Pyrazine- and pyridine-based π-conjugated σ-donor molecules, such as 4,4′-bipyridine, 1,2-di(4-pyridyl)ethylene, 3,5-dipyridyl-1,2,4-triazole, N,N′-bis(4-pyridylmethylidene)benzene-1,4-diamine, 2,5-di(pyridylmethylidene)cyclopentanone, 2,6-di(4-pyridylmethylidene)cyclohexanone (LL, 2a–2g) can successfully be used to span heterobimetallic π-tweezer units of the type [{[Ti](μ-σ,π-CCSiMe3)2}M]+ ([Ti]=(η5-C5H4SiMe3)2Ti; M=Cu, Ag). The thus accessible di-cationic species [{[Ti](μ-σ,π-CCSiMe3)2}MLLM{(Me3SiCC-μ-σ,π)2[Ti]}]2+ (4), which are formed via the formation of [{[Ti](μ-σ,π-CCSiMe3)2}MLL]+ (3) complexes, can be isolated in yields between 66% and 99%.However, when C5H4NCHCHC6H4CHCHNC5H4 (5a) and C5H4NCHNC6H4CHCHNC5H4 (5b), respectively, are reacted with {[Ti](μ-σ,π-CCSiMe3)2}AgBF4(1c) in a 1:1 molar ratio, then the silver(I) ion is released from the organometallic π-tweezer 1c and coordination polymers [AgBF4·5a]n (6a) and [AgBF4·5b]n (6b) along with [Ti](CCSiMe3)2 (7) are formed in quantitative yield.

The synthesis and characterization of the pyrazine-bridged tweezer complex [{[Ti](CdropCSiMe(3))(2)}Ag(OClO3)](2)pz (4) and bimetallic {[Ti](CdropCSiMe(3))(2)}Ag(OClO3)(pz) (3) {[Ti] = (eta(5)-C5H4SiMe3)(2)-Ti; pz = 1,4-pyrazine, C4H4N2} is discussed. The solid-state structure of 4 is reported. The structure of 4 consists of two heterobimetallic titanium-silver tweezer fragments of type {[Ti](mu-sigma, pi-CdropCSiMe(3))(2)}Ag(OClO3) which are spanned by pyrazine. Each silver(I) ion is thereby tetra-coordinated by two pi-coordinated alkynes and the datively bound pyrazine and OClO3 moieties. Cyclic voltammetric studies of 3 and 4 are reported. (C) 2003 Elsevier B.V. All rights reserved.

The synthesis and reaction chemistry of heterobi- (TiM) (5) tri- (Ti2M) (3) and pentanuclear (Ti2MFe2) (4) complexes (M=Cu, Ag) is described. The X-ray structure analysis as well as the electrochemical behaviour of one example is reported.

Treatment of [Ti](Cl)(C=CSiMe3) (1) {[Ti] = (eta(5)-C5H5)(2)Ti} with Ni(CO)(4) (2) in a 1:1 molar ratio produces the heterobimetallic early-late transition metal complex {[Ti](Cl)(CdropCSiMe(3))} Ni(CO) (3), which features a low-valent Ni(CO) entity stabilized by a datively bonded Cl and a eta(2)-coordinated Me(3)SiCdropC ligand. As side-products [Ti]Cl-2 (8) and {[Ti](CdropCSiMe(3))(2)}Ni(CO) (5) are formed. The latter complex can also be synthesized by the reaction of [Ti](CdropCSiMe(3))(2) (4) with equimolar amounts of 2. If 3 is reacted with stoichiometric amounts of P(OR)(3) (6a, R = C6H5; 6b, R = C6H4CH3-2; 6c, R = (C6H4Bu)-Bu-t-2) the bis(alkynyl) titanocene 4, (CO)(2)Ni[P(OR)(3)](1) (7a, R = C6H5; 7b, R = C6H4CH3-2; 7c, R = (C6H4Bu)-Bu-t-2), complex 8, {[Ti](mu,eta(1):eta(2)-CdropSiMe(3))}(2) (9) along with Me(3)SiCdropC-CdropCSiMe(3) (10) is produced. A possible mechanism for the formation of these species is presented. The solid-state structure of 7b is reported. Complex 7b crystallizes in the tetragonalic space group P-42(1)c with the following parameters: a = 14.8 52(2), b = 14.8 52(2), c = 19.410(4) Angstrom, V = 4281.5(12) Angstrom(3), Z = 4 and p = 1.271 g cm(-3). Mononuclear 7b features a Ni(0) centre in a pseudo-tetrahedral environment, caused by the CO and P(OC6H4CH3-2)(3) ligands. (C) 2002 Elsevier Science B.V. All rights reserved.

Based on the bis(alkinyl) titanocene [Ti](CCSiMe3)2 (1) a series of mixed early–late metal complexes of the general type {[Ti](CCSiMe3)2}ML {[Ti]=(η5-C5H4SiMe3)2Ti; M=Ni, Pd; L=PPh3, P(OMe)3, P(OPh)3} was prepared. The solid-state structure of {[Ti](CCSiMe3)2}Pd(PPh3) (2) is reported. Complex 2 exhibits typical features of early–late heterobimetallic tweezer complexes: (i) a pseudo-tetrahedrally coordinated Ti(IV) centre; (ii) a trigonal-planar coordination sphere around the Pd(0) centre, comprised of the two η2-coordinated Me3SiCC entities and the datively-bound PPh3 ligand; (iii) a lengthening of the CC triple bonds upon their η2-coordination to Pd(0); and (iv) a trans-deformation of the TiCCSiMe3 units due to the tweezer effect. Cyclic voltammetric studies on 2 and {[Ti](CCSiMe3)2}NiL [3, L=CO; 4a, L=P(OCH3)3] reveal an electron donating character of the coordinated M(0) centres, which is demonstrated by the shift of the Ti(IV)/Ti(III) reduction to a more negative potential. This reductive process also exhibits a dependence on the π-acidity of the respective Lewis-base L.

With bis(alkynyl) titanocene ligands acting as chelating moieties for low-valent transition-metal heterobimetallic Ti(IV)M(I) (M=Cu, Ag), complexes of the type {[Ti](CCSiMe3)}MX ([Ti](η5-C5H4SiMe3)2Ti; X=Cl; Br; I) were prepared. On reacting such complexes with {[Ti](CCSiMe3)2}MX′ (X′=OClO3) tetrametallic species of the type [{[Ti](CCSiMe3)2}MXM{(Me3SiCC)2[Ti]}]+ are formed. Their redox electrochemistry was studied with cyclic voltammetry (CV); for comparison, data of related bimetallic complexes were obtained. Results indicate a strong intramolecular interaction between the Group 11 metal held in place by the organometallic {[Ti](CCSiMe3)2} π-tweezers.

The preparation of heterobimetallic transition metal complexes of the type {[Ti](C CR)(2)}Ni(CO) {R = SiMe3: 3a, [Ti] = (eta(5)- C5H5)(2)Ti; 3b: [Ti] = (eta(5) -C5H5)(eta(5) -C5H4SiMe3)Ti; 3c, [Ti] = (eta(5)-C5H4SiMe3)(2)Ti; R = 'Bu: 3d, [Ti] = (eta(5) -C5H4SiMe3)(2)Ti; R = Ph: 3e, [Ti] = (eta(5) -C5H4SiMe3)(2)Ti}, in which next to an early (Ti) a late (Ni) transition metal is present, is described. Additionally, the reaction chemistry of 3a and 3c towards P(OR')(3) (4a, R' = CH3; 4b, R' = C6H5; 4c, R' = C6H4Me-2; 4d, R' = C6H4'Bu-2) is reported. In these reactions the nickel-bound carbonyl ligand is replaced by P(OR')(3) producing {[Ti](C CSiMe3)(2)}Ni[P(OR')(3)] {[Ti] = (eta(5)- C5H5)(2)Ti: 5a, R'= CH3; 5b, R'= C6H5; 5c, R= C6H4Me-2; [Ti] = (eta(5)-C5H4SiMe3)(2)Ti: 5d, R'= CH3; 5e, R'= C6H5} along with Ni(CO)(2)[P(OR')(3)](2) (6a, R' = C6H5; 6b, R' = C6H4Me-2; 6c, R' = (C6H4Bu)-Bu-t-2). It appeared that the latter reaction strongly depends on the sterical demand, Tolman cone angle, of the respective phosphites used: while, in the reaction of 3a or 3c with 4a selectively 5a and 5d is formed, with more bulky substituents R', e.g. R'= C6H5 and C6H4Me-2, complexes 5b and 5c along with 6a and 6b are produced. Changing to even more sterical demanding groups such as R' = (C6H4Bu)-Bu-t-2 than exclusively 6c is formed. The dynamic behaviour of 5 in solution is discussed. When 3a is treated with equimolar amounts of PPh3 (7) the titanium-nickel alkynyl species [Ti](mu-eta(1):eta(2)-C CSiMe3)Ni(PPh3)(mu-eta(1):eta(2)-C CSiMe3) {8a, [Ti] = (eta(5)-C5H5)(2)Ti} is accessible via an alkynyl-transfer reaction from titanium to nickel. However, on treatment of 3c with 7 no reaction occurs. Arguments for the different behaviour of 3a-3c towards 4 and 7 will be presented. The result of the X-ray structure analysis of complexes 5d and 5e are reported. Both complex crystallize in the monoclinic space group P2(1)/n. Cell parameters for 5d: a = 10.9390(10), b = 15.585(4), c = 22.950(3) Angstrom, beta = 92.861(7)degrees, V= 3907.7(14) Angstrom(3), Z = 4 and delta = 1.189 g mol(-1). 5e: a = 17.694(9), b = 22.620(10), c = 24.510(10) Angstrom, beta = 103.90(4)degrees, V= 9523(8) Angstrom(3), Z = 8 and delta = 1.236 g mol(-1). In both complexes a low-valent Ni[P(OR')(3)] building block (5d, R'= CH3; 5e, R'= C6H5) is stabilised by the chelating effect of the organometallic pi-tweezer [Ti](C CSiMe3)2, giving rise to a trigonal-planar environment at the nickel atom. The early (Ti) and late (Ni) transition metal centers are thereby bridged via the sigma- and pi-bound alkynyl groups Me3SiC C.The influence of the different sterical demanding phosphites onto the [Ti](C CsiMe(3))(2) framework will be discussed. (C) 2002 Elsevier Science B.V. All rights reserved.

The synthesis and reaction chemistry of bi- and oligometallic transition metal complexes containing Groups 4, 8, 10 as well as 11 metal atoms are presented. The respective metals are thereby linked by carbon rich π-conjugated organic units, mainly σ- and π-bonded alkynyls. The structural aspects and electrochemical properties of the corresponding complexes will be discussed.

The reaction of [Ti](CCPh)2 (1) {[Ti]=(η5-C5H4SiMe3)2Ti} with equimolar amounts of CuBr, Ni(PPh3)3 or Pd(PPh3)4 produces the heterobimetallic early–late transition metal complexes of general type {[Ti](CCPh)2}MX [2: MX=CuBr, 3: MX=Ni(PPh3), 4: MX=Pd(PPh3)} in which the respective transition metal atoms are linked by σ,π-bound alkynyl ligands. The solid-state structure of 1 and 4 is reported. In heterobimetallic 4 the Pd(0) centre possesses a trigonal–planar environment caused by the two η2-coordinated Me3SiCC ligands and the datively bonded PPh3 group. The PPh3 moiety is thereby located out of the best Ti(CCSi)2Pd plane. Comparative cyclic voltammetric studies on complexes 1–4 as well as {[Ti](CCPh)2}Ni(CO), for comparison, are presented. These studies reveal a strong influence of the η2-coordinated low-valent transition metal complex fragments MX on the reduction behaviour of the Ti(IV) centre.

The synthesis of the titanocene dichlorides (eta (5)-C5H5)(eta (5)-C5H4SiMe3)TiCl2 (3) and (eta (5)-C5H4CO2R)(2)TiCl2 (8a, R = CH3; 8b, R = CH2CH3), which contain either electron-donating or electron-withdrawing substituents at the cyclopentadienyl fragments is discussed. While the reaction of 8a or 8b with LiC drop CR' in different stoichiometric ratios leads only to product mixtures from which no pure components could be isolated, treatment of 3 with two equivalents of LiC drop CR' (9a, R'= C6H5; 9b, R'='Bu; 9c, R'= SiMe3) selectively produces the bis(alkinyl) titanocenes (eta (5)-C5H5)(eta (5)-C-5-C5H(4)SiMe(3))Ti(C drop CR')(2) (10a, R'= C6H5; 10b, R' =Bu; 10c, R'= SiMe3). However, it was found that when complex 10c is stirred in tetrahydrofuran solutions, Me3SiC dropC is eliminated and Me3SiC dropC-C drop CSiMe3 (11) along with [(n(5)-C5H5)(eta (5) -C5H4SiMe3)Ti(C drop CSiMe3)(2) (12) is formed. The solid-state structure of (eta (5)-C5H5)(eta (5)-C5H4SiMe3)Ti(C drop CSiMe3)(2) (10c) is reported. Complex 10c crystallises in the monoclinic space group Pc with two independent molecules in the asymmetric unit and with the cell constants a 20.8131(6), b = 10.6615(3), e = 12.2543(4) Angstrom, beta = 101.12(3)degrees, V = 2668.14(14) Angstrom (3) and Z = 4. 10c exhibits a pseudotetrahedrally coordination sphere around the Ti(IV) centre comprised of the two sigma -bonded alkynyl ligands Me3SiC dropC and the eta (5)-coordinated cyclopentadienyl. moieties C5H5 as well as C5H4SiMe3. (C) 2001 Elsevier Science B.V. All rights reserved.

The reaction of [Ti]Cl-2 (1) ([Ti]=(eta(5)-C5H4SiMe3)(2)Ti) with two equivalents of LiC=C-SiMe2-C=CSiMe3 (2) produces [Ti](C=C-SiMe2-C=CSiMe3)(2) (3). On treatment with [MX] (M = Cu: 4a X = Cl, 4b X = Br; M = Ag: 5a X = Cl, 5b X = Br) the tweezer complexes ([Ti](C=C-SiMe2-C=CSiMe3)(2)) MX (M = Cu: 6a X = Cl, 6b X = Br; M = Ag: 7a X = Cl, 7b X = Br) are formed in which the Ti-C=C-Si units are eta(2)-coordinated to a monomeric copper(I) or silver(I) halide moiety. When 6b is further reacted with [CuBr] (4b), oligomeric ([Ti](C=C-SiMe2-C=CSiMe3)(2)(CuBr)(3))(n) (8) is formed. This molecule contains a (eta(2)-TiC=CSi)(2)CuBr entity next to two (eta(2)-SiC=CSi)CuBr moieties, of which the latter building blocks are responsible for the oligomeric structure. In addition, 8 can be prepared by the direct reaction of 3 with an excess of 4b, respectively. However, when an excess of [AgX] is used, the only formed products are 7; no polymeric material is obtained. A Group 11 metal exchange reaction is noticed, when 7a or 7b are reacted with [CuX]: depending on the amount of [CuX] used, monomeric 6 or oligomeric 8 is produced. An explanation is given by a better bonding synergysmus for the alkyne-to-copper interaction. The result of the X-ray structure analysis of compound Tb is reported. The compound 7b crystallizes in the monoclinic space group C2/c with cell constants a = 25.097(8), b = 11.327(3), c = 19.014(6) Angstrom, beta = 122.36(3)degrees V = 4566(2) Angstrom(3) and Z = 4. The compound 7b contains a monomeric (eta(2)-alkyne)(2)AgBr moiety in which the silver(I) center possesses a trigonal-planar environment, caused by the eta(2)-coordinated TiC=CSi units as well as a eta(1)-bonded bromine atom. However this differs from the behavior of compounds 6 and 7 in solution, where all four C=C building blocks of the TiC=CSi as well as the SiC=CSi units are complexed by the transition metal entities MX (M = Cu, Ag X = Cl, Br). (C) 1999 Elsevier Science S.A. All rights reserved.