Palladium vacataporphyrin reveals conformational rearrangements involving hueckel and moebius macrocyclic topologies

Published on Web 04/16/2008

Palladium Vacataporphyrin Reveals Conformational

Rearrangements Involving Hückel and Möbius Macrocyclic

Topologies

Ewa Pacholska-Dudziak, Janusz Skonieczny, Miłosz Pawlicki, Ludmiła Szterenberg,

Zbigniew Ciunik, and Lechosław Latos-Graz˙yn´ski*

Department of Chemistry, UniVersity of Wrocław, Joliot-Curie 14, 50 383 Wrocław, Poland

Received December 12, 2007; E-mail: llg@wchuwr.pl

Abstract: 5,10,15,20-Tetraaryl-21-vacataporphyrin (butadieneporphyrin, an annulene-porphyrin hybrid)

which contains a vacant space instead of heteroatomic bridge acts as a ligand toward palladium(II). The

metal ion of square-planar coordination geometry is ﬁrmly held via three pyrrolic nitrogen atoms where the

fourth coordination place is occupied by a monodentate ligand or by an annulene part of vacataporphyrin.

The macrocycle reveals the unique structural ﬂexibility triggered by coordination of palladium. The structural

rearrangements engage the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment which serves as a linker between

two pyrrolic rings of vacataporphyrin albeit the signiﬁcant rufﬂing of the tripyrrolic block is also of importance.

Two fundamental modes of interactions between the palladium ion and annulene moiety have been

recognized. The ﬁrst one resembles an η²-type interaction and involves the C(2)C(3) unit of the butadiene

part. Alternatively the profound conformational adjustments allowed an in-plane coordination through the

deprotonated trigonally hybridized C(2) center of butadiene. The coordinated vacataporphyrin acquires

Hückel or extremely rare Möbius topologies readily reﬂected by spectroscopic properties. The palladium

vacataporphyrin complexes reveal Hückel aromaticity or Möbius antiaromaticity of [18]annulene applying

the butadiene fragment of vacataporphyrin as a topology selector. The properties of speciﬁc conformers

were determined using ¹H NMR and density functional theory calculations.

Chart 1. Vacataporphyrin, (VPH₂-1)H, Ar ) 4-Metoxyphenyl (Anis),

p-Tol

Introduction

An aza-deﬁcient porphyrin 5,10,15,20-tetraaryl-21-vacatapor-

phyrin (butadieneporphyrin), (VPH₂-1)H can be classiﬁed as

an annulene-porphyrin hybrid (Chart 1).^1,2In more general terms

vacataporphyrin is a representative example of the subclass of

modiﬁed porphyrins described by a general formula [n]triphy-

rin(n.1.1) (n ) 1,2, ...).^3–6

Such a molecule provides the fundamental structural and

spectroscopic features of parental 5,10,15,20-tetraarylporphyrin.¹

Actually this macrocycle has an expanded coordination core,

comparable by size and shape with texaphyrin,^7,8and related

(1) Pacholska, E.; Latos-Graz˙yn´ski, L.; Ciunik, Z. Chem. Eur. J. 2002,

8, 5403–5406.

(2) From now on we will use the symbol (VPH₂-n) to denote the

hypothetical monoanion obtained from the vacataporphyrin by

abstraction of a pyrrolic NH proton. The hypothetical dianion formed

by abstraction the C-bound proton H(2) is noted as (VPH-n). The

group attached to C(2/3) will appear as a sufﬁx. Finally the appro-

priate macrocyclic conformers are systematically numbered (n) as

occur in the text. The neutral structure is described by the following

acronym (VPH₂-n)H.

to theoretically investigated secophyrin.⁹As illustrated by

comparison to texaphyrin^10–12the annulene fragment of vacat-

aporphyrin is suitably exposed to coordination or interaction

withinitiallynitrogen-coordinatedmetalions.Consequentlyvacata-

(3) Franck, B.; Nonn, A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1795–

1811.

(8) Hannah, S.; Lynch, V. M.; Gerasimchuk, N.; Magda, D.; Sessler,

J. L. Org. Lett. 2001, 3, 3911–3914.

(4) Krivokapic, A.; Cowley, A. R.; Anderson, H. L. J. Org. Chem. 2003,

68, 1089–1096.

(9) Waluk, J.; Michl, J. J. Org. Chem. 1991, 56, 2729–2745.

(10) Sessler, J. L.; Hemmi, G.; Mody, T. D.; Murai, T.; Burrell, A.; Young,

S. W. Acc. Chem. Res. 1994, 27, 43–50.

(5) Inokuma, Y.; Kwon, J. H.; Ahn, T. K.; Yoo, M.-C.; Kim, D.; Osuka,

A. Angew. Chem., Int. Ed. 2006, 45, 961–964.

(11) Lisowski, J.; Sessler, J. L.; Lynch, V.; Mody, T. D. J. Am. Chem.

Soc. 1995, 117, 2273–2285.

(6) Kobayashi, N.; Takeuchi, Y.; Matsuda, A. Angew. Chem., Int. Ed.

2007, 46, 758–760.

(12) Hannah, S.; Lynch, V.; Guldi, D. M.; Gerasimchuk, N.; MacDonald,

C. L. B.; Magda, D.; Sessler, J. L. J. Am. Chem. Soc. 2002, 124,

8416–8427.

(7) Sessler, J. L.; Murai, T.; Lynch, V.; Cyr, M. J. Am. Chem. Soc. 1988,

110, 5586–5588.

9

6182 J. AM. CHEM. SOC. 2008, 130, 6182–6195

10.1021/ja711039c CCC: $40.75

2008 American Chemical Society

Hückel and Möbius Conformational Rearrangements

A R T I C L E S

Chart 2. Stereoisomers of (VPH₂-n)M^IICl (M ) Zn, Cd, Ni)

ion is susceptible to phenyl coordination in a cyclopalladation

process.^30–32Once the carbon donor is built into macrocylic³³

or pincer ligands the palladium(II) ion forms robust organo-

metallic compounds.^30,33–36Markedly a peculiar coordination

mode has been detected for a Pd₂²⁺couple sandwiched between

two helical bilindione-palladium moieties. The compound

reveals η²-coordination of a palladium center to the π system

at one of the C_meso-C_Rbonds.³⁷

In the course of our present investigations we have realized

that coordination of palladium(II) to three pyrrolic nitrogens of

vacataporphyrins triggers a spectacular sequence of conforma-

tional changes, which have been readily visualized by ¹H NMR

spectroscopy as the respective spectra reﬂect distinct diatropic

or moderate paratropic effects. These spectroscopic features

made us receptive to the [18]annulene model of a porphyrin,³⁸

which, emphasizing the presence of an 18-electron main

π-conjugation pathway in the molecule, invites the application

porphyrin may eventually reveal its carbaporphyrinoid nature.

In general carbaporphyrinoids provide a unique macrocyclic

platform which is suitable to explore organometallic chemistry

conﬁned to a peculiar macrocyclic environment.^13–18Such an

entrapment enforces frequently an unusual coordination geo-

metry involving M· · ·(H-C), M· · ·(CH)₂or M-C fragments.

As we have previously demonstrated vacataporphyrin forms

diamagnetic (VPH₂)Zn^IICl and (VPH₂)Cd^IICl and paramagnetic

(VPH₂)Ni^IICl complexes.¹⁹A metal ion is bound in the

macrocyclic cavity by three pyrrolic nitrogens. Coordination

imposes a steric constraint on ligand geometry and leads to

stereoisomers with the annulene part oriented outward (VPH₂-

1)M^IICl or toward (VPH₂-2)M^IICl the macrocyclic center (Chart

2). The (VPH₂-1)Cd^IICl, (VPH₂-1)Zn^IICl complexes and the free

1

of Hu¨ckel rule to the tetrapyrrolic macrocycle. In H NMR

spectroscopic terms this electronic feature is typically reﬂected

by a well-deﬁned diatropic effect detectable even for the most

severe distortion of [18]porphyrins(1.1.1.1).³⁹Signiﬁcantly such

an effect has been detected even for folded porphyrin derivatives

of C_ssymmetry: 21,23-ditelluraporphyrin which contains a

single ﬂipped tellurophene ring,⁴⁰nonplanar p-benziporphy-

rin,^16,41–43and vacataporphyrin complexes containing inverted

butadiene fragment.¹⁹

Extending the [18]annulene model of porphyrin, in direct

analogy to [n]annulenes we have put forward a hypothesis that

twisted vacataporphyrin may acquire Hu¨ckel aromaticity or

Mo¨bius antiaromaticity.^44,45The special energetics of π-systems

for Mo¨bius annulenes was ﬁrst considered over 40 years ago.⁴⁶

The ﬂexible systems, such as higher annulenes, prefer to adopt

untwisted Hu¨ckel topologies, which are normally more

stable.^44,47,48While computational data indicate that Mo¨bius

conformations might contribute to chemistry of higher [4n]an-

nulenes,^49,50so far they have not been identiﬁed experimentally.

1

base (VPH₂-1)H share a common H NMR spectral pattern as

the basic structural features are preserved after metal ion

insertion. The ¹H NMR spectra of (VPH₂-2)Cd^IICl and (VPH₂-

2)Zn^IICl reﬂect a considerable decrease of aromaticity accounted

by inverted geometry. The annulene-metal ion proximity induces

direct couplings between the spin-active nucleus of the metal

1

(

^111/113Cd) and the adjacent H nuclei of annulene.

To explore coordinating properties of vacataporphyrin here

we have focused on coordination chemistry of palladium. A

variety of palladium(II) complexes with porphyrins,²⁰expanded

porphyrins,²¹heteroporphyrins,²²and carbaporphyrins^23–29have

been isolated and characterized. Importantly, the palladium(II)

(30) Steenwinkel, P.; Gossage, R. A.; van Koten, G. Chem. Eur. J. 1998,

4, 759–762.

(31) Steenwinkel, P.; Gossage, R. A.; Maunula, T.; Grove, D. M.; van

Koten, G. Chem. Eur. J. 1998, 4, 763–768.

(32) Jolliet, P.; Gianini, M.; von Zelewsky, A.; Bernardinelli, G.; Stoeckli-

Evans, H. Inorg. Chem. 1996, 35, 4883–4888.

(13) Harvey, J. D.; Ziegler, C. J. Coord. Chem. ReV. 2003, 247, 1–19.

(14) Srinivasan, A.; Furuta, H. Acc. Chem. Res. 2005, 38, 10–20.

(15) Chmielewski, P. J.; Latos-Graz˙yn´ski, L. Coord. Chem. ReV. 2005,

249, 2510–2533.

(33) Giesbrecht, G. R.; Hanan, G. S.; Kickham, J. E.; Loeb, S. J. Inorg.

Chem. 1992, 31, 3286–3291.

(34) Rietveld, M. H. P.; Grove, D. M.; van Koten, G. New J. Chem. 1997,

21, 751–771.

(16) Ste¸pien´, M.; Latos-Graz˙yn´ski, L. Acc. Chem. Res. 2005, 38, 88–98.

(17) Maeda, H.; Furuta, H. Pure Appl. Chem. 2006, 78, 29–44.

(18) Pawlicki, M.; Latos-Graz˙yn´ski, L. Chem. Rec. 2006, 6, 64–78.

(19) Pacholska-Dudziak, E.; Skonieczny, J.; Pawlicki, M.; Latos-Graz˙yn´-

ski, L.; Szterenberg, L. Inorg. Chem. 2005, 44, 8794–8803.

(20) Stolzenberg, A. M.; Schussel, L.; Summers, J. S.; Foxman, B. M.;

Petersen, J. L. Inorg. Chem. 1992, 31, 1678–1686.

(35) van Koten, G. Pure Appl. Chem. 1989, 61, 1681–1694.

(36) van der Boom, M. E.; Liou, S. Y.; Ben David, Y.; Shimon, L. J. W.;

Milstein, D. J. Am. Chem. Soc. 1998, 120, 6531–6541.

(37) Lord, P.; Olmstead, M. M.; Balch, A. L. Angew. Chem., Int. Ed.

1999, 38, 2761–2763.

(38) Vogel, E. Pure Appl. Chem. 1993, 65, 143–152.

(39) Medforth, C. J. NMR Spectroscopy of Diamagnetic Porphyrins. In

The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,

R., Eds.; Academic Press: San Diego, CA, 2000; pp 1-80.

(40) Pacholska, E.; Latos-Graz˙yn´ski, L.; Ciunik, Z. Angew. Chem., Int.

Ed. 2001, 40, 4466–4469.

(21) Shimizu, S.; Osuka, A. Eur. J. Inorg. Chem. 2006, 1319–1335.

(22) Latos-Graz˙yn´ski, L.; Lisowski, J.; Chmielewski, P. J.; Grzeszczuk,

M.; Olmstead, M. M.; Balch, A. L. Inorg. Chem. 1994, 33, 192–

197.

(23) Ste¸pien´, M.; Latos-Graz˙yn´ski, L.; Lash, T. D.; Szterenberg, L. Inorg.

Chem. 2001, 40, 6892–6900.

(41) Ste¸pien´, M.; Latos-Graz˙yn´ski, L. J. Am. Chem. Soc. 2002, 124, 3838–

3839.

(24) Pawlicki, M.; Latos-Grazyn´ski, L. Chem. Eur. J. 2003, 9, 4650–

4660.

(42) Ste¸pien´, M.; Latos-Graz˙yn´ski, L.; Szterenberg, L.; Panek, J.; Latajka,

Z. J. Am. Chem. Soc. 2004, 126, 4566–4580.

(25) Ste¸pien´, M.; Latos-Graz˙yn´ski, L. Chem. Eur. J. 2001, 7, 5113–5117.

(26) Lash, T. D.; Colby, D. A.; Graham, S. R.; Ferrence, G. M.; Szczepura,

L. F. Inorg. Chem. 2003, 42, 7326–7338.

(43) Ste¸pien´, M.; Latos-Graz˙yn´ski, L.; Szterenberg, L. Inorg. Chem. 2004,

43, 6654–6662.

(44) Rzepa, H. S. Chem. ReV. 2005, 105, 3697–3715.

(45) Warner, P. M. J. Org. Chem. 2006, 71, 9271–9282.

(46) Heilbronner, E. Tetrahedron Lett. 1964, 5, 1923–1928.

(47) Herges, R. Chem. ReV. 2006, 106, 4820–4842.

(48) Castro, C.; Isborn, C. M.; Karney, W. L.; Mauksch, M.; Schleyer,

P. Org. Lett. 2002, 4, 3431–3434.

(27) Liu, D.; Lash, T. D. Chem. Commun. 2002, 2426–2427.

(28) Venkatraman, S.; Anand, V. G.; Pushpan, S. K.; Sankar, J.;

Chandrashekar, T. K. Chem. Commun. 2002, 462–463.

(29) Furuta, H.; Maeda, H.; Osuka, A.; Yasutake, M.; Shinmyozu, T.;

Ishikawa, Y. Chem. Commun. 2000, 1143–1144.

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A R T I C L E S

Pacholska-Dudziak et al.

The Mo¨bius π-systems were proposed as transition states^49–52

and reactive intermediates.⁵³The ﬁrst examples of stable, neutral

Mo¨bius π systems, that is, biantraquinodimethane modiﬁed

[16]annulenes, have been synthesized only recently by Herges

and co-workers.^54,55In those molecules, the twist was achieved

by combining normal and in-plane conjugation. The effect of

benzannelation on Mo¨bius [4n]annulene aromaticity in these

compounds was analyzed by Schleyer and co-workers.⁵⁶

Recently we have presented the ﬁrst example of dynamic

switching between Hu¨ckel and Mo¨bius topologies in a conju-

gated molecule. This effect is documented for an expanded

porphyrin analogue containing para-phenylene rings A,D-di-p-

benzi[28]hexaphyrin.⁵⁷Subsequently Osuka and co-workers

demonstrated that Mo¨bius aromatic molecules are spontaneously

formed upon metalation of meso-aryl-substituted expanded

porphyrins ([36]octaphyrin, [32]heptaphyrin, [26]heptaphyrin,

and N-fused [24]pentaphyrin).^58,59

Figure 1. The electronic spectra of (VPH₂-1)Pd^IICl (black line), (VPH-

3)Pd^II(blue line), [(VPH₂-4)Pd^II]BF₄(green line), and [(VPH₂-5)Pd^II]BF₄

(red line) in CH₂Cl₂.

The present contribution describes the synthesis and properties

of palladium vacataporphyrin complexes which reveal Hu¨ckel

aromaticity or Mo¨bius antiaromaticity of [18]annulene applying

a butadiene fragment of vacataporphyrin as a topology selector.

Their behavior was studied using spectroscopic and computa-

tional methods.

Results and Discussion

Formation and Characterization of (VPH₂-1)Pd^IICl. Reaction

of Pd(PhCN)₂Cl₂with vacataporphyrin, (VPH₂-1)H in chloro-

form results in the formation of the four-coordinate (VPH₂-

1)Pd^IICl (Scheme 1).

Figure 2. ¹H NMR spectrum of (VPH₂-1)Pd^IICl, Ar ) p-Tol (CDCl₃,

298 K).

Scheme 1. Synthesis of (VPH₂-1)Pd^IICl. Ar ) 4-Metoxyphenyl

The vacataporphyrin acts as a monoanionic ligand coordinat-

ing through the three nitrogen donors. To complete the coordina-

tion sphere of metal dication one additional ligand is required.

Originally the identity of (VPH₂-1)Pd^IICl was conﬁrmed by high

resolution mass spectrometry and NMR spectroscopy. The

electronic absorption spectrum of (VPH₂-1)Pd^IICl is shown in

Figure 1 (solid black line). A porphyrin-like pattern is clearly

present with a distinct Soret-like band (440 nm) and a set of

bands in the Q region. The spectral pattern resembles that of

(VPH₂-1)Cd^IICl and (VPH₂-1)Zn^IICl providing that the reference

spectra were collected under conditions where the (VPH₂-

1)M^IICl stereoisomer prevailed in solution.¹⁹

(Anis), p-Tol

To obtain a single well-deﬁned product it is absolutely

necessary to carry out the insertion in the dark. It has been also

found that (VPH₂-1)Pd^IICl undergoes demetalation in acidic

solutions.

(VPH₂-1)Pd^IICl has an effective C_ssymmetry, with the mirror

plane passing through the metal ion, chloride, and pyrrolic N(23)

1

nitrogen. As a consequence, the H NMR spectrum (Figure 2)

contains three pyrrole resonances (H(7)/H(18), H(8)/H(17), and

H(12)/H(13)) and two multiplets of butadiene resonances (H(1)/

H(4) and H(2)/H(3)). Nonequivalence of two macrocyclic sides

causes each aryl ring to have two distinct ortho and two meta

resonances, unless the rotation around the C_meso-C_ipsobond is

sufﬁciently fast. The (VPH₂-1)Pd^IICl stereoisomer has been

unambiguously identiﬁed as a single product of insertion in the

(49) Castro, C.; Karney, W. L.; Valencia, M. A.; Vu, C. M. H.; Pemberton,

R. P. J. Am. Chem. Soc. 2005, 127, 9704–9705.

(50) Pemberton, R. P.; McShane, C. M.; Castro, C.; Karney, W. L. J. Am.

Chem. Soc. 2006, 128, 16692–16700.

(51) Zimmerman, H. E. J. Am. Chem. Soc. 1966, 88, 1564–1565.

(52) Moll, J. F.; Pemberton, R. P.; Gutierrez, M. G.; Castro, C.; Karney,

W. L. J. Am. Chem. Soc. 2007, 129, 274–275.

(53) Mauksch, M.; Gogonea, V.; Jiao, H.; Schleyer, P. v. R. Angew. Chem.,

Int. Ed. Engl. 1998, 37, 2395–2398.

1

dark (Scheme 1). Thus (VPH₂-1)Pd^IICl shares a common H

(54) Ajami, D.; Oeckler, O.; Simon, A.; Herges, R. Nature 2003, 426,

819–821.

NMR spectral pattern with the free base or (VPH₂-1)Cd^IICl.¹⁹

In particular the AA′XX′ multiplets are consistent with an

extended structure of C(1)C(2)C(3)C(4) annulene moiety (Figure

(55) Ajami, D.; Hess, K.; Ko¨hler, F.; Na¨ther, C.; Oeckler, O.; Simon, A.;

Yamamoto, C.; Okamoto, Y.; Herges, R. Chem. Eur. J. 2006, 12,

5434–5445.

(56) Castro, C.; Chen, Z.; Wannere, C. S.; Jiao, H.; Karney, W. L.;

Mauksch, M.; Puchta, R.; Hommes, N. J. R.; Schleyer, P. J. Am.

Chem. Soc. 2005, 127, 2425–2432.

1

2, Scheme 1). Actually the determined H NMR parameters

closely resemble those of [18]annulene.⁶⁰On the other hand

¹H NMR spectrum of the pyrrolic part of (VPH₂-1)Pd^IICl

exhibits values of J and δ which are typical for tetraaryl-21-

(57) Ste¸pien´, M.; Latos-Graz˙yn´ski, L.; Sprutta, N.; Chwalisz, P.; Szterenberg,

L. Angew. Chem., Int. Ed. 2007, 46, 7869–7874.

(58) Tanaka, Y.; Saito, S.; Mori, S.; Aratani, N.; Shinokubo, H.; Shibata,

N.; Higuchi, Y.; Yoon, Z. S.; Kim, K. S.; Noh, S. B.; Park, J. K.;

Kim, D.; Osuka, A. Angew. Chem., Int. Ed. 2008, 47, 693–696.

(59) Park, J. K.; Yoon, Z. S.; Yoon, M.-C.; Kim, K. S.; Mori, S.; Shin,

J.-Y.; Osuka, A.; Kim, D. J. Am. Chem. Soc. 2008, 130, 1824–1825.

(60) Stevenson, C. D.; Kurth, T. L. J. Am. Chem. Soc. 2000, 122, 722–

723.

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Hückel and Möbius Conformational Rearrangements

A R T I C L E S

Figure 4. ¹H NMR spectrum of (VPH-3)Pd^II, Ar ) p-Tol (CDCl₃, 298

K). Resonance assignments follow the numbering given in Scheme 2.

fragment is remote from the palladium (Pd · · ·C(1), 3.31;

Pd· · ·H(1), 2.89; Pd· · ·C(2), 4.19; Pd· · ·H(2), 5.05 Å). The

distinct features of the structure are the planar (approximately)

butadiene moiety displaced above the C_mesoplane (C(5)-

C(10)C(15)C(20)) and the very pronounced bending of the

chloride ligand toward the annulenic unit. Signiﬁcantly the

N(22)-Pd-N(24) fragment (170.0(1)°) is only slightly distorted

from linearity, while N(23)-Pd-C(l) is bent (155.4(1)°) as a

consequence of strain induced by incorporation of a metal-

chloride bond into a macrocycle with a limited core. The angle

between the butadiene plane and the plane of the four meso

carbons (C_butadiene/C_meso) is relatively small and equals 3.5°.

Actually the nearly parallel position of the C_butadieneand C_meso

planes seen at the Figure 3 (the side-on projection) is nicely

reﬂected by the displacement the C(1), C(2), C(3), C(4) atoms

from the C_mesoplane (C(1) 0.74, C(2) 0.69, C(3) 0.64, C(4)

0.55 Å). The bond distances within the tripyrrolic and annulenic

framework of complexes indicate that the macrocycle is highly

conjugated following the free base pattern.¹

Figure 3. The crystal structure of (VPH₂-1)Pd^IICl (top, perspective view;

bottom, side view; phenyl groups omitted for clarity). The thermal ellipsoids

represent 50% probability.

Scheme 2. Formation of Palladium(II) Vacataporphyrin Complex

Containing the Pd-C Bond, (VPH-3)Pd^II

Formation and Characterization of (VPH-3)Pd^II. The pho-

tochemical isomerization (VPH₂-1)M^IICl a (VPH₂-2)M^IICl was

previously detected for cadmium(II) and zinc(II) complexes.¹⁹

Initially a sample of (VPH₂-1)Pd^IICl has been exposed to light

in an attempt to generate an expected (VPH₂-2)Pd^IICl stereo-

isomer. The progress of the reaction has been followed by

electronic spectroscopy. After one hour the spectrum of (VPH₂-

1)Pd^IICl has been completely replaced by a spectrum of aromatic

(VPH-3)Pd^II, differentiated from the original one by batochromic

shifts of major bands (Figure 1). In particular the Soret band

becomes more intense and shifts batochromically to 461 nm.

The reaction proceeds with well-deﬁned isosbestic points

proving that the conversion has not included any abundant

intermediate form. The novel organopalladium(II) complex

(VPH-3)Pd^II(Scheme 2) results from an intramolecular carba-

palladation.

heteroporphyrins.⁶¹The δ_H(2)- δ_H(1)chemical shift difference

equal to 8.01 ppm (for Ar ) p-Tol, see Scheme 1), considered

as the experimental criterion of aromaticity,^62–64is signiﬁcantly

smaller than the 12.15, 10.77, and 9.28 ppm values determined

for (VPH₂-1)H, (VPH₂-1)Cd^IICl, and (VPH₂-1)Zn^IICl, respec-

tively.¹⁹The coordination imposes the macrocyclic nonplanarity

which is revealed by the diminished ring current.

The structure of (VPH₂-1)Pd^IICl has been determined by

X-ray crystallography. Perspective views of the molecule are

shown in Figure 3. The geometry of the complex reﬂects the

balance among constraints of the macrocycle ligand, the size

of the palladium ion, and the predisposition of the palladium

for square planar geometry.

The ¹H NMR spectrum of (VPH-3)Pd^II(Figure 4) differs from

that determined till present for any stereoisomer of vacatapor-

phyrin complexes. Speciﬁcally the increased multiplicity of

resonances indicates the full asymmetry of the acting ligand.

The most notable feature of (VPH-3)Pd^IIis coordination of

palladium(II) in the plane of the butadiene fragment through

the unprotonated trigonally hybridized C(2) center. The H(2)

resonance of (VPH₂-1)Pd^IICl, located at 9.14 ppm, is absent in

the ¹H NMR spectrum of (VPH-3)Pd^II. Consistently the AA′XX′

spin system, H(1)H(2)H(3)H(4), of (VPH₂-1)Pd^IICl, has been

replaced by the AMX, H(1)H(3)H(4), of (VPH-3)Pd^II. Impor-

The palladium atom is bound to three nitrogen atoms. The

Pd-N bond lengths (Pd-N(23): 2.021(4), Pd-N(24): 2.069(4),

Pd-N(22): 2.073(3) Å) fall in the range of distances (2.00-2.08

Å) seen for palladium porphyrins.^{20,22,65–67}The annulene

(61) Latos-Graz˙yn´ski, L. Core Modiﬁed Heteroanalogues of Porphyrins

and Metalloporphyrins. In The Porphyrin Handbook; Kadish, K. M.,

Smith, K. M., Guilard, R., Eds.: Academic Press: New York, 2000;

pp 361-416.

(62) Konig, H.; Eickmeier, C.; Moller, M.; Rodewald, U.; Frank, B.

Angew. Chem., Int. Ed. Engl. 1990, 29, 1393–1395.

(63) Chmielewski, P. J.; Latos-Graz˙yn´ski, L.; Rachlewicz, K. Chem. Eur.

J. 1995, 1, 68–73.

3

tantly the J_H(3)H(4)coupling constant equals 9.2 Hz and is

consistent with the cis geometry of C(2)C(3)C(4)C(5). Partial

(64) Sprutta, N.; Latos-Graz˙yn´ski, L. Chem. Eur. J. 2001, 7, 5099–5112.

(65) Golder, A. J.; Milgrom, L. R.; Nolan, K. E.; Povey, D. C. Chem.

Commun. 1987, 1788–1790.

(66) Vogel, E.; Bro¨ring, M.; Erben, C.; Demuth, R.; Lex, J.; Nendel, M.;

Houk, K. N. Angew. Chem., Int. Ed. Engl. 1997, 36, 353–357.

(67) McGhee, E.; Hoffman, B. M.; Ibers, J. A. Inorg. Chem. 1991, 39,

2162–2165.

9

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A R T I C L E S

Pacholska-Dudziak et al.

Scheme 3. Alkylation and Protonation of (VPH-3)Pd^II

Figure 6. Drawing of the [(VPHMe-4)Pd^II]⁺skeleton as obtained from

the DFT (B3LYP) optimization. Meso-aryl rings were omitted for clarity.

Arrows indicate the selected contacts, which are crucial in the interpretation

of the NOE relays. The distances equal (a) 3.53 Å, (b) 3.67 Å.

lation. Thus [(VPH₂-4)Pd^II]⁺has been generated by addition

of HBr in CH₃COOH or HBF₄in Et₂O to (VPH-3)Pd^II(Scheme

3, Figure 5). An HBF₄titration has been carried out in CD₂Cl₂

(200 K) and the progress of this reaction has been monitored

1

by H NMR. The feasible palladium-centered hydrogenation

to form [(VPH-4)Pd^IVH]⁺has not been detected. The protona-

tion is reversible. Thus heating the solution of [(VPH₂-4)Pd^II]⁺

with addition of methanol recovers (VPH-3)Pd^II(Scheme 3).

The analogous deprotonation of [(VPHMe-4)Pd^II]⁺follows the

similar path producing the appropriate C-methylated aromatic

derivative (VPMe-3)Pd^II(Scheme 3).

assignments of ¹³C resonances have been carried out using

¹H-¹³C correlation techniques (HMQC, HMBC). The chemical

shift found for C(2) (150.8 ppm) is indicative of the sp²

hybridization. Thus the NMR data provide an evidence for the

coordination of the aromatic dianionic ligand derived from

(VPH₂-1)H after the dissociation of NH and C(2)H protons.

Alkylation and Protonation of (VPH-3)Pd^II. Of particular

importance is the observation that (VPH-3)Pd^IIis susceptible

to methylation at the C(2) atom. Thus (VPH-3)Pd^IIrapidly reacts

with methyl iodide but only in presence of AgBF₄to yield a

unique organopalladium(II) complex [(VPHMe-4)Pd^II]BF₄

(Scheme 3).

The representative ¹H NMR spectral patterns of [(VPH₂-

4)Pd^II]⁺and [(VPHMe-4)Pd^II]⁺are shown in Figure 5. The

unambiguous assignments of resonances, which are given above

selected groups of peaks have been made on the basis of rela-

tive intensities and detailed two-dimensional NMR. Importantly

the 2D NMR studies have been accompanied by crucial

assignments achieved via speciﬁc deuterium labeling of vacat-

aporphyrin complexes. The deuterated derivative (VPH₂-d_x)H

(x > 2), where deuterium atoms are bound to C(1) and C(4)

and to some extent to ꢀ-pyrrolic carbon atoms, has been

synthesized as described previously.¹The localization and

degree of deuteration has been readily determined for (VPH₂-

d_x-1)Pd^IICl. The deuteration has resulted in diminished intensity

of the H(1) and H(4) resonance(s) for (VPH₂-d_x-1)Pd^IICl and

subsequently for each compound generated in the sequence of

processes (Scheme 2, 3). The peculiar paratropicity of [(VPH₂-

4)Pd^II]⁺and [(VPHMe-4)Pd^II]⁺is evident from unusual features

1

Judging by the H NMR spectrum of the reaction mixture,

the process is nearly quantitative provided that there are no traces

of water. The selectively deuterated [(VPH(Me-d₃)-4)Pd^II]⁺has

been obtained using methyl-d₃iodide. Protonation causes

spectral changes which resemble those resulting from methy-

1

of their H NMR spectra (discussed here for the 10,15-di-(p-

tolyl) derivative). The ꢀ-H signals of both species are shifted

upﬁeld relative to nonaromatic fully conjugated systems which

contain pyrrole moieties.^25,68The ¹H NMR spectrum of [(VPH₂-

4)Pd^II]⁺exhibits one AB (at 5.89 and 5.33 ppm) and one A₂

(4.95 ppm) patterns assigned to pyrrole protons. The reversed

order of meso-aryl resonances has been observed (p-H > m-H

> o-H). In particular the upﬁeld relocation of the ortho protons

(phenyl 7.15, p-tolyl 7.01 ppm) seems to be of signiﬁcance. In

the case of the 10,15-bis(4-methoxyphenyl) derivative, the

inﬂuence of methoxy substituent obscures the paratropic effect.

The peculiar position of the methine H(2), H(3) resonance at

8.98 ppm has also been noted. Signiﬁcantly, the adjacent H(1)

hydrogen produces the resonance at 6.18 ppm. The spectroscopic

pattern observed for [(VPH₂-4)Pd^II]⁺is apparently consistent

with a pronounced folding of the macrocyle and palladium η²-

alkene coordination (vide infra) and indicates Mo¨bius antiaro-

maticity of this 4n+2 π-electron system.^44,46,47At the same time

Figure 5. ¹H NMR spectra (CD₂Cl₂, 298 K, Ar ) Anis) of (A) (VPHCH₃-

4)Pd^II, (B) [(VPH₂-4)Pd^II]⁺, Ar ) Anis. Resonance assignments follow the

numbering given in Scheme 3. Inset presents the details of the H(2,3)

multiplet.

(68) Mys´liborski, R.; Latos-Graz˙yn´ski, L. Eur. J. Org. Chem. 2005, 5039–

5048.

9

6186 J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008

Hückel and Möbius Conformational Rearrangements

A R T I C L E S

Figure 7. ¹H NMR spectra (250 K, CDCl₃, Ar ) Anis) showing the

transformation of [(VPH₂-1)Pd^II(H₂O)]⁺, prepared from (VPH₂-1)Pd^IICl and

AgBF₄in CDCl₃. The mixture held in 298 K for (A) 90 s, (B) 150 s, (C)

2 h; then cooled to 250 K. Labeling: green triangle, [(VPH₂-1)Pd^II(H₂O)]⁺;

red dot, [(VPH₂-5)Pd^II]⁺; blue square, [(VPH₂-4)Pd^II]⁺.

Figure 8. Geometries of vacataporphyrin stereoisomers (VPH₂-n)H ob-

tained in a DFT optimization for level B3LYP/6-31G**. Projections emphasize

the conformations of macrocycle. The conﬁguration of C(20)C(1)C(2)C(3)-

C(4)C(5) is described as in the text.

the ¹³C chemical shift of C(2) (100.1 ppm) is consistent with

the carbon coordination (the chemical shift for noncoordinated

C(1) equals 125.3 ppm). Actually the change of the coordination

mode of vacataporphyrin to palladium determined for [(VPH₂-

4)Pd^II]⁺can be readily appreciated by analysis of coordination

shifts deﬁned here as a ¹³C chemical shift difference between

the (VPH₂-1)Pd^IICl and [(VPH₂-4)Pd^II]⁺at chosen positions.

These values equal C(1) 10.8, C(2) 37.9, C(7) 5.6, C(8) -2.4,

C(12) 5.0 ppm and clearly point out C(2) and C(3) acting as a

coordinating unit.^69–75

The degeneracy of [(VPH₂-4)Pd^II]⁺resonances is removed

for [(VPHMe-4)Pd^II]⁺. Thus three pyrrolic AB patterns have

been detected even though they are in the similar region as

determined for [(VPH₂-4)Pd^II]⁺(5.0-6.0 ppm). The atypical

¹H chemical shift values resemble those of [(VPH₂-4)Pd^II]⁺.

The peculiar downﬁeld position of the H(3) resonance (8.04

ppm) has been preserved but the effect is evidently smaller as

compared to the nonmethylated counterpart.

1

The H NMR spectrum of [(VPH₂-4)Pd^II]⁺complies with

the C₂molecular symmetry. The twisted structure of [(VPH₂-

4)Pd^II]⁺(Scheme 3) accounts for the presence of the paratropic

effect. Such a macrocyclic conformation affords the palladium

η²-C(2)C(3) interaction. The C(2) and C(3) atoms are located

at the opposite sides of the N₃plane owing to the perpendicular

orientation of the C(2)C(3) bond. Actually the C(2)C(3)

fragment acquires the common orientation of the η²-alkene for

(69) Elschenbroich, C.; Salzer, A. Organometallics-A concise Introduction;

VCH: Weinheim, Germany, 1989.

(70) Hahn, C.; Vitagliano, A.; Giordano, F.; Taube, F. Organometallics

1998, 2960–2066.

Figure 9. Geometries of palladium vacataporphyrin complexes obtained

in a DFT optimization for level B3LYP/LANL2D. Projections emphasize

the conformations of the macrocycle. The conﬁguration of C(20)C(1)C(2)-

C(3)C(4)C(5) is described as in the text.

(71) Bernard, G. M.; Wasylishen, R. E.; Phillips, A. D. J. Phys. Chem.

2000, 104, 8131–8141.

(72) Hahn, C.; Morvillo, P.; Vitagliano, A. Eur. J. Inorg. Chem. 2001,

419–429.

the square planar palladium complexes containing the -CdC-

moiety.⁷⁰For instance, a similar coordination geometry, involv-

ing chelate rings, was reported for a cyclometallated hydridoos-

mium complex of a phosphine-based pincer ligand.⁷⁶There the

η²-alkene moiety is encompassed in the pincer ligand (E)-

(73) Hahn, C.; Morvillo, P.; Herdtweck, E.; Vitagliano, A. Organome-

tallics 2002, 21, 1807–1818.

(74) Cavallo, L.; Macchioni, A.; Zucaccia, C.; Zuccaccia, D.; Orabona,

I.; Ruffo, F. Organometallics 2004, 23, 2137–2145.

(75) Ghebreyessus, K. Y.; Ellern, A.; Angelici, R. J. Organometallics

2005, 24, 1725–1736.

9

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A R T I C L E S

Pacholska-Dudziak et al.

Scheme 4. Alternative Route of Conversion of (VPH₂-1)Pd^IICl into

[(VPH₂-4)Pd^II]⁺

Ph₂P(CH₂)₂CHdCH(CH₂)₂PPh₂allowing the meridional coor-

dination via two phosphine and η²-{CHdCH} centers. Similarly

the chiral palladium(0) complexes with 15-membered trioleﬁnic

macrocyclic ligands present the comparable coordination ge-

ometry consistent with the trans stereochemistry in the palla-

dacyclopropane rings.⁷⁷

One has to notice that a fundamental chain of transformations,

(VPH₂-1)Pd^IICl f (VPH-3)Pd^IIf [(VPH₂-4)Pd^II]⁺(Schemes

2 and 3), requires a series of profound conformational rear-

rangements. Primarily the structural changes involve the C(20)-

C(1)C(2)C(3)C(4)C(5) annulene fragment. For the sake of

convenience a conformation with respect to each bond is

symbolically described as t (trans) or c (cis) respectively. Thus

this sequence of reactions is associated with the following

conformational changes [ttctt] f [ccttc] f [cctcc]. Here it is

important to recall that the AA′XX′ pattern of H(1), H(2), H(3),

and H(4) resonances of (VPH₂-1)Pd^IICl presents the NMR

parameters which resemble those of [18]annulene.^20,21In

particular ³J₁₂) 12.6 Hz and ³J₂₃) 9.8 Hz coupling constants

are consistent with trans and cis arrangement of the respective

hydrogens. In noticeable contrast the AA′XX′ pattern (H(1)-

H(2)H(3)H(4)) of [(VPH₂-4)Pd^II]⁺gives a markedly different

3

set of coupling constants: J₁₂) 5.8 Hz and J₂₃) 14.9 Hz.

These values are, however, consistent with a trans arrangement

of H(2), H(3) hydrogens and are diagnostic of the S-like skeleton

which is unique for the [(VPH₂-4)Pd^II]⁺structure.⁷⁸The

structural models, generated by DFT calculations (vide infra),

have been used to visualize the suggested structures of [(VPH₂-

4)Pd^II]⁺and [(VPHMe-4)Pd^II]⁺(Figures 6 and 9) and to

evaluate the degree of the porphyrin distortion and the proximity

of hydrogens.

All spectroscopic evidence indicated that the [(VPH₂-4)Pd^II]⁺

complex has C₂symmetry and contains an S-like C(20)C(1)C(2)-

C(3)C(4)C(5) structural motif. [(VPHMe-4)Pd^II]⁺preserves the

identical structural feature, albeit the molecular symmetry due

to the substitution is lowered. Concurrently the structural

analysis has been carried out using NOE. The relay of observed

and assigned NOE connectivities for [(VPHMe-4)Pd^II]⁺is

presented in Figure 6. The analytically decisive set of the NOE

effects, unique for the S-like geometry, results from through-

space interactions: 2-CH₃· · ·H(7) (a) and 2-CH₃· · ·5-o-Ph (b).

in solution once protected from light. In contrast, the [(VPH₂-

1)Pd^II(H₂O)]⁺species is unstable at 298 K as data in Figure 7

show. Evidently the fourth coordination position becomes

available to be occupied by C(2)C(3) moiety after a removal

of weakly bound water. In trace A (Figure 7) one can see the

spectrum of [(VPH₂-1)Pd^II(H₂O)]⁺at 250 K. In standing at 298

K for 150 seconds the peaks due to the starting species decrease

in intensity as shown in trace B (the spectrum measured at 250

K to prevent the further transformation). As [(VPH₂-1)Pd^II-

(H₂O)]⁺decomposes an unstable intermediate, [(VPH₂-5)Pd^II]⁺,

forms which eventually decays after 4 h (298 K) into indepen-

dently identiﬁed (see above) [(VPH₂-4)Pd^II]⁺being the sole

stable product of the reaction (Scheme 4). The time of decay

(298 K) of [(VPH₂-5)Pd^II]⁺can be hastened to a few minutes

by the presence of light. At applied conditions the transformation

of [(VPH₂-5)Pd^II]⁺into [(VPH₂-4)Pd^II]⁺is irreversible.

Alternative Route to [(VPH₂-4)Pd^II]⁺. The addition of AgBF₄

to the solution of (VPH₂-1)Pd^IICl in CDCl₃results in formation

of the complex cations [(VPH₂-1)Pd^II(PhCN)]⁺or [(VPH₂-

1)Pd^II(H₂O)]⁺. Traces of benzonitrile (introduced in the course

of insertion) or water are sufﬁcient to introduce respective

ligands to the coordination sphere. The methathesis reaction,

carried out in dark, has been followed by ¹H NMR spectroscopy.

The spectroscopic patterns of [(VPH₂-1)Pd^II(PhCN)]⁺or [(VPH₂-

1)Pd^II(H₂O)]⁺(Figure 7, trace A) resemble maternal (VPH₂-

1)Pd^IICl (Figure 2). Characteristically, the upﬁeld relocated

resonances of coordinated benzonitrile have been readily

identiﬁed at 6.19, 6.97, and 7.19 ppm for o-H, m-H and p-H,

respectively (298 K, CDCl₃). The resonance of the coordinated

water molecule has been detected at -1.17 ppm (250 K). It

has also been found that [(VPH₂-1)Pd^II(PhCN)]⁺remains stable

The ¹H NMR spectrum of [(VPH₂-5)Pd^II]⁺(Figure 7B)

resembles the basic features of [(VPH₂-4)Pd^II]⁺(Figures 7C

and 5B, discussed for the 10,15-bis(4-methoxyphenyl) deriva-

tives) consistent with residual Mo¨bius antiaromaticity, albeit the

effect is markedly less pronounced ([(VPH₂-5)Pd^II]⁺: δ_H(2)-δ_H(1)

) 1.63 ppm, [(VPH₂-4)Pd]⁺: δ_H(2)-δ_H(1)) 2.67 ppm). The ¹H

NMR spectrum of [(VPH₂-5)Pd^II]⁺(250 K) exhibits one AB

(6.77, 6.35 ppm) and one A₂(6.47 ppm) patterns assigned to

pyrrole resonances. The reversed order of meso-phenyl reso-

nances has been observed (p-H > m-H > o-H). In particular

the upﬁeld relocation of the ortho protons (7.30 ppm) seems to

be of signiﬁcance. The remote position of the methine H(2)

resonance at 7.67 ppm has also been noted. The AA′XX′ pattern

assigned to H(1), H(2), H(3), and H(4) of [(VPH₂-5)Pd^II]⁺gives

the following coupling constants: ³J₁₂) 6.4 Hz and ³J₂₃) 10.2

Hz, which suggests the strongly twisted conformation of the

butadiene part.⁷⁸One has to realize that deceptively similar

spectra of [(VPH₂-5)Pd^II]⁺and (VPH₂-2)Zn^IICl¹⁹demonstrate

(76) Liu, S. H.; Huang, X.; Lin, Z.; Lau, C. P.; Jia, G. Eur. J. Inorg.

Chem. 2002, 1697–1702.

(77) Pla-Quintana, A.; Roglans, A.; de Julián-Ortiz, J. V.; Moreno-Man´as,

M.; Parella, T.; Benet-Buchholz, J.; Solans, X. Chem. Eur. J. 2005,

11, 2689–2697.

(78) Gernel, C.; Mereiter, K.; Schmid, R.; Kirchner, K. Organometallics

1997, 16, 2623–2626.

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Hückel and Möbius Conformational Rearrangements

A R T I C L E S

Scheme 5. Transformations of Palladium Vacataporphyrin (Transient Species in Braces)

the reversed position of H(1) and H(2) resonances as accounted

by differences in the macrocyclic conformation.

an arenium form is involved in the analogous transient

stage.^{79–81,86–90}The following dissociation of H⁺from {[(iso-

VPH₂-3′)Pd^II]⁺} yields (VPH-3)Pd^II. An alternative route

involves a 1,2-shift affording the hydridopalladium(IV) complex

{[(VPH-3)Pd^IVH]⁺}. The reductive elimination eventually

produces (VPH-3)Pd^II. Importantly the replacement of strongly

bound chloride in (VPH₂-1)Pd^IICl by relatively weakly bound

water triggered an independent route to form [(VPH₂-4)Pd^II]⁺

which involves a new intermediate [(VPH₂-5)Pd^II]⁺but omits

the (VPH-3)Pd^IIspecies.

Reaction Mechanisms. The formation of (VPH-3)Pd^IIfrom

(VPH₂-1)Pd^IICl or [(VPH₂-4)Pd^II]⁺can be considered as a

special case of C-H activation assisted by intramolecular

palladium coordination. Actually such a process can be classiﬁed

as a classical cyclometalation but being conﬁned into a macro-

cyclic cavity. Several examples of such chemistry have been

elaborated in the pincer ligand environment.^79–81In fact the

formation of palladium(II) carbaporphyrinoids^{16,23–26,28,29,82–84}

or palladium(II) hexaphyrin⁸⁵provides the appropriate examples

as well.

The mechanism suggested for methylation of (VPH-3)Pd^II

is presented in Scheme 6. An initial oxidative addition of MeI

in the presence of AgBF₄may afford a methylpalladium(IV)

species {[(VPH-3)Pd^IVMe]⁺}.⁹¹This step should be followed

by a 1,2-shift from palladium(II) to the C(2) carbon yielding

the palladium(II) complex of 2-Me-iso-vacataporphyrin {[(iso-

VPHMe-3′)Pd^II]⁺} where the tetrahedral C(2) carbon atom binds

palladium(II). The next transformation seems to be unique to a

macrocyclic environment as the methylation product is forced

to remain in the adjacency of palladium(II). The appropriately

prearranged structure of {[(iso-VPHMe-3′)Pd^II]⁺} undergoes a

smooth slippage to afford the η²-complex [(VPHMe-4)Pd^II]⁺.

The ﬁrst step (Scheme 5) is conformational rearrangement

[(VPH₂-1)Pd^II]⁺a {[(VPH₂-2)Pd^II]⁺} (from now on acronyms

of transient species are given in braces), affording the η²-

interaction in {[(VPH₂-2)Pd^II]⁺T [(VPH₂-2)Pd^IV]⁺}. Subse-

quently an electrophilic process may produce a transient

palladium(II) complex of iso-vacataporphyrin {[(iso-VPH₂-

3′)Pd^II]⁺} affording a σ-coordination of tetrahedral C(2) carbon

atom, where iso-vacataporphyrin (iso-VPH₃), is a hypothetical

tautomer of vacataporphyrin formed by relocation of NH

hydrogen on the C(2) atom. In the case of relevant pincer ligands

(86) Grove, D. M.; van Koten, G.; Louwen, J. N.; Noltes, J. G.; Spek,

A. L.; Ubbels, H. J. J. Am. Chem. Soc. 1982, 104, 6609–6616.

(87) Terheijden, J.; van Koten, G.; Vinke, I. C.; Spek, A. L. J. Am. Chem.

Soc. 1985, 107, 2891–2898.

(79) Canty, A. J.; van Koten, G. Acc. Chem. Res. 1995, 28, 406–413.

(80) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750–

3781.

(81) van der Boom, M. E.; Milstein, D. Chem. ReV. 2003, 103, 1759–

(88) Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am.

Chem. Soc. 2000, 122, 11822–11833.

1792.

(82) Graham, S. R.; Ferrence, G. M.; Lash, T. D. Chem. Commun. 2002,

894–895.

(89) Albrecht, M.; Spek, A. L.; van Koten, G. J. Am. Chem. Soc. 2001,

123, 7233–7246.

(83) Liu, D.; Ferrence, G. M.; Lash, T. D. J. Org. Chem. 2004, 69, 6079–

6093.

(90) Vigalok, A.; Milstein, D. Acc. Chem. Res. 2001, 34, 798–807.

(91) DFT optimizations demonstrated that the putative methylpalladium

IV) [(VPH-3)Pd^IVMe]⁺is highly unstable relatively to [(VPHMe-

4)Pd^II]⁺as the respective energy difference equals 38.40 kcal/mol.

Thus [((VPH-3)Pd^IVMe]⁺may play at best a role of a transient

species (see Tables S14, S15 of the Supporting Information).

(84) Furuta, H.; Kubo, N.; Maeda, H.; Ishizuka, T.; Osuka, A.; Nanami,

H.; Ogawa, T. Inorg. Chem. 2000, 39, 5424–5425.

(85) Mori, S.; Shimizu, S.; Taniguchi, R.; Osuka, A. Inorg. Chem. 2005,

44, 4127–4129.

9

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A R T I C L E S

Pacholska-Dudziak et al.

Scheme 6. Mechanism of Alkylation

Table 1. Selected Calculated Geometrical Parameters, Relative

Energies, and Nucleus-Independent Chemical Shift (NICS) for

Vacataporphyrin Conformers

A protonation of (VPH-3)Pd^IIwill follow the identical sequence

of reactions forming eventually [(VPH₂-4)Pd^II]⁺(Scheme 5).

Contrary to methylation, the protonation is reversible (Scheme

3). The conversion from [(VPH₂-4)Pd^II]⁺to (VPH-3)Pd^II

includes a rearrangement to {[(iso-VPH₂-3′)Pd^II]⁺} followed

by dissociation. An alternative path of the transformation (VPH-

3)Pd^IIf [(VPH₂-4)Pd^II]⁺involves a direct protonation or

methylation of (VPH-3)Pd^IIat the activated C(2) carbon.

DFT Calculations: Vacataporphyrin. Density functional

theory (DFT) has been successfully used to describe the

properties of porphyrins and related systems, providing informa-

tion on their conformational behavior, tautomerism, and aro-

maticity.⁹²We have previously applied DFT modeling to study

the relationship between aromaticity, tautomerism, and coor-

dinating capabilities of carbaporphyrinoids.^{23,42,43,93–96}In par-

ticular, we demonstrated the energetic preference of a free

vacataporphyrin for a nearly planar conformation.^1,19In direct

analogy to an unorthodox geometry of 21,23-ditelluraporphy-

rin,⁴⁰the inverted geometry of the free ligand was considered.

The hypothetical inverted geometry of vacataporphyrin has been

trapped by coordination of nickel(II), zinc(II), or cadmium(II)

ions.¹⁹

(VPH₂-1)H (VPH₂-2)H (VPH₂-3)H (VPH₂-4)H (VPH₂-5)H (VPH₂-6)H

E^a

0

7.02

8.05 19.68 14.05

8.18

-1.7

-9.1

b

NICS_B3LYP

-13.6 -7.5

-12.6 -3.9

-7.7

+6.0

+4.4

+4.0

+4.6

c

NICS_KMLYP

-2.0

C(20)-C(1)^d

C(1)-C(2)^d

1.384 1.377 1.373 1.471 1.357 1.391

1.405 1.426 1.432 1.351 1.457 1.402

1.398 1.369 1.366 1.453 1.348 1.385

1.405 1.426 1.435 1.351 1.457 1.399

1.384 1.377 1.369 1.471 1.357 1.439

0.011 0.028 0.035 0.063 0.057 0.021

C(2)-C(3)^d

C(3)-C(4)^d

C(4)-C(5)^d

standard deviation^e

^aE relative to (VPH₂-1)H, kcal/mol, calculated using the B3LYP/

6-31G**//B3LYP/6-31G** approach. ^bNICS in ppm, calculated by GIAO-

B3LYP method for B3LYP/6-31G** geometries. ^cNICS in ppm, calcula-

ted by GIAO-B3LYP method for KMLYP/6-31G** geometries. ^dIn Å,

for B3LYP/6-31G** geometries. ^eBond length standard deviation along

the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment.

The ﬁnal B3LYP geometries are shown in Figure 8. To present

a complete picture, the previously optimized structures (VPH₂-

1)H and (VPH₂-2)H are included.

A genuine energy minimum was obtained for all structures.

The calculated total energies, using the B3LYP/6-31G**//

B3LYP/6-31G** approach (Table 1) demonstrate that the planar

conformer is the most stable, which accounts for the preference

for (VPH₂-1)H in the solution and in the solid state. The relative

energy of conformers increases in a series (VPH₂-1)H < (VPH₂-

2)H < (VPH₂-3)H e (VPH₂-6)H < (VPH₂-5)H < (VPH₂-4)H.

Actually these values are relatively low. An analysis of the

calculated relative stabilities of conformers provides some

additional insight into the coordinating properties of vacatapor-

phyrin. Previously, considering the mechanism of metal ion

insertion into the coordination core of inverted porphyrin or

2-oxybenziporphyrin we included a preorganization step into

the mechanistic route.^23,97Thus, to satisfy the requirements

imposed by the inserted metal ion the molecular and electronic

structures of the ligand have to be prearranged. Such a step

may contribute to an overall activation energy of metal insertion.

Speciﬁc coordination constraints of palladium(II) may pro-

mote a variety of plausible macrocyclic conformations in

accordance with the fascinating conformational ﬂexibility of an

annulene moiety. Consequently we have considered several

novel geometries of vacataporphyrin being evidently inspired

by the experimentally determined ligand structures supported

by palladium coordination. These conformers were subjected

to a DFT optimization at the B3LYP/6-31G** level of theory.

(92) Ghosh, A. Quantum Chemical Studies of Molecular Strucutres and

Potential Energy Surafaces of Porphyrins and Hemes. In The

Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.;

Academic Press: San Diego, CA, 2000; pp 1-38.

(93) Ste¸pien´, M.; Latos-Graz˙yn´ski, L.; Szterenberg, L. J. Org. Chem. 2007,

72, 2259–2270.

(94) Mys´liborski, R.; Latos-Graz˙yn´ski, L.; Szterenberg, L.; Lis, T. Angew.

Chem., Int. Ed. 2006, 45, 3670–3674.

(95) Pawlicki, M.; Latos-Graz˙yn´ski, L.; Szterenberg, L. Inorg. Chem. 2005,

44, 9779–9786.

(96) Pawlicki, M.; Latos-Graz˙yn´ski, L.; Szterenberg, L. J. Org. Chem.

2002, 67, 5644–5653.

(97) Szterenberg, L.; Latos-Graz˙yn´ski, L. Inorg. Chem. 1997, 36, 6287–

6291.

9

6190 J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008

Hückel and Möbius Conformational Rearrangements

A R T I C L E S

Table 2. Selected Calculated (B3LYP/LANL2DZ) Geometrical Parameters and Relative Energies for Palladium Complexes of

Vacataporphyrin

[(VPH₂-1)Pd^II]⁺

[(VPH₂-2)Pd^II]⁺

(VPH-3)Pd^II

[(VPH₂-4)Pd^II]⁺

[(VPH₂-5)Pd^II]⁺

E^a

15.12

3.229

4.136

2.450

5.100

1.391

1.418

1.400

1.418

1.391

4.5

5.96

3.336

2.480

2.481

4.130

2.656

1.370

1.449

1.391

1.449

1.370

46.3

0

6.21

Pd-C(1)^b

Pd-C(2)^b

Pd-C(3)^b

Pd· · ·H(1)^b

Pd· · ·H(2)^b

C(20)-C(1)^b

C(1)-C(2)^b

C(2)-C(3)^b

C(3)-C(4)^b

C(4)-C(5)^b

angle C_butad\N₃

3.024

2.025

3.105

3.991

3.137

2.290

2.277

4.046

2.772

1.373

1.468

1.414

1.467

1.373

90.1

3.188

2.431

3.957

2.800

1.367

1.467

1.385

1.465

1.367

1.385

1.448

1.403

1.420

1.388

30.9

c

90.0

0.051

standard deviation^d

0.014

0.040

0.026

0.047

^aE relative to [(VPH₂-4)Pd]⁺, kcal/mol, calculated using the B3LYP/LANL2DZ//B3LYP/LANL2DZ approach. ^bIn Å. ^cIn deg. ^dBond length

standard deviation along the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment.

Energies of a direct preorganization evaluated for (VPH₂-

1)H f (VPH₂-n)H (n > 1), are equal to the relative energy of

the conformers (Table 1). These values are comparable with

activation energies determined for metal ion insertion to

porphyrins and N-methylporphyrins.^98–101Alternatively a step-

wise conversion can be considered as a route facilitating

conformational transformations of (VPH₂-1)H into any given

conformer. For instance the most energetically demanding

transformation (VPH₂-1)H f (VPH₂-4)H can be realized in

sequence (VPH₂-1)H [ttctt] f (VPH₂-5)H [ctttc] f (VPH₂-

4)H [cctcc] where the energy difference for each subsequent

step is below 15 kcal/mol. The relatively small calculated ligand

preorganization energy explains why vacataporphyrin coordi-

nates acquiring several different macrocyclic conformations.

DFT Calculations: Palladium Vacataporphyrin. The experi-

mentally observed conformers of palladium vacataporphyrin

raised the question of their relative stability. To approach this

problem we have applied the density functional theory (DFT)

in the similar way as previously described for metallocarba-

porphyrinoids and zinc(II) and cadmium(II) vacataporphy-

rin.^{19,23,42,43,95,102}The principal geometries of palladium(II)

vacataporphyrin complexes have been already schematically

presented above in Schemes 3 and 5. The appropriate structures

were subjected to a DFT optimization at the B3LYP/LANL2DZ

level of theory. The ﬁnal geometries corresponding to [(VPH₂-

1)Pd^II]⁺, [(VPH₂-2)Pd^II]⁺, (VPH-3)Pd^II, [(VPH₂-4)Pd^II]⁺, and

[(VPH₂-5)Pd^II]⁺are shown in Figure 9. In each case a

genuine energy minimum was obtained (B3LYP/LANL2DZ).

Selected geometrical parameters and relative energies are

given in Table 2.

condition for palladium(II) coordination. Here it is important

to realize that the computational results correspond to cationic

molecules where one coordination site is readily accessible once

chloride of (VPH₂-1)Pd^IICl has been removed. The direct energy

comparison excludes (VPH-3)Pd^IIwhich is formally a product

of the proton dissociation from [(VPH₂-1)Pd^II]⁺or [(VPH₂-

4)Pd^II]⁺.

The interaction involving the palladium(II) and the butadiene

fragment in [(VPH₂-2)Pd^II]⁺, [(VPH₂-4)Pd^II]⁺, and [(VPH₂-

5)Pd^II]⁺is of special interest. The dihedral angle between the

butadiene plane and the plane of the three pyrrolic nitrogens

(C₄/N₃) at the hypothetical [(VPH₂-2)Pd^II]⁺is similar as found

from DFT optimized structures of zinc(II) or cadmium(II)

vacataporphyrins.¹⁹The interaction, as quantiﬁed by the dis-

tances M· · ·H, M· · ·C (Table 2), is indeed affected by the

conformational changes under study. For instance, in the

[(VPH₂-2)Pd^II]⁺conformer, the separation between palladium

and C(2) or C(3) (2.481 Å) is smaller than the expected van

der Waals contact (ca. 3.3 Å)¹⁰³but still larger than normally

observed Pd-C bond lengths as the typical values for Pd^II-C(η²-

alkene) complexes are in the order of 2.18 Å,^104,105although

the longer bond distance have been reported as well (e.g.,

Pd(II)-C, 2.273(7) Å;⁷⁰Pd(I)-C, 2.388(3)-2.495(4)¹⁰⁶). The

projection of the palladium(II) ion onto the C(1)C(2)C(3)C(4)

plane (C₄plane) lies close to the center of the C(2)-C(3) bond,

so the metal ion interacts in an η²fashion although in peculiar

position of the ethylene fragment with respect of the nearly

coplanar molecular fragment containing tripyrrolic unit and

palladium(II). The conformer [(VPH₂-5)Pd^II]⁺reveals a weak

η²interaction, with Pd-C(2) and Pd-C(3) distances being equal

at 2.431 Å.^70,105The zigzag shape of C(20)C(1)C(2)C(3)C(4)C(5)

orients the central C(2)C(3) fragment coplanarly with the PdN₃

plane ﬁlling the fourth corner of the square-planar structure.

Eventually the conformational transformation afforded [(VPH₂-

4)Pd^II]⁺, allowing η²interactions where the Pd-C(2) and

Pd-C(3) distances equal 2.290 and 2.277 Å, respectively. These

distances are still longer than the upper limits of the regular

palladium(II)-carbon bond.^70,105The S-like shape of C(20)-

C(1)C(2)C(3)C(4)C(5) orients the central C(2)C(3) bond per-

The energy ordering of [(VPH₂)Pd^II]⁺stereoisomers is

[(VPH₂-4)Pd^II]⁺<[(VPH₂-2)Pd^II]⁺e[(VPH₂-5)Pd^II]⁺<[(VPH₂-

1)Pd^II]⁺. Most importantly the experimentally detected sequence

of reactions results eventually in the most stable molecular state.

Unexpectedly, the strongly folded macrocyclic structure of

vacataporphyrin provides energetically the most favorable

(98) Bain-Ackerman, M. J.; Lavallee, D. K. Inorg. Chem. 1979, 18, 3358–

3364.

(99) Hambright, P.; Chock, P. B. J. Am. Chem. Soc. 1974, 96, 3123–

3131.

(100) Longo, R. F.; Brown, E. M.; Quimby, D. J.; Adler, A. D.; Meot-

Ner, M. Ann. N.Y. Acad. Sci. 1973, 206, 420–442.

(101) Longo, R. F.; Brown, E. M.; Rau, G. W.; Adler, A. D. In The

Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; p

459-481.

(103) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.

(104) Binotti, B.; Bellachioma, G.; Cardaci, G.; Macchion, A.; Zuccaccia,

C.; Foresti, E.; Sabatino, P. Organometallic 2006, 21, 346–354.

(105) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.

J. Chem. Soc., Dalton Trans. 1989, S1–S83.

(102) Chmielewski, M. J.; Pawlicki, M.; Sprutta, N.; Szterenberg, L.; Latos-

Graz˙yn´ski, L. Inorg. Chem. 2006, 45, 8664–8671.

(106) Dotta, P.; Kumar, P. G. A.; Pregosin, P. S. Organometallics 2004,

23, 4247–4254.

9

J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008 6191

A R T I C L E S

Pacholska-Dudziak et al.

Scheme 7. Building Blocks of Vacataporphyrin^a

Figure 10. Strategy to construct (A) Mo¨bius and (B) Hu¨ckel conformers

of vacataporphyrin.

^aThe n identiﬁes the conformer of (VPH₂-n)H; locations of linkage are

in red.

single molecular structure (Figure 10, trace A). The s-trans-

butadiene moiety is perpendicular to the C₂axis of tripyrrin.

Consequently the Mo¨bius structure can be achieved by com-

bining normal and in-plane conjugation. One can readily notice

that an alternative mode of tripyrrin and butadiene connections

(both fragments are located in parallel planes) creates a molecule

acquiring the Hu¨ckel topology (Figure 10, trace B).

pendicularly to the PdN₃plane completing the fourth corner of

the square-planar structure. The vector of the C(2)-C(3) bond

is nearly orthogonal to the coordination plane, bisected by it.

Thus the ﬁrst coordination spheres of [(VPH₂-4)Pd^II]⁺and

[(VPH₂-5)Pd^II]⁺resemble d⁸palladium complexes containing

the ethylene ligand.^70,72

Hückel and Möbius Macrocyclic Topologies. In the confor-

mational analysis of vacataporphyrin and its complexes it is

signiﬁcant to notice that molecule combines two fundamental

building blocks: a tripyrrin with terminal C(5) and C(20) meso

carbon atoms and a butadiene fragment in its compact (s-cis)

or extended (s-trans) conformations shown in Scheme 7. These

two fundamental conformations of butadiene can be accom-

modated by tripyrrin and eventually preserved in the resulting

structure acquiring suitable macrocyclic conformations.

The tripyrrin part has to adjust its geometry to constraints

imposed by butadiene. The planar geometry of tripyrrin is

preserved in (VPH₂-1)H, (VPH₂-2)H, and (VPH₂-6)H where the

s-cis-butadiene (ﬁrst two) or s-trans-butadiene (the last) unit

can be readily identiﬁed. To join the tripyrrin fragment and

s-trans-butadiene moiety to get an architecture of (VPH₂-3)H,

(VPH₂-4)H, or (VPH₂-5)H, the terminal carbon atoms C(5) and

C(20) have to occupy positions above and below the plane

deﬁned by the N(23) and two adjacent meso carbon atoms. This

adjustment imposes a pronounced helical twist creating a

nonplanar structure of tripyrrin which resembles helical con-

formations identiﬁed for a number of metallobiliverdins^107,108

and a palladium tripyrrin complex.¹⁰⁹The extent of the twist is

readily reﬂected by an improper torsion angle C(5)-C(10)-

C(15)-C(20). These corresponding values equal (VPH₂-1)H,

3.1°; (VPH₂-2)H, 0.1°; (VPH₂-3)H, 42.6°; (VPH₂-4)H, 42.8°;

(VPH₂-5)H, 55.4°; (VPH₂-6)H, 11.5°. Due to the marked helicity

of the tripyrrin chain the conformers (VPH₂-3)H, (VPH₂-4)H,

(VPH₂-5)H are chiral. Thus in Figures 8 and 9 only single

enantiomers have been shown.

Herges and co-workers in their inspiring contribution on

synthesis of the ﬁrst Mo¨bius annulenes^54,55noticed that these

molecules exhibit two types of conjugation: (1) “normal”

conjugation with p orbitals perpendicular to the ring plane and

sp²carbon atoms in their preferred trigonal planar conﬁguration

and (2) in-plane conjugation with pyramidalized carbon atoms.

Their successful synthetic strategy involved a connection of a

pyramidalized building block that is kept in its strained

conﬁguration by a rigid molecular frame and a regular π system.

In our analysis we have noticed that the two structural motives

of vacataporphyrin may introduce “normal” (tripyrrin) and in-

plane (s-trans-butadiene) conjugations once comprised in a

In more general terms, conjugated cyclic molecules can be

classiﬁed as Hu¨ckel or Mo¨bius systems, depending on the

number of half-twists in the ring. The molecules with an even

number of half-twists are predicted to follow the Hu¨ckel rule,

whereas those with an odd number of half-twists (Mo¨bius

systems) follow the reverse Hu¨ckel rule.⁴⁷Alternatively, the

topology can be determined by counting the number of trans

bonds along the [18]annulenoid conjugation pathway.⁵⁰Sig-

niﬁcantly, the Hu¨ckel rule is reversed and 4n-electron Mo¨bius

annulenes are aromatic whereas the (4n + 2)-electron systems

are antiaromatic.^44,46,47

In the case of vacataporphyrin or its palladium complexes

the distinction between Mo¨bius and Hu¨ckel topology is

facilitated by the fact that despite the differences in helicity

of tripyrrin moiety these fragments conserve an even number

of half-twists for any (VPH₂-n)H conformer. Consequently

the straightforward structural analysis focuses solely on

C(20)C(1)C(2)C(3)C(4)C(5). As inferred from our consid-

erations (VPH₂-1)H, [ttctt]; (VPH₂-2)H, [ctctc]; (VPH₂-3)H

[cttcc]; (VPH₂-6)H [ttttc] have the Hu¨ckel topology. Notably

(VPH₂-4)H [cctcc] and (VPH₂-5)H [ctttc] acquire the Mo¨bius

topology. Evidently the most stable stereoisomer of a free

vacataporphyrin belongs to the familiar group of Hu¨ckel

annulenes.^47,110The evidently large energy differences

between the most stable Hu¨ckel (VPH₂-1)H and other con-

formers rule out their contribution to any conformational

equilibria.

The coordination of palladium preserves the fundamental

structural features of free base vacataporphyrin including the

extent of the tripyrrin twist deﬁned as above [(VPH₂-1)Pd^II]⁺,

0.0°; [(VPH₂-2)Pd^II]⁺, 0.5°; (VPH-3)Pd^II, 45.2°; [(VPH₂-

4)Pd^II]⁺, 51.7°; [(VPH₂-5)Pd^II]⁺, 57.9°. Importantly [(VPH₂-

4)Pd^II]⁺and [(VPH₂-5)Pd^II]⁺conformers have Mo¨bius topology

where the remaining palladium complexes are described by

Hu¨ckel topology. Signiﬁcantly the torsional angles along the

macrocyclic skeleton of [(VPH₂-4)Pd^II]⁺vary in the range

0°-42.8° (176.3-151.6°) which is in the limit 0°-50°

(180°-130°) required for the efﬁcient π conjugation in the

Mo¨bius band.⁴⁴Predictably^{44,48,111,112}the stronger alternations

of bond lengths have been found for antiaromatic molecules

with Mo¨bius topology in comparison to aromatic Hu¨ckel ones

(107) Balch, A. L.; Mazzanti, M.; Noll, B. C.; Olmstead, M. M. J. Am.

Chem. Soc. 1994, 116, 9114–9122.

(110) Rzepa, H. S. Org. Lett. 2005, 7, 4637–4639.

(111) Pohl, M.; Schmickler, H.; Lex, J.; Vogel, E. Angew. Chem., Int. Ed.

Engl. 1991, 30, 1693–1697.

(108) Nguyen, K. T.; Rath, S. P.; Latos-Graz˙yn´ski, L.; Olmstead, M. M.;

Balch, A. L. J. Am. Chem. Soc. 2004, 126, 6210–6211.

(109) Bro¨ring, M.; Brandt, C. B. J. Chem. Soc., Dalton Trans. 2002, 1391–

1395.

(112) Cissel, J. A.; Vaid, T. P.; Yap, G. P. A. Org. Lett. 2006, 8, 2401–

2404.

9

6192 J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008

Hückel and Möbius Conformational Rearrangements

A R T I C L E S

(Tables 1, 2) as reﬂected by larger values of mean standard

deviations of bond lengths along the C(20)C(1)C(2)C(3)C(4)C(5)

route.

Signiﬁcantly the stability order is reversed in comparison to

the free base vacataporphyrin (Tables 1 and 2). In contrast to

vacataporphyrin conformers the Mo¨bius structure [(VPH₂-

4)Pd^II]⁺can be considered as a fundamental state for the whole

series. Thus the coordination of palladium(II) imposes additional

structural constrains on vacataporphyrin and affords the remark-

able stabilization of Mo¨bius geometries, for example, [(VPH₂-

4)Pd^II]⁺(0) versus [(VPH₂-1)Pd^II]⁺(15.12 kcal/mol). The most

importantly two Mo¨bius [(VPH₂-5)Pd^II]⁺and [(VPH₂-4)Pd^II]⁺

conformers have been experimentally identiﬁed. Despite the

obvious energetic preferences for the Mo¨bius form [(VPH₂-

4)Pd^II]⁺as determined by DFT optimization, two Hu¨ckel

conformers (VPH₂-1)Pd^IICl and (VPH-3)Pd^IIhave been detected

as well. Their stabilization results from coordination of chloride

to yield (VPH₂-1)Pd^IICl or from formation of the Pd-C bond

as seen for (VPH-3)Pd^II.

Chemical Shifts and NICS Calculations. Nucleus-independent

chemical shifts (NICS) have been proposed by Schleyer et al.

as a computational measure of aromaticity that is related to

experimental magnetic criteria.^113,114A NICS is deﬁned as the

negative shielding value computed at the center of a ring. The

NICS method has been used to assess the aromaticity of

annulenes or porphyrins^115,116and their analogues.^93,117NICS

values were calculated using GIAO-B3LYP method for (VPH₂-

n)H molecules for the central 17-membered ring (Table 1).

Comparison of the center ring NICS (calculated for B3LYP/6-

31G** geometries) values shows that they are good indicators

of macrocyclic aromaticity. For the Hu¨ckel aromatic systems

(VPH₂-1)H and (VPH₂-3)H the central NICS values are negative

(-13.6 and -7.7 ppm, respectively) and comparable with that

reported for a porphyrin (-16.5 ppm).¹¹⁵For the Mo¨bius

antiaromatic conformers (VPH₂-4)H and (VPH₂-5)H the center

ring NICS are positive (+6.0 and +4.0 ppm, respectively).

These values are smaller than the NICS obtained for [20]an-

nulene (+12.1 ppm)⁴⁸but similar to values determined previ-

ously for antiaromatic tautomer of 22-hydroxybenziporphyrin

(+5.0 ppm).⁹³

The NICS calculations (by the same, GIAO-B3LYP method)

have been also done using KMLYP/6-31G** geometries (Table

1 and Supporting Information, Table S2) to limit an overestima-

tion of π contribution. Using these geometries, the NICS values

for (VPH₂-4)H and (VPH₂-5)H equal +4.4 and +4.6, respec-

tively. Thus they are comparable to those found for B3LYP/

6-31G** geometries (+6.0 and +4.0). Evidently they are

consistent with the Mo¨bius antiaromaticity resulting from the

Mo¨bius topology of these crucial structures. The NICS values

calculated for KMLYP/6-31G** optimized Hu¨ckel structures

(VPH₂-1)H, (VPH₂-2)H, (VPH₂-3)H, and (VPH₂-6)H reﬂect

their aromatic nature although these values are notably less

negative than found for B3LYP/6-31G** geometries (Table 1).

Figure 11. Linear correlation between calculated (GIAO-B3LYP//B3LYP/

6-31G**) and experimental values of chemical shifts for the representative

Hu¨ckel and Mo¨bius conformers (A) (VPH₂-1)Pd^IICl (B) [(VPH₂-4)Pd^II]⁺.

¹H NMR chemical shifts have been calculated using the

GIAO-B3LYP method for the B3LYP/LANL2DZ geometries

of [(VPH₂-n)Pd^II]⁺and (VPH₂-1)Pd^IICl which acquire Hu¨ckel

or Mo¨bius macrocyclic topologies (Figure 11 and Figure S1).

The presence of a paratropic ring current is a commonly

accepted indicator of antiaromatic character.^118,119Paratropicity

causes shielding of the protons located on the outside of the

ring and deshielding, often substantial, of the protons inside

the ring. The paratropic ring current is thus a reversal of the more

common diatropic current associated with aromaticity which yields

the deshielding of the perimeter protons and the substantial

shielding of the internal ones. It is signiﬁcant to make a point

that the Hu¨ckel conformers of vacataporphyrin are aromatic but

the Mo¨bius conformer systems are antiaromatic as discussed

above. The paratropicity is expected to generate an upﬁeld

relocation of ꢀ-H resonances and the reversed order of meso-

aryl resonances (meso-phenyl, p-H > m-H > o-H; meso-p-tolyl,

m-H > o-H). The analysis of the calculated chemical shifts

reveals that indeed the chemical shift pattern reﬂects quantita-

tively the features predicted for Hu¨ckel or Mo¨bius topologies

of [(VPH₂-n)Pd^II]⁺listed above.

In the case of the described palladium vacataporphyrin

complexes we have been in a unique position to compare the

experimental and calculated chemical shifts assigned to Hu¨ckel

and Mo¨bius geometries. We have concluded that there is a

satisfactory qualitative agreement for each considered set of

theoretical and experimental data readily demonstrated by linear

correlations between the calculated and experimental chemical

shifts shown for two representative examples of Hu¨ckel [(VPH₂-

1)Pd^II]Cl (aromatic) or Mo¨bius [(VPH₂-4)Pd^II]⁺(antiaromatic)

topologies (Figure 11). Still the calculated paratropicity or

diatropicity are overestimated. For instance in the case of

Mo¨bius topology the differences in calculated (B3LYP) and

(113) Schleyer, P. v. R.; Meaerker, C.; Dransfeld, A.; Jiao, H.; Hommes,

N. J. R. v. E. J. Am. Chem. Soc. 1996, 118, 6317–6318.

(114) Chen, Z.; Wannere, C. S.; Corminboeuf, C.; Puchta, R.; Schleyer,

P. v. R. Chem. ReV. 2005, 105, 3842–3888.

(115) Cyran´ski, M. K.; Krygowski, T. M.; Wisiorowski, M.; Hommes,

N. J. R. v. E.; Schleyer, P. v. R. Angew. Chem., Int. Ed. Engl. 1998,

37, 177–180.

1

experimental H NMR shifts reach maximally 0.9 ppm. The

visible differences have been detected for indisputably aromatic

(VPH₂-1)PdCl (max 1.1 ppm for H(2,3)). This effect is likely

(116) Juse´lius, J.; Sundholm, D. J. Org. Chem. 2000, 65, 5233–5237.

(117) Furuta, H.; Maeda, H.; Osuka, A. J. Org. Chem. 2001, 66, 8563–

8572.

(118) Pople, J. A.; Untch, K. G. J. Am. Chem. Soc. 1966, 88, 4811–4815.

(119) Lazzeretti, P. Phys. Chem. Chem. Phys. 2004, 6, 217–223.

9

J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008 6193

A R T I C L E S

Pacholska-Dudziak et al.

Data for (VPH₂-1)Pd^II, Ar ) p-Tol. UV-vis, CH₂Cl₂λ(logε):

394 (4.5), 442 (4.7), 616 (sh), 663 (4.3). ¹H NMR: δ 9.14 (AA′XX′,

³J_1,2) 12.6 Hz, ³J_2,3) 9.8 Hz, ⁴J_1,3) -1.5 Hz, 2H, 2,3), 8.09 (d,

J ) 4.6 Hz, 2H, 8,17), 7.99 (s, 2H, 12,13), 7.96 (d, J ) 4.6 Hz,

2H, 7,18), 7.89 (d, J ) 7.7 Hz, 2H, o-Tol), 7.73 (d, J ) 7.1 Hz,

4H, o-Ph), 7.64 (d, J ) 7.7 Hz, 2H, o-Tol), 7.52 (t, J ) 7.1 Hz,

4H, m-Ph), 7.47 (m, 4H, p-Ph, m-Tol), 7.40 (d, J ) 7.7 Hz, 2H,

m-Tol), 2.61 (s, 6H, p-Tol-CH₃), 1.13 (AA′XX′, 2H, 1,4). ¹³C

NMR: δ 165.7 (6,19), 142.3 (11,14), 140.8 (9,16), 138.6 (ipso-

Ph), 138.2 (p-Tol), 138.0 (2,3), 136.9 (ipso-Tol), 136.8 (5,20), 136.1

(1,4), 133.9 (8,17), 133.1 (o-Tol), 132.7 (10,15), 132.5 (o-Ph), 132.1

(o-Tol), 130.8 (7,18), 130.5 (12,13), 128.2 (m-Ph), 127.9, and 127.8

(m-Tol, p-Ph), 127.7 (m-Tol), 21.5 (CH₃-Tol). HRMS (EI, m/z):

734.1801 (734.1782 for C₄₆H₃₄N₃¹⁰⁶Pd⁺).

due to the known propensity of the B3LYP functional to

overestimate π-conjugation.¹²⁰The calculated chemical shifts

of hypothetical [(VPH₂-2)Pd^II]⁺resemble those found for

appropriate conformer of (VPH₂-2)Zn^IICl or (VPH₂-2)Cd^IICl

complexes.¹⁹

Conclusions

Vacataporphyrin can be described as an annulene-porphyrin

hybrid. The macrocycle contains the relatively rigid tripyrrin

moiety which affords the limited ﬂexibility resembling regular

porphyrins. In contrast the butadiene fragment can readily adopt

several conformations consistent with the annulene-like nature

of vacataporphyrin. Thus the combination of both structural

components allows a unique ﬂexibility of the whole conjugated

macrocycle albeit constrained by tripyrrin fragment and eventu-

ally by coordination. In this contribution we have demonstrated

that vacataporphyrin acts as ligand to palladium which is ﬁrmly

held via three pyrrolic nitrogen atoms. Two fundamental ways

of interactions between palladium and annulene fragment have

been recognized. The ﬁrst one resemble an η²-type interaction

and involves a C(2)C(3) unit of the butadiene part. The second

one imposed the profound conformational changes allowed to

create the regular Pd-C bond demonstrating that vacatapor-

phyrin acts as “true” carbaporphyrinoid.

Vacataporphyrin, applied as a ligand toward palladium(II)

and previously to zinc(II) and cadmium(II), reveals the peculiar

plasticity of its molecular and electronic structure. This important

feature has enabled us to investigate and eventually to control

the subtle interplay between their structural ﬂexibility and

aromaticity. In particular the coordinated vacataporphyrin

may acquire the relatively planar Hu¨ckel or extremely rare

twisted Mo¨bius topologies which are reﬂected by aromatic

or antiaromatic properties of 18-electron π-system, respec-

tively. The properties of speciﬁc conformers were studied using

¹H NMR and supported by DFT calculations. Actually we have

experimentally identiﬁed the very ﬁrst example of Mo¨bius

antiaromaticity. Thus vacataporphyrin provides a stimulating

environment to investigate coordination chemistry of the Mo¨-

bius-type macrocyclic ligand. Efforts to explore such coordina-

tion chemistry are underway.

(VPH-3)Pd^II. A solution of (VPH₂-1)PdCl (10 mg, 0.016 mmol)

in dichloromethane (50 mL) was stirred in the daylight for 1 day.

The solution was concentrated to 1-2 mL and chromatographed

on a short silica gel or alumina grade III column. The ﬁrst, green

fraction eluted with CH₂Cl₂contained the desired product. The

solvent was evaporated, the green solid (VPH-3)Pd^IIwas dried in

vacuum. Yield 80%.

Data for (VPH₂-3)Pd^IICl, Ar ) p-Tol. UV-vis, CH₂Cl₂

λ(logε): 345 (4.4), 376 (4.4), 428 (sh), 461 (5.0), 542 (3.7), 600

1

4

3

(sh), 653 (4.3). H NMR: δ 9.75 (dd, J_1,3) 0.9 Hz, J_3,4) 9.2

4

3

Hz, 1H, 3), 8.96 (d, J_1,3) 0.9 Hz, 1H, 1), 8.25 (d, J ) 4.7 Hz,

1H, 13), 8.08 (d, J ) 9.2 Hz,1H, 4), 8.02 (d,³J ) 5.0 Hz,1H, 8),

3

7.89 (d, 2H, 20-o-Ph), 7.89 (s, 2H, 17, 18), 7.87 (d,³J ) 5.0 Hz,

3

1H, 12), 7.76 (d, J ) 4.7 Hz, 1H, 7), 7.72 (d, J ) 7.5 Hz, 2H,

15-o-Tol), 7.66 (d, J ) 7.3 Hz, 2H, 10-o-Tol), 7.59 (t,³J ) 7.5

3

Hz, 2H, 20-m-Ph), 7.54 (d, ³J ) 7.3 Hz, 2H, 5-o-Ph), 7.50 (t,³J )

7.3 Hz, 1H, 20-p-Ph), 7.44 (t, ³J ) 7.1 Hz, 2H, 5-m-Ph), 7.40 (m,

3H, 15-m-Tol, 5-p-Ph), 7.35 (d, ³J ) 8.0 Hz, 2H, 10-m-Tol), 2.57

(s, 3H, 15-p-Tol (CH₃)), 2.54 (s, 3H, 10-p-Tol (CH₃)). ¹³C NMR:

δ 155.5, 151.7, 151.0, 145.1, 143.5, 142.3, 142.0, 141.1, 139.9,

139.2, 138.0, 137.8, 137.7, 137.3, 136.7 136.0, 135.3, 133.6, 132.7,

132.6, 132.4, 132.2, 131.8, 131.7, 131.3, 131.0, 129.5, 128.3, 128.2,

127.9, 127.8, 127.6, 127.4, 127.3, 127.1, 122.0, 21.40 21.41. HRMS

(EI, m/z): 733.1734 (733.1704 for C₄₆H₃₃N₃¹⁰⁶Pd⁺).

[(VPH₂-4)Pd^II]BF₄. A 5 equiv portion of HBF₄(diethyl ether

solution) was added to the (VPH-3)Pd^IIsolution in CH₂Cl₂. The

green solution turns immediately brown. The solvents were

evaporated, and the brown solid, pure [(VPH₂-4)Pd^II]BF₄, was dried

in vacuum.

Data for [(VPH₂-4)Pd^II]BF₄, Ar ) p-Tol. UV-vis, CH₂Cl₂

1

λ(logε): 376 (sh), 423 (4.6), 525 (sh). H NMR: δ 8.98 (AA′XX′,

Experimental Section

3

4

5

³J_1,2) 5.8 Hz, J_2,3) 14.9 Hz, J_1,3) -1.3 Hz, J_1,4) 1.1 Hz

2H, 2,3), 7.37 (t, ³J ) 7.5 Hz, 2H, p-Ph), 7.30 (t, ³J ) 7.6 Hz, 4H,

Solvents and Reagents. Dichloromethane-d₂(CIL) was used as

received. Chloroform-d (CIL) was passed through basic Al₂O₃.

Vacataporphyrin (VPH₂-1)H derivatives (5,20-diphenyl-10,15-di(p-

tolyl)-21-vacataporphyrin, 10,15-bis(4-metoxyphenyl)-5,20-diphe-

nyl-21-vacataporphyrin, and their deuterated analogues) have been

obtained according to the previously described procedures.^1,19

(VPH₂-1)PdCl. A 6.5 mg (0.010 mmol) portion of (VPH₂-1)H,

was dissolved in 15 mL of CHCl₃with 5 µL of triethylamine.

Nitrogen was bubbled through the solution for 20 min. The ﬂask

was carefully protected from light with black paper. Subsequently

6.8 mg (0.018 mmol) of Pd(PhCN)₂Cl₂was added. The mixture

was stirred at room temperature for 20 min. The progress of the

reaction was checked by UV-vis spectroscopy, and if necessary

another portions of Pd(PhCN)₂Cl₂and triethylamine were added.

The resulting mixture was evaporated to dryness with a vacuum

rotary evaporator (the ﬂask was always wrapped in the black paper).

Typically the insertion is quantitative. Recrystallization was

performed from CH₂Cl₂and CH₃OH. If the product was contami-

nated with (VPH-3)Pd^II, it was chromatographed in dark on a short

silica gel 40 column with CH₂Cl₂(second, green fraction).

3

m-Ph), 7.15 (m, 8H, o-Ph, m-Tol), 7.01 (br. d, J ) 7.2 Hz, 4H,

3

o-Tol) 6.18 (AA′XX′, 2H, 1,4), 5.89 (d, J ) 4.9 Hz, 2H, 8,17),

3

5.33 (d, J ) 4.9 Hz, 2H, 7,18), 4.95 (s, 2H, 12,13), 2.36 (s, 6H,

p-Tol-CH₃); ¹³C NMR: δ 136.3 (8,17), 129.8 (p-Ph), 128.6 (m-

Ph), 128.6 (m-Tol), 128.4 (o-Tol), 127.4 (o-Ph), 125.5 (12,13), 125.3

(1,4), 125.2 (7,18), 100.1 (2,3), 21.5 (p-Tol-CH₃). HRMS (ESI,

m/z): 734.1801 (734.1782 for C₄₆H₃₄N₃¹⁰⁶Pd⁺).

¹H NMR Data for [(VPH₂-4)Pd^II]BF₄, Ar ) Anis. δ 8.91

(AA′XX′, ³J_1,2) 5.8 Hz, ³J_2,3) 14.9 Hz, ⁴J_1,3) -1.3 Hz, ⁵J_1,4

) 1.1 Hz, 2H, 2,3), 7.36 (t, ³J ) 7.4 Hz, 2H, p-Ph), 7.29 (t, ³J )

7.7 Hz, 4H, m-Ph), 7.18 (d,³J ) 7.3 Hz, 4H, o-Ph), 7.10 (d, J )

3

8.4 Hz, 4H, o-Anis), 6.87 (d,³J ) 8.7 Hz, 4H, m-Anis), 6.24

(AA′XX′, 2H, 1,4), 5.98 (d, ³J ) 4.9 Hz, 2H, 8,17), 5.37 (d, ³J )

4.9 Hz, 2H, 7,18), 5.01 (s, 2H, 12,13), 3.83 (s, 6H, OCH₃).

[(VPH₂-5)Pd^II]BF₄. An NMR sample of (VPH₂-1)PdCl in CDCl₃

was treated in dark with excess of AgBF₄and shaked for a few

minutes. The product was not isolated. The degree of conversion

was up to 90%. The solution of [(VPH₂-5)Pd^II]BF₄is very unstable

in light, moderately stable in the dark (298 K). All the 2D NMR

spectra were obtained in low temperature. The full description was

not possible because of the gradual conversion into [(VPH₂-

4)Pd^II]BF₄.

(120) Wannere, C. S.; Sattelmeyer, K. W.; Schaefer, H. F., III; Schleyer,

P. v. R. Angew. Chem., Int. Ed. 2004, 43, 4200–4206.

9

6194 J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008

Hückel and Möbius Conformational Rearrangements

A R T I C L E S

Data for [(VPH₂-5)Pd^II]BF₄, Ar ) Anis. UV-vis, CH₂Cl₂

recorded on spectrometers using the electrospray and electron

impact techniques.

1

λ(logε): 404 (4.3), 444 (4.3), 616 (3.8), 815 (4.0). H NMR (250

K): δ 7.67 (AA′XX′, ³J_1,2) 6.4 Hz, ³J_2,3) 10.2 Hz, ⁴J_1,3) -1.4

Hz, 2H, 2,3), 7.41 (m, 6H, m,p-Ph), 7.35 (d, 4H, o-Anis), 7.31 (d,

X-Ray Analysis. X-ray quality crystals (VPH₂-1)PdCl·CH₂Cl₂

were prepared by diffusion of methanol into dichloromethane

solution contained in a thin tube stored in the refrigerator. Data

were collected at 100 K on Kuma KM4CCD diffractometer using

Cu KR radiation (λ ) 1.54180 Å). The data were corrected for

Lorentz and polarization effects. An analytical absorption correction

was applied. Crystal data are compiled in the CIF ﬁle in the

Supporting Information. The structure was solved using direct

methods with SHELXS-97 and reﬁned against F²using SHELXL-

97 (G. M. Sheldrick, University of Go¨ttingen, Germany, 1997) with

anisotropic thermal parameters for the non-hydrogen atoms. Scat-

tering factors were those incorporated in SHELXS-97.^121,122

Theoretical Calculations. DFT calculations were performed with

the Gaussian 03 program.¹²³Geometry optimizations were carried

out within unconstrained C₁symmetry. Starting geometries were

obtained using molecular mechanics and semiempirical calculations.

B3LYP^124,125and KMLYP^126,127hybridization of the exchange

funtional DFT were used with the LANL2DZ basis set for metal

complexes and 6-31G** for the free ligands.¹²⁸The structures were

found to have converged to a minimum on the potential energy

surface; the resulting zero-point vibrational energies were included

in the calculation of relative energies. NICS values were calculated

at the GIAO-B3LYP/6-31G** level for the optimized structures.

Calculations of NMR properties for investigated systems were

performed for the optimized structures.

3

4H, o-Ph), 6.99 (d, J ) 8.8 Hz, 4H, m-Anis), 6.77 (d, J ) 4.9

Hz, 2H, 8,17), 6.47 (s, 2H, 12,13), 6.35 (d, ³J ) 4.9 Hz, 2H, 7,18),

6.04 (AA′XX′, 2H, 1,4), 3.89 (s, 6H, OCH₃). ¹³C NMR: δ 142.2

(1,4), 140.4 (8,17), 133.1 (o-Anis), 130.6 (m-Ph), 130.4 (12,13),

129.8 (p-Ph), 129.3 (o-Ph), 125.8 (7,18), 120.8 (2,3), 114.0 (m-

Anis).

[(VPHMe-4)Pd^II]BF₄. A toluene solution of (VPH-3)Pd^II(3 mg,

0.005 mmol) was heated under nitrogen. Twenty-ﬁve µL of methyl

iodide (0.40 mmol) was added and the solution was reﬂuxed for 3

min. Then 20 mg of AgBF₄(0.10 mmol) was added. AgI precipitated

immediately and the solution turned slowly brown. After 5 more

minutes of boiling, the mixture was ﬁltered through glass wool and

the solution was dried. Typically the insertion is quantitative and

does not require any further puriﬁcation.

Data for [(VPHMe-4)Pd^II]BF₄, Ar ) Anis. UV-vis, CH₂Cl₂

λ (logε): 379 (sh), 430 (4.7). ¹H NMR, (CD₂Cl₂): δ 8.04 (d, ³J )

6 (5.7 Hz) Hz, 1H, 3), 7.67 (d, ³J ) 8.0 Hz, 1H, o-Ph), 7.62 (t, ³J

) 7.8 Hz, 1H, p-Ph), 7.48 (t, 2H, m-Ph), 7.41 (m, 6H, Ar), 7.22

(d, ³J ) 8.1 Hz, 2H, 5-o-Ph), 7.14 (m, 5H, m-Anis, Ph), 6.90 (2d,

3

³J ) 8.8 Hz, 4H, o-Anis), 6.34 (d, J ) 4.8 Hz, 1H, 17) 6.25 (d,

³J ) 4.8 Hz, 1H, 8), 6.11 (d, ³J ) 5.7 Hz, 1H, 4), 5.67 (s, 1H, 1),

3

5.61 (d, J ) 4.8 Hz, 1H, 18), 5.54 (d, J ) 4.8 Hz, 1H, 7), 5.31

3

(d, J ) 4.5 Hz, 1H, 12), 5.24 (d, J ) 4.5 Hz, 1H, 13), 3.81 (s,

6H, OCH₃), 3.17 (s, 3H, 2-CH₃). ¹³C NMR: δ 136.8 (17), 136.8

(8), 132.2 (m-Ph), 130.6 (m-Anis), 129.6 (Ph), 128.8 (o-Ph), 128.4

(Ph), 127.4 (1), 127.3 (Ph), 126.8 (Ph), 126.0 (12), 125.2 (18), 125.0

(13), 123.1 (7), 120.9 (4), 113.5 (o-Anis), 91.8 (3), 55.1 (OCH₃),

24.5 (2-CH₃). HRMS (ESI, m/z): 780.1852 (780.1859 for

C₄₇H₃₆N₃O₂¹⁰⁶Pd⁺).

Acknowledgment. Financial support from the Ministry of

Science and Higher Education (Grant PBZ-KBN-118/T09/2004)

is kindly acknowledged. Quantum chemical calculations have

been carried out at the Poznan´ Supercomputer Center (Wro-

cław).

(VPCH₃-3)Pd^II. [(VPHMe-4)Pd^II]BF₄(3 mg) was dissolved in

a methanol solution of Et₄NCl (25 mg) and stirred for 30 min at

50 °C. The solvent was evaporated and the solid was dissolved in

1 mL of CH₂Cl₂and ﬁltered through a short column of Al₂O₃grade

III. Yield ca. 60%.

Supporting Information Available: Complete ref 123; tables

of computational results (Cartesian coordinates); ﬁgures present-

ing correlations between calculated and experimental ¹H NMR

chemical shifts for (VPH-3)Pd^II, [(VPH₂-5)Pd^II]⁺, [(VPH₂-

1)Pd^II]⁺, and [(VPH₂-4)Pd^II]⁺; tables of selected calculated (with

KMLYP functional) geometrical parameters for [(VPH₂-

1)Pd^II]⁺, [(VPH₂-4)Pd^II]⁺, and (VPH₂-n)H; crystallographic data

in CIF format for (VPH₂-1)PdCl. This material is available free

of charge via the Internet at http://pubs.acs.org.

Data for (VPMe-3)Pd^II, Ar ) Anis. UV-vis, CH₂Cl₂λ(logε):

1

351 (4.5), 381 (4.5), 465 (4.9), 669 (4.4). H NMR: δ (CD₂Cl₂)

3

8.46 (s, 1H, 1), 8.06 (d, J ) 4.8 Hz, 1H, 17), 7.77 (d, J ) 4.9

3

Hz, 1H, 8) 7.75 (d, J ) 7.3 Hz, 2H, 20-o-Ph), 7.68 (d, J ) 8.4

Hz, 2H, 15-o-Anis), 7.64 (d, ³J ) 8.8 Hz, 2H, 10-o-Anis), 7.62 (d,

³J ) 4.3 Hz, 1H, 12), 7.59 (d, ³J ) 4.5 Hz, 1H, 13), 7.58 (d, ³J )

4.7 Hz, 1H, 18), 7.51 (t, ³J ) 7.7 Hz, 2H, 20-m-Ph), 7.46 (m, 4H,

7, 20-p-Ph, 5-o-Ph), 7.39 (m, 4H, 5-m,p-Ph), 7.40 (s, 4), 7.11 (d,

JA711039C

3

³J ) 8.8 Hz, 2H, 15-m-Anis), 7.07 (d, J ) 8.8 Hz, 2H, 10-m-

(121) Sheldrick, G. M. SHELXS97: Program for Crystal Structure Solution;

University of Go¨ttingen: Go¨ttingen, Germany, 1997.

(122) Sheldrick, G. M. SHELXL97: Program for Crystal Structure Reﬁne-

ment; University of Go¨ttingen: Go¨ttingen, Germany, 1997.

(123) Frisch, M. J., et al. Gaussian 03, revision C.01; Pittsburgh, PA, 2004.

(124) Becke, A. D. Phys. ReV. A 1988, 38, 3098–3100.

(125) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.

(126) Kang, J. K.; Musgrave, C. B. J. Chem. Phys. 2001, 115, 11040–

11051.

Anis), 3.93 (s, 3H, 15-p-OCH₃), 3.91 (s, 3H, 10-p-OCH₃), 3.41 (s,

3H, 2-CH₃). ¹³C NMR: δ 159.6, 159.4, 154.8, 153.8, 144.1, 143.2,

142.7, 141.0, 140.8, 140.7, 139.5, 137.3, 135.5, 134.8, 134.8, 134.4,

133.4, 133.2, 132.9, 131.4, 131.0, 131.0, 130.5, 130.2, 129.0, 128.5,

127.9, 127.5, 127.1, 127.0, 125.5, 112.3, 112.2. HRMS (ESI, m/z):

780.1821 (780.1859 for C₄₇H₃₅N₃O₂¹⁰⁶Pd + H⁺).

Instrumentation. NMR spectra were recorded on a highﬁeld

spectrometer (frequencies: ¹H 500.13 MHz, ¹³C 125.77 MHz)

equipped with either a broadband inverse gradient probehead or a

direct broadband probehead. Absorption spectra were recorded on

a diode array spectrometer. High resolution mass spectra were

(127) Senosiain, J. P.; Han, J. H.; Musgrave, C. B.; Golden, D. M. Faraday

Discuss. 2002, 119, 173–189.

(128) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270–283;. 284-298;

299-310.

9

J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008 6195

Article Doi

Palladium vacataporphyrin reveals conformational rearrangements involving hueckel and moebius macrocyclic topologies

DOI: 10.1021/ja711039c

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Article abstract of DOI:10.1021/ja711039c

Full text of DOI:10.1021/ja711039c

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