Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity, and Structural Flexibility

2,5-Disilylsubstituted germole dianions <b>1</b> react with hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]­hexene germylenes <b>3</b>. Their formation proceeds via hafnocene-germylene complexes <b>2</b> that were identified by NMR and UV spectroscopy. Germylenes <b>3</b> are stabilized by homoconjugation between the empty 4p­(Ge) orbital and the π-bond of the innercyclic C²C³ double bond. This interaction can be understood as σ², π-coordination of the butadiene part to the dicoordinated germanium atom that leaves the 16e^– hafnocene moiety electronically unsaturated. We demonstrate that this new class of germylenes might serve as ligand to a variety of low-valent transition-metal complexes. The structure of the germylene ligand in complexes with Fe(0), Ni(0), and Au­(I) and in reaction products with N-heterocyclic carbenes showed an intriguing structural flexibility that allows to accommodate different electronic situations at the ligating germanium atom. The origin of this structural adaptability is the interplay between the topological flexible unsaturated germanium ring and the hafnocene group

Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity, and Structural Flexibility

2,5-Disilylsubstituted germole dianions <b>1</b> react with hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]­hexene germylenes <b>3</b>. Their formation proceeds via hafnocene-germylene complexes <b>2</b> that were identified by NMR and UV spectroscopy. Germylenes <b>3</b> are stabilized by homoconjugation between the empty 4p­(Ge) orbital and the π-bond of the innercyclic C²C³ double bond. This interaction can be understood as σ², π-coordination of the butadiene part to the dicoordinated germanium atom that leaves the 16e^– hafnocene moiety electronically unsaturated. We demonstrate that this new class of germylenes might serve as ligand to a variety of low-valent transition-metal complexes. The structure of the germylene ligand in complexes with Fe(0), Ni(0), and Au­(I) and in reaction products with N-heterocyclic carbenes showed an intriguing structural flexibility that allows to accommodate different electronic situations at the ligating germanium atom. The origin of this structural adaptability is the interplay between the topological flexible unsaturated germanium ring and the hafnocene group

Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity, and Structural Flexibility

2,5-Disilylsubstituted germole dianions <b>1</b> react with hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]­hexene germylenes <b>3</b>. Their formation proceeds via hafnocene-germylene complexes <b>2</b> that were identified by NMR and UV spectroscopy. Germylenes <b>3</b> are stabilized by homoconjugation between the empty 4p­(Ge) orbital and the π-bond of the innercyclic C²C³ double bond. This interaction can be understood as σ², π-coordination of the butadiene part to the dicoordinated germanium atom that leaves the 16e^– hafnocene moiety electronically unsaturated. We demonstrate that this new class of germylenes might serve as ligand to a variety of low-valent transition-metal complexes. The structure of the germylene ligand in complexes with Fe(0), Ni(0), and Au­(I) and in reaction products with N-heterocyclic carbenes showed an intriguing structural flexibility that allows to accommodate different electronic situations at the ligating germanium atom. The origin of this structural adaptability is the interplay between the topological flexible unsaturated germanium ring and the hafnocene group

Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity, and Structural Flexibility

2,5-Disilylsubstituted germole dianions <b>1</b> react with hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]­hexene germylenes <b>3</b>. Their formation proceeds via hafnocene-germylene complexes <b>2</b> that were identified by NMR and UV spectroscopy. Germylenes <b>3</b> are stabilized by homoconjugation between the empty 4p­(Ge) orbital and the π-bond of the innercyclic C²C³ double bond. This interaction can be understood as σ², π-coordination of the butadiene part to the dicoordinated germanium atom that leaves the 16e^– hafnocene moiety electronically unsaturated. We demonstrate that this new class of germylenes might serve as ligand to a variety of low-valent transition-metal complexes. The structure of the germylene ligand in complexes with Fe(0), Ni(0), and Au­(I) and in reaction products with N-heterocyclic carbenes showed an intriguing structural flexibility that allows to accommodate different electronic situations at the ligating germanium atom. The origin of this structural adaptability is the interplay between the topological flexible unsaturated germanium ring and the hafnocene group

Hafnocene-based Bicyclo[2.1.1]hexene Germylenes – Formation, Reactivity, and Structural Flexibility

2,5-Disilylsubstituted germole dianions <b>1</b> react with hafnocene dichloride to give hafnocene-based bicyclo[2.1.1]­hexene germylenes <b>3</b>. Their formation proceeds via hafnocene-germylene complexes <b>2</b> that were identified by NMR and UV spectroscopy. Germylenes <b>3</b> are stabilized by homoconjugation between the empty 4p­(Ge) orbital and the π-bond of the innercyclic C²C³ double bond. This interaction can be understood as σ², π-coordination of the butadiene part to the dicoordinated germanium atom that leaves the 16e^– hafnocene moiety electronically unsaturated. We demonstrate that this new class of germylenes might serve as ligand to a variety of low-valent transition-metal complexes. The structure of the germylene ligand in complexes with Fe(0), Ni(0), and Au­(I) and in reaction products with N-heterocyclic carbenes showed an intriguing structural flexibility that allows to accommodate different electronic situations at the ligating germanium atom. The origin of this structural adaptability is the interplay between the topological flexible unsaturated germanium ring and the hafnocene group

Trialkylsilyl-Substituted Silole and Germole Dianions

The synthesis of dipotassio-2,5-bis­(trialkylsilyl)­silacyclopentadienediides K2[3] and germacyclopentadienediides K2[4] is reported. The prepared silole dianions, 3, are characterized by a significantly deshielded silicon nuclei with 29Si NMR signals at very unusual low field positions for silicon anions (δ29Si = 148–169). The results of DFT calculations revealed that this deshielding is a consequence of the silylene-like frontier orbitals of silole dianions 3 and efficient hyperconjugation between the trialkylsilyl-substituents and the cyclic delocalized π-system. Solid-state structure determinations of potassium salts of silole and germole dianions revealed a novel polymeric bis-η5,bis-η1-coordination mode between heterole and potassium ions

Evidence for a Single Electron Shift in a Lewis Acid–Base Reaction

The Lewis acid–base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B­(C6F5)3) to give the conventional B–Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV–vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium­(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid–base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs

Evidence for a Single Electron Shift in a Lewis Acid–Base Reaction

The Lewis acid–base reaction between a nucleophilic hafnocene-based germylene and tris-pentafluorophenylborane (B­(C6F5)3) to give the conventional B–Ge bonded species in almost quantitative yield is reported. This reaction is surprisingly slow, and during its course, radical intermediates are detected by EPR and UV–vis spectroscopy. This suggests that the reaction is initiated by a single electron-transfer step. The hereby-involved germanium radical cation was independently synthesized by oxidation of the germylene by the trityl cation or strong silyl-Lewis acids. A perfluorinated tetraarylborate salt of the radical cation was fully characterized including an XRD analysis. Its structural features and the results of DFT calculations indicate that the radical cation is a hafnium­(III)-centered radical that is formed by a redox-induced electron transfer (RIET) from the ligand to the hafnium atom. This valence isomerization slows down the coupling of the radicals to form the polar Lewis acid–base product. The implications of this observation are briefly discussed in light of the recent finding that radical pairs are formed in frustrated Lewis pairs