The reaction in water of Cu(NO3)2·2.5H 2O with 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen), or 1,10-phenanthroline-5-amine (phenam), and sodium pyrophosphate (Na 4P2O7), at various pHs, afforded three new copper(II)-pyrophosphate complexes, namely, [Cu(bipy)(cis-H2P 2O7)]2·3H2O (1a), [Cu(phen)(H2O)]4(HP2O7) 2(ClO4)2·4H2O (2), and [Cu2(phenam)2(P2O7)] 2·25H2On (3). A solvent free crystalline phase of 1a was also isolated with formula [Cu(bipy)(trans-H 2P2O7)]2 (1b), which can be regarded as a pseudo-polymorph of 1a. Single crystal X-ray analyses revealed these compounds to have uncommon molecular architectures, with 3 being an unprecedented pyrophosphate-containing two-dimensional (2D) polymer. Compounds 1a/1b and 2 are discrete di- and tetra-nuclear complexes, respectively. The cationic [Cu(phen)(H2O)]4(HP2O 7)22+ unit in 2 presents a unique quasi-flat structure, held together by solely in-plane pyrophosphate bridging modes (short Oeq-P-Oeq and long Oeq-P-O-P-Oeq pathways), a coordination arrangement also not previously reported. A different tetranuclear copper(II)-pyrophosphate arrangement is found in 3, with two classically bridged dimers (Oeq-P-Oeq pathway) joined together by auxiliary equatorial-axial μ-O pyrophosphate bridges. Here, the bidimensionality is reached through bridging phenam ligands, which provide further inter-"tetramer" metal-metal connections [(N,N′) eq-(N″)ax pathway], leading to the formation of an expanded covalent network based on the [Cu2(phenam) 2(P2O7)]2 moiety. Variable-temperature magnetic susceptibility measurements on polycrystalline samples of 2 and 3 revealed net antiferromagnetic coupling between metal centers with J2a = -7.9(2) cm-1, J2b = -46.9(3) cm-1, J2c = 0 cm-1 in 2 (H = -J 2a[SCu(1)·SCu(2) + S Cu(1a)·SCu(2a)] - J2b[S Cu(1)·SCu(2a) + SCu(1a)·S Cu(2)] - J2cSCu(2)·SCu(2a)), and J3a = -87.9(2) cm-1, J3b = -5(1) cm -1 and J3c = +5(3) cm-1 in 3 (H = -J 3a[SCu(1)·SCu(2) + S Cu(1a)·SCu(2a)] - J3b[S Cu(1)·SCu(2a) + SCu(1a)·S Cu(2)] - J3cSCu(2)·SCu(2a)). For 1a, a net ferromagnetic coupling is observed with J1a = +0.86(1) cm-1 (H = -J SA·SB + S A·D·SB + βH (gASA + gBSB). This is the first example of ferromagnetic coupling in pyrophosphate-complexes reported to date. A structure-function correlation study focusing on magnetic exchange across the observed diverse pyrophosphate-bridges is described with density functional theory (DFT) calculations included to support the stated observations. © 2010 American Chemical Society.
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|Titolo:||Pyrophosphate-mediated magnetic interactions in Cu(II) coordination complexes|
|Data di pubblicazione:||2011|
|Appare nelle tipologie:||1.1 Articolo in rivista|