N-heterocyclic phosphenium, arsenium, and stibenium ions as ligands in transition metal complexes: A comparative experimental and computational study

Article abstract of DOI:10.1002/zaac.200400538

Reaction of 2-chloro-1,3,2-diazaarsolenes and -diazaphospholenes with Tl[Co(CO)₄] gives instable complexes of type [Co(ER ₂)(CO)₄] which decarbonylated to yield [Co(ER ₂)(CO)₃]. Spectroscopic and X-ray diffraction studies revealed that the tetracarbonyl complexes can be formulated as ion pair for E = P and as covalent metalla-arsine for E = As, and the tricarbonyl complexes as carbene-like species with a formal E = Co double bond. A similar reactivity towards Tl[Co(CO)₄] was also inferred for 1,3,2-diazastibolenes although the products were not isolable and their constitution remained uncertain. Evaluation of structural and computational data suggests that the weak and polarized Co - As bond in [Co(AsR₂)(CO)₄] can be characterized as an inverse M→L donor-acceptor bond. The computational studies disclosed further η²(EN)-coordination of the EN₂C₂ heterocycle as an alternative to the formation of a carbene-like structure for [Co(ER₂)(CO)₃]. The η2-complex is less stable for E = P but close in energy for E = As and more stable than the carbene-like complex for E = Sb.

Full text of DOI:10.1002/zaac.200400538

N-Heterocyclic Phosphenium, Arsenium, and Stibenium Ions as Ligands in
Transition Metal Complexes: A Comparative Experimental and Computational
Study
Sebastian Burck, Jörg Daniels1), Timo Gans-Eichler2), Dietrich Gudat*, Kalle Nättinen1), 3), Martin Nieger1)
Stuttgart, Institut für Anorganische Chemie der Universität
Received December 27th, 2004.
Dedicated to Professor Gerd Becker on the Occasion of his 65th Birthday
Abstract. Reaction of 2-chloro-1,3,2-diazaarsolenes and -diaza-
phospholenes with Tl[Co(CO)4] gives instable complexes of type
[Co(ER2)(CO)4] which decarbonylated to yield [Co(ER2)(CO)3].
Spectroscopic and X-ray diffraction studies revealed that the tetra-
carbonyl complexes can be formulated as ion pair for E ϭ P and
as covalent metalla-arsine for E ϭ As, and the tricarbonyl com-
plexes as carbene-like species with a formal EϭCo double bond. A
similar reactivity towards Tl[Co(CO)4] was also inferred for 1,3,2-
diazastibolenes although the products were not isolable and their
constitution remained uncertain. Evaluation of structural and com-
putational data suggests that the weak and polarized CoϪAs bond
in [Co(AsR2)(CO)4] can be characterized as an “inverse” MǞL
donor-acceptor bond. The computational studies disclosed further
η2(EN)-coordination of the EN2C2 heterocycle as an alternative to
the formation of a carbene-like structure for [Co(ER2)(CO)3]. The
η2-complex is less stable for E ϭ P but close in energy for E ϭ As
and more stable than the carbene-like complex for E ϭ Sb.
Keywords: Carbene analogues; Phosphenium complexes; Arsenium
complexes; π-Acceptor ligands; Metal-ligand multiple bonding
Introduction
We have recently reported on the synthesis of stable
N-heterocyclic arsenium and stibenium cations III (E ϭ As,
Sb) [15, 16]. Knowing that the corresponding phosphenium
analogues (III, E ϭ P) form transition metal complexes
(IV, E ϭ P) that are stabilized by a high degree of metal-
phosphorus double bonding [17], we conceived that exten-
sion of this chemistry to ligands with heavier group 15 ele-
ments might allow to access as yet unprecedented stable
arsenium and stibenium complexes. In this work we report
on the generation of 1,3,2-diazaphospholenium, -arsolen-
ium, and -stibolenium complexes IV (E ϭ P, As, Sb; M ϭ
Co(CO)3), the first structural characterization of an arsen-
ium complex IV and a precursor V where the metal-ligand
interaction is best described as “inverse” dative bond in-
volving MǞL charge transfer, and on computational
studies aiming at a comparison of the ligand properties of
the phosphenium, arsenium, and stibenium ligands in the
complexes IV and V, respectively.
Phosphenium cations (I, E ϭ P) have since their discovery
some thirty years ago constantly remained in the focus of
research [1]. A great deal of the interest in these species was
Ϫ and still is Ϫ initiated by their ability to act as σ-donor/
π-acceptor ligands in transition metal complexes II (E ϭ P)
with similar molecular and electronic structures as Fischer-
carbene complexes [1, 2], and to stabilize electrophilic com-
plexes with late metal atoms [3].
Stable cations with heavier dicoordinate group-15 ele-
ments such as arsenic (I, E ϭ As [4Ϫ7]), antimony (I, E ϭ
Sb [8]), and bismuth (I, E ϭ Bi [8]) have likewise been pre-
pared, but coordination chemical studies involving these
species are largely confined to the formation of complexes
with Lewis bases [5, 9Ϫ11]. Well characterized carbene-ana-
logue transition metal complexes II (E ϭ As, Sb) with for-
mal double bonds to As and Sb are as yet unknown, even
if complexes [CpMo(CO)2ϭEMe2] (E ϭ As, Sb) were ident-
ified by matrix isolation techniques [12], and metalla-arsine
and -stibine complexes with pyramidal coordination at the
pnicogen atom and formal M-E single bonds have been
described [13, 14].
* Prof. Dr. Dietrich Gudat
Institut für Anorganische Chemie der Universität Stuttgart
Pfaffenwaldring 55
70550 Stuttgart
Fax: ϩ49 (0)711 685 4241
E-mail: gudat@iac.uni-stuttgart.de
1) Institut für Anorganische Chemie der Universität Bonn
2) current address: Institut für Chemie, Universität Hohenheim
3) Department of Chemistry, University of Jyväskylä, Finland
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S. Burck, J. Daniels, T. Gans-Eichler, D. Gudat, K. Nättinen, M. Nieger
Results and Discussion
ting of the single IR-active F2 ν(CO) mode, the spectral
data point to a very low degree of intermolecular interac-
tion which might perturb the local Td symmetry in the
anion [20]. This suggests that the solution structure of 3a is
presumably best described as a solvent separated ion pair.
The cleavage of CO from 3a was accompanied by the
appearance of a new 31P NMR signal at lower field (δ31P
234.7) and two new IR bands at 2015 and 1948 cmϪ1. These
data suggest conversion of 3a into a single product which
was formulated as the carbene-analogue tricarbonyl com-
plex 4a. Direct formation of 4a was observed when 1a and
Tl[Co(CO)4] were reacted at ambient temperature in tolu-
ene, and treatment of 1b with Tl[Co(CO)4] under the same
conditions yielded the analogous complex 4b. Both 4a,b
were isolated as red, slightly air and moisture sensitive crys-
talline solids and their constitution was confirmed by ana-
lytical and spectroscopic data and single-crystal X-ray dif-
fraction studies.
The reaction of Tl[Co(CO)4] with the 2-chloro-1,3,2-di-
azaarsolene 5b at room temperature in MeCN proceeded,
in the same way as with 1a, via elimination of TlCl and
formation of a red solution which decomposed slowly under
evolution of CO if stored for prolonged time above Ϫ30 °C.
In contrast to the reaction of 1a, crystallization at Ϫ25 °C
allowed to isolate a moderate yield of the initial product.
Since the reaction occurred without liberation of CO and
the IR spectrum displayed a pattern of three ν(CO) bands
(ν¯ ϭ 2004, 1934, 1928 cmϪ1) which is in accord with an
assignment to the three IR-active modes (2 a1 ϩ e) of a
complex [Co(CO)4(L)] having local C3v symmetry, the prod-
uct was formulated as the metalla-1,3,2-diazaarsolene 7b.
The marked deviation of the IR data of 7b from those of
[Co(CO)4]Ϫ suggest that, unlike as in 3a, a substantial co-
valent interaction between As and Co is present. This pro-
posal was confirmed by a single-crystal X-ray diffraction
study of 7b.
Complex syntheses
Although earlier work by Nakazawa et al. demonstrated the
use of strongly back-donating 16 valence electron [M(CO)3-
(bipy)] fragments (M ϭ Mo, W; bipy ϭ 2,2Ј-bipyridine) for
a highly efficient stabilization of phosphenium complexes
[2, 18], we have previously established that N-tBu substi-
tuted 1,3,2-diazaphospholenes fail to react with [W(CO)3-
(bipy)(MeCN)], presumably because of sterical reasons [17].
The desired 1,3,2-diazaphospholenium complexes were,
however, easily accessible when N-aryl substituted diaza-
phospholene precursors were used [17]. Surprisingly, neither
N-tBu nor N-aryl substituted 2-chloro-1,3,2-diazaarsolenes
or Ϫstibolenes nor the corresponding cations reacted with
[M(CO)3(bipy)(MeCN)]. Similar negative results were ob-
served when other neutral transition metal complexes such
as [W(CO)5(C8H14)] or [Pt(C2H4)(PPh3)2] were employed.
Facing this general failure to prepare complexes of the As-
and Sb-centered cations with neutral metal fragments, we
turned to reactions with metal carbonylate anions, antici-
pating that enhanced electrostatic attraction between the re-
actants might provide an additional driving force for com-
plex formation. To this end, the reactions of the 2-chloro-
substituted heterocycles 1, 5, 9 (scheme 2) with Tl[Co(CO)4]
were studied.
Reaction of equimolar amounts of the solid P-chloro-
diazaphospholene 1a and Tl[Co(CO)4] in CH2Cl2 at Ϫ70 °C
and slow warming to ambient temperature produced solid
TlCl and a red solution which was stable for a short time
before decomposing under evolution of CO. As the 31P{1H}
NMR spectrum of the initial solution displayed the signal
of the cation 2a formed by loss of chloride from 1a [19],
and both IR (ν(CO) at 1889 cmϪ1) and 59Co NMR spectra
(sharp signal at δ59Co Ϫ2979) revealed the presence of
[Co(CO)4]Ϫ [20], the initial product was formulated as ionic
1,3,2-diazaphospholenium cobaltate 3a. Considering the
absence of any broadening of the 59Co NMR line or split-
In solution, 7b decarbonylated easily to afford the car-
bene-analogue complex 8b which was identified by spectro-
Scheme 2 (R ϭ tBu (1a, 3a, 4b), Mes (1b, 4b-9b)).
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N-Heterocyclic Phosphenium, Arsenium, and Stibenium Ions as Ligands in Transition Metal Complexes
˚
scopic data and a single-crystal X-ray diffraction study. The
same species was also identified by 1H NMR and IR studies
as product of the reaction of Tl[Co(CO)4] with the diazaar-
solenium triflate 6b [16]. The most significant spectral data
Table 1 Selected bond distances (in A) and angles (in °) for the
complexes 4a,b, 7b, and 8b. The numbering scheme for all three
compounds is identical to the one used in Figures 1 and 2.
4a
4b
7b
8b
comprise the blue shift for the ν(CO) bands (ν ϭ 2023,
¯
1964 cmϪ1) with respect to 4b (ν¯ ϭ 2015, 1945 cmϪ1).
Finally, reaction of Tl[Co(CO)4] with the 2-chloro-1,3,2-
diazastibolene 9b in d8-toluene or hexane at Ϫ78 °C af-
forded a precipitate of TlCl and a red solution whose color
first faded and then turned black when the solution was
warmed to room temperature. Although no isolable product
Co(1)-P(1)/As(1)
Co(1)-C(1A)
Co(1)-C(1B)
Co(1)-C(1C)
Co(1)-C(1D)
P(1)/As(1)-N(2)
P(1)/As(1)-N(5)
N(2)-C(3)
2.0450(5)
1.760(2)
1.772(2)
1.775(2)
2.0018(4)
1.784(2)
1.773(2)
1.781(2)
2.7263(4)
1.789(3)
1.790(3)
1.796(3)
1.822(3)
1.846(2)
1.835(2)
1.394(3)
1.330(3)
1.396(3)
2.129(1)
1.777(1)
1.774(4)
1.774(4)
1.6835(14)
1.6838(13)
1.387(2)
1.330(2)
1.388(2)
1.6720(12)
1.6733(13)
1.391(2)
1.332(2)
1.394(2)
1.811(2)
1.807(3)
1.381(4)
1.355(5)
1.381(4)
1
was obtained, H NMR spectra disclosed formation of a
C(3)-C(4)
C(4)-N(5)
transient species which decayed rapidly to afford a diazadi-
ene, (MesNϭCH-)2, and further unidentifiable products. IR
spectra showed two strong (ν¯ ϭ 2062, 1977 cmϪ1) and two
N(5)-As(1)/P(1)-N(2)
N(5)-As(1)/(P(1)-Co(1)
N(2)-As(1)/P(1)-Co(1)
Σangles(P1/As1)
90.33(7)
135.10(5)
134.50(5)
359.9(2)
89.17(6)
133.83(5)
136.59(5)
359.6(2)
85.40(8)
111.59(6)
103.15(6)
300.1(2)
85.5(1)
141.6(1)
132.9(1)
360.0(3)
further weak ν(CO) bands (ν ϭ 2002, 1942 cmϪ1) attribu-
¯
table to transient products. By analogy to the reactions of
1a,b and 5b, we interpret the available data in terms of the
presence of a mixture of a tetracarbonyl complex (minor
product) and a tricarbonyl complex (major product) which
undergo dynamic exchange via CO-transfer on the NMR
time scale (thus explaining the presence of a single set of
signals) but are distinguishable on the IR time scale. Evi-
dence for a similar dynamic equilibrium between the As-
complexes 7b and 8b was provided by line broadening and
PϪN and NϪC bonds in the diazaphospholene rings are
slightly longer and the CϭC bonds shorter than in the
˚
˚
cation 2a (PϪN 1.658 Ϫ 1.665 A, NϪC 1.368 Ϫ 1.377 A,
˚
CϭC 1.341 Ϫ 1.353 A [19]). Although these deviations are
small (approx. 2 pm for PϪN and 1 pm for NϪC and
CϭC bonds), their direction and magnitude matches the
complexation induced changes in bond lengths in cationic
diazaphospholenium tungsten complexes [17] and may in
line with the generally acknowledged model of bonding in
aminophosphenium complexes [2] be interpreted by as-
suming that the strong M-P π-interaction introduces a
massive reduction in cyclic intraligand π-delocalization.
1
coalescence effects in the H NMR spectra of solutions of
partially decarbonylated 7b.
Crystal structure studies. The molecular structures of the
1,3,2-diazaarsolene complexes 7b and 8b are displayed in
Figures 1 and 2, respectively. The molecular structures of
the diazaphospholenium complexes 4a,b are very similar to
the latter and are not shown. All crystals consist of isolated
mononuclear complexes that exhibit no significant intermo-
lecular interactions. Selected bond distances and angles for
all four complexes are listed in Table 1.1)
¯
The crystals of the metalla-arsine 7b (space group P1) are
composed of molecular complexes with a central cobalt
atom in a distorted trigonal bipyramidal coordination
sphere (Fig. 1). All equatorial and one axial coordination
sites are occupied by carbonyl groups and the second axial
site by the As-atom of the diazaarsolene unit. The equa-
torial CoϪC bonds are equal and by approx. 4 pm shorter
The phosphenium complexes 4a,b crystallize in the space
group P21/n and P21/c, respectively. The phosphorus and
cobalt atoms have distorted trigonal planar and tetrahedral
coordination, respectively. The CoϪP bond in 4a is slightly
˚
longer than in 4b (2.0450(5) vs. 2.0018(4) A), reflecting
presumably the larger steric bulk of N-tBu as compared to
N-Mes groups and
a concomitantly increased steric
congestion between the phosphenium fragment and
the ancillary CO ligands. Both bond lengths deviate
not significantly from those in the known complexes
˚
[(CO)3CoϭP(O-C6H2tBu3)(Cp*)] (CoϪP 2.010(19) A [21])
and [(CO)3CoϭP(O-C6H2tBu3)(C2(Ph)Co2(CO)6)] (CoϪP
˚
2.009(3) A [22]) and suggest that the metal-ligand interac-
tion may be described as in other neutral phosphenium
complexes [1, 2] as a formal double bond. The remaining
bond distances and angles in 4a,b (apart from those in
N-tBu and N-Mes substituents) differ not significantly. The
1) In order to facilitate the discussion of structural features, we ap-
plied a universal numbering scheme for all compounds which is in
some cases different from the one used in the cif-files.
Fig. 1 Molecular structure of 7b. Thermal ellipsoids are drawn at
50 % probability level, and H atoms have been omitted for clarity.
Selected bond lengths and angles are given in Table 1.
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S. Burck, J. Daniels, T. Gans-Eichler, D. Gudat, K. Nättinen, M. Nieger
than the axial one; all CoϪC bonds are longer than in
[Co(CO)4]Ϫ (avg. distance in known tetracarbonyl co-
baltates 1.75(4) pm [23]) but similar as in metalla-germanes
of the type [R3GeϪCo(CO)4] or [R2Ge(Co(CO)4)2], respec-
tively [24, 25]. A most remarkable feature of 7b is the ex-
˚
tremely long CoϪAs bond (2.7264(4) A) whose distance
substantially exceeds those in cobalt complexes of tertiary
˚
arsines (AsϪCo 2.33(4) A [23]) or cobalta-arsines
˚
[LnCoϪAsR2] (CoϪAs 2.39(3) A [23]). The As(1) atom ex-
hibits a pyramidal coordination geometry (sum of bond
angles 300°) and the diazaarsolene ring is not exactly planar
as in 4a [16] but exhibits a very flat envelope conformation
similar to that in the P-chloro-diazaphospholene 1a [19].
Fig. 2 Molecular structure of 8b. Thermal ellipsoids are drawn at
50 % probability level, and H atoms have been omitted for clarity.
Selected bond lengths and angles are given in Table 1.
˚
The endocyclic AsϪN (avg. 1.840(2) A) and CϪN bonds
˚
˚
(avg. 1.395(2) A) are longer and the CϭC bond (1.330(3) A)
is shorter than in the cation 6b (avg. distances: AsϪN
as the lighter congeners. Apart from the AsϪCo and AsϪN
bond lengthening and the decrease of the NϪAsϪN angle
which reflect the larger atomic radius of an arsenic as com-
pared to a phosphorus atom, all bond distances in the
EN2C2-rings (E ϭ P, As) and Co(CO)3 moieties of 8b and
4b are equal within experimental error, and all atoms in the
AsN2C2 ring of 8b still display planar coordination (sum of
˚
1.832(2), CϪN 1.349(2), CϭC 1.362 A [16]), thus indicating
a higher π-bond localization [16, 19] in 7b.
If one considers all structural features, the bonding situ-
ation in 7b resembles that in 2-chloro-diazaphospholenes 1.
The unique PϪCl bond lengthening in these compounds
was attributed to ionic bond polarization (PδϩϪClδϪ) and
lead to a description as donor/acceptor complexes of a diaz-
aphospholenium cation and a chloride anion [19]. The pola-
rization may be rationalized in the VB picture by assuming
that the high cation stability of a diazaphospholenium frag-
ment stabilizes the ionic canonical structures 1Љ, 1ٞ (scheme
3) relative to the covalent structure 1Ј and thus emphasizes
ionic bonding contributions and charge transfer to the
chlorine atom. Adapting this picture suggests that the
bonding in 7b can be likewise described by bond/no-bond
resonance between the canonical structures 7Ј ؊ 7ٞ. As-
suming that the observed bond lengthening points as in the
case of 1 to a considerable contribution by the ionic struc-
tures, the CoϪAs bond adopts substantial character of an
“inverse” donor-acceptor bond involving charge transfer
from cobalt atom into the empty p(As)-orbital of a diazaar-
solenium cation.
˚
angles 360°). The AsϪCo bond in 8b (2.129(1) A) is by
36 pm (11 %) shorter than the average single bond in
˚
[LnCoϪAsR2] (2.39(3) A [23]), and by 60 pm (22 %) shorter
than the unusually long bond in 7b. Evaluation of further
structural variations in the diazaarsolene units of 8b and 7b
reveals a small but distinguishable shortening of AsϪN
bonds in the former by approx. 3 pm whereas shifts in CϪN
and CϭC distances are not considered significant.
All features suggest clearly that the bonding between the
cobalt and arsenic atoms in 8b may be described as in phos-
phenium complexes [1, 2, 17] as a formal double bond re-
sulting from superposition of a dative LǞM σ-bonding and
a retro-dative MǞL π-bonding interaction between a 16VE
Co(CO)3Ϫ and a diazaarsolenium fragment. The shortening
of the AsϪN bonds in 8b with respect to 7b suggests that
as in diazaphospholenium complexes [17] residual intrali-
gand π-delocalization may contribute to stabilize the posi-
tive charge on the As-atom.
Although the formation of dative/retro-dative double
bonds has precedence in phosphenium complexes, the mo-
lecular structures of the dimeric dication 10 [26] and the
complex 11 [13] (Scheme 4) suggest that the lone-pair of an
adjacent sulfur or nitrogen atom may provide a more favor-
able donor site for interaction of an arsenium moiety with
a Lewis acid than the lone-pair at arsenic atom. In con-
sideration of these results it cannot be ruled out that the
preference for a carbene-analogue structure for 8b is largely
determined by the steric influence of the N-aryl substituents
which renders the nitrogen lone-pairs inaccessible for attack
by a Lewis acid.
Scheme 3
Computational studies
Crystals of the arsenium complex 8b (space group P21/c)
In order to permit a profound discussion of structure and
contain molecules (Fig. 2) with similar structural features
bonding in N-heterocyclic phosphenium, arsenium, and sti-
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N-Heterocyclic Phosphenium, Arsenium, and Stibenium Ions as Ligands in Transition Metal Complexes
ation functions at all heavy atoms. The final structures of
the metalla-diazaphospholene 3c, -arsolene 7c and -sti-
bolene 13c are displayed in Fig. 3, and those of the carbene-
analogue complexes 4c, 8c, 14c and their isomers 4Јc, 8Јc,
14Јc with η2-bound heterocyclic ligands in Fig. 4. All struc-
tures shown represent according to vibrational analyses lo-
cal minima on the energy hypersurface. Selected bond dis-
tances of the complexes and the free cations 2c, 6c, and 12c
are listed in Table 2 and results of NBO population analyses
[29] in Table 3.
The computed bond lengths of 4c and 7c (Tab. 2) are,
regardless of the simplified structures of the model com-
pounds, in close agreement with the experimental data of
4a,b and 7b (Tab. 1); the only notable discrepancy com-
prises the overestimation of the CoϪAs distance in 7c. We
attribute this deviation to the neglect of inductive substitu-
ent effects in the model compounds and the inappropri-
benium complexes, computational studies were carried out.
To cut down computation times, model compounds carry-
ing N-H substituents were used and interaction between
ionic species with counter-ions or solvent molecules were
neglected. Molecular structures of all compounds were ob-
tained from energy optimizations, using the B3LYP density
functional method [27] employed in the GAUSSIAN 98 su-
ite [28] with sdd basis sets augmented by one set of polariz-
Fig. 3 Representation of the computed molecular structures of the tetracarbonyl complexes 3c (a), 7c (b), and 13c (c). Selected bond
lengths and angles are listed in Table 2.
Fig. 4 Representation of the computed molecular structures of the tricarbonyl complexes 4c (a), 8c (b), and 14c (c) and the η2-complexes
4Јc (d), 8Јc (e), and 14Јc (f). Selected bond lengths and angles are listed in Table 2.
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˚
Table 2 Computed bond distances (in A) and angles (in °) for the cationic ligands 2c, 6c, 12c, the tetracarbonyl complexes 3c, 7c, 10c,
and the tricarbonyl complexes 4c/4Јc, 8c/8Јc, and 11c/11Јc. The numbering scheme for all compounds is identical to the one used in Figures
1 and 2.
2c
P
C2v
3c
P
C1
4c
P
Cs
4Јc
P
C1
6c
As
C2v
7c
As
Cs
8c
As
Cs
8Јc
As
C1
12c
Sb
C2v
13c
Sb
Cs
14c
Sb
Cs
14Јc
Sb
C1
E
Symm.
Co-E
Co-Ceq
2.885
1.788
1.775
1.771
1.792
1.708
2.008
1.778
1.781
2.255
1.769
1.775
1.793
2.885
1.791
1.776
2.142
1.774
1.780
2.359
1.764
1.777
1.787
2.960
1.791
1.780
2.376
1.761
1.796
2.524
1.756
1.790
1.801
Co-Cax
E-N
1.788
1.840
1.788
2.037
1.685
1.375
1.378
1.704
1.394
1.361
359.8
1.777
1.737
1.426
1.391
1.354
1.817
1.364
1.385
1.835
1.387
1.366
359.0
1.912
1.873
1.421
1.387
1.357
2.028
1.358
1.392
2.052
1.378
1.375
331.9
2.140
2.078
1.418
1.378
1.363
N-C
1.398
1.359
302.1
1.397
1.360
299.4
1.399
1.361
290.9
C-C
Σang(E)
279.4
249.1
231.5
Table 3 Natural populations of electrons on the Co(CO)x moieties and selected Wiberg Bond Indexes (WBI) for the cationic ligands 2c,
6c, 12c, the tetracarbonyl complexes 3c, 7c, 10c, and the tricarbonyl complexes 4c/4Јc, 8c/8Јc, and 11c/11Јc.
2c
3c
4c
4Јc
6c
7c
8c
8Јc
12c
13c
14c
14Јc
q(Co(CO)x
WBI(E-Co)
WBI(E-N)
Ϫ0.49
0.24
0.92
Ϫ0.50
0.67
0.88
Ϫ0.28
0.49
0.82
0.83
1.04
1.13
1.73
Ϫ0.47
0.24
0.83
Ϫ0.52
0.60
0.80
Ϫ0.30
0.47
0.75
0.75
1.05
1.15
1.71
Ϫ0.47
0.25
0.73
Ϫ0.58
0.52
0.69
Ϫ0.29
0.43
0.65
0.65
1.07
1.20
1.67
1.06
1.21
1.55
0.97
1.26
1.49
0.85
1.31
1.44
WBI(N-C)
WBI(C-C)
1.11
1.70
1.12
1.68
1.12
1.69
1.15
1.65
1.13
1.69
1.20
1.58
WBI(Co-N): 4Јc: 0.18, 8Јc: 0.19, 14Јc 0.25
ateness of calculations referring to isolated molecules in the
gas phase to model EϪX bond polarization by intermolecu-
lar interactions in condensed phases; both effects have con-
siderable influence on PϪCl distances in 2-chloro-1,3,2-di-
azaphospholenes [19]. The failure to reproduce an ion pair
structure (which had been inferred in the case of 3a) for
3c may again be attributable to the overestimation of the
electrostatic attraction between differently charged species
in gas phase calculations.
The complexes 3c, 7c, 13c (Fig. 3) display all very similar
molecular structures whose shapes resemble that of the co-
balta-diazaarsolene 7b. Even though the CoϪE distances
are much longer than normal single bonds and, surpris-
ingly, fail to reflect the change in the size of E, the results of
NBO population analyses (Tab. 3) suggest a marked charge
equalization between the [Co(CO)4]Ϫ and [E(NH)2(CH)2]ϩ
fragments. All these features are best explained by the pres-
ence of a covalent CoϪE interaction with low bond order
which results (as judged by the Wiberg Bond Index, WBI)
from population of the σ*(CoϪE) orbital as a consequence
of hyperconjugative interaction with the adjacent nitrogen-
centered lone-pairs. Both the trends in computed bond
lengths in the EN2C2 rings and the electron population re-
veal a higher degree of π-bond localization than in the cat-
ions 2c, 6c, 12c. On the whole, all data further substantiate
the description of the highly polarized CoϪE bond in terms
of bond/no-bond resonance between the canonical struc-
tures 7Ј ؊ 7ٞ (scheme 3).
The tricarbonyl complexes 4c, 8c, and 14c (Fig. 4) con-
tain planar heterocyclic ligands and cobalt atoms with
tetrahedral and pnicogen atoms with planar (4c, 8c) or pyr-
amidal (14c) coordination spheres that are connected by
rather short CoϪE bonds. The values of bond lengths and
Wiberg Bond Indices are in accord with a similar bonding
situation as in Fischer-type carbene-complexes, which is
characterized by a superposition of EǞM charge transfer
(σ-bonding contribution) and MǞE back-donation (π-
bonding contribution) to yield formal double bonds. As in
diazaphospholene-complexes [17], MǞE back-donation at-
tenuates π-interactions across the EϪN bonds and supports
the formation of a butadiene-dianion analogous 4π-system
in the N2C2 part of the heterocycle.
The complexes 4Јc, 8Јc, 14Јc (Fig. 4) feature metal atoms
that are bound to three carbonyls and one η2-bound hetero-
cyclic ligand in a Ψ-tetrahedral arrangement. The coordi-
nation of the pnicogen atom E is pyramidal and the CoϪE
bonds are 6 to 12 % longer than in the carbene-like isomers
4c, 8c, 14c, but still much shorter than in the tetracarbonyl
complexes. Evaluation of structural data and NBO results
(Tab. 3) suggests a decrease in EϪN and NϪC bond orders
and an increase in CϭC bond order with respect to 4c, 8c,
14c which implies a further π-bond localization; the obser-
1408
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
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Z. Anorg. Allg. Chem. 2005, 631, 1403Ϫ1412

N-Heterocyclic Phosphenium, Arsenium, and Stibenium Ions as Ligands in Transition Metal Complexes
vation of identical WBI values for uncoordinated and me-
tal-bridged EϪN bonds suggests that the larger bond
lengths of the latter reflect mainly rehybridization at the
nitrogen atom and not a lower π-bond order. In summary,
the η2-complexes can be described as metallacyclic struc-
tures with polar CoϪN and CoϪE bonds that may be ra-
tionalized in terms of resonance between the canonical
structures shown in scheme 5.
force for the pyramidal coordination of the Sb-atom in 14c
and the stabilization of the η2-complexes 8Јc, 14Јc relative
to the carbene-like isomers 8c, 14c, respectively (reaction
(3), scheme 6).
Although the implication of the energies of reaction (3)
Ϫ that P- and As-centered cations (2c, 6c) form preferably
Co(CO)3-complexes with carbene-like structures and the
Sb-centered cation 10c η2-complexes Ϫ is in accord with
the observed constitution of 4a,b and 8b, generalization of
this prediction is prevented by the present unfeasibility to
assess the impact of steric and electronic factors associated
with the presence of bulkier substituents at the nitrogen
atoms. Still, the presence of a sizeable energy difference be-
tween the isomeric species 4c and 4Јc in combination with
the abundance of known carbene-like phosphenium com-
plexes [1, 2] suggests that the formation of η2-phosphenium
complexes may not be likely. This situation may be different
for 1,3,2-diazaarsolenium ions where the preference for a
certain coordination mode is less obvious and the existence
of species as the complex 11 [13] suggests that the order of
relative energies may be adjustable by careful tuning of
steric and electronic requirements of substituents. The same
arguments hold for the Sb-centered cations where it is con-
ceivable that the energetic preference by some 4 kcal/mol in
favor of the η2-complex can be overcome by steric blocking
of the nitrogen donor sites by bulky substituents.
Scheme 5
Insight into the energetic relation between the different
types of complexes is feasible from inspection of the com-
puted energies for the quasi-isodesmic reactions displayed
in Scheme 6. Comparison of the energies of reactions (1)
and (2) involving formal transfer of Co(CO)n units between
different cationic ligands suggests that the tetracarbonyl
complexes (3c, 7c, 13c) become slightly more stable and the
tricarbonyl complexes (4c, 8c, 14c) less stable when the
atomic number of E increases. The first trend parallels pre-
vious findings of 1,3,2-diazaarsolenium and -diazastibolen-
ium ions being better chloride acceptors than analogous di-
azaphospholenium ions and reflects presumably a greater
Lewis-acidity with increasing size and polarizability of the
cationic center [15, 16]. The lower stability of carbene-like
complexes with As- and Sb-centered ligands (8c, 14c) may
be seen in the context of a growing reluctance of these ele-
ments to undergo isovalent orbital hybridization which is
illustrated in a rising degree of s-character for the lone-pair
at the pnicogen atom that is observed upon going from 3c,
4Јc (E ϭ P) to 7c, 8Јc (E ϭ As) and 13c, 14Јc (E ϭ Sb). As
this effect disfavors formation of trigonal-planar frame-
works of σ-bonds, it provides obviously a strong driving
Conclusions
It was shown that 1,3,2-diazaarsolene derivatives react with
Tl[Co(CO)4] in the same way as the analogous phosphorus
compounds to form complexes of formulae [Co(ER2)(CO)4]
and [Co(ER2)(CO)3], respectively. A similar reactivity was
also inferred for 1,3,2-diazastibolenes although the prod-
ucts were not isolable and their constitution remains uncer-
tain. In spite of these general analogies, a close inspection
of structure and bonding in the products formed reveals
differences that reflect the changing bonding preferences of
Scheme 6 Computed energies (in kcal/mol at the b3lyp/sdd(pol) ϩ zpe level) for quasi-isodesmic ligand transfer reactions.
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 1403Ϫ1412
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
1409

S. Burck, J. Daniels, T. Gans-Eichler, D. Gudat, K. Nättinen, M. Nieger
1,3-Di-tert.-butyl-1,3,2-diazaphospholenium tetracarbonyl cobaltate
(3a). A solution of Tl[Co(CO)4] (375 mg, 1.00 mmol) in CH2Cl2
(5 ml) was cooled to Ϫ78 °C and a solution of 1a (235 mg,
1.00 mmol) in CH2Cl2 (10 ml) was added drop wise. A white pre-
cipitate formed immediately. The mixture was slowly warmed to
ambient temperature and the product formed characterized by IR
and NMR spectroscopy. 31P{1H} NMR: δ ϭ 200.1. Ϫ 59Co NMR:
the individual elements. The complexes [Co(ER2)(CO)4] ex-
ist as ion pairs for E ϭ P and as covalent metalla-arsines
for E ϭ As; the CoϪAs bond may be represented as “in-
verse” MǞL donor-acceptor bond, leading to a similar
bonding description as in 2-chloro-1,3,2-diazaphospholenes
or -diazaarsolenes and highlighting the importance of li-
gand electrophilicity as driving force for complex forma-
tion. The trend in CoϪE interactions parallels previous fin-
dings of diazaarsolenium ions being better chloride ac-
ceptors than diazaphospholenium ions and is attributed to
a growing Lewis-acidity with increasing size and polariz-
ability of the ligand atom.
The tricarbonyl complexes [R2EϭCo(CO)3] (E ϭ P, As)
can be described as carbene-like species with a formal
double bond arising from superposition of EǞM charge
transfer (σ-bonding contribution) and MǞE back-do-
nation (π-bonding contribution). Computational studies
disclose that η2(EN)-coordination of the EN2C2 heterocycle
provides an alternative bonding mode to the formation of
a carbene-like structure which is less favorable for phos-
phorus containing rings but close in energy for arsenic and
energetically even more favorable for antimony derivatives.
The decreasing preference for the carbene-like coordination
mode correlates with the reluctance of the heavier pnicogen
elements to adopt a trigonal planar coordination geometry
and is thus essentially attributable to the decreasing propen-
sity for isovalent orbital hybridization and the presence of
a lone-pair with an increasing degree of s-character.
δ ϭ Ϫ2979. Ϫ IR (NaCl, CH Cl ): ν ϭ 1889 cmϪ1 (ν CO).
¯
2
2
1,3-Di-tert.-butyl-1,3,2-diazaphospholenium tricarbonyl cobaltate
(4a). Tl[Co(CO)4] (750 mg, 2.00 mmol) and 1a (470 mg, 2.00 mmol)
were dissolved in toluene (20 ml). Evolution of CO was observed.
The mixture was stirred for 2 h, the precipitate formed removed by
filtration, and the solvents evaporated in vacuum. Recrystallization
of the residue from MeCN (25 ml) at Ϫ25 °C afforded 532 mg
(78 %) of 4a as red crystals, mp.126 °C.
Anal. for C13H20CoN2O3P: calcd. C 45.63 H 5.89 N 8.19; found C
45.12 H 5.94 N 8.06 %.
3
1H NMR (C6D6): δ ϭ 6.35 (d, 2 H, JPH ϭ 5.0 Hz, NCH), 1.40 (s, 18 H,
2
CCH3). Ϫ 13C{1H} NMR (C6D6): δ ϭ 211.3 (br), 120.0 (d, JPC ϭ 4.2 Hz,
3
PNCH), 60.2 (d, 2JPC ϭ 7.9 Hz, NCCH3), 31.0 (d, JPC ϭ 5.5 Hz, NCCH3).
Ϫ
31P{1H} NMR (C6D6): δ ϭ 234.7 (br). Ϫ IR (KBr, nujol mull): ν¯ ϭ 2015,
1948 cmϪ1 (ν CO).
1,3-Di-Mesityl-1,3,2-diazaphospholenium tricarbonyl cobaltate (4b).
The reaction of Tl[Co(CO)4] (750 mg, 2.00 mmol) and 1b (720 mg,
2.00 mmol) was carried out as described above for 3a to 860 mg
(92 %) of 4b as red crystals, mp.199 °C.
Anal. for C23H24CoN2O3P: calcd. C 59.24 H 5.19 N 6.01, found:
C 58.93 H 5.20 N 6.01 %.
1H NMR: δ ϭ 6.74 (s, 4 H, m-CH), 5.92 (d, 2 H, 3JPH ϭ 4.7 Hz, NCH), 2.10
(s, 12 H, o-CH3), 2.05 (s, 6 H, p-CH3).
4
13C{1H} NMR: δ ϭ 208.9 (br), 139.1 (d, JPC ϭ 1.9 Hz, m-C), 135.5 (d,
2
5
3JPC ϭ 3.9 Hz, o-C), 133.6 (d, JPC ϭ 7.0 Hz, i-C), 129.7 (d, JPC ϭ 1.2 Hz,
p-C), 124.2 (d, 2JPC ϭ 3.8 Hz, PNCH), 21.0 (d, 6JPC ϭ 0.7 Hz, p-CH3), 17.8
Acknowledgement. We thank Dr. Joachim Opitz, Institut für Or-
ganische Chemie, Universität Stuttgart, for the recording of mass
spectra.
4
(d, JPC ϭ 1.1 Hz, o-CH3).
31P{1H} NMR: δ ϭ 233.0 (br). Ϫ IR (NaCl, CH3CN): ν¯ ϭ 2015, 1945 cmϪ1
(ν CO). Ϫ MS (EI, 70 eV, 380 K): m/e(%) ϭ 466 (13) [Mϩ], 438 (1)
[MϩϪCO], 410 (39) [MϩϪ2 CO], 323 (100) [MϩϪCo(CO)3].
1,3-Di-Mesityl-1,3,2-diazaarsolenium tetracarbonyl cobaltate (7b).
A solution of Tl[Co(CO)4] (750 mg, 2.00 mmol) in MeCN (10 ml)
was added drop wise to a solution of 5b (800 mg, 2.00 mmol) in
MeCN (10 ml). The precipitate formed was filtered off and the fil-
trate concentrated in vacuum. Storing the solution overnight at
Ϫ30 °C gave a dark red crystalline precipitate which was collected
by filtration to yield 400 mg (42 %) of 7b, mp. 60 °C.
Experimental Section
General Remarks: All manipulations were carried out under dry
argon. Solvents were dried by standard procedures. NMR spectra:
Bruker Avance 400 (1H: 400.13 MHz, 13C: 100.6 MHz, 31P
161.9 MHz, 59Co 94.95 MHz) in thf-D8 at 30 °C; chemical shifts
Anal. for C23H24AsCoN2O3: calcd. C 54.12 H 4.7 N 5.49, found:
C 53.62 H 4.79 N 5.6 %.
referenced to ext. TMS (1H, 13C), 85 % H3PO4 (Ξ
ϭ
40.480747 MHz, 31P) 0.56 M K3Co(CN)6 (Ξ ϭ 23.727074 MHz,
59Co); positive signs of chemical shifts denote shifts to lower fre-
quencies, coupling constants are given as absolute values. MS:
Kratos Concept 1H, Xe-FAB, m-NBA matrix. IR spectra: Nicolet
FT-IR Magna 550 or Perkin-Elmer Paragon, NaCl cells. Elemental
analysis: Perkin-Elmer 2400CHSN/O Analyser. Melting points
were determined in sealed capillaries.
1H NMR (C6D6): δ ϭ 6.73 (s, 4 H, m-CH), 6.31 (s, 2 H, NCH), 2.04 (s, 18 H,
o/p-CH3). Ϫ 13C{1H} NMR: δ ϭ 208.7 (br, CO), 139.1, 134.9, 135.9, 130.1
(m-CH), 127.2 (s, AsNCH), 21.0 (p-CH3), 18.0 (o-CH3). Ϫ IR (NaCl,
CH CN): ν ϭ 2004, 1934, 1928 cmϪ1 (ν CO).
¯
3
1,3-Di-Mesityl-1,3,2-diazaarsolenium tricarbonyl cobaltate (8b). A
solution of 7b in hexane was stored for several h at ambient tem-
perature. All volatiles were evaporated and the residue dissolved in
little MeCN. Storing the solution overnight at Ϫ30 °C gave a small
amount of a red crystalline precipitate of 8b, mp. 60-65 °C.
1H NMR (CD2Cl2): δ ϭ 7.40 (s, 2 H, NCH), 7.11 (s, 4 H, m-CH), 2.40 (s,
6 H, p-CH3), 2.26 (s, 12 H, o-CH3). Ϫ 13C{1H} NMR: δ ϭ 204 (br, CO),
139.2 (i-C), 135.6(p-C), 134.2 (o-C), 129.6 (m-CH), 130.0 (AsNCH), 20.7 (p-
CH3), 17.5 (o-CH3). Ϫ IR (NaCl, hexane): ν¯ ϭ 2023, 1964 cmϪ1 (ν CO).
DFT calculations (B3LYP) were carried out with the Gaussian 98
program package [28] using the SDD basis set augmented by one
set of polarisation functions on all heavy atoms. The basis set em-
ploys a D95V split-valence basis set for the lighter atoms, and
Stuttgart/Dresden ECPs at As and Sb. Harmonic vibrational fre-
quencies and zero-point vibrational energies (ZPE) were calculated
at the same level. All structures reported here are minima on the
potential energy surface (only positive eigenvalues of the Hessian
matrix).
Reaction of Tl[Co(CO)4] with 9b. Tl[Co(CO)4] (38 mg, 0.10 mmol)
was added to a cooled (Ϫ78 °C) solution of 9b (55 mg, 0.10 mmol)
in d8-toluene (1ml) or hexane (1 ml), respectively. The solution was
1410
© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim
zaac.wiley-vch.de
Z. Anorg. Allg. Chem. 2005, 631, 1403Ϫ1412

N-Heterocyclic Phosphenium, Arsenium, and Stibenium Ions as Ligands in Transition Metal Complexes
first warmed to Ϫ40 °C until a red color was observed, and then
further to ambient temperature. The red product formed decom-
posed slowly, and the color of the solution turned black. The tran-
sient product was characterized in the reaction mixture byNMR
(in d8-toluene) or IR spectroscopy (in hexane solution). Isolation
of a stable product was not feasible. Ϫ 1H NMR: δ ϭ 6.73 (s, 4 H,
m-CH), 6.23 (s, 2 H, NCH), 2.16 (s, 12 H, o-CH3), 2.36 (s, 6 H, p-
2σ(I)) ϭ 0.038, wR2 ϭ 0.071, largest diff. peak and hole 0.472 and
Ϫ3
˚
Ϫ0.841 e A
.
Crystallographic data (excluding structure factors) for the struc-
tures reported in this work have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication no.
CCDC-258388 (4a), 258389 (4b), 258390 (7b), 258391 (8b). Copies
of the data can be obtained free of charge on application to The
Director, CCDC, 12 Union Road, Cambridge DB2 1EZ, UK (Fax:
int.codeϩ(1223)336-033; e-mail: deposit@ccdc.cam.ac.uk).
CH ). Ϫ IR (NaCl, hexane): ν ϭ 2062 (s), 1977 (s), 2002 (w), 1942
¯
3
(w) cmϪ1 (ν CO).
Crystal structure determinations
4a: red crystals, C13H20CoN2O3P, M ϭ 342.2, crystal size 0.20 x
0.15 x 0.10 mm, monoclinic, space group P21/n (No. 14): a ϭ
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V ϭ 1615.3(1) A , Z ϭ 4, ρ(calcd) ϭ 1.41 Mg mϪ3, F(000) ϭ 712,
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μ ϭ 1.17 mmϪ1, 8490 reflexes (2 θmax ϭ 55°) measured on a Nonius
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˚
(λ ϭ 0.71073 A), 3624 unique [Rint ϭ 0.027] used for structure solu-
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.
4b: red crystals, C23H24CoN2O3P, M ϭ 466.3, crystal size 0.50 x
0.30 x 0.20 mm, monoclinic, space group P21/c (No. 14): a ϭ
˚
˚
˚
19.9890(2) A, b ϭ 7.8653(1) A, c ϭ 15.1176(3) A, β ϭ 101.619(1)°,
V ϭ 2328.1(1) A , Z ϭ 4, ρ(calcd) ϭ 1.33 Mg mϪ3, F(000) ϭ 968,
3
˚
μ ϭ 0.83 mmϪ1, 36218 reflexes (2 θmax ϭ 55°) measured on a Non-
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ius Kappa-CCD diffractometer at 123(2) K using MoKα radiation
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(λ ϭ 0.71073 A), 5281 unique [Rint ϭ 0.059] used for structure solu-
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trix least-squares on F2, SHELXL-97 [31]) with 277 parameters,
empirical absorption correction from multiple reflections, H-atoms
with a riding model, R1 (I > 2σ(I)) ϭ 0.030, wR2 ϭ 0.081, largest
Ϫ3
˚
diff. peak and hole 0.286 and Ϫ0.298 e A
.
[8] (a) M. Veith, B. Bertsch, Z. Anorg. Allg. Chem. 1988, 557, 7;
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7b: red crystals, C24H24AsCoN2O4, M ϭ 538.3, crystal size 0.30 x
0.20 x 0.10 mm, triclinic, space group P1 (No. 2): a ϭ 10.6396(3) A,
˚
¯
˚
˚
b ϭ 10.7829(3) A, c ϭ 11.2066(4) A, Ͱ ϭ 112.236(2)°, β ϭ
3
˚
96.521(2)°, γ ϭ 93.104(2)°, V ϭ 1175.8(1) A , Z ϭ 2, ρ(calcd) ϭ
1.52 Mg mϪ3, F(000) ϭ 548, μ ϭ 2.16 mmϪ1, 22309 reflexes (2
θmax ϭ 55°) measured on a Nonius Kappa-CCD diffractometer at
˚
123(2) K using MoKα radiation (λ ϭ 0.71073 A), 5252 unique
[Rint ϭ 0.074] used for structure solution (Direct Methods,
SHELXS-97 [30]) and refinement (full-matrix least-squares on F2,
SHELXL-97 [31]) with 295 parameters, empirical absorption cor-
rection from multiple reflections, H-atoms with a riding model, R1
(I > 2σ(I)) ϭ 0.032, wR2 ϭ 0.063, largest diff. peak and hole 0.561
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Ϫ3
˚
and Ϫ0.503 e A
.
8b: red crystals, C23H24AsCoN2O3, M ϭ 466.3, crystal size 0.10
x 0.13 x 0.34 mm, monoclinic, space group P21/c (No. 14): a ϭ
˚
˚
˚
8.2259(9) A, b ϭ 15.0722(16) A, c ϭ 19.7199(19) A, β ϭ 101.76(1)°,
V ϭ 2393.6(4) A , Z ϭ 4, ρ(calcd) ϭ 1.42 Mg mϪ3, F(000) ϭ 1040,
3
˚
μ ϭ 2.11 mmϪ1, 11912 reflexes (2 θmax ϭ 50°) measured on a Non-
ius Kappa-CCD diffractometer at 223(2) K using MoKa radiation
˚
(λ ϭ 0.71073 A), 4175 unique [Rint ϭ 0.079] used for structure solu-
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