Square-Planar Cobalt(III) Pincer Complex - Inorganic Chemistry (ACS

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Square-Planar Cobalt(III) Pincer Complex Paraskevi O. Lagaditis,‡ Bastian Schluschaß,‡ Serhiy Demeshko, Christian Würtele, and Sven Schneider* Institut für Anorganische Chemie, Georg-August-Universität, Tammannstraße 4, 37077 Göttingen, Germany S Supporting Information *

ABSTRACT: A series of square-planar cobalt(II) complexes with pincer ligands {N(CH 2 CH 2 PtBu 2 ) 2 } − ({L 1 t B u } − ), {N(CH 2 CH 2 PtBu 2 )(CHCHPtBu 2 )} − ({L 2 t B u } − ), and {N(CHCHPtBu2)2}− ({L3tBu}−) was synthesized. Ligand dehydrogenation was accomplished with a new, high-yield protocol that employs the 2,4,6-tri-tert-butylphenoxy radical as hydrogen acceptor. [CoCl{LntBu}] (n = 1−3) were examined with respect to reduction, protonation, and oxidation, respectively. One-electron oxidations of [CoCl(L1tBu)] and [CoCl(L2tBu)] lead to ligand-centered radical reactivity, like amide disproportionation into cobalt(II) amine and imine complexes. In contrast, oxidation of [CoCl{L3tBu}] with Ag+ enabled the isolation of thermally stable, square-planar cobalt(III) complex [CoCl{L3tBu}]+, which adopts an intermediate-spin (S = 1) ground state with large magnetic anisotropy. Hence, pincer dehydrogenation gives access to a new platform for high-valent cobalt in square-planar geometry.

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derived from parent {HL1R}.6 Complexes with unsaturated ligand backbones (Figure 1), like [RuCl{L2tBu}] (1b), [MCl{L3tBu}] (M = Ru (1c), Rh (2), Ir (3)), or [IrCl{L3tBu}]+ (4), are accessible upon oxidation of parent {L1} complexes with benzoquinone.7 This simple ligand functionalization offers subtle tailorability of the donor properties, as demonstrated for 1a vs 1b/1c, which adopt low-spin vs intermediate-spin ground states, respectively.7c The {Ln} ligand series (n = 1−3) also provides a platform for the stabilization of high-valent, late transition metal complexes with multiply bonded ligands, such as [RuN{L1tBu}], [RhN{L3tBu}], [IrN{L3tBu}], and [IrN{L3tBu}]+.8 Moreover, {L2R}− and {L3R}− represent aliphatic analogues of two other widely utilized PNP amido pincer ligand types, i.e., the pyridine-based dearomatized ligands popularized by Milstein and the phenylene-bridged PNP pincer ligands.9 However, compared with parent amido ligand {L1R}−, the reactivity of dehydrogenated {L2R}− and {L3R}− is not well examined. Arnold and co-workers reported the synthesis of several CoI and CoII amine and amido complexes with the {L1iPr} ligand.10 Furthermore, the groups of Hanson and Jones utilized [CoII(CH2SiMe3){HL1Cy}] as an efficient de/hydrogenation catalyst. 11 However, square-planar complexes in higher oxidation states (Con>+II) were not reported. In this context, we were interested in whether the {LntBu} ligand series can stabilize low-coordinate, high-valent cobalt complexes. Here we report the synthesis and characterization of the cobalt(II) dialkyl-, alkylvinyl-, and divinylamido series [CoCl{LntBu}] (n = 1−3). While the oxidation of [CoCl(L1tBu)] and [CoCl(L2tBu)] results in ligand disproportionation, [CoCl(L3tBu)]+ could be isolated and represents a rare square-planar cobalt(III) complex

INTRODUCTION Monoanionic, rigid, tridentate ligands (“pincer ligands”) have become highly popular in recent years. Their tailorable steric and electronic properties enable the stabilization of platforms that mediate selective chemical transformations. In addition, their reactivity can be further controlled by ligand functional groups that undergo reversible transformations. This use of cooperating pincer ligands that serve as reversible proton (“bifunctional ligands”) and/or electron (“redox noninnocent ligands”) relays has proven particularly useful in base metal catalysis to maintain redox leveling in multiproton and -electron reactions.1 The aminodiphosphine HN(CH2CH2PR2)2 (HL1R, Figure 1) was popularized as a cooperating ligand in catalysis, e.g.,

Figure 1. PNP pincer ligands derived from parent {HL1R}.

challenging de/hydrogenation reactions.2,3 Furthermore, this ligand also stabilizes electronically highly unsaturated, squareplanar complexes with unusual electronic structures, such as [RuCl{L1tBu}] (1a).4 Their stability can be attributed to the accessibility of bulky PR2 substitutents and the strong amido N → M π-donation. Notably, dialkylamido complex 1a undergoes β-H-elimination in solution, and one-electron oxidation sometimes leads to PNP ligand disproportionation into amine and imine complexes, as shown for [Ir(L)(L1iPr)] (L = PMe3, COE, CO).5 These oxidative ligand degradation processes offer synthetic access to dehydrogenated pincers © XXXX American Chemical Society

Received: February 12, 2016

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DOI: 10.1021/acs.inorgchem.6b00369 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

low-spin.13 The molecular structure of 6a was confirmed by Xray crystallography (Figure 2). The cobalt ion adopts an approximately square-planar coordination geometry with some distortion mainly arising from the P−Co−P pincer bite angle (171.519(18)°). All bond distances and angles around the metal are almost identical with Arnold’s [CoCl{L1iPr}].10b Compared with 5, the considerable shortening of the Co−N bond (Δd = 0.34 Å) is attributed to the stronger donation of the amide and the change from high-spin to low-spin configuration, as evidenced by concomitant shortening of the Co−P (mean Δd = 0.36 Å) and Co−Cl (Δd = 0.10 Å) bonds. Starting from 6a, the PNP ligand can be modified by oxidative dehydrogenation. The reaction of 6a with 1,4benzoquinone resulted in half-dehydrogenated cobalt(II) vinyl amido complex [CoCl{L2tBu}] (6b) (Scheme 2) in moderate

with an intermediate-spin (S = 1) ground state and large magnetic anisotropy.

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RESULTS AND DISCUSSION Synthesis of [CoIICl{LntBu}]. [CoCl2{HL1tBu}] (5) was synthesized in almost quantitative yield by reaction of HL1tBu with CoCl2 in THF (Scheme 1). Susceptibility measurement by Scheme 1. Synthesis of Cobalt(II) Amido Complex 6a

the Evans method (μeff = 4.1 ± 0.1 μB) at room temperature reveals a high-spin configuration with three unpaired electrons and some orbital contribution as reported by Arnold and coworkers for the PiPr2 analogue.10a However, the molecular structure that was obtained by X-ray crystallography (Figure 2) exhibits distinct differences. While [CoCl2{HL1iPr}] can be described as distorted trigonal-pyramidal with axial N and one Cl, 5 is closer to square-pyramidal cobalt coordination with Cl2 in apical position. The different conformation is attributed to the bulkier PR2 substituents in 9 as indicated by the longer Co−P distances (mean Δd = 0.17 Å) and larger P1−Co−P2 angle (Δα = 26°). Notably, a short intermolecular contact between the N−H proton and Cl1 is found (2.62 Å). Arnold and co-workers reported the deprotonation of [CoCl2{HL1iPr}] with nBuLi to cobalt(II) amido complex [CoCl{L1iPr}] in 42% yield.10b Reduction to cobalt(I) compound [CoCl(HL1iPr)] was also obtained in considerable amounts, and Chirik and co-workers later demonstrated that the use of MeLi (2 equiv) results in reduction, exclusively.12 The cobalt(II) amido complex [CoCl{L1tBu}] (6a) is easily synthesized in high yield upon deprotonation of 5 with KOtBu in benzene (Scheme 1). Crystallization from pentanes affords dark green crystalline 6a in 94% isolated yield. Like the PiPr2 analogue, complex 6a exhibits an electronic low-spin configuration in solution (μeff = 1.9 ± 0.1 μB), as expected for square-planar-coordinated cobalt(II) in strong ligand field. In contrast, the related cobalt(II) complexes [CoX{N(SiMe2CH2PPh2)2}] (X = Cl, Br, I) adopt distorted tetrahedral structures with electronic high-spin configuration, while [CoCl{N(SiMe2CH2PtBu2)2}] shows spin-transition behavior and square-planar [CoN3{N(SiMe2CH2PtBu2)2}] is strictly

Scheme 2. Synthesis of Cobalt(II) Complexes 6b/c, Reduction to Cobalt(I) Complexes 7a−c, and Protonation with Brønsted Acid to 8a−c

isolated yield around 37%. Significant decomposition during the reaction becomes evident from a fair amount of gray precipitate and the formation of free {HL1tBu} detected by 31P NMR spectroscopy. Accordingly, full dehydrogenation of the PNP ligand with two or more equivalents of 1,4-benzoquinone

Figure 2. Molecular structures of complexes 5 (left) and 6a (right) in the solid state derived by single-crystal X-ray diffraction (ellipsoids set at 50% probability, hydrogen atoms of the tBu groups are omitted for clarity). Selected bond lengths (Å) and angles (deg) for 5: Co1−N1 2.1772(16), Co1−P1 2.5761(6), Co1−P2 2.5789(6), Co1−Cl1 2.3264(5), Co1−Cl2 2.2873(6); P1−Co1−P2 151.90(2), N1−Co1−Cl1 152.07(5), N1−Co1− Cl2 99.98(5). 6a: Co1−N1 1.8337(13), Co1−P1 2.2200(5), Co1−P2 2.2156(5), Co1−Cl1 2.2291(5); P1−Co1−P2 171.519(18), N1−Co1−Cl1 174.90(5). B


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Inorganic Chemistry

Figure 3. Molecular structures of complexes 6b (left) and 6c (right) in the solid state derived by single-crystal X-ray diffraction (ellipsoids set at 50% probability, hydrogen atoms of the tBu groups are omitted for clarity). Selected bond lengths (Å) and angles (deg) for 6b: Co1−N1 1.8646(15), Co1−P1 2.2410(6), Co1−P2 2.2508(6), Co1−Cl1 2.2118(5), N1−C1 1.448(2), N1−C11 1.387(2), C1−C2 1.476(3), C11−C12 1.370(3); P1− Co1−P2 170.17(2), N1−Co1−Cl1 178.00(5). 6b: Co1−N1 1.893(2), Co1−P1 2.2561(5), Co−Cl1 2.2026(7), N1−C1 1.382(2), C1−C2 1.343(3); P1−Co1−P2 169.51(3), N1−Co1−Cl1 180.0.

bonding compared with the cobalt(I) PNP pincer complexes [Co(CO){N(CH2CH2PiPr2)2}] (1882 cm−1),10c [Co(CO){N(SiMe2CH2PtBu2)2}] (1885 cm−1),13b and [Co(CO){N(C6H32-PiPr2-4-CH3) 2}] (1901 cm−1).14 Importantly, ligand dehydrogenation results in hypsochromic shifts of the CO stretching vibrations in 7b (1882 cm−1) and 7c (1890 cm−1), respectively, relative to parent 7a. This shift can be attributed to reduced π-donation by the vinyl- and divinylamido ligands compared with the dialkylamide. Accordingly, an identical hypsochromic shift (28 cm−1) of the CO stretching vibration was found for the five-coordinate series [RuCl(CO){LntBu}] (n = 1−3).7c Hence, the stepwise ligand dehydrogenation enables versatile tuning of the donor properties irrespective of the nature of the metal or the coordination geometry. Protonation of [CoIICl{LntBu}]. Since the protonated ligands {HLntBu} (n = 1−3) could adopt different tautomeric structures, protonation of 6a−c was examined to identify the basic sites. As expected, 6a is protonated by H[BArF4] at the nitrogen atom to give the four-coordinate cobalt(II) amine complex 8a (Scheme 2). In contrast to 6a, the reactions of 6b and 6c with H[BArF4] result in C-protonation, giving the imine and enimine tautomers 8b and 8c, respectively (Scheme 2). As for the parent amido complexes, the magnetic moments in solution (8a: μeff = 2.1 ± 0.1 μB; 8b/c: μeff = 2.0 ± 0.1 μB) are in agreement with electronic low-spin configurations. In the 1H NMR spectrum of 8a, four signals are found at δ = 10.0 (tBu), 12.2 (tBu), 20.2 (CH2), and 39.8 ppm (CH2), respectively (Figure 4). In comparison, the two chemically inequivalent tBu groups are resolved for 8b (9.9 and 7.3 ppm), yet not for 8c (8.4 ppm), and all four expected 1H NMR signals for the pincer backbones of both 8b and 8c were found, respectively. The molecular structures of 8a and 8c were derived by single-crystal X-ray diffraction (Figure 5). All crystallization attempts for 8b resulted in disordered structures with respect to the CN double bond in the pincer backbone, preventing meaningful refinement. Complexes 8a and 8c feature the cobalt ions in approximate square-planar coordination in agreement with the electronic low-spin configurations. Accordingly, the Co−N, Co−P, and Co−Cl bond lengths are considerably shorter than in high-spin 5. Instead, the Co−Cl and Co−P bonds are very close to low-spin cobalt(II) amides 6a−c, while the Co−N bonds of the amine (8a) and enimine (8c) are longer by about 0.15 Å than in the respective neutral parent complexes 6a and 6c. The bond metrics in the ligand backbone of 8c confirm the enimine formulation. A short N−H···F hydrogen bridge with the BF4 anion was found (2.09 Å).

was unsuccessful and gave only 6b in even lower yields. Instead, high isolated yields (70%) of fully dehydrogenated divinylamido cobalt(II) complex [CoCl{L3tBu}] (6c) are obtained upon reaction of 6a with 4.5 equiv of the 2,4,6-tert-butylphenoxy radical (Scheme 2), which defines a new route to {L3} complexes. Like 6a, red-purple 6b and wine-red 6c exhibit low-spin electronic configurations at room temperature in solution (both μeff = 1.9 ± 0.1 μB). In the 1H NMR the two sets of signals expected for the PtBu2 groups of 6b are too broad to be resolved. Only one signal is found at a chemical shift of δ = 4.85 ppm, which is in fact right between the tBu signals found for 6a (δ = 1.41 ppm) and for 6c (δ = 11.9 ppm), respectively. 6b and 6c were also structurally characterized by single-crystal X-ray diffraction (Figure 3). Compared with 6a the coordination around the metal ion is almost invariant. The main difference arises from slight lengthening of the Co−N bond in 6b (Δd = 0.03 Å) and 6c (Δd = 0.06 Å). This trend is tentatively attributed to weaker N → Co π-donation of the vinyl and divinyl amides, respectively, compared with the dialkylamide. Accordingly, the π-conjugation within the vinyl amido moieties is evidenced by C−N bond shortening by around 0.07 Å as compared with the alkyl amide groups. Importantly, ligand dehydrogenation is further demonstrated by shortening of the C−C bonds in the backbone by more than 0.15 Å. Reduction of [CoIICl{LntBu}]. Electrochemical examination of 6a−c in THF by cyclic voltammetry (CV, Supporting Information) reveals irreversible reduction waves at low potential (Epa = −3.0 V (6a), −2.6 V (6b and 6c) vs Fc/ Fc+), presumably due to dissociation of chloride. Arnold reported the formation of amido-bridged dimer [(Co{L1iPr})2] upon reduction of [CoCl{L1iPr}].10b In comparison, electrochemical reduction of [IrCl{L3tBu}] in THF was reversible and [K(15-cr-5)2][IrCl{L3tBu}] could be isolated.7b While the products that result from reduction of 6a−c were not isolated, reaction with Na/Hg under a CO atmosphere was used to introduce a spectroscopic probe for the electronic influence of backbone dehydrogenation. The cobalt(I) carbonyl complexes 7a−c are obtained in high isolated yield (Scheme 2). The diamagnetic complexes were structurally characterized by NMR spectroscopy, and complex 7a was also characterized by singlecrystal X-ray diffraction (see Supporting Information), confirming the square-planar geometries and respective degrees of backbone dehydrogenation. The IR band assignable to the CO stretching vibration of 7a was found at 1863 cm−1, indicating stronger M → CO backC


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Scheme 3. Chemical Oxidation of Cobalt(II) Complexes 6a− c

Figure 4. 1H NMR spectra of isolated 8a−c and of the reaction mixtures after oxidation of 6a and 6b with [FeCp2]PF6, respectively. All spectra were recorded in CD2Cl2.

4) and ESI-MS spectrometry allowed for the identification of 8b as product in about 70% spectroscopic yield. Further products could not be identified, and it remains unclear whether the formation of 8b results from ligand disproportionation or from hydrogen or proton transfer from the solvent or serendipitous water. In contrast to 6a and 6b, the immediate product of divinyl amide 6c oxidation can be isolated. Due to the slightly positive oxidation potential of 6c vs FeCp2/FeCp2+, AgX (X = SbF6 or PF6) in CH2Cl2 can be used as oxidant. 1H NMR spectroscopy suggests selective oxidation, and the cobalt(III) complex [CoCl{L3tBu}]SbF6 (9) could be isolated in over 80% yield. While the peaks assignable to tBu (δ = −7.12 ppm) and the two CH (δ = 158.9, 1.32 ppm) groups are strongly paramagnetically shifted, the line width its much narrower compared with all cobalt(II) complexes described above. The magnetic properties of a crystalline sample of 9 were derived by SQUID magnetometry (Figure 6). The data could be satisfactorily fit using a spin-Hamiltonian (eq 1) for an electronic intermediate-spin state (S = 1) with unquenched orbital momentum (gx = gy = 2.28; gz = 2.41) and substantial axial zero-field splitting (ZFS, D = +79 cm−1) that leads to a nonmagnetic ground state (Ms = 0). Notably, the analogous

Oxidation of [CoIICl{LntBu}]. As for electrochemical reduction, the CV (see Supporting Information) of 6a in THF features irreversible oxidation at Epc = −0.3 V (vs Fc/ Fc+). In contrast, the vinyl- (6b) and divinylamido (6c) complexes undergo reversible oxidation (v = 100 mV s−1), indicating enhanced chemical stability under oxidative conditions after ligand dehydrogenation. With respect to 6a the oxidation half-wave potentials of 6a/b are shifted to higher potentials by 0.14 (6b) and 0.31 V (6c), respectively, which is attributed to the weaker donor properties of the vinyl and divinyl amido ligands {L2tBu}− and {L3tBu}− (see above). To rationalize this result, complexes 6a−c were oxidized chemically. Monitoring the reaction of 6a with [FeCp2]PF6 in CH2Cl2 by 1H NMR spectroscopy (Figure 5) reveals immediate conversion to equimolar amounts of 8a and 8b. This observation confirms ligand disproportionation after oneelectron oxidation, accounting for the electrochemical EC process. The same observation was previously reported for [Ir(L)(L1iPr)] (L = CO, PMe3, C2H4).5 Despite reversible electrochemical oxidation, the chemical oxidation of 6b with [FeCp2]PF6 both in CH2Cl2 and in THF also results in ligandcentered reactivity (Scheme 3). 1H NMR spectroscopy (Figure

Figure 5. Molecular structures of complexes 8a (left) and 8c (right) in the solid state derived by single-crystal X-ray diffraction (ellipsoids set at 50% probability; hydrogen atoms of the tBu groups are omitted for clarity). Selected bond lengths (Å) and angles (deg) for 8a: Co1−N1 1.9835(11), Co1−P1 2.2694(4), Co1−P2 2.2775(4), Co1−Cl1 2.2045(4), N1−C1 1.4909(17), N1−C11 1.4933(16), C1−C2 1.5148(19), C11−C12 1.517(2); P1−Co1−P2 170.783(14), N1−Co1−Cl1 176.08(4). 8c:* Co1−N1 1.941(4)/1.928(4), Co1−P1 2.2478(15)/2.2519(15), Co1−P2 2.2493(15)/ 2.2550(15), Co1−Cl1 2.2025(15)/2.2013(15), N1−C1 1.392(7), N1−C11 1.319(7)/1.312(7), C1−C2 1.340(9)/1.350(8), C11−C12 1.429(8)/ 1.437(8); P1−Co1−P2 170.16(6)/170.24(6), N1−Co1−Cl1 178.63(14)/178.41(14). *The asymmetric unit contains two complex molecules. D


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Figure 6. (Left) Temperature dependence of the experimental χMT product (circles) at 0.5 T. (Right) Variable-temperature variable-field magnetization measurements as Mmol vs B/T. Solid lines represent the global fit using the spin-Hamiltonian given in the text (S = 1; gx = gy = 2.21; gz = 2.41; D = +76 cm−1; TIP = 10.0 × 10−4 cm3 mol−1; PI denotes the contribution from an S = 3/2 paramagnetic impurity (2.8%)).

iridium(III) complex [IrCl{L3tBu}]PF6 adopts a low-spin ground state.7a

Cl (>8 Å) and Co−F (>6 Å) distances are all far beyond the sum of van der Waals radii. Compared with parent 6c, the bond angles around the metal ion are almost identical. The Co−P bonds are slightly elongated (mean Δd = +0.02 Å), and Co−N (mean Δd = −0.04 Å) and Co−Cl (mean Δd = −0.04 Å) bonds indicate a small effect of oxidation state and charge on the cobalt ionic radius.

Ĥ = μB (Sxgx Bx + SygyBy + Szgz Bz ) ⎤ ⎡ 2 1 + D⎢Sẑ − S(S + 1)⎥ ⎦ ⎣ 3

(1)

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Comparably large magnetic anisotropy was also found for other square-planar coordinated 3d6 triplet ions, such as cobalt(III) or iron(II).15,16 Hence, the magnetic data exclude ligand-centered oxidation, as reported for oxidation of the square-planar cobalt(II) and nickel(II) complexes [Co{C6H101,2-(NCHC6H2-1-NH-4,6-tBu2)2}] and [NiCl{N(C6H3-2PiPr2-4-CH3)2}] or for the superoxo complex [Co(O2){N(C6H4NC(O)iPr)2}]−.17 Large g- and D-anisotropy was also found for [RuCl{L3tBu}], and detailed magnetic, spectroscopic, and computational analysis confirmed the out-of-state angular momentum due to splitting of the S = 1 ground term upon mixing with excited states through spin−orbit coupling.7c However, for cobalt(III) square-planar coordination remains rare compared with the predominant octahedral coordination, which usually features low-spin (S = 0) or sometimes high-spin (S = 2) electronic configurations. The square-planar cobalt coordination in 9 was confirmed by single-crystal X-ray diffraction (Figure 7). Intermolecular Co−

CONCLUSIONS In summary, this paper describes a systematic study to establish a Co(PNP) pincer platform that stabilizes coordinatively unsaturated cobalt ions in oxidation states beyond +II. With this goal, a series of square-planar cobalt(II) PNP pincer complexes was synthesized with varying degrees of ligand backbone saturation, from fully saturated (L1tBu) to halfsaturated (L2tBu) and fully unsaturated (L3tBu). Full ligand dehydrogenation is accomplished employing a new protocol with 2,4,6-tert-butylphenoxy radical as hydrogen acceptor. This route seems to be widely applicable for the synthesis of {ML3tBu} complexes in high yield,18 providing access to aliphatic analogues of the well-established phenylene-bridged PNP amido pincer ligands, which are not available with bulky PR2 substituents. Adjustable steric bulk and ligand donor properties can have decisive effects on molecular and electronic structure. This is showcased by the structure of [CoCl2{HL1tBu}] (5) vs [CoCl2{HL1iPr}] or the low-spin configurations of 6a−c and [CoCl{L1iPr}] in comparison with high-spin [CoCl{N(SiMe2CH2PPh2)2}] or the high-spin/lowspin transition reported for [CoCl{N(SiMe2CH2PtBu2)2}]. The steric bulk by the PtBu groups, the strong donation by the alkyl amido group, and the increasing ligand backbone rigidity all stabilize the strictly square-planar low-spin configuration found for 6a−c. The reactivity of 6a−c toward reduction, protonation, and oxidation was examined due to the relevance of related pincer ligands in cooperative catalysis. Facile chemical reduction of 6a−c to cobalt(I) carbonyl complexes allows for probing the change in ligand donor properties upon ligand dehydrogenation. The trend of CO stretching vibrations reflects the decreasing π-donor within the pincer ligand series. While complex 6a is protonated at the amido functional group, the vinyl carbon atoms define the basic sites of 6b and 6c, resulting in imine and enimine complexes 8b and 8c. The one-electron oxidation of 6a is irreversible owing to rapid pincer ligand disproportionation to a mixture of 8a and 8b. Despite reversible

Figure 7. Molecular structure of 9 (with PF6 instead of SbF6 anion) in the solid state derived by single-crystal X-ray diffraction (ellipsoids set at 50% probability; hydrogen atoms of the tBu groups are omitted for clarity). Selected bond lengths (Å) and angles (deg): Co1−N1 1.8518(11)/1.8552(11), Co1−P1 2.2813(4)/2.2915(4), Co1−P2 2.2854(4)/2.2904(4), Co1−Cl1 2.1651(3)/2.1693(4), N1−C1 1.3910(16)/1.3929(17), N1−C11 1.3901(17)/1.3913(16), C1−C2 1.3410(19)/1.341(2), C11−C12 1.3431(19)/1.3420(19); P1−Co1− P2 171.053(14)/170.775(14), N1−Co1−Cl1 176.83(4)/179.03(4). The asymmetric unit contains two complex molecules. E


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Inorganic Chemistry

μeff = 4.1 ± 0.1 μB. 1H NMR (CD2Cl2, 300 MHz): 43.69 (CH2), 16.32 (CH2), −5.40 (tBu) Synthesis of [CoCl{N(CH2CH2PtBu2)2}] (6a). A vial was charged with 5 (400 mg, 0.81 mmol), KOtBu (110 mg, 0.98 mmol), and benzene (10 mL). The slurry was allowed to stir for 1 h, by which time the solution turned dark green. The slurry was filtered through a sintered glass crucible, and the solvent was removed in vacuo. The green residue was dissolved in pentane, filtered into a vial, and placed at −38 °C for 2 days to yield a dark green crystalline solid. The filtrate was decanted into a second vial, concentrated in vacuo, and placed back in the freezer for 2 days to yield a second batch of crystalline solid (total yield: 350 mg, 94%). Crystals suitable for X-ray diffraction were obtained from a saturated pentane solution at −38 °C in an NMR tube. Anal. Found (Calcd) for C20H44ClCoNP2: C, 53.31 (52.81); H, 10.33 (9.75); N, 2.97 (3.08). μeff = 1.9 ± 0.1 μB. 1H NMR (C6D6, 300 MHz, ppm): 1.41 (tBu), −17.23 (CH2); the third signal was not found. Synthesis of [CoCl{N(CHCHPtBu2)(CH2CH2PtBu2)}] (6b). A vial was charged with 6a (135 mg, 0.30 mmol), 1,4-benzoquinone (32 mg, 0.30 mmol), and benzene (10 mL). The green-yellow solution turned dark pink with a black precipitate. After stirring for 1 h, the black precipitate was filtered off and the solvent was removed. The redbrown residue was taken up with pentane (∼15 mL) and filtered again. Upon removal of the solvent a dark pink residue was isolated. The residue was taken up with pentane and placed at −38 °C for 2 days to yield a dark pink crystalline solid. The solution was decanted into a second vial, concentrated, and stored at −38 °C for 2 days to yield a second batch of crystalline solid (50 mg, 37%). Crystals suitable for Xray diffraction were obtained from slow evaporation of a saturated pentane solution at room temperature. Anal. Found (Calcd) for C20H42ClCoNP2: C, 52.97 (53.04); H, 10.18 (9.35); N, 3.11 (3.09). μeff = 1.9 ± 0.1 μB. 1H NMR (C6D6, 300 MHz, ppm): 4.85 (tBu), −1.71 (CH2), −57.71 (CH); the remaining signals were not found. Synthesis of [CoCl{N(CHCHPtBu2)2}] (6c). A vial was charged with 6a (150 mg, 0.33 mmol), 2,4,6-tert-butylphenoxy radical (387 mg, 1.48 mmol), and benzene (10 mL). After stirring overnight, the solution was transferred to a Schlenk tube, and the solvent was removed to yield a bluish-purple residue. The Schlenk tube was equipped with a coldfinger, and the excess radical and phenol byproduct were removed by sublimation at 75 °C for 2 h. The coldfinger was replaced with a new coldfinger, and trace byproduct and radical were further removed by sublimation for an additional 2 h. The remaining pink residue in the Schlenk tube was taken up with pentane (∼20 mL), transferred to a preweighed vial, and placed in a freezer (−38 °C) for 2 days. The solution was decanted and dried in vacuo to yield a wine-red-colored crystalline solid (100 mg, 67%). Crystals suitable for X-ray diffraction were obtained from a saturated pentane solution in an NMR tube stored at −38 °C. Anal. Found (Calcd) for C20H40CoNP2: C, 53.14 (53.28); H, 8.96 (8.94); N, 3.12 (3.11). μeff = 1.9 ± 0.1 μB. 1H NMR (C6D6, 300 MHz, ppm): 11.88 (tBu), −40.45 (CH), −89.07 (CH). Synthesis of [Co(CO){N(CH2CH2PtBu2)2}] (7a). A mixture of 6a (40 mg, 0.09 mmol) and Na/Hg (1.32 g, 0.10 mmol) in THF (8 mL) was degassed by two freeze−pump−thaw cycles and stirred under a CO atmosphere for 14 h. The solvent was removed in vacuo, and the orange-gray residue was extracted with pentane (3 × 4 mL), filtrated over Celite, and dried in vacuo. The residue was dissolved in pentane (4 mL) and placed in a freezer (−32 °C) for 2 days. The solution was decanted and dried in vacuo to yield an orange-colored crystalline solid (29 mg, 74%). Anal. Found (Calcd) for C21H44CoNOP2: C, 55.93 (56.37); H, 9.67 (9.91); N, 2.97 (3.13). 1H NMR (C6D6, 300 MHz, ppm): 3.22 (m, 4 H, NCH2), 1.91 (m, 4 H, PCH2), 1.36 (A18XX′A′18, N = |3JHP + 5JHP| = 12.0 Hz, 36 H, PtBu2). 31P{1H} NMR (C6D6, 121 MHz, ppm): 117.7 (br). IR (KBr, cm−1): 1863 (CO). Synthesis of [Co(CO){N(CH2CH2PtBu2)(CHCHPtBu2)}] (7b). A mixture of 6b (40 mg, 0.09 mmol) and Na/Hg (1.43 g, 0.10 mmol) in THF (8 mL) was degassed by two freeze−pump−thaw cycles and stirred under a CO atmosphere for 14 h. The solvent was removed in vacuo, and the greenish-gray residue was extracted with pentane (3 × 4 mL), filtrated over Celite, and dried in vacuo. The residue was

electrochemical oxidation, imine complex 8b was the only product that could be identified upon reaction of 6b with [FeCp2]PF6. Importantly, full ligand dehydrogenation afforded the isolation of stable complex 9, which represents a rare example for a cobalt(III) complex with square-planar geometry with intermediate-spin configuration and large magnetic anisotropy expressed in axial ZFS (D = +76 cm−1). Hence, the oxidative PNP ligands’ functionalization gives rise to a platform that is capable of stabilizing coordinatively highly unsaturated, high-valent cobalt.

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EXPERIMENTAL SECTION

Materials and Methods. All experiments were carried out using Schlenk and glovebox (argon atmosphere) techniques. All solvents were dried by passing through columns packed with activated alumina. Deuterated solvents were obtained from Euriso-Top GmbH, dried over Na/K (d6-benzene and d8-THF) or CaH2 (CD2Cl2), respectively, distilled by trap-to-trap transfer in vacuo, and degassed by three freeze−pump−thaw cycles. HL1tBu, H[BArF4], and 2,4,6-tert-butylphenoxy radical were synthesized following literature procedures.19−21 KOtBu (VWR) and 1,4-benzoquinone (VWR) were sublimed prior to use. All other chemicals were used as purchased from either Aldrich, ABCR, or VWR. Elemental analyses were obtained from the analytical laboratories at the Georg-August University on an Elementar Vario EL 3. NMR spectra were recorded on Bruker Avance III 300 MHz spectrometers and were calibrated to the residual solvent proton resonance (d6benzene: δH = 7.16 ppm; d8-THF: δH = 3.58 ppm; CD2Cl2: δH = 5.32 ppm). Signal multiplicities are abbreviated as s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet), and br (broad). Magnetic moments in solution (d6-benzene) were determined at room temperature by Evans’ method as modified by Sur and corrected for diamagnetic contribution.22 ESI-MS measurements were carried out on a Bruker HTC Ultra (ESI-MS) mass spectrometer. IR spectra were recorded as KBr pellets using a Thermo Scientific Nicolet iZ10 FT/IR spectrometer. Temperature-dependent magnetic susceptibility measurements were carried out with a Quantum-Design MPMS-XL-5 SQUID magnetometer equipped with a 5 T magnet in the range from 210 to 2 K at 0.5 T applied magnetic field. The crystalline sample was contained in a gel bucket, covered with a few drops of low-viscosity perfluoropolyether-based inert oil Fomblin Y45 to fix the crystals, and fixed in a nonmagnetic sample holder. The maximum measuring temperature of 210 K was chosen because of the pour point of the oil, in order to keep the oil in the frozen state and to avoid therefore the orientation of the crystals parallel to the magnetic field. Each raw data file for the measured magnetic moment was corrected for the diamagnetic contribution of the gel bucket and of the inert oil according to Mdia = χgmH, with the experimentally obtained gram susceptibility of the gel bucket (χg = −5.70 × 10−7 emu/(g·Oe) and of the oil (χg = −3.82 × 10−7 emu/(g·Oe)). The molar susceptibility data were corrected for the diamagnetic contribution according to χMdia(sample) = −0.5M × 10−6 cm3·mol−1.23 A Curie-behaved paramagnetic impurity (PI) with spin S = 3/2 and temperatureindependent paramagnetism (TIP) were included according to χcalc = (1 − PI)χ + PIχmono + TIP. Simultaneous calculation of the experimental temperature-dependent and VTVH data was performed with the julX_2S program: E. Bill, Max-Planck Institute for Chemical Energy Conversion, Mülheim/Ruhr, Germany. Before simulation, the experimental data were corrected for TIP. Synthesis of [CoCl2{HN(CH2CH2PtBu2)2}] (5). A vial was charged with CoCl2 (0.327 g, 2.51 mmol), (tBu2PC2H4)2NH (0.907 g, 2.51 mmol), and THF (20 mL). The purple-blue slurry was allowed to stir for 2 days. The solid was collected via a sintered glass crucible, washed with pentane (∼20 mL), and dried in vacuo. Yield: 1.12 g (90%). Crystals suitable for X-ray diffraction were obtained from slow diffusion of pentane into CH2Cl2 at −38 °C. Anal. Found (Calcd) for C20H45Cl2CoNP2: C, 48.19 (48.89); H, 9.19 (9.23); N, 2.78 (2.85). F


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Inorganic Chemistry dissolved in pentane (4 mL) and placed in a freezer (−32 °C) for 2 days. The solution was decanted and dried in vacuo to yield a dark green colored crystalline solid (35 mg, 89%). Anal. Found (Calcd) for C21H42CoNOP2: C, 56.19 (56.62); H, 8.82 (9.50); N, 2.93 (3.14). 1H NMR (THF-d8, 300 MHz, ppm): 6.90 (ddd, 3JHP = 45.2 Hz, 3JHH = 5.1 Hz, JHP = 2.1 Hz, 1 H, NCH), 3.63 (d, 3JHH = 5.1 Hz, 1 H, PCH), 3.19 (m, 2 H, NCH2), 2.00 (dt, 2JHP = 9.2 Hz, 3JHH = 6.8 Hz, 2 H, PCH2), 1.43 (d, 3JHP = 9.3, 18 H, PtBu2), 1.39 (d, 3JHP = 9.1, 18 H, PtBu2). 31P{1H} NMR (THF-d8, 121 MHz, ppm): 110.0 (br), 92.1 (br). IR (KBr, cm−1): 1882 (CO), 1532 (CC). Synthesis of [Co(CO){N(CHCHPtBu2)2}] (7c). A mixture of 6c (30 mg, 0.07 mmol) and Na/Hg (995 mg, 0.07 mmol) in THF (8 mL) was degassed by two freeze−pump−thaw cycles and stirred under a CO atmosphere for 14 h. The solvent was removed in vacuo, and the greenish-gray residue was extracted with pentane (3 × 4 mL), filtrated over Celite, and dried in vacuo. The residue was dissolved in pentane (4 mL) and placed in a freezer (−32 °C) for 2 days. The solution was decanted and dried in vacuo to yield an orange-colored crystalline solid (26 mg, 88%). Anal. Found (Calcd) for C21H40CoNOP2: C, 57.07 (56.88); H, 9.96 (9.09); N, 3.06 (3.16). 1H NMR (C6D6, 300 MHz, ppm): 6.88 (m, 2 H, NCH), 4.12 (d, 3JHH = 5.1 Hz, 1 H, PCH), 1.42 (A18XX′A′18, N = |3JHP + 5JHP| = 13.2 Hz, 36 H, PtBu2). 31P{1H} NMR (C6D6, 121 MHz, ppm): 95.3 (br). IR (KBr, cm−1): 1890 (CO), 1526 (CC). Synthesis of [CoCl{HN(CH2CH2PtBu2)2}][BArF4] (8a). A solution of H[BArF4] (111 mg, 0.11 mmol) in Et2O (6 mL) was added dropwise at 0 °C to 6a (50 mg, 0.11 mmol) in Et2O (12 mL). After warming to RT and stirring for 30 min the solvent was removed in vacuo. The red-orange residue was washed with pentane (2 × 5 mL) and dried in vacuo. Yield: 136 mg, 95%. Crystals suitable for X-ray diffraction were obtained from the analogous reaction with HBF4 and crystallization upon layering a saturated CH2Cl2 solution with toluene. Anal. Found (Calcd) for C52H57BClCoF24NP2: C, 47.15 (47.35); H, 4.43 (4.36); N, 0.99 (1.06). μeff = 2.1 ± 0.1 μB. 1H NMR (CD2Cl2, 300 MHz, ppm): 39.76 (CH2), 20.23 (CH2), 12.18 (tBu), 10.02 (tBu), 7.46 (BArF4), 7.27 (BArF4). 11B{1H} NMR (CD2Cl2, 96 MHz, ppm): −6.8. 19F{1H} NMR (CD2Cl2, 282 MHz, ppm): −63.2. Synthesis of [CoCl{N(CHCH2PtBu2)(CH2CH2PtBu2)}][B(ArF)4] (8b). H[BArF4] (81 mg, 0.08 mmol, 0.90 equiv) in Et2O (4 mL) was added dropwise to a solution of 6b (40 mg, 0.09 mmol) in Et2O (8 mL) at 0 °C. After warming to RT and stirring for 30 min the solvent was removed in vacuo. The red-orange residue was washed with pentane (2 × 5 mL) and dried in vacuo. Yield: 108 mg, 94%. Anal. Found (Calcd) for C52H55BClCoF24NP2: C, 47.61 (47.42); H, 4.23 (4.21); N, 1.02 (1.06). μeff = 2.0 ± 0.1 μB. 1H NMR (CD2Cl2, 300 MHz, ppm): 24.03 (CH2), 9.91 (tBu), 7.55 (BArF4), 7.37 (BArF4) 7.32 (tBu), −4.34 (CH2), −13.96 (CH2), −60.81 (CH) ppm. 11B{1H} NMR (CD2Cl2, 96 MHz, ppm): −6.8. 19F{1H} NMR (CD2Cl2, 282 MHz, ppm): −63.1. MS (ESI, THF, m/z+): 453.19 (M+). Synthesis of [CoCl{N(CHCH2PtBu2)(CHCHPtBu2)}][BArF4] (8c). A solution of H[BArF4] (65 mg, 0.07 mmol) in Et2O (4 mL) was added dropwise at 0 °C to a solution of 6c (32 mg, 0.07 mmol) in Et2O (5 mL). After warming to RT and stirring for 30 min the solvent was removed in vacuo, and the red-orange residue washed with pentane (2 × 4 mL) and dried in vacuo. Yield: 84 mg, 90%. Crystals suitable for X-ray diffraction were obtained from the analogous reaction with HBF4 and crystallization upon layering a saturated CH 2 Cl 2 solution with Et 2 O. Anal. Found (Calcd) for C52H53BClCoF24NP2: C, 47.80 (47.49); H, 4.26 (4.06); N, 0.94 (1.07). μeff = 2.0 ± 0.1 μB. 1H NMR (CD2Cl2, 300 MHz, ppm): 8.43 (tBu), 7.55 (BArF4), 7.33 (BArF4), −0.60 (CH2), −13.20 (CH), −38.22 (CH), −48.36 (CH). 11B{1H} NMR (CD2Cl2, 96 MHz, ppm): −6.8. 19F{1H} NMR (CD2Cl2, 282 MHz, ppm): −63.1. Oxidation of 6a with FeCp2+. A vial was charged with 6a (15 mg, 0.03 mmol) and CH2Cl2 (2 mL). While stirring, a solution of [Cp2Fe][PF6] (11 mg, 0.03 mmol) in CH2Cl2 (4 mL) was added dropwise. The solution immediately turned red and was allowed to stir for 10 min. The solution was filtered, the solvent was removed, and the residue was washed with pentane (∼5 mL) and Et2O (∼5 mL). The orange filtrate was decanted, and an additional 5 mL of Et2O was

added to wash the residue and subsequently decanted. The red residue was dried in vacuo (18 mg). 1H NMR (CD2Cl2, 300 MHz, ppm): 13.34 (tBu, 8a), 10.39 (tBu, 8a + 8b), 7.66 (tBu, 8b). Oxidation of 6b with FeCp2+. A vial was charged with 6b (25 mg, 0.06 mmol) and CH2Cl2 (5 mL). While stirring a solution of [Cp2Fe][PF6] (18 mg, 0.06 mmol) in CH2Cl2 (5 mL) was added dropwise. The solution, which immediately turned red, was stirred for 10 min and filtered. After removal of the solvent the residue was washed with pentane (∼5 mL) and twice with Et2O (∼5 mL). The red residue was dried in vacuo (26 mg). Spectroscopic characterization confirmed the formation of 8b in about 70% overall yield. 1H NMR (CD2Cl2, 300 MHz, ppm): 10.72 (tBu, 8b), 7.98 (tBu, 8b). MS (ESI, THF, m/z+): 453.19 (8b). Synthesis of [CoCl{N(CHCHPtBu2)2}]SbF6 (9). A vial was charged with 6c (100 mg, 0.22 mmol) and CH2Cl2 (5 mL). While stirring, solid AgSbF6 (76 mg, 0.22 mmol) was added. The reaction mixture immediately turned to a yellowish-brown slurry. After filtration the solution was concentrated to ∼2 mL. The brown crude product was precipitated by addition of Et2O (∼10 mL) and pentane (∼10 mL). Filtration with a sintered glass crucible, washing with Et2O (∼5 mL), and drying in vacuo yielded yellow-brown 9. Yield: 132 mg (86%). The PF6 salt was obtained following the same procedure with AgPF6 (56 mg, 0.22 mmol) with 90% isolated yield. Anal. Found (Calcd) for C20H40CoF6NP2Sb: C, 34.98 (34.38); H, 5.75 (5.87); N, 1.93 (2.04). 1H NMR (CD2Cl2, 300 MHz, ppm): 158.9 (CH), 1.32 (CH), −7.12 (tBu). Crystallographic Details. Suitable single crystals for X-ray structure determination were selected from the mother liquor under an inert gas atmosphere and transferred in protective perfluoro polyether oil on a microscope slide. The selected and mounted crystals were transferred to the cold gas stream on the diffractometer. Intensity data for 6c were collected on a STOE IPDS II diffractometer (graphite-monochromated Mo Kα radiation, λ = 0.710 73 Å) at −140 °C. Face-indexed absorption corrections were performed numerically with the program X-RED.24 The diffraction data for 6b were collected at 100 K with a Bruker D8 three-circle diffractometer equipped with a SMART APEX II CCD detector and an INCOATEC microfocus source with Quazar mirror optics (Ag Kα radiation, λ = 0.560 86 Å). Intensity data for 5, 6a, 8a, 8c, and 9 were obtained at 100 K on a Bruker D8 three-circle diffractometer, equipped with a PHOTON 100 CMOS detector and an INCOATEC microfocus source with Quazar mirror optics (Mo Kα radiation, λ = 0.710 73 A). The data obtained with a Bruker diffractometer were integrated with SAINT, and a multiscan absorption correction with SADABS was applied. All structures were solved and refined using the Bruker SHELX 2014 software package.25 All non-hydrogen atoms were refined with anisotropic displacement parameters. All C−H hydrogen atoms were refined isotropically on calculated positions by using a riding model with their Uiso values constrained to 1.5Ueq of their pivot atoms for terminal sp3 carbon atoms and 1.2 times for all other carbon atoms. The N−H hydrogen atoms of 5 and 8a were found and isotropically refined. CCDC 1452220−1452226 (5, 6a−c, 8a, 8c, 9) and CCDC 1452371 (7a) contain the supplementary crystallographic data for this paper. This data can be obtained free of charge via http://www.ccdc. cam.ac.uk/products/csd/request/ (or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK; fax: + 44-1223-336-033; e-mail: [email protected]).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00369. Cyclic voltammograms of complexes 6a−c and full crystallographic details of complexes 5, 6a−c, 7a, 8a, 8c, and 9 (PDF) Crystallographic data file of 5 (CIF) G


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Inorganic Chemistry

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(9) (a) Ozerov, O. The Chemistry of Pincer Compounds; MoralesMorales, D.; Jensen, C. M., Eds.; Elsevier: Amsterdam, 2007; pp 287− 309. (b) Khusnutdinova, J. R.; Milstein, D. Angew. Chem., Int. Ed. 2015, 54, 12236. (10) (a) Rozenel, S. S.; Kerr, J. B.; Arnold, J. Dalton Trans. 2011, 40, 10397. (b) Rozenel, S. S.; Padilla, R.; Arnold, J. Inorg. Chem. 2013, 52, 11544. (c) Rozenel, S. S.; Padilla, R.; Camp, C.; Arnold, J. Chem. Commun. 2014, 50, 2612. (d) Camp, C.; Naested, L. C. E.; Severin, K.; Arnold, J. Polyhedron 2016, 103, 157. (11) (a) Zhang, G.; Scott, B. L.; Hanson, S. K. Angew. Chem., Int. Ed. 2012, 51, 12102. (b) Xu, R.; Chakraborty, S.; Yuan, H.; Jones, W. D. ACS Catal. 2015, 5, 6350. (12) Schaefer, B. A.; Margulieux, G. W.; Small, B. L.; Chirik, P. J. Organometallics 2015, 34, 1307. (13) (a) Fryzuk, M. D.; Leznoff, D. B.; Thompson, R. C.; Rettig, S. J. J. Am. Chem. Soc. 1998, 120, 10126. (b) Ingleson, M. J.; Pink, M.; Fan, H.; Caulton, K. G. Inorg. Chem. 2007, 46, 10321. (14) Fout, A. R.; Basuli, F.; Fan, H.; Tomaszewski, J.; Huffman, J. C.; Baik, M.-H.; Mindiola, D. J. Angew. Chem., Int. Ed. 2006, 45, 3291. (15) (a) Birkek, P. J. M. W. L.; Bour, J. J.; Steggerda, J. J. Inorg. Chem. 1973, 12, 1254. (b) van der Put, P. J.; Schilperoord, A. A. Inorg. Chem. 1974, 13, 2476. (c) Collins, T. J.; Richmond, T. G.; Santarsiero, B. D.; Treco, B G. R. T. J. Am. Chem. Soc. 1986, 108, 2088. (d) GarciaMonforte, M. A.; Ara, I.; Martin, A.; Menjon, B.; Tomas, M.; Alonso, P. J.; Arauzo, A. B.; Martinez, J. I.; Rillo, C. Inorg. Chem. 2014, 53, 12384. (16) (a) Goff, H.; La Mar, G. N.; Reed, C. A. J. Am. Chem. Soc. 1977, 99, 3641. (b) Hawrelak, E. J.; Bernskoetter, W. H.; Lobkovsky, E.; Yee, G. T.; Bill, E.; Chirik, P. J. Inorg. Chem. 2005, 44, 3103. (17) (a) Kochem, A.; Gellon, G.; Jarjayes, O.; Philouze, C.; Leconte, N.; van Gastel, M.; Bill, E.; Thomas, F. Chem. Commun. 2014, 50, 4924. (b) Adhikari, D.; Mossin, S.; Basuli, F.; Huffman, J. C.; Szilagyi, R. K.; Meyer, K.; Mindiola, D. J. J. Am. Chem. Soc. 2008, 130, 3676. (c) Corcos, A. R.; Villanueva, O.; Walroth, R. C.; Sharma, S. K.; Bacsa, J.; Lancaster, K. M.; MacBeth, C. E.; Berry, J. F. J. Am. Chem. Soc. 2016, 138, 179610.1021/jacs.5b12643. (18) Schneck, F.; Würtele, C.; Schneider, S. Manuscript in preparation. (19) Meiners, J.; Friedrich, A.; Herdtweck, E.; Schneider, S. Organometallics 2009, 28, 6331. (20) Abdur-Rashid, K.; Fong, T. P.; Greaves, B.; Gusev, D. G.; Hinman, J. G.; Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2000, 122, 9155. (21) Manner, V. W.; Markle, T. F.; Freudenthal, J. H.; Roth, J. P.; Mayer, J. M. Chem. Commun. 2008, 256. (22) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (23) Kahn, O. Molecular Magnetism; VCH Publishers Inc.: New York, 1993. (24) X-RED; STOE & CIE: Darmstadt, Germany, 2002. (25) APEX2 v2014.9-0 (SAINT/SADABS/SHELXT/SHELXL); Bruker AXS Inc.: Madison, WI, USA, 2014.

6a (CIF) 6b (CIF) 6c (CIF) 7a (CIF) 8a (CIF) 8c (CIF) 9 (CIF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

P. O. Lagaditis and B. Schluschaß contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.O.L. thanks the Alexander-von-Humboldt Foundation for a postdoctoral scholarship. REFERENCES

(1) Recent review articles: (a) Chirik, P. J.; Wieghardt, K. Science 2010, 327, 794. (b) van der Vlugt, J. I. Eur. J. Inorg. Chem. 2012, 2012, 363. (c) Luca, O. R.; Crabtree, R. H. Chem. Soc. Rev. 2013, 42, 1440. (d) Bullock, R. M. Science 2013, 342, 1054. (2) Schneider, S.; Meiners, J.; Askevold, B. Eur. J. Inorg. Chem. 2012, 2012, 412. (3) Selected examples: (a) Clarke, Z. E.; Maragh, P. T.; Dasgupta, T. P.; Gusev, D. G.; Lough, A. J.; Abdur-Rashid, K. Organometallics 2006, 25, 4113. (b) Nielsen, M.; Alberico, E.; Baumann, W.; Drexler, H.-J.; Junge, H.; Gladiali, S.; Beller, M. Nature 2013, 495, 85. (c) Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.; Yang Wan, K.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136, 1367. (d) Chakraborty, S.; Dai, H.; Bhattacharya, P.; Fairweather, N. T.; Gibson, M. S.; Krause, J. A.; Guan, H. J. Am. Chem. Soc. 2014, 136, 7869. (e) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem. Soc. 2014, 136, 8564. (f) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.; Würtele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S. J. Am. Chem. Soc. 2014, 136, 10234. (g) Chakraborty, S.; Lagaditis, P. O.; Förster, M.; Bielinski, E. A.; Hazari, N.; Holthausen, M. C.; Jones, W. D.; Schneider, S. ACS Catal. 2014, 4, 3994. (h) Glüer, A.; Förster, M.; Celinski, V. R.; Schmedt auf der Günne, J.; Holthausen, M. C.; Schneider, S. ACS Catal. 2015, 5, 7214. (4) Askevold, B.; Khusniyarov, M. M.; Herdtweck, E.; Meyer, K.; Schneider, S. Angew. Chem., Int. Ed. 2010, 49, 7566. (5) Askevold, B.; Friedrich, A.; Buchner, M. R.; Lewall, B.; Filippou, A. C.; Herdtweck, E.; Schneider, S. J. Organomet. Chem. 2013, 744, 35. (6) Friedrich, A.; Drees, M.; Käss, M.; Herdtweck, E.; Schneider, S. Inorg. Chem. 2010, 49, 5482. (7) (a) Meiners, J.; Scheibel, M. G.; Lemeé-Cailleau, M.-H.; Mason, S. A.; Boeddinghaus, M. B.; Fässler, T. F.; Herdtweck, E.; Khusniyarov, M. M.; Schneider, S. Angew. Chem., Int. Ed. 2011, 50, 8184. (b) Kinauer, M.; Scheibel, M. G.; Abbenseth, J.; Heinemann, F. W.; Stollberg, P.; Würtele, C.; Schneider, S. Dalton Trans. 2014, 43, 4506. (c) Askevold, B.; Khusniyarov, M. M.; Kroener, W.; Gieb, K.; Müller, P.; Herdtweck, E.; Heinemann, F. W.; Diefenbach, M.; Holthausen, M. C.; Vieru, V.; Chibotaru, L. F.; Schneider, S. Chem. - Eur. J. 2015, 21, 579. (8) (a) Askevold, B.; Nieto, J. T.; Tussupbayev, S.; Diefenbach, M.; Herdtweck, E.; Holthausen, M. C.; Schneider, S. Nat. Chem. 2011, 3, 532. (b) Scheibel, M. G.; Askevold, B.; Heinemann, F. W.; Reijerse, E. J.; de Bruin, B.; Schneider, S. Nat. Chem. 2012, 4, 552. (c) Scheibel, M. G.; Wu, Y.; Stückl, A. C.; Krause, L.; Carl, E.; Stalke, D.; de Bruin, B.; Schneider, S. J. Am. Chem. Soc. 2013, 5, 17719. (d) Scheibel, M. G.; Abbenseth, J.; Kinauer, M.; Heinemann, F. W.; Würtele, C.; de Bruin, B.; Schneider, S. Inorg. Chem. 2015, 54, 9290. H


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