Olefin Substitution in (silox)₃M(olefin) (silox = ^tBu₃SiO; M = Nb, Ta):  The Role of Density of States in Second vs Third Row Transition Metal Reactivity

Abstract

Abstract:The substitution chemistry of olefin complexes (silox)3M(ole) (silox ) t Bu3SiO; M ) Nb (1-ole), Ta (2-ole); ole ) C2H4 (as 13 C2H4 or C2D4), C2H3Me, C2H3Et, cis-2-C4H8, iso-C4H8, C2H3Ph, c C5H8, c C6H10, c C7H10 (norbornene)) was investigated. For 1-ole, substitution was dissociative (∆G q diss), and in combination with calculated olefin binding free energies (∆G°bind), activation free energies for olefin association (∆G q assoc) to (silox)3Nb (1) were estimated. For 2-ole, substitution was not observed prior to rearrangement to alkylidenes. Instead, activation free energies for olefin association to (silox)3Ta (2) were measured, and when combined with ∆G°bind (calcd), estimates of olefin dissociation rates from 2-ole were obtained. Despite stronger binding energies for 1-ole vs 2-ole, the dissociation of olefins from 1-ole is much faster than that from 2-ole. The association of olefins to 1 is also much faster than that to 2. Linear free energy relationships (with respect to ∆G°bind) characterize olefin dissociation from 1-ole, but not olefin dissociation from 2-ole, and olefin association to 2, but not olefin association to 1. Calculated transition states for olefin dissociation from (HO)3M(C2H4) (M ) Nb, 1′-C2H4; Ta, 2′-C2H4) are asymmetric and have orbitals consistent with either singlet or triplet states. The rearrangement of (silox)3Nb(trans-Vy,Ph-c Pr) (1-VyPh c Pr) to (silox)3NbdCHCHd CHCH2CH2Ph (3) is consistent with a diradical intermediate akin to the transtion state for substitution. The disparity between Nb and Ta in olefin substitution chemistry is rationalized on the basis of a greater density of states (DOS) for the products (i.e., (silox) 3M + ole) where M ) Nb, leading to intersystem crossing events that facilitate dissociation. At the crux of the DOS difference is the greater 5dz2/6s mixing for Ta vs the 4dz2/5s mixing of Nb. This rationalization is generalized to explain the nominally swifter reactivities of 4d vs 5d elements.

Lewis Bases Trigger Intramolecular CH–Bond Activation: (tBu3SiO)2W=NtBu [rlhar2] (tBu3SiO)(κO,κC‐tBu2SiOCMe2CH2)HW=NtBu

“…That promotional energy is modest for Nb with its high DOS, but for Ta it represents the bulk of the enthalpic activation energy. [17,22] Related findings have been noted in the swift oxygen atom transfer (OAT) reactivity of transient (silox) 3 Nb versus the slow OAT activity of the stable (silox) 3 -Ta. [16] The promotional energies also factor into the relative stabilities of (silox) 3 MCCM(silox) 3 (M = Nb, Ta).…”

“…[1] The electronic state [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] of the activating species is a consideration that is often overlooked, mostly because its importance is often difficult to discern by experimental methods. Previous research on group 5 d 2 systems has shown that the course of olefin substitution in (silox) 3 M(ole) (M = Nb, Ta; ole = C 2 H 4 , C 2 H 3 Me, C 2 H 3 Et, cis-2-C 4 H 8 , iso-C 4 H 8 , C 2 H 3 Ph, c-C 5 H 8 , c-C 6 H 10 , norbornene; silox = OSi t Bu 3 ) is dependent on the density of states (DOS).…”

“…Previous research on group 5 d 2 systems has shown that the course of olefin substitution in (silox) 3 M(ole) (M = Nb, Ta; ole = C 2 H 4 , C 2 H 3 Me, C 2 H 3 Et, cis-2-C 4 H 8 , iso-C 4 H 8 , C 2 H 3 Ph, c-C 5 H 8 , c-C 6 H 10 , norbornene; silox = OSi t Bu 3 ) is dependent on the density of states (DOS). [17] The high DOS in the Nb system renders the transition states for olefin dissociation/association energetically flat, whereas in the low DOS Ta cases, the transition states are dependent on the nature of the olefin. The relatively low DOS for the Ta system is a product of far greater nd/(n+1)s mixing [16][17][18][19][20][21][22][23] for the third row transition element (n = 5) relative to its second row congener (n = 4).…”

Molybdenum and Tungsten Structural Differences are Dependent onndz2/(n+ 1)s Mixing: Comparisons of (silox)3MX/R (M = Mo, W; silox =tBu3SiO)

“…For example, different rates of olefin dissociation in (silox) 3 M(olefin) and association to (silox) 3 M (M ) Nb, Ta; silox ) t Bu 3 SiO) were traced to the higher density of states of niobium versus tantalum. 10 The stability of (silox) 3 Ta relative to (silox) 3 Nb -a plausible species that has never been directly observed -may stem from the relatively lower energy of 5d z 2 , whose torus is attenuated from mixing with the 6s. Investigations of oxygen atom transfer to d 2 (silox) 3 M (M ) V, NbL (L ) 4-picoline, PMe 3 ), Ta revealed orbital symmetry constraints in which the energy of the d z 2 orbital plays a prominent role.…”

“…In three-coordinate molecules, changes in the torus of d z 2 do not effect the general trigonal geometry observed. 10,11 In higher-coordinate species, the orbital is σ* and empty, and its filled, bonding counterpart is unlikely to be the origin of a geometric difference. For example, in coordination chemistry studies of biological relevance, molybdenum and tungsten are often complementary in model complexes in part because the metric parameters in relevant 5-and 6-coordinate compounds are so similar.…”