Stabilization of diborane(4) by transition metal fragments and a novel metal to π Dewar–Chatt–Duncanson model of back donation

Abstract

The feasibility of using transition metal fragments to stabilize B2H4 in planar configuration by donating 2 electrons to the boron moiety is investigated. Building upon the existing theoretical and experimental data and aided by the isolobal analogy, the model transition metal complexes Cr(CO)4B2H4 (6), Mn(CO)CpB2H4 (7), Fe(CO)3B2H4 (8) and CoCpB2H4 (9) are chosen to illustrate this unique bonding feature--bond strengthening with π-back donation. Other possible types of complexes with B2H4 and the metal fragment are also explored and the energies are compared. One of the low energy isomers wherein the planar B2H4 interacts with the metal fragment in an in-plane fashion represents a unique case study for the Dewar-Chatt-Duncanson model. In this complex the back-donation from the metal fills the π bonding orbital between the two boron atoms thus forming a B=B double bond.

Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)2}2{μ‐η2:η2‐B2H4}] and [{Cp*M(CO)2}2B2H2M(CO)4], M=Mo,W

“…Several theoretical studies on B 2 H 4 isomers showed that the relative energy difference between various isomers lies below 5.0 kcal mol −1 (Scheme a) . Although such energy differences are small, transition‐metal fragments can be utilized to stabilize these different diborane(4) species …”

“…NBO (natural bonding orbital) analysis showed dissimilar charge distributions around the boron centres (NBO charges: B1=−0.234 and B2=0.134; see Table S2), thus supporting the significantly different 11 B NMR values at −40 °C. This result is also in tune with the computed low barriers for the interconversion of similar B–H–M bridged structures …”

“…The diborane(4) structure in 2 resembles to a singly bridged C s minimum energy structure of diborane(4), higher in energy than the doubly bridged C 2 v global minimum only by 4.06 kcal mol −1 . However, computational studies have shown that complexes of the doubly bridged C 2 v structure are higher in energy than the bridged C s structure . The diborane(4) structure as in 2 is particularly rare and only one such example is hypothesised in cobaltaborane [Co 2 (CO) 6 B 2 H 4 ] …”

Thermodynamic Hydricity of Small Borane Clusters and Polyhedral closo-Boranes

“…In the Li 2 [B n H 3n−2 ] series formed by the consecutive addition of BH 3 to Li 2 BH ( Scheme S6 ), the HDA MeCN values decrease by 20–52 kcal/mol relative to the corresponding neutral boranes B 3 H 3n−2 ( Table 1 and Scheme 2 ). Both [B 2 H 4 ] 2− [ 99 , 102 , 104 , 111 ] and [B 3 H 7 ] 2− [ 82 , 112 , 113 , 114 ] are used as ligands in transition metal complexes. The most stable structures, [B 4 H 12 ] 2− and [B 5 H 15 ] 2− , are based on the geometry of [B 3 H 7 ] 2− , whereas cyclic structures are energetically less favorable by 7.4 and 16.8 kcal/mol (∆G° MeCN ), respectively ( Table S7 ).…”

“…B2H4 can be obtained by the reaction of B2H6 with F radicals in the gas phase [98]. Due to its high electrophilicity (HDA MeCN = 105.4 for B2H4 is comparable to HDA MeCN = 108.4 for BH3), there is a lack of transition metal complexes with B2H4 [99]; however, it can be stabilized in the form of a Lewis acid-base complex (Me3P)2B2H4 [100][101][102][103][104]. In turn, (Me3P)2B2H4 can act as a bidentate ligand in transition metal complexes [102][103][104][105].…”