Experimental and theoretical study of a gold(I) aminonitrene complex in the gas phase.

Metal-coordinated nitrenes are recognized to be reactive intermediates in aziridination and insertion reactions. Among other metals, the coinage metal triad (Cu, Ag, Au) was demonstrated to be active in catalyzing these important transformations. In particular, He and co-workers recently showed that nitrene transfer could efficiently be accomplished relying on gold. While copper nitrene intermediates have been structurally characterized, we are not aware of any reports on monometallic gold analogues. Having successfully applied a phosphorus ylid-based strategy for the preparation of transient gold(I) benzylidene carbenes inside a modified Finnigan MAT TSQ-700 mass spectrometer, we were motivated to implement a similar approach to the gas-phase synthesis of a metal-bound nitrene species. The triflate salt of IMes-supported (IMes=1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) phosphazene gold adduct 1-OTf was prepared as an airand moisture-stable solid in 72% yield (Scheme 1). Upon electrospray, the thermalized parent ion current (m/z 821) was directed into the argon-filled chamber for the collision-induced dissociation (CID) event (Figure 1). Detachment of the labile PPh3 group proceeds smoothly as the only reaction channel to afford a signal with 559 m/z ratio, which we ascribe to the singlet (vide infra) gold aminonitrene 2s. Energy-resolved reactive cross-section measurements under near-single-collision conditions were conducted for this process in order to quantitatively determine the N P bond strength (Figure 2). The L-CID algorithm for the cross-section data fitting requires an input assumption about the properties of the transition state. It generally holds true that simple dissociation, without prior rearrangement, of a neutral fragment from an ion occurs via a loose transition state, that is, with no reverse activation barrier. Standard L-CID treatment of the process 1!2s + 3 gave a value of 45.0 2.7 kcalmol 1 assuming a loose transition state. We subsequently modeled the PPh3 detachment at the DFT level of theory (see Computational Section and Supporting Information for details) to confirm the assignment of the daughter ion channel to 2s and to rule out possible rearrangements and spin isomerism. A priori a number of events could precede nitrene formation (Scheme 2). One may speculate that a 1,2-triphenylphosphine shift connects 1 and 4s and formation of 2s could then be accomplished via PPh3 dissociation from the tricoordinate gold species. However, 1,1-dimethylnitrene 6s is only weakly bound to the gold atom in 4s with an Au N distance of 2.867 and thus is predisposed to dissociation producing 5. Since species 5 is not observed experimenScheme 1. Synthesis of the triflate adduct 1-OTf. Figure 2. Energy-resolved cross sections recorded at various Ar gas pressures, linear extrapolation to zero pressure (red circles) and L-CID fit (black line) for loss of PPh3 from 1.