A series of the extremely crowded triarylpnitogens, tris(2,4,6-triisopropylphenyl)phosphine (1), arsine (2), stibine (3), and bismuthine (4), were synthesized by the reaction of 2,4,6-triisopropylphenylcopper(I) with the corresponding pnictogen trichlorides. Introduction [corrected] of the three bulky aryl groups resulted in the unusual structures and redox properties, which were studied by X-ray crystallography and cyclic voltammetry. The triarylpnictogens 1, 2, and 3 had extremely large bond angles around pnictogen atoms (1: 111.5 degrees , 2: 109.2 degrees , 3: 106.7 degrees ), and not only 1 and 2, but also stibine 3 displayed a reversible redox wave in the cyclic voltammograms at very low potentials (1: 0.16 V, 2: 0.50 V, 3: 0.57 V vs Ag/Ag+), which suggests considerable stability of the corresponding cation radicals.

Crowded triarylphosphines possessing ferrocenyl groups [(4-ferrocenyl-2,6-diisopropylphenyl)(n)(2,4,6-triisopropylphenyl)(3-n)P (5a, n = 1; 5b, n = 2; 5c, n = 3)] were synthesized by the reaction of the corresponding arylcopper(I) reagents with the diarylchlorophosphines. Structures of the triarylphosphines were studied by 1H, 13C, and 31P NMR spectroscopies, and the characteristic patterns of the proton signals of the 2,6-isopropyl groups and upfielded 31P chemical shifts suggest structural similarities of the triarylphosphine moiety to the previously reported tris(2,4,6-triisopropylphenyl)phosphine (2). X-ray crystallography of 5c also revealed that the structure around the phosphorus is similar to that of 1, where the average bond angle and length around the phosphorus atom are 110.8 degrees and 1.854 A, respectively. According to the electrochemical measurements, phosphines 5a, 5b, and 5c are reversibly oxidized in two, three, and four steps, respectively, which suggests significant electronic interaction among the triarylphosphine and the ferrocene redox centers as well as weak interaction among the ferrocene redox centers. The EPR spectra obtained at cryogenic temperature after oxidation of phosphines 5a, 5b, and 5c are superpositions of those for the cation radicals of the crowded triarylphosphine and ferricinium. The solution spectra obtained at 293 K, which consist of two lines typical of the cation radical of the crowded triarylphosphines, become weaker as the number of the ferrocenyl groups increases and the cation radical of 5c does not show EPR signals. These findings suggest not only instability of the tetra(cation radical) of 5c but also the course of oxidation where the ferrocenyl groups in the periphery of the molecules are oxidized at first.

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Dedicated to Professor Dr. Dieter Seebach on the occasion of his 65th birthday Bis{4-[bis(4-methoxyphenyl)amino]-2,6-bis(2,4,6-triisopropylphenyl)phenyl}diphosphene (1), possessing two bis(4-methoxyphenyl)amino groups as redox sites as well as electron-donating sources, was synthesized and isolated as a red solid. The cyclic voltammogram of 1 at À 788 consisted of three reversible redox waves corresponding to two-step oxidation of the triarylamine moieties and reduction of the diphosphene moiety. Introduction of the two amino groups also contributed to a red shift of the absorption maximum in the UV/VIS spectrum, which was responsible for the intense red color of 1.Introduction. ± Sterically protected diaryldiphosphenes [1] generally have an intense orange color and are reduced to the corresponding radical anion at a moderate potential [2]. Recently, we have been interested in molecules that have a PP bond as a redox center as well as chromophore, and prepared the model compound, {2,6-dimesityl-4[bis(4-methoxyphenyl)amino]phenyl}[2,4,6-tri(tert-butyl)phenyl]diphosphene (2), which has the bis(4-methoxyphenyl)amino group as a reversible redox center [3]. Diphosphene 2 was revealed to be a two-step reversible redox system at low temperature due to oxidation to the aminium radical cation and reduction to the diphosphene radical anion. In addition, a red shift of the electronic spectra was observed as compared with typical diphosphenes. However, oxidation to the aminium radical cation became irreversible at room temperature, and the 2,6-dimesityl-4-[bis(4-methoxyphenyl)amino]phenyl group did not allow isolation of a symmetrical diphosphene. To further stabilize the redox system as well as the PP bond, we employed the 4-[bis(4-methoxyphenyl)amino]-2,6-bis(2,4,6-triisopropylphenyl)phenyl group, since the 2,6-bis(2,4,6-triisopropylphenyl)aryl group has been utilized as a sterically protecting group for diphosphenes [4] and is expected to be more sterically demanding than 2,6-dimesitylaryl groups. Herein, we report the synthesis, redox properties, and electronic spectra of bis{4-[bis(4-methoxyphenyl)amino]-2,6-bis(2,4,6-triisopropylphenyl)phenyl}diphosphene (1).