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Electronic structures of bent lanthanide(III) complexes with two N-donor ligands.

ABSTRACT: Low coordinate metal complexes can exhibit superlative physicochemical properties, but this chemistry is challenging for the lanthanides (Ln) due to their tendency to maximize electrostatic contacts in predominantly ionic bonding regimes. Although a handful of Ln2+ complexes with only two monodentate ligands have been isolated, examples in the most common +3 oxidation state have remained elusive due to the greater electrostatic forces of Ln3+ ions. Here, we report bent Ln3+ complexes with two bis(silyl)amide ligands; in the solid state the Yb3+ analogue exhibits a crystal field similar to its three coordinate precursor rather than that expected for an axial system. This unanticipated finding is in opposition to the predicted electronic structure for two-coordinate systems, indicating that geometries can be more important than the Ln ion identity for dictating the magnetic ground states of low coordinate complexes; this is crucial transferable information for the construction of systems with enhanced magnetic properties.

SUBMITTER: Nicholas HM

PROVIDER: S-EPMC7020784 | biostudies-literature | 2019 Dec

REPOSITORIES: biostudies-literature

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Electronic structures of bent lanthanide(III) complexes with two N-donor ligands.

Nicholas Hannah M HM Vonci Michele M Goodwin Conrad A P CAP Loo Song Wei SW Murphy Siobhan R SR Cassim Daniel D Winpenny Richard E P REP McInnes Eric J L EJL Chilton Nicholas F NF Mills David P DP

Chemical science 20190918 45

Low coordinate metal complexes can exhibit superlative physicochemical properties, but this chemistry is challenging for the lanthanides (Ln) due to their tendency to maximize electrostatic contacts in predominantly ionic bonding regimes. Although a handful of Ln<sup>2+</sup> complexes with only two monodentate ligands have been isolated, examples in the most common +3 oxidation state have remained elusive due to the greater electrostatic forces of Ln<sup>3+</sup> ions. Here, we report bent Ln<s ...[more]

PMID: 32110338

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Project description:Complexes featuring lanthanide (Ln)-Si bonds represent a highly neglected research area. Herein, we report a series of open-shell Ln(II+) and Ln(III+) complexes bearing ?-bonded silyl and base-stabilized N-heterocyclic silylene (NHSi) ligands. The reactions of the Ln(III+) complexes Cp3Ln (Ln = Tm, Ho, Tb, Gd; Cp = cyclopentadienide) with the 18-crown-6 (18-cr-6)-stabilized 1,4-oligosilanyl dianion [(18-cr-6)KSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2K(18-cr-6)] (1) selectively afford the corresponding metallacyclopentasilane salts [Cp2Ln({Si(SiMe3)2SiMe2}2)](-)[K2(18-cr-6)2Cp](+) [Ln = Tm (2a), Ho (2b), Tb (2c), Gd (2d)]. Complexes 2a-2d represent the first examples of structurally characterized Tm, Ho, Tb, and Gd complexes featuring Ln-Si bonds. Strikingly, the analogous reaction of 1 with the lighter element analogue Cp3Ce affords the acyclic product [Cp3CeSi(SiMe3)2SiMe2SiMe2Si(SiMe3)2-Cp3Ce](2-)2[K(18-cr-6)](+) (3) as the first example of a complex featuring a Ce-Si bond. In an alternative synthetic approach, the aryloxy-functionalized benzamidinato NHSi ligand Si(OC6H4-2-tBu){(NtBu)2CPh} (4a) and the alkoxy analogue Si(OtBu){(NtBu)2CPh} (4b) were reacted with Cp*2Sm(OEt2), affording, by OEt2 elimination, the corresponding silylene complexes, both featuring Sm(II+) centers: Cp*2Sm ? :Si(O-C6H4-2-tBu){(NtBu)2CPh} (6) and Cp*2Sm ? :Si(OtBu){(NtBu)2CPh} (5). Complexes 5 and 6 are the first four-coordinate silylene complexes of any f-block element to date. All complexes were fully characterized by spectroscopic means and by single-crystal X-ray diffraction analysis. In the series 2a-2d, a linear correlation was observed between the Ln-Si bond lengths and the covalent radii of the corresponding Ln metals. Moreover, in complexes 5 and 6, notably long Sm-Si bonds are observed, in accordance with a donor-acceptor interaction between Si and Sm [5, 3.4396(15) Å; 6, 3.3142(18) Å]. Density functional theory calculations were carried out for complexes 2a-2d, 5, and 6 to elucidate the bonding situation between the Ln(II+) or Ln(III+) centers and Si. In particular, a decrease in the Mayer bond order (MBO) of the Ln-Si bond is observed in the series 2a-2d in moving from the lighter to the heavier lanthanides (Tm = 0.53, Ho = 0.62, Tb = 0.65, and Gd = 0.75), which might indicate decreasing covalency in the Ln-Si bond. In accordance with the long bond lengths observed experimentally in complexes 5 and 6, comparatively low MBOs were determined for both silylene complexes (5, 0.24; 6, 0.25) .

Project description:The chiral nonaazamacrocyclic amine L, which is a reduction product of the 3 + 3 Schiff base macrocycle, wraps around the lanthanide(III) ions to form enantiopure helical complexes. These Ce(III), Pr(III), Nd(III), Eu(III), Gd(III), Tb(III), Er(III), Yb(III) and Lu(III) complexes have been isolated in enantiopure form and have been characterized by spectroscopic methods. X-ray crystal structures of the Ln(III) complexes with L show that the thermodynamic product of the complexation of the RRRRRR-isomer of the macrocycle is the (M)-helical complex in the case of Ce(III), Pr(III), Nd(III) and Eu(III). In contrast, the (P)-helical complex is the thermodynamic product in the case of Yb(III) and Lu(III). The NMR and CD spectra show that the (M)-helicity for the kinetic complexation product of the RRRRRR-isomer of the macrocycle is preferred for all investigated lanthanide(III) ions, while the preferred helicity of the thermodynamic product is (M) for the early lanthanide(III) ions and (P) for the late lanthanide(III) ions. In the case of the late lanthanide(III) ions, a slow inversion of helicity between the kinetic (M)-helical product and the thermodynamic (P)-helical product is observed in solution. For Er(III), Yb(III) and Lu(III) both forms have been isolated in pure form and characterized by NMR and CD. The analysis of 2D NMR spectra of the Lu(III) complex reveals the NOE correlations that prove that the helical structure is retained in solution. The NMR spectra also reveal large isotopic effect on the 1H NMR shifts of paramagnetic Ln(III) complexes, related to NH/ND exchange. Photophysical measurements show that L(RRRRRR) appears to favor an efficient 3pipi*-to-Ln energy transfer process taking place for Eu(III) and Tb(III), but these Eu(III)- and Tb(III)-containing complexes with L(RRRRRR) lead to small luminescent quantum yields due to an incomplete intersystem crossing (isc) transfer, a weak efficiency of the luminescence sensitization by the ligand, and/or efficient nonradiative deactivation processes. Circularly polarized luminescence on the MeOH solutions of Eu(III) and Tb(III) complexes confirms the presence of stable chiral emitting species and the observation of almost perfect mirror-image CPL spectra for these compounds with both enantiomeric forms of L.

Project description:The three isomorphous [3.3.1] metallacryptate complexes bis-(pyridinium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydroxim-ato(1-)]hexa-aluminiumgadolinium-pyridine-water (1/7.396/1), (C5H6N)2[GdAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.396C5H5N·H2O or [Hpy]2[GdAl6(H2shi)2(shi)7(py)1.855(H2O)2]·7.396py·H2O, 1, bis-(pyridinium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydroxim-ato(1-)]hexa-aluminiumdysprosium-pyridine-water (1/7.429/1), (C5H6N)2[DyAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.429C5H5N·H2O or [Hpy]2[DyAl6(H2shi)2(shi)7(py)1.891(H2O)2]·7.429py·H2O, 2, and bis-(pyrid-in-ium) di-aqua-dipyridine-hexa-kis-[μ3-salicyl-hydroximato(3-)]bis-[μ2-salicyl-hydrox-imato(1-)]hexa-aluminiumytterbium-pyridine-water (1/7.386/1), (C5H6N)2[YbAl6(C7H6NO3)2(C7H4NO3)7(C5H5N)1.855(H2O)2]·7.429C5H5N·H2O or [Hpy]2[YbAl6(H2shi)2(shi)7(py)1.818(H2O)2]·7.386py·H2O, 3, where Hpy+ is pyridinium, shi3- is salicyl-hydroximate, and py is pyridine, consist of an aluminium-based metallacryptand that captures an Ln III ion in the central cavity. The metallacryptand portions are comprised of an Al-N-O repeat unit; thus, they can be considered three-dimensional metallacrowns. The encapsulated Ln III ions are nine-coordinate with a spherical capped-square-anti-prism geometry, while the six AlIII ions are all octa-hedral. Four of the AlIII ions are chiral centers with 2 Δ and 2 Λ stereoconfigurations. The remaining two AlIII ions have trans chelate rings from two different shi3- ligands. For 1-3, a section of the main mol-ecule is disordered induced by the presence or absence of a pyridine ligand coordinated to one of the AlIII ions. In the absence of the pyridine moiety, an H2shi- ligand moves into the space otherwise occupied by the pyridine and the phenol oxygen atom coordinates to the AlIII ion. The movement of the H2shi- ligand induces movement for the Ln III ion, for another AlIII ion that also binds the same H2shi- ligand, and for one of the shi3- ligands coordinated to the latter AlIII ion. For 1-3 the occupancy ratio of the metallacryptand portions refined to 0.8550 (13):0.1450 (13), to 0.8909 (13):0.1091 (13), and to 0.8181 (14):0.1819 (14), respectively.

Dataset Information

Electronic structures of bent lanthanide(III) complexes with two N-donor ligands.

Publications

Electronic structures of bent lanthanide(III) complexes with two N-donor ligands.

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