Exploring the Intricacies of Weak Interactions in Metal–Metal Bonds Using an Unsymmetrical Carbonyl Precursor and a Triple-Bonded W26+ Paddlewheel

Stepwise reaction of W­(CO)6 with tetramethylated bicyclic guanidinate ligands, characterized by a central C­(N)3 unit joining two fused six-membered rings with CH2CMe2CH2 units spanning two of the nitrogen atoms, allowed isolation of W2(μ-CO)2(μ-TMhpp)2(η2-TMhpp)2, 1, a precursor of W2(TMhpp)4Cl2 (J. Am. Chem. Soc. 2013, 135, 17889; TMhpp = [(CH2CMe2CH2)2(C­(N)3)]). Subsequent heating of 1 followed by reaction with TlPF6 generates [W2(TMhpp)4]­(PF6)2, 2. Compound 1 has an edge-sharing bioctahedral (ESBO) arrangement with a W2(μ-CO)24+ core having semibridging carbonyl groups, while 2 has a paddlewheel structure with a W26+ core spanned by four tetramethyl-substituted bicyclic guanidinate ligands. This compound also has hexafluorophosphate anions along the metal–metal bond that are nestled within methylene groups with the aid of a network of weak C–H···F interactions that prevent a close approach of the fluorine atoms to the dimetal unit. Theoretical computations were carried out on ditungsten model complexes supported by three ligand sets: bicyclic guanidinate, guanidinate, and formamidinate. The computations show that the π-accepting ability of the carbonyl groups significantly lowers the energy of the σ* orbital, and thus, the energy falls below that of the δ orbital. This information along with the diamagnetism of both 1 and 2as shown by the sharp signals in the 1H NMR spectra that support a lack of unpaired electrons (S = 0)is consistent with the electronic configuration of σ2π2σ*2δ2 (π2δ2) and thus a formal bond order of 2 for 1 and σ2π4 for the triple-bonded W26+ core in 2. A comparison of the W–W bond lengths in 2, its chloro precursor W2(TMhpp)4Cl2, and the corresponding analogue W2(hpp)4Cl2 shows a substantial effect from the axially coordinated ligand, distal lone pair in determining the length of the metal–metal bond for these paddlewheel species. The importance of the ligands in tuning the energy level of the metal–metal bonds that may lead to dramatic changes in physical properties is also discussed. It is noteworthy that bicyclic guanidinates with the strongest π-donating ability push upward the energy level of the δ orbital, thus allowing the compounds to be easily oxidized.