1,4-Diphosphabutadyne:A Realistic Target for Synthesis? A Theoretical Investigation of C_2P_2, C_2N_2, [Cr(CO)_5PCCP],and [(CO)_5cR(PCCP)Cr(CO)_5]

摘要

To assess the viability of 1,4-diphosphabutadiyne (PCCP,2a)as a target for synthesis, we have investigated C_2P_2 isomers 2, the end-on complexes [Cr(CO)_5PCCP],(3) and [(CO)_5cR(PCCP)Cr(CO)_5](4),as well as their side-on analogues 6 and 7,respectively,using nonlocal density functional theory and a large, doubly polarized triple -#zeta# STO basis (BP86/TZ2P);C_2N_2 isomers 1 were included for comparison. The PCCP molecule 2a turns out to be a thermodynamically stable, linear CP dimer with a 298 K carbon-carbon bond dissociation enthalpy (BDE) of 152.2 kcalmol~(-1). The relatively high BDE and a C-C bond length of 1.336 A, together with the results of a careful bond analysis, show that the PC-CP bond is best conceived as having partial triple bond character (i.e.,Pident toC-Cident toP<->ident toC-P) similar to the NC-CN bond. However, the relatively low HOMO-LUMO gap of 2.5eV in the #pi# system of 2a [versus 5.6 eV in NCCN (1a)]is indicative of a low kinetic stability:2a is likely to have a strong tendency toward polymerization. A conceivable strategy to protect and stabilize the evasive target molecule is coordinatioin of the terminal P atoms to a transition metal fragment, for example, {Cr(CO)_5}. The (first) Cr-P bond dissociation enthalpy (for 298 K)in our model systems 3 and 4 amounts to 20 kcalmol~(-1);side-on coordination (6 and 7)leads to an additional stabilization of about 1-2kcalmol~(-1). In a preliminary investigation, the effect of double side-on coordination of the Pident toC bond of 2a to {Co_2(CO)_6} was also briefly explored and found to furnish considerable stabilization. Furthermore, the differences in geometry (e.g.,linear versus nonlinear) and bonding in CP and CN dimers are discussed and related to the electronic structures of the monomers. The bonding inthe linear CC-NN and CC-PP codimers is also analyzed. We also address the question why flash vacuum thermolysis (FVT) of norbornadienonazine,although it does contain the CNNC unit,, yields nearly exclusively CNCN (1b)and only a trace of CNNC (1c).