The dinuclear complex (Me4N)2[Pd2(μ-SPh)2Cl4] is solvolyzed upon reaction with AgClO4·H2O in acetone. The X-ray crystal structure of (Ph4As)2[Pd2(μ-SPh)2Cl4] showed that this dinuclear complex is bridged by two thiolate ligands with Cl- ions occupying terminal sites on the palladium(II) atoms. The dinuclear solvolyzed species [Pd2(μ-SPh)2(sol)4](ClO4)2, in which sol is H2O or acetone, binds to methionine side chains in AcMet-X, where X is Gly, Ala, Leu, Phe, or Val. It then catalyzes hydrolysis of the amide bond involving the carboxyl group of methionine. No prior activation of the amide bond is required for hydrolysis. Dipeptides with regular amide bonds are hydrolyzed in nonaqueous solvents, under mild conditions. The reactions were followed by 1H NMR spectroscopy. Turnover was achieved with the following N-acetylated dipeptides:  AcMet-Gly, AcMet-Val, AcMet-Phe, and AcMet-Ala. One equivalent of [Pd2(μ-SPh)2(sol)4](ClO4)2 cleaves 6−14 equiv of dipeptide. The turnover number depends on the steric bulk of the leaving amino acid. Hydrolysis kinetics were studied for AcMet-Gly, AcMet-Ala, AcMet-Val, AcMet-Phe, and AcMet-Leu. The reaction proceeds very rapidly, with a half-life of less than 7 min for AcMet-Ala at 50 °C. The half-lives at 40 °C for most of the dipeptides are shorter than 30 min. Because the rate of the reaction also depends on the volume of the leaving amino acid, the catalyst is potentially sequence-selective. The effects of temperature on the hydrolysis of AcMet-Ala were also studied. This study is a step toward the use of transition-metal complexes as reagents for the hydrolysis of lipophillic peptides and proteins.

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The syntheses and properties of tetra- and pentanuclear vanadium(IV,V) carboxylate complexes are reported. Reaction of (NBzEt(3))(2)[VOCl(4)] (1a) with NaO(2)CPh and atmospheric H(2)O/O(2) in MeCN leads to formation of (NBzEt(3))(2)[V(5)O(9)Cl(O(2)CPh)(4)] 4a; a similar reaction employing (NEt(4))(2)[VOCl(4)] (1b) gives (NEt(4))(2)[V(5)O(9)Cl(O(2)CPh)(4)] (4b). Complex 4a.MeCN crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -148 degrees C: a = 13.863(13) Å, b = 34.009(43) Å, c = 12.773(11) Å, and Z = 4. The reaction between (NEt(4))(2)[VOBr(4)] (2a) and NaO(2)CPh under similar conditions gives (NEt(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6a), and the use of (PPh(4))(2)[VOBr(4)] (2b) likewise gives (PPh(4))(2)[V(5)O(9)Br(O(2)CPh)(4)] (6b). Complex 6b crystallizes in space group P2(1)2(1)2(1) with the following unit cell dimensions at -139 degrees C: a = 18.638(3) Å, b = 23.557(4) Å, c = 12.731(2) Å, and Z = 4. The anions of 4a and 6b consist of a V(5) square pyramid with each vertical face bridged by a &mgr;(3)-O(2)(-) ion, the basal face bridged by a &mgr;(4)-X(-) (X = Cl, Br) ion, and a terminal, multiply-bonded O(2)(-) ion on each metal. The RCO(2)(-) groups bridge each basal edge to give C(4)(v)() virtual symmetry. The apical and basal metals are V(V) and V(IV), respectively (i.e., the anions are trapped-valence). The reaction of 1b with AgNO(3) and Na(tca) (tca = thiophene-2-carboxylate) in MeCN under anaerobic conditions gives (NEt(4))(2)[V(4)O(8)(NO(3))(tca)(4)] (7). Complex 7.H(2)O crystallizes in space group C2/c with the following unit cell dimensions at -170 degrees C: a = 23.606(4) Å, b = 15.211(3) Å, c = 23.999(5) Å, and Z = 4. The anion of 7 is similar to those of 4a and 6b except that the apical [VO] unit is absent, leaving a V(4) square unit, and the &mgr;(4)-X(-) ion is replaced with a &mgr;(4),eta(1)-NO(3)(-) ion. The four metal centers are now at the V(IV), 3V(V) oxidation level, but the structure indicates four equivalent V centers, suggesting an electronically delocalized system. Variable-temperature magnetic susceptibility data were collected on powdered samples of 4b, 6a, and 7 in the 2.00-300 K range in a 10 kG applied field. 4b and 6a both show a slow increase in effective magnetic moment (&mgr;(eff)) from approximately 3.6-3.7 &mgr;(B) at 320 K to approximately 4.5-4.6 &mgr;(B) at 11.0 K and then a slight decrease to approximately 4.2 &mgr;(B) at 2.00 K. The data were fit to the theoretical expression for a V(IV)(4) square with two exchange parameters J = J(cis)() and J' = J(trans)() (H = -2JS(i)()S(j)()): fitting of the data gave, in the format 4b/6a, J= +39.7/+46.4 cm(-)(1), J' = -11.1/-18.2 cm(-)(1) and g = 1.83/1.90, with the complexes possessing S(T) = 2 ground states. The latter were confirmed by magnetization vs field studies in the 2.00-30.0 K and 0.500-50.0 kG ranges: fitting of the data gave S(T) = 2 and D = 0.00 cm(-)(1) for both complexes, where D is the axial zero-field splitting parameter. Complex 7 shows a nearly temperature-independent &mgr;(ef...

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