A Unique Barium−Carbon Bond:  Mechanism of Formation and Crystallographic Characterization

Abstract

Abstract:The reaction of tetrakis(tetrahydrofuran-O)barium bis[bis(trimethylsilyl)phosphanide] with diphenylbutadiyne yields dimeric (tetrahydrofuran-O)barium 2,5-diphenyl-3-(1,4-diphenylbutene-3-yne-2-ide-1-yl)-4-trimethylsilyl-1-phosphacyclopentadienide 1. The alkenide moiety bridges two barium atoms, with Ba-C bond lengths of 2.881 and 3.071 Å, thus forming a unique three-center two-electron Ba-C-Ba bond. Furthermore, the coordination sphere of the alkaline earth metal atom contains the phosphacyclopentadienide moiety as well as one tetrahydrofuran molecule. A loose side-on coordination of the alkyne moiety completes the coordination sphere of the barium center. A similar reaction with magnesium bis[bis(trimethylsilyl)-phosphanide] gives the dimeric addition product magnesium bis(trimethylsilyl)phosphanide 1,4-diphenyl-1-bis(trimethylsilyl)phosphanyl-but-1-ene-3-yne-2-ide 2. A reaction mechanism for the formation of the barium derivative is suggested.Thus far, crystallographically characterized molecules with barium-carbon bonds have been limited to barocenes 1 or sideon coordinated aromatic systems. 2 Up to now, σ bonds between the heavier alkaline earth metals and carbon atoms have only been structurally characterized for calcium. Lappert and coworkers 3 inserted a calcium atom into a C-Br bond by cocondensation of the alkaline earth metal and bromobis-(trimethylsilyl)methane; the addition of 1,4-dioxane led to the crystallization of (diox) 2 Ca[CH(SiMe 3 ) 2 ] 2 with Ca-C distances of 2.48 Å. Smith, Eaborn and co-workers 4 prepared bis[tris-(trimethylsilyl)methyl]calcium with a bent C-Ca-C fragment of 150°and Ca-C bond lengths of 2.46 Å via the metathesis reaction. The bridging phenylacetylide 5 between two calcium atoms in [(C 5 i Pr 4 H)Ca-CtC-Ph] 2 shows Ca-C bond lengths of 2.52 and 2.55 Å. Similar calcium-carbon distances were found in the tetrakis(tetrahydrofuran-O)calcium and -strontium 2,3-dimethyl-1,4-diphenylbutadiene complex A with a η 4 -bonded ligand. 6 2,4-Di(tert-butyl)pentadienide coordinates in a η 5 -fashion to calcium (B, Scheme 1). 7 Here we report for the first time a crystallographically characterized alkenylbarium bond. Results and DiscussionSynthesis. The reaction of (tetrahydrofuran-O)barium bis-[bis(trimethylsilyl)phosphanide] 8 with diphenylbutadiyne in toluene at room temperature yields dimeric (tetrahydrofuran-O)barium 2,5-diphenyl-3-(1,4-diphenylbutene-3-yne-2-ide-1-yl)-4-trimethylsilyl-1-phosphacyclopentadienide 1. The proposed reaction mechanism is presented in eq 1. The first reaction step is the addition of a Ba-P bond to the CtC bond. If magnesium bis[bis(trimethylsilyl)phosphanide] 9 is used instead of the barium derivative, the product 2, which represents the first step of the reaction sequence, precipitates from a toluene solution. The subsequent 1,3-trimethylsilyl shift is well-known for the addition reaction of barium bis[bis(trimethylsilyl)phosphanide] with benzonitrile 10 where barium bis[1,3-bis(trimethylsilyl)-2-phenyl-1-aza-3-phosphapropenide] is isolable. Th...

Phospha‐Organic Chemistry: Panorama and Perspectives

“…[264] In addition, several h 5 complexes of Group 1 and 2 metals have been structurally characterized, including Na, [265] K, [265] Cs, [266] Ca, [267] and Ba. [268] The chemistry of phosphaferrocenes and phosphacymantrenes has been studied in some detail. They are still the only phosphorus heterocycles displaying an extensive aromatic chemistry.…”

Arylcalcium‐Verbindungen: Synthesen, Strukturen, physikalische Eigenschaften und chemisches Verhalten

“…Ein Beispiel für die erhöhte Reaktivität ist bereits für die Erdalkalimetallphosphanide beobachtet worden, die wegen der Schrägbe-ziehung im Periodensystem große Gemeinsamkeiten mit den Kohlenstoffanaloga aufweisen sollten. [45] Während Mg[P(SiMe 3 ) 2 ] 2 nur an eine CC-Bindung des Diphenylbutadiins addiert, [46] gehen die Derivate von Calcium, Strontium und Barium Folgereaktionen ein, und es bilden sich Phospholide (Phosphacyclopentadienide). [46,47] Diese Arbeiten wurden finanziell von der Deutschen Forschungsgemeinschaft unterstützt.…”

“…[45] Während Mg[P(SiMe 3 ) 2 ] 2 nur an eine CC-Bindung des Diphenylbutadiins addiert, [46] gehen die Derivate von Calcium, Strontium und Barium Folgereaktionen ein, und es bilden sich Phospholide (Phosphacyclopentadienide). [46,47] Diese Arbeiten wurden finanziell von der Deutschen Forschungsgemeinschaft unterstützt. M. Gärtner dankt dem Fonds der Chemischen Industrie für ein Promotionsstipendium.…”

Synthesis and Structure of Solvent-Free Hexameric Magnesium Tri(tert-butyl)silylphosphandiide

“…5 This equilibrium explains the high reactivity for example toward multiple bonds such as of nitriles and alkynes. 6 The ether adducts are monomeric in solution and the solid state. 7 Here, we report a solvent-free magnesium phosphandiide.…”