Recent advances in mixed β-pyrrole substituted meso-tetraphenylporphyrins

Tetrahedron Letters 57 (2016) 5150–5167

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Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet

Digest paper

Recent advances in mixed b-pyrrole substituted meso-tetraphenylporphyrins Bhyrappa Puttaiah Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India

a r t i c l e

i n f o

Article history: Received 19 July 2016 Revised 28 September 2016 Accepted 3 October 2016 Available online 5 October 2016 Keywords: Synthesis Porphyrins Electrochemical redox Crystal structures Optical absorption Tunable properties NSD analysis

a b s t r a c t The modulation of physicochemical properties of the porphyrin macrocycle could find applications in nonlinear optics, dye-sensitized solar cells, photodynamic therapy etc. The synthesis of such porphyrins is challenging and gaining considerable importance. By appending appropriate substituents at the b-pyrrole or meso-carbon positions of the macrocycle could modulate the frontier orbitals of the porphyrin p-system. The present review discusses the reported synthetic routes employed for various mixed b-pyrrole substituted meso-tetraphenylporphyrins. Further, earlier work over the past 15 years on the influence of mixed b-substituent pattern in the electronic spectral, electrochemical redox and stereochemistry of the porphyrin macrocycle will be briefly discussed in this review article including their future prospects. Ó 2016 Elsevier Ltd. All rights reserved.

Contents Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Syntheses of mixed b-substituted TPPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mixed b-tri and b-penta substituted TPPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Unsymmetrically mixed b-hepta and b-octa-substituted TPPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Symmetrically and unsymmetrically mixed b-octasubstituted TPPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electronic absorption spectral properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrochemical redox properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural chemistry of mixed b-substituted porphyrins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Normal-coordinate structural decomposition analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References and notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction Synthetic porphyrins are of remarkable interest owing to their usefulness as model compounds of biological tetrapyrrole pigments.1–3 Of the synthetic porphyrin analogues, 5,10,15,20tetraphenylporphyrin, H2TPP (Fig. 1) and its derivatives have been widely used for its ease of preparation, large p-conjugation and E-mail address: [email protected] http://dx.doi.org/10.1016/j.tetlet.2016.10.010 0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

5150 5151 5151 5151 5151 5151 5156 5158 5165 5166 5166 5166

facile functionalization at both the meso-phenyl and b-pyrrole positions. Moreover, it can incorporate a variety of metal ions with variable oxidation states and diverse coordination geometry.3 From the past few decades there has been a shift in the derivatization of porphyrins with suitable substituents at both the meso-carbons and/or b-pyrrole positions for their use in potential material applications.4–6 The synthetic strategies with varying yields for the variety of meso-tetraarylporphyrins have been reported by various groups.7,8 Earlier methods involve the reaction

P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167

2

3

1

18

4 5

20 19

N 21 H N 24

17 16 15

6 7 N 22

H 23 N

14

8 9 10

11 13

5151

b-mono-formyl18 or b-mono-nitro19 H2TPP to generate regioselectively brominated H2TPP(R)Br2 (R = NO2 or CHO). The H2TPP(NO2) (CN)2 and other Suzuki or Stille cross-coupled products, H2TPP(R) X2 (X = Ph, PE and 20 -thienyl) were isolated in very good yields (Scheme 1a).18,19 Kadish group20 reported the synthesis of the b-penta substituted TPP series (Scheme 1b) by formylation or nitration of NiTPP(Ph)4 followed by demetallation to generate their H2TPP(X)(Ph)4 (X = CHO and NO2). The more electron rich H2TPP (CH2OH)(Ph)4 was obtained by the reduction of H2TPP(CHO)(Ph)4 with NaBH4.

12

1,4,6,9,11,14,16,19 = α -pyrrole carbon = Ca 2,3,7,8,12,13,17,18 = β-pyrrole carbon = Cb 5,10,15,20 = meso-carbon = Cm Figure 1. Molecular structure of H2TPP and its IUPAC numbering scheme of the porphyrin ring.

of b-substituted pyrroles with aldehydes under appropriate experimental conditions to produce some mixed b-substituted porphyrins.7,8 Other synthetic routes for mixed meso-substituted porphyrins have been published in the literature.8 The TPPs with similar b-octasubstituents revealed unusual physicochemical properties.3 The porphyrins with similar substituents such as b-octafluoro-TPP,9a,b and b-octachloro-TPP,9c b-octabromo-mesotetrakis(20 ,60 -chlorophenyl)porphinato iron(III) chloride9d have been reported in the literature. The mixed b-substituted systems generally offer unique changes in physicochemical properties when compared to meso-substituted porphyrins. The modulation of such properties of the porphyrin ring is vital in some material applications.1 For example, introducing appropriate substituents (electron donor and acceptor) at the peripheral positions of the macrocycle induce ‘push–pull’ effect on the porphyrin p-system and they find applications in nonlinear optics.10–12 Further, b-substituents/extended p-conjugation can shift the electronic absorption bands to the near infrared region and such systems may find use in photodynamic therapy13,14 and dye-sensitized solar cells.15–17 In this digest article, an account on the role of mixed b-pyrrole substituents (here after referred as mixed b-substituents) of TPPs on some tunable physicochemical properties will be discussed briefly. Modulation of the properties of the porphyrin ring can be induced by appending appropriate substituents which are in direct conjugation with the porphyrin p-system. This article discusses the reported syntheses of different types of mixed b-substituted TPPs followed by their optical absorption spectral and electrochemical redox properties. Furthermore, the some crystal structures of the free base porphyrins and their metal complexes have been reviewed. Syntheses of mixed b-substituted TPPs Mixed b-tri and b-penta substituted TPPs These mixed substituted porphyrins and their metal complexes have been reported by few research groups and they were synthesized using various strategies.18–28 The synthetic routes for the unsymmetrically mixed b-tri and b-penta-substituted TPPs (Scheme 1) were reported by employing Cu(II) and Ni(II)-porphyrins as versatile precursors because of their greater stability relative to free base porphyrins under strong acidic/ oxidative reaction conditions. In some of these schemes, the bromination, metallation and demetallation reactions are often used because of their utility in the synthesis of other derivatives. Mixed b-trisubstituted porphyrins were synthesized using

Unsymmetrically mixed b-hepta and b-octa-substituted TPPs The tri-mixed b-hepta substituted porphyrin was generated by bromination of NiTPP(NO2)(Ph)4 with liq. Br2/py yielded NiTPP (NO2)(Ph)4Br220 (Scheme 2i) and its acid demetallation produced very good yields of the H2TPP(NO2)(Ph)4Br2. It was further cyanated to generate NiTPP(NO2)(Ph)4(CN)2 in 44% yield. Bhyrappa and Bhavana21 reported the bromination of CuTPP(NO2) followed by acid demetallation resulted in H2TPP(NO2)Brn (n = 6 and 7) and unexpected H2TPPBr8 in moderate yields (Scheme 2ii).21 CuTPP(NO2)Br6 was also prepared using NBS induced bromination of CuTPP(NO2) by Kumar and Sankar.19b Further, NiTPP(NO2)Cl7 was isolated by the chlorination of NiTPP(NO2) with NCS in refluxing 1,1,2,2-tetrachloroethane (TCE) and the desired H2TPP (NO2)Cl7 was isolated along with small amounts of NiTPPCl8.22 The Suzuki and Stille cross-coupling reaction of H2TPP(NO2)Br6 produced another class of H2TPP(NO2)X6 (X = Ph, PE and 20 -thienyl) porphyrins in moderate-to-high yields.19b Symmetrically and unsymmetrically mixed b-octasubstituted TPPs In 2006, Bhyrappa et al., reported the synthesis of two series of antipodal symmetrically mixed b-substituted TPPs, H2TPP (R)4X4s.23 These derivatives were prepared using H2TPPBr4 as the precursor (Scheme 3).24 The first step involves preparation of H2TPP(R)4 (R = CH3 and Ph) derivatives in high yields by Suzuki cross-coupling reactions.23 The halogenation of NiTPP(R)4 with N-halosuccinimide produces corresponding NiTPP(R)4(X)4 (X = Br and Cl) in very good yields. Further, H2TPP(R)4(CN)4 derivatives were prepared by the cyanation of NiTPP(R)4Br4 followed by acid demetallation reaction. In these cyanation reactions, small amounts of NiTPP(R)4(CN)3 were also reported (15–20%). Other Stille cross-coupled products, H2TPP(R)4X4 (X = PE and 20 -thienyl) were prepared by facile reaction of H2TPP(R)4Br4 with (n-Bu)3Sn (X) using Pd(0) catalyst.25a In 2015, another class of unsymmetrically mixed b-octasubstituted H2TPP(Ph)3X5 (X = H, Cl, Br and CH3) were reported using H2TPPBr326 as the precursor (Scheme 4).27a The antipodal b-trifluoromethyl-TPP has also been reported in the literature.27b Suzuki cross-coupling reaction of H2TPPBr3 generates H2TPP(Ph)3 in high yields. Its Ni(II)-complex, NiTPP(Ph)3 on facile halogenation produced moderate-to-high yields of NiTPP(Ph)3X5 (X = Br and Cl).27a Suzuki cross-coupling reaction of H2TPP (Ph)3Br5 generates desired H2TPP(Ph)3(CH3)5 in 41% yield. The more electron-deficient, NiTPP(Ph)3(CN)5 was also prepared by cyanation of its NiTPP(Ph)3Br5.28 Some divalent metal complexes of these mixed substituted TPPs reported in Schemes 1–4 have been reported in the literature.19–28 Electronic absorption spectral properties The electronic absorption properties of substituted porphyrins have been examined by various groups.29 Gouterman’s29a

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P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167

D

1

1&

1 1L 1&

12 1

1

%U 1L,, LQVHUWLRQ &X&14XLQROLQH %U R&KU

1

1 + + 1

&RQF+ 62 DT1+

1 1&

1 +

%X 6Q5 3G33K  GLR[DQ$UΔ  KU

5 1 5

12 1

; &+2RU12 5 3K+ 733; 3K   5 3(+ 733; 3(   5  WKLHQ\O+ 733;  WKLHQ\O  

1 + + 1

; 1

12 $F2+$F  2

1 1L

1

1

12

1

&X12  + 2

&RQF+ 62

1 1L 1

PLQ57

DT1+

1 

1L7333K 

1L73312 3K 

32&O '0) + 62

E

+ 1

; 1

; &+2+ 733&+2 ; 12+ 73312

+ 73312 &1  

1

1 +

1%6&+&O 57

1

+ 1

1

3K%2+  3G33K  . &2WROXHQH$UΔ RU

; &+2+ 733&+2 %U  ; 12 + 73312 %U

1L73312 &1 

1&

;

1 1+

+1 1

 + 73312 3K 

1D2+ &+2

;

1 1+

1 +1

1

 + 733&+2 3K 

1D%+   (W2+

1+

+1 1

 ; &+ 2++ 733&+ 2+ 3K  Scheme 1. Synthesis of mixed b-(tri and penta)substituted TPPs.

four-orbital model proposed nominally degenerates highest occupied p-molecular orbitals (a1u, a2u, HOMOs) and lowest unoccupied p-orbitals (e⁄g, LUMOs) to explain the electronic spectra of porphyrins. The a2u and a1u orbitals have p-electron density contribution from the pyrrolic nitrogens and meso-carbon, and b-pyrrole carbons, respectively. Further, their HOMO–LUMO gap and the relative order of HOMOs were altered by introduction of appropriate substituents at the periphery of the porphyrin macrocycle.29b The absorption spectra of porphyrins is influenced by the number and nature of the substituents, nonplanarity of the macrocycle, core metal ion and axial ligand at the metal centre. The molecular symmetry, d2d (saddled), as in some b-substituted MTPPs, and the density functional theoretical (DFT) calculations show that the frontier p-molecular orbitals, HOMO and LUMO are being (b1, b2) and e⁄, respectively.29e The b-pyrrole functionalization of TPP with different electron donor and electron acceptor groups induces ‘push–pull’ effect on the porphyrin p-system and hence alters the physicochemical

properties. The absorption spectral data of various mixed substituted TPPs generally showed an intense Soret, ‘B’ and three-to-four less intense ‘Q’ bands. The ‘B’ and the longest wavelength band, Qx(0,0) (hereafter referred as Q1) data of various mixed substituted H2TPPs are listed in Table 1. Because of the solvent dependent absorption spectral band shifts in nonplanar free base porphyrins,39,40 the data in less polar CH2Cl2 at 298 K are taken for analysis. For comparison, the data for the selected TPPs with similar substituents are also listed in Table 1. Instead of metal-complexes, the data comparison is made for free base porphyrins because Q1 band is generally observable in these systems. The mixed b-(tri, penta and hepta) substituted TPPs revealed red-shift of absorption bands with increase in number of b-substituents. The general trend in red-shift of the absorption bands of these systems relative to their parent porphyrin can be calculated, for example, H2TPP(CHO)X2 relative to H2TPP(CHO).18 Then the relative shifts from the parent porphyrins depend on X (Br, Ph, 2-thienyl and PE) and the range of red-shift in bands

5153

P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167

NO2

NO2 N (i)

N

1. Liq. Br 2 / 4hr 2. Pyridine / 6hr

N Ni N

NO2

N Br

NH

2. aq. NH 3

N

57% NiTPP(NO2)(Ph) 4

N

1. Conc. H 2SO 4

N Ni N

HN N

Br

Br Br H 2TPP(NO2)(Ph) 4(Br) 2; 79%

NiTPP(NO2)(Ph)4Br 2; 84% CuCN / Quinoline 180oC / 3hr NO2

NO2

N

NC

N Br

Br

Br N H H N

CN

H 2TPP(NO2)(Ph) 4(CN)2; 70%

NiTPP(NO2)(Ph) 4(CN)2; 44%

Br

HN N

CN

NC

(ii)

NH

2. aq. NH 3

N

Br

N

1. Conc. H2SO 4

N Ni N

Cl N

1. Liq. Br2 / 4hr Pyridine / 6 hr 2. H 2SO4 3. aq. NaHCO3

X N Y

M

N

NO2 N

N

TCE / 2.5 hr

N

N

Cl

NCS /

Cl

Br

Ni

X N Cl

N Cl

M = Cu(II), CuTPP(NO2) M = Ni(II), NiTPP(NO2)

Cl

X = NO2, NiTPP(NO2)Cl7; 23% X = Cl, NiTPPCl8; 19%

3

1. N 8 2. 0 o BS C C /D /a o n / 1 C E q. c . 6 Na H hr H 2 SO CO 4

X = NO2, Y = H, H2TPP(NO 2)Br6; 30% X = NO2, Y = Br, H 2TPP(NO2)Br7; 30% X = Y = Br, H2TPPBr 8; 40%

Cl

Br

Br

R

R PhB(OH) 2/ Pd(PPh 3)4

Br N Br

N H H N

NO2

K2CO3/ toluene/Ar/Δ or

N

R

(Bu) 3Sn(PE or 2'-thienyl)/ R Pd(PPh 3)4 /1,4-dioxan/

N

N H H N

NO2 N

Ar/Δ /15 hr Br

Br

92% H 2TPP(NO2)Br6

R R R = Ph, H 2TPP(NO2)(Ph) 6; 78% R = PE, H 2TPP(NO2)(PE) 6; 54% R = 2'-thienyl, H 2TPP(NO2)(2'-thienyl) 6; 51%

Scheme 2. Preparation of mixed b-(hepta and octa)substituted TPPs.

(in nm) follow the order, H2TPP(CHO)X2 (B, 5–14; Q1, 13–22) < H2TPP(NO2)X2 (B, 10–18; Q1, 19–31) < H2TPP(NO2)X6 (B, 42–68; Q1, 65–90).18,19 Generally, the reported H2TPP(CHO)X2, H2TPP (NO2)Xn (n = 2, and 6) showed red-shift in B band which also depend on ‘X’ and follow the order: H < Br < Ph < 20 thienyl < PE.18,19 But the trend in ‘Q1’ band differs depending on X and nature of the porphyrin ring. Interestingly, the energy of the ‘B’ and ‘Q1’ bands of H2TPP(NO2)(Ph)4 are intermediate in energy of H2TPP(NO2)(Ph)n (n = 2 and 6).19,20 The mixed substituted H2TPPs showed red-shift of the absorption bands with the increase in number of substituents and the Q1 band follows the order: H2TPP(CH2OH)(Ph)4 < H2TPP(NO2)(Ph)4 <

H2TPP(CHO)(Ph)4 < H2TPP(NO2)(Ph)4Br2 < H2TPP(NO2)(Ph)4(CN)2. Generally, divalent metal complexes of b-(tri, penta and hepta) substituted porphyrins show blue-shifted absorption bands relative to their free base derivatives and trend in shift of absorption band differs based on the b-substituted porphyrin and the core metal ion. The reported H2TPP(NO2)X6 (X = Br, Ph and PE) revealed absorption bands (B and Q1) comparable to that of their corresponding H2TPPX8 derivatives.37,32,31 The DFT calculations on the ground state geometry optimized structures of H2TPP(NO2)Xn (n = 2 and 6) showed increase in nonplanarity when n = 6 relative to n = 2.19b The time dependent density functional theoretical (TD-DFT) calculations

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P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167

Br

Br N

1. RB(OH) 2 / Pd(0) HN

N

N

M

N

N

or NCS / TCE / Δ

R

N Br

Ni

R N R

N

X X X = Cl or Br, R = Ph; NiTPP(Ph) 4(X) 4; 90-92 % X = Br, R = CH 3; NiTPP(CH 3)4Br4; 75%

R R R = Ph or CH 3 H 2TPP(Ph) 4; 85% H 2TPP(CH 3)4; 90% NiTPP(R)4; 90 %

X = Br CuCN / p y / Δ

2.

R = Br, H2TPPBr 4

N

R

NBS / CHCl3 / Δ

3

Br

N

K2CO3 / Super base 2. Ni(II) insertion

X

1. C o a q nc .N . H aH 2 S CO O4

NH

X

R

R

N

R NH R

R HN R

N X

X

R

R

N

R N

HN

NH

/ 1,4-dioxan / Ar / Δ / 20 hr

R = Ph or CH3 X = 2'-thienyl, H 2TPP(R) 4(2'-thienyl)4; 54-66% X = PE, H2TPP(R)4(PE) 4; 40-51%

N

R

(Bu) 3Sn(X) / Pd(PPh 3)4

N

CN

NC

X

X

X

X

R

X X R = Ph or CH 3; X = Br, H 2TPP(R)4Br4; 85-90 % R = Ph, X = Cl, H 2TPP(Ph)4Cl4; 90%

R N

Ni

R

R

N NC

CN

R = Ph, NiTPP(Ph) 4(CN)4; 56% R = CH 3, NiTPP(CH 3) 4(CN)4; 40%

Scheme 3. Synthesis of symmetrically mixed b-octasubstituted TPPs from H2TPPBr4.

X

X

Br Br

N H N N H N

1. PhB(OH)2 / Pd(0)

Br K2 CO3 / Super base 2. Ni(II) insertion

1. NBS/CHCl 3 /RT or NCS / TCE / 6 hr 2. Conc. H2 SO4 / aq. NaHCO3

N N M N N

M = 2H, H2TPP(Ph)3; 80% M = Ni(II), NiTPP(Ph)3; 90%

H 2TPPBr3

N HN N

X X X = Br, H 2TPP(Ph)3Br5; 82% X = Cl, H2 TPP(Ph)3 Cl5; 85% X = Br CH3 B(OH)2 / Pd(0) K2 CO3 / Super base

NBS/CHCl 3 /RT

NC N

CN

N Ni N

2 days

N NC

CuCN/py/ Δ

N

NiTPP(Ph)3 (CN)5 ; 30%

N

Br

CH 3 HN

NH

N Ni N

N

N Br

CN

CH3

H3 C

Br

Br

CN

X

NH

H3 C

Br

NiTPP(Ph)3 Br5; 89%

CH 3

H2 TPP(Ph)3(CH 3)5 ; 41%

Scheme 4. Preparation of unsymmetrically mixed b-octasubstituted TPPs from H2TPPBr3.

on the geometry optimized structures of CuTPP(X)(Ph)4 (X = H, CHO, NO2 and CH2OH) and CuTPP(NO2)(Ph)4X2 (X = Br and CN) were performed by Kadish group20 and showed an interesting trend in their properties. Theoretical calculations on the low symmetry of mixed b-substituted CuTPP(R)(Ph)4 (R = CHO and NO2) and (CuTPP(NO2)(Ph)4X2) (X = Br and CN) indicate stabilization of b1 (a1u, d4h symmetry) relative to b2 (a2u, d4h symmetry) due to electron withdrawing nature of the substituents at the b-pyrrole carbons.20 The shift in the HOMO and LUMO levels calculated from the absorption spectral and electrochemical redox data shows expected trend in the energy difference in the HOMO (b1 and b2)

levels relative to e⁄ (LUMOs). The red shifts of the transitions (B and Q1) were attributed to the nonplanar geometry of the macrocycle, conjugative and inductive effect of the b-substituents. Generally, symmetrically (H2TPP(R)4X4) and unsymmetrically (H2TPP(Ph)3X5) mixed substituted derivatives feature red-shifted absorption bands when compared to the corresponding parent TPP(R)n derivatives (Table 1).23,25,27a The H2TPP(Ph)4X4 (X = Br, PE, 20 -thienyl and CN) feature about 7–14 nm red-shifted absorption in both B and Q1 transitions when compared to corresponding H2TPP(CH3)4X4 derivatives. However, for both the series, the observed red-shift of transitions depend on ‘X’ and the

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P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167 Table 1 Selected electronic absorption spectral data of various mixed substituted TPPs in CH2Cl2 at 298 K Entry

Porphyrin

B, band nm

Q1, band nm

Ref.

Entry

Porphyrin

B, band, nm

Q1, band, nm

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

TPP TPP(CHO) TPP(CHO)Br2 TPP(CHO)(Ph)2 TPP(CHO)(20 -thienyl)2 TPP(CHO)(PE)2 TPP(NO2) TPP(NO2)(Ph)2 TPP(NO2)(20 -thienyl)2 TPP(NO2)(PE)2 TPP(NO2)Br2 TPP(NO2)(CN)2 TPP(CHO)(Ph)4 TPP(CH2OH)(Ph)4 TPP(NO2)(Ph)4 TPP(NO2)(Ph)6 TPP(NO2)(20 -thienyl)6 TPP(NO2)(PE)6 TPP(NO2)Br6 TPP(NO2)Br7 TPP(NO2)Cl7 TPP(NO2)(Ph)4Br2 TPP(NO2)(Ph)4(CN)2 TPP(PE)8 TPP(PE)4

414 431 436 438 441 445 426 439 440 444 436 440 468 439 454 470 481 494 468 474 461 452 470 506 452

646 664 678 677 686 685 664 686 695 687 688 702 702 690 697 729 754 751 739 747 732 735 740 761 690

30 18a 18a 18a 18a 18a 19a 19b 19b 19b 19a 19b 20 20 20 19b 19b 19b 21 21 22 20 20 31 19b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

TPP(Ph)4 TPP(Ph)4(CH3)4 TPP(Ph)4(20 -thienyl)4 TPP(Ph)4(PE)4 TPP(Ph)4Br4 TPP(Ph)4Cl4 TPP(Ph)4(CN)4 TPP(CH3)4 TPP(CH3)4(20 -thienyl)4 TPP(CH3)4(PE)4 TPP(CH3)4Br4 TPP(CH3)4(CN)4 TPP(Ph)3 TPP(Ph)3(CH3)5 TPP(Ph)3Br5 TPP(Ph)3Cl5 TPP(Ph)8 TPPBr4 TPPBr8 TPPCl8 TPP(CH3)3 TPP(CH3)8 TPP(20 -thienyl)4 TPP(CN)4

434 457 476 486 470 463 464 420 469 476 461 457 429 458 472 463 468 436 469 452 419 447 445 449

677 704 740 745 734 717 778 641 733 732 722 764 662 707 741 722 724 685 743 720 641 691 701 729

23 23 25a 25a 23 23 23 23 25a 25a 23 23 26 27a 27a 27a 32 24 37 38 34 35 33 36

porphyrins align in the order: H2TPP(R)4(CN)4 > H2TPP(R)4(PE)4 P H2TPP(R)4(20 -thienyl)4 > H2TPP(R)4Br4 for Q1 band and H2TPP (R)4(PE)4 > H2TPP(R)4(2-thienyl)4 > H2TPP(R)4Br4 > H2TPP(R)4(CN)4 for ‘B’ band. Among these systems, both H2TPP(R)4(PE)4 (B, 52–56 nm; Q1, 68–91 nm) and H2TPP(R)4(CN)4 (B, 30–37 nm; Q1, 101–123 nm) revealed largest shift in B and Q1 transitions relative to their corresponding H2TPP(R)4s.23 This suggests that the CN and PE groups contribute greater inductive and conjugative interactions with the porphyrin p-system although H2TPP (Ph)4(CN)440 is nonplanar and H2TPP(Ph)4(PE)425a anticipated to be nonplanar. The absorption bands of H2TPP(Ph)3X5 (X = H, Cl, Br and CH3) exhibited slight red-shift when compared to the corresponding H2TPP(Ph)4X4 derivatives. The magnitude of the red-shift in B (29–43 nm) and Q1 (45–79 nm) bands of H2TPP (Ph)3X5 (X = Cl, Br and CH3) are significant in contrast to absorption bands of the H2TPP(Ph)3. The divalent metal

complexes of b-octa-substituted TPPs show an intense ‘B’ and two visible (Q) bands. Further, Q1 is more intense than Q(1,0) in MTPP(R)4(CN)4 and the red-shift of ‘B’ band follows the order: Zn (II) > Cu(II) > Ni(II) relative to MTPP(R)4. Both the H2TPP(R)4X4 and H2TPP(Ph)3X5 series feature unique trend in absorption bands depending on the size, shape and nature of the substituents. Generally, these porphyrins reveal their ‘B’ and ‘Q1’ transitions which are intermediate in energy relative to their corresponding similar b-octa-substituted H2TPPs (Fig. 2). This suggests the tunable absorption spectral properties of porphyrins with mixed b-substituents. The large red-shift in absorption bands of nonplanar porphyrins mainly suggested to the destabilization of the HOMOs as evidenced from density functional theoretical, DFT calculations.29e,f b-Tri, penta and hepta-substituted TPPs showed progressive red-shift of the bands with an increase in number of substituents by modulating the frontier orbitals of the porphyrin

Figure 2. Plot of Q1 band energy versus various b-octasubstituted H2TPPs. The blue and red bars represent the mixed b-octa-substituted and similar b-octa-substituted TPPs, respectively.

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p-system. The photophysical properties of free base and Zn 19b,20,22

(II)-complexes of mixed b-substituted MTPPs feature red-shifted emission with generally low fluorescence quantum yield and lifetime when compared to the corresponding similarly b-substituted porphyrins. This has been suggested largely due to the substituent effect and nonplanarity of the macrocycle. Electrochemical redox properties Previously, Kadish group has reviewed the electrochemical redox data of numerous substituted porphyrins and metallopor-

phyrins.42 Generally, the b-octasubstituted porphyrin p-system is known to exhibit two successive one-electron oxidation and two successive one-electron reductions and the electrochemical oxidation arises from the highest occupied (HOMO) and reduction from lowest unoccupied p-molecular orbitals (LUMOs). In some cases, the metal centre itself is redox active and shows additional redox couples. The electrochemical first ring oxidation and reduction potentials (Vs Ag/AgCl) for the mixed b-substituted TPPs in CH2Cl2 with 0.1 M tetrabutylammonium hexafluorophosphate, TBAPF6 at scan rate of 0.1 V/sec are listed in Tables 2 and 3. The redox data of some divalent metal complexes of these free base porphyrins

Table 2 The electrochemical first ring redox potentialsa (Ag/AgCl, in V) and Q1(0,0) band energy of asymmetrically mixed b-(tri, penta and hepta) substituted TPPs in CH2Cl2 at 298 K

a b c d

Entry

Porphyrin

IOx.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

TPP TPP(CHO) TPP(CHO)Br2 TPP(CHO)(Ph)2 TPP(NO2) TPP(NO2)(Ph)2 TPP(NO2)(20 -thienyl)2 TPP(NO2)(PE)2 TPP(NO2)Br2 TPP(NO2)(CN)2 TPP(Ph)4 TPP(CHO)(Ph)4 TPP(CH2OH)(Ph)4 TPP(NO2)(Ph)4 TPP(NO2)(Ph)6 TPP(NO2)(20 -thienyl)6 TPP(NO2)(PE)6 TPP(NO2)Br6d TPP(NO2)Br7d TPP(NO2)Cl7 TPP(NO2)(Ph)4Br2 TPP(NO2)(Ph)4(CN)2 TPP(20 -thienyl)4

1.00 1.05 1.11 1.00 1.10 1.01 1.01 1.12 1.12 1.29 0.83 0.89c 0.79 0.94c 0.80c 0.81c 1.10c 1.05c 1.05 1.11 0.89c 0.91c 0.92

DIOx.b

0.06 0.05 0.09 0.09 0.02 0.02 0.19 0.06 0.04 0.11 0.21 0.29 0.00 0.05 0.05 0.01

IRed. 1.23 0.99 0.89 1.00 0.87 0.85 0.80 0.75 0.75 0.52 1.19 1.02 1.10 0.88 0.92c 0.80 0.66 0.65c 0.58c 0.54 0.76 0.64 1.02

DIRed.b

0.10 0.01 0.02 0.07 0.12 0.12 0.35 0.17 0.09 0.31 0.05 0.07 0.21 0.22 0.29 0.33

DE½, V

Q1, eV

Ref.

2.23 2.04 2.00 2.00 1.97 1.86 1.81 1.87 1.87 1.81 2.02 1.91 1.89 1.82 1.72 1.61 1.76 1.70 1.63 1.65 1.65 1.55 1.94

1.92 1.87 1.83 1.83 1.87 1.81 1.78 1.80 1.80 1.77 1.83 1.77 1.80 1.78 1.70 1.64 1.65 1.67 1.66 1.70 1.68 1.67 1.77

23 18b 18b 18b 19b 19b 19b 19b 19b 19b 23 20 20 20 19b 19b 19b 21 21 22 20 20 33

Using 0.1 M TBAPF6 with a scan rate of 0.1 V/s at 298 K. DIox. and DIred. refers to potential shift relative to parent porphyrin Ist oxidation and Ist reduction potentials, respectively. Irreversible potential. Potential versus SCE.

Table 3 The electrochemical first ring redox potentialsa (Ag/AgCl, in V) and Q1(0,0) band energy of mixed b-octasubstituted TPPs in CH2Cl2 at 298 K

a b c d

Entry

Porphyrin

IOx.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 22

TPP TPP(Ph)4 TPP(Ph)4(CH3)4 TPP(Ph)4(20 -thienyl)4 TPP(Ph)4(PE)4 TPP(Ph)4Br4 TPP(Ph)4Cl4 TPP(Ph)4(CN)4 TPP(CH3)4 TPP(CH3)4(20 -thienyl)4 TPP(CH3)4(PE)4 TPP(CH3)4Br4 TPP(CH3)4(CN)4 TPP(Ph)3 TPP(Ph)3(CH3)5 TPP(Ph)3Br5 TPP(Ph)3Cl5 TPP(Ph)8 TPP(PE)8 TPPBr8 TPP(20 -thienyl)4 TPPBr4 TPP(CN)4d

1.00 0.83 0.59 0.89 1.00 0.82 0.86 1.13 0.83 0.65 1.03 0.85 1.11 0.90 0.78c 0.97c 0.98c 0.56c 0.56 1.08c 0.92c 0.98 1.43

DIOx.b

0.24 0.06 0.17 0.01 0.03 0.30 0.18 0.20 0.02 0.28 0.12 0.07 0.08

IRed. 1.23 1.19 1.20 0.99 0.66 0.88 0.91 0.28 1.27 1.08 0.59 0.91 0.36 1.21 1.05 0.71c 0.75 1.28 1.11 0.56 1.02c 0.86 0.23

DIRed.b

0.01 0.20 0.53 0.31 0.28 0.91 0.19 0.68 0.36 0.91 0.16 0.50 0.46

using 0.1 M TBAPF6 with a scan rate of 0.1 V/s. DIOx. and DIRed. refer to potential shift relative to the corresponding reference porphyrin potentials, respectively. Irreversible potential. Redox data versus SCE.

DE½, V

Q1, eV

Ref.

2.23 2.02 1.79 1.88 1.66 1.70 1.77 1.41 2.10 1.73 1.62 1.76 1.47 2.11 1.83 1.68 1.73 1.84 1.67 1.64 1.94 1.84 1.66

1.92 1.83 1.76 1.67 1.66 1.69 1.73 1.59 1.93 1.69 1.69 1.72 1.62 1.87 1.75 1.67 1.72 1.71 1.63 1.67 1.77 1.81 1.70

23 23 23 25a 25a 23 23 23 23 25a 25a 23 23 27a 27a 27a 27a 43 31 37 33 45 36

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Ist Reduction (V)

-1.6

-1.2 CH3 -0.8

H Cl Br

2'-thienyl

r2 = 0.86

PE -0.4 CN 0.0

Ist oxidation (V)

are also available in the literature.19–23,27 The effect of substituent ‘X’ on the shift in first oxidation potential (DIOx.) and first reduction potential (DIRed.) of the H2TPP(NO2)(X)n (n = 2 and 6), H2TPP(X) (Ph)4, H2TPP(R)4X4, were calculated relative to the potentials of H2TPP(NO2), H2TPP(Ph)4 and H2TPP(R)4, respectively (Table 3). Moreover, this also permits us to investigate the effect of other substituents on the shift in redox potentials of the porphyrin p-system. The reported first ring redox potentials of mixed b-substituted porphyrins and their metal complexes follow the Hammett parameter (rp) of the ‘X’ group and follow the order: CN or Br  PE > 20 -thienyl  Ph. The extent of (DIOx. and DIRed.) potential shifts (in mV) depends on substituent ‘X’ and span a range as in H2TPP (NO2)X2 [(90)-to-190; 20-to-350], H2TPP(NO2)X6 [0-to-(290); (50)-to-220], H2TPP(X)(Ph)4 [(40)-to-(110); 90-to-310], H2TPP (R)4X4 [(240)-to-300; (10)-to-910)] and MTPP(Ph)3X5 [(120)to-80; 160-to-500]. The Ist ring redox potentials of MTPP(NO2)X2 are cathodically shifted relative to the corresponding H2TPP(NO2) X2 derivatives and the general trend follow the order: Zn(II) > Ni (II) > Cu(II) while an anodic shift in Ist redox potentials (Ni(II) > Cu(II) > Zn(II)) is reported for MTPP(NO2)X6 series. Similarly, the MTPP(NO2)Br7 (M = 2H, Cu(II) and Zn(II)) and H2TPP(NO2)Cl7 showed anodic shift (80–120 mV) in redox data relative to MTPPX8.22 Available redox data for MTPP(R)4X4 (R = Ph and CH3), permitted us to investigate the Hammett plot (E½ = 4rq) for the first ring redox potentials versus Hammett parameter (rp)44 of the substituents ‘X’ which showed reasonable linear relationship (Fig. 3). Further, the reaction constant ‘q’ and correlation coefficient (r2) for Ist ring oxidation (IOx.) and Ist reduction (IRed.) potentials are (130–190 mV; r2 = 0.60–0.82) and (290–330 mV; r2 = 0.77–0.93), respectively. The reported ‘q’ and r2 of H2TPP(NO2)X6 are (117 mV, r2 = 0.23) in oxidation and (194 mV, r2 = 0.95) in reduction. Many H2TPP(R)4X4s show higher reaction constant ‘q’ values when compared to other reported mixed substituted porphyrins.19,23 The moderate or poor correlation coefficient values in oxidation suggest the possible influence of distortion of the macrocycle. The mixed b-substituted TPPs revealed significant anodic shift in first oxidation potentials depending on X (X = Br, PE, and 2-thienyl) relative to their corresponding parent b-substituted

0.4

CH3 H

0.8

Cl Br

2'-thienyl PE

1.2

r² = 0.73 1.6 -1.0

CN

0.0 1.0 2.0 Hammett Parameter (4σp)

Figure 3. Plot of first ring redox potentials of H2TPP(Ph)4X4 derivatives versus the Hammett parameter of the substituents.

H2TPP derivatives (Tables 2 and 3) and follow the trend: H2TPP(Ph)3X5 > H2TPP(Ph)4X4 > H2TPP(NO2)X2 > H2TPP(R)(Ph)4 > H2TPP(NO2)X6. Further, a comparison of absorption data (Q1 band) with that of their respective Ist ring redox potentials of some of the most electron deficient porphyrins showed smallest HOMO–LUMO gap for H2TPP(Ph)4(CN)4 among the various mixed substituted free base porphyrins (Fig. 4, Tables 2 and 3). The redox potentials of these porphyrins are distinctly different from the H2TPP(R)4 (R = Br, CN, Ph and 20 -thienyl) (Table 3). Moreover, the reported longest wavelength electronic absorption band (Q1) of the mixed substituted porphyrins correlate reasonably well with their DE½ = (IOx.  IRed.).47 Kadish group20 correlated the electronic absorption spectral data with the Ist ring redox potentials of

Ist Reduction (V)

-1.4 TPP -1.0 TPP(NO2)(Ph)4Br2

TPP(Ph)4(PE)4 TPP(NO2)Br7

TPP(NO2)(CN) 2

-0.6

TPP(Ph)4(CN)4 -0.2 0.0

1.65 V

1.81V

1.41 V

1.63 V

1.66 V

2.23 V

0.2

Ist Oxidation (V)

3.0

0.6

1.0

1.4 Figure 4. Comparison of HOMO–LUMO gap derived from electrochemical redox data for some representative mixed b-substituted H2TPPs.

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CuTPP(R)(Ph)4 and CuTPP(NO2)X2 relative to CuTPP(Ph)4. The shift (Db1) of the b1 orbital in the HOMOs relative to LUMOs in these complexes was calculated as a function of b-substituents. The observed third redox couple in these complexes was assigned to M(II)/M(III) (M = Ni(II) or Cu(II)) and it was further supported by spectroelectrochemical studies.20 The reported trend in redox behaviour of mixed b-substituted MTPPs was ascribed to the nonplanarity of the macrocycle, nature of the substituent and the core metal ion.19b,20,23 These porphyrins showed varying degrees of nonplanarity as evidenced from their crystal structures.19b,23,25,27 Distortion of the macrocycle leads to destabilization of the HOMO energy levels which predominantly affect the oxidation potentials while LUMO energy levels are less influenced by it.46 The reported work on mixed substituted porphyrins demonstrates the tunable absorption and electrochemical redox properties by modulation of the frontier p-molecular orbitals (Fig. 4). Interestingly, the difference in potential, DEox (mV) = IIOx.  IOx. for various mixed b-substituted porphyrins on the basis of oxidation (in mV) are as follows: H2TPP(NO2)X2 (90– 185) < H2TPP(Ph)4X4 (110–250) < H2TPP(NO2)X6 (90–290) < H2TPP (X)(Ph)4 (130–590) < H2TPP(CH3)4X4 (130–780). Some of these systems show much smaller DEox value for the oxidation potentials and their available crystal structures also show significant decrease in the dihedral angles for the meso-phenyl groups indicating their possible p-conjugation with the porphyrin p-system. As reported recently, the meso-tetraarylporphyrins (H2TArP) bearing fivemembered rings showed smaller DEox (110–270 mV) in TBAPF6 in CH2Cl2 which was suggested to the possible p-conjugation of five-membered meso-arenes with the porphyrin p-system compared to H2TPP (DEox., 350 mV).48 Structural chemistry of mixed b-substituted porphyrins The reviews by Scheidt and Lee49a and Senge50 provided wealth of structural information on the variety of porphyrins and metalloporphyrins. The structural variations of highly substituted meso-tetraphenylporphyrins bearing similar b-substituents were examined by Senge.50 As suggested by Scheidt and Lee,49a the porphyrin macrocycle shows mainly four different distortion modes (Fig. 5). Unlike porphyrins with similar b-substituents, the mixed b-substituents could induce different degree of electronic factor and steric strain which can change structural parameters along the transannular pyrrole directions. The structural chemistry of mixed b-tri-substituted MTPPs is fairly different relative to planar b-tetra-substituted H2TPPs.49b,51 The available structural and conformational parameters of the reported crystal structure data of the mixed b-trisubstituted TPPs and some of their metal complexes are listed in Table 4.52 The H2TPP(CHO)(Ph)2 shows imino hydrogens on the unsubstituted pyrrolic nitrogens with Ca–N–Ca > Ca0 –N–Ca0 , surprisingly, the reported H2TPP(NO2)(20 -thienyl)219a has imino hydrogens on the electron-deficient pyrrolic nitrogens although Ca–N–Ca > Ca0 –N– Ca0 . The latter structure shows significant nonplanarity relative to former structure (Fig. 6). Further, the individual Cb0 –Cb0 bond distance is short (1.327(9) Å) on the pyrrole ring having nitro group while the other transannular pyrrole with 20 -thienyl groups show much longer Cb0 –Cb0 , 1.360(9) Å, however, H2TPP (CHO)(Ph)2 revealed similar Cb0 –Cb0 bond length on both the substituted pyrroles.18a The reported five-coordinated ZnTPP(CHO)(CH3OH), ZnTPP (NO2)(PE)2(CH3OH), ZnTPP(NO2)Br2(CH3OH) and non-solvated NiTPP(NO2)(Ph)2(py) revealed (M–N)av < (M–N0 )av and Cb–Cb < Cb0 – Cb0 indicating the electron-deficient nature of the mixed substituted pyrroles. The aldehyde group has slight disorder on the adjacent b-carbon and is tilted from the plane of the pyrrole ring by 23°

in H2TPP(CHO)(Ph)2 and 6° in ZnTPP(CHO)(CH3OH).18a The nitro group is bent from its attached pyrrole ring plane and the bent angle span a range of 33–67° in different structures (Table 4).19b The steric crowding around the substituted pyrrole quadrant leads the substituents to push the adjacent phenyl group which results in a general trend Cb0 –Cb0 > Cb–Cb and Ca0 –Cm–Cph > Ca–Cm–Cph by 1–4° (Table 4).53 The NiTPP(NO2)(Ph)2(py) showed an unusual five-coordinate geometry at the Ni(II) centre and the enhanced (Ni–N and Ni–N0 )av bond length (2.080(3) Å) is comparable to the six-coordinated54 (2.040(2)–2.089(2) Å) but it is longer than that reported for four-coordinated (1.896–1.916 Å)55,41 and other Ni(II)-porphyrins in Table 5 of this article. The mean displacement of the 24-atom core (D24) of the structures in Table 4 indicates that the H2TPP(NO2)(20 -thienyl)2 is most nonplanar while ZnTPP(NO2)(PE)2(CH3OH) is the least distorted structure. H2TPP(NO2)(20 -thienyl)2 shows enhanced mean displacements (DCb and DCm) values and this suggests they have mainly saddle (sad) distortion combined with very small degree of ruffled (ruf) and domed (dom) distortions. The ZnTPP(NO2)(PE)2(CH3OH) exhibit mainly very minimal ruf combined with very small degree of sad distortion. The degree of nonplanarity is reflected from the decrease in meso-phenyl and increase in pyrrole ring dihedral angles relative to N4 core. Molecular packing of the structures in Table 4 revealed mainly slipped stack orientation in the solid-state. The highly distorted and solvated H2TPP(NO2)(20 -thienyl)2 shows notable [(C–H. . .O) Cph–H. . .O(NO2) = 2.52 Å, Cpyrrole. . .O(NO2) = 3.219 Å] intermolecular short contacts. The enhanced distortion of the structure was ascribed to steric crowding and intermolecular interactions. The moderately distorted H2TPP(CHO)(Ph)2 shows that CHO group is situated above the pyrrole of the adjacent porphyrin to induce O(CHO). . .Cpyrrole = 3.141 Å, C(CHO). . .Npyrrole = 3.158 Å, p. . .p (Cph. . .Cph = 3.266 Å)] close contacts. The ZnTPP(CHO)(CH3OH) complex shows C(CHO). . .Cpyrrole = 3.357 Å and Cpyrrole. . .Csolvate = 3.367 Å intermolecular short contacts.18a The solvated ZnTPP(NO2) (PE)2(CH3OH) exhibited ((NO2)O. . .O(CH3OH)axial = 2.87 Å) close contact. The presence of intermolecular close contacts or crystal packing forces could also influence the variation in distortion of the macrocycle. As reported earlier, C–H. . .O short contact induces

Saddle (sad)

Domed (dom)

Ruffled (ruf )

Wave (wav)

Figure 5. Depicts four main distortion modes of porphyrin macrocycle. The major displacements of the atoms (+) and () refer above the mean plane and below the porphyrin ring mean plane, respectively. The pyrrolic nitrogens are not shown for simplicity.

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P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167 Table 4 Structural and conformational parameters of solvated mixed b-tri-substituted MTPPs

Cb CPh Ca Cm R Cb' Cb' R

Cb Ca

M = 2H, R = Ph, X = CHO; H2TPP(CHO)(Ph)2

N

M = 2H, R = 2'-thienyl, X = NO2; H2TPP(NO2)(2'-thienyl)2

Ca' N'

M = Zn(II), R = H, X = CHO; ZnTPP(CHO)(CH3OH) M

N'

Ca'

X N

M = Zn(II), R = Br, X = NO2; ZnTPP(NO 2)Br2(CH3OH) M = Ni(II), R = Ph, X = NO2; NiTPP(NO2)(Ph)2(Py)

H2TPP(CHO)(Ph)2

H2TPP(NO2) (20 -thienyl)2(CH3OH) (H2O)

ZnTPP(CHO) (CH3OH)0.5 (C7H8)

ZnTPP(NO2) (PE)2(CH3OH)(H2O)2

ZnTPP(NO2) Br2(CH3OH)H2O

NiTPP(NO2) (Ph)2(py)

18a

19b

18a

19b

19b

19b

2.042(3) 2.089(2) 2.173(3) 1.339(5) 1.370(4)

2.043(10) 2.087(9) 2.154(12) 1.298(15) 1.334(15)

2.048(3) 2.112(3) 2.156(3) 1.344(5) 1.349(5)

Ref. Bond length (Å) M–N M–N0 M–Lax Cb–Cb Cb0 –Cb0

1.352(3) 1.358(3)

1.360(6) 1.344(9)

2.059(5) 2.069(5) 2.159(4) 1.343(9) 1.359(9)

Bond angle (°) Ca–N–Ca Ca0 –N0 –Ca0 Ca–Cm–Cph Ca0 –Cm–Cph Ca0 –Cb0 –X

109.6(2) 106.7(2) 115.2(2) 119.1(2) 130.9(2)

109.9(4) 107.2(4) 116.9(5) 118.8(5) 130.2(5)

107.3(5) 106.5(5) 118.2(5) 116.9(5) 136.5(6)

106.9(2) 108.0(2) 116.6(3) 118.4(3) 128.2(3)

107.5(1) 108.0(1) 116.5(1) 119.3(1) 128.0(1)

106.3(3) 107.5(3) 116.1(3) 118.7(3) 130.8(3)

0.751 0.723 0.778 0.273 0.051 0.109 0.368

0.239 0.312 0.166 0.092 0.052 0.038 0.124 0.331 2.045

0.072 0.040 0.104 0.043 0.061 0.072 0.060 0.294 2.052

0.157 0.143 0.171 0.067 0.029 0.090 0.083 0.237 2.053

0.166 0.117 0.216 0.063 0.069 0.044 0.095 0.307 2.051

75.2 — 2.8 9.1

76.6 — 2.6 5.7

78.9 — 4.2 5.8

76.4 85.4 6.8 7.1

Structural parameter (Å) DC b a 0.253 DC b 0.263 DCb0 0.242 DCaa 0.085 a DN 0.031 DCm 0.075 D24 0.130 DM  2.072

2.056

Dihedral angle (°) relative to N4 core mean plane meso-Ph 63.7 45.1 b-Ph 75 56.6 Pyrrole-H 8.2 19.2 Pyrrole-(X) 6.3 20.1 a

M = Zn(II), R = PE, X = NO2; ZnTPP(NO 2)(PE)2(CH3OH)

Average value along both the transannular pyrrole directions.

nonplanar geometry of the 5,10,15,20-tetrakis(40 -methoxy phenyl)porphinato copper(II) structure.56 Bhyrappa et al. reported various symmetrically23,25a,41 and unsymmetrically27a,28 mixed b-octasubstituted H2TPP structures. The structural and conformational parameters of the reported structures are listed in Table 5 including the data for planar solvated H2TPP(CN)4 structure. The structural data listed are comparable to symmetrically substituted b-octa(bromo or phenyl)tetraphenylporphyrin structures.59,32 The structural data in Table 5 show that the hydrogens are located on the pyrrole having phenyl rings with Ca0 –N–Ca0 > Ca–N–Ca angle by 2–4°. Moreover, the presence of Ph/CH3 group pushes the adjacent phenyl by enhancing the mean value of (Cb0 –Ca0 –Cm and Ca–Cb–Cm)av and decrease in the Ca–Cm–Ca0 angle relative to the corresponding angles of the nearly planar H2TPP(CN)4. The size of the N4 core () increases with increase in size of the substituents; CN < Cl < Br for H2TPP(Ph)4X4 structures.41 The side views of the free base porphyrins and their linear displacement of the 24-atoms core revealed significant changes in distortion of the macrocycle as the size of the ‘X’ group increases (Fig. 7). The solvates in H2TPP(Ph)4X4 (X = Br, Cl and CN)41 are

occupying void space above and below the N4H2 core. Their severe nonplanarity is evidenced from the tilting of the pyrroles alternatively up and down from the mean plane of the porphyrin ring. The displacement values (DCb or DCb0 ) generally show an increase along the antipodal direction with larger size substituents. The linear displacement of 24 atoms core in the solvated H2TPP(Ph)4X4 structures suggests mainly sad distortion combined with very small varying degree of ruf, dom and wav distortion of the macrocycle. The displacement value of one of the Ca was reported incorrectly as 0.964 Å and now it is appropriately corrected as 0.426 Å in the solvated H2TPP(Ph)3(Cl)5 structure.28 The increase in nonplanarity of the porphyrin ring is also evident from the increase in dihedral angle of the pyrroles and b-phenyl rings with the concomitant decrease in dihedral angles of meso-phenyl rings (Table 5). The mixed b-octasubstituents produced varying degree of nonplanarity of the macrocycle indicating the tunable distortion of these systems. The macrocyclic nonplanarity of the solvated structures in Table 5 depend on D24 and align in the order: H2TPP(CN)4 < H2TPP(Ph)4(20 -thienyl)4 < H2TPP(Ph) 4 (CN) 4 < H 2 TPP(Ph) 4 Cl 4 6 H2 TPP(CH 3) 4 (20 -thienyl) 4 < H 2 TPP(Ph)3 Cl5 < H2TPP(Ph)4Br4. Based on the longest wavelength band Q1

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Figure 6. ORTEPs of the macrocyclic ring with displacement of atoms in Å (left column). The linear displacement of core atoms is depicted on the right column. The hydrogens, phenyl groups, b-pyrrole substituents and lattice solvates are not shown for clarity. The disordered aldehyde group is omitted for simplicity.

Table 5 Structural and conformal parameters of mixed b-octasubstituted H2TPPs

5

5 &E

&E

; &E

&D

&D

&P

1

;

&D

&E

;

1

1

&D

< 1

5

Ref. Bond length (Å) Ca–N Ca0 –N0 Ca–Cb Ca0 –Cb0 Cb–Cb Cb0 –Cb0 Ca–Cm Ca0 –Cm Bond angle (°) Cb–Ca–Cm Cb0 –Ca0 –Cm N–Ca–Cm

5 &1; < ++ 733&1  5 &1; < 3K+7333K  &1  5 &O; < 3K+ 7333K &O 5 %U; < 3K+ 7333K %U 5  WKLHQ\O; < 3K+7333K   WKLHQ\O  5  WKLHQ\O; <  &+ + 733&+  WKLHQ\O  5 < &O; 3K+ 7333K &O

5

H2TPP (CN)4C6H14

H2TPP (Ph)4(CN)4 (C2H4Cl2)3

H2TPP (Ph)4Cl4 (CH3OH)2

H2TPP (Ph)4Br4 2THF(CH3OH)1.5

H2TPP(Ph)4(20 -thienyl)4 (C2H4Cl2)2

H2TPP(CH3)4 (20 -thienyl)4 (C2H4Cl2)2

H2TPP(Ph)3Cl5 (C2H4Cl2)2

58

41

41

41

25a

25a

28

1.364(5) 1.377(5) 1.448(5) 1.424(5) 1.371(5) 1.353(6) 1.404(5) 1.397(5)

1.361(5) 1.373(5) 1.450(5) 1.444(6) 1.375(5) 1.380(5) 1.415(6) 1.397(5)

1.360(4) 1.379(4) 1.458(5) 1.437(5) 1.345(5) 1.380(5) 1.409(5) 1.412(5)

1.362(6) 1.359(6) 1.447(7) 1.442(7) 1.352(8) 1.385(7) 1.413(7) 1.416(7)

1.362(6) 1.357(6) 1.465(5) 1.443(5) 1.362(5) 1.384(5) 1.413(5) 1.405(6)

1.373(4) 1.381(4) 1.472(4) 1.446(4) 1.373(5) 1.380(5) 1.422(4) 1.415(4)

1.357(9) 1.363(9) 1.435(10) 1.439(10) 1.340(10) 1.365(10) 1.403(10) 1.400(10)

123.7(3) 126.2(4) 126.2(3)

124.0(3) 128.9(3) 126.0(3)

128.2(3) 127.9(3) 122.7(3)

128.7(5) 129.1(6) 122.7(5)

125.6(3) 130.0(3) 123.6(3)

125.9(3) 130.4(3) 123.3(3)

126.4(7) 130.3(7) 124.2(7)

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P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167 Table 5 (continued)

N0 –Ca0 –Cm Ca–N–Ca Ca0 –N0 –Ca0 Ca–Cm–Ca0

H2TPP (CN)4C6H14

H2TPP (Ph)4(CN)4 (C2H4Cl2)3

H2TPP (Ph)4Cl4 (CH3OH)2

H2TPP (Ph)4Br4 2THF(CH3OH)1.5

H2TPP(Ph)4(20 -thienyl)4 (C2H4Cl2)2

H2TPP(CH3)4 (20 -thienyl)4 (C2H4Cl2)2

H2TPP(Ph)3Cl5 (C2H4Cl2)2

127.1(3) 106.9(3) 109.7(3) 125.3(3)

124.4(3) 107.2(3) 111.0(3) 124.0(3)

125.3(3) 107.8(3) 110.4(3) 122.3(3)

123.4(3) 108.2(5) 110.8(5) 121.2(6)

123.5(3) 106.0(3) 112.0(3) 123.4(3)

123.3(3) 106.1(3) 111.2(3) 123.6(3)

123.8(7) 107.2(7) 112.3(6) 123.3(7)

0.805 0.867 0.743 0.281 0.089 0.183 0.407 2.068

1.130 1.080 1.180 0.367 0.043 0.026 0.510 2.092

1.301 1.322 1.280 0.429 0.039 0.051 0.591 2.103

0.332 0.314 0.350 0.197 0.282 0.304 0.274 2.048

1.070 1.099 1.042 0.390 0.070 0.092 0.514 2.063

1.071 1.086 1.056 0.401 0.076 0.098 0.520 2.048

29.2 35.9 54.8 41.7

39.8 36.7 51.4 26.4

30.6 25.5 55.5 45.5

29.1 26.7 56.4 42.7

30.3 24.4 55.8 44.7

Structural parameter (Å) DC b a 0.039 DC b 0.027 0 DC b 0.052 a DC a 0.029 a DN 0.050 DC m 0.023 D24 0.035  2.073

Dihedral angle (°) relative to N4 core mean plane Pyrrole (R4) 5.7 25.4 Pyrrole (X3Y) 8.8 18.6 b-Ph — 64.1 76.6 51.6 meso-Ph/CH3 a

Average value along both the transannular pyrrole directions.

Figure 7. Side views of the crystal structures of the solvated free base porphyrins. The hydrogens and lattice solvates are not shown for clarity. The right side panel shows the corresponding skeletal deviations (in Å) relative to porphyrin ring mean plane.

(Table 1), HOMO–LUMO gap of these porphyrins follow the trend: H2TPP(CN)4(Ph)4 (778 nm) < H2TPP(20 -thienyl)4(Ph)4 (740 nm) < H2TPP(Ph)4Br4 (734 nm) 6 H2TPP(CH3)4(20 -thienyl)4 (733 nm)

< H2TPP(CN)4 (729 nm) < H2TPP(Ph)3Cl5 (722 nm) < H2TPP(Ph)4Cl4 (717 nm). The absence of correlation between the macrocyclic ring distortion and red-shift in absorption data in these complexes may

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P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167

The available structural data of some mixed b-substituted Ni (II)-porphyrins are listed in Table 6. The solvated NiTPP(Ph)4(CN)4 shows the shortest mean bond length of 1.892(2) Å for (Ni–N and Ni–N0 )av when compared to other Ni(II)-porphyrins (Table 6). In general, Ni(II)-porphyrins shows that the pyrrole with electron donor b-substituents show longer M–N bond length than other antipodal pyrrole with electron-withdrawing groups. This suggests the influence of steric strain is more predominant than electronic factors. The nonplanarity of the porphyrin ring is evidenced from side view and linear displacement of the 24-atom macrocyclic skeleton (Fig. 8). The degree of nonplanarity of the macrocycle depends on DCb value and follows the order: NiTPP(Ph)4(CN)4(py)2 (±0.044 Å)23 < NiTPP(CN)4(py)2 (±0.13 Å)54e < NiTPP(CN)4Br4(py)2 (±0.625 Å).54d The b-hepta substituted four-coordinate NiTPP(NO2)Br6 has Ni–N > Ni–N0 with shortest Cb0 –Cb0 when compared to other Ni(II)-porphyrins in Table 6. In these systems, the stereochemistry of the macrocycle is largely influenced by the steric strain imposed by the size and shape of the substituents along the transannular pyrrole direction. The NiTPP(Ph)3Cl528 is predominantly saddle while NiTPP(Ph)4(CN)4(py)241 shows near planar geometry. This

be attributed to steric crowding of the peripheral substituents largely in the former, however, the latter is influenced by both substituent effects and distortion of the macrocycle. The molecular packing diagram of the free base porphyrin structures showed that they are arranged in an offset fashion with the lattice solvates located in between the molecules. The solvated H2TPP (CN)458 and H2TPP(Ph)4(CN)441 structures showed mainly weak close contacts [Cph–H. . .C(CN) = 2.73 Å, Cph–H..C(CN) = 2.77 Å] and [(C–H. . .N) Csolvate  Npor = 3.308(13) Å], respectively. The H2TPP (Ph)4Cl4 structure shows hydrogen-bonding interactions, N–H  O [N  OCH3OH = 3.018(3) Å, 3.052(4) Å] and OCH3OH. . .OCH3OH = 2.664 (4) Å. These contacts positioned along unit cell ‘c’ axis leading to one-dimensional supramolecular structure. For the solvated H2TPP (Ph)4Br441 structure, the closest intermolecular contacts are (N–H  O) Npor  OTHF = 3.00(4) Å and Npor  OCH3OH = 2.79(3) Å. The solvated H2TPP(CH3)4(2-thienyl)4 structure shows essentially [Cph. . .C2-thienyl = 3.341 Å] and H2TPP(Ph)4(2-thienyl)4 [C2-thienyl–H. . .Cph = 2.80 Å; Cph–H. . .Clsolvate = 2.78 Å; Csolvate–H. . . C2-thienyl = 2.54 Å] close contacts.25a H2TPP(Ph)3Cl5 shows CPh–H... Clpyrrole (2.588–2.941 Å) CPh...Clpor (3.387 Å) and CPh–H...CPh (2.817–2.844 Å) intermolecular close contacts.28

Table 6 Selected structural and conformational parameters of mixed substituted NiTPPs

R

R Cb

Cb ' C b' X

Ca

Ca

Cm X

Cb

C a'

R = Z = Cl, X = Y = Ph; NiTPP(Ph)3 Cl5 N'

R = Z = CN, X = Y = Ph; NiTPP(Ph) 3(CN) 5

N'

Ni

Ca'

Z N

R = CN, X = Y = Z = Ph; NiTPP(Ph) 4(CN) 4 R = CN, X = Y = Z = Ph; NiTPP(Ph) 4(CN)4 (py) 2 R = X = Br, Y = H, Z = NO2; NiTPP(NO2 )Br6

R

NiTPP (Ph)3

NiTPP (Ph)3(CH3)5(C7H8)

NiTPP(Ph)3 Cl5(C2H4Cl2)2

NiTPP (Ph)3(CN)5(C7H8)2

NiTPP(Ph)4(CN)4(C6H14) (C2H4Cl2)0.5

NiTPP(Ph)4(CN)4(py)2) (C2H4Cl2)

NiTPPBr6 (NO2)

Ref. Bond length (Å) M–N M–N0 Cb–Cb Cb0 –Cb0

27a

27a

28

28

41

23

19b

1.903(3) 1.925(3) 1.346(5) 1.360(5)

1.912(3) 1.906(3) 1.361(5) 1.368(5)

1.907(4) 1.905(5) 1.350(8) 1.352(6)

1.925(4) 1.923(4) 1.363(9) 1.360(10)

1.883(4) 1.901(3) 1.378(6) 1.362(8)

2.051(2) 2.089(2) 1.371(3) 1.356(3)

1.921(6) 1.919(6) 1.345(10) 1.335(12)

Bond angle (°) Ca–N–Ca Ca0 –N0 –Ca0 Ca–Cm–Ca0

105.6(3) 105.5(3) 121.4(3)

106.1(3) 105.6(3) 121.4(3)

106.6(4) 105.9(4) 120.1(7)

106.7(4) 105.4(5) 120.5(6)

108.3(3) 106.2(3) 119.8(3)

108.4(2) 107.3(2) 125.7(2)

106.8(6) 106.0(6) 120.5(7)

1.050 1.079 1.020 0.435 0.168 0.395 0.589 0.048 1.909

1.189 1.195 1.183 0.488 0.181 0.026 0.594 0.016 1.906

0.895 0.870 0.920 0.368 0.145 0.230 0.483 0.024 1.924

0.766 0.749 0.782 0.406 0.115 0.716 0.529 0.032 1.892

0.044 0.057 0.031 0.030 0.040 0.033 0.037 0.000 2.069

1.097 1.088 1.107 0.453 0.171 0.111 0.567 0.035 1.919b

42.5 54.4 27.6 28.9

55.2 57.0 22.6 21.5

57.5 62.8 30.7 27.9

80.4 80.0 3.1 3.8

41.5 — 27.2 25.3

Geometrical parameter (Å) DC b a 0.402 DC b 0.380 DCb0 0.425 DCaa 0.320 DNa 0.063 DCm 0.550 D24 0.343 DM 0.014  1.915b

Dihedral angle (°) relative to N4 core mean plane meso-Ph 65.0 52.3 b-Ph 76.3 55.3 Pyrroles (R)4 22.7 25.7 Pyrroles (X2YZ) 21.0 29.5 a

R = Z = CH3 , X = Y = Ph; NiTPP(Ph) 3(CH 3 )5

Y

R

b

R = Z = H, X = Y = Ph; NiTPP(Ph) 3

N

Average value along both the transannular pyrrole directions. Mean value of the centroids of two porphyrins in the asymmetric unit.

P. Bhyrappa / Tetrahedron Letters 57 (2016) 5150–5167

5163

Figure 8. Side views of the mixed substituted Ni(II)-porphyrins (left panel). The hydrogens and solvates are omitted for simplicity. Skeletal displacements of the core atoms are shown on the right panel.

is reflected by variation in key bond lengths, bond angles and structural parameters of the 24 atom core (D24 and Ca–N–Ca, Ca0 –N0 –Ca0 and Ca–Cm–Ca0 ). The Ni(II)-porphyrins revealed varying degrees of nonplanar land scapes (Table 6). Other structures mainly exhibit varying saddles combined with ruffled distortions. The parameter  is highest in solvated six-coordinated NiTPP (Ph)4(CN)4(py)223 and least in four coordinated NiTPP(Ph)4(CN)4 structure41 (Table 6). One of the Cm displacement value was reported as erroneous (+ve) and now it is corrected as –ve for the solvated NiTPP(Ph)4(CN)4 structure.41 Notably, the macrocyclic distortion (D24) in four-coordinated mixed b-octasubstituted Ni(II)-porphyrin structures shows slight anomaly with the size of substituent ‘X’ group and follows the order: NiTPP(Ph)3Cl5 > NiTPP(Ph)3(CH3)5 > NiTPP(Ph)4(CN)4 > NiTPP(Ph)3 (CN)5 > NiTPP(Ph)3. The intermolecular interactions may also influence the distortion of the porphyrin macrocycle. In general, the porphyrin units (Table 6) are arranged in an offset-fashion with intermolecular interactions largely influenced by weak close contacts. Notably, the major intermolecular short contacts are listed for few structures; NiTPP(Ph)3Cl5 [Cph. . .Cph = 3.298 Å and C–H. . .Clpyrrole = 2.76–2.91 Å], NiTPP(Ph)4 (CN)4 [Csolvate. . .Cpyrrole = 3.149 Å, N(CN). . . Cpyrrole = 3.108 Å], NiTPP(Ph)4(CN)4(py)2 [Cph. . .Cpyrrole = 3.295 Å] and NiTPP(NO2)Br6 [Cph. . .Br = 3.473–3.517 Å]. The severe nonplanarity of the

macrocycle in the four coordinated Ni(II)-porphyrins is largely due to steric crowding of the peripheral substituents but it is less so in NiTPP(Ph)3.27a However, the related larger size Zn(II) ion containing ZnTPP(Ph)3(1,4-dioxan) structure shows near planarity of the macrocycle.26 Intermolecular interactions56 or crystal packing forces may also influence their stereochemical features.57 Several available single crystal XRD data on the Cu(II) and Zn(II) mixed b-octasubstituted TPPs allowed to determine the stereochemical differences with the reported metal complexes of 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin, MOETPPs.59,32 The selected structural and conformational parameters of mixed b-octasubstituted MTPPs are listed in Table 7 and the data are compared with the planar CuTPP(Ph)4 and ZnTPP(Ph)4(py) structures. The planar CuTPP(Ph)460 shows Cu–N0 > Cu–N and their mean bond length are higher than the other nonplanar Cu(II)-porphyrins. Moreover, the M–N distances are shorter for the pyrroles bearing electron donor b-substituents relative to pyrroles with electron withdrawing b-substituents. The CuN4 core is planar in CuTPP(Ph)4 but deviates by 5–8° in the other nonplanar Cu(II)porphyrins. The nonplanar five-coordinated Zn(II)-porphyrins revealed longer mean Zn–Npor bond lengths relative to planar four-coordinated Zn(II)-porphyrins.25b,33 The Zn–Laxial bond lengths are longer than the (Zn–Npor)av bond distances and Zn is situated above the N4 plane in the range of 0.260–0.403 Å.49a,57A

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Table 7 Some selected structural and conformational parameters of solvated mixed b-octasubstituted MTPPs

5

5 &E

&E

; & E

&E

;

1

<

& D

1

0

1

& D

; 1

5

5

CuTPP(Ph)4

CuTPP (CH3)4 (PE)4(C2H4Cl2)2

CuTPP (CH3)4(Ph)4) (CHCl3)2

CuTPP (Ph)3(CH3)5 C2H4Cl2

ZnTPP (Ph)4(py)

ZnTPP(CH3)4 (PE)4(CH3OH) (C2H4Cl2)0.5

ZnTPP(Ph)4 Cl4(Py)(C2H4Cl2)2

ZnTPP(Ph)4 Br4(CH3OH) (CH3OH)

Ref. Bond length (Å) M–N M–N0

60

25a

23

27a

25b

25a

41

23

1.959(1) 2.060(1)

Ca–N Ca0 –N0 Ca–Cb Ca0 –Cb0 Cb–Cb Cb0 –Cb0 Ca–Cm Ca0 –Cm

1.382(2) 1.384(2) 1.435(2) 1.451(3) 1.342(3) 1.365(2) 1.395(2) 1.396(2)

1.997(3) 1.986(3) — 1.381(5) 1.375(5) 1.456(5) 1.447(5) 1.361(6) 1.378(6) 1.397(6) 1.401(6)

1.946(2) 1.962(2) — 1.373(3) 1.376(3) 1.452(4) 1.454(4) 1.363(4) 1.371(4) 1.404(4) 1.410(4)

1.983(4) 1.975(4) — 1.382(6) 1.379(5) 1.452(6) 1.448(6) 1.359(7) 1.374(6) 1.399(6) 1.406(6)

2.007(2) 2.130(2) 2.239(8) 1.375(6) 1.377(6) 1.432(9) 1.465(9) 1.351(9) 1.352(9) 1.403(7) 1.403(8)

2.052(3) 2.055(3) 2.129(4) 1.372(5) 1.368(5) 1.459(5) 1.443(5) 1.352(5) 1.384(6) 1.404(5) 1.406(6)

2.104(6) 2.051(6) 2.142(4) 1.370(4) 1.374(4) 1.449(7) 1.455(6) 1.350(6) 1.367(6) 1.404(6) 1.410(6)

2.102(7) 2.000(7) 2.044(8) 1.407(10) 1.401(10) 1.505(12) 1.426(11) 1.294(11) 1.375(11) 1.326(12) 1.449(11)

Bond angle (°) (N-M-N)adja (N–M–N)opp (N0 –M–N0 )opp M–N–Ca M–N0 –Ca0 Cb–Ca–Cm Cb0 –Ca0 –Cm Ca–N–Ca Ca0 –N0 –Ca0 N–Ca–Cm N0 –Ca0 –Cm Ca–Cm–Ca0

90.0(1) 180.0(1) 180.0(1) 127.3(1) 127.0(1) 122.8(2) 125.7(2) 105.4(1) 105.9(1) 127.3(2) 124.2(1) 124.2(2)

90.2(1) 172.5(2) 174.6(2) 126.1(3) 125.0(3) 126.8(3) 126.0(4) 105.8(3) 107.5(3) 123.0(4) 124.5(4) 123.6(4)

90.2(2) 173.7(2) 174.4(1) 125.1(2) 124.4(1) 127.7(2) 128.1(2) 107.6(2) 107.5(2) 122.9(2) 122.2(2) 122.4(2)

90.2(1) 172.9(2) 173.0(1) 126.0(3) 125.0(3) 127.5(4) 126.7(4) 106.1(3) 105.6(2) 122.6(4) 123.3(4) 123.4(4)

89.1(2) 164.4(1) 167.0(1) 126.6(3) 126.1(3) 123.2(5) 127.1(2) 106.5(4) 107.3(4) 127.4(4) 123.7(5) 125.8(5)

89.2(1) 161.9(1) 169.5(1) 126.4(2) 123.6(2) 126.7(3) 126.5(3) 106.9(3) 108.3(3) 123.8(3) 124.5(4) 125.1(4)

88.5(2) 164.4(3) 157.9(2) 121.7(2) 125.9(2) 128.0(4) 125.9(4) 108.8(2) 107.0(2) 124.0(3) 124.4(1) 124.1(3)

88.6(3) 156.8(3) 166.1(3) 119.6(6) 126.9(5) 132.7(9) 127.4(9) 113.3(7) 113.1(7) 123.8(8) 120.8(8) 124.6(8)

0.873 0.873 0.873 0.345 0.113 0.020 0.011 0.428 1.991

0.900 0.878 0.922 0.357 0.103 0.598 0.027 0.536 1.953

0.941 0.913 0.970 0.373 0.121 0.203 0.040 0.492 1.979

0.062 0.097 0.028 0.029 0.048 0.045 0.030 0.046 2.052

0.718 0.744 0.692 0.267 0.069 0.225 0.260 0.377 2.037

0.866 0.800 0.932 0.313 0.063 0.015 0.368 0.406 2.050

1.070 0.984 1.156 0.397 0.082 0.043 0.403 0.510 2.024

53.3 66.6 27.7 31.4

51.7 61.0 21.8 25.2

79.5 78.3 3.8 2.5

57.8 20.2 17.9

57.0 63.7 19.8 24.9

48.4 60.4 23.4 31.9

Structural parameter (Å) DC b a 0.009 DC b 0.012 DCb0 0.006 a DC a 0.009 a DN 0.020 DC m 0.023 DM 0.015 D24 0.013  2.010

Dihedral angle (°) relative to N4 core mean plane meso-Ph 70.7 46.7 b-Ph 73.1 Pyrrole (R4) 2.5 20.6 Pyrrole (X3Y) 1.5 21.2 a

0 &X,,  5 + ; < 3K&X7333K  5 &+; < 3(&X7333K 3(  5 &+ ; < 3K&X7333K  &+   5 < &+ ; 3K&X7333K  &+   0 =Q,, 5 +; < 3K=Q7333K  S\ 5 &+ ; < 3(=Q733&+   3( &+ 2+ 5 &O; < 3K=Q7333K &OS\ 5 %U; < 3K=Q7333K  %U &+ 2+

&D

&D

&P

Mean value along both the transannular pyrrole directions.

comparison of almost planar CuTPP(Ph)460 and ZnTPP(Ph)4(py)25b structural data with the corresponding mixed b-octasubstituted porphyrins shows a significant increase in 2–6° for \Cb–Ca–Cm = (\Cb–Ca–Cm and \Cb0 –Ca0 –Cm)av with the concomitant decrease of 2–4° in \N–Ca–Cm = (\N–Ca–Cm and \N0 –Ca0 –Cm)av and 2–4° in \M–N–Ca = (\M–N–Ca and \M–N0 –Ca0 )av. In the case of the mixed b-octasubstituted Cu(II)-porphyrins, the severe nonplanarity is also evidenced from decrease of 6–19° in \(N–M–N)opp = [\(N–

M–N)opp and \(N0 –M–N0 )opp]av relative to corresponding angle in CuTPP(Ph)4 (180°). This arises largely due to steric crowding around the periphery of the porphyrin ring. The available structural data of mixed b-octasubstituted MTPPs (M = Ni(II), Cu (II) and Zn(II)) allowed to investigate the effect of size of the metal ion on the nonplanar distortion of the macrocycle. It is worthy to note that the extent of steric crowding at the b-pyrrole positions differs in these structures listed in Table 8. The

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ment (doop) and in-plane displacement (dip) of core atoms. Based on the minimum basis set, the out-of-plane distortion decomposes into B2u (sad), B1u (ruf), A2u (dom), Eg (wav(x,y)) and A1u (propellering) type of deformation modes.62 The inplane-displacement (dip) values have also been reported in the literature for many of these structures listed in Table 9. Majority of the crystal structures revealed mainly four deformation modes (sad, ruf, dom and wav(x,y)) as shown in Figure 5. The magnitude of the doop value indicates the degree of nonplanarity of the porphyrin macrocycle. The percentage contribution from all the five individual deformation modes is listed in Table 9. A large majority of the mixed b-substituted H2TPPs showed sad distortion with very small contributions from other distortion modes (10–15%). Interestingly, some Ni(II)-porphyrins exhibited significant contribution mainly from both sad and ruf distortions with very small additional contribution from other distortions (4– 10%). The planar H2TPP(CN)4 and CuTPP(Ph)4 show mostly negligible wav distortion modes. It is worthy to note that the Ni(II)porphyrins have varying degrees of nonplanar distortion modes when compared to free base or Zn(II) or Cu(II)-porphyrins (Table 9). These data also show the unsymmetrical MTPP (Ph)3X5 (M = Ni(II) or Cu(II)) structures have greater saddling than ruffling of their macrocycle in contrast to symmetrically substituted structures (MTPP(Ph)4X4).

structures bearing similar steric demand show the anticipated trend in stereochemistry of the macrocycle with increase in size of the core metal ion. The key structural/conformational parameters influenced by core size of the metal ion, Ni(II) < Cu (II) < Zn(II) and the general trend is an increase in average M–N, Ca–Cm, \Ca–N–Ca, \Ca–Cm–Ca0 ,  and decrease in N–Ca, \M–N– Ca and D24.61 As anticipated the five-coordinated Zn(II)-porphyrin structures (Table 7) show an increase in distortion of the macrocycle depending on the size of the b-substituents, X (Cl < Br). Some of these structures show weak intermolecular interactions. The major close contacts revealed for CuTPP(Ph)4(PE)4 [Cpyrrole. . .CPE = 3.331 Å, 3.312 Å and Cph. . .Cph = 3.392 Å], CuTPP (Ph)4(CH3)4 [Cl. . .Cl = 3.164 Å] and ZnTPP(CH3)4(PE)4(CH3OH) [Cph. . .CPE = 3.376 Å, Cpyrrole. . .Clsolvate = 3.324 Å] structures. The macrocyclic nonplanarity is mainly influenced by the steric crowding with saddle distortion predominating over other modes in these structures (Table 7). Normal-coordinate structural decomposition analysis Normal-coordinate the crystal structures been reported in the phyrin macrocycle is

structural decomposition analysis, NSD of of various mixed substituted MTPPs have literature. The NSD analysis of the porknown to exhibit out-of-plane displace-

Table 8 Comparison of structural and conformational parameters of various solvated/non-solvated crystal structures of mixed b-substituted MTPPsa

a

Porphyrin

Ref.

M–N

N–Ca

Ca–Cm

M–N–Ca

Ca–N–Ca

Ca–Cm–Ca

D24



NiTPP(Ph)4(CN)4 NiTPP(Ph)3(CN)5 NiTPP(Ph)3Cl5 NiTPP(Ph)3(CH3)5 NiTPPBr6(NO2) CuTPP(Ph)4(PE)4 CuTPP(Ph)4(CH3)4 CuTPP(Ph)3(CH3)5 ZnTPP(CH3)4(PE)4(CH3OH) ZnTPP(Ph)4Cl4(py) ZnTPP(Ph)4Br4(CH3OH)

41 28 28 27a 19b 25a 23 27a 25a 41 23

1.892(3) 1.924(4) 1.906(4) 1.909(3) 1.920(6) 1.992(3) 1.954(2) 1.979(4) 2.054(3) 2.078(5) 2.051(7)

1.377(6) 1.373(6) 1.380(7) 1.387(5) 1.382(10) 1,378(5) 1.375(3) 1.381(6) 1.370(5) 1.372(4) 1.404(10)

1.398(7) 1.389(10) 1.395(7) 1.400(5) 1.393(10) 1.399(6) 1.407(4) 1.403(6) 1.405(5) 1.407(6) 1.388(11)

125.7(3) 126.1(4) 125.2(3) 125.8(2) 125.3(5) 125.6(3) 124.8(2) 125.5(3) 125.0(2) 123.8(2) 123.3(5)

107.3(3) 106.1(5) 106.3(5) 105.9(3) 106.4(6) 106.7(3) 107.6(2) 105.9(2) 107.6(3) 107.9(2) 113.2(7)

119.8(3) 120.5(6) 120.1(7) 121.4(3) 120.5(7) 123.6(4) 122.4(2) 123.4(4) 125.1(4) 124.1(3) 124.6(8)

0.529 0.483 0.593 0.589 0.564 0.428 0.536 0.492 0.377 0.406 0.510

1.892 1.924 1.906 1.909 1.919 1.991 1.953 1.979 2.037 2.050 2.024

Mean value along both the transannular pyrrole directions.

Table 9 Normal-coordinate structural decomposition analysis data of the crystal structures of various mixed b-substituted porphyrins and their metal complexes Entry

Crystal structure

Doop, (Å)

B2u, sad, %

B1u, ruf, %

A2u, dom, %

Eg (x,y) wav, %

A2u, pro, %

Ref.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

H2TPP(CHO)(Ph)2 H2TPP(CN)4 H2TPP(Ph)4CN)4 H2TPP(Ph)4Cl4 H2TPP(Ph)4Br4 H2TPP(Ph)4(20 -thienyl)4 H2TPP(CH3)4(20 -thienyl)4 H2TPP(Ph)3Cl5 NiTPP(Ph)3 NiTPP(Ph)4(CN)4 NiTPP(Ph)3Cl5 NiTPP(Ph)3(CN)5 NiTPP(Ph)3(CH3)5 CuTPP(Ph)3(CH3)5 CuTPP(Ph)4(CH3)4 CuTPP(CH3)4(PE)4 CuTPP(Ph)4 ZnTPP(CHO)(CH3OH) ZnTPP(Ph)4(py) ZnTPP(CH3)4(PE)4(CH3OH) ZnTPP(Ph)4Cl4(Py)

0.7895 0.1825 2.4628 3.3055 3.8113 3.2240 3.2223 3.1287 1.9890 3.0910 3.6513 2.8296 3.4270 2.9280 3.2119 2.6601 0.0327 0.7651 0.2635 2.2493 2.5970

57.7 0 69.5 93.7 91.3 91.2 81.5 85.7 40.7 50.5 94.1 73.7 68.1 77.5 59.7 93.0 0.2 65.4 26.5 73.6 85.0

11.6 0.1 14.5 0.4 3.1 3.6 6.2 7.5 51.3 43.1 1.9 17.2 23.6 15.4 36.9 1.9 0.2 9.2 23.1 21.2 1.4

4.4 0.1 7.0 4.8 2.3 4.9 2.6 5.3 1.8 1.3 0.7 1.9 2.6 3.2 2.0 0.3 0.6 20.2 30.6 2.8 5.7

26.0 99.7 7.5 0.9 2.6 0.0 8.2 1.0 5.5 3.6 1.4 5.2 5.4 3.5 0.0 2.5 98.5 3.6 15.3 1.8 6.7

0.4 0 1.1 0.2 0.6 0.3 1.4 0.4 0.7 1.4 1.9 2.0 0.3 0.5 1.4 2.3 0.4 1.5 4.4 0.7 1.2

18a 58 41 41 41 25a 25a 28 27a 41 28 28 27a 27a 60 25a 60 18a 25b 25a 41

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Conclusion In this review, synthesis and physicochemical properties of variety of mixed substituted H2TPPs and their metal complexes have been discussed. Because of the direct p-conjugation, the mixed b-substituents are able to dramatically modulate the properties of the porphyrin p-system and nonplanarity of the macrocycle including its ligand field strength. The HOMO–LUMO gap of the porphyrin p-system were tunable by appending different mixed substituents at the b-pyrrole positions of TPP and the electronic absorption spectral data of the free base porphyrins are correlated well with their electrochemical redox data (DE½). The degree of nonplanarity of the macrocycle has been modulated by varying the size, shape and number of the b-substituents at the antipodal positions of the MTPPs. The porphyrins having electron donor and electron acceptor groups which are directly linked to the p-system could induce large dipole moment in the molecule. The synthesis of these ‘‘push–pull” porphyrins could be useful in applications such as nonlinear optics and dye-sensitized solar cells. The review suggests that by appending appropriate functional groups at the periphery of the macrocycle, one could generate porphyrins with desired properties and they may find use in specific material applications. Acknowledgment This work was supported by the grant from the Department of Science and Technology (Govt. of India), (Grant No. SR/S1/IC30/2007) to Dr. B.P. References and notes 1. (a) Kadish, K. M.; Smith, K. M.; Guilard, R. Handbook of Porphyrin Science In ; World Scientific: London, 2000–2013; Vol. 1–35, and references therein; (b) Milgrom, L. R. The Colours of Life: An Introduction to the Chemistry of Porphyrins and Related Compounds; Oxford University Press: New York, 1997; (c)The Porphyrins; Dolphin, D., Ed.; Academic Press: San Diego, 1979. Vols. 1–8. 2. Warren, M.; Smith, A. Tetrapyrrole: Birth, Life and Death (Molecular Biology Intelligence Unit); Springer, Landes Bioscience, 2009. 3. The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, 2000; Vols. 1–10,. and references cited therein. 4. Chou, J.-H.; Kosal, M. E.; Nalwa, H.; Rakow, N. A.; Suslick, K. S. In Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds., 2000; Vol. 6, p 43. 5. Suslick, K. S.; Bhyrappa, P.; Chou, J. H.; Kosal, M. E.; Nakagaki, S.; Smithenry, D. W.; Wilson, S. R. Acc. Chem. Res. 2005, 38, 283. 6. Handbook of porphyrin Science; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific: London, 2013; Vols. 12 and 18,. and references therein. 7. (a) Lindsey, J. S. Acc. Chem. Res. 2010, 43, 300. and references therein; (b) Syeda, H. H. Z.; Fico, R. M., Jr.; Lindsey, J. S. Org. Pro. Res. Dev. 2006, 10, 118. 8. (a) Senge, M. O. Chem. Commun. 2011, 1943; (b) Takeda, J.; Sato, M. Tetrahedron Lett. 1994, 35, 3565; (c) Senge, M. O.; Gerstung, V.; Ruhlandt-Senge, K.; Runge, S.; Lehmann, I. J. Chem. Soc., Dalton Trans. 1998, 4187. 9. (a) Woller, E. K.; DiMagno, S. G. J. Org. Chem. 1997, 62, 1588; (b) Leroy, J.; Bondon, A.; Toupet, L.; Rolando, C. Chem. Eur. J. 1997, 3, 1890; (c) Wijesekara, T. P.; Matsumoto, A.; Dolphin, D.; Lexa, D. Angew. Chem., Int. Ed. Engl. 1990, 29, 1028; (d) Traylor, T. G.; Tsuchiya, S. Inorg. Chem. 1987, 26, 1338. 10. (a) Senge, M. O.; Fazekas, M.; Notaras, E. G. A.; Blau, W. J.; Zawadzka, M.; Locos, O. B.; Mhuircheartaigh, E. M. N. Adv. Mater. 2007, 19, 2737; (b) Ronchi, M.; Biroli, A. O.; Marinotto, D.; Pizzotti, M.; Ubaldi, M. C.; Pietralunga, S. M. J. Phys. Chem. C 2011, 115, 4240. and references therein. 11. Zhang, C.; Chen, P.; Dong, H.; Zhen, Y.; Liu, M.; Hu, W. Adv. Mater. 2015, 27, 5379. 12. Tanaka, T.; Osuka, A. Chem. Soc. Rev. 2015, 44, 943. 13. Tyler, G. St. D.; Ying-Ying, H.; Michael, H. R. In Handbook of Porphyrin Science; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; World Scientific: London, 2014; Vol. 27, p 255. 14. (a) Sunaina, S.; Aggarwal, A.; Bhupathiraju, N. V. D. K.; Arianna, G.; Tiwari, K.; Drain, C. M. Chem. Rev. 2015, 115, 10261. and references therein; (b) Heidi, A.; Hamblin, M. R. Biophys. J. 2016, 473, 347. 15. Bella, F.; Gerbaldi, C.; Barolo, C.; Grätzel, M. Chem. Soc. Rev. 2015, 44, 3431. and references therein. 16. Urbani, M.; Grätzel, M.; Nazeeruddin, K.; Torres, T. Chem. Rev. 2014, 114, 12330. and references cited therein. 17. (a) Li, L.-L.; Diu, E. W.-G. Chem. Soc. Rev. 2013, 42, 291; (b) Higashino, T.; Imahori, H. Dalton Trans. 2015, 44, 448.

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