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In chemistry, tetradentate ligands are 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 method of classifying ligands.
Tetradentate ligands are common in nature in the form of chlorophyll, which has a core ligand called chlorin, and heme, which has a core ligand called porphyrin. They are responsible for the colour observed in plants and humans. Phthalocyanine is an artificial macrocyclic tetradentate ligand that is used to make blue and green pigments.
The ligand can bend so that one donor atom is at the pole and the remaining three are on the equator of the central atom. This is called cis-β (beta). The remaining octahedral positions are cis (adjacent) to each other. The triangles of coordinating atoms and the central atom have two coplanar atoms, and one perpendicular atom. This arrangement is chiral, so there are two possible mirror images. The arrangement where the chain goes down and clockwise is termed lambda, Λ, and where it goes down and anticlockwise is called delta, Δ. If the chain is not symmetrical, then different isomers can be produced by the end of the ligand that has the bend. If three donor atoms are the same at one end of the chain, the mer- and fac- prefixes used for tridentate ligands can be used. If the three donor atoms are arranged on a meridian, β- mer- is used; if the three donor atoms are arranged on the face of an octahedron, β- fac is used.
The chain can have two bends, with one donor at a pole, two on the equator and one at the opposite pole. None of the triangles of coordinating atoms and the central atom are coplanar. This is termed cis-alpha (α). This arrangement is chiral, so there are two possible mirror images. The arrangement where the chain goes down and clockwise and down is termed lambda, Λ, and where is goes down and anticlockwise and down is called delta (Δ).
Atoms with five coordinate positions are usually trigonal bipyramidal or square pyramid geometry. A symmetric tripodal tetradentate ligand can form two isomers on a square pyramid, depending on whether the bridging donor is on the apex or the base of the pyramid. The extra vacant position on the square pyramid is on the base. Square pyramidal coordination tends to occur where a six-member ring is formed with the bridgehead, bridge, feet donor atom and central atom. The longer leg (with three bridging atoms) connects to the apex of the pyramid, and symmetry is lost.
For the trigonal bipyramid, the tripod shaped ligand has its most symmetrical position with the bridging donor at one of the apexes, and the feet of the tripod are arranged around the base, leaving a vacant position at the opposite apex, resulting in C3 v symmetry. Trigonal bipyramidal coordination tends to occur where five member rings are formed with the bridgehead, bridge, feet donor atoms and central atom.
In four coordination a tripodal ligand would fill all the positions available, the geometry is trigonal pyramid. The shape is distorted from the tetrahedron due to the non-symmetry of the tripod.
One further characteristic is the size of the rings formed by the central metal with two donor atoms and the intervening chain of the ligand. Usually these rings have five or six members, but sometimes seven atoms. For ring shaped ligands, the total number of atoms in the ring is important, as it is a determiner of the hole size for the central atom. Each additional atom in the ring enlarges the hole radius from 0.1 to 0.15 Å.
Ligands are also characterized by charge. Tetradentate ligands can be neutral so that the charge of the whole complex is the same as the central atom. A tetradentate monoanionic (TMDA) ligand has one donor atom with a negative charge. A tetradentate dianionic ligand has a double negative charge, and tetradentate trianionic ligands have a triple negative charge. The maximal charge is on tetradentate tetraanionic ligands, which can stabilize metals in high oxidation states, however such ligands also have to resist oxidation by the highly oxidizing metal centre.
Chlorophyll comes in several forms and is important in plant photosynthesis. Bacteria may use variants called bacteriochlorophylls.
Tripodal ligands
Tripodal ligand tetradentate ligands have a donor atom connected via three chains to other donor atoms. The top of the tripod is called the apex, and a donor atom in that position is apical, or also known as the bridging atom. The other three donor atoms are on the "feet" of the tripod. Tripodal tetradentate ligands can have three identical chains attached to an atom (such as nitrogen, phosphorus, or arsenic) in tertiary arrangement. Molecules containing phosphorus, or arsenic donor atoms remain stiff at the P or As and can hold their shape, unlike nitrogen compounds which rapidly racemize. If all the feet of the tripod are symmetrical and identical to each other, there will be only one way to attach in an octahedral coordination. However, there are two non-equivalent positions left on the central atom, so if two different monodentate ligands or an unsymmetrical bidentate ligand attaches, there will be two possible isomers. If the feet differ, there are more isomers. When two feet are the same, and one is different there are three arrangements, two of which are enantiomers of each other. When there are three different legs, there are six possible isomers, but two are enantiomers of another pair and two are symmetric. open access
Classification
In addition to shape, tetradentate ligands can be classified by the ligating atoms on the ligand. For linear ligands the order can be given. The ligand may have a negative charge when it is in a complex with the central atom. This may develop through the loss of hydrogen ions when the substance is dissolved.
List
Chlorin ring NNNN –2 312.3678 Mg Corrin ring NNNN –1 306.40 Co 1,4,7,10-tetraoxacyclododecane 12-crown-4 (C2H4O)4 ring OOOO 0 176.21 Li 1,4,8,11-tetraazacyclotetradecane cyclam (NHCH2CH2NHCH2CH2CH2)2 ring NNNN 200.33 transition metals 1,4,7,10-tetraazacyclododecane cyclen ring N4 172.271 Zn Dibenzotetramethyltetraaza14annulene tmtaa ring NNNN 2- UO2 N,N-ethylenediaminediacetic acid NH2C2H4N(CH2COOH)2 tripodal NNO2 2– N,N'-ethylenediaminediacetic acid (-CH2NHCH2COOH)2 linear ONNO 2– N-hydroxyimino-2,2'-dipropionic acid H3HIDPA HON(CH(CH3)CO2H)2 linear ONOO 3– V4+ diethylenetriamineacetic acid DTMA NH2C2H4NHC2H4NHCH2COOH linear NNNO 1– Co iso-diethylenetriamineacetic acid i-DTMA (NH2C2H4)2NCH2COOH tripodal NN2NO 1– Co Jäger's N2O2 ligand linear acacen ONNO N2O2 Ni Naphthalocyanine C48H26N8 ring NNNN 714.79 Nitrilotriacetic acid NTA N(CH2CO2H)3 tripodal NO3 3– 191.14 Ca2+, Cr, Cu2+, and Fe3+, Ni Phthalocyanine H2Pc C32H18N8 ring NNNN 2– Cu, Co PorphyrinNumerous derivatives of porpyrin are known. See the list in (1995). 9783540592815, Springer. ISBN 9783540592815 ring NNNN Mg, V, Fe, Ni Rhodotorulic acid C14H24N4O6 I shape OOOO 344.36 Fe3+ Salen ligand linear ONNO N2O2 268.31 salpn ligand salpn linear ONNO 2− 282.34 Cr, Cu, Fe, Ni tetars (meso and racemic isomers) (CH3)2As(CH2)3As(C6H5)CH22 linear AsAsAsAs 0 Co2+ 1,1,4,7,10,10-hexaphenyl-1,4,7,10-tetraphosphadecane
tetraphostet-1 linear PPPP 0 670.68 Fe+ Ru+ Os+ Re3+ Pd2+ Pt2+ 1,4,7,10-tetrathiadodecane (1990). 9780444888419, Elsevier. ISBN 978044488841912-ane-S4 ring SSSS 0 Cu2+ 1,4,7,10-tetrathiatridecane 13-ane-S4 ring SSSS 0 Cu2+ 1,4,8,11-tetrathiatetradecane 14-ane-S4 ring SSSS 0 Cu2+ 1,4,8,12-tetrathiapentadecane 15-ane-S4 ring SSSS 0 Cu2+ 1,5,9,13-tetrathiahexadecane 14-ane-S4 ring SSSS 0 Cu2+ 2,5,8-trithia9(2,5)thiophenophane ring SSSS 0 Cu2+ Triethylene glycol dimethyl ether TG3 CH3(OCH2CH2)3OCH3 linear OOOO 0 178.23 neutral Na, K Triethylenetetramine TETA
trienCH2NHCH2CH2NH22 linear NNNN 146.24 Cu2+ tris-(dimethylarsinopropyl)-arsine (1973). 9780333136287, Macmillan Education. ISBN 9780333136287 AsCH2CH2CH2As(CH3)23 tripod AsAs3 0 Fe2+ Ni2+ Co3+ oct
Ni3+ tbptris-( o-dimethylarsinophenyl)-arsine As o-C6H4As(CH3)23 tripod AsAs3 0 Pt2+ Pd2+ Ni2+ tbp
Ru2+ octtris-( o-diphenylarsinophenyl)-arsine As o-C6H4As(C6H5)23 tripod AsAs3 0 Pt2+ Pd2+ Ru0 Rh+ Ni2+ tbp
Re2+ Ru2+ Os2+ Rh3+ Pd4+ Pt4+ oct CH3As(CH3) o-C6H43AsCH(3)2 linear As4 0 Pd2+ square pyramydal As(C6H5)2 o-C6H4As(C6H5)CH22 linear As4 0 Ni2+ 4 coordinate
Ni2+ Co2+ five coordinatetris-( o-diphenylphosphinophenyl)-phosphine tripod tetraphosphine of Venanzi P o-C6H4P(C6H5)23 tripod PP3 0 Pd2+ Pt2+ Ru0 Ru2+ Os2+ Cr0 Cr+ Cr3+ Mn+ Co3+ oct
Ni2+ Fe2+ Co+ Co2+ tbpTris(2-pyridylmethyl)amine TPA tripodal NN3 290.37 Cu 2,2′-bi-1,10-phenanthroline BIPHEN linear N4 0 Cd Sm Am Quaterpyridine qtpy linear N4 0
Biomolecules
Heme is a heterocyclic macrocycle ring shaped tetradentate ligand. It is an important molecule in red blood cells.
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