Genesis of Tetrathiooxalate-like C2S4 and C2Se4 Units. A Multiform Experimental Study and the Relevant Theoretical Implications for the Role of Transition-Metal Fragments

Article abstract of DOI:10.1021/ja00235a029

Binuclear complexes of general formula <(triphos)M(μ-C2X4)M(triphos)>Y2-, BPh4^-, BF4^-, PF6^-; M = Ir, X = S, Y = Cl^-, BPh4^-; triphos = MeC(CH2PPh2)3> can be obtained from (triphos)RhCl(η²-CS2)-C6H6 (1), (triphos)RhN3(η²-CS2) (2), (triphos)IrCl(η²-CS2) (3), and (triphos)RhCl(η²-CSe2)-C6H6 (4) after elimination of chloride or azide ligands and addition of suitable anions.The synthetic strategy favors the head-to-head dimerization of the dihapto-coordinated heteroallene molecules.The crystal structures of one of the precursors, 4, and one of the products, <(triphos)Rh(μ-C2S4)Rh(triphos)>(BPh4)2*CH2Cl2 <10-CH2Cl2 in text>, have been determined by X-ray methods.Crystal data for 4 are as follows: a = 21.151 (6) Angstroem, b = 16.450 (5) Angstroem, c = 13.500 (3) Angstroem, β = 90.81 (3)deg, monoclinic, P2₁/n, Z = 4, R = 0.077.Crystal data for 10-CH2Cl2 are as follows: a = 15.496 (4) Angstroem, b = 28.736 (7) Angstroem, c = 14.216 (4) Angstroem, β = 106.02 (2)deg, monoclinic, P2₁/a, Z = 4, R = 0.079.Compound 4 is a typical complex of the type L4M(η-CX2) (M = d₈).The dication in 10-CH2Cl2 has a dimeric framework containing a C2S4 bridge between two L3M fragments.The double bond value of the C-C linkage <1.37 (3) Angstroem> suggests the formulation of the ligand as an ethenetetrathiolate.The electrochemical behavior in nonaqueous solvents (MeCN, CH2Cl2) of the complexes <(triphos)Rh(μ-C2X4)Rh(triphos)>(PF6)2 (X = S,Se) shows a surprisingly rich redox chemistry.The electron-transfer sequences 4+/2+, 2+/1+, 1+/0, 0/1-, and 1-/2- are thermodynamically characterizable.The compound <(triphos)Rh(μ-C2S4)Rh(triphos)>(BF4)4 has been also synthesized by chemical oxidation with NOBF4 of the 2+ derivative, whereas reduction of the latter with LIHBEt₃ yields the neutral, paramagnetic complex (triphos)Rh(μ-C2S4)Rh(triphos).The present family of μ-C2X4 complexes displays noticeable electron-transfer properties.In particular, the neutral derivative transfers electrons to 7,7,8,8-tetracyanoquinodimethane, TCNQ, to give the paramagnetic compound <(triphos)Rh(μ-C2S4)Rh(triphos)>(TCNQ)2, whereas <(triphos)Rh(μ-C2S4)Rh(triphos)>(BF4)4 is able to extract electrons from tetrathiafulvalene to yield the 2+ derivative and the salt (TTF)3(BF4)2.The electronic structure of the dimeric complexes has been investigated by using the extended Hueckel method.There are two relevant ?_<*> interactions between the metals and the C2S4 ligand, one of which delocalizes electron density over a C2S4 orbital which is strongly C-C ? bonding.The ?_<*> nature of three frontier MOs and their extended delocalizattion over the Rh(μ-C2S4)Rh core accounts for the rich electrochemistry of this series of compounds.The genesis of the C2S4 unit from two dihapto coordinated CS2 molecules is discussed in terms of perturbation theory arguments.The relevant keystep seems to be the cleavage of the Rh-C bond in a homolytic fashion.Walsh-type diagrams show that along some possible pathways there is an avoided crossing...