Tripodal ligand

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Motifs for complexation of tri- and tetradentate tripodal ligands TripodCoordModes.png
Motifs for complexation of tri- and tetradentate tripodal ligands

Tripodal ligands are tri- and tetradentate ligands. They are popular in research in the areas of coordination chemistry and homogeneous catalysis. Because the ligands are polydentate, they do not readily dissociate from the metal centre. Many tripodal ligands have C3 symmetry.

Coordination chemistry

In their coordination complexes with an octahedral molecular geometry the tridentate tripod ligands occupy one face, leading to a fixed facial (or fac) geometry. The tetradentate tripodal ligands occupy four contiguous sites, leaving two cis positions available on the octahedral metal center. When bound to four- and five-coordinate metal centres, these ligands impose C3 symmetry, which can lead to uncommon ligand field splitting patterns. Tripodal ligands are often able to coordinately saturate metal ions with lower coordination numbers.

One tripodal ligand of commercial significance is nitrilotriacetate, N(CH2CO2)3 because it is cheaply produced and has a high affinity for divalent metal ions. Other tripodal triamine ligands include tren (N(CH2CH2NH2)3) and cis-1,3,5-triaminocyclohexane. [1] Certain triphosphines such as RC(CH2PPh2)3 are also tripodal. Many kinds of donor groups have been incorporated into the arms of tripodal ligands, including amido (R2N), [2] and N-heterocyclic carbenes.

Structure of [Ni(TACH)(H2O)3] (color code: blue = nitrogen, red = oxygen, dark blue = nickel). BAVNAN01.png
Structure of [Ni(TACH)(H2O)3] (color code: blue = nitrogen, red = oxygen, dark blue = nickel).

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Scorpionate ligand

The term scorpionate ligand refers to a tridentate ligand which would bind to a metal in a fac manner. The most popular class of scorpionates are the hydrotris(pyrazolyl)borates or Tp ligands. These were also the first to become popular. These ligands first appeared in journals in 1966 from the then little-known DuPont chemist of Ukrainian descent, Swiatoslaw Trofimenko. Trofimenko called this discovery "a new and fertile field of remarkable scope".

Octahedral molecular geometry Molecular geometry

In chemistry, octahedral molecular geometry describes the shape of compounds with six atoms or groups of atoms or ligands symmetrically arranged around a central atom, defining the vertices of an octahedron. The octahedron has eight faces, hence the prefix octa. The octahedron is one of the Platonic solids, although octahedral molecules typically have an atom in their centre and no bonds between the ligand atoms. A perfect octahedron belongs to the point group Oh. Examples of octahedral compounds are sulfur hexafluoride SF6 and molybdenum hexacarbonyl Mo(CO)6. The term "octahedral" is used somewhat loosely by chemists, focusing on the geometry of the bonds to the central atom and not considering differences among the ligands themselves. For example, [Co(NH3)6]3+, which is not octahedral in the mathematical sense due to the orientation of the N−H bonds, is referred to as octahedral.

Salen ligand Chemical compound

Salen refers to a tetradentate C2-symmetric ligand synthesized from salicylaldehyde (sal) and ethylenediamine (en). It may also refer to a class of compounds, which are structurally related to the classical salen ligand, primarily bis-Schiff bases. Salen ligands are notable for coordinating a wide range of different metals, which they can often stabilise in various oxidation states. For this reason salen-type compounds are used as metal deactivators. Metal salen complexes also find use as catalysts.

In inorganic chemistry, the trans effect is the increased lability of ligands that are trans to certain other ligands, which can thus be regarded as trans-directing ligands. It is attributed to electronic effects and it is most notable in square planar complexes, although it can also be observed for octahedral complexes. The analogous cis effect is most often observed in octahedral transition metal complexes.

Trispyrazolylborate

In inorganic chemistry, the trispyrazolylborate ligand, abbreviated Tp, is an anionic tridentate and tripodal ligand. Trispyrazolylborate refers specifically to the anion [HB(C3N2H3)3], but the term trispyrazolylborate refers to derivatives substituted at on the pyrazolyl rings. This family of compounds are sometimes called scorpionate ligands.

Metal nitrosyl complex

Metal nitrosyl complexes are complexes that contain nitric oxide, NO, bonded to a transition metal. Many kinds of nitrosyl complexes are known, which vary both in structure and coligand.

Tris(2-aminoethyl)amine Chemical compound

Tris(2-aminoethyl)amine is the organic compound with the formula N(CH2CH2NH2)3. This colourless liquid is soluble in water and is highly basic, consisting of a tertiary amine center and three pendant primary amine groups. Abbreviated tren or TREN it is a crosslinking agent in the synthesis of polyimine networks and a tripodal ligand in coordination chemistry.

Denticity

Denticity refers to the number of donor groups in a single ligand that bind to a central atom in a coordination complex. In many cases, only one atom in the ligand binds to the metal, so the denticity equals one, and the ligand is said to be monodentate. Ligands with more than one bonded atom are called polydentate or multidentate. The word denticity is derived from dentis, the Latin word for tooth. The ligand is thought of as biting the metal at one or more linkage points. The denticity of a ligand is described with the Greek letter κ ('kappa'). For example, κ6-EDTA describes an EDTA ligand that coordinates through 6 non-contiguous atoms.

Iron tetracarbonyl dihydride Chemical compound

Iron tetracarbonyl dihydride is the organometallic compound with the formula H2Fe(CO)4. This compound was the first transition metal hydride discovered. The complex is stable at low temperatures but decomposes rapidly at temperatures above –20 °C.

Dichlorotris(triphenylphosphine)ruthenium(II) Chemical compound

Dichlorotris(triphenylphosphine)ruthenium(II) is a coordination complex of ruthenium. It is a chocolate brown solid that is soluble in organic solvents such as benzene. The compound is used as a precursor to other complexes including those used in homogeneous catalysis.

Metal acetylacetonates are coordination complexes derived from the acetylacetonate anion (CH
3
COCHCOCH
3
) and metal ions, usually transition metals. The bidentate ligand acetylacetonate is often abbreviated acac. Typically both oxygen atoms bind to the metal to form a six-membered chelate ring. The simplest complexes have the formula M(acac)3 and M(acac)2. Mixed-ligand complexes, e.g. VO(acac)2, are also numerous. Variations of acetylacetonate have also been developed with myriad substituents in place of methyl (RCOCHCOR′). Many such complexes are soluble in organic solvents, in contrast to the related metal halides. Because of these properties, acac complexes are sometimes used as catalyst precursors and reagents. Applications include their use as NMR "shift reagents" and as catalysts for organic synthesis, and precursors to industrial hydroformylation catalysts. C
5
H
7
O
2
in some cases also binds to metals through the central carbon atom; this bonding mode is more common for the third-row transition metals such as platinum(II) and iridium(III).

Trisoxazolines

Trisoxazolines are a class of tridentate, chiral ligands composed of three oxazoline rings. Despite being neutral they are able to form stable complexes with high oxidation state metals, such as rare earths, due to the chelate effect. The ligands have been investigated for molecular recognition and their complexes are used in asymmetric catalysts and polymerisation.

Trisoxazolinylborate

Tris(oxazolinyl)borate compounds are a class of tridentate ligands; often abbreviated ToR, where R is the substituent on the oxazoline ring. Most commonly the substituent is either a methyl, propyl, tert-butyl or hydrogen. The formation of anionic boron backbone with addition of a phenyl group on boron allows the ligand to strongly bind to the metal center. It results in a more robust complex.

Hexahydro-1,3,5-triazine

In chemistry, hexahydro-1,3,5-triazine is a class of heterocyclic compounds with the formula (CH2NR)3. They are reduced derivatives of 1,3,5-triazine, which have the formula (CHN)3, a family of aromatic heterocycles. They are often called triazacyclohexanes or TACH's but this acronym is also applied to cis,cis-1,3,5-triaminocyclohexane

Tetradentate ligands are ligands that bind four donor atoms to a central atom to form a coordination complex. This number of donor atoms that bind is called denticity and is a way to classify ligands. Tetradentate ligands are common in nature in the form of chlorophyll which has a core ligand called chlorin, and heme with a core ligand called porphyrin. They add much of the colour seen in plants and humans. Phthalocyanine is an artificial macrocyclic tetradentate ligand that is used to make blue and green pigments.

In homogeneous catalysis, C2-symmetric ligands refer to ligands that lack mirror symmetry but have C2 symmetry. Such ligands are usually bidentate and are valuable in catalysis. The C2 symmetry of ligands limits the number of possible reaction pathways and thereby increases enantioselectivity, relative to asymmetrical analogues. C2-symmetric ligands are a subset of chiral ligands. Chiral ligands, including C2-symmetric ligands, combine with metals or other groups to form chiral catalysts. These catalysts engage in enantioselective chemical synthesis, in which chirality in the catalyst yields chirality in the reaction product.

Transition metal amino acid complexes are a large family of coordination complexes containing the conjugate bases of the amino acids, the 2-aminocarboxylates. Amino acids are prevalent in nature, and all of them function as ligands toward the transition metals. Not included in this article are complexes of the amides and ester derivatives of amino acids. Also excluded are the polyamino acids including the chelating agents EDTA and NTA.

<i>cis</i>,<i>cis</i>-1,3,5-Triaminocyclohexane Chemical compound

cis,cis-1,3,5-Triaminocyclohexane is an organic compound with the formula (CH2CHNH2)3. It is a triamine. Of the many isomers possible for triaminocyclohexane, the cis,cis-1,3,5-derivative has attracted attention because it is a common tripodal ligand, abbreviated as tach. It is a colorless oil. It is a popular tridentate ligand in coordination chemistry.

Transition metal dithiocarbamate complexes

Transition metal dithiocarbamate complexes are coordination complexes containing one or more dithiocarbamate ligand, which are typically abbreviated R2dtc. Many complexes are known. Several homoleptic derivatives have the formula M(R2dtc)n where n = 2 and 3.

1,1,1-Tris(diphenylphosphinomethyl)ethane Chemical compound

1,1,1-Tris(diphenylphosphinomethyl)ethane, also called Triphos, is an organophosphorus compound with the formula CH3C[CH2PPh2]3. An air-sensitive white solid, it is a tripodal ligand ("three-legged") of idealized C3v symmetry. It was originally prepared by the reaction of sodium diphenylphosphide and CH3C(CH2Cl)3:

References

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  2. Verkade, J. G. (1993). "Atranes: New Examples with Unexpected Properties". Acc. Chem. Res. 26: 483. doi:10.1021/ar00033a005.
  3. Schwarzenbach, Gerold; Bürgi, Hans-Beat; Jensen, William P.; Lawrance, Geoffrey A.; Mønsted, Lene; Sargeson, Alan M. (1983). "Acid Cleavage of Nickel(Ii) Complexes Containing cis,cis-1,3,5-Cyclohexanetriamine (TACH), Crystal Structure of [Ni(tach)(H2O)3](NO3)2, and a Correlation Between the Structure and Reactivity of Nickel-Polyamine Complexes". Inorganic Chemistry. 22 (26): 4029–4038. doi:10.1021/ic00168a042.
  4. Saouma, C. T., Peters, J. C. (2011). "M-E and M=E Complexes of Iron and Cobalt that Emphasize Three-Fold Symmetry (E = O, N, NR)". Coord. Chem. Rev. 255: 920. doi:10.1016/j.ccr.2011.01.009. PMC   3103469 .CS1 maint: multiple names: authors list (link)
  5. Blackman, A. G. (2008). "Tripodal Tetraamine Ligands Containing Three Pyridine Units: the other polypyridyl ligands". Eur. J. Inorg. Chem.: 2633. doi:10.1002/ejic.200800115.
  6. Parkin, G. (2000). "The Bioinorganic Chemistry of Zinc: Synthetic Analogues of Zinc Enzymes that Feature Tripodal Ligands". Chemical Communications: 1971-1985. doi:10.1039/B004816J.