Approaching a “naked” boryl anion: amide metathesis as a route to calcium, strontium, and potassium boryl complexes

Amide metathesis has been used to generate the first structurally characterized boryl complexes of calcium and strontium, {(Me3Si)2N}M{B(NDippCH)2}(thf)n (M=Ca, n=2; M=Sr, n=3), through the reactions of the corresponding bis(amides), M{N(SiMe3)2}2(thf)2, with (thf)2Li- {B(NDippCH)2}. Most notably, this approach can also be applied to the analogous potassium amide K{N(SiMe3)2}, leading to the formation of the solvent-free borylpotassium dimer [K{B(NDippCH)2}]2, which is stable in the solid state at room temperature for extended periods (48 h). A dimeric structure has been determined crystallographically in which the K+ cations interact weakly with both the ipso-carbons of the flanking Dipp groups and the boron centres of the diazaborolyl heterocycles, with K⋅⋅⋅B distances of >3.1 Å. These structural features, together with atoms in molecules (QTAIM) calculations imply that the boron-containing fragment closely approaches a limiting description as a “free” boryl anion in the condensed phase

N‐nacnac stabilized tetrelenes: formation of an N,P‐Heterocyclic germylene via C–C bond Insertion

The use of an amino‐functionalized β‐diketiminate (“N‐nacnac”) ligand in low‐valent germanium chemistry is reported, with a view to comparison with “conventional” nacnac systems. Transmetallation of the N‐nacnac ligand from lithium allows access to a versatile chlorogermylene system, and subsequent substituent exchange processes are used to generate related hydrido‐, and phosphaketenyl‐germylenes. The latter undergoes photolytically‐induced cleavage of the P–CO bond to yield an unusual imine‐coordinated N,P‐heterocyclic germylene. On the basis of DFT calculations this transformation is proposed to occur via concerted attack by the electron‐rich carbon–carbon bond of the N‐nacnac backbone accompanying CO loss, rather than via the generation of a free phosphinidene

N-H cleavage vs. Werner complex formation: reactivity of cationic group 14 tetrelenes towards amines

β-Diketiminate ligands featuring backbone NMe2 groups have been exploited to access a series of two-coordinate cations of the type [(N-nacnac)E]+ (E = Si, Ge, Sn), whose reactivity towards N-H bonds has been investigated. While the heavier group 14 systems react via simple adduct formation, N-H oxidative addition occurs for E = Si consistent with differences in EII/EIV redox potentials. The structurally characterized Ge/Sn adducts can be viewed as models for the corresponding (transient) Si systems [(N-nacnac)Si·(NH2R)]+ (R = H, tBu) - which are potential intermediates in the formation of [(N-nacnac)Si(H)(NHR)]+ via a proton-shuttling mechanism

N‐nacnac stabilized tetrelenes: formation of an N,P‐Heterocyclic germylene via C–C bond Insertion

The use of an amino‐functionalized β‐diketiminate (“N‐nacnac”) ligand in low‐valent germanium chemistry is reported, with a view to comparison with “conventional” nacnac systems. Transmetallation of the N‐nacnac ligand from lithium allows access to a versatile chlorogermylene system, and subsequent substituent exchange processes are used to generate related hydrido‐, and phosphaketenyl‐germylenes. The latter undergoes photolytically‐induced cleavage of the P–CO bond to yield an unusual imine‐coordinated N,P‐heterocyclic germylene. On the basis of DFT calculations this transformation is proposed to occur via concerted attack by the electron‐rich carbon–carbon bond of the N‐nacnac backbone accompanying CO loss, rather than via the generation of a free phosphinidene

Reactions of a diborylstannylene with CO2 and N2O: diboration of carbon dioxide by a main group bis(boryl) complex

The reactions of the boryl-substituted stannylene Sn{B(NDippCH)2}2&nbsp;(1) with carbon dioxide have been investigated and shown to proceed&nbsp;via&nbsp;pathways involving insertion into the Sn&ndash;B bond(s). In the first instance this leads to formation of the (boryl)tin(II) borylcarboxylate complex Sn{B(NDippCH)2}{O2CB(NDippCH)2} (2), which has been structurally characterized and shown to feature a &kappa;2&nbsp;mode of coordination of the [(HCDippN)2BCO2]&minus;&nbsp;ligand at the metal centre.&nbsp;2&nbsp;undergoes B&ndash;O reductive elimination in hexane solution (in the absence of further CO2) to give the boryl(borylcarboxylate)ester {(HCDippN)2B}O2C{B(NDippCH)2} (3)&nbsp;i.e.&nbsp;the product of formal diboration of carbon dioxide. Alternatively,&nbsp;2&nbsp;can assimilate a second equivalent of CO2&nbsp;to give the homoleptic bis(borylcarboxylate) Sn{O2CB(NDippCH)2}2&nbsp;(4), which can be prepared&nbsp;via&nbsp;an alternative route from SnBr2&nbsp;and the potassium salt of [(HCDippN)2BCO2]&minus;, and structurally characterized as its DMAP (N,N-dimethylaminopyridine) adduct. Structural and reactivity studies also point to the possibility for extrusion of CO from the [(HCDippN)2BCO2]&minus;&nbsp;fragment to generate the boryloxy system [(HCDippN)2BO]&minus;, a ligand which can be generated directly from&nbsp;1via&nbsp;reaction with N2O. The initially formed unsymmetrical species Sn{B(NDippCH)2}{OB(NDippCH)2} has been shown to be amenable to crystallographic study in the solid state, but to undergo ligand redistribution in solution to generate a mixture of&nbsp;1&nbsp;and the bis(boryloxy) complex Sn{OB(NDippCH)2}2.</p

A β-diketiminate-stabilized sila-acyl chloride: systematic access to base-stabilized silicon analogues of classical carbonyl compounds

An oxidation/substitution strategy for the synthesis of silicon analogues of classical organic carbonyl compounds is reported, by making use of a novel β-diketiminate-supported sila-acyl chloride-the first example of such a compound isolated without the use of a stabilizing Lewis acid. Nucleophilic substitution at the SiIV center allows direct access to the corresponding sila-aldehyde and sila-ester. An alternative approach utilizing the reverse order of synthetic steps is thwarted by the facile rearrangement of the corresponding SiII systems featuring either H or OR substituents. As such, the isolation of (N-nacnac)Si(O)Cl represents a key step forward in enabling the synthesis of sila-carbonyl compounds by a synthetic approach ubiquitous in organic chemistry

Contrasting reactivity of anionic boron- and gallium-containing NHC analogues: E-C vs. E-M bond formation (E = B, Ga).

The anionic Group 13 NHC analogues [(CHNDipp)(2)E](-) (E = B or Ga) display contrasting reactivity towards the half-sandwich titanium imido complex Cp*TiCl(NtBu)py; while the gallium system undergoes salt metathesis yielding the first example of a titanium gallyl compound, the more nucleophilic boryl anion generates a dearomatized pyridyl fragment via attack at the ligand 2-position

Oxidative bond formation and reductive bond cleavage at main group metal centers: reactivity of five-valence-electron MX₂ radicals.

Monomeric five-valence-electron bis(boryl) complexes of gallium, indium, and thallium undergo oxidative M-C bond formation with 2,3-dimethylbutadiene, in a manner consistent with both the redox properties expected for M(II) species and with metal-centered radical character. The weaker nature of the M-C bond for the heavier two elements leads to the observation of reversibility in M-C bond formation (for indium) and to the isolation of products resulting from subsequent B-C reductive elimination (for both indium and thallium)

Reduction of carbon oxides by an acyclic silylene: reductive coupling of CO

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO2 to [C2 O2 ]2-

Reduction of carbon oxides by an acyclic silylene: reductive coupling of CO

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO2 to [C2 O2 ]2-