M₂(hpp)₄Cl₂ and M₂(hpp)₄, Where M = Mo and W: Preparations, Structure and Bonding, and Comparisons with C₂, C₂H₂, and C₂Cl₂ and the Hypothetical Molecules M₂(hpp)₄(H)

The reaction between M2Cl2(NMe2)4, where M = Mo or W, and Hhpp (8 equiv) in a solid-state melt reaction at 150 °C yields the compounds M2(hpp)4Cl2 1a (M = Mo) and 1b (M = W), respectively, by the elimination of HNMe2 [hpp is the anion derived from deprotonation of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine, Hhpp]. Purification of 1a and 1b is achieved by sublimation of the excess Hhpp and subsequent recrystallization from either CH2Cl2 or CHCl3 (or CDCl3). By single-crystal X-ray crystallography, the structures of 1a and 1b are shown to contain a central paddlewheel-like M2(hpp)4 core with Mo−Mo = 2.1708(8) Å (from CH2Cl2), 2.1574(5) Å (from CDCl3), W−W = 2.2328(2) Å (from CDCl3), and M−N = 2.09(1) (av) Å. The Cl ligands are axially ligated (linear Cl−M−M−Cl) with abnormally long M−Cl bond distances that, in turn, depend on the presence or absence of hydrogen bonding to chloroform. The quadruply bonded compounds M2(hpp)4, 2a (M = Mo), and 2b (M = W), can be prepared from the reactions between 1,2-M2R2(NMe2)4 compounds, where R = iBu or p-tolyl, and Hhpp (4 equiv) in benzene by ligand replacement and reductive elimination. The compounds 2a and 2b are readily oxidized, and in chloroform they react to form 1a and 1b, respectively. The electronic structure and bonding in the compounds 1a, 1b, 2a, and 2b have been investigated using gradient corrected density functional theory employing Gaussian 98. The bonding in the M−M quadruply bonded compounds, 2a and 2b, reveals M−M δ2 HOMOs and extensive mixing of M−M π and nitrogen ligand lone-pair orbitals in a manner qualitatively similar to that of the M2(formamidinates)4. The calculations indicate that in the chloride compounds, 1a and 1b, the HOMO is strongly M−Cl σ antibonding and weakly M−M σ bonding in character. Formally there is a M−M triple bond of configuration π4σ2, and the LUMO is the M−M δ orbital. An interesting mixing of M−M and M−Cl π interactions occurs, and an enlightening analogy emerges between these d4−d4 and d3−d3 dinuclear compounds and the bonding in C2, C2H2, and C2Cl2, which is interrogated herein by simple theoretical calculations together with the potential bonding in axially ligated compounds where strongly covalent M−X bonds are present. The latter were represented by the model compounds M2(hpp)4(H)2. On the basis of calculations, we estimate the reactions M2(hpp)4 + X2 to give M2(hpp)4X2 to be enthalpically favorable for X = Cl but not for X = H. These results are discussed in terms of the recent work of Cotton and Murillo and our attempts to prepare parallel-linked oligomers of the type [[bridge]−[M2]−]n.