Heterometallic Cube‐Type Molecular Nitrides

Abstract

Polynuclear transition metal nitrido complexes constitute a class of molecular cage compounds with fascinating structures and interesting bonding properties. However, there is a lack of systematic strategies for the rational construction of aggregates with desired structure and composition. This article provides a brief overview of the structure and bonding modes of polynuclear nitrido complexes, the most common synthetic approaches used to generate such aggregates, and a systematic review of the development of a family of heterometallic nitrido complexes with [MTi3N4] cube-type cores. The rational entry to those well-defined systems is based on the incorporation of transition, lanthanides, and main-group metals into the incomplete cube structure of the trinuclear titanium(IV) imido-nitrido complex [{TiCp*(-NH)}3(3-N)] (Cp* =  5 -C5Me5). The great versatility of [{TiCp*(-NH)}3(3-N)] as a preorganized metalloligand is also confirmed by the possibility of preparing complexes with heterometals in low (e.g. Mo 0 , Ir I and Pt 0 ) or high oxidation states (e.g. Zr IV , Ta V and Pt IV ).

Molecular Zirconium Nitride Super Base from a Mononuclear Parent Imide

“…31 Despite this, the 7 Li NMR spectrum of 4-15 N shows a doublet (in addition to a broad shoulder) due to coupling with nitrogen, 1 J NLi = 43.2 Hz (bottom right, Figure 2). Therefore, we are confident that coupling of the nitride with lithium ( 7 Li, I = 3/2, 92.58%) leads to significant broadening of the 15 N NMR spectroscopic signal. A 7 Li NMR spectrum of unlabeled 4 indeed shows a singlet and further corroborates the nitrogen/lithium coupling.…”

“…A weak ν(NH) stretch in IR spectrum at 3392 cm −1 could be assigned, which red-shifted to 3385 cm −1 for 3-15 N. 31 With 50% enriched 3-15 N, we confirmed the presence of the parent imido moiety at 351.4 ppm (referenced to liquid ammonia at 0.0 ppm, bottom left, Figure 2) using an INEPT 15 N NMR spectroscopic experiment. Without signal enhancement, this resonance was too broad to conclusively assign due to 91 Zr (I = 5/2, 11.22%) in addition to 50% loss of enriched 15 N. There are scarce examples of terminal parent imides of titanium reported in the literature, 22,24,25,34 and their 1 J NH values ( 1 J NH = 64.0 Hz) are coincident with those observed for 3-15 N. 22,25 A bridging Zr 2 (μ 2 -NH) moiety also has a nearly identical 1 J NH value to 3 or 3-15 N when prepared with enriched 15 N. 19 The XRD of 3 established a C 2 symmetric complex (Figure 1, right) with the phosphine and anilide moiety pairs in a trans orientation and with the metal ion being confined between square pyramidal and trigonal bipyramidal geometries (τ 5 = 0.69). 35 The imido H-atom was located on a difference map and refined isotropically with an N−H distance of 0.86(3) Å.…”

“…1 H and 31 P NMR spectra of 4 are in accordance with a C 2 symmetric system, but now in absence of the broad feature at 5.11 ppm ( 1 H NMR, vide supra) for the formal ZrNH moiety in 3. Surprisingly, attempts to observe a 15 N NMR spectroscopic feature for 50% labeled isotopomer {(PN) 2 Zr≡ 15 N[μ 2 -Li(THF)]} 2 , (4)- 15 N, were inconclusive. Computational studies suggest the NMR spectroscopic signal to reside quite downfield (745 ppm (B3LYP) and 684 ppm (TPSSh)), as expected, whereas a Zr≡N stretch in the IR was estimated to be at 884 cm −1 .…”

“…Although we recently reported molecular precursors to monomeric titanium nitrides, precursors for molecular zirconium nitrides, a class of compounds that can allow us to understand the bonding and structure for this ligand, have remained elusive. The challenge in accessing a multiply bonded zirconium nitride moiety is attributed to both the propensity of titanium and zirconium nitrides to bridge and oligomerize, − and also the lack of suitable N-atom transfer reagents that deliver a single N-atom selectively. Although one class of reagent that fits such criteria is the ubiquitous azide ligand, the lack of L n Zr­(II) precursors that can promote reduction with concurrent disproportionation of N 3 – to N 2 and N 3– make this approach more of an encumbrance.…”

Dinitrogen Reduction to Ammonium at Rhenium Utilizing Light and Proton-Coupled Electron Transfer

“…First, molecular systems capable of splitting the strong bond of N 2 into nitrides remain rare. One way to render NN bond splitting thermodynamically favorable is to form nitrides that bridge multiple metal centers. ,− Alternatively, transition metal complexes with the appropriate electronic structure to support strong metal–nitride multiple bonds can form terminal nitrides from N 2 . Following the lead discovery of Cummins ( A , Figure ), several molybdenum complexes have proven capable of thermal and photochemical N 2 cleavage. − ,− More recently, Schneider reported the first examples of thermal and photochemical N 2 splitting reactions at rhenium to form nitride complexes B and C (Figure ), respectively. , …”