The nature of supermolecular bonds: Investigating hydrocarbon linked beryllium solvated electron precursors

Abstract

Beryllium ammonia complexes Be(NH3)4 are known to bear two diffuse electrons in the periphery of a Be(NH3)42+ skeleton. The replacement of one ammonia with a methyl group forms CH3Be(NH3)3 with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH3)4. Two CH3Be(NH3)3 species stick together to form strongly bound beryllium ammonia complexes, (NH3)3Be(CH2) nBe(NH3)3, n = 1-6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analogue of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H2 in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H2. The biradical character of the system is analyzed and the singlet-triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of direct covalent bond between the two diffuse electrons as happens for two Li(NH3)4 complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic.

Introducing Novel Materials with Diffuse Electrons for Applications in Redox Catalysis and Quantum Computing via Theoretical Calculations

“…We first focus on the electronic structure of two lithium SEPs connected with a diamine of various carbon chain lengths, LiAm-LSEP(n) with n = 1–10. We compare our results with similar species we studied recently (NH 3 ) 3 BeCH 2 (CH 2 ) n H 2 CBe(NH 3 ) 3 , where the lithium atoms are replaced with beryllium and the NH 2 units with CH 2 to keep one outer diffuse electron around each metal center . The frontier peripheral orbitals occupied by the diffuse electrons are of identical nature for the two species; the orbitals of LiAm-LSEP(4) are depicted in Figure .…”

“…Additionally, we examined the next four excited states of LiAm-LSEPs . Consistent with our previous works on BeMe-LSEPs and BMe-LSEPs , , these electronic states involve a promotion of one electron to the π and π* orbitals of Figure . These are the constructive (+) and destructive (−) combinations of the 1p x and 1p y orbitals of the SEP termini.…”

“…Two systems were studied, (NH 3 ) 3 Be(CH 2 ) 1−6 Be(NH 3 ) 3 and (NH 3 ) 3 B(CH 2 ) 3−6 B-(NH 3 ) 3 , with each metal center offering one and two diffuse electrons for Be and B, respectively. 14,38 Our main observation was the bridged SEP centers act as two distinct atoms fixed at a distance determined by the carbon chain length. For example, the two electrons of (NH 3 ) 3 Be(CH 2 ) 1−6 Be(NH 3 ) 3 (one around each Be center) may couple to form either a triplet or singlet state.…”

“…It should be mentioned that all carbon−carbon bonds have a staggered conformation. For similar Be species (see below and ref 38), we saw in the past that allowing other conformations (eclipsed or Gauche) for the singlet spin states may lead to (slightly) lower energy curved structures. Table S9 of Supporting Information shows that such structures are higher in energy within a few kcal/mol.…”

“…We compare our results with similar species we studied recently (NH 3 ) 3 BeCH 2 (CH 2 ) n H 2 CBe(NH 3 ) 3 , where the lithium atoms are replaced with beryllium and the NH 2 units with CH 2 to keep one outer diffuse electron around each metal center. 38 The frontier peripheral orbitals occupied by the diffuse electrons are of identical nature for the two species; the orbitals of LiAm-LSEP(4) are depicted in Figure 3. (NH 3 ) 3 BeCH 2 (CH 2 ) n H 2 CBe(NH 3 ) 3 can be seen as the linked analogue of two (NH 3 ) 3 BeCH 3 moieties.…”