pyrazine amide ligands. Some noteworthy results

Coordination Chemistry Reviews 257 (2013) 350–368

Contents lists available at SciVerse ScienceDirect

Coordination Chemistry Reviews journal homepage: www.elsevier.com/locate/ccr

Review

Coordination chemistry with pyridine/pyrazine amide ligands. Some noteworthy results Amit Rajput, Rabindranath Mukherjee ∗,1 Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India

Contents 1. 2.

3.

4.

Introduction: purpose and scope of the review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The ligands and the complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. One pyridine carboxamide unit (bidentate ligands) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. One pyridine carboxamide unit and appended (2-pyridyl)alkanes (tridentate, tetradentate, and pentadentate ligands) . . . . . . . . . . . . . . . . . 2.3. One pyridine carboxamide unit with phenol/thiol/azo/bis(2-pyridyl)amine functionality (tridentate and pentadentate ligands) . . . . . . . 2.4. Two pyridine carboxamide units (tridentate ligands) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5. Two pyridine carboxamide units with aliphatic amine (tetradentate behavior) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.6. Two pyridine carboxamide units (tetradentate ligands) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.7. Two pyridine carboxamide units (pentadentate ligands) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8. One pyridine carboxamide unit (bidentate bridging ligands) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.9. Two pyridine/pyrazine carboxamide units (tridentate bridging ligands) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Noteworthy results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Role of axial ligands on the spin-state of iron(III) of tetradentate ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Effect of the donor atom type, geometry, and spin-state on CoIII –CoII redox potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Nickel is stabilized in its bivalent, trivalent, and tetravalent state with a common tridentate ligand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. Effect of CuII N4 geometry on CuII -CuI redox potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. C S bond cleavage and cyclometallation reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Note Added in Proof . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

a r t i c l e

i n f o

Article history: Received 30 January 2012 Received in revised form 16 March 2012 Accepted 21 March 2012 Available online 7 April 2012 Dedicated to Professor Edward Solomon on his 65th birthday. Keywords: Pyridine/pyrazine-2-carboxamide ligands Pyridine-2,6-dicarboxamide ligands Coordination complexes Crystal structures Redox properties Notable properties

351 351 351 354 355 357 357 358 360 364 364 366 366 366 366 366 366 367 367 367 367

a b s t r a c t Pyridine-2-carboxamide and pyridine-2,6-dicarboxamide-based chelating ligands form a variety of coordination complexes with a number of metal ions, providing varying coordination geometry and nuclearity. Recent years have seen considerable interest in the designing of this class of ligands and to study their structural properties to serve specific stereochemical requirement of a particular metalbinding site. Notably, this class of ligands has been extensively utilized by Mascharak and co-workers to provide low-molecular-weight representations of metallo-proteins/enzymes such as bleomycins, nitrile hydratase. Moreover, the transition metal complexes of this class of ligands are being used as various exogenous nitric oxide (NO) donors. Using over 60 this class of chelating carboxamide ligands, the stereochemical properties of over 150 discrete coordination complexes, studied by single-crystal X-ray crystallography have been analyzed. Various bonding modes for a given chelating ligand are involved, and are reviewed with reference to ligand structure and the resulting coordination complexes. It is shown that the complexes synthesized have served to address notable issues such as effect of ligand structure/donor atom on metal-centered redox potentials, change of spin-state of iron(III), ligand-radical coordinated metal-complexes, interesting chemical reactivity studies, and catalytic potential. The ligands are introduced systematically as a function of their denticity, making easy access to information on specific type of ligands and coordination complexes thereof. X-ray crystallographically determined

∗ Corresponding author. Tel.: +91 512 2597437; fax: +91 512 2597436. E-mail addresses: [email protected], [email protected] (R. Mukherjee). 1 Present address: Indian Institute of Science Education and Research Kolkata, Mohanpur Campus, Mohanpur – 741 235, India. 0010-8545/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ccr.2012.03.024

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bond lengths of various donor atoms/groups are collected in a table, thus providing an accessible source for reference purposes. Source material for the review amounts to about 90 references. © 2012 Elsevier B.V. All rights reserved.

1. Introduction: purpose and scope of the review The coordination chemistry of bispyridylamides and applications of such complexes in catalysis have been reviewed in 2005 in this journal [1]. There has been an explosion of growth in the coordination chemistry of pyridine/pyrazine amide ligands in both the nonbiological and biological areas. There is thus a scope and this article therefore attempts to highlight some notable results in the coordination chemistry of pyridine/pyrazine amide ligands, in their deprotonated forms. Deprotonated pyridine amide groups usually coordinate via the nitrogen atom whereas coordination via oxygen is more frequent for neutral ligands [2]. The anionic ligand is a strong ␴-donor capable of stabilizing metal ions in high (≥3) oxidation states. The properties of metal–ligand coordination compounds are determined in large measure by the nature of ligands bound to the metal ion. Focus is therefore placed on the design of chelating ligands whose metal-binding sites differ in the nature of the donor atom. The purpose of this article is (i) to update the progress made in the design of pyridine/pyrazine amide ligands and (ii) to highlight some notable results of the coordination chemistry developed, using deprotonated forms of the chelating ligands. A large amount of structural data for coordination complexes of selected transition metal ions with various ligand types has been accumulated, giving rise to an opportunity to look into their structural varieties in a systematic manner. Emphasis has been directed to the structural patterns, redox properties, and reactivity aspects. Importance and relevance of coordination chemistry of pyridine carboxamide ligands in the context of bioinorganic chemistry has been kept outside the scope of this article. However, relevant literature references of such chemistry have been provided for ready reference [3–7]. The subject matter has been broadly divided into two parts. First part deals with the ligand design, the coordination complexes synthesized and their structural characterization, and compilation of their metal–ligand bonding parameters in a systematic manner. Ligand design is organized by ligand denticity, with further subdivision into ligand donor type, the nature of spacers between two chelating units, and the nature of heterocycle ring (pyridine or pyrazine). The sections dealing with specific ligand donor types cut across several structural types. For each subgroup of ligands the following facts will be presented: the types of coordination compounds, molecular structural aspects, and selected properties. Second part deals with specific coordination complexes for discussion because they exemplify important concepts concerning the relationship of ligand structure to properties of the metal complexes. Notable contributions from author’s laboratory and by others have been presented. Specifically, attention is placed to highlight (i) the spin-state variation of iron(III) complexes as a function of the nature of axial ligands, (ii) the generality and versatility of deprotonated pyridine amide ligands to highly stabilize trivalent state of iron, cobalt, and ruthenium, (iii) the versatility of a common tridentate pyridine-2,6-dicarboxamide ligand to stabilize bivalent, trivalent, and tetravalent states of nickel, (iv) the noninnocence nature of 1,2-phenylenediaminebased tetradentate ligand to stabilize ligand-radical coordinated low-spin iron(III) and tridentate pyridine-2,6-dicarxamide ligandradical coordinated low-spin ruthenium(III), (v) base-assisted C S bond cleavage of SCH2 CH2 S spacer and Co C bond formation with SCH2 CH2 CH2 S spacer in the case of cobalt complexes, and

(vi) effect of donor atom type and stereochemistry at the metal center on the trends of E1/2 values of FeIII –FeII , CoIII –CoII , CoII –CoI , NiIV –NiIII , NiIII –NiII , and CuII –CuI redox processes. Although there has been much interest in the variety of bonding possibilities associated with pyridine-2-carboxamide- and pyridine-2,6-dicarboxamide-based chelating ligands, in their neutral and deprotonated form, and the amount of structural, spectral, redox, and reactivity properties of their coordination complexes in general and bioinorganic perspectives in particular reported in the literature, so far there is no general review article on the present theme. Since 1990 an extensive variety of coordination complexes with such ligands, and their structures and properties have been published by our group. Therefore we now undertake an attempt to present an overview of these compounds. The purpose of this review is to update the progress made (i) in the design of chelating ligands having pyridine-2-carboxamideand pyridine-2,6-dicarboxamide functionality and (ii) in the development of systematic coordination chemistry thereof, utilizing deprotonated form of such ligands. Understandably, every reference cannot be cited and selection has had to be severely limited. Priority is given mainly to work related to structural chemistry, redox properties, and reaction chemistry. Only those structures that exist as discrete molecules, as opposed to a polymeric network extending throughout the crystal lattice, will be considered. The present review also does not cover categorically multinuclear (more than three metal centers have been excluded) cluster compounds, neither does it attempt to explore specifically organometallic compounds, extensive supramolecular networks, helical structures, and work of an essentially magnetic and catalytic nature. This review concentrates on notable chemistry developed, with coverage until end of 2011, and it is hoped that it will make researchers in this area aware of a wider range of available ligands and aid them in selecting appropriate ligands for their specific requirements. 2. The ligands and the complexes In all the complexes described in this article the metal ion is bound to a large variety of chelating ligands containing at least one pyridine/pyrazine amide moiety. The relevant metric parameters for all the structurally characterized complexes of pyridine/pyrazine-2-carboxamide- and pyridine-2,6dicarboxamide-based chelating ligands considered in this article, are collected in Table 1. 2.1. One pyridine carboxamide unit (bidentate ligands) The ligands HL1 –HL6 are considered in this section (Fig. 1). The complex [AuIII (L1 )Cl2 ] (1) (S = 0) is nearly square-planar with the two rings being almost coplanar [8]. The copper(II) geometry in [CuII (L2 )2 ] (2) (S = 1/2) is distorted toward tetrahedral. In fact, the dihedral angle between the two CuII N2 planes is ∼40◦ [9]. Compared to the structurally characterized square-pyramidal structure of [CuII (L27 )(H2 O)] [10] (see below) it is revealed that two L2 (−) ligands exert a measurable degree of geometric control over the coordination sphere of 2 (distorted toward tetrahedral). The CuII –CuI redox potentials in N,N -dimethylformamide (DMF) solution were determined by cyclic voltammetry. Upon replacement of the tetradentate ligand L27 (2−) by two bidentate ligands

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Table 1 ˚ of the structurally characterized complexes. Selected average bond distances (A) Complexes III

1

[Au (L )Cl2 ] (1) [CuII (L2 )2 ] (2) [PtII (L3 )2 ] (3) [PtII (L3 )(HL3 )Cl] (4) [RuIII (L4 )3 ] (5) [CoIII (L4 )3 ] (6) [RuIII (L5 )(HL5 )Cl2 ] (7) [CoII (L6 )2 (H2 O)]·H2 O (8) [CuII (L6 )2 (H2 O)]·H2 O (9) [CuII (L7 )(4-MePy)(H2 O)][ClO4 ]·H2 O (10) [CuII (L7 )(4-MeImH)(H2 O)][ClO4 ]·H2 O (11) [CuII (L8 )(3-MePy)(H2 O)][ClO4 ] (12) [CuII (L8 )(4-MePy)(H2 O)][ClO4 ] (13) [CuII (L8 )(4-MeImH)(H2 O)][ClO4 ]·H2 O (14) [MnIII (L7 )2 ][PF6 ] (15) [FeIII (L7 )2 ][BF4 ] (16) [CoIII (L7 )2 ][ClO4 ] (17) [CoIII (L9 )2 ][ClO4 ]·MeOH (18) [FeII (L9 )2 ]·H2 O (19) [FeIII (L9 )Cl2 (H2 O)]·Me2 CO (20) [FeIII (L9 )2 ][NO3 ]·1.67H2 O (21) [CuII (L10 )Cl] (22) [CuII (L10 )(MeOH)Cl] (23) [CuII (L10 )(EtOH)Cl] (24) [CuII (L10 )Cl]2 (25) [CuII (L10 )]2 [ClO4 ]2 (26) [FeII (L10 )2 ]·0.5MeCO2 H (27) [FeIII (L11 )Br2 (MeOH)] (28) [FeIII (␮-OH)2 (L11 )2 Br2 ]·2MeOH (29) 2 [FeIII (␮-OMe)2 (L12 )2 Br6 ] (30) 4 Na[FeIII (L13 )2 ]·2.5MeCN (31) [Et3 NH][(VV O(L13 )(␮-O)(VIV O(L13 )] (32) [Et3 NH][VIII (L13 )2 ] (33) [Et4 N][FeIII (L13a )2 ] (34) [NiII (L14 )2 ] (35) [FeIII (L15 )(MeCN)][ClO4 ]2 (36) [FeIII (L15 )Cl][ClO4 ] (37) [FeIII (L15 )(CN)][ClO4 ] (38) [CuII (L16 )(py)]2 (39) [CuII (L16 )(py)(H2 O)] (40) [CuII (L16 )(bpy)] (41) [CuII (L16 )(Me2 -bpy)]·NaClO4 ·MeCN (42) [CuII (L16 )(o-phen)]·NaClO4 ·1.5MeCN (43) [CuII (L16 )(Me2 phen)]·NaClO4 ·1.2MeCN·0.4toluene (44) [Et4 N][FeIII (L17 )2 ]·1.5H2 O (45) [Et4 N][CoIII (L17 )2 ]·2H2 O (46) [Et4 N]2 [NiII (L17 )2 ]·H2 O (47) [NiIV (L17 )2 ]·0.75H2 O (48) Na[CuII2 (L17 )2 (OH)]·2H2 O (49) K[CuII2 (L17 )2 (OH)]·2H2 O (50) [Et4 N][RuIII (L17 )2 ]·H2 O (51) [PdII (L17 )(NCMe)] (52) [RuII (trpy)(L18 )] (53) [RuIII (trpy)(L19 )][ClO4 ] (54) [NiII (L20 )(H2 O)] (55) [PdII (L21 )(NCMe)] (56) [(L21 )PdII (qd)PdII (L21 )] (57) [PdII (L22 )(3,5-diethynylpyridine)] (58) [Et4 N]2 [CuII (L23 )(O2 CMe)Cl]·1.5MeCN (59) [Et4 N]2 [CuII (L23 )(Cl)Cl]·MeCN (60) [CuII (L24 {H}2 )Cl2 ]·H2 O (61) [RuII Cl2 (PPh3 )(L25 {H}2 )] (62) [RuII (NO2 )(NO)(PPh3 )(L25 )] (63) [NiII (L26 ){H}][ClO4 ] (64) [VOIV (L27 )(H2 O)]·0.5DMSO·0.36MeOH (65) [CoIII (L27 )Et(H2 O)] (66) [CoIII (L28 )(CH2 CH2 CMe CH2 )(H2 O)] (67) [RhIII (L27 )(py)2 ][ClO4 ] (68) [Et4 N][FeIII (L28 )(MeCO2 )2 ]·CHCl3 (69) Na[CoIII (L28 )(CN)2 ]·MeOH (70) Na[FeIII (L27 )(CN)2 ] (71) [(n-Bu)4 N][FeIII (L27 )(N3 )2 ]·1/2toluene (72) [(n-Bu)4 N][FeIV (␮-N)(L27 )2 (CN)2 ] (73) 2 [(n-Bu)4 N][FeIV (␮-N)(L27 )(N3 )2 ] (74) 2 [FeIII (L28 )(l-MeIm)2 ][ClO4 ] (75) [FeIII (L28 )Cl(DMF)] (76)

Average MII/III –NPy /MII/III –NPz

Average MII/III –Nam

Average MII/III –X

Reference

2.086(6) 2.006(2) 2.081(6) 2.068(7) 2.070(17) 1.9496(2) 2.0615(4) 2.1095(7) 1.9935(5) 2.0215(5) 2.033(6) 2.042(4) 2.046(4) 2.035(1) 2.147(12) 1.978(3) 1.993(2) 1.9421((14) 1.9425(8) 2.137(4) 1.958(6) 2.021(2) 2.0297(14) 2.0341(18) 1.990(15) 2.017(2) 2.1675(2) 2.131(4) 2.129(4) 2.1005(6) 2.178(5) 2.094(2) 2.1141(19) 2.000(3) 2.087(3) 1.972(3) 1.986(17) 1.984(2) 1.923(2) 1.9330(13) 1.925(4) 1.929(4) 1.928(2) 1.942(2) 1.874(3) 1.845(3) 1.994(7) 1.846(8) 1.909(1) 1.927(1) 1.945(9) 1.953(8) 1.9650(4) 1.9820(2) 2.009(3) 1.924(3) 1.922(8) 2.0045(4) 1.945(4) 1.944(3) 1.9505(18) 1.947(4) 2.016(3) 1.818(19) 2.149(2) 1.975(4) 1.970(4) 2.0925(3) 2.202(2) 1.9755(1) 1.9965(1) 2.186(4) 2.0105(2) 2.0125(2) 2.000(4) 2.168(9)

1.969(6) 1.931(2) 2.081(7) 1.968(9) 2.046(16) 1.950(2) 1.999(4) 2.010(7) 2.0055(5) 1.904(4) 1.910(4) 1.931(4) 1.937(3) 1.950(1) 1.917(11) 1.888(4) 1.881(3) 1.8944(14) 1.9235(7) 2.076(4) 1.908(5) 1.914(2) 1.9223(15) 1.9289(15) 1.9209(15) 1.9079(19) 2.079(8) 2.038(4) 2.046(4) 2.031(4) 2.064(4) 2.037(2) 2.068(18) 1.954(2) 2.001(3) 1.826(3) 1.8559(17) 1.858(2) 1.998(2) 2.0054(13) 1.996(5) 2.0105(4) 1.990(2) 1.999(2) 1.971(3) 1.964(4) 2.131(8) 1.918(8) 2.023(1) 2.0145(1) 2.053(7) 2.0065(8) 2.074(4) 2.054(2) 2.077(3) 2.0235(3) 2.025(9) 2.0275(4) 2.028(4) 2.0375(3) 2.8810(18) 2.1475(4) 2.115(3) 1.924(16) 2.014(2) 1.875(2) 1.877(4) 1.971(3) 2.064(2) 1.9065(1) 1.8925(1) 2.0585(4) 1.908(4) 1.906(2) 1.896(4) 2.044(8)

2.269(2)

[8] [9] [11] [11] [12] [13] [14] [15] [15] [17] [17] [18] [18] [18] [19] [20] [20] [20] [21] [21] [21] [22] [22] [22] [22] [22] [23] [23] [23] [23] [24] [25] [25] [26] [27] [28] [28] [28] [29] [29] [29] [29] [29] [29] [30] [30] [31] [31] [32] [32] [33] [34] [35] [35] [36] [37] [38] [39] [40] [41] [41] [42] [42] [43] [44] [16] [16] [47] [48] [50] [50] [50] [50] [50] [51] [52]

2.039(6) 2.266(4) 2.368(4) 2.380(5) 2.361(4) 2.405(3) 2.380(1)

2.2312(8) 2.2253(5), 2.3297(14) 2.2260(5), 2.3182(14) 2.7029(5)

1.962(4) 1.611(18), 1.7496(17) 2.3675(7) 2.229(1) 1.982(3) 2.2833(5) 1.974(3)

1.899(1) 1.913(1) 2.017(9)

2.014(3) 1.998(8) 2.052(4) 1.949(3), 2.819(4) 2.2564(9), 2.7483(10) 2.3580(12) 2.4907(8) 2.4374(10), 1.745(3), 2.144(3) 1.940(17) 1.607(2), 2.241(2) 1.977(4), 2.137(3) 1.994(7), 2.136(5) 2.055(3) 2.972(2) 1.92(1) 1.969(2) 2.0325(5) 2.0655(2) 1.975(4) 2.272(3), 2.068(7)

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Table 1 (Continued) Complexes III

28

[{Fe (L )(H2 O)}2 O]·3H2 O (77) [CoIII (L30 )(prldn)2 ][ClO4 ] (78) [NiII (L30 )] (79) [CuII (L30 )] (80) [RuII (NO)(L31 )Cl]·2H2 O (81) [RuII (NO)(L32 )Cl]·2H2 O (82) [CuII (L33 )(H2 O)]·3H2 O (83) [CuII (L34 )]·0.5EtOH·0.5MeOH (84) II [Ni2 (L35 )2 (H2 O)2 ] · 6H2 O (85) [CuII2 (L35a )2 ] · H2 O (86) [CuII (L35b )]·3H2 O (87) [CuII (L36 )] (88) Na[FeIII (L37 )2 ]·1.5MeCN·0.5DMF (89) Na[FeIII (L38 )2 ]·DMF (90) [NiII (L37 )] (91) [CoIII (L37 )(HL37 )] (92) II [Ni3 (L37 )2 (O2 CMe)2 (MeOH)2 ] (93) [CuII (L38 )] (94) [CuII (L39 )]·2H2 O (95) [CuII (L40 )]·2MeOH (96) [NiII (L41 )]·CHCl3 (97) [CuII (L41 )] (98) [ZnII (L42 )]·toluene-d8 (99) Na[FeIII (L43 )(1-MeIm)2 ]·3MeCN (100) Na3 [FeIII (L43 )(NCS)2 ]·3DMF2MeCN (101) [Et4 N][FeIII (L43a )2 ]·2H2 O (102) [Et4 N]2 [FeII (L44 )] (103) [Et4 N][FeIII (L44 )] (104) [Et4 N][FeIII (L44a )2 ]·1.7MeCN Et2 O (105) [Et4 N][CoIII (L44a )2 ] (106) [FeIII (L45 )Cl(H2 O)]·MeOH (107) [FeII (HL45 )(Cl)(MeOH)] (108) [FeII (L45 )(MeOH)2 ]·MeOH (109) [FeIII (L45 )(1-MeIm)]2 O·2MeCN (110) [FeII (L46 )(MeOH)] (111) [FeII (L46 )(MeOH)2 ]·MeOH (112) [FeIII (L46 )(DMSO)]2 O·2DMSO (113) [CuII (L47 )(H2 O)][ClO4 ]·H2 O (114) [NiII (L47 )]·H2 O (115) [CoIII (L47 )][ClO4 ]0.75 [Cl]0.25 ·2.5H2 O (116) [CoIII (L49 )][ClO4 ]·H2 O (117) [CoII (L51 )] (118) [NiII (L51 )] (119) [CuII (L51 )] (120) [CuII (L52 )] (121) [CuII (L53 )]·DMSO (122) [NiII (L52 )] (123) [NiII (L53 )] (124) [FeII (L52 )] (125) [FeII (L54 )]·CH2 Cl2 (126) [CoIII (L* )(L** )]·2MeOH (127) [CoII (L55 )]·C6 H6 (128) [CoIII (L55 )][ClO4 ] (129) [CoIII ((L55 )] (130) [NiII (L54 )]·MeOH (131) [CuII (L54 )]·CH2 Cl2 (132) [CoIII,II (L56 )3 (Cl)]Cl·MeOH (133) 2 [CoIII,II (L56 )3 (Br)]Br·MeOH (134) 2 [CoIII ZnII (L56 )3 (Cl)]Cl·MeOH·5H2 O (135) [CoIII (L57 )3 ]·19H2 O (136) 2 [CuII2 (L58 )Cl2 ]·MeCN (137) [CuII2 (L59 )(MeCN)2 (H2 O)2 ][BF4 ]2 ·H2 O (138) [CuII2 (L60 )(MeCN)2 (H2 O)2 ][BF4 ]2 (139) [CuII2 (L59 )(H2 O)4 ][SiF6 ] (140) [CuII2 (L60 )(H2 O)4 (BF4 )2 ]·2H2 O (141) [CuII3 (L61 )2 (␮2 -O2 CMe)2 ]·CHCl3 ·0.25H2 O(142) [CuII3 (L62 )2 (␮2 -O2 CMe)2 ]·1.5MeCN·0.25H2 O (143) [CoIII (L2a )3 ]·DMF (144) [CoIII (L24 )2 ]·H2 O (145) [CoIII (L2a )3 ZnII Cl2 ]·MeCN (146) [CoIII (L24 )2 (ZnII Cl2 )(ZnII (Cl)(DMA)] (147) [CoIII (L24 )2 {CuI (MeCN)}2 ][ClO4 ] (148) [FeIII (L24 )2 {CuI (MeCN)}2 ][ClO4 ] (149) [CoIII (L24 )2 {CdII }2 (NO3 )3 (DMF)2 ] (150) [CoIII (L24 )2 {HgII }2 (NO3 )3 (DMSO)2 ] (151)

Average MII/III –NPy /MII/III –NPz

Average MII/III –Nam

Average MII/III –X

Reference

2.153(5) 1.9955(3) 1.935(2) 2.023(3) 2.123(3) 2.188(6) 2.049(3) 1.9815(3) 2.1425(17) 2.1315(4) 2.1409(19) 1.928(2) 1.881(3) 1.875(2) 1.882(2) 1.930(4) 2.009(3) 2.067(2) 1.929(6) 1.950(2) 1.926(15) 1.975(2) 2.022(3) 2.224(4) 2.063(5) 1.875(3) 2.129(2) 2.099(3) 2.081(3) 1.8468(16) 2.193(4) 2.1999(11) 2.203(2) 2.197(4) 2.1714(4) 2.180(6) 2.193(4) 2.011(3) 2.139(2) 1.938(5) 1.9464(12) 2.0795(3) 2.049(2) 2.0495(4) 2.155(4) 2.056(3) 2.0665(2) 2.063(3) 1.9728(18) 1.938(2) 1.9965(5) 2.041(3) 1.9345(5) 1.996(3) 2.085(3) 2.1797(18) 1.940(5), 2.019(5) 1.953(6), 2.033(6) 1.9367(6), 2.018(6) 1.9450(5) 2.003(2) 1.999(3)/2.044(3) 2.0015(3)/2.024(2) 2.007(5)/2.030(4) 1.981(4)/2.021(4) 2.0327(3) 2.038(10) 1.9467(3) 1.871(7) 1.949(4), 2.036(5) 1.872(7), 2.046(8) 1.852(4), 1.965(4) 1.872(5), 1.968(5) 1.8565(4), 2.2917(5) 1.859(12), 2.1570(14)

2.086(5) 1.892(3) 1.844(2) 1.930(3) 1.9815(3) 2.0365(6) 1.9445(3) 1.9295(3) 2.031(16) 1.9345(4) 1.9622(18) 1.9905(2) 1.9555(3) 1.962(2) 1.884(2) 1.929(4) 2.183(3) 1.994(3) 2.037(7) 1.985(1) 1.998(14) 2.060(2) 2.069(3) 2.1375(4) 2.092(6) 1.979(3) 2.1695(2) 2.038(3) 2.093(3) 1.9578(18) 2.156(4) 2.0837(11) 2.189(3) 2.1865(5) 2.2148(14) 2.209(6) 2.1715(4) 1.907(3) 2.005(1) 1.882(5) 1.904(11) 2.007(3) 1.979(2) 1.946(5) 1.977(3) 1.952(30 2.0255(2) 2.020(3) 2.2518(6) 1.965(2) 1.945(5) 1.944(3) 1.9265(6) 1.939(3) 2.0288(3) 1.9754(18) 1.945(5) 1.962(6) 1.944(60 1.958(4) 1.9305(2) 1.906(3) 1.933(2) 1.904(4) 1.946(4) 2.009(3) 2.0437(10) 1.9463(3) 1.9857(8) 1.955(4) 1.9615(7) 1.9585(4) 1.968(4) 1.958(4) 1.9755(12)

2.145(4), 1.779(11) 2.002(3)

[53] [54] [55] [56] [57] [57] [58] [59] [60] [61] [61] [62] [63] [63] [64] [64] [64] [65] [66] [66] [67] [67] [67] [68] [68] [69] [70] [70] [69] [71] [72] [73] [73] [73] [73] [73] [73] [74] [74] [74] [76] [77] [77] [77] [78] [78] [78] [78] [79] [79] [80] [80] [80] [80] [81] [81] [82] [82] [82] [83] [84] [85] [85] [85] [85] [56] [56] [86] [86] [86] [86] [87] [87] [88] [88]

2.3833(12), 1.742(4) 2.3683(19), 1.732(6) 2.354(3) 2.0993(16)

2.142(3)

2.1015(3) 2.0955(7) 2.050(1) 2.067(15) 1.995(2) 2.0735(3)

2.3938(8) 2.3125(13) 1.958(14) 2.3573(13), 2.041(3) 2.2249(13), 2.3842(8) 2.325(3), 2.277(3) 2.2982(5), 1.816(6), 2.2585(5) 2.3072(14), 2.2374(13) 2.3725(6), 2.216(5) 2.265(4), 2.0138(6) 2.375(3)

2.205(4) 2.434(4) 2.253(19) 2.5195(7) 2.503(1) 3.006(1) 2.4058(7) 2.418(1) 2.2372(9) 2.2697(3) 2.3617(12) 2.2165(2) 2.001(4), 2.2017(16) 2.4002(18) 2.4934(8) 2.226(2) 2.3752(14) 2.2928(17) 2.4257(9) 1.972(3), 2.201(3) 1.934(2), 2.310(3) 2.110(5) 2.2385(5) 1.989(3) 1.976(9)

2.2418(19) 2.2185(4), 1.946(12) 1.9515(5) 1.965(5) 2.392(7) 2.522(17)

354

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Fig. 1. Ligand structures (bidentate: HL1 –HL6 ).

L2 (−), of similar donor set, a marked anodic shift (ca. 0.6–0.7 V) was observed, implying the predominance of structural effects. Using HL3 two complexes [PtII (L3 )2 ] (3) (S = 0) and [PtII (L3 )(HL3 )Cl] (4) (S = 0) were synthesized and structurally characterized. One of the two ligands in 3 undergoes a fast ring-opening reaction to give 4 in the presence of Cl− . In 4 only the pyridine ring of HL3 coordinates. A cis–trans isomerization reaction has occurred during the conversion from complex 3 to 4. Complex 4 demonstrated notable activity against human and murine leukemia cells (HL-60 and P388), but rather weak potency against lung adenocarcinoma A-549 [11]. The stereochemistry at the metal center in [RuIII (L4 )3 ] (5) (S = 1/2) is distorted from the ideal octahedral geometry. The relative disposition of the pyridine and amide nitrogens shows that the ligand coordinates in the meridional stereochemistry [12]. Four cobalt(III) complexes ([CoIII (L4 )3 ] (6) was structurally characterized) (S = 0) have high catalytic activities and excellent selectivities in the oxidation of ethylbenzene to acetophenone using O2 as oxidant, without need of solvent [13]. The epoxidation of cyclic alkenes with molecular oxygen was efficiently completed in excellent epoxide yield using [RuIII (L5 )(HL5 )Cl2 ] (7) (S = 1/2), as catalyst under mild reaction conditions [14]. Using HL6 two new five-coordinate complexes of composition [MII (L6 )2 (H2 O)]·H2 O [M = CoII (8) (S = 3/2) or CuII (9) (S = 1/2)] have been structurally characterized. The coordination geometry at CoII and CuII is approximately trigonal-bipyramidal, being more distorted in the case of CuII [15]. The effect of geometry on CoIII –CoII redox potential of 8 has been compared with that of CoII (L27 )·H2 O [16] and systematically analyzed. Similarly, the CuII –CuI redox potential of 9 was compared with that of 2 [9] and [CuII (L27 )(H2 O)] [10]. 2.2. One pyridine carboxamide unit and appended (2-pyridyl)alkanes (tridentate, tetradentate, and pentadentate ligands) The ligands HL7 –HL15 are considered in this section (Fig. 2). Four mixed–ligand copper(II) complexes (S = 1/2) of square pyramidal geometry [CuII (L7 )(4-MePy)(H2 O)][ClO4 ]·H2 O (10) (4-MePy = 4methylpyridine), [CuII (L7 )(4-MeImH)(H2 O)][ClO4 ]·H2 O (11) (4MeImH = 4-methylimidazole), [CuII (L8 )(3-MePy)(H2 O)][ClO4 ] (12) (3-MePy = 3-methylpyridine), [CuII (L8 )(4-MePy)(H2 O)][ClO4 ] (13), and [CuII (L8 )(4-MeImH)(H2 O)][ClO4 ]·H2 O (14) were reported [17,18]. The complex [MnIII (L7 )2 ][PF6 ] (15) (S = 2) undergoes oneelectron oxidation and reduction at 1.05 V and −0.20 V (versus Ag/AgCl electrode), respectively. Density functional theory (DFT) calculations show that both redox processes have significant ligand character. DFT calculations also show that there is strong electronic coupling between the central metal ion and the amide ligands, which leads to higher ligand-to-metal charge-transfer and thus

higher metal–ligand covalency with increasing oxidation state on the central metal ion. This implies that the amide ligand L7 (−) is redox non-innocent [19]. The complexes [FeIII (L7 )2 ][BF4 ] (16) and [CoIII (L7 )2 ][ClO4 ] (17) have been prepared. The CH2 moiety of L7 (−) in [MIII (L7 )2 ]+ [M = Fe and Co] is very reactive and is readily converted to carbonyl group, upon exposure to dioxygen. Such conversion results in [MIII (L9 )2 ][ClO4 ] complexes (M = Fe and Co). The complex [CoIII (L9 )2 ][ClO4 ]·MeOH (18) was structurally characterized [20]. The complexes [FeII (L9 )2 ]·H2 O (l9) (S = 0), [FeIII (L9 )Cl2 (H2 O)]·Me2 CO (20) (S = 5/2), and [FeIII (L9 )2 ][NO3 ]·1.67H2 O (21) (S = 1/2) were also reported [21]. The novel tripodal ligand HL10 afforded monomeric and dimeric copper(II) complexes. The complex [CuII (L10 )Cl] (22) remains monomeric and planar with a pendant pyridine. When 22 was dissolved in alcohols, square-pyramidal alcohol adducts [CuII (L10 )(MeOH)Cl] (23) and [CuII (L10 )(EtOH)Cl] (24) were readily formed. In 23 and 24, the ROH molecules are bound at axial site of copper(II) and the weak axial binding of the ROH molecule is strengthened by intramolecular hydrogen-bonding between ROH and the pendant pyridine nitrogen. Two ligand-shared dimeric species [CuII (L10 )Cl]2 (25) and [CuII (L10 )]2 [ClO4 ]2 (26) were also synthesized in which the pendant pyridine of one [CuII (L10 )] unit completes the coordination sphere of the other [CuII (L10 )] neighbor. These ligand-shared dimers were obtained in aqueous solutions or in complete absence of chloride in the reaction mixtures [22]. Using HL10 the syntheses of a number of complexes were achieved: [FeII (L10 )2 ]·0.5MeCO2 H (27) (S = 2), [FeIII (L11 )Br2 (MeOH)] (28) (S = 5/2), [FeIII (␮-OH)2 (L11 )2 Br2 ] · 2MeOH (29), and 2 (␮-OMe)2 (L12 )2 Br6 ] (30). In these complexes L10−11 (−) [FeIII 4 and L12 (2−) act as a meridional tridentate ligand. In 27 the two L10 (−) ligands are coordinated to the FeII center via only three nitrogen atoms in a meridional fashion, and one of the pyridyl arms of the bis(2-pyridyl)methyl amine remains uncoordinated. This observed ligation mode suggests that the anionic ligand prefers to maintain a planar pyridine-2-carboxamidate moiety, preventing the coordination of the remaining pyridine. In 28 the third pyridine ring in the ligand is uncoordinated but engages in a hydrogen-bonding interaction with the proton of the coordinated ˚ An unexpected feature of this MeOH (N· · ·O distance = 2.67 A). complex is the replacement of the tertiary hydrogen atom of the ligand with a methoxy group. Complex 29 assumes a dimeric 4+

structure with an {FeIII (␮-OH)2 } core. The tetrameric complex 2 30 was obtained as a by-product in the preparation of 29. In this case, the tertiary C H hydrogen atom on the ligand HL9 was replaced by an oxygen atom, presumably derived from residual water in the solvent. The crystal structure of 30 shows that the resulting L12 (2−) ligand behaves as a pentadentate ligand [23].

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355

Fig. 2. Ligand structures (tridentate with one carboxamide unit: HL7 – HL15 ).

2.3. One pyridine carboxamide unit with phenol/thiol/azo/bis(2-pyridyl)amine functionality (tridentate and pentadentate ligands) The complex Na[FeIII (L13 )2 ]·2.5MeCN (31) (S = 5/2) reveals two L13 (2−) ligands coordinated to an FeIII center in a mer geometry. In the solid state, 31 forms an elaborate network of [FeIII (L13 )2 ] and Na+ ions bridged through carbonyl and phenolato oxygens with additional coordination of MeCN to the Na+ center [24]. Reaction of the disulfide-bridged form of H2 L13a with [VOCl2 (THF)2 ] leads to reductive scission of the disulfide bond and formation of the mixedvalence complex [Et3 NH][{VV O(L13 )}(␮-O){(VIV O(L13 )}] (32), with the dianionic ligand coordinating through the pyridine-N atom, the deprotonated amide-N atom, and thiophenolate-S atom. Reaction between [VCl3 (THF)3 ] and H2 L13 yielded [Et3 NH][VIII (L13 )2 ] (33) (S = 1) [25]. The coordination geometry around iron in [Et4 N][FeIII (L13a )2 ] (34) (S = 1/2) is distorted octahedral with two deprotonated carboxamido nitrogens, one pyridine N, and a thiolato S constituting the basal plane while the other pyridine N and the second thiolato S occupy the axial positions. The two L13a (2−) ligands are coordinated in a mer fashion. In DMF, the half-wave potential (E1/2 ) of 34 is −1.12 V versus SCE (saturated calomel electrode) [26]. A new potentially tridentate ligand HL14 consisting of 2pyridinecarboxamide unit and azo functionality has been used to prepare [NiII (L14 )2 ] (35) [27]. The two L14 (−) ligands bind to the NiII center in a mer configuration. The relative orientations within the

pairs of pyridyl-N, deprotonated amido-N, and azo-N atoms are cis, trans, and cis, respectively. The NiII N2 (pyridyl)N2  (amide)N2  (azo) coordination environment is severely distorted from ideal octahedral geometry. The Ni Nam (am = amide) bond lengths are the shortest and the Ni Nazo bond lengths are the longest. Complex 35 exhibits a quasireversible NiIII –NiII redox process and displays two ligand-centered (azo group) quasireversible redox processes. Spectroscopic (absorption and EPR) properties have been studied on coulometrically generated nickel(III) species. To understand the nature of metal–ligand bonding interactions DFT calculations have been performed at the B3LYP level of theory. Calculations have also been done for closely related nickel(II) complexes of deprotonated pyridine amide ligands and comparative discussion has been made using observed results. A new pentadentate ligand HL15 was synthesized. The iron(III) complexes [FeIII (L15 )(MeCN)][ClO4 ]2 (36), [FeIII (L15 )Cl][ClO4 ] (37), and [FeIII (L15 )(CN)][ClO4 ] (38) were isolated. All three complexes are low-spin (S = 1/2) and exhibit rhombic EPR signals around g = 2. Complex 36 reacts with H2 O2 in MeCN at low-temperature to afford [Fe(L15 )(OOH)]+ (g = 2.24, 2.14, 1.96). When cyclohexene was allowed to react with 36/H2 O2 at room-temperature, a significant amount of cyclohexene oxide was produced along with the allylic oxidation products. Analysis of the oxidation products indicated that the allylic oxidation products arose from a radical-driven autoxidation process while the epoxidation was carried out by a distinctly different oxidant. No epoxidation of cyclohexene was observed with 36/TBHP [28].

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Fig. 3. Ligand structures (tridentate with two carboxamide units: H2 L16 – H2 L25 ).

A. Rajput, R. Mukherjee / Coordination Chemistry Reviews 257 (2013) 350–368

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2.4. Two pyridine carboxamide units (tridentate ligands) The ligands H2 L16 –H2 L25 are considered in this section (Fig. 3). Six copper(II) complexes with the [CuII (L16 )] moiety and ligands like pyridine, water, substituted and unsubstituted o-phenanthroline, and 2,2 -bipyridine [CuII (L16 )(py)]2 (py = pyridine) (39), [CuII (L16 )(py)(H2 O)] (40), [CuII (L16 )(bpy)] (41) (bpy = 2,2 -bipyridine), [CuII (L16 )(Me2 -bpy)]·NaClO4 ·MeCN (Me2 bpy = 6,6 -dimethyl-2,2 -bipyridine), (42) [CuII (L16 )(ophen)]·NaClO4 ·1.5MeCN (43) (o-phen = 1,10-phenanthroline), and [CuII (L16 )(Me2 -phen)]·NaClO4 ·1.2MeCN·0.4toluene (44) (Me2 -phen = 2,9-dimethyl-1,10-phenanthroline) have been isolated and structurally characterized [29]. The basal angles of these structurally related five-coordinate CuII complexes correlate well with the EPR hyperfine splitting parameter A|| . The ligand H2 L17 afforded [Et4 N][FeIII (L17 )2 ]·1.5H2 O (45) (S = 1/2) and [Et4 N][CoIII (L17 )2 ]·2H2 O (46) (S = 0) having nonmacrocyclic tetraamido N-coordination and two very short metal–pyridine bonds [30]. Cyclic voltammograms of 45 and 46 in MeCN solution at a glassy-carbon electrode show a one-electron MIII –MII reductive response with E1/2 values of −0.91 V (45) and −1.10 V (46) versus SCE. In MeCN solution 45 and 46 display an additional oxidative response at 1.05 V and 1.17 V versus SCE, respectively. Using H2 L17 nickel complexes in three consecutive oxidation states [Et4 N]2 [NiII (L17 )2 ]·H2 O (47), [Et4 N][NiIII (L17 )2 ]·H2 O, and [NiIV (L17 )2 ]·0.75H2 O (48) were prepared [31]. These X-ray structures represent first crystallographically characterized NiN6 coordination sphere, with a common pyridine bis-amide ligand. The complex [Et4 N][NiIII (L17 )2 ]·H2 O exhibits a rhombic EPR signal (g values: 2.149, 2.115, and 2.034), showing that the metal center is the primary residence site of the unpaired electron. Cyclic voltammetric measurements of 47 in MeCN solution at a glassy-carbon electrode exhibit two chemically reversible and electrochemically quasireversible oxidative responses: a NiIII –NiII couple (E1/2 ) 0.05 V versus SCE) and a NiIV –NiIII couple (E1/2 ) 0.51 V versus SCE). A one-electron chemical oxidation of yellowish brown [Et4 N]2 [NiII (L17 )2 ]·H2 O was achieved in a two-phase solvent mixture H2 O CH2 Cl2 with [Fe(␩5 -C5 H5 )2 ][PF6 ], which led to the isolation of reddish brown [Et4 N][NiIII (L17 )2 ]·H2 O. A two-electron chemical oxidation of 47 was readily achieved in MeCN with (NH4 )2 Ce(NO3 )6 to afford dark violet crystals of 48. Two monohydroxo-bridged dicopper(II) complexes M[CuII2 (L17 )2 (␮-OH)] · 2H2 O [M = Na+ (49) and K+ (50)], in which each copper(II) ion is terminally coordinated by one pyridyl and two amide nitrogen donors, were prepared [32]. The two copper(II) centers are bridged by a hydroxo group, with each copper(II) center assuming a distorted square-planar geometry. Interestingly, each cation Na+ /K+ is coordinated to − units through the amide four different [CuII2 (L17 )2 (␮-OH)] O-donors, in an uncommon distorted tetrahedral coordination environment (cf. complex 31). Temperature-dependent magnetic susceptibility measurements revealed that the compounds have S = 0 ground state with singlet–triplet energy separation, 2J = −334 and −296 cm−1 for 49 and 50, respectively. The larger Cu OH Cu bridge angle in 49 [131.1(6)◦ ] causes better antiferromagnetic exchange coupling than that in 50 [125.7(6)◦ ]. Structural analysis on [Et4 N][RuIII (L17 )2 ]·H2 O (51) (S = 1/2) revealed that the RuN6 coordination comprises four deprotonated amide-N in the equatorial plane and two pyridine-N donors in the axial positions, imparting a tetragonally compressed octahedron around Ru [33]. Complex 51 displays in MeCN/CH2 Cl2 solution three chemically/electrochemically reversible redox processes: a metal-centered reductive RuIII –RuII couple (E1/2 = −0.84/−0.89 V versus SCE) and two ligand-centered oxidative responses (E1/2 = 0.59/0.60 and 1.05/1.05 V versus SCE). Isolation of a dark blue one-electron oxidized counterpart of

Fig. 4. X-ray crystal structure of [PdII (L22 )(3,5-diethynylpyridine)]. Adapted from Ref. [39].

51, [RuIII (L2− )(L•− )]·H2 O, was achieved. The palladium coordination environment in [PdII (L17 )(NCMe)] (52) is approximately square-planar [34]. Two mixed-ligand complexes [RuII (trpy)(L18 )] (53) (trpy = 2,2 ,2 -terpyridine) and [RuIII (trpy)(L19 )][ClO4 ] (54) were also prepared [35]. The four-coordinate complex [NiII (L20 )(H2 O)] (55) [36] and an approximately square-planar complex [PdII (L21 )(NCMe)] (56) were reported [37]. The conjugated homobimetallic palladium(II) complex [(L21 )PdII (qd)PdII (L21 )] (57) (qd = 1,4-quinonediimine) was obtained in a one-pot reaction by the in situ oxidative complexation of 1,4-phenylenediamine with the complex 56 [38]. Synthesis of a novel interlocked system [PdII (L22 )(3,5-diethynylpyridine)] (58) (Fig. 4) has also been reported [39]. Using C2-symmetric ligands containing hydrogenbond donors the complexes [Et4 N]2 [CuII (L23 )(O2 CMe)Cl]·1.5MeCN (59), [Et4 N]2 [CuII (L23 )(Cl)Cl]·MeCN (60) were synthesized [40]. Notably, one MeCO2 − /Cl− binds trans to the pyridyl nitrogen to complete coordination in the equatorial plane. The remaining chloride ion binds to a second sphere-binding site, which is formed by two convergent hydrogen-bonding amide groups. Long Cu· · ·Cl bonds distances of >2.75 A˚ were observed. The complex [CuII (L24 {H}2 )Cl2 ]·H2 O (61) was reported, in which the two pendant pyridine nitrogens are protonated and involved in hydrogen-bonding interaction with the oxygens of amide groups [41]. The complex [RuII Cl2 (PPh3 )(L25 {H}2 )] (62), in which the metal ion is coordinated to the N atoms of the two deprotonated amides and the central pyridine of L25 (2−) was reported [42]. The two pendant pyridines are both protonated, and hydrogen bonds are formed to the coordinated chloride positioned in the molecular cleft between these two groups. Treatment of 62 with NO2 − resulted in displacement of the chlorides. The protons on the pendant pyridines were lost and the nitrosyl-containing complex [RuII (NO2 )(NO)(PPh3 )(L25 )] (63) was formed [42]. 2.5. Two pyridine carboxamide units with aliphatic amine (tetradentate behavior) The ligand H2 L26 is considered here (Fig. 5). In the complex [NiII (L26 ){H}][ClO4 ] (64) (S = 0), both of the amide groups are

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deprotonated and are bound to the nickel ion with a trans-amide configuration and that the remaining coordination sites are occupied by the pyridine nitrogen atom and one of the amine arms. The second amine arm is protonated and participates in an array of hydrogen-bonds in the solid-state [43]. In solution intramolecular exchange of the arms is observed on the NMR timescale. 2.6. Two pyridine carboxamide units (tetradentate ligands)

Fig. 5. Ligand structure (pentadentate with two carboxamide units but acting as tetradentate): (H2 L26 ).

The ligands H2 L27 –H2 L34 are considered in this section (Fig. 6). The coordination geometry around vanadium in [VOIV (L27 )(H2 O)]·0.5DMSO·0.36MeOH·0.13H2 O (65) is distorted octahedral [44]. The equatorial positions are occupied by L27 (2−), while the axial positions are occupied by the oxo group and a water molecule. The vanadium atom sits above the best mean plane of the four nitrogen atoms of the planar L27 (2−) ligand, 0.30 A˚ toward the oxo group. Che and Cheng [45a] demonstrated the epoxidation of cyclohexene, styrene, and toluene in MeCN using the manganese(III) complexes [MnIII (L27 )X] (X = Cl− or N3 − ) in the presence of iodosylbenzene. The complexes [CrIII (L27 )Cl]·xH2 O, [CrIII (L28 )Cl]·H2 O and [MnIII (L28 )(O2 CMe)]l were synthesized [45b]. The complexes [CrIII (L27 )(H2 O)2 ][ClO4 ], [CrIII (L27 )Cl(MeOH)], and [MnIII (L28 )(O2 CMe)] catalyzed olefin epoxidation by PhIO. With [MnIII (L28 )(O2 CMe)], catalytic oxidation of alkanes by PhIO was

Fig. 6. Ligand structures (tetradentate with two carboxamide units: H2 L27 – H2 L34 ).

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also observed [45b]. The synthesis and solution properties of [MnIII (L27 )X] [X = Cl− , N3 − , or SCN− ; S = 2] were reported. In DMF solution the complexes exhibit a quasi-reversible MnIII –MnII redox process. For the complexes [MnIII (L27 )Cl] and [Mn(L27 )(N3 )] accessibility of MnIV –MnIII redox process was suggested [46a]. Cobalt(III)-alkyl complexes [CoIII (L27 )Et(H2 O)] (66) and [CoIII (L28 )(CH2 CH2 CMe CH2 )(H2 O)] (67) were reported [16]. Electrochemical studies on the one-electron oxidation of these complexes suggest the involvement of the equatorial ligand in these processes. A series of diamagnetic cobalt(III) complexes of composition trans-[CoIII (L27 /L28 )X2 ]− with axial ligands (Cl− , N3 − , SCN− , NO2 − or MeCO2 − ) were reported but not structurally characterized [46b]. In MeCN solution the complexes exhibit an irreversible CoIII –CoII couple [Epc = −0.42 to −1.18 V versus SCE] and a quasireversible CoII –CoI couple (E1/2 = −1.12 to −1.27 V versus SCE). When X = SCN− this couple is irreversible (Epc = −1.40 V versus SCE). These complexes displayed an additional quasireversible oxidative response (0.69–0.92 V versus SCE) of primarily ligand-oxidation origin. When X = NO2 − and SCN− this couple is irreversible [Epa = 0.88 V (NO2 − ); Epa = 1.00 V (SCN− )]. A linear spectroelectrochemical correlation was obtained between the ligand-field strength of the axial ligands and the cathodic peak potential for the CoIII –CoII couple. A series of organo- and non-organo rhodium and iridium complexes of L27 (2−) and L28 (2−) were synthesized [47]. The complex [RhIII (L27 )(py)2 ][ClO4 ] (68) was structurally characterized. These complexes displayed reversible one-electron oxidation processes. Stable one-electron-oxidized species were generated both chemically and electrochemically. The potentials of the oxidation processes were affected dominantly by the charge effect but are relatively independent of the nature of the central metal ions and axial ligands. The involvement of the equatorial ligand in the oxidation of the L27 (2−) and L28 (2−) complexes was suggested. Mononuclear iron(III) complexes trans-[FeIII (L27 )X2 ]+/− [X = py (pyridine) (S = 1/2), N3 − (S = 5/2), MeCO2 − (S = 5/2) or CN− (S = 1/2)] and trans-[FeIII (L28 )X2 ]+/− [X = py (S = 1/2), Cl− (S = 5/2) or MeCO2 − (S = 5/2)] were synthesized and characterized [48]. The complex [Et4 N][FeIII (L28 )(O2 CMe)2 ]·CHCl3 (69) was structurally characterized. Cyclic voltammetric measurements of trans[FeIII (L27 )(py)2 ][ClO4 ] in pyridine and trans-Na[FeIII (L27 )(CN)2 ] in DMF revealed a quasi-reversible FeIII –FeII reduction with E1/2 = −0.06 V and E1/2 = −0.83 V versus SCE, respectively. The high-spin complexes exhibit an electrochemically irreversible but chemically reversible FeIII –FeII reduction (E1/2 = −0.44 to −0.83 V) in MeCN solution. Complexes trans-[FeIII (L27 )(N3 )2 ]− , trans-[FeIII (L27 )(CN)2 ]− , and [FeIII (L28 )Cl2 ]− showed a reversible one-electron oxidation (E1/2 = +0.66 to +1.05 V). Chemical oxidation of Na[FeIII (L27 )(CN)2 ] and Na[FeIII (L29 )(CN)2 ]·H2 O (S = 1/2) [synthesized from a new iron(III) complex [FeIII (L29 )(py)2 ][ClO4 ] (S = 1/2)] by (NH4 )2 Ce(NO3 )6 afforded isolation of two novel complexes [FeIII {L27 (ox1)}(CN)2 ] and [FeIII {L29 (ox1)}(CN)2 ]·H2 O, respectively [49]. The collective evidence from infrared, electronic, Mössbauer, and 1 H NMR spectroscopies, from temperature-dependent magnetic susceptibility data, and from cyclic voltammetric studies provides unambiguous evidence that [FeIII {L27 (ox1)}(CN)2 ] and [FeIII {L29 (ox1)}(CN)2 ]·H2 O are low-spin iron(III) ligand cation-radical complexes rather than iron(IV) complexes. The Mössbauer data for [FeIII {L27 (ox1)}(CN)2 ]·H2 O are almost identical with those of the parent compound Na[FeIII (L27 )(CN)2 ], providing compelling evidence that oxidation has occurred at the ligand in a site remote from the iron atom. Strong antiferromagnetic coupling (J ≈ −450 cm−1 ) of the S = 1/2 iron atom with the S = 1/2 ligand ␲-cation radical leads to an effectively S = 0 ground state of [FeIII {L27 (ox1)}(CN)2 ] and [FeIII {L29 (ox1)}(CN)2 ]·H2 O. The oxidized complexes displayed

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NMR spectra (in CDCl3 solution), characteristic of diamagnetic species [49]. Notably, photolysis of [(n-Bu)4 N][FeIII (L27 )(N3 )2 ] (S = 5/2) in acetone solution at room-temperature generated the diamagnetic dinuclear complex [(n-Bu)4 N][FeIV (␮-N)(L27 )2 (N3 )2 ]; the 2

(␮-N)(L27 )2 (CN)2 ] corresponding cyano complex [(n-Bu)4 N][FeIV 2 − − was prepared via N3 substitution by CN [49]. The crystal structures of Na[CoIII (L28 )(CN)2 ]·MeOH (70), Na[FeIII (L27 )(CN)2 ] (71), [(n-Bu)4 N][FeIII (L27 )(N3 )2 ]·1/2toluene (72), [(n-Bu)4 N] [FeIV (␮-N)(L27 )2 (CN)2 ] (73), and [(n-Bu)4 N][FeIV (␮-N)(L27 )(N3 )2 ] 2 2 (74) were determined by single-crystal X-ray diffraction. The ligands L27/28 (2−) are “noninnocent” in the sense that in coordination compounds they can exist in their radical one- and diamagnetic two-electron-oxidized forms L27/28 (ox1)(−) and L27/28 (ox2)(0). Wieghardt and co-workers [50] also reported that the mononuclear complexes [(n-Bu)4 N][FeIII (L27 )(CN)2 ] (S = 1/2), [Et4 N][CoIII (L27 )(CN)2 ], and Na[CoIII (L28 )(CN)2 ]·3MeOH can be electrochemically or chemically one-electron-oxidized to give [FeIII {L27 (ox1)}(CN)2 ] (S = 0), [CoIII {L27 (ox1)}(CN)2 ] (S = 1/2), and [CoIII {L28 (ox1)}(CN)2 ] (S = 1/2). The complex trans-[FeIII (L28 )(l-MeIm)2 ][ClO4 ] (75) (S = 1/2) (1-MeIm = 1methylimidazole) was reported. This complex displayed reversible one-electron oxidation and reduction couples [51]. The controlled nucleophilic halide displacement reaction of [Et4 N][FeIII (L28 )Cl2 ] with AgClO4 in MeCN afforded a crystalline iron(III) complex [FeIII (L28 )Cl(H2 O)] (S = 5/2). The mixed chloroDMF axially ligated complex [FeIII (L28 )Cl(DMF)] (76) (obtained during recrystallization of [FeIII (L28 )Cl(H2 O)] from DMF; however, it loses DMF quite readily to revert back to [FeIII (L28 )Cl(H2 O)]) was structurally characterized [52]. The iron(III) center is coordinated in the equatorial plane by two pyridine nitrogens and two deprotonated amide nitrogens of the ligand, and two axial sites are coordinated by a chloride ion and a DMF molecule. The metal atom has a distorted octahedral geometry. Reaction of [FeIII (L28 )Cl(H2 O)] with [n Bu4 N][OH] in MeOH afforded a ␮oxo-bridged diiron(III) complex, [FeIII (L28 )]2 O·DMF·2H2 O. The two iron(III) centers (S = 5/2) in [FeIII (L28 )]2 O·DMF·2H2 O are antiferromagnetically coupled (J = −117.8 cm−1 ) and the bridged dimeric structure is retained in DMF solution. Bridge-cleavage reactions of [FeIII (L28 )]2 O·DMF·2H2 O were demonstrated by its ready reaction with mineral acids such as HCl and MeCO2 H to generate authentic S = 5/2 complexes, [FeIII (L28 )Cl2 ]− and [FeIII (L28 )(O2 CMe)2 ]− , respectively. Reaction of [FeIII (L28 )Cl(H2 O)] with cyanide under stoichiometric conditions produced the ␮-oxodiiron(III) complex [{FeIII (L28 )(H2 O)}2 O]·3H2 O (77) [53]. This suggests that the role of the cyanide salt in this reaction is as a base rather than as a ligand. Temperature-dependence of the magnetic susceptibility showed the existence of a strong antiferromagnetic coupling between the iron(III) centers (S = 5/2) [J = –108.10(3) cm−1 ; see above]. Redox properties of the complex [CoIII (L30 )(pridn)2 ][ClO4 ] (78) (pridn = pyrrolidine) were investigated [54]. The complex [NiII (L30 )] (79) exhibits distorted square-planar NiN4 coordination with two short and two long Ni N bonds [55]. The electrochemical behavior was investigated with the goal of evaluating the structural effects on the redox properties (metal-centered and ligand-centered). The complex [CuII (L30 )] (80) exhibits distorted square-planar CuN4 coordination with two short and two long Cu N bonds [56]. Two ruthenium-nitrosyl complexes [RuII (NO)(L31 )Cl]·2H2 O (81) and [RuII (NO)(L32 )Cl]·2H2 O (82) were characterized [57]. The complex [CuII (L33 )(H2 O)]·3H2 O (83) with pyrazine amide ligand was reported [57]. A new distorted square-planar (two CuN2 planes making an angle of ∼43o ) copper(II) complex [CuII (L34 )]·0.5EtOH·0.5MeOH (84) was reported [59].

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2.7. Two pyridine carboxamide units (pentadentate ligands) The ligands H2 L35 –H2 L46 are considered (Fig. 7). Using a potentially pentadentate ligand a dimeric NiII complex [NiII (L35 )(H2 O)]2 ·6H2 O (85) was synthesized [60]. Each nickel(II) center resides in a octahedral geometry generated by the Npyridyl , Namido , Namine portion of one ligand and one pyridine2-carboxamido end of the other. The hexa-coordination of each NiII ion is achieved by a water molecule. In the dimeric complex [CuII (L35a )]2 ·H2 O (86) each copper(II) center resides in a distorted square-pyramidal geometry generated by the Npy –Namido –Namine portion of one ligand and one pyridine-2-carboxamido end of the other [61]. The base of the square-pyramid around each copper is completed by the Npy –Namido –Namine portion of one ligand and Namido donor center of the sharing ligand while the apical position is occupied by the second Npy of the shared ligand. The carboxamido nitrogen donors occupy trans positions in the basal plane of each copper center. The two copper(II) centers, connected by the pendant ethylene linkages, are 5.3433(14) A˚ apart. In the monomeric complex [CuII (L35b )]·3H2 O (87) the geometry around copper is distorted square-pyramidal [61]. The base of the squarepyramid is comprised of one pyridine-2-carboxamido moiety, the tertiary amine nitrogen, and the carboxamido nitrogen of the second pyridine-2-carboxamide group which lies perpendicular to the basal plane. The pyridine ring nitrogen of this second pyridine-2carboxamide group occupies the apical position to complete the square-pyramid geometry. The coordination geometry around copper in [CuII (L36 )] (88) is distorted square-pyramidal [62]. In the complexes Na[FeIII (L37 )2 ]·1.5MeCN·0.5DMF (89) (S = 1/2) and Na[FeIII (L38 )2 ]·DMF (90) (S = 1/2) the iron atoms are coordinated by two ligands in a mer geometry and the four deprotonated carboxamido nitrogens reside in the equatorial plane [63]. The pendant pyridine arms are oriented in space in a propeller-like fashion. The overall structure is more compact in the former case. The ligand L37 (2−) shows limited tendency to form 1:1 M–L complexes with labile metal ions [64]. However, a five-coordinate complex [NiII (L37 )] (91) was reported. A mononuclear 1:2 M:L complex [CoIII (L37 )(HL37 )] (92) with two ligands acting as tridentate ligands, one coordinated by the central pyridine, and its two flanking deprotonated amido groups, and the other by the central pyridine, one amido, and one terminal pyridine group, with the remaining poorly coordinating protonated amide remaining unbound along with other terminal pyridine groups. The most remarkable tendency of L37 (2−), however, is toward the formation of robust double helical complexes: a dinuclear CuII complex [CuII (L37 )]2 forms, as well as a trinuclear NiII complex [NiII3 (L37 )2 (O2 CMe)2 (MeOH)2 ] (93). A mononuclear [CuII (L38 )] (94) was reported [65]. The coordination geometry around the metal ion is square-pyramidal. Two deprotonated amido nitrogens and two nitrogens from the pyridine rings reside in the basal plane while one pyridine nitrogen occupies the axial position. The copper ion in [CuII (L39 )]·2H2 O (95) and [CuII (L40 )]·2MeOH (96) is coordinated to five nitrogen atoms in both complexes [66]. It was revealed that introduction of the secondary amino substituent is sufficient to change the geometry around CuII from square-pyramidal toward trigonal-bipyramidal in the crystalline state. EPR spectra in frozen methanol solutions at 77 K as well as visible absorption spectra indicate that the distortion of the geometry around the copper is reduced by the introduction of an alkylamine substituent on the pyridine of the ligand and that the substituted complexes distort toward trigonal-bipyramidal geometry compared to the unsubstituted one in solution. The complexes with helical ligands [NiII (L41 )]·CHCl3 (97), [CuII (L41 )] (98), and [ZnII (L42 )]·toluene-d8 (99) revealed that the coordination rigidifies the structure in a manner that affords significantly higher, solvent-dependent helical interconversion barriers [67].

The structure of Na[FeIII (L43 )(1-MeIm)2 ]·3MeCN (100) consists of [FeIII (L43 )(1-MeIm)2 ]− units that comprise iron(III) center in a pentagonal-bipyramidal geometry. The iron atom is coordinated to the deprotonated L43 (2−) ligand through two phenolic oxygens, two carboxamido nitrogens, and the pyridine nitrogen in the pentagonal plane, while the nitrogens of two 1-MeIm ligands occupy the axial positions. In the solid state, the structure of 100 consists of extended networks of Na+ , [FeIII (L43 )(1MeIm)2 ]− , and the lattice solvent molecules. The structure of Na3 [FeIII (L43 )(NCS)2 ]·3DMF·2MeCN (101), is more extended and complex [68]. The [FeIII (L43 )(NCS)2 ]3− unit consists of an iron(III) center coordinated by five donors of L43 (2−) with additional axial coordination by the nitrogen atoms of two thiocyanate groups. The seven donor atoms are arranged around iron in a pentagonalbipyramidal fashion. In DMF solutions with additional 1-MeIm, 100 exhibits a reversible FeIII –FeII redox process with E1/2 = −1.01 V versus SCE [68]. In [Et4 N][FeIII (L43a )2 ]·2H2 O (102) the coordination geometry around the FeIII center in [FeIII (L43a )2 ]− is distorted octahedral with two L43a (2−) ligands coordinated in a mer fashion [69]. The complex consists of an FeIII N6 chromophore with two axial pyridine nitrogens and four equatorial carboxamido nitrogens in its coordination sphere. The iron center in [Et4 N]2 [FeII (L44 )] (103) (S = 2) and in [Et4 N][FeIII (L44 )] (104) (S = 5/2) is in a trigonal-bipyramidal geometry with two deprotonated carboxamido nitrogens, one pyridine nitrogen, and two thiolato sulfurs as donors [70]. Complex 103 is stable in water and binds a variety of Lewis bases at the sixth site at low temperature to afford green solutions. The iron(III) centers in these six-coordinate species are low-spin. In DMF, 103 exhibits a reversible cyclic voltammogram due to the FeIII –FeII redox process with half wave potential (E1/2 ) of −0.65 V versus SCE. In [Et4 N][FeIII (L44a )2 ]·1.7MeCN·Et2 O (105) the coordination geometry around the FeIII center is distorted octahedral, with two L44a (2−) units ligated in a mer fashion [69]. Interestingly, the FeIII N5 O chromophore consists of two pyridine nitrogens, three deprotonated carboxamido nitrogens, and the carbonyl oxygen of the fourth deprotonated carboxamido moiety. This observed mode of binding of the two L44a (2−) ligands in 105 is not accidental and was also noted in the corresponding complex [Et4 N][CoIII (L44a )2 ] (106) [71]. The last carboxamido nitrogen moiety fails to bind through nitrogen due to the steric repulsion that builds up between the pendant thioether groups as an increasing number of carboxamido nitrogens become ligated to the MIII center. The complex with a pentadentate macrocyclic ligand [FeIII (L45 )Cl(H2 O)]·MeOH (107) (S = 5/2) adopted a pentagonalbipyramidal geometry, where an equatorial plane is occupied by the pyridine nitrogen, two deprotonated amide nitrogens, and two secondary amines from L45 (2−), and two axial positions are available for Cl− and H2 O coordination [72]. The rigid, planar iron-amide building blocks are linked in a 3D-network via a system of hydrogen bonds. The FeIII state is reasonably stabilized [E1/2 (FeIII –FeII couple) = −0.57 V versus SCE]. Variable-temperature 57 Fe Mössbauer spectroscopy and ac and dc magnetization studies indicate slow paramagnetic relaxation and a crossover to long-range antiferromagnetic order at T < ∼3.2 K. In [FeII (HL45 )(Cl)(MeOH)] (108) the metal ion has a distorted octahedral coordination sphere, in which HL45 (−) provides the pyridine nitrogen, a deprotonated nitrogen, and two secondary amines [73]. The metal center in [FeII (L45 )(MeOH)2 ]·MeOH (109), [FeIII (L45 )(1-MeIm)]2 O·2MeCN (110), [FeII (L46 )(MeOH)] (111), [FeII (L46 )(MeOH)2 ]·MeOH (112), [FeIII (L46 )(DMSO)]2 O·2DMSO (113) adopts a pentagonal-bipyramidal geometry. Detailed kinetic studies of oxygenation of the iron(II) complexes showed that the deprotonation state of the complex has a profound effect on the reactivity with dioxygen. Oxygenation of the dideprotonated

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Fig. 7. Ligand structures (pentadentate with two carboxamide units: H2 L35 – H4 L46 ).

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Fig. 8. Ligand structures (hexadentate with two carboxamide units: H2 L47 – H2 L55 ).

complex of iron(II), FeII (L45 ), in aprotic solvents proceeds via a path that is analogous to that of iron(II) porphyrins: via iron(III) superoxo and diiron(III) peroxo species, as evidenced by the spectral changes during the reaction, which is second-order in the concentration of the iron(II) complex, and with an inverse dependence of the reaction rate on the concentration of dioxygen. The final products of oxygenation are crystallographically characterized iron(III) ␮-oxo dimers. The authors also found that the presence of 1-methylimidazole stabilizes the diiron peroxo intermediate. The reaction of FeII (L45 ) with dioxygen in MeOH is distinctly different under the same conditions. The reaction is first-order in both iron(II) complex and dioxygen, and no intermediate is spectroscopically observed. Similar behavior was observed for the monodeprotonated complex FeII (HL45 )(Cl). The presence of an accessible proton either from the solvent (reactions in MeOH) or from the complex itself (in FeII (HL45 )(Cl)) proves sufficient to alter the oxygenation pathway in these macrocyclic systems, which is reminiscent of the properties of iron(II) porphyrin complexes. The macrocyclic ligands L45 (2−) and L46 (2−) can be considered as new members of the “expanded porphyrin analogue” family.

Three hexadentate non-macrocyclic ligands H2 L47 –H2 L55 (Fig. 8) were reported [74–76]. The ligand H2 L50 was also synthesized. Although H2 L47 is a potentially hexadentate ligand, the complex [CuII (L47 )(H2 O)][ClO4 ]·H2 O (114) has a distorted squarepyramidal geometry [74]. Four nitrogen atoms from the pyridine2-caboxamido moiety and the two central secondary amines form the base of a square pyramid with a water molecule occupying the apical position. Thus, one of the two pyridine-2-carbamoyl groups remained uncoordinated. The metal ions in [NiII (L47 )]·H2 O (115) and [CoIII (L47 )][ClO4 ]0.75 [Cl]0.25 ·2.5H2 O (116) [74] have octahedral coordination geometry. The ligand encapsulates the metal atom through the six ligating nitrogen atoms: two pyridyl, two amido, and two secondary amine nitrogen atoms. Though several isomers can be isolated with a linear hexadentate ligand, the one observed here has the pyridine nitrogen atoms in the cis position and the amido nitrogen atoms in the trans position. The metal center in [CoIII (L49 )][ClO4 ]·H2 O (117) is distorted octahedral with the two pyridyl groups in cis-position [76]. Four new complexes, [CoII (L51 )] (118) and its one-electron oxidized counterpart [CoIII (L51 )][NO3 ]·2H2 O, [NiII (L51 )] (119), and

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[CuII (L51 )] (120), have been synthesized [77]. Structural analyses revealed that the CoII center in 118 and the NiII center in 119 are sixcoordinate, utilizing all the available donor sites and the CuII center in 120 is effectively five-coordinated (one of the ether O atoms does not participate in coordination). The structural parameters associated with the change in the metal coordination environment have been compared with corresponding complexes of thioethercontaining hexadentate ligands. Cyclic voltammetric experiments in CH2 Cl2 revealed quasireversible CoIII –CoII , NiIII –NiII , and CuII –CuI redox processes. In going from ether O to thioether S coordination, the effect of the metal coordination environment on the redox potential values of CoIII –CoII (here the effect of spin-state as well), NiIII –NiII and CuII –CuI processes have been systematically analyzed. The complexes [CuII (L52 )] (121), [CuII (L53 )]·DMSO (122), [NiII (L52 )] (123) (S = 1), and [NiII (L53 )] (124) (S = 1) were prepared [78]. The CuII center in 121 assumes axially compressed octahedral geometry. However, this coordination geometry changes to an axially elongated one in solution. The coordination geometry of the CuII ion in 121 can also be described as a distorted octahedral with cis-open sites and two S atoms interact weakly on the open sites. The powder spectrum of 121 is typical of an axial pattern with g⊥ > g|| . This spectrum shows that the unpaired electron lies on the dz2 orbital in solid state (X-ray structure). However, the CHCl3 glass spectrum of 121 is of an axial pattern (g|| > g⊥ ), contrary to the powder spectrum. This fact indicates that the structure of 121 in solid state changes in the CHCl3 solution from an equatorially elongated octahedron to an axially elongated one. The powder spectrum of 122 shows a rhombic pattern with g1 = 2.22, g2 = 2.10, and g3 = 2.03, justifying the crystal structure of this compound. No structural change like compound 121 was observed. In DMSO the CuII –CuI redox process for 121 and 122 are ∼−1.2 V and the NiIII –NiII process for 123 and 124 at ∼0.3 V versus [Fe(␩5 -C5 H5 )2 ]+ /Fe(␩5 -C5 H5 )2 couple. The ligands H2 L52 and H2 L54 afforded isolation of purple complexes [FeII (L52 )] (125) (S = 0) and [FeII (L54 )]·CH2 Cl2 (126) (S = 0) [79]. Chemical oxidation of 125 by [Fe(␩5 -C5 H5 )2 ][PF6 ] or (NH4 )2 Ce(NO3 )6 led to the isolation of green complexes [FeIII (L52 )][PF6 ] (S = 1/2) or [Fe(L52 )][NO3 ]·H2 O (S = 1/2), and oxidation of 126 by (NH4 )2 Ce(NO3 )6 afforded [FeIII (L54 )][NO3 ]·H2 O (S = 1/2). X-ray crystal structures of 125 and 126 revealed that L54 (2−) binds more strongly than L52 (2−), affording distorted octahedral MII N2 (pyridine/pyrazine)N2  (amide)S2 (thioether) coordination. The iron(III) complexes display rhombic EPR spectra. Each complex exhibits in CH2 Cl2 /MeCN a reversible to quasireversible cyclic voltammetric response, corresponding to the FeIII –FeII redox process. The E1/2 value of 126 is more anodic by ∼0.2 V than that of 125, attesting that compared to pyridine, pyrazine is a better stabilizer of iron(II). Moreover, the E1/2 value of 125 is significantly higher (∼1.5 V) than that of 34 [the FeIII complex of L13a (2−) with the tridentate pyridine-2-carboxamide ligand incorporating thiolate donor site] [26]. Anaerobic reaction of Co(O2 CMe)2 ·4H2 O with the thioethercontaining acyclic pyrazine amide hexadentate ligand (H2 L54 ) furnished [CoII (L54 )]·MeOH (S = 1/2) [80]. A similar reaction in air, however, furnished [CoIII (L*)(L**)]·2MeOH (127) [L* is a tridentate thiolate-containing pyrazine amide ligand (cf. L13a (2−)); L** is a tridentate vinylthioether-containing pyrazine amide ligand)] (S = 0), resulting from a C S bond cleavage reaction triggered by an acetate ion as a base. The complex has CoIII N2 (pyrazine)N2  (amide)S(thioether)S (thiolate) coordination (Fig. 9). On the other hand, the reaction of Co(O2 CMe)2 ·4H2 O with H2 L55 in air afforded a cobalt(II) complex [CoII (L55 )]·MeOH (S = 1/2); its structurally characterized variety has the composition [CoII (L55 )]·C6 H6 (128). Interestingly, [CoII (L55 )]·MeOH undergoes facile metal-centered oxidation by aerial O2 /H2 O2 /[Fe(␩5 C5 H5 )2 ][PF6 ], which led to the isolation of the corresponding

363

Fig. 9. X-ray crystal structure of [CoIII (L*)(L**)]·2MeOH (127)]. Adapted from Ref. [80].

Fig. 10. X-ray crystal structure of [CoIII (L55 )] (130). Adapted from Ref. [80].

cobalt(III) complex [CoIII (L55 )][ClO4 ] (129). When treated with methanolic KOH, 129 affords an organocobalt(III) complex [CoIII ((L55 )] (130) (S = 0) (Fig. 10). A five-membered chelate-ring forming ligand L54 (2−) effects C S bond cleavage and a sixmembered chelate-ring forming ligand L55 (2−) gives rise to Co C bond formation, in cobalt(III)-coordinated thioether functions due to ␣ C H bond activation by the base. A rationale has been provided for the observed difference in the reactivity properties [80]. The crystal structures of [NiII (L54 )]·MeOH (131) and [CuII (L54 )]·CH2 Cl2 (132) revealed that in these complexes the ligand coordinates in a hexadentate mode, affording examples of distorted octahedral MII N2 (pyrazine)N2  (amide)S2 (thioether) coordination [81]. Complexes 131 and 132 exhibit in CH2 Cl2 a reversible to quasireversible cyclic voltammetric response, corresponding to the NiIII –NiII and CuII –CuI redox process, respectively. The E1/2 values reveal that the complexes of L53 (2−) are uniformly more anodic by ∼0.2 V than those of the corresponding complexes with the analogous pyridine ligand (H2 L51 ), attesting that compared to pyridine, pyrazine is a better stabilizer of the NiII or CuI state. Coulometric oxidation of [NiII (L52 )] and 131 generates [NiIII (L52 )]+ and [NiIII (L54 )]+ species, which exhibit axial EPR spectra corresponding to a tetragonally elongated octahedral

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Fig. 11. Ligand structures (bridging ligands with one and two carboxamide units: HL56 – H2 L62 ).

geometry. Complex 132 exhibits EPR spectra characteristic of the dz2 ground state. 2.8. One pyridine carboxamide unit (bidentate bridging ligands) The ligands HL56 –H2 L62 are considered in this section (Fig. 11). Using a potentially bridging ligand HL56 binuclear mixed-valent homonuclear bimetallic complexes [CoIII,II (L56 )3 (X)]X·S [X = Cl (133) (CoIII (S = 0), CoII (S = 3/2)) 2 (Fig. 12); X = Br, S = MeOH (134)] and heteronuclear bimetallic complex [CoIII ZnII (L56 )3 (Cl)]Cl·MeOH·5H2 O (135) were synthesized [82]. Structural analysis revealed that trivalent cobalt is in distorted octahedral and bivalent cobalt/zinc is in distorted tetrahedral environment. Three L56 (−) ligands provide sixcoordination by utilizing three pyridine amide units (pyridine N and amide N donor set) in a facial mode, which in turn places three

4-methylpyridine nitrogens to coordinate to another metal center, which completes four-coordination by a chloride/bromide ion. (L57 )3 ]·19H2 O (136) reveals that it is made of The structure of [CoIII 2 triple stranded metallocryptand [83]. 2.9. Two pyridine/pyrazine carboxamide units (tridentate bridging ligands) In [CuII2 (L58 )Cl2 ]·MeCN (137) both CuII ions adopt a (4 + 1) distorted square-pyramidal geometry [84]. One copper forms a longer apical bond to an adjacent carbonyl oxygen atom, whereas the second copper is chelated to a neighboring Cu–Cl chloride ion to afford a ␮-Cl-bridged dimer [CuII2 (L58 )Cl2 ]2 . Pyrazine amide ligands with bridging capability were also synthesized [85]. The copper(II) coordination sphere in the centrosymmetric dinuclear complex [CuII2 (L59 )(MeCN)2 (H2 O)2 ]

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365

Fig. 13. X-ray crystal structure of [Cu2 II 3 (L61 )2 (␮2 -O2 CMe)2 ]·CHCl3 ·0.25H2 O (142). Adapted from Ref. [56].

Fig. 12. X-ray crystal structure of [CoIII,II (L56 )3 (Cl)]Cl (133). 2 Adapted from Ref. [82].

[BF4 ]2 ·H2 O (138) is N4 O square-pyramidal, exhibiting only a slight distortion from the perfect square-pyramidal geometry. Complex [CuII2 (L60 )(MeCN)2 (H2 O)2 ][BF4 ]2 (139) features two crystallographically independent but chemically very similar centrosymmetric dinuclear molecules. In each molecule L58 (2−) acts as an (N3 )2 bis(terdentate) chelate. Each copper(II) center is in a distorted N4 O square-pyramidal coordination environment. The coordination sphere is made up of the equatorially coordinating N3 terdentate coordination site of the deprotonated amide ligand, an H2 O in the remaining equatorial position and a MeCN at the apex. The molecular structure of the hexafluorosilicate complex [CuII2 (L59 )(H2 O)4 ][SiF6 ] (140) is very similar to the structure of the analogous tetrafluoroborate complex 138. In the centrosymmetric complex 140 a N3 O2 distorted square-pyramidal coordination environment about the CuII ion is achieved by equatorially and axially coordinated H2 O molecules. Hydrogen-bonding, involving the amide oxygen atom and the equatorial H2 O co-ligand of two ˚ and both of the H2 O neighboring subunits [Oamide · · ·Owater 2.603 A] ˚ is co-ligands with the SiF6 2− anions [Fanion · · ·Owater 2.760–3.377 A], also a feature of this structure. The overall molecular structure of the centrosymmetric complex [CuII2 (L60 )(H2 O)4 (BF4 )2 ] · 2H2 O (141) is very similar to that of complex 139. The major difference is the N3 O2 F distorted octahedral coordination environment about the copper(II) ions in 141, the two axial positions being elongated and occupied by an H2 O and a BF4 − co-ligand. Crystal structure of [CuII3 (L61 )2 (␮2 -O2 CMe)2 ]·CHCl3 ·0.25H2 O (142) reveals that two L61 (2−) hold three CuII ions [56]. The central CuII center is bound by the nitrogen atoms from the central pyridyl rings of each L61 (2−), while terminal Cu centers are chelated by the carboxamide and pyridyl nitrogen atoms of each L61 (2−). Two ␮2 -acetato groups bridging adjacent copper centers complete the molecular structure. Terminal copper(II) ions possess distorted trigonal-bipyramidal N4 O donor sets (Fig. 13). The central CuII ion has approximately square-planar cis-N2 O2 geometry. The complex [CuII3 (L62 )2 (␮2 -O2 CMe)2 ]·1.5MeCN · 0.5H2 O (143) is isostructural with 142. There are two independent molecules of 143 in the unit cell. The complex [CoIII (L2a )3 ]·DMF (144) is reported. The bischelate complex [CoIII (L24 )2 ]·H2 O (145) is also reported. In the latter case the CoIII center is coordinated octahedrally by four deprotonated amide nitrogens in the equatorial plane and two

pyridyl nitrogens in the axial positions. The pyridyl arms remain uncoordinated. The complex [CoIII (L2a )3 –ZnII Cl2 ]·MeCN (146) has been structurally characterized. Here the ZnII ion is coordinated to two of the hanging pyridine rings emerging from the central octahedral CoIII complex. The remaining two coordinations come from the Cl− groups completing the tetrahedral geometry around the ZnII ion. When Na[CoIII (L24 )2 –(ZnII Cl2 )2 ] was recrystalized from N,N -dimethylacetamide (DMA) and DMF two complexes [CoIII (L24 )2 –(ZnII Cl2 )(ZnII (Cl)(DMA)] (147) (Fig. 14) or [CoIII (L24 )2 –(ZnII Cl2 )(ZnII (Cl)(DMF)] were isolated. Two ZnII ions are situated in two clefts created by the hanging pyridine rings originating from the central cobalt core. One ZnII ion is externally coordinated by two Cl− ions, whereas other ZnII ion has one Cl− and one DMA/DMF molecule. These heterobimetallic complexes catalyze the Beckmann rearrangement of the aldoximes and ketoxime to their respective amides [86]. The complexes [CoIII (L24 )2 –{CuI (MeCN)}2 ][ClO4 ] (148) and [FeIII (L24 )2 –{CuI (MeCN)}2 ][ClO4 ] (149) are isostructural [87]. Two deprotonated tridentate ligands were arranged meridionally around the central MIII ion. The central MIII ion is coordinated by four deprotonated Namide atoms in the equatorial basal plane while two Npyridine atoms occupy the axial positions. Two CuI ions are situated in the pre-organized clefts created by the hanging pyridine rings originated from the building block. Both secondary CuI ions have trigonal-planar geometry, where two coordinations come from the hanging Npyridine atoms while the remaining one coordination is provided by the coordinated MeCN molecule. The accessible

Fig. 14. X-ray crystal structure of [CoIII (L24 )2 –(ZnII Cl2 )(ZnII (Cl)(DMA)] (147). Adapted from Ref. [87].

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CuII –CuI redox potential and presence of labile site on the copper center has been utilized for the oxidation of hindered phenols in the presence of molecular oxygen. Hindered phenols were oxidized to the C C-coupled products in most cases, however, de-alkylation resulted in the case of 2,4,6-trisubstituted phenols. Interestingly, when H2 O2 was used as oxidant, de-alkylation was not observed; suggesting the uniqueness of the active species generated in the presence of catalyst and molecular oxygen. The heterobimetallic complexes [CoIII (L24 )2 –{CdII }2 (NO3 )3 –(DMF)2 ] (150) and [CoIII (L24 )2 –{HgII }2 (NO3 )3 –(DMSO)2 ] (151), have been used for the catalytic cyanosilylation of imines and ring-opening reactions of oxiranes and thiiranes. The results suggest peripheral metal-selective catalytic reactions [88]. 3. Noteworthy results In this section we highlight some of the noteworthy results of complexes of pyridine/pyrazine amide ligands of varying denticity. 3.1. Role of axial ligands on the spin-state of iron(III) of tetradentate ligands Using tetradentate ligand system H2 L27 –H2 L29 complexes of the type trans-[FeIII (L27 /L28 /L29 )X2 ]z [X = py or 1-MeIm (z = 1+); X = Cl− , Me2 CO2 − , N3 − , CN− (z = 1−)] and also the complex trans[FeIII (L28 )Cl(DMF)] were synthesized. This group of complexes encompass both high-spin (S = 5/2) and low-spin (S = 1/2) states of iron(III) [48–52]. This was achieved keeping the in-plane ligand invariant and changing only the axial ligands. Representative monomeric complexes were structurally characterized: 69, 71, 72, and 75. The ␮-oxo-bridged diiron(III) complex 77 was also synthesized [52,53]. Barring iron(III) complexes of macrocyclic (including porphyrins and phthalocyanins) and Schiff base ligand systems this is a notable result using deprotonated pyridine amide ligand system [48]. The synthesis and thorough characterization of antiferromagnetically coupled low-spin iron(III) (S = 1/2) and ligand-radical (S = 1/2) coordinated complexes [FeIII {L27 /L29 (ox1)}(CN)2 ] (S = 0) could be considered as a notable achievement. It is worth mentioning here that it is the noninnocent 1,2-phenylenediamine spacer that is prone to get oxidized. To the best of our knowledge, barring the porphyrin system such a result was not achieved with other ligand systems. 3.2. Effect of the donor atom type, geometry, and spin-state on CoIII –CoII redox potential The stereochemical flexibility provided by two bidentate L2 (−) ligands in 8 (S = 3/2) is expected to stabilize the CoII state more than in CoII (L27 )·H2 O (S = 1/2), where the cobalt(II) center is coordinated by a rigid tetradentate ligand L27 (2−). The low-spin state of the resulting cobalt(III) species, generated at the electrode surface, is expected to be attained more easily in the case of low-spin CoII (L27 )·H2 O than in the case of high-spin complex 8. It is understandable that for 8 an additional energy is necessary to bring about spin-reorganization [15]. In order to identify the effect of donor atom type, coordination geometry, and also the spin-state of the cobalt(II) center on the CoIII –CoII redox potentials, the result of 118 was compared to that of reported complexes 8 (S = 3/2) [15], [CoII (L54 /L55 )]·MeOH [80] (distorted octahedral; S = 1/2) [80]. For [CoII (L54 /L55 )]·MeOH the type [notably, these ligands are pyrazine amide-based instead of pyridine amide-based, the spacer is CH2 CH2 in L54 (2−) and CH2 CH2 CH2 in L55 (2−)], the number of donor atoms and geometry [grossly octahedral around cobalt(II)], the spin-state of cobalt(II) and the charge of the complexes (neutral) are all

invariant. Therefore, these complexes gave a unique opportunity to pinpoint the effect of donor atom type, geometry, and spin-state on CoIII –CoII redox potential values [77]. It is interesting to note that the complexes 8 and CoII (L27 )·H2 O display in DMF an additional CoII –CoI redox process at −1.66 V and −1.26 V, respectively [15]. For 118 in CH2 Cl2 , no redox response attributable to the CoII –CoI redox process was observed [77]. Here the redox potential values clearly indicate that the CoI state is better stabilized in the complex CoII (L27 )·H2 O than in 118. It is understandable that the d8 configuration of the CoI state would prefer a planar coordination. The ligand L51 (2−) may not support such a stereochemical adjustment required for stabilization of the CoI state. Moreover, the addition of an extra electron to a high-spin CoII may not occur easily and hence the CoII –CoI redox process will be at a more cathodic potential, which is outside the cathodic window of the medium (CH2 Cl2 ). 3.3. Nickel is stabilized in its bivalent, trivalent, and tetravalent state with a common tridentate ligand Stabilization of nickel in its higher (>2) oxidation state was a challenge to inorganic chemists during seventies and eighties [31]. Using a common tridentate bis-amide ligand H2 L17 nickel complexes in three consecutive oxidation states 48 (S = 1), [Et4 N][NiIII (L)2 ]·H2 O (S = 1/2), and 49 (S = 0) were achieved. These complexes displayed reversible electron-transfer series [31]. This is beyond doubt a noteworthy result. 3.4. Effect of CuII N4 geometry on CuII -CuI redox potential Owing to the differing stereochemical preferences of fourcoordinate copper(II) (square-based geometry) relative to CuI (tetrahedral/trigonal planar) ready interconversions of these two oxidation states is expected to be facilitated by use of flexible ligands which can adjust their coordination geometry to the differing demands of the two oxidation states. The crystal structure of [CuII (L27 )(H2 O)] (square pyramidal) [10], 2 (distorted tetrahedral) [9], 9 (distorted trigonal bipyramidal) [15], and 84 (distorted square planar) [59] are reported in the literature. To reveal the effect of stereochemical changes around the CuII center caused by ligand-structure modification these complexes provide a unique opportunity without changing the number, gross identity or electronic properties of the donor atoms. In the present series the ligand modification was systematically tuned to identify the effect of ligand flexibility/rigidity which in turn is expected to contribute to the stabilization of CuI state (more flexible ligand imparting tetrahedral twist) over CuII (more rigid ligand providing square-based geometry). The chosen complexes convincingly demonstrate the structural effect on the redox properties of CuII N4 units. 3.5. C S bond cleavage and cyclometallation reaction The low-spin complexes [CoII (L54 )]·MeOH and [CoII (L55 )]·MeOH exhibited marked differential reactivity toward base when reacted with O2 [80]. A rationale for the chelate-ring size dependent reactivity of CoII complexes with a base under aerobic conditions is provided (Fig. 15). During facile oxidation of CoII to the CoIII state initial activation of expected low acidity of a C H bond ␣ to a coordinated thioether unit appended to a strong-field ligand framework will be facile due to the strong-field nature of L54 (2−)/L55 (2−). In the case of [CoII (L54 )]·MeOH the coordinated ligand cleaves into two unsymmetrical ligand components via scission of the C S bond affording the CoIII complex 127. It has a CoIII (N2 S)(N2 S ) coordination sphere involving both thiolate (S ) and thioether (S) sulfur, the possible formation of an unfavorable and more strained

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Fig. 15. The C S bond cleavage and cyclometallation reaction. Adapted from Ref. [80].

four-membered metallacycle is bypassed via migration of the anionic charge to a sulfur site with concomitant ligand cleavage. The ligand L55 (2−), capable of providing N4 S2 coordination, has a CH2 group stereochemically suitably positioned such that deprotonation, with subsequent carbanion coordination, could lead to an N4 CS donor set around CoIII with all five-membered chelate-rings. Indeed it happens in the synthesis of 130. In essence, an activated five-membered chelate-ring leads to C S bond cleavage whereas a six-membered chelate-ring affords the organocobalt(III) complex. 4. Conclusions In this review the coordination potential of an extensive series of pyridine/pyrazine-2-carboxamide- and pyridine-2,6dicarboxamide-based chelating ligands in the development of extensive coordination chemistry has been surveyed. The bonding properties of about hundred fifty-crystal structures in connection with the metal coordination geometry have been discussed. The structures described in this review give a clear demonstration of the flexibility of the geometries around the metal ion, provided by the chosen class of chelating peptide ligands. It was demonstrated that the geometries, nuclearities, and reactivity properties of a particular compound can be controlled by the change in the coordination mode of chelating ligands in the coordination sphere of metal ions; by the nature of spacer between carboxamide moieties. Many ligands with bridging potential have also been designed and the nature of the coordination complexes discussed. It is evident from the extant literature that synthesis and characterization of coordination complexes of this class of ligands has been a primary emphasis of research, and that somewhat less attention has been directed toward detailed examination of their reactivity. Future exploitation of the unique bonding characteristics of a particular compound for discovering and understanding reactivity properties is anticipated. In view of the breadth of the coordination chemistry already uncovered by pyridine/pyrazine-based chelating ligands, it is anticipated that the extent of future designing of new ligands will depend on the imagination of the researcher, based on the nature of the research problem.

Note Added in Proof During the preparation of this manuscript, two relevant references missed out [D. Huang, R.H. Holm, J. Am. Chem. Soc. 132 (2010) 4693; D. Huang, O.V. Makhlynets, L.L. Tan, S.C. Lee, E.V. Rybak-Akimova, R.H. Holm, Proc. Natl. Acad. Sci. (USA) 108 (2011) 1222]. The 2010 paper reported mononuclear planar NiII complexes [Et4 N][NiII (L20* )X] (H2 L20* has two methyl groups in the place of two isopropyl groups; X = OH− , CN− , HCO2 − , HCO3 − ). X = OH− complex reacts with ethyl formate and CO2 to form HCO2 − and HCO3 − products. The 2011 paper reports kinetics and mechanistic analysis of an extremely rapid carbon dioxide fixation reaction by X = OH− complex. Acknowledgments Research on coordination chemistry of pyridine/pyrazine amide ligands carried out in author’s laboratory has been supported by the Council of Scientific & Industrial Research (CSIR) and Department of Science & Technology (DST), Government of India. RM gratefully acknowledges the award of JC Bose fellowship by DST. The author thanks the present and past members of his research group, who have worked in this area. Their names appear in the appropriate literature citations. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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