Modulating the frontier orbitals of L(X)Ga-substituted diphosphenes [L(X)GaP] (X = Cl, Br) and their facile oxidation to radical cations

• Published in: Chemical science (Cambridge) 2022-11, Vol.13 (43), p.12643-1265
• Main Authors: Sharma, Mahendra K, Chabbra, Sonia, Wölper, Christoph, Weinert, Hanns M, Reijerse, Edward J, Schnegg, Alexander, Schulz, Stephan
• Format: Article
• Language: English
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/d2sc04207j

Modulating the electronic structures of main group element compounds is crucial to control their chemical reactivity. Herein we report on the synthesis, frontier orbital modulation, and one-electron oxidation of two L(X)Ga-substituted diphosphenes [L(X)GaP] 2 (X = Cl 2a , Br 2b ; L = HC[C(Me)N(Ar)] 2 , Ar = 2,6- i- Pr 2 C 6 H 3 ). Photolysis of L(Cl)GaPCO 1 gave [L(Cl)GaP] 2 2a , which reacted with Me 3 SiBr with halide exchange to [L(Br)GaP] 2 2b . Reactions with Me NHC ( Me NHC = 1,3,4,5-tetramethylimidazol-2-ylidene) gave the corresponding carbene-coordinated complexes L(X)GaPP( Me NHC)Ga(X)L (X = Cl 3a , Br 3b ). DFT calculations revealed that the carbene coordination modulates the frontier orbitals ( i.e. HOMO/LUMO) of diphosphenes 2a and 2b , thereby affecting the reactivity of 3a and 3b . In marked contrast to diphosphenes 2a and 2b , the cyclic voltammograms (CVs) of the carbene-coordinated complexes each show one reversible redox event at E 1/2 = −0.65 V ( 3a ) and −0.36 V ( 3b ), indicating their one-electron oxidation to the corresponding radical cations as was confirmed by reactions of 3a and 3b with the [FeCp 2 ][B(C 6 F 5 ) 4 ], yielding the radical cations [L(X)GaPP( Me NHC)Ga(X)L]B(C 6 F 5 ) 4 (X = Cl 4a , Br 4b ). The unpaired spin in 4a (79%) and 4b (80%) is mainly located at the carbene-uncoordinated phosphorus atoms as was revealed by DFT calculations and furthermore experimentally proven in reactions with n Bu 3 SnH, yielding the diphosphane cations [L(X)GaPHP( Me NHC)Ga(X)L]B(C 6 F 5 ) 4 (X = Cl 5a , Br 5b ). Compounds 2-5 were fully characterized by NMR and IR spectroscopy as well as by single crystal X-ray diffraction (sc-XRD), and compounds 4a and 4b were further studied by EPR spectroscopy, while their bonding nature was investigated by DFT calculations. Carbene-coordination allowed for one-electron oxidation of diphosphenes [LGa(X)P] 2 to P-centered radicals cations 4a (X = Cl) and 4b (X = Br), in which the unpaired spin mainly reside at the carbene uncoordinated P-atoms.