Metal dithiolene complex

Dithiolene metal complexes can be found in coordination compounds where the metal centre is coordinated by one, two, or three dithiolene ligands. The tris(dithiolene) complexes were the first examples of trigonal prismatic geometry in coordination chemistry. One example is {{chem2|Mo(S2C2Ph2)3}}. Similar structures have been observed for several other metals.{{cite journal | vauthors = Eisenberg R, Gray HB | title = Trigonal-prismatic coordination. Crystal and Molecular Structure of Tris (cis-1,2-diphenylethylene-1,2-dithiolato)vanadium | journal = Inorg. Chem. | year = 1967 | volume = 6 |issue=10 | pages = 1844–9 | doi = 10.1021/ic50056a018}}

File:(Et4N)2Ni(mnt)2.jpg)2}}, illustrating the intense color that typifies many dithiolene complexes]]

Because of the unusual redox and intense optical properties of dithiolenes, the electronic structure of dithiolene complexes has been the subject of intense studies. 1,2-Dithiolene ligands can exist in three oxidation states: the dianionic "ene-1,2-dithiolate", the neutral "1,2-dithioketone," and a monoanionic radical intermediate between these two.{{cite journal | vauthors = Aragoni MC, Caltagirone C, Lippolis V, Podda E, Slawin AM, Woollins JD, Pintus A, Arca M | title = Diradical Character of Neutral Heteroleptic Bis(1,2-dithiolene) Metal Complexes: Case Study of [Pd(Me₂timdt)(mnt)] (Me₂timdt = 1,3-Dimethyl-2,4,5-trithioxoimidazolidine; mnt^2- = 1,2-Dicyano-1,2-ethylenedithiolate) | journal = Inorganic Chemistry | volume = 59 | issue = 23 | pages = 17385–17401 | date = December 2020 | pmid = 33185438 | pmc = 7735710 | doi = 10.1021/acs.inorgchem.0c02696 | url =}} When the latter two are complexed to a metal centre, the oxidation state of the ligand (and therefore the metal centre) cannot be easily defined. Such ligands are therefore referred to as non-innocent. The substituents on the backbone of the dithiolene ligand, R and R', affect the properties of the resulting metal complex in the expected way. Long chains confer solubility in less polar solvents. Electron acceptors (e.g. cyanide {{chem2|CN−}}, acetate {{chem2|CH3CO2−}}) stabilize reduced and anionic complexes. Derivatives are known where the substituents are the same, symmetrical dithiolenes (R = R') are more common than unsymmetrical.

Due to their delocalized electronic structure, 1,2-dithiolene complexes undergo reversible redox reaction. When oxidized, dithiolene complexes have greater 1,2-dithioketone character. In reduced complexes, the ligand assumes more ene-1,2-dithiolate character. These descriptions are evaluated by examination of differences in C-C and C-S bond distances. The true structure lies somewhere between these resonance structures. Reflecting the impossibility to provide an unequivocal description of the structure, McCleverty introduced the term 'dithiolene' to give a general name for the ligand that does not specify a particular oxidation state. This suggestion was generally accepted, and 'dithiolene' is now a universally accepted term. Only more recently the radical nature of monoanionic 1,2-dithiolene ligands has been pointed out. While few examples of authentic dithiolene radicals have been reported, diamagnetism in neutral bis(1,2-dithiolene) complexes of divalent transition metal ions should be considered as a consequence of a string antiferromagnetic coupling between the two radical ligands.

:File:Resonance_structures_of_dithiolene_complex.png, CN or organyl.]]

1,2-Dithiolene metal complexes occur widely in nature in the form of the molybdopterin-bound Mo and W-containing enzymes.

File:Activesite.jpg features two pyranopterindithiolene ligands.{{cite journal | vauthors = McEwan AG, Ridge JP, McDevitt CA, Hugenholtz P | title = The DMSO Reductase Family of Microbial Molybdenum Enzymes; Molecular Properties and Role in the Dissimilatory Reduction of Toxic Elements | year = 2002 | journal = Geomicrobiology Journal | pages = 3–21 | volume = 19 | issue = 1 | doi=10.1080/014904502317246138| bibcode = 2002GmbJ...19....3M | s2cid = 85091949}}]]

1,2-Dithiolene complexes applications are numerous, and span from superconductivity, to linear and non linear optics, to biochemistry. Commercial applications of 1,2-dithiolene complexes are limited. A few dithiolene complexes have been commercialized as dyes in laser applications (Q-switching, mode-locking). 1,2-Dithiolene complexes have been discussed in the context of conductivity, magnetism, and nonlinear optics. It was proposed to use dithiolene metal complexes that bind unsaturated hydrocarbons at the sulfur centers for industrial olefin (alkene) purifications.{{cite journal | vauthors = Wang K, Stiefel EI | title = Toward separation and purification of olefins using dithiolene complexes: an electrochemical approach | journal = Science | volume = 291 | issue = 5501 | pages = 106–9 | date = January 2001 | pmid = 11141557 | doi = 10.1126/science.291.5501.106 | bibcode = 2001Sci...291..106W}} However, the complexities within such systems became later apparent, and it was argued that more research would be needed before using metal dithiolene complexes in alkene purifications may become practical.{{cite journal | vauthors = Harrison DJ, Nguyen N, Lough AJ, Fekl U | title = New insight into reactions of Ni(S2C2(CF3)2)2 with simple alkenes: alkene adduct versus dihydrodithiin product selectivity is controlled by [Ni(S2C2(CF3)2)2]- anion | journal = Journal of the American Chemical Society | volume = 128 | issue = 34 | pages = 11026–7 | date = August 2006 | pmid = 16925411 | doi = 10.1021/ja063030w}}

Most dithiolene complexes are prepared by reaction of alkali metal salts of 1,2-alkenedithiolates with metal halides. A thiolate is the conjugate base of a thiol, so alkenedithiolate is, formally speaking, the conjugate base of an alkenedithiol. Common alkenedithiolates are 1,3-dithiole-2-thione-4,5-dithiolate{{cite journal| vauthors = Dietzsch W, Strauch P, Hoyer E |title=Thio-oxalates: Their Ligand Properties and Coordination Chemistry|journal=Coord. Chem. Rev.|year=1992|volume=121|pages=43–130|doi=10.1016/0010-8545(92)80065-Y}} and sodium maleonitriledithiolate:{{cite book | vauthors = Holm RH, Davison A | title = Inorganic Syntheses | chapter = Metal Complexes Derived from cis ‐1,2‐dicyano‐1,2‐ethylenedithiolate and Bis(Trifluoromethyl)‐1,2‐dithiete | author-link1 = R. H. Hol | year = 1967 | volume = 10 | pages = 8–26 | doi = 10.1002/9780470132418.ch3| isbn = 978-0-470-13241-8}}

:{{chem2|Ni(2+) + 2 (NC)2C2S2(2−) → Ni[S2C2(CN)2]2(2−)}}

Some alkenedithiolates are generated in situ, often by complex organic reactions:

:{{chem2|cis\-H2C2(SCH2Ph)2 + 4 Na → cis\-H2C2(SNa)2 + 2 NaCH2Ph}}

Once generated, these anions are deployed as ligands:

:{{chem2|NiCl2 + 2 cis\-H2C2(SNa)2 → Na2[Ni(S2C2H2)2] + 2 NaCl}}

Often the initially formed, electron-rich complex undergoes spontaneous air-oxidation:

:{{chem2|2 [Ni(S2C2H2)2](2−) + 4 H+ + O2 → 2 Ni(S2C2H2)2 + 2 H2O}}

File:CAZYOR.png

An early and still powerful method for the synthesis of dithiolenes entails the reaction of α-hydroxyketones, acyloins, with phosphorus pentasulfide followed by hydrolysis and treatment of the mixture with metal salts. This method is used to prepare {{chem2|Ni[S2C2Ar2]2}} (Ar = aryl).

Although 1,2-dithiones are rare and thus not useful precursors, their valence isomer, the 1,2-dithietes are occasionally used. One of the more common dithiete is the distillable 3,4-Bis(trifluoromethyl)-1,2-dithiete. This electrophilic reagent oxidatively adds to many low valent metals to give bis- and tris(dithiolene) complexes.

:{{chem2|Mo(CO)6 + 3 (CF3)2C2S2 → [(CF3)2C2S2]3Mo + 6 CO}}

:{{chem2|Ni(CO)4 + 2 (CF3)2C2S2 → [(CF3)2C2S2]2Ni + 4 CO}}

Species of the type {{chem2|Ni[S2C2Ar2]2}} were first prepared by reactions of nickel sulfides with diphenylacetylene. More modern versions of this method entail the reaction of electrophilic acetylenes such as dimethyl acetylenedicarboxylate with well defined polysulfido complexes.

Early studies on dithiolene ligands, although not called by that name until the 1960s,{{cite book | vauthors = McCleverty J | title = Progress in Inorganic Chemistry | chapter = Metal 1,2-Dithiolene and Related Complexes | year = 1968 | volume = 10 | pages = 49–221 | doi = 10.1002/9780470166116.ch2 | isbn = 978-0-470-16611-6}}{{rp|58}}{{cite book | vauthors = Arca M, Aragoni MC | title = Handbook of Chalcogen Chemistry | chapter = Chapter 12.3. 1,2-Dithiolene Ligands and Related Selenium and Tellurium Compounds | year = 2007 | pages = 797–830 |doi=10.1039/9781847557575-00797| isbn = 978-0-85404-366-8 }} focused on the quinoxaline-2,3-dithiolates and 3,4-toluenedithiolates, which form brightly colored precipitates with several metal centres. Such species were originally of interest in analytical chemistry. Dithiolenes lacking benzene backbones represented an important development of the area, especially maleonitrile-1,2-dithiolate ("mnt"), {{chem2|(NC)2C2S2(2−)}}, and ethylenedithiolene, {{chem2|H2C2S2(2−)}}.

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{{Coordination complexes}}

Category:Coordination complexes

Category:Thiols

Category:Chelating agents