Experimental and Computational Studies of Binding of Dinitrogen, Nitriles, Azides, Diazoalkanes, Pyridine, and Pyrazines to M(PR₃)₂(CO)₃ (M = Mo, W; R = Me, ⁱPr).

Abstract

The enthalpies of binding of a number of N-donor ligands to the complex Mo(P(i)Pr(3))(2)(CO)(3) in toluene have been determined by solution calorimetry and equilibrium measurements. The measured binding enthalpies span a range of approximately 10 kcal mol(-1): DeltaH(binding) = -8.8 +/- 1.2 (N(2)-Mo(P(i)Pr(3))(2)(CO)(3)); -10.3 +/- 0.8 (N(2)); -11.2 +/- 0.4 (AdN(3) (Ad = 1-adamantyl)); -13.8 +/- 0.5 (N(2)CHSiMe(3)); -14.9 +/- 0.9 (pyrazine = pz); -14.8 +/- 0.6 (2,6-Me(2)pz); -15.5 +/- 1.8 (Me(2)NCN); -16.6 +/- 0.4 (CH(3)CN); -17.0 +/- 0.4 (pyridine); -17.5 +/- 0.8 ([4-CH(3)pz][PF(6)] (in tetrahydrofuran)); -17.6 +/- 0.4 (C(6)H(5)CN); -18.6 +/- 1.8 (N(2)CHC( horizontal lineO)OEt); and -19.3 +/- 2.5 kcal mol(-1) (pz)Mo(P(i)Pr(3))(2)(CO)(3)). The value for the isonitrile AdNC (-29.0 +/- 0.3) is 12.3 kcal mol(-1) more exothermic than that of the nitrile AdCN (-16.7 +/- 0.6 kcal mol(-1)). The enthalpies of binding of a range of arene nitrile ligands were also studied, and remarkably, most nitrile complexes were clustered within a 1 kcal mol(-1) range despite dramatic color changes and variation of nu(CN). Computed structural and spectroscopic parameters for the complexes Mo(P(i)Pr(3))(2)(CO)(3)L are in good agreement with experimental data. Computed binding enthalpies for Mo(P(i)Pr(3))(2)(CO)(3)L exhibit considerable scatter and are generally smaller compared to the experimental values, but relative agreement is reasonable. Computed enthalpies of binding using a larger basis set for Mo(PMe(3))(2)(CO)(3)L show a better fit to experimental data than that for Mo(P(i)Pr(3))(2)(CO)(3)L using a smaller basis set. Crystal structures of Mo(P(i)Pr(3))(2)(CO)(3)(AdCN), W(P(i)Pr(3))(2)(CO)(3)(Me(2)NCN), W(P(i)Pr(3))(2)(CO)(3)(2,6-F(2)C(6)H(3)CN), W(P(i)Pr(3))(2)(CO)(3)(2,4,6-Me(3)C(6)H(2)CN), W(P(i)Pr(3))(2)(CO)(3)(2,6-Me(2)pz), W(P(i)Pr(3))(2)(CO)(3)(AdCN), Mo(P(i)Pr(3))(2)(CO)(3)(AdNC), and W(P(i)Pr(3))(2)(CO)(3)(AdNC) are reported.

Coordination-Mode Control of Bound Nitrile Radical Complex Reactivity: Intercepting End-on Nitrile−Mo(III) Radicals at Low Temperature

“…The amide N exhibits trigonal planar geometry in 1- [NCNMe 2 ] for both η 1 and η 2 conformers. This behavior has been previously reported for other complexes such as trans -[Mo(N 2 )(NCNEt 2 )(dppe) 2 ], W(P[ i Pr] 3 ) 2 (CO) 3 (Me 2 NCN), Cr(CO) 5 (NCNEt 2 ), and trans -[Fe(Et 2 PCH 2 CH 2 PEt 2 ) 2 (NCNEt 2 ) 2 ][BF 4 ] 2 …”

“…The amide N exhibits trigonal planar geometry in 1-[NCNMe 2 ] for both η 1 and η 2 conformers. This behavior has been previously reported for other complexes such as trans-[Mo(N 2 )(NCNEt 2 )(dppe) 2 ], 15 W(P[ i Pr] 3 ) 2 (CO) 3 (Me 2 -NCN), 16 Cr(CO) 5 (NCNEt 2 ), 37 38 DFT calculations at the BP86/6-311G(d,p) (MWB28 with an additional set of f functions for Mo) level predict a ∆E 0 ) -16.3 kcal mol -1 for formation of 1-[η 1 -NCPh] which compares to the experimentally determined value of ∆H ) -14.5 ( 1.5 kcal mol -1 . For the formation of 1-[η 2 -NCPh], ∆E 0 ) -10.3 kcal mol -1 , implying that for PhCN the η 2 binding motif is less stable than is the η 1 .…”

“…While there is no doubt that steric factors play a dominant role in ligand binding to 1 , electronic factors may be even more important. For the complexes 1- [η 1 -NCAd] and 1- [η 1 -CNAd] the steric ligand profile must be similar, yet the difference in enthalpies of binding, ≈ 24 kcal mol −1 is a much larger difference than typically seen in comparative thermochemistry of nitriles and isonitriles in binding to transition metals. ,, …”

“…Despite the importance of the metal-nitrile bond there are surprisingly few solution phase studies of bond energetics even for bonding in the terminal mode to diamagnetic complexes. We recently reported thermochemical and computational study of bond energetics of a range of nitriles and other N donor ligands to Mo(CO) 3 (P[ i Pr] 3 ) 2 . While nitriles bound ∼6 kcal mol −1 more strongly than N 2 , changes in R that caused significant changes in IR frequencies and dramatic changes in color resulted in no real difference in bond strength for the nitriles in eq : Δ H = 0.0 ± 0.4 kcal mol −1 , AdCN = 1-adamantyl nitrile, Ar = 4-NMe 2 C 6 H 4 ; 2,4,6-Me 3 C 6 H 2 ; 2,6-F 2 C 6 H 3 ; 4-CF 3 C 6 H 4 ; F 5 C 6 .…”

“…The lone pair of electrons on the dialkyl amino group of R 2 N-CtN can become involved in the bonding and reactivity of mixed dinitrogen organocyanamide complexes of Mo(0) as reported also by Pombeiro and co-workers. 15 Notably, the crystal structures of both trans-[Mo(N 2 )(NCNEt 2 )(dppe) 2 ] 15 and W(P[ i Pr] 3 ) 2 (CO) 3 (NCNMe 2 ) 16 have trigonal planar geometry of the amide group.…”