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.

B-substituted group 1 phosphides: synthesis and reactivity

1-Boryl-8-phosphinonaphthalenes 1-BCy2-8-PCl2-C10H6 (1) and 1-BCy2-8-PPhCl-C10H6 (2) were prepared and used as starting materials for the synthesis of B-substituted phosphides. The reduction of 1 and 2 by Mg provided neutral compounds [1-BCy-8-PCy-C10H6]2 (3) and [1-BCy2-8-PPh-C10H6]2 (4). Compound 3 represents the dimer of phosphinoborane 1-BCy-8-PCy-C10H6 while complex 4 is a rare example of a discrete B ← P coordinated diphosphine. The reduction of 2 by Na or K in THF yielded B-substituted group 1 phosphides [Na(THF)3]+[1-BCy2-8-PPh-C10H6]- (5) and {[K(THF)2]+[1-BCy2-8-PPh-C10H6]-}∞ (6), which structurally resembled bulky group 1 phosphides. Complex 5 showed easy activation of elemental chalcogens E (E = O, S, Se) to give B-substituted chalcogenophosphinites {[Na(THF)2]+[1-BCy2-8-P(E)Ph-C10H6]}2 (E = O (7), S (8), Se (9)) as the products of chalcogen insertion into the P-Na bond. Importantly no oxidation to dichalcogenophosphinates was observed. Compound 5 is tolerant of the CO polar bonds in organic substrates and the reactions of 5 with 2,3-butanedione or an acyl chloride provided {[Na(THF)2]+[1-BCy2-8-P{CHC(O)C(Me)O}Ph-C10H6]-}2 (10) and [1-BCy2-8-P{C(O)tBu}Ph-C10H6] (11). Finally, B-coordinated phosphatetrylenes [1-BCy2-8-P(SnL)Ph-C10H6] (12) and [1-BCy2-8-P(PbL)Ph-C10H6] (13) (L is {2,6-(Me2NCH2)C6H3}-) were also prepared by substitution reactions of 5.

Phosphido-borane-supported stannates

The reactions between SnCl₂ and three equivalents of the alkali metal phosphido-borane complexes [R₂P(BH₃)]M yield the corresponding tris(phosphido-borane)stannate complexes [L_nM{R₂P(BH₃)}₃Sn] [R₂ = iPr₂, L_nM = (THF)₃Li (2Li), (Et₂O)Na (2Na), (Et₂O)K (2K); R₂ = Ph₂, L_nM = (THF)Li (3Li), (THF)(Et₂O)Na (3Na), (THF)(Et₂O)K (3K); R₂ = iPrPh, L_nM = (THF)₄Li (4Li)]. In each case X-ray crystallography reveals an anion consisting of a trigonal pyramidal tin centre coordinated by the P atoms of the phosphido-borane ligands. These tris(phosphido-borane)stannate anions coordinate to the alkali metal cations via their BH₃ hydrogen atoms in a variety of modes to give monomers, dimers, and polymers, depending on the alkali metal and the substituents at the phosphorus centres. In contrast, reactions between SnCl₂ and three equivalents of [tBu₂P(BH₃)]M (M = Li, Na) gave the known hydride [M{tBu₂P(BH₃)}₂SnH], according to multinuclear NMR spectroscopy.

Mechanochemical synthesis and structural analysis of trivalent lanthanide and uranium diphenylphosphinodiboranates

Phosphinodiboranates (H₃BPR₂BH₃^-) are a class of borohydrides that have merited a reputation as weakly coordinating anions, which is attributed in part to the dearth of coordination complexes known with transition metals, lanthanides, and actinides. We recently reported how K(H₃BP^tBu₂BH₃) exhibits sluggish salt elimination reactivity with f-metal halides in organic solvents such as Et₂O and THF. Here we report how this reactivity appears to be further attenuated in solution when the ^tBu groups attached to phosphorus are exchanged for R = Ph or H, and we describe how mechanochemistry was used to overcome limited solution reactivity with K(H₃BPPh₂BH₃). Grinding three equivalents of K(H₃BPPh₂BH₃) with UI₃(THF)₄ or LnI₃ (Ln = Ce, Pr, Nd) allowed homoleptic complexes with the empirical formulas U(H₃BPPh₂BH₃)₃ (1), Ce(H₃BPPh₂BH₃)₃ (2), Pr(H₃BPPh₂BH₃)₃ (3), and Nd(H₃BPPh₂BH₃)₃ (4) to be prepared and subsequently crystallized in good yields (50-80%). Single-crystal XRD studies revealed that all four complexes exist as dimers or coordination polymers in the solid-state, whereas ¹H and ¹¹B NMR spectra showed that they exist as a mixture of monomers and dimers in solution. Treating 4 with THF breaks up the dimer to yield the monomeric complex Nd(H₃BPPh₂BH₃)₃(THF)₃ (4-THF). XRD studies revealed that 4-THF has one chelating and two dangling H₃BPPh₂BH₃^- ligands bound to the metal to accommodate binding of THF. In contrast to the results with K(H₃BPPh₂BH₃), attempting the same mechanochemical reactions with Na(H₃BPH₂BH₃) containing the simplest phosphinodiboranate were unsuccessful; only the partial metathesis product U(H₃BPH₂BH₃)I₂(THF)₃ (5) was isolated in poor yields. Despite these limitations, our results offer new examples showing how mechanochemistry can be used to rapidly synthesize molecular coordination complexes that are otherwise difficult to prepare using more traditional solution methods.

Regioselective benzannulation of allylic sulfur ylides with ynones: a rapid access to substituted thioanisoles

The efficiency and selectivity of annulation reactions are often difficult to control in the presence of multiple potential reactive centers, like in the case of allylic sulfur ylides (ASY). Here, we describe a novel base mediated [3+3] benzannulation of ASY and readily available alkynones, which accomplishes the regioselective formation of multisubstituted thioanisoles, highly sought after chemical scaffolds. A new reactivity pattern of ASY has been unearthed, where it acted as both a 3C component and sulfur source in benzannulation. Use of a widely available base, operational simplicity and broad substrate scope are the additional salient features of the conversion.

Homoleptic uranium and lanthanide phosphinodiboranates

The synthesis and structures of a new class of homoleptic f-metal borohydride complexes (phosphinodiboranates) are described with U, Nd, and Er.

Formation of a unique ‘unsupported’ hydridic stannate(ii)

The reaction of the amido-stannate LiSn(NMe₂)₃ with the phosphine–borane ^tBu₂PHBH₃ gives the Sn^II hydride [(Me₂NH)₂Li{BH₃P(^tBu)₂}₂Sn(H)] (1); the first example of a hydridic stannate(ii) that is not supported by transition metal or ligand bonding.