Covalent palladium-zinc bonds and their reactivity

Article abstract of DOI:10.1039/b709364k

Photolysis of (^FPNP)Pd-Et in the presence of Et₂Zn leads to the formation of (^FPNP)Pd-Zn-Pd(PNP^F), the first example of a compound with a covalent Pd-Zn bond. The Royal Society of Chemistry.

Full text of DOI:10.1039/b709364k

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Covalent palladium–zinc bonds and their reactivity{
Claudia M. Fafard, Chun-Hsing Chen, Bruce M. Foxman and Oleg V. Ozerov*
Received (in Berkeley, CA, USA) 20th June 2007, Accepted 24th July 2007
First published as an Advance Article on the web 22nd August 2007
DOI: 10.1039/b709364k
Photolysis of (FPNP)Pd–Et in the presence of Et2Zn leads to
the formation of (FPNP)Pd–Zn–Pd(PNPF), the first example of
a compound with a covalent Pd–Zn bond.
Chemical bonds are the fabric of chemistry. Bonds between many
combinations of elements in the periodic table have been described,
yet a number of possible element–element bonds still await
discovery. Covalent bonds between various transition metals and
zinc have been previously reported.1–4 Several examples involving
bonds between early transition metals (groups 4 and 5) and zinc
are known.1 The majority of the rest contain bonds between Zn
and a group 6–9 metal fragments that carry (with some
exceptions)4 multiple CO ligands,2 including Zn[Co(CO)4]2,2a the
first such reported compound. Compounds with Zn–Zn bonds
have also been reported recently.3 A distinctly different group of
complexes with M–Zn interactions is comprised of adducts of
divalent zinc compounds (ZnX2) as Lewis acids with transition-
metal Lewis bases.5 To the best of our knowledge, no example of a
Pd–Zn bond has been reported. The lack of prior Pd–Zn examples
is particularly interesting given the notoriety of the Negishi
coupling of organozinc reagents with aryl halides catalyzed by Pd
complexes.6 In this Communication we report a well-defined
example of compound containing a covalent Pd–Zn bond that was
discovered in the course of our studies of the complexes of rigid
PNP pincer-type ligands.7,8
Scheme 1 Photolysis and other approaches to complex 5.
this case, 5 is only produced in ,50% yield as part of a mixture of
several compounds. Notably, 3 does not react with metallic Zn
dust, even at 65 uC in THF.
Complex 5 is an orange solid, thermally stable in C6D6 solution
under argon. Solutions of 5 give rise to NMR spectra with time-
averaged D2d symmetry (local C2v symmetry for each PNP
fragment) at 22 uC on the NMR timescale, similar to 3.9 A single
31P NMR resonance is observed at d 53.9 ppm and a single 19F
NMR resonance at d 2134.2 ppm, consistent with D2d symmetry.
The solid-state structure of 5 was determined in the course of an
X-ray study (Fig. 1).{ The Pd–Zn–Pd linkage is linear (as it lies on
the two-fold axis). The environment about Pd is approximately
square-planar with the Pd atom in plane with the P, N and P
atoms of the FPNP ligand. The two planes defined by the P, N and
P atoms are close to perpendicular (the P1–Pd1–Pd2–P2 dihedral
is 86.01(6)u). The structure does not show the distortions of the
PNP ligand observed in 4;9 the conformation of the FPNP ligand
and the environment about Pd in 5 is similar to the sterically
unencumbered (FPNP)PdMe (7, Fig. 2).11 Presumably, the
separation of the two PNP ligands in 5 is sufficiently large that
the steric repulsion between the two PNP ligands is not severe.
From the previously reported structure of (FPNP)PdMe (7)11
We have recently reported that UV irradiation of 1 or 2 leads to
the formation of dipalladium dimers 3 and 4 with a single metal–
metal bond (Scheme 1).9 3/4 display interesting reactivity, for
instance, splitting the NH3 molecule into a terminal hydride and a
terminal amide. Here we report that irradiation of 1 in the presence
of Et2Zn leads instead to a new and unusual compound (5) that
possesses two covalent Pd–Zn bonds.
Photolysis of 1 in the presence of .1 equiv. Et2Zn leads to
.85% conversion to 5 (NMR evidence). 5 can be isolated in 74%
yield (based on Pd). Photolysis of 1 with lower quantities of Et2Zn
leads to a mixture of 3 and 5. Irradiation ostensibly serves to cleave
the Pd–C and the Zn–C bonds in a homolytic fashion. UV
homolysis of Zn–C bonds in dialkylzinc compounds has been
documented,10 including in the context of making transition
metal–zinc bonds.2j The exact nature of the selectivity leading to
the high yield of 5 is not clear. Intermediate formation of 3 is not a
problem for the formation of (and perhaps a pathway to) 5 since,
in a separate experiment, 5 can be produced (88%, NMR evidence)
by photolysis of isolated 3 in the presence of 1 equiv. of Et2Zn. 5
can also be produced by reacting (PNP)PdH (6) with Et2Zn. In
˚
and the 0.77 A covalent radius of CH3, it is possible to estimate the
F
covalent radius of ( PNP)Pd at 1.29 A. Similarly, from the
˚
determination of the structure of Me2Zn,12 the covalent radius of
Department of Chemistry, MS 015, Brandeis University, 415 South
Street, Waltham, Massachusetts, USA. E-mail: ozerov@brandeis.edu
{ Electronic supplementary information (ESI) available: Experimental
details and characterization. See DOI: 10.1039/b709364k
˚
1.16 A for divalent zinc in a linear environment can be extracted.
˚
This predicts a covalent Zn–Pd bond length of 2.45 A. The
˚
observed Pd–Zn distances in 5 (2.3793(10) and 2.3723(10) A) are
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4465–4467 | 4465

View Article Online
Fig. 1 ORTEP drawing§ (50% thermal ellipsoids) of 5 with selected
atom labeling. H, F atoms as well as Me groups are omitted for clarity.
Primed atoms are related to unprimed by a crystallographic C2 axis
passing through atoms N2–Pd2–Zn1–Pd1–N1.
Scheme 3 Reactions of 5.
NH3 or H2 at 95 uC in C6D6. 5 displays rather unexpected
reactivity towards MeI. Allowing 5 to react with 2 equiv. of MeI at
22 uC initially leads to ca. 90% formation of (FPNP)PdI (11) and
Me2Zn. When this mixture is allowed to stand for 5 d, 11 is slowly
methylated to increase the content of (FPNP)PdMe (7) to ca. 60%.
11/Me2Zn are thus kinetically preferred products. It is perhaps
surprising that the less electronegative carbon partner in Me–I is
selectively coupled with the less electronegative partner (Zn) in the
Pd–Zn bond in 5. In contrast, (CO)5Mn–Zn–Mn(CO)5 was
reported to react with MeI to give only Me–Mn(CO)5.2d,e Addition
of MeI across the Pd–Zn bond may be formally regarded as an
example of binuclear oxidative addition.16 On the other hand, the
valence17 of Pd in 5, 7 or 11 and of Zn in 5, Me2Zn or ZnI2
remains 2 and the reaction can also be formally viewed as a
metathesis of single, covalent bonds.
Fig. 2 Metric parameters from the structure of 5 and of (FPNP)PdMe
˚
(7). Esds lie in the range of 0.001–0.005 A and 0.01–0.06u.
˚
slightly (ca. 0.07 A) shorter than this calculated value, consistent
with a covalent single Pd–Zn bond. The Pd–Zn distance in 5 is also
considerably shorter than the Pd–Zn distance in the ZnPd alloy
13
˚
(2.65 A).
In the initial stages of the photolysis of a mixture of 1 and
Et2Zn, we have observed an intermediate (19F NMR (C6D6):
d 2133.8; 31P{1H} NMR (C6D6): d 61.7 ppm) that ostensibly
evolves into 5. We tentatively assign this compound as (FPNP)Pd–
Zn–Et (8, Scheme 2). The same compound can be obtained in situ
upon allowing 5 to react with Et2Zn. The fraction of 8 increases
with the increasing concentration of Et2Zn, presumably owing to
the establishment of the Schlenk-type equilibrium14 shown in
Scheme 2. Removal of volatiles (which includes Et2Zn) leaves only
5, thus precluding isolation of 8 in a pure form. A similarly
behaving pair of compounds [CpMo(CO)3]2Zn and CpMo(CO)3-
ZnEt in equilibrium with Et2Zn was reported by Thorn in 1991.2j
Since 5 possesses two Pd–Zn bonds, one might formally view
this compound as a potential four-electron reductant and expect its
reactivity be governed by the drive for the formation of ZnX92 and
(PNP)PdX02 products (Scheme 3). Indeed, 5 reacts with .4 equiv.
of Ph3CCl to cleanly produce (FPNP)PdCl (9) (as well as
Gomberg’s dimer15 and presumably ZnCl2). Exposure of a C6D6
solution of 5 to room air at ambient temperature leads primarily to
the formation of (FPNP)PdOH (10) after 24 h. Thermolysis of 5
with 35 equiv. of H2O in THF (6 h, 95 uC) leads to the clean
formation of (FPNP)PdH (6). Unlike 3/4,9 5 does not react with
In summary, we report the first example of a compound
containing a covalent Pd–Zn bond. Structural studies are
consistent with the presence of a single covalent Pd–Zn bond.
Reactions of this compound (5) generally lead to square planar
(FPNP)PdX complexes.
Acknowledgment is made to Brandeis University, the US
National Science Foundation (grant CHE-0517798), the donors of
the Petroleum Research Fund (administered by the American
Chemical Society), the Sloan Foundation (Research Fellowship to
O. V. O.), and the Dreyfus Foundation (Camille Dreyfus Teacher
Scholar Award to O. V. O.) for support of this research.
Notes and references
{ Crystal data for 5,18–20 C48H68F4N2P4Pd2Zn1, M = 1151.15, monoclinic,
˚
space group: C2/c; a = 22.7195(17), b = 16.3813(9), c = 18.0772(10) A, b =
127.920(6)u; U = 5307.4(7) A ; Z = 4; Dc = 1.441 g cm23, T = 294 K,
3
˚
m(Mo-Ka) = 1.286 mm21, prismatic habit, orange, 0.14 6 0.14 6 0.24 mm.
Data collected on an Enraf-Nonius CAD-4 Turbo diffractometer, R =
0.0395; Rw = 0.0340 [3161 data, I . 1.96s(I), 278 parameters], Rw = 0.0666
(structure factor calculation for all 5385 unique data). CCDC 651085. For
crystallographic data in CIF or other electronic format see DOI: 10.1039/
˚
b709364k. Selected distances (A) and angles (u): Pd1–P1 2.2684(13), Pd1–
Zn1 2.3793(10), Pd1–N1 2.149(5), Pd2–P2 2.2545(12), Pd2–N2 2.133(5),
Pd2–Zn1 2.3723(10); P1–Pd1–P19 164.48(6), P2–Pd1–P29 167.19(6).
§ Created using Persistence of Vision Ray Tracer (POV-Ray, http://
www.povray.org/) and Ortep-3 for Windows.21
1 (a) P. H. M. Budzelaar, J. Boersma, G. J. M. van der Kerk, A. L.
Spek and A. J. M. Duisenberg, Inorg. Chem., 1982, 21, 3777; (b) P. H.
M. Budzelaar, K. H. den Haan, G. J. M. van der Kerk and A. L. Spek,
Organometallics, 1984, 3, 156; (c) P. H. M. Budzelaar, A. A. H. van
der Zeijden, J. Boersma, G. J. M. van der Kerk, A. L. Spek and
A. J. M. Duisenberg, Organometallics, 1984, 3, 159; (d) Y. Wang,
Scheme 2 Schlenk-type equilibrium for the Pd–Zn compounds 5 and 8.
4466 | Chem. Commun., 2007, 4465–4467
This journal is ß The Royal Society of Chemistry 2007

View Article Online
B. Quillian, C. S. Wannere, P. Wei, P. v. R. Schleyer and G.
H. Robinson, Organometallics, 2007, 26, 3054.
H. A. Mayer and W. C. Kaska, J. Organomet. Chem., 2003, 682, 149;
(c) L. Fan, B. M. Foxman and O. V. Ozerov, Organometallics, 2004, 23,
326.
2 (a) W. Hieber and U. Teller, Z. Anorg. Allg. Chem., 1942, 249, 43; (b)
J. M. Burlitch, J. Am. Chem. Soc., 1969, 91, 4562; (c) J. M. Burlitch and
A. Ferrari, Inorg. Chem., 1970, 9, 563; (d) A. T. T. Hsieh and M. J. Mays,
J. Chem. Soc. A, 1971, 729; (e) A. T. T. Hsieh and M. J. Mays, J. Chem.
Soc. A, 1971, 2648; (f) J. S. Denis, W. Butler, M. D. Glick and
J. P. Oliver, J. Am. Chem. Soc., 1974, 96, 5427; (g) D. E. Crotty and
J. P. Oliver, Inorg. Chem., 1977, 16, 2501; (h) J. M. Burlitch, S. E. Hayes
and G. E. Whitwell, Organometallics, 1982, 1, 1074; (i) S. Alvarez,
M. Ferrer, R. Reina, O. Rossell and M. Seco, J. Organomet. Chem.,
1989, 377, 291; (j) D. L. Thorn, J. Organomet. Chem., 1991, 405, 161; (k)
C. A. Wright and J. R. Shapley, Inorg. Chem., 2001, 40, 6338.
3 (a) I. Resa, E. Carmona, E. Gutierrez-Puebla and A. Monge, Science,
2004, 305, 1136; (b) Y. Wang, B. Quillian, P. Wei, H. Wang, X.-J. Yang,
Y. Xie, R. B. King, P. V. R. Schleyer, H. F. Schaefer, III and
G. H. Robinson, J. Am. Chem. Soc., 2005, 127, 11944; (c) Z. Zhu,
R. J. Wright, M. M. Olmstead, E. Rivard, M. Brynda and P. P. Power,
Angew. Chem., Int. Ed., 2006, 45, 5807; (d) A. Grirrane, I. Resa,
A. Rodriguez, E. Carmona, E. Alvarez, E. Gutierrez-Puebla, A. Monge,
A. Galindo, D. del Rio and R. A. Andersen, J. Am. Chem. Soc., 2007,
129, 693; (e) X.-J. Yang, J. Yu, Y. Liu, Y. Xie, H. F. Schaefer, Y. Liang
and B. Wu, Chem. Commun., 2007, 2363.
8 (a) L.-C. Liang, Coord. Chem. Rev., 2006, 250, 1152; (b) O. V. Ozerov, in
The Chemistry of Pincer Compounds, ed. D. Morales-Morales and
C. Jensen, Elsevier, Amsterdam, 2007, pp. 287–309; (c) D. J. Mindiola,
Acc. Chem. Res., 2006, 39, 813.
9 C. M. Fafard, D. Adhikari, B. M. Foxman, D. J. Mindiola and
O. V. Ozerov, J. Am. Chem. Soc., DOI: 10.1021/ja0731571.
10 (a) P. F. Seidler, J. Phys. Chem., 1994, 98, 2095; (b) G. A. Razuvaev,
R. F. Galiulina and G. G. Petukhov, Zh. Obshch. Khim., 1968, 38, 3.
11 L. Fan, L. Yang, C. Guo, B. M. Foxman and O. V. Ozerov,
Organometallics, 2004, 23, 4778.
12 A. Almenningen, T. U. Helgaker, A. Haaland and S. Samdal, Acta
Chem. Scand., Ser. A, 1982, 36, 159.
13 J. A. Rodriguez and M. Kuhn, J. Phys. Chem., 1996, 100, 381.
14 A. J. Blake, J. Shannon, J. C. Stephens and S. Woodward, Chem. Eur.
J., 2007, 13, 2462.
15 (a) M. Gomberg, J. Am. Chem. Soc., 1900, 22, 757; (b) H. Lankamp,
W. T. Nauta and C. MacLean, Tetrahedron Lett., 1968, 2, 249; (c)
J. M. McBride, Tetrahedron, 1974, 30, 2009.
16 R. H. Crabtree, The Organometallic Chemistry of the Transition Metals,
Wiley-Interscience, New York, 4th edn, 2005, pp. 160–161.
17 For a cogent and relevant discussion concerning the valence and
oxidation state of Zn in Cp*Zn–ZnCp*,3 see: G. Parkin, Science, 2004,
305, 1117.
18 L. H. Straver, CAD4-EXPRESS, Enraf-Nonius, Delft, The
Netherlands, 1992.
19 A. Altomare, G. Cascarano, G. Giacovazzo, A. Guagliardi, M. C. Burla,
G. Polidori and M. Camalli, J. Appl. Crystallogr., 1994, 27, 435.
20 P. W. Betteridge, J. R. Carruthers, R. I. Cooper, K. Prout and
D. J. Watkin, J. Appl. Crystallogr., 2003, 36, 1487.
4 (a) M. Ohashi, K. Matsubara and H. Suzuki, Organometallics, 2007, 26,
2330; (b) S. Seebald, B. Mayer and U. Schubert, Inorg. Chem., 1995, 34,
5285; (c) P. H. M. Budzelaar, J. Boersma, G. J. M. van Der Kerk,
A. L. Spek and A. J. M. Duisenberg, J. Organomet. Chem., 1985, 287,
C13; (d) P. H. M. Budzelaar, J. Boersma, G. J. M. Van der Kerk,
A. L. Spek and A. J. M. Duisenberg, Organometallics, 1985, 4, 680.
5 (a) W. Chen, G. Liu, T. Nishioka and K. Matsumoto, Eur. J. Inorg.
Chem., 2003, 4234.
6 E. Negishi, Bull. Chem. Soc. Jpn., 2007, 80, 233.
7 (a) L.-C. Liang, J.-M. Lin and C.-H. Hung, Organometallics, 2003, 22,
3007; (b) A. M. Winter, K. Eichele, H.-G. Mack, S. Potuznik,
21 L. J. Farrugia, J. Appl. Crystallogr., 1997, 30, 565.
This journal is ß The Royal Society of Chemistry 2007
Chem. Commun., 2007, 4465–4467 | 4467