Institut deQuímicaComputacional |
UdGUniversitat de Girona |
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Institut deQuímicaComputacional |
UdGUniversitat de Girona |
|---|
A.1 Molecular Orbital Analysis
Chromium carbenes such as 1 can be considered as formed by bonding a singlet Cr(CO)5 fragment and a singlet carbene fragment, i.e., bonding datively using singlets, like in a metal carbonyl [35]. Figure 1 displays the frontier orbital diagram for the valence configuration of the (CO)5Cr fragment, CO, and the C(OH)(C2H3) carbene. As can be seen, the 1-hydroxy-2-propenilidene ligand is a better s -donor than CO (because the former has a higher-lying occupied MO of local s symmetry than the latter), and a weaker Figure 1, the carbene ligand has a single p -acceptor orbital, which is better than either of the two p -acceptor orbitals of CO (lying higher in energy). However, the CO ligand is a stronger p -acceptor than the carbene ligand because it can accept electron density into two different 36]. The bonding is similar to that of a metal carbonyl; the carbene donates in a s fashion to the metal, and the metal donates to the carbene ligand in a p fashion. One might expect a decrease in the carbene 3].
The aforementioned facts are in agreement with the experimental dipole moment value of 4 Debye [(CO)5Crd - ¬ Cd +] measured for (CO)5Cr=C(OCH3)CH3, which indicates a lack of charge on the carbene carbon atom, and suggests an electrophilic reactivity of the carbene site [37]. To our knowledge, no experimental dipole moment is available for compound 1. Our calculations yield a dipole moment of 5.3 Debye, and reveal that electron charges on the carbene fragment are smaller than charges on the CO ligands (Table 1). These results predict therefore a similar electrophilic reactivity for 1, and confirms the assigned s -donor/A.2 Structure of the chromium carbene complex
The optimized geometry of complex 1, together with the definitions of structural parameters, is displayed in Figure 2. We can distinguish four different cis CO ligands (labeled 1-4), and one trans CO ligand (labeled 5). As far as the carbene fragment is concerned, calculations indicate that in the optimized structure 1 the H atom bonded to O6 prefers to adopt a cis-conformation. In fact, we expect free rotation around the C6-O6 bond axis at room temperature as it has been experimentally observed for a similar carbene complex [38]. In the solid state, the methoxy(methyl)carbene complex is found to be in its trans-conformation (Scheme 5), whereas in solution at -40oC and higher temperatures, both cis and trans forms occur, the rotational energy barrier [38] being only Bond lengths and bond angles obtained at the nonlocal level of theory for the most stable conformation of 1 are collected in Table 2. We can split our discussion here into three parts:
Two arrangements can be in principle differentiated for the (CO)5Cr=C(OH)(C2H3) complex, namely a staggered conformation S (the plane of carbene bisects two carbonyls' plane), and an eclipsed conformation E. These configurations are related by a rotation of the carbene ligand around the Cr=C6 axis of the complex. Previous studies on metal carbene complexes [25,40] have revealed that this rotation is essentially free, with rotational barriers being ca. 2 kJ mol-1. The preference for an E or S arrangement is determined by the substituents of the carbene ligand as well as by the M=C bond distance. In particular, complex 1 adopts an S-like structure. The torsional angle Ð C1CrC6O6 is -29.30o, and the dihedral angle The aforementioned slight distortions from an ideal octahedron can be rationalized on the basis of molecular orbital arguments [25,26]. When the carbene ligand is rotated into the staggered conformation, the steric repulsion is reduced. Moreover, a small reduction in orbital interaction is also observed, since the overlap of mainly the empty 2p orbital at carbon with the chromium 3d orbital decreases, causing an elongation of the Cr=C bond. As pointed out by Jacobsen and Ziegler [25], the preference for the E or S conformation depends on the balance of these two effects (steric repulsion and orbital interaction). Thus, the bulkier the carbene ligand, the bigger the steric repulsion, and therefore, an S arrangement is expected to be more favored. This is the case of our system, where the former effect dominates. Steric repulsions counterbalance orbital interactions. The net result is a more stable S-like arrangement. If a simpler carbene complex had been considered, then Pauli repulsions would have been likely weaker, allowing a smaller rotation angle, and favoring a more eclipsed conformation. This is the case of (CO)5Cr=CH(OH), whose E structure was reported to be more stable than its S structure [41].