A Homoleptic Diphosphatetrahedrane Nickel(0) Complex

stable diphosphatetrahedrane

In 1970 Ginsberg and Lindsell reported the first transition metal complexes of white phosphorus (P 4 ) [RhClL 2 (η 2 -P 4 )] (L = PPh 3 , P(p-Tol) 3 , P(m-Tol) 3 , AsPh 3 ; Tol = C 7 H 8 ). [1] Since then, a wide variety of coordination compounds have resulted from coordination studies of P 4 . [2] These complexes often contain reduced polyphosphide units resulting from a formal electron transfer from the metal atom to P 4 . In many cases, this effects the cleavage of PÀ P bonds (as observed in the above-mentioned rhodium complexes), [3] which may be followed by a redistribution of the P atoms forming various polyphosphido ligands of the general composition P n xÀ . In contrast to this, the number of transition metal complexes featuring an intact, neutral P 4 tetrahedron has remained limited. Figure 1a shows the structure of [(np 3 )Ni(η 1 -P 4 )] (A, np 3 = tris(2-diphenylphosphinoethyl)amine) reported by Sacconi and co-workers in 1979, the first structurally characterized transition metal complex in which the P 4 moiety coordinates end-on via a single phosphorus atom. [4] Further examples for this end-on coordination mode have been reported for a variety of transition metals such as Fe, Ru, Os, Mn, and W. [5] In addition, several complexes with intact, side-on coordinated P 4 molecules have been reported. [3c,6] Such complexes are typically stabilized by different ancillary ligands, while the coinage metal complexes [M(P 4 ) 2 ]X (B; M = Cu, Ag, X = Al{OC(CF 3 ) 3 } 4 ; M = Au, X = GaCl 4 ) displayed in Figure 1a are the only isolated homoleptic P 4 complexes to the best of our knowledge. [3c,6a,b,d] These rare homoleptic complexes are stabilized by weakly coordinating anions. The coordinated PÀ P bonds are only slightly elongated  [3c,4,6a,b] b) coordination of intact phosphatetrahedranes. [7,9] by 0.10 to 0.20 Å with respect to those in the free P 4 molecule, indicating the presence of intact P 4 tetrahedra.
These initial results promise a versatile coordination chemistry of phosphatetrahedranes, which is expected to be similar to that of P 4 and may also be comparable to the coordination behavior of related low-coordinate organophosphorus compounds such as phosphaalkynes and phosphabenzenes. [2,11] However, in contrast to these wellinvestigated compound classes, reactivity studies of phosphatetrahedranes towards transition metal carbonyl complexes have not yet been reported. In extension to our recent studies of ditert-butyldiphosphatetrahedrane towards NHC-stabilized Ni(0) complexes, we studied its reaction towards the highly reactive tetracarbonyl nickel(0) complex Ni(CO) 4 . Herein we report the isolation and characterization of [Ni{η 2 -(tBuCP) 2 } 3 ] (1), which features three intact di-tert-butyldiphosphatetrahedrane moieties that coordinate to the Ni(0) core via their PÀ P bonds.

Figure 2. Solid
Dipp = 2,6-di-isopropylphenyl, IPr = 1,3-bis(2,6-di-iso-propylphenyl)imidazolin-2ylidene). [17] The Ni complex 1 was isolated in 23 % yield as an indigocolored solid. The relatively low yield can likely be attributed to the thermal instability of 1 at room temperature. 1 is characterized by a 31 P{ 1 H} NMR singlet at À 338.4 ppm, which is significantly shifted to low field in comparison to the resonance of free diphosphatetrahedrane (tBuCP) 2 (δ( 31 P) = À 468.2 ppm). The low-field shift is far less pronounced for the Ag complex C (δ( 31 P) = À 446.8 ppm). [7] Again, this points to a stronger interaction between metal core and ligand for 1. The 1 H NMR spectrum of 1 displays the singlet of the tert-butyl groups at 1.33 ppm, while the 13 C{ 1 H} NMR spectrum shows three resonances at 29.7 ppm, 32.0 ppm, and 45.5 ppm. These are assigned to the quaternary carbon atoms of the tert-butyl groups, the methyl groups, and the carbon atoms in the tetrahedral core, respectively. The signal at 45.5 ppm shows the expected multiplet structure due to coupling with the 31 P nuclei. In conclusion, the multinuclear NMR spectra of 1 are fully consistent with the molecular structure observed in the solid state. The UV-Vis absorption spectrum of 1 dissolved in nhexane displays three absorption bands in the UV region at 225 nm, 280 nm and 315 nm in addition to two absorptions in the visible region at 415 nm and 680 nm, which presumably cause the blue color of 1.
While complex 1 can be isolated as a reasonably pure compound according to elemental analysis, it is noteworthy that it decomposes at ambient temperature, forming a clear brown mixture within 48 h which contains free (tBuCP) 2 and its dimerization product, the ladderane (tBuCP) 4 , according to 31 P{ 1 H} NMR monitoring. [7] The reaction proceeds without formation of side products as NMR spectra of the crude reaction mixture illustrate (see Figures S4 and S5 in the SI), yet the thermal decomposition hampers the isolation of higher yields of 1. The structurally comparable Ni(0) complex tris(ethylene)nickel(0) is also thermolabile, decomposing above 0°C. [12] The bonding situation in 1 was analyzed by calculating intrinsic bond orbitals (IBOs) on the BP86/def2-TZVP level of theory. [18] Three filled orbitals forming 3-center-2-electron bonds between Ni1/P1/P2, Ni1/P3/P4 and Ni1/P5/P6 could be identified (see Figure 3 for a representative IBO 175, IBOs 173 and 174 are similar and represent the other NiP 2 moieties; these orbitals and further relevant IBOs are displayed in Figure S7, SI). Such 3-center-2-electron bonds are also characteristic for olefin complexes as described by the Dewar-Chatt-Duncanson model, which describes the structure of tris(ethylene)nickel(0) well. [19] Indeed, calculations constructing IBOs for tris(ethylene)nickel(0) on the same level of theory confirmed the existence of three filled orbitals forming 3-center-2-electron bonds between Ni1/ C1/C2, Ni1/C3/C4 and Ni1/C5/C6 (see Figure S8 in the SI for a graphical representation of these orbitals). Similarly, the binding mode of η 2 -coordinated intact P 4 tetrahedra is analogous to the Dewar-Chatt-Duncanson model. [6g] Moreover, a 3d 10 configuration for the nickel could be derived from the IBO analysis of 1, confirming the oxidation state Ni(0) (see Figure S7 in the SI for a depiction of the orbitals).
In summary, complex 1 is only the third reported example of coordination of an intact phosphatetrahedrane other than P 4 and the first homoleptic complex among these compounds. In the structure of 1, diphosphatetrahedrane acts as a bidentate ligand coordinating in an η 2 -fashion via the PÀ P bond to nickel. So far, mainly coinage metal cations stabilized by weakly coordinating anions have been reported to side-on coordinate intact tetrahedra of phosphatetrahedranes and the isoelectronic P 4 molecule. Prior to this work, only one example of the side-on coordination of an intact P 4 tetrahedron to a Ni core was proposed, however, this compound was neither isolated nor confirmed by an X-ray structure. [6c] Quantum chemical calculations support the oxidation state Ni(0) in 1 and indicate the formation of 3-center-2-electron bonds between the P atoms of (tBuCP) 2 and the Ni center. The bonding situation appears to be comparable to that of tris(ethylene)nickel(0) and the few known examples in the literature showing side-on coordination of an intact P 4 tetrahedron. [6g,19] Further studies exploring the coordination chemistry of (tBuCP) 2 and related molecules are currently underway in our laboratory.

Experimental Section
General: All reactions and manipulations were performed under an atmosphere of dry argon using standard Schlenk line techniques or in a MBraun UniLab glovebox under an atmosphere of dry argon. n-Hexane, and toluene were dried and degassed with an MBraun SPS-800 solvent purification system. Toluene was stored under argon over activated 3 Å molecular sieves and n-hexane was stored under argon over a potassium mirror. Deuterated toluene was purchased from Eurisotop and used as received. (tBuCP) 2 was prepared according to the previous reported procedure. [7] Ni(CO) 4 in toluene (c = 1.2 M) was kindly provided by the group of Manfred Scheer.

Elemental analysis:
The elemental analysis was determined by the analytical department of the University of Regensburg with a Micro Vario Cube (Elementar).

UV-Vis spectroscopy:
The UV/Vis absorption spectrum was recorded on an Ocean Optics Flame Spectrometer with the corresponding light source (DH-2000-BAL/UV-Vis-NIR light source).

X-ray diffraction:
The single-crystal X-ray diffraction data was recorded on a Rigaku XtaLAB Synergy DW R (DW system, HyPix-Arc 150) diffractometer with microfocus Cu-Kα radiation (λ = 1.54184 Å). Crystals were selected under mineral oil, mounted on micromount loops and quench-cooled using an Oxford Cryosystems open flow N 2 cooling device. Either semi-empirical multi-scan absorption corrections [20] or analytical ones [21] were applied to the data. Using Olex2, [22] the structure was solved with the SHELXT [23] structure solution program using Intrinsic Phasing and refined with the SHELXL [24] refinement package using Least Squares refinements on F 2 . The hydrogen atoms were located in idealized positions and refined isotropically with a riding model. Crystallographic data for the structure in this paper has been deposited with the Cambridge Crystallographic Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK. Copies of this data can be obtained free of charge on quoting the depository number: 2158460 for compound 1; E-mail: deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).