Novel dinuclear and trinuclear palladium β-diiminate complexes containing amido-chloro double-bridges

Article abstract of DOI:10.1039/b804792h

We report the formation of an unexpected trinuclear palladium β-diiminate complex from the decomposition of [Pd(Ph₂nacnac)(Cl) (4-H₂NC₆H₄-^tBu)] (nacnac = β-diiminate derived from acetylacetone), the p

Full text of DOI:10.1039/b804792h

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www.rsc.org/dalton | Dalton Transactions
Novel dinuclear and trinuclear palladium b-diiminate complexes containing
amido–chloro double-bridges†
Alen Hadzovic, John Janetzko and Datong Song*
Received 25th March 2008, Accepted 17th April 2008
First published as an Advance Article on the web 16th May 2008
DOI: 10.1039/b804792h
We report the formation of an unexpected trinuclear pal-
ladium b-diiminate complex from the decomposition of
[Pd(Ph2nacnac)(Cl)(4-H2NC6H4-tBu)] (nacnac = b-diiminate
derived from acetylacetone), the proposed reaction pathway,
and the synthesis of the first dinuclear palladium complex
with an amido–chloro double-bridge.
Despite the considerable development and interests in the chem-
istry of dinuclear palladium compounds, dinuclear Pd(II) com-
plexes with amido or mixed amido–X bridges are scarce in
the literature (X = halide or OH−).1 The previous work has
concentrated on Pd2(l-OH)2 bimetallic complexes in which the
hydroxo bridge can be cleaved in the presence of an amine.2–10 The
coordinated amine can be deprotonated by one of the OH− groups,
producing complexes with [Pd2(l-OH)(l-NHR)] structural mo-
tifs. Ruiz et al. have used [{Pd(jC,N-ortho-C6H4CH2NMe2)} (l-
OH)(l-Br)] to prepare [{Pd(jC,N-ortho-C6H4CH2NMe2)}22(l-
Br)(l-NHR)] (R = Ph, p-MeC6H4 and p-MeOC6H4), however
without X-ray structural characterization.11 A series of related
dinuclear Pd(II) complexes with acetylacetonato (acac−) ligands of
general formula [{Pd(acac)}2(l-NHR)2] (R = aryl) was reported
in the early 1980s.12
Scheme 1 Synthesis of 1 and formation of complex 3.
The palladium–nacnac (nacnac = b-diiminate derived from
acetylacetone) chemistry is still in its infancy compared to other
metal–nacnacs.13 Apart from our recent work,14 only a few exam-
ples exist in the literature.15–17 We have recently reported the synthe-
sis and characterization of [Pd(Ph2nacnac)(Cl)(4-H2NC6H4-tBu)]
1 starting from the dimeric complex [Pd(Ph2nacnac)(Cl)]2 2 and
two equivalents of 4-tert-butylaniline (Scheme 1).12
Complex 1 was one in a series of new Pd–nacnac complexes of
general formula [Pd(Ph2nacanc)(Cl)(L)], where L = N-methyl-4,5-
diphenylimidazole, CO, and 4-tert-butylaniline. Although 1 gave
Fig. 1 The molecular structure of complex 3. All hydrogen atoms (except
for NHs) and phenyl carbon atoms (except for the ipso carbons) on
1
satisfactory elemental analysis, as well as H and 13C NMR and
(Ph2nacnac)− are removed for clarity. Selected bond lengths (A): Pd1–N3
˚
IR spectra in accordance to the proposed structure in Scheme 1,
attempts‡ to obtain X-ray quality crystals of 1 produced an
unexpected trimetallic palladium complex 3 (Scheme 1 and Fig. 1)§
along with small amount of a pale yellow material. The formation
of 3 is a result of NH bond activation on the coordinated amine
and takes place at room temperature without the need to exclude
either oxygen or moisture.
2.087(5), Pd1–Cl1 2.3351(19), Pd2–N3 2.100(5), Pd2–Cl1 2.3941(18),
Pd3–C3 2.119(6), N1–C2 1.288(7), N2–C4 1.297(7), N4–C29 1.337(8),
N5–C31 1.327(8). Selected bond angles (◦): Pd1–Cl1–Pd2 78.96(5),
Pd1–N3–Pd2 91.81(19), N1–Pd1–N2 87.2(2), N4–Pd2–N5 90.8(2).
two (Ph2nacnac)− ligands bound to the third palladium through
its b-carbon (C3 in Fig. 1). The two six-membered chelate rings
have different conformations as a consequence of the additional
Pd–C bond. The ring containing Pd1 has a boat conformation
folding along N1 · · · N2 and C2 · · · C3 axes with a tetrahedral
geometry around the b-C (C3), while the Pd2-containing ring has
Complex 3 consists of two [Pd(Ph2nacnac)]+ moieties doubly
bridged by an amide nitrogen atom and a chloride, with one of the
Davenport Chemical Research Laboratories, Department of Chemistry,
University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S
3H6. E-mail: dsong@chem.utoronto.ca; Fax: +1 416 978 7013; Tel: +1 416
978 7014
† CCDC reference numbers 680460 and 680461. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b804792h
˚
a half-chair conformation with Pd2 lying ∼0.54 A out of the plane
defined by the ligand backbone. The corresponding bond lengths
within the rings are different as well. For example, N1–C2 and N2–
2
2
=
C4 bonds are similar to the N(sp ) C(sp ) double bonds found
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in jN,Nꢀ-diimine palladium complexes18–20 and shorter than N4–
C29 and N5–C31 in the other ring. Besides the carbon donor
atom, Pd3 is further coordinated by an aniline and two chloride
ligands. A related complex in which [Pd(CH3CN)3]2+ cation is
coordinated to the b-C of [(Arꢀ2nacnac)Pd(CH3CN)2]+ (Arꢀ = 2,6-
diisopropylphenyl) fragment has been reported previously.15,16 The
formation of complex 3 further indicates that the b-C of nacnac
backbone retains sufficient nucleophilicity even in the charge-
neutral Pd–nacnac complexes.
The structure of 4 has also been confirmed by X-ray crystallog-
raphy (Fig. 2).†† The two [Pd(Ph2nacnac)]+ moieties are doubly
bridged by an amido nitrogen atom and a chloride. The two six-
membered chelate rings have similar half-chair conformations
˚
with the Pd(II) ions ∼0.54 A out of the planes defined by the
remaining atoms in the rings. The metrical parameters of the
bridge are similar to those found in 3.
To understand the formation of 3, we monitored the 1H NMR of
a freshly prepared sample of 1 in CD2Cl2 over a period of 14 days.
During the course of the reaction a pale yellow precipitate of trans-
[PdCl2(p-H2NC6H4-tBu)2] 5 formed slowly.¶ The spectra show the
formation of free [Ph2nacnac]H ligand identified by the distinct
downfield shift at 12.71 ppm from the NH as well as the resonance
at 4.88 ppm from H(b-C). Therefore, it appears that the amido
bridge in 3 is a result of the deprotonation of the aniline ligand
by (Ph2nacnac)− and the [Pd(H2NAr)Cl2] motif in complex 3 is
a result of the nacnac-ligand loss from 1 due to the protonation.
As such, the addition of a base to deprotonate the aniline should
block this reaction pathway and prevent the loss of the nacnac
ligand from 1 and in turn, the formation of 3.
To test this hypothesis we treated 1 eq. of in situ generated
1 (from 2 and the aniline in 1 : 2 ratio) with 0.5 eq. of KOtBu.
As expected, the formation of 3 or [Ph2nacnac]H was not
observed. Instead, the reaction produced complex 4 and free
aniline (Scheme 2).* The use of two equivalents of the aniline
with respect to 2 is crucial since using only one equivalent lowers
the yield of 4 by about 50% indicating that the full conversion of
2 to 1 is required prior to the treatment with a base. When Et3N
instead of KOtBu was used, the formation of 4 and free aniline
were also observed. However, the reaction proceeded at a slower
rate, similar to the decomposition timescale for 1 alone. In both
cases neither free [Ph2nacnac]H in the solution nor 5 as precipitate
was observed.
Fig. 2 The molecular structure of complex 4. For clarity all the hydrogen
atoms (except the amido hydrogen) are omitted and phenyl rings of
(Ph2nacnac)− backbone have been reduced to their ipso carbons. Selected
˚
bond lengths (A): Pd1–N5 2.072(4), Pd2–N5 2.011(4), Pd1–Cl1 2.3241(17),
Pd2–Cl1 2.3237(16). Selected bond angles (◦): Pd1–N5–Pd2 97.67(17),
Pd1–Cl1–Pd2 84.39(6).
The proposed reaction pathway for the formation of 3 on
the basis of the above observations is shown in Scheme 3. The
condensation of two molecules of 1 produces 4 and eliminates
the aniline salt, ArNH3Cl. In the absence of an exogenous base,
the acidic ArNH3Cl protonates the nucleophilic b-carbon of the
nacnac backbone in 1 to yield 1a and the free aniline. The neutral b-
diimine ligand in 1a can then be displaced by either the free aniline
to afford the insoluble 5, or the nucleophilic b-C on the nacnac
Scheme 2 Synthesis of complex 4 (Ar = p-tBu-C6H4).
1
The H NMR spectra of 4 are consistent with the proposed
structure in Scheme 2. Thus, the methyl groups on the ligand
backbone show two singlets at 1.43 and 1.57 ppm while the tert-
butyl group gives a singlet at 1.22 ppm, integrated to six, six,
and nine protons, respectively, against the resonance at 4.67 ppm
from two H(b-C). These resonances are also observed in the NMR
experiments monitoring the degradation of 1 in solution.
Scheme 3 Proposed reaction pathways (Ar = p-tBu-C6H4).
3280 | Dalton Trans., 2008, 3279–3281
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backbone of 4 to give 3. The formation of 5 in this system is slow
but irreversible because of the insolubility of 5. It appears that 3 is
only a metastable intermediate, trapped during the crystallization,
while 4 and 5 are the thermodynamic products.
benzene–hexanes gives the analytically pure sample. X-Ray quality single
crystals of 4 as benzene solvate were grown by slow evaporation of
benzene–hexanes solution. 1H NMR (C6D6, 400 MHz, d): 1.22 (s, 9H,
tert-Bu), 1.43 (s, 6H, CH3), 1.57 (s, 6H, CH3), 4.67 (s, 2H, H(b-C)), 6.36 (d,
2H, aniline, 3JH–H = 8.8 Hz), 6.60–6.62 (m, 4H, Ph), 6.69 (d, 2H, aniline,
3JH–H = 8.8 Hz), 6.86–6.95 (m, 12H, Ph), 7.06–7.09 (m, 4H, Ph). 13C (C6D6,
100 MHz, d): 24.17 (CH3), 24.38 (CH3), 31.90 (CH3, tert-butyl), 98.03 (b-
C, nacnac), 123.9, 124.9, 125.0, 125.2, 127.8, 128.1, 128.5, 129.4, 129.6
(phenyl and aniline rings), 149.9, 151.0, 151.7 (ipso-C on Ph and aniline
rings), 157.1, 158.1 (C-N, nacnac backbone). Elemental analysis, calc. for
C50H54N5ClPd2 (4·C6H6): C, 61.71; H, 5.59; N, 7.20%. Found: C, 62.16; H,
5.85; N, 6.96%.
In conclusion, we have observed a facile NH activation
in [Pd(Ph2nacnac)(Cl)(4-H2NC6H4-tBu)], producing an unusual
trimetallic complex 3 containing a unique structural motif, [Pd2(l-
Cl)(l-HNR)], where the mixed amido–chloro double-bridge links
the two [Pd(Ph2nacnac)]+ moieties. The formation of 3 has been
rationalized through a series of experiments, during the course
of which a convenient synthetic procedure for the novel amido–
chloro doubly bridged bimetallic complex, 4, has been developed.
The reactivities of 4 and its analogues with other amines and their
potential applications in catalysis are being investigated in our
laboratory.
†† Selected crystallographic data for complex 4: The data were collected22
on a Nonius Kappa-CCD diffractomter and processed with the DENZO-
SMN package.23 The structures were solved and refined with SHELXTL
V6.10.24 Empirical formula: C44H48ClN5Pd2·C6H6; FW 973.23; T 150(2)
¯
˚
˚
˚
K; triclinic, P1; a = 12.5418(4) A, b = 13.5019(4) A, c = 15.5413(5) A,
a = 106.2391(18)◦, b = 105.9472(18)◦, c = 97.1865(16)◦, V = 2370.66(13)
A , T = 150(2) K, Z = 2, Dc = 1.363 g cm−3; GOF on F2, 1.048; final R
3
˚
indices [I > 2r(I)]: R1 = 0.0549, wR2 = 0.1533; R indices (all data): R1 =
0.0812, wR2 = 0.1756.
Acknowledgements
1 V. K. Jain and L. Jain, Coord. Chem. Rev., 2005, 249, 3075–3197.
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Organometallics, 2006, 25, 4596–4604.
This research is supported by grants to D.S. from the Natural Sci-
ence and Engineering Research Council of Canada, the Canadian
Foundation for Innovation, the Ontario Research Fund and the
University of Toronto (Connaught Foundation).
4 L. Diez, P. Espinet and J. A. Miguel, J. Chem. Soc., Dalton Trans., 2001,
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5 M. S. Driver and J. F. Hartwig, Organometallics, 1997, 16, 5706–5715.
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Notes and references
‡ The crystals of 3, as CH2Cl2 solvate, have been produced in ∼40% yield
by top-layering a concentrated CH2Cl2 solution of 1 with hexanes and
further slow evaporation of the solvent after the layers mix for about 5 d.
Unfortunately the bulk sample produced this way always contains trace
amounts of 4 and 5. Therefore, no satisfactory elemental analysis data were
obtained. 1H NMR (C6D6, 400 MHz, d): 1.12 (s, 9H, CH3, tert-Bu), 1.19
(s, 9H, CH3, tert-Bu), 1.53 (s, 3H, CH3, nacnac), 1.59 (s, 3H, CH3, nacnac),
1.90 (s, 3H, CH3, nacnac), 1.95 (s, 3H, CH3, nacnac), 3.55 (br, 2H, NH),
4.53 (s, 1H, H(b-C), nacnac), 4.68 (s, 1H, H(b-C), nacnac), 6.01–6.05 (4H,
m, aniline aromatic), 6.32–6.37 (4H, aniline aromatic), 6.65–7.91 (20H,
Ph, nacnac).
11 J. Ruiz, N. Cutillas, J. Sampedro, G. Lopez, J. A. Hermoso and M.
´
Martınez-Ripoll, J. Organomet. Chem., 1996, 526, 67–72.
12 S. Okeya, S. H. Yoshimatsu, Y. Nakamura and S. Kawaguchi, Bull.
Chem. Soc. Jpn., 1982, 55, 483–491.
´
§ Selected crystallographic data for complex 3: The data were collected22
on a Nonius Kappa-CCD diffractometer and processed with the DENZO-
SMN package.23 The structures were solved and refined with SHELXTL
V6.10.24 Empirical formula, C54H63Cl3N6Pd3·CH2Cl2; FW 1306.58; tri-
13 L. Bourget-Merle, M. F. Lappert and J. R. Severn, Chem. Rev., 2002,
102, 3031–3065.
14 A. Hadzovic and D. Song, Organometallics, 2008, 27, 1290–1298.
15 X. Tian, R. Goddard and K.-R. Porschke, Organometallics, 2006, 25,
5854–5862.
¯
˚
˚
˚
clinic, P1◦; a = 11.9402(3)◦A, b = 15.3099(7) A, c = 15.5713(5) A, a =
¨
◦
3
˚
85.923(2) , b = 78.728(3) , c = 88.962(2) , V = 2784.49(17) A , T =
150(2) K, Z = 2, Dc 1.370 g cm−3; GOF on F2, 1.040; final R indices [I >
2r(I)]: R1 = 0.0634, wR2 = 0.1592; R indices (for all data): R1 = 0.1021,
wR2 = 0.1845.
16 J. Feldman, S. J. McLain, A. Parthasarathy, W. J. Marshall, J. C.
Calabrese and S. D. Arthur, Organometallics, 1997, 16, 1514–1516.
17 M. Kang and A. Sen, Organometallics, 2005, 24, 3508–3515.
18 D. Domin, D. Benito-Garagorri, K. Mereiter, C. Hametner, J. Frohlich
and K. Kirchner, J. Organomet. Chem., 2007, 692, 1048–1057.
19 D. N. Coventry, A. S. Batsanov, A. E. Goeta, J. A. K. Howard and T. B.
Marder, Polyhedron, 2004, 23, 2789–2795.
¶ Complex 5: 1H NMR (400 MHz, dmso-d6, d): 1.19 (s, 9H, tert-Bu), 4.79
(br, s, 2H, H2N), 6.48 (d, 2H, aniline aromatic, 3JH–H = 8.0 Hz), 7.02 (d, 2H,
aniline aromatic, 3JH–H = 8.0 Hz). These data are in accordance with the
spectrum obtained from a sample of 5 prepared according to the literature
procedure for the synthesis of trans-[PdCl2(H2NAr)2].21
20 E. K. Cope-Eatough, F. S. Mair, R. G. Pritchard, J. E. Warren and R. J.
Woods, Polyhedron, 2003, 22, 1447–1454.
* Synthesis of 4: 4-tert-butylaniline (32 mg, 0.2 mmol) was added to a
stirred solution of 2 (78 mg, 0.1 mmol) in THF (5.0 mL). The reaction
changed colour from green to red within minutes. The reaction was left
stirring for 20 min and solid KOtBu (11.5 mg, 0.1 mmol) was added. After
2.5 h the solution was filtered over a pad of Celite and solvent removed
in vacuo. The red residue was re-dissolved in THF–hexanes (2.0 mL) and
left in the freezer overnight. The red crystalline product was decanted
and dried in vacuum to afford 4 (65 mg, 70%). Recrystallization from
21 M. A. J. Jungbauer and C. Curran, Spectrochim. Acta, 1965, 21, 641–
648.
22 Collect. Data collection software, B. V. Nonius, Delft, The Netherlands,
1997–2002.
23 Z. Otwinowski and W. Minor, Methods Enzymol., 1997, 276, 307.
24 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
64, 112.
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The Royal Society of Chemistry 2008
Dalton Trans., 2008, 3279–3281 | 3281
©