Heterobimetallic Nitride Complexes from Terminal Chromium(V) Nitride Complexes: Hyperfine Coupling Increases with Distance

Abstract

Terminal nitride complexes of rhenium, osmium and molybdenum can form complexes with either alkylating agents, Lewis acidic metal halides, or low-valent, coordinatively unsaturated metal complexes. [1][2][3][4][5][6][7] The few reactions of this type with a first-row transition-metal complex are limited to vanadium. [8,9] Recently, the nitride chemistry of the chromium(V) cation has been significantly expanded by introduction of a preparative route which is based on nitrogen transfer from [Mn(N)(salen)] (salen = N,N'-bis(salicylidene)ethylenediamine) to the chromium(V) cation. [10][11][12] With a range of chromium nitride complexes at hand we have investigated their reactivity and found that nucleophilicity is a general property which can be observed during formation of imide complexes with, for example, the trityl cation, tris-(pentafluorophenyl)boron, and methyl triflate. In addition we report that terminal chromium(V) nitride complexes coordinate through the nitride ligand to low-valent complexes of the platinum metals. These compounds are possible precursors to bimetallic nitride phases which are gaining in importance as heterogeneous catalysts in, for example, the Haber-Bosch process. [13] Solutions of terminal chromium nitride complexes in noncoordinating solvents treated with electrophiles such as B(C 6 F 5 ) 3 or C(C 6 H 5 ) 3 + quickly yield intensely colored orangered or green solutions. The reactions proceed cleanly as shown by EPR spectra which display a signal from a single S = 1 = 2 spin species. Similar reactivity was observed in reactions with either [Rh(cod)Cl] 2 or cis-[PtCl 2 (dmso) 2 ] (cod = 1,5-cyclooctadiene, dmso = dimethyl sulfoxide). Structures of some of these systems, characterized by single-crystal X-ray diffraction, are shown in Scheme 1.Experimental and crystallographic details such as ORTEP drawings and metric parameters of complexes 1-5 (Scheme 1) are available in the Supporting Information (Tables S1 and S1 a). Inspection of the structures reveals a number of general aspects: there is a strong propensity for the chromium center to increase its coordination number from five to six upon coordination of the nitride ligand. This propensity is expected and a consequence of the trans influence of either an imide or a bridging nitride ligand which is significantly lower than that of a terminal nitride ligand. Accompanying this, the displacement of Cr out of the plane spanned by the equatorial ligators is diminished from about 0.5 to about 0.2 . The Cr À N bond length is elongated from 1.55 in the terminal nitride complexes to approximately 1.60-1.62 in the functionalized systems. Comparison of structure 1 with that of [Cr(N)-(salen)] reveals that the metal-salen ligand bonds are significantly shorter when the nitride ligand is functionalized, as expected when two ligands compete for electron donation. However, for the systems derived from [Cr(N)(dbm) 2 ] the situation is less clear (dbm = dibenzoylmethanolate). In complex 2 all the Cr-dbm bonds are longer than in the parent terminal nit...

Carbide complexes as π-acceptor ligands

“…Along these lines, the identification of {Os(N)} 3+ and {Cr(N)} 2+ as back-bonding ligands towards electron-rich metal centres was partially based on structural evidence. 28 , 59 …”

“…Similarities between carbide and nitride complexes as ligands might be expected based on the isolobal relationship between M C: and M N: moieties. A direct comparison is made possible by the fact that 1 , 59 , 69 – 72 2 , 71 , 73 , 74 8 , 59 , 75 and 11 ( ref. 40 ) have nitride-bridged analogues, [L n MN–RhCl(cod)], [L n MN–IrCl(cod)], [L n MN–PtCl 2 (dmso- S )], and [L n MN–AuCl], where the ligand spheres only differ by RuC being replaced by a terminal nitride complex ( Table 4 ).…”

“…Along these lines, the identication of {Os(N)} 3+ and {Cr(N)} 2+ as back-bonding ligands towards electron-rich metal centres was partially based on structural evidence. 28,59 The Cambridge Structural Database (CSD) allows a quanti-cation of whether the metal-carbide bonds in 1-12 are short. Fig.…”

“…8,59,75 and 11 (ref 40). have nitride-bridged analogues, [L n MN-RhCl(cod)], [L n MN-IrCl(cod)], [L n MN-PtCl 2 (dmso-S)], and [L n MN-AuCl], where the ligand spheres only differ by RuC being replaced by a terminal nitride complex (Table4).…”

Catalytic aziridination with alcoholic substrates via a chromium tetracarbene catalyst

“…The most prominent studies for Cr(V) have featured auxiliary ligands such as salen, porphyrin, or corroles. 34, 35 Notably, our recent research with a similar tetracarbene Cr(IV) imide complex demonstrates non-catalytic imide transfer. 36 We suspect that 1 is more reactive both due to the stronger donor NHCs and the key trans chloride ligand.…”