Highly distorted d(8)-rhodium(+I) and d(10)-rhodium(-I) complexes: synthesis, reactivity, and structures in solution and solid state

A synthesis of the new ligand 5-diphenylphosphanyl-10-methyl-5H-dibenzo[a,d ]cycloheptene ((me)tropp(ph)) was developed in order to prepare highly dist orted tetra-co-ordinated rhodium(+I) complexes. The ligand (me)tropp(ph) co ntains a cycloheptatriene ring in a rigid boat conformation such that a Ph2 P and an olefinic binding site are perfectly arranged for transition metal complexation. Four equivalents of (me)tropp(ph) react with [Rh-2(mu -Cl)(2) (cod)(2)] in the presence of KPF6 to yield almost quantitatively [Rh(+I)((m e)tropp(ph))(2)]PF6, which was isolated in the form of dark red-violet crys tals. A crystal structure analysis reveals that the co-ordination sphere of the rhodium centre in [Rh(+I)((me)tropp(ph))(2)](+) deviates strongly from a square planar arrangement (phi =42 degrees). One methyl group in [Rh(+I) ((me)tropp(ph))(2)](+) can be deprotonated by KOBut to give the allyl compl ex [Rh((allyl)tropp(ph))((me)tropp(ph))]. This complex has a structure that may be best described as a distorted trigonal bipyramid. The boron hydride [BEt3H](-) and organolithium reagents, LiR, react with [Rh((allyl)tropp(ph ))((me)tropp(ph))] in allylic alkylation reactions to yield anionic rhodate complexes [Rh((RCH2)tropp(ph))((me)tropp(ph))](-) (R=H, Me, n-Bu, Ph) that formally have a valence electron configuration. The rhodate [Rh((me)tropp( ph))(2)](-) can be obtained directly by reduction of cation [Rh((me)tropp(p h))(2)](+) with alkali metals. In a sym-proportionation reaction [Rh((me)tr opp(ph))(2)](+) and [Rh((me)tropp(ph))(2)](-) give the neutral -[Rh((me)tro pp(ph))(2)] radical (K=1.1x10(7)), which is not stable but decomposes with loss of H-2 to give the allyl complex [Rh((allyl)tropp(ph))((me)tropp(ph))] . The structures in solution of [Rh((me)tropp(ph))(2)](+), [Rh((allyl)tropp (ph))((me)tropp(ph))], and [Rh((nBuCH2)tropp(ph))((me)tropp(ph))](-) were d etermined by NMR techniques, which reveal that (i) they match the solid sta te structures and (ii) are rather similar to each other. This fact may expl ain the remarkable electronic flexibility of the rhodium centre, which chan ges reversibly its formal oxidation state from +I to 0 to -I at rather low negative potentials (DeltaE(01)=-0.882 V; DeltaE(02)=-1.298 V).