Substituent Effects on the Stability of Thallium and Phosphorus Triple Bonds: A Density Functional Study

Abstract

Three computational methods (M06-2X/Def2-TZVP, B3PW91/Def2-TZVP and B3LYP/LANL2DZ+dp) were used to study the effect of substitution on the potential energy surfaces of RTl≡PR (R = F, OH, H, CH3, SiH3, SiMe(SitBu3)2, SiiPrDis2, Tbt (=C6H2-2,4,6-(CH(SiMe3)2)3), and Ar* (=C6H3-2,6-(C6H2-2, 4,6-i-Pr3)2)). The theoretical results show that these triply bonded RTl≡PR compounds have a preference for a bent geometry (i.e., ∠R⎼Tl⎼P ≈ 180° and ∠Tl⎼P⎼R ≈ 120°). Two valence bond models are used to interpret the bonding character of the Tl≡P triple bond. One is model [I], which is best described as TlP. This interprets the bonding conditions for RTl≡PR molecules that feature small ligands. The other is model [II], which is best represented as TlP. This explains the bonding character of RTl≡PR molecules that feature large substituents. Irrespective of the types of substituents used for the RTl≡PR species, the theoretical investigations (based on the natural bond orbital, the natural resonance theory, and the charge decomposition analysis) demonstrate that their Tl≡P triple bonds are very weak. However, the theoretical results predict that only bulkier substituents greatly stabilize the triply bonded RTl≡PR species, from the kinetic viewpoint.

Is It Possible To Prepare and Stabilize Triple-Bonded Thallium≡Antimony Molecules Using Substituents?

“…These theoretical results suggest the bonding characters of the triple-bonded R′Tl≡SbR′ species can be diagrammatically represented in Figure 2[II], which somewhat differs from Figure 2[I]. 81,82 In other words, the triplet R′–Tl unit and the triplet R′–Sb moiety can combine to produce a triple-bonded R′Tl≡SbR′ molecule at the singlet ground state, i.e., [R′–Tl] 3 + [R′–Sb] 3 → [R′Tl≡SbR′] 1 . From Figure 2, the bonding picture based on the valence bond theory strongly suggests model [II], , is better to interpret the nature of the Tl≡Sb triple bond in the R′Tl≡SbR′ molecule.…”

Simulations Suggest Possible Triply Bonded Phosphorus≡E13 Molecules (E13 = B, Al, Ga, In, and Tl)

“…: E13 = PR2) are studied. The respective computational results for RB☰PR [28], RAl☰PR [29], RGa☰PR [30], RIn☰PR [31], and RTl☰PR [32] are…”

“…That is to say, it is easy for the RE13PR species to migrate to the corresponding doubly bonded R2E13 = P: or: E13 = PR2 isomers rather than to the triply bonded RE13 ☰ PR molecules. The theoretical evidence strongly suggests that the experimental detection of RE13☰PR that features small groups is very unlikely so they are not discussed in this section [28][29][30][31][32].…”

“…In order to avoid the London dispersion forces [36], the dispersion-corrected M06-2X/Def2-TZVP level of theory [37] is used to compute geometrical parameters and some properties. The respective results for RB☰PR [28], RAl☰PR [29], RGa☰PR [30], RIn☰PR [31], and RTl☰PR [32] are shown in Tables 1-5. The same level of theory is also used to determine the feasibility of producing triply bonded R'E13 ☰ PR´compounds (Scheme 1 and Tables 1-5).…”

“…Also, molecules that feature a phosphorus double bond have been the subject of many experimental and theoretical studies of structure and reactivity [15][16][17][18][19][20][21][22][23][24][25][26][27]. However, little is known about the molecules that feature a phosphorus triple bond [28][29][30][31][32]. In particular, whether it is possible to anticipate the stability of the R-E13 ☰ phosphorus-R (E13 = B, Al, Ga, In, and Tl) species based on the effects of substituents, since the R-E13 ☰ phosphorus-R systems are isoelectronic to the R-E14 ☰ E14-R (E14 = C, Si, Ge, Sn, and Pb) compound from the valence electron viewpoints.…”