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Pyrophosphate-Mediated Magnetic Interactions in Cu(II) Coordination Complexes

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journal contribution
posted on 03.01.2011, 00:00 by Nadia Marino, Oluwatayo. F. Ikotun, Miguel Julve, Francesc Lloret, Juan Cano, Robert P. Doyle
The reaction in water of Cu(NO3)2·2.5H2O with 2,2′-bipyridine (bipy), 1,10-phenanthroline (phen), or 1,10-phenanthroline-5-amine (phenam), and sodium pyrophosphate (Na4P2O7), at various pHs, afforded three new copper(II)-pyrophosphate complexes, namely, {[Cu(bipy)(cis-H2P2O7)]2}·3H2O (1a), {[Cu(phen)(H2O)]4(HP2O7)2}(ClO4)2·4H2O (2), and {[Cu2(phenam)2(P2O7)]2·25H2O}n (3). A solvent free crystalline phase of 1a was also isolated with formula {[Cu(bipy)(trans-H2P2O7)]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(HP2O7)2}2+ 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 = −J2a[SCu(1)·SCu(2) + SCu(1a)·SCu(2a)] − J2b[SCu(1)·SCu(2a) + SCu(1a)·SCu(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 = −J3a[SCu(1)·SCu(2) + SCu(1a)·SCu(2a)] − J3b[SCu(1)·SCu(2a) + SCu(1a)·SCu(2)] − J3cSCu(2)·SCu(2a)). For 1a, a net ferromagnetic coupling is observed with J1a = +0.86(1) cm−1 (H = −J SA·SB + SA·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.