Synthesis of and evidence for electronic communication within heteromultimetallic Tetrakis(NCN-pincer metal) - (Metallo)porphyrin Hybrids

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Synthesis of and Evidence for Electronic Communication within

Heteromultimetallic Tetrakis(NCN-pincer metal)-(Metallo)porphyrin

Hybrids

Bart M. J. M. Suijkerbuijk,^†Duncan M. Tooke,^‡Martin Lutz,^‡Anthony L. Spek,^‡

Leonardus W. Jenneskens,^†Gerard van Koten,^†and Robertus J. M. Klein Gebbink*^,†

^†Organic Chemistry & Catalysis, Debye Institute for Nanomaterials Science, Faculty of Science,

Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, and ^‡Crystal and Structural

Chemistry, Bijvoet Center for Biomolecular Research, Faculty of Science, Utrecht University, Padualaan 8,

3584 CH Utrecht, The Netherlands

r.j.m.kleingebbink@uu.nl

Received November 19, 2009

Several heteromultimetallic pincer-porphyrin hybrids have been prepared in excellent yields by

stepwise metalation of a general precursor, [2H(Br(NCN))₄], which was designed in such a way so as

to guarantee selectivity for either the porphyrin or pincer sites during the metalation steps. First, a metal

was introduced in the porphyrin cavity using a metal(II) salt, followed by metalation of the pincer units

through oxidative addition to an appropriate metal(0) complex. The resulting multimetallic complexes

show an appreciable amount of intramolecular communication between the meso-pincer metal groups

and the central metalloporphyrin component. This was manifested in changes of the optical and ligand-

binding properties of the metalloporphyrin part upon reactions at the peripheral pincer sites.

Introduction

effectuate this is to alter the ligands that coordinate to the

metal ion. Within a certain group of ligands, further fine-

tuning may be accomplished by attaching electron-withdrawing

or -donating,^2-8or even redox-active substituents^9-11to

the ligand. The success of this approach hinges on the extent

In general, the electron density on the metal center of

transition metal complexes is of great importance for their

properties. It determines several key features of the com-

plex, such as the rate of ligand exchange, its optical proper-

ties, and redox characteristics.¹The ability to fine-tune the

electron density on the metal center of a complex is therefore

of great value as a tool for the modulation of its coordination

and physical properties, and therefore of, e.g., its cata-

lytic activity. One of the most straightforward ways to

(2) Abashkin, Y. G.; Collins, J. R.; Burt, S. K. Inorg. Chem. 2001, 40,

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*To whom correspondence should be addressed. Phone: þ31-30-2531889.

Fax: þ31-30-2523615.

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John Wiley & Sons: New York, 1988.

1534 J. Org. Chem. 2010, 75, 1534–1549

Published on Web 01/29/2010

DOI: 10.1021/jo902402g

r

2010 American Chemical Society

Suijkerbuijk et al.

JOCArticle

CHART 1. General Formula of the Heteromultimetallic

Pincer-Porphyrin Hybrid Complexes Presented Here

FIGURE 1. Schematic representation of para-substituted NCN-

pincer ML complexes [ML(NCN-Z)]: Z = H₂N, HO, t-Bu, Me₃Si,

H, I, HOOC, OHC, O₂N; M = Ni, Pd, Pt; L = Cl, Br, I, OH₂,

Lewis base.

(MX = NiBr or PtCl) depends on the electron-donating

or -withdrawing properties of the para-substituent Z, as

indexed by its σ_pHammett substituent constant. Further-

more, it has been demonstrated that the catalytic activity of

several para-substituted NCN-pincer Pd aqua complexes in

the double Michael addition reaction of ethyl R-cyano acetate

to methyl vinyl ketone changes when the electronic properties

of the para-substituent are varied.¹⁸In these cases, however,

modularity of the tuning of the electron density on the metal

center is precluded by the need for extensive synthetic chemi-

stry to introduce each different para-substituent with its

specific electronic properties onto the ligand.

Kim et al. synthesized a series of meso-mono- and -bis-

(ferrocenyl)-substituted β-octakis(alkyl) metalloporphyrins

and very elegantly showed that the (metallo)porphyrin unit

can be used as a highly modular substituent to tune the

electronic properties of the attached ferrocene groups

(Figure 2, left).^19,20It was found that a change of the metal

in the porphyrin cavity was communicated to the meso-

ferrocene units and consequently led to changes in their

redox potentials. Those changes could be rationalized by

taking the electronic properties of the (metallo)porphyrin into

consideration. Arnold and co-workers synthesized (metallo)-

porphyrins that act as monodentate ligands for palladium(II)

and platinum(II) complexes (Figure 2, right).^21-28They

were able to introduce several metals inside and outside

of the porphyrin ring and showed that ligand exchange at

the peripheral palladium and platinum sites is possible,

and that multidentate ligands could be introduced. Impor-

tantly, they showed that both metal centers influence each

other.

to which the electronic properties of the metal are coupled to

those of its ligand(s). In most cases each member of a series of

electronically modified ligands has to be synthesized inde-

pendently, which may be very time-consuming. Ideally, one

would like to be able to more readily change the electron

density at the metal center. One strategy would be to

covalently attach a modular substituent to, or merge it with,

a metal complex. In this case, the electronic properties of the

substituent should be easily, or maybe even in situ, tunable to

bring about changes in the properties of the metal complex as

a whole. By doing so, a series of electronically diverse metal

complexes may be obtained readily from one common

starting compound in, ideally, one step. This paper describes

the exploration of this modular strategy by studying a series

of hybrid complexes¹²that combine NCN-pincer-type orga-

nometallics with (metallo)porphyrins in a covalent manner

(Chart 1).

For NCN-pincer-type organometallic complexes¹³(with

the NCN-pincer being the monoanionic, potentially triden-

tate ligand [2,6-(Me₂NCH₂)₂C₆H₃]^-, Figure 1), the electron

density on the ligated metal is known to be influenced

strongly by the aryl-substituent at the para-position relative

to the carbon-to-metal σ-bond. We have recently shown that

the electron density on the chelated metal in para-substituted

NCN-pincer Ni^14,15and Pt^16,17complexes [MX(NCN-Z)]

In analogy to these examples, it was anticipated that

a metalloporphyrin could be a highly modular pincer

para-substituent. Not only do the electronic properties of

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Suijkerbuijk et al.

JOCArticle

an aryl bromide to an appropriate M(0) center would

provide us with an orthogonal metalation protocol (see the

Discussion section for a more in-depth discussion of this

matter). To this end, we aimed at synthesizing porphyrin

[2H(Br(NCN))₄], which has 3,5-bis[(dimethylamino)methyl]-

4-bromophenyl groups at its meso-positions (Chart 1). To

obtain [2H(Br(NCN))₄] we used a similar approach as during

the synthesis of the previously described [2H(H(NCN))₄], in

which an Adler-type cyclo-condensation of an appropriately

functionalized benzaldehyde was used to form the porphyrin

ring.³³In addition, benzylic methoxide groups were used as

masking groups for benzylic bromide functionalities, as the

latter were expected to be too reactive to be carried through-

out the whole synthetic route.

FIGURE 2. meso-Ferrocenyl (metallo)porphyrins prepared by

Kim et al. (left; R = phenyl or ferrocenyl; M = 2H, MnCl, Ni,

Zn) and meso-η¹-metalloporphyrins developed by Arnold and co-

workers (right; R = H, phenyl, PdX_nL_n, PtX_nL_n; M¹= 2H, MnCl,

Co, Ni, Zn; M²X_nL_n= PdX_nL_n, PtX_nL_n).

The functionalized benzaldehyde required for the Adler

procedure, i.e., 3,5-bis(methoxymethyl)-4-bromobenzalde-

hyde (4, Scheme 1), was prepared from 3,5-bis(hydroxy-

methyl)-4-bromoiodobenzene (1), which itself was obtained

from 3,5-dimethylaniline in six steps.³⁴The direct prepara-

tion of 3,5-bis(methoxymethyl)-4-bromoiodobenzene (3)

from 1 by treatment with NaH/MeI was hampered by partial

reduction of the aryl bromide bond leading to an inseparable

product mixture. Instead, 3,5-bis(chloromethyl)-4-bromoio-

dobenzene (2) was prepared in 93% yield by treatment

of 1 with methane sulfonylchloride in triethylamine/THF

(Scheme 1).³⁵Subsequent nucleophilic substitution of the

chloride groups with use of sodium methoxide in methanol

proceeded readily to give 3 in 90% isolated yield. Lithiation

of 3 at the iodo-position with t-BuLi (2 equiv) in THF at

-100 °C and reaction of the resulting phenyl lithium species

with DMF gave 4 in an optimized yield of 86%. During

the synthesis of 4 it was noted that the selective lithiation

of 3 at the iodo-position was impeded by a competing

lithiation at the bromo-position. This was caused by the

presence of the two hard benzylic oxygen donors, which can

intramolecularly stabilize the 4-lithio species.³⁶By perform-

ing the reaction at lower temperatures for shorter times, the

formation of the thermodynamic 4-lithio intermediate could

be successfully suppressed in favor of the kinetic 1-lithio

intermediate.

metalloporphyrins strongly depend on the incorporated

metal,^29-31in addition most metals can be introduced in

the porphyrin system in a single step with use of mild reaction

conditions. Rather than having to go through elaborate

syntheses to introduce each new para-substituent in the

classical para-substituted metallopincer systems, a metal

ion introduction step would suffice to change its electronic

properties and consequently those of the pincer metal com-

plex. The viability of this approach was shown recently by

Osuka and co-workers.³²

Conversely, the modulation of the electronic properties of

(metallo)porphyrin systems, which can lead to variations in

their electrochemical and photophysical properties, and

coordination chemistry,^29,31has also attracted considerable

attention. The attachment of derivatized phenyl groups at

the meso-position(s), for example, has been proven to be a

powerful tool in this respect. The use of metal complexes for

this purpose creates the possibility of, for example, redox

coupling between the two moieties or electronic interaction

in general.

Here, we present our efforts toward the development of a

hybrid ligand system that will allow for the exploration of the

concept of modular substituents in catalysis. A synthetic

study toward meso-tetrakis(NCN-pincer M²X)-porphyrin-

(M¹) hybrids ([M¹(M²X(NCN))₄]: M¹=2H, Ni, Zn; M²X=

Br, PdBr, PtCl) of the type depicted in Chart 1, and, in

particular, ways to address each coordination site, i.e.,

NCN-pincer ligand and porphyrin, independently and thus

orthogonally are presented. We, furthermore, studied quali-

tatively the effect of alterations at one metal site on the

properties of the second metal site in these hybrid systems.

The resulting bis(methoxymethyl)-4-bromobenzaldehyde

(4) was used in an Adler-type condensation reaction³⁷with

pyrrole in refluxing propionic acid to give meso-tetrakis[3,5-

bis(methoxymethyl)-4-bromophenyl]porphyrin (5) in 20%

yield (Scheme 2). The procedures developed by Lindsey

et al., which rely on the formation of a porphyrinogen,

catalyzed by for example BF₃OEt₂/NaCl or trifluoroacetic

3

Results

acid,³⁸were also attempted for the synthesis of 5. However,

the former conditions led to cleavage of the benzylic ether

I. Synthesis. A successful protocol for the synthesis of the

targeted heteromultimetallic molecules requires a building

block consisting of a free-base porphyrin with NCN-pincer

ligands attached to its four meso-positions. Selective metala-

tion of either ligand site relies on the availability of ortho-

gonal methods to address each moiety selectively and

independently. Since porphyrins are often metalated using

M(II) sources, we anticipated that metalation of the periph-

eral pincer ligands through oxidative addition of for instance

(33) Suijkerbuijk, B. M. J. M.; Lutz, M.; Spek, A. L.; van Koten, G.;

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SCHEME 1. Synthetic Route toward Functionalized Benzaldehyde 4

SCHEME 2. Synthetic Route toward [2H(Br(NCN))₄]

fragments (cf. Dijkstra et al.³⁹), whereas the latter conditions

led to difficulties during purification. Next, the methoxy-

methyl groups of 5 were converted into bromomethyl func-

tionalities by using a fresh solution of HBr in acetic acid to

give tetrakis[3,5-bis(bromomethyl)-4-bromophenyl]porphy-

rin (6) in 82% yield (Scheme 2).^33,40During this step we

noted that the use of Br₂-free HBr/HOAc is imperative, since

traces of Br₂impurities lead to partial β-bromination as

evidenced by MALDI-TOF MS and ¹H NMR spectroscopy.

Since the products of Adler-type porphyrin syntheses are

invariably contaminated with the corresponding chlorines,

crude 6 was treated with 2,3-dichloro-5,6-dicyanobenzoqui-

none (DDQ). This oxidation step was performed after the

bromo-demethoxylation reaction because the separation of

the polar polyether porphyrin 5 from DDQ and DDQH₂

proved to be difficult. Fortunately, the less polar nature of 6

allows for its straightforward isolation and purification by

column chromatography on silica gel. The benzylic bromine

atoms of 6 can be readily replaced through nucleophilic

substitution as was shown earlier by us in the synthesis of a

series of tetrapincer-porphyrin hybrids.³³The ditopic ligand

meso-tetrakis(3,5-bis[(dimethylamino)methyl]-4-bromophenyl)-

porphyrin [2H(Br(NCN))₄] was isolated in 99% yield from

the reaction of 6 with an excess of dimethylamine in CH₂Cl₂

(Scheme 2).

Because of the sensitivity of benzylic dimethylamino

groups toward singlet oxygen,⁴¹this compound was stored

and handled in the absence of molecular oxygen and light.

The structural identity and product homogeneity of all

1

compounds were confirmed by H NMR and ¹³C NMR

spectroscopy, elemental analysis, mass spectrometry, and

UV/vis spectroscopy. Porphyrin [2H(Br(NCN))₄] was sub-

sequently subjected to several metalation procedures to

selectively metalate the peripheral pincer groups and the

central porphyrin core, to finally obtain the heteromultime-

tallic pincer-porphyrin hybrids [M¹(M²X(NCN))₄].

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J. Org. Chem. Vol. 75, No. 5, 2010 1537

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SCHEME 3. Synthetic Routes Leading to [2H(M²X(NCN))₄], [M¹(Br(NCN))₄], and [M¹(M²X(NCN))₄]

Selective Peripheral Metalation. In principle, the aryl

bromide bonds of the peripheral NCN-pincer ligand groups

of [2H(Br(NCN))₄] can oxidatively add to suitable metal

[M(0)] sources to give the peripherally metalated complexes

[2H(M²X(NCN))₄]. We chose to focus on the metals of the

nickel triad, since the corresponding NCN-pincer metal

complexes are applicable as catalysts, sensors, and as Lewis

acids in materials chemistry.^14,16,18,42

acetone),⁴³which proved superior among several other plati-

nation reagents (vide infra), to give [2H(PtX(NCN))₄]

(X=halide) in 90% yield (Scheme 3). Benzene was used as

a solvent for this reaction since the use of THF⁴⁴gave rise

to the formation of black impurities, which were insepara-

ble from the product. [2H(PtX(NCN))₄] is almost exclu-

sively soluble in chlorinated solvents like CH₂Cl₂and

CHCl₃. Solubilization of [2H(PtX(NCN))₄] in these sol-

vents, however, is accompanied by fast halide exchange to

give meso-tetrakis(NCN-pincer PtX)porphyrins in which

The peripheral platination of [2H(Br(NCN))₄] was per-

formed with [Pt₂dipdba₃] (dipdba=4,4⁰-diisopropylbenzylidene

(42) Singleton, J. T. Tetrahedron 2003, 59, 1837–1857.

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1538 J. Org. Chem. Vol. 75, No. 5, 2010

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X is a mixture of Br and Cl. This process seems to be sped

up when a solution of [2H(PtX(NCN))₄] is subjected to

ambient light, but also occurs in its absence. To obtain a

pure product, [2H(PtX(NCN))₄] was converted into the

corresponding tetrachloride complex [2H(PtCI(NCN))₄].

Halide exchange on NCN-pincer platinum halide com-

plexes is usually accomplished by halide abstraction with

AgBF₄followed by treatment with a halide source. Un-

fortunately, treatment of a solution of [2H(PtX(NCN))₄] in

CH₂Cl₂with AgBF₄led to the formation of a sticky

precipitate, which could not be analyzed and did not

redissolve upon treatment with LiCl or NaCl. Therefore,

[2H(PtX(NCN))₄] was treated several times with aqueous

NH₄Cl in the presence of ambient light, which finally gave

[2H(PtCI(NCN))₄] in an overall yield of 67%.

[2H(PtCl(NCN))₄] with Ru₃(CO)₁₂in 1,2-dichloroben-

zene,⁴⁶[Au(tht)₂]BF₄in CH₂Cl₂,⁴⁷or Pd(OAc)₂in CHCl₃

did not lead to the formation of the respective porphyrin

chelates. We next turned our attention to porphyrin meta-

lation prior to pincer metalation.

Selective Core Metalation. Nickel(II) and zinc(II) could be

selectively introduced in the porphyrin part of [2H(Br-

(NCN))₄]. Nickel(II) was introduced by using Ni(acac)₂in

boiling toluene,⁴⁸which gave [Ni(Br(NCN))₄] in quantitative

yield after recrystallization from hexanes. The introduction

of zinc(II) was performed with Zn(OAc)₂2H₂O in a mixed

3

solvent system of MeOH and CH₂Cl₂, which readily gave

[Zn(Br(NCN))₄] in 99% yield (Scheme 3). Both compounds

were fully characterized and their structures were also con-

firmed by single-crystal X-ray crystallography. Whereas the

introduction of Zn(II) and Ni(II) in [2H(Br(NCN))₄] pro-

ceeded smoothly, attempts to introduce the 4d and 5d metals

mentioned above also failed in this case. Zinc porphyrin

[Zn(Br(NCN))₄] was found to form infinite coordination

polymers through hexacoordination of the zinc atom includ-

ing bis(apical) coordination of dimethylamino groups of

adjacent [Zn(Br(NCN))₄] molecules. The resulting solid state

topology is of interest with respect to the structures of

biological photosynthetic systems^49,50and has recently been

described by us in detail.⁵¹

Palladation of the peripheral ligand sites of [2H-

(Br(NCN))₄] was achieved with use of [Pd₂dba₃] CHCl₃in

3

benzene, to give [2H(PdBr(NCN))₄] in 90% yield (Scheme 3).

Although the solubility profile of this compound is identical

with that of [2H(PtX(NCN))₄], halide exchange in chlori-

nated solvents is sufficiently slow to allow its isolation as an

analytically pure compound. In fact, substantial amounts of

halide exchange were only visible after exposure of a CDCl₃

solution to ambient light for several days. Exchange of the

bromide ligands for chloride ligands via the AgBF₄route led

to similar problems as for [2H(PtX(NCN))₄], while the

NH₄Cl route employed in the synthesis of [2H(PtCl-

(NCN))₄] did not proceed to completion. Therefore, the

compound was fully characterized as the tetrakis-PdBr

product.

Single crystals of [Ni(Br(NCN))₄] were grown by slow

concentration of a saturated solution in n-hexane. The

crystal structure shows that the nickel(II) ion is located on

an exact, crystallographic inversion center at the midpoint of

the N₄coordination plane formed by the pyrrolic nitrogen

atoms, with N_pyrrole-Ni distances of 1.947(4) and 1.951(4) A

for Ni-N1 and Ni-N2, respectively (Figure 3).

Whereas in most cases the incorporation of the small

Ni(II) ion in the porphyrin cavity leads to nonplanar distor-

tions of the macrocycle in order to accommodate it,^52-58this

structure is one of the few examples wherein a planar

conformation is observed.^59,60Contrary to its zinc(II) coun-

terpart, [Ni(Br(NCN))₄] does not form supramolecular

aggregates through intermolecular interactions of the nickel

Unfortunately, nickelation of the peripheral pincer sites

with Ni(COD)₂in THF¹⁴did not lead to the desired product.

Although strict anaerobic procedures were employed,

the high sensitivity of the NCN-pincer nickel(II) complex

toward oxygen in combination with the close proximity of

the porphyrin might have facilitated (partial) oxidation of

the anticipated [2H(NiBr(NCN))₄] product.

The selectivities of the palladation and platination reac-

tions for the peripheral NCN-pincer ligand sites were corro-

1

borated by H NMR spectroscopy. Similar ratios between

the integrals corresponding to the benzylic and pyrrolic

protons were found for the parent [2H(Br(NCN))₄] ligand

and for the oxidative addition products [2H(PdBr(NCN))₄]

and[2H(PtCl(NCN))₄]. The selectivity was further supported

by mass spectrometry and UV/vis analysis of the products.

To arrive at the targeted heterobimetallic complexes, it

was subsequently attempted to introduce metals in the

porphyrin core of [2H(PdBr(NCN))₄] and [2H(PtCl(N-

CN))₄]. In order to later use ¹⁹⁵Pt NMR spectroscopy as a

tool to study the influence of the porphyrin on the peripheral

metal centers, it was anticipated that core metalation with

metals that would give rise to diamagnetic metalloporphyr-

ins would be of interest. Reactions of [2H(PdBr(NCN))₄]

(46) Poriel, C.; Ferrand, Y.; Juillard, S.; Le Maux, P.; Simonneaux, G.

Tetrahedron 2004, 60, 145–158.

(47) Chambron, J.-C.; Heitz, V.; Sauvage, J.-P. New J. Chem. 1997, 21,

237–240.

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Lehmann, I. J. Chem. Soc., Dalton Trans. 1998, 4187–4200.

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(50) Huijser, A. M.; Suijkerbuijk, B. M. J. M.; Klein Gebbink, R. J. M.;

Savenije, T. J.; Siebbeles, L. D. A. J. Am. Chem. Soc. 2008, 130, 2485–2492.

(51) Suijkerbuijk, B. M. J. M.; Tooke, D. M.; Spek, A. L.; van Koten, G.;

Klein Gebbink, R. J. M. Chem. Asian J. 2007, 2, 889–903.

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

Soc., Chem. Commun. 1987, 1788–1789.

(53) Golder, A. J.; Nolan, K. B.; Povey, D. C.; Traylor, T. G. Inorg. Chim.

Acta 1988, 143, 71–75.

(54) Hayvali, M.; Gunduz, H.; Gunduz, N.; Kilic, Z.; Hokelek, T. J. Mol.

Struct. 2000, 525, 215–226.

(55) La, T.; Richards, R. A.; Lu, R. S.; Bau, R.; Miskelly, G. M. Inorg.

Chem. 1995, 34, 5632–5640.

(56) Maclean, A. L.; Foran, G. J.; Kennedy, F. J.; Turner, P.; Hambley,

T. W. Aust. J. Chem. 1996, 49, 1273–1278.

and [2H(PtCl(NCN))₄] with Ni(OAc)₂4H₂O⁴⁵or Zn-

3

(OAc)₂2H₂O⁴⁵in MeOH/CH₂Cl₂only gave moderate

3

yields (∼30% isolated yield). Unfortunately, reactions

with sources of the 4d and 5d metals, which usually readily

give access to the corresponding metalloporphyrins, did

not succeed either: treatment of [2H(PdBr(NCN))₄] or

(57) Nurco, D. J.; Smith, K. M.; Fajer, J. Chem. Commun. 2002, 2982–

2983.

(58) Richard, P.; Rose, E.; Boitrel, B. Inorg. Chem. 1998, 37, 6532–6534.

(59) Boyd, P. D. W.; Burrell, A. K.; Campbell, W. M.; Cocks, P. A.;

Gordon, K. C.; Jameson, G. B.; Officer, D. L.; Zhao, Z. Chem. Commun.

1999, 637–638.

(45) Padmaja, K.; Wei, L.; Lindsey, J. S.; Bocian, D. F. J. Org. Chem.

2005, 70, 7972–7978.

(60) Sugiura, K.-I.; Iwasaki, K.; Umishita, K.; Hino, S.; Ogata, H.;

Miyajima, S.; Sakata, Y. Chem. Lett. 1999, 841–842.

J. Org. Chem. Vol. 75, No. 5, 2010 1539

Suijkerbuijk et al.

JOCArticle

FIGURE 3. ORTEP plot of [Ni(Br(NCN))₄] in the crystal with displacement ellipsoids at the 50% probability level (left). Hydrogen atoms and

disordered solvent molecules have been omitted for clarity. Symmetry operation a: -x, 1 - y, 1 - z. On the right is depicted a view along the

N1a-Ni-N1 direction, wherein all meso-phenyl groups have been omitted.

atom with NMe₂groupings⁶¹of neighboring [Ni(Br-

(NCN))₄] molecules.

Zn-N bonds) became sharp after intramolecular coordina-

tion of the dimethylamino groups to Pd or Pt. Halide

scrambling at the metal centers (Br/Cl) was observed in all

cases after long reaction times in chlorinated solvents such as

CH₂Cl₂or CHCl₃. This transformation was inferred from

both ¹H NMR spectroscopy and (MA)LDI-TOF mass

Heteromultimetallic Systems. To arrive at the targeted

heteromultimetallic systems [M¹(M²X(NCN))₄] (M¹= Ni,

Zn; M²X = PdBr, PtCl), the core-metalated porphyrins

[Ni(Br(NCN))₄] and [Zn(Br(NCN))₄] were peripherally

metalated with palladium and platinum according to the

procedures employed for [2H(PdBr(NCN))₄] and [2H-

(PtCl(NCN))₄] (vide supra). Peripheral palladation of

[Ni(Br(NCN))₄] and [Zn(Br(NCN))₄] proceeded readily with

1

spectrometry. In the H NMR spectrum all peaks for the

chloride and bromide complexes coincide except for the

signals belonging to the NMe₂protons. In the case where

X = Cl those signals are shifted ∼0.04 ppm upfield with

respect to the case where X=Br. For all complexes, mass

spectrometric analyses were performed by MALDI-TOF

(with 2,5-dihydroxybenzoic acid as matrix or without

matrix). In every case, peaks corresponding to the molecular

ion were found. Additional information about the composi-

tion of the [M¹(M²X(NCN))₄] compounds was obtained by

the fragmentation pattern of the molecular ion. Each mole-

cular ion peak ([MI]^þ) was accompanied by peaks corre-

sponding to loss of one halide ([MI - X]^þ), one metalloha-

lide group ([MI - M²X]^þ), one halide and one metallohalide

group ([MI - M²X - X]^þ), and so forth. When MALDI-

TOF samples of [M¹(PdBr(NCN))₄] were prepared in

CH₂Cl₂the peaks corresponding to (partially) halogen-

scrambled complexes were always found.

Single crystals of the pyridine adduct of [Zn(PdBr-

(NCN))₄], {[Zn(PdBr(NCN))₄](pyridine)}, were obtained by

slow evaporation of a solution of [Zn(PdBr(NCN))₄] in a

mixed solvent system comprising CH₂Cl₂, hexanes, and one

drop of pyridine. The molecular structure as obtained by

X-ray crystallography is depicted in Figure 4. It comprises a

[meso-tetraphenylporphyrinato]zinc(II) unit of which the

zinc(II) ion is pentacoordinate in a distorted tetragonal

pyramidal fashion.

a slight excess of [Pd₂dba₃] CHCl₃in benzene at room

3

temperature (Scheme 3). The resulting peripherally palla-

dated pincer-porphyrin hybrids [Ni(PdBr(NCN))₄] and

[Zn(PdBr(NCN))₄] were isolated as analytically pure com-

pounds in high yields. The corresponding tetraplatinated

compounds [Ni(PtCl(NCN))₄] and [Zn(PtCl(NCN))₄] were

obtained after treatment with [Pt₂dipdba₃] in refluxing ben-

zene followed by halide exchange with NH₄Cl. In the case of

[Ni(PtCl(NCN))₄], the Br/Cl exchange could also be

achieved by treatment with AgBF₄and LiCl, most probably

owing to the relative inertness of nickel porphyrins.

Clear evidence for the 4-fold oxidative addition of the

pincer-(metallo)porphyrin hybrids [M¹(Br(NCN))₄] (M¹=

2H, Ni, Zn) to Pd(0) and Pt(0) (to give the corresponding

meso-tetrakis(NCN-pincer MX) complexes [M¹(M²X(NC-

1

N))₄]) was obtained from a comparison of their H NMR

spectra. Whereas the benzylic protons and the dimethyl-

amino-protons of the ligands resonate at δ ≈ 3.8 and

2.4 ppm, respectively, they are shifted to δ ≈ 4.2 and

3.2 ppm upon coordination of the nitrogen atoms to the

metal. Peripheral metalation of [Zn(Br(NCN))₄] was espe-

cially clear because the four broad resonances of the starting

material (caused by self-aggregation through intermolecular

The structure does not contain any symmetry elements.

The four pyrrolic nitrogen atoms of the porphyrin form the

basal plane with Zn-N pyrrole distances ranging from

(61) Jia, S.-L.; Jentzen, W.; Shang, M.; Song, X.-Z.; Ma, J.-G.; Scheidt,

W. R.; Shelnutt, J. A. Inorg. Chem. 1998, 37, 4402–4412.

1540 J. Org. Chem. Vol. 75, No. 5, 2010

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FIGURE 4. ORTEP plot of {[Zn(PdBr(NCN))₄](pyridine)} in the crystal with displacement ellipsoids at the 50% probability level. Hydrogen

atoms and solvent molecules (CHCl₃and toluene) present in the crystal lattice have been omitted for clarity. On the right are shown a view along

the N5-Zn axis (top) and along the N2-N4 axis (bottom).

TABLE 1. ¹⁹⁵Pt NMR Chemical Shifts for [2H(PtCl(NCN))₄],

2.072(3) to 2.082(3) A. The nitrogen atom of a pyridine mole-

cule occupies the apical fifth ligand position at 2.100(3) A

[Ni(PtCl(NCN))₄], and [Zn(PtCl(NCN))₄]^a

compd

δ

_Pt(ppm)

from the zinc atom, which is displaced from the least-squares

plane of the porphyrin by 0.3927(4) A toward the pyridine

molecule. The Zn-N5 distance compares to the literature

values for ZnTPP-N_pyridineadducts, which cover quite a

broad range between 2.10 and 2.20 A.^62-66The four meso-

phenyl groups are all part of an NCN-pincer bromido-

palladio(II) moiety with the Pd atoms at the positions para

with respect to the porphyrin ring. The Pd-Pd distances

across the porphyrin structure amount to 19.035(5) A for

Pd1-Pd3 and 19.055(5) A for Pd2-Pd4, respectively. Each

distorted square-planar Pd(II) center is ligated by a

κ³-coordinating NCN-pincer moiety provided by the pincer-

porphyrin hybrid and by a halide ion at the position trans with

respect to the Pd-C σ-bond. The halide position was refined

with a partial occupation by chloride (54%) and by bromide

(46%). The Pd-C_ipsodistances are in the range of 1.905(3) to

1.924(4) A. The benzylic nitrogen atoms reside at distances of

2.090(3)-2.111(3) A from the Pd atoms. The meso-phenyl

groups are connected to the porphyrin at angles ranging from

58.45(16)° to 84.57(16)°.

II. Spectroscopic and Physicochemical Characterization.

There are several ways to probe intramolecular interaction in

the present [M¹(M²X(NCN))₄] molecules, since the

(metallo)porphyrin and the NCN-pincer metallohalide each

display their own characteristic responses to external stimuli.

The intramolecular influence of the (metallo)porphyrin part

and the NCN-pincer PtCl moieties on each other was studied

by means of ¹⁹⁵Pt NMR spectroscopy and UV/vis spectro-

scopy. Additionally, it was also examined by means of its

chemical reactivity, i.e., selective pyridine binding to zinc

[2H(PtCl(NCN))₄]

[Ni(PtCl(NCN))₄]

[Zn(PtCl(NCN))₄]

[PtCl(NCN)]

-3162

-3170

-3176

-3147

^aChemical shifts were referenced to an external standard (1 M Na₂-

[PtCl₆] in D₂O) before each measurement.

porphyrin moieties and selective SO₂and CuCl₂reactivity

toward the NCN-pincer platinum groups.

¹⁹⁵Pt NMR Spectroscopy. In general, the ¹⁹⁵Pt nucleus is

highly sensitive to changes in its magnetic environment.⁶⁷

Particularly in para-substituted NCN-pincer PtX complexes

([PtX(NCN-Z)]), the ¹⁹⁵Pt chemical shift has been proven to

be highly sensitive to either the electron-releasing or -with-

drawing properties of the para-substituent Z.^16,68In the

current systems, the para-substituent of each NCN-pincer

PtX group is a (metallo)porphyrin. The electronic properties

of the latter are strongly dependent on the complexed metal²⁹

and thus it was investigated whether the nature of the central

(metallo)porphyrin influences the chemical shift of the peri-

pheral Pt centers in our hybrids. ¹⁹⁵Pt NMR spectra were

recorded of [M¹(PtCl(NCN))₄] (M¹=2H, Ni, Zn) and their

δ_Ptvalues are listed in Table 1 (samples were measured at

different concentrations (2-10 mM) and it was found that

for each compound Δδ_Pt< 1 ppm within this concentration

range). Whereas the differences between the δ_Ptvalues of

these compounds are relatively small, they are detectable and

reproducible and, therefore, indicate a difference in the

electron density at Pt as a direct consequence of a change in

the porphyrin metal. In comparison with the p-H-substituted

[PtCl(NCN)], the (metallo)porphyrins cause an upfield shift

of the Pt nuclei of the peripheral NCN-pincer platinum

systems.

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J. Org. Chem. Vol. 75, No. 5, 2010 1541

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FIGURE 5. Comparison of the normalized (Soret band) electronic spectra of [M¹(Br(NCN))₄] (blue), [M¹(PdBr(NCN))₄] (green), and

[M¹(PtCl(NCN))₄] (red) for the M¹= 2H (left) and M¹= Ni (right) complexes.

Along the series M¹=2H, Ni, Zn, the Pt nuclei gradually

become more shielded. Similarly, the porphyrin part of the

molecule becomes a better electron donor along the same

series by virtue of an increasing destabilization of its HOMO

(from ca. -11.7 and ca. -11.0 eV for the free-base and the

nickel complex, respectively, to ca. -10.6 eV in the case of

zinc).⁶⁹This trend follows the redox potentials of the corre-

sponding tetraphenylporphyrin (TPP) complexes as a mea-

sure of the porphyrin ring’s electron-donating power. Hence,

these values indicate that the electron density on the por-

bathochromically and increased in intensity with respect to

the Soret band. For peripheral platination the effects are

even more pronounced: the absolute bathochromic shifts in

comparison with the nonperipherally metalated pincer por-

phyrin hybrids are 13, 14, and 9 nm for [2H(PtCl(NCN))₄],

[Ni(PtCl(NCN))₄], and [Zn(PtCl(NCN))₄], respectively.

With respect to the corresponding Pd compounds the differ-

ences are 8, 8, and 7 nm. It was noted that exchange of peri-

pheral bromide ligands for chloride in [Ni(PtCl(NCN))₄] had

no significant effect on its electronic spectrum.

Pyridine Binding at Zinc. Pyridine is known to bind to

zinc(II) porphyrins to give distorted square-pyramidal co-

ordination complexes.⁷⁸The binding constant of this inter-

action depends on the interaction of an unoccupied

molecular orbital with zinc character on the porphyrin with

the pyridine lone pair.⁷⁹The binding constants of pyridine

with [tetraphenylporphyrinato]zinc(II) (ZnTPP),⁸⁰[Zn(Pd-

Br(NCN))₄], and [Zn(PtCl(NCN))₄] in CH₂Cl₂at room

temperature were determined by UV/vis spectroscopy

(Figure 6).

phyrin increases in the order of M¹=2H < Ni < Zn [E_1/2

-

(0/1)=0.97, 0.95, and 0.71 V, respectively].⁷⁰

UV/Vis Spectroscopy. The electronic spectra of

(metallo)porphyrins are dominated by the high energy Soret

(B) band and lower energy Q bands. The energies of the

transitions are largely governed by the incorporated metal

and by the substitution pattern at the porphyrin periphery.

For meso-tetrakis(aryl)porphyrins, it is known that these

energies are also dependent on the electronic properties of

the meso-aryl groups, i.e., on the nature of their substi-

tuents.^71-77It was therefore expected that a possible influ-

ence of the peripheral NCN-metallopincer groups of

[M¹(M²X(NCN))₄] on the (metallo)porphyrin core could

be deduced from their UV/vis spectra. Figure 5 shows the

UV/vis absorption spectra of the 2H- (left) and Ni-series

(right) of [M¹(M²X(NCN))₄], respectively. For these series,

the spectra of the [M¹(Br(NCN))₄] species were very similar

to those of the corresponding TPP analogues. When Pd was

incorporated at the peripheral NCN-ligand sites, however,

bathochromic shifts, amounting to 5 nm for [2H(PdBr-

(NCN))₄], 6 nm for [Ni(PdBr(NCN))₄], and 2 nm for [Zn-

(PdBr(NCN))₄], were observed. The Q-bands also shifted

For ZnTPP a binding constant of 8100 ( 200 M^-1was

determined, which corresponds well with the value found by

Mizutani et al. (7720 M^-1).⁸¹Interestingly, the binding

constants decreased substantially upon peripheral meta-

lation: for [Zn(PdBr(NCN))₄], a value of 6000 ( 200 M^-1

was found, whereas [Zn(PtCl(NCN))₄] exhibits an even

lower affinity for pyridine at 4700 ( 150 M^-1. These findings

indicate that peripheral metalation of the NCN-pincer

groups in the hybrids destabilizes the unoccupied orbital

with zinc character of their porphyrin moieties. This effect is

larger in the case when platinum occupies the peripheral

positions than when palladium does.

Reactions at Pt. The experiments described above demon-

strate that there is an electronic interaction between the

peripheral metallopincer groups and the central porphyrin

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(70) Fuhrhop, J.-H. Reversible Reactions of Porphyrins and Metallopor-

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Netherlands, 1974.

(71) Chang, D.; Malinski, T.; Ulman, A.; Kadish, K. M. Inorg. Chem.

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latter’s coordination behavior with pyridine is obscured by a competing,

intermolecular Zn-NMe₂R self-assembly process (ref 51).

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1542 J. Org. Chem. Vol. 75, No. 5, 2010

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FIGURE 6. Spectral changes of solutions of [ZnPdBr(NCN))₄] (left) and [Zn(PtCl(NCN))₄] upon titration with pyridine. The insets show the

corresponding Benesi-Hildebrand plots.

SCHEME 4. Reactions Performed on NCN-Pincer PtCl Sites^a

^an = 4, R = [porphyrinato]metal(II)-5,10,15,20-tetrayl (metal = 2H, Ni, Zn).

group. The rich chemistry of the NCN-pincer Pt species

prompted us to investigate the effect of chemistry at the

peripheral NCN-pincer PtCl sites on the UV/vis spectra of

the porphyrin core.

Gaseous SO₂is known to bind to NCN-pincer Pt halide

complexes via a Lewis base/Lewis acid interaction in which

Pt acts as the Lewis base (through its filled d_z₂orbital) and

SO₂as the Lewis acid (Scheme 4). During this transforma-

tion, a diagnostic color change from colorless to orange

occurs, which led to the use of these complexes as SO₂

sensors.⁸²

Whereas the electronic spectra of all [M¹(Br(NCN))₄]

compounds are completely insensitive to the presence of an

excess of SO₂(Δλ_max< 0.1 nm), dissolution of [M¹(Pt-

FIGURE 7. Comparison of the normalized electronic spectra of

[Ni(Br(NCN))₄] (blue), [Ni(PtCl(NCN))₄] (red), and [Ni(Pt(IV)-

Cl₃(NCN))₄] (green).

Cl(NCN))₄] in a saturated CH₂Cl₂solution of SO₂leads to an

immediate change in λ_maxof the Soret band leading to

hypsochromic shifts of ∼4 nm with concomitant broadening

of the band. Color changes of the solutions were not ob-

served due to the intense absorption bands of the porphyrins

in the visible. The two UV/vis absorption bands of the NCN-

pincer PtCl-SO₂complex itself [λ_max≈ 350 nm (ε ≈ 7000 M^-1

cm^-1), 410 (ε ≈ 2000 M^-1cm^-1)]⁸³were not observed; most

probably they are obscured by the dominant Soret band.

Upon decomplexation of SO₂and subsequent evapo-

ration from the sample, the Soret band returned to its ini-

tial position. The complexation/decomplexation could be

repeated numerous times, without a change in the response

of the system, suggesting the process is reversible.

in a mixture of CH₂Cl₂and EtOH,⁸⁴which gave the almost

insoluble [Ni(Pt(IV)Cl₃(NCN))₄] species. Conversion of the

Pt(II) centers into Pt(IV) centers exerts a dramatic effect on

the electronic spectrum of the hybrid (Figure 7). The Soret

band exhibits a substantial hypsochromic shift of 12 nm

when the peripheral NCN-pincer Pt(II)Cl sites are irrever-

sibly oxidized to NCN-Pt(IV)Cl₃. Control experiments with

NiTPP showed no reactivity toward CuCl₂. Unfortunately,

the same reaction could not be applied to [2H(PtCl(NCN))₄]

and [Zn(PtCl(NCN))₄] due to side reactions, which were not

investigated further.

Discussion

In addition to the reversible reaction of the NCN-pincer

PtCl groups with SO₂, we also explored the irreversible

reaction of the peripheral platinum pincer centers with CuCl₂

I. Synthesis. We have previously described the synthesis of

[2H(H(NCN))₄],³³whose selective 4-fold metalation at the

phenyl-positions para with respect to the porphyrin ring

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J. Org. Chem. Vol. 75, No. 5, 2010 1543

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[ortho with respect to both (dimethylamino)methyl donor

groupings] should give direct access to the family of

[M¹(M²X(NCN))₄] hybrid compounds described here.

Among other reasons, we anticipated that this strategy

would be hampered by the selectivity of a direct cyclopalla-

dation reaction for the positions meta with respect to the

desired para position.⁸⁵An alternative route to arrive at

[M¹(M²X(NCN))₄] via [2H(H(NCN))₄] would involve a

lithiation/transmetalation sequence.^13,86This idea was

abandoned for a number of reasons. First of all, it would

involve a protection step of the porphyrin ring (metalation)

prior to lithiation/transmetalation and, to obtain the free-

base porphyrin, an inevitable deprotection step (demeta-

lation) thereafter. The conditions used for demetalation of

the porphyrin ring can also lead to demetalation of the NCN-

metallopincer fragment, especially in the case when the metal

in the pincer unit is palladium. In addition, the necessary

equimolarity of organolithium reagent and NCN-pincer

fragment is generally difficult to achieve practically⁸⁷as a

result of the small quantities in which [2H(H(NCN))₄] is

available. Therefore, a different route that relies on meta-

lation of the NCN-pincer fragments via oxidative addition

was opted for.¹³This requires the presence of a group that

can oxidatively add to Pd or Pt centers, e.g., a carbon-

bromine bond.¹³However, if M(II) sources (M=Pd, Pt) are

used in combination with a free-base porphyrin, metalation

of the porphyrin may also occur.³¹For example, when

[{Pt(p-tol)₂SEt₂}₂] in benzene⁸⁸was used as a platinating

agent¹⁶for [2H(Br(NCN))₄], not only the NCN-pincer

groups were platinated but also the porphyrin core. This

was independently confirmed by the reaction of tetraphe-

nylporphyrin (TPP) with half an equivalent of [{Pt(p-tol)₂-

SEt₂}₂] in refluxing benzene. After 3 h at reflux, the crude

reaction mixture consisted of 55% TPP, 38% PtTPP,⁸⁹and

7% of an unidentified, platinated porphyrin. The peripheral

metalation reactions of [2H(Br(NCN))₄] were eventually

analysis of the product mixture showed substantial amounts of

unidentified impurities. The impurities composed 50% of the

total and could be removed by column chromatography. Mass

1

spectrometry and H NMR unfortunately did not provide

conclusive evidence as to their nature. Although to a lesser

extent, the same outcome was noted when attempting to

introduce Zn(II) into the platinated porphyrin. A metalation

sequence in which the porphyrin is metalated prior to the

peripheral pincer ligands was therefore used for the synthesis

of heterobimetallic hybrids [M¹(M²X(NCN))₄].

After the peripheral metalation steps, analysis of the

products obtained after a workup involving solubilization

in chlorinated solvents (CH₂Cl₂, CHCl₃) revealed that halide

scrambling took place at the peripheral metal atoms (Br/Cl).

We found that this process is much faster for the peripherally

platinated porphyrins than for their palladium analogues.

The [M¹(PdBr(NCN))₄] complexes could be readily isolated

as analytically pure solids; halide scrambling only took place

when solutions were exposed to ambient light for several

days (as judged from ¹H NMR spectroscopy). For the

platinum compounds [M¹(PtX(NCN))₄], however, this ex-

change reaction proved to be very fast. Even in degassed and

deacidified solutions, and with a minimum exposure to light,

halide scrambling always seemed to take place. For their

novel meso-η¹-metalloporphyrins,^21-24,26,27Arnold et al.

also found that halide exchange occurred in chlorinated

solvents. They ascribed this to the high trans-effect of the

σ-bound porphyrin ligand.²¹Although this might be a viable

explanation, reference experiments conducted by us on the

parent NCN-pincer PdBr and PtBr compounds showed

that these complexes underwent halide scrambling much

more slowly, if at all, than the corresponding multimetallic

pincer-porphyrin hybrids. This observation would make a

trans-effect explanation quite drastic and rather seems to

point to another effect of the porphyrin on halide exchange

at the pincer-metal sites. Recently, Perutz et al. found that

photoinduced electron transfer from a metalloporphyrin to

an intramolecular Re center led to fast ligand exchange at

Re.^91-94This phenomenon might also be of importance

when considering halide exchange on palladium and plati-

num complexes connected to a (metallo)porphyrin system.

II. Intramolecular Influence between the Complexes. The

influence of the (metallo)porphyrin on the electronic proper-

ties of the peripheral platinum groups was investigated by

several means. ¹⁹⁵Pt NMR spectroscopy indicated that, for

the [M¹(PtCl(NCN))₄] compounds (M¹=2H, Ni, Zn), the

electron density at Pt increases in the order M¹=2H < Ni <

Zn. Cyclovoltammetric studies on the analogous metal

tetraphenylporphyrins have shown that a decrease of the

first oxidation potential of the (metallo)porphyrin, i.e., the

increase in HOMO energy, follows the same order (vide

supra). The correlation between the increments of δ_Ptin the

[M¹(PtCl(NCN))₄] series and those of E_1/2(0/1) of the

M¹TPP series shows that the electronic nature of the metal-

loporphyrin in the hybrids has an effect on the electron

performed with the M(0) complexes [Pd₂(dba)₃] CHCl₃

3

and [Pt₂dipdba₃]. This strategy readily yields the desired

peripherally metalated porphyrins [2H(M²X(NCN))₄] with-

out affecting the central porphyrin cavity.

In analogy to the meso-metalated porphyrins of Arnold

et al.²⁷heteromultimetallic complexes derived from [2H(Br-

(NCN))₄] can be synthesized in two ways in which either the

porphyrin or the pincer is metalated before the other.

Whereas the selectivity and yield of the palladation and

platination reactions at the periphery were always high,

metalation of the porphyrin cavity after peripheral meta-

lation sometimes led to the formation of side products. For

example, when [2H(PtCl(NCN))₄] was reacted with Ni(OAc)₂

4H₂O in a mixture of CH₂Cl₂/MeOH at 40 °C,^{90 1}H NMR

3

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

4, 759–762.

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

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

(87) Kleij, A. W.; Gossage, R. A.; Klein Gebbink, R. J. M.; Brinkmann,

N.; Reijerse, E. J.; Kragl, U.; Lutz, M.; Spek, A. L.; van Koten, G. J. Am.

Chem. Soc. 2000, 122, 12112–12124.

(88) Canty, A. J.; Patel, J.; Skelton, B. W.; White, A. H. J. Organomet.

Chem. 2000, 599, 195–199.

(89) Milgrom, L. R. Polyhedron 1984, 3, 879–882.

(90) Since the starting material is virtually insoluble in toluene, the

Ni(acac)₂/toluene procedure could not be employed. In addition, Ni(II)

introduction using Ni(OAc)₂is milder and also the nickel(II) insertion

into [2H(Br(NCN))₄] using the same method yielded quantitatively

[Ni(Br(NCN))₄] without any problems.

(91) Aspley, C. J.; Lindsay Smith, J. R.; Perutz, R. N. J. Chem. Soc.,

Dalton Trans. 1999, 2269–2271.

(92) Aspley, C. J.; Lindsay Smith, J. R.; Perutz, R. N.; Pursche, D.

J. Chem. Soc., Dalton Trans. 2002, 170–180.

(93) Gabrielsson, A.; Hartl, F.; Lindsay Smith, J. R.; Perutz, R. N. Chem.

Commun. 2002, 950–951.

(94) Gabrielsson, A.; Hartl, F.; Zhang, H.; Lindsay Smith, J. R.; Towrie,

ꢂ

M.; Vlcek, J., A.; Perutz, R. N. J. Am. Chem. Soc. 2006, 128, 4253–4266.

1544 J. Org. Chem. Vol. 75, No. 5, 2010

Suijkerbuijk et al.

JOCArticle

density on the peripheral platinum centers. In fact, the

(metallo)porphyrin part acts as an electron donor in these

systems.

Pyridine binds stronger to zinc tetraarylporphyrins with

electron-withdrawing groups than to those with electron-

releasing groups. This arises from the fact that electron-

withdrawing substituents (e.g., pentafluorophenyl)¹⁰⁰lower

the LUMO with zinc character on the porphyrin, which

makes its interaction with the pyridine lone pair energetically

more favorable. For our systems the interaction between the

pyridine and zinc porphyrin becomes weaker along the series

ZnTPP > [Zn(PdBr(NCN))₄] > [Zn(PtCl(NCN))₄], which

points to a destabilization of the unoccupied porphyrin

molecular orbital that is responsible for the binding of a

ligand to the central zinc atom upon peripheral metalation.

Chemistry that is selective for the NCN-pincer Pt moiety

also influences the metalloporphyrin’s electronic properties.

Upon SO₂binding to the four peripheral NCN-pincer

PtCl moieties of [Ni(PtCl(NCN))₄], its Soret band shows a

hypsochromic shift of about 4 nm. The oxidation of the

platinum(II) centers of [Ni(PtCl(NCN))₄] to platinum(IV)

has an even stronger effect on the electronic absorption

spectrum of the resulting [Ni(Pt(IV)Cl₃(NCN))₄]. As a result

of this reaction the Soret band almost shifts back to the

position of the Soret band of [Ni(Br(NCN))₄]. Complexation

of SO₂to the d_z₂orbital of platinum is expected to modulate

the energies of its other d orbitals, including those interact-

ing with the π-system of the phenyl ring. This apparently

leads to a decreased interaction between the porphyrin

and the meso-phenyl rings by increasing the gap between

the interacting energy levels of the platinum center and

its phenyl ligand. Oxidation of the platinum centers to

Pt(IV) completely alters the orbital system at Pt,¹which,

as inferred from the hypsochromic shift of the Soret band

of [Ni(PtCl(NCN))₄] as a result of this reaction, apparently

leads to even smaller interaction of the porphyrin with the

meso-phenyl groups.

The absorption maxima as observed in their UV/vis

spectra for [2H(Br(NCN))₄] and [Ni(Br(NCN))₄] parallel

those found for their TPP analogues 2HTPP and NiTPP.

This shows that the meso-3,5-bis[(dimethylamino)methyl]-4-

bromophenyl groups exert relatively little electronic influ-

ence on the porphyrin chromophores compared to that of

phenyl groups. The insertion of palladium or platinum

atoms in the carbon-bromine bonds, however, leads to

bathochromic shifts in the electronic spectra. The extent to

which this occurs for peripheral palladation is 4 nm, while

peripheral platination leads to bathochromic shifts amount-

ing to 12 nm, indicating a reduction of the S₀f S₂gap by

peripheral metalation. Arnold and co-workers noted similar

effects for their meso-η¹-palladio(II) and -platino(II) por-

phyrins, but attributed those to nonplanar distortions of the

porphyrin macrocycle (although they did ascribe shifts in the

redox potentials of the peripherally metalated porphyrins to

the strong electron-donating capacity of the palladium and

platinum groups).²¹In our case, distortions of the porphyrin

core are not expected upon metalation of the rather remote

NCN-pincer ligand sites. For example, the domed structure

of the zinc porphyrin part of the crystal structure of

{[Zn(PdBr(NCN))₄](pyridine)} (Figure 4) is not different

from those of pyridine adducts of other zinc tetraaryl-

porphyrins.^62,63,65,66In a related tetrakis(SCS-pincer Pd-

Cl)porphyrin, the porphyrin core did not show significant

distortions from planarity in comparison with other meso-

tetraarylporphyrins.³³

From previous research it is known that metalation of the

NCN-pincer ligand fragment leads to release of electron

density from the metal to the pincer aryl ring.^16,95As to the

extent of electron-donation of the metal to the meso-aryl

rings of the pincer-porphyrin hybrids, or electronic cou-

pling between them, Parshall found that MX groups (M=

Pd, Pt; X = halide) connected to a σ-phenyl group are

electron donating to the aromatic ring and increase the

phenyl-HOMO in energy.^96,97These effects are significantly

more pronounced for platinum than for palladium. When

these findings are compared to the results presented here,

this also accounts for the stronger bathochromic shifts of

the Soret bands induced by peripheral platination in com-

parison with those of peripheral palladation. These interac-

tions likely occur by virtue of interaction between the

π-orbitals on the meso-phenyl rings and the π-orbitals on

the porphyrin core.^76,98Although single-crystal X-ray

analysis has shown that the angles between the meso-

phenyl rings and the porphyrin range from 58° to 84° (see

Figure 4 {[Zn(PdBr(NCN))₄](pyridine)}), it should be noted

that in solution rotation around the carbon-carbon bond

connecting the meso-phenyls to the porphyrin is still feasi-

ble.⁹⁹Hence, this will allow for the occurrence of intra-

molecular interactions that are mediated through the

π-system.

In another part of our research, we have synthesized and

investigated the closely related 5-(3,5-bis[(dimethylamino)-

methyl]-4-bromophenyl)-10,15,20-tris(p-tolyl)porphyrin

and its heterobimetallic complexes.¹⁰¹For these complexes,

the effects of peripheral palladation, platination, and chemi-

stry on the peripheral NCN-pincer PtCl sites are reduced

4-fold when compared with those of the tetrapincer-

porphyrin systems discussed here. These findings demon-

strate the cumulativity of the effects exerted by the peri-

pheral metalated pincer groups. Arnold and co-workers

noted a similar, additive effect for the incorporation of Pt

atoms at one or two meso-positions of their meso-η¹-metal-

loporphyrins.²⁷

Conclusions

A novel meso-tetrakis(NCN-pincer ligand)porphyrin hy-

brid, [2H(Br(NCN))₄], was synthesized via a 13-step proce-

dure in a total yield of 5.3%. By design, orthogonal

metalation procedures could be employed to metalate either

the pincer or porphyrin moiety. M(II) salts were employed

for metal insertion into the tetrapyrrolic macrocyle whereas

the M(0) complexes [Pd₂dba₃] CHCl₃and [Pt₂dipdba₃] were

3

used for metalation of the peripheral NCN-pincer ligand

€

(95) Kocher, S.; van Klink, G. P. M.; van Koten, G.; Lang, H.

J. Organomet. Chem. 2006, 691, 3319–3324.

(96) Parshall, G. W. J. Am. Chem. Soc. 1966, 88, 704.

(97) Parshall, G. W. J. Am. Chem. Soc. 1974, 96 (8), 2360–2366.

(98) Ghosh, A. J. Mol. Struct. 1996, 388, 359–363.

(99) Anderson, H. L. Chem. Commun. 1999, 2323–2330.

(100) Kashiwagi, Y.; Imahori, H.; Araki, Y.; Ito, O.; Yamada, K.;

Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2003, 107, 5515–5522.

(101) Suijkerbuijk, B. M. J. M.; Schamhart, D. J.; Kooijman, H.; Spek,

A. L.; van Koten, G.; Klein Gebbink, R. J. M. Submitted for publication.

J. Org. Chem. Vol. 75, No. 5, 2010 1545

Suijkerbuijk et al.

JOCArticle

units, which gave the desired heteromultimetallic complexes

[M¹(M²X(NCN))₄] in excellent yields. ¹⁹⁵Pt NMR spectro-

scopy convincingly showed the influence of the metal in-

corporated in the porphyrin core on the electron density at

the Pt centers in the peripheral NCN-pincer complexes.

UV/vis spectroscopic measurements furthermore esta-

blished that the metallopincers influence the metallopor-

phyrin moiety.

Since we have found direct evidence for the possibility

of fine-tuning the electron density of peripheral platinum

centers by changing the para-metalloporphyrin substituent,

we envisage that this effect is also exerted on the peripheral

Pd centers. The use of this insight in the design of (ECE-

pincer Pd)-porphyrin hybrids as tunable catalysts in

C-C bond forming reactions is the subject of further

investigations.^101,102

(100%, [C₈H₆⁷⁹Br³⁵Cl³⁷ClI]^þþ [C₈H₆⁸¹Br³⁵Cl₂I]^þ), 382 (40%,

[C₈H₆⁸¹Br³⁵Cl³⁷ClI]^þ). Further characterization of this com-

pound can be found in ref 35.

3,5-Bis(methoxymethyl)-4-bromoiodobenzene (3). A solution

of sodium methoxide in methanol was prepared from sodium

(3.39 g, 147 mmol) and MeOH (250 mL). Compound 2 (21.90 g,

57.66 mmol) was added to this clear solution at once and the

resulting white suspension was stirred and heated. Just before

the suspension reached reflux temperature, all solids had dis-

solved and after 5 min of continued stirring at reflux tempera-

ture a white microcrystalline solid began to precipitate (NaCl).

After 4 h, the mixture was allowed to reach room temperature

and all volatiles were subsequently evaporated. The remaining

white residue was partitioned between Et₂O (200 mL) and H₂O

(100 mL), the organic layer was isolated, and the aqueous layer

was extracted with Et₂O (2 ꢀ 100 mL). The combined organic

layers were dried (MgSO₄) and filtered, and the solvents were

evaporated. The remaining white solid was recrystallized from

hot methanol (7 mL/g) to yield white crystals, which were

collected, washed with cold methanol, and dried in vacuo. Yield

19.20 g (90%). ¹H NMR (CDCl₃) δ 7.72 (s, 2H, ArH), 4.47 (s,

4H, CH₂O), 3.48 (s, 6H, OCH₃) ppm; ¹³C{¹H} NMR (CDCl₃)

δ 140.1, 136.4, 122.1, 93.1, 73.5, 59.0 ppm; GC-MS m/z

370 (100%, [C₁₀H₁₂⁷⁹BrIO₂]^þ), 372 (96%, [C₁₀H₁₂⁸¹BrIO₂]^þ).

Anal. Calcd for C₁₀H₁₂BrIO₂: C 32.37, H 3.26. Found: C 32.46,

H 3.23.

3,5-Bis(methoxymethyl)-4-bromobenzaldehyde (4). A white,

turbid mixture of 3 (5.07 g, 13.7 mmol) in dry THF (200 mL)

was treated dropwise with t-BuLi (18.2 mL, 1.5 M in pentane,

27.3 mmol) at -100 °C during 5 min. After 5 min of additional

stirring at -100 °C, the resulting bright yellow solution was

treated dropwise with dry DMF (2.8 mL, 36 mmol). After 5 min

the light yellow suspension was allowed to reach room tempera-

ture. H₂O (10 mL) was subsequently added, which was followed

by evaporation of all volatiles. Et₂O (150 mL) and H₂O (50 mL)

were added, the mixture was vigorously stirred for 30 min, and

the layers were separated. The water layer was extracted with

Et₂O (2 ꢀ 50 mL) and the combined organic layers were washed

with brine (300 mL), dried (MgSO₄), filtered, and concentrated

to leave a yellow solid. Purification by column chromatography

(Et₂O/hexanes 2:1, v/v) yielded 4 as a colorless crystalline

material. Yield 3.26 g (86%). Further purification can be

achieved by recrystallization at room temperature from hot

hexanes (∼16 mL/g). ¹H NMR (CDCl₃) δ 10.02 (s, 1H, CHO),

7.91 (s, 2H, ArH), 4.58 (s, 4H, CH₂O), 3.51 (s, 6H, OCH₃) ppm;

¹³C{¹H} NMR (CDCl₃) δ 191.7, 139.6, 135.4, 128.9, 128.3, 73.8,

59.0 ppm; GC-MS m/z 272 (100%, [C₁₁H₁₃⁷⁹BrO₃]^þ), 274 (98%,

[C₁₁H₁₃⁸¹BrO₃]^þ). Anal. Calcd for C₁₁H₁₃BrO₃: C 48.37, H

4.80. Found: C 48.28, H 4.84.

Experimental Section

I. General. All air-sensitive reactions were performed under a

nitrogen atmosphere with standard Schlenk techniques. All

reactions involving porphyrin compounds were shielded from

ambient light with aluminum foil. Et₂O and THF were carefully

dried and distilled from sodium/benzophenone prior to use.

CH₂Cl₂and N,N-dimethylformamide (DMF) were distilled

from CaH₂. Pyrrole was distilled from CaH₂and stored at

-30 °C. 3,5-Bis(hydroxymethyl)-4-bromoiodobenzene (1),³⁴

[Zn(Br(NCN))₄],⁵¹[Pd₂dba₃] CHCl₃,¹⁰³and [Pt₂dipdba₃]⁴³

3

were synthesized according to literature procedures. Column

chromatography was performed with use of silica gel for column

chromatography, 0.060-0.200 mm, pore diameter ca. 6 nm. ¹H

and ¹³C{¹H} NMR spectra were recorded at 300 and 75 MHz,

respectively, at 298 K and were referenced to residual solvent

signal. The ¹³C NMR signals of the free-base porphyrin R- and

β-carbon atoms were not resolved due to extreme line-

broadening.¹⁰⁴MALDI-TOF measurements were carried out

with either 9-nitroanthracene (9NA) or 2,5-dihydroxybenzoic

acid (DHB) as the matrix.

3,5-Bis(chloromethyl)-4-bromoiodobenzene (2). A mixture

of 3,5-bis(hydroxymethyl)-4-bromoiodobenzene 1 (23.81 g,

69.42 mmol) and NEt₃(24.4 mL, 174 mmol) in dry CH₂Cl₂

(350 mL) was cooled to 0 °C and subsequently treated with

methanesulfonyl chloride (13.4 mL, 174 mmol) in a dropwise

manner. At the end of the addition, all solids had dissolved to

give a slightly yellow solution. This solution was allowed to

warm to room temperature during which time a white precipi-

tate formed. The suspension was then heated to reflux for 22 h,

and allowed to cool to room temperature. Subsequently, H₂O

(200 mL) was added to the resulting yellow solution, and stirring

was continued for another 15 min. The layers were separated

and the organic phase was washed with aqueous HCl (4 M, 200

mL) and brine (200 mL), dried (MgSO₄), and filtered. The

filtrate was concentrated to dryness to leave an off-white solid.

The solid was extracted with boiling hexane (6 mL/g) and the

extract was evaporated to dryness. The resulting white fluffy

solid was washed with cold hexanes (100 mL) and dried in

vacuo. Yield: 24.40 g (93%). Note: This compound is an extre-

mely potent lachrymator and should be used immediately after its

5,10,15,20-Tetrakis[3,5-bis(methoxymethyl)-4-bromophenyl]-

porphyrin (5). A refluxing, colorless solution of freshly prepared

4 (8.30 g, 30.4 mmol) in propionic acid (120 mL) was treated

with freshly distilled pyrrole (2.12 mL, 30.4 mmol) and refluxing

was continued for 40 min. The resulting black solution was

stored at 6 °C overnight (refrigerator) and the resulting purple

crystals were filtered off and washed with MeOH until the

washings remained colorless. Yield 1.96 g (20%). ¹H NMR

(CDCl₃) δ 8.83 (s, 8H, β-H), 8.25 (s, 8H, ArH), 4.85 (s, 16H,

CH₂O), 3.55 (s, 24H, OCH₃), -2.84 (s, 2H, NH) ppm; ¹³C{¹H}

NMR (CDCl₃) δ 141.4, 136.5, 133.9, 123.1, 119.2, 74.5, 59.0

ppm; UV/vis [λ_max(rel intensity), CH₂Cl₂] 420 (1.000), 516

(0.038), 552 (0.017), 592 (0.011), 647 (0.008) nm; ESI-HRMS

m/z 1279.1068 ([M þ H]^þ), calcd for C₆₀H₅₉Br₄N₄O₈1279.1066.

Anal. Calcd for C₆₀H₅₈Br₄N₄O₈: C 56.18, H 4.56, N 4.37.

Found: C 56.24, H 4.45, N 4.31.

1

isolation! H NMR (CDCl₃) δ 7.78 (s, 2H, ArH), 4.66 (s, 4H,

CH₂Cl) ppm; GC-MS m/z 378 (60%, [C₈H₆⁷⁹Br³⁵Cl₂I]^þ), 380

ꢁ

(102) Suijkerbuijk, B. M. J. M.; Herreras Martinez, S. D.; van Koten, G.;

Klein Gebbink, R. J. M. Organometallics 2008, 27, 534–542.

(103) Komiya, S. Synthesis of Organometallic Compounds; Wiley:

New York, 1997.

(104) Shaw, S. J.; Shanmugathasan, S.; Clarke, O. J.; Boyle, R. W.;

Osborne, A. G.; Edwards, C. E. J. Porphyrins Phthalocyanines 2001, 5, 575–

581.

5,10,15,20-Tetrakis[3,5-bis(bromomethyl)-4-bromophenyl]-

porphyrin (6). A red solution of 5 (0.98 g, 764 μmol) in CH₂Cl₂

(200 mL) was treated with HBr/HOAc (FRESH! 300 mL),

1546 J. Org. Chem. Vol. 75, No. 5, 2010

Suijkerbuijk et al.

JOCArticle

yielding a green solution, which was stirred at room temperature

for 5 h. The resulting green suspension was cooled to 0 °C with

use of an ice/water bath and ice water (500 mL) was added. After

5 min of vigorous stirring the layers were separated and the

water layer was extracted once with CH₂Cl₂(100 mL). The

combined organic layers were vigorously stirred with 6 M

K₂CO₃solution in H₂O (500 mL) until the whole mixture

attained a purple color and all solids had dissolved (due to the

low solubility of [6(H^þ)₂]^2þin H₂O and CH₂Cl₂this can take

approximately 2 h). The layers were again separated and the

organic layer was dried (MgSO₄), filtered, and concentrated to

40 mL. Further purification with column chromatography

(CH₂Cl₂/hexane 1:1, v/v), collection of the first purple band,

and evaporation of the solvent yielded crude 6 as a purple solid.

Yield 1050 mg (82%). The remaining chlorine contamination of

this product can be oxidized to the corresponding porphyrin:

the product was dissolved in CH₂Cl₂(50 mL), DDQ (0.96 g,

4.23 mmol) was added, and the orange mixture was stirred for

2 h at room temperature. The mixture was filtered through a

short pad of silica gel (CH₂Cl₂/hexane 4:1, v/v), which gave the

title porphyrin as a purple, crystalline solid. ¹H NMR (CDCl₃) δ

8.91 (s, 8H, β-H), 8.28 (s, 8H, ArH), 4.93 (s, 16H, CH₂Br), -2.90

(s, 2H, NH) ppm; ¹³C{¹H} NMR (CDCl₃) δ 141.8, 137.4, 136.9,

126.8, 118.1, 33.9 ppm; UV/vis [λ_max(rel intensity), CH₂Cl₂] 422

(1.000), 517 (0.046), 551 (0.021), 591 (0.016), 648 (0.013) nm;

MALDI-TOF MS (DHB) m/z 1674.44 ([M þ H]^þ), calcd for

C₅₂H₃₅Br₁₂N₄1674.72. Anal. Calcd for C₅₂H₃₄Br₁₂N₄: C 37.32,

H 2.05, N 3.35. Found: C 37.39, H 1.98, N 3.31.

C 56.57, H 5.59, N 11.64, Ni 4.07. Found: C 56.50, H 5.68, N

11.51, 4.15. Crystals suitable for X-ray crystallographic analysis

were obtained by slow evaporation of a saturated solution of

[Ni(Br(NCN))₄] in hexane at room temperature.

Standard procedure for 4-fold palladation: A solution of

[M¹(Br(NCN))₄] (M¹= 2H, Ni, Zn) in dry benzene (2 mM)

was treated with [Pd₂dba₃] CHCl₃(4.5-4.8 equiv) and the

3

solution was stirred for 16 h at room temperature under an N₂

atmosphere, yielding a turbid mixture. HNEt₂(1 mL) was then

added to decompose the unreacted [Pd₂dba₃] CHCl₃and after

3

3 h of subsequent stirring at room temperature, all volatiles were

removed in vacuo. The dark residue was dissolved in CH₂Cl₂

(5 mM) and the mixture was filtered over Celite into a centrifuge

vessel and the Celite was washed with fresh CH₂Cl₂until the

washings were colorless (the Celite retains Pd⁰). The volume of

the filtrate was reduced to ∼10 mL and hexane (50 mL) was

added, which resulted in (partial) precipitation of the product.

The suspension was then concentrated to ∼20 mL and the

now fully precipitated product was collected by centrifugation

(10 min at 2400 rpm).

5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-bromido-

palladio(II)phenyl)porphyrin ([2H(PdBr(NCN))₄]). Purple solid.

Yield 90%. ¹H NMR (CDCl₃) δ 8.86 (s, 8H, β-H), 7.60 (s, 8H,

ArH), 4.27 (s, 16H, CH₂N), 3.16 (s, 48H, N(CH₃)₂), -2.79 (s, 2H,

NH) ppm; ¹³C{¹H} NMR (CDCl₃) δ 157.4, 143.5, 138.8, 131.1,

126.5, 120.6, 75.1, 53.7 ppm; UV/vis [λ_max(rel intensity), CH₂Cl₂]

426 (1.000), 522 (0.034), 558 (0.028), 594 (0.011), 653 (0.015) nm;

MALDI-TOF MS (DHB) m/z 1811.04 ([M þ H]^þ), calcd for

C₆₈H₈₃Br₄N₁₂Pd₄1811.99. Anal. Calcd for C₆₈H₈₂Br₄N₁₂Pd₄: C

45.05, H 4.56, N 9.27. Found: C 45.12, H 4.63, N 9.12.

5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-bromo-

phenyl)porphyrin ([2H(Br(NCN))₄]). Porphyrin 6 (500 mg,

299 μmol) was dissolved in dry CH₂Cl₂(100 mL) under a

nitrogen atmosphere. The solution was treated with Me₂NH

(1.0 mL, excess), and the resulting clear solution was stirred for

90 min. Degassed H₂O (60 mL) was added while stirring and

after 30 min the two layers were separated. The organic layer

was again washed with degassed H₂O (100 mL), dried (MgSO₄),

filtered, and concentrated to 10 mL. Hexane (60 mL) was added

and the purple solution was concentrated to 20 mL, whereupon

crystallization of the product started to set in. The mixture was

stored overnight at -30 °C, the supernatant was decanted, and

the resulting purple, glistening crystals were dried in vacuo and

[5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-bromido-

palladio(II)phenyl)porphyrinato]nickel(II)

([Ni(PdBr(NCN))₄]).

Orange solid. Yield 87%. ¹H NMR (CDCl₃) δ 8.76 (s, 8H, β-H),

7.39 (s, 8H, ArH), 4.22 (s, 16H, CH₂N), 3.16 (s, 48H, N(CH₃)₂)

ppm; UV/vis [λ_max(rel intensity), CH₂Cl₂] 423 (1.000), 531 (0.088)

nm; MALDI-TOF MS (DHB) m/z 1789.73 ([M - Br]^þ), calcd for

C₆₈H₈₀Br₃N₁₂NiPd₄1788.98. Anal. Calcd for C₆₈H₈₀Br₄N₁₂-

NiPd₄: C 43.69, H 4.31, N 8.99, Ni 3.14. Found: C 43.84, H

4.38, N 8.78, Ni 3.30. The small quantity in which this com-

pound was available did not allow for a satisfactory ¹³C NMR

measurement.

1

stored under a nitrogen atmosphere. Yield 412 mg (99%). H

[5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-bromido-

palladio(II)phenyl)porphyrinato]zinc(II) ([Zn(PdBr(NCN))₄]).

NMR (CDCl₃) δ 8.84 (s, 8H, β-H), 8.19 (s, 8H, ArH), 3.85 (s,

16H, CH₂N), 2.43 (s, 48H, N(CH₃)₂), -2.81 (s, 2H, NH) ppm;

¹³C{¹H} NMR (CDCl₃) δ 140.7, 137.3, 135.5, 127.3, 119.4, 64.4,

46.0 ppm; UV/vis [λ_max(rel intensity), CH₂Cl₂] 421 (1.000), 518

(0.038), 552 (0.020), 592 (0.011), 650 (0.010) nm; ESI-HRMS m/z

692.1800 ([M þ 2H]^2þ), calcd for ¹/₂C₆₈H₈₄Br₄N₁₂692.1838.

1

Purple/greenish solid. Yield 90%. H NMR (CDCl₃) δ 8.97

(s, 8H, β-H), 7.61 (s, 8H, ArH), 4.27 (s, 16H, CH₂N), 3.17

(s, 48H, N(CH₃)₂) ppm; ¹³C{¹H} NMR (CDCl₃) δ 156.5,

150.4, 143.2, 139.4, 132.0, 126.3, 121.4, 75.0, 53.7 ppm; UV/

vis [λ_max(log ε), CH₂Cl₂] 429 (5.75), 554 (4.35), 596 (4.00) nm;

ESI-MS m/z 1797.20 ([M - Br]^þ), calcd for C₆₈H₈₀Br₃-

Anal. Calcd for C₆₈H₈₂Br₄N₁₂C₆H₁₄: C 60.33, H 6.57, N 11.41.

3

Found: C 60.39, H 6.35, N 11.29.

N₁₂Pd₄Zn 1796.25. Anal. Calcd for C₆₈H₈₀Br₄N₁₂Pd₄Zn: C

[5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-bromo-

phenyl)porphyrinato]nickel(II) ([Ni(Br(NCN))₄]). To a purple

solution of [2H(Br(NCN))₄] (33.0 mg, 23.8 μmol) in dry toluene

(10 mL) was added Ni(acac)₂(122 mg, 476 μmol) and the

resulting mixture was heated at reflux temperature for 3 h. After

the reaction had completed (monitored by UV/vis spectroscopy)

the volatiles were evaporated. The remaining red solid was

partitioned between CH₂Cl₂(10 mL) and water (10 mL) and

the organic layer was isolated, dried (MgSO₄), and filtered.

Hexane (20 mL) was added and the solution was concentrated

to 10 mL and stored at -30 °C for 4 days. The resulting red/

orange crystals were isolated by centrifugation. Yield 35 mg

(100%). ¹H NMR(CDCl₃) δ 8.72 (s, 8H, β-H), 7.99(s, 8H, ArH),

3.78 (s, 16H, CH₂N), 2.38 (s, 48H, N(CH₃)₂) ppm; ¹³C{¹H}

NMR (CDCl₃) δ 142.9, 139.6, 137.5, 134.7, 132.1, 127.2, 118.4,

64.3, 45.9 ppm; UV/vis [λ_max(rel intensity), CH₂Cl₂]: 417 (1.000),

528 (0.090) nm; LDI-TOF MS m/z 1438.91 ([M]^þ), calcd for

C₆₈H₈₀Br₄N₁₂Ni 1438.27. Anal. Calcd for C₆₈H₈₀Br₄N₁₂Ni:

43.53, H 4.30, N 8.96, Zn 3.49. Found: C 43.65, H 4.42, N

9.06, Zn 3.43. Crystals suitable for X-ray crystallographic

analysis were obtained by slow evaporation of a solution of

[Zn(PdBr(NCN))₄] (4 mg) in a mixture of CH₂Cl₂and hexanes

(2:1, v/v, 1.5 mL) and a drop of pyridine.

Standard procedure for 4-fold platination: A solution of

[M¹(Br(NCN))₄] (M¹= 2H, Ni, Zn) in dry benzene (1 mM)

was treated with [Pt₂dipdba₃] (5 equiv) and the solution was

stirred for 16 h at reflux temperature under an N₂atmosphere,

yielding a turbid mixture. HNEt₂(1 mL) was then added to

decompose the unreacted [Pt₂dipdba₃] and after 3 h of sub-

sequent stirring at room temperature, all volatiles were removed

in vacuo. The black residue was stirred in CHCl₃(2 mM) with a

saturated solution of NH₄Cl in H₂O for 48 h and the layers were

separated. The organic layer was dried (MgSO₄) and filtered

over Celite into a centrifuge vessel, and the Celite was washed

with fresh CH₂Cl₂until the washings were colorless (the Celite

retains Pt⁰). The volume of the filtrate was reduced to ∼10 mL

J. Org. Chem. Vol. 75, No. 5, 2010 1547

Suijkerbuijk et al.

JOCArticle

and hexane (50 mL) was added, which resulted in (partial)

precipitation of the product. The suspension was then con-

centrated in vacuo to ∼20 mL and the now fully preci-

pitated product was collected by centrifugation (10 min at

2400 rpm).

in CH₂Cl₂. The absorbance of the resulting complex was

monitored as a function of the concentration of pyridine. When

{1/Δabsorbance} is then plotted against {1/[pyridine]}, the

intercept with the X-axis is equal to -[binding constant].^105,106

X-ray Crystal Structure Determinations. Reflections were

measured on a Nonius Kappa CCD diffractometer with rota-

ting anode (graphite monochromator, λ = 0.71073 A) at a

temperature of 150 K. Intensities were integrated with Eval-

CCD,¹⁰⁷using an accurate description of the experimental setup

for the prediction of the reflection contours. The structures were

refined with SHELXL-97¹⁰⁸against F²of all reflections. Non-

hydrogen atoms were refined with anisotropic displacement

parameters. Hydrogen atoms were introduced in calculated

positions and refined with a riding model. Geometry calcula-

tions and checking for higher symmetry was performed with the

PLATON program.¹⁰⁹

5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-chlorido-

platino(II)phenyl)porphyrin ([2H(PtCl(NCN))₄]). Purple micro-

1

crystalline solid. Yield 67%. H NMR (CD₂Cl₂) δ 8.95 (s, 8H,

β-H), 7.63 (s, 8H, ArH), 4.31 (s, 16H, CH₂N), 3.27 (s, 48H,

N(CH₃)₂), -2.77 (s, 2H, NH) ppm; ¹⁹⁵Pt NMR (CDCl₃) δ -3162

ppm; UV/vis [λ_max(relintensity), CH₂Cl₂] 433 (1.000), 522 (0.058),

564 (0.056), 657 (0.026) nm; MALDI-TOF MS (DHB) m/z

1989.80 ([M þ H]^þ), calcd for C₆₈H₈₃Cl₄N₁₂Pt₄1989.41; m/z

1953.55 ([M - Cl]^þ), calcd for C₆₈H₈₂Cl₃N₁₂Pt₄1953.44. Anal.

Calcd for C₆₈H₈₂Cl₄N₁₂Pt₄CHCl₃: C 41.03, H 4.45, N 7.66.

3

Found: C 40.54, H 4.10, N 7.56. A satisfactory ¹³C{¹H} spectrum

could not be recorded since the compound quickly crystallized

from the solution.

Compound [Ni(Br(NCN))₄]: C₆₈H₈₀Br₄N₁₂Ni þ disordered

solvent, Fw = 1443.79, red-orange block, 0.35 ꢀ 0.06 ꢀ 0.02

mm³, monoclinic, C2/c (no. 15), a = 35.4380(10) A, b =

9.8494(10) A, c = 29.2457(10) A, β = 126.113(1)°, V =

8246.6(9) A³, Z = 4, D_x= 1.163 g/cm³(derived parameters do

not contain the contribution of the disordered solvent), μ =

2.21 mm^-1(derived parameters do not contain the contribution

of the disordered solvent). A total of 77763 reflections were

measured up to a resolution of (sin θ/λ)_max= 0.54 A^-1. The

reflections were corrected for absorption and scaled on the basis

of multiple measured reflections with the program SADABS¹¹⁰

(0.61-0.91 correction range). A total of 5378 reflections were

unique (R_int=0.1217). The structure was solved with the program

SHELXS-86,¹¹¹using Direct Methods. The crystal structure con-

tains voids (2260 A³/unit cell) filled with disordered solvent

molecules. Their contribution to the structure factors was secured

by back-Fourier transformation, using the routine SQUEEZE of

the program PLATON,¹⁰⁹resulting in 551 electrons/unit cell. A

total of 393 parameters were refined with 72 restraints. R1/wR2

[I > 2σ(I)]: 0.0647/0.1303. R1/wR2 [all reflections]: 0.1117/0.1463.

S=1.070. Residual electron density between -0.68 and 0.98 e/A³.

[5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-chlo-

ridoplatino(II)phenyl)porphyrinato]nickel(II) ([Ni(PtCl(NCN))₄]).

Orange solid. Yield 94%. ¹H NMR (CDCl₃) δ 8.80 (s, 8H, β-H),

7.40 (s, 8H, ArH), 4.21 (s, 16H, CH₂N), 3.23 (s, 48H, N(CH₃)₂)

ppm; ¹³C{¹H} NMR (CDCl₃) δ 145.0, 142.8, 142.0, 136.1, 132.1,

125.2, 120.1, 78.0, 54.8 ppm; ¹⁹⁵Pt NMR (CDCl₃) δ -3170 ppm;

UV/vis [λ_max(rel intensity), CH₂Cl₂] 431 (1.000), 534 (0.089) nm;

MALDI-TOF MS (DHB) m/z 2011.79 ([M - Cl]^þ), calcd for

C₆₈H₈₀Cl₃N₁₂NiPt₄2012.36. Anal. Calcd for C₆₈H₈₀Cl₄N₁₂Ni-

Pt₄: C 39.91, H 3.94, N 8.21, Ni 2.87. Found: C 40.11, H 4.08,

N 8.07, Ni 2.75.

[5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-chlo-

ridoplatino(II)phenyl)porphyrinato]zinc(II) ([Zn(PtCl(NCN))₄]).

Purple/green microcrystalline solid. Yield 87%. ¹H NMR

(CDCl₃) δ 9.01 (s, 8H, β-H), 7.63 (s, 8H, ArH), 4.29 (s, 16H,

CH₂N), 3.29 (s, 48H, N(CH₃)₂) ppm; ¹³C{¹H} NMR (CDCl₃) δ

150.3, 144.7, 141.7, 138.1, 132.0, 126.0, 122.1, 78.1, 54.9 ppm;

¹⁹⁵Pt NMR (CDCl₃) δ -3176 ppm; UV/vis [λ_max(rel inten-

sity), CH₂Cl₂] 435 (1.000), 555 (0.056), 599 (0.036) nm;

MALDI-TOF MS (9NA) m/z 2017.70 ([M - Cl]^þ), calcd

Compound {[Zn(PdBr(NCN))₄](pyridine)}: C₇₃H₈₅Br_1.84

-

for C₆₈H₈₀Cl₃N₁₂ZnPt₄2017.36.

A completely satisfac-

Cl_2.16N₁₃Pd₄Zn 3C₇H₈2CHCl₃þ disordered CHCl₃, Fw =

3

tory elemental analysis could not be obtained. Unfortunately,

this compound was too immobile on either silica gel or

alumina to allow for further purification by chromatographic

means.

[5,10,15,20-Tetrakis(3,5-bis[(dimethylamino)methyl]-4-trichlorido-

platino(IV)phenyl)porphyrinato]nickel(II) ([Ni(Pt(IV)Cl₃(NCN))₄]).

A deep orange solution of ([Ni(PtCl(NCN))₄]) (14 mg, 6.8 μmol) in

2374.25 (derived parameters do not contain the contribution

of the disordered solvent), purple block, 0.39 ꢀ 0.24 ꢀ 0.18 mm³,

triclinic, P1(no. 2), a=15.126(3) A, b=19.255(4) A, c=20.943(5) A,

R=111.226(3)°, β=95.283(3)°, γ=101.042(3)°, V=5493(2) A³,

Z=2, D_x=1.435 g/cm³(derived parameters do not contain the

contribution of the disordered solvent), μ=1.77 mm^-1(derived

parameters do not contain the contribution of the disordered

solvent). A total of 125122 reflections were measured up to a

resolution of (sin θ/λ)_max= 0.65 A^-1. The reflections were

corrected for absorption and scaled on the basis of multiple

measured reflections with the program SADABS¹¹⁰(0.61-0.73

correction range). A total of 25199 reflections were unique

(R_int= 0.0329). The structure was solved with the program

DIRDIF99,¹¹²using automated Patterson Methods. The crys-

tal structure contains voids (824 A³/unit cell) filled with

dry CH₂Cl₂(5 mL) was treated with a solution of CuCl₂2H₂O

3

(9.3 mg, 54.7 μmol) in EtOH (2 mL), which led to the immediate

precipitation of a dark orange solid. After 30 min of continued

stirring, the solid was filtered off, then washed with EtOH (5 mL),

H₂O (2 ꢀ 10 mL), Et₂O (2 ꢀ 10 mL), and finally CH₂Cl₂(2 ꢀ

10 mL) to give 15.9 mg of the title compound (quantitative yield).

The solubility of this compound, even in hot DMSO, is so low that it

precluded the recording of a satisfactory ¹H NMR spectrum. UV/vis

[λ_max(rel intensity), CH₂Cl₂] 419 (1.000), 530 (0.083) nm. Anal.

Calcd for C₆₈H₈₀Cl₁₂N₁₂NiPt₄: C 35.05, H 3.46, N 7.21, Ni 2.52.

Found: C 35.18, H 3.60, N 7.17, Ni 2.50.

(105) Benesi, H. A.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703–

2707.

(106) Schneider, H.-J.; Yatsimirsky, A. Principles and Methods in Supra-

molecular Chemistry; John Wiley & Sons: New York, 1999.

(107) Duisenberg, A. J. M.; Kroon-Batenburg, L. M. J.; Schreurs,

A. M. M. J. Appl. Crystallogr. 2003, 36, 220–229.

II. Complexation and Decomplexation of SO₂to

[M¹(PtCl(NCN))₄] (M¹=2H, Ni, Zn). Dry SO₂gas was bubbled

through a solution of the appropriate [M¹(PtCl(NCN))₄] in dry

CH₂Cl₂until no further changes in the absorption spectrum

were noted. Decomplexation was accomplished by allowing the

solution to stand open to air for at least 16 h, or alternatively by

bubbling dry air through the solution of the SO₂complex for at

least 30 min.

€

(108) Sheldrick, G. M. SHELXL-97; University of Gottingen:

€

Gottingen, Germany: 1997.

(109) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7–13.

€

(110) Sheldrick, G. M. SADABS; University of Gottingen: Gottingen,

€

Germany: 1999.

€

(111) Sheldrick, G. M. SHELXS-86; University of Gottingen: Gottingen,

Germany: 1986.

III. Determination of Binding Constants. Solutions were pre-

pared containing the zinc porphyrin of interest (∼1 μM) and

the appropriate amount of pyridine (200-1 000 000 equiv)

(112) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.;

Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. DIRDIF99;

University of Nijmegen: Nijmegen, The Netherlands, 1999.

1548 J. Org. Chem. Vol. 75, No. 5, 2010

Suijkerbuijk et al.

JOCArticle

disordered CHCl₃solvent molecules. Their contribution to the

structure factors was secured by back-Fourier transformation,

using the routine SQUEEZE of the program PLATON,¹⁰⁹

resulting in 190 electrons/unit cell. The halogenide position

was refined with a partial occupation by chlorine (54%) and by

bromine (46%) and the Pd-halogen distances were restrained,

respectively. A total of 1129 parameters were refined with 175

restraints. R1/wR2 [I > 2σ(I)]: 0.0408/0.1150. R1/wR2 [all

reflections]: 0.0619/0.1230. S=1.141. Residual electron density

between -1.42 and 1.55 e/A³.

Acknowledgment. The Dutch council for scientific research

(NWO) is gratefully acknowledged for financial support

(D.M.T., M.L., and A.L.S.) and for a “Jonge Chemici”

scholarship (B.M.J.M.S. and R.J.M.K.G.).

Supporting Information Available: ¹H NMR and mass

spectra of all reported compounds, ¹³C{¹H} spectra of selected

compounds, and crystallographic information files (CIF) for

[Ni(Br(NCN))₄] and {[Zn(PdBr(NCN))₄](pyridine)}. This material

is available free of charge via the Internet at http://pubs.acs.org.

J. Org. Chem. Vol. 75, No. 5, 2010 1549

Article Doi

Synthesis of and evidence for electronic communication within heteromultimetallic Tetrakis(NCN-pincer metal) - (Metallo)porphyrin Hybrids

DOI: 10.1021/jo902402g

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Article abstract of DOI:10.1021/jo902402g

Full text of DOI:10.1021/jo902402g

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