Stephanie R. Fiedler

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

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.

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.

Synthesis and Characterization of an Asymmetric, Linear, Trinuclear Manganese(II) Complex

After prolonged heating in acetonitrile, a highly asymmetric, trinuclear manganous complex self-assembles from MnCl(2) and bis(2-pyridylmethyl)-1,2-ethanediamine (bispicen). The central Mn(II) ion is bridged to the terminal metal ions in the molecule by single chloride anions. The organic ligands each bind to a single Mn(II) ion. The central Mn(II) and only one of the terminal Mn(II) ions are six-coordinate and bound to bispicen ligands. The remaining terminal Mn(II) ion is coordinated by a tetrahedral array of chloride anions, endowing the trinuclear cluster with a high degree of asymmetry. Variable temperature magnetic measurements are consistent with an S = 5/2 system, indicating net antiferromagnetic coupling.

Solvent Dependent Spin-State Behaviour via Hydrogen Bonding in Neutral FeII Diimine Complexes

We report the syntheses, structures, and magnetic properties of cis-[Fe(pizR)2(NCS)2] complexes based on the pyridyl imidazoline ligands 2-(2′-pyridinyl)-4,5-dihydroimidazole (pizH, 1) and 2-(2′-pyridinyl)-4,5-dihydro-1-methylimidazole (pizMe, 2). The ligands, complexes, and magnetic measurements are chosen to separate hydrogen-bonding and intrinsic ligand field properties, so as to improve our understanding of the effect of hydrogen-bonding interactions on spin-state switching. In the solid state, both complexes are high spin between 5 and 300 K. In deuterated methanol and acetonitrile solutions, both complexes show gradual thermal spin crossover. Complex 1, capable of hydrogen bonding, shows solvent-sensitive spin crossover, whereas spin crossover in the methylated analogue 2 is insensitive to solvent identity.