Free and metal-coordinated (N-isocyanimino)triphenylphosphorane: X-ray structures and selected reactions

Article abstract of DOI:10.1002/ejic.200500196

An improved procedure for the synthesis of (N-isocyanimino)- triphenylphosphorane, C≡N-N=PPh₃ (3), is described. The X-ray structure analysis reveals an unusually small N-N=P angle [115.2(2)°] and an N-N bond order of only about 1.5, which indicates considerable C≡N-N ^--P⁺ participation and electronically more-isolated functional groups (CN, P=N) in the isocyanide than, for example, in the isomeric N≡C-N=PPh₃ (4) [C-N=P = 123.0(4)°, C-N bond order = 2.0]. In order to gain insight into the stereochemical consequences of metal coordination of 3, an X-ray structural study of [Cr-(CO)₅C≡N-N= PPh₃] (5) was also undertaken. Surprisingly, the central bond lengths (C-N, N-N) and angles (C-N-N) remain practically unchanged with noticeable coordination effects occurring only at the periphery of 5, with the N-N-P angle [112.3(2)°] further decreased by 15σ, the elongated (by 7σ) P-N bond, the somewhat shortened (by 4σ) P-C(Ph) bonds and even shorter C-H(Ph) bonds on the one side, and the well-known Cr-C(O)_trans contraction on the other. Treatment of 5 or its tungsten derivative with anhydrous Bronstedt and Lewis acids such as CF₃COOH, HCl, COS, phosgene or, most efficiently, [PdCl₂(1,5-COD)] causes CN→NC isomerisation to give [M(CO)₅N≡C-N=PPh₃] [M = Cr (6), W (7)]. In solution, [PdCl₂(CNNPh₃)₂] and Ph₃BCNNPPh₃ (8) slowly isomerize even without additional acid to give both free and Pd-coordinated 4 and Ph₃BNCNPPh ₃ (9), respectively. In the presence of catalytic amounts of [PdCl₂(1,5-COD)], 3 is converted into 4 and the dimer Ph ₃PN-C(CN)=N-NPPh₃ (10) in an almost 1:1 ratio. The optimised geometries of the methyl derivatives of 3 and 4, namely Me ₃P=N-N≡C (3c) and Me₃P=N-C≡N (4c), are in excellent agreement with the experimental data; major differences between the isomers (P-N-N angle, N-N bond length) are explained by the higher electronegativity of the isocyano group as compared to the CN substituent, which, in turn, is a better π-acceptor). The reaction path of the isomerisation of 3 to 4 (3c to 4c) has also been studied computationally and been found to proceed via an [(P)=N^A-N≡C^A(N ^A-C^A)] cyclic transition state. The overall process is exothermic by 50 kcal mol^-1. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500196

FULL PAPER
Free and Metal-Coordinated (N-Isocyanimino)triphenylphosphorane: X-ray
Structures and Selected Reactions
Heribert Stolzenberg,[a][‡] Bernd Weinberger,[a] Wolf Peter Fehlhammer,*[b,c]
Frank G. Pühlhofer,[d][‡‡] and Robert Weiss[d][‡‡]
Keywords: Chromium / Isocyanide ligands / Isomerisation / Phosphoranes / Tungsten / Quantum chemical calculations
An improved procedure for the synthesis of (N-isocyanimino)- most efficiently, [PdCl2(1,5-COD)] causes CNǞNC isomeri-
triphenylphosphorane, CϵN–N=PPh3 (3), is described. The
X-ray structure analysis reveals an unusually small N–N=P
angle [115.2(2)°] and an N–N bond order of only about 1.5,
which indicates considerable CϵN–N––P+ participation and
electronically more-isolated functional groups (CN, P=N) in
the isocyanide than, for example, in the isomeric NϵC–
N=PPh3 (4) [C–N=P = 123.0(4)°, C–N bond order = 2.0]. In
order to gain insight into the stereochemical consequences
of metal coordination of 3, an X-ray structural study of [Cr-
(CO)5CϵN–N=PPh3] (5) was also undertaken. Surprisingly,
the central bond lengths (C–N, N–N) and angles (C–N–N)
remain practically unchanged with noticeable coordination
effects occurring only at the periphery of 5, with the N–N–P
angle [112.3(2)°] further decreased by 15σ, the elongated (by
7σ) P–N bond, the somewhat shortened (by 4σ) P–C(Ph)
bonds and even shorter C–H(Ph) bonds on the one side, and
the well-known Cr–C(O)trans contraction on the other. Treat-
ment of 5 or its tungsten derivative with anhydrous Brønstedt
and Lewis acids such as CF3COOH, HCl, COS, phosgene or,
sation to give [M(CO)5NϵC–N=PPh3] [M = Cr (6), W (7)]. In
solution, [PdCl2(CNNPh3)2] and Ph3BCNNPPh3 (8) slowly
isomerize even without additional acid to give both free and
Pd-coordinated 4 and Ph3BNCNPPh3 (9), respectively. In the
presence of catalytic amounts of [PdCl2(1,5-COD)], 3 is con-
verted into 4 and the dimer Ph3PN–C(CN)=N–NPPh3 (10) in
an almost 1:1 ratio. The optimised geometries of the methyl
derivatives of 3 and 4, namely Me3P=N–NϵC (3c) and
Me3P=N–CϵN (4c), are in excellent agreement with the ex-
perimental data; major differences between the isomers (P–
N–N angle, N–N bond length) are explained by the higher
electronegativity of the isocyano group as compared to the
CN substituent, which, in turn, is a better π-acceptor). The
reaction path of the isomerisation of 3 to 4 (3c to 4c) has also
been studied computationally and been found to proceed via
an [(P)=NA–NϵCA(NA–CA)] cyclic transition state. The over-
all process is exothermic by 50 kcalmol–1
.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
this very labile substance.[2] Staudinger, in search of di-
isocyan, CN–NC, obviously was the first to get hold of 1,
and very remarkably assigned it the correct structure of an
isocyanoamine.[3] Later, for more than 30 years, alternative
constitutions were favoured, and it was not until 1968 that
Müller et al. returned to Staudinger’s formula.[2,4] In the
mid-1970s, the first metal complexes of 1 were synthesised
in our laboratory both from Müller’s isodiazomethane solu-
tions and, much more elegantly, from a stable, storable de-
rivative, Wiberg’s CϵN–N(SiMe3)2 (2), by hydrolysis at the
protecting metal.[5,6] The only disadvantage of 2 was its
problematic accessibility, requiring many steps and costly
low-temperature techniques. In 1980 we reported a novel
precursor of 1 which is synthesised in a one-step procedure
from cheap starting materials and actually proved to be a
key substance in N-isocyanide chemistry.[7,8] In the follow-
ing we present an improved preparation of CϵN–NPPh3
(3), its X-ray structure in comparison with that of its penta-
carbonylchromium complex, and its metal-catalysed CN/
NC-isomerisation and dimerisation reactions. In addition,
quantum chemical calculations have been carried out to get
more insight into the electronic and structural properties of
Introduction
Despite occasional reports for almost 100 years now, N-
isocyanides, i.e. species with the structural element CϵN–
N, still remain an exotic class of compounds. This holds
particularly true of the parent molecule, isodiazomethane,
CϵN–NH2 (1).[1] Its mere naming as something which it is
not, viz. diazomethane, reflects the uncertainties and diffi-
culties connected with the discovery and understanding of
[a] Institut für Anorganische Chemie der Universität Erlangen-
Nürnberg,
Egerlandstraße 1, 91054 Erlangen, Germany
[b] Institut für Anorganische und Analytische Chemie der Freien
Universität Berlin,
Fabeckstraße 34–36, 14195 Berlin, Germany
[c] Present address: Department Chemie und Biochemie der Lud-
wig-Maximilians-Universität München,
Butenandtstr. 5–13, 81377 München, Germany
E-mail: wpfehlhammer@gmx.de
[d] Institut für Organische Chemie der Universität Erlangen-
Nürnberg,
Henkestraße 42, 91054 Erlangen, Germany
[‡] X-ray structure determinations.
[‡‡] Quantum chemical computations.
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
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DOI: 10.1002/ejic.200500196
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compounds 3 and 4 and the reaction path of their respective
isomerisation.
Results
(N-Isocyanimino)triphenylphosphorane: Synthesis and X-ray
Structure
The title compound was prepared by reaction of N-for-
myl hydrazine with two equivalents of Appel’s three-compo-
nent reagent, which serves two purposes, viz. dehydration
of the formamido group and masking of the primary amine
function [Equation (1)].[7]
Figure 1. ORTEP drawing of the molecular structure of 3 (30%
probability). The effect on the thermal ellipsoids of the functional
group of a refinement cycle with interchanged scattering factors for
C19 and N2 (“CN/NC test”) is also shown. Both the abnormal
blowing up of C19 (treated as an N atom) and the odd shrinkage
of N2 (calculated as C) clearly exclude the cyanide form of 3, as
does the R value which increased by 0.34%. Selected bond lengths
[Å] and angles [°]: P–N1 1.604(2), P–Cav. 1.797, N2–N1 1.345(4),
N2–C19 1.154(4); P–N1–N2 115.2(2), N1–N2–C19 173.4(3).
Interestingly, the corresponding P=N–C angle [123.0(4)°]
in the isomeric cyanoiminophosphorane 4 (Scheme 1) is
much wider, while the CϵN and P=N bond lengths are
identical with those in 3 within one or two standard devia-
tions.[9] Furthermore, the CN distances agree perfectly with
the rated value for an ideal CN triple bond, and both PN
bonds come very close to a true PN double bond (1.57 Å)
as derived from experimental data.[10] Another major differ-
ence occurs in the remaining bonds, however. The N1–N2
bond distance in 3 [1.345(4) Å] lies halfway between an NN
single (1.44 Å) and an NN double bond (1.24 Å), while the
C–N(imine) bond in 4, which is formally a single bond, is
only slightly longer than a typical CN double bond
(1.27 Å). The two functional groups (CϵN and P=N) are
thus electronically more isolated in the isocyanide 3 than in
the cyano isomer 4, an observation which has been made
(1)
Repetition of the procedure as published, however,
turned out to be difficult, and in only a few cases could
yields of the claimed order of magnitude be reached. As
reasons presumably responsible for this problem we have
identified an incomplete reaction and decomposition of the
desired product during work-up (vide infra). Taking this
into account, in the modified procedure refluxing of the
reaction mixture is prolonged and immediately followed by
basic hydrolysis (with aqueous Na2CO3) of the highly acidic
phosphorus() intermediates (e.g. Ph3PCl+Cl–) and separa-
tion of the organic layer (containing 3) from the aqueous
one (see Experimental Section).
Single crystals suitable for an X-ray structure analysis
were grown by slowly cooling a solution of 3 in warm etha-
nol. The molecule shows the expected bent structure at the
imino nitrogen N1 (Figure 1). Structural investigations of
iminophosphoranes generally reveal a considerable flexibil-
ity of this nitrogen as regards its hybridisation. P=N–X
angles ranging from 119° up to 137° indicate that this is
primarily sp2 with a tendency to higher s participation. The
unusually small angle in 3 [115.2(2)°] conversely points to
more p-character (i.e. some C–ϵN+–N––P+Ph3 form),
which causes the phosphorus and the isocyano nitrogen
atoms to get as close as 2.494 Å which is actually the short-
many times in systems of the type CϵN(NϵC)–X–Y. A
ត
more realistic description of the bonding in 4 – although
not in 3 (!) – might therefore make use of a further reso-
nance formula such as 4a.
Scheme 1. Schematic formulae with selected bonding parameters
(bond lengths in Å): a = 1.154(5), b = 1.345(4), c = 1.604(2) Å, α =
115.2(2)°; d = 1.151(9), e = 1.301(7), f = 1.595(4) Å, β = 123.0(4)°.
Deviations from a tetrahedral configuration at the P
est nonbonding distance between non-hydrogen atoms in atom in 3 are marginal. The phenyl rings show the well-
this crystal structure. known propeller-like arrangement with dihedral angles rel-
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Free and Metal-Coordinated (N-Isocyanimino)triphenylphosphorane
FULL PAPER
ative to the P–N1–N2–C19 plane of 18.3° (Ph1), 69.8° (Ph2)
and –69.6° (Ph3).
The intermolecular contacts (Յ 3.5 Å) were also calcu-
lated. With the exception of a weak N···H–C bridge be-
tween a phenyl hydrogen and N1, which also shows strong
intramolecular interactions with two hydrogen atoms, none
of the contacts lies substantially below the sum of the corre-
sponding van der Waals radii.
Figure 2. ORTEP drawing of the molecular structure of 5 (30%
probability). The isocyanide nature of the ligand was ascertained
by a “CN/NC test” (cf. caption of Figure 1), which resulted in unre-
alistic temperature factors and an increase of the R value by 0.66%.
Selected bond lengths [Å] and angles [°]: P–N1 1.618(2), P–Cav.
1.787, N2–N1 1.345(3), N2–C19 1.151(4), Cr–C19 2.031(3), Cr–
C20trans 1.861(3), (Cr–Ccis av. 1.895, (C20–O20)trans 1.140(4),
(C–O)cis,av. 1.135; P–N1–N2 112.3(2), N1–N2–C19 174.2(2),
Cr–C19–N2 176.9(2).
)
X-ray Structure of Pentacarbonyl[(N-isocyanimino)-
triphenylphosphorane]chromium (5): Stereochemical
Consequences of Metal Coordination
The Cr–CN bond length is 2.031(3) Å, which fits well
into the range of 2.00–2.05 Å determined so far for penta-
carbonyl(N-isocyanide)chromium complexes. This is long in
comparison with the corresponding values for [Cr(CO)5(C-
isocyanide)] complexes [d(Cr–CNC) Ͻ 2.00 Å] and clearly
indicates a lower degree of π back-bonding.
Metal coordination of a stable isocyanide is routine, and
CϵN–N=PPh3 (3) is no exception. Preparation of the chro-
mium complex 5 proceeds by substitution of THF in the
photolytic intermediate [Cr(CO)5THF];[7,8] suitable crystals
for the X-ray study were obtained from dichloromethane.
Turning to the new ligand itself, which, according to
what we know from the stereochemistry of the CrC6 sur-
rounding, is a markedly better donor than common C-iso-
cyanides, we were surprised to find its central skeleton prac-
tically unchanged: the CϵN and N–N bond lengths are ab-
solutely identical in 3 and 5, and the 3σ deviation in the
quasi-linearity of the C–N–N atomic sequence is insignifi-
cant; again, coordination effects are noticed only at the pe-
riphery of CNNPPh3 [as they appeared – most distinctly –
at the other end of the molecule, namely the remote trans-
CO ligand (see above)]: the more acute N–N–P angle of
112.3(2)° [–2.9° (15σ)], which brings the P and N2 atoms
even closer together, the slight elongation [by 0.014 Å (7σ)]
of the P–N bond and, remarkably, the somewhat shortened
P–C(Ph) bonds [–0.010 Å (4σ)]. All these data point to a
considerable Ph3P+–N––N+C[M–] participation in the over-
all bonding in 5 by which extra electronic charge is transfer-
red to M(CO)5. Obviously, the N–N bond is not as insulat-
ing as inferred from a comparison with the nitrile 4 (4a)
earlier on. These findings are perhaps best understood in
terms of Gutmann’s “pile up” and “spill over” effects ac-
companying Lewis acid–Lewis base interactions.[15]
Figure 2 presents the overall geometry as expected for
the combination of the functional N-isocyanide ligand 3
and the metal complex fragment Cr(CO)5, together with se-
lected bonding parameters. The first point of interest is the
stereochemistry of the Cr(CO)5 entity, which reflects the li-
gand properties of the incoming group. While all of the CO
ligands experience the donating capacity of a typical isocya-
nide ligand, it is the trans-CO ligand which has to take the
lion’s share of the electronic charge transported onto the
metal. In the present case the result is particularly striking:
the Cr–(CO)trans bond in 5 is shortened to 1.861(3) Å as
compared, for example, to that in Cr(CO)6 (1.918 Å) or
those of Cr(CO)5 adducts of several other functional iscocy-
anides
such
as
[Cr(CO)5CNCH2C(Me)(CH2NC)2]
[1.882(2) Å],[11] [Cr(CO)5CNCϵN] [1.913(4) Å],[12] [Cr(CO)5-
CNC(=O)Ph] [1.899(3) Å][13] and [Cr(CO)5CNCϵCPh]
[1.896(4) Å],[14] while the average Cr–(CO)cis distance in 5
only drops to 1.895 Å. A much less significant effect, yet
one pointing in the same direction, is the corresponding
lengthening of the C–O bonds, which again is more pro-
nounced for (C–O)trans [1.140(4) Å] than for the averaged
(C–O)cis (1.135 Å).
Selected Reactions
CN/NC Isomerisations of (N-Isocyanimino)-
triphenylphosphorane Complexes
The action of aqueous trifluoroacetic acid on CNNPPh3
coordinated to pentacarbonyl–group VI metal fragments
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H. Stolzenberg, B. Weinberger, W. P. Fehlhammer, F. G. Pühlhofer, R. Weiss
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proceeds with hydrolysis of the (imino)N–P bond to give taneously and smoothly they occur under the applied mild
isodiazomethane complexes, occasionally in excellent yield reaction conditions. It is true that these kinds of isomerisa-
(group VI metal = chromium).[7,8] A totally different reac- tions are very common in cyanometal coordination poly-
tion occurs, however, when [Cr(CO)5CNNPPh3] (5) is mers and are known to happen immediately by what has
treated with anhydrous acids: when allowed to stand for 1– been termed a “flip-over” mechanism, however, the organic
2 days a solution of 5 in dichloromethane containing a few synthesis of nitriles by isomerisation of free C-isocyanides
drops of CF3COOH undergoes complete conversion of the requires severe heating over a longer period.[19] Obviously,
isocyanide ligand into its nitrile isomer to form 6.
the involvement of at least one metal (or metalloid, e.g.
SiMe3[18,20]) is essential for the activation energy to be suf-
ficiently small. Strangely enough, this is particularly born
out by the functional N-isocyanide under consideration,
which, as a free ligand, cannot be transformed into its iso-
mer at all, either thermally, say by refluxing in THF or
chloroform, or by protonation.
If we accept a synchronous mechanism with a “rotating”
CϵN species between the fixed metal and NPPh3 groups,
or, which comes down to the same thing, a migration of
these groups about a stationary CϵN, perhaps via some
bicyclic transition state of the type A, it seems plausible that
the addition of an acid (Brønsted or Lewis) to the imino
nitrogen greatly enhances the (less pronounced) migration
tendency of the iminophosphorane group (as HN=PPh3).
At any rate, in the absence of an acid the [M(CO)5-
CNNPPh3] complexes do not show any sign of isomeris-
ation even after standing for weeks in various solvents.
The same effect can be achieved by passing a stream of
dry HCl through a solution of 5. While protic acids seem
not to prompt the corresponding tungsten complex to iso-
merize, Lewis acids like COS and COCl2 are effective with
both metal complexes. Even more sophisticated Lewis acids
such as [PdCl2(1,5-COD)] work if the reaction is carried
out under a CO atmosphere. Thus, under 1 bar of CO 5
isomerizes quantitatively within 3–4 days, whereas in a rot-
ating autoclave under a CO pressure of 10 bar the transfor-
mation is complete in 24 h. Without carbon monoxide, Pd
metal separates slowly from the solution, which then turns
deep green. Complexes 6 and 7 were adequately character-
ised by their analytical and IR spectroscopic data (see Ex-
perimental Section); the data of 6 are in full agreement with
the data reported for the compound which has earlier been
synthesised directly by photolysis of hexacarbonylchro-
mium in the presence of NCNPPh3.[16]
At palladium(), 3 isomerizes without the need for any
additional acid. After two weeks two strong new bands ap-
pear at 2230 and 2180 cm–1 in the IR spectrum of a solution
of [PdCl2(CNNPPh3)2][8] in dichloromethane in place of the
very weak original ν(CN) (2200 cm–1). Of these, the absorp-
tion at 2180 cm–1 can unambiguously be assigned to the
free NCNPPh3, while the band at 2230 cm–1 fits into the
range of palladium-coordinated nitrile species.
With equimolar amounts of triphenylboron, 3 forms an
almost colourless crystalline adduct 8 which is air-stable and
soluble in chlorinated hydrocarbons. In the IR spectrum, the
weak CN stretch of the free ligand at 2067 cm–1 has been
replaced by a new weak band at much higher wavenumbers
(2230 cm–1). As frequency shifts even of this order of magni-
tude are quite possible for isocyanide–BPh3 adducts − for
the corresponding (N-isocyanodialkylamine)triphenylboron
A closely related case of a CN Ǟ NC isomerisation to
the one presented here is the rearrangement of a coordi-
nated fulminate into an isomeric isocyanate ligand. This re-
action occurred on attempted protonation of difulminato-
bis(triphenylphosphane)platinum to produce the then un-
known isofulminic acid (CϵN–OH) stabilized in a com-
plex.[21] Later, we studied the spontaneous isomerisation of
[W(CϵN–O)(CO)5]– and found it to be strongly solvent-
dependent.[22]
Catalytic Isomerisation and Dimerisation of 3
From the findings discussed above, it was obvious that
we should study the possibility of a catalytic isomerisation
of 3 with traces of a metal component. As such, [PdCl2(1,5-
COD)] was chosen and found to produce complete disap-
pearance of the weak ν(CN) band at 2067 cm–1 within 20 h
at ambient temperature. What appeared, however, were two
absorptions at 2180 and 2220 cm–1 instead of the only one
expected. Work-up finally resulted in two products: the iso-
meric (N-cyanoimino)triphenylphosphorane (4), which has
already been synthesised by Appel and co-workers and was
identified on the basis of its elementary analysis, its melting
point and its IR data,[23,24] and a second pale -yellow com-
pound 10, which was recrystallised from dichloromethane/
species Ph3B–CN–NR2, ∆ν(CN) [ν(CN)complex – ν(CN)free
]
amounts to 135 cm–1[17] − we can be sure that we are still
dealing with the isocyanide isomer. In the end, this was un-
equivocally confirmed by its slow conversion in chloroform
into 9, which exhibits a very strong IR absorption at even
higher wavenumbers (2280 cm–1).
CN h NC isomerisations in both directions are by no diethyl ether and showed the same analytical composition
means rare.[18] What is surprising here, though, is how spon- but double the molecular mass. The IR spectrum with a
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Free and Metal-Coordinated (N-Isocyanimino)triphenylphosphorane
FULL PAPER
weak to medium intensity band at 2200 cm–1 [ν(CϵN)], a
ν(C=N) absorption at 1520 cm–1 and two ν(P=N) features
at 1110 [ν(P=N–N)] and 1300 cm–1 [ν(P=N–C)], as well as
the 31P NMR spectrum, which contains two singlets at δ =
24 and 11 ppm, are in perfect agreement with the suggested
structure for 10.
Discussion
Table 1 contains selected stereochemical parameters of
compounds 3, 3c, 4 and 4c. In general, the differences be-
tween the bond lengths and angles of the phenyl-substituted
systems 3 and 4, as obtained from X-ray analyses, and the
data computed for the corresponding methyl species (3c
and 4c) are minor and easily explained by the exchange at
P1 of phenyl for methyl (cf. Figure 3 and Table 1). Thus,
the elongation of the P1–N1 bonds in 3c and 4c is clearly
a consequence of the decreased electron-acceptor capacity
of the trimethylphosphonio moiety. Other points of interest
are the deviations between 3c and 4c (3 and 4, respectively),
especially those of the P1–N1–X angles (3, 3c: X = NC; 4,
4c: X = CN).
A catalytic dimerisation of isocyanides has so far only
been reported in one case, that of tert-butyl isocyanide with
trace amounts of BF3 to give 11.[25] On the other hand,
there are numerous examples of stoichiometric C-C coup-
lings of C-bonded species in the ligand spheres of metal
complexes,[26] with Lippard’s reductive dimerisation of tert-
butyl isocyanide ligands in [Mo(CNtBu)6I]I to give B being
among the first.[27] Still, for the case at hand it seems most
plausible to assume a mechanism C in analogy to that of
the well-known metal-catalysed α-additions to isocyanides
(or isocyanide insertions), which proceed via a primary nu-
cleophilic attack at the metal-bonded isocyano carbon.[28]
Actually, all the necessary “instruments” are there: thus, we
have shown that N-isocyanides are prone to nucleophilic
attack even in less activating metal complexes,[8,29] 3 carries
a strong nucleophile in the form of the imino nitrogen, and
N–N cleavage is a common occurrence in N-isocyanide
chemistry.[17,30,31]
Table 1. Comparison of the experimental bonding parameters of 3
and 4 with those obtained from geometry optimisation (B3LYP/6-
311+G**) (3c, 4c).
Bond length [Å]
3
3c (Cs)
4
4c (Cs)
P1–N1
N1–N2
N1–C1
C1–N2
1.604
1.345
1.620
1.342
1.595
1.600
1.301
1.151
1.318
1.167
1.154
1.176
Bond angle [°]
P1–N1–N2
P1–N1–C1
115.2
114.5
123.0
121.8
Figure 3. Representative structural elements in 3 (3c) (left) and 4
(4c) (right).
The exchange of N2 by the less electronegative C1 in 3c
(to give 4c) changes the electronic nature of N1. NBO
analyses assume N1 in both compounds to carry a lone
pair of p-character oriented perpendicular to the symmetry
plane of 3c and 4c. This results in an overall hybridisation
Computational Section
Quantum chemical calculations were performed to gain of sp2 for the remaining three substituents of N1 in this
a detailed insight into the electronic and structural proper- plane: the PMe3 group, the second lone pair and the -NC
ties of the compounds 3 and 4. To simplify the problem, (3c) or -CN (4c) group. The specific breakdown of the over-
the phenyl substituents in 3 and 4 were replaced by methyl all sp2 hybridisation into the contributions of N1 to each
groups, and the geometries of the resulting compounds 3c bond results in the data listed in Table 2. The most notice-
and 4c were optimised at the B3LYP/6-311+G** level of able changes are obviously caused by very different hybrid-
theory. ZPEs were determined at the same level. The fre- isations of the lone pair and the bond to the (iso)cyano
quencies were also analysed at this level to confirm that the group, whereas the hybridisation of the N1–P1 bond re-
optimised geometries correspond to minima (NIMAG = 0) mains nearly unchanged. The former can easily be ex-
or first-order transition states (NIMAG = 1). A transition- plained by Bent’s rule:[33] electronegative substituents force
state search was performed using the QST3 method. NBO the central atom (N1) to increase the p-character of the cor-
analyses were performed at the above-mentioned level of responding bond. The isocyano substituent in 3c is clearly
theory. All computations were run using the Gaussian98W more electronegative than the cyano function in 4c, which
suite of programs and NBO 3.1, as implemented in results in the very different orbital hybridisations of N1 in
G98W.[32]
the two N1–X bonds (Table 2). As a result, the P1–N1–N2
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angle in 3c is smaller than the P1–N1–C1 angle in 4c. In much better π-acceptor than the isocyano function in 3c.
3c, both bonds (P1–N1 and N1–N2) have contributions The respective donor–acceptor interactions were computed
from N1 hybridisations with p coefficients close to 3.0, at the B3LYP/6-311+G** level resulting to be about
hence the P1–N1–N2 angle is expected to be way below 63 kcalmol–1 for 4c and 33 kcalmol–1 for 3c. The experi-
120° (computed: 114.5°). In 4c, the average hybridisation of mentally observed pronounced multiple-bond character of
the respective two bonds is sp2.1 (cf. Table 2), which should the N1–CN bond in 4 is thus confirmed computationally.
give rise to an angle of about 120° (computed: 122°). These
Finally, the reaction path of the isomerisation of 3 to 4
values are in excellent agreement with the corresponding (3c to 4c) was studied computationally using the QST3
X-ray data of the phenyl derivatives 3 (115°) and 4 (123°; method as implemented in Gaussian98W. The reaction pro-
Table 1).
file is shown qualitatively in Figure 4, quantitative details
are given in Table 3, and geometric parameters of TS1, TS2
and IM are given in Figure 5 (see also Supporting Infor-
mation).
Table 2. Orbital hybridisation of N1 based on NBO analyses
(B3LYP/6-311+G**).
3c (X = NC)
4c (X = CN)
Table 3. Relative energies of the species in Figure 3 (B3LYP/6-
311+G** incl. uncorrected ZPE).
N1–PMe3
N1–X
Lone pair (in plane)
sp2.8
sp2.9
sp1.1
sp2.5
sp1.7
sp1.9
Relative energy [kcalmol–1]
3c
+50.3
+81.7
+79.8
+82.1
0.0
TS1
IM
TS2
4c
As described above, the N1–N2 bond in 3 lies halfway
between an N-N single and an N-N double bond, whereas
the N1–C1 bond in 4 is much closer to a C-N double bond.
The same observations are made in the case of the com-
puted structures 3c and 4c, and can again be explained on
the basis of NBO analyses: both lone pairs of N1 interact
with the two mutually perpendicular π*-orbitals of the C1–
N2 bond. Since the π-bonds between C1 and N2 are polar-
ized towards the nitrogen atom, the π*-orbitals have larger
coefficients at C1, and hence the cyano group in 4c is a
Figure 5. Geometrical data of TS1, IM and TS2 (bond lengths in
Å). TS1: P1–N1 1.631, N1–N2 1.556, N1–C1 2.018, N2–C1 1.215;
N1–N2–C1 92.7°. IM: P1–N1 1.646, N1–N2 1.613, N1–C1 1.681,
N2–C1 1.234; N1–N2–C1 71.0°, N1–C1–N2 65.1°. TS2: P1–N1
1.629, N1–N2 1.861, N1–C1 1.597, N2–C1 1.215; N1–C1–N2
81.7°.
As shown in Figure 4, the reaction proceeds with forma-
tion of a new σ-bond between the 2pz lone pair at N1 and
the terminal carbon atom, via the transition state TS1 (Ea
= 31.4 kcalmol–1), to the cyclic intermediate IM. The latter
reacts with cleavage of the N–N bond via TS2 (Ea
=
2.3 kcalmol–1) to the rearrangement product 4c. The overall
process is exothermic by 50.3 kcalmol–1 (cf. Table 3).
The experimentally observed lowering of the activation
energy for the isomerisation by addition of Lewis acids can
also be explained on this basis: the formation of donor–
acceptor complexes between the intermediate IM and Lewis
acids stabilizes the former by decreasing the repulsive inter-
actions of lone pairs (see Figures 4 and 5).
Experimental Section
General Remarks: All experiments were performed under argon in
dry, argon-saturated solvents. The (N-isocyanimino)triphenylphos-
phorane metal complexes [M(CO)5CNNPPh3] [M = Cr (5), W] and
[PdCl2(CNNPPh3)2] were prepared according to literature pro-
cedures.[7,8] IR spectra were recorded on a Perkin–Elmer 621 or a
Beckman IR 12 spectrometer. NMR spectra (31P) were recorded
on a Jeol JNM-PS-100. Mass spectra were recorded on a Varian
CH-5 (excitation energy 70 eV). Microanalyses (C,H,N) were ob-
tained with a Heraeus CHN Rapid-Elementanalysator. Molecular
Figure 4. Reaction profile of the isomerisation of 3c to 4c at the
B3LYP/6-311+G** level (including uncorrected ZPE at the same
level).
4268
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Free and Metal-Coordinated (N-Isocyanimino)triphenylphosphorane
FULL PAPER
weights were determined in CH2Cl2 solution using a Knauer
Dampfdruck Osmometer, and melting or decomposition points
were determined with a Büchi Model 510 melting point apparatus
and are uncorrected.
(940 mg, 1.5 mmol) in 30 mL of CH2Cl2. After stirring for 2 h at
room temp., the solvent was removed in vacuo and the raw product
was dissolved in a few millilitres of CH2Cl2. Cooling to about
–35 °C gave 432 mg (48%) of 7 as yellow-green crystals, m.p.
125 °C (dec.). IR (KBr): ν = 2230 cm–1 s [ν(NC)]; 2060 m, 1905 vs,
˜
Preparation of (N-Isocyanimino)triphenylposphorane (3). Revised
Procedure: CH2Cl2 (600 mL), PPh3 (157.4 g, 0.6 mol), NEt3 (freshly
distilled from KOH; 50.6 g, 0.5 mol) and formylhydrazine (dried in
high-vacuum at 45 °C; 15.3 g, 0.25 mol) were placed in a reaction
flask which had been dried with a heat-gun under high-vacuum.
The slurry was then heated to 50–60 °C, and CCl4 (77.0 g, 0.5 mol)
was added dropwise over a period of about 30 min. The mixture
was kept at 50–60 °C for at least 5–6 h. After cooling to room
temp., 250 mL of a saturated aqueous Na2CO3 solution was added,
the layers were separated, and the aqueous layer was washed with
two 50-mL portions of CH2Cl2. The combined organic phases were
dried with Na2SO4 and filtered. After evaporation of the solvent,
the residue was dried under high vacuum, pulverised, stirred in
200 mL of ethanol/water (1:1.5) and collected on a frit. Recrystalli-
sation from hot ethanol yielded 30.2 g (40%) of an orange-brown
br., 1870 sh [ν(CO)]; 1280 s, br. [ν(PN)]. C24H15N2O5PW (626.22):
calcd. C 46.03, H 2.41, N 4.47; found C 45.89, H 2.39, N 4.62.
CN/NC-Isomerisation of (N-Isocyanimino)triphenylphosphorane at
BPh3
Preparation of [(N-Isocyanimino)triphenylphosphorane]triphenylbor-
ane (8): A solution of CNNPPh3 (3; 906 mg, 3.0 mmol) in 35 mL
of CH2Cl2 was added dropwise to a solution of BPh3 (730 mg,
3.0 mmol) in 30 mL of CH2Cl2 at 0 °C. After stirring for 1.5 h at
0 °C the product was precipitated with petroleum ether and stored
overnight at –18 °C to give 8 (1.2 g, 74%) as off-white crystals, m.p.
153 °C (dec.). IR (KBr): ν = 2225 cm–1 w [ν(CN)]; 1190 sh, 1150
˜
sh, 1120 vs, br. [ν(PN)]. C37H30BN2P (544.5): calcd. C 81.63, H
5.55, N 5.15; found C 81.63, H 5.87, N 4.88. Compound 8 is ther-
mally stable in the solid state and can be kept under nitrogen at
room temp.
crystalline material (m.p. 159–160 °C, dec.). IR (KBr): ν =
˜
2067 cm–1 w [ν(CN)]; 1117 s, 1099 sh [ν(PN)]. C19H15N2P (302.32):
CN/NC Isomerisation of [(N-Isocyanimino)triphenylphosphorane]tri-
phenylborane. Preparation of [(N-Cyanimino)triphenylphosphorane]-
triphenylborane (9): A solution of 8 (1.09 g, 2.0 mmol) in 10 mL of
CHCl3 was stored for 2 weeks at room temp. After evaporation of
the solvent the crude material was treated with ethanol. The insolu-
ble white powder (9) was filtered off and dried in high vacuum.
calcd. C 75.49, H 5.00, N 9.27; found C 75.69, H 4.99, N 9.40.
CN/NC-Isomerisation of Pentacarbonyl[(N-isocyanimino)triphenyl-
phosphorane]chromium (5) with Protic Acids and COS: A few drops
(8–10) of CF3COOH were added, with stirring, to a solution of
[Cr(CO)5CNNPPh3] (5; 1.48 g, 3.0 mmol) in dry CH2Cl2 (25 mL).
After standing for 48 h at room temp. the volatile components were
removed under high vacuum and the residue was dissolved in a
small amount of CH2Cl2 and chromatographed on silica gel in n-
hexane (3.5×20 cm column) with a CH2Cl2/n-hexane (3.5:1) eluent.
Slow evaporation of the bright yellow phase resulted in the cyano
isomer 6 as yellow to green crystals.
Yield: ca. 600 mg (55%). IR (KBr): ν = 2280 cm–1 vs, 2085 sh
˜
[ν(NC)]; 1350 s, 1320 sh [ν(PN)]. C37H30BN2P (544.5): calcd. C
81.63, H 5.55, N 5.15; found C 80.03, H 5.80, N 5.14.
Catalytic Isomerisation and Dimerisation of (N-Isocyanimino)tri-
phenylphosphorane (3). Preparation of (N-Cyanimino)triphenylphos-
phorane (4) and (CNNPPh3)2 (10):
A catalytic amount of
[PdCl2(1,5-COD)] (29 mg, 0.1 mmol) was added, with stirring, to a
solution of CNNPPh3 (3; 3.25 g, 10.75 mmol) in 40 mL of CH2Cl2.
Stirring was continued for 20 h at room temp., after which time the
solvent was removed in vacuo and the residue stirred in 30 mL of
ethanol for 1 h. The insoluble yellow product was filtered off and
recrystallised from CH2Cl2/Et2O to give 1.6 g (50%) of pure 10,
Similar results were obtained by (a) passing a rapid stream of dry
HCl through the above solution for a few seconds and keeping it
at room temp. for 14 h, and (b) by treating a solution of [Cr(CO)5-
CNNPPh3] (1.48 g, 3.0 mmol) in THF (30 mL) with an excess of
COS at 45 °C for 24 h and applying the same work-up procedure
to the reaction mixtures. The yields (non-optimised) varied between
30 and 70% (see also below).
m.p. 221 °C (dec.). IR (KBr): ν = 2220 cm–1 w [ν(NC)]; 1300 s, br.,
˜
1275 sh, 1130 sh, 1110 s, br. [ν(PN)]; 1540 s, 1480 w, 1140 s, 1040
sh, 1030 s. C38H30N4P2 (604.60): calcd. C 75.49, H 5.00, N 9.27, P
10.25; found C 75.81, H 5.05, N 9.21, P 10.14. Mol. mass calibrated
against PPh3: 616.
CN/NC-Isomerisation of Pentacarbonyl[(N-isocyanimino)triphenyl-
phosphorane]chromium (5) with [PdCl2(1,5-COD)]. Preparation of
Pentacarbonyl[(N-cyanimino)triphenylphosphorane]chromium
(6):
(a) [Cr(CO)5CNNPPh3] (5; 990 mg, 2 mmol) and [PdCl2(1,5-COD)]
(290 mg, 1 mmol) were dissolved in 90 mL of toluene/CH2Cl2 (2:1)
under a CO atmosphere and stirred for one week at room temp.
Every two days the CO atmosphere was renewed. After evaporation
of the solvent, the residue was extracted with diethyl ether. The
extract was filtered and concentrated, and the product 6 crystallised
on cooling to about –35 °C as yellow-green crystals. Yield: 60%.
The brown ethanolic solution was evaporated to dryness and the
solid residue was dissolved in 10 mL of warm CH2Cl2. Compound
4 (1.45 g, 45%) separated in the form of a light-brown powder at
0 °C, m.p. 188 °C (dec.). [ref.[23,24] 192–196 °C (dec.)]. IR (KBr): ν
˜
= 2180 cm–1 vs [ν(NC)]; 1270 s, br. [ν(PN)]. C19H15N2P (302.3):
calcd. C 75.49, H 5.00, N 9.27; found C 74.73, H 5.07, N 9.18.
X-ray Structure Determinations: Single crystals of 3 were obtained
by slowly cooling a warm ethanol solution of 3; those of 5 were
grown from dichloromethane. Crystallographic data of 3 and 5
were collected on a Philips PW1100 diffractometer in the ω–2θ scan
mode using Ag-Kα radiation (λ = 0.5596 Å) and a graphite mono-
chromator (Table 4). The phase problem in 3 was solved by direct
methods, and that of 5 by applying the heavy atom approach. Nei-
ther absorption nor extinction corrections were carried out. In both
cases, all hydrogen positions could be obtained from ∆F syntheses
yet were not included in the final cycles of refinement. All calcula-
tions were carried out using the program system SHELX-76[34] with
(b) A mixture of 5 (1.98 g, 4.0 mmol) and [PdCl2(1,5-COD)]
(580 mg, 2.0 mmol) in 60 mL of toluene/CH2Cl2 (2:1) was placed
in a 500-mL rotation autoclave and reacted under a CO pressure
of 10 atm for 2 d. The work-up procedure was the same as in (a)
and yielded 1.4 g (71%) of yellow-green crystalline 6, m.p. 107 °C
(dec.; ref.[16] 102–106 °C). IR (KBr): ν = 2240 cm–1 s [ν(NC)]; 2065
˜
s, 1920 vs, 1870 sh [ν(CO)]; 1260 s, br. [ν(PN)]. C24H15CrN2O5P
(494.4): calcd. C 58.31, H 3.06, N 5.67; found C 58.49, H 3.07, N
5.55.
CN/NC-Isomerisation of Pentacarbonyl[(N-isocyanimino)triphenyl-
phosphorane]tungsten. Preparation of 7: COCl2 (30 mL of a 0.1  scattering factors for neutral atoms taken from the literature.[35]
solution in THF) was added to a solution of [W(CO)5CNNPPh3]
The molecular plots were produced with the ORTEP program.[36]
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H. Stolzenberg, B. Weinberger, W. P. Fehlhammer, F. G. Pühlhofer, R. Weiss
FULL PAPER
Table 4. Crystal data and structure refinement of 3 and 5.
3
5
Empirical formula
Formula mass
Crystal size [mm]
Crystal system
Space group
a [Å]
C19H15N2P
302.3
0.4×0.3×0.15
monoclinic
P21/n
9.680(2)
16.602(4)
10.356(2)
110.55(1)
C24H15CrN2O5P
494.4
0.4×0.3×0.2
monoclinic
P21/a
12.817(5)
15.431(3)
12.536(4)
116.03(4)
2227.86
b [Å]
c [Å]
β [°]
V [Å3]
1558.38
Z
4
4
ρcalcd. [Mgm–3]
1.288
0.94
1.473
2.80
µ(Ag-K ) [cm–1]
α
F(000)
632
293
13.62–49.42
9661
2180
1008
233
3.10–47.64
10721
4921
Temperature [K]
2θ range for data collection [°]
Reflections collected
Independent reflections
Rint [%]
0.0511
0.0419
Observed reflections
Weighting scheme
R1/wR2 [F Ͼ 4σ(F)]
1835 [I Ն 2.5σ (I)]
w = 1/[σ2(Fo) + 9×10–5 Fo
0.0439/0.0426
3691 [I Ն 2.5σ (I)]
w = 1/[σ2(Fo) + 10–6 Fo
0.0409/0.0397
2
2
]
]
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CCDC-279551 (for 3) and -279552 (for 5) contain the supplemen-
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Financial support for this work by the Fonds der Chemischen In-
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Fuchs of the Institut für Anorganische und Analytische Chemie der
Freien Universität Berlin for his assistance in the X-ray structure
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Chemie und Biochemie der Ludwig-Maximilians-Universität
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Free and Metal-Coordinated (N-Isocyanimino)triphenylphosphorane
FULL PAPER
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Received: March 4, 2005
Published Online: September 19, 2005
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