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A Molecular Zirconium Nitride Super Base from a Mononuclear Parent Imide Lauren N. Grant, Balazs Pinter, Jun Gu, and Daniel J. Mindiola J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11198 • Publication Date (Web): 03 Dec 2018 Downloaded from http://pubs.acs.org on December 3, 2018
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Journal of the American Chemical Society
A Molecular Zirconium Nitride Super Base from a Mononuclear Parent Imide. Lauren N. Grant,† Balazs Pinter,‡ Jun Gu,† and Daniel J. Mindiola†* †Department ‡Department
of Chemistry, University of Pennsylvania, Philadelphia, PA 19104, USA. of Chemistry, Universidad Técnica Federico Santa María, Valparaíso 2390123, Chile.
Supporting Information Placeholder ABSTRACT: In this work we prepared, isolated and structurally characterized a zirconium complex having a terminally bound imide motif, (PN)2Zr≡NH (PN− = (N-(2-iPr2P-4-methylphenyl)2,4,6-trimethylanilide)), along with the zirconium nitride complex {(PN)2Zr≡N[μ2-Li(THF)]}2. (PN)2Zr≡NH was prepared by reduction of trans-(PN)2Zr(N3)2 with KC8. Isotopic labeling and spectroscopic studies were conducted using the respective 15N enriched isotopologues, whereas solid-state structural studies confirmed some of the shortest Zr≡N distances known to date (Zr≡NH, 1.830(3) Å; Zr≡N‒, 1.822(2) Å). It was found that the nitride in {(PN)2Zr≡N[μ2-Li(THF)]}2 is super basic and in the
have remained elusive. The challenge in accessing a multiply bonded zirconium nitride moiety is attributed to both the propensity of titanium and zirconium nitrides to bridge and oligomerize,15-21 and also the lack of suitable N-atom transfer reagents that deliver a single N-atom selectively. Although one class of reagent that fits such criteria is the ubiquitous azide ligand, the lack of LnZr(II) precursors that can promote metal oxidation with concurrent disproportionation of N3− to N2 and N3− make this approach more of an encumbrance.
range of −36 to −43 pKb units. Computational studies have been applied to probe the bonding and structure for this new class of zirconium-nitrogen multiple bonds.
Group 4 transition metal nitrides represent an important category of materials due to their inherent properties such as extraordinary
Scheme 1. Synthesis of precursor 2, parent imide 3, nitride 4, and protonation back to 3.
hardness, anticorrosiveness, and heat-resistance which make them useful for coating or protective purposes.1-3 Molecularly, group 4 metal nitrides have been proposed inasmuch as isolated from the reductive splitting of atmospheric nitrogen,4-13 as well as in the ring-opening, denitrogenation and hydrodenitrogenation of pyridines and quinolines.14 Although we recently reported molecular precursors to monomeric titanium nitrides, precursors for molecular zirconium nitrides, a class of compounds which can allow us to understand the bonding and structure for this ligand,
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Figure 1. Molecular structures of 3 (left) and 4 (center and expanded core on right) with ellipsoids shown at the 50% probability level. Hatoms and residual 3 (pentane) and 4 (toluene) have been omitted for clarity. We recently reported how the d1 species (L)(tolyl2N)Ti(N3) could undergo formation of the parent imide (L)(tolyl2N)Ti≡NH (L‒ = [ArNC(R)]2CH; R = CH3 or tBu; Ar = 2,6-iPr2C6H3, tolyl = p-H3CC6H4) via a titanium nitridyl, whereas the more stable azide precursor (PN)2Ti(N3) (PN− = N-(2-iPr2P-4-methylphenyl)-2,4,6trimethylanilide) could be reduced with KC8 to directly yield the titanium nitride salt.22-25 Realizing coordinatively unsaturated Zr(III) precursors are unstable,26,27 and a parent imide should be a synthon to a nitride, we hypothesized that a Zr(IV) azide precursor could be instead reduced to a radical species. We focused on the PN− ligand since it supports an ideal geometry for metal-ligand multiple bonding, while providing flexibility and robustness.23,24,26,28,29 Herein we report the first mononuclear example of a zirconium parent imide, namely (PN)2Zr≡NH (3). Using a strong base, complex 3 can be deprotonated to afford a molecular monoanionic nitride complex {(PN)2Zr≡N[μ2-Li(THF)]}2 (4). Utilizing 50% αN enriched 15N3−, we report NMR and IR spectroscopic data for the isotopologues of 2-4. We estimate the super basicity of the nitride ligand in 4 and report theoretical studies for this new class of moiety in zirconium coordination chemistry. We recently reported the trivalent complexes (PN)2MCl, prepared from one electron reduction of (PN)2MCl2 (M = Zr (1), Hf),26 but these compounds are metastable and readily undergo disproportionation in solution especially with NaN3. Intuitively, a cleaner and more convenient entry to 2 involved vigorous stirring of 1 with excess (4 equiv.) NaN3 in 1,4-dioxane over 72 hours at 75 oC (Scheme 1). A notable feature is the characteristic N ‒ vibration 3 at 2094 cm‒1 in the IR spectrum which has been identified unambiguously through the isotopologue (PN)2Zr(15N3)2 (2)-15N, red-shifted to 2081 cm‒1 (Table 1). The 15N NMR spectrum of 215N also revealed two resonances at 154.6 and 114.0 ppm, akin to two resonances observed for the reported 15N-enriched bisazide trans,trans,trans-[Pt(N3)2(OH)2(py)2].30 A single-crystal X-ray diffraction (XRD) study of 2 confirmed the presence of two azido ligands, oriented trans to each other with the two PN ligands occupying the basal plane.31 Having the azido ligand on zirconium(IV), complex 2 was treated with an excess of KC8 (2.2 equiv) in Et2O (or THF), in hopes of producing the nitride salt {(PN)2Zr≡N[μ2-K(solvent)x]}2 like that found for the titanium congener.23,24 Upon work-up of the reaction mixture, the monomeric zirconium parent imide, 3, was instead isolated in 42% yield (Scheme 1). Mononuclear parent imides are unknown for zirconium and examples of bridging imides are exceptionally rare.19-21,32,33 Complex 3 displays a singlet resonance (Δν1/2 = 9.0 Hz) in the 1H NMR spectrum corresponding to the parent imido ZrNH residue at 5.11 ppm (top left, Figure 2). The latter spectroscopic feature resolved into a doublet accompanied with ~50% of the signal for unlabeled 3 with a 1JNH
= 63.5 Hz for the isotopologue (PN)2Zr≡15NH, (3)-15N (top right, Figure 2). A weak ν(NH) stretch in IR spectrum at 3392 cm‒1 could be assigned, which red-shifted to 3385 cm‒1 for 3-15N.31 With 50% enriched 3-15N, we confirmed the presence of the parent imido moiety at 351.4 ppm (referenced to liquid ammonia at 0.0 ppm, bottom left, Figure 2) using INEPT 15N NMR spectroscopic experiment. Without signal enhancement, this resonance was too broad to conclusively assign, due to 91Zr (I = 5/2, 11.22%) in addition to 50% loss of enriched 15N. There are scarce examples of terminal parent imides of titanium reported in the literature,22,24,25,34 and their 1JNH values (1JNH = 64.0 Hz) are coincident with those observed for 3 or 3-15N.22,25 A bridging Zr2(μ2-NH) moiety also has a nearly identical 1JNH value to 3 or 3-15N when prepared with enriched 15N.19 The XRD of 3 established a C2 symmetric complex (Figure 1, right) with the phosphine and anilide moiety pairs in a trans orientation and with the metal ion being confined between square pyramidal and trigonal bipyramidal geometries (τ5 = 0.69).35 The imido H-atom was located on a difference map and refined isotropically with an N‒H distance of 0.86(3) Å. However, the most salient metrical parameter for compound 3 is the very short Zr‒Nimide distance of 1.830(3) Å (Figure 1). Formation of 3 from precursor 2 most likely traverses through a putative Zr(III) azide (PN)2Zr(N3) (A) species, followed by N2 extrusion to form a nitridyl (PN)2Zr≡N• (B), which then abstracts an H-atom to generate 3. Performing the reduction in isotopically enriched solvent, THF-d8, did not result in any observable deuteration of the imido residue in 3, thus confirming the PN ligand to be the source of the H-atom. The yield of
