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. The few reactions of this type with a first-row transition-metal complex are limited to vanadium. 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. 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. Solutions of terminal chromium nitride complexes in noncoordinating solvents treated with electrophiles such as B(C6F5)3 or C(C6H5)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= =2 spin species. Similar reactivity was observed in reactions with either [Rh(cod)Cl]2 or cis-[PtCl2(dmso)2] (cod= 1,5cyclooctadiene, 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 andmetric parameters of complexes 1–5 (Scheme 1) are available in the Supporting Information (Tables S1 and S1a). 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 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 nitride complex, while they are shorter or similar within the limits of uncertainty in complex 5. The B N and C N bonds in 1, 4, and 5 are unexceptional but the N Rh and N Pt bond lengths in 2 and 3 are at about 1.970 and 1.906 , respectively, and very short; the first value belongs to the top 5% of the shortest Rh N bonds and the second belongs to the top 1% of the shortest Pt N bonds. Table 1 compares the Pt N bond of 3 with Pt N bonds of other cis[PtCl2(dmso)L] structures. Scheme 1. Schematic representation of the chromium(V) imide and chromium(V) bridging-nitride complexes.