Ambident σ- and π-donor ability of a neutral 10π-aromatic phosphoniobenzophospholide

Article abstract of DOI:10.1039/b005130f

1-Triphenylphosphoniobenzo[c]phospholide 2 which is accessible via NaBH₄ reduction of 1,3-bis(triphenylphosphonio)benzophospholide 1 displays an ambident coordination ability similar to a phosphinine; this is shown by its reactions with (cyclooctene)Cr(CO)₅ and (naphthalene)Cr(CO)₃ which gave (σ(P)-2)Cr(CO)₅ and (η⁵-2)Cr(CO)₃, the first π-complex of a 10π-aromatic phosphorus heterocycle with a d-block metal.

Full text of DOI:10.1039/b005130f

Ambident s- and p-donor ability of a neutral 10p-aromatic
phosphoniobenzophospholide†
Dietrich Gudat,* Stefan Ha¨p, Laszlo Szarvas and Martin Nieger
Institut fu¨r Anorganische Chemie der Universita¨t Bonn, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany.
E-mail: dgudat@uni-bonn.de
Received (in Cambridge, UK) 27th June 2000, Accepted 18th July 2000
Published on the Web 9th August 2000
1-Triphenylphosphoniobenzo[c]phospholide 2 which is ac-
cessible via NaBH4 reduction of 1,3-bis(triphenylphospho-
nio)benzophospholide 1 displays an ambident coordination
ability similar to a phosphinine; this is shown by its reactions
with (cyclooctene)Cr(CO)5 and (naphthalene)Cr(CO)3
which gave (s(P)-2)Cr(CO)5 and (h5-2)Cr(CO)3, the first p-
complex of a 10p-aromatic phosphorus heterocycle with a d-
block metal.
frontier orbital gaps in both molecules. As the effect of the
positive charge in 1 should shift all orbitals to more negative
energies,8 neutral 2 is thus expected to behave as a better p-
donor but less effective p-acceptor than 1.
To survey the coordination behaviour of 2, we explored
reactions with equimolar amounts of [(cyclooctene)Cr(CO)5]
and [(naphthalene)Cr(CO)3], respectively. Both reactions pro-
ceeded according to an 31P NMR spectroscopic assay with
quantitative formation of a single phosphorus containing
species. The products were isolated after precipitation with
hexane and recrystallisation from toluene, and their nature as
the complexes 3, 4 was established by microanalytical and
spectroscopic data and single crystal X-ray diffraction stud-
ies.9
Phosphinines I and phospholides II are aromatic phosphorus
heterocycles whose use as ligands in transition metal complexes
receives substantial current interest,1 in particular in connection
with possible applications in catalysis.2 As compared to their
organic analogues, viz. arenes and cyclopentadienides, I, II are
more versatile ligands which may bind to metals not only via the
Complex 3 displays a moderate negative coordination shift
for the endocyclic phosphorus atom [dcoord = d(ligand) 2
d(complex) = 229] while coordination shifts for the benzo-
phospholide carbon atoms are insignificant (dcoord < ± 3) and
the proton at C3 is slightly deshielded (dcoord = 0.3). All these
features are characteristic for complexes featuring s(P)-
coordinated low valent phosphorus species.1 The carbonyl
region of the IR spectrum of 3 displays the expected pattern of
a M(CO)5 moiety with the band at highest energy (n = 2064
cm21) appearing at slightly lower wavenumbers than in
[(Ph3P)Cr(CO)5] (n˜ = 2070 cm21) or the phosphinine complex
5
6
p-electron system (h /h -coordination) but also via the phos-
phorus lone-pair (s(P)-coordination), or a combination of
both.1 For phosphinines, formation of s(P)-complexes in which
the ligand displays s-donor/p-acceptor-properties similar as a
phosphane or phosphite is generally more favourable with
6
metals in low oxidation states, although complexes with h -
coordinated phosphinines are also accessible for various metal
centres.1 Phospholides prefer, in contrast, p-coordination and
5
the vast majority of complexes feature h -bound ligands; pure
s(P)-phospholide complexes are known,3 but remain rare.
We have recently established that cationic 1,3-bis-triphenyl-
phosphonio-benzophospholide 1 binds to various transition
metals in a s(P)- or m2(P)-coordination mode.4,5 The donor
ability of the p-electron system in 1 was found to be inferior to
that of the phosphorus lone-pair, and on the whole the structural
features of the complexes resembled more closely those of
topologically related phosphinine complexes rather than genu-
ine phospholide complexes.4 These findings suggested that the
reduction of p-nucleophilicity and enhancement of p-electro-
philicity induced by the phosphonio groups is of pivotal
importance for the complexation regioselectivity and lead us to
conclude that mono-phosphonio-substituted benzophospho-
lides might display a balanced ligand behaviour with prospects
for both s(P)- and p-coordination, similar to the case of
phosphinines. Here, we report on the synthesis and coordination
studies of the phosphonio-benzophospholide 2 which lead to the
isolation and structural characterisation of the first p-complex
of a 10p-aromatic phosphaarene with a d-block transition
metal.6
[s-(2,4,6-triphenylphosphinine)Cr(CO)5]
5
(n˜
=
2071
cm21 10). Although comparison of this vibrational frequency in
LM(CO)5 complexes has been used to relate the p-acceptor
qualities of L,2 a more common and quantitative approach is
provided by evaluation of Tolman’s electronic parameter c
which can be obtained from vibrational spectra of LNi(CO)3
complexes.11 We have therefore recorded IR spectral data of the
Ni(CO)3 complex of 2 (prepared in situ from equimolar
amounts of 2 and Ni(CO)4) and determined a value of c = 13
indicating that the p-acceptor ability of 2 should match that of
PPh3 (c = 1311) but is lower than that of P(OMe)3 (c = 2311)
or 2,4,6-triphenylphosphinine (c = 232).
The 1H, 13C and 31P NMR spectra of complex 4 reveal large
negative coordination shifts for the endocyclic phosphorus
(dcoord = 2158), the hydrogen at C3 (dcoord = 22.6) and the
carbon atoms in the five membered ring (dcoord = 217 to 224)
whereas the remaining nuclei in the benzophospholide unit
exhibit similar chemical shifts as in free 2. Altogether, these
During our studies of reductive fragmentation reactions of the
cation 17 we discovered that 1[Cl] reacts selectively with excess
NaBH4 via cleavage of PPh3 to form neutral species 2 (Scheme
1). The latter was isolated in satisfactory yield after re-
crystallisation from THF–isopropyl alcohol and its constitution
established by analytical and spectroscopic data.† Interestingly,
the first UV–VIS absorption band of 2 [lmax (CH2Cl2) = 352
nm] which is attributable to a p–p* transition of the benzophos-
pholide chromophore occurs at nearly the same wavelength as
for 1 [lmax (CH2Cl2) = 354 nm5], thus indicating similar
† Electronic supplementary information (ESI) available: analytical and
spectroscopic data for 2-4. See http://www.rsc.org/suppdata/cc/b0/
b005130f/
Scheme 1 Reagents and conditions: i, excess NaBH4, THF, 24 h, r.t.; ii, 1
equiv. [(cyclooctene)Cr(CO)5], THF, 6 h, r.t.; iii, 1 equiv. [(naph-
thalene)Cr(CO)3], THF, 6 h, r.t.
DOI: 10.1039/b005130f
Chem. Commun., 2000, 1637–1638
This journal is © The Royal Society of Chemistry 2000
1637

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data indicate p-coordination of 2 via the five-membered ring
closest distances observed for the carbons atoms C2, C9
which has likewise been observed for indenyl complexes such
adjacent to P1 which are presumably the centres of highest p-
electron density,8 and the Cr–P distance amounts to 2.407(4) Å.
The distances between the metal and the carbonyl C-atoms
[1.806(3)–1.837(3) Å] and the centroid of the phospholide ring
[1.874(3) Å], respectively, compare well with the correspond-
5
as [(h -indenyl)Cr(CO)3]2 6.12 The carbonyl stretching vibra-
tions in the IR spectrum of 4 (n˜ = 1925, 1835, 1826 cm21
)
appear at higher wavenumbers than in both 6 (n˜ = 1895, 1791
5
cm21 12 and [h -(C5H4PPh3)Cr(CO)3] 7 (n˜ = 1900, 1805, 1785
cm21 13), indicating that the amount of p-electron density
transferred to the metal is lower for 2 than for the carbocyclic
ligands C9H72 and C5H4PPh3, respectively.
ing values of [h -(C5H4PPh3)Cr(CO)3] 7 (Cr–CO 1.77–1.83,
5
6
Cr–Cent. 1.862 Å13) and [(h -2,4,6-triphenylphosphini-
ne)Cr(CO)3] (Cr–CO 1.80–1.82, Cr–Cent. 1.686 Å14). The
exocyclic P2–C2 bond in 4 [1.755(3) Å] is similar than in 3 but
the endocyclic P–C [1.744(2), 1.805(2) Å] and adjacent C–C
bonds [1.426(3), 1.460(3) Å] are lengthened as expected for a p-
complex. The extent of this lengthening is somewhat more
pronounced for the P–C than for the C–C bonds which agrees
with the finding that the largest coefficients in the frontier
orbitals of benzo[c]phospholides are found at the phosphorus
and the two adjacent carbon atoms.8
The results of the X-ray structure analysis of complex 3 [Fig.
1(a)] confirms the presence of a s(P)-coordinated benzophos-
pholide ligand with a planar annulated ring system. Unlike as in
known complexes of s(P)-coordinated phospholides,3 the
metal-bound phosphorus atom lacks any evidence for pyr-
amidalisation (sum of bond angles 360°). The exocyclic P–C
bond [P2–C2 1.744(1) Å] is similar as in complexes of 1 (1.75
± 1 Å4) whereas of the endocyclic P–C distances the one to the
protonated carbon atom is shortened [P1–C9 1.691(2) Å] and
the other lengthened [P1–C2 1.767(2) Å]. The P1–Cr1 bond
[2.376(5) Å] matches that in the phosphinine complex 5
[2.372(15) Å4]. The Cr–C distances in 3 [Cr–Ctrans 1.862(4) Å,
Cr–Ccis 1.895–1.912, av. 1.902 Å] are generally longer than
those in 5 [Cr–Ctrans 1.822(12) Å, Cr–Ccis 1.825–1.865, av.
1.843 Å10]. Although interpretation of these data in terms of
electronic effects is difficult in view of sterically induced
distortions in both structures, the more pronounced shortening
of the trans-relative to the cis-Cr–C bonds in 3 agrees with a
lower p-acceptor ability of 2 as compared to 2,4,6-Ph3H2C5P
which was predicted on grounds of the spectroscopic data.
The molecular structure of 4 [Fig. 1(b)] displays a three-
legged piano-stool geometry with the coordinated five mem-
bered ring featuring a flat twist conformation. The Cr–C
distances range between 2.229(2) and 2.341(3) Å with the
Preliminary studies indicate that analogous complexes as 3, 4
are likewise accessible with molybdenum and tungsten. In
conclusion, our reported findings strongly suggest that the
electron withdrawing effect of the PPh3 group induces a
balanced coordination behaviour which enables the benzophos-
pholide 2 to act both as a phosphine-like s-donor/p-acceptor
ligand in s(P)-complexes and a phosphaarene like ligand in p-
complexes. A similar switching between different coordination
modes was previously known for phosphinines,1 but scarcely
for phospholide type species. Further exploration of this feature
as well as the possible exploitation of the chiral nature of 4 are
currently under investigation and may possibly add new facets
to the coordination chemistry of phospholide ligands.
We thank the Fonds der Chemischen Industrie and Deutsche
Forschungsgemeinschaft for financial support.
Notes and references
1 F. Mathey, Coord. Chem. Rev., 1994, 137, 1; P. Le Floch and F. Mathey,
Coord. Chem. Rev., 1998, 179–180, 771; K. B. Dillon, F. Mathey and
J. F. Nixon, Phosphorus: The Carbon Copy, John Wiley, Chichester,
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2 B. Breit, Chem. Commun., 1996, 2071; B. Breit, J. Mol. Catal. A, 1999,
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3 F. Mercier, L. Ricard and F. Mathey, Organometallics, 1993, 12, 98.
4 D. Gudat, M. Nieger and M. Schrott, Chem. Ber., 1995, 128, 259; D.
Gudat, M. Schrott and M. Nieger, J. Chem. Soc., Chem. Commun., 1995,
1541; D. Gudat, M. Schrott, V. Bajorat, M. Nieger, S. Kotila, R.
Fleischer and D. Stalke, Chem. Ber., 1996, 129, 337; D. Gudat, A. W.
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B, 1999, 54, 1244.
5 D. Gudat, Coord. Chem. Rev., 1997, 173, 71.
6 p-Complexes of a benzo[b]phospholide with Li+ and Sm2+ have been
described, see: E. Niecke, M. Nieger and P. Wenderoth, Angew. Chem.,
1994, 106, 362; Angew. Chem., Int. Ed. Engl., 1994, 33, 353; F. Nief and
L. Ricard, J. Organomet. Chem., 1994, 464, 149.
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Chem., 1999, 1169.
8 D. Gudat, V. Bajorat and M. Nieger, Bull. Chim. Soc. Fr., 1995, 132,
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9 Crystal data: for 3: C31H20CrO5P2, M = 586.4, triclinic, space group
¯
P1, (no. 2), a = 10.2695(4), b = 11.0128(4), c = 13.7391(4) Å, a =
111.151(2), b = 93.476(2), g = 110.126(2)°, U = 1330.03(8) Å3, Z =
2, Dc = 1.46 g cm23, m(Mo-Ka) = 0.59 mm21, 18922 reflections
measured, 6253 unique which were used in all calculations, wR2(F2) =
0.084 (all data), R1 = 0.035 [I > 2s(I)]. For 4: C29H20CrO3P2, M =
530.4, monoclinic, space group P21/n (no. 14), a
= 8.5429(4),
b = 13.2026(9), c = 21.3056(14) Å, b = 97.658(4)°, U = 2381.6(3)
Å3, Z = 4, Dc = 1.48 g cm23, m(Mo-Ka) = 0.65 mm21, 21481
reflections measured, 4163 unique which were used in all calculations,
wR2(F2) = 0.082 (all data), R1 = 0.036 [I > 2s(I)]. CCDC 182/1721.
See http://www.rsc.org/suppdata/cc/b0/b005130f/ for crystallographic
files in .cif format.
Fig. 1 Molecular structures of 3 (a) and 4 (b). Thermal ellipsoids are drawn
at 50% probability level, and H atoms have been omitted for clarity.
Selected bond lengths (Å); for 3, Cr–P1 2.3760(5), P1–C9 1.6910(17), C9–
C8 1.414(2), C–C7 1.422(2), C7–C6 1.370(3), C6–C5 1.398(3), C5–C4
1.379(2), C4–C3 1.417(2), C3–C8 1.430(2), C3–C2 1.456(2), C2–P1
1.7670(18), C2–P2 1.7438(17), Cr1–C1D 1.9030(19), Cr–C1B 1.912(2),
Cr–C1E 1.8953(19), Cr–C1A 1.862(2). For 4: P1–C9 1.744(4), C9–C8
1.426(0), C8–C7 1.429(3), C7–C6 1.354(0), C6–C5 1.418(1), C5–C4
1.367(3), C4–C3 1.419(0), C3–C8 1.436(1), C3–C2 1.460(2), C2–P1
1.805(0), C2–P2 1.755(3), Cr–C1A 1.826(2), Cr–C1B 1.806(1), Cr–C1C
1.837(1).
10 J. Deberitz and H. No¨th, J. Organomet. Chem., 1973, 49, 453; H.
Vahrenkamp and H. No¨th, Chem. Ber., 1973, 106, 2227.
11 C. A. Tolman, Chem. Rev., 1977, 77, 313.
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13 J. C. Kotz and D. G. Pedrotty, J. Organomet. Chem., 1970, 22, 425.
14 J. Deberitz and H. No¨th, Chem. Ber., 1970, 103, 2541; H. Vahrenkamp
and H. No¨th, Chem. Ber., 1972, 105, 1148.
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