T-shaped cationic CuI complexes with hemilabile PNP-type ligands

Article abstract of DOI:10.1021/ic800298a

The versatile coordination behavior of the PNP ligands 1A (2,6-bis[(di-tert-butylphosphino)methyl]pyridine) and 1B (2,6- bis[(diphenylphosphino)methyl]pyridine) to Cu^I is described, whereby a hemilabile interaction of the pyridine N-donor atom to the copper center resulted in a rare T-shaped complex with 1A, while with 1B also a tetracoordinated species could be isolated. Theoretical calculations support the weak interaction of the pyridine N donor in 1A with the Cu center.

Full text of DOI:10.1021/ic800298a

Inorg. Chem. 2008, 47, 4442-4444
T-Shaped Cationic CuI Complexes with Hemilabile PNP-Type Ligands
Jarl Ivar van der Vlugt,*,† Evgeny A. Pidko,† Dieter Vogt,† Martin Lutz,‡ Anthony L. Spek,‡
and Auke Meetsma§
Schuit Institute of Catalysis, Laboratory of Homogeneous Catalysis, Department of Chemistry and
Chemical Engineering, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen,
The Netherlands, Department of Crystal and Structural Chemistry, Utrecht UniVersity,
Padualaan 8, 3584 CH Utrecht, The Netherlands, and Department of Chemical Physics, Zernike
Institute for AdVanced Materials, UniVersity of Groningen, Nijenborgh 4,
9747 AG Groningen, The Netherlands
Received February 15, 2008
Scheme 1. Synthesis of Complexes 2-4 with Ligands 1A/1B as Well
as Reactivity of 3B toward Additional Donor Ligands
The versatile coordination behavior of the PNP ligands 1A (2,6-
bis[(di-tert-butylphosphino)methyl]pyridine) and 1B (2,6-bis[(diphe-
nylphosphino)methyl]pyridine) to CuI is described, whereby a
hemilabile interaction of the pyridine N-donor atom to the copper
center resulted in a rare T-shaped complex with 1A, while with
1B also a tetracoordinated species could be isolated. Theoretical
calculations support the weak interaction of the pyridine N donor
in 1A with the Cu center.
The formation of so-called T-shaped (late) transition metal
complexes has been reported for a growing number of ligand
systems and metal centers, and these inherently electron-
deficient complexes often display interesting reactivity.1
Pincer ligands based on 2,6-bis(methylene-E)benzene
(monoanionic upon coordination) or -pyridine (neutral),
denoted as ECE and ENE, wherein E is a (hetero)donor atom,
have become a common structural motif for the synthesis
of a variety of transition metal complexes. It is therefore
surprising that complexes with a strict trigonal T-shaped
arrangement, wherein no additional ligand is present to stabilize
the metal center, are very rare and, to the best of our knowledge,
have not been reported for E ) phosphorus at all.
therefore set out to explore the coordination of the neutral
ligands PNPtBu (1A)6 and PNPPh (1B)2a with CuI, anticipating
the formation of T-shaped complexes.
Reaction of an equimolar amount of 1 with CuBr(SMe2)
in diethyl ether yielded a bright (A) or intense yellow (B)
solid after workup (Scheme 1). Complex 2A displayed a
singlet at δ 46.2 (acetone-d6) in the 31P NMR spectrum (δ
-3.0 for 2B). Notably, the IR spectrum indicated that the
(2) For the PPh2-based PNP-ligand, see: (a) Dahlhoff, W. V.; Nelson,
S. M. J. Chem. Soc. A 1971, 218, 4–2190. (b) Sacco, A.; Vasapollo,
G.; Nobile, C. F.; Piergiovanni, A.; Pellinghelli, M. A.; Lanfranchi,
M. J. Organomet. Chem. 1988, 356, 397–409. (c) Hahn, C.; Cucciolito,
M.; Vitagliano, A. J. Am. Chem. Soc. 2002, 124, 9038–9039. (d)
Cochran, B. M.; Michael, F. E. J. Am. Chem. Soc. 2008, 130, 2786–
2792.
Recently, the 2,6-bis(phosphinomethyl)pyridine skeleton
has attracted attention within the family of neutral pincer
ligands.2,3 However, to date, no single report on the chemistry
of such neutral PNP ligands with copper(I) exists.4,5 We
(3) For PR2-based PNP-ligands, see: (a) Trovitch, R. J.; Lobkovsky, E.;
Chirik, P. J. Inorg. Chem. 2006, 45, 7252–7260. (b) Kloek, S. M.;
Heinekey, D. M.; Goldberg, K. I. Angew. Chem., Int. Ed. 2007, 46,
4736–4738. (c) Feller, M.; Ben-Ari, E.; Gupta, T.; Shimon, L. J. W.;
Leitus, G.; Diskin-Posner, Y.; Weiner, L.; Milstein, D. Inorg. Chem.
2007, 46, 10479–10490.
* Author to whom correspondence should be addressed. E-mail:
j.i.v.d.vlugt@tue.nl. Tel.: +31 40 2475028. Fax: +31 40 2455054.
†
Eindhoven University of Technology.
Utrecht University.
University of Groningen.
(4) One previous report mentions the intended formation of a CuI complex
with a PNP ligand, but phosphine and metal oxidation occurred and
a CuII-oxide complex was obtained: Lang, H.-F.; Fanwick, P. E.;
Walton, R. A. Inorg. Chim. Acta 2002, 329, 9–12.
‡
§
(1) (a) Ingleson, M.; Fan, H.; Pink, M.; Tomaszewski, J.; Caulton, K. G.
J. Am. Chem. Soc. 2006, 128, 1804–1805. (b) Eckert, N. A.; Dinescu,
A.; Cundari, T. R.; Holland, P. L. Inorg. Chem. 2005, 44, 7702–7704.
(c) Braunschweig, H.; Radacki, K.; Rais, D.; Scheschkewitz, D. Angew.
Chem., Int. Ed. 2005, 44, 5351–5354. (d) Yamashita, M.; Hartwig,
J. F. J. Am. Chem. Soc. 2004, 126, 5344–5345. (e) Stambuli, J. P.;
Incarvito, C. D.; Bühl, M.; Hartwig, J. F. J. Am. Chem. Soc. 2004,
126, 1184–1194.
(5) Tetrahedral coordination of CuI to
a supermesityl substituted
bis(phosphaethenyl)pyridine ligand was recently reported, see: Hayashi,
A.; Okazaki, M.; Ozawa, F.; Tanaka, R. Organometallics 2007, 26,
5246–5249.
(6) (a) Kawatsura, M.; Hartwig, J. F. Organometallics 2001, 20, 1960–
1964. (b) Hermann, D.; Gandelman, M.; Rozenberg, H.; Shimon,
L. J. W.; Milstein, D. Organometallics 2002, 21, 812–818.
4442 Inorganic Chemistry, Vol. 47, No. 11, 2008
10.1021/ic800298a CCC: $40.75  2008 American Chemical Society
Published on Web 04/29/2008

COMMUNICATION
Figure 2. Displacement ellipsoid plots (50% probability level) of the
cationic part of complex 3A, [Cu(1A)]SbF6. The noncoordinated SbF6
counterion and hydrogen atoms are omitted for clarity. Selected atom
distances (Å) and angles (deg): Cu1-P1, 2.2211(8); Cu1-P2, 2.2215(8);
Cu1-N1, 2.091(2); P1-C6, 1.858(3); P2-C15, 1.865(3); Cu1 · · ·Sb1, 5.4520(5);
Cu1 · · · F3, 4.400(3); P1-Cu1-P2, 172.44(3); N1-Cu1-P1, 87.22(7);
N1-Cu1-P2, 87.49(7); Cu1-P1-C6, 98.47(10); Cu1-P2-C15, 98.69(10);
Cu1 · · ·N1 · · ·C3, 161.13(15).
Figure 1. Displacement ellipsoid plots (50% probability level) of complex
2A, Cu(1A)Br. Hydrogen atoms are omitted for clarity. Selected distances
(Å) and angles (deg): Cu-P1, 2.3150(5); Cu-P2, 2.3104(5); Cu · · ·N,
2.8938(17); Cu-Br, 2.4376(3); P1-Cu-P2, 140.75(2); P1-Cu-Br,
106.555(16); P2-Cu-Br, 112.658(16); P1-Cu· · ·N, 70.43(5); P2-Cu· · ·N,
70.37(5); Br-Cu· · ·N, 174.03(4); Cu-P1-C6, 114.56(7); Cu-P2-C15,
115.42(7); Cu · · ·N· · ·C3, 133.34(8).
additional solvent molecule. Recrystallization of complex 3A
from THF/pentane furnished suitable single crystals that were
analyzed by X-ray diffraction (Figure 2). Complex 3A was
pyridine nitrogen showed variable coordination behavior
toward CuI, depending on the ligand environment. For 2A,
bands were observed at ν 1584 (shoulder) and 1576 cm-1
that are assigned to an uncoordinated pyridine fragment,7
similar as seen for ligand 1, while for 2B, the well-separated
bands at ν 1587 and 1567 cm-1 are indicative of a true Cu-N
bonding interaction (see the Supporting Information for the
molecular structure of 2B, with an intramolecular Cu-N
distance of 2.160(3) Å, confirming this statement).
Recrystallization from Et2O at -20 °C yielded yellow block-
shaped single crystals suitable for X-ray diffraction (Figure 1)
for complex 2A, which crystallized in the monoclinic space
group P21/c, and with Cu-P atom distances of 2.3104(5) and
2.3150(5) Å. The P1-Cu-P2 angle was found to be 140.75(2)°,
leading to a distorted equilateral triangular geometry for the
copper ion. The nitrogen atom of the pyridine ring is well
outside the bonding distance at ∼2.89 Å. Furthermore, the
pyridine ring is tilted out of the P-Cu-P plane, as deduced
from the angle Cu · · ·N1 · · ·C3 of 133.34(8)°.
j
shown to crystallize in the triclinic space group P1.
Coordination of the pyridine N atom is evident from the
Cu-N bond length of 2.091(2) Å, which results in a large
P1-Cu-P2 angle of 172.44(3)° and N-Cu-P angles of
87.22(7) and 87.49(2)°. The closest Cu contact to the SbF6
counterion is with fluorine-atom F3, but the distance of 4.400(3)
Å is clearly outside of the bonding range. The CuI environment
in complex 3A has approximate local C2V symmetry with a
T-shaped geometry around copper, somewhat related to the CuI
complex reported by Halcrow et al. featuring a bis(pyra-
zolyl)pyridine ligand.9 To the best of our knowledge, this kind
of trigonal, T-shaped arrangement is unprecedented for copper(I)
complexes with a P-N-P donor set, judging from a search of
the Cambridge Crystal Structure Database.10
From the side view of complex 3A, shown in Figure 2, the
nonplanar arrangement of the pyridine ring with the CuI center
is evident, which leads to an out-of-plane distortion of ∼19°.
Arguably, this conformation may lead to unusual reactivity, due
to weaker (i.e., hemilabile) binding of the pyridine N atom with
the Cu ion, as the π orbital of the nitrogen donor does not
overlap perfectly with the dxz orbital of the copper center. Niecke
et al. reported a diphosphine macrocycle incorporating two
pyridine rings that showed a distorted tetrahedral or “seesaw”
geometry upon coordination to CuI.11 Ligand 1A appears to be
unable to enforce a similar stable tetracoordinate geometry; the
addition of tBuNC to a solution of 3A in THF did not induce
stable coordination of the neutral coligand in the solid state.11
This most likely relates to the steric hindrance induced by the
phosphine groups and the resulting unfavorable crystal packing,
We wondered whether the pyridine ring might be included
in the coordination environment of the CuI ion in the absence
of the strongly σ-donating halide coligand. This would open
up a new coordination mode if additional exogenous ligation
could be repressed. The reaction of AgSbF6 with complex 2
in THF yielded complex 3 as a bright green (3A) or light-
yellow solid (3B) after solvent removal. Surprisingly, these
compounds are not very susceptible to decomposition by air.8
The 31P NMR spectrum showed signals at δ 42.6 (3A) and
δ 0.5 (3B) in acetone-d6, while IR spectroscopy revealed
that the pyridine ring is coordinated to the CuI ion, with bands
at 1591 and 1566 cm-1 (3A) or 1596 and 1567 cm-1 (3B).
1
The H NMR spectrum did not indicate ligation of any
(9) Foster, C. L.; Kilner, C. A.; Thornton-Pett, M.; Halcrow, M. A.
Polyhedron 2002, 21, 1031–1041.
(7) (a) Hahn, C.; Vitagliano, A.; Giordano, F.; Taube, R. Organometallics
1998, 17, 2060–2066. (b) Mu¨ller, G.; Klinga, M.; Leskela¨, M.; Rieger,
B. Z. Anorg. Allg. Chem. 2002, 628, 2839–2846, and references
therein.
(10) Cambridge Crystallographic Database, February 2008.
(11) Ekici, S.; Nieger, M.; Glaum, R.; Niecke, E. Angew. Chem., Int. Ed.
2003, 42, 435–438.
(8) Solid samples of complexes 2 and 3 appear to be stable in air. A full
investigation of the electrochemical properties of these and related
compounds will be reported elsewhere.
(12) Coordination of tBuNC was indicated by NMR and IR spectroscopy,
but recrystallization of the green solid led to reorganization, as
indicated by NMR spectroscopy (see the Supporting Information).
Inorganic Chemistry, Vol. 47, No. 11, 2008 4443

COMMUNICATION
Figure 4. Displacement ellipsoid plots (50% probability level) of the
cationic part of complex 5. The noncoordinated SbF6 counterion and
hydrogen atoms are omitted for clarity. Selected atom distances (Å) and
angles (deg): Cu-P1, 2.2765(8); Cu-P2, 2.2792(7); Cu-P3, 2.2391(9),
Cu-N, 2.157(2); P1-C25, 1.853(3); P2-C31, 1.853(3); P1-Cu-P2, 133.64(3);
P1-Cu-P3, 110.46(3); P2-Cu-P3, 115.84(3); N-Cu-P1, 80.99(6);
N-Cu-P2, 82.63(6); N-Cu-P3, 114.13(6), Cu-P1-C25, 97.35(8);
Cu-P2-C31, 95.44(8); Cu-N· · ·C28, 166.30(12).
Figure 3. Frontier orbital plots for the (a) HOMO and (b) LUMO of
complex 2A and the (c) HOMO and (d) LUMO for complex 3A, based on
DFT calculations.
as indicated also by the results of DFT B3LYP/6-31G(d)
calculations.13
The observed out-of-plane tilting of the pyridine ring is
reproduced in the calculations. There is significant σ-bonding
character of the Cu-Br bond in 2A, and the Cu-N σ
interaction is nonbonding. The highest occupied molecular
orbital (HOMO) for both complex 2A and 3A (Figure 3)
shows strongly bonding Cu-P interactions, whereas anti-
bonding character for the Cu-N interaction is observed for
the cationic fragment in 3A. The LUMO for 3A indicates a
strongly bonding interaction between Cu and N. This
indicates the hemilabile character of the Cu-N bond.
With strongly donating anionic ligands, the above T-
shaped geometry is not retained. Thus, the addition of 1 equiv
of KSCN to a light-green solution of 3A in THF resulted in
the rapid formation of a deep-green solution. Characterization
of the corresponding green solid showed that the hemilabile
Cu-NPy bond is disrupted, indicated by the diagnostic IR band
for the free pyridine fragment (ν 1574 cm-1) and a bound
thiocyanate ligand (ν 2067 cm-1) in the solid state. The product
was formulated as Cu(κ2-P,P-1)(NCS), complex 4.14
Figure 5. Structural correlation of T-shaped complex 3A and distorted
trigonal-pyramidal species 5 with relevant literature precedents.5,9
pyramidal CuI center. The assumed T-shaped geometry
present in 3B clearly shows flexible behavior, but a true
“seesaw” arrangement was not observed. Figure 5 sum-
marizes some notable structural comparisons of complexes
3A and 5 with CuI species based on bis(carbene)pyridine5
and bis(phosphaalkenyl)pyridine.9
In summary, ligand class 1 exhibits hemilabile coordina-
tion of the pyridine nitrogen donor toward CuI. The selective
formation of T-shaped complexes 3 is demonstrated. The
addition of thiocyanate disrupts the Cu-NPy bond and
regenerates a trigonal planar geometry for the CuI ion. The
coordination of neutral donor ligands is governed by steric
factors; only in the case of 3B was stable coordination of
PMe3 and tBuNC observed, also in the solid state. The built-
in hemilabile coordinationsswitch of the PNPsligand will
prove useful for catalytic applications of these CuI complexes,
and initial screening studies are ongoing.
As we were unable to crystallographically characterize a
tetracoordinated CuI complex supported by ligand 1A, we
turned our attention to the diphenylphosphino analogue 1B.
Instantaneous fading of the yellow color to leave a colorless
solution was evident upon the addition of 1 equiv of PMe3
(5) or tBuNC (6) to a solution of 3B in THF (Scheme 1), as
Acknowledgment. This research has been made possible
by an Innovation grant (VENI) from the Dutch Research
CouncilsChemical Sciences (NWO-CW) to J.I.v.d.V. and
support from Eindhoven University of Technology.
1
deduced from the respective H NMR spectra. Complex 6
showed an IR band at ν 2177 cm-1. Furthermore, the
coordination around the CuI center in complex 5 (Figure 4)
was elucidated by an X-ray crystal structure determination.
Complex 5 crystallized in the monoclinic space group P21/n
with four formula units per unit cell.
The N-Cu-P1 and P1-Cu-P2 angles of 80.99(6)° and
133.64(3)°, respectively, and the observed dihedral angle
P1-P2-Cu-P3 of 176.69(6)° give rise to a distorted trigonal-
Supporting Information Available: Experimental details for
complexes 2-6, molecular structures of complexes 2B and 4,
computational calculations (including full citation of ref 13),
crystallographic details and CIF files for compounds 2A, 2B, 3A,
4, and 5. This material is available free of charge via the Internet
at http://pubs.acs.org.
IC800298A
(14) We confirmed this by an independent synthesis of 4 starting from 1A
and commercially available Cu(SCN). This complex shows distorted
trigonal planar geometry around the CuI, analogous to complex 2. See
Figure S2 in the Supporting Information.
(13) Calculations were performed at B3LYP/6-31G(d) level: Frisch, M. J.;
et al. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh PA, 2003
(see Supporting Information for full details).
4444 Inorganic Chemistry, Vol. 47, No. 11, 2008