Unusual properties of the first copper complex containing a π(η2)-coordinated phosphorus-carbon double bond moiety

Article abstract of DOI:10.1039/b204144h

The first stable phosphaarene π-complex of copper displays unusual ³¹P NMR data that suggest a novel interpretation of ³¹P coordination shifts in π-complexes of phosphorus containing multiple bonds.

Full text of DOI:10.1039/b204144h

Unusual properties of the first copper complex containing a
p(h2)-coordinated phosphorus–carbon double bond moiety†
Dietrich Gudat,* Martin Nieger, Katja Schmitz and Laslo Szarvas
Universität Bonn, Institut für Anorganische Chemie, Gerhard-Domagk-Str. 1, 53121 Bonn, Germany.
E-mail: dgudat@uni-bonn.de; Fax: ++49 228 73 5327
Received (in Cambridge, UK) 1st May 2002, Accepted 3rd July 2002
First published as an Advance Article on the web 19th July 2002
The first stable phosphaarene p-complex of copper displays
unusual 31P NMR data that suggest a novel interpretation of
31P coordination shifts in p-complexes of phosphorus
containing multiple bonds.
Copper( )–olefin complexes play an important role in modern
I
organic and biochemistry; in particular, valuable transforma-
tions such as Cu-catalysed addition of carbanions to a,b-
unsaturated carbonyls and alkene cyclopropanation/aziridina-
tion involve these compounds as catalytically active species or
resting states.1 Recently, interaction between copper and
Fig. 1 ORTEP style drawing of 2a (thermal ellipsoids at the 50% probability
level; hydrogen atoms omitted). Selected bond lengths (Å) and angles (°):
I1–Cu1 2.5218(6), Cu1–P1A 2.2262(12), Cu1–C9 2.312(4), Cu1–P1
2.4332(12), P1–C9 1.749(4), P1A–C9A 1.704(4); I1–Cu1–P1A 125.47(4),
P1A–Cu1-Cent(P1,C9) 116.5(1), I1–Cu1–Cent(P1,C9) 116.9(1).
aromatic p-systems has gained attention, and some Cu( )–arene
I
complexes have been characterised.2 Despite the analogy
between CNC and PNC double bonds and the ability of the latter
to form p-complexes with transition metals,3 stable copper p-
complexes of phosphaarenes or phosphaalkenes are still
unknown, although Cu-mediated coupling of phosphaalkenes
has been successfully applied in synthesis.4 During our studies
on zwitterionic benzo[c]phospholides such as 1a,5 we prepared
complex 2a (Scheme 1) comprising the first example of a p-
bound PNC unit at a copper atom. Apart from the rare trapping
of different coordination modes for identical ligands at one
metal, a solid state NMR study revealed an unusually low 31P
coordination shift for the p-bound ligand. Computational
analysis of this effect lead to a new interpretation of coordina-
tion shifts that allows novel insight into the nature of metal–
ligand bonding in p-complexes of phosphorus containing
multiple bond systems.
(sum of intra-ligand angles 360(2)°), and other intra-ligand
distances and angles match those in free 1a.8 The modest
structural distortion of the p-bound phosphaarene in 2a is
similar to Cu(
)–arene complexes2 and suggests a lower degree
I
of p-bond localization and M?L(p*) charge-transfer than in
other benzophospholide p-complexes.5,9
The inequivalence of the benzophospholide ligands in 2a is
as well reflected in solid state 31P MAS NMR spectra. The metal
bound phosphorus nuclei give rise to two distinct signals with
nearly identical isotropic chemical shift, but different spin–spin
couplings to the 63Cu (69.1%, I = 3/2) and 65Cu (30.9%, I =
3/2) nuclei (Fig. 2): one resonance is split into an asymmetric
1+1+1+1 quartet by the combined effects of scalar and residual
dipolar coupling,10 while the second one is an unresolved
singlet that is superimposed to one quartet line. All J(P,P)
couplings are unresolved. The quartet shows a similar habit as
Reaction of CuI (1 equiv.) with 1a5 (2 equiv.) in CH2Cl2 gave
a yellow solution whose work-up yielded orange crystals of
2a.‡ A single crystal X-ray diffraction study§ revealed the
presence of complexes [(1a)2CuI] whose metal atom is
coordinated in a Y-trigonal geometry defined by the iodide, the
was found in Cu(
)–phosphine complexes11 and is assigned to
I
1
2
1
phosphorus atom of a h (P)-bound benzophospholide, and a h -
coordinated PNC double bond of another molecule of 1a (Fig.
1). The Cu1–I1 (2.5218(6) Å) and Cu1–P1A (2.2262(12) Å)
distances match those in Cu-complexes of bis-phosphonio-
benzophospholides (Cu–I 2.501(2), Cu–P 2.20–2.25 Å6). The
the h (P)-coordinated phosphorus atom, and the singlet to that
2
in the h (PNC) unit. This interpretation was supported by
spectral simulation12 (Fig. 2) which yielded d31P = 149.5/153.8
1
2
and 1J(63Cu, 31P) = 1460/125 Hz for the h (P)/h (PNC) sites,
1
respectively. The relative magnitudes of J(M,P) agree with
2
1
distance between the metal and the h (PNC)-moiety (Cu–
earlier assignments in h (P)- and p-complexes of phosphaalk-
enes,3,13 but—quite surprisingly—the p-bound ligand in 2b
fails to show a large negative coordination shift which was up to
now considered an unmistakable characteristic for p (PNC)-
coordination.3,9,13 The signals of the phosphonio groups appear
as a single broad line at d31P = 10.7.
Centroid 2.206(2) Å) exceeds analogous distances in olefin
complexes (Cu–Centroid 2.00 ± 0.117). The p-coordinated P1–
C9 bond (1.749(4) Å) is longer than the corresponding bonds in
1
the h (P)-bound ligand in 2a (P1A–C9A 1.704(4) Å) and free 1a
(1.7171(14) Å8), but still shorter than those in h -complexes of
2
Solution 31P{1H} NMR spectra of 2a in CH2Cl2 display at
290 °C an [AX]2 pattern with resolved couplings J(A,X) = 78
phosphoniobenzophospholides with Co( ) or Mo(0) (1.81–1.83
I
Å9). In contrast to these, the geometry at the C9 atom is planar
Scheme 1 R = Ph (1a, 2a), H (1b, 2b).
† Electronic supplementary information (ESI) available: experimental
procedure for 2a, details on computational studies and simulation of the
solid state NMR data of 2a. See http://www.rsc.org/suppdata/cc/b2/
b24144h/
Fig. 2 Expansion of the 31P{1H} MAS NMR spectrum of 2a (161.9 MHz,
rotor frequency 9 kHz) showing the isotropic line (bottom trace), and results
of a spectral simulation (top trace).
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CHEM. COMMUN., 2002, 1820–1821
This journal is © The Royal Society of Chemistry 2002

Hz (intra-ligand) and J(A,AA) = 105 Hz (P–M–P). At higher
temperatures, the multiplets broaden and finally collapse due to
intermolecular ligand exchange. Even if one cannot rule out that
the equivalence of both benzophospholides at low temperature
owes to dynamic interchange of s- and p-bound ligands, the
large value of J(A,AA) agrees better with a static structure with
Duncanson formalism, in accord with a lower back-donating
ability of Cu( ) than of Co( ) or Mo(0).
I
I
In principle, the described approach to the analysis of 31P
coordination shifts should be more general, and we believe it
likewise to be applicable to analysis of the bonding in p-
complexes of other types of unsaturated phosphorus ligands.
We thank Dr W. Hoffbauer (Universität Bonn) for the
recording of solid state NMR spectra.
1
two h (P)-bound ligands. A low preference of 2a for p-
coordination in solution resembles the behavior of Cu( )–arene
I
complexes,2 and p/s-coordination shifts are known for phos-
phaalkene complexes.3
Notes and references
In order to rationalise the unusual spectroscopic and
structural features associated with phosphaarene p-coordination
in 2a, we initiated a DFT computational study of the model
complexes [(1b)2CuI].14 A survey of coordination isomers
‡ Characterization data for 2a: yield 75%, mp 149 °C (decomp.); (+)-FAB–
MS: m/z = 851 [M 2 I]+, 457 [1a + Cu]+, 395 [1a + H]+.
§ Crystal data for 2a at 123 K: C52H40CuP4I, M = 979.2, triclinic, space
¯
group P1, a = 10.5944(2), b = 11.4496(2), c = 20.0383(15) Å, a =
2
1
revealed almost equal energies for [{h (p)-1b}{ h (P)-1b}CuI]
88.5390(10), b = 78.5760(10), g = 64.676(2)°, V = 2148.8(2) Å3, Z = 2;
7328 reflections collected, R1 = 0.042 (I > 2s(I)), wR2 = 0.109, the H
atoms attached to C9 and C9A were located and refined freely; CCDC
reference number 184981. See http://www.rsc.org/suppdata/cc/b2/
b204144h/ for crystallographic data in CIF or other electronic format.
1
(2b) and the C2-symmetric complexes [{h (P)-1b}2CuI] (DE =
20.1 kcal mol21) and [{h (p)-1b}2CuI] (DE = 20.4 kcal
2
mol21), thus providing a rationale for conformational flux-
ionality of 2a and suggesting that its solid state structure may be
determined by crystal packing effects. Comparison of geometric
1 (a) Modern Organocopper Chemistry, ed. N. Krause, Wiley-VCH,
Weinheim, 2002; (b) G. van Koten, S. L. James and J. T. B. H.
Jastrzebski, in Comprehensive Organometallic Chemistry II, ed. E. W.
Abel, F. G. A. Stone and G. Wilkinson, Pergamon, New York, 1995, vol
3, p. 57ff; (c) M. P. Doyle, in ref. 1(b), vol 12, p. 387ff.
2 (a) R. W. Turner and E. L. Amma, J. Am. Cem. Soc., 1966, 88, 1877; (b)
M. B. Dines and P. H. Bird, Chem. Commun., 1973, 12; (c) H.
Schmidbaur, W. Bublak, B. Huber, G. Reber and G. Müller, Angew.
Chem., Int. Ed. Engl., 1986, 25, 1089; (d) R. R. Conry and W. S.
Striejewske, Chem. Commun., 1998, 555; (e) Y. Shimazaki, H.
Yokoyama and O. Yamauchi, Angew. Chem., Int. Ed., 1999, 38,
2401.
3 K. B. Dillon, F. Mathey and J. F. Nixon, Phosphorus: The Carbon Copy,
Wiley & Sons, Chichester, 1998, pp. 88–127, 258–358 and references
therein.
4 (a) S. Ito and M. Yoshifuji, Chem. Lett., 2000, 1390; (b) S. Ito, K.
Toyota and M. Yoshifuji, J. Organomet. Chem., 1998, 553, 135.
5 D. Gudat, S. Häp, L. Szarvas and M. Nieger, Chem. Commun., 2000,
1637.
6 (a) D. Gudat, M. Schrott and M. Nieger, J. Chem. Soc., Chem.
Commun., 1995, 1541; (b) D. Gudat, A. W. Holderberg, N. Korber, M.
Nieger and M. Schrott, Z. Naturforsch., Teil B, 1999, 54, 1244.
7 Result of a quest in the CCSD database.
8 S. Häp, L. Szarvas, M. Nieger and D. Gudat, Eur. J. Inorg. Chem., 2001,
2763.
9 (a) D. Gudat, S. Häp and M. Nieger, Z. Anorg. Allg. Chem., 2001, 627,
2269; (b) D. Gudat, S. Häp and M. Nieger, J. Organomet. Chem., 2002,
643, 181.
10 E. M. Menger and W. S. Veeman, J. Magn. Reson., 1982, 46, 257.
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M. Brodie, J. Magn. Res., 1998, 135, 208; (b) J. H. Nelson, Conc. Magn.
Reson., 2002, 14, 19.
2
features of 2b revealed that the h -coordinated PNC bond (1.753
1
Å, cf. 1.719 Å in h (P)-bound and 1.721 Å in free 1b) displays
only moderate lengthening and weak deviations from planarity
(sum of bond angles at C = 358.5°). A charge decomposition
analysis15 of M–L donor–acceptor interactions in 2b yielded
values of 0.28/0.20 and 0.27/0.23 electrons for charge donation/
2
1
back-donation involving h (p)- and h (P)-1b, respectively,
suggesting that the ligand is a weaker p-acceptor towards Cu( )
I
when present in p- rather than in s(P)-coordination mode.
Computed 31P chemical shifts for 2b obtained with energy
optimised or experimental geometries yield even smaller
negative coordination shifts Dd31P for h (p)- than for h (P)-
bound 1b (Table 1), reproducing the unusually low observed
coordination shift for the p-bound ligand in 2a.
2
1
To explain this effect, one has to consider that trends of
chemical shifts among non-hydrogen nuclei arise generally
from changes in the paramagnetic shielding term whose
magnitudes depend on the availability of excited states that are
connected with the ground state by magnetic-dipole allowed
transitions. For a phosphorus atom in a multiple bond system,
the dominant contribution is associated with a n?p* excitation,
and low transition energies correlate with large deshieldings.16
1
A decrease of d31P upon h (P)-coordination is in this context
attributable to a higher n–p* transition energy which results
from combined lowering of the n- and raising of the p*-orbital
by the effects of L(n)?M and M?L(p*) charge-transfer.
While p-coordination renders little stabilisation of the n-orbital,
the p*-orbital may here face severe destabilisation due to p-
bond pyramidalisation and concurrent s/p-rehybridisation ef-
fects associated with M?L charge-transfer. Trends in 31P
coordination shifts in p-complexes reflect thus mainly changes
in M?L charge-transfer and should allow to gauge the degree
of M?L back-donation. In this respect, the lower Dd31P and
12 Program WSolids1, K. Eichele and R. E. Wasylishen, Dalhousie Univ.,
Halifax, Canada; further details are given as ESI†.
13 A. H. Cowley, in, Phosphorus-31 NMR Spectroscopy in Stereochemical
Analysis, ed. J. G. Verkade and L. D. Quin, VCH Publishers, Deerfield
Beach, Fl, 1987, p. 621 and references therein.
2
structural distortion of the (h )p-bound phosphoniobenzophos-
14 All calculations were performed with Gaussian 98 (Rev. A.7), M. J.
Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, V. G. Zakrzewski, J. A. Montgomery, R. E. Stratmann, J.
C. Burant, S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M.
C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick, A. D.
Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz, B.
B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.
Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y.
Peng, A. Nanayakkara, C. Gonzalez, M. Challacombe, P. M. W. Gill, B.
G. Johnson, W. Chen, M. W. Wong, J. L. Andres, M. Head-Gordon, E.
S. Replogle and J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1998. Details
on methods and basis sets are given as ESI†.
15 S. Dapprich and G. Frenking, J. Phys. Chem., 1995, 99, 9352.
16 (a) D. Gudat, W. Hoffbauer, E. Niecke, W. W. Schoeller, U. Fleischer
and W. Kutzelnigg, Am. Chem. Soc., 1994, 116, 7325; (b) N. Burford,
T. S. Cameron, J. A. C. Clyburne, K. Eichele, K. N. Robertson, S.
Sereda, R. E. Wasylishen and A. W. Whitla, Inorg. Chem., 1996, 35,
5460.
pholide in 2a as compared to similar Co(
) or Mo(0) complexes9
I
indicate a reduced degree of M?L charge-transfer and thus a
lesser metallacycle character in the frame of the Dewar-Chatt-
Table 1 Experimental (2a) and computed (2b) values of d31P and Dd31P for
1
2
the h (P) and h (p)-coordinated phosphorus nuclei in 2a,b
2b: Exptl.
geometrya
2b: Optimized
geometryb
2a
d31
P
Dd31Pcd d31
P
Dd31Pce d31
P
Dd31Pcf
1
h (P)
149.5
238.3
234.0
173.9
197.7
262.9
239.1
185.1
215.4
270.2
239.3
2
h (PNC) 153.8
a Based on atomic coordinates from the X-ray structure of 2a. b Energy
c
optimised molecular geometry of 2b. Dd31
P =
d31P(complex) 2
d31P(ligand). d d31P(1a) 187.8. e d31P(1b) 236.8. f d31P(1b) 255.3.
CHEM. COMMUN., 2002, 1820–1821
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