Larry R. Falvello

Synthesis, characterisation and crystal structure of cis-dioxomolybdenum(VI) complexes of some potentially pentadentate but functionally tridentate (ONS) donor ligands

Abstract Neutral cis-dioxomolybdenum(VI) complexes with potentially pentadentate ONSNO donor Schiff bases of thiocarbodihydrazone of salicylaldehyde (H3L1), 5-bromo (H3L2), 5-nitro salicylaldehyde (H3L3) and 2-hydroxyacetophenone (H3L4) acting as tridentate ONS donor ligands have been synthesised. The complexes are found to be of the form MoO2LH(ROH) (R=CH3) where, LH=LIH, L2H, L3H and L4H. The complexes were characterised by elemental analyses, UV, IR, and 1H NMR spectroscopy, magnetic susceptibility measurement, molar conductivities in solution and by cyclic voltammetry. Two of the complexes [MoO2(L2H)(MeOH)] and [MoO2(L4H)(MeOH)] were crystallographically characterised. The structures reveal that the molybdenum acceptor centre is present in a distorted octahedral NO4S donor environment. The presence of a substituent either on the aromatic ring of the salicylaldehyde moiety or on the carbon atom of its carbonyl group is found to exhibit little effect on the corresponding metal ligand bond distances and angles. The sixth coordination site of the complexes harboring the weakly coordinated ROH moiety is found to act as the binding site for various neutral monodentate Lewis bases.

Cyclopalladation versus hydroxylation. A case of pH dependence

Alkyl-2-(naphthyl--azo)imidazoles (NaiR, 2 )( C 10H7NNC3H2N21R; R= Me (a), Et (b), PhCH2 (c)) have been reacted with Na2PdCl4 in MeOH or MeCN solutions of Pd(MeCN)2Cl2 to synthesise Pd(NaiR)Cl2 (3). The reaction of Pd(OAc)2 in boiling benzene with NaiR followed by the addition of LiCl has resulted in the synthesis of the cyclopalladated complex Pd(NaiRH)Cl (4). The ligand, NaiR, acts as a N,N-bidentate chelator while NaiRH acts as a tridentate N,N,C-cyclometallat- ing ligand. The infrared spectra of 3 exhibit two PdCl stretches correspond to a cis-PdCl2 geometry, and a single (PdCl) band in 4 suggests one PdCl bond. Cyclopalladation is supported by a single crystal X-ray crystal structural study of Pd(NaiEtH)Cl (4b) and the metallation takes place at C(8)-position. The solution of Pd(NaiR)Cl2 (3) is also irreversibly transformed into Pd(NaiRH)Cl (4) when the pH is adjusted to 4.5-6.0 by NaOAc or other bases (NaOMe, NaOH, LiOH, Li2CO3 etc.). At higher pH values (8-10) the reaction shows the chelative hydroxylation at the C(2)-site to synthesise Pd(NaiRO)Cl (5). The structure of the hydroxylated blue product is also supported by a single crystal X-ray crystal structure of Pd(NaiEtO)Cl (5b). The reaction of Pd(NaiR)Cl2 in MeCN with dilute sodium hydroxide in air, or aqueous silver nitrate under boiling conditions, or its treatment with Tollens reagent in MeCN solution under ambient conditions has also yielded the hydroxylated product. All the compounds have been characterised by elemental analyses, IR, UV-vis and 1 H-NMR data. The solution spectral behaviour has been interpreted by EHMO calculations.

Transition metal (Mn, Co) and zinc formamidinate compounds having the basic beryllium acetate structure, and unique isomeric iron compounds

Abstract Oxidation of Co 2 (DPhF) 3 or hydrolysis of Co 2 (DPhF) 4 (DPhF = N , N ′-diphenylformamidinate) gives Co 4 O(DPhF) 6 ( I ). This tetranuclear compound consists of an oxygen atom centered in a tetrahedron of four-coordinate Co atoms with a DPhF bridge along each edge of the tetrahedron. An idealized T d symmetry is also found for zinc, II , and manganese, III , analogs. The latter, Mn 4 O(DPhF) 6 , crystallizes in a disordered fashion so as to appear as two interpenetrating tetrahedra. Two α 4 -O tetrairon compounds, namely, Fe 4 O(DPhF) 6 ( IV ) and Fe 4 O(DBiPhF) 4 ( V ) (DBiPhF = N , N ′-bisbiphenylformamidinate) are also described. These molecules are isomers of the Co, Zn and Mn molecules in that they have the ‘tetrahedron’ of metal atoms badly distorted by a different distribution of the formamidinate ligands. Two opposite FeFe edges are doubly bridged, another two opposite edges are singly bridged and the remaining two edges are unbridged. The lengths of these phree pairs of opposite edges are short, medium and long and the idealized molecular symmetry is only C 2 . Crystal data for I ·toluene at −60°C are: triclinic, space group P 1 , a = 13.426(3), b = 13.491(3), c = 22.600(5) A , α = 99.55(3), β = 95.39(3), γ = 111.80(3)°, Z = 2 . For II · 1.45C 6 H 14 at −60°C: orthorhombic, space group Pbca , a = 23.8113(8), b = 23.406(2), c = 30.956(1) A , Z = 8 . For III at −150°C: trigonal, space group R 3, a = 23.440(1), c = 10.708(1) A , Z = 3 . For IV · 1.5 toluene at −60°C: triclinic, space group P 1 , a = 14.920(3), b = 15.539(3), c = 19.027(4) A , α = 84.63(3), β = 85.67(3), γ = 63.30(3)°, Z = 2 .

Trigonal-lantern dinuclear compounds of diiron(I,II): the synthesis and characterization of two highly paramagnetic Fe2(amidinato)3 species with short metal-metal bonds ☆

Two examples of a new type of trigonal-lantern compound in which a formal Fe23+ unit is bridged by three amidinato groups are formed by reacting FeCl2(HDPhF)2 (HDPhF = N,N′-diphenylformamidine) and FeCl2(HDPhBz)2 (HDPhBz = N,N′-diphenylbenzamidine) with NaEt3BH and methyllithium in THF. The isolated crystalline products, Fe2(amidinato)3, have been structurally characterized by X-ray crystallography at −60°C. Crystal data for Fe2(DPhF)3 (I): monoclinic, space group C2/c, , β = 93.38(1)°, Z = 4; and for Fe2(DPhBz)3·2C6H6 (II): trigonal, space group , a = 11.237(2), . The FeFe bond distances, 2.2318(8) and 2.198(2) A for compounds I and II, respectively, are the shortest known for any diiron compound. The room temperature magnetic susceptibilities of 7.81 and 7.53 B.M. for I and II and the EPR spectrum of I in a toluene glass (10 K) which shows two signals at g values of 1.99 and 7.94 are consistent with the presence of seven unpaired electrons in an axially symmetric environment.

The hydrogen bond, front and center.

The hydrogen bond has for some years been “edging in” toward the geometric center of transition-metal complexes. This largely noncovalent interaction has long been observed at the periphery of transition-metal complexes, and more recently has been studied closer in, where the presence of a metal atom influences the donor or acceptor properties of one of the direct H-bond participants. Even more intimate interactions have been reported in which C H or N H bonds, commonly at the peripheries of metal-bound ligands, donate a hydrogen atom to a nominally empty coordination site of a transition metal. Now Kozelka et al. report the preparation of a molecular solid in which a molecule of water acts as a hydrogen-bond donor to an axial coordination site of a square-planar platinum center. There is no room for doubt about the location of all of the atoms involved, including hydrogen: For the first time, the accuracy and fidelity of a high-resolution single-crystal neutron-diffraction analysis has captured water interacting through one of its hydrogen atoms—its electropositive frontside—with a transition metal. The concept of hydrogen bonding was proposed far from the realm of coordination chemistry. Some time before they were named, hydrogen bonds drew the attention of experimentalists and theorists. In the context of a study on oxyacids, ionization, and solvent polarity, it was noted that “a free pair of electrons on one water molecule might be able to exert sufficient force on a hydrogen held by a pair of electrons on another water molecule to bind the two molecules together... Such an explanation amounts to saying that the hydrogen nucleus held between 2 octets constitutes a weak bond .” The involvement of electronegative heteroatoms—N, O— in biological molecules suggested that hydrogen bonding should play an important role in establishing macromolecular conformation. It is not surprising in retrospect that one of the early landmark discoveries in which hydrogen bonding was implicated was the prediction of protein secondary structure on the basis of covalently rigid, conformationally flexible amino acid building blocks held into secondary structures— helices and sheets—by hydrogen bonding. It was here that Pauling et al. placed the hydrogen bond in its biological thermodynamic context: “The energy of an N H···O=C hydrogen bond is of the order of 8 kcalmol , and such great instability would result from the failure to form these bonds that we may be confident of their presence.” Just a few years later the essential role of the hydrogen bond in DNA structure became clear. Decades afterward, and with hundreds of thousands of crystal structures established, the evident structural importance of the “classical” hydrogen bonds, largely involving N and O donors and acceptors, accredited the concept that these interactions could be used in designing molecular solids with specific structural and even physical properties. This “crystal engineering” has its own rubric, based largely on graph sets, which has served as an aid in systematizing the known hydrogen-bonded constructs. Motivated by such developments and by improvements in X-ray diffraction techniques, many researchers today dedicate much effort to characterizing molecular solids with hydrogen-bonded superstructures, supplying the readers of crystal structure journals with detailed descriptions of molecular aggregation patterns mediated by both the classical interactions and the more recent additions to the natural toolkit—hydrogen bonds with C H donors or p-cloud acceptors. Apart from the acquisition of vast quantities of new data regarding structures containing hydrogen bonds, there is still an ongoing effort—no less important albeit significantly less extensive—to achieve a new understanding of hydrogen bonding itself. Some of that effort has involved transition metals as possible hydrogen-bond acceptors. A thermodynamic imperative such as that described by Pauling et al. is not the argument for hydrogen bonding at the center of transition-metal complexes. Nor is it clear that these hydrogen bonds will ever be considered as structure-directing elements for extended supramolecular arrays. The role of such interactions in chemical processes has not yet been as clearly established as the role that the hydrogen bond fulfills in the structures and dynamics of living chemistry. Hydrogen bonds with transition-metal acceptors have been studied, rather, as intriguing extensions—neither mere analogues nor simply further examples—of what is known about the hydrogen-bonding phenomenon itself, about its possible complicity in the mechanisms of reactions at transition-metal centers, and about its place among the several roles that hydrogen plays at the core of transition-metal compounds. [*] Prof. L. R. Falvello Dpto. de Qu mica Inorg nica and Instituto de Ciencia de Materiales de Arag n, Universidad de Zaragoza—C.S.I.C. Pedro Cerbuna, 12, 50009 Zaragoza (Spain) Fax: (+ 34)976-761-187 E-mail: falvello@unizar.es

Novel Copper(I) Complexes Containing 1,1‘-Bis(diphenylphosphino)ferrocene (dppf) as a Chelate and Bridging Ligand: Synthesis of Tetrabridged Dicopper(I) Complexes [Cu2(μ-η1-C⋮R)2(μ-dppf)2] and X-ray Crystal Structure of [Cu2(μ-η1-C⋮CC6H4CH3-4)2(μ-dppf)2]

Binuclear copper(I) complexes [Cu(κ2-P,P-dppf)(CH3CN)2][BF4] (1), [Cu(κ2-P,P-dppf)(bipy)][BF4] (2) containing the chelating dppf ligand (dppf = 1,1‘-bis(diphenylphosphino)ferrocene) have been prepared by substitution reactions of the acetonitrile ligands from the complexes [Cu(CH3CN)4][BF4] and (1) with dppf and bipy, respectively. Similarly, the treatment of the complex [Cu2(μ-dppm)2(CH3CN)2][BF4]2 with dppf in CH2Cl2 at room temperature gives the tetranuclear complex [Cu2(μ-dppm)2(κ2-P,P-dppf)2][BF4]2 (3). The analogous bridging chloride tetranuclear complex [Cu2(μ-Cl)2(κ2-P,P-dppf)2] (4) has been also prepared by the addition of dppf to a solution in THF containing an equimolar mixture of CuCl and tetramethylethylenediamine. Complex 4 has been used as a precursor for μ-η1-alkynyl bridging dicopper(I) complexes containing the framework [Cu2(μ-dppf)2]. Complexes [Cu2(μ-η1-C⋮CR)2(μ-dppf)2] (R = C6H4CH3-4 (5), C6H5 (6), CH2OCH3 (7), CH2CH2CH3 (8), (η5-C5H4)Fe(η5-C5H5) (9)) are obtained by the treatment of ...

A new class of dinuclear compounds: The synthesis and x-ray structural characterization of tris(μ-diphenyl-formamidinato) diiron

Abstract A new type of dinuclear metal-metal bonded compound is formed by reacting FeCl 2 (HDPhF) 2 , HDPhF=diphenylformamidine, with butyllithium in THF. The isolated crystalline product, Fe 2 (DPhF) 3 , contains the mixed-valent dinuclear unit bridged by only three formamidinato groups. It has been structurally characterized by X-ray crystallography at −60 °C. Crystal data: monoclinic, space group C 2/ c , a = 17.154(4), b = 8.255(1), c = 23.400(5) A, β = 93.38(1)°, Z = 4. The FeFe bond distance is 2.2318(8) A. The EPR spectrum in a toluene glass (10 K) show two signals at g values of 1.99 and 7.94.

Mixed-ligand systems containing quadruple bonds. Capture and structural characterization of an intermediate in the ligand exchange process leading to new carboxylates of the dimolybdenum(4+ unit. Synthesis and X-ray crystallographic and electrochemical studies of Mo2[(η5-C5H4CO2)Fe(η5-C5H5)]2(O2CCH3)2(C5H5N)2 and [Mo2](η5-C5H4CO2)Fe(η5-C5H5)]4(ax-CH3CN)(ax-DMSO)](DMSO)2

Abstract The compounds Mo2[(η5-C5H4CO2)Fe(η5-C5H5)]2(O2CCH3)2(C5H5N)2 and [Mo2[(η5-C5H4CO2)Fe(η5-C5H5)]4(ax-CH3CN)(ax-DMSO)](DMSO)2 have been prepared by ligand exchange on Mo2(O2CCH3)4 by ferrocenemonocarboxylic acid. The first compound, an intermediate in the complete carboxylate exchange process used in the synthesis of more exotic carboxylates of the quadruply-bonded dimolybdenum(4+) unit crystallizes in the orthorhombic system space group Pbca (no. 61), with a 8.063(1), b 20.653(2), c 21.095(1) A, V 3513(3) A3 and Z = 4. The structure was refined to discrepancy indices R1 = 0.057 and R2 = 0.068. The compound is (a) the first reported tetracarboxylate of dimolybdenum(4+) possessing two different carboxylate ligands; (b) an example of the relatively rare trans geometry seen only infrequently in dimers containing a mixture of bridging ligands. The second compound is the final product for the ligand exchange process. It crystallizes in the triclinic system, space group P 1 (no. 2), with a 11.877(7), b 13.491(11), c 9.922(12) A. α 105.57(2), β 105.62(2), γ 101.86(2)°, V 1407(6) A3 and Z = 1. The compound possesses both eclipsed and staggered ferrocene moieties. The structure was refined to discrepancy indices R1 = 0.0632 and R2 = 0.107. Both compounds exhibit one-electron oxidations with potentials very close to that of the ferrocene-ferrocenium couple itself. Attempts to further oxidize either of the compounds led to their destruction.

Reversible single-crystal-to-single-crystal cross-linking of a ribbon of cobalt citrate cubanes to form a 2D net.

An unprecedented 1-D polymer of cobalt citrate cubanes, 1, has been prepared by wet chemistry techniques and isolated as single crystals with two units of formula Cs2Co7(citr)4(H2O)21 in the unit cell [H4citr = C6H8O7, citric acid], as characterized by X-ray diffraction at T = 278 K. In addition to the four Co atoms in the cubane unit, there are four independent peripheral citrate-bound Co sites. Upon warming at T = 303 K, the polymer undergoes cross-linking through a complex substitution/addition reaction at one of the peripheral Co centers to produce a new 2-D polymer based on the same Co citrate cubane building block. In the transition a second peripheral Co atom undergoes a substitution reaction and separates from the polymer. The new phase, 2, analyzed by X-ray diffraction from the same crystalline sample as 1, has two units of formula Cs2Co7(citr)4(H2O)13.5 in the unit cell. Water egress in the transition amounts to 35.7 mole percent. The volume of the unit cell diminishes by 12.4%. The Co center at...

Synthesis and characterization of PdII complexes containing cyclic bis-ylides

The reaction of Ph2PCH2PPh2 (dppm) with ClCH2C(O)CH2Cl (1:1 molar ratio) in refluxing CHCl3 gives the new ylide-phosphonium salt 1, through quaternization of the two P atoms of the dppm and spontaneous loss of HCl. Compound 1 reacts with NEt3 in CHCl3 or CH2Cl2 solution (1:1 molar ratio) giving a mixture of the bis-ylide compound 2a and the ylide-methanide 2b (molar ratio 2a:2b=1:2). This mixture (prepared in situ) reacts with PdCl2(NCMe)2 to give the corresponding dichloride(bis-ylide) complex 3a (coordination through the two ylidic carbons) and the dichloride(ylide-methanide) complex 3b (coordination through the ylide carbon and the methanide carbon), in a molar ratio 3a:3b=1:2. The reaction of the mixture 3a:3b with AgClO4 (1:2 molar ratio, NCMe or Me2CO) in the presence of neutral ligands L gives the corresponding mixtures of the dicationic complexes [Pd(L)2(C,C-bis-ylide)](ClO4)2 (4a, L=NCMe; 5a, L=pyridine) and [Pd(L)2(C,C-ylide-methanide)](ClO4)2 (4b, L=NCMe; 5b, L=pyridine) (molar ratio 4a:4b=1:2; molar ratio 5a:5b=3:1). On the other hand, the reaction of Ph2PCH2PPh2 (dppm) with ClCH2C(CH2)CH2Cl (1:1 molar ratio) in refluxing ClCH2CH2Cl gives the bis-phosphonium salt 6. This salt reacts with LitBu and PdCl2(NCMe)2 (1:2.2:1 molar ratio) in THF affording the ylide-methanide complex 7. The crystal structures of complexes 3b·3dmso and 7·CHCl3 have been determined by X-ray diffraction.