Linear Finite “Mers”—Homoleptic Polynuclear Heavy Alkaline Earth Metal Pyrazolates

Abstract

The increased demand for highly volatile precursor molecules for the production of Group 2 solid-state materials sparked our interest in alkaline earth metal pyrazolate (pz) chemistry. Well explored for di-and trivalent rare earth metal derivatives, [1, 2] the pyrazolate ligand system is uniquely capable to induce a multitude of metal-ligand binding modes. [2, 3] This structural flexibility, when used in conjunction with donor molecules, enabled the isolation of several families of monomeric alkaline earth metal pyrazolates. [4,5] Studies probing the utility of the pyrazolates as precursor molecules showed that not all compounds sublime intact, [5,6] and frequently loss of donors and consequent reduction in volatility is observed. As an example, Winter and co-workers reported that [Ca(tBu 2 pz) 2 (thf) 2 ] (tBu 2 pz = 3,5-di-tert-butylpyrazolate; thf = tetrahydrofuran) sublimes with partial decomposition (200 8C, 0.1 mm Hg) into a white solid of the composition [Ca(tBu 2 pz) 2 ] n but with an unknown structure.[5]Herein we present a family of coligand-free, heavy alkaline earth metal pyrazolates, rare examples of homoleptic linear oligomers with a noteworthy array of metal-ligand binding modes. These feature a metal-size dependent degree of association, namely, the trimeric [Ca 3 (tBu 2 pz) 6 ] 1, the tetrameric [Sr 4 (tBu 2 pz) 8 ] 2 and the unprecedented hexameric [Ba 6 (tBu 2 pz) 12 ] 3. The only other structurally characterized homoleptic alkaline earth metal pyrazolate is the dimeric [Mg 2 (tBu 2 pz) 4 ] 4, [3g] which is now shown to be the junior member of the series.Compounds 1-3 were prepared by the direct treatment of 3,5-di-tert-butylpyrazole (tBu 2 pzH) with the appropriate metal at 250 8C [Eq(1)]. X-ray quality crystals were obtained by recrystallization from a nondonating high-boiling solvent or sublimation.The solid-state structures of 1-3 were established by low temperature X-ray crystallography.[7] The structures of compounds 1-3 are presented in Figures 1-3

Linear Finite “Mers”—Homoleptic Polynuclear Heavy Alkaline Earth Metal Pyrazolates

“…However, extraction of crude 1-3 with THF and crystallization yield dimeric [{M(tBu 2 pz) 2 (thf) n } 2 ], which are being structurally characterized. [13] Isolation of 1-3 together with the earlier 4 shows that in the series [{M(tBu 2 pz) 2 } n ], the degree of association increases with the ionic radius of the metal with n = 2 (Mg), [3g] 3 (Ca), 4 (Sr), 6 (Ba). Although this is not associated with a major change in formal coordination number, except from Mg (CN = 4) to Ca (CN = 7,8), the points of attachment increase on descending the group, from 1 (AEh n = 9 Ca(1,3), 8 Ca(2)) to 2 (AEh n = 12 Sr(1), 9 Sr(2)) and 3 (AEh n = 12 Ba(1), 10 Ba(2,3)), thus supporting the lengthening of the chain.…”

The Synthesis, Structures and Polymorphism of the Dimeric Trivalent Rare‐Earth 3,5‐Dimethylpyrazolate Complexes [Ln(Me2pz)3(thf)]2

“…[2e–2g,2k] Of particular interest is the observation of μ‐η 2 :η 5 coordination in complexes of the larger lanthanoids. This binding mode is quite uncommon, but has been observed for lanthanoid complexes with bulkier pyrazolate substituents, such as [Eu( t Bu 2 pz) 2 ] 4 ,5a [Ln 3 (Ph 2 pz) 9 ] (Ln = La, Nd),5d and [K(PhMe)Ln( t Bu 2 pz) 4 ] (Ln = La, Sm, Tb, Yb) {η 5 (K):η 2 (Ln)},5c and has also been observed in some alkaline earth pyrazolate complexes 4c,4d,11. It should be noted that η 5 ‐pz‐Nd binding has been observed in [Nd 3 (pz) 9 (pzH) 2 ] as part of a unique μ 3 ‐η 5 :η 1 :η 1 binding mode,5e and there is η 5 ‐binding of pyrazole ligand (pzH) in [Eu(pz) 2 (pzH) 2 ] ∞ 5e,6c.…”

Highly Fluorinated Tris(indazolyl)borate Silylamido Complexes of the Heavier Alkaline Earth Metals: Synthesis, Characterization, and Efficient Catalytic Intramolecular Hydroamination

“…With the exception of the agostic interactions, the only previous example of a trinuclear homoleptic Ca complex is [Ca 3 (tBu 2 pz) 6 ], [61] which exhibits a similar variable-temperature 1 H NMR behavior. Trinuclear homoleptic Mg compounds displaying two different ligand environments in a 1:2 ratio for the terminal and bridging positions are more common.…”