Ground-State and Excited-State Structures of Tungsten–Benzylidyne Complexes

Abstract

The molecular structure of the tungsten-benzylidyne complex trans-W(≡CPh)(dppe)(2)Cl (1; dppe = 1,2-bis(diphenylphosphino)ethane) in the singlet (d(xy))(2) ground state and luminescent triplet (d(xy))(1)(π*(WCPh))(1) excited state (1*) has been studied using X-ray transient absorption spectroscopy, X-ray crystallography, and density functional theory (DFT) calculations. Molecular-orbital considerations suggest that the W-C and W-P bond lengths should increase in the excited state because of the reduction of the formal W-C bond order and decrease in W→P π-backbonding, respectively, between 1 and 1*. This latter conclusion is supported by comparisons among the W-P bond lengths obtained from the X-ray crystal structures of 1, (d(xy))(1)-configured 1(+), and (d(xy))(2) [W(CPh)(dppe)(2)(NCMe)](+) (2(+)). X-ray transient absorption spectroscopic measurements of the excited-state structure of 1* reveal that the W-C bond length is the same (within experimental error) as that determined by X-ray crystallography for the ground state 1, while the average W-P/W-Cl distance increases by 0.04 Å in the excited state. The small excited-state elongation of the W-C bond relative to the M-E distortions found for M(≡E)L(n) (E = O, N) compounds with analogous (d(xy))(1)(π*(ME))(1) excited states is due to the π conjugation within the WCPh unit, which lessens the local W-C π-antibonding character of the π*(WCPh) lowest unoccupied molecular orbital (LUMO). These conclusions are supported by DFT calculations on 1 and 1*. The similar core bond distances of 1, 1(+), and 1* indicates that the inner-sphere reorganization energy associated with ground- and excited-state electron-transfer reactions is small.

Pump—probe XAS investigation of the triplet state of an Ir photosensitizer with chromenopyridinone ligands

“…26,27 For W and Re halide complexes, the photo-induced structural changes are smaller. 28,29 Finally, complexes with a 5d 6 central atom ground state electron configuration as well N or C in the first coordination sphere display only minor changes in their local structure (∼0.02 Å) upon photoexcitation and the X-ray absorption spectra in the near-edge region (XANES) became sensitive to the electronic structure only. Os photosensitizers were the first examples in this category 30,31 and the interest for Ir has increased since then.…”

Pentadiynylidyne and Pentacarbido Complexes

“…Although the WC 5 Si spine is slightly bowed, this distortion is not reproduced in the optimized geometry (DFT:B3LYP-LANL2DZ, gas phase,S PA RTAN-18), indicating that it is as oft response to crystal packing forces.T he frontier molecular orbitals derived from this geometry ( Figure 2) suggest considerable electronic delocalization along the WC 5 spine.B ased on these orbitals,t he color of complex 2 (Figure 3) would appear to arise from two absorptions of interest. Thef irst weak absorption (l max = 562 nm) corresponds to ad xy !p* WC transition [10] whilst the second considerably more intense absorption (l max = 403 nm) is attributed primarily to p WC !p* WC ,that is,from the W C p-bonding orbital to the WC p-antibonding orbital, both of which are extensively delocalized along the C 5 spine.T hese are both to higher wavelength of the corresponding absorptions for [W(CSiMe 3 )(CO) 2 (Tp*)] and [W(CÀCCSiMe 3 )-(CO) 2 (Tp*)]. [4a] Theenergies,calculated at the same level of theory,ofthe corresponding orbitals for the known "C 1 "[ W( CSiMe 3 )-(CO) 2 (Tp*)] and "C 3 "[ W(CCCSiMe 3 )(CO) 2 (Tp*)] congeners are also presented in Figure 2, from which it is clear that 1) the d xy orbital, which is primarily associated with W(CO) 2 retro-donation, is essentially invariant across the series such that increases in the l max for the d xy !p* WC transition are due to progressive conjugative stabilization of the p* WC orbitals (LUMO, LUMO + 1), and 2) the pair of p WC orbitals (HOMOÀ1, HOMOÀ2) increase in energy with chain extension resulting in an increase in both the intensity (e)a nd wavelength of the p WC !p* WC absorption(s).…”

Dihydrogen activation by a tungsten–alkylidyne complex: toward photoredox chromophores that deliver renewable reducing equivalents

“…A density functional theory (DFT) calculation of the geometry of 1H + in the gas phase was performed using Gaussian 09. 6 The calculation employed the B3P86 functional, 7,8 which has been benchmarked as providing accurate geometries for tungstenalkylidyne complexes 3,9 and third-row transition-metal compounds. 10 The LANL2DZ basis and effective core potential 11 was used for tungsten, and the 6-31G* basis set 12,13 was used for other atoms.…”