Unexpectedly high barriers to M–P rotation in tertiary phobane complexes : PhobPR behavior that is commensurate with tBu2PR

The four isomers of 9-butylphosphabicyclo[3.3.1]nonane, s-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl, have been prepared. Seven isomers of 9-butylphosphabicyclo[4.2.1]nonane (a5-PhobPBu, where Bu = n-butyl, sec-butyl, isobutyl, tert-butyl; a7-PhobPBu, where Bu = n-butyl, isobutyl, tert-butyl) have been identified in solution; isomerically pure a5-PhobPBu and a7-PhobPBu, where Bu = n-butyl, isobutyl, have been isolated. The σ-donor properties of the PhobPBu ligands have been compared using the JPSe values for the PhobP(═Se)Bu derivatives. The following complexes have been prepared: trans-[PtCl2(s-PhobPR)2] (R = nBu (1a), iBu (1b), sBu (1c), tBu (1d)); trans-[PtCl2(a5-PhobPR)2] (R = nBu (2a), iBu (2b)); trans-[PtCl2(a7-PhobPR)2] (R = nBu (3a), iBu (3b)); trans-[PdCl2(s-PhobPR)2] (R = nBu (4a), iBu (4b)); trans-[PdCl2(a5-PhobPR)2] (R = nBu (5a), iBu (5b)); trans-[PdCl2(a7-PhobPR)2] (R = nBu (6a), iBu (6b)). The crystal structures of 1a–4a and 1b–6b have been determined, and of the ten structures, eight show an anti conformation with respect to the position of the ligand R groups and two show a syn conformation. Solution variable-temperature 31P NMR studies reveal that all of the Pt and Pd complexes are fluxional on the NMR time scale. In each case, two species are present (assigned to be the syn and anti conformers) which interconvert with kinetic barriers in the range 9 to >19 kcal mol–1. The observed trend is that, the greater the bulk, the higher the barrier. The magnitudes of the barriers to M–P bond rotation for the PhobPR complexes are of the same order as those previously reported for tBu2PR complexes. Rotational profiles have been calculated for the model anionic complexes [PhobPR-PdCl3]− using DFT, and these faithfully reproduce the trends seen in the NMR studies of trans-[MCl2(PhobPR)2]. Rotational profiles have also been calculated for [tBu2PR-PdCl3]−, and these show that the greater the bulk of the R group, the lower the rotational barrier: i.e., the opposite of the trend for [PhobPR-PdCl3]−. Calculated structures for the species at the maxima and minima in the M–P rotation energy curves indicate the origin of the restricted rotation. In the case of the PhobPR complexes, it is the rigidity of the bicycle that enforces unfavorable H···Cl clashes involving the Pd–Cl groups with H atoms on the α- or β-carbon in the R substituent and H atoms in 1,3-axial sites within the phosphabicycle

Anatomy of phobanes. diastereoselective synthesis of the three isomers of n-butylphobane and a comparison of their donor properties.

Three methods for the large scale (50-100 g) separation of the secondary phobanes 9-phosphabicyclo[3.3.1]nonane (s-PhobPH) and 9-phosphabicyclo[4.2.1]nonane (a-PhobPH) are described in detail. Selective protonation of s-PhobPH with aqueous HCl in the presence of a-PhobPH is an efficient way to obtain large quantities of a-PhobPH. Selective oxidation of a-PhobPH in an acidified mixture of a-PhobPH and s-PhobPH is an efficient way to obtain large quantities of s-PhobPH. The crystalline, air-stable phosphonium salts [s-PhobP(CH(2)OH)(2)]Cl and [a-PhobP(CH(2)OH)(2)]Cl can be separated by a selective deformylation with aqueous NaOH. a-PhobPH is shown to be a(5)-PhobPH in which the H lies over the five-membered ring. The isomeric a(7)-PhobPH has been detected but isomerizes to a(5)-PhobPH rapidly in the presence of water. s-PhobPH is more basic than a-PhobPH by about 2 pK(a) units in MeOH. Treatment of s-PhobPH with BH(3).THF gives s-PhobPH(BH(3)) and similarly a-PhobPH gives a(5)-PhobPH(BH(3)). Isomerically pure s-PhobPCl and a(5)-PhobPCl are prepared by reaction of the corresponding PhobPH with C(2)Cl(6). The n-butyl phobane s-PhobPBu is prepared by nucleophilic (using s-PhobPH or s-PhobPLi) and electrophilic (using s-PhobPCl) routes. Isomerically pure a(5)-PhobPBu is prepared by treatment of a-PhobPLi with (n)BuBr and a(7)-PhobPBu is prepared by quaternization of a-PhobPH with (n)BuBr followed by deprotonation. From the rates of conversion of a(7)-PhobPBu to a(5)-PhobPBu, the DeltaG(double dagger) (403 K) for P-inversion is calculated to be 38.1 kcal mol(-1) (160 kJ mol(-1)). The donor properties of the individual isomers of PhobPBu were assessed from the following spectroscopic measurements: (i) (1)J(PSe) for PhobP(Se)Bu; (ii) nu(CO) for trans-[RhCl(CO)(PhobPBu)(2)], (iii) (1)J(PtP) for the PEt(3) in trans-[PtCl(2)(PEt(3))(PhobPBu)]. In each case, the data are consistent with the order of sigma-donation being a(7)-PhobPBu > s-PhobPBu > a(5)-PhobPBu. This same order was found when the affinity of the PhobPBu isomers for platinum(II) was investigated by determining the relative stabilities of [Pt(CH(3))(s-PhobPBu)(dppe)][BPh(4)], [Pt(CH(3))(a(5)-PhobPBu)(dppe)][BPh(4)], and [Pt(CH(3))(a(7)-PhobPBu)(dppe)][BPh(4)] from competition experiments. Calculations of the relative stabilities of the isomers of PhobPH, [PhobPH(2)](+), and PhobPH(BH(3)) support the conclusions drawn from the experimental results. Moreover, calculations on the frontier orbital energies of PhobPMe isomers and their binding energies to H(+), BH(3), PdCl(3)(-), and PtCl(3)(-) corroborate the experimental observation of the order of sigma-donation being a(7)-PhobPR > s-PhobPR > a(5)-PhobPR. The calculated He(8) steric parameter shows that the bulk of the isomers increases in the order a(7)-PhobPR < s-PhobPR < a(5)-PhobPR. The crystal structures of [a-PhobP(CH(2)OH)(2)][s-PhobP(CH(2)OH)(2)]Cl(2), cis-[PtCl(2)(a(5)-PhobPCH(2)OH)(2)], trans-[PtCl(2)(s-PhobPBu)(2)], and trans-[PtCl(2)(a(7)-PhobPBu)(2)] are reported

Photochemical Route to Actinide-Transition Metal Bonds: Synthesis, Characterization and Reactivity of a Series of Thorium and Uranium Heterobimetallic Complexes

A series of actinide-transition metal heterobimetallics has been prepared, featuring thorium, uranium and cobalt. Complexes incorporating the binucleating ligand N[-(NHCH2PiPr2)C6H4]3 and Th(IV) (4) or U(IV) (5) with a carbonyl bridged [Co(CO)4]- unit were synthesized from the corresponding actinide chlorides (Th: 2; U: 3) and Na[Co(CO)4]. Irradiation of the isocarbonyls with ultraviolet light resulted in the formation of new species containing actinide-metal bonds in good yields (Th: 6; U: 7); this photolysis method provides a new approach to a relatively rare class of complexes. Characterization by single-crystal X-ray diffraction revealed that elimination of the bridging carbonyl is accompanied by coordination of a phosphine arm from the N4P3 ligand to the cobalt center. Additionally, actinide-cobalt bonds of 3.0771(5) and 3.0319(7) for the thorium and uranium complexes, respectively, were observed. The solution state behavior of the thorium complexes was evaluated using 1H, 1H-1H COSY, 31P and variable-temperature NMR spectroscopy. IR, UV-Vis/NIR, and variable-temperature magnetic susceptibility measurements are also reported