Linda H. Doerrer

Steric and electronic effects in metallophilic double salts

This Perspective highlights our efforts to assemble infinite chains of metal atoms in double salts of the form [M](+)[M](-) under the complementary influences of metallophilic interactions and electrostatic attraction. Our design strategy necessarily incorporates the significant steric constraints of one-dimensional assemblies as well as the more subtle and challenging electronic tuning of weak dispersion forces.

High-Spin Square-Planar CoII and FeII Complexes and Reasons for Their Electronic Structure†

More than half a century of intense investigation in coordination compounds has laid a firm foundation for our understanding of the ligand fields in transition-metal complexes. Complexes of the heavier 4d and 5d metals are generally low spin, whereas the spin of 3d metal complexes can be high or low, depending on ligand characteristics. The number and type of donor atoms, ligand substituents, and the presence or absence of chelate rings all influence metal spin states. A combination of data-mining and detailed computational study have quantified recently these empirical observations. In spite of such variety, there are still some types of metal complexes that are rarely observed. The stereospinomers of high-spin, square-planar complexes, for example, are extremely rare because the large separation of the dx2 y2 orbital from the rest of the d-manifold favors low-spin electron configurations for d with n> 4, and four-coordinate compounds are rare for d systems which could have all four low-lying d orbitals half filled. Known d examples include multiple Cr species, a Mn species, and one Nb complex. The rarity of this geometry and spin-state combination is demonstrated by only a handful of examples with late 3d metals. An interesting {NiO2N2} d 8 system is known whose high-spin examples are subtly dependent on ligand substitution. Until the structure was confirmed as tetrameric with octahedral coordination at the Co center, [Co(acac)2] was postulated to have square-planar geometry based on magnetic and spectroscopic data that differed from tetrahedral complexes. A search of the Cambridge Structural Database (V 5.32) for Fe and Co complexes in a fourcoordinate environment with t4 parameter [11] < 0.25 and magnetic susceptibility data revealed five high-spin d Fe complexes with macrocyclic or chelating N4 [12, 13] and O4 [14,15] coordination, and three high-spin d Co complexes 16,17] with varied ligand systems. A complete description of the CSD search and results can be found in the Supporting Information. Herein we report a unique pair of high-spin, square-planar {MO4} species. Our group has prepared several families of homoleptic fluorinated aryloxide and alkoxide complexes of 3d metals. We have extensively investigated the high-spin aryloxide compounds [M(OAr)4] 2 , M = Fe, Co, Ni, and Cu and [M(OAr)5] 2 , M = Fe, in which OAr = OC6F5 or 3,5OC6H3(CF3)2, as well as the high-spin alkoxide compounds [M(OC4F9)3] 1 , M = Fe, Co, Cu and [M(OC4F9)4] 2 , M = Co, Ni. Spectroscopic and computational work have shown that these fluorinated ligands are medium field ligands, on par with OH and F , and stronger than NCO . The electron-withdrawing power of extensively fluorinated ligands reduces the p-donor character of the O atom, such that bridging is not observed and mononuclear species are readily prepared. More recently, we have begun studies of the chelating perfluoropinacolate ligand, ddfp . Magnetic susceptibility and elemental analysis data were reported for K2[M(ddfp)2], (M = Mn, Ni, Cu) for which square-planar geometry was proposed. An octahedral bis-H2O adduct, (Me4N)2[Co(OH2)2(ddfp)2] has been proposed based on elemental analysis data. Despite the relative ease in making the [M(ddfp)2] 2 complexes with first-row transition metals, no examples of M = Co or Fe have been published. We now report a highspin, square-planar Co complex, {K(DME)2}2[Co(ddfp)2] (1), and the analogous high-spin, square-planar Fe complex {K(DME)2}2[Fe(ddfp)2] (2). We also provide a discussion of three other square-planar {MO4} species from the recent literature whose composition and spin-state characteristics clarify the ligand requirements for the highly unusual highspin, square-planar combination in late row 3d metals. Compound 1 has been prepared as pale pink crystals as shown in Equation (1), and is stable in an inert atmosphere and in various organic solvents, but yields a brown oil upon prolonged exposure to air. Iron-containing 2, and the Zn derivative, {K(DME)2}2[Zn(ddfp)2] (3), were similarly prepared as purple-pink, and colorless crystals, respectively. No [*] S. A. Cantalupo, Prof. Dr. L. H. Doerrer Department of Chemistry, Boston University 590 Commonwealth Avenue, Boston, MA 02215 (USA) E-mail: doerrer@bu.edu

Stability of superparamagnetic iron oxide nanoparticles at different pH values: Experimental and theoretical analysis

The detection of superparamagnetic nanoparticles using NMR logging has the potential to provide enhanced contrast in oil reservoir rock formations. The stability of the nanoparticles is critical because the NMR relaxivity (R(2) ≡ 1/T(2)) is dependent on the particle size. Here we use a molecular theory to predict and validate experimentally the stability of citric acid-coated/PEGylated iron oxide nanoparticles under different pH conditions (pH 5, 7, 9, 11). The predicted value for the critical surface coverage required to produce a steric barrier of 5k(B)T for PEGylated nanoparticles (MW 2000) was 0.078 nm(-2), which is less than the experimental value of 0.143 nm(-2), implying that the nanoparticles should be stable at all pH values. Dynamic light scattering (DLS) measurements showed that the effective diameter did not increase at pH 7 or 9 after 30 days but increased at pH 11. The shifts in NMR relaxivity (from R(2) data) at 2 MHz agreed well with the changes in hydrodynamic diameter obtained from DLS data, indicating that the aggregation behavior of the nanoparticles can be easily and quantitatively detected by NMR. The unexpected aggregation at pH 11 is due to the desorption of the surface coating (citric acid or PEG) from the nanoparticle surface not accounted for in the theory. This study shows that the stability of the nanoparticles can be predicted by the theory and detected by NMR quantitatively, which suggests the nanoparticles to be a possible oil-field nanosensor.

METALLOPHILIC INTERACTIONS IN DOUBLE SALTS: TOWARD 1D METAL ATOM CHAINS

A new family of double salts has been developed that can display infinite one-dimensional chains of metallophilic interactions in the solid state. These salts are composed of cationic and anionic components that are designed to participate in metallophilic interactions with one another. These building blocks incorporate Au(III), Au(I), and Pt(II) atoms in the forms [L3PtX]+, [L2AuX2]+, [AuX4]−, and [AuX2]−. Five different metallophilic structural motifs have been observed in these double salts and their monomeric precursors. The particular structure observed is affected by metal oxidation state, the electronic nature of the neutral (L) and anionic (X) substituents, the synthetic conditions employed, as well as the solvent present.

Antiferromagnetic coupling across a tetrametallic unit through noncovalent interactions

Three paramagnetic heterobimetallic lantern complexes of the form [PtM(tba)4(OH2)] (M = Fe, 1; Co, 2; Ni, 3; tba = thiobenzoate) have been prepared in a single-step, bench-top procedure. In all three cases, a lantern structure with Pt–M bonding is observed in solution and in the solid state. Compound 1 is a monomer whereas 3 exists as a dimer in the solid state via a Pt⋯Pt metallophilic interaction. Compound 2 has been characterized in forms with (2a, purple) and without (2b, yellow) Pt⋯Pt metallophilic interactions. The dimers 2a (J = −10 cm−1, based on the spin Hamiltonian Ĥ = −2J(SA·SB)) and 3 (J = −60 cm−1) exhibit antiferromagnetic coupling between the two first-row metal ions in the solid state via a Pt⋯Pt non-covalent metallophilic interaction. The electronic structure of C4v [PtM(tba)4], C2 [PtM(tba)4(OH2)], (M = Fe, Co, Ni) and D2 symmetry [PtM(tba)4(OH2)]2 M = Co, Ni, units have been studied with DFT calculations, confirming the relative spin-state energies observed and the antiferromagnetic exchange pathway through four dz2 orbitals. The compounds 2a and 3 are the first examples of antiferromagnetic coupling through an unbridged M⋯M contact.

Structural and Electronic Properties of Old and New A2[M(pinF)2] Complexes

Seven new homoleptic complexes of the form A2[M(pin(F))2] have been synthesized with the dodecafluoropinacolate (pin(F))(2-) ligand, namely (Me4N)2[Fe(pin(F))2], 1; (Me4N)2[Co(pin(F))2], 2; ((n)Bu4N)2[Co(pin(F))2], 3; {K(DME)2}2[Ni(pin(F))2], 4; (Me4N)2[Ni(pin(F))2], 5; {K(DME)2}2[Cu(pin(F))2], 7; and (Me4N)2[Cu(pin(F))2], 8. In addition, the previously reported complexes K2[Cu(pin(F))2], 6, and K2[Zn(pin(F))2], 9, are characterized in much greater detail in this work. These nine compounds have been characterized by UV-vis spectroscopy, cyclic voltammetry, elemental analysis, and for paramagnetic compounds, Evans method magnetic susceptibility. Single-crystal X-ray crystallographic data were obtained for all complexes except 5. The crystallographic data show a square-planar geometry about the metal center in all Fe (1), Ni (4), and Cu (6, 7, 8) complexes independent of countercation. The Co species exhibit square-planar (3) or distorted square-planar geometries (2), and the Zn species (9) is tetrahedral. No evidence for solvent binding to any Cu or Zn complex was observed. Solvent binding in Ni can be tuned by the countercation, whereas in Co only strongly donating Lewis solvents bind independent of the countercation. Indirect evidence (diffuse reflectance spectra and conductivity data) suggest that 5 is not a square-planar compound, unlike 4 or the literature K2[Ni(pin(F))2]. Cyclic voltammetry studies reveal reversible redox couples for Ni(III)/Ni(II) in 5 and for Cu(III)/Cu(II) in 8 but quasi-reversible couples for the Fe(III)/Fe(II) couple in 1 and the Co(III)/Co(II) couple in 2. Perfluorination of the pinacolate ligand results in an increase in the central C-C bond length due to steric clashes between CF3 groups, relative to perhydropinacolate complexes. Both types of pinacolate complexes exhibit O-C-C-O torsion angles around 40°. Together, these data demonstrate that perfluorination of the pinacolate ligand makes possible highly unusual and coordinatively unsaturated high-spin metal centers with ready thermodynamic access to rare oxidation states such as Ni(III) and Cu(III).

Thiocyanate-Ligated Heterobimetallic {PtM} Lantern Complexes Including a Ferromagnetically Coupled 1D Coordination Polymer

A series of heterobimetallic lantern complexes with the central unit {PtM(SAc)4(NCS)} have been prepared and thoroughly characterized. The {Na(15C5)}[PtM(SAc)4(NCS)] series, 1 (Co), 2 (Ni), 3 (Zn), are discrete compounds in the solid state, whereas the {Na(12C4)2)}[PtM(SAc)4(NCS)] series, 4 (Co), 5 (Ni), 6 (Zn), and 7 (Mn), are ion-separated species. Compound 7 is the first {PtMn} lantern of any bridging ligand (carboxylate, amide, etc.). Monomeric 1-7 have M(2+), necessitating counter cations that have been prepared as {(15C5)Na}(+) and {(12C4)2Na}(+) variants, none of which form extended structures. In contrast, neutral [PtCr(tba)4(NCS)]∞ 8 forms a coordination polymer of {PtCr}(+) units linked by (NCS)(-) in a zigzag chain. All eight compounds have been thoroughly characterized and analyzed in comparison to a previously reported family of compounds. Crystal structures are presented for compounds 1-6 and 8, and solution magnetic susceptibility measurements are presented for compounds 1, 2, 4, 5, and 7. Further structural analysis of dimerized {PtM} units reinforces the empirical observation that greater charge density along the Pt-M vector leads to more Pt···Pt interactions in the solid state. Four structural classes, one new, of {MPt}···{PtM} units are presented. Solid state magnetic characterization of 8 reveals a ferromagnetic interaction in the {PtCr(NCS)} chain between the Cr centers of J/kB = 1.7(4) K.

K⋅⋅⋅F/O Interactions Bridge Copper(I) Fluorinated Alkoxide Complexes and Facilitate Dioxygen Activation

Seven E[Cu(OR)2] copper(I) complexes (E = K(+), {K(18C6)}(+) (18C6 = [18]crown-6), or Ph4P(+); R = C4F9, CPhMe(F)2, and CMeMe(F)2) have been prepared and their reactivity with O2 studied. The K[Cu(OR)2] species react with O2 in a copper-concentration-dependent manner such that 2:1 and 3:1 Cu/O2 adducts are observed manometrically at -78 °C. Analogous reactivity with O2 is not observed with the {K(18C6)}(+) or Ph4P(+) derivatives. Solution conductivity data demonstrate that these K[Cu(OR)2] complexes do not behave as 1:1 electrolytes in solution. The K(+) ions induce aggregation of multiple [Cu(OR)2](-) units through K⋅⋅⋅F/O interactions and thereby effect irreversible O2 reduction by multiple Cu centers. Bond valence analyses for the potassium cations confirm the dominance of the fluorine interactions in the coordination spheres of K(+) ions. Intramolecular hydroxylation of ligand aryl and alkyl C-H bonds is observed. Nucleophilic reactivity with CO2 is observed for the oxygenated Cu complexes and a Cu(II) carbonate has been isolated and characterized.

Pt···Pt vs Pt···S contacts between Pt-containing heterobimetallic lantern complexes.

A trio of Pt-based heterobimetallic lantern complexes of the form [(py)PtM(SAc)4(py)] (M = Co, 1; Ni, 2; Zn, 3) with unusual octahedral coordination of Pt(II) was prepared from a reaction of [PtM(SAc)4] with excess pyridine. These dipyridine lantern complexes could be converted to monopyridine derivatives with gentle heat to give the series [PtM(SAc)4(py)] (M = Co, 4; Ni, 5; Zn, 6). An additional family of the form [PtM(SAc)4(pyNH2)] (M = Co, 7; Ni, 8; Zn, 9) was synthesized from reaction of [PtM(SAc)4(OH2)] or [PtM(SAc)4] with 4-aminopyridine. Dimethylsulfoxide and N,N-dimethylformamide were also determined to react with [PtM(SAc)4] (M = Co, Ni), respectively, to give [PtCo(SAc)4(DMSO)](DMSO), 10, and [PtNi(SAc)4(DMF)](DMF), 11. Structural and magnetic data for these compounds and those for two other previously published families, [PtM(tba)4(OH2)] and [PtM(SAc)4(L)], L = OH2, pyNO2, are used to divide the structures among three distinct categories based on Pt···Pt and Pt···S distances. In general, the weaker donors H2O and pyNO2 seem to favor metallophilicity and antiferromagnetic coupling between 3d metal centers. When Pt···S interactions are favored over Pt···Pt ones, no coupling is observed and the pKa of the pyridine donor correlates with the interlantern S···S distance. UV-vis-NIR electronic and (1)H NMR spectra provide complementary characterization as well.

Heterobimetallic lantern complexes that couple antiferromagnetically through noncovalent Pt···Pt interactions.

A series of Pt-based heterobimetallic lantern complexes of the form [PtM(SAc)4(OH2)] (M = Co, 1; Ni, 2; Zn, 3) were prepared using a facile, single-step procedure. These hydrated species were reacted with 3-nitropyridine (3-NO2py) to prepare three additional lantern complexes, [PtM(SAc)4(3-NO2py)] (M = Co, 4; Ni, 5; Zn, 6), or alternatively dried in vacuo to the dehydrated species [PtM(SAc)4] (M = Co, 7; Ni, 8; Zn, 9). The Co- and Ni-containing species exhibit Pt-M bonding in solution and the solid state. In the structurally characterized compounds 1-6, the lantern units form dimers in the solid state via a short Pt···Pt metallophilic interaction. Antiferromagnetic coupling between 3d metal ions in the solid state through noncovalent metallophilic interactions was observed for all the paramagnetic lantern complexes prepared, with J-coupling values of -12.7 cm(-1) (1), -50.8 cm(-1) (2), -6.0 cm(-1) (4), and -12.6 cm(-1) (5). The Zn complexes 3 and 6 also form solid-state dimers, indicating that the formation of short Pt···Pt interactions in these complexes is not predicated on the presence of a paramagnetic 3d metal ion. These contacts and the resultant antiferromagnetic coupling are also not unique to heterobimetallic lantern complexes with axially coordinated H2O or the previously reported thiobenzoate supporting ligand.