀN) for the free isocyanide (in THF): CNtBu (2134), CNiPr (2144), CN(2-Pent) (2141), CN(2,6-Me2C6H3) (2122,
KBr), CNCH2-Tos (2154, KBr), CNCH2C(O)OMe (2149 cm−1).
c The ionic formulation has been chosen on the basis of conductivity measurements.
tantalum(V) to the +II state prior to coordination of
isocyanide.
tions model the isocyanide reductase/ligase activity of
nitrogenases as well as the role of low-valent Group 5
metals, generated in situ in the system MCln/Zn/H+, in
the in vitro coupling reactions involving organic car-
bonyls [18,19], alkynes and nitriles [20], or alkynes [21].
Whether or not the first reaction step of the conversion
of 4 to 5 is an electrophilic attack of protons to the
nitrogen of the isocyanide, with the formation of a
carbyne intermediate as proposed for coupling reac-
tions with external electron donors [4,5], remains to be
clarified and is presently under investigation in our
laboratory.
[VCl3(THF)3]+LiCꢀCPh+2CNtBu
“[Cl2V(CꢀCPh)(CNtBu)2] (12)+LiCl
(5)
(6)
TaCl5+3Na+4CNcHex
“[Cl2Ta(CNcHex)4] (15)+3NaCl
3. Conclusions
The isolation and structural characterisation of the
carbonyl–isocyanide–niobium(I) complex 4a, and its
conversion to the aminoalkyneniobium(III) complex 5a
in the presence of small amounts of water but without
an external reducing agent, has shown that type 4
complexes can be considered intermediates in reductive
C–C coupling of isocyanides. In this reaction, NbI acts
both as an activator (by coordination) of the substrate
and an internal two-electron transfer agent for the
substrate. This is different from the coupling reactions
reported for {ClNbI(CNR)} and {IMoII(CNR)} sys-
tems and probably reflects the greater basicity of the
{INbI(CNR)} centre. The reaction is reminiscent of the
conversion of the Cr0 complex [Cr(CNR)6] to the CrII
complex [ICr(CNR)4RNHCꢀCNHR]I [17]. A corre-
sponding aminoalkyne–VIII complex, 8, has also been
obtained by direct interaction of [V(CO)6]− with iso-
cyanide and iodine in the presence of water. The reac-
4. Experimental
All operations were carried out under nitrogen, em-
ploying the common Schlenk techniques, and in abso-
lute solvents, if not noted otherwise. THF was dried
with lithium alanate and distilled in a N2 stream. Mix-
tures of ethanol and liquid nitrogen were employed as
cold baths. CO released during the reactions was al-
lowed to evolve through a mercury valve (15 mm). All
compounds were dried at room temperature (r.t.) under
high vacuum prior to elemental analyses.
Starting materials were purchased (CNcHex, CNtBu,
CNCH2Tos, CNCH2C(O)OMe, CN(2,6-Me2C6H3),
tolane, dppe, PPhMe2, NbCl5, VCl3, TaCl5) or prepared
according to literature procedures (CNiPr, CN2–Pent

D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
301
Table 4
Selected conductivity data for some isocyanide–niobium complexes in MeCN a
Complex
1a
c (mmol l−1
)
\
(S cm2 mol−1
)
\
(S cm2 mol−1
0
)
s (S cm2 l−1/2 mol−3/2
)
M
0.724
0.329
0.692
0.314
0.835
0.379
0.761
0.346
218
228
217
223
226
242
195
211
245
235
274
244
32.3
21.7
57.3
57.1
2a
1d
3a·THF
a The quantities in this table are connected by the relation \M=\0−s×c1/2
.
[22], [Et4N][Nb(CO)6] [10c,23], [INb(CO)4dppe] [10c,11],
[Et4N][{Nb(CO)4}2(m-I)3][8c,24],[Et4N][V(CO)6] [8b,23],
[Et4N][Ta(CO)6] [25]).
not presented in detail for this reason. l(13C) values
for methyl groups of coordinated tBuNC cover the
range 30–32 ppm, the shift ranges for the phenyl
rings of tolane and the phosphines amount to
l(13C)=126–135. l(13C) resonances for selected com-
pounds are provided in the respective experimental
sections. The 51V-NMR spectra, obtained at 94.73
MHz, are referenced against VOCl3.
IR spectra were obtained in KBr, nujol or THF
(0.1 mm CaF2 cuvette) on a Perkin–Elmer spectrome-
ter 1720 FT. Characteristic data are summarised in
Table 3 (Nb complexes) and Table 5 (V and Ta com-
plexes). For the cyclovoltammetric measurement of
4a, an EG&G Princeton Applied Research poten-
tiostat 273A was employed. The working electrode
was a platinum foil and the counter electrode a plat-
inum wire. The potential was internally standardised
against Fc/Fc+, and referenced against the saturated
calomel electrode (SCE). The measurements were car-
ried out in MeCN at concentrations of approximately
1 mM and with 0.2 M TBAP as conducting salt.
Conductivity measurements were carried out with a
WTW type LF 315/Set 300230 electrode in MeCN at
25°C (constant) under N2. Magnetic measurements
were carried out with a Johnson Matthey Chemicals
Ltd. susceptibility balance in 4 mm diameter vials.
The calculated magnetic moments were corrected for
diamagnetic contributions.
Data for the X-ray structure analyses were col-
lected in the q/2q scan mode on a Hilger & Watts
diffractometer
monochromator) Mo–Ka irradiation. The structure of
4a was determined with Mo–Ka and also, on a CAD
diffractometer, with monochromatic (graphite
with
monochromatic
(graphite
-
4
monochromator) Cu–Ka irradiation. The quality of
the structure determination did not significantly differ
in these two measurements. The program packages
SHELXS-86 and SHELXL-93 were used throughout. All
non-hydrogen atoms were refined anisotropically. Hy-
drogen atoms were put into calculated positions and
included with isotropic displacement factors in the
last cycles of the structure refinement. Absorption
corrections were carried out for 4a and 9a (see Table
1 for details on the structure solution and refine-
ment).
NMR spectra were scanned on a Bruker AM 360
spectrometer with the usual instrument settings. Due
to the quadropolar nature of the nuclei 93Nb (nuclear
spin I=9/2, quadrupole moment Q= −0.28×10−28
m2), 51V (I=7/2, Q= −0.052×10−28 m2) and 181Ta
(I=7/2, Q=3.4×10−28 m2), and the non-cubic sym-
metry of the complexes, NMR spectroscopy for the
nuclei directly bonded to these nuclei (13C, 31P) usu-
ally show, if any, complex and broad signals which
are of limited analytical significance for the complexes
[26]. Typically, 31P resonances are ca. 35–50 (dppe)
1
or centred at ca. 25 ppm (alkyl phosphines). The H-
NMR data of the coordinated ligands are of re-
stricted diagnostic value since they do not differ
significantly from those of the free ligands (e.g.
l(1H)=1.29–1.77 for CH2 in cHex, 2-Pent, dppe;
3.50–3.92 for CH in cHex and 2-Pent; 1.3–1.5 for
CH3 in tBu; 7.1–7.4 for the phenyls of tolane and
phosphines (type 3, 6 and 13b complexes)); they are
Fig. 4. Graphical presentation of conductivity data for the complexes
1a, 2a, 1d and 3a (see Table 4).

302
D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
Table 5
IR data [w(CꢀN) and w(CꢀO) region] of isocyanide–vanadium and –tantalum complexes a,b
a
Compound
w(CꢀN) (cm−1
)
Other
[XV(CO)2(CNtBu)2dppe] (6)
X=I (6a)
Br (6b)
2168vs
2159vs
2098vs
2105vs
1889vs, 1842vs; w(CO)
1887vs, 1841vs; w(CO)
[ITa(CO)3(CNtBu)L2] (13)
L=CNtBu (13a)
2L=dppe (13b)
2145vs
2135vs
2173br
2148br
2208
2125vs
1887s, 1863s, 1824s; w(CO)
1955vs, 1868vs, 1840vs; w(CO)
[I2V(CNtBu)6]I (7)
[I2Ta(CNtBu)6]I (14) c
[I2V(CNtBu)4(tBuHNCꢀCNHtBu)]I (8)
[I2V(CNR)4] (9)
3204; w(NH), 1561; w(CꢁN)
R=2,6-Me2C6H3 (9a)
CH2Tos (9b)
2137br
2136br
[X2V(CNtBu)2(PR3)2] (10)
X=I, PR3=PPh3 (10a)
PPhMe2 (10b)
X=Br, 2PR3=dppe (10c)
[V(CO)4(PPhMe2)2]− (11)
[Cl2V(CꢀCPh)(CNtBu)2] (12) f
[Cl2Ta(CNcHex)4] (15)
2171
2156 d
2155vs
2145sh
2174vs
1891s, 1774vs, 1760sh, 1724m; w(CO) e
2207vs
2151br
a For abbreviations, see legend to Table 3.
b In KBr or nujol, if not noted otherwise.
c Isolated as the 1:1 adduct with [Et4N]I.
d In THF.
e w(CO) in THF: 1896s, 1786vs, 1762sh, 1731m.
f Isolated as the 1:1 adduct with LiCl.
4.1. [I2Nb(CNR)6]I (1)
The same procedure was also applied for the prepara-
tion of [Br3Nb(CNcHex)4] (2c), from [Et4N][Nb-
(CO)6] (230 mg, 0.59 mmol), CNcHex (290 ml, 2.36
mmol) and bromine (61 ml, 1.19 mmol). Reaction temper-
ature in this case was −80°C (toluene-N2 cold bath), and
reaction time 24 h. Yield of red–brown 2c, 354 mg (82%).
IR (KBr) w(CH) 2931 and 2854, w(CN) 2205vs and 2164s,
l(CH) 1448 cm−1. Anal. Calc. for C28H44Br3N4Nb
(M=769.30 g mol−1): C, 43.72; H, 5.76; N, 7.28; Br,
31.16; Nb, 12.08. Found: C, 43.22; H, 5.62; N, 6.85; Br,
30.3; Nb, 12.4%.
The type 1 complexes were prepared as described
previously for 1a (R=tBu) [8a]. Colour, IR data, yields
and elemental analyses are summarised in Table 6.
4.2. [NbI3(CNtBu)3] (2a)
[Et4N][Nb(CO)6] (604 mg, 1.54 mmol), dissolved in dry
THF (30 ml), was treated with CNtBu (1.05 ml, 6.04
mmol) and after cooling to −60°C, with iodine (784 mg,
3.09 mmol) in eight successive portions, keeping the
temperature constantly at −60°C. The solution, while
stirred continuously, was slowly warmed to r.t. within 30
h. [Et4N]I was filtered off, the filtrate was evaporated to
dryness (r.t., vacuum), washed with two 5 ml portions of
pentane and dried under high vacuum to yield a red–
4.3. [I2Nb(CNtBu)3tolane]I·THF (3·THF)
[Et4N][Nb(CO)6] (210 mg, 0.54 mmol) was dissolved in
THF (25 ml). To this solution, CNtBu (182 ml, 1.61
mmol) and tolane (96 mg, 0.54 mmol) were added. The
solution was cooled to −60°C and treated with iodine
(273 mg, 1.07 mmol). This reaction mixture was stirred
for 12 h while warmed to r.t., and stirred at r.t. for an
additional two days. [Et4N]I was filtered off, the filtrate
evaporated to dryness, washed with three 5 ml portions
of pentane and dried to yield an orange–red powder.
Yield 489 mg (93% based on [Et4N][Nb(CO)6]).
IR (KBr): w(CH)arom 3051, w(CH) 2981, 2933 and
brown powder. Yield 1.030
g
(93% based on
[Et4N][Nb(CO)6]). IR (KBr) w(CH) 2981 and 2933,
w(CN) 2203, l(CH) 1450 cm−1. Anal. Calc. for
C15H27I3N3Nb (M=723.02 g mol−1): C, 24.92; H, 3.76;
N, 5.81. Found: C, 25.76; H, 3.92; N, 6.17%.
Ochre [I3Nb(CNcHex)3]·THF (2b·THF) was prepared
accordingly from [Et4N][Nb(CO)6] (240 mg, 0.61 mmol),
CNcHex (226 ml, 1.83 mmol) and iodine (311 mg, 1.23
mmol) in 82% yield. IR (KBr) w(CH) 2931 and 2854,
w(CN) 2190, l(CH) 1448 cm−1. Anal. Calc. for
C25H21I3N3NbO (M=873.24 g mol−1): C, 34.39; H,
4.73; N, 4.81. Found: C, 34.63; H, 4.90; N, 5.15%.
2868, w(CN) 2202 cm−1 13C-NMR (THF-d8) l 30.7
.
[C(CH3)3], 58.5 [C(CH3)3], 128.3 to 129.5 (Ph), 207.7
(CꢀC), 253.0 (CNtBu). Anal. Calc. for C33H45I3N3NbO
(M=973.36 g mol−1): C, 40.72; H, 4.66; N, 4.32; I,

D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
303
39.11; Nb, 9.54. Found: C, 40.49; H, 4.44; N, 5.15; I,
4.4.3. Method C
39.1; Nb, 9.7%.
A 25 ml sample of a toluene solution containing
[Et4N][{Nb(CO)4}2(m-I)3] (589 mg, 0.64 mmol) was
cooled to −40°C and treated with CNtBu (578 ml,
5.12 mmol). The solution was stirred for 16 h while
warmed back to r.t. and filtered. The filtrate was kept
at 4°C to yield orange–red crystals of 4a·2toluene,
suitable for X-ray analysis. Yield 584 mg (68% based
on [Et4N][{Nb(CO)4}2(m-I)3]).
Alternatively, 3 can be prepared by adding, at r.t.,
tolane to a THF solution of 2, reaction time of two
days and work-up as described above.
4.4. [INb(CO)2(CNtBu)4] (4a)
[INb(CO)2(CNcHex)4] (4b), [INb(CO)2{CN(2,6-
Me2C6H3}4] (4c) and [BrNb(CO)2(CNtBu)4] (4d), were
prepared accordingly. 4b: IR (KBr) w(CH) 2933 and
2854, w(CN) 2198s, 2159s and 2125sh, w(CO) 1868m
and 1814m cm−1. Anal. Calc. for C30H44IN4NbO2
(M=712.52 g mol−1): C, 50.57; H, 6.22; N, 7.86.
Found: C, 51.10; H, 6.35; N, 8.06%. 4c: IR (KBr)
w(CN) 2146vs and 2107vs, w(CO) 1870s and 1809s
cm−1. Anal. Calc. for C38H36IN4NbO2 (M=800.6 g
mol−1): C, 57.01; H, 4.53; N, 7.00. Found: C, 58.35;
H, 5.03; N, 7.48%. 4d: IR (KBr): w(CH) 2978, 2933
and 2871, w(CN) 2204m, 2133m and 2111s, w(CO)
4.4.1. Method A
[INb(CO)4dppe] (630 mg, 0.86 mmol), dissolved in
THF (25 ml) was cooled to −40°C and treated,
while stirred vigorously, with CNtBu (390 ml, 3.45
mmol). The solution was warmed to r.t. within 24 h.
During this time, the colour gradually changed from
brown to light red. The solution was concentrated to
about one fifth of its original volume and allowed to
stand at −20°C. Compound 4a precipitated as a
dark yellow powder, which was filtered off and dried.
Yield 274 mg (52% based on [INb(CO)4dppe]). IR
(KBr) w(CH) 2981 and 2934, w(CN) 2198m and
2155vs, w(CO) 1874m and 1815m cm−1. Anal. Calc.
for C22H36IN4NbO2 (M=608.36 g mol−1): C, 43.43;
H, 5.96; N, 9.21. Found: C, 43.40; H, 6.01; N, 9.23%.
1871s
and
1822
cm−1
.
Anal.
Calc.
for
C22H36BrN4NbO2 (M=561.36 g mol−1): C, 47.07; H,
6.46; N, 9.98. Found: C, 47.56; H, 6.71; N, 9.93%.
4.5. [I2Nb(CNtBu)4tBuHNCꢀCNHtBu]I (5a)
4.4.2. Method B
[Et4N][Nb(CO)6] (510 mg, 1.30 mmol) dissolved in
30 ml of THF was treated with tBuNC (440 ml, 3.9
mmol) and cooled to −70°C. Upon addition of I2
(330 mg, 1.3 mmol), vigorous CO formation was ob-
served. The solution was warmed to r.t. within 15 h,
filtered, treated with 3 ml of pentane and stored at
−20°C. After 4 h, orange, crystalline 4a was ob-
tained which was filtered off and dried under vacuum
4.5.1. Method A (see also Ref. [8a])
[Et4N][Nb(CO)6] (426 mg, 1.09 mmol) was dissolved
in 25 ml of THF (Merck, proanalysis, water content
4.2 mmol in 25 ml) and treated with CNtBu (491 ml,
4.4 mmol). The solution was stirred, cooled to
−40°C and treated with iodine (369 mg, 1.45 mmol).
The reaction mixture was warmed to r.t. within 28 h,
accompanied with
a colour change from yellow
for
4
h. Yield: 500 mg (63% based on
to red–brown. A solid residue consisting of [Et4-
N]I and NbIx (x, according to elemental analyses
of the vacuum-dried residue, varies between 2.1
and 2.3) was filtered off. In order to obtain crys-
talline material, pentane (5 ml) was added to
[Et4N][Nb(CO)6]). IR (nujol) w(CN) 2190vs and
2150vs, w(CO) 1875s and 1817s. Anal. Calc. for
C22H36IN4O2Nb (M=608.4 g mol−1): C, 43.43; H,
5.96; N, 9.21. Found: C, 43.63; H, 6.08; N, 9.11%.
Table 6
Experimental details for the compounds [I2Nb(CNR)6]I (1)
R
Complex w(CH) (cm−1
)
Colour
Yield (%) M (g mol−1
)
Formula
Anal. (Calc./Found) (%)
C
H
N
tBu
iPr
1a
1b
1c
1d
2978, 2933
2936, 2880
2939, 2877
3011, 2922
2953
red
red
red
87
81
82
972.42
888.26
1056.58
1645.07
1068.16
C30H54I3N6Nb
C24H42I3N6Nb
C36H66I3N6Nb
C54H54I3N6NbO12S6 39.43/39.72 3.31/3.50 5.11/5.28
C24H30N6NbO12 26.99/27.34 2.83/3.01 7.87/8.11
37.06/36.57 5.60/5.55 8.64/8.43
32.45/32.70 4.77/5.01 9.46/9.84
40.92/41.16 6.30/6.58 7.95/8.09
2-Pent
CH2Tos
CH2C(O)OMe 1e
dark brown 92
black 88

304
D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
the filtrate. This solution was refluxed for 30 min and
cooled back to r.t. Orange–red crystalline 5a separated
with bromine (77 ml, 1.50 mmol) and stirred overnight.
[Et4N]Br was filtered off, the filtrate evaporated to
dryness, and the residue washed with pentane. The
yellow–brown powder thus obtained contained
[Br2V(CNtBu)4], which was extracted with a pentane/
THF mixture. From the extract, yellow crystals with
the cell parameters reported for [Br2V(CNtBu)4] [8b]
were isolated. The remaining powder (6b) was dried.
Yield 517 mg (46%). Anal. Calc. for C33H42BrN2O2P2V
(M=751.56 g mol−1): C, 60.73; H, 5.63; N, 3.73.
Found: C, 60.52; H, 5.89; N, 3.70%. A 51V-NMR signal
was not detected, probably due to residual admixtures
of paramagnetic [Br2V(CNtBu)4].
during
[Et4N][Nb(CO)6]). IR (KBr) w(NH) 3196, w(CH) 2978,
2933 and 2871, w(CꢀN) 2201, w(CꢁN) 1557 cm−1
13C-NMR (THF-d8) l 31.2 [–NH–C(CH3)3], 30.1
[CNC(CH3)3], 58.8 [–NH–C(CH3)3)], 49.9
3 days. Yield 478 mg (45% based on
.
[CNC(CH3)3]. Anal. Calc. for C30H56I3N6Nb (M=
974.44 g mol−1): C, 36.50; H, 5.77; N, 8.73. Found: C,
36.27; H, 5.77, N, 8.61%.
4.5.2. Method B
Compound 4a (121 mg, 0.20 mmol; prepared by
method A) was dissolved in toluene-d8 (20 ml; source:
Deutero GmbH; not specifically dried) and allowed to
stand at r.t. for five days. Crystalline 5a formed, which
was filtered off the yellow solution and dried under
high vacuum. Yield: 59 mg (31% based on 4a). Spectral
parameters and analytical data corresponded to those
of 5a prepared according to method A.
4.9. [I2V(CNtBu)6]I (7)
A solution of [Et4N][V(CO)6] (349 mg, 1 mmol) and
CNtBu (678 ml, 6 mmol) in 20 ml of THF was cooled
to −70°C and treated with iodine (508 mg, 2 mmol).
The mixture immediately turned red and CO developed
vigorously. The finally green–black solution was
warmed to r.t. within 20 h. After filtration, the filtrate
was treated with 10 ml of pentane and allowed to stand
at −20°C for two days. The green powder that precip-
itated during this time was recovered and dried. Yield
631 mg (68%). Anal. Calc. for C30H54I3N6V (M=
930.45 g mol−1): C, 38.71; H, 5.85; N, 9.03. Found: C,
36.34; H, 5.61; N, 8.62%.
4.6. [Br2Nb(CNtBu)4tBuHNCꢀCNHtBu]Br·THF
(5b·THF)
The compound was prepared as described for 5a
(method A), and isolated as a brown, microcrystalline
powder in 42% yield. IR (KBr) w(NH) 3189, w(CH)
2976, 2933 and 2871, w(CꢀN) 2213 and 2194, w(CꢁN)
1564 cm−1. Anal. Calc. for C34H64Br3N6NbO (M=
905.54 g mol−1): C, 45.10; H, 7.12; N, 9.28. Found: C,
44.95; H, 7.04; N, 8.91%.
4.10. [I2V(CNtBu)4(tBuHNCꢀCNHtBu)]I (8)
[Et4N][V(CO)6] (508 mg, 1.45 mmol) was dissolved in
30 ml of THF and treated with water (150 ml, 8.3
mmol) and CNtBu (657 ml, 5.82 mmol). The solution
was cooled to −50°C, and iodine (508 mg, 2.0 mmol)
was added. The mixture was stirred at −50°C for 12 h
to yield a black brown solution which was warmed to
r.t. and refluxed for 1 h. The still-hot solution was
filtered. The red–brown residue consisted of [Et4N]I
and an unidentified isocyanide complex with a charac-
teristic IR absorption at 2161 cm−1. The brown–red
filtrate (ca. 28 ml) was treated with 7 ml of pentane and
allowed to stand at −20°C for three days. Bordeaux-
red, microcrystalline 8 was then recovered by filtration.
Yield 410 mg (30%). 13C-NMR (THF-d8) l 31.4 [–
NH–C(CH3)3], 30.4 [CNC(CH3)3], 58.6 [–NH–
C(CH3)3)], 49.5 [CNC(CH3)3]. Anal. Calc. for
C30H56I3N6V (M=932.47 g mol−1): C, 38.64; H, 6.05;
N, 9.01. Found: C, 38.34; H, 6.39; N, 8.88%.
4.7. [IV(CO)2(CNtBu)2dppe] (6a)
[Et4N][V(CO)6] (708 mg, 2.03 mmol), dppe (808 mg,
2.03 mmol) and tBuNC (459 ml, 4.06 mmol) were
dissolved in 30 ml of THF and cooled to −60°C.
Addition of iodine (515 mg, 2.03 mmol) immediately
produced a red colour ([V2(CO)8(m-I)3]−). The solution
was warmed to 8°C, stirred for 12 h, and further at r.t.,
until the IR spectrum indicated complete conversion to
the isocyanide complex (ca. 6 h). The black solution
was then filtered, the filtrate evaporated to dryness and
washed with pentane to yield, after drying, 910 mg
(1.14 mmol, 56%) of brown, powdery 6a. 13C-NMR
(THF-d8) l 22.8 (CH2), 30.1 [C(CH3)3], 49.8 [C(CH3)3],
127.8 to 133.2 (Ph). Anal. Calc. for C38H42IN2O2P2V
(M=798.56 g mol−1): C, 57.16; H, 5.30; N, 3.51.
Found: C, 56.82; H, 5.51; N, 3.74%. l(51V) (THF-d8)=
−849, W1/2=470 Hz.
4.8. [BrV(CO)2(CNtBu)2dppe] (6b)
4.11. [I2V(CNR)4], R=2,6-Me2C6H3 (9a) and CH2Tos
(9b)
[Et4N][V(CO)6] (524 mg, 1.50 mmol), dppe (597 mg,
1.50 mmol) and tBuNC (339 ml, 3.00 mmol), dissolved
in 30 ml of THF and cooled to −70°C, were treated
Preparation of 9a: [Et4N][V(CO)6] (349 mg, 1.00
mmol) and 2,6-dimethylphenylisocyanide (523 mg, 4.00

D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
305
mmol) were dissolved in 20 ml of THF and cooled to
−50°C. After addition of I2 (381 mg, 1.50 mmol), the
solution was warmed to r.t. within 12 h, during which
time the colour gradually changed from red to black
and finally green. A yellowish to light green precipitate
(mainly [Et4N]I) was filtered off, the filtrate treated with
2 ml of pentane and cooled to −20°C. Green–black
crystals, suitable for X-ray analysis, separated within
three days and were collected, washed with pentane and
dried. Yield 280 mg (34%). Anal. Calc. for
C36H36I2N4V (M=829.50 g mol−1): C, 52.13; H, 4.38;
N, 6.76. Found: C, 52.36; H, 4.91; N, 6.31%.
Compound 9b was prepared accordingly. In contrast
to 9a, 9b is only sparingly soluble in THF. Compound
9b was separated from [Et4N]I by extraction with hot
methanol. Evaporation and drying yielded 387 mg
(27%) of brown, powdery 9b. Anal. Calc. for
C36H36I2N4O8S4V (M=1085.72 g mol−1): C, 39.82; H,
3.34; N, 5.16. Found: C, 39.55; H, 3.51; N, 5.37%.
and dried. Yield 345 mg (66%). l(51V) (THF-d8)= −
1710 (t), J(51V–51P)=216 Hz, W1/2=59 Hz. Anal.
Calc. for C28H42NO4P2V (M=569.54 g mol−1): C,
59.05; H, 7.43; N, 2.50. Found: C, 58.74; H, 7.29; N,
2.64%. Crystals suitable for X-ray analysis formed in a
concentrated solution of [Et4N]-11 in THF after stand-
ing at 2°C for several weeks.
For a conversion of 11 to 10b, 310 mg (0.54 mmol) of
[Et4N]-11 and 123 ml (1.1 mmol) of CNtBu were dis-
solved in 15 ml of THF, cooled to −60°C and treated
with I2 (406 mg, 1.6 mmol). The solution was stirred for
10 h, while gradually warmed to r.t., and further 12 h at
r.t. After filtration, 15 ml of pentane was added to the
filtrate and this mixture was allowed to stand at 2°C for
two days. The dark green precipitate of 10b thus
formed was filtered off and dried. Yield 306 mg
(75.4%). Anal. Calc. for C26H40I2N2P2V (M=747.32 g
mol−1): C, 41.79; H, 5.40; N, 3.75; P, 8.29. Found: C,
41.90; H, 4.32; N, 3.85; P, 8.11%.
4.12. [I2V(CNtBu)2(PPh3)2] (10a) and [Br2V(CNtBu)2-
dppe] (10c)
4.14. [Cl2V(CꢀCPh)(CNtBu)2]·LiCl (12·LiCl)
[VCl3(THF)3] [28] (950 mg, 2.54 mmol), dissolved in
25 ml of THF and cooled to −60°C, was treated
dropwise within 1 h with 2.54 ml of a 1 M THF
solution of LiCCPh, followed by CNtBu (0.86 ml, 7.62
mmol). The black solution was warmed to r.t. within 15
h. A 6.5 ml sample of pentane was added, and the
mixture stored for 2 days at −20°C to yield brown
12·LiCl, which was filtered off and dried. Yield 515 mg
(47%). Anal. Calc. for C18H23Cl2N2V·LiCl (M=431.64
g mol−1): C, 50.09; H, 5.37; N, 6.49. Found: C, 49.68;
H, 5.65; N, 6.51%.
Preparation of 10a: [Et4N][V(CO)6] (349 mg, 1
mmol), PPh3 (525 mg, 2 mmol) and CNtBu (226 ml, 2.2
mmol) were dissolved in 25 ml of THF, cooled to
−45°C and treated with I2 (381 mg, 1.5 mmol). The
solution was warmed to r.t. within 12 h and stirred for
30 min on a water bath at 50°C. [Et4N]I was filtered
off, the filtrate evaporated to dryness and the residue
washed with hot pentane. The green powder thus ob-
tained was dried to yield 760 mg (77%) of 10a. Anal.
Calc. for C46H48I2N2P2V (M=995.60 g mol−1): C,
55.50; H, 4.86; N, 2.81. Found: C, 55.61; H, 5.25; N,
2.89%.
Compound 10c was prepared accordingly. Br2 was
added at −70°C, and the green–black product isolated
at r.t. Anal. Calc. for C36H42Br2N2P2V (M=775.44 g
mol−1): C, 55.76; H, 5.46; N, 3.61. Found: C, 55.41; H,
5.41; N, 3.63%.
4.15. [ITa(CO)3(CNtBu)3] (13a) and [ITa(CO)3-
(CNtBu)dppe] (13b)
Preparation of 13a: [Et4N][Ta(CO)6] (148 mg, 0.31
mmol) and CNtBu (105 ml, 0.93 mmol) were dissolved
in 25 ml of THF and cooled to −45°C. Addition of
iodine (79 mg, 0.31 mmol) caused a slow colour conver-
sion from yellow to red–orange. The mixture was
warmed to r.t. within 15 h, filtered and the filtrate
evaporated to dryness and washed with pentane. Yield
130 mg (68%). Anal. Calc. for C18H27IN3O3Ta (M=
641.28 g mol−1): C, 33.71; H, 4.24; N, 6.55. Found: C,
33.61; H, 4.51; N, 6.86%.
4.13. [Et4N][V(CO)4(PPhMe2)2] ([Et4N]-11) and its
con6ersion to [I2V(CNtBu)2(PPhMe2)2] (10b)
[Et4N]-11 was prepared in analogy to other phos-
phine substituted carbonylvanadates (Ref. [27] and lit-
erature cited therein): [Et4N][V(CO)6] (328 mg, 0.94
mmol) was dissolved in 20 ml of THF, treated with
PPhMe2 (268 ml, 1.88 mmol), and irradiated with an
externally installed UV source (high pressure mercury
lamp Philips HPK 125). Care was taken to maintain the
reaction temperature below 40°C. After 12 h of irradia-
tion time, the mixture was stirred at r.t. for one day,
filtered, the filtrate concentrated to 10 ml and treated
with an equal volume of pentane. The red precipitate
was filtered off, washed twice with 10 ml of pentane
Compound 13b was obtained analogously by em-
ploying one equivalent each of isocyanide and dppe.
Yield 210 mg (58%). 13C-NMR (THF-d8) l 23.2 (CH2),
31.6 [C(CH3)3], 58.9 [C(CH3)3], 128.5 to 136.8 (Ph).
Anal. Calc. for C34H33INO3P2Ta (M=873.44
g
mol−1): C, 46.76; H, 3.81; N, 1.60. Found: C, 47.56; H,
4.02; N, 1.84%.

306
D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
References
4.16. [I2Ta(CNtBu)6]I·[Et4N]I (14·[Et4N]I)
[1] C.T. Lam, P.W.R. Corfield, S.J. Lippard, J. Am. Chem. Soc.
99 (1977) 617.
[2] R.W. Miller, in: M.J. Dilworth, A.R. Glenn (Eds.), Biology
and Biochemistry of Nitrogen Fixation, Elsevier, Amsterdam,
1991, pp. 20–23.
[Et4N][Ta(CO)6] (327 mg, 0.76 mmol), dissolved in
30 ml of THF and cooled to −78°C was treated
with I2 (386 mg, 1.52 mmol) to yield a red–brown
solution containing the [{Ta(CO)4}2(m-I)3]− anion.
Thirty minutes after addition of the iodine, CNtBu
(680 ml, 6.1 mmol) was added. The reaction mixture
was warmed to r.t. within 16 h and filtered. The
beige residue on the filter plate was washed with
THF and pentane and dried. According to the ele-
mental analysis, its composition corresponded to
14·[Et4N]I. Yield 450 mg (55%). Anal. Calc. for
C30H54I3N6Ta·C8H20IN (M=1317.62 g mol−1): C,
34.64; H, 5.66; N, 7.44. Found: C, 34.44; H, 5.62; N,
7.59%.
[3] R.N. Vrtis, S. Liu, C.P. Rao, S.G. Bott, S.J. Lippard,
Organometallics 10 (1991) 275.
[4] (a) R.N. Vrtis, C.P. Rao, S. Warner, S.J. Lippard, J. Am.
Chem. Soc. 110 (1988) 2669. (b) S. Warner, S.J. Lippard,
Organometallics 8 (1989) 228. (c) E.M. Carnahan, Lippard, J.
Chem. Soc. Dalton Trans. (1991) 699. (d) B.S. Bronk, J.D.
Protasiewicz, L.E. Pence, S.J. Lippard, Organometallics 14
(1995] 2177.
[5] (a) A.C. Filippou, W. Gru¨nleitner, C. Vo¨lkl, P. Kiprof, Angew.
Chem. Int. Ed. Engl. 30 (1991) 1167. (b) A.C. Filippou, C.
Vo¨lkl, W. Gru¨nleitner, P. Kiprof, J. Organomet. Chem. 434
(1992) 201.
[6] (a) For a review, see: A. Mayr, C.M. Bastos, Progr. Inorg.
Chem. 40 (1992) 1. (b) Y. Wang, J.J.R. Frau´sto da Silva,
A.J.L. Pombeiro, M.A. Pellinghelli, A. Tiripicchio, R.A. Hen-
derson, R.L. Richards, J. Chem. Soc. Dalton Trans. (1995)
1183.
[7] (a) E.M. Carnahan, S.J. Lippard, J. Am. Chem. Soc. 112
(1990) 3230. (b) E.M. Carnahan, S.J. Lippard, J. Am. Chem.
Soc. 114 (1992) 4166.
[8] (a) C. Collazo, D. Rodewald, H. Schmidt, D. Rehder,
Organometallics 15 (1996) 4884. (b) C. Bo¨ttcher, D. Rodewald,
D. Rehder, J. Organomet. Chem. 496 (1995) 43. (c) F. Calder-
azzo, M. Castellani, G. Pampaloni, P.F. Zanazzi, J. Chem. Soc.
Dalton Trans. (1985) 1989.
[9] F.A. Cotton, W.J. Roth, J. Am. Chem. Soc. 105 (1983) 3734.
[10] (a) C. Felten, D. Rodewald, W. Priebsch, F. Olbrich, D. Re-
hder, J. Organomet. Chem. 480 (1994) 51. (b) D. Rodewald, C.
Schulzke, D. Rehder, J. Organomet. Chem. 498 (1995) 29. (c)
F. Calderazzo, C. Felten, G. Pampaloni, D. Rehder, J. Chem.
Soc. Dalton Trans. (1992) 2003.
4.17. [Cl2Ta(CNcHex)4] (15)
TaCl5 (500 mg, 1.4 mmol) was added cautiously,
and in several portions to 30 ml of THF, previously
cooled down to −80°C. This solution was treated
with 1 ml of 10% sodium amalgam (4.35 mmol of
Na) and warmed to 0°C within 3 h. After addition of
CNcHex (690 ml, 5.6 mmol) the formerly beige solu-
tion took on a dark brown colour. After 10 min of
reaction time, the solution was poured off the de-
posit. Addition of 10 ml of pentane precipitated dark
brown solid, which was dried to yield 450 mg (47%)
of 15. Anal. Calc. for C28H44Cl2N4Ta (M=688.54 g
mol−1): C, 44.84; H, 6.44; N, 8.14. Found: C, 45.06;
H, 6.32; N, 7.99%.
[11] F. Calderazzo, G. Pampaloni, G. Pelizzi, F. Vitali,
Organometallics 7 (1988) 1083.
[12] C. Felten, J. Richter, W. Priebsch, D. Rehder, Chem. Ber. 122
(1989) 1617.
[13] R. Hoffmann, B.F. Bayer, E.L. Muetterties, A.R. Rossi, Inorg.
Chem. 16 (1977) 511.
[14] E.M. Carnahan, R.L. Rardin, S.G. Bott, S.J. Lippard, Inorg.
Chem. 31 (1992) 5193.
If the isocyanide is added to the THF solution of
TaCl5 prior to Na/Hg, the insertion product
[Cl4TaC(Cl)NcHex] is formed (identified by elemental
analysis and its essential characteristics, identical to
those of [Cl4TaC(Cl)NtBu] [29]), which is resistant to
reduction by Na/Hg.
[15] (a) R. Talay, D. Rehder, Z. Naturforsch. 36b (1981) 451. (b) D.
Rodewald, F. Su¨ßmilch, D. Rehder, Anal. Quim. 92 (1996) 269.
[16] M.L. Luetgens Jr., J.C. Huffman, A.P. Sattelberger, J. Am.
Chem. Soc. 105 (1983) 4474.
[17] J. Acho, S.J. Lippard, Organometallics 13 (1994) 1294.
[18] J. Park, S.F. Pedersen, Tetrahedron 48 (1992) 2069.
[19] T. Hirao, T. Hasegawa, Y. Maguruma, I. Ikeda, Org. Chem. 61
(1996) 366.
5. Supplementary material
Crystallographic data for the structural analyses
have been deposited with the Cambridge Crystallo-
graphic Data Centre, CCDC Nos. CSD-8/07562J,
CSD-115347 and CSD-115348 for compounds 4a, 9a,
and [Et4N]-11, respectively.
[20] P. Lorente, C. Carfagna, M. Etienne, B. Donnadieu,
Organometallics 15 (1996) 1090.
[21] F.A. Cotton, L.R. Falvello, R.C. Najjar, Organometallics 1
(1982) 1640.
[22] W.P. Weber, G.W. Gokel, Phase Transfer Catalysis in Organic
Synthesis, Springer, Heidelberg, 1977, p. 18.
Acknowledgements
[23] F. Calderazzo, U. Englert, G. Pampaloni, G. Pelizzi, R. Zam-
boni, Inorg. Chem. 22 (1983) 1865.
[24] F. Calderazzo, G. Pampaloni, J. Chem. Soc. Chem. Commun.
(1984) 1249.
This work was supported by the Deutsche
Forschungsgemeinschaft and the Fonds der Chemi-
schen Industrie.
.

D. Rehder et al. / Journal of Organometallic Chemistry 585 (1999) 294–307
307
[25] J.E. Ellis, G.F. Warnock, M.V. Barybin, M.K. Pomije, Chem.
Eur. J. 1 (1995) 521.
[27] F. Su¨ßmilch, W. Glo¨ckner, D. Rehder, J. Organomet. Chem.
388 (1990) 95.
[26] D. Rodewald, D. Rehder, in: M. Gielen, R. Willem, B.
Wrackmeyer (Eds.), Advanced Applications of NMR to
Organometallic Chemistry, Wiley, Chichester, 1996, Ch. 10.
[28] L.E. Manzer, Inorg. Synth. 21 (1982) 135.
[29] M. Behnam-Dahkordy, B. Crociani, R.L. Richards, J. Chem.
Soc. Dalton Trans. (1977) 2015.
.
.