Communication pubs.acs.org/Organometallics
Formation of a Uranium-Bound η1‑Cyaphide (CP−) Ligand via Activation and C−O Bond Cleavage of Phosphaethynolate (OCP−) Christopher J. Hoerger,† Frank W. Heinemann,† Elisa Louyriac,§ Laurent Maron,§ Hansjörg Grützmacher,‡ and Karsten Meyer*,† †
Friedrich-Alexander-University of Erlangen-Nürnberg (FAU), Department of Chemistry and Pharmacy, Inorganic Chemistry, Egerlandstraße 1, 91058 Erlangen, Germany ‡ Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog Weg 1, Hönggerberg, 8093 Zürich, Switzerland § Université de Toulouse et CNRS INSA, 135 avenue de Rangueil, 31077 Toulouse, France S Supporting Information *
A BSTRA CT: Reaction of t he trivalent uranium co mplex [((Ad,MeArO)3N)U(DME)] with [Na(OCP)(dioxane)2.5] and 2.2.2-crypt yields the μ-oxo-bridged, diuranium complex [Na(2.2.2-crypt)][{((Ad,MeArO)3N)U(DME)}(μ-O){((Ad,MeArO)3N)U(CP)}] (1). Complex 1 features an asymmetric, dinuclear UIV−O−UIV core structure with a cyaphide (CP−) anion η1-CP bound to one of the U ions, and a κ2-O DME coordinated to the other. The CP− ligand is unprecedented in uranium chemistry and is formed through reductive C−O bond cleavage of the phosphaethynolate anion (OCP−). An analogous reaction was performed starting from the tetravalent uranium halide complex [((Ad,MeArO)3N)U(DME)(Cl)]. This salt metathesis approach with [Na(OCP)(dioxane)2.5] results in formation of the mononuclear complex [((Ad,MeArO)3N)U(DME)(OCP)] (2) with an OCP− anion bound to the uranium(IV) center via the oxygen atom in an η1-OCP fashion.
R
phosphorus-containing heterocycles.17−22 Salt metathesis of uranium and thorium halides was also successfully achieved, resulting in coordination of the phosphaethynolate anion to the metal centers via the oxygen atom, η1-OCP, yielding [(amid)3UIV(OCP)] and [(amid)3ThIV(OCP)] (amid = N,N′-bis(trimethylsilyl)benzamidinate).23,46 In order to investigate the reactivity of the ambident OCP− nucleophile to uranium, coordination to the UIII precursor complex [((Ad,MeArO)3N)UIII(DME)] ((Ad,MeArO)3N3− = trianion of tris(2-hydroxy-3-(1-adamantyl)-5-methylbenzyl)amine) was explored. Within the series of tris-aryloxide ligands employed in our laboratory, namely (R,R′ArO)3tacn3−,24−26
esearch interest in the long-sought and highly reactive cyaphide anion (CP−), the phosphorus-containing analogue of the well-explored cyanide (CN−) anion, has been ongoing for over a decade.1 Several cyaphide-containing compounds have been synthesized, including a range of phosphaalkynes and metal complexes with bridging cyaphide anions.2−4 Reports of terminally bound, metal-coordinated cyaphide anions, however, are scarce and remain limited to two crystallographically characterized examples. The first example, namely [(dppe)2Ru(H)(CP)], featuring a terminal M−CP moiety, emerged in 2006 and was reported by Grützmacher and co-workers,5 followed by closely related examples reported by Crossley and co-workers in 2014, the cyaphide-alkynyl complexes [(dppe) 2 Ru(CCR)(CP)] (R = CO 2 Me, C6H4OMe).6 Mechanistically, these compounds are formed via base-induced desilylative rearrangement of the M−PCSiR3 (R = Ph, Me) moiety and subsequent elimination of Ph3SiOPh and Me3SiOtBu, respectively.6,7 Here we report the direct generation of a cyaphide anion through C−O bond cleavage of the phosphaethynolate anion (OCP−) accompanied by coordination to a uranium metal center. The OCP− anion, known to be an ambident nucleophile,8 is quickly emerging as a multifaceted reagent for coordination chemistry owing to its recently developed multigram-scale preparation.9,10 Examples for versatile OCP− coordination chemistry include the P atom transfer to metal centers via reductive decarbonylation11−16 and the synthesis of © XXXX American Chemical Society
(R,R′ArO)3mes3−,27−29 and (Ad,MeArO)3N3−,30−32 the single Nanchored chelate is among the most versatile in small-molecule activation chemistry and thus was chosen for the envisioned OCP chemistry. Addition of [Na(OCP)(dioxane)2.5] to [((Ad,MeArO)3N)UIII(DME)] in DME, followed by the addition of 2.2.2-crypt (2.2.2-crypt = 4,7,13,16,21,24-hexaoxa-1,10diazabicyclo[8.8.8]hexacosane), leads to precipitation of the yellow μ-oxo-bridged, dinuclear complex [Na(2.2.2-crypt)][{(( A d , M e ArO) 3 N)U I V (DME)}(μ-O){(( A d , M e ArO) 3 N)UIV(CP)}] (1), with an isolated yield of 80% (Scheme 1, top). The X-ray structure analysis of single crystals of 1 (Figure 1, Received: August 1, 2017
A
DOI: 10.1021/acs.organomet.7b00590 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
Merely, the U−μ-O distances do not follow the trend of increasing bond distances (increasing ionic radii) with increasing CN. The d(U−μ-O) bond lengths were determined to be 2.173(4) Å for U1 with the coordinated cyaphide anion (CN 6) and 2.078(4) Å for U2 (CN 7) with the bound DME. For comparison, the symmetric dinuclear UIV/UIV complex [{((Ad,MeArO)3N)UIV(DME)}2(μ-O)], in which both U ions are seven-coordinate,47 possesses U−μ-O bond lengths of 2.1036(2) Å and average U−OAr bond distances of 2.206(3) Å.32 VT-SQUID magnetization measurements of 1 (2−300 K at 1 T, Supporting Information) reproducibly reveal a temperature-dependent increase of the effective magnetic moment, μeff, varying from 0.61 μB at 2 K to 4.46 μB at 300 K, with no indication of magnetic exchange. Dinuclear 1 is structurally related to a number of diuranium(IV/IV) complexes. Although its magnetic behavior at low temperatures compares well with these species reported in the literature,32,33 its magnetic moment at 300 K is appreciably higher. The 31P NMR spectrum of paramagnetic 1 exhibits one signal, centered at δ 265.8 ppm, which can be assigned to the bound cyaphide anion (see the Supporting Information). This resonance is significantly shifted to higher frequencies in comparison to the values reported for diamagnetic Ru−CP complexes (31P, δ 165−159.5 ppm). The IR vibrational spectrum of 1 was recorded, but a signature C−P stretching frequency could not be identified in the region around 1200−1300 cm −1 (Supporting Information).5,6 The corresponding vibrational band most likely is superimposed by intense and broad ligand signals in the fingerprint region of the spectrum. Analogous to previous reactivity and mechanistic studies on various low-valent U complexes toward COS,34,35 CS2,35−37 and CO2,38−40 the computational studies reported herein (see Figure S13 in the Supporting Information) reveal that the formation of 1 proceeds via two successive one-electrontransfer steps. The initial one-electron step is the reaction of the trivalent precursor with Na(OCP) to yield a mononuclear UIV intermediate, with the OCP dianion bound η1-OCP to one U (formation is almost athermic, +1.6 kcal mol−1). Subsequent coordination of a second equivalent of [((Ad,MeArO)3N)UIII(DME)] to the O atom of the intermediate η2-OCP entity and an ensuing one-electron reduction of the UIII center lead to the formation of the favored (−26.6 kcal mol−1) key intermediate with (OCP)3− sandwiched between two UIV ions, in which the (OCP)3− is η2-coordinated to one U center and μ-oxo bridged to the other. This η2 activation allows for facile O−CP bond cleavage (activation barrier of 4.4 kcal mol−1) and the formation of the thermodynamically stable μoxo-bridged diuranium(IV/IV) species (−31.2 kcal mol−1) with the CP− ligand η1-bound to the now six-coordinate U center (U1). As observed for [{((Ad,MeArO)3N)UIV(DME)}2(μ-O)],32 the coordinately unsaturated U2 ion seeks additional coordination that is provided by the DME oxygen atoms. Cleavage of the C−O-bondrather than the C−P bondof the OCP− anion has been ascribed to the oxophilicity and reducing power of UIII.41 The proposal also is in agreement with theoretical calculations of the OCP− anion, showing that the phosphaethynolate’s resonance structure with a negatively charged O atom and weaker C−O bond is favored (PC−O−, 51.45%) in comparison to that of the phosphaketenide (−P CO, 38.43%).42−44 In order to study potential reactivity differences of tri- and tetravalent uranium complexes toward the OCP− anion, a salt metathesis reaction with a UIV halide complex was also carried
Scheme 1. Synthesis of Complexes 1 and 2
Figure 1. Solid-state molecular structure of the complex anion 1 in crystals of 1·6 C6H6 (left), coordination environment of the sevencoordinate U2 ion (top right), and dinuclear core structure (bottom right). Hydrogen atoms, the [Na(2.2.2-crypt)] counterion, and cocrystallized solvents are omitted for clarity. Thermal ellipsoids are at the 50% probability level.
left), grown from a concentrated benzene solution, revealed a dinuclear, μ-oxo-bridged structure with a linear U−O−U (174.7(2)°) entity, bridging six- and seven-coordinate uranium ions (Figure 1, bottom right). The seven-coordinate U center (U2) is bound to the tetradentate chelate (d(U−N) = 2.551(5) Å, d(U−OAr)av = 2.248 Å), the two oxygens of a bound DME (d(U−ODME)av = 2.667 Å), and the bridging μ-oxo ligand O7 (Figure 1, top right). The coordination polyhedron is best described as distorted octahedral with the two oxygen atoms of the DME molecule occupying two coordination sites trans to the anchoring N atom. Six-coordinate U1 is also coordinated in an idealized octahedral geometry and features the cyaphide anion bound trans to a slightly elongated U−N bond (2.643(5) Å) of the supporting N-anchored tris-aryloxide (d(U−OAr)av = 2.198 Å), with U−C and C−P bond lengths of 2.570(7) and 1.523(8) Å, respectively. The observed C−P distance falls within the range of those found in [(dppe)2Ru(H)(CP)] and [(dppe)2Ru(CCC6H4OMe)(CP)] (d(C−P) = 1.573(2) and 1.544(4) Å).5,6 Also, the close to linear U−C−P angle of 177.5(4)° in 1 is comparable to those found for the Ru complexes (177.9(1) and 172.3(2)°). On the basis of chargebalance considerations, both U ions in dinuclear, monoanionic 1 appear to be in the tetravalent oxidation state. The different U coordination numbers (CN) in 1 are reflected in diverging U− OAr bond lengths. The average U−OAr bond distance to the aryloxide O atoms is significantly shorter for the six-coordinate (2.198 Å) in comparison to the seven-coordinate ion (2.248 Å). B
DOI: 10.1021/acs.organomet.7b00590 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics ORCID
out. Addition of [Na(OCP)(dioxane)2.5] to a stirred suspension of [((Ad,MeArO)3N)UIV(DME)(Cl)] leads to the straightforward formation of [((Ad,MeArO)3N)UIV(DME)(OCP)] (2) in 85% yield (Scheme 1, bottom). Recrystallization from a concentrated DME solution yields single crystals suitable for X-ray diffraction analysis. The molecular structure of 2, in crystals of 2·0.5 DME, represents the second example of a uranium phosphaethynolate complex and features the OCP− anion coordinated in an η1-OCP fashion. The U−O bond (2.345(4) Å) is longer than the U−O distance reported for [(amid)3UIV(OCP)] (2.297(3) Å), while the C−P bond length of 1.559(6) Å is slightly shorter (1.576(5) Å).23 The close to linear O−C−P− ligand (∠(O−C−P) = 177.9(5)°) of 2 is coordinated to the U center in a slightly bent fashion with an U−O−C angle of 164.5(4)°, whereas for [(amid)3UIV(OCP)] it is less bent with an angle of 170.9(3)°. Analysis of 2 by 31P NMR reveals one sharp signal at δ −300 ppm, which is shifted slightly upfield in comparison to the resonance observed for [(amid)3UIV(OCP)], centered at δ −285 ppm.23 Temperaturedependent SQUID magnetization measurements confirm the oxidation state assignment of UIV, f 2, in complex 2. Microcrystalline samples of 2 show a magnetic moment that is increasing from 0.38 μB at 2 K to 2.50 μB at 300 K. The absolute values and temperature dependence compare well to those of other mononuclear U IV compounds of the (Ad,MeArO)3N3− ligand system, namely [((Ad,MeArO)3N)UIV(DME)(EH)] (E = S, Se, Te), with magnetic moments ranging from 0.26−0.30 μB (2 K) to 2.56−2.6 μB (300 K), respectively. The C−O stretching vibration of the OCP− ligand is observed at 1688 cm−1 and thus is comparable to the analogous stretch reported for [(amid)3UIV(OCP)] (1685 cm−1).23 In summary, we have reported the diverging reactivity of phosphaethynolate (OCP−) anion toward UIII and UIV. The dinuclear complex 1 features a unique example of a CP− anion coordinated to U, formed through reductive, two-electron C− O bond cleavage of the OCP− anion. The synthesis of complex 2, starting from a UIV halide, proceeds via salt metathesis and results in the isolation of a mononuclear complex with an η1OCP bound anion.
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Frank W. Heinemann: 0000-0002-9007-8404 Laurent Maron: 0000-0003-2653-8557 Karsten Meyer: 0000-0002-7844-2998 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by funds of the German Federal Ministry of Education and Research (BMBF 2020+ support codes 02NUK012C and 02NUK020C), the Joint DFG-ANR projects (ME1754/7-1, ANR-14-CE35-0004-01), and the FAU Erlangen-Nürnberg. We thank Dr. Andreas Scheurer for assistance with the NMR measurements.
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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.organomet.7b00590. Synthetic details, 1H, 31P NMR, UV/vis/NIR, and IR spectroscopic, magnetochemical, and CHN elemental analysis data, computational details, and crystallographic data of 1 and 2 (PDF) Cartesian coordinates of calculated structures (XYZ) Accession Codes
CCDC 1558613−1558614 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail for K.M.:
[email protected]. C
DOI: 10.1021/acs.organomet.7b00590 Organometallics XXXX, XXX, XXX−XXX
Communication
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DOI: 10.1021/acs.organomet.7b00590 Organometallics XXXX, XXX, XXX−XXX
