Searching for precursors to metal-metal bonded dipalladium species: A study of Pd24+ complexes

Article abstract of DOI:10.1021/ic050876c

Reactions of Pd₃(OAc)₆ with lithium salts of mononegative bidentate N,N-ligands, L, of various types, such as formamidinates, benzamidinates, triazinates, and guanidinates, were investigated in a search for ways to obtain Pd₂⁴⁺ compounds that could serve as precursors to paddlewheel complexes with a metal-metal bond and a Pd ₂⁵⁺ core. It was found that the reactions are complex and that either square planar mononuclear or dinuclear species may be formed depending on the reaction conditions or the method of isolation. For Pd ₂L₄ compounds, α and β isomers were sometimes obtainable. In the α isomer, all N,N-ligands serve as bridges, whereas in the β isomer, two ligands bridge the Pd₂⁴⁺ unit and each of the other two chelate to a metal atom. Electrochemistry shows that the paddlewheel compounds Pd₂(TolNC(H)NTol)₄, Pd ₂(PhNC(Ph)NPh)₄, and Pd₂(PhNC(Ph)NPh) ₃(OAc) and the orthometalated complex cis-Pd₂[η ²-C₆H₄NC(Ph)N(H)Ph]₂(μ-OAc) ₂ have reversible oxidation waves between 0.70 and 0.92 V vs Ag/AgCl, which makes them good candidates for chemical oxidation.

Full text of DOI:10.1021/ic050876c

Inorg. Chem. 2005, 44, 6129−6137
Searching for Precursors to Metal−Metal Bonded Dipalladium Species:
4+
A Study of Pd2 Complexes
John F. Berry, F. Albert Cotton,* Sergey A. Ibragimov, Carlos A. Murillo,* and Xiaoping Wang
Department of Chemistry and Laboratory for Molecular Structure and Bonding, P.O. Box 30012,
Texas A & M UniVersity, College Station, Texas 77842-3012
Received May 31, 2005
Reactions of Pd3(OAc)6 with lithium salts of mononegative bidentate N,N-ligands, L, of various types, such as
formamidinates, benzamidinates, triazinates, and guanidinates, were investigated in a search for ways to obtain
4
+
5+
Pd2 compounds that could serve as precursors to paddlewheel complexes with a metal
core. It was found that the reactions are complex and that either square planar mononuclear or dinuclear species
may be formed depending on the reaction conditions or the method of isolation. For Pd2L4 compounds, and
isomers were sometimes obtainable. In the isomer, all N,N-ligands serve as bridges, whereas in the isomer,
−metal bond and a Pd2
R
â
â
R
4+
two ligands bridge the Pd2 unit and each of the other two chelate to a metal atom. Electrochemistry shows that
the paddlewheel compounds Pd2(TolNC(H)NTol)4, Pd2(PhNC(Ph)NPh)4, and Pd2(PhNC(Ph)NPh)3(OAc) and the
2-C6H4NC(Ph)N(H)Ph]2(
orthometalated complex cis-Pd2[η µ-OAc)2 have reversible oxidation waves between 0.70
and 0.92 V vs Ag/AgCl, which makes them good candidates for chemical oxidation.
Scheme 1
Introduction
Palladium and its complexes are widely used in catalysis,1
medicine,2 and many other fields of research such as H2
storage3 and thin film deposition.4 The divalent state is the
most common, and the electron configuration produces
almost invariably square planar complexes.5
* To whom correspondence should be addressed. E-mail: cotton@tamu.edu
(F.A.C.); murillo@tamu.edu (C.A.M.).
(1) (a) Raminelli, C.; Prechtl, M. H.; Santos, L. S.; Eberlin, M. N.;
Comasseto, J. V. Organometallics 2004, 23, 3990. (b) Nishimura, T.;
Nishiguchi, Y.; Maeda, Y.; Uemura, S. J. Org. Chem. 2004, 69, 5342.
(c) Stahl, S. S. Angew. Chem. 2004, 43, 3400. (d) Mueller, J. A.;
Goller, C. P.; Sigman, M. S. J. Am. Chem. Soc. 2004, 126, 9724. (e)
Goj, L. A.; Cisneros, G. A.; Yang, W.; Widenhoefer, R. A. J.
Organomet. Chem. 2004, 689, 2845.
(2) See, for example: (a) Phillips, A. D.; Gonsalvi, L.; Romerosa, A.;
Vizza, F.; Peruzzini, M. Coord. Chem. ReV. 2004, 248, 955. (b)
Bincoletto, C.; Tersariol, L. S.; Oliveira, C. R.; Dreher, S.; Fausto, D.
M.; Soufen, M. A.; Nascimento, F. D.; Caires, C. F. Bioorg. Med.
Chem. 2005, 13, 3047. (c) Jackson, A.; Davis, J.; Pither, R. J.; Rodger,
A.; Hannon, M. J. Inorg. Chem. 2001, 40, 3964.
(3) (a) Kishore, S.; Nelson, J. A.; Adair, J. H.; Eklund, P. C. J. Alloys
Compd. 2005, 389, 234. (b) Sun, Y.; Tao, Z.; Chen, J.; Herricks, T.;
Xia, Y. J. Am. Chem. Soc. 2004, 126, 5940. (c) Souza, E. C.; Ticianelli,
E. A. J. Braz. Chem. Soc. 2003, 14, 544. (d) Higuchi, K.; Yamamoto,
K.; Kajioka, H.; Toiyama, K.; Honda, M.; Orimo, S.; Fujii, H. J. Alloys
Compd. 2002, 330, 526. (e) Fujii, H.; Orimo, S. J. Phys. B 2003, 328,
77. (f) Watari, N.; Ohnishi, S.; Ishii, Y. J. Phys. C 2000, 12, 6799.
(4) (a) Love, J. C.; Wolfe, D. B.; Haasch, R.; Chabinyc, M. L.; Paul, K.
E.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 2003, 125,
2597. (b) Yurakov, Yu. A.; Kashkarov, V. M.; Kushchev, S. B.;
Balashova, V. Yu. PoVerkhnost 2003, 6, 10.
This research group has been interested in transition-metal
compounds containing metal-metal bonds. Although an
enormous variety of dimetal compounds with metal-metal
bonds has been described in the literature,6 dipalladium
compounds are very scarce. A generalized molecular orbital
diagram for D4h compounds containing bonded pairs of metal
atoms is shown on the left of Scheme 1, where it can be
n+
seen that, for elements with d8 configurations of the M2
core, both bonding and antibonding orbitals are filled, leading
to an M-M bond order of zero. Therefore, it is necessary
(5) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999.
(6) Multiple Bonds Between Metal Atoms; Cotton, F. A., Murillo, C. A.,
Walton, R. A., Eds.; Springer Science and Business Media, Inc.: New
York 2005.
10.1021/ic050876c CCC: $30.25
Published on Web 07/29/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 17, 2005 6129

Berry et al.
Table 1. Interpalladium Separations in Some Pd2L4 Paddlewheel Compounds
compound
distance (Å)
ref
Pd2(DTolF)4
Pd2(DPhBz)4
Pd2(hpp)4
2.622(3)
2.576(1)
2.555(1)
2.715(3)
2.738(1)
2.559(3)
2.547(6)
2.563(1)
2.5626(7)
2.745(1)
8
9
7
13
14
15
15
15
16
17
-
Pd2(dtpa)4 (dtpa ) C6H5CH2CS2
)
-
Pd2(dta)4 (dta ) CH3CS2
)
cis-Pd2(mhp)4 [mhp ) (6-methyl-2-oxypyridine)]
trans-Pd2(mhp)4
Pd2(chp)4 [chp ) (6-chloro-2-oxypyridine)]
Pd2(DPhTA)4 (DPhTA ) 1,3-diphenyltriazene)
Pd2(bttz)4 [bttz ) tetrakis(µ-1,3-benzothiazole-2-thiolato-N,S)]
to look to higher oxidation states to find metal-metal bonds
in paddlewheel complexes.
palladium atoms in compounds with the general formula
Pd2L4 (where L is a bidentate-bridging, singly charged
+
anionic ligand). The strategy that we shall employ, which is
similar to that used in previous work,7-9 is schematically
shown in Scheme 1. Starting with Pd2L4 paddlewheel
compounds, one electron will be removed from an antibond-
ing molecular orbital by chemical oxidation. This should lead
(if oxidation occurs from a mainly metal-based molecular
orbital) to the formation of a net one-electron metal-metal
bond.12
Up to now, there has been only one dipalladium paddle-
wheel compound known with an unequivocal metal-metal
single bond.7 This Pd2(hpp)4Cl2 compound (where hpp is the
anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine)
was prepared almost a decade ago. Four hpp ligands are
6+
embracing the singly bonded Pd2 unit, and the molecule
possesses 4-fold symmetry. The two chlorine atoms are
located in axial positions on the 4-fold axis. This molecule
was obtained by oxidation of Pd2(hpp)4 with several oxidizing
agents. Two other binuclear dipalladium compounds with
A first stage in the present work is to identify new
precursors to metal-metal bonded dipalladium species,
thereby expanding the limited pool for which structures are
1
presumed bond orders of /2 between the palladium atoms
have also been described; they have Pd25+ cores.8,9 Oxidation
of Pd2(DTolF)4 (DTolF ) N,N′-di-p-tolylformamidinate) with
1.6 equiv of AgPF68 gave Pd2(DTolF)4PF6. The metal-metal
distance in the oxidized compound did not differ significantly
available (see Table 1).7-9,13-17 In this report, we describe
4+
optimized syntheses for seven Pd2
compounds and one
mononuclear compound. We have focused our attention on
the ligands shown in Scheme 2, where the abbreviations used
for the ligands are also defined. We note also that Bear and
co-workers9 have mentioned some species that they made
but were not able to characterize unambiguously while
preparing Pd2(DPhBz)4 and Pd2(µ-DPhBz)2(η2-DPhBz)2.
Therefore, we also re-examined more thoroughly the Pd2+/
DPhBz- system. Numerically, the compounds described here
4+
from that in the Pd2 precursor, which does not have a
Pd-Pd bond. Because oxidation is expected to give rise to
a bond within the Pd25+ unit, shortening of the metal-metal
distance might have been expected. However, there is also
an increase in electrostatic repulsion between the two
positively charged metal atoms which may explain the
invariance in bond distance, as has been observed before in
some dirhenium10 and ditechnetium11 compounds. Calcula-
tions done for the oxidized species suggest that the unpaired
electron resides mainly in a metal-based orbital, but an EPR
(7) Cotton, F. A.; Gu, J.; Murillo, C. A.; Timmons, D. J. J. Am. Chem.
Soc. 1998, 120, 13280.
(8) Cotton, F. A.; Matusz, M.; Poli, R.; Feng, X. J. Am. Chem. Soc. 1988,
110, 1144.
(9) Yao, C.-L.; He, L.-P.; Korp, J. D.; Bear, J. L. Inorg. Chem. 1988, 27,
4389.
(10) Cotton, F. A.; Dunbar, K. R.; Falvello, L. R.; Tomas, M.; Walton, R.
A. J. Am. Chem. Soc. 1983, 105, 4950.
+
spectrum of the Pd2(DTolF)4 species showed only one
isotropic peak, suggesting that perhaps the unpaired electron
is ligand-based rather than metal-based.
Bear and co-workers9 synthesized and structurally char-
acterized two Pd24+ isomers containing four DPhBz ligands
(DPhBz ) the anion of N,N′-diphenylbenzamidine). One was
shown to be the R isomer where all four ligands bridge the
dimetal unit to form a paddlewheel. In the â isomer, there
are two bridging and two chelating ligands. Electrochemical
oxidation of the R isomer, followed by an EPR spectrum of
the singly oxidized species, revealed anisotropy of the
g-value and hyperfine splitting of the signal consistent with
(11) (a) Nefedov, V. I.; Koz′min Inorg. Chim. Acta 1982, 64, L177. (b)
Cotton, F. A.; Gage, L. D. NouV. J. Chim. 1977, 1, 441. (c) See also:
Sattelberger, A. P. Chapter 7. In Multiple Bonds Between Metal Atoms;
Cotton, F. A., Murillo, C. A., Walton, R. A., Eds.; Springer Science
and Business Media, Inc.: New York, 2005.
(12) It should be noted that with N,N-ligands other than hpp a one-electron
oxidation is far more likely to occur than a two-electron oxidation.
For a discussion of the extraordinary ability of the guanidinate ligand
hpp to stabilize high-oxidation states in dimetal complexes, see, for
example: (a) Cotton, F. A.; Gruhn, N. E.; Gu, J.; Huang, P.;
Lichtenberger, D. L.; Murillo, C. A.; Van Dorn, L. O.; Wilkinson, C.
C. Science 2002, 298, 1971. (b) Cotton, F. A.; Daniels, L. M.; Murillo,
C. A.; Timmons, D. J.; Wilkinson, C. C. J. Am. Chem. Soc. 2002,
124, 9249. (c) Berry, J. F.; Cotton, F. A.; Huang, P.; Murillo, C. A.
J. Chem. Soc., Dalton Trans. 2003, 1218.
5
the nuclear spin of /2 of the 105Pd isotope, which suggest
that the unpaired electron is in the dimetal unit. It is important
to note, however, that no direct structural evidence for the
(13) Bonamico, M.; Dessy, G.; Fares, V. J. Chem. Soc., Dalton Trans.
1977, 2315.
(14) Piovesana, O.; Bellitto, C.; Flamini, A.; Zanazzi, P. F. Inorg. Chem.
1979, 18, 2258.
+
presence of an R-Pd2(DPhBz)4 species was provided.
As the above summary shows, information about com-
pounds containing palladium-palladium bonds is fragmen-
tary and somewhat contradictory. It is a goal of our research
group to explore bond formation (if any) between two
(15) Bancroft, D. P.; Cotton, F. A.; Falvello, L. R.; Schwotzer, W. Inorg.
Chem. 1986, 25, 1015.
(16) Corbett, M.; Hoskins, B. F.; McLeod, N. J.; O’Day, B. P. Aust. J.
Chem. 1975, 28, 2377.
(17) Kubiak, M. Acta Crystallogr., Sect. C 1985, 41, 1288.
6130 Inorganic Chemistry, Vol. 44, No. 17, 2005

4+
Pd2 Complexes as Precursors to Dipalladium Species
Scheme 2
synthesized according to published procedures. Palladium acetate
was either purchased from Aldrich or was synthesized from PdCl2.20
A new procedure used for the synthesis of Pd2(DPhTA)4 is given
below.21
Physical Measurements. The IR spectra were recorded on a
Perkin-Elmer 16PC FTIR spectrometer using KBr pellets. Cyclic
voltammograms were measured on a CH Instruments electrochemi-
cal analyzer using dichloromethane solutions with 1 M NBu4PF6
and 0.1 mM analyte; the electrodes were Pt disk (working), Pt wire
(auxiliary), and Ag/AgCl (reference). The redox couple for fer-
rocene/ferrocenium consistently appeared at +450 mV under these
conditions. Elemental analyses were carried out by Canadian
Microanalytical Services in British Columbia, Canada. Samples
were vacuum-dried prior to elemental analyses to remove the
interstitial solvent molecules of the crystals. 1H and 13C NMR
spectra were obtained on VXR-300 and VXR-500 NMR spectrom-
eters. Mass spectrometry data (electrospray ionization) were
recorded in the Laboratory for Biological Mass Spectrometry at
Texas A & M University, College Station, Texas, using an MDS
Series Qstar Pulsar with a spray voltage of 5 keV. Visible spectra
were obtained on either a Shimadzu UV-2501 PC spectrophotometer
or a Cary 17D spectrophotometer.
Synthesis of R-Pd2(DAniF)4, 1. To a solution of Pd3(OAc)6 (300
mg, 0.446 mmol, 1.34 mmol of Pd2+) in 20 mL of THF was added
LiDAniF dissolved in 20 mL of THF. The latter was prepared in
situ by deprotonation of HDAniF (686 mg, 2.68 mmol) with a 1.6
M solution of BunLi in n-hexane (1.68 mL, 2.7 mmol). The solution
was stirred overnight at room temperature. The solvent was then
removed under vacuum. The resulting orange solid was washed
several times with water and dried under vacuum with a yield of
0.759 g (92%). X-ray quality crystals were grown by dissolving
Pd2(DAniF)4 in 10 mL of CH2Cl2 and layering with 45 mL of
hexane. Anal. Calcd for C60H60N8O8Pd2: C, 58.44; H, 4.87; N,
1
9.09. Found: C, 58.35; H, 4.85; N, 9.11. H NMR (CDCl3, 300
MHz; δ, ppm): 3.767 (s, OCH3, 6H), 6.674-6.865 (m, aromatic,
8H), 7.035 (s, CH, 1H). IR (KBr, cm-1): 2948 (w), 2832 (w), 1618
(s), 1695 (m), 1499 (vs), 1459 (w), 1438 (w), 1345 (w), 1295 (w),
1243 (s), 1222 (s), 1179 (w), 1108 (w), 1035 (m), 968 (w), 828
(m), 766 (w). ESI+ mass spectrum (m/z, amu): 1232, M+. UV-
vis (CH2Cl2 solution; λ, nm; ꢀ, M-1cm-1): 494, 2 × 103.
are R-Pd2(DAniF)4 (1), â-Pd2(TPG)2(η2-TPG)2 (2), R-Pd2-
(DPhTA)4 (3), Pd(DPhBz)2 (4), â-Pd2(DPhBz)2(η2-DPhBz)2
(5), Pd2(DPhBz)3(OAc) (6), R-Pd2(DPhBz)4 (7), and cis-Pd2-
[η2-C6H4NC(PH)N(H)Ph]2(µ-OAc)2 (8). The structures of 3,16
5,9 and 79 have been described earlier, and the rest are given
here along with electrochemical data for all compounds to
help in the evaluation of the best candidates for chemical
oxidation, which will be carried out in a separate study.
Synthesis of â-Pd2(TPG)2(η2-TPG)2, 2. To a solution of Pd3-
(OAc)6 (300 mg, 0.446 mmol, 1.34 mmol of Pd2+) in 20 mL of
THF was added LiTPG dissolved in 20 mL of THF. The latter
was prepared in situ by deprotonation of HTPG (769 mg, 2.68
mmol) with a 1.6 M solution of BunLi in n-hexane (1.68 mL, 2.7
mmol). After the reaction mixture was stirred overnight at room
temperature, the solvent was removed under vacuum. The resulting
orange solid was washed several times with water and dried under
vacuum with a yield of 0.799 g (88%). X-ray quality crystals were
grown by dissolving Pd2(µ-TPG)2(η2-TPG)2 in 8 mL of CH2Cl2
and layering with 47 mL of hexane. Anal. Calcd for C76H66N12-
OPd2: C, 66.33; H, 4.83; N, 12.21. Found: C, 66.34; H, 4.87; N,
Experimental Section
General Comments. All manipulations were carried out under
an atmosphere of dry nitrogen gas using standard Schlenk tech-
niques, unless otherwise specified. Solvents were either distilled
over appropriate drying agents in a nitrogen atmosphere or purified
using a Glass Contour solvent system. The organic amines N,N′,N′′-
triphenylguanidine (HTPG) and N,N′-diphenyltriazine (HDPhTA)
were purchased from TCI and Acros, respectively, and used as
received. The formamidine18 N,N′-di-p-anisylformamidine (HDAniF)
and the benzamidine N,N-diphenylbenzamidine19 (HDPhBz) were
1
12.06. H NMR (C6D6, 300 MHz; δ, ppm): 5.461 (s, NH, 1H),
5.638 (d, aromatic, 2H), 5.887 (s, NH, 1H), 6.4-7.1 (m, aromatic,
28H). IR (KBr, cm-1): 3393 (w), 3054 (w), 1602 (m), 1592 (m),
1540 (vs), 1472 (s), 1443 (m), 1423 (s), 1396 (m), 1380 (s), 1307
(m), 1278 (w), 1263 (m), 1247 (w), 1206 (w), 1173 (w), 1074 (w),
1027 (w), 922 (w), 774 (w), 754 (m), 747 (m), 733 (w), 691 (m),
(18) Lin, C.; Protasiewicz, J. D.; Smith, E. T.; Ren, T. Inorg. Chem. 1996,
(20) Bakhmutov, V. I.; Berry, J. F.; Cotton, F. A.; Ibragimov, S.; Murillo,
C. A. J. Chem. Soc., Dalton Trans. 2005, 1989.
35, 6422.
(19) Kakimoto, M.; Ogata, S.-I.; Mochizuki, A.; Imai, Y. Chem. Lett. 1984,
(21) A synthesis for this compound was reported earlier. See: Dwyer, F.
P. J. Am. Chem. Soc. 1941, 63, 78.
821.
Inorganic Chemistry, Vol. 44, No. 17, 2005 6131

Berry et al.
518 (vw), 483 (vw). UV-vis (CHCl3 solution; λ, nm; ꢀ, M-1cm-1):
485, 2 × 103; 392, 3 × 103.
resulting in a dark red solution. The solvent was removed under
vacuum, and the dark red residue was washed with 40 mL of MeOH
and dried under vacuum with a yield of 0.752 g (75%). H NMR
1
Synthesis of R-Pd2(DPhTA)4, 3. To a solution of Pd3(OAc)6
(300 mg, 0.446 mmol, 1.34 mmol of Pd2+) in 20 mL of THF was
added LiDPhTA dissolved in 20 mL of THF. The latter was
prepared in situ by deprotonation of HDPhTA (528 mg, 2.68 mmol)
with a 1.6 M solution of BunLi in n-hexane (1.68 mL, 2.7 mmol).
The solution was stirred overnight at room temperature, and the
solvent was then removed under vacuum. The reddish-brown
residue was washed several times with methanol and had a yield
of 0.460 g (69%). 1H NMR (CDCl3, 300 MHz; δ, ppm): 7.051 (t,
1H), 7.154 (t, 2H), 7.572 (d, 2H). IR (KBr, cm-1): 1593 (m), 1483
(m), 1456 (w), 1400 (vs), 1330 (vw), 1292 (vw), 1267 (vw), 1226
(vw), 1211 (m), 1167 (w), 1076 (vw), 1024 (vw), 953 (vw), 902
(vw), 826 (vw), 759 (s), 697 (s), 662 (m), 522 (w), 502 (w). UV-
vis (CHCl3 solution; λ, nm; ꢀ, M-1cm-1): 574, 5 × 103.
(CDCl3, 300 MHz; δ, ppm): 1.946 (s, OCH3, 3H), 6.486-7.270
(m, aromatic, 45H). 13C NMR (CDCl3, 300 MHz; δ, ppm): 25.0,
122.8, 123.1, 127.4, 127.7, 127.589, 128.0, 128.4, 131.2, 131.9,
135.6, 135.7, 151.9, 171.8, 172.1, 184.5. IR (KBr, cm-1): 1655
(w), 1580 (vs), 1543 (vw), 1500 (s), 1483 (w), 1440 (m), 1408
(w), 1265 (m), 1215 (m), 1074 (vw), 1026 (vw), 792 (m), 695 (s),
513 (vw). ESI+ mass spectrum (m/z): 1086 (M+). UV-vis (λ,
nm; ꢀ, M-1cm-1): 550 (very broad).
Synthesis of R-Pd2(DPhBz)4, 7. A round-bottomed flask was
charged with Pd3(OAc)6 (100 mg, 0.45 mmol), HDPhBz (607 mg,
2.23 mmol), Et3N (1 mL), and 40 mL of THF. The mixture was
refluxed in air for 3.5 h. After the removal of THF under vacuum,
the residue was dissolved in CH2Cl2. The mixture was separated
by chromatography on a TLC plate. The first bright orange band
was collected, and a yield of 0.035 g (30%) was obtained. 1H NMR
(22 °C; CDCl3, 500 MHz; δ, ppm): 6.239 (br, 8H), 6.381 (br, 8H),
6.649-6.752 (m, 20H), 6.786 (br, 16H), 7.046 (br, 8H). 1H NMR
(-45 °C; CDCl3, 300 MHz; δ, ppm): 6.215 (br, 8H), 6.456 (br,
Synthesis of Pd(DPhBz)2, 4. To a solution of HDPhBz (729
mg, 2.67 mmol) in 20 mL of toluene was added a 1.6 M solution
of BunLi in n-hexane (1.68 mL, 2.7 mmol). The mixture was stirred
at room temperature for 10 min, and then, it was added to a solution
of Pd3(OAc)6 (300 mg, 0.446 mmol, 1.34 mmol of Pd2+) in 20 mL
of toluene. The mixture was stirred overnight at room temperature,
resulting in a dark red solution and a dark red precipitate. After
removal of the solvent under vacuum, the residue was washed first
with 2 × 25 mL of water and then with 2 × 10 mL of ether, leaving
a bright red product that was analytically pure as shown by NMR
in a yield of 0.648 g (74%). 1H NMR (CDCl3, 300 MHz; δ, ppm):
6.401 (m, 4H), 6.856 (m, 6H), 6.993 (d, 2H), 7.113 (t, 2H), 7.226
(t, 1H). 13C NMR (CDCl3; δ, ppm): 123.0, 125.1, 128.4, 128.4,
129.3, 129.9, 131.5, 144.4 (all aromatic). IR (KBr, cm-1): 3057
(vw), 1592 (m), 1581 (m), 1494 (vs), 1477 (s), 1433 (s), 1298 (w),
1274 (w), 1211 (w), 1170 (w), 1072 (w), 1025 (w), 958 (w), 794
(w), 777 (w), 706 (m), 693 (m), 499 (w). ESI+ mass spectrum
(m/z): 699 (M+). UV-vis (λ, nm; ꢀ, M-1cm-1): 378 nm, 2 ×
105; 496 nm, 2 × 103.
1
8H), 6.685-6.837 (m, 20H), 6.894 (br, 16H), 7.147 (br, 8H). H
NMR (+45 °C; CDCl3, 300 MHz; δ, ppm): 6.402 (br, 16H),
6.742-6.830 (m, 20H), 6.900 (br, 16H), 7.100 (br, 8H). ESI+ mass
spectrum (m/z): 1086 (M+). UV-vis (λ, nm; ꢀ, M-1cm-1 ): 500,
398, 350.
Synthesis of cis-Pd2[η2-C6H4NC(Ph)N(H)Ph]2(µ-OAc)2, 8. To
a solution of HDPhBz (729 mg, 2.68 mmol) in 25 mL of CH2Cl2
was added a 1.6 M solution of BunLi in n-hexane (1.67 mL, 2.68
mmol), and the mixture was allowed to reach room temperature.
This was added to a solution of Pd3(OAc)6 (300 mg, 0.450 mmol)
in 25 mL of CH2Cl2. The mixture was stirred overnight at room
temperature, resulting in a dark red solution. The solvent was
removed under vacuum, and the residue was extracted with one
40 mL portion of MeOH. The MeOH solution was transferred into
a Petri dish and allowed to evaporate in air. A mixture of pale
yellow and dark red crystals formed. The yellow crystals of 8 were
separated manually from the red ones of 6, with a yield of 0.058 g
Synthesis of â-Pd2(DPhBz)2(η2-DPhBz)2, 5. A solution contain-
ing 100 mg of Pd(DPhBz)2 in 50 mL of methanol was heated to
reflux for 30 min. The color of the solution darkened, and a small
amount of palladium metal was observed. Prolonged heating was
avoided to prevent further decomposition. The solvent was removed,
and the residue was extracted with 15 mL of CH2Cl2. To this extract
was added 15 mL of hexanes, and the mixture was separated on a
silica gel column using a 1:1 mixture of CH2Cl2 and hexanes. The
first bright orange band corresponded to â-Pd2(DPhBz)2(η2-
DPhBz)2. Removal of the solvent gave a bright orange powder in
1
(10%) of yellow crystals. H NMR (CDCl3, 300 MHz; δ, ppm):
1.463 (s), 6.442 (d), 6.600 (d), 6.739-7.223 (m), 7.858 (d). 13C
NMR (CDCl3, 300 MHz; δ, ppm): 23.5, 114.3, 120.9, 122.4, 124.8,
124.9, 127.1, 127.3, 127.6, 128.1, 128.2, 129.4, 134.1, 134.8, 136.8,
147.4, 153.9, 179.9. IR (KBr, cm-1): 2363 (w), 2344 (w), 1655
(m), 1625 (s), 1583 (vs), 1535 (s), 1509 (m), 1490 (m), 1462 (w),
1414 (w), 1346 (m), 1283 (s), 1265 (w), 1135 (w), 1028 (w), 833
1
a yield of 0.020 g (40%). H NMR (22 °C; CDCl3, 300 MHz; δ,
(w), 803 (w), 754 (m), 698 (m), 512 (vw). UV-vis (λ, nm; ꢀ, M-1
cm-1): 305, 6 × 103.
-
ppm): 5.915 (d), 6.138 (d), 6.338 (d), 6.558 (d), 6.629-7.036 (m),
6.729-7.036 (m), 7.141 (t). 1H NMR (-45 °C; CDCl3, 300 MHz;
δ, ppm): 5.712 (d), 5.951 (d), 6.048 (d), 6.195 (t), 6.355 (d), 6.474
(t), 6.614-6.949 (t), 7.024 (t), 7.166 (t), 7.522 (t), 7.695 (d), 8.081
(d). 13C NMR (CDCl3; δ, ppm): 122.1, 122.5, 122.6, 123.0, 125.3,
125.9, 126.9, 127.1, 127.3, 128.1, 128.2, 128.4, 129.3, 130.6, 131.2,
131.4, 132.2, 136.027, 146.1, 146.5, 150.1, 152.1, 152.7, 170.0,
174.6. IR (KBr, cm-1): 1655 (vs), 1638 (s), 1561 (s), 1542 (vs),
1508 (m), 1492 (m), 1432 (w), 1263 (w), 1210 (w), 1027 (w), 793
(w), 695 (m). ESI+ mass spectrum (m/z): 1298 (M+). UV-vis
(λ, nm; ꢀ, M-1cm-1): 486, 8 × 103.
X-ray Crystallography. Single crystals of each of the com-
pounds 1, 2‚5C6H6, 4, 6‚2/3 acetone, and 8‚2CH3OH were mounted
and centered on the goniometer of a Bruker SMART 1000 CCD
area detector diffractometer and cooled to -60 °C. Geometric and
intensity data were collected using SMART software.22 The data
were processed using SAINT software,23 and corrections for
absorption were applied using the program SADABS.24
All structures were solved using the Patterson method available
in the SHELX-97 software package.25 Crystal data are shown in
Table 2, and Table 3 lists pertinent bond distances and angles.
Synthesis of Pd2(DPhBz)3(OAc), 6. To a solution of HDPhBz
(729 mg, 2.68 mmol) in 25 mL of CH2Cl2 was added a 1.6 M
solution of BunLi in n-hexane (1.67 mL, 2.68 mmol) at 0 °C, and
then the mixture was allowed to reach room temperature. This was
added to a solution of Pd3(OAc)6 (300 mg, 0.450 mmol) in 25 mL
of CH2Cl2. The mixture was stirred overnight at room temperature,
(22) SMART V 5.618 Software for the CCD Detector System; Bruker
Analytical X-ray Systems, Inc.: Madison, WI, 1998.
(23) SAINTPLUS V 6.45A Software for the CCD Detector System; Bruker
Analytical X-ray System, Inc.: Madison, WI, 1998.
6132 Inorganic Chemistry, Vol. 44, No. 17, 2005

4+
Pd2 Complexes as Precursors to Dipalladium Species
Table 2. Crystal Data for 1, 2·5C6H6, 4, 6·2/3CH3C(O)CH3, and 8·2CH3OH
compound
formula
fw
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
1
2‚5C6H6
C106H94N12Pd2
1748.73
triclinic
P1h
11.676(2)
17.172(3)
22.102(3)
93.980(3)
100.526(3)
95.357(3)
4320.7(1)
2
4
6‚2/3 acetone
C61H52N6O2.67Pd2
1130.55
orthorhombic
P212121
11.527(4)
22.807(7)
58.40(2)
90
90
90
15353(8)
12
1.467
8‚2 methanol
C88H88N8O12Pd4
1875.26
orthorhombic
Pnn2
11.027(1)
18.183(2)
10.062(1)
90
90
90
2017.4(4)
2
1.544
C60H60N8O8Pd2
1233.96
triclinic
C38H30N4Pd
649.06
triclinic
P1h
P1h
10.306(3)
10.337(3)
13.774(5)
81.103(5)
76.778(5)
81.146(6)
1400.7(8)
1
11.584(6)
11.651(6)
13.222(7)
105.079(9)
112.883(8)
96.779(9)
1538.5(1)
2
Z
d (calcd) (g cm-3
)
1.463
0.0337, 0.0890
0.0353, 0.0910
1.344
0.0762, 0.1586
0.1389, 0.1875
1.401
0.0509, 0.1353
0.0590, 0.1445
R1,a wR2b (I > 2σI)
0.0494, 0.1141
0.0608, 0.1210
0.0207, 0.0473
0.0276, 0.0512
R1,a wR2b (all data)
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. b wR2 ) [∑[w(Fo - Fc )2]/∑[w(Fo )2]]1/2 and w ) 1/[σ2(Fo ) + (aP)2 + bP], where P ) [max(0 or Fo ) + 2(Fc )]/3.
2
2
2
2
2
2
Table 3. Selected Bond Distances (Å) and Angles (deg) for 1, 2·5C6H6, 4, 6·2/3 Acetone, and 8·2 Methanola
1
2‚5C6H6
4
6‚2/3 acetone
8‚2 methanol
Pd‚‚‚Pd
2.6486(8)
2.042[2]
2.8971(9)
2.5634[9]
2.021[5]
2.070[5]
2.9435(4)
2.013(2)
2.114[2]
Pd-N
2.045[4]
Pd-O
Pd-Nbridge
Pd-Nchel.
2.043[6]
2.054[6]
Pd-C
1.960(3)
79.02[6]
Pd‚‚‚Pd-N
Pd‚‚‚Pd-O
trans-N-Pd-N
cis-N-Pd-N
N-Pd‚‚‚Pd-N
torsion angle
O-Pd‚‚‚Pd-O
torsion angle
Pd‚‚‚Pd-Nbridge
Pd‚‚‚Pd-Nchel.
C-Pd-N
85.90[6]
85.7[2]
86.1[1]
171.2[2]
90.3[2]
14.4
171.75[8]
89.71[9]
0
116.3[1]
63.8[1]
13.5
78.5[2]
107.5[2]
90.7(1)
a Values in square brackets are averages.
Results and Discussion
methine region of the 1H NMR spectrum at δ of 7.035 ppm,
recorded on noncrystalline samples obtained from the reac-
tion mixture, is diagnostic of a single configuration having
D4h symmetry because two signals in a 1:1 ratio would be
expected for the â isomer. The M+ peak at 1232 amu in the
mass spectrum is consistent with the presence of a dinuclear
species, as is the crystal structure which is shown in Figure
Reactions of trinuclear palladium(II) acetate with a variety
of mononegative N,N-ligands, which include the formamidi-
nate DAniF, the guanidine-type ligand TPG, the triazine
DPhTA, and the benzamidine DPhBz, may be complex. Such
reactions afford mononuclear or dinuclear species depending
on the reaction conditions or the method of isolation.
Additionally, the dinuclear species Pd2(N,N)4 may produce
isomeric compounds. In the R isomer, there are four bridging
N,N-ligands forming a paddlewheel, and in the â isomer,
there are two bridging and two chelating groups. In all cases,
however, the palladium atoms are in an essentially square
planar environment as shown in Scheme 3.
Scheme 3
Whether there were mononuclear and/or dinuclear species
in the reaction mixture was easily distinguished by using
mass spectrometry, whereas 1H and/or 13C NMR were useful
techniques in distinguishing between the R and the â isomers
or their mixtures. Our studies show that with DAniF the only
product observed under the experimental conditions reported
above is the R form. The presence of a single signal in the
(24) SADABS program for absorption correction using SMART CCD data
based on the method of Blessing: Blessing, R. H. Acta Crystallogr.,
Sect. A 1995, 51, 33.
(25) Sheldrick, G. M. SHELXTL97; University of Go¨ttingen: Go¨ttingen,
Germany, 1997.
Inorganic Chemistry, Vol. 44, No. 17, 2005 6133

Berry et al.
Figure 2. Core structure of Pd2(TPG)2(η2-TPG)2 with displacement
ellipsoids drawn at the 30% probability level. For simplicity, only the
attached carbon atom (shown in red) for each of the three phenyl groups of
each TPG ligand is shown. The remaining carbon atoms and all hydrogen
atoms have been omitted. Selected bond distances: Pd1-N9 ) 2.024(6),
Figure 1. Paddlewheel 1 having a fully eclipsed configuration. Displace-
ment ellipsoids are drawn at the 30% probability level, and hydrogen atoms
have been omitted. Selected bond distances: Pd1-N2 ) 2.033(2),
Pd1-N3
) 2.040(2), Pd1-N1 ) 2.045(2), Pd1-N4 ) 2.050(2),
Pd1‚‚‚Pd1A ) 2.6486(8) Å.
Pd1-N4
) 2.051(7), Pd1-N3 ) 2.059(6), Pd1-N1 ) 2.069(7),
Pd1‚‚‚Pd2 ) 2.8971(9), Pd2-N6 ) 2.037(6), Pd2-N10 ) 2.039(6),
Pd2-N11 ) 2.049(6), Pd2-N7 ) 2.058(6) Å.
1. The structure is that of a typical tetragonal paddlewheel
8
similar to those of Pd2(DTolF)4 and Pd2(DPhBz)4.9 One
mamidinate in which Ar is either phenyl or tolyl.27 These
reports indicate that the R isomer can sometimes be obtained
by heating the â isomer in nucleophilic solvents for a short
time. However, care must be exercised as prolonged periods
in boiling solvents may lead to metal deposition, as we have
observed in unpublished results using Pd2(hpp)4. Another
example was found when the guanidine-type compound 2
was heated in MeOH for 1 h; this led to complete
decomposition and formation of Pd metal. Also, when
heating was attempted using the reaction mixture of Pd3-
(OAc)6 and LiTPG, orthometalation of one TPG ligand was
observed with formation of Pd2(µ-TPG)(η2-TPG)[η2-C6H4N-
(CNHPh)N(H)Ph](µ-OAc), as determined by X-ray crystal-
lography.28
important feature is that the DAniF- ligands wrap around
the dimetal unit so that a fully eclipsed structure is formed
with a dihedral angle N-Pd‚‚‚Pd-N of 0°. This contrasts
with Pd2(DTolF)4 where the torsion angle is 15.1(6)°8 and 6
(vide infra) where the N-Pd‚‚‚Pd-N and O-Pd‚‚‚Pd-O
torsion angles are 15.4 and 14.5°, respectively. In the R
isomer of Pd2(DPhBz)4, there is also a considerable torsion
angle of 14°.9
Similarly, results from the reaction of Pd3(OAc)6 with
LiDPhTA show that the only observed product corresponds
to the dinuclear R form. A synthesis21 and structure16 of Pd2-
(DPhTA)4 have been described. The compound was made
in unspecified yield in a two-step reaction. First, an unstable
diphenyltriazine-palladium species was reported to be
obtained by mixing sodium tetrachloropalladate, sodium
acetate, and HDPhTA, which then yielded a brown precipi-
tate after standing in a solution of acetone at 50 °C. Our
method of preparation using Pd3(OAc)6 and LiDPhTA gives
the pure product straightforwardly in one step and in 69%
yield.
In contrast to 1 and 3, reaction with the guanidine-type
ligand26 TPG yields exclusively the â isomer 2. The 1H NMR
spectrum is characteristically complex because of the ap-
pearance of signals from both the chelating and bridging
groups, which when resolved are in a ratio of 1:1. For
example, there are two singlets of equal intensities at 5.461
and 5.887 which have been assigned to signals of the -NH
hydrogen for the chelating and bridging ligands. The mass
spectrum shows the presence of the peak for the dinuclear
species, which is also consistent with the solid-state structure
shown in Figure 2.
Palladium/Diphenylbenzamidinate System. The reaction
of Pd3(OAc)6 and Li(DPhBz) was first studied in the Bear
group that reported the crystal structures of both the R and
â isomers of Pd2(DPhBz)4 and also the possible existence
+ 9
of a species containing the cation Pd2(DPhBz)4 . Because
the latter was of special interest to us, we tried to make the
precursor R-Pd2(DPhBz)4. It was quickly found that this
system is far more complex than originally recognized.
Furthermore, some of the reported spectroscopic data devi-
ated significantly from what we were finding. A decision
was made to reinvestigate this system very carefully to
resolve the inconsistencies.
To put the problem in perspective, we begin with a
summary of the published report that is relevant to the
understanding of our results. The reaction of Pd3(OAc)6 and
Li(DPhBz) was carried out in CH2Cl2 at room temperature
(27) Cotton, F. A.; Matonic, J. H.; Murillo, C. A. Inorg. Chem. 1996, 35,
498.
It should be pointed out that the presence of both R and
â isomers was reported earlier in the Pd/DPhBz system9 and
also in Pt2(DArF)4 compounds, where DArF is a diarylfor-
(28) Crystallographic data for Pd2(µ-TPG)(η2-TPG)[η2-C6H4N(CNHPh)N-
(H)Ph](µ-OAc)‚2CD3CN: triclinic; P1h; a ) 11.209(1) Å, b ) 12.0459-
(1) Å, c ) 23.011(3) Å; R ) 74.877(2)°, â ) 89.529(2)°, γ )
71.256(2)°; V ) 2830.3(5) Å3; R (all data) ) 0.0410; Pd‚‚‚Pd )
2.9202(4) Å, average Pd-N ) 2.037[2] Å, Pd-C ) 2.483(3) Å,
Pd-O ) 2.105[2] Å.
(26) For a review on guanidine-type ligands see: Bailey, P. J.; Pace, S.
Coord. Chem. ReV. 2001, 214, 91.
6134 Inorganic Chemistry, Vol. 44, No. 17, 2005

4+
Pd2 Complexes as Precursors to Dipalladium Species
for 14 h.9 The solvent was then removed, and the solid was
dissolved in a minimum amount of CH2Cl2. Methanol was
then added. After the mixture had been stirred overnight, an
orange precipitate was isolated. Following “extensive puri-
fication” of the solid, it was assigned the formula of the â
form, Pd2(µ-DPhBz)2(η2-DPhBz)2, on the basis of a crystal
structure; a 14% yield was reported. The â isomer was said
to give a UV-vis spectrum with two absorption peaks, at
378 and 496 nm, on the edge of a strong band rising into
the UV region. The CV (cyclic voltammetry) in CH2Cl2
showed two irreversible waves at 1.02 and 1.26 V vs SCE
(1.06 and 1.30 V vs Ag/AgCl).
It was further reported that, when a methanol solution of
this compound was refluxed for 2 h, a small amount of Pd
metal formed and it was removed. Elimination of the solvent
from the remaining solution, followed by chromatography,
produced another product in a reported yield of 40%. An
X-ray study of a crystal showed that it was the isomer R-Pd2-
(DPhBz)4, in which all benzamidinate ligands bridge the
4+
Pd2 unit. This complex was reported to give a broad
absorbance in the visible region centered at 500 nm and a
reversible electrochemical wave at 0.695 V from CV or 0.745
V vs Ag/AgCl, as obtained from bulk controlled-potential
electrolysis. No additional characterization for any of these
compounds, such as elemental analyses29 or NMR data, was
provided. Also, it was stated that other products were
observed during chromatography, but they were not identi-
fied.
When trying to reproduce the synthesis of Pd2(µ-DPhBz)2-
(η2-DPhBz)2, we noticed that addition of methanol to the
reaction mixture, as reported, consistently gave an oily
substance instead of the reported solid. This oil prevented
the isolation of any crystalline material even when chro-
matographic separation was attempted. We then decided to
modify the synthetic procedure with the hope of overcoming
the synthetic difficulties.
A similar reaction of Pd3(OAc)6 and Li(DPhBz) was
carried out in dichloromethane but without methanol. Metha-
nol was used only later to wash the solid obtained after
elimination of CH2Cl2 from the reaction mixture, and this
procedure led to an analytically pure product in high yield
that was shown by NMR to contain a ratio of benzamidate
to acetate of 3:1. The ratio suggested that the formula is Pd2-
(DPhBz)3(OAc), and this was confirmed by the structure,
shown in Figure 3, where all ligands bridge the dipalladium
unit. The CV showed a reversible oxidation at 0.776 V vs
Ag/AgCl, a value that is similar to that reported for the R
isomer.9
Figure 3. Core structure of the unsymmetrical compound Pd2(DPhBz)3-
(OAc). Only one of the phenyl carbon atoms for each of the bridging DPhBz
ligands is shown. Displacement ellipsoids are drawn at the 30% probability
level, and hydrogen atoms have been omitted. Selected bond distances:
Pd1-N4 ) 2.002(5), Pd1-N1 ) 2.027(5), Pd1-N7 ) 2.032(5), Pd1-O1
) 2.079(5), Pd1‚‚‚Pd2 ) 2.5678(9), Pd2-N6 ) 2.012(5), Pd2-N8 ) 2.027-
(5), Pd2-N2 ) 2.036(5), Pd2-O2 ) 2.092(5) Å.
is no acetate. Furthermore, it is consistent with the presence
of two types of phenyl rings in a 2:1 ratio. This is consistent
with the presence of a highly symmetrical species and only
one kind of benzamidinate group. The 13C NMR spectrum,
which shows eight sharp peaks in the region between 123
and 145 ppm, is simple and also consistent with equivalent
benzamidinate groups. Four of the eight peaks correspond
to the unique carbon atoms in the phenyl rings bonded to
the nitrogen atoms, and the other four are from the phenyl
ring bonded to the bridgehead carbon atom. There are only
four unique carbon atoms in each ring because each ring is
free to rotate. The signal for the carbon bridgehead was not
observed, but this is not unusual as it is bound to two nitrogen
atoms, each with a large quadrupole moment. The mass
spectrum showed only a [M+] peak consistent with that of a
mononuclear molecule of formula Pd(DPhBz)2. This mo-
lecular formula is also supported by the crystal structure,
shown in Figure 4, which shows two chelating benzamidinate
groups around a single Pd atom in a distorted square planar
environment. The CV showed two irreversible oxidation
processes at 1.005 and 1.465 V (vs Ag/AgCl). Interestingly,
the UV-vis spectrum of Pd(η2-DPhBz)2 with two bands at
376 and 496 nm is very similar to that reported for the â
isomer.9
By changing the reaction solvent from CH2Cl2 to toluene,
another product was formed, as shown by NMR and mass
spectrometry. The 1H NMR spectrum is complex; however,
it indicates that benzamidinate ligands are present, and there
Boiling of mononuclear Pd(η2-DPhBz)2 in MeOH followed
by chromatographic separation produces an orange product.
Its 1H NMR spectrum is consistent with the existence of only
one type of DPhBz group, but it is significantly more
complex than that of its precursor. The 13C NMR spectrum
is also very complex. Upon slow evaporation of the NMR
solvent, all of the solute was recovered as orange crystals.
(29) It should be mentioned that we have had serious difficulties obtaining
elemental analyses for all compounds having benzamidinate ligands.
In all cases, the experimental percentage of C was well below the
calculated value. It is our belief that this is due to the formation of
carbides that could form upon heating of the samples because
orthometalation to form Pd-C bonds occurs, for example, in
compound 8.
Inorganic Chemistry, Vol. 44, No. 17, 2005 6135

Berry et al.
Figure 4. Structure of the mononuclear compound Pd(η2-DPhBz)2 showing
the distorted square planar arrangement. Displacement ellipsoids are drawn
at the 30% probability level, and hydrogen atoms have been omitted.
Selected bond distances: Pd1-N1 ) 2.037(3), Pd1-N4 ) 2.041(3),
Pd1-N3 ) 2.048(4), Pd1-N2 ) 2.053(4) Å.
Figure 5. Core structure of the chiral orthometalated compound cis-Pd2-
(η2-C6H4NC(Ph)NPh)2(µ-OAc)2, 8. Only one carbon atom of each of the
non-orthometalated phenyl groups is shown. Displacement ellipsoids are
drawn at the 30% probability level, and all hydrogen atoms have been
omitted. Selected bond distances: Pd1-C21 ) 1.960(3), Pd1-N1 ) 2.013-
(2), Pd1-O5 ) 2.057(2), Pd1-O4 ) 2.170(2), Pd1-Pd1 ) 2.9435(4) Å.
The structure was determined by single-crystal X-ray dif-
fraction, and it is the same as that described earlier for the
â isomer, Pd2(µ-DPhBz)2(η2-DPhBz)2.9 The complexity of
the 1H and 13C NMR spectra may be attributed to restricted
rotation of the phenyl groups in each of the benzamidinate
ligands, as shown by the changes in the spectra when the
temperature is modified. The yield of 40% is significantly
higher than that reported earlier (14%). However, there are
again some discrepancies in the spectroscopic data. The CV
taken by dissolving crystalline samples shows two irrevers-
ible processes at 0.927 and 1.291 V (vs Ag/AgCl). These
values are somewhat different from those reported earlier
(1.065 and 1.305 V vs Ag/AgCl). Also, the electronic
spectrum, with only one band at 486 nm, is different from
that reported earlier.9
greater rotational freedom of the phenyl rings at high
temperatures. The pattern and integration of these signals
are consistent with the presence of a highly symmetrical
species in solution. The NMR spectrum, supported by the
mass spectrum ([M+] of 1086 amu), is consistent with a
molecule with close to D4h symmetry. Indeed, crystallization
of this compound produced crystals in ca. 30% yield, and
they have the same structure as that reported earlier for the
R isomer.31 In this case, the electrochemistry and UV-vis
data were the same as those reported.9
We were also interested in the byproducts formed during
the synthesis of Pd2(DPhBz)3(OAc). Washing the red powder
formed after reaction of Pd3(OAc)6 with Li+DPhBz- in CH2-
Cl2 yielded a dark solution. After slow and complete
evaporation of MeOH, a mixture of yellow and red crystals
was obtained. The dark red crystals were identified as Pd2-
(DPhBz)3(OAc), 6, and the yellow crystals were identified
as cis-Pd2[η2-C6H4NC(Ph)N(H)Ph]2(µ-OAc)2, 8. In this
compound, the two metal atoms are bridged by two cis
acetate linkers and there are two chelating orthometalated
DPhBz- ligands which envelop the palladium centers, as
shown in Figure 5. Six-membered rings that consist of three
carbon atoms, two nitrogen atoms, and a palladium atom
are formed. In these rings, the nitrogen atom not bound to a
palladium atom has a hydrogen atom. Its presence is strongly
supported by a distance of 2.876 Å between that nitrogen
atom and the oxygen atom of an interstitial methanol
All attempts to prepare the R-Pd2(DPhBz)4 from the â
isomer by boiling in MeOH were unsuccessful. Once again,
a different synthetic approach was sought, and the one used
resembled that for the preparation of Ru2(DMBz)4Cl2 (DMBz
) N,N′-dimethylbenzamidinate),30 in which Ru2(OAc)4Cl
was refluxed with 5 equiv of HDMBz in THF in the presence
of an excess of LiCl and Et3N. By analogy, refluxing of a
THF mixture of Pd3(OAc)6 with 5 equiv of HDMBz in the
presence of Et3N followed by chromatography yielded a dark
purple mixture. After separation of a bright orange band by
1
TLC, an H NMR spectrum showed peaks only in the
aromatic region, as expected for a compound with only
benzamidinate ligands. The spectrum changed as the tem-
perature was varied, giving a pattern that was significantly
simpler at 45 °C than at -45 °C. This is consistent with
(31) Another way to synthesize the R isomer in low yield is by reaction in
(30) Xu, G.; Campana, C.; Ren, T. Inorg. Chem. 2002, 41, 3521.
6136 Inorganic Chemistry, Vol. 44, No. 17, 2005
THF of Pd2(DPhBz)3(OAc) and one equivalent of a DPhBz anion.

4+
Pd2 Complexes as Precursors to Dipalladium Species
Table 4. E1/2 (V vs Ag/AgCl) Values for Compounds 1-8
electron-donating ability of the bridging ligand with the very
basic DAniF- ligand exhibiting the lowest oxidation poten-
tial. Interestingly, Pd2(DAniF)4 can be oxidized reversibly
four times, and this will be discussed more thoroughly in a
later report where we will explore the electronic structure
using DFT calculations.
1
2
3
4
compound
E1/2
0.70
E1/2
E1/2
1.12
E1/2
1.27
1
2
3
4
5
6
7
8
0.97
0.729 irra
1.3 irra
1.005 irra
0.927 irra
0.776
1.031 irra
1.465 irra
1.291 irra
0.698
0.920
Acknowledgment. We thank the National Science Foun-
dation and the Welch Foundation for support. J.F.B. thanks
the National Science Foundation for a predoctoral research
fellowship. We also thank Lindsay R. Riddle for help in the
synthesis of some ligands and Dr. V. I. Bakhmutov for help
in interpreting NMR spectra.
a Irreversible.
molecule, a separation that is only consistent with the
presence of a hydrogen bond. Thus, the orthometalated
4+
groups are mononegative and the dimetal unit has a Pd2
1
core. For this compound, the H NMR and 13C NMR are
Supporting Information Available: Crystallographic data in
CIF format for 1, 2‚5C6H6, 4, 6‚2/3 acetone, and 8‚2 methanol, as
well as for Pd2(µ-TPG)(η2-TPG)[η2-C6H4N(CNHPh)N(H)Ph](µ-
OAc). Displacement ellipsoid plots of the entire molecules in 2, 6,
8, Pd2(µ-TPG)(η2-TPG)[η2-C6H4N(CNHPh)N(H)Ph](µ-OAc), and
Pd2(µ-TPG)(η2-TPG)[η2-C6H4N(CNHPh)N(H)Ph](µ-OAc)‚2CD3-
CN in pdf format. This material is available free of charge via the
Internet at http://pubs.acs.org.
very complex. The CV and differential pulse voltammogram
show one reversible oxidation wave at 0.930 V (vs Ag/AgCl).
A Look Ahead. Because compounds 1, 6, 7, and 8 have
reversible oxidation potentials, as shown in Table 4, these
compounds appear to be good candidates for chemical
oxidation and, thus, good candidates to serve as precursors
to metal-metal bonded dipalladium species. This will be
done as a second part of this study. It should be noted that
the E1/2 values in these compounds are dependent on the
IC050876C
Inorganic Chemistry, Vol. 44, No. 17, 2005 6137