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C-H bond activation was observed in a novel PCO ligand 1 (C(6)H(CH(3))(3)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))) at room temperature in THF, acetone, and methanol upon reaction with the cationic rhodium precursor, [Rh(coe)(2)(solv)(n)()]BF(4) (solv = solvent; coe = cyclooctene). The products in acetone (complexes 3a and 3b) and methanol (complexes 4a and 4b) were fully characterized spectroscopically. Two products were formed in each case, namely those containing uncoordinated (3a and 4a) and coordinated (3b and 4b) methoxy arms, respectively. Upon heating of the C-H activation products in methanol at 70 degrees C, C-C bond activation takes place. Solvent evaporation under vacuum at room temperature for 3-4 days also results in C-C activation. The C-C activation product, ((CH(3))Rh(C(6)H(CH(3))(2)(CH(2)OCH(3))(CH(2)P(t-Bu)(2))BF(4)), was characterized by X-ray crystallography, which revealed a square pyramidal geometry with the BF(4)(-) anion coordinated to the metal. Comparison to the structurally similar and isoelectronic nonchelating Rh-PC complex system and computational studies provide insight into the reaction mechanism. The reaction mechanism was studied computationally by means of a two-layer ONIOM model, using both the B3LYP and mPW1K exchange-correlation functionals and a variety of basis sets. Polarization functions significantly affect relative energetics, and the mPW1K profile appears to be more reliable than its B3LYP counterpart. The calculations reveal that the electronic requirements for both C-C and C-H activation are essentially the same (14e intermediates are the key ones). On the other hand, the steric requirements differ significantly, and chelation appears to play an important role in C-C bond activation.

1,8-Bis(diphenylphosphino)anthracene (1) was prepared in a three-step synthesis in 51 % overall yield starting from 1,8-dichloro-9,10-anthraquinone (8). Compound 8 was converted by chlorinefluorine exchange and reduction with zinc into 1,8-difluoroanthracene (7) from which 1 was obtained by reaction with potassium diphenylphosphide. The conversion of 7 with lithium dimethylamide to 1,8-bis(dimethylamino)anthracene (10) clearly showed that in the case of 7 direct nucleophilic displacement (addition-elimination mechanism) dominates over aryne formation (elimination-addition mechanism). Single crystal X-ray structure analyses are reported for 1 and 10. Previouslywe reported the synthesis of the 1,8-bis(diphenylphosphino)anthracene (1) and its reaction with nickel or palladium dichloride forming cyclometallated and hence extremely stable complexes 2 and 3. 2 As a result of the cyclometallation at the anthracene C-9 atom, the diphosphine 1 is acting as a tridentate PCP ligand in these complexes. Whereas we first had failed to prepare the diphosphine 1 from either 1,8-dichloro-or 1,8-dibromoanthracene, 1 was obtained in moderate yield of 33 % by reaction of potassium anthracenedisulfonate (4) with two equivalents potassium diphenylphosphide (Ph 2 PK) in diethylene glycol diethyl ether (DEGDEE) under harsh reaction conditions (180°C, 20 h). 2,3 The disulfonate 4 needed for this conversion was prepared by zinc reduction of potassium 9,10-anthraquinone-1,8-disulfonate. 4 However, since the latter compound apparently is not commercially available anymore, it was desirable to search for a new and improved synthesis of the diphosphine 1.In a recent paper we described the reaction of 4,5-difluoroacridine (5) with potassium diphenylphosphide (Ph 2 PK) giving 4,5-bis(diphenylphosphino)acridine (6) in 79 % yield. 5 This prompted us to attempt the synthesis of 1 from 1,8-difluoroanthracene (7) and to investigate the use of fluoro substituted polycyclic aromatic hydrocarbons in nucleophilic aromatic substitution reactions with alkali metal phosphides and other nucleophiles. 6The unknown 1,8-difluoroanthracene (7) was prepared from the commercially available 1,8-dichloro-9,10-anthraquinone (8) in a twostep synthesis. Following a literature procedure 8 was converted by reaction with cesium fluoride in dimethyl sulfoxide under anhydrous conditions into 1,8-difluoro-9,10-anthraquinone (9) [m.p. = 228°C (DSC, lit. 7 m.p. = 228-229°C), 71 %]. 7 Reduction of 9 with zinc powder in aqueous ammonia (75°C, 4 h) followed by an acidic treatment (aqueous HCl, 2-propanol, reflux, 3 h) yielded 7 [yellow needles, m.p. = 142°C (DSC), 84%]. 8,9 As anticipated compound 7 proved to be highly reactive for nucleophilic displacement of the fluoro substituents by alkali metal phosphides. Reaction with two equivalents potassium diphenylphosphide (Ph 2 PK, dioxane/THF 5:1 reflux, 3 h) 10 converted 7 in excellent yield to the diphosphine 1 [yellow needles, m.p. = 234°C (lit. 2 m.p. = 233-235°C), 86 %]. For the three-step synthesis starting from commercial 8...

Thermostability is “conditio sine qua non” (a necessity) if organometallic compounds are used in the homogeneous catalysis of endothermic processes such as the dehydrogenation of alkanes to alkenes and hydrogen (see scheme). This thermostability and other requirements for such a catalyst are fulfilled by the anthraphos iridium(III)dihydride complex 1.