Synthesis of 2,2′,3,3′-Tetramethyl- and 2,2′,3,3′-Tetra-tert-butylfulvalene: Attractive­ Platforms for Dinuclear Transition Metal Fragments, as Exemplified by (η 5:η 5-2,2′,3,3′-t-Bu4C10H4)M2(CO) n (M = Fe, Ru, Os, Mo) and First X-ray Crystal Structures of Fulvalene Diiron and Diosmium Complexes

2,2',3,3'-Tetramethylfulvalene (4) and 2,2',3,3'-tetra-tert-butylfulvalene (7) were synthesized by two different routes. The potential of these systems to function as ligands to transition metals was explored with the more readily made 7, resulting in the preparation of the dinuclear complexes (η 5 :η 5 -t-Bu 4 C 1 0 H 4 )M 2 (CO) 4 (M = Fe, Ru, Os, 8-10) and (η 5 :η 5 -t-Bu 4 C 1 0 H 4 )Mo 2 (CO) 6 (11). The molecular structures of complexes 8 and 10 were determined by X-ray diffraction. Complexes 9 and 10 rearranged on irradiation to their (μ 2 -η 1 :η 5 -cyclopentadienyl) 2 M 2 (CO) 4 isomers 12 and 13, respectively, thermally reversible for 12, while 8 was photoinert.