Head-to-head cross-linked adduct between the antitumor unit bis(mu-N,N'-di-p-tolylformamidinato)dirhodium(II,II) and the DNA fragment d(GpG)

Reactions of the compound cis-[Rh2(DTolF)2(CH3CN)6](BF4)2, a formamidinate derivative of the class of antitumor compounds [Rh2(O2CR)4] (R=Me, Et, Pr), with 9-ethylguanine (9-EtGuaH) or the dinucleotide d(GpG) proceed by substitution of the acetonitrile groups, with the guanine bases spanning the Rh--Rh bond, in a bridging fashion, through sites N7/O6. In the case of 9-EtGuaH, both head-to-head (HH) and head-to-tail (HT) isomers are formed, whereas with the tethered bases in d(GpG), only one right-handed conformer HH1R [Rh2(DTolF)2{d(GpG)}] is present in solution. For both cis-[Rh2(DTolF)2(9-EtGuaH)2](BF4)2 and [Rh2(DTolF)2{d(GpG)}], the absence of N7 protonation at low pH and the substantial decrease of the pKa values for N1-H deprotonation, support N7/O6 binding of the bases to the dirhodium core. The N7/O6 binding of the bases is further corroborated by the downfield shift by Deltadelta approximately 4.0 ppm of the 13C NMR resonances for the C6 nuclei as compared to the corresponding resonances of the free ligands. The HH arrangement of the guanine bases in [Rh2(DTolF)2{d(GpG)}] is indicated by the intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. Complete characterization of the [Rh2(DTolF)2{d(GpG)}] conformer by 2D NMR spectroscopy supports anti-orientation and N (C3'-endo) conformation for both deoxyribose residues. The N-pucker for the 5'-G base is universal in such cross-links, but it is very unusual for platinum and unprecedented for dirhodium HH cross-linked adducts to have both deoxyribose residues in the N-type conformation. The bulk, the nonlabile character, and the electron-donating ability of the formamidinate bridging groups spanning the dirhodium core affect the nature of the preferred dirhodium DNA adducts. Molecular modeling studies performed on [Rh2(DTolF)2{d(GpG)}] corroborate the structural features obtained by NMR spectroscopy.     

Unprecedented head-to-head right-handed cross-links between the antitumor bis(mu-N,N'-di-p-tolylformamidinate) dirhodium(II,II) core and the dinucleotide d(ApA) with the adenine bases in the rare imino form

Reactions of the anticancer active compound cis-[Rh2(DTolF)2(CH3CN)6](BF4)2 with 9-ethyladenine (9-EtAdeH) or the dinucleotide d(ApA) proceed with bridging adenine bases in the rare imino form (A*), spanning the Rh-Rh bond at equatorial positions via N7/N6. The inflection points for the pH-dependent H2 and H8 NMR resonance curves of cis-[Rh2(DTolF)2(9-EtAdeH)2](BF4)2 correspond to N1H deprotonation of the metal-stabilized rare imino tautomer, which takes place at pKa approximately 7.5 in CD3CN-d3, a considerably reduced value as compared to that of the imino form of 9-EtAdeH. Similarly, coordination of the metal atoms to the N7/N6 adenine sites in Rh2(DTolF)2{d(ApA)} induces formation of the rare imino tautomer of the bases with a concomitant substantial decrease in the basicity of the N1H sites (pKa approximately 7.0 in CD3CN-d3), as compared to the imino form of the free dinucleotide. The presence of the adenine bases in the rare imino form, due to bidentate metalation of the N6/N7 sites, is further corroborated by DQF-COSY H2/N1H and ROE N1H/N6H cross-peaks in the 2D NMR spectra of Rh2(DTolF)2{d(ApA)} in CD3CN-d3 at -38 degrees C. Due to the N7/N6 bridging mode of the adenine bases in Rh2(DTolF)2{d(ApA)}, only the anti orientation of the imino tautomer is possible. The imino form A* of adenine in DNA may result in AT-->CG transversions or AT-->GC transitions, which can eventually lead to lethal mutations. The HH arrangement of the bases in Rh2(DTolF)2{d(ApA)} is indicated by the H8/H8 NOE cross-peaks in the 2D ROESY NMR spectrum, whereas the formamidinate bridging groups dictate the presence of one right-handed conformer HH1R in solution. Complete characterization of Rh2(DTolF)2{d(ApA)} by 2D NMR spectroscopy and molecular modeling supports the presence of the HH1R conformer, anti orientation of both sugar residues about the glycosyl bonds, and N-type conformation for the 5'-A base.     

Unprecedented head-to-head conformers of d(GpG) bound to the antitumor active compound tetrakis (mu-carboxylato)dirhodium(II,II)

The N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) with the DNA fragment d(GpG) have been unambiguously determined by NMR spectroscopy. Previous X-ray crystallographic determinations of the head-to-head (HH) and head-to-tail (HT) adducts of dirhodium tetraacetate with 9-ethylguanine (9-EtGH) revealed unprecedented bridging N7/O6 guanine nucleobases that span the Rh-Rh bond. The absence of N7 protonation at low pH and the notable increase in the acidity of N1-H (pK(a) approximately 5.7 as compared to 8.5 for N7 only bound platinum adducts), suggested by the pH dependence titrations of the purine H8 (1)H NMR resonances for Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2-)[d(GpG)],are consistent with bidentate N7/O6 binding of the guanine nucleobases. The pK(a) values estimated for N1-H (de)protonation, from the pH dependence studies of the C6 and C2 (13)C NMR resonances for the Rh(2)(OAc)(2)(9-EtG)(2) isomers, concur with those derived from the H8 (1)H NMR resonance titrations. Comparison of the (13)C NMR resonances of C6 and C2 for the dirhodium adducts Rh(2)(OAc)(2)(9-EtG)(2) and Rh(2)(OAc)(2)[d(GpG)] with the corresponding resonances of the unbound ligands [at pH 7.0 for 9-EtGH and pH 8.0 for d(GpG)], shows substantial downfield shifts of Deltadelta approximately 11.0 and 6.0 ppm for C6 and C2, respectively; the latter shifts reflect the effect of O6 binding to the dirhodium centers and the ensuing enhancement in the acidity of N1-H. Intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum of Rh(2)(OAc)(2)[d(GpG)] indicate head-to-head arrangement of the guanine bases. The Rh(2)(OAc)(2)[d(GpG)] adduct exhibits two major right-handed conformers, HH1 R and HH2 R, with HH1 R being three times more abundant than the unusual HH2 R. Complete characterization of both adducts revealed repuckering of the 5'-G sugar rings to C3'-endo (N-type), retention of C2'-endo (S-type) conformation for the 3'-G sugar rings, and anti orientation with respect to the glycosyl bonds. The structural features obtained for Rh(2)(OAc)(2))[d(GpG)] by means of NMR spectroscopy are very similar to those for cis-[Pt(NH(3))(2))[d(GpG)]] and corroborate molecular modeling studies.     

Novel binding interactions of the DNA fragment d(pGpG) cross-linked by the antitumor active compound tetrakis(mu-carboxylato)dirhodium(II,II)

Insight into the N7/O6 equatorial binding interactions of the antitumor active complex Rh(2)(OAc)(4)(H(2)O)(2) (OAc(-) = CH(3)CO(2)(-)) with the nucleotide 5'-GMP and the DNA fragment d(pGpG) has been obtained by one- (1D) and two-dimensional (2D) NMR spectroscopy. The lack of N7 protonation at low pH values and the significant increase in the acidity of N1-H (pK(a) approximately 5.6 as compared to 8.5 for N7 only bound platinum adducts), indicated by the pH dependence study of the H8 (1)H NMR resonance for the HT (head-to-tail) isomer of Rh(2)(OAc)(2)(5'-GMP)(2), are consistent with bidentate N7/O6 binding of the guanine. The H8 (1)H NMR resonance of the HH (head-to-head) Rh(2)(OAc)(2)(5'-GMP)(2) isomer, as well as the 5'-G and 3'-G H8 resonances of the Rh(2)(OAc)(2) [d(pGpG)] adduct exhibit pH-independent titration curves, attributable to the added effect of the 5'-phosphate group deprotonation at a pH value similar to that of the N1 site. The enhancement in the acidity of N1-H, with respect to N7 only bound metal adducts, afforded by the O6 binding of the bases to the rhodium centers, has been corroborated by monitoring the pH dependence of the purine C6 and C2 (13)C NMR resonances for Rh(2)(OAc)(2)(5'-GMP)(2) and Rh(2)(OAc)(2) [d(pGpG)]. The latter studies resulted in pK(a) values in good agreement with those derived from the pH-dependent (1)H NMR titrations of the H8 resonances. Comparison of the (13)C NMR resonances of C6 and C2 for the dirhodium adducts Rh(2)(OAc)(2)(5'-GMP)(2) and Rh(2)(OAc)(2) [d(pGpG)] with the corresponding resonances of the unbound ligands at pH 8.0, showed substantial downfield shifts of Deltadelta approximately 11.0 and 6.0 ppm, respectively. The HH arrangement of the bases in the Rh(2)(OAc)(2) [d(pGpG)] adduct is evidenced by intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. The presence of the terminal 5'-phosphate group in d(pGpG) results in stabilization of one left-handed Rh(2)(OAc)(2) [d(pGpG)] HH1 L conformer, due to the steric effect of the 5'-group, favoring left canting in cisplatin-DNA adducts. Complete characterization of the Rh(2)(OAc)(2[d(pGpG)] adduct revealed notable structural features that resemble those of cis-[Pt(NH(3))(2) [d(pGpG)]]; the latter involve repuckering of the 5'-G sugar ring to C3'-endo (N-type) conformation, retention of C2'-endo (S-type) 3'-G sugar ring conformation, and anti orientation with respect to the glycosyl bonds. The superposition of the low energy Rh(2)(OAc)(2) [d(pGpG)] conformers, generated by simulated annealing calculations, with the crystal structure of cis-[Pt(NH(3))(2) [d(pGpG)]], reveals remarkable similarities between the adducts; not only are the bases almost completely destacked upon coordination to the metal in both cases, but they are favorably poised to accommodate the bidentate N7/O6 binding to the dirhodium unit. Unexpectedly, the two metal-metal bonded rhodium centers are capable of engaging in cis binding to GG intrastrand sites by establishing N7/O6 bridges that span the Rh-Rh bond.     

Binding interaction of [Re(H2O)3(CO)3]+ with the DNA fragment d(CpGpG)

Insights into the interaction of the [Re(H2O)3(CO)3]+ complex (1) with the DNA fragment d(CpGpG) have been obtained by one- (1D) and two-dimensional (2D) NMR spectroscopy. The H8 resonances of the single major [Re(H2O)d(CpGpG)(CO)3]- adduct (2) exhibit pH-independent chemical shift changes attributable to metal N7 binding. The structure of this adduct has been characterized by molecular modeling studies based on 1D and 2D NMR data. In solution, 2 shows the presence of two N7-coordinated guanine moieties in a head-to-head (HH) orientation as evidenced by G2H8/G3H8 cross-peaks in the 1H-1H NOESY NMR spectrum. The presence of the 5'-bridging phosphodiester appears to stabilize the HH1 L conformer, as was previously described for related Pt and Rh complexes.     

NMR studies on the novel heterobimetallic complexes [M(dppm)(Ph(2)PCH(2)PPh(2)PPPP) {Pt(PPh3)2}]OTf (M = Rh, Ir) derived from the stepwise activation of white phosphorus

The solution structures of the novel heterobimetallic complexes [Ir(dppm)(Ph(2)PCH(2)PPh(2)PPPP){Pt(PPh(3))2}]OTf and [Rh(dppm)(Ph(2)PCH(2)PPh(2)PPPP){Pt(PPh(3))(2)}]OTf derived from the reaction of Rh and Ir--P(5) precursors with [Pt(C2H4)(PPh3)2] have been unambiguously assigned on the basis of 1H NMR and 31P{1H} NMR data. The results are in agreement with the regio-selective insertion of the {Pt(PPh3)2} moiety resulting in a new pentaphosphorus topology which agrees with the formal formation of a unique phosphonium(+)-tetraphosphabutadienide(2-) ligand.     

Metal-promoted synthesis of amidines containing the model nucleobases 1-methylcytosine and 9-methyladenine

The amidine complexes cis-[L(2)PtNH==C(R){1-MeCy(-2H)}]NO(3) (R = Me, 1a; Ph, 1b, Me(3)C, 1c; Ph(2)(H)C, 1d) and cis-[L(2)PtNH==C(R){9-MeAd(-2H)}]NO(3) (R = Me, 2a; Ph, 2b; Me(3)C, 2c; Ph(2)(H)C, 2d), are formed when cis-[L(2)Pt(μ-OH)](2)(NO(3))(2) (L = PPh(3)) reacts with 1-methylcytosine (1-MeCy) and 9-methyladenine (9-MeAd) in solution of MeCN, PhCN, Me(3)CCN and Ph(2)(H)CCN. Reaction of 1a,b and 2a,b with HCl affords the protonated amidines [NH(2)==C(R){1-MeCy(-H)}]NO(3) (R = Me, 3a; Ph, 3b) and [NH(2)==C(R){9-MeAd(-H)}]NO(3) (R = Me, 4a; Ph, 4b) and cis-(PPh(3))(2)PtCl(2) in quantitative yield. Treatment of 3b and 4b with NaOH allows the isolation of the neutral benzimidamides NH(2)-C(Ph){1-MeCy(-2H)} (5b) and NH(2)-C(Ph){9-MeAd(-2H)} (6b). In the solid state 3b shows a planar structure with the hydrogen atom on N(4) cytosine position involved in a strong H-bond with the NO(3)(-) ion. Intermolecular H-bonds between the oxygen of the cytosine ring and one of the H atoms of the amidine-NH(2) group allow the dimerization of the molecule. A detailed analysis of the spectra of 3b in DMF-d(7) at -55 °C indicates the presence of an equilibrium between the species [NH(2)==C(R){1-MeCy(-H)}]NO(3) and [NH(2)==C(R){1-MeCy(-H)}](2)(NO(3))(2), exchanging with trace amounts of water at 25 °C. [(15)N,(1)H]-HMBC experiments for 5b and 6b indicate that the amino tautomer H(2)N-C(Ph){nucleobase(-2H)}, is the only detectable in solution and such structure has been confirmed in the solid state. The reaction of 5b and 6b with cis-L(2)Pt(ONO(2))(2) (L = PPh(3)), in chlorinated solvents, determines the immediate appearance of a pale yellow colour due to the coordination of the neutral amidine, likely in its imino form HN==C(Ph){nucleobase(-H)}, to give the adducts cis-[L(2)PtNH==C(Ph){nucleobase(-H)}](2+). In fact, addition of "proton sponge" leads to the immediate deprotonation of the amidine ligand with formation of the starting complexes 1b and 2b.     

Optimizing dirhodium(II) tetrakiscarboxylates as chiral NMR auxiliaries

Thirteen enantiopure paddlewheel-shaped dirhodium(II) tetrakiscarboxylate complexes have been checked for their efficiency in the dirhodium method (differentiation of enantiomers by NMR spectroscopy); six of them are new. Their diastereomeric dispersion effects were studied and compared via so-called key numbers KN. Adducts of each complex were tested with five different test ligands representing all relevant donor properties from strong (phosphane) to very weak (ether). Only one of them, the dirhodium complex with four axial (S)-N-2,3-naphthalenedicarboxyl-tert-leucinate groups (N23tL), showed results significantly better for all ligands than the conventional complex Rh* [Rh(II)(2)[(R)-(+)-MTPA](4); MTPA = methoxytrifluoromethylphenylacetate]. On the basis of (1)H{(1)H} NOE spectroscopy and X-ray diffraction, a combination of favourable anisotropic group orientation and conformational flexibility is held responsible for the high efficiency of N23tL in enantiodifferentiation. Both complexes, Rh* and N23tL, are recommended as chiral auxiliaries for the dirhodium experiment.     

Formation of adenine-N3/guanine-N7 cross-link in the reaction of trans-oriented platinum substrates with dinucleotides

The reactions of the anticancer complex trans-[PtCl2{(E)-HN=C(OMe)Me}2] (trans-EE) with a series of ribo and deoxyribodinucleotides have been studied by HPLC and 2D [1H, 15N] HMQC NMR spectroscopy and compared with those of the inactive trans isomer of cisplatin, trans-[PtCl2(NH3)2] (trans-DDP). Reactions of trans-EE with r(ApG) and d(ApG) take place through solvolysis of the starting substrate and subsequent formation of trans G-N7/monochloro and G-N7/monoaqua adducts. Slowly, the monofunctional adducts evolve to a bifunctional adduct forming an unprecedented and unexpected A-N3/G-N7 platinum cross-link spanning two trans positions. For stereochemical reasons, trans platinum complexes cannot form N7/N7 cross-links between adjacent purines in di- or polynucleotides. For the reverse sequence r(GpA), no chelate structure was formed even after a two-week reaction. The reaction of trans-DDP with r(ApG) produces many more products than the analogous reaction with trans-EE. One of these products was identified as the A-N3/G-N7 trans-chelate.     

Rhodium hydride formation in the presence of a bulky monophosphite ligand: a spectroscopic and solid-state investigation

A study has been carried out on rhodium catalyst preforming when modified with the bulky tris(2,4-di-tert-butylphenyl) phosphite, P(Obtbp)(3). X-Ray crystal structure determinations of a tropolone-type precursor complex [Rh(TropBr(3))(CO){P(Obtbp)(3)}].P(Obtbp)(3).CH(3)COCH(3)(TropBr(3)= 3,5,7-tribromotropolonate) and the free P(Obtbp)(3) ligand are reported. Systematic in situ IR and NMR studies of the particular rhodium phosphite modified catalyst and its precursors have led to the identification of two distinct rhodium hydride species. A {(1)H,(31)P} HMBC NMR experiment afforded clarity on the (31)P NMR spectra observed under hydroformylation conditions. The species were identified as [HRh(CO)(3){P(Obtbp)(3)}] and [HRh(CO)(2){P(Obtbp)(3)}(2)]. Attention was also given to the rate of catalyst formation when starting from different rhodium precursors.     

Pentasubstituted ferrocene and dirhodium(II) tetracarboxylate as building blocks for discrete fullerene-like and extended supramolecular structures

The synthesis of a penta(1-methylpyrazole)ferrocenyl phosphine oxide ligand (1) [Fe(C(5)(C(3)H(2)N(2)CH(3))(5))(C(5)H(4)PO(t-C(4)H(9))(2))] is reported together with its X-ray crystal structure. Its self-assembly behavior with a dirhodium(II) tetraoctanoate linker (2) [Rh(2)(O(2)CC(7)H(15))(4)] was investigated for construction of fullerene-like assemblies of composition [(ligand)(12)(linker)(30)]. Reaction between 1 and 2 in acetonitrile resulted in the formation of a light purple precipitate (3). Evidence for the ligand-to-linker ratio of 1:2.5 expected for a fullerene-like structure [Fe(C(5)(C(3)H(2)N(2)CH(3))(5))(C(5)H(4)PO(t-C(4)H(9))(2))](12)[Rh(2)(O(2)CC(7)H(15))(4)](30) was obtained from (1)H NMR and elemental analysis. IR and Raman studies confirmed the diaxially bound coordination environment of the dirhodium linker by comparing the stretching frequencies of the carboxylate group and the rhodium-rhodium bond with those in model compound (5), [Rh(2)(O(2)CC(7)H(15))(4)](C(3)H(3)N(2)CH(3))(2), the bis-adduct of linker 2 with 1-methylpyrazole. X-ray powder diffraction and molecular modeling studies provide additional support for the formation of a spherical molecule topologically identical to fullerene with a diameter of approximately 38 ? and a molecular formula of [(1)(12)(2)(30)]. Dissolution of 3 in tetrahydrofuran (THF) followed by layering with acetonitrile afforded purple crystals of [(1)(2)(2)](∞) (6) [Fe(C(5)(C(3)H(2)N(2)CH(3))(5))(C(5)H(4)PO(t-C(4)H(9))(2))][Rh(2)(O(2)CC(7)H(15))(4)](2) with a two-dimensional polymeric structure determined by X-ray crystallography. The dirhodium linkers link ferrocenyl units by coordination to the pyrazoles but only four of the five pyrazole moieties of the pentapyrazole ligand are coordinated. The ligand-to-linker ratio of 1:2 in 6 was confirmed by (1)H NMR spectroscopy and elemental analysis, while results from IR and Raman are in agreement with the diaxially coordinated environment of the linker observed in the solid state.     

Enantiomer recognition of amides by dirhodium(II) tetrakis[methyl 2-oxopyrrolidine-5(S)-carboxylate

Association constants of the chiral dirhodium(II) carboxamidate Rh(2)(5S-MEPY)(4) with Lewis bases including acetonitrile and amides have been determined by UV-vis titration experiments. With chiral lactams and acyclic acetamides in their R- and S-configurations equilibrium constants with chiral dirhodium carboxamidates are measures of chiral differentiation, and equilibrium constant ratios as high as three have been determined. From equilibrium associations with acetamide, N-methylacetamide, and N,N-dimethylacetamide, as well as equilibrium constants for lactams and acyclic amides, higher values occur when both the amide carbonyl oxygen and N-H are bound to Rh(2)(5S-MEPY)(4). This cooperative bonding mode is confirmed by NMR measurements that show a distinctive shift of a N-H absorption, as well as perturbation of the ligands on dirhodium compound, and they suggest N-H association with a ligated oxygen of Rh(2)(5S-MEPY)(4). Measurements were made on the dirhodium(II) compound from which protective axial ligands have been removed to enhance their reliability.     

The rich chemistry of [Zn2Cp*2]: trapping three different types of zinc ligands in the PdZn7 complex [Pd(ZnCp*)4(ZnMe)2{Zn(tmeda)}

The synthesis, structural characterization, and bonding situation analysis of a novel, all-zinc, hepta-coordinated palladium complex [Pd(ZnCp*)(4)(ZnMe)(2){Zn(tmeda)}] (1) is reported. The reaction of the substitution labile d(10) metal starting complex [Pd(CH(3))(2)(tmeda)] (tmeda = N,N,N',N'-tetramethyl-ethane-1,2-diamine) with stoichiometric amounts of [Zn(2)Cp*(2)] (Cp* = pentamethylcyclopentadienyl) results in the formation of [Pd(ZnCp*)(4)(ZnMe)(2){Zn(tmeda)}] (1) in 35% yield. Compound 1 has been fully characterized by single-crystal X-ray diffraction, (1)H and (13)C NMR spectroscopy, IR spectroscopy, and liquid injection field desorption ionization mass spectrometry. It consists of an unusual [PdZn(7)] metal core and exhibits a terminal {Zn(tmeda)} unit. The bonding situation of 1 with respect to the properties of the three different types of Zn ligands Zn(R,L) (R = CH(3), Cp*; L = tmeda) bonded to the Pd center was studied by density functional theory quantum chemical calculations. The results of energy decomposition and atoms in molecules analysis clearly point out significant differences according to R vs L. While Zn(CH(3)) and ZnCp* can be viewed as 1e donor Zn(I) ligands, {Zn(tmeda)} is best described as a strong 2e Zn(0) donor ligand. Thus, the 18 valence electron complex 1 nicely fits to the family of metal-rich molecules of the general formula [M(ZnR)(a)(GaR)(b)] (a + 2b = n ≥ 8; M = Mo, Ru, Rh; Ni, Pd, Pt; R = Me, Et, Cp*).     

Head-to-head (HH) and head-to-tail (HT) conformers of cis-bis guanine ligands bound to the [Re(CO)3]+ core

We have prepared four complexes of the type [Re(guanine)(2)(X)(CO)(3)] (guanine = 9-methylguanine or 7-methylguanine, X = H(2)O or Br) in order to understand the factors determining the orientation of coordinated purine ligands around the [Re(CO)(3)](+) core. The 9-methylguanine ligand (9-MeG) was chosen as the simplest N(9) derivatized guanine, and 7-methylguanine (7-MeG) was chosen because metal binding to N(9) does not impose steric hindrance. Two types of structures have been elucidated by X-ray crystallography, an HH (head-to-head) and HT (head-to-tail) conformer for each of the guanines. All complexes crystallize in monoclinic space groups: [Re(9-MeG)(2)(H(2)O)(CO)(3)]ClO(4) (2) in P2(1)/n with a = 12.3307(10) A, b = 16.2620(14) A, c = 13.7171(11) A, and beta = 105.525(9) degrees, V = 2650.2(4) A(3), with the two bases in HT orientation and its conformer [Re(9-MeG)(2)(H(2)O)(CO)(3)]Br (3) in P2(1)/n with a = 15.626(13) A, b = 9.5269(5) A, c = 15.4078(13) A, and beta = 76.951(1) degrees, V = 2234.5(3) A(3), and the two bases in an HH orientation. Similarly, [Re(7-MeG)(2)(H(2)O)(CO)(3)]ClO(4) (4) crystallizes in P2(1)/c with a = 13.0708(9) A, b = 15.4082(7) A, c = 14.316(9) A, and beta = 117.236(7) degrees, V = 2563.5(3) A(3), and exhibits an HT orientation and [ReBr(7-MeG)(2)(CO)(3)] (5) in P2/c with a = 17.5117(9) A, b = 9.8842(7) A, c = 15.3539(1) A, and beta = 100.824(7) degrees, V = 2610.3(3) A(3), and shows an HH orientation. When crystals of any of these complex pairs are dissolved in D(2)O, the (1)H NMR spectrum shows a single peak for the H(8) resonance of the respective coordinated purine indicating a rapid equilibrium between HH and HT conformations in solution. DFT calculations simulating the rotation of one ligand around its Re-N bond showed energetic barriers of less than 8.7 kcal/mol. We find no hypochromic effect in the Raman spectrum of 3, which showed base stacking in the solid state. Neither steric interactions nor hydrogen bonding are important in determining the orientation of the ligands in the coordination sphere.     

Isolation of the novel dirhodium(II/II) thiolate compound Rh(2)(eta(1)-C(6)H(5)S)(2)(mu-C(6)H(5)S)(2)(bpy)(2)

The reaction of the anticancer active compound [Rh(2)(mu-O(2)CCH(3))(2)(bpy)(2)(CH(3)CN)(2)][BF(4)](2) (1) (bpy = 2,2'-bipyridine) with NaC(6)H(5)S under anaerobic conditions yields Rh(2)(eta(1)-C(6)H(5)S)(2)(mu-C(6)H(5)S)(2)(bpy)(2).CH(3)OH (2), which was characterized by UV-visible, IR, and (1)H NMR spectroscopies as well as single-crystal X-ray crystallography. Compound 2 crystallizes as dark red platelets in the monoclinic space group C2/c with cell parameters a = 20.398(4) A, b = 11.861(2) A, c = 17.417(4) A, beta = 108.98 degrees, V = 3984.9(14) A(3), Z = 4. The main structural features are the presence of a [Rh(2)](4+) core with a Rh-Rh distance of 2.549(2) A bridged by two benzene thiolate ligands in a butterfly-type arrangement. The axial positions of the [Rh(2)](4+) core are occupied by two terminal benzene thiolates. Cyclic voltammetric studies of 2 reveal that the compound exhibits an irreversible oxidation at +0.046 V in CH(3)CN, which is in accord with the fact that the compound readily oxidizes in the presence of O(2). The fact that this unusual dirhodium(II/II) thiolate compound is formed under these conditions is an important first step in understanding the metabolism of dirhodium anticancer active compounds with thiol-containing peptides and proteins.     

Amidoximes provide facile platinum(II)-mediated oxime-nitrile coupling

The nucleophilic addition of amidoximes R'C(NH(2))═NOH [R' = Me (2.Me), Ph (2.Ph)] to coordinated nitriles in the platinum(II) complexes trans-[PtCl(2)(RCN)(2)] [R = Et (1t.Et), Ph (1t.Ph), NMe(2) (1t.NMe(2))] and cis-[PtCl(2)(RCN)(2)] [R = Et (1c.Et), Ph (1c.Ph), NMe(2) (1c.NMe(2))] proceeds in a 1:1 molar ratio and leads to the monoaddition products trans-[PtCl(RCN){HN═C(R)ONC(R')NH(2)}]Cl [R = NMe(2); R' = Me ([3a]Cl), Ph ([3b]Cl)], cis-[PtCl(2){HN═C(R)ONC(R')NH(2)}] [R = NMe(2); R' = Me (4a), Ph (4b)], and trans/cis-[PtCl(2)(RCN){HN═C(R)ONC(R')NH(2)}] [R = Et; R' = Me (5a, 6a), Ph (5b, 6b); R = Ph; R' = Me (5c, 6c), Ph (5d, 6d), correspondingly]. If the nucleophilic addition proceeds in a 2:1 molar ratio, the reaction gives the bisaddition species trans/cis-[Pt{HN═C(R)ONC(R')NH(2)}(2)]Cl(2) [R = NMe(2); R' = Me ([7a]Cl(2), [8a]Cl(2)), Ph ([7b]Cl(2), [8b]Cl(2))] and trans/cis-[PtCl(2){HN═C(R)ONC(R')NH(2)}(2)] [R = Et; R' = Me (10a), Ph (9b, 10b); R = Ph; R' = Me (9c, 10c), Ph (9d, 10d), respectively]. The reaction of 1 equiv of the corresponding amidoxime and each of [3a]Cl, [3b]Cl, 5b-5d, and 6a-6d leads to [7a]Cl(2), [7b]Cl(2), 9b-9d, and 10a-10d. Open-chain bisaddition species 9b-9d and 10a-10d were transformed to corresponding chelated bisaddition complexes [7d](2+)-[7f](2+) and [8c](2+)-[8f](2+) by the addition of 2 equiv AgNO(3). All of the complexes synthesized bear nitrogen-bound O-iminoacylated amidoxime groups. The obtained complexes were characterized by elemental analyses, high-resolution ESI-MS, IR, and (1)H NMR techniques, while 4a, 4b, 5b, 6d, [7b](Cl)(2), [7d](SO(3)CF(3))(2), [8b](Cl)(2), [8f](NO(3))(2), 9b, and 10b were also characterized by single-crystal X-ray diffraction.     

Cyclo- and carbophosphazene-supported ligands for the assembly of heterometallic (Cu2+/Ca2+, Cu2+/Dy3+, Cu2+/Tb3+) complexes: synthesis, structure, and magnetism

The carbophosphazene and cyclophosphazene hydrazides, [{NC(N(CH(3))(2))}(2){NP{N(CH(3))NH(2)}(2)}] (1) and [N(3)P(3)(O(2)C(12)H(8))(2){N(CH(3))NH(2)}(2)] were condensed with o-vanillin to afford the multisite coordination ligands [{NC(N(CH(3))(2))}(2){NP{N(CH(3))N═CH-C(6)H(3)-(o-OH)(m-OCH(3))}(2)}] (2) and [{N(2)P(2)(O(2)C(12)H(8))(2)}{NP{N(CH(3))N═CH-C (6)H(3)-(o-OH)(m-OCH(3))}(2)}] (3), respectively. These ligands were used for the preparation of heterometallic complexes [{NC(N(CH(3))(2))}(2){NP{N(CH(3))N═CH-C(6)H(3)-(o-O)(m-OCH(3))}(2)}{CuCa(NO(3))(2)}] (4), [{NC(N(CH(3))(2))}(2){NP{N(CH(3))N═CH-C(6)H(3)-(o-O)(m-OCH(3))}(2)}{Cu(2)Ca(2)(NO(3))(4)}]·4H(2)O (5), [{NC(N(CH(3))(2))}(2){NP{N(CH(3))N═CH-C(6)H(3)-(o-O)(m-OCH(3))}(2)}{CuDy(NO(3))(4)}]·CH(3)COCH(3) (6), [{NP(O(2)C(12)H(8))}(2){NP{N(CH(3))N═CH-C(6)H(3)-(o-O)(m-OCH(3))}(2)}{CuDy(NO(3))(3)}] (7), and [{NP(O(2)C(12)H(8))}(2){NP{N(CH(3))N═CH-C(6)H(3)-(o-O)(m-OCH(3))}(2)}{CuTb(NO(3))(3)}] (8). The molecular structures of these compounds reveals that the ligands 2 and 3 possess dual coordination pockets which are used to specifically bind the transition metal ion and the alkaline earth/lanthanide metal ion; the Cu(2+)/Ca(2+), Cu(2+)/Tb(3+), and Cu(2+)/Dy(3+) pairs in these compounds are brought together by phenoxide and methoxy oxygen atoms. While 4, 6, 7, and 8 are dinuclear complexes, 5 is a tetranuclear complex. Detailed magnetic properties on 6-8 reveal that these compounds show weak couplings between the magnetic centers and magnetic anisotropy. However, the ac susceptibility experiments did not reveal any out of phase signal suggesting that in these compounds slow relaxation of magnetization is absent above 1.8 K.     

Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine)

[Rh(nbd)(PCyp(3))(2)][BAr(F) (4)] (1) [nbd = norbornadiene, Ar(F) = C(6)H(3)(CF(3))(2), PCyp(3) = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp(3) ligand in CH(2)Cl(2) solution to form an equilibrium mixture of cis-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 a) and trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H(2) to 2 a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp(3))(2)(H)(2)(eta(2)-H(2))(2)][BAr(F) (4)] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2 a/b, passing through an intermediate which has undergone dehydrogenation of only one PCyp(3) ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(PCyp(3))(NCMe)][BAr(F) (4)] (6). Dehydrogenation of a PCyp(3) ligand also occurs on addition of Na[BAr(F) (4)] to [RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give [Rh(eta(6)-C(6)H(5)X){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] 9 is also reported. Rapid ( approximately 5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp(3))] with Na[BAr(F) (4)] to give [Rh(dppe){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (10), which reacts with H(2) to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H(2))][BAr(F) (4)] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy(2)(Cyp) show that [RhCl(nbd){PCy(2)(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(eta(6)-C(6)H(5)X){PCy(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2 a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.     

Synthesis and characterisation of group nine transition metal complexes containing new mesityl and naphthyl based azaindole scorpionate ligands

Two novel boron-based flexible scorpionate ligands based on 7-azaindole, Li[HB(azaindolyl)(2)(1-naphthyl)] and Li[HB(azaindolyl)(2)(mesityl)] {Li[(Naphth)Bai] and Li[(Mes)Bai] respectively}, have been prepared (mesityl = 2,4,6-trimethylphenyl). These salts have been isolated in two forms, either as dimeric structures which contain bridging hydride interactions with the lithium centres or as crystalline material containing mono nuclear bis-acetonitrile solvates. The newly formed ligands have been utilised to prepare a range of group nine transition metal complexes with the general formula [M(COD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where M = rhodium, iridium; Ar = 1-naphthyl, mesityl; COD = 1,5-cyclooctadiene) and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(Ar)}] (where NBD = 2,5-norbornadiene; Ar = 1-naphthyl, mesityl). These new complexes have been compared to the previously reported compounds which contain the related scorpionate ligands Li[HB(azaindolyl)(2)(phenyl)] and K[HB(azaindolyl)(3)] {Li[(Ph)Bai] and K[Tai] respectively}. Structural characterisation of the complexes [Rh(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}], [Ir(COD){κ(3)-NNH-HB (azaindolyl)(2)(mesityl)}] and [Rh(NBD){κ(3)-NNH-HB (azaindolyl)(2)(naphthyl)}] confirm the expected κ(3)-NNH coordination mode for these new ligands. Spectroscopic analysis suggests strong interactions of the B-H functional group with the metal centres in all cases.     

Unravelling the reaction path of rhodium-MonoPhos-catalysed olefin hydrogenation

The mechanism of the asymmetric hydrogenation of methyl (Z)-2-acetamidocinnamate (mac) catalysed by [Rh(MonoPhos)(2)(nbd)]SbF(6) (MonoPhos: 3,5-dioxa-4-phosphacyclohepta[2,1-a:3,4-a']dinaphthalen-4-yl)dimethylamine) was elucidated by using (1)H, (31)P and (103)Rh NMR spectroscopy and ESI-MS. The use of nbd allows one to obtain in pure form the rhodium complex that contains two units of the ligand. In contrast to the analogous complexes that contain cis,cis-1,5-cyclooctadiene (cod), this complex shows well-resolved NMR spectroscopic signals. Hydrogenation of these catalyst precursors at 1 bar total pressure gave rise to the formation of a bimetallic complex of general formula [Rh(MonoPhos)(2)](2)(SbF(6))(2); no solvate complexes were detected. In the dimeric complex both rhodium atoms are ligated to two MonoPhos ligands but, in addition, each rhodium atom also binds to one of the binaphthyl rings of a ligand that is bound to the other rhodium metal. Upon addition of mac, a mixture of diastereomeric complexes [Rh(MonoPhos)(2)(mac)]SbF(6) is formed in which the substrate is bound in a chelate fashion to the metal. Upon hydrogenation, these adducts are converted into a new complex [Rh(MonoPhos)(2){mac(H)(2)}]SbF(6) in which the methyl phenylalaninate mac(H)(2) is bound through its aromatic ring to rhodium. Addition of mac to this complex leads to displacement of the product by the substrate. No hydride intermediates could be detected and no evidence was found for the involvement at any stage of the process of complexes with only one coordinated MonoPhos. The collected data suggest that the asymmetric hydrogenation follows a Halpern-like mechanism in which the less abundant substrate-catalyst adduct is preferentially hydrogenated to phenylalanine methyl ester.