Delocalized Metal–Metal and Metal–Ligand Multiple Bonding in a Linear RuRuN Unit: Elongation of a Traditionally Short RuN Bond

Abstract

Binuclear metal-metal bonded complexes such as [Rh 2 -(OAc) 4 ] (Scheme 1, A) are celebrated because of their unmatched ability to catalyze reactions that directly functionalize CÀH bonds.[1] These metal-metal bonded catalysts function by assisting the transfer of a carbene or nitrene group (CR 2 or NR, respectively) to an organic substrate. [1] The key intermediates in these C À H activation reactions are proposed to have structures such as B (Scheme 1) that feature both a metal-metal bond and a metal-ligand multiple bond. Despite many years of mechanistic study on these reactions, no multiply bonded species, such as B, has, to our knowledge, ever been isolated and characterized.[2] In order to synthesize an M À M = E metal-metal/metal-ligand multiply bonded system, we chose to target a RuÀRuN nitrido complex (Scheme 1, C) for which no structural precedents exist. This species could be synthesized from thermal or photolytic decomposition of the appropriate RuÀRuÀN 3 azido precursor.[3] This experimental strategy is attractive because synthetically useful [Ru 2 (L) 4 X] compounds (L = ligand) are well known [4] and also because Ru is known to stabilize mononuclear nitrido Ru VI complexes [5] that form a useful comparison to C.We used the previously reported azido compound [Ru 2 -(dPhf) 4 N 3 ] (2, dPhf = N,N'-diphenylformamidinate) [6] as a precursor for the photoreaction (Scheme 2), since it has intense absorption bands at 520 nm and 660 nm.[6] The nitrido complex, [Ru 2 (dPhf) 4 N] (1) was detected in positive-ion mode MALDI-TOF experiments on 2. At low laser power, only one set of peaks with the characteristic distribution of natural Ru isotopes can be seen, centered at 984.1, which is assigned as [2ÀN 3 ] + whereas, at higher laser power, a new set of peaks is detected at 14 mass units higher (998.1), that is ascribed unequivocally to 1 + , as is further supported by the calculated isotopic pattern. [7] Owing to its quartet ground state, the X-band EPR spectrum of 2 at 8 K features a distinct S = 3/2 signal at g eff = 4.22. Photolysis of 2 at À40 8C produces a new sharp axial S = 1/2 EPR signal, described below, that is unmistakably due to a highly reactive intermediate species because it decays rapidly at À40 8C.Photolysis of frozen samples of 2 in CH 2 Cl 2 at 77 K allowed us to maximize the yield of this new species, and to observe an accompanying color change from purple to pink upon its formation (UV/Vis spectroscopic data are given in the Supporting Information). The new axial EPR signal (Figure 1) clearly indicates an S = 1/2 species that we suggest is due to the photooxidation product [Ru 2 (dPhf) 4 N] (1), because photooxidation formally removes two electrons from the {Ru 2 } moiety, leaving only one unpaired electron located in an Ru 2 d* orbital (see below). Simulation of the EPR data yielded g tensor components, g ? = 2.189 and g k = 1.900, consistent with axial molecular symmetry. The g k value < 2.00 suggests spin-orbit coupling with vacant d-orbital-based molecular orbitals, that is, a higher ...

Terminal Nitrido and Imido Complexes of the Late Transition Metals

“…We have recently used this concept to design a new type of reactive Ru-nitrido complex, 5, which has an axial RuÀRu bond trans to the nitride, resulting in an extraordinarily weak RuN bond. [50] The resulting structure of this RuÀRuN complex has a RuN bond length that lies between what is observed for mononuclear Ru(VI) nitrido complexes and for N atoms bound to a bulk Ru metal surface ( Figure 3, Scheme 6).…”

“…It is suggested that the Rh=NR bonding in these intermediates is weakened via electron delocalization, similar to what occurs in 5. [50] …”

“…Consequently, structural information on 5 was only obtainable by EXAFS methods. [50] Although decomposition products from 5 have not yet been identified, it is likely that they result from reactivity associated with the weakly-bound nitrido ligand.…”

“…[51,[252][253][254][255] Regardless of the specific catalyst used, however, a nitrenoid species such as XVIII is invariably invoked as being the key reactive intermediate in the catalytic cycle. [51,252] Although such a species has never been isolated, recently we have reported a metal-metal bonded nitrido complex with a related linear RuÀRuN structure; [50] this compound is the closest structural analog to XVIII that has yet been prepared. Coordinated nitrenoid intermediates of unspecified geometry have also been proposed in amidation reactions catalyzed by palladium(II) acetate, [256] and in the reduction of organo-nitrogen compounds by palladium clusters.…”

Diruthenium σ-iminophenylacetylide complexes from on-complex Schiff base condensation

“…While the research during the ensuing 40þ years has focused on the structure and bonding [26,27], recent years have witnessed new development in molecular magnetism [28e31], small molecule activation [32,33] and catalysis [34e36]. Described in this contribution, as the first step toward organometallic wires containing an imine moiety, are the preparation of compounds 3 and 4 (Scheme 1) by the condensation reaction between [Ru 2 (ap) 4 ]e(C^CeC 6 H 4 e4/3eNH 2 ) (compounds 1/2) [37] and arylaldehydes and their characterization.…”