Synthesis and Homomolecular Metalation of Trialkylsilylphosphanides of Calcium and Barium

Barium phosphidoboranes and related calcium complexes

The attempted synthesis of [{Carb}BaPPh2] (1) showed this barium-phosphide and its thf adducts, 1·thf and 1·(thf)2, to be unstable in solution. Our strategy to circumvent the fragility of these compounds involved the use of phosphinoboranes HPPh2·BH3 and HPPh2·B(C6F5)3 instead of HPPh2. This allowed for the synthesis of [{Carb}Ae{PPh2·BH3}] (Ae = Ba, 2; Ca, 3), [{Carb}Ca{(H3B)2PPh2}·(thf)] (4), [{Carb}Ba{PPh2·B(C6F5)3}] (5), [{Carb}Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}·thf] (6), [Ba{O(B(C6F5)3)CH2CH2CH2CH2PPh2}2·(thf)1.5] (7) and [Ba{PPh2·B(C6F5)3}2·(thp)2] (8) that were characterised by multinuclear NMR spectroscopy (thp = tetrahydropyran). The molecular structures of 4, 6 and 8 were validated by X-ray diffraction crystallography, which revealed the presence of Ba⋯F stabilizing interactions (ca. 9 kcal mol-1) in the fluorine-containing compounds. Compounds 6 and 7 were obtained upon ring-opening of thf by their respective precursors, 5 and the in situ prepared [Ba{PPh2·B(C6F5)3}2]n. By contrast, thp does not undergo ring-opening under the same conditions but affords clean formation of 8. DFT analysis did not highlight any specific weakness of the Ba-P bond in 1·(thf)2. The instability of this compound is instead thought to stem from the high energy of its HOMO, which contains the non-conjugated P lone pair and features significant nucleophilic reactivity.

A Structural Diversity of Molecular Alkaline-Earth-Metal Polyphosphides: From Supramolecular Wheel to Zintl Ion

A series of molecular group 2 polyphosphides has been synthesized by using air-stable [Cp*Fe(η5 -P5 )] (Cp*=C5 Me5 ) or white phosphorus as polyphosphorus precursors. Different types of group 2 reagents such as organo-magnesium, mono-valent magnesium, and molecular calcium hydride complexes have been investigated to activate these polyphosphorus sources. The organo-magnesium complex [(Dipp BDI-Mg(CH3 ))2 ] (Dipp BDI={[2,6-i Pr2 C6 H3 NCMe]2 CH}- ) reacts with [Cp*Fe(η5 -P5 )] to give an unprecedented Mg/Fe-supramolecular wheel. Kinetically controlled activation of [Cp*Fe(η5 -P5 )] by different mono-valent magnesium complexes allowed the isolation of Mg-coordinated formally mono- and di-reduced products of [Cp*Fe(η5 -P5 )]. To obtain the first examples of molecular calcium-polyphosphides, a molecular calcium hydride complex was used to reduce the aromatic cyclo-P5 ring of [Cp*Fe(η5 -P5 )]. The Ca-Fe-polyphosphide is also characterized by quantum chemical calculations and compared with the corresponding Mg complex. Moreover, a calcium coordinated Zintl ion (P7 )3- was obtained by molecular calcium hydride mediated P4 reduction.

Alkaline earths as main group reagents in molecular catalysis

The past decade has witnessed some remarkable advances in our appreciation of the structural and reaction chemistry of the heavier alkaline earth (Ae = Mg, Ca, Sr, Ba) elements. Derived from complexes of these metals in their immutable +2 oxidation state, a broad and widely applicable catalytic chemistry has also emerged, driven by considerations of cost and inherent low toxicity. The considerable adjustments incurred to ionic radius and resultant cation charge density also provide reactivity with significant mechanistic and kinetic variability as group 2 is descended. In an attempt to place these advances in the broader context of contemporary main group element chemistry, this review focusses on the developing state of the art in both multiple bond heterofunctionalisation and cross coupling catalysis. We review specific advances in alkene and alkyne hydroamination and hydrophosphination catalysis and related extensions of this reactivity that allow the synthesis of a wide variety of acyclic and heterocyclic small molecules. The use of heavier alkaline earth hydride derivatives as pre-catalysts and intermediates in multiple bond hydrogenation, hydrosilylation and hydroboration is also described along with the emergence of these and related reagents in a variety of dehydrocoupling processes that allow that facile catalytic construction of Si-C, Si-N and B-N bonds.

Understanding the Conformational Dependence of Spin-Spin Coupling Constants: Through-Bond and Through-SpaceJ(31P,31P) Coupling in Tetraphosphane-1,4-diides [M(L)x]2[P4R4]

The characteristic dependence of J(31P,31P) spin-spin coupling constants of alkali metal tetraphosphane-1,4-diides on structure and composition has been analyzed by density functional methods. The computations confirm that the structure of the contact ion pairs is conserved in solution. Calculations on model systems M2P4H4, on naked P4H4(2-) anions, and on models including point charges, show that the role of the cations is mainly structural and to a smaller extent electrostatic. Three of the four J(P,P) coupling constants depend characteristically on the conformation of the anion, which in turn is determined by the substituents R and by cation-anion interactions. Several couplings exhibit a large through-space component and are thus strongly dependent on the relative orientation of nonbonding electron pairs on the phosphorus atoms involved. This is shown by visualization of coupling pathways using the recently introduced coupling energy density (CED), in combination with the electron localization function (ELF).

Synthesis and crystal structure of [tBu3SiPAg2]8: A novel Ag16-cluster featuring a remarkable symmetrical structure

The synthesis of [tBu3SiPAg2]8 which features an Ag16-cluster in the solid state was achieved by metathesis reaction of AgOCN with tBu3SiPNa2.

Complexes of the Heavier Alkaline Earth Metals Ca, Sr, and Ba with O-Functionalized Phosphanide Ligands

Metathesis reactions between either SrI(2) or BaI(2) and 2 equiv of the potassium phosphanide [[(Me(3)Si)(2)CH]-(C(6)H(4)-2-OMe)P]K yield, after recrystallization, the complexes [[([Me(3)Si](2)CH)(C(6)H(4)-2-OMe)P](2)M(THF)(n)] [M = Sr, n = 2 (5); Ba, n = 3 (6)]. Similar metathesis reactions between MI(2) and 2 equiv of the more sterically demanding potassium phosphanide [[(Me(3)Si)(2)CH](C(6)H(3)-2-OMe-3-Me)P]K yield the chemically isostructural complexes [[([Me(3)Si](2)CH)(C(6)H(3)-2-OMe-3-Me)P](2)M(THF)(2)] [M = Ca (9), Sr (7), Ba (8)]. Compounds 5-9 have been characterized by multi-element NMR spectroscopy and X-ray crystallography. Complex 9 is thermally unstable and decomposes at room temperature to give the tertiary phosphane [(Me(3)Si)(2)CH](C(6)H(3)-2-OMe-3-Me)P(Me) and an unidentified Ca-containing product. Compounds 5 and 6 also decompose at elevated temperatures to give the corresponding tertiary phosphane [(Me(3)Si)(2)CH](C(6)H(4)-2-OMe)P(Me) and intractable metal-containing products. The decomposition of 5, 6, and 9 suggests that these compounds undergo an intramolecular methyl migration from the O atom in one phosphanide ligand to the P atom of an adjacent phosphanide ligand to give species containing dianionic alkoxo-phosphanide ligands.

A homologous series of alkaline earth phosphanides: syntheses, crystal structures, and unusual dynamic behavior of (THF)(n)M[P(CH(SiMe(3))(2))(C(6)H(4)-2-CH(2)NMe(2))](2) (M = Mg, Ca, Sr, Ba)

The secondary phosphine R(Me(2)NCH(2)-2-C(6)H(4))PH reacts with Bu(2)Mg to give the homoleptic complex Mg[PR(C(6)H(4)-2-CH(2)NMe(2))](2) (1) [R = CH(SiMe(3))(2)]. The analogous heavier alkaline earth metal complexes (THF)(n)Ae[PR(C(6)H(4)-2-CH(2)NMe(2))](2) [Ae = Ca (2), n = 0; Ae = Sr (3), Ba (4), n = 1] have been synthesized by metathesis reactions between K[PR(C(6)H(4)-2-CH(2)NMe(2))] and 0.5 equiv of the respective alkaline earth metal diiodide. Compounds 1-4 have been characterized by X-ray crystallography and multielement NMR spectroscopy. In the solid state, compounds 1-4 are monomeric, complexes 1 and 2 adopting a distorted tetrahedral geometry and complexes 3 and 4 adopting a distorted square pyramidal geometry (1: orthorhombic, P2(1)2(1)2(1), a = 11.413(3) A, b = 12.072(3) A, c = 32.620(11) A, Z = 4. 2: monoclinic, P2(1)/c, a = 9.5550(4) A, b = 17.4560(7) A, c = 24.5782(10) A, beta = 91.673(2) degrees, Z = 4. 3: monoclinic, C2/c, a = 15.0498(9) A, b = 13.0180(8) A, c = 24.3664(14) A, beta = 104.593(2) degrees, Z = 4. 4: monoclinic, C2/c, a = 15.2930(10) A, b = 13.0326(9) A, c = 24.6491(17) A, beta = 105.542(2) degrees, Z = 4). In toluene solution, compounds 2-4 are subject to dynamic processes which are attributed to a monomer-dimer equilibrium for which bridge-terminal exchange of the phosphanide ligands in the dimer may be frozen out at low temperatures.

Polyhedral members of the Mg2nP2m family derived from dimeric magnesium trialkylsilylphosphandiide

The magnesiation of tri(tert-butyl)silylphosphane in THF yields tetrameric (tetrahydrofuran-O)magnesium tri(tert-butyl)silylphosphandiide 1. The central moiety is a slightly distorted Mg4P4 cube with tetracoordinate magnesium and phosphorus atoms. The reaction of dibutylmagnesium with H2PSitBu3 in toluene gives tetramagnesium tetrakis[mu-tri(tert-butyl)silylphosphanide] bis[mu 4-tri(tert-butyl)silylphosphandiide] 2. The central fragment is a Mg4P2 octahedron with the phosphorus atoms in a trans position. The Mg...Mg edges are bridged by the phosphanide substituents. Crystallographic data of 1: C68H148Mg4O5P4Si4, monoclinic, P2(1)/c, a = 13.454(1) A, b = 26.123(1) A, c = 24.539(2) A, beta = 96.53(1) degrees, Z = 4; crystallographic data of 2: C72H166Mg4P6Si6, monoclinic, P2(1)/n, a = 13.951(1) A, b = 14.269(1) A, c = 24.209(2) A, beta = 102.415(1) degrees, Z = 2.