Synthesis of [2]ferrocenophanes containing trivalent diphosphine units

Three (PR)2-bridged [2]ferrocenophanes (R = tBu (10a), nBu (10b), and Ph (10c)) in which the two C5H4 rings of the ferrocene unit were bridged with a P(III)—P(III) bond were synthesized via Mg-mediated reductive coupling between the phosphorus centers of corresponding [Fe{C5H4P(Cl)R}2]. The P(Cl)R groups of [Fe{C5H4P(Cl)R}2] were derived from the P(NEt2)2 groups of [Fe{C5H4P(NEt2)2}2] as follows: the P(NEt2)2 group was first converted to P(Cl)NEt2 by HCl, and then R (R = tBu, nBu, and Ph) was introduced to give P(R)NEt2, which was again allowed to react with HCl to form the P(Cl)R group. Of the three diphospha[2]ferrocenophanes, (PtBu)2-bridged [2]ferrocenophane (10a) could be isolated as free diphosphine, while because of difficulty in their isolations as free ligands, the nBu and Ph analogs (10b and 10c, respectively) were characterized as bis(pentacarbonylmetal) complexes (μ-10b)-[W(CO)5]2 (11) and (μ-10c)-[Cr(CO)5]2 (12), in which the respective phosphorus centers were coordinated to the pentacarbonylmetal fragments. X-ray analysis was performed for the three diphospha[2]ferrocenophanes, 10a, 11, and 12, demonstrating that their P—P bond lengths are 2.3093(7), 2.261(5), and 2.2502(8) Å, respectively. Because these tether P—P lengths are smaller than an ideal distance between the two parallel C5H5 rings of ferrocene, the two C5H4 rings of each of 10a, 11, and 12 are inclined from the parallel by 12.9°, 9.7°, and 13.6°, respectively. DFT calculations were performed on (PH)2-bridged [2]ferrocenophane as a model-compound, and showed that there are two conformational isomers. One, named the parallel form, features a P—P bond that is almost parallel to the axis passing through the centers of the two C5H4 rings, while the P—P bond of the other, named the twist form, leans with the two C5H4 rings twisted. The smaller inclined angle of 9.7° observed for 11 was found to result from its parallel-form conformation, while the C5H4 rings of 10a and 12 were more inclined due to adoption of the twist-form conformation.