Possible strategies toward the elusive tetraaminodisilene

In this paper we predict, using quantum mechanical calculations, which diaminosilylenes would dimerize to produce strongly bound tetraaminodisilenes, which so far have proven to be elusive. The central idea is that diaminosilylenes with a small singlet-triplet energy difference would dimerize to strongly bonded disilenes. Calculations at the B3LYP/6-311++G(3df,2p)//MP2/6-31G(d) level of theory showed that the energy difference between the singlet and the triplet states (DeltaE(ST)) of diaminosilylenes (R(2)N)(2)Si: (1) strongly depends on (i) the twist angle varphi between the SiN(2) and the R(2)N planes and (ii) the NSiN bond angle alpha at the divalent silicon. DeltaE(ST) decreases with increased twisting (larger varphi) and with widening of alpha. DeltaE(ST) is reduced from 70.7 kcal mol(-1) for planar (H(2)N)(2)Si: (1a) to DeltaE(ST) = 21.7 kcal mol(-1) when varphi is held at 90 degrees. Likewise, the bicyclic diaminosilylenes 1,4-diaza-7-silabicyclo2.2.1]hepta-7-ylidene and 1,5-diaza-9-silabicyclo3.3.1]nona-9-ylidene (4a,b), with the nitrogens in the bridgehead positions (varphi = 90 degrees), have DeltaE(ST) values of 45.1 and 38.3 kcal mol(-1), respectively. When dimerized, these silylenes form strongly bonded disilenes 5 (E(dim) = -32.2 kcal mol(-1) (4a) and -41.3 kcal mol(-1) (4b)) with Si=Si bond lengths of 2.239 A (4a) and 2.278 A (4b) (MP2/6-31G(d)//MP2/6-31G(d)). These theoretical predictions pave the way for the synthesis of the first strongly bonded tetraaminodisilene. Due to the steric requirements, also silyl substitution at nitrogen has a significant effect on DeltaE(ST) and (H(3)Si)(2)N](2)Si: (1d) is predicted to form a stable Si=Si bonded dimer (E(dim)= -24.1 kcal mol(-1)). However, the larger size of the Me(3)Si substituent prevents the formation of a Si=Si bonded dimer of (Me(3)Si)(2)N](2)Si: (1e).