Paramagnetic organometallic complexes of chromium, iron and cobalt
Creators: name: Leznoff, Daniel Bernard
Publisher: name: University of British Columbia
Year: 2009

description: The chemistry of the amidodiphosphine ligand ⁻N(SiMe₂CH₂PPh₂)₂ with chromium, iron and cobalt metal centres in the +2 and +3 oxidation states is explored. Metal-chloride complexes MCl[N(SiMe₂CH₂PPh₂)₂] are high-spin and tetrahedral for iron(II) and cobalt(II) and dinuclear with bridging chlorides for chromium(II). Alkylation reactions give high-spin, square-planar CrR[N(SiMe₂CH₂PPh₂)₂] (R = Me, CH₂Ph, SiMes₂H); the benzyl fragment in Cr(r] ²⁻ CH₂Ph)[N(SiMe₂CH₂PPh₂)₂ ] is n²-bound. The analogous cobalt(II) alkyl complexes, CoR[N(SiMe₂CH₂PPh₂)₂] (R = Me, CH₂Ph, CH₂SiMe₃), are square-planar and low-spin; the benzyl fragment is n²-bound. CrMe[N(SiMe₂CH₂PPh₂)₂] reacts with H₂ to give {[(Ph₂PCH₂SiMe₂)₂N]Cr)₂(μ-H)₂, which is isostructural with the bridging chloride. Variable temperature magnetic susceptibility measurements give J = -12.4 cm⁻1 for the chloride and J = -139 cm⁻1 for the hydride. Reaction of CrMe[N(SiMe₂CH₂PPh₂)₂] with PhSSPh yields a rare five-coordinate chromium(III) complex CrMe(SPh)[N(SiMe₂CH₂PPh₂)₂]. The one-electron oxidation reaction of CrR[N(SiMe₂CH₂PPh₂)₂] with alkyl halides gives Cr(R)X[N(SiMe₂CH₂PPh₂)₂]. Bulky alkyl groups stabilize chromium(III) dialkyl complexes, an example being Cr(CH₂SiMe₃)₂[N(SiMe₂CH₂PPh₂)₂]. Reaction of CoX[N(SiMe₂CH₂PPh₂)₂] with PhCH₂X gives five-coordinate CoX₂ [N(SiMe₂CH₂PPh₂)₂] complexes. Solid state variable temperature magnetic susceptibility measurements confirm the presence of an S = 1 cobalt(III) centre with a zero field splitting parameter of D = 32.6 cm⁻1 . The S = 3/2 cobalt(II) complex CoI[N(SiMe₂CH₂PPh₂)₂] exhibits zero-field splitting with D = 20 cm⁻1 . Alkyl complexes of cobalt(III) can not be prepared; Co(R)X[N(SiMe₂CH₂PPh₂)₂] decomposes via Co(III)-C bond homolytic cleavage, loss of alkyl radical and formation of CoX[N(SiMe₂CH₂PPh₂)₂]. Addition of MeI to CoX[N(SiMe₂CH₂PPh₂)₂] gives CoIX[N(SiMe₂CH₂PPh₂)-(SiMe₂CH₂PPh₂CH₂Ph)], a zwitterionic complex, by intermolecular reaction of RX with fluxional phosphine arms. Conversely, Mel reacts with CoMe[N(SiMe₂CH₂PPh₂)₂] to give CoI[N(SiMe₂CH₂PPh₂)₂], likely via Co(Me)I[N(SiMe₂CH₂PPh₂)₂]. Reaction of CoMe[N(SiMe₂CH₂PPh₂)₂] with P h C H ₂ X gives a mixture of CoX₂[N(SiMe₂CH₂PPh₂)₂ ] and CoX₂ [N(SiMe₂CH₂PPh₂)(SiMe₂CH₂PPh₂Me)]. A mechanism to account for intramolecular methyl-transfer, based on methyl-phosphoranyl radical formation from an organometallic radical cage, is presented. Alkylation of the iron complex FeCl[N(SiMe₂CH₂PPh₂)₂] gives low-spin Fe(n⁵- C₅H₅)[N(SiMe₂CH₂PPh₂)₂], isostructural with the analogous low-spin chromium(II) complex, Cr(n⁵-C₅H₅)[N(SiMe₂CH₂PPh₂)₂] and high-spin, tetrahedral FeR[N(SiMe₂CH₂PPh₂)₂] (R = CH₂SiMe₃, CH(SiMe₃)₂). The reaction of iron(II) complexes with alkyl halides yields a mixture of zwitterions as well as FeX₂ [N(SiMe₂CH₂PPh₂)₂]; the iron(III) complex can be prepared independently from FeX₃. FeB₂r[N(SiMe₂CH₂PPh₂)₂] is trigonal-bipyramidal and is an example of a spin-admixed S = 3/2, 5/2 iron(III) system. Anti - Co[PhP(CH₂SiMe₂NSiMe₂CH₂)₂PPh] (Co[P₂N₂]) is low-spin and square-planar; reaction with I₂ gives five-coordinate anti - CoI[P₂N₂]. Syn- and anti - Co[P₂N₂] react with 0₂ to yield cobalt phosphine-oxide complexes.