412 - Inorg. Chem. 2007, 46, 55675575 A New N,N,O Chelate...

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A New N,N,O Chelate for Transition Metal Chemistry: Fe 5 and Fe 6 Clusters from the Use of 6-Hydroxymethyl-2,2 -bipyridine (hmbpH) Rashmi Bagai, Khalil A. Abboud, and George Christou* Department of Chemistry, Uni V ersity of Florida, Gaines V ille, Florida 32611-7200 Received January 30, 2007 The initial use of the anion of 6-hydroxymethyl-2,2 -bipyridine (hmbpH) as a chelate in coordination chemistry is described. The syntheses, crystal structures, and magnetochemical characterization are reported of four new iron- (III) clusters [Fe 5 O 2 (OH)(O 2 CMe) 5 (hmbp) 3 ](ClO 4 ) 2 ( 1 ) and [Fe 6 O 2 (OH) 2 (O 2 CR) 6 (hmbp) 4 ](NO 3 ) 2 (R ) Ph ( 2 ), Me ( 3 ), Bu t ( 4 ); hmbpH ) 6-hydroxymethyl-2,2 -bipyridine). The reaction of Fe(ClO 4 ) 3 , hmbpH, and sodium acetate in a 1:1: 4 ratio in EtOH gave 1 , and the reaction between [Fe 3 O(O 2 CR) 6 (H 2 O) 3 ](NO 3 )(R ) Ph, Me, Bu t ) and hmbpH in a 1:1 ratio in MeCN gave 2 - 4 , respectively. The core of 1 consists of a [Fe 4 ( μ 3 -O) 2 ] 8 + butterfly unit to which is attached a fifth Fe atom by bridging O atoms. The core of 2 - 4 also consists of a [Fe 4 ( μ 3 -O) 2 ] 8 + butterfly unit to which are attached an Fe atom on either side by bridging O atoms. Variable-temperature ( T ) and -field ( H ) solid- state DC and AC magnetization ( M ) studies were carried out on complexes 1 - 4 in the 5.0 - 300 K range. Fitting of the data revealed that 1 has an S ) 5/2 ground state spin whereas 2 - 4 possess an ) 5 ground state. Fitting of the / N μ B vs / data by matrix diagonalization and including only axial zero-field splitting (ZFS) gave values of the axial ZFS parameter | D | of 0.75, 0.36, 0.46, and 0.36 cm - 1 for 1 - 4, respectively. Introduction There continues to be a great interest by many groups around the world in the synthesis and study of 3d transition metal cluster compounds, not least for the structural aesthetics possessed by such species. Other reasons for this interest are varied. For iron(III) chemistry, for example, there are bioinorganic areas of relevance such as the great desire to understand and model the assembly of the polynuclear iron core of the iron storage protein ferritin. 1 - 3 There is also a materials interest in that high-nuclearity iron compounds can sometimes exhibit unusual and occasionally novel magnetic properties, with some of them even being examples of single- molecule magnets (SMMs); 4 - 6 the latter are molecules with a combination of a relatively large ground state spin and a significant magnetoanisotropy of the easy-axis (Ising) type. 6,7 That Fe(III) is one area where high-nuclearity species are often encountered is as expected from the high charge-to- size ratio of this oxidation state and the resulting propensity to favor oxide-bridged multinuclear products. Indeed, the formation of the Fe/O/OH core of ferritin that was mentioned as a bioinorganic area of interest is merely an extreme example of such polynuclear chemistry. As a result, many large Fe(III) clusters have been reported to date with
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412 - Inorg. Chem. 2007, 46, 55675575 A New N,N,O Chelate...

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