An array of neutral borabenzene−ligand complexes (ligand = pyridine, 2,6-lutidine, NEt3, PMe3, CN-t-Bu) have been synthesized in three straightforward steps from commercially available 1-(trimethylsilyl)-1,4-pentadiyne. An X-ray crystal structure of borabenzene−PMe3 is reported.

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Stimulated by its relationship to the ubiquitous triphenylphosphine ligand (1), 1 we have prepared 2 and begun to explore the coordination chemistry of the diphenylphosphidoboratabenzene anion (DPB; 2). DPB may be viewed as a negatively charged, essentially isosteric, variant of PPh 3 ; within this context, comparative studies of PPh 3 and DPB complexes, both for a given metal and for metals which are adjacent in the periodic table, could lead to useful new insights into reactivity. 3 In this communication, we report the first stage of our investigation into the chemistry of DPB, specifically, synthetic and structural work which establishes the viability of DPB as a ligand for an array of transition metals.A wide range of DPB adducts can be generated through treatment of transition metal halides with potassium diphenylphosphidoboratabenzene (K-DPB). For example, reaction of Cp 2 ZrHCl with K-DPB in the presence of PMe 3 leads to displacement of the chloride ligand and formation of Cp 2 ZrH-(DPB)(PMe 3 ) (3; eq 1). A single-crystal X-ray diffraction study of this complex ( Figure 1a; Table 1a) 4 reveals a structure very similar to that found for Cp 2 ZrH(SiPh 3 )(PMe 3 ). 5 The P-C bonds of the DPB ligand are ∼0.12 Å shorter than the P-B bond, and the ligand adopts a slightly distorted tetrahedral geometry.We have also established that the iodide of CpFe(CO) 2 I is readily substituted by DPB, producing CpFe(CO) 2 (DPB) (4, eq 2). The X-ray crystal structure of 4 ( Figure 1b; Table 1b) 6 displays a three-legged piano-stool geometry typical of CpFeL 2 X complexes, with a nearly staggered conformation about the Fe-P bond (dihedral angle [Cp centroid-Fe(1)-P(1)-C(31)]) -168°). 7 As in the case of zirconium complex 3, the P-C bonds of the DPB ligand of 4 are shorter than the P-B bond (by ∼0.13 Å).[CpFe(CO) 2 (PPh 3 )] + and CpFe(CO) 2 (PPh 2 ) are isoelectronic with complex 4. 8,9 Comparison of the C-O stretching frequencies (Table 2) suggests that the iron atom of [CpFe(CO) 2 -(PPh 3 )] + is the least electron-rich and that of CpFe(CO) 2 (PPh 2 ) is the most electron-rich. It is important to note that the diphenylphosphido group is unique among the three phosphorus ligands in that it bears a "lone pair" which can contribute electron density to the metal. The IR data as well as the Fe-(1) For a review of phosphine complexes of transition metals, see: Dias, P. B.; de Piedade, M. E. M.; Simoes, J. A. M. Coord. Chem. ReV. 1994, 135, 737-807. (2) Qiao, S.; Hoic, D. A.; Fu, G. C. J. Am. Chem. Soc. 1996, 118, 6329-6330.(3) This general approach has proved to be extremely interesting in early transition metal metallocene chemistry. For examples and leading references, see: (a) Crowther, D. J.; Baenziger, N. C.; Jordan, R. F. Data for compound 3: A wheat-colored plate (0.08 × 0.09 × 0.39 mm; grown from toluene/reaction-mixture at -35°C for three weeks) was mounted with a glass fiber on a Siemens SMART/CCD three circle diffractometer ( fixed at 54.78°). Data collection was done at -120°C using Mo KR radiation. The crystal was ...

We recently reported that treatment of several transition metal halides with potassium diphenylphosphidoboratabenzene (K-DPB) affords phosphorus-bound σ complexes, whereas otherwise identical reactions of potassium diphenylamidoboratabenzene (K-DAB) do not provide the corresponding nitrogen-bound σ complexes. 1-3 Earlier investigations of aminoboranes and phosphinoboranes have clearly established that nitrogen is more prone than phosphorus to participate in π-bonding with boron (1 T 2). 4 It therefore seemed likely that the dichotomy in reactivity between K-DPB and K-DAB has its origin in a dichotomy in bonding. In this communication we furnish support for this hypothesis in the form of structural studies that reveal that the phosphorus lone pair of K-DPB engages in little, if any, π-bonding with boron, whereas the nitrogen atom of K-DAB adopts a geometry that permits a π interaction. This is, to the best of our knowledge, the first crystallographic comparison of P-B versus N-B π-bonding between two molecules that differ only in the group 15 atom. 4,5 X-ray quality crystals of K-DPB‚(18-crown-6)‚(toluene) 1/2 (3) were obtained upon cooling a saturated solution of K-DPB in toluene/18-crown-6 to -35°C (Figure 1a, Table 1a). 6 The phosphorus atom of K-DPB‚(18-crown-6)‚(toluene) 1/2 is pyramidal, with ∠B-P-C12 ) 106.2(3)°, ∠B-P-C6 ) 102.5(3)°, and ∠C12-P-C6 ) 101.4(2)°(∑ ) 310°). The C6-P-B-C1 torsion angle of -66.2(5)°and the P-B bond distance of 1.968(7) Å 7 are inconsistent with the presence of significant π-bonding between phosphorus and boron.