Single hydroxo-bridged group 13 metalloporphyrin dimers: Solution studies and solid-state structures

Journal of Porphyrins and Phthalocyanines

J. Porphyrins Phthalocyanines 2019; 23: 969–989

DOI: 10.1142/S1088424619500834

Published at http://www.worldscinet.com/jpp/

Single hydroxo-bridged group 13 metalloporphyrin dimers:

Solution studies and solid-state structures

a,c

b,c

c

Orde Q. Munro* , Athanassios G. Coutsolelos* , Beisong Cheng

c

and W. Robert Scheidt*

a

School of Chemistry, University of the Witwatersrand, Johannesburg, PO WITS 2050, South Africa

Department of Chemistry, University of Crete, Voutes Campus, 70013 Heraklion-Crete, Greece

Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, USA

b

c

Received 5 June 2019

Accepted 15 July 2019

ABSTRACT: The syntheses of indium, gallium and aluminum porphyrin dimers with a single hydroxo-

+

bridge, {[M(Porph)] (OH)} , are described. Emphasis is given to indium and gallium derivatives. The

2

X-ray structures for {[Ga(OEP)] (OH)}ClO and {[In(OEP)] (OH)}ClO (two forms) are presented.

2

4

2

4

The dimeric molecules can be synthesized by the acid-treatment of the corresponding hydroxo-ligated

monomeric complexes [M(OEP)(OH)] and [M(TPP)(OH)]. The nature of the starting material (the

hydroxo-ligated monomer) was ﬁrst suggested by IR spectroscopy and further proved by proton-

1

deuterium exchange followed by H NMR spectroscopy. The structure of a monomeric indium hydroxide

complex, [In(OEP)(OH)], is also presented. The synthesis of the dimer for all metals can be monitored

by UV-vis spectroscopy, which clearly demonstrates that a blue-shift of the Soret band accompanies

formation of the dimer from the monomer. A strong p–p interaction between the two porphyrin rings

1

of these m-hydroxo-bridged dimers is conﬁrmed both by solution state studies ( H NMR and UV-vis

spectroscopy) and the X-ray structures of {[M(OEP)] (OH)}ClO (M = In, Ga). In addition, exposure

2

4

of methylene chloride solutions of these bridged complexes to white light afforded the corresponding

chloro derivatives, [M(Porph)Cl]. The stereochemistry of a range of m-hydroxo dimers is discussed and

DFT simulations at the HSEH1PBE/SDD level of theory provide suitable structural models and further

+

electronic structure insights on selected [Ga(Porph)(OH)] and {[Ga(Porph)] (OH)} derivatives.

2

KEYWORDS: group 13 metalloporphyrin, hydroxo dimer, condensation, X-ray crystallography,

DFT.

the syntheses and structural characterization of another class

of metalloporphyrin dimers in which the two porphyrins

INTRODUCTION

A number of strategies have been described for the forma-

tion of metalloporphyrin dimers linked by a bridging group.

These include (1) complexes of the type M(Porph)(Porp

are linked by a single m-hydroxo bridge (Fig. 1), including

{

[Fe(OEP)] (OH)}ClO [9, 10], {[Mn(OEP)] (OH)}ClO

2 4 2 4

h′),

[

11], and {[Mn(TPP)] (OH)}ClO [12].

2 4

where Porph or Porph′ and M is a olyvalent metal

=

≠

p

All of these M(III) m-hydroxo dimers have been

ion from the lanthanide or actinide series that serves as the

bridging group between the macrocycles [1]–[6], and (2)

dimers with the molecular formula (Porph)M−M′(Porph′),

obtained by the stoichiometric proton-dependent con-

densation of their corresponding monomeric hydroxo

complexes (Eq. 1),

where Porph = or

≠

Porph′

, M = or ≠ M′, and a metal–metal

b

ond links the monomers [7, 8]. We ha ve recently reported

1

+

[

M(Porph)(OH)] + [H O]

3

2

*

Correspondence to: Orde Q. Munro, email, Orde.Munro@

(1)

1

wits.ac.za; Athanassios G. Coutsolelos, email, acoutsol@uoc.

gr; W. Robert Scheidt, email; Scheidt@nd.edu.

+









{[M(Porph)] (OH)} + H O

2

970

O. Q. MUNRO ET AL.

the low-temperature X-ray structure of the gallium(III)

and indium(III) derivatives {[M(OEP)] (OH)}ClO .

2

4

The preparation and crystal structure of two additional

Group 13 hydroxo-bridged species have been reported in

the literature [15], however, the gallium derivative dif-

fers from all other species described herein, with one of

the two gallium centers coordinated to a water molecule

to form a six-coordinate species. DFT simulations at the

HSEH1PBE/SDD level of theory both in vacuo and in a

chloroform solvent continuum were used to model the

+

structures of selected {[Ga(Porph)] (OH)} derivatives,

2

their electronic spectra, and their NMR shielding param-

eters. Selected DFT-calculated properties are reported

alongside the experimental data where relevant to faci-

litate insightful comparisons. A deeper understanding

of the relationship between molecular and electronic

structure and the spectroscopic behavior of this class of

compounds has consequently been achieved.

Fig. 1. Metalloporphyrin structures and a schematic illustration

of the geometries of the 5-coordinate hydroxo monomers and

m-hydroxo dimers described in this work

We have found that {[Fe(OEP)] (OH)}ClO₄may

2

also be prepared by protonation of the m-oxo species,

[

Fe(OEP)] O [9, 13]. Rath and co-workers have extended

RESULTS

2

the iron studies with a numberof different porphyrins and

counterions [14].

NMR Spectroscopy

As a continuation of our study of this novel class of

dimeric metalloporphyrin derivatives, we have syn-

thesized main group trivalent m-hydroxo dimers to test

the generality of the synthetic method and to compare

the chemical behavior of these systems with that of

their transition metal counterparts. We have also stud-

ied the solution structures of {[Ga(OEP)] (OH)}ClO

1

The temperature dependence of the 1D H and

1

3

1

C( H) spectra for the hydroxide-bridged complexes

{[Ga(OEP)] (OH)}ClO and {[Ga(TPP)] (OH)}ClO

4

2

4

2

revealed that both are stable over the range of -40 to

+90°C. The hydroxide bridge protons of these two

gallium porphyrin derivatives resonate in the high ﬁeld

region between -12.5 to -15.0 ppm. The presence of the

OH bridge was further supported by the fact that after

2

4

1

13

and {[Ga(TPP)] (OH)}ClO using H NMR, C NMR,

2

4

7

1

and Ga NMR spectroscopy. The dynamic structures of

these diamagnetic dimers are compared with their X-ray

structures. Solid state structural data are provided by

the addition of a drop of D

ﬁeld signal corresponding to one proton disappeared due

O to the solutions, the high

2

Fig. 2. Illustration of the different molecular regions, protons, and carbon atoms relevant to interpreting the NMR spectra of

+

{

[Ga(Porph)] (OH)} derivatives. Hydrogen atoms appended tocarbon have been omitted from the DFT-calculated structures where

2

calc

necessary for clarity. DFT-calculated chemical shifts (CHCl solvent continuum, d ), are in ppm. Selected bond angles and bond

distances for the solution phase structures are indicated

3

H

J. Porphyrins Phthalocyanines 2019; 23: 970–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

971

1

Table 1. H NMR chemicals shifts, d (ppm), for hydroxo-bridged Ga(III) porphyrin dimers and their monomeric precursors

H

measured in CDCl solution at ambient temperature. DFT-calculated chemical shifts for the dimers are given in Fig. 2

3

Complex

beta meso

ortho

endo

meta

endo

para

CH₂

CH₃

OH

exo

{

[Ga(OEP)] (OH)}ClO₄

9.36

10.30

4.024/4.16 1.70 -13.87

2

[

Ga(OEP)(OH)]

[Ga(TPP)] (OH)}ClO

4.17

1.97

-7.18

a

{

8.64

8.05

ortho

8.11/8.39

7.64

7.82

meta, para

8.12

7.64 7.85

-14.51

2

4

[

Ga(TPP)(OH)]

9.06

-6.63

a

Recorded at -20.0°C.

7

1

to the rapid exchange between hydrogen and deuterium

Supporting Information, Fig. S1). The OH protons of the

monomeric precursors [Ga(OEP)(OH)] and [Ga(TPP)

OH)] resonate at about 7 ppm to lower ﬁeld. The

1D Ga NMR spectra (ambient temperature) were mea-

sured for the m-hydroxo-bridged gallium(III) porphyrin

dimers and their ﬁve-coordinate hydroxo precursors. Only

the precursor complexes, [Ga(OEP)(OH)] and [Ga(TPP)

(OH)], afforded observable resonance signals in the

spectrum at 72.8 and 77.1 ppm, respectively.

(

expected chemical and magnetic environments (regions)

for the protons of the m-OH species are shown using the

DFT-calculated geometries (CHCl solvent continuum)

3

of the complexes in Fig. 2.

Mass spectrometry

The solution structure and dynamics of the hydroxo-

bridged gallium OEP and TPP derivatives were studied

in detail by NMR methods over the temperature range

The molecular formula, {[Ga(Porph)] (OH)}ClO ,

2

4

for this class of hydroxo-bridged metalloporphyrins

was conﬁrmed by mass spectrometry. The molecular

ion peaks observed for both dinuclear complexes cor-

-

20 to +20°C. It was found that all phenyl groups in the

TPP derivative and all ethyl groups in the OEP deriva-

tive are magnetically equivalent. However, all individual

protons and carbons of a phenyl group give rise to the

separate resonances, i.e. they are magnetically nonequiv-

alent. Furthermore, the methylene protons of the ethyl

groups of {[Ga(OEP)] (OH)}ClO were found to be

+

·

responds to the entity [Ga(Porph)] (OH) , with relative

2

intensities of 22% and 1% for the OEP and TPP deriva-

tives, respectively. The fragmentation patterns of these

dimeric gallium(III) derivatives are intrinsically similar

to the patterns observed for the porphyrin free bases. The

expected isotopic composition (comprising the isotopes

2

4

anisochronous. The pyrrolic and meso protons exhibited

single resonance lines for the TPP and OEP derivatives,

respectively. The proton resonances for both hydroxo-

bridged gallium porphyrins and their corresponding

precursors were unambiguously assigned to individual

hydrogen atoms based on the interpretation of the DQF-

COSY and NOESY spectra (Table 1). The signals of all

carbons with directly attached protons were assigned

by analyzing HETCOR spectra. Assignments of signals

derived from quaternary carbon nuclei are based on their

6

9

71

Ga and Ga) of the bimetallic compound was con-

ﬁrmed in each case.

Electrochemistry

Electrochemical studies of {[Ga(OEP)] (OH)}ClO

2

4

and {[Ga(TPP)] (OH)}ClO were carried out under

2

4

argon in CH Cl containing 0.1 M TBAP as the support-

2

ing electrolyte. It is to be noted that dissolution of the

m-hydroxo dimers in the electrochemical solvent mixture

(CH Cl /0.1 M TBAP) favored partial dissociation of the

1

3

chemical shifts. The C chemical shifts are summarized

in Table S1.

2

dimer. (This observation was conﬁrmed by UV-vis spec-

troscopy). The dissociation equilibrium was observed

for both the dinuclear OEP and TPP derivatives. How-

ever, the extent of dissociation in pure organic solvents

was negligible. Otherwise, the complexes are stable on

the cyclic voltammetry time-scale and no loss of the

m-hydroxo bridge was detected during oxidation cycles.

Table 2 summarizes the electrochemical data for

the gallium(III) complexes investigated. The E₁_/₂for

the ﬁrst reduction in CH Cl measured -1.43 V for

1

3

C spin-lattice relaxation times, T , and NOE enhan-

1

cements were measured for carbons with directly atta-

ched protons at a magnetic ﬁeld strength B of 11.74 T

0

1

3

(

125.69 MHz, C) for {[Ga(OEP)] (OH)}ClO and

2 4

{

[Ga(TPP)] (OH)}ClO over the temperature range of

2 4

-

20 to +20°C. Both the relaxation times, T , (Table S2)

1

and NOE parameters (Table S3) of all measured car-

bons increased with temperature. The values of the NOE

parameters for {[Ga(TPP)] (OH)}ClO , however, did

2

4

2

not reach their theoretical maximum of 1.988 at temper-

{[Ga(TPP)] (OH)}ClO , while the OEP congener was

2 4

atures below 0°C.

reduced at -1.77 V. The second reduction in CH Cl

2 2

J. Porphyrins Phthalocyanines 2019; 23: 971–989

972

O. Q. MUNRO ET AL.

Table 2. Half-wave peak potentials (V vs. SCE) of m-OH

dimers and their monomeric precursors in CH Cl (0.1 M

2

TBAP) at ambient temperature

Complex Oxidation

nd 1st

Reduction

1st 2nd

2

{

[Ga(TPP)] (OH)}ClO₄

1.34 1.08 -1.04 -1.43

1.42 1.18 -1.16 -1.54

2

a

[

Ga(TPP)](OH)]

{

[Ga(OEP)] (OH)}ClO₄1.31 1.04 -1.32 -1.77

2

a

[

Ga(OEP)(OH)]

1.44 1.01 -1.37 -1.88

a

Similar data are available in the literature for these com-

plexes [16].

ranges from -1.04 V for {[Ga(TPP)] (OH)}ClO to

2

4

-

1.32 V for {[Ga(OEP)] (OH)}ClO . Two reversible

2 4

oxidations were observed for both {[Ga(OEP)] (OH)}

2

ClO and {[Ga(TPP)] (OH)}ClO (refer to Table 2).

4

2

4

X-ray crystallography

The crystal structures of ﬁve-coordinate Group 13

metalloporphyrins with hydroxide as the axial ligand have

been determined and are reported here. The structures

are those of the monomeric hydroxide [In(OEP)(OH)]

(Supporting Information, Fig. S6) and the dinuclear salts

{

[Ga(OEP)] (OH)}ClO and {[In(OEP)] (OH)}ClO ,

2

4

2

4

which are the primary focus of our current structural

studies. Two different crystalline forms (monoclinic

and triclinic) of the indium species have been char-

acterized. The structures of the dinuclear cations

Fig. 3. Edge-on thermal ellipsoid views of the cations of

{[Ga(OEP)] (OH)}ClO and {[In(OEP)] (OH)}ClO (mono-

clinic form). Atoms are contoured at the 50% probability level.

Hydrogen atoms have been omitted for clarity, except for the

hydrogen atom of the m-hydroxo bridging group in the gallium

derivative. The hydrogen atom in the indium derivative was not

+

2

4

2

4

{

[Ga(OEP)] (OH)} and {[In(OEP)] (OH)} (monoclinic

2

polymorph) are shown in Fig. 3. The triclinic polymorph

of {[In(OEP)] (OH)}ClO had two independent molecules

2

4

in the asymmetric unit; the structures of the unique cat-

ions are shown in Fig. 4. The salient structural feature in

each case is the bent geometry of the M–(OH)–M bridge,

which exceeds 140° and clearly cants the porphyrin rings

within the dimer toward one another such that the separa-

tion between the ring edges measures between 3.3–3.5 Å.

The ﬁve-coordinate geometry of the metal ion results in

a typical square pyramidal coordination group for each of

the µ-hydroxo dimers, with perpendicular displacements

…

located experimentally in the E map, but the O O (water) dis-

tance is consistent with the presence of a hydrogen bond between

the hydroxide ion H atom donor and a solvate water O atom

–

4

acceptor. For simplicity, the ClO counterions are not shown

DFT simulations

of the metal ion from the 24-atom

p

orphyrin mean plane

We have used DFT simulations at the HSEH1PBE/

SDD level of theory to provide some theoretical insight

on the experimental structures, electrochemical data, and

spectroscopic features (NMR and UV-vis spectroscopy)

of the Ga(III) m-hydroxo dimers investigated here. An

exhaustive treatise on all of the DFT-calculated parame-

ters for the compounds is beyond the scope of the present

article.

ranging from 0.32 Å (Ga) to 0.66 Å (In). The displace-

ment vector, in all cases, is toward the bridging hydroxyl

group and the porphyrins adopt domed conformations.

Primarily because of the larger ionic radius of In(III)

…

(0.94 Å) compared with Ga(III) (0.76 Å) [17], the M M

separation in the indium dimers (3.927–3.966 Å) exceeds

that for the gallium derivative (3.677 Å). The mean M–O

(

2.093 (7) Å) and M–N (2.134 (4) Å) bond distances for

Molecular structures. The structures of [Ga(Porph)

p

+

the indium derivatives structurally elucidated here are

consequently also longer than those of the gallium deriva-

tive (1.911 (0) Å and 2.019 (2) Å, respectively).

(OH)] and {[Ga(Porph)]

OEP, as well as [Ga(OEP)(OH

2

(OH)} , where Porph = TPP and

+

)

] , were calculated both

2

in vacuo and in a chloroform solvent continuum. Several

J. Porphyrins Phthalocyanines 2019; 23: 972–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

973

parallel arrangement for the porphyrin rings. The increase

in steric bulk of the porphyrin ring substituents under-

…

pins the increase in the Ga Ga distance from 3.77 Å

in the porphine derivative to 3.87 Å in the TPP derivative

(

Fig. S9). The hallmark coordination group parameters

+

of the DFT-calculated structure of {[Ga(OEP)] (OH)}

2

in a chloroform solvent continuum (Ga–O, Ga–O–Ga,

and Ga–N ) are within 2–13% of their experimental values

p

(

X-ray crystallography), offering a measure of conﬁ-

dence in the DFT method and basis set used in this study

taking into account the effect of the solvent continuum).

(

One noteworthy difference is the out-of-plane displace-

ment of the metal ion, ∆M24, which measures 0.439 (1)Å

+

for the solution phase structure of {[Ga(OEP)] (OH)}

2

and is signiﬁcantly larger than that observed crystallo-

graphically (0.32 Å). This accounts for the longer Ga–N_p

distance in the solution structure (2.047 (14)Å) compared

with the X-ray structure (2.019 (1) Å) and likely reﬂects

the lack of axial compression of the µ-hydroxo dimer in

the CHCl solvent continuum (i.e. the absence of crystal

3

packing constraints).

FMOs and electronic spectra. The four frontier MOs

+

(

FMOs) of {[Ga(OEP)] (OH)} are shown in Fig. 5. All

2

four FMOs evenly span both porphyrin rings in the dimer

despite the lack of any true symmetry elements in the

complex. The Ga(III) ions are positioned at nodes in

the MO wave functions and no components of the four

FMOs are located on the m-hydroxo bridge. The HOMO

and HOMO–1 are p-symmetry MOs that correspond to

two non-degenerate wave functions of similar charac-

ter that match the classical a₁_uMO of a D -symmetry

4h

metalloporphyrin [18–20]. The FMO energy gap ∆E_F_M_O

(E_L_U_M_O–E_H_O_M_O) is signiﬁcantly affected by the presence

of the solvent, which modulates the splitting relative to

the gas phase value, as evidenced by the ∆E data for

FMO

+

Fig. 4. Edge-on thermal ellipsoid views of the two independent

{

[Ga(OEP)] (OH)} in vacuo, in CHCl , and in CH Cl

2 3 2 2

cations of {[In(OEP)] (OH)}ClO (triclinic polymorph). Atoms

2

4

(2.394, 1.781, and 2.410 eV, respectively).

are contoured at the 50% probability level. Hydrogen atoms

have been omitted for clarity, except for the hydrogen atom of

^t^h^e^m^-^h^y^d^r^o^x^o^b^rⁱ^d^gⁱⁿ^g^g^r^o^u^p^s^.^F^o^r^sⁱ^m^p^lⁱ^cⁱ^t^y^,^t^h^e^C^l^O4 coun-

The porphyrin ring substituent groups and nature of

the species (monomeric ﬁve-coordinate hydroxo com-

plex vs. m-hydroxo dimer), however, have by far the larg-

est effect on the electronic spectra of the complexes. In

all cases, the Soret band shifts to a shorter wavelength for

the m-hydroxo dimer relative to the 5-coordinate hydroxo

monomer (see Fig. S7). From Table 3, the OEP deriva-

tive exhibits the largest blue shift (∆λ ~45 nm), with the

TPP derivative displaying a more modest shift of 14 nm.

Also noteworthy is the calculated energy order of the

substituent effect. Speciﬁcally λ_m_a_xfollows the order

OEP < porphine < TPP for this series of Ga(III) hydroxo

monomers and dimers, with the effect being especially

pronounced in the m-hydroxo dimers (the Soret band of

the OEP derivative is blue-shifted by 38 nm relative to

the TPP derivative). Signiﬁcantly, the DFT-calculated

spectra have allowed us to assign various experimen-

tally-observed solution phase species with conﬁdence,

facilitating conﬁrmation of the mechanistic pathway pro-

posed for the synthesis of the m-hydroxo dimers reported

–

terions are not shown

simulations were also effected in a dichloromethane

solvent continuum to facilitate direct comparison with

experimental electronic spectra. Representative solution

phase structures of the µ-hydroxo dimers are shown in

Fig. 2; key structural and electronic parameters for the

complexes are summarized in Table 3. The solution phase

+

structure of{[Ga(OEP)] (OH)} isbroadlyconsistentwith

2

the X-ray structure of the complex, displaying the same

characteristic 5-coordinate metal ion geometry, canted

porphyrin rings, domed macrocycle conformations, and

…

intramolecular ring ring contacts. The solution phase

conformations of the TPP and porphine µ-hydroxo dimers

are generally similar, though intramolecular steric repul-

sion effects in the TPP derivative evidently result in a near

J. Porphyrins Phthalocyanines 2019; 23: 973–989

974

O. Q. MUNRO ET AL.

J. Porphyrins Phthalocyanines 2019; 23: 974–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

975

dynamic signal averaging that occurs experimentally.

DFT-calculated proton chemical shift data for the gal-

lium m-hydroxo dimers are given in Fig. 2 along with the

structures of the complexes.

Of particular interest, the calculated chemical shifts

of the OH proton range from -5.47 ppm in [Ga(TPP)

+

(

OH)] to -12.88 ppm in {[Ga(P)] (OH)} , consistent

2

with the experimental trend that the bridged hydroxyl

proton in the m-hydroxo dimers experiences strongly

enhanced shielding relative to the axial hydroxo ligand

of the relevant ﬁve-coordinate monomer. The calculated

OH proton chemical shifts are, however, less shielded by

1

.1–1.6 ppm relative to the experimental data. (Although

the largest deviations occur for the OH protons of the

m-hydroxo dimers, other proton chemical shifts are gen-

erally more accurately calculated.)

13

Calculated C chemical shifts are tabulated alongside

the experimental chemical shifts for the m-hydroxo dimers

in Table S1. The pyrrole β-C (135.2–146.9 ppm) and all

substituent group chemical shifts are within 5 ppm of the

experimental chemical shifts. The calculated pyrrole α-C

chemical shifts (150.8–153.6 ppm) span a narrow range,

but are more deshielded (by up to 8 ppm) relative to the

experimental chemical shifts.

The fractional atomic charge distribution for the

atoms of the coordination group was calculated with the

NBO model (Table 3) [21] which gives results that are

independent of the basis set used for the simulations.

The mean charges on the hydroxyl group O atom

(

-1.19 ± ±0.01), H atom (+0.51 ± 0.02), and Ga(III) ion

(

1.93 ± ±0.03) are essentially independent of the struc-

ture of the complex (monomer vs. m-hydroxo dimer).

The porphyrin nitrogen atoms (N ) are similarly charged

p

for all of the complexes (-0.74

± 0.01), despite their dif-

fering peripheral substituent groups and the nature of the

species (monomer vs. dimer). The m-hydroxo dimers,

however, all have slightly more negatively charged

(~0.03 units) porphyrin nitrogen atoms relative to the

ﬁve-coordinate hydroxo monomers.

Fig. 5. Views of the frontier molecular orbitals (FMOs) for

+

{

[Ga(OEP)] (OH)} . The DFT- calculated structure represents

2

the geometry of the cation optimized in a chloroform solvent

continuum

DISCUSSION

Compound synthesis and electronic spectra

here (this is discussed later in relation to Fig. 6). The

relevant DFT-calculated electronic spectra of [Ga(OEP)

We have previously demonstrated the syntheses of

a number of single m-hydroxo bridged metalloporphy-

rin complexes including {[Fe(OEP)] (OH)}ClO [9],

+

(

OH)], [Ga(OEP)(OH ) ] , and {[Ga(OEP)] (OH)} have

2 2 2

been included alongside the experimental spectra in

Fig. 8 to further facilitate our discussion of the synthe-

sis of the present class of metalloporphyrin m-hydroxo

dimers.

NMR and electrostatic parameters. To compare

the experimental and theoretical NMR chemical shift

data, the calculated isotropic shieldings (GIAO method,

2

4

{[Mn(OEP)] (OH)}ClO [11], and {[Mn(TPP)] (OH)}

2

4

2

ClO [12], through the condensation of their corre-

4

sponding monomeric aqua complexes rather than the

protonation of their unprotonated m-oxo forms. As a

continuation of our studies on monomeric and dimeric

congeners within this broad class of compounds, we car-

ried out the analogous syntheses with trivalent Group

13 metalloporphyrins to test the generality of such a

synthetic method, as well as to compare their chemi-

cal behavior. Figure 6 summarizes our approach to the

CHCl solvent continuum) of all chemically equivalent

3

nuclei were averaged to remove magnetic inequiva-

lences associated with the static nature of the geometry-

optimized DFT structures. This procedure is akin to the

J. Porphyrins Phthalocyanines 2019; 23: 975–989

976

O. Q. MUNRO ET AL.

Fig. 6. Reaction scheme summarizing the interconversion pathways between ﬁve-coordinate Group 13 metalloporphyrin species

(

M = Al, Ga, In). Porphyrin substituents have been omitted for clarity. Species assignments are readily made from the electronic

spectra of the compounds

synthesis of these m-hydroxo-bridged metalloporphyrin

dimers, the postulated reaction intermediate, and the

behavior of the dimers toward white light.

compounds. The quite different physical characteris-

tics of the starting and product complexes facilitates

puriﬁcation of the desired dinuclear species by recrystal-

lizations from mixtures of dry CH Cl and hexanes. No

In the reaction mixture (acidiﬁed solution) the equi-

librium between the monomeric 5-coordinate hydroxo

species and the m-hydroxo dimer is strongly dependent

2

equilibrium between the dinuclear species and their cor-

responding monomers has been detected in the absence

of traces of water or electrolyte salt in the solutions. All

efforts to synthesize m-hydroxo-bridged dinuclear com-

plexes with two different porphyrin macrocycles, e.g.

on the amount of HClO present (Eq. 1). The precision of

4

the added equivalent of HClO in conjunction with step-

4

wise analysis of the reaction mixture by UV-vis spectro-

scopy affords the level of control required to manage the

equilibrium distribution of species. The approach allows

one to carefully optimize the amount of the m-hydroxo

dimer generated, as evidenced by the electronic spectra

shown in Figs 7 and 8. (Analogous spectroscopic data

are available for the indium and aluminum derivatives

in the supporting information, see Figs S2–S4.) Sig-

niﬁcantly, the DFT-calculated electronic spectra for the

postulated reactant, intermediate, and m-hydroxo dimer

product involved in the equilibrium (Fig. 6) afford an

acceptable match of the experimentally observed spectra

+

{[Ga(OEP)](m-OH)[Ga(TPP)]} were unsuccessful. For

instance, we found that the condensation of a mixture

of [Ga(OEP)(OH)] and [Ga(TPP)(OH)] gave rise only

to the more thermodynamically favored dimer, namely

{[Ga(OEP)] (OH)}ClO . Furthermore, the monomeric

2

4

complex with TPP as the porphyrin macrocycle is stead-

fastly unreactive. In principle, this class of asymmetric

dinuclear species would provide an interesting family of

complexes in which selective oxidation or reduction of

only one of the porphyrin macrocycles might be possible

(if the complexes can be made), thereby facilitating the

possible formation of mixed-valence complexes.

(

after appropriate, uniform scaling of the absolute energy

values). The calculations cleanly conﬁrm the identity of

the postulated intermediate, namely the six-coordinate

Electrochemical and DFT data for

µ-hydroxo dimers

+

bis(aqua) complex [Ga(OEP)(OH ) ] .

2

Method 1 and Method 2 (described in the Experimen-

tal Section) are two slightly different approaches for the

above mentioned synthetic route. We always observe,

after following the appropriate synthetic route, a small

amount of the starting material, [M(Porph)(OH)], with

the crude solid product (i.e. the desired m-hydroxo

dimer). For this reason, one or two recrystallizations

are required in order to obtain analytically pure dimeric

The electrochemical behavior of {[Ga(OEP)] (OH)}

2

ClO and {[Ga(TPP)] (OH)}ClO was investigated in

4

2

4

CH Cl with 0.1 M TBAP as the supporting electrolyte

2

under argon. Two quasireversible waves are observed

both in the reduction and oxidation cycles. All elec-

trochemical processes take place in the macrocycle

and clearly correspond to a transfer of two electrons.

J. Porphyrins Phthalocyanines 2019; 23: 976–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

977

Fig. 7. Electronic spectra monitoring the conversion of mono-

meric [Ga(OEP)(OH)] to {[Ga(OEP)] (OH)}ClO₄by treat-

2

Fig. 8. Electronic spectra recorded in CH Cl at ambient

ment with aqueous HClO in CH Cl at ambient temperature.

2

4

2

temperature illustrating the difference in the Soret band for the

The spectrum of the m-hydroxo complex, {[Ga(OEP)] (OH)}

2

=

ClO , was obtained by dissolving clearly identiﬁed crystals of

^s^t^a^r^tⁱⁿ^g^m^a^t^e^rⁱ^a^l^[^G^a⁽^O^E^P⁾⁽^O^H⁾^]^,^t^h^e^“^a^q^u^a^c^o^m^p^l^e^x^”⁽^λmax

4

3

96 nm) and {[Ga(OEP)] (OH)}ClO (λ = 380 nm). The

the complex. Hence, the absorption scale is not equivalent to that

of the other species

2

4

max

DFT-calculated spectra of the relevant species (in a CH Cl

2

solvent continuum) are shown in the inset. A correction factor

λ = +25 nm) was uniformly applied to the calculated energies

to match the experimental spectra. The bandwidth for plotting

(

Experimentally discrete oxidation or reduction of

the individual porphyrin rings of the present group of

m-hydroxo dimers was not observed. This type of behav-

ior has been seen in some, but not all, covalently linked

bisporphyrins [22].

Half-wave potentials for oxidation and reduction are

given in Table 2 where the redox potentials of their pre-

cursors are also included [23]. The absolute potential

difference between the ﬁrst two reduction potentials for

-

1

the calculated spectra was 1500 cm (FWHM)

system, typically CV measurements conducted in non-

aqueous media (e.g. CH CN with 0.1 M supporting

3

electrolyte and ferrocene, Fc, as the internal reference

compound) [24]:

E_H_O_M_O= -(E_[_o_n_s_e_t_,_o_x_v_s_._F_c

+

+ 5.1 – 0.40) eV

(2)

(3)

/Fc]

{

[Ga(TPP)] (OH)}ClO is 400 mV while the value for

2 4

[Ga(OEP)] (OH)}ClO is 450 mV. These separations

E_L_O_M_O= -(E_[_o_n_s_e_t_,_r_e_d_v_s_._F_c

+

2

4

/Fc]

between the reduction waves are in good agreement with

the 420 50 mV generally observed for ring-centered

+

±

In these equations, the formal potential of the Fc /Fc

redox couple (in acetonitrile) is approximately -5.1 eV

in the Fermi scale. We’ve added the +0.40 V correction

to the equations above to facilitate calculations with

reductions of different metalloporphyrin complexes [23].

The reported range of successive ring-centered oxida-

tions is 240

± 50 mV [23], consistent with the observed

+

values; this is not the case for the corresponding mono-

meric species [16].

redox data measured against SCE (as opposed to the Fc /

Fc standard redox couple). The experimental peak poten-

tials for the ﬁrst oxidation and reduction of the Ga(III)

complexes listed in Table 2 were used with Eqs. 2 and

3 to calculate the gas phase HOMO and LUMO ener-

gies of the complexes. The empirical estimates are

tabulated with the in vacuo DFT-calculated MO ener-

gies in Table S4. Although the absolute energies of the

HOMO and LUMO exhibit some marked differences

between experiment and theory (due to the approxima-

tions associated with Eqs. 2 and 3), the most important

parameter, ∆E_F_M_O, is notably congruent across this small

series of Ga(III) hydroxo derivatives. The maximum

deviation between the empirical and DFT-calculated

values amounts to <8% for [Ga(OEP)(OH)]. As far as

the µ-hydroxo dimers are concerned, our simulations

explain why all reversible electron transfers involve the

pair of porphyrin macrocycles equally (i.e. no step-wise

From our DFT simulations, which are unprecedented

for m-hydroxo metalloporphyrin dimers, it is clear that the

HOMO and LUMO of the m-hydroxo dimers evenly span

both porphyrin rings (Fig. 5) and correspond to the typi-

cal p-character a and e frontier MOs of D symmetry

1

u

g

4h

metalloporphyrins [18–20]. Notably, the Ga(III) ions are

located at nodes in these MO wave functions, succinctly

accounting for the concerted ring-centered oxidations

and reductions observed in the experimental CV scans.

The fact that the FMOs (especially the antibonding p*

MOs) do not encompass the m-hydroxo bridge suggests

that the dinuclear species will be redox-stable, consistent

with experiment.

The energy of the gas phase HOMO and LUMO

may be estimated from the experimentally determined

ﬁrst oxidation and ﬁrst reduction peak potentials of the

J. Porphyrins Phthalocyanines 2019; 23: 977–989

978

O. Q. MUNRO ET AL.

Fig. 9. Molecular overlay (least-squares ﬁt) of the upper

Ga(III) porphyrin rings within the m-hydroxo dimers

+

{

[Ga(OEP)] (OH)} and {[Ga(porphine)] (OH)} . The ﬁgure

2

highlights the effect of the peripheral groups on the overall

architecture of the dimer. Most hydrogen atoms have been

omitted from the tube model for clarity

1

Fig. 10. Ambient temperature

[

H NMR spectra for

Ga(OEP)(OH)] and {[Ga(OEP)] (OH)}ClO recorded in

2

4

CDCl . Signal assignments are indicated along with integral

3

electron transfer to separate porphyrin rings is theo-

retically possible given the nature of the HOMO and

LUMO).

ratios. The DFT-calculated structure of the m-hydroxo dimer

highlights the two inequivalent magnetic environments for the

methylene protons of the ethyl groups

The decrease in the magnitude of ∆E_F_M_Oupon dimer-

ization of the hydroxo monomer to form the m-hydroxo

dimer (Eq. 1) is largest for the OEP system (0.25 eV,

DFT data) and suggests that changing the porphyrin sub-

stituents represents one practical method for lowering the

energy gap in m-hydroxo dimers of Ga(III) porphyrins

should this be desired for testing the applications of these

(

OH)] causes an upﬁeld shift of all proton signals for

both dimers (OEP and TPP) relative to their monomeric

precursors. For [Ga(OEP)(OH)] and {[Ga(OEP)] (OH)}

ClO , the signal from the meso protons is shifted upﬁeld

to 9.36 ppm in the dimer from 10.30 ppm in the mono-

mer (Fig. 10). The signal from the pyrrole β-protons of

the TPP complex exhibits an upﬁeld shift of 0.4 ppm.

Similar changes in the chemical shifts of the meso and

pyrrole β proton signals have also been observed for the

2

4

complexes in OLEDs [25, 26] or solar cells [27]. The

+

decrease in ∆E_F_M_Ofor {[Ga(TPP) ](OH)} is less signiﬁ-

2

cant (0.04 eV) presumably because the p–p interaction

between the porphyrin rings is sterically limited by the

bulky meso-phenyl groups. While repulsive steric inter-

actions between substituent groups appended to the por-

phyrin rings undoubtedly affect the ring–ring separation,

this is not the only geometrical change mediated by intra-

molecular peripheral group steric effects. Thus, the Ga–O

bond distances, Ga–O–Ga bond angle, and N–M–M–N

twist angle governing the relative orientations of the rings

(

[

OEP)Rh–In(OEP) and (OEP)Rh–Tl(OEP) complexes

16]. This upﬁeld shift in d can be attributed to signiﬁ-

H

cant aromatic ring current induced shielding [28] in the

intra dimer region between porphinato rings that are in

close proximity.

In µ-hydroxo dimers, a further consequence of two por-

phyrin ring currents intersecting in the intra dimer space

is that any peripheral group protons located in the region

between the two macrocycles (i.e. the outer reaches of the

endo region depicted in Fig. 2) are effectively “doubly

deshielded”, while protons outside that region positioned

toward the solvent (the exo region depicted in Fig. 2),

are only “singly deshielded” due to the operation of a

single conical shielding cone and peripheral deshielding

zone on the solvent-exposed outer face of each porphyrin

macrocycle of the dimer. The net effect is that periph-

eral group exo protons in Group 13 µ-hydroxo metallo-

porphyrin dimers are relatively shielded compared with

their chemically-equivalent counterparts located within

the endo region of the complex. The magnetic anisotropy

within the dimer also change, as evidenced by the super-

+

imposed gas phase structures of {[Ga(OEP)] (OH)}

2

+

and [Ga(porphine)] (OH)} shown in Fig. 9. All of these

2

factors in unison with the p–p interaction and electronic

character of the porphyrin substituents (e.g. inductively

electron-releasing ethyl groups) are likely to modulate

the FMO energy gap in the dimer and thus the net change

in the gap relative to the monomer.

NMR spectroscopy

1

H NMR spectroscopy. The formation of the

m-hydroxo bridge upon condensation of [M(Porph)

J. Porphyrins Phthalocyanines 2019; 23: 978–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

979

is especially pronounced in {[Ga(TPP)] (OH)}ClO , as

endo region (m-endo) resonate at a slightly higher mag-

netic ﬁeld relative to the meta carbons in the exo region

(m-exo). In the H NMR spectrum by contrast, both

2

4

summarized in Table 1, and accounts for the chemical

shift difference of 0.41 ppm between the o-endo and

o-exo protons of the meso-phenyl groups.

1

the ortho and meta protons in the exo region resonate

at a higher magnetic ﬁeld relative to the corresponding

protons in the endo region.

In the case of {[Ga(OEP)] (OH)}ClO , the ring cur-

2

4

rent-induced magnetic anisotropy in the slow exchange

limit accounts for the split signal and chemical shift

1

3

C spin-lattice relaxation data in Table S2 (mea-

inequivalence of the β-methylene (CH ) protons of the

sured at B = 11.74 T) are presented as NT , where N

2

0

1

ethyl groups appended to the porphyrin rings (Table 1,

is the number of protons directly attached to an indi-

Fig. 10). Magnetic anisotropy for the β-CH protons of

vidual carbon atom. Listing the data as NT accounts

2

1

[

Ga(OEP)(OH)] was, however, not observed at ambi-

for the different numbers of protons present in methine

(CH), methylene (CH ), and methyl (CH ) groups, and

ent temperature, despite the 5-coordinate geometry

of the monomeric complex, due to free rotation of the

ethyl groups and thus signal averaging (as evidenced

2

3

enables one to directly compare the dynamics of various

molecular segments within the m-hydroxo dimers. For

{[Ga(TPP)] (OH)}ClO , the NT values for the pyrrole

1

3

by the C spin-lattice relaxation rates in Table S2, vide

infra). This behavior contrasts that of [Ga(TPP)(OH)]

which has magnetically inequivalent o-phenyl protons

in the slow exchange limit due to asymmetry (structur-

ally distinct faces of the macrocycle engendered by the

single hydroxo ligand) and a signiﬁcant barrier to free

rotation of the meso-phenyl rings [29] (vide infra). Our

observations for {[Ga(OEP)] (OH)}ClO regarding the

2

4

1

β-carbons are, within experimental error, equal to the

NT values of the phenyl para carbons. The NT values

1

of the ortho and meta carbons of the phenyl rings are

similarly equivalent within experimental error. However,

they are higher than those of both the pyrrole β-carbons

and the meso-phenyl para-carbons. This fact demon-

strates that the phenyl rings of the TPP dimer undergo

dynamic internal reorientation (rotation) about their two-

fold symmetry axes.

2

4

anisochronous β-methylene protons are consistent with

other studies on monomeric metalloporphyrins contain-

ing Tl(III) [30–32], ﬁve-coordinate [Ga(OEP)X] deriva-

The relaxation rates of the methylene and methyl

carbon nuclei of {[Ga(OEP)] (OH)}ClO are higher than

tives (where X = halogen) [33], and [Sc(OEP)] (O) (a

2

4

structurally-related m-oxo-dimer) [34].

the rate observed for the TPP meso-phenyl carbons, con-

sistent with the overall higher internal mobility of the

sterically less bulky peripheral ethyl groups in the OEP

dimer.

Regarding the DFT-calculated isotropic shieldings, the

1

calculated H chemical shifts are in good agreement with

the experimental chemical shifts. The smallest deviations

1

3

(

< 0.4 ppm) are for the porphyrin β- and meso-protons as

We have analyzed C NMR relaxation data of

carbon atoms with directly attached protons for

+

well as the ethyl group protons of [Ga(OEP)] (OH)} . The

2

1

3

1

chemical shift pattern and relative shielding for the endo

and exo ortho-phenyl ring protons of {[Ga(TPP)] (OH)}

{[Ga(TPP)] (OH)}ClO assuming direct C– H dipolar

2 4

+

interactions to be the dominant relaxation mechanism.

Using NMR relaxation theory [35, 36], the spin-lattice

2

is similarly well modeled, the deviation between experi-

ment and theory amounting to <0.8 ppm. A similar level

of agreement holds for the para and meta protons. As

noted above, the hydroxo group protons of the ﬁve-

coordinate monomers, [Ga(Porph)(OH)], are shielded by

virtue of their location within the ring current shielding

zone above the porphyrin macrocycle. The calculated

isotropic shieldings for the monomer OH protons (-5.4 to

-1

relaxation rates R = (NT1) and NOE factors can be

1

expressed as a combination of spectral densities of

motion J (ω ) in the following manner [37],

i

j

2

1



γ γ

h



Η

C

3

R₁=

{J₀(ω - ω )





H

C

(4)

2

+

0  2pr



CH

3J (ω ) + 6J (ω + ω )

}

1

C

2

H

C

-6.1 ppm, Table 3) are in broad agreement with the exper-

imental chemical shifts (-6.6 to -7.2 ppm) and correlate

with the charge on the H atom and ∆E_F_M_O(Fig. S8).

Importantly, the enhanced shielding brought about by

µ-hydroxo dimer formation and sandwiching of the OH

proton between the bridged porphyrin rings is conﬁrmed

by the DFT simulations, which predict a strong upﬁeld

NOE = 1+ (γ /γ )

H

C

6J (ω + ω )-J (ω - ω )

}

(5)

{

2

H

C

0

H

C

{ ₀

J (ω - ω )+3J (ω ) + 6J (ω + ω )

}

H

C

1

C

2

H

C

1

shift in the d values to -12.0 to -12.9 ppm. (The experi-

where γ and γ are the magnetogyric ratios of the H and

H

C

1

3

mental chemical shifts are -13.9 and -14.5 ppm for the

C nuclei, respectively, h is Planck’s constant and r_C_His

TPP and OEP dimers, respectively).

the C–H bond length. ω and ω are the Larmor frequen-

H

C

1

3

13

1 13

C NMR spectroscopy. The C chemical shifts

cies for H and C, respectively. The spectral densities

for both µ-hydroxo Ga(III) porphyrin dimers are sum-

marized in Table S1. For the TPP complex, the phenyl

carbons are, similar to the phenyl protons, magnetically

inequivalent (Table 1). However, the meta carbons in the

of motion, J (ω )(i = 0, 1, 2; ω = ω –ω , ω , ω + ω ),

i

j

H

C

H

C

are Fourier transforms of the corresponding correlation

functions G (t). The correlation functions G (t) describe

i

1

3

ﬂuctuations in the C–H bond orientation with respect

J. Porphyrins Phthalocyanines 2019; 23: 979–989

980

O. Q. MUNRO ET AL.

to the static magnetic ﬁeld B in terms of the second

rank tensor components which characterize the dipolar

coupling [35].

~1400 ppm. This is due to a dominance of the paramag-

0

7

1

netic term in the nuclear screening constant. The Ga

NMR resonance signals of the monomeric precursors of

both dimers are at a lower magnetic ﬁeld relative to the

In order to analyze the experimental R and NOE data,

1

the correlation functions G (t) must be derived from a

reference signal from Ga(NO ) . The chemical shifts for

i

3 3

speciﬁc physical model describing ﬂuctuations of the

[Ga(OEP)(OH)] and [Ga(TPP)(OH)] measured 72.8 and

77.1 ppm, respectively. These d_G_avalues are signiﬁcantly

shielded relative to the chemical shifts (d_G_a~110–171

ppm) observed for aliphatic triazamacrocycle chelates of

the metal ion, which are of considerable interest as poten-

tial biomedical imaging agents [38, 39]. Interestingly, the

octahedral Ga(III) complex [Ga(H ppma) ][NO ] [40],

1

3

1

dipolar coupling associated with a given C– H bond.

Based on the crystal structure of {[Mn(TPP)] (OH)}ClO

2

4

[

12] we have employed a simple model of overall (isotro-

pic) molecular tumbling for the phenyl para- and pyrrole

β-carbons, and a model of composite motion consisting

of overall molecular tumbling and a jump motion about

the two-fold symmetry axis of the phenyl ring between

two equally populated positions (rotamers) for the ortho

and meta carbons. Calculation of the correlation func-

tions leads to the spectral densities of the form [37]

3

2

3 3

where H ppma = tris(4-(phenylphosphinato)-3-methyl-

3

3-azabutyl)amine, has 6 neutral oxygen donor atoms and

7

1

exhibits a signiﬁcantly more shielded Ga NMR chemi-

cal shift (-62.3 ppm) relative to the [Ga(Porph)(OH)]

derivatives.

2

j

2

iso

J (ω ) = 2[t / (1 + ω t )]

(6)

No observable signal was detected for the correspond-

ing µ-hydroxo dimers despite scanning a wide region of

i

j

iso

7

1

the Ga NMR chemical shift range (-1000 ppm to +1000

ppm around the reference signal). The signal is most

likely very broad due to the interaction of the quadrupole

for the isotropic reorientation and









t









iso

2

J

71

J (ω ) = 2 (1 -C)

moment of Ga with the electric ﬁeld gradient created

i

j



2

iso

(

1 + ω t )





by the negative charge localized on the oxygen atom of

the hydroxo bridge (Table 3), and thus undetectable with

a high-resolution NMR spectrometer. (High-resolution

instruments generally have limited hardware capabilities

to measure broad resonance signals of ~100 KHz.)

(7)

-

1

-

1

-1







(

t

iso + tjump

)

+

C

-

2

-1

1

+ ω_j

(

t

iso + tjump

)





for the composite motion, where

X-ray crystallography

C = 3/4[sin2β(1 − cos2q)][2 − sin2β(1 − cos2q)] (8)

The structures of the Group 13 µ-hydroxo metal-

loporphyrin dimers elucidated here are broadly similar

to the µ-hydroxo-bridged species of other M(III) por-

and t_i_s_oand t_j_u_m_pare the correlation times corresponding

to the isotropic reorientation and the jump about the two-

fold symmetry axis of the phenyl ring, respectively. β is

phyrins determined previously [9, 11, 12]. Side-on

+

views of the X-ray structures of {[Ga(OEP)] OH } and

2

1

3

1

+

the angle between the relaxation vector ( C– H bond)

and the axis about which the vector jumps, while the

angle 2q is the extent of the jump.

{[In(OEP)] OH } depicted in Figs 3 and 4 (for the tri-

2

clinic polymorph of the indium derivative) indicate the

structural features to be considered, namely the M–N_p

and M–O(OH) bond distances, the M–O(OH)–M bond

angle, and the dihedral angle between the two porphyrin

rings (among others).

The results of the analysis of the experimental T and

1

NOE data in terms of the motional models (Eqs. 1–5,

vide supra) are presented in Tables S2 and S3. The model

of isotropic reorientation ﬁts the experimental data for

the pyrrole β carbon nuclei and the phenyl para carbons

at all temperatures, with the exception of the NOE at

The M–O(OH)–M bond angles differ substantially

3

from the ideal value of ~104.48° for an sp -hybridized

oxygen atom in water [41] and reﬂect both the bonding

requirements of the central metal ion, including the

M–O(OH) bond length, and the steric requirements of

the porphyrin ligand. These views also show an unusual

feature found in all three unique indium µ-hydroxo spe-

cies, namely that the hydrogen atom of the bridging OH

ligand is hydrogen-bonded to a solvate water molecule

with normal hydrogen bonding distances. The appear-

ance of these hydrogen bonds may reﬂect reduced

basicity of the OH bridging group in the indium deriva-

tives, a postulate that is consistent with the more posi-

tive H and less negative O atom charges calculated for

the µ-hydroxo dimers relative to the [Ga(Porph)(OH)]

2

(

0°C. In this case, the calculated NOE value is outside

above) the error range of the experimental NOE. The

calculated T values for the meso-phenyl ring ortho and

1

meta carbons using the model of composite motion for

these nuclei are consistent with those measured experi-

mentally, further validating the model.

7

1

Ga NMR spectroscopy. Gallium possesses two

6

9

71

stable isotopes, Ga and Ga, that have a nuclear spin

I = 3/2 and a quadrupole moment [35]. As Ga has higher

7

1

receptivity and provides sharper resonance lines than

6

9

Ga, we have measured the NMR spectra of the former

7

1

isotope. The chemical shift range of the Ga nucleus is

J. Porphyrins Phthalocyanines 2019; 23: 980–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

981

+

Fig. 11. Mean plane diagrams for the two rings of {[Ga(OEP)] (OH)} (top line), monomeric [In(OEP)(OH)] (center), and the two

2

+

rings of the monoclinic polymorph of {[In(OEP)] (OH)} (bottom line). Displayed on each diagram are the displacements of each

2

atom (in units of 0.01 Å) from its respective 24-atom mean plane. Also shown are the averaged values (4-fold symmetry) of the bond

distances and bond angles in the core. The numbers in parentheses are the estimated standard uncertainties based on the assumption

that all values are drawn from the same population

monomers (Table 3). Also revealed in these two ﬁgures

is that the hydrogen atom of the OH group in three of

the four dinuclear complexes was experimentally located

values for the Fe, Mn, and Ga derivatives reﬂect values

for the porphyrin rings with least strain. The much larger

values for the indium derivatives (~2.08 Å) reﬂect the

(

difference Fourier synthesis). The values of the coordi-

larger size of the indium(III) ion, as do the values of the

nation group in the reported complexes along with previ-

ously reported species are given in Table 4.

In–N bond distances. The large size of the indium ion is

p

also reﬂected in the increased displacement of the metal

ion from both the mean plane of the 24-atom core and the

plane of the 4N core. The difference in these two values,

especially pronounced for the indium species, also

reveals that substantial doming of the porphyrin macro-

cycle occurs, which is another stereochemical feature to

accommodate larger metal ions [43, 44]. Moreover, the

differences in the value of the dihedral angles between

the 24-atom mean planes and four nitrogen atom mean

planes indicate that distortions from planarity accommo-

date the close approach of the porphyrin rings.

Regarding the metalloporphyrin conformations for

this group of structurally-related derivatives, Fig. 11 dis-

plays the perpendicular displacements of the ring atoms

from the 24-atom mean plane of the porphyrin core.

The diagram also presents the average values of bond

distances and bond angles in the core. The differences

…

Table 4 s

h

ows that the M M separations displa

y

muc smaller

h

variation than other quantitative descrip-

tors, reﬂecting the importance of repulsion between the

porphyrin ligands in determining the overall stereochem-

istry of the m-hydroxo dimers. The porphyrin–porphy-

rin interactions also favor “outward” orientations of the

peripheral ethyl groups (i.e. most of the ethyl groups are

oriented away from the molecular center with an exo-

conﬁguration). These interactions may also affect the

N–M–M–N twist angles (Table 4). Diagrams illustrating

the overlap between the rings are given in Fig. 12.

The values of the M–N and M–O(OH) bond dis-

p

tances are in general accord with values typical of the

respective metal of the dinuclear species. Additional

stereochemical parameters include the ring size, which is

given by the average ring center (Ct) to N distance. The

p

J. Porphyrins Phthalocyanines 2019; 23: 981–989

982

O. Q. MUNRO ET AL.

J. Porphyrins Phthalocyanines 2019; 23: 982–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

983

Fig. 12. Thermal ellipsoid diagrams illustrating the overlap between the two porphyrin rings in each of the four dinuclear species

reported here. All atoms are contoured at the 50% probability level. In all diagrams, the top ring is shown with unshaded bonds.

Top left: {[Ga(OEP)] (OH)}ClO , Top right: {[In(OEP)] (OH)}ClO , Bottom left: {[In(OEP)] (OH)}ClO (molecule 1, triclinic

2

4

2

4

2

4

phase), Bottom right: {[In(OEP)] (OH)}ClO (molecule 2, triclinic phase). All H atoms except those located on the bridging oxygen

2

4

atom (when located) have been omitted for clarity; the perchlorate counterions are also not shown.

1

in the core metrical parameters between those of the

gallium species (top line) and those of the indium spe-

cies (bottom line) are the result of the core expansion

accommodating metal ions of substantially different

sizes. In the gallium/indium comparison, the C –C –C

Although solution state evidence ( H NMR and elec-

tronic spectroscopy) conﬁrms the formation of monomeric

hydroxide complexes of all Group 13 species, wehaveonly

obtained crystals of [In(OEP)(OH)]. The ﬁve-coordinate

species is illustrated in Fig. S6 (Supporting Information).

The complex forms an extended structure with weak

a

m

a

bond angle opens up by ~3° in the indium species along

with an increase of ~2.5° in the C –N–C bond angle.

p–p interactions between porphyrin rings (Fig. 13). For

a

…

An increase in the bond length of the C –C bonds

the centrosymmetric inversion pair, the In In distance

measures 8.60 Å, while the interplanar spacing is close

to the graphite spacing of 3.44 Å [42]. These metrics are

consistent with a weak edge-to-edge type p–p interaction,

as reviewed earlier by Scheidt and Lee [43].

a

m

also contributes to increasing the size of the porphyrin

ring. Similar patterns are seen in the remaining indium

derivatives that are reported in Fig. S5 of the Supporting

Information.

J. Porphyrins Phthalocyanines 2019; 23: 983–989

984

O. Q. MUNRO ET AL.

Although we have experienced no problem with the

procedures described in dealing with systems contain-

ing perchlorate ion, they can detonate spontaneously and

should be handled only in small quantities; in no case

should such a system be heated above 30°C and other

safety precautions are also warranted [46].

Compound synthesis

Synthesis of [Al(OEP)Cl]. [Al(OEP)Cl] was synthe-

sized by metallation of H (OEP) with anhydrous AlCl in

2

3

reﬂuxing benzonitrile according to the literature method

-1

[

47]. IR (cm ): 440(s) (ν_A_l₋_C_l).

Synthesis of [Ga(OEP)Cl] and [In(OEP)Cl]. The

procedure used to prepare [Ga(OEP)Cl] and [In(OEP)Cl]

was essentially that of Bhatti et al. [48]. H OEP (500 mg)

2

and GaCl (InCl ) (1.0 g) were added to a hot solution of

3

glacial acetic acid (300 mL) containing 5 g of sodium

acetate. The solutions were then reﬂuxed for about 3 h

until the UV/vis spectra showed the disappearance of

the characteristic bands of octaethylporphyrin free base.

The solutions were cooled down to room temperature

and then to lower temperatures with an ice-water bath.

Crystalline [Ga(OEP)Cl] or [In(OEP)Cl] were obtained

by vacuum ﬁltration of the cooled solutions and washed

Fig. 13. Two views highlighting the distinct edge-to-edge p–p

interactions between molecules of ﬁve-coordinate [In(OEP)

(

OH)]. Key interplanar spacings (based on the distance between

-1

with hexane. IR cm : (νM−Cl⁾³³³⁽^s⁾⁽^[^G^a⁽^O^E^P⁾^C^l^]⁾^aⁿ^d

parallel 24-atom porphyrin mean planes) are depicted for the

edge-on view (top), while the lower image (perpendicular view)

highlights the relationship between the inversion pair and the

closest adjacent molecule (right most structure) in the lattice.

H atoms have been omitted for clarity; thermal ellipsoids are

rendered as 40% probability surfaces. Darkly shaded objects

are closest to the viewer.

3

08(s) ([In(OEP)Cl]).

Syntheses of [M(OEP)(OH)], M = Al, Ga and In.

A slightly modiﬁed literature method for [Al(OEP)

(

[

OH)] [23] was employed to synthesize [Al(OEP)(OH)],

Ga(OEP)(OH)]and[In(OEP)(OH)].Inatypicalprepara-

tion, crystalline [Ga(OEP)Cl] or [In(OEP)Cl] (~200 mg)

were dissolved in ~50 mL of CH Cl and chromato-

2

graphed on alumina (80–200 mesh) column. Elution

EXPERIMENTAL SECTION

with CH Cl removed a small amount of free base; the

2 2

metalloporphyrin complexes were washed out from

the column by a mixture of CH Cl and methanol (4:1

General methods

2

H OEP was purchased from Midcentury Chemicals

v:v), and taken to dryness under vacuum. Spectroscopic

2

-

1

and H TPP was prepared according to the literature

data for [Al(OEP)(OH)]: IR (cm ): (νO−H) 3655 (w).

UV/vis (nm, CH Cl ): λmax 332, 377, 397 (Soret), 531,

570. [Ga(OEP)(OH)]: IR (cm ): (νO−H) 3663 (w). UV/

vis (nm, CH Cl ): λmax 336, 382, 402 (Soret), 534, 571.

[In(OEP)(OH)]: IR (cm ): (νO−H) 3658 (w). UV/vis (nm,

CH Cl ): λmax 344, 388, 408 (Soret), 540, 577.

2

[

45]. Dichloromethane, hexane, glacial acetic acid,

2

-

1

sodium acetate, and alumina adsorbent (80–200 mesh)

were bought from Fisher; perchloric acid (~70%), gal-

lium chloride, indium chloride, and aluminum chloride

were purchased from Aldrich. All materials were used

as received unless otherwise noted. Dichloromethane

for electrochemical studies was distilled over CaH₂

and subsequently over P O . Tetrabutylammonium per-

2

-

1

2

The indium derivative synthesized above was recrys-

tallized by diffusion of hexane into a toluene solution of

the complex. A single-crystal X-ray analysis conﬁrmed

that the product complex was [In(OEP)(OH)]. Moreover,

an IR spectrum for the single crystal sample displayed

2

5

chlorate, TBAP, was purchased from GFS Chemicals

and was always stored in a vacuum oven at +40.0°C.

Aqueous perchloric acid was prepared by diluting the

original concentrated acid 1000-fold prior to deter-

mining its exact concentration by standard acid-base

titrations against potassium acid phthalate. IR spectra

were recorded on a Perkin-Elmer 883 Infrared Spectro-

photometer as Nujol mulls with CsBr disks; electronic

spectra were recorded on a Perkin Elmer Lambda 19

UV/vis/near-IR Spectrometer. Caution!

-1

an identical O−H stretch (at 3658 cm ) as that for the

powder sample.

Synthesis of [Ga(TPP)(OH)]. To H

0.376 mmol) and sodium acetate (5 g) was added a solu-

tion of GaCl (525 mg, 2.98 mmol) in 250 mL of gla-

TPP (231 mg,

2

3

cial acetic acid. The resulting dark green solution was

reﬂuxed under argon for 4 h until deep purple in color.

TLC on silica gel (1:1 ether:benzene) indicated about

J. Porphyrins Phthalocyanines 2019; 23: 984–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

985

9

5% conversion of H TPP (R = 0.64) to [Ga(TPP)Cl]

with a Soret band maximum at 388 nm only. An X-ray

2

f

(

R = 0.46). 250 mL of distilled water was added to the

analysis identiﬁed the compound as {[In(OEP)] (OH)}

f

2

cooled solution and the metalloporphyrin extracted into

50 mL of toluene in a separatory funnel. The toluene

ClO in a different crystal form (monoclinic polymorph).

4

2

UV/vis (nm, CH Cl ) for the single crystal sample of

2

extract was washed twice with distilled water (200 mL),

dried over anhydrous calcium chloride, ﬁltered, and taken

to dryness on a rotary evaporator. [Ga(TPP)Cl] was con-

verted to the hydroxide derivative by anion exchange on

an aluminum oxide column (17.8 × 3.8 cm, CH Cl ) as

{[In(OEP)] (OH)}ClO : λ 345 (sh), 388 (Soret), 407

2

4

max

(sh), 542, 580.

Synthesis of {[Ga(TPP)] (OH)}ClO . [Ga(TPP)

2

4

(OH)] (54 mg, 77.2 µmol) was dissolved in 20 mL

of CH Cl in a separatory funnel and titrated with a

2

described above. The pink–purple solution of [Ga(TPP)

OH)] was then concentrated by rotary evaporation to

yield a purple microcrystalline product. Spectroscopic

17.2 mM solution of HClO . The mixture was vigor-

4

(

ously shaken after each 200–500 µL addition of titrant.

The progress of the reaction was followed spectrophoto-

metrically by diluting an aliquot (1 µL) of the reaction

mixture in ca. 2 mL of CH Cl in a 1 cm path length

-1

data for [Ga(TPP)(OH)]: IR (cm ): 3664 (w) (ν_O₋_H), 444

(

w) (ν_G_a₋_O). UV/vis (nm, CH Cl ): λ 275, 314, 399

2

max

2

N), 419.5 (Soret), 550, and 590.

cuvette; the intensity of the Soret band of [Ga(TPP)(OH)]

(420 nm) decreased relative to a new Soret band from

{[Ga(TPP)] (OH)}ClO at 399 nm. The concentrationof

Synthesis of {[Ga(OEP)] (OH)}ClO . Method 1:

2

4

In a separatory funnel, a CH Cl solution (~20 mL) of

2

4

[

Ga(OEP)(OH)] (~50 mg) was treated with various

{[Ga(TPP)] (OH)}ClO , as judged from the spectra of the

2 4

-2

amounts of aqueous HClO (C = 1.36 × 10 M). When

dilute solutions used for the spectroscopic assay, reached

a maximum after ca. 1.3 molar equivalents of acid (in the

aqueous phase) had been added. Addition of excess acid

resulted in the loss of the 399-nm band and the appear-

ance of a new Soret band at 415 nm; the process was

reversible upon addition of dilute NaOH solution. The

titration was stopped when the intensity of the Soret band

of {[Ga(TPP)] (OH)}ClO (399 nm) was approximately

4

approximately one-half of an equivalent of acid was

used, the Soret band at 402 nm gradually diminished to

a shoulder and a new Soret band at 380 nm appeared.

The CH Cl phase was separated, dried with solid Na SO

2

4

and then taken to dryness under vacuum. The powder

material so obtained was recrystallized from CH Cl₂

and hexane. After about ﬁve days single crystals were

isolated. An X-ray structure determination identiﬁed the

crystal form of the complex as {[Ga(OEP)] (OH)}ClO .

2

4

40% of that of the monomer, [Ga(TPP)(OH)]. The wine-

red CH Cl phase was separated, dried over anhydrous

2

4

2

UV/vis (nm, CH Cl ) for the single-crystal sample of

magnesium sulfate, ﬁltered, and slowly concentrated

on a rotary evaporator. Addition of hexane to the con-

centrated solution afforded a plum-colored precipitate

which was isolated by ﬁltration and washed with hexane.

2

{

[Ga(OEP)] (OH)}ClO : λ 335 (sh), 380 (Soret), 400

2

4

max

(

sh), 539, 572, and 585 (sh).

Method 2: If the initial CH Cl solution of [Ga(OEP)

2

-1

(

OH)] was treated with excess aqueous HClO (50 mL,

Spectroscopic data for {[Ga(TPP)] (OH)}ClO : IR (cm ):

4

2 4

6

% v:v), a Soret band was observed at 396 nm. After

1099(s) (ν_C_l_O₄). UV/vis (nm, CH Cl ): λ 380 (N), 399

2

max

separation, the CH Cl phase was washed with 20 mL

(Soret), 548, 591.

Synthesis of {[Al(OEP)] (OH)}ClO . The identical

2

of distilled water. The UV/vis spectrum displayed a

blue shift of the Soret band to 380 nm. Single crystals

were obtained with the same procedure described above.

These crystals show identical electronic spectra to the

material prepared by Method 1. A single crystal unit cell

determination by X-ray diffraction also conﬁrmed that

the product had the same cell parameters as that prepared

by Method 1.

2

4

procedures of Method 1 and Method 2 for {[Ga(OEP)]₂-

(OH)}ClO were applied for the synthesis of {[Al(OEP)] -

4

2

(OH)}ClO , starting from [Al(OEP)(OH)]. With Method 1,

4

+

after ~0.5 equivalent of H was introduced, a blue-shift of

the Soret band (from 397 to 379 nm) was observed and

a species with a Soret band at 379 nm became dominant

in the reaction system. With Method 2, a species with a

Soret band at 379 nm was also generated. Unfortunately,

all attempted crystallizations of this interesting compound

were unsuccessful.

Synthesis of {[In(OEP)] (OH)}ClO . The iden-

2

4

tical procedures of Method 1 and Method 2 for

[Ga(OEP)] (OH)}ClO were applied for the synthesis

{

2

4

of {[In(OEP)] (OH)}ClO , starting from [In(OEP)(OH)].

2

4

With Method 1, the blue-shift of the Soret band maximum

from 408 nm ([In(OEP)](OH)) to 388 nm was observed.

Crystallization of the product from a toluene solution of

the product layered with hexane yielded crystals with a

Soret band maximum at 388 nm only. An X-ray structure

Physical measurements

1

13

71

NMR spectroscopy. H, C, and Ga NMR spec-

tra were recorded on Varian UNITYplus and Varian

VXR–500S spectrometers operating at magnetic ﬁeld

analysis identiﬁed the compound as {[In(OEP)] (OH)}

strengths B of 7.04 and 11.74 T, respectively. The

2

0

ClO (triclinic form). With Method 2, the blue-shift of the

Soret band from 406 nm (aqua/perchlorato complex) to

experiments were carried out over a -40 to +90°C tem-

4

-5

-7

perature range using 1 × 10 to 7 × 10 M solutions

in CDCl (-40.0 to +30.0°C) and CDCl CDCl (+20.0 to

388 nm was observed. Crystallization from a toluene solu-

3

2

tion of the product layered with hexane also gave crystals

+90.0°C). Relatively low concentrations were used for

J. Porphyrins Phthalocyanines 2019; 23: 985–989

986

O. Q. MUNRO ET AL.

{

[Ga(OEP)] (OH)}ClO because of solubility limitations

point obtained and to generate all normal modes of vibra-

tion for the compound. NMR shielding tensors were

calculated by the GIAO method without spin–spin inter-

actions [57, 58]. Electronic spectra were calculated using

the TD-DFT method of Bauernschmitt andAhlrichs [59];

75 singlet excited states were calculated in each case

to generate the key spectral bands over a wide spectral

range (down to λ ~ 200 nm for the highest energy excited

states). Calculations were also performed in solvent con-

tinua (CDCl or CH Cl ) by fully optimizing the geom-

etry of the complex in the solvent (SCRF PCM model)

[60] prior to calculating the normal modes of vibration,

NMR shielding tensors, and singlet excited states to

permit accurate comparisons with experimental data.

Fractional atomic charges were calculated using natural

population analysis in NBO 3.0 [21], which gives distri-

butions that are independent of the basis set employed for

2

4

at lower temperatures. In the case of {[Ga(TPP)] (OH)}

ClO , however, a 2.2 × 10 M solution in CDCl was

used to maximize the concentration of the dimer.

DQF-COSY and NOESY (mixing time 300 ms) spec-

tra were recorded in phase sensitive mode using standard

pulse sequences [49, 50]. HETCOR experiments were

performed using the pulse sequence described by Bax

2

-

4

3

1

3

and Morris [51]. C spin-lattice relaxation times, T₁,

were measured with proton decoupling using the inver-

sion recovery technique [52]. At least fourteen different

t values were used and the waiting period between suc-

cessive acquisitions was at least 2–3 times the longest T₁

time measured. The values of the relaxation times were

obtained from three-parameter ﬁts to the raw data [53].

3

2

1

3

1

C{ H} NOE enhancements were measured using gated

decoupling techniques [54]. Chemical shifts are reported

in d (ppm) using TMS as an internal standard for all

the calculations.

1

13

+

H and C spectra. The resonance signal of Ga(NO₃)₃

Simulations on {[Ga(porphine)] (OH)} were attemp-

2

obtained in D O solution was used as an external stan-

dard for all Ga NMR spectra.

ted ﬁrst. The reﬁned geometry of the simplest member

2

7

1

of the series was subsequently used to generate the input

+

Electrochemistry. Cyclic voltammetry (CV) experi-

ments were performed using a Cypress System Model

CS–1090 Computer-Controlled Electroanalytical Sys-

tem. Current-voltage curves were recorded on a HP

Laser printer, or EPSON Model FX–850 printer. A three-

electrode system was used with a platinum button work-

ing electrode, a platinum wire counter electrode, and a

saturated calomel reference electrode (SCE). The solvent

was dry, degassed CH Cl containing 0.10 M TBAP as

structure of {[Ga(OEP)] (OH)} in which the ethyl group

2

conﬁgurations of each ring were 4 exo/4 endo (++++− −

−−) as this conﬁguration is common in several bridged

+

M(OEP) complexes, including {[In(OEP)] (OH)} [15].

2

The closest structure to those reported here, {[Ga(OEP)]

+

(OH)[Ga(OEP)(H O)]} [15], however, has a 5 exo/3

2

endo conﬁguration. Because rotation of the ethyl groups

is likely facile in solution (giving averaged signals from

the CH CH groups), we averaged the shielding ten-

2

3

the supporting electrolyte. All CV experiments were con-

ducted under an argon atmosphere.

sors of their constituent atoms to facilitate compari-

sons with experiment. Simulations on the congener,

+

Mass spectrometry. Fast Atom Bombardment

{[Ga(TPP)] (OH)} , were effected similarly.

2

(FAB) mass spectra were acquired with a JEOL JMS–

Allgeometryoptimizationsconvergedonstableenergy

AX505HA mass spectrometer. Xenon gas (>99.995%

purity, Air Products) was used with the JEOL FAB gun

which was operated at 5 kV primary beam energy and

minima with non-imaginary normal modes of vibration,

+

with the exception of the structure of {[Ga(TPP)] (OH)}

2

in a CHCl solvent continuum, which converged on a sta-

3

-1

1

0 mA emission current. The instrument was operated

with an accelerating voltage of 3 kV and a resolution of

000 (10% valley deﬁnition). Samples for mass spec-

tionary point with one imaginary frequency at -5.5 cm .

Examination of the vibrational mode indicated that it

involvedpurerotationabouttheN – Ga – Ga – N dihedral

2

p

trometry were prepared with 1–3 µL of a dichlorometh-

ane solution and were transferred to the FAB probe tip

which had been coated with 2-nitrophenyloctyl ether

angle. Based on the small magnitude of the eigenvalue

and its nature, the calculated geometry was an accept-

able solution phase minimum. All simulated spectra were

-1

1

(

Sigma Grade, Sigma). Data were acquired over the mass

plotted using spectralwidthsof4cm (IR),0.025ppm( H

-1

range 500–1500 at 15 s/scan cycle or 500–2300 at 22 s/

scan cycle. Results represent the average of 5–10 scans.

Samples were initially dissolved in methylene chloride.

NMR), and 2000 cm (full-width at half-maximum, UV-

vis), unless otherwise indicated.

X-ray structure determinations

DFT simulations

The four crystal structure determinations were carried

out on an Enraf-Nonius FAST area-detector diffracto-

meter with a Mo rotating anode source (λ = 0.71073 Å).

Our detailed methods and procedures for small mole-

cule X-ray data collection with the FAST system have

been described previously [61]. Data collections were

performed at room temperature (20 ± 2°C) for[In(OEP)

(OH)] and low temperature (-146 ± 2°C) for the three

DFT simulations at the HSEH1PBE [55]/SDD [56]

level of theory (default convergence criteria and integra-

tion grid) were performed using 64-bit Gaussian 16W

and GaussView 6. After initial structures were created

in GaussView, their geometries were optimized to sta-

tionary points in vacuo. This was followed by a full fre-

quency analysis to establish the nature of the stationary

J. Porphyrins Phthalocyanines 2019; 23: 986–989

SINGLE HYDROXO-BRIDGED GROUP 13 METALLOPORPHYRIN DIMERS

987

µ-hydroxo dimer structures. Crystal decay was excluded

Crystal data for the four structures are listed in

Table S5. Complete crystallographic details, anisotropic

thermal parameters and the ﬁxed hydrogen atom coordi-

nates are available as CIF ﬁles (Supporting Information).

by comparison of the relative scale factors of individual

sweep data [62]. Data were corrected for Lorentz and

polarization factors and at the ﬁnal stages of analysis, a

modiﬁed version [63] of the absorption correction pro-

gram DIFABS [64] was applied.

SUMMARY AND CONCLUSIONS

All four structures were solved using the direct meth-

ods program SHELXS [65]; subsequent difference

Fourier syntheses led to the location of all the remain-

ing nonhydrogen atoms. All structures were reﬁned

The synthesis, solution and solid-state characteri-

zation of 5-coordinate Group 13 (aluminum, gallium,

and indium) metalloporphyrin hydroxide derivatives

are reported. These derivatives include both the

2

against F with the program SHELXL [66] where all

data collected were used, including negative intensi-

ties. Hydrogen atoms of the porphyrin ligands and the

ordered solvent molecules were idealized with the stan-

dard SHELXL idealization methods; those belong to the

disordered solvates were ignored. The hydroxo hydrogen

monomeric [M(OEP)(OH)] species and their dinuclear

+

{

[M(OEP)] (OH)} counterparts. The mononuclear

2

[

In(OEP)(OH)] complex has been structurally character-

ized by single crystal X-ray diffraction and spectroscopy.

The reaction of the 5-coordinate monomeric hydroxides

in CH Cl with a slight stoichiometric excess of aque-

atoms in {[Ga(OEP)] (OH)}ClO and the triclinic poly-

2

4

2

morph of {[In(OEP)] (OH)}ClO were directly located

2

4

ous perchloric acid permits the synthesis of the 5-coordi-

from the difference Fourier map and included in the

least-squares reﬁnement as isotropic contributors; how-

ever, the hydroxo hydrogenatomsinthemonoclinicpoly-

nate µ-hydroxo-bridged complexes {[M(Porph)] (OH)}

2

ClO via loss of water (condensation). The µ-hydroxo

4

1

13

71

dimers were characterized by H, C, and Ga NMR

spectroscopy, UV-vis spectra, electrochemistry, and

X-ray structure determinations of the gallium(III) and

indium(III) OEP derivatives. DFT simulations afforded

suitable solution phase structures (geometries) and sig-

nature spectroscopic features (NMR shielding tensors, IR

vibrational modes, and electronic spectra) for the exper-

imentally-characterized mono- and dinuclear deriva-

tives. The DFT-calculated frontier MOs (FMOs) of the

µ-hydroxo dimers (localized on the porphyrin macro-

cycles) accounted for both their redox behavior and sta-

bility in solution. Finally, TD-DFT calculated electronic

spectra conﬁrmed the nature of the species involved in

the synthetic equilibrium for the Ga(III) OEP deriva-

morph of {[In(OEP)] (OH)}ClO and [In(OEP)(OH)]

2

4

could not be located.

Solvent disorder was observed in all three µ-hydroxo

structures. For {[Ga(OEP)] (OH)}ClO , two CH Cl

2

4

2

molecules were found per asymmetric unit, one of which

has two orientations: Cl(4)–C(2)–Cl(5a) and Cl(4)–

C(2)–Cl(5b) with occupancies of 0.835 (8) and 0.165

(

{

8) for Cl (5a) and Cl(5b), respectively. For triclinic

[In(OEP)] (OH)}ClO , there are two independent

2

4

µ-hydroxo molecules (A and B) in the asymmetric unit

and a total of three toluene molecules, two water mol-

ecules, and one methylene chloride molecule as solvent

components. Among these solvent molecules, two of the

three toluene molecules and one water molecule were

ordered and fully occupied, the remaining species were

only partially present. One of the two perchlorate anions

was disordered at two positions (a and b). Based on con-

sideration of the non-bonding interactions, the perchlo-

rate disorder was judged to be associated with the partial

occupancy of the solvent components. Thus, reﬁnement

of group occupancies was applied, which led to occu-

pancy factors 0.663 (3) for perchlorate (a) and the par-

tial toluene molecule, 0.337 (3) for perchlorate (b) and

+

tives; six-coordinate [Ga(OEP)(OH ) ] serves as the

2

key intermediate in the conversion of [Ga(OEP)(OH)] to

[Ga(OEP)] (OH)}ClO .

{

2

4

Acknowledgments

We thank the National Institutes of Health for support

of this research under Grant GM–38401.We thank Dr. Bill

Boggess of the Mass Spectrometry Facility and Mr. Don

Schifferl of the NMR Spectrometry Facility for assis-

tance (University of Notre Dame). Funds for the purchase

of the FAST area detector diffractometer were provided

through NIH Grant RR–06709 to the University of Notre

Dame. Part of this work was funded by the South African

Research Chairs Initiative of the Department of Science

and Technology and National Research Foundation of

South Africa (Grant No 64799 to OQM).

the partial CH Cl molecule. The occupancy for the par-

2

tial water molecule (O(w2)) was reﬁned independently

to be 0.18 (2). For monoclinic {[In(OEP)] (OH)}ClO ,

2

4

three toluene molecules and one water molecule were

found in the asymmetric unit with each µ-hydroxo dimer.

Two of the three toluene molecules have an inversion

center at the center of the six-membered ring, and thus

they are equally shared between two asymmetric units.

The water molecule was disordered at two positions,

O(w1) and O(w2) with occupancy factors of 0.46(3)

and 0.54(3), respectively. Rigid-group descriptions were

applied for the disordered toluene molecules in both

Supporting information

Electronic spectra monitoring the conversion of mono-

meric [Ga(OEP)(OH)] to {[Ga(OEP)] (OH)}ClO . Elec-

2

4

{

[In(OEP)] (OH)}ClO structures.

tronic spectra illustrating the difference in the Soret

2

4

J. Porphyrins Phthalocyanines 2019; 23: 987–989

988

O. Q. MUNRO ET AL.

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2

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1

3

13

DFT calculated C NMR, C spin relatation times,

1

3

C NOE parameters, frontier molecular orbital ener-

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Article Doi

Single hydroxo-bridged group 13 metalloporphyrin dimers: Solution studies and solid-state structures

DOI: 10.1142/S1088424619500834

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Article abstract of DOI:10.1142/S1088424619500834

Full text of DOI:10.1142/S1088424619500834

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