Coexistence of Three Different Spin States in a Cyanido-Bridged Trinuclear Iron(III) Complex with an Unusual Fe III to Ligand Charge Transfer.

We report a trinuclear iron(III) cyanido-bridged complex trans-[CpMe ₃ Fe ^III (dppe)(CN)] ₂ [Fe ^III (L _N4 )][PF ₆ ] ₄ (2[PF ₆ ] ₄ ) as the oxidation product of binuclear complex [CpMe ₃ (dppe)Fe ^II CN-Fe ^III (L _N4 )][PF ₆ ] (1[PF ₆ ]) (CpMe ₃ =1, 2, 4-trimethyl-1,3-cyclo-pentadienyl, dppe=1,2-bis(diphenylphosphino)ethane, L _N4 =pentane-2,4-dione-bis(S-methylisothiosemicarbazonato). Complex 1[PF ₆ ] possesses an intermediate-spin five-coordinated Fe ^III (S=3/2) which couples antiferromagnetically to the π-radical ligand (L⋅ _N4 ) ^2- and shows a LMCT (ligand to metal charge transfer) transition from (L⋅ _N4 ) ^2- to Fe ^III and the Fe ^II →Fe ^III MMCT transition. Upon oxidation of 1[PF ₆ ], (L⋅ _N4 ) ^2- loses one electron to be the strong electron-attracting ligand (L ^Ox _N4 ) ^- and the intermediate-spin five-coordinated Fe ^III (S=3/2) becomes a low-spin six-coordinated Fe ^III (S=1/2) in 2[PF ₆ ] ₄ . Also interestingly, 2[PF ₆ ] ₄ presents the coexistence of three different spin states (one S=3/2 and two S=1/2) and an uncommon Fe ^III →(L ^Ox _N4 ) ^- MLCT transition _, confirmed by the experimental results and supported by the TDDFT calculations.
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