| Summary: | 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. === Science, Faculty of === Chemistry, Department of === Graduate
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