Quan Du

Analogies between binuclear phospholyl and cyclopentadienylmanganese carbonyl complexes: seven-electron donor bridging phospholyl rings

Phosphacymantrene, (η5-C4H4P)Mn(CO)3, is a known stable compound and a potential precursor to binuclear (C4H4P)2Mn2(CO)n derivatives analogous to isovalent (C5H5)2Mn2(CO)n derivatives. In this connection the structures and energetics of binuclear derivatives have been investigated by density functional theory. The lowest energy (C4H4P)2Mn2(CO)n (n = 5, 4) structures have seven-electron donor η5,η1-C4H4P ligands bridging the two metal atoms, which are too far apart for a direct Mn–Mn bond. However, the lowest energy (C4H4P)2Mn2(CO)3 structure is predicted to be a triply carbonyl bridged structure with a formal MnMn triple bond of length 2.17 A analogous to (η5-C5H5)2Mn2(μ-CO)3, which has been synthesized and structurally characterized by X-ray diffraction. The lowest energy (C4H4P)2Mn2(CO)2 structures have one terminal η5-C4H4P ring and one bridging seven-electron donor η5,η1-C4H4P ring. However, only slightly higher energy (C4H4P)2Mn2(CO)2 structures are found with two η5-C4H4P rings and thus analogous to previously found (η5-C5H5)2Mn2(CO)2 structures. Such a triplet (η5-C4H4P)2Mn2(CO)2 structure is predicted to have a formal MnMn triple bond of length 2.20 A. The two analogous singlet (η5-C4H4P)2Mn2(CO)2 structures have shorter MnMn bonds of length 2.07 A, which are interpreted as the formal quadruple bonds required to give each Mn atom the favored 18-electron configuration.

Binuclear iron carbonyl complexes of thialene

Thialene (C8H6S) is an isomer of benzothiophene and related to azulene by replacement of a CC unit in the seven-membered ring by a sulfur atom. The geometries of binuclear iron carbonyl complexes of thialene (C8H6S)Fe2(CO)n (n = 6, 5, 4) have been investigated using density functional theory for comparison with the corresponding azulene derivatives. The lowest energy (thialene)Fe2(CO)6 structures have a bis(tetrahapto)-η4,η4-thialene ligand bonded to two separate Fe(CO)3 units without involvement of the sulfur atom. This differs from the isomeric (benzothiophene)Fe2(CO)6 structure known experimentally in which an iron atom has inserted into a carbon–sulfur bond to give a ferrathianaphthalene system. The only low-energy (thialene)Fe2(CO)5 structure has a pentahapto-trihapto-η5,η3 thialene ligand bonded to an Fe2(CO)5 unit, again without involvement of the sulfur atom. This structure is related to the experimental structure of azulene diiron pentacarbonyl, (η5,η3-C10H8)Fe2(CO)5 by replacement of the uncomplexed CC double bond in the seven-membered ring with the sulfur atom in the thialene six-membered ring. The potential energy surface of the unsaturated (thialene)Fe2(CO)4 is very complicated but does not include any low-energy structures with formal FeFe double bonds. Thermochemical considerations suggest (thialene)Fe2(CO)5 as a realistic synthetic objective, which is potentially accessible from reactions of thialene with Fe2(CO)9 or (benzalacetone)Fe(CO)3 under mild conditions.

Theoretical studies on the desulfurization of benzothiophene (thianaphthene) and thienothiophene (thiophthene) by carbon–sulfur bond cleavage: binuclear iron carbonyl intermediates

Thiophene is known experimentally to be desulfurized by Fe3(CO)12 under mild conditions to give the tricarbonyl ferrole (η4,η2-C4H4)Fe2(CO)6. A similar reaction of benzothiophene (thianaphthene) with Fe3(CO)12 gives a (C8H6S)Fe2(CO)6 complex in which an iron carbonyl moiety has inserted into the thiophene ring to give a thiaferranaphthalene ligand. Density functional theory shows this experimental structure to be the lowest energy structure. Furthermore, the lowest energy structures of the diiron pentacarbonyl (C8H6S)Fe2(CO)5 are simply derived from this (C8H6S)Fe2(CO)6 by loss of a CO group retaining the thiaferranaphthalene ligand. However, a higher energy isomeric (η6,η2-C8H6S)Fe2(CO)5 structure retains the original benzothiophene ligand with the C6 ring bonded to an Fe(CO)2 moiety as a hexahapto ligand and the CC double bond of the C4S ring bonded to an Fe(CO)3 moiety as a dihapto ligand with an Fe→Fe dative bond between the iron atoms. Similar insertion of an iron atom into a thiophene ring to give a thiaferrabenzene ring is predicted to occur in the lowest energy (C6H4S2)Fe2(CO)6 structure derived from either the anti or syn isomers of thienothiophene. However, the bonding of the exocyclic iron atom to the resulting thiaferrabenzothiophene ligand involves atoms in both rings in contrast to the (C8H6S)Fe2(CO)6 complex where the benzene ring is not involved in the ligand–iron bonding.

Effect of metal complexation on the equilibrium between methylphosphepine and methylphosphanorcaradiene and their benzo analogues

Theoretical studies are reported on methylphosphepine, methylbenzophosphepine, their norcaradiene isomers, and their metal complexes with Fe(CO)3 and CpCo (Cp = η5-C5H5) units. Both methylphosphepine and methylphosphanorcaradiene are C6H6PCH3 species existing as anti/syn stereoisomer pairs with the methylphosphepine structures at slightly higher energies. The transition states for the interconversion of these isomers and their benzo derivatives lie ∼20 kcal mol−1 in energy above the methylphosphanorcaradiene isomers. Complexation of either C6H6PCH3 ligand with the transition metal units Fe(CO)3 and CoCp leads to energetically closely spaced η4-tetrahapto and η3-trihapto isomers of the methylphosphepine complexes and η4-tetrahapto isomers of the methylphosphanorcaradiene complexes. However, the bis(dihapto) (η2,2-C6H6PCH3)Fe(CO)3 and (η2,2-C6H6PCH3)CoCp complexes involving coordination of non-adjacent CC double bonds lie at significantly higher energies. Fusion of a benzene ring to the C6H6PCH3 rings in methylphosphepine and methylphosphanorcaradiene leads to significantly different structures of their lowest energy metal complexes. Thus the lowest energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures are η2,2 and η4 methylbenzophosphepine complexes, which avoid using any carbon atoms of the benzene ring in the ligand for metal complexation. Higher energy (C10H8PCH3)Fe(CO)3 and (C10H8PCH3)CoCp structures have tetrahapto ligands with one or both CC double bonds of the benzene ring complexed with the metal atom.