title: M-M bond-stretching energy landscapes for M2(dimen)4(2+) (M = Rh, Ir; dimen = 1,8-diisocyanomenthane) complexes.
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)).
authors.affiliation: California Institute of Technology, Pasadena, California 91125, USA.
authors.forename: Bryan M
authors.initials: BM
authors.lastname: Hunter
authors.forename: Randy M
authors.initials: RM
authors.lastname: Villahermosa
authors.forename: Christopher L
authors.initials: CL
authors.lastname: Exstrom
authors.forename: Michael G
authors.initials: MG
authors.lastname: Hill
authors.forename: Kent R
authors.initials: KR
authors.lastname: Mann
authors.forename: Harry B
authors.initials: HB
authors.lastname: Gray
mesh_terms: D003485:Cyanates / Q000737:chemistry*; D007495:Iridium / Q000737:chemistry*; D017953:Isocyanates; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D012211:Rhenium / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D003485:Cyanates; D017953:Isocyanates; D009942:Organometallic Compounds; D007495:Iridium; C059330:methylene diisocyanate; D012211:Rhenium