No abstract

An improved preparation of 4-(dichlorophosphino)-2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphole (1) is described. Replacement of the two chlorine substituents with two fluorine (2), dimethylamino (3), diethylamino (4), bis(n-propyl)amine (5), pyrazole (9), 3,5-dimethylpyrazole (10), 2,2,2-trifluoroethoxy (11), phenoxy (12), pentafluorophenoxy (13), 2,6-difluorophenoxy (14), and pentafluorobenzoxy (15) substituents has been accomplished to create a large suite of potentially bifunctional phosphorus(III) ligands with two- and three-coordinate P centers spanning a range of basicity and steric bulk at the exo-phosphorus center. Bulky secondary amines (such as diisopropylamine, dibenzylamine, and iminodibenzyl) replaced only one chlorine atom to give asymmetric 4-(chloroaminophosphino)-2,5-dimethyl-2H-1,2,3sigma(2)-diazaphospholes (6, 7, and 8, respectively). The asymmetric substitution creates a diastereotopic center in both 6 and 7 which is observed as fluxional NMR behavior at room temperature. Similar diastereotopic induced behavior was observed in the substituent methylene protons of 11. Coordination studies of the fluorinated phosphole (L = 2) with Cr(0) and Mo(0) gave Cr(CO)(5)L (16), cis-Mo(CO)(4)L(2) (17), and fac-Mo(CO)(3)L(3) (18) (where L = 4-(difluorophosphino)-2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphole). The fluoro ligand displays a behavior which is similar to that of PF(3) and phosphites.

Metal complexes of a series of diphosphorus ligands, 4-(difluorophosphino)-2,5-dimethyl-2H-1,2,3σ2-diazaphosphole (1), 4-(bis(dimethylaminophosphino)-2,5-dimethyl-2H-1,2,3σ2-diazaphosphole (2), and 4-bis(1,1,1-trifluoroethoxyphosphino)-2,5-dimethyl-2H-1,2,3σ2-diazaphosphole (3), were prepared. Ligand 1 reacted with CpRu(PPh3)2Cl to give the diastereotopic complex CpRu(PPh3)(1)Cl (4). With CpRh(CO)2 this same ligand gave CpRh(1)2 (5), which was structurally characterized. The Cp−rhodium center carries two difluorophosphinodiazaphole ligands. The P−N bond distances, (1.670(4) and 1.672(3) Å), suggest partial multiple-bond character. [Cp*Rh(Cl)2] with (1) gave Cp*Rh(Cl)2(1). Ligand 2 with [Rh(CO)2Cl]2 gave trans-Rh(CO)Cl(2)2 (7), which was structurally characterized. The structure reveals two square-planar isomers in 75:25 ratio differing only in the interchange of Cl and CO. The two diazaphosphole ligands lie trans to each other and the planar diazaphosphole rings are oriented perpendicular to the square plane, stacked so that a mirror plane exists through the Cl−Rh−CO plane. The phosphorus−rhodium distance is 2.33 Å. The average P−N bond distance of the exo-phosphorus center and the dimethylamino groups is 1.683 Å, shorter than the normally accepted single-bond length. The dimethylamino nitrogen atoms on the exo-phosphorus are planar. Similarly, ligand 3 with [Rh(CO)2Cl]2 gave trans-[Rh(CO)Cl(3)2] (8). The ligand action ranges in reactivity from a similarity to phenylphosphines through to PF3, reflecting the variation in basicity induced by substituent changes on the exo-phosphorus.

The substituted-exo-phosphine (X = F, NMe(2), OCH(2)CF(3)) diazaphospholephosphines are exclusively oxidized at this center with either chalcogens (S, Se) or azides to phosphoranodiazaphospholes. Oxidation imparts a dramatic upfield shift of the phosphorus NMR signals and an increase in the (1)J(PC) coupling constants within the ring. (Difluorophosphino)diazaphosphole was also oxidized with selected amines using diethyl azodicarboxylate (DAD) as the coupling agent. Bulky amines (e.g., 2,4,6-tri-tert-butylaniline (mes)) gave the monomeric iminophosphorane whereas less bulky amines (p-toluidine) formed mostly the cyclic diazadiphosphetidine. The crystal and molecular structure of 4-(difluoro((p-cyanotetrafluorophenyl)imino)phosphorano)-2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphole was determined: triclinic, P&onemacr; (No. 2), a = 7.2744(15) Å, b = 10.087(4) Å, c = 10.566(2) Å, alpha = 66.62(2) degrees, beta = 77.60(2) degrees, gamma = 78.14(3) degrees, V = 688.8(4) Å(3), Z = 2. Final indices are R(1) = 0.0368 and wR(2) = 0.0968, and for all data, R(1) = 0.0478, wR(2) = 0.1033, and GOF = 1.067. The structure revealed two planar ring systems consisting of the diazaphosphole and the p-tetrafluorophenyl (tfbn) ring with an angle of 26.3 degrees between the rings. The angle about the phosphine imine nitrogen (i.e., P=N-tfbn) is relatively open (141.2(2) degrees ), and the P=N bond length is relatively short (1.514(2) Å). (((Trimethylsilyl)imino)(bis(dimethylamino))phosphorano)diazaphosphole gave, with CpTiCl(3), [(eta(5)-C(5)Me(5))TiCl(2)(N=P(NMe(2))(2)(2,5-dimethyl-2H-1,2,3sigma(2)-diazaphosphol-4-yl))], which was also characterized structurally: monoclinic, P2(1) (No. 4), a = 11.9477(11) Å, b = 8.4757(6) Å, c = 12.7567(11) Å, beta = 108.824(8) degrees, V = 1222.7(2) Å(3), Z = 2. Final indices are R(1) = 0.0630 and wR(2) = 0.1593, and for all data, R(1) = 0.0768, wR(2) = 0.1973, and GOF = 1.081. The Ti-N-P angle of 161.0(5) degrees was large, and the P=N distance (1.592(6) Å) and the Ti-N distance (1.781(6) Å) were both slightly shorter than those in similar titanium complexes. The P-N single bond distances between the exo-phosphorus atom and the attached dimethylamino groups were also short (1.649 Å (average)). These short values suggest delocalized bonding character throughout the metal-ligand framework, possibly a consequence of additional conjugation through the diazaphosphole ring.

Selected platinum(II) and palladium(II) complexes react with 4-(difluorophosphino)-2,5-dimethyl-2H-1,2,3σ2-diazaphosphole (LF) to form the expected X2M(LF)2 complexes. In some cases, redox processes yield stable, solid, M0(LF)4 complexes; thus Pd(cod)Cl2 with LF gave a mixture of the Pd0 and PdII complexes whereas Pt(cod)Cl2 gave only the Pt(LF)2Cl2 complex. With limited LF, (cod)Pt(Me)Cl gave the expected (LF)2Pt(Me)Cl complex, but when the reaction was carried out with excess LF, PtII was reduced to Pt0 to form the Pt0(LF)4 complex. This complex was structurally characterized. Crystal data for C16H28F8N8O2P8Pt: tetragonal, I4̄2m (No. 121), a = 11.961(2) Å, c = 15.034(3) Å, V = 2150.8(6) Å3, Z = 2. Final indices:  GOF = 1.299 [F o 2 ≥ −3σ(F o 2)] and R 1 = 0.0381, wR 2 = 0.1180 (for F o 2 > 2σ(F o 2)) and R 1 = 0.0395, wR 2 = 0.1188 for all data. The structure around the Pt0 atom was revealed as a slightly squashed tetrahedron surrounded by four phospholylphosphine ligands coordinated through the exocyclic phosphine. The Pt−P distance is 2.237(3) Å. The Pd(LF)4 analogue was also isolated in pure form from the reaction of LF with Pd2(dba)3. The related 4-(bis(dimethylamino))-2,5-dimethyl-2H-1,2,3σ2-diazaphosphole (LN) displaced cod from M(cod)Cl2 (M = Pd, Pt) to give only the (LN)2MCl2 complexes (M = Pd and Pt) with no evidence for reduction of the metal center. The larger steric bulk of LN is presumably also the reason this ligand forms the trans complex instead of the cis complexes which were formed by LF from the same precursors in analogous reactions. The reaction of LN with (cod)Pt(Me)Cl produced only cis-(LN)2Pt(Me)Cl; there was no evidence of reduction to Pt0 in contrast to the behavior of LF. With LF, the stereochemistry of the final product was the same as that of the starting material. The third ligand, 4-(bis(trifluoroethoxy)phosphino)-2,5-dimethyl-2H-1,2,3σ2-diazaphosphole (LO) gave the expected platinum cis complex (LO)2MCl2 from the (cod)MCl2 precursor, but palladium rearranged during the complexation reaction to form the trans complex. Again, no evidence for reduction of either Pt or Pd was observed with LO.