Palladium N(CH2CH2PiPr2) 2-Dialkylamides: Synthesis, structural characterization, and reactivity

Article abstract of DOI:10.1021/ic802286u

Palladium(II) aminodiphosphine PNP pincer complexes [PdR(PNP^H )]PF6 (1^R; R ) Cl Me, Ph; PNP^H ) HN(CH₂CH ₂P'Pr₂)₂) were prepared. Deprotonation with KO'Bu affords dialkylamides [P

Full text of DOI:10.1021/ic802286u

Inorg. Chem. 2009, 48, 3699-3709
Palladium N(CH2CH2PiPr2)2-Dialkylamides: Synthesis, Structural
Characterization, and Reactivity
Alexander N. Marziale, Eberhardt Herdtweck, Jo¨rg Eppinger, and Sven Schneider*
Technische UniVersita¨t Mu¨nchen, Department Chemie, Lichtenbergstrasse 4,
D-85747 Garching b Mu¨nchen, Germany
Received December 2, 2008
Palladium(II) aminodiphosphine PNP pincer complexes [PdR(PNPH)]PF6 (1R; R ) Cl Me, Ph; PNPH
)
HN(CH2CH2PiPr2)2) were prepared. Deprotonation with KOtBu affords dialkylamides [PdR(PNP)] (2R; R ) Cl Me,
Ph; PNP ) (NCH2CH2PiPr2)2) in high yield which are stable toward ꢀ-H elimination. While AgPF6 oxidizes the
amides, cationic amido complexes [PdL(PNP)]PF6 (3L; L ) CNtBu, PMe3) were obtained upon chloride abstraction
from 1Cl with TlPF6. The reaction of amide 2Cl with MeOTf results in N-methylation yielding [PdCl(PNPMe)]OTf (5)
quantitatively. N-H acidities of the amino complexes 1Me (pKa ) 24.2(1)) and 1Ph (pKa ) 23.2(1)) were determined
in dmso. Complexes 1Cl, 1Me, 2Cl 2Me, 3CNtBu, and 5 were structurally characterized by single crystal X-ray diffraction.
The amido complexes feature pyramidal nitrogen atoms in the solid state. The molecular structures, high N-basicity,
and reactivity of the amido complexes can be explained with Pd-Namido bonding that is characterized by strong
NfPd σ-donation and repulsive dπ-pπ π-interactions. This interpretation was confirmed by density functional theory
(DFT) calculations of 2Cl.
Introduction
coordinate palladium(II) amido species has been identified
as the crucial step determining the product selectivity.6
Hartwig and co-workers found that the rate of amine
reductive elimination from [(dppf)Pd(Ar)(NMeAr′)] (dppf
) [(η5-C5H4PPh2)2Fe]) correlates qualitatively with the pKa
of the corresponding free amine.7 Recently, Buchwald and
co-workers have examined competetive C-N coupling of
aryl- and alkylamine substrates with PhCl catalyzed by
[(SPhos)PdCl]2 (SPhos ) 2-dicyclohexylphosphino-2′,6′-
dimethoxy-1,1′-biphenyl) in the presence of NaOCMe2Et as
base, resulting in high chemoselectivity toward aryl-ary-
Palladium catalysis has considerably contributed to the
remarkable progress in C-N coupling reactions, as in
Hartwig-Buchwald cross-coupling, hydroamination, or oxi-
dative amination of olefins.1-3 Hartwig-Buchwald type
reactions have been studied experimentally and with quantum-
chemical calculations.4,5 In the generally accepted mecha-
nism, arylamine reductive elimination from a three- or four-
*
To whom correspondence should be addressed. E-mail:
sven.schneider@ch.tum.de.
(1) (a) Kosugi, M.; Kameyama, M.; Toshihiko, M. Chem. Lett. 1983, 927–
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5969–5970. (c) Guram, A.; Rennels, R. A.; Buchwald, S. L. Angew.
Chem. 1995, 107, 1456–1459. Angew. Chem., Int. Ed. 1995, 34,
1348-1350. (d) Louie, J.; Hartwig, J. F. Tetrahedron Lett. 1995, 36,
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Hartwig, J. F. J. Am. Chem. Soc. 2006, 128, 8134–8135. (c) Johns,
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10.1021/ic802286u CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 8, 2009 3699
Published on Web 03/12/2009

Marziale et al.
lamine coupling. However, if lithium amides were used as
nitrogen source, the chemoselectivity was reversed toward
aryl-alkylamine coupling.8 These results were rationalized
with the higher pKa values of alkyl- versus arylamines.
However, despite the importance of palladium amido
complexes in catalysis, structural and thermodynamic data
about this class of compounds, for example, pKa values of
amines coordinated to PdII, remains scarce.9 Since the
pioneering work by Fryzuk et al. on disilylamido com-
plexes,10 studies about palladium amides have mostly focused
on arylamides.11-13 The reactivity of electron rich complexes
bearing covalently bound π-donating ligands, such as amides,
is generally attributed to the high ligand nucleophilicity.14
Both repulsive filled-filled pπ-dπ interactions and high M-N
σ-bond polarity have been stressed as an explanation.15,16
While the π-acceptor ability of silyl and aryl substituents
stabilizes the high electron density on the nitrogen by
delocalization of the lone pair, terminal alkylamido deriva-
tives bearing ꢀ-hydrogens typically suffer from decomposi-
tion to metal hydrides.14,17 Hence, isolable alkylamides of
palladium are particularly rare.11b,18
In our efforts to study the chemistry of electron-rich
alkylamido complexes we have recently reported the isolation
of iridium(I) amino and amido complexes with the pincer
ligands HN(CH2CH2PiPr2)2 (PNPH) and N(CH2CH2PiPr2)2
(PNP).19 In contrast to the stable iridum(I) dialkylamides,
for RuIIPNP-complexes reversible ꢀ-H elimination processes
were observed.20 In this context, we were interested in
palladium PNP dialkylamido complexes to obtain thermo-
dynamic and structural information that can be correlated
with reactivity patterns which are important to understand
catalytic reaction mechanisms. In this manuscript we describe
the synthesis and structural characterization of palladium(II)
PNP dialkylamido complexes which are stable toward ꢀ-H
elimination. The reactivity toward electrophiles and oxidizing
agents will be reported.
Experimental Section
Materials and Methods. All experiments were carried out under
an atmosphere of argon using Schlenk and glovebox techniques.
Benzene and tetrahydrofuran (THF) were dried over Na/benzophe-
none, distilled under argon and deoxygenated prior to use. Dieth-
ylether and pentane were dried and deoxygenized by passing
through columns packed with activated alumina and Q5, respec-
tively. Deuterated solvents were dried by distillation from Na/K
alloy (C6D6 and d8-THF) or stirring over 4 Å molecular sieves and
distillation from B2O3 (d6-acetone), respectively, and deoxygenated
by three freeze-pump-thaw cycles. KOtBu was purchased from
VWR and sublimed prior to use. Palladiumdichloride (ABCR),
AgPF6 (ABCR), TlPF6 (ABCR), MeOTf (Aldrich), PMe3 (Aldrich),
CNtBu (Fluka) were used as purchased. Pyrrol and 2-pyrrolidinone
(ACROS) were dried over CaH2 and distilled prior to use.
HN(CH2CH2PiPr2)2, [PdCl2(CH3CN)2], and [Pd(I)Ph(tmeda)] were
prepared as reported in the literature.21-23
Analytical Methods. Elemental analyses were obtained from
the Microanalytical Laboratory of Technische Universita¨t Mu¨nchen.
The IR spectra were recorded on a Jasco FT/IR-460 PLUS
spectrometer as nujol mulls between KBr plates. NMR spectra were
recorded on Jeol Lambda 400 and Bruker DPX 400 spectrometers
at room temperature and were calibrated to the residual proton
resonance and the natural abundance 13C resonance of the solvent
(C6D6, δH ) 7.16 and δC ) 128.06 ppm; d6-acetone, δH ) 2.05
and δC ) 29.84 + 206.26 ppm). 31P NMR chemical shifts are
reported relative to external phosphoric acid (δ 0.0 ppm). Signal
multiplicities are abbreviated as: s (singlet), d (doublet), t (triplet),
vt (virtual triplet), sp (septet), m (multiplet), br (broad).
Syntheses. [PdCl(PNPH)][PF6] (1Cl). HN(CH2CH2PiPr2)2 (0.100
g; 0.327 mmol) is added to a solution of PdCl2(NCMe)2 (0.085 g;
0.327 mmol) in THF (5 mL) and stirred for 30 min. AgPF6 (0.081
g; 0.327 mmol) is added to the solution, and the reaction is stirred
at room temperature overnight. After filtration the product is
precipitated with diethylether and pentanes, washed twice with
pentanes, and dried in vacuo to give a yellowish solid. Yield: 0.189
g (0.319 mmol; 99%). Anal. Calcd for C16H37ClF6NP3Pd (592.26):
C, 32.50; H, 6.14; N, 2.37. Found: C, 32.18; H, 6.43; N, 2.40. IR
(8) Biscoe, M. R.; Barder, T. E.; Buchwald, S. L. Angew. Chem. 2007,
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1
(cm-1) ν ) 3239 (N-H). NMR (d6-acetone, r.t., [ppm]) H NMR
(13) (a) Fan, L.; Foxman Ozerov, O. V. Organometallics 2004, 23, 326–
328. (b) Fan, L.; Yang, L.; Guo, C.; Foxman, B. M.; Ozerov, O. V.
Organometallics 2004, 23, 4778–4787. (c) Ozerov, O. V.; Guo, C.;
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Fafard, C. M.; Chen, C.-H.; Foxman, B. M.; Ozerov, O. V. Chem.
Commun. 2007, 4465–4467. (e) Fafard, C. M.; Adhikari, D.; Foxman,
B. M.; Mindiola, D. J.; Ozerov, O. V. J. Am. Chem. Soc. 2007, 129,
10318–10319.
(14) (a) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163–1188. (b)
Fryzuk, M. D.; Montgomery, C. D. Coord. Chem. ReV. 1989, 95, 1–
40. (c) Fulton, J. R.; Holland, A. W.; Fox, D. J.; Bergman, R. G. Acc.
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(399.8 MHz): δ 1.34 (dvt, 6H, 3JHH ) 7.0 Hz, JPH ) 8.7 Hz, CH3),
1.39-1.49 (m, 18H, CH3), 2.20-2.33 (m, 2H, NCH2CH2), 2.38-2.49
(m, 4H, CH(CH3)2+ NCH2CH2), 2.58 (m, 2H, CH(CH3)2), 3.09
(m, 2H, NCH2), 3.43 (m, 2H, NCH2), 6.77 (br, 1H, NH). 13C {1H}
NMR (100.6 MHz): δ 17.0 (s, CH3), 17.4 (s, CH3), 18.0 (s, CH3),
18.2 (s, CH3), 22.9 (vt, JCP ) 9.9 Hz, PCH2), 23.6 (vt, JCP ) 12.6
Hz, CH(CH3)2), 25.0 (vt, JCP ) 10.7 Hz, CH(CH3)2), 56.3 (d, 2JCP
) 9.2 Hz, NCH2). 31P {1H} NMR (161.8 MHz): δ 64.6 (s, PiPr2),
-143.6 (sp, 1JPF ) 709 Hz, PF6). Assignments were confirmed by
1
1H-1H COSY and H-13C HETCOR spectra.
[PdMe(PNPH)][PF6] (1Me). To a solution of 1Cl (0.030 g, 0.051
mmol) in THF (5 mL) is added a solution of MeMgBr in THF
(0.170 mL, 0.3M) dropwise, and the solution is stirred at room
(21) Danopoulos, A. A.; Wills, A. R.; Edwards, P. G. Polyhedron 1990, 9,
2413–2418.
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121, 922–924; Angew. Chem. Int. Ed. 2009, 48, 905-907.
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1629–1638.
3700 Inorganic Chemistry, Vol. 48, No. 8, 2009

Palladium (NCH2CH2PiPr2)2-Dialkylamides
temperature overnight. Two milliliters of Dioxane are added to the
solution, and the reaction is stirred until a white precipitate forms.
The reaction is filtered, and the solvent is evaporated in vacuo.
The residue is extracted with dichloromethane, and another 2 mL
of dioxane are added. The solution is filtered again and evaporated
in vacuo. The residue is extracted with THF, and the product is
precipitated with diethylether and pentanes. The precipitate is
washed twice with pentanes and dried in vacuo to give a yellowish
solid. Yield: 0.029 g (0.048 mmol; 95%). Anal. Calcd for
C17H40F6NP3Pd (571.84): C, 35.70; H, 7.05; N, 2.45. Found: C,
35.49; H, 6.91; N, 2.49. IR (cm-1) ν ) 3271 (N-H). NMR (d6-
CH3), 23.7 (vt, JCP ) 9.6 Hz, PCH2), 24.1 (vt, JCP ) 10.7 Hz,
CH(CH3)2), 62.6 (s, NCH2). 31P {1H} NMR (161.8 MHz): δ 70.1
(s, PiPr2). Assignments were confirmed by a 1H-13C HSQC
spectrum.
[PdMe(PNP)] (2Me). KOtBu (0.014 g, 0.115 mmol) is added to
a solution of 1Me (0.066 g, 0.115 mmol) in THF (10 mL) at room
temperature. The reaction is stirred at room temperature for 30 min.
until a white precipitate is formed. After filtration the solution is
evaporated in vacuo to give a yellow solid. The solid is dissolved
in pentanes and stored at -18 °C for 24 h. A white precipitate is
formed. After filtration the solution is evaporated and dried in vacuo
to give a yellow solid. Yield: 0.034 g (0.071 mmol; 62%). Anal.
Calcd for C17H39NP2Pd (425.87): C, 47.95; H, 9.23; N, 3.30. Found:
C, 46.81; H, 8.88; N, 3.00. NMR (C6D6, r.t., [ppm]) 1H NMR (399.8
MHz): δ 0.28 (t, 3JPH ) 5.6 Hz, 3H, PdCH3), 1.02 (dvt, 12H, 3JHH
) 6.6 Hz, JPH ) 6.8 Hz, CH3), 1.16 (dvt, 12H, 3JHH ) 7.4 Hz, JPH
1
3
acetone, r.t., [ppm]) H NMR (399.8 MHz): δ 0.44 (t, JPH ) 5.6
Hz, 3H, PdCH3), 1.21-1.37 (m, 24H, CH3), 2.13-2.23 (m, 2H,
NCH2CH2), 2.33-2.51 (m, 6H, CH(CH3)2+ NCH2CH2), 2.66-2.78
(m, 2H, NCH2), 3.34-3.49 (m, 2H, NCH2), 4.93 (br, 1H, NH).
13C {1H} NMR (100.6 MHz): δ -19.5 (t, 2JCP ) 5.4 Hz, PdCH3),
16.9 (s, CH3), 17.5 (s, CH3), 18.4 (s, CH3), 18.5 (s, CH3), 22.8 (vt,
JCP ) 12.7 Hz, PCH2), 24.7 (vt, JCP ) 10.0 Hz, CH(CH3)2), 24.9
(vt, JCP ) 11.1 Hz, CH(CH3)2), 51.7 (s, NCH2). 31P {1H} NMR
) 7.6 Hz, CH3), 1.89 (br, 8H, NCH2CH2 + CH(CH3)2), 3.37 (m,
2
4H, NCH2CH2). 13C {1H} NMR (100.6 MHz): δ -23.0 (t, JCP
)
9.7 Hz, PdCH3), 17.7 (s, CH3), 19.0 (s, CH3), 24.0 (vt, JCP ) 10.4
Hz, PCH2), 24.7 (vt, JCP ) 10.7 Hz, CH(CH3)2), 59.5 (s, NCH2).
31P {1H} NMR (161.8 MHz): δ 65.2 (s, PiPr2).
1
(161.8 MHz): δ 52.0 (s, PiPr2), -143.6 (sp, JPF ) 709 Hz, PF6).
[PdPh(PNPH)][PF6] (1Ph). To a solution of Pd(I)Ph(tmeda)
(0.670 g, 1.684 mmol) in THF (20 mL) is added a solution of
(PNP)H (0.514 g, 1.684 mmol) in THF (2 mL) dropwise and stirred
at room temperature for 30 min. A yellow solid is formed and
filtered off. The precipitate is washed with three times diethylether
and dried in vacuo. The yellow solid is dissolved in dichloromethane
(15 mL), and a solution of AgPF6 (0.185 g, 0.731 mmol) in
dichloromethane (5 mL) is added dropwise. The solution is stirred
for 15 min until a yellowish-white solid is formed. The precipitate
is filtered off and washed twice with diethylether. The product is
precipitated from the filtrates with diethylether and pentanes, washed
twice with diethylether, and dried in vacuo to give a yellowish solid.
Yield: 0.412 g (0.651 mmol; 39%). Anal. Calcd for C22H42F6NP3Pd
(633.15): C, 41.70; H, 6.68; N, 2.21. Found: C, 42.37; H, 6.26; N,
2.11. IR (cm-1) ν ) 3277 (N-H). NMR (d6-acetone, r.t., [ppm])
1H NMR (399.8 MHz): δ 1.02-1.11 (m, 12H, CH3), 1.24 (dvt,
6H, 3JHH ) 7.0 Hz, JPH ) 8.3 Hz, CH3), 1.31 (dvt, 6H, 3JHH ) 7.0
Hz, JPH ) 7.9 Hz, CH3), 2.19 (m, 4H, NCH2CH2), 2.36 (m, 2H,
CH(CH3)2), 2.48 (m, 2H, CH(CH3)2), 2.79-2.88 (m, 2H, NCH2),
[PdPh(PNP)] (2Ph). A solution of KOtBu (0.026 g, 0.229 mmol)
in THF (5 mL) is added to a solution of 1Ph (0.145 g, 0.229 mmol)
in THF (10 mL) at - 80 °C. The reaction is stirred at -80 °C for
15 min. The solvent is removed in vacuo to give a yellow solid.
The solid is dissolved in pentanes, and the solution is filtered. The
filtrate is evaporated in vacuo to give a yellow solid. Yield: 0.083
g (0.169 mmol; 74%). Anal. Calcd for C17H39NP2Pd (487.94): C,
54.15; H, 8.47; N, 2.87. Found: C, 53.46; H, 8.52; N, 2.83. NMR
1
(C6D6, r.t., [ppm]) H NMR (399.8 MHz): δ 0.95 (m, 24H, CH3),
1.80 (br, 4H, NCH2CH2), 1.89 (m, 4H, CH(CH3)2), 3.33 (m, 4H,
3
NCH2CH2), 7.03 (t, 1H, JHH ) 7.1 Hz, para-C6H5), 7.22 (t, 2H,
3
3JHH ) 7.5 Hz, meta-C6H5), 7.70 (d, 2H, JHH ) 6.6 Hz, ortho-
C6H5). 13C {1H} NMR (100.6 MHz): δ 16.9 (s, CH3), 17.9 (s, CH3),
23.5 (vt, JCP ) 10.8 Hz, PCH2), 24.7 (vt, JCP ) 9.2 Hz, CH(CH3)2),
62.5 (s, NCH2), 120.7 (s, para-C6H5), 125.9 (s, meta-C6H5), 139.5
(s, ortho-C6H5), 138.6 (t, 2JCP ) 5.8 Hz, ipso-C6H5). 31P {1H} NMR
(161.8 MHz): δ 62.7 (s, PiPr2).
[Pd(CNtBu)(PNP)][PF6] (3CNtBu). A solution of TlPF6 (0.035
g; 0.10 mmol) in THF (10 mL) and CNtBu (0.0084 g; 0.011 mL;
0.10 mmol) are added to a solution of 2Cl (0.045 g; 0.10 mmol) in
THF (10 mL) at -78 °C. The solution is stirred at -78 °C for 1 h
and warmed to room temperature. After evaporation of the solvent
in vacuo the orange-grayish solid is extracted with benzene and
filtered. The product is precipitated upon addition of diethylether
and pentanes, washed twice with pentanes, and dried in vacuo to
give a yellow solid. Yield: 0.057 g (0.09 mmol; 90%). Anal. Calcd
for C21H45F6N2P3Pd (638.93): C, 39.48; H, 7.09; N, 4.38. Found:
C, 40.41; H, 7.15; N, 4.32. IR (cm-1) ν ) 2186 (Ct NtBu). NMR
3.40-3.54 (m, 2H, NCH2), 5.08 (br, 1H, NH), 6.86 (t, 1H, 3JHH
)
7.4 Hz, para-C6H5), 7.02 (t, 2H, 3JHH ) 7.5 Hz, meta-C6H5), 7.33
3
(d, 2H, JHH ) 7.1 Hz, ortho-C6H5). NMR (d3-acetonitrile, r.t.,
[ppm]) 13C {1H} NMR (100.6 MHz): δ 16.9 (s, CH3), 17.1 (s, CH3),
17.8 (s, CH3), 18.1 (s, CH3), 22.7 (vt, JCP ) 9.2 Hz, PCH2), 23.8
(vt, JCP ) 11.5 Hz, CH(CH3)2), 25.1 (vt, JCP ) 10.9 Hz, CH(CH3)2),
2
56.3 (d, JCP ) 10.0 Hz, NCH2), 123.0 (s, para-C6H5), 127.7 (s,
2
meta-C6H5), 136.9 (s, ortho-C6H5), 143.2 (t, JCP ) 6.9 Hz, ipso-
C6H5). 31P {1H} NMR (161.8 MHz): δ 49.6 (s, PiPr2), -144.4 (sp,
1JPF ) 709 Hz, PF6).
[PdCl(PNP)] (2Cl). HN(CH2CH2PiPr2)2 (0.105 g; 0.340 mmol)
is added to a solution of PdCl2(NCMe)2 (0.089 g; 0.340 mmol) in
THF (5 mL) and stirred for 30 min. KOtBu (0.038 g; 0.340 mmol)
is added to the solution, and the reaction is stirred at room
temperature for 2 h. After filtration the solution is evaporated in
vacuo to give an orange solid. The solid is dissolved in pentanes
and KCl is filtered off. The filtrate is evaporated and dried in vacuo
to give an orange solid. Yield: 0.148 g (0.33 mmol; 98%). Anal.
Calcd for C16H36ClNP2Pd (446.28): C, 43.06; H, 8.13; N, 3.14.
(d6-acetone, r.t., [ppm]) H NMR (399.8 MHz): δ 1.26-1.35 (m,
1
24H, CH3), 1.60 (s, 9H, CH3), 2.24 (m, 2H, NCH2CH2), 2.53 (m,
4H, CH(CH3)2), 3.22 (m, 4H, NCH2CH2). 13C {1H} NMR (100.6
MHz): δ 17.5 (s, CH3), 18.9 (s, CH3), 24.2 (vt, JCP ) 11.5 Hz,
PCH2), 25.0 (vt, JCP ) 12.3 Hz, CH(CH3)2), 29.1 (s, C(CH3)3), 59.6
(s, C(CH3)3), 61.4 (s, NCH2), 209.1 (s, CNtBu). 31P {1H} NMR
1
(161.8 MHz): δ 89.9 (s, PiPr2), -143.6 (sp, JPF ) 710 Hz, PF6).
1
19F NMR (376.2 MHz): δ -72.5 (d, JPF ) 712 Hz, PF6).
[Pd(PMe3)(PNP)][PF6] (3PMe3). A solution of TlPF6 (0.040 g;
0.114 mmol) in THF (5 mL) is added to a solution of 2Cl (0.050 g;
0.112 mmol) in THF (5 mL) at room temperature and stirred for 5
min. A 1 M solution of PMe3 in THF (0.200 mL; 0.20 mmol) is
added dropwise to the reaction, which is stirred at room temperature
for 3 h. The solution is filtered, and the filtrate is evaporated in
1
Found: C, 43.84; H, 7.71; N, 2.73. NMR (C6D6, r.t., [ppm]) H
3
NMR (399.8 MHz): δ 1.03 (dvt, 12H, JHH ) 7.0 Hz, JPH ) 7.3
3
Hz, CH3), 1.38 (dvt, 12H, JHH ) 7.4 Hz, JPH ) 8.4 Hz, CH3),
1.55 (m, 2H, NCH2CH2), 2.03 (m, 4H, CH(CH3)2), 2.83 (m, 4H,
NCH2CH2). 13C {1H} NMR (100.6 MHz): δ 17.6 (s, CH3), 18.8 (s,
Inorganic Chemistry, Vol. 48, No. 8, 2009 3701

Marziale et al.
vacuo to give an orange-grayish solid. The solid is dissolved in
benzene, and the solution is filtered again. After removing the
solvent the orange solid is dissolved in THF, and the product is
precipitated with diethylether and pentanes, washed twice with
pentanes, and dried in vacuo to give a yellow-orange solid. The
solid is dissolved in benzene and the product is precipitated with
diethylehter and pentanes, washed twice with diethylehter and dried
in vacuo to give an orange solid. Yield: 0.024 g (0.039 mmol; 35%).
Anal. Calcd for C19H45F6NP4Pd (631.87): C, 36.11; H, 7.17; N,
2.22. Found: C, 35.86; H, 7.04; N, 2.39. NMR (d6-benzene/CD2Cl2,
Hz, CH(CH3)2), 46.5 (s, NCH3), 66.4 (s, NCH2). 31P {1H} NMR
(161.8 MHz): δ 60.3 (s, PiPr2). 19F NMR (376.2 MHz): δ -78.7
(s, SO3CF3).
pKa Determinations. The palladium amido compound 2R (R )
CH3, Ph) and an equimolar amount of the corresponding acid (2Me
:
2-pyrrolidinone; 2Ph: pyrrole) were dissolved in 0.6 mL of d6-dmso.
Because of fast proton exchange on the NMR time scale one peak
resulted in the 31P NMR with a chemical shift δmeas that was located
between those of pure 1R (δ1R) and 2R (δ2R) in d6-dmso. The
equilibrium molar fractions of 2R (x2R) and the corresponding 1R
(x1R) conjugate acid cation were derived using
1
r.t., [ppm]) H NMR (399.8 MHz): δ 0.84-0.95 (m, 24H, CH3),
2
1.28 (d, JPH ) 9.2 Hz, 9H, P(CH3)3), 1.63 (m, 4H, NCH2CH2),
1.87 (m, 4H, CH(CH3)2), 2.80 (m, 4H, NCH2CH2). 13C {1H} NMR
δmeas)x1Rδ1R+x2Rδ2R
x1R+x2R ) 1
(1)
(2)
1
(100.6 MHz): δ 17.8 (s, CH3), 18.7 (d, JCP ) 28.3 Hz, P(CH3)3),
20.2 (s, CH3), 23.0 (vt, JCP ) 12.2 Hz, PCH2), 26.2 (vt, JCP ) 11.9
Hz, CH(CH3)2), 60.6 (s, NCH2). 31P {1H} NMR (161.8 MHz): δ
81.3 (d, 2JPP ) 48.5 Hz, PiPr2), -22.2 (t, 2JPP ) 50.5 Hz, P(CH3)3),
pKa values were calculated from the equilibrium constant (Keq) using
the following equations (pKa(2-pyrrolidinone) ) 24.2; pKa(pyrrole)
) 23.0):24
1
-143.1 (sp, JPF ) 710.9 Hz, PF6).
Reaction of 2Ph with AgPF6. 2Ph (8.8 mg; 18 µmol) is dissolved
in THF (0.5 mL) in a J-Young NMR tube, and AgPF6 (4.6 mg; 18
µmol) added at room temperature. A black precipitate immediately
forms, and the yellow solution turns colorless. After 5 min the
starting material is quantitatively converted to a mixture of
1Ph (∼ 80%) and [PdPh{N(CHCH2PiPr2)(CH2CH2PiPr2)}]PF6 (4;
∼ 20%) by NMR. Selected NMR data of 4 (d6-acetone, r.t., [ppm]):
1H NMR (399.8 MHz): δ 3.89 (m, 2H, N ) CHCH2), 8.48 (d, 3JHP
) 25.3 Hz 1H, N ) CHCH2). 31P {1H} NMR (161.8 MHz): δ 54.3
Keq)x22R/x21R
(3)
(4)
pKa(1R) ) pKa(ref) - log(Keq)
X-ray Crystal Structure Determinations. General Proce-
dures. Crystal data and details of the structure determination are
presented in Table 1. Suitable single-crystals for the X-ray
diffraction studies were grown by diffusion of pentane into THF
solutions (1Cl, 1Me, 3CNtBu, 5) or slow cooling of saturated pentane
solutions to -30 °C (2Cl, 2Me). Crystals were stored under
perfluorinated ether, transferred in a Lindemann capillary, fixed,
and sealed. Preliminary examinations and data collection were
carried out with area detecting systems and graphite-monochromated
Mo KR radiation (λ ) 0.71073 Å). The unit cell parameters were
obtained by full-matrix least-squares refinements during the scaling
procedures. Data collection was performed at low temperatures
(OXFORD CRYOSYSTEMS), each crystal was measured with a
couple of data sets in rotation scan modus with either ∆ꢁ/∆ω )
0.5° or 1.0°. Intensities were integrated, and the raw data were
corrected for Lorentz, polarization, and arising from the scaling
procedure for latent decay. The structures were solved by a
combination of direct methods and difference Fourier syntheses.
All non-hydrogen atoms were refined with anisotropic displacement
parameters. Full-matrix least-squares refinements were carried out
by minimizing ∑w(Fo2 - Fc2)2 with the Shelxl-97 weighting scheme
and stopped at shift/err <0.001. The final residual electron density
maps showed no remarkable features. Neutral atom scattering
factors for all atoms and anomalous dispersion corrections for the
non-hydrogen atoms were taken from International Tables for
Crystallography. All calculations were performed on an Intel
Pentium 4 PC, with the WinGX system, including the programs
PLATON, SIR92, SIR97, and SHELXL-97.25
2
2
(d, JPP ) 351 Hz, PiPr2), 50.1 (d, JPP ) 351 Hz, PiPr2), -143.6
1
(sp, JPF ) 707.4 Hz, PF6).
[PdCl(PNPMe)][OTf] (5). MeOTf (0.018 g; 0.012 mL; 0.110
mmol) is added to a solution of 2Cl (0.049 g; 0.110 mmol) in toluene
(5 mL) and stirred for 10 min until a yellow solid is formed. The
solid is filtered off, washed three times with diethylether, and dried
in vacuo to give microcrystalline, yellow 5. Yield: 0.066 g (0.109
mmol; 99%). Anal. Calcd for C18H39ClF3NO3P2PdS (610.39): C,
35.42; H, 6.44; N, 2.29. Found: C, 35.49; H, 6.47; N, 2.32. NMR
1
(d6-acetone, r.t., [ppm]) H NMR (399.8 MHz): δ 1.32-1.52 (m,
24H, CH3), 2.51 (m, 4H, NCH2CH2), 2.59-2.79 (m, 4H,
CH(CH3)2), 2.99 (s, 3H, NCH3), 3.16-3.28 (m, 2H, NCH2), 3.46
(m, 2H, NCH2). 13C {1H} NMR (100.6 MHz): δ 16.9 (s, CH3),
17.5 (s, CH3), 18.1 (s, CH3), 18.2 (s, CH3), 21.9 (vt, JCP ) 8.8 Hz,
PCH2), 23.9 (vt, JCP ) 12.3 Hz, CH(CH3)2), 25.6 (vt, JCP ) 11.9
(24) (a) Bordwell, F. G.; Drucker, G. E.; Fried, H. E. J. Org. Chem. 1981,
46, 632–635. (b) Bordwell, F. G.; Fried, H. E. J. Org. Chem. 1991,
56, 4218–4223.
(25) (a) APEX suite of crystallographic software. APEX 2, Version 2008.4.;
Bruker AXS Inc.: Madison, WI, 2008. (aa) SAINT, Version 7.56a and
SADABS Version 2008/1.; Bruker AXS Inc.: Madison, WI, 2008. (b)
Data Collection Software and Data Processing Software for Stoe IPDS
2T diffractometer, X-ARERA, Version 1.26; Stoe & Cie: Darmstadt,
Germany, 2004. (c) Data Processing Software for Stoe IPDS 2T
diffractometer, XRED, XSHAPE, Version 1.26; Stoe & Cie: Darmstadt,
Germany, 2004. (d) CrysAlis Data Collection Software and Data
Processing Software for Oxford Xcalibur diffractometer, Version
1.171; Oxford Diffraction Ltd.: Oxfordshire, U.K., 2005. (e) Farrugia,
L. J. J. Appl. Crystallogr. 1999, 32, 837–838; WinGX Version 1.70.01,
January 2005. (f) Altomare, A.; Cascarano, G.; Giacovazzo, C.;
Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M. SIR92, J. Appl.
Crystallogr. 1994, 27, 435–436. (g) Altomare, A.; Cascarano, G.;
Giacovazzo, C.; Guagliardi Moliterni, A. G. G. A.; Burla, M. C.;
Polidori, G.; Camalli, M.; Spagna, R. J. Appl. Crystallogr. 1999, 32,
115–119; SIR97, A New Tool for Crystal Structure Determination
and Refinement. (h) International Tables for Crystallography; Wilson,
A. J. C., Ed.; Kluwer Academic Publishers: Dordrecht, The Nether-
lands, 1992; Vol. C, Tables 6.1.1.4, 4.2.6.8, and 4.2.4.2. (i) Spek, A.
L. PLATON, A Multipurpose Crystallographic Tool; Utrecht Univer-
sity: Utrecht, The Netherlands, 2001. (j) Sheldrick, G. M. SHELXL-
97, Universita¨t Go¨ttingen: Go¨ttingen, Germany, 1998.
Specific Details. 1Cl: Data collection was performed with an
Oxford Diffraction device (Xcalibur, κ-CCD; sealed tube, Enhance
X-ray Source, Spellman, DF3) at T ) 153 K. Intensities were
corrected for absorption effects during the scaling procedure (Tmax
) 0.8600, Tmin ) 0.5695). Methyl hydrogen atoms were calculated
as a part of rigid rotating groups, with dC-H ) 0.98 Å and Uiso(H)
) 1.5Ueq(C). All other hydrogen atoms were placed in ideal positions
and refined using a riding model, with methylene and methyne dC-H
distances of (0.99 and 1.00 Å) and Uiso(H) ) 1.2Ueq(C). In contrast
the amine hydrogen atom could be located in the final difference
Fourier map and was allowed to refine freely. 1Me: Data collection
3702 Inorganic Chemistry, Vol. 48, No. 8, 2009

Palladium (NCH2CH2PiPr2)2-Dialkylamides
Table 1. Crystallographic Data for 1Cl, 1Me, 2Cl, 2Me, 3CNtBu, and 5
1Cl
1Me
2Cl
2Me
3CNtBu
5
formula
fw
C16H37ClF6NP3Pd
592.25
C17H40F6NP3Pd
571.83
C16H36ClNP2Pd
446.27
C17H39NP2Pd
425.85
C21H45F6N2P3Pd
638.88
C18H39ClF3NO3P2PdS
610.38
color/habit
cryst dimensions
(mm3)
cryst syst
space group
a, Å
yellowish/fragment
yellowish/fragment
orange/fragment
yellow/fragment
yellow/plate
yellow/fragment
0.14 × 0.34 × 0.56 0.20 × 0.30 × 0.30 0.18 × 0.18 × 0.51 0.08 × 0.20 × 0.56 0.25 × 0.53 × 0.63 0.25 × 0.25 × 0.31
monoclinic
P21/c (No. 14)
8.6831(1)
15.4168(2)
18.4350(2)
90
95.210(1)
90
2457.62(5)
4
monoclinic
P21/c (No. 14)
8.6804(3)
15.6177(5)
18.5296(8)
90
94.552(3)
90
2504.10(16)
4
monoclinic
P21/n (No. 14)
8.5012(4)
11.6523(7)
21.1295(10)
90
90.537(4)
90
2092.97(19)
4
triclinic
orthorhombic
Pbcm (No. 57)
9.5853(1)
15.7998(2)
19.5797(2)
90
monoclinic
P21/c (No. 14)
8.4064(2)
25.1998(4)
13.0821(2)
90
98.8013(8)
90
2738.68(9)
4
j
P1 (No. 2)
7.6469(6)
8.5660(6)
17.8499(14)
76.172(6)
83.808(6)
69.326(6)
1061.88(15)
2
b, Å
c, Å
R, deg
ꢀ, deg
90
90
γ, deg
V, Å3
2965.26(6)
4
Z
T, K
153
1.601
1.107
1208
173
1.517
0.981
1176
173
1.416
1.163
928
173
1.332
1.021
448
153
1.431
0.837
1320
293
1.480
1.007
1256
Dcalcd, g cm-3
µ, mm-1
F(000)
θ range, deg
index ranges (h, k, l)
no. of rflns collected
no. of indep
rflns/Rint
2.84-25.38
(10, ( 18, ( 22
45861
4.57-26.83
(11, ( 19, ( 23
33072
3.99-25.30
(10, ( 14, ( 25
15075
4.71-25.30
(9, ( 10, ( 21
11610
2.78-25.61
(11, ( 19, ( 23
52425
2.89-25.34
(9, ( 30, ( 15
34614
4500/0.029
5282/0.039
3695/0.068
3471/0.044
2881/0.025
4615/0.031
no. of obsd
rflns (I > 2σ(I))
no. of data/
restraints/params
R1/wR2
4049
4895
3207
3016
2551
4317
4500/0/265
0.0276/0.0623
0.0327/0.0669
5282/0/266
0.0312/0.0769
0.0347/0.0785
3695/0/198
0.0241/0.0655
0.0293/0.0664
3471/0/199
0.0215/0.0477
0.0281/0.0489
2881/0/219
0.0353/0.0782
0.0419/0.0828
4615/0/280
0.0272/0.0661
0.0298/0.0687
(I > 2σ(I))a
R1/wR2
(all data)a
GOF (on F2)a
largest diff
peak and hole (e Å-3
a
1.153
+0.59/-0.60
1.080
+0.87/-0.64
1.011
+0.49/-0.66
0.948
+0.37/-0.34
1.158
+1.28/-0.69
1.080
+0.44/-0.32
)
R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo - Fc2)2]/∑w(Fo )2}1/2; GOF ) {∑[w(Fo - Fc2)2]/(n - p)}1/2
.
2
2
2
was perfomed with a Stoe device (IPDS 2T, rotating anode, Nonius
FR591) at T ) 173 K. Intensities were corrected for absorption
using the DELABS procedure (Tmax ) 0.819, Tmin ) 0.422). Methyl
hydrogen atoms were calculated as a part of rigid rotating groups,
with dC-H ) 0.98 Å and Uiso(H) ) 1.5Ueq(C). All other hydrogen
atoms were placed in ideal positions and refined using a riding
model, with methylene and methyne dC-H distances of (0.99 and
1.00 Å) and Uiso(H) ) 1.2Ueq(C). In contrast the amine hydrogen
atom could be located in the final difference Fourier map and was
allowed to refine freely. The compounds 1Cl and 1Me appear to be
isostructural. 2Cl: Data collection was perfomed with a Stoe device
(IPDS 2T, rotating anode, Nonius FR591) at T ) 173 K. Intensities
were corrected for absorption (numerical after crystal shape
optimization, Tmax ) 0.9149, Tmin ) 0.7087). Methyl hydrogen
atoms were calculated as a part of rigid rotating groups, with dC-H
) 0.98 Å and Uiso(H) ) 1.5Ueq(C). All other hydrogen atoms were
placed in ideal positions and refined using a riding model, with
methylene and methyne dC-H distances of (0.99 and 1.00 Å) and
Uiso(H) ) 1.2Ueq(C). 2Me: Data collection was perfomed with a Stoe
device (IPDS 2T, rotating anode, Nonius FR591) at T ) 173 K.
Intensities were corrected for absorption (numerical after crystal
shape optimization, Tmax ) 0.9215, Tmin ) 0.7806). Methyl
hydrogen atoms were calculated as a part of rigid rotating groups,
with dC-H ) 0.98 Å and Uiso(H) ) 1.5Ueq(C). All other hydrogen
atoms were placed in ideal positions and refined using a riding
model, with methylene and methyne dC-H distances of (0.99 and
1.00 Å) and Uiso(H) ) 1.2Ueq(C). 3CNtBu: Data collection was
performed with an Oxford Diffraction device (Xcalibur, κ-CCD;
sealed tube, Enhance X-ray Source, Spellman, DF3) at T ) 153
K. Intensities were corrected for absorption effects during the
scaling procedure (Tmax ) 0.8100, Tmin ) 0.5537). Methyl hydrogen
atoms were calculated as a part of rigid rotating groups, with dC-H
) 0.98 Å and Uiso(H) ) 1.5Ueq(C). All other hydrogen atoms were
placed in ideal positions and refined using a riding model, with
methylene and methyne dC-H distances of (0.99 and 1.00 Å) and
Uiso(H) ) 1.2Ueq(C). A disorder of the PF6-anion could be resolved
clearly. 3CNtBu is sited on a crystallographic mirror plane passing
N1, Pd1, N2, C18, and C20 with a symmetry code for equivalent
atoms of x, y, 1/2 - z. 5: Data collection was perfomed with a
Bruker Axs device (APEX II, rotating anode, Bruker Axs FR591)
at T ) 293 K. Intensities were corrected for absorption effects
during the scaling procedure (Tmax ) 0.7750, Tmin ) 0.7312). Methyl
hydrogen atoms were calculated as a part of rigid rotating groups,
with dC-H ) 0.96 Å and Uiso(H) ) 1.5Ueq(C). All other hydrogen
atoms were placed in ideal positions and refined using a riding
model, with methylene and methyne dC-H distances of (0.97 and
0.98 Å) and Uiso(H) ) 1.2Ueq(C)
.
Density Functional Theory (DFT) Calculations. DFT calcula-
tions were performed with GAUSSIAN 03 using the PBE func-
tional.26,27 Geometry optimizations were run without symmetry or
internal coordinate constraints using the Stuttgart-RSC-ECP and
corresponding valence basis set for palladium and all-electron split
valence double-ꢂ basis set 6-31+G** for all other elements.28,29
The optimized structures were verified as being true minima by
the absence of negative eigenvalues in the vibrational frequency
analysis. Hirshfeld charges were obtained by single-point calcula-
tions on the geometry optimized minimum structures using the all-
electron DGDZVP basis-set for palladium.30,31 Orbital expressions
were visualized with GaussView via cube files generated from
formatted checkpoint files.32 The orbital energies were obtained
from the Mulliken population analysis.
Results and Discussion
Syntheses. Reaction of HN(CH2CH2PiPr2)2 (PNPH) with
[PdCl2(NCMe)2] and subsequent chloride abstraction with
AgPF6 gives palladium(II) amino complex [PdCl(PNPH)]PF6
(1Cl) in quantitative yield (Scheme 1). Methyl complex
Inorganic Chemistry, Vol. 48, No. 8, 2009 3703

Marziale et al.
Facile deprotonation of the amino complexes with KOtBu
gives the corresponding neutral palladium(II) amides [PdR(P-
NP)] (2R; R ) Cl, Me, Ph), which are well soluble in non-
polar solvents such as pentane. Alternatively, 2Me can be
prepared from 2Cl with MeMgBr in THF, albeit in lower
yield. Owing to the chelating pincer ligand, the highly air
sensitive amido complexes are stable in the solid state and
in solution toward ꢀ-hydride elimination.
Scheme 1. Syntheses of Palladium(II) PNP Amino and Amido
Complexes 1-3 (PNPH ) HN(CH2CH2PiPr2)2; R ) Me, Ph; L )
CNtBu, PMe3)
Clean chloride abstraction from amide 2Cl was ac-
complished with TlPF6. In the presence of CNtBu or PMe3
the cationic amido complexes [PdL(PNP)]PF6 (3L; L )
CNtBu, PMe3) are obtained in moderate (3PMe3) to good
yields (3CNtBu), while the corresponding carbonyl complex
could not be isolated under CO atmosphere (1 bar). However,
the synthesis of cationic PdII carbonyl complexes typically
requires the use of highly non-nucleophilic anions and
handling at low temperatures because of labile binding of
CO to electrophilic palladium centers.33
In contrast to the halide abstraction with TlPF6, the
addition of silver salts to amido complexes 2R at room
temperature results in immediate precipitation of silver metal.
While the reaction of 2Cl with AgPF6 gives several products
by 31P NMR, including amino complex 1Cl, oxidation of the
organic derivatives 2R (R ) Me, Ph) proceeds much more
selective. 2Ph reacts with AgPF6 in THF to give around 80%
of the amino complex 1Ph and one other product (4)
accounting for the remaining yield (Scheme 2). While 4 could
not be obtained analytically pure, the NMR spectra are in
agreement with imino complex [PdPh{N(CHCH2PiPr2)-
(CH2CH2PiPr2)}]PF6, resulting from dehydrogenation of the
PNP pincer in R-position to the nitrogen donor.34 The
presence of the two products suggests that the radical cation
[PdR(PNP)]+• is formed as the initial oxidation product.
Decomposition pathways via hydrogen atom addition and
hydrogen abstraction (Scheme 2) could then account for the
formation of 1Ph and 4, respectively. When the reaction is
carried out in d8-THF, the product distribution of 1Ph/4 is
retained (1:0.25 by 31P NMR). However, the signal for the
1Ph N-H proton (4.55 ppm) is strongly diminished, now
integrating over 0.2 protons, and deuteration of the amino
[PdMe(PNPH)]PF6 (1Me) is easily prepared from 1Cl by salt
metathesis with MeMgBr, while reaction with LiMe yields
intractable mixtures of several products. Likewise, the
reaction of 1Cl with phenyl Grignard-solution suffers from
low selectivity. However, phenyl complex [PdPh(PNPH)]PF6
(1Ph) was isolated in modest yields starting from [Pd(I)Ph(T-
MEDA)] (TMEDA ) Me2NCH2CH2NMe2).
(26) Frisch, M. J. Trucks, G. W. Schlegel, H. B. Scuseria, G. E. Robb, M.
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Burant, J. C. Millam, J. M. Iyengar, S. S. Tomasi, J. Barone, V.
Mennucci, B. Cossi, M. Scalmani, G. Rega, N. Petersson, G. A.
Nakatsuji, H. Hada, M. Ehara, M. Toyota, K. Fukuda, R. Hasegawa,
J. Ishida, M. Nakajima, T. Honda, Y. Kitao, O. Nakai, H. Klene, M.
Li, X. Knox, J. E. Hratchian, H. P. Cross, J. B. Bakken, V. Adamo,
C. Jaramillo, J. Gomperts, R. Stratmann, R. E. Yazyev, O. Austin, A.
J. Cammi, R. Pomelli, C. Ochterski, J. W. Ayala, P. Y. Morokuma,
K. Voth, G. A. Salvador, P. Dannenberg, J. J. Zakrzewski, V. G.
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Rabuck, A. D. Raghavachari, K. Foresman, J. B. Ortiz, J. V. Cui, Q.
Baboul, A. G. Clifford, S. Cioslowski, J. Stefanov, B. B. Liu, G.
Liashenko, A. Piskorz, P. Komaromi, I. Martin, R. L. Fox, D. J. Keith,
T. Al-Laham, M. A. Peng, C. Y. Nanayakkara, A. Challacombe, M.
Gill, P. M. W. Johnson, B. Chen, W. Wong, M. W. Gonzalez, C.
Pople, J. A. Gaussian03, Rev. C.02; Gaussian Inc.: Wallingford, CT,
2004.
2
function was confirmed by H NMR spectroscopy (δ N-D
) 4.20 ppm).35 These results are in agreement with the
formation of [PdPh(PNPD)]+ (∼60%) by deuterium atom
transfer from the solvent, and recombination of two [PdR(P-
(31) (a) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can.
J. Chem. 1992, 70, 560–571. (b) Sosa, C.; Andzelm, J.; Elkin, B. C.;
Wimmer, E.; Dobbs, K. D.; Dixon, D. A. J. Phys. Chem. 1992, 96,
6630–6636.
(32) Dennington, R., II.; Keith, T.; Millam, J. GaussView V4.1; Semichem
Inc.: Shawnee Mission, KS, 2007.
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Can. J. Chem. 1993, 71, 1532–1536. (b) Weber, L. Angew. Chem.
1994, 106, 1131–1133; Angew. Chem., Int. Ed. Engl. 1994, 33,
1077-1078. (c) Rix, F. C.; Brookhart, M.; White, P. S. J. Am. Chem.
Soc. 1996, 118, 4746–4764. (d) Ara, I.; Fornie´s, J.; Navarro, R.; Sicilia,
V.; Urriolabeitia, E. P. Polyhedron 1997, 16, 1963–1970. (e) Popeney,
C.; Guan, Z. Organometallics 2005, 24, 1145–1155. (f) Burns, C. T.;
Jordan, R. F. Organometallics 2007, 26, 6737–6749.
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A. R.; Rees, L. H. J. Chem. Soc., Dalton Trans. 2001, 3384–3395.
(35) Some deuteration was also obtained for the iso-propyl methyne protons
of the pincer ligand by 2H NMR. We tentatively attribute this
observation to H-/D-exchange with THF radicals.
(27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865–3868.
(28) (a) Andrae, D.; Ha¨uꢀermann, U.; Dolg, M.; Stoll, H.; Preuꢀ, H. Theor.
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2257–2261. (b) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley,
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3704 Inorganic Chemistry, Vol. 48, No. 8, 2009

Palladium (NCH2CH2PiPr2)2-Dialkylamides
Scheme 2. Oxidation of 2Ph with AgPF6 in THF and Proposed Pathway of Product Formation via a Radical Cation Intermediate
Scheme 3. Reaction of 2Cl with MeOTf
For example, iridium(I) amido complex [Ir(CO)(PNP)]
features a carbonyl stretching frequency at particularly low
wavenumbers (1908 cm-1) which can be explained by strong
IrfCO back bonding reinforced by a pN-dIr-π*CO push-pull
π-interaction.15,19 As the palladium analogue [Pd(CO)(P-
NP)]+ could not be prepared, 3CNtBu was synthesized. Ct N
stretching vibrations of palladium(II) tert-butylisocyanide
complexes are typically found at about 50 to 100 cm-1 higher
wavenumbers with respect to the free ligand (2136 cm-1),
and cationic complexes are located at the upper end of that
range.41,42 Therefore, the frequency of the IR band of 3CNtBu
assignable to the Ct N stretch (2186 cm-1) suggests strong
electron donation by the dialkylamido ligand, for example,
as compared with trans-[Pd(C6F5)(CNtBu)(PEt3)2]ClO4 (2230
cm-1). However, the molecular structure of 3CNtBu in the solid
state, which features a pyramidal amido nitrogen atom (see
below), is indicative of predominant NfPd σ-donation. Note
that dialkylamines (pKadmso ∼ 44) and C6F5H (pKadmso ) 29.0)
exhibit a large difference in proton acidity as well.43,44
Solution NMR spectra of amino complexes 1R (R ) Cl,
Me, Ph) and 5, featuring one 31P NMR signal and two sets
of chemically inequivalent iso-propyl substituents with
diastereotopic methyl groups (1H and 13C NMR), are in
agreement with a square planar coordination geometry around
NP)]+• radicals accounting for an equimolar amount of 4
(∼20%) and [PdPh(PNPH)]+ (∼20%). Nitrogen centered
aminyl radicals are typically transient species, representing
pivotal intermediates in radical based C-N coupling reac-
tions,36 and only few persistent late metal complexes that
were described as metal stabilized aminyl radicals have been
reported.37 However, proton coupled electron transfer (PCET)
reactions of metal complexes have attracted considerable
interest, owing to their importance in various chemical and
biological processes,38 and the oxidative dehydrogenation
of amines coordinated to transition metals has been studied
extensively.39
The reactivity of 2Cl toward electrophiles was probed by
the reaction with methyl triflate. Quantitative conversion to
N-methyl amino complex [PdCl(PNPMe)]OTf (5; PNPMe
)
MeN(CH2CH2PiPr2)2) is observed (Scheme 3), as can be
expected for a highly nucleophilic amido ligand.40
Spectroscopic Characterization. Palladium(II) amino
complexes 1R (R ) Cl, Me, Ph) exhibit N-H stretching
vibrations between 3239-3277 cm-1 in the IR spectra, very
close to the free ligand (3285 cm-1), suggesting the absence
of N-H hydrogen bonding. The IR stretching vibrations of
strong π-acceptor ligands, such as CO or isocyanides, can
serve as a probe for the electron density at the metal center.
1
the Pd center and a pyramidal nitrogen atom. The 31P, H,
and 13C NMR spectra of amido complexes 2R (R ) Cl, Me,
Ph) and 3L (L ) PMe3, CNtBu) indicate C2V symmetry on
the NMR time scale with a mirror plane defined by the
coordination plane of palladium, as was found for isoelec-
(41) Treichel, P. M. AdV. Organomet. Chem. 1973, 11, 21–86.
(42) (a) Otsuka, S.; Nakamura, A.; Yoshida, T. J. Am. Chem. Soc. 1969,
91, 7196–7198. (b) Otsuka, S.; Tatsuno, K.; Ataka, K. J. Am. Chem.
Soc. 1971, 93, 6705–6706. (c) Empsall, H. D.; Green, M.; Shakshooki,
S. K.; Stone, F. G. A. J. Chem. Soc. (A) 1971, 3472–3476. (d) Uson,
R.; Fornies, J.; Espinet, P.; Lalinde, E. J. Organomet. Chem. 1981,
220, 393–399. (e) Uson, R.; Fornies, J.; Espinet, P.; Garcia, A.; Sanau,
A. Transition Met. Chem. 1983, 8, 11–13. (f) Langrick, C. R.; Pringle,
P. G.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1984, 1233–1238.
(g) De Munno, G.; Bruno, G.; Arena, C. G.; Drommi, D.; Faraone, F.
J. Organomet. Chem. 1993, 450, 263–267. (h) Ruiz, J.; Martinez,
M. T.; Vincente, C.; Garcia, G.; Lopez, G.; Chaloner, P. A.; Hitchcock,
P. B. Organometallics 1993, 12, 4321–4325. (i) Veya, P.; Floriani,
C.; Chiesi-Villa, A.; Rizzoli, C. Organometallics 1994, 13, 441–450.
(j) Samar, D.; Fortin, J.-F.; Fortin, D.; Decken, A.; Harvey, P. D.
J. Inorg. Organomet. Polym. Mater. 2005, 15, 411–429. (k) Cadierno,
V.; Diez, J.; Garcia-Alvarez, J.; Gimeno, J.; Nebra, N.; Rubio-Garcia,
J. Dalton Trans. 2006, 5593–5604.
(36) (a) Bowman, W. R.; Bridge, C. F.; Brookes, P. J. Chem. Soc., Perkin
Trans. 1 2000, 1–14. (b) Bowman, W. R.; Cloonan, M. O.; Krintel,
S. L. J. Chem. Soc., Perkin Trans. 1 2001, 2885–2902.
(37) (a) Penkert, F. N.; Weyhermu¨ller, T.; Bill, E.; Hildebrandt, P.; Lecomte,
S.; Wieghardt, K. J. Am. Chem. Soc. 2000, 122, 9663–9673. (b)
Bu¨ttner, T.; Geier, J.; Frison, G.; Harmer, J.; Calle, C.; Schweiger,
A.; Scho¨nberg, H.; Gru¨tzmacher, H. Science 2005, 307, 235–238. (c)
Adhikari, D.; Mossin, S.; Basuli, F.; Huffmann, J. C.; Szilagyi, R. K.;
Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676–3682.
(d) Tejel, C.; Ciriano, M. A.; Pilar del Rio, M.; Hetterscheid, D. G. H.;
Tsichlis i Spithas, N.; Smits, J. M. M.; de Bruin, B. Chem.sEur. J.
2008, 14, 10932–10936.
(38) (a) Cukier, R. I.; Nocera, D. G. Annu. ReV. Phys. Chem. 1998, 49,
337–369. (b) Hammes-Schiffer, S. Acc. Chem. ReV. 2001, 34, 273–
281. (c) Meyer, J. M. Annu. ReV. Phys. Chem. 2004, 55, 363–390. (d)
Huynh, M. H. V.; Meyer, T. J. Chem. ReV. 2007, 107, 5004–5064.
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W. Coord. Chem. ReV. 2003, 245, 95–106.
(43) pKa values of dialkylamines in dmso are typically beyond that of dmso
(35.1) and were extrapolated: ref 24a.
(44) Predicted from quantum chemical calculations, Shen, K.; Fu, Y.; Li,
J.-N.; Liu, L.; Guo, Q.-X Tetrahedron 2007, 63, 1568–1576.
(40) Park, S.; Rheingold, A. L.; Roundhill, D. M. Organometallics 1991,
10, 615–623.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3705

Marziale et al.
Scheme 4. Acid-Base Equilibrium Utilized for the Determination of
the 1R (R ) Me, Ph) pKa Values
Me, Ph) range between those of free dialkylamines (∼44)
and dialkylammonium cations (∼10).51 Low proton acidity
(pKa 31.5 in THF) was reported by Bergman and co-workers
for octahedral ruthenium(II) ammonia complex [RuH(NH3)-
(dmpe)2]+ (dmpe ) Me2PCH2CH2PMe2).52 Gru¨tzmacher’s
group has developed a family of dialkylamino and diamino
ligands with benzanellated biscycloheptatrienyl substituents,
which form stable group 9 amido complexes. Despite the
lower metal oxidation state, four-coordinate [Rh(trop2dach)]-
OTf(pKa )15.7)andfive-coordinate[Rh(HNtrop2)(H2Ntrop)]-
OTf (pKa ) 20.6) and [Rh(HNtrop2)(bipy)]OTf (pKa ) 18.7)
are considerably more acidic than 1R in dmso (HNtrop2 )
Bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine,H2Ntrop)5-amino-
5H-dibenzo[a,d]cycloheptene, trop2dach ) N,N′-bis(5H-
dibenzo[a,d]cyclohepten-5-yl)-1,2-diaminocyclohexane, bipy
) 2,2′-bipyridine).53-55 Likewise, iridium(I) complexes
[Ir(L)(PNPH)]PF6 (L ) CO: 14.8; cyclooctene: 16.0) exhibit
much lower pKa values as compared with 1R.45 While the
small amount of pKa data in non-protic solvents prevents a
systematic analysis, regarding the influence of oxidation state,
coordination number, and auxiliary ligands, it is noteworthy
that the palladium(II) complexes 2R are the most basic as
compared with a series of isoelectronic group 9 dialkylamido
complexes.
tronic iridium compounds [Ir(L)(PNP)] (L ) CO, C2H4,
PMe3).19,45 While the iridium amides feature almost perfectly
planarized nitrogen atoms in the solid state, the molecular
structures of palladium complexes 2R show strong pyrami-
dalization of the N-donor (see below). However, cooling a
sample of 2Cl in d8-toluene did not result in peak broadening
1
in the H NMR spectrum down to -40 °C, suggesting low
barriers for nitrogen inversion, as typically found for organic
amines with electropositive substituents and metal dialky-
lamides.46,47
pKa Measurements. pKa values of the amino complexes
were determined in d6-dmso to ensure efficient ion separa-
tion.48 The equilibrium constants from the reaction of 2R (R
) Me, Ph) with a suitable acid (Scheme 4, Href) were
derived by NMR and pKa values calculated by referencing
to the pKa of Href. The right reference was found by
screening a range of acids with known pKa’s in dmso.49 For
1
example, 2Me showed no reaction (31P and H NMR) with
equimolar amounts of p-toluidine (pKa > 30) or 2-aminopy-
ridine (pKa ) 27.7), minor protonation with diphenylamine
(pKa ) 25.0), almost complete conversion to 1Me with pyrrole
(pKa ) 23.0), and complete protonation with indene (pKa )
20.1). Quantitative derivations of the pKa’s were carried out
with 2-pyrrolidinone (2Me; pKa ) 24.2) and pyrrole (2Ph),
respectively. The pKa of 1Cl could not be obtained in this
way as 2Cl is not stable in dmso, presumably because of
halide versus solvent exchange. For the alkyl and aryl
complexes pKa values of 24.2(1) (1Me) and 23.2(1) (1Ph) were
derived indicating only a minor influence of the ligands in
trans-position, that is, the purely σ-donating methyl or the
potentially π-accepting phenyl ligands.
The structural analysis of the amino and amido complexes
suggests, that the flexible PNPH and PNP ligands are
relatively unstrained (see below). Therefore, we believe that
these values can serve as a good general estimate for the
acidity of dialkylamines coordinated to palladium(II). To the
best of our knowledge this is the first time that pKa values
of palladium alkylamino complexes were measured.
It is well established that metal coordination acidifies
amine protons.40,50 Accordingly, the pKa values of 1R (R )
X-ray Structure Determinations. The molecular struc-
tures of 1Cl, 1Me, 2Cl, 2Me, 3CNtBu, and 5 in the crystal were
derived by X-ray crystallography (Figures 1 and 2, Table
2). 2Cl, 2Me, and 3CNtBu are the first structurally characterized
palladium dialkylamido complexes. All structures feature the
palladium centers in a distorted square-planar coordination
geometrywithpincerbiteanglesintherangeof165.8°-170.2°.
P-Ir-P angles found for [Ir(L)(PNP)H]PF6 and [Ir(L)(PNP)]
(L ) CO, C2H4, PMe3) were in the same range.19,45 The
structural parameters of the Pd(PNPH) fragments in 1Cl and
1Me are almost identical except for the significantly longer
Pd-N1 distance of 1Me (∆D ) 0.08 Å) reflecting the stronger
methyl trans-influence. This observation confirms the struc-
tural flexibility of the (PNP)H pincer which can easily adjust
to accommodate different bonding situations around the metal
center, particularly in comparison with more rigid PNP
pincers, such as Ozerov’s phenylene bridged chelate
ligands.13 Therefore, the chelating nature of the (PNP)H
ligand should have a minor effect on chemical properties of
the amino moiety, such as pKa values, and 1R represent
reasonable general models for dialkylamines coordinated to
palladium(II). Furthermore, the N-methylated amino complex
5 exhibits only minor structural differences as compared to
N-H analogue 1Cl.
(45) Friedrich, A.; Buchner, M. R.; Herdtweck, E.; Schneider, S., manuscript
in preparation.
(46) Stevenson, P. E.; Burkey, D. L. J. Am. Chem. Soc. 1974, 96, 3061–
3064.
(47) Dewey, M. A.; Bakke, J. M.; Gladysz, J. A. Organometallics 1990,
9, 1349–1351.
(48) (a) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456–463. (b) Abdur-
Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman, J. G.;
Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2000,
122, 9155–9171.
(51) Liu, W.-Z.; Bordwell, F. G. J. Org. Chem. 1996, 61, 4778–4783.
(52) The pKa value was not corrected for ion pairing effects, Fulton, J. R.;
Bouwkamp, M. W.; Bergman, R. G. J. Am. Chem. Soc. 2000, 122,
8799–8800.
(53) Maire, P.; Breher, F.; Scho¨nberg, H.; Gru¨tzmacher, H. Organometallics
2005, 24, 3207–3218.
(49) For an extensive compilation of acidities in dmso, cf. http://
www.chem.wisc.edu/areas/reich/pkatable/ (accessed Nov 25, 2008).
(50) (a) Park, S.; Roundhill, D. M.; Rheingold, A. L. Inorg. Chem. 1987,
26, 3972–3974. (b) Joslin, F. L.; Johnson, M. P.; Mague, J. T.;
Roundhill, D. M. Organometallics 1991, 10, 2781–2794.
(54) Bu¨ttner, T.; Breher, F.; Gru¨tzmacher, H. Chem. Commun. 2004, 2820–
2821.
(55) Ko¨nigsmann, M.; Donati, N.; Stein, D.; Scho¨nberg, H.; Harmer, J.;
Sreekanth, A.; Gru¨tzmacher, H. Angew. Chem. 2007, 119, 3637–3640.
Angew. Chem. Int. Ed. 2007, 46, 3567-3570.
3706 Inorganic Chemistry, Vol. 48, No. 8, 2009

Palladium (NCH2CH2PiPr2)2-Dialkylamides
Figure 1. DIAMOND plot of the cations of 1Cl (above), 1Me (middle), and
5 (below) in the crystal with thermal ellipsoids drawn at the 50% (1Cl and
1
Me) and 40% (5) probability levels, respectively. Hydrogen atoms other
than amino protons are omitted for clarity.
Deprotonation of the amino ligand results in shorter
covalent Pd-N bonds (∆DPd-N(1Cl/2Cl) ) 0.03 Å; ∆DPd-N(1Me/
2
Figure 2. DIAMOND plot of 2Cl (above), 2Me (middle), and the cation of
3CNtBu (below) in the crystal with thermal ellipsoids drawn at the 50%
probability level. Hydrogen atoms are omitted for clarity.
Me) ) 0.07 Å), and the shortest Pd-N bond was found for
cationic 3CNtBu. The Pd-N bond distances (2Cl: 2.033(2) Å,
2Me: 2.075(2) Å, 3CNtBu: 2.019(3) Å) compare well with those
reported for palladium(II) arylamido and silylamido com-
plexes, which typically range around 2.03-2.09 Å.10b,12,13a,56
Therefore, the Pd1-N1 distances in 2Cl, 2Me, and 3CNtBu
provide no evidence for a stronger N-Pd π-interaction
because of the higher basicity of dialkylamines. Furthermore,
amine deprotonation results in an elongation of the bond in
trans position to the nitrogen atom (∆DPd-Cl(1Cl/2Cl) ) 0.07
Å; ∆DPd-C(1Me/2Me) ) 0.04 Å), resulting from an increased
amido ligand trans-influence.
343.2°). This observation contrasts sharply with molecular
structures of palladium complexes bearing non-bridging aryl-
or silylamido ligands, which generally feature planar amido
donor atoms.57,58 Amido complexes with pyramidal nitrogen
atoms are particularly rare. Few examples of octahedral
rhenium and ruthenium d6 complexes have been reported.59
Since in these complexes no empty orbital on the metal center
(57) Out of the structurally characterized palladium complexes LnPd-NRR′
with terminal aryl- and silylamido ligands (total 311 hits on the CSD
database), most complexes exhibit Pd-N-R-R′ torsional angles θ
between 160°-180°, which is considerably higher as compared with
2Cl (θ ) 129.0°), 2Me (θ ) 138.9°), and 3CNtBu (θ ) 131.8°). Nine
complexes were found with 150° < θ < 160°,12,56c,58a-e and only one
with θ ) 126.6°.58f
(58) (a) Kim, Y.-J.; Choi, J.-C.; Osakada, K. J. Organomet. Chem. 1995,
491, 97–102. (b) Braunstein, P.; Naud, F.; Rettig, S. J. New J. Chem.
2001, 25, 32–39. (c) Coleman, K. S.; Green, M. L. H.; Pascu, S. I.;
Rees, N. H.; Cowley, A. R.; Rees, L. H. J. Chem. Soc., Dalton Trans.
2001, 3384–3395. (d) Pascu, S. I.; Anderson, G. D. W.; Green,
M. H. L.; Green, J. C.; Rees, N. H.; Cowley, A. R. Inorg. Chim. Acta
2006, 359, 3677–3692. (e) Michon, C.; Ellern, A.; Angelici, R. J. Inorg.
Chim. Acta 2006, 359, 4549–4556. (f) Angurell, I.; Mart´ınez-Ruiz,
I.; Rossell, O.; Seco, M.; Go´mez-Sal, P.; Mart´ın, A.; Font-Bardia, M.;
Solans, X. J. Organomet. Chem. 2007, 692, 3882–3891.
Most significantly, upon deprotonation of the amino ligand
the PNP pincer conformation is retained resulting in pyra-
midal amido nitrogen atoms, as evidenced by the sum of
bond angles around N1 (2Cl: 337.4°, 2Me: 345.7°, 3CNtBu
:
(56) Examples:(a) Coleman, K. S.; Green, M. L. H.; Pascu, S. I.; Rees,
N. H.; Cowley, A. R.; Rees, L. H. J. Chem. Soc., Dalton Trans. 2001,
338, 4–3395. (b) Santra, P. K.; Byabartta, P.; Chattopadhyay, S.;
Falvello, L. R.; Sinha, C. Eur. J. Inorg. Chem. 2002, 112, 4–1131.
(c) Hooper, M. W.; Hartwig, J. F. Organometallics 2003, 22, 3394–
3403. (d) Huang, M.-H.; Liang, L.-C. Organometallics 2004, 23, 2813–
2816. (e) Liang, L.-C.; Chien, P.-S.; Huang, M.-H. Organometallics
2005, 24, 353–357.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3707

Marziale et al.
Table 2. Selected Bond Lengths and Angles of the Cations of 1Cl, 1Me, and 5 and of 2Cl, 2Me, 3CNtBu in the Crystal and of the [PdCl(PNPH)]+ (1Cl
)
calc
and [PdCl(PNP)] (2Clcalc) DFT models (PBE/6-31+G**)a
1Cl
1Cl
1Me
2Cl
2Cl
2Me
3CNtBu
5
calc
calc
Bond Lengths (Å)
2.033(2)
Pd-N1
Pd-P1
Pd-P2
Pd-R
2.064(2)
2.100
2.326
2.334
2.296
2.146(2)
2.2960(7)
2.2970(7)
2.075(3)c
2.038
2.308
2.310
2.373
2.075(2)
2.2711(6)
2.2729(7)
2.109(2)c
2.019(3)
2.089(2)
2.3108(7)
2.3060(7)
2.3045(6)
2.2988(6)
2.2898(8)b
2.2927(7)
2.2923(6)
2.2923(6)d
1.956(5)e
2.2886(7)
2.3553(7)b
2.2925(8)b
Bond Angles (deg)
168.71(2)
176.12(5)b
85.09(6)
P1-Pd-P2
N1-Pd-R
N1-Pd-P1
N1-Pd-P2
C2-N1-C4
C2-N1-Pd
C4-N1-Pd
170.15(2)
179.39(7)b
84.84(6)
85.35(6)
111.2(2)
113.4(2)
113.5(2)
170.7
178.8
85.3
169.23(3)
179.4(1)c
84.73(7)
84.52(7)
112.2(2)
111.9(2)
112.5(2)
168.0
177.5
84.5
165.82(2)
171.8(1)c
83.82(6)
83.73(6)
109.8(2)
118.2(2)
117.7(2)
168.55(3)f
177.2(2)e
84.31(2)
84.31(2)g
108.2(3)h
115.7(2)
115.7(2)i
168.77(3)
173.87(7)b
85.92(6)
85.78(6)
107.8(2)
111.3(2)
110.6(2)
85.8
83.62(6)
108.5(2)
114.7(2)
114.1(2)
83.7
112.8
111.6
113.1
109.9
115.6
114.3
a 3CNtBu is sited on a crystallographic mirror plane with a symmetry code for equivalent atoms _a: x, y, 1/2 - z. b R ) Cl. c R ) CH3. d Pd-P1_a; R )
CNtBu. e Pd-P1_a; R ) CNtBu. f P1-Pd-P1_a. g N1-Pd-P1_a. h C2-N1-C2_a. i C2_a -N1-Pd.
is available for NfM π-donation, pyramidalization of the
amido moiety is a consequence of minimizing repulsive
filled-filled pπ-dπ interactions, resulting in a M-N single
bond.15 A similar bonding picture should arise in square-
planar d8 and in d10 amido complexes, but higher bond orders
can be expected in three-coordinate d8 amido complexes
because of NfM π-bonding with an empty in-plane metal
d-orbital.14d,60 While palladium(II) complexes with strongly
π-basic dialkylamido ligands were not structurally character-
ized prior to this work, isoelectronic four-coordinate gold(III)
dialkylamides in fact exhibit pyramidal nitrogen atoms.61 The
comparison with [IrL(PNP)] (L ) CO, C2H4, PMe3), reveals
a much higher degree of planarization of the dialkylamido
nitrogen atom bound to iridium(I) (Σangles around N ) 355.7°
(L ) CO); 356.7° (L ) C2H4); 353.0° (L ) PMe3)).19,45
This observation can be rationalized by delocalization of
Figure 3. Relative frontier orbital energies of 2Cl from DFT calculations
electron density into π*-orbitals of the strong (CO, C2H4)
or moderate (PMe3) π-acceptor ligands bound to iridium in
(PBE/6-31+G**) with occupied (solid lines) and unoccupied (dashed lines)
orbitals. A contour plot of the HOMO and schematic representations of Cl,
trans-position to the amido moiety, resulting in a stabilizing
push-pull-type π-interaction.15 Although 3CNtBu bears a
potentially strong π-acceptor in trans-position to the amido
ligand, a pyramidal nitrogen atom is observed, comparable
to the complexes with a pure σ-donor (2Me: CH3) or a σ-
and π-donor ligand (2Cl: Cl) in this position, respectively.
Therefore, the molecular structures of 2Cl, 2Me, and 3CNtBu
suggest that N-M π-interactions are generally strongly
reduced for PdII amides as compared with isoelectronic group
9 metal centers.
DFT Calculations. DFT models of [PdCl(PNP)] and
[PdCl(PNPH)]+ were calculated to gain further insights into
the nature of the Pd-Namido bond. The calculated geometries
are in good agreement with the experimentally derived
molecular structures in the solid state (Table 2), including
Pd, and N contributions are depicted in the box.
pyramidalization of the 2Cl nitrogen atom (Σangles around N
)
339.8 °). The frontier orbital scheme of complex 2Cl exhibits
a particularly high lying highest occupied molecular orbital
(HOMO, Figure 3, MO 102) with high nitrogen pz character
2
and small contributions from palladium (4dz , 5s), chlorine
(py), and the axial NCH2 hydrogen atoms. Hirshfeld charges
of the nitrogen (-0.21e), palladium (0.25e), and chlorine
atoms (-0.43e) in 2Cl are reduced to -0.05e (N), 0.28e (Pd),
and -0.33e (Cl) in 1Cl with a charge of 0.12e found for the
amino proton. Consequently, protonation of the nitrogen atom
results in overall charge transfer to the proton (∆q ) -0.88e)
from N (∆q ) 0.16e) and Cl (∆q ) 0.10e) rather than from
palladium (∆q ) 0.03e), confirming a high concentration of
charge at the nitrogen atom in the amido complexes.
Therefore, our results explain the nitrogen centered reactivity
of 2Cl, for example, with Brønstedt acids, nucleophiles, and
oxidizing agents. While the repulsive π-interactions are
avoided by N-pyramidalization it is evident that efficient
charge transfer is accomplished via the σ-bonding framework
between the two trans-ligands N and Cl. This picture strongly
resembles the bonding model that was proposed by Pud-
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Boyle, P. D.; White, P. S. J. Am. Chem. Soc. 2003, 125, 9435–9441.
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Inorg. Chem. 2007, 46, 1361–1368.
3708 Inorganic Chemistry, Vol. 48, No. 8, 2009

Palladium (NCH2CH2PiPr2)2-Dialkylamides
dephattandco-workersforAuIII amidocomplex[AuCl{N(CH2-
Available kinetic data reveal, that N-Caryl reductive
elimination rates from palladium(II) are higher for electron-
poor aryls and more basic nitrogen centers.1,6,7 Hence,
Hartwig proposed, that although the reason for the obserVed
effect is likely to be complex, one can consider that this trend
could result from a faVorable pairing of a nucleophilic
nitrogen ligand and an electrophilic aryl group.1i Regarding
C-N reductive elimination as intramolecular nucleophilic
attack of the amido ligand at the aryl ipso carbon suggests
that a general reluctance of palladium(II) for π-bonding could
be prerequisite for its predominant role in catalytic C-N
coupling reactions.
o-C5H4N)}]+.61c
Conclusions
We have reported the synthesis and first structural
characterization of palladium(II) dialkylamido complexes.
The stability of 2R and 3L toward ꢀ-hydrogen elimination
can be attributed to the chelating effect of the PNP pincer
ligand, preventing phosphine dissociation. The molecular
structures of the amido complexes, which feature pyramidal
nitrogen atoms, and the DFT calculations of 2Cl indicate
repulsive Pd-Namido π-interactions, determining the nitrogen-
centered reactivity with electrophiles and oxidizing agents.
Unlike for isoelectronic iridium(I) amido complexes, even
with a potentially strong π-acceptor in trans-position (CNtBu)
N-pyramidalization indicates a lack of stabilizing push-pull
Acknowledgment. The authors are grateful to Prof. W. A.
Herrmann for generous support. We thank the DFG (Emmy-
Noether Programm (SCHN950/2-1): S.S.), the Stifterverband
fu¨r die deutsche Wissenschaft (Projekt-Nr. 11047 (Fors-
chungsDozentur Molekulare Katalyse): J.E.), the Elite-
netzwerk Bayern (graduate fellowship for A.N.M.), and the
IDK NanoCat for funding. Dr. S. D. Hoffmann and Dr. F.
Kraus are acknowledged for operating the Xcalibur diffrac-
tometer.
π-interactions in 3CNtBu
.
The pKa values found for 2R (R ) Me, Ph) do not disagree
with Buchwald’s proposal that the chemoselectivity of PhCl
coupling with alkyl- versus arylamines can be related to pKa
arguments.8 However, our results show that even dialky-
lamines are sufficiently acidified by coordination to palladium
to be fully deprotonated by alkoxides in non-protic solvents,
raising the question if further effects must be considered to
explain the observed chemoselectivity, for example, alkoxide
coordination to the metal center as proposed by Hartwig and
co-workers.62
Supporting Information Available: Crystallographic informa-
tion for 1Cl, 1Me, 2Cl, 2Me, 3CNtBu, and 5 in CIF format. NMR spectra
of 1Cl, 1Me, 1Ph, 2Cl, 2Me, 2Ph, 3CNtBu, 3PMe3, 4 and 5 and coordinates
for the optimized geometries of 1Cl and 2Cl in PDF format. This
materialisavailablefreeofchargeviatheInternetathttp://pubs.acs.org.
(62) (a) Alcazar-Roman, L. M.; Hartwig, J. F. J. Am. Chem. Soc. 2001,
123, 12905–12906. (b) Shekhar, S.; Hartwig, J. F. Organometallics
2007, 26, 340–351.
IC802286U
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