Transient Triplet Metallopnictinidenes M-Pn (M = PdII, PtII; Pn = P, As, Sb): Characterization and Dimerization

Abstract

Nitrenes (R-N) have been subject to a large body of experimental and theoretical studies. The fundamental reactivity of this important class of transient intermediates has been attributed to their electronic structures, particularly the accessibility of triplet vs singlet states. In contrast, electronic structure trends along the heavier pnictinidene analogues (R-Pn; Pn = P-Bi) are much less systematically explored. We here report the synthesis of a series of metallodipnictenes, {M-Pn═Pn-M} (M = Pd^II, Pt^II; Pn = P, As, Sb, Bi) and the characterization of the transient metallopnictinidene intermediates, {M-Pn} for Pn = P, As, Sb. Structural, spectroscopic, and computational analysis revealed spin triplet ground states for the metallopnictinidenes with characteristic electronic structure trends along the series. In comparison to the nitrene, the heavier pnictinidenes exhibit lower-lying ground state SOMOs and singlet excited states, thus suggesting increased electrophilic reactivity. Furthermore, the splitting of the triplet magnetic microstates is beyond the phosphinidenes {M-P} dominated by heavy pnictogen atom induced spin-orbit coupling.

Figure 1

Selected examples of nitrenes, R–N (a) and heavier pnictinidenes, R–Pn (b) versus the metallopnictinidenes presented here (c).

Scheme 1. Photoinduced Pnictide Coupling; Reaction Conditions: (a) 2^Pd,P: λ = 456 nm, Benzene, 3 h, Room Temperature, 87% Yield; (b) 2^Pt,P: see ref (28); (c) 2^Pd,As: λ = 525 nm, THF, 90 min, 0 °C, 82% Yield; (d) 2^Pt,As: λ = 525 nm, Toluene, 30 min, −30 °C; 87% Yield; (e) 2^Pt,Sb: λ = 370 nm, 2-Methyl THF, 6 h, −196 °C; 84% Yield; (f) 2^Pt,Bi: λ = 370 nm, Toluene, 16 h,–40 °C, 25% Yield

Figure 2

Molecular structure of 1^Pt,As (left) and 1^Pt,Bi (right) with thermal ellipsoids at the 50% probability level. H atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]:1^Pt,As Pt1–As1:2.455(1); Pt1–As1–C21:99.0(3); 1^Pt,Bi: Pt1–Bi1:2.7323(3); Pt1–Bi1–C21:99.69(15), Pt1–Bi1–C22:117.53(14), C21–Bi1–C22:91.7(2).

Figure 3

Molecular structures and selected structural parameters of the dipnictenes with thermal ellipsoids at the 50% probability level. H atoms and disordered atoms are omitted and ^tBu-groups represented as sticks for clarity. Structural parameters of 2^Pt,P, [2^Pt,P]⁺, and [2^Pt,P]²⁺ taken from ref (28).

Figure 4

(a) Synthesis of radical complex [2^Pt,As]⁺. (b) Cyclic voltammogram of 2^Pt,As in PhF. (c) Continuous wave Q-band EPR spectrum (black) and simulation (red with simulation parameters) of [2^Pt,As]⁺ in MeTHF at 40 K. (d) NPA spin population of [2^Pt,As]⁺.

Figure 5

Overlays of the molecular structures of the precursor complexes and the respective metallopnictinidenes from crystal-to-crystal transformation experiments; thermal ellipsoids at the 50% probability level. H atoms are omitted and ^tBu-groups drawn as sticks for clarity. Selected bond lengths [Å]: 3^Pd,As Pd1–As1A, 2.349(13); 3^Pt,P Pt1–P3A, 2.25(4); 3^Pt,As Pt1–As1A 2.36(3).

Figure 6

(a) UV/vis spectra of the metallopnictinidenes 3^Pt,Pn after photolysis of the precursors 1^Pt,Pn in frozen MeTHF at 77 K. (b) DFT computed transition difference densities (green: electron depopulation, red: electron population) of the respective transitions of 3^Pt,P. (c) Assignment of the experimental data for the series 3^Pt,Pn (Pn = N–Sb) with TD-DFT computed data in parentheses.

Figure 7

(a) DFT computed spin density of 3^Pt,As and selected NLMOs (cf. Supporting Information for other pnictinidenes). The doubly occupied NLMOs are obtained by averaging over the α and β spin orbitals. (b) NPA charges and spin populations of the pnictide ligands of 3^M,Pn.

Figure 8

(a) SQUID magnetometric characterization of in situ formed 3^Pd,P, 3^Pd,As and 3^Pt,P, 3^Pt,As; left: experimental (circles) and simulated (colored lines) χ_mT vs T data; right: comparison of simulated (colored lines) and ab initio computed (dotted lines) χ_mT vs T data. (b) THz-EPR data (black lines) and numerical simulations (red lines; cf. Supporting Information for details) of 3^Pd,As. (c) THz-EPR data (black lines) and numerical simulations (red lines; cf. Supporting Information for details) of 3^Pt,As. (d) Comparison of axial zero-field splitting parameters D from SQUID magnetometry and THz-EPR (in paratheses) with ab initio computed values (cf. Supporting Information for details).