Zachary D. Brown

A germanium isocyanide complex featuring (n → π*) back-bonding and its conversion to a hydride/cyanide product via C-H bond activation under mild conditions.

Reaction of the diarylgermylene Ge(Ar(Me(6)))(2) [Ar(Me(6)) = C(6)H(3)-2,6-(C(6)H(2)-2,4,6-(CH(3))(3))(2)] with tert-butyl isocyanide gave the Lewis adduct species (Ar(Me(6)))(2)GeCNBu(t), in which the isocyanide ligand displays a decreased C-N stretching frequency consistent with an n → π* back-bonding interaction. Density functional theory confirmed that the HOMO is a Ge-C bonding combination between the lone pair of electrons on the germanium atom and the C-N π* orbital of the isocyanide ligand. The complex undergoes facile C-H bond activation to produce a new diarylgermanium hydride/cyanide species and isobutene via heterolytic cleavage of the N-Bu(t) bond.

Reversible, Photoinduced Activation of P4 by Low‐Coordinate Main Group Compounds

Two unique systems based on low-coordinate main group elements that activate P4 are shown to quantitatively release the phosphorus cage upon short exposure to UV light. This reactivity marks the first reversible reactivity of P4, and the germanium system can be cycled 5 times without appreciable loss in activity. Theoretical calculations reveal that the LUMO is antibonding with respect to the main group element-phosphorus bonds and bonding with respect to reforming the P4 tetrahedron, providing a rationale for this unprecedented activity, and suggesting that the process is tunable based on the substituents.

Mechanisms of reactions of open-shell, heavier group 14 derivatives with small molecules: n-π* back-bonding in isocyanide complexes, C-H activation under ambient conditions, CO coupling, and ancillary molecular interactions.

The main themes of this review are the mechanisms of the reactions of germanium and tin analogues of carbenes with isocyanides, CO, ammonia, and related molecules. The treatment of Ge(Ar(Me6))2 (Ar(Me6) = C6H3-2,6(C6H2-2,4,6-Me3)2) with MeNC or Bu(t)NC afforded 1:1 complexes, but the increase in the electron density at germanium leads to C-H activation at the isocyanide methyl or tert-butyl substituents. For MeNC, the initial adduct formation is followed by a migratory insertion of the MeNC carbon into a Ge-C(ipso) bond of an aryl substituent. The addition of excess MeNC led to sequential insertions of two further MeNC molecules. The third insertion led to methylisocyanide methyl group C-H activation, to afford an azagermacyclopentadienyl species. The Bu(t)NC complex (Ar(Me6))2GeCNBu(t) spontanously transforms into (Ar(Me6))2Ge(H)CN and isobutene with C-H activation of the Bu(t) substituent. The germylene Ge(Ar(Me6))(Ar(Pr(i)4)) [Ar(Pr(i)4) = C6H3-2,6(C6H3-2,6-Pr(i)2)2] reacted with CO to afford α-germyloxyketones. The initial step is the formation of a 1:1 complex, followed by migratory insertion into the Ge-C bond of the Ar(Pr(i)4) ligand to give Ar(Me6)GeC(O)Ar(Pr(i)4). Insertion of a second CO gave Ar(Me6)GeC(O)C(O)Ar(Pr(i)4), which rearranges to afford α-germyloxyketone. No reaction was observed for Sn(Ar(Me6))2 with RNC (R = Me, Bu(t)) or CO. Spectroscopic (IR) results and density functional theory (DFT) calculations showed that the reactivity can be rationalized on the basis of Ge-C (isocyanide or CO) Ge(n) → π* (ligand) back-bonding. The reaction of Ge(Ar(Me6))2 and Sn(Ar(Me6))2 with ammonia or hydrazines initially gave 1:1 adducts. However, DFT calculations show that there are ancillary N-H---N interactions with a second ammonia or hydrazine, which stabilizes the transition state to form germanium(IV) hydride (amido or hydrazido) products. For tin, arene elimination is favored by a buildup of electron density at the tin, as well as the greater polarity of the Sn-C(ipso) bond. Germanium(IV) products were observed upon reaction of Ge(Ar(Me6))2 with acids, whereas reactions of Sn(Ar(Me6))2 with acids did not give tin(II) products. In contrast to reactions with NH3, there is no buildup of negative charge at tin upon protonation, and its subsequent reaction with conjugate bases readily affords the tin(IV) products.

Mechanistic Study of Stepwise Methylisocyanide Coupling and C–H Activation Mediated by a Low-Valent Main Group Molecule

An experimental and DFT investigation of the mechanism of the coupling of methylisocyanide and C-H activation mediated by the germylene (germanediyl) Ge(Ar(Me6))2 (Ar(Me6) = C6H3-2,6(C6H2-2,4,6-Me3)2) showed that it proceeded by initial MeNC adduct formation followed by an isomerization involving the migratory insertion of the MeNC carbon into the Ge-C ligand bond. Addition of excess MeNC led to sequential insertions of two further MeNC molecules into the Ge-C bond. The insertion of the third MeNC leads to methylisocyanide methyl group C-H activation to afford an azagermacyclopentadienyl species. The X-ray crystal structures of the 1:1 (Ar(Me6))2GeCNMe adduct, the first and final insertion products (Ar(Me6))GeC(NMe)Ar(Me6) and (Ar(Me6))GeC(NHMe)C(NMe)C(Ar(Me6))NMe were obtained. The DFT calculations on the reaction pathways represent the first detailed mechanistic study of isocyanide oligomerization by a p-block element species.

Transition Metal Functionalization of P4 Using a Diarylgermylene Anchor

The manipulation of white phosphorus (P4) has been a long-standing challenge for chemists. While the holy grail remains at finding a method to catalytically activate and functionalize P4 to yield new organophosphorus compounds, fundamental research lies in developing procedures to control the reactivity of elemental phosphorus. In this work, Lewis acidic transition metal moieties M(CO)5 (M = Cr, Mo, W) and AuCl react with P4 derivatized with a low valent germanium compound. For both M(CO)5 and AuCl, bis-functionalized products can be formed; however the monosubstituted derivatives are found to be more stable, and the decomposition can be monitored by 31P{1H} NMR spectroscopy. The selective reactivity of white phosphorus, once a P-P bond has been activated, is a key step in yielding new organophosphorus compounds.