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

The reaction in water of Cu(NO3)(2)center dot 2.5H(2)O 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)}center dot 3H(2)O (1a), {[Cu(Phen)(H2O)](4)(HP2O7)(2)}(ClO4)(2)center dot 4H(2)O (2), and {[Cu-2(phenam)(2)(P2O7)](2)center dot 25H(2)O}(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 la. 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 O-eq-P-O-eq and long O-eq-P-O-P-O-eq pathways), a coordination arrangement also not previously reported. A different tetranuclear copper(II)-pyrophosphate arrangement is found in 3, with two classically bridged dimers (O-eq-P-O-eq pathway) joined together by auxiliary equatorial-axial mu-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 [Cu-2(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 J(2a) = -7.9(2) cm(-1), J(2b) = -46.9(3) cm(-1), J(2c) = 0 cm(-1) in 2 (H = -J(2a)[S-Cu(1)center dot S-Cu(2) + S-Cu(1a)center dot S-Cu(2a)]-J(2b)[S-Cu(1)center dot S-Cu(2a)+ S-Cu(1a)center dot S-Cu(2) - J(2c)S(Cu(2))center dot S-Cu(2a)], and J(3a) = -87.9(2) cm(-1), J(3b) = -5(1) cm(-1) and J(3c) = +5(3) cm(-1) in 3 (H= -J(3a)[S-Cu(1)center dot S-Cu(2) + S-Cu(1a)center dot S-Cu(2a)] - J(3b)[S-Cu(1)center dot S-Cu(2a) + S-Cu(1a)center dot S-Cu(2)] - J(3c)S(Cu(2))center dot S-Cu(2a)). For 1a, a net ferromagnetic coupling is observed with J(1a) = +0.86(1) cm(-1) (H = -J S-A center dot S-B + S-A center dot D center dot S-B + beta H (g(A)S(A) + g(B)S(B)). 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 he stated observations.