Reaction of [RhCl(PPh3)2]2 with parahydrogen revealed that the binuclear dihydride [Rh(H)2(PPh3)2mu-Cl)2Rh(PPh3)2] and the tetrahydride complex [Rh(H)2(PPh3)2(mu-Cl)]2 are readily formed. While magnetisation transfer from free H2 into both the hydride resonances of the tetrahydride and [Rh(H)2Cl(PPh3)3] is observable, neither transfer into [Rh(H)2(PPh3)2(mu-Cl)2Rh(PPh3)2] nor transfer between the two binuclear complexes is seen. Consequently [Rh(H)2(PPh3)2(mu-Cl)]2 and [Rh(H)2(PPh3)2(mu-Cl)2Rh(PPh3)2] are not connected on the NMR timescale by simple elimination or addition of H2. The rapid exchange of free H2 into the tetrahydride proceeds via reversible halide bridge rupture and the formation of [Rh(H)2(PPh3)2(mu-Cl)RhCl(H)2(PPh3)2]. When these reactions are examined in CD2Cl2, the formation of the solvent complex [Rh(H)2(PPh3)2(mu-Cl)2Rh(CD2Cl2)(PPh3)] and the deactivation products [Rh(Cl)(H)PPh3)2(mu-Cl)(mu-H)Rh(Cl)(H)PPh3)2] and [Rh(Cl)(H)(CD2Cl2)(PPh3)(mu-Cl)(mu-H)Rh(Cl)(H)PPh3)2] is indicated. In the presence of an alkene and parahydrogen, signals corresponding to binuclear complexes of the type [Rh(H)2(PPh3)2(mu-Cl)(2)(Rh)(PPh3)(alkene)] are detected. These complexes undergo intramolecular hydride interchange in a process that is independent of the concentration of styrene and catalyst and involves halide bridge rupture, followed by rotation about the remaining Rh-Cl bridge, and bridge re-establishment. This process is facilitated by electron rich alkenes. Magnetisation transfer from the hydride ligands of these complexes into the alkyl group of the hydrogenation product is also observed. Hydrogenation is proposed to proceed via binuclear complex fragmentation and trapping of the resultant intermediate [RhCl(H)2PPh3)2] by the alkene. Studies on a number of other binuclear dihydride complexes including [(H)(Cl)Rh(PMe3)2(mu-H)(mu-Cl)Rh(CO)(PMe3)], [(H)2Rh(PMe3)2(mu-Cl)2Rh(CO)(PMe3)] and [HRh(PMe3)2(mu-H)(mu-Cl)2Rh(CO)(PMe3)] reveal that such species are able to play a similar role in hydrogenation catalysis. When the analogous iodide complexes [RhIPPh3)2]2 and [RhI(PPh3)3] are examined, [Rh(H)2(PPh3)2(mu-I)2Rh(PPh3)2], [Rh(H)2(PPh3)2(mu-I)]2 and [Rh(H)2I(PPh3)3] are observed in addition to the corresponding binuclear alkene-dihydride products. The higher initial activity of these precursors is offset by the formation of the trirhodium phosphide bridged deactivation product, [[(H)(PPh3)Rh(mu-H)(mu-I)(mu-PPh2)Rh(H)(PPh3)](mu-I)2Rh(H)2PPh3)2]

Complexes of the form RhX(CO)(PR 3 ) 2 [X = Cl, Br or I; R = Me or Ph] reacted with H 2 to form a series of binuclear complexes of the type (PR 3 ) 2 H 2 Rh(µ-X) 2 Rh(CO)(PR 3 ) [X = Cl, Br or I, R = Ph; X = I, R = Me] and (PMe 3 ) 2 (X)-HRh(µ-H)(µ-X)Rh(CO)(PMe 3 ) [X = Cl, Br or I] according to parahydrogen sensitised 1 H, 13 C, 31 P and 103 Rh NMR spectroscopy. Analogous complexes containing mixed halide bridges (PPhare detected when RhX(CO)(PPh 3 ) 2 and RhY(CO)(PPh 3 ) 2 are warmed together with p-H 2 . In these reactions only one isomer of the products (PPh 3 ) 2 H 2 Rh(µ-I)(µ-Cl)Rh(CO)(PPh 3 ) and (PPh 3 ) 2 H 2 Rh(µ-I)-(µ-Br)Rh(CO)(PPh 3 ) is formed in which the µ-iodide is trans to the CO ligand of the rhodium() centre. When (PPh 3 ) 2 H 2 Rh(µ-Cl)(µ-Br)Rh(CO)(PPh 3 ) is produced in the same way two isomers are observed. The mechanism of the hydrogen addition reaction is complex and involves initial formation of RhH 2 X(CO)(PR 3 ) 2 [R = Ph or Me], followed by CO loss to yield RhH 2 X(PR 3 ) 2 . This intermediate is then attacked by the halide of a precursor complex to form a binuclear species which yields the final product after PR 3 loss. The (PPh 3 ) 2 H 2 Rh(µ-X) 2 Rh(CO)(PPh 3 ) systems are shown to undergo hydride self exchange by exchange spectroscopy with rates of 13.7 s Ϫ1 for the (µ-Cl) 2 complex and 2.5 s Ϫ1 for the (µ-I) 2 complex at 313 K. Activation parameters indicate that ordering dominates up to the rate determining step; for the (µ-Cl) 2 system ∆H ‡ = 52 ± 9 kJ mol Ϫ1 and ∆S ‡ = Ϫ61 ± 27 J K Ϫ1 mol Ϫ1 . This process most likely proceeds via halide bridge opening at the rhodium() centre, rotation of the rhodium() fragment around the remaining halide bond and bridge re-establishment. If the triphenylphosphine ligands are replaced by trimethylphosphine distinctly different reactivity is observed. When RhX(CO)(PMe 3 ) 2 [X = Cl or Br] is warmed with p-H 2 the complex (PMe 3 ) 2 (X)HRh(µ-H)(µ-X)Rh(CO)(PMe 3 ) [X = Cl or Br] is detected which contains a bridging hydride trans to the rhodium() PMe 3 ligand. However, when X = I, the situation is far more complex, with (PMe 3 ) 2 H 2 Rh(µ-I) 2 Rh(CO)(PMe 3 ) observed preferentially at low temperatures and (PMe 3 ) 2 (I)HRh(µ-H)-(µ-I)Rh(CO)(PMe 3 ) at higher temperatures. Additional binuclear products corresponding to a second isomer of (PMe 3 ) 2 (I)HRh(µ-H)(µ-I)Rh(CO)(PMe 3 ), in which the bridging hydride is trans to the rhodium() CO ligand, and (PMe 3 ) 2 HRh(µ-H)(µ-I) 2 Rh(CO)(PMe 3 ) are also observed in this reaction. The relative stabilities of related systems containing the phosphine PH 3 have been calculated using approximate density functional theory. In each case, the (µ-X) 2 complex is found to be the most stable, followed by the (µ-H)(µ-X) species with hydride trans to PH 3 .

The extracellular matrix protein EMILIN1 (elastin microfibril interface located protein 1) is implicated in maintaining blood pressure homeostasis via the N-terminal elastin microfibril interface domain and in trophoblast invasion of the uterine wall via the globular C1q (gC1q) domain. Here, we describe the first NMR-based homology model structure of the human 52-kDa homotrimer of the EMILIN1 gC1q domain. In contrast to all of the gC1q (crystal) structures solved to date, the 10-stranded ␤-sandwich fold of the gC1q domain is reduced to nine ␤ strands with a consequent increase in the size of the central cavity lumen. An unstructured loop, resulting from an insertion unique to EMILIN1 and EMILIN2 family members and located at the trimer apex upstream of the missing strand, specifically engages the ␣4␤1 integrin. Using both Jurkat T and EA.hy926 endothelial cells as well as site-directed mutagenesis, we demonstrate that the ability of ␣4␤1 integrins to recognize the trimeric EMILIN1 gC1q domain mainly depends on a single glutamic acid residue (Glu 933 ). Static and flow adhesion of T cells and haptotactic migration of endothelial cells on gC1q is fully dependent on this residue. Thus, EMILIN1 gC1q-␣4␤1 represents a unique ligand/receptor system, with a requirement for a 3-fold arrangement of the interaction site.EMILIN1 (elastin microfibril interface located protein 1) is a secreted extracellular matrix multidomain glycoprotein (1, 2). It is characterized by a unique arrangement of structural domains, including the elastin microfibril interface domain at the N terminus, an ␣-helical domain predicted to form a coiledcoil structure in the central part of the molecule, a short collagenous sequence, and a region homologous to the globular domain of C1q (gC1q domain) 4 at the C-terminal end (3, 4). Although the role of the coiled-coil region has not yet been elucidated, it has conclusively been demonstrated that EMILIN1 interacts with pro-tumor growth factor-␤ (5) through the elastin microfibril interface domain (6). EMILIN1 deficiency causes systemic arterial hypertension, and the expression of EMILIN1 at physiological levels by binding to pro-tumor growth factor-␤ prevents its maturation by protein convertases (5). Thus, EMILIN1 favorably located at the subendothelium of blood vessels is a new specific antagonist of tumor growth factor-␤, and the function of this constituent of elastic tissues is linked to the pathogenesis of hypertension. The C-terminal gC1q domain is involved in the oligomerization of EMILIN1 (7), in cell adhesion and migration via interaction with the ␣4␤1 integrin (8), and in trophoblast invasion (9).The gC1q signature is found in a variety of proteins, and the essential features of the specific structure-function relationship were recognized with the elucidation of the crystal structure of the homotrimeric gC1q domain of mouse ACRP30 (adipocyte complement-related protein of 30 kDa) (10). It suggested a structural and evolutionary link between the tumor necrosis factor and the gC1q domains and le...

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