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Project Title:

Binding and generation of molecular hydrogen through mono- and dinuclear metal complexes

Ref.No.: 137

Project Type and Category:

Basic research

Project Duration:

See ref. No.: 73

Project Participants:

Wolfgang Kaim, Stefan Greulich, Frank Baumann, Christoph Titze, Ralf Reinhardt, Institut für Anorganische Chemie , Universität Stuttgart

Sponsor:

# See ref. No.: 73

Project Budget and
Funding:

# See ref. No.: 73

Project Description and Objectives:

The recent discovery [1] of a first stable molecular coordination compound of dihydrogen, H2, bonded in a side-on fashion to a mononuclear metal center in W(CO)3(PiPr3)2(H2) (1) has caused an enormous increase in experimental and theoretical studies related to the interaction of H2 with coordinatively unsaturated metal sites [2-4]. While the principles of the bonding situation seem to be well understood [3], the significance of dihydrogen-binding and -releasing metal complexes in homogeneous solution ractions is still far from clear.

Technical Goals:

In a fundamental approach we have previously studied the classical H2 binding systems M(CO)3(PR3)2 [5]. Realizing the frequent necessity for association of two or more redox reaction centers for efficient multielectron catalysis [6] such as the conversion 2H+ + 2e- H2, we set out
(i) to understand the functions of the components in the well-used H2-releasing organometallic catalysts [(C5Me5)RhCl(L)]+ (2) [7,8], and
(ii) to explore the possibility of intramolecular coupling of electron and atom transfer reactivity in corresponding dinuclear compounds {[(C5Me5)RhCl]2(L)}2+ bridged by ligands .

Project Status

#

Preliminary or Final Results:

(i) Cyclic voltammetric and spectroelectrochemical analyses [9] of the crucial intermediates (C5Me5)Rh(L) in the above cycle (2) have suggested a major role of the chelate ligand L in bringing about the two-electron processes required for hydride generation and eventual transfer. Apparently, the cooperation of reducible L with the metal is strong, creating an      electron-delocalized situation with the capacity of intermediate electron buffering.
Studies on analogous complexes [(C5Me5)IrCl(L)]+ and (C5Me5)Ir(L) with the heavier homologue of rhodium., i.e. iridium [10], revealed a similar pattern, however, the final hydride transfer step to H+ (to produce H2) proved to be much slower in this case. Stable and reducible hydrides [(C5Me5)IrH(L)]+ were thus obtained. With the lighter homologue of rhodium, i.e cobalt, the corresponding complexes are less persistent and exhibit two separated one-electron processes which allows for the production of unwanted radical species by reaction with the Co(II) intermediate [11].
In summary, the [(C5Me5)RhCl(L)]+/(C5Me5)Rh(L) systems have the benefit of two-electron reactivity (in cooperation with L) but do not excessively stabilize the penultimate hydridic intermediate: an ideal situation for the intended catalysis.
(ii) Coupling of two formally identical reaction or even catalysis centers by an electron exchange-mediating molecular bridge was accomplished (3) using new synthetic methodology [11]. The ECE mechanism underlying the first activating step of the catalytic cycle (2) was thus duplicated and coupled, just as simpler electron transfer processes were duplicated and coupled in that landmark compound for pure electron transfer, the Creutz-Taube ion (4) [5].
With three different ligand bridges, three different coupling patterns were observed [11]. One pattern was a simple splitting of the two ECE processes [12] by about 0.5 V in the anodic and cathodic direction relative to the one observed for the mononuclear compoud with isolated reaction centers (bpym compounds; ECE/ECE ® ECE + ECE). The two other systems (bptz and bpip), however, showed a qualitatively different kind of interaction:
ECE/ECE ® E + EC + ECE (bptz) and ECE/ECE ® E + EC + EC + E (bpip).
In both cases there is a ligand-based electron reservoir function [12] preceding the first chloride loss, a type of reaction which is essential for electron transfer catalysis. Current efforts are under way to make use of these materials for improved hydrogen-producing catalysis.

Related Reference Papers and Other Publications:

[1] Kubas, G.J. Acc. Chem. Res. 1988, 21, 120.
[2]Saillard, J.-Y.; Hoffmann, R. J. Am. Chem.            Soc. 1984, 106, 2006.
[3]Jessop, P.G.; Morris, R.H. Coord. Chem. Rev. 1992, 121, 155.
[4]Reinhardt, R.; Fees, J.; Klein, A.; Sieger, M.; Kaim, W. in Wasserstoff als Energieträger, VDI Verlag, Düsseldorf, 1994, p 133.
[5]Bruns, W.; Kaim, W.; Waldhör E.; Krejcik, M. Inorg. Chem 1995, 34, 663.
[6] Tributsch, J. J. Eletroanal. Chem. 1992, 331, 783.
[7](a) Kölle U.; Grätzel, M. Angew. Chem. 1987, 99, 572; Angew. Chem. Int. Ed. Engl. 1987, 26, 568. (b) Kölle, U.; Kang. B.-S.; Infelta, P.; Comte, P.; Grätzel, M. Chem. Ber. 1989, 112, 1869.
[8] Cosnier, S.; Deronzier, A.; Vlachopoulos, N. J. Chem. Soc., Chem. Commun. 1989, 1259.
[9] (a) Ladwig, M.; Kaim, W. J. Organomet. Chem. 1991, 419, 233. (b) Reinhardt R.; Kaim, W. Z. Anorg. Allg. Chem. 1993, 619, 1998. (c) Greulich, S.; Kaim, W.; Stange, A.; Stoll, H.; Fiedler, J.; Zalis, S. Inorg. Chem., in Teilprojekt von sfb 270 Nr. 73 print.
[10] Ladwig, M.; Kaim, W. J. Organomet. Chem. 1992, 439, 79.
[11]Reinhardt, R. Ph.D. Thesis, University of Stuttgart, 1995.
[12]Astruc, D. Electron Transfer and Radical Processes in Transition-Metal Chemistry, VCH, Weinheim, 1995.

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Project Title: Binding and generation of molecular hydrogen through mono- and dinuclear metal complexes		Ref.No.: 137
Project Type and Category:	Basic research
Project Duration:	See ref. No.: 73
Project Participants:	Wolfgang Kaim, Stefan Greulich, Frank Baumann, Christoph Titze, Ralf Reinhardt, Institut für Anorganische Chemie , Universität Stuttgart
Sponsor:	# See ref. No.: 73
Project Budget and Funding:	# See ref. No.: 73
Project Description and Objectives:	The recent discovery [1] of a first stable molecular coordination compound of dihydrogen, H2, bonded in a side-on fashion to a mononuclear metal center in W(CO)3(PiPr3)2(H2) (1) has caused an enormous increase in experimental and theoretical studies related to the interaction of H2 with coordinatively unsaturated metal sites [2-4]. While the principles of the bonding situation seem to be well understood [3], the significance of dihydrogen-binding and -releasing metal complexes in homogeneous solution ractions is still far from clear.
Technical Goals:	In a fundamental approach we have previously studied the classical H2 binding systems M(CO)3(PR3)2 [5]. Realizing the frequent necessity for association of two or more redox reaction centers for efficient multielectron catalysis [6] such as the conversion 2H+ + 2e- H2, we set out (i) to understand the functions of the components in the well-used H2-releasing organometallic catalysts [(C5Me5)RhCl(L)]+ (2) [7,8], and (ii) to explore the possibility of intramolecular coupling of electron and atom transfer reactivity in corresponding dinuclear compounds {[(C5Me5)RhCl]2(L)}2+ bridged by ligands .
Project Status	#
Preliminary or Final Results:	(i) Cyclic voltammetric and spectroelectrochemical analyses [9] of the crucial intermediates (C5Me5)Rh(L) in the above cycle (2) have suggested a major role of the chelate ligand L in bringing about the two-electron processes required for hydride generation and eventual transfer. Apparently, the cooperation of reducible L with the metal is strong, creating an electron-delocalized situation with the capacity of intermediate electron buffering. Studies on analogous complexes [(C5Me5)IrCl(L)]+ and (C5Me5)Ir(L) with the heavier homologue of rhodium., i.e. iridium [10], revealed a similar pattern, however, the final hydride transfer step to H+ (to produce H2) proved to be much slower in this case. Stable and reducible hydrides [(C5Me5)IrH(L)]+ were thus obtained. With the lighter homologue of rhodium, i.e cobalt, the corresponding complexes are less persistent and exhibit two separated one-electron processes which allows for the production of unwanted radical species by reaction with the Co(II) intermediate [11]. In summary, the [(C5Me5)RhCl(L)]+/(C5Me5)Rh(L) systems have the benefit of two-electron reactivity (in cooperation with L) but do not excessively stabilize the penultimate hydridic intermediate: an ideal situation for the intended catalysis. (ii) Coupling of two formally identical reaction or even catalysis centers by an electron exchange-mediating molecular bridge was accomplished (3) using new synthetic methodology [11]. The ECE mechanism underlying the first activating step of the catalytic cycle (2) was thus duplicated and coupled, just as simpler electron transfer processes were duplicated and coupled in that landmark compound for pure electron transfer, the Creutz-Taube ion (4) [5]. With three different ligand bridges, three different coupling patterns were observed [11]. One pattern was a simple splitting of the two ECE processes [12] by about 0.5 V in the anodic and cathodic direction relative to the one observed for the mononuclear compoud with isolated reaction centers (bpym compounds; ECE/ECE ® ECE + ECE). The two other systems (bptz and bpip), however, showed a qualitatively different kind of interaction: ECE/ECE ® E + EC + ECE (bptz) and ECE/ECE ® E + EC + EC + E (bpip). In both cases there is a ligand-based electron reservoir function [12] preceding the first chloride loss, a type of reaction which is essential for electron transfer catalysis. Current efforts are under way to make use of these materials for improved hydrogen-producing catalysis.
Related Reference Papers and Other Publications:	[1] Kubas, G.J. Acc. Chem. Res. 1988, 21, 120. [2]Saillard, J.-Y.; Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 2006. [3]Jessop, P.G.; Morris, R.H. Coord. Chem. Rev. 1992, 121, 155. [4]Reinhardt, R.; Fees, J.; Klein, A.; Sieger, M.; Kaim, W. in Wasserstoff als Energieträger, VDI Verlag, Düsseldorf, 1994, p 133. [5]Bruns, W.; Kaim, W.; Waldhör E.; Krejcik, M. Inorg. Chem 1995, 34, 663. [6] Tributsch, J. J. Eletroanal. Chem. 1992, 331, 783. [7](a) Kölle U.; Grätzel, M. Angew. Chem. 1987, 99, 572; Angew. Chem. Int. Ed. Engl. 1987, 26, 568. (b) Kölle, U.; Kang. B.-S.; Infelta, P.; Comte, P.; Grätzel, M. Chem. Ber. 1989, 112, 1869. [8] Cosnier, S.; Deronzier, A.; Vlachopoulos, N. J. Chem. Soc., Chem. Commun. 1989, 1259. [9] (a) Ladwig, M.; Kaim, W. J. Organomet. Chem. 1991, 419, 233. (b) Reinhardt R.; Kaim, W. Z. Anorg. Allg. Chem. 1993, 619, 1998. (c) Greulich, S.; Kaim, W.; Stange, A.; Stoll, H.; Fiedler, J.; Zalis, S. Inorg. Chem., in Teilprojekt von sfb 270 Nr. 73 print. [10] Ladwig, M.; Kaim, W. J. Organomet. Chem. 1992, 439, 79. [11]Reinhardt, R. Ph.D. Thesis, University of Stuttgart, 1995. [12]Astruc, D. Electron Transfer and Radical Processes in Transition-Metal Chemistry, VCH, Weinheim, 1995.