The influence of cis/trans isomerism on the physical properties of a cyano-bridged dinuclear mixed valence complex

Abstract

The cyano-bridged complexes cis-[L 14 Co III NCFe II (CN) 5 ] Ϫ and cis-[L 14 Co III NCFe III (CN) 5 ] (L 14 = 6-methyl-1,4,8,11tetraazacyclotetradecan-6-amine) are prepared and characterised spectroscopically, electrochemically and structurally: Na{cis-[L 14 Co III NCFe II (CN 1), b = 14.709(2), c = 18.760(3) Å, Z = 4. In both complexes, the pendant amine is cis to the bridging cyanide ligand. An analysis of the metal-to-metal charge transfer (MMCT) transition in these systems with Hush theory has been carried out. This has revealed that the change in the configuration of the macrocycle both decreases the redox isomer energy difference (∆E 1/2 ) and increases the reorganisational energy (λ) of the cis-[L 14 Co III NCFe II -(CN) 5 ] Ϫ complex with respect to the trans-[L 14 Co III NCFe II (CN) 5 ] Ϫ complex, the result being that both isomers display an MMCT transition of similar energy. The variation in redox isomer energy differences of the configurational isomers has been related to strain energy differences by molecular mechanics analysis of the [CoL 14 Cl] 2ϩ/ϩ precursor complexes. DALTON

Geometric isomers of chloro(6-methyl-1,4,8,11-tetraazacyclotetradecane-6-amine)cobalt(III) tetrachlorozincate(II)

“…In addition, the bond lengths are not markedly different from those in the N-based diastereomer trans-III-[CoLCl](ClO 4 ) 2 , (III) (Hambley et al, 1992). Molecular mechanics calculations (Bernhardt et al, 2002) have reproduced the variations in CoÐN and CoÐCl bond lengths observed in cis-, trans-I-and trans-III-[CoLCl] 2+ . The folded cis con®guration is best able to accommodate an expansion of the CoÐN bond lengths, whereas the trans isomers tend to enforce shorter coordinate bond lengths.…”

Tuning the metal-to-metal charge transfer energy of cyano-bridged dinuclear complexes

“…2,3 In fact the trans-I N-based diastereomeric form has been seen in all but one structurally characterised, pentadentate-coordinated Co III complex of L 14 . 21,22 The coordinate bonds around the Ru atom are the same within experimental error and consistent with those found in other hexacyanoruthenate(II) containing structures.…”

“…S4 †). The spectrum of [Ru(CN) 6 ] 3− exhibits three intense broad absorption bands from 250-370 nm ( = 1.5-2.0 × 10 3 dm 3 mol −1 cm −1 ) 27,28 and a smaller band with a maximum at 460 nm ( ~ 1000 dm 3 The MMCT transition of both complexes also vanishes upon reduction of the Co III centre. In this case, the lability and airsensitivity of the putative Co II -M II (M = Fe or Ru) complexes necessitated the rapid acquisition of an electronic spectrum following reduction.…”

“…Previously, we found that the influence of geometric isomerism on the MMCT energy of the closely related compounds trans- [L 14 CoNCFe(CN) 5 ] − and cis- [L 14 CoNCFe(CN) 5 ] − led to differences in both G o and . 3 However, as these changes were opposite in sign, the overall MMCT energies of the isomers were essentially the same. Earlier we had also found that changing the macrocyclic ring size from fourteen (trans-[L 14 CoNCFe(CN) 5 ] − ) to fifteen gave essentially the same values of all three parameters.…”

“…[1][2][3][4] As an example, the complex anion trans-[L 14 CoNCFe(CN) 5 ] − (Chart 1) and its analogues fall into the Robin and Day 5 Class II group of mixed valence compounds, where weak to moderate electronic coupling between the metal centres is present and electronic transitions from the individual Co III and Fe II chromophores are apparent. and acceptor (Co III/II ) redox potentials i.e.…”