Ligand design for site-selective installation of Pd and Pt centers to generate homo- and heteropolymetallic motifs

Article abstract of DOI:10.1039/c003163a

The modular synthesis of a series of nitrogen-rich polydentate ligands that feature a common pincer-type framework is reported. These ligands allow for site-selective installation of palladium and platinum to give rise to bi- and trimetallic complexes that have d⁸-d⁸ interactions.

Full text of DOI:10.1039/c003163a

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COMMUNICATION
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Ligand design for site-selective installation of Pd and Pt centers to
generate homo- and heteropolymetallic motifsw
Daniel L. M. Suess and Jonas C. Peters*
Received 15th February 2010, Accepted 18th June 2010
DOI: 10.1039/c003163a
The modular synthesis of a series of nitrogen-rich polydentate
ligands that feature a common pincer-type framework is
reported. These ligands allow for site-selective installation of
palladium and platinum to give rise to bi- and trimetallic
complexes that have d8–d8 interactions.
these ligands to poison the Pd cross-coupling catalyst. A non-
adentate, trinucleating ligand (9) is prepared in an analogous
synthesis from 5 and 8-amino-2-methylquinoline (see ESIw).
The installation of Pd into the anionic binding site of 6 is
readily achieved by reaction with (COD)Pd(Me)Cl and catalytic
Et3N to form red, neutral L1OMePdCl 10 with concomitant loss
of methane. Alternatively, 10 may be generated using
(COD)PdCl2 and stoichiometric Et3N; however, the use of
(COD)Pd(Me)Cl obviates the need to purify 10 from Et3NÁ
HCl. 1H NMR spectroscopy of 10 shows the disappearance
of the bis(quinolinyl)amine resonance. The solid-state structure6
of 10 (determined by X-ray crystallography, Fig. 1) confirms the
installation of Pd into the bis(quinolinyl)amido site; the neutral
bis(2-methoxyethyl)amine binding site remains open.
Natural and synthetic polymetallic reaction sites mediate a
wide range of small molecule transformations that have not
been realized in monometallic analogs.1 For example, syn-
thetic bimetallic complexes that contain d8–d8 interactions
display unusual photochemical reactivity2 and have interesting
electronic structures3 owing to the metal–metal orbital inter-
action. Studies of these systems—and of polymetallic com-
plexes more generally—are enhanced by the synthetic ability
to rationally generate specific homo- and heteropolymetallics
of interest.4 We are beginning to explore the synthesis and site-
selective metallation of polynucleating ligands in order to
generate such complexes. Reported herein are initial efforts
toward the preparation of nitrogen-rich ligands that feature a
common pincer-type framework. We establish the utility of
this family of ligands for the site-selective installation of Pd
and Pt centers which can result in bi- and trimetallic systems
featuring d8–d8 interactions in the solid-state and in solution.
The four ligands described in this communication feature two
or three tridentate binding sites. Common to each ligand is a
monoanionic, bis(quinolinyl)amide (BQA) pincer-type binding
site. Decoration of the pincer scaffold with one or two neutral
binding sites affords access to the polymetallic complexes of
interest herein. We have chosen in this initial investigation to
target Pd and Pt species due to our prior experience with the
chemistry of group 10 BQA complexes,5 and due to their low
lability by comparison to first row late metals.
Reaction between 7, (COD)PdCl2, and Et3N affords the red
salt [L1imPd]Cl 11, the solid-state structure7 of which demon-
strates that one imidazole occupies the coordination site trans
to the amide, thereby forming an eight-member ring (Fig. 1).
1
Variable-temperature H NMR spectroscopy reveals that the
solution-state structure in CDCl3 is consistent with the solid-
state structure. At low temperature (ca. À50 1C), the two
imidazole rings are inequivalent and each methylene proton is
coupled to its diastereotopic partner; coalescence is observed
near room temperature and the averaged signals can be seen at
higher temperature (ca. 50 1C; see ESIw).
For 8, the kinetically preferred site for metal uptake is the
neutral bis((pyridin-2-yl)methyl)amine pocket; reaction of 8 with
one equivalent (COD)Pd(Me)Cl and 1 equiv. TlPF6 thus
provides yellow [L1py(H)PdMe][PF6] 12 with precipitation of
TlCl. 1H NMR spectroscopy of 12 reveals that the bis(quinolinyl)-
amine NH proton is still present and that the methylene
protons between the pyridine rings and the tertiary amine
are diastereotopic. Both observations are consistent with
selective metallation of the bis((pyridin-2-yl)methyl)amine
binding pocket. The solid-state structure8 confirms this assign-
ment (Fig. 1).
The ligand synthesis of the dinucleating L1RH series
(Scheme 1) is accomplished in three steps from 8-bromo-2-
methylquinoline (1). Oxidation of 1 with SeO2 provides access
to aldehyde 2. Reductive amination of 2 with NaHB(OAc)3 and
the appropriate secondary amine installs the neutral tridentate
binding sites in amine intermediates 3, 4, and 5. The BQA
binding pocket is generated by Pd-mediated C–N cross-coupling
of 3, 4, and 5 with 8-aminoquinoline to give yellow ligands 6, 7,
and 8,5a respectively. The latter transformation proceeds to
remarkably high conversion (>90%) given the potential for
We were gratified to find that the BQA site of 12 is readily
metallated with an additional equiv. of (COD)Pd(Me)Cl and
catalytic Et3N to give purple-red [L1py(PdCl)PdMe][PF6] 13.
The solid-state structure9 of 13 (Fig. 2) reveals a Pd–Pd d8–d8
interaction with an average Pd–Pd distance of 3.1909(7) A,
which is less than the sum of the van der Waals radii (3.26 A).10
This favorable zero bond order interaction has been explained
by mixing of 4dz2 and 5pz orbitals between the interacting
metals.11 Among the DFT-calculated occupied orbitals are
two orbitals that have Pd–Pd s and s* character (Fig. 3).12
The 1H NMR spectrum of 13 in CDCl3 is consistent with a Cs
symmetric structure at room temperature; however, cooling the
solution results in decoalescence of the signals at ca. À40 1C and
a spectrum that requires a C1 symmetric structure at À58 1C
Department of Chemistry, Massachusetts Institute of Technology,
Cambridge, MA 02139, USA and Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena, CA 91125,
USA. E-mail: jcpeters@caltech.edu
w Electronic supplementary information (ESI) available: Synthetic
details, crystal structure diagrams, and spectroscopic data. CCDC
766171–766176. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c003163a
c
6554 Chem. Commun., 2010, 46, 6554–6556
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Scheme 1 Synthesis of 6, 7, 8, and 9.
Fig. 1 Structures of monometallic complexes 10, 11, and 12. Ellipsoids are shown at the 40% probability level and hydrogen atoms, solvent
molecules, and counteranions (for 11 and 12) are omitted for clarity.
1
(see ESIw). Thus, the H NMR behavior of 13 appears to be
consistent with its solid-state structure. We examined the
possibility of a s*(dz2) - s(pz) transition resulting from a
d8–d8 interaction in solution by comparing the electronic
absorption spectrum of 13 with those of its independently
prepared monometallic fragments, [(N-methyl-bis(pyridin-2-yl-
methyl)amine)PdMe][PF6] (see ESIw) and (bis(quinolin-8-yl)-
amido)PdCl.5a Unfortunately, no such interaction could be
rigorously identified, possibly due to overlap with intense
ligand-centered p–p* transitions as suggested elsewhere.13
Given that the bimetallic metallation of 8 proceeds in a step-
wise and selective manner we sought to extend this synthetic
strategy to the generation of heterobimetallics. As an illustration,
the platinum-containing analog of 11 can be generated by
reaction of 8 with 1 equiv. (COD)Pt(Me)Cl and 1 equiv. TlPF6
to give [L1py(H)PtMe][PF6] 14, the solid- and solution-state
structure of which is analogous to that of 11 based on 1H
NMR spectroscopic studies. Further metallation with
(COD)Pd(Me)Cl and catalytic Et3N provides red hetero-
bimetallic [L1py(PdCl)PtMe][PF6] 15, which also contains a
d8–d8 interaction in the solid-14 and solution-states. The Pd–Pt
distance of 3.1668(3) A is shorter than the sum of the van der
Waals radii, 3.35 A.10 DFT calculations support the presence
of a d8–d8 interaction (Fig. 4).12 The 195Pt NMR chemical shift
of 15 is downfield of that of 14 (À3,038 and À3,059 ppm vs.
K2PtCl6, respectively), which is consistent with donation of
electron density to the Pd atom in the solution phase. Ligand 8
offers the potential for the generation of a range of homo- and
heterobimetallics and the study of weak metal–metal inter-
actions between metals in different ligand fields.
The procedure for installing Pd into ligand 8 may be applied to
the related trinucleating ligand 9. The room temperature addition
of 3 equiv. [(CH3CN)4Pd][BF4]2 to 9 results in the immediate
metallation at the two neutral bis((pyridin-2-yl)methyl)amine sites
as determined by 1H NMR spectroscopy. The final palladation of
the bis(quinolinyl)amine occurs upon the addition of Et3N and
the application of heat. Purple [L2py(PdBr)3][BF4]2 16 is formed
when the putative intermediate [L2py(Pd(CH3CN))3][BF4]5 is
treated with 3 equiv. TBABr. The solid-state structure15 of 16
reveals three unique Pd atoms (Fig. 2); the Pd1–Pd2 distance
(3.2447(10) A) is shorter than the sum of the van der Waals radii
whereas the Pd2–Pd3 distance (3.4411(10) A) is longer. These
bond lengths are consistent with the limiting formulation of two
square pyramidal Pd centers and one that is square planar.
Similarly to 13 and 15, 16 is fluxional in solution as demonstrated
by the broad resonances in its 1H NMR spectrum.
The complexation chemistry presented here demonstrates that
selective metallation of a polynucleating ligand demands careful
tuning of the binding pockets. For example, the BQA site in 7 is
metallated with 1 equiv. of a Pd(II) source whereas metallation of 8
occurs most readily at the neutral bis((pyridin-2-yl)methyl)amine
site. The modular synthesis of these ligands, in which a multitude
c
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Fig. 2 Structures of polymetallic complexes 13, 15, and 16. Ellipsoids are shown at the 40% probability level and hydrogen atoms, solvent
molecules, and counteranions are omitted for clarity.
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6 Crystal data for compound 10: C25H27ClN4O2Pd, M = 743.14,
monoclinic, a = 5.1225(2) A, b = 17.5023(7) A, c = 24.8571(10) A,
Fig. 3 Selected calculated molecular orbitals of 13.12
b = 90.583(2)1, U = 2228.46(15) A3, T = 173 K, space group P21/c
(no. 14), Z = 4, 42087 reflections measured, 3997 unique (Rint =
0.0226), which were used in all calculations. The final wR(F2) was
0.0215 (I > 2s(I)) and 0.0216 (all data). CCDC 766171.
7 Crystal data for compound 11Á2H2OÁ2CHCl3: C31H33Cl7N8O2Pd,
M = 904.20, triclinic, a = 7.3314(7) A, b = 13.8105(13) A, c =
18.3307(18) A, a = 84.743(2)1, b = 88.483(2)1, g = 82.262(2)1,
3
U = 1831.2(3) A , T = 173 K, space group P1 (no. 2), Z = 2,
ꢀ
47 572 reflections measured, 9810 unique (Rint
= 0.0404),
which were used in all calculations. The final wR(F2) was 0.0341
(I > 2s(I)) and 0.0414 (all data). CCDC 766172.
8 Crystal data for compound 12ÁCH3CN: C34H32F6N7PPd, M =
790.04, monoclinic, a = 14.6795(13) A, b = 13.2225(11) A, c =
16.7504(14) A, b = 91.096(2)1, U = 3250.7(5) A3, T = 173 K, space
group P21/c (no. 14), Z = 4, 74757 reflections measured, 9886
unique (Rint = 0.0502), which were used in all calculations. The final
wR(F2) was 0.0332 (I > 2s(I)) and 0.0493 (all data). CCDC 766173.
Fig. 4 Selected calculated molecular orbitals of 15.12
of pendant binding pockets can be prepared, allows for such
tuning. Further work is directed at studying the electrochemical
properties of these and related systems as well as the generation of
heterofunctional systems that contain basic and acidic sites for the
binding and polarization of non-polar small molecules.
9 Crystal data for compound 13Á1.5DMSO: C35H28ClF6N6O1.5PPd2S1.5
,
M = 997.94, monoclinic, a = 16.542(3) A, b = 32.618(5) A, c =
14.722(2) A, b = 109.131(2)1, U = 7505(2) A3, T = 173 K, space
group C2/c (no. 15), Z = 8, 58 464 reflections measured, 7673 unique
(Rint = 0.0500), which were used in all calculations. The final wR(F2)
was 0.0430 (I > 2s(I)) and 0.0551 (all data). CCDC 766174.
10 A. Bondi, J. Phys. Chem., 1964, 68, 441–451.
This work was funded in part by the NSF (CHE-0750234).
DLMS is grateful for funding from the NSF-GRFP and the
MIT Presidential Fellowship Program. Dr Peter Muller is
¨
acknowledged for insightful discussions.
11 K. R. Mann, J. G. Gordon II and H. B. Gray, J. Am. Chem. Soc.,
1975, 97, 3553.
12 See Supplementary Informationw for details. Frisch et al., Gaussian
03, Revision C.02, Gaussian, Inc., Wallingford CT, 2004.
13 J. E. Bercaw, A. C. Durrel, H. B. Gray, J. C. Green, N. Hazari,
J. A. Labinger and J. R. Winkler, Inorg. Chem., 2010, 49, 1801.
14 Crystal data for compound 15ÁCH2Cl2: C33H30Cl3F6N6PPdPt,
M = 1063.44, triclinic, a = 7.3949(4) A, b = 15.6616(8) A, c =
Notes and references
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15.8870(9) A, a = 70.729 (2)1, b = 81.541(2)1, g = 86.647(2)1,
3
U = 1717.88(16) A , T = 173 K, space group P1 (no. 2), Z = 2,
ꢀ
2 Selected examples: (a) D. M. Roundhill, H. B. Gray and
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33 056 reflections measured, 6077 unique (Rint
= 0.0318),
which were used in all calculations. The final wR(F2) was 0.0232
(I > 2s(I)) and 0.0234 (all data). CCDC 766175.
15 Crystal data for compound 16Á4CH3CN: C52H50B2Br3F8N13Pd3,
M = 1589.60, orthorhombic, a = 12.8582(13) A, b = 19.3783(19) A,
c = 45.245(4) A, U = 11 273.8(19) A3, T = 173 K, space group Pbca
(no. 61), Z = 8, 221 928 reflections measured, 15 158 unique (Rint =
0.0708), which were used in all calculations. The final wR(F2) was
0.0905 (I > 2s(I)) and 0.1043 (all data). CCDC 766176.
c
6556 Chem. Commun., 2010, 46, 6554–6556
This journal is The Royal Society of Chemistry 2010