Organometallic Strontium Borohydrides: Synthesis, X‐ray Structures, Catalytic Polymerization of ε‐Caprolactone, and Density Functional Calculations

Abstract

In the last few years a number of research groups started significant activity in the exploration of the coordination and organometallic chemistry of the heavier alkaline-earth metals. [1] Most of this work is focused on calcium, [2] an inexpensive and non-toxic element present in significant amounts in the human body. Indeed, synthetic access to such large alkalineearth metal complexes is often limited by their kinetic lability with detrimental Schlenk-type equilibria. [3] Within this reserach not only the synthesis of new compounds but also the structures, [4] the reactivity, [5] and some catalytic applications were investigated. [6] Among these latter the ring-opening polymerization (ROP) of cyclic esters such as e-caprolactone (e-CL) and lactide (LA), allows access to polymers with controlled molar mass and molar mass distribution and especially, in some cases, controlled stereochemistry. [2a, 7] In contrast to this well-established chemistry, the application of structurally characterized strontium compounds as initiators in ROP remains very rare. Besides ill-defined amino isopropoxyl strontium, [8] phenoxide ligated cationic strontium complexes [9] and some phenoxide strontium amides [10] were only very recently used for the ROP of LA.Whereas in related rare-earth chemistry, borohydrides such as [Ln(BH 4 ) 3 (thf) 3 ] [11] or its derivatives have become well-established initiators for the ROP of cyclic esters and carbonates, [12] comparable chemistry of alkaline-earth elements was only first published recently by Cushion and Mountford (e.g. on calcium). [13] This is rather surprising because [Ca(BH 4 ) 2 (thf) 2 ] is commercially available for applications in materials science. [14] In contrast, such borohydride chemistry involving strontium is even less developed. Although, the synthesis and the solidstate structure of [Sr(BH 4 ) 2 (thf) 2 ] (1) were already reported in 1995, [15] there is to date, to the best of our knowledge, no structurally characterized coordination or organometallic strontium borohydride compound known.Herein, we describe the synthesis and structural characterization of the half-sandwich [Cp*Sr(BH 4 )(thf) 2 ] 2 (Cp* = h 5 -C 5 Me 5 ; 2) and the bis(phosphinimino)methanide [{(Me 3 SiNPPh 2 ) 2 CH}Sr-(BH 4 )(thf) 2 ] (3) complexes, as well as their application as initiators for the ROP of e-CL. The starting material 1 was prepared according to a literature procedure from [Sr(OEt) 2 ] and a solution of BH 3 in THF. [15] Reaction of 1 with KCp* and K[(Me 3 SiNPPh 2 ) 2 CH] [16] resulted in the corresponding heteroleptic monoborohydride derivatives 2 and 3, respectively (Scheme 1).The new complexes 2 and 3 have been characterized by standard analytical/spectroscopic techniques and the solidstate structures were established by single-crystal X-ray analysis. The 1 H NMR spectra show the expected resonance for the Cp* ligand of 2 at d = 1.98 ppm and a broad signal for the methine group of the bis(phosphinimino)methanide ligand of 3 at d = 1.72 ppm. The BH 4 À groups are observed as...

(Iminophosphoranyl)(thiophosphoranyl)methane rare-earth borohydride complexes: synthesis, structures and polymerization catalysis

“…Both ligands have been extensively used in main group, 1-16 transition [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and f-element 5,13,15,19,[35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] chemistry. Both ligands have been extensively used in main group, 1-16 transition [16][17][18][19][20][21][22][23][24][25][26][27][28][29]…”

“…Calculation studies revealed that the negative charge is mainly localized on the carbene carbon atom of the ligand backbone and that the charge delocalization from carbon to nitrogen is less important. 6,14,16,20,[36][37][38][39][40][41][42][43][44][45][46][47][48][49] In this context, some very efficient rare-earth metal initiators ligated by {CH(PPh 2 NSiMe 3 ) 2 } − have been unveiled for polymerization reactions. Mostly six membered metallacycles (N-P-C-P-N-M) are formed by chelation of the two imine groups to the metal center, adopting a pseudo-boat conformation.…”

Methanediide Formation via Hydrogen Elimination in Magnesium versus Aluminium Hydride Complexes of a Sterically Demanding Bis(iminophosphoranyl)methanediide

“…This, however, yielded a product mixture with stoichiometric HBpin, including some unreacted 1, and a product mixture with one major L-containing species ( 31 P{ 1 H} NMR resonance: δ 22.8 ppm) if an excess of HBpin is used. As a previously used alternative pathway to an MgH species on a sterically demanding monomeric tris(pyrazolyl)methanide MgnBu complex [35] (10), N(1)-P(1)-C(1)101.55 (13), N(2)-P(2)-C(1) 104.52 (14), P(4)-C(2)-P(3) 125.13 (19), N(3)-P(3)-C(2) 100.99 (14), N(4)-P(4)-C(2) 104.70 (14). (13), N(2)-P(2)-C(1) 104.52 (14), P(4)-C(2)-P(3) 125.13 (19), N(3)-P(3)-C(2) 100.99 (14), N(4)-P(4)-C(2) 104.70 (14).…”

“…Methanediides show several bonding modes containing typically one or two coordinated metal centres [2][3][4][5][6][7][8]. Over the past years, several examples of alkaline earth metal complexes of substituted bis(phosphoranyl)methanides and -methanediides have been forthcoming [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] that show several coordination types B-E, see Figure 1. Most common is a dimeric Most common is a dimeric structure (B) with central M2C2 four-membered ring and additional M-E coordination, and monomeric complexes (C) with an N,C,N′-chelating methanide ligand and additional donors coordinating to the metal centre.…”

Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

“…The calcocene [Cp (SiMe3)2 2 Ca] reacts with oxygen to give a biradical [82w] . [Cp*Sr(μ‐BH 4 )(thf) 2 ] 2 with rarely found bridging BH 4 groups acts as an initiator in ring‐opening polymerization of cyclic esters [82k] . The Cp Me4 Mg‐alkynyls catalyze the methyl methacrylate polymerization [82l] .…”