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4 Summary and Outlook This thesis reports the electrochemical investigation, synthesis and characterization of novel TM compounds containing low valent aluminum, gallium and substituent free Ga + compounds. Since the rst syntheses of AlCp* 62 63 and GaCp* around 20 years ago the chemistry of low valent group 13 elements with steric bulky groups R has seen substantial progresses. 17, 20, 21, 127, 188 The chemistry of low valent group 13 elements with steric less bulky groups however is much less developed. There are few reports about GaMe as a ligand 43, 163 or substituent free Ga + 27, 28 + 27 and In containing compounds. In addition, while experimental proof for the interesting redox properties was available, 73, 75 no systematic investigation of these properties has yet been carried out. The focus of this thesis lies on the systematic investigation of the redox properties of TM I complexes containing E R (E = Al, Ga; R = Me, Cp*) ligands as well as the formation + of Ga containing compounds. This is investigated by the comparison of the reaction of F + the gallium transfer reagent [Ga2 Cp*][BAr4 ] and the in-situ generation of Ga by one F electron oxidation with [FeCp2 ][BAr4 ]. 4.1 Redox Chemistry of [Ni(ECp*)x(PPh3)y ] Compounds (E = Al, Ga; x = 2, 4; y = 0, 2) 4.1.1 Summary Paramagnetic Ni I complexes are used in a variety of catalytic processes. 115121, 189 The ligands used for the stabilization of the paramagnetic nickel complexes are homoleptic 137 116, 134 or heteroleptic phosphine complexes. Whereas all of the complexes, except I the three coordinated [Ni (PPh3 )3 ][BF4 ], used for the catalysis reactions are either unstable intermediates or neutral complexes. In general the eld of open shell 15 or 17 Ve compounds is not a very intensively investigated eld of transition metal chemistry. The compounds synthesized in this thesis are an interesting extension to the chemistry of open shell compounds. The synthesis and characterization of the rst indenitely in 67 Chapter 4. Summary and Outlook solution or in solid state (under argon) stable cationic tetra-coordinated paramagnetic I homoleptic and heteroleptic Ni complexes with ECp* (E = Al, Ga) ligands could be F achieved. [Ni(AlCp*)4 ][BAr4 ] (1) the rst example of a homoleptic group 13 complex of monovalent nickel could be synthesized in very low yields. The use of the heteroleptic F educt [Ni(AlCp*)2 (PPh3 )] for the oxidation with [FeCp2 ][BAr4 ] leads to the quantitaI I F tive synthesis of the heteroleptic monovalent Ni compound [Ni (AlCp*)2 (PPh3 )][BAr4 ] F + + (2). The reaction of [Ni(GaCp*)2 (PPh3 )] with [C][BAr4 ] (C = FeCp2 or Ga2 Cp* ) interestingly did not yield the corresponding [NiGa(GaCp*)2 (PPh3 )] or a cluster. Instead the Ni center I is oxidized forming the paramagnetic compound F [Ni (GaCp*)2 (PPh3 )][BAr4 ] (3) in quantitative amounts. This reaction is further proof + that Ga can react as an oxidizing agent and secondly the variation of the ligands at the metal center can switch the oxidation from the ligand to the metal center. 4.1.2 Outlook The use of Paramagnetic Ni indicate that the Ni I I complexes as catalysts in dierent catalytic processes complexes presented in this thesis should be investigated in relation to their catalytical properties. The stability of the heteroleptic [Ni(AlCp*)2 (PPh3 )] in comparison to the homoleptic [Ni(AlCp*)4 ] in the oxidized state indicates the stabilizing eect of the phosphines. According to this reactions of the chemically inert [Ni(AlCp*)4 ] with dierent phosphine ligands may be achieved by the reaction of a catalytic amount F of [FeCp2 ][BAr4 ] with [Ni(AlCp*)4 ] in the presence of an excess of the phosphine. AnI F other promising eld of activity may be the reactions of the [Ni (ECp*)2 (PPh3 )2 ][BAr4 ] (E = Al, Ga) complexes with oxygen. Both of these complexes react pyrophoric in the solid state towards oxygen, but slow diusion of oxygen into a uorobenzene solution of the compounds 2 and 3 leads to clear colorless solutions without decomposition. Also slow diusion of oxygen into peruoropolyalkylether containing crystals of 2 and 3 leads to slow oxidation beginning from the edges of the crystals without decomposition yielding colorless amorphous compounds. I F This may indicate that the [Ni (ECp*)2 (PPh3 )2 ][BAr4 ] (E = Al, Ga) complexes are usable for oxygen activation and should be investigated. 68 Chapter 4. Summary and Outlook 4.2 The Coordination Chemistry of Ga+ towards d10 Transition Metal Complexes 4.2.1 Summary The coordination chemistry of substituent free Ga been widely explored. F [PtGa(GaCp*)4 ][BAr4 ] The obtained only by examples the + towards d are reaction F of m 10 -TM complexes has not the terminal the gallium + Ga adduct transfer agent F [Ga2 Cp*][BAr4 ] with [Pt(GaCp*)4 ] and [(GaCp*)4 Pt-( 2 -Ga)-PtH(GaCp*)3 ][(BAr4 )2 ] + with a linear bridging Ga unit synthesized by protolysis of the same educt with F 27 [H(OEt2 )2 ][BAr4 ]. In the course of this thesis the one electron oxidation by F F [FeCp2 ][BAr4 ] and the reaction of [Ga2 Cp*][BAr4 ] towards the homoleptic compounds m [M(GaCp*)4 ] (M = Ni, Pd, Pt) and [Pd3 ( 2 -GaCp*)4 (GaCp*)4 ] have been investi- F gated. [NiGa(GaCp*)4 ][BAr4 ] (4) isformed in the reactions of [Ni(GaCp*)4 ] with F F [C][BAr4 ] (C = Ga2 Cp*, FeCp2 ) as a congener to [PtGa(GaCp*)4 ][BAr4 ]. m m In con- F trast to this [Pd2 ( 2 -Ga)2 ( 2 -GaCp*)2 -(GaCp*)4 ][(BAr4 )2 ] (5) is obtained by the re- F actions of [C][BAr4 ] with [Pd(GaCp*)4 ] independently from stoichiometry. Compound 5 exhibits the longest Pd-GaR bond distance known so far (2.4704(11) Å). No con- m m F gener of [MGa(GaCp*)4 ][BAr4 ] (M = Ni, Pt) could be synthesized. [Pd3 ( 2 -Ga)2 ( 2 - m 3 m2 F GaCp*)( 3 -GaCp*)2 -(GaCp*)3 ][(BAr4 )2 ] (7) was synthesized by one electron oxidation of [Pd ( -GaCp*)4 (GaCp*)4 ]. In comparison to the linear solid state structure of the parent compound it possesses a trigonal bipyramidal geometry. The formation of both Pd complexes 5 and 7 indicate that palladium compounds prefer the coordination of two substituent free Ga + in the bent edge bridging bonding mode. In contrast to this F the reaction of [Pt(GaCp*)4 ] with [FeCp2 ][BAr4 ] does not lead to the gallium transfer F product [PtGa(GaCp*)4 ][BAr4 ]. Instead compound 6 forms the trigonal bipyramidal m m m F Pt trimer[Pt3 2 -Ga( 2 -GaCp*)( 3 -GaCp*)2 (GaCp*)3 ][(BAr4 )2 ] independent from the stoichiometry of the reaction. This is noteworthy as it is the only example so far of the formation of dierent products between the reactions of in-situ generation of Ga F + by F [FeCp2 ][BAr4 ] and the reaction of the gallium transfer reagent [Ga2 Cp*][BAr4 ]. 4.2.2 Outlook The formation of compounds 5 and 7 indicate the ability of TM-complexes to accom- + modate more than one Ga in one compound. The selection of a suitable two electron oxidation agent or the use of a ferrocenium salts with higher charged anions should give 69 Chapter 4. Summary and Outlook access to compounds with more than two Ga + ligands. Another interesting aspect is the reduction of the presented compounds which may lead to the formation of compounds containing gallium in the oxidation state 0. 4.3 The Coordination Chemistry of Ga+ towards Ruthenium Complexes 4.3.1 Summary The behaviour of + Ga towards [Ru(H)2 (GaCp*)2 (PCy3 )2 ] is known. 28 the ruthenium With the addition of Ga + hydride complex a reductive elimination F of the hydrides occur and hydrogen is evolved aording [RuGa(GaCp*)2 (PCy3 )2 ][BAr4 ] (10). Herein the synthesis of the congener [Ru(H)2 (GaCp*)2 (PPh3 )2 ] (8) and the be- F F haviour towards [FeCp2 ][BAr4 ] and [Ga2 Cp*][BAr4 ] is reported. Compound 8 is synthesized by the substitution of two PPh3 ligands from [Ru(H)2 (PPh3 )4 ] by GaCp*. F Treatment of [Ru(H)2 (GaCp*)2 (PPh3 )2 ] with [C][BAr4 ] (C = Ga2 Cp*, FeCp2 ) leads to the addition of + Ga without the loss of the hydrides yielding F [RuGa(H)2 (GaCp*)2 (PCy3 )2 ][BAr4 ] (9). The second dierence between compound 9 and 10 besides the retaining of the hydrides is the great dierence in the bond length + of the terminal Ga (9: 2.684(1) Å; 10: 2.300(2) Å). This dierence is due to the higher 8 6 basicity of the d -Ru center in compound 10 compared to the d -Ru center in compound 9 and the corresponding better bonding to the strong sv p + / -acceptor properties of Ga . The hydride positions in the crystal structure could not be rened from single crystal X-Ray data and were determined by theoretical calculations. These calculations reveal a low energy dierence between the possible isomers 9A (hydrides trans to GaCp), 9B (hydrides trans to PMe3 ) and 9C (GaH + RuH) but model complex 9B can be disregarded due to it having the wrong structure. Model 9A and 9C can be distinguished by the dierence in the bond length of the Ga + to the Ru center and the better tting of the calculated IR-frequencies of 9A to the measured IR-spectrum. This indicates that the model compound 9A may be seen as the most probable model for the positions of + the hydrides which are situated trans to the CaCp* ligands and surrounding the Ga . The formation of [{Ru(GaCp*)3 ( h3 m F -(CH2 )2 C(CH2 ( -Ga)))}2 ][(BAr4 )2 ] (11) was ini- tially achieved by Thomas Cadenbach by the reaction of [Ru(GaCp*)3 (TMM)] with F 28 [Ga2 Cp*][BAr4 ]. In this work a second synthesis method involving the oxidation of F [Ru(GaCp*)3 (TMM)] by [FeCp2 ][BAr4 ] is presented. 70 Chapter 4. Summary and Outlook 4.3.2 Outlook The striking dierence between [Ru(H)2 (GaCp*)2 (PCy3 )2 ] and + [Ru(H)2 (GaCp*)2 (PPh3 )2 ] after the addition of Ga , is the loss of the hydrides in the rst case and the retention of the hydrides in the second case. Because convenient hydrogen storage and release is currently a hot topic in research, it may be an interesting challenge to tune the properties of a ruthenium hydride complex by choosing the ap- + propriate phosphines. The addition of Ga to this complex may allow reversible release of hydrogen at slightly elevated temperatures and the uptake of hydrogen at elevated pressure. 71