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. 2020 Jul 27;39(14):2594-2601.
doi: 10.1021/acs.organomet.0c00167. Epub 2020 Apr 24.

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes

Jan Pecak  1 , Wolfgang Eder  1 , Berthold Stöger  2 , Sara Realista  3 , Paulo N Martinho  4 , Maria José Calhorda  4 , Wolfgang Linert  1 , Karl Kirchner  1
Affiliations

Synthesis, Characterization, and Catalytic Reactivity of {CoNO}8 PCP Pincer Complexes

Jan Pecak et al. Organometallics. .

Abstract

The reaction of coordinatively unsaturated Co(II) PCP pincer complexes with nitric oxide leads to the formation of new, air-stable, diamagnetic mono nitrosyl compounds. The synthesis and characterization of five- and four-coordinate Co(III) and Co(I) nitrosyl pincer complexes based on three different ligand scaffolds is described. Passing NO through a solution of [Co(PCPNMe-iPr)Cl], [Co(PCPO-iPr)Cl] or [Co(PCPCH2-iPr)Br] led to the formation of the low-spin complex [Co(PCP-iPr)(NO)X] with a strongly bent NO ligand. Treatment of the latter species with (X = Cl, Br) AgBF4 led to chloride abstraction and formation of cationic square-planar Co(I) complexes of the type [Co(PCP-iPr)(NO)]+ featuring a linear NO group. This reaction could be viewed as a formal two electron reduction of the metal center by the NO radical from Co(III) to Co(I), if NO is counted as NO+. Hence, these systems can be described as {CoNO}8 according to the Enemark-Feltham convention. X-ray structures, spectroscopic and electrochemical data of all nitrosyl complexes are presented. Preliminary studies show that [Co(PCPNMe-iPr)(NO)]+ catalyzes efficiently the reductive hydroboration of nitriles with pinacolborane (HBpin) forming an intermediate {CoNO}8 hydride species.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Chart 1
Chart 1. Examples of Group 9 Nitrosyl Pincer Complexes
Figure 1
Figure 1
Structural view of 1b showing 50% displacement ellipsoids (H atoms omitted for clarity). Selected bond lengths [Å] and angles [°]: Co1–C1 1.9131(9), Co1–Cl1 2.2258(4), Co1–P1 2.1879(4), Co1–P2 2.1898(3), P1–Co1–P2 162.90(1), C1–Co1–Cl1 178.88(5).
Figure 2
Figure 2
Structural view of 1c showing 50% displacement ellipsoids (H atoms and a second independent complex omitted for clarity). Selected bond lengths [Å] and angles [°]: Co1–C1 1.955(1), Co1–Br1 2.3764(2), Co1–P1 2.2077(4), Co1–P2 2.2124(4), P1–Co1–P2 169.39(2), C1–Co1–Br1 177.49(4).
Scheme 1
Scheme 1. Synthesis of Complexes 2a–c
Figure 3
Figure 3
Structural view of 2a showing 50% displacement ellipsoids (H atoms omitted for clarity). Selected bond lengths [Å] and angles [°]: Co1–Cl1 2.2947(12), Co1–C1 1.959(2), Co1–P1 2.227(1), Co1–P2 2.236(1), Co1–N3 1.736(2), N3–O1 1.164(3), P1–Co1–P2 155.88(2), C1–Co1–Cl1 151.25(6), Co1–N3–O1 140.1(2).
Figure 4
Figure 4
Structural view of 2b showing 50% displacement ellipsoids (H atoms omitted for clarity). Selected bond lengths [Å] and angles [°]: Co1–Cl1 2.2823(5), Co1–C1 1.939(2), Co1–P1 2.2298(5), Co1–P2 2.2070(5), Co1–N1 1.726(1), N1–O3 1.176(2), P2–Co1–P1 154.58(2), C1–Co1–Cl1 150.69(5), Co1–N1–O3 140.2(1).
Figure 5
Figure 5
SEC-IR experiments with [Co(PCPNMe-iPr)(NO)Cl] (2a, 5 mM in CH2Cl2) at 1.00 V vs Ag pseudoreference electrode. The absorption at 1654 cm–1, typical for a bent NO alignment, gradually shifts to a new resonance at 1771 cm–1 typical of a linear NO coordination mode (OCP = Open Circuit Potential).
Figure 6
Figure 6
HOMO of [Co(PCPNMe-iPr)(NO)Cl] (2a, left) and the calculated spin density in open-shell [Co(PCPNMe-iPr)(NO)Cl]+ obtained after electrochemical oxidation (right).
Scheme 2
Scheme 2. Synthesis of Complexes 3a and 3b
Figure 7
Figure 7
Structural view of 3a showing 50% displacement ellipsoids (H atoms and counterion omitted for clarity). Selected bond lengths [Å] and angles [°]: Co1–C1 1.929(4), Co1–P1 2.2086(11), Co1–P2 2.202(1), Co1–N3 1.631(3), N3–O1 1.173(4), P2–Co1–P1 164.57(4), C1–Co1–N3 176.0(2), Co1–N3–O1 176.6(3).
Figure 8
Figure 8
Structural view of 3b showing 50% displacement ellipsoids (H atoms and counterion omitted for clarity). Selected bond lengths [Å] and angles [°]: Co1–C1 1.932(2), Co1–P1 2.2042(8), Co1–P2 2.2023(7), Co1–N1 1.635(2), N3–O1 1.162(2), P2–Co1–P1 159.68(3), C1–Co1–N1 175.1(1), Co1–N1–O3 177.3(2).
Scheme 3
Scheme 3. Alternative Synthesis of Complex 3a
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