The Influence of Peripheral Substituent Modification on PV, MnIII, and MnV(O) Corrolazines: X-ray Crystallography, Electrochemical and Spectroscopic Properties, and HAT and OAT Reactivities

DOI: 10.1021/acs.inorgchem.6b01219

Source and publish data:

Inorganic Chemistry p. 8646 - 8660 (2016)

Update date:2022-08-10

Topics:

Authors:

Joslin, Evan E.

Zaragoza, Jan Paulo T.

Baglia, Regina A.

Siegler, Maxime A.

Goldberg, David P.

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Article abstract of DOI:10.1021/acs.inorgchem.6b01219

The influence of remote peripheral substitution on the physicochemical properties and reactivity of phosphorus and manganese corrolazine (Cz) complexes was examined. The substitution of p-MeO for p-t-Bu groups on the eight phenyl substituents of the β-car

Full text of DOI:10.1021/acs.inorgchem.6b01219

Article

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The Inﬂuence of Peripheral Substituent Modiﬁcation on P^V, Mn^III, and

Mn^V(O) Corrolazines: X‑ray Crystallography, Electrochemical and

Spectroscopic Properties, and HAT and OAT Reactivities

Evan E. Joslin,^†Jan Paulo T. Zaragoza, Regina A. Baglia, Maxime A. Siegler, and David P. Goldberg*

Department of Chemistry, The Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, United States

* Supporting Information

ABSTRACT: The inﬂuence of remote peripheral substitution

on the physicochemical properties and reactivity of phospho-

rus and manganese corrolazine (Cz) complexes was examined.

The substitution of p-MeO for p-t-Bu groups on the eight

phenyl substituents of the β-carbon atoms of the Cz ring led to

changes in UV−vis transitions and redox potentials for each of

the complexes. The oxygen atom transfer (OAT) and

hydrogen atom transfer (HAT) reactivity of the Mn^V(O)

complexes was also inﬂuenced by p-MeO substitution. The

OAT reactivity of Mn^V(O)(MeOP₈Cz) (MeOP₈Cz = octakis-

(p-methoxyphenyl)corrolazinato³⁻) with triarylphosphine (PAr₃) substrates led to second-order rate constants from 10.2(5) to

3.1(2) × 10⁴M⁻¹s⁻¹. These rates of OAT are slower than those seen for Mn^V(O)(TBP₈Cz) (TBP₈Cz = octakis(p-tert-

butylphenyl)corrolazinato³⁻). A Hammett study involving para-substituted PAr₃substrates reveals a Hammett ρ-value for

Mn^V(O)(MeOP₈Cz) that is more negative than that observed for Mn^V(O)(TBP₈Cz), consistent with a less electrophilic Mn

center. The HAT reactivity of Mn^V(O)(MeOP₈Cz) with C−H substrates was examined and revealed second-order rate constants

from 6.8(5) × 10⁻⁵to 1.70(2) × 10⁻¹M⁻¹s⁻¹. The rate constants varied with the C−H bond strength of the substrate. Slightly

faster HAT rates with C−H substrates were observed with Mn^V(O)(MeOP₈Cz) compared to Mn^V(O)(TBP₈Cz), indicating that

the basicity of the putative [Mn^IV(O)]⁻intermediate likely compensates for the more negative redox potential in the driving

force for HAT. In addition, the complete, large-scale synthesis of the para-phenyl-substituted porphyrazines RP₈PzH₂(R = p-tert-

butylphenyl (TB), p-methoxyphenyl (MeO), and p-isopropylphenyl) and corrolazines RP₈CzH₃(TBP₈CzH₃and MeOP₈CzH₃)

is presented. The crystal structures of the monoprotonated, metal-free corrolazine [(TBP₈CzH₃)(H)]⁺[BArF]⁻, P^V(OMe)₂-

(MeOP₈Cz), and Mn^III(MeOP₈Cz)(MeOH) are presented. This work provides the ﬁrst insights into the inﬂuence of electronic

substituent eﬀects on the corrolazine periphery.

electronegativity arguments, while also adding stability to the

macrocyclic ring. Following the initial synthesis of the ﬁrst

corrolazine, it has been shown that the corrolazine ligand does

stabilize a number of high-valent metal complexes, including

copper(III), vanadium(V),¹⁸cobalt(IV),¹⁹iron(IV),^20,21iron-

(III),²²rhenium(V),²³chromium(V),²⁴and an isolable

manganese(V)-oxo complex²⁵(Figure 1). The Mn^V(O)

complex exhibited oxidative reactivity in oxygen atom transfer

(OAT) and hydrogen atom transfer (HAT) reactions, and this

complex provided a means to study the inﬂuence on reactivity

of a range of biomimetic modiﬁcations, including the addition

of axial ligands,²⁶⁻²⁸and the addition of protons and Lewis

acids.²⁹⁻³²However, the eﬀect of modifying the peripheral

substituents on the β-carbon atoms of the corrolazine ring was

not examined.

INTRODUCTION

■

Over a decade ago, our lab published the ﬁrst examples of ring-

contracted porphyrazines, which we named corrolazines in

analogy with both porphyrazines and corroles.¹We envisioned

that these porphyrinoid ligands would provide access to a

variety of new metal complexes and exhibit novel spectroscopic

features and reactivity that would be quite distinct from

porphyrins, porphyrazines, and corroles. One anticipated

property of particular interest to us was the stabilization of

high oxidation states at the central metal, which was also seen

in the analogous corrole²⁻¹⁰complexes. This property would

allow us to target species that mimic reactive intermediates in

both heme enzymes and synthetic catalysts, including high-

valent metal-oxo species analogous to Compound-I (Fe^IV(O)-

(porph^•+)) and Compound-II (Fe^IV(O)(porph)) in the

Cytochrome P450s, peroxidases, and catalases.¹¹⁻¹⁷We

hypothesized that the corrolazine ligand would also be useful

in new metalloporphyrinoid catalysts, especially for reactions

involving the oxidation of organic substrates. The inclusion of

nitrogen atoms at the meso positions was expected to aﬀord

enhanced reactivity of high-valent species based on simple

The variation of the substituents on the meso and β-positions

of porphyrins has been well-studied for their inﬂuence over the

properties of both the internal metal and the macrocyclic

Received: May 18, 2016

DOI: 10.1021/acs.inorgchem.6b01219

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Figure 1. General motif for porphyrazine and corrolazine complexes.

ring.³³⁻³⁶The spectroscopic eﬀects of varying the substituents

on the β-positions of porphyrazines,^37,38phthalocyanines,³⁹

tetrapyrazinocorrolazines,⁴⁰and triazatetrabenzcorroles⁴¹⁻⁴³

has also been examined. These kinds of substitutions were

made as part of a wide range of eﬀorts, including the

development of new dye-sensitized solar cells (DSSCs)⁴⁴⁻⁴⁶

and the synthesis of molecules for photodynamic therapy

(PDT).³⁴In addition, varying the substituents at the peripheral

positions allowed for the study of their inﬂuence on a wide

range of metal-mediated reactions, including the oxidation of

organic substrates.

Herein we describe the inﬂuence of remote substitution at

the peripheral β-carbon substituents on the spectroscopic

properties, redox potentials, and reactivity of phosphorus and

manganese corrolazines. The remote p-t-Bu groups on the eight

peripheral phenyl rings were substituted with p-MeO groups.

The HAT and OAT reactivity of Mn^V(O) complexes for the p-

MeO and p-t-Bu derivatives was examined. Even though the

para substituents on the β-carbon aryl rings are remote from

the metal center, it was found that they still exert a signiﬁcant

inﬂuence on the spectroscopic and reactivity properties of these

porphyrinoid compounds. The complete synthesis of the

peripherally substituted, metal-free corrolazine ligands

TBP₈CzH₃(TBP₈Cz = octakis(p-tert-butylphenyl)-

corrolazinato³⁻) and MeOP₈CzH₃(MeOP₈Cz = octakis(p-

methoxyphenyl)corrolazinato³⁻; Figure 1) is also presented.

The synthesis of the TBP derivative was described in brief in an

earlier report,⁴⁷but in this work we present the improved,

large-scale synthesis of this compound from commercially

available starting materials in one practical report. The same

synthetic strategy was used to prepare the para-methoxy

derivative, and these syntheses are described in suﬃcient detail

so as to make it easy for other laboratories to prepare the

corrolazine ligand. During the course of this work, new

porphyrazines (MeOP₈PzH₂, iPr₈PzH₂) were also synthesized,

as well as the new P^V, Mn^III, and Mn^V(O) corrolazines (Figure

1).

Innovative Technologies. Dichloromethane-d₂and chloroform-d₁

were obtained from Cambridge Isotopes, Inc. The synthesis of 9,10-

dihydro-10-methylacridine (AcrH₂),⁴⁹[H(OEt₂)₂][B{C₆H₃(m-

CF₃)₂}₄] (HBArF),⁵⁰Mn^III(TBP₈Cz), and Mn^V(O)(TBP₈Cz)⁴⁷

followed literature procedures. Xanthene (Xn) was obtained from

commercial sources and puriﬁed by recrystallization with ethanol, and

1,4-cyclohexadiene was ﬁltered through neutral alumina prior to

kinetics experiments. All other reagents were purchased from

commercial sources and used without further puriﬁcation.

Instrumentation. H NMR spectra were recorded on a Bruker

300 MHz or a Bruker Avance 400 MHz spectrometer. ¹³C NMR

spectra were recorded on a Bruker 300 MHz or a Bruker 400 MHz

spectrometer (operating frequency 101 or 151 MHz, respectively). All

¹H and ¹³C NMR spectra are referenced against residual proton signals

(¹H NMR) or the ¹³C resonances of the deuterated solvent (¹³C

NMR). ³¹P NMR spectra were obtained on a Bruker 400 MHz

(operating frequency 161.9 MHz) spectrometer and referenced against

an external standard of H₃PO₄(δ = 0). Elemental analyses were

performed at Atlantic Microlab, Inc, in Norcross, GA. Electrochemical

experiments were performed under an argon atmosphere using a BAS

100B electrochemical analyzer. Cyclic voltammograms were recorded

in CH₂Cl₂using a standard three-electrode cell with a glassy carbon

working electrode and tetrabutylammonium hexaﬂuorophosphate

(TBAPF₆, 0.1 M) as electrolyte unless otherwise noted. All potentials

are reported versus Fc⁺/Fc. UV−vis measurements were performed on

a Hewlett-Packard 8453 diode-array spectrophotometer with a 3.5 mL

air-free quartz cuvette (path length = 1 cm) ﬁtted with a glass stopper

or rubber septum. A 400 nm long-pass ﬁlter was employed to prevent

photo-decomposition by placing the ﬁlter directly between the sample

quartz cuvette and the spectrophometer light source. For reactions

with total reaction time of less than 10 s, stopped-ﬂow experiments

were performed using HiTech SHU-61SX2 (TgK Scientiﬁc Ltd.) with

a xenon light source and Kinetic Studio software. Laser desorption/

ionization mass spectrometry (LDI-MS) was collected on a Bruker

Autoﬂex III/TOF/TOF instrument equipped with a nitrogen laser at

335 nm using an MTP 384 ground steel target plate.

Single Crystal X-ray Crystallography. Reﬂection intensities

were measured at 110(2) K for P^V(MeOP₈Cz)(OMe)₂and

[(TBP₈CzH₃)(H)]⁺[BArF]⁻or 93(2) K for Mn^III(MeOP₈Cz)-

(HOMe) using a SuperNova diﬀractometer (equipped with Atlas

detector) with Cu Kα radiation (λ = 1.541 78 Å) under the program

CrysAlisPro (Version 1.171.36.32 Agilent Technologies, 2013). The

same program was used to reﬁne the cell dimensions and for data

reduction. The structure was solved with the program SHELXS-2013

and was reﬁned on F²with SHELXL-2013.⁵¹Analytical numeric

absorption correction based on a multifaceted crystal model was

applied using CrysAlisPro. The temperature of the data collection was

EXPERIMENTAL SECTION

■

General Procedures. All reactions were performed using dry

solvents and standard Schlenk techniques unless otherwise noted.

Dichloromethane, tetrahydrofuran (THF), diethyl ether, and methanol

were puriﬁed via a Pure-Solv solvent puriﬁcation system from

DOI: 10.1021/acs.inorgchem.6b01219

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controlled using the system Cryojet (manufactured by Oxford

Instruments). The H atoms were placed at calculated positions

using the instructions AFIX 43 or AFIX 137 with isotropic

displacement parameters having values 1.2 or 1.5 U_eqof the attached

C atoms.

was treated as an idealized CH₃group using the AFIX 137 instruction.

Most likely, the positions of those H atoms which are slightly diﬀerent.

The occupancy factors for all lattice solvent molecules were reﬁned

freely, and there is ca. 0.9 × CHCl₃and 1.39 × MeOH in the

asymmetric unit.

[(TBP₈CzH₃)(H)]⁺[BArF]⁻·4.18 Toluene Crystallographic Data.

The positions of the H atoms attached to N3, N4, N6, and N7 were

unambiguously determined by reconstructing the contoured diﬀerence

Fourier maps within the plane deﬁned by the three neighboring C−

N−C atoms (e.g., C5−N3−C8 for the H atom attached to N3). The

structure of [(TBP₈CzH₃)(H)]⁺[BArF]⁻is partly disordered. For the

corrolazine, two t-butyl groups (the six others are ordered) were found

to be disordered over two orientations, and the occupancy factors of

the major components of the disorder reﬁne to 0.588(7) and 0.683(4).

The BArF counterion is also disordered, as three −CF₃groups can be

found in two diﬀerent orientations. The occupancy factors of the

major components of the disorder reﬁne to 0.723(10), 0.663(9), and

0.787(5). The crystal lattice contains some solvent molecules. There

are ﬁve (but not all are fully occupied) toluene molecules in the

asymmetric unit, and their occupancy factors were either reﬁned freely

or were ﬁxed. Three of those molecules are treated as ordered, and

their occupancy factors reﬁne to 0.977(4), 0.909(4), and 0.793(4). A

fourth solvent molecule (fully occupied) is disordered over two

orientations, and the occupancy factor of the major component reﬁnes

to 0.823(4). A ﬁfth solvent molecule is found at sites of inversion

symmetry, and consequently the occupancy factor must be constrained

to 0.5 by symmetry. Overall, the ratio of corrolazine/BArF/toluene in

the crystal is 1:1:4.18.

Data for Mn^III(MeOP₈Cz)(HOMe): Fw = 1383.77, thin dark

brown plate, 0.61 × 0.24 × 0.02 mm³, triclinic, P1 (No. 2), a =

10.0064(5), b = 18.3946(9), c = 19.5794(9), α = 69.721(4), β =

86.165(4), γ = 80.387(4)°, V = 3332.9(3) Å³, Z = 2, D_x= 1.379 g

cm⁻³, μ = 3.158 mm⁻¹, T_min−T_max: 0.349−0.936. 42 229 Reﬂections

were measured up to a resolution of (sin θ/λ)_max= 0.62 Å⁻¹. 12 960

Reﬂections were unique (R_int= 0.0574), of which 9476 were observed

[I > 2σ(I)]. 1006 Parameters were reﬁned using 382 restraints. R1/

wR2 [I > 2σ(I)]: 0.0695/0.1949. R1/wR2 [all reﬂ]: 0.0920/0.2181. S =

1.042. Residual electron density found between −0.75 and 1.21 e Å⁻³.

Bis(4-methoxyphenyl)fumaronitrile. The synthesis of bis(4-

methoxyphenyl)fumaronitrile was previously reported by an alter-

native procedure, and all characterization data match the reported

literature values.⁵³An amount of (4-methoxyphenyl)acetonitrile (10.0

g, 0.0679 mol) and I₂(17.2 g, 0.0679 mol) was dissolved in 35 mL of

Et₂O. NaOMe in MeOH (55 mL, 3.1 M) was slowly added by an

addition funnel to the stirring solution at −78 °C. The reaction was

allowed to stir at −78 °C for 1.5 h, and at 0 °C for an additional 2 h.

The dark red solution slowly changed to yellow, and a precipitate

formed. The yellow solid was collected by ﬁltration through a Buchner

funnel, washed with 1:1 MeOH/H₂O, and dried under vacuum to

yield a bright yellow solid (5.98 g, 61%).

Bis(4-tert-butylphenyl)fumaronitrile. The synthesis of bis(4-

tert-butylphenyl)fumaronitrile was previously reported by an alter-

native procedure, and all characterization data match the reported

literature values.^48,54An amount of (4-tert-butylphenyl)acetonitrile

(20.6 g, 0.119 mol) and I₂(30.3 g, 0.119 mol) was dissolved in 500

mL of Et₂O. NaOMe in MeOH (3.04 M, 100 mL) was slowly added

by an addition funnel to the stirring solution at 0 °C. The dark red

solution slowly changed to yellow, and a precipitate formed. The white

Data for [(TBP₈CzH₃)(H)]⁺[BArF]⁻follow: Fw = 2608.10, dark

brown block, 0.45 × 0.24 × 0.12 mm³, triclinic, P1 (No. 2), a =

16.2718(3), b = 19.8352(3), c = 23.4803(3) Å, α = 99.8050(11), β =

109.3816(13), γ = 91.7165(12)°, V = 7013.9(2) Å3, Z = 2, D_x= 1.235

g cm⁻³, μ = 0.782 mm⁻¹, T_min−T_max: 0.796−0.927. 93 671 Reﬂections

were measured up to a resolution of (sin θ/λ_max) = 0.62 Å⁻¹. 27 531

Reﬂections were unique (R_int= 0.0249), of which 23 693 were

observed [I > 2σ(I)]. 1999 Parameters were reﬁned using 1005

restraints. R1/wR2 [I > 2σ(I)]: 0.0586/0.1689. R1/wR2 [all reﬂ]:

0.0664/0.1779. S = 1.025. Residual electron density found between

−0.58 and 0.78 e Å⁻³.

solid was collected via ﬁltration through a Buchner funnel and washed

with MeOH and dried under vacuum. A second batch of product

could be isolated after concentration of the ﬁltrate by approximately

half and then placing in a freezer (−20 °C) for 12 h. The second batch

was isolated in the same way as the ﬁrst batch to yield a white solid

(15.3 g, 75% yield).

P^V(MeOP₈Cz)(OMe)₂Crystallographic Data. The structure for

P^V(MeOP₈Cz)(OMe)₂is partly disordered. The methoxyphenyl

groups C17→C23, C24→C30, and C31→C37 are found to be

disordered over either two or three orientations, and the occupancy

factors of the major components of the disorder reﬁne to 0.653(7),

0.803(5), and 0.526(3), respectively. The crystal lattice contains some

very disordered solvent molecules (CH₂Cl₂), and their contribution

was taken out in the ﬁnal reﬁnement (SQUEEZE details are provided

in the CIF ﬁle).⁵²

Bis(4-isopropylphenyl)fumaronitrile. An amount of (4-

isopropylphenyl)acetonitrile (10 g, 0.063 mol) and I₂(16 g, 0.063

mol) was dissolved in 250 mL of Et₂O. NaOMe in MeOH (80 mL, 1.6

M) was slowly added by an addition funnel to the stirring solution at 0

°C. The reaction was stirred at −78 °C, and a solid immediately

precipitated out. The white solid was collected by ﬁltration through a

Buchner funnel, washed with MeOH, and dried under vacuum to yield

Data for P^V(MeOP₈Cz)(OMe)₂: Fw = 1240.27, dark lath, 0.45 ×

a white solid. A second batch of product could be isolated by placing

0.05 × 0.03 mm³, triclinic, P1 (No. 2), a = 14.8549(6), b =

the ﬁltrate in the freezer for 12 h (7.3 g, 74%). H NMR (400 MHz,

CDCl₃) δ 7.77 (d, ³J_HH= 8 Hz, 4H, Ar-H), 7.38 (d, ³J_HH= 8 Hz, 4H,

Ar-H), 2.99 (hept, ³J_HH= 7 Hz, 2H, CH(CH₃)₂), 1.29 (d, ³J_HH= 7 Hz,

12H, CH(CH₃)₂). ¹³C NMR (101 MHz, CDCl₃) δ 153.15, 129.79,

128.89, 127.46 (Ar-C), 124.61 (alkene-C), 117.19 (CN), 34.29

(CH(CH₃)₂), 23.80 (CH(CH₃)₂).

15.6119(18), c = 16.6521(7), α = 87.211(4), β = 66.674(4), γ =

74.628(4)°, V = 3412.4(5) Å³, Z = 2, D_x= 1.207 g cm⁻³, μ = 0.869

mm⁻¹, T_min−T_max: 0.822−0.977. 43 404 Reﬂections were measured up

to a resolution of (sin θ/λ)_max= 0.62 Å⁻¹. 13 452 Reﬂections were

unique (R_int= 0.0635), of which 9906 were observed [I > 2σ(I)]. 1103

Parameters were reﬁned using 1150 restraints. R1/wR2 [I > 2σ(I)]:

0.0573/0.1466. R1/wR2 [all reﬂ]: 0.0798/0.1623. S = 1.014. Residual

electron density found between −0.38 and 0.76 e Å⁻³.

3,4-Bis(2-methoxy)pyrroline-2,5-diimine. Bis(4-

methoxyphenyl)fumaronitrile (5.01 g, 0.0173 mol) was placed in

330 mL of ethylene glycol and heated to 115 °C. An amount of

sodium metal (0.0530 g, 0.00268 mol) was added to the

heterogeneous reﬂuxing solution. Ammonia gas was rapidly bubbled

through the solution for 1.25 h, and the reaction mixture turned a dark

blue and became homogeneous. The solution was ﬁltered hot through

Mn^III(MeOP₈Cz)(HOMe)·0.9CHCl₃·1.39MeOH Crystallo-

graphic Data. The H atoms from the MeOH molecules could not

be successfully retrieved. The structure is partly disordered. The

methoxyphenyl group C24→C30 and coordinated methanol molecule

are found to be disordered over two orientations, and the occupancy

factors of the major components of the disorder reﬁne to 0.765(12)

and 0.742(10). The crystal lattice contains some amount of lattice

solvent molecules (ordered MeOH and disordered CHCl₃). The B

level alert suggests one unusually short H···H contact. H74C belongs

to one partially occupied (occupancy factor: 0.607(10)) lattice MeOH

solvent molecule. Thus, it is very diﬃcult to determine accurately the

positions of the H atoms attached to C74 from X-ray data, and C74

a Buchner funnel to remove any unreacted starting material. The

ﬁltrate was added to ice water to induce precipitation and allowed to

sit for 45 min. The solid was collected on a Buchner funnel and

washed with copious amounts of water. The solid was left to dry for 12

h under vacuum to yield a light yellow solid (3.90 g, 73% yield).

3,4-Bis(2-tert-butylphenyl)pyrroline-2,5-diimine. The com-

pound was synthesized via the same procedure as 3,4-bis(2-

methoxy)pyrroline-2,5-diimine starting with bis(4-tert-butylphenyl)-

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fumaronitrile (4.72 g, 0.0138 mol). The product was isolated as a pale

yellow solid (2.55 g, 51% yield). All characterization data match the

literature values.^48,54

An amount of Mg^II(TBP₈Pz) (6.88 g, 0.00492 mol) was dissolved in

CH₂Cl₂(20 mL) and TFA (30 mL) and stirred for 3 h, in which time

the solution turned dark blue. CH₂Cl₂was removed by evaporation

under a stream of air. The reaction mixture was poured into ice water,

2,3,7,8,12,13,17,18-Octakis(4-methoxyphenyl) porphyrazine

(MeOP₈PzH₂). Route 1. MeOP₈PzMg was synthesized previously via

an alternative procedure.³⁷Magnesium metal (0.0446 g, 1.85 mmol)

and a small chip of I₂were reﬂuxed in n-BuOH (10 mL) under argon

until all the Mg metal had been consumed (∼2 h), yielding a white

suspension. An amount of 3,4-bis(2-methoxy)pyrroline-2,5-diimine

(1.41 g, 4.60 mmol) was added, and the reaction mixture was reﬂuxed

for 12 h. The solution immediately turned dark yellow and then slowly

converted to a dark green-blue. The n-BuOH was removed under

vacuum, and the residue was dissolved in CH₂Cl₂(60 mL). Excess

CF₃CO₂H (TFA; 14 mL) was added, and the reaction mixture turned

from dark green-blue to a dark blue. The reaction mixture was stirred

in the dark for 12 h, the CH₂Cl₂was removed by a stream of air, and

the remaining blue slurry was poured over ice water (200 mL) and

neutralized with concentrated ammonium hydroxide. The solid was

collected by ﬁltration and washed thoroughly with water and MeOH

and then dried under vacuum to yield MeOP₈PzH₂as dark purple

solid (0.984 g, 53%).

neutralized with NH OH, and ﬁltered through a Buchner funnel. The

solid was washed with copious amounts of water and MeOH and dried

under vacuum to yield a dark green solid (3.50 g, 52%).

2,3,7,8,12,13,17,18-Octakis(4-isopropylphenyl) porphyra-

zine (iPrP₈PzH₂). Magnesium metal (0.153 g, 0.00637 mol) was

added to 13 mL of a 5:1 1-octanol/1-pentanol solution. The reaction

mixture was reﬂuxed until all the metal had reacted, yielding a viscous

gray suspension. Bis(4-isopropylphenyl)fumaronitrile (2.00 g, 0.00636

mol) was added, and the reaction mixture was reﬂuxed for 2 h, in

which time the solution turned dark green-blue. The solvent was

removed via vacuum distillation to yield a blue-green solid (UV−vis

(CH₂Cl₂) λ_max[nm] 379, 640; LDI-MS [M⁺] m/z = 1281.97 calcd for

C₈₈H₈₈N₈Mg 1282.30). The solid was dissolved in CH₂Cl₂(20 mL)

and TFA (10 mL), and the solution immediately turned to a dark blue

and was left to stir in the dark for 12 h. The CH₂Cl₂was evaporated,

and the blue solution was poured on ice and neutralized with NH₄OH.

The reaction mixture was ﬁltered through a Buchner funnel and

washed with copious amounts of MeOH. The solid was dried under

vacuum to yield a dark blue-green solid. (1.6 g, 82%). Well-resolved

NMR spectroscopic data could not be obtained because iPrP₈PzH₂is

poorly soluble in common solvents. UV−vis (CH₂Cl₂) λ_max[nm] 371,

454, 604, 670. LDI-MS [M]⁺m/z = 1259.74 calcd for C₈₈H₉₀N₈

1259.74.

Route 2. Magnesium metal (0.367 g, 0.0151 mol) was added to 30

mL of a 5:1 1-octanol/1-pentanol solution. The reaction mixture was

reﬂuxed until all the metal had reacted, yielding a viscous gray

suspension. Bis(4-methoxyphenyl)fumaronitrile (4.00 g, 0.0138 mol)

was added to the reaction mixture and allowed to reﬂux for 12 h. The

reaction mixture turned dark brown-purple. The reaction mixture was

poured into ∼500 mL of MeOH to induce precipitation and ﬁltered

[P^V(MeOP₈Cz)(OH)][OH]. MeOP₈PzH₂(1.09 g, 0.869 mmol) was

dissolved in pyridine (200 mL) and heated to reﬂux and then PBr₃

(1.8 mL, 19 mmol) was added by syringe directly into the reﬂuxing

reaction. The reaction was reﬂuxed 12 h, in which time the solution

turned from dark green to dark red. An additional 30 equiv of PBr₃

(2.8 mL, 30 mmol) was added, the reaction was reﬂuxed for an

additional 6 h, then a ﬁnal addition of 30 equiv of PBr₃(2.8 mL, 30

mmol) was added, and the reaction mixture was reﬂuxed for 12 h. The

reaction mixture was cooled to room temperature, methanol was

added via syringe to quench the reaction, and the solution became

dark green. The reaction mixture was reduced under vacuum to

dryness; the remaining dark brown-green solid was dissolved in

CH₂Cl₂and ﬁltered through a silica plug, and the plug was washed

with 8:2 CH₂Cl₂/EtOAc. A small amount of solid silica was added to

the ﬁltrate and this mixture was reduced to dryness. The product fused

on silica was puriﬁed on a silica gel column with 8:2 CH₂Cl₂/EtOAc

(R_f= 0.82). The solid was collected, washed with CH₃CN, and dried

through a Buchner funnel, and the solid was washed with copious

amounts of methanol until the ﬁltrate was colorless. The blue-green

solid was washed with pentane and dried under vacuum (2.60 g, 65%

yield). H NMR, UV−vis, and LDI-MS matched previously reported

data for Mg^II(MeOP₈Pz).³⁷

Mg^II(MeOP₈Pz) (2.60 g, 0.002 19 mol) was dissolved in CH₂Cl₂

(50 mL) and excess TFA (20 mL). The solution instantly turned to a

dark blue and was left to stir in the dark for 3 h. The CH₂Cl₂was

evaporated, and the blue solution was poured into 400 mL of ice water

and neutralized with NH₄OH. The reaction mixture was ﬁltered

through a Buchner funnel and washed with copious amounts of

MeOH. The solid was dried under vacuum to yield a dark purple solid

(1.41 g, 55%). Well-resolved NMR spectroscopic data could not be

obtained because MeOP₈PzH₂is poorly soluble in common solvents.

UV−vis (pyridine) λ_max[nm] 381, 522, 636, 692, 719. Anal. Calcd for

C₇₂H₅₈N₈O₈: C, 74.34; H, 4.63; N, 8.05. Found: C, 73.99; H, 5.03; N,

9.63%. LDI-MS [M⁺] m/z = 1163.34 Calcd for C₇₂H₅₈N₈O₈1163.30.

2,3,7,8,12,13,17,18-Octakis(4-tert-butylphenyl) porphyra-

zine (TBP₈PzH₂). Route 1. TBP₈PzH₂was synthesized previously

via an alternative procedure.^37,47,55,56Magnesium metal (0.0420 g, 1.72

mmol) and a small chip of I₂were reﬂuxed in n-BuOH (12 mL) until

all the Mg metal had been consumed, yielding a white suspension. An

amount of 3,4-bis(2-tert-butylphenyl)pyrroline-2,5-diimine (2.00 g,

5.84 mmol) was added to the reaction mixture, and the reaction

immediately turned dark yellow and then slowly converted to a dark

green-blue. The reaction was reﬂuxed for 24 h. The n-BuOH was

removed under vacuum, and the residue was dissolved in excess TFA

(15 mL) and stirred in the dark for 12 h. The reaction mixture was

poured over ice water and neutralized with concentrated NH₄OH. The

solid was collected by ﬁltration, washed thoroughly with water and

MeOH, and dried under vacuum to yield a dark green solid (1.20 g,

63%).

under vacuum to yield a dark green solid (0.555 g, 49%). H NMR

(300 MHz, CD₂Cl₂) δ 8.37−8.24 (m, 8H, Ar-H), 8.13 (d, J_HH= 8.8

Hz, 4H, Ar-H), 7.26−7.17 (m, 8H, Ar-H), 7.10 (d, ³J_HH= 8.9 Hz, 4H,

Ar-H), 7.04 (vb s, 4H, Ar- H), 6.68 (d, ³J_HH= 8.3 Hz, 4H, Ar-H), 4.00

(s, 6H, Ar-OCH₃), 3.99 (s, 6H, Ar-OCH₃), 3.94 (s, 6H, Ar-OCH₃),

3.91 (s, 6H, Ar-OCH₃). ¹³C NMR (101 MHz, CDCl₃) δ 160.00,

159.88, 159.61, 146. 01, 144.85, 142.33, 139.13, 136.10, 134.55,

134.42, 134.08, 132.33, 126.08, 125.77, 114.06, 113.90 (all s, all related

to Ar-carbons, eight aryl-carbon signals missing due to coincidental

overlap), 55.56 (s, Ar−OCH₃), 55.49 (s, Ar−OCH₃), 55.07 (s, Ar−

OCH₃), one methoxy carbon signal missing due to coincidental

overlap. ³¹P NMR (162 MHz, CDCl₃) δ −111.35. Cyclic voltammetry

(CV; CH₂Cl₂, 50 mV): E_1/2= 0.65 V, E_1/2= −1.27 V, E_1/2= −1.78 V.

UV−vis (CH₂Cl₂) λ_max[nm] (ε × 10⁻⁴M⁻¹cm⁻¹) 461 (8.2), 557

(0.96), 594 (1.2), 644 (4.5). Anal. Calcd for C₇₂H₅₈N₇O₁₀P: C, 71.34;

H, 4.82; N, 8.09. Found: C, 71.13; H, 4.98; N, 7.94%. LDI-MS m/z =

[M − OH]⁺1194.73 calcd for C₇₂H₅₇N₇O₉P 1193.39.

Route 2. Magnesium metal (0.594 g, 0.0245 mol) was added to 53

mL of a 5:1 1-octanol/1-pentanol solution. The reaction mixture was

reﬂuxed until all the metal reacted, yielding a viscous gray suspension.

An amount of bis(4-tert-butylphenyl)fumaronitrile (8.21 g, 0.0240

mol) was added, and the reaction mixture was reﬂuxed for 3 h. The

solvent was removed via vacuum distillation, and the residual solid was

P^V(MeOP₈Cz)(OMe)₂. Crystals of P^V(MeOP₈Cz)(OMe)₂were

obtained via slow diﬀusion of MeOH into a CH₂Cl₂solution of

[P^V(MeOP₈Cz)(OH)][OH]. The sample was allowed to sit in the

dark for ∼2 d. The solid was collected on a Buchner funnel, washed

with MeOH and hexanes, and allowed to dry under vacuum yielding a

washed with H O, collected on a Buchner funnel, washed with MeOH

dark purple solid. ¹H NMR (400 MHz, benzene-d₆) δ 8.71 (d, ³J_HH

8.8 Hz, 3H, Ar-H), 8.66 (d, ³J_HH= 8.8 Hz, 3H, Ar-H), 8.58 (d, ³J_HH

and pentane, and dried under vacuum to give Mg^II(TBP₈Pz) as a blue-

green solid (6.88 g, 83% yield). UV−vis and LDI-MS matched

previously reported data for Mg^II(TBP₈Pz).^37,57

8.8 Hz, 3H, Ar-H), 7.48 (d, ³J_HH= 8.5 Hz, 4H, Ar-H), 7.12 (s, 2H, Ar-

H), 7.05 (d, ³J_HH= 8.9 Hz, 3H, Ar-H), 6.59−6.54 (m, 4H, Ar-H), 3.47

DOI: 10.1021/acs.inorgchem.6b01219

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Article

Scheme 1. Synthetic Routes for the RP₈PzH₂

(two s, 12H, each a Ar-OCH₃), 3.43 (s, 6H, Ar-OCH₃), 3.37 (s, 6H,

the reaction mixture, and the solution was allowed to slowly warm to

room temperature to remove the ammonia. The reaction solution was

Ar-OCH₃), −1.69 (d, J_HP= 21.9 Hz, 6H, axial-OCH₃). ³¹P NMR

(162 MHz, C₆DC₆) δ −192.7. UV−vis (CH₂Cl₂) λ_max[nm] 451, 594,

644. LDI-MS m/z = [M − OMe]⁺1208.69 calcd for C₇₃H₅₉N₇O₉P

1208.41.

ﬁltered through a Buchner funnel, and the ﬁltrate was removed in

vacuo. A ﬂash silica column was run with 40:60 hexanes/CH₂Cl₂(R_f=

0.75) to yield a ﬂat green solid (0.067 g, 47%). All characterization

data match the previously reported literature.¹

[P^V(TBP₈Cz)(OH)]OH. Two reactions were run simultaneously and

combined prior to puriﬁcation. To a reﬂuxing solution of compound

TBP₈PzH₂(1.20 g, 0.875 mmol and 0.900 g, 0.656 mmol) in pyridine

(250 mL) was added PBr₃(2.85 and 2.14 mL, 35 equiv) by syringe

directly into the reaction solution and reﬂuxed for 24 h, and the

reaction mixture changed from dark green to dark red. Additional PBr₃

(2.85 and 2.14 mL, 35 equiv) was added, and the reaction was reﬂuxed

another 24 h. The reaction mixture was cooled to room temperature,

and methanol was added by syringe to quench the reaction, upon

which the solution became dark green. The two reaction mixtures were

combined and reduced under vacuum to dryness. The remaining dark

brown-green solid was dissolved in CH₂Cl₂and ﬁltered through a

[(TBP₈CzH₃)(H)][BArF]. To a solution of the TBP₈CzH₃(15 mg,

11 μmol) in 750 μL toluene was added 1.5 equiv of HBArF. A color

change from green to brown was observed. This solution was layered

with n-heptane, and X-ray quality crystals were obtained after a week.

¹H NMR (400 MHz, CD₂Cl₂) δ (ppm) 12.68 (vbs, 1H, Cz-H), 7.91

(d, ³J_HH= 8 Hz, 4H, Ar-H), 7.82 (d, ³J_HH= 8 Hz, 12H, Ar-H), 7.70 (d,

³J_HH= 8 Hz, 4H, Ar-H), 7.61 (d, ³J_HH= 8 Hz, 4H, Ar-H), 7.53 (d, ³J_HH

= 8 Hz, 4H, Ar-H), 7.24 (d, ³J_HH= 8 Hz, 4H, Ar-H), 2.47 (s, 2H, Cz-

H), 1.49, 1.48, 1.44, 1.30 (each a s, each 18H, t-Bu-H). UV−vis

(CH₂Cl₂): λ_max(nm) 450, 740. LDI-MS m/z = [M]⁺1360.82 calcd for

C₉₆H₁₀₈N₇1359.88.

Mn^III(HOMe)(MeOP₈Cz). MeOP₈CzH₃(0.0175 g, 0.0152 mmol)

was dissolved in 10 mL of CH₂Cl₂and 1 mL of pyridine. Mn(acac)₃

(0.0543 g, 0.154 mmol; acac = acetylacetonate) was added to the

reaction mixture and was stirred for 12 h in the dark under air. The

solvent was removed in vacuo to yield a brown solid. The crude brown

solid was puriﬁed by ﬂash chromatography on silica gel with 8:2

CH₂Cl₂/EtOAc (R_f= 0.80). The product was dried and crystallized

from CHCl₃:MeOH to yield Mn^III(HOMe)(MeOP₈Cz) as a brown

crystalline solid (0.0081 g, 43% yield). CV data (CH₂Cl₂, 100 mV)

E_1/2= 0.198 V, E_1/2= 0.678 V, E_1/2= −1.71 V. UV−vis (CH₂Cl₂) λ_max

[nm] (ε × 10⁻⁴M⁻¹cm⁻¹) 446 (5.50), 690 (2.48) with methanol as

the axial ligand. UV−vis (CHCl₃) λ_max[nm] (ε × 10⁻⁴M⁻¹cm⁻¹) 454

(5.96), 646 (1.12), 699 (1.53) with pyridine as the axial ligand. Anal.

Calcd for C₇₂H₆₀N₇O₉Mn·0.5CHCl₃: C, 69.01; H, 4.51; N, 7.77.

Found: C, 69.29; H, 4.85; N, 7.68%. LDI-MS m/z = [M − HOMe]⁺

1202.49 calcd for C₇₂H₅₆N₇O₈Mn 1202.22.

Buchner funnel, and the ﬁltrate was reduced to dryness. The residual

solid was puriﬁed via chromatography with 99:1 CH₂Cl₂/MeOH on

silica gel (R_f= 0.55) and dried to yield a lustrous green solid (1.34 g,

62%). All characterization data match the previously reported

literature.¹

MeOP₈CzH₃. To a solution of [P^V(MeOP₈Cz)(OH)][OH] (0.105

g, 0.0862 mmol) in 30 mL of liquid ammonia at −78 °C was added

sodium metal (0.0595 g, 2.59 mmol), and the mixture was stirred for

30 min. THF (20 mL) was added to the reaction mixture, and the

solution was stirred for an additional 40 min at −78 °C. The reaction

solution turned from dark green to a dark purple. NH₄Cl (0.641 g,

12.0 mmol) was added to the reaction mixture and was allowed to

warm to room temperature to remove the ammonia. The reaction

solution was ﬁltered through a Buchner funnel, and the ﬁltrate was

removed in vacuo. A ﬂash silica column was run with 95:5 CH₂Cl₂/

EtOAc (R_f= 0.93) to separate the desired product from unreacted

starting material. A ﬂat green solid (0.0175 g, 17.6%) was obtained.

The solid was used without any further puriﬁcation. UV−vis

(pyridine): λ_max(nm) 467, 671. UV−vis (CH₂Cl₂): λ_max(nm) 452,

623, 664, 684. LDI-MS (pyridine) m/z = 1150.8 calcd for C₇₂H₅₉N₇O₈

1150.31.

Mn^V(O)(MeOP₈Cz). Mn^III(MeOP₈Cz)(HOMe) (0.0065 g, 0.0053

mmol) was dissolved in 2 mL of CHCl₃. PhIO (0.0028 g, 0.013 mmol)

was added, and the reaction mixture was stirred for 15 min, during

which time the solution slowly went from a dark brown to a dark

green. The solution was ﬁltered through glass wool and dried. The

dark green-brown solid was washed with NCMe and dried. The

remaining solid was puriﬁed by ﬂash chromatography on silica column

with 9:1 CHCl₃/EtOAc (R_f= 0.76). The product was dried under

TBP₈CzH₃. To a mixture of [P(TBP₈Cz)(OH)][(OH)] (0.150 g,

0.106 mmol) in 15 mL of liquid ammonia cooled to −78 °C was

added sodium metal (0.090 g, 3.9 mmol) and the mixture was stirred

for 30 min. THF (10 mL) was added and the mixture was stirred for

an additional 30 min at −78 °C. The reaction solution turned from

dark green to a red-brown. NH₄Cl (0.750 g, 14.0 mmol) was added to

vacuum to yield a dark green solid (0.0014 g, 22% yield). H NMR

(400 MHz, CD₂Cl₂) δ 8.42 (d, ³J_HH= 8 Hz, 4H, Ar-H), 8.36 (d, ³J_HH

= 8 Hz, 4H, Ar-H), 8.21 (d, ³J_HH= 8 Hz, 4H, Ar-H), 7.42 (bs, 4H, Ar-

DOI: 10.1021/acs.inorgchem.6b01219

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Article

Scheme 2. General Synthesis of RP₈CzH₃

H), 7.22 (d, ³J_HH= 8 Hz, 8H, Ar-H), 7.12 (d, ³J_HH= 8 Hz, 4H, Ar-H),

6.68 (d, ³J_HH= 8 Hz, 4H, Ar-H), 4.01, 4.00, 3.95, 3.91 (each a s, each

6H, Ar-OCH₃). CV (CH₂Cl₂, 25 mV) E_1/2= 0.07 V, E_1/2= 0.45 V,

E_1/2= −0.57 V. UV−vis (CH₂Cl₂) λ_max[nm] (ε × 10⁻⁴M⁻¹cm⁻¹)

436 (4.53), 636 (1.54). LDI-MS m/z = 1217.008 calcd for

C₇₂H₅₆MnN₇O₉1217.352.

As shown in Scheme 1, the ﬁrst step involves preparing the

dinitrile precursor by an oxidative coupling of the phenyl-

acetonitrile derivative. The reaction of 4-R-phenylacetonitrile

(R = t-Bu, MeO, or iPr) in the presence of excess NaOMe and

I₂leads to the dinitrile product.^48,53This product crashes out of

solution when cooled, and is then collected by ﬁltration,

thoroughly washed with methanol and water to remove any

unreacted starting materials and impurities, and dried under

vacuum to yield the dinitrile as a white or yellow solid (t-Bu:

75%; MeO: 61%; iPr: 74%).

Two diﬀerent methods to synthesize the metal-free

porphyrazine from the dinitrile are shown in Scheme 1. The

conventional method for synthesizing metal-free porphyrazine

involves converting the dinitrile starting material into the

pyrroline-2,5-diimine (blue pathway, Scheme 1), which was the

original synthesis employed for TBP₈PzH₂.^1,48,54This method

involves a high-temperature reaction with ethylene glycol and a

large excess of ammonia gas, giving the pyrroline product in

reasonable yields (t-Bu: 51%; MeO: 73%). The pyrroline is

then cyclized in reﬂuxing n-BuOH with a Mg^IItemplate. The

mechanism of the cyclization has been discussed previously for

phthalocyanines and triazatetrabenzcorroles.⁵⁷The cyclized

product, RP₈PzMg, is isolated by vacuum distillation and

demetalated by treatment with excess TFA to form the metal-

free porphyrazine as a ﬂuﬀy solid, RP₈PzH₂(t-Bu: 63%; MeO:

53%). Although this pathway yields the desired porphyrazines,

it has signiﬁcant drawbacks. The synthesis of the pyrroline

requires the use of large quantities of highly reactive and toxic

NH₃(g), and the yields of pyrroline are highly variable.

Puriﬁcation of the pyrroline is hampered by the diﬃculties of

removing viscous and high-boiling ethylene glycol. The yields

of porphyrazine also vary greatly, depending upon the quality of

the Mg/butanol suspension, which is sensitive both to moisture

and the quality of the magnesium source.

A new, more eﬃcient strategy to synthesize the symmetri-

cally substituted porphyrazines was devised that avoids the

pyrroline step and can be performed on a much larger scale.

Our new synthetic strategy is based on the synthesis of certain

phthalocyanine and mixed phthalocyanine-porphyrazine hy-

brids.⁵⁷The method involves direct cyclization of dinitrile,

avoiding the preparation of the pyrroline precursor. The

dinitrile is cyclized in a mixture of 1-octanol and 1-pentanol

(5:1 v/v) around a Mg^IItemplate at high temperature (180

°C). Heating of dinitrile in this mixture gives Mg^IIporphyrazine

for the iPr and t-Bu derivatives, after 2−3 h, and the MeO

derivative, after 24 h (Scheme 1). The Mg^IIporphyrazine is

then isolated by vacuum distillation and puriﬁed by simple

solvent extraction (MeOH) without the need for column

chromatography. We optimized this procedure to work on a

Kinetics Studies. General procedure. The pseudo-ﬁrst-order

rate constants k_obswere obtained by nonlinear least-squares ﬁtting of

the plots of both the growth of Mn^III(MeOP₈Cz) and decay of

Mn^V(O)(MeOP₈Cz) absorbances (Abs) versus time (t) according to

the equation Abs_t= Abs_f+ (Abs₀− Abs_f)exp(−k_obst), where Abs₀and

Abs_fare initial and ﬁnal absorbance, respectively. Second-order rate

constants (k) were obtained from the slope of the best-ﬁt line from a

plot of k_obsversus substrate concentration. The same kinetics analysis

was employed for the stopped-ﬂow UV−vis studies. Pseudo-ﬁrst-order

k_obsvalues were obtained and exhibited a linear correlation with

substrate concentrations.

Reaction of Mn^V(O)(MeOP₈Cz) with C−H Substrates.

Mn^V(O)(MeOP₈Cz) (30 μM, CH₂Cl₂) was reacted with 1,4-

cyclohexadiene (1,4-CHD; 0.20−1.0 mM), Xn (0.26−1.5 mM), or

AcrH₂(0.31−6.7 mM). The spectral changes showed isosbestic

conversion of Mn^V(O)(MeOP₈Cz) (λ_max= 436 and 636 nm) to

Mn^III(MeOP₈Cz) (λ_max= 446 and 689 nm).

Reaction of Mn^V(O)(MeOP₈Cz) with Triarylphosphines.

Mn^V(O)(MeOP₈Cz) (16−30 μM, CH₂Cl₂) was reacted with varying

amounts of substituted triarylphosphine (0.20−1.5 mM). The spectral

changes showed isosbestic conversion of Mn^V(O)(MeOP₈Cz) (λ_max

436 and 636 nm) to Mn^III(MeOP₈Cz)(OPAr₃) (λ_max= 444 and 690

nm).

RESULTS AND DISCUSSION

■

Synthesis of Symmetrically Substituted Porphyra-

zines. Both symmetrically and asymmetrically substituted

porphyrazines have been synthesized by using a variety of

strategies.^{37,48,55,58−60}The synthesis of porphyrazines typically

begins with an appropriate dinitrile precursor, which is then

either directly cyclized into the porphyrinoid product, or ﬁrst

converted to a more reactive pyrroline intermediate. In our

earlier work, we relied upon a general procedure that followed

the pyrroline method for the synthesis of the p-tert-butyl-

substituted porphyrazine,¹although we did not report the step-

by-step details of this synthesis. Herein we describe the detailed

syntheses of symmetrically substituted metal-free porphyrazines

with eight p-substituted phenyl groups attached at the β-carbon

atoms, RP₈PzH₂(R = p-tert-butylphenyl (t-Bu), p-methox-

yphenyl (MeO), p-isopropylphenyl (iPr)), on a large (3−7 g)

scale. These syntheses involve a new method that does not

require the pyrroline step, as shown in Scheme 1. Note that the

p-methoxyphenyl derivative was reported previously with Mg^II

or P^Vas the central atom, but these compounds were made by

the more tedious pyrroline method on a much smaller scale,

and no report of the metal-free compound was made.^37,54

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large scale, typically 5−7 g of dinitrile, compared to the ∼1 g

maximum scale of the pyrroline pathway.¹The ﬁnal metal-free

porphyrazine was obtained by stirring MgPz in TFA followed

by workup to yield R₈PzH₂as a dark solid in good yield (t-Bu:

52%; MeO: 55%; iPr: 82%.) This method takes 2−3 days,

compared to the pyrroline pathway, which takes at least one to

two weeks to convert the dinitrile to metal-free porphyrazine.

Synthesis of TBP₈CzH₃. The preparation of the ﬁrst metal-

free corrolazine, TBP₈CzH₃, was reported by us in 2001.¹

However, a complete detailed synthesis of the corrolazine,

beginning from commercially available reagents, was not given

in a single, convenient report. In the current work we ﬁll in this

gap by describing the complete, detailed synthesis of metal-free

corrolazine from commercial precursors (Scheme 2). The

synthesis has also been signiﬁcantly improved since the initial

report, including the improved method of porphyrazine

synthesis described above. The corrolazine synthesis begins

with the “ring contraction” of metal-free porphyrazine by

reﬂuxing with excess PBr₃in pyridine for 2 d. The PBr₃needs

to be added in successive amounts (35 equiv), with a 24 h

period of reﬂux between each addition. These conditions lead

to extrusion of a meso-N atom and reclosure of the ring around

a phosphorus template, which is inserted in the internal cavity.

The mechanism of the ring contraction has been discussed, and

it is proposed that the ﬁrst step is insertion of the phosphorus

to yield a phosphorus porphyrazine (observed in LDI-MS),

followed by ring contraction via nucleophilic attack of PBr₃at

the meso nitrogen to form [PNBr₃]⁻and the desired

phosphorus corrolazine.⁶¹After workup, the crude product is

puriﬁed on a silica gel column (eluent 99:1 CH₂Cl₂/MeOH),

yielding the phosphorus(V) corrolazine [P^V(TBP₈Cz)(OH)]-

OH (conﬁrmed by ³¹P NMR spectroscopy) as a shiny, dark

green solid (62%).

Figure 2. UV−vis spectrum of TBP₈CzH₃(blue line) and

[TBP₈CzH₄][BArF] (red line) in CH₂Cl₂.

Removal of the P^Vion is challenging, since the usual methods

of porphyrin demetalation involving strong acids do not work.¹

However, [P^V(TBP₈Cz)(OH)]OH can be demetalated via

reduction. Reaction of the P^Vcompound with sodium metal in

NH₃/THF at −78 °C followed by quenching with NH₄Cl and

warming to room temperature yields a bright green solution of

crude metal-free corrolazine. Puriﬁcation on silica with 60:40

CH₂Cl₂/hexanes yields TBP₈CzH₃as a dark green solid (47%).

Protonation and Crystal Structure of TBP₈CzH₃.

Numerous attempts to crystallize the metal-free corrolazine

led only to precipitation of amorphous solid. However, we

knew from previous work that Mn^III(TBP₈Cz) can be mono-

and diprotonated at the meso-nitrogen positions by addition of

a strong acid source, and these protonated forms were

amenable to crystallization for single crystal X-ray diﬀraction

(XRD) studies.³²Thus, we attempted the protonation of the

metal-free compound, and found that addition of HBArF (1

equiv) (BArF = tetrakis[3,5-bis(triﬂuoromethyl)phenyl)borate)

to a CH₂Cl₂solution of TBP₈CzH₃resulted in a color change

from green to red, with distinct shifts in the UV−vis spectrum

(Figure 2). Both Soret and Q-bands are red-shifted, with a

more dramatic shift observed for the Q-band. These shifts are

consistent with protonation of a meso-nitrogen atom on the

corrolazine ring.^{23,32,56,62−64}Layering of a toluene solution of

[TBP₈CzH₄][BArF] with n-heptane gave dark-colored crystals

after one week. The crystal structure of the monoprotonated

metal-free [TBP₈CzH₄][BArF] is shown in Figure 3. Only a

few crystal structures of metal-free porphyrazine⁶⁵or

corrole^2,66−71have been reported, and the structure in Figure

3 is the ﬁrst example of a crystallographically characterized

Figure 3. (a) Displacement ellipsoid plot (50% probability level) of

the [TBP₈CzH₄]⁺cation at 110(2) K. Hydrogen atoms, except for N−

H hydrogen, and disorder are omitted for clarity. (b) Edge-on view

down the plane of the Cz ring, with hydrogen atoms, except for N−H

hydrogen, and tert-butylphenyl groups omitted for clarity.

metal-free corrolazine. A single BArF counterion was found in

the asymmetric unit, which, by charge considerations, indicate

monoprotonation of the corrolazine ring. The C−C and C−N

bond distances of the corrolazine ligand in [TBP₈CzH₄][BArF]

are similar to those of Mn^III(TBP₈Cz), [Mn^III(TBP₈Cz(H))]-

[BArF], and [Mn^III(TBP₈Cz(H)₂)][BArF]₂,¹indicating that

the aromaticity of the ligand is not disrupted upon protonation

(Figure S4 and Table S1). The four acidic protons associated

with [TBP₈CzH₄]⁺were located unambiguously from diﬀer-

ence Fourier maps. Surprisingly, only two of the pyrrole N−H

protons were located inside the corrolazine cavity. The third

pyrrole N−H proton was located on a meso-N atom, along with

the additional proton from HBArF, which was found on the

opposite meso-nitrogen. An edge-on view of the structure is

shown in Figure 3, revealing a planar macrocycle, with the two

NH protons on opposite sides of the cavity. In contrast, the

structures of metal-free corroles are signiﬁcantly puckered due

to the steric strain of the three pyrrolic N−H protons in the

internal cavity.^2,66−71

DOI: 10.1021/acs.inorgchem.6b01219

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Article

Synthesis of MeOP₈CzH₃. The new corrolazine

[P^V(MeOP₈Cz)(OH)]OH was prepared by a route similar to

[P^V(TBP₈Cz)(OH)]OH, with a slight modiﬁcation of the PBr₃

ring-contraction step (see Experimental). Puriﬁcation on silica

with CH₂Cl₂/EtOAc (8:2 v/v) yields the ring-contracted

product as a ﬂat green solid (49%; Scheme 2). The UV−vis

spectrum for [P^V(MeOP₈Cz)(OH)]OH yields two diagnostic

peaks expected for a corrolazine ring, a Soret band at 461 nm (ε

= 8.2 × 10⁴M⁻¹cm⁻¹), and a Q-band at 644 nm (ε = 4.5 × 10⁴

redox potentials, with the largest shift being 80 mV for the

second reduction wave. The data show that substitution of p-t-

Bu for p-MeO groups inﬂuences both ring-based and metal-

based redox potentials, indicating that the MeO derivative is

more electron-rich than the t-Bu compound.

Crystals of P^V(MeOP₈Cz)(OMe)₂suitable for XRD were

obtained via slow diﬀusion of MeOH into a CH₂Cl₂solution of

[P^V(MeOP₈Cz)(OH)]OH. The structure of P^V(MeOP₈Cz)-

(OMe)₂reveals the expected ring-contracted product, with an

octahedral phosphorus that has two methoxy groups bound in

axial positions (P−O9 = 1.6996(19) Å; P−O10 = 1.6792 (19)

Å; Figure 6, Table 1). The phosphorus atom is nearly coplanar

M⁻¹cm⁻¹; (Figure 4). The H NMR spectrum (Figure S9)

Figure 4. UV−vis spectrum of [P^V(TBP₈Cz)(OH)]OH (blue dashed

line) and [P^V(MeOP₈Cz)(OH)]OH (green solid line) in CH₂Cl₂.

supports the formation of a symmetric product, with four

singlets found upﬁeld (∼4 ppm) that can be assigned to the p-

MeO groups and eight resonances in the aromatic region, each

integrating to 2H as expected for the diﬀerent phenyl protons.

A single resonance is observed at −111.35 ppm in the ³¹P NMR

spectrum, which is in agreement with other ﬁve coordinate

phosphorus corrole and corrolazine complexes.^1,41,42Both LDI-

MS and elemental analysis were also consistent with the

structure of the ﬁnal product.

Figure 6. Displacement ellipsoid plot (50% probability level) of

P^V(MeOP₈Cz)(OMe)₂at 110(2) K. Hydrogen atoms and disorder are

omitted for clarity.

with the corrolazine ligand, sitting 0.012 Å above the plane

deﬁned by the pyrrole-N atoms, and 0.051 Å above the plane

deﬁned by the 23-atom macrocyclic core. This is more in the

plane of the corrolazine by 0.022 and 0.014 Å compared to the

P^V(TBP₈Cz)(OMe)₂. The N6−P−N7 bond angle of

83.23(9)°, which includes the pyrroles involved in the Cα−

Cα linkage, is ca. 12° smaller than the opposite angle N2−P−

N4, as seen for the t-Bu derivative. The average P−N bond

distance (1.78 Å) is also comparable to P^V(TBP₈Cz)(OMe)₂

(1.77 Å).

Further characterization of [P^V(MeOP₈Cz)(OH)]OH via

cyclic voltammetry revealed two reversible waves, and one

quasi-reversible wave, at −1.78, −1.27, and 0.65 V versus Fc⁺/

Fc, respectively (Figure 5). These data can be compared to the

electrochemical data for [P^V(TBP₈Cz)(OH)]OH, which

exhibits three waves at −1.70, −1.25, and 0.71 V versus Fc⁺/

Fc.¹The p-MeO substituents cause a negative shift in all three

The ³¹P NMR of P^V(MeOP₈Cz)(OMe)₂shows an upﬁeld

signal at −192.7 ppm compared to −111.4 ppm for the ﬁve-

coordinate complex before crystallization. This upﬁeld shift is

well-documented for six-coordinate phosphorus(V) porphyr-

azines and corrolazines.^1,37,41,72Addition of water converts the

six-coordinate complex back to the ﬁve-coordinate

[P^V(MeOP₈Cz)(OH)]OH (along with some side products)

as conﬁrmed by ³¹P NMR.

Manganese Derivatives of p-Methoxyphenyl-Substi-

tuted Corrolazine. The complex [P^V(MeOP₈Cz)(OH)]OH

was demetalated with liquid NH₃in the presence of excess

sodium metal at −78 °C. The desired metal-free product could

be partly puriﬁed by column chromatography on silica gel

(eluent 95:5 CH₂Cl₂/EtOAc), yielding material of suﬃcient

purity to be carried on to the next step. Metal-free MeOP₈CzH₃

was reacted with Mn(acac)₃in CH₂Cl₂/pyridine (10:1 v/v) by

Figure 5. Cyclic voltammogram for [P^V(MeOP₈Cz)(OH)]OH (0.7

mM) in CH₂Cl₂with 0.1 M TBAPF₆electrolyte.

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The cyclic voltammogram for Mn^III(MeOP₈Cz)(HOMe) in

CH₂Cl₂yields three quasi-reversible waves at 0.68, 0.20, and

−1.71 V versus Fc⁺/Fc (Figure 8). The wave at 0.20 V can be

Table 1. Selected Bond Lengths (Å) and Bond Angles (deg)

for P^V(MeOP₈Cz)(OMe)₂and P^V(TBP₈Cz)(OMe)₂.¹

P^V(MeOP₈Cz)(OMe)₂

P^V(TBP₈Cz)(OMe)₂

P1−N2

P1−N4

P1−N6

P1−N7

1.7903(19)

1.788(2)

1.7761(19)

1.777(2)

0.012

1.75(2)

1.82(2)

1.75(2)

1.77(2)

0.034

P1−(N_pyrrole

)

plane

P1−(23-atom)_core

Cα−Cα

P1−O9

0.051

0.065

1.423(3)

1.6996(19)

1.6792(19)

174.71(9

95.39(9)

90.73(9)

83.23(9)

90.64(9)

90.07(9)

85.32(9)

89.76(9)

93.79(9)

86.26(9)

91.21(9)

94.29(9)

90.07(9)

1.42(2)

1.69(1)

1.72(2)

175.7(8)

93.5(7)

91.8(9)

82.8(8)

91.9(9)

85.6(8)

89.6(8)

94.6(8)

91.9(9)

91.4(8)

87.5(8)

88.6(8)

91.3(8)

P1−O10

O9−P1−O10

N2−P1−N4

N4−P1−N6

N6−P1−N7

N7−P1−N2

N2−P1−O9

N4−P1−O9

N6−P1−O9

N7−P1−O9

N2−P1−O10

N4−P1−O10

N6−P1−O10

N7−P1−O10

Figure 8. Cyclic voltammogram for Mn^III(MeOP₈Cz) (0.3 mM) in

CH₂Cl₂with 0.1 M TBAPF₆electrolyte.

assigned to a metal-based oxidation, whereas the other two

processes are likely ring-based. The wave at −1.71 V can be

assigned to one-electron reduction of the corrolazine ring. For

comparison, the cyclic voltammogram for Mn^III(TBP₈Cz) was

obtained in the same solvent, CH₂Cl₂(Figure S12). Three

quasi-reversible waves were observed for the t-Bu derivative at

potentials of 0.76, 0.28, and −1.67 V. The shift in potentials for

the MeO derivative is indicative of this system being more

electron-rich compared to the t-Bu derivative, as expected for

substitution of t-Bu by MeO on the eight aryl peripheral

substituents.

stirring for 12 h in the dark (Scheme 2). The metalated

product, Mn^III(MeOP₈Cz), was puriﬁed via ﬂash chromatog-

raphy and recrystallized from layering of a CHCl₃solution of

Mn^III(MeOP₈Cz) with MeOH to yield Mn^III(MeOP₈Cz)-

(HOMe) as a dark brown crystalline solid (43%). The UV−

vis spectrum of Mn^III(MeOP₈Cz)(HOMe) reveals a Soret band

at 446 nm (ε = 5.50 × 10⁴M⁻¹cm⁻¹) and a Q-band at 690 nm

(ε = 2.48 × 10⁴M⁻¹cm⁻¹) as seen in Figure 7, together with

Crystals of Mn^III(MeOP₈Cz)(MeOH) led to the structure

shown in Figure 9. Selected bond lengths and angles are given

in Table 2 together with those of Mn^III(TBP₈Cz)(MeOH) for

comparison. The complex crystallizes with one axial methanol

ligand (Mn−O = 2.130(5) Å). The bond distances and angles

around the metal ion for Mn^III(MeOP₈Cz)(MeOH) are similar

Figure 7. UV−vis spectrum of Mn^III(TBP₈Cz) (blue dashed line) and

Mn^III(MeOP₈Cz) (green solid line) in CH₂Cl₂.

the t-Bu derivative for comparison (vide infra). Addition of

pyridine causes a signiﬁcant shift in the Soret and Q-bands to

454 (ε = 5.96 × 10⁴M⁻¹cm⁻¹) and 699 nm (ε = 1.53 × 10⁴

M⁻¹cm⁻¹), respectively, consistent with axial coordination of

pyridine to Mn^III(MeOP₈Cz). The pyridine spectrum also

matched those seen for aliquots taken from the reaction

mixture during metalation.

Figure 9. Displacement ellipsoid plot (50% probability level) of

Mn^III(MeOP₈Cz)(MeOH) at 93(2) K. Hydrogen atoms and disorder

are omitted for clarity.

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Table 2. Selected Bond Lengths (Å) and Bond Angles (deg)

for Mn^III(MeOP₈Cz)(MeOH) and Mn^III(TBP₈Cz)(MeOH)·

MeOH⁴⁷

Mn^III(MeOP₈Cz)(MeOH) Mn^III(TBP₈Cz)(MeOH)

Mn−N2

Mn−N4

Mn−N6

Mn−N7

1.890(3)

1.887(3)

1.886(3)

0.337

1.892(4)

1.876(3)

1.885(3)

1.884(3)

Mn−(N_pyrrole

)

0.373(2)

plane

Mn−(23-atom)_core

Cα−Cα

Mn−O

0.477

0.582(1)

1.394(5)

2.130(5)

94.16(12)

89.25(13)

80.45(13)

88.80(13)

96.6(2)

1.445(6)

2.107(3)

N2−Mn−N4

N4−Mn−N6

N6−Mn−N7

N7−Mn−N2

N2−Mn−O9

N4−Mn−O9

N6−Mn−O9

N7−Mn−O9

92.76(15)

88.58(14)

80.59(14)

89.10(14)

96.92(14)

100.92(14)

106.45(14)

101.83(14)

Figure 10. UV−vis spectra of Mn^V(O)(TBP₈Cz) (blue dashed line)

and Mn^V(O)(MeOP₈Cz) (green solid line) in CH₂Cl₂.

102.0(2)

104.5(2)

107.0(5)

The inﬂuence of the p-MeO substituents on the redox

properties of the Mn^V(O) complex was assessed. The cyclic

voltammogram of Mn^V(O)(MeOP₈Cz) is shown in Figure 11.

to the phosphorus derivative P^V(MeOP₈Cz)(MeO)₂and

Mn^III(TBP₈Cz)(MeOH),⁴⁷but the Cα-Cα linkage is smaller

than that seen for P^V(MeOP₈Cz)(MeO)₂(1.423(3) Å) or the

t-Bu derivative (1.445(6) Å). The origin of this Cα−Cα linkage

shortening for the MeO derivative is not known.

The complex Mn^III(MeOP₈Cz)(MeOH) was converted into

a high-valent Mn^V(O) complex by oxidation with iodosylben-

zene. Mn^III(MeOP₈Cz)(MeOH) was mixed with 2.5 equiv of

PhIO, as shown in Scheme 3. The heterogeneous reaction

mixture went from a dark brown to a bright green in color,

indicating formation of Mn^V(O)(MeOP₈Cz), which was

puriﬁed by column chromatography and isolated as a stable

dark green solid (22%). The UV−vis spectrum for Mn^V(O)-

(MeOP₈Cz) is similar to that seen for Mn^V(O)(TBP₈Cz), with

a Soret band at 440 nm (ε = 4.52 × 10⁴M⁻¹cm⁻¹) and a blue-

shifted, broad, and relatively weak Q-band at 636 nm (ε = 1.54

× 10⁴M⁻¹cm⁻¹; Figure 10).⁴⁷The complex Mn^V(O)-

Figure 11. Cyclic voltammogram for Mn^V(O)(MeOP₈Cz) (0.3 mM)

in CH₂Cl₂with 0.1 M TBAPF₆electrolyte.

(MeOP₈Cz) was characterized by H NMR spectroscopy and

LDI-MS. A diamagnetic H NMR spectrum (Figure S13) was

There are two major redox processes that can be seen at E_1/2=

observed, with four singlets appearing at 4.00−3.95 ppm for the

p-MeO substituents, and a total of seven resonances between

8.4 and 6.7 ppm assigned to the eight peripheral aryl

substituents. The NMR spectrum conﬁrms the diamagnetic,

low-spin, d²conﬁguration of the metal. The stability of the p-

MeO-substituted Mn^V(O) complex is the same as the t-Bu

derivative, allowing for long-term storage of Mn^V(O)-

(MeOP₈Cz).

0.45 and −0.57 V, which can be assigned to ring oxidation and

metal reduction, respectively, based on previous redox

assignments for metallocorrolazines.⁴⁷There is also a small

redox couple centered at E_1/2= 0.07 V, which likely arises from

a small amount of an unidentiﬁed impurity. The Mn^V/Mn^IV

redox couple is shifted in the negative direction by 40 mV when

compared to Mn^V(O)(TBP₈Cz), and the corrolazine ring

oxidation has shifted by 60 mV in the same direction.²⁴The

Scheme 3. Synthesis of Mn^V(O)(MeOP₈Cz)

DOI: 10.1021/acs.inorgchem.6b01219

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Article

shifts indicate that the remote p-MeO substituents increase the

electron density at the metal center.

changes in redox potentials from para substitution in the

corrolazine complexes are relatively insensitive to the identity

of the internal metal, although the substituents for the

corrolazine compounds are attached at the β-carbon atoms

and not the meso positions. To our knowledge, there is only

one study describing the para-R substitution (R = H, F) of aryl

rings attached at the β-carbon atoms of a porphyrin. In this

case, the manganese(III) octaaryl porphyrins exhibited a shift in

both metal- and porphyrin-based redox potentials (50−80 mV)

that is similar in magnitude to that seen for the corrolazines.⁷⁸

Reactivity of Mn^V(O) Corrolazines. Hydrogen-Atom

Transfer with C−H Substrates. The reactivity of Mn^V(O)-

(MeOP₈Cz) with a variety of substrates containing C−H bonds

was examined to determine how changing the redox potential

at the metal center eﬀects the rate of hydrogen atom transfer

(HAT) (Scheme 4). The reaction of Mn^V(O)(MeOP₈Cz) with

excess C−H substrates in CH₂Cl₂was monitored by UV−vis

spectroscopy (Figure 12). The Q-band for the Mn^V(O)

Comparison of Spectroscopic and Redox Properties

with Other Porphyrinoid Compounds. A comparison of

the UV−vis data for the complexes in this study together with

other porphyrinoid compounds is useful to understand the

electronic inﬂuence of the peripheral aryl groups on the

corrolazine ring. In all cases, a red shift was observed for both

the Soret and Q-bands for the MeO-substituted corrolazines as

compared to the t-Bu-substituted compounds, and the

extinction coeﬃcients for the MeO compounds are found to

be smaller for both bands (Table 3). This observation can be

Table 3. Spectroscopic UV−vis Comparison of para-

Substituted Corrolazines

Soret band λ_max(nm)

Q-band λ_max(nm)

complex

(ε × 10^‑4M⁻¹cm⁻¹

)

(ε × 10⁻⁴M⁻¹cm⁻¹

)

[P^V(MeOP₈Cz)

(OH)]OH

[P^V(TBP₈Cz)

(OH)]OH

461 (8.2)

644 (4.5)

451 (13.7)

641 (8.0)

Δ_MeO‑tBu

10 nm

3 nm

Mn^III(MeOP₈Cz)

Mn^III(TBP₈Cz)

Δ_MeO‑tBu

446 (5.50)

435 (5.85)

11 nm

690 (2.48)

685 (3.49)

5 nm

Mn^V(O)

436(4.53)

636 (1.54)

(MeOP₈Cz)

Mn^V(O)(TBP₈Cz)

418 (6.29)

18 nm

634 (1.99)

2 nm

Δ_MeO‑tBu

compared to what was reported by Kobayashi for a series of

related magnesium and phosphorus octa-aryl-porphyrazines, in

which it was shown that an increase in the electron-donating

ability of the peripheral aryl groups caused a red shift in the Q-

bands.³⁷These observations are in contrast to what is seen for

both meso-aryl-^34,73and β-aryl-substituted porphyrins,^35,74

which show no clear correlation with the electron-donating

properties of the substituents and the UV−vis transitions.

However, the magnitude of the shifts observed for the Q-bands

in porphyrazines are slightly larger than those seen for the

corrolazines, whereas the opposite trend is seen for the Soret

bands, in which the corrolazines show a more pronounced

eﬀect.

With regard to redox potentials, the MeO corrolazine

complexes uniformly show shifts to more negative potentials

when compared to the analogous t-Bu corrolazines. The redox

potentials of metalloporphyrins are also sensitive to substitution

of the para-position on aryl groups attached at the meso

positions, but the magnitude of the shifts are highly dependent

on the identity of the metal center.^{35,73,75−77}In contrast, the

Figure 12. (a) Time-resolved UV−vis spectral changes observed in the

reaction of Mn^V(O)(MeOP₈Cz) (26 μM) with P(p-CF₃−Ph)₃(0.63

mM) in CH₂Cl₂at 25 °C. (b) Changes in absorbance vs time for the

growth of Mn^III(MeOP₈Cz) (red ) and decay of Mn (O)-

●

■

(MeOP₈Cz) (blue ) with the best ﬁt lines (black). (c) Plot of

pseudo-ﬁrst-order rate constants (k_obs) vs [P(p-CF₃−Ph)₃] (R²

0.99).

Scheme 4. Hydrogen Atom Transfer (Right) And Oxygen Atom Transfer (Left) Of Mn^V(O)(MeOP₈Cz) with Substrate

DOI: 10.1021/acs.inorgchem.6b01219

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Table 4. Comparison of Rate Constants for HAT by Mn^V(O) Corrolazines

substrate

Mn^V(O)(MeOP₈Cz) k (M⁻¹s⁻¹

)

Mn^V(O)(TBP₈Cz) k (M⁻¹s⁻¹

)

ratio k_MeO/k_TBP

BDFE (kcal mol⁻¹

)

1,4-cyclohexadiene

xanthene

6.8(4) × 10⁻⁵

2.3(1) × 10⁻³

3.3(1) × 10⁻⁵

1.8(2) × 10⁻³

1.66(1) × 10⁻¹

1.3

1.0

AcrH₂

1.70(2) × 10⁻¹

Reference 26. BDE reported in the gas phase in ref 79. BDFE values reported in DMSO in ref 79. Rate constants normalized per reactive C−H

bond n (e.g., n = 2 for xanthene).

complex at 636 nm decays over time along with the isosbestic

growth of a peak at 689 nm corresponding to the formation of

Mn^III(MeOP₈Cz)(H₂O). Plots of absorbance versus time for

both species, Mn^V(O)(MeOP₈Cz) and Mn^III(MeOP₈Cz), yield

single exponential curves that are well-ﬁt by pseudo-ﬁrst-order

kinetics, yielding the pseudo-ﬁrst-order rate constants (k_obs).

The rate constants increased with increasing concentration of

substrate, and plots of k_obsversus [C−H substrate] provided a

linear relationship whose slope gives the second-order rate

constants (Table 4).

Table 5. Second-Order Rate Constants for the Oxidation of

Triarylphosphines by Mn^V(O) Corrolazine

Mn^V(O)(MeOP₈Cz) Mn^V(O)(TBP₈Cz)

ratio

k_TBP/k_MeO

substrate

(M⁻¹s⁻¹(M⁻¹s⁻¹

)

P(o-CH₃-Ph)₃

P(p-CH₃-Ph)₃

P(p-H-Ph)₃

1.02(5) × 10¹

3.1(2) × 10⁴

1.04(6) × 10⁴

2.3(1) × 10³

2.3(1) × 10²

1.6(1) × 10¹

5.8(2) × 10⁴

1.43(6) × 10⁴

4.7(4) × 10³

6.0(6) × 10²

1.6

1.9

1.3

2.0

2.6

P(p-Cl-Ph)₃

P(p-CF₃-Ph)₃

Reference 83.

As seen in Table 4, the largest diﬀerence in HAT rates for the

MeO and t-Bu derivatives is observed with 1,4-CHD, in which

there is a twofold rate enhancement for the MeO-substituted

complex. For the other substrates (Xn, AcrH₂), there is little or

no change in reaction rates for the two corrolazine derivatives.

For Mn^V(O) complexes, the HAT rates decrease with

increasing C−H bond strength,⁷⁹as expected for an HAT

mechanism.²⁶The driving force for HAT can be related to the

bond strength of the O−H bond being formed in the MnO−H

product. This bond strength can in turn be related to the redox

potential and basicity of the one-electron-reduced [Mn^IV(O)]⁻

species through eq 1:

second-order rate constant is seen for the most electron-poor

phosphine (P(p-CF₃−Ph)₃). A linear relationship (ρ =

−1.01(2)) was observed in the Hammett plot for the reaction

of the p-X-substituted triarylphosphines with Mn^V(O)-

(MeOP₈Cz) (Figure 13). A more negative ρ-value is seen for

BDFE(Mn^IVO − H) = 1.37pK_a+ 23.06E° + C_G

(1)

where BDFE = bond dissociation free energy of the O−H bond

formed, the pK_aand E° equal the basicity of [Mn^IV(O)]⁻and

the Mn^V/IV(O) couple, respectively, and C_Gis the free energy of

formation of a hydrogen atom.⁸⁰⁻⁸²The lower redox potential

of the MeO derivative should contribute to a weaker Mn^IVO−

H bond by ∼1 kcal/mol and hence lead to a slightly weaker

driving force for HAT. However, the data in Table 4 show that

the rates of HAT for MeO versus t-Bu complexes are slightly

greater for the MeO derivative. These data suggest that the

basicity of the [Mn^IV(O)]⁻species is enhanced for the MeO

derivative, thereby compensating for the lower redox potential

and increasing the driving force for HAT.

Figure 13. Hammett plot for OAT reactions between PAr₃and

Mn^V(O)(MeOP₈Cz) (red line) and Mn^V(O)(TBP₈Cz) (blue line) in

CH₂Cl₂at 25 °C.

the MeO complex as compared to the t-Bu complex (ρ =

−0.91(5)), and this observation is consistent with a less

electrophilic p-MeO complex. The weakened electrophilicity

correlates nicely with the lower redox potential of the latter

complex. The steric inﬂuence of the substrate was studied by

the use of the tri(o-tolyl)-substituted phosphine. A large

decrease of ∼3000-fold in the second-order rate constant was

observed going from the tri(p-tolyl)phosphine to tri(o-tolyl)-

substituted phosphine, which is similar to the ∼3700-fold

decrease seen for Mn^V(O)(TBP₈Cz). This rate change provides

strong evidence that Mn^V(O)(MeOP₈Cz) reacts via a

concerted oxygen-atom transfer mechanism, as does Mn^V(O)-

(TBP₈Cz).⁸³Overall, the kinetics show that the para-methoxy

derivative in general is a weaker oxygen atom transfer agent

than the tert-butyl derivative.

Oxygen-Atom Transfer with Triarylphosphines. The

reaction of Mn^V(O)(MeOP₈Cz) with excess triarylphosphine

derivatives in CH₂Cl₂was monitored by UV−vis spectroscopy

(Figure 12). The disappearance of Mn^V(O)(MeOP₈Cz) (436,

636 nm) was seen together with the isosbestic growth of

Mn^III(MeOP₈Cz)(OPAr₃) (444, 690 nm), consistent with the

two-electron oxidation of the triarylphosphine to phosphine

oxide. The formation of phosphine oxide was conﬁrmed by ³¹

NMR spectroscopy. The coordination of the phosphine oxide

to the Mn ion is supported by the appearance of a split Soret

band.⁸³Plots of absorbance versus time and subsequent ﬁtting

of the data yielded the pseudo-ﬁrst-order rate constants (k_obs),

and plotting of the k_obsversus [PAr₃] gives a linear relationship,

which provides a measure of the second-order rate constants

(Table 5).

The rate constants of OAT for Mn^V(O)(MeOP₈Cz) with the

substituted triarylphosphines are slower than the corresponding

rate constants for Mn^V(O)(TBP₈Cz). The greatest diﬀerence in

SUMMARY AND CONCLUSIONS

This work provides a general one-pot synthesis, from dinitrile

to metal-free porphyrazine, for symmetrical octa-arylporphyr-

■

DOI: 10.1021/acs.inorgchem.6b01219

Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

Article

azines, RP₈PzH₂(R = t-Bu, MeO, and iPr). Furthermore, the

optimized and complete synthesis of metal-free corrolazine

from commercially available reagents is presented. The detailed

synthetic protocol given here should allow for easy

reproducibility of corrolazine synthesis on a practical scale by

other laboratories. New manganese(III) and manganese(V)-

oxo corrolazines with p-MeO-phenyl substituents are also

reported. Characterization of the p-MeO corrolazines by UV−

vis spectroscopy and CV revealed clear eﬀects induced by the

modiﬁcation of the peripheral substituents on the β-carbon

atoms. Substitution of MeO in place of t-Bu leads to a red shift

in the Soret and Q-bands in the UV−vis spectrum, which is

consistent with a more electron-rich corrolazine. An increase in

electron density at the metal center is also indicated by the CV

data, where a negative shift (20−80 mV) for the Mn^V/IVcouple

and the corrolazine ring oxidation is observed.

on modiﬁcations to the Cz periphery to further enhance

reactivity at the metal center.

ASSOCIATED CONTENT

* Supporting Information

■

The Supporting Information is available free of charge on the

ACS Publications website at DOI: 10.1021/acs.inorg-

chem.6b01219.

UV−vis absorption spectral data, illustrated bond lengths

in macrocyclic core, electron density diﬀerence Fourier

maps, NMR spectra, cyclic voltammogram, product

analysis, kinetics analyses (PDF)

X-ray crystallographic data (CIF)

The relative hydrogen atom transfer and oxygen atom

transfer chemistry of Mn^V(O)(MeOP₈Cz) versus Mn^V(O)-

(TBP₈Cz) was determined. The Mn^V(O)(MeOP₈Cz) complex

was found to abstract hydrogen atoms from activated C−H

bonds. The rate of HAT was either the same or slightly faster

for Mn^V(O)(MeOP₈Cz) than for Mn^V(O)(TBP₈Cz). The rate

acceleration for HAT observed for the p-MeO derivative

contradicts the prediction that slower rates of HAT should

follow from the lower Mn^V/IVredox potential for this complex.

A reasonable explanation for the HAT reactivity is that the p-

MeO substituents enhance the basicity of the putative

[Mn^IV(O)]⁻, thereby increasing the driving force for HAT,

which compensates for the lower redox potential. This result

can be compared to previous studies from our group that

showed that the addition of anionic axial donors to Mn^V(O)-

(TBP₈Cz) also showed enhanced rates of HAT, and this

increase was attributed to the same increase in basicity and

overall driving force.²⁶These studies provide support for the

notions that the basicity of high-valent metal-oxo porphyrinoid

species plays a major role in determining reactivity and that

biological systems can use diﬀerent strategies to tune this

parameter involving both porphyrinoid structure and axial

ligand eﬀects.

AUTHOR INFORMATION

Corresponding Author

*E-mail: dgp@jhu.edu.

■

Present Address

^†Faculty at Sewanee: The University of the South; 735

University Avenue, Sewanee, Tennessee 37383, United States

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

■

This work was supported by the NIH (GM101153 to D.P.G).

E.E.J. is grateful for The Johns Hopkins Dean’s Science

Postdoctoral Teaching Fellowship. J.P.T.Z. acknowledges the

Glen E. Meyer ’39 Fellowship for Research Support. R.A.B. is

grateful for the Harry and Cleio Greer Fellowship.

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The OAT reactivity for Mn^V(O)(MeOP₈Cz) is lower than

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Inorg. Chem. XXXX, XXX, XXX−XXX

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