Aggregation and Degradation of White Phosphorus Mediated by N‐Heterocyclic Carbene Nickel(0) Complexes

Abstract The reaction of zerovalent nickel compounds with white phosphorus (P4) is a barely explored route to binary nickel phosphide clusters. Here, we show that coordinatively and electronically unsaturated N‐heterocyclic carbene (NHC) nickel(0) complexes afford unusual cluster compounds with P1, P3, P5 and P8 units. Using [Ni(IMes)2] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene], electron‐deficient Ni3P4 and Ni3P6 clusters have been isolated, which can be described as superhypercloso and hypercloso clusters according to the Wade–Mingos rules. Use of the bulkier NHC complexes [Ni(IPr)2] or [(IPr)Ni(η6‐toluene)] [IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene] affords a closo‐Ni3P8 cluster. Inverse‐sandwich complexes [(NHC)2Ni2P5] (NHC=IMes, IPr) with an aromatic cyclo‐P5 − ligand were identified as additional products.

Reactions of transition metal complexes with white phosphorus present a powerful strategy to access binary metal phosphide frameworks, and the structural motifs of the resulting compounds are highly diverse. [1,2] On the one hand, degradation of P 4 to products containing one to four phosphorus atoms is of tremendous industrial relevance, in order to improve the processes used in the production of organophosphorus compounds. [3] On the other hand, the aggregation of P 4 to polyphosphorus compounds with five or more phosphorus atoms is essential for understanding the structure and bonding in metal phosphides. [4] The use of nickel as a metal for P 4 activation may result in unique nickel phosphide clusters. Besides a few reactions of P 4 with Ni II species, for example, the formation of the sandwich compound [{(triphos)Ni} 2 (m 2 ,h 3:3 -cyclo-P 3 )](BF 4 ) 2 (triphos = Me(CH 2 CH 2 PPh 2 ) 3 ), [5] known examples typically involve Ni in the + I oxidation state. Cyclopentadienyl-substituted Ni I radicals are particularly versatile, as the outcome of photolysis or thermolysis reactions of nickel complexes of the type [Cp x Ni(CO) 2 ] 2 with P 4 is highly dependent on the size of the Cp ligand used. [6] Relatively small cyclopentadienyl ligands such as Cp*, Cp'' (1,3-tBu 2 C 5 H 3 ), or Cp''' (1,2,4- [7] In contrast to Ni I compounds, only a few examples of P 4 activation using Ni 0 sources have been reported ( Figure 1). [8][9][10] In seminal work dating back to 1979, Sacconi and co-workers reported the formation of the complex [(k 3 -P,P,P-NP 3 )Ni(h 1 -P 4 )] (A, NP 3 = tris(2-diphenylphosphinoethyl)amine) containing an intact, end-on coordinated P 4 tetrahedron. [8] Moreover, Le Floch and MØzailles reported on the use of [Ni(cod) 2 ] (cod = 1,4-cycloocta-1,5-diene) for the synthesis of nickel phosphide nanoparticles. [9] More recently, the group of Radius reported the synthesis of the butterfly compound [{Ni(ImiPr 2 ) 2 } 2 (m,h 2:2 -P 2 )] (C, ImiPr 2 = 1,3-bis(isopropyl)imidazolin-2-ylidene) by reaction of codstabilised Ni(ImiPr 2 ) 2 fragments with P 4 . [10] While these examples demonstrate both the coordination and degradation of P 4 by 14 valence electron (VE) and 18 VE Ni 0 compounds, examples of P 4 aggregation using Ni 0 appear to be unknown, Figure 1. a) Overview of products resulting from P 4 activation using Ni 0 sources; [8][9][10] b) P 4 activation and aggregation products described herein.
The molecular structure of 1 is reminiscent of the distorted kite-like cyclo-P 4 complex [(Cp'Fe) 2 (m-P 4 )] reported by Walter and co-workers. [16] However, 1 can be described as a bicapped trigonal bipyramid featuring a Ni 3 triangle with one short Ni2ÀNi3 bond (2.3720(3) ) and two long nickelnickel bonds (Ni1ÀNi2: 2.7533(3) and Ni1ÀNi3: 2.6528-(3) ). Ni 3 triangles are a common structure motif, for example, in carbonyl-or phosphine-stabilised clusters. [17] The Ni 3 triangle is capped by two phosphorus atoms P1 and P4. The P4 atom is part of a P 3 -chain with PÀP bond lengths of 2.1671(5) (P2ÀP3) and 2.1754(5) (P3ÀP4), which are in the range commonly observed for PÀP single bonds. Notably, the P 4 plane and the Ni 3 plane are almost perpendicular with a plane twist angle of 89.68.
Compound 1 can be isolated in pure form as a black crystalline solid in 20 % yield. As expected from analysis of the initial reaction mixture, 31 P{ 1 H} NMR measurements of pure 1 dissolved in C 6 D 6 revealed two signals at chemical shifts of 463.1 ppm (P1/P4) and 105.6 ppm (P2/P3, averaged J PP = 67.0 Hz), which are assigned to 1. Notably, the observation of just two 31 P{ 1 H} NMR resonances is in apparent contrast with the presence of four distinct P atom positions in the solid-state XRD structure of 1. An additional minor signal is observed at 134.0 ppm. This signal is assigned to an unidentified species, which may be an isomer of 1. A variable temperature (VT) NMR study showed that the integral ratio of signal P1/P4 to P2/P3 remains constant at 1:1, whereas the intensity of the signal at 134.0 ppm increases with higher temperatures and disappears upon cooling the solution to 283 K (see the Supporting Information for spectra). In order to understand this dynamic behaviour, DFT calculations were performed on a truncated model compound, where the mesityl substituents at the NHC moieties were replaced by phenyl groups. The calculations reproduce the asymmetric molecular structure of 1, but also reveal an isoenergetic isomer (DE = À0.3 kcal mol À1 ) with a more symmetrical Ni 3 P 4 core (see the Supporting Information for details). The fluxional behaviour observed by NMR spectroscopy can presumably be attributed to an exchange process between P1/ P4 and P2/P3, which proceeds via this symmetrical isomer or a symmetrical transition state with a low energy (DE = 2.6 kcal mol À1 ). The 1 H NMR spectra are in good agreement with these findings, exhibiting three different signal sets for the IMes ligand and similar thermal dependence of the integral ratios.
Analysis of 1 by liquid field ionisation desorption mass spectrometry (LIFDI-MS) revealed a molecular ion peak at m/z = 1212.2952 in good agreement with the calculated molecular ion peak (1212.2784). The cyclic voltammogram of 1 (THF/[nBu 4 N]PF 6 , Figure S18, Supporting Information) features two reversible redox events at E 1/2 = À1.07 and À2.76 V (vs. Fc/Fc + ), which may be assigned to the reversible oxidation and reduction of the complex, respectively.
The bonding situation in 1 was analysed by means of localised orbitals. In particular, intrinsic bond orbitals (IBO)  were constructed starting from a BP86/def2-TZVP wavefunction. Looking at the composition of those orbitals, six filled orbitals involving multicentre bonds between the Ni and P atoms could be identified along with a 3d 10 configuration for each Ni atom (see the Supporting Information for a depiction). This is consistent with classical electron-counting rules. [18] Thus, the cluster may be defined as a superhypercloso-cluster (12 = 2(nÀ1), n = 7, number of cluster atoms).
The reaction of [Ni(IMes) 2 ] with P 4 is significantly less selective when THF is used as a solvent instead of toluene. Besides 1, two other products formed could be identified by 31 P{ 1 H} NMR spectroscopy and X-ray crystallography. After work-up, brown crystals of the trinuclear cluster [(IMes) 3 Ni 3 P 6 ] (2) were obtained from n-hexane (Figure 3). Structural analysis of 2 reveals a distorted tricapped trigonal prism (or, equivalently, two facial Ni 3 P 3 octahedra sharing a common Ni 3 face). Notably, compounds featuring pnictogen (P, As) prisms with iron or cobalt are usually stabilised by anionic cyclopentadienyl ligands. [19] Similar to 1, an unsymmetrical Ni 3 -triangle is observed (Ni1ÀNi2 2.4835(3) , Ni1À Ni3 2.4882(3) , Ni2ÀNi3 2.6429(3) ). The PÀP bond lengths range from 2.2055(4) to 2.2700(4) consistent with P À P single bonds. The 31 P{ 1 H} NMR spectrum in C 6 D 6 shows a broad resonance at À8.6 ppm. The bonding situation in 2 was analysed similarly to that in cluster 1. In accordance with electron-counting rules, nine doubly occupied orbitals of multicentre bonds between the cluster atoms were identified (see the Supporting Information for a depiction). Thus, due to its closed deltahedral structure (distorted tricapped trigonal prism) and fulfilment of the 2n cluster electron rule (n = 9), 2 can be described as a 9-vertex hypercloso-cluster. Additionally, a 3d 10 configuration for each Ni atom in 2 could be derived from the analysis of the IBO (see the Supporting Information for details).
Moreover, we were able to identify [(IMes) 2 Ni 2 P 5 ] (3 a) as a side product. This compound co-crystallises with 2 from the mother liquor of the reaction mixture of [Ni(IMes) 2 ] with P 4 . Structural analysis of crystals of the composition [(IMes) 3 Ni 3 P 6 ]·[(IMes) 2 Ni 2 P 5 ] (2·3 a) revealed that compound 3 a features a dinuclear inverse sandwich structure in the solid state with a bridging cyclo-P 5 À ligand (Figure 3). The Ni1 À Ni1' distance is 2.6339(13) and the P À P bond lengths range from 2.182(8) to 2.211 (9) , which is in the common range observed for dinuclear 3d transition metal complexes with bridging cyclo-P 5 À ligands. [20,21] The pentaphosphacyclopentadienyl ligand is frequently observed in transition metal mediated P 4 activation. [1] However, most complexes comprising such a cyclo-P 5 À ligand feature group 8 metals and there are only a few examples of other transition metal complexes. [21] Furthermore, all known cyclo-P 5 À complexes additionally contain cyclopentadienyl ligands, while complex 3 a is stabilised by an L-type ligand.
Having established the ability of [Ni(IMes) 2 ] to act as a precursor to interesting Ni/P clusters, we proceeded with performing the analogous reactions using the bulkier carbene complex [Ni(IPr) 2 ] in order to examine if there is any difference in product distribution (Scheme 2). And, indeed, in contrast to observations made using [Ni(IMes) 2 ], 31 P-{ 1 H} NMR spectroscopy revealed no resonances. Nevertheless, the 1 H NMR spectrum clearly showed the formation of free IPr and one new distinct diamagnetic IPr environment.

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Ni2'). The P 8 -framework contains short PÀP bonds ranging from 2.201(3) to 2.288(3) (P1ÀP3, P1ÀP2, P2ÀP3'), and long P À P bonds with bond lengths from 2.349(4) to 2.459(3) (P4 À P4', P3 À P4', P2 À P4). 1 H and 13 C{ 1 H} NMR spectra of crystals of 4 dissolved in C 6 D 6 showed only one set of IPr signals despite the presence of two distinct IPr environments in the solid-state structure. This evidence for fluxionality in solution was further confirmed by variable-temperature 31 P{ 1 H} NMR spectroscopy ( Figure 5). Coincidentally, the spectrum recorded at room temperature exhibits an extremely broad signal that could not be resolved. However, heating up the solution results in one broad resonance, whereas cooling the solution to 193 K afforded three signals with an integral ratio of 4:2:2, at chemical shifts of À136.2 (P2, P2', P3, P3'), 97.0 (P1, P1' or P4, P4') and 124.6 ppm (P1, P1' or P4, P4'), which is in agreement with the presence of three different P environments as suggested by the crystallographic study. Even at 193 K, the couplings could not be resolved completely.
Unfortunately, separation of free IPr from compound 4 proved to be challenging. The use of [(IPr)Ni(h 6 -toluene)] as an attractive precursor was therefore pursued and led to the isolation of pure 4 as a dark green powder in 41 % yield. The cyclic voltammogram of 4 (THF/[nBu 4 N]PF 6 , Figure S20) shows one reversible oxidation wave at E 1/2 = À0.76 V (vs. Fc/ Fc + ). Analysis of the IBO reveals 12 orbitals that involve bonding between the cluster atoms again being in accordance with established electron-counting rules. Thus 4 obeys the 2(n + 1) (n = 11) electron count rule of a 11-vertex closocluster (see the Supporting Information for a depiction of the IBO). The same analysis additionally allows for the assignment of a d 8 -configuration for the Ni1 atom and d 10 -configurations for Ni2/Ni2'.
Apart from 4, the reaction of [(IPr)Ni(h 6 -toluene)] with P 4 also affords green crystals of [(IPr) 2 Ni 2 (m-P 5 )] (3 b), which were obtained from the n-hexane washing solution and identified by X-ray crystallography. Complex 3 b is isostructural with 3 a and features similar Ni À Ni and P À P bond lengths (see the Supporting Information for further details).
The electronic structure of a slightly truncated model complex 3' ([(IPh) 2 Ni 2 P 5 ], IPh = 1,3-diphenylimidazolin-2ylidene) was calculated at the TPSSh/IGLO-III (CP(PPP) on Ni) level of theory. [25] This method was chosen since it has proven to yield reliable results for the calculation of magnetic properties. Significant interactions between the Ni atoms (Mayer bond order: 0.8) as well as the Ni atoms and the aromatic P 5 ring were found (Mayer bond order: 0.5). The Xband EPR spectrum of 3 b (Figure 6) recorded in a toluene glass at 20 K reveals an axial signal pattern for an S = 1 = 2 system showing hyperfine interactions with all five phosphorus atoms. A satisfactory simulation of the experimental spectrum was obtained assuming hyperfine interactions with five equivalent phosphorus atoms (g 11     Supporting Information). Given the good agreement between the measured and DFT calculated EPR parameters, the calculated and the true electronic structure should resemble each other closely. Thus, according to our DFT calculations, the spin density in 3 is evenly distributed between the Ni atoms ( Figure 6).
To conclude, reactions of N-heterocyclic carbene nickel-(0) complexes with P 4 afford unprecedented nickel phosphorus clusters. These reactions clearly show an impact of the size of the NHC ligand on the products obtained. Upon increasing the steric demand from IiPr 2 to IMes, di-and trinuclear complexes with Ni 3 P 4 (1), Ni 3 P 6 (2) cores as well as Ni 2 P 5 (3 a) were obtained. Notably, 3 a represents the first nickel pentaphosphacyclopentadienyl complex. The bulky NHC IPr again changes the outcome of the reaction to afford a Ni 3 P 8 (4) closo-cluster with a novel homoquadricyclane-like P 8 framework. Bulky substituents on the NHC ligands presumably facilitate the formation of monocarbene nickel fragments observed in the molecular structures of 1-4. However, the mechanism of formation of these products is obviously complex, and the details of the initial P 4 activation process and the subsequent transformations of the resulting intermediates must be revealed by further studies. Moreover, we are currently investigating the use of 1-4 as single-source precursors for the preparation of nickel phosphides as electrocatalysts for hydrogen evolution. [26]