[n]Ferrocenophanes (n = 2, 3) with Nitrogen and Phosphorus in Bridging Positions

The in situ prepared dilithio derivative of the known species 1-bromo-1′-(trimethylsilylamino)­ferrocene (1) reacted with tBuPCl2 to form the first example of a [2]­ferrocenophane ([2]­FCP) bridged by nitrogen and phosphorus (2). Sulfurization of 2 followed by column chromatography on silica gel gave the expected [2]­FCP with a tBu­(S)­PN­(SiMe3) bridging moiety (3a) and its desilylated counterpart with a tBu­(S)­PNH bridging moiety (3b). The molecular structure of 3b was determined by single-crystal X-ray analysis (α = 18.40(11)°). Using a common synthetic methodology, two new 1-amino-1′-bromoferrocene derivatives were prepared, one species with a PhCH2 (6a) and another with a tBuCH2 group (6b) on nitrogen. Dilithiation of 6a followed by addition of tBuPCl2 gave a mixture of three constitutional isomers: the targeted [2]­FCP (7a), the 1,1′-disubstituted ferrocene derivative (tBuPH)­(PhCHN)­fc (8a), and the [3]­FCP bridged by a (NH)­(CHPh)­PtBu moiety (9a). NMR spectroscopy revealed that 8a is the precursor for 9a. The salt-metathesis reaction of the dilithio derivative of 6b with tBuPCl2 exclusively gave the 1,1′-disubstituted ferrocene derivative (tBuPH)­(tBuCHN)­fc (8b), which does not isomerize to the respective [3]­FCP. DFT calculations at the M06/6-311+G­(d,p) level were used to better rationalize these unexpected results.