Metalloporphyrins with All the Pyrrole Nitrogens Replaced with Phosphorus Atoms, MP(P)4 (M = Sc, Ti, Fe, Ni, Cu,

Zn)

Dr. Aleksey Kuznetsov, Visiting Professor,

Department of Chemistry, UFSCar1

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Motivation: What Are Porphyrins and Why Are We

Interested in Them?

Important Features of Metalloporphyrins

Four pyrrole rings Nearly degenerate HOMOs and degenerate LUMOs

Aromaticity

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Metalloporphyrins and Related Compounds

(a) chromophores (b) electron transfer agents

Biological Functions: O2 transport and storagelight energy collection/transportsolar energy ⇒ chemical energy conversion (photosynthetic reaction centers) electron transfer (cytochromes) etc.

Technological Applications: catalysis phototherapymolecular electronicsartificial photosynthesis sensitizers for dye-sensitized solar cells (inexpensive, effective, and environmentally friendly chromophores) etc.

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4

Sterically demanding/electron-

withdrawing or donating substituents in - or

meso-positions; annulation

Central metal

oxidationAlteration of

the conjugated

system

Replacement of N(s) with

heteroatom(s)

Macrocycle “strapping” via covalent linkage

Degree of

reduction

Cation-radical formati

on

Axial ligands

N-substitutio

n

Factors capable of affecting the porphyrin macrocycle geometry (distortions from planarity) and electronic

structure/delocalization (Senge, M. O. Chem. Commun. 2006, 243)

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Tuning the porphyrin size, shape, charge, and properties by replacing pyrrole nitrogen(s) with other replacing pyrrole nitrogen(s) with other elements:elements: a highly promising approach for tuning optical, electrochemical, and coordinating properties, and reactivity (For example: I. Gupta, M. Ravikanth, Coord. Chem.

Rev. 250 (2006) 468–518). Numerous efforts devoted to the pyrroles N’s

replacement with C, Si, O-Se. Until recent time: no significant attempts to replace the

pyrrole nitrogens with phosphorus. Replacement of 1 or 2 pyrrole nitrogens by P studied for several porphyrins and their derivatives (see references below).

D. Delaere, M.T. Nguyen, Chem. Phys. Lett. 376 (2003) 329-337.Y. Matano, T. Nakabuchi, H. Imahori, Pure Appl. Chem. 82 (2010) 583-593.T. Nakabuchi, Y. Matano, H. Imahori, Org. Lett. 12 (2010) 1112-1115.T. Nakabuchi, M. Nakashima, S. Fujishige, et al. J. Org. Chem. 75 (2010)

375-389.Y. Matano, H. Imahori, Acc. Chem. Res. 42 (2009) 1193-1204. Y. Matano, M. Nakashima, T. Nakabuchi, et al. Org. Lett. 10 (2008) 553-556. Y. Matano, T. Nakabuchi, S. Fujishige, et al. J. Am. Chem. Soc. 130 (2008)

16446-16447.Y. Matano, T. Nakabuchi, T. Miyajima, et al. Org. Lett. 8 (2006) 5713-5716. Y. Matano, T. Miyajima, N. Ochi, et al. J. Am. Chem. Soc. 130 (2008) 990-

1002. Y. Matano, T. Miyajima, T. Nakabuchi, et al. J. Am. Chem. Soc. 128 (2006)

11760-11761.

Motivation: What Is Interesting for Us?

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No studies of free porphyrins or their metal complexes with all pyrrole nitrogens replaced with phosphorus atoms (P4-porphyrins, or P(P)4) have been performed so far except

our recent report: J. Barbee, A.E. Kuznetsov, Comp. Theoret. Chem. 2012, 981, 73-85.

Motivation: What Is Interesting for Us?

Design of new compounds with potential applications in:CatalysisAlternative Energetics (Solar Energy Conversion)OpticsElectronicsMedicine????

Motivation: Why Is It Interesting for Us?

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What Happens when ALL Pyrrole N’s Replaced with P’s?

∡Cm-Ni-Cm', o:146.5

∡Cm-Ni-Cm', o:167.5

0.79-0.71

-0.11

0.26

J. Barbee, A.E. Kuznetsov, Comp. Theoret. Chem. 981 (2012) 73-

85.

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Recent Work

Computational studies extended to include the tetraphosphaporphyrins of Sc, Ti, Fe, Cu, and Zn.

Both the neutral species, ScIIP(P)4, TiIIP(P)4, FeIIP(P)4, CuIIP(P)4, ZnIIP(P)4,

and cations, ScIIIP(P)4+, TiIVP(P)4

2+, FeIIIP(P)4+, studied.

In ScIIP(P)4 and TiIIP(P)4 both transition metals (TM) exist in non-characteristic oxidation states, but studies of these species performed for uniformity sake.

Thus, systematic investigation of TM-substituted P4-porphyrins with increasing number of d-electrons done, following the sequence of electronic configurations: 3d14s2 (Sc) → 3d24s2 (Ti) → 3d64s2 (Fe) → 3d84s2 (Ni) → 3d104s1 (Cu) → 3d104s2 (Zn).

Kuznetsov, A. E. Chem. Phys. submitted for publication

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Geometries optimized with the C2 symmetry constraints, structures then checked to be real global minima by vibrational frequencies analysis.

For all the species under investigation, thorough global minimum search performed and different possible spin states calculated.

Calculations performed in the gas phase (vacuum) using the split-valence 6-31G* basis set and the hybrid B3LYP functional as implemented in the Gaussian 09.

Time-dependent DFT calculations of the optical gap values for the MP(P)4 species done using the TDB3LYP approach.

The vertical ionization potential and electron affinity (IPv/EAv) values computed in single-point calculations from the energies of systems with N and N±1 electrons, using geometries of the N-electron systems. The adiabatic IPs and EAs (IPad/EAad) obtained from the energies of the systems with N and N±1 electrons, calculated using optimized geometries of the N±1 electron species.

Charge analysis done using the Natural Bond Orbital (NBO) scheme implemented in the Gaussian 09 with the keyword ‘pop=NBO’.

Molecular orbitals and structures visualized using Molden 5.0 and Molekel 5.4.0.8 visualization software.

Computational Details

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Structural Features of MP(P)4 CompoundsAll the neutral and cationic MP(P)4 species studied show strong deformation of the whole MP(P)4 molecule which adopts pronounced bowl-like shape, compared to generally planar or slightly distorted from planar shapes of tetrapyrrole counterparts.

Calculated gas-phase structures (B3LYP/6-31G* level of theory) of the neutral ScIIP(P)4 (C1, 2A) (a), FeIIP(P)4 (C1, 3A) (b), NiIIP(P)4 (C2, 1A) (c), and ZnIIP(P)4 (C2, 1A) (d) compounds, along with the labeling scheme (shown at the ScIIP(P)4 structure).Kuznetsov, A. E. Chem. Phys. Submitted for publication.

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Species Bond length, Å, and angles, deg.

M -P P-C C-C C-C C-Cm Cm-M -Cm' C-P-C P-M -P P5-P4-P3-M

ScIIP(P)4

(C1, 2A)

2.43,

2.53

1.77,

1.80

1.43,

1.45

1.37,

1.39

1.40,

1.42

119.36

92.39,

95.39

107.76,

137.97

46.42

TiIIP(P)4

(C2, 3B)

2.32

1.78

1.44

1.38

1.41

123.24 92.31

130.73

32.97

FeIIP(P)4

(C1, 3A)

2.15,

2.20

1.78,

1.79

1.43,

1.45

1.37,

1.39

1.40,

1.42

141.79 93.01,

93.34

166.03,

169.73

9.74

NiIIP(P)4

(C2, 1A)

2.12

1.78

1.44

1.38

1.41

146.47

92.94

174.13 4.15

CuIIP(P)4

(C2, 2A)

2.24 1.79 1.44 1.38 1.41 144.94 91.55 178.18 -1.29

ZnIIP(P)4

(C2, 1A)

2.37 1.79 1.45 1.37 1.41 144.55 91.39 173.83 -4.36

Table 1. Gas-phase calculated (B3LYP/6-31G* level of theory) principal structural parameters of the neutral MP(P)4 species, M = Sc, Ti, Fe, Ni, Cu, Zn.

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Species Bond length, Å, and angles, deg.

M -P P-C C-C C-C C-Cm Cm-M -Cm' C-P-C P-M -P P5-P4-P3-M

ScIIIP(P)4

(C1, 1A)

2.43,

2.55,

2.56

1.77,

1.79

1.44,

1.45

1.38 1.40,

1.41

119.26

92.32,

95.00

105.18,

136.25

47.69

TiIVP(P)4

(C1, 3A)

2.35,

2.36

1.79

1.43

1.39

1.40,

1.41

122.03,

122.04

93.12,

93.34

124.53,

126.02

36.72

FeIIIP(P)4

(C1, 4A)

2.19

1.79 1.43 1.39 1.41 139.50,

139.51

92.80,

92.81

165.20,

165.24

10.38

Table 2. Gas-phase calculated (B3LYP/6-31G* level of theory) principal structural parameters of the cationic MP(P)4 species, M = Sc, Ti, Fe.

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Species NBO charges, e

M P C C Cm H

ScIIP(P)4

(C1, 2A)

0.79 0.42,

0.50

-0.35,

-0.39

-0.23 -0.18

0.24

TiIIP(P)4

(C2, 3B)

0.36 0.54

-0.37

-0.22 -0.16

0.23

FeIIP(P)4

(C1, 3A)

0.20 0.55,

0.62

-0.37,

-0.38

-0.22 -0.17 0.24

NiIIP(P)4

(C2, 1A)

-0.19 0.68

-0.38

-0.22

-0.17

0.24

CuIIP(P)4

(C1, 2A)

0.24 0.54

-0.37 -0.22 -0.17 0.24

ZnIIP(P)4

(C2, 1A)

0.61 0.45

-0.35 -0.22

-0.17 0.24

Table 3. Gas-phase calculated NBO charges for the neutral MP(P)4 species, M = Sc, Ti, Fe, Ni, Cu, Zn, at the B3LYP/6-31G* level of theory.

Electronic Features of MP(P)4 Compounds

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Negative NBO charges on the metal calculated only in the Ni-substituted derivative.

NBO charges on the metal (B3LYP/6-31G* level of theory) found to change as:

from the high positive value at ScIIP(P)4, +0.79e, dropping to the significant negative value, -0.19e, for NiIIP(P)4, and then again increasing to significant positive values at ZnIIP(P)4,

+0.61e.

This is generally in line with changes of electronegativies (Sc: 1.36; Ti: 1.54; Fe: 1.83; Ni: 1.91; Cu: 1.90; Zn: 1.65) of the transition metals under consideration and thus with their polarization ability.

The unusually small ionic radius of Ni2+ (0.69 Å) leads to its unusually high polarization ability and thus very significant charge transfer from the phosphorus centers to Ni2+.

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Species

Gap, eV IPv//IPad, eV

EA v//EA ad, eV

(HOM O/ LUM O)a

Optical gapb

ScIIP(P)4 (C1, 2A) : 1.45 : 1.97

2.48 5.24 (1A)// 5.13 (1A)

1.69 (1A)// 1.80 (1A)

TiIIP(P)4 (C2, 3B) : 1.35 : 1.95

2.34 5.47 (2A)// 5.39 (2A)

1.74 (2A)// 1.87 (2A)

FeIIP(P)4 (C1, 3A) : 2.59 : 1.74

2.59 6.52 (2A)// 6.40 (2A)

1.87 (2A)// 1.98 (2A)

NiIIP(P)4 (C2, 1A) 2.57 3.04 6.49 (2A)// 6.36 (2A)

1.44 (2A)// 1.61 (2A)

CuIIP(P)4 (C1, 2A) : 1.38 : 2.67

2.77 5.54 (1A)// 5.36 (1A)

1.44 (1A)// 1.69 (1A)

ZnIIP(P)4 (C2, 1A) 1.97 2.91 6.23 (2A)// 6.10 (2A)

1.68 (2B)// 1.74 (2B)

Table 4. Gas-phase calculated HOMO/LUMO gaps (eV), optical gaps (eV), and IPs/EAs (eV) for the neutral MP(P)4 species, M = Sc, Ti, Fe, Ni, Cu, Zn, at the B3LYP/6-31G* level of theory.

aGaps, eV, given as HOMO-LUMO energy differences.bValues calculated using the TDB3LYP approach.

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(i) All the MP(P)4 species studied posses the strongly pronounced bowl-like shape, compared to generally planar or slightly distorted from planar tetrapyrrole counterparts. In all the neutral and cationic MP(P)4 species all the bond distances used as the structural deformation measure (see Tables 1 and 2) are essentially the same.

The only bond distance which changes significantly is the M-P bond: it decreases from 2.53 Å in ScIIP(P)4 to 2.12 Å in NiIIP(P)4 and then increases to 2.37 Å for ZnIIP(P)4. The P5-P4-P3-M dihedral angles characterize the shape of the MP(P)4 ‘bowl bottom’: either with a metal center protruding outwards (Sc, Ti, Fe, and Ni), with the positive P5-P4-P3-M values decreasing from Sc (46.42o) to Ni (4.15o), or with a metal center protruding inwards the ‘bowl’, as in the case of Cu (-1.29o) and Zn (-4.36o). The cationic species have the structures very similar to their neutral counterparts.

The structures of the MP(P)4 species can be explained by the smaller hybridization of the P-atom valence orbitals and by the pyramidalization of the P-atom bonds. The changes in the M-P bond distances and in the P5-P4-P3-M dihedral angles are generally in line with M electronegativies, Sc, 1.36; Ti, 1.54; Fe, 1.83; Ni, 1.91; Cu, 1.90; Zn, 1.65, and thus with M polarization abilities, but not with the M cationic radii.

Findings and Conclusions

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(ii) The NBO charges on the central metal of MP(P)4 were calculated to have the large positive value, +0.79e, in ScIIP(P)4, then drop to the noticeable negative value, -0.19e, in NiIIP(P)4, and then again increase to the positive value in ZnIIP(P)4, +0.61e (Table 3). The significant positive charge, The significant positive charge, from +0.42e up to +0.68efrom +0.42e up to +0.68e, was found to accumulate on the P-atoms, which is opposite to the negative charge on nitrogens in the MP(P)4 tetrapyrrole counterparts and can result in different reactivity patterns of the MP(P)4 compounds and their potential novel applications.

(iii) The MP(P)4 optical gaps drop quite sharply from the both ends of the transition metals range under study and then rise noticeably again with the highest value, 3.04 eV, for the singlet NiIIP(P)4. Similar pattern is observed for the HOMO/LUMO gaps, with the largest gap value calculated for the triplet FeIIP(P)4 compound. The computed IPs vary within the 1.39 eV range for all the species studied, and the computed EA values lie within even narrower range of 0.54 eV. The triplet FeIIP(P)4 species shows both the highest IPv/IPad and EAv/EAad values which is in line with its highest HOMO/LUMO gap values.

Findings and Conclusions

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Further questions to be answered:

How will different polar and nonpolar solvents commonly used in porphyrin synthesis and characterization (water, benzene, acetonitrile, etc.) influence MP(P)4 structures and properties?How strong will different metals (both transition and non-transition metals) interact with P4-porphyrins and how stable

will those compounds be compared to their tetrapyrrole counterparts?How will different counterions (Cl-, N3

-, CH3COO-, etc.) affect structures and properties of cationic MP(P)4 species?How will different meso- and -substituents influence structures, electronic and optical properties, and reactivities of MP(P)4 compounds?How will the annulation with various aromatic/nonaromatic rings influence structures and properties of MP(P)4 compounds?

I gratefully acknowledge:

the organizing committee of the Conference and the University of Mauritius

the computational resources of Duke University and specifically of the research group of Professor David N. Beratan at Duke University

The financial support of this research partially coming from the DOE grant ER 46430 and partially coming from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) grant “Estudo Teórico Computacional de Sistemas Nanoestruturados com Potencial Aplicação Tecnológica”, number 402313/2013-5, approved in the call N° 70/2013 Bolsa de Atração de Jovens Talentos - BJT - MEC/MCTI/CAPES/CNPq/FAPs /Linha 2 - Bolsa de Atração de Jovens Talentos – BJT.

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Acknowledgements

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Thank youThank youfor your attentionfor your attention

For any questions, please contact me by:

aleksey73kuznets@gmail.com