The spectroscopic, electrochemical and photophysical effects of the b1/a2 π* lowest unoccupied molecular orbital switching in [M(CO)4(N,N)] (M = Cr or W; N,N = 1,10-phenanthroline or 3,4,7,8-tetramethyl-1,10-phenanthroline). An experimental and DFT computational study

Abstract

DFT calculations on the complexes [M(CO)₄(N,N)] (M = Cr or W; N,N = 1,10-phenanthroline (phen) or 3,4,7,8-tetramethyl-1,10-phenanthroline (tmp)) have revealed that the phen and tmp complexes have different LUMOs: b₁ and a₂, respectively. Nevertheless, the character of the low-lying MLCT electronic transitions, calculated by time-dependent (TD) DFT, hardly changes on going from phen to tmp since the b₁(d_xz) → b₁(phen/tmp) transition is the strongest, whether the b₁(phen/tmp) orbital is the LUMO or not. The switching of LUMO orbitals is manifested by the following features exhibited by the tmp complexes, as compared with their phen counterparts: slightly lower IR ν(CO) frequencies, larger solvatochromism, higher relative resonance enhancement of the A₁² Raman ν(CO) peak and larger shifts of electrochemical reduction potentials from the “free” ligand value. The similar shapes and intensities of the visible absorption bands of the tmp and phen complexes and similarity of their resonance Raman spectra support the TD-DFT prediction of an essentially identical character of the electronic transition(s) responsible. Reduction of the [M(CO)₄(N,N)] complexes produces the corresponding radical anions [M(CO)₄(N,N)]˙⁻, which were characterized by EPR, IR and UV-Vis spectroelectrochemistry. In contrast with the neutral species, the properties of the radical-anionic tmp and phen complexes are very different due to difference between their SOMOs: a₂ and b₁, respectively. This is manifested by the profoundly different EPR hypefine splitting (hfs) patterns observed: [M(CO)₄(phen)]˙⁻ complexes show large hfs from the ¹⁴N donor atoms and from the pairs of ¹H atoms at C3,8 and C4,7 positions. On the other hand, EPR spectra of [M(CO)₄(tmp)]˙⁻ show large hfs from ¹H atoms of a pair of CH₃ groups at C4,7 positions and two pairs of ¹H atoms at C2,9 and C5,6, while the ¹⁴N splitting is rather small. Reasonable agreement between experimental and DFT-calculated hfs was obtained. The switching of LUMO character between b₁ and a₂ can have important implications for constructing molecular devices based on phen complexes.