The o-phenylene bridged phosphine-borane iPr2P­(o-C6H4)­B­(Fxyl)2 2 was prepared. Despite ring strain, it adopts a closed form, as substantiated by NMR, XRD, and DFT analyses. However, the corresponding open form is only slightly higher in energy. The dormant Lewis pair 2 proved to efficiently catalyze the dehydrogenation of a variety of amine- and diamine-boranes under mild conditions. The corresponding phosphonium-borate iPr2PH­(o-C6H4)­BH­(Fxyl)2 3 was authenticated as a key intermediate of these dehydrogenation reactions. The propensity of 3 to release H2 plays a major role in the catalytic turnover.

Exposure of a solution of the square pyramidal tungstacyclopentane complex W­(NAr)­(OSiPh3)2(C4H8) (Ar = 2,6-i-Pr2C6H3) to ethylene at 22 °C in ambient (fluorescent) light slowly leads to the formation of propylene and the square pyramidal tungstacyclobutane complex W­(NAr)­(OSiPh3)2(C3H6). No reaction takes place in the dark, but the reaction is >90% complete in ∼15 min under blue LED light (∼450 nm λmax). The intermediates are proposed to be (first) an α methyl tungstacyclobutane complex (W­(NAr)­(OSiPh3)2(αMeC3H5)), and then from it, a β methyl version. The TBP versions of each can lose propylene and form a methylene complex, and in the presence of ethylene, the unsubstituted tungstacyclobutane complex W­(NAr)­(OSiPh3)2(C3H6). The W–Cα bond in an unobservable TBP W­(NAr)­(OSiPh3)2(C4H8) isomer in which the C4H8 ring is equatorial is proposed to be cleaved homolytically by light. A hydrogen atom moves or is moved from C3 to the terminal C4 carbon in the butyl chain as the bond between W and C3 forms to give the TBP α methyl tungstacyclobutane complex. Essentially, the same behavior is observed for W­(NCPh3)­(OSiPh3)2(C4H8) as for W­(NAr)­(OSiPh3)2(C4H8), except that the rate of consumption of W­(NCPh3)­(OSiPh3)2(C4H8) is about half that of W­(NAr)­(OSiPh3)2(C4H8). In this case, an α methyl-substituted tungstacyclobutane intermediate is observed, and the overall rate of formation of W­(NCPh3)­(OSiPh3)2(C3H6) and propylene from W­(NCPh3)­(OSiPh3)2(C4H8) is ∼20 times slower than in the NAr system. These results constitute the first experimentally documented examples of forming a metallacyclobutane ring from a metallacyclopentane ring (ring contraction) and establish how metathesis-active methylene and metallacyclobutane complexes can be formed and reformed in the presence of ethylene. They also raise the possibility that ambient light could play a role in some metathesis reactions that involve ethylene and tungsten-based imido alkylidene olefin metathesis catalysts, if not others.

Upon addition of 5−15% PhNMe 2 H + X − (X = B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 or B(C 6 F 5 ) 4 ) to Mo(NAr)(styrene)(OSiPh 3 ) 2 (Ar = N-2,6-i-Pr 2 C 6 H 3 ) in C 6 D 6 an equilibrium mixture of Mo(NAr)-(styrene)(OSiPh 3 ) 2 and Mo(NAr)(CMePh)(OSiPh 3 ) 2 is formed over 36 h at 45 °C (K eq = 0.36). A plausible intermediate in the interconversion of the styrene and 1-phenethylidene complexes is the 1-phenethyl cation, [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 ] + , which can be generated using [(Et 2 O) 2 H][B(C 6 F 5 ) 4 ] as the acid. The interconversion can be modeled as two equilibria involving protonation of Mo(NAr)(styrene)(OSiPh 3 ) 2 or Mo(NAr)(CMePh)(OSiPh 3 ) 2 and deprotonation of the α or β phenethyl carbon atom in [Mo(NAr)-(CHMePh)(OSiPh 3 ) 2 ] + . The ratio of the rate of deprotonation of [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 ] + by PhNMe 2 in the α position versus the β position is ∼10, or ∼30 per H β . The slow step is protonation of Mo(NAr)(styrene)(OSiPh 3 ) 2 (k 1 = 0.158(4) L/(mol• min)). Proton sources such as (CF 3 ) 3 COH or Ph 3 SiOH do not catalyze the interconversion of Mo(NAr)(styrene)(OSiPh 3 ) 2 and Mo(NAr)(CMePh)(OSiPh 3 ) 2 , while the reaction of Mo(NAr)(styrene)(OSiPh 3 ) 2 with pyridinium salts generates only a trace (∼2%) of Mo(NAr)(CMePh)(OSiPh 3 ) 2 and forms a monopyridine adduct, [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 (py)] + (two diastereomers). The structure of [Mo(NAr)(CHMePh)(OSiPh 3 ) 2 ] + has been confirmed in an X-ray study; there is no structural indication that a β proton is activated through a CH β interaction with the metal. W(NAr)(CMePh)(OSiPh 3 ) 2 is also converted into a mixture of W(NAr)(CMePh)(OSiPh 3 ) 2 and W(NAr)(styrene)(OSiPh 3 ) 2 (K eq = 0.47 at 45 °C in favor of the styrene complex) with 10% [PhNMe 2 H][B(C 6 F 5 ) 4 ] as the catalyst; the time required to reach equilibrium is approximately the same as in the Mo system.