Tetrazines tetrazine ligands

Finally, it should be mentioned that the tetrazine ligands remain attractive for organometallic chemistry as useful building blocks (14CC12542, 14DT6513,14DT7958). 

The dinuclear Pt complex with the 3,6-bis(2-pyridyl)-l,2,4,5-tetrazine ligand 879 undergoes one-electron reduction and one-electron oxidation electrochemically. The oxidation product is composed of a Pt"-Pd" mixed-valence state, although the value is small due to the <5 -orbital character of the redox orbitals. 

Bis(2-pyridyl)-l,2,4,5-tetrazine (bptz, (846)) can act as a dinucleating ligand towards Nin. 2056 From the reaction of [Ni(MeCN)6]2+ with bptz, tetranuclear squares [Ni4(bptz)4(MeCN)8]8+ or metallapentagons [Ni5(bptz)5(MeCN)10]10+ have been isolated, depending on the choice of the templating counteranion (C1C)4 or BF4- vs. SbF6-).2057,2058... 

Complexes involving the 3,6-bis(2-pyridyl)-l,2,4,5-tetrazine (bptz) ligand have been obtained <06JA5895 06MI1498>. 

This review, which complements an earlier one (Part I) dealing with transition metal complexes of triazenes, tetrazenes, tetraazadienes, and pentaazadienes, examines the coordination chemistry of related cyclic catenated nitrogen ligands. Six-membered rings containing three, four, or five adjacent nitrogen atoms — 1,2,3-triazines, 1,2,3,4-tetrazines, and pentazines, respectively — are either unknown or are relatively unstable species whose coordination chemistry has yet to be explored. 

Dihydrotetrazine 32 was oxidized with nitric acid in acetic acid to yield tetrazine 45 which suffered cycloaddition with acetylene in refluxing DMF followed by the extrusion of nitrogen to give pyridazine 46 which was used as bis-tridentate bridging ligand <02OL1253>. 

A rich coordination chemistry of aromatic diazine (N-N), especially pyridazine and phthalazine related ligands has emerged over the last three decades,1-72 and recently open-chain diazine (N-N) coordination chemistry has been well developed, especially by Thompson and others.62-113 Many types of aromatic heterocyclic compounds contain a 1,2-diazine (N-N) moiety, e.g., pyridazine and its 3,6-disubstituted derivatives (Scheme 1, Type 1), phthalazine, condensed phthalazines and their substituted derivatives (Scheme 1, Type 2), and other compounds such as pyrazole, triazole, thiadiazole, tetrazole, indazole, 1,2,4-triazine, 1,2,4,5-tetrazine, and thiadiazepines. Alternatively, the 1,2-diazine (N-N) moiety also exists as an open-chain entity in some related compounds, e.g., A-substituted-amide hydrazonimidates (Scheme 1, Type 3), A-substituted-amide hydrazonidates (Scheme 1, Type 4), A-substituted hydrazides (Scheme 1, Type 5), A-substituted amidrazones (Scheme 1, Type 6), and A-sub-stituted hydrazidates (Scheme 1, Type 7). 

Tetrazine derivatives have been used widely in making pyridazines by inverse electron-demand Diels-Alder reactions with dienophiles (see Scheme 10), followed by loss of nitrogen from the usually unstable intermediate if the dienophile has a triple bond or potential triple bond properties, the final pyridazines will be aromatic. From the coordination chemistry point of view 3,6-dipyridin-2 -yl-l,2,4,5-tetrazine and dimethyl 1,2,4,5-tetrazinedicarboxylate and their analogs are probably the most useful starting compounds for making relevant ligands. 

A large number of homologous ligands ((40), Scheme 11) have been prepared from a tetrazine (39)4-9 through so-called inverse electron-demand Diels-Alder reactions . 

Several reports have dealt with the 3,6-bis(2-pyridyl)-l,2,4,5-tetrazine (bptz) ligand reactions of bptz with first-row transition metals have been explored <05JA12909> the synthesis and crystallographic study of a dinuclear iridium complex have been published <051CA1317> and a new molecular propeller compoimd prepared from the reaction of bptz with Ag[AsFe] has been reported <05CC46>. 

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