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M–M Bond-Stretching Energy Landscapes for M_2(dimen)_(4)^(2+) (M = Rh, Ir; dimen = 1,8-Diisocyanomenthane) Complexes

Hunter, Bryan M. and Villahermosa, Randy M. and Exstrom, Christopher L. and Hill, Michael G. and Mann, Kent R. and Gray, Harry B. (2012) M–M Bond-Stretching Energy Landscapes for M_2(dimen)_(4)^(2+) (M = Rh, Ir; dimen = 1,8-Diisocyanomenthane) Complexes. Inorganic Chemistry, 51 (12). pp. 6898-6905. ISSN 0020-1669. http://resolver.caltech.edu/CaltechAUTHORS:20120710-133619920

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Abstract

Isomers of Ir_2(dimen)_(4)^(2+) (dimen = 1,8-diisocyanomenthane) exhibit different Ir–Ir bond distances in a 2:1 MTHF/EtCN solution (MTHF = 2-methyltetrahydrofuran). Variable-temperature absorption data suggest that the isomer with the shorter Ir–Ir distance is favored at room temperature [K = ~8; ΔH° = −0.8 kcal/mol; ΔS° = 1.44 cal mol^(–1) K^(–1)]. We report calculations that shed light on M_2(dimen)_(4)^(2+) (M = Rh, Ir) structural differences: (1) metal–metal interaction favors short distances; (2) ligand deformational-strain energy favors long distances; (3) out-of-plane (A_(2u)) distortion promotes twisting of the ligand backbone at short metal–metal separations. Calculated potential-energy surfaces reveal a double minimum for Ir_2(dimen)_(4)^(2+) (4.1 Å Ir–Ir with 0° twist angle and ~3.6 Å Ir–Ir with ±12° twist angle) but not for the rhodium analogue (4.5 Å Rh–Rh with no twisting). Because both the ligand strain and A_(2u) distortional energy are virtually identical for the two complexes, the strength of the metal–metal interaction is the determining factor. On the basis of the magnitude of this interaction, we obtain the following results: (1) a single-minimum (along the Ir–Ir coordinate), harmonic potential-energy surface for the triplet electronic excited state of Ir_2(dimen)_(4)^(2+) (R_(e,Ir–Ir) = 2.87 Å; F_(Ir–Ir) = 0.99 mdyn Å^(–1)); (2) a single-minimum, anharmonic surface for the ground state of Rh_2(dimen)_(4)^(2+) (R_(e,Rh–Rh) = 3.23 Å; F_(Rh–Rh) = 0.09 mdyn Å^(–1)); (3) a double-minimum (along the Ir–Ir coordinate) surface for the ground state of Ir_2(dimen)_(4)^(2+) (R_(e,Ir–Ir) = 3.23 Å; F_(Ir–Ir) = 0.16 mdyn Å^(–1)).


Item Type:Article
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http://dx.doi.org/10.1021/ic300716qDOIArticle
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Additional Information:© 2012 American Chemical Society. Publication Date (Web): May 23, 2012. We thank Jay Winkler for helpful discussions. Our work was supported by the NSF Center for Chemical Innovation (Grant CHE-0802907) and by the David & Lucille Packard Foundation Initiative for Interdisciplinary Research. B.M.H. is an NSF Graduate Fellow.
Group:CCI Solar Fuels
Funders:
Funding AgencyGrant Number
NSFCHE-0802907
David and Lucile Packard FoundationUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
Record Number:CaltechAUTHORS:20120710-133619920
Persistent URL:http://resolver.caltech.edu/CaltechAUTHORS:20120710-133619920
Official Citation:M–M Bond-Stretching Energy Landscapes for M2(dimen)42+ (M = Rh, Ir; dimen = 1,8-Diisocyanomenthane) Complexes Bryan M. Hunter, Randy M. Villahermosa, Christopher L. Exstrom, Michael G. Hill, Kent R. Mann, and Harry B. Gray Inorganic Chemistry 2012 51 (12), 6898-6905
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:32334
Collection:CaltechAUTHORS
Deposited By: Jason Perez
Deposited On:10 Jul 2012 23:12
Last Modified:23 Aug 2016 10:14

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