Stereoelectronic Effects in Cl₂ Elimination from Binuclear Pt(III) Complexes

Abstract

Halogen photoelimination is the critical energy-storing step of metal-catalyzed HX-splitting photocycles. Homo- and heterobimetallic Pt(III) complexes display among the highest quantum efficiencies for halogen elimination reactions. Herein, we examine in detail the mechanism and energetics of halogen elimination from a family of binuclear Pt(III) complexes featuring meridionally coordinated Pt(III) trichlorides. Transient absorption spectroscopy, steady-state photocrystallography, and far-infrared vibrational spectroscopy suggest a halogen elimination mechanism that proceeds via two sequential halogen-atom-extrusion steps. Solution-phase calorimetry experiments of the meridional complexes have defined the thermodynamics of halogen elimination, which show a decrease in the photoelimination quantum efficiency with an increase in the thermochemically defined Pt-X bond strength. Conversely, when compared to an isomeric facial Pt(III) trichloride, a much more efficient photoelimination is observed for the fac isomer than would be predicted based on thermochemistry. This difference in the fac vs mer isomer photochemistry highlights the importance of stereochemistry on halogen elimination efficiency and points to a mechanism-based strategy for achieving halogen elimination reactions that are both efficient and energy storing.

Can Serendipity Still Hold Any Surprises in the Coordination Chemistry of Mixed-Donor Macrocyclic ligands? The Case Study of Pyridine-Containing 12-Membered Macrocycles and Platinum Group Metal ions PdII, PtII, and RhIII

“…Indeed, an examination of the data reported in Figure 9 shows that reportedly mixedvalence d n -d m complexes are quite rare, with only five examples known in the literature, among the 505 items found [49][50][51][52][53] beside the complexes [Pt 2 I,III (tfepma) 2 X 4 ] (X = Cl, Br) [46,47]. The category with the higher number of items (212) is that of complexes formally featuring two penta-coordinated Pt II ions, which can be envisaged as two squareplanar complexes interacting through long Pt-Pt bonds ranging from 2.53 to 3.41 Å, with a mean value as long as 2.94 Å and an overall distorted square-based pyramidal environment for both metal ions.…”

“…The complex [Pt(L 3 )(µ-1,3-MeCONH)PtCl(MeCN)](BF 4 ) 2 •H 2 O belongs to the very unusual category of discrete Pt-dimers, featuring one square-planar and one octahedral platinum center connected by a metal-metal bond (d 9 -d 7 , Pt4-Pt6 red column in Figure 9) and formally considered as mixed-valence Pt I -Pt III systems. It is interesting to note that the only two examples known of binuclear complexes formally sharing a Pt I -Pt III bond, namely, [Pt 2 I,III (tfepma) 2 X 4 ] [X = Cl, Br; tfepma = bis(bis(trifluoroethoxy)-phosphino)methylamine], contain the same trifluoroethyl-imidophosphito ligand bridging the metal centers counterbalanced by halides that complete the platinum coordination spheres [46,47]. Our compound would be the first example supported by a macrocyclic ligand that does not bridge the two metal centers.…”

“…Within the variety of DFT methods available, the broken symmetry approach (DFT-BS) proposed by Noodleman [65][66][67], widely used in the field of molecular magnetism for the calculation of exchange coupling constants in multinuclear complexes [68], allows unrestricted calculations for low-spin open-shell molecular systems, in which the α and β electrons are allowed to be localized on different atomic centers. This approach was applied to the complex cation [Pt(L 3 )(µ-1,3-MeCONH)PtCl(MeCN)] 2+ and the previously reported related system [Pt 2 (tfepma) 2 Cl 4 ] [46,47] in an attempt to elucidate the oxidation state of the platinum ions in these species. For the latter, a mixed-valence d 9 -d 7 Pt I -Pt III nature was proposed for the binuclear system.…”

“…Theoretical calculations were performed on the complex cation [Pt(L 3 )(µ-1,3-MeCONH) PtCl(MeCN)] 2+ and on [Pt 2 (tfepma) 2 Cl 4 ] (tfepma = ((CF 3 CH 2 O) 2 P) 2 NCH 3 ) [46,47] at the density functional theory (DFT) [56] level with the Gaussian 16 (Rev. B.01) suite of programs [78], on a IBM x3755 server with four 12-core processors and 64 Gb of RAM (OS: SUSE Linux Enterprise Server 11 SP3).…”

Photohalogen elimination chemistry in low-valent binuclear nickel complexes

“…[1][2][3] Although HXsplitting is only a two electron and two proton processes, the design of a catalytic system that can simultaneously execute the photoelmination of X 2 from M-X bonds and H 2 evolution from HX efficiently on one discrete metal complex constitutes a difficult challenge owing to the more thermodynamically and kinetically demanding halogen photoelimination step, which is the bottleneck to closing HX-splitting photochemical cycles. 4 Successful M-X photoactivation has primarily been relegated to 2 nd and 3 rd row mono-and bi-nuclear metal complexes, [5][6][7][8][9][10][11][12] and main group complexes, [13][14][15][16][17][18] with more limited success involving metal complexes of the 1 st row. [19][20][21][22] Moving to the 1 st row transition elements for photochemical HX-splitting platforms presents the challenge that their complexes often exhibit short excited state lifetimes because of fast, energy wasting, internal conversion processes.…”