PHOSPHOLE COMPLEXES OF SILVER(I) - INVESTIGATIONS OF STRUCTURE AND BONDING BY X-RAY CRYSTALLOGRAPHY, INFRARED-SPECTROSCOPY, AND CP/MAS AND SOLUTION P-31 NMR-SPECTROSCOPY

A series of complexes of 1-phenyl-3,4-dimethylphosphole (DMPP) and 1-phenyldibenzophosphole (DBP), of the type [L(n)AgX]m (n = 1, m = 4, L = DMPP; n = 2, m = 2, L = DMPP, DBP; n = 3, m = 1, L = DBP; X = Cl, Br, I), have been prepared and characterized. The structure of [(DBP)2AgCl]2 has been determined by X-ray crystallography. It crystallizes in the space group P1BAR with a = 10.831 (3) angstrom, b = 11.683 (3) angstrom, c = 11.243 (3) angstrom, alpha = 82.73 (2)-degrees, beta = 89.39 (2)-degrees, gamma = 78.01 (2)-degrees, and Z = 1. The structure was refined by least-squares methods with R(F) = 0.066 for 2273 unique reflections with I/sigma(I) greater-than-or-equal-to 3.0. The complex crystallizes as a dimer with a structure similar to that which has previously been observed for 2:1 complexes of PPh3 with silver(I) halides. In this structure both of the silver atoms are tetracoordinated through bonds to the P atoms of two phosphole ligands and two mu-2-chlorine atoms, which bridge the two silver atoms. The dimer lies on a crystallographic inversion center. The bond lengths in the P2Ag2Cl2 core are Ag-P = 2.504 (3), 2.525 (4), Ag-Cl = 2.643 (3), 2.634 (4), and Ag---Ag = 3.750 angstrom, and the bond angles are Ag-Cl-Ag = 90.5 (1), Cl-Ag-Cl = 89.4 (1), P-Ag-Cl = 108.5 (1), 122.4 (1), 121.5 (1), 102.6 (1), and P-Ag-P = 111.5 (1)-degrees. The Ag2Cl2 unit in this complex is thus almost perfectly square, with a difference of less than 0.01 angstrom between the symmetrically inequivalent Ag-Cl bond lengths. This contrasts with the structure of the corresponding PPh3 complex, [(PPh3)2AgCl]2, where the Ag-Cl bond lengths differ by nearly 0.15 angstrom. The structures of the other members of the phosphole/AgX series were deduced from their far IR and cross-polarization magic-angle-spinning (CP/MAS) P-31 NMR spectra. Thus, the 1:1 complexes (n = 1), which exist only for L = DMPP, are shown to have the tetrameric cubane structure. The 2:1 complexes all have the halogen-bridged dimeric structures, [L2AgX]2, which was proved for the L = DBP, X = Cl case by X-ray diffraction. Their far-IR spectra appear to be very sensitive to distortion of the Ag2X2 core. The 3:1 complexes, which exist only for L = DBP, have mononuclear [L3AgX] structures. The CP/MAS P-31 NMR spectra show splitting due to 1J(Ag-P) coupling, which progressively decreases in magnitude from the 1:1 complexes (450-580 Hz) to the 3:1 complexes (ca. 250 Hz). A further splitting of about 100 Hz was observed for some of the 2:1 and 3:1 complexes, and this is assigned to 2J(P-P) coupling between crystallographically inequivalent phosphorus atoms that are bound to a common silver atom in the complex. The ionic 4:1 complexes [L4Ag]BF4 (L = DMPP, DBP) were also prepared and characterized by CP/MAS P-31 NMR spectroscopy. The Ag-P bond length r may be estimated with reasonable accuracy from 1J(Ag-P) via the empirical relation 1/(r/angstrom)3 = (3.707 x 10(-5)J/Hz + 4.788 x 10(-2). The solution P-31 NMR spectrum of [LAgCl]4 (L = DMPP) in CH2Cl2 at -80-degrees-C shows a signal due to [L4Ag]+, as well as that due to the parent [LAgCl]4. Likewise, [L2AgX]2 complexes (L = DMPP; X = Cl, Br) show signals due to [LAgCl]4, [L3AgCl], and [L4Ag]+. Solutions of [L3AgX] (L = DBP; X = Cl, Br, I) show signals due to [L4Ag]+ and uncoordinated L, as well as to the parent [L3AgX]. Thus, although the binuclear complexes [L2AgX]2 are the prevalent ones in the solid state, they do not exist in solution in detectable amounts. The nonexistence of [LAgX]4 (L = DBP) and [L3AgX] (L = DMPP) in the solid state is consistent with the absence or, in the latter case, high lability of these species in solution.