Experimental and Theoretical EPR Study of Jahn-Teller-Active [HIPTN3N]MoL Complexes (L = N2, CO, NH3)

The trigonally symmetric Mo(III) coordination compounds [HIPTN3N]MoL (L = N-2, CO, NH3; [HIPTN3N]Mo = [(3,5-(2,4,6-i-Pr3C6H2)(2)C6H3NCH2CH2)(3)N]Mo) are low-spin d(3) (S = 1/2) species that exhibit a doubly degenerate E-2 ground state susceptible to a Jahn-Teller (JT) distortion. The EPR spectra of all three complexes and their temperature and solvent dependences are interpreted within a formal two-orbital model that reflects the ground-state configuration, describes the vibronic interactions that lead to the JT distortions, and addresses whether these complexes exhibit static or dynamic JT distortions. The electronic and vibronic properties of these complexes are then analyzed through ab initio quantum chemical computations. It is not possible to interpret the spectroscopic properties of the orbitally degenerate [HIPTN3N]MoL with DFT methods, so we have resorted to multi-reference wavefunction approaches, the entry level of which is the complete active space self-consistent field (CASSCF) method. Overall, the experimental and computational studies provide new insights into the role of trigonal coordination, as enforced by the [HIPTN3N](3-) ligand, in activating the Mo ion for the binding and reduction of N-2.